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Data will be made available on request.Due to the dependence on the non-renewable energy sources, there is a serious increase in environmental problems. With increasing environmental problems and decreasing fossil fuels, it is essential to seek alternative energy sources. In this context, hydrogen, which is a sustainable, low-cost, environmentally friendly, and highly efficient energy source, stands out as a green energy carrier [1–7].In order to use hydrogen actively today, it must be produced, stored, and transported efficiently. At this point, promising improvements have been experienced with the use of chemical hydrides such as NaBH4, LiBH4, NH3BH3, LiH, NaH, CaH2, MgH2, and AlH3. Among these hydrides, NaBH4 comes forward as a hydrogen storage material due to its advantages such as high hydrogen capacity (10.8%), recyclability of byproducts, non-flammability, chemical stability, and sustainability [2,8,9]. The release of stored NaBH4 occurs by hydrolysis and methanolysis reactions [10–13]. (1) Na BH 4 + 2 H 2 O → catalyst Na BO 2 + 4 H 2 ↑ + heat − 300 kj / mol (2) Na BH 4 + 4 CH 3 OH → NaB OCH 3 4 + 4 H 2 Hydrolysis of NaBH4 for hydrogen production is quite cheap and simple, but the reaction temperature can limit this application. For this reason, methanol is frequently used as an alternative method for hydrogen release by a methanolysis reaction due to its suitable reaction rate and high activity at low temperatures (≤ 0 °C) [14–16].In NaBH4 hydrolysis and methanolysis reactions, the lack of a catalyst slows down the reaction to a great extent. Thus, researchers focused on new types of catalysts to accelerate the reaction. These catalysts were homogeneous catalysts containing acid or metal complexes and heterogeneous catalysts containing metal or non-metal catalysts [17,26,27]. Although both catalyst types accelerate the reaction as reported in these studies, heterogeneous catalysts have become more advantageous due to the fact that homogeneous catalysts are not deactivated; they are irreversible, and their industrial usage are limited [18–25]. In addition to all these, catalysts such as carbon nanotubes, carbon spheres and particles, etc. which do not contain metal and are generally carbon-based, are frequently preferred today due to their environmentally friendly, low cost and stable catalytic activity nature [28–32].Carbon spheres are attracting a lot of attention due to their smooth surfaces, unique structures, and potential applications (such as in drug release, hydrogen storage, catalyst supports and lithium batteries). Researchers have developed different methods for the preparation of carbon spheres, such as hydrothermal synthesis, chemical vapor deposition, and carbonization pathways [33–35].However, alternative sources are needed for the preparation of carbon spheres. Among them, starch is one of the most abundant natural biopolymers in nature. It contains two different polysaccharides, namely linear (1, 4)-linked α-D-glucan amylose and highly (1, 6)-branched α-D-glucan amylopectin. Additionally, it has advantages such as being available from plant sources, being biodegradable and being a cheap product. Starch is generally produced from raw materials such as wheat, corn, peas, and potatoes. Among them, potato starch is a very suitable material due to its spherical morphology and the preparation of micron-sized carbon spheres [36–41]. Starch is used in a wide range of areas such as food, paper, textile, and pharmaceutical industries. It has also been used as a template for synthesizing alloy and metal nanoparticles in hydrogen production with NaBH4 [42,43].There are many carbon based catalysts in the literature such as CNTs, graphene, C60, carbon quantum dots, carbon fibers, activated carbon, carbon black, carbon nitride, boron carbon nitride, covalent organic frameworks, metal-organic frameworks etc. In this study, starch being one of the natural, inexpensive, accessible polysaccharide biopolymer was used as a carbon source for the production of carbon spheres via hydrothermal synthesis. Hydrothermal carbonization has received much attention as a promising large-scale application. The method is simple and inexpensive. Also modifications were made with simple mechanisms and increased the performance of the catalyst. Modified forms of carbon spheres used as the catalysts in hydrogen production by NaBH4 methanolysis was reported for the first time with this study. The synthesized catalysts were characterized by using Fourier-transform infrared spectrometer (FT-IR), thermogravimetric analyzer (TGA), zeta potentiometry (ZP), and scanning electron microscopy (SEM). The parameters affecting the methanolysis such as ambient temperature, amount and type of catalyst and NaBH4 concentration were investigated. Activation parameters such as activation energy (Ea), activation enthalpy (ΔH#) and activation entropy (ΔS#) and also hydrogen (H2) production rate were determined in the temperature range of 273–303 K. In addition, catalysts reuses, and regeneration efficiencies were investigated.In the synthesis of carbon spheres (CSs), potato starch (Sigma-Aldrich) as a carbon source and potassium hydroxide (KOH, ≥ 85%, Sigma-Aldrich) for the preparation of alkaline conditions were used. Iron(II)chloride tetrahydrate (FeCl2.4H2O, Sigma-Aldrich) and ammonia (25%, Merck) were used for magnetic property. Ethylenediamine (EDA, 99.0%, Sigma-Aldrich), taurine (TA, 99.0%, Sigma-Aldrich), and poly(ethyleneimine) (PEI, 50% solution in water, Sigma-Aldrich) and epichlorohydrin (ECH, 99%, Sigma-Aldrich) were used as modifying agents, and dimethylformamide (DMF, ≥ 99.0%, Isolab) was used as reaction medium. Sodium hydroxide (NaOH, 98%, Sigma-Aldrich) was used for the deprotonation before modification. Hydrochloric acid (HCl, 36.5–38%, Sigma-Aldrich) was used for protonation of modified CSs. Sodium borohydride (NaBH4, 98%, Merck) was used as hydrogen carrier/source and methanol (≥ 99.8%, Tekkim) was used as reaction medium in hydrogen gas production. Ethanol (≥ 99.9%, Isolab) and distilled water (DW) were used for cleaning/purification throughout the experiments.Natural resources like starch were used as a carbon source when creating carbon spheres. Aqueous solutions of starch were prepared freshly before hydrothermal synthesis. Optimum condition for the spherical uniform CS synthesis were studied via altering all effective parameters. CS synthesis carried out in alkali condition. Starch (0.3 M) was put into KOH solution (10 mM, 70 mL) and then the mixture was immersed in oil bath pre-heating at 80 °C. After 30 min, the obtained solutions were put into Teflon autoclaves and placed in an oven at chosen temperature. To ensure the dehydration process, these prepared solutions/mixtures were maintained at the working temperature for the enough time. The autoclave was taken out of the oven after this time and quickly left to cool. The cooled autoclave was opened, and the solid portion was removed using centrifugation or decantation. The obtained product (CS) was cleaned by soaking in DW for 2 days. The washing water was renewed every 3–4 h to remove unreacted species. After the cleaning process, the CSs were dried in an oven at 50 °C for one day and taken in a closed tube.To decide mass production condition, effect of the same parameters as starch concentration (0.1–0.4 M), base (KOH) concentration (1.0–100.0 mM), reaction temperature (120–200 °C), and reaction time (6–24 h) was evaluated. After CS synthesis, second hydrothermal process was applied for the production of mag-CSs. For this, a method similar to the method described in the literature was followed [44]. For the preparation of mag-CS, 0.3125 g FeCl2.4H2O was dissolved in 10.0 mL water and stirred for 10–15 min; then 25% ammonia (1.6 mL) was added and stirring continued for another 10–15 min. Then, 0.625 g of CS was added to the mixture and mixed for another 5 min. The mixture was then transferred to the Teflon autoclave. The autoclave was placed in the oven at 180 °C for 3 h. At the end of the period, mag-CS was cleaned with a water-ethanol (1:1 v/v) mixture. Obtained mag-CS was dried in an oven or lyophilizer and then put the magnetic particles in the sealed vial for later use.CSs were modified using different sources having different functional groups used as catalysts in the methanolysis of NaBH4, as previously reported in the modification of Hal nanotube catalysts [9]. 4.0 g of CS was mixed with 0.3 M in 200 mL of NaOH (aq.) solution for 1 h. Then, the CS-containing solution was centrifuged three times at 4500 rpm for 10 min, followed by redispersion in distilled water to remove the residue NaOH after filtration. After base treatment, Na cation containing CS (CS-Na) was dried in a 50 °C oven for 1 day. The resulting solid (CS-Na) was used in other step to modification with EDA, TA, and PEI by using linking agent as ECH.Chemical modifications of CS with both EDA and PEI were carried out similarly to each other, with some changes in the literature [9]. The synthesis was briefly started by dispersing 4.0 g of EDA-Na in a 100 mL single-necked flask containing 55.0 mL of DMF. In order to get a good dispersion, the reaction flask was closed with a rubber septum and stirred at 600 rpm for about 15 min at RT. Then, the round bottom flask was dipped in an oil bath, and the temperature, which was set at 90 °C, was allowed to reach equilibrium for 5 min, and 3.0 mL of ECH added dropwise to the CS-Na dispersion with a syringe and mixed at 700 rpm for 1 h. Then, 3.0 mL of ethylene diamine mixed with 5.0 mL of DMF and dropped into the mixture through a syringe, with the reaction continuing for 20 more min after the final drop was injected. To get rid of contaminants like unreacted EDA, ECH, and NaCl, the EDA-CS was centrifuged at 4500 rpm, washed once with DMF, and then several times with an ethanol/water mix solution (1/1, v/v). The resulting EDA-CS was dried in an oven at 50 °C for 24 h.The modification of CS with PEI was also performed under identical circumstances as the EDA-CS modification procedure. The same quantity of CS-Na and ECH as used in the EDA-CS modification reaction was added to the DMF solution. Instead of EDA, 3.0 mL of PEI was added dropwise via syringe, and the reaction was continued for 1 h to establish linkages among CS and PEI with ECH to form PEI-CS. The washing and drying procedures were also performed using the same method as used in the EDA-CS approach described above. After dispersing 3.0 g of EDA-CS in 50.0 mL of acid solution (1.0 M HCl) for 1 h at room temperature with steady stirring at 400 rpm, the cleaned and dried modified CS was protonated with HCl. After that process, the protonated EDA-CSs were cleaned using excess water and continued with centrifugation several times at 4500 rpm before drying in an oven at 50 °C. The prefix “H” was then used to define samples as H-EDA-CS and H-PEI-CS. Both products were stored in closed tubes for the characterization and the catalysis reactions after drying.CSs were also modified with TA by using same technique given in Section 2.3.1. Briefly, 50.0 mL of DMF and 100 mL of reaction flask containing 1.0 g of CSs and 3.0 mL of ECH were added. The mixture was stirred at 90 °C for 1 h at 750 rpm. 3.0 mL of DW was used to dissolve 0.2 g of TA in a separate reaction vial. The mixture was stirred for an additional hour after the addition of the TA aqueous solution drop by drop. The elimination of unreacted ECH and TA was completed by washing TA-CS with an ethanol-water solution (1:1 v/v). The produced TA-CSs were put into closed vials for characterization and catalysis reaction after drying in an oven at 50 °C.Modified CS as called H-EDA-CS was used in the hydrogen production reaction as a catalyst via NaBH4 methanolysis. Catalyst performance was determined by measuring the amount of H2 gas released for the period of reaction using the water replacement method. The work was carried out in a 50.0 mL single neck reaction flask immersed in a temperature controlled 25 °C water bath. The reaction initiated by dispersing 50.0 mg of catalyst (H-EDA-CS) in 20.0 mL of methanol. Then 125 mM NaBH4 was added and the displacement of the released H2 gas with water at a stirring speed of 1000 rpm was continuously noted. Additionally, Hydrogen Generation Rate (HGR) were calculated changing some parameters such as NaBH4 concentration, type and amount of catalyst. To calculate activation parameters, the reaction was realized at different temperatures. Obtained results were given by comparing with each other and with the literature.Reuse of the H-EDA-CS catalyst was tested by using conversion% and catalytic activity%. The process started with dispersing 50.0 mg of H-EDA-CS catalyst in 20.0 mL of methanol and followed by the addition of 0.0965 g of NaBH4, and the reaction was continued until the hydrogen production finished. After the first hydrogen production finished, the same amount of NaBH4 was added to the reaction medium without adding a new amount of catalyst. This process was repeated 10 times in total. After 10 repeated uses, the catalyst washed tree times with an ethanol-water mixture (1:1 v/v), centrifuged at 4500 rpm for 10 min, and dried in an oven at 50 °C. Then the catalyst was regenerated according to the literature [9] by 30.0 mL HCl (1.0 M) for 0.5 h. After regeneration, the catalyst was reused in hydrogen production for three repetitive uses.To determine surface morphology of CS and its modified forms, a scanning electron microscope (SEM, Hitachi Regulus 8230) working at 1.0–10 kV was used. The sample was placed on carbon band-attached aluminum SEM stubs suspended with a drop of ethanol, after drying process the samples coated with several nanometers gold under vacuum, and the SEM image of the freeze-dried sample was determined. To characterize the magnetic properties of the mag-CS, Vibrating Sample Magnetometer (VSM, Lake Shore, 7407 model) was used. The FT-IR analyses of the CS-based catalyst and modified-CS were performed by using a Bruker Tensor FT-IR spectrophotometer in the 4000–400 cm−1 spectral range. A thermogravimetric analyzer was used to complete the thermal analyses of CS-based catalysts (TGA, SII EXSTAR 6000, Japan). The same amount of starch, bare CS, and CS-based catalysts (5 mg) were put in a TGA pan, and the weight loss against temperature was recorded by increasing the temperature from 30 to 1000 °C with a heating rate of 15 °C min−1 and a gas flow rate of 200 mL.min−1. Zeta potential measurements on CS-based catalyst before and after chemical modification were performed by dispersing the CS-based catalyst in 1 × 10−3 M KCl aqueous solution at approximately 0.1 mg mL−1 concentration at 25 °C using a zeta potential analyzer (Malvern Inst.).Starch was chosen because it is an environmentally benign, low-cost, and being biocompatible substance that may be employed as the catalyst after modification of the CS via different functional group containing agents. Hydrothermal synthesis was used to obtain CSs. The reaction conditions and SEM images of obtained CSs were given in the Table 1 . To determine optimum conditions, the effect of the starch concentration, base concentration, reaction temperature, and reaction time was systematically studied. In reactions carried out at temperatures below 160 °C, the CS yield was either very low or no product could be obtained. Polydisperse spheres were obtained when the starch concentration was >0.3 M and the base concentration was <10 mM. The results were evaluated based on the yield and the homogeneity. Optimum conditions in CS preparation by hydrothermal synthesis were to keep the mixture which was prepared with 0.3 M starch and 10 mM KOH in an oven at 200 °C for 18 h. Thus, the CSs used in all of the studies were synthesized by the method given above.Magnetic CSs were prepared in two steps and optimized conditions were used for the first step. Subsequently, the obtained CSs were mixed with the production medium of Fe3O4 particles to obtain mag-CSs. Fe3O4 magnetic particles were freshly formed in the basic medium of CSs applying second time hydrothermal treatment. Obtained mag-CSs digital photo in aqueous media by applying magnetic field and VSM result was given in supplementary file as Fig. S1. The synthesized magnetic CS particles were dispersed in water. The magnet was located close to this vial, and it was observed that the mag-CS were rapidly oriented towards the magnet (Fig. S1a). This sensitivity will help to ensure that the application potential of the particles is high, and when used as a catalyst, it will help to slow down the reaction rate or to stop it. VSM result is given in the Fig. S1b. The results for mag-CS, Hci: 68.929 Oe, Ms.: 0.22891 emu, Mr.: 25.841E-3 emu, slope at Hc: 393.83E-6 emu/Oe were obtained.The SEM images of obtained CS were shown in Fig. 1a with different magnifications. It is apparent that the synthesized CS is in spherical shape, smooth and has a diameter smaller than 8 μm. New functional groups were also introduced on the CS by using different modifying agent. Epoxy groups are known to be reactive in alkaline conditions [9,45]. Therefore, the activation of CSs was achieved by using alkaline solution (0.3 M NaOH, RT, 1 h), thus increasing the density of –O−Na+ (deprotonation of –OH) on the CS surface. Fig. 1b illustrates CS formation and its modifications. Accordingly, amine modification of CS in DMF occurred in two steps: The first step was the rapid reaction of hydroxyl groups forming CS-Na dispersion with ECH, and the second step was the addition of amine sources to the medium to form modified CSs.Additionally, some modified CSs were protonated by HCl treatment, and the samples were coded as using the prefix H-. FT-IR spectroscopy, TGA and ZP measurements were used to characterize the CS and its modified forms. FT-IR spectra of both CS and modified CS were shown in Fig. S2 (see Supplementary Information). In CS spectra, the broad band between 3600 and 3000 cm−1 correspond to hydroxyl group vibration. A peak at 2926 cm−1 can be attributed −CH vibrations. The peak at 1208 cm−1 shows the C − O stretching. In modified forms, the peaks/band at 3400–3200 cm−1 for NH stretching and 2930 cm−1 for -CH stretching from modified amine sources.TGA was used to determine the thermal stability of starch, bare CS, and modified CS. Fig. S3 shown the TGA thermogram. The thermogram showed that bare CS had high thermal stability and a primary decomposition temperature of between 350 and 550 °C. The residual mass at 900 °C was determined as 50% for bare CS. The degradation temperature of the modified CS and starch (precursor) was found to be 38 and 20% at 900 °C for H-EDA-CS and starch, respectively. Because of the presence of amine groups, main degradation occurred earlier (200–450 °C) in modified CS.Zeta potential measurements were performed following CS changes in order to understand the catalytic performance of the synthesized catalyst and as a control modification. Table 2 shows the recorded zeta potential values for all of the catalysts. The zeta potential values of the samples were determined by mixing the CS with 1.0 mM KCl at 25 °C.Due to the -OH group on the surface, CS had a negative ZP value (−63.4 mV), but after amine derivatization, the ZP values for PEI and EDA modification agents resulted with a change from negative to positive. Because of the presence of sulfonic acid, the ZP of TA modified CS is slightly higher when compared to bare CS. When the bare CS had −63.4 mV ZP value, after second step modification, protonation of EDA and PEI modified CS, ZP values increased too much as +45.5 mV and + 48.3 mV for H-PEI-CS and H-EDA-CS, respectively.In the current study, CSs obtained systematic modification using a variety of amine group-containing resources before being protonated with HCI. In the methanolysis of NaBH4, these amine modified CSs acted as a catalyst in the production of H2. From the H2 evolution curves, the HGR, measured in mL.min−1 or mL.min−1.g−1, was calculated. Based on the graphs of the H2 generation vs time, the rate of reaction was determined at the half point of the NaBH4 conversion slopes (r50).The effects of CSs modified with different amine groups on H2 production were determined by using them as catalysts in the NaBH4 dehydrogenation reaction in methanol. To clarify activity of the catalyst, in a parallel reaction settings control experiment was performed in the absence of the catalyst. Self-dehydrogenation of the NaBH4 and modified CS-based catalysts generated the equal quantity of hydrogen gas from NaBH4 dehydrogenization in methanol and achieved 100% conversions at varied rates of reaction, as seen in Fig. 2 .As shown in Fig. 2, the hydrogen production rate was calculated to be 19 mLmin−1 for self-methanolysis and 26 mL.min−1 for CS (HGR: 529 mL.min−1.g−1). Hydrogen production rates for mag-CS, TA-CS, PEI-CS, H-PEI-CS, EDA-CS and H-EDA-CS were calculated to be 33, 46, 24, 36, 30 and 85 mL.min−1, respectively, while HGR values were calculated as 668, 930, 481, 713, 608, and 1705 mL.min−1.g−1. When modified CS was used as a catalyst, the reaction rate increased, and the reaction was completed faster than self-one by 100% conversion. The dehydrogenation reaction of NaBH4 in methanol is considered an acid-catalyzed reaction [16]. When the catalytic effects or reaction rates of the modified CS catalysts in the NaBH4 dehydrogenation in methanol were evaluated, it was realized that the reaction rate increased with the use of the catalyst with a high acid character. When the measured zeta potential values of the catalysts are examined, it is seen that the highest value belongs to H-EDA-CS with +48.3 mV (Table 2). According to the literature, the existence of NaCl in the reaction medium increases the rate of a reaction by increasing the concentration of CH3OH2 +. Therefore, catalysts protonated with HCl (H-PEI-CS and H-EDA-CS) performed better than PEI and EDA modified CS (non-protonated form) and bare CS. Among CS and its modified forms, H-EDA-CS spheres were found to be the most efficient, therefore, the study continued with the use of H-EDA-CS spheres as catalysts and the impact of different factors on the dehydrogenation of NaBH4 in methanol was examined. Activation parameters of the reaction were calculated in the range of 273–303 K by using H-EDA-CS catalyst.To evaluate catalyst effect on the dehydrogenation of NaBH4, the reaction was carried out using 12.5, 25, 50, and 75 mg of H-EDA-CS catalyst while holding all other parameters constant. Fig. 3a shows that changing the catalyst amount had no effect on the amount of H2 produced. The reaction rate increased from 43 mL.min−1 to 96 mL.min−1 with the addition of 75 mg of catalyst. In Fig. 3b, however, as catalyst amount increases from 12.5 mg to 75 mg, contrary to the reaction rate, the HGR decreases from 3460 mL.min−1.g−1 to 1282 mLmin−1 g−1, respectively. Slope of ln(H2 production rate) vs ln(NaBH4 conc.) graph was calculated to be 0.4741. Based on the amount of H-EDA-CS catalyst, the reaction kinetic can be assumed to be between pseudo-zero-order and pseudo-first-order. For the catalysis of such metholysis reaction, various catalys have been reported as the catalysts providing both pseudo-zero order and first order kinetic results. A condition that could lead to zero order rates; when two or more reactants are involved, One is used in a small amount and the others in higher amount. This situation commonly occurs when a reaction is catalyzed on a solid surface (heterogeneous catalysis).Five different NaBH4 concentrations were studied using 50 mg H-EDA-CS catalyst with the given condition in order to investigate the initial NaBH4 concentration effect on the rate of hydrogen production in NaBH4 dehydrogenation.The plot of H2 produced volume as a consequence of time shows that the amounts of H2 produced and the rate of H2 production increase proportionally when the concentration of NaBH4 increases, as shown in Fig. 4a. As seen in Fig. 4b, when the NaBH4 concentration is increased from 50 mM to 250 mM, the HGR increases from 673 mL.min−1.g−1 to 2933 mL.min−1.g−1. With the increase in the amount of NaBH4, the volume of H2 produced per unit time increases. The slope of the graph of ln(H2 production rate) vs ln(NaBH4 concentration) was found to be 0.9201 (Fig. 4c). This value confirms that the reaction kinetic is first order with respect to NaBH4 concentration.Temperature effect on NaBH4 dehydrogenation reaction catalyzed by CS and modified CS in methanol was examined by carrying out the reaction at four different temperatures, 273, 283, 293 and 303 K, using 125 mM NaBH4 in 20 mL methanol solution. The rate constants at different temperatures (273 K–303 K) were calculated by using R50 value. Then the calculated “k” values for the dehydrogenation of NaBH4 catalyzed by CS and modified CS catalysts were used to construct graphs as lnk - 1/T from Arrhenius Eq. (3) and ln(k/T) - 1/T from Eyring Eq. (4). Activation parameters (Ea, ΔH#, and ΔS#) for the dehydrogenation reaction of NaBH4 with the use of H-EDA-CS as a catalyst were calculated. (3) ln k = ln A − Ea RT (4) ln k T = ln k B h + ∆ S # R − ∆ H # R 1 T where, k is the rate constant, Ea is the activation energy, R is the gas constant (8.314 J.K−1.mol−1), T is temperature, kB is Boltzmann constant (1.381 × 10−23 J.K−1), h is Planck constant (6.626 × 10−34 J.s), ΔH# is activation enthalpy, ΔS# is the activation entropy. Fig. 5a shows a graph of reaction temperature vs produced volume of hydrogen for NaBH4 dehydrogenation in methanol. When the effect of temperature on the dehydrogenation of NaBH4 with methanol is examined, the reaction time shortens with the increase in temperature, and all of the reactions at various temperatures produced approximately 250 mL H2. When the reaction temperature was increased from 273 K to 303 K, the reaction rate increased from 27 mL.min−1 to 91 mL.min−1. Again with the same temperature increase, the HGR value increased from 534 mL.min−1.g−1 to 1824 mL.min−1.g−1 (Fig. 5b). At other temperatures, the rates of generated H2 were between these two limits.The lnk - 1/T graph using Eq. (3) and the ln(k/T) - 1/T graph using Eq. (4) were plotted for the H-EDA-CS catalyst at temperatures up from 273 K to 303 K. The related graphs are shown in Fig. 6a and b, respectively. A relatively low activation energy of 26.14 kJ.mol−1 was calculated between 273 and 303 K temperature values and the comparison of the result obtained with the literature is given in Table 3 .As seen in Table 3, the activation energy of the H-EDA-CS (26.14 kJ.mol−1) catalyst described in this study is comparable to the activation energies of metal-containing and metal-free catalysts published in the literature. For example, the Ea value for H-EDA-CS is quite close to the Ea of 21.7 kJ.mol−1 for the MGCell-PEI+ catalyst [31], 18.94 kJ.mol−1 for C-KOH-S-P catalyst [48], 20.84 kJ.mol−1 for p(2-VP)++C6 catalyst [49], 26.20 kJ.mol−1 for Co–B-50GO catalyst [57], 24.01 kJ.mol−1 for p(CO) catalyst [53], 24.29 kJ.mol−1 for AC@Pt–Ru–Ni catalyst [58], 24.03 kJ.mol−1 for SiO2@PAA catalyst [62] and 24.10 kJ.mol−1 for p(MTMA) catalyst [50]. Furthermore, the Ea for NaBH4 dehydrogenation in methanol catalyzed by H-EDA-CS is considerably lower than the other previously reported activation energies, for example 34.80 kJ.mol−1 for the CAP catalyst [54], 38.41 kJ.mol−1 for the Co-Cr-B/NG catalyst [59], 38.80 kJ.mol−1 for the Co1B/GNS catalyst [60].Because of their high reactivities, primary alcohols can be used instead of water in the production of hydrogen with NaBH4. Methanol is known to have the highest reactivity of all of the primary alcohols. Therefore, methanol has become a feasible alternative for H2 generation [63,64]. Also, among other alcohols, methanol has a higher acidity than water. The carbon atom linked to the oxygen in the methoxy ion is less electropositive than the hydrogen, resulting in a more stable structure [65]. (5) NaBH 4 ↔ Na + + BH 4 − (6) BH 4 − + H + ↔ BH 3 + H 2 (7) BH 3 + 3 CH 3 OH → B CH 3 O 3 + 3 H 2 (8) B CH 3 O 3 + CH 3 OH ↔ B CH 3 O 4 − + H 2 (9) 4 B CH 3 O 4 − + 2 H + + 7 H 2 O ↔ B 4 O 7 2 − + 16 CH 3 OH Eq. (2) makes assumptions the reaction between NaBH4 and methanol. Eqs. (5), (6), (7), and (8) explain the mechanism of methanol and NaBH4 in details. The recoverability of methanol by hydrolysis of byproducts is defined by Eq. (9).The reaction mechanism of the H-EDA-CS catalyst with NaBH4 is shown in Fig. 7 . Hereunder the reaction first starts with the dehydrogenation of NaBH4, and an active intermediate is formed. As a result of the reaction of the intermediate product formed afterwards with methanol, as shown in Eq. (7), 4 mol of H2 and a tetramethoxyborate anion are released as by-products. As mentioned before, these released by-products make the reaction environment more basic and cause methanolysis to slow down.The repeated use of catalyst in the methanolysis of NaBH4 is a very important parameter to determine its catalytic stability and feasibility for industrial applications. As a consequence, ten consecutive uses of H-EDA-CS as a catalyst in the production of H2 from the dehydrogenation of NaBH4 in methanol were performed, and the obtained results were given in Fig. 8a. As shown in Fig. 8a, the activity of the H-EDA-CS catalyst decreased to 82% after the 1st use, and it continued to decrease after the 2nd and subsequent uses, reaching 43% after the 10th usage. It is important to emphasize that 100% conversion is obtained at every usage, and the H-EDA-CS catalyst demonstrated little decrease in activity even after the 10th cycle. The activity decrease can be solved by regenerating the used catalyst by treating it with HCI. Fig. 8b represents the three reuse cycles after regeneration of the H-EDA-CS catalyst with HCl.The catalyst was treated with 30.0 mL of 1.0 M HCl and used three times in a row after three repeated uses. In the first/initial use, the activity of the catalyst was higher. With the repeated use of the catalyst, the activity decreased when the conversion was 100% at each use, as seen in Fig. 8a. Fig. 8b, the experiments were repeated and the catalyst was regenerated after the first (3 repetitions) use, and it was used again 3 times after each generation. In Fig. 8b obtained with the new test after correction, all values were given in comparison with the catalyst pre-regeneration (first use) activity. Activity % was calculated as 100, 91, 80 and 79% for initial use, 1st regeneration, 2nd regeneration and 3rd regeneration, respectively. According to the literature, the decrease in the activity percentage of the H-EDA-CS catalyst can be related to the facile reaction of BH4 − anions with positively charged amine groups and the tetramethoxyboron generated in the environment as the reaction progressed [3,45]. As a result of this study, it is evaluated that the CS-based catalyst will be a useful catalyst in industrial-scale hydrogen gas production since it can be regenerated and has high reusability performance, which is critical for practical applications.CS was synthesized systematically via hydrothermal method and optimum conditions were determined. Then CS was modified and used as a catalyst for dehydrogenation of NaBH4 in methanol to produce H2 gas, a clean fuel source. Zeta potential, thermal analysis, and FT-IR spectroscopy measurements were used to confirm the chemical modification of CS. When used directly as a catalyst in the dehydrogenation of NaBH4, the CS HGR value was 529 mL.min−1.g−1. Surprisingly, the protonation of EDA modified CS (H-EDA-CS) had significantly better catalytic performance than bare CS and maximum HGR value was determined to be 3460 mL.min−1.g−1.The calculated HGR value of H-EDA-CS showed an increase with an increase on temperature and NaBH4 concentration but a decrease with increasing amount of catalyst. Moreover, the calculated activation energy value was 26.14 kJ.mol−1 for H-EDA-CS. When compared to conventional catalysts, the calculated Ea values of the CS catalyst are relatively low in the methanolysis of NaBH4. These values outperform the majority of noble metal-based catalysts reported in the literature. Moreover, reusability studies of the H-EDA-CS catalyst indicated that the activity of the catalyst was more than 50% after 7th usage with reaching 100% conversion in each usage. Additionally, after various usage the loss of activity can be easily recovered by regeneration. As a result, starch, one of the world renewable, natural, and plentiful biopolymers, can be easily synthesized and changed using the hydrothermal process, and used as an efficient catalyst in the synthesis of a greener energy source, H2, from the dehydrogenation of NaBH4 in methanol. Sultan Butun Sengel: Conceptualization, Methodology, Project administration, Writing – review & editing, Visualization, Supervision, Funding acquisition. Hatice Deveci: Resources, Data curation, Formal analysis, Writing – original draft, Visualization. Harun Bas: Resources, Visualization, Data curation. Vural Butun: Supervision, Funding acquisition, Writing – review & editing.The authors report no declarations of interest.This work was supported by the Commission of Scientific Research Projects of Eskisehir Osmangazi University (ESOGU BAP, FBA-2021-1620). Supplementary material Image 5 Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2023.106650.

Carbon sphere (CS) was successfully synthesized by using starch via hydrothermal process with optimization of all conditions and parameters, systematically. The optimized CS was modified and used as a catalyst in the dehydrogenation of sodium borohydride in methanol for hydrogen production. Factors affecting the H2 production rate such as reaction temperature, catalyst type and amount, NaBH4 amount were investigated. Activation parameters for the dehydrogenation reaction of NaBH4 with the use of amine modified and protonated CS (H-EDA-CS) catalyst were calculated to be 26.14 kJ.mol−1, 23.75 kJ.mol−1, −192.19 J.mol−1.K−1, for Ea, ΔH# and ΔS#, respectively. Maximum HGR value was calculated as 3460 mL.min−1.g−1 at 25 °C. Moreover, reusability studies of the H-EDA-CS catalyst were made and the activity of the catalyst was found to be above 50% even after the 7th use. The catalyst was also regenerated 3 times, and the % activity results for initial use, 1st, 2nd and 3rd regeneration were calculated as 100%, 91%, 80% and 79%, respectively.

In order to improve the petroleum refining efficiency, various catalysts are widely used in refinery industry. The fluid catalytic cracking (FCC) catalyst is widely used in the petroleum refineries worldwide (Muddanna and Baral, 2019). FCC catalyst tends to lose the catalytic activity with the metal deposition from crude oil. After repeated use, FCC catalyst will be deactivated and become spent FCC catalyst (SFCCC). SFCCC contribute large amounts of solid wastes in petrochemical industry, and about 200,000–400,000 tons of SFCCC are produced worldwide every year (Alonso-Fariñas et al., 2020).Currently, SFCCC is mostly treated by landfills, although it contains a variety of metals (Ni, V, Sb, La, etc.). However, SFCCC has been considered to be toxic to environment (Alonso-Fariñas et al., 2020). SFCCC has been identified as hazardous waste in China since 2016 (Xue et al., 2020), and was subject of directive regulations in Europe (EC Commission Decision, 2014). However, the risk of SFCCC is still controversial. SFCCC is not included in the “Hazardous Waste Listings” by United States Environmental Protection Agency (US EPA) (USEPA, 2016). Studies have evaluated the risk and pollution characteristics of SFCCC in China. Zhang et al. (2019) found that SFCCCs have no ignitability, reactivity, corrosivity and toxicity to mice and rabbit. Study of Liu et al. (2016) showed that SFCCC did not belong to the hazardous solid waste. Unfortunately, few studies have been conducted on the evaluation of ecological and environmental hazards of SFCCC, and the main pollution components of SFCCC are still unclear. In the process of stacking and landfill of SFCCC and under the action of rainfall, the metal elements in SFCCC (Ni, Sb, V, La, etc.) will be leached with the rain and flow into groundwater, rivers, lakes and other water bodies, thus polluting the aquatic environment. However, to the best of our knowledge, there are no reports on the toxicity evaluation of SFCCC on aquatic organisms.The freshwater green microalgal species Raphidocelis subcapitata (formerly known as Pseudokirchneriella subcapitata or Selenastrum capricornutum) has been frequently employed to evaluate the toxicity of chemicals and wastes as the test organism (Fu et al., 2017; Sousa et al., 2018), and was recommended as assay microalga by Organization for Economic Co-operation and development (OECD), US EPA, or International Organization for Standardization (ISO) (OECD, 2011; ISO, 2012; USEPA, 2012).In this study, the ecotoxicity assays using R. subcapitata were conducted to evaluate the effect caused by metals in SFCCC leachates. To achieve this experiment, we collected 17 SFCCC samples from different petroleum refineries and prepared SFCCC leachates. Pearson's correlation analysis was conducted to discover the relationships between the metal concentrations of leachates and the values of toxicity. Furthermore, we designed the toxicity prediction models of the SFCCCs by multiple linear and non-linear regression models. Through our study, the ecotoxicity and key toxigenic factors of SFCCC can be first clarified, which provides reference for the management of SFCCC and formulation of measures to reduce toxicity.17 Spent FCC catalyst samples were collected from spent catalyst warehouses in different petroleum refineries. All SFCCCs were FCC catalysts deposed with metals from crude oil. FCC catalysts are Y molecular sieve, which is supported by SiO2-Al2O3 and loaded with rare earth metals. The label of those SFCCCs were Ha, Ji, Sha, Yan, Sh, Go, Ba, Qi, QD, Ur, Zho, ZD, Zhe, Fu, So, Ya-1, and Ya-2. The collected SFCCC samples were oven-dried at 75 °C for 24 h to remove moisture and stored in desiccators for 2 h.In this study, the mixed solution of nitric acid and sulfuric acid was used as the leaching agent based on the Chinese solid waste extraction procedure for leaching toxicity – sulfuric acid and nitric acid method (HJ/T299-2007) (Wang et al., 2019; Li et al., 2021). The method simulated the leaching process of metal components from SFCCC into the environment under the influence of acidic precipitation during the nonstandard landfill disposal or stockpiling of SFCCC.Aqueous extraction treatment was carried out on the dried SFCCCs samples. The mixture of concentrated sulfuric acid and concentrated nitric acid with a mass ratio of 2:1 was added to the MilliQ water (about 2 drops in 1 L water) to prepare the extract with the pH of 3.20 ± 0.05 (Wang et al., 2019). The 150 g SFCCC was mixed with 1500 mL extract (1:10, m/v) in 2000 mL PTFE bottle. The bottles were fixed on the flip oscillator (Polyfutai, ZKF-WF, Beijing, China) and oscillated for 18 ± 2 h at 30 ± 2 rpm and 23 ± 2 °C. The leachates were filtered by 0.45 µm cellulose acetate membrane washed with dilute nitric acid and the filtered leachates were stored at 4 °C. The concentration of metals in leachates was measured using an inductively coupled plasma-optical emission spectrometer (ICP-OES) (PE, OPTIMA 8000, USA). Metal concentration data were averaged for at least three measurements. Raphidocelis subcapitata FACHB-271 was obtained from the Freshwater Algae Culture Collection at the Institute of Hydrobiology (FACHB) in China. Microalga was acclimatized in ISO 8692 standard algal medium (ISO, 2012) under continuously illumination of white fluorescence light at 100 ± 4 μmol m-2 s-1 at a temperature of 24 + 1 °C in the illumination incubator (Jiangnan, GXM-508F-4, Ningbo, China) for two weeks.The fresh algal medium was prepared by adding the stock solution of medium to MilliQ water supplemented with different SFCCC leachate concentrations ( Table 1).Generally, these dosing levels were designed to be a geometric progression after finishing a range-finding preliminary test. The tests were carried out in 250 mL flasks containing 100 mL of medium. An initial inoculum (1 × 104 cells mL−1) in the exponential growth phase of R. subcapitata was added to medium. Microalgae incubated under the same condition, but without added SFCCC leachates, were used as a control. All assays were carried out in triplicate for 96 h. Microalgal cultures were maintained under the same conditions as were used for microalgal acclimatization in incubator. The microalgal cell density measurements were performed every 24 h. The cell density in the inoculum was determined by counting under microscope (Olympus, CKX53, Japan) in the haemocytometer. The pH of all test solutions was measured at test initiation and termination using a pH meter (Sartorius, PB-10, Germany).The growth curves of R. subcapitata under different initial concentrations of SFCCC leachates were plotted versus time, and the area (A) under the growth curve of microalgae based on cell density for each treatment was estimated using the following equation (USEPA, 2012): (1) A = ∑ i = 1 n ( N i − 1 + N i − 2 N 0 2 ) ( t i − t i − 1 ) where N 0 and N i are the cell densities at test initiation (t 0), time of the i th counting (t i) after test initiation.The percent of inhibition (% I) of each concentration of SFCCC leachate relative to control was estimated using the following equation (USEPA, 2012): (2) % I = A C − A T A C × 100 where A C is the area under the microalgal growth curve of the control, and A T is the area of SFCCC leachate treatment group.The values of percentage of inhibition were used to calculate the median effective concentration values for 96 h (96 h EC50) using the means of a probit analysis (Finney, 1971).For each treatment and control, means and standard deviations were calculated from three replicated determinations. The relationships between the toxicity and the metal factors were analyzed by Pearson correlation using SPSS 19.0 software at significance levels of p < 0.05 and p < 0.01. Some measured metal concentrations in the leachates were below the detection limits, so the statistical analysis of those data was performed using the half value of the detection limit (Farnham et al., 2002). Before the correlation analysis, a logarithmic transformation was conducted for the factors that were not normally distributed.In this study, SPSS 19.0 was used to perform multiple linear regression (MLR) and non-linear regression (MNLR), which are used to derive a mathematical relationship between the 96 h EC50 values and the concentrations of metals which are significantly correlated with EC50.The linear equation containing all those variables can be constructed in: (3) y = ∑ i = 1 n β i x i + C where, y, β i, x i and C represent ln(EC50), parameters of the model, concentrations of metals and constant of the model, respectively.The best-fit MNLR model was built using the stepwise selection method. The exponential model was introduced based on the fitting result of MLR: (4) y = α 1 + α 2 × exp ( ∑ i = 1 n β i x i + C ) where, α 1 and α 2 are constants of the model.A total of 27 elements of 17 SFCCC leachates were determined, of which Fe, Cu, Zn, P, As, Be, Ti, Cr, Cd, Ag, Pb, Ba, Zr, and Se were not detected in all samples. The concentrations of the other 13 metals are shown in Table S1. The results shown that, V, Sb, La, Ce, Na, and Ca are the metal elements with high concentration in the leachates. The concentrations of Ni, Al, Cu, Zn, Mo, Mn, and Co are relatively low, and the concentrations in most samples are less than 1 mg L−1. The concentrations of metals vary greatly among different SFCCC leachates.The effects of different concentrations of 17 SFCCC leachates on the growth curves of R. subcapitata are illustrated in Fig. S1. The growth lag period of R. subcapitata was about 24 h, and tiny changes between the control and the treatments were observed in the early stage of test (Fig. S1). The pH in each tests experienced a trend of change from weakly acidic (5.3–6.1) to weakly alkaline (7.7–8.2). The calculated 96 h EC50 values based on the area under the growth curve of microalgae are listed in Table 2.The 96 h EC50 values of the 17 SFCCC leachates varies greatly due to the large differences in metal concentrations. Qi SFCCC was the most toxic to R. subcapitata with a 96 h EC50 value of 1.38%. Yan, ZD, Ba, and Ji SFCCC also exhibit high toxicity to R. subcapitata, with 96 h EC50 value of 2.35%, 2.79%, 3.11% and 3.81%, respectively. Ha SFCCC was the least toxic, with 96 h EC50 values exceeding 100% to R. subcapitata.In order to identify the toxicogenic factors of SFCCC leachate, bivariate correlation analysis was conducted between SFCCC toxicity (96 h EC50 value) and concentrations of metals in SFCCC leachates, and the results are shown in Table 3. It is clear that the concentration of Ni (C Ni) (r = −0.751, p = 0.001) and La (C La) (r = −0.726, p = 0.001) showed inversely significant correlation with EC50 value at the 0.01 level, which suggests that an increase in C Ni and C La generates a more toxic leachate. In addition, EC50 value was inversely correlated with the concentration of Mn (C Mn) (r = −0.581, p = 0.014), Co (C Co) (r = −0.565, p = 0.018), Ce (C Ce) (r = −0.568, p = 0.017), and Ca (C Ca) (r = −0.523, p = 0.031) at 0.05 level. The concentration of other metals, V, Sb, Al, Co, Zn, Mo, and Na, did not correlate with the toxicity of SFCCC leachates.In order to establish the prediction model of the toxicity of SFCCC, MLR model was used to analyze the contribution of each metal to the toxicity. In this model, the linearity relationship between metals and EC50 values is assumed. The concentration of metals with significant correlation with EC50 values were selected for MLR analysis. The collinearity diagnostic test confirmed that C Ce and C Ca are highly intercorrelated with C La and C Mn, respectively. Therefore, C Ni, C La, C Mn, and C Co were selected for MLR analysis, and none had a variance inflation factor (VIF) more than 1.9. The Result of MLR analysis was shown in Table 4. When P of the independent variable is greater than 0.05, the variable should be excluded from the model. As shown in Table 4, only the significance test results of C Ni and C La meet the criteria, therefore the regression model is defined as: (5) ln ( E C 50 ) = 3.518 − 1.112 × C N i − 0.107 × C L a Fig. 1 shows the relation between the estimated values of ln(EC50) using MLR model (Eq. (5)) and the experimental ln(EC50) values. Although the MLR model can describe the trend of EC50 values, however the prediction accuracy is not high (R2 = 0.783). In addition, it can be clearly seen from Fig. 1 that the predicted value and experimental value are more in line with the logarithmic model. Therefore, this study considered to introduce a logarithmic model based on MLR to simulate the regression relationship between metal concentrations and EC50 values of SFCCC leachates.In order to construct a more fitting model to better simulate the response relationship of metal concentrations and 96 h EC50 values of SFCCC leachates, the multiple non-linear regression (MNLR) model was tested. The logarithmic equation (Eq. (4)) was introduced and fitted with the results of toxicity and metal concentrations. The MNLR model was constructed as follows: (6) l n ( E C 50 ) = 0.817 + e x p ( 1.356 - 1.736 × C N i - 0.262 × C L a ) The analysis of variance (ANOVA) for MNLR model was provided in Table 5. The sum of squares metric from Eq. (6) is 92.703 and the residual is 1.667. Low mean squares for residuals indicate the robustness of the proposed MNLR model for predicting the toxicity of SFCCC.The ln(EC50) values calculated by MNLR model was fitted with the experimental ln(EC50) values. The fitting result was shown in Fig. 2. As can be seen in Fig. 2, the predicting result of MNLR model (R2 = 0.926) are better than that of MLR model (R2 = 0.783). The MLNR model established in this study can be used as a toxicity prediction model for SFCCC.Ni, V, Sb, Mo, Mn, Co, La, and Ce are the characteristic metals in SFCCC leachates, which is similar to the previous studies (Zhou et al., 2020; Aung and Ting, 2005). Among the 27 metal elements, C Ni, C Mn, C Co, C La, C Ce, and C Ca have significant correlation with the 96 h EC50 values of R. subcapitata. The Ni, Co, Ca, and Mn in SFCCC are from the deposition of metals in crude oil. La and Ce are the main catalytic active components of fresh FCC catalyst. In this study, the main toxic ingredients of SFCCC to microalgae were identified for the first time. Researches should pay more attention to these elements in the development of SFCCC treatment and detoxification technology.At present, many studies have adopted various methods to remove or recover La (Mouna and Baral, 2019; Zhao et al., 2017; Lu et al., 2020; Innocenzi et al., 2015), Ni (Aung and Ting, 2005; Muddanna and Baral, 2019; Bayraktar, 2005), and Ce (Zhao et al., 2017; Lu et al., 2020; Innocenzi et al., 2015) in SFCCC. However, metals such as Al, V, and Sb concerned in SFCCC metal removal study did not show toxic effects on R. subcapitata. Satoh et al. (2005) assayed the toxicity of Cu, As, Sb, Pb, and Cd to eight microalgae. The results showed that the 72 h IC50 value of Sb to microalgae was 7.9–45.9 mg L-1, which was significantly higher than that of other metals (Cu: 4.2–11.7 mg L−1; As: 1.6–12.0 mg L−1; Pb: 2.5–21.4 mg L−1; Cd: 2.9–13.8 mg L−1). The results of low toxicity of Sb to microalgae in the research of Satoh et al. (2005) are consistent with the results of this study. Meisch and Bielig (1975) studied the effects of pentavalent vanadium on the growth of Scenedesmus obliquus and Chlorella pyrenoidosa, and found that V had no toxic effects on microalgae. Furthermore, vanadium was able to overcome completely a limited iron-deficiency in the algae and the chlorophyll formation was stimulated in Scenedesmus obliquus in presence of vanadium (Meisch and Bielig, 1975).In this study, the concentrations of Ni and La were significantly correlated with the 96 h toxicity of SFCCC to R. subcapitata (P = 0.001). The high toxicity of Ni and La to microalgae has been reported. Deleebeeck et al. (2009) researched the toxicity of Ni (Ⅱ) to R. subcapitata. Under different conditions, the 72 h EC50 value of Ni to R. subcapitata was 0.082–1.120 mg L-1. For the toxicity of La (Ⅲ) on microalgae, the 96 h EC50 value of La (Ⅲ) to Chlorella vulgaris and Phaeodactylum tricornutum is 10.077 mg L−1 and 5.665 mg L−1 respectively, in the research of Sun et al. (2019). The toxicity category of Ni and La according to “Hazardous Substances (Classification) Notice” by USEPA (2017) is highly toxic (0.1–10) mg L-1.According to the results of this study, C Ni and C La can be used as indicators of toxicity of SFCCC leachate. The prediction model for toxicity of SFCCC was established according to C Ni and C La in leachate (Eq. (6)) (R2 = 0.926). In the future, when evaluating the ecotoxicity of SFCCC, the 96 h EC50 value of SFCCC to R. subcapitata can be roughly obtained by preparing the leachate and measuring the C La and C Ni. The work in this study can provide a quick method for the determination of SFCCC ecotoxicity and provide assist for the hierarchical management and control of SFCCC.However, it should be noted that this study only explored the toxic effects and toxic factors of SFCCC on microalgae. However, metals are not equally toxic to different organisms. For example, some studies have shown that Sb has a high toxicity to large plants (Feng et al., 2020; Park et al., 2021). More studies are needed to investigate the toxic effects of SFCCC on a variety of organisms.The ecotoxicity of 17 SFCCC leachates to R. subcapitata was assayed in this study. The toxicity of the 17 SFCCCs varied enormously with the range of 96 h EC50 values are 1.38%- > 100%. The concentration of Ni (p = 0.001), La (p = 0.001), Mn (p = 0.014), Ce (p = 0.017), Co (p = 0.018), and Ca (p = 0.031) in SFCCC leachates showed significant correlation with 96 h EC50 values. The predictive models for the toxicity of SFCCCs were established with the concentrations of Ni and La in leachates by multiple linear and non-linear regression models. The main toxic ingredients of SFCCC to microalgae were identified for the first time in this work. This study is significance for toxicity determination and management of SFCCC.This work was supported by the SINOPEC Ministry of Science and Technology Basic Prospective Research Project (CN) through a research scheme (A-531). Yue-jie Wang: Conceptualization, Methodology, Software, Writing - original draft. Chen Wang: Data curation, Writing - review & editing, Supervision. Ling-ling Li: Formal analysis, Investigation, Resources. Yan Chen: Visualization, Investigation. Chun-hong He: Investigation. Lu Zheng: Investigation.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.We are grateful to Engineer Mingzhe Li and Engineer Huaji Wang, our colleague of Key Laboratory, who contributed a lot to the measurement of metal concentration.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.ecoenv.2021.112466. Supplementary material .

The 17 spent fluid catalytic cracking refinery catalysts (SFCCCs) from different petroleum refineries were collected and the leachates of SFCCCs were prepared. The ecotoxicity of SFCCC leachates to Raphidocelis subcapitata was assayed. The results showed that the toxicity of the 17 SFCCCs differ greatly. Ji SFCCC was the most toxic to R. subcapitata with a 96 h EC50 value of 1.38%, while Ha SFCCC was the least toxic, with the EC50 value was >100%. The relationships between the toxicity of SFCCCs and the metal concentrations in leachates were analyzed. The concentration of Ni (p = 0.001), La (p = 0.001), Mn (p = 0.014), Ce (p = 0.017), Co (p = 0.018), and Ca (p = 0.031) in leachates showed significant correlation with EC50 values. The predictive model for the ecotoxicity of SFCCCs were established with the concentrations of Ni and La in leachates as: ln(EC50) = 0.817 + exp(1.356 – 1.736 × C Ni - 0.262 × C La) (R2 = 0.926). The main toxic ingredients of SFCCC to microalgae were identified for the first time in this work. The results and predictive model of this study are significance for toxicity determination and management of SFCCCs.

Biomass-derived energy cycle is one of the most sustainable alternatives to existing fossil fuel-derived energy platform [1]. A key intermediate for biomass conversion, 5-hydroxymethylfurfural (HMF) is of great interest as it can be obtained from the most earth-abundant organic materials, cellulosic matter, and can be converted to various kinds of important chemicals and fuels [2]. Among them, its oxidation product, 2,5-furandicarboxylic acid (FDCA) is receiving a great attention as a precursor for producing biomass-derived polymer, polyethylene 2,5-furandicarboxylate (PEF). PEF is a renewable candidate for replacing petroleum-derived polymer, polyethylene terephthalate (PET) [3].Typically, the FDCA has been produced by thermochemical oxidation of HMF under high pressure O2 or air (3–20 bar) at 30–130 °C usually using precious metal catalysts such as Pd [4], Ru [5], Pt [6], and Au [7]. As a promising alternative route, electrochemical oxidation of HMF in aqueous solution has several advantages over the conventional oxidation approaches. (i) It doesn't require harmful oxidant as the water serves as an oxygen donor. (ii) It can be performed at ambient temperature and pressure. (iii) Tunable applied potential can control the selectivity thereby providing mechanistic insights. (iv) Various important reduction reactions like CO2 reduction reaction and hydrogen evolution reaction could be integrated, significantly increasing the worth of electrochemical biomass upgrading. (v) There remains a room for increment in economics when the electricity is replaced with renewable energy sources. In the same vain, electrocatalytic HMF oxidation reaction (HMFOR) represents a desirable strategy to replace kinetically unfavored oxygen evolution reaction (OER) in water electrolysis, increasing the overall energy efficiency.Electrochemical HMFOR also adopted noble metal-based nanoparticles like Au, Pd, PdAu alloys as catalytic anodes. For example, Chadderdon and co-workers exploited carbon-supported Au and Pd nanoparticles as the electrocatalyst [8]. They showed high conversion yield (∼100% at 0.9 V vs reversible hydrogen electrode, RHE), but the selectivity was only 83%. In a recent work, Choi and co-workers achieved a high Faradaic efficienty (∼100% at 1.54 V vs RHE) over gold anode by adopting an mediator (2,2,6,6,-tetramethylpiperidine-1-oxyl, TEMPO) [9]. However, use of the organic mediator would increase the separation/recovery cost. To replace the precious metals, diverse non-noble metal-based electrodes including nickel oxide/hydroxide [10], Co-P [11], Ni2P/Ni foam [12], Ni3S2/Ni foam [13], Porous Ni [14], NiB x [15], NiCo2O4 [16], Ni3N [17], and NiFe layered double hydroxide (LDH) [18] have been studied. Interestingly, most active electrocatalysts include Ni species, which are both active for HMFOR and OER. In particular, the introduction of Fe into layered nickel hydroxide has been effective strategy to enhance performance of OER as well as HMFOR [19]. However, reducing the activity toward the OER is essential to result in high selectivity for the HMFOR. Considering that HMFOR and OER are competitive with each other, it is hardly understandable how Fe insertion into nickel hydroxide catalyst simultaneously enhances both reactions. Previosuly, the Fe-related species have been addressed only in individual reactions. Boettcher group revealed that incorporation of Fe-impurities existed in KOH electrolyte increased OER activity for NiOOH [19]. Choi group reported that the FeOOH itself showed the inferior HMFOR activity compared to the NiOOH [20]. The role of Fe in Ni-based composite under conditions considering both oxidation reactions remains an unresolved issue.To address this issue, herein we comparatively studied the role of Fe in the layered Ni(OH)2 catalysts in both oxidation conditions; HMFOR and OER. For this purpose, layered Ni(OH)2 catalysts were prepared with controlled Fe content (0 to 5 at.%). Their structural properties were scrutinized with X-ray spectroscopic analyses. Their general morphologies, crystalline phases and chemical states were well retained after insertion of Fe, but with increasing Fe content, the crystallite size and layer number decrease due to the peeling of layers promoted by Fe intercalation. Electrochemical analysis performed in Fe-free KOH solution revealed that the OER activity increases up to 0.4 at.% with increasing Fe content, and then decrease showing volcano relation with the Fe content. Combined analysis based on the spectroscopic and electrochemical results suggested that traces of Fe penetrate the interlayer of Ni(OH)2 and enlarge the interlayer distances, resulting in a formation of NiFe LDH by replacing the Ni site with Fe during the electrochemical reaction. Although this Fe-promoted peeling of the Ni(OH)2 layers and facilitated OER activity up to optimal Fe content, the performance of the HMF conversion to FDCA was gradually decreased with Fe content due to charge consumption to OER. The HMFOR activity was the highest in the Fe-free Ni(OH)2, showing HMF conversion (99.9%), FDCA yield (94.2%), and Faraday efficiency (FE) for FDCA (98.0%). This comparative study offers a way of controlling selectivity and provides a guideline for rational design of Ni-based HMFOR-selective electrocatalysts.Fe(X)-Ni(OH)2 (X = 0.4 to 5 at.%) was synthesized by a simple microwave-assisted method [21], with some modifications. Ni(NO3)2·6H2O (99.999%, Sigma-Aldrich) and Fe(NO3)3·9H2O (≥99.95%, Sigma-Aldrich) were dissolved in 10 mL of deionized (DI) water. NaOH pellet (98%, Samchun) was dissolved in 40 mL of DI water. Above tow solutions were mixed and stirred for 10 min. The resulting slurry was centrifugated at 8000 rpm for 15 min, and washed with DI water several times. They are dispersed in DI water again and undergone microwave heating at 90 °C for 1 h. The product was collected with centrifugation at 12000 rpm for 20 min and dried. The synthesis of Ni(OH)2 was carried out in the same manner as Fe(X)-Ni(OH)2, but without Fe precursor.The Fe content was analyzed using an inductively coupled plasma optical emission spectrometry (ICP-OES) analyzer (iCAP 6000 Series, Thermo, US). Transmission electron microscope (TEM) images were taken with a transmission electron microscope (Tecnai F20 G2, FEI, USA) operated at 200 kV. X-ray diffraction (XRD) was measured on a diffractometer (D8 Advance, Bruker AXS, Germany) with a Cu K α radiation using a LynxEye line detector. X-ray photoelectron spectroscopy (XPS) analysis was performed to analyze the chemical states of the samples with a spectrometer (Nexsa, Thermo Fisher Scientific, USA) equipped with a monochromatic Al K α X-ray source (1486.6 eV).Polypropylene (PP) centrifuge tubes were cleaned with a H2SO4 solution (0.5 M). 2 g of Ni(NO3)2·6H2O was dissolved in 4 mL of DI water. 20 mL of 1 M KOH was added to precipitate Ni(OH)2, which serves as adsorbent for Fe impurities. The mixture was shaken and cetrifugated, and the supernatant was decanted. The Ni(OH)2 was washed with 20 mL of DI water and 2 mL of 1 M KOH. The tube was filled with 100 mL of 0.1 M KOH solution for purification. The solid was redispersed and mechanically agitated at least 10 min, followed by at least 3 h of resting for adsorption of Fe in KOH solution. The mixture was centrifugated, and the purified KOH supernatant was kept in a H2SO4-cleaned PP bottle for storage before use.Electrochemical experiments were performed using a Biologic VSP multichannel potentiostat electrochemical analyzer at room temperature (RT) and atmospheric pressure in a three-electrode configuration in a H-cell divided by Fumasep FBM-PK membrane. Hg/HgO and Pt mesh were used as the reference electrode and counter electrode, respectively. Catalyst inks were prepared by mixing 5 mg of catalyst with 40 μL of Nafion (5 wt% in a mixture of isopropanol and water, Sigma-Aldrich) in a solution of 800 μL DI water, 200 μL of isopropanol (99.9%), and the mixture was sonicated for 30 min to produce homogeneous slurry. Afterwards, 42 μL of the catalyst ink was drop-cast onto carbon paper (CP, 1 cm2), and dried at RT. The resulting catalyst loading on CP was 200 μg·cm −2. A Fe-free 0.1 M KOH (pH 13) was used as the electrolyte, and HMF (5 mM) was added only for HMFOR test. Linear sweep voltammetry (LSV) for the HMFOR was conducted from 0.9 V to 1.9 V (vs RHE) at a scan rate of 10 mV s −1 in Fe-free 0.1 M KOH. For OER test, LSVs were conducted without HMF following the same procedure. The electrochemical impedance spectra (EIS) were recorded at 1.1 V (vs RHE) and an AC potential amplitude of 10 mV from 200 kHz to 100 mHz. Series resistance (Rs) arises from a combination of resistance in solution, resistance in the catalysts themselves, and resistance in the glassy carbon substrate [22]. The LSVs in this paper were plotted after compensating the ohmic drops with Rs values. The electrochemical double-layer capacitance (C dl) was determined from cyclic voltammetry (CV) measured in the non-Faradaic window with a series of scan rates (12.5, 25, 50, 100, and 200 mV·s −1). Theoretically, the charging current (i c) is equal to the product of C dl and scan rate (v), as the following equation: i c = v·C dl. Hence, the C dl is the slope obtained from a linear fitting of i c as a function of v.HMF and its oxidation products were quantified by high-performance liquid chromatography (HPLC). After constant potential electrolysis performed at different applied potentials of 1.39 V, 1.44 V, and 1.54 V (vs RHE), 100 μL solution aliquots were taken from the anode compartment before and after the HMFOR and diluted with 1500 μL of 5 mM H2SO4 for HPLC analysis (YL9100 HPLC system containing a PDA detector). As the intermediates and FDCA exhibit a different light absorption profile, different detection wavelengths were chosen. 266 nm, 285 nm, 258 nm, 289 nm and 284 nm were set as detection wavelengths for FDCA, HMF, HMFCA, DFF and FFCA, respectively. 5 mM H2SO4 (99.999%, Sigma-Aldrich) was used as the mobile phase in isocratic mode with a flow rate of 0.5 mL min−1 at 40 °C. Aliquots of the diluted samples (20 μL) were injected into a Coregel 87H3 column (Concise Separations). Product identities and concentrations were determined from calibration curves obtained using standard solutions of known concentrations.The following equations were used to calculate HMF conversion (%), product yields (%), and FE for FDCA (%). Here, F represents Faraday constant, 96485C mol−1. HMF conversion % = HMF consumed (mol) Initial HMF ( mol ) × 100 % Product yield % = Product formed (mol) Initial HMF ( mol ) × 100 % FE for FDCA % = Charge consumed for producing FDCA Total charge passed × 100 % = FDCA formed (mol) × 6 × F Total charge passed × 100 % Layered Ni(OH)2 catalysts containing controlled Fe contents were prepared by a simple microwave-assisted synthesis at low temperature (90 °C for 1 h). The detailed synthetic procedure is described in experimental section. Since excess amounts of Fe can produce FeO x and FeOOH, which are catalytically less active for the HMOER and OER [20,23], the Fe content was controlled from 0.4 to 5 at.%. Successful incorporation of desired amount of Fe were confirmed with ICP–OES (Table S1). Hereafter, these samples are denoted as Fe(X)-Ni(OH)2 (X = Fe content). TEM images show that Ni(OH)2 and Fe(X)-Ni(OH)2 have a morphology of quasi-hexagonal nanoplate with lateral sizes about 30–50 nm (Fig. 1 ).The Fe(X)-Ni(OH)2 nanostructures were further scrutinized with XRD and XPS. Their XRD patterns are in good agreement with the Ni(OH)2 standard (JCPDS card no: 14–0117) (Fig. 2a). The edge plane direction of (001) diffraction peak at 2θ = 19.3–22.1° became broader with increased Fe content. Scherrer analysis for the diffraction demonstrates that the crystallite size and layer number in Ni(OH)2 nanosheets decrease with increasing Fe content (Fig. 2b). Introduction of Fe species between the Ni(OH)2 layers promoted the peeling of nanosheets [19,24]. Their chemical structures were accessed with XPS analysis (Fig. 2c,d and Fig. S1). Ni 2p XPS spectra of the Fe(X)-Ni(OH)2 samples almost overlapped with that for Ni(OH)2 (0% Fe) (Fig. 2c). The predominant peaks centered around 855.3–855.6 eV (Ni 2p3/2) and 873.0–873.2 eV (Ni 2p1/2) well matched with the Ni2+ [25]. Marginal shift to lower binding energy with increased Fe content indicates slight reduction possibly due to formation of Fe3+ species, as confirmed with Fe 2p XPS spectra for Fe(4)-Ni(OH)2 and Fe(5)-Ni(OH)2 (Fig. 2d) [18,26]. These peaks at 713.1 eV (Fe 2p3/2) and 725.4 eV (Fe 2p1/2) correspond to the Fe3+ species in FeOOH, which are catalytically less active [20,23]. It is rarely detectable when the Fe content is too small (< 3 at.%). Overall, XRD and XPS suggested the decreased crystallite size, decreased layer number, reduced oxidation states of Ni species, and increased amount of less active FeOOH with increased Fe content in Ni(OH)2.Electrocatalytic HMFOR and OER activities were measured on three-electrode setup using the catalyst-loaded carbon paper as working electrode. Fe-free 0.1 M KOH solution was used to exclude the effect of Fe existed in the base electrolyte only concerning the Fe species in catalytic material. The OER activity was measured without HMF dispersion in the electrolyte. The OER activities were compared with the OER currents at 1.7 V (vs RHE), which are not overlapped with the oxidation currents of Ni(II) species appeared in the potential rage of 1.4–1.5 V (vs RHE). The OER activity has a volcano relation with Fe content in the catalysts (Fig. 3a,b). The OER current is the highest in Fe(0.4)-Ni(OH)2; the current densities derived at a potential of 1.7 V (vs RHE) were 2.99, 7.01, 6.57, 5.39, and 3.95 mA cm−2 for the Ni(OH)2, Fe(0.4)-Ni(OH)2, Fe(2)-Ni(OH)2, Fe(4)-Ni(OH)2, and Fe(5)-Ni(OH)2, respectively (Fig. 3a). The Tafel analysis showed the same activity trend (Fig. 3b); the Tafel slopes are 192, 143, 155, 159, and 186 mV dec−1 for the Ni(OH)2, Fe(0.4)-Ni(OH)2, Fe(2)-Ni(OH)2, Fe(4)-Ni(OH)2, and Fe(5)-Ni(OH)2, respectively. The OER activity parameters are summarized in Table S2. Interestingly, the onset potentials for the oxidation of Ni(II) to Ni(III) shifted to higher potential with increasing Fe content, indicating unfavorable transition of Ni(II) to Ni(III), resulting in decrease of active species for the OER. However, the OER current (at 1.7 V vs RHE) rather increased with Fe content up to 0.4 at.%. The combined analysis from structural and electrochemical characterizations suggest that even traces of Fe impurity can penetrate the interlayer of Ni(OH)2 and form NiFe LDH by replacing the Ni site with Fe during the electrochemical reaction [19], facilitating the OER up to optimal Fe content (0.4 at.%).The HMFOR activity was evaluated with HMF dispersion in the electrolyte. In the presence of HMF, the following reactions can occur together and compete. OER: 4OH −   ➔ 2H 2 O + O 2 + 4e − HMFOR: HMF C 6 H 6 O 3 + 6OH − ➔ FDCA C 6 H 4 O 5 + 4H 2 O + 6e − Unlike the activity trend in OER, the kinetics of HMFOR enhances with increasing pH, thereby resulting in high rates and yields for FDCA. However, further increase of pH over 14 could induce polymerization of HMF and produce insoluble humins, thereby decreasing the effective HMF concentration [27]. Therefore, designing non-noble metal-based catalysts, which can be active at pH < 13, is of prime importance. For this reason, we adopted Fe-free 0.1 M KOH (pH ∼ 13) as an electrolyte solution for both HMFOR and OER. The HMFOR activities were compared with the HMFOR currents at 1.5 V (vs RHE), which can neglect the effect of OER and observe only the effect of Fe content on the HMFOR. In the series of Fe(X)-Ni(OH)2, the HMFOR currents decreased with increasing Fe content (Fig. 3c); the current densities derived at a potential of 1.5 V (vs RHE) were 2.53, 2.36, 1.63, 1.26, and 1.04 mA cm−2 for the Ni(OH)2, Fe(0.4)-Ni(OH)2, Fe(2)-Ni(OH)2, Fe(4)-Ni(OH)2, and Fe(5)-Ni(OH)2, respectively. The higher HMFOR current density in Ni(OH)2 were likely attributed to the abundance of Ni(II) species in surface, which are the main source for the Ni(III) species, initiating the indirect HMFOR process [20]. In electrooxidation of organic compound, Ni(II) is directly oxidized to Ni(III) in electrode surface and then the Ni(III) oxidize organic reactant by reducing themselves [28]. They showed similar Tafel slopes, might indicating the reaction rates of the Fe(X)-Ni(OH)2 were limited by the same step, which occured on Ni(OH)2 (Fig. 3d). To show the involvement of Ni(III) species in the indirect HMFOR, the surface electrochemistry was investigated with CV measurements in absence and in presence of HMF (Fig. S2). The oxidation current for Ni2+ to Ni3+ increased with addition of HMF due to continuous regeneration of Ni2+ by the indirect HMFOR. In this regard, the reduction peak area for Ni3+ to Ni2+ decreased with addition of HMF as the Ni3+ species already reduced to Ni2+ by the indirect HMFOR. Additionally, double-layer capacitance (C dl) of Ni(OH)2 was determined by measuring a series of CVs at various scan rates (12.5–200 mV·s−1) to evaluate electrochemically active surface area of electrodes (Fig. S3). The C dl obtained from the linear regression of current densities plotted against the scan rates was determined to be 561 uF·cm−2, which was comparable to previous C dl values of Ni(OH)2-based electrodes (100 and 576 uF·cm−2) [29,30]. The HMFOR activity parameters are summarized in Table S3.Constant potential electrolysis for HMF conversion to FDCA was performed to analyze the production yield and Faradaic efficiency (FE) for FDCA. For full conversion of HMF, stoichiometric amount of charge was passed (20.84C). As the applied potential strongly influences on the selectivity toward HMFOR, the constant potential electrolysis was performed at a series of potentials (1.39 V, 1.44 V, and 1.54 V vs RHE), where the OER currents are very minor (Fig. 3c and Fig. S4) [18,20]. The reaction time required to consume the stoichiometric amount of charge for the full conversion of HMF decreased with increased applied potentials as the higher potential results in the higher current (Fig. S4). At the potentials of 1.39 V and 1.44 V (vs RHE), HMF was almost completely converted (> 99.9%) with FDCA yields of 94.2% and 90.4%, respectively (Fig. 4a). At an applied potential of 1.54 V (vs RHE), even the stoichiometric amount of charge for full conversion of HMF was passed, the HMF conversion was 95.5%, lower than those of 1.39 V and 1.44 V. Also, the FDCA yield decreased to 71.3% and unreacted HMF (4.5%) was found in the product solution, indicating that the passed charge was not used only for the HMF conversion (Fig. 4a). With increasing potentials, the FE decreases mainly due to charge consumption to the competing OER (Fig. 4b). For the HMF conversion at 1.54 V, 87.1% of the passed charge was used for the HMF conversion, while 12.9% of the charge was consumed for the competing OER. Additionally, quantitative analysis based on the HPLC measurement revealed that the yields of the other intermediate compounds are 15.7%, 1.0%, and 6.9% for FFCA, HMFCA, and DFF, respectively. The HMF conversion, production yield, and FE were summarized in Tables S4–S6. The gap between HMF conversion (95.5%) and products yield (94.9%) could originate from the crossover of organic compounds through the membrane to the counter electrode compartment [31], and the uncontrollable degradation of HMF and intermediates due to the base-induced polymerization of aldehydes [32], which can make it difficult to assess the true production yield.Herein, layered Ni(OH)2 catalysts with controlled Fe content (0 to 5 at.%) were prepared for comparative investigation on the role of Fe in competing oxidation reactions; HMFOR and OER. While the OER activity increases up to 0.4 at.% and then decrease showing a volcanic relation, the HMFOR activity decrease with the introduced Fe content. It is explained with structural characterization that even traces of Fe largely affected to enlarge the interlayer spacing of Ni(OH)2 and form NiFe LDH by replacing the Ni site with Fe during the electrochemical reaction, boosting the OER performance and lowering the selectivity toward HMFOR. Among the samples, the Fe-free Ni(OH)2 showed the highest HMF conversion (99.9%), FDCA yield (94.2%), and FE for FDCA (98.0%). This finding offers a promising way of controlling selectivity and provides guideline for rational design of Ni-based HMFOR electrocatalysts. Bora Seo: Conceptualization, Methodology, Data curation, Investigation, Writing – original draft. Jongin Woo: Investigation, Data curation. Eunji Kim: Investigation. Seok-Hyeon Cheong: Investigation. Dong Ki Lee: Writing – review & editing, Funding acquisition. Hyunjoo Lee: Writing – review & editing, Supervision, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by C1 Gas Refinery Program through the National Research Foundation (NRF) of Korea (NRF-2015M3D3A1A01065435) and KIST Institutional Program (Atmospheric Environment Research Program, Project No. 2E31690). This work was also supported by NRF grant funded by the Ministry of Science and ICT (No. 2022M3H4A1A02091720). The XAS experiments performed at Beamline 1D of the Pohang Accelerator Laboratory (PAL) were supported in part by the Ministry of Education and Pohang University of Science of Technology (POSTECH). Supplementary material Image 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2022.106501.

5-hydroxymethylfurfural oxidation reaction (HMFOR) has been considered as promising anodic reaction alternating oxygen evolution reaction (OER). The introduction of Fe into layered nickel hydroxide has been effective strategy to enhance performance of OER as well as HMFOR. However, considering that HMFOR and OER are competitive with each other, it is hardly understandable how Fe simultaneously enhances both reactions. Herein, we provide an insight toward the role of Fe in the layered Ni(OH)2 in both oxidation reactions. While the OER activity of the layered Ni(OH)2 electrocatalyst has volcanic relation with Fe content, the HMFOR activity was highest in the Fe-free Ni(OH)2.

Data will be made available on request.Plastic has become an indispensable product in modern society. Likewise, plastic waste is ubiquitous and accumulates in huge quantities. The production of plastic in the EU has been reported to reach 55 Mt per year in 2020, accompanied by over 29.5 Mt of plastic waste (Soup, 2022). Unfortunately, the Covid-19 pandemic has further intensified this issue (Behera, 2021), and it is estimated that the global daily plastic waste generated since the outbreak began is 1.6 million tons (Benson et al., 2021).Despite this, only a small fraction of plastic waste is recycled, with the vast majority being either discarded, landfilled, or incinerated (Soup, 2022). In addition to causing severe environmental damage, discarding and landfilling plastic waste is a huge waste of resources. The energy potential in 1 ton of plastic waste is substantial; it equals that of more than 1 ton of coal or more than 4 barrels of oil (Themelis et al., 2011). Although incineration can convert the chemical energy in plastic waste into power (Al-Salem, 2019), it is crucial to note that plastic is a carbon-based material and releases over 2.8 kg of CO2 per kilogram of plastic burned, contributing significantly to carbon emissions (No-burn, 2021).In order to reach ‘net zero emissions’, low-carbon processes that convert plastic waste into high-value products are urgently needed. A number of thermochemical processes, including pyrolysis (Qureshi et al., 2020), hydrothermal liquefaction (Veksha et al., 2020), gasification (Nanda and Berruti, 2021), and chemolysis (R. X. Yang et al., 2022a, 2022b), have been extensively studied to convert waste plastics into more valuable products, such as syngas and plastic oil. Syngas obtained from the gasification of plastics has a LHV of 16–25 MJ/m3 (Lopez et al., 2018). Additionally, plastic oil obtained from pyrolysis or hydrothermal liquefaction has a HHV of 40–45 MJ/kg (Sharuddin et al., 2016), which is comparable to that of gasoline (HHV: 42.5 MJ/kg). However, these energy products still contribute to greenhouse gas emissions when they are combusted (Mani et al., 2009).Based on the elemental composition of plastic waste, the complete conversion of plastic waste into H2 and value-added carbon products via pyrolysis and catalytic cracking is recognized as the most promising and attractive solution for plastic recovery. Here, H2 has been recognized as one of the most promising energy carriers for the future (Ishaq et al., 2022). The generation of carbon products instead of COx, results in no CO2 emissions. More importantly, as high-value products, carbon products have critical applications in many industries, such as metallurgy (Zhang et al., 2016), batteries (Zhang et al., 2021), catalysis (Yoon et al., 2005), adsorbents (Wong et al., 2018), refractories (Thethwayo and Steenkamp, 2020), and so on.Numerous studies have also been conducted to co-produce carbon and hydrogen from plastic wastes. The main strategy is to deploy a metallic catalyst for reforming/cracking plastic pyrolysis volatiles. For instance, Wu and Williams reported a hydrogen yield of 133 mmol/g using Ni–Mg–Al catalyst for the pyrolytic reforming of PP while obtaining filamentous carbon on the catalyst surface (Wu and Williams, 2010). Yao et al. used Ni/ZSM5-30 catalysts for the pyrolytic reforming of HDPE and obtained a hydrogen yield of 30.11 mmol/g (without steam) (H. Yang et al., 2022a, 2022b). Aboul-Enein et al. obtained 27.8% CNT yield by pyrolysis of LDPE by using a 10% Ni–Mo/Al2O3 catalyst (Aboul-Enein et al., 2017).The construction of a highly efficient and stable metallic catalysts remains a major challenge in the process. The metallic catalyst could deactivate quickly as carbon (coke) is deposited on the catalyst surface (Wong et al., 2022). Furthermore, Barbarias et al. reported that in a continuous HDPE pyrolytic reforming experiments, the catalytic effect of Ni-based catalysts decreased significantly, and a slight irreversible deactivation persisted even after regeneration (Barbarias et al., 2019). The separation of carbon and metallic catalysts is another major challenge. Carbon applications generally require a high carbon purity, especially in the battery industry (Wissler, 2006). To obtain carbon with a high purity, it is necessary to purify the carbon to remove the residual metals in carbon products by using strong acids (Ma et al., 2022). However, the use of strong acids would inevitably create new environmental impacts (Shin et al., 2014).Biocarbon catalysts derived from a woody biomass pyrolysis serve as ideal catalysts for the thermal cracking of plastic waste pyrolysis volatiles to co-produce carbon and hydrogen. On one side, a metal-free carbon framework is thermally stable and has a high tolerance with respect to a coke deposition (Konwar et al., 2014; Wang et al., 2020). In this way, the catalyst's lifetime could be significantly extended. More importantly, spent biocarbon catalysts (with plastic waste derived carbon coating on the surface) could be directly used as functional carbon products with no need for an additional separation process (He et al., 2021). On the other side, woody biocarbon catalyst has a hierarchical porous structure originating from wood (Chen and Pilla, 2021). The tortuous and complex pore channels have been reported to increase the tortuosity of gas flow, which increases the residence time of the gas in the high-temperature region and significantly enhances thermal cracking reactions (Wang et al., 2017).In this study, biocarbon catalysts are fabricated by pyrolysis and subsequent carbonization of woody sawdust waste at a specific temperature. Thereafter, the catalysts are applied to catalytic cracking of plastic waste pyrolysis volatiles to produce metal-free carbon and hydrogen-rich gases. Specifically, the influence of the catalyst to plastic waste ratio (mass), and the influence of the plastic types have been investigated.Three types of sphere plastic samples (from Goodfellow), i.e. PP, LDPE, and HDPE, with the average particle size around 2–4 mm were used in this study. The elemental compositions of the plastic samples are listed in Table S1.The biocarbon catalyst used in this study was prepared by pyrolysis followed by a subsequent carbonization of woody sawdust, which consists of a mixture of pine and spruce bought from Svenska Cellulosa AB (SCA). Before each experiment, the sawdust was sieved, and only particles with a size larger than 1.25 mm were used. The aim was to retain the porous structure of biocarbon and to lower the pressure drop of the biocarbon catalyst bed. The pyrolysis and carbonization of sawdust were conducted at a temperature of 900 °C for a duration of 2 h in a nitrogen atmosphere and using a vertical electrical heating furnace.In this study, the experiments were carried out by using a two-stage reactor, as shown in Fig. 1 . And the detailed information of the models in the experiments was provided in Table S2. The whole system consists of a carrier gas supply section, a two-stage fixed bed, a cooling system, Micro GC, and a gas collection system. For the reactors, the furnace on the left was the pyrolysis reactor, and the one on the right was the catalytic cracking reactor. Also, ball valves were installed at the top of the pyrolysis reactor to drop the raw materials.Each test used 5 g of plastics and different amounts of a biocarbon catalyst, based on the cases described in Table 1 . Before the test, biocarbon was placed in the middle of the cracking reactor. Nitrogen at a flow rate of 200 ml/min was injected into the reactor as the carrier gas, and the cooling bath was set at −15 °C. Then two reactors were heated up to the set temperature shown in Table 1.Once the temperature is reached, plastic samples were quickly dropped into the pyrolysis reactor from the top valve. Hot gaseous compounds from the cracking reactor first passed through a series of condensed bottles in the cooling bath. Condensable liquids were condensed and collected in the bottles. Non-condensable gases passed through a Micro-GC instrument for fast online determinations of the chemical compositions. The total volume of the gases was measured by using a gas clock. Finally, the gases were collected in a gas bag for further chemical composition determinations. The experiment was terminated when only nitrogen is detected by the Micro-GC. After the reactor was cooled down, the spent biocarbon catalysts together with plastic-derived carbon were collected from the cracking reactor.For the continuous feeding experiment, 5 g of plastic was used for each testing cycle with 10 g of biocarbon catalyst placed in the cracking reactor. The Micro-GC was used to monitor the gas composition changes in the continuous feeding experiment. When the hydrogen peak was lower than the nitrogen peak, this test ends, and the gas collection was stopped. After changing the gas bag, the plastic sample is re-fed and a new test is started.The gas yield is calculated by multiplying the density of a single component by its volume, and then summing the results for all gas components. Futhermore, the liquid yield is calculated by measuring the weight increase of the cooling bottles before and after analysis.The ultimate composition analysis of biomass was conducted by using a “Vario EL cube” elemental combustion analyzer (Elementar Analysensysteme GmbH, German) located at KTH. The oxygen content in all cases was determined by difference (100 %-C %-H %-S %) on a dry basis. Also, a Raman test was conducted by using Tyrode I located at KTH. TGA of plastics and carbon products were performed by using a NETZSCH STA 449 F3 Jupiter thermogravimetric analyzer located at KTH. SEM test was conducted by Ultra 55 in KTH. The composition of the syngas was determined by using a micro gas chromatograph with thermal conductivity detector (Micro GC-TCD, Agilent). The surface area and pore size distribution were tested by BET Surface Area Instrument ASAP 2060 in Uppsala University.In this study, the performance of the biocarbon catalyst is first investigated, which is quantified in terms of the mass ratio of the catalyst to the plastic. Specifically, PP, the common material to make personal protective equipment (PPE) (Bratovcic, 2021), is used as feedstock, and three different C/P ratios i.e. 0, 1, 2 (see Table 1, cases 0,1,2) are used. The results are shown in Fig. 2 .Carbon, liquid, and gases are three major types of products which are obtained in all cases. For the non-catalytic case, i.e. case 0, the yields of the carbon, liquids, and gases are 51.58 wt%, 7.93 wt%, and 40.49 wt%, respectively. The carbon yield (weight percent) is calculated by difference, i.e., carbon wt.% = 100%-liquid wt.%-gas wt.%. Because a certain percentage of carbon is flushed away by nitrogen and thereby hard to collect during the test. Liquids are mainly believed to be wax, adhering to the glass bottle walls. Due to the relatively low yield, the composition of liquid is not analyzed.H2 and CH4 are two major components of the gas, which contain some C2, C3 compounds. The yield of CH4 (15.32 mmol/g) is slightly higher than that of H2 (14.09 mmol/g). The case using a C/P ratio of 1, i.e. case 1, shows an 11.03 wt% higher carbon yield, a 17.14 mmol/g higher H2 yield, and a 5.26 mmol/g lower CH4 yield compared to case 0. The case using a C/P ratio of 2, i.e. case 2, shows that the hydrogen yield further increases to a value as high as 34.31 mmol/g (68.6 mg/gplastic). Meanwhile, the C2 yield decreased from 3.17 mmol/g (case 1) to 2.17 mmol/g. The result indicates that the increase of the C/P ratio from 1 to 2 further enhances the formation of H2 by facilitating the cracking of C2 compounds. Notably, the slight increase of the CH4 yield indicates that a further increase of the catalyst loading seems not to be favorable for the CH4 cracking. Compared to the non-catalytic case, all catalytic cases show higher carbon yield, but lower liquid and gas yields. The result indicates that the deployment of the biocarbon catalyst promotes a cracking of hydrocarbons derived from plastic pyrolysis into carbon and gas compounds. The transfer of carbon from gases to solids results in a reduced mass yield, but an increased molar yield of gases. The carbon and hydrogen distribution results are shown in Fig. S1 in the supplement. Compared to the non-catalyst case, catalyst case 1, with a C/P ratio of 1, the amount of hydrogen in the PP converted to H2 increased from 22.7% to 54.3%. Furthermore, the amount of carbon in the PP converted to carbon increased from 57.1% to 76.3%. Case 2, with a C/P ratio of 2, showed slightly lower conversion rates of the hydrogen and carbon in the PP to H2 (53.5%) and carbon (68.7%) compared to Case 1. However, Case 2 showed the lowest distribution ratios of hydrogen and carbon for C2–C4 compounds, indicating that the addition of biocarbon catalyst promoted the cracking of the C2–C4 compounds. In general, the use of the biocarbon catalyst significantly promotes the conversion of carbon in plastic into carbon products and hydrogen in plastic into hydrogen. A carbon yield as high as 580.6 mg/gplastic and a H2 yield as high as 68.6 mg/gplastic are obtained when using a C/P ratio of 2, which further confirms the advantages of using a biocarbon catalyst. Compared to similar studies, showed in Table S3, using biocarbon catalyst have a higher carbon yield and a desirable hydrogen yield.Another two types of commonly used plastics i.e. LDPE and HDPE (see Table 1, cases 3,4) have also been tested to show the biocarbon catalytic performance for different types of plastic waste. Based on the above results, the C/P ratio was fixed at a value of 2. The corresponding results are shown in Fig. 3 .Carbon, liquid, and gases are still the main products of the process, but the product yields vary for different plastic types. Compared to the results of Case 2, Case 3, using LDPE as the raw material, shows a higher carbon yield of 64.12 wt%, but a lower gas yield of 31.93 wt%. The H2 and CH4 yields decrease to 28.4 mmol/g and 8.78 mmol/g, respectively, while the C2 yield increase to 5.48 mmol/g. Case 4, which uses HDPE as the raw material, shows a higher gas yield (46.4 wt%) with an increase in CH4 yield to 14.31 mmol/g. This demonstrates that, for different types of plastics, the catalytic effect of biocarbon is slightly different. The best catalytic cracking effect is found in the test using PP where the highest H2 yield (34.3 mmol/g) is obtained. Moreover, the carbon and hydrogen distribution results, shown in the Supplement Fig. S2, indicate that for PP, the percentage of hydrogen in plastic convert into H2 is the highest (53.5%). The overall cracking effect of biocarbon catalysts for LDPE and HDPE is slightly weaker compared to that for PP. In terms of carbon and hydrogen distributions, the catalyst effect for LDPE appears to be better, as a higher percentage of carbon in the plastic is converted into carbon, and a higher percentage of hydrogen in the plastic is transformed into hydrogen, compared to the results obtained for HDPE. TGA curves of different plastic waste are provided in the Supplement Fig. S3. It can be seen that the decomposition temperature of the plastic waste is in following the order: PP, LDPE, HDPE. It could be seen that the catalytic effect of the biocarbon catalyst decreases with the higher decomposition temperature of the plastic, resulting in less hydrogen in plastic being converted into hydrogen and less carbon in plastic being converted into carbon products.Tests with continuous feeding of the plastic sample (PP, case 5) are conducted to study the stability of the biocarbon catalyst. As mentioned in the experimental section, 5 g of plastic is fed for each testing cycle and 10 g of biocarbon catalyst is placed in the cracking reactor. A complete collection of all produced gases for each cycle is challenging, as some gases can be retained in the reactor. However, based on the previous experiment, the peak area of gases detected by GC-MS have the same trend during the experiment, which means the gas fraction remained stable during the experiment. Therefore, the yield of CH4 has been verified to be closely related to the yield of other gaseous products. And a stable gas component to CH4 volumetric ratio for each cycle is calculated as the indicator of the catalyst performance. The results are shown in Fig. 4 . As seen, during the nine-cycle experiment, the H2 to CH4 ratios are maintained at values between 1.65 and 2 and the C2 compounds to CH 4 ratios are maintained at values between 0.3 and 0.4. The result indicates that the catalyst remained relatively stable during this test period. In total 45 g of plastic is used which indicates that more than 20 g of carbon is produced and mixed with 10 g of biocarbon catalysts. The relatively stable performance also indicates that the carbon products from plastics are prone not to cause a severe blockage of the pores in the biocarbon catalyst. If a more detailed analysis is performed, it can be seen that from the sixth test to the seventh test, the H2 to CH4 ratio plummets from a value of approximately 2.0 to about 1.7 and stabilizes at a value of approximately 1.65 in the subsequent tests. This means the accumulation of carbon products (from plastic) still has a certain effect on the biocarbon performance. Specifically, the production of CH4 seems to be promoted which indicates a certain decrease in the deep cracking ability of the catalyst. A more detailed characterization of the spent biocarbon catalyst will be conducted to understand the potential reason.The fresh biocarbon catalyst appearance is characterized by using SEM determinations. The corresponding SEM images are shown in Fig. 5 . Fig. 5a and b shows the appearance of biocarbon particles: Fig. 5a shows the aggregation state of the biocarbon particles with irregular shapes, and Fig. 5b focuses on the appearance of the individual biocarbon particles. In general, biocarbon pieces have a longitudinal length of about 1 mm or more, which is the result of the use of sawdust with particle size larger than 1.25 mm. Moreover, from Fig. 5a and b, the biocarbon pieces retain the original appearance of the woody sawdust, which is consistent with literature findings (Bridgwater et al., 1999). Correspondingly, elongated channels in the length direction could be observed on the surface of the biocarbon pieces, as shown more clearly in Fig. 5b. Also, Fig. 5c and d shows enlarged views of a specific biocarbon particle in the x-axis and y-axis directions. It can be seen that the biocarbon catalyst has a hierarchical and regular macroporous structure (Fig. 5c) with open channels through the length direction (Fig. 5d), which have been verified to be originated from the vessels in natural wood (Wang et al., 2017). Specifically, the open channels have inner diameters ranging between 10 and 30 μm and outer diameter between 13 and 35 μm. Instead of straight channels, channels with a certain tortuosity have been observed (Fig. 5d). Moreover, additional pores have been detected along the channel walls.Porous properties of the fresh biocarbon catalyst are characterized by applying a Nitrogen adsorption-desorption analysis. As shown in Table S4, the fresh biocarbon catalyst has a surface area of approximately 40 m2/g and a micro-pore surface approximately 37 m2/g indicating a micropore-rich structure of the biocarbon catalyst. N2 adsorption-desorption isotherm, shown in Fig. S4a, shows a IV-type isotherm, which confirms the coexistence of micropores and mesopores. Notably, owing to a micropore-rich structure, the adsorption and desorption isotherms does not overlap. The pore size distribution figure, i.e. Fig. S4b shows that the pore size of the pores inside the biocarbon catalyst. Specifically, pore sizes are concentrated in the range between 8 and 20 Å, which belongs to the micropore size range. This indicates that, in addition to the regular elongated macropores that formed channels, micropores that cannot be detected by the SEM also existed inside the biocarbon catalyst. The channel walls seem to be the only place where the micropores exist.From the catalyst perspective, a hierarchical macro- and micro-porous structure with elongated channels could provide a large surface area for reactions (Yang et al., 2019). A regular macroporous structure (Fig. 5c) with open channels could significantly promote a diffusion of the intermediates (diffusion into and outside of the catalyst) while maintaining a relatively low pressure drop (Custodis et al., 2016; Trogadas et al., 2016). Moreover, the channels with certain tortuosity have been reported to be able to extend the residence time of intermediates along the channel walls (Wang et al., 2017). Macropores and micropores on the channel walls could further promote the diffusion of intermediates and increase the tortuosity of the intermediates pathway (Bai et al., 2016). On a macroscopic level, the catalyst bed assembly using a biocarbon catalyst with a certain particle size can also form channels with a certain degree of tortuosity. Combining all of these, the residence time of intermediates i.e., the components that forms the plastic pyrolysis volatiles in the biocarbon catalyst bed would be significantly increased. Also an increased residence time at high-temperature catalyst bed would inevitably promote the cracking reactions of the intermediates (Hu et al., 2016). Correspondingly, the excellent performance of biocarbon catalysts for plastic volatiles cracking could be attributed to this. Additionally, due to the steric hindrance effect caused by the pore size (Jae et al., 2011), large organic molecules may encounter obstacles when accessing the catalytic site in the pore (Zhang et al., 2019), resulting in the micropores on the channel walls exhibiting selective catalytic properties for intermediates of different sizes. This could potentially explain the differences in product yield distributions among different cases, due to the different pyrolysis intermediates of PP, LDPE, and HDPE (Honus et al., 2018).The surface morphology and textural property of spent biocarbon catalyst are further characterized to show the catalyst change during the reactions. Specifically, the spent catalyst from the continuous feeding tests is used. Fig. 6 shows the SEM images of the spent biocarbon catalyst. As shown in Fig. 6a and b, compared with the fresh biocarbon catalyst, there is no significant change in the particle size for the spent catalyst. It implies that the rapid flushing and reaction of plastic pyrolysis volatiles does not result in a significant breakage of biocarbon particles during the experiment. However, the catalyst surface, especially the exposed open channels is partially covered, as seen in Fig. 6b. To observe the surface of the spent biocarbon more clearly, a higher magnification is chosen, and the enlarged views are displayed in Fig. 6c and d. Extra carbon products covering the surface of the elongated channels are observed clearly. Undoubtedly, these extra carbon products are generated from a plastic pyrolysis and a subsequent cracking of the pyrolysis volatiles. With a further magnification, the carbon products do not have regular morphologies, as shown in Fig. 6d. The results indicate that carbons derived from plastic could remain on the surface and inner pores of the biocarbon catalyst, which may further lead to a pore blockage of the catalyst. The XRD analysis results of both fresh and spent catalyst are provided in Fig. S5. It can be observed that the peak representing the graphite 002 crystal plane has shifted towards the right and has become sharper in the XRD figure. This shift indicates that the catalyst tends towards graphite gradually after use due to prolonged heating and the deposition of carbon products in the pores of the biocarbon catalyst. However, the peak corresponding to the graphite 100 crystal plane does not show significant changes. Table S4 also provides the textural property characterization results for the spent catalyst sample. It can be seen that the spent catalysts have a surface area value lower than 1. This, in turn, indicates a non-porous structure. Correspondingly, it makes no sense to show adsorption-desorption isotherm. The result indicates that the micropores on the channel walls were almost completely blocked. This further confirmed that the carbon derived from the plastic could cause blockage of the catalyst, and thereby cause a deactivation of the catalyst.After the continuous feeding test, plenty of individual carbon products that do not adhere to the biocarbon catalysts are also obtained. This indicates that only a certain percentage of carbon could remain on the surface or inside the inner pores of the catalyst. This is also the reason why the catalyst performance is kept relatively stable after 9 testing cycles. SEM images of these individual collected carbon products are given in Fig. 7 . As shown in Fig. 7a and b, the overall size of the carbon products are relatively small (<50 μm) and the vast majority have a spherical shape. A clear comparison view between biocarbon particles and carbon spheres are provided in Fig. 7b, where several fine biocarbon particles are mixed with carbon products. More apparent carbon spheres could be observed from enlarged views shown in Fig. 7c and d. It can be seen that some of these carbon spheres are independent of each other, and that some of them are stacked to form larger carbon spheres or even carbon strips. More specifically, the diameters of the carbon spheres varies from 10 μm to 50 μm or larger. This is because the catalysts used in the present study are metal-free, it is believed the cracking of the pyrolytic volatiles results in the agglomerated carbon product in the absence of the metal catalyst (Veksha et al., 2022). This mechanism is similar to the nucleation process observed in the formation of carbon black, where hydrocarbon molecules crack at high temperatures and form spherical carbon on the surface (Jun et al., 2022; Smith, 1982).Raman determinations of the individual carbon products have also been conducted to show the degree of order of the carbon atoms. The corresponding Raman spectra result is shown in Fig. S6. A D-peak represents carbon atoms in a disordering state, and a G-peak represents carbon atoms in the ordering state. These are the two major peaks for Raman spectra (Muzyka et al., 2018). Also, the D-peak is defined as the band between 1320 cm−1 and 1365 cm−1. Furthermore, the G-peak is defined as the band between 1520 cm−1 and 1600 cm−1 (Ferrari, 2007). The ID/IG ratio represents the degree of material defects and disordered structures (Cheng et al., 2021). The ID/IG value of the carbon products is around 1. This demonstrates that the carbon atoms in the individual carbon products have a certain degree of order. Moreover, a clear 2D peak that represents the graphite layer structures could also be observed. This also indicates a certain degree of order of carbon atoms. According to the literature, carbon products from PP plastic belong to soft carbon, which could be used for a series of applications such as batteries (Yaqoob et al., 2022), adsorbents (Shen et al., 2022), and industrial additives (Asl et al., 2018). The regular spherical shape observed by SEM, and the certain degree of order of carbon atoms detected by Raman spectra both indicate the formation of promising carbon products that may be utilized in a variety of applications. Notably, biocarbon derived from biomass is hard carbon, which is not graphitizable (Alvira et al., 2022). To improve the electrochemical performance of hard carbon, many studies have been reported attempts to coat additional soft carbon in the surface and internal pores of hard carbon (He et al., 2021; Lee et al., 2007). This is practically the same as done in the current study. Therefore, the spent biocarbon catalyst also has promising application potential. A further exploration of the applications of the individual carbon products as well the spent biocarbon catalysts mixed with plastic derived carbon will be the focus of our future research.A hierarchical porous biocarbon catalyst has been prepared via the combination of using a biomass pyrolysis and a subsequent carbonization at a certain temperature. The catalyst performance for a waste plastic pyrolysis and an in-line catalytic cracking has been studied to coproduce hydrogen-rich gases and carbon products. The main conclusions are as follows:Compared to the non-catalytic case, all catalytic cases show higher carbon and H2 yields. The deployment of the biocarbon catalyst promotes a cracking of hydrocarbons derived from the plastic pyrolysis into carbon and gas compounds. An increae of the catalyst-to-plastic (C/P) ratio from 0 to 2, leads to an increase of the conversion rate of H in plastics to H2 from 22.7% to 53.5%. Also, a maximum H2 yield of 34.31 mmol/g plastic (68.6 mg/gplastic) could be obtained by pyrolysis (550 °C) and catalytic cracking (C/P = 2, 900 °C) of PP. The biocarbon catalyst is suitable for different plastics and all applications result in carbon yields of more than 50 wt% (500 mg/gplastic) as well as high hydrogen yields above 28 mmol/g plastic (56 mg/gplastic). Moreover, in the continuous experiments, the catalytic activity of the biocarbon catalyst does not decrease significantly, which indicates that the catalyst has a longer lifetime.Biocarbon catalysts have a hierarchical macro- and micro-porous structure consisting of elongated and tortuous channels. The tortuous channels and the existence of the macropores and micropores on the channel walls could significantly increase the tortuosity of the gas flow. Combined with tortuous channels formed by the accumulation of biocarbon of a certain size, the residence time of gases could be significantly promoted. This allows the biocarbon catalyst to effectively facilitate the cracking reactions resulting in an increase of the hydrogen and carbon yields.A spent biocarbon catalyst contains a certain amount of plastic derived carbons coating on the surface of the catalyst. The individual carbon products have regular spherical shapes and the carbon atoms have a highly ordering state. Both the individual carbon products and the spent catalyst have the potential to be used in a variety of applications.Yanghao Jin: Investigation, Validation, Formal analysis, Visualization, Writing – original draft. Hanmin Yang: Investigation, Validation, Formal analysis, Visualization, Supervision. Yanghao and Hanmin contribute equally to this article, Guo Shuo: Investigation, Conceptualization., Ziyi Shi: Validation, Formal analysis, Visualization. Tong Han: Conceptualization, Methodology, Data curation, Supervision, Writing-Reviewing and Editing, Formal analysis. Ritambhara Gond: Investigation, Weihong Yang: Resources, Writing-Reviewing and Editing, Supervision, Project administration. Pär G. Jönsson: Supervision, Writing-Reviewing and Editing.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Yanghao Jin reports financial support was provided by Sweden's Innovation Agency. Hanmin reports financial support was provided by Chinese scholarship Concil. Tong Han reports a relationship with Chinese Scholarship Council that includes: funding grants.The supply of raw materials for the experiments by Envigas is greatly appreciated. The financial support by VINNOVA-Swedish Innovation Agency with the project number 2021–03735 is higher appreciated. Tong Han and Hanmin Yang, would also like to acknowledge funding from Chinese Scholarship Council (CSC).The following is the Supplementary data to this article. Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2023.136926.

Carbon and H2 recoveries from plastic waste enable high value-added utilizations of plastic waste while minimizing its GHG emissions. The objective of this study is to explore the use of a metal-free biocarbon catalyst for waste plastic pyrolysis and in-line catalytic cracking to produce H2-rich gases and carbon. The results show that the biocarbon catalyst exhibits a good catalytic effect and stability for various plastic wastes. Increasing the C/P ratio from 0 to 2, induce an increase in the conversion rate of C and H in plastics to carbon and H2 from 57.1% to 68.7%, and from 22.7% to 53.5%, respectively. Furthermore, a carbon yield as high as 580.6 mg/gplastic and an H2 yield as high as 68.6 mg/gplastic can be obtained. The hierarchical porous structure with tortuous channels of biocarbon extends the residence time of pyrolysis volatiles in the high-temperature catalytic region and thereby significantly promotes cracking reactions.

With the reduction of industrial hydrogen production costs and the development of hydrogen fuel cell technology, hydrogen has gradually become one of the most promising clean energy in the 21st century [1]. Noticeably, hydrogen storage is a crucial link in rolling out infrastructure construction to build a “hydrogen economy,” especially in terms of the extensive applications in hydrogen compressor, fuel cell vehicle (FCV), as well as grid-scale hydrogen energy storage [2–5]. More specifically, it is still urging to develop hydrogen storage technologies with the characteristics of high gravimetric capacity, low cost, high safety, and reliability. Compared with the high-pressure gaseous and low-temperature liquid storage technologies, the solid-state hydrogen storage technology is highly promising, due to its relatively higher gravimetric or volumetric density, safety, and economy [6,7]. Owing to the high theoretical gravimetric capacity (7.6 wt.% for pure MgH2) and relatively abundant resources, Mg-based materials have been receiving widespread coverage from researchers as one of the most promising carriers for on-board hydrogen storage devices [8–10]. But the relatively high desorption temperature (>300 °C) and slow sorption kinetics make them difficult to meet the requirements of the fuel cell module (<85 °C) and on-board hydrogen storage devices. Besides, Mg-based materials still have the problem in terms of the capacity fade during the ab/desorption cycles.Doping additives or catalysts, such as transition metal-based catalysts (such as Nb, Ti, V, and Ni, etc.), has been considered as one of the most effective strategies to improve the hydrogen storage performance of Mg/MgH2. It can effectively accelerate the dissociation and combination of hydrogen atoms and decrease the activation energy for hydrogen desorption [11–14], thus improving the dehydrogenation kinetics and lowering the dehydrogenation temperatures [15–21]. For example, Wang et al. [15] demonstrated that the NNb2O5 (10 wt.%)-doped MgH2 composite has an excellent desorption performance, releasing 5.0 wt.% H2 at 250 °C within 3 min. By exfoliating the Ti3AlC2 powders to synthesize 2D Ti3C2 (MXene), Liu et al. [20] obtained MgH2 containing 5 wt.% Ti3C2 that can release 6.2 wt.% H2 at 300 °C within 1 min, exhibiting superior dehydrogenation kinetics to counterparts doped with other Ti-based catalysts. In particular, metallic Ni is also an efficient and low-cost catalyst to significantly improve the hydrogen storage performance of Mg/MgH2. To name a few, Liu et al. [21] added the porous Ni@rGO to MgH2 by ball milling, and the formed MgH2+5 wt.% Ni@rGO nanocomposite can still release 6 wt.% H2 at 300 °C within 10 min after the 9th cycle. Although the excellent catalytic effects, the amount of doped catalyst should be limited largely, especially for some heavy elements since it would lower the practical hydrogen storage capacity of the Mg/MgH2 system. Therefore, the catalytic activity of the doped catalyst should be effectively improved to promote the dehydrogenation kinetics of MgH2 as much as possible, so as to maintain the high hydrogen storage capacity of Mg/MgH2.Recently, it is interesting to find that the reduction in size and improvement of dispersity would increase the catalytic activity of the catalyst [22–25]. For instance, Zhang et al. [23] employed a wet-chemical method to prepare the NbH x (∼10–50 nm-sized nanoparticles) and doped it into MgH2 to improve its hydrogen storage properties, which can release 7.0 wt.% H2 within 9 min at 300 °C. In addition, their experimental results also concluded that the smaller the particle size of the NbH x was, the better catalytic effect on hydrogen storage performance of MgH2 would be. In this regard, Chen et al. [24] reported that the well-distributed Ni nanoparticles (NPs) can provide more active catalytic sites for the absorption and desorption cycles. Specifically, the homogeneous distribution of super Ni NPs (uniform size of ∼10–20 nm) on the surface of MgH2 was achieved by breaking the 1D fibrous Ni via ball milling. Accordingly, the MgH2 doping with 4 mol% Ni NPs composites can dehydrogenate 7.02 wt.% H2 within 11 min at 325 °C. More impressively, the ultrafine catalyst with homogeneous dispersity can be tailored from some precursors, such as some transition metal MXene and metal organic frameworks (MOF) [26–30]. For example, Jia et.al [27] obtained ultrafine Ni NPs (2–3 nm) from Ni-MOF-74 in the MgH2 matrix by a mechanochemical-force-driven procedure, which improved the hydrogen absorption/desorption processes of Mg/MgH2 and was proven by theoretical calculations and experiments. Huang et al. [30] used MOF as a precursor to homogeneously disperse metallic Ni on Ti3C2. The synthesized MgH2+10 wt.% Ni@C-MXene composite can release about 5.6 wt.% H2 within 2 min at 300 °C and absorb approximately 5 wt.% H2 within 2 min under 3.2 MPa at 150 °C, possessing an excellent cycling stability (e.g., without obvious decay for both capacity and kinetics after 10 cycles). Particularly, a common Ni-based metal-organic complex named nickel acetylacetonate (Ni(acac)2) has been often used as a precursor for high-efficiency catalysts [31]. In comparison with other Ni-based compounds (e.g., NiCl2 and NiF2), Ni(acac)2 as catalyst precursor has a lower melting point (238 °C) [32,33], which is beneficial for the smaller particle size and better dispersion [34]. Nonetheless, many ultrafine catalysts-doped Mg/MgH2 systems still show the obvious degradation of cycle stability. [35].It has been found that the addition of carbon can be used as a grinding aid to inhibit the grain aggregation and growth of Mg/MgH2 during cycle-life (kinetics). Various carbon-based materials, such as activated carbon, carbon nanotubes, graphite, graphene, and its derivatives, are considered as additives [36–40], among which carbon nanotubes and graphite are typical representatives for the ideal candidates. For example, Liu et al. [41] supported the Co/Pd catalysts on bamboo-shape carbon nanotubes to obtain MgH2 Co/Pd@B-CNTs composite, which can absorb 6.68 wt.% H2 at 250 °C within 10 s. Wang et.al [42] developed a graphene-guided and growth process to prepare N-doped Nb2O5@C nanorods and the MgH2 with 10 wt.% N-doped Nb2O5@C can release 6.2 wt.% H2 from 170 °C to 270 °C, which has a capacity retention of 98% after 50 cycles. It should be noted that expanded graphite (EG) is one of the cheapest and most efficient carbon materials [37].The above descriptions indicate that a suitable transition metal catalyst precursor and carbon materials (especially the EG) could be introduced into the Mg/MgH2 matrix to in situ form the ultrafine and well-dispersed catalyst with high catalytic activity, thereby improving the dehydrogenation kinetics and cycle stability of Mg/MgH2 system while maintaining the high hydrogen capacity (e.g., over 7 wt.%) for the target of the on-board application. Herein, a facile one-step high-energy ball milling technique has been developed to in situ form ultrafine Ni nanoparticles catalyst in the MgH2 matrix, combining the nickel acetylacetonate as a precursor and EG. On one hand, the in situ formed ultrafine Ni nanoparticles catalyst from the Ni(acac)2 can significantly improve the desorption kinetics of MgH2. On the other hand, the cycle performance of Mg/MgH2 is improved by the low-cost and effective EG. Consequently, the formed MgH2 Ni-EG nanocomposite with the optimized doping amounts of Ni and EG can release 7.03 wt.% H2 within 8.5 min at 300 °C after 10 cycles. The exceptional hydrogen storage performance was credited to a 26.9% decrease in the dehydrogenation activation energy in comparison with pure MgH2. In addition, the evaluation process of Ni(acac)2 and was revealed on the basis of the microstructural characterization analysis.The high-purity Ni(acac)2 (99%, Aladdin), MgH2 (98%, Aladdin), Ni powder (99%, Maclin), and expandable graphite (XingRuiDa Graphite manufacturing Co., Ltd.) were used as raw materials. The received expandable graphite was annealed in an Ar atmosphere at 1300 °C for 2 h, and then sintered in a H2 atmosphere at 400 °C for 4 h to obtain EG. Ni(acac)2 was doped into the commercial MgH2 at mass percentages of x = 1 wt.%, 3 wt.%, 5 wt.%, 7 wt.%, and 10 wt.%, respectively, via a vibration-type ball mill (QM-3C, Nanjing, China) at 1200 rpm for 5 h under H2 pressure of 1.5 MPa. Further, the EG was introduced into MgH2 with Ni(acac)2 together by ball milling. The mass ratio of MgH2, Ni(acac)2, and EG is 97:1.5:1.5 (denoted as MgH2 Ni-EG). For the comparison, pure Ni powder was also doped into MgH2 (donated as MgH2 Nip-EG) by ball milling for 5 h The ball-to-sample radio was around 50:1 during the milling process, which was conducted for 30 min after every 30 min pause.The X-ray diffraction (XRD) equipped with Cu Kα radiation (λ = 0.15,418 nm) operated at 45 kV and 40 mA was used to identify the phase of the samples. The XRD data was captured in a 2θ range of 15°∼85° with a step of 0.026°. A scanning electron microscope (Zeiss Supra-40) and a transmission electron microscope (JEM-2100, Japan) were used for observing morphologies and microstructures of the samples. X-ray photoelectron spectroscopy (XPS) spectra (Thermo Fisher Scientific K-Alpha) were performed with a monochromatic Al Kα X-ray source at a base pressure of 5 × 10−9 mbar to obtain the relevant valence information about the sample. The XPS data were fitted using Avantage software.The hydrogen sorption properties of materials were measured by using PCT Pro2000, in which the sample with a mass of 180 ± 5 mg was loaded into a stainless-steel sample holder. For non-isothermal dehydrogenation tests, the sample was heated at a heating rate of 2 K/min under a vacuum environment for desorption. For isothermal measurements, the sample was heated to the preset temperature at a heated rate of 5 K/min and then start the next dehydrogenation experiments. In addition, the sample with the mass of 9.5 ± 0.5 mg was loaded into an alumina crucible for thermal analysis (DSC, Setaram SENSYS Evolution) and it was heated from room temperature to 450 °C at different rates (2, 7, 10 and 15 K/min, respectively) under an argon atmosphere. Before the measurement for DSC, the MgH2 Ni-EG and MgH2 Nip-EG composites were activated via dehydrogenation and hydrogenation procedures at 300 °C.Ni(acac)2 precursors with different mass fractions were added to MgH2 via vibratory-type high-energy ball milling to investigate the effect of Ni(acac)2 addition on the hydrogen storage performance of MgH2. Fig. 1 shows the XRD results of MgH2+x wt. Ni(acac)2 (x = 1, 3, 5, 7, and 10) samples after high-energy ball milling which can provide sufficient energy for the reaction between them. Obviously, the diffraction peaks associated with the β-MgH2, γ-MgH2, and a little amount of MgO phases can be well indexed in the ball-milled MgH2+x wt. Ni(acac)2 samples. Characteristic peaks related to Ni(acac)2 cannot be found, indicating the chemical reaction between MgH2 and Ni(acac)2 or the decomposition of Ni(acac)2 during the ball milling process. However, there are no diffraction peaks of Ni and/or Ni-based compounds in the XRD patterns. It may be due to the small content of the Ni phase or the small size in situ formed Ni particles from Ni(acac)2 (as evidenced by the HRTEM observations in the following section), which might result in the corresponding diffraction peaks being too weak to detect. In addition, it should be noted that the peak intensity of the MgO phase become stronger with the increase of the mass percent of Ni(acac)2 (Fig. S1), which suggests that the O element might come from the C = O group of Ni(acac)2 to facilitate the formation of the MgO phase.Non-isothermal dehydrogenation curves of MgH2+x wt.% Ni(acac)2 samples (x = 1, 3, 5, 7, and 10) were firstly tested to explore the influence of Ni(acac)2 precursor on the hydrogen storage performance of MgH2, which was also compared with the pure MgH2 treated under the same conditions of ball milling. Specifically, the temperature at which the sample releases 0.1 wt.% H2 was used as the initial dehydrogenation temperature during the non-isothermal dehydrogenation test. As shown in Fig. 2 (a), it can be clearly seen that the initial dehydrogenation temperature of the MgH2+x wt.% Ni(acac)2 samples decrease with the increase of the addition of Ni(acac)2, i.e., around 260 °C, 254 °C, 245 °C, 243 °C and 234 °C for x = 1, 3, 5, 7, and 10, respectively. These initial hydrogen release temperatures of all MgH2+x wt.% Ni(acac)2 samples are lower than 265 °C of as-milled pure MgH2. In addition, the kinetics of MgH2+x wt.% Ni(acac)2 samples in the subsequent dehydrogenation are better than that of the as-milled MgH2 sample. It should also be noted that the hydrogen storage capacity of the MgH2+x wt.% Ni(acac)2 samples is decreased with the increase of the addition of Ni(acac)2.Furthermore, the first and second-cycle isothermal dehydrogenation curves of the MgH2+x wt.% Ni(acac)2 samples and as-milled MgH2 at 300 °C under initial H2 pressure of 0.05 bar were given and compared in Fig. 2(b) and (c), respectively. Obviously, all the MgH2+x wt.% Ni(acac)2 samples demonstrate the much better dehydrogenation kinetics than the as-milled MgH2 sample. Fig. 2(b) shows that the MgH2+x wt.% Ni(acac)2 samples can release H2 ranging from 7.26 wt.% to 6.66 wt.% at 300 °C in the first cycle, with increasing the doping amount of Ni(acac)2 from 1 wt.% to 10 wt.%. And the increment in the doping amount of Ni(acac)2 could also speed up the dehydrogenation kinetics of MgH2 in the first dehydrogenation process. Noticeably, the dehydrogenation kinetics of the MgH2+x wt.% Ni(acac)2 are further accelerated in the second cycle (Fig. 2(c)), especially for the MgH2+x wt.% Ni(acac)2 samples with relatively small Ni(acac)2 amount. For example, the MgH2+1 wt.% Ni(acac)2 sample spends ∼ 1580s to release 6.9 wt.% H2 in the first dehydrogenation process, as shown in Fig. 2(b-c), which is significantly reduced to be ∼ 758 s in the second dehydrogenation process. When the addition of Ni(acac)2 exceeds a certain value (i.e., higher than 3 wt.%) at 300 °C, the dehydrogenation kinetics of MgH2+x wt.% Ni(acac)2 samples would keep unimproved in the second dehydrogenation cycle, while the hydrogen capacity is decreased correspondingly, as shown in Fig. 2(c). In other words, the MgH2+3 wt.% Ni(acac)2 sample may exhibit the best combination of the dehydrogenation kinetics and hydrogen storage capacity (∼ 6.9 wt.% H2 for the second dehydrogenation test) at 300 °C, showing the sufficient catalytic effect without scarifying the hydrogen capacity. This optimized doping amount is very important for designing the subsequent experiments, which will be discussed later.In addition, the isothermal dehydrogenation kinetics curves of the MgH2+x wt.% Ni(acac)2 samples were measured at a lower temperature of 275 °C under initial H2 pressure of 0.02 bar to further understand the effect of Ni(acac)2 addition on the dehydrogenation kinetics of MgH2, as shown in Fig. 2(d), exhibiting the significantly enhanced dehydrogenation kinetics performance in comparison with the as-milled MgH2 sample. Noticeably, when the added amount of Ni(acac)2 is increased to 5 wt.%, the improvement in the dehydrogenation kinetics of MgH2 will be not obvious anymore. For instance, the MgH2+5 wt.% Ni(acac)2 sample releases 6.27 wt.% H2 within 10 min and 6.5 wt.% H2 within 13 min, respectively. Similarly, the 10 wt.% Ni(acac)2-doped sample can release the slightly less hydrogen capacity of 6.15 wt.% within 10 min, which might be caused by the more addition of Ni(acac)2. Different from that at 275 °C, it is noted that the dehydrogenation kinetics of the MgH2+10 wt.% Ni(acac)2 sample can be significantly faster than MgH2+5 wt.% Ni(acac)2 sample at two lower temperatures of 250 °C under initial H2 pressure of 0.002 bar and 225 °C under initial H2 pressure of 0 bar, respectively, as shown in Fig. 3 . It is reasonable to see in Fig. 3(a) that MgH2+10 wt.% Ni(acac)2 sample with more addition results in a lower reversible hydrogen storage capacity than the MgH2+5 wt.% Ni(acac)2 sample at 250 °C. However, Fig. 3(b) shows that both MgH2+5 wt.% Ni(acac)2 and MgH2+10 wt.% Ni(acac)2 sample can desorb equivalent H2 capacity of 5.6 wt.% within 120 min at the lower temperature of 225 °C, but the dehydrogenation kinetics of the former is slower than the latter. For example, the MgH2+5 wt.% Ni(acac)2 sample releases 3.6 wt.% H2 with 1 h, significantly lower than 4.6 wt.% H2 for the MgH2+10 wt.% Ni(acac)2 sample. The above dehydrogenation curves of MgH2+x wt.% Ni(acac)2 tested at various temperatures indicate that appropriately controlling the amount of the additive could achieve the optimal combination of the fast desorption kinetics and high hydrogen storage capacity at a certain temperature.The results of dehydrogenation tests suggest that MgH2+3 wt.% Ni(acac)2 sample might have the optimal combination of reversible hydrogen storage capacity and desorption kinetics at 300 °C. Subsequently, the cycle stability of the MgH2+3 wt.% Ni(acac)2 sample was further measured at 300 °C under initial H2 pressure of 0.05 bar using isothermal dehydrogenation mode, as shown in Fig. 4 (a). It is found that the capacity of MgH2+3 wt.% Ni(acac)2 sample is decayed rapidly from 7.15 wt.% in the first cycle to 6.73 wt.% in the fifth cycle during the dehydrogenation cycle-life (kinetics), showing the relatively poor cycle stability. This phenomenon may be related to the agglomeration of Mg/MgH2 and catalysts during high-temperature cycles [25,40,43], leading to the incomplete hydrogenation of Mg (also evidenced by our XRD results in Fig. 8(b) in the later section). Therefore, the prepared EG with the same weight fraction was introduced to improve the cycle stability of the MgH2, denoting as MgH2+3 wt.% EG. And the XRD pattern and SEM image of EG were shown in Fig. S2 and Fig. S3, respectively. Owing to the huge improvement in the cycle stability from EG, as shown in Fig. 4(b) and (c), the MgH2+3 wt.% EG sample can maintain the hydrogen capacity of 7.05 wt.% after 5 cycles. To balance the dehydrogenation kinetics and cycle stability, the Ni(acac)2 and EG were added to MgH2 together with a mass ratio of 1:1, namely MgH2:Ni(acac)2:EG=97:1.5:1.5 (donated as MgH2 Ni-EG)).Accordingly, the hydrogen storage performance, including the isothermal hydrogenation and dehydrogenation measurements, was characterized for the MgH2 Ni-EG nanocomposite. Fig. 5 (a) shows the isothermal dehydrogenation curves of MgH2 Ni-EG nanocomposite at different temperatures of 275 °C, 300 °C, and 320 °C, respectively. Excitingly, the MgH2 Ni-EG nanocomposite can release 7.0 wt.% H2 within 9.3 min at 300 °C and within 4.2 min at 320 °C, respectively. To the best of our knowledge, this should be the best performance in the literature in terms of the dehydrogenation hydrogen capacity of 7.0 wt.% for the Ni catalyst. The final dehydrogenation capacity of the MgH2 Ni-EG nanocomposite is reduced to 6.8 wt.% at 275 °C, which is still significantly better than pure MgH2 that only can desorb 0.55 wt.% H2 at 300 °C within the same period of dehydrogenation time. Fig. 5(b) shows the hydrogen absorption performance of the MgH2 Ni-EG nanocomposite at several different temperatures ranging from 50 °C to 125 °C under initial H2 pressure of 50 bar It turns out that MgH2 Ni-EG nanocomposite can absorb 5.6 wt.% H2 within 60 min at 100 °C, and the finally reach 6.3 wt.% within 2 h When the temperature increases to 125 °C, the hydrogen absorption kinetics of the sample is greatly accelerated, absorbing 6.0 wt.% H2 within 40 min and 6.51 wt.% H2 within 75 min. Even at a low temperature of 50 °C, 2.42 wt.% and 3.44 wt.% H2 can still be absorbed within 1 h and 2 h, respectively, which is much better than that of the pure as-milled MgH2. In addition, the hydrogen absorption kinetics of MgH2 Ni-EG and MgH2+3 wt.% Ni(acac)2 samples are given and compared in Fig. S4, which shows that the hydrogen absorption kinetics of MgH2 Ni-EG sample are slower than MgH2+3 wt.% Ni(acac)2 sample at relatively low temperatures of 50 °C and 100 °C. When the temperature increasing to 125 °C, the MgH2 Ni-EG sample exhibits a better hydrogen absorption kinetics and higher hydrogen absorption capacity than that of the MgH2+3 wt.% Ni(acac)2 sample, consistent with the dehydrogenation kinetics performance. Fig. 5(c) gives the dehydrogenation kinetics curves of MgH2 Nip-EG composite at the same temperatures of 275 °C, 300 °C, and 320 °C, respectively, to compare with the MgH2 Ni-EG nanocomposite using Ni(acac)2 as a precursor. Since the content of Ni in Ni(acac)2 is ∼22.6 wt.%, the actual Ni content in the MgH2 Ni-EG nanocomposite is 0.33 wt.%. Therefore, the metallic Ni powder with the amount of 0.33 wt.% and EG with 1.5 wt.% were selected to prepare the MgH2 Nip-EG sample for comparison. Fig. 5(c) shows that the MgH2 Nip-EG composite needs 25.5 min to desorb 7.0 wt.% H2 at 300 °C, obviously longer than that 9.3 min for the MgH2 Ni-EG nanocomposite. In addition, the MgH2 Ni-EG nanocomposite also shows faster dehydrogenation kinetics at both 275 °C and 320 °C. As a concluding point, Fig. 5(d) compares the dehydrogenation kinetics curves of different samples at 300 °C. Although the MgH2+3 wt.% Ni(acac)2 sample shows the best dehydrogenation kinetics, the poor cycle stability and the loss of theoretical hydrogen storage capacity due to the reaction of Ni(acac)2 with MgH2 lead to the necessity to further optimize its performance. Thanks to the excellent catalytic performance of the catalyst, the MgH2 Ni-EG nanocomposite can not only speed up the dehydrogenation kinetics rate but also increase the dehydrogenation capacity, compared to the MgH2+3 wt.% EG, MgH2 Nip-EG, and as-milled MgH2 samples. Thus, replacing part of Ni(acac)2 by EG to improve the cycle stability of the composite is necessary, as indicated by our experimental results.Further, the effects of Ni(acac)2 and EG on the dehydrogenation process of MgH2 Ni-EG nanocomposite was analyzed by DSC in Fig. 6 (a), which indicates that the dehydrogenation peak temperatures of MgH2 Ni-EG nanocomposite are 293.20 °C, 321.35 °C, 329.20 °C, and 340.04 °C at the heating rates of 2, 7, 10, and 15 K/min, respectively. And the pure MgH2 and MgH2 Nip-EG were also studied by the DSC method, as shown in Fig. S5 and Fig. S6. Based on DSC curves at different heating rates, the dehydrogenation activation energy (Ea ) can be calculated using Kissinger's method as follows [44]: (1) d ( l n β T m a x 2 ) d ( 1 T m a x ) = − E a R where Ea is the apparent activation energy (kJ·mol−1), β is the heating rate(K/min), Tmax is the absolute temperature for the maximum reaction rate(K), and R is the gas constant(J/(K·mol)), respectively. Based on Eq. (1), the Ea can be obtained by linearly fitting the slope of the plot with ln(β/T2 max ) versus 1/T (Fig. 6(b)). Accordingly, the dehydrogenation activation energy (Ea ) of MgH2 Ni-EG nanocomposite is linearly fitted to be 114.7 kJ·mol−1, which is lower than 133.0 kJ·mol−1 for MgH2 Nip-EG sample and 157.1 kJ·mol−1 for pure MgH2, respectively. On the other hand, Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation can be expressed as [42]: (2) ln [ − ln ( 1 − α ) ] = η ln k + η ln t where α is the dehydrogenation reaction fraction at time t, η is Avrami index, k is the dehydrogenation rate constant and t is the reaction time, respectively. Combining the Arrhenius equation, the dehydrogenation activation energy of the MgH2 Ni-EG sample can also be obtained by a linearly fitting plot with lnk versus 1000/T. Fig. 6(c) shows the fitting results of the dehydrogenation kinetics curves for the MgH2 Ni-EG sample at 275 °C, 300 °C, and 320 °C, respectively, contributing to a dehydrogenation activation energy of 118.1 kJ·mol−1 in Fig. 6(d). It should be noticed that the dehydrogenation activation energy of the MgH2 Ni-EG sample calculated by the above two methods is very close. Both the high reversible hydrogen storage capacity of 7 wt.% and the faster dehydrogenation kinetics for the MgH2 Ni-EG sample can be understood by the decreased dehydrogenation activation energy.The excellent reversible hydrogen storage capacity of the MgH2 Ni-EG sample over 7.0 wt.% was also confirmed by the dehydrogenation PCI curves (Fig. 7 ) at 300 °C, 320 °C, and 340 °C, respectively. Consistently, the dehydrogenation PCI curves in Fig. 7(a) show that MgH2 Ni-EG nanocomposite has a hydrogen storage capacity of ∼7.1 wt.% at all temperatures. In addition, based on the dehydrogenation plateau pressure corresponding to the different temperatures (Table S1), the linearly fitted slope of the van't Hoff curve (Fig. 7(b)) denotes the dehydrogenation reaction enthalpy (ΔH) of 76.5 kJ mol−1 for the MgH2 Ni-EG sample, which is almost equivalent to that of pure MgH2, indicating that the addition of Ni(acac)2 and EG hardly influences the thermodynamic property of MgH2.The XRD patterns at different states of MgH2 Ni-EG nanocomposite during ball milling and the cycle-life(kinetics) process were obtained in Fig. 8 to clarify the evolution process of Ni(acac)2. Fig. 8(a) shows that the diffraction peaks of β-MgH2, γ-MgH2, and trace amount MgO phases can be found in the as-milled MgH2 Ni-EG sample. As mentioned earlier, the formation of the MgO phase may be due to the reaction between MgH2 and Ni(acac)2 during ball milling (Fig. 8(c)). However, it should be noted that neither metallic Ni nor Ni-based compounds were detected from the XRD patterns in the as-milled, dehydrogenated, and rehydrogenated samples. It might be due to the low concentration of Ni being ultrafine particles. In this regard, the XRD pattern of as-milled, dehydrogenated, and rehydrogenated MgH2+10 wt.% Ni(acac)2 sample with a higher amount of Ni were obtained in Fig. 8(b), in which Mg2Ni and Mg2NiH4 phases can be well-indexed. Admittedly, the in situ formed Mg2Ni/Mg2NiH4 can actively affect the re/dehydrogenation process of MgH2 as a “hydrogen pump” [45].XPS was also used to analyze the chemical states of Ni in the sample after ball milling. Similarly, an effective XPS signal related to the Ni might still not be detected in the MgH2 Ni-EG nanocomposite because of the too low concentration of Ni. Therefore, the high-resolution spectrum of the Ni 2p was obtained from the as-milled MgH2+10 wt.% Ni(acac)2 sample, which is shown in Fig. 8(d). The Ni 2p spectrum exhibits two 2p1/2 and 2p3/2 contributions located at 852.38 and 869.98 eV, respectively, which can be assigned to Ni0. And the characteristic peak at 873.78 eV is the satellite peak of Ni0 2p1/2, also implying that the valence state of metal Ni is 0. In addition, Fig. 9 gives the HRTEM images of the microstructure and distribution of the in situ formed ultrafine Ni particles in the as-milled MgH2 Ni-EG nanocomposite. As marked by the yellow lines, two typical diffraction rings in SAED patterns can be indexed to (211) planes of MgH2 (PDF#01–074–0934) and (220) planes of metallic Ni (PDF#01–070–0989) phases, respectively, in the as-milled MgH2 Ni-EG sample. In addition, the HRTEM image of the as-milled MgH2 Ni-EG nanocomposite in Fig. 9(c) also confirms the existence of these two phases. More specifically, the interplanar spacing of d (111) = 0.287 nm for γ-MgH2, d (101) = 0.255 nm for □-MgH2, and d (111) = 0.204 nm for Ni are measured, respectively. In addition, a Fast Fourier Transform (FFT) pattern of the area circled by the red circle in Fig. 9(c) is also given in its insert, showing the (111) plane of the metallic Ni phase. It should be noted that the ultrafine metallic Ni (4–5 nm) is in situ formed and dispersed uniformly in the MgH2 matrix, as shown in Fig. 9(c), which is consistent with the reference work [27]. Therefore, the evolution process of Ni(acac)2 can be described as follows: (3) Mg H 2 + Ni ( acac ) 2 → MgO + Ni (4) Mg H 2 + Ni → M g 2 Ni + H 2 (5) M g 2 Ni + H 2 → M g 2 Ni H 4 The cycling stability of MgH2 Ni-EG nanocomposite has been evaluated in Fig. 10 for the purpose of the potential practical application. During the cycling testing, the MgH2 Ni-EG nanocomposite was operated to absorb H2 at 300 °C under initial H2 pressure of 50 bar for 25 min and then started the dehydrogenation kinetics test at the same temperature under initial H2 pressure of 0.05 bar Fig. 10 shows that the MgH2 Ni-EG nanocomposite is able to maintain a high hydrogen storage capacity of 7.03 wt.% even after 10 hydrogen ab/desorption cycles. It achieves a hydrogen capacity retention of up to 97.2% (Fig. S7), which is referred to the second-cycle capacity. Surprisingly, the MgH2 Ni-EG nanocomposite exhibits faster dehydrogenation kinetics with the increase of the cycle number. There is no doubt that the in situ formed ultrafine and uniformly dispersed metallic Ni from Ni(acac)2 precursor and the EG combined in the MgH2 Ni-EG nanocomposite can significantly improve the hydrogen storage performance of MgH2, including the dehydrogenation kinetics, high hydrogen capacity, and cycle stability. Fig. 11 shows the TEM results of the MgH2 Ni-EG sample after 10th dehydrogenation. It can be proved from the SAED patterns (Fig. 11(a)) that the main phases of the dehydrogenated MgH2 Ni-EG sample are Mg and Mg2Ni phases, which is consistent with the XRD results (Fig. 8(b)). The interplanar spacing d (101) = 0.245 and d (200) = 0.245 can be also well-indexed for Mg (PDF#01–089–7195) and Mg2Ni (PDF#01–075–1249) phases, respectively, in the HRTEM image (Fig. 11(b)). Note that the in situ formed Mg2Ni distributed around the Mg particles can act as catalytic active sites. In addition, the EDX analysis of the dehydrogenated MgH2 Ni-EG sample (Fig. 11(c-f)) shows that the Ni and C elements are still homogeneously distributed on the MgH2 matrix, in lieu of aggregation after 10 dehydrogenation/hydrogenation cycles. Therefore, the TEM observations reveal that the in situ formed Mg2Ni/Mg2NiH4 phase has been well-maintained with the cycles, in which the high-dispersibility of Ni and C elements might contribute to such superior cycling stability of the MgH2 Ni-EG system. Fig. 12 schematically summarizes the roles of the in situ formed Mg2Ni/Mg2NiH4 and EG in improving the hydrogen storage performance of MgH2 and the catalytic mechanism. More specifically, the Mg2Ni/Mg2NiH4 converted from the ultrafine metallic Ni and EG are homogeneously dispersed on the interface of the MgH2 particles. First, the highly dispersed Mg2Ni/Mg2NiH4 provides a large number of active sites for the hydrogen absorption/desorption reactions of MgH2, which can reduce the dehydrogenation activation energy and accelerate its hydrogen de/absorption kinetics. Secondly, the presence of EG can inhibit the grain agglomeration and growth of Mg/MgH2 at high temperatures and thus improve the cycle stability of the MgH2 Ni-EG nanocomposite. With the combined action of the in situ formed Mg2Ni/Mg2NiH4 and EG, the MgH2 Ni-EG nanocomposite not only can ensure suitable dehydrogenation kinetics but also show good cycle stability.In this work, a facile one-step high-energy ball milling process is developed to in situ form ultrafine Ni nanoparticles with uniform dispersity from the nickel acetylacetonate precursor in the MgH2 matrix. On one hand, the in situ formed ultrafine Ni nanoparticles catalyst from the Ni(acac)2 can significantly improve the desorption kinetics of MgH2. On the other hand, the cycle performance of Mg/MgH2 is improved by the low-cost and effective EG. After tailoring the amounts of the catalyst, the MgH2 Ni-EG nanocomposite can combine the individual functions from the ultrafine metallic Ni catalyst and EG to release 7.03 wt.% H2 within 8.5 min after 10 cycles, showing an exceptional hydrogen storage performance. The activation energy of dehydrogenation is reduced to 115 kJ/mol from 157.1 kJ/mol for pure MgH2. Additionally, the MgH2 Ni-EG sample can absorb 2.42 wt.% H2 within 1 h at a temperature close to room temperature (50 °C). As a result, the ultrafine metallic Ni (<5 nm) in situ formed and the Mg2Ni/Mg2NiH4 generated in the subsequent hydrogen absorption and desorption process play a critical role in the improvement of the hydrogen ab/desorption kinetics of MgH2. Our work provides a methodology to significantly improve the hydrogen storage performance of MgH2 by combining the in situ formed and uniformly dispersed ultrafine metallic catalyst from precursor and EG.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to acknowledge financial support from the National Basic Research Program of China (2018YFB1502100). XSY acknowledges the support from the PolyU grant (No. G-YW5N).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jma.2021.12.003. Image, application 1

It has been well known that doping nano-scale catalysts can significantly improve both the kinetics and reversible hydrogen storage capacity of MgH2. However, so far it is still a challenge to directly synthesize ultrafine catalysts (e.g., < 5 nm), mainly because of the complicated chemical reaction processes. Here, a facile one-step high-energy ball milling process is developed to in situ form ultrafine Ni nanoparticles from the nickel acetylacetonate precursor in the MgH2 matrix. With the combined action of ultrafine metallic Ni and expanded graphite (EG), the formed MgH2 Ni-EG nanocomposite with the optimized doping amounts of Ni and EG can still release 7.03 wt.% H2 within 8.5 min at 300 °C after 10 cycles. At a temperature close to room temperature (50 °C), it can also absorb 2.42 wt.% H2 within 1 h It can be confirmed from the microstructural characterization analysis that the in situ formed ultrafine metallic Ni is transformed into Mg2Ni/Mg2NiH4 in the subsequent hydrogen absorption and desorption cycles. It is calculated that the dehydrogenation activation energy of the MgH2 Ni-EG nanocomposite is also reduced obviously in comparison with the pure MgH2. Our work provides a methodology to significantly improve the hydrogen storage performance of MgH2 by combining the in situ formed and uniformly dispersed ultrafine metallic catalyst from the precursor and EG.

Data will be made available on request.The search for sustainable and clean energy sources has become one of the most important topics as the global crisis on energy and environmental issues is accompanied with unknown uncertainties. Hydrogen is endowed with clean features and possibility of large-scale productions via sustainable routes. Among several approaches for obtaining hydrogen, Fujishima and Honda devised a promising strategy through water splitting on a photosensitive semiconductor device [1], which has aroused great interest in the production of hydrogen by semiconductor photocatalysis [2].Many semiconductor photocatalysts have been explored so far. They are represented by not limited to metal oxides (TiO2, ZnO, WO3 and Bi2O3) [1,3–6], metal sulfides (CdS, ZnS and MoS2) [7–12], Bi-based compounds [13–15], Ag-based photocatalysts and g-C3N4 and etc [16–18]. Among these composites and their variants, ternary metal sulfides have been featured by their photocatalytic properties, tunable band gap, and good visible light absorption ability. As a typical member, ZnIn2S4 has received extensive attentions from scientists due to its suitable visible light absorption band gap (2.34–2.48 eV) and its reaction-friendly electrical properties [19–23]. It is associated with three different crystal polymorphs including trigonal, cubic, and hexagonal, with latter two active and suitable for photocatalytic hydrogen production under light irradiation [24]. With the existence of a surfactant, Bai et al. created a series of ZnIn2S4 that resembled flowers. The pH of the reactant was crucial in achieving the highest production of H2, which was 1545 μmol/g/h [25]. However, the weak ability to separate and transfer photo-induced charges and unavoidable photocorrosion are associated with the sulfide yet hindering practical application in photocatalysis. As a result, attempts have been made to limit carrier recombination and design an effective strategy to mitigate photocorrosion.To suppress high recombination rates of photogenerated charge carriers, numerous efforts have been taken place through modifications of photocatalysts, including morphology control [26,27], elemental doping [28], defect engineering [29], and heterojunction construction [30–32]. Among them, heterojunction construction is considered as a highly effective method to enhance photocatalytic performance. So far, various heterojunctions have been engineered and grouped to type-I, type-II, p-n type, Schottky junction, Z-type heterojunction, and S-type heterojunction etc. Despite of endeavors to separate photoinduced charges and facilitate charge transfers to heterosites, the mitigations to severe photocorrosion happening on photosensitive semiconductors have been overlooked. Very recently, intensive attentions were emphasized to this field [33–36]. For instance, with the “two birds with one stone” composite photocatalyst, Chun successfully solved the two pressing issues of S-metal bonding of sulfide readily oxidized by photogenerated holes and Ag+ of Ag-based photocatalyst [17]. According to Hao’s team, the photocorrosion of CdS can be effectively suppressed through the photocorrosion-recrystallization process [37].To reach low-cost and potential industrial applications, water splitting photocatalysts are further developed by substituting noble metal with transition metal (TM) complexes [38–40]. The Co(III)-dimethylglyoxime (dmgH) complex is considered as an essential unit for hydrogen production in cobalt oxime, according to earlier researches [41,42]. However, the complex’s low stability makes it unsuitable for sustained usage. A similar compound of nickel butanedione oxime (Ni(dmgH)2) has been noticed as a catalyst for the electrooxidation of methanol. This is ascribed to the cheap raw materials and simple synthesis method, and moreover the increased electrochemical reactivity of nickel ions [43]. By combining Ni(dmgH)2 with graphitic carbon nitride (g-C3N4) submicron lines and using the triethanolamine as a sacrificial agent, the complex catalyst designed by Cao’s team exhibited efficient hydrogen production (1.18 μmol/h) under visible light irradiation [44]. This indicates that it has considerable potential in the field of photocatalysis. Despite these prior arts, the Ni(dmgH)2 has been less commonly used for photocatalysis compared with other TM complexes, and its potential in the field is yet to be explored.Aiming to improvement of charge transfer in heterojunction, inhibition of sulfide photocorrosion, and facile preparation of photocatalysts, we have designed and constructed a novel ZnIn2S4/Ni(dmgH)2 (ZIS/NID) composite system for vigorous hydrogen evolution under UV–visible illumination. A high hydrogen evolution rate of 36.3 mmol/g/h was reached and associated with a large amount bubbles visually observable, in competence to hydrogen gas evolution seen in electrolysis [45]. It is further found that the sulfur ions generated by the photocorrosion of sulfide got involved in an in situ formation of the Ni-S intermediate which behaves as a co-catalyst and an electron trap to influence the electron transfer path. Thus, the photocatalytic hydrogen evolution capacity of the catalyst was greatly enhanced.Zinc chloride (ZnCl2), nickel nitrate hexahydrate (Ni(NO3)2·6H2O), ammonium hydroxide solution (NH3·H2O, 25.0 ∼ 28.0%), absolute alcohol (CH3CH2OH), hydrochloric acid (HCl, 36.0 ∼ 38.0%) and barium chloride (BaCl2) were purchased from Sinopharm Chemical Reagent Co., Ltd. Indium chloride (InCl3) and Thioacetamide (TAA) were purchased from Shanghai Maclean Biochemical Technology Co., Ltd. Dimethylglyoxime (dmgH) was purchased from Shanghai Aladdin Chemistry Technology Co., Ltd. All chemicals were analytical grade and used without further purification.The synthesis of ZIS followed protocol as documented in the literature [46]. An excess amount of thioacetamide (TAA, 6 mmol) was dissolved in 100 mL deionized water by adding 1 mmol ZnCl2 and 2 mmol InCl3 at a stoichiometric ratio. The solution was adjusted to pH 1 ∼ 3 with hydrochloric acid, and the flask was put in a 353 K water bath without stirring for 6 h. The flask naturally cooled down to room temperature when the reaction was completed. Centrifugation was used to collect the product, which was then washed several times with deionized water and anhydrous ethanol. For characterizations, the final sample was dried under vacuum for 6 h at 333 K to obtain ZIS.A simple chemical precipitation method was used to synthesize Ni(dmgH)2 and ZnIn2S4/Ni(dmgH)2 composites. First, Ni(NO3)2·6H2O (2.6 mmol) and ZIS (2.6 mmol) were dispersed in 60 mL of 95% ethanol and sonicated for 30 min to obtain solution A. Dimethylglyoxime (5.2 mmol) was dissolved in 60 mL of 95% ethanol to obtain solution B. Liquid A was added to liquid B alternately drop by drop under stirring and heating at 70 °C. Then the pH of the mixed solution was adjusted to 8–10 with 25%-28% NH3 ·H2O and the solution was mixed continuously under a water bath at 70 °C for 2 h to form a red suspension. The product was collected after centrifugation, washed repeatedly with water and ethanol, and then dried in a vacuum oven. Here, the amount of Ni(dmgH)2 in the composite was calculated based on the amount of Ni(NO3)2·6H2O.Pure Ni(dmgH)2 (NID) was produced by using a similar strategy only without the addition of ZIS. The final catalyst was named as ZnIn2S4/Ni(dmgH)2 with a molar ratio of 1:1 (abbreviated as ZIS/NID-1). Similarly, different molar ratios of ZnIn2S4/Ni(dmgH)2 (i.e., 0.5 and 2) were defined as ZIS/NID-0.5 and ZIS/NID-2 by changing the amount of ZnIn2S4, respectively. The synthesis process of the ZnIn2S4/Ni(dmgH)2 was shown in Scheme 1 .X-ray diffraction (XRD) measurements were carried out with a 0.02° step on a Bruker D8 ADVANCED diffractometer (Cu-Kα, λ = 0.15406 nm; 40 kV; 40 mA). Fourier transform infrared (FT-IR) spectra were collected on a Nicolet 6700 spectrophotometer. A JSM-6610LV scanning electron microscope (SEM), and a 200 kV JEM-2100F transmission electron microscope (TEM) were employed to study surface morphology evolution. The x-ray photoelectron spectra (XPS) measurements were recorded using a Perkin-Elmer PHI 5000C spectrometer with a monochromatic Al Kα incident source. A UV–vis spectrophotometer (UV-2550, Japan) was used to obtain the diffuse reflectance spectra. Steady-state photoluminescence (PL) spectra were recorded on a fluorescence spectrometer (F-4600 FL Spectrophotometer) with an excitation wavelength of 320 nm. Electrochemical measurements were carried out on an electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Corporation, China). During these experiments, samples were dry-pressed into thin films.A photocatalytic online analysis system (Lab Solar-III AG, Beijing Perfect Light) was used in conjunction with a gas chromatograph to perform the usual PHP test (GC-6890A). Firstly, 20 mg of photocatalyst was added into 100 mL aqueous solution, which was comprised of 10 vol% triethanolamine (TEOA) as the sacrificial reagent, then kept stirring until the reaction was completed. The above reactor was bubbled with nitrogen for 30 min before the photocatalytic experiment to guarantee that the reaction system was anaerobic. A 300 W Xe lamp (Beijing Perfect Light, PLS-SXE300) was used to irradiate the reactor. Meanwhile, a circulating condensate system was used to keep that the reaction was at 6 °C. Finally, the amount of hydrogen generated was measured by using a gas chromatograph with a thermal conductive detector (TCD). Visible light was simulated by equipping the xenon lamp with a 420 nm cut-off filter. By using different band-pass filters on the Xenon lamp (DT420 and DT475), under the identical circumstances, the apparent quantum efficiencies (AQEs) were obtained. The following equation was used to calculate the quantum efficiency. (1) In a three-electrode electrode cell, the photoelectrochemical analysis was conducted in a 0.5 M Na2SO4 aqueous solution. The reference electrode was Ag/AgCl chloride, while the counter electrode was Pt., and electrolyte was Na2SO4 (0.5 mol L-1) aqueous solution. Drop coating method was used to make the work electrode. The specific steps were as follows. 10 mg of the photocatalyst was dispersed in 20 μL of nafion, 0.5 mL of ethanol, and 0.5 mL of distilled water. The suspension was then sonicated for 0.5 h to form a homogeneous dispersion. 50 μL of the suspension was dropwise injected onto ITO and then dried at room temperature. For the photocurrent measurements, the light source was a 300 W Xe lamp. Electrochemical impedance spectra (EIS) were obtained in the frequency range from 105 to 0.1 Hz (applied potential of −0.2 V). The steady-state photocurrent density of the photoanode was further checked by using transient photocurrent measurements with chopper illumination. The Mott-Schottky plots were performed at 1.0, and 0.5 kHz with Na2SO4 aqueous solution as the electrolyte (0.5 M). The linear sweep voltammetry (LSV) was carried out at a scanning speed of 0.05 V/s.The crystal structure and phase compositions of the ZIS/NID, NID, and ZIS composites were evaluated by X-ray diffraction (XRD) as depicted in Fig. 1 (a-b). For the pure ZnIn2S4, the diffraction peaks at 21.1°, 27.7°, 47.3°, 52.4°, and 56.4° are respectively indexed to (006), (102), (110), (116), and (022) crystallographic planes in the hexagonal ZnIn2S4 (JCPDS No.65–2023) [47,48]. (110), (200), (130), (002), (112), and (240) crystal facets are found situating at 9.98°, 10.6°, 26.3°, 27.5°, 29.3°, and 36.0°, respectively (Fig. 1(a)), illustrating that the Ni(dmgH)2 produced has the same diffraction peaks as the published results (JCPDS No.55–1252) [44,49–52]. All characteristic signals can be observed in the sign of ZnIn2S4 and Ni(dmgH)2 for the ZIS/NID sample, showing that the composite has been properly constructed. Furthermore, as the amount of ZnIn2S4 integrated with the composite grows, the peak intensity of ZnIn2S4 increases, showing the presence of the combination.Similar findings were also confirmed by FTIR spectra where the surface functional groups are revealed in Fig. 1(c). For pure Ni(dmgH)2, IR spectra showed characteristic Ni-N vibrations at 428 and 520 cm−1 [53,54], while the peaks at 989,1101, 1238 and 1367 cm−1 may be attributed to NO stretching vibrations [55,56]. The characteristic peak at 1571 cm−1 can be attributed to CN stretching vibrations, the weak band at 1650–1823 cm−1 to OH bending vibrations [56], and the CH symmetric stretching vibrations were located at 2920 cm−1 [55]. The peaks in the spectra of pure ZnIn2S4 at 1616 cm−1 and 3430 cm−1 could be connected to the stretching vibrations of the adsorbed hydroxyl group and the HO-H group of the adsorbed water molecule [48]. The above XRD and FTIR results denote successful synthesis of the ZnIn2S4/Ni(dmgH)2 photocatalyst.SEM and TEM were used to characterize the morphology and microstructure of the synthesized photocatalysts. In Fig. 2 (a-b), the NID owns long but thin microrods morphology with the length ranging from 1 μm to 20 μm (both just grown and fully grown), and pure ZIS (Fig. 2(c-d)) exhibits a microsphere morphology with a rough surface. Fig. 2(e-f) show the SEM images of ZIS/NID, and it can be observed that ZIS/NID exhibits micron-level “rod” and “sphere” interaction structures associated with ZIS and NID. Fig. 2(g-i) depict the TEM images of ZIS/NID. The ZIS/NID exhibits a three-dimensional structure of ZIS microspheres loaded on the smooth surface of NID microrods. In addition, according to the composite’s HR-TEM image (Fig. 2(l)), for ZIS/NID, distinct lattice stripes at a distance of 0.32 nm and 0.29 nm can be seen, which agrees well with the value of the (102) and (104) plane in hexagonal ZnIn2S4 [24,57,58]. The result is also confirmed by the selected area electron diffraction (SAED) pattern in Fig. 2(j). Interestingly, the contact between ZIS and NID can be seen clearly. This feature is critical for charge transfer between the two components to be effective.The composites contained C, O, Ni, Zn, S, In, and N components, as determined by energy dispersive spectroscopy (EDS) (Fig. S1). Spatial distributions of elements in the ZnIn2S4/ Ni(dmgH)2 sample were investigated using elemental mapping analysis, as shown in Fig. S2(a-h). C, N, O, and Ni elements were consistently distributed throughout the composite, while S, In, and Zn components spread in the microsphere.The surface compositions and chemical states of the ZnIn2S4/Ni(dmgH)2 composites were investigated by X-ray photoelectron spectroscopy (XPS) measurements. The XPS survey spectrum of the ZIS/NID-1 sample (Fig. 3 (a)) shows that it consists mainly of C, N, O, Ni, Zn, In, and S elements, which agrees well with the EDS results. As shown in Fig. 3(b), the C 1s binding energy peak at 285.78 eV can be attributed to sp2 CC bonds in Ni(dmgH)2, and the peak of 284.8 eV is assigned to adventitious carbon. The remaining weak peaks at 288.27 eV can be assigned to the CO bonds, possibly from the absorbed CO2 and defective sample surface [59–61]. The peak at 531.17 eV is related to oxygen in water adsorbed on the catalyst surface, and the O 1s peak at 532.32 eV is related to the OH group in Ni(dmgH)2 (Fig. 3(c)). The peaks of 400.45 eV and 403.16 eV in the N 1s spectrum (Fig. 3(d)) are resulted from the CN (pyrrolic-type N) bond and its satellite peaks, respectively [62]. The Ni 2p spectrum shows four peaks at 854.8 eV, 872.1 eV, 858.78 eV, and 876.08 eV (Fig. 3(e)), attributing to Ni 2p3/2, Ni 2p1/2 and their satellite peaks, which indicate the presence of Ni2+ in the complex [18,51,63]. According to Fig. 3(f), S 2p can be split into two separate signals at 161.3 eV and 162.5 eV, which correspond to S 2p3/2 and S 2p1/2, respectively [64]. Fig. 3(g) shows two peaks at 444.7 eV (In 3d5/2) and 452.23 eV (In 3d3/2) in In 3d high-resolution XPS spectra, denoting existence of In3+ cation. Two main peaks can be assigned to Zn 2p3/2 (1021.61 eV) and Zn 2p1/2 (1044.64 eV), respectively in Zn 2p XPS figure (Fig. 3(h)), demonstrating the existence of Zn2+ ions [65,66].In addition to ascertaining the chemical valence states of surface elements, X-ray photoelectron spectroscopy (XPS) characterization facilitates the investigation of charge transfers among elements following their bonding schemes. Generally, a positive shift in binding energy implies a reduction in electron density, while a negative shift signifies an increase in electron density [67]. Thus, the migration pathways of electrons in heterojunction photocatalysts can be assessed by analyzing the shifts in binding energy observed in XPS spectra for heterogenous systems [68,69]. A noticeable shift towards lower binding energy of the peaks of S 2p, In 3d, and Zn 2p complex were found in the XPS of ZnIn2S4/Ni(dmgH)2–1 compared to those of ZnIn2S4 (Fig. 3(f-h)). Contrarily, the Ni 2p peak in the ZnIn2S4/Ni(dmgH)2–1 complex exhibits a significant shift towards higher binding energy, as compared to pure Ni(dmgH)2 (Fig. 3(e)). These changes in binding energy suggest that electron charge transfers happen from Ni(dmgH)2 to ZnIn2S4 in the ZnIn2S4/Ni(dmgH)2 composite. Overall, the XPS results indicate a robust interfacial coupling effect between Ni(dmgH)2 and ZnIn2S4, which may facilitate separation and migration of photogenerated carriers, ultimately leading to an improved photocatalytic performance of the ZnIn2S4/Ni(dmgH)2 complex [48,57].As shown in Fig. 3(i), the UV–vis diffuse reflectance spectroscopy (DRS) was used to assess the optical absorption parameters of pure Ni(dmgH)2, ZnIn2S4, and ZnIn2S4/Ni(dmgH)2 composites. Ni(dmgH)2 microrods exhibit a broad visible spectral absorption range, with an absorption edge of roughly 580 nm, whereas pure ZnIn2S4 has a shorter absorption range. When comparing pure ZnIn2S4 with ZnIn2S4/ Ni(dmgH)2, the light absorption edge of the ZnIn2S4/Ni(dmgH)2 composite displays a clear red shift, showing that the coupling of Ni(dmgH)2 and ZnIn2S4 leads to the formation of a photocatalyst complex that exhibits a broader light absorption range compared to pure ZnIn2S4, as supported by relevant literature [70,71]. The Kubelka-Munk formula was employed to calculate the band gap values (Eg) of Ni(dmgH)2 and ZnIn2S4 in general [64]. (2) α h v = A ( h v - E g ) n / 2 The absorption coefficient, Planck constant, optical frequency, constant value, and band gap energy of the photocatalyst are denoted by α, h, v, A, and Eg, respectively. ZnIn2S4, ZIS/NID-0.5, ZIS/NID-1, ZIS/NID-2, and Ni(dmgH)2 had Eg values of 2.04, 1.63, 1.48, 1.63, and 1.52 eV, respectively, as shown in Fig. 3(j).Under light irradiation, the photocatalytic hydrogen evolution (PHE) performance of Ni(dmgH)2, ZnIn2S4, and ZnIn2S4/Ni(dmgH)2 composites with various ratios were examined using 10% triethanolamine (TEOA) as sacrificial agents. According to the experimental findings (Fig. 4 (a-b)), a single fraction of Ni(dmgH)2 displayed a minimal PHE activity. With a hydrogen evolution rate of 7.4 mmol/g/h, similar results were seen when utilizing pure ZnIn2S4. The fast recombination of photogenerated electron and hole pairs is primarily responsible for the pure photocatalyst’s inactive PHE reaction. The application of ZIS/NID composites, however, offers the possibility of significantly boosting PHE activity, and the component ratio of a single catalyst has a significant impact on its PHE performance. The Ni(dmgH)2-ZnIn2S4 composites (ZIS/NID-0.5, ZIS/NID-1, and ZIS/NID-2) own different PHE rates of 13.6, 36.3, and 25.4 mmol/g/h, respectively. A comparison with other recent works on photocatalytic hydrogen evolution was summarized in Table S1.To study possible reaction sites in PHE, 100 mL of 10% vol methanol solution and 50 mL of 0.35 M Na2S/0.25 M Na2SO3 solution were used as sacrificial agents for comparison experiments. From Fig. 4(c), the amount of hydrogen evolution was greatly reduced, respectively. This is not surprising since the methanol was volatile, causing the solution’s methanol content to decrease. It was easily oxidized by h+ to produce formic acid, which would corrode the photocatalyst. In the Na2S/Na2SO3 case, too many sulfur ions would interrupt the reaction equilibrium and be hazardous to the formation of NiS intermediate. Furthermore, different sacrificial agents also had varying redox potentials, which would affect their consumption of cavities and thus lead to different amounts of hydrogen evolution. Moreover, it was discovered that the H2 production rate of pure ZnIn2S4 grew slowly after 3 h of photoreaction while the H2 production rate of ZIS/NID-1 increased after 6 h (Fig. 4(d)). As can be seen in Fig. 4(e), the AQE of the ZIS/NID-1 was also tested under single wavelength irradiations of 420 nm and 475 nm. The AQE reached a surprising value of 20.45%at 420 nm, and although it significantly decreased as input light wavelength increased, it still reached 5.02%at 475 nm. Four cycles of PHE experiment, each lasting three hours, were carried out to examine the stability of the ZIS/NID-1 composite in the photocatalytic hydrogen evolution reaction. As shown in Fig. 4(f), no discernible deactivation of PHE performance was observed after four additional runswithout anyaddition of hole scavengers, demonstrating the high stability of the ZIS/NID-1 composite photocatalyst. After the photocatalytic process, the stability of this photocatalyst was further supported by XRD, SEM and XPS, which revealed no changes to the crystal structure and elements (Fig. S3).It is important to note that the color of the photocatalyst, water, and triethanolamine suspension changed significantly before and after light irradiation. As shown in Fig. 5 (a-c), the suspension system color changes from red to black and back to red imply a photochromic phenomenon, whereas the black color during the light irradiation denotes the enhancement of the light absorption and maximization of conversion of photon energy to other forms. When the photocatalytic reaction was completed and left overnight, its color surprisingly returned to red again. The color transition under light irradiation was reversible to some extent, which confirmed its considerable hydrogen evolution activity after four cycles of experiments. A comparison of the color of pure ZnIn2S4 before and after the experiment was also performed. In Fig. 5(d-f), the original pale yellow color became partly cloudy after the reaction and remained almost unchanged after standing all night, which may be caused by the widely reported photocorrosion of the sulfide catalyst [17,36]. By coupling ZnIn2S4 and Ni(dmgH)2, the photocorrosion problem associated with the sulfide was effectively mitigated, thus greatly enhancing the photocatalytic hydrogen evolution performance.The experiments were also repeated to compare the performance of photocatalytic hydrogen evolution under visible and UV–visible light irradiation by equipping a 420 nm cut-off filter (Fig. S4). From Fig. S4(a-b), the ZIS/NID-1 composite achieved a hydrogen evolution rate of 4.4 mmol/g/h, which was about 3.4 times higher than that of pure ZnIn2S4. Also after four cycles, the rate did not decrease significantly, revealing its good stability (Fig. S4(c)). The excellent hydrogen evolution performance under both visible and UV–vis irradiation confirmed the applicability of this photocatalyst under a wide range of conditions. In addition, visible hydrogen bubbles produced by simulated sunlight during and after irradiation and real sunlight irradiation recorded by the picture and movie (Fig. S5 and movie1, 2 and 3) verified this statement.To verify the photoelectrochemical properties of the synthesized ZIS/NID-1 complexes, relevant characterizations were performed. The steady-state PL spectra of ZnIn2S4, Ni(dmgH)2, and ZIS/NID-1 were measured at the excitation wavelength of 320 nm. It can be seen that there are strong emission peaks around 470 nm and the ZIS/NID-1 catalyst exhibited the lowest fluorescence intensity compared to pure ZnIn2S4 and Ni(dmgH)2, indicating that it was more favorable for the suppression of recombination of photogenerated carriers via radiative decay (Fig. 6 (a)). The photocurrent of the ZIS/NID-1 composite was much larger than those of ZnIn2S4 and Ni(dmgH)2. The current increased with time, in line with the increase of hydrogen evolution rate of this photocatalyst with time up to the threshold. The increase of photocurrent and PHE rate indicate substantial enhancement of the charge transfer at the interface and suppressions of the recombination rate of photoinduced electron and hole pairs (Fig. 6(b)). Additionally, it can be seen from Fig. 6(c) of the electrochemical impedance spectrum (EIS) that the ZIS/NID-1 photocatalyst has a smaller semicircle than these for pure ZnIn2S4 and Ni(dmgH)2. The ZIS/NID-1 electrolyte interface’s charge transfer resistance was lower, resulting in a faster interfacial charge transfer and better carrier separation. Fig. 6(d) shows the linear scanning voltammetry (LSV) curves of the three prepared samples. It is clear that ZIS/NID-1 displays the lowest hydrogen evolution overpotential, which means that ZIS/NID-1 is the most favorable candidate where hydrogen precipitation reactions can occur.The band energies of Ni(dmgH)2 and ZnIn2S4 were also studied to understand possible transport paths for photo-generated carriers during photocatalysis. At AC frequencies of 1.0 and 0.5 kHz, the Mott-Schottky (M−S) curves of Ni(dmgH)2 and ZnIn2S4 were recorded. Positive slopes were seen in both ZnIn2S4 microspheres and Ni(dmgH)2 microrods, as illustrated in Fig. 6(e-f), which correlated to n-type semiconductor characteristics. In general, n-type semiconductors have a conduction band potential (ECB) that is approximately 0.1–0.3 V lower than their flat band potential (Efb) [72–74]. From the tangent intercept of the curve, the Efb of ZnIn2S4 microspheres and Ni(dmgH)2 microrods were determined to be −0.57 and −0.89 V, respectively (vs. Ag/AgCl.PH = 7). For ZnIn2S4 microspheres and Ni(dmgH)2 microrods, respectively. The observed flat-band potentials were −0.27 V and −0.59 V (vs. NHE), according to the Nernst equation. According to the literature [22,27], n-type semiconductors’ Fermi levels (Ef) are close to their CB. There, the VB potentials (EVB) of ZnIn2S4 microspheres and Ni(dmgH)2 microrods were calculated using the equation (EVB = ECB + Eg) and were determined to be + 1.77 and + 0.93 V (vs. NHE), respectively, based on the band gap energy obtained from UV–vis DRS. Fig. 6(g) showed the DMPO spin-trapping ESR spectra of ⋅O2 − over ZIS/NID-1 photocatalyst under dark and after 10 min light irradiation. The ⋅O2 − was not detected under dark or light conditions. In addition, the VB of Ni(dmgH)2 was calculated to be 0.94 V based on E NHE /V = Φ + 0.78 eV − 4.44 (E NHE : potential of normal hydrogen electrode and Φ of 4.6 eV: electron work function of the analyzer), as shown in Fig. 6(h). It was approximately the same as the valence band calculated by the Mott-Schottky (M−S) curves.Based on the above experimental results and characterizations, we systematically investigated the possible photocatalytic mechanisms leading to high HER performances of the present ZnIn2S4/Ni(dmgH)2 system. A possible direct Z-scheme for the current system was first presumed according to the band alignment as determined by the above PL and M−S method. Charge transfer mechanism was shown in Fig. 7 (a). Under light irradiation, the photogenerated e- in ZnIn2S4 CB will combine with the photogenerated h+ in Ni(dmgH)2 VB, leaving h+ in ZnIn2S4 VB and the e- in Ni(dmgH)2 CB active for hydrogen evolution. Then the h+ in ZnIn2S4 VB was consumed by TEOA and the e- in Ni(dmgH)2 CB react with the H+ to form H·, and sequentially the H2 [75]. However, since the CB potential of Ni(dmgH)2 was more negative (-0.59 eV, vs. NHE), the aggregated electrons on the CB of Ni(dmgH)2 can convert O2 to ⋅O2 – (-0.33 eV, vs. NHE). This, however, was not consistent with the DMPO spin-trapping ESR spectra results, vetoing the presumption of the Z-scheme in the current system at the beginning of the photocatalysis.Alternatively, the charge migration of the type-II scheme was then envisaged and considered more reasonable than in the Z-scheme at the interface between Ni(dmgH)2 and ZnIn2S4 (Fig. 7(b)). Due to their respective narrow band gaps, Ni(dmgH)2 microrods and ZnIn2S4 microspheres were both stimulated to produce electron-hole pairs when exposed to light irradiation. The photogenerated electrons in the Ni(dmgH)2 CB will be spontaneously injected into the CB of ZnIn2S4 through the type II heterojunction formed when the ZnIn2S4/Ni(dmgH)2 heterojunction was constructed. The majority of photogenerated holes in the ZnIn2S4 VB will be transported to the Ni(dmgH)2 VB. Therefore, the recombination of photoexcited electrons and holes can be greatly suppressed by the combination of Ni(dmgH)2 microrods and ZnIn2S4 microspheres, enhancing the photocatalytic activity. As a result, the holes on the VB of ZnIn2S4 were moved to Ni(dmgH)2 VB, where they could be consumed. The electrons were focused on the CB of ZnIn2S4, which could react with H ions to generate H2 and greatly suppress photocorrosion.Despite the successful inferring of type-II heterojunction for the current photocatalytic system, origins photochromic-like phenomenon that occurred during the photocatalysis still remain elusive. It has been noticed the suspension system became dark in color, but both the photocurrent and PHE ability enhanced with light irradiation time. These three evidences denote a new chemical composition was created in the heterojunctions, with the properties of i) facilitating charge migrations between hetero-sites, ii) color changes, and iii) reversable photochemical reaction process to return original semiconductors. In fact, the color appearance in the heterojunction is in line with the color changes for the pure ZnIn2S4 with both systems are less favorable for light transmission (Fig. 5(b) v.s. Fig. 5 (e)). This infers photocorrosion happens on the sulfide, regardless its combination with heterojunctional sites. Therein, the sulfur tends to be oxidized, also meaning more electronegativity for the sulfur element itself during the photocorrosion process. Meanwhile, the coordination complex Ni(dmgH)2 also suffers from photostability under irradiation, as can be deduced from the Co(dmgH)2 with similar structures [76]. The hydrogen bond joining the organic ligands behave low photostability and lead to changes of Ni coordination stages. A possible Ni-S composition was then formed between a chemically oxidative S and weakly coordinated Ni from both sides. The cross-interfacial formation has indeed been observed between sulfide and bare nickel even with a noble metal buffer [77]. The resulted Ni-S component has the pigmentary color of black as the NiS and acts as the interface to bridge charge transfer between Ni(dmgH)2 and ZnInS4, as denoted in Fig. 7(c). After light irradiation, the complex returns to the original coordination and the color of the heterojunction returns to the apparent color of red as the Ni(dmgH)2, while the sulfur may be leached or transformed to other chemical groups in the aqueous system. The suspension system is thus owning a combined feature of type-II and all-solid-state Z-scheme during the photocatalysis thanks to the appearance of the Ni-S interfacial component. Due to the facilitation of electron-hole annihilation at the interface, the electrons in the CB at the Ni(dmH)2 site and holes at the VB of ZnIn2S4 became active during the photocatalysis and moreover with larger redox activities compared to these in the previous type-II structures due to redox potentials (Fig. 7). A single ZnIn2S4/Ni(dmgH)2 heterojunction, thus, has the possibility to transfer from the type-II to Z-scheme photocatalyst where additional but more active photocatalytic sites are created, resulting in the boosting of hydrogen evolution (Fig. 4).The above proposition was supported by the analysis of ions in the suspensions during the photocatalysis. The experimental details can be found in Supporting experimental section of the SI. The Ni2+ and SO4 2- in the black suspension (NiS was not completely oxidized and decomposed) were qualitatively detected and in the suspension that had faded to red to restore the intrinsic color of the Ni(dmgH)2. As shown in Fig. S6(a), A, B and C were blank without a detector, adding dmgH to the black suspension after centrifugation, adding dmgH to the red suspension after centrifugation, respectively. The color became darker from A, B to C, and no obvious red precipitate can be seen in solution B, indicating that the Ni2+ present in the solution at this time was a trace, and the majority of Ni2+ was located in NiS and Ni(dmgH)2, the red precipitate (Ni(dmgH)2) in the C solution was visible. And as seen in Fig. S6(b), the solution became increasingly turbid from D, E to F, probably because of the increasing number of fine white precipitates (BaSO4) in the solution.The possible hydrogen-producing reactions of ZIS/NID heterojunctions under light irradiation with TEOA as a sacrificial agent are shown in Eqs. (3)–(14) below: (3) ZnI n 2 S 4 + h v → Z n I n 2 S 4 ( e - - h + ) (4) N i ( d m g H ) 2 + h v → N i ( d m g H ) 2 ( e - - h + ) (5) TEOA + h + → T E O A + (6) ZnI n 2 S 4 + 8 h + → Z n 2 + + 2 I n 3 + + 4 S 0 (7) 2 H 2 O + 4 h + → O 2 + 4 H + (8) (9) S 0 + 2 H 2 O + O 2 + 2 h + → S O 4 2 - + 4 H + (10) N i 2 + + S 0 + 2 e - → hv N i S (11) Z n 2 + + 2 I n 3 + + 4 S 0 + 8 e - → Z n I n 2 S 4 (12) 2 H + + 2 e - → H 2 (13) N i S + 4 h + + 2 H 2 O + O 2 → N i 2 + + S O 4 2 - + 4 H + (14) The above heterojunction type and ion detections during the photocatalysis allows us to detail the chemical reaction paths as follows. When the reaction solution is black in color, NiS has not yet been completely oxidized and decomposed into Ni2+ and SO4 2- ions. As to why its weak ion quantity can also be detected. This is also due to the reaction course described by Eqs. (3)–(14) has been in dynamic equilibrium, or it may be because when the black suspension is separated into solid and liquid. The whole system is inevitably contacting oxygen, and then a very small portion of NiS is thus oxidized. In contrast to the black faded red suspension, relatively high amounts of Ni2+ and SO4 2- ions are present in the solution that can be detected due to the complete oxidation of the in situ generated Ni-S active intermediate during the reaction, and this result verifies the rationality of the proposed mechanism from the side. The amount of Ni2+ free in the solution is still very small compared with the amount of Ni(dmgH)2 generated again after adding the dmgH. It worth noting that the loss of catalyst is not much, which is known from the fact that the amount of hydrogen evolution does not drop significantly after several cycles. It is important to note that the solubility product constants of NiS and Ni(dmgH)2 are about 3.2 × 10-19 and 4 × 10-24, respectively. So theoretically Ni(dmgH)2 is more stable than Ni-S weakly bonded in a squeezed space at the interface [78]. Being exposed to air at room temperature and pressure, Ni-S composition will tend to be broken and the coordinating group next to Ni convert it back to Ni(dmgH)2 and returns its color.In summary, we have successfully synthesized pure ZnIn2S4 microspheres and ZnIn2S4/Ni(dmgH)2 heterojunction photocatalysts using a simple low-temperature and low-pressure co-precipitation technique. The optimized ZnIn2S4/Ni(dmgH)2 heterojunction photocatalyst exhibited a remarkable photocatalytic hydrogen evolution activity of 36.3 mmol/g/h under simulated solar illumination, which was approximately 4.9 times higher than that of pure ZnIn2S4. The enhanced photocatalytic performance was attributed to several synergistic effects: (i) the addition of ZnIn2S4 microspheres onto the surface of Ni(dmgH)2 microrods improved light absorption; (ii) a well-designed type-II heterojunction structure facilitated the separation of photogenerated charge carriers and successfully mitigated the issue of photocorrosion commonly found in sulfide photocatalysts; and (iii) the in-situ formation of NiS active intermediate under simulated solar irradiation effectively enhanced the light absorption and utilization of the catalyst. Our work presents a novel approach for improving the photocatalytic performance of sulfides and provides a reference for the design and construction of sulfide composite systems for various photocatalytic applications in the future. Shangshu Liu: Investigation, Formal analysis, Validation. Feng Li: Conceptualization, Project administration, Funding acquisition, Data curation, Writing – review & editing. Taohai Li: Methodology, Supervision, Data curation, Resources. Wei Cao: Supervision, Data curation, Resources, Writing – review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Financial supports from the National Natural Science Foundation of China (21601149), China Scholarship Council, University of Oulu, Finland and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 101002219) are acknowledged. Data availability. All data generated or analyzed during this study are included in this published article (and its supplementary information files) and available from the corresponding author upon reasonable request.Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2023.03.123.The following are the Supplementary data to this article: Supplementary video 1 Supplementary video 2 Supplementary video 3 Supplementary data 4

In this work, a novel photocatalyst of ZnIn2S4/Ni(dmgH)2 was designed by a simple chemical precipitation method and used to enhance hydrogen evolution under visible light irradiation. Along with vigorous discharges of hydrogen bubbles, an optimal rate of 36.3 mmol/g/h was reached under UV–Vis light for hydrogen evolution, nearly 4.9 times of the one from pure ZnIn2S4. The heterojunction exhibits steady hydrogen evolution capability and owns a high apparent quantum efficiency (AQE) of 20.45% under the monochromatic light at 420 nm. By coupling ZnIn2S4 with Ni(dmgH)2, an extraordinary photochromic phenomenon was detected and attributed to the active Ni-S component in situ formed between the nickel and sulfur composites under light irradiation. The emerging sulfide benefits light absorption of the system and separation of photogenerated electron and hole pairs. Besides providing a promising photocatalyst for visible light hydrogen production, the present work is hoped to inspire new trends of catalytic medium designs and investigations.

Within the last few decades, rapid technological development has led to major climate changes, which are the most urgent issues that currently need to be faced. Burning carbon-derived fuels releases enormous amounts of carbon dioxide into the atmosphere, which constitutes more than 60% of global warming [1,2]. Currently, remarkable effort is being made to decrease CO2 emissions as well as to effectively capture and store CO2. Carbon dioxide can be used as an environmentally friendly carbon source to produce synthetic fuels and other chemicals (methane, methanol, ethanol, etc.), tackling both CO2 capture issues and resource depletion.Solid Oxide Electrolysis Cells (SOECs) are electrochemical devices that are able to convert water into hydrogen and oxygen by an electrolysis process occurring at the triple-phase boundary of the fuel electrode [3]. When CO2 is fed to the electrode as an additional fuel, the co-electrolysis of CO2/H2O may occur. During this process, both steam and carbon dioxide undergo parallel splitting reactions resulting in the formation of a mixture of CO and H2 (syngas) on one side and the formation of oxygen on the opposite side, which can be further utilized to produce useful chemicals [4,5]. However, due to the presence of both CO2 and H2 from H2O electrolysis, the reverse Water-Gas Shift reaction (rWGS, Equation (1)) occurs simultaneously. (1) CO2 + H2 ↔ CO + H2O ΔH298K = 41 kJ mol−1 So far, there is no agreement on the influence of rWGS on CO production: some studies show that carbon monoxide production is mainly dependent on the electrolysis, while others state that rWGS produces a significant amount of CO [6] [–] [12]. Furthermore, the total amount of CO produced depends on the temperature, voltage applied to the cell, and the composition of the inlet gases [5]. The current studies involves the modified construction of the conventional SOECs for more efficient performance when in co-electrolysis mode as well as fabrication of novel materials that will replace e.g. Ni-YSZ cermet [13–16].The methanation reaction, in which carbon oxides are converted into CH4, is a promising method of CO2 utilization and also provides a solution for the transportation of low-grade energy [17]. Methanation, also known as the Sabatier reaction, can be expressed by Equation (2). For SOECs working in co-electrolysis mode, reactions of CO methanation can occur simultaneously (Equations (3) and (4)) [17] [–] [19]. As one can see from Equations (1)–(4) the methanation process is exothermic, while rWGS is endothermic, and the selectivity may be adjusted by changing the working temperature [20]. (2) CO2 + 4H2 → CH4 + 2H2O ΔH298K = −165 kJ mol−1 (3) CO + 3H2 → CH4 + H2O ΔH298K = −206 kJ mol−1 (4) 2CO + 2H2 → CH4 + CO2 ΔH298K = −247 kJ mol−1 Although the methanation process is thermodynamically favorable, it is an eight-electron process, which results in the presence of a kinetic barrier [17,18,21]. To transcend this limit, the use of proper catalysts is required, among which Ni is the most commonly used, due to its high catalytic activity, selectivity, and low cost [20,22,23]. Several studies proved that the methanation is most efficient at temperatures not exceeding 400 °C, which stands in contrast with the SOEC working regime [24] [–] [27]. Moreover, at temperatures above 450 °C, the co-electrolysis reaction is favored, decreasing the selectivity of CH4 [26,28]. In the low-temperature regime of methanation, the degradation of Ni catalysts is not significant. However, at higher temperatures of the working SOEC, Ni catalysts tend to agglomerate and coke, leading to a decrease in their performance [29] [–] [32]. J. Gao et al. showed that the addition of steam to the feed gas slightly, but not significantly, decreases the CO2 conversion and CH4 selectivity [24].Other catalysts with promising properties for CO2 electrochemical conversion, such as coking resistance, are noble metals such as Pt, Rh, and Ru [20,33]. However, their high cost considerably restricts their use for methanation [23,34,35].Besides using monometallic catalysts, many studies are focused on incorporating a second metal, to e.g. Ni-based catalysts, to form a bimetallic system that couples the advantages of both metals. Bimetallic catalysts exhibit different catalytic properties due to modification of the electronic structure and geometry [35]. Among the most popular doping approaches, one can find the incorporation of other transition metals such as Fe or Co. Cobalt and iron can be easily dissolved in Ni metal, what can result in the formation of Ni–Co and Ni–Fe alloys and intermetallics [21]. What is more, the addition of a secondary metal may lead to increased stability and resistance to deactivation at higher temperatures [32]. Monometallic iron catalysts exhibit a high reaction rate, but low methanation selectivity, while NiFe alloys have an improved CO2 conversion rate [21,36]. The unarguable advantage of Fe doping is its low price and high abundance. The promotion of Ni catalytic properties under the influence of Fe strongly depends on the Ni/Fe weight ratio. The work of C. Mebrahtu et al. has shown that the optimal Ni/Fe weight ratio on an (Mg, Al)Ox substrate is 0.1, while the studies by D. Pandey et al. revealed that the greatest enhancement of CH4 selectivity and CO2 conversion was observed for 75 wt% of Ni and 25 wt% of Fe on an alumina substrate [37,38]. The enhancement of the catalytic properties can be associated with iron ions acting as a protective element to the nickel, as suggested by M. A. Serrer et al. [39].Another element studied as a secondary metal introduced into the Ni lattice is Co. The catalytic properties of cobalt are similar to those of nickel and its addition was shown to improve the dispersion of Ni and resistance to deactivation. Although the price of Co is higher than Ni, it is still much lower than the noble metals [22]. M. Guo et al. have shown that a small amount of Co (0.2 M Co/Ni ratio) added by impregnation to Ni/SiO2 leads to a decrease in catalytic properties, while molar ratios higher than 0.4 enhanced the low-temperature methanation catalysis [40]. Studies by C. Jia et al. revealed that NiCo@TiO2/SiO2 nearly doubled the turnover frequency (TOF) compared to monometallic catalysts [41]. The enhancement in TOF values indicates that cobalt promoted the intrinsic catalytic activity as a result of a synergistic effect. Furthermore, the CH4 selectivity was above 95% and the CO2 conversion exceeded 50% for the aforementioned bimetallic catalysts. L. Xu et al. have synthesized NiCo on Al2O3 with different Co/(Ni + Co) ratios [42]. They proved that the addition of 20% Co to the total metal amount exhibits the best catalytic properties. Moreover, the synergistic effect between nickel and cobalt resulted in a decrease in the activation energy for CO2 methanation, which further increased the CO2 conversion rate. The addition of Co may also have a beneficial effect on the reducibility of the Ni and metal dispersion, as suggested by B. Arlafei et al. [22] However, they observed a positive influence of Co only in the case of low Ni loading (not exceeding 10% weight). It is noteworthy that long-term tests were performed on novel NiCo catalysts, proving their high stability.So far, there has been a lack of work on the bimetallic catalysts for high-temperature methanation on Solid Oxide Electrolysis Cells. The study most similar to those presented in this article is the work of H.Y. Jeong et al., in which Fe ions were incorporated into the Ni/YSZ electrode by the wet infiltration method. They observed an increase in CO selectivity and an increase in the rWGS reaction while the SOEC was working in co-electrolysis mode [43].Herein, a series of Solid Oxide Electrolysis Cells modified by impregnation with Co was prepared, resulting in the samples containing 1.8, 3.6, and 5.4 wt% Co metal after reduction. The changes in the phase composition and electronic structure were studied. It was found that a small amount of Co impregnated into the conventional Ni-8YSZ fuel electrode greatly enhances the performance of the SOEC for co-electrolysis of H2O/CO2 mixtures with direct methanation. The addition of 3.6 wt% Co resulted in a nearly 3-times higher methane peak concentration at the outlet compared to the unmodified cell. The wide variety of the characterization techniques made it possible to determine the reasons behind the enhancement being three-fold: the Co-YSZ interface increases the basicity of the cell, the formation of NiCo2O4 delivers active sites for the reactions, and the formed structures highly develop the active surface area of the electrocatalyst.All electrolysers were modified by the wet impregnation method. For this purpose, a 1 m solution of Co(NO3)2 6H2O (Merck, 99.9%) in 10 v/v% EtOH in DI was prepared by dissolving the exact amount of nitrate salt in the solvent (5.82 g in 20 cm3 of the solvent). According to our previous experience in catalyst preparation, β-cyclodextrin (βCD, Sigma Aldrich, ≥97%) was added into the precursor solution at an amount equal to 0.05 mol βCD per every mole of Co2+ cations (1.135 g). The native cyclodextrin acted as an ion capping agent, altering the size and dispersion of the forming nanoparticles [44].The SOECs used in the study were delivered by the S.-F. Wang group from National Taipei University of Technology, Taiwan. The 1-inch diameter half cells were composed of a 400 μm NiO/YSZ cermet support with two different porosity levels and a 15 μm thick YSZ electrolyte. Prior to each modification step, the half cells were reduced at 850 °C under an H2 atmosphere to further increase the porosity of the cermet layer by NiO reduction. The cells were then impregnated using 100 μL of the precursor solution, which was found to be the maximum sorption volume. The half cells were transferred to a vacuum chamber for 15 min to ensure good penetration of the precursor solution throughout the fuel electrode. The cells were dried at 120 °C and sintered under an air atmosphere at 400 °C for 4 h to decompose all nitrates and organics. A series of three samples were prepared by repeating the impregnation steps 1, 2, and 3 times, which corresponds to 1.8, 3.6, and 5.4 wt% of Co0 in the metallic phase. A reference sample was prepared according to the same heat treatment routine but omitting the impregnation steps.X-ray diffraction patterns of the impregnated electrodes were collected using a Bruker D2 PHASER XE-T with a Cu-Kα radiation source before and after the testing procedure. The cross-sectional morphology of the pristine and spent fuel electrodes was verified using a Scanning Electron Microscope (SEM, FEI Quanta FEG 250) with an Energy-Dispersive X-ray spectroscope (EDX, EDAX Genesis APEX 2i) and Apollo X SDD detector. TEM imaging was performed using a JEOL 2100 F (Tokyo, Japan) microscope operating at 200 kV coupled with energy-dispersive X-ray spectroscopy (EDS, Oxford Instruments, UK). The X-ray Photoelectron Spectroscopy (XPS) spectra were collected using an Omnicron NanoTechnology X-ray photoelectron spectrometer with a 128-channel collector. XPS measurements were undertaken in ultra-high vacuum conditions, below 1.1 × 10−8 mbar. Photoelectrons were excited by an Mg-Kα X-ray source with the anode operating at 15 keV and 300 W. The obtained spectra were deconvoluted using the XPSPEAK41 software. X-ray Absorption Spectroscopy (XAFS) and novel Scanning Transmission X-ray Microscopy (STXM) measurements were performed at the PIRX (former PEEM/XAS) and DEMETER beamline, respectfully, at the SOLARIS National Synchrotron Radiation Centre, Kraków, Poland [45]. The used synchrotron energy range ensured the collection of the Ni-/Co-L2,3 and O–K edges of the as-prepared and spent cells. Powdered samples were dispersed onto carbon tape and placed on measurement plates for spectra collection. The measurements were performed using total electron yield (TEY) and/or partial fluorescence yield (PFY) using an SDD window C2 detector from Amptek depending on the self-absorption of the samples under high vacuum conditions. The energy resolution was 200 meV and better, and the beam size (h x v) was 250 μm × 40 μm. The STXM imaging was performed on powdered samples dispersed onto an Si3N4 membrane. The series of image stacks of the samples were collected under a He atmosphere. Detection of the transmitted radiation was performed using a photomultiplier tube. The collected images were analyzed using the aXis2000 software. Elemental maps were formed as the difference between the signals collected at the absorption peak and pre-peak energies presenting the global distribution of the selected element.The H2-TPR (temperature-programmed reduction) and O2-TPO (temperature-programmed oxidation) measurements were performed using an in-house-built apparatus equipped with a TCD detector (Buck Scientific, USA), coldtrap, and a heated gas transfer line. Each time, the same amount of the sample was placed in a quartz reactor with an internal measurement of the bed temperature. The powders were degassed at 200 °C for 20 min in a stream of 5 N He prior to the measurements. The samples were reduced under a flow of 40 ml min−1 5 vol%H2 in an Ar gas mixture. The tests were performed up to 900 °C with a heating rate of 10 °C min−1. Afterwards, the samples were cooled down to RT and the gas stream was switched to a mixture of 5 vol% O2 in He. The powders were equilibrated for 1 h in an O2–He stream and followed by oxygen uptake tests carried out using the same regime as during the TPR measurements. The profiles were collected using the PeakSimple software with a frequency of 1 Hz.Prior to each SOEC test, the LSCF (La0.6Sr0.4Co0.2Fe0.8O3-δ , Electro-Science Laboratories 4421A, USA) and the LSC (La0.6Sr0.4CoO3, Fiaxell SOFC Technologies, Switzerland) layers were screen-printed onto the air-facing side of the cell with an intermediate drying step at 120 °C between each layer. The deposited pastes gave as a result of a ∼30 μm thick porous base layer of LSCF underneath a ∼10 μm thick LSC layer used for better electron transfer to the Au current collector. The air electrode was sintered in-situ during SOEC start-up to protect the prereduced Ni-8YSZ side from reoxidation and possible breakdown. The active surface area of the air electrode was equal to 0.78 cm2. The SOECs were mounted onto the in-house-built measuring rig. The exact scheme of the unit and prior-measurement cell preparation steps were described in detail in our previous paper [46]. In brief, the modified SOEC was mounted onto an alumina tube and sealed using Ag-based conductive paste and dielectric ceramic adhesive (552–1105, Aremco). Au and Pt wires were used to maintain the electrical connection. The air electrode was contacted by an Au mesh and spring-loaded alumina interconnector. The set-up was placed inside a high-temperature furnace. The SOEC was each time heated up under flowing N2 to 850 °C and then the feeding gas was switched to 47 mLSTP min−1 H2 to perform the reduction of the modified fuel electrode material. The cell was held for 45 min at this temperature and then cooled down to 700 °C. The SOEC was equilibrated under flowing H2 for 12 h prior to electrocatalytic tests.After 12 h of SOEC reduction, the gas stream was switched to a CO2/H2O/H2 mixture. All of the gas components were dosed using electronic flowmeters. Hydrogen was supplied from the H2 generator while the other gases were provided from pressure tanks (Air Liquide) with purity over 99.9%. The water vapor was introduced by a controlled H2–O2 mixture burning in an external reactor heated up to 700 °C loaded with Pt catalyst. The H2O vapor concentration in the inlet mixture was set to 20 vol% for all runs. The rest of the inlet mixture was composed of CO2/H2 at a ratio of 1:2 by vol. for comparison tests. The mixture composition was chosen to be far from the equilibrium ratio (H2:CO2 = 4) to obtain a stronger response in CH4 concentration change for each of the fabricated SOECs. The gas mixture was supplied at the total flow rate of 28 mLSTP min−1. After switching to the testing mixture, the SOEC underwent conditioning at OCV until a stable voltage was achieved and later with a 1.3 V applied potential unless a stable current was observed. The resulting exhaust gases were analyzed using the FTIR-based measuring unit described in our previous work [46]. The spectrophotometer (PerkinElmer Spectrum 100) was equipped with a 10 cm length heated gas cell (60 °C) and ZnSe optical windows. The inlet/outlet flow rates were controlled using electronic flow meters. The exhaust stream was passed through a coldtrap (3 °C) followed by Nafion dryer tubing for complete water vapor removal prior to entering the FTIR. The measurements of the concentrations of gases in the outlet mixture were carried out from 700 °C down to 500 °C with a 20 °C step and 60 min delay for thermal stabilization and reaction equilibration. The spectra were collected every 5 min in the range of 4000–500 cm−1 with a spectral resolution of 4 cm−1. Each scan was composed of five accumulations. The concentration of CO, CO2, and CH4 was retrieved via spectra integration performed at 3760–3520 cm−1 for CO2, 2226–2143 cm−1 for CO, and 3250–2650 cm−1 for CH4 using the SpectraGryph software and calibration curves specially designated for our set-up and presented previously elsewhere [46,47]. The amount of H2 was determined as the difference between 100% and the sum of the CO2, CO, and CH4 concentrations. Simultaneously, continuous electrical measurement of the current flowing through the cell was carried out using a Gamry potentiostat/galvanostat under 1.3 V. A set of additional tests under various potentials and gas mixture ratios (maintaining 20 vol% of H2O) was also performed. The applied potential was changed in the range of 1.1–1.6 V and the H2/CO2 volume ratio at 1.3 V was switched within 0.25 and 7.The quality of the prepared cells for efficient co-electrolysis and methane production was described by means of the CO2 conversion ( X C O 2 ), CH4/CO yields ( Y i ) and CH4 selectivity ( S C H 4 ) catalytic coefficients calculated from the measured molar flow values using Equations (5)–(8). The yields were calculated considering the CO2 input stream. (5) X C O 2 ( % ) = n ˙ C O 2 i n − n ˙ C O 2 o u t n ˙ C O 2 i n × 100 (6) S C H 4 ( % ) = n ˙ C H 4 o u t n ˙ C O 2 i n × X C O 2 100 × 100 (7) Y C H 4 ( % ) = n ˙ C H 4 o u t n ˙ C O 2 i n × 100 (8) Y C O ( % ) = n ˙ C O o u t n ˙ C O 2 i n × 100 where: n ˙ i o u t and n ˙ i i n are the molar flow rates of a specified gas ( i ) at the outlet and inlet of the reactor, respectively. The reference equilibrium compositions were calculated with the usage of the Gem module from the HSC Chemistry™ software. The calculations were performed under free-flow reactor conditions and atmospheric pressure for an idealized system using the Gibbs energy minimalization method disregarding the electrocatalytic reactions (OCV catalytic measurements).The XRD patterns of the prereduced and modified SOEC fuel electrodes are presented in Fig. S1. As the NiO/YSZ composite underwent the reduction step prior to the modification steps, the phase composition consisted of ionic conductor 8YSZ (8 mol.% Yttria Stabilized Zirconia) and metallic Ni. The impregnation steps from the novel precursor solution required cyclic firing of the cermet electrode at 400 °C to decompose the cobalt nitrate and organics existing in the precursor solution. This resulted in the formation of a small amount of surficial NiO/NiOOH layer via low-temperature reoxidation of the Ni metal. Despite the introduction of the Co ions into the electrode, no additional phases were clearly distinguished from XRD measurements. It is caused by the fact that the amount of Co species present in the electrode is below the detection limit of the XRD equipment used for pattern collection. Additionally, the created species were most likely forming highly nanocrystalline objects, which in fact would produce even broader and lower in intensity diffraction peaks that could be lost within the noise. There was also no clear evidence of shifting in the peak position of the NiO/Ni lattices, even though the formation of the mixed NiO–Co3O4 oxide was very possible. A slight increase in the lattice parameters was observed according to the Rietveld refining method of the peaks assigned to NiO, but due to the low crystallinity of the resultant phase and broad peaks, the outcomes were designated as unreliable. Co is characterized by an infinite solubility in the Ni lattice when the metallic form is considered. The results obtained during the studies on Ni–Co alloy nanoparticles and their oxidation in the work of L. Han et al. followed by the presentation of the Ellingham and NiOx-CoOx phase diagram clearly indicated that within the used amounts of Co, it is highly probable to obtain mixed (Ni,Co)O monoxide with traces of metastable NiCo2O4 spinel structure [48]. Even though the XRD studies gave little information regarding the internal changes in the phase composition, they revealed the partially nanometric nature of the modified electrode due to the highly broadened peaks coming from the NiO phase.The cross-section SEM analysis of the unmodified cell presented in Fig. S2 revealed that the microstructure of the pristine prereduced electrode consisted of micrometric grains of well-sintered 8YSZ and Ni. The NiO-YSZ composite was primarily designed to reach around 35–40% of total open porosity after the reduction. There were also two clearly distinguishable levels of porosity in the outer and functional layer (FL). The reduction step allowed for easier penetration of the infiltrant solution. The SEM images of the fractures of the reference and modified SOECs after 1, 2, and 3 impregnation procedures are presented in Fig. 1 .The structure of the electrodes differs greatly depending on the amount of introduced Co precursor solution. There are two new species clearly distinguishable in the images. The first is composed of nanoparticles of the Co oxides-hydroxides formed on the surface of the 8YSZ grains formed via non-reactive deposition. The new Co species are rather well-dispersed and clearly distinguishable on the 8YSZ grains forming uniform nanoparticles (NPs). Secondly, the paper ball-like structures that were determined to be a surficial oxide formed during the sintering steps under ambient air atmosphere. The latter one was described as Co species delivered during reactive deposition. Similar structures were also present in the reference electrode material, but of slightly less developed microstructure. Based on the simple EDX point analysis, the formed oxide scale consisted of Ni, O, and a small amount of Co. This indicated that a new structure of mixed composition formed on the surface of the Ni metal grains. The oxide scale seemed to exhibit a highly developed surface area of offbeat microstructure. The look of the scale may also stands for the formation of the spinel-like structures, as those tend to form powders of complicated morphology when in nanometric form [49,50]. Spinels are a group of materials that have been widely studied for electrochemical application by the members of our team and other international groups with groundbreaking properties followed by promising potential always being uncovered [51] [–] [53]. The arrangement of the nanoparticles on the 8YSZ grains was different for all three samples. With the increasing amount of introduced Co, the separated NPs started to form a rather continuous layer with bigger distinguishable objects. The coalescence of the repeatedly deposited Co oxides is best seen for the 5.4 wt% Co sample (Fig. 1D). In the case of reactive deposition, each additional cycle of impregnation tends to further modify the microstructure of the paper ball-like structures creating bigger, clumped-up agglomerates of Co–Ni oxides. The previously mentioned NiOx-CoOx phase diagram diminishes the fact of spinel formation in the homogeneous mixture. Even so, the surficial character of the Co deposition can lead to a reaction at the Ni–Co interface and form a spinel-like contact mixed layer as those two transition metals exhibit rather high reactivity [54]. To better understand the internal structure of the modified samples, a series of TEM images (Fig. 1E and F) were captured using lamellae cut from the cells. An exemplary image of the 3.6 wt% Co sample accurately represents the general structure after Co incorporation. The imaging proved the bimodal behavior of the Co ions, i.e. reactive and non-reactive deposition. Co partially dissolves into reoxidized Ni and simultaneously forms homogeneous nanoparticles on the surface of 8YSZ. Additionally, a highly developed structure consisting of spherical polycrystalline nanoparticles was formed. The HRTEM was further utilized to analyze this porous structure deeply. From the image, we can see that the outer surface of the structure is nearly amorphous. This was caused by the low sintering temperature coupled with the addition of the cyclodextrin and most likely implies a high external active surface area [48,55]. Deeper inside, three exemplary interplanar distances were marked. The interplanar distance of well-defined lattice fringes equal to 0.46 nm was extremely close to the values observed by L. Huang et al. and A. Cetin et al., and assigned to (111) planes of NiCo2O4 [56,57]. The interplanar distances of 0.26 nm and 0.16 nm corresponded to the (220) planes of NiCo2O4 and (311) planes of Co3O4, respectively [58]. Slight deviation of the interplanar distances in the ideal NiCo2O4 phase arose from the nonideal stoichiometry of the compound and very probable coexistence of intermixed NiOOH–CoOOH layer double hydroxide (denoted as NiCo2(OH)6) [59]. Parallel to the mixed oxides, free CoxOy nanoparticles were also formed closer to the surface as the Ni ions were bound by previously deposited Co, and diffusion in the surface proximity was slowed down [60,61]. Based on those results it was stated that Ni ions are being consumed by reacting with Co ions and the surficial layer consists of finely dispersed spinel-like particles and free CoxOy nanoparticles. The NiO unreacted layers can be found most likely closer to the surface of the pristine Ni grains.To quantitively study the surface composition and identify the valence state of the elements, a series of XPS measurements was performed on the as-prepared and spent electrodes (after SOEC tests). The exemplary results of the 3.6 wt% Co as-prepared sample presented in Fig. 2 shows that for both the core level spectra of Ni2p and Co2p, the peaks were fitted to the two spin-orbit doublets corresponding to 2p3/2 and 2p1/2. For each of the elements, two bands were deconvoluted after Lorentzian-Gaussian fitting and ascribed to the valence state of either +2 or +3. For Ni (Fig. 2A), the peaks located at 853.7 eV and 871.8 eV were assigned to Ni2+, while the doublet around 855.4 eV and 873.2 eV to Ni3+. These values are in agreement with the variety of the research dedicated to the NiCo2O4-based structures in electrocatalysis [60,62] [–] [64]. There are also two clearly visible shake-up satellite peaks within the Ni2p spectra. The slight positive shift (∼0.4 eV) of the Ni2+ peaks in the modified samples compared to the reference also indicates the formation of a new mixed phase [60,65,66]. Moving forward to the Co2p spectra (Fig. 2B), the Co3+ oxidation state was indexed to the doublet located at 779.6 eV and 795.2 eV, while Co2+ to the one located at 781.4 eV and 797.0 eV. The position of the peaks is similar to the studies of X. Tong et al. on urchin-like NiCo2O4 and the wide range of data retrieved by other groups [60,63,67,68]. In the case of the Co2p core-level spectra, the 2p3/2 peaks are shifting towards higher binding energies of ∼0.3–0.4 eV per modification step. This is also an indication of the mixed oxide phase formation and substantial decrease of Co oxidation state, i.e., taking up more electrons [69]. It is due to the higher amount of available Co ions and prolonged sintering time allowing for better interdiffusion. Well-fitted doublets of both elements determined the ratio of Ni3+ to Ni2+ and Co3+ to Co2+. The changes in those ratios, depending on the amount of Co introduced, are depicted in Fig. 2E. With the increasing concentration of surficial Co ions, the ratio of Ni3+ to Ni2+ increased drastically from 1.22 up to 2.22. This directly indicates the formation of the mixed Ni–Co oxide layer, as in the case of the introduction of a minor amount of Co into NiOx where the structure can become self-doped with Ni3+. It was previously described that the Co doping of NiO results in reaching higher Ni valence states as it facilitates the formation of an Ni2O3 component, playing the role of p-type dopant [70] [–] [72]. A further increase of the Co amount over 3.6 wt% resulted in a slight lowering of the concentration of Ni3+ within the structure. A similar issue was described elsewhere and stated that the increase in the amount of free CoxOy clusters formed on the surface acts as a carrier trap and limits the charge transfer from Ni to Co [71]. In the case of Co, the ratio of Co3+/Co2+ increased only slightly over the following cycles of impregnation (1.29–1.48). The same behavior was further observed and described thanks to the XAS measurements. The increase in the amount of Ni3+ has a substantial influence on the electrochemical performance of catalysts in oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) though similar explanations may also apply in this research [73] [–] [76]. In the case of post-mortem measurements (Fig. 2C and D) it is clearly visible that an Ni metallic form came up in the XPS spectra and overpowering peaks of Ni3+ and Co3+ appeared. This is due to the presence of water vapor content in the stream during the cooling routine. It also indicates the high activity of the metals towards reacting with H2O and the formation of NiOOH–CoOOH phases, which can have a crucial impact on the water molecules bonding during electrolysis [77].The electrocatalytic activity of the modified cells towards co-electrolysis with simultaneous methanation was tested under the conditions of a working SOEC. The measurements were performed under 20 vol% water vapor CO2 stream with H2 to prevent the oxidation of the metallic phase and ensure a higher FTIR response due to the changes in the CH4 concentration. The testing runs were performed both under OCV and a thermoneutral bias of 1.3 V. The CH4 concentration changes for the reference and the modified Co-impregnated cells versus SOEC temperature are presented in Fig. 3 A and B. The obtained current densities shown in Fig. 3C indicated that, surprisingly, only a slight difference in the SOEC electrical efficiency was visible between the samples.Despite their similar electrical efficiency, the CH4 outlet concentration for the presented cells changes drastically after the incorporation of those minimal amounts of Co into the structure. The outcomes indicate superbly promising properties of the modified cells. The peaking methane concentration at OCV for the sample containing 3.6 wt% Co in the reduced state reached an outstanding 2.1% being over 2.5 times the value of the unmodified cell (∼0.8%). In the case of the 1.8 and 3.6 wt% Co-impregnated cells, the maximum concentration peak shifted towards a slightly higher temperature compared to the reference. It was caused by higher efficiency in the electrocatalytic CO2/H2O mixture splitting overcoming the existing thermodynamic limitations of the methanation reaction at high temperatures. Generally speaking, those samples exhibited elevated catalytic activity at the temperature range suitable for the proper work of an SOEC. To better represent the increase in catalytic activity, a series of describing parameters (CH4 yield, CO2 conversion, CH4 selectivity, and CO yield) were calculated, and the results are depicted in Fig. 4 . All of the calculations were done for the samples subjected to the tests at 1.3 V applied bias. The dashed lines were added as a visual guide to better represent the general trends. The efficiency of methane generation was determined by means of the CH4 yield parameter. At 1.3 V and a temperature equal to around 640 °C, all of the new cells easily reached the thermodynamic equilibrium compositions simulated for the idealized systems, in contrast to the reference cell. The reason was most probably two-fold. Firstly, the modified cells in fact reached a higher level of electrochemical efficiency, which can be best seen in Fig. 4. Secondly, the methane-formation kinetics improved as the CO2 conversion coefficient greatly increased. As the CO2 conversion went significantly over the level of the simulated thermodynamic equilibrium, the direct CO2 hydrogenation reaction in particular had to be facilitated by the novel composition of the working electrode. At the same time, relatively comparable currents indicated the tendency of the Co-impregnated samples to alter the ratio of electrolyzed H2O and CO2 towards the second component. In the case of the modified samples, there was an outstanding, over twofold increase in methane yield. The efficiency of the methanation was clearly enhanced with subtle shifts in the maximum point. The cells with 1.8, and 3.6 wt% Co revealed altered temperature-dependent catalytic response. Further addition of the Co into the structure did not increase the methane yield greatly but definitely changed the general activity look of the profile to be more like the non-modified sample. The aim of this study was to increase the high-temperature co-electrolysis and methanation efficiency (>600 °C), so the 3.6 wt% Co-impregnated sample was chosen as the most promising one. Moving on to the CH4 selectivity, the samples were following similar trends as in the case of the CH4 yield. There is clear and visible evidence of the increased efficiency of methane formation at elevated temperatures. Even more pronounced is the activity of the 5.4 wt% Co impregnated cell, but the shifting to the lower temperature range is also noticeable. This could be further explored regarding the methanation catalysts working under conventional conditions. For both the reference and modified samples, the evidence of electrolysis is indisputable. In all cases, the CO2 conversion and CO yield parameters are placed high above the thermodynamic equilibrium point, pointing out the role of electrochemical H2O/CO2 splitting with the further formation of CH4 being independent of electrochemical reactions on the electrode. There are still several disputes over the impact of direct CO2 electrolysis and rWGS on the final outcome of the SOEC [78]. In the case of these studies, it is believed that the modified electrode material is responsible for the increased CO2 direct electrolysis in parallel to the rWGS reaction. The minor increase in the currents flowing through the electrolyte followed by the comparable CO yields, despite the higher CH4 yields, is basic evidence for the aforementioned statement. The increased rate of CO2 electrolysis on the surface of the electrode may be mostly due to the higher basicity and higher tendency to react with water to form oxyhydroxides of the Co species compared to the NiOx. This was predicted based on the high tendency of Co to undergo oxidation in a wet atmosphere and to further react the Co(O)OH species with CO2 forming a corresponding metal carbonate. A series of reaction enthalpies and Gibbs energy changes (Rea module) were calculated and followed by the thermodynamic equilibrium composition (Gem module) simulation using HSC Chemistry for Ni and Co under the working SOEC idealized inlet gas mixture composition. The results of the simulations are presented in the Supplementary Materials (Figs. S3 and S4). The higher ability of Co to form oxides-hydroxides as well as adsorb and bond the CO2 should increase the overall basicity of the electrode and increase the retention time of CO2 on the surface [79]. This would result in two things: A) increase of the CO2 electrolysis rate, and B) increase of the possibility of CO2 hydrogenation to form CH4 [80]. The obtained results prove the proposed mechanism of the increase in catalytic activity after Co addition. To develop the description of the routes leading to the enhanced efficiency of the Co-impregnated Ni-YSZ SOECs, a series of more sophisticated measurements were performed, and are discussed further in the text.To further examine the changes in the material properties after the introduction of the guest Co ions, a series of XAS and STXM measurements followed by a throughout spectra analysis with the assistance of the beamline specialists were performed at the SOLARIS synchrotron facility (Cracow, Poland). During the energy scans, the spectra for Ni-, Co-L2,3 and O K absorption edges were collected for the as-prepared and spent samples. Due to the high amount of Ni in the base structure, the spectra of the Ni-L2,3 edges were very similar for all of the samples with only slight evidence of Ni3+ forming on the surface after the tests (Figs. S5 and S6) [81]. The fluorescence signal was recognized as unreliable due to the strong self-absorption of Ni. A minor shift of the Ni edge and post-edge distortions may correspond to the formation of Ni–Co species. Much more reliable results with clear evidence of the electronic structure changes are presented in Fig. 5 in the form of L2,3-edges of Co both in the total electron yield (TEY) and fluorescence (PFY) signal. To assist in the analysis, a series of certified reference materials were also subjected to the spectra collection. The XAS spectra are mostly dominated by the Co2p core-hole spin-orbit coupling, splitting the spectra into two regions – L3, L2 white lines parts. The changes in the shape of the absorption edge are clearly noticeable. The total TEY signal increased with the increase in the amount of Co in the structure. When taking the PFY signal as a reference point, it was observed that the ratio of the Co ions dissolved deeper into the oxide scale is higher for lower Co amounts. After repeated the impregnation steps, the intensity of both signals reached a ratio of ∼1:1, showing that Co ions are much more likely to form surficial CoxOy oxides than to further dissolve into the NiOx. This finding is in line with XPS results and analysis of the TEM images described previously. Disregarding the absorption intensity, the shape of the edges also slightly changed depending on the Co concentration. Looking at the reference spectra of CoO and Co3O4, there is a clear difference between the pre- and post-edge shape. The higher relative intensity and slight distortions of the pre-edge part, coming mostly from the CoO edge, can be easily observed in the case of the 1.8 wt% Co-impregnated sample. An even more pronounced difference in this region of the spectra can be seen in the PFY signal. As the Co concentration increased, its oxidation state slowly increased as the shape of the absorption edge started to very closely resemble that of Co3O4. According to research by Zhang B. et al., where the edge of the Co strongly resembled our result, it was stated that the Co2+/Co3+ admixture varied around the 1:1 ratio [82]. Based on experimental and simulational research by Chang C.-F. et al., it was established that the created substructure of Co–Ni oxides contains mostly low spin Co3+ and high spin Co2+ ions [83]. The absorption edges of the samples after the SOEC testing revealed that the cobalt was reduced on the start-up procedure of the cell (resembling the Co0 edge). Due to the presence of residual water vapor and noninert storage of the samples after the tests, a nanometric layer of the CoxOy oxide built up on the surface of the metal, with a higher share of Co3O4 in the case of the 5.4 wt% Co-impregnated sample. This indicated a lower stability and higher tendency of this sample to get oxidized, mostly due to the higher amount of initial free Co oxide species.The O K absorption edges were measured and the results for the as-prepared 3.6 wt% Co-impregnated sample are presented in Fig. 6 . The additional absorption edges of the reference materials were also included to resolve the composition of the surficial layer as the NiO, Co3O4, and NiCo2O4 compounds are characterized by slight shifts in the edge energy. The precise description of the pure NiCo2O4 preparation is detailed in the Supplementary Materials. Fortunately, the 8YSZ O K edge and the features coming from the carbon tape are located at an energy range that is not interfering with the shapes of the edges of Ni/Co-based compounds. Based on the obtained shape of the edge, three regions in superposition were highlighted and corresponded well with the placement of the absorption edges of the NiCo2O4, Co3O4, and NiO, respectively. The existence of the mixed Ni–Co spinel-like structure was identified based on the small pre-edge feature peaking at 529 eV, which indicated the existence of an increased share of Ni3+ ions in the surficial layer of the electrode [84]. The position of the Co edge pre-peak feature is also clearly visible and surely represents the Co3O4 nanoparticles formed within the spinel structure and on the surface of the 8YSZ ionic conductor.The formation of NiCo2O4 increases the abundance of the Ni2+/Ni3+ and Co2+/Co3+ redox couples and through this can deliver a high number of active sites for performing chemical reactions. The recent studies focus on the use of NiCo2O4 normal and inverse spinel structure as the electrocatalyst. The position of the Ni and Co cations in the tetrahedral (Td) or octahedral (Oh) sites determines the final performance of the catalyst [85]. The degree of the inversion in the Ni–Co spinel greatly changes the DOS and alters the electronic properties of the complex oxide. This further affects the SOEC performance as the purely reduced state is not most likely to happen. A high amount of water vapor increases the pO 2 and establishes an equilibrium between the formation and disintegration of the Ni–Co oxide mixed compound. It is generally agreed that the existence of a high share of Ni3+ ions plays a crucial role in the electrochemical enhancement of the catalyst. The feature located at ∼529 eV (Fig. 6) can be taken as newly appearing with the unoccupied e g state of Ni3+ (3 d7) hybridized with O2p [86,87]. Studies conducted by M. Cui et al. revealed that Co3+ located at Td sites is thermodynamically unstable and tends to get reduced into the more stable Co2+ valence [88]. As a result of this change, the Ni2+ gets oxidized into Ni3+ to reach the charge neutrality of the lattice. It leads to the generation of a new hole state of Ni3d-character and shifting of the E F closer to the valence band. The synergistic interaction between the Co and Ni may substantially increase the electronic conductivity and, in extreme cases, induce a metallic character of the NiCo2O4. The measurements at the O K edge can give a clear description of the interactions between the elements for the spinel-like structures that evolved in future samples.To better represent the distribution of the elements regarding the properties of their electronic structure, a series of STXM images was collected. A representative set of the images of the as-prepared sample containing 3.6 wt% Co is presented in Fig. 7 . This novel method of examination of the space distribution of the elements of the given absorption edge energy sheds new light on the results obtained during SEM imaging. Clear evidence was obtained of the above-mentioned statements regarding the formed structures of the Co-8YSZ facet and the Ni–Co intermixed oxide species. A reference scan performed much below the absorption edges of Ni and Co resembled the TEM image presented in Fig. 1. Elemental maps were obtained as the difference between the energy scan below and at the absorption edge. After reaching the energy of the Co-L3 edge, the Co nanoparticles and NiCo2O4-like structure come up within the imaging region. Moving up to the higher scanning energy located at the Ni-L3 edge, clear evidence of the bimodal state of the Ni was observed. The dark area is ascribed to the 8YSZ particle being impermeable to the radiation. The Co was homogeneously distributed over the sponge-like surficial structure of the reoxidized Ni particle, which indicates the presence of the NiCo2O4-like structure of reacted oxides. Additionally, not clearly visible in the TEM and SEM pictures, nanoparticles of CoxOy were identified both on the surface of the YSZ creating no obvious interlayer and as the inclusions of spherical nanoparticles embedded within the homogeneous structure of the mixed Ni–Co oxide. Minor bigger agglomerates were also noticed in the images, which indicated that the impregnation with the addition of βCD is a promising method for homogeneous modification of SOC cells. According to the Ni maps, the ions are found in the homogeneous surficial oxide scale. Interestingly, the formed structure resembles a core-shell particle where the internal part is mostly composed of metallic Ni (slight shift of the absorption edge). The gradient-like distribution of Ni ions beautifully revealed the layer-by-layer buildup of the structure where the metallic core is surrounded by a NiCo2O4-like structure and peripheral Co-enriched phase with CoxOy inclusions. The complex structure greatly represents the diffusion limitations of the Co ions, but at the same time brings more difficulties for the formulation of final conclusions regarding the primary reasons for the enhanced operation of the SOEC in co-electrolysis accompanied by methanation.To better understand the interaction of Co and Ni species in the samples on start-up of the cell and reduction under flowing H2, a series of H2-TPR measurements were performed using our in-house-built TPx system. The corresponding reduction profiles are shown in Fig. 8 . The reduction kinetics of all of the samples is beyond the obvious, as the process ends much before 400 °C indicating the formation of the highly nanometric and porous structures which greatly reduce the reduction temperature. In general, for all of the samples, the shape of the reduction profile is composed of two distinctive regions, denoted as the lower temperature region (α) and higher temperature region (β). The first H2 consumption peak (α) located at around 250 °C was related to the existence of the mesoporous surficial structure of paper ball-like oxide scale with an amorphous structure (see Fig. 1) as it is more likely to undergo fast oxygen uptake and release than the layers underneath. According to the literature and previous findings, the α peak can be attributed to the reduction of surface-active oxygen species adsorbed onto the surface of the mixed (Co,Ni) oxyhydroxides and/or smaller nanoparticles of (Ni,Co) oxides as the low temperature is sufficient to overcome the energy barrier of their release [89,90]. As a similar α peak appeared in the reference sample, it indicates a significant amount of Ni3+ ions, presumably both in the form of pure and Co-doped NiOOH. The low temperature during the sintering step of the prereduced samples under an ambient air atmosphere resulted in the formation of copious amounts of surficial oxyhydroxides of low crystallinity. This is in agreement with previously analyzed TEM images (Fig. 1).The splitting of the β reduction peak, visible mostly for the 1.8 wt% Co-impregnated sample, is related to the coexistence of a NiCo2O4 spinel-like phase and embedded CoxOy nanoparticles. To better represent the deconvolution of the overlapping reduction processes, a series of H2-TPR measurements was also performed for the 8YSZ particles impregnated with NiO, Co3O4, and NiCo2O4. The detailed procedure is included in Supplementary Materials and simulated the preparation steps covered in this research. The results are presented in Fig. 9 . By comparison, it is clearly visible that the resultant H2-TPR profile of the Co-containing cells is a superposition of the NiO-YSZ and Co3O4-YSZ reduction profiles and highly resembles the one of NiCo2O4-YSZ. The high-temperature β region was further deconvoluted into two subregions, namely β1 and β2. The first was attributed to the reduction of the NiO subphase being in strong interaction within Ni–Co spinel-like structure. Due to this interaction between the Ni and Co, the reduction temperature was significantly lower for the Co-impregnated samples. Based on research by Y. Yi et al., the reason for this shift is bidirectional [91]. Firstly, the synergistic effect of the coexistence of Ni–Co spinel-like compound facilitates the reaction between Ni3+ and Co2+ through the charge transfer reaction N i 3 + + C o 2 + → N i 2 + + C o 3 + , which facilitates the easier reduction of Ni ions. The second reason is the higher dispersion of the NiO subphase throughout the whole structure and increased mesoporosity of the oxide scale after the introduction of Co. As in this research, the well-defined peak around 300 °C was attributed to the reduction of the bulk Ni2+ to Ni0 based on the general data on the reduction behavior of nanometric NiO. The secondary process rising within the β1 peak is the simultaneous reduction of Co3+ to Co2+ [92]. The β1 was shown to undergo a slight shift towards higher temperatures, but still lower than for the unmodified sample. This indicates that the additional heating steps increase the crystallinity and average particle size of the scale.The β2 peak at around 340 °C was likened to the reduction of Co species. The position of the assigned process is also in agreement with the profile of the Co3O4-YSZ reference material shown in Fig. 9. According to the findings of Niu J. et al. on NiCo2O4-based catalysts for toluene conversion at a temperature similar to that observed during this study, the reduction of Co2+ to metallic Co occurs [93]. Considering the reference material of Co3O4-YSZ, the main reduction peak of the aforementioned process was also shifted towards lower temperatures. The modified structure of the cells is hereby experiencing the synergistic effects of the mixing of the transition metals. On the one hand, the Co ions help to reduce the Ni ions at the lower temperature, while the metallic Ni speeds up the reduction of Co ions into the metallic form, most likely by the hydrogen spillover mechanism [94,95].With the increasing number of impregnation steps, the β1 and β2 start to fully overlap each other, creating a profile that highly resembles that of the reference material NiCo2O4-YSZ (Fig. 9). This indicates that the abundance of the Co ions and longer diffusion time facilitated the formation of the proper Ni–Co mixed compound. For the 5.4 wt% Co-impregnated sample, the bimodal and enhanced reductivity are extinguished. It seems that the existence of the layer of lower crystallinity is of major importance.O2-TPO experiments were also performed to resolve the changes in the redox chemistry of the electrode due to the composition of the altered catalyst. The results of the measurements are presented in Fig. 8 and additional profiles of the reference materials are shown in Fig. 9. Agreeing with the Gibbs free energy calculations for the oxidation of Ni and Co metals and the TPO profiles presented in Fig. 9, the higher the amount of Co added into the electrode, the lower its oxidation temperature. This is direct evidence of the higher susceptibility of the modified material towards oxygen uptake. Taking into account that SOECs work under high water vapor pressures and the previous conclusions over the role of oxyhydroxides and Me3+/Me2+ pairs, the higher ability to bind oxygen and form the surficial layer of the Ni–Co mixed oxide is of great importance for the final performance of the cell.A novel behavior, previously undescribed in the literature, of the NiCo2O4-YSZ material was discovered and recorded. The behavior on reduction of the NiCo2O4-YSZ reference catalyst material was in line with other reports. Even so, the further O2-TPO measurements revealed that the material, despite being subjected to the oxidizing atmosphere, was able to evolve additional oxygen from the lattice at elevated temperature (Fig. 9, right). A cyclic heating-cooling experiment was performed to ensure a reliable result. During the heating, the NiCo alloy supported on the YSZ substrate underwent full oxidation until ∼450 °C. A further increase of the temperature over 600 °C caused the evolution of the oxygen from the catalyst material into the oxidant stream, even though it contained 5 vol% O2 of He. This was recorded as a negative TCD detector signal. The cycle was completed with the cooling of the catalyst bed in the same stream of the gas mixture. On cooling, the process was fully reversed at around 600 °C again showing a positive TCD peak of a similar area. This interesting feature of the NiCo2O4 compound will be further studied to understand the chemical and physical causes of the discovery. The primary hypotheses are threefold. The first states that the compound disintegrates into two major compounds – NiO and CoO – with the simultaneous release of the redundant oxygen. The second covers the issues concerning the change of the crystallographic structure of the spinel. And finally, the most promising one: the reduction on oxidation may directly correspond to the oxygen activity coefficient and oxygen bond strength. When subjected to a slightly oxygen-depleted atmosphere, the lower pO 2 causes the partial reduction of the surface of the spinel causing the kinetics of the lattice oxygen release to overedge the kinetics of oxygen uptake. The results of those analyses will be of significant scientific value to the field of materials science and catalysis. Furthermore, according to this research, the behavior of the spinel structure may explain the increased catalytic activity of the electrode after the introduction of Co. The increase in the oxygen exchange rate should highly influence especially the electrocatalytic activity in the context of water splitting in the SOEC.To better illustrate the dependencies between the actual inlet mixture composition and the final output of the SOEC, a series of tests under various H2/CO2 ratios and operating voltages were performed. The results are presented in Fig. 10 A–C. The representative temperature of 640 °C and the 3.6 wt% Co-impregnated sample were chosen for all tests. When the influence of voltage changes was examined, the inlet mixture was set to 2:1 H2 to CO2, and for the mixture change-related test, a thermoneutral bias of 1.3 V was selected. While increasing the bias applied to the SOEC, the CH4 concentration increased linearly, followed by a similar trend observed for the CO concentration (Fig. 10A). Due to the increased conversion, the corresponding concentration of CO2 dropped in the same manner. Despite this, the cell maintained the same linear dependence of the current increase with the increase in voltage (Fig. S7). This indicates that the cell performed well, even though a relatively low temperature was maintained. The increase in the CH4 concentration was a result of the shifting of the thermodynamic equilibrium by increasing the amount of the reactants – mainly CO and H2 – which enabled the high rates of the methanation. When changing the H2/CO2 vol. ratio at a fixed bias (1.3 V), an increase in the CH4 concentration was observed until the 4:1 vol ratio was reached. After that point, the CH4 yield remained almost unchanged due to the highly depleted carbon source for the methanation reaction to happen. An interesting view of the electrochemical aspects of the work of the SOEC was seen when the change of the CH4 in the outlet stream (Fig. 10B) and the current change (Fig. 10C) were compared. After the increase of the share of H2 in the inlet mixture, the CH4 easily reached the thermodynamic equilibrium concentration while the current flowing through the SOEC kept decreasing. The fixed amount of H2O vapor followed by the decreasing share of CO2 limited the electrolysis efficiency due to the lack of reactants to be reduced. This is proof that the CO2 can also be directly electroreduced on the electrode material, in parallel to the fast, ongoing rWGS reaction.To better represent the stability of the SOEC with additional modifications, a prolonged test for 12 h was carried out. The cell was subjected to the same start-up procedure and after the gas switch to the H2/CO2 mixture, the concentration changes were monitored every 15 min. The Time-On-Stream measurements are shown in Fig. 10D. The results gave us clear evidence that the Co impregnation does not change the stability of the pristine cell, while increasing the catalytic activity and maintaining its performance for over 12 h. During the first hours of electrolysis, a slight decrease was observed mostly due to Ni particle growth and sintering. This is a normal behavior for SOCs, and after a few hours the CH4 concentration remained nearly unchanged. A similar trend was noticed for the CO and CO2 concentration changes, where after 6 h of testing, the value oscillated around 18% and 23.5%, respectively. This shows that the introduction of Co into the structure enormously enhances the catalytic performance with no negative impact on the cell stability issues.Post-mortem imaging was done to observe the structural changes after reduction and 12 h of operation in SOEC mode. The SEM and TEM images of the spent electrodes are depicted in Fig. 11 . An additional collection of elemental maps using μEDS was performed for the 3.6 wt% Co-impregnated sample. After the reduction step, the Ni and Co got fully reduced and the bimetallic alloy formed on the surface of the Ni particles. The Ni grains looked homogeneous and no clear evidence of the formation of secondary phases was found. This is due to an interesting aspect of the binary Ni–Co phase diagram, which states that both elements can form solid solutions throughout the whole range of concentrations, due to their close proximity on the periodic table and similar atomic structures [96,97]. In contrast, the Co in its metallic form created a distinctive structure of spherical nanoparticles on the 8YSZ grains. It demonstrates nearly no solubility of the Co in 8YSZ with the tendency of the metal to undergo dewetting and form a network of homogeneous nanoparticles. This is true in the case of the 1.8, and 3.6 wt% Co-impregnated samples, while for the 5.4 wt% Co-impregnated sample the amount of Co was so significant that a strong agglomeration process occurred. Also, a higher tendency towards oxidation led to the possible formation of the secondary flake-like phase visible in Fig. 11. Based on the previous XAS measurements, it may be composed of cobalt oxides that were prereduced and not fully dissolved into the Ni matrix, further oxidized during the cell cooldown scheme. The highest homogeneity was assigned to the sample containing 3.6 wt% Co in the structure. The TEM images and elemental maps are in line with the aforementioned analysis showing the Ni–Co homogeneous mixture and Co nanoparticles anchored to the surface of the 8YSZ. As to the cell's performance, the existence of both the Co-YSZ system resulting in higher basicity, and NiCo2O4 spinel-like oxide for providing active sites and the increase in specific surface area can be ascribed as the major causes of its enhancement.The purpose of this work was to successfully modify conventional Solid Oxide Electrolysis Cells via Co-impregnation and characterize the changes it caused to the internal microstructure, phase composition, and electronic structure in detail. The modifications consisted of the introduction of small amounts of Co into the Ni-YSZ cermet material of the cells via the wet impregnation method. The addition of βCD resulted in the homogeneous dispersion of the Co ions throughout the material bulk. It was observed that the Co ions formed three types of substructures, namely nanoparticles of CoxOy supported on the surface of the 8YSZ, Ni–Co mixed spinel-like compound on the interface between the Ni core and outer layer, and CoxOy nanoparticles embedded into the spinel scale. The XPS results indicated that Co induced the formation of a high amount of Ni3+ ions. This directly increased the number of the available catalytic sites through the active Ni3+/Ni2+ and Co3+/Co2+ couples for reactions to happen. The performed modifications increased the CH4 concentration in the outlet stream over 2.5 times and ensured better efficiency of the H2O/CO2 co-electrolysis and conversion. The addition of Co increased the CO2 conversion from 47% up to 57% at 700 °C. The search for the possible causes of the enhancement through XAS and STXM measurements resulted in direct proof of the existence of a significant amount of the intermixed Ni–Co compound which induced changes in the shift of the E F band energy, generated the inverse spinel structure, and introduced a significant amount of active surface species. The STXM measurements clearly evidenced a Co-graduated structure of core-shell-like Ni grains. The H2-TPR and O2-TPO studies revealed that highly developed and nanometric structures were formed after the Co introduction. The modified cells were characterized by a lower reduction temperature due to the synergistic effects of the coexistence of Ni and Co. A highly novel discovery concerning the interesting behavior of the NiCo2O4 supported on the 8YSZ was made. Even though the TPO experiment was carried out in an oxidizing atmosphere, the powder evolved additional oxygen at high temperatures, only to reverse this process when cooling down. Tests under various operating voltages and H2/CO2 inlet ratios led to the general proof of simultaneous direct and indirect (rWGS) routes of CO2 electroreduction. The 3.6 wt% Co-impregnated sample was characterized by the most homogeneous distribution of Co species across the cermet material and small, spherical nanoparticles developed within the structure. The addition of the secondary metal into the Ni-YSZ conventional cermet material revealed highly promising results to be further applied in the field of H2O/CO2 co-electrolysis with the simultaneous single-process of methanation for the buildup of advanced conversion systems. Further studies on the bimetallic synergy and strange spinel behavior under different pO 2 and elevated temperatures are planned to better understand the phenomena.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the 5th Polish-Taiwanese/Taiwanese-Polish Joint Research Project PL-TW/V/4/2018 granted by the National Centre for Research and Development of Poland and the Ministry of Science and Technology of Taiwan. This publication was developed under the provision of the Polish Ministry of Education and Science project: “Support for research and development with the use of research infrastructure of the National Synchrotron Radiation Centre SOLARIS” under contract nr 1/SOL/2021/2. We acknowledge SOLARIS Centre for the access to the Beamline PIRX and Beamline DEMETER, where the measurements were performed.The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2022.08.057.

To study the synergy between the transition metals for enhancing the electrochemical and chemical activity, a series of SOECs were modified with a small amount of Co ions, namely 1.8, 3.6, and 5.4 wt% in the reduced state. The addition of βCD into the precursor solution allowed for extremely fine dispersion of Co species across the Ni-YSZ cermet structure. The sample containing 3.6 wt% Co reached an outstanding over 2.5-times-higher concentration of CH4 in the outlet stream. At the same time, the Co greatly enhanced the electrochemical efficiency of water and CO2 co-electrolysis. Full characterization involving STXM imaging allowed for better understanding of the synergy between the Ni and Co host metal and made it possible to find the causes of the increased activity. It revealed the complexity of the substructures formed within the electrode. A novel discovery was described regarding the NiCo2O4 spinel structure subjected to the O2-TPO measurements. Despite the applied oxidizing atmosphere, the catalyst evolved oxygen at elevated temperatures in a reversible manner. The performance tests indicated the roles of both rWGS and direct electrolysis of CO2 in the electroreduction process. The addition of Co did not influence the prolonged degradation of the cell.

The market for proton exchange membrane fuel cells (PEMFCs) is growing as the EU has highlighted hydrogen as a key factor in the energy market in their Green Deal plan [1]. One of the ways in which hydrogen shall be used is to power heavy-duty vehicles, where large storage capacity and fast refueling are crucial. To increase the competitivity of such fuel cell electric vehicles, the production cost of fuel cells needs to be reduced. Platinum-based electrocatalysts are one of the main cost-determining components, leading to an enormous research interest in the reduction of Pt content. Among the various strategies towards this goal, the alloying of Pt is one of the most promising and advanced approaches [2].Along with the strive for cost reduction, the durability of the electrocatalysts is a major issue to be tackled. The widely used, commercially available Pt/C catalysts show a number of disadvantages with respect to oxygen reduction reaction (ORR) performance at the cathode of the PEMFC. Apart from their poor efficiency with respect to the Pt loading, their limited durability is a major drawback for reliable long-term application and sustainability [3]. Pt itself, however, is crucial for ORR especially in acidic environments such as the PEMFC. The most active transition metals at ORR and therefore worth considering for alloying with Pt are Cu, Ni, and Co [4]. The most investigated and most promising Pt-based nanoparticles (NPs) tackling durability issues are Pt–Co and Pt–Ni NPs, often in the form of core-shell NPs with a Pt shell [5,6]. Further effort to simultaneously improve both ORR activity and durability are sought by investigation of ternary and quaternary systems [7,8]. Apart from the composition of the electrocatalyst, a high number of active sites is targeted to achieve high catalytic efficiency [9]. For this reason, the incorporation of NPs and even single-metal atom catalysts in carbonaceous matrices is currently a subject of intense investigation [10,11].One of the challenges, and an important factor influencing the fuel cell performance, is the synthesis route of the catalyst, which also adds up to the production cost. In most approaches, the catalyst NPs are synthesized on a carbon support, mixed with a binder and ionomer, and sprayed onto the membrane, which leads to NPs being deposited in locations where they are not accessible for the catalytic reactions [12]. Using direct electrodeposition of Pt alloy NPs onto the gas diffusion layer (GDL) of the PEMFC, the NPs are deposited on the most active sites of the carbon support because the metal ions should occupy the locations where the local electric field is highest during the electrodeposition—locations on the substrate where the path is short and the local charge density is high. Furthermore, this approach combines both synthesis and fixation (or distribution) of NPs into a single step, leading to a cost reduction of the synthesis route [13].Fundamental research on the mechanisms of NP formation by electrodeposition has been carried out by Ustarroz et al., including the electrodeposition of Pt NPs for potential application at ORR [14–16]. The electrodeposition of Pt alloy NPs has also been shown promising for alkaline ORR and other catalytic reactions [17,18].A common synthesis route for metallic NPs is by pulse electrodeposition. An extensive study on the pulse electrodeposition of Pt NPs for ORR was carried out by Huang et al. [19]. A pulse reverse process was employed by Sriwannaboot et al. for the codeposition of Pt–Co alloy onto carbon cloth substrates [20]. Egetenmeyer et al. thoroughly investigated the effects of pulse electrodeposition parameters for Pt, Pt–Ni, and Pt–Co NPs deposited onto GDLs, and determined optimum electrodeposition parameters for each alloy [21]. Santiago et al. were able to reduce the rather high Pt NP size commonly obtained by electrodeposition down to below 10 nm by deposition onto a rotating disk electrode (RDE) [22]. Wang et al. achieved Pt particle sizes of 3–10 nm via the use of a complexing agent [23]. Remarkably, pulse electrodeposition offers several advantages compared with other NP fabrication schemes. First, synthesis and anchoring to a substrate such as GDL is achieved in one step. Second, NPs nucleate onto the most active sites of the GDL, as aforementioned, as opposed to other post-synthesis approaches for which the catalyst is distributed on the GDL irrespective of local electrical properties and depth, so that many NPs may be located at inaccessible or unfavorable locations. The main disadvantage of pulse electrodeposition compared with conventional liquid-phase synthesis and high-temperature calcination [24] is that an increase in the catalyst loading is achieved at the expense of increasing particle size, which is often undesirable for catalytic purposes.Interestingly, Liu et al. synthesized a Pt–Ni alloy with low Pt content via a carbothermal shock method, which was shown to be effective at ORR in acidic media and PEMFC testing [25]. This is in contrast to most ORR electrocatalysts investigated for PEMFC, which usually rely on high-Pt content alloy NPs.Although the synthesis of Pt–Ni NPs for ORR [26,27], as well as the electrodeposition of Pt–Ni alloys in general [28,29] is well advanced, Mo-containing Pt alloys have been scarcely investigated. Huang et al. showed that the doping of Pt3Ni NPs with Mo led to extraordinary ORR performance, making the ternary Pt–Ni–Mo system an interesting candidate for ORR studies [30]. These NPs were obtained by decomposition of acetonate and hexacarbonyl precursors. The electrochemical co-deposition of Mo, which is usually achieved using sodium molybdate [31], may lead to the formation of intermediate Mo oxide species due to partial reduction of Mo(VI) [32]. However, even NPs containing Mo oxide show improved electrocatalytic properties [8,33]. The coordination of Mo with electronegative elements like oxygen or nitrogen can move the d-band center of Mo so that its binding capacity with reaction intermediates (O∗, OH∗, and OOH∗) increases, thus making Mo atoms moderately active towards ORR [34]. Therefore, the introduction of oxidized Mo atoms to Pt–Ni should not be, in principle, deleterious for the ORR performance.Considering the above results, it seems that (i) Pt–Ni is both an adequate candidate for ORR and (ii) the incorporation of a third alloying element can improve electrocatalytic performance. Yet, the expected improvement is not always attained. For example, Sorsa et al. electrodeposited Pt–Ni from liquid crystalline solution onto GDL substrates, but they did not observe any significant improvement of the electrodeposited Pt–Ni over commercial Pt/C [35]. Hence, further investigation on this topic is still to be performed.In this work, pulse electrodeposition is used to deposit Pt–Ni and Pt–Ni–Mo(O) particles directly onto the microporous layer of a commercial GDL. The GDL is composed of a woven carbon cloth with a microporous, PTFE-coated carbon layer, where the catalysts shall be applied. For such a complex three-dimensional substrate, electrodeposition is an especially suitable method [36]. In the deposition process, all particles are deposited on the most active sites of the carbon support's surface where they guarantee excellent contact with both the support and the proton exchange membrane (PEM) in the fuel cell. Although the use of Pt–Ni NPs for ORR, and the electrodeposition of Pt–Ni alloys are rather developed, the combination of both, in addition to the direct deposition onto the GDL, has been reported very scarcely. The Pt–Ni and Pt–Ni–Mo(O) NPs with different compositions are chemically and structurally characterized, and ORR in acidic media is investigated. Finally, fuel cell performance and durability tests in a PEMFC prototype are carried out after hot-pressing the obtained cathodes with the PEM and a commercial Pt/C electrode as anode.The synthesis of Pt–Ni and Pt–Ni–Mo(O) NPs was performed by pulse electrodeposition from aqueous solution in a three-electrode electrochemical cell. An Autolab PGSTAT204 potentiostat/galvanostat was used with a Pt wire as counter electrode (CE), an Ag|AgCl (3 M KCl) reference electrode (RE) and a working electrode (WE). The WE consisted of a 2 cm by 2 cm GDL, supplied by Freudenberg, mounted on a Cu support to ensure electrical connection and a homogeneous charge distribution over the entire area of the GDL during electrodeposition. The excessive area of the Cu support was isolated with polyimide tape (Fig. 1 ).The aqueous electrolytes were loaded with nickel chloride, sodium hexachloroplatinate, boric acid, and ammonium chloride, based on an earlier study [28]. For the deposition of Pt–Ni–Mo(O), sodium molybdate (as Mo precursor) as well as citric acid as complexing agent were added (Table 1 ). All chemicals for electrolyte preparation were of analytical grade and had been supplied by Merck. The pH was adjusted to 2.7 by addition of sulfuric acid solution.The electrodeposition was carried out in stagnant conditions at 30 °C with pulses of varying current density to obtain different compositions. The choice of pulse deposition parameters was initially orientated on the study on the electrodeposition of Pt–Ni NPs carried out by Egetenmeyer et al. [21] and then altered by empirically optimizing pulse deposition parameters with respect to particle size, catalyst loading, and composition. It was observed that lower particle sizes were obtained at the expense of the catalyst loading, so that finally the process parameters were chosen as a compromise between particle size and catalyst loading. Pulse on-time and off-time were kept constant at 5 ms and 70 ms, respectively, whereas the number of cycles was changed with the current density to obtain identical deposited charges and, assuming similar Faradaic current efficiencies for each electrolyte, similar catalyst loading (Table 2 ). The total charge was increased for the Pt–Ni–Mo(O) deposition due to the addition of citric acid, which was observed to compromise the current efficiency. This is in accordance with previous studies, where the addition of citrate was found to increase Mo content in the deposits while lowering the current efficiency [37]. It was also observed during initial studies that higher current densities were needed for the ternary NPs to obtain the desired particle size and composition.The same three-electrode set-up used for NP electrodeposition was used for cyclic voltammetry (CV) studies of the NP/C assemblies in 0.5 M H2SO4 to activate the catalysts and to determine their electrochemically active surface area (ECSA). To this end, 30 cycles at 200 mV/s and 5 cycles at 50 mV/s were recorded in a potential window between 0 and 1.3 V versus reversible hydrogen electrode (RHE). ORR was studied by CV in O2-saturated 0.1 M HClO4 after recording a background CV in N2-saturated electrolyte, both at 10 mV/s between 0 and 1.1 V versus RHE and in static conditions. Potentials applied against Ag|AgCl were converted to RHE scale. The curves were corrected for Ohmic drop (iR-correction) after determination of the instrumentation resistance by electrochemical impedance spectroscopy (EIS) [38]. A Pt/C GDE with a Pt loading of 0.3 mg/cm2 was tested as a reference. It should be noted that the procedure for ORR measurements employed here differs from the usual approach using RDE since the electrocatalysts are directly deposited onto the GDL substrates and cannot be deposited directly onto an RDE. As a result, a quantitative determination of ORR parameters, such as the half-wave potential, is not feasible. Nevertheless, the measurements give qualitative information to show trends between catalysts of different compositions, in addition to the comparison with a commercial GDE.All deposited NPs, as well as the unloaded GDL substrate, were analyzed by scanning electron microscopy (SEM) on a Zeiss Merlin electron microscope to evaluate particle size, distribution, and loading of the substrates, using an acceleration voltage of 1 kV. For energy-dispersive X-ray spectroscopy (EDX), an acceleration voltage of 20 kV was used. However, quantification of the electrocatalyst NPs was not feasible by EDX due to the very low loading of NPs. For this reason, a chemical analysis method was employed (see below). Particle sizes were determined by image analysis of the SEM micrographs using ImageJ, taking the average of 100–200 particles per sample. SEM was also used for post analysis after PEMFC testing on a TESCAN LYRA3, after the cathode was delaminated from the membrane electrode assembly (MEA). For cross-sectional observation, one MEA was cut, embedded in epoxy resin, mechanically ground and polished and subsequently ion milled by Ar ions on a Gatan PECS II.Transmission electron microscopy (TEM) was conducted on a JEOL JEM-2011 in high resolution and diffraction mode to study the crystalline structure of the NPs. Samples for TEM were prepared by scratching the catalyst NPs, together with part of the carbon support, off the electrodes, dispersing the samples in ethanol, and dropping them onto the carbon film of a TEM copper grid. Electron energy loss spectroscopy (EELS) was performed on a FEI Tecnai G2 F20 STEM. Both TEM were working at an acceleration voltage of 200 kV.For determination of both composition and loading of the substrates, NPs were dissolved in aqua regia, consisting of hydrochloric acid and nitric acid in a ratio of 3:1 in volume, and the concentration of Pt, Ni, and Mo in the solution was determined by inductively coupled plasma mass spectrometry (ICP-MS) on an Agilent 7500ce spectrometer for each variation of electrodeposition parameters. In addition, the Faradaic current efficiency was determined by relating the total charge converted during electrodeposition to the absolute masses of Pt, Ni, and Mo and assuming that all metals had been fully reduced from their initial oxidation state.X-ray photoelectron spectroscopy (XPS) was performed on a PHI 5500 spectrometer to study the surface state of the NPs, recording the core spectra for C1s, Ni2p, Pt4f, and Mo2p, before and after Ar ion sputtering for 30 s, as well as on the disassembled MEAs after PEMFC testing. Peak fitting of the spectra was done by the software XPSPEAK. Energy calibration was done by positioning the main C1s peak at 284.5 eV.For the PEMFC testing, the cathodes containing the electrodeposited Pt–Ni and Pt–Ni–Mo(O) NPs were hot-pressed together with a Nafion 212 membrane (Chemours) and a commercial GDE with a Pt loading of 0.3 mg/cm2 as anode using a pressure of 0.5 MPa at 110 °C for 3 min to form the MEA.All MEAs were tested in a single-cell fuel cell tester using coated stainless steel flow plates. The active area of the fuel cell was 2.9 cm2. A Greenlight G20 test station was used for PEMFC tests, with a temperature of 80 °C for both gases and a dewpoint temperature of 65 °C. The flow of H2 and air were 0.042 and 0.10 l n /min (normal liter per minute), respectively.The cells were activated by cycling between 0.9 V and 0.6 V for 2 000 cycles before a polarization curve was obtained. For durability testing, an accelerated stress test (AST) consisting of 20 000 additional cycles were performed, with polarization curves obtained after 10 000 cycles and at end of test (EoT).For all NPs, the amplitude of the deposition potential increases with the amplitude of the current density (Fig. 2 ). During pulse deposition of Pt–Ni NPs, the amplitude ranges initially between −1.4 and −2.0 V versus Ag|AgCl depending on the applied current density. This potential drops towards the end of the deposition to within −1.0 to −1.3 V. The potential established during off-time changes from just below 0 V to slightly positive potential. In turn, the potential amplitudes for the deposition of ternary Pt–Ni–Mo(O) NPs ranges from −2.0 V to −4.0 V due to the higher current densities applied. Those latter potentials do seem uncommonly high, however they are based on an empirical determination of the process parameters. They may thus be the result of a significant resistivity of the GDL substrate. The potential does not change significantly until the end of deposition, whereas the potential during off-time changes to −0.6 V towards the end, showing a trend which is inverse to the one observed for binary NPs.The results of the ICP-MS shows that all four depositions of Pt–Ni resulted in a Pt content of 67–80 at% (Table 3 ) and a Pt loading of 3.7–4.1 μg/cm2. Assuming that the metal is completely reduced, which is fairly true for this case, the current efficiency is ca. 25% for all Pt–Ni depositions. However, Pt–Ni–Mo(O) appeared to pass a threshold as the current density increased from 77 to 100 mA/cm2, where the two lower current densities resulted in similar composition to the Pt–Ni with 78 and 66 at% Pt and low amounts of 1 and 2 at% Mo, respectively. The higher current densities on the other hand resulted in very low Pt content of only 5 at% while Ni became the main constituent. The Mo content increased to 24 and 21 at%. Note that the composition of the ternary NPs is given in atomic percentage disregarding the oxygen content. The occurrence of oxygen in the Mo-containing NPs originating from incomplete reduction of Mo(VI) precursor was deduced by XPS, as shown later on. The total catalyst loading was less uniform for the Pt–Ni–Mo(O), ranging from 2.5 to 8.1 μg/cm2. The current efficiency was also significantly reduced when comparing the ternary Pt–Ni–Mo system to the binary Pt–Ni, with current efficiencies as low as 2–12%.The GDL substrates show the typical homogeneous microporous layer, consisting of aggregates of globular carbon particles (Fig. 3 a). Fluorine emissions in EDX confirmed the presence of PTFE (not shown).After the pulse electrodeposition, Pt–Ni and Pt–Ni–Mo(O) NPs are homogeneously distributed on the surface and near-surface region of the microporous layer, which appears darker in the SEM images compared with the deposited NPs (Fig. 3). All Pt–Ni NPs show a uniform, spherical shape. The particle size is spread between 20 nm and 80 nm, with an average particle size between 40 nm and 50 nm independent of composition or process parameters (Table 3), and are agglomerated in very few cases (Fig. 3b–e). The Pt–Ni–Mo(O) NPs have the tendency to a spherical structure, although less defined and with higher roughness than the Pt–Ni NPs. While the particle size of Pt78Ni21Mo1 is around 80 nm (Fig. 3f), the other ternary NPs lie between 40 nm and 50 nm. For Pt5Ni74Mo21, a mixture of large (>100 nm) and small (<50 nm) NPs is appreciated (Fig. 3i). This observation is in agreement with the large distribution in particle size determined for this composition, while all other compositions show relatively narrow size distributions (Fig. 4 a–h). While commercial Pt/C has significantly smaller particle size, Fouda-Onana et al. electrodeposited Pt particles with a size of 50 nm and found that despite the larger particle size, the utilization rate was high due to all particles having triple phase boundaries (TPBs) [39].In TEM observations, the Pt–Ni and Pt–Ni–Mo(O) NPs are clearly distinguished from the C support by phase contrast (in bright field TEM, the NPs appear darker than the carbon-based support; Fig. 4i,m). The binary NPs are of well-defined, globular form, whereas the ternary NPs are more irregular (Fig. 4j,n). The high-resolution mode reveals the nanocrystalline structure of Pt–Ni NPs, indicated by diffraction planes with different orientations (Fig. 4k). This nanocrystallinity is also clearly observed in the selected area electron diffraction (SAED) pattern of a single NP with a diameter (ø) <50 nm, where almost any diffraction direction is present, confirmed by the quasi-continuous rings observed in the pattern (Fig. 4l). The corresponding d-spacings match the face-centered cubic (fcc) phase for a Pt–Ni alloy. For Pt67Ni33, the average cell parameter determined from SAED was 3.88 Å, which corresponds well with the d(111) spacing of 2.2 Å measured in the high-resolution image.In contrast to the Pt–Ni NPs, the Pt–Ni–Mo(O) NPs show more diffuse diffraction rings, which are more characteristic of an amorphous material, even when the Mo content is very low (Fig. 4p). Some additional diffraction spots indicate the presence of Mo oxide species such as MoO3 or other non-stoichiometric oxides [40]. Diffraction planes in high-resolution TEM are only appreciated in extremely few occasions at the NPs’ surfaces, showing crystals smaller than 5 nm (Fig. 4o). Other than that, diffraction planes are not observed, leading to the conclusion that most Pt–Ni crystals are too small to diffract and that they are completely mixed in with the molybdenum oxide species in a sort of composite.From SAED data, the existence of a homogeneous alloy is clear for the Pt–Ni NPs, the ternary Pt–Ni–Mo(O) NPs were analyzed by EELS to confirm the distribution of Pt, Ni, and Mo. However, even for the NPs with the highest Mo content (Pt5Ni74Mo21), Mo was not detectable by EELS due to the high delay of the Mo signal, and the high amount of carbon causing a large background signal (and resulting in a very low Mo content with respect to carbon). In contrast, Pt and Ni are clearly distributed evenly over the NPs (Fig. 5 ).The Pt4f spectra of the surfaces of the deposited Pt–Ni NPs, exemplary shown for Pt73Ni27, show emissions of metallic Pt at 71.4 eV and of Pt(I) at 72.3 eV, which can be assigned to platinum hydroxide [41]. In the Ni2p spectra, no contribution of metallic Ni is observed and most superficial Ni is bound in Ni(OH)2 (Fig. 4q,r).After Ar ion sputtering, the XPS spectra show a contribution of metallic Ni, as well as a higher fraction of metallic Pt with respect to the hydroxide species. It must be noted that, in contrast to bulk material, the contribution of the NPs' surface cannot be eliminated by sputtering. This means that after sputtering, there is still a significant contribution of the surface state, and the actual bulk state at the NPs’ cores is assumed to contain even higher fractions of metallic and less oxidized species than represented in the XPS spectra after Ar ion sputtering.For the ternary Pt–Ni–Mo(O) NPs, very similar observations are made with respect to the Pt4f and Ni2p emissions (Fig. 4s and t). The Mo3d detail spectra do not show any evidence of metallic Mo at 228.0 eV [41]. The initial surface state shows the occurrence of Mo(VI) exclusively, which points to the presence of MoO3. After sputtering, the lower oxidation state Mo(IV) is found, which may correspond to MoO2 (Fig. 4u). Therefore, Mo(VI) was not fully reduced to Mo(0) in spite of the presence of Ni(II) and citrate in the electrolyte. It can be argued that sufficient complexing of molybdate species with citrate, and therefore the full discharge of Mo(VI) to Mo(0), does not occur at the rather low pH of the electrolyte of 2.7. At this pH, the citrate mostly exists in its protonated form, thus hindering complexation of metal ions.For both binary and ternary NPs, a drastic reduction of O1s emissions (not shown) were observed after sputtering, indicating that the NPs exhibit an oxidized surface and a metallic core. Due to the fact that contributions of the NPs' surfaces are present even after sputtering, O1s emissions are not completely suppressed, nor can it be completely ruled out that there remain low amounts of residual oxygen in the NPs’ cores.Combining all knowledge obtained on the ternary Pt–Ni–Mo system, the following conclusions can be drawn. Taking into account the existence of Pt(0), Ni(0), Mo(IV), and Mo(VI) shown by XPS, the absence of concrete diffraction rings for the Pt–Ni fcc phase in SAED, the very small (<5 nm) crystalline regions in high resolution TEM, and the homogeneous distribution of Pt and Ni throughout the NPs as evidenced by EELS, it is concluded that Pt–Ni crystals and molybdenum oxide species are coexistent and randomly distributed among the NPs. Thus, the Pt–Ni–Mo(O) NPs consist of a heterogeneous compound of Pt–Ni and molybdenum oxide, which due to their nanosized nature and random distribution, appear as a homogeneous compound even on the nanoscale.The GDL substrates exhibit a rather low electric conductivity, leading to a relatively high double-layer capacitance observed in CV. Nevertheless, the hydrogen adsorption (H ads ) and desorption peaks (H des ) used for the determination of ECSA are well defined, as shown exemplarily for Pt73Ni27 (Fig. 6 ).The ECSA of both the Pt–Ni and Pt–Ni–Mo(O) is comparable with what is found in literature (Table 3) [2], showing that despite the electrodeposited catalysts having a large particle size, the Pt utilization is high.The electrodeposited Pt–Ni and Pt–Ni–Mo(O) NPs all show lower reduction potentials at ORR compared with the Pt/C (Fig. 7 a). Among them, the Mo-containing NPs are inferior to the binary alloy NPs with comparable Pt/Ni ratio. Interestingly, the best performance among the electrodeposited NPs is observed for Pt67Ni33. A diffusion-controlled region is not observed for any of the samples; this is related to the fact that kinetics is limited because the measurements were carried out in stagnant conditions, and also due to the GDL substrates which are not perfect conductors.The advantage of electrodeposited Pt–Ni and Pt–Ni–Mo(O) NPs becomes apparent when the Pt mass activity at ORR is considered (Fig. 7b). Pt67Ni33 shows the highest Pt mass activity among the binary NPs. In addition, the Ni-rich ternary NPs, which also have significantly higher amounts of Mo, show the highest mass activity with respect to Pt content. This shows that the addition of Mo does not compromise the catalytic activity and may indeed lead to improvement, however, the stability at ORR of these low-Pt content NPs in acidic conditions may be compromised. Liu et al. recently found that a Ni-rich Ni–Pt alloy electrocatalyst can be employed successfully in PEMFC with only little performance loss over 30 h of constant operation [25]. Among the Pt-rich ternary NPs, Pt66Ni32Mo2 shows the highest Pt mass activity, corresponding to the Ni/Pt ratio of the binary Pt67Ni33. Zalitis et al. showed that ultrathin catalyst layers of 200 nm thickness can be employed to overcome limitations by internal resistances such as experienced here with the use of the GDL. In this way, the mass activity can be further increased by one order of magnitude at comparable Pt loading [42].An increase in total load of electrodeposited NPs on the GDL is expected to increase the half-wave potential at ORR. However, the electrodeposition of higher amounts of catalyst could easily lead to NP growth rather than nucleation of more particles.Another way of improving the ORR of the electrodeposited NPs is by addition of an ionomer such as Nafion, which can lead to a significant increase in ECSA and ORR performance by improving the wettability of the GDL [43].In terms of kinetics, the determined Tafel slopes (b) show a significant improvement with respect to the commercial Pt/C electrode, for which a Tafel slope of 103 mV is obtained. Interestingly, all binary Pt–Ni binary alloy NPs exhibit an almost identical Tafel slope of 65–66 mV (cf. Table 3). The ternary Pt–Ni–Mo(O) NPs show a higher spread in b, where Pt66Ni32Mo2 has the lowest Tafel slope of 62 mV (Fig. 7c). The measured Tafel slopes correspond well with the one determined by Fortunato et al. on pure Pt NPs synthesized by electrodeposition [44].The cross-sectional image of a hot-pressed MEA prior to testing in the fuel cell shows that the catalyst layer (CL) formed by the electrodeposited NPs is homogeneous and well adhered to the membrane, ensuring proton conductivity (Fig. 7d).The polarization curves show that the low catalyst loading results in low current density (not shown). However, the catalysts activity is similar independent of composition, both initially and after AST. Interestingly, the Pt67Ni33 and Pt66Ni32Mo2 again show the highest activity. The major drop in PEMFC performance takes place during the first 10 000 cycles, whereas there is not as significant drop from 10 000 to 20 000 AST cycles (Fig. 8 ). This is in accordance with literature [2] and can be explained by the dissolution of unstable catalytic sites, such as surface Ni and Mo in the initial half of the AST, leaving the particles more stable in the second half of the AST. No decrease in activity could be seen for the commercial Pt/C, which after 20 000 AST cycles show a peak power density of 0.377 W/cm2.In general, the SEM micrographs of the NPs after AST in the PEMFC show that the particle size is reduced (cf. Table 3), related to a loss of material by dissolution in the acidic environment (Fig. 8). The exception is Pt80Ni20, where the particle size is slightly increased (Table 3). This increase in particle size can be explained by Pt dissolution from smaller, less stable particles, and re-deposition onto larger particles. It should be noted that a non-negligible amount of particles remained attached to the membrane rather than the GDL after disassembling the MEA for post-analysis. The Pt–Ni particles have kept their spherical morphology, while the Pt–Ni–Mo(O) NPs have increased roughness after testing. It is also clear that the two Ni-rich samples with very low Pt content experience the most severe dissolution of catalyst material. However, the PEMFC performance at EoT is similar to the other samples. This may be due to the high surface area endowed by the rough morphology of these NPs.The XPS spectra obtained from the catalyst layers after PEMFC testing are compromised by the aforementioned loss of material due to particles remaining on the membrane; however, it was generally observed that in comparison with the initial surface state, both Pt(0) and Ni(0) were present in higher fractions with respect to their oxidized forms (Fig. 9 ). For the Mo-containing catalysts, only Mo(VI) was detected after the PEMFC tests. Contrarily to what might be expected, Ni2p emission levels are close to their initial values (cf. Fig. 4q–u) while Pt emissions are significantly lower.Although electrodeposition is a very common and widely used synthesis method, its employment in the synthesis of ORR electrocatalysts for PEMFC is rather unexplored. Few studies have investigated this topic in recent years, and the processes need to be further optimized in view of the electrocatalysts’ properties and performance, and especially in terms of actual testing in PEMFC (Table 4 ).Binary Pt–Ni and ternary Pt–Ni–Mo(O) NPs were successfully synthesized by pulse electrodeposition from aqueous media directly onto the GDL of a PEMFC. The so-prepared carbon-supported catalysts show high specific activities at ORR and the applicability in PEMFC was demonstrated in a single-cell, with Pt67Ni33 and Pt66Ni32Mo2 showing the highest activity both in ORR measurements and in the PEMFC.NP particle sizes range around 50 nm, and Tafel slopes at ORR of around 65 mV are achieved at low catalyst loadings of 4 μg/cm2. Most importantly, very high ORR mass activities up to 10 mA/μg Pt are reached at half-cell electrochemical tests in 0.1 M HClO4, owing to the favorable distribution of electrocatalyst NPs along the catalyst layer as a result of the electrodeposition process. In the MEAs produced for PEMFC testing, almost all NPs are expected to contribute to the ORR reaction due to their direct contact with both carbon support and PEM, resulting in extremely high efficiency with respect to Pt utilization.For industrial application in PEMFC, the total power output of the fuel cell would need to be increased. To this respect, a higher total amount of catalyst may be needed, achieved by either increasing the Pt load in electrodeposition or by increasing the fuel cell's active area. An increase of loading by electrodeposition is easily achievable. However, by simply increasing the deposition time, additional deposition will occur on already deposited material and lead to particle growth. This would most likely improve half-wave potential and obtainable currents, but adversely affect mass activity. Further optimization is possible by tuning the NP particle size, or by optimizing the electrical properties by the use of thin carbon layers adapted to electrodeposition, to reduce intrinsic resistances.With respect to the ternary system NPs, an obvious advantage over binary Pt–Ni NPs is not observed. Since Mo was mostly found in an oxidized state and the crystallinity of the ternary NPs as observed by SAED was lower, the question remains open as to whether a true, metallic ternary alloy would yield superior ORR activity.Overall, the demonstrated electrodeposition process provides a promising alternative to the conventional methods of ORR electrocatalyst synthesis. In addition to the facile synthesis which applies the catalyst NPs directly onto the GDL, the utilization of the metal electrocatalyst can be seen as close to 100%. Konrad Eiler: conceptualization, validation, formal analysis, investigation, data curation, writing—original draft, writing—review & editing, visualization. Live Mølmen: conceptualization, software, validation, formal analysis, investigation, data curation, writing—original draft, writing—review & editing, visualization. Lars Fast: conceptualization, methodology, validation, resources, supervision, project administration. Peter Leisner: conceptualization, resources, supervision, project administration, funding acquisition. Jordi Sort: conceptualization, resources, writing—review & editing, supervision, funding acquisition. Eva Pellicer: conceptualization, validation, resources, writing—review & editing, supervision, project administration, funding acquisition.The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations. Specific data can be obtained from the corresponding authors on request.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 764977, the Generalitat de Catalunya under project 2017-SGR-292, and the Spanish government under project PID2020-116844RB-C21. The authors want to express their thanks to Freudenberg, Germany, who gladly supported the GDL material for this study.

Proton exchange membrane fuel cells (PEMFCs) are an important alternative to fossil fuels and a complement to batteries for the electrification of vehicles. However, their high cost obstructs commercialization, and the catalyst material, including its synthesis, constitutes one of the major cost components. In this work, Pt–Ni and Pt–Ni–Mo(O) nanoparticles (NPs) of varying composition have been synthesized in a single step by pulse electrodeposition onto a PEMFC's gas diffusion layer. The proposed synthesis route combines NP synthesis and their fixation onto the microporous carbon layer in a single step. Both Pt–Ni and Pt–Ni–Mo(O) catalysts exhibit extremely high mass activities at oxygen reduction reaction (ORR) with very low Pt loadings of around 4 μg/cm2 due to the favorable distribution of NPs in contact with the proton exchange membrane. Particle sizes of 40–50 nm and 40–80 nm were obtained for Pt–Ni and Pt–Ni–Mo(O) systems, respectively. The highest ORR mass activities were found for Pt67Ni33 and Pt66Ni32–MoO x NPs. The feasibility of a single-step electrodeposition of Pt–Ni–Mo(O) NPs was successfully demonstrated; however, the ternary NPs are of more amorphous nature in contrast to the crystalline, binary Pt–Ni particles, due to the oxidized state of Mo. Nevertheless, despite their heterogeneous nature, the ternary NPs show homogeneous behavior even on a microscopic scale.

Currently, fossil fuel resources, such as petroleum, coal, and natural gas, play a crucial role in meeting the growing demand for energy and chemicals [1]. However, these resources are unsustainable, and the environmental problems caused in their utilization are also very serious, especially the greenhouse effect and fog-haze weather [2,3]. Biomass, which consists mainly of cellulose, hemicellulose, and lignocellulose, has the potential to replace fossil fuels owing to its wide distribution, abundance, low cost, and renewable nature [4–7]. 5-hydroxymethylfurfural (HMF) is described as an important biomass-based platform compound and a versatile intermediate, derived from cellulose and C6 sugars, for connecting biomass and the chemical industry. The C–O, C=O, and furan ring of HMF make it very flexible and enable it to be transformed into fuel molecules, such as 2,5-diformylfuran (DFF), 2,5-furandicarboxylic acid (FDCA), ethyl levulinate (EL), 2,5-dimethylfuran (DMF), and 2,5-dihydroxymethylfuran (DHMF) by various methodologies [8–13]. In particular, DMF, formed by selective hydrogenolysis of HMF, is proposed as a promising and sustainable liquid fuel for transportation. Compared to bio-ethanol and bio-butanol, DMF possesses higher energy density (31.5 MJ/L), a higher octane number (RON = 119), a higher boiling point (92–94 °C), lower volatility, and water immiscibility [14]. In addition, DMF can react with ethylene to form p-xylene in a Diels-Alder reaction, which is a potent pathway towards biomass transformation [15].The selective hydrogenolysis of HMF to DMF is a key process in the efficient utilization of biomass resources and has garnered considerable attention [16]. However, the heterogeneous metal-catalyzed conversion of HMF produces various products, including 5-methylfurfural (MF), 5-methylfurfurylalcohol (MFA), DMF, and DHMF. Thus, there is an urgent need to achieve high DMF selectivity. To date, several catalysts, mainly including noble metal or supported noble metal catalysts, have been employed for the selective hydrogenolysis of HMF [17–19]. In 2007, Roman-Leshkov et al. initiated research on HMF selective hydrogenolysis and showed that the DMF yield from fructose could reach 71% over a modified Cu-Ru/C catalyst with the assistance of hydrochloric acid (HCl) and sodium chloride (NaCl) [20]. Subsequently, Chidambaram and Bell first introduced the ionic liquid 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) as the solvent in the HMF-to-DMF reaction and achieved 32% selectivity in an H2 atmosphere with Pd/C as the catalyst, but the poor solubility of H2 in the ionic liquid led to an unsatisfactory result [21]. Chatterjee et al. developed the hydrogenation of HMF in supercritical carbon dioxide using a Pd/C catalyst under milder conditions (80 °C, 1 MPa H2, 2 h) with high conversion (99%) and selectivity (99%) [22]. Zu et al. reported that a Ru/Co3O4 catalyst exhibited outstanding catalytic performance for the production of DMF and showed that Ru and CoO x were responsible for hydrogenation and breaking the C–O bond, respectively [23]. More recently, the CoFe layered double oxide (CoFe-LDO) has been widely applied on account of its controllable acid sites and higher surface area, and the catalyst Ru/CoFe-LDO has been shown to allow selective hydrogenation of HMF to DMF in the presence of H2 [24]. With the continuous development of catalytic science, a series of novel catalysts such as Pd-Co9S8/S-CNT, Pt/rGO, Ru/NaY and Pd-GVL/C have also been used in the conversion of HMF into DMF [25–28]. Nonetheless, the high price and low availability of precious metals limit their application in industrial production.The non-noble metal catalysts Ni, Co, Fe, and Cu have gradually become the focus of intense research for the conversion of HMF into DMF. However, some results have implied that the selectivity of monometallic catalysts for DMF is low [29]. Kong et al. used a commercial Raney Ni catalyst to achieve the complete conversion of HMF at 180 °C and 1.5 MPa H2 pressure in 1,4-dioxane. The poor DMF selectivity was due to the high hydrogenation ability of Ni, which would lead to the generation of some by-products [30]. On the other hand, Gorte's group pointed out that HMF selective hydrogenolysis is a sequential reaction, and DMF may continue to react as an intermediate to form either ring-hydrogenated (e.g. 2,5-dimethyltetrahydrofuran, DMTHF) or ring-opened (e.g. 2-hexanone and 2,5-hexanedione) as by-products in the presence of monometallic catalysts [31,32]. Based on these conclusions, the yield of DMF depends on the relative rates of DMF formation and consumption. It is important to develop a catalyst with excellent C=O/C–O hydrogenation ability but to inhibit C=C/C–C reaction ability. Metal alloying or designing bimetallic catalysts for the hydrogenolysis of HMF to DMF is a promising option. For example, a Ni–Fe alloy formed on the surface of carbon nanotubes (Ni–Fe/CNTs) favors C–O bond breaking and was employed to catalyze the selective hydrogenolysis of HMF to DMF by Yuan et al. [33]. Resasco et al. showed that a Ni–Fe bimetallic catalyst showed better performance than monometallic catalysts because of the oxyphilic Fe atoms [34]. Fang et al. prepared a NiZn catalyst by a coprecipitation method using hydrotalcite-derived mixed oxides as raw materials for selective conversion of HMF to DMF. The excellent effect of the catalyst was attributed to the formation of the alloy and the electronic modification of Ni [35]. A NiCu3/C nanocrystal designed with a core-shell structure exhibited efficient hydrogenation ability, achieving 98.7% DMF yield at 180°C and 3.3 MPa H2 pressure [36]. Moreover, given that FeCoNi/hexagonal–BN and Ni–MoS2/mAl2O3 catalysts had achieved idealized results under certain conditions, the synergetic catalysis of polymetals and the structure-activity relationship between carriers and metals were well recognized [37,38]. Although these Ni-based catalysts have obtained good results for the catalytic hydrogenolysis of HMF, the preparation of the catalysts is complex and hydrogen is needed as a reducing agent, and improvement will be needed for industrial demand in the future. Therefore, it is necessary to use cheap raw materials and to prepare efficient catalysts in a simple way.With the aim of constructing a highly reactive non-noble catalytic system for the selective hydrogenolysis of HMF to DMF, mesoporous TS-1-supported Ni–Cu bimetallic catalysts were prepared through a solid-phase grinding synthesis method. In this method, the reduction was effected by the gas generated during the roasting of the precursor [54]. To thoroughly evaluate the performance of the catalyst, the effect of reaction conditions, such as reaction temperature, H2 pressure, and catalyst dosage, was also investigated. It is shown that the catalysis will not proceed in the direction of side reactions owing to a strong interaction between Ni and Cu, ensuring high selectivity for the target product. Importantly, a possible reaction route and mechanism over the 40%Ni–5%Cu/TS-1 catalyst were obtained.5-Hydroxymethylfurfural (≥98.0%) was purchased from Shanghai D&B Biological Science and Technology Co., Ltd. (Shanghai, China). 2,5-Dimethylfuran (99.0%) was purchased from Nine-Dinn Chemistry Co., Ltd. (Shanghai, China). 5-Methylfurfural (98.0%) was purchased from Shanghai Yien Chemical Technology Co., Ltd. (Shanghai, China). 2, 5-Dihydroxymethylfuran (98.0%) was purchased from Shanghai Macklin Biochemical Co. Ltd. (Shanghai, China). 5-Methylfurfuryl alcohol (95.0%) was purchased from Shanghai Bidepharm Technology Co., Ltd. (Shanghai, China). Nickel nitrate hexahydrate (A.R. grade), cupric nitrate trihydrate (A.R. grade), citric acid monohydrate (A.R. grade), tetrahydrofuran (A.R. grade), and octane (C.P. grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).The hollow-structured TS-1 was synthesized by the classical hydrothermal synthesis method [39]. Ni–Cu/TS-1 catalysts were prepared by a simple solid-phase grinding synthesis method. Typically, nickel nitrate hexahydrate (10 mmol, 2.91 g), cupric nitrate trihydrate (1.15 mmol, 0.279 g) were taken in a mortar and ground into a powder. Then, citric acid monohydrate (12 mmol, 2.52 g) was mixed with the above powder by continuous grinding until homogeneous. Afterwards, TS-1 was added to the mixture and ground for 30 min to a viscous paste. The resulting Ni–Cu/TS-1 precursor was dried at 120 °C for 10 h. Next, the dried precursor was calcined in a tubular furnace under nitrogen atmosphere at 370°C for 3 h. The heating rate from 50 °C to 370 °C was 3 °C/min. The as-prepared catalyst was defined as 40%Ni–5%Cu/TS-1. Ni–Cu/TS-1 catalysts with different metal contents, single-metal catalysts, Ni–Cu/MCM-22, Ni–Cu/Al2O3, Ni–Cu/H-Beta and Ni-Cu/SiO2 catalysts were prepared through the same procedure.The X-ray diffraction (XRD) patterns of the catalysts were recorded by a Bruker diffractometer with Cu Kα radiation and diffraction angle (2θ) ranging from 10° to 80°. Fourier-transform infrared spectra (FT-IR) of the catalysts were collected by the KBr pellet technique on a Nicolet 370 infrared spectrophotometer in the range 400–4000 cm–1. Thermogravimetric and derivative thermogravimetric (TG-DTG) measurements were made on a Netzsch Model STA 409 PC instrument. The heating rate from room temperature to 800 °C was 20 °C/min using α-Al2O3 as the standard material. X-ray photoelectron spectroscopy (XPS) was performed using monochromatic Al Kα (1486.6 eV) as the radiation source (Thermo Scientific K-Alpha+, USA). All binding energies (±0.2 eV) of samples were recalibrated based on the sp2 hybridized C1s line of graphitic carbon at 284.8 eV. The XPS Peak 4.1 program was used for curve fitting after a Shirley-type background subtraction. The nitrogen adsorption measurements of the catalysts were performed on a Micromeritics ASAP 2460 sorption analyzer. The catalysts were out-gassed at 200 °C for 4 h before measurement. The surface acidity of the catalysts was measured by temperature-programmed desorption of ammonia (NH3-TPD) using AutoChem II 2920 equipment. NH3 was used as probe molecule to estimate acidity and the TPD data were collected from 50 °C to 700 °C. The surface morphology of the catalyst was characterized by scanning electron microscopy (SEM) using a Quanta600F instrument and X-ray energy spectrometer (EDX, IE350). The electron beam accelerating voltage was 20 kV and the surfaces of the materials were sprayed with gold.In a typical procedure, HMF (1 mmol, 0.126 g), the catalyst (0.050 g), and THF (10.0 mL) were added to a 100-mL autoclave with Teflon liner, which was sealed and purged with H2 several times. The hydrogenolysis of HMF was conducted under the chosen temperature and pressure conditions with stirring. After the reaction, the reactor was quenched in ice-water, and the liquid products were analyzed by GC (Nexis GC-2030) with an FID detector and a capillary column (Shimadzu SH-Rtx-1701). The structural characteristics of the products were further identified by GC-MS and comparison with retention times of pure chemicals. The reused catalyst was washed with THF and then vacuum dried at 50 °C for the next run.The necessary parameters for GC were as follows: injection volume 1.0 μL, split ratio 1:40, temperatures of the injection port and detector both 250 °C. The temperature program was set as 40 °C for 3 min, 40–100 °C (10 °C/min), 100 °C for 1 min, 100–250°C (40 °C/min), 250 °C for 2 min. All compounds were quantified based on the internal standard method using octane as the internal standard. The equations were as follows: HMF conversion ( % ) = ( 1 − M o l e s o f H M F I n i t i a l m o l e s o f H M F ) × 100 % DMF selectivity ( % ) = M o l e s o f D M F I n i t a l m o l e s o f H M F − m o l e s o f H M F × 100 % The XRD patterns of TS-1 and as-synthesized supported catalysts with different metal content are displayed in Fig. 1 . The characteristic peaks at 2θ = 7.9°, 8.9°, 23.2°, and 24.4°, which are observed for TS-1, are due to diffraction by crystalline MFI zeolite. The intensity of the MFI contribution is weakened due to the metal doped in samples b to g while the original crystal remained, indicating that the structure of MFI prepared by the solid-phase grinding synthesis method is undamaged. Based on the XRD patterns of the monometallic 40%Ni/TS-1 catalyst, the peaks at 2θ = 37.2° (1 1 1) and 43.7° (2 0 0) can be indexed as NiO (JCPDS card No. 47-1049) and the reflection peaks at 44.5° (1 1 1) and 51.8° (2 0 0) can be assigned to Ni (JCPDS card No. 04-0850), which indicates that a small amount of NiO was formed along with Ni0 during the decomposition of the catalyst precursor. This is consistent with the conclusions reached by Abu-Zied and Asiri [40]. In the XRD analysis of the 40%Cu/TS-1 catalyst, the evident peaks at 43.3° and 50.4° are reflections of Cu0, corresponding to the (1 1 1) and (2 0 0) planes, respectively. Interestingly, a diffraction peak is found at 36.4°, which corresponds to a Cu2O phase. The diffraction peaks at 35.2° and 38.5° are attributed to CuO. These phenomena can be explained by the equation: Cu+O2→Cu2O, CuO. In the case of the as-prepared Ni–Cu/TS-1 catalysts, new peaks appear at 2θ = 44.3°, and 51.8° and their width increases gradually with increase in the Ni:Cu ratio. The latter two characteristic peaks are located between Cu0 and Ni0, indicating formation of a Ni–Cu alloy structure [41]. On the other hand, the diffraction peaks of CuO x are not obvious for some reason, possibly because the reducing gas generated by the decomposition of citric acid during calcination of the precursor occupies a dominant position, so the reduction reaction takes precedence. In addition, the content of copper in the bimetallic catalysts is less, and the formation of a Cu-Ni alloy inhibits the oxidation process.The FT-IR spectra of the catalysts are shown in Fig. 2 . In these as-synthesized samples, the peaks at 3450 cm–1 and 1630 cm–1 are assigned to the stretching and bending vibrations of H2O. The peak observed at 970 cm–1 arises from the stretching vibration of Si–O–Ti bonds in the framework of sample a, not observed for b-g samples, which can be assigned to the presence of metal species side-on bound to the silicon of Si–O–Ti unit, thus weakening the [Ti–O–Si] bond [42]. The strong bands at 1100 cm–1 and 1230 cm–1 are ascribed to the Si-O-Si asymmetric stretching region. The band at 806 cm–1, which originates from the Si-O-Si symmetric stretching vibration of TS-1, is wider than the others, indicating that the loaded nickel and copper species have replaced Si in the framework. The shoulder bands at 545 cm–1 and 455 cm−1 are respectively attributed to the typical structure of MFI TS-1 zeolite and the Si-O bending vibration. Based on the above phenomena and similar characteristic peaks of each sample, it can be concluded that the metal has been successfully loaded onto the carrier.Citric acid, nickel nitrate, and copper nitrate produce complexes during the preparation of catalysts by the solid-phase grinding synthesis method. Subsequently, when they are calcined under nitrogen, the reducing gas generated by the decomposition of the precursor converts Ni2+ and Cu2+ into Ni and Cu, and the Ni–Cu alloy structure is formed due to complexation. The decomposition process and the thermal stability of the samples are proved by the thermo-gravimetric (TG) analysis experiments. The results are shown in Fig. 3 . It can be seen that the curves of TS-1 have only a small mass loss with the increase in temperature, indicating that TS-1 has high thermal stability. In the case of the precursor, major weight loss, amounting to about 32%, is observed in the TG-DTG plots from 350 °C to 400 °C. This shows that rapid decomposition of the complex occurs to form NiO and CuO x , with release of CO or CO2. For the 40%Ni–5%Cu/TS-1 catalyst, the weight increase from 200 °C to 400 °C is owing to the oxidation of the metals according to the previous report [37], whereas the weight loss at approximately 400–500 °C is reasonably could be attributed to the destruction of the Ni–Cu alloy structure.To obtain further understanding of the surface composition and chemical state of the 40%Ni–5%Cu/TS-1 catalyst, X-ray photoelectron spectroscopy (XPS) was conducted. All the measurements were calibrated by the C1s binding energy at 284.8 eV. The XPS spectra are shown in Fig. 4 . It is worth noting that the catalyst may cause the oxidation of some metals when transferred to the XPS chamber [43]. It can be observed in the survey spectra (Fig. 4 (a)) that the main peaks appeared at the positions of 532.8, 853.8, 103.6, 459.9, 931.6, and 283.7 eV, corresponding to O, Ni, Si, Ti, Cu, and C, respectively. This is consistent with the energy dispersive spectroscopy (EDS) mapping. In the Ni 2p XPS spectrum, the peaks at 852.3 eV and 870.1 eV are assigned to metallic Ni 2p3/2 and Ni 2p1/2, respectively [44]. Two other satellite peaks can be discerned with binding energies at 861.4 eV and 879.5 eV. Simultaneously, the two main peaks located at 855.0 eV (Ni2+ 2p3/2) and 856.9 eV (Ni2+ 2p1/2) demonstrate the presence of NiO on the surface of the catalyst [45].The peaks at binding energy values of 934.8 eV and 953.7 eV with strong satellite peaks, which correspond to Cu2+ 2p3/2 and Cu2+ 2p1/2, respectively, signify the existence of CuO (Fig. 4 b) [41]. Peaks due to Cu metal or CuO species can be seen centered at around 932.4 eV (Cu 2p3/2) and 952.8 eV (Cu 2p1/2). In the Cu 2p XPS analysis, Cu0 and Cu+ cannot be clearly identified because of their overlapping signals, so the Auger spectra of Cu are used for recognition. The peak with kinetic energy 918.0 eV is attributed to Cu0, indicating that this is part of the catalyst [55]. Based on the above introduction and previous reports, the phenomenon of partial migration of the characteristic peaks of Ni 2p and Cu 2p in the 40%Ni–5%Cu/TS-1 catalyst indicates the formation of a Ni–Cu alloy [46].The O 1s spectrum of 40%Ni–5%Cu/TS-1 can be deconvoluted into three underlying peaks. The resolved peaks with the binding energies around 529.6 eV and 531.5 eV may belong to surface lattice oxygen and adsorbed oxygen, respectively, which can transfer electrons to Ni2+ and Cu2+. The Ni–Cu interaction can be enhanced by this electronic effect, which is favorable to the formation of an alloy [47]. Based on previous report [48], the geometry of the bimetallic structure is different from that of the origin metals, metal Ni has more d band holes than metal Cu, d electrons of Cu0 can flow to the unoccupied d orbit of Ni0 which leads to the d electron density of metal Ni active site increase. The high electron density of metal Ni active site favors the electron transfer from metal site to the lower unoccupied molecular orbital of C-O and C=O, which can effectively promote the activation of C-O and C=O bonds, and improve the HMF conversion. Furthermore, the peak at 533.6 eV is attributed to lattice defect species of the catalyst, which the surface-adsorbed oxygen species on oxygen vacancies belonging to defect-oxide and surface hydroxyl-like groups adsorbed on metal ions [49].N2 adsorption-desorption isotherms and pore diameter distribution curves of as-prepared catalysts are depicted in Fig. 5 . All samples showed type IV Langmuir adsorption-desorption isotherms with a type H1 hysteresis loop, which is characteristic of an ordered mesoporous structure according to the IUPAC classification [50]. The TS-1 isotherm closured at a lower relative pressure (P/P0=0.42), and the Ni–Cu/TS-1 catalysts moved to a higher value, indicating the production of larger pores during the catalyst preparation process. This is consistent with the result of the pore size distribution. The main physicochemical properties of the different materials are summarized in Table 1 . The TS-1 support possessed a high specific surface area, 371.21 m2/g, as determined from the Brunauer-Emmett-Teller (BET) equation. With increasing metal loading, the surface areas of catalysts b, c, d, and e were reduced to 239.00, 233.72, 233.22 and 221.62 m2/g, respectively. The total pore volumes were calculated to be 0.37, 0.35, 0.34 and 0.24 cm³/g for b, c, d and e, respectively. These values indicate that the metal phase not only covered the surface of the carrier, but was also inserted into the hollow structure of TS-1, resulting in the blockage of available pores. Compared with TS-1, the pore volumes of these samples also followed a similar decreasing trend, indicating filling of the pores by alloy particles.The surface acidities of TS-1 and 40%Ni–5%Cu/TS-1 were determined using NH3‐TPD experiments. The profiles are shown in Fig. 6 . It is well known that the surface acidity can be divided into weak (<250 °C), medium (250–400 °C), and strong (>400 °C) acidic sites according to the desorption temperature of ammonia. For TS-1 and 40%Ni–5%Cu/TS-1, one peak around 100 °C was observed, which could be ascribed to weak acidity. On the other hand, the maximum desorption strength increased with the introduction of Ni and Cu in the 40%Ni–5%Cu/TS-1 catalyst, resulting in the formation of a strongly acidic site (407 °C, 0.483 mmol/g). The actual situation is that nickel atoms enter the framework of titanium silicalite-1 and replace silicon and titanium atoms in the process of catalyst preparation, and because nickel presents a positive bivalent state, it can provide lone electron pairs to form Lewis acid sites. In addition, NiO increases the Lewis acidity of the catalysts. The proper acid strength is helpful for the hydrogenolysis of the C–O bond and ensures the specificity of its target product DMF; this is discussed later.The surface morphology and structure of the catalysts were characterized by the SEM technique, with the images of the TS-1 and 40%Ni–5%Cu/TS-1 samples shown in Fig. 7 . It can be noticed that the materials exhibited a similar spherulite nanostructure. Comparison of images (a) and (d) shows that the introduction of Ni and Cu had caused a significant change in the morphology of the catalyst. Some grooves and cracks appeared in the 40%Ni–5%Cu/TS-1 catalyst, which indicates that the acting force of Ni–Cu alloy affected the hollow structure of TS-1 (Fig. 7. b and e). Elemental mapping along with the EDS spectra were employed to investigate the distribution of atoms on TS-1. As shown in Fig. 8 , the results showed that the 40%Ni–5%Cu/TS-1 catalyst contained elements of C, O, Si, Ti, Ni, and Cu. Considering the corresponding EDS analysis, it is evident that Ni and Cu were uniformly incorporated into the TS-1 framework without partial agglomeration.The catalytic performance of various metal catalysts and the support for the selective hydrogenolysis of HMF was investigated, carrying out the reaction in THF at 180 °C and 0.5 MPa H2 pressure for 7 h in a 50 mL Teflon-lined stainless steel autoclave. The results are summarized in Table 1. For control purposes, a blank experiment without any catalyst was first conducted, and it showed 3% HMF conversion (Table 2 , entry 1). This phenomenon may be caused by the hydrogen atmosphere in the enclosed environment. The carrier material was also submitted to the same evaluation for comparison, the result showing that HMF conversion of 24.4% with negligible DMF selectivity can be obtained under the given conditions (Table 2, entry 2). Moreover, the main by-product of this reaction was MF. It is well known that the Lewis acid sites of TS-1 are conducive to the activation of C–O bonds, thus facilitating C–O bond rupture. When the 40%Ni–5%Cu catalyst was used, the selectivity for the target product DMF reached 41.1% (Table 2, entry 3). On account of the poor hydrogenolysis capacity of this catalyst, the product distribution was not specific. To verify the synergistic relationship between metal sites and acid sites, physically mixed 40%Ni–5%Cu and TS-1 catalysts were introduced into the selective conversion of HMF to DMF (Table 2, entry 4). It can be seen that the conversion of HMF increased to 77.2% and the selectivity for DMF increased to 68.8%. This result suggests that co-operation between the metal and carrier plays a pivotal role in determining catalytic performance. However, because of the simple mixing, excellent activity had not yet been achieved. The activity of monometallic catalysts was also researched. The 40%Ni/TS-1 catalyst achieved high conversion (100%) but with low selectivity (67.4%) (Table 2, entry 5). This could be ascribed to the excessive hydrogenation ability of Ni metal, with the DMF being used as an intermediate to generate some by-products. Thus, the selectivity of this catalyst for DMTHF, 2-methylfuran (MeF) and 2,5-hexandione (HD) accounted for 32%. In contrast, the 40%Cu/TS-1 catalyst showed lower catalytic activity and its product distribution remained at the intermediate stage of DHMF, MF and MFA, suggesting that the individual Cu species has insufficient ability for selective hydrogenation of HMF to DMF (Table 2, entry 6). Therefore, further investigation of the catalytic performance of Ni–Cu/TS-1 focused on different Ni/Cu ratios. Surprisingly, the incorporation of Cu was beneficial to improve the selectivity of the target product, which may be because tilted furan ring formed on the Cu surface inhibited the side reactions of DMF [51]. In the case of the 40%Ni–1%Cu/TS-1 catalyst, 100% HMF conversion was obtained but the selectivity for DMF was 85.3% (Table 2, entry 7). Over-hydrogenated products were the main by-products with a selectivity of 14.7%, which may have been due to the low Cu content. In fact, by changing the Ni/Cu ratio, the product distribution was changed. The best results were obtained when the Cu load was increased from 3% to 5%, which increased the DMF selectivity from 90.2% to 97.3% (Table 2, entry 8–9). In light of this, it would appear that an appropriate Cu content may be beneficial by inhibiting the excessive hydrogenation capacity of the catalyst. The rate of C=C/C–C bond breaking was reduced, which may be caused by the interaction force formed between Ni and Cu. This was consistent with the results of XRD and other characterization methods. Hence, the HMF to DMF conversion process could be synergistically accelerated by the combination of Ni, Cu, and TS-1. This was also confirmed by the result for the 40%Ni–7%Cu/TS-1 catalyst, indicating that an excess of the Cu source can worsen the activity of the catalyst (Table 2, entry 10). Therefore, the 40%Ni–5%Cu/TS-1 catalyst was selected as the appropriate catalyst in all subsequent experiments.The reaction temperature affects the product distribution to some extent; therefore, the influence of different temperatures on the selective hydrogenolysis of HMF to DMF was investigated using the 40%Ni–5%Cu/TS-1 catalyst. The results are shown in Fig. 9 . Clearly, the conversion of HMF and the selectivity for DMF are positively correlated with the reaction temperature. When the reaction occurred at the lower temperature of 120 °C, conversion of HMF was considerably lower (27.8%) with very little DMF selectivity (3.4%), and DHMF and MF selectivity accounted for a large proportion. DHMF and MF can further generate DMF through the cleavage of C–O/C=O. The hydrogenolysis of HMF was evidently not complete. When the reaction temperature increased to 140 °C, 51.8% HMF conversion and 20.7% DMF selectivity were obtained. At this point, the highest activity of the catalyst was not realized. With a further increase in the reaction temperature to 160 °C, the conversion of HMF and the selectivity for DMF increased to 90.6% and 78.1%, respectively. As usual, the selectivity for the main intermediates DHMF, MF, and MFA continued to decrease. It is noteworthy that higher temperature favors activation of the C–O/C=O bonds, thereby promoting the cleavage of carbonyl and aldehyde groups [53]. To achieve the highest reactivity, the reaction was conducted at 180 °C. The HMF was fully converted and the selectivity for DMF reached 97.3%. Surprisingly, the percentage decrease in the selectivity of the target product DMF was negligible at 200 °C. This may be because the addition of Cu species and the strong interaction in the Ni–Cu alloy increase the adsorption of the furan ring, thus preventing the occurrence of some side reactions. Overall, the most suitable reaction temperature over the 40%Ni–5%Cu/TS-1 catalyst was chosen to be 180 °C.The performance of the 40%Ni–5%Cu/TS-1 catalyst for the selective hydrogenolysis of HMF to DMF was also examined at various H2 pressures at 180 °C for 7 h. As shown in Fig. 10 , catalytic activity is closely related to H2 pressure. Initially, the reaction rate of HMF and the selectivity for DMF gradually increased with increase in H2 pressure. When the reactor was in hydrogen atmosphere, but the initial pressure was 0 MPa, the conversion of HMF was 40.6% and the selectivity for DMF was 22.5%. As the H2 pressure reached 0.25 MPa, the HMF conversion and DMF selectivity were, respectively, 82.5% and 70.6%. In terms of dynamics, the increase in the concentration of dissolved hydrogen promotes the transformation of intermediates DHMF, MF and MFA to DMF. With an increase in H2 pressure to 0.5 MPa, the conversion of HMF increased to 100% and the selectivity for DMF was nearly equivalent (97.3%). When the H2 pressure was further increased to 0.75 or 1.0 MPa, no noticeable change in the conversion of HMF and the selectivity for DMF was obtained. In general, higher reaction pressure may cause opening or excessive hydrogenation of the furan ring [52]. In this study, even though hydrogen pressure increases to a certain extent, the Ni–Cu alloy structure in the catalyst still shows lower reactivity to DMF, meaning that the surface oxygen can effectively prevent deep hydrogenolysis of the furan ring. Based on the survey of H2 pressures, 0.5 MPa was deemed appropriate for this catalytic system.As demonstrated, the dosage of catalyst is a significant parameter for selective conversion of HMF to DMF, and should accelerate the mass transfer of the reaction mixture. The dosage effect was investigated by varying the catalyst weight between 20 and 60 mg. As shown in Fig. 11 , when the dosage of the catalyst was increased from 20 mg to 50 mg, the conversion of HMF rapidly increased from 46.1% to 100%, and the selectivity for DMF increased from 62.1% to 97.3%. The reason is mainly that the generation of more active sites accelerates the hydrogenolysis process, so that the –CHO and –CH2OH of HMF are continuously activated. The almost complete conversion of DHMF (4.3%), MF (18.7%) and MFA (14.9%) further implied the necessity of an appropriate amount of catalyst. However, on further increasing the catalyst amount to 60 mg, the DMF selectivity fell to 92.5%. It is possible that the excess of catalyst may have caused some side reactions, which generated by-products like DMTHF, HD and MeF that affected the selectivity for DMF. Based on the above discussion, the amount of 40%Ni–5%Cu/TS-1 catalyst was precisely controlled at 50 mg for the further studies.The selective hydrogenolysis of HMF to DMF is a sequential reaction in which DMF may continue to produce secondary by-products. Under normal conditions, HMF first generates some O-containing intermediate products (IP) through the hydrogenolysis process, including DHMF, MF and MFA. The IP then form DMF. Finally, DMF produces over-hydrogenated products (OP), such as DMTHF, HD, MeF and so on. The product distribution in the selective hydrogenolysis of HMF to DMF at 180 °C over 40%Ni/TS-1, 40%Cu/TS-1, and 40%Ni–5%Cu/TS-1 catalysts is shown in Fig. 12 . It is obvious that the bimetallic catalyst is more advantageous than the monometallic catalyst. From the trend of product distribution (Fig. 12 a), the reactivity of the 40%Cu/TS-1 catalyst was relatively low. The maximum conversion of HMF reached 88.7% by extending the reaction time, but the selectivity for DMF was only 70.0%. Overall, the products mainly stayed in the stage of O-containing intermediates. This phenomenon is consistent with the results of some Cu-based catalysts used in hydrogenolysis reactions [56]. Results for the performance of the 40%Ni/TS-1 catalyst are presented in Fig. 12 (b). DMF selectivity initially increased with the reaction time. When the reaction time increased to 5 h, HMF was almost completely converted, with an 80.4% maximum selectivity for DMF. Notably, intermediate products were formed in the minimal reaction time and then quickly diminished. Subsequently, over-hydrogenation or ring opening of the furan ring may have occurred during the reaction. In summary, a challenging problem that arises in this domain is that the single-metal catalysts (40%Ni/TS-1, 40%Cu/TS-1) cannot ensure the specificity of the target product DMF. Broadly speaking, as shown in Fig. 12 (c) and Table S1, the 40%Ni–5%Cu/TS-1 catalyst prepared by the solid-phase grinding synthesis method exhibited excellent properties. When the reaction time was 1 h, the intermediates DHMF, MF and MFA accounted for a large proportion of products. After 1 h, it was found that the selectivity for DMF increased almost linearly with time, and a maximum DMF selectivity of 97.3% was obtained after 7 h. At this point, the HMF conversion reached 100%. In addition to HMF and DMF, the proportion of intermediates also decreased gradually in the reaction mixture. Within the limits of the experiment, the 40%Ni–5%Cu/TS-1 catalyst exhibited little activity for DMF; however, when the reaction time was 9h, the selectivity for DMF still remained at 97.0%. This behavior of inhibiting side reactions is the main reason for maintaining the high selectivity for DMF, the alloying metals preventing the furan ring from lying down on the surface of the 40%Ni–5%Cu/TS-1 catalyst.From the discussion regarding Table 2, it is known that the acidic sites of TS-1 are conducive to the hydrogenolysis of the hydroxyl group, while the metal sites of Ni and Cu are favorable for the hydrogenation of the aldehyde group. To further explore the role of the support in bifunctional catalyst, the selective hydrogenolysis of HMF was evaluated under the conditions of different catalyst supports. The comparison is summarized in Fig. 13 . For the Ni–Cu/MCM-22, Ni–Cu/Al2O3, and Ni–Cu/H-Beta catalysts, the conversion of HMF and the selectivity for DMF were lower. In the case of Ni–Cu/SiO2, the selectivity for DMF decreased because of side reactions. This indicates poor synergism between the hydrogenolysis reaction and active sites of the bifunctional catalyst. Additionally, the overall acidity of various catalysts can be calculated from the NH3-TPD experiments, as shown in Table S2. Appropriate Lewis acid sites played an important role in the selective hydrogenolysis of HMF into DMF. The catalysts with acid amount of about 1.50 mmol/g were beneficial to the production of DMF.Nowadays, the preparation of catalysts with good stability is of great significance for commercial applications. Recycling of the 40%Ni–5%Cu/TS-1 catalyst was performed to investigate the reusability of the catalyst under the optimum reaction conditions. The catalyst was recovered by centrifugation after the reaction, washed at least five times with THF, and used directly for the next run without further reactivation. The results are shown in Fig. 14 . It was surprising that the 40%Ni–5%Cu/TS-1 catalyst still maintained good catalytic performance after six consecutive experiments. There was a slight decrease in HMF conversion (85.6%) and DMF selectivity (80.9%) in the fourth cycle. At the same time, the selectivity for the intermediates DHMF, MF and MFA was 4.2%, 4.3%, and 10.6%, respectively. To verify the leaching of metal ions in the catalyst, a thermal filtration test was conducted. The 40%Ni–5%Cu/TS-1 catalyst was removed from the reaction solution by filtration after 3 h, and the filtrate without catalyst was then allowed to react for 4 h. As shown in Fig. 15 , there was no obvious change in the yield of DMF after the removal of the catalyst, which indicates that the active sites were not leached. Next, the catalyst that had been used four times was recalcined under nitrogen atmosphere and used for a fifth and sixth reaction. HMF conversion and DMF selectivity reached 100% and 96.5%, respectively. A possible explanation is that organic matter in the reaction liquid covers the active sites of the catalyst. To confirm the stability of the catalyst after the selective hydrogenolysis of HMF, the XRD patterns and FT-IR spectra of the used catalyst were obtained, and are shown in Fig. 16 . There were almost no differences in the XRD patterns and FT-IR spectra between the spent and fresh catalysts, suggesting that the structure of the catalyst was still intact.Based on the abovementioned experimental results and previous publications, the following detailed reaction pathways for selective hydrogenolysis of HMF to DMF over Ni–Cu/TS-1 catalysts are proposed. As illustrated in Scheme 1 and scheme S1, the –CHO of HMF is first hydrogenated to form the intermediate DHMF on the metal sites and the –CH2OH of HMF is deoxygenated on acid sites to transform it to MF. Then, DHMF and MF generate intermediate MFA through hydrogenolysis of the C–O bond and hydrogenation of the C=O bond, respectively. Finally, the –CH2OH of MFA is further hydrogenated to DMF.Thus, a plausible reaction mechanism was proposed. First, H2 and substrate HMF are adsorbed onto the surface of the Ni–Cu/TS-1 catalyst. In a closed environment, hydrogen is dissociated into hydrogen atoms by interaction with the (1 1 1) surface of the metal. In the reaction of HMF to give DHMF, the carbonyl carbon atom is attacked by a hydrogen atom, such that the aldehyde group of HMF becomes a hydroxyl group. On the other hand, the oxygen atom of the HMF hydroxyl group is stimulated by the Lewis acid site of the catalyst, and is automatically dehydrated after being attacked by a hydrogen atom, leading to the formation of the intermediate MF. In addition, the Cu in the Ni–Cu/TS-1 catalysts is more favorable for the reaction of C–O and C=O and inhibits the excessive hydrogenolysis of C–C. The possible explanation is that, owing to the overlap of the 3d band of the surface Cu atoms and the aromatic furan ring, a repulsion occurs between the Cu (1 1 1) plane of Ni–Cu/TS-1 catalysts and the furan ring [50]. Similarly, the intermediates DHMF and MF are further transformed into MFA through the above process. Ultimately, MFA is easily deoxygenated on the acidic sites to produce DMF.In summary, the selective hydrogenolysis of HMF to DMF takes place over Ni–Cu/TS-1 catalysts with an appropriate Ni/Cu ratio in the presence of THF solvent under H2 atmosphere. Detailed characterization of the catalysts was performed to unravel the Ni–Cu alloy species formed on the surface of the carrier material. Control experiments revealed that there is synergy between the hydrogenolysis of the hydroxyl group in HMF over the Lewis acid TS-1 and the hydrogenation of the aldehyde group over metal particles during the reaction. Reusability studies further showed that the Ni–Cu/TS-1 catalysts are relatively stable. Based on the concept of economy and sustainability, this study provides an approach for the efficient and targeted hydrogenation of biomass-based furan derivatives into fine chemicals and fuel additives.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (21606082), China Postdoctoral Science Foundation (2019M662787), Scientific Research Fund of Hunan Provincial Education Department (20B364), Hunan Provincial Innovation Foundation for Postgraduate (CX20200522) and the Opening Fund of Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education & Hubei Key Laboratory of Catalysis and Materials Science (CHCL21003).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ceja.2020.100081. Image, application 1

Production of biofuels from biomass resources has received wide attention because of the current energy crisis. Recently, significant interest has been directed towards the selective hydrogenolysis of 5-hydroxymethylfurfural (HMF) to produce biofuel 2, 5-dimethylfuran (DMF). In this study, non-noble Ni–Cu catalysts with various Ni/Cu ratios, supported on titanium silicalite-1 (TS-1), were prepared through a solid-phase grinding method and used to catalyze the hydrogenolysis of HMF to DMF under a hydrogen atmosphere. The structure and surface morphology of the catalysts were characterized by X-ray diffraction, Fourier-transform infrared, thermogravimetry, X-ray photoelectron spectroscopy, temperature-programmed desorption of ammonia, scanning electron microscopy, and N2 adsorption-desorption techniques. Under the optimized reaction conditions, the conversion of HMF and the selectivity for DMF over a 40%Ni–5%Cu/TS-1 catalyst could reach 100.0% and 97.3%, respectively. Importantly, because the strong interaction of the Ni–Cu alloy structure prevents further reaction of the furan ring, almost no by-products are produced. The metal sites of Ni and Cu and the acid sites of TS-1 combine to provide a synergistic effect, which is beneficial to the hydrogenolysis of HMF. In addition, reusability experiments showed that the catalyst maintained good activity and stability.

All data related to this study included in the article and supplemental information will be provided by the lead contact upon request.Oxygen evolution reaction (OER), a four-electron-involved anodic process, normally suffers from sluggish kinetics, which thus is the bottleneck in various electrochemical applications. 1–7 It has always been a formidable challenge to design outstanding OER catalysts with low costs and high activity ahead of the proposal of single-atom catalysts (SACs). 8–12 Thanks to maximal atomic-utilization efficiency, abundant active species, and inspiring catalytic activity, SACs have been regarded as one of the most promising solutions. 13–19 M−N4, M−N2O2, and M−N3C1 (M = transition metals) represent common coordination modes in SACs, which are validated to be capable of tuning the electronic structures of the central metal atom and thus the catalytic performance. 20–25 For instance, Jiang et al. demonstrated that a low N coordination number could favor the formation of COOH∗ intermediates of the Ni SA–N2–C, leading to superior CO2RR activity. 26 Zhou and co-workers employed density functional theory (DFT) computation and machine learning and found that both metals and coordination modes have a great impact on the OER descriptor (ΔG∗O−ΔG∗OH). 27 Compared with other commonly used transition metals such as cobalt (Co) or nickel (Ni), iron (Fe) is more abundant and environmentally friendly. It has attracted particular interest not only because of its high abundance but also because it shows intriguing interaction capability with coordinating atoms/centers, which may lead to unusual coordination modes. 28 Therefore, it is encouraged to explore more Fe-SACs with novel coordination configurations by prudent structural design and to unveil their OER catalytic behavior.Covalent organic frameworks (COFs) with well-defined and tailorable structures could provide a platform for the variable coordination of isolated metal atoms by the confinement effect and coordination interaction with specified atoms of the organic skeleton. 29–35 For example, a series of dioxin-linked metallophthalocyanine COFs could construct a typical M−N4 coordination mode. 36–38 The Salen-COFs derived from the ortho-hydroxybenzaldehyde and ethylenediamine through Schiff-base condensation reactions could lead to M−N2O2 moieties. 27 An M−N2 site can be anchored on a 2, 2′-bipyridine-based COF. 39 , 40 Thus, by tuning the backbones and functional groups as well as the coordinating geometry, unusually coordinated Fe-SACs could be constructed, which remains a great challenge in practice. Meanwhile, those SACs with specific coordinating structures are a necessity to perform reliable theoretical computations, which is conducive to understanding the catalytic mechanism at the molecular level and to developing highly efficient OER catalysts.Herein, we confine Fe single-atom species at a low operation temperature (−60°C) in a two-dimensional (2D) COF, which is achieved based on the Schiff-base condensation reaction between the 1,3,5-triformylphloroglucinol (Tp) and 4,4′,4''-(1,3,5-triazine-2,4,6-triyl)trianiline (Tta) ligands. Extended X-ray absorption fine structure spectroscopy (EXAFS) and aberration-corrected high-angle annular dark-field scanning TEM (AC HAADF-STEM) reveal that the metal species are dispersed uniformly and isolated on the COF carriers with an unusual Fe–NO atomic arrangement in the skeleton. As a result, the as-prepared Fe single atom embedded in the COF catalyst (Fe-SAC@COF) demonstrates a fairly low overpotential of 290 mV at 10 mA cm−2 and a Tafel slope of 40 mV dec−1, which, to the best of our knowledge, surpasses all reported atomically dispersed Fe-based OER electrocatalysts. Moreover, DFT calculations reveal that the Fe–NO-coordinated Fe-SAC@COF processes a much lower energy difference of potential-determining step than that of Ni species and thus behaves superiorly toward OER. This work may inspire more novel coordinated SACs in judicious COFs in achieving highly active electrocatalysis.The SAFe-COF is synthesized by a two-step process, as schematically demonstrated in Figure 1A. First, Tp-Tta COF was prepared through the Schiff-base condensation reaction between the two ligands under a solvothermal condition 41 (see experimental details in the experimental procedures). Then, the as-prepared Tp-Tta COF powder was used as the Fe loading carrier, where the process was performed at a low temperature of −60°C to prevent the species from aggregating. 42 Subsequent treatment in N2H4·H2O-containing alkaline reducing agent was conducted to produce the targeted Fe-SAC@COF.Powder X-ray diffraction (PXRD) patterns of the Tp-Tta COF sample fit with the simulation results, which implies successful synthesis (Figure 1B). The intense peak at 5.8° (2θ) corresponds to the reflection of the (100) plane, while the broad peak at ∼26° is assigned to the (001) plane, which is attributed to π-π stacking between successive layers of the 2D Tp-Tta COF. The Fourier transform infrared spectroscopy (FT-IR) peak at 3,000–3,500 cm−1 of the Tta ligand, corresponding to –NH2 bonds, disappears, while two strong peaks at 1,623 and 1,286 cm−1 can be assigned to the –C=O and –C–N bonds, respectively (Figure 1C). This indicates the formation of the β-ketoenamine-linked framework. In the solid-state 13C nuclear magnetic resonance (NMR) spectrum, the characteristic peaks at 182.0 and 105.1 ppm can be assigned to –C=O and –C=C, which confirms the success of the condensation reaction between the two ligands (Figure S1). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show a fibrous morphology for the Tp-Tta COF (Figures S2 and S3). Moreover, the morphology, PXRD, and FT-IR patterns are retained after soaking in 1 M KOH electrolyte solution for the Tp-Tta COF, which demonstrates decent stability and meets the basic prerequisites for alkaline OER electrocatalysis (Figures S4 and S5).No impurities were detected in the PXRD patterns and FT-IR spectra after the coordination of Fe, which indicates that the Fe-SAC@COF sample retains crystallinity and has sturdy chemical bonds (Figures 1B and 1C). Meanwhile, the fibrous-like morphology is maintained in the SEM image, implying the stable structure of Fe-SAC@COF (Figure 1D). The N2 adsorption-desorption isotherm of Fe-SAC@COF exhibits a smaller Brunauer-Emmett-Teller (BET) surface area of 470 m2 g−1 compared with pristine Tp-Tta COF (716 m2 g−1), which could be attributed to the increment of mass of the Fe atoms (Figure S6). Meanwhile, the pore size distribution of Fe-SAC@COF (M=Fe, Ni) by the non-local DFT (NLDFT) indicates that the size is sub-1 nm, which is similar to that of pristine COF (Figures S7, S8, and S11). Additionally, the mass ratio of Fe in Fe-SAC@COF is 1.0 wt %, detected by the inductively coupled plasma atomic emission spectroscopy (ICP-AES) results (Table S1). To visualize the relationship between coordination configuration and catalytic activity, Ni-SAC@COF was prepared through a similar synthetic approach (Figures S9–S11).The composition analysis by X-ray photoelectron spectroscopy (XPS) reveals that metals in Fe-SAC@COF and Ni-SAC@COF are in approximate +3 and +2 valence states, respectively (Figures S12 and S13). No clusters or nanoparticles are displayed in the TEM images of the as-obtained Fe-SAC@COF and Ni-SAC@COF samples, while the metal elements show a homogeneous distribution (Figures 2A, S14, and S15). This implies that the metal atoms are probably dispersed in the single-atom form. AC HAADF-STEM images were further performed to identify the distribution state of the coordinated atoms. As shown in Figures 2B and 2C, uniform and high-density bright dots are observed, which verifies the atomic dispersion of the metal atoms in the COF skeleton.X-ray absorption fine structure (XAFS) spectra were performed to reveal the interface structure at the atomic level. The rising-edge position of the X-ray absorption near-edge structure spectra (XANES) of both Fe-SAC@COF and Ni-SAC@COF samples are between the corresponding metallic foils and metal oxides (Figures 2D and 2G). This suggests that the metal atoms in COFs are in oxidized states, which are consistent with the XPS results. Furthermore, the FT-EXAFS spectrum was applied to confirm the metal coordination environment (Figures 2E and 2H). Fe-SAC@COF presents only one main peak at about 1.47 Å, which is attributed to the Fe–N/O scattering path, and no Fe–Fe bond (2.17 Å) can be detected, confirming the single-atomic state of Fe in Fe-SAC@COF. Similarly, Ni-SAC@COF also shows one peak in the R space (at a shorter length than the typical M–M distance), confirming no metal-metal bonds. Besides, the wavelet transforms (WTs) were carried out to analyze metal-atom, K-edge EXAFS oscillations (Figures 2F, 2I, S16, and S17). The WT maximum is observed only at ∼4 Å−1 for M-SAC@COF (M=Fe, Ni), which could be assigned to M-N/O bonds. No WT maximum, corresponding to the M–M bond, is detected as in the metallic foil and metal oxide, suggesting the presence of mononuclear metals. EXAFS fitting was performed to extract the structure parameters and the quantitative chemical configuration of metal atoms (Figures S18 and S19; Tables S2 and S3), which gives a coordination number of 4 for both Fe and Ni centers. The coordination of Fe with O atoms is confirmed by the O1s spectrum, where Fe-SAC@COF shows an emerging deconvoluted Fe–O peak (Figure S20). Moreover, the interlayer spacing of Fe-SAC@COF is calculated to be ∼3.4 Å based on the (001) reflection peak at ∼26° in the XRD pattern. As there is one O in each layer and the measured Fe–O/N bond distance is 1.97 Å, the three O atoms are all bound within the framework can be ruled out. Besides, if Fe binds with 2 N and 2 O atoms from the adjacent layers, the valence state would be 2+, which is inconsistent with the XPS and EXAFS results. Thus, Fe is most likely coordinated with one O and one N of the COF, while the other 2 O are coordinated with the O of the acetate. The Fe–NO3 coordination mode agrees with the EXAFS fitting results (Figure S18) as well as the geometry optimization (see details in the supplemental information).The electrocatalytic performance of pristine Tp-Tta COF, Fe-SAC@COF, and Ni-SAC@COF were evaluated in 1 M KOH electrolyte in a typical three-electrode testing configuration. Fe oxide nanoparticles with an average size of 5 nm were prepared on the Tp-Tta COF (denoted as Fe-NP/COF) for comparison (Figures S21–S23). As revealed in the representative linear sweep voltammetry (LSV) curves, the Fe-SAC@COF electrode exhibits superior catalytic activity with a low overpotential of 290 mV at a current density of 10 mA cm−2 (Figure 3A), which outperforms the Ni-SAC@COF (337 mV), Fe-NP/COF (359 mV), and pristine Tp-Tta COF electrodes (430 mV). Though Fe-SAC@COF and Ni-SAC@COF have the same coordination mode, the former, as visualized in Figure 3B, exhibits much higher activity toward OER. Accordingly, the Fe-SAC@COF electrode has the lowest Tafel slope of 40 mV dec−1 among all samples (Ni-SAC@COF, 45 mV dec−1; Fe-NP/COF, 51 mV dec−1; pristine COF, 129 mV dec−1), suggesting its fast kinetics for OER (Figure S24). Moreover, Co-SAC@COF was also synthesized and tested, which is inferior to Fe-SAC@COF (onset potential, 359 mV; Tafel slope, 48 mV dec−1), further verifying the influence of metal centers on catalytic activity (Figures S25 and S26). Correspondingly, Fe-SAC@COF exhibits a larger turnover frequency (TOF; 1.27 s−1 at 1.63 V) than the other two samples (Ni-SAC@COF, 0.68 s−1 and Fe-NP/COF, 0.39 s−1), implying its high intrinsic catalytic activity. The mass activity of Fe-SAC@COF is 9.20 A mg−1 at 1.63 V, which is about 1.95 and 5.05 times higher than Ni-SAC@COF and Fe-NP/COF, respectively. Figure 3C shows the comparison of the Tafel slopes, overpotentials, and TOFs with other Fe-based SACs for OER catalysis. Apparently, these values of the Fe–NO-coordinated Fe-SAC@COF are state of the art among single-atom Fe-based OER electrocatalysts (see specific values in Table S4). The superiority of Fe-SAC@COF might be attributed to its highly effective atomic utilization and unique metal-coordination species. Besides, the overpotential is significantly increased when introducing potassium thiocyanate in the electrolyte, which confirms that the single Fe atoms are the electrocatalytic active center (Figure S27).Furthermore, electrochemical double-layer capacitances (Cdls) were carried out to evaluate the electrochemical surface areas (ECSA) using cyclic voltammetry (CV) measurements (Figures S28–S30). The Fe-SAC@COF electrode shows a larger Cdl value of 17.9 mF cm−2 than those of Ni-SAC@COF (6.5 mF cm−2) and Fe-NP/COF (4.3 mF cm−2), demonstrating more exposed active atoms for Fe-SAC@COF (Figure 3D). On the other hand, electrochemical impedance spectroscopy (EIS) plots were conducted to reveal the charge-transfer kinetics. The Fe-SAC@COF electrode shows the smallest semicircle, indicating the fastest charge-transfer rate on the Fe-SAC@COF catalyst (Figures 3E and S31). In addition, both Fe-SAC@COF and Ni-SAC@COF samples show a negligible change in LSV curves after 2,000 cycles (Figures S32 and S33). Meanwhile, the Fe-SAC@COF electrode maintains a stable current density at around 10 mA cm−2 for 24 h consecutive operation, indicating outstanding stability during long-term OER operations (Figure S34). The XPS spectrum of Fe-SAC@COF after OER catalysis clearly shows that the composition and electronic structure are maintained (Figure S35). The TEM images of Fe-SAC@COF and Ni-SAC@COF show original fibrous-like morphologies and uniform distribution of metal elements (Figures S36 and S37). These characters demonstrate the excellent structural stability of the two samples.To reveal the impact of metal species and their coordination modes on the catalytic performance, the OER mechanism models were studied using DFT calculations implemented on the Gaussian 16 program. As the bonding mode between metal ions and the COF is the chelation with N and O in keto-enamine form, the cluster model is used as depicted in Figures 4A and S38A (calculation details are described in the supplemental information). The associated four-electron-involved reaction pathways for the Fe-SAC@COF and Ni-SAC@COF cluster models (that are Fe–NO3 and Ni–NO3, respectively) under alkaline conditions are illustrated in Figures 4A and S38B, respectively. Specifically, step A shows the adsorption of OH− on the Fe metal center; step B indicates the formation of Fe–O from Fe–OH; step C represents the subsequent formation of Fe–OOH; and step D is the process of decomposition of adsorbed –OOH to O2. Accordingly, the free energy diagram and the Gibbs free energy change (ΔG) of all steps are summarized in Figure S39. The potential determining step (PDS), i.e., the step that has the largest ΔG, is the one to form M−O intermediates (step B) for both Fe-SAC@COF and Ni-SAC@COF catalysts. The ΔG of the PDS for the Fe-SAC@COF is 0.79 eV, which is much lower than that of Ni species (1.35 eV). This explains why the former catalyst shows faster OER kinetics. Furthermore, Fe-SAC@COF with a different coordination model (Fe-NO4) is calculated, whose PDS (the step to form M−OH) is distinct from that of Fe-SAC@COF (Fe-NO3). The ΔG of the PDS is 1.10 eV, which is 0.31 eV higher than Fe-SAC@COF (Fe-NO3). The overpotential versus difference between the ΔGO∗ and ΔGOH∗ for these three models shows a volcano-like shape (Figure 4B). The Fe–NO3 model of Fe-SAC@COF is at the summit (the lowest overpotential), which means that it has the most moderate interaction with the oxygenated intermediates during OER. The Fe–NO4 and Ni–NO3 models, instead, exhibit too strong and too weak interactions, respectively. Therefore, it can be concluded that both coordination modes and the metal species of the same coordination mode could have a great impact on the PDS and thus the catalytic activity of OER.We have developed stable and atomically dispersed Fe-SACs coordinated on Tp-Tta COFs with novel and precise coordination modes alleviating pyrolysis treatment. AC HAADF-STEM and EXAFS confirm the atomic dispersion and specific coordination environment. Fe-SAC@COF, with the uncommon coordination configuration, shows outstanding OER activity with the overpotential and Tafel slope higher than the other atomically dispersed Fe-based OER electrocatalysts. DFT calculations reveal that Fe-SAC@COF processes lower ΔG of the PDS for OER than that of the Ni-SAC@COF catalyst, which explains its faster OER kinetics. This work demonstrates that COFs may serve as variable coordination supports for the design and synthesis of novel-coordinated and stable SACs, which might lead to the exploration of highly active electrocatalysts.Further information and requests for resources and materials should be directed to and will be fulfilled by the lead contact, Dr. Wei-Qiao Deng (dengwq@sdu.edu.cn).This study did not generate new unique reagents.All solvents and materials in this study were purchased from commercial sources without further purification. Fe(II) acetate, Ni(II) acetate tetrahydrate, potassium hydroxide, methanol, ethanol, 1,4-dioxane, mesitylene, acetone, tetrahydrofuran, and hydrazine monohydrate were obtained from Aladdin (Shanghai, China). Tp and Tta were purchased from Jilin Chinese Academy of Sciences - Yanshen Technology.To prepare the Tp-Tta COF, Tp (0.3 mmol) and Tta (0.3 mmol) were dispersed in 3 mL mixture solution with 1.5 mL 1,4-dioxane and 1.5 mL mesitylene in a Pyrex tube. After being sonicated for 5 min, 0.3 mL aqueous acetic acid (6 M) was added, and the mixture was sonicated to afford a homogeneous dispersion. Subsequently, the mixture was subjected to three freeze-pump-freeze cycles, then the tube was sealed off and heated at 120°C for 72 h. The precipitate was collected by centrifugation and washed with tetrahydrofuran (THF; 3 × 40 mL) and acetone (3 × 40 mL). The collected powder was dried at 60°C under vacuum for 12 h to afford the Tp-Tta COF.Taking Fe-SAC@COF as an example, the Fe precursor was prepared by the dispersion of Fe(OAc)2 in 20 mL mixed solution of water and ethanol (v/v = 1:9) under stirring being held for 1 h at −60°C. Subsequently, 50 mg COF material was added into the mixture and continuously stirred for 12 h. Then, 5 M N2H4 H2O+0.05 M KOH solution (20 mL) was injected quickly. This mixture was allowed to react for another 12 h, and the resulting powder was washed at −60°C and naturally dried at 25°C. The metal content is confirmed by ICP-AES. Ni-SAC@COF was prepared using a similar procedure except using Ni(OAC)2·4H2O.In a 50 mL round flask, 50 mg Tp-Tta COF and 2 mg Fe(OAc)2 were dispersed in 20 mL methanol, and the mixture was stirred for 72 h at 50°C under N2 atmosphere. The resulting solid was fully washed with THF and methanol. Then, the obtained material was dried at 60°C for 12 h.FT-IR spectra were obtained from a VERTEX 70v spectrometer (Bruker) in a range of 4,000–500 cm−1. PXRD patterns were taken using a D-MAX 2500 diffractometer (Rigaku) with Cu-Kα radiation. Solid-state 13C CP/MAS NMR spectra were taken in a Bruker 300 MHz NMR. TEM images were carried out with Thermo Fisher Scientific Talos F200X G2, and the samples were prepared by drop-casting the powder from ethanol on copper grides. SEM images were obtained with a SEU8010 scanning electron microscope. Gas adsorption and desorption were recorded on a Quantachrome instrument. XPS measurements were carried out on a Thermo ESCALAB 250XI spectrometer. ICP (Optima 2100DV) analyses were used to determine the mass concentration of metal. AC HAADF-STEM was performed using a FEI Themis Z microscope.XAS spectra of all catalysts were recorded under ambient conditions in a transition mode at beamline 1W1B of Beijing Synchrotron Radiation Facility (BSRF), using a Si (111) double-crystal monochromator. The acquired EXAFS data were processed according to standard procedures using the ATHENA module implemented in the IFEFFIT software packages. 43 For all XAS data, the experimental absorption coefficients as a function of energies μ(E) were processed by background subtraction and normalization procedures. For EXAFS modeling, EXAFS of the metal foil is fitted, and the obtained amplitude reduction factor S0 2 value was set in the EXAFS analysis to determine the coordination numbers (CNs) in M-SAC@COF (M=Fe or Ni) catalysts.Electrochemical measurements were carried out in a three-electrode system connected on an electrochemical workstation (Bipotentiostat model CS2350) in 1 M KOH at room temperature. The working electrode was prepared by coating the catalyst ink onto a Ni foam (NF; 1 ∗ 1.5 cm). An Hg/HgO electrode and a platinum foil were used as the reference and counter electrodes, respectively. In a typical process to prepare the catalyst ink, 10 mg catalyst (0.5 mg carbon nanotube) was suspended in 2 mL mixed solvent containing ethanol and 5% Nafion (v/v = 9/1) to form a homogeneous ink. After sonication for 10 min, the catalyst ink was dripped on the two sides of the working electrode. The optimal mass loading is measured to be about 1 mg cm−2. Moreover, the optimal metal content for all SAC samples is measured to be ∼1.0 wt %. All potentials were converted versus reversible hydrogen electrodes (RHEs) based on the Nernst equation: E (versus RHE) = E (versus Hg/HgO) + 0.098 V + 0.059∗pH. The electrolyte was pre-saturated with Ar gas for 30 min before OER tests. During OER testing, the catalyst was first subjected to CV activation for 20 cycles with a scan rate of 50 mV s−1, and polarization curves were performed with LSV mode at a scan rate of 5 mV s−1. The electrochemical impedance measurement (EIS) was measured in frequency ranges from 100 kHz to 0.1 Hz at a potential of 300 mV (versus RHE). ESCA was carried out by testing C dl in non-Faradaic potential regions with various scan rates from 20 to 100 mV s−1. No iR compensation was employed in all tests.The TOF was calculated by the following equation: TOF= (J × A)/(4 × F × n), in which J (A cm−2) is the current density at a given overpotential, A (cm−2) is the surface area of the electrode, F stands for the Faraday constant (96,485 C mol−1), and n represents the number of active sites (mol). In this study, the metal Fe atom is regarded as the active site. Mass activity (mA mg−1) values were calculated from the electrocatalyst metal loading m and the measured current density j (mA cm−2) at η = 400 mV: mass activity = J/m.Geometry optimizations and frequency calculations were carried out at the B3LYP/6-31G (d,p) level. For each geometric stationary point, the single-point energy calculations were performed using an extended 6-31++G(d,p) basis set, as well as the SMD solvation model, considering the solvent (water) effect. 44 Only reactant and product states in the OER process, as well as intermediate states, are proposed and evaluated. The overall reaction scheme of OER reaction in an alkaline environment is: (Equation 1) 4 OH − → O 2 ( g ) + 2 H 2 O ( l ) + 4 e − We consider four elementary steps for OER, with each consisting of a single-electron transfer step reaction written as the following equations. (Equation 2) M + OH − → M - OH + e − (Equation 3) M - OH + OH − → M - O + H 2 O ( l ) + e − (Equation 4) M - O + OH − → M - OOH + e − (Equation 5) M - OOH + OH − → M + O 2 ( g ) + H 2 O ( l ) + e − Here, the asterisk refers to the active site of metal-decorated calculation models. The free energy change of each elementary step is described by the following expressions: (Equation 6) Δ G A = G M - OH − G OH − − G M − e U , (Equation 7) Δ G B = G M - O + G H 2 O ( l ) − G M - OH − G OH − − e U , (Equation 8) Δ G C = G M - OOH − G M - O − G OH − − e U , and (Equation 9) Δ G D = G M + G H 2 O ( l ) + G O 2 ( g ) − G M - OOH − G OH − − e U , where U denotes the applied electrode potential and G represents the free energy for each species.The free energy of H2O(l) can be derived from the gas state of water as the following equation: (Equation 10) G H 2 O ( l ) = G H 2 O ( g ) + R T ln ( p / p 0 ) , where GH2O(g) is the free energy of H2O(g), which can be directly obtained by DFT calculations. R is the ideal gas constant. Because it is hard to calculate the electronic energy of an oxygen molecule in a high-spin ground state exactly, the free energy of an oxygen molecule in gas (GO2(g)) was derived as the following equation: 45 (Equation 11) G O 2 ( g ) = 2 G H 2 O ( g ) − 2 G H 2 ( g ) + 4.92 eV The free energy of OH– was derived as (Equation 12) G OH = G H 2 O ( l ) − G H + , (Equation 13) G H + = 1 2 G H 2 ( g ) − R T ln 10 × pH. The thermal corrections to the free energy of each reactant, product, and intermediate state were calculated by frequency calculations at room temperature (298.15 K) based on the optimized geometry. The other parameters were set: T = 298.15 K, p = 0.035 bar, and P0 = 1 bar.This work was supported by the National Key Research and Development Program of China (no. 2017YFA0204800) and the Natural Science Foundation of Shandong Province (no. YDZX2021001). H.W. thanks the financial support from the Program of Qilu Young Scholars of Shandong University (no. 62460082163005), the Natural Science Foundation of Shandong Province (no. ZR2021QB201), and the Science Foundation for Outstanding Young Scholars of Shandong Province.Conceptualization, H.W. and W.-O.D.; methodology, X.W., L.Y., and L.S.; investigation, X.W., W.Z., and G.R.; writing – original draft, X.W. and L.S.; writing – review & editing, H.W. and W.-O.D.; funding acquisition, H.W. and W.-O.D.; resources, X.W. and L.S.; supervision, H.W. and W.-O.D.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.xcrp.2022.100804. Document S1. Figures S1–S39 and Tables S1–S4 Document S2. Article plus supplemental information

The exploration of environmentally friendly and highly efficient oxygen evolution reaction (OER) catalysts is vital to large-scale, electrochemical renewable-fuels generation. Here, we report an iron single-atom catalyst (SAC) confined in a covalent organic framework (Fe-SAC@COF), which constitutes an unusual Fe–NO coordination in the skeleton. The as-prepared Fe-SAC@COF exhibits a high mass activity of 9.20 A mg−1, which is 1.95 times higher than Ni species of the same coordination and 5.05 times higher than nanoparticulate Fe counterpart. Moreover, it shows, to the best of our knowledge, a record-low overpotential (290 mV) and Tafel slope (40 mV dec−1) among the reported atomically dispersed Fe-based catalysts and surpasses the benchmark Ir/C catalyst. The density functional theory calculation shows that the Fe–NO coordination exhibits low binding energy of oxygenated intermediates, which leads to an outstanding electrocatalytic OER performance. This work provides design strategies toward unusually coordinated SACs by prudent COF confinement for advanced electrocatalysis.

The increasing number of automobile vehicles on roads therefore the concentration of pollutants gasses emitted is also increases. The unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and sulfur oxides (SO2) are main pollutants produced from the internal combustion engines of an automobile vehicle. These gases are mainly produced due to the incomplete combustion of regions in oxygen deficiency of the engine. The application of catalytic converter in automobile vehicles for reduces the toxic gasses emissions from the internal combustion engines [1,2]. The catalysts present in a catalytic converter are reduction catalyst and an oxidation catalyst. The catalytic reaction is the reaction between catalyst surfaces and remaining gases present in the automobile vehicle exhaust. The activity of catalytic converter is highly dependent upon the types of catalyst was used. Carbon monoxide (CO) is one of the very poisonous gases present in the atmosphere and also represents as the silent killer of 21st century. The CO is produced into the atmosphere by partial combustion of carbon-containing compounds [3,4]. The CO decreases the amount of oxygen gas that reaches the blood and might because sleepiness, slow reflexes, impaired vision and decision and even it may cause personal death as CO gas replace oxygen by binding to the iron atom presented in blood hemoglobin(Hb) and so Hb becomes failure to carry out oxygen to the brain. The more exposure to CO can diminish the amount of oxygen absorbed by the brain so much that the victim may become unconscious and may suffer from the brain damage or person death from hypoxia [5,6]. The low-temperature catalytic oxidation of CO is very important process for all life support in enclosed atmospheres such as submarines and spacecraft. Due to increasing the cost of noble metals the catalytic conversion of CO over transition metal oxide catalysts (TMOs) has been much interest [7,8].The transitional metal oxides (TMOs) constructions, buildings, designing and structures have been represented as one of the most important useful elements in the area of catalysis, fuel cells, energy storage and air pollution control and so on. These materials structured units frequently contains certain physical and chemical properties that are very helpful for their efficient applications. The TMOs catalysts are novel types of mixed oxide that surface area chemistry can be considered in the similar way as the effective oxide catalysts [9,10]. It's a cheaper cost, environmentally friendly and simply available catalyst for ambient conditions CO oxidation. TMOs represents a huge structural differences due to their capacity to produced phases of different metal to oxygen ratios showing several steady oxidation states of the metal ions. The structural defects in TMO are highly influence by their catalytic performances. The location of surface ions will change from the massive structure [11,12]. The surface metal oxides are highly influenced by the coordination of metal cation and oxygen anion, which modify the catalytic activity of these compounds. The nanosized metal oxides are represented by special and crucial applications particularly in the catalysis field. The superior activity of TMOs nanoparticles in CO oxidation was associated with the smaller particle size, higher surface area and closely covered the surface coordination unsaturated sites [13,14]. In the TMO catalysts the nickel oxides (NiOx) are novel types of mixed oxide catalyst that surface area chemistry can be considered in the similar way as the effective oxide catalysts. There are different preparation conditions have been developed for the production of nickel oxide catalyst with unique structure and higher activity. The better catalytic activity of nickel oxides has been suggested for particles with the smaller sizes [15,16].Nickel oxide (NiO) is an earth-abundant transition metal oxide with superior redox property, electrochemical performance and gas sensing property. The Ni-based catalysts are commonly studied for their potential capacity to catalyze the dry-reforming reaction on an industrial scale. In addition, the several researchers have studied NiO catalysts with various morphologies for CO oxidation and found that ring-like and flower-like NiO demonstrated high activity. The Ni is the best catalyst for reaction of virtue of its life, high activity and selectivity towards CO oxidation at a lower cost. The Ni nano-particle catalysts have been represented as an excellent catalyst from lower cost, thermally, activity and selectivity point of view [17,18]. The influence of NiO particle size on catalytic activity has been subjected of continuous interest due to its importance from both primary and practical viewpoints. The high performances of NiO are associated to the presence of both intra and inter-particle porosity of catalysts and highly Ni metal particles dispersion on the catalyst surfaces. The NiO has a bimodal pore structure, those represents that huge performances for CO oxidation. It will provide more favorable pore size for the chemisorptions of reactants. The massive amorphous NiO have formed and highly dispersed on the catalyst surface, resulting in the formation of abundant surface Ni2+ ions [19,20].The potential chemisorptions of Ni have transitional binding energies; therefore, it has an intermediate value of the heat adsorption. The performance of NiO catalysts also depends upon the calcinations conditions of precursors and subsequent pretreatment of the catalysts. The synergetic effect between Ni and other metal oxides over bimetallic catalysts can reduce nickel size to improve the metal particle dispersion and accelerate the activation of adsorbed CO, thereby improving the catalytic activity. The reaction of NiO and O2 is influenced by the oxygen concentration and temperature. The Ni–Co composite oxides could be potential catalysts for CO oxidation at low temperatures. The synergetic interaction between Ni and Co affecting the catalytic physicochemical properties and activity taking into account the diverse morphologies of bimetallic oxide catalysts and catalytic mechanism is worth in the further explanation [21,22]. The unique Ni0, Ni2+ and Ni3+ species for the nickel-containing samples are make them better catalytic activity. The valence of nickel for all catalysts was maintained as Ni2+ after the reaction. The Ni deposited on CeO2 has highly low even any activity for CO oxidation reaction between 20 and 120 °C. The addition of nickel, which is in a good agreement with the argument of metallic gold, is active species for CO oxidation. The addition of nickel component, benefit the catalytic performance of ceria supported catalysts for CO oxidation. The different oxidation state of Au by the addition of Ni and compared the activity of Ce-supported Au catalysts with after the normalization by Au amount [23,24].The nickel-based catalysts such as Ni/Al2O3, Ni-CeO2/Al2O3, Ni/CeO2 and Ni/C have been studied. Under ultra-high vacuum conditions the NiO-catalysts showed high reactivity for CO oxidation below room temperature. The Ni nanoparticles are initially more reactive for CO oxidation at room temperature; however the CO oxidation reactivity decreases with reaction time due to the structural instability of Ni nanoparticles causing agglomeration of nanoparticles with time under reaction conditions. The lithium doping NiO catalysts also improved their performances for CO oxidation. The increase in catalytic activity with Ni3+ concentration improves the performances of catalysts for CO oxidation. The CO adsorption is a slow processes and O2 adsorption is a fast processes. The NiFe2O4 catalysts also showed improved performances for CO oxidation at 120–180 °C [25,26]. The TiO2 is a reducible metal oxide with several crystal structures and titanium-supported metal catalysts have also been used for converting CO into CO2. The titanium supported nickel oxide catalysts are more active in CO oxidation than silica or alumina supported catalysts. The surface oxygen present on the catalyst was responsible for formation of both CO and CO2 for there is no oxygen in the feed gas and catalyst was not pre-reduced. The different nickel loadings and reaction conditions are highly impact on the performance of catalysts for CO oxidation. The NiO and TiO2 may react in the solid phase at the presence of oxygen gas to form NiTiO3 (NiO + TiO2→NiTiO3) [27,28].The reduction of NiO to Ni0 on the surface, since there was no oxygen in the feed, the formation of CO and CO2 represents that the catalyst itself supplies the oxygen. The NiO lattice oxygen O2−and/or oxygen from the TiO2 support participated in the oxidation of CO. The correlation between Ni phase structures and surface acidity of Al2O3 supports calcinations at different temperatures are very important for CO oxidation reaction. The interaction between Ni nanoparticles and Al2O3 supports due to the rapid decrease of specific surface area and acidity of Al2O3 supports [29,30]. A lot of efforts have been made to improve the stability of Ni/Al2O3 catalysts by increasing the Ni loadings. The variation of Al2O3 properties will affect the dispersion of active Ni particles and metal-support interaction, thus significantly impacting their catalytic activity and stability. All the reduced catalysts mainly consisted of Ni2+, possibly in oxidic, hydrated or carbonated form, with smaller fraction of Ni0. The small and highly dispersed Ni particles (about 3 − 8 nm) were formed on it, which can strongly interact with the support. The increase of calcinations temperature, both surface area and acidic sites of Al2O3 decreased, forming weaker interaction between metal nanoparticles and support. As a result, the Ni oxide particles can be easily reduced to highly active Ni particles (20−40 nm). The particle size and interaction of Ni and Al2O3 support highly influences on the performance of catalysts [31,32].The Physico-chemical property of the support should simultaneously influence the catalytic performance of catalysts reduction in the surface area and acid sites of Al2O3 supports, Ni oxide particle size increases with the weakening of metal-support interaction and becomes more and more reducible conditions. The activation energy for CO oxidation reaction on NiO is not affected at lower temperature by the addition of foreign ions to the nickel oxide lattice. The reason of catalytic oxidation of CO on NiO and Cu2O surfaces electron transfer processes occur between the gas and solid phase, as evidenced by the solid-phase reaction. The excess oxygen contents of NiO would probably influenced the initial rate of oxidation [33,34]. The monovalent cation increases the valence state therefore the activation energy of the processes was decreases. The activation energy of reaction is the interaction of CO with oxide surfaces and controlling step in the high-temperature interval of reaction. The addition of foreign ions into the NiO lattice might modify the concentration and distribution of holes and electrons by suitable changes of the Fermi level in semi-conductor. The catalytic activity of amorphous NiCuO2 alloys depends on some surface changes. The Ni-based alloys with HCL acid treatment also observed very high activity for CO oxidation. After HCL treatment the alloy surfaces were covered with a thick oxide film, which could not be reduced during the CO oxidation [35,36].The oxide layer on the as-received alloy was thicker than that on the HCL treated alloy. The acid treatment and oxidation the surface becomes porous and surface areas are increased by 10 times. The increase of surface area leads to an increase in the number of active sites per unit area, and their activity. The formation of additional active sites is possibly associated to the presence of an oxide phase therefore; the catalytic activity of activated alloys was higher than that of the untreated alloys. The additional formation of active sites is associated to the presence of an oxide phase [37,38]. In catalytic activity, the major role is played by the structure and orbital conformity of the active centers with reagent molecules. At high temperatures, the oxidation of CO occurs by the reaction of adsorbed CO molecule with a lattice oxygen atom and oxide surface is initially reduced then oxidized by the atmospheric oxygen. The influence of particle size on catalytic activity has been subjected of continuous interest due to its importance in both primary and practical viewpoints. Future studies will focus on the development and utilization of Ni oxide catalysts for high catalyst selectivity. The achievement of NiO catalysts has encouraged huge amount of basic work dedicated to use the role played by each element and nature of active sites. The objective of present study is to review the factors that influence the CO oxidation reaction on special attention under the catalyst compositions, crystal size, pre-treatment and preparation conditions [39,40]. Table 1 Nickel (II) oxide is the chemical compound with the formula NiO and its principal oxide of nickel. NiO is a basic metal oxide. The mineralogical form of NiO, bunsenite is very rare. NiO adopts the NaCl structure with octahedral Ni2+ and O2− sites. The simple structure is commonly known as the rock salt structure. Like many other binary metal oxides, NiO is often non-stoichiometric, meaning that the Ni:O ratio deviates from 1:1. The NiO was also as component in the nickel-iron battery, also known as the Edison Battery, and is a component in the fuel cells. Ni (III) oxide is the inorganic compound with the formula Ni2O3 and also referred to as black nickel oxide. Nickel oxide hydroxide is the inorganic compound with chemical formula NiO(OH). It's a black solid that is insoluble in all the solvents but attacked by base and acid [41,42]. The crystalline structures of various NiO and NiCo2O4 unit cells are shown in the Fig. 1 .Nickel oxide is highly insoluble and thermally stable so that it suitable for glass, optic and ceramic applications. It is a green crystalline solid and primary oxide of nickel. Although it is rare in nature, several million kilograms are produced annually. The certain perovskite structured oxides are electronically conductive application in the cathode of solid oxide fuel cells and oxygen generation systems. They are compounds containing at least one oxygen anion and one metallic cation. Ni oxide compounds are basic anhydrides and react with acids and strong reducing agents in redox reactions. Nickel oxide can be reacted with acids to form salts and other compounds e.g. nickel sulfamate for electroplating and nickel molybdate for hydrodesulfurization catalysts. The gold doped nickel oxide films can be used as transparent electrodes in an optoelectronic devices. The NiO is an attractive conversion reaction-based on anode material for its lower cost, nontoxicity and high theoretical capacity [43–45].Nickel oxide usually taken the relatively simple rock salt lattice. The natural cleavage plane of NiO is (100), and studies have shown that the resulting surfaces are high quality, relaxing the slightly ideal bulk terminated (100) surface. Structural determinations of adsorbents have been performed on both this surface and the polar (111) surface. To circumvent surface charging problems almost all of these studies have been performed on highly oriented NiO thin films. The NiO(100) thin film on the surface is the single crystals structures. The film preparation involved oxidation of Ni(100) surface at increasing temperature. The NiO(100) thin films are known to contain a high density of surface defects, which could drastically affect the adsorption properties [46,47].Nickel is more familiar because of its use in pure metal or in the form of alloys for its many domestic and industrial applications. The nickel constitutes about 0.007 percent of Earth's crust and fairly common constituent of igneous rocks. The most important sources of nickel are pentlandite found with nickel-bearing pyrrhotite of which certain varieties 3 to 5% nickel. In Ni2O3(with nickel in the +3 oxidation state) which is roasted in air to give nickel oxide, NiO (+2 state), which is then reduced with carbon to obtain the metal. Nickel (atomic number 28) has high electrical and thermal conductivity and it's shown in the Fig. 1. More than half of the nickel produced is used in alloys with iron (particularly in stainless steels). Nickel is also used in electrically resistive, magnetic and many other kinds of alloys, such as nickel silver. Finely divided nickel is employed to catalyze the hydrogenation of unsaturated organic compounds (e.g. fats and oils). Natural nickel consists of five stable isotopes: nickel-58 (68%), nickel-60 (26%), nickel-61 (1.20%), nickel-62 (3.60%) and nickel-64 (0.91%). It has a face-centered cubic crystal structure. Nickel is ferromagnetic up to 358 °C or 676°F (Curie point). The metal is uniquely resistant to the action of alkalies and frequently used for containers to concentrated solutions of sodium hydroxide [48–50].Nickel reacts slowly with strong acids under ordinary conditions to liberate hydrogen and form Ni2+ ions. In its compounds nickel exhibits oxidation states of −1, 0, +1, +2, +3 and +4 though the +2 state is by far the most common. The Ni2+ produced large number of coordination numbers 4, 5, and 6 and all main structures are octahedral, trigonal bipyramidal, tetrahedral and square shapes. Nickel in the +2 state have a variety of applications like NiCl2, Ni(NO3)2·6H2O, Ni(SO3NH2)2·4H2O in electroplating baths. The NiSO4 or Ni2O3 highly used in nickel plating, fuel cells, storage batteries and electroplating baths and preparation of catalysts. In the sulfide nickel is in the +2 oxidation state, but in other compounds cited in the +3 state. The nickel carbonyl compound, in which nickel exhibits a zero oxidation state, is used primarily as a carrier of CO in the synthesis of acrylates (plastics). It is colorless volatile liquid is formed by the action of CO on finely divided nickel and is characterized by an electronic configuration in which the nickel atom is surrounded by 36 electrons. This type of configuration is quite comparable to that of the noble-gas atoms [51,52].The size and morphologies of primary particles are often crucial factors in determining the catalytic performance of nickel oxides in structure-sensitive reactions. The small particles sizes are resultant more exposed surfaces are desirable in the catalytic oxidation reactions because sufficient activity sites can be accessed by the reactants. The oxygen movement of catalysts as shown in the Fig. 2 , which can be reflected by the reducibility, is decisive for initiating the oxidation reactions. The NiO catalyzed oxidation of CO and suggested that high concentration of Ni2+ could result in the weak Ni–O bonds, which might be ensure that the good catalytic activity. The nano-sized Ni oxide catalysts with controlled surface properties such as size, shape, morphology, coordination, atomic arrangement and orientations are an important key to understand the catalytic reactions [53,54].Nickel is sometimes found free in nature but is more commonly found in the ores. The nickel atom has a radius of 124pm and a Vander Waals radius of 184pm. The Ni0/NiO core-shell nanostructures are synthesized through a facile combustible redox reaction. The hetero-phase boundary with different crystalline orientations offered dual properties, which helped in bifunctional catalysis. Hexagonal Ni/NiO nanostructures as showed in the Fig. 3 manifested ferromagnetic behavior and catalyst could be collected easily at the end of catalytic reduction. The Ni/NiO core-shell catalysts at nanoscale had outstanding catalytic performance (reduction of 4-nitrophenol to 4-aminophenol) compared with pure NiO catalysts beyond a reaction time of ~9 min [55,56].The Ni nanocubes 15–40 nm diameter and high surface area contributes to the high turnover rate. From the activity order of various Ni nanocatalyst observed that the nanorods shape with highest catalytic activity and nanocubes shape Ni catalyst has shown lower catalytic activity. The activity order of CO oxidation over various Ni nanoparticle catalysts was as follows: Nanorods> Nanobelt> Nanowires> Nanoflowers> Nanospheres> Nanocubes. The high catalytic activity of nano NiO or Ni2O3 catalyst indicates that the oxidation state of Ni species may not be only cause for catalytic performances in CO oxidation [57,58].The Ni2O3 has high surface defects such as situation cluster, pits and more number of surface irregularity represents better activity for CO oxidation. The various Ni/O coordination ratios affect the number of deficiency sites, produced in various nano shapes. The lattice development can be represented by the occurrence of Ni2+ ions in this ionic lattice. Finally, observed that the Ni nanoparticles with small index surface planes and simple deficit production are attractive for the catalytic applications. The average diameter of Ni nanowires was varying from 80 to 90 nm, while the average length of Ni nanowires is equal to 5 μm. Nickel nanorods are complete particles ranging from 20 to 110 nm with specific surface area (SSA) in the 30–60m2/g range. The nanoflower look constituted of small grains of approximately 50–65 nm which are assembled in a flower-shaped structure. The nanoflower size determined from the TEM investigations as discussed in the Table 2 follows normal distribution with a mean diameter of 42 nm [59,60].The nickel nanobelt prepared by the hydrothermal method and their morphology was confirmed by the TEM images (Fig. 4 e), also indicating size of 15–30 nm in thickness, 30–160 nm in width and variable lengths up to microns. The Ni2O3 nanocubes (25 nm size) inhomogeneous dispersions over the Ni2O3 nanocubes at the graphene nanosheets. The nickel spheres are broad size particles ranging from 24 to 30 nm with specific surface area in the 40–50m2/g range. The transformation of nanospheres to nanorods seems to be caused by the irreversible binding of surfactant on the central region of growing nanoparticles. In summary nickel nanocubes, nanorods and nanowires were successfully synthesized by one-step reduction approach in a solvothermal environment. The reactions occur on the surfaces of NiO nanoparticles. Increasing the surface area of nanoparticles usually increases the rate of chemical reactions. TEM image of NiO nanoparticles represents that the non-spherical particle shape with smooth and uniform particle morphology, Fig. 3F with average diameter (taken as average particle diameter) is nearly equal to 32 nm. The physicochemical properties of nickel nanoparticles are different as compared to the bulk counterparts owing to the fact that surface area to volume ratio increases and quantum effects as the size is decreases [61,63].The TEM analysis is used for the identification of internal composition of nanoparticles including their shape, size, distribution and defects. The NiO nanoparticles are studied broadly because of their electro-catalysis, high chemical stability, super conductance characteristics and electron transfer capability. The NiO is a p-type semiconductor metal oxide having a bandgap ranging from 3.6 to 4.0 eV depending upon the nature of defects and their density. It is an antiferromagnetic material having lower temperature TN of ~523 K and besides a high isoelectric point of ~10.7, it also shows high ionization. The development of a facile preparation process that allows convenient production of NiO nanoparticles is necessary for practical application. The oxidation of Ni2+ to NiO2 in initial conditions then transforming of NiO by treating with ethanol in the presence of a surfactant at room temperature. Nickel oxide nanoparticle is prepared in rod and hexagonal shape by hydrolysis precipitation method from the solution of nickel chloride in the aqueous solution as a dissolving agent. The NiO nanoparticles, nanodots or nanopowder are white spherical high surface area metal particles. Nanoscale nickel oxide particles are typically 10–30 nanometers with specific surface area in the 130–150m2/g range. The NiO films composing of nanoparticles maintained the porous microstructure and represents best electro chromic performance. The NiO associated with the injection or extraction of ions and electrons corresponding with the transformation between Ni2+ and Ni3+. The NiO films composing of nanoparticles maintained the porous microstructure and represents excellent catalytic performance [64,65].The catalytic oxidation of CO has long been studied on various NiO catalysts due to their high CO oxidation capacity and low cost in contrast to the expensive noble metal catalysts. The various Ni oxide catalysts have been observed for CO oxidation since has importance in the environmental protection. The catalytic activity for CO oxidation strongly depends upon the metal ion concentration on the surface and surface crystalline. Carbon dioxide is produced by the reaction of CO with oxygen adsorbed on the metal ions of catalysts surface. A surface concentration of oxygen was monitored by the subsequent reaction and partial pressures of the reactants. The oxygen species associated with Ni in the Ni2O3 catalyst are very active and may be dominated by the low-temperature catalytic oxidation of CO. The higher activity of Ni2O3 nanoparticles in CO oxidation was attributed to the small particle size, high surface area, high concentration of hydroxyl groups and more densely populated surface coordination to the unsaturated sites [66–68].In the study of CO oxidation over various bulks NiOx at low temperature represented that the ranking for CO oxidation in mixture of unit ratio of O2/CO by decreasing activity followed the sequence: Ni2O3 > NiO. The CO oxidation over NiO and Ni2O3 occurred via Langmuir–Hinshelwood mechanism while over NiO in MvK mechanism represents. The more energy was needed for activating the CO–Ni2+ bond in the reaction which accounted for low activity of NiO. By opposition, the high reactivity of Ni2O3 was applied not only for the moderate strength of CO–Ni3+ bond but also abundant defects/oxygen vacancies on its surface phase [69,70]. The best catalytic activity of Ni2O3 nanorods might be associated with the high oxygen ad-species concentration and low-temperature reducibility. The surface morphology effect of Ni2O3 having hollow and solid sphere morphologies for CO abatement. The better catalytic activity by using hollow spheres of Ni2O3 was a result from beneficial effects such as morphology leading to the high surface area, higher Oads/Olatt molar ratio, believed to be proportional to the active oxygen and higher Ni average oxidation state. The NiO represents strong oxygen storage/release capacity due to the fact that they can easily undergo a rapid reduction-oxidation cycle through the interaction with reducing or oxidant agents accompany by the formation of nickel ions in the various oxidation states [71,72].The addition of lattice oxygen of catalysts incomplete oxidation of CO and strong correlation between labile lattice oxygen and catalytic activity suggest that the reaction could proceed via the MvK model. To improve the efficiency of NiO catalysts many strategies can be pursued, such as external morphology control, doping, optimization of the active Ni phase with the nature of support. Interestingly, the effect of surface morphology of Ni2O3 which was easily tune has to be deeply investigated may have a significant impact on the density of vacancies, of defects, as well as on the Ni average oxidation state and textural properties of the materials. The reaction of surface oxygen species with gas-phase CO is considered to be the rate-determining step in CO oxidation on the Ni2O3 catalyst. In Table 3 the light-off characteristic was representing that the activity of catalysts with the increase of temperature. The characteristic temperature T10, T50 and T100 represents that the initial oxidation of CO, half conversion and full conversion of CO respectively. The nickel oxide catalyst is also able for oxidation of CO at a low temperature due to the presence of lattice oxygen on their surfaces. The nickel forms very complex species with oxygen in the presence of cations and water make a characterization of supported nickel oxide catalysts. The surface area of Ni2O3 is more active than that of NiO in CO oxidation, whereas it failed insufficient surface oxygen species due to its lower specific surface area. Therefore, the redox properties and total activities of nickel oxides were influenced by both the crystal phases and textural properties [73–75].The NiO have produced highly dispersed on the catalyst surface, resulting in the formation of abundant surface Ni2+ ions. The Ni2+ ions partially substitute Co3+ ions to form Ni–Co spinel catalysts, generating an abundance of surface oxygen vacancies, which are vital for CO oxidation. The Ni0.8Co0.2 catalyst represents highest catalytic activity and good stability for CO oxidation at 120 °C. The more cobalt content results in decline its activity, suggesting that the amorphous NiO phase is dominant active phase in place of Co3O4 for CO oxidation. The introduction of cobalt in Ni0.8Co0.2 catalyst can alter the morphology of catalyst from plate-like to flower-like structure and then to dense granules [20]. The Ce-supported Ni-Au catalyst has taken much interest due to its high reactivity on CO oxidation. The easily transformation between Ce3+ and Ce4+ nanosized cerium oxide applied to reducible oxide support to deposit the different metals or oxide. The introduction of Au and Ni ions are uniformly converted to deposited species on the surface of CeO2 nanorods during synthesis. The valence of Ni in AuNiCe catalysts remained same as Ni2+ after CO oxidation reaction. The reduction temperatures for Ni-containing species were much lowers in AuNiCe than the gold-free NiCe catalyst, representing that the interaction between Au and Ni oxide to form Au-O-Ni structure. The Ni deposited on CeO2 has completely lower level in CO oxidation at 20 and 120 °C and addition of metallic Au0 species favor for CO oxidation reaction over Ni-Au-Ce-O catalyst at 100 °C. The order of reaction for CO oxidation over Ni-Au-Ce-O catalyst is AuNiCe> AuCe> NiCe. The initial small-size Auδ+ species interacting with Ni oxide and ceria support effectively kept the active Au atoms/clusters survived after the catalytic measurement. The main contribution of Ni oxide is to tune the electronic structure of active Au species in AuNiCe catalyst [23].All the nickel oxide catalysts, such as Ni/Al2O3, Ni-CeO2/Al2O3 and Ni/CeO2 are highly active for CO oxidation. The CO oxidation increased with the increase in temperature. The NiAl2O4 spinel structure is composite metal oxide produced by bonding between alumina as the support and NiO as the active material at high temperatures. It has been reported that the Ni metal sites on the Ni-oxide catalysts have high catalytic activity for CO oxidation. The CeO2 which has redox properties was added to the Ni-oxide catalysts to act as a promoter. As the content of CeO2 was increased, the surface morphology of catalyst has been changed; thus the CeO2 does cover the nickel surface [19]. The NiO catalyst showed high reactivity for CO oxidation below room temperature. The CO molecules reacted with oxygen at NiO to form reaction intermediates such as carbonate species can diffuse to the uncovered Al2O3 sites and these can be either reside on the uncovered Al2O3 or catalytically active NiO for gaseous CO2 conversion. With a lower NiO density, the reaction intermediates reside at the uncovered Al2O3 sites increase. The average NiO particle size of 1 nm or less than 1 nm and particle size was suggested to increase with increasing pre-annealing temperature. The chemisorptions of Ni2O3 catalysts are shown in the Fig. 4. The initial catalytic activity and time-dependent change in activity of catalysts are highly dependent on the pre-annealing temperature at a reaction temperature of 30 °C [22].The rate of CO oxidation increases with lithium doping in Ni-oxide catalyst as compared to iridium doping. The preparation conditions and composition of catalyst is not positive in all the cases. The improved in catalytic activity with higher Ni3+concentration and relate to the activity with electronic property of the solids. The addition of lithium increases in the hole concentration of nickel oxide. The small amount of CO2 could be produced due to pre-adsorbed of oxygen on the catalysts surfaces. Therefore the precipitation lattice oxygen was moved out. To reduce the availability of holes at nickel sites that participates in the rate-determining step of reactions [24]. In NiFe2O4 catalyst the electron transfer between Fe2+ and Fe3+ ions and between Ni3+ and Ni2+ ions in the material. The smaller particle size in composite phase increases surface area with wide range of pore size distribution in the composite materials. The bi-modal size distribution in the composite phase with respect to mono-modal distribution over pure materials. This is probably due to the asymmetry in particle size with two different crystal structures [17].The TiO2 is a reducible metal oxide with several crystal structures. Titanium possesses a variety of oxidation states and titanium-supported nickel oxide catalysts have also used for converting CO into CO2. The Ni–TiOx interaction can promote the catalytic activity and stability. Titanium supported nickel catalysts are more active in oxidation than silica or alumina supported catalyst. The surface oxygen present on the (NiO + TiO → NiTiO3) catalyst was responsible for the formation of both CO and CO2. The NiO lattice oxygen (O2−) and/or oxygen from the TiO2 support participated in the oxidation of CO to CO2. Pure TiO2 cannot oxidized CO but additional of reduced nickel oxide improved the CO oxidation process. The 8 wt.% Ni/TiO2 catalyst is complete oxidation of CO done at 50 °C. The side reaction producing CO2 from CO and oxygen was fast reacts [16]. The correlation between phase structures and surface acidity of Al2O3 supports over NiO catalyst is very important for their catalytic reactions. The high calcinations temperature not only affects the growth in Ni particle size, but also weakened the interaction between Ni nanoparticles and Al2O3 supports due to rapid decrease of the specific surface area and acidity of Al2O3 supports. The Ni/Al2O3 catalysts suffer from a series of drawbacks, such as sintering of the active Ni nanoparticles and supports due to the exothermic nature of CO oxidation reaction.A lot of efforts have been made to improve the stability of Ni/Al2O3, including increasing Ni loadings. As compared to α, β the γ-Al2O3 could act as effective supports for nickel oxide catalysts. Additionally, the variation of Al2O3 properties will further affect the dispersion of active particles of Ni and their metal-support interaction. The pore size distribution was suggesting that the growth of particle size and formation of large pores due to the inter-particle voids. All reduced catalysts mainly consisted of Ni2+with smaller fraction of Ni. The Ni particles were smaller in size are better dispersed on γ-Al2O3 support than the larger size of Ni particles. The intensity of Ni 2p3/2 increased as the calcinations temperature of Al2O3 support was increased. The Ni particles are dispersed into the porous channels on low-temperature calcinations supports and also the oxidation of surface atoms screened some of Ni atoms. The CO oxidation is a structure sensitive reaction; the metal crystallite size together with the Physico-chemical property of support and simultaneously influences the catalytic performance of catalysts. The best catalytic performance due to the moderate particle size (20–40 nm) and metal-support interactions. Future study should be focused on the developing higher surface area α-Al2O3 with higher Ni loadings and promoters to further improve the catalytic performance [25]. The self-propagating high-temperature synthesis method has been produced highly active Ni-Cu–Cr–O mixed spinel catalyst for CO oxidation. The CuO reduction may be moderated by the presence of Cr ions on the surface. The various range of compositions reaction temperatures and catalytic activity of each material were measured for each particular process including the oxidation of CO. The catalytic activity may be improved if their surface area was increased. The high heating and cooling rates can produce defect structures with large lattice strains, often relieved by the formation of defects in the bulk or on the surface. The strong influence of point defects on catalytic behavior and oxide surfaces as showed in the Fig. 5 , point defects have been shown to act as active centers. The relative ease of preparation, high thermal and chemical stability and good catalytic activities of new oxide catalysts offer promise for environmental applications [18]. The catalytic activity of amorphous NiCuO2 catalysts also depends on the some surface changes and morphology of untreated alloy also changes the following activation. The catalytic reactions occur on thin oxide film, which is not destroyed after the catalytic reaction. After the acid treatment and oxidation over surface becomes porous and surface areas were increased. The increase of surface area leads to an increase in the number of active sites per unit area and affects the activity. The formation of additional active sites is probably associated to the presence of an oxide phase and therefore, the catalytic activity of the activated alloys is higher than that of the untreated alloys. The additional formation of active sites is probably associated to the presence of an oxide phase [15].The correlation between phase structures and surface acidity of Al2O3 supports calcined at different temperatures are very important in the catalytic performance of Ni/Al2O3 catalysts for CO oxidation. The phase structures and surface acidity of Al2O3 supports are adjusted by their calcinations conditions. The high calcinations temperature not only led to the growth in Ni particle size, but also weakened the interaction between Ni nanoparticles and Al2O3 supports due to the rapid decrease of the specific surface area and acidity of Al2O3 supports. The Ni-O catalysts supported on Al2O3 calcinations at 1200 °C represents that the best catalytic activity for CO oxidation under the different reaction conditions [21]. The more oxygen content of nickel oxide and probably influences the initial rate of oxidation. The impurity affects directly the oxidation reactions by changing the overall activation energy. The monovalent cation increases, while cations with a higher valence than decreases the activation energy of processes. The addition of nickel over CuMnOx catalyst highly effects on their structural properties and catalytic performance for ambient conditions CO oxidation. The promoted NiO has an efficient catalyst for CO oxidation. The addition of small amount of Ni in CuMnOx catalyst exhibited improvements in the carbon tolerance properties and activity also. Addition of Ni can reduce the size of CuMnOx catalysts; therefore, the huge amorphous NiO phases have been produced and mostly dispersed over the catalyst surfaces, resultant in the production of rich surface. The higher deviation in surface areas and total mesopores volume is affected by the catalytic performances. The complete conversion of CO was achieved at 75 °C over CuMnNiOx catalyst, which was lower by 10 °C over CuMnOx catalyst. The performance of catalysts synthesized in RC conditions for CO oxidation was as follows: CuMnNiOx > CuMnOx. The CuMnNiOx catalyst has represented that the best catalytic activity towards CO oxidation at ambient conditions. The occurrence of uniform pore size distribution on CuMnNiOx catalyst surface was the main reason of increasing catalytic activity [26].The efficiency of nickel oxide catalysts for reactions with CO molecules is strongly dependent on the chemisorptions process. The chemisorptions of CO gasses is very important step, which increases the concentration of reactant on the catalyst surfaces which chemisorbed on CO molecules applying on the more energy to be easily obtain the chemical reactions. The distinct reaction mechanisms are stable with the observed kinetics. The initial reaction mechanism represents that the highly accepted CO oxidation reaction on catalyst surface that participants the O2 adsorption to produced O2* precursors, which divide on a vicinal vacancy. In the second mechanism, the O2 activation occur via the kinetically applicable with CO*-assisted O2 dissociation step lacking of the definite conditions of stable O2* precursors. In the CO oxidation process, the oxygen was first adsorbed on the catalyst surfaces with the energy of activation. When the temperature is increases in certain amount so that the adsorption of oxygen reaches on certain proportions, therefore any CO passing over the catalyst surfaces either reacts directly with the adsorbed oxygen or initially adsorbed then reacts, after which the CO2 being produced was desorbed. The performance of Ni2O3 catalysts for CO oxidation reaction is measured on the activation energy of procedure [50–52]. The activation energy data are very helpful for modeling and planning of catalytic converter. The CO molecules and O atoms initiate to disperse on the catalysts surface and once a CO molecule and O atom combines each other, they recombine and produced CO2. A catalytic reaction on the surface of catalyst and gas surface interface includes the observation of gas adsorption, dissociation, diffusion and desorption. The mechanisms of CO oxidation on the surfaces of nickel oxide catalysts are top tactic in nature as shown in the Fig. 6 , thus responsible for loss and uptake of bulk oxygen for production and disappearance of vacancies on these systems as attractive oxidation catalysts [53,54].The amount of CO2 molecules chemisorbed corresponded to the amount of oxygen atoms pre-adsorbed on the catalyst surfaces. The equal concentration of oxygen atoms in the gas phase over surface, therefore, the heterogeneous exchange reaction was taking places. In stable oxides the rate-determining step will be the reaction between COads and Oads, while for nickel oxide that has an inferior M-O bond energy, the rate-determining step will be the adsorption or dissociation of molecular oxygen on the metal surface [55,56]. (1) O2   +  2 *   →  2Oads (2) CO + *   →  COads (3) COads   +  Oads   →  CO2   +  2 * The molecular chemisorptions of CO discussed in the Eqn (1-3) can be done at the higher temperatures, which represents that the appearance of reactive oxygen forms. Where * represent a free site on the metal surface. The CO2 produced is simply adsorbed and does not influence the rate significantly, since it's rapidly desorbed into the gas phase. The reaction rate will be proportional to the surface exposure of Oads and COads. The CO is reacting with chemisorbed oxygen either by adding on it from the gas phase or produced an adsorbed state subsequently to the adsorbed oxygen. In the reaction conditions, the rate was proportional to the O2 pressure and independent of CO pressure. The rate of CO oxidation was determined by following the rate of production of CO2 when the CO was adsorbed and rate of disappearance of CO. The COad was oxidized by the adsorbed oxygen (Oad), which was the rate-determining process. Therefore, the mobility of active oxygen (Oad and/or OL) is crucial for CO oxidation. The more surface area and huge mesoporosity of nickel oxide catalyst can be easily access of reactants to the active sites and diffusion of products thus from CO2 on the catalyst surface. The CO molecule is an electron-donor probe during the adsorption, so that the active interface oxygen species may adsorbs the CO molecule [57,58].The intensity of Ni2+/3+–CO at ambient conditions is slightly weaker than that of only CO adsorption as represents in the Fig. 7 , which could be the preferential adsorption of O2 molecules on the surface of catalyst to form O2− species in the oxygen vacancies, restraining in the adsorption of CO. This process of electron transfer activates the lattice oxygen availability on the Ni species. The surface capping oxygen and lattice oxygen vacancy are main oxygen sources for this reaction. An oxygen vacancy was created when the adsorbed CO picks of oxygen from the Ni2O3 surface, probably capping oxygen. In the kinetic measurements the mass of carbon was also balanced i.e. decrease in CO corresponds to the formation of CO2. In the Eley–Rideal mechanisms the dissociative adsorption of O2 on the active Ni sites, followed by the reaction of surface oxygen with gaseous CO and producing CO2. Furthermore, a mean-field model was constructed for several modeling and simulation of CO oxidation, as well as calculation of the Ni2O3 surface coverage. The most important reactions Eqn (4-5) for CO oxidation over the catalysts as followed. Another redox reaction is highly exothermic reaction with the aluminum oxide support. The function of substrate is not constant the small supported particles, but smooth to the advance adsorption and activation of oxygen. The exact reaction mechanism for two-stage reduction behavior of Ni2O3 is still ambiguous. The Ni2O3 has tetragonal distorted spinel structure, which contained different types of Ni-O bonds. The two-stage reduction of Ni2O3 could be attributed to the different reducibility of Ni-O bonds [58–65]. (4) COads   +  Oads   →  CO2 (g) (5) Ni2O3   +  3 CO   →  2 Ni   +  3 CO2 For non-reducible supports, such as SiO2 and Al2O3, the catalytic performances were much strongly dependent on the Ni2O3 diffusion as represents in the Eqn (6); in fact, the oxygen adsorption has done over the metal sites. Thermodynamic results represent that the total oxidation of CO into CO2 is always done in the present conditions, provide the proofs of kinetic restriction. (6) 2 Al + Ni2O3 → 2 Ni + Al2O3 On the basis of above explanation, the reduction of most reducible Ni-O bonds, the crystal structure of neighboring atoms would immediately undergo some transformation and postulate that the transformed structures could be more stable, therefore requiring a higher temperature for further reduction. After the surface layer or sub-layers of the Ni2O3 particles are initially reduced then formed relatively stable NiOx species would cover the inner core as shown in the Fig. 8 . Despite of the Eley–Rideal mechanism in particularly CO oxidation, the Langmuir–Hinshelwood mechanism was considered kinetically favored from the fundamental standpoint [65–68].The high activity could be associated with the huge specific surface area, abundant surface oxygen species and excellent low-temperature reducibility. The activity of Ni2O3 catalysts with similar crystal structures decrease significantly with increase in the calcinations temperature. The Mars-van-Krevelen mechanism is operational in the Ni2O3 catalysts for oxidation reactions, which involves the process of releasing and replenishing lattice oxygen. Therefore, the reducibility of catalyst will be closely related to the catalytic activity. The surface defect sites represent that high oxidation activity because of catalyst closely related to the catalytic activity. The dissolution was assigned Ni3+ to Ni2+ surface reduction and reconstruction was same for all nickel oxides because all oxides and metal ions in electrolyte follow the same thermodynamic standard potentials. A pure nickel oxide metal may not be suitable for practical applications because of its defects. However, the property of metal oxide can be improved by introducing foreign metal cations into its lattice. There will be also an interaction between different metal oxides [69–72].Nickel mainly acts as adsorption center for CO. The Ni2+ receives an electron from CO or CO2 and will be reduced into Ni0. Then the reduced Ni0 would be restored into Ni2+ by extra oxygen then the next cycle starts. To obtain the independent kinetic parameters for adsorption and desorption of O2 and CO2 were modeled numerically. In Ni oxides the most important challenges is to obtain a single-phase because in all the procedures to obtained result is significant for core-shell structures. To improve the activity of Ni-oxides the oxygen defects have been introduced by thermal reduction which reduces Ni3+ to more active Ni2+ and improves the conductivity as shown in the Fig. 9 . The model of Eley–Rideal mechanism represented that the CO oxidation by a Ni2O3-SiO2/Al2O3 catalyst in the absence of O2. Furthermore, the quantum mechanical calculations from the (100) surface, the CO oxidation on nanosized Ni2O3 occurs through an Eley–Rideal mechanism, whereas on the (011) plane, a Langmuir–Hinshelwood type mechanism contributes. Despite of the aforementioned arguments for Eley–Rideal type mechanism in particularly CO oxidation, the Langmuir–Hinshelwood mechanism was considered kinetically favored from a fundamental standpoint [73–75]. The surface complexes are produced by interaction of CO2 with surface oxygen in close proximity to Ni sites. The present kinetic model is based on the different types of active nickel sites are supposed to be equal. The adsorbed CO was oxidized by the high valence state Nin+ cations, on the catalysts' surface to form adsorbed monodentate nitrate (Ni( n  − 1)+–O–CO) and metal cations are reduced as Ni( n  − 1)+. The lower valence state of Ni oxides is also active for CO oxidation; the Ni3O4 is rich in electrochemical properties due to the mixed-valence of Ni. The dependence of OO bond length on the oxidation state of Ni given the limited number of structurally characterized of Ni2O3 compounds. The Ni compounds in such higher oxidation states are usually unstable and decompose to release oxygen atoms. The mechanism of NiO catalysts for CO oxidation is shown in the Fig. 10 . To obtain the independent kinetic parameters for adsorption and desorption of oxygen and carbon dioxide were modeled numerically [76–78].In order to further investigate the nature of surface reaction mechanism of NiO catalysts, the in situ of CO and O2 co-adsorption are obtained under the CO + O2 reaction conditions. As far as the catalyst development was concerned, it is critical to explore the structure-activity correlation of catalysts. When the surface–adsorbed CO reacts with activated O over the Ni-oxide catalysts, it follows the Langmuir–Hinshelwood (L–H) mechanism. The O2 molecules preferentially adsorb on the nickel oxide catalyst surface in an oxygen-enriched atmosphere, forming surface active oxygen species (such as O2 or O), which occupy the surface vacancies. The CO molecules are adsorbed on the surface of NiO to form Ni2+–CO species and adsorbed CO reacts with the active oxygen species on surface oxygen vacancies and transformed into gaseous CO2. Finally, the surface oxygen vacancies are regenerated by gaseous O2 and completing the catalytic cycle direct surface oxidation mechanism. The reaction over reduced Ni/TiO2 catalyst takes place via the direct surface oxidation reaction mechanism, in which the adsorbed CO and oxygen species are involved. The CO can be oxidized by oxygen in NiO or by active oxygen within the TiO2 support via the non-selective oxidation mechanism over oxidized Ni/TiO2 which may contain NiO and NiTiO3 [79,80]. The CO is oxidized via a direct oxidation mechanism only when the nickel is reduced to Ni0 or when its surface oxygen species concentration is very low. The CO is oxidized by lattice oxygen in NiO or by active oxygen in the TiO2 support via the non-selective mechanism over oxidized Ni/TiO2, while it's efficiently converted into CO2 via a direct oxidation mechanism when Ni/TiO2 is reduced or partially reduced. The highly dispersed Ni particles were formed on it, which can strongly interact with the support. The CO oxidation process is highly exothermic and thermodynamically favorable reaction. The α-Al2O3 is a very promising support candidate for CO oxidation catalysts even at low Ni loadings [81,82].The NiAl2O4 spinel structure is a composite metal oxide produced by bonding between alumina as the support and NiO as the active material at high temperatures. It has been reported that the Ni metal sites on the Ni-oxide catalysts have a high activity for reforming hydrocarbons. The kinetic equations which were found to describe the reaction on pure nickel oxide are also operative in the case of nickel oxide catalysts, containing foreign atoms. In the low-temperature regions all catalysts operates practically with the same value of activation energy. This indicates that in these temperature intervals the added ions do not affect directly the catalytic processes. The modification of excess oxygen contents of nickel oxides and influence the initial rate of oxidation. The higher temperatures impurity presence directly influences the initial rate of oxidation reaction by changing the activation energy [83,84]. The value of activation energy depends upon the functions of various ions added. The introduction of monovalent cations increases, while cation with a valency higher than two decreases the activation energy of the processes. The reverse effects are obtained by introducing foreign anions into the nickel oxide lattice. The CO behaves like as an electron donor and possible to conclude from the effect of impurities on the activation energy of oxidation reaction that the interaction of CO with oxide surface is the controlling step in the high-temperature interval of the reaction. The addition of foreign ions into the nickel oxide lattice might be modifications the concentration and distribution of electrons by suitable changes of the Fermi level of the catalysts [85,86].This can be associated to the experimentally observations by the additions of activation energy in the heterogeneous processes involving formation and/or destruction of acceptor and donor levels in the catalysts. Although the effect has been observed on the overall activation energy of heterogeneous reactions process occurred on the catalytic materials. In the nickel oxide catalysts treated with HNO3 acid shows very high activity in CO oxidations. During acid treatment the nickel alloy surfaces were covered with a thick oxide film, which could not be reduced during the CO oxidation. The increase of surface area leads to an increase in the number of active sites per unit area and also affects the activity. The different treatment leads to a more surface area and higher number of active sites per unit area. The additional formation of active sites is probably associated to the presence of an oxide phase. The creation of more active sites is probably associated to the presence of an oxide phase and catalytic activity of activated alloys is higher than that of the untreated alloys. The amorphous nickel alloys represent unique and original compositions and surface structures to the reacting molecules unlike conventional crystalline metals [87,88].They possess several properties: a high reactivity due to their metastable structure, a high density of low co-ordination sites and defects, chemical homogeneity and easy reproducibility, which make them interesting materials in the heterogeneous catalysis. Most of the amorphous alloys show exceptional activity and selectivity in the CO oxidation reactions. The activation energy not only depends on the surface composition and structure, but it mainly depends on the number of active sites, whose nature is similar. The main reasons for high CO oxidation activities measured on both the actual composition of products as well as the highly defective structure of new materials [89,90]. The high heating and cooling rates can produce the defect structures with large lattice strains, often relieved by the formation of defects in the bulk or on the surface. The strong influence of point defect improved on the catalytic behavior. The relative ease of preparation, high thermal and chemical stability and good catalytic activities of the nickel oxide catalysts present assure for the environmental applications. The angular orientation of CO on NiO(l11)(1  ×  1) has also been dependent of the π*/σ* resonances in the CO was adsorbed on the (100) nanofaceted octopolar reconstruction, with its orientation found on NiO(100). This geometry gives a tilt angle for CO away from the NiO(111) surface normal, which is within the uncertainty of the experimental value. The assessment of synthesis conditions and properties of small Ni-O clusters is at the limits of capabilities of contemporary experimental technique. To understanding and interpretation of the available and emerging experimental results must be based on the theoretical studies to represents the physical and chemical mechanisms behind observed properties of such systems [90–92].The performances and selectivity of nickel oxide catalysts in catalytic converter are crucial for CO oxidation reaction. The catalyst deactivation and failure over time in catalytic performances is creating problem in the practice of catalytic process. The major reason for catalyst deactivation is divided into three parts: Chemically, mechanically and thermally. The lead, sulfur poisoning, carbon formation and sintering is the major cause of catalyst deactivation. The dispersion of active phase rapidly decreases, which is one of the major reason for catalyst deactivation. The initial decrease in catalytic activity can be recognized to the formation of carbonate species on the catalyst surface and occupation of active sites on the catalyst by CO2 and moisture. The deactivation is main reason for failure of catalytic surface area due to the crystalline development of catalytic phase [93,94].The main reason of poisoning is due to the highly adsorption of feed impurities; therefore, the poisoned catalysts are very tough to regenerate. The sulfur poisoning and thermal degradation of catalyst is one of the main reasons for Ni-oxide catalysts deactivation. The poisoning is highly effects of reactants, impurities present on the sites or existing for the catalysis. The deposition of chemical poisoning on nickel oxide catalysts surfaces cause of deactivation is shown in the Fig. 11 . The nickel oxide catalysts become “poisoned” when their surfaces are covered by carbon species formed during the reactions with carbon-containing molecules, such as when the CO dissociates into carbon and oxygen. Carbon deposited on the catalyst surface blocks the active sites and prevents further reactions from taking place, thus “poisoning” and ultimately deactivating the catalyst. Due to consumption of SO2 caused by the formation of sulfate is one of the main reason of Ni2O3 catalyst deactivation [95,96].The other important sources of nickel oxide catalysts deactivation are ammonia in high temperature. In high-temperature oxidation of ammonia represented that the catalyst undergoes phase and chemical transformations mainly to the formation of low-selective nickel oxide catalysts. The deactivation processes are showed in the Fig. 12 and it was accompanied by structural changes: recrystallization and decrease in the specific surface area of system. Under the critical conditions of reaction (catalyst decay), recrystallization processes and decrease in the specific surface area of catalyst diminish its limiting load. The deactivation of catalyst can be used in developing theoretical and practical foundations for design of high-performance catalysts for CO oxidation [97,98].The deactivation caused by water vapor can be contributed to the competitive adsorption. The installing of reactor downstream of the desulfurized and precipitator is an excellent way to avoid the deactivation. When the excess O2 present in the flue gas, the trace residual SO2 can be oxidized into SO3, a reaction catalyzed by the metal active sites. The transformation of NiO into NiSO4 on the NiOx/Al2O3 catalyst significantly deactivated the catalyst's activity. The deactivation process of SO2 would be improved in the case of moisture vapor. The deactivation of Ni-oxide catalysts may be explained by the chemical interaction of Ni-oxides and support materials resulting in the formation of inactive phases of Ni cations [99,100]. The Ni/Al2O3 catalysts suffer from a series of drawbacks, such as sintering of the active Ni nanoparticles and supports due to the exothermic nature of CO oxidation reaction and severe choking. Therefore, a lot of efforts have been made to improve the stability of Ni/Al2O3, including increasing Ni loadings. The variation of Al2O3 properties will further affect the dispersion of active particles and metal-support interaction. The activity of reused Ni catalysts highly depends on the catalyst's chemical composition and recycling conditions. Ideally, a Ni catalyst should be reused in various times as possible without any treatment. The Ni particles are severely sintered, as the crystal size was increased from about 3 nm to 16 nm. The small Ni nanoparticles are easily deactivation of supported metal catalysts is due to severe coke formation in catalytic CO oxidation processes, which is highly influenced by the nature of active metal and support, the dispersion and particle size of active component, metal-support interaction, as well as the reaction conditions [101,102]. The carbon deposition occurs faster on small particles and more readily on Ni step planes. For Ni/α-Al2O3, due to the reduction in surface area and acid sites of Al2O3 supports, Ni oxide particle size increases with the weakening of metal-support interaction and becomes more and more reducible. As a result, Ni/α-Al2O3 is the most active, stable and coke-resistant catalyst among all the Ni/α-Al2O3catalysts, which can be attributed to its stable, nonporous and non-acidic support with low surface area, as well as easily reducible Ni species. The α-Al2O3 is a very promising support candidate for CO oxidation catalysts even at low Ni loadings [103,104].The nickel oxide catalysts regenerations processes are shown in the Fig. 13 as applied for total removal of blinding layers, SO2 to SO3 conversion rate, mechanical constancy and deactivation rate in all the regenerated catalyst are better than the other catalysts. The off-site regeneration processes are more sophisticated and demanding than on-site rejuvenation processes; it offer more efficient cleaning and reconstitution of catalyst with complete improvement of activity—sometimes better than the fresh catalyst performances. The fresh catalyst activities by removing the carbon deposit and returning the sintered Ni2O3 catalyst particles close to the optimum size. Sintering is highly removed by reducing and controlling the temperature of reaction, although the new developments have pay attention on the encapsulating metal crystallites to remove the mobility, while remains allowing for the entrance of reactants and products [105–107]. To reduce the deactivation of nickel oxide catalyst, added a certain amount of support materials like SiO2, TiO2 and Al2O3 into the nickel oxide catalyst and also increases the lifetime of catalyst. The regeneration treatment processes by which the carbon was burning off the Ni2O3 catalyst with the aid of oxidized gasses. However, the oxidative regeneration of Ni2O3 catalyst if not carefully controlled with respect to the oxidation conditions may frequently placed an undesirable over-oxidation of the Ni component of catalyst. To regenerate the spent Ni-oxide catalyst at the higher temperatures in the presence of reducing gas in a fluid solids regenerator. The self-regenerating effect of catalyst with steams as dilutions agents for the reactants. The nitrate NO3–, sulfate SO4 2– and ammonium NH4 + are the three main components on the poisoned of Ni-oxide catalysts. The water washing, thermal regeneration and reductive regeneration were used to regenerate the catalytic activity of Ni/α-Al2O3 catalyst. The regeneration of deactivated Ni2O3 catalysts is highly dependent on the chemical, economical and environmental factors [108–110].The nickel oxide is one of the best transition metal oxide catalysts for low-temperature CO oxidation. The synthesis and calcinations strategies are highly influences the performance of Ni2O3 catalysts for CO oxidation. The relative ease of preparation, lower cost, high thermal and chemical stability and more activity of Ni2O3 catalysts offer better performances for automobile vehicle pollution control applications. The addition of suitable promoters and would results to improvement in the performances of nickel oxide catalysts towards CO oxidation. The CO oxidation is highly influenced by the crystal size of nickel oxide catalysts and increases with reducing the crystal size of catalyst till certain limit and further CO oxidation conversion decreases. After the review of all papers observed that the Ni/TiO2 catalyst is highly active for total oxidation of CO at 50 °C temperature. The results analysis show that the catalytic activity of pure Ni2O3 is not very high in the CO oxidation, but the addition of Co3O4/Fe2O3/CeO2/TiO2 significantly increases the catalytic activity at low temperatures. On the basis of studying the isotope exchange of oxygen and position of the Fermi level in the system MiO−Ni1−xO, an explanation was proposed for the compensation effect in the reaction of CO oxidation on nickel oxide catalyst. In deactivation analysis observed that the Ni-oxide catalyst easily regenerated without loss of catalytic activity and gave equal turnover rate as the fresh catalyst. There are lots of papers available for CO oxidation over Ni-oxide catalysts, but this review paper provides important information about the pure and substituted Ni-oxide catalysts for CO emissions control.None.

The low-temperature catalytic oxidation of carbon monoxide (CO) is very important process for all human health protection systems. The major sources of CO produced into the environment are automobile exhaust, so that the various types of catalysts are used in the catalytic converter for oxidation of CO. As compared to noble metal oxide catalysts the transitional metal oxide catalysts are very active, lower cost, easily available and fast regenerated. Among the various transitional metal oxide catalysts the nickel oxide (NiO) is one of the best catalysts for CO oxidation at a lower temperature. A small amount of NiO was deposited on mesoporous Al2O3 using atomic layer deposition and subsequently oxidation at different temperatures. Furthermore, as the pre-annealing temperature increased and improved resistance towards poisoning due to the CO oxidation was observed. The Ni/TiO2 catalyst show that the best efficiency in selectivity, performances and stability in the heterogeneous catalysis. The performances of NiO nanoparticles are highly dependent on the crystallite size, surface area and pore volume of the catalysts. The certain attention has been paid on recovery of Ni nanoparticle catalysts from reaction systems and their reuse for several times without losses of catalytic activity in CO oxidation. This investigation will shown scientific basis for potential design of Ni nano-particle catalysts for CO oxidation.

1. Introduction Waste plastic is a kind of organic polymer solid waste rich in carbon and hydrogen, and its typical types are polyethylene and polypropylene [1]. Compared with the traditional processing technology of waste plastics, pyrolysis technology has prominent advantages. Pyrolysis technology is the process of depolymerization and reforming of waste plastics under thermochemical action to produce hydrogen-rich gas [2]. This technology can provide the harmless and resource-based treatment of waste plastic, with outstanding advantages of safety, environmental protection, high efficiency and energy savings [3]. In recent years, the catalytic pyrolysis of plastics to produce hydrogen-rich gases has attracted extensive attention [4,5]. Catalysts play an important role in the reforming process of plastics to produce hydrogen, which can help the long chain break into short chains and break chemical bonds to promote the formation of small molecular gas products [6,7]. Therefore, an efficient and stable catalyst is of great significance, but a much larger problem exists that is closely related to the catalyst itself [8]. In the reaction, numerous olefins generated by pyrolysis easily form coke and deposit on the surface of the catalyst; this results in catalyst inactivation, also known as catalyst poisoning [9]. Therefore, to effectively avoid catalyst poisoning, slow the catalyst inactivation rate and find a simple method of catalyst regeneration are important research directions for hydrogen production from waste plastics [10]. Moreover, achieving effective bond breaking of macromolecular organic compounds such as olefins and generating more small gaseous molecules are the key steps to increasing hydrogen production [11].In terms of catalysts, precious metals such as platinum and rhodium have good catalytic performance, but their high price limits their wide application [12]. Inexpensive transition metals such as iron and nickel also have excellent catalytic activity [13], and their catalytic activity increases with increasing reduction degree [14,15]. The application of transition metals in the cracking of plastics has also attracted attention [16,17]. Yao et al. [18] compared the catalytic activity of iron and nickel in the cracking of plastics, and they found that iron catalysts have better gas and carbon production performance for single metal catalysts, with hydrogen and carbon nanotube production rates of 22.9 mmol H2/gplastic and 195 mg/gplastic, respectively. Cai et al. [19] specifically studied the conversion of different types of plastics by iron catalysts; their study showed that various plastics can be converted into high-value hydrogen, liquid fuels and carbon nanotubes with iron catalysts. In another paper [20], the catalytic cracking of polypropylene with an iron catalyst supported by alumina was described in detail, and the effect of iron active substances on the yield of gas, liquid oil and solid carbon was explored. Ann et al. [21] also confirmed that the presence of Fe in the catalyst could promote the selectivity of deoxidation products, promote demethoxidation, demethylation and deoxidation reactions in the process of volatile catalytic reforming, and significantly improve the selectivity of phenol and H2 formation. Many studies have shown that the active material iron has excellent catalytic performance in the catalytic reaction of hydrogen production from plastic cracking [22–24].The catalytic performance depends not only on the active components but also on the performance of the catalyst support, especially the physical and chemical properties and surface structure of the catalyst support [25,26]. Molecular sieves are usually selected as cracking catalyst carriers due to their conventional pore structure and high specific surface area, but they have poor hydrothermal stability and high cost [27]. Most pyrolysis of plastics for the production of hydrogen occurs under the condition of water vapour. Therefore, the hydrothermal stability of the catalyst support is an important parameter of the catalyst [28]. Activated carbon has a large specific surface area and developed pore structure. Additionally, due to the stability of the activated carbon structure, it shows strong chemical stability and hydrothermal stability in the reaction; therefore, it is widely used in the study of hydrogen production from methane [29,30]. The surface chemistry of activated carbon determines the initial catalytic rate, and the pore structure determines the stability of catalysis [31]. Therefore, mesoporous carbon with a larger specific surface area and pore capacity shows better catalytic activity and stability in the catalytic hydrogen production reaction [32].Based on the good performance of transition metal iron and carrier activated carbon in catalytic hydrogen production, this study develops a new activated carbon-based iron catalyst for the catalytic cracking of polymer waste plastic to produce hydrogen. The supported activated carbon-based iron catalyst was prepared by using the wet impregnation method. Amorphous carbon was chosen as the carrier because it contains two-dimensional graphite layers or three-dimensional graphite microcrystals with very small diameters and a large number of irregular bonds on the edges of the microcrystals [33]. This characteristic is conducive to its combination with active substances. Compared with the regular form of carbon, the prepared catalyst has better catalytic performance and stability. The experimental instrument used is a self-developed two-stage tubular furnace. The effects of the amount of iron, the ratio of raw material to catalyst and the amount of water vapour on the production of hydrogen from plastic catalytic cracking were studied. Surface microscopic analysis, material structure analysis and thermogravimetric analysis of the new activated carbon-based iron catalyst were carried out. Additionally, the related characterization of the spent catalyst was carried out to analyse its carbon deposition and deactivation. 2. Experimental Raw polypropylene (PP) with a particle size less than 5 mm was purchased from Shanghai Myrell Chemical Technology Co., Ltd. (China). The catalyst support amorphous activated carbon was in powder form and purchased from Guangzhou Xiting Experimental Equipment Co., Ltd.Iron nitrate hexahydrate (Fe(NO3)3·9H2O, AR, 98%), the experimental cylinder gas nitrogen (N2, purity>99.99%) and the chemical reagent isopropyl alcohol (AR, 99.5%) were purchased from Shanghai MacLean Biochemical Co., Ltd., Guangzhou Shengying Gas Co., Ltd., and Guangzhou Chemical Reagent Co., Ltd., respectively.The powdered activated carbon (AC) was initially sieved to obtain a powder with a diameter of 0.15-0.18 mm. Its physical and chemical properties are shown in Table 1 . The contents of ash, volatile and fixed carbon are 25.26 wt.%, 14.07 wt.% and 60.66 wt.%, respectively. AC is mainly carbon and contains a small amount of hydrogen and oxygen. The preparation method of the activated carbon-based catalyst was the wet impregnation method. The detailed experimental method was as follows: a certain proportion of AC and Fe(NO3)3·9H2O were dissolved in 200 ml of deionized water, and the two were completely dissolved after stirring thoroughly. The solution was placed on a magnetic stirrer and stirred at a speed of 600 rpm for 5 hours. The stirred solution was then placed in a drying oven for evaporation of water to obtain the catalyst precursor. After grinding, the precursor was calcined in a tube furnace under an inert atmosphere. The catalyst was obtained by calcination at 900 °C for 2 hours. The catalysts are named 5Fe/AC, 10 Fe/AC, 15 Fe/AC and 20 Fe/AC according to the proportion of active material iron.Thermogravimetric mass spectrometry (TGMS) (NETZSCH STA 449 F5, NETZSCH QMS 403) was used to detect the weight loss characteristics of the catalyst. The surface morphology of the catalyst was characterized using a Hitachi S-4800 scanning electron microscope (SEM). The inside structure of the catalyst was examined with a high-resolution transmission electron microscopy (HR-TEM) using a JEOL JEM-2100HR (200 kV). Powder X-ray diffraction (XRD) analysis of the catalysts was performed by using an X’Pert Pro MPD operated at 40 kV and 40 mA with Cu Kα radiation. The XRD patterns were recorded at a diffraction angle of 2θ between 10° and 80°, and Jade 6.5 software was used for data analysis. The graphitization and purity degree of the obtained solid carbon materials were examined by using Raman spectroscopy (labRAM HR800, HORIBA JY, France) with an excitation wavelength of 532 nm.A two-stage pyrolytic-catalytic vertical tube furnace is shown in Fig. 1 . Before the beginning of the experiment, the PP and catalyst were placed in a stainless steel reactor in a certain proportion from bottom to top, and then the reactor was sealed. Before the reaction was heated, nitrogen was pumped into the reactor to expel air. After the nitrogen atmosphere was stabilized in the reactor, the temperature of the catalyst section was initially raised to 900 ℃. After the temperature of the catalytic section was stabilized, the temperature in the PP pyrolysis section was heated. The temperature in the pyrolysis stage rose to 500 ℃ after 15 minutes, and water vapour was added at the same time. Gas collection began when the temperature of the pyrolysis section reached 450 ℃, and the continuous collection time was 30 minutes. The reaction gas was passed through a wash cylinder containing isopropyl alcohol and then condensed and dried for collection. After the reaction was complete, nitrogen flow was continued until the reactor temperature dropped to room temperature.Polypropylene is mainly decomposed into tar, carbon and hydrogen during pyrolysis (Eq. (1), (2), (3)). In the catalyst stage, under the action of catalyst and water vapour, tar underwent a catalytic cracking reaction and further decomposed into carbon monoxide, carbon dioxide, hydrogen and other small molecular gases (Eq. (4)), but tar and carbon dioxide and carbon and water vapour also underwent other side reactions (Eq. (5), (6)). Finally, after pyrolysis and catalytic reforming reactions, PP produced gas products dominated by hydrogen and solid products dominated by carbon. In our previous study, it was found that C3H6 was the main gas produced by PP pyrolysis. In a two-stage reaction unit, CH4 is the main gas produced by PP after pyrolysis at a high-temperature stage without catalyst. This result shows that even if no catalyst is added in the two-stage reaction device, the secondary high temperature will cause the decomposition of C3H6 into CH4, and the catalyst mainly acts on the further decomposition of CH4. (1) ( C 3 H 6 ) n → C + H 2 (2) ( C 3 H 6 ) n → C X H y ( T a r ) + H 2 (3) C X H y ( T a r ) → C + H 2 (4) C X H y T a r + H 2 O ↔ C O + H 2 + CO 2 (5) C X H y T a r + CO 2 ↔ C O + H 2 (6) C + H 2 O ↔ C O + H 2 The calculation method of each value is as follows:The content of the produced gases (CO, H2, CO2, CH4 and C2H4) was detected with a gas chromatograph (GC, Agilent 7890A).The mol% of CO, H2, CO2, CH4 and C2H4 were calculated using Eq. (7): (7) mol % i = % i % co + % H 2 + % C O 2 + % C H 4 + % C 2 H 4 100 % where i represents CO, H2, CO2, CH4, and C2H4, and % c o , % H 2 , % C O 2 , % C H 4 , and % C 2 H 4 are the percentage of the specified gas obtained from the GC results.The H2 yield was calculated using Eq. (8): (8) H 2 y i e l d m m o l g P P = ( % H 2 × C N 2 × t ) / ( 22.4 × % N 2 ) where % H 2 and % N 2 represent the percentage of gas obtained from the GC results. t represents the gas collection time, and C N 2 represents the volume flow of nitrogen.The theoretical H2 yield was calculated using Eq. (9): t h e o r e t i c a l H 2 y i e l d m m o l g P P = M H , P P + M H , H 2 O × t × 1000 / 2 where M H , P P and M H , H 2 O are the moles of hydrogen in PP and water, respectively.The H conversion (%) was calculated using Eq. (10): (10) H c o n v e r s i o n ( % ) = H 2 y i e l d / t h e o r e t i c a l H 2 y i e l d × 100 % 1.1 Pyrolysis-catalytic reforming of PP for hydrogen productionIn the hydrogen production from PP pyrolysis and steam reforming experiment, the loading of iron in the catalyst, the addition ratio of catalyst to PP and the addition amount of water vapour are all important factors. Therefore, the influence of each parameter on gas production and hydrogen production was analysed. Fig. 2 shows the influence of iron loading on the experimental results. The range of iron loading is 0%-25%, and 0% loading is the blank comparison experiment. Pure activated carbon also has a certain ability to catalyse the cracking of plastics and depends on its developed pore structure and surface oxygen-containing functional groups, especially the content of ash. The addition of active material iron significantly improved the performance of the catalyst. With increasing iron loading, the gas production and hydrogen production of the plastic cracking continued to increase. When the iron content was 15%, the gas production and hydrogen production reached peak values of 172.09 mmol/gPP and 112.71 mmol/gPP, respectively. At this time, the catalytic capacity of the composite catalyst was approximately 3 times that of activated carbon. Furthermore, the hydrogen production performance was not improved by continued addition of iron loading (more than 15%). This was potentially due to the excessive iron bearing on the catalyst surface, which led to a decrease in the pore patency and a reduction in the catalytic contact area. Fig. 2 also shows the composition of the produced gas. Due to the addition of water vapour, hydrogen is the most abundant gas, followed by carbon monoxide. Carbon dioxide and small molecule hydrocarbons are rarely produced. Carbon deposition is inevitable in the cracking reaction of polymer compounds. The formation of carbon oxides and hydrocarbons indicates that the addition of water can release the deposited carbon in the form of gas, thus reducing the carbon deposition on the surface of the catalyst and ensuring the catalytic stability over a long time.The key to ensuring hydrogen production and reducing cost is to explore the appropriate ratio of PP and catalyst. Fig. 3 shows the effect of the PP-to-catalyst ratio on gas production, hydrogen production and gas composition. With increasing catalyst proportion, gas production and hydrogen production gradually increase. When the ratio of PP to catalyst is 1:0.75, gas production and hydrogen production basically reach the equilibrium state. When the ratio increases to 1:1, the gas production increases slightly, but the hydrogen production does not increase further. Considering the expense, the ratio of 1:0.75 is determined to be the appropriate addition ratio of PP and catalyst. Within a certain range, increasing the amount of catalyst can improve the reflection efficiency. When the contact point between the catalyst and PP is saturated, the increase in excessive catalyst affects the performance of catalysis. Additionally, the change in the ratio of PP to catalyst has no significant effect on the gas composition. Under the most appropriate conditions, the proportion of hydrogen is 65.5% and that of carbon monoxide is 22.8%.In the steam reforming reaction, the amount of water vapour added is an important factor in determining the hydrogen production capacity. Fig. 4 shows the effect of the amount of water vapour added on the reaction result. In the reaction without water, the gas and hydrogen production of polypropylene cracking were 57.66 mmol/gPP and 38.73 mmol/gPP, respectively. Although the anhydrous cracking reaction of polypropylene produced little gas, the content of hydrogen in the gas was considerable. The addition of water greatly increased gas production and hydrogen production. The maximum gas yield (172.09 mmol/gPP) was achieved at a water content of 6 ml/h. When the amount of water reached 6 ml/h, the hydrogen yields reached a maximum peak (112.71 mmol/gPP). However, an additional increase in water content did not lead to a higher hydrogen yield. First, too much water vapour surrounded the catalyst and prevented the contact of the hydrocarbon molecules with the active site. In addition, a large amount of water vapour formed an excessive scour effect on the catalyst, which caused a decreased the catalytic performance. The addition of water had a certain saturation point, and supersaturation could have a negative effect. The determination of the equilibrium point among water, PP and catalyst in the reaction was the key to ensuring a low cost and high yield of hydrogen.The produced hydrogen can be traced back to the hydrogen in polypropylene and water. To understand the transformation degree of hydrogen in the reaction, the conversion rate of hydrogen was calculated, as shown in Table 2 . Table 2 calculates the hydrogen conversion rate based on the actual and theoretical hydrogen production data. In the absence of water, 54.24% of the hydrogen in polypropylene can be successfully decomposed and reformed into hydrogen molecules. Other hydrogens are converted into large or small hydrocarbon molecules. With increasing water addition, the hydrogen conversion rate gradually decreases. This indicates that most of the water is not involved in the reaction, and only a small portion of the water successfully participates in the cleavage reaction and produces hydrogen.To analyse the contribution of water in the reaction of hydrogen production by steam reforming of polypropylene more directly, the transformation of hydrogen in water is analysed separately. The premise of this analysis is to assume that all hydrogen elements in polypropylene are converted to hydrogen. The difference in water content corresponds to different hydrogen contribution amounts. When the water content is 6 ml/h, its contribution degree to hydrogen is the largest, showing that the water is at its maximum strength in the reaction. This result also matches the water content choice for the optimal experimental conditions. Thus, for water to function optimally, it is necessary to add the proper amount of water. Too much or too little affects its optimal response capacity.Based on the above analysis results, the optimal experimental conditions are an iron loading of 15%, a PP to catalyst ratio of 1:0.75 and a water content of 6 ml/h. To better evaluate the catalytic stability of the catalyst under long-term operation, 10 cycle experiments were carried out on the reaction under optimal conditions, and the results are shown in Fig. 5 . With increasing catalyst frequency, the hydrogen production performance of the catalyst decreased gradually, and the hydrogen content decreased from 65% to 47%. The selectivity of hydrocarbons and carbon dioxide increased, while the selectivity of carbon monoxide did not change significantly. As the catalyst was reacted for a longer time, the more inclined it was to produce hydrocarbons, and its catalytic capacity for small molecular hydrogen clearly decreased. The degradation of the catalyst performance in the process of recycling was due to the accumulation of carbon on the catalyst surface. With increasing reaction time, carbon deposition further increased, the initial active site slowly lost its activity, and a new active site was generated on the discontinuous graphite layer, whose activity was not as high as that of the original active site; thus, the overall catalytic activity was reduced.1.2 Fresh and used catalyst microscopic and textural characterizationThe physical and chemical properties of catalysts directly affect the production performance of the catalytic reforming reaction. To understand the catalyst change from catalysis, fresh and used catalysts were characterized and analysed with a focus on the surface microstructure and material composition changes of the catalysts; also, the inevitable carbon deposition phenomenon was detected and analysed. Table 3 shows the specific surface area data of the activated carbon and catalyst. The specific surface area, pore volume and pore size of the activated carbon were 839.38 m2/g, 0.35 cm3/g and 4.82 nm, respectively. Compared with activated carbon, the specific surface area of the synthesized catalyst decreased from 562.27 m2/g to 619.24 m2/g, which was caused by the addition of iron and the covering of surface holes. However, the catalyst still had good pore structure data, and the excellent specific surface area and pore volume could ensure the catalytic performance.Fig. 6. shows the SEM images of fresh and used catalysts of 15Fe/AC. The used 15Fe/AC catalyst was recycled 10 times. Fig. 6 (a) and (b) show the microscopic SEM images of the fresh 15Fe/AC catalyst. The surface of the fresh catalyst shows nanosized spherical particles, which are densely distributed on the surface of the activated carbon support. In addition, numerous tiny pore structures are observed on the surface of the activated carbon. This shows that a large number of iron active sites are widely distributed on the surface of the catalyst, ensuring the development of catalytic performance. Fig. 6 (c), (d), (e) and (f) show the microscopic images of the surface of the used 15Fe/AC catalyst. The surface of the used catalyst is covered with a layer of graphite carbon, some of which are carbon nanospheres and some are carbon nanotubes. The carbon nanospheres are approximately 50-100 nm in diameter and are from the incomplete graphitization of carbon nanospheres; carbon nanotubes are approximately 20-100 nm in diameter. Under the coupling effect of many factors, two kinds of carbon nanostructures were produced. Morphological differences in carbon nanomaterials are closely related to the size of the metal particles on the surface of the catalyst and the difference in the force between the metal and the support. Relevant studies have shown that larger metal particles on the surface of the catalyst correlate to a larger inner diameter of the generated carbon nanotubes, and stronger interaction between the metal active components and the support correlated to a smaller the diameter of the generated carbon nanotubes [34].TEM was used to further observe the morphology and size of carbon deposition, as shown in Fig. 7 . The resulting carbon nanotubes are approximately 50 nm in diameter, with the smallest being 20 nm in diameter. Additionally, the nanotube is a hollow structure and has a relatively thick wall. The wall of the tube is relatively smooth and basically free of other impurities. The darker black spots in the image are catalyst particles, and very few particles are observed embedded in the tube wall. The active metal is distributed in the middle and end of the carbon tube, and the growth pattern of carbon nanotubes is a typical apex growth pattern. The active component of the metal appears quasiliquid during the growth of the carbon nanotubes and migrates continuously with the growth of the carbon nanotubes. Fig. 8 shows the XRD test results of fresh and used catalyst. The activated carbon support is composed of amorphous carbon, and the diffraction peaks of new substances such as Fe2O3, Fe, Fe5C2, FeC2, and Fe3O4 appear in the catalyst after metal iron is loaded; the iron active material in the catalyst is present in these four forms. The diffraction peak intensity of iron is the highest, while those of iron oxides and carbides are relatively small. As the iron loading increases, the diffraction peak of iron-related active substances gradually increases. The used catalyst structure is shown in Fig. 8 (b). Compared with the fresh catalyst, the diffraction peaks of Fe3C and C appear in the used catalyst without water, and the intensity of the diffraction peak of iron decreases. This indicates that carbon deposition appears on the surface of the catalyst after the reaction, and carbon deposition covers most of the active sites. For the used catalyst with water in the reaction, the diffraction peak of iron is completely covered, only the diffraction peak of Fe2O3 remains, and the diffraction peak of carbon deposition is also increased. This shows that the catalytic performance of the catalyst is mainly derived from the role of Fe2O3 during long-term use, and Fe2O3 has better stability in the catalytic process.The microscopic image and material structure analysis showed that the surface of the catalyst was a graphite carbon deposit. To further determine the amount of carbon deposition and the degree of graphitization, TG, TDA and Raman spectroscopy were carried out on the used catalyst 15Fe/AC.The TG and DTA curves are shown in Fig. 9 . Fig. 9 (a) shows the thermogravimetric curve. The fresh catalyst began to lose weight at approximately 500 °C and continued to lose weight until 660 °C, with a weight loss ratio of approximately 50%. The weight loss ratio of the used catalyst (65%-70%) was significantly higher than that of the fresh catalyst. From Fig. 9 (b), compared with fresh catalyst, the used catalyst showed a wider weight loss temperature range, and a larger interval between the start and end temperatures of oxidation indicated a greater diversity of carbon structures. The used catalyst contained more ordered graphite structure carbon materials, so its weight loss began at a higher temperature and a wider weight loss interval. Moreover, the weight loss rates of the used catalysts with water content 0 and water content 6 were approximately 70% and 65%, respectively, which further indicated that the addition of water could inhibit the formation of carbon deposition.To understand the degree of graphitization of carbon deposition, Raman spectroscopic analysis was performed on the catalyst containing the carbon deposition, and the results are shown in Fig. 10 . The Raman spectra near 1350 cm-1 and 1580 cm-1 correspond to D and G peaks, respectively. The Raman-active D peaks are observed due to defects (such as branches, openings, bends), amorphous carbon or the edge of the lamellae's crystal surface activating the vibration absorption mode of the six-member ring. The G peak is the Raman characteristic peak corresponding to the C-C bond stretching vibration (E2g vibration mode) of sp2 hybrid carbon atoms between graphite lamellae. In the literature, the ratio of the D peak to G peak intensity, ID/IG, is used to calculate the graphitization degree and crystallization index [35]. Theoretically, the ratio is zero for pure carbon nanotubes, and a smaller ratio correlated to a higher graphitization degree [36]. Analysis of the ratios obtained from peaks in the figure shows that the degree of graphitization of carbon deposition without water is higher, and the addition of water increases the number of defects in graphite carbon and the amount of amorphous carbon, reducing the degree of graphitization and crystallinity of carbon deposition. In addition, the Raman G' peak at 2700 cm-1 is related to the elastic scattering of the two phonons, and the ratio of IG'/IG can also indicate the purity of graphite carbon to a certain extent. A higher IG'/IG ratio correlates to a higher purity of the carbon nanomaterial. The purity of the carbon nanomaterials prepared by the reaction without water is higher.1.3 Comparison with literature-reported studiesTo better present the experimental results of this study, a comparison is performed with the results from similar studies, as shown in Table 4 . The H2 yield (38.73 mmol/g) was relatively significant in the reaction without water participation. Similarly, the H2 yield of the Fe-γAl2O3 catalyst is 22.9 mmol/g with PP as the raw material. This result further indicates that, to some extent, the catalytic support performance of the disordered carbon structure is better than that of the ordered molecular sieve structure. In the catalytic conversion with added water, the H2 yield was 112.71 mmol/g, which was outstanding in comparison other reported studies. This result also shows the feasibility and superiority of the experimental method and technology in this study.2 ConclusionsIn this study, an iron catalyst supported by activated carbon was prepared and applied to research on hydrogen production by catalytic reforming of plastic polypropylene. The experimental apparatus was a two-stage pyrolysis catalytic furnace. The characterization test showed that there were some structures on the surface of the prepared catalyst, such as Fe2O3, Fe, Fe5C2, and Fe3O4, which acted as active sites in the catalytic reaction. The experimental results showed that the 15Fe/AC catalyst had good hydrogen production performance, and the H2 yield could reach 38.73 mmol/gPP in the catalytic reforming reaction without water and 112.71 mmol/gPP in the reaction with water. After the analysis of the water contribution degree, the optimal water content was determined to be 6 ml/h. In addition, the carbon deposition of the used catalyst was analysed in depth. The results showed that the carbon deposited by the reaction without water had a higher graphitization degree and purity. The addition of water reduced the degree of graphitization of carbon deposition but effectively increased the yield of H2. This study provides a novel method for hydrogen production from plastic thermal conversion.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This paper is financially supported by National Key R&D Program of China (2019YFC1906803), and CAS Project for Young Scientists in Basic Research (YSBR-044).

The purpose of this study is to explore a method for the high-yield production of hydrogen by pyrolysis and steam reforming of polymer plastics. The developed Fe-based catalyst supported on activated carbon was applied to reactions with polypropylene for hydrogen production. The effects of iron loading (%) in the catalyst, the total catalyst amount, and the water content in the reaction atmosphere on the performance of hydrogen and gas production were investigated. Under the optimal conditions, the hydrogen yield without water added reached 38.73 mmol/gPP, and this yield was significantly improved by adding water into the reaction atmosphere. By optimizing the amount of water added, the hydrogen yield reached 112.71 mmol/gPP. The surface morphology and structural components of the fresh and used catalysts were characterized, and the morphology and quantity of carbon deposition on the catalyst were analysed. The catalytic stability of the 15Fe/AC catalyst was determined by repeating the test 10 times under the optimal reaction conditions. As the reaction time increased, the selectivity of the catalyst for hydrogen decreased and that for hydrocarbons increased. Moreover, the experimental method used in this study had excellent hydrogen production capacity. Thus, this study provided a novel method for the high-efficiency production of hydrogen by pyrolysis and steam reforming of polymer plastics.

Data will be made available on request.Dry Reforming of Methane (DRM) has great potential to contribute to current efforts towards a sustainable energy future. Beyond that, this catalytic reaction turns two of Earth’s most abundant greenhouse gases, CO2 and CH4, into valuable synthesis gas [1], thus mitigating global warming. Both products of DRM, H2 and CO, are building blocks in the synthesis of various fuels and chemicals via heterogeneous catalysis, e.g. generation of hydrocarbons via Fischer-Tropsch synthesis or methanol production [2,3]. Especially the Fischer-Tropsch reaction benefits from the low H2/CO ratio obtained by DRM [4]. DRM is represented by following Eq. (1): (1) CH 4 ( g ) + CO 2 ( g ) ⇌ 2 H 2 ( g ) + 2 CO g ( Δ H r 298 = + 247 kJ mol − 1 ) The endothermic nature of DRM necessitates high operating temperatures in order to achieve high conversions, usually between 650 °C and 1000 °C [5]. Unfortunately, DRM is still not a mature industrial process, despite its great environmental potential [4]. High operating temperatures and associated deactivation phenomena of sintering and coke formation are the biggest obstacles. Steam reforming, partial oxidation, or autothermal reforming of methane are the dominant technologies for syngas production from methane on an industrial level; however, they mainly yield hydrogen-rich products [6]. In contrast, due to the introduction of an additional carbon source, DRM leads to CO-rich syngas, beneficial for certain downstream processes (e.g. production of acetic acid [6]). Consequently, the interest in DRM is still extremely high, and further developments are necessary to obtain effective catalyst materials that are stable at the required operation temperatures.Various catalyst materials have been extensively studied for their capability of promoting DRM, with Ni-supported catalysts mostly utilised for industrial processes due to their high activity and low cost [7,8]. Unfortunately, they suffer from deactivation by carbon deposition and/or Carbon Nano Tube (CNT) formation [9]. Noble metal-based materials (e.g. Pt, Pd, Rh, Ru…) are less prone to such adverse side reactions, although their expensive nature makes them unsuitable for large-scale applications [1,5]. By adding small amounts of noble metals to e.g. Ni-based catalysts with subsequent alloy formation, both drawbacks, carbon formation and high cost, can be alleviated [10]. Furthermore, use of bimetallic catalysts enables tuning of particle size and dispersion, thus increasing overall performance [11]. Wang et al. calculated the thermodynamically limiting temperatures in DRM for carbon formation (carbon is not stable above this limit) as a function of the educt ratio and found that in a CO2 excess this temperature decreases, meaning that this excess can prevent carbon formation at temperatures below 700 °C [12]. During the search for highly active materials, Co- and Fe-based materials were tested as well [9,13] with Co reaching almost the activity level of Ni [4]. Moreover, perovskite-type materials have lately drawn much attention as promising substitutes to conventional catalysts, as they allow a design approach, making cost effective and highly active materials available. For example, Dama et al. have demonstrated high performance for CaZr0.8Ni0.2O3-δ [14]. Additionally, perovskites are often used as precursors for DRM catalysts that release the active metal upon reductive treatment, which is accompanied by partial decomposition of the perovskite into the respective oxides [15,16]. Unfortunately, the rich redox and defect chemistry of the perovskite surface – which is highly beneficial for DRM – is lost during this process.Although numerous studies in the past intensively investigated DRM on supported metal catalysts, the reaction mechanism is still debated without general agreement [17]. One reason for this may be related to the fact that the mechanism depends significantly on the utilised materials and the combination of active metal and nature of the oxide support [6,18]. In principle, the reaction mechanism can be divided in the following major steps: The first step is the dissociative adsorption of methane, which occurs on the metal and is established as the rate limiting step for e.g. Ni [4]. Dissociative adsorption of CO2 occurs in parallel on the surface of the support, which is considered a fast reaction step. It has to be emphasised that CO2 activation is strongly dependent on the used materials, with basic or redox-active supports or surface defects strongly enhancing this process (e.g. oxygen vacancies on perovskite surfaces are extremely active for CO2 activation already at low temperatures around 400 °C) [14,19]. In a subsequent reaction step, the formed H from CH4 dissociation can either desorb as H2 or move to the support to form OH-groups that can be observed below 800 °C [4]. In the latter case, a competing reverse Water Gas Shift (rWGS) type reaction is dominating – especially as it is thermodynamically favoured compared to the DRM-pathway at lower reaction temperatures. Both pathways result in the formation of NiO and NiC which can recombine and lead to the desorption of CO. These two possible pathways are depicted in Fig. 1. OH-groups can react with activated CO2 (forming e.g. formate) or with CHx groups to form CHxOH. The most complex part of the reaction mechanism is the oxidation of the intermediates and the subsequent CO and H2 desorption with many different possible pathways reported in literature. For instance, Iglesia and Wei outlined different routes of CHx oxidation via surface oxygen [20]. Most models were established for Ni-based materials, but they are claimed to be transferable to noble metal systems in literature (Wittich et al. [6].)The H2/CO ratio obtained by DRM (which ideally should be 1, according to Eq. 1) depends strongly on the reaction temperature: the rWGS-type pathway occurs under similar reaction conditions and reduces the ratio as can be seen in the rWGS equation: (2) H 2 ( g ) + CO 2 ( g ) ⇌ H 2 O ( g ) + CO g ( Δ H r 298 = + 42.1 kJ mol − 1 ) Aside from rWGS, a large number of additional possible side reactions influence the DRM process, a brief summary of which is given by Aramouni et al. [4]. Higher operating temperatures are generally favourable for DRM, as CO and CO2 hydrogenation or the Boudouard reaction, for example, occur at lower reaction temperatures. The latter, together with methane dehydrogenation, which is more pronounced at high reaction temperatures (>730 °C), is the main source for carbon deposition on DRM catalysts. The degree of catalyst deactivation by coking depends on the utilised materials. The choice of support has a strong influence on coking resistance, with highly redox-active oxides promoting the removal of formed carbon deposits during DRM [5,14]. Basic oxides and perovskites have been reported to exhibit increased ability for carbon gasification [21]. Perovskites in particular are promising alternatives due to their rich surface redox chemistry and thermal stability [19]. In addition, perovskites provide the opportunity to incorporate dopant materials on both A- and B-site, enabling the synthesis of materials that contain additional promoters and catalytically active elements. For instance, addition of Ca to DRM catalysts has been reported to improve carbon removal from the surface [14].Doping Ni or Co on the B-site of perovskites leads to the formation of metallic nanoparticles upon reduction or in reducing reaction environments via exsolution as was reported previously [22]. Unlike the precursor method mentioned above, exsolution preserves the perovskite structure – meaning its beneficial properties are not lost. Due to their high dispersion and strong anchoring to the surface, the metallic nanoparticles provide an ideal system for DRM. Additionally, Neagu et al. showed that small nanoparticles (around 20 nm) that were produced via exsolution are still stable in high temperature reducing conditions, even though the surface area of the perovskite was around 1 m2 g−1 [23]. High metal dispersion in combination with available surface oxygen enhances coking resistance of these type of system as reported by Dama et al. [14].This was the motivation to use Co- and Ni-doped perovskites for the present study, which was focussed on comparing the effect of the formation of the metal nanoparticles during DRM (in-situ exsolution) to the effect of pre-reduction with H2/H2O (with the formation of the nanoparticles prior to the catalytic reactions). Both phenomena were studied by catalytic testing as well as with in-situ surface chemical analysis by Near Ambient Pressure X-Ray Photoelectron Spectroscopy (NAP-XPS) and operando X-Ray Diffraction (XRD). For this purpose, perovskites with the nominal compositions Nd0.6Ca0.4FeO3-δ (B-site undoped), Nd0.6Ca0.4Fe0.9Co0.1O3-δ (B-site Co-doped), and Nd0.6Ca0.4Fe0.97Ni0.03O3-δ (B-site Ni-doped) were investigated. The obtained results were supplemented by Scanning Electron Microscopy (SEM) combined with Energy Dispersive X-Ray analysis (EDX).The catalysts (Nd0.6Ca0.4FeO3-δ, Nd0.6Ca0.4Fe0.9Co0.1O3-δ, and Nd0.6Ca0.4Fe0.97Ni0.03O3-δ) were prepared via a modified Pechini synthesis method, as outlined in detail in previous works [22,24]. To that end, the respective starting materials Nd2O3 (99.9 %, Strategic Elements), CaCO3 (99.95 %, Sigma-Aldrich), Fe (99.5 %, Sigma-Aldrich), Ni(NO3)30.6 H2O (98 %, Alfa Aesar), and Co(NO3)30.6 H2O (99.999 %, Sigma-Aldrich) were mixed in stochiometric amounts and dissolved in either HNO3 (65 % Merck) or H2O, both doubly distilled. The salts were complexed using citric acid (99.9998 % trace metal pure, Fluka) in excess of 20 mol% with regard to the cations. After evaporation of the liquid on a heater, the resulting gels were ignited by further heating, leading to the formation of powders. These powders were calcined for 3 h at 800 °C. The catalyst Nd0.6Ca0.4Fe0.97Ni0.03O3-δ had a reduced Ni content and underwent an additional annealing step at 1200 °C, as phase impurities were observed even after the calcination step of the original synthesis. The products were ground to ensure homogeneity. Respective surface areas and morphological characterisation of the pristine perovskites can be found in reference [22]. The weights used for the synthesis are listed in the Supporting Information (SI) in Tables S1 to S3.The powder XRD measurements were carried out on a PANalytical X′Pert Pro diffractometer (Malvern Panalytical, Malvern, UK) in Bragg-Brentano geometry using a mirror for separating the Cu Kα1,2 radiation and an X′Celerator linear detector (Malvern Panalytical, Malvern, UK). For the operando experiments, an Anton Paar XRK 900 chamber (Anton Paar, Graz, Austria) was used. After sample preparation, the catalysts were pre-treated with oxygen (600 °C, 40 min, 0.5 L min−1 O2) before switching to the reaction atmosphere. The DRM reaction was carried out in CH4 excess (48 mL min−1 CH4, 16 mL min−1 CO2, and 50 mL min−1 Ar at ambient pressure) at increasing temperatures (going from 300 °C to 700 °C with 50 °C steps). The methane excess was chosen for two reasons: Firstly, to simulate real biogas conditions, as – dependent on the feedstock – biogas generally consists of around 66 % CH4 and 32 % CO2 [25,26]. Secondly, CH4 excess leads to a reducing atmosphere and, therefore, may promote in-situ exsolution. At each step, reaction conditions were held for 10 min (to achieve equilibrium) before an operando XRD measurement (about 30 min) was carried out. For interpretation of the data and assignment of the diffraction peaks, the PDF-4 + 2019 database (International Centre for Diffraction Data) [27] in combination with the HighScore Plus [28] software (PANalytic) was used. Assignment was performed by comparison with database structures and measurements and validated by performing Rietveld refinements.A Quanta 250 FEGSEM (FEI Company) microscope was used to record SEM images for morphology examination. Additionally, EDX was performed with an Octane Elite X-Ray detector (EDAX Inc). To obtain a satisfactory surface sensitivity, an acceleration voltage of 5 kV was used for imaging and of 10 kV for EDX measurements. In-situ NAP-XPS experiments were performed on 200 nm thick catalyst films, which were grown on Yttria Stabilised Zirconia (YSZ) single crystals in (100) orientation with a size of 5×5x 0.5 mm (CrysTec) via Pulsed Laser Deposition (PLD). The detailed manufacturing process is described in the SI and in previous work [24]. The exact setup of the samples as well as the NAP-XPS sample stage is described in previous work as well [29]. Monochromatic Al Kα radiation with a spot size of 350 µm was used for excitation achieving an energy resolution of around 0.2 eV. Heating was done using a near-IR laser with a wavelength of 970 nm and a maximum output of 100 W, the temperature was monitored with a S-type thermocouple mounted on the sample stage as well as a pyrometer (LumaSense Technology). Both temperature measurement methods were previously calibrated utilising electrochemical impedance spectroscopy and the known temperature dependent resistivity of the YSZ substrate. Therefore, the high frequency offset could be attributed to the ohmic resistance of YSZ, which then was used to calculate the temperature [30]. Similar to the other experiments, all catalysts were initially oxidised in 1 mbar of O2 at 600 °C. NAP-XPS spectra of all relevant core levels including carbon, sulphur, and the fermi edge were recorded simultaneously. After ensuring equal initial states for all samples, the gas phase was changed to reaction conditions (CH4:CO2 = 2:1) at a total pressure of 1 mbar. The reaction temperature was increased in steps of 50 °C from 400 °C to 700 °C. At each step, a full set of spectra was collected (survey, Nd4d, Ca2p, Fe2p, Co2p, Ni2p, O1s, C1s, S2p, fermi edge). To evaluate the recorded XPS spectra, the software Casa XPS was employed. The background was approximated with a Shirley background and binding energies were calibrated using a combination of the Fermi edge and the Ca2p3/2 peak (364.1 eV). The peaks were fitted with a “Gaussian/Lorentzian product form 30″ function as implemented in Casa XPS (“GL(30)”) without asymmetry restriction. Two constraints were applied to the fits of the Ca2p spectra: the difference in binding energy between 2p1/2 and 2p3/2 was fixed at 3.55 eV, and the area ratio of the 2p1/2 and the 2p3/2 was set to 1:2. Fe2p spectra were fitted with four components, namely Fe(II) and Fe(III) for the 2p1/2 and 2p3/2 transition each. Spin orbit splitting for the 2p1/2 and 2p3/2 fits were set to 13.6 eV and the difference in binding energy was fixed at 1.5 eV between Fe(II) and Fe(III). For the area ratio of the Fe 2p1/2 and 2p3/2 transition, a fixed value of 0.4:1 was used. This deviates from the theoretical value of 0.5:1, but results in a better fit most likely due to uncertainties in the background determination.To test the activity of the catalysts, a fixed bed reactor system operating at ambient pressure was used as described in previous works [22,24]. In short, it consists of a home-built gas mixing system made from steel tubes (Burde & Co, Vienna, Austria) and fittings (Swagelok, Solon, USA) and an optional saturator filled with water. The catalyst is fixed in a quartz glass tube with 6 mm diameter (4 mm inner diameter) in an oven, and a Micro-Gas Chromatography system (Micro-GC, Fusion 3000 A, Inficon) is used to analyse the gas composition of the reaction gas mixture after passing through the reactor every 2–3 min to monitor the catalytic activity. The amounts of the catalysts were chosen in such a way to yield conversions between 10 % and 20 % at 700 °C, ensuring that the thermodynamic equilibrium is not reached. The K-type thermocouple was placed inside the catalyst bed to ensure that measured temperature matches the actual temperature of the catalyst. To achieve comparable starting conditions for all catalysts, an oxidative pre-treatment in pure oxygen was performed before each test (10 mL min−1 O2, 600 °C, 30 min). For experiments with metallic nanoparticle exsolution prior to the actual DRM experiment, an additional reducing pre-treatment in humidified H2 was performed for one hour: pure H2 was bubbled through a water-filled saturator at room temperature at a flow rate of 10 mL min−1 (leading to a H2/H2O ratio of ~32:1). The ideal temperature for the reducing pre-treatment was determined in previous experiments and was chosen such that exsolution of the B-site dopant occurs without decomposition of the remaining perovskite [22]. The chosen temperatures were 625 °C for the Ni-doped catalyst, 575 °C for the Co-doped material, and 700 °C for the undoped sample. The order of the “exsolution willingness” of the B-site metals – with Co at lowest temperatures followed by Ni and Fe only at the highest temperatures – is confirmed by Temperature Programmed Reduction (TPR) experiments in the SI (Fig. S6).Afterwards, the catalyst was cooled to 400 °C in Ar (total flow of 12 mL min−1). The gas phase was then changed to the reaction mixture, with flows of 3.0 mL min−1 CH4, 1.5 mL min−1 CO2, and 6.0 mL min−1 Ar (CH4:CO2 = 2:1). With this gas mixture, a temperature ramp from 400 °C to 700 °C with a rate of 1 °C min−1 was performed. In case of the purely oxidatively pre-treated samples, the temperature ramps were performed twice to check for irreversible changes during the first ramp (e.g. exsolution, deactivation). Comparing the catalytic performance directly to known materials is difficult, as quantities that could be used for such comparisons strongly depend on the experimental setup: In literature, the catalytic activity is often quantified by the conversion (a comprehensive summary of different catalysts used for DRM for a variety of different conditions and setups is given in Ref. [4]); however values one gets for conversion vary heavily with experimental parameters (like weight of the catalyst, active surface, space time velocities…). Another quantity commonly used is the Turn Over Frequency (TOF). For this method to be applicable, however, the surface structure and number of active centres have to be known very well and should not change during the reaction [31]. As our perovskite-type oxide catalysts are dynamic systems, the number of active centres is changing under reaction conditions. Especially when exsolution occurs, it is not straightforward to determine TOF values. To account for this fact, the catalysts were compared with respect to their specific activity as outlined in previous work [19,32]. This means that the produced amount of CO was normalised to the gas flow and the surface area of the catalyst. The respective BET areas and a detailed explanation of how the calculation of the specific activity was performed can be found in the SI. For a series of high temperature measurements, a different setup was employed. A Pfeiffer PrismaPro QMG 250 mass spectrometer was used to monitor the composition. The catalysts were again pre-treated by oxidation prior to the catalytic reaction (20 mL min−1 O2, 600 °C, 30 min), which was followed by a reducing pre-treatment at the respective temperature specified above (20 mL min−1 , H2/H2O of ~32:1, 60 min). Afterwards, the catalyst was cooled to 400 °C in Ar and the gas atmosphere was switched to the reaction mixture. In contrast to the other catalytic experiments, the gas flow was four times higher to account for the increased activities of the catalysts at higher temperatures. For these high temperature experiments, the temperature was raised from 400 °C to 950 °C with a rate of 2 °C min−1.Two sets of experiments were performed as catalytic tests for each catalyst: During the first set, measurements were performed immediately after an oxidising pre-treatment as described in the experimental section – samples treated that way will be designated “oxidised”. For the second set, an additional reducing pre-treatment step before measurements was conducted (the corresponding samples are labelled “reduced”). These different catalysts pre-treatments led to two different starting conditions: either a fully oxidised perovskite surface or a reduced material with oxygen vacancies and exsolved nanoparticles embedded in the parent oxide. In the first case (only oxidising pre-treatment), the possibility of in-situ exsolution at higher DRM reaction temperatures still exists, which may lead to an increase in catalytic performance. In case of the latter experiment, exsolution already occurred during pre-reduction in H2/H2O, and the formed nanoparticles are present throughout the whole catalytic measurement.For the B-site undoped sample (Nd0.6Ca0.4FeO3-δ), the catalytic results for the two experiments are shown in Figs. S1–S3 (see SI). Reduction prior to the test clearly led to an earlier onset of CO formation, with the start of CO formation being already observable at 400 °C. In contrast, the oxidised sample exhibited CO production only above 500 °C. Moreover, for the reduced sample, the CO concentration reached a first local maximum at 450 °C, before an intermediate drop. It started to rise again around 520 °C with the CO concentration constantly increasing. Interestingly, the H2 concentration was not rising during the first CO production peak. H2 was only produced at the highest reaction temperatures. This means, that two different processes for CO formation occur: (i) The first CO formation peak at low temperatures took place as a consequence of the reductive pre-treatment, during which oxygen vacancies were formed in the perovskite lattice. CO2 can react with those vacancies – refilling them while releasing CO in the process. (ii) The second increase of the CO concentration (at higher temperatures), which also was observed in the oxidised sample, was accompanied by H2 production. This means that at higher temperatures DRM took place. The first CO formation peak at low temperatures took place as a consequence of the reductive pre-treatment, during which oxygen vacancies were formed in the perovskite lattice. CO2 can react with those vacancies – refilling them while releasing CO in the process.The second increase of the CO concentration (at higher temperatures), which also was observed in the oxidised sample, was accompanied by H2 production. This means that at higher temperatures DRM took place.However, as can be seen in both Fig. S1a and d in the SI, the H2/CO ratio was not reaching the theoretical value of 1, indicating side reactions (as discussed in the introduction). The main reason for a value below 1 is that the competing rWGS reaction dominates at lower to medium temperatures. As shown in previous works, perovskite-type oxides can also be efficient for rWGS reaction [19]. The SEM images (SI, Fig. S3) taken after the catalytic reactions did not display any metallic nanoparticles for both experiments, but in case of the pre-reduced sample, particles with a diameter of more than 100 nm are visible on the surface, which were identified as CaCO3. As highlighted in previous works [19], the investigated materials can form CaCO3 particles at higher temperatures under CO2-rich reaction conditions. This CaCO3 phase could also be confirmed by operando XRD experiments, as discussed in Section 3.3 below. In Fig. S4 a SEM image with visible CaCO3 crystallites is shown, and additional EDX analysis confirms their chemical nature. This formed CaCO3 phase blocks catalytically active sites and, therefore, leads to deactivation of the catalyst. The difference in activity at 700 °C between the two pre-treatments was also significant as the activity increased roughly five-fold (Fig. S2a).For the Co-doped catalyst Nd0.6Ca0.4Fe0.9Co0.1O3-δ, a clear activation effect of the reductive pre-treatment could be confirmed as well. CO formation started already at 400 °C with the reduced Co-doped catalyst, while in case of the oxidised perovskite the CO formation onset was at 560 °C (SI, Fig. S1b and e). The two consecutive runs for the oxidised Co-doped perovskite are nearly identical (SI, Fig. S2b), with only a slight decrease (83 % of the first run) in activity at 700 °C. A slight deactivation effect may be connected to the formation of CaCO3 and CaO as seen in the operando XRD measurements (see Section 3.3) and in the SEM image with EDX analysis (SI, Fig. S4). The activity of the reduced sample was about one order of magnitude higher. The SEM images taken after the catalytic reaction (SI, Figs. S3a and S3d) also show a clear difference between the samples. Nanoparticles with a diameter of around 33 nm could be found on the surface of the reduced sample, while on the oxidised sample no nanoparticles could be observed.In Fig. 2, the catalytic results for Nd0.6Ca0.4Fe0.97Ni0.03O3-δ are displayed. The onset of CO production occurred at approximately 500 °C for the reduced perovskite, while the oxidised sample showed CO formation only above 550 °C. In case of the oxidised sample, a small initial increase of the CO production could be observed around 550 °C, followed by a drop between 570 °C and 590 °C. This resembles the behaviour of the undoped sample; however, in this case no pre-formed oxygen vacancies were present. In this case it is a side effect of the onset of in-situ nanoparticle exsolution as explained in detail in the SI. Regarding the selectivity of the Ni-doped samples (Fig. S1c and f, SI), predominantly CO formation by rWGS was observed at low temperatures. Onset of significant H2 formation via DRM was observable only at higher reaction temperatures (see also the explanation presented in Section 3.2).The in-situ formation of Ni-nanoparticles could also be confirmed via SEM images taken after the catalytic experiments. In Fig. 2b, nanoparticles are shown after the runs with an oxidic pre-treatment. In contrast, the nanoparticles for the pre-reduced catalyst are substantially bigger (Fig. 2a). Their mean diameter (26 nm) was around twice the size of the particles on the oxidised catalyst (14 nm). The exact distribution of the nanoparticles is displayed in Fig. S5 (SI). The sample with bigger nanoparticles exhibited an increase in catalytic activity. These bigger nanoparticles appear to be beneficial for the activation of methane.When comparing the three catalysts after application of reducing pre-treatments, it becomes apparent that the Ni-doped perovskite performed best with a specific activity for CO of 1.5∙10−6 mol s−1 m−2, while the Co-doped catalyst only showed an activity of 0.75∙10−6 mol s−1 m−2. The undoped sample exhibited an activity of only 0.37∙10−6 mol s−1 m−2. The measurements are compared in Fig. S7 (SI).The tested perovskite catalysts exhibit significant activity with respect to the rWGS reaction at intermediate temperatures (~500–700 °C), a known competitive side reaction of DRM [19]. This causes a shift of the product ratio within this temperature range from an equal distribution to a CO excess of about 10:1 in the present study. It is, however, known that rWGS becomes less dominant for DRM applications above 800 °C [1]. As the main intention of this study was to examine the exsolution behaviour and its impact on the DRM activity, the initial focus was put on the intermediate temperature region. To get more insights into the DRM capabilities of the investigated perovskite catalysts, a second set of catalytic reactions up to 950 °C was conducted to check if the H2/CO ratio of the product gas stream increases at higher temperatures. An overview of the results of these high temperature measurements is given in Fig. 3.During the high temperature measurements, onset of DRM occurred later than in the above-mentioned catalytic tests. This delayed onset was caused by the lower sensitivity of the MS towards low concentrations compared to the micro-GC. Also, the higher flow rates used for these experiments led to lower overall conversion. After onset of DRM, the H2/CO ratio increased steadily up to a value of around 0.5 in case of the undoped and Ni-doped samples, and it even exceeded 0.6 for the Co-doped catalyst at 950 °C. The potential to tune the desired H2/CO ratio by varying the composition of the perovskite lattice becomes evident. Powder XRD measurements after the reactions were performed, which confirmed that the perovskite structure was intact even after the high temperature reactions (Fig. S9, SI). Additionally, a SEM image of the used Ni-doped catalyst (Fig. 3b) shows particles with an average diameter of 60.9 nm, which is larger than in the sample that was exclusively used at lower temperatures (26 nm, cf. Fig. 2a). An EDX mapping (Fig. 3c, possible due to the larger particle size) revealed that Ni is accumulated within the particles, supporting their Ni-rich composition. It should be noted that the achieved H2/CO ratio is still not as high as in recent literature (e.g. from Ignacio de Garcia et al. [33]). The reason for this lies in the pronounced oxygen vacancy formation and stabilisation of our perovskite oxide samples which results in a high rWGS activity and therefore decreases the H2/CO ratio [19].To investigate structural changes that occur during DRM and upon in-situ exsolution, operando XRD measurements were performed. For each catalyst, experiments with both pre-formed nanoparticles and in-situ exsolution were conducted. For the undoped catalyst (Nd0.6Ca0.4FeO3-δ), the perovskite lattice structure ( Figs. 4a as well as S11, SI) was preserved even at high temperatures and even if a reducing pre-treatment in wet H2 was applied before reaction. This highlights the stability of the perovskite host lattice, which does not decompose under reaction conditions and holds true for the doped samples as well. However, it should be noted that exsolution as well as Ca segregation were observed meaning that the perovskite structure is a dynamic one and does undergo changes, but it remains the predominant phase. This dynamic structure, including its rich oxygen chemistry of the perovskite surface is beneficial for DRM (e.g. by preventing coking). For the undoped sample, metallic Fe formed upon pre-reduction, which was oxidised to Fe3O4 (peak at 35.4°) during DRM at lower temperatures. Between 650 °C and 700 °C, Fe3O4 was transformed back into metallic Fe (peak at 44.2°). For the oxidised sample, the formation of Fe3O4 could be observed above 550 °C, which transitioned into Fe above 650 °C as well. A comparison of the intensities of pre-reduced and oxidised samples indicates that pre-reduction led to more metallic Fe being formed during DRM (noticeable by a stronger Fe metal signal). However, in case of the pre-reduced sample, also a stronger CaCO3 peak (29.4°) was observed which transformed into CaO (36.8°) above 650 °C. This may be a consequence of the more pronounced Fe exsolution; however, due to the substantially higher activity observed on the pre-reduced catalysts, a significant loss of active perovskite surface due to Ca surface segregation can be compensated. Additionally, a small diffraction peak corresponding to graphite formation was detected at 26.3° with no significant difference between the two experiments. Fig. S12 summarises results for the Co-doped perovskite (Nd0.6Ca0.4Fe0.9Co0.1O3-δ) with only oxidative treatment. Exsolution of B-site elements are represented by a diffraction peak forming at 44.5°. This peak can be attributed either to a Co hcp phase or a Fe bcc phase. Alternatively, a mixed bcc phase consisting of both B-site metals is possible [34]. However, alloy formation cannot be assessed with XRD alone, as the diffraction peaks are too close to each other. The second peak of the bcc phase, located at around 64.5°, begins to form at 650 °C, which might indicate additional Fe exsolution taking place. Furthermore, the formation of CaCO3, with a diffraction peak at 29.3°, could be observed starting at 550 °C. The intensity of this signal increased up to 650 °C, but it vanished at 700 °C, were CaCO3 transformed into CaO, shown by the new peak at 31.2°. Even though the second peak of CaO (at 53.5°) overlaps with a perovskite peak, a change of the intensity ratios of the perovskite peaks in this region further indicated formation of CaO. Additionally, a signal corresponding to graphite started forming at 550 °C, thus indicating that at least some carbon deposition occurred. However, as the peak remained small, it can be assumed that the carbon deposition was not severe. This assumption is further supported by the fact that no CNTs could be seen in the SEM images. Alternatively, the formation of a graphite layer around the exsolved nanoparticles would be possible, but since the diffraction peak is not growing, ongoing coking was not observed. The pre-reduced sample showed similar behaviour upon heating (Fig. 4b). Exsolution was visible already at 500 °C and the corresponding diffraction peak continued to grow with rising temperature. Similar to the oxidised catalyst, Co and Fe could not be separated and appear in a combined peak at 44.5°. CaCO3, CaO, and graphite were also observed.In case of the oxidised Ni-doped catalyst (Nd0.6Ca0.4Fe0.97Ni0.03O3-δ, Fig. S13), sharper XRD reflexes were observed, as this sample had to be sintered at higher temperatures to achieve phase purity. In the measurement of the reduced Ni-doped catalyst, the formation of an additional phase similar to a perovskite phase was observed. At 700 °C the perovskite diffraction peaks had shoulders on their left edges (Fig. 4c). This is most likely a Ruddlesden-Popper phase (marked with “RP”). In case of the oxidised sample, peaks corresponding to Fe and Ni appear at 44.8° and 44.2°, respectively, at 650 °C. This indicates in-situ exsolution of the dopant and the B-site cation. The Ni particles (43.5°) on the surface of the pre-reduced sample (exsolved during the reductive pre-treatment) were observed to switch back to an oxidic state upon exposure to the DRM reaction environment at lower temperatures (i.e. the nanoparticles on the surface are oxidised). Only at higher temperatures (700 °C), the metallic state re-emerges due to the reducing reaction environment. Furthermore, a diffraction peak at 44.1° possibly corresponding to Fe is present above 500 ° in the oxidised sample. In contrast to the measurements with Nd0.6Ca0.4FeO3-δ, neither CaCO3 nor CaO could be observed. In both Ni-doped samples, graphite was observed in small amounts.Using refinement techniques, the peak widths of the undoped and the Co-doped samples were analysed. This is related to crystallite sizes, however only relative trends are reported here, as reasonable absolute quantification would require further knowledge about broadening caused by the XRD instrument. The Ni-doped perovskite (which was additionally sintered during synthesis) exhibited very narrow diffraction peaks in the XRD data (visible broadening is mainly caused by the XRD setup itself), indicating larger perovskite crystallites and high order. The broadening of the perovskite phase itself was similar (and much more pronounced than in the case of Ni-doping) for the undoped and Co-doped samples. In both cases, the catalytic reaction led to a narrowing of the perovskite phase peaks. This indicates that the pristine samples consist of relatively small perovskite crystallites, but the morphology slowly changes towards bigger and less imperfect crystallites at high reaction temperatures. Furthermore, the peak width of the metallic phase in the Co-doped sample confirms that pre-reduction led to larger crystallites (indicated by a narrower peak) – this agrees with the SEM results.To further investigate the changes of the catalyst surfaces during DRM reactions, in-situ NAP-XPS experiments were performed. Special attention was paid to the chemical state of the elements on the B-site of the catalyst as the formation of a metallic B-site species in XPS spectra is an indicator for exsolution.The spectroscopic results for the B-site elements for the oxidised samples are summarised in Fig. 5 (the full series is shown in the SI, Figs. S14–S16). In case of the undoped Nd0.6Ca0.4FeO3-δ, no formation of a metallic phase occurred even at 700 °C. The same experiment with the corresponding pre-reduced catalyst revealed that even at 700 °C under DRM condition or in H2/H2O atmosphere no Fe exsolution could be triggered (Fig. S14d).For the oxidised Co-doped catalyst Nd0.6Ca0.4Fe0.9Co0.1O3-δ, the Co2p3/2 peak exhibited a gradual shift into a metallic state (from 780.2 eV to 777.5 eV [29], Fig. S15a). First indications of a metallic Co2p contribution, corresponding to in-situ exsolution, were found at 500 °C. The amount of metallic species was increasing with the temperature up to 700 °C in DRM gas atmosphere. The in-situ pre-reduction of the catalyst revealed that metallic Co formed on the sample surface during the reduction step. When switching to DRM conditions at 400 °C, however, the Co turned oxidic again. This indicates that the formed nanoparticles are present in an oxidic form at lower temperatures. They are transformed back into a metallic state between 550 °C and 600 °C (Fig. S15b). Interestingly, the ratio of metallic to oxidic Co is significantly larger than in the oxidised sample. This could suggest that the reductive pre-treatment led to the formation of bigger or more nanoparticles on the surface as already shown above (Fig. 2).For the oxidised Ni-doped catalyst, a shift of the Ni2p signal was observed from the oxidic state at 854.7 eV into a metallic state at 852.2 eV between 600 °C and 650 °C (Fig. S16a) [35]. The in-situ measurement on the related pre-reduced catalyst demonstrated that the reduction step led to the formation of metallic Ni (Fig. S16b). Switching to DMR conditions at 400 °C caused the re-oxidation of the exsolved nanoparticles. Above 600 °C, the nanoparticles became metallic again. The Fe2p signal does not show the formation of metallic Fe during the course of any experiment (Fig. S14). This is a discrepancy to the results of the XRD measurements. Possible reasons for this are discussed in the summary.The different behaviour between the Ni- and Co-doped catalysts, respectively, (in the Ni2p and Co2p spectra a fast switch to the metallic state within one temperature step was observed for the Ni-doped sample, while the formation of Co metal occurred more gradually in the Co-doped material) could be explained by the amount of B-site doping. Whereas the amount of Co-doping was 10 %, only 3 % Ni could successfully be incorporated into the perovskite. The Ni reservoir in the sub-surface region is therefore much smaller. Thus, Ni depletes much faster upon exsolution. Moreover, remaining trace amounts of Ni within the host lattice may be below the detection limit of XPS. In case of Co-doping, the larger reservoir allows for a more gradual growth of the nanoparticles with increasing reaction temperature. The importance of the concentration of the exsolving element for exsolution properties was already proven by Gao et al. [36]. As no metallic Fe could be detected even at 700 °C, the measurements indicate that the formed nanoparticles are indeed pure Ni and Co, respectively. TPR results (shown in the SI) support that there is a temperature window, where solely the more easily reducible dopant element is reduced, while reduction of Fe starts only at even higher temperatures (the exact temperatures depend on the conditions).The key finding of an analysis of the C1s spectra for all materials was the clear absence of dominant coking, as no strong carbon signals occurred throughout the reaction (Fig. S17). The observed carbon species were mostly present at lower temperatures and, in fact, vanished at higher reaction temperatures. According to Dama et al., the NiCx peak, indicating the coking of the nanoparticles, forms at binding energies around 280 eV [14]. This peak was entirely absent during all measurements, and no carbonaceous structures were observed in any post-reaction SEM images, proving that all tested catalysts exhibited improved coking resistance. This is probably a consequence of the rich oxygen surface chemistry of perovskites, which hinders the formation of carbon deposits on the surface. Furthermore, it was reported that alkaline earth metals are promoting the re-oxidation of surface carbon on Ni-based catalysts [37] and Ca is known to be an exceptionally good promoter [14]. Additionally, with respect to NiCx, Dama et al. distinguished between C-C compounds with binding energies around 285 eV and COx compounds with higher binding energies around 290 eV. In both these energy regions, peaks could be observed, however, no intense signal occurred, indicating that only traces of carbon were present on the surface. Concerning the C1s spectra and the carbon surface chemistry, the investigated perovskites show a quite different behaviour:The experiment with the pre-reduced undoped catalyst (Nd0.6Ca0.4FeO3-δ) showed no C1s signals during the pre-treatments. When switching to DRM conditions, four species appeared ( Fig. 6a). At ~292.5 eV, the gas phase peak becomes visible. This signal grew weaker as the temperature increased. This indicates that the CO2 near the surface was reacting. The CO3 2- species (289.2 eV) is present above 400 °C up to 700 °C. Its amount decreased with rising temperature as well. Between 285.5 eV and 286.0 eV, adventitious carbon is present; however, the signal decreased upon further heating. The species with the lowest binding energy (283.9 eV) is only visible at the 550 °C and 600 °C steps and corresponds to graphitic carbon.The pre-reduced Co-doped catalysts displayed only two contributions to the C1s signal at lower temperatures (Fig. 6b). The carbonate species at 289.6 eV was present between 400 °C and 550 °C and vanished at higher temperatures. Adventitious carbon could be detected below 550 °C between 284.8 eV and 285.6 eV. Above 600 °C, no carbon signals could be detected.When a reductive pre-treatment was applied to the Ni-doped catalyst, three different carbon species were visible (Fig. 6c). The species at the highest binding energy (around 292.5 eV) could be assigned to the gas phase signal. It was only visible at 400 °C and 500 °C, indicating increased CO2 conversion at higher reaction temperatures. Between 289.3 eV and 289.7 eV, a carbonate species could be observed at the two lowest temperatures. Below 550 °C, adventitious carbon could be detected between 285.1 eV and 285.6 eV.The O1s peak fitting revealed two relevant species ( Fig. 7). The main peak, in case of all three materials between 528.3 and 528.9 eV, can be attributed to lattice oxygen of the perovskites host lattice, as reported in literature [38,39]. The smaller peak at higher binding energies consists of a carbonate component and a hydroxide component. Both of these species are important for the mechanism of DRM [40]. Even though the resolution of the lab-based XPS system is not sufficient to separate them clearly [38], the analysis of the C1s region confirms the presence of carbonate as discussed above. The signal from the C1s spectra can be used to calculate and fit the corresponding O1s contribution. The remaining peak intensity can be assigned to surface hydroxyl groups. The details for this calculation can be found in the SI (Tables S6–S12). It was observed that the carbonate amount is largest below 600 °C in each measurement. This means that surface carbonate at lower temperature most likely derives from adsorbed CO2, which is reacting faster at higher temperatures. Alternatively, the formed CaCO3 is transforming into CaO as observed in the XRD experiments (Section 3.3).In the experiment with the undoped pre-reduced catalyst, the lattice oxygen did not shift under DRM reaction conditions; its position stayed constant at 528.9 eV (Fig. S18d). The hydroxyl component did not shift as well and stayed at 530.4 eV [41]. Interestingly, the signal contribution assigned to the carbonate shifted to higher binding energies during reaction. As the temperature increased, the carbonate species shifted from 531.4 eV to 532.0 eV [42]. A closer look into the carbonate amount contributing to the shoulder of the main oxygen peak reveals that the carbonate amount peaks at 600 °C at 39 % (similar to the maximum amounts found for the oxidised sample as seen in Fig. S18a). In contrast to the latter, there was, however, carbonate present at higher temperatures as well. The carbonate species at lower temperatures, stemming from adsorbed CO2, is most likely reacting similarly to the measurement with the oxidised catalyst. The carbonate remaining at higher temperatures most likely corresponds to CaCO3 which was already observed in the operando XRD measurements.In case of the oxidised Co-doped catalyst, the hydroxyl contribution was constant at 531.0 eV and a carbonate peak was present at 531.5 eV. Interestingly, in contrast to the undoped oxidised catalyst, carbonate is present in all measurements. The amount of carbonate in this sample was the highest for all measured catalysts. At 500 °C, the amount peaked at 64 % before declining again to around 40 % at 700 °C (Table S9). The general trend of the carbonate amount with temperature was similar to the other experiments (i.e. rising at first but declining at higher temperatures). The decrease of the amount of carbonate coincided with the temperature at which metallic Co could be observed in the measurements of the oxidised catalyst (Fig. 4). However, the Co-doped catalyst appears to exhibit a tendency to form more CaCO3, as the carbonate amount on the surface was also significant at higher temperatures. During the measurement with the pre-reduced catalyst, the hydroxyl component shifted from 530.5 eV to 531.1 eV. The carbonate species was only present up to 550 °C (at 531.5 eV). As seen in Table S10 (SI), the carbonate amount remained constant at around 47 % between 400 °C and 550 °C. At higher temperatures, the carbonate amount dropped to 0, as no carbonate could be observed in the C1s region. One has to keep in mind that the areas fitted for the C1s of carbonate were already very low, the decrease of surface carbonate (through the start of DRM reaction) led to the signal dropping below the detection limit and made fitting not feasible anymore.In case of the corresponding reduced catalyst with Ni-doping, the reductive pre-treatment caused formation of surface carbonate visible at 400 °C and 500 °C. At these temperatures, the amount of carbonate in the region with high binding energy was nearly 50 % (Fig. S18f). However, at higher temperatures (>550 °C), it completely vanishes due to the increased catalytic activity. The binding energy of the OH- species remain unchanged at 531.0 eV during the whole measurement.The catalytic results confirm that reductive pre-treatment increased the catalytic activity significantly for all three perovskite catalysts. Compared to samples with only an oxidation step, SEM images revealed that in case of B-site doped pre-reduced perovskites the exsolved nanoparticles were significantly bigger on average, e.g. 14.1 nm compared to 25.5 nm in case of the Ni-doped perovskite, thus providing a possible explanation for the increased activity. This clearly shows that to achieve enhanced catalytic activity, pre-reduction is necessary. The Ni-doped perovskite exhibited the highest catalytic activity of all tested materials after nanoparticle exsolution. A pronounced rWGS activity could be observed at intermediate reaction temperatures. The selectivity shifts towards the desired DRM reaction only at high reaction temperatures above 800 °C. For the Ni- and Co-doped perovskites, a H2/CO ratio of 0.5 and 0.6, respectively, could be observed at 950 °C. Kapokova et al. observed similar behaviour in their studies [43]. They reported an increase in the H2/CO ratio with rising temperature. However, the increase in H2 formation in that work was not as high as in our case, most likely due to the fact that perovskites used in their studies are extremely active for the rWGS reaction [19].XRD and operando XRD measurements confirmed host lattice stability even at the highest reaction temperatures (950 °C) for all materials and experiments. Deactivating phases such as CaCO3 and graphite were observed, but no severe coking and CNT formation as SEM images revealed. The B-site doped catalysts exhibited in-situ exsolution under DRM conditions – the Co-doped sample showed an exsolution onset of 550 °C, exsolution on the Ni-doped material started above 600 °C.Comparing operando XRD and in-situ NAP-XPS results, some discrepancies stand out at first glance. In the XRD results, the undoped catalyst exhibited formation of metallic Fe which could not be observed in the NAP-XPS studies. Moreover, the XRD measurements show a graphitic phase at high reaction temperatures which could not be observed in any XPS spectra. An explanation for this deviation could be the difference in the respective operating pressures: While operando XRD was performed at ambient pressure (1 bar), only a pressure of 1 mbar was accessible in case of NAP-XPS. This, of course, leads to different reaction rates on the surface. The pressure dependence of the chemical potential of the reaction environment plays a role as well. Additionally, more educt is reacting on the surface during operando XRD measurements due to larger partial pressures. This increasing reaction rate in turn leads to different rates at which oxygen vacancies are formed and side products (such as carbonaceous species) are deposited on the surface. The increased rate of the oxygen vacancy formation can, of course, cause preferred exsolution of B-site elements as a way to increase stability of the material [45]. Another difference between operando XRD and in-situ NAP-XPS is the sensitivity of the respective method. While XRD is a bulk method, XPS only probes surfaces. As it is also possible for nanoparticles to form at grain boundaries within the bulk, more exsolved metal can potentially be detected with XRD. Furthermore, two different types of samples were used for the different methods. While powders were used directly for the XRD measurements, thin film samples had to be prepared with PLD for the XPS measurements.The graphical scheme in Fig. 8 summarises the generally accepted parts of the DRM mechanism. The scheme depicts the steps of H2 and CO2 activation as well as the hydrogen spill-over. The most complex and not yet fully understood step, the oxidation of the intermediates, is not shown. The formed COx species constitute the catalytically active carbonate species observed in the XPS measurements at lower temperatures. As mentioned above, the most complex part of the reaction mechanism is the reaction of the COx groups with either OH or adsorbed hydrogen to form CHxOH, which is subsequently oxidised and desorbs as CO and H2. For this step, a lot of different pathways can be found in the literature [20]; however, no fully agreed upon pathway has been established. Based on our findings, we propose a switch of the reaction mechanism depending on the temperature – the main property affected by this switching is the behaviour of the adsorbed hydrogen: At temperatures below 600 °C, we observed a low H2/CO ratio, which indicates that hydrogen is not recombining to H2, but is instead spilling over to the oxide where it forms OH groups with lattice oxygen atoms. These OH groups on the surface can form water and subsequently desorb from the surface, leaving behind an oxygen vacancy that can be refilled by an adsorbed CO2. This pathway is also shown in Fig. 1 as the “rWGS-type pathway” due to its similarity to the rWGS reaction – albeit with a different source for the adsorbed hydrogen. At higher temperatures, we observed an increase of the H2/CO ratio which suggests a change in the system. The NAP-XPS analysis also revealed that the OH contribution in the O1s region is decreasing with rising temperature. This supports the assumption that the adsorbed hydrogen is recombining directly on the nanoparticles instead of spilling over to the support. This pathway is depicted in Fig. 1 as “DRM-type pathway”.The correlation between in-situ nanoparticle exsolution during DRM and exsolution by pre-reduction in wet H2, respectively, and the subsequent catalytic performance was studied for three different perovskites: Nd0.6Ca0.4FeO3-δ (B-site undoped), Nd0.6Ca0.4Fe0.9Co0.1O3-δ (B-site Co-doped) and Nd0.6Ca0.4Fe0.97Ni0.03O3-δ (B-site Ni-doped). Although both exsolution scenarios led to the formation of Co or Ni nanoparticles on the surface of the B-site doped materials, the catalytic results show that nanoparticle exsolution by pre-reduction is enhancing the surface activity significantly more compared to in-situ exsolution. SEM investigations and determinations of particle size distributions reveal a possible reason for this performance difference: Nanoparticles formed during the reducing pre-treatment were bigger on average than their counterparts that were exsolved in-situ. At intermediate reaction temperatures, rWGS is a significant side reaction leading to reduced H2 production. At high temperatures, the selectivity changes and DRM is the dominant pathway leading to an obtained H2/CO ratio of 0.5 and 0.6 for the Ni- and Co-doped perovskites, respectively. These results correlate with the observed amount of hydroxyl groups on the perovskite surface (formed by H-spill over) by NAP-XPS. Of all tested perovskites, the Ni-doped catalysts showed the highest total activity.In addition to catalytic tests, operando XRD and in-situ NAP-XPS measurements were performed during DRM. The host perovskite lattice was stable up to the highest reaction temperatures for all tested materials. Metallic phases, corresponding to the exsolved nanoparticles, could be detected by both methods in case of the B-site doped catalysts. At high reaction temperatures, the formation of trace amounts of CaCO3 and graphite was observed. Both processes are undesired, as they can lead to surface deactivation. Interestingly, no formation of carbon nanotubes or big amounts of carbon deposits could be observed in case of the Ni-doped catalyst. The rich oxygen chemistry of the perovskite is a likely reason, as it facilitates effective removal of undesired carbon species, as observed by NAP-XPS.This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement n° 755744 / ERC - Starting Grant TUCAS). Florian Schrenk: Conceptualization, Investigation, Formal analysis, Validation, Writing – original draft, Writing – review & editing. Lorenz Lindenthal: Investigation, Formal analysis, Visualization, Writing – review & editing. Hedda Drexler: Investigation, Formal analysis, Software. Gabriele Urban: Investigation, Formal analysis, Raffael Rameshan: Data curation, Investigation, Formal analysis. Harald Summerer: Investigation, Formal analysis, Resources. Tobias Berger: Investigation, Formal analysis, Software. Thomas Ruh: Data curation, Validation, Writing – review & editing. Alexander K. Opitz: Supervision, Validation, Writing – review & editing. Christoph Rameshan: Conceptualization, Funding acquisition, Project administration, Supervision, Writing – original draft, Writing – review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The X-ray measurements were carried out within the X-Ray Center of TU Wien; SEM images were recorded at the USTEM, TU Wien. The authors acknowledge TU Wien Bibliothek for financial support through its Open Access Funding Programme.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcatb.2022.121886. Supplementary material .

Nanoparticle exsolution is regarded as a promising alternative to classical catalyst synthesis routes. In this work, we compare the catalytic performance of nanoparticles formed by in-situ exsolution during dry reforming of methane with particles pre-formed by reductive pre-treatment. The experiments were conducted on three perovskite-type oxides. Using a combination of in-situ and operando spectroscopic investigations (x-ray diffraction, near ambient pressure x-ray photoelectron spectroscopy) and the correlation to the obtained catalytic results, we could highlight that pre-formed nanoparticles strongly enhance the activity compared to in-situ exsolution. Scanning electron microscope images recorded after catalytic tests revealed that nanoparticles formed during reductive pre-treatment are bigger on average than particles formed in-situ. Furthermore, B-site doping with Co or Ni significantly enhanced the catalytic activity. Importantly, the perovskite host lattice was stable in all experiments, thus providing the necessary enhanced oxygen surface chemistry which is the key to the coking resistance of the investigated materials. Additionally, we observe a temperature dependent change of mechanism leading to different product ratios.

An attempt has been made to investigate and optimize the recovery of Ni and Al through sulphuric acid (3.0−5.5 mol/L) leaching under different operating conditions. From the leaching experiments, it was possible to extract 98.5% of NiO and 40.7% of Al2O3 under the conditions of 5.5 mol/L H2SO4, reaction time of 4 h, solid-to-liquid ratio 0.2 g/mL, temperature of 358 K, particle size <100 μm, 200−250 r/min with 5.0 g catalyst dosage. The leached liquor Al was separated by selective crystallization using 1.4 mol/L KOH and Ni was separated by selective precipitation using 0.3 mol/L H2C2O4. From the studies, it is possible to recover around 97.9% of NiO having 98.3% purity, around 25% of Al2O3 was also recovered as alum-(K) having 99% purity and 14.7% of Al2O3 as a salt of Al−K−C2O4−SO4.Sulphuric acid was found to be a suitable leaching agent for selective leaching and it was also observed that alum-(K) can be selectively crystallized from sulphate solutions. The study also indicated the effective extraction and recovery of nickel and aluminium which were well supported by characterization studies using TG-DTA/DTG and XRD techniques.

In recent decades, stringent environmental regulations have strictly limited the sulfur contents in engine fuels to below 10 µg•g−1 with increasing global concerns about the contamination of the eco-environment. Organosulfur compounds in engine fuels have been confirmed to be one of the main sources of eco-environmental contaminants since sulfur dioxide, which damages both the eco-environment and human health. These compounds are emitted after combustion in engines [27,46,16,3,71]. Thus, it becomes a significant challenge for oil refineries to produce road engine fuels with ultralow or nearly zero sulfur content. Various efforts have been made to develop desulfurization techniques, such as oxidation desulfurization, adsorption desulfurization, biodesulfurization and hydrodesulfurization (HDS). Among all these techniques, HDS has been concretely confirmed to be the most efficient, economical and practical desulfurization technique [6,26,56,49,25]. After decades of development, the efficient design of HDS catalysts with high catalytic performances has been confirmed as the core of the HDS technique. The most successful commercial HDS catalyst is bimetallic Ni(Co)Mo(W)S2 supported on γ-Al2O3 due to its relatively low cost and comparable activities to noble metal supported catalysts [47,32,52,19,51]. However, with the increasing contents of highly refractory sulfides derived from dibenzothiophene in the harmonic blends, the activities of these kinds of catalysts cannot easily meet the requirements for the production of road engine fuels with ultralow sulfur contents [8,50].Massive literatures have demonstrated that the higher homologues of dibenzenthiophene, especially sulfides similar with 4,6-dimethyldibenzothiophene(4,6-DMDBT), in road engine fuels are the most difficult to remove [21,17]. Thus, the effective removal of sulfur atoms from this kind of sulfide is the bottleneck for the improving the performances of HDS catalysts [34,70]. 4,6-DMDBT can undergo HDS via either direct desulfurization (DDS) pathway or the hydrogenation desulfurization (HYDS) pathway. Because the methyl groups severely restricts the DDS pathway due to the sterically hindering of the C-S-C bond in the thiophene ring adsorbing on the active sites. and the latter one has been acknowledged as the main pathway. the methyl groups located at the 4- and 6-positions sterically hindering the C-S-C bond, which severely [7,38,11]. Thus, efforts should be made to improve the hydrogenation activity of HDS catalysts to further enhance their catalytic performance.Our previous experimental work and the density functional theory (DFT) calculation results have already proven that the selectivity for both the DDS pathway and the HYDS pathway are highly correlated to the morphology of the active sites [12,45,69,68]. We found that the activity and selectivity of the DDS pathway correlates to the coordination unsaturated Mo sites (CUS) from the corner of the NiMoS crystals, and the activity and selectivity of the HYDS pathway correlates to the CUS from the edge sites of the NiMoS crystals. The formation of both corner CUS and edge CUS is affected by the composition and structure of support materials because the morphology of the NiMoS2 slabs can be modulated by the MSI [2,5]. For this purpose, the modification of γ-Al2O3 surfaces by other oxides, metal oxides and nonmetal oxides has been widely investigated. Although the incorporated modifier could greatly disperse on the γ-Al2O3 surfaces at the initial stage, it agglomerates after a long period of usage, which would lead to a quick deactivation of the corresponding HDS catalysts. In situ synthesized binary oxides, such as Ga2O3-Al2O3 [10,24], SiO2-Al2O3 [9,55,57,54], ZrO2-Al2O3 [10], MgO-Al2O3 [53], B2O3-Al2O3 [43,33] and TiO2-Al2O3 [37,40], have been explored as HDS catalyst supports to overcome this problem. Among these investigated binary oxides, TiO2-Al2O3 attracts the most scientific attention due to its improved hydrogen transfer capacity caused by the existence of Ti3+ species, which results in the improved hydrogenation activity of HDS catalysts. V. Santes [20] prepared TiO2-Al2O3 binary oxides via three different preparation methods and found that the sol-gel method provides the highest surface area and excellent pore structures. The mesopores of the prepared TiO2-Al2O3 binary oxides is disordered, and its pore size distribution is relatively wide. Duan and coauthors [13] also successfully synthesized TiO2-Al2O3 composites which were used as HDS catalyst support. They proposed that the incorporated TiO2 weakened the MSIs of a NiW supported catalyst since less strong Al-O-W linkages were formed. Morris and coauthors [36] proposed a solvent evaporation-induced self-assembly strategy to synthesize TiO2-Al2O3 composites with highly ordered mesopores. Recently, we also synthesized TiO2-Al2O3 composites with highly ordered mesostructures and used them as HDS catalyst support to uncover the effect of the incorporation of Ti species on the NiMo/TiO2-Al2O3 catalyst [72]. All these studies have established that the type of active phase and the dispersion, morphology, MSIs and sulfidation degrees are closely related to the incorporated TiO2 species. However, few literatures have shed light on the effect of solvent evaporation temperature on the surface states of highly ordered mesoporous TiO2-Al2O3 composites, and the effect of solvent evaporation temperature on the catalytic performance of highly refractory sulfides with the corresponding HDS catalysts has not been published.Here, we report the strategy of synthesizing highly ordered TiO2-Al2O3 composites at different solvent evaporation temperatures to test the hypothesis that the synthesis temperature influences the surface states of TiO2-Al2O3 composites and the corresponding NiMo supported catalysts as well as their HDS performance. Several advanced characterization techniques were performed on both the synthesized TiO2-Al2O3 binary oxides and the corresponding catalysts. Finally, the catalytic performance of the catalysts were assessed.Highly ordered TiO2-Al2O3 with TiO2 content uniformed at 20 wt.% were synthesized at different solvent evaporation temperatures according to a reported method [1,61,65]. Here is an example: 1.76 g of aluminum isopropoxide (99.8%, Rhawn) was precisely weighed and dissolved in 10.0 mL of anhydrous ethanol, dropwise addition of 1.6 mL of fuming HNO3 (68.0%, Xi'an Sanpu Fine Chemical Co. Ltd.) was followed. Simultaneously, 2.00 g of P123 (98.0%, Sigma–Aldrich) was dissolved in 10.0 mL of anhydrous ethanol, this solution was added dropwise into the flask within 5 min. After suspended for 4 h, 0.47 g of titanium tetraisopropanolate (98.0%, Aladdin) was added within 5 min. The mixture was transferred to a porcelain vessel after being stewed for 4 h, which was then placed into an oven to evaporate the solvent at the required solvent evaporating temperature for 2 days. Finally, the obtained powder was dehydrated in a 120 °C oven and calcined at 550 °C in a muffle furnace. Such obtained products were denoted TA-x, where x is the solvent evaporation temperature in Celsius with values of 50, 60, 70 or 80.The Ni and Mo precursors were loaded via the proposed incipient wetness coimpregnation method [36,52,71,72]. The pelleted and crushed grains between 20 and 40 mesh were collected. Then, the coimpregnation solution containing both the Ni precursors and Mo precursors was impregnated onto the aforementioned grains. The prepared samples were dehydrated in a dry-air flow overnight at room temperature and dried at 120 °C for 6 h before it was calcined at 550 °C in a muffle furnace. The prepared catalysts with uniform NiO loadings of 4 wt.% and MoO3 loadings of 12 wt.% were denoted NiMo/TA-x.Bruker D8 Advance powder diffractometer was used for performing both the wide-angle and small-angle XRD characterization of the synthesized TA-x serial composites. For all the tested samples, small angle XRD patterns with 2θ values from 0.5° to 6° were recorded at scanning rate of 0.02°s−1, and wide angle XRD patterns with 2θ values from 20° to 80° were also recorded at scanning rate of 0.5°s−1. The N2 physical adsorption-desorption characterization of the synthesized TA-x serial composites was performed on a Micromeritics ASAP 2020 volumetric analyzer. Prior to the test, the samples were completely degassed at 400 °C for 12 h at vacuum. The BET method was employed to calculate the specific surface areas and the BJH method was employed to determine the pore sizes distribution. A JEOL JEM-2100 instrument, whose acceleration voltage is 200 kV, was used to observe the mesopore arrangement of the synthesized TA-x serial samples, and images were taken. An Magna 560 FT-IR instrument was used to perform the pyridine absorbed FTIR characterizations of the synthesized TA-x serial composites. The tested TA-x sample (0.1 g) was completely degassed for no less than 4 h under vacuum conditions at 350 °C, after cooled to 50 °C, the saturated pyridine vapor was pulsed for 30 min. The FTIR spectra were recorded after the pyridine desorbed at 200 °C for 0.5 h. Then, the pyridine was further desorbed at 350 °C for another 0.5 h, and the FTIR spectra were recorded again.The prepared NiMo/TA-x serial oxide catalysts were temperature programmed and reduced by H2 on a self-built device. The pretreated catalyst (0.3 g) was loaded onto a quartz tube and the reduction gas (5 v% H2 loaded by 95 v% N2) was pulsed into the quartz tube with temperature programming to 800 °C at heating rate of 10 °C/min; simultaneously, the H2-TPR profile was recorded on a mass spectrograph. The Fourier transformed Raman (FT-Raman) spectra for the NiMo/TA-x serial catalysts were recorded on a Renishaw inVia Reflex apparatus in the wavenumber range of 300–1400 cm−1 to disclose the effect of the solvent evaporation temperature on the coordination states of the NiMo precursors on the TA-x composite surface.After the catalysts were fully sulfided by CS2 cyclohexane solution with concentration of 2.2 wt.% at 320 °C for no less than 6 h, an FEI Tecnai G2 F20 instrument was used to observe the morphologies of the Ni-promoted MoS2 crystals. Statistical works referred to reported methods were performed based on the HRTEM images of the Ni-promoted MoS2 slabs [18,15,70–72]: (1) L ¯ = ∑ i = 1 n n i l i / ∑ i = 1 n n i (2) N ¯ = ∑ i = 1 n n i N i / ∑ i = 1 n n i (3) D M o = ( Moe + Moc ) / Mot = ∑ i = 1 t 6 ( m i − 1 ) / ∑ i = 1 t ( 3 m i 2 − 3 m i + 1 ) (4) f M o e = Moe / Mot = ∑ i = 1 t 6 ( m i − 2 ) / ∑ i = 1 t ( 3 m i 2 − 3 m i + 1 ) (5) f M o c = D M o − f M o e Here, L ¯ is the statistical average slab length of MoS2 crystals, li is the length of slab i, ni is the number of MoS2 slabs whose slab length is li; N ¯ is the statistical average stacking number of the MoS2 crystals, and Ni is the stacking number of slab i. Mot, Moe and Moc are the numbers of the total Mo atoms, the edge Mo atoms and the corner Mo atoms, respectively. mi is calculated from the slab length (L = 3.2(2mi - 1) Å). XPS characterizations of the sulfide NiMo/TA-x serial catalysts were performed on a PHI-5000 Versaprobe III spectrometer whose radio source was Al Kα radiation, and the C 1s peak with a binding energy of 284.6 eV was used to calibrate the binding energy (BE) scales. Both the recorded Mo 3d XPS spectra and the recorded Ni 2p XPS peaks were differentiated to check the effects of the solvent evaporation temperature on the covalent states of both Mo and Ni species as well as on the formation of NiMoS phases over the investigated sulfide catalysts.A model oil containing 4,6-DMDBT was used to assess the effect of the solvent evaporation temperature on the catalytic performance of the prepared NiMo/TA-x serial catalysts. Prior to the test, the catalysts were completely dehydrated at a temperature of 200 °C in an oven for no less than 6 h. Then, precisely weighed 1.0 g of the dehydrated catalyst was loaded into the reactor with both ends sealed by quartz sand. The temperature of the reactor was increased to 320 °C, and the sulfidation solution composed of 97.8 wt.% cyclohexane and 2.2 wt.% CS2 was pumped into the reactor at a flow rate of 10 mL•h−1 by an Eldex-Optos 1LM micropump. H2 with a pressure of 4 MPa was simultaneously fed to the reactor at a flow rate of 1000 mL•h−1. After complete sulfidation for 5 h, the sulfidation solution was switched with a reaction solution composed of 99.5 wt.% cyclohexane and 0.5 wt.% 4,6-DMDBT, and the reactor temperature was adjusted. The weight hourly space velocity (WHSV) of the reaction solution was adjusted, and the volumetric ratio of H2/oil was fixed at 150. The liquid products were carefully collected after being fully stabilized for no less than 6 h and were off-line analyzed by means of GC–MS. The activation energies and the reaction rate constants for the overall 4,6-DMDBT HDS reaction, DDS pathway and HYDS pathway over the investigated catalysts were calculated by the following equations reported elsewhere [41,18,72]: (6) E a = R T l n ( x 1 / x 2 ) / ( T 1 − T 2 ) (7) k HDS = W · ω · ln ( 1 / 1 − x ) / M (8) k HDS = k H D S · S D D S (9) k HYDS = k HDS − k DDS Here, W is the WHSV counted in h−1. ω, x (x 1, x 2) and M are the concentration, conversion and molar mass of 4,6-DMDBT, respectively. T(T1, T2) is the reaction temperature. SDDS is the product selectivity of the DDS pathway, namely, the selectivity of 3,3′-DMDBT in the products. The turn over frequencies (TOFs) with 4,6-DMDBT conversion lower than 15% were also calculated [39]: (10) C = B × S / ( f × m ) Here, F is the molar flow rate of 4,6-DMDBT, nMo is the molar quantity of the Mo species loaded on the supported catalyst and SMo  is the sulfidation degree of Mo species.The solvent evaporation temperature is normally considered to be a crucial factor for the successful synthesis of TiO2-Al2O3 composites. XRD characterizations of the synthesized TA-x serial composites were performed, and both the wide-angle and the small-angle XRD patterns are displayed in Fig. 1 . Fig. 1A show that there is a broad diffraction peak with a 2θ degree centered at approximately 30°, which is attributed to the (101) facet of γ-Al2O3 for sample TA-50 [72]. This result suggests that the main crystal phase for the synthesized TA-50 sample is γ-Al2O3. There were barely any obvious diffraction peaks observed for the synthesized TA-60, TA-70 and TA-80 samples. These results indicate that the prepared TA-60, TA-70 and TA-80 samples are amorphous TiO2-Al2O3 with TiO2 species highly dispersed in the aluminum oxide phases.The small-angle XRD patterns displayed in Fig. 1B clearly show that there are two different diffraction peaks for the investigated TA-50, TA-60 and TA-70 samples. The diffraction peak with a 2θ degree of approximately 0.75° is assigned to the (100) facet, and the diffraction peak with a 2θ degree of approximately 1.5° is assigned to the (110) facet of the two-dimensional hexagonal P6mm symmetric group [65,36,28,61]. This indicates the existence of ordered two-dimensional hexagonal mesopores in samples TA-50, TA-60 and TA-70. The highest intensity in the diffraction peak centered at 0.75° was observed for sample TA-60, indicating that 60 °C is the most suitable solvent evaporating temperature for the synthesis of highly ordered mesoporous TiO2-Al2O3 binary composites among the four investigated temperatures. The absence of the diffraction peak at 0.75° for sample TA-80 suggests failure in the synthesis of highly ordered TiO2-Al2O3 at a solvent evaporation temperature of 80 °C.The N2 physical adsorption-desorption characterization results of the investigated TA-x serial composites are displayed in Fig. 2 , and the surface areas, the pore volumes and the average pore diameters of the investigated samples are summarized in Table 1 . Fig. 2A clearly shows that the N2 physical adsorption-desorption isotherms exhibit a large H1-type hysteresis loop for samples TA-50 and TA-60. This indicates the existence of abundant, highly ordered cylindrical mesopores with narrow pore distributions for these two samples [59,66,58]. The hysteresis loop for sample TA-70 becomes flatter than those observed for TA-50 and TA-60, indicating a decrease in the amount of mesopores for sample TA-70 compared to samples TA-50 and TA-60. Although the shape of the hysteresis loop for sample TA-70 remained H1 type, the parallelism of the adsorption isotherm and the desorption isotherm declined, suggesting the existence of ordered mesopores and the lower orderliness in the mesopores [67]. The shape of the hysteresis loop for sample TA-80 is observed as H1 type in the low pressure region and H4 type in the high pressure region, suggesting the existence of both ordered cylindrical mesopores and disordered slit mesopores between different particles. Moreover, the relative pressure range of sample TA-80 is much wider than that of the other three samples, suggesting a wider pore size distribution for sample TA-80 than for the other three samples. Fig. 2B clearly shows that the pore size distribution profoundly changed with the evaporation temperature of the solvent. Table 1 shows that the specific surface areas declined in the order of TA-60 (241 m2•g−1) > TA-70 (215 m2•g−1) >TA-50 (214 m2•g−1) > TA-80 (165 m2•g−1), and the pore volumes of the synthesized samples declined in the order of TA-50 (0.41 cm3•g−1) ≈ TA-60 (0.40 cm3•g−1) > TA-80 (0.36 cm3•g−1) > TA-70 (0.30 cm3•g−1). The pore size distribution became narrower, and the calculated average pore diameter decreased with increasing solvent evaporation temperature in the solvent evaporation temperature range of 50 to 70 °C. Although the most likely pore diameters for samples TA-70 and TA-80 are almost the same, the pore size distribution for sample TA-80 became wider, and the calculated average pore diameter increased to approximately 7.6 nm from that of approximately 4.6 nm for sample TA-70. All the observed reversed changing trends between samples TA-70 and TA-80 could be attributed to the formation of disordered slit mesopores between different particles in sample TA-80.The mesopore arrangements of the synthesized TA-x serial samples were investigated via the TEM method, and representative TEM images are displayed in Fig. 3 . It clearly shows that the shape of the mesopores for the synthesized TA-50 sample is hexagonal, and the arrangement of the highly ordered mesopores is honeycomb-like. The observed pore walls became thicker, and the orderliness of the mesopores increased when the solvent evaporation temperature increased from 50 °C to 60 °C. The orderliness of the mesopores for sample TA-70 became much poorer, and the orderliness of the mesopores declined and even became disordered for sample TA-80. Which in line with those observed from XRD and N2 physical adsorption-desorption. All these changes in the morphology of the mesopores can be explained by the following facts. The self-assembly rates for both the Ti4+ species and the Al3+ species by the P123 micellar are relatively slow at 50 °C, which results in a relatively thin pore wall and weak linkage between the TiO2 species and the Al2O3 species. This leads to a wider pore diameter and lower stability of the synthesized TA-50 sample. The acceleration in the self-assembly rate of Ti and Al species with P123 favors the formation of highly ordered TiO2-Al2O3 composites, and thus, the specific surface area increases. On the other hand, the acceleration in the evaporation rate of ethanol favors the deposition of Ti hydroxides and Al hydroxides on the P123 micellar; thus, the thickness of the pore wall increased, and the pore diameter slightly decreased. When the solvent evaporation temperature further increased to 70 °C or even 80 °C, the evaporation rate was too high to efficiently form highly ordered mesopores with relatively high specific surface areas; although the thickness of the mesopores further increased. Moreover, the generated ethanol steam blast partially destroyed a proportion of the formed ordered mesostructures and resulted in a relatively wide pore size distribution; thus, a relatively large calculated average mesopore diameter for the TA-80 sample was observed [65,61,72].It is well accepted that the acidity of the solid is always closely related to the bond linkages between different components, which are often affected by the imposed conditions, such as the solvent evaporation temperature. Thus, Py-FTIR characterization of the TA-x serial composites was performed, and the details of the acidity property changes are displayed in Fig. 4 . The three observed IR bands from Fig. 4 are designated Lewis acid sites (LAS, with a wavenumber of 1453 cm−1), both LAS and Brønsted acid sites (BAS, with a wavenumber of 1490 cm−1) and BAS (with a wavenumber of 1540 cm−1). The amounts of LAS and BAS were calculated according to the reported equation [23,64], and the results are summarized in Table 2 . (11) C = B × S / ( f × m ) Here, C is the concentration of the acid sites, S is the integrated absorbance, B is the surface area of the tested sample, f is the extinction coefficient with values of 3.03 cm•mmol−1 for the calculation of the BAS amount and 3.80 cm•mmol−1 for the calculation of the LAS amount, and m is the mass of the tested sample.The results from Table 2 show that the amount of weak LAS detected for sample TA-60 increased with the solvent evaporating temperature to approximately 82.1 µmol•g−1 from that of approximately 52.0 µmol•g−1 for sample TA-50. It then sharply decreased to approximately 56.3 µmol•g−1 for sample TA-70 with a further increase in the solvent evaporating temperature. A similar variation trend was observed for the amount of strong LAS. The variation trends in the LAS are caused by the fact that the deposition rate of both the Al hydroxide precursors and the Ti hydroxide precursors increased as the solvent evaporating temperature increased from 50 °C to 60 °C, resulting in more naked Al species, which are considered the sources of LAS. With the solvent evaporating temperature further increased to 70 °C, the thickness of the pore wall profoundly increased (proved by the relatively small mesopore diameter for sample TA-70), resulting in fewer Al species located on the surface of the synthesized composite; thus, the detected amounts for both weak and strong LAS declined. For sample TA-80, the detected amount of weak LAS profoundly increased to approximately 74.2 µmol•g−1, and the amount of strong LAS increased to 35.0 µmol•g−1 since the collapse of the mesopore wall formed amorphous TiO2-Al2O3 composites whose acidity property profoundly changed and was totally different from the highly ordered mesoporous TiO2-Al2O3 composites. This can be attributed to the faster deposition of titanium hydroxide and aluminum hydroxide on the P123 micelles, the more Ti-OH-Al groups existing on the surface, and the generation of H+ from the surface Ti-OH-Al groups being the main source of B acid. Thus, the higher solvent evaporating temperature favors the formation of BAS.The effect of the solvent evaporating temperature on the changes in the MSI over the NiMo/TA-x serial catalysts was checked by H2-TPR characterization, and the results are displayed in Fig. 5 .From the recorded H2-TPR profile for sample NiMo/TA-50, four different H2 reduction peaks appeared at different reduction temperatures. The first broad H2 reduction peak with a reduction temperature from 300 °C to 400 °C was assigned to the reduction of NiO species, and the existence of this H2 consumption peak suggests that NiO precursors and MoO3 precursors do not form the so-called active NiMoO precursors very well. The peak at approximately 430 °C represents the H2 consumed by the reduction of Mo(VI) to the Mo(IV) species. The H2 reduction peak at approximately 650 °C was assigned to the reduction of the higher coordinated Mo(VI) species to the lower coordinated Mo(IV) species from the NiMoO precursors that strongly interacted with the Al2O3 and TiO2 species from the support materials. The peak at a reduction temperature higher than 700 °C was assigned to the reduction of the tetra-coordinated Mo(IV) species [14,71,29]. The H2 consumption peak with a reduction temperature lower than 400 °C disappeared with the solvent evaporation temperature higher than 60 °C, suggesting that the higher solvent evaporation temperature favors the high dispersion of NiO precursors into MoO3 precursors. The largest area for the H2 consumption peak with a reduction temperature of approximately 430 °C was observed for catalyst NiMo/TA-60, indicating the highest efficiency for the formation of NiMoO precursors with a solvent evaporating temperature of 60 °C. Both the reduction temperature and the integrated area for the H2 consumption peak at approximately 650 °C did not change much for catalysts NiMo/TA-50 and NiMo/TA-60. This result suggests that the MSIs for these two samples are similar. The integrated area for the H2 consumption peaks of catalysts NiMo/TA-70 and NiMo/TA-80 increased compared to those of catalysts NiMo/TA-50 and NiMo/TA-60, indicating intensified MSIs over catalysts NiMo/TA-70 and NiMo/TA-80. Moreover, the reduction temperature of the H2 consumption peak at approximately 650 °C for catalyst NiMo/TA-80 is relatively lower, which can be attributed to the poorer uniformity between the TiO2 and Al2O3 components caused by the relatively fast deposition rates for the Ti hydroxide precursors and the Al hydroxide precursors.To further confirm the existence states of the Mo species over the oxide NiMo/TA-x serial catalysts, FT-Raman characterization on the prepared catalysts were performed, the recorded spectra are displayed in Fig. 6 . After decomposition, three different vibration peaks were observed, as shown in Fig. 6. The relatively weak vibration peak at a Raman shift of approximately 325 cm−1 and the broad vibration peak at a Raman shift of 850 cm−1 were attributed to MoO4 2− precursors, which is believed to be difficult in transferring into NiMoS active phases [22,63,62]. The peak at approximately 945 cm−1 is assigned to the Mo7O24 6− precursors, which is believed to be easy in transferring into NiMoS active phases [21]. The proportions of the MoO4 2− precursors and the Mo7O24 6− precursors were summarized in Table 3 . The proportion of the Mo7O24 6− precursors increases in the line of NiMo/TA-80 (53%) < NiMo/TA-50 (55%) < NiMo/TA-70 (58%) < NiMo/TA-60 (62%), suggesting that the superior solvent evaporating temperature (with a value of 60 °C) is favorable for the formation of the active precursors, which is highly consistent with the H2-TPR characterization results.The physicochemical properties of the support materials and the oxide precursors affect the morphologies of the resulting NiMoS2 nanoclusters and the covalent states of the active metals of the corresponding sulfide-supported catalysts [35,44]. HRTEM images of the investigated NiMo/TA-x serial catalysts were taken to observe the changes in the morphology of the MoS2 crystals, and the statistical results based on the HRTEM images are listed in Table 4 . Fig. S1 reveals that most of the MoS2 slabs from the investigated NiMo/TA-x serial catalysts were monolayer and bilayer MoS2 slabs with relatively short MoS2 slab lengths, resulting in a relatively high dispersion of Mo species. There is an obvious decline in the MoS2 slab length when the evaporation temperature of the solvent increased from 50 °C to 60 °C to 70 °C, while the stacking of the MoS2 crystals slightly declined. The slab length of MoS2 crystals increased to approximately 3.1 nm over the other two catalysts. Thus, the dispersion degrees of Mo species over catalysts NiMo/TA-60 and NiMo/TA-70 are higher. Moreover, the distributions of MoS2 slab length over these two catalysts seems much narrower than the other two catalysts. Suggesting relatively higher uniformity in the dispersion of active metals over catalysts NiMo/TA-60 and NiMo/TA-70 than over catalysts NiMo/TA-50 and NiMo/TA-80. This can be attribute to the moderate MSI between the active metals and the support materials revealed by H2-TPR characterization results and the higher proportions of Mo7O24 6− precursors revealed by Raman characterization results.The calculated fraction of corner Mo atoms (fMoc) and the fraction of edges Mo atoms (fMoe) are summarized in Table 4.XPS characterization of the investigated sulfide NiMo/TA-x serial catalysts was performed to check the effect of solvent evaporation temperature on the covalent states of the active metals. Both the XPS peaks for the Mo 3d orbital and the XPS peaks for the Ni 2p orbital were recorded and decomposited. The corresponding decomposition details for the Mo 3d orbitals are displayed in Fig. 7 , and the Ni 2p orbitals are displayed in Fig. 8 . The Mo sulfidation degrees and the NiMoS active phases proportions calculated from the decomposition results are listed in Table 5 .The Mo 3d orbital are composed of Mo 3d5/2 (228.9 eV) and Mo 3d3/2 (231.7 eV) orbitals for Mo4+(MoS2) species, Mo 3d5/2 (230.5 eV) and Mo 3d3/2 (233.6 eV) orbitals for Mo5+(MoOxSy), Mo 3d5/2 (232.7 eV) and Mo 3d3/2 (236.0 eV) orbitals for Mo6+(MoO3). The decomposition results for the Ni 2p orbital are composed of decomposited peaks for NiSx species (853.5 eV), NiMoS phases (855.5 eV) and NiO species (857.3 eV) [42,62,15,71]. The sulfidation degrees of both the Mo species (Mosul) and the Ni species (Nisul) reached their summits over catalyst NiMo/TA-60 (with values of 58.4% and 87.8%, respectively), given that Mosul and Nisul had values of approximately 51.7% and 81.2%, respectively, over catalyst NiMo/TA-50. Then, Mosul and Nisul decreased to 53.6% and 84.2%, respectively, over catalyst NiMo/TA-70 and were even lower over catalyst NiMo/TA-80 with a further increase in the solvent evaporation temperature.The above observed changes can be explained by that the MSIs of catalysts NiMo/TA-50 and NiMo/TA-60 are weaker than those of the other two catalysts, and it is more difficult for the active metals strongly interact with the support to be sulfided. Thus, the lower sulfidation degrees were observed over catalysts NiMo/TA-70 and NiMo/TA-80. For catalyst NiMo/TA-50, it has been proven by the H2-TPR results that NiO precursors and MoO3 precursors cannot form the so-called active NiMoO precursors very well; thus, lower sulfidation degrees of active metals were observed. Because the NiMoS phase is acknowledged as the real active phase for the sulfide NiMoS supported catalyst, the proportions of the NiMoS phase were also listed in Table 5. The details show that it increases in the order of NiMo/TA-80 (45.9%) < NiMo/TA-50 (51.8%) < NiMo/TA-70 (54.2%) < NiMo/TA-60 (57.5%), which is in line with both the MSIs between the active metals and the support materials and the proportion of Mo7O24 6− precursors over the oxide NiMo/TA-x serial catalysts evidenced by both the H2-TPR and Raman characterization results. These results suggest that the solvent evaporation temperature not only affects the sulfidation process but it also affects the formation efficiency of the NiMoS phases.The above results and discussions clearly demonstrate that the solvent evaporation temperature greatly influenced the physicochemical properties of the synthesized TA-x serial materials, which further played important roles on the corresponding NiMo/TA-x serial catalysts. However, which solvent evaporating temperature is the most moderate needs to be further confirmed. Thus, the HDS performances were evaluated at different reaction temperature. The results displayed in Fig. 9 show that the 4,6-DMDBT conversions increase with the reaction temperature over all four investigated NiMo/TA-x serial catalysts. At the defined reaction temperature, The 4,6-DMDBT conversions increase in line of NiMo/TA-80 < NiMo/TA-50 < NiMo/TA-70 < NiMo/TA-60 at each defined temperature, suggesting the same order of HDS activity of these investigated catalysts.The calculated activation energies over the investigated NiMo/TA-x serial catalysts summarized in Table 6 reveal an increasing order in the activation energies: NiMo/TA-60 (108 kJ•mol−1) < NiMo/TA-70 (118 kJ•mol−1) < NiMo/TA-50 (135 kJ•mol−1) < NiMo/TA-80 (137 kJ•mol−1), suggesting the superior catalytic activity over catalyst NiMo/TA-60 and the poorest catalytic activity over catalyst NiMo/TA-80, which on the same level with those reported ones [60,72]. This result is highly consistent with the NiMoS phase proportions determined by the aforementioned XPS characterizations.The HDS performances at different WHSVs were also determined, and the overall HDS reaction rate constants (kHDS) were calculated. The details from Fig. 10 clearly show that the conversions of 4,6-DMDBT decrease with increasing of WHSV. At each investigated WHSV, the 4,6-DMDBT conversions increased in the same order observed at different reaction temperatures. The calculated kHDS increases in the order of NiMo/TA-80 (303 µmol•h−1) < NiMo/TA-50 (405 µmol•h−1) < NiMo/TA-70 (510 µmol•h−1) < NiMo/TA-60 (566 µmol•h−1), with the same standard reported elsewhere [65,61]. This suggests that catalysts NiMo/TA-80 and NiMo/TA-50 are far less active than catalyst NiMo/TA-70, which is less active than catalyst NiMo/TA-60. TOF values vary in the line of NiMo/TA-80 (1.8 h−1) << NiMo/TA-50 (2.4 h−1) < NiMo/TA-70 (2.7 h−1) < NiMo/TA-60 (2.9 h−1). Moreover, the TOF values for catalyst NiMo/TA-80 are only approximately 3/4 of those for catalyst NiMo/TA-50, suggesting that the existence of highly ordered mesopores favors the hydroconversion of refractory sulfides. The product distributions with a 4,6-DMDBT conversion of 50±1% over each investigated catalyst were calculated to further confirm the influence of the solvent evaporation temperature on the selectivity of the individual HDS pathways over the NiMo/TA-x serial catalysts, and the details are summarized in Table 7 .In the typical 4,6-DMDBT HDS theory, 3,3′-DMBP is recognized as the only product of the DDS pathway, and other products are believed to be either the desulfurization products or the intermediates for the HYDS pathway [60,34,72]. The selectivity of 3,3′-DMBP decreases in the order of NiMo/TA-60 (18%) > NiMo/TA-70 (15%) ≈ NiMo/TA-50 (15%) > NiMo/TA-80 (12%), indicating the same trend in DDS selectivity of the investigated catalysts. This variation trend is highly related to the acidity properties of the corresponding TA-x samples except for catalyst NiMo/TA-80 and its corresponding support. This result suggests that the DDS selectivity for the 4,6-DMDBT HDS reaction is not only related to the morphology of the MoS2 nanoclusters but also highly related to the cracking activity provided by the acid sites from the support. The calculated specific reaction rate constants for the DDS pathway (kDDS) over the NiMo/TA-x serial catalysts varied more profoundly than the selectivity of 3,3′-DMBP. It profoundly increased to 85 µmol•h−1 over catalyst NiMo/TA-60 from approximately 56 µmol•h−1 over catalyst NiMo/TA-50 and then decreased to approximately 71 µmol•h−1 over catalyst NiMo/TA-70. The relatively low kDDS with a value of only approximately 28 µmol•h−1 over catalyst NiMo/TA-80 can be attributed to the destruction of highly ordered mesopores of the synthesized TA-80 sample. In the present work, the kDDS values for catalysts NiMo/TA-50, NiMo/TA-60 and NiMo/TA-70 were correlated to the product of fMoc and Mosul, and the result is displayed in Fig. 11 . It reveals that the rate constants of the DDS pathway can be correlated to the product of the fMoc values and the Mo sulfidation degrees linearly with R2 of 0.98893 and the slope for the fitted line of 1302. Since the loadings of Mo species on the investigated NiMo/TA-x serial catalysts were uniform at 12 wt.% counted by MoO3, the product of fMoc and Mosul is the actual amount of corner Mo atoms of the Ni-promoted MoS2 nanoclusters. Thus, the results demonstrate that the DDS reaction of 4,6-DMDBT can only take place over the corner active sites of the MoS2 nanoclusters. Finally, the kHYDS values for catalysts NiMo/TA-50, NiMo/TA-60 and NiMo/TA-70, fMoc, fMoe, Mosul and the proportion of the NiMoS phase (PNiMoS) were correlated. The correlation result is displayed in Fig. 12 .It is well acknowledged by the reported works that all the corner Mo atoms and only a portion of the Mo atoms located at the edge sites of the NiMoS phase are active sites [31,48,30,4]. Based on this theory, fMoc was taken as an independent variable, while the product of fMoe and PNiMoS was taken as another variable in the correlation works in the present work. The results show that kHYDS is linearly related to the product of 85fMoc+fMoe•PNiMoS and Mosul with an R2 of 0.99966 and a slope for the fitted line of 78. The coefficient for fMoc is 85 times that for the product of fMoe and PNiMoS in the correlating equation, suggesting that the activity of a single corner active site for the HYDS pathway of 4,6-DMDBT is approximately 85 times that of a single edge active site. Moreover, the slope for the fitted line of kDDS and factor fMoc•Mosul is approximately 17 times that of kHYDS and the factor (85fMoc+fMoe•PNiMoS)•Mosul, suggesting that the activity of the DDS of 4,6-DMDBT is approximately 17 times that of a single edge active site for the HYDS of 4,6-DMDBT over a single corner site. Thus, the overall HDS activity for 4,6-DMDBT of a single corner active site is approximately 100 times that of a single edge active site. Moreover, the activity for the HYDS of 4,6-DMDBT is approximately 5 times that for the DDS of 4,6-DMDBT over the corner active sites. These correlation results well explained the HDS product distribution for 4,6-DMDBT over the NiMo/TA-x serial catalysts summarized in Table 7.Hexagonal TA-x serial composites with a TiO2 content of 20 wt.% were successfully synthesized at different solvent evaporating temperatures. The corresponding NiMo/TA-x serial catalysts were prepared. A suitable solvent evaporating temperature (60 °C) favors the formation of narrow dispersed highly ordered mesopores with a relatively high specific surface area and excellent acidity properties due to the high match in the hydrolyzation rate of the precursor salts and the self-assembly efficiency of the mesopore structure directing agent and the hydroxides. Moderate MSIs and a high proportion of active Mo7O24 6− species were obtained with a moderate solvent evaporating temperature (60 °C), which led to a higher dispersion degree, higher fMoc value, higher sulfidation degree and higher proportion of the NiMoS phase, resulted in the highest activity of catalyst NiMo/TA-60. The kinetic studies show that catalyst NiMo/TA-60 exhibits superior catalytic performance due to the relatively high proportion of corner Mo sites. DDS reaction can take place over the corner sites and HYD reaction can take place over both the corner sites and the edge sites. The activity for the HYDS reaction of a single corner active site is approximately 85 times that of a single edge active site, and the activity for the overall HDS of a single corner active site is approximately 100 times that of a single edge active site. These promising findings demonstrate that the direction for the design and development of highly active HDS catalysts for the removal of highly refractory organosulfides such as 4,6-DMDBT is to increase the proportion and amount of corner active sites.This work is funded by the National Natural Science Foundation of China (No. 22,178,283 and No. 21,908,174), the National Postdoctoral Program for Innovative Talents (BX20190280), the Postdoctoral Research Foundation of China (2019M663778), the State Key Laboratory of Heavy Oil Processing (SKLOP201902002, SKLOP202102004), the Natural Science Foundation of Shaanxi Province (2022GY-136, 2019JLP-10, 2020JM-517). Guangheng Wang: Investigation, Data curation, Methodology, Writing – original draft. Zegao Zhao: Investigation, Data curation. Wenwu Zhou: Conceptualization, Supervision, Methodology, Project administration, Funding acquisition, Writing – review & editing. Zhiping Chen: Methodology, Funding acquisition, Formal analysis, Supervision. Anning Zhou: Funding acquisition, Supervision, Writing – review & editing, Formal analysis, Supervision. Yating Zhang: Writing – review & editing, Supervision. Xingyu Yang: Funding acquisition, Investigation. Fei Yao: Investigation.The authors declare that they have no known competing interests on this work. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors greatly acknowledge the financial support from the National Natural Science Foundation of China (Nos. 22178283 and 21908174), the Key Research and Development Program of Shaanxi(Program No. 2022GY-136), the National Postdoctoral Program for Innovative Talents (BX20190280), the Postdoctoral Research Foundation of China (2019M663778), the State Key Laboratory of Heavy Oil Processing (SKLOP201902002, SKLOP202102004), the Natural Science Foundation of Shaanxi Province (2019JLP-10, 2020JM-517).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ceja.2022.100319. Image, application 1

Binary TiO2-Al2O3 serial composites(TA-x) and the NiMo supported catalysts(NiMo/TA-x) were successfully synthesized at different solvent evaporation temperatures,. The materials were characterized by several advanced characterization techniques, and the catalytic hydrodesulfurization (HDS) performances were evaluated. The results revealed that the synthesized TA-x serial composites are amorphous mixtures of TiO2 and Al2O3 with highly ordered two-dimensional hexagonal mesopores. Both the mesostructures and the acidities of the synthesized TA-x composites changed at different evaporation temperature of the solvent. The metal-support interaction(MSI), the existence states and the coordination states of the Mo precursors can be modulated by the changes in the physicochemical property of the TA-x composites caused by the solvent evaporating temperature. A moderate solvent evaporation temperature is favorable for the doping of Ni species into the MoS2 slab and Mo sulfidation degree, thus enhancing the proportion of the NiMoS active phases. Catalyst NiMo/TA-60 exhibits superior activity and the highest direct desulfurization(DDS) pathway selectivity due to the excellent acidity and pore structure property, the highest dispersion and the highest Mo sulfidation degree, a moderate MSI, the highest efficiency in the formation of NiMoS phase and the highest proportion of corner Mo atoms. Moreover, the rate constants of the specific HDS pathways can be correlated with the specific types of active sites: the DDS pathway (kDDS) is closely related to the proportion of corner active sites and the hydrogenation desulfurization pathway (kHYDS) is closely related to the proportion of brim sites. The activity of a single corner active site is found to be approximately 100 times that of a single edge active site. These findings are promising in the design of HDS catalysts for inferior distillates.

Extensive use of fossil fuels has led to excessive emission of CO2 to the atmosphere, causing global warming and rising sea levels [1]. The electrocatalytic conversion of CO2 into valuable chemicals and fuels would be an effective approach to reduce the atmospheric CO2 concentration, reduce harm to ecological environments, and mitigate the energy crisis [2,3]. Among the products of the CO2 electroreduction reaction (CO2RR), the two-electron transfer product CO is one of the most important, as it can be utilized in well-established gas-to-liquid conversion technologies such as the Fischer–Tropsch process [3–5]. To date, metal-based materials [6–13], molecular complexes [14–20], and carbon-based catalysts [21–23] have been designed and applied in the CO2RR to produce CO. Noble metals such as Au, Ag, and Pd are favorable for the conversion of CO2 to CO [6,12], but their high cost and scarcity limit their commercial application. Homogeneous molecular catalysts such as metalloporphyrin and metalophthalocyanine complexes have well-defined active sites and structures. However, they usually work in organic electrolytes, and their low current density and poor stability impede their application [24–26]. Although carbon materials doped with non-metal atoms show some activity for the CO2RR, their performance still needs to be improved. It is therefore urgent to fabricate highly efficient CO2RR electrocatalysts that have high selectivity and appreciable current density to meet commercial application standards.Recently, single-atom catalysts (SACs), wherein single metal atoms are dispersed on carbon supports, have shown high activities for electrocatalysis and organic catalysis due to their high atom efficiency and unique coordination environment. In particular, single-atom Ni catalysts with Ni-Nx centers anchored on carbon supports have been developed for the CO2RR towards CO production [27–35]. Nevertheless, these Ni SACs supported on carbon materials such as graphene [36–39], N-doped nanosheets [40–43], and N-doped carbon nanotubes [44,45] have been unable to achieve high current density while maintaining high Faradaic efficiency (FE) for the CO2RR. This is probably because the microporous supports cannot completely expose the single-atom active sites. Thus, excellent SACs with high exposure of the single-atom active sites are needed to achieve current densities suitable for commercial applications.Carbon aerogels with 3D crosslinked networks are known to have a hierarchically porous structure, large specific surface areas, and high electron conductivity [46–51]. These unique features endow them with great electrocatalytic potential for improving current density by exposing accessible active sites, facilitating mass transport, and accelerating electron transfer [52–55]. Although a multitude of carbon-based aerogel materials have been applied for the hydrogen evolution reaction (HER) [56,57], oxygen evolution reaction [58], and oxygen reduction reaction [59], rarely have carbon aerogels supporting single-atom sites for the CO2RR been reported.In the past several years, using metal-organic frameworks (MOFs) as precursors to prepare carbon-supported SACs has attracted much attention because MOFs can inherit the unique properties of the intrinsic materials and achieve high surface areas as well as uniform distribution of metal centers [60–65]. However, the pyrolysis of MOFs usually results in microporous carbonaceous materials, which limit mass transportation and hinder the accessibility of active sites during catalysis. In contrast, SACs supported on hierarchical carbon materials containing micro-, meso-, and macropores, such as carbon aerogels, may circumvent this problem. So far, however, no report has been published on using a MOF as a precursor to prepare carbon aerogel-supported SACs.Herein, Ni SACs supported on carbon aerogels were successfully prepared for the first time by pyrolyzing aerogels composed of a Ni/Zn bimetallic zeolitic imidazolate framework-8 (Ni/Zn-ZIF-8) and carboxymethylcellulose (CMC) under N2 flux at 1000 ​°C for 4 ​h. The obtained Ni SACs are denoted as Ni-NCA-X (X ​= ​10, 20), where X is the weight ratio of CMC in the Ni/Zn-ZIF-8/CMC. During the annealing process, the ZIF frameworks and CMC were transformed into porous carbon aerogels with numerous N atoms, which served as anchoring sites to bind single Ni atoms. The Zn species were removed by evaporation at high temperatures (> 970 ​°C) [30,66]. For comparison, single-atom Ni supported on microporous N-doped carbon (Ni-NC) was also prepared by pyrolyzing Ni1/Zn2-ZIF-8 under the same conditions. Compared with Ni-NC, the optimal Ni-NCA-10 exhibited excellent CO2RR activity, with a more positive onset potential of −0.466 ​V vs. the reversible hydrogen electrode (RHE) and a CO Faradaic efficiency (FECO) above 95% over a wide potential range from −0.5 ​V to −1.0 ​V. It should be noted that all the potentials mentioned in this work were with reference to the RHE. Importantly, an industrial current density of 226 ​mA ​cm−2 with a high FE of 95.6% was achieved for Ni-NCA-10 in a flow-cell reactor at −1.0 ​V. Moreover, after 20 ​h of continuous electrocatalytic measurement, the Ni-NCA-10 showed little decline in FECO, suggesting its relative electrochemical stability.All reagents and chemicals were obtained commercially and used without further purification: 2-Methylimidazole (Aladdin), carboxymethylcellulose (Adamas), Ni(NO3)2·6H2O (Adamas), Zn(NO3)2·6H2O (Adamas), KHCO3 (99%, SCR), carbon paper (Toray).The Ni/Zn bimetallic zeolitic imidazolate framework (Ni/Zn-ZIF-8) was prepared using a modified method of preparing ZIF-8. First, 0.3921 ​g of Zn(NO3)2·6H2O and 0.1916 g of Ni(NO3)2·6H2O (the molar ratio of Zn metal ions to Ni metal ions remained 2:1) were dissolved in 15 ​mL methanol. Then, 1.297 ​g of 2-methylimidazole (2-MeIM) dissolved in 15 ​mL methanol was added to the above solution under stirring. The molar ratio of Zn and Ni metal ions to 2-MeIM remained 1:8. The mixed solution was transferred to a 100 ​mL Teflon-lined autoclave. After an ultrasonic bath for 15 ​min, the autoclave was kept at 100 ​°C for 12 ​h and then cooled down to room temperature. The precipitate was collected by centrifugation, and the obtained violet solid was washed thoroughly with methanol (3 ​× ​20 ​mL) and then dried at 70 ​°C in a vacuum for 24 ​h. The dried violet solid is denoted as Ni1/Zn2-ZIF-8. Materials with different ratios of Zn and Ni were prepared by the same method and are denoted as Ni1/Zn1-ZIF-8 and Ni4/Zn1-ZIF-8. Ni1/Zn2-ZIF-8 was selected as the optimal material for the subsequent synthesis of aerogel materials.The violet Ni1/Zn2-ZIF-8 powder (0.9 ​g) was dispersed in water/acetone (10 mL/0.5 ​mL, V/V ​= ​20:1) and sonicated for 2 ​h to form a homogeneous solution. Carboxymethylcellulose (CMC, 0.1 ​g) was dispersed in water (10 ​mL) under stirring at 80 ​°C, then was stirred for 3 ​h to form a homogeneous jelly-like solution at room temperature. The as-prepared Ni1/Zn2-ZIF-8 solution was mixed with the CMC solution and stirred for 3 ​h, then sonicated for 3 ​h. The Ni1/Zn2-ZIF-8/CMC gel was frozen at–19 ​°C for 12 ​h and freeze-dried under vacuum for 48 ​h to obtain Ni1/Zn2-ZIF-8/CMC aerogel, denoted as Ni1/Zn2-ZIF-8/CMC-10 (where 10 is the weight ratio of CMC in the Ni1/Zn2-ZIF-8/CMC). Ni1/Zn2-ZIF-8/CMC-20 and pure CMC aerogel were synthesized using the same method.The as-prepared Ni1/Zn2-ZIF-8/CMC-10 was placed in an alumina crucible and transferred to a vacuum tube furnace. Before carbonization, the air in the furnace was expelled by blowing nitrogen for 1 ​h. Then the temperature was heated to 1000 ​°C and kept there for 4 ​h under a N2 atmosphere. After naturally cooling to room temperature, Ni-NCA-10 was obtained. C-CMC, Ni-NC, and Ni-NCA-20 were obtained by using the pure CMC aerogel, Ni1/Zn2-ZIF-8, or Ni1/Zn2-ZIF-8/CMC-20 as the respective precursor under the same pyrolysis conditions. The residual Zn species in all the samples were etched with dilute HCl; this was followed by washing with distilled water and drying to yield purified samples.As shown in Scheme 1 , bimetallic zeolitic imidazolate frameworks (Ni/Zn-ZIF-8) with different ratios of Ni and Zn were synthesized by solvothermal reactions of Zn(NO3)2·6H2O, Ni(NO3)2·6H2O, and 2-methylimidazole. Then the Ni1/Zn2-ZIF-8 was chosen as the optimal material for the subsequent synthesis of aerogels. CMC was utilized as an agglomerant and mixed with Ni1/Zn2-ZIF-8 to form Ni1/Zn2-ZIF-8/CMC aerogel after freeze-drying. The aerogels with different contents of CMC are denoted as Ni1/Zn2-ZIF-8/CMC-X (X ​= ​10 and 20, where X is the weight ratio of CMC in the Ni1/Zn2-ZIF-8/CMC). Finally, Ni SACs supported on carbon aerogels (denoted as Ni-NCA-X, X ​= ​10, 20) were successfully synthesized by pyrolyzing the aerogels composed of Ni1/Zn2-ZIF-8/CMC-X under N2 flux at 1000 ​°C for 4 ​h. For comparison, pure carbon aerogel (C-CMC) and single-atom Ni supported on microporous N-doped carbon (Ni-NC) were also prepared by pyrolyzing the pure CMC and Ni1/Zn2-ZIF-8 under the same conditions.As shown in Fig. S1, the powder X-ray diffraction (PXRD) patterns of the Ni1/Zn2-ZIF-8 and Ni1/Zn2-ZIF-8/CMC-X aerogels were consistent with those of the simulated ZIF-8, suggesting that both had the same crystalline phase as ZIF-8. Additionally, with the introduction of Ni2+, the color of the materials changed from white to violet, indicating that a portion of the zinc nodes were replaced by nickel atoms (Fig. S2) [64]. These results indicated that Ni had replaced a portion of the Zn with the Ni becoming trapped by N atoms and subsequently forming Ni-Nx sites during the annealing process. The Ni content in Ni-NCA-10 and Ni-NCA-20 was 0.209 ​wt% and 0.153 ​wt%, respectively, according to inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements (Table S1). Fig. S3a shows that the four samples C-CMC, Ni-NC, Ni-NCA-10, and Ni-NCA-20 exhibited similar PXRD patterns, and only two broad peaks were observed, centered at 2θ ​= ​25.1° and 43.8°. The two peaks were assigned to the (002) and (101) planes of graphitic carbon [67]. No diffraction peaks for Zn-based species or Ni nanoparticles appeared, indicating that Ni SACs were formed and elemental Zn had been vaporized during pyrolysis above 1000 ​°C. The Raman spectra in Fig. S3b demonstrate that all the carbon-based samples had high ratios of D-band and G-band intensity (ID/IG) with values of 0.98 for Ni-NCA-10, 0.97 for Ni-NCA-20, 1.0 for Ni-NC, and 0.99 for C-CMC. The high ID/IG pointed to the presence of numerous defects due to the insertion of Ni metal and elemental N [68]. These results indicated these materials would be beneficial for the CO2RR.X-ray photoelectron spectroscopy (XPS) measurement was conducted to investigate the surface compositions and chemical states of Ni-NCA-10, Ni-NCA-20, and Ni-NC. It can be seen from the XPS survey spectra (Fig. 1 a) that C, N, O, and Ni elements were present in the corresponding carbon aerogels. The high-resolution XPS spectra of elemental Ni (Fig. 1b) showed that the binding energy of the Ni 2p3/2 peak was observed at 855.04 ​eV in the Ni-NC, Ni-NCA-10, and Ni-NCA-20 samples, located between the peaks for Ni(II) metal (856.0 ​eV) and Ni (0) metal (854.3 ​eV). This phenomenon demonstrated that the valence state of single-atom Ni in Ni-NCA-10 and Ni-NCA-20 was situated between Ni (0) and Ni(II) [44]. The XPS N 1s spectra of Ni-NC, Ni-NCA-10, and Ni-NCA-20 permitted the differentiation of pyridinic N (398.2 ​eV), Ni–N (398.9 ​eV), pyrrolic N (399.7 ​eV), graphitic N (401.1 ​eV), and oxidized N (402.7 ​eV) (Fig. 1c and Figs. S4a–b) [69]. Further XPS analysis demonstrated that Ni-NCA-10 contained a high ratio of pyrrolic N to pyridinic N (Table S2), which would facilitate the adsorption of CO2 and enhance the CO2RR activity.The coordination environment of the nickel atoms in the samples was further analyzed by synchrotron-based X-ray absorption spectroscopy (XAS). As shown in Fig. 1d, the Ni K-edge X-ray absorption near edge structure (XANES) curves of Ni-NCA-10 demonstrated that the Ni absorption edge was located between Ni foil and NiO, implying the positively charged single Ni atoms were between Ni(0) and Ni(II). This result was consistent with the XPS (Fig. 1b). A weak pre-edge peak positioned at 8333 ​eV corresponded to the dipole-forbidden but quadrupole-allowed transition from 1s to 3d [70]. There was also a strong absorption peak at about 8339 ​eV, associated with the 1s to 4p electronic transition [70].To further investigate the Ni–N structure of Ni-NCA-10, the Ni K-edge X-ray absorption fine structure (EXAFS) was also analyzed. As presented in Fig. 1e, a main peak centered at ca. 1.35 ​Å, ascribed to the characteristic Ni–N coordination path, was observed for Ni-NCA-10 and the Ni-phthalocyanines (NiPc) molecule [33,71]. The peak at 2.17 ​Å for Ni–Ni coordination was absent in the Ni K-edge EXAFS curve of Ni-NCA-10, revealing there were no Ni particles. According to the fitting result based on the EXAFS curve in Fig. 1f and Table S3, the Ni species in Ni-NCA-10 were coordinated with four nitrogen atoms and one oxygen atom from the coordinated water molecule. All these results strongly confirmed the atomic dispersion of single Ni sites in the Ni-NCA-10.The morphologies of the samples were characterized by scanning electron microscopy (SEM). As shown in Fig. 2 a and Fig. S5a, the Ni-NCA-10 and Ni-NCA-20 clearly displayed a 3D network structure with interconnected hierarchical micro-, meso-, and macropores, while the Ni-NC inherited the dodecahedral morphology of the Ni1/Zn2-ZIF-8 (Fig. S5b). Only micropores could be observed. Compared with the Ni-NCA-10, the pure C-CMC showed an irregular, lumpy morphology with an uneven distribution of pores (Fig. S5c). The fine structure of Ni-NCA-10 was further investigated using transmission electron microscopy (TEM); Fig. 2b shows a large number of pores with an average size of 156 ​nm (Fig. S5d). Notably, no metal nanoparticles (NPs) were visible in the TEM or high-angle annular dark-field scanning TEM (HAADF-STEM) images of Ni-NCA-10 (Figs. 2b and c). This result was consistent with the PXRD (Fig. S3a). A number of bright atomic-size dots related to the heavier Ni elements were clearly visible in the aberration-corrected HAADF-STEM image (Figs. 2c and d). The 3D crosslinked porous network structure was particularly obvious in the HAADF-STEM image in Fig. 2e. The corresponding EDS mapping revealed the homogeneous distribution of Ni, C, and N elements. All these results indicated that interconnected, hierarchically porous N-doped carbon aerogels supporting Ni single atomic sites had been successfully fabricated.To further investigate the porous structure of these carbon-based materials, N2 adsorption–desorption isotherms were tested at 77 ​K. As shown in Fig. 2f, typical IV isotherms with obvious hysteresis loops in the P/P0 range of 0.45–1.0 were observed for Ni-NCA-10 and Ni-NCA-20. This phenomenon indicated the presence of meso- and macropores. In addition, the pore size distribution of these materials (Fig. S6) showed that the Ni-NCA-10 and Ni-NCA-20 had large amounts of meso- and macropores, with pore sizes ranging from 2 to 65 ​nm. In contrast, the Ni-NC had only micropores of ca. 0.6 ​nm (Fig. S6). Moreover, Ni-NCA-10 had a Brunauer–Emmett–Teller (BET) area of 1200 ​m2 ​g−1, which was much larger than that of Ni-NC (786 ​m2 ​g−1) and Ni-NCA-20 (555 ​m2 ​g−1) (Table S4). Thus, it is reasonable to infer that the highly porous Ni-NCA-10 had a hierarchical micro-, meso-, and macroporous structure, which would be favorable for expediting mass and electron transport and further promoting the catalytic activity of the CO2RR. The interaction between the carbon aerogels and the CO2 molecules was demonstrated by the CO2 adsorption measurements. As displayed in Fig. 2g, the Ni-NCA-10 and Ni-NCA-20 exhibited high CO2 adsorption capacities of 44–56 ​cm3 ​g−1 at room temperature, implying that the Ni sites supported on highly porous carbon aerogel materials interacted strongly with CO2 molecules and would thus promote catalytic conversion.We next investigated the CO2RR properties of the Ni SACs supported on hierarchically porous carbon aerogel. The CO2RR measurements were conducted in a two-compartment electrochemical H-cell separated with a Nafion-117 proton exchange membrane. Linear sweep voltammetry (LSV) demonstrated that Ni-NCA-10 displayed a larger current density in CO2-saturated (1 ​atm) 0.5 ​M KHCO3 solution than in an Ar-saturated electrolyte (Fig. S7), suggesting the activity may have originated from CO2 reduction. Notably, no liquid products were detected by off-line 1H NMR spectroscopy (Fig. S8) and only CO and H2 were found via gas chromatography (GC).Ni-NCA-10 also showed an excellent FECO of > 95% in a wide potential range from −0.5 ​V to −1.0 ​V, reaching a maximum of 99.7% at −0.8 ​V (Fig. 3 a). The highest current density for Ni-NCA-10 was 47165.0 ​mA ​mg−1(Ni), achieved at −1.2 ​V; this was about 1.2 and 3.4 times larger than for Ni-NCA-20 (39673.2 ​mA ​mg−1(Ni) and Ni-NC (13762.3 ​mA ​mg−1(Ni)), respectively (Fig. 3b). The corresponding CO partial current density (j CO) of the Ni-NCA-10 was 34688.9 ​mA ​mg−1(Ni), which was about 1.38 and 5.2 times larger than for Ni-NCA-20 (25163.3 ​mA ​mg−1(Ni)) and Ni-NC (6650.2 ​mA ​mg−1(Ni)), as shown in Fig. 3c. The Ni-NCA-10 outperformed most of the reported Ni SACs (Table S5). Its excellent current density can reasonably be attributed to its high porosity, which exposed more active sites to substrates. Moreover, the hierarchical micro-, meso-, and macropores promoted the diffusion of the electrolyte and CO2 to the active sites.To further confirm whether the activity originated from CO2 reduction, we performed 13C-labeled CO2 isotope experiments, and the products were determined by gas chromatography–mass spectrometry (GC-MS). When using 13C-labeled CO2 to replace 12CO2 in 0.5 ​M KH12CO3 during the CO2RR process, the signals for 13CO (m/z ​= ​29) and 12CO (m/z ​= ​28) were observed. However, when conducting the CO2RR with 13C-labeled CO2 in 0.5 ​M KCl electrolyte, only 13CO was detected, and no signal for an m/z of 28 was observed. These results indicated that the CO originated from the CO2 in equilibrium with bicarbonate anions in a CO2-saturated KHCO3 aqueous solution (Fig. S9) [70,72].To confirm whether the atomically dispersed Ni sites were the active centers and exclude the influence of elemental Zn on the performance, N-doped carbon (NC) was prepared by pyrolyzing the ZIF-8/CMC aerogel without Ni sites. The Ni and Zn element contents in the NC are provided in Table S1. Unlike with the Ni-NC, Ni-NCA-10, and Ni-NCA-20, H2 was the major product for NC, implying that the atomically dispersed Ni sites were the active centers for the CO2RR, and the presence of Zn did not affect the CO2 reduction performance (Fig. S10).To investigate the stability of the aerogel materials, electrochemical stability testing was conducted. As shown in Fig. 3d, there was a slight decay in current density and FECO efficiency at an applied potential of −0.9 ​V for 20 ​h in CO2-saturated 0.5 ​M KHCO3 solution, indicating Ni-NCA-10 had electrochemical stability. Moreover, no Ni NPs were observed in the TEM image of Ni-NCA-10 after electrocatalysis (Fig. S11a), nor had the Ni-NCA-10 morphology changed after testing (Fig. S11b).To achieve an industrial current density for the CO2RR, a flow-cell reactor was assembled using a gas diffusion electrode (Fig. 4 a and Fig. S12). In this electrochemical test, the optimal catalyst, Ni-NCA-10, was selected as a typical sample to evaluate its CO2RR performance. Due to the faster diffusion of CO2 to the catalyst, the current densities for Ni-NCA-10 in the flow-cell significantly exceeded those in the H-cell. As shown in Fig. 4b and Fig. S13, it was clearly demonstrated that the j CO in the flow-cell achieved an industrial-level value of 226 ​mA ​cm−2 at −1.0 V. Surprisingly, the current density was greatly enhanced in the flow-cell, while the FECO remained > 90% at the applied potentials. The calculated turnover frequency (TOF) of Ni-NCA-10 reached 271810 ​h−1 ​at −1.0 ​V, which was much larger than that of Ni-NC (Fig. 4c). This result proved that the carbon aerogel structure containing hierarchical micro-, meso-, and macropores played a significant role in improving the activity for the CO2RR. Moreover, the excellent TOF surpassed the values for most reported catalysts toward CO2-to-CO conversion (Fig. S14 and Table S6). The long-term durability test also indicated only slight changes in the applied potential on the Ni-NCA-10 aerogel at a high current density of 100 ​mA ​cm−2, indicating its good CO2RR stability (Fig. 4d and Fig. S15).We took electrochemical impedance spectroscopy (EIS), electrochemical active surface area (ECSA), and Tafel slope measurements to understand the intrinsic catalytic activity of the carbon aerogels with Ni single atoms. As shown from the EIS Nyquist plots in Fig. S16a, Ni-NCA-10 and Ni-NCA-20 exhibited smaller semicircles in comparison with that of Ni-NC, which may partially explain the high electron conductivity and excellent CO2RR activity of these aerogel materials. Subsequently, the double-layer capacitance was calculated as a measure of the ECSA. The results showed that the Ni-NCA-10 had a much higher Cdl value of 31.0 ​mF ​cm−2 than Ni-NC, demonstrating that the Ni-NCA-10 aerogel had a far greater number of active sites (Figs. S16b and S17). In addition, the Ni-NCA-10 showed a lower Tafel slope of 149.3 ​mV dec−1 than the Ni-NC (189.1 ​mV dec−1, Fig. S16c), which indicated that faster kinetics occurred in the Ni-NCA-10. Based on the analysis, the 3D crosslinked aerogel structure tended to expose more active sites and accelerate the reaction kinetics, thereby achieving high TOF.To identify the reaction intermediates and mechanism of the CO2RR, in situ electrochemical Fourier transform infrared spectroscopy (FTIR) measurements of Ni-NCA-10 were conducted. The experiment was performed at a potential of −0.8 ​V in 0.5 ​M KHCO3. Figs. 5 a and b show bands located at 1395 ​cm−1 and 1386 ​cm−1 in the spectra, assigned to a bidentate ∗COO− intermediate and a carboxyl intermediate of ∗COOH, respectively, which implied the coexistence of these two intermediates [72,73]. The ∗COO− intermediate was active in the system and easily hydrogenated to form ∗COOH, which is the key intermediate for the formation of CO [74].Density functional theory (DFT) was used to calculate the free energy to further understand the catalytic mechanism. Using the XANES and EXAFS results, we first calculated the water desorption energy from the Ni centers. This value (0.08 ​eV) was too small to affect subsequent calculations. We therefore built a stable model with Ni–N4 coordination (Fig. 5c) to calculate the free energies for the CO2RR and HER processes. In the typical pathway for the CO2RR on Ni sites (Fig. S18), the CO2 molecule was first absorbed on the Ni–N4 site and formed the ∗COOH intermediate by a proton-coupled electron transfer process. Subsequently, the ∗COOH was converted to ∗CO via another proton-coupled electron transfer process, and finally, the ∗CO was desorbed from the Ni–N4 to obtain CO. It is worth noting from the DFT that the formation of the absorbed intermediate, ∗COOH or ∗H, was the rate-determining step for the CO2RR and HER [67], respectively. Additionally, the free energy of ∗COOH formation on Ni–N4 was 1.48 ​eV, which was smaller than for the formation of ∗H (1.55 ​eV) in the HER process (Figs. 5d and S19). This result indicated that the CO2RR occurred preferentially at the Ni–N4 site rather than the HER, which concurred with the experiments.We have successfully designed and prepared Ni single atomic sites supported on N-doped carbon aerogels as catalysts, via the pyrolysis of Ni/Zn-ZIF-8/CMC aerogel to expose a greater number of active sites for CO2 molecules and electrolytes. The electrocatalytic performance for the electrochemical reduction of CO2 was thereby enhanced. Compared with the single-atom Ni supported on microporous N-doped carbon, the Ni-NCA-10 aerogel exhibited a high CO Faradaic efficiency of over 95% in a wide potential range of −0.5 to −1.0 ​V vs. RHE in 0.5 ​M KHCO3 electrolyte. Notably, the as-prepared Ni-NCA-10 achieved an industrial current density of 226 ​mA ​cm−2 with excellent CO Faradaic efficiency in a flow-cell reactor. The exceptional electrocatalytic performance of Ni-NCA-10 can reasonably be attributed to its hierarchical porous structure, high surface area, and uniformly accessible dispersed active Ni sites. The control experiments and theoretical calculations demonstrated that the formation of ∗COOH over the Ni–N4 sites was the rate-determining step for the CO2RR. This work might shed light on promising perspectives for designing excellent electrocatalysts containing hierarchical micro-, meso-, and macropores to achieve the electrochemical reduction of CO2 with industrial-level current densities.R. Cao and Y.-B. Huang proposed the concept. H. Guo performed the experiments. D.-H. Si conducted the DFT calculation. H. Guo and Y. B. Huang co-wrote the manuscript. All authors participated in data analysis and manuscript discussion.The authors declare no competing financial interests.The work was supported by the National Key Research and Development Program of China (2018YFA0208600, 2018YFA0704502), NSFC (21871263, 22071245, and 22033008), and Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (2021ZZ103). The authors thank the beamline BL14W1 station for XAS measurements at the Shanghai Synchrotron Radiation Facility, China.The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.esci.2022.03.007.

Finding highly efficient electrocatalysts for the CO2 electroreduction reactions (CO2RR) that have high selectivity and appreciable current density to meet commercial application standards remains a challenge. Because their reduction potentials are similar to that of the associated competitive hydrogen evolution reaction and the CO2 activation kinetics are sluggish. Although single-atom catalysts (SACs) with high atom efficiency are one class of promising candidates for the CO2RR to produce CO, single-atom active sites supported on microporous carbons are not fully exposed to substrates and thus lead to low current density. Carbon aerogels with interconnected channels and macropores can facilitate mass transport. But few reports describe utilizing them as supports to anchor SACs for efficient electrocatalysis. Herein, N-doped carbon aerogels supporting Ni single atomic catalyst sites (denoted as Ni-NCA-X, X ​= ​10, 20) were fabricated by pyrolyzing Ni/Zn bimetallic zeolitic imidazolate framework (Ni/Zn-ZIF-8)/carboxymethylcellulose composite gels. Owing to abundant hierarchical micro-, meso-, and macropores and high CO2 adsorption, the Ni single active sites in the optimal Ni-NCA-10 were readily accessible for the electrolyte and CO2 molecules and thus achieved an industrial-level CO partial current density of 226 ​mA ​cm−2, a high CO Faradaic efficiency of 95.6% at −1.0 ​V vs. the reversible hydrogen electrode, and a large turnover frequency of 271810 h−1 in a flow-cell reactor at −1.0 ​V. Such excellent CO2RR performance makes Ni-NCA-10 a rare state-of-the-art electrocatalyst for CO2-to-CO conversion. This work provides an effective strategy for designing highly efficient electrocatalysts toward the CO2RR to achieve industrial current density via anchoring single-atom sites on carbon aerogels.

The recent discovery of shale gas reserves has caused a decrease in the price of natural gas, encouraging its use as feedstock for the production of valuable chemicals [1]. Ethylene is, by far, the most important chemical feedstock for Petrochemistry, being directly used in the production of a wide number of commodity chemicals [2]. Currently, the main industrial route to obtain ethylene is steam cracking, which is an energy intensive and non-catalytic process [2,3]. In fact, steam cracking is considered as the most energy consuming process of the chemical industry, due to its endothermic character and the need for high reaction temperatures. Moreover, the absence of catalysts leads to the formation of many reaction products so that the separation costs are also high. Among all the possibilities, the oxidative dehydrogenation (ODH) of ethane is established as one of the most interesting alternatives to steam cracking, being an exothermic process with lower energy requirements and no thermodynamic limitations [3–5]. The energy consumption of ODH is expected to be substantially lower than any of the current alkene production technologies due to its exothermic nature. Furthermore, the deposition of coke is prevented provided that the presence of oxygen can oxidize coke to form carbon oxides. Despite the large amount of research efforts, industrial scale application of the ODH of ethane has not been implemented up to date due to the relatively low ethylene selectivity shown by the catalysts currently available. The main problem with most of the catalysts studied for the ODH of ethane is the excessive formation of carbon oxides (COx) which limits the selectivity to ethylene [3–6]. In this sense, among all catalytic systems based on reducible metal oxide catalysts, the most promising ones are multicomponent MoV(Te,Sb)NbO mixed oxides [7,8] and modified NiO materials [9–21]. In the latter case, it has been reported that pure NiO exhibits an important formation of CO2 and low ethylene selectivity [9–20]. However, the role of promoters [9–15] and/or supports/diluents [16–25] in NiO based catalysts is still under discussion. In this way, several investigations have shown the effect of different promoters on the catalytic behavior of NiO-based materials [9–25], with the presence of many promoters reducing the formation of carbon dioxide. Nevertheless, at this moment, Nb-promoted catalysts are the most effective ones [9–15], although the presence of other elements, such as Sn4+, W6+, Zr4+, Ti4+ [10–15], with dopant contents lower than 10 at.%, favor small changes in the characteristics of NiO particles, thus leading to the best catalytic performance (selectivity to ethylene up to 80–90 %).Alternatively, supported/diluted NiO catalysts also show a high ethylene selectivity, especially by using Al2O3 [16–19], porous clays [20] or other supports based on transition metal oxides [21–25]. In this case, after the incorporation of NiO contents of ca. 10−30 wt%, changes in both physico-chemical characteristics and catalytic performance are observed. These changes have been related to a decrease in crystal size of NiO particles but, in addition, some interaction between NiO and the support (decreasing the reducibility of Ni-O bonds) could be also necessary. Thus, it is known that the decrease of the NiO crystallite size [9,21], the elimination of non-stoichiometric oxygen species, a decrease in the reducibility of Nin+ sites, an increased Lewis acidity [15,26,27] or a lower electron conductivity [28] can give rise to high selectivity to ethylene during the ODH of ethane [9,11,12,17]. Interestingly, a similar increasing effect in the selectivity to the olefin in the ODH of ethane has been observed when oxalic acid is incorporated during the preparation procedure [15].Generally, nickel oxide catalysts with small NiO particle size (below 20 nm, although less than 10 nm is preferred) present optimum catalytic performance in the ODH of ethane [15]. However, the drastic change observed in the redox properties of NiO must rely on the modification of the chemical nature of NiO (coordination, surface environment, oxidation state) [9,15]. This could be achieved by decreasing NiO particle size. However, it is possible to increase the selectivity to ethylene by optimizing the active phase-support interaction, without substantially decreasing NiO particle size, as observed in TiO2-supported nickel oxide catalysts [28]. Thus, and according to these observations, it seems that a small particle size might not be a sufficient requirement to improve the catalytic performance of NiO-based catalysts in ODH.In order to shed some light into the chemical nature of selective NiO catalysts for the oxidative dehydrogenation of ethane, we have synthesized Al2O3-supported nickel oxide catalysts, but with varying degrees of nickel oxide-support interaction by using modifying the catalyst preparation procedure. In this way, it will be possible to determine the influence of the interaction between nickel oxide and the support, minimizing the possible interference of both the NiO crystal size effect and the support employed.Accordingly, we have followed two different synthetic approaches for a series of NiO/Al2O3 catalysts: i) addition of oxalic acid as an organic additive to NiO/Al2O3 system; and ii) incorporation of Nb5+ as a dopant during the preparation of the Al2O3-supported materials. In the latter case, the synthesis of Nb5+-promoted NiO/Al2O3 catalysts has been carried out by incorporating Nb5+ in one or two steps: a) using a Ni2+/Nb5+-containing solution to be directly impregnated on Al2O3; and b) Nb5+ is firstly impregnated on Al2O3, and a Ni2+-containing solution is subsequently added on the NbOx-Al2O3 support. The results are discussed in terms of the specific chemical and structural features found in selective and unselective materials.Al2O3-supported NiO catalysts were prepared by wet impregnation of γ-Al2O3 (SBET =210 m²/g, ABCR) with aqueous solutions of nickel nitrate Ni(NO3)2·6H2O (Sigma-Aldrich, 99 %). The catalysts are named as xNi/AL, where x is the NiO wt%.Alternatively, oxalic acid was added to the nickel nitrate solution, with Ni/oxalic acid molar ratios of 1/1 and 1/3 (i.e. 15Ni/AL-o1 and 15Ni/AL-o3 catalysts, respectively). For comparison, a 15 NiO wt% Al2O3 catalyst was prepared by a mechano-chemical procedure, by mixing and grinding the corresponding amounts of nickel oxide and alumina in an agate mortar (i.e. Ni+AL(PM) sample).Nb-containing alumina-supported nickel oxide catalysts were prepared by following two strategies, using an aqueous solution of C4H4NNbO9·xH2O (Sigma–Aldrich): i) direct impregnation of γ-Al2O3 by an aqueous solution of promoting compounds and subsequent impregnation with an aqueous solution of nickel nitrate (15 wt% NiO) named as Ni/Nb/AL; ii) γ-Al2O3-supported Ni-Nb-O mixed oxides were prepared by wet impregnation method using aqueous solutions of nickel nitrate and the niobium compound, with a Nb/(Ni + Nb) atomic ratio of 0.1; which has been named as (Ni+Nb)/AL. All catalysts were dried overnight at 100 °C and finally calcined at 500 °C for 2 h (5 °C/min). The characteristics of these catalysts are shown in Table 1 .Catalytic tests were carried out under steady state conditions in a fixed bed quartz reactor (i.d. 20 mm, length 400 mm) at temperatures in the 300−500 °C range. Feed consisted of an ethane/O2/He mixture with 3/1/26molar ratio. The total flow and the catalyst weight were varied (25−100 mL min−1, 0.1–1.0 g of catalyst and 0.3–0.5 mm particle size) in order to achieve several contact times (W/F). For some selected experiments an ethane/O2/He mixture with 3/3/24molar ratio was employed.Reactants and products were analyzed by gas chromatography using two packed columns [20]: (i) molecular sieve 5A (2.5 m); and (ii) Porapak Q (3 m).N2-adsorption isotherms were recorded in a Micromeritics ASAP 2000 device. The materials were degassed in vacuum at 300 °C prior to N2 adsorption. Surface areas were estimated by the Brunauer-Emmet-Teller (BET) method.X-ray diffraction patterns were collected in an Enraf Nonius FR590 diffractometer with a monochromatic CuKα1 source operated at 40 keV and 30 mA.Raman spectra were obtained in an inVia Renishaw spectrometer, equipped with an Olympus microscope, using a wavelength of 325 nm (UV-Raman), generated with a Renishaw HPNIR laser with a power of approximately 15 mW.UV–vis diffuse reflectance spectroscopy measurements of the solids were carried out within the 200−800 nm range using a Varian spectrometer model Cary 5000. The value of band gap Eg is calculated by extrapolating the linear fitted region at [F(R(∞))hυ]2 = 0 in the plot of [F(R(∞)) hυ]2 versus hυ. Additional information in supporting information.Temperature-programmed reduction experiments (H2-TPR) were performed in an Autochem 2910 (Micromeritics) equipped with a TCD detector. The reducing gas composition was 10 % H2 in Ar, with a total flow rate of 50 mL min−1. The materials were heated until 800 °C, with a heating rate of 10 °C min−1.X-ray photoelectron spectroscopy (XPS) measurements were performed on a SPECS spectrometer equipped with a Phoibos 150 MCD-9 detector using a monochromatic Al Kα (1486.6 eV) X-ray source. Spectra were recorded using an analyzer pass energy of 50 eV, an X-ray power of 100 W, and an operating pressure of 10−9 mbar. Spectra treatment was performed using CASA software. Binding energies (BE) were referenced to C 1s at 284.5 eV.Selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM) and Scanning-TEM (STEM)-Energy-Dispersive Spectroscopy (EDS) maps were performed on a JEOL JEM300 F electron microscope by working at 300 kV (point resolution of 0.17 nm). Crystal-by-crystal chemical microanalysis was performed by energy-dispersive X-ray spectroscopy (XEDS) in the same microscope equipped with an ISIS 300 X-ray microanalysis system (Oxford Instruments) with a detector model LINK “Pentafet” (resolution 135 eV). Samples for transmission electron microscopy (TEM) were ultrasonically dispersed in n-butanol and transferred to carbon coated copper grids.The catalytic performance of supported nickel oxide catalysts during the oxidative dehydrogenation (ODH) of ethane at 400 °C is summarized in Table 1. As mentioned in the experimental section, the catalysts tested consist of a set of nickel oxide materials supported on γ-alumina, with Ni-loadings from 5 to 30 wt% NiO. In addition, catalysts with a 15 wt% NiO prepared with different amounts of oxalic acid and promoted with Nb5+ were also evaluated.The ethane conversion and the selectivity to ethylene strongly depend on NiO-loading and the catalyst preparation procedure. For comparison, it has been also included the catalytic results of a mechano-chemical NiO-Al2O3 mixture, named as Ni+AL(PM). In all cases, ethylene was the main reaction product. In addition, CO2 was the only product detected from oxidation reactions.The variation of the selectivity to ethylene with the ethane conversion during the ethane ODH on representative Al2O3-supported nickel oxide catalysts (xNi/AL series) is shown in Fig. 1 a. On the other hand, Fig. 1b allows the comparison of catalytic results over catalysts with 15 wt% NiO with or without additional synthetic modifications: i) Nb-promoted catalysts, i.e. Ni/Nb/AL and (Ni+Nb)/AL samples; ii) unpromoted catalysts prepared from synthesis gels containing oxalic acid (i.e. 15Ni/AL-o1 and 15Ni/AL-o3 samples).As it can be observed in Fig. 1a and Table 1, the selectivity to ethylene has a clear dependence on the NiO-loading. Thus, the selectivity to ethylene gradually increases with NiO-loading (in the range of 88–95 %), reaching a maximum of ca. 95 % over sample 15Ni/AL (i.e. 15 wt% NiO). On the contrary, further increasing NiO-loading on Al2O3 (up to 30 wt% NiO) leads to a decrease in the selectivity to ethylene during the ODH of ethane (down to ca. 72 %).In the case of Nb-promoted catalysts, the (Ni+Nb)/AL sample (synthesized in one step) presents a slight decrease in the selectivity to ethylene (ca. 86 %) compared to that achieved using the reference sample, 15Ni/AL. Moreover, Ni/Nb/AL catalyst (prepared in two steps) presents a remarkably lower selectivity (ca. 71 %).The method used to prepare the catalysts, and especially the presence of oxalic acid in the synthesis gel, may also have an important influence in the catalytic performance. Fig. 1b shows the variation of selectivity with ethane conversion of samples with 15 wt% of NiO. Lower selectivity to ethylene is observed over samples prepared with oxalic acid in the synthesis gel (see 15Ni/AL-o1 and 15Ni/AL-o3), especially notorious when high oxalic acid contents are used. Thus, both Nb-promoted NiO/γ-Al2O3 catalysts and Nb-free NiO/γ-Al2O3 catalysts (where the oxalic acid has been directly added) show a poorer selectivity to ethylene than the corresponding catalyst of xNi/Al series (i.e. 15Ni/AL sample). The selectivity to ethylene for catalysts prepared with a 15 wt% NiO decreases according to the following trend: 15Ni/AL > (Ni+Nb)/AL > 15Ni/AL-o1 > Ni/Nb/AL > 15Ni/AL-o3 > Ni+AL(PM). Fig. 2 shows the variation of the space-time yield for ethylene formation of studied catalysts. For xNi/AL series, the formation of ethylene per nickel site increases with the NiO-loading, achieving its maximum value for 20Ni/AL catalyst (Fig. 2). However, the highest space-time yield for ethylene formation was observed for samples 15Ni/AL-o1 and (Ni+Nb)/AL (Fig. 2). This is so because of the higher C2H6 reaction rates for the latter cases (Figure S1, supporting information).The present study has been undertaken using a low concentration of oxygen (C2H6/O2/He = 3/1/26 M ratio) so achieving high conversions without running out of oxygen is complicated. Then, we have carried out a few experiments with the optimal catalyst 15Ni/AL using more oxygen in the feed (C2H6O2/He = 3/3/24) and higher contact times to reach higher conversions. Thus, an ethane conversion of 51.2 % and a selectivity to ethylene of 71.7 %, so that the ethylene yield obtained was 36.7 %, was obtained when working at 450 °C and a contact time, W/F, of 205 gcat h (molC2H6)−1 (Table S1, supporting information). This yield could be enhanced by optimizing the reaction conditions.In order to understand their catalytic performance, the catalysts were characterized by several physicochemical techniques. Table 1 summarizes their main physicochemical characteristics. Fig. 3 displays the XRD patterns of Al2O3-supported NiO catalysts synthesized in the absence or the presence of increasing amounts of oxalic acid or Nb-oxalate in the synthesis gel. For comparison, XRD pattern of the mechano-chemically mixed Ni+AL(PM) sample is presented in Figure S2-A (supporting information).All the materials show diffraction lines corresponding to face-centered cubic NiO phase, space group Fm-3m (JCPDS: 78−0643), and γ-alumina (JCPDS: 10−0425). However, depending on the catalyst preparation procedure (the NiO-loading and/or the presence/absence of Nb5+ as promoter), differences in the relative intensity of diffraction maxima of the two crystal phases were observed. X-ray diffraction patterns of the catalysts from xNi/AL series show a progressive increase in the relative intensity of NiO diffraction maxima as the nickel oxide loading increases (see Fig. 3, patterns a to e). For the same nickel oxide content (we have considered the optimal Ni/Al ratio), the incorporation of increasing amounts of oxalic acid leads to an increase in the relative intensity of the NiO peaks (Fig. 3, diagrams c, h and i).Likewise, both the use of niobium as a promoter and the method followed to incorporate it, modify the relative intensity and peak width of NiO maxima, for a fixed Ni/Al content (again, we have considered the optimal NiO content of 15 wt%). Thus, NiO diffraction maxima appear to be narrower in Ni/Nb/AL sample than in (Ni+Nb)/AL ( Fig. 3, patterns f and g, respectively), which could be related to differences in NiO crystal size and/or degree of crystallinity, depending on the extent of interaction with γ–Al2O3 support.In order to get further insight into the crystalline nature of NiO-based catalysts, these materials were analyzed by high-resolution transmission electron microscopy. Catalysts of the xNi/AL series are made up of γ–Al2O3 and NiO crystallites of 5−10 nm in size up to the composition 15Ni/AL, NiO showing a good dispersion over the support (Figs. 4 a and 4b). In the 20Ni/AL catalyst, platelet-like NiO crystals of 30−50 nm begin to appear (Fig. 4c), which are very abundant, and in the form of agglomerates in 30Ni/AL sample (Fig. 4d).The addition of oxalic acid tends to modify the microstructure of the catalysts. After adding oxalic acid in 1:1 Ni/oxalic acid molar ratio (catalyst 15Ni/AL-o1), crystallinity, the degree of dispersion on the support and the average size of the NiO crystallites, remain almost unchanged with respect to that observed in catalyst 15Ni/AL. However, for 1: 3 Ni/oxalic acid molar ratio (i.e. 15Ni/AL-o3 sample), NiO particles display higher crystallinity, even though the average crystallite size hardly changes (Fig. 4e). Note that despite the similarity between X-ray patterns of 15Ni/AL-o3 and 30Ni/AL catalysts (Fig. 3, patterns i and e, respectively), their microstructure is quite different.In the case of niobium promoted catalysts, microstructural differences are found depending on the preparation method. The Nb-promoted NiO/γ-Al2O3 catalyst prepared in two steps (Ni/Nb/AL sample) is formed by NiO crystallites of 10−15 nm distributed on the support, as well as agglomerates of large crystals of ∼ 50 nm. EDS maps (Figure S3) show a homogeneous distribution of niobium on the γ-Al2O3 support and the absence of niobium in areas where NiO crystals are observed. Interestingly, well-dispersed crystallites of approximately 5−10 nm size with a characteristic d spacing of approximately 7 Å can be also observed in this catalyst. Provided the chemical composition of the sample and the elemental distribution, this periodicity is compatible with the (002) d spacing of Ni2O3 (d 002 = 7.29 Å). These crystallites are clearly visible in Fig. 4f. It is important to mention that this is the only catalyst in which this type of crystals have been observed, and the details of the preparation method in this particular case must be the origin of the formation of this species.For Nb-promoted NiO/γ-Al2O3 prepared in one step, (Ni+Nb)/AL, NiO crystallites of 10−15 nm distributed on the support with agglomerates of larger NiO crystals of ∼ 50 nm were observed. These large crystals are less abundant than in the catalyst prepared in two steps. EDS maps performed (Figure S4) show that, although in low concentration, niobium is effectively distributed and associated with nickel on the support.Al2O3-supported NiO catalysts were also characterized by UV Raman spectroscopy (using an excitation wavelength of 325 nm) in order to elucidate the spin-phonon interaction in the materials [29,30]. Fig. 5 shows the UV Raman spectra of alumina-supported nickel oxide catalysts. For comparison, the UV Raman spectrum of Ni+AL(PM) sample is presented in Figure S2 (pattern B).As reported in the literature, the UV Raman spectra of bulk NiO are characterized by the presence of five Raman bands [29,30]: i) two bands at ca. 510 and 580 cm−1, assigned to one-phonon (1P) transverse optical (TO) and longitudinal optical (LO) modes; ii) two weak bands at ∼740 cm-1 and ∼900 cm−1 and an intense band at ca. 1100 cm−1 related to two-phonon modes, i.e. the second-order transverse optical mode (2TO), the combination of TO + LO modes and the second-order longitudinal optical (2LO) modes, respectively. Among all these bands, the most intense ones are those located at 580 and ca. 1100 cm−1.We must notice that, when NiO is antiferromagnetically ordered or defect-rich, the intensity of one-phonon scattering (1P, LO and TO modes) increases significantly [29,31]. In addition, a very low intensity of the band at ca. 1124 cm−1 has been observed in silica-supported nickel oxide (with 3 wt% NiO), which has been attributed to the presence of very small NiO crystals [32]. In addition, the presence of only one band at ca. 570 cm-1 has been recently reported for NiO supported on Nb5+-containing siliceous porous clay heterostructure catalysts [33]. This observation was attributed to a high NiO dispersion as a consequence of an effective active phase-support interaction, what would lead to a decrease in NiO particle size and/or the generation of defects.As expected, the intensity of the most characteristic bands (1P LO band at ∼580 cm−1 and 2P 2LO band at ∼1100 cm−1) increases when the nickel oxide loading increases (Fig. 5). However, the relative growth of both bands differs depending on the specific structural and chemical features of the catalyst. Thus, an increase in the intensity of 2P 2LO band (I1100) higher than that of 1P LO band (I580) is observed at high NiO-loading. Therefore, the relative intensity of 1P LO band simultaneously increases with the decrease of NiO particle size and/or the presence of oxygen defects [31]. This fact would mean that the increase in the nickel oxide loading favors an increase of the NiO crystal size and/or a decrease in the concentration of oxygen defects.UV Raman spectra of catalysts with the same NiO-loading (with or without Nb5+ or oxalic acid in the synthesis gel) are comparatively shown in Fig. 5 (spectra c, f to i). Differences are observed in the relative intensity of the LO band (I580) and 2LO band (I1100) depending on the catalyst preparation procedure. Catalysts 15Ni/AL and (Ni+Nb)/AL present an I580/I1100 ratio higher than 1; whereas Ni/Nb/AL, 15Ni/AL-o1 and 15Ni/AL-o3 catalysts present I580/I1100 ratios lower than 1. These results could be partly explained in terms of the presence of NiO particles with different crystal sizes. However, the distribution and the mean crystal size of NiO in the reference catalyst (15Ni/AL) and sample 15Ni/AL-o1 are very similar, despite being the relative I580/I1100 ratio much higher in the reference catalyst.In the same way, Nb-promoted NiO/Al2O3 catalysts prepared in either one or two synthesis steps ((Ni+Nb)/AL and Ni/Nb/AL, respectively) also present significant differences in their UV Raman profiles (Fig. 5, spectra f and g, respectively). Despite showing similar NiO crystal size, both catalysts display different relative I580/I1100 ratio, being higher in the material prepared in one-step (i.e. (Ni+Nb)/AL sample). In addition, this would also underline the low capability of niobium oxide in the dispersion of NiO [34].The DR-UV–vis spectra of prepared catalysts are shown in Fig. 6 , in which spectra a to e correspond to Al2O3-supported NiO catalysts with different Ni-loadings, while spectra f to i are those corresponding to catalysts prepared in the presence of oxalate anions, Nb-promoted and unpromoted catalysts. For comparison, DR-UV–vis spectrum of NiO-Al2O3 mixture, Ni+AL(PM) sample, has been also recorded (Figure S2-C).Bulk NiO shows bands at 715 nm and 377 nm, which can be assigned to octahedrally coordinated Ni2+ species in the NiO lattice [17,19,23,35,36]. Additionally, a band at 510 nm can be also assigned to charge transfer in NiO crystals [36,37]. On the other hand, it has been reported that supported nickel oxide catalysts, such as NiO-Al2O3 [38,39], NiO/Silica-Alumina [39] or NiO/Al2O3 [17,19], can also present a doublet (at 600−645 nm) and a band at 416–430 nm, which were attributed to Ni2+ species with tetrahedral (Td) and octahedral (Oh) coordination, respectively. Nevertheless, a band at 630 nm has been also observed in NiO supported on TiO2/γ-Al2O3 [40], which has been attributed to the absorption of surface-dispersed nickel oxide species in tetrahedral coordination. We must note that the accommodation of Ni2+ species in both tetrahedral and octahedral coordination could lead to nickel aluminate as surface spinel phase in samples calcined at higher temperatures [17,24,36,38–40].According to our results, the DR-UV–vis spectra of samples of xNi/AL series with NiO-loading up to 10 wt% NiO suggest the presence of Ni2+ species with tetrahedral (doublet at 600 and 640 nm) and octahedral (band at 410 nm) coordination, without the appearance of the band at 715 nm, typical of bulk NiO (Fig. 6 a–b). These results could indicate the existence of highly dispersed nickel species with high interaction between support and part of Ni-containing crystallites (Ni2+ tetrahedral diffused into the γ-Al2O3 lattice) [17,24,36,38,39]. At this point it is important to mention that NiAl2O4 spinel was not detected neither by X-ray diffraction nor by electron microscopy. When nickel loading increases, the interaction with the support reaches a maximum (15 wt% NiO or below), and from there, NiO crystals begin to grow, reaching larger size. Then, the bands corresponding to Ni2+ in octahedral coordination in NiO begin to be observed in the spectrum of the 15Ni/AL catalyst, thus indicating the simultaneous presence of nickel sites linked to the support as well as bulk NiO (Fig. 6, spectra c–e). The above analysis is in strong agreement with what was observed by electron microscopy, where NiO crystals of 5−10 nm size are observed in 10Ni/AL and 15Ni/AL, in the last one co-existing with agglomerates of larger polygonal NiO crystals.In Nb-promoted catalysts, the nature of Ni2+-species depends on the catalyst preparation procedure. Spectrum of Ni/Nb/AL sample shows an intense band at 715 nm, Ni2+(Oh), in addition to a small band at 630 nm, Ni2+(Td) (Fig. 6, spectrum g). Thus, the presence of niobium species on the surface of support, as in the Ni/Nb/AL catalyst, limits the NiO-support interaction, thus favoring NiO crystallization. However, the distribution of niobium associated to nickel in (Ni+Nb)/AL sample (one-step synthesis), limits the growth of NiO crystals as the band at 715 nm shows lower intensity (Fig. 6, spectrum f). Accordingly, agglomerates of large NiO crystals are more abundant in Ni/Nb/AL sample, as observed by electron microscopy.Bands corresponding to NiO at 377 and 715 nm are particularly intense in catalysts 15Ni/AL-o1 and 15Ni/AL-o3, indicating that crystallized NiO has a weak interaction with the support. This fact is in agreement with electron microscopy data, where an increase in the crystallinity of NiO is observed although the size of the crystallites does not increase significantly. Table 1 summarizes the energy band gaps (Eg, in eV) of supported nickel oxide catalysts calculated from Kubelka-Munk function (Figure S5). In general, Eg values decrease when increasing the Ni-loading. In the same way, for a fixed NiO loading (15 wt%), the lowest band gap values are observed in Nb-containing catalysts (3.50 and 3.58 eV) and catalysts prepared in the presence of oxalic acid in the synthesis gel (3.25 and 3.55 eV). Samples from xNi/AL series with NiO contents in the range 5−15 wt% display the highest Eg among the samples analyzed (4.05−3.81 eV) (Table 1), which are also the most selective catalysts in the ODH of ethane. In our case, small differences are observed for all the catalysts, however, these band gap values alone cannot explain their catalytic properties as it may be influenced by various factors such as crystallite size, structural parameter, carrier concentrations, presence of impurities and lattice strain [41–44].In order to study the reducibility of catalysts and NiO-support interaction, TPR-H2 experiments were performed. Fig. 7 shows the TPR-H2 profiles of xNiO/AL catalysts with different NiO-loading (Fig. 7, patterns a to e) and catalysts with 15 wt% NiO, with or without promoters, and synthesized by different preparation procedures (Fig. 7, patterns f to i). For comparative purposes, TPR-H2 profile of a mechano-chemical mixture, Ni+AL(PM) sample (Fig. S2), is also included.The shape of reduction profiles depends on the strength of NiO-support interaction. In the prepared catalysts, profiles show three main features corresponding to (in order of decreasing reduction temperature) (Fig. 7): i) the reduction of small and highly dispersed NiO particles for which the above interaction is strong [17–19,36], which gives a peak at ca. 520−550 °C (observed in all catalysts); ii) the reduction of NiO particles that tend to form small agglomerates with medium strength interaction with the support, that originates a reduction peak slightly above 450 °C (observed in sample with Ni-loading of 15 wt%); and iii) the reduction of large NiO crystals with weak interaction with the support, which gives a peak at ca. 340 °C, similar to that observed in pure NiO [17–25] (observed in sample 30Ni/AL).Additionally, differences in reducibility can be clearly observed as a function of the catalyst preparation procedure when comparing catalysts with 15 wt% NiO (Fig. 7, patterns c, f-i). In this sense, Nb-promoted catalysts show a peak of high reducibility at 330 °C, which is more intense and constitutes the main feature of the reduction profile in Ni/Nb/AL. On the other hand, (Ni+Nb)/AL catalyst shows lower reducibility, with the main peak appearing at 530 °C. These results are consistent with the structural characterization. Thus, higher reducibility of Ni-species in Ni/Nb/AL catalyst can be interpreted on the basis of a low interaction between NiO and support (NbOx/Al2O3), which facilitates the growth of crystals and the formation of agglomerates that are more easily reduced. In this way, it has been proposed that Nb2O5 has not shown good properties as a NiO diluter/support, being unable in these conditions to eliminate a large proportion of non-selective sites [34].In catalysts prepared with oxalic acid in the synthesis gel, the reduction profiles show a unique peak around 400−450 °C, in contrast with the xNi/AL series with Ni-loading below 15 wt% NiO, that presents a reduction peak at ca. 520 °C. According to TEM data, catalysts with 15 wt% of NiO and different amounts of oxalic acid (i.e. 15Ni/AL-o1 and 15Ni/AL-o3) present a similar size distribution of the NiO crystals, although crystallinity clearly improves compared to 15Ni/AL when increasing amounts of oxalic acid are used. A poor interaction with the support is the reason of this change as well as the decrease in reducibility. This is also in agreement with results from DR-UV–vis spectra.It is worth mentioning that nickel aluminate-like species present a reduction peak at ∼ 800 °C, as reported in the literature [17,38]. This peak is not observed in the reduction profiles of the catalysts under study. This is in agreement with the structural and microstructural characterization of the samples, where the formation of NiAl2O4 has not been observed in any case. However, the possible presence of this phase cannot be completely ruled out, although, if present, it should be in low concentration in samples with low NiO-loading.According to the TPR-H2 experiments (Fig. 7), the catalyst 15Ni/AL presents the highest NiO-support interaction among the samples with a Ni-loading of 15 wt% NiO, also showing the maximum relative intensity of LO (1 P) band in UV Raman spectra (Fig. 5). Fig. 8 shows Ni 2p3/2 core-level XPS spectra for selected Al2O3-supported NiO catalysts, whereas Figures S4 and S5 displays the XPS results of additional NiO-based materials. Ni 2p3/2 core level spectra present the characteristic features of NiO, i.e. a main peak (ca. 856 eV) together with two satellites at 1.5–2.0 and 7.0 eV over the main peak (Sat I and Sat II, respectively) [13,15,33]. Sat I can be attributed to the presence of a wide variety of defects, such as Ni2+ vacancies, Ni3+ species or surface Ni2+-OH species; while Sat II is usually assigned to ligand-metal charge transfer. Changes in the relative intensity of Sat I signal have been related to the variations in the concentration of defects or in the particle size, which can be favored when an effective NiO-support interaction takes place [9,15,17–21,33]. Unfortunately, not a clear relationship between the relative intensity of Sat I / Main peak and the selectivity to ethylene has been observed.O 1s core level spectra are shown in Fig. 9 and Figure S6. In general, the O 1s signal of alumina appeared at higher binding energy (532.2 eV) than the signal for nickel oxide (530.5 eV) in all cases, as seen elsewhere [45]. The contribution of the alumina O 1s signal is bigger for the catalysts with Ni-loading lower than 20 wt% (Fig. 9), with a symmetric display of the peak. However, a shift of the band to lower binding energy is observed for catalysts with 30 wt% of NiO (Fig. S6), samples prepared with Nb (especially for Ni/Nb/AL sample (Fig. 9)) or a mechano-chemically treated sample (Fig. S6), in agreement with a worst dispersion of the NiO. A strong interaction of the oxygen anions with the Ni2+ cations for these catalysts was also suggested as a shift to lower binding energy occurred.Al 2p and Ni 3p spectra for the catalysts with different Ni amount (Fig. 10 ), show a single peak at 74.8 eV for Al 2p, corresponding to the Al3+ species in octahedral coordination, together with a signal at 68.5 eV, attributed to Ni 3p core level [45]. This latter Ni 3p core level peak increases in intensity as the NiO-loading increases. On the other hand, a small shift to lower binding energies when increasing the Ni loading is observed, likely associated with a progressively higher interaction of Al3+-bonded oxygen sites with Ni2+ cations, resulting in a distortion of the Al2O3 octahedral network [45].In Nb 3d XPS spectra (Fig. S7), Nb-containing catalysts presented a classical doublet with a split spin-orbit of the components of 2.78 eV which is related to the unique presence of dispersed Nb5+ [33].The TPR experiments undertaken confirm the close relationship between reducibility of nickel oxide species and the interaction of NiO particles with the support, which seems to determine the selectivity to ethylene during ethane ODH. Fig. 11 plots the relative hydrogen consumption of the band at 330 °C (reduction degree), related to NiO with low interaction with the alumina support, and the selectivity to ethylene at isoconversion conditions. It can be observed that the presence of NiO with low interaction with the alumina support must be avoided in order to achieve high ethylene formation, since an inverse relationship between the hydrogen consumption of the NiO reduction peak and the selectivity to the olefin has been observed. Accordingly, a higher interaction between NiO particles and support decreases the reducibility of Nin+ species, and the concentration of electrophilic oxygen species, thus favoring a more controlled oxygen supply during catalytic cycles, and a higher selectivity to the olefin.This NiO-support interaction determines not only NiO crystal size, but also the definition/crystallinity of the crystals (and the concentration of defects in the active phase). As discussed in the UV Raman spectra, an increase in the relative intensity of the 1 P LO band (I580) with respect to that for 2 P 2LO band (I1100) means that the NiO crystallite size decreases and/or the amount of oxygen defects increases. As the crystal size in all the catalysts is not constant, an accurate estimation of the number of defects cannot be undertaken through this technique. Interestingly, as observed by TEM, the reference catalyst 15Ni/AL and those with oxalic acid (samples 15Ni/AL-o1 and 15Ni/AL-o3) present very similar NiO crystal size (Fig. 4). However, the I580/I1100 ratio in the corresponding UV Raman spectrum of 15Ni/AL (Fig. 5, spectrum c) is remarkably higher than that observed for 15Ni/AL-o1 and 15Ni/AL-o3 catalyst (Fig. 5, spectra h and i). Accordingly, a different concentration of oxygen species can be proposed for the reference catalyst (15Ni/AL).Similarly, both Nb-containing catalysts present similar NiO crystal size (Fig. 4), but the catalyst prepared in one step (Ni+Nb)/AL displays a higher I580/I1100 ratio in the corresponding UV Raman spectra than the one prepared in two steps (Ni/Nb/AL) (Fig. 5, spectra f and g), which also results in a higher ethylene selectivity.Al2O3-supported nickel oxide catalysts prepared by a conventional wet impregnation method (without oxalic acid in the synthesis gel) showed high selectivity to ethylene during the ethane ODH. In this way, catalysts with NiO-loadings of 15–20 wt% NiO display interesting catalytic properties (with a selectivity to ethylene higher than 90 %), in agreement with previous results from other authors [16–19]. The slightly lower selectivity to ethylene observed in the catalysts with 5–10 wt% NiO could be related to the presence of highly dispersed Ni2+(Td) species but also to available unselective alumina sites, which present poor catalytic activity under our reaction conditions. The lower selectivity to ethylene observed over catalysts with Ni-loading higher than 20 wt% NiO can be related to the presence of big crystals of NiO, as deduced from TEM (Fig. 4), UV Raman (Fig. 5) and DR-UV–vis (Fig. 6) spectra, with weak NiO-support interaction, presenting high reducibility (Fig. 7).It is especially noteworthy that the positive effect of the incorporation of Nb5+ in unsupported Nb-promoted NiO catalysts, which has been widely reported in the scientific literature [9–15], is not observed in supported Nb-promoted NiO/Al2O3 catalysts, regardless of the preparation method. In fact, both (Ni+Nb)/AL and Ni/Nb/AL catalysts present lower selectivity to ethylene than that observed for Al2O3-supported NiO catalysts displaying a NiO-loading from 10 to 20 wt%. The characterization results of these catalysts, as well as those prepared in the presence of oxalic acid in the synthesis gel, clearly indicate the presence of NiO particles with low interaction with the support, whose reducibility and crystallinity increases when increasing the concentration of oxalic acid in the synthesis gel.For Al2O3-supported nickel oxide catalysts, the presence of oxalic acid during the synthesis has a deleterious effect on the catalytic performance, i.e. the higher the amount of oxalic acid in the synthesis gel, the lower is the selectivity to ethylene. This observation can be interpreted in terms of an increase in the reducibility of Nin+ sites due to a lower NiO-support interaction, which is not only related to crystallite size, but also to the crystallinity.The addition of an organic additive, such as oxalic acid, in the synthesis gel, during the preparation of unsupported materials, could help to decrease NiO particle size, thus leading to a material with lower amount of electrophilic oxygen, as observed in bulk SnO2-NiO catalysts [15]. However, the presence of a support can hinder the interaction of the organic additive with the active phase, due to a favored coupling between the additive and the support. This fact can lead, as in the case of NiO/Al2O3 system, to NiO particles presenting a bigger particle size and a higher crystallinity, even when high amounts of oxalic acid are used, thus leading to a lower selectivity to the olefin in the ODH of ethane. Moreover, this is in agreement with the characterization results by XRD, diffuse reflectance UV–vis and UV Raman.On the other hand, from XPS results (Figs. 8–10), it can be concluded that the samples with low reducibility (i.e. samples of xNi/AL series with NiO-loading lower than 20 wt. %) present: i) the characteristic features of NiO (with band at ca. 856 eV) and two satellites, Sat I and Sat II (at 1.5–2.0 and 7.0 eV, respectively, over the main peak) [9,15,17–21,33]; ii) a bigger contribution of the alumina O1 s signal (according to a higher dispersion of NiO); and iii) a shift to lower binding energy in the Al 2p + Ni 3p signal, confirming also the interaction between NiO particles and Al2O3 support [45]. All of these characteristics suggest the correlation between the selectivity to ethylene and a higher and more effective NiO-Al2O3 interaction.It must be noted that the amount of carbon detected on the catalysts after use is negligible. This positive aspect is due to the oxidative conditions employed since the possible coke formed is oxidized into carbon dioxide. Moreover, TPR experiments of representative used catalysts were undertaken. As it can be seen in Figure S8 the differences between the fresh and the used catalysts are hardly perceptible. Then, the most characteristic features that define a TPR assay: the hydrogen consumption, the onset temperature and the temperature for the maximum hydrogen consumption, seem to keep unaltered after the reaction.A tight correlation has been found between the NiO-Al2O3 interaction (as concluded from H2-TPR) and the selectivity to ethylene during the oxidative dehydrogenation of ethane on alumina-supported nickel oxide catalysts. This interaction depends not only on the NiO crystal size and Ni-loading, but also on the synthesis method employed. Interestingly, NiO crystals with similar size have shown remarkably different interactions with the support and, consequently, different levels of ethylene formation.The preparation method has been shown to be of capital importance to synthesize selective catalysts. Then, the use of oxalic acid in the preparation of NiO/γ-Al2O3 catalysts leads to lower NiO-alumina interaction, higher crystallinity of NiO particles and, subsequently, to a lower selectivity to ethylene during the ethane ODH.The presence of Nb5+ in alumina-supported Ni-Nb-O catalysts (i.e. (Ni+Nb)/AL) has not improved the selectivity to ethylene with respect to the corresponding Nb-free catalyst (i.e. 15Ni/AL), in contrast with the positive effect reported in bulk NiO catalysts. Nevertheless, the alumina-supported Ni-Nb-O catalysts, also prepared in presence of oxalate anions, are more selective to ethylene than the corresponding Nb-free samples prepared with oxalic acid in the synthesis gel (i.e. 15Ni/AL-o1) or than the nickel oxide supported on NbOx/γ-Al2O3 (i.e. Ni/Nb/AL). Therefore, the presence of oxalate anions in the synthesis gel hinders the interaction between NiO and γ-Al2O3, favoring the formation of supported NiO particles with high crystallinity. Yousra Abdelbaki, Investigation, Discussing, Writing - review & editing. Agustín de Arriba, Investigation, Discussing, Writing - review & editing. Benjamín Solsona, Supervision, Conceptualization, Validation, Writing - review & editing, Funding. Daniel Delgado, Investigation, Discussing, Writing - review & editing. Ester García-González, Investigation, Discussing, Writing - review & editing. Rachid Issaadi, Methodology, Investigation, José M. López Nieto, Supervision, Conceptualization, Writing - review & editing, Funding.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Authors would like to acknowledge the Ministerio de Ciencia, Innovación y Universidades in Spain through projects CRTl2018-099668-B-C21, MAT2017-84118-C2-1-R and PID2019-106662RB-C44 and Ministry of Higher Education and Scientific Research of Algeria for the National Exceptional Program for the fellowships. A.A. acknowledges Severo Ochoa Excellence Program for his fellowship (BES-2017-080329). EGG acknowledges Dr. E. Urones for the valuable assistance in the use of electron microscopy facilities as well as to the CNME (Spain).Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcata.2021.118242.The following is Supplementary data to this article:

Nickel oxides supported on γ-alumina (Ni-loading from 5 to 30 wt% NiO) have been synthesized and tested in the oxidative dehydrogenation (ODH) of ethane in order to determine the importance of the NiO-support interaction. The best performance was achieved by the catalyst with 15 wt% NiO; higher NiO-loadings lead to the formation of unselective bulk-like NiO and lower Ni-loadings present high proportion of free alumina surface sites. The presence of oxalic acid and/or niobium in the synthesis gel resulted in the formation of NiO particles with similar size, but higher crystallinity and reducibility than the standard 15 wt% NiO catalyst. The obtained results have revealed that, in addition to NiO crystal size, the nickel oxide-support interaction determines the catalytic performance of these catalysts.

Fast depletion of conventional crude oil reserves and increasing demand for clean transportation fuels greatly stimulate the research and development of heavy oil upgrading technologies (Bellussi et al., 2013; Browning et al., 2021; Saab et al., 2020). Efficient conversion of vacuum residue (VR), the heaviest fraction of crude oil, into lighter fractions such as gasoline and diesel is considered as one of the greatest challenges in the modern petroleum processing industry (Omajali et al., 2017; Prajapati et al., 2021). VR is mainly comprised of high boiling point polycyclic aromatic hydrocarbons with large amount of sulfur (S), nitrogen (N), and metals (usually vanadium (V) and nickel (Ni)) (Pham et al., 2022; Prajapati et al., 2022), resulting in coke formation on both catalyst and equipment in the refining process (Tsubaki et al., 2002; Fortain et al., 2010; Kim et al., 2017). Among the various VR conversion technologies developed up to date, slurry-phase hydrocracking technology is considered as the most efficient and economic one because of its great feedstock flexibility, high conversion efficiency, and high light distillates yield. It is recognized that catalyst is the key in this process, because it determines the feedstock conversion and the liquid product yield (Saab et al., 2020; Morawski and Mosiewski, 2006; Looi et al., 2012).Slurry-phase hydrocracking catalysts include homogeneously dispersed catalysts of oil-soluble dispersed catalysts and water-soluble dispersed catalysts and heterogeneous solid powder catalysts (Purón et al., 2013; Nguyen et al., 2015). Oil-soluble dispersed catalysts exist as organometallic compounds can effectively depress the gas and coke formation due to its homogeneously dispersed heavy oil to adequately contact with reactant molecule (Chen et al., 2022; Kang et al., 2019). Water-soluble dispersed catalysts are prepared with multiple steps such as dispersion, emulsification and dehydration, which greatly increase the operation complexity and cost (Liu et al., 2009; Luo et al., 2011). Natural mineral catalysts as solid powder catalysts, which have the advantages of low cost and broad sources, were extensively used in the early stage of slurry-phase hydrocracking technology. Nevertheless, they were replaced gradually by other types of catalysts due to their inferior catalytic activities and property instability (Yue et al., 2016, 2018; Manek and Haydary, 2017). Supported metal catalysts are composed of active metal and support, in which the active metals are usually Mo, Co, Ni or their binary/trinary combinations, and the supports are commonly acidic materials such as alumina, silica-alumina, zeolites and even natural minerals (Looi et al., 2012; Leyva et al., 2007, 2009). As compared with natural mineral catalyst, supported metal catalyst has the advantages on the enhancement of hydrocracking reactivity and adjustable performance, thus it has attracted increasing attention in slurry-phase hydrocracking.In supported metal catalysts, the metal species are considered as the active centers to hydrogenate the polycyclic aromatic hydrocarbons and olefins, and to quench the free radicals in the feedstock and intermediate products to avoid the over-cracking reactions and condensation reactions to form gas and coke. In addition, the support also plays a crucial role in determining the catalytic performance by affecting the metal dispersion on the catalyst surface and thus influencing the feedstock conversion. There were many reports on supported catalysts employed in the slurry-phase hydrocracking process. The MoS2/SiO2–ZrO2 bifunctional catalyst was applied in the slurry-phase hydrocracking of decalin-phenanthrene mixture to study the effect of Si/Zr molar ratio on performance of catalyst, the analysis results revealed that Brönsted acid on SiO2–ZrO2 support was mainly contributed to the catalytic performance (Ma et al., 2021). Looi et al. (2012) prepared a series of catalysts using alumina supports with different pore sizes and investigated their catalytic performance in residual oil hydrocracking, the result showed that the residue oil conversion was about 50 wt% and the highest yield of liquid products was 97 wt% at 400 °C, and more acid sites benefited to the residue oil conversion. Yue et al. (2018) prepared the slurry-phase hydrocracking catalyst by using a hydrothermally treated natural bauxite mineral as the support and assessed their performance by using a high temperature coal tar as the feedstock, and they found that the high feedstock conversion and high liquid yield were attributed to the suitable support acidity and the weaker interaction between the active metal and the support. Sánchez et al. (2018) investigated the catalytic performance of a bifunctional MoS2/ASA (amorphous silica-alumina) catalyst in the slurry-phase hydroconversion, and found that the presence of moderate Brönsted acid sites promoted the cracking, isomerization and ring-opening reactions. Despite numerous reports available on supported slurry-phase hydrocracking catalysts, there is a lack of systematic and deep understanding on the influences of support properties such as composition, pore structure and acidity on catalytic performance.Herein, we present a thorough study on the effects of support properties on the catalytic performance of supported metal catalysts in the VR slurry-phase hydrocracking process. A series of Mo catalysts supported on SiO2, γ-Al2O3, amorphous silica-alumina (ASA) and ultra-stable Y (USY) zeolite, respectively, were prepared by the conventional impregnation method. The pore structure and acidity of the different supports were examined by N2-adsorption-desorption and pyridine adsorbed Fourier transform infrared (Py-FTIR) spectroscopy, the morphology of metal sulfide species on the corresponding catalysts were investigated by high resolution transmission electron microscopy (HRTEM), and the catalytic performances of the different catalysts were compared in the VR slurry-phase hydrocracking.In the present study, SiO2, γ-Al2O3, ASA and USY zeolite with Si/Al ratio of 2.7 were used as the supports to prepare slurry-phase hydrocracking catalysts. SiO2, γ-Al2O3 and USY zeolite were obtained from Shanghai Aladdin Bio-Chem Technology Co. Ltd., Fujian Yucheng Environmental Protection Technology Co. Ltd. and Nankai University, respectively. ASA was prepared as follows: 25 mL of an aluminum nitrate solution with a concentration of 2 mol/L and 30 mL ammonia water (25 wt% of ammonia) were simultaneously and slowly added into 50 mL water to maintain pH value of 8–9 at 60 °C under agitation. Then, 2.7 g of sodium silicate (27.68% SiO2 and 8.95% Na2O) was added into the above solution to obtain a mixture with a SiO2/Al2O3 molar ratio of 1:1. Finally, the resulting mixture was aged for 1 h, filtered with distilled water, dried at 120 °C for 10 h, and calcined at 500 °C for 3 h to obtain ASA sample.A series of catalysts supported on the different materials were prepared by the conventional incipient wetness impregnation method with an aqueous solution of ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O, 98%, Adamas). The resultant samples were aged at 30 °C for 12 h, dried at 120 °C for 10 h, and calcined at 600 °C for 4 h to obtain the corresponding supported Mo oxide catalysts, which are designated as Mo/SiO2, Mo/USY, Mo/γ-Al2O3 and Mo/ASA, respectively. The MoO3 content in each catalyst is 5 wt% according to the literature (Kim et al., 2018; Ancheyta et al., 2003).N2 adsorption-desorption measurement was taken on a Micromeritics ASAP 2460 apparatus at −196 °C. The surface area (S BET) of sample was determined by using Brunauer-Emmett-Teller (BET) equation, and the pore volumes (V total) and average pore diameters (D p) were obtained by Barrett-Joyner-Halenda (BJH) method. Adsorbed pyridine Fourier transform infrared spectroscopy (Py-FTIR) measurement was carried out on a MAGNAIR 560 FTIR instrument, and the spectra were recorded at 250 and 350 °C, respectively. H2 temperature programmed reduction (H2-TPR) was conducted on an ASAP-2920 equipment using a thermal conductivity detector (TCD). Around 20 mg of sample was firstly pretreated in Ar steam at 300 °C for 30 min, and then cooled to 50 °C. The H2-TPR profiles were acquired from 50 to 950 °C at a heating rate of 10 °C/min in a 10 vol% H2/Ar stream.High resolution transmission electron microscopy (HRTEM) images of MoS2 slabs were collected on a Tecnai G2 F20 instrument at 200 kV, and at least 200 slabs were measured to calculated for each sample. The average lengths ( L ¯ ) and stacking numbers ( M ¯ ) according to the Eqs. (1) and (2) (Liu et al., 2017; Escobar et al., 2018): (1) Average slab length : L ¯ = ∑ i = 1 n x i L i ∑ i = 1 n x i (2) Average stack number : M ¯ = ∑ i = 1 n x i m i ∑ i = 1 n x i where L i , x i and m i denote the length, the number and the layer number in a stack of MoS2 slabs.The MoS2 dispersion (D Mo) was acquired by Eq. (5), f Moe is the ratio of Mo atoms located at edge sites of MoS2 slabs, and f Moc is the fraction of Mo atoms at corner sites, which were estimated by Eqs. (3) and (4) (Hensen et al., 2001; Kasztelan et al., 1984): (3) f Moe = Mo edge Mo total = ∑ i = 1 t 6 ( x i − 2 ) ∑ i = 1 t ( 3 x i 2 − 3 x i + 1 ) (4) f Moc = Mo corner Mo total = 6 ∑ i = 1 t ( 3 x i 2 − 3 x i + 1 ) (5) D Mo = f Moe + f Moc = Mo edge + Mo corner Mo total = ∑ i = 1 t 6 ( x i − 1 ) ∑ i = 1 t ( 3 x i 2 − 3 x i + 1 ) where Moedge, Mocorner and Mototal are the numbers of Mo atoms along on edge sites, corner sites and total Mo atoms on MoS2 slabs, t represents the total number of slabs, and x i denotes the number of Mo atoms on edge sites of each MoS2 slab, acquired by L i  = 3.2 × (2x i –1).X-ray photoelectron spectroscopy (XPS) was taken on a Thermo ESCALAB 250 spectrometer with a monochromatic Al Kα source. C 1s peak at 284.6 eV was used as reference to calibrate the binding energy. The XPSPEAK41 software was employed to analyze the experimental results. The relative contents of each species of MoS2, MoS x O y and Mo6+ oxide for an individual sulfide catalyst were determined through their peak areas. For instance, the relative MoS2 content [MoS2] (%) was calculated by Eq. (6): (6) [ MoS 2 ] ( % ) = A MoS 2 A MoS 2 + A MoS x O y + A Mo 6 + × 100 % where A X represents the peak area of species X in Mo 3d XPS envelope.VR provided by SINOCHEM Quanzhou PetroChemical Co., Ltd. was employed as the feedstock to assess the slurry-phase hydrocracking performance of catalysts, and its properties are shown in Table 1 .The catalyst assessment for VR hydrocracking was carried out in a 300 mL stainless-steel autoclave equipped with a stirrer. 40 g of VR, 1.2 g of catalyst and 0.88 g of sulfur powder were added into the reactor. Prior to the hydrocracking reaction, the sulfuration of catalyst in situ was conducted at 350 °C under an initial H2 pressure of 11 MPa for 5 h, subsequently, the VR hydrocracking reaction was performed at 430 °C with a volumetric H2 to oil ratio of 850 (v/v) for 3 h under a stirring rate of 600 rpm. After reaction, the mixture of the reaction product and catalyst was collected after the autoclave rapidly cooled to room temperature and was separated by centrifugalization and filtration. The liquid product was divided into four fractions in a SYD-9168 vacuum distillation apparatus according to the boiling point (BP) range, the four fractions of naphtha, middle distillate, vacuum gas oil (VGO) and VR are in the range of BP < 180 °C, 180–350 °C, 350–500 °C and BP > 500 °C. Additionally, the solid residue including coke and the used catalyst was washed with toluene. The VR conversion and the yields of gas, naphtha, middle distillate, VGO and coke were acquired by the following Eqs. (7)–(12): (7) VR conversion ( wt % ) = M f − M p M f × 100 % (8) Gas yield ( wt % ) = M g M t × 100 % (9) Naphtha yield ( wt % ) = M n M t × 100 % (10) Middle distillate yield ( wt % ) = M m M t × 100 % (11) VGO yield ( wt % ) = M v M t × 100 % (12) Coke yield ( wt % ) = M c M t × 100 % where M f, M p are mass of >500 °C fraction in the feed and product, M g, M n, M m, M v and M c denote mass of gas, <180 °C fraction, 180–350 °C fraction, 350–500 °C fraction, and coke in product, meanwhile, M t means the total mass of feed.VR slurry-phase hydrocracking performance of Mo catalysts supported on the different supports was assessed, and the resulting product was distilled to obtain the different distillate fractions. The VR conversions and the yields of naphtha and middle distillate obtained over different catalysts are shown in Fig. 1 . It can be seen that the VR conversion over Mo/ASA is 75.2%, only slightly higher than those (73.9% and 73.5%) over Mo/γ-Al2O3 and Mo/USY, but significantly higher than that (67.4%) over Mo/SiO2. The results demonstrate that, as compared with the other catalysts, Mo/ASA can effectively convert the heavy fraction with large molecules in VR into lighter fractions with smaller molecules. The yields of naphtha and middle distillate over the different catalysts are in the order of Mo/ASA > Mo/γ-Al2O3 ≈ Mo/USY > Mo/SiO2, indicating that Mo/ASA favors the production of naphtha and middle distillate. Fig. 2 displays the yields of naphtha, middle distillate, VGO, gas and coke obtained over the different catalysts. The naphtha yield over Mo/ASA is 19.5 wt%, slightly lower than that (21.6 wt%) over Mo/USY, but obviously higher than those (14.3 wt% and 15.9 wt%) over Mo/SiO2 and Mo/γ-Al2O3, while the middle distillate yield over Mo/ASA is 35.2 wt%, much higher than those over the others. The VGO yields obtained over the different catalysts follow the order of Mo/γ-Al2O3 (22.8 wt%) ≈ Mo/SiO2 (22.7 wt%) > Mo/ASA (18.3 wt%) > Mo/USY (16.4 wt%). The yields of unconverted residue decrease in the order of Mo/SiO2 (21.5 wt%) > Mo/USY (17.5 wt%) ≈ Mo/γ-Al2O3 (17.2 wt%) > Mo/ASA (16.4 wt%). The yields of gas are in the order of Mo/USY (15.6 wt%) > Mo/SiO2 (11.7 wt%) > Mo/γ-Al2O3 (10.2 wt%) ≈ Mo/ASA (10.1 wt%), and the yields of coke increase in the order of Mo/ASA (0.5 wt%) ≈ Mo/γ-Al2O3 (0.6 wt%) < Mo/SiO2 (0.8 wt%) < Mo/USY (1.7 wt%). By comparing the above results, it is concluded that Mo/ASA exhibits the best overall performance among all the catalysts due to its highest VR conversion, highest yield of naphtha and middle distillate, and relatively lower yields of gas and coke.The hydrogen consumption can be considered as an index of catalyst hydrogenation activity, because VR slurry-phase hydrocracking reaction is accompanied with hydrogen consumption (Bianco et al., 1994; Kang et al., 2020). Fig. 3 shows the H2 pressure profiles during the VR slurry-phase hydrocracking process involving the different catalysts. Before the reaction, the initial H2 pressure in the reactor was 11.0 MPa at room temperature, when the reaction temperature was increased to 430 °C, the pressure in reactor increased up to 21.5 MPa, then the pressure in reactor gradually decreased with the prolonging reaction time, and the decreasing tendencies were different for the reaction systems involving the different catalysts. After the hydrocracking reaction was terminated by cooling the reactor to about 200 °C, the pressures in the reactors loaded with different catalysts were dramatically dropped, the residual pressures were in the order of Mo/ASA < Mo/USY < Mo/γ-Al2O3 < Mo/SiO2. This indicated that Mo/ASA has the highest hydrogenation activity among the four catalysts. The Mo catalysts on the different support present different slurry-phase hydrocracking performances, thus it is necessary to deeply analyze properties and pore structures of these supports and their derived catalysts to understand the underlying reasons.The textural properties of catalysts can significantly impact their VR hydrocracking performance. It is widely accepted that macro-/meso-porous structure in supported catalysts benefits the diffusion of bulkier molecules in VR and thereby provides higher accessibility of active sites to reactants molecules, promoting feedstock conversion and improving the selectivity to target products (Leyva et al., 2014; Zheng et al., 2019). To understand the effects of pore structures of the supports on the catalytic performance of their derived catalysts, N2 adsorption-desorption measurements were conducted to compare the textural properties of the different supports, and the results are shown in Fig. 4 . It can be seen that the N2 adsorption-desorption isotherms of USY zeolite and γ-Al2O3 belong to type II ones with a H4 hysteresis loop, but those of SiO2 and ASA belong to type IV ones with a H1 hysteresis loop and a H3 hysteresis loop, respectively, indicating that the different supports have different pore structures. The surface areas, pore volumes and average pore diameters of the different supports calculated from the N2 adsorption-desorption data are summarized in Table 2 . It is seen that the surface area (585 m2/g) of USY zeolite is much larger than those (386 m2/g, 198 m2/g, and 226 m2/g) of ASA, γ-Al2O3 and SiO2, whereas ASA has the largest external surface area (357 m2/g) among all the supports, with USY zeolite having the smallest external surface area (52 m2/g). The average pore volumes of different supports are in the order of SiO2 (0.89 cm3/g) > ASA (0.76 cm3/g) > USY zeolite (0.34 cm3/g) > γ-Al2O3 (0.25 cm3/g), but their mesoporous volumes are in the order of SiO2 (0.88 cm3/g) > ASA (0.75 cm3/g) > γ-Al2O3 (0.17 cm3/g) > USY zeolite (0.07 cm3/g). These results suggest that, among different supports, ASA that simultaneously has the largest external surface area, larger average pore volume and mesoporous volume should be the most suitable for the preparing slurry-phase hydrocracking catalyst, because its larger average pore volume and mesoporous volume are beneficial for the diffusion of bulkier molecules in VR and their generated intermediate molecules onto the active sites of catalyst, and its largest external surface area favors metal dispersion and thereby generates more active metal sites to restrain gas and coke formation, this can provide the highest VR conversion, highest total yield of naphtha and middle distillate, and lowest yields of coke and gas obtained over Mo/ASA, as shown in Figs. 1 and 2.The acid properties of the different supports were characterized by Py-FTIR, the results measured at 250 and 350 °C are shown in Fig. 5 . The bands at 1540 and 1450 cm−1 are attributed to Brönsted and Lewis acid sites, respectively (Schweitzer et al., 2022). The Py-FTIR spectrum of USY zeolite has a stronger adsorption peak at 1540 cm−1 and a weaker adsorption peak at 1450 cm−1, indicating that USY zeolite has a large amount of B acid sites, ascribed to the bridging Si–OH–Al groups because of the replacement of Si4+ in the crystallite framework by Al3+ (Tang et al., 2019). However, it has only a very small amount of L acid sites. The spectrum of γ-Al2O3 has an adsorption peak at 1450 cm−1 but no apparent peak at 1540 cm−1, and that of ASA has two weaker peaks at 1450 and 1540 cm−1 for both 250 and 350 °C, illustrating the existence of small amounts of B and L acid sites in ASA. No obvious pyridine adsorption peak is observed for SiO2, indicating its negligible acid sites. The amounts of acid sites calculated according to Py-FTIR spectra measured at 250 and 350 °C for the different supports are summarized in Table 3 . Notably, the acid sites determined at 250 °C are considered as weak ones, while the acid sites determined at 350 °C can be taken as moderate and strong ones (Gafurov et al., 2015; Phung and Busca, 2015). No acid site exists in SiO2, as shown in Table 3, and the USY zeolite has the largest amount of acid sites including weak, moderate and strong ones, especially B acid sites. Thus, γ-Al2O3 and ASA present the total acid amounts standing between those of SiO2 and the USY zeolite, with the former having only L acid sites and the latter having only a smaller amount of L acid sites but a larger amount of B acid sites, which is ascribed to the bridging hydroxyls in connection with tetrahedrally coordinated Al species on silica (Valla et al., 2015). It indicates the amounts of acid sites in the different supports are in the order of USY zeolite > γ-Al2O3 > ASA > SiO2.By comparing the acidity characterization results and the hydrocracking reaction results in Figs. 1 and 2, it is found that Mo/SiO2 prepared from SiO2 with the largest average pore volume and mesoporous volume but without acid sites gives the lowest VR conversion among the different catalysts. Mo/USY prepared from USY zeolite with the smallest external surface area and mesoporous volume but with the largest amount of acid sites displays a higher VR conversion than Mo/SiO2. Mo/γ-Al2O3 prepared from γ-Al2O3 with a relatively larger external surface area and a slightly larger mesoporous volume but with a larger amount of L acid sites, gives a VR conversion comparable to that of Mo/USY but much lower yields of gas and coke than Mo/USY. Mo/ASA prepared from ASA with the largest external surface area, larger mesoporous volume and more B and L acid sites presents the highest VR conversion, the highest yields of naphtha and middle distillate, and the yields of gas and coke comparable to that of Mo/Al2O3 but much lower than those of Mo/SiO2 and Mo/USY. Therefore, it can be concluded that both support acidity and pore structure can significantly impact VR conversion and the distribution of various distillates of hydrocracking product. By further comparing the acidity properties of ASA and γ-Al2O3, and the catalytic performance of their corresponding supported catalysts, it is interesting to note that, despite of the lower amount of acid sites of ASA than γ-Al2O3, Mo/ASA shows a slightly higher VR conversion, because ASA has abundant B acid sites and a larger mesoporous volume that can promote the hydrocracking reactions of VR following the carbenium ion mechanism (Weitkamp, 2012).The reducibility of Mo species on the four supported catalysts were investigated by H2-TPR and the results are given in Fig. 6 . In the H2-TPR profiles, two H2 reduction peaks are observed: the low-temperature peak can be attributed to the reduction of octahedrally coordinated Mo6+ species to tetrahedrally coordinated Mo4+ species, and the high-temperature peak can be assigned to the further reduction of tetrahedrally coordinated Mo4+ species to Mo (Wang et al., 2002; Liu et al., 2012; Lv et al., 2018; Zhang et al., 2019). The low-temperature peaks of the four catalysts are in the range of 400–500 °C and shift to high temperatures in the order of Mo/ASA < Mo/γ-Al2O3 < Mo/USY < Mo/SiO2. It is also noted that the high temperature reduction peaks of Mo/ASA, Mo/SiO2 and Mo/γ-Al2O3 are centered at 816, 820 and 857 °C, respectively, whereas no obvious high temperature reduction is observed for Mo/USY. This suggests that the reducibilities of Mo species in the four catalysts are different, possibly due to the different metal-support interaction (Fan et al., 2007). Generally, it is considered that the ability of a metal oxide to adsorb and activate hydrogen is related to the reduction temperature, and a metal oxide that can be easily reduced by hydrogen usually has a higher hydrogenation activity (Cheng et al., 2020). Therefore, the higher yields of naphtha and middle distillate and the lower yields of gas and coke obtained over Mo/ASA can be attributed to the easier reduction of Mo species in Mo/ASA.HRTEM characterization was performed to observe the morphology of MoS2 slabs as the active phase in the hydrocracking reaction, the representative images obtained are shown in Fig. 7 . The statistical results of the lengths and stacking numbers of MoS2 slabs are presented in Fig. 8 and Table 4 . The two-dimensional thread-like fringes with layer stacking spacing of about 0.65 nm can be assigned as MoS2 slabs (Zheng et al., 2019). The lengths of MoS2 slabs are mainly from 3 to 7 nm for all catalyst, as shown in Fig. 8a. In addition, the average length of MoS2 slabs on different catalysts reduces following of Mo/SiO2 (6.2 nm) > Mo/USY zeolite (5.2 nm) > Mo/γ-Al2O3 (4.8 nm) > Mo/ASA (4.3 nm). It is found that the MoS2 slabs on Mo/SiO2, Mo/γ-Al2O3 and Mo/ASA are mainly stacked in 1–2 layers, but those on Mo/USY are mainly stacked in 2–5 layers, as shown in Fig. 8b. The average stacking number of MoS2 slabs on Mo/USY catalyst is 3.0, obviously higher than those on the others (1.7 for Mo/SiO2, 1.5 for Mo/γ-Al2O3 and 1.6 for Mo/ASA). The results reveal that Mo/ASA exhibits small MoS2 particles with the lowest stacking number and the shortest slab length among all catalysts, implying the more exposure active sites and the weaker space resistance that benefit to improve the hydrocracking activity (Liu et al., 2019). The dispersion degrees of Mo species (D Mo) and the proportion of Mo species located along edge sites of MoS2 slabs (f Moe) with high hydrogenation activity were also estimated based on the HRTEM results. The values of D Mo and f Moe for the different catalysts increase as Mo/SiO2 (0.17 and 0.19) < Mo/USY (0.20 and 0.23) < Mo/γ-Al2O3 (0.21 and 0.25) < Mo/ASA (0.23 and 0.27). The result indicates that the highest dispersion and the largest proportion of Mo species located along edge sites of MoS2 slabs on Mo/ASA, possibly due to the appropriate interaction of ASA with Mo species. Generally, the D Mo and f Moe values of MoS2 slabs can determine the hydrogenation activity of catalyst, because MoS2 slabs with larger D Mo and f Moe values can expose more hydrogenation active sites, favoring the hydrogenation reaction to avoid over-cracking reaction and the condensation reaction of polycyclic aromatic hydrocarbons to yield gas and coke (Jiang et al., 2017). Therefore, the different yields of naphtha and middle distillate, gas and coke over the four catalysts can be attributed to their different D Mo and f Moe values.The obtained Mo 3d XPS spectra and their deconvolution results are shown in Fig. 9 and Table 5 . The Mo 3d XPS envelope includes three Mo 3d doublets, the doublet with binding energies at 229.0 ± 0.2 and 232.1 ± 0.2 eV for Mo 3d5/2 and Mo 3d3/2 are assigned to MoS2 species (Mo4+), the binding energies at 230.9 ± 0.2 and 234 ± 0.2 eV for Mo 3d5/2 and Mo 3d3/2 are related to MoS x O y oxysulfide compounds (Mo5+), and the binding energies at 232.6 ± 0.2 and 235.8 ± 0.2 eV for Mo 3d5/2 and Mo 3d3/2 are attributed to MoO3 species (Mo6+) (Ninh et al., 2011). Table 5 lists the relative content of MoS2 species that represents the sulfuration degree of Mo/SiO2 (63%) < Mo/USY (66%) < Mo/γ-Al2O3 (70%) < Mo/ASA (74%). In view of the fact that the active Mo sites are generally located at edge and corner sites, and the hydrogenation reaction mainly occurs along the edge sites of MoS2 crystals (Alsalme et al., 2016; Zheng et al., 2021), the contents of Mo atoms located at edge sites (Moe) in the different catalysts were calculated by Moe = MoS2 × f Moe, the results are summarized in Table 5. It can be seen that the contents of Mo atoms located at edge sites are in the order of Mo/SiO2 (10.7) < Mo/USY (13.2) < Mo/γ-Al2O3 (14.7) < Mo/ASA (17.0), indicating the hydrogenation activity increases following as the above tendency, it is consistent with HRTEM result. The XPS result demonstrates that the hydrocracking product distributions over the different catalysts are related with the sulfuration degrees and the contents of Mo atoms located at edges sites of Mo species. Mo/ASA catalyst with the highest sulfuration degree and content of Mo atoms located at edge sites of Mo species presents the highest hydrogenation activity to restrain the over-cracking reaction producing gas and the condensation reaction forming coke.In this study, four Mo catalysts supported on four supports (ASA, γ-Al2O3, USY zeolite and SiO2) were compared by investigating the effects of their pore structure, acidity and Mo species properties on their surface on the VR slurry-phase hydrocracking performance. The hydrocracking reaction results show that the VR conversions obtained over the four catalysts are in the order of Mo/ASA > Mo/γ-Al2O3 ≈ Mo/USY > Mo/SiO2, which is closely related to the acid site amounts and pore structures of supports. Moreover, the highest VR conversion over Mo/ASA is attributed to the larger mesoporous volume of ASA that is beneficial for the diffusion of the large molecules in VR to contact with the active sites on catalyst. Additionally, the appropriate amount of acid sites, especially B acid sites, of ASA enhances the catalytic cracking of VR. Therefore, both the appropriate amount of acid sites and larger mesoporous volume of supports are essential to catalytic activity. Importantly, the catalyst Mo/ASA exhibits the highest yield (54.7 wt%) of naphtha and middle distillate and the lowest yields (10.1 wt% and 0.5 wt%) of gas and coke among all catalysts, because the Mo species in Mo/ASA possess simultaneously the highest sulfuration degree, highest dispersion degree of MoS2 slabs, and largest proportion of Mo atoms located at the edges sites. These features endow Mo/ASA with the highest hydrogenation activity, and thereby it can effectively restrain the over-cracking reaction of intermediate products and the condensation of polycyclic aromatic hydrocarbon to reduce the yields of gas and coke.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors acknowledge National Key Research and Development Program of China (2018YFA0209403), National Natural Science Foundation of China (21908027) and Qingyuan Innovation Laboratory Program (00121002) for financing this research.

To deeply understand the effects of support properties on the performance of Mo-based slurry-phase hydrocracking catalysts, four Mo-based catalysts supported on amorphous silica alumina (ASA), γ-Al2O3, ultra-stable Y (USY) zeolite and SiO2 were prepared by the incipient wetness impregnation method, respectively, and their catalytic performances were compared in the vacuum residue (VR) hydrocracking process. It is found that the Mo/ASA catalyst exhibits the highest VR conversion among the different catalysts, indicating that both the appropriate amount of acid sites, especially B acid sites and larger mesoporous volume of ASA can enhance the VR hydrocracking into light distillates. Furthermore, Mo catalysts supported on the different supports show quite different product distributions in VR hydrocracking. The Mo/ASA catalyst provides higher yields of naphtha and middle distillates and lower yields of gas and coke compared with other catalysts, it is attributed to the highest MoS2 slab dispersion, the highest sulfuration degree of Mo species, and the most Mo atoms located at the edge sites for the Mo/ASA catalyst, as observed by HRTEM and XPS analyses. These features of Mo/ASA are beneficial for the hydrogenation of intermediate products and polycyclic aromatic hydrocarbons to restrict the gas and coke formation.

Data will be made available on request.Considering the current status of the global energy policy, the coming years will be marked by important challenges, associated with the desired energy transition to a circular bioeconomy. In this scenario, gasification of biomass is of particular interest since it can convert a variety of low-value feedstocks into a valuable gas product, providing a flexible renewable source for the production of baseload electricity, transportation fuels and various chemicals [1–3]. During biomass thermal conversion, additional contaminants are also produced which require cleaning and conditioning of the raw gas.Despite the recognized potential of biomass gasification, its transition to an industrial context still faces technical shortcomings, mostly resulting from the formation of undesired tar which causes several operating problems [4,5]. In addition, the H2:CO molar ratio of the biomass-derived gas shows typical values < 2.0, which implies further adjustments to fulfil the quality demands of different end-use applications [6]. Accordingly, the integration of heterogeneous catalysts as a part of the gasification concept is crucial to connect the divergence between the conversion of biomass-derived gas with profitable products.Catalytic hot gas upgrading has been widely investigated, as reported in previous reviews provided by e.g. Guan et al. [7], Sutton et al. [8], Zhang et al. [9] and Shahbaz et al. [10]. Ni-based materials revealed greater activity for the conversion of tar to CO and H2 but are relatively expensive and require continuous regeneration. In addition, deactivation by sulfur chemisorption on Ni active sites and microstructural ageing are other recognized limitations [11,12], making these type of materials unattractive in an industrial context that present severe process conditions.On the other hand, Fe-based materials has shown a growing interest for gasification applications because of their low-cost, lower environmental impact and effective performance toward tar cracking and H2 production [13,14]. However, complications resulting from carbon deposition is expected for in-situ applications due to unconverted char in the gasifier bed and the catalytic mechanisms involving tar side reactions [15]. There is also evidence that redox changes of iron active sites, induced by the thermochemical conditions of the biomass-derived raw gas, might result in a progressive decline of catalytic activity [16]. Gas-solid interactions with S-containing compounds may also have implications on the selectivity of chemical reactions [17]. The design of suitable operating conditions is therefore crucial for extending catalyst lifetime, which is a crucial issue in the context of process economics.Thermodynamic modelling has been extensively applied to support the optimization of biomass gasification technologies [18–22], giving qualitative and quantitative information about the operational limits. Though the relevance of such approach, theoretical analysis specifically addressing catalyst performance are limited in literature. Studies are mostly focused on the evaluation of specific reactor parameters, such as the gasifier agent, temperature and composition of biomass feedstocks, with the objective of improving producer gas quality and overall gasification efficiency. Subsequently, there is a particular need of understanding the interactions of iron-based materials with biomass-derived gas within the gasifier, as well as simplified modelling approaches to evaluate the operating conditions required for efficient operation.Therefore, the present study aimed to develop a graphical approach to support the operation of iron-based materials during biomass gasification. A combination of experimental data and thermodynamic modelling was applied as guidelines to elucidate the dependence of catalyst performance on the process conditions. This is expected to provide comprehensive guidelines for the selection of proper operating conditions for catalysts, which is still dictated mainly by empiricism, and to minimize the impact of some underestimated deactivation mechanisms. In addition, a mathematical model for gasification of biomass is provided by applying mass balances, energy balances and thermodynamic equilibrium predictions, aiming to establish appropriate conditions for the betterment of carbon conversion. Though the fundamentals discussed here are of general applicability, the work is focused on autothermal gasification of biomass, where primary measures are preferable to enhance process efficiency.A biomass gasification model was employed to support the design of temperature diagrams. It was based on thermodynamic equilibrium approach that consists in evaluating the composition of the biomass-derived products using minimization Gibbs energy. The proposed model is based on steady-state calculations of energy associated with all the intervenient species and includes the following general assumptions: i) perfect mixing and uniform temperature and pressure; ii) heat losses through the gasifier are neglected; iii) the biomass feedstock is represented by an equivalent molecule comprising carbon (C), hydrogen (H) and oxygen (O). The presence of nitrogen (N) and sulfur (S) is neglected, and the fraction of ashes in the biomass feedstock is only considered for its impact on the overall mass balance; iv) the model assumes that gasification reaction rates are fast and residence time is long enough to reach chemical equilibrium; v) reaction products mainly consist of H2, CO, CO2, CH4, H2O, N2 and unconverted carbon (char). Details concerning the thermodynamic equilibrium approach was provided in previous investigations [18,19].Based on the aforementioned considerations, mass and energy balances, per unit of biomass on a dry and ash-free basis (daf), were implemented. The principle of mass conservation applied to the gasification process can be described by a single global equation and expressed as follows: (1) ( 1 1 − W w ) ∙ ( 1 + W A 1 − W A ) + W G A − Y C h a r − Y G a s − ( W A 1 − W A ) = 0 where W w is the fraction of moisture in biomass ( k g H 2 O ∙ k g f u e l − 1 ), W A is the fraction of ash in dry biomass ( k g A s h ∙ k g d r y , f u e l − 1 ), W G A is the ratio between the gasifier agent and biomass on a dry and ash-free basis ( k g G A ∙ k g d a f , f u e l − 1 ), Y C h a r and Y G a s are the mass yields of char and producer gas, respectively. The W G A may comprise different components, being formulated according to the following equation: (2) W G A = E R · W s · [ 1 + M N 2 M O 2 ∙ ( 1 X O 2 , G A − 1 ) ] + W H 2 O where ER is the equivalence ratio, W s and W s t e a m are the stoichiometric amount of gas mixture required for a complete combustion of biomass ( k g A i r / O 2 ∙ k g d a f , f u e l − 1 ) and the steam to biomass mass ratio ( k g H 2 O ∙ k g d a f , f u e l − 1 ), respectively, X O 2 , G A is the molar fraction of O2 in W s , M N 2 and M O 2 are the molar mass ( k g ∙ m o l − 1 ) of N2 and O2, respectively. The Y G a s and Y C h a r relates to the abundances of the biomass-derived products and can be defined as follows: (3) Y C h a r = 1 − Y G a s (4) Y G a s = ∑ i n Y i , G a s i = H 2 , C O , C O 2 , C H 4 , N 2 a n d H 2 O Energy calculations take into account reference conditions, at normal room temperature (Tref = 298 K) and pressure (Pref = 1 atm). Considering the biomass on a dry and ash-free basis, one easily obtains the following relation for the enthalpy balance: (5) Δ H p r o d u c t s − Δ H r e a c t a n t s − Q = 0 where Δ H p r o d u c t s and Δ H r e a c t a n t s denote the enthalpy changes of biomass-derived products and reactants ( k J ∙ k g d a f , f u e l − 1 ) at reference conditions, according to: (6) Δ H r e a c t a n t s = ( c p ‾ f u e l + E R ∙ W s ∙ c p ‾ s + W H 2 O ∙ c p ‾ H 2 O ) ∙ ( T a d b − T r e f ) + L H V f u e l (7) Δ H p r o d u c t s = Δ H p , G a s + Δ H p , C h a r (8) Δ H p , G a s = Y G a s ∙ [ c p ‾ G a s ∙ ( T a d b − T r e f ) + L H V G a s ] (9) Δ H p , C h a r = Y C h a r ∙ [ c p ‾ C h a r ∙ ( T a d b − T r e f ) + L H V C h a r ] The parameter Q denotes the heat exchanged between the reactor and the environment, being negative, positive or zero in cases where the gasification process is exothermic, endothermic or adiabatic, respectively. The thermodynamic data, such as the average specific heat ( c p ‾ ) and the lower heating value (LHV) of reactants and products, was obtained through empirical formulas [23]. In the case of the W s and Y G a s parameters, the c p ‾ values were determined as follows: (10) c p ‾ W s = 1 M W s ∙ ( c p ‾ O 2 ∙ X O 2 , G A ∙ M O 2 + c p ‾ N 2 ∙ ( 1 − X O 2 , G A ) ∙ M N 2 ) (11) c p ‾ G a s = ∑ i c p ‾ i ∙ X i , G a s Depending on the reaction to be promoted, oxidation or reduction of the active sites can significantly affect the performance of metal catalysts; this can be described by reaction (12), for a generic metal active site, which depends on partial pressure of oxygen ( p O 2 ). Based on this principle, the behaviour of the metal active sites under gasification conditions will depend on the redox conditions imposed by the biomass-derived gas. (12) 2 ( x y ) M e + O 2 ⇌ ( 2 y ) M x O y The partial conversion of biomass through gasification involves a set of parallel reactions, including onset of fully oxidized species (reactions 13 and 14), whose extension is dictated by the working conditions of the gasifier [24]. The Water-Gas-Shift (WGS) reaction (15) may also play a role in the pO2 associated with biomass-derived. WGS reaction can be expressed as the sum of reaction (13) and (14) and, although it suggests the absence of oxygen, kinetic restriction may affect this redox-type mechanisms [18]. (13) 2 C O + O 2 ⇌ 2 C O 2 (14) 2 H 2 + O 2 ⇌ 2 H 2 O (15) C O + H 2 O ⇌ C O 2 + H 2 Still, one may estimate ideal redox conditions and dependence of p O 2 on temperature or producer gas composition on assuming gas phase equilibrium. An ideal condition may be defined in the gas phase, as follows: (16) p C O p C O 2 = 1 K 13 ∙ p O 2 (17) p H 2 p H 2 O = 1 K 14 ∙ p O 2 where p i (i = CO, CO2, H2 and H2O) represents the partial pressure of gas species, K13 and K14 denote the equilibrium constants of reaction (13) and (14), respectively, calculated from thermodynamic data ( K i = exp [ − Δ G i / R T ] ). On the other hand, to analyse risks of carbon precipitation on catalyst surface, or onset of metal carbides at sufficiently high temperatures, one should consider the carbon activity ( a C ) in the gas phase which is mainly imposed by the Boudouard reaction and described as follows: (18) 2 C O ⇌ C + C O 2 (19) a C = K 18 ∙ p C O 2 p C O 2 Risks of carbon deposition correspond to conditions when a C ≥ 1 or K 18 ∙ p C O 2 ≥ p C O 2 , i.e. for CO-rich conditions, and/or low temperatures, as K 18 rises with decreasing temperature.Onset of the carbide phase may also occur at the onset of carbon (reaction 20) or on reaching sufficiently high activity of carbon (reaction 21), mainly at relatively high temperatures. Though this may be interpreted as a negative impact on catalytic performance, carbide catalysts have also been proposed for relevant gas phase processes, namely Mo carbides proposed as catalysts for reverse water gas shift [25] or utilization of CO2 [26]. Note also that this concept has been extended to the so-called MXenes, after previous functionalization of carbides of different transition metal elements (Ti, V, Cr, …) [27]. Thus, one must also examine the thermochemical conditions when the carbide phase may be present in biomass gasification. (20) 3 F e + C ⟺ F e 3 C (21) 3 F e O + C ⟺ F e 3 C + 1.5 O 2 The experimental data used to determine the a C and p O 2 values associated with biomass derived gas was compiled by collecting and organizing experimental results from the literature [14,28–53], regarding gasification experiments with distinct biomass feedstocks and different operation conditions.Thermodynamic calculations were applied as guidelines to investigate the stability range of iron-based catalysts and their compatibility with the thermochemical conditions of biomass gasification. The analysis was performed on assuming simplified model systems and computing diagrams in a form of planar representations [54]; this is based on derivation of representative reactions for 2-phase equilibria, and then extracting the relevant values of oxygen partial pressure ( p O 2 ), carbon dioxide partial pressure ( p C O 2 ), water partial pressure ( p H 2 O ), hydrogen sulphide partial pressure ( p H 2 S ) and/or activity values ( a i ) to establish stability ranges for expected phases. For example, interactions of iron oxides with biomass-derived contaminants such as H2S corresponds to a quaternary system Fe–O–S–H and phase equilibrium at constant temperature and total pressure will still depend simultaneously on H2S, p O 2 and p H 2 (or p H 2 O ). Early studies of the mechanism of reaction of iron with H2S at high temperatures [55] reported linear dependence on time and suggested that kinetics relies on mixed transport of cation vacancies and holes in a dense non-stoichiometric F e 1 − δ S scale, combined on migration of H2 in a top porous layer. One may then assume ready re-equilibration with H2O, under the thermochemical conditions of gasification, depending on oxygen partial pressure (14). Direct formation of H2O is expected for reaction of H2O2 with iron oxides, as depicted for FeO. Thus, one analysed the relevant 2-phase equilibrium reactions vs p O 2 , to account for changes in redox conditions and vs the p H 2 S : p H 2 O ratio, to account for the combined effects of other gases. The corresponding reactions are shown in Table 1 .In the case of solid carbon interactions (graphite is the stable carbon form) involving metallic iron, its carbide F e 3 C and oxides (FeO, F e 3 O 4 , F e 2 O 3 ), one may describe the corresponding 2-phase equilibrium as a function of carbon activity (Table 2 ).The dependence of p H 2 S : p H 2 O ratio and p O 2 , or on a C and p O 2 are determined numerically for given values of p O 2 , being the outcomes used to plot the corresponding stability diagrams. Log scales are applied for their closer relation with corresponding chemical potential differences from the reference state Δ μ O 2 = R T l n ( p O 2 ) and Δ μ C = R T l n ( a C ) . The same approach was applied to obtain the equilibrium conditions for other iron-containing catalytic systems. The FactSage software package (version 7.3) has been used to support the development of the stability diagrams. The thermodynamic properties required for the analysis, such as standard enthalpies of formation ( Δ H i ° ), standard entropies ( S i ° ), and specific heat ( c p i ), were taken from the FactPS, SGTE 2017 and FToxide databases. The thermodynamic analysis was conducted at a temperature range of 600–1000 °C that is typical for biomass gasification processes.Under direct gasification conditions, where air is used to partially convert the biomass feedstock, p O 2 will be mainly dictated by the CO:CO2 molar ratio (Eq. (16)) in the producer gas. A higher contribution of the H2:H2O molar ratio to the p O 2 (Eq. (17)) is expected for biomass steam gasification. In this case, steam will behave as a mild oxidant and promotes both the conversion of CO and higher yield of H2. Thus, a commonly used parameter to characterize the redox environment of a catalytic process is the reduction factor [56], which is defined as the quotient between the contents of both reductive gases and the contents of fully oxidized gases, as follows: (22) R = p C O + p H 2 p C O 2 + p H 2 O Iron-based materials tend to lose activity over operation time due to carbon deposition which leads to blocking of active sites and, eventually formation of iron carbide (Fe3C), with consequent deterioration of catalyst performance by metal dusting [57]. In the case of primary catalysts, these phenomena can be controlled to some extent by the betterment of carbon conversion, which is strongly affected by the biomass properties and the gasifier operating parameters such as residence time, bed temperature, gasification agent and equivalence ratio [58]. Although residence time cannot be directly controlled, the bed temperature and amount of oxidant are monitoring during operation and can be adjusted to guarantee thermodynamic conditions for complete carbon conversion. Note that the discrepancies between theoretical and experimental measurements decreases in the case of solid carbon [59], suggesting that the accumulation of carbon in the gasifier bed can be predicted with a reasonable accuracy. Fig. 1 shows the evolution of the gasification adiabatic temperature ( T a d b ) and minimum temperature for complete conversion of carbon ( T 0 , C h a r ), as a function of the equivalence ratio (ER). The analysis was performed for atmospheric air, using model biomass compounds (cellulose and lignin) and real biomass feedstocks (eucalyptus and rape seed). The crossing of both temperatures in the diagrams allows the definition of different operating windows for the gasifier. The conversion of biomass under carbon-free conditions is achievable for zones II and III, avoiding excessive carbon precipitation on the surface of catalysts. Actually, gasification should be driven across Zone III, where autothermal conditions are guaranteed. It is also clear from the results that the conversion of carbon is affected by the O:C molar ratio associated with the biomass feedstock. The minimum ER required to attain complete conversion of cellulose (O:C = 0.83) inside the gasifier is ER ≈ 0.085 (Fig. 1-a); this value increases to ER ≈ 0.33 in the case of lignin (O:C = 0.22), because the low O:C molar ratio requires additional supply of oxygen, to reach the stoichiometric ratio, and shifts the carbon-free operating windows to higher equivalence ratio. A similar behaviour is observed when eucalyptus (O:C = 0.73) and rape seed (O:C = 0.30) are considered as biomass feedstocks. Note that direct gasification of biomass is typically carried out at temperatures between 700 and 900 °C with ER ranging from 0.15 to 0.30 (shaded area on diagrams) [3]; this means that the use of lignin-rich feedstocks will result in higher accumulation of unwanted carbon in the gasifier bed, with subsequent negative impact on gasification efficiency. Higher tar content can also be expected in biomass-derived gas, consisting of more stable compounds such as polyaromatic hydrocarbons, since lignin is more difficult to decompose compared to other biomass components (cellulose and hemicellulose) [60,61]. Accordingly, the selection of proper gasification conditions, as well as suitable biomass feedstocks, are critical to minimize the formation of unwanted products. In this regard, it should also be taken into consideration that performance indexes are in trade-off relationship. For example, increasing the ER to improve carbon conversion will reduce the cold gas efficiency, due to increasing fractions of fully oxidized gases (Eqs. (13) and (14)). A reasonable compromise can be obtained by driving the process through Zone 3 (Fig. 1), near the intersection between T a d b and T 0 , C h a r .On the other hand, highly basic alkaline oxides (K2O, Na2O …) and alkaline earth oxides (BaO, SrO, CaO, MgO) interact with acidic gases (CO2, HCl …), as indicated by free energy of carbonation reactions (Table 3 ) and are likely to promote the catalytic activity of water-gas-shift and reforming reactions and oppose methanation [62]. There is also convincing evidence that the fractions of alkaline and alkaline earth metals (AAEMs) in the solid fraction (ashes) of gasification may assist this catalytic activity [63,64]. However, the volatility and low melting temperatures of alkaline oxides (e.g. ≈ 740 °C for K2O), or low eutectic temperatures in relevant systems (e.g. K2O − SiO2 and Na2O − SiO2 [65]) raise major difficulties, unless one considers alternative concepts based on alkaline-containing compounds, as reported for sodium titanates [66], which were proposed to upgrade the H2:CO ratio and to remove tars.Iron-based materials can exhibit distinct catalytic behaviour during biomass gasification due to the variable oxidation state of their active sites. Previous studies have reported metallic iron (Fe) as the main active phase for tar decomposition [28,67] because of its higher ability to break C–C and C–H bonds in aromatic hydrocarbon compounds compared to the corresponding iron oxides. Still, when the purpose of the catalyst is to increase the production of H2 through the WGS reaction, the spinel magnetite (Fe3O4) shows enhanced catalytic activity, which relies on the reducibility of the Fe3+ ↔ Fe2+ redox couple in the octahedral sites of Fe3O4 [56,68]. In the case of chemical looping gasification, where transition metal oxides are applied as oxygen carriers to promote oxidation reactions, performance of Fe-based catalysts relies on cycling between oxidation to hematite (Fe2O3), which provides higher oxygen storage, and reduction to lower valence states in the gasifier [69,70]. Fig. 2 shows thermodynamic predictions for the Fe–O–C system, which is presented vs p O 2 and a C . The corresponding thermochemical conditions of biomass gasification were calculated from experimental data, and were superimposed in this diagram (symbols); this comprises gasification experiments with different gasification agents (air, steam and O2-steam mixtures). Note that relatively small variations of p O 2 in the gasifier atmosphere can have practical consequences on the prevailing phase of Fe-based catalysts, ranging from a prevailing relevance of wustite (FeO) for gasification at the highest temperatures, and gradual shift to a distribution from magnetite (Fe3O4), wustite and metallic Fe at lower temperatures. Risks of carbon deposition at relatively low temperatures (600 °C) may be minimized by maintaining the redox conditions in the Fe3O4 range, which also lowers the risk of collapse by excessive volume changes on reducing magnetite to wustite (−16%) or wustite to metallic Fe (−42%). Fe3O4 shows the widest redox window, whereas the redox window of wustite (FeO) narrows with decreasing temperature [71]. The reduction factor of the producer gas (R) is shown in the secondary vertical axis and is also a useful guideline to prevent deposition of carbon, by keeping R > 1, as shown in Fig. 2 for 600 °C. Otherwise, one may design structural changes in the active sites of magnetite-based catalysts, as pointed out for WGS catalysts [56]. One may also consider the incorporation of promoters (e.g. La, Sr, Ce, …) into iron oxides catalysts for H2-enriched gas production, mainly when it is unfeasible to adjust R, and to seek enhanced oxygen storage [72].Risks of carbon deposition decrease with increasing temperatures, as shown in Fig. 2, at the highest temperatures. Note that the chemical potential differences Δ μ C = R T l n ( a C ) and Δ μ O 2 = R T l n ( p O 2 ) which separate the average experimental conditions from onset of carbon (at a C  = 1) increase with temperature. In fact, carbon deposition is not expected at temperatures ≥900 °C, even when the reducing factor is high. Onset of carbide (Fe3C) also seems unlikely under typical conditions of gasification because the activity of carbon is shifted to sufficiently low values below the Fe/Fe3C equilibrium. Still, the experimental conditions may reach the Fe/Fe3C boundary at intermediate temperatures, as shown for 700 °C in Fig. 2. Thus, unconverted char in the gasifier bed may still shift the a C to higher values, raising concerns about their impact on Fe3C formation. The tendency to carbon precipitation with decreasing operating temperature may contribute to the formation of iron carbide (Fe3C) resulting from interactions of carbon with metallic iron [73]. The risk of metal dusting and their negative effects on catalytic activity during long-term operation might be minimized by alloying Fe with other elements, promoting the formation of a protective oxide layer [74]. Formation of iron carbonate (FeCO3) is unlikely under biomass gasification conditions, since FeCO3 is unstable at temperatures above ≈ 600 K, even in pCO2-rich atmospheres [57].It is well-know that sulfur impurities in biomass-derived gas is one of the major concerns associated with the use of metal-based catalysts. Though some mechanistic investigations showed that adsorption of H2S onto iron surface can induce oxide-metal bond scission with negative impact on WGS performance [75], the influence of gas-phase sulfur on the catalytic behaviour of iron species during exposure to biomass-derived gas is still poorly understood. The poisoning effect of sulfur on the catalytic activity of iron active sites was generally explained by a simple site-blocking mechanism, leading to formation of iron sulphide (FeS) which causes a sharp drop in the rate of catalytic conversion, as well as poorer product selectivity [17]. Fig. 3 shows gas-solid thermodynamic predictions for the sulfur tolerance of iron species, and superimposed calculations for H 2 S : H 2 O vs p O 2 values associated with producer gas compositions. The phase stability diagrams are analysed for the combined effects of hydrogen sulphide and water vapour ( p H 2 S : p H 2 O ) , at fixed temperatures, for complete description of the quaternary system Fe–O–S–H. In this case, the analysis is not restricted to specific values of pH2O, as proposed earlier to assess the sulfur tolerance of Fe-based oxygen storage materials for chemical looping combustion [76]. Dependence on ( p H 2 S : p H 2 O ) also allows one to emphasize that operation in steam-rich conditions offer prospects to upgrade sulfur tolerance in biomass gasification, as found on comparing the average results from air gasification (black triangles in Fig. 3) and results from steam gasification (green squares) or oxy-steam gasification (red diamonds). Thus, one may expect substantial gains in tolerance to H2S when the fraction of p H 2 O in the producer gas is high. For example, sulfur tolerance up to p H 2 S ≈ 30 ppm is expected at 800 °C and log ⁡ ( p O 2 ) = −17.8 atm if one assumes a gas composition with p H 2 O = 0.10 atm, whereas this value increases to p H 2 S ≈ 75 ppm when p H 2 O = 0.25 atm. Fig. 3 also suggests that sulfur tolerance is poorest for wustite (FeO) and may the optimized for Fe3O4-based catalysts, mainly in the intermediate range of p O 2 . However, this does not translate in real advantages if one considers biomass gasification catalysts, since most operation conditions fall in a narrow range of redox conditions, usually near the FeO/Fe3O4 borderline; this conclusion may also be extended for potential applications in chemical looping gasification, since the oxidising step requires complete conversion to Fe2O3, under conditions when sulphates become highly stable, even for sulfur contents below the ppm range. Note that a typical standard for atmospheric air quality is in the order of 0.2 ppm of SO2, at room temperature, and this corresponds to similar contents of H2S at higher temperatures [77]. Thus, one should not expect regeneration on cycling between H2S-contaminated reducing producer gas and the oxidising step in fairly cleaner air.The so-called Fe/CaO catalysts in the Fe–Ca–O–C system have received special attention because of their high activity towards tar conversion and H2 promotion during steam gasification. It is often based on the activity of Fe3O4 for the enhancement of WGS reaction [56], combined with the ability of calcium oxide (CaO) to favour in-situ CO2 absorption, shifting the reaction equilibrium to higher H2 yields. The CaO is also active in reforming reactions but is easily deactivated by biomass tar, resulting in the decline of catalytic performance [78]. Thus, significant cumulative formation of calcium carbonate (CaCO3) results in the suppression of the CO2-sorption ability, requiring regeneration cycles at high temperatures, which causes particle coarsening or agglomeration and severe pore blockage [79]. To overcome these constraints, the promotion of brownmillerite phase (Ca2Fe2O5) is a proposed option, which is expected to retain the catalyst performance by enhancing its thermal stability and redox tolerance over multiple operation cycles. Catalytic performance of Ca2Fe2O5 has been ascribed to co-existence of octahedral and tetrahedral sites of the brownmillerite structure, and the lower coordination was interpreted as O-vacancies facilitating the mobility of oxygen [80]. However, this interpretation is somewhat arguable taking into account that direct measurements of oxygen permeability are lower than for ferrite perovskites and also because significant changes in oxygen stoichiometry are related mainly to reductive decomposition rather than changes in occupation of the structural tetrahedral positions of Ca2Fe2O5 [81]. Therefore, one revised the extended phase stability of the Ca–Fe–O–C system Fig. 4 as a guideline for coexistence of Ca2Fe2O5 with other phases, and oxygen storage or CO2 storage ability related to onset of secondary phases, including formation of carbonate.Ready onset of CaCO3 at 600 °C implies greater risks of CaO deactivation for log (pCO2) > −0.86 atm, due to the limited thermodynamic stability of the brownmillerite structure (A2B2O5). Higher pO2 values results in improved CO2 tolerance relative to carbonation of CaO, but these gains are insufficient to guarantee Ca2Fe2O5 stability under typical gasification gas compositions, as indicated by the experimental data points (symbols). Increasing the gasification temperature (T ≥ 700 °C) provides CO2 tolerance of Ca2Fe2O5, even in CO2-rich atmospheres, and minimize the risks of massive decomposition of the brownmillerite structure when exposed to biomass-derived gas, in close agreement with evidence in relevant literature [82]. The wide redox range for Ca2Fe2O5 extends from the actual range of biomass-derived gas up to oxidising conditions. In fact, the compiled information from a wide variety of experimental data on biomass gasification falls almost entirely within the thermochemical phase boundary of the brownmillerite phase, even at 700 °C. Thus, the Ca2Fe2O5 phase allows prospective operation under much wider redox ranges, compared to pure metallic Fe or its oxides (FeO or Fe3O4). Note that the Fe/FeO and FeO/Fe3O4 boundaries are clearly located inside the stability range of Ca2Fe2O5. Thus, this phase delays onset of metallic Fe, and minimizes its catalytic promotion of carbon deposition, raising prospects for higher H2 production at moderate reaction temperatures (≈700 °C), enhanced gasification efficiency [83] and catalyst stability during long-term operation.Chemical looping gasification by means of transition metal oxides provides an alternative option for biomass thermal conversion. Biomass is partially converted by the lattice oxygen of metal oxide and steam, aiming to obtain N2-free producer gas with a low tar content [84]. Tar conversion through oxidation reactions is also expected, namely earlier precipitation of metallic particles and their impact on C–C bonds, increasing the carbon conversion efficiency. Particular attention has been given to the application of ferrite materials such as NiFe2O4, CuFe2O4, MnFe2O4 and CoFe2O4, as oxygen carriers, [85,86]. Ni- and Co-based compounds are known for their higher catalytic activity but also raise the highest environmental concerns during operation [86], including carcinogenic effects at least in the case of Ni. Ni- and Co-based compounds are also less affordable than corresponding Mn-based compounds. Manganese ferrite is less expensive and also raises lower concerns about safety. Reduced (Mn,Fe)xOy nanoparticles were successfully tested in biomass gasification with impact on tar conversion [14].The reducibility of spinels and their reversibility in reduction/reoxidation cycles can be related to the high flexibility of the AB2O4 spinel structure which allows incorporation of diverse combinations of divalent and trivalent transition metal ions in both tetrahedral A-sites and octahedral B-sites.Thermodynamic modelling of the Cu–Fe–O, Ni–Fe–O, Co–Fe–O and Mn–Fe–O systems (Fig. 5 ) show that the stability windows of spinels, in terms of temperature-pO2 ranges differ significantly but does not reach the conditions of biomass gasification (circles); these results combine a wide range of experimental data of producer gas compositions, obtained by gasification at different temperatures, using air, steam and O2-steam mixtures as gasifying agent. Thus, one observes a significant gap between the redox conditions of producer gas and the phase boundary of ferrites in the reducing side, and the widest gap is observed for CuFe2O4.Decomposition of ferrites under conditions of biomass gasification is a gradual multistep process, as detailed for C o F e 2 O 4 : C o F e 2 O 4 → C o O + 2 / 3 F e 3 O 4 + x 1 O 2 ↑ → ( 1 + δ ) ( C o , F e ) + ( 2 / 3 − δ ) F e 3 O 4 + x 2 O 2 ↑ → ( 1 + δ ) ( C o , F e ) + ( 2 − δ ) F e O + x 2 O 2 ↑ (23) → 3 ( C o , F e ) + x 3 O 2 ↑ Volume changes induced by these reduction steps are relatively high (Table 4 ), with corresponding risks of mechanical disintegration upon redox cycling, mainly if one considers complete reduction of both oxide components. These risks are somewhat minimized if one considers only reduction of cobalt, while retaining magnetite as the main oxide phase; this minimizes the volume changes and also maintains structural similarity between magnetite and C o F e 2 O 4 . However, this step of reduction only covers a relatively small fraction of the experimental results reported for biomass gasification, as shown in Fig. 5. The effective oxygen supply in this early reduction step is also relatively small.Decomposition of other ferrites follow a similar sequence of decomposition steps, except for the first step which only occurs at sufficiently high temperatures in the case of N i F e 2 O 4 (>800 °C) and is not observed in the case of C u F e 2 O 4 . The final stage yields complete reduction to a bimetallic alloy and is slightly displaced from the corresponding conditions for reduction of pure wustite (FeO) to metallic Fe, as shown by a dotted blue line for the Ni–Fe–O and Co–Fe–O systems. In these cases, one observes a significant fraction of gasification experiments within the redox range of complete reduction; this indicates higher oxygen supply ability for chemical looping within the redox range of gasification. Thus, one cannot find a clear advantage of CuFe2O4 relative to other ferrites, in what concerns the oxygen storage ability and redox conditions for charge/discharge. Effective application of Cu-based materials as oxygen carriers should also take into consideration greater risks of microstructural ageing derived from the low melting point of metallic Cu (≈1085 °C) and readier sintering. Greater risks of microstructural degradation may also be caused by contaminants such as alkaline, which may induce low melting eutectics [87]. Thus, gasification temperatures should be limited to minimized these risks [88]. Ni- and Co-based compounds also offer greater catalytic potential for a wide variety of processes, such as the production of H2 through the promotion of WGS reaction and degradation of tar compounds [89], except possibly for their simultaneous promotion of carbon deposition.MnFe2O4 follows a somewhat different multistep reduction, mainly because MnO is hardly reduced by fuels: M n F e 2 O 4 → M n O + ( 2 3 − δ ) F e 3 O 4 + x 1 O 2 ↑ → M n O + 2 F e O + x 2 O 2 ↑ → 2 F e + M n O + x 2 O 2 ↑ (24) → ( 2 + δ ) ( M n , F e ) + ( 1 − δ ) M n O + x 3 O 2 ↑ In addition, MnFe2O4 shows limited stability under oxidising conditions, undergoing complete oxidation to trivalent state of both oxide components, as follows: (25) M n F e 2 O 4 → O 2 1 / 3 M n 3 O 4 + F e 2 O 3 → O 2 0.5 M n 2 O 3 + F e 2 O 3 Thus, the spinel phase is not retained in both limiting conditions of chemical looping cycles, except possibly for less common processes when a specific redox pair (e.g. CO2/CO) may still allow an oxidation step within the intermediate redox range. For example, M n F e 2 O 4 was proposed for a chemical looping reaction between methane and CO2 [90].In the present study, one re-examined the thermodynamics of iron-based catalysts under the experimental conditions of biomass gasification. A combination of experimental data and thermodynamic modelling allows one to assess the dependence of catalyst performance on the thermochemical conditions of biomass gasification, by superimposing these results on phase stability diagrams of Fe-based catalysts. Thermodynamic modelling of biomass conversion showed that conversion of carbon inside the reactor is strongly dependent on the O:C molar ratio associated with the biomass feedstock and gasifier temperature. Lignin-rich feedstocks lead to higher accumulation of unconverted carbon in the gasifier bed, with expected negative impact on catalyst stability and process efficiency. This risk can be minimized with sufficient equivalence ratio to ensure operation of the gasifier at a temperature slightly above the theoretical value required for complete carbon conversion, and by selecting appropriate biomass feedstocks.Thermodynamic predictions for the Fe–O–C system indicated that changes in the redox atmosphere of the gasifier can have significant impact on the catalytic behaviour of Fe active sites. Greater redox tolerance of Fe3O4 phase is expected at 600 °C. At higher gasification temperatures, the catalytic promotion of H2 through the WGS reaction requires precise control of the reduction factor (R < 1), and modification of Fe-based catalysts to retain the redox tolerance of active sites. Conversion of tars over metallic Fe is challenging because the required oxygen partial pressure may cause reoxidation.Coke deposition and sulfur contamination of iron active sites can be assessed by suitable stability diagrams, with planar representations in p O 2 vs activity of carbon or vs the partial pressure ratio p H 2 S : p H 2 O in the gas atmosphere. Experimental conditions of biomass gasification were superimposed in the diagrams, and confirm that carbon precipitation on Fe surface is expected under gasification conditions at relatively low temperatures. Accumulation of unwanted carbon in the reactor bed may raise concerns about the impact of Fe3C formation at higher temperatures. Thermodynamic modelling of the Fe–O–S system revealed that poisoning by H2S can cause degradation of Fe-based catalysts, with tolerance limits differing according to process conditions, including significant differences between gasification with air and with steam.The Ca–Fe–O–C system was examined as guideline for Ca2Fe2O5; this shows ready carbonation at 600 °C, while enhancing the stability at higher temperatures. The corresponding results suggest that thermodynamic stability of brownmillerite phase at 600 °C requires higher redox potential in the biomass-derived gas to avoid decomposition of Ca2Fe2O5 structure, with subsequent formation of carbonate phases. Accordingly, the in-situ application of those materials may involve higher gasification temperatures, as suggested by the wide gap between the upper and lower limits of resistance to CO2 for the Ca2Fe2O5 phase at temperatures above 700 °C.Stability phase diagrams of typical ferrites (AB2O4, with A = Cu, Ni, Co and Mn) were also computed to evaluate their reactivity at gasification conditions, and prospects for chemical looping. These systems provide conditions for onset of bimetallic (Fe,Cu), (Fe.Ni) or (Co,Fe) particles. Similar conditions were also observed in terms of reduction steps and corresponding oxygen supply, except for slight differences in the conditions for complete reduction of both oxide components, and greater risks of microstructural ageing of oxygen storage materials in the Cu–Fe–O system. The Mn–Fe–O system shows a more complex sequence of reduction/oxidation steps in chemical looping. Luís Ruivo: Conceptualization, Methodology, Investigation, Formal analysis, Visualization, Validation, Writing – original draft, Writing – review & editing, Tiago Silva: Conceptualization, Investigation, Formal analysis, Writing – review & editing, Daniel Neves: Conceptualization, Investigation, Formal analysis, Writing – review & editing, Luís Tarelho: Conceptualization, Visualization, Validation, Writing – review & editing, Supervision, Jorge Frade: Conceptualization, Investigation, Formal analysis, Visualization, Validation, Writing – review & editing, Supervision, Project administration.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors acknowledge the financial support through projects NOTARGAS (ref. POCI-01-0145-FEDER-030661) and CHARCLEAN (PCIF/GVB/0179/2017). Thanks to the Portuguese Foundation for Science and Technology (FCT)/Ministry of Science, Technology and Higher Education (MCTES) for the financial support to CESAM (UIDP/50017/2020, UIDB/50017/2020, LA/P/0094/2020), and CICECO – Aveiro Institute of Materials (UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020), through national funds. The authors also acknowledge the Portuguese Foundation for Science and Technology for providing financial support to the PhD scholarship granted to Luís Ruivo (ref. SFRH/BD/129901/2017).The following are the supplementary data related to this article: Multimedia component 1 Multimedia component 1 Multimedia component 2 Multimedia component 2 Multimedia component 3 Multimedia component 3 Multimedia component 4 Multimedia component 4 Multimedia component 5 Multimedia component Multimedia component 6 Multimedia component Multimedia component 7 Multimedia component Multimedia component 8 Multimedia component Supplementary data related to this article can be found at https://doi.org/10.1016/j.energy.2023.126641.

The present work intended the development of a graphical approach to support the operation of conventional iron-based catalysts under gasification conditions. A combination of experimental data and thermodynamic modelling was used as guidelines to elucidate the dependence of catalyst performance on the thermochemical conditions of producer gas. The outcomes are represented by stability diagrams in a form of planar representations for easier identification of appropriate operating windows. Attention was focused not only on potential deactivation mechanisms resulting from gas-solid interactions, but also on the stability of relevant catalytic phases when exposed to biomass-derived gas atmospheres at temperatures in the range 600–900 °C. The results suggest that controlled process parameters contributes to enhance the tolerance of iron-based materials to deactivation by carbon deposition, H2S poisoning and/or carbonation. Selected examples also show that the redox potential imposed by producer gas can have a significant impact on the stability of relevant active phases, with subsequent impact on catalyst performance. To overcome these constrains, one should considerer suitable composition changes to enhance their redox properties, possibly combined with microstructural or nanostructural development during materials processing.

The oxygen evolution reaction (OER) is one of the most relevant anodic reactions within electrochemical cells, where it is coupled to the hydrogen evolution reaction (HER) 1–6 or the CO2 reduction reaction (CO2RR) to energy-dense carbon compounds at the cathode. 7–9 It is, therefore, of high relevance for electrochemical energy conversion and storage technologies. Oxygen generation from water oxidation at the anode is typically carried out in acidic or alkaline conditions. Operation in alkaline conditions allows the use of cheap, efficient, and stable non-precious-metal catalysts, in contrast to acidic conditions, in which only expensive and scarce noble-metal-based catalysts such as IrO2 and RuO2 exhibit significant stability. 10 In alkaline conditions, the best performance and highest stabilities were observed for Ni-based multimetallic catalysts, 11–15 which led to their widespread use as OER catalysts. 10 , 16–19 However, sluggish kinetics of the four-electron OER requires a significant anodic overpotential to achieve relevant geometric current densities, reducing the efficiency of the conversion of electrical to chemical energy. Hence, identifying efficient, cheap, and stable OER catalysts comprising earth-abundant elements is of fundamental importance and has been a prominent field of research during the last 20 years. 20–23 Among non-noble multimetallic metal-based OER catalysts reported so far, mixed nickel/iron/cobalt oxides in particular have shown stable low overpotentials at relevant geometric current densities.Benchmarking these novel OER electrocatalysts is of utmost importance but remains highly challenging, given that methods for evaluating performance (activity and stability) are non-standardized, making fair and reliable comparison extremely difficult. Notably, different catalyst supports are used, to which OER is very sensitive, 24 , 25 and variation in characterization methods and experimental setups further adds to disparities between reported performances. As a matter of fact, very few benchmarking studies have been carried out so far. The last significant efforts in providing meaningful benchmarking studies were carried out in 2012, 26 2013, 27 and 2015. 28 These studies, conducted on OER electrocatalysts deposited on Au/Ti or glassy carbon electrodes, performed water oxidation at a current density of 10 mA cm− 2 with observed overpotentials higher than 300 mV. A relevant figure of merit from these studies is the overpotential (denoted as η 10) required to achieve 10 mA cm− 2 current density per geometric surface area at ambient temperature and 1 atm O2. The η 10 value is indeed the benchmarking criterion generally used in literature. 26–28 Some OER electrocatalysts reported during the last five years exhibit η 10 overpotentials much closer to 200 mV, representing a significant advancement in the field. We, thus, found it timely to provide a fair comparison of the performances of the most active catalysts of this new generation.Here, we establish a protocol to benchmark a range of anodes, consisting of various catalysts synthesized on the same nickel foam (NF) support. Because of its conductivity, mechanical strength, relative inertness at alkaline pH, and low cost, nickel is an efficient current collector and a good support for active material deposition. Furthermore, nickel foam shows extended geometric surface areas and fine three-dimensional structures, which make it attractive as a support for heterogeneous catalysts. 25 , 29 Although various porous metallic foams have been used in the past for water oxidation, 30–32 a recent resurgence in the usage of NF as a support material has occurred. This was partly driven by the development of energy-storage electrochemical systems in alkaline conditions, such as solar-driven water splitting and electrocatalytic CO2RR using gas-fed flow cells. We have followed this trend and used NF as the catalyst support exclusively. 17–19 For this study, we selected nine promising multimetallic, non-precious-metal catalysts reported in the literature (Table 1 ). Several selection criteria were adopted such as the η 10 overpotential in alkaline conditions, the Tafel slope, the maximum current density, and the long-term stability, as these data were available in previous publications. 33–41 We also made sure to select materials composed of a variety of transition metals with different morphologies and synthetic procedures in order to broaden the scope of our comparison. These catalysts were preferentially chosen on the basis that they display low η 10 overpotentials (below 300 mV). However, we also included the Co-based catalyst developed by Nocera et al. (CoPi), 34 despite it exhibiting a η 10 value greater than 300 mV in 1 M KOH, given that it is widely used in literature. The NiMoFe-O catalyst was included as it was reported to be the highest performing catalyst at the time of writing. 27 , 42 CoV-O and CoV-OOH were chosen to provide a comparison between two catalysts based on cobalt and vanadium that were synthesized by using different methods. In the publications, these catalysts were characterized under very different conditions. In particular, various electrode supports were used (glassy carbon, Cu plate, or Ni foam), illustrating the difficulty of comparing them just on the basis of the literature data. In our study, we exclusively used the same Ni foam as the conductive support. The syntheses followed the reported procedures as strictly as possible with slight adaptations to enable deposition on a 1 cm2 NF support.The materials were characterized and their kinetic performances for the OER in alkaline conditions analyzed. Geometric current densities of OER catalysts must attain several hundreds of milliamperes per square centimeter to facilitate CO2 reduction and solar-driven water splitting under lab-scale conditions. 1 , 2 , 43 Therefore, we not only benchmarked the catalysts at the commonly reported value of 10 mA cm2 but also at a more relevant current density of 100 mA cm− 2, 17 , 44–47 according to a standardized protocol. Catalysts must be compared in terms of intrinsic activities, which requires normalization of the current densities by the effective electrochemical surface areas (ECSAs). This is in general a very challenging issue, quite often incompletely addressed. We have, thus, been careful in providing an array of experimental procedures for the determination of concentrations of active sites and specific activities, and we propose a best-practice protocol for researchers in the field. Finally, we report a novel NF-based support with different morphology and increased structuration, leading to significant improvements in the performance of almost all studied catalysts. By doing so, we report two OER catalysts with overpotential values of 195 and 198 mV required for a current density of 10 mA cm− 2, and overpotentials of 247 mV required for 100 mA cm− 2, among the lowest values reported so far. We demonstrate the outstanding electrochemical and compositional stability of these two catalysts, evaluated by long-term electrolysis in a flow cell (Figure S1). Furthermore, we stress the importance and necessity of performing galvanostatic tests as well as surface leaching quantification in order to assess the stability of the catalyst.Nine catalysts were selected among the most active materials reported in the literature (Table 1). Cu-O and NiMoFe-O were cathodically electrodeposited on Ni foam (Figure 1 A) from aqueous solutions of the metallic precursors. This simple method resulted in the formation of dendritic structures as shown by SEM analysis (Figures 1B, 1C, S2, and S4). NiMoFe-O showed a particularly strong adhesion to the substrate because the presence of nickel in the as-deposited catalyst ensures a continuous interface with NF in terms of composition and, thus, a small lattice mismatch. EDX was used to confirm the elemental composition (Figures S3 and S5). CoPi was anodically electrodeposited, which resulted in the slow formation of large dendrites with a poor adhesion to the NF support (Figures 1D, S6, and S7) . FeCoW (Figures 1E, S8A, S8B, and S9) and CoV-OOH (Figures 1F, S10, and S11) were synthesized from nanomaterial dispersions mixed with a Nafion ink and drop-cast and dried onto the surface of the support to form thick layers, which remained well attached to the support despite large cracks (Figures S8B and S10C). The major limitation of this method is the hydrophobicity of dry Nafion, which promotes the formation of an air film trapped at the catalyst/electrolyte interface and limits their contact area. In order to reach a stable OER activity, these catalysts must be kept under oxidative conditions (10 min at 5 mA cm− 2) in an aqueous electrolyte in order for the Nafion’s hydrophilic domains to swell and become predominant in the bulk. 48 A modification in the nanostructure of FeCoW was observed after this process, revealing the formation of a thin layer structure (Figures S8C and S8D). We conclude that once the Nafion network is fully hydrated the metallic sites are exposed to the electrolyte. This allows dissolution/precipitation equilibria at the interface, resulting in the formation of highly nanostructured surfaces. NiFe-OOH (Figures 1G, S12, and S13) was made through galvanic exchange of the Ni-based substrate with a Fe3+ precursor, followed by the deposition of a bimetallic oxy-hydroxide. As the support is the only source of nickel, it is etched during the reaction. This not only ensures a very high adhesion of the catalyst on the support but also slightly modifies the material by etching its Ni backbone. Nevertheless, this method is particularly interesting as it is very simple. NiFeSe- and CoFeSe-derived oxides were synthesized by a more complex three-step procedure involving the hydrothermal formation of layered double hydroxides (LDHs), followed by selenization and subsequent reoxidation. The SEM images and EDX elemental analysis of the resulting NiFeSe-dO (Figures 1H, S14, and S15) and CoFeSe-dO (Figures 1I, S16, and S17) catalysts revealed the formation of very dense, thick, and mechanically stable deposits at the surface of NF with fine nanostructures in the range of 30–100 nm. However, the formation of a selenide involves hazardous synthesis steps and its reoxidation leads to toxic selenite waste products. CoV-O was synthesized by a simple one-step hydrothermal procedure involving the coprecipitation of Co and V in a mixed-phase composed of a fine LDH nanostructure (Figures 1J, S18, and S19)—this method is simple but results in a very low loading on NF.It should be noted that the atomic compositions of the catalysts differed slightly from those reported in some cases. For example, in the case of NiMoFe-O (Figure S5), the use of NF as the support led to small modifications in the chemical composition of the film. The tungsten content in FeCoW is also lower than expected (Figure S9). Additionally, as the procedure for CoFeSe-dO synthesis could not be reproduced, it was modified in order to reach a Co:Fe ratio comparable with the Ni:Fe ratio in NiFeSe-dO (Figures S15 and S17). In spite of the modifications we had to make in the procedures, the morphologies of the catalysts were comparable with those in the literature, as illustrated by the SEM images in Figures S14 and S16.The catalytic activity of each material was measured under alkaline conditions, in an aqueous 1 M KOH electrolyte solution. Chronopotentiometric steps (CP steps) were performed at different fixed current densities (j = 0, 5, 10, 25, 50, and 100 mA cm −2) for 5 min each under stirring. This method is better than linear sweep voltammetry as it ensures that exclusively the OER response is measured and other contributions to the current are eliminated, such as the oxidation of Ni(OH)2 to NiOOH in Ni-containing materials. 49 Furthermore, these CP steps give some information regarding the stability of the potential measured at different current densities on a short timescale. A possible drawback resides in some additional ohmic drop associated with the accumulation of oxygen bubbles at the surface of the electrode. 16 , 50 The j-η profiles and overpotentials at j = 10 and 100 mA cm −2 of the tested catalysts are displayed in Figures 2A and 2B, respectively (see Figure S20 for error bars), with the exact values of the overpotentials at j = 10 mA cm −2 given in Table S1. Additionally, the electrodes were held at a fixed current density (j = 50 mA cm −2) for 30 min in static conditions to verify that measured potentials were stable over a longer reaction time. In all cases, this confirmed no clear degradative reactions (see Figure S21 for stability data).The overpotential η 10 value is considered a figure of merit for OER catalysts. All the catalysts characterized in this study have η 10 values between 211 and 347 mV, a significant 136 mV range (Figure 2; Table 2 ). The lowest overpotentials were obtained for NiFeSe-dO (η 10 = 211 mV) and CoFeSe-dO, (η 10 = 212 mV). The highest overpotentials were obtained for CoV-O (η 10 = 331 mV) and Cu-O (η 10 = 347 mV). Comparison with literature data is shown in Table S1.Oxygen evolution must be carried out at higher current densities in order to meet the requirements for the electrochemical conversion and storage of renewable energy, such as solar-driven water splitting and CO2 reduction technologies. 17 , 43–47 , 51 We, therefore, focus on the catalytic activities of the nine catalysts at 100 mA cm–2 (Figure 2; Table 2). Although NF requires an overpotential as high as 563 mV, the nine catalysts enable an important drop in η 100 as compared with that of their support. Despite differences in the slopes, the trend in OER activity is the same at 10 and 100 mA cm–2. NiFeSe-dO remains at the head of the group with a low overpotential of η 100 = 264 mV, followed by CoFeSe-dO and NiFe-OOH with the same overpotential value of 289 mV, and by FeCoW with η 100 = 293 mV. CoV-O (η 100 =397 mV) and Cu-O (η 100 =432 mV) show the lowest performances at all current densities.The dependence of the OER kinetics on the applied potential for the catalysts is well illustrated through Tafel analysis (Figure S22; Table S1). The fastest increase in current density upon potential increase occurs at the surface of NiFe-OOH, with a slope as low as 36 mV dec −1. NiFeSe-dO and FeCoW also showed low Tafel slope values of 55 and 56 mV dec− 1. The obtained values for the other catalysts ranged from 63 to 83 mV dec −1 (Figure S22). We systematically observed larger Tafel slopes with respect to reported ones, except for NiFe-OOH (Table S1). This is likely because chronopotentiometric steps were used to measure Tafel slopes in place of linear sweep voltammetry (LSV) scans, which is the general methodology employed in the recent literature. Also, as a consequence of the CP steps, the accumulation of O2 bubbles at the surface of the electrodes might contribute some extra resistance, especially at higher current densities, resulting in increased Tafel slope values. 16 , 52 , 53 Comparing catalysts with different morphologies in terms of their intrinsic activities requires the determination of the density of electrochemically active sites, a very important yet challenging analysis. 54 , 55 Depending on parameters such as their nanostructure, porosity, and lattice structure, catalysts can show very different interactions with the surrounding electrolyte. 56–59 The density of electrochemically active and accessible sites can vary a lot from one catalyst to another. A range of techniques can be used to relate the total OER activity of a catalyst to the intrinsic activity of each active site. 54 , 55 Two main methodologies are considered here in order to estimate the density of accessible active sites. The first is through the determination of the ECSA whereas the second involves analysis of the pre-OER redox peaks. Both methods have serious limitations; however, they are complementary techniques, therefore combined analysis enables general trends to be established.OER catalysts behave as capacitors: upon application of a potential, a charge build-up is observed at the catalyst-electrolyte interface. The capacitance of a catalyst in the absence of any Faradaic process is the double-layer capacitance C DL. ECSAs can theoretically be calculated from C DL values; however, they are difficult to obtain accurately. 27 , 60–66 Indeed, ECSA = C DL /C S where C S is the specific capacitance of the material, which corresponds to the capacitance of an atomically smooth planar surface of the same material per unit area under identical electrolyte conditions. Although C DL can be experimentally determined by measuring the non-Faradaic capacitive current associated with double-layer charging from the scan-rate dependence of the cyclic voltammograms (CVs), it is almost impossible to determine reliable values of C S for each sample. 60 , 66 Note that in previous studies for determination of ECSAs of various OER catalysts, the same value of C S = 0.040 mF cm− 2 in 1 M NaOH, based on typical reported values for metallic surfaces, was applied. 27 , 28 However, one should be aware that this only gives an estimation of the ECSAs given that the C S value varies significantly from one material to another. 27 In our case, C DL values were measured by cyclic voltammetry in the range +0.95 – +1.05 V versus RHE, as it is a non-Faradaic region for all our catalysts (except Cu-O, which was characterized between +0.66 and +0.76 V versus RHE). However, OER catalysts can show potential-dependent conductivity variations, therefore inaccuracies can arise from lower conductivities in potential regions prior to water oxidation. 54 , 55 , 67 Here, we report the C DL values for the nine materials and for the NF support, measured in 1 M KOH by using electrodes with 1 cm2 geometric areas (Figures 3A and S23). We indeed observed large differences: some of the studied catalysts have C DL values in the range of 1 mF, slightly larger than that of the Ni foam (0.9 mF), whereas CoFeSe-dO, NiMoFe-O, and NiFeSe-dO have much larger C DL values of 3.65, 2.40, and 2.35 mF, respectively. This result is in line with the observation of extremely fine nanostructures for these three catalysts (Figures 1I, 1H, and 1C).Most OER catalysts show redox features at potentials below the OER onset. These processes are generally attributed to the oxidation of the metal sites. For instance, NiII hydroxide oxidizes to NiIII oxyhydroxide prior to OER catalysis. 54 , 65 , 66 The amount of charge (Q) necessary to oxidize all the electrochemically active metal sites in a catalyst can be estimated through the integration of the oxidation wave. Q is directly proportional to the number of electrochemically accessible active sites (N) in the material according to the relation N = Q/(n e ·Q e ), where n e is the number of electrons of the oxidation process observed and Q e is the charge of an electron. It is generally assumed that this oxidation is a one-electron process, although this is a very strong assumption because the initial state of the catalyst is likely to be a mixed-valence state, with delocalized energy bands. 68 Therefore, electron transfers are more complex than isolated one-electron transfers—here, we chose to compare Q values without assuming an arbitrary value for n e . However, one should be aware that Q can account for bulk sites, which might not participate in the OER reaction, especially given that recent studies have shown the prominent role of surface sites in OER catalysis. 69 , 70 Another limitation of this method comes from the superposition of the oxidation wave and the OER onset in some cases—the deconvolution of these two contributions can be challenging. The simultaneous OER wave can also block a fraction of the active sites because of the formation of O2 bubbles and interference from mass transport limitations at high current density.The estimated oxidation charges Q for each of our nine catalysts with 1 cm2 geometric areas are displayed in Figure 3 A. Most catalysts have Q values comprised between 200 and 480 mC, except NiMoFe-O, NiFeSe-dO, and CoFeSe-dO, which stand out once again with values as high as 600, 800, and 980 mC, respectively. NiFe-OOH has a surprisingly low value of 60 mC. Despite some discrepancies (e.g., Cu-O and NiFe-OOH), it is interesting to observe that the C DL values correspond quite well with the obtained Q values. Among the nine catalysts, CoFeSe-dO shows the highest density of active sites, followed by NiFeSe-dO and NiMoFe-O.The current densities measured for each catalyst at a fixed overpotential of 250 mV are displayed in Figure 3B. We observe that NiFeSe-dO has the highest activity, followed by CoFeSe-dO, FeCoW, and NiFeOOH. The correlation of these data with those in Figure 3A provides the following conclusions. First, NiFeSe-dO has a much higher intrinsic activity than CoFeSe-dO, given that the current density is significantly higher than that of CoFeSe-dO, despite exhibiting a lower density of active sites. Therefore, NiFeSe-dO is the most active catalyst thanks to a combination of a high density of active sites and high intrinsic activity of these sites, whereas the high activity of CoFeSe-dO is mainly due to the large density of active sites. Second, FeCoW and NiFe-OOH display quite high activities but have a low density of active sites, as shown by the very low C DL and Q values. This suggests that these two catalysts have relatively high intrinsic activities.The evaluation of the density of active sites of metal-based (oxy)hydroxide materials is a highly challenging task. The two methods used here both suffer from limitations. Estimating ECSA by using C DL measurements requires the use of specific capacitances and strongly depends on the conductivity of the material, which can vary upon application of a potential. The integration of pre-OER oxidation peaks relies on the assumption that single electron transfer steps operate, and calculation is often complicated by the deconvolution of OER catalytic wave. We show a correlation between the results of these two methods and stress the need to cross-check data by using two complementary techniques. 54 , 66 This should be considered as a best-practice protocol for researchers aiming to evaluate intrinsic catalytic activities.The evaluation of the total number of moles of metal atoms (n M) in a catalyst on a 1 cm2 geometric area electrode can provide access to other useful information, namely the mass activity or molar activity. A high molar activity effectively translates into a lower cost, as a lower number of metal atoms are required to perform OER catalysis at a given overpotential.A n M value can be obtained through the dissolution of the catalyst layer and analysis by using ICP-MS. The metal content of the nine catalysts are displayed in Figure S24A. We observe that NiFeSe-dO and CoFeSe-dO have the lowest metal content whereas FeCoW has the highest. The metals molar activity was calculated by dividing the current density at η = 250 mV by the metal content of each catalyst (Figure S24B). The data clearly show that not only do NiFeSe-dO and CoFeSe-dO display the largest current densities but they do so with the lowest amount of metals; therefore, they show extremely high metal molar activities (520 and 634 mA cm−2 mmol−1, respectively). In contrast, all other catalysts have much lower molar activities, in the 20–50 mA cm−2 mmol−1 range. In the case of FeCoW, although the number of metals is high, only a small fraction is involved in the OER. Consequently, an improved exposition of the metal sites to the electrolyte might be key in the enhancement of the OER activity of this catalyst.The NF support used in this study benefits from a relatively high ECSA of approximately 15 cm2 cmgeo −2 (per geometric square centimeter), estimated by using the C DL measurement for NF and a C S measurement for a Ni plate electrode (Figure S25). In order to further increase the surface area of this support, we used a straightforward method for the electrodeposition of nickel dendrites on NF. 71 The deposition of the metallic branched structures in the presence of protons at very high current density generates H2 bubbles at the surface of the electrode, creating a porous dendritic morphology (Figures 4 A and S26). As a result of this increased structuration, the double-layer capacitance of NF was greatly increased from 0.9 to 4.9 mF (Figure S27), leading to an estimated surface area of approximately 82 cm2 cmgeo −2.In line with our prior conclusion regarding the benefit of a higher surface area and a larger density of accessible active sites for OER activity, a significantly higher activity was observed for this dendritic nickel foam (NiNF) than for NF. A 80 mV decrease of its η 10 value (η 10|NiNF = 331 mV), and a 129 mV decrease of its η 100 value (η 100|NiNF = 434 mV) were measured. This marked increase in activity makes NiNF a highly interesting anodic support material for electrolytic cells. NiNF was used as a new support for the deposition of the OER catalysts described above. We first detail the results obtained with the most active catalysts, namely NiFeSe-dO and CoFeSe-dO, deposited on NiNF (Figures 4B and 4C). From the SEM images, it appears that the nanostructure of NiFeSe-dO and CoFeSe-dO was maintained on this support, resulting in a hierarchical porous structure composed of three levels of porosity: the large pores of the NF (≈ 500 μm), the pores formed by the nickel dendrites (≈ 1–10 μm), and the meso- and/or macropores resulting from the layered structure of the catalysts (≈ 30–100 nm) (Figures 4B and 4C). The C DL value of NiFeSe-dO on NiNF was a factor of four greater than on the NF, reaching a high C DL value of 9.6 mF (Figures S27–S27A). The C DL value for CoFeSe-dO was roughly three times higher, giving an extremely large C DL value of 10.3 mF (Figures S27–S27B). Therefore, the use of this highly porous support enables a significant increase in the density of accessible active sites. To determine how the increase in C DL impacted the catalytic activity, the NiNF-deposited catalysts were evaluated by using our standard electrochemical characterization procedure detailed in the previous sections (Figure 5 ). For both catalysts, the η 10 and η 100 values decreased upon substitution of NF with NiNF. For NiFeSe-dO, replacing NF with NiNF decreased η 10 from 211 to 198 mV and η 100 from 264 to 247 mV (Figure 5A). With CoFeSe-dO, the improvement was even more significant with η 10 decreasing from 212 to 195 mV and η 100 decreasing from 289 to 247 mV (Figure 5B). This represents a substantial improvement in the performance of these OER catalysts (Table 2). The η 10 obtained for CoFeSe-dO and NiFeSe-dO are among the lowest values reported in the literature so far. The Tafel slope decreased from 55 to 54 mV dec−1 in the case of NiFeSe-dO and from 72 to 63 mV dec−1 in the case of CoFeSe-dO (see Figures S28 and S29; Table 3 ).For the other catalysts, we also observed a large increase in the C DL by shifting from NF to NiNF as the support (Figures S30 and S31). In all cases, apart from FeCoW, the C DL values increased by a factor between 2.4 and 5, reflecting the improvement in the specific surface area provided by NiNF. FeCoW shows a unique 9.7-fold increase in its C DL, larger than that of the support itself (Figure S31). This is likely due to an altered morphology of this catalyst when moving from NF to NiNF, leading to an increased density of accessible active sites. Additionally, a decrease of the overpotentials η 10 (by 5 to 80 mV) and η 100 (by 14 to 129 mV) (Figures S32–S38; Table 2) and a decrease of the Tafel slopes (by 1 to 30 mV dec− 1, except for Cu-O) (Figures S32–S38; Table 3) was observed. The gain of 20 to 30 mV dec− 1 in most cases is significant as it induces a large decrease in the overpotential at high current densities. In particular, NiFe-OOH displays a remarkably low Tafel slope of 20 mV dec− 1 when deposited on NiNF.Stable operation under continuous flow conditions at high current density is an important property of OER systems. We designed a water-splitting experiment under flow conditions (Figure S1), in order to test our best catalysts (NiFeSedO-NiNF and CoFeSedO-NiNF) under conditions closer to industrial applications. The catalyst was loaded in a two-compartment cell separated by a Nafion membrane with a platinum mesh-based cathode. The anolyte and catholyte were 1 M KOH aqueous solutions. These solutions were recirculated in each compartment from separate containers. A current density of 100 mA cm–2 was applied for 8 h and the potential response as well as FEO2 were measured over time. Online monitoring of the elements present in solution during electrolysis also allowed quantitative measurement of metal leaching from the catalysts.The flow experiment described above was performed by using NF, NiFeSedO-NiNF (Figure 6 A), and CoFeSedO-NiNF (Figure 6B) as the anodes. As illustrated by Figure 6, these two catalysts show extremely stable potentials over 8 h of electrolysis at a high current density of 100 mA cm–2. NF performs oxygen evolution at 1.96 ± 0.06 V versus RHE. The initial increase in potential is attributed to the oxidation of nickel, which occurs at the surface of the foam but also reaches its subsurface during the first hour of electrolysis under such a high current density. Both NiFeSedO-NiNF and CoFeSedO-NiNF perform oxygen evolution at potentials as low as 1.58 ± 0.02 V versus RHE, with outstanding stability, which represents a major improvement as compared with that of NF. The Faradaic efficiency for O2 production (FEO2 ) was evaluated by measuring the amount of oxygen produced at the anode (Figure 6). During the first hour of the experiment, the headspace of the anolyte container was saturated in gas. After this equilibration period, FEO2 was very stable, with mean values of 98.0% ± 1.5%, 97.8% ± 3.0%, and 98.5% ± 1.9% for NF, NiFeSedO-NiNF, and CoFeSedO-NiNF, respectively. This confirms that oxygen evolution was the only process occurring at the surface of these catalysts.The concentrations of Ni, Co, and Fe in the anolyte were measured every hour by ICP-MS (Figure 6). In the case of NF, the Ni concentration is in the range 40–120 ppb. This concentration does not increase over time, which proves the very high stability of this support in 1 M KOH under a high current density. The concentration of Fe was comprised between 250 and 450 ppb and remained stable over time. This Fe content in a KOH electrolyte is common. 72 , 73 In the case of NiFeSedO-NiNF, we observe a small increase in the Ni concentration at the beginning of the experiment, as it reaches 300 ppb. After this small increase, the concentration slowly stabilized at around 100 ppb, which corresponds to the background concentration measured in the case of NF. No additional Ni was dissolved over the course of the reaction (Figure 6A). As a result, NiFeSedO-NiNF is an extremely stable catalyst in these conditions. As expected, no Co was detected for NF or NiFeSedO-NiNF. In the case of CoFeSedO-NiNF, Ni, and Co concentrations in the range 200–700 ppb were measured (Figure 6B). This means that some Co and some Ni are dissolved from the surface of the catalyst at the beginning of the experiment, but do not accumulate in the solution, thus revealing the absence of continuous dissolution over the course of the electrolysis. The Fe concentrations remained constant for both catalysts and correspond to the background concentration measured with bare NF. In conclusion, these two catalysts have shown a high chemical and mechanical stability over 8 h under continuous flow conditions, at a high current density of 100 mA cm–2 and an electrolyte flow of 9 mL min–1. For practical development of these catalysts, evaluation of their stability during electrolysis over months would be required.For the first time, some of the most active OER catalysts reported during the last 4 years have been compared under identical reaction conditions after deposition on the same Ni foam support. For the reliability of the comparison, we used a standardized protocol to characterize the catalysts in terms of activity, the density of active sites, and stability. Specifically, j–E curves were not obtained from LSVs, but instead from chronopotentiometric steps in 1 M KOH at different fixed current densities, up to 100 mA cm–2, to determine overpotentials. A range of complementary techniques was used in order to evaluate the density of accessible active sites of each anode. These procedures allowed for a reliable comparison of the catalysts.The highest activities were observed for Se-doped bimetallic oxides, NiFeSe-dO and CoFeSe-dO, with remarkably low overpotentials of 211 and 212 mV at 10 mA cm− 2, and 264 and 289 mV at 100 mA cm− 2 on a NF support, respectively. Their excellent overall OER activity is due to a combination of a high intrinsic activity and a high density of accessible active sites (Figure 3). The importance of selenium in precursor materials for OER has been previously discussed. 37 , 38 , 74 , 75 During OER, Se is removed in solution, in the form of selenate and selenite ions, whereas oxygen atoms are incorporated. The metal selenide, thus, serves as a templating precursor to oxides and/or hydroxides that are the actual active species. Such Se-derived oxides display greater activity than oxides/hydroxides prepared by other methods. Furthermore, it is likely that the substitution of Se by O allows more active sites to be exposed, in line with the significantly higher densities of available active sites for the two Se-doped materials as compared with the other materials (Figure 3A). Moreover, in all cases, including here, significant amounts of Se atoms (0.5–2 mol %) are retained in the material after a prolonged reaction. These atoms, which can be referred to as Se-doping, seem to improve the OER activity of the catalytic sites. A recent theoretical study indeed showed that Se-doping resulted in a substantial decrease of the energy barrier of the rate-determining step of the OER. 74 NiFeSe-dO and CoFeSe-dO are unique, thanks to very high metal molar activities (Figure S24): these catalysts can perform OER at high current density while consuming few metal resources.The combination of Ni and Fe sites provides the highest activity as has been observed in previous studies. 11 , 75–78 In keeping with the very high performances measured for CoFeSe-dO, a similar beneficial association between Co and Fe is illustrated by the high catalytic activity of FeCoW, just below that of NiFeSe-dO and CoFeSe-dO, with a η 10 value of 235 mV and a η 100 value of 293 mV on the NF support. Analysis using NF indicates that FeCoW displays the highest intrinsic activity of the catalytic sites (Figure 3), but a very poor metal molar activity (Figure S24). Therefore, further structuration leading to higher active surface areas of the deposit would be needed to improve the OER activity of FeCoW. Similar reasoning applies to NiFeOOH.Catalysts based on the association of Co and V are less active than the materials discussed above. CoV-OOH shows a strikingly higher OER activity than CoV-O, which we attribute to the 50-fold difference in mass loadings. Indeed, the hydrothermal synthesis procedure used for CoV-O led to the deposition of only 0.5 mg cm–2, whereas 25 mg cm–2 were drop-cast in the case of CoV-OOH. We do not discuss further the other catalysts, CoPi, Cu-O, and NiMoFe-O, as they show considerably lower activities than the most active catalysts discussed above.We improved the structuration and the porosity of the electrodes through modification of the NF support by depositing dense dendritic and porous Ni structures. This was achieved by using a fast and very simple electrodeposition procedure, where H2 bubbles act as templates for the formation of pores. A large increase in the double-layer capacitance and greater OER performance than the untreated NF was observed. We tested this novel high-surface-area support, NiNF, with 9 different catalysts, and in all cases, we obtained increased double-layer capacitances, reflecting higher accessibility of active sites. We measured enhanced performances, as shown from decreased overpotentials and Tafel slopes. Specifically, the overpotentials of NiFeSe-dO and CoFeSe-dO further decreased to 198 and 195 mV at 10 mA cm −2 respectively, and to 247 mV at 100 mA cm− 2. These are among the lowest overpotentials ever reported in the literature.NiFeSedO-NiNF and CoFeSedO-NiNF are the most active catalysts studied in this work. We decided to test their long-term stability. Most reports assess the stability of OER catalysts by applying a constant current and measuring the potential response. This method is relevant for preliminary tests because it enables fast analysis of extremely unstable catalysts. However, a constant potential response is not sufficient to claim that a catalyst is stable. Indeed, Kibsgarrd et al. 79 underline that catalyst corrosion can not only impair its activity but also improve it in certain cases because of factors, including corrosion, that can lead to increased roughness. Moysiadou et al. 80 draw attention to the observation that a catalyst can maintain stable macroscopic features during electrolysis, such as a stable potential, while experiencing a loss of mass because of a partial decomposition of its surface. As a result, activity monitoring does not provide a reliable measure of the overall stability of a catalyst. Transition metals at the surface of an oxygen evolution catalyst experience dissolution/redeposition equilibria during catalysis. 56 , 81 Depending on the solubility of the metals considered, irreversible dissolution might occur. Moreover, the strong flow of electrolyte hitting the surface and the large amount of O2 bubbles generated can add to mechanical instability. For these reasons, leaching of metals from the surface of the catalyst is considered as a major route of decomposition, and a catalyst can only be claimed stable if leaching is negligible. This phenomenon can be detected by analyzing the composition of the electrolyte after electrolysis by inductively coupled plasma-mass spectrometry (ICP-MS). 82 , 83 Both electrochemical and compositional stabilities must be evaluated if one wants to prove the stability of a catalyst. This work proposes a methodology to perform stability evaluation by measuring galvanostatic features, O2 Faradaic efficiency, and surface leaching all at once. This rigorous stability check, using a water-splitting system, showed that NiFeSedO-NiNF and CoFeSedO-NiNF perform oxygen evolution in flow conditions, at a high current density of 100 mA cm–2, with outstanding stability, be it on the macroscopic or on the microscopic scale. Constant potentials as low as 1.58 V versus RHE were measured at the anode. Extremely small amounts of transition metals were leached from the catalysts’ surfaces, resulting in Ni, Co, and Fe concentrations generally comprised between 100-700 ppb in the electrolyte. These results thus prove the excellent chemical and catalytic stability of the two catalysts.The benchmarking study presented here provides a reliable comparison between OER anodes comprised of catalysts deposited on porous supports. Nine of the most active and relevant precious-metal-free multimetallic OER catalysts were synthesized on NF and compared in alkaline conditions (1 M KOH) using a standardized protocol. The overpotentials at 10 and 100 mA cm–2 showed similar trends in OER activities. We identified NiFeSe-dO and CoFeSe-dO as the two best catalysts on NF, both showing η 10 values of ≈ 210 mV and η 100 values of 264 and 289 mV, respectively. We propose a protocol to assess the density of available active sites by using complementary techniques (combining double-layer capacitance measurements and pre-OER oxidation wave integration) and provide a qualitative comparison between intrinsic activities. NiFeSe-dO, in particular, stands out as a catalyst with high intrinsic activity, exhibiting higher currents than CoFeSe-dO despite displaying a comparatively lower density of active sites. With insights from the relationship between available active sites and overall activity, we further enhanced the surface area of NF by using an electrodeposition technique to texture the surface and form NiNF. OER catalysts displayed better performances (decreased overpotentials and Tafel slopes) when deposited on this new substrate, in the best case giving η 100 values of 247 mV for CoFeSe-dO and NiFeSe-dO. Electrochemical and compositional stabilities of these catalysts were both evaluated. They perform OER catalysis in flow conditions at 100 mA cm–2 maintaining a constant potential without undergoing significant leaching. The information gained from this study not only enables fair and reliable comparison of the best reported OER catalysts on NF but also highlights the important role of the support in oxygen evolution seeing as the enhanced performance was observed after structuration of the NF to form NiNF. The integration of these improved NiNF-based catalysts in electrolytic cells can increase overall energy efficiency and enhance the viability of electrically driven energy conversion and storage.Further information and requests should be directed to and will be fulfilled by the lead contact, Marc Fontecave (marc.fontecave@college-de-france.fr).There are restrictions to the availability of catalysts described in this work due to an ongoing patent submission.All data in this manuscript are available upon request to the lead contact.Chemical reagents were purchased in reagent grade from Alfa Aesar and Merck. Nickel foam (1.6 mm in thickness, purity 99.5%, density 0.45 g cm–3, 95% porosity, and 20 pores cm–2) was purchased from Goodfellow. Oxygen 5.0 was purchased from Linde. All electrochemical experiments were performed with a VSP300 BioLogic potentiostat and the Biologic EC-Lab software was used for data analysis. Hydrothermal syntheses were performed in a Carbolite Gero CWF1213 furnace. SEM images were collected on a SU-70 Hitachi FEGSEM equipped with a X-max 50 mm2 Oxford spectrometer for energy dispersive X-ray spectroscopy (EDX) measurements. Inductively coupled plasma-optical emission spectrometry (ICP-OES) measurements were performed on a Thermo Scientific iCAP 6300 duo device. Inductively coupled plasma-mass spectroscopy (ICP-MS) measurements were performed on a ICP-QMS 7900 Agilent apparatus.NF was used as the support for all the catalysts. 1 cm2 square foams were cut, and an additional section was left for electrical contact. This area was partially covered with epoxy glue in order to limit the 1 cm2 area as precisely as possible. These foams were pre-treated through soaking in a 3 M HCl solution for 10 min, in order to remove the nickel oxide layer formed at the surface when in contact with air. Then the substrates were sonicated for 5 min in ethanol, 5 min in water, and dried with compressed air before use. The synthesis procedures of the different catalysts and of the dendritic Ni support are described in the supplemental information. Mass loadings between 0.5 and 40 mg cm–2 were obtained.The catalysts surface morphologies were examined using SEM, and their elemental compositions were measured using SEM-EDX. In some cases, this analysis was complemented with elemental analysis using ICP-OES. This study was performed by dissolving the powder sample in an aqueous nitric acid solution.A two-compartment cell separated by a glass frit was used for electrochemical measurements. The electrolyte was an aqueous solution of 1 M KOH. A three-electrode arrangement was used with a platinum mesh counter electrode (Goodfellow, 2.25 cm2) and a Ag/AgCl/KClsat reference electrode (BioLogic), which was very regularly calibrated against potassium ferrocyanide in order to ensure the absence of any shift in its potential. The potentials were reported versus RHE according to the following equation: ERHE = EAg/AgCl + 0.197 + 0.059·pH. The working electrode was positioned in the cell in order to minimize the distance to the reference electrode (≈ 1 mm), thus avoiding a large contribution from the cell in the ohmic drop (the resistance was always between 0.1 and 0.25 Ω). Before each set of experiments, O2 was flowed through the working electrode’s compartment for 20–30 min. This is an important step as it prevents any contribution from the O2 partial pressure pO2 in the thermodynamic potential calculation: ENernst = EH2O /O2 – 0.059·log pH + 0.015·log pO2 . In an O2-saturated solution pO2  = 1, and the thermodynamic potential is given by the following equation: ENernst = EH2O /O2 – 0.059·log pH.Each material was characterized in 10 mL of 1 M KOH aqueous solution following a precise protocol divided into 3 steps. Step 1: consecutive LSV scans were performed at a scan rate of 10 mV s −1 until the response was stable. Step 2: in order to study the OER kinetics, it is important to avoid any transient oxidation process such as the oxidation of Ni(OH)2 to NiOOH. For this purpose, chronopotentiometric steps (CP steps) were performed at different fixed current densities (j = 0, 5, 10, 25, 50, and 100 mA cm–2) for 5 min each with stirring. In some cases, a stable potential was not obtained after 5 min, so the CP steps were extended by 5 additional min. The (j, E j ) data points were collected. The overpotential at a given current density j (η j ) was calculated according to the following equation η j  = E j –1.23 with E j the potential measured at the current density j, in V versus RHE. The (j,η j ) points were plotted in a j-η graph. Tafel slopes were obtained by plotting η against log j. The linear fit of these plots: η = a + b·log j gives the Tafel slope b. Step 3: the short-term stability of the different samples in each electrolyte was tested by running electrolysis at a fixed current density j of 50 mA cm −2 for 30 min under stirring. The pH of the electrolyte in the anode compartment did not change during electrolysis.The double-layer capacitance C DL values were determined electrochemically in an aqueous solution of 1 M KOH. All measurements were conducted in the voltage range +0.95 – +1.05 V versus RHE as it is a non-Faradaic region for most of the studied samples as well as for the NF support. An exception was made for Cu–O, which shows a Faradaic process in this region and therefore the double-layer capacitance was measured in the range +0.66 to +0.76 V versus RHE for this sample. The difference between the anodic and cathodic charging currents Δj was obtained from CV scans at different scan rates (from 20 to 600 mV s−1). The double-layer capacitance is given by Δj/2 = v·CDL where v is the scan rate. ECSAs could theoretically be obtained by using the relation ECSA = C DL/C S where C S is the specific capacitance of the sample, which corresponds to the capacitance of an atomically smooth planar surface of the same material per unit area under identical electrolyte conditions. However, it is impossible to determine reliable values of C S for each sample. Note that in previous studies, for determination of ECSAs of various OER catalysts, the same value of C S = 0.040 mF cm −2 in 1 M NaOH, based on typical reported values for metallic surfaces, was applied. 27 , 28 However, one should be aware that this only gives an estimation of the ECSAs given that the C S value varies significantly from one material to another. 27 The charge passed during the pre-OER oxidative process was evaluated. Each sample was maintained at a reducing potential (+0.4 V versus RHE) for 20 min in order to start from a fully reduced catalyst. LSV scans were recorded from +0.40 to +1.62 V versus RHE, with scan rates ranging from 1 to 100 mV/s. Between each scan rate, the sample was kept at +0.4 V for 5 min. An exponential background was subtracted from the i = f(E) curve in order to deconvolute the contributions from the pre-OER redox process and the OER catalysis, which often occur in similar ranges of potential. The charge Q passed during the pre-OER process was calculated from the integration of the oxidative wave: Q = (1/v)·∫i(E)dE with v the scan rate in V/s. The charge value was taken in a range where Q is independent from the scan rate.Each catalyst deposited on NF with a 1 cm2 geometric area was carefully removed from the support. Note that this operation was not possible in the case of NiFe-OOH and CoV-O. The collected powders were dissolved in 65% HNO3 at room temperature for 10 days in Teflon tubes. The solutions were diluted with 2% HNO3 and analyzed by using inductively coupled plasma-mass spectroscopy (ICP-MS). The quantity of each metal in each catalyst was calculated. The total number of moles of metals n M is the sum of the number of moles of each metal contained in the catalyst. For instance, in the case of NiMoFe-O, n M = n Ni + n Mo + n Fe. This assumes that all the metals are potentially OER active, which is only a rough estimation as the exact nature/composition of the active sites is unknown.Stability measurements were performed in a 2-electrode electrochemical flow cell FLC-Standard purchased from Sphere Energy (Figure S1). The potential was measured by using a leak free Ag/AgCl/KCl3.4 M micro reference (Innovative instruments). The gas produced in the anodic compartment was analyzed by gas chromatography every 30 min by using a SRI 8610C gas chromatograph equipped with a packed Molecular Sieve 5 Å column for permanent separation. Argon (Linde 5.0) was used as carrier gas, the flow rate was regulated by using a mass flow controller (Bronkhorst). A thermal conductivity detector (TCD) was used to quantify O2. The FEO2 was calculated by dividing the measured amount of oxygen by the theoretical amount of O2 expected: FEO2  = nO2,measured/nO2,expected = nO2,measured·4F/Q where nO2,measured and nO2,expected are the measured and expected amounts of O2, Q the charge passed, and F the Faraday constant. An aliquot of anolyte was collected every hour. The aliquots collected were analyzed via inductively coupled plasma-mass spectrometry (ICP-MS).This work was supported financially by funding from TOTAL S.A. Parts of this work were supported by Institut de Physique du Globe de Paris (IPGP) multidisciplinary program PARI and by Paris–IdF region SESAME grant number 12015908.Conceptualization A.P. and M.F.; methodology A.P., C.E.C., D.K., H.N.T., and M.F.; investigation A.P., C.E.C., and D.K.; writing – original draft A.P. and M.F.; writing – review & editing A.P., C.E.C., D.K., H.N.T., M.S., and M.F.; funding acquisition, M.S. and M.F.; supervision C.E.C. and M.F.A European patent has been submitted in relation to this work (EP21315007.1).Supplemental information can be found online at https://doi.org/10.1016/j.joule.2021.03.022. Document S1. Supplemental experimental procedures, Figures S1–S38, Table S1, and Supplemental references Document S2. Article plus supplemental information

Active and inexpensive oxygen evolution reaction (OER) electrocatalysts are needed for energy-efficient electrolysis applications. Objective comparison between OER catalysts has been blurred by the use of different supports and methods to evaluate performance. Here, we selected nine highly active transition-metal-based catalysts and described their synthesis, using a porous nickel foam and a new Ni-based dendritic material as the supports. We designed a standardized protocol to characterize and compare the catalysts in terms of structure, activity, density of active sites, and stability. NiFeSe- and CoFeSe-derived oxides showed the highest activities on our dendritic support, with low overpotentials of η 100 ≈ 247 mV at 100 mA cm–2 in 1 M KOH. Stability evaluation showed no surface leaching for 8 h of electrolysis. This work highlights the most active anode materials and provides an easy way to increase the geometric current density of a catalyst by tuning the porosity of its support.

The constantly rising worldwide energy requirement with environmental alarms has become reason to search the clean and renewable energy to alternate outdated non-renewable fossil fuels [1,2]. The solar and wind energy sources may be transformed into electrical energy and are regarded as alternating and unpredictable founded over natural weather. All-weather exploitation requires dominant significance for efficient energy storage as chemical energies [3]. Because of null carbon footprint release, great gravimetric energy density, and recyclable advantages, green hydrogen is often considered a talented energy vector for viable energy systems for future research. Due to the availability of ample water resources, electrocatalysis of water-splitting may alter the produced electricity as hydrogen storage with great purity in insignificant situations is an attractive and accessible energy conversion approach. It is sustainable and elevated for overall energy productivity, thereby is still more excessive than outdated thermocatalytic reactions originated from the techno-economic analysis [4–9].The effectual photocatalytic materials exhibit extended solar response, ample catalytic sites, and proper band alignment positions. The assortment of a suitable photocatalytic candidate is imperative in developing an innovative photocatalytic system [10,11]. In the past, the researchers employed two measures to investigate a suitable band structure of the photocatalysts. The first measure comprises the fabrication of micro-heterostructures via elemental doping and heterojunction construction [12,13]. The second measure involves developing macroscopic structures with numerous dimensions like hierarchical, 0D, 1D, 2D, and 3D structures [14,15]. Heterojunction, which comprises an interface between two photocatalytic materials, can hinder the unification of charge carriers by supporting their transportation to various constituents [16]. 2D heterostructures with abundant unique features possess more benefits than bulk materials, accelerating photocatalytic efficiency. 2D architectures demonstrate atomic geometry for thickness and the highest surface-to-volume ratio compared to alternative dimensional nanomaterials [17–19]. Commonly, a monolayer of 2D materials composes of atomically thick, covalent bonds of lattice, which in turn contain dangling-bond of free nanosheets, may show strange electronic and optical merits. Moreover, van der Waals (vdW) forces are often identified between nearby layers in 2D layered-structures [20]. Due to the lack of directly chemical bond proficiency, the gathering for 2D derivatives of vdW architectures may be regarded as out of restrictions from lattice matching [19].Heterostructures, commonly composed of various components associated with significant interfaces [21–24], are widely analyzed to avoid the difficulty endowed with the hybrid containing sole functioning with exotic properties [25] in the form of tunneling and confinement effects [26]. A strong approach to accelerate the HER activity is suggested to produce suitable heterogeneous interfacial contacts in tailoring the adsorption/desorption energy for critical reactions to promote the kinetics of chemical reactions [27]. Certainly, as a good platform, 2D-materials are suggested to be illustrated by graphene, as they belong to larger surface area and have higher electron transfer capability with amusing active sites, thereby leveraging towards electrochemical water splitting process [28]. However, in utmost 2D layers, the non-defective basal planes become inactive during catalytic processes [29–32]. Thus, the rational architecture of 2D heterostructures, as well as deuterogenic heterogeneous contacts, may create newly generations to optimize the electrochemical reactions with the adjustment of electron density having higher density of active sites, and in turn furnish an interfacial-created electric field, creation of a strong synergism to promote the kinetics attributed to surface catalysis [33].Very recently, effective progress has been made in build-up 2D heterostructures for electrocatalytic water-splitting [34–36]. Numerous reviews focusing the combinatorial approaches for 2D heterostructures systems as the building blocks of hybridized nanoparticles, nanorods, and nanocubes, respectively, which may be classified as 0D–2D [37], 1D–2D [38], and 3D–2D [39] depending on several dimensions [40–42]. Among 2D-based heterostructures, 2D/2D structure is the most effectual due to exhibiting high surface area and low transmission resistance [16,43–47]. However, essential progress significantly acknowledged in recent literature attributing to 2D vertical- and lateral-heterostructures are still extensive to be realized for electrocatalytic processes. Hence, an extensive study emphasizes 2D manifold heterostructures like stacking, lateral, vertical, and core-shell architectures. The electrocatalytic water-splitting implementations are suggested as important facilitation for researchers to be better comprehension in recent studies for this field.The current paper lies in its comprehensive analysis of the latest advancements in the field of catalyst design; offers insights into the rational design of 2D heterostructured materials for efficient and sustainable water splitting. It also provides a critical review of the various techniques employed in synthesizing and characterizing these materials, highlighting the advantages and limitations of each approach. The paper further explores the mechanisms underlying the improved performance of 2D heterostructured photo- and electro-catalysts for HER, providing a valuable resource for researchers in this field. Overall, the paper offers a holistic approach to designing and optimizing 2D heterostructured photo- and electro-catalysts for HER, potentially advancing the development of clean and renewable energy sources. The focus on rational design provides a novel approach to HER electrocatalysis that has the potential to accelerate the development of efficient and sustainable hydrogen production technologies.Photocatalysis and electrocatalysis are two different processes that involve using a catalyst to speed up a chemical reaction.Photocatalysis is a process in which a catalyst absorbs light energy and uses it to initiate a chemical reaction. The process typically involves a semiconductor material such as titanium dioxide, which absorbs light energy and generates excited electrons and holes. These electrons and holes can then react with water and other molecules to form reactive species, such as hydroxyl radicals, which can be used to degrade pollutants or produce hydrogen gas [48]. The overall reaction can be summarized as: Light energy + Catalyst → Excited electrons and holes Excited electrons and holes + Reactants → Products + Reactive species Light energy + Catalyst → Excited electrons and holesExcited electrons and holes + Reactants → Products + Reactive speciesElectrocatalysis, on the other hand, involves using a catalyst to facilitate an electrochemical reaction. The catalyst typically speeds up the reaction by providing an alternate pathway for the reaction to occur with lower energy barriers. For example, in water electrolysis, to produce hydrogen gas, a catalyst such as platinum can be used to speed up the reaction at the electrodes [49]. The overall reaction can be summarized as follows: Water + Electrical energy + Catalyst → Hydrogen gas + Oxygen gas Water + Electrical energy + Catalyst → Hydrogen gas + Oxygen gasIn both photocatalysis and electrocatalysis, the catalyst plays a crucial role in facilitating the reaction and increasing the efficiency of the process.The chemical vapor deposition approach (CVD) is gaining considerable interest as an effectual technology that may be employed to prepare high-quality, low-defect transition-metal dichalcogenides (TMDCs) nanostructures or growth of thin films proceeding for numerous substrates [50–52]. Developments in vapor-phase deposition comprise the adjustment of precursors ratio. Hence final product components may be easily regulated, and well-contacted nanostructure-substrate interfaces are favorable for charge migration. The exploration of the HER mechanism may also be executed by high-quality catalysts produced by the CVD method. The metal starting materials, comprising metal films [53,54], metal oxides [55,56], metal halides [57,58], or metalorganics [59], are deposited in the furnace in the middle of the heating condition. In contrast, Te, Se, or S powder is sited upstream for directional flow of gas carrier. This is a standard CVD synthesis technique (generally Ar or N2 combined with a particular concentration of H2). The vapor of sulfur, selenium, or tetraethyl will move downstream by increasing T, causing the metal precursor to being converted into the TMDCs corresponding to the high T zone (Fig. 1a –c) [50]. Current research reported that 2D monolayer heterostructures could be synthesized by reversing the carrier gas flow direction [60]. Generally, the lateral heterostructures comprising two distinctive TMDCs monolayers are delicate, and it may be difficult to survive the multistep development. Zhang et al. suggested an advanced step-by-step synthesis method for preparing different 2D TMDCs (in-plane) micro heterostructures. This technique encompasses switching the direction in which the gas is flowing.The monolayer TMDCs nanosheet needed to be produced on the substrate using the CVD technique before implementing this strategy. For the sequential procedure, previously formed monolayer was positioned downstream of Ar movement that moved from the opposite orientation throughout T fluctuation, cooling the current single layer TMDCs substances and averting thermal deterioration. During this period, the flow of Ar in the opposite direction assisted in inhibiting an uncontrolled nucleation before sequential development stage (Fig. 1d). This powerful CVD strategy may be employed to prepare a range of in-phase 2D adjacent heterostructures (i.e. WS2– WSe2, WSe2−MoS2, and so on, see Fig. 1d, e), in addition to numerous-heterojunctions (i.e., WS2–WSe2−MoS2, WS2−MoSe2–WSe2). As a result, the synthesis of superiority TMDCs, or heterostructures, by the CVD process compels the thoughtful regulation of various fundamental factors in which artificial T is the foremost one. Generally, a higher T will enhance crystallinity; however, the nanostructure may become unstable under extreme conditions.In conclusion, the T is the most significant characteristic that has to be carefully selected. Another contributing factor is the distance between the source of the chalcogen and the metal. Furthermore, if CVD apparatus contained two independent heat sources, it would not be possible to modify the distance. Ar or N gas carrier flow should be attuned to a reasonable rate to place TMDCs nanosheets on the substrates. It is mandatory to calibrate the T gradients inside the tube beforehand if there is just one heating source to categorize where the reactants should be placed. In this specific situation, carrier gas flow and distance should be regulated at once. Research on graphic layout of CVD tools has a long history and is currently resurgent with prospects of precise distance among substrates and reactants. This makes it challenging to duplicate experimental findings because of minor changes in CVD set up.CVD has several advantages over other synthesis methods for producing 2D heterostructures. One significant advantage is the ability to precisely control the growth process, including the thickness and composition of the films, as well as the orientation and alignment of the layers. This control allows for producing high-quality films with well-defined interfaces, which is critical for the performance of heterostructures. Additionally, CVD can be easily scaled up for large-scale production, making it an attractive option for commercial applications. CVD can also synthesize a wide range of materials, including those that are difficult to synthesize using other methods. Moreover, CVD enables the synthesis of complex heterostructures with unique properties, such as tunable bandgaps, magnetic properties, and electrical conductivity. Overall, the advantages of CVD make it a valuable method for synthesizing 2D heterostructures with tailored properties and potential applications in various fields.The un-exfoliated TMDCs, like MoS2 [61], MoSe2 [62,63], and WS2 [64], are semiconducting materials comprising of 2H phase. Most of the unpredicted catalytic active sites were explored in these substances, which significantly confines their catalytic activity. By exfoliating the bulk TMDCs, layered TMDCs nanosheets can be prepared with a greater surface area and active sites [65,66]. Mechanical exfoliation is a direct technique that may be employed to synthesize single-layer TMDC nanosheets. The synthetic procedure of mechanical exfoliation is correlated to the fabrication of 2D graphene [67]. This method produces single- or multiple-layer TMDC nanosheets at yield rates that are too low for electrocatalysis, though appropriate for device fabrication or mechanism research. Li insertion has been accessed as an efficacious technique for attaining layered TMDCs materials substantially. As a result, it can accomplish the necessities of practical catalytic applications. However, Li insertion decreases the layer number of bulk TMDCs and tunes their crystal structure; thus, it causes to improve catalytic progress headed for HER. Further, three different lithiation techniques can often be utilized to exfoliate large TMDCs substance. Among them, the preliminary technique is chemical exfoliation procedure, which uses organolithium compounds like butylLi (BuLi) [68–70], MeLi [71], or LiBH4 [72] (Fig. 2a -c). This strategy includes the saturation of TMDCs powder in the solution that enclosed the Li bases and the associated organic solvent. At the same time, continuous ultra-sonication would be applied to the mixture for an extended period (normally more than two days), which would enhance the efficiency of the exfoliation procedure. However, investigations are required to obtain a large quality yield of single-layer nanosheets and better control over the Li insertion process. A large-quality yield electrochemical Li inclusion technique was explored to prepare 2D one-layer nanomaterials using a regulated lithiation process to address the preceding restrictions [73–75].The noteworthy discrepancy lies in the fact that the Li inclusion was executed in a Li battery cell (Fig. 2d). During the discharge procedure, large TMDCs at the cathode of a cell were progressively exfoliated to layer nanosheets when Li was introduced into the cathode from the anode. Several nanosheets containing monolayers or a few layers were recovered after washing, sonicating, and centrifuging the samples (Fig. 2e-g). The yield attained via electrochemical Li insertion is considerably greater than that obtained by chemical exfoliation (normally 10–20%) and can reach over 90% for MoS2, TaS2, and TiS2 [76]. Though, this technology entails certain shortcomings, like a challenging technique that entails assembling battery cells. These drawbacks make this method less desirable than others. Furthermore, additional additives that are often employed throughout the fabrication process of electrodes have the potential to introduce impurities into the goods that are eventually synthesized [18]. An innovative liquid ammonia-assisted lithiation (LAAL) technique has been designed for TMDCs exfoliation. This technique is an effective source of ultrathin 2D nanosheets [61,77]. Before beginning the LAAL technique (depicted in Fig. 2h), Li metal must be encircled in a quartz tube and shielded from the atmosphere with Ar. Afterward, the tube is emptied of its contents and immersed in a bath of liquid N. Simultaneously, exceptionally pristine gaseous ammonia is injected, and it slowly transforms into a liquid state during the process. Once the powder is submerged into the liquid ammonia, its color progressively varies from blue tint to colorless due to lithiation process, hence reaction's progression is monitored. Evaporation is utilized to eliminate ammonia gas from the tube once the “blue hue” has been totally eradicated from the tube.Furthermore, the ultrathin 2D nanosheets may be fabricated via introducing water in Li intercalated system (Fig. 2i–k). The LAAL method depicted three clear advantages relative to above mentioned techniques: 1) Time required to accomplish this process will be less, normally falling within an hour. Furthermore, a notable shift made it possible to intuitively estimate the response process without any additional signal; 2) 1T phase TMDCs nanosheets exhibiting a single layer or a few layers gained with a large quality yield (∼ 82%); 3) The intense lithiation technique will result in plenty of Sulphur vacancies (S-vacancies) and large amount of edges, both of which will enhance the electrochemical efficacy of exfoliated TMDCs and nanosheets. Due to strong reaction that takes place when water and metal Li encounter one another, as well as the usage of liquid ammonia, it is significant that each stage of the process be executed with care to assure the user's wellbeing. It is possible to create a thinner sheet of TMDCs material by exfoliating the bulk substances. As a result, exfoliation of the bulk materials may be employed. Due to complex reaction procedure in liquid environment, it is difficult to prepare TMDCs nanosheets with preferred features, such as definite structure and layer quantity, making electrocatalysis challenging to explore.Exfoliation is another popular method for synthesizing 2D heterostructures with several advantages. One of the main advantages is that exfoliation is a simple and cost-effective method that can be performed using standard laboratory equipment. This method involves the mechanical or chemical exfoliation of bulk materials to obtain thin 2D sheets with unique properties that can be stacked to create heterostructures. Additionally, exfoliation can produce a wide range of 2D materials, including those that are difficult to synthesize using other methods, and the resulting heterostructures have sharp and well-defined interfaces. Exfoliated 2D heterostructures have shown promising properties for applications in catalysis. Furthermore, the ability to combine different 2D materials through exfoliation opens new possibilities for the synthesis of novel heterostructures with unique properties. Therefore, exfoliation is a valuable method for synthesizing 2D heterostructures with tailored properties and potential applications in various fields.Hydro/solvothermal is an efficient and applicable technique to prepare TMDCs-based nanomaterials at a large scale. Materials with various structures and phases may be prepared by tuning T, reaction duration, metal precursors, surfactants, and other factors, making hydro/solvothermal an appropriate technique to prepare nanostructured materials [78–80]. Currently, MoSe2 nanosheets prepared via hydrothermal method are an effective HER catalytic material. By varying the reaction T, combined with the quotient of NaMoO4•2H2O and Se starting materials to the reductant (NaBH4), the products revealed dissimilar crystal structure and disordered degree (Fig. 3a ) [62]. It showed that an increased concentration of NaBH4 would raise 1T MoSe2 ratio, which displayed enhanced HER efficiency (Fig. 3b). Conversely, slower reaction T carried the high active sites, though went beside the 1T surface creation. The MoSe2 nanosheets with excellent HER catalytic performance was generated by cautiously controlling the disordered degree and 1T surface ratio. Though hydrothermal technique was commonly employed to prepare nanostructured materials that might easily be oxidized during synthesis, the process could affect the product's purity. Manufacturing TMDCs containing nanomaterials also utilizes the solvothermal technique to avert potential oxidation [81,82]. The selectivity of the final products is a prominent advantage of solvothermal reactions. For instance, 2H WS2 would be generated by carefully adding hexamethyl disilazane, and 1T WS2 would finally be fabricated (Fig. 3d) [83]. The resultant products will exhibit lots of active sites. The numerous uniting forms of reactants will give advantage to the creation of heterostructured nanomaterials, like MoS2/CuS [84], MoS2/CdS [85], MoS2-graphene [86], and MoS2/CoSe2 [87], with extra interfaces and acceptable level structures (Fig. 3e-g).Hydrothermal or solvothermal synthesis is another method that has gained attention for synthesizing 2D heterostructures due to several advantages. One advantage of this method is that it can be performed at relatively low temperatures and pressures, making it a more environmentally friendly and energy-efficient method compared to other synthesis methods. Additionally, hydrothermal or solvothermal synthesis can produce high-quality 2D heterostructures with excellent crystallinity and uniformity, essential for the optimal performance of heterostructures. This method can also be used to synthesize a wide range of materials, including complex compositions and heterostructures with well-defined interfaces. Furthermore, the hydrothermal or solvothermal method allows for controlling the size, shape, and morphology of the resulting 2D heterostructures, which can impact their physical and chemical properties. Overall, the hydrothermal or solvothermal method is a promising approach for synthesizing 2D heterostructures with controlled properties and potential applications in various fields.Template transformation is one of the most effective methods to attain desired superior topological materials. The 2D heterostructures prepared using template-assisted synthetic approaches can hold a targeted size, morphology, and composition [88]. Depending on the category of templates, the template-assisted approaches can be categorized into two methods: hard-template method and soft-template method [89].The hard-template method is considered one of the most common approaches for preparing hybrids. The hard template is a rigid material that can directly evaluate the final sample's size, structure, morphology, and components [90]. For 2D heterostructure fabrication, the material/compound precursor with 2D geometry is generally chosen as the hard template. Depending on considerably various solubilities (Ksp) between metal sulfide and hydroxide, Zhang et al. prepared the CoNi hydroxide sheets in advance as a hard template to partially reconstruct into an ultrathin CoNi hydroxysulfide shell using ethanol-modified surface sulfurization route, preparing a 2D Co1.8Ni(OH)5.6@Co1.8NiS0.4 (OH)4.8 core–shell heterostructure (Fig. 4a ) [91]. As the Co1.8Ni(OH)5.6 sheets were immersed in Na2S ethanol solution under room temperature, Co1.8Ni(OH)5.6 endured an interfacial reaction with hydrolyzed HS¯ and transformed into Co1.8Ni(OH)0.4(OH)5.6 shell. The resultant Co1.8Ni(OH)5.6@Co1.8NiS0.4 (OH)4.8 sample displayed a well-maintained hexagonal plate-like structure of the Co1.8Ni(OH)5.6 precursors (Fig. 4b). Hou et al. synthesized a confined carburization-engineered technique to prepare the unique 2D NiOx/Ni ultrathin heterostructure nanosheets (Fig. 4c), which can be demonstrated as a complete hard template transformation [92]. First, ultrathin Ni(OH)2 nanosheets were used as the starting materials (Fig. 4d). Later, an oxidative self-polymerization reaction of the dopamine precursor to polydopamine (PDA) was induced on the surface of Ni(OH)2, producing a core–shell 2D Ni(OH)2@PDA heterostructure (Fig. 4e). Ultimately, the 2D Ni(OH)2@PDA as the hard template was subject to annealing treatment in nitrogen environment to entirely convert into 2D ultrathin NiOx/Ni heterostructure nanosheets (Fig. 4f,g).Wang et al. prepared the holey 2D transition metal carbide/nitride heterostructure nanosheets (h-TMCN) using controlled thermal annealing of the Mo/Zn bimetallic imidazolate frameworks (Mo/Zn BIFs) (Fig. 4h) [93]. In their work, Mo/Zn BIF precursors with 2D thin flake-like structures were fabricated using coordination and complexation of Zn2+, [MoO4]2¯, and 2-methylimidazole (Fig. 4i). Subsequently, they were annealed in nitrogen environment at 800–900 °C to assist the total pyrolysis of Mo/Zn BIF and the development of Mo2C and Mo2N nanocrystals (Fig. 4j). Li et al. prepared a 2D hierarchical Mo2C/C nanosheet hybrid by water-soluble sodium chloride cube crystal-template approach as depicted in Fig. 4k [94]. Obtained results displayed sheet-like Mo2C that were evenly attached on the surface of carbon nanosheets (Fig. 4l–n). The resultant Mo2C/C hybrids revealed noteworthy HER performance in both alkaline and acid medium, associated with the strong synergistic catalytic effect and charge transfer ability.Xu et al. employed the electrostatic assembly of positively charged β-Ni(OH)2 nanosheets and negatively charged functionalized single-layer graphene (FGR), resulting in face-to-face hybridization between Ni(OH)2 and FGR [95]. Later, the Ni(OH)2-FGR heterotemplate was conversed into Ni2P-FGR by a low-temperature phosphorization reaction (Fig. 4o). TEM micrograph of Ni(OH)2-FGR showed a wrinkled sheet structure of FGR combined with ultrathin Ni(OH)2 nanosheets (Fig. 4p). The TEM micrograph of Ni2P-FGR illustrated that ultrathin Ni2P nanosheets covered the FGR with wrinkles with 3.2 nm in thickness (Fig. 4q). Depending on controllable experiments, the stress transfer-induced mechanism was proposed to rationalize the FGR assisted growth of ultrathin Ni2P nanosheets from Ni(OH)2 nanosheets during phosphorization (Fig. 4r) [95].The soft templates comprise micelles or vesicles, macro- and microemulsions, some polymers, and biological molecular assemblies [96]. The advantages of the soft-template technique usually involve comparatively moderate experimental conditions and simple execution [97]. Combining various template approaches has also been applied to prepare 2D heterostructures. For instance, Zhuang et al. coupled soft template with hard-template approaches to prepare targeted 2DPC-RuMo heterostructure with RuMo nanoalloy-embedded 2D porous carbon (PC) nanosheets for high-performance alkaline HER activity [98].In situ/operando characterization techniques have been employed to understand the working mechanism of these catalysts. This section will discuss these techniques and their application in studying the working mechanism of 2D heterostructured photo- and electro-catalysts for HER. In situ/operando characterization techniques allow us to monitor the catalyst's structure and activity under reaction conditions. These techniques provide information about the catalyst's surface chemistry, electronic properties, and reaction intermediates [99,100]. Some commonly used in situ/operando characterization techniques for 2D heterostructured photo- and electro-catalysts for HER: (i) X-ray photoelectron spectroscopy (XPS): XPS is a powerful technique for investigating catalysts' chemical composition and electronic structure. In situ, XPS can be used to study the changes in the surface chemistry of the catalyst during the reaction. This technique can also provide information about the oxidation states of the catalyst and the adsorbed species [101,102]. (ii) Transmission electron microscopy (TEM): TEM can be used to study the structural changes of the catalyst during the reaction. In situ TEM can provide information about the catalyst's morphology, size, and crystal structure under reaction conditions. This technique can also be used to study the formation and evolution of reaction intermediates [101]. (iii) Fourier-transform infrared spectroscopy (FTIR): FTIR can study the adsorption and desorption of gasses on the catalyst surface. In situ, FTIR can provide information about the surface species and reaction intermediates formed during the reaction [103]. (iv) Raman spectroscopy: Raman spectroscopy can be used to study the chemical and structural changes of the catalyst during the reaction. In situ Raman spectroscopy can provide information about the formation and evolution of reaction intermediates [104]. X-ray photoelectron spectroscopy (XPS): XPS is a powerful technique for investigating catalysts' chemical composition and electronic structure. In situ, XPS can be used to study the changes in the surface chemistry of the catalyst during the reaction. This technique can also provide information about the oxidation states of the catalyst and the adsorbed species [101,102].Transmission electron microscopy (TEM): TEM can be used to study the structural changes of the catalyst during the reaction. In situ TEM can provide information about the catalyst's morphology, size, and crystal structure under reaction conditions. This technique can also be used to study the formation and evolution of reaction intermediates [101].Fourier-transform infrared spectroscopy (FTIR): FTIR can study the adsorption and desorption of gasses on the catalyst surface. In situ, FTIR can provide information about the surface species and reaction intermediates formed during the reaction [103].Raman spectroscopy: Raman spectroscopy can be used to study the chemical and structural changes of the catalyst during the reaction. In situ Raman spectroscopy can provide information about the formation and evolution of reaction intermediates [104].The working mechanism of 2D heterostructured photo- and electro-catalysts for HER can be understood by studying the changes in the surface chemistry, electronic properties, and reaction intermediates during the reaction. In situ/operando characterization techniques can provide valuable insights into the working mechanism of these catalysts. For example, in situ XPS can be used to study the changes in the oxidation states of the catalyst and the adsorbed species during the reaction. In situ TEM can be used to study the formation and evolution of reaction intermediates and the structural changes of the catalyst. In situ, FTIR and Raman spectroscopy can study the surface species and reaction intermediates formed during the reaction.The electronic structure is a single aspect that regulates heterogeneous catalysts' adsorption capabilities in all intermediates categories [105–108]. Therefore, modifying the electrical catalyst band is an ideal strategy to influence its catalytic progress. Such as, disrupting electronic arrangement of a catalyst commonly intends to enhance its hydrogen binding energy (HBE) for alkaline HER, comparable to the approaches employed in acid conditions.Hydrogen adsorptive hydrogen bonds are reinforced when the d-band center of metallic catalytic substance shifts closer to Fermi energy, and vice versa when the center of D-band transfers away from Fermi level [108–110]. The most effectual technique for creating the D-band vacancy of a metallic catalyst is to alloy one metal with another. The HBE of most metallic catalysts can be changed by alloying with another metal, as depicted in Fig. 5a , generally due to mass electron shifting among two separate metal sites [111,112]. In certain belongings, this kind of substantial electron movement can offer an electronic structure of non-metallic sites that work with metal alloys to improve whole catalytic activity. A succession of RuCo alloys enclosed in an N-doped graphene sheet is one such example [113]. The alkaline HER is catalyzed by very active meal Ru, while stability and efficiency of RuCoC alloy are considerably higher (Fig. 5b). Additionally, DFT studies demonstrate higher activity is strongly associated with the unusual electron arrangement of carbon shell. Because of alloying of RuCo, the variation of electron transport from atom to carbon shell happened (Fig. 5c). Consequently, the CH bond of carbon heightened, causing to decrease HBE of catalyst; therefore, enhancing entire efficiency. Alike findings were attained aimed at numerous alloy catalysts. Relative to Pt/C, a PtNi/NiS nanowire prepared by Huang et al. showed 5.58 times advanced current density values at −0.07 V vs RHE [114]. Zheng et al. developed PtNi alloy in the form of hexagonally packed nano-multipods which displayed considerably greater HER activity than Pt/C [115]. Remarkably, alloying causes to trigger of non-noble metal substances, which ordinarily have reduced HER activity by modifying the electronic structure [116–118]. Furthermore, exceptional HER progress was revealed by Chen et al. for Cu-Ti bimetallic electrocatalysts [117]. The electronic arrangement of CuTi was attuned by modifying Ti's content in the catalyst. Meanwhile, HBE of catalytic material remained modified to a suitable level by placing them at uppermost mark of volcano plot. The Cu-Ti catalyst showed incredible activity, even though neither Cu nor Ti is a favorable HER candidate [117]. A comparable development was presented for MoNi4 alloy [119]. MoNi4 alloy shows an enhanced electronic structure that stemmed into surprising water dissociation aptitude because of rebuilt electronic arrangement of alloy, compared to Mo, Ni, and MoO2, which show relatively slow alkaline HER kinetics. Such characteristics of MoNi4 assured their significant HER efficiency using alkaline conditions.2D nanomaterials have attained considerable attention because of their extraordinary properties relative to bulk counterparts. Numerous 2D nanomaterials have been prepared to date, but specifically, TMDCs and graphene have attracted significant interest owing to their exceptional properties [120–122]. Earth-rich and reasonably priced TMDCs, phosphides chalcogenides, and nitrides exhibiting numerous structures have been synthesized in large quantities during the past few decades due to fast-evolving fabrication methods [65,123,124]. Various TM-based materials were perceived to reveal exceptionally high interaction with H by precise electronic morphological engineering [123,125].Carbon-doped MoS2 (C-MoS2), which may be developed by sulfurization of Mo2C has substantial alkaline HER efficiency that is relatively comparable to Pt/C (Fig. 6a ) [126]. The materials' electrical structure is tuned by including MoS2 with carbon, which results in exceptional activity. The carbon in the catalyst did not function as a dual active site; instead, it induced vacant 2p orbitals perpendicular to the MoS2 basal plane. This generated an environment that was beneficial for the adsorption of water (Fig. 6b). Therefore, the catalyst shows an improved inclusive performance in addition to an enhanced rate of water dissociation while operating in alkaline settings. Defect engineering and incorporation of heteroatom are considered effective methods for carbon-possessing catalysts [106,127–129]. Further, defect engineering and the incorporation of heteroatom can be directly utilized to increase valence orbital energy for carbon sites, which eventually results in an enhancement of interaction among H* and carbon intermediates. It has been proven especially for alkaline HER. Though, the efficacy of carbon-based materials is often enhanced when they are combined with extra catalysts rather than functioning as an only catalytic phase in HER using alkaline conditions [130,131]. Because of this, a comprehensive examination of these resources will not be delivered during this assessment. However, modifying the charge density of the catalytic surface is another route commonly utilized to increase the natural capacity of transition metals to absorb electrons. Conjoining catalysts based on transition metals with other substances is a significant method normally used to attain this objective. Zou et al. conducted a study on regulating the charge shifting of Mo2C-based HER catalytic materials [132]. By coating Mo2C with N-rich carbon, superior progress for catalytic HER was acquired for entire pH scales (Fig. 6c). Aforementioned effect is recognized as lonely effect for enhanced H adsorption capability of catalyst was accomplished only by covering Mo2C with N-rich carbon.Later, studies revealed that substantial electron-extracting capacity of N sites created an electron shifting that drew electrons from Mo2C to N via C sites. This transfer happened because Mo2C sites could attract electrons. This mechanism activated the C atoms and N sites, which stemmed into the production of HER-active C sites responsible for working in conjunction with Mo2C (Fig. 6d). Hence, the total activity of HER might be improved in all situations irrespective of pH. Similar strategies were also designated for other basic HER catalytic substances. These approaches are effective for altering H adsorption proficiency of the catalysts and improving interaction between the catalyst and water (OH) [133–135]. The MoP@C catalyst is an excellent depiction of a distinctive case since it showed exceptional HER efficiency in a basic environment (Fig. 6e) [135]. In catalyst, development of Mo-C bond was caused by the presence of carbon on MoP surface, which in turn had a substantial influence on electronic arrangement of molybdenum. In conclusion, the recently upgraded Mo site is now appropriate for the dissociation of water. In contrast, the adjacent P sites were accountable for the recombination of hydrogen (Fig. 6f). It was revealed that such a structure delivered an appropriate platform for the processing of alkaline HER. Its forthright construction, which may speed up two processes simultaneously, makes it the most operative approach for generating high-quality HER activity.Though the part that OH– plays in alkaline HER has not been computed so far, it is apparent that the interaction between catalyst and OH– is one of the most momentous features in evaluating the catalyst's activity. On the other hand, it is highly thought-provoking to attain stability among desorption and adsorption abilities headed for H* and OH* on a single site because of faintly understood scaling connection among H* and OH* intermediates binding energy. Fabricating dual active sites responsible for individually presenting exceptional functionalities is one of the most broadly acknowledged routes for gaining control over OH* and H* in a distinctive way. A sequence of catalysts was revealed by Markovic et al. via decorating bare Pt with metal/metal hydroxide sites [136–138]. This was accomplished according to perception described above. Fabrication of a hybrid material is predominant aim of having standard HBE values for HER via espousing a material over top of an acidic HER volcano plot. It is just like Pt with an oxophilic metal that offers adequate OH interaction sites in place of water dissociation. Fig. 7a shows a comparison of oxophilicity of several regularly used materials; a larger oxophilicity anticipates a stronger OH binding energy, and vice versa [139]. For HBE, the OH binding energy of a catalyst must be moderate to generate an appropriate interaction between the catalyst and the water. Consequently, a few choices are notable for their sufficient oxophilicity, such as Ni, Ru, and Co. These materials reveal extraordinary activity; hence, they are employed widely in developing catalysts with dual active sites [130,140,141].Based on the achievements of the alloys with dual sites, in preference to Pt the non-noble materials have been used as H* interaction sites to design worthwhile catalysts. In HER technique, it has been verified that MoS2 and g-C3N4 can absorb hydrogen completely, like Pt. Yang et al. investigated that MoS2 can be used as the layered double hydroxide (LDH) and H-active material as OH-active sites for excellent HER efficiency (Fig. 7b) [142]. DFT calculations reported that MoS2/LDH heterostructures exhibit excellent progress that may be associated with decreased activation energy (Fig. 7c) which is the consequence of water dissociation procedure. MoS2/LDH system suggests separate sites for dissociation of water (on LDH) and hydrogen adsorption (on MoS2), similar to the Pt/Ni(OH)2 system. These sites correspond to the overall alkaline HER procedure by acting as synergists (Fig. 7d) [142].Thickness and the lateral dimension of 2D materials exhibit an abundant impact on electrocatalytic aptitudes of these resources [143]. (1) When the lateral dimension of 2D materials is lowered, supplementary defects and edge locations appear; (2) the width of 2D materials may be tuned to modify their electrical structures, which results in amendable catalytic activity; (3) catalytic progress augmentation might rise from deficiencies (in-plane) produced through structural disorder when the thickness is abridged to the atomic level; (4) monolayered 2D materials have the highest hypothetical surface area, which provides maximum prospective for catalytic activity. The control of morphological aspects of MoS2 was offered by Kibsgaard et al., nanoscale that reveals these effects may alter the surface structure at nanoscale, enhancing HER efficiency (Fig. 8 ) [144]. The lattice modification in many 2D materials provides enormous vacancies for catalytic progress when thicknesses are abridged to sub-nanometer level. Thickness management also enables using 2D morphologies to reveal a catalyst's maximum area and generate large electrochemical surface area (ECSA). Moreover, the development of freestanding Pd nanosheets was offered by Huang et al. for catalytic and plasmonic activities [145]. The electrocatalytic oxidation process of formic acid involves superior ECSA for Pd nanosheets, which stemmed into higher density (than commercial Pd black). Consequently, a number of approaches have been designed to increase the ECSA of metal and metal alloy nanosheets (for instance, Pt−Cu, Rh, and Pd−Cu) [146,147].Developing a Z-scheme heterojunction may increase the photogenerated carriers' split-up and retain a photocatalytic reaction's high redox capacity. These aspects have considerable benefits for Z-scheme heterojunction. The development of 2D/2D stacked heterojunction causes prolonged contact area among two semiconductors, increases the number of charge migration channels, and decreases the charge transfer distance, stimulating the migration and split-up of photogenerated exciton pairs. In the meantime, the higher specific surface area of these heterojunctions can give additional reactive sites, which is beneficial to reactant adsorption. Prominently, developing a 2D/2D heterojunction (Z-scheme) may completely integrate and exploit previously described benefits of Z-scheme and 2D/2D heterojunction. Furthermore, the 2D/2D heterojunction (Z-scheme) photocatalysts can be used in various photocatalytic processes. This section discusses photocatalytic H2 generation by 2D/2D heterojunction (Z-scheme) [148].Hydrogen has been proposed as an excellent energy vector to replace existing fossil fuels due to its high energy density (237.2 kJmol−1), pure combustion product (H2O), and long-term sustainability [149,150]. Among the different strategies for producing H2, sunlight-driven water splitting is a promising option since it can turn unlimited solar energy into chemical energy [151,152]. The photocatalyst must satisfy the necessary thermodynamic conditions for overall water splitting, which are that the CB (conduction band) position be more negative than the reduction potential of H +/H2 (0 V vs NHE at pH = 0) and the VB (valance band) position be more positive than the oxidation potential of O2/H2O (1.23 V vs NHE at pH = 0). However, most photocatalytic H2 production reactions were carried out with sacrificial agents (lactic acid, methanol, triethanolamine, Na2S, Na2SO3, and so on) to install the problematic water oxidation reaction with the hole-sacrificial agent reaction, which raises the price of H2 production. Many semiconductor photocatalysts have been created and used in photocatalytic H2 generation up to this point. However, their activities need to be enhanced since they fall far short of the desired value of solar-to-hydrogen efficiency (STH) of 20% [153–155]. A promising H2 generation photocatalyst should have enough redox ability for water splitting, broad and strong light absorption, and effective separation of photogenerated carriers, according to the preceding description of the photocatalysis mechanism (Fig. 9 ). The 2D/2D Z-scheme heterojunction can theoretically fulfill all of these requirements. Zhu et al. created a 2D/2D black phosphorus (BP)/BiVO4 Z-scheme heterojunction and achieved total water splitting for H2 generation [156]. BP nanosheets were created by ultrasonic exfoliation of bulk BP in an N-methyl-2-pyrrolidone solution, whereas BiVO4 nanosheets were created using a hydrothermal technique with sodium dodecylbenzene sulfonate as a template. Electrostatic interactions easily hybridized the thin BP and BiVO4 nanosheets to generate a 2D/2D BP/BiVO4 heterojunction. Dark-colored BiVO4 nanosheets were constructed on the surface of BP nanosheets, as illustrated in Fig. 9a, b. The closely connected BP ((040), d-spacing of 0.26 nm) and BiVO4 ((−121), D-spacing of 0.31 nm) heterojunction is visible in HR-TEM images (Fig. 9c, d). In all photocatalytic H2 and O2 generation processes, the 2D/2D BP/BiVO4 heterojunction outperformed BP and BiVO4 alone, with or without sacrificial agents (Fig. 9e, F).The increased photocatalytic H2 and O2 generation activity showed that the charge transfer mechanism was Z-scheme; the authors confirmed the Z-scheme charge transfer process using femtosecond Time-domain reflectometry spectroscopic analysis. Photogenerated electrons in the CB of BiVO4 were swiftly transported to the VB of BP and merged with its photogenerated holes via the intimately connected 2D/2D interface when exposed to visible light (λ > 420 nm). Consequently, electrons in the CB of BP and holes in the VB of BiVO4 with sufficient redox capacity could achieve total water splitting with H2 and O2 production rates of 53.34 and 34 μmol g−1 h−1, respectively, with an apparent quantum efficiency of 0.89% at 420 nm (Fig. 9g). With the assistance of a co-catalyst Co3O4, the H2 and O2 production rates were increased to 260 and 153.34 μmol g−1 h−1, respectively (Fig. 9h). Neither BP nor BiVO4 can accomplish total water splitting on their own. These findings highlight the benefits and efficiency of the 2D/2D Z-scheme heterojunction in photocatalytic water splitting for H2 generation.Li et al. created a sulfur-vacancy-confined in ZnIn2S4 (Vs-ZnIn2S4)/WO3 2D structure in the Janus bilayer Z-scheme heterojunction with higher visible light photocatalytic H2 generation [157]. Vs-ZnIn2S4 nanosheets were created using a low-temperature refluxing approach followed by a desulfurization process between lithium and ZnIn2S4. Heat treatment of liquid exfoliated WO32H2O nanosheets from bulk WO3 ·2H2O produced WO3 nanosheets. The Janus bilayer Vs-ZnIn2S4/WO3 heterojunction was created by electrostatic self-assembly by including WO3 nanosheets into the Vs-ZnIn2S4 precursor, and the remaining steps were the same as for Vs-ZnIn2S4 fabrication. The authors opted to experiment with Vs-ZnIn2S4 nanosheets because introducing sulfur vacancies might considerably increase the photocatalytic H2 generation activity of ZnIn2S4 nanosheets by enhancing light usage and charge separation. Fig. 10a , b indicates that WO3 nanosheets were closely adhered to the surface of ZnIn2S4, showing that the 2D/2D Vs-ZnIn2S4/WO3 heterojunction was successfully synthesized. Additionally, a new peak associated with WS bonds developed in the Vs-ZnIn2S4/WO3 heterojunction's Raman spectra indicated strong interfacial adhesion between the two components. Fig. 10c depicts the as-prepared samples having visible light (λ > 400 nm) photocatalytic H2 generation activities. Compared to Vs-ZnIn2S4, all Vs-ZnIn2S4/WO3 exhibited increased H2 production activity; however, WO3 showed no H2 production activity. The best H2 production activity of 7.81 mmol g−1 h−1 was achieved by the WO3 (10 wt%)/Vs-ZnIn2S4 composite, which was about 2.9 and 1.57 times greater than the plain Vs-ZnIn2S4 (2.68 mmol g−1 h−1) and the WO3 (10 wt.%)/ZnIn2S4 composite (4.97 mmol g−1 h−1). Furthermore, increased loading of NiS quantum dots as a co-catalyst on the WO3 (10 wt%)/Vs-ZnIn2S4 composite may boost its photocatalytic H2 generation activity, which reached 11.09 mmol g−1 h−1 at the optimal NiS (1.0 wt%) loading level. Because of the low CB position of WO3, the energy band positions of WO3 and Vs-ZnIn2S4 (Fig. 10d) indicate that photogenerated carrier transfer in the WO3/Vs-ZnIn2S4 composite accompanied the Z-scheme mechanism rather than the type II mechanism; instead that, the photocatalytic H2 production activity could not be enhanced (0.06 eV). To further evaluate the Z-scheme mechanism, radical intermediate detection tests were performed. Because of their restricted redox capacity, WO3 could only produce •OH (Fig. 10e) and Vs-ZnIn2S4 could only produce •O2 (Fig. 10f) under visible light irradiation; however, both the •O2 and •OH could be identified with relatively strong signals in the 2D/2D Vs-ZnIn2S4/WO3 heterojunction, which strongly validated the Z-scheme charge transfer mechanism. Advantages Efficient charge separation: The 2D/2D Z-Scheme heterojunction structure can facilitate efficient charge separation, which benefits HER. Electrons and holes generated in one layer can be quickly transferred to the other layer, reducing recombination and enhancing photocatalytic activity [158]. Broad range of visible light absorption: 2D materials such as graphene, MoS2, WS2, etc., have a wide range of visible light absorption capabilities. This enables the heterojunctions to absorb more light and increase the energy available for HER. High stability: 2D materials are generally more stable than their 3D counterparts, making them ideal for photocatalytic applications [159]. Limitations Limited active sites: The 2D/2D Z-Scheme heterojunctions can have a limited number of active sites for HER. The active sites are the locations where hydrogen evolution occurs, and if there are not enough active sites, the photocatalytic activity will be limited [159]. Difficult to fabricate: Fabricating 2D/2D Z-Scheme heterojunctions can be challenging and require specific techniques; it can limit their widespread use in HER applications. Low HER efficiency (compared to some traditional Z-schemes): Despite their advantages, the HER efficiency of 2D/2D Z-Scheme heterojunctions can still be low due to the limited active sites and other factors. This means more research is needed to optimize their performance for practical applications [148].The theory of heterostructure originates from semiconductor physics. The heterostructures consist of several heterojunctions; interfaces among dissimilar components, and significantly the heterostructures are semiconductor materials in which chemical configuration changes with position. The theory of heterostructure has led the system over semiconductor physics owing to integration and intercrossing of knowledge network. Generally, heterostructures can also be defined as composite structures consisting of interfaces aimed at dissimilar solid-state materials, covering semiconductors, insulators, and conductors [160]. Heterostructures are regarded as a crucial aspect of developing catalytic advancement of 2D materials since they overcome every material's intrinsic limits and generate innovative features. In this regard, assemblage and creation of heterostructures, usually based on 0D, 1D, and 2D materials, is an effective approach to attain higher electrocatalytic improvement. During past years, advancement. Previous fundamental studies have focused on synthesizing 2D material-based heterostructures to produce advanced electrocatalysts. Hence, this section highlights the fabrication and development of 2D-heterostructures.2D materials can show significant catalytic advancement due to their remarkable features. However, these catalytic yields cannot strive with catalysts dependent on noble metals owing to higher restacking problems. Currently, vdWs heterostructures could propose novel methods to achieve the entire response of 2D materials. Two different 2D materials can be merged to produce 2D/2D heterostructures to compensate for particular weaknesses and abridged interfacial contact resistances, generating enhanced catalytic performances [161–163]. Yang et al. prepared 2D rGO (reduced graphene oxide) and WS2 heterostructure by hydrothermal method that resulted in developed HER activity due to improved charge transfer kinetics [164]. Hereafter, Tang et al. [165] developed a vdW heterostructure comprising nitrogen and graphene-doped MoS2 by mesoporous magnesia as a template. The fabrication of porous graphene skeleton was employed by CVD and integration of Mo/S/N sources for the growth of nitrogen-doped MoS2 nanosheets over graphene skeleton (G@N-MoS2) as shown in Fig. 11a , b. The fabrication technique of material endorsed for active regulation towards electronic and physical structures of each component as well as the hybrid material to hold stronger interfacial interactions. Furthermore, the presence of N-MoS2 was confirmed by HR-TEM having layer to layer distance of 0.62 nm as displayed in Fig. 11c. Additionally, the micrographs reveal the dispersion of N-MoS2 nanosheets over graphene to prepare face-to-face vdW heterostructures as depicted in Fig. 11d. The resultant heterostructure produce effectual multifunctional electrocatalytic developments due to their excellent electronic and structural features.The HER was also explored for N-MoS2 relative to pristine MoS2 in which G@N-MoS2 catalyst provides superb HER activity in acidic media having a current density of 10 mA cm−2 and low overpotential (243 mV) as illustrated in Fig. 11e. Moreover, an onset potential (100 mV) which is higher relative to resultant counterparts in alkaline media as depicted in Fig. 11f. The exceptional ORR (oxygen reduction reaction) and OER (oxygen evolution reaction) activity (Fig. 11g and h respectively) was also presented by G@N-MoS2 catalyst by alkaline media. The partial current density was explored for G@N-MoS2 catalysts that reveal current density findings were quite close to Pt/C for ORR and half-wave potential was relatively reduced relative to other catalysts that hold overpotential (20 mV) of fabricated catalyst by current density (10 mA cm−2) that is lower relative to Ir/C catalysts. Developments in electrocatalytic activities in this study are attributed to various factors; primarily the electronic structures of MoS2 can be proficiently controlled by nitrogen doping to provide shortened bandgap energies [166], larger spin densities [167] that stemmed from supporting interfacial charge transfer. Then, interfacial interaction between MoS2 and graphene can increase adsorption energy, and finally, the resultant 3D mesoporous structures may improve proton transport in addition to active site exposure. Regardless of valuable tri-functional progress of G@N-MoS2 catalyst, the vital routes remain undefined, and further research is mandatory. 2D/2D heterostructures can also be used as self-sustaining electrodes like 1D/2D heterostructures towards direct energy conversion systems. Duan et al. worked on [168] advancement of flexible film by integrating porous C3N4 nanosheets (PCN) using nitrogen-doped graphene (PCN@-N-graphene) by adopting simple vacuum filtration approach.Currently, catalytic experiments endorse flexible film with porous structure (hierarchical) for higher mass transport. The layered structure of graphene and C3N4 exhibits superior interfaces to improve charge transfer. Afterward, more advancement was carried out for self-supporting PCN@-N-graphene electrode with slight onset potential (− 0.008 V) closer to commercial Pt, an unusual exchange current density (0.43 mA cm−2), along with exceptional durability after 5000 cycles. These essential features allow this 2D/2D heterostructure to have superior flexibility, conductivity, and catalytic development as promising material for advantageous electrocatalysis applications. Voiry et al. [169] presented the electronic coupling among gold (Au) substrate and MoS2 that causes reduced contact resistance of systems and increases the electron injection to catalyst active sites from substrate. The basal plane is usually less active for 2H MoS2 relative to 1T MoS2 towards HER due to its inferior charge transfer kinetics and poor conductivities. Hence, charge transfer facilitation is acknowledged as a practical approach to improving basal planes of 2H MoS2 [170]. Furthermore, Au substrates have plenty of d electrons which can be caused to enhance charge transfer in 2H MoS2 by coupling between them. Hereafter, the electrons introduced from the Au substrate (towards 2H MoS2 basal plane) resulted in increasing charge transfer accompanied by adsorption of hydrogen reactants onto 2H MoS2 basal planes, thus, promoting the electrocatalytic development of 2H MoS2. Normally, this research provides innovative findings on the part of charge transport and contact resistance on catalytic advancement of 2D materials; the results of this section expose that catalytic development of 2D materials can be considerably improved by preparing 2D/2D heterostructures that provide pathways for the progress of 2D materials in electrocatalytic applications.This type of heterostructure offers several advantages, such as enhanced charge transfer, tunable bandgap, and high surface area. The enhanced charge transfer between the two materials can lead to faster reaction rates and improved performance for HER. The tunable bandgap allows for the optimization of the heterostructure's performance for HER. Moreover, the high surface area of 2D materials provides a large number of active sites for HER, further increasing their efficiency.However, some disadvantages to using 2D/2D heterostructures for HER exist. Firstly, synthesizing these heterostructures can be challenging and may require specialized equipment. Additionally, the stability of the heterostructures can be limited, and they may be prone to degradation over time. Furthermore, their performance can be sensitive to environmental conditions, such as temperature and humidity, affecting their efficiency. Finally, the toxicity of some materials fabricating 2D/2D heterostructures can pose environmental risks if not handled properly.In summary, 2D/2D heterostructures have several advantages that make them attractive for improving the efficiency of HER in electrochemical water splitting. However, their synthesis can be challenging, and their stability and performance can be sensitive to environmental conditions. Overall, further research is needed to address these challenges and fully realize the potential of 2D/2D heterostructures for HER.The preparation of 1D/2D heterostructures involves the fabrication of exceptional development of electrocatalysts that allow optimally sized pores for gas diffusion or mass transfer. Additionally, a large diversity of 1D/2D heterostructures have been designed to enhance their features and microstructures [171,172]. For instance, Li et al. [173] prepared (carbon nanotubes) CNT/graphene (CNT/G) heterostructures by oxidation of few-walled CNTs that was employed as catalyst in ORR. The exfoliation approach was exposed to external walls of CNTs to prepare nano-sized graphene. Graphene exhibiting massive amounts of defects assist in advancement of ORR catalytic sites after annealing in ammonia. Moreover, internal walls of CNTs remained together that aids as conductors for charge transfer. Thus, CNT/G catalyst yields exceptional production for ORR experiments having half-wave potential (∼ 0.76 V) and onset overpotential (∼ 0.89 V). Chen et al. [174] prepared metal-free NG (N-doped graphene) with N-doped CNT (NG-NCNT) heterostructured that comprises four-electron mechanisms for ORR experiments, which shows that entire component of the heterostructured catalyst delivers active sites to intensify electrochemical experiment. While the utilization of CNTs assists in isolating graphene layers that produce pores in its structure and result in increasing gas diffusion. Consequently, these characteristics reveal that synergistic effects can boost catalytic development of 1D/2D heterostructures. Furthermore, outstanding findings were obtained for as-prepared hydrogel relative to some transition-metal complex and noble metal oxides (i.e., IrO2) catalysts [175–177]. Hence, 1D/2D heterostructures that behave as self-supporting electrodes displays exceptional properties and superb development so that they can be directly employed in energy conversion devices.These heterostructures offer several advantages, such as high efficiency, enhanced stability, faster reaction rates, flexibility in design, and low cost. Due to their unique structure and properties, 1D/2D heterostructures can facilitate the transfer of charge carriers, promote faster reaction rates, and reduce corrosion and degradation. Furthermore, their flexible design enables the optimization of their performance for HER. In some cases, 1D/2D heterostructures can be synthesized using low-cost and abundant materials, making them more attractive for large-scale applications.However, there are also some disadvantages to using 1D/2D heterostructures for HER. Firstly, the fabrication and synthesis of these structures can be complex and challenging, which limits their widespread use. Additionally, while they can enhance the stability of some materials, 1D/2D heterostructures can be unstable and prone to degradation over time. Moreover, their performance can be sensitive to environmental conditions, such as temperature and humidity, affecting their performance. Scaling up the production of 1D/2D heterostructures for large-scale industrial applications can also be difficult. Finally, some materials fabricating 1D/2D heterostructures, such as heavy metals, can be toxic and pose environmental risks if not handled properly.In summary, 1D/2D heterostructures have several advantages that make them promising candidates for enhancing the efficiency of HER in electrochemical water splitting. However, their complexity, limited stability, sensitivity to environmental conditions, lack of scalability, and potential toxicity are essential considerations that must be addressed to realize their potential fully.For electrocatalysis, the dispersion of single-metal atoms can be made over 2D materials to prepare 0D/2D heterostructures. Fei et al. [178] prepared minor numbers of discrete cobalt atoms dispersed on NG (Co-NG) for HER electrocatalyst that display excellent electrocatalytic development for HER in conjunction with little overpotential (∼ 170 mV) at 10 mA cm−2. Similarly, Cheng et al. [179] synthesized 0D/2D heterostructures, which consist of distinct Pt atom clusters on N-doped graphene nanosheets (Pt/NGNs). The dispersion and size of Pt atom clusters were specifically measured by atomic layer deposition (ALD), as shown in Fig. 12a . To explore the adsorption abilities of Pt atoms, both theoretical and experimental techniques were implemented that show the absorption of Pt atoms over nitrogen sites in NG. Findings showed extraordinarily higher catalytic progress accompanied by excellent HER stability obtained for Pt/NGNs catalysts relative to Co-NG electrocatalysts and considerably commercial Pt/C. TMDCs can also be hosted as supports to obtain excellent catalytic potential of 0D/2D heterostructures. For instance, Cheng et al. [180] made a study on the preparation of Rh/MoS2 heterostructures via MoS2 nanosheets (as rapid H2-desorbing element) morphology as depicted in Fig. 12b-F, and Rh nanoparticles (as strong H-adsorbing element). Resultant heterostructures displayed a slight Tafel slope (24 mV dec−1) and low overpotential (47 mV) at 10 mA cm−2, along with good stability (Fig. 12g-j). The extraordinary development of catalysts was attributed to the presence of Rh atoms that depicts prompt capturing of hydronium ion since these atoms behave as strong H-adsorbing element. Moreover, the strength of MoS2/rGO catalyst was monitored for 1000 cycles which resulted in minor cathodic current losses. Large current density is another noteworthy factor in the performance of electrocatalysts which is normally unnoticed. In this regard, a research group fabricated an electrocatalyst based on 0D/2D heterostructure composed of MoS2 nanosheets and Mo2C nanoparticles (MoS2/Mo2C) via carbonization of MoS2 (in situ) as depicted in Fig. 12k-m [181]. The resultant heterostructure exhibit rough surfaces and exceptionally exposed active sites at micro- and nanoscale. Further research is mandatory regarding industrial/practical applications. Normally, 0D/2D heterostructures are advantageous for electrocatalysis; but the poor long-term strength impedes practical/industrial application and further signs of progress are essential (Fig. 12n-o).Due to their unique structure and properties, 0D/2D heterostructures can improve the stability of materials during HER, promote faster reaction rates, and reduce corrosion and degradation. These heterostructures offer several advantages, such as high efficiency, enhanced stability, facilitated charge transfer, versatility in design, and low cost. Moreover, their flexible design enables optimization of their performance for HER. In some cases, 0D/2D heterostructures can be synthesized using low-cost and abundant materials, making them more attractive for large-scale applications.However, there are also some disadvantages to using 0D/2D heterostructures for HER. Firstly, the fabrication and synthesis of these structures can be complex and challenging, which limits their widespread use. Additionally, while they can enhance the stability of some materials, 0D/2D heterostructures themselves can be unstable and prone to degradation over time. Moreover, their performance can be sensitive to environmental conditions, such as temperature and humidity, affecting their performance. Scaling up the production of 0D/2D heterostructures for large-scale industrial applications can also be difficult. Finally, some materials fabricating 0D/2D heterostructures, such as heavy metals, can be toxic and pose environmental risks if not handled properly.In conclusion, 0D/2D heterostructures have several advantages that make them promising candidates for enhancing HER efficiency in electrochemical water splitting. However, their complexity, limited stability, sensitivity to environmental conditions, lack of scalability, and potential toxicity are important considerations that must be addressed to realize their potential. Tables 1 and 2 contain various 2D Heterostructures for HER performances.This brief overview discusses a variety of 2D materials and their heterostructures, including improved photo and electrocatalysis for H2 evolution reactions and the design of catalysts. Different heterostructure types, such as 2D/2D, 1D/2D, and 0D/2D, are explained regarding the water-splitting process. The author's suggestions in conformity with future problems and prospects will clear the path for readers by thoroughly addressing the fundamental features, recent advancements, and related challenges. Some perspectives are given below:While photocatalysis and electrocatalysis have shown great potential in various applications, there is still a need for further optimization and improvement of the catalysts. This includes improving the catalysts' efficiency, selectivity, stability, and cost-effectiveness.There is a need for more fundamental research to understand the mechanisms behind photocatalysis and electrocatalysis better. This includes studying the surface chemistry, reaction kinetics, and charge transfer processes involved in these reactions.The development of photocatalysis and electrocatalysis has led to new opportunities in energy conversion, environmental remediation, and chemical synthesis. However, there is still a need for more interdisciplinary collaborations between researchers from different fields to realize the potential of these catalysts fully.There is a need to explore new materials and synthesis methods for photocatalysts and electrocatalysts. This includes developing new types of materials such as metal-organic frameworks, perovskites, and 2D materials and exploring novel synthesis methods such as atomic layer deposition and plasma-enhanced CVD.The wrinkle/buckle broadly occurs in self-supporting 2D materials. Yet, relevant research on the interface's uniformity in 2D architecture has become unique and is suggested to be critical in interface architecture to modulate the catalytic efficiency. Moreover, by increment in complexity for catalyst components, thereby necessary reaction process for 2D heterostructure in the electrocatalytic mechanism requires detailed studies, particularly for the OER. Depending on in situ or operando characterization techniques like Raman, FTIR, TEM, and XANES spectroscopy are required to control the momentary engineering of reconstruction and trace out the exact active sites for the desired reactions. In this way, comprehensive reaction kinetics and thermodynamic mechanisms offer valuable support to design efficiently 2D heterostructures towards electrocatalytic reactions rationally. Further, despite DFT provision for the reaction mechanism of specific active sites for single 2D nanomaterials founded over abridged models, still is challenging to curate a decisive route towards the 2D heterostructure containing multifunctional active sites.Secondly, solar-based water-splitting strategy for effective implementation, an STH (10%) of broad commercialization standard, becomes desirable. The greatest STH transformation performance is evaluated as 30%, achieved by the InGaP/GaAs/GaInNAsSb triple-connection solar cell associated with two series-junction polymeric electrolyte-membrane-electrolyzers. Recently, numerous standard PV-EC (integrated photovoltaic electrolyzer) device systems commonly face great Ohmic resistance, massive connections, and less integrated designs, reducing competence. Additionally, depending on standard formula, the magnitude of STH becomes parallel to close relationship between chemical-energy output and solar-energy input. In addition, reducing solution resistance, accelerating the movement and stability of catalysts, and optimizing mass transfer for the electrolysis device become achievable core strategies for gaining substantial STH performance when the performance of solar cells accelerates towards theoretical value.Thirdly, 2D heterostructures show unlimited potential for electrocatalytic water-splitting systems, while application-based research is still beginning. It is believed that the knowledge of the catalytic nature of nanomaterials is associated with the aid of in situ characterizations and DFT outcomes and producing rationally prepared technologies to improve the massive efficiency and stability. Consequently, large-scale execution for electrocatalytic implementations may be realized.2D heterostructures have revealed great perspectives in water-splitting but are resurgent in application-oriented research. We assume that by considering the catalytic nature of these materials with the aid of in situ characterizations and DFT calculations and designing more rational synthesis technologies for enhancing the overall efficiency and stability, the large-scale implementation of electrocatalytic applications will be acquired.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors thank all individuals and organizations who permitted them to republish their figures and other relevant information.

Two-dimensional (2D) structures show atomic geometry of ultrahigh thickness and have the highest surface-to-volume ratio. A monolayer of 2D materials commonly comprises atomically thick covalent lattice bonds, which contain free nanosheets with dangling bonds that might exhibit odd electrical and optical properties. The gathering for 2D derivatives of vdW architectures may be viewed as being outside the bounds of lattice matching due to the lack of directly chemical bond proficiency. Heterostructures are frequently composed of multiple parts associated with significant interfaces and are extensively studied to prevent problems caused by hybrids with unique functions, such as tunneling and confinement effects. To optimize the adsorption/desorption energy for important reactions and to advance the kinetics of chemical reactions, heterostructures formation is a strong strategy to accelerate Hydrogen evolution reaction (HER) activity is proposed. This mini-review deals with various 2D material and their heterostructures, from catalysts design to enhanced photo and electrocatalysis for H2 evolution reactions. Various forms, including 2D/2D, 1D/2D, and 0D/2D of heterostructures, are explained in water splitting reaction. By thoroughly addressing the fundamental aspects, recent developments, and associated challenges ⁠— the author's recommendation in compliance with future contests and prospects will pave the way for readers.

Lithium–oxygen (Li–O2) batteries have received wide attention because they have much higher energy density (3500 Wh kg–1) than current Li-ion batteries [1-7]. Based on their electrolytes, Li–O2 batteries can be categorized into four types: aqueous, aprotic, hybrid, and all-solid state. Among these, aprotic Li–O2 batteries are regarded as promising candidates for energy storage systems owing to their simple construction and excellent reversibility. A typical Li–O2 battery includes a lithium foil anode, a separator, and a cathode electrocatalyst (Fig. 1 ), with the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) occurring during the discharge and charge processes, respectively [8, 9].Though Li–O2 batteries are still in their infancy, research on them has already attracted wide attention around the world (Fig. 2 ). The first Li–O2 battery was demonstrated by Abraham and Jiang, laying a solid foundation for investigating the first Li–air batteries [10]. However, the development of Li–air batteries was at a standstill for the next ten years because the electrochemistry process remained unclear. In 2006, mass spectrometry confirmed the evolution of O2 during the oxidation process of Li2O2 [11]. After that, primary research to understand the electrochemistry processes during Li2O2 formation and decomposition attracted worldwide attention to the reversibility of Li–O2 batteries.During the discharge process (ORR), O2 accept an electron to generate the intermediate (LiO2). This is followed by electrochemical reduction to form Li2O2 films (the surface growth model) or, in a disproportionation reaction, to produce Li2O2 discs (the solution growth model). To be specific, under a solvent with a high donor number, LiO2 can dissolve in the solvent and undergo a disproportionation reaction to form toroidal Li2O2 in the solution (Fig. 3 ). In a solvent with a low donor number, LiO2 can be stabilized on the catalyst surface. A continuous surface disproportionation reaction or a second reduction reaction then occurs to form Li2O2 films (Fig. 3). Usually, Li2O2 formed by the solution growth model better promotes the discharge capacity and rate performance than that which is formed by the surface growth model. (1) O2 + Li+ + e- = Li (2) LiO2* + e- + Li+ = Li2O2 (3) LiO2* + LiO2* = Li2O2 + O2 (4) O2 + Li + e- = LiO2(sol) (5) 2LiO2(sol) = Li2O2 + O2 For the charge process (OER), Li2O2 is electrochemically oxidized to release O2 and Li+. Two different charge mechanisms — one-step oxidation and two-step oxidation — have been proposed to explain the OER process. During one-step oxidation, Li2O2 can be decomposed by two electrons [11]. During two-step oxidation, a one-electron oxidation process can occur at a low charge voltage, followed by the one-step oxidation process at a high charge potential [21]. However, a conclusion has not been reached on whether LiO2 exists during the OER process, since an intermediate (LiO2) cannot be identified by in situ surface enhanced Raman spectroscopy. Further study suggests that the formation of LiO2 depends on the donor number of the solvent during the charge process (Fig. 4 ) [22]. Soluble LiO2 can be formed in a high donor-number solvent, where crystalline Li2O2 is generated through a disproportionation reaction. In contrast, a solvent with a low donor number is beneficial for forming Li2-xO2, which is further oxidized to release O2.Despite their high energy density, the development of Li–O2 batteries is currently prevented by several disadvantages, such as serious charge polarization and solvent degradation [23-26]. The formation of insulating Li2O2 can inhibit the transportation of O2 and passivate the cathode electrocatalyst, leading to high resistance, low rate performance, and restricted cyclic stability. Moreover, the decomposition of solvents and carbon-based cathodes results in the formation of by-products, which can cause cell polarization and unsatisfactory performance [27-30]. Furthermore, the high activities of intermediates (e.g., LiO2) can lead to parasitic reactions with the cathode and electrolyte, triggering serious corrosion.To decrease the ORR and OER polarizations and enhance the catalytic performance, research has focused on constructing efficient cathode electrocatalysts to improve the discharge and charge kinetics. According to previous reports, the rational construction of electrocatalysts such as carbon materials [31], noble metals [32, 33], and transition metal oxides [34-36] are confirmed to be efficient strategies for promoting catalytic activity. Within the scope of this review, we examine recent progress on cathode electrocatalysts for fast ORR and OER kinetics. Therefore, we first review recent progress in oxygen catalysts for Li–O2 batteries using advanced carbon materials, transition metal oxides, noble metal-based catalysts, and conductive metal–organic frameworks (MOFs). Then we discuss the discharge and charge mechanisms of new photo-assisted and single-atom catalysts for improving catalytic performance. These two aspects have the ability to facilitate the high-speed development of Li–O2 batteries. Finally, we identify future challenges in developing cathode electrodes for Li–O2 batteries.Carbon materials are potential candidates due to their abundant porosity as well as excellent electric conductivity. Numerous carbon-based catalysts (carbon nanotubes [37-40], carbon nanocubes [41], carbon nanofibers [42, 43], carbon spheres [44, 45], etc.) have been widely developed as electrocatalysts, promoting the discharge process to enhance the discharge capacity via their high surface areas. Unfortunately, the pore structure of carbon materials becomes blocked by the insulating discharge products of Li2O2, causing solvent decomposition and inferior cycling stability. To alleviate this issue, well-designed architectures [46-49] have been constructed to obtain high-energy Li–O2 batteries. Recently, porous carbon nanospheres were developed as efficient electrocatalysts, demonstrating a high capacity of 20300 mAh g–1 [31]. Discharge products were observed on the surface of the nanospheres due to strong adsorption between LiO2 and functional groups. At the same time, the open pores of the nanospheres facilitated the transport of Li+ ions and O2 and did not become blocked with extensive Li2O2 deposition (Fig. 5 a). Since a low amount of surface functional groups can obviously inhibit the carbon oxidation side reaction, the carbon nanospheres’ abundant porosity and the absence of oxygenic groups led to good cycling stability for 330 cycles (Figs. 5b and c).Though a porous carbon-based catalyst can promote performance, the sluggish OER kinetics of Li2O2 with porous carbon still hinder Li–O2 battery development [50]. Density functional theory (DFT) calculations show that the catalytic activities of porous carbon materials can be enhanced by heteroatom doping, due to strong adsorption between the lone-pair electrons of heteroatoms and carbon materials [51]. In particular, the introduction of N atoms with high electronegativity can capture electrons from C atoms, indicating the high oxidation charge state of the adjacent C atoms compared to pure carbon materials [50]. O2 tends to adsorb on the positively charged carbon atoms, which promotes breakage of the O–O bond and the reduction of O2 to Li2O2 for enhanced catalytic performance. In addition, pyridinic N doping can effectively promote the deposition of Li2O2 better than graphitic or pyrrolic N due to fast electron transfer between Li2O2 and pyridinic N, which differs from active oxygen sites such as C–O and C=O in carbon materials [52, 53]. In a typical demonstration of N-doped carbon nanotube fabrication on stainless-steel mesh (N-CNTs@SS) for use as an electrode [54], the electrode deliver an OER overpotential below 1.0 V and outstanding cycling performance for 232 cycles due to the high electronic conductivity and superior mechanical strength conferred by N doping. Under bending and stretching stress, the light-emitting diode (LED) display screen of N-CNTs@SS-based Li-O2 batteries remained unchanged (Fig. 6 a), with negligible variation in the discharge–charge curves and open-circuit voltage (Figs. 6b–d). Furthermore, the N-CNTs@SS yielded little LiOH compared to pure hydrophilic catalysts, suggesting its excellent resistance to H2O (Figs. 6e and f).Compared to N-doped carbon catalysts, whereby O2 tends to interact with C atoms near the N atoms, B atoms near C atoms are positively charged due to their low electronegativity, which can facilitate the adsorption of O2 on B sites [52]. Furthermore, theoretical calculations reveal that the B–O group as an active site via B implantation can activate the π electrons of carbon, enhancing the charge transfer and decreasing the OER energy barrier [55]. A novel B-doped reduced graphene oxide (B-rGO) has been proven to substantially improve the catalytic performance of Li–O2 batteries, delivering a high discharge capacity of 18000 mAh g–1 and a low charge overpotential of 0.85 V at 0.1 A g–1 [56]. DFT calculations confirmed that the doping of B atoms into the rGO activated its electrons, resulting in good rate performance. Additionally, the B-rGO displayed a strong affinity towards Li5O6 clusters, enhancing the decomposition of Li2O2. This explained the good performance of B-rGO cathodes.As mentioned above, though carbon material-based Li–O2 batteries can exhibit high discharge capacity, carbon materials are easily corroded by oxygen intermediates and singlet oxygen, which can promote the degradation of carbon catalysts. In addition, carbon materials can also be oxidized when the charge voltage is higher than 3.5 V [57]. As a result, parasitic products can passivate the surface of carbon catalysts, leading to high interface resistance as well as low charge polarization, causing limited cycle life.Noble metals are critical for Li–O2 batteries due to their tunable d orbital states, which allow manipulation of the interactions between intermediates and noble metals, thereby affecting OER catalytic activity [18, 33, 49, 58-66]. Usually, pure noble metals show high charge overpotentials, since the high d band center leads to strong adsorption interactions with LiO2 [67]. A strategy for enhancing the OER kinetics of noble metals is to form high-index planes. Anisotropic Pt with abundant high-index planes was developed as an efficient electrocatalyst, delivering outstanding electrochemical performance relative to commercial Pt catalyst [68]. The high-index [411] facets and atomic steps of anisotropic Pt were confirmed by high-resolution transmission electron microscopy (HR-TEM) images and fast Fourier transformed (FFT) patterns (Figs. 7 a–d). In addition, theoretical calculations suggested that high-index Pt showed higher binding energy towards intermediates than commercial Pt, thereby promoting the OER process with a low energy barrier. Anisotropic Pt electrocatalyst thus displayed a larger discharge capacity of 12,985 mAh g–1 and a lower overall overpotential of 0.51 V than pure Pt (capacity: 6272 mAh g–1; overall potential: 1.36 V, Fig. 7e).Another tactic to enhance OER performance is alloying with different noble metals. Pt-based alloys have shown great potential for fast OER kinetics due to their tunable electron states. PtIr alloy was confirmed as an efficient OER electrocatalyst. The low Lewis acidity of Pt atoms on the PtIr surface indicated a downshifting of the d-band center, weakening the adsorption interaction with LiO2 and leading to low overpotentials for the ORR (0.11 V) and OER (0.33 V). Later, a PtAu alloy was applied as an electrocatalyst to improve OER electrochemical performance [69]. Due to its high electronegativity, the Au atom tends to receive electrons from Pt atoms, leading to low eg electrons of Pt in PtAu (Figs. 8 a and b). The lower eg electrons in PtAu can result in an upward shift of the Pt d-band center, leading to a stronger affinity for LiO2 than a PtRu alloy (Fig. 8c). Benefiting from the strong adsorption strength between LiO2 and PtAu electrode, Li2O2 nanosheets can be formed by the surface growth model. As a result, a good Li2O2/catalyst interface can possess rapid charge kinetics and a low OER energy barrier (0.84 eV in Fig. 8d). In contrast, due to the weak adsorption energy towards LiO2, Li2O2 discs can be generated by the solution growth model on the PtRu electrode (Fig. 8f). The inferior Li2O2/PtAu interface results in a high OER energy barrier (1.01 eV in Fig. 8e) and high charge overpotentials, delivering poor cycling stability. Therefore, the electron occupancy of Pt can tune the d-band position and thus change the adsorption strength towards LiO2 as well as the OER overpotential (Fig. 8g).Though utilizing noble metal-based catalysts can obviously facilitate the OER kinetics, the commercial application of noble metals is limited by their high cost and possible side reactions arising from solvent degradation [70-72]. Therefore, designing low-cost, high-performance electrocatalysts with high OER catalytic activities is crucial for Li–O2 batteries.Currently, various non-noble metal electrocatalysts containing metal nitrides and transition metal oxides (Co3O4 [3, 73-75], MnO2 [76-78], NiCo2O4 [79, 80], perovskites [81-86], etc.) are used as advanced electrocatalysts in Li–O2 batteries. Compared to transition metal-based catalyst such as Fe2O3 and MnO2, Co-based electrocatalysts exhibit higher OER and ORR kinetics, which entail high discharge capacity and cycling stability [87, 88]. DFT calculations further confirm that the charge overpotentials of transition metal oxides show volcano relationships with surface acidity [89]. Among all the transition metal oxides, Co-based catalysts with medium surface acidity at the vertex position of the volcano plot can decrease the activation energy of the potential-determining step, thereby achieving a high OER activity [89]. In addition, due to mild fabrication procedures, the catalytic activities of Co-based electrocatalysts are easily manipulated by various strategies, such as morphological [90, 91], facet [92, 93], and doping engineering [94]. Open-structured Co9S8 has been confirmed as an excellent catalyst, simultaneously achieving an ideal storage matrix for discharge products as well as superior discharge and charge performance [95]. During the discharge process, hydrangea-like Li2O2 were observed on the surface of a CoO-PCF electrode, leading to a high charge overpotential and poor cycle life caused by insufficient Li2O2/electrode interfaces (Figs. 9 a–c). In comparison, hydrangea-shaped Li2O2 was homogeneously deposited on the edge of Co9S8 nanorods, suggesting enough interface contacts were present between Li2O2 and the Co9S8 electrode (Figs. 9d–f). DFT calculations suggested that the (440), (311), and (111) facets of Co9S8 nanorods showed stronger oxygen adsorption than the (111), (220), and (200) facets of CoO-PCF cathodes (Figs. 9g–l), which was beneficial for heterogeneous nucleation that formed Li2O2 by the surface growth model, thereby enhancing the Co9S8 cathode’s cycling stability.Though Co-based catalysts have been confirmed to accelerate the ORR and OER kinetics, vacancy engineering on Co-based electrocatalysts can further improve their catalytic performance. Theoretical calculations show that the vacancy defects in Co-based electrocatalysts optimize the adsorption interaction with LiO2 and delocalize the surrounding electrons, which is critical for improving ORR/OER performance. Recently, cobalt oxides with Co vacancies (Co3−xO4) have been demonstrated as cathode electrocatalysts; tuning the electronic state of Co3O4 and thus manipulating the potential-determining step in the charge process enhanced the OER catalytic performance [96]. The differential charge density distributions confirmed charge redistribution on the (111) and (220) slabs of Co24O32 and Co23O32 (Figs. 10 a–d). Additionally, for the (111) slabs of Co24O32 and Co23O32, the obvious overlap of electron density promoted rapid charge transportation, facilitating the ORR and OER processes (Figs. 10a and b). The Gibbs free energy curves of Co23O32 further confirmed the low ORR (0.31 V) and OER (0.45 V) overpotential for the (220) slab and (111) slab, respectively (Figs. 10e and f). Co3−xO4 thus displayed a high discharge capacity of 13,331 mAh g–1 and a high overall discharge/charge overpotential (1.38 V). This research provides a way to increase the energy efficiency by engineering vacancies in transitional metal oxides.As discussed above, though the utilization of Co-based catalysts has resulted in remarkable performance improvement, there are gaps in our understanding of their OER and ORR mechanisms, impeding the development and design of highly efficient catalyst. In addition, the structure–activity relationship between the electrocatalysts’ reaction processes and function should be further established by analyzing and detecting intermediates.Besides transition metal oxides, flexible metal sites in conductive metal–organic frameworks can modulate the adsorption strength towards oxygen intermediates, making conductive MOFs efficient electrocatalysts. Copper tetrahydroxyquinone (Cu-MOF) was employed as a cathode to remarkably enhance the OER and ORR kinetics of Li–O2 batteries [97]. TEM images and electron energy loss spectroscopy (EELS) indicated that the discharge product was nanocrystalline Li2O2 with amorphous regions (Figs. 11 a and b). According to DFT calculations, heteromorphic Li2O2 had higher electronic conductivities and was thermodynamically favorable for the formation of Li2O2, which promoted the formation of amorphous Li2O2. Therefore, Cu-MOF displayed a low charge overpotential (below 3.7 V) and an excellent cycle life of up to 300 cycles at 1000 mAh g–1. Elsewhere, a high-valence conductive nickel catecholate framework (NiIII-NCF) was constructed as a cathode to further improve the redox kinetics via spin manipulation (Figs. 11c and d) [98]. Compared to a low-valence nickel catecholate framework (NiII-NCF), NiIII-NCF showed a smaller energy difference between the d-band center of Ni and the p-band center of O, which greatly promoted electron exchange between Ni and oxygen intermediates. Therefore, during the discharge process, NiIII-NCF displayed strong adsorption to LiO2, leading to the formation of Li2O2 nanosheets (Figs. 11e–i) with high discharge voltage. NiIII-NCF electrocatalysts with abundant Li2O2/NiIII-NCF interfaces delivered low ORR/OER polarization (0.81 V at 200 mA g–1) and high energy efficiency — specifically, a high discharge capacity of 16,800 mAh g–1 and good reversibility for 200 cycles at 500 mAh g–1.Compared to conventional solid catalysts, single-atom catalysts have unique electronic states, unsaturated coordination environments [99-101], and controllable single atom–support interactions, enabling tuning of the discharge–charge mechanism [102-104]. Additionally, the atomic dispersion of metals indicates high atom utilization and accelerated redox kinetics [105-107]. Single-atom catalysts incorporated into Li–O2 batteries presently include Se single atoms supported on Ti3C2 [108], Co single atoms supported on nitrogen-doped carbon [109, 110], Ru single atoms on nitrogen-doped carbon [111], and Pt single atoms on g-C3N4 nanosheets [112]. Novel Co single-atom catalysts on hollow N-doped carbon spheres (N-HP-Co SACs) were found to obviously enhance the redox kinetics for Li–O2 batteries [109]. In the OER process, UV–vis spectra showed an absorption peak centered at 260 nm, suggesting that charging was dominated by a single-electron oxidation process (Figs. 12 a and b). DFT calculations further confirm that the N-HP-Co SACs demonstrated weaker adsorption towards LiO2 than a Pt/C electrode did, indicating LiO2 tended to dissolve in the solvent rather than in the cathode (Figs. 12c and h). The one-electron process of Li2O2 oxidation delivered rapid kinetics and excellent catalytic performance. Therefore, the N-HP-Co SACs yielded a low OER overpotential (0.29 V).Aside from Co SACs, Se SACs also have shown excellent performance, accelerating the redox kinetics of Li2O2 formation and decomposition. Recently, Se single atoms supported on Ti3C2 MXene (SASe-Ti3C2) have been reported as an electrocatalyst [108]. According to DFT calculations, the SASe-Ti3C2 delivered a stronger LiO2 affinity (0.98 eV) than Ti3C2 (0.43 eV). The charge difference density further confirmed more abundant electron transfer by SASe-Ti3C2 than by Ti3C2; the Se–C bond can be regarded as a highway for transferring electrons (Figure 13 a–f). The strong adsorption strength between LiO2 and SASe-Ti3C2 can lead to the formation of Li2O2 nanoarrays by the surface growth model, thereby reducing the OER overpotential. In comparison, the weak interaction between LiO2 and Ti3C2 caused the formation of Li2O2 nanodisks, triggering high charge overpotentials. The Gibbs free energy further proved that the SASe-Ti3C2 exhibited a lower OER overpotential (0.88 V, Fig. 13h) than the Ti3C2 (1.6 V, Fig. 13g). The SASe–Ti3C2 electrode thus displayed a high discharge capacity of 17260 mAh g−1 and excellent cycling stability (170 cycles) with a low overall discharge–charge overpotential (1.10 V).Though single-atom catalysts show great potential, they have major stability issues that need to be addressed. Since single atoms have higher surface energies than the corresponding metal particles, large particles can form due to accumulation. Therefore, the loading mass of single atoms should be enough low to prevent agglomeration. In this regard, efficient preparation strategies for various single atoms are necessary for the development of single-atom catalysis.Though a solid catalyst can decrease the charge overpotential, it is still high in terms of applications. In response, photocatalysis has been introduced to further decrease the charge overpotential. Under light illumination, the electron can be excited from the valence band (VB) to the conductive band (CB), enhancing ORR and OER performance. A suitable photocatalyst should yield a redox potential of O2/Li2O2 between the CB and VB. During the ORR process under light illumination, the electron will transfer from the VB to the CB, resulting in the reduction of O2 to Li2O2 (Fig. 14 a). Furthermore, the hole in the VB can be reduced by the electron from the external circuit. The discharge potential is the voltage between the VB and the potential of Li+/Li. In comparison, the oxidation of Li2O2 is conducted via the holes in the VB to form O2 and Li+, and the electron will transfer from the VB to the external circuit (Fig. 14a). The charge overpotential is the voltage between the CB and the potential of Li+/Li.While photocatalysts can enhance energy efficiency, only the ultraviolet region, which occupies 4% of the solar radiation spectrum, can be absorbed by the semiconductors currently used [113]. In addition, high electron-hole recombination rates can cause incompatibility between the carrier lifetime and charge–discharge rate [4]. Therefore, manipulating the band gap of semiconductor catalysts to allow absorption of a large cross-section of visible light and achieve long carrier lifetime is the principle behind photo-assisted Li–O2 batteries; this involves defect engineering, nanostructure design, and heterojunction interface design. To date, a variety of photocatalysts have been developed, including TiO2 [114], siloxene nanosheets [115], Au/C3N4 [116], ZnS [117], polyterthiophene [118], and Fe2O3 [119]. In particular, the siloxene nanosheets were developed as an efficient photocatalyst with extremely high energy efficiency [115]. Under light illumination, siloxene nanosheets with abundant sites aided in the formation and oxidation of Li2O2. As a consequence, a siloxene nanosheets oxygen electrocatalyst delivered a high discharge voltage of 3.51 V and a high energy efficiency of 185% (Figs. 14b and d). Furthermore, the siloxene nanosheets showed excellent rate performance (129% energy efficiency at 1 mA cm–2, Fig. 14c), offering a possible strategy to design efficient catalysts for photoconversion and storage systems.Though photocatalysts can decrease the OER overpotential, the high recombination of electrons and holes can compete with the OER and ORR processes, leading to decreased energy efficiency. To suppress the recombination of electrons/holes and increase absorption in the visible light range, Au-supported C3N4 with N vacancies (Au/Nv-C3N4) was used as a photocatalyst [116]. O2 tended to be adsorbed on the Nv of Nv-C3N4 rather than on C3N4, which benefited O2 activation (Figs. 15 a and b). The projected density of state (PDOS) further confirmed strong orbital hybridization between O2 and Nv (Fig. 15c). When Au was loaded on the Nv-C3N4, obvious hybridization between the Au cluster and Nv occurred (Fig. 15d), promoting the ORR catalytic performance. Therefore, during the discharge process, plasmonic Au NPs adsorbed visible light to form hot electrons and holes. The electrons and holes were separated by the interface between Au and Nv-C3N4, which prolonged the lifetime. The electron migrated from Au to C3N4, then to the NV-induced defect band to achieve the reduction of O2 to Li2O2. In a typical charge process, the hole in Au NPs oxidized the Li2O2 to release O2 and lithium ions (Fig. 15e). The achieved discharge and charge voltages were 3.16 and 3.26 V, respectively, indicating a high energy efficiency of 97%. In addition, the batteries cycled stably for 50 cycles. In the future, tuning the loading mass of metal particles should further enhance the utilization efficiency of solar light and improve the redox kinetics.With the increasing requirement for high energy density during the last decade, Li–O2 batteries have received wide attention. Yet despite their ultrahigh energy density, Li–O2 batteries also face challenges before they can be used in practical applications, including sluggish OER and ORR kinetics, and solvent degradation. In this review, we have discussed recent progress in developing highly efficient cathode electrocatalysts. We elucidated the research process and explored the main challenges with respect to carbon materials, noble metals, non-noble metals, SACs, and semiconductor photocatalysts. Systematic strategies for developing cathode structures to solve the corresponding issues were explored in depth, and we highlighted the intrinsic relationship between catalytic performance and catalyst electronic structure. Though great progress has been reported in the design and fabrication of advanced catalysts, Li–O2 battery development remains in the early stages, and numerous challenges must be overcome before commercialization. In what follows, we summarize several obstacles that should be tackled in future research.Given the significant effect of cathode catalysts on the performance, optimizing catalysts’ electronic structure at the molecular level should be further investigated to tune the adsorption energy towards LiO2, thereby manipulating the overpotential and cycling stability. Li2O2 films can be formed by the surface growth model due to the strong adsorption strength towards LiO2 in solvents with a low donor number, delivering a low discharge capacity and inferior rate stability. In contrast, weak adsorption strength towards LiO2 is obtained by using an electrolyte with a high donor number, entailing the formation of Li2O2 discs with high discharge capacity. Adsorption between LiO2 and cathode electrocatalysts can alter the reaction energy barrier and tune the ORR/OER kinetics of the rate-determining step (RDS), thereby affecting the overpotential, energy efficiency, and long-term cycling stability. In addition to the influence on the discharge growth model and OER/ORR reaction kinetics, accumulation of side products and high charge transfer resistance can also be triggered due to the degradation of cathode catalysts when oxygen-containing intermediates are produced. The superoxide radicals formed during the cycling processes can attack the defect sites of cathode electrocatalysts, while singlet oxygen formed in the charge process can further cause severe degradation of cathode catalysts. The parasitic products Li2CO3 and LiOH originating from singlet oxygen can passivate the catalyst electrode and result in high charge voltage and low energy efficiency. Therefore, suitable catalysts should be developed to convert singlet oxygen to 3O2, which inhibits the side reactions and remarkably enhances the cycling stability.Further, although great improvements have been achieved by using advanced cathode electrocatalysts, the lack of direct evidence proving the OER and ORR mechanisms restricts the development of highly active catalysts. To reveal the catalytic mechanisms, in situ characterization techniques such in situ Fourier transform infrared (FTIR) spectroscopy and differential electrochemical mass spectrometry (DEMS), X-ray absorption fine-structure (XAFS), Raman spectroscopy, electrochemical quartz crystal microbalance (EQCM), ultraviolet-visible (UV-vis) absorption spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy should be established to monitor intermediates and confirm the reversibility under working conditions.Apart from issues with cathode electrocatalysts, the stability of the Li metal anode has been a serious issue for Li–O2 batteries. Due to the complicated solvent components of Li–O2 batteries, the Li anode can react with O2 and solvents, leading to Li anode corrosion and the formation of lithium dendrites, and thereby causing inferior anode stability and battery short-circuiting. Therefore, having a good understanding of Li stripping and plating mechanisms as well as protecting the Li anode via strategies such as forming Li-based alloys, constructing SEI layers, and introducing a lithium host should be developed for Li metal batteries. The solvent is also an important component determining the stability and cycle life, since it can be attacked by oxygen-containing intermediates. We propose exploiting advanced liquid electrolytes to achieve high ionic conductivities and inhibit attacks by oxygen intermediates. Solid-state electrolytes may be potential candidates because of their high stability towards intermediates at a high charge voltage. Unfortunately, solid-state electrolytes usually exhibit poor cycle life and high polarization compared to liquid electrolytes due to the former’s low ionic conductivity and interface issues between the electrolytes and electrodes. By fully utilizing the advantages of liquid and solid-state electrolytes, mixtures of liquid and solid-state electrolytes could be explored to guarantee high ionic conductivity as well as high interface and solvent stability for realizing high-performance Li–O2 batteries.The authors declare no competing interests.This study was financially supported by National Science Fund for Distinguished Young Scholars (No. 52025133), Tencent Foundation through the XPLORER PRIZE, and the Fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP202004), China Postdoctoral Science Foundation (No. 2021M700211).

Lithium–oxygen (Li–O2) batteries have great potential for applications in electric devices and vehicles due to their high theoretical energy density of 3500 Wh kg–1. Unfortunately, their practical use is seriously limited by the sluggish decomposition of insulating Li2O2, leading to high OER overpotentials and the decomposition of cathodes and electrolytes. Cathode electrocatalysts with high oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities are critical to alleviate high charge overpotentials and promote cycling stability in Li–O2 batteries. However, constructing catalysts for high OER performance and energy efficiency is always challenging. In this mini-review, we first outline the employment of advanced electrocatalysts such as carbon materials, noble and non-noble metals, and metal–organic frameworks to improve battery performance. We then detail the ORR and OER mechanisms of photo-assisted electrocatalysts and single-atom catalysts for superior Li–O2 battery performance. Finally, we offer perspectives on future development directions for cathode electrocatalysts that will boost the OER kinetics.

Data will be made available on request.In recent years, with the tremendous dependency on oil as a valuable feedstock, there has been an urge and interest for researchers in biomass utilization for the production of fuels and fine chemicals [1]. This urge has led to increase in the research for an appropriate substitute to petroleum with lignocellulose being the most abundant, carbon-sustainable and bio-renewable biomass. In variety of these lignocellulose one of the examples being furfural which can undergo a great variety of reactions to yield a wide range of chemicals [1,2]. Taking these points into consideration, industrially these reactions can be carried out either in liquid phase or vapour phase. Liquid phase reactions are carried out in high pressure reactors wherein high conversion, selectivity and yield can be achieved [2,3]. However, these liquid phase reactions give best activity, the use of solvent and high pressure may limits its industrial application and alternatively vapour phase reactions play a major role for some of these reactions [3]. The industrial applications of the vapour phase can be employed as the reaction can be carried out in vapour phase reactor in fixed bed down flow reactor at atmospheric pressure via heterogeneous catalyst [4]. However, among various catalytic vapour phase reactions, hydrogenation of furfural and dehydrogenation of cyclohexanol are two industrially important reactions which mainly produce furfural alcohol and cyclohexanone respectively [1–4].The catalytic vapour phase hydrogenation of furfural can yield furfuryl alcohol, 2-methyl furan, tetrahydrofurfural, tetrahydrofurfuryl alcohol, 2-pentanol and few more by-products among which furfural alcohol is widely used in manufacturing various synthetic fibers, rubbers and resins [4]. It is also an important chemical intermediate for the production of lysine, plasticizers and lubricant [5]. The industrial application of hydrogenation of furfural, copper chromite catalyst have been employed widely as commercial catalysts [6]. However, the toxic nature of chromium based catalysts greatly hinders its use in industries. Therefore, in order to develop Cr free catalytic system focus has been shifted to metal-based monometallic and bimetallic catalysts such as Au, Pd, Ru, Cu, Ni, Pd–Cu, Cu–Co, Pt–Sn catalysts [5,7] Additionally, amorphous alloys such as Ni–P, Ni–B, Ni–P–B, Co–B, Cu–Al layered double hydroxide (LDH) [4,8] and Cu on different supports Al2O3, SiO2 and SBA-15 as catalyst for hydrogenation of furfural have been studied. For instance, hydrogenation of the furan ring to THFA, hydrogenolysis of the CO bond to methylfuran, decarbonylation to furan, and further hydrogenation to THF are observed with furfural alcohol [2]. Hence, more advance, selective, stable and economically feasible catalytic systems are required for this reaction.Meanwhile, the catalytic dehydrogenation of cyclohexanol to cyclohexanone is another industrially important reaction since cyclohexanone can be further transformed into caprolactam and adipic acid which are two major raw materials in synthesizing polyamide fibre [9]. The heterogeneous catalytic dehydrogenation of cyclohexanol may lead to various products namely cyclohexanone, phenol, cyclohexene, cyclohexenyl and cyclohexanone. The commercial catalysts reported for the dehydrogenation of cyclohexanol are either Cu–Zn–Al or Cu–Mg catalysts, for which the method of preparation is very crucial and tedious to obtain equilibrium conversion of cyclohexanol, which is in the range of 60–68% and temperature varying from 493 to 533 K [4,5,10–12]. Hence, various other catalytic systems need to be designed for improved cyclohexanol conversion and selectivity towards cyclohexanone. Precisely, cyclohexanol dehydrogenation on precious metal catalysts [2–4] has received much attention however, are highly expensive. Taking into consideration all the downsides of already reported catalysts, we developed Cu–MgO co-precipitated catalyst in our previous study which proved to be very efficient in catalyzing both furfural hydrogenation and cyclohexanol dehydrogenation independently and simultaneously as well [13–15]. The activity of copper catalysts depends mainly on the nature of support, method of preparation, dispersion and crystallite size of the active component.In the present work we have focused on the co-precipitation method with different precipitating agents as the process involves simple reaction set up, scalability and homogeneity with controlled morphology at atomic scale [16,17]. Additionally, the diversity in the morphology, particle size, type of reductant and ability for interfacial tension allows the developed material for the target reaction in efficient way [16,17]. The present work is a continuation of our previous study in understanding the influence of different precipitating agent's on the characteristics of Cu–MgO catalysts and their activity towards hydrogenation of furfural and dehydrogenation of cyclohexanol, respectively [12–14]. Co-precipitated Cu–MgO catalysts were prepared using a series of five different precipitating agents. The as-synthesized catalysts were characterized by various analytical and spectroscopic techniques to investigate the effect of precipitating agents on morphology and properties of Cu–MgO catalyst. The study revealed that the catalyst prepared by using potassium carbonate as precipitating agent displayed very good activity towards hydrogenation of furfural to lead to a selectivity of about 99% to furfural alcohol and dehydrogenation of cyclohexanol to lead to a selectivity of 100% to cyclohexanone.Furfural (99% purity, ACS grade), Cyclohexanol (99% purity, ACS grade), Furfuryl alcohol (99% purity, ACS grade) and Cyclohexanone (99% purity, ACS grade) were purchased from Sigma Aldrich. Cu(NO3)2.3H2O (99% purity), Mg(NO3)2.6H2O (99.5% purity) are also purchased from Sigma Aldrich. K2CO3.3H2O, KOH, Na2CO3, (COOH)2 and NH3 were purchased from M/s. LOBA Chemie, India. All chemicals in this work are of reagent grade and used as received without further purification.The Cu–MgO catalysts with 16 wt % Cu were synthesized by co-precipitation method using different precipitating agents. For instance, aqueous solution containing requisite amounts of 1 M each of Cu (NO3)23H2O and Mg (NO3)2.6H2O were prepared and mixed together. The mixed solution was precipitated using an aqueous solution containing 1 M K2CO3.3H2O till pH of the solution reaches approximately 9. The co-precipitated mass was thoroughly washed, filtered and dried at 393 K for 12 h. The dried sample was then calcined in air at 723 K for 4 h to obtain CuO–MgO and reduced in H2 flow at 523 K for 4 h to obtain Cu–MgO prior to catalytic reaction. This Cu–MgO co-precipitated catalyst is designated as CM-A. In a similar manner, other Cu–MgO catalysts were prepared using different precipitating agents- Na2CO3, KOH, NH3 as well as (COOH)2 and are designated as CM-B, CM-C, CM-D and CM-E respectively.The XRD pattern of both, calcined and reduced catalysts were recorded on an M/S. Rigaku's Miniflex diffractometer with Ni filtered Cu Kα as a radiation source at a 2θ scan speed of 2omin−1. The crystallite size of Cu was calculated by XLB method on the same instrument. The catalysts were characterized for specific surface area by N2 adsorption at 77 K by BET method using a Micrometrics Pulse Chemisorb 2700 instrument. Before measurements, the samples were dried in oven at 393 K for 12 h and flushed in-situ with Helium gas for 2 h. Surface morphologies of as-synthesized and calcined catalysts were examined by field emission scanning electron microscope (FE-SEM, Carl Zeiss Sigma VP FE-SEM). FT-IR spectra were recorded using a Varian 2000 IR spectrometer (Scimitar series) for the functional group analysis of calcined materials. Temperature programmed reduction (TPR) studies of the catalyst were performed on an indigenous pulse reactor with 6% H2–Ar as reducing and carrier gas respectively. The temperature was increased linearly at a ramp of 5 K min−1 from room temperature to 973 K where the isothermal conditions were maintained for 30 min. The change in the H2 concentration was monitored by micro TCD and recorded on GC work station. The elaborated experimental details of TPR are discussed elsewhere [13]. M/S. Kratos Axis 165 XPS spectrometer, with Mg-Kα radiation (1253.6 eV) was used for obtaining XPS data. XPS analysis was used to study the chemical composition and oxidation state of the catalyst surfaces. In the XPS study, C 1s line binding energy value of 285 eV (accuracy with in ±0.2 eV) as a reference level and the relative atomic sensitivity factors of 4.871 and 0.168 for Cu2p3/2 and Mg 2p, respectively for determining Cu/Mg surface composition were chosen. Prior to the ESCA studies, all the catalysts has been reduced in 6% H2 balance He flow at 523 K for 4 h. During the data acquisition the background pressure was kept slow at 10 bar. DTA/TGA profiles of Cu–MgO samples (in dried form) were recorded on M/S. Metler Toledo (Switzerland) instrument at a heating rate of 10 Kmin-1.Vapour phase hydrogenation of furfural to furfural alcohol and dehydrogenation of cyclohexanol to cyclohexanone were carried out separately in a fixed bed quartz reactor (200 mm long and 8 mm i. d.). About 1 g of catalyst packed at the center of reactor between two plugs of quartz wool was reduced in a flow of 6% H2 in He mixture at 523 K for 4 h followed by lowering the temperature of the reactor to 453 K and replacing the H2/He mixture with ultra-pure H2 (99.9% H2 which was further purified by passing it through de-oxo and molecular sieve traps in a series to remove oxygen and moisture, etc.). Hydrogenation reaction of furfural was carried out by injecting furfural at a flow rate of 1.2 mL/h with the help of a syringe pump (Secura FT, M/S. B. Braun, Germany) with H2/furfural molar ratio = 2.5 at a reaction temperature of 453 K. For dehydrogenation of cyclohexanol the reaction temperature of 523 K was maintained. For both the reactions the gas hourly space velocity (GHSV) was maintained at 0.05 mol h−1gcat−1. The product mixture was collected every hour in an ice-cold trap. The reaction products were analyzed by gas chromatography (GC-7820 A (M/s. Agilent, USA) equipped with a flame ionization detector and a capillary column HP-5, 19091J-413 (30 m length, 0.32 mm inner diameter and 0.25 μm film thickness) and GC-MS (QP-5050, M/S. Shimadzu instruments, Japan) using a DB-5MS capillary column of 0.32 mm dia. and 25 m long (M/S. J & W Scientific Instruments, USA).The conversion of furfural to furfural alcohol and cyclohexanol to cyclohexanone is currently been studied over various catalytic system majorly metal based, metal oxide based and bimetallic catalyst to obtain high conversion and selectivity due to availability of active sites [4,5,7,8]. In our previous reports we have studied the activity of Cu–MgO catalyst prepared by co-precipitation method using K2CO3 as a precipitating agent which gave high conversion for furfural to furfural alcohol in vapour phase [18]. In present study, we have selected five different precipitating agents namely K2CO3, KOH, Na2CO3, (COOH)2 and NH3 and synthesized the catalysts following the same method. To study and correlate the influence of these precipitating agents on structural and chemical properties of Cu–MgO catalyst, the as-synthesized catalyst were characterised by various modern analytical and spectroscopic techniques.The results of XRD analysis of both the calcined and reduced samples of copper catalysts are presented in Table 1 .The predominant phases observed in the calcined catalysts (Fig. 1 a) were CuO (ICDD file No.5-661) and MgO (ICDD file No. 4-829). The Cu2O peaks have matched with the ICDD file No. 5-667 was observed in all the catalysts. The XRD results of Cu/MgO reduced catalysts, CM-A, CM-B, CM-C, CM-D and CM-E exhibited the formation of CuO phase matches with the ICDD file No. 4-836.The XRD pattern of CM-A catalyst revealed the presence of Cu metallic particles to be either in amorphous or in microcrystalline form as shown from Fig. 1b. Thus, the higher surface area of CM-A catalyst may be due to the interacted species formed between Cu and MgO. It was interesting to observe that the catalysts prepared with alkali metal containing precipitating agents consisted of smaller crystallites of Cu. The surface areas of these catalysts were relatively higher as a result of the smaller Cu-crystallites. The other catalysts namely CM-D and CM-E, showed bigger crystallites of CuO. In these catalysts, the intensity of d lines of Cuo phase reduces at the expense of CuO phase. This may be due to incomplete reduction of the bulk CuO or due to re-oxidation of the Cuo species to Cu+ and Cu2+ formed over other catalysts. Only in Cu–MgO co-precipitated catalyst using K2CO3 as a precipitating agent, MgO particles were in microcrystalline form indicating a possible interaction of MgO with Cuo/Cu+ species. It is reported that MgO has the ability to stabilize the metal particles, prevent sintering and volatilization [19,20].The crystallite size of Cuo as represented in Table 1 was calculated by X-ray line broadening technique using Debye-Scherrer equation mentioned in Equation (1). The crystallite size of Cu from XRD in CM-A catalyst was lower. The XRD pattern of CM-A showed that Cu and MgO phases were in micro crystalline form, as a consequence of which the Cu dispersion and metal surface area was higher [18]. The catalysts CM-D and CM-E showed bigger crystallites of Cu and the copper dispersed in bulk on the surface of the support as shown in Table 1. These catalysts has smaller surface areas due to bigger crystallites on the surface of the support. This explains that copper crystallite size is dependent on the type of precipitating agent employed in the synthesis process. 1 D = k λ / β Cos θ where,D = Crystalline Sizek = Scherer's constant (0.94).λ = X-ray Wavelength (1.54178 A).β = Full width at half maximum (FWHM).θ = Bragg angle corresponding to (hkl) reflection. Table 1 represents the BET-surface area and the crystalline phases of various Cu–MgO co-precipitated catalysts using different precipitating agents studied for the hydrogenation of furfural and dehydrogenation of cyclohexanol. The surface area of MgO was found to be 38 m2g-1 and the surface areas of the Cu–MgO co-precipitated catalysts were more or less same when the precipitating agent containing alkali metal was used. Higher surface area of Cu/MgO co-precipitated catalyst is reported in our previous communication [13]. However, when the precipitating agent was either oxalic acid or ammonia the resulting catalyst exhibited very low surface area. This indicated that addition of Cu to MgO reduced the surface area of the catalyst probably due to the coverage of the MgO surface by bigger crystals of Cu.The TPR profiles of Cu–MgO catalysts prepared by different precipitating agents are shown in Fig. 2 . Their corresponding H2 uptakes calculated from the TPR peaks has been presented in Table 1 . Two stages of reduction of CuO to Cuo were observed in CM-A and CM-C catalyst. The standard CuO was found to reduce at a Tmax of 606 K (profile is not shown in Fig. 2). Most of the acidic Cu in the Cu–MgO co-precipitated catalysts seems to reduce at Tmax in the range of 523–563 K. But a complete reduction seemed to occur at ∼673–713 K. Thus, it appears that the reduction of Cu–MgO samples took place in two stages with the first peak corresponding to the reduction of CuO and the second H2 consumption peak at high temperature may be due to the reduction of Cu2O to Cuo. XRD results of calcined catalysts indicated the presence of both CuO and Cu2O.Many literature reports suggest that the reduction peak of bulk CuO to metallic Cu directly takes place in a single step [21–23]. The reduction patterns of CM-A and CM-C co-precipitated catalysts using K2CO3 and Na2CO3 as precipitating agents showed peaks at 610 K and 722 K in CM-A and peaks at 654 K and 713 K in CM-C. The low temperature reduction peak indicated the presence of easily reducible Cu+2 species. A similar observation is reported with Cu/Al2O3 [11]. The defect sites known to be present in MgO probably contribute to the formation of surface interacted species with copper in the CM-A and CM-C catalysts. These interacted species got reduced at lower temperatures compared to the reduction of bulk CuO (i.e., larger crystallites) which reduce at higher reduction temperatures [21]. The XRD patterns also confirmed that the CM-A co-precipitated catalyst showed poorly crystalline CuO phase and Table 1 shows the CM-A catalyst has higher surface area. Wang et al. reported the presence of three peaks in different temperature regions in the impregnated Cu/SiO2 catalysts [23]. The major peak corresponds to the reduction of larger CuO clusters and the minor peaks reduced to of small CuO clusters and highly dispersed Cu (II) species, respectively.The Fig. 3 represents the thermal changes and behaviour of the various Cu–MgO dried samples in the temperature range of 298–1273 K with the help of their corresponding DTA patterns. The endotherms in the low temperature region of 343–473 K correspond to the removal of physically adsorbed water and dehydroxylation of some hydroxyls associated with Cu and Mg cations showing a weight loss of 2–10% in almost all the catalysts except CM-D which showed only one major endotherm at ∼608 K that may be attributed to the dissociation of oxalates of Cu and Mg forming their corresponding oxides.Jongen et al. found that copper oxalate shows a very small endotherm between 303 and 523 K corresponding to dehydration process (with a 3% wt. loss) and a major endotherm at 523–573 K (with ∼40% wt. loss) attributed to the decomposition of copper oxalate to CuO and CO/CO2 [24]. The samples CM-A, B and C showed a minor endotherm ∼303–543 K corresponding to partial dehydroxylation of the hydroxyl groups of Cu and Mg. The CM-B catalyst prepared using KOH showed a major endotherm at ∼633 K corresponding to the transformation of the hydroxides of Cu and Mg to their corresponding oxides by complete dehydroxylation. However, the samples obtained from carbonate precipitating agents; CM-A and CM-C showed the major peak at ∼703 K and 623 K respectively. The major peaks along with shoulder peaks of these samples were found to exist at 653 K and 728 K in CM-A and 703 K in CM-C. The DTA patterns of dried samples of single oxides namely, CuO and MgO showed endotherms at 493–533 K and ∼633 K respectively. Thus, it clearly indicated that the major endotherms of CM-A and CM-C correspond to the dehydroxylation process and the shoulder peaks associated with them may be ascribed to subsequent decarboxylation (CO2 elimination). The transitions, which occur in the range of 673–773 K were attributed to the removal of CO2 associated with Cu containing samples [25,26]. The high temperature endotherm observed at 1073–1123 K in the DTA patterns of CM-B and CM-C was probably due a partial transformation of CuO to Cu2O [25,26]. CM-E sample showed two endotherms at 573 K and 673 K, which were attributed to the dehydroxylation of hydroxyls associated with Cu and Mg with a corresponding percentage weight loss of 28% and 5%, respectively.The XPS patterns of reduced catalysts for Cu 2p and Mg 2p are shown in Fig. 4 . The binding energy values of Cu 2p3/2, Mg 2p and O 1s along with the Cu/Mg atomic ratios, Cus/Cup (intensity ratio of the Cu 2p satellite peak to Cu 2p parent peak) of catalysts are given in Table 2 . XPS analysis of Cu–MgO catalysts prepared by co-precipitation, impregnation and solid-solid wetting method has been discussed in our earlier work [14]. The binding energy values of Cu 2p3/2 in metallic copper and in CuO are reported to be 932.7 and 933.6, respectively [27]. The presence of another minor band of C 1s at 289 eV in some of the samples indicates carbon contamination due to CO3 2−species with the exposure of samples to air. The presence of significant amount of CuO species on the surface in the reduced samples may be due to the re-oxidation of Cuo on exposure to air.The higher Cus/Cup observed in case of CM-B, CM-C, CM-D and CM-E samples probably indicate the presence of more amounts of Cu2+ species. The satellite peaks observed in case of Cu+2 compounds are due to the shake-up transitions by ligand to metal 3 d charge transfer [28]. These satellite peaks are not seen in Cu+ compounds or in metallic Cu because of completely filled 3 d orbital. In fact, transition metal ions with unfilled 3 d orbital are well reported to show satellite peaks in the core level XPS spectra due to electron shake-up [29]. The higher intensity of CuO phase (Fig. 2) observed in the case of CM-B, CM-C, CM-D and CM-E (reduced) catalysts support the above finding. The Cu/Mg ratio is although higher with CM-A catalyst, the lower Cus/Cup in this sample suggests the surface enrichment of other species viz., Cuo or Cu2O apart from Cu2+. Our earlier observations in the hydrogenation of furfural to furfural alcohol over Cu–MgO co-precipitated catalyst prepared with K2CO3 as a precipitating agent indicated that the presence of more Cuo species in this catalyst may be responsible for higher activity and selectivity towards the formation of furfural alcohol [18]. Secondly, O 1s spectra show the binding energy values in the range of 530.8–532.2 eV for the Cu–MgO catalysts which is a characteristic feature of metal oxides. The Mg 2p binding energy values are almost the same in all the catalysts. Fig. 5 shows the FE-SEM images of uncalcined and calcined Cu–MgO catalysts prepared by five different precipitating agents. FE-SEM analysis revealed that changing the precipitating agent could affect the morphology of the final catalyst [30]. The sample morphology differs from each other to some degree in terms of shape, size and distribution of copper over the catalyst surface. In CM-A catalyst, employing K2CO3 as the precipitating agent led to the formation of a very peculiar rod like structure, which is retained in calcined sample as well. It can be seen that Cu particles are very well dispersed on the rod like MgO (CM-A; a-d). Hence there is less aggregation of copper particles on MgO support. The rods seem to be approximately of 4–5 μm in length. Since the morphology of synthesized material varies according to the functional group of the precipitants (carbonates or hydroxides) used in synthesis method, catalyst prepared by using Na2CO3 as precipitating agent (CM-B) also displayed rod like morphology similar to that of CM-A and the same morphology was observed even after calcinations (CM-B; a-d). This perhaps may be due to presence of same functional group (CO3−) in precipitating used for synthesizing both the catalyst. These observations are in good agreement with those reported by Jung et al. [31]. Even in this case the active species was dispersed on rod shaped surface having length may be between 5 and 6 μm.CM-C catalyst prepared by KOH as precipitating agent showed better dispersion of Cu and Mg species which was observed in this case and is well supported by BET results which showed better surface area as compared to rest of the samples. While for CM-D catalyst prepared by NH3 as precipitating agent led to aggregation of both copper and magnesium on catalyst surface (CM-D; a-d). The low surface area reported for this catalyst can be attributed to poor dispersion of active Cu species on catalyst surface which is clearly seen in SEM image. Later on when oxalic acid was used as a precipitating agent, catalyst (CM-E; a-d) showed spherical morphology. Additionally, BET analysis of this catalyst showed lowest surface area. The blockage of pores by Cu and the compact structure can be one of the reasons for this observation.Moreover, the information regarding the formation of the Cu–MgO was obtained by FT-IR analysis. Fig. 6 presents the FT-IR spectra of the uncalcined (Fig. 6.1) and calcined (Fig. 6.2) catalysts. The broad band observed in range 3300–3650 cm−1is assigned to O–H stretching due to the presence of hydroxyl as well as both adsorbed and interlayer water in uncalcined Cu–MgO. These broad bands are observed for all the uncalcined catalyst prepared by different precipitating agents with a slight shift in band in each case. Two distinct peaks in finger print region is attributed to metal-OH stretching vibrations [32]. The presence of C O 3 2 − from precipitating agents K2CO3 and Na2CO3 contributes to the carbonyl stretching at around 1380 cm−1 in uncalcined FT-IR spectra of CM-A and CM-B respectively [32]. Supporting, the FT-IR spectrum recorded for all the calcined catalysts showed absence of O–H stretching peaks. The disappearance of M-O-H stretch and appearance of metal-O stretching in finger print region of calcined spectra suggests successful synthesis of Cu–MgO catalysts [33]. The finger print region shown separately gives clear insight picture of the respective characteristic peaks of M − OH and M − O for uncalcined and calcined samples.After acquiring in-depth knowledge of prepared catalyst by characterisation, we have tested all prepared catalysts for their catalytic activity in hydrogenation and dehydrogenation reaction respectively. Fig. 7 shows the hydrogenation activity data for furfural to furfuryl alcohol. The graph displayed shows that the CM-A catalyst showed higher conversion of furfural and selectivity of furfural alcohol, 98% and 99% respectively and with 97% yield of furfural alcohol.The presence of more Cuo species has been attributed to the presence of defect sites of MgO which are reported to be more reactive and that the adsorption properties for metal species can be qualitatively different from those of regular surface sites [34,35]. It is also reported that the defect sites at the metal support interfacial region in case of Pt–TiO2 system is helpful to coordinate the oxygen atom of the CO group via lone pair of electrons and thus activate the hydrogenation of CO group [36]. The higher conversion of CM-A catalyst compared to other catalysts was due to the presence of small crystallites of Cu as found from XRD, correspondingly higher surface area as shown in Table 1. However, the BET surface area was comparable in case of CM-A and CM-C, the uptake of H2 gas was impactful in case of the CM-A catalyst and hence affected positively in the catalytic activity. The higher conversion of CM-A catalyst was also attributed due to the presence of small crystallite size of Cu as seen in the XRD data in Table 1. Fig. 8 a, b and c represents the conversion, selectivity and yield with time on stream over various Cu–MgO co-precipitated catalysts studied. The CM-A catalyst showed a steady and higher conversion of 99% and selectivity of 99% and therefore higher yields of furfuryl alcohol throughout the 300 min operation. CM-B catalyst showed an initial conversion of 45.5%.Additionally, from the second hour onwards, the conversion goes on decreasing and in the fifth hour of the activity run, it drastically decreased to 11.6%. Over CM-C catalyst, the initial conversion of 63.9% decreased to 43.1%, whereas over CM-D catalyst, initially the conversion was59.6% which decreased to 47.1% in 300 min run. CM-E catalyst showed a conversion of 57.4% and ended to 53% in 5 h. The selectivity towards furfuryl alcohol over all the catalysts is almost 99%. Fig. 9 shows the activity data for cyclohexanol conversion towards cyclohexanone over the Cu–MgO catalysts prepared by different precipitating agents. The CM-A and CM-C catalysts shows a 64.3% cyclohexanol conversion with 100% selectivity towards cyclohexanone. The selectivity towards cyclohexanone is reported to be governed mainly by metal-support interaction, electronic and steric influence of the support, morphology of the metal particles, selective poisoning, influence and nature of second metal, effect and pressure and steric effects of substituents at the conjugated double bond [37]. Dehydrogenation of cyclohexanol is known to be an equilibrium-controlled reaction showing a maximum conversion of 68.89% at 523 K [38]. In the present investigation, Cu–MgO catalyst prepared using K2CO3 as a precipitating agent resulted in relatively smaller Cu particles compared to CM-B, CM-C, CM-D and CM-E catalysts. The defective sites at Cu and MgO interfacial region and suitable particle size of Cu appears to be the key factor in governing the selectivity towards cyclohexanone. Fig. 10 a, b and c show the activity data relating to the conversion, selectivity and yield against time on stream over the catalyst prepared by different precipitating agents in the dehydrogenation of cyclohexanol.The CM-A and CM-C catalyst show a consistent activity of 64% conversion of cyclohexanol up to 5 h but the conversion over CM-B, CM-D and CM-E dropped down to 10%, 4.8% and 2.5% from 42.7%, 45% and 60.2% respectively. The large number of Cuo species on the surface of CM-A and CM-C catalysts which were responsible for higher interaction with the defect sites of MgO may have helped in yielding superior activity for cyclohexanol conversion.It is reported that various kinds of defects like steps, kinks, edges etc., impurities and vacancies at the surface of supports can interact with metal species [13,39]. There are reports with experimental evidence that the growth of metal clusters and films is initiated at the defect sites, in particular on surface vacancies. The adsorption of Cu and Pd moieties on MgO support with exposed (001) planes has been reported [39,40]. The high activity exhibited by the catalyst CM-A is due to the large number of smaller Cu crystallites at the surface evidenced by XRD data as shown in Table 1 . Thus, the CM-A catalyst showed higher activity for dehydrogenation of cyclohexanol to cyclohexanone compared to other catalysts. Hence, K2CO3 was found to be a preferable precipitating agent in the preparation of highly active Cu–MgO catalyst for both hydrogenation of furfural and dehydrogenation of cyclohexanol. Presence of smaller crystallites of Cu and surface enrichment by Cuo/Cu+ species in the Cu–MgO catalyst prepared using K2CO3 appeared to be the reason for good hydrogenation and dehydrogenation activities.The literature available reveals that in Cu–MgO catalysts, Cuo species acts as a catalytically active site [15,38–40]. Based on XPS analysis, the present of surface basic sites (oxygen vacancies) in present in the catalysts can be close contact with metallic Cuo sites and play a projecting role in facilitating the hydrogenation of aldehyde and dehydrogenation of alcohols significantly [8,18,41–43]. Considering above point, we propose a plausible mechanism for hydrogenation of furfural and dehydrogenation of cyclohexanol over Cu–MgO co-precipitated catalyst. It is well represented in Scheme 1 .The hydrogenation pathway starts with adsorption of hydrogen molecule over Cu–MgO surface where dissociation of molecular hydrogen occurs on Cuo species followed by stepwise interaction of furfural with metallic copper and basic site of catalyst to yield furfuryl alcohol. Initially, carbonyl carbon of aldehydic group in furfural interacts with oxygen vacancies (basic sites) on the Cu–MgO catalyst surface which highly activates the CO bond. This CO bond with enhanced nucleophilic character can be easily attacked by the dissociated hydrogen atom present on the surface of Cuo species. Therefore, the carbonyl oxygen (CO) gets one H atom while the second hydrogen is abstracted by carbonyl carbon (CO). Finally the desired product, furfuryl alcohol is formed which gets desorbed from the catalyst surface. This proposed mechanism well agrees with mechanism suggested in various published literature [41,44].In dehydrogenation pathway, basic sites on the Cu–MgO catalyst surface play a crucial role in conversion of cyclohexanol to cyclohexanone. The oxygen vacancies (basic sites) act as a nucleophile and abstracts a proton from O–H of cyclohexanol to form a negatively charged alkoxide intermediate. This intermediate undergoes β-H elimination to produce cyclohexanone which finally gets desorbed from the catalyst surface along with H2. This proposed mechanism is well supported by reported literature work [42,45]. Hence, in our case, reason for high conversion and selectivity for both the reactions over Cu–MgO catalyst can be attributed to uniform distribution and high dispersion of active small crystallites of Cuo species over MgO support, as well as high synergistic interaction between active Cuo and MgO support.Highly efficient Cu–MgO catalysts were prepared by co-precipitation method using five different precipitating agents such as K2CO3, KOH, Na2CO3, (COOH)2. 2H2O and NH3. The as-synthesized catalysts were investigated for their hydrogenation activity towards furfural to furfuryl alcohol and dehydrogenation activity of cyclohexanol to cyclohexanone individually. K2CO3 was found to be superior precipitating agent in yielding a Cu–MgO catalyst that is highly active and stable towards both the mentioned reactions. The catalytic activity of all Cu–MgO catalyst were tested for time on stream analysis and it was observed that the CM-A catalyst was active for 300 min yielding 96.71% furfuryl alcohol in furfural hydrogenation at 453 K and 64.3% cyclohexanone in cyclohexanol dehydrogenation at 523 K. The studied different analytical techniques revealed that the catalysts developed by different precipitating agents affected the reaction in different effective ways and solely depended on the physiochemical properties of as-synthesized catalyst. The physiochemical properties depicted by the XRD showed almost same crystallite size and BET surface area for CM-A and CM-C with the enrichment of Cu0/Cu + species as the results of higher catalytic activity. However, the results obtained from the TPR analysis depicted that the H2 uptake is better in case of CM-A which might have enriched the catalytic activity giving better results when compared with other catalysts.I hereby declare that I am submitting this manuscript on behalf of my co-author. My co-author is aware of this submission. This manuscript has not been previously published, is not currently submitted for review to any other journal, and will not be submitted elsewhere before a decision is made by this journal.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by Centre for Nano and Material Sciences (CNMS), JAIN (Deemed-to-be University), Bangalore and funding support was through the basic research grant of JAIN JU/MRP/CNMS/11/2022 and (11(39)/17/005/2017SG). Authors would like to thank Nano Mission, DST, Government of India, for partial financial support SR/NM/NS-20/2014. Meanwhile, authors thank to Director, Indian Institute of Chemical Technology, Hyderabad for his keen interest.

The major industrial application and study of hydrogenation of furfural-to-furfural alcohol and dehydrogenation of cyclohexanol to cyclohexanone, over different precipitating agents namely K2CO3, Na2CO3, KOH, NH3 and (COOH)2.2H2O was carried out in vapour phase reactor. Efforts were made to study the effect of these different precipitating agents on Cu–MgO catalyst and its activity towards the hydrogenation and dehydrogenation reactions due to its industrial applications. The prepared co-precipitated Cu–MgO catalysts were characterized by using various modern analytical and spectroscopic techniques which include FE-SEM, BET, XRD, XPS, TPR and DTA. The characterization data revealed that different precipitating agents strongly influenced the physiochemical properties of the developed heterogeneous catalysts. Additionally, FE-SEM images revealed that employing different precipitating agents resulted in various morphologies for the final catalysts. The hydrogenation and dehydrogenation reactions over the Cu–MgO catalyst revealed that the catalyst prepared by K2CO3 as precipitating agent exhibited high catalytic activity. Meanwhile, The presence of more Cuo/Cu+ species on this catalyst with smaller Cu crystallite size as evidenced by XPS and XRD results seems to be accountable for its high activity towards the formation of furfural alcohol and cyclohexanone compared to the other catalysts with different precipitating agents. Additionally, the time on stream (T.O.S) studies performed and it revealed that the catalyst was fairly stable for 300 min showing consistency in its activity towards both reactions. The yield obtained for furfural alcohol and cyclohexanone was 97% and 64%, respectively.

Deliberate consideration of materials for photoelectrochemical (PEC) water splitting is crucial, given that photoelectrodes must absorb the wide range of the solar spectrum to maximize photogenerated charge carriers, must have long carrier diffusion length and lifetime to minimize charge recombination, and must be cost-effective for practical application. 1 , 2 Silicon (Si), which has been extensively used in the photovoltaic (PV) industry, is a promising candidate for the photoanode because of its long carrier diffusion length, abundance, and well-established technologies to obtain highly crystalline and large-area wafers. 3 , 4 However, the utilization of silicon as a photoelectrode material raises the question in the view of relatively negative valence band position compared to the water oxidation potential, poor catalytic activity, and instability in aqueous solution. Silicon suffers from two main deleterious mechanisms, which contribute to the low stability. 5 The first is the self-oxidation of Si into SiOx, which suppresses the charge transfer at the Si/electrolyte interface because of electrical insulating property. The second mechanism is that at high pH electrolyte, silicon is chemically etched naturally. 6 The research to date has been devoted to solving the challenges of silicon by introducing chemically stable and catalytic active materials as protection layers. 7 , 8 Since Kenney et al. 9 first reported the Ni/n-Si photoanode by e-beam evaporator in 2013, various transition metals and their oxides and transparent conductive oxides have been used as both catalytic and passivation layers by high-vacuum processes such as e-beam evaporator and atomic layer deposition (ALD). 10 , 11 The introduction of a single passivation layer of silicon such as CoOx, 12 NiOx, 13–15 and MnO 16 thin film successfully protected the Si photoanode against surface corrosion and facilitated oxygen evolution reaction (OER) kinetics. Recent studies have shown that introducing heterogeneous catalysts on silicon via high vacuum processes (e.g., ALD, sputtering) exerts synergistic effects on PEC performance. Yang et al. 17 introduced biphasic Co3O4/Co(OH)2 by the plasma-enhanced ALD method and showed the effects of multifunctional catalysts on OER activity. Oh’s group 18 adjusted a similar strategy, in which double-layered CoO/Co3O4 films were formed using ALD to derive the collective characteristics in both high photovoltage and catalytic properties. However, those methods require the consumption of a large amount of energy and high cost, which conflict with the ultimate goal of sustainable energy production. The electrodeposition method, which is a facile and cost-effective method, has been applied to synthesizing various nanostructured photoactive materials and electrocatalysts by substituting the high vacuum processes. 19 In designing Si-based photoanodes, Switzer’s group 20 first designed inhomogeneous Co nanoparticles (NPs) on a silicon surface by electrodeposition in 2015, which showed high water oxidation performance. This strategy was followed by the work of several groups to fabricate pinched-off photoanodes; NPs with core-shell structures exhibited improved photovoltage 21 , 22 and alloy NPs 23 , 24 exhibited excellent PEC properties compared to single metal NPs. Despite that the electrodeposition method has the merit of synthesizing materials with various nanostructures, there are limited reports using spherical NPs as catalysts for silicon photoanodes. Controlling the electrodeposition reaction to obtain various morphologies is quite difficult for the silicon substrate compared to other conductive substrates (e.g., Ni foam, fluorine-doped tin oxide glass) because of its fast corrosive behavior in the electrolyte.Another challenge of Si photoanodes is that additional bias is required for driving OER spontaneously. The photovoltage generated in silicon is usually <0.7 V, which is far below the voltage for spontaneous water splitting. It is estimated that at least 1.6 V is needed to meet the thermodynamic voltage of 1.23 V for water splitting and to compensate over 0.35 V for the energy losses and kinetics overpotentials at the semiconductor/liquid interfaces. 2 , 25 Because of insufficient photovoltage output, efforts have been made to design integrated devices such as PEC tandem photoanodes, 26 , 27 integrated PV/PEC devices, 28 and PV/electrolyzer devices. 29 Although high solar-to-hydrogen (STH) efficiency >25% is estimated for tandem-based light absorbers when Si is integrated with 1.6–1.8 eV band gap materials, 30 , 31 an additional complicated buried junction is necessary to minimize charge recombination and facilitate charge transfer. 32 In terms of tandem device fabrication, the utilization of PEC cells and PV devices is attractive because of device fabrication costs, solar light use, and the possibility of achieving relatively high STH efficiency.In this article, we report the incorporation of heterogeneous Ni-based catalysts synthesized by the electrodeposition method on Si substrates and investigate their synergetic effects on PEC water splitting. We show that the morphology and water oxidation property of Ni-based catalysts can be easily tuned by changing the precursors in the electrolyte, resulting in Ni NPs and Ni(OH)2 thin films. A systematic study reveals that heterogeneous Ni NPs/Ni(OH)2/n-Si shows 2.5 times higher stability and higher enhanced charge transfer kinetics than Ni(OH)2/n-Si and 130 mV lower onset potential than Ni NPs/n-Si. One step further, by combining heterogeneous Ni NPs/Ni(OH)2/n-Si photoanodes and perovskite/Si tandem solar cells, the wired tandem cell device shows a photocurrent of 9.8 mA cm−2 without external bias corresponding to the STH conversion efficiency of 12%.The electrodeposition of Ni-based catalysts used in this study was conducted in the three-electrode system as shown in Figure 1 A. Recently, the electrodeposition of distinctive morphologies of Co-based catalysts on the n-Si surface has been conducted by controlling the kinds of additives that affect the pH of the electrolyte. 33 We used different electrolytes to control the reaction accurately in real time to synthesize different morphologies of the Ni-based catalysts. The electrodeposition of Ni(OH)2 was conducted under nickel nitrate electrolytes by applying a constant current of −0.15 mA cm−2 versus a saturated calomel electrode (SCE) (Figure 1B). It has been reported that in the presence of water, nitrate ions are reduced to nitrite ions, generating hydroxide ions. The hydroxide ions near the working electrode surface combine with the nickel cations by an electrochemical (EC) reaction to form Ni(OH)2 films. 34 The inset indicates the schematic of the Ni(OH)2 film on the silicon surface. Similarly, Ni NPs were pulse electrodeposited under nickel sulfate electrolyte containing boric acid as an additive; Figure 1C shows the duration and amplitude of the deposition potential versus SCE. The fabrication of heterogeneous Ni NPs/Ni(OH)2 on top of the silicon surface was conducted by a two-step electrodeposition, as described above. Figure 1D presents the photographs of electrodeposited Ni(OH)2 (orange), Ni NPs (green), and Ni NPs/Ni(OH)2 (blue), respectively, on the silicon surface. It is well known that the local pH near the electrode surface can increase during the electrodeposition of iron-group metals, generating hydroxides. 35 In this respect, the addition of boric acid in the electrolyte can suppress the formation of hydroxides as a buffer reagent. The surface morphologies of Ni-based catalysts were analyzed by field emission scanning electron microscopy (FESEM). Figures 1E–1G show the scanning electron microscopy (SEM) images of Ni(OH)2, Ni NPs, and Ni NPs/Ni(OH)2, respectively. The SEM images reveal that various surface morphologies of Ni-based catalysts can be obtained through electrodeposition by controlling the reaction parameters. The shape of nickel particles was tuned by electrodeposition in a nickel chloride electrolyte containing glycine as an additive. As shown in Figure S1, the morphology of the Ni NPs became branch shaped, which reveals the merits of electrodeposition as a simple and facile synthetic approach to control morphology. To investigate the effects of conditions of applying deposition currents in detail, we performed 3 types of electrodeposition of Ni NPs. We reported the pulse electrodeposition of Ni particles, in which the number of deposition cycles affects the PEC performance. 21 Compared to the continuous electrodeposition, it is possible to promote more nucleation and obtain uniform films because ions can be replenished during pulse-off time, decreasing the concentration variation. Figure S2A shows the different electrodeposition modes of Ni NPs. By applying a direct current (DC) of −0.8 mA cm−2, Ni NPs showed size variations (Figure S2B). However, the more uniform size distribution of Ni NPs was observed when pulse electrodeposition (denoted as pulse-1) was conducted with the duration (ton/ton+toff) of 0.5 in the same electrolytes and applied current density (Figure S2C). This tendency is observed when applying a high current density of −8 mA cm−2 for a few seconds and conducting the same pulse electrodeposition (denoted as pulse-2). As shown in Figure S2D, the increased number of Ni NPs with decreased particle size was formed uniformly on silicon substrates. We conducted successive electrodepositions of Ni NPs on Ni(OH)2/n-Si using 3 types of deposition mode. Since the electrodeposition of catalysts occurs uniformly on the conductive substrates, where the resistance of the surface is constant, the size and coverage of the Ni NPs on Ni(OH)2/n-Si were quite different from those of Ni NPs/n-Si (Figures S2E–S2G). It was observed that there were no significant morphological differences between DC and pulse-1 electrodeposition. However, pulse-2 electrodeposition showed an increased number of nickel particles whose size was smaller than that of DC and pulse-1 electrodeposition. By applying high current density in the initial step during pulse electrodeposition, Ni(OH)2 films were partially etched, and more nucleation of Ni NPs occurred.Transmission electron microscopy (TEM) was conducted to clarify the element distributions and morphologies of electrodeposited Ni-based catalysts. To prevent the surface from the damage induced by a focused ion beam (FIB), the application of carbon and Pt coating was introduced. Energy-dispersive spectroscopy (EDS) was carried out to investigate the elements of the fabricated Ni-based catalysts on the silicon surface. As shown in Figures 2A and 2B, Ni(OH)2 films were uniformly formed, showing a thickness of 50 nm, and the size of Ni NPs corresponded to 50 nm. EDS analysis revealed the difference in the presence of oxygen between Ni(OH)2 and Ni NPs. As shown in Figure S3, high-resolution TEM images and the electron patterns indicate the amorphous Ni(OH)2 thin films and crystalline Ni NPs. With continuous electrodeposition, the thickness of the Ni(OH)2 films significantly decreased to 3 nm because of electrical and chemical etching during the electrodeposition of Ni NPs. Uniform oxygen distribution on the silicon surface shows the existence of Ni(OH)2 thin layers, as shown in Figure 2C. X-ray photoelectron spectroscopy (XPS) was carried out to identify the chemical state of the electrodeposited Ni-based catalysts. Figure 2D shows the Ni 2p spectrums of Ni(OH)2//n-Si, Ni NPs/n-Si, and Ni NPs/Ni(OH)2/n-Si, which result from the spin-orbit splitting of the p orbital (Ni 2p3/2 and Ni 2p1/2). The Ni(OH)2/n-Si photoanode showed the main peak of Ni 2p3/2 at 855.2 eV and Ni 2p1/2 at 872.8 eV and the binding energy difference (Ni 2p3/2, Ni 2p1/2) of 17.6 eV, which is assigned to nickel hydroxide. 19 , 36 The Ni 2p peaks of Ni NPs/n-Si indicated Ni0 (852.4 eV, 869.7 eV), where the binding energy difference between the Ni 2p3/2 and Ni 2p1/2 of 17.3 eV is indicative of metallic nickel and Ni2+ or nickel hydroxide (855.4 eV, 873.4 eV). 37 , 38 Heterogeneous Ni NPs/Ni(OH)2/n-Si photoanodes showed the peaks of 853.5 and 854.8 eV, which correspond to the metallic Ni and Ni2+, respectively. 39–41 Figure 2E shows the O 1 s spectrums of Ni NPs/n-Si, Ni(OH)2/n-Si, and Ni NPs/Ni(OH)2/n-Si, in which different chemical states of oxygen were identified. The O 1 s peaks of Ni NPs/n-Si indicated typical metal-oxygen bonds (529.3 eV) and surface hydroxide groups (531.8 eV), and that of Ni(OH)2/n-Si showed metal hydroxides (530.5 eV) and surface –OH groups (531.6 eV). Combined Ni NPs/Ni(OH)2/n-Si revealed the peaks of 531 and 532.1 eV, which correspond to metal hydroxides and surface –OH groups, respectively. 19 , 37 , 42 , 43 The PEC water oxidation behavior of n-Si coated with Ni-based catalysts was investigated under 100 mW cm−2 AM 1.5 G irradiation in 1 M NaOH electrolyte. Figure 3 A shows the current density versus potential (J-V) curves of Ni(OH)2/n-Si, Ni NPs/n-Si, and Ni NPs/Ni(OH)2/n-Si photoanodes. The light-limited photocurrent density of Ni(OH)2/n-Si at 1.23 V versus reversible hydrogen electrode (RHE) was 31.4 mA cm−2, Ni NPs/n-Si of 11.44 mA cm−2, and Ni NPs/Ni(OH)2/n-Si of 29.6 mA cm−2. As shown in Figure S4A, the Ni NPs (branched)/n-Si photoanode also showed a high photocurrent density of 25.8 mA cm−2 at water oxidation potential. The onset potential of the photoanode in the J-V curves was defined as the potential at which the photocurrent density recorded 1 mA cm−2. Compared to the onset potential for Ni NPs/n-Si of 1.15 V versus RHE, both the Ni(OH)2/n-Si and the Ni NPs/Ni(OH)2/n-Si photoanode showed the onset potential of 1.02 V versus RHE. The morphology-controlled Ni NPs (branched)/n-Si photoanode recorded the onset potential of 1.03 V versus RHE. For comparison with the previously reported Si-based photoanodes, the current densities at water oxidation potential, onset potentials, and stability values are summarized in Table S1. Applied bias photon-to-current efficiency (ABPE) of the Si-based photoanodes, which ascertains the external bias dependence of actual PEC devices, was calculated based on the linear sweep voltammetry (LSV) curves, as shown in Figure 3A. Compared to the Ni NPs/n-Si photoanodes, which showed the maximum value of 0.18% at 1.2 V versus RHE, both Ni(OH)2/n-Si and Ni NPs/Ni(OH)2/n-Si photoanodes recorded the maximum value of 1.76% at 1.14 V versus RHE and 1.75% at 1.13 V versus RHE, respectively, which is 9.7 times higher than that of Ni NPs/n-Si (Figure S6A). The Ni NPs (branched)/n-Si photoanode recorded the maximum ABPE value of 1.23% at 1.14 V versus RHE (Figure S4B). XPS analysis was performed to scrutinize the oxidation states of the Ni-based catalysts after LSV measurements. As shown in Figure S5, after PEC measurements, Ni 2p peaks of all of the Ni-based catalysts exhibited the presence of Ni2+ (855.2 and 872.9 eV). 44 The J-V behavior of Ni-based catalysts/p++-Si electrode, in which p++-Si is not photoactive material, was measured in a 1 M NaOH electrolyte under dark conditions to evaluate the oxygen-evolving activity of the catalysts. Compared to the PEC performance of n-Si photoanodes, the EC performance of Ni-based catalysts showed contradictory behavior (Figure 3B). The overpotential to produce the current density of 10 mA cm−2 of Ni(OH)2 was 300 mV, which is 60 mV higher than that of Ni NPs. With the integration of both catalysts, Ni NPs/Ni(OH)2 requires an overpotential of 250 mV for producing 10 mA cm−2, which is quite similar to that of Ni NPs. Using the J-V curve of Figure 3B, the Ni NPs/Ni(OH)2 showed the Tafel slope of 52.45 mV dec−1. We calculated the photovoltage generated at n-Si photoanodes, where photovoltage can be derived by the difference in onset potential between n-Si under light illumination and metallic p++-Si under dark conditions with the same catalysts. The calculated photovoltage generated at Ni(OH)2/n-Si in 1 M NaOH was 520 mV, and Ni NPs/n-Si showed the photovoltage as 150 mV lower than Ni(OH)2/n-Si. The integrated Ni NPs/Ni(OH)2/n-Si photoanodes showed a photovoltage of 500 mV (Figure 3C). By combining the EC and PEC characteristics of different Ni-based catalysts on silicon, their contradictory behaviors came together to exert synergistic effects on both catalytic activity and junction behavior at the interfaces. The electrochemical impedance spectroscopy (EIS) was performed to elucidate the effects of Ni-based catalysts on the kinetics of charge transfer during the water oxidation reaction (Figure 3D). The EIS analysis was conducted at an external bias near the onset potential to exclude any possible intricate factor. The equivalent circuits used to fit the measured EIS data consisted of charge transfer resistance (Rct) and capacitance (C) elements, where Rct,1 indicates the contact resistance of silicon, Rct,2 the resistance between silicon and Ni-based catalysts, and Rct,3 the resistance between Ni-based catalysts and electrolytes. In the Nyquist plots, small semicircular arcs mirror the low charge transfer resistance at the interfaces. As summarized in Table 1 , the Ni(OH)2/n-Si photoanode showed the high charge transfer resistance of 16.1 Ω cm2 at the n-Si/catalyst interface and low charge transfer resistance of 2.7 Ω cm2 between the catalyst/electrolyte interface. Ni NPs/n-Si showed the opposite results, in which the resistance between the catalyst/electrolyte interface was significantly higher than that of the n-Si/catalyst. By introducing both catalysts, the charge transfer resistances of the Ni NPs/Ni(OH)2/n-Si photoanode were buffered, in which the Rct,2 value is similar to that of Ni NPs/n-Si and the Rct,3 value decreased compared to the Ni(OH)2/n-Si photoanode.To scrutinize the PEC performance of fabricated Si-based photoanodes, we calculated and evaluated the efficiencies of photoanodes. In the case of photoanodes that oxidize water to generate oxygen, the amount of hole injected to the catalyst/electrolyte interface represents the charge injection efficiency. Na2SO3 was used as a hole scavenger; the oxidation of sulfite is thermodynamically and kinetically more favorable than the oxidation of water, resulting in eliminating the injection barrier. Ni(OH)2/n-Si and Ni NPs/Ni(OH)2/n-Si photoanodes showed similar charge injection efficiencies up to 1.3 V versus RHE; however, at potentials >1.3 V versus RHE, the Ni NPs/Ni(OH)2/n-Si photoanode exhibited almost 100% compared to the Ni(OH)2/n-Si of 90% (Figure S6B). The chronoamperometric measurements of Ni(OH)2/n-Si, Ni NPs/n-Si, and Ni NPs/Ni(OH)2/n-Si photoanodes were carried out under chopped light (on/off) at 1.5 V versus RHE to investigate the charge recombination at the transient state (Figure S6C). All of the photoanodes instantly reacted to the light irradiation to the same extent, which indicates negligible leakage current in the light-off condition. 8 The Ni(OH)2/n-Si photoanode revealed the weak current spike and a slight decrease in photocurrent spectra when the light was switched on, which indicates that accumulated carriers at the transient state would lead to the electron-hole recombination. 45 By introducing Ni NPs, both Ni NPs/n-Si and Ni NPs/Ni(OH)2/n-Si photoanodes showed the constant photocurrent density, which would indicate the reduced charge recombination. Incident photon-to-current efficiency (IPCE), which reveals the overall efficiency of water splitting the photoanodes, was measured at a bias of 1.23 V versus RHE in a 1 M NaOH electrolyte (Figure 3E). Compared with the Ni NPs/n-Si, which exhibited lower than 10% in the wavelength range of 400–800 nm, both the Ni(OH)2/n-Si and the Ni NPs/Ni(OH)2/n-Si photoanode reached the efficiency of ∼80% at nearly 800 nm. When the sufficient bias of 1.5 V versus RHE was applied for reaching the saturated current density of all photoanodes, Ni NPs/n-Si photoanodes showed the significantly improved value in the range of 400–800 nm, which is similar to the Ni(OH)2/n-Si and Ni NPs/Ni(OH)2/n-Si photoanode (Figure S6D). All photoanodes can absorb visible light under the sufficient applied voltage and are highly reactive to the visible spectrum. The chronoamperometric measurements were conducted to determine whether the Ni-based catalysts could behave as a passivation layer. As shown in Figure S7A, although the Ni(OH)2/n-Si showed the cathodic onset potential shift of 130 mV compared to Ni NPs/n-Si, Ni(OH)2/n-Si was able to withstand 5,000 s and degradation of the photocurrent density was observed; at the same time, Ni NPs/n-Si maintained its performance for 25,000 s without degradation. Ni NPs (branched)/n-Si operated up to 2,000 s and drastically lost performance, as shown in Figure S4C. After the stability test, the surface of Ni(OH)2/n-Si and Ni NPs/n-Si was partially peeled off, resulting in exposure of the silicon surface to the alkaline electrolyte, which accelerates the degradation of photoanodes (Figures S7C–S7H). By introducing relatively stable Ni NPs on highly active Ni(OH)2/n-Si, the stability of Ni NPs/Ni(OH)2/n-Si was improved 2.5 times higher than that of Ni(OH)2/n-Si. Based on these results, integrating both Ni NPs and Ni(OH)2 displays their synergetic effects in catalytic activity and stability. The long-term stability test of heterogeneous Ni NPs/Ni(OH)2/n-Si was then conducted in a mild alkaline K-borate buffer (K-Bi) solution (pH 9.5). As shown in Figure 3F, a remarkable 6-day operation duration of Ni NPs/Ni(OH)2/n-Si was observed without any decay. By comparing the stability of the photoanode in high alkaline 1 M NaOH and mild 1 M K-Bi electrolyte, fascinating long-term stability can be attributed to the suppression of Ni NPs/Ni(OH)2 to be cracked and chemical etching of silicon. After a 6-day operation in 1 M K-Bi, the Ni2+/3+ peak disappeared and showed activated J-V characteristics (Figure S7B). The morphology changes in Ni NPs/Ni(OH)2, such as cracks and pinholes, were observed as shown in Figures S7I–S7K.The PEC performance of photoelectrodes is significantly suppressed when the rate constant of the surface charge transfer (ktrans) is lower than that of the charge recombination (krec). 46 , 47 We conducted intensity-modulated photocurrent sinusoidal spectroscopy (IMPS) to further investigate charge transfer and recombination kinetics, unveiling photocurrent in regard to sinusoidal modulation of DC illumination. IMPS is a useful method for deriving the ktrans and krec of PEC devices, where the apex of the semicircle indicates ktrans + krec and the ratio of the normalized real photocurrent intercepts at low frequency and high frequency reveals a charge transfer efficiency defined as ktrans/(ktrans + krec). Through the apex of the semicircle and the normalized real photocurrent intercepts, it is possible to calculate both ktrans and krec. Figures 4A and 4B show Nyquist plots consisting of the complex photocurrent of Ni(OH)2/n-Si and Ni NPs/Ni(OH)2/n-Si photoanodes at various applied potentials. In both photoanodes, two semicircles were observed due to the presence of surface states. 48 The calculated ktrans and krec of Ni(OH)2/n-Si and Ni NPs/Ni(OH)2/n-Si at water oxidation potential are shown in Figures 4C and 4D. The values of ktrans and krec of Ni(OH)2/n-Si at 1.25 V versus RHE are 0.51 and 12.26 s−1, corresponding to 196.2 and 8.15 ms, and those of Ni NPs/Ni(OH)2/n-Si are 0.8 and 17.71 s−1, corresponding to 124.9 and 5.64 ms. The increase in ktrans and similar krec were observed, which demonstrates that introducing Ni NPs on Ni(OH)2/n-Si expedites charge transfer at the surface. When the n-type semiconductor is illuminated, photoexcited charges are separated in the presence of the space charge layer. The migration of photoexcited charges induces the inverse potential called photopotential in the electrode, reducing the potential across the space charge layer. The band bending of n-type semiconductors is significantly affected by the charge recombination, photoexcited hole accumulation, and intrinsic potential of a semiconductor-liquid junction. Since the extent of band bending for n-type semiconductors serves as a driving force for charge separation and transport, the increase of band bending is beneficial in terms of PEC operation. 22 The open-circuit potential (OCP) difference between dark (OCPdark) and sunlight irradiation (OCPlight) of heterogeneous Ni-based catalysts on Si photoanodes was obtained in 1 M NaOH electrolytes to compare the extent of band bending. As shown in Figure S8, the value of |OCPlight − OCPdark| was 0.21 V for Ni(OH)2/n-Si and 0.18 V for Ni NPs/n-Si. The electrodeposition of Ni(OH)2 and Ni NPs on silicon induce band bending at the semiconductor/catalyst junction. Ni(OH)2 has an electrolyte-permeable structure, in which electronic charges can be balanced by solution ion movement, resulting in a decrease in the electrostatic potential drop at the catalyst/electrolyte interface. 49 Depositing Ni(OH)2 onto Si leads to an adaptive semiconductor/electrocatalyst junction, which can enlarge the effective junction barrier height. 50 In the case of Ni NPs, when metal NPs with a high work function are introduced to the semiconducting silicon surface, inhomogeneous barrier heights are formed by the pinch-off effect. This inhomogeneous contact induces band bending at the interface, where photogenerated charges are moved to a low barrier region to oxidize water. 5 Heterogeneous Ni NPs/Ni(OH)2/n-Si photoanodes showed a value of 0.25 V, which implies that introducing both Ni NPs and Ni(OH)2 enlarges the band bending of the Si photoanodes. Considering that the krec of both Ni(OH)2/n-Si and Ni NPs/Ni(OH)2/n-Si was similar, thin Ni(OH)2 between partially formed Ni NPs and n-Si affects the intrinsic potential of the Si/electrolyte junction. There was a discrepancy between the value of OCP and photovoltage, which is defined as the onset potential difference between photoactive n-Si and metallic p++-Si. The extent of band bending is determined by the difference between the Fermi level of the photoanode and the redox level of the electrolyte, called the built-in potential (Vbi). Although the ideal OCP should be the same as Vbi, the photovoltage is defined by the quasi-Fermi-level difference between electrons and holes. The OCP value is influenced by the PV losses from charge separation, recombination, and so on. 51 Whether catalysts are ion permeable or ion impermeable can affect the barrier height change and OCP. With ion-permeable catalysts, an adaptive junction is formed, in which the barrier height changes depending on the oxidation level of the catalysts. Dense catalysts form a buried junction with constant barrier height. 50 Considering the IMPS results, fast charge recombination kinetics probably affected the smaller OCP value. The oxygen evolution of the Ni NPs/Ni(OH)2/n-Si photoanode was measured by gas chromatography (GC) at 1.5 V versus RHE. As shown in Figure S9A, the blue dots indicate the faradic efficiency value >90%, demonstrating that most of the photogenerated charges are consumed for the OER. The generated amounts of O2 were close to the theoretical value, which is calculated based on the passed charge (Figure S9B). To assess the catalytic ability of Ni NPs/Ni(OH)2 as oxygen-evolving catalysts, turnover frequency (TOF) was calculated by integrating the cyclic voltammetry (CV) curves of Ni2+/3+ to derive the number of Ni active sites (Figure S9C). The calculated TOF of Ni NPs/Ni(OH)2/n-Si was 6.648 × 103 h−1. In summary, heterogeneous Ni NPs/Ni(OH)2 catalysts facilitate charge transfer kinetics and enlarge the band bending at the interfaces, leading to enhanced water oxidation property.The fabricated heterogeneous Ni NPs/Ni(OH)2/n-Si photoanode generated the photovoltage of 500 mV; however, it is not ample enough for driving the spontaneous water splitting reaction. We fabricated a wired tandem cell device consisting of Ni NPs/Ni(OH)2/n-Si photoanodes as a light absorber and a perovskite/Si tandem solar cell as a voltage supplier. Although the ultimate goal of designing a tandem device is a wireless tandem configuration for commercial applications due to the ease of assembly, wireless tandem devices to date still have difficulty in achieving high STH efficiency >10% for commercial applications. 26 , 52 Therefore, we wired high-performance Si photoanodes and perovskite/Si tandem solar cells under parallel illumination to obtain high STH efficiency. The two-electrode system measurement was conducted to evaluate the performance of the front Ni NPs/Ni(OH)2/n-Si photoanodes and the rear perovskite/Si tandem solar cell in 1 M NaOH electrolyte under 1-sun irradiation. The structure of perovskite/Si tandem cells is shown in Figure S10A. Under the two-electrode system, the operating current density (Jop) and operating voltage (Voc) of the cell can be defined. As shown in Figure S10B, the fabricated perovskite/Si tandem solar cell device showed a short-circuit current density (Jsc) of 19.31 mA cm−2 and open-circuit voltage of 1.802 V. The corresponding fill factor was recorded at 77.9% and a power conversion efficiency (PCE) of 27.1% was achieved, as recently reported. 53 The schematic of the wired tandem cell device is shown in Figure 5 A. The active area ratio of the front photoanode to the rear perovskite cell was 1:1. Based on the intersection of the J-V curves of heterogeneous Ni NPs/Ni(OH)2/n-Si photoanodes and a perovskite/Si tandem solar cell, it can be assumed that the photocurrent of 8.8 mA cm−2 can be achieved without external bias (Figure 5B). The actual wired tandem cell device performed for 2 h in chopped light, with an interval of 5 min without a sign of degradation. As shown in Figure 5C, the average photocurrent density of the combined tandem cell was 9.8 mA cm−2, which corresponds to 12% STH efficiency.We successfully demonstrated the synthesis of heterogeneous Ni-based catalysts via electrodeposition on Si substrates and investigated their PEC performances. The morphology of Ni-based catalysts was controlled by changing the precursors in the electrolyte; Ni NPs with spherical and branched shapes and Ni(OH)2 films were obtained. We showed that a combination of Ni NPs and Ni(OH)2 showed synergistic effects on PEC water oxidation by taking advantage of the high catalytic activity of Ni(OH)2 and the high stability of Ni NPs. The high photocurrent density of 29.6 mA cm−2 at 1.23 V versus RHE and stability >140 h was achieved. These results showed the possibility of synthesizing multifunctional heterogeneous catalysts via the electrodeposition method, which permits efficient interfacial charge transport kinetics at the catalyst-electrolyte interface, replacing the high vacuum processes in existence. Finally, we fabricated wired tandem cell devices and evaluated their water oxidation properties. Without external bias, the wired tandem cell generated a photocurrent of 9.8 mA cm−2, which corresponds to an STH efficiency of 12%. Our systematic study reveals that introducing heterogeneous catalysts on Si photoanodes by the low-cost electrodeposition method is an underlying strategy to enhance the PEC characteristics of Si photoanodes. Although silicon photoanodes covered with electrodeposited catalysts may not exert sufficient stability compared to the photoanodes prepared using high vacuum processes, the large photovoltage and high catalytic activity can be achieved without the buried junction. We believe that this study can be a breakthrough in view of the synthesizing catalysts for silicon, opening the doors for the fabrication of high-performance PEC water splitting devices without additional bias.The lead contact for this paper is Ho Won Jang (hwjang@snu.ac.kr).This study did not generate new unique reagents.The authors declare that the data supporting the findings of this study are available within the article and Supplemental Information. All other data are available from the Lead Contact upon reasonable request.Nickel sulfate hexahydrate (NiSO4⋅6H2O), nickel chloride hexahydrate (NiCl2⋅6H2O), nickel nitrate hexahydrate (Ni(NO3)2⋅6H2O), hydrogen peroxide solution (H2O2, 35%), hydrochloric acid (HCl, 35%), sulfuric acid (H2SO4, 95%), potassium nitrate (KNO3), glycine (C2H5NO2), 1 N sodium hydroxide standard solution (1 N NaOH), and 1 N standard potassium hydroxide solution (1 N KOH) were procured from Daejung Chemical. The buffer oxide etchant (6:1) was purchased from J.T. Baker. Boric acid (H3BO3) and dipotassium phosphate (K2HPO4) were sourced from Junsei. Poly(triaryl amine) (PTAA), dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), polyethylenimine (PEIE), and toluene were purchased from Sigma-Aldrich. Formamidinium iodide (FAI), methylammonium bromide (MABr), phenethylammonium iodide (PEAI), and phenethylammonium thiocyanate (PEASCN) were procured from GreatCell Solar. Lead iodide (PbI2) and lead bromide (PbBr2) were sourced from TCI Chemicals.On top of the silicon bottom cell with a 20-nm-thick ITO recombination layer, a PTAA/perovskite/C60 layer was sequentially deposited. PTAA solution (5 mg/mL in toluene) was spin coated at 6,000 rpm for 25 s and annealed at 100°C for 10 min. Perovskite solutions were prepared by dissolving FAI, MABr, CsI, PbI2, and PbBr2. Their molar ratios were adjusted to form stoichiometric (FA0.65MA0.20Cs0.15)Pb(I0.8Br0.2)3 in a DMF and NMP (4:1 [v/v]) mixed system. An additive 2D perovskite solution was prepared by adding 2 mol% Pb(SCN)2 and 2 mol% PEAX (X = I, SCN) to the 3D perovskite solution. The solution was spin coated at 4,000 rpm for 20 s on PTAA film. Subsequent immersing of spin-coated film in diethyl ether (DE) for 30 s was conducted. The color of the films turned dark brown, which indicates that perovskite films were crystallized. Then, the films were annealed at 100°C for 10 min. C60 layers (C60, bathocuproine [BCP], Ag electrode) were deposited by the thermal evaporator. A 0.2 wt% of PEIE (80% ethoxylated) solution in methylalcohol was spin coated at 6,000 rpm for 30 s. ITO films were deposited on the C60/PEIE layer using radiofrequency sputtering at room temperature (working pressure: 2 × 10−3 mTorr). A 150-nm-thick Ag metal grid was deposited using a thermal evaporator on the ITO film.An n-Si (100) wafer (1–10 Ω cm) was cut into 1.5 × 1.5 cm2 pieces. The wafers were cleaned with acetone, isopropanol alcohol, and ultrapure water by ultrasonication. To remove residual contaminants, Si pieces were cleaned using a piranha etching process; soaked in 3/1 v/v concentrated H2SO4/H2O2 solution for 10 min, and immersed in a buffered HF etchant for 30 s. Then, wafers were soaked in 5/1/1 (by volume) concentrated H2O, HCl, and H2O2 at 80°C for 30 min. The Si pieces were rinsed with ultrapure water and dried under a N2 flow.Before electrodeposition, ohmic contact was formed by scratching the backside of silicon and applying an InGa alloy (Sigma-Aldrich). Then, copper wire was attached by depositing silver paste on the InGa alloy. After the silver paste dried, the Si surface was covered with adhesive tape, except for the active area (1 × 1 cm2), to prevent contact with the electrolyte. n-Si was soaked for 30 s in the buffer oxide etchant (6:1, J.T. Baker) to remove the residual SiO2 layer and organic solvent from the surface. Then, Ni-based catalysts were deposited on the silicon surface by electrodeposition. The electrodeposition of Ni-based catalysts was conducted in a standard three-electrode system: an encapsulated Si electrode as the working electrode, a Pt mesh as the counter electrode, and a SCE as the reference electrode. In the case of Ni NPs (sphere), Ni aqueous plating solution was prepared by dissolving 0.1 M nickel sulfate hydrate (NiSO4⋅6H2O, Daejung) and 0.1 M boric acid (H3BO3, Junsei). Pulsed electrodeposition was conducted by applying −8 mA cm−2 for 2 s and −0.8 mA cm−2 for 20 s. For the deposition of Ni NPs (branched), 0.1 M nickel chloride hydrate, 0.3 M glycine, and 0.6 M K2HPO4 were dissolved in deionized (DI) water, and a plating solution was kept at 65°C. The electrodeposition of Ni NPs (branched) was conducted by applying −0.5 mA cm−2 for 40 s. Ni(OH)2 films were synthesized by dissolving 0.004 M nickel nitrate hydrate (NiNO3⋅6H2O, Daejung) and 0.01 M KNO3 (Daejung). The electrodeposition of Ni(OH)2 was conducted by applying −0.15 mA cm−2 for 100 s. After electrodeposition, the Si pieces were rinsed with DI water, dried under nitrogen gases, and adhesive tapes were detached.After electrodeposition of catalysts on the Si surfaces, Si pieces were processed to fabricate the electrodes. The backside of the Si surface was scratched, and InGa eutectic was applied to establish ohmic contact. The silver paste was deposited on the contact and Cu wire. After the silver paste layer dried, the whole surface of the silicon except the active area was covered with an epoxy-based resin. The samples were dried for 12 h in air to cure the resin.PEC measurements (Ivium Technologies, Nstat) were performed with a three-electrode system using Ag/AgCl (saturated) reference electrode and a Pt plate as a counter electrode in 1 M NaOH electrolyte (pH 14). A Xe arc lamp (Abet Technologies, LS150) was used as a light source, and the light intensity was calculated to 1 sun (100 mW cm−2, AM 1.5 G) using a reference photodiode. For measuring PEC performance, the potential was swept toward the anodic direction and the quartz vessel was used to avoid UV absorption. The IPCE was carried out using a monochromator and light source. The applied potential was 1.23 and 1.5 V versus RHE to compare the effect of applied voltage on samples. EIS was conducted near the onset potential (1.02 V versus RHE for Ni(OH)2/n-Si and Ni NPs/Ni(OH)2/n-Si, 1.15 V versus RHE for Ni NPs/n-Si). The EIS data were fit to the equivalent circuits, which were discussed in the text, using the Z plot 2.x software. The sweeping frequency ranged from 250 kHz to 0.1 Hz using alternating current with an amplitude of 10 mV. The measured potential versus Ag/AgCl was converted to the RHE scale according to the Nernst equation: (Equation 1) E R H E = E A g / A g C l + 0.059 pH × E A g / A g C l O where E RHE is the converted potential versus RHE, E O Ag/AgCl  = 0.198 V at 25°C and E Ag/AgCl is the experimentally measured potential versus the Ag/AgCl reference. The GC system (Agilent GC 7890B) was used to calculate the faradic efficiency and amount of generated oxygen. Two-compartment electrochemical cells consisting of glass body and quartz glass windows was used for GC measurements. The amounts of H2 and O2 were measured. The LSV curves in 1.0 M NaOH with or without 0.5 M Na2SO3, which was used as the hole scavenger, were used to calculate the charge injection efficiency (Φinj) at the electrode-electrolyte interface: (Equation 2) J P E C = J a b s × Φ s e p × Φ i n j (Equation 3) J N a 2 S O 3 = J a b s × Φ s e p where JPEC is the observed photocurrent density and Jabs is the photocurrent density, assuming that absorbed photons are converted to current completely. The TOF was calculated by the produced moles of oxygen and the number of moles of the active sites; produced moles of oxygen were determined by GC, and the number of moles of the active sites was calculated by integrating the CV curve of Ni2+/3+ of 0.78–0.9 V versus RHE. A scan rate of 10 mV s−1 was used. The turnover number (TON) and TOF are defined as follow: (Equation 4) T O N = n o x y g e n n N i (Equation 5) T O F = T O N t IMPS measurements were conducted under white light illumination, with 10% modulation intensity using a potentiostat (PP211, Zahner) and EC workstation (Zennium, Zahner) in the three-electrode configuration. The frequency of the modulation was swept from 100 kHz to 0.1 Hz. The ktrans and krec can be calculated as follows: (Equation 6) k t r a n s + k r e c = 2 π f (Equation 7) k t r a n s = t r a n s f e r e f f i c i e n c y × ( k t r a n s + k r e c ) STH efficiency (η STH ) was calculated under the two-electrode measurement as follows: (Equation 8) η S T H = J o p × 1.23 V × η F P i n where J op is the current density (mA cm−2) at zero bias and η F is the faradic efficiency. The STH efficiency was calculated using the faradic efficiency of the Si photoanodes.The morphology of the Ni-based catalysts was examined by FESEM (MERLIN Compact, JEISS) and TEM (JEM-2100F, JEOL). XPS (AXIS-His, Kratos) analysis was carried out to confirm the surface bonding of Ni(OH)2, Ni NPs, and Ni NPs/Ni(OH)2 on a Si substrate. To clarify the exact bonding form of Ni and O, the narrow Ni and O spectrum was analyzed using CASA XPS.This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Ministry of Science and ICT (MSIT) (2019M3E6A1103818), the Korean Ministry of Science, ICT and Future Planning (MSIP) (2012R1A3A2026417), the Basic Science Research Program through an NRF grant funded by MSIP (2017R1A2B3009135), the Creative Material Discovery Program through an NRF grant funded by the MSIT (2018M3D1A1058793), the Basic Research Laboratory of the NRF funded by the MSIT (2018R1A4A1022647), and Korea Hydro & Nuclear Power Co., Ltd. (no. 2018-Tech-21). S.A.L. acknowledges the Global Ph.D. Fellowship Program through the National Research Foundation of Korea funded by the Ministry of Education (2017H1A2A1044293).S.A.L. fabricated all of the Si photoanodes and wrote the manuscript. I.J.P. fabricated the perovskite/Si tandem solar cells. J.W.Y. assisted in the PEC measurements. J.P. measured the IMPS of the photoanodes. T.H.L. carried out the TEM analysis. C.K. performed the GC measurements. J.M. and J.Y.K. participated in the discussion of the data and wrote the manuscript. H.W.J. led the overall project.The authors declare no competing interests.Supplemental Information can be found online at https://doi.org/10.1016/j.xcrp.2020.100219. Document S1. Figures S1–S10 and Table S1 Document S2. Article plus Supplemental Information

Selecting moderate semiconducting materials for photoelectrochemical (PEC) cells is essential to achieve high solar-energy conversion. Despite the advantageous features of silicon, such as earth abundance and narrow band gap, silicon suffers from severe photocorrosion. Here, heterogeneous nickel-based catalysts produced via electrodeposition are investigated to expedite water oxidation and protect the silicon from corrosion. The morphology of the catalysts and their PEC performances are demonstrated in detail. Synergistic Ni nanoparticles (NPs)/Ni(OH)2/n-Si photoanode shows a high photocurrent density of 29.6 mA cm−2 at 1.23 V versus RHE and operates over 140 h. Owing to the insufficient photovoltage generated by a single photoanode, we introduce a perovskite/Si tandem solar cell as a voltage supplier. The fabricated wired tandem device shows a photocurrent density of 9.8 mA cm−2, corresponding to a solar-to-hydrogen (STH) conversion efficiency of 12% without external bias. Our work may present a promising pathway toward a design of spontaneous energy conversion devices.

Data will be made available on request.Nowadays, chemical production is heavily dependent on fossil resources. Alternatives are required for sustainable development and the use of renewable carbon sources such as biomass is an attractive option. In particular, lignin is interesting for this purpose, as nowadays it is mainly used for energy generation and thus highly underutilized. Due to its oxygenated propylphenolic backbone structure, it could serve as an attractive source for phenol and alkylphenols, which are currently produced from fossil resources on a large scale [1]. Examples of strategies that have been proposed and explored to obtain low molecular weight phenols from lignin are acid/base catalyzed depolymerizations [2–5], fast pyrolysis [6], and reductive catalytic fractionation [7,8]. However, the obtained monomeric products are usually highly functionalized, for example by methoxy groups, hindering their direct utilization [9]. As a result, demethoxylation is needed to funnel the product mixtures into products with reduced complexity and increased value [10,11].A well-known demethoxylation strategy involves catalytic hydrotreatment with metal-based catalysts in combination with hydrogen. Exploratory catalyst studies for demethoxylation have been reported using heterogeneous catalysts in metallic, oxide, sulfide, phosphide, nitride or carbide form. Precious metal catalysts, such as Au/TiO2 and carbon-supported Pt and Pd catalysts, show excellent performance for the conversion of (propyl)guaiacol to demethoxylated phenols and 80 + % selectivities have been reported at conversions > 97% [12–14]. However, based on green chemistry and engineering principles, there is a need for the use of cheaper and more abundantly available metal catalysts [15]. A number of studies have been reported for the demethoxylation of guaiacols using Ni, Fe and Mo bases catalysts and an overview is given in Table 1. Both batch and continuous set-ups have been used and typical temperatures are between 285 and 400 °C, with hydrogen pressures between 1 and 90 bar. Ni catalysts, either supported on TiO2 or SiO2, particularly gave good results for the demethoxylation of 4-n-propylguaiacol (89% selectivity at 95% conversion, 285 °C, 1 bar) whereas worse results were obtained using guaiacol [16,17]. Various types of Mo based catalysts have also been used for catalytic demethoxylation, ranging from bulk MoO3 to Mo-oxides, sulphides, nitrides and phosphides on various supports (AC, SiO2, and SBA-15). When considering bulk MoO3, good performance was obtained when using guaiacol in batch set-ups though performance dropped considerably in continuous fixed-bed reactors. In addition, selectivity to phenols was limited and large amounts of fully deoxygenated products (e.g., benzene and toluene) were obtained [18,19]. When considering supported Mo- catalysts, excellent performance was found for MoP/SiO2 when using 4-n-propylguaiacol as the feed (350 °C, 90 bar) [20–22]. Good results were also achieved with the Fe based catalysts such as FeOx/CeO2 with conversion and selectivity levels similar to those for Ni catalysts, though catalyst stability was fair with up to 41% reduction in activity over a time on stream (TOS) of 10 h (400 °C, 1 bar) [23].The use of supported Cu catalysts for the guaiacol demethoxylation with high selectivity is limited (Table 1). The best results were reported in batch using Cu/AC as the catalyst [33]. Performance was reasonable, with 79% selectivity to demethoxylated products though at a low conversion of 24% (350 °C, 50 bar). Only one study reports the use of a continuous reactor set-up in combination with Cu/C, though only 3.6% guaiacol conversion was reported (350 °C, 40 bar, Table 1). Furthermore, support studies, stability testing, and demethoxylation of crude bioliquids (enriched in guaiacols) in continuous set-ups has not been explored systematically for supported Cu catalysts. To overcome this knowledge gap, a series of supported Cu catalysts were investigated for the demethoxylation of guaiacol. The effect of inorganic supports (SiO2, ZrO2, TiO2 (various forms), MoO3-ZrO2, and MoO3-TiO2), solvent (toluene, n-octane), WHSV, and temperature on catalyst performance (activity, selectivity, and stability) was studied. In addition to common inorganic supports (viz. SiO2, ZrO2, and TiO2), MoO3-ZrO2 and MoO3-TiO2 were also investigated because the introduction of MoO3 on ZrO2 or TiO2 is known to have a positive effect on catalyst performance for the hydrodeoxygenation of lignin-derived monomers (e.g., m-cresol, anisole, and guaiacol) [36–38]. The Mo species are supposed to form coordinatively unsaturated sites during reactions, which promote the HDO pathway, possibly via an oxygen vacancy-driven mechanism [36]. Besides guaiacol, also other relevant feeds such as 4-n-propylguaiacol, and a guaiacol mixture isolated from pyrolysis oil were tested using the best Cu catalyst in the series.SiO2 (99.5% trace metals basis, 10–20 nm particle size), ZrO2 (< 100 nm particle size), TiO2 (P25, ≥ 99.5% trace metals basis, 21 nm primary particle size), anatase TiO2 with a low surface area (denoted as LSA, ≥ 99%, average diameter of 156 nm), anatase TiO2 with a high surface area (denoted as HSA, 99.7% trace metals basis, < 25 nm particle size), rutile TiO2 (99.5% trace metals basis, < 100 nm particle size), silicon carbide (about 200 mesh particle size), copper nitrate trihydrate (99–104%), ammonium molybdate tetrahydrate (99.98% trace metals basis), guaiacol (≥ 99%), 4-n-propylguaiacol (≥ 99%), 4-n-propylphenol (99%), 2-sec-butylphenol (98%), p-propyl anisole (≥ 99%), benzene (≥ 99.7%), and cyclohexene (99%) were supplied by Sigma-Aldrich. n-Octane (≥ 98%) was obtained from Alfa Aesar. Toluene (≥ 99%) and tetrahydrofuran (THF) stabilized with BHT (for analysis) were obtained from Avantor and Boom B.V., respectively. Phenol (> 99.5%), 4-methylphenol (> 99.0%), 2,3-dimethylphenol (> 98.0%), 2,3,6-trimethylphenol (> 98.0%), 1,2-dimethoxybenzene (> 99.0%), 4-methylguaiacol (> 98.0%), catechol (> 99.0%), n-propylbenzene (> 99.0%), n-decane (> 99.0%), and n-dodecane (> 99.0%) were purchased from TCI. p-Xylene (99%), m-xylene (99%), and o-xylene (99%) were obtained from abcr GmbH. High purity (> 99.99 mol%) N2, He, and H2 were purchased from Linde. Certified gas mixtures used in this study, including 1 vol% O2/N2 5 vol% H2 in Ar, and 10 vol% NH3 in He were also supplied by Linde. A guaiacols enriched feed from pyrolysis oil fractionation was provided by the Biomass Technology Group B.V. [39].Supported Cu catalysts (5 wt%) were prepared using an incipient wetness impregnation method by the following procedure. Copper nitrate trihydrate (0.1053 g) was dissolved in an amount of Milli-Q water equal to the measured pore volume for water of the support. Then, the solution was added dropwise to the support (2.000 g) with mixing at room temperature. After 6 h, the mixture was dried at 100 °C for 12 h, followed by calcination at 400 °C (2 °C min−1) for 2 h in static air. The catalysts were pelletized, crushed, sieved, and the 100–200 µm fraction was used in the experiments. For catalyst characterization purposes, the Cu catalysts were reduced ex situ under a H2 flow (200 mL/g Cat, 350 °C, 2 h) and then passivated under 1% O2/N2 (room temperature, 2 h). The MoO3-ZrO2 and MoO3-TiO2 (P25) mixed oxides supports with ‘sub-monolayer’ coverage of MoO3 (about 4 Mo/nm2) were also synthesized by an incipient wetness impregnation method [38]. This involved dissolving the appropriate amount of ammonium molybdate tetrahydrate in Milli-Q water, support impregnation, drying, and calcination at 550 °C (2 °C min−1) for 4 h in static air. The nominal MoO3 content in the MoO3-ZrO2 and MoO3-TiO2 (P25) are 3 and 5 wt%, respectively.The catalytic hydrotreatment experiments with lignin-derived model compounds and a guaiacols enriched feed were carried out in a continuous down-flow fixed-bed reactor (stainless steel) with an outer diameter of 6.35 mm and an inner diameter of 4.55 mm. Typically, the supported Cu catalyst (100 mg) was mixed with SiC (200 mg). The mixture was loaded into the reactor and reduced in situ at 350 °C for 2 h under a H2 flow (20 mL min−1). An experiment was started by heating the reactor to the desired temperature, followed by increasing the pressure to 10 bar using a back pressure valve using a H2 flow (20 mL min−1). Subsequently, the H2 flow rate was set to the desired value (typically 10 mL min−1) and feeding of the reactant (5 wt% guaiacol in n-octane or toluene, 5 wt% 4-n-propylguaiacol in toluene, or 5 wt% crude feed in toluene) at the desired Weighted Hourly Space Velocity (WHSV, h-1) was started using an HPLC pump. The WHSV was calculated based on the feed flow rate (reactant and solvent) and the catalyst intake as shown in Eq. (1). (1) WHSV = F g / w Where Fg is the flow rate of the feed (g/h) and w is the weight of catalyst (set at 0.100 g). Aspen plus simulations indicate that all reactions were carried out in the vapor phase, see supporting information for details.The reaction products were separated in a gas-liquid separator at room temperature, and condensable products were collected at 1 h intervals. The feed and liquid product composition were analyzed offline using GC-MS (Agilent 6890 series GC system equipped with an HP973 mass detector) and GC-FID [Agilent 8860 gas chromatograph equipped with a flame ionization detector and an HP-5 capillary column (30 m x 320 µm x 0.25 µm)]. n-Dodecane was added to the feed and used as an internal standard (IS). The liquid products were diluted about 20 times using THF containing 500 ppm n-decane as an IS before analyses.Conversion of feed component g and selectivity for a product i were calculated on a molar basis using Eqs. (2) and (3). (2) X g % = ( C g , 0 − C g ) / C g , 0 × 100 (3) S i % = C i / ( C g , 0 − C g ) × 100 Here Cg,0, is the molar fractions of guaiacol or 4-n-propylguaiacol in the feed, Cg is those molar fractions in the product mixture, and Ci is the molar fraction of a product i.The molecular weight distributions of the feed and liquid product after hydrotreatment were determined by gel permeation chromatography (GPC) analyses using an Agilent HPLC 1100 system equipped with three MIXED-E columns (length 300 mm, i.d. 7.5 mm) in series and a GBC LC 1240 refractive index detector (RID). All samples were diluted by THF to a concentration of about 10 mg mL-1, and toluene was used as internal reference.The reproducibility of a selected set of experimental conditions was determined and shown to be good (Fig. 5).Inductively coupled plasma-optical emission spectrometry (ICP-OES) was used to quantify the Cu content in the supported Cu catalysts. Before analyses, the samples were dissolved in aqua regia (HF was used for Cu/SiO2), and then the Cu content was determined using an PerkinElmer Optima 7000 DV equipped with a solid-state CCD array detector.H2-TPR measurements were conducted using a Micromeritics AutoChem 2920 system, equipped with a thermal conductivity detector (TCD). Samples (about 100 mg) were pretreated under Ar at 400 °C for 1 h and then cooled to 50 °C. After that, the H2-TPR profiles were collected under 5 vol% H2 in Ar (30 mL min-1) from 50 °C to 800 °C using a temperature ramp of 10 °C min-1.Surface acidity was quantified by NH3-TPD measurements, which were carried out on a Micromeritics AutoChem 2920 system. The passivated Cu catalysts (about 100 mg) were reduced under 5 vol% H2 in Ar at 350 °C for 1 h. Then a gas mixture containing 10 vol% NH3 in He (50 mL min-1) was used to saturate the acid sites at 100 °C for 1 h. After that, the samples were purged under a He flow until the baseline was stable. TPD measurements were carried out from 100° to 600°C at a rate of 10 °C min−1 under a He flow (50 mL min-1).X-ray diffraction (XRD) patterns were collected using a Bruker D8 Advance diffractometer, operating at 40 kV and 40 mA using Cu-Kα radiation (λ = 1.5544 Å). Data were collected using a coupled 2θ / θ configuration, between 2θ values of 5–80° with a step size of 0.02 and a scan time of 1.000 s.The specific surface area and pore properties of the supported Cu catalysts were determined by N2 physisorption, conducted at 77 K using a Micromeritics ASAP 2420 system. Before analysis, the samples were degassed at 250 °C under vacuum for 6 h. The specific surface area was determined using the Brunauer-Emmett-Teller (BET) method at relative pressures (P/P0) ranging from 0.05 to 0.25. The total pore volume was obtained from the single-point desorption point at a relative pressure of 0.98. The pore diameter was calculated using the adsorption branch from the N2 isotherm according to the Barrett-Joyner-Halenda (BJH) method. This approach eliminates tensile strength effects, which lead to an artificial peak at 4 nm in the pore size distribution [40].The microstructure of the Cu/TiO2 catalysts was examined with a probe and image aberration corrected Themis Z microscope (Thermo Fisher Scientific) operating at 300 kV equipped with a Ceta camera. The samples were first ultrasonically dispersed in ethanol and then deposited on a carbon-coated gold grid. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the Cu/TiO2 catalysts were obtained using the same microscope in STEM mode with a convergence semi-angle of 26 mrad and a probe current of 100 pA. Energy dispersive X-ray spectroscopy (EDS mapping) results were achieved with a Dual X EDS system (Bruker) with a probe current of 250 pA. Data acquisition and analysis were done using Velox software (version 2.8.0).The coke amount on the spent catalyst were determined using thermogravimetric analysis (TGA) by a TGA 4000 from PerkinElmer. The samples were heated in an air flow (30.0 mL min-1) from 50° to 900°C with a heating rate of 10 °C min-1, and the coke amount was obtained from the TG curves.The oxygen vacancy concentration of the support was quantified by oxygen storage capacity measurements on a Micromeritics AutoChem 2920 system. Firstly, the sample (0.06–0.34 g) was pretreated in a 5 vol% H2 in Ar stream (25 mL min-1) at 550 °C for 2 h. Afterward, the sample was purged with He until a constant baseline was obtained. Then, successive pulses of 10 vol% O2 in He were injected into the sample at 550 °C until O2 consumption ceased. The O2 uptake was used to calculate the concentration of oxygen vacancies on the sample by assuming that one O2 molecule saturates two oxygen vacancies [25,36].A series of supported Cu catalysts on non-reducible (SiO2 and ZrO2) and reducible supports (TiO2, MoO3-ZrO2, and MoO3-TiO2) was prepared using an incipient wetness impregnation method, all with about 5 wt% Cu (determined by ICP-OES, see Table S1). The focus of this study is the use of TiO2 supported Cu catalysts (Cu/TiO2-P25, Cu/TiO2-A HSA, Cu/TiO2-R, and Cu/MoO3-TiO2), which were characterized in detail using H2-TPR, NH3-TPD, XRD, N2 physisorption, HAADF-STEM, and TGA (for spent catalyst) to rationalize catalyst performance ( Table 2).The N2 physisorption data reveals that the TiO2 supported Cu catalysts have a specific surface area of 27–61 m2/g, pore volume of 0.25–0.37 cm3/g, and pore diameter 19–21 nm. The Cu/TiO2-A HSA shows the highest specific surface area as well as lowest pore volume and pore diameter among these four catalysts [41].H2-TPR was used to determine the reducibility of catalysts and the results are given in Fig. 1. Two or three peaks are present in all samples in the 100–300 °C region, which are associated with the reduction of Cu-oxides. The lower temperature peak between 136 and 198 °C is assigned to the reduction of monomeric Cu species directly interacting with the support, whereas the second peak at higher temperatures (195–296 °C) is believed to be due to the reduction of bulk CuO on the support [42,43]. For Cu/TiO2 on P25, two peaks are observed in this region due to reduction of bulk CuO with different particle sizes [42,44]. For Cu/TiO2-A HSA, the reduction peak for bulk CuO shifted to lower temperature and merged with that of monomeric Cu species, indicating the presence of low amounts of bulk CuO and a high dispersion of Cu species. Based on these data, it was deemed sufficient to reduce the catalysts before reaction at 350 °C for 2 h (pure H2) to ensure quantitative reduction of Cu species to metallic Cu.A distinct reduction peak centered at 383 °C is observed for Cu/TiO2-A HSA, which is associated to reduction of support (TiO2-A HSA) [45]. This reduction step is promoted by the presence of Cu, as the support alone shows only a high temperature reduction peak at about 674 °C (Fig. S1). The Cu/MoO3-TiO2 also showed a clear peak at 737 °C, which is due to the reduction of Mo species [46].XRD was used to examine the various phases and crystallinity of the Cu catalysts (reduced at 350 °C for 2 h under pure H2), and the XRD patterns are shown in Fig. 2. Clear peaks of the various TiO2 crystal phases are present at 2θ = 25.3° (anatase (101)) and 2θ = 27.4° (rutile (110)). Integration shows that the TiO2-P25 contains about 16 wt% of the rutile phase, in line with literature data [47–49].In three of the samples (Cu/TiO2-P25, Cu/TiO2-R and Cu/MoO3-TiO2-P25), clear peaks from Cu species were present at 43.3° and 50.4°, corresponding to the (111) and (200) planes of Cu, respectively (JCPDS PDF No. 04–0836). For Cu/TiO2-A HSA Cu peaks are absent, indicating that the Cu species are highly dispersed, which is in line with the TPR results.The average crystallite size of the Cu species in the samples showing clearly visible Cu peaks were determined using the Scherrer equation and the results are given in Table 2 [50]. The average Cu size varies between 27 and 81 nm, the smallest for Cu/TiO2-P25, and the largest for Cu/MoO3-TiO2. The differences in Cu crystal sizes may be due differences in the metal-support interactions as well as the specific surface area of the materials (Table 2).Surface acidity was quantified by NH3-TPD, and the profiles are shown in Fig. 3. The surface acidity decreases in the order: Cu/TiO2-A HSA > Cu/TiO2-P25 > Cu/MoO3-TiO2 > Cu/TiO2-R. The Cu/TiO2-P25, Cu/TiO2-A HSA, and Cu/TiO2-R mainly show weak acidic sites, while the Cu/MoO3-TiO2 mainly contains strong acidic sites.High-angle annular dark-field STEM (HAADF-STEM) was conducted to investigate the morphologies and particle size of Cu species for the fresh Cu/TiO2-P25 and Cu/TiO2-A HSA, and distinct differences were found between these two samples. The Cu nanoparticles (small bright ones) are highly dispersed on the TiO2-P25 ( Figs. 4a and 4b), with an average diameter of 2.6 nm. Beside these small Cu particles, larger ones (> 10 nm) are also present (confirmed by the EDS mapping) on the fresh Cu/TiO2-P25 (Fig. S2). The presence of these larger Cu particles is consistent with the XRD results, which showed an average particle size of 26 nm. Based on these measurements we conclude that the Cu species on the support have a bimodal distribution.In contrast to Cu/TiO2-P25, the HAADF-STEM images of Cu/TiO2-A HSA reveal the presence of hollow TiO2 spheres and small Cu nanoparticles (Fig. S3). The average diameter of the Cu nanoparticles is about 1.6 nm, which is somewhat smaller than that in Cu/TiO2-P25. In addition, the XRD data for Cu/TiO2-A HSA show that larger Cu particles are absent, and imply that the Cu size distribution is not bimodal and that only smaller Cu particles are present.A series of supported Cu catalysts on non-reducible (SiO2 and ZrO2) and reducible supports (TiO2, MoO3-ZrO2, and MoO3-TiO2) was prepared. Four different types of TiO2 were investigated: anatase (both high and low surface area, denoted as TiO2-HSA and TiO2-LSA, respectively), rutile (TiO2-R) and P25. Initial catalytic screening experiments to assess support effects were performed using guaiacol in octane (5 wt%) as the feed in a continuous fixed bed reactor at 300 °C, a WHSV of 16 h-1 and 10 bar pressure. Experiments were run for a TOS of at least 3 h to ensure steady state operation and the conversions at 3 h were used for evaluation. The conversion of guaiacol and the selectivity to individual products are shown in Fig. 5 (see also Fig. S4).The main products are the desired (alkylated)phenols such as the parent phenol, methyl- and dimethylphenols formed by demethoxylation and methyl transfer, with minor amounts of methoxybenzene and dimethoxybenzene ( Scheme 1). The balance closure on molar basis is not quantitative, and this is likely due to the formation of some gas-phase components, and GC-undetectable oligomers. The presence of the latter was confirmed by GPC measurements showing a small peak at higher molecular weight values (Fig. S5).Strong support effects were observed on catalyst performance (300 °C, 10 bar, and WHSV of 16 h-1). The conversion for Cu/SiO2, Cu/TiO2-A LSA (low surface area anatase TiO2), Cu/TiO2-R (rutile), and Cu/MoO3-ZrO2 are all below 15%. Better results were obtained using Cu/ZrO2, Cu/TiO2-P25, and Cu/TiO2-A HSA (high surface area anatase TiO2) with conversion levels between 20% and 30%. The by far most active catalyst is Cu/MoO3-TiO2 with a guaiacol conversion of 70%.Blank experiments with support only (TiO2-P25 or MoO3-TiO2) were performed and the results are shown in Fig. S6. It was found the TiO2-P25 has limited activity (conversion of 8%), while the MoO3-TiO2 (P25) is much more active (conversion of 31%). However, performance of the bare supports is considerably less than for the supported Cu catalysts, indicating that Cu plays a major role in the catalytic cycle (vide infra).It is of interest to compare the activity data for the TiO2 supported catalysts in the series. Among them, guaiacol conversion increases in the order anatase (LSA) < rutile < anatase (HSA) < P25. The latter consists of a mixture of anatase and rutile phases and this appears to be favored when considering activity. Mo doping has a very positive effect on conversion, which is particularly evident when comparing Cu/TiO2-P25 (conversion of 29%) and Cu/MoO3-TiO2 (conversion of 70%). The promoting effect of Mo doping on titania supports is in line with recent results from our group [38] and was speculated to be due to i) the formation of additional oxygen vacancies which promote the deoxygenation reaction and/or ii) the formation of active Mo oxycarbide or Mo oxycarbohydride phase.However, activity is not the only catalyst performance criterium and selectivity should be considered as well. Regarding product distribution, demethoxylated phenols like phenol, methylphenols and dimethylphenols are the major products. In addition, small amounts of methoxybenzene and dimethoxybenzene were detected. This product distribution indicates that demethoxylation in combination with methyl transfer are the dominant reactions (Scheme 1). Selectivity to non-aromatics is low, indicating limited overhydrogenation of the aromatic rings [51,52]. Methyl transfer to form methylated phenols is assumed to be catalyzed by acid sites on the catalyst via Friedel−Crafts-type reactions with a carbonium ion (CH3 +) as an intermediate [23,53,54].For a proper comparison of the best catalysts in the series in terms of selectivity at equal conversion, the WHSV was tuned to obtain a conversion of about 40% for three of the most active catalysts (300 °C, 10 bar). The results are given in Fig. 5 (right). The highest selectivity to alkylated phenols (76%) was obtained using Cu/TiO2-P25, and only minor amounts of dimethoxybenzene was formed. The parent phenol selectivity in this case is up to 55%. The amounts of dimethoxybenzene are considerably higher for the Cu/MoO3-TiO2 catalyst. As such, we can conclude that i) the TiO2 based catalysts (e.g., TiO2-P25, TiO2-LSA, and MoO3-TiO2 supported Cu catalysts) perform best and ii) the best catalyst in terms of selectivity towards alkylated phenols is Cu/TiO2-P25.When considering the catalytic performance data for the TiO2 supported catalysts, guaiacol conversion increases in the order anatase (LSA) < rutile < anatase (HSA) < P25. Thus, a mixture of anatase and rutile phases as in P25 TiO2 appears to be favored when considering catalyst activity. Friedrich et al. have proposed that the interface and particularly the disorder of the TiO2 lattice between the anatase and rutile phase has a positive effect on catalyst performance and plays a major role [55]. In addition, mixed-phase TiO2 was shown to have superior photocatalytic activity [56,57], which was ascribed a larger charge separation of the anatase-rutile phase junction [57,58]. It could be one of the possible explanation for the good performance of the P25 based catalyst, as there is close contact between the anatase and rutile phases in P25 TiO2 [49].However, catalyst characterization studies showed that other properties like surface area, Cu particle size, and acidity are also different for the set of catalysts and may affect performance. Attempts have been undertaken to find correlations between these catalyst features and performance like conversion and selectivity. Unfortunately, no clear relations between these relevant properties and performance was obtained. This implies that other factors play a role as well and may interfere. Oxygen vacancies (Ov) have been proposed to play a role in the molecular mechanism for the demethoxylation of guaiacol (or anisole) when using supported catalysts with reducible supports like MoO3 and TiO2 [18,45,59,60]. For instance, Liu et al. performed studies on Ag/TiO2 as a demethoxylation catalyst and proposed that Ov are formed by the reduction of the TiO2 surface by spillover hydrogen [45].Unfortunately, the amounts of oxygen vacancies on the supports of the Cu based catalysts could not be quantified by oxygen storage capacity measurement due to the presence of Cu. To get some information on the possible involvement of oxygen vacancies, the oxygen storage capacity of the support (TiO2-P25, TiO2-A HSA, TiO2-R, and MoO3-TiO2) without Cu was quantified. A clear trend was found between the oxygen vacancies of the support and the conversion of guaiacol at a WHSV of 16 h-1 ( Fig. 6), indicating these oxygen vacancies may play an important role in the catalytic cycle.In addition, the presence and levels of impurities in the different types of titania supports may also affect performance. P25 TiO2 shows the presence of about 180 ppm of trace metals, among others K, Ag, Pd, Pt and Sb (Table S2). The total amount of trace elements in TiO2-A HSA is much higher (1018 ppm), the majority being Na. The presence of Na in anatase TiO2 was shown to be detrimental for the photocatalytic degradation of certain substrtaes (malachite green oxalate and 4-hydroxy benzoic acid) [61]. A such, differences in type and levels of impurities in the titania supports may also affect catalyst performance for the demethoxylation reactions reported in this study.Based on literature precedents and the experimental correlation between oxygen vacancies and conversion [45], a mechanism is proposed for the guaiacol demethoxylation on Cu based catalysts ( Fig. 7). It involves two active sites, viz., Cu and oxygen vacancies close to the Cu sites. It assumes that hydrogen is activated and chemosorbed on reduced Cu sites, and after spill over create oxygen vacancies on the TiO2 surface. Interaction of the substrate with these oxygen vacancies leads to demethoxylation [23,45]. The mechanism for the formation of alkylated phenols is still under debate. Either the initially formed phenol reacts with guaiacol to form methylated phenols and catechol [45,51] or Friedel-Crafts-type reactions occur with the involvement of CH3 + species that are known to be active in intra- or inter-molecular methylation reactions [23,53]. The formation of oligomers, minor byproducts, may be related to the presence of acidic sites on the surface.The stability of the Cu/TiO2-P25 catalyst was determined in the continuous set-up for an experiment at a TOS of 100 h (300 °C, 10 bar, and WHSV of 16 h-1). Excellent stability was demonstrated with a near constant conversion level of about 27% ( Fig. 8, detailed selectivity data are given in Fig. S7). The high stability of the Cu catalyst is remarkable as catalyst deactivation is significant for the catalytic hydrodeoxygenation of lignin-derived guaiacols to phenols or BTX [14,23,29,53,62,63]. As such, these Cu-based catalysts combine good activity and selectivity with stability which is highly beneficial when considering potential industrial application.The desired demethoxylated phenols like phenol and methylated phenols are the major products with a selectivity of about 79% throughout the run. The only major side product was dimethoxybenzene, in line with the exploratory runs at much shorter TOS’s (vide supra). The selectivity towards the latter increases slightly over time, indicating some changes in the catalyst structure.As such, the spent Cu/TiO2-P25 after 100 h-on-stream was characterized by several techniques (HADDF-STEM, ICP-OES, and TGA) to identify changes in composition and texture. HADDF-STEM (Fig. 4) reveal that the original small Cu nanoparticles (< 3 nm) are still present, though qualitatively the amount is lower than in the original samples. The latter is indicative for some aggregation to larger particles (Fig. 4f). Aggregation is typical for Cu based catalysts due to the high mobility of Cu species at elevated temperatures [64]. Besides aggregation, also some leaching of Cu also occurred as revealed by ICP-OES, showing that the Cu contents decreased from 5.0 wt% for the fresh to 3.7 wt% for the spent catalyst. Carbon deposition on the catalyst was quantified by TGA (Fig. S8). Based on these data, it can be concluded that carbon deposition on the catalyst is minimal.The effect of the solvent on catalyst performance and particularly product selectivity was determined for the Cu/TiO2-P25 catalyst in the continuous set-up (300 °C, 10 bar) using toluene and n-octane at different conversion levels (18–40%). The latter was varied by operating the continuous set-up at different WHSV values. The selectivity versus the conversion in both solvents is given in Fig. 9a. Distinct differences in selectivity trends are observed for both solvents. n-Octane gives the highest selectivity to phenol and demethoxylated phenols at low conversion, whereas the trend is opposite at higher conversions. Thus, toluene is favored for the reaction and best results were a 58% selectivity of phenol, and 81% of demethoxylated phenols at a conversion level of 35%. Possible reactivity of toluene was checked by performing a blank experiment with toluene alone and the catalyst at 300 °C. Clear peaks from toluene derived products were not detected in the product mixtures, indicating that toluene is inert at the prevailing reaction conditions.The effect of reaction temperature was studied in the continuous set-up for Cu/TiO2-P25 with toluene as the solvent at a fixed WHSV of 21 h−1 and a pressure of 10 bar. The results are shown in Fig. 9b. The guaiacol conversion increases with temperature and values up to 98% were obtained at 360 °C. The selectivity of demethoxylated phenols also increased to 87%, though the selectivity to the parent phenol dropped from 54% to 48%. These values correspond to a yield of demethoxylated products of about 85%, which is by far better than reported for Cu/AC in batch (79% selectivity at 24% conversion giving a yield of 19%, Table 1) and other Cu based catalysts (yields between 1.2% and 11%).The gas-phase product was collected for the experiment at 360 °C (10 bar, WHSV = 21 h−1), with 5 wt% guaiacol in toluene as reactant. CH4 was the only product identified, though present in only very minor amounts (0.28%). In addition, the carbon balance closure considering gas and liquid phase products, as identified and quantified with GC, was calculated for this experiment and was found to be reasonably good (86.6%).Catalytic demethoxylation using the Cu bases catalysts was extended to another model compound (4-n-propylguaiacol) and a crude bioliquid (a pyrolysis oil fraction enriched in guaiacols). The former was selected as it is a major component of lignin oils obtained by the reductive catalytic fractionation (RCF) of lignocellulosic biomass [20]. The demethoxylation of 4-n-propylguaiacol using Cu/TiO2-P25 was carried out in the continuous set-up at 360 °C, 10 bar, with 5 wt% 4-n-propylguaiacol dissolved in toluene as a reactant and a WHSV of 21 h−1. The results are given in Table 3. The conversion level of 4-n-propylguaiacol was 94%, which is slightly lower when compared to guaiacol (98%) at similar conditions. The main products were higher alkylated phenols (i.e., propylphenols and methylated propylphenols) with a total selectivity to demethoxylated phenols of 69%. The latter is somewhat lower than found for guaiacol (87%), though comparison is cumbersome as the substrate conversion is different for both experiments.A bioliquid enriched in guaiacols, obtained by pyrolysis oil fractionation was used as the feed to extent the synthetic strategy to more complex feeds. The feed is enriched in guaiacols, and like guaiacol (16 wt%), methylguaiacols (35 wt%), 4-ethylguaiacol (9 wt%), eugenol (9 wt%), and 4-n-propylguaiacol (1 wt%), as shown in Table S3. The reaction was carried out in the continuous set-up at 360 °C, 10 bar, with 5 wt% crude feed dissolved in toluene as a reactant and a WHSV of 23 h−1. The catalytic reaction was conducted without major operational issues for a TOS of 6 h and a total of 14.1 g of the product was obtained. The product was analyzed in detail using GC-FID and was shown to contain mainly (methylated)-phenols like 2-methylphenol, 4-methylphenol, and 2,4-dimethylphenol ( Fig. 10). The presence of mainly methylated phenols is not surprising as the feed contains high amounts of methyl substituted guaiacols. Based on the analyses data, the total guaiacol conversion was estimated at 87% and the selectivity to demethoxylated phenols was about 81%. Overall, it can be concluded that demethoxylation of a complex bioliquid is well possible using the Cu/TiO2-P25 catalyst.A series of novel non-precious metal catalysts based on Cu and supported on inorganic supports (TiO2, SiO2 and ZrO2) was tested for the demethoxylaton of guaiacols. Cu/TiO2-P25, was shown to be the best catalyst in the series when considering selectivity at equal conversion levels. A clear correlation was found between the oxygen storage capacity of the support (TiO2-P25, TiO2-A HSA, TiO2-R, and MoO3-TiO2) and guaiacol conversion, indicating that this property plays an important role in the catalytic cycle. Further optimization studies showed that the selectivity to demethoxylated phenols can be up to 87% at 98% conversion. In addition, the catalyst shows excellent stability for a 100 h TOS experiment. The synthetic methodology was successfully extended for the demethoxylation of 4-n-propylguaiacol and a guaiacols enriched pyrolysis oil fraction, showing high substrate flexibility. Further studies are in progress to obtain biobased phenol, a valuable bulk chemical, now primarily obtained from fossil resources, by selective dealkylation of the alkylated phenols formed in this study. These studies will be reported in due course. Huiazhou Yang: Investigation, Writing – original draft. Xiatian Zhu: Investigation, Writing – original draft. Helda Wika Amini: Investigation, Writing – original draft. Majid Ahmad: Investigation, Writing – original draft. Gert H. ten Brink: Investigation, Writing – original draft. Boy Fachri: Supervision, Writing – review & editing. P. Deuss: Supervision, Validation, Writing – review & editing. H.J. Heeres: Conceptualization, Funding acquisition, Supervision, Validation, Writing – review & editing.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Huaizhou Yang and Xiaotian Zhu reports financial support was provided by China Scholarship Council.H.Y. and X.Z. acknowledge the China Scholarship Council for funding their PhD studies (grant number 201706160156 and 201707040079, respectively). The authors thank Hans Heeres (Biomass Technology Group, BTG) for providing the crude feed isolated from pyrolysis oil. Leon Rohrbach, Erwin Wilbers, Marcel de Vries, and Hans van der Velde are acknowledged for technical and analytical support. We also thank Jos Winkelman for performing the Aspen plus simulations.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcata.2023.119062. Supplementary material. .

Lignin is an attractive feedstock for low molecular weight biobased phenols using depolymerization strategies such as reductive catalytic fractionation or fast pyrolysis. Such strategies often yield a product mixture enriched in lignin-derived monomers with methoxy substituents. Selective catalytic hydrodeoxygenation (HDO) is an effective methodology to demethoxylate these monomers into valuable alkylated phenols. Here, we report the use of non-precious Cu based catalysts supported on SiO2, ZrO2, TiO2 (various forms), MoO3-ZrO2, and MoO3-TiO2 in a continuous fixed-bed reactor at elevated temperature and pressure (300–360 °C, 10 bar) for the selective demethoxylation of guaiacol. Among the various catalysts, Cu/TiO2-P25 was found to be an effective and highly stable catalyst (100 h on stream) with a selectivity of 87% to demethoxylated compounds like phenol and cresols at a guaiacol conversion of 98%. A correlation was found between the oxygen storage capacity of the support (TiO2-P25, TiO2-A HSA, TiO2-R, and MoO3-TiO2) and guaiacol conversion, indicating that this property plays a role in the catalytic cycle. Besides, the demethoxylation of 4-n-propylguaiacol and a realistic guaiacol-rich feed isolated from a representative pyrolysis oil was successfully demonstrated. 87% of the guaiacols present in the feed were converted to demethoxylated phenols with a selectivity of 81%.

Data will be made available on request.The replacement of fossil fuels is of paramount importance. For renewable energy to be sustainable, it must be limitless and provide net-zero CO2 emissions. This demand still depends on several factors including biomass source, regional location and available technologies. Lignin, as a key component of the plant cell wall, has been identified as a major potential source of aromatic renewable resources. From an energy point of view, it has a high C/O ratio and accounts for 40 % of the carbon-based energy content in biomass [1]. However, a vast amount of generated lignin, from paper and bio-ethanol production, is predominantly incinerated and converted into heat or thermal energy and less than 2 % was sold for the production of value-added chemicals [1].Lignin consists of polymerized three-dimensional monomers, including syringyl (S), p-hydroxyphenyl (H), and guaiacyl (G) units, which have been investigated as model compounds for catalytic hydroconversion [2]. The most common linkages of carbon–oxygen (β-O-4) and carbon–carbon (β-5, 5–5, β-1 and β-β) are formed from these 3D monomers, particularly from S and G monomer units. The β-O-4 bonds are the most abundant interunit linkages, making up almost 50 % of all intermonomer linkages in softwoods and 60 % in hardwoods [3]. The cleavage of two β-O-4 bonds, which are located one on each side of the aromatic unit, is required to produce one monomer unit [4]. Moreover, it has been observed that a higher S/G ratio gives a higher yield of monomers during catalytic depolymerization. It has been reported that during catalytic depolymerization at 250 °C of birch lignin (S/G = 3) a monomer yield of 50 mol % is found, whereas a poplar lignin (S/G = 1.5) produced yields of 44 mol % [5]. Interestingly, Shuai et al. observed a monomer yield of 78 mol % using high-syringyl transgenic poplar lignin (S/G = 38) [6].With lignin being an integral part of the plant cell wall, its extraction is one of the challenges to achieve a suitable S/G ratio with a high quality. The extraction, depending on its process conditions, may add further structural complexity to the native physicochemical properties of the lignin. The lignin product is often contaminated or not fully extracted with a significant amount of residual carbohydrates or process chemicals [7]. These challenges have created ambiguities to understand the structural composition of lignin, and hence their chemical reaction network during the depolymerization of processed lignins. Consequently, identifying practical extraction and pretreatment conditions yielding a high quality lignin suitable for facile depolymerization requires further attention and development [2,7,8].Numerous approaches and methodologies have been investigated for Kraft lignin depolymerization. The catalytic reductive depolymerization with hydrogen has been studied extensively as a means to liquify Kraft lignin and selectively produce monomers. Typically, conventional hydrotreating catalysts based on supported molybdenum sulfides (MoS2), promoted by cobalt (Co), nickel (Ni) or iron (Fe) have been studied [9,10]. Recent studies of these transition metal based catalysts showed that they exhibited high activity and selectivity for the CO bond cleavage [1,11,12]. However, the sulfate pulping process contributes to high ash and sulfur contents in Kraft lignin [13]. The major problem, along with the coke deposition via bimolecular condensation reactions, is that inorganic contaminants can lead to catalyst deactivation during the catalytic depolymerization of Kraft lignin [14].Compared to Kraft lignin, hydrolysis lignin, containing less ash and sulfur, can be produced from conversion of lignocellulosic biomass during enzymatic hydrolysis, which results in solid lignin (≥60 wt%), and unreacted cellulose [15]. However, limited studies have investigated catalytic reductive depolymerization of enzymatic hydrolysis lignin. Tymchyshyn et al. used a MoRu/AC catalyst to depolymerize hydrolysis lignin in acetone solvent and obtained low molecular weight bio-oils (380 g/mol) with high yields of around 85 wt% at 340 °C [15]. It was also reported that an aromatic monomer yield of 12.1 wt% was obtained for hydrolysis lignin over a 5 wt% Ni/AC catalyst at 240 °C for 4 h with 30 bar H2 in methanol [16]. Bai et al. used 15 wt% Ni/Al2O3 to depolymerize hydrolysis lignin and obtained a yield of 10.3 wt% of aromatic monomers at 320 °C after 7.5 h under 28 bar H2 in ethanol [17]. The effect of reaction conditions on the depolymerization of hydrolysis lignin has been investigated in a semi-continuous process over a sulfided NiMo/γ-Al2O3 catalyst [18]. A full conversion was achieved and the liquid products, mainly aromatics, naphthenes, and phenols increased under the severe reaction conditions of 380 °C, and 70 bar H2. Recently, Sang et al. examined the depolymerization of hydrolysis lignin over an unsupported Ni catalyst in supercritical ethanol and achieved complete liquefication, with the highest monomer yield of 28.9 % at 280 °C for 6 h with 20 bar H2 [19]. Importantly, these unsupported catalysts decrease the mass transfer limitations inherent to supported catalysts, to achieve a more complete liquefication and prevent char formation during the depolymerization [19]. More recently, the direct conversion of hydrolysis lignin into cycloalkanes over a NiMo/γ-Al2O3 catalyst was carried out in a single step at 320 °C for 7.5 h [20]. The highest obtained overall yield of cycloalkanes was 104 mg/g enzymatic hydrolysis lignin, with a ethyl-cyclohexane selectivity of 44 wt% [20].However, there are to the best of our knowledge, no published studies where the efficiency of catalytic valorization in reducing conditions have been compared using Kraft and hydrolysis lignin, which is the objective of the current work. In this work, we introduce a facile preparation method for an unsupported NiMoS catalyst that is highly active with a high surface area. This catalyst can be a key factor in enhancing the hydrodeoxygenation and hydrogenation capabilities, and simultaneously decreasing the unwanted repolymerization reactions producing char. In addition, the reaction pathways for both lignins are proposed and discussed to reveal the key steps in their depolymerization and how they differ.Two different lignins were investigated. Kraft lignin is a three-dimensional polymer that has undergone a hydrolytic degradation process [21,22]. It was supplied by Sigma-Aldrich as a brown dry powder. The enzymatic hydrolysis lignin was kindly provided by Sekab (Sweden). Prior to all experiments, the lignin samples were dried at 80 °C in an oven. The chemicals used were of analytical grade and were not further purified. The reagents used can be found in Supplementary Information (SI).Unsupported NiMoO4 catalysts were synthesized by a nanocasting method using mesostructured silica as a hard template. SBA-16 and MCM-41, consisting of only silica, were used as templates. Hard templating is an important strategy to synthesis crystalline mesoporous materials. The unique structure of the hard template restricts the crystallization or aggregation of the precursors, and a mesoscopic phase having a structure opposite to that of the template can be obtained with the removal of template material by the appropriate method [23].A mixture of ammonium heptamolybdate tetrahydrate and nickel nitrate hexahydrate, in a molar ratio of 1:1, was dissolved in ethanol. The aqueous mixture was added to the mesoporous silica and stirred for 2 h at room temperature. Subsequently, ethanol was evaporated gradually using a water bath at 65 °C. The obtained paste was then calcined at 200 °C for 6 h. The resulting solid was re-impregnated again, followed by calcination at 450 °C for 6 h at a heating rate of 6 °C/min. Lastly, the silica template was removed from the mesoporous composite by 0.5 M NaOH using a vacuum filtration process. The solid products were washed with deionized water several times and then dried at 110 °C. Elemental analysis using ICP confirmed the absence of templates and showed the successful removal of the template. The absence of silica was also confirmed with XPS, XRD and TEM-EDS. The oxide forms of the unsupported catalysts will be denoted NiMo-SBA and NiMo-MCM, respectively, according to the mesostructured SBA-16 and MCM-41 templates used in their synthesis.Elemental composition of the catalysts was determined by using an inductively coupled plasma (ICP) and was performed by ALS Scandinavia AB after digestion of the solids in an acid solution.Powder X-ray diffraction (XRD) was used to examine the crystallinity of the catalysts. This was done using a Bruker D8 Advance (40 kV; 40 mA; Cu Kα radiation (λ = 1,542 Å); 2θ range of 20-80°; 1°/min scan speed). The textural properties of the samples, such as pore volume, surface area and pore size were determined by nitrogen physisorption using a TriStar 3000 analyzer. Prior to N2-physisorption measurements, approximately 300 mg sample was degassed overnight at 300 °C under a continuous flow of nitrogen gas. After drying, the N2-physisorption isotherms were collected at −195 °C under vacuum. The specific surface area and pore size were determined using the Brunauer-Emmett-Teller equation (BET) and the Barret − Joyner − Halenda equation (BJH), respectively.The lignins and catalysts were thermally characterized by TGA with a TGA/DSC 3+ Star system (Mettler Toledo, Switzerland) featuring automated temperature and weight control as well as data acquisition. The samples were analyzed, without any pre-treatment, from 25 to 800 °C (to 900 °C for catalysts) with a 5 °C/min heating rate. Dry air and nitrogen were used as carrier gases for comparison.Scanning electron microscopy (SEM) was used to study the surface structure and morphologies of the sulfided catalyst samples with a JEOL JSM6400, operating at 25 kV. The shape and size of the metal species in the catalysts were examined using transmission electron microscopy (TEM), with a FEI Titan 80–300 microscope (field emission gun; a probe Cs corrector; Gatan image filter Tridium; 300 kV). X-ray photoelectron spectra of the fresh sulfided catalyst was recorded using a PerkinElmer PHI 5000 Versa Probe III scanning XPS Microprobe apparatus equipped with a monochromatic Al Kα source with a binding energy of 1486.6 eV and the beam size diameter of 100 µm. The reference used is so-called adventitious carbon (AdC) using the C 1 s peak from the surface contamination layer and its binding energy (BE) is set to 284.6 eV.The NH3 temperature-programmed desorption (NH3-TPD) experiments were conducted in a Differential Scanning Calorimeter (DSC, Setaram Sensys), where the gas flow was regulated with mass flow controllers (MFC, Bronkhorst), and the outlet gases detected with a mass spectrometer (MS, Hiden Analytical HPR 20). 50 mg of NiMoO4 and NiMoS unsupported catalysts were pre-treated at 400 °C for 2 h in an argon flow of 20 mL/min. This was followed by exposing the catalyst to 4 vol% NH3 in Ar (10 mL/min) for 2 h at 120 °C. Thereafter the catalyst was flushed with Ar for 6 h and then the NH3 desorption was studied while increasing the temperature from 120 to 700 °C at a ramp rate of 5 °C/min.Elemental analyses (EA) were performed to determine the C, H, N, and S content in the feed lignins, and lignin oils using a vario MICRO cube analyzer. The MICRO cube analyzes the CHNS content of organic compounds in one single run. The amount of oxygen was determined by difference from the CHNS content. All analyses were performed twice and the average value is given.The water content in organic samples was determined by Karl Fischer (KF) titration using a Metrohm Titrino 807 titration equipment. The sample was added to a glass container with Hydranal® (Riedel de Haen) and the titrations were performed with Karl Fischer titrant Composite 5 K (Riedel de Haen). All analyses were performed twice, and the average value is given. 31P NMR technique was used for the characterization and quantification of hydroxyl and carboxylic acid groups in lignin oils using an earlier method involving a prior derivative phosphitylation step [24]. The amount of different hydroxyl groups (mmol OH/g) in lignin oil samples was calculated according to [24]. The 13C solid-state NMR was conducted on a Bruker Avance III 500 MHz spectrometer equipped with a 4 mm MAS BB/1H probe. The rotor was spun at 10 kHz and a cross-polarization time of 1.5 ms was used.The GC-TCD technique was used to characterize gases formed during lignin hydroconversion. The gas samples were analyzed by a calibrated GC (450-GC, Varian) that was equipped with a TCD detector. A GS-GASPRO column (30 m, 0.32 mm) was used to separate and quantify the concentration of H2, CO, CO2, CH4, and C 2 + light hydrocarbons. The quantification was performed from the calibration of each gas using reference mixtures. All measurements were carried out in triplicate and the average value is provided.GCxGC-MS analysis was performed on organic liquid samples using an Agilent 7890B apparatus equipped with a closed cycle cryogenic jet modulation (ZX2 Model) from Zoex Corporation, two parallel detectors, an FID and a quadrupole MSD, and two columns. The first column was a moderately polar VF1701ms column (30 m × 0.25 mm × 0.25 μm) and the second column was a nonpolar DB-5 MS UI column (1.2 m × 0.15 mm × 0.15 μm). The temperature of the injector port and MS interface were set at 250 °C. The column flow was set at 0.8 mL/min. The oven temperature program was set initially at 40 °C and held for 1 min, then heated up to 280 °C at 3 °C/min. The modulation time was 6 s. The data was processed via GC image 2.5 (Zoex Corporation) using GC Project and Image Investigator functions. The 2014 NIST library (Match Factor >700), together with high resolution mass spectral data, were used for compound identification. To assess the relative standard deviation of each analysis, an internal standard was loaded before analysis by GCxGC. From GC Image Investigator, the relative standard deviations (RSD) in the first-dimension retention time, the second-dimension retention time and peak volume were less than 0.1 %.The hydroconversion experiments were carried out in a 450 mL stainless-steel Parr reactor. The influence of catalyst loadings (ranging from 5 to 20 wt% based on a constant 5 g of dry lignin always loaded into reactor), pressures (50–80 bar H2) and temperatures (330–400 °C) have been investigated (Table 1 ). In a typical experiment, the reactor was loaded with catalyst and lignin (5 g) in 70 mL of hexadecane as a co-processing solvent to reduce the exothermic heat effects. To create a baseline for the study, reaction experiments using only hexadecane with lignin were included in the experimental plan. The sulfidation of the NiMoO4 catalyst was performed in-situ in the liquid phase, with the lignin feedstock and with the addition of (0.25–0.75 mL) dimethyl disulfide (DMDS). In-situ sulfidation of the unsupported NiMoO4–SBA was carried out simultaneously with hydroconversion of Kraft and hydrolysis lignins. The advantage of in-situ activation is the simplification of the startup procedure. The amount of DMDS added depends on the catalyst amount used. During the sulfiding step for all experiments, a mixture of lignin, catalyst, and solvent were blended. Meanwhile, a small amount of DMDS (250, 500 or 750 μL) was added to the corresponding reaction mixture to maintain the sulfidity of the catalyst (5, 10 or 20 wt%, respectively). Under identical operating conditions, some experiments were performed to compare products yields from in-situ sulfided versus pre-sulfided NiMoO4..After the reactor was closed, it was flushed for 3–5 times with nitrogen to remove the air and thereafter purged three times with hydrogen. After leak testing, the reactor was initially pressurized with 16–25 bar H2 (depending on the target pressure for reaction conditions) and heated up to the designated temperature at an approximate heating rate of 5 °C/min while stirring at 1200 rpm. The time zero was set once the desired reaction temperate and pressure were reached. In the case of our study, duplicate reaction experiments were repeated for many of our experiments and the relative standard deviation (%, RSD) was determined to be within 9.0 %.After each experiment, different fractions of lignin products were recovered and analyzed. The typical products are gas, liquid bio-oil, unconverted lignin and solid char, as depicted in the product recovery protocol (Fig. S1, Supplementary Information (SI)). The pressure was recorded after the reactor was cooled to room temperature. The hydrogen consumption was determined from the difference in the initial and final pressures at room temperature [25,26]. Hydrogen was considered as an ideal gas and other gases produced during the reaction, were accounted for in the calculation of the hydrogen consumption. The pressure was released to atmospheric pressure and the gas products were collected in a 1 L Tedlar gas bag to determine its composition by using GC-TCD. The blended organic product was recovered, filtered and labelled as lignin oil. Subsequently, the reactor was rinsed with acetone, filtered, and this product fraction was labelled as the acetone soluble phase. Moreover, the solid phase (char, unconverted lignin, and catalyst) was washed with acetone to remove any adsorbed organics.The unconverted lignin was determined by suspending 0.5 g of solid residue in 15 mL of DMSO and thereafter stirring for 24 h at room temperature. After filtration of the solution, the solids were washed with acetone and dried. The weight difference between the solids before and after the washings was assumed to be the weight of unconverted lignin. The conversion, char, and monomer yields were determined using Eqs. (1) to (3). (1) Conversion wt.\% = initial lignin feed g - unconverted lignin g initial lignin feed (g) x 100 (2) Char yield wt.\% = solid fraction g -(unconverted lignin (g) + catalyst g ) initial lignin feed (g) x 100 (3) Monomer yield wt. \% = monomer g initial lignin feed (g) x 100 The elemental composition was measured using ICP and the unsupported NiMoO4-SBA catalyst had a Mo/Ni ratio of 1.08). The pore structural parameters are summarized in Table 2, which displays high surface areas in the range of 155–207 m2/g for none sulfided catalysts. A sulfidation treatment of the unsupported NiMoO4-SBA leads to an inevitable decrease of the catalyst surface area from 155 to 53 m2/g and a pore volume decreased from 0.19 to 0.08 cm3 g−1, which may be due to the inclusion of sulfur to form NiS2 and MoS2 new phases. These catalysts exhibit the type-IV isotherm, confirming their ordered mesostructured morphology. Moreover, the pore size distributions correlate with those of the templates used and exhibited a narrow pore size distribution at around 5 nm for both as prepared and pre-sulfided catalysts. While using MCM-41 as a template, relatively larger cages and mesoporous channels, corresponding to the MCM-41 template, were observed. The pore diameter of the unsupported NiMoO4-MCM catalyst enlarged to 11.8 nm with a significantly larger pore volume of 0.61 cm3 g−1.The crystal structures of the unsupported NiMoO4 were characterized by XRD (Fig. 1 ). The broad peaks indicate a small crystallite size of the samples. Comparison of the NiMoO4 fabricated via the two different hard templates, SBA and MCM, reveals that there are distinct differences in the XRD patterns (Fig. 1a). α-NiMoO4 and β-NiMoO4 phases have a monoclinic crystal structure (group space C12/m1). The characteristic peak of the β phase is 2θ = 26.4˚and for the α phase it is 2θ = 28.7˚. From the structural point of view, the relevant important differences between both phases are the molybdenum ion coordination in the crystal structure, being octahedral clusters, [MoO6], for the α-NiMoO4 and tetrahedral [MoO4], for the β-NiMoO4 powder [27]. It has been found that for the preparation of highly active unsupported NiMo catalysts, the β-NiMoO4 phase is most favorable for hydrodesulfurization (HDS) reactions [28–31]. In this work, the unsupported NiMoO4 fabricated from the SBA hard template which exhibited the β-NiMoO4 phase was therefore selected for the catalytic hydroconversion.The XRD patterns of the pre-sulfided NiMoO4-SBA are depicted in Fig. 1b. The sulfided NiMoS showed strong diffraction peaks consistent with the relatively good crystalline structure of MoS2 (JCPDS-ICDD 371492), NiS2 (JCPDS-ICDD 89–3058), and β-NiS (JCPDS-ICDD 12–0041). The presence of peaks at 2θ value of 14.5, 33, 39 and 59 corresponded to the (002), (100), (103) and (110) planes of MoS2 [32]. As revealed by the XRD patterns, peaks due to NiS2 and β-NiS species were also detected. Some peaks at 2θ value of 27, 31, 35, 38, 45, and 53 were observed, matching well with the NiS2 [33]. Moreover, the presence of peaks can be clearly indexed to β-NiS at 2θ value of 33, 37, 41, 49, 51, and 57 corresponding to (300), (220), (221), (131), (410) and (330) planes [34]. Obvious diffraction peaks from other compounds, such as Ni3S2 and Ni3S4, were also observed at 2θ value of 43.5, and 48 respectively [35,36]. No obvious ternary Ni-Mo-S and NiMoO4 oxide patterns were observed. It is noteworthy to mention that before XRD analysis, the pre-sulfided catalyst was passivated under a flow of 25 mL/min of 2 % O2 in Ar for 2 h and thereafter transferred into another N2 atmospheric bottle to avoid air contact.The SEM image of the sulfided NiMoS-SBA catalyst showed two major structural features (Fig. S2a, SI). Small pore sizes can be seen in the material with an average pore size of 5–10 nm (white insert). These smaller pores are the fine intra-aggregate pores of the material. The larger sized pores (>50 nm) may be attributed to the secondary pores formed by the combination of primary particles. The TEM result of the freshly sulfided NiMoS-SBA catalyst is given in Fig. S2b (SI). Areas with black thread-shape fringes that have spacings of about 0.5 nm indicate a high purity of the active components and are characteristic of the (002) basal planes of crystalline MoS2 (XRD pattern Fig. 1b). This has been confirmed by Yoosuk et al. [37]. As well-known from the intercalation model, slab growth occurs in parallel and perpendicular directions [2]. But for NiMoS in Fig. S2b, the slabs seem curved. These results are in good agreement with the recent study of Yoosuk et al. [37], who also suggested a reduction in the slab length and form when Ni was incorporated into the Mo sulfide. In these directions, the promoting Ni atoms are bonded (in intercalation positions) by Van der Walls forces [36]. It is generally accepted that the best hydrotreating MoS2 catalysts are promoted with Ni or Co atoms located at the edges of MoS2 slabs [2], which is discussed later in connection to the lignin depolymerization. In good agreement with the literature data, it can be assumed that the formation of NixSy active sites occurred at the rims of MoS2 sheet crystals. Fig. 2 displays the TGA and DTG profiles of the pre-sulfided and as prepared unsupported NiMoO4-SBA catalysts. For the unsulfided NiMoO4-SBA catalyst, it shows two weight loss areas, totaling about 6.4 wt% up to the reaction temperature of 400 °C, together with corresponding endothermic peaks (Fig. 2b). The first weight loss was about 3.4 wt% from 75 to 170 °C, based on a calculation of the first derivative of the weight loss curve at 70.4 °C and might be attributed to the loss of physically absorbed and chemically bonded water. The following weight loss was about 3.0 wt% from 210 to 415 °C, which can be due to the total phase transformation of α-NiMoO4 (octahedral MoO6) to pure β-NiMoO4 (tetrahedral MoO4) above 286 °C, which was reported by Pillay et al. by measuring the XRD pattern at high temperature to characterize the α-NiMoO4 and β-NiMoO4 phase transitions [38].After the sulfidation of the unsupported NiMoO4-SBA catalyst, the TGA thermogram showed a 6.5 wt% weight loss up to 400 °C, which was similar as for the unsulfided NiMoO4-SBA. Besides, it also shows two endothermic weak peaks (pre-sulfided NiMoS, Fig. 2b) that may refer to a decomposition of the α-NiMoO4 and β-NiMoO4 formed due to oxygen contact. At higher temperature, a weight loss was observed from 400 to 650 °C and thereafter another weight loss a higher rate in the 650–900 °C range. It is likely that these weight losses are originating from decomposition of different sulfur species on the catalyst since they were not found on the non-sulfided catalyst (Fig. 2b, blue line).In order to investigate the nature of the surface species, XPS analysis of the sulfided catalyst was conducted and shown in Fig. 3 . Based on XPS analysis, the atomic percentage of various elements present at the surface of the catalyst is given in Table 3 . Note that the sulfided sample could be partially oxidized due to oxygen contact when storing and transferring the sample to the XPS instrument, which could possibly explain the high oxygen content (Table 3). XPS was used to investigate the chemical states of Mo, Ni and S in the pre-sulfided NiMoS, shown in Fig. 3. The XPS spectra showed that the binding energies of Mo 3d5/2 and Mo 3d3/2 were located at 228.7 eV and 232.1 eV respectively, owing to the Mo4+ in MoS2 [39,40] and there is also a weak peak located at 235.2 eV assigned to Mo6+ 3d5/2 of MoO3 formed due to oxidation of Mo (Fig. 3b). Another weak peak at the binding energy at 225.9 eV is ascribed to S2s of S2 in MoS2 [39,40]. Besides, there are two strong peaks at 161.5 and 162.7 eV in the S2p spectrum (Fig. 3c), which are assigned to the S2p3/2 and S2p1/2 binding energies for S2− of MoS2 and NiS2. Moreover, there are three peaks present in the Ni 2p spectrum (Fig. 3d) at 853.3, 856.4, and 861.7 eV, which could be assigned to NiS2, nickel oxide and nickel hydroxide, respectively [36,40].NH3-TPD profiles of NiMoO4-SBA and NiMoS-SBA catalysts are presented in Fig. S3 (SI) to evaluate their surface acidity. Both catalysts exhibited moderate (200–400 °C) and strong (>400 °C) acid sites. The NH3-TPD profile of NiMoO4-SBA displayed a relatively wide peak at 300 °C and a minor peak at 550 °C, which were attributed to moderate and strong acid sites, respectively. Whereas after sulfidation, it can be observed that there is an important decrease in the number of moderate acid sites, while an increase in the acid strength for sulfided NiMoS-SBA.Chemical structures and thermal characterization of lignins were carried out through thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC), elemental analysis (CHONS), and solid state 13C NMR. TGA curves, under oxidizing atmosphere (Fig. 4 ), showed that the water content corresponds to the weight loss of about 2.3 wt% for Kraft lignin and 1.5 wt% for hydrolysis lignin at 100 °C, whereas the remaining weight at the end corresponds to the ash content of 3.5 wt% for Kraft lignin and only 0.5 wt% for hydrolysis lignin. Elemental CHONS analyses, ash, water content as well as the H/C and O/C atomic ratios are reported in Table 4 .The chemical structures of Kraft and hydrolysis lignin were analyzed by means of solid-state 13C NMR (Fig. 5 ). Both lignins showed small signals in the area 200–160 ppm, indicating a low amount of CO structure, attributed to CO bonds in Ar-CHO or R-O-CO-CH3 at 177.5 ppm and 181 ppm, respectively [41]. In the Carom region (110–150 ppm), the intense peak at 150 ppm corresponds to Carom-O, whereas three weak signals were observed at 115, 123 and 127 ppm corresponding to Carom-C and Carom-H [42,43]. The 13C NMR spectrum of hydrolysis lignin showed characteristics of typical lignocellulosic biomass, which is primarily composed of cellulose (62–110 ppm) and lignin [44]. Fu et al. showed that hydrolysis lignin is covalently attached onto cellulosic moieties, indicating that the cellulose from lignocellulosic biomass is not fully isolated [15,45–47]. Note that the high O elemental content result for hydrolysis lignin (38.1 wt%) may be partly due to the presence of cellulosic oxygen content. Moreover, methoxy groups were detected at 58.0 ppm for both lignin types. The CH, CH2, and CH3 saturated aliphatic signals (<50 ppm) could not be clearly observed and distinguished. Some identical peaks were observed in both lignins, but the peak intensities were higher for Kraft lignin than the hydrolysis lignin. By comparing these different analytical results, we can observe that both lignins are connected randomly through β-β or β-aryl ester, G-type β-O-4 and β-5 linkages, and cellulosic units for hydrolysis lignin.TGA and DSC curves for hydrolysis and Kraft lignin under inert atmosphere are given in Fig. 6 . TGA analysis showed that Kraft and hydrolysis lignins start to decompose in the range of 175–460 °C and 250–405 °C, respectively. Thus, reactions involving lignins are already expected to take place to a certain extent when heating up the reactor to the final chosen temperature, in our case 400 °C [48]. Moreover, under inert atmosphere at 550 °C, the char yields of Kraft (43 wt%) and hydrolysis lignin (31 wt%) were higher than in an oxidizing environment (see Fig. 4 and Fig. 6). These results show that depolymerization of lignin without a catalyst to facilitate hydrogenation and deoxygenation reactions, is likely to yield a high char formation, particularly for Kraft lignin. It can also be seen that the thermal properties of the lignin depend on their source due to their structural variations. However, under catalytic hydroconversion, the conversions of the Kraft and hydrolysis lignins were found to be higher than 90 %, which was confirmed by performing DMSO extraction of the unconverted lignin. In the subsequent sections, we examine and discuss in detail the individual product yields of the hydrotreated lignins.For each lignin, the results of non-catalyzed and catalyzed hydrotreatment reactions are presented in Table 5 . The conversions of both lignins were in the range of 91.0–99.5 wt%. The lowest lignin-oil yields were observed for the non-catalyzed experiments (KES0, and HES6), which correlate well with the least water content formed upon the reaction (0.3 and 0.7 wt%) and the highest char formation (52.9 and 38.6 wt%), showing a low degree of deoxygenation without the catalyst. These results also provide clear evidence that thermal depolymerization reactions play a role. Interestingly, the hydrolysis lignin results in significantly less char for uncatalyzed reactor experiments than Kraft lignin, which is consistent with the TGA results in Fig. 6. When the unsupported NiMoS-SBA catalyst (in situ sulfided) was used under the same conditions, the lignin-oil yield increased to 65.1 and 83.7 wt%, whereas the char yields were repressed to 20.6 and 8.3 wt% for Kraft (KES5: 400 °C, 80 bar H2, 10 % catalyst, 5 h) and hydrolysis lignin (HES8: 400 °C, 80 bar H2, 10 % catalyst, 5 h), respectively. The characterization results demonstrated that the sulfided catalyst had two separated sulfide phases rather than a trinary Ni-Mo-S phase. Thus, the presence of both phases of MoS2 and NixSy likely improved the lignin conversion. This is mainly due to a synergism between NixSy and MoS2, evidently reflected by the major XRD peaks of pre-sulfided NiMoO4-SBA (Fig. 1b). Moreover, the XPS analysis confirms the presence of both phases. It was also reported by Wang et al. that a synthesis of NiS2/MoS2 has higher surface area, resulting in the exposure of more active sites [49]. The hydrodeoxygenation activity is considered enhanced in the presence of both NiS2 and MoS2 which could be described by a Remote Control (RC) model via hydrogen spillover [49]. According to the RC model the two separated sulfide phases of the catalyst are described as a donor phase (promoter, NiS2) and an acceptor phase (active component, MoS2), and thus spillover hydrogen was created on NiS2 which then migrated to MoS2 [49]. It was also reported that a Ni-Mo binary sulfide phase is more active than either of the single Mo or Ni sulfide phases and the maximum synergy depends on Ni/(Mo + Ni) ratio to achieve a well dispersed active phase [37].A comparison is made between pre-sulfidation and in-situ sulfidation method (see Table S1) and it is found that both methods give similar results. Based on these results, we consider that the DMDS added was adequate to activate both in-situ or pre-sulfided NiMoO4-SBA catalysts.The influence of temperature, pressure, residence time and catalyst loadings were investigated. The effect of temperature (330 and 400 °C) on the product yield was investigated in experiments KES1 (330 °C, 50 bar H2, 5 % catalyst, 5 h) and KES2 (400 °C, 50 bar H2, 5 % catalyst, 5 h) using Kraft lignin. Higher temperature led to a slight increase in lignin liquefaction yields from 48.7 to 49.7 wt% and a clear decrease in char residue from 40.5 to 30 wt%. We also observed that both the water and gas contents increased to >2.0 wt%, indicating that the removal of oxygen from the biomass starts above 330 °C. Moreover, a longer residence time of the reaction (KES3 (400 °C, 50 bar H2, 5 % catalyst, 12 h) led to further suppression of the char yield to 27 wt%, and large increases in lignin oil, water and gas content were observed. Hydrogen consumption was measured for the 5 and 12 h residence times and it increased from 1.5 to 3.9 mmol per g of Kraft lignin, simultaneously as the water yield increased from 2.3 % to 7.9 %, due to a higher degree of deoxygenation being achieved.At a constant temperature of 400 °C, the pressure was increased from 50 to 80 bar (KES2: 400 °C, 50 bar H2, 5 % catalyst, 5 h) versus (KES4: 400 °C, 80 bar H2, 5 % catalyst, 5 h). This higher pressure ensured a higher solubility of hydrogen in the oil, which resulted in an increase from 49.7 to 61.5 wt% of the oil yield and thereby a higher availability of hydrogen in the vicinity of the catalyst. As seen in Table 5 (KES2 vs KES4), this increases the reaction rate and further decreases the unreacted lignin from 9.0 to 4.7 wt%. Furthermore, higher degrees of deoxygenation are favored by increasing the catalyst loadings from 5 to 10 wt%. Consequently, higher lignin-oil yields of 65.1 and lower char yields of 20.6 wt% were achieved for Kraft lignin (KES4 vs KES5) with increased catalyst loading. Similarly, for hydrolysis lignin (compare HSE7, HSE8 and HSE10), higher loadings of catalyst resulted in increased oil yield and decreased char formation. This could be explained by that when increasing the loading more active sites are available for the adsorption of lignin and in addition, an enhanced hydrogen spillover from NixSy could occur, resulting in a promotion of the hydrogenation-dehydration reactions.A comparison of lignin-oil and char yields for Kraft and hydrolysis lignins is presented in Fig. 7 . Under identical operating conditions, hydrolysis lignin results display higher lignin oil formation and lower char yields in comparison to Kraft lignin. One of the reasons is that hydrolysis lignin was obtained from enzyme catalytic conditions, making it more active and free from ash and sulfur contents (Table 4), than the lignin materials obtained from chemical processes [20,50]. Generally, additional central factors may be the different chemical structures and compositions of the lignin-types, and in particular their different S/G ratios [51,52]. Moreover, the hydrolysis lignin consisted of both lignin and cellulose (see Fig. 5) and thereby the hydrolysis lignin contains less lignin per mass, and this could also be a factor for producing less char since it is known that lignin often gives large amount of char.To investigate the stability of the sulfided NiMoO4-SBA catalyst, three consecutive conversions of hydrolysis lignin were conducted using the recycled catalyst (Table S4). Upon completion of the reaction (HES10), the catalyst used was regenerated by calcining the spent catalyst in air at 500 °C for 5 h to burn off any remaining unseparated char or solids. The recycled catalyst was then used for the next reaction cycles as described in the Reaction experiments section 2.5. Table S4 shows that for the experiments with recycled catalyst the conversion and oil yields are similar during repeated cycles. A catalyst weight loss of 10.6 wt% was observed for the first cycle, which is similar to the 8.5 wt% from the TGA thermogram results (Fig. 2a). The second and third cycles were further performed and resulted in negligible weight losses up to 2.0 wt%. Thus, the spent NiMoS-SBA catalyst was found to have good stability and performance comparable to a duplicated HES10 experiment (Table 5).The gaseous phase composition was quantified, and the results are given in Table 6 . The results show a maximum of 5.6 wt% gas product yield measured at room temperature after the reaction. The dominant gas products were CH4 (0.1–3 wt% on lignin), and CO2 (0.1–2.4 wt%), with small quantities of olefins C 2-C4 (<0.7 wt%). The gas-phase formation may be explained by reactions occurring during lignin hydroconversion, along with gas phase reactions. The olefins are derived from the CC cleavage of alkyl chains or via dehydration of intermediate small alcohols derived from the cleavage of the β-O-4 ether linkage. The formation of CH4 can be explained by demethylation, which is favored under our conditions. Another possible pathway for methane formation is the reaction of the released CO2 and CO with H2. This was also demonstrated for model compounds such as formic and acetic acid over a Ru/TiO2 catalyst [53,54]. Moreover, the formation of CO2 and traces of CO can result from decarboxylation and water gas shift reactions. The decarboxylation of –COOH to form larger amounts of CO2 product was observed for the catalytic hydroconversion at 400 °C (KES2), but not at 330 °C (KES1). These results are consistent with those reported previously, suggesting that decarboxylation of carboxylic acids occur at a temperature higher than 350 °C [52,55].When increasing the temperature (KES1 versus KES2) and residence time (KES2 versus KES3) for Kraft lignin over the NiMoS-SBA catalyst, the yields of CH4 and CO2 increased, whereas it was similar when increasing the pressure (KES2 versus KES4). With increasing catalyst content from 5 wt% (KES4) to 10 wt% (KES5), the yields of CH4 and CO2 were similar. However, a significant decrease in gas yield was observed for hydrolysis lignin while increasing the catalyst loadings from 5, 10 and up to 20 wt% (HES7, HES8, and HES10). In contrast to our results, Chowdari et al. [56] found that the total yield of gases slightly increased when increasing the catalyst loading for a 20NiMoP/AC from 5 wt% to 10 wt%, respectively from 8.6 to 9.4 % at 400 °C [56]. This can be due to several differences, such as much higher selectivity for the decarboxylation and demethylation with their supported 20NiMoP/AC catalyst and the conditions under which it was used.The oxygen and hydrogen contents in the lignin-oils are displayed in the form of Van Krevelen diagrams (Fig. 8 ). The hydrolysis lignin used in this study has a relatively high O/C ratio of 0.51, while that of Kraft lignin is considerably lower, 0.39. This is likely due to differences in the extraction methods, feedstock origins and their different compositions (e.g. cellulose in hydrolysis lignin). Based on the van Krevelen diagrams, the lignin oils show significantly lower O/C ratios (0.02–0.11) and an increase in H/C ratios (1.97–2.26), suggesting that the hydroconversion reactions have occurred to a large extent. It is interesting to note that the estimated Higher Heating Values (HHV) (Tables S1 and S2) are high and similar to those of traditional petroleum-based fuels [57]. Despite the higher starting O/C ratio, hydrolysis lignin showed overall lower oxygen contents in the oils, which possibly can allow easier conversion into high-quality fuels compared to the oil from Kraft lignin. Also, the presence of hydrogen and catalyst resulted in a reduction in sulfur content in the lignin-oils (<0.02 wt%), showing the efficiency of the catalyst for HDS reactions (see Tables S1 and S2). In the absence of catalyst (KES0 and HES6), the values of oxygen were higher (8.3–11.6 wt%) than for all experiments with unsupported NiMoS-SBA catalyst (1.9–6.3 wt%), which clearly demonstrates the beneficial effect of the catalyst. The above results suggest that NiMoS-SBA is an effective catalyst for hydrogenation, deoxygenation and desulfurization of lignin under the selected conditions. At high temperature and pressure (KES4), the H/C ratio increases, and it can further increase with extended reaction time as was evident for KES3 versus KES2. For reactions performed at 400 °C and 80 bar, an increase in catalyst loading (KES5 versus KES4) leads to a decrease in the O/C ratio, which results in that >87 % of the oxygen was removed, compared with the initial feedstocks used. 31P NMR analysis on the lignin-oils was performed to help elucidate the changes that occurred during the reaction. This offers the unique ability to distinguish hydroxyl groups attached to p-hydroxyphenyl, guaiacyl, and syringyl units. Fig. 9 shows 31P NMR spectrums of lignin oils obtained under the selected conditions. A compressed compilation of hydroxyl groups in lignin oils and their typical chemical integration ranges are summarized in Fig. 9 along with the quantitative data (Table 7 ) clearly shows that Kraft and hydrolysis lignin oils were rich in p-hydroxyphenyl and guaiacyl OH groups, while syringyl and condensed OH groups (C5-substituted OH) were observed in small amounts for the highly catalyzed Kraft lignin (10 wt% loaded, KES5) and less catalyzed (5 wt% loaded, HES7) and uncatalyzed hydrolysis lignin (HES6 and HES7). Small amounts of carboxylic acid (133.6–136 ppm) and aliphatic OH (145–150 ppm) can be distinguished for Kraft and hydrolysis lignin oils as well.The total quantity of monomer in the lignin-oils is of high interest to indicate the effect of the unsupported catalyst and various operating conditions on the target product classes in this study. Therefore, all lignin-oils were subjected to GCxGC analysis, with a correction for the hexadecane solvent contribution (Fig. 10 ). The lignin-oil phase product comprises a complex mixture of monomeric compounds. The monomer composition detected by GCxGC is quite similar for both lignins when other variables are kept constant (listed in Fig. 10). For noncatalyzed reactions shown in the 13P NMR spectrum, the total OH content (Fig. 9, uncatalyzed) in the lignin-oils was lower and dominated by p-hydroxyphenyl (phenolic OH groups). This implies that hydrogenolytic cleavage of aryl-O-aryl and aryl-O-aliphatic linkages in the lignin only partially occurred. A series of operating conditions were examined with the presence of the unsupported NiMoS-SBA catalyst and exhibited an important impact on lignin depolymerization, namely enhanced cleavage of COC linkages between lignin units. 13P NMR spectrums showed the presence of a larger amount of p-hydroxyphenyl and guaiacyl in the area 138–140 ppm (Fig. 9, catalyzed). By increasing the temperature to 400 °C for Kraft lignin, the methoxyphenol were converted to alkylphenolics by O-demethylation reactions, as confirmed by the GCxGC analysis (KES1 to KES2, Fig. 10). This suggests that most of the O-demethylation (–OCH3) and hence higher CH4, CO2 and water yields (Table 5 and Table 6) resulted from cleavage of guaiacol, syringyl and C5-substituted OH groups occurring at the higher temperature.By increasing the residence time (KES2 to KES3), both alkylated phenolic and aliphatic OH were increased for Kraft lignin (Table 7 and Fig. 10), whereas the carboxylic acid (COOH) was significantly suppressed, leading to the enhancement of CO2 formation (Table 6). This is probably due to the reduction of the carboxylic acids accompanied by an increase in solubility of the Kraft lignin in the fluid phase [58]. A similar effect on content of carboxylic acid was observed for hydrolysis lignin while increasing the residence time from 5 to 12 h at 400 °C (HES8 to HES9), however; this resulted in slightly decreased phenolic and aliphatic OH contents according to NMR results (Table 7). It is noteworthy to mention that part of the phenolic OH detected by 31P NMR includes those in oligomeric compounds which are not detected by GCxGC analysis. Thus, the enhancement of alkylphenolic and aromatic compounds when increasing the residence time from 5 h to 12 h, as shown in the GCxGC results (HES8 to HES9, Fig. 10), can be explained by a deep cleavage of the oligomeric compounds (Table 7). As can be seen in Fig. 9, the C5-substituted OH units for uncatalyzed (0 wt%, KES0) and less catalyzed Kraft lignin (5 wt%, KES4) were not observed, whereas it was observed for the highly catalyzed Kraft lignin (10 wt%, KES5). For hydrolysis lignin, the uncatalyzed (0 wt%, HES6) and less catalyzed (5 wt%, HES7) lignin showed small amounts of C5-substituted OH units, however; none were observed at the higher catalyst loading (10 wt% HES8). Although the reaction conditions were nearly identical, the depolymerization and cleavage of CO bonds differed to a certain extent between the lignins.Upon increasing the catalyst loading from 10 wt% (HES8) to 20 wt% (HES10) for hydrolysis lignin, all hydroxylic groups and particularly the phenolic OH content decreased by eightfold (Table 7), which was accompanied by a significant reduction in the char yield and an enhancement in lignin-oil yields (HES10, Table 5). These changes also resulted in an increase in aromatic, and a decrease in alkylated phenolics and aliphatic OH/Ketones yields (HES10, Fig. 10). As shown by comparison between the Kraft and hydrolysis lignins in Fig. 9, the high correlation between alkyl phenolics and aromatics is strongly dependent on the composition and structural complexity of the lignins which influences their hydroconversion reactivity.The total monomer yield ranged from 25.1 to 47.0 wt% for Kraft lignin (KES0 to KES5, Fig. 10) and from 32.7 to 76.0 wt% for hydrolysis lignin (HES6 to HES10) under the various reaction conditions. Alkylphenolics are the dominant chemical group from Kraft (7.0–22.7 wt%) and hydrolysis (11.9–24.8 wt%) lignin oil, except at higher than 10 wt% of catalyst loading for both the Kraft and hydrolysis lignins in which case the aromatics were the major compound from Kraft (18.3 wt%) and hydrolysis lignin oil (39.4 wt%). However, the proportion of the oil products composed of aromatics/naphthalenes varies between the lignin oils, where higher aromatic yields of 14.7 wt% were obtained for hydrolysis lignin (HES7) compared to 7.3 wt% for Kraft lignin oil (KES4) at the same operating conditions. When comparing the 10 wt% loading of catalyst at the same conditions (HES8 vs KES5), it is apparent that hydrolysis lignin produces higher lignin-oil yields of 83.7 wt% compared to Kraft lignin-oil of 65.1 wt% (Table 5), corresponding to 64.3 and 47.0 wt% of monomer yields respectively (Fig. 10). This has been previously confirmed using model compounds over NiS2/MoS2 [50]. It was reported that only an appropriate proportion of donor phase (NiS2) to acceptor phase (MoS2) could produce the maximum HDO activity. More specifically, the hydrogenation activity was enhanced in the presence of NiS2, leading to an increase in cyclohexane derivative selectivity and deoxygenation degree. The authors claimed that an optimal Ni/(Ni + Mo) molar ratio of 0.3 was important to achieve the highest activity with 99.8 % deoxygenation degree [50]. In our study, this ratio, determined from XPS analysis, was about 0.33, which is in good agreement with the reported optimum.In a recent study, Chowdari et al. [56] reported a total monomer yield of 45.7 wt% over 10 wt% bimetallic 20NiMoP/AC for Kraft lignin at 400 °C and 100 bar with no added solvents. These findings are in line with our results for Kraft lignin, with monomer yields in the range of 42.3–47.0 wt%. However, the aromatics were reported to be lower (8.7 wt%) for the 20NiMoP/AC catalyst in comparison to the unsupported NiMoS-SBA (18.3 wt%), whereas opposite levels in the yields of alkylphenolics resulted (25.0 wt% for 20NiMoP/AC and l6.1 wt% for NiMoS-SBA). A higher solid yield was obtained for our operating conditions, but notably at lower pressures of 50–80 bar. The adjustment of pressure, residence time (from 5 to 12 h) and catalyst loadings can likely further reduce the char formation and significantly increase lignin-oil and monomeric yields. Interestingly by increasing the catalyst loading from 5 % to 10 % (KES4 to KES5), the degree of deoxygenation is strongly increased from about 46.0 to 60.5 % and therefore resulted in an increment of aromatics from 7.3 to 18.3 wt% respectively for Kraft lignin oil. This was also evident by a reduction of the calculated oxygen content of the oils (Tables S2 and S3) and indicating that the unsupported catalyst exhibits better selectivity for deoxygenated products under nearly identical conditions.In the case of hydrolysis lignin, the effect of catalyst loading (from 5 to 20 wt%) on lignin oil yields and composition was investigated at 400 °C using the same unsupported NiMoS-SBA catalyst. By increasing the catalyst loading from 5 to 10 wt%, the monomer yield was significantly enhanced from 46.6 to 64.3 wt% with a suppression of char to 8.3 wt% (HES8). While increasing the residence time (from 5 to 12 h), the monomer yield was further enhanced from 64.3 to 70.6 wt% with a suppression of char from 8.3 down to 3.9 wt% for HES8 and HES9 respectively. Upon increasing the catalyst loading to 20 wt%., the total monomer yield was still further increased to 76.0 wt%. The char formation was suppressed considerably at the highest catalyst loading from 8.3 to 4.6 wt% (HES10) due to the depolymerization reactions involving the bimetallic NiMoS-SBA catalyst. These results are also in agreement with data reported by Chowdari et al. [56], suggesting that the repolymerization reactions leading to char are likely thermal and not catalytic, while the depolymerization reactions are catalytic. However, Chowdari et al. found that higher temperature (>400 °C) leads to the formation of more char and less oil yield [56]. In our study, the main objective was to explore reaction conditions that are favorable using an unsupported NiMoS-SBA catalyst. The results obtained at 20 wt% loading of catalyst for hydrolysis lignin showed that over 87 wt% of lignin can be depolymerized, which consists of 39 wt% of aromatics/naphthalenes with the lowest alkylphenolic yield of 10.1 wt%.Like a previously published work [59], TGA analysis was used to evaluate the volatility of lignin-oils resulting from biomass conversion. Comparison of the volatilities of Kraft (KES5) and hydrolysis (HES8) lignin-oils were performed with oils produced under the same operating conditions (400 °C, 80 bar, 5 h). Fig. 11 shows that a complete weight loss for both lignin-oils was observed when increasing the temperature to 500 °C under N2 flow. The hydrolysis lignin oil contains less thermally stable compounds, as indicated by its higher volatilization rate at 200 °C. In addition, it requires more energy (higher temperature) to completely volatilize the produced Kraft lignin-oil (>300 °C). This difference may be due to the influence of the origin of the lignins and their extraction processes. More importantly, both lignin-oils followed the TGA curve of diesel oil, particularly for hydrolysis lignin-oil [59]. This implies that the hydrolysis lignin-oil contains a higher share of low boiling point compounds than the Kraft lignin-oil.Based on the above analysis and discussion, lignin hydroconversion to produce lignin-oils, gas and residual solids involves various reaction pathways depending on the lignin composition, and operating conditions. Numerous studies have proposed and summarized schemes for the conversion of Kraft lignin by depolymerization [56,60–63]. In contrast to Kraft lignin, limited studies have investigated reaction pathways of the depolymerization of enzymatic hydrolysis lignin. Chudakov et al. and Pikovskoi et al. [64,65] proposed a unique macrostructural composition of hydrolysis lignin consisting of about 7000 peaks of deprotonated molecules [M—H]−. The largest detected molecules were decamers with molecular weights up to 1600 Da, containing up to 10 aromatic units with an average molecular weight of 150 Da per structural unit [64,65]. Pikovskoi et al. also claimed that depolymerization of hydrolysis lignin released coniferous lignin corresponding to the abundant guaiacyl structural unit of about 196 Da. In addition to Chudakov and Pikovskoi suggestions, the hydrolysis lignin in this work showed the presence of cellulosic units by means of solid-state 13C NMR (Fig. 5), as depicted in Scheme 1 .According to the reported data in Table 5, Table 6 and Fig. 10 the non-catalytic experiments favored the formation of solid-char. We suggest that the highly reactive intermediates (oxygenated compounds) formed during the thermal decomposition of lignin results in condensation (Scheme 1A) and large char formation. Generally, the alkylation reactions predominantly have a significant role to restrain the condensation reaction by multiple substitutions of the abundantly formed positively charged species (e.g., R—+O—CH3) with electron-rich active species (negatively charged Aryl, Ar–—O—R) that jointly affect the electron distribution of both lignins [67,68]. Without the presence of catalyst (Scheme 1A), a negatively charged aromatic, ring rich in electrons, is mostly formed (Ar–—O—R) and subjected to substitution of phenol-ether and/or alkylphenolic (Ar—R—+OH and Ar—+O—R). In addition to the resonance effect, the inductive effect of the positively charged moieties (R—+O—R) leads to higher electron densities, which are involved in lignin condensation by forming benzylic carbocations and thereby enhance char formation [67,68]. This suggestion is supported by the fact that the formed alcohols/ketones, olefins and CH4 were present in the lowest amounts and only account respectively for less than 0.2 and 0.3 wt% of products for both lignins during uncatalyzed reactions (KES0 and HES6).In the presence of the unsupported NiMoS-SBA catalyst (Scheme 1B–E), the hydrotreatment of both lignins becomes relevant and contributes to heterogeneous catalyzed processes at higher temperatures. Due to the difference in structural and chemical composition of hydrolysis lignin, a higher yield of small ketone and aliphatic alcohols in hydrolysis lignin-oils (13.2 wt%) were obtained due to the depolymerization and ring opening of cellulose and furan units present in hydrolysis lignin, as was evident from the 13C NMR spectrum and GCxGC results (Fig. 5 and Fig. 10). According to Shuai and Saha [69] alcohols, ketones and aldehydes can block the electron-rich sites on the aromatic ring and the benzylic cation on the side chain, which would reduce the lignin condensation. The larger amount of alcohols and ketones formed in hydrolysis lignin (Fig. 9) could be one reason for the lower char amount and enhanced monomeric yields in the liquid phase (Scheme 1B). Huang et al. [68] examined lignin depolymerization in the presence of ethanol and suggested that the C-alkylation and O-alkylation reactions are important reactions for decreasing the condensation and thereby the char formation. We therefore suggest that during the depolymerization of hydrolysis lignin, higher stability of the aromatics (negatively charged moieties) via C- and O-alkylation reactions mainly come from cleaved and dehydration of the aliphatic OH and ketones. Additionally, these small ketones/alcohols derived from lignin can be hydroconverted to small alkenes via cracking dehydration (Scheme 1C). The primary stable monomers obtained from lignin (Scheme 1D) may undergo secondary hydrogenation reactions of oxygenates to form more stable monomer products (Scheme 1E).Generality across different mechanisms can be accepted considering the complexity of the types of lignin depolymerized and may play a role in the identification of effective catalysts. In this study, evidence suggests that a combination of mechanisms hinders the formation of oligomeric CC linkages and thus blocks the re-condensation reactions. From our set of experiments and the literature [6,66–68], Scheme 2 summarizes the most important aspects hindering the repolymerization over the unsupported NiMoS-SBA catalyst. We suggest that the condensation reaction rate was slowed down by blockage of the electron-rich sites (aromatic ring) via hydrogen spillover generated on NixSy, which prevented the formation of benzylic cations. Thereafter, the negatively charged aryl molecules either undergo hydrogenolysis interactions in series or in parallel reactions with the supplied hydrogen (H+) and/or with alkylated species present in the lignin-oils. These alkylated species are present in greater quantities from hydrolysis lignin (13.2 wt%) and thus serve as promoters for depolymerization compared to the lower quantities of alkylated species from Kraft lignin (6.8 wt%).We also suggest that the stabilization of aromatic rings is enhanced by a combination of interunit C(aryl)—C(alkyl) linkages and the supplied hydrogen species as shown for hydrolysis lignin (Scheme 2B), whereas these factors are less dominant in the case of Kraft lignin (Scheme 2A). This accounts for the higher resulting monomeric yields and lower char formation from hydrolysis lignin compared to Kraft lignin. In this study, our hypothesis is built on the importance of the inductive effect that provides higher electron densities on methoxy groups (R—+O—CH3) due to the formation of interunit C(aryl)—C(alkyl) linkages, which create higher electron densities on the aromatic ring to further stabilize it and block the reactive benzylic positions.Based on literature, our proposal agrees with that of by Shuai et al. [6,69] that found that formaldehyde acted as a lignin stabilizer by blocking the reactive benzylic positions of intermediates [6]. Also in agreement with our results, Huang et al. [68] suggested that ethanol is a capping agent acting as a scavenger for formaldehyde formed by removal of methoxy groups from the lignin, that thereby suppresses repolymerization reactions involving formaldehyde [68].To summarize, lignin valorization is more efficient with hydrolysis lignin compared to Kraft lignin, which could be seen by a significantly higher amount of monomer bio-oil produced and lower char formation. This is be explained by (i) differences in the structure of hydrolysis lignin, which facilitates thermal decomposition, (ii) less lignin per mass unit in hydrolysis lignin, due to the presence of cellulose (iii) less inorganic ash in the hydrolysis lignin which could negatively affect the catalytic reactions and (iv) suppressing the repolymerization by reactions with components formed from the cellulose.In this study, we have for the first time according to our knowledge compared the reductive catalytic lignin depolymerization using Kraft and hydrolysis lignin and we found large differences. We have synthesized a highly active unsupported NiMoS catalyst that was used in this work. The use of the unsupported NiMoS-SBA demonstrated a potentially promising approach to obtain bio-oils with a high proportion of aromatic and alkylphenolic compounds. The influence of the operating conditions (temperatures, pressure and time) with various catalyst loadings of 5–20 wt%, were evaluated in terms of product yields and composition.Catalytic hydrotreatment experiments with hydrolysis lignin exhibited deeper deoxygenation performance in comparison to Kraft lignin. The increase in the reaction temperature, between 330 and 400 °C, dramatically enhanced the cleavage of COC bonds, especially increasing alkylated phenolics from 7.0 to 20.1 wt% yield respectively. On the other hand, it was observed that as the pressure increased from 50 to 80 bar and residence time increased, the yields in oil and monomeric compounds also increased, and char formation could be suppressed to a certain extent. Interestingly, the monomers yield was the highest and the char was the lowest for hydrolysis lignin at comparable reaction conditions. In addition, the NMR and GCxGC analysis demonstrated that with an increase in catalyst loading, the phenolic OH groups were decreased in the product oil which resulted in an increment in the aromatics yield. Comparing the volatilities, lignin-oils from Kraft and hydrolysis lignin showed a complete volatilization which indicates a high content of low boiling compounds, particularly for hydrolysis lignin-oil.The unsupported NiMoS catalyst displayed notable deoxygenation activity, with 87 wt% lignin-oil yield from hydrolysis lignin, with less than 5 wt% char yield. Remarkably the significant reduction of char from hydrolysis lignin compared to Kraft lignin led to an increase in water and lignin-oil yields, resulting in higher selectivity and yield of aromatics. These results highlighted the importance of the chemical stability and the nature of processed lignins that arise directly from the oligomer composition of the lignin, and their correlation between depolymerization yields and the ratio of COC and CC linkages in the lignins. From the experimental results, the higher monomeric yield from hydrolysis lignin can be explained by that the hydrolysis lignin more easily depolymerizes. Moreover, the hydrolysis lignin also contains less lignin (since it contains both lignin and cellulose) and less ash, which also are important reasons for the lower char production and higher bio-oil yield when using hydrolysis lignin. In addition, the molecular binding mechanism of hydrolysis lignin is a key to generate higher-value small molecules through depolymerization. Results suggested that the high electron densities of the formed small molecules from cellulose decomposition could interact with the aromatic ring and influence the reactivity of the benzylic carbocations formation. Apart from effective hydrotreating cleavage, the catalyst demonstrates a good activity and stability over multiple regeneration cycles; however, long testing will be required in a pilot scale reactor to assess its total lifetime in the future.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.We would like to acknowledge the collaboration with Preem, Borealis and RISE in this project. We would like to thank Sekab for kindly providing us the hydrolysis lignin. We would like to acknowledge Vinnova (Vinnväxt) and Swedish Energy Agency (P47511-1) for the funding. We would like to thank the Swedish NMR Centre for the access to NMR facilitates and Chalmers Material Characterization Laboratory (CMAL) for CHONS, XRD, XPS, SEM and TEM access/measurements.Supplementary data to this article can be found online at https://doi.org/10.1016/j.cej.2022.139829.The following are the Supplementary data to this article: Supplementary data 1

Catalytic hydroconversion of Kraft and hydrolysis lignins was for the first time compared in a batch reactor over an unsupported NiMoS-SBA catalyst. We also report the effect of key reaction parameters on the yields and properties of the products. The results obtained at 20 wt% catalyst loading for hydrolysis lignin showed the highest monomer yield of 76.0 wt%, which consisted of 39 wt% aromatics with the lowest alkylphenolics yield of 10.1 wt%. Identical operating conditions, 400 °C, 80 bar, 5 h at 10 wt% catalyst loading, were used to compare both lignins and the highest monomer yield (64.3 wt%) was found for the hydrolysis lignin, consisting of 16.0 wt% alkylphenolics and 20.1 wt% aromatic compounds. These values are considerably higher than those for Kraft lignin with its 47.0 wt% monomer yield. We suggest that the reason for high yields of monomeric units from hydrolysis lignin is that it is more reactive due to its lower ash and sulfur contents and the chemical structural differences compared to the Kraft lignin. More precisely, the bio-oil from hydrolysis lignin contained higher yields of small molecules, sourced from ring-opening of cellulose in the hydrolysis lignin, which could stabilize the reactive oligomeric groups. These yields were two to seven times higher from kraft and hydrolysis lignin, respectively, compared to those obtained without catalyst. The results showed that the NiMoS-SBA catalyst is a promising catalyst for reductive depolymerization of lignin and in addition that the regenerated catalyst had good stability for multiple reaction cycles.

The increasing growth and modernisation of the economy intensify the demand for transportation fuel and power consumption rapidly in the last few decades (Seifi and Sadrameli, 2016). Dependence on fossil energy as the ultimate energy source has resulted in the exhaustion of the world’s petroleum reserves and has led to energy price crisis (Charusiri and Vitidsant, 2017; Fazril et al., 2020). The shortage and price hike of fossil fuels accompanied by vast CO2 emissions have triggered a worldwide search for alternative and sustainable resources (Sousa et al., 2018). A wide consumption of fossil fuel in combustion largely contributes to the increase in greenhouse gas emissions in the atmosphere, thereby resulting in global climate change, acid rain and ozone layer depletion (Wang et al., 2017). Thus, the reduction of anthropogenic CO2 emissions is important to mitigate global warming (Arstad et al., 2014). These environmental issues have inspired researchers to find an alternative green fuel from renewable resources (Asikin-Mijan et al., 2018). Green fuel consists of free-oxygenated hydrocarbon compound, which is also known as clean-burning fuel with significantly low sulphur and aromatics contents and high cetane number, lubricity and renewability (Alsultan et al., 2017). Green fuel is carbon neutral because the CO2 released by its combustions is neutralised by the CO2 utilisation for plant growth, which can be used back as a raw material for biofuel generation; as a result, the net CO2 level in the atmosphere is unaffected (Pattanaik and Misra, 2017).Extensive research on biofuel generation from many renewable feedstocks, such as edible biomass and triglyceride-based biomass, has gained momentum in recent years. Triglyceride-based sources have a molecular network similar to hydrocarbon, which plays a substantial role in biofuel production (Seifi and Sadrameli, 2016). WCO can be a promising feedstock for biofuel production because the main composition in waste cooking oil (WCO) is triglyceride, and the fatty acid fractions are derived from cooking oil such as sunflower, palm, soybean, and coconut (Wang et al., 2017). These sources are considered economically viable because of their low cost and high availability, and their valorisation can also solve the issues associated with their disposal. The disposal of WCO is important in terms of the economy, environment protection and personnel safety. The abundance of WCO produced annually throughout the world has increased the demand for its rational disposal and reutilisation (Chen et al., 2014). The lack of a proper disposal collection system can result in significant disposal, odour and pollution problems (Chang et al., 2017). In the United States, 10 million tons of WCO are produced each year, whilst China generates 5 million tons of WCO annually (Lam et al., 2016). The WCO collected from restaurants, hotels, university cafeterias, hospitals, and refectories are disposed to landfills, evacuated to sewers, and utilised in the soap industry (Trabelsi et al., 2018). The use of WCO as the second generation biofuel feedstock not only improves the quality of the environment but also solves the waste disposal problem (Romero et al., 2016; Hafriz et al., 2018). Furthermore, the cost of raw material is a dominant part of the overall cost of green fuel production. Thus, the use of non-expensive, sustainable and non-value-added WCO could improve the overall financial viability (Wako et al., 2018) and promote a circular economy. However, the direct use of WCO as fuel in a diesel engine can cause problems and air pollution because of particulate matter deposits (Trabelsi et al., 2018). Therefore, several pre-treatment processes, including pre-heating, mixing with gas oil with low viscosities, emulsification, pyrolysis and transesterification, have been used to improve the properties of these edible oils (Trabelsi et al., 2018). Amongst the pre-treatment methods, pyrolysis deoxygenation (DO) is considered an inexpensive and effective way to convert WCO into lighter fractions of gasoline boiling range (Wako et al., 2018). Pyrolysis DO is a thermochemical method that breaks the chemical bonds of materials and converts them into a potential fuel product in three phases, which are liquid product, carbon-rich solid residues, and gaseous product, under oxygen-absence circumstances at a rapid heating rate (Chen et al., 2014; Lam et al., 2016; Dong and Zhao, 2018). Pyrolysis product is predominantly composed by alkanes, alkenes, dienes, aromatic compound, carboxylic acids with carbon ranging from 4 to 20 and other unsaturated compounds (Trabelsi et al., 2018). In addition to pyrolysis DO, hydrodeoxygenation (HDO) technologies have been used extensively to produce hydrocarbon-based fuel (Asikin-Mijan et al., 2016). HDO is a cracking process under high pressure in a H2 gas environment, which produces water as a by-product (Asikin-Mijan et al., 2018). However, it requires a high amount of H2 gases and high reaction pressure. Hence, due to the considerable consumption of H2, complicated equipment and high reaction operating conditions in HDO process, DO reaction is preferred because it is more economical, effective, more flexible in the choice of raw materials and suitable for industrial practices (Wang et al., 2017; Alsultan et al., 2017).The pyrolysis DO of edible oils using a selected heterogeneous catalyst is a potential candidate for renewable process-based industrialisation instead of a homogeneous catalyst. The heterogeneous catalyst is easily recycled and regenerated and is environmentally friendly, thereby increasing the product yield and decreasing the cost of liquid fuels (Wako et al., 2018). In recent years, as an environmentally friendly and low-cost value-added industrial mineral, dolomite is mainly used in construction and agricultural fields as a fertilizer. Recently, dolomites have attracted much attention as a promising basic solid catalyst for biofuel production (Mao et al., 2017; Shahruzzaman et al., 2018). Dolomites, CaMg(CO3)2, which consist of calcium carbonate (CaCO3), magnesium carbonate (MgCO3) and a very small percentage of other compounds, is widely found in sedimentary rocks deposited in marine and continental lacustrine settings (Shajaratun Nur et al., 2014). Based on recent studies by Mohammed et al. (2013) and Zhou et al. (2017) with focus on the calcination behaviour of dolomite, dolomite with high calcite content requires temperature above 900 °C for calcination, which contributes to the excellent performance in catalytic reactions.Low-cost transition metal oxides (TMOs) (e.g., Ni, Co, W, Mo, Cu, Fe and Zn) that has been extensively investigated as alternative to expensive noble metals (e.g., Pt and Pd) proven to successfully enhance the catalytic activity in DO process thus increase the yield of hydrocarbon fractions (Alsultan et al., 2017). Asikin-Mijan et al. (2018) reported that Ni-based catalyst was proven to be a good potential catalyst in improving the yield of pyrolysis oil via DO reaction. However, the presence of high acidity sites resulted in extensive deactivation due to catalyst coking and tar formation. Meanwhile, Co-based catalyst respond differently because its acidity was lower than that of Ni catalyst. Nevertheless, this paper (Asikin-Mijan et al., 2018) also reported the synergy effect of the acidity and basicity of Ca/CaO catalyst and improved the quality of green fuel and reduction in coke formation during DO reaction. The conjugation of basic and acid metals is needed to become highly selective towards the high formation of light hydrocarbon fractions (Alsultan et al., 2017). To the best knowledge of the author, no other report has covered in-depth studies of using a low-cost Malaysian dolomite via catalytic pyrolysis of WCO, to date. The present study aims to synthesise Malaysian dolomite-based catalyst, which has a basic characteristic and has been doped using selected transition metals, such as nickel (Ni), zinc (Zn), copper (Cu), cobalt (Co), and iron (Fe), which has an acidic characteristic. The physicochemical properties of the synthesised catalyst were examined. The present work also investigates the activity of the synthesised catalysts in the DO reaction of WCO. The yield and the characteristic of the liquid products were also evaluated in this study.Analytical grade nitrate salts of various metals, including iron(III) nitrate nanohydrate, (Fe(NO3)3·9H2O) (99.95%), Cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O) (99.95%), nickel(II) nitrate hexahydrate, (Ni(NO3)26H2O) (99.95%), Copper(II) nitrate trihydrate (Cu(NO3)2·3H2O) (99.95%) and zinc nitrate hexahydrate (Zn(NO3)2·6H2O) (99.95%) were purchased from Systerm Chemicals Sdn. Bhd. Grinded Malaysian dolomite (CaMg(CO3)2) was obtained from Northern Dolomite Sdn. Bhd. (Perlis, Malaysia). The grinded Malaysian dolomite was calcined at 900 °C for 4 h and was denoted as CMD900. Malaysian dolomite contained 31.0% of CaO and 20.0% of MgO. The WCO was collected from residential area in Selangor, Malaysia and treated by centrifugation at 6000 rpm for 30 min to remove the sediments from the WCO. Subsequently, the treated WCO was filtered using a wire sieve strainer (250 µ mesh size) to decant the food residue. The composition of the WCO used was analysed using GC–MS analysis with 1.97% hydrocarbon and 98.02% oxygenated compound. The composition of carboxylic acid in WCO is presented in Table 1 . Industrial-grade nitrogen gas, N2, (99.95%) was supplied by Smart Biogas Sdn. Bhd. GC analytical-grade n-Hexane (>98%) was acquired from Merck.The 5 wt% of x/CMD900 catalyst was prepared by using simple liquid–liquid blending (precipitation technique) with 5 wt% loading of a transition metal x = Fe, Co, Ni, Cu and Zn onto CMD900. A known amount of metal nitrate was dissolved in deionised water. A total of 14 g of CMD900 was dispersed into 100 ml of deionised water under vigorous stirring at 60 °C. Subsequently, the aqueous solution of a metal nitrate was added dropwise into the slurry of CMD900 with agitation for 4 h. The mixture was stirred vigorously (with 400 rpm) and heated at 60 °C for 4 h. The sample was recovered by filtration and rinsed several times with deionised water. The sample was dried at 120 °C overnight. Subsequently, the dried sample was grinded and sieved using a mesh with a size of 250 µm. Finally, the sample was calcined at 900 °C for 4 h under a continuous flow of N2 gas. The aforementioned procedure was repeated using Fe(NO3)3.9H2O, Co(NO3)2·6H2O, Ni(NO3)2 ·6H2O, Cu(NO3)2·3H2O and Zn(NO3)2·6H2O and will be denoted as Fe/CDM900, Co/CDM900, Ni/CDM900, Cu/CDM900 and Zn/CDM900, respectively.The XRD analysis was performed to identify the phase and chemical composition of the synthesised catalysts using Shimadzu XRD-600 with scan ranges from 2θ = 20–80° and scanning rate at 2°/min. Cu Kα radiation source was generated at 40 kV and 40 mA with a broad focus of 2 kW and 7 kW, respectively. The XRD patterns were compared in accordance with the Joint Committee on Powder Diffraction Standard (JCPDS) file. The surface area and porosity of the synthesised catalysts were determined by the Brunauer–Emmett–Teller (BET) method using a quantachrome instrument (model Autosorb-1). The sample was degassed at 150 °C overnight and was flown with N2 gas in a vacuum chamber at −196 °C for the desorption and adsorption processes. The basicity of the synthesised catalysts was investigated using temperature-programmed desorption (TPD) with CO2 as probe molecules. The TPD-CO2 was performed using a Thermo Finnigan TPD/R/O 1100 instrument equipped with a thermal conductivity detector (TCD). The samples were pretreated with N2 gas flow at 30 min at 250 °C. Subsequently, the samples were exposed to CO2 for 1 h to allow the adsorption process. The desorption of CO2 was detected by TCD under helium gas flow from 50 °C to 900 °C for 30 min. The surface morphology of the synthesised catalysts was examined by scanning emission microscopy (SEM) using JEOL SEM (JSM-6400). The sample was dispersed on the stub coated with a thin layer of gold by using BIO-RAS sputter.The DO reactions were conducted in a fractionated cracking system with two condensers as depicted in Fig. 1 . In a typical experiment, 150.0 g of WCO and 7.5 g of catalyst amount were added to the reactor. The reaction was performed under inert N2 flow at a flow rate of 150 cm3/min. The DO reaction was performed at a reaction temperature of 390 °C ± 5 °C for 30 min. The vapour generated during the DO reaction flowed through a Graham condenser (250–270 °C) and condensed into a liquid product in a water-cooling condenser (25 °C). The liquid product and residual oil–coke were collected from the collecting and reaction flasks, respectively. The gas product was released through the gas outlet without conducting further analysis. Two liquid fractions and a small amount of soap were observed in aqueous (top layer) and organic phases (bottom layer) of the collecting flask. These phases were then separated by decantation. The soap was removed by filtration using a filter paper while the pyrolysis oil was analysed with GC–MS. The yield of the pyrolysis oil and the conversion of WCO was calculated by using mass balance (Eq. (1) and (2)) (Choi et al., 2018). (1) Yield o f o i l p r o d u c t ( % ) = mass o f o i l p r o d u c t ( g ) mass o f W C O ( g ) × 100 % (2) Conversion o f W C O % = mass o f W C O g - m a s s o f c o k e ( g ) mass o f o i l W C O ( g ) × 100 % The composition of pyrolysis oil products was analysed by gas chromatography–mass spectrometry (GC–MS, Shimadzu QP2010) equipped with non-polar ZB-5MS column (30 m × 0.25 mm × 0.25 µm) in a split mode. The identification of unsaturated hydrocarbons was performed by interpreting GC–MS data and by comparing with the National Institute of Standards and Testing (NIST) library. The hydrocarbon fraction (%) was determined by the total area of the chromatogram of saturated (n-alkane) and unsaturated (n-alkene) straight-chain hydrocarbons (C8-C20), as shown in Eq. (3). Meanwhile, product selectivity was determined by Eq. (4) (Dong and Zhao, 2018), and the percentage removal of oxygenated compound was calculated by Eq. (5): (3) Hydrocarbon % = ∑ A r e a o f a l k e n e ( C 8 - C 20 ) + ∑ A r e a o f a l k a n e ( C 8 - C 20 ) ∑ A r e a o f t o t a l p r o d u c t (4) Product s e l e c t i v i t y % = Area o f d e s i r e d p r o d u c t Total a r e a o f t h e p r o d u c t × 100 % (5) Percentage r e m o v a l o f o x y g e n a t e d c o m p o u n d % = Σ A r e a o f o x y g e n a t e d c o m p o u n d o f W C O - Σ A r e a o f o x y g e n a t e d c o m p o u n d o f p y r o l y s i s o i l Σ A r e a o f o x y g e n a t e d c o m p o u n d o f W C O × 100 % Pyrolysis oil generated using Ni/CMD900 was selected, and its properties were analysed. The analysis behaviour of the biofuel properties has been compared with the other properties of fuel, such as pyrolysis oil from palm oil, diesel fuel, and hydrocarbon biofuel, based on literatures. The property analysis was carried out in accordance to the pre-established American Society for Testing and Materials (ASTM) standard. The pyrolysis oil density was measured with pycnometer using ASTM D 4052-09 standard, and dynamic viscosity was measured using the Fenske routine viscometer type of tube model 150 L938 (ASTM D 445-09). The acid value was tested by simple titration using ASTM 974-08e1 standard. Cloud and pour points were tested by a Petrotest machine using standard ASTM D 2500 – 66 and ASTM D97-87, respectively. Fig. 2 shows the XRD pattern of undoped dolomite, CMD900, and that with dopant catalysts (Fe/CMD900, Co/CMD900, Ni/CMD900, Cu/CMD900, and Zn/CMD900) after calcination at 900 °C for 4 h. The XRD pattern of calcined CMD900 was mainly composed of CaO and MgO. The intensified peaks that corresponded to CaO at 2θ were 32.4°, 37.6°, 54.1°, 64.3°, and 67.6° (JCPDS File: 37-1497) and MgO peaks at 2θ were 43.1°, 62.5°, 74.9°, and 78.8° (JCPDS File: 71-1176), which were in agreement with those in (Zhou et al., 2017). The high intensities of the diffractograms indicated the high crystallinity of CaO–MgO. The characteristic diffraction peaks of Fe/CMD900 exhibited at 2θ were 44.6°, 58.2°, and 77.1° (JCPDS File: 01-089-6466). For Co/CMD900, the major phases of CoO at 2θ were 34.5° and 77.2° (JCPDS File: 01-042-1300). For Ni/CMD900, the diffraction pattern clearly observed at 2θ were 43.2° and 48.5° (JCPDS File: 01-089-5881). The characteristic CuO peaks observed 2θ were 37.6°, 43.8°, and 51.2° (JCPDS File: 01-077-1898). A peak in the Zn/CMD900 pattern corresponded to ZnO, which was slightly observed at 2θ = 37.8° and 48.6° (JCPDS File: 01-036-1451). The result shows that the reflection peaks of CaO and MgO were slightly shifted along with the reduction in intensity with the presence of metal oxide that was well dispersed onto the CMD900 (Shajaratun Nur et al., 2014). This finding was consistent with the crystallinity calculation, in which the crystallite size of the modified dolomite catalyst was slightly altered after loading with transition metal (Table 2).The average crystallite sizes of the CMD900 and all doped CMD900 samples were estimated by Debye–Scherer’s equation based on the significant peaks of CaO and MgO (Table 2 ). The crystallite sizes of doped CDM900 were in the order of Fe/CMD900 > Zn/CMD900 > Ni/CMD900 > Co/CMD900 > Cu/CMD900 > CMD900. The average crystallite size increase upon the addition of metal oxide resulted in the expansion of the crystalline structure.The specific surface area, pore volume, and average pore size for CMD900 and doped CMD900 catalysts with various transition metals are presented in Table 2. The surface area of CMD900 was 12.02 m2/g. Meanwhile, all the doped CMD900 catalysts show the least improvement in terms of textural properties because the surface area has a low porous structure. The surface area was in the order of Fe/CMD900 > Cu/CMD900 > Zn/CMD900 > Ni/CMD900 > Co/CMD900 > CMD900. The low surface area of all synthesised catalysts might be caused by the sintering effect of the catalyst after calcination, which can create severe particle agglomeration (Waheed et al., 2016). Table 2 reveals that the pore sizes in CMD900 are macroporous (e.g., 63.07 nm). Nevertheless, the pore diameter of all doped CMD900 catalysts exhibited slight reduction, and they mainly consisted of mesoporous structure with a pore diameter that was within the range of 2–50 nm, except for Co/CMD900, which was still in the macroporous range size (Shajaratun Nur et al., 2014). The pore size of all the synthesised catalysts increase in the order of CMD900 > Co/CMD900 > Ni/CMD900 > Zn/CMD900 > Cu/CMD900 > Fe/CMD900. However, this increase can still provide a wide channel for the diffusion of reactant for the catalytic activity (Asikin-Mijan et al., 2018). In addition, the reduction of pore volume after dispersion with metal dopant onto CMD900, except Fe/CMD900, were shown in Table 2. The trend of the pore volume was presented as follows: Fe/CMD900 > CMD900 > Zn/CMD900 > Co/CMD900 > Ni/CMD900 > Cu/CMD900. The decrement in pore size might be due to the pore blockage caused by the metal dopant particles that filled inside the CMD900 pore (Waheed et al., 2016).The basicity characteristic of all synthesised catalysts was summarised in Table 2 and Fig. 3 . The basicity trend was increased in the order of Ni/CMD900 > CMD900 > Cu/CMD900 > Co/CMD900 > Zn/CMD900 > Fe/CMD900. The transition metal doped CMD900 catalyst (expect Ni/CMD900) showed low intensity on CO2 desorption peaks at low temperature (624 °C to 702 °C) when compared with the CDM900 catalyst (Table 2 and Fig. 3). This finding suggested that the presence of a considerable amount of weak acid sites on the CDM900 catalyst surface was due to the successful dispersion of 5 wt% of transition metals on the CMD900 catalyst surface. According to Asikin-Mijan et al. (2018) the presence of acid sites was attributed to the Bronsted acid sites associated with the bridging of OH groups and/or the Lewis acid sites associated with the presence of transition metal ions. The TPD profiles of CMD900 and all doped CMD900 catalysts shown in Fig. 3 exhibited considerably high basic strength with CO2 desorption peak at a temperature of more than 500 °C. The basic strength distribution from the catalyst active sites is expressed in terms of CO2 desorption temperature. The basic strength (Tmax = 822 °C) and basic density (7306.37 μmol/g) of Ni/CMD900 catalyst significantly increased because of the synergy effect promoted by the interaction between NiO-CaO/MgO and the increment in active sites that resulted from the dispersion of NiO on the CMD900 surface. The characteristic of basicity is important to inhibit coke formation and enhance the cracking reaction (Kay Lup et al., 2017).The morphological features of CMD900 and doped CMD900 catalysts at 60,000× magnification were shown in Fig. 4 . The SEM images of CMD900 showed an agglomeration structure of particles and appeared in finer and smaller uniform sizes because of the sintering effect of metal oxides during calcination. The individual grain’s segregation can be clearly observed and identified. However, the SEM morphology of the active metal-doped catalyst rendered significant changes in morphology for surface structure into larger irregular aggregate and rough surface. The active metal-doped catalysts also displayed a more agglomerated structure, which indicates well-dispersed metal oxide on the dolomite catalyst surface. The segregated grains of dolomite become increasingly invisible. As evidence, the crystallite size of active metal-doped catalysts increased with the dispersion of transition metals, thereby implying the growth of the catalyst crystal. A similar observation was reported by Azri et al. (2020) when metal was added to dolomite as an effective catalyst for glycerol hydrogenolysis. In addition, the active metal-doped catalysts comprised small cluster, which resulted in high surface area of the catalyst (Table 2).The conversion of WCO and WCO deoxygenated product distribution for all synthesised catalyst are shown in Fig. 5 . The conversion of WCO followed the sequence: Ni/CMD900 > Fe/CMD900 > Co/CMD900 > Zn/CMD900 > Cu/CMD900 > CMD900. The DO of WCO will lead to the production of the liquid product via decarboxylation and decarbonylation with the release of gas and acid phase as the by-products, respectively. The liquid products obtained from all DO reactions were divided into two separate fractions, namely, pyrolysis oil and acid phase with insignificant soap formation. The pyrolysis oil produced were in the order of Ni/CMD900 > Zn/CMD900 > Fe/CMD900 > Co/CMD900 > Cu/CMD900 > CMD900. This finding shows that TMO is a good promoter and has played an important role in tuning product selectivity towards monofunctional hydrocarbon intermediates, which further converted into the desired liquid product (pyrolysis oil).The acid phase composed of water and a high amount of carboxylic compound was detected at the bottom phase of the liquid product. This finding suggested that the decarboxylation reaction was more preferred than the decarbonylation reaction. Ni/CMD900 catalyst produced a lesser amount of acid phase (0.1%) in the liquid product as compared with other catalysts. Nevertheless, white precipitate, (or soap) was observed in all liquid products catalysed by doped CMD900 catalysts. Ni/CMD900 shows the lowest amount of soap formed (6.7%) but still slightly higher than the un-doped CMD900 catalyst (4.8%). Supposedly, the formation of undesirable by-product (coke) must be avoided during the DO reaction for an ideal reaction. However, all the reactions with all catalysts tested yielded substantial coke formation in the following order: CMD900 > Cu/CMD900 > Zn/CMD900 > Co/CMD900 > Fe/CMD900 > Ni/CMD900. Asikin-Mijan et al. (2018) reported that the coke generated via polymerisation reaction in the DO of triolein was expected to occur because of the poor acidic properties of active-doped metal catalyst in CaO surface. The mass balance profile suggested that Ni/CMD900 catalyst can perform actively for the DO reaction of WCO because of the high amount of basic site on the catalyst surface (Table 1). Based on a previous study (Romero et al., 2016), the composition of gas produced via DO of WCO was analysed using GC-TCD. The gas was collected for certain reaction time at a certain temperature, and the gaseous product consisted of hydrocarbon gas, CO2, CO, CH4, H2 and O2 (except calcined dolomite), using various catalysts, such as calcined Malaysian dolomite and commercial acid catalyst (e.g. FCC, Zeolite NaY, HZSM-5). Fig. 6 shows the main composition determined by GC–MS analysis for the produced pyrolysis oil catalysed by all synthesised catalysts with different catalyst dopants. This study showed that WCO was thermally cracked to a liquid hydrocarbon product and a small quantity of oxygenated compounds. As shown in Fig. 6, the trend of liquid hydrocarbon yield was in the order of Ni/CMD900 > CMD900 > Co/CMD900 > Fe/CMD900 > Zn/CMD900 > Cu/CMD900. The oxygenated compound yielded in the following order: Cu/CMD900 > Zn/CMD900 > Fe/CMD900 > Co/CMD900 > CMD900 > Ni/CMD900. Based on these findings, the DO performance of Ni/CMD900 resulted in the highest yield of liquid hydrocarbon and the lowest yield of the oxygenated compound, suggesting that the synergistic energy effect from acid–base interaction between NiO and CaO-MgO performed actively in the DO of WCO as mixed metal oxides. NiO improved the yield production of liquid products comprising mainly hydrocarbons, thereby presenting a potentially high-value chemical feedstock or fuel source. Based on Fig. 6, CMD900 catalyst contains the unique basic properties of CaO-MgO, which helps in oxygen removal by absorbing more CO2 in the gas phase. This finding is line with the result shown by Lin et al. (2010) in the pyrolysis of biomass using dolomite (MgO-CaO) catalyst as related to the reduction of tar formation with the increase in H/C ratio. The observation showed that the presence of MgO and CaO will produce a better route for oxygen removal through catalytic DO. The reduction in oxygenated compounds in pyrolysis oil product by synthesised Ni/CMD900 catalysts were also due to the existence of a large density of basic site Ni-promoted catalyst (4.40 × 1021 atom/g) as compared with other metal-promoted catalysts and hence improved catalyst stability during DO reaction. The efficiency of DO reaction has been calculated on the basis of the reduction of oxygenated compound present in pyrolysis oil generated using synthesised transition metal-doped dolomite catalyst through area of GC–MS analysis data (Eq. (5)). The trend of oxygenated compound removal was in the order of Ni/CMD900 (79.8%) > CMD900 (76%) > Co/CMD900 (71.9%) > Fe/CMD900 (67.9%) > Zn/CMD900 (65.6%) > Cu/CMD900 (62.4%). This finding showed that the catalysts can reduce oxygenated compound via desired catalytic DO pathway whilst improving the quality of the final fuel product. Based on Zhang et al. (2016), strong basic site generated from oxygen on the metal oxides created stronger forces via the abstraction of alpha hydrogen in carbonyl compound and followed by C-O scission to form hydrocarbon compound. Fig. 7 shows a formation of a high number of oxygenated by-products, such as alcohol and carboxylic acid. Fe/CMD900, Cu/CMD900, Co/CMD900 and Zn/CMD900 are amongst the catalysts with high alcohol value (e.g. 15.1%, 13.8%, 13.8% and 7.1%, respectively). For carboxylic acid, the alcohol value of the aforementioned catalysts is 4.6%, 11.0%, 5.4% and 11.7%, respectively. Ni/CMD900 catalyst contains less carboxylic acid (3.6%) with a minor quantity of ketone (2.6%), and the absence of aldehydes indicates a slight occurrence of such oxidation reaction in the DO reaction as compared with other transition metal-promoted catalysts. High acid value was obtained with presence of high amount of carboxylic acid group in the oxygenated compound. The produced pyrolysis oil contains a lower acid value with the presence of these catalysts, starting with CMD900 (33 mg KOH/ g) followed by Fe/CMD900 (40 mg KOH/ g) < Ni/CMD900 (47 mg KOH/ g) < Co/CMD900 (64 mg KOH/ g) < Cu/CMD900 (75 mg KOH/ g) < Zn/CMD900 (78 mg KOH/ g) as compared with the 186 mg KOH/g acid value of pyrolysis oil generated from non-catalytic WCO pyrolysis (Hafriz et al., 2018). The presence of transition metals doped on dolomite catalyst increased the acid value of pyrolysis oil produced as compared with the CMD900 catalyst. This phenomenon is in accordance with the low reduction of carboxylic acid upon the incorporation of transition metals on CMD900. Other researchers have suggested that carboxylic acid groups are more difficult to hydrogenate with a present transition metal relative to other functional types in bio-oil, such as ketone, aldehyde carbonyls and alkenes (Laurent et al., 1999). As reported by Li et al. (2013), lower acid value had relationship with good cold flow properties, such as the cold filter plugging point and freezing point. Fig. 8 presents the further breakdown of the main compounds into individual chemical groups, as determined by GC–MS analysis pyrolysis oil produced for CMD900 and transition metal-doped CMD900 catalyst. The main chemical groups can be classified into seven components according to their structure, namely, alkanes, cycloalkane, alkene, cycloalkene, diene, alkyne, and aromatic. The figure shows that the hydrocarbon product is dominated by aliphatic hydrocarbon, namely, alkene and alkane, for all synthesised catalysts. Evidently, aliphatic alkene was dominating the composition as compared with alkane in this liquid product with the order of Ni/CMD900 > Co/CMD900 > CMD900 > Fe/CMD900 > Zn/CMD900 > Cu/CMD900. Meanwhile, the aliphatic alkane yielded in the following order: Ni/CMD900 > Co/CMD900 > Zn/CMD900 > Fe/CMD900 > CMD900 > Cu/CMD900. In the case of Ni/CMD900 catalyst, the highest concentration in alkane was detected as tetradecane (C14H30) and tridecane (C13H28).Meanwhile, 8-heptadecene (C17H38) and 1-tridecene (C13H26) were the most abundant in alkene. The aliphatic alkane for Fe/CMD900, Zn/CMD900 and Cu/CMD900 were mostly tetradecane and tridecane, except in Mo/CMD900, in which tetradecane and dodecane (C12H26) were detected in the produced pyrolysis oil. The alkenes present for all tested metals promoted catalysts and were mostly from 8-heptadecene and 1-tridecene. The detailed characterisation of hydrocarbon fractions distribution for the deoxygenated liquid product conversion (Fig. 9 ) showed that the hydrocarbon fractions for Ni/CMD900 catalyst was composed of a mixture of n-C14 and n-C17. Meanwhile, Co/CMD900 catalyst contained n-C14 and n-C10 fractions. Zn/CMD900 and Fe/CMD900 were mostly composed by n-C13 and n-C12, whereas Cu/CMD900 contained the shortest C range, which was n-C13 and n-C10. Fig. 10 shows the composition of biofuel as determined by the carbon number of the gasoline, kerosene and diesel fraction as the petroleum product. Interestingly, CMD900 catalyst exhibits a high degree of selectivity (48.9%) of gasoline range (C4-C12). Doped CMD900 catalyst for gasoline fraction were presented in the following order: CMD900 > Co/CMD900, Fe/CMD900 > Cu/CMD900 > Ni/CMD900 > Zn/CMD900. However, catalytic DO over Ni/CMD900 catalyst were found predominantly selective towards diesel range (55.2%) when compared with other doped metal in the order of Ni/CMD900 > Zn/CMD900 > Co/CMD900 > Fe/CMD900 > Cu/CMD900 > CMD900. In addition, Ni/CMD900 also shows higher fraction towards kerosene (38.8%). The DO reactivity towards kerosene was in the order of Ni/CMD900 > CMD900 > Co/CMD900 > Fe/CMD900 > Zn/CMD900 > Cu/CMD900.The properties of the pyrolysis oil derived from Ni/CMD900 were investigated, and the results are shown in Table 3 . For comparison purposes, Table 3 also displays the specified values for the pyrolysis oil (palm oil), diesel and hydrocarbon biofuel. The results show that the fuels derived from WCO using Ni/CMD900 as catalysts possessed acceptable values for the given properties when compared with other fuels, excluding the comparison of acid value with diesel. Acid value is referred to as the oil quality indicator to monitor the oil degradation during the storage period. According to the ASTM standard for fuel application, the maximum value of acid number is 0.5 mgKOH/g (Yasin et al., 2013). The number of the acid value of pyrolysis oil is high, and pyrolysis oil can be degraded at extensive storage period when compared with mineral diesel. Therefore, studies on reducing the acid value of pyrolysis oil can be conducted by modifying the synthesised catalyst and the ratio of blending with diesel in the future. However, Ni/CMD900 is an excellent low-cost catalyst for the DO of WCO; its typical bifunctional (NiO-CaO/MgO) properties could focus on the distribution of the product and improved the fuel properties.The distribution and composition of the product were obtained through mass balance and GC–MS analysis, respectively. The general catalytic pyrolysis of WCO pathway under atmosphere free oxygen over modified Malaysian dolomite catalysts can be established in Fig. 11 , as adapted from Lestari et al. (2009). The reaction pathways consist of liquid hydrocarbon, gas and solid and soap phase reactions. The liquid phase reactions of the WCO DO process dominantly involve decarboxylation and decarbonylation reactions to produce alkanes and alkenes by releasing carbon dioxide, carbon monoxide and water, as illustrated in Reactions (1) and (2) (Hafriz et al., 2018; Kamil et al., 2020; Snare et al., 2008). Based on Reaction (3), the triglyceride of WCO also undergone hydrogen abstraction at higher temperatures to produce diene formation. Meanwhile, alkenes and diene will also undergo Diels–Alder reaction to produce cycloalkenes, as shown in Reaction (4). Idem et al. (1996) mentioned that cycloalkenes are the main hydrocarbons used for synthesising the cycles and aromatic hydrocarbons. In a hydrogenation reaction (Reaction (5)), two hydrogen atoms are added across the double bond of cycloalkenes, thereby resulting in a stable form product called saturated cycloalkanes. A side reaction, such as dehydrogenation, could occur during the removal of hydrogen from of cycloalkenes and alkenes to produce a small amount of aromatic and alkynes in a liquid hydrocarbon product, as mentioned in Reaction 6.Gas is a secondary product that is generated via the DO of WCO. According to Idem et al. (1996), the hydrocarbon radicals Ru or Rs are formed by eliminating of ethylene molecules during secondary cracking. The straight and branched-chain hydrocarbons of which in the C1–C5 range in the gas phase product were yielded through the successive elimination of ethylene molecules from the saturated and unsaturated hydrocarbon radicals followed by disproportionation, isomerisation, and subsequent hydrogen transfer reactions, as illustrated in Reaction 7. Dandik and Aksoy (1998) reported that the gaseous product generated from the catalytic pyrolysis of canola oil consisted mostly of hydrocarbon gases in the C1–C5 range. In addition, the methanation reaction (Reaction 8) could occur as a pathway because of the conversion of carbon monoxide and carbon dioxide to methane (CH4) through hydrogenation. In a review of the DO of fatty acid Hermida et al. (2015) also mentioned the water gas shift in Reaction 9 due to the presence of carbon monoxide and water via decarbonylation reaction in producing hydrogen and carbon dioxide.Solid and soap phase reactions are the side reactions involved. In the solid-phase reaction, the polymerisation of aromatic hydrocarbon and the condensation of WCO generate the coke formation, as mentioned in Reactions 10 and 11, respectively. In the soap phase reaction, calcium carbonate is produced during the absorption of CO2 by CaO because of the high formation of CaO composition in calcined Malaysian dolomite. The oxygenates, such as carboxylic acid (RCOOH), are expected to react with calcium carbonate (CaCO3) in producing fatty acid soap (Ca(RCOO)2), as illustrated in Reaction 12.This study demonstrated that the calcined Malaysian dolomite (CMD900) was successfully dispersed with various transition metals, such as Fe, Co, Ni, Cu and Zn. The result indicated that amongst all of the doped catalyst, Ni/CMD900 catalyst demonstrated the best performance in terms of the DO reaction of WCO with high conversion (68.0%), high yield pyrolysis oil (36.4%) and low coke formation (32.0%). The high availability of active basic sites on the Ni/CMD900 catalyst surface plays an important role in affecting the catalytic behaviour in the DO reaction of WCO. CMD900 can also remove high oxygen content with only 19.8% of oxygenated compound detected and produced the maximum yield of hydrocarbon product (80.2%) of C13 and C17 fractions. In addition, Ni/CMD900 catalysed reaction rendered higher selectivity towards diesel when compared with other doped CMD900 catalysts.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors acknowledge the financial support from the Ministry of Higher Education of Malaysia for Fundamental Research Grant Scheme (FRGS/11/TK/UPM/02) and AAIBE Chair of Renewable Energy Grant No. 201801 KETTHA for funding this research publication.

In the present work, nickel (Ni), zinc (Zn), copper (Cu), cobalt (Co) and iron (Fe) are tested as catalyst dopants on Malaysian dolomite calcined at T = 900 °C (CMD900). The physicochemical properties of all synthesised catalyst are investigated by X-ray diffraction, Brunauer–Emmett–Teller surface area, temperature-programmed desorption of carbon dioxide and scanning emission microscopy. The synthesised catalysts are tested on the basis of the deoxygenation (DO) reaction of waste cooking oil to produce liquid fuels under N2 atmosphere. The chemical composition of the liquid product is identified by gas chromatography–mass spectroscopy. The overall study suggests that Ni/CMD900 catalyst exhibits the highest performance with over 67.0% conversion and high selectivity (80.2%) with a high proportion of saturated linear hydrocarbons that corresponds to green diesel. Result indicates that Ni/CMD900 is a highly potential DO catalyst with 19.8% oxygenated compound, which is favourable for decarboxylation and/or decarboxylation predominates.

Proper control of atmospheric CO2 content to fight climate change is one of the central challenges of mankind. Anthropogenic CO2 is mainly produced via combustion of fossil fuels and is currently producing a notable environmental impact, such as in global warming [1]. Fortunately, CO2 can be captured and transformed into other valuable chemicals (e.g., formaldehyde, methane, methanol or bicarbonate, among others [2–6]) with the help of transition metal (TM)-based catalysts [7], mainly through Au, Pd, Cu, Ru, Ni, Pt, Pd, Rh or Fe [8–14]. However, since some of those TMs are expensive and scarce, it is still imperative to develop better catalysts to increase the efficiency and reduce the cost of the CO2 conversion.In the last few years, single-atom catalysts (SACs) finely dispersed into different supports have emerged as new promising materials for catalysis [15,16]. SACs aim to combine the large activity and selectivity of homogeneous catalysts but with the separation and reutilization capabilities of a heterogeneous catalyst [17,18]. Supported SACs have a higher activity/mass relationship due to a better metal utilization than nanoparticles, which makes them also more cost-efficient for catalysis. Some of the early successful reactions were carried out in oxide and graphene supported SACs including CO oxidation [19], water-gas shift reaction [20,21], hydroformylation of olefins [22] and methanol and benzene oxidations [23,24]. The great activity of SACs is correlated to their low coordination numbers, which means they can be partially deactivated if they diffuse through the support and aggregate forming nanoparticles. For this reason, it is not only important to find a good SAC for a given application, but it is also critical to disperse it into a support that can stabilize it (i.e., prevent the metal atoms from clustering). In that sense, many efforts are devoted to preventing SAC surface migration by depositing the TM in surface vacancies [25–28], or spatially confining them in microporous materials (e.g., zeolites or metal-organic frameworks) [29–32].From all those promising supports, we have turned our attention to zeolites [33], where important successes were achieved in the last years by encapsulating different TM atoms in structures with different Si/Al ratio to carry out CO oxidation [34], methane conversion to higher hydrocarbons [35], to methanol and to acetic acid [36] or n-hexane isomerization [37], among others [16]. Pd SACs were also anchored to mesoporous silica SBA-15 [38] and used to hydrogenate alkynes. Finally, Ru and Rh SACs were recently encapsulated in the fully silicated MFI structure (i.e., TM1@Silicalite-1 or simply TM1@S-1) [39,40] and the resulting catalysts were promising for H2 production from ammonia borane hydrolysis and ammonia synthesis, respectively. The encapsulation of metal atoms in zeolites requires a strict control of experimental conditions, since high pH and/or temperature might lead to particle aggregation [41]. Available synthesis techniques include direct synthesis from inorganic or ligand-assisted metal precursors, multistep post-synthesis encapsulation (e.g., two-step dry-gel-conversion) or ion-exchange followed by reduction, as described by Chai et al., [42]. From the very large number of potential SAC + support combinations, only less than 10 TMs have actually been synthesized as SACs. Such a small number evidences the need of a systematic screening for catalysts with large activity whilst still being stable at operative conditions. In a previous study [43], we computationally assessed the structure and stability of all period IV-VI TM1@S-1 (except for Tc due to its radioactive nature), showing that TMs can be encapsulated via dispersion + electrostatic interactions in the MFI framework, which can be quite strong since the adsorption energies range from -0.48 eV (for Cu and Zn) to -1.67 eV (for Pt).Herein, we evaluate the potential activity of this set of SACs for CO2 conversion. Due to the large number of systems included in this screening study and the large amount of possible reaction products, it was not feasible to build the full reaction profiles for each SAC. Instead, we focus on the first steps of CO2 activation, which involve the adsorption of reactants (CO2 and H2), direct CO2 dissociation through the redox mechanism (CO2 → CO + O), H2 dissociation (H2 → H + H) and hydrogen-assisted CO2 dissociation through the associative mechanism, either via formate (CO2 + H → HCOO) or carboxylate (CO2 + H → COOH) intermediates.Note that the experimental viability of TM1@S-1 synthesis was already proven for Rh1@S-1 and Ru1@S-1 [39,40], so the results from this study will serve to assess how adequate are those catalysts in comparison to other non-synthesized TM1@S-1 and for proposing novel catalysts capable of adsorbing and converting CO2. The results obtained here will provide a solid theoretical background from which potential catalytic activity can be predicted, paving the road for further experimental and computational studies on this topic.The MFI Silicalite-1 has a microporous Si96O192 unit cell composed by SiO4 tetrahedra (T) units positioned at 12 non-equivalent T sites (i.e., T1-T12). The framework O atoms are located at 26 distinct O sites (i.e., O1 - O26). This arrangement leads to a 3D pore system with straight ten-membered-ring (10-MR) channels in the [010] direction intersected by sinusoidal 10-MR channels in the [100] direction, as shown in Fig. 1 . The most stable structures of each one of the 29 TM1@S-1 SACs were taken from our previous study [43]. In summary, we employed Density Functional Theory (DFT) calculations to find the most stable site to adsorb each TM in the pristine S-1 structure by optimizing the geometry of each TM placed in all possible non-equivalent positions of all pores. With this procedure, we located only 6 preferred sites in S-1 by the entire set of TM adatoms. Most TM atoms adsorb via van der Waals (vdW) interactions into the sinusoidal 10-MR channels at distances larger than 2.7 Å. Among all possible positions in these channels, only 3 specific adsorption sites are preferred by the TM atoms, which are denoted as Channel X (XA, B or C) sites (see Fig. 1). The only exceptions are group 3 T Ms (i.e., Sc, Y and La), which prefer to adsorb in the middle of the quadrilaterals formed by two O16 and two O25 atoms (i.e., O16( × 2) - O25( × 2) site); Group 10 T Ms (i.e., Ni, Pd and Pt) and Ru atoms, which are found closely coordinated with O18 and O23 atoms (i.e., O18 - O23 site) and weakly coordinated to O16( × 2); and finally, Rh atoms, which are located in the middle of the quadrilaterals formed by O8, O11, O18 and O26 atoms (i.e., O8 - O11 - O18 - O26 site). In all those systems the porous support provides protection against SAC sintering via 3D confinement, except in group 3 TMs where the S-1 also accepts part of their electron density changing their electronic structure. Notice that, Ru and Rh are the only TMs experimentally encapsulated in Silicalite-1, and their coordination according to EXAFS fittings agree with our DFT-based predictions, both suggesting TM coordination with four O atoms. However, DFT results slightly overestimates the average Ru - O distance (r (DFT) = 2.15 Å) by a 7% [40] and the Rh - O distance (r (DFT) = 2.40 Å) by a 18% [39] with differences in TMO bonds lower than 0.01 Å between DFT and DFT-D3 geometries. For a more complete description of those sites, the reader is referred to the original work [43]. Additionally, images of each TM location are compiled in Fig. 1.To be consistent with our previous study [43], the Vienna Ab Initio Simulation Package (VASP) [44] was used to perform all periodic DFT calculations by employing the Perdew-Burke-Ernzerhof [45] exchange-correlation functional, plus the Grimme D3 dispersion correction (PBE-D3) [46]. The valence electron density was expanded in a 600 eV kinetic energy plane-wave basis set, which gave total energy variations below 0.01 eV. The effect of core electrons on the valence electron density was accounted through the Projected Augmented Wave (PAW) method [47], as implemented in VASP by Kresse and Joubert [48]. Spin-polarization was taken into consideration to reflect the TM1@S-1 magnetic properties. Due to the large size of the simulation cells (i.e., around 290 atoms with a = 20.09 Å, b = 19.74 Å and c = 13.37 Å), only the Γ -point was used to sample the Brillouin zone.The most stable site for the adsorbed species was obtained by screening several initial geometries with different positions and orientations. The tolerance for the conjugate gradient algorithm to minimize energy and forces on atoms was set to 10−5 eV and 0.01 eV/Å, respectively. Adsorption energies ( Δ E a d s , i ) were calculated as: (1) Δ E a d s , i = E i - S A C - E S A C - E i ( g ) where E i - S A C is the total energy of adsorbed i species in the TM1@S-1 SAC, E S A C is the energy of the clean SAC (i.e., the relaxed pristine TM1@S-1 structure) and E i ( g ) is the energy of species i in gas-phase and in its ground electronic state. With this definition, negative values of Δ E a d s , i indicate favorable adsorption. The latter term was calculated in a simulation cell with the same parameters than the TM1@S-1 using only the Γ -point. The energy barriers ( Δ E ≠ ) and reaction energies ( Δ E r ) were calculated as: (2) Δ E ≠ = E T S - S A C - E R - S A C (3) Δ E r = E P - S A C - E R - S A C where E T S - S A C is the energy of the transition state (TS), E R - S A C is the energy of the initial configuration (i.e., adsorbed reactants), and E P - S A C the energy of final configuration (i.e., adsorbed products). All TSs were located by using the Climbing-Image Nudged Elastic Band (CI-NEB) method [49]. The initial guesses for the employed intermediate images were created through the Image Dependent Pair Potential (IDPP) interpolation procedure [50] as implemented in the Atomic Simulation Environment (ASE) [51]. All adsorption minima and TSs were characterized through frequency calculations by computing the elements of the Hessian matrix as finite differences of 0.03 Å length and considering only displacements of the adsorbate. Note that the Zero Point Energy (ZPE) term is included in all reported energy values unless otherwise indicated. Additionally, the Gibbs free energy of adsorption, reaction and free energy barriers were calculated by correcting the respective E values in Eq. 1–3 with the corresponding temperature/pressure correction to the free energy. For gas-phase species, the correction was obtained using the ideal gas approximation, whereas for adsorbed species the harmonic oscillator model was used for all degrees of freedom [52].Finally, to further understand the interaction between metal atoms, reactant molecules and the support, we computed the atomic charges on the supported transition metal atoms ( Q T M ), the total net charges on the zeolite ( Q Z e o ) and on the adsorbed CO2 and H2 species ( Q C O 2 and Q H 2 ) through a Bader analysis of the electron density [53].Prior to CO2 conversion by either the redox or associative mechanisms, CO2 and H2 species must adsorb on the SAC and, for H2, it must also dissociate to give rise to adsorbed H species that are required in the associative pathway. In this section, we study the molecular adsorption of CO2 and the adsorption and subsequent dissociation of H2 species, evaluate their geometry and their electronic structure.CO2 physisorbs on the pristine S-1 structure mainly by vdW interactions, leading to a weak Δ E a d s , C O 2 (see Fig. 2 a–d and Table S1 in the SI). However, the presence of single TM atoms in general leads to a stronger chemisorption of CO2, which typically interacts with the metal atom through the C - O bond (i.e., η(CO) configuration in Fig. 3 ) with an average C - TM distance of around 2 Å. During this adsorption step, the C - O bond is elongated from the gas-phase value of 1.18 Å to 1.22 - 1.46 Å and the O - C - O angle is bent from 180° to 120 - 150°. This activated configuration leads to a significantly strong adsorption energy (i.e.,   Δ E a d s , C O 2 ≲ - 1 eV). In general, the binding strength of CO2 on the supported TMs decreases along a period and moving up along a group (Fig. 2a), with a limit on group 11 and 12 TMs, where CO2 weakly physisorbs with no noticeable perturbation from its gas-phase geometry. Other exceptions are those TMs with semi-occupancy of d orbitals (i.e., Mn and Re with s2d5 ) and group 4 TMs, where CO2 does not have a stable configuration (i.e., Hf) or where it binds in a η(O) configuration instead of η(CO) (i.e., Ti and Zr). Notice that, for Hf, Δ E a d s , C O 2 is calculated by considering CO + O as the adsorbed state.During the adsorption process, there is a significant TM-to-CO2 charge transfer, which is evidenced by the final oxidation state of the metal atom (i.e., Q T M > 0 ) and the negative net charge on CO2, as shown in Fig. 2b. The atomic Bader charges on the TM atoms decrease along a period, which in part can be rationalized due to the higher electronegativity of the metal atom. On group 11 and 12 T Ms, the charge transfer is negligible, due to the weak CO2 – TM physisorbed interaction, and therefore the net charges on the TM and CO2 are almost zero. Despite the fact that in most cases there is only charge transfer between the TM atom and CO2 (i.e., Q T M + Q C O 2 ≈ 0 ), in the case of group 3 TMs, there is also a significant TM-to-zeolite charge transfer evidenced by the negative net Bader charge on the zeolite. This charge transfer occurs due to the very strong interaction between group 3 TM atoms and the zeolite support, as discussed in our previous work [43]. When this occurs, part of the charge density still remains in the zeolite after CO2 adsorption. Interestingly, stronger adsorption energies are correlated with longer C - O bond lengths, while more negative Q C O 2 are correlated with longer C - O bond lengths and lower O - C - O angles (see Fig. S1 in the SI).H2 also physisorbs into the pristine S-1 via vdW interactions leading to a weak   Δ E a d s , H 2 (see Fig. 2g–i and Table S2 in the SI). In the presence of TMs from groups 3, 4, 5, 9 and 10, as well as for Os, H2 chemisorbs with Δ E a d s , H 2 values up to - 0.99 eV and in a η(HH) configuration, as illustrated in Fig. 3. In those TMs, the equilibrium H - TM distance lays between 1.5 Å and 2.5 Å and the H - H bond is elongated from 0.75 Å (gas-phase) up to 1.00 Å. Within this sub-set, the H2 binding strength increases when moving right along a period and down in a group. Additionally, H2 breaks spontaneously (with no energy barrier) in Ru, Rh and Pt. In these cases, Δ E a d s , H 2 values are calculated by considering H + H as the adsorbed state. In the remaining TMs (i.e., groups 5, 6, 7, 8, 11 and 12, except Os and Ru), H2 weakly physisorbs and no relevant change with respect the gas-phase geometry or charge transfer is observed. Noticeably, Δ E a d s , H 2 and Q H 2 are correlated with the H - H distance in the adsorbed state, as shown in Fig. S2 in the SI.The reaction energies and energy barriers for H2 dissociation are compiled in Fig. 2j,k and in Table S2 in the SI. The pristine S-1 framework does not dissociate H2, since the weak binding with the framework atoms results in a gas-phase-like highly endoergic reaction with a prohibitive energy barrier larger than 5 eV. However, H2 dissociates in almost all considered SACs with low energy barriers, being barrierless for Ru, Rh and Pt, as mentioned above. Interestingly, H2 dissociation is exoergic in almost all group 3–9 TM SACs, with Δ E r up to - 2.20 eV and Δ E ≠ ≤ 1.02 eV. Those barriers are comparable to other good H2 dissociation catalysts such as CeO2 [54], TM1/CeO2 [55], (Ni2, Cu2)/MgO [56] or TM carbides [57], which show Δ E ≠ values from almost zero to 0.8 eV. On the other hand, in TMs from groups 11 and 12 the reaction becomes endoergic and Δ E ≠ increases significantly, with prohibitive values for group 12 SACs. The most common dissociation pathway for TMs where H2 adsorbs in η(HH) configuration involves further H - H bond elongation, yielding to the coadsorbed H + H on the TM atom. Otherwise, when H2 only physisorbs around 3 Å from the TM, it first needs to get closer to the metal, relocate as η(HH) and then, break.It is worth noting that for TM SACs adsorbed very close to the S-1 zeolite wall (i.e., group 3, group 10, Ru and Rh SACs) the CO2 adsorption typically promotes a small surface reconstruction, where the TM + CO2 pair separates slightly from the zeolite wall to reduce the repulsive CO2 - zeolite interactions. Similarly, the H2 adsorption only promotes a small surface reconstruction on top of Ni and Pd TM SACs due to the H2 smaller size and weaker H2 - zeolite repulsion. However, the cleavage to H + H leads to an equivalent reconstruction of group 3, Rh and Pt TMs. For a more precise picture of the surface reconstruction the reader is referred to Section S4 of the SI, where the original TM1@S-1 structure and the deformation caused by CO2/H2 is compiled for the full set of TMs. Also, all geometry files for each stationary point characterized in this work have been uploaded to a public repository (see Appendix A).The redox mechanism involves the C - O bond breaking by direct CO2 dissociation to CO + O. The calculated reaction energies and energy barriers for this step are compiled in Fig. 2e-f and Table S1 in the SI. For the un-aided CO2 dissociation in the pristine S-1 with no TM, the weak interactions among the support, the reactants and the products lead to a gas-phase-like highly endoergic reaction with a prohibitive energy barrier ( Δ E ≠ > 6 eV). However, the presence of single TM atoms stabilizes the reaction products, yielding to affordable reaction barriers and even barrierless dissociations in a few cases. Δ E r and Δ E ≠ follow a similar trend than the Δ E a d s , C O 2 along the periodic table, meaning that the reaction is more favourable in TM SACs from the bottom left of the periodic table and less favourable when moving right along a period and up along a group. As shown in Fig. 2e, the reaction is highly exoergic (i.e., high negative values up to - 2.8 eV) for early TMs, and proceeds with very low reaction barriers (i.e., 11 TM1@S-1 catalysts belonging to groups 3 - 8 break CO2 with Δ E ≠ < 0.30 eV, 5 of them with Δ E ≠ < 0.10 eV). These values contrast with the typical Δ E ≠ ranging from 0.38 eV to 0.90 eV reported for other catalysts [58], such as flat metal surfaces [59–61] or supported metal clusters [62–64], placing many TM1@S-1 from groups 3 - 8 as extremely active towards direct C - O bond cleavage. In those TMs, CO2 is expected to be transformed via redox pathway, because it follows a single unimolecular step with a very low reaction energy barrier. In contrast, TM1@S-1 of groups 10 - 12 are poor catalysts for the direct CO2 dissociation, with prohibitive energy barriers due to their inability to stabilize the reaction products and their weak interaction with CO2. This implies that hydrogen-assisted associative pathways could dominate in these cases.As suggested by Pallasana and Neurock [65] and popularized by Nørskov et al. [66], the correlation between reaction energies and energy barriers in heterogeneous catalysed reactions is expected to follow the Brønsted-Evans-Polanyi (BEP) [67,68] relationship. Specifically, the transition state energy ( E T S ) and the dissociative reaction energy ( E r ) defined by Wang et al. [69] as Eqs. 4 and 5 (without ZPE) are used to compare with the universal BEP relation for extended TM surfaces. (4) E T S = E T S - S A C - E S A C - E i ( g ) (5) E r = E P - S A C - E S A C - E i ( g ) Indeed, a quantitative BEP relationship for CO2 dissociation on TM1@S-1 emerges when plotting E T S in front of E r as shown in Fig. 4 . The obtained scaling line has a similar slope than the universal BEP relation [69], but with E T S values about 1 eV lower. This fact evidences that S-1 encapsulated TM SACs are much more active for C - O bond scission than their extended TM counterparts. Note that several TM1@S-1 feature negative E T S and E r values, unachievable by extended metal surfaces, which only get as low as E T S = 0.88 eV and E r = - 2.00 eV. Unfortunately, the activation of H2 does not seem to follow a clear BEP relationship as shown in Fig. S3 in the SI.The most common dissociation pathway involves further C - O bond elongation of the adsorbed CO2 in η(CO) configuration, yielding to the reaction products CO and O, coadsorbed on the TM atom. There are, however, a few alternative dissociation pathways followed by some TM1@S-1, which we describe below. In the case of La1@S-1, the CO2 + La pair needs to slightly separate from the S-1 wall in order to have enough space to accommodate the products. During this translation, the CO2 molecule dissociates in an apparently barrierless process, with the imaginary frequency of the located TS corresponding to the translation of the CO2 + La pair moving away from the S-1 wall. For Ti and Zr, where CO2 is adsorbed in η(O) configuration, it breaks with no energy barrier before reaching the η(CO) configuration, and the imaginary frequency of the TS corresponds to a rotation from the η(O) to the η(CO) configuration, which also includes an elongation of the C - O bond. On the other hand, Hf spontaneously breaks the CO2 into CO + O. Next, for TMs of group 11, the reaction is highly endoergic, and the system must overcome a very large endothermicity to bring the CO2 closer from its initial physisorbed state to a η(CO) configuration. Then, the C - O bond breaks and the O atom is first adsorbed onto the TM. Since the coadsorbed CO + O products are not stable, the adsorbed O species diffuses to the opposite side of the TM and then CO stays adsorbed through the C atom. Finally, for the case of group 12 TMs, the reaction is even more endoergic due to the low stability of the reaction products. The CO2 dissociation proceeds through a direct gas-phase-like elongation of the C - O bond from its η(C) physisorbed state leading to adsorbed O and a weakly physisorbed CO in a η(C) configuration. Further information along with snapshots of the reaction pathway are found in Section S4 of the SI.As mentioned above, many TMs exhibit extremely low reaction energy barriers to convert CO2 through the redox pathway. Since the dissociation is a unimolecular step, it is not expected that bimolecular steps (such as hydrogenations in the associative pathway) will be kinetically relevant, even in the case of low energy barriers. For this reason, we have decided to consider this alternative pathway only in a subset of TM1@S-1 in which the redox mechanism has a significant energy barrier. This includes the late TMs in group 10 (i.e., Ni, Pd and Pt), where CO2 adsorbs strongly but it is hard to break, and also Ru and Rh, which have been already synthesized experimentally, proven to be highly active catalysts, and can dissociate H2 spontaneously.In those five TMs, CO2 adsorbed as η(CO) can react with a coadsorbed H species by forming an O - H bond leading to the carboxyl intermediate (i.e., COOH), which is bonded to the TM via the C atom (see Fig. 3). Alternatively, the H atom can react with the C atom forming a C - H bond leading to the formate intermediate (i.e., HCOO), which is first produced in its monodentate configuration (m−HCOO), bonded to the metal through one of its O atoms. Then, m−HCOO rearranges itself leading to the lower energy bidentate configuration (b−HCOO), in which the formate is bonded to the metal through both its O atoms. The reaction energies and energy barriers for both pathways (plus the redox for comparison) are compiled in Fig. S4 and Table S3 in the SI. To compare those pathways in actual operative conditions, the Gibbs free energy diagrams at 600 K and 1 atm were built and shown in Fig. 5 . To our knowledge, CO2 conversion in TM1@S-1 is currently not assessed experimentally, so those conditions were selected as they are generally used in similar systems [40,70] or other CO2 conversion catalysts [71]. Finally, the geometry for each minimum and TS are shown in Section S4 of the SI.The present results suggest that CO2 conversion steps on group 10 TMs (i.e., Ni, Pd and Pt) should follow the associative mechanism via COOH or HCOO intermediates, as the free energy barrier for their formation are much lower than for the direct CO2 dissociation (Fig. 5). In the case of Ni, the HCOO pathway is much more favoured than the COOH pathway. However, after H2 dissociation, the H + H recombination is barrierless, implying that the activity of the associative mechanism will be limited by the small amount of H species present in the catalyst. For Pd and Pt, the dissociation of H2 is very favoured and there will be a strong competition between the HCOO and COOH pathways, as both exhibit similar free energy barriers. For this reason, kinetic modelling techniques such as kinetic Monte Carlo [61] or microkinetic modelling should be employed to truly identify which pathway is more dominant. However, developing an accurate model for the TM1@S-1 catalysts requires further study due to their uncommon 3D morphology and is out of the scope for this work. Finally, we show that Rh and Ru would follow the redox mechanism, since, apart from being unimolecular, have lower free energy barriers compared to COOH or HCOO associative pathways.CO2 adsorption and subsequent activation have been investigated by means of periodic DFT calculations for the full set of 3d, 4d, and 5d TM SACs supported on zeolite S-1, namely TM1@S-1. The steps considered include adsorption and dissociation of CO2 and H2 species, as well as CO2 reaction with adsorbed H to produce COOH or HCOO intermediates. The catalytic properties are mainly controlled by the encapsulated TM atoms, since CO2 and H2 interact very weakly with the S-1 framework structure. In general, CO2 binds strongly to TM atoms and receives electron density from the TM, ending up being negatively charged. These effects are particularly strong for those TMs in the bottom left of the periodic table and become less pronounced when moving right along a period or up in a group. On the other hand, H2 chemisorbs on TMs from groups 3, 4, 5, 9, 10 and Os with negligible charge transfer from the TM. Noticeably, TMs from groups 11 - 12 hardly interact with CO2 and H2, where both species are physisorbed. TMs in groups 3 - 9 exhibit very low energy barriers for direct CO2 dissociation, suggesting that CO2 conversion reactions on these SACs would follow the redox mechanism. However, the COOH or HCOOmediated associative mechanism is more favoured in group 10 TMs, according to the free energy profiles. Finally, we predict group 11 - 12 TMs SACs to have a very poor catalytic activity for CO2 conversion, due to their weak interaction with CO2 and H2 as well as high energy barriers.Combining the present results with the stability assessment of supported TM SACs in our previous contribution [43], we conclude that groups 3 and 10 TM SACs, as well as Rh1@S-1 and Ru1@S-1 are very promising candidates for CO2 conversion reactions. All these catalysts exhibit low energy barriers and are predicted to have a strong resistance to aggregation/sintering due to their strong interactions with the zeolite wall. These results are in agreement with experimental observations stating that Ru and Rh were encapsulated in S-1 and used in different catalytic processes [39,40] with good stability and superior catalytic activity. With this contribution, we hope to narrow down the materials space for novel CO2 conversion SACs and provide a solid theoretical background from which observed experimental features can be interpreted, understood, and discussed. Gerard Alonso: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Estefanía López: Methodology, Investigation, Writing - review & editing. Fermín Huarte-Larrañaga: Methodology, Investigation, Writing - review & editing. Ramón Sayós: Project administration, Funding acquisition, Writing - review & editing. Hector Prats: Conceptualization, Methodology, Formal analysis, Writing - review & editing. Pablo Gamallo: Supervision, Methodology, Investigation, Funding acquisition, Writing - review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Support to this research is granted by the Spanish Ministry of Science, Innovation and Universities (Grants RTI2018-094757-B-I00, MCIU/AEI/FEDER, UE and MDM-2017-0767) and by the Generalitat de Catalunya (Grant 2017SGR0013 and P.G. Serra Hunter Associate Professorship). Authors thank to the Red Española de Supercomputación (RES) for the supercomputing time granted (QS-2021-1-0035 and QS-2020-3-0023).Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2021.101777.The following is Supplementary data to this article:

Zeolite-supported single-atom catalysts (SACs) have emerged as a novel class of cheap and tuneable catalysts that can exhibit high activity, selectivity and stability. In this work, we conduct an extensive screening by means of density functional theory calculations to determine the usefulness of 3d, 4d and 5d transition metal (TM) SACs-supported in MFI-type Silicalite-1 zeolite for CO2 conversion. Two reaction mechanisms are considered, namely the redox - direct CO2 dissociation - and associative - hydrogen-assisted CO2 dissociation - mechanisms. Early TM SACs exhibit the lowest energy barriers, which follow the redox mechanism. These energy barriers raise when going right in the periodic table up to group 10, where they become prohibitive and the associative mechanism should dominate. By also considering their resistance to aggregation, we support the use of Sc, Y, La, Ru, Rh, Ni, Pd and Pt as potentially active and stable catalysts for CO2 conversion, given their low energy barriers and strong interaction with the zeolite framework.

During the past decades, the selective hydrogenation of alkynes/alkynols has drawn significant attention. The partial hydrogenation of carbon-carbon triple bonds (C≡C) in the presence of carbon-carbon double bonds (C=C) is primarily applied in the purification of pyrolysis gas, which is of paramount significance in industrial sectors. 1 Likewise, it is an efficient tactic for the preparation of fine chemicals and pharmaceuticals like linalool, vitamins, and natural products through the selective hydrogenation of unsaturated alkynes or alkynols. 2–5 In this review, we discuss the origin and evolution of efficient catalysts for the hydrogenation of alkynes/alkynols, for example, modified Pd-based catalysts, 2 , 3 , 6–15 alloy/intermetallic catalysts, 16–21 and single-atom catalysts (SACs) with corresponding representatives. 22–39 Analogously, some existing issues including “run-away” 40 , 41 and “green oil” 21 , 42 problems in the gas phase, as well as hydrogen solubility 43 and metal leaching 44 in the liquid phase are analyzed. In a similar fashion, several kinds of typical hydrogenation mechanisms like the Horiuti-Polanyi mechanism (so-called dissociative mechanism), 40 , 45 the associative mechanism, 45 and the Eley-Rideal mechanism 46 are discussed, and their applicability in hydrogenation reactions are compared. Multiple techniques, including in situ Fourier transform infrared spectroscopy (FTIR), 29 , 32 , 33 , 47 , 48 temperature-programmed desorption (TPD), 32–35 , 40 , 48 , 49 H2-D2 exchange, 50–52 solid-state nuclear magnetic resonance (NMR) 48 , 53 , 54 spectroscopy, as well as density functional theory (DFT) calculations, 29 , 31–33 , 36 are proposed for insight into the mechanism and structure-performance relationship in selective hydrogenations.Exploring an efficient catalyst to obtain both high activity and good selectivity toward alkenes remains a key challenge in the semi-hydrogenation process. 55 Two types of catalysts, namely homogeneous and heterogeneous catalysts, are the major contributors. Despite the high selectivity and explicit mechanism of homogeneous catalysts, the difficulties in separation and recycling restrict their industrial applications to some extent. 56 Compared with homogeneous catalysts, heterogeneous catalysts not only share the advantages of recyclability and regeneration but also the explicit and uniform location of active metal sites in some heterogeneous catalysts provide visualized models favoring the investigation of the structure-performance relationships. 57 , 58 On this basis, various heterogeneous catalysts, including traditional Pd-based catalysts, intermetallic compounds or alloys, and SACs, gradually spring up as promising candidates for selective hydrogenations (Figure 1 and Table 1 ).Pd is one of the most efficient components in hydrogenation processes, inspired by its superior ability in dihydrogen dissociation. 106 , 107 However, the selectivity toward C=C bonds exhibits a huge fluctuation on account of the desorption energy of alkenes on the Pd sites. 108 Specifically, ethylene has the following three adsorption patterns: ethylidyne adsorbed on Pd-trimers, di-σ-bonded C2H4 adsorbed on Pd-dimers, and π-bonded C2H4 adsorbed on Pd single atoms, respectively 108 (Figure 2A). Noteworthily, the desorption energy of ethylene is lower than that for the over-hydrogenation only when C2H4 is π-bonded on Pd single atoms, thus leading to high selectivity toward ethylene. In contrast, when the ethylene is bonded on Pd species in ethylidyne or di-σ-bonded C2H4 modes, the excessive hydrogenation is more favorable than the desorption of ethylene, which is detrimental to the selectivity and causes the production of the over-hydrogenation product, ethane. In this regard, despite the excellent activity of Pd-based catalysts, the selectivity toward alkenes is severely deteriorated under the ensemble effect of Pd species. Correspondingly, several components, like Pb, 6 S, 2 , 7–10 C (subsurface carbon), 11 CO, 12 , 13 and other p-block elements 14–16 have been adapted to promote the selectivity toward alkenes by covering the corner of edge sites of Pd counterparts.The Lindlar catalyst, typically Pd/CaCO3 modified by both lead and quinoline, has been regarded as the benchmark in the selective hydrogenation reaction. 6 The selectivity toward alkenes is promoted since the Pd species are partially poisoned by lead and quinoline. Similarly, sulfur-containing substances like thiols can be exploited as a “toxicant” in Pd-complex systems. 1 , 7–11 , 50 Anderson and co-workers discovered a particular palladium sulfide phase (Pd4S) that was eligible in the semi-hydrogenation reactions. 7–9 The Pd4S active sites not only exhibited high conversion and selectivity in the semi-hydrogenation of dienes and alkynes but also showed high endurance even at high pressure (up to 18 bar), which is a challenging topic but is overlooked in the hydrogenation industry. 8 Inspired by the promising performance of the Pd4S phase in the high-pressure hydrogenation reactions, Javier and co-workers designed a nanostructured Pd3S phase with controlled crystallographic orientation, denoted as Pd3S@C3N4, 2 which showed unparalleled performance in the liquid-phase hydrogenation reactions. In fact, the above-mentioned sulfur-modified Pd catalysts 7–9 mimic enzyme catalysts by imitating the tactics of ensemble and electronic density control. The stable phase of Pd-S compounds provides plenty of space for tailoring the active site with the most selective ensembles at the molecule level. Recently, Zheng and co-workers demonstrated a distinct Pd-sulfide/thiolate interface, denoted as Pd@SPhF2, showing good performance in the selective hydrogenation process. 10 On the premise of the Eley-Rideal mechanism, the Pd@SPhF2 catalyst showed negative adsorption capacity of alkynes or alkenes and therefore exhibited high selectivity in the hydrogenation of 1-phenyl-1-propyne (>97% selectivity toward 1-phenyl-1-propene at full conversion).The above strategies provide us some enlightenment about how the surface coordination environment regulates the reaction performance. In addition to organic sulfides mentioned above, various metal sulfides show a promotion effect on alkene selectivity. 53 , 94 Li and co-workers disclosed that CuS nanoplates contributed to the dispersion of Pd elements by forming stable anchored active sites, accordingly attaining high selectivity, activity, and stability simultaneously. 94 Similarly, Zheng et al. demonstrated that the modification of rhodium sulfide over Pd nanosheets facilitated the formation of surface PdS x ensembles, which not only promoted the hydrogenation activity but also accounted for the isomerization of cis alkenes to trans alkenes. 53 Likewise, the subsurface carbonous compounds can promote the selectivity in a similar manner. Schlögl et al. demonstrated that the near-surface region of Pd played an important role in the selective hydrogenation process. 11 The carbonaceous substance from feeding organic molecules could occupy the interstitial lattice sites and separated the adjacent Pd ensembles, accounting for the enhancement in selectivity (Figure 2B). The importance of subsurface chemistry is evident, and is beneficial for the in-depth understanding of surface and subsurface dynamics as well as the rational design of catalysts.Other p-block elements, like boron (B), can also boost alkene selectivity by covering the redundant Pd active sites. 14 , 15 Edman Tsang and co-workers employed BH3THF as a reagent and designed a novel catalyst, Pdint-B, Pd nanoparticles modified with interstitial boron atoms. 14 The catalyst exhibited good chemical and thermal stability owing to the strong electronic interaction within the host-guest sites between Pd and BH3THF. Meanwhile, Philip and co-workers compared the selective hydrogenation activity over boron-modified Pd catalysts using DFT calculations. 15 It is clear that the Pd(111)-B, Pd(111) surface modified with boron atoms, shows higher activity than clean Pd(111) in the hydrogenation of acetylene and 1,3-butadiene, validating the feasibility of boron modification for the purpose of increasing the hydrogenation performance. Apart from the above inorganic elements, CO has been selected to be a good promoter in the hydrogenation process, especially in industrial applications. Javier et al. investigated the activity of Pd-based catalyst in the absence and presence of CO with the assistance of DFT calculations. 12 It was demonstrated that the CO tends to reduce the Pd ensembles by forming densely packed overlayers. Thus, the over-hydrogenation and the polymerization can be prevented by suppressing the adsorption of unsaturated reactants, dihydrogen, and the hydrocarbon intermediates. However, the ratio of CO/H2 should be controlled within a certain limit (usually below 0.1) to prevent the formation of oligomers. 13 The above poison strategy aims at promoting the selectivity by covering partial Pd active sites. However, due to sacrificing a mass of active phases and the utilization of toxic compounds, the strategy is deemed to be environmental unfriendly, with low atomic efficiency. Therefore, an alternative method employing a second or third metal, called alloys or intermetallics (Figure 3A), was proposed. 16–21 , 60 , 61 , 71 , 93 , 105 On this basis, various Pd-based alloys/intermetallics like PdAg, 16 PdCu, 17 PdAu, 18 Pd-Zn, 19 Pd-Ga, 20 and Pd-In 21 have been investigated. Some non-precious metal alloys have also been developed on the premise of the site separation strategy. Under the construction of alloys/intermetallics, the continuous bulk metal ensembles can be separated or isolated by the second metal, promoting the selectivity toward alkenes.Zhang and co-workers reported a series of IB-metal-alloyed Pd-based catalysts, namely PdAu/SiO2, 18 PdAg/SiO2, 16 and PdCu/SiO2, 17 and compared their activities in the selective hydrogenation of acetylene. The similar activation energies among the three catalysts indicated an analogous mechanism, but shared different selectivity toward ethylene, which might imply different electronic effects between Pd and the second metals (Figures 3B and 3C). Specifically, the PdCu/SiO2 catalyst possessed the highest electron density of Pd species, and thus suppressed the adsorption of C=C bonds and resulted in the higher selectivity toward ethylene (Figure 3D). In addition to the elements of group IB, Zhang and co-workers constructed PdZn intermetallic nanostructure through alloying Pd with Zn. 19 The Pd species in the PdZn catalyst weakened the π-bonding adsorption of ethylene and further prevented its over-hydrogenation, resulting in high selectivity toward ethylene. The ingenious arrangement of Pd species provided two adjacent but isolated sites, which was feasible for the adsorption and activation of acetylene through moderate σ-bonding patterns, and responsible for the superior activity thereof (Figures 2A and 3E). Recently, Su and co-workers found that adding a Ga phase could destroy successive Pd atom ensembles, and designed supported Pd2Ga intermetallic catalysts 20 showing high thermal stability under employed reaction conditions by taking the advantage of covalent interactions between nanocrystals. However, the selectivity toward ethylene over the PdGa catalyst remained to be improved, probably due to the nonuniformity of nanoparticles prepared by a wet impregnation method. Recently, Fan and co-workers developed a calcite-supported PdBi intermetallic compound, PdBi/calcite. Owing to the isolated and electron-rich Pd sites, the PdBi/calcite catalyst showed weak adsorption of ethylene and superior stability (over 99% ethylene selectivity at full conversion) over a wide range of reaction temperatures (423–573 K). 105 In addition to Pd-based alloy catalysts, some non-precious intermetallics have been investigated to replace the use of noble metals. Nørskov and co-workers screened about 70 bimetallic compounds, including Fe, Ni, Co, Cu, Pd, Pt, Ag, Au, Zn, Cd, Hg, Ga, Tl, Ge, Sn, and Pb, for the hydrogenation process (Figures 3F and 3G). 60 Among these catalyst models, the low-cost Ni-Zn catalyst exhibited comparable activity (turnover rate) and selectivity. Theoretical predictions provided a novel and efficient strategy for the rational design of alloy hydrogenation catalysts free of noble metals. In addition, Javier et al. developed a ternary Cu-Ni-Fe catalyst that exhibited good performance in the semi-hydrogenation of propyne (selectivity of 80% at full conversion). 71 The three metal phases performed their own function, where Cu was used for a basement, Fe severed as the structural promoter to enhance propylene selectivity, and Ni facilitated the spillover of hydrogen and further prevented the oligomerization. The ternary catalyst was a great achievement, promoting the selectivity without the use of noble metal species and the potential poisoning step. Under the guidance of the site-isolation concept, a low-cost replacement for Pd-based catalyst, Al13Fe4, was constructed by Armbrüster et al. 61 The electron structure of both Fe and Al was changed by the tight chemical bonding, and thus exhibited remarkable performance in the semi-hydrogenation of acetylene. Inspired by the above results, Wang et al. synthesized intermetallic NixGay and NixSny nanocrystals via a solution-based co-reduction strategy, 93 favoring the formation of alloys with uniform sizes. The isolated sites between Ni and Sn/Ga as well as the electronic effect within the active sites accounted for the good catalytic performance, making NixMy good candidates for the Pd-based catalysts.Compared with the “selective poison” strategy, the construction of alloys/intermetallics promotes the atomic utilization efficiency to a great extent. However, multi-element alloys are usually prepared through wet impregnation, leading to inhomogeneous active sites and showing negative impacts on catalytic performance. Thereupon, the single-atom strategy with the maximal economic efficiency emerged. 22 By means of advanced techniques like Cs-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray adsorption spectroscopy (XAS), and FTIR spectroscopy with CO adsorption, the explicit structure of SACs can be revealed. On this basis, the single-atom catalyst systems can be modeled by DFT calculations and the structure-performance relationship can be further interpreted.Graphene, nitrogen-rich carbon (C3N4), aluminum oxide (Al2O3), and silicon oxide (SiO2) are widely utilized as relatively inert supports for metal species. 23–29 , 109 , 110 These inert supports can modify the configuration of metal species’ morphologic and electronic aspects, forming metal/support interfaces, which are important to selective hydrogenation. Lu and co-workers prepared atomically dispersed Pd on graphitic carbon nitride (g-C3N4), Pd1/C3N4, through an atomic layer deposition (ALD) technique, which showed 95% ethylene selectivity in the semi-hydrogenation of acetylene. 109 The Pd1/C3N4 also exhibited a higher stability (more than 100 h) in either reducing or oxidizing conditions than g-C3N4-supported Pd NP catalysts, and thus appearing to be a promising candidate for promoting selectivity as well as coking resistance in hydrogenation systems. A similar structure-performance relationship was also disclosed over Pd1/graphene where the alkyne adsorbed through mono-π-adsorption rather than a di-π-adsorption manner and thus showed excellent selectivity. 23 , 24 In addition, Javier and co-workers established a stable single-site Pd catalyst, namely [Pd]mpg-C3N4, which was confirmed to be efficient for the three-phase hydrogenation of alkynes and thus provide more potential in industry applications. 28 Recently, Ma et al. constructed the single Pd atoms supported on nanodiamond-graphene, Pd1/ND@G. 26 The atomically dispersed Pd atoms over Pd1/ND@G were strongly anchored on the support through Pd-C bonds, which not only avoided the formation of β-H species but also favored the desorption of ethylene, and thus showed remarkable selectivity. Similarly, Li and co-workers demonstrated a mesoporous N-doped carbon nanosphere, Pd/MPNC, which had high activity, excellent ethylene selectivity, and good long-term stability in the semi-hydrogenation of acetylene. 111 Considering the high cost of noble metals, Lu and co-workers designed a novel metal trimer catalyst (Ni1Cu2/g-C3N4) exhibiting high efficiency in the semi-hydrogenation of acetylene. 29 The Cu atomic grippers could boost the loading of Ni species through dynamic and synergetic metal-support interactions, providing an atom-by-atom fabrication approach for the rational design of catalysts. Similar performance was also disclosed on Cu0.5/Al2O3 25 and Cu1/ND@G 27 when applied in the semi-hydrogenation of acetylene.Porous materials such as zeolites, metal-organic frameworks (MOFs), 30 and zeolite imidazolate frameworks (ZIFs) 91 with ordered channel structure as well as unique confinement effects have become indispensable supports in heterogeneous catalysis. 57 These porous supports can provide accessible space to stabilize metal species and construct single-site catalysts. For example, Corma et al. designed FeIII-(OH) single sites embedded within MOFs, which exhibited good acetylene semi-hydrogenation performance under simulated front-end industrial conditions. 30 Inspired by the channel confinement effect of the zeolite, Gong and co-workers demonstrated that Pd clusters encapsulated within sodalite (SOD) zeolite, namely Pd@SOD, could catalyze the semi-hydrogenation of acetylene. 31 The narrow channels of SOD cages (0.28 × 0.28 nm) restricted the free diffusion of acetylene and ethylene, but allowed dihydrogen to enter the pore channels smoothly. Under such circumstances, the dihydrogen molecules underwent cleavage on encapsulated Pd clusters, accompanied by OH species transferring to the surface of SOD cages through spillover and reacting with acetylene. Recently, Li and co-workers constructed Ni(II) species confined within different zeolites, Ni@FAU 32 and Ni@CHA, 33 respectively. These two zeolite catalysts exhibited comparable hydrogenation performance but followed different mechanism patterns, which might originate from the faint difference of the confining environments between faujasite and chabazite zeolites (Figure 4A). Owing to the stronger local electric field of chabazite than that of faujasite, the Ni(II) sites were more tightly confined in Ni@CHA and triggered the direct dissociation of dihydrogen. By contrast, Ni@FAU with a lower coordination number possessed higher affinity for acetylene and further induced the acetylene-promoted hydrogenation mechanism.Intermetallics or alloys provide an alternative strategy toward selective hydrogenation and thus inspired the construction of the single-atom-alloy catalysts. 16–21 , 60 , 61 , 71 , 93 , 105 In essence, the single-atom-alloy catalysts can be rationally designed by regulating the formation and aggregation energies within host/guest metals in equilibrium, 34–39 thus reducing the dosage of noble metals to a great extent. Equipped with simple and specific configurations, the single-atom alloys accommodate the ideal catalyst models for surface science investigations and DFT calculations, beneficial to in-depth investigations of the structure-performance relationship. 112 As for the selective hydrogenation of alkynes, the facile dissociation of dihydrogen and the easy desorption of hydrocarbon intermediates are crucial factors controlling the selectivity, which are still difficult to realize simultaneously. To solve the problem, Sykes and co-workers constructed the Pd/Cu(111) single-atom-alloy catalyst with isolated Pd atoms over a Cu(111) surface. 34 Through low-temperature scanning tunneling microscopy (STM), it was disclosed that molecular hydrogen first dissociated on the single-dispersed Pd sites among 0.01 ML Pd/Cu(111) catalyst. Then the individual H adatoms underwent transfer from Pd sites to bare Cu(111) terraces (that is, hydrogen spillover) (Figure 4B), providing the first direct observation of hydrogen spillover. The lower energy barrier for the dissociation of dihydrogen and the easy desorption of alkenes over Pd/Cu(111) than over bare Cu(111) surfaces were also confirmed via TPD and DFT calculations. Similarly, the Pt/Cu(111) single-atom-alloy catalyst with low concentration of Pt doped on Cu(111) surface was also found to be active for the facile dissociation of dihydrogen and the subsequent hydrogenation of 1,3-butadiene. 35 Inspired by the unique characteristics of trace amount metal supported on Cu(111), Zheng et al. designed Pd atoms supported on different crystal faces, Pd1/Cu(100) and Pd1/Cu(111), and their hydrogenation behaviors were compared. 36 It was disclosed that extremely diluted Pd1/Cu(111) was inert for the hydrogenation of phenylacetylene unless Cu(100) was introduced (Figure 4C). That is, the spillover of dihydrogen could only occur on the Cu(100) face despite the clear evidence of H adatoms spillover from Pd single sites to the bare Cu(111) face. 36 The essence of dihydrogen spillover over Pd/Cu single-atom alloy remains a controversy. Nevertheless, under the guidance of the single-atom-alloy strategy, diverse metal species in the form of single-dispersed atoms stabilized on peculiar crystal facets, for example Rh/Cu(111), 37 Pd/Au(111), 38 and Pt/Ag(111), 39 were constructed for various reactions not limited to selective hydrogenations.Apart from inert supports as mentioned above, many reducible metal oxides, like titanium oxides (TiO2), 113 zinc oxide (ZnO), 102 ferric oxide (FexOy), 50 , 114 cerium oxide (CeO2), 99 , 114 and Ga2O3, 115 can be employed as support materials in the selective hydrogenation reactions. The metal oxide support can interact strongly with the active metal sites under certain conditions, called strong metal-support interaction (SMSI). For example, Francisco and co-workers found that moderate thermal treating of Pt/TiO2 favored the deep diffusion of Pt phase into the bulk TiO2 structure. 113 Correspondingly, the strong intimate relationship between Pt and TiO2 could be induced, facilitating the following hydrogenation process. Similarly, Wang et al. reported that Pd/In2O3 synthesized through facile wet impregnation was a promising alternative for the hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to the corresponding alkenol 2-methyl-3-buten-2-ol (MBE), a key component for the fabrication of vitamin E. 102 The In2O3 support or substrate could form a loose layer to cover the exposed Pd active site under employed conditions on the premise of SMSI, ensuring the high selectivity toward alkenols. On the basis of SMSI, Choi et al. disclosed a novel mechanism, dynamic metal-polymer interactions (DMPI), which can be regarded as the organic version of SMSI. 50 The organic support polyphenylene sulfide was flexible and could cover Pd active sites under employed conditions via the strong metal-polymer interactions. This controllable interface could regulate the passages of reactants and prevent the over-hydrogenation. Apart from the concept of SMSI and DMPI, Zhu and co-workers designed a porous yolk-shell structure via reverse strong metal-support interaction. 104 The fully encapsulated core-shell configuration Pd-FeOx nanoparticles transformed into a porous yolk-shell structure Pd-Fe3O4-H under H2 treatment. Being exposed to the reactants, the Pd active sites exhibited excellent catalytic performance in the selective hydrogenation of acetylene with a turnover frequency of (TOF) of 6.46 s−1 under the rule of SMSI in a reverse route. In addition to the common reducible support mentioned above, Ga2O3 exhibited strong electron transfer as well as the decaying adsorption of CO and H2, which should be a promising candidate for the SMSI. 115 On this basis, Xu and co-workers constructed Pd/Ga2O3 and disclosed that the formation of SMSI was triggered by the coexistence of PdGa alloy as well as the Ga2O3, thus providing both high propylene selectivity and propyne conversion under mild reaction conditions (303 K, atmospheric pressure). Noteworthily, the unique SMSI feature was also disclosed over layered double hydroxide (LDH). 116 Feng et al. reported that the Pd/MgAl-LDH-Al2O3 exhibited high activity and selectivity in semi-hydrogenation of acetylene owing to the reduced acidity as well as the SMSI.Generally, the hydrogenation of alkynes is a two-phase system, where the gaseous acetylene/propyne/vinylacetylene and dihydrogen react over the solid catalyst. 1 , 42 , 60 The target products ethylene/propylene/1,3-butadiene are key components for the manufacture of polyolefins. 117 Alkenes are obtained in large scale through the cracking of naphtha, which contains a considerable number of impurities; i.e., 0.5%–8% alkynes. 1 These highly unsaturated compounds may cause serious damage in olefin polymerization; for example, the break of polymerase chains and the deactivation of the Ziegler Natta catalyst. 118 Thus, the content of triple-bond chemicals in an ethylene/propene stream must be reduced to an acceptable level (<5 ppm). Typically, the front-end and tail-end semi-hydrogenation is supposed to be the most efficient route to the elimination of trace alkynes. 17 , 119 The front-end hydrogenation, which requires high concentration of dihydrogen in the feed gas, easily causes the over-hydrogenation of acetylene and the “run-away” of reaction temperature, and thus makes it more challenging to obtain high selectivity in the front-end semi-hydrogenation. 40 In contrast, the ethylene feed gas in the tail-end process has been purified before the hydrogenation of trace amounts of alkynes. Therefore, the tail-end hydrogenation generally requires near-stoichiometric dihydrogen (dihydrogen/alkynes = 1–2), which is easier to realize and more popular in practice. 120 For the selective alkyne hydrogenation in the gas phase, the selectivity toward the target alkene products is affected by many factors; for example, the drastic reaction temperature, 65 the excess amount of hydrogen, and the inner construction of catalysts. 91 On this basis, multiple hydrogenation catalysts have been investigated to avoid the over-hydrogenation process, which is not explained in detail here. Besides, some side reactions, such as couple crossing or polymerization of alkynes, are nonnegligible. Under certain circumstances, especially on the catalysts that have strong affinity toward C≡C bonds, alkynes tend to form coke and C4–C6 hydrocarbons (green oils) in the presence of alkenes. 121 The above adverse reactions may cause a huge waste of alkene cuts and will lead to catalyst deterioration as well as breaking off the polymer chains. Thus, it is urgent to design suitable catalysts that can suppress the side reactions and ensure high selectivity toward alkenes.The selective hydrogenation of C≡C bonds can also occur in the liquid phase to produce fine chemicals. Nowadays, the hydrogenation of alkynols is considered a fundamental process for the synthesis of fine chemicals and intermediate chemicals. 48 One important example is the synthesis of intermediate substance like linalool and MBE, which are prevalently utilized in the production of vitamin E, vitamin K, and provitamin β-carotene. 4 , 5 , 14 The demand of efficient hydrogenation catalysts in the liquid phase is now at a high pitch. Generally, the durability of hydrogenation catalysts in the liquid phase does not draw much attention owing to the relatively mild conditions compared with the hydrogenation in the gas phase. 55 However, there are some severe problems to solve in liquid-phase hydrogenations. For example, the dihydrogen solubility in liquid phase is limited, thus hampering the conversion of alkynes. 43 On the other hand, the selectivity toward alkenes/alkanols remains to be promoted since the active sites exhibit stronger adsorption affinity of liquid medium than that of dihydrogen in the gas phase. Therefore, the alkenes/alkanols tend to be over-hydrogenated to saturated alkanes/alcohols instead of being desorbed as products, corresponding to low selectivity. Besides, it is important to identify whether the active sites are heterogeneous or not, which is crucial for industry applications, and this may require hot filtration test in batch reaction or long-term fixed-bed reaction. However, many liquid-phase hydrogenation catalysts equipped with organic modifiers have a huge risk of leaching, which further leads to declines in catalytic activity and/or selectivity when applied in practical operation. That is to say, the exploitation of hydrogenation catalysts with anchored active sites appropriate for hydrogenation in the liquid phase is still a challenging task.The adsorption and activation of dihydrogen is considered the crucial step in the hydrogenation process, which can be classified as homolytic dissociation and heterolytic cleavage, respectively. 11 The homolytic dissociation of dihydrogen generally happens on the surface of VIII group metals such as Pt, 122 Pd, 123 and Rh. 37 These metals have segmental occupied d orbitals, where the σ electrons from dihydrogen can be accommodated. On the other hand, metals can donate d electrons to the antibonding orbits of dihydrogen (Figure 5A). Accordingly, two hydrides are formed by homolytic cleavage of the weak H–H bond. It is acknowledged that metal ensembles, where more than two metal atoms are present in the vicinity, are more beneficial for the homolytic cleavage of dihydrogen than single-atom metal species. 96 However, this might lead to over-hydrogenation, since the hydride formed after homolytic dissociation will migrate to the near-surface region of metal counterparts, forming the subsurface hydride, which is detrimental to the selectivity. 11 Analogously, hydrogen species can be heterolytically cleaved into H+/H− through the concerted effect of metal species and basic materials such as supports. The formed H+ and H− then bond with metal atoms and a proton acceptor, for example N, 125 O, 33 and C 23 atoms, respectively. It is well-known that heterolytic dissociation of dihydrogen usually happen in homogeneous catalytic systems like enzymes, 125 the frustrated Lewis pairs (FLPs), and the classical Lewis pairs (CLPs). 99 , 126 , 127 However, some heterogeneous catalysts, like supported metal oxides, 98 , 128 , 129 SACs, 28 , 96 , 98 , 130 and metal encapsulated zeolites, 33 , 131 are also prone to dissociate dihydrogen heterolytically. For example, Li et al. demonstrated that dihydrogen underwent heterolytic dissociation by Ni (II) sites and the adjacent zeolite framework oxygen atoms, forming hydride (Ni-H) and proton (O-H), respectively. 33 The heterolytic cleavage of dihydrogen was confirmed by in situ FTIR spectroscopy of H2/D2 activation (νO−H = 3,610 cm−1, νO−D = 2,600 cm−1, νO−H/νO−D ≈ 1.4) as well as the Bader charges of Ni–H (−0.31 eV) and O–H (1.0 eV) by DFT calculations. Lu and co-workers compared the Bader charge of H in Pd-H (−0.30 eV) and O-H (0.62 eV), which provided strong evidence for the heterolytic cleavage of hydrogen. 24 Unlike the homolytic dissociation of dihydrogen, which needs adjacent metal atoms, heterolytic dissociation of hydrogen requires the joint efforts of both metal and support, which serve as the hydride acceptor and proton acceptor, respectively. Lacking metal ensembles, the analogous FLPs/CLPs patterns of hydrogen dissociation enable the higher selectivity toward alkenes in alkyne hydrogenations, and therefore become a hot spot in heterogeneous hydrogenation systems. 99 , 126 , 131 On the premise of CLPs, Guo and co-workers demonstrated that the Ni-doped ceria, namely Ni@CeO2(111), exhibited high activity in acetylene hydrogenation. 99 DFT calculations revealed that the oxygen vacancies facilitated the heterolytic dissociation of dihydrogen, producing the Ce-H and O-H, respectively. The CLPs pattern of dihydrogen activation avoided the overstabilization of C2H3 ∗ intermediate and corresponded to the low barrier for the formation of ethylene. Similar structure-performance relationship was disclosed over Pt@Y zeolite, confirming the wide applicability of CLPs and the corresponding selective specificity in hydrogenation reactions. 131 For insight into the mechanism of alkyne hydrogenation, some unique catalyst systems and their corresponding reaction pathways are discussed in the following sections. The Langmuir-Hinshelwood mechanism 124 , 132 with the co-adsorption of reactants on surface plays a dominating role in heterogeneous hydrogenation catalysis. In contrast, the Eley-Rideal mechanism, 133 featuring the direct reaction between a molecular reactant with an adsorbed one, is rarely reported for alkyne hydrogenation.The Langmuir-Hinshelwood mechanism, which remains the most common one among heterogeneous catalytic systems, can be divided into two routes (Figure 5B). Proposed in 1934, the Horiuti-Polanyi mechanism, or the so-called dissociative mechanism, plays a primary role in hydrogenation processes. 134 It entails dihydrogen homolytic dissociation on the metal surface, followed by the successive addition of H atoms to the adsorbed alkynes. This routine is firstly disclosed on the surface of Ni 86 and Pd 135 with intrinsic ability to split dihydrogen, leaving the hydrogen atoms bonded with metal and establishing the stable metal hydrides (M-H). Subsequently, the so-called dissociative mechanism was discovered over Au nanoparticles by Javier and co-workers. 42 Despite the filled d orbitals of Au-based catalyst and the restricted activity toward dihydrogen dissociation, the catalyst exhibited decent performance in the selective hydrogenation of propyne with ∼90% conversion and selectivity to propylene. Additionally, the Horiuti-Polanyi mechanism was found to be applicable over alloys and metal phosphides such as Pd3Ga7 69 and Ni2P, 92 involving molecular hydrogen dissociation followed by the successive addition of hydrogen atoms to the tri-bonded compounds.However, for SACs without adjacent metal-metal pairs available for the homolytic dissociation of dihydrogen, the dissociation of dihydrogen must take place via an alternative pathway, namely heterolytic dissociation. 96 Typically, the dihydrogen molecules undergo heterolytic cleavage, leaving one of the hydrogen atoms bound to the metal atom and the second one to the heteroatom of the support like N, C, or O. Inspired by this kind of dissociation routine, various SACs, e.g., Pd1-O/graphene, 23 Pd4S, 2 Ni@CHA, 33 and Pd/mpg-C3N4, 28 were rationally designed. For example, in a typical system of Ni(II)-encapsulated zeolite, namely Ni@CHA, the dihydrogen firstly undergoes heterolytic cleavage under the effect of coordinately unsaturated Ni(II) site and the surrounding oxygen atoms in the six-membered ring of chabazite zeolite. 33 This type of hydrogen heterolytic dissociation was verified not only via DFT calculations but also through isotope-labeled FTIR spectra, providing strong experimental evidence for the CLPs. As shown in Figure 5C, the stretching pattern of bridging hydroxy ions (νO-H = 3,610 cm−1) shifted to the red region (νO-D = 2,600 cm−1) in the deuterium labeling experiment, according with the theoretical ratio of the frequencies for harmonic oscillation of H2 and D2 molecules against a rigid wall (1.41). 136 Similarly, Lu and co-workers demonstrated that the hydrogenation of 1,3-butadiene over Pd1-O/Graphene followed the typical Horiuti-Polanyi mechanism where the heterolytic dissociation of dihydrogen was found to be the rate-determining step. 23 Considering the different dissociation patterns of dihydrogen with the Horiuti-Polanyi mechanism, the hydrogenation with heterolytic cleavage of dihydrogen was denoted as pseudo-Horiuti-Polanyi mechanism by Lu et al.Noteworthily, the Horiuti-Polanyi mechanism occurs over metal surfaces in most cases, which is feasible for the cleavage of dihydrogen. However, this kind of hydrogenation mechanism generally leads to mostly cis alkene products in the hydrogenation of internal alkynes, even though the trans alkenes are thermodynamically more stable than the cis ones. 137 , 138 On this basis, Zheng et al. reported that the defective Rh2S3-x exhibited high selectivity toward trans alkenes in the hydrogenation of internal alkynes. 53 The dihydrogen underwent dissociation at the defects of the solid surface and formed the frustrated hydrogen pair, which could modulate the cis-to-trans isomerization without over-hydrogenation. That is to say, the isomerization of cis/trans alkenes can be modulated by altering the hydrogenation mechanism, providing a new thought for the rational design of novel catalysts.Despite the most crucial step of dihydrogen dissociation in the hydrogenation process, there are some relatively inert metal sites that are invalid for the direct splitting of dihydrogen. In this sort of catalyst, the hydrogen cannot be dissociated by active center independently but with the assistance of alkynes, denoted as associative mechanism (Figure 5B). The alkyne-assisted pathway was first disclosed on Ag nanoparticles by Javier et al. in 2013. 45 The adsorption sites of alkynes and the energy barriers over Ag(211) under a different mechanism were compared using DFT calculations. As shown in Figure 5D, the associative mechanism required relatively lower activation barriers than the classical Horiuti-Polanyi mechanism. Later, this acetylene-promoted associative mechanism was reported with over Pd@SOD 37 and Cu1/ND@G 27 systems. On the premise of the associative mechanism, it was found that the above SACs exhibited higher selectivity than the corresponding metal ensembles.However, the associative mechanism is merely confirmed by DFT calculations, lacking direct experimental evidence. 45 This can be ascribed to the fact that the intermediate phases of Horiuti-Polanyi mechanism and associative mechanism are identical or similar; for example, (C=CH∗, CH–CH2∗), making it difficult to distinguish by means of spectroscopic protocols. 32 , 33 Recently, Li et al. conducted the H2-D2-C2H2 pulse-response experiments, providing the first in-depth evidence on the alkyne-associative mechanism. 32 The signals of HD (m/z = 3) in the absence and presence of acetylene were compared over Ni@CHA and Ni@FAU catalysts, respectively. As shown in Figure 5E, no HD signal was detected over Ni@FAU under the mixture flow of H2-D2 at rational temperature, indicating that the dihydrogen could not be efficiently activated under employed condition. However, it appeared immediately upon the introduction of acetylene, confirming that the dihydrogen was dissociated after the injection of acetylene molecules. That is, the hydrogen and deuterium underwent dissociation and formed HD only after the combination of Ni(II) sites and alkynes. The so-called alkyne-promoted/assisted mechanism is different from the conventional Horiuti-Polanyi mechanism, 134 which is rarely seen in heterogeneous hydrogenation but widely observed in homogeneous systems. 89 , 139 , 140 For the Ni@CHA catalyst, the HD signals were clearly detected in the H2-D2 stream at the very beginning and gradually decreased upon the introduction of acetylene due to the hydrogenation of acetylene by HD, deriving a typical Horiuti-Polanyi mechanism. In short, the H2-D2-C2H2 pulse-response experiments provide direct evidence to distinguish the associative mechanism and Horiuti-Polanyi mechanism. Under the guidance of the associative mechanism, considerable amounts of trans alkenes could be obtained from the hydrogenation of internal alkynes like di-phenylacetylene and 1-phenylpropyne, in accordance with the views of Zheng et al. 53 That is, the associative mechanism may lead to the formation of trans alkenes in the selective hydrogenation of internal alkynes.The associative mechanism, where the dihydrogen dissociated heterolytically, is widely observed in organometallic chemistry and homogeneous catalysis. For example, the associative mechanism has been found to boost the trans-alkene selectivity in various metal complexes like (IMes)Ag∗Rp, 89 Ni∗Ln 139 and Cp∗Ru. 140 However, this mechanism is rarely seen in a heterogeneous hydrogenation system. The alkyne-promoted mechanism provides a specific hydrogenation process that compensates for the conventional Horiuti-Polanyi mechanism. The associative pattern was also disclosed to be suitable in heterogeneous ammonia synthesis by Li et al. 141–143 The associative mechanism was also found to be efficient in ammonia synthesis over Ru/H-ZSM-5 142 or Fe3/θ-Al2O3 143 catalyst, further proving its wide application in hydrogenation reactions.Hu and co-workers summarized the Horiuti-Polanyi mechanism and non-Horiuti-Polanyi mechanisms in hydrogenation catalysis from DFT calculations. 144 , 145 The universality of the Horiuti-Polanyi routine, proposed 100 years ago, has been confirmed over various types of catalysts. For the catalysts showing weak adsorption or dissociation ability of dihydrogen, for example Ag(211) and Ni(111), the prevalence of associative mechanism may occupy the dominating role.Proposed by Eley and Rideal in 1938, the Eley-Rideal mechanism illustrates the route that only one of the reactant molecules adsorbs on catalyst surface and the other one participates in the reaction without adsorption (usually from the gas phase). 46 , 146 The reaction scheme of C≡C bond hydrogenation is briefly described in Figure 5B. Typically, the Eley-Rideal mechanism can be divided into two categories: (1) adsorbed alkynes molecules, ∗C≡H reacting with dihydrogen in the gas phase, H2(g); (2) adsorbed hydrogen molecules, ∗H2 reacting with alkynes in the gas phase, C≡H(g). (1) ∗C≡H reacting with H2(g) ∗C≡H reacting with H2(g) It is acknowledged that the Eley-Rideal mechanism is extensively applied in the hydrochlorination of acetylene, in which the ∗C2H2 reacting with HCl(g). 147 However, as for the selective hydrogenation process, the activation and dissociation of H2 molecules remains very significant in the hydrogenation process. 54 Thus, the reaction pattern of ∗C2H2 reacting with H2(g) is rarely seen in alkyne hydrogenation. In a very early study, 148 Butt and co-workers investigated the performance of benzene hydrogenation over supported Ni catalysts. The kinetic results of benzene hydrogenation matched well with a typical Eley-Rideal mechanism, proceeding via the molecular addition of dihydrogen to adsorbed benzene.However, there remain some controversies about the feasibility of Eley-Rideal mechanism (∗C≡H reacting with H2(g)), especially in terms of alkyne hydrogenation. The pathway of adsorbed acetylene reacting with molecular dihydrogen from the gas phase was found to be inapplicable by Hafner et al. using DFT calculations. 149 , 150 The high activation energy of 200 kJ/mol as well as the strong Pauli repulsion between dihydrogen and acetylene molecules suppressed the impending hydrogenation process theoretically. Therefore, more straightforward approaches to distinguish the peculiar Eley-Rideal mechanism and to investigate the origin thereof are urgent required. (2) C≡H (g) reacting with ∗H2 C≡H (g) reacting with ∗H2 As mentioned above, the dissociation of dihydrogen represents an essential step in the hydrogenation of alkynes, whether in homolytic or heterolytic patterns. 11 Therefore, the hydrogenation pattern of adsorbed dihydrogen molecules reacting with alkynes in gas phase is easier to accept in the Eley-Rideal mechanism. For example, Zheng et al. demonstrated that the poisoned Pd-based catalyst, Pd4S@SPhF2, might follow the typical reaction scheme. 10 The direct participation of internal alkynes from the gaseous phase might be speculated since neither PhC≡CCH3 nor PhCH = CHCH3 could adsorb on the catalyst surface.There remain some controversies about the feasibility of the Eley-Rideal mechanism. Hafner et al. assumed the pathway of adsorbed acetylene reacting with molecular dihydrogen from the gas phase using DFT calculations. 149 , 150 This route was claimed to be inapplicable in alkyne hydrogenation process limited by the high activation energy of 200 kJ/mol as well as the strong Pauli repulsion between dihydrogen and acetylene molecules. Therefore, more straightforward means are required to distinguish the peculiar Eley-Rideal mechanism and to investigate the origin thereof.For a better understanding of the selective hydrogenation reaction, comprehensive characterization of the catalytic configuration and reaction process is highly desired, which is a hot and challenging topic. For the structure and configuration of catalysts, several advanced characterization strategies, such as Cs-corrected HAADF-STEM; 26 , 27 FTIR spectroscopy with CO adsorption; 22 and XAS, including extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES), 17 , 18 , 22–33 , 59 , 91 have been extensively discussed, especially for SACs. 151 Herein, some representative characterization techniques relevant to the hydrogenation process and structure-performance relationship investigation are summarized, including in situ FTIR, 32 , 33 TPD, 30 pulse-response experiments, 32 , 50 H2-D2 exchange, 24 , 30 and solid-state NMR. 53 , 54 In situ FTIR is supposed to be a fast and sensitive characterization technique, since it provides accessible information on intermediate species through the vibrations of organic functional groups. 31 , 47 Li et al. reported the in situ temperature-dependent FTIR spectroscopy of acetylene hydrogenation over Ni@CHA catalyst. 33 The competitive adsorption of reactants, i.e., dihydrogen and acetylene, could be analyzed, providing useful information on the detail reaction pathway. 32 , 33 As shown in Figure 6A, the chemisorbed acetylene on Ni@CHA (3,010 and 2,925 cm−1) could be weakly captured only after the pretreatment with helium but completely disappeared after the pretreatment of dihydrogen. That is, the Ni (II) sites showed stronger affinity of dihydrogen that acetylene and subsequently hindered the adsorption of latter. On the contrary, the stretching bands of C–H (acetylene) were strongly bonded to Ni(II) sites in Ni@FAU whether pretreatment was in helium or dihydrogen. The distinct adsorption behaviors toward reactants over Ni@CHA and Ni@FAU may imply the diverse hydrogenation mechanisms.As mentioned above, the Ni(II) species confined within faujasite and chabazite displayed different adsorption affinity of acetylene and dihydrogen shown by FTIR spectra (Figure 6A). 32 , 33 The intrinsic competitive adsorption behaviors on Ni@FAU and Ni@CHA can be interpreted from TPD experiments. As shown in Figure 6B, the desorption temperature of dihydrogen was higher than that of acetylene over Ni@CHA catalyst, indicating the stronger affinity of dihydrogen than acetylene. It revealed that the dihydrogen could be easily dissociated over Ni@CHA with trace hydrogen spillover around the zeolite (small peak at ∼500 K). In contrast, the Ni@FAU catalyst only exhibited a moderate desorption peak of acetylene but no desorption signals of dihydrogen, suggesting the very weak adsorption of dihydrogen. On the premise of the different desorption behaviors between Ni@FAU and Ni@CHA, two different hydrogenation mechanisms were disclosed. 32 , 33 , 57 Similarly, on the premise of the TPD investigation, Zheng et al. disclosed that the Pd@SPhF2 exhibited sore adsorption of dihydrogen but no desorption peak of alkynes (PhC≡CCH3), suggesting a typical Eley-Rideal mechanism. 10 The dissociation of dihydrogen is acknowledged to be prerequisite in most hydrogenation reactions. The kinetic isotopic effect can provide strong evidence that the dissociation of dihydrogen is the rate-determining step, which also complies with the strong Pauli repulsion between dihydrogen and acetylene. 149 , 150 Additionally, the dissociation sites can be investigated by comparing the HD formation rate. Lu and co-workers compared the HD formation rate over Pd1-O/graphene and Pd-NPs/graphene via H-D exchange. 23 As shown in Figure 6C, the HD formation rate over Pd-NPs/graphene was about 12 times higher than that over Pd1-O/graphene, indicating that the hydrogen dissociation over Pd1 single atoms was extremely hindered and the rate-determining step could be identified. The HD formation rates decreased on both catalysts after the introduction of butadiene, which could be attributed to the stronger adsorption of butadiene on Pd1 single atoms and Pd-NPs than dihydrogen.Choi and co-workers performed the H-D isotope exchange over Pd/PPS and Pd/SiO2 catalysts. 50 As shown in Figure 6D, the absence of hydrogen activation ability over Pd/PPS was verified since no HD formation could be detected when feeding an H2-D2 mixture to the catalyst. The signal of HD appeared immediately upon the feeding of acetylene, revealing the activation of dihydrogen in the presence of co-adsorbed acetylene. The H-D isotope exchange results provided strong evidence for the alkyne-promoted hydrogenation process over Pd/PPS catalyst (i.e., the associative mechanism). Similarly, Li et al. disclosed the strong affinity of acetylene over Ni@FAU catalyst and confirmed the alkyne-promoted mechanism though H2-D2-C2H2 pulse-response experiments as discussed previously (Figure 5E). 32 The heterolytic dissociation of dihydrogen favors the high selectivity toward ethylene owing to the lack of β-H species on metal surface. However, it is difficult to capture the M-H− species limited by the characterization methods. Theoretically, the solid-state 1H magic-angle spinning (MAS) NMR spectroscopy can provide clear evidence on the heterolytic cleavage of dihydrogen. Geoffrey and co-workers demonstrated that the dihydrogen underwent heterolytic cleavage on the In2O3-x(OH)y catalyst, forming In-OH2 + and In-H−, respectively. 54 The stretching bands of In-OH2 + and In-H− attributed to 1,220 and 1,300 cm−1 could be clearly captured through FTIR spectroscopy. In the 1H MAS NMR measurements (Figure 6E), the chemical shifts at 4.05 and 1.14 ppm, attributed to the In-OH2 + and In-H−, respectively, could be captured at room temperature, in good consistency with FTIR results. On these grounds, the NMR measurement makes a quantitative compensation for the characterization of dihydrogen hydrolysis. Recently, Zheng and co-workers demonstrated that the dihydrogen could be heterolytically dissociated into the frustrated hydrogen pair over defective Rh2S3-x catalyst. 53 The frustrated hydrogen pair could stereo-selectively mediate the cis-to-trans isomerization of alkene via 1H MAS NMR. As shown in Figure 6F, the chemical shift signals at 6.25 and 6.13 ppm corresponding to the α-C-H (Hc) and β-C-H (Hd) confirmed the formation of trans-1-phenyl-1-propene, with weak signals at 6.32 and 5.65 ppm attributed to the α-C-H (Ha) and β-C-H (Hb). The deuterium labeling experiments (Figure 6F) demonstrated that the intensity of Hd decreased drastically after the D2 was charged while the intensities of others specie kept nearly unchanged. In such a way, the isomerization process was illustrated where the alkene inserted into the metal-D bond with the elimination of the original β-C-H (Hd) after the rotation of C–C single bond.In addition to the above-mentioned experimental protocols, DFT calculations play a significant role in studying the selective hydrogenation process, providing comprehensive information from precise structure of catalyst to the reaction mechanism as well as structure-performance relationship. First, the structure and configuration of heterogeneous catalysts such as zeolite, 33 oxides, 86 , 128 carbon-nitride, 28 and alloys 36 can be optimized and modeled. Li et al. optimized the structure of Ni@CHA by calculated energies, and Ni2+ sites were found to sit stably in the six-membered rings with Al atoms in the para or meta position (Figure 7A). Second, various modes of reactant adsorption as well as the dissociation of dihydrogen among multiple active sites can be interpreted. For instance, Zheng et al. measured different energy barriers of dihydrogen spillover among different Cu sites in Pd/Cu 86 catalyst 36 (Figure 7B). The small energy barriers (0.11–0.25 eV) verified the facile spillover of H atoms. Third, the spectroscopy signature of adsorption and reaction intermediates can be validated. As shown in Figure 7C, the stretching bands of CH2∗ and CH3∗ were well predicted via DFT calculations, in accordance with the experimental observations and assignments. 33 Noteworthily, with the optimized structures, the hydrogenation routine can be predicted accurately. For example, Li et al. compared the energy barrier of Horiuti-Polanyi mechanism and associative mechanism in acetylene hydrogenation over Ni@FAU, revealing the priority of associative mechanism (Figure 7D). This is in line with the H2-D2-C2H2 pulse-response experiments where dihydrogen is activated with the assistance of adsorbed acetylene molecules.Finally, DFT calculations can provide multiple insights for the rational design of hydrogenation catalysts. Nørskov et al. performed DFT calculations to identify relations in heats of adsorption of hydrocarbon molecules and fragments on metal surfaces. 60 As a result, cheap Ni-Zn alloys were predicted to be efficient catalysts for acetylene semi-hydrogenation (Figures 3F and 3G), which were successfully verified by experimental studies. Hopefully this strategy might change the current trial-and-error mode of catalyst development.In this review article, we have summarized recent progress in the selective hydrogenation of C≡C bonds to the corresponding C=C bonds. This is not only an industrially relevant process known as semi-hydrogenation but also a popular model reaction. Efficient heterogeneous catalysts are being pursued and the detailed mechanism is still hotly discussed. Various strategies, including covering/poisoning the corner/edge active sites by organic compounds, segregation of active sites by adding the second metal, and the site-isolation approach by forming SACs, were discussed with concrete examples. Then, some key issues in the semi-hydrogenation process, for example the thermal run-away in front-end hydrogenation, the formation of green-oil and coke in tail-end hydrogenation, the solubility of dihydrogen, and the potential leaching of active sites in liquid-phase hydrogenation, are listed, which remain key challenges in industrial semi-hydrogenations. Finally, the detailed reaction mechanism of selective hydrogenation was discussed. Typical mechanisms, including the Horiuti-Polanyi mechanism, associative mechanism, and Eley-Rideal mechanism, are compared and the origin thereof discussed. Understanding reaction mechanisms and the further structure-performance relationships undoubtedly gives huge benefits to the rational design of robust catalysts for semi-hydrogenations. To achieve this goal, some important research methodologies, including spectroscopic investigations and theoretical calculations, were briefly detailed.The semi-hydrogenation of acetylene to ethylene in the gas phase and the semi-hydrogenation of MBY to MBE in the liquid phase have been industrialized on a large scale for decades, both using Pd-based catalysts. The innovation of catalytic materials offers great opportunities to construct a new generation of semi-hydrogenation catalysts with improved performance and economy. SACs appear to be ideal solutions to the new semi-hydrogenation processes. However, there is still a long way from the laboratory to industry. Objectively speaking, new semi-hydrogenation catalysts with comprehensive performance (substrate conversion, product selectivity, catalytic stability, and catalyst cost) surpassing the commercial ones are yet to be reported and confirmed. Apart from the thermo-catalytic semi-hydrogenation systems, electrocatalytic and photocatalytic or photothermal catalytic semi-hydrogenations are attracting growing attention in recent studies. For example, layered double hydroxide (LDH)-derived copper catalysts 152 and Cu dendrites deposited through an electrochemical method 117 show remarkable performance in the electrocatalytic semi-hydrogenation of acetylene to ethylene. Pd1/TiO2, 153 Au-Pd/C-TiO2, 154 and Pd1/N-graphene 155 catalysts show good performance in the semi-hydrogenation of acetylene under photothermal irradiation. Interestingly, Pt/TiO2 photocatalyst shows high substrate conversion and styrene selectivity in the hydrogenation of phenylacetylene under 385-nm monochromatic light irradiation, in significant contrast to the thermo-catalytic process. 156 These achievements might pave the way to new semi-hydrogenation processes, especially in the liquid phase.This work was supported by the National Natural Science Foundation of China (21872072, 22025203), the Frontiers Science Center for New Organic Matter, Nankai University (63181206), and Haihe Laboratory of Sustainable Chemical Transformations, Tianjin.X.D. and L.L. proposed the outline and completed the writing. J.W. arranged the table and collected the copyright. N.G. and L.L. revised the manuscript.The authors declare no competing interests.

The selective hydrogenation of carbon-carbon triple bonds to the corresponding double bonds, the semi-hydrogenation process, plays a very important role in polymer and fine chemical industry. Various heterogeneous catalysts have been exploited for the selective hydrogenation of alkynes and alkynols in the gas and liquid phase. Herein, the recent progress in developing semi-hydrogenation catalysts, from traditional Pd-based monometallic catalysts to intermetallic compounds and single-atom catalysts, is summarized. The activation of dihydrogen during hydrogenation and the full hydrogenation mechanism, along with relevant research methodologies, are discussed. This review provides a comprehensive overview on the catalysts and mechanisms of industrially relevant semi-hydrogenation processes, addresses some existing debates, and sheds light on future catalyst design for hydrogenation.

3D metal-embedded microporous carbocatalystsPre-exponential factorsPristine biocharBrunauer-Emmett-TellerBarret–Joyner–HalendaCatalytic fast pyrolysisCatalytic fast co-pyrolysisCarbon NanotubesCorn strawDeionizedDerivative thermogravimetryActivation energyEnergy dispersive X-ray spectroscopyFe-embedded microporous carbon catalystFourier transform-infraredHydrogen to carbon effective ratioHigher heating valueLow-density polyethyleneLower heating valueMonocyclic aromatic hydrocarbonsNi-embedded microporous carbon catalystPolycyclic aromatic hydrocarbonsPellet biocharPolyethylene terephthalatePolypropylenePolystyrenePolyvinyl chloridePlastic wasteThermogravimetryThermogravimetric analysisTemperature programmed oxidationScanning electron microscopyTransmission electron microscopeWheat strawWS and PE blendWS, PE, and catalysts blendsWS and PW blendWS, PW, and catalysts blendsX-ray diffractometerX-ray photoelectron spectroscopyZn-embedded microporous carbon catalystBiomass is regarded as one of the most renewable and sustainable energy sources with huge reserves [1]. Exploration of biofuels and well-defined chemicals from biomass through fast pyrolysis has received extensive attentions [2]. At present, most efforts were devoted to further upgrading bio-oil due to the high oxygen content, low heating value, low pH value, etc.[3]. Alternatively, catalytic fast pyrolysis (CFP) is one of the most prevailing and promising techniques for the conversion of biomass directed toward valuable biofuels and chemicals [4].However, the CFP technology is commonly plagued by the low yield of target products and rapid deactivation of catalysts [5,6]. These huge challenges are mainly related to the low hydrogen to carbon effective (H/Ceff) ratio of biomass [7–9]. To mitigate these issues, it is reasonable to incorporate hydrogen-rich co-reactant with biomass in CFP process to modify the reactions of oxygen elimination by substituting decarbonylation and decarboxylation with dehydration [7,10]. Tremendous quantities of plastic waste produced each year can represent a cheap and abundant hydrogen source to be co-fed with biomass during the CFP process [11]. It is discerned that co-feeding of biomass with plastic waste in CFP process is remarkably beneficial for the environment and energy recapture [5,6,12].Zeolites and metal oxides are the most extensively used and the highest-efficiency catalysts to manufacture considerable petrochemicals (aromatics and olefins) [13]. However, these catalysts are expensive and still suffer from relatively severe deactivation due to the low anti-deactivation abilities [5,14]. A lot of efforts have been devoted to developing alternative carbonaceous catalysts due to the adequate environmentally benign nature, biocompatibility, and great tolerance to coke deposition [15,16]. Diverse carbonaceous materials, including activated carbon, carbon nanotubes, carbon dots, and graphene, and graphene oxide, are recognized as the very promising carbocatalysts [17,18]. Yet, the commonly used carbonaceous precursors for the synthesis of carbocatalysts present the huge limitations considering the economic feasibility, sustainability, and scalable production [16].Alternatively, biochar as the emerging carbonaceous material from biomass pyrolysis or gasification has attracted considerable attention in catalysis due to its low cost, tunable pore structure, and high stability [19]. For instance, biochar has been used as a promising catalyst for CFP of biomass [20], which could contribute to the generation of aromatics by catalyzing the deoxygenation reaction to eliminate the oxygen content in the resulting liquid product [21]. Besides, biochar has been reported as an inexpensive catalyst with fair performance in tar removal [22]. The deactivated biochar catalyst could be readily disposed by gasification or combustion for the recoveries of energy and loaded metals [22]. However, biochar usually possesses a low specific surface area, pore volume, and thermal stability, hindering the applications of biochar for effective catalysis [23].Recently, many efforts have been also devoted to developing three-dimensional (3D) porous structures and improving the catalytic abilities of biochar by incorporating metals species on the surface of biochar [22,24]. Metal chlorides (e.g., ZnCl2) with pore-forming ability are scalable and green porogens to fabricate biochar with 3D porous structures [24]. The metal dopant on biochar was able to create efficient active sties (Lewis acid sites), and the 3D porous structure allowed more active sites to be exposed, both of which were commendably desired for catalysis [25]. Thus, 3D porous biochar doped with the catalytically active sites (metal or metal oxides nanoparticles) could endow biochar with a significant increase in catalytic abilities [26].It is discerned that biochar is a hard carbon material mainly containing amorphous carbon atoms [23]. In contrast to amorphous carbon, graphitic carbon is usually composed of a hexatomic ring lattice, giving rise to a high specific surface area [27] and thereby increasing the number of active sites for catalysis [28]. If biochar can be modulated with a graphitic structure, the properties of biochar will be significantly improved [29]. Recently, some studies have also introduced graphitic carbon into amorphous carbon matrix, which exhibited excellent catalytic activities for electrocatalysis [30]. More importantly, it was reported that the 3D porous graphitic biochar exhibits an outstanding advantage of combining high stability and porous structure [31]. The high graphitization degree and abundant porosity of 3D porous graphitic biochar indicated an outstanding photocatalytic activity [32]. Biochar with highly-perfect graphitization and porous structure could also serve as a good support for the introduction of metal nanoparticles, which was beneficial for the effective photocatalysis [32].It should be also noted that the catalyst pore structure play a very crucial role in the CFP of biomass, micropores with the pore size of c.a. 0.52–0.59 nm was conducive to the higher aromatic yield, while the mesopores and macropores resulted in the higher coke and lower aromatic production [33]. Our previous study has also evidenced that the microporous structure of the catalyst contributed to the generation of monocyclic aromatic hydrocarbons [6]. Interestingly, it has been found that biomass pelletization is a promising pretreatment way for biochar production with compressed structures [34]. Biomass pelletization could rearrange the same particles and the original cell structure could be compressed [35], resulting in pellet biochar (PBC) developed with more compressed microporous structures.Hence, developing 3D metal-embedded microporous carbocatalysts (denoted as 3DMeMCs) with excellent catalytic activities has motivated our curiosity. Prior to the carbonization of biomass pellets, corn stover was selected as the carbonaceous precursor for pelletization due to its extensive abundance in agricultural wastes. Then we develop a single-step energy-efficient strategy, using three widely used metal chlorides (ZnCl2, FeCl3, and NiCl2), to fulfil the synchronous pore-forming, metal-doping, and graphitization for synthesizing 3DMeMCs. The simplified and facile synthesis route can remarkably reduce the consumptions of energy and chemicals. The intrinsic (surface chemistry, degree of graphitization, etc.) and extrinsic (specific surface area, morphology, etc.) characteristics of the as-synthesized catalysts were instigated by using a series of characterization techniques including SEM, EDX, TEM, TPO, TGA, FTIR, XRD, XPS, and Raman.To the best of our knowledge, the development of 3DMeMCs for catalytic fast co-pyrolysis of biomass and plastic waste has not been researched yet. It is discerned that H2 emerging as a clean and eco-friendly energy carrier; and syngas as a mixture of H2 and CO is receiving extensive attention due to its high calorific value and board applications as a precursor for the Fischer-Tropsch synthesis [36]. Carbon nanotubes derived from plastic waste possesses a variety of applications due to unique physicochemical properties [37]. Therefore, the catalytic abilities of the as-synthesized catalysts were subsequently examined in the on-line and ex-situ catalytic fast co-pyrolysis of biomass and plastic waste for aromatics, syngas, as well as valuable carbons under different scenarios. To further explore the catalytic performances and synergistic effects during co-pyrolysis, the thermal decomposition behaviors of individual reactant and co-reactants were investigated by thermogravimetric analysis (TGA), followed by the calculations of kinetic parameters. Eventually, a plausible reaction mechanism regarding practical ex-situ catalytic fast co-pyrolysis of biomass and plastic waste over 3DMeMCs were proposed in terms of the experimental observations.Corn stover (CS) and wheat straw (WS) were supplied by the Shangzhuang experimental station of the China Agricultural University. The proximate and ultimate analysis of CS and WS were given in Table S1. Both CS and WS were initially dried at 105 °C to remove the physically bound moisture until the weights were constant. Afterwards, they were pulverized using a high-speed miller (RT-34, Taiwan RongCong Precison Technology Co., China) into the particle size of 20 – 40 mesh. Low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET) and polyvinyl chloride (PVC) in the form of 100 mesh powder were all purchased from Huachuang Plastic Ltd., China. 40 wt% of LDPE, 35 wt% of PP, 18 wt% of PS, 4 wt% of PET and 3 wt% of PVC were thoroughly mixed to simulate real-word plastic waste. All chemicals and reagents, including HCl (12 mol/L), ethanol (>99.7%), ZnCl2, FeCl3·6H2O, and NiCl2·6H2O, were supplied by Beijing Lanyi Chemical Co., Ltd. (Beijing, China).Biomass pellets (approximately 4 mm in diameter and 12 – 14 mm in length) were manufactured with the raw ground CS by a 9KLP-125 pelleter (Yongfeng Machinery Equipment Co., Ltd, Henan, China). The biomass pellets were carbonized at 500 °C under N2 atmosphere with a heating rate of 10 °C/min and maintained for 1 h. After cooling to room temperature, the pellet biochar was crushed and sieved to 20 – 30 mesh for the future use. The pulverized pellet biochar was named as PBC. The raw ground CS was also carbonized at the same condition to achieve pristine biochar (denoted as BC) for comparison.The synthesis procedures of 3DMeMCs are depicted in Fig. 1 . PBC was leveraged as the precursor of 3DMeMCs. Zn-embedded microporous carbon catalyst (termed as Zn@C) was synthesized starting with ZnCl2 dissolved in deionized (DI) water, and PBC was then immersed into the ZnCl2 solution with the ZnCl2 to PBC weight ratio of 1:1. Inspired by Zhu et al., both ZnCl2 and FeCl3 were used as the activating agent and functional material [38]. Similarly, the Fe-embedded microporous carbon catalyst (Fe@C) and Ni-embedded microporous carbon catalyst (Ni@C) were prepared in light of the procedure but using FeCl3 and NiCl2 as substrates, respectively. The three slurries were stirred for 4 h at 1000 rpm and set for overnight. The slurries were subsequently air-dried at 105 °C for 12 h; and then these solid mixtures were annealed at 700 °C for 4 h under N2 flow of 100 mL/min at a heating rate of 10 °C/min. The obtained solid mixtures were successively with 0.1 mol/L HCl, ethanol (washing the constituents that undissolved in water), and DI water to remove the metal chlorides left and organic matters on the surface, prior to ultimately being dried at 105 °C. Finally, these composites defined as 3DMeMCs were cooled to room temperature under N2 flow and the 3DMeMCs were stored for the subsequent catalytic tests.The hyphenated technique of Pyrolyzer (Py-3030D, Frontier Laboratories Ltd., Japan) and gas chromatography/mass spectrometry (7890A/5975C inert, Agilent technology) were applied to conduct the catalytic fast pyrolysis and analyze the products synchronously. WS was well mixed with PW or PE in a mass ratio of 1:1 for co-pyrolysis. As such the reactants and catalysts were also blended conformably at the mass ratio of 1:1 for catalytic fast co-pyrolysis (CFcP). 2 mg of samples were subjected to the on-line catalytic fast co-pyrolysis system each time. The reaction temperature for all runs was implemented at 500 °C and reaction time was kept at 20 s. High-purity He (99.99%) was utilized as the inert gas can carried gas at a constant flow of 1 mL/min, and the split ratio was set at 1:150. The temperature of gas transmission pipeline between the micro-pyrolyzer and GC injector was maintained at 250 °C, while the GC/MS injector temperature was set at 270 °C. An HP-5MS capillary column (30 m × 0.25 mm × 0.25 um) was used to identify the hot pyrolysates. For MS analysis, the mass-to-charge ratio was kept in the range of 35 – 550 m/z, electron ionization was set at 70 eV. Compounds were identified by comparing the spectral data with that in the NIST Mass Spectral library.The ex-situ CFcP of WS and PW was performed to evaluate the catalytic capacities of as-synthesized PBC and 3DMeMCs for desired products by using a two-stage tube furnace reactor, as shown in Fig. S1. 1.5 g of WS and 1.5 g of PW in a mass ratio of 1:1 by sharking to obtain the mixture as constant feedstock for fast co-pyrolysis; approximately 1.5 g of catalyst was first placed in the catalytic fixed-bed into the quartz tube (inner diameter 20 mm; length 550 mm). The quartz wool was utilized to separate and hold the catalyst and the blend in place. Prior to all tests, the ex-situ CFcP system was purged with Ar at a flow rate of 30 mL/min for 20 min to remove the air present in the system. The region of fast pyrolysis was set at 500 °C, while the catalytic region was maintained at either 500 or 800 °C. To achieve relatively consistent temperatures, the pyrolysis and catalysis beds were pre-heated and equilibrated at the set temperature for 10 min. Thereafter, the pyrolysis bed was pushed into the furnace to initiate the experiments and kept for 10 min. After the experiment, the condensers and adaptors were then washed with ethyl acetate (10 mL) in small aliquots. The ethyl acetate-soluble organic phase was named as bio-oil, while the water-soluble fraction was the aqueous phase product. In the meantime, bio-oils were recovered by flowing the air over the ethyl acetate-soluble phase at room temperature for 4 h to ensure the complete evaporation of ethyl acetate. In this study, the aqueous phase products were not considered for analysis due to the small amounts. The non-condensable pyrolytic vapors escaped as gas at the end of the condensers were collected for analysis. All ex-situ CFcP experiments were operated in triplicate; the product yields were calculated as the mean values of three tests, and standard deviations were all less than 5%.Thermogravimetric analysis (TGA) of WS, plastics (PE, PP, PS, PET, PVC, and PW) as well as their blends was carried out by using a thermogravimetric analyzer (SDT Q600, TA Instruments, USA) to investigate their thermal degradation behaviors. For each experiment, ∼5 mg of the sample was placed in an alumina crucible and heated from room temperature to 600 °C at a heating rate of 20 °C/min with a N2 flow rate of 50 mL/min. TGA of WS or plastics alone were first conducted. As for the measurements concerning thermal degradation of WS and PW or PE blends, the WS to PW/PE mass ratio was set as 1:1. In addition, the same thermogravimetric analyzer was employed to conduct TGA for catalytic co-pyrolysis of co-reactants with the catalysts. The co-reactants to catalyst mass ratio was also set at 1:1.The elemental compositions (C, H, N, and S) of the reactants, carbonaceous materials, and bio-oils were tested by using an elemental analyzer (Elementar Vario ELIII, Germany). The amount of moisture, volatile matter, and ash were analyzed by using a fully automatic measuring industrial analyzer (YX-GYFX 7705B, U-Therm, China). The fixed carbon was determined by the subtraction method.The surface morphology of these carbonaceous materials was characterized by a scanning electron microscopy (SEM, SU3500, Hitachi Ltd., Japan). The elemental compositions and distributions in these carbonaceous materials were analyzed through the energy dispersive X-ray spectroscopy (EDX, SDD3310, IXRF Systems, USA) connected with the SEM. A transmission electron microscope (TEM, FEITecnaiG2F30) operated at 200 kV was used to further characterize the surface morphology of the carbonaceous materials. The textural properties of the carbonaceous catalysts were measured with N2 adsorption–desorption isotherms at a liquid nitrogen temperature of 77 K using TristarII3020 (Micromeritics, USA). All the catalysts were degassedin vacuum at200 °Cfor6 h. The Brunauer-Emmett-Teller (BET) equation was used to calculate the surface area, the desorption branch of the isotherm was utilized to calculate the pore size distribution, and the pore volume was measured according to theBarret–Joyner–Halenda(BJH) method.Temperature programmed oxidation (TPO) of these carbonaceous materials were operated by using the thermogravimetric analyzer (SDT Q600, TA Instruments, USA) to investigate the thermal stabilities and char-inorganics quantifications of the carbonaceous materials. All TPO experiments were conducted in the synthetic air (N2:O2 = 80/20 v/v%) flowing at 50 mL/min with a temperature ramping rate of 20 °C/min up to 600 °C. The TGA of these carbonaceous materials was also carried out by using the thermogravimetric analyzer under N2 atmosphere to further evaluate the thermal stabilities of the carbonaceous materials that would be used as catalysts during the CFcP process. The chemical structures and metal species on the carbonaceous materials were determined by X-ray diffractometer (XRD, XD-3, Persee Ltd., Beijing, China) operated at 36 kV and 20 mA, 2θ scale of 10–80° at a step size of 0.02°, and a scanning speed of 2°/min. The graphitization degree of the carbonaceous catalysts was analyzed by using a Raman spectrometer (SENTERRA Ⅱ, BRUKER, Germany) at ambient temperature. The spectra (Raman shift from 200 to 3500 cm−1) were recorded with spectrograms at a wavelength of 532 nm.The functional groups of the carbonaceous materials were characterized by Fourier transform-infrared (FTIR) spectroscopy (Spectrum 400, PerkinElmer, USA). The spectra were recorded at a range of 400 – 4000 cm−1. Surface chemical compositions were characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB 250xi, ThermoFisher Scientific) equipped with a dual anode monochromatic Kα excitation source. The binding energies of all elements were corrected against the adventitious carbon C 1s core level at 284.8 eV. The deconvolution of the XPS spectra was performed by using the XPS peak4.1 software [39].The chemical compositions of bio-oils were analyzed by a GC–MS (QP2010 SE, Shimadzu, Japan) with RTX-5MS (30 m × 0.25 mm × 0.25 µm) capillary column. The GC was initially heated to 60 °C for 5 min and then programmed to heated to 270 °C at a rate of 5 °C/min and maintained at 250 °C for 5 min. The injection volume was 1 µL and injection temperature was held at 300 °C. The interface temperature was adjusted to 280 °C and the ion source temperature was set at 200 °C for the mass selective detector. High-purity helium (99.999%) was used as the carrier gas at a stable flow of 3 mL/min, and the split radio was 1:10. The identification of bio-oils was determined according to NIST database of MS spectra library. The peak area percentages of chemical compounds based on GC/MS results were applied to predict the product selectivity. A mixture of C6 – C12 aromatic hydrocarbons ((≥99.5%, Macklin, China) was used calibrated standards to quantify the main aromatic hydrocarbons in the bio-oils. All the GC/MS measurements were conducted in triplicated to assure reproducibility.All gaseous products were collected by a 2 L gas bag and then offline characterized by a Shimadzu GC-2014C gas chromatography (GC, Shimadzu Corp., Kyoto, Japan) with a thermal conductivity detector (TCD). A standard gas mixture consisting of H2, CO, CH4, CO2, C2H4, C2H6, C3H6, and C3H8 was employed to calibrate the proportion (vol.%) of gaseous compositions. The gas fractions (≥C3) were not observed or negligible in this study. All GC measurements were also implemented in triplicate to ensure reproducibility.The weight of bio-oils was determined on the basis the weight difference of the container before and after the test. The mass of biochar was calculated by the left residue in the pyrolysis bed, while the mass of gaseous products was measured by combining the total gas volume and the gas density of each gaseous fraction [39]. The mass of carbon (coke) deposited on the catalyst and waxy loss (CW) was defined as the following equation: (1) Mass o f C W = f e e d s t o c k m a s s - b i o o i l m a s s - b i o c h a r m a s s - g a s m a s s Yields (wt%) of the bio-oil, biochar, and gas were determined according to the following formula: (2) Yieldwt % = mass o f a p r o d u c t g mass o f d r i e d f e e d s t o c k g × 100 % The carbon yield of bio-oils was defined as the following equations. (3) Carbon y i e l d C % = moles o f c a r b o n i n b i o - o i l moles o f c a r b o n f e d i n × 100 % H2 and syngas (H2 and CO) yields were expressed as the volumes of H2 and syngas generated divided by the total mass of mixed feedstock (eqs. (4) and (5)). (4) Y i e l d H 2 NmL g = volume o f H 2 m L mass o f d r i e d f e e d s t o c k g (5) Yiel d syngas NmL g = volumes o f ( H 2 + C O ) m L mass o f d r i e d f e e d s t o c k g The higher heating value (HHV) of bio-oils were determined by the modified Dulong formula as follow [40]: (6) H H V MJ / k g = 0.3578 × C + 1.1357 × H - 0.0845 × O + 0.0594 × N + 0.119 × S The lower heating value (LHV) of the gaseous products was evaluated using the following equation [39]. (7) L H V M J / N m 3 = 0.126 × C O + 0.108 × H 2 + 0.358 × CH 4 + 0.665 × C n H m where CO, H2, CH4 and CnHm (≥C2) are the volume fractions of those species.The morphologies and microstructures of the as-synthesized materials were initially examined by using SEM images, as shown in Fig. 2 a–e. BC reveals an irregular morphology and smooth surface with a very limited porous structure (Fig. 1a). With the aid of pelletization of biomass prior to carbonization, more exfoliation and pores started to emerge in resulting PBC (Fig. 1b), suggesting that pelletization could act as a pore-forming step in the carbonization process. Fig. 2c-e shows the representative SEM images of Zn@C, Fe@C, and Ni@C, respectively. The three as-synthesized catalysts were found to be comprised of a great number of interconnected sheets crosslinking each other, generating the apparent 3D porous structures. Compared to PBC, more abundant pores were generated within the 3DMeMCs. Interestingly, Fe@C and Ni@C had microspheres half-embedded or completely enclosed in the porous carbon matrix (Fig. 2d and e). Obviously, the well-developed porous structures of 3DMeMCs are beneficial to accelerate mass transfer, adsorption, and exposure of more active sites for catalysis.Further insight into the morphology and structures of as-synthesized materials was observed by the typical TEM images (Fig. 2f-i). TEM analysis substantiates the crosslinking of the three-dimensional structures of as-synthesized materials, giving rise to thin nanosheets of diverse thickness. Obviously, 3DMeMCs showed a typical 3D network with open-hollow and interconnected micropores (Fig. 2g-i), which were expected to provide more active sites for catalytic conversions. More specifically, Zn@C was endowed with a randomly stacked graphite-like feature with a high level of disorder, related to a high porosity and specific surface area. TEM images of Fe@C (Fig. 2h) and Ni@C (Fig. 2i) reveal a light-colored carbon matrix embedded with highly-dense and dark-colored fine particles distributed all over the carbon matrix. For the sake of Fe@C, the light-colored carbon matrix was thin, especially on its edges; the dark-colored particles were homogeneously distributed with the nanoscales. TEM image of Ni@C indicates that the Ni-impregnated biochar matrix consisted of film-like porous materials and nanoparticles. Based on the high-resolution TEM (HRTEM) images as shown in Fig. S2, the surface of PBC (Fig. S2a) was not decorated with nanoparticles, but only revealed microporous networks; Zn@C (Fig. S2b) presented a sp3 -dominated structure with highly disorder. A relatively apparent lattice fringes with a distance of 0.24 nm appeared along the edges of Fe@C (Fig. S2c) and a much more obviously lattice fringes with a distance of 0.20 nm were observed throughout Ni@C (Fig. S2d), implying the high degree of crystalline of the synthesized Fe@C and Ni@C.The SEM-EDX (Fig. 2j-n) and elemental mappings (Fig. 2o-r) demonstrate the main coexistence of C and O in the as-synthesized materials. Fig. S3 show the extra elemental mappings, suggesting the presence of small amounts of trace minerals, i.e., Mg, P, K, Ca, etc. (for details, please see Table S2). As the metallic nanoparticles were prone to agglomerating to decrease the surface energy, thereby rendering a significant decrease in catalytic performances [41]. In this study, the EDX and elemental mappings have evidenced that the Zn, Fe, and Ni nanoparticles were homogeneously dispersed over the microporous carbon network of 3DMeMCs, contributing to the improvement of catalytic efficiencies. Interestingly, more non-agglomerated metal nanoparticles homogeneously penetrated insides the micropores or channels of the highly microporous carbon skeleton were obviously discerned for Ni@C than Zn@C and Fe@C. The subtle differences among the morphologies of 3DMeMCs were caused by different metallic precursors, resulting in different metallic nanoparticles.Further information on the porous structures of these materials were probed by N2 adsorption–desorption isotherms at 77 K. The N2 adsorption–desorption isotherms and pore size distribution of these carbonaceous materials are presented in Fig. 3 a and b, respectively. BC possess an IV-type isothermal curve with a small hysteretic loop of desorption branch in the relative pressure (P/P0 ) range of 0.45 – 0.98 (Fig. 3a). The hysteresis loop of BC, extending from the medium relative pressure region, was attributed to the capillary condensation in the mesopores [42]. The observations strongly showed the coexistence of micropores and mesopores in BC. In contrast, PBC and 3DMeMCs show a type-I adsorption–desorption isotherm with sharp adsorption inflections at low relative pressures and well-developed plateaus (Fig. 3a), suggesting the predominance of micropores within the carbonaceous materials. The rich microporous structures of 3DMeMCs might be derived from the defects or pores and slits among the graphite-like sheets [23]. It is also noted that the isotherms of Fe@C showed a sharper increase of N2 uptake at the low relative pressure region (P/P0  < 0.1) than others, affirming the formation of a larger number of micropores with the activation of FeCl3 [31]. As shown in Fig. 3b, BC exhibited a broad pore size distribution, with a maximum at approximately 4 nm, indicating the co-existence of micropores and mesopores. However, PBC and 3DMeMCs showed narrow pore size distributions in the range of 0.7 – 1.3 nm with a sharp maximum at nearly 1.0 nm, substantiating the presence of well-developed microporous structures of PBC and 3DMeMCs. These results were in agreement with the adsorption–desorption isotherms.The textural characteristics of these materials are summarized in Table 1 . The BET surface area and porosity of these materials were entirely different. Due to the rugged surface feature, BC displays a low BET surface area (about 105 m2/g) and total pore volume (0.13 cm3/g), while the BET surface area and total pore volume of PBC were dramatically augmented to 539 m2/g and 0.27 cm3/g, respectively. Apparently, PBC formed undergoing biomass pelletization favored the generation of larger surface area and pore volume as compared to BC. Notably, the apparent BET surface area of 3DMeMCs was significantly dependent upon the species of metal ions. Ni@C with the modulation by NiCl2 integration possesses a lower BET surface area (645 m2/g) as compared to Zn@C (714 m2/g) and Fe@C (964 m2/g), which was ascribed to the dispersion and penetration of more active Ni species into the micropores of the carbonaceous skeleton, thereby resulting in the obstruction of the generation of micropores [41].Besides, the microporosity of 3DMeMCs could be manipulated by varying the types of metal ions. The micropore volumes of Zn@C, Fe@C, and Ni@C were 0.30, 0.42, and 0.28 cm3/g, respectively; the total pore volumes of these materials were 0.36, 0.48, 0.33 cm3/g, indicating that the proportions of micropores to the total porosity were determined to be 83, 88, 85%, respectively. There results suggested that 3DMeMCs exhibited well-developed microporosities, which were essential to accelerate the pyrolytic volatiles access to the micropores for rapid adsorption or desorption. Noted that Fe@C showed the highest porosity than Zn@C and Fe@C. The higher microporosity of Fe@C was attributed to more volatile matters and gaseous fraction under the high carbonization temperature (700 °C). Nevertheless, Ni@C exhibited the lowest porosity. This could be explained that the micropores induced by the NiCl2 and pore channels were blocked by the formed metallic Ni and nickel oxides. The collapse of the well-defined microporous structure caused by the catalytic graphitization of amorphous carbon also resulted in the decreases of surface area and pore volume. As demonstrated in Table 1, the average pore diameter (2.04 nm) of Ni@C was larger than that of Zn@C (2.01 nm) and Fe@C (1.99 nm), which validated the obstruction of micropores in Ni@C. Overall, the BET specific surface area, total pore volume, and micropore volume obey the following order: BC < PBC < Ni@C < Zn@C < Fe@C. Table 1 also lists the average pore diameter of these materials. The average pore diameter of BC (5.12 nm) was larger than that of PBC and 3DMeMCs. The average pore diameter (∼2.0 nm) of 3DMeMCs, most of which was derived from micropores, were beneficial for the diffusion and transport of pyrolytic volatiles.Thermal stability of these materials was tested by TGA, the plots of weight and derivate weight with regard to oxidation temperature are presented in Fig. 3c and d, respectively. As shown in Fig. 3c, the weight losses between 300 and 600 °C were caused by the oxidation of carbonaceous materials; however, the remaining residues were originated from minerals and incorporated metal oxides. PBC exhibits the largest weight loss (94.4%) during oxidation, indicating the least ash (including metal oxides) in the materials. The curve of Zn@C at the end experienced a weight loss of 91.7%, indicating that Zn@C was virtually free of impurities. However, the curves of Fe@C and Ni@C showed that 76.5 and 56.1% of weight were combusted, respectively. It was inferred that the large amounts of Ni and Fe components were retained after the catalyst synthesis. These trends were in line with the observations by XPS, EDX, and elemental analysis.The oxidation peaks in derivate weight plots represent the type of carbons and thermal stabilities, as shown in Fig. 3d. It has been reported that the oxidation peak at lower temperature is related to amorphous carbon, whereas the peak at higher temperatures is ascribed to graphitic carbon that is more stable and less reactive [43]. BC and PBC contained two distinct oxidation peaks. Compared to BC, PBC showed an increased decomposition temperature at around 400 and 430 °C, indicating the decomposition of oxygen-containing functional groups and more carbon content [44]. All 3DMeMCs revealed only one decomposition step with inflection temperatures; and the maximum temperatures of weight loss in the derivate peaks of 3DMeMCs shifted to higher oxidation temperatures as compared to PBC, indicating the high crystallinity of carbon nanomaterials in 3DMeMCs [45]. Zn@C and Ni@C, the onset and end temperatures were shifted to higher temperature (∼575 °C) than Fe@C (470 °C), implying that the amorphous carbon in Ni@C and Zn@C was transformed into semi-crystalline carbon with higher thermal stabilities [46]. This was well aligned with the observation by XRD and TEM that higher degree of graphitization was gained for Ni@C. Overall, the degree of graphitization according to TPO results follows the order of Ni@C ≈ Zn@C > Fe@C > PBC > BC. Note that the carbon content in Zn@C was comparatively higher than that of Ni@C according to DTG intensity at around 575 °C. There was no weigh loss found in these materials after 600 °C, confirming that there was no secondary decomposition or weight loss of active products. Fig. 3e further illustrates a TG and DTG curves of as-synthesized materials in N2 atmosphere and the curves (particularly Zn@C) was almost unchanged (less than 5% of weight loss) up to 600 °C, suggesting that 3DMeMCs presented superior thermal stabilities which enable them for utilization in the wide range of temperatures. However, the TG curve of PBC involved in Fig. 3e showed a higher weight loss (∼10%) due to the further pyrolysis of PBC in the range of 500 – 600 °C. Thus, 3DMeMCs are proper for catalytic pyrolysis under the oxygen-free atmosphere.During the synthesis with relatively high temperature for 3DMeMCs, the initial amorphous carbon configuration (sp3 -dominated) of PBC could be rearranged for graphitization in terms of its low-energy and stable properties [26], giving rise to the transformation of carbon configuration. More intuitive evidences were shown by XRD and Raman analysis (Fig. 3f and g). As demonstrated in Fig. 3f, there is no distinct peak found in the XRD pattern of BC, which is probably due to the destruction of its intrinsic structure. The baseline deflection (2θ < 30°) was obviously found in the XRD patterns of PBC and Zn@C, evidencing the amorphous feature. Note that there was no Fe phase that was discerned in the XRD patterns of Fe@C, which probably resulted from the limited detection depth of XRD over encapsulated Fe species with thick carbon nanoshells or the amorphous Fe species on the surface [41].Upon modulation with NiCl2 followed by carbonization/thermal treatment, the diffraction peaks around 44.5, 51.3, and 76.4° were associated with crystallin planes of metallic Ni [41], indicating the successful reduction of nickel ions to metallic state during the synthesis, which was in accordance to other studies reported elsewhere [47,48]. The finding was also in line with the HRTEM observation (Fig. S2d) that the lattice spacings of ∼0.20 nm [48]. In addition, the distinct diffraction peak at approximately 44.1° are observed, indicating the presence of crystalline planes of graphite carbon [49]. The graphitic carbon was probably produced by the catalytic graphitization of amorphous carbon by the anchored Ni nanoparticles. That’s because the catalytically active metals could effectively lower the energy barrier of the solid-state transformation from amorphous carbon to graphitic carbon. In this study, the peak attributed to graphitic carbon (especially in Ni@C) was strong at 700 °C, which might be due to the fact that the transformation of the energetically less favorable amorphous carbon to a more favorable phase of graphitic carbon by the catalytically active metal could be significantly enhanced above 600 °C [50]. However, a weak amorphous peak of Ni@C was still found, suggesting that most of amorphous carbon were transformed into graphitic carbon, but not completely. It should be noted that the transition of pure sp3 -carbon in tetrahedral amorphous (ta-C) into thoroughly graphitic (sp2 ) carbon (g-C) can be categorized into three basic steps: 1) from tetrahedral amorphous (ta-C) to amorphous carbon; 2) from a-C to nanocrystalline graphite (ng-C) with apparent ordered aromatic clusters; 3) from ng-C to complete graphite (g-C) [26].Generally speaking, the enhanced porous structure is usually accompanied with the improvement of the defected carbon structures [25]. In this study, the defected carbon structures of as-synthesized materials were further characterized by Raman spectra. As depicted in Fig. 3g, three distinct peaks of the carbonaceous materials were detected. Typically, the D band at ∼1350 cm−1 is associated with the defect sites or disordered carbon, and G band at ∼1580 cm−1 is assigned with E2g mode stretching vibration of the sp2 -hybridized carbon network of graphite [32]. The 2D band at ∼2700 cm−1 is ascribed to the two phonon lattice vibration, which is typical symbol of graphitic carbon [31]. The intensity ratio of D band to G band (ID/IG ) indicates the degree of crystallization or defect density of carbon materials [31]. Overall, the ID/IG value of PBC and 3DMeMCs were in the order of PBC < Ni@C < Fe@C < Zn@C. The ID/IG ratio of PBC was calculated to be 0.82 and an intense 2D band related to the two-phonon double resonance was clearly found, confirming that the pretreatment of pelletization could endow PBC with a considerable degree of graphitization. However, the increased ID/IG ratio of Zn@C was estimated to be 0.99, resulting from the formation of micropores and enhanced structure disorder during the carbonization [51] It is noted that the Raman spectra of Fe@C and Ni@C presented narrow D and G bands as well as the gradually decreased ID/IG value, affirming the enhanced transformation of amorphous carbon into graphitic carbon. The above-mentioned findings indicated that 3DMeMCs (especially Fe@C and Ni@C) presented highly microporous structures with a high degree of graphitization. In addition, the ID/IG value of standard graphite was much lower than that of all 3DMeMCs [52], implying that there were still large amounts of sp3 carbon present in 3DMeMCs.Moreover, the surface functional groups in PBC and 3DMeMCs were characterized by FTIR analysis (Fig. 3h). The typical characteristic peak of porous graphitic carbon were manifested at 1637 cm−1 corresponding to the skeletal vibration of CC bonds in aromatic ring [32]. The peak range of 1470 – 1580 cm−1 could be assigned to ketonic CO stretching vibration, and the peak at 1072 cm−1 ascribed to C–O/C–O–C stretching vibration [32]. Upon fabrication by metal chlorides, the new stretch appeared at the range of 2280 – 2380 cm−1 was derived from the production of metal oxides. There results were in line with the observations from the high-resolution XPS spectra of C 1s and O 1s (Fig. 4 ). As compared to the FTIR bands of PBC, some bands decreased in intensities or even disappeared for all 3DMeMCs. Such variations explained the decompositions of the surface functionalities during the one-step thermal process [53]. Obviously, the decrease of disappearance of the bands between 950 and 1710 cm−1 implied that most oxygenated functionalities on the surface of 3DMeMCs were removed at 700 °C.To further elucidate the surface chemical composition and the valence state of elements, XPS measurements were conducted as shown in Fig. 4. The XPS full survey spectra (Fig. 4a) of PBC and 3DMeMCs exhibited that all as-synthesized materials primarily consisted of C and O. The strong peaks of C 1s, O 1s of PBC suggested that PBC was in high purity and the characteristic peaks of Zn 2p, Fe 2p, and Ni 2p peaks were observed in Zn@C, Fe@C, and Ni@C, respectively. The summary of XPS atomic distribution is listed in Table S3. According to the XPS data (Table S3), PBC contained 84.27 at% C and 15.73 at% O, and no other impurity elements were detected. The C atom ratio of Zn@C was found to considerably increase through the surface fabrication of ZnCl2, while the C content was reduced in Fe@C and Ni@C because of the incorporation of metal components. The atomic percentages of Zn (0.90%), Fe (14.35%), and Ni (6.81%) were achieved for Zn@C, Fe@C, and Ni@C, respectively. This result indicated that successful incorporation of Zn, Fe, and Ni elements into carbon skeleton, which was consistent with the EDX (Table S2) and CHNS elemental analysis (Table S4). Besides, the proportion of O was found to increase from 15.73 at% to 34.68 at% and 26.03 at% in Fe@C and Ni@C, suggesting that more metal oxides were present on the surfaces of Fe@C and Ni@C.The high-resolution XPS scans of the C 1s, O 1s, Zn 2p, Fe 2p, and Ni 2p regions, including the curve-fitting spectra for the representative catalysts, namely PBC and 3DMeMCs, are demonstrated in Figs. 4 and S3. The high-resolution C 1s XPS spectra of PBC and 3DMeMCs can be deconvoluted into four dominant peaks. As depicted in Fig. 4b-e, the main peak of C1s around 284.7 eV was attributed to C–C, CC, and graphite in the carbon matrix [26]. The peak around 285.4 eV was ascribed to C–OH, C–O–C, and COOR [26]; while the adjacent peak at around 287.7 eV belonged to CO [54]. Noteworthily, a peak (290.5 eV) originating from the π–π* shake up was found [54], indicating the presence of graphitization. According to the deconvoluted peak area calculation (as shown in the inserted tables), PBC and Zn@C had higher CC/CC and aromatic C atomic ratios than Fe@C and Ni@C. The high-resolution O 1s spectra of these materials are shown in Fig. 4f-i, and they all contained three dominant peaks. Isolated OH or carbonyl group (CO) was the dominant oxygen-containing functional groups, followed by CO groups such as phenolic hydroxyl group (C–OH) in PBC. 3DMeMCs with the modulation by metal chlorides (e.g., ZnCl2) could enhance the abundance of COOH, acting as the main oxygen-containing acidic functional group on the surface of the catalysts [55]. CO groups like C–OH could offer binding sites to the carbon substrate for metals (like Zn) [56]. The Fe@C retained more oxygen atoms on the carbon surface, forming abundant oxygen-containing functional groups.The high-resolution spectra of Zn 2p, Fe 2p and Ni 2p are displayed in Fig. S4. The given Zn 2p XPS survey profile (Fig. S4a) suggested the two spin orbitals Zn 2p3/2 and Zn 2p1/2 at around 1022.2 eV and 1045.4 eV, respectively [57]. The trace amount of Zn composition was left in Zn@C even though a thorough washing step was performed. The melting ZnCl2 was intercalated into carbon layer with the formation of ZnO or Zn complexes in the high-temperature region, triggering the difficult removal [58]. The XPS spectrum of Fe 2p spectra regions was delineated in Fig. S4b, The peak at around 712.0 eV (Fe 2p3/2) was the characteristic of Fe(III) and Fe(II) [59], another main peak at 725.9 eV (Fe 2p1/2) was assigned to Fe(III) [59]. That was because the surface metallic Fe was readily oxidized to generate Fe(III) [60]. The presence of metallic Fe was also obtained with the distinctive satellite peak at approximately 720.2 eV, which was probably accompanied with the formation of Fe3C in the carbon matrix [61]. In the Ni 2p spectrum (Fig. S4c), the binding energy of Ni 2p3/2 peak at 855.1 eV and the satellite peak at 861.2 eV indicated the presence of Ni (II) bound to oxygen [62,63], affirming the observation by O 1 s analysis (Fig. 3i). The broad and intense photoelectron Ni 2p1/2 at the binding energy of 872.7 eV and the satellite peak (878.9 eV) could be assigned to metallic Ni, which was in agreement with XRD observations. Typically, the binding energy value of metallic Ni located at 872.7 eV was higher than other studies reported elsewhere, indicating an extremely strong Ni-support interaction [41]. Thus, most Ni present on the surface of Ni@C was in the metallic form instead of oxidized sates.To first get insight into the catalytic abilities of 3DMeMCs in the CFcP of WS and PW blends, the on-line micro-pyrolyzer integrated with GC/MS analysis was applied to determine the relative contents of each chemical group in the condensable products obtained from fast co-pyrolysis of WS and PW blends with/without catalysts (Fig. 5 ). The detailed chemical compounds could be categorized into the following groups: aliphatic olefins, cyclic olefins, aliphatic alkanes, cyclic alkanes, aromatic hydrocarbons, phenolics, and other oxygenates. The relative contents of these groups are presented in Fig. 5a, 5d and Table S5. The chemical compounds in the condensable product from fast pyrolysis of WS alone were abroad, containing acids, furans, phenols, phenolics, aldehydes, ketones, alcohols, esters, and aromatic hydrocarbons. As shown in Fig. 5a, the primary categories were phenolics and other oxygenates with the high relative contents of 19.0% and 70.6%, respectively.Importantly, the oxygenated compounds significantly decreased with a concomitant increase in hydrocarbon contents when WS was co-pyrolyzed with PW. A very low relative content (7.4%) of oxygenates was obtained and the relative content of phenolics was even decreased to zero during the fast co-pyrolysis of WS and PW. These phenomena indicated that the introduction of hydrogen-sufficient feedstock could be beneficial for suppressing the formation of oxygenates, especially phenolic compounds [64]. A comparison between the experimental and theoretical (the average of the experimental data gained from the fast pyrolysis of WS or PW alone) results are also presented in Fig. 5a. The relative content of oxygenates were much lower in the experimental case, which suggested that the addition of PW to fast pyrolysis of WS could result in an additional deoxygenation reaction. Conversely, the relative content of aromatic hydrocarbons was augmented from 18.2% to 36.9%, suggesting that an apparent synergistic effect took place for the generation of aromatic hydrocarbons by co-pyrolyzing WS with PW.Based on the chromatograms (Fig. 5c) of the condensable products originating from the fast co-pyrolysis of WS and PW blends with/without catalysts, the chemical components in condensable products gained from CFcP of WS and PW blend over PBC and 3DMeMCs were concluded in Fig. 5d and Table S5. In general, aromatic hydrocarbons were the most abundant in each case. More specifically, the CFcP of WS with PW over Fe@C and Ni@C resulted in a noticeable selectivity (over 35%) toward aromatic hydrocarbons. A more profound content (46.9%) of aromatic hydrocarbons was achieved by introducing Zn@C in the CFcP process, whereas the content (24.5%) was approximately halved in the case of CFcP by using PBC. Hence, 3DMeMCs was in favor of the formation of aromatic hydrocarbons as compared to PBC, indicating their higher aromatization activities.Yet a higher content of aliphatic and cyclic hydrocarbons was achieved using PBC as the catalyst. These observations implied that PBC solely containing carbon matrix cannot present sufficient efficiency for aromatic hydrocarbons formation, even though it had offered measurable catalytic activity. The higher efficiency of 3DMeMCs can be explained by its larger BET surface area, larger pore volume, and more metal active sites than PBC. The microporous structures of 3DMeMCs could accelerate the formation of aromatic hydrocarbons since the reactants in the micropores had a higher chance to collide with the active sites than in the large-pore catalysts, resulting in the improved conversion efficiencies [65]. It is also observed that the higher catalytic activities related to deoxygenation were differentiated by the species of metal active sites. Note that there were high relative contents (26.2%) of oxygenates (phenolics and others) in the condensable product resulting from CFcP of WS and PW by using Fe@C as the catalyst. This suggested that Fe@C showed a weaker deoxygenated ability than Zn@C and Ni@C.In terms of aromatic hydrocarbons that were mostly monocyclic aromatic hydrocarbons (MAHs), their yields were varied rather marginally. Apparently, styrene was the dominant aromatic hydrocarbon in all cases. As demonstrated in Fig. 5b, co-pyrolyzing WS with PW showed a higher yield of MAHs than fast pyrolysis of WS or PW alone. Similar to the chemical compositions in the condensable products, the experimental value was more than 2 times than theoretical value when WS and PW were co-fed in the fast pyrolysis process. These observations further confirmed the synergistic effect between WS and PW on the formation of MAHs occurring in the process.According to the pyrograms (GC/MS area: abundance versus time) as shown in Fig. 5c, MAHs were predominant in all cases by using carbonaceous catalysts for the fast co-pyrolysis process. This result was strongly related to the shape selectivity imposed by the microporous structures of the as-synthesized catalysts, which was in favor of the formation of molecules with six-membered carbon rings [66]. As given in Fig. 5e and Table S6, the strong abundances of MAHs in condensable products from CFcP of WS and PW over PBC, Zn@C, Fe@C, and Ni@C were 6.08 × 107, 15.8 × 107, 7.30 × 107, 19.4 × 107, respectively. Compared with PBC, 3DMeMCs gave rise to an enhanced formation of MAHs because of the increase amounts of metal active sites available for cyclization, oligomerization, aromatization, etc. Indeed, MAHs were enhanced during the CFcP process via Diels-Alder reaction between WS-derived furans and PW-derived olefins over the active sites of 3DMeMCs [5,10]. Among the 3DMeMCs, the yields of MAHs presented strong dependence upon the metal species. Ni@C showed the largest quantity of MAHs from CFcP of WS and PW than the other catalyst; and Fe@C was less effective for the production of MAHs, obeying the order of Ni@C > Zn@C > Fe@C. That’s because the metal active sites in the CFcP might function differently. The higher yield of MAHs for Ni@C and Zn@C should be also ascribed to more metal actives and/or higher degree of graphitization, as evidenced by the EDX, XPS, and TPO results described in Section 3.1 . Accordingly, 3DMeMCs can be selected as appropriate catalysts for the production of aromatic hydrocarbons especially MAHs through CFcP of WS and PW.3DMeMCs was thereafter examined during practical ex-situ CFcP process to determine their catalytic abilities. The product yields from practical ex-situ CFcP of WS and PW over PBC and 3DMeMCs at 500 °C are listed in Table 2 . The product yield from fast co-pyrolysis of WS and PW mixture in the absence of catalysts were not provided in Table 2 because waxy products were hard to collect and calculate their yields. With the use of these carbonaceous catalysts, both the yields of liquid and gaseous products remarkably went up due to the active acid sites of the carbonaceous catalysts. Compared with PBC, 3DMeMCs were superior to the bio-oil production, peaking at 64.16 wt% when Zn@C was used as the catalyst. In general, these as-synthesized showed comparable gaseous and biochar yields at approximately 14 wt% and 17 wt%, respectively.Inspired by Williams et al. [37,43], the higher catalytic temperature (namely 800 °C) was in favor of the gaseous production and valuable carbon. In this study, we also augment the catalytic temperature to 800 °C for H2 or syngas production (Table S7), together with valuable carbons. Indeed, the gas yield was dramatically enhanced to around 48 wt% when 3DMeMCs was employed as the catalysts. Interestingly, PBC exhibited a higher gas yield at 52.78 wt%, and PBC also revealed a higher bio-oil yield (23.85 wt%) than 3DMeMCs.The carbon yield and qualities of the bio-oils produced from ex-situ CFcP process at 500 °C were investigated by CHNS elemental analysis, HHV and GC/MS. Representative results of CHNS elemental analysis, carbon yield, HHV, and characterization of chemical compounds by GC/MS are listed in Table 2 and Fig. 6 . The carbon content (73.18 wt%) of the bio-oil derived from the ex-situ CFcP process in the presence of Ni@C was higher than that by using the other carbonaceous catalysts. In contrast, the oxygen content in the bio-oil from the process over Ni@C showed the lowest value. These findings suggested that the presence of metals embedded on the carbon matrix might be beneficial for the deoxygenations during the process [67], which in turn elevated both the carbon and hydrogen contents. Table 2 also demonstrated the carbon yield and HHV of bio-oils produce from the ex-situ CFcP process at 500 °C. Given the high mass yield and carbon content of bio-oils in the presence of 3DMeMCs, the carbon yield of the bio-oils produced from the process over 3DMeMCs was drastically promoted when comparing with the carbon yield (37.54 C%) by using PBC as the catalyst. Of these 3DMeMCs, Zn@C revealed the highest carbon yield (60.38 C%) than Fe@C and Ni@C, which was due to the highest mass yield (64.16 wt%) of bio-oil. However, the HHV of bio-oil from the process using Ni@C as the catalysts was 38.52 MJ/kg, which was higher than that (greater than31 MJ/kg) using PBC, Zn@C, and Fe@C. This finding suggested that the bio-oils generated from ex-situ CFcP of WS and PW at 500 °C in the presence of as-synthesized catalysts are a promising replacement of petroleum-derived fuels.The chemical compounds in the bio-oils are characterized and categorized, as shown in Fig. 6a and b. The bio-oil from CFP of WS alone was applied as a control to examine the synergistic effect between biomass and plastic waste during the ex-situ CFcP process at 500 °C. The oxygenated compounds such as furans, phenols, alcohols, ketones, aldehydes, aromatic hydrocarbons, etc. were the dominate species in the control. With the introduction of PBC and 3DMeMCs as the catalysts during the ex-situ CFcP process at 500 °C, the O-species content was reduced to less than 7%; particularly, Fe@C presented a lowest oxygenated content (less than 3%). Moreover, there was no oxygenates found in the bio-oils when the catalytic temperature was elevated to 800 °C over 3DMeMCs (Fig. 7 c).On the contrary, the hydrocarbon content reached over 93% by using 3DMeMCs at 500 °C; and Fe@C showed the highest content (97.52%) of hydrocarbons in the bio-oil. More importantly, it was found that more than 50% of aromatic hydrocarbons were present in the bio-oils when the PBC and 3DMeMCs were used as catalysts at 500 °C; while over 90% of aromatic hydrocarbons in the bio-oils were achieved over the carbonaceous catalysts at 800 °C (Fig. 7c). The abovementioned results were comparable with the observations reported elsewhere, which were found that approximately 70% of aromatics were enriched in the bio-oils [68,69]. The rest hydrocarbons in the bio-oils obtained at 500 °C were olefins, occupying around 32%. Regarding the hydrocarbons especially aromatic hydrocarbons, their carbon numbers were primarily ranged from C8 to C16, which are lumped in the jet fuel range [70]. As shown in Fig. 6b, Fe@C was superior to other carbonaceous catalysts for the production of C8 – C16 MAHs, with the selectivity reaching 59.50%. The concentrations of the dominant MAHs (i.e., ethylbenzene and styrene) are summarized in Table 2. It was noticed that styrene was the most abundant compound in all bio-oils, which mainly originated from the decomposition of PS; the concentration of styrene was all over 18 mg/mL, maximizing at 35.99 mg/mL when PBC was applied as the catalyst. These outcomes implied that the metals embedded on the carbon matrix contributed to the Diels-alder and aromatization reactions. Moreover, these results are similar with the findings observed from on-line CFcP tests, reaffirming the stable capacities of 3DMeMCs. Yet, most (over 50%) of the aromatic hydrocarbons in bio-oils gained at 800 °C were polycyclic aromatic hydrocarbons (PAHs), and PBC was in favor of the polycyclic aromatic hydrocarbons with up to 65% of selectivity (Fig. 7d). According to the concentration of the aromatic hydrocarbons (Table S7), styrene and naphthalene were the two main aromatic compounds found in the bio-oils obtained at 800 °C.It is well known that the gaseous product was more of potential value comparing to liquid product during CFP process [45]. In this study, the gaseous product was quantified by relatively pure gas fractions with highly thermodynamic stabilities.[71] The releasing properties of the gaseous products from the CFcP process catalyzed at 500 and 800 °C are correspondingly explained in Fig. 6c, d and Fig. 7a, b. As for the fast pyrolysis of WS, CO and CO2 were the predominant gaseous fraction through the cleavage of COC, CO, and OCO groups, which are enriched in the biomass [72]. Even though the fast co-pyrolysis of WS and PW could benefit the yields and qualities of bio-oils, gas as the byproduct was slightly influenced in terms of evolved gaseous compositions (Fig. 6c) as well as H2 and syngas yields (Fig. 6d). That was due to the incomplete decomposition of PW, which was confirmed by the high amounts of waxy products. Notably, the percentages of CO and CO2 were sharply declined by using the PBC and 3DMeMCs at 500 °C. Due to the co-feeding of biomass and plastic waste, O-species were expelled by reacting with intermediate olefins through Diels-Alder reaction followed by dehydration, instead of decarboxylation and decarbonylation reactions [6].Importantly, Fe@C catalyzed at 800 °C exhibited the lowest CO (17.56 vol%) and CO2 (8.90 vol%) proportions, as shown in Fig. 7a. The use of catalysts both at 500 and 800 °C improved the H2 proportion. There observations were in agreement with the results reported by Gupta’s group that the introduction of catalysts during catalytic fast co-pyrolysis/gasification of biomass and plastic wastes could efficiently enhance H2 yield and syngas production [73]. In particular, H2 proportion was found to ascend in the most pronounced way from 1.58 vol% in the absence of catalysts to over 24 vol% when using Fe@C and Ni@C as the catalysts at 800 °C (Fig. 7a). It was proven that Fe and Ni were the effective elements to enhance dehydrogenation reaction to produce H2 and valuable carbon [43,45]. In addition, the proportion of light hydrocarbons especially CH4 and C2H4 were observed to significantly increase over the carbonaceous catalysts up to 28.27 vol% and 20.15 vol% when Zn@C served as the catalyst at 800 °C, implying that the catalysts could be conducive to the catalytic waxy intermediates into light hydrocarbons.The LHV of gaseous products was calculated and shown in Fig. 6c and 7a. According to Eq. (7), the more share of CO and H2, but the less proportion of CO2 would contribute to the LHV of gas [74]. The gaseous products from CFcP process over the carbonaceous catalysts possessed the higher LHV than gas obtained from the process without the use of catalyst. 3DMeMCs were the more advantageous catalysts to gain the higher LHV both at 500 and 800 °C due to the higher proportions of CO, H2 but the lower CO2 proportion. More importantly, Zn@C and Fe@C at 800 °C showed the higher LHV (around 30 MJ/Nm3), presenting comparable value with natural gas.To further detect the catalytic capacities for the production of H2 and/or syngas (H2 plus CO), the H2 and syngas yields as a function of the catalyst species at 500 and 800 °C are illustrated in Fig. 6d and 7b, respectively. It was apparently found that the H2 and syngas yields remarkably went up to the high levels especially from the tests conducted at 800 °C (Fig. 7b). Among these 3DMeMCs, Ni@C was the optimal catalyst to yield the highest H2 and syngas yields at 156.81 and 272.54 NmL/gfeedstock, respectively.Since the slight and negligible changes of carbonaceous catalysts at 500 °C after experiments, and there was no obvious carbon deposition found on the catalysts. Accordingly, these spent catalysts at 500 °C were not considered for detailed analysis. In this regard, the characteristics of carbon deposited on PBC and 3DMeMCs at 800 °C were demonstrated in Fig. 8 . Apparently, there were obvious carbon deposition on the surface of the spent carbonaceous catalysts (Fig. 8a – d). Particularly, it was observed that the presence of a dense entangled growth of filamentous carbons covering the surface of Ni@C (Fig. 8d and e).According to the high-resolution SEM image as shown in Fig. 8f, a lattice fringes with a distance of 0.34 nm was found throughout spent Ni@C, suggesting the formation of multi-walled carbon nanotubes (CNTs). These observations evidenced that Ni@C favored the decomposition of hydrocarbons into CNTs and H2, which also confirmed by the higher H2 yield from Ni@C at 800 °C, as described in Section 3.3.3 . Based on the SEM-EDX analysis (Fig. 8i-k), there were some Ni nanoparticles found on the surface of Ni@C as well, which was probably explained by the reason that the nickel oxides were reduced by C and/or active pyrolytic vapors into metallic Ni.The spent carbonaceous catalysts (800 °C) were also analyzed by TPO to determine the properties of carbon deposition. As shown in Fig. 8g, the TPO profiles of spent catalysts were comparable with fresh catalysts. However, it was found that all spent catalysts presented single oxidation peaks at over 500 °C in the derivate TPO profiles (Fig. 8h), indicating the presence of high crystallinity of CNTs in spent carbonaceous catalysts [45]. The onset and end temperatures of spent Ni@C were shifted to higher temperature (∼550 °C) than other catalysts, suggesting the large amounts of CNTs formed on the Ni@C. In this regard, the fresh, spent Ni@C at 500 and 800 °C were compared in terms of derivate TPO profiles, as shown in Fig. 8l. It was confirmed that spent Ni@C at 800 °C revealed higher oxidation temperature peak and higher carbon oxidation rate than the comparable fresh Ni@C and spent Ni@C at 500 °C.Additionally, biochar generated from co-pyrolysis of biomass and plastics can be used as inexpensive absorbent, solid fuel, carbon sequestration, and soil amendment [10]. In this study, the thermal stabilities of biochar were tested by TPO, the profiles of weight and derivate weight on the basis of oxidation temperature are depicted in Fig. S5a and b. The weight losses occurred in the oxidation temperature range of 300–600 °C, the remaining residues were recognized by the minerals and/or incorporated metal oxides. Compared with WS-derived biochar, biochar from co-pyrolysis was shifted to higher oxidation temperatures, suggesting its higher thermal stability. There were no weight losses found in both biochar samples after 600 °C, verifying that there was no secondary decomposition or weight losses taking place.TGA was finally conducted to investigate the relationships between weight variation and temperature, which could further determine the thermal degradation behaviors and reaction mechanisms. Fig. 9 presents the TG and DTG curves of WS, PW, WS and PW blend (WS + PW), and WS, PW, and catalysts blends (WS + PW + CA). As plotted in Fig. 9a, the thermal degradation of WS mainly happened at between 150 and 400 °C and gradually continued up to 600 °C, which was due to devolatilization and most lignocellulosic fractions (cellulose, hemicellulose, and lignin) were degraded in the temperature range [75]. The decomposition of PW displayed a sequence of stages in the temperature from 300 to 500 °C. Considering the complex mixtures of PE, PP, PS, PET, and PVC, the TGA of these five model components of PW were also studied, the thermal degradation behaviors of these plastics are illustrated in Fig. S6a. The thermal degradation behavior of PVC was different from that of other plastics. Apparently, the thermal degradation of WS + PW blend was much more complicated than that of WS or PW alone.The DTG curve (Fig. 9c) of WS was peaked at approximately 320 °C, which was assigned to the time when the maximum degradation rates of hemicellulose and cellulose [76]. There was no an clear peak assigned to lignin decomposition because lignin was a small fraction in WS and the degradation of lignin took place in a wide temperature range of 200 – 500℃ [76]. The DTG curve of PW showed that PW was primarily degraded from 400 to 500 °C, which was evidenced by the DTG curves of individual plastic components as shown in Fig. S6b. As expected, the DTG curves of WS + PW blend revealed distinct peaks, which was due to the different characteristics of individual plastic components.The thermal degradation of WS + PW + CA were significantly different from that of WS + PW blend (Fig. 9b), suggesting that the introduction of catalysts had an essential influence in the thermal degradation behaviors. In particular, the TG curve of WS + PW + BC was almost linear and the amount of residue after thermal degradation of WS + PW blend over BC was nearly 40%, suggesting that BC was not stable during the thermal degradation stage and it could be further decomposed at the high temperature range. In contrast, WS + PW blend with PBC or 3DMeMCs exhibited the stable and comparable thermal degradation behaviors. It was reaffirmed that PBC with high degree of graphitization can be considered as a more promising carbonaceous material for direct use or the synthesis of catalysts than BC. To gain more insight into the effect of the types of catalysts on the synergistic effect between biomass and PW, TGA of WS and PE (typical model component of PW) blend were performed as shown in Fig. S7. Likewise, the TG curves of WS, PE, and catalysts blends (WS + PE + CA) showed stable and comparable thermal degradation behaviors.The peaks of DTG curves (Fig. 9d) of WS + PW + CA was shifted slightly to the lower temperature regions as compared to the peaks of the DTG curves of WS + PW blend without the introduction of a catalyst. The observation indicated that the catalysts except BC could decrease the degradation temperatures and be good for the decomposition of WS + PW blend, which was in line with the CFcP results [76]. It is noted that the DTG curves of WS + PE + CA experienced two distinct peaks (Fig. S7d). Compared to the DTG curve of WS and PE blend (WS + PE), the DTG curve of WS + PE + CA (especially WS + PE + Fe@C) was shifted to the lower temperatures. In addition, the results regarding kinetic studies were specifically described in Supplementary S2. The kinetic parameters including activation energy (E), pre-exponential factor (A), and correlation coefficients (R2 ) were calculated and provided in Figs. S8–S10 and Tables S8–10. It was consolidated that there was a synergistic effect during co-pyrolysis and the introduction of PBC and 3DMeMCs in the thermal degradation of biomass and plastics could significantly reduce the activation energy (E). Notably, PBC became a highly porous carbonaceous material due to the treatment of pelletization of corn stover. That’s possibly due to the fact that large amounts of gaseous species (e.g., H2O, CO2, and CO) generated during the carbonization that was not prone to escape due to the compacted structure of corn stover, the gradually accumulated gas retained might affect the carbon structure, which would finally evaporate and leave more pores (especially micropores) in PBC [25]. Additionally, in the pelletization process, the fraction of lignin was molten during the grinding process with moderate temperature, which severed as the binder for pelletization. Accordingly, the grinding together with pelletization for manufacturing biomass pellets could etch biomass structure, which possibly endowed the carbon structure with more micropores after carbonization. These findings were evidenced by N2 sorption analysis in Fig. 3a, 3b and Table 1. Accordingly, PBC exhibited suitable properties to be applied as the carbon skeleton.In the synthesis of 3DMeMCs, metal chlorides (i.e., ZnCl2, FeCl3, and NiCl2) were utilized as both the activating agent and the catalysts to fulfil the synchronous carbonization and graphitization of biomass carbon, according to the following equations, but they did not necessarily happen in a sequential order. The produced metal oxides and CO2 could react with carbon over 700 °C; thus, the carbon lattices expanded irreversibly and led to high microporosity. As for Zn@C, PBC with decent porosity offered more channels for the storage of ZnCl2 in the impregnation step [77]. ZnO derived from the decomposition of ZnCl2 was formed into the pores of Zn@C (eqs. (8)–(9)), which might be further reduced into metallic Zn by carbon, H2, and CO (Eq. (10)). (8) ZnCl 2 + H 2 O → H Z n C l 2 O H → Z n O H 2 + H C l T ≤ 105 ° C (90 Zn O H 2 → Z n O + H 2 O T ≥ 105 ° C (10) ZnO + C , H 2 , C O → M e t a l l i c Z n T ≥ 700 ° C With regard to Fe@C, the changes of the chemical states for Fe@C are illustrated in the following equations. On the basis of eqs. (11)–(14), amorphous Fe species such as Fe(OH)3, FeOOH were initially converted into Fe2O3 at 400 °C [22], and then reduced into Fe3O4 and/or FeO by a carbon matrix, and reduced gases (H2 and CO) generated from biomass pyrolysis at 500 – 700 °C [78,79]. The iron oxides might be further reduced into metal atom by carbon, H2, and CO (Eq. (14)). The carbon consumed for reduction would contribute to the generation of porous carbonaceous structure. (11) Fe Cl 3 + H 2 O → F e O H 3 + H C l T ≤ 105 ° C (12) Fe O H 3 → F e O O H → Fe 2 O 3 T ≥ 400 ° C (13) Fe 2 O 3 + ( C , H 2 , C O ) → Fe 3 O 4 / F e O T ≥ 500 ° C (14) Fe 2 O 3 / Fe 3 O 4 / F e O + ( C , H 2 , C O ) → M e t a l l i c F e T ≥ 700 ° C Likewise, amorphous Ni species such as Ni(OH)2 was first formed at the low temperature (Eq. (15)), which were thereafter transformed into NiO and/or Ni2O3 (Eq. (16)) [22]. The nickel oxides were further reduced into metallic Ni (Eq. (17)) by carbon, H2 and CO at the elevated temperature (700 °C), which could eventually act as the catalyst for the conversion of amorphous carbon into graphitic carbon [22,80]. (15) Ni Cl 2 + H 2 O → N i O H 2 + H C l T ≤ 105 ° C (16) Ni ( O H ) 2 → N i O / Ni 2 O 3 T ≤ 650 ° C (17) NiO / N i 2 O 3 + ( C , H 2 , C O ) → M e t a l l i c N i T ≥ 700 ° C These observations revealed that the formation of micropores and charring reactions took place during the synthesis of 3DMeMCs. The Lewis acid sites were produced in the one-step thermal treatment by introducing metal chlorides with the synergistic effect of metal cations (Zn2+, Fe3+, and Ni2+) and Cl-, which could enhance the charring reaction and develop intricate microporous structure [81]. Additionally, the generation of metal oxides was conductive to the formation of microporous structures. Overall, PBC could be fabricated and applied as a promising carbonaceous material for the synthesis of 3DMeMCs, and metal chlorides could serve as the activating agents and catalysts for micropore development.According to these observations, the plausible reaction mechanisms regarding ex-situ catalytic fast co-pyrolysis of biomass and plastics over 3DMeMCs are outlined in Fig. 10 . Broadly speaking, large amounts of biomass-derived oxygenates or plastics-derived large molecular hydrocarbons from individual catalytic fast pyrolysis of biomass or plastics were transformed into light hydrocarbons, which could form the hydrocarbon pool inside the micropores and/or on the surface of 3DMeMCs and were ultimately converted into aromatic hydrocarbons through oligomerization and aromatization over the acid sites [65,75,82]. Although the formation of aromatic hydrocarbon could take place in several routes, it was mainly ascribed to the interactions of biomass-derived furans and plastics-derived light olefins [5,6]. The Diels-Alder reaction between furans and light olefins was the most dominant reaction followed by dehydration to produce MAHs.More specifically, metal chlorides (e.g., ZnCl2) used to modulate biochar after the one-step thermal process could generate Lewis acid sites [83,84]. Importantly, the Lewis acid sites associated with metal species (e.g., Zn species) contributed to facilitating the Diels-Alder reaction [85], which was the dominant effect on the formation of aromatic hydrocarbons. These Lewis acid sites could also improve the dehydrogenation and dehydrocyclization of alkane and olefins meanwhile promoting the rate of aromatization reaction [83,85]. Meanwhile, it was clearly found that the aromatic hydrocarbons obtained at 500 °C were present in the form of monocyclic alkyl-aromatic hydrocarbons. That could be explained that the alkylation of aromatic hydrocarbons was accelerated by the Lewis acid sites, producing alkyl-aromatic hydrocarbons at the expense of olefins and aromatic hydrocarbons [86].Among the 3DMeMCs, Ni@C and Fe@C showed the highest share of metal species, as evidenced by EDX, XPS, and elemental analysis. The higher share of Ni and Fe species would form more Lewis acids sites than those of Zn species, which therefore promoted aromatization reactions to produce the higher content of aromatic hydrocarbons when using Ni@C and Fe@C as the catalyst during the ex-situ CFcP process at both 500 and 800 °C. In this study, Ni@C at 500 °C exhibited the best catalytic performance for deoxygenation to produce higher HHV bio-oils. Furthermore, Ni@C at 800 °C was also in favor of gaseous products (e.g., H2 and/or syngas) along with carbon nanotubes at the cost of intermediate hydrocarbons through dehydrogenation and cracking reactions.Additionally, it has been reported that the mineral components in biochar matrix played an vital role in acid sites during the catalysis [87]. It was reported that Al, S, and P species (like AlPO4) were found to show high surface acidity with a mixture of both Lewis and BrØnsted acid sites [87,88]. In this study, some strong acid sites associated with Al, P and Si species (inherent elements in carbon matrix) should be formed on the surface of 3DMeMCs, which could enhance the hydrogen transfer reaction to achieve more aromatic hydrocarbons and alkanes in the consumption of olefins [86]. As a result, the incorporation of metal chlorides coupled with the one-step thermal process to modulate biochar not only introduced many external Lewis active sites inside the micropores and on the surface of 3DMeMCs, but also fabricated physicochemical properties of 3DMeMCs. The presence of these catalysts was conducive to altering the product distribution and enhancing the production of aromatic hydrocarbons, together with dehydrogenation of intermediate hydrocarbons (CnHm) to generate H2 and valuable carbons especially at the high catalytic temperature (800 °C) [37]. Therefore, 3DMeMCs with microporous structures possessed enough active acid sites, enhancing the catalytic conversion of biomass and plastic waste into valuable aromatic hydrocarbons, syngas, and carbons.To summarize, the 3D metal-embedded microporous carbocatalysts were successfully synthesized from pellet biochar incorporated with metal chlorides in a sing-step energy-efficient thermal process, fulfilling the synchronous pore-forming, metal-doping, and graphitization. The physicochemical characteristics of the as-synthesized catalysts were comprehensively investigated by a sequence of techniques. These as-synthesized catalysts were further tested in the online and ex-situ catalytic fast co-pyrolysis of wheat straw and plastic waste to evaluate their catalytic abilities. As expected, an apparent synergistic effect happened for the production of aromatic hydrocarbons during ex-situ fast co-pyrolysis of biomass and plastic waste; and these catalysts (especially Fe@C) at 500 °C favored the formation of MAHs with the high relative content up to 60%. Besides, Ni@C at 800 °C exhibited the better catalytic performances than the other catalysts for the production of H2 (157 NmL/gfeedstock), syngas (273 NmL/gfeedstock), and carbon nanotubes. In addition, the thermal degradation behaviors and kinetic analysis regarding co-pyrolysis of wheat straw and plastic waste over the as-synthesized catalysts were determined in detail. A plausible reaction mechanism was elucidated for ex-situ catalytic fast co-pyrolysis over these catalysts. Overall, this study developed a simple, rapid, and facile method to synthesize the green, promising, and highly efficient 3D metal-embedded microporous carbocatalysts for catalytic fast co-pyrolysis/gasification of biomass and plastic waste for the production of aromatic hydrocarbons, syngas, and valuable carbons. Xuesong Zhang: Writing – review & editing, Conceptualization, Methodology, Project administration, Funding acquisition. Ruolan Xu: Investigation, Conceptualization, Methodology, Software, Formal analysis. Quan Liu: Data curation. Ge Kong: Formal analysis. Hanwu Lei: Writing – review & editing. Roger Ruan: Writing – review & editing. Lujia Han: Writing – review & editing, Supervision, Project administration, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by Start-up Funding for High-end Talents of China Agricultural University, Chinese Universities Scientific Fund (10092001), and China Agriculture Research System of MOF and MARA.Supplementary data to this article can be found online at https://doi.org/10.1016/j.ecmx.2021.100176.The following are the Supplementary data to this article: Supplementary data 1

This study explored an energy-efficient and cost-effective method to synthesize three-dimensional metal-embedded microporous carbocatalysts. Pellet biochar manufactured with compressed and porous structure was used as the carbonaceous precursor, which was modulated by diverse metal chlorides in the single-step thermal process, fulfilling the synchronous pore-forming, metal-doping, and graphitization. The as-synthesized carbocatalysts were characterized in detail by using N2 physisorption, SEM, TEM, EDX, XRD, TPO, TGA, FTIR, XPS, Raman, CHNS elemental analysis, etc. It was found that the metal-embedded carbocatalysts possessed well-developed 3D microporous structures with the highest specific surface area of 964 m2/g. The catalytic activities of these catalysts were investigated during on-line and ex-situ catalytic fast co-pyrolysis of wheat straw and plastic waste. It was observed that the carbon yield of bio-oils could reach over 60 C% by using Zn@C as the catalyst at 500 °C, and the HHV of bio-oils peaked at 38.52 MJ/Kg in the presence of Ni@C at 500 °C. Moreover, these carbcatalysts at 500 °C favored production of hydrocarbons with a relative content up to 98%; in particular, monocyclic aromatics presented the highest selectivity (nearly 60%). Among metal-embedded carbcatalysts, Ni@C at 800 °C was in favor of H2 (157 NmL/gfeedstock) and syngas (273 NmL/gfeedstock) production; importantly, Ni@C also promoted the generation of carbon nanotubes. Additionally, the thermal degradation behaviors and kinetics of non-catalytic and catalytic co-pyrolysis of biomass and plastic waste over the as-synthesized catalysts were also tested by thermogravimetric analysis. Finally, a rational reaction mechanism regarding ex-situ catalyst fast co-pyrolysis of biomass and plastic waste over catalytically active sites on the as-synthesized catalysts was elucidated. Accordingly, this work provides a great potential of using the promising carbocatalysts to co-valorize biomass and plastic waste into the integrated harvests of monocyclic aromatics, syngas, and valuable carbons.

The published article includes all datasets generated or analyzed during this study. All data reported in this paper will be shared by the lead contact upon request. This paper does not report original code. The published article includes all datasets generated or analyzed during this study.All data reported in this paper will be shared by the lead contact upon request.This paper does not report original code.Electrocatalytic overall water splitting, including hydrogen-evolving reaction (HER) and oxygen-evolving reaction (OER), is a promising strategy that converts electrical energy into chemical energy. Pt- and Ir-based noble metals are generally known as the state-of-the-art catalysts for HER and OER, respectively. However, the low abundance and high prices of the catalytic materials limit the large-scale commercial application of water-splitting technologies. Despite many efficient noble-metal-free catalysts for OER and HER, such as metal nitrides, 1 metal chalcogenides, 2 , 3 metal phosphides, 4 and metal carbides, 5 there is still a big gap in performance between these catalysts and noble metal-based materials. It is highly desirable to explore efficient and cost-effective bifunctional electrocatalysts for water splitting.Alloying Pt or Ir with 3d transition metals (e.g. Ni, Co, or Cr) has been regarded as a promising strategy to concurrently reduce the noble metal loading and promote catalytic activity. 6 , 7 , 8 However, these traditional alloys tend to degrade and dissolve into electrolytes during electrochemical cycling due to the segregation of the transition metal atoms toward the catalyst surface. 9 A new class of high-entropy alloys (HEAs), defined as materials containing five or more near-equimolar principle metal components, 10 has been substantiated as efficient and stable electrocatalysts recently. 11 , 12 In most cases, the performance of HEAs is simply attributed to the synergetic effect between multiple elements. 13 The mechanism of such multi-component synergy in HEAs remains elusive. In-depth understanding of the structure-performance relationship should be elucidated.Herein, we synthesize PtIrCuNiCr HEA electrocatalysts by a simple laser scanning ablation (LSA) strategy developed by our group. 12 Ni are electron-rich atoms with both paired and unpaired d electrons, while Cu and Cr are atoms with all the d orbitals of full and half-empty, respectively. On the basis of the Brewer-Engel valence bond theory, 14 the electrode surface with both pairs of d electrons and half-empty d orbitals will help the electron transfer as well as the adsorption and desorption of intermediates during the electrocatalytic reaction process. Consequently, the noble-metal mass activity of PrIrCuNiCr for HER and OER is 13.0 and 9.3-fold higher than those of Pt/C and Ir/C, respectively. As bifunctional electrocatalysts for overall water splitting, PtIrCuNiCr achieves an overpotential of ca. 190 mV at 10 mA cm−2, far surpassing the reported catalysts. Aberration-corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) and X-ray diffractometer (XRD) manifest that the incorporation of Cu, Ni, and Cr atoms in HEAs induces remarkable shrunk surface strain. Theoretical calculations reveal that the effective strain can push the HEA toward the top of volcano plots in electrocatalytic HER and OER due to the improved adsorption kinetics of reaction intermediates (i.e., ∗H, ∗OH, and ∗O). The lattice strain effect of HEA can be reasonably engineered by tailoring the particle radius and configurational entropy, thereby achieving optimal interaction with the intermediates. This work enables us to better understand the relationship between radius, entropy, strain, and electrocatalytic activities of HEAs. It also provides a systematic strain regulation strategy for designing high-performance HEA catalysts for overall water splitting.The HEA nanoparticles (NPs) were fabricated on graphene substrates by the LSA strategy (Figure 1 A). 12 Typically, various salt precursors with the same atom ratios were firstly loaded onto graphene and then transferred in hexane and exposed under laser with a pulse duration of 5 ns (the laser parameters are described in Table S1 and Figure S1). Due to the coupling of laser photons and salt electrons, the electron temperature immediately rises. 15 The surface microns of the salts layer are transformed into a melting pool when the fluence of laser light exceeds that of the melting threshold (Figure 1B). As the laser pulse duration is much longer than the electron cooling rate (∼ in the order of 1 ps), the excited electrons can keep transmitting their energy to the salt lattices, leading to the sublimation, ionization, erosion, and/or explosion of the molten salts. 16 As a result, highly compressed plasma containing a mass of electrons, ions, atoms, vapors, and clusters forms. The rapid heating of the salt layer and the medium in the vicinity of the plasma plume lead to the generation of cavitation bubbles which play essential roles in the subsequent NPs formation due to their confinement effect. 17 During plasma or bubble expansion and collapse, significant shock waves are created behind the ablation surface and expanded inside the salt layers with mechanical solid energy, resulting in the ejection of the salt layer into NPs. Although a laser pulse lasts only 5 ns, the ablation steps take place over several orders of magnitude in time, from electronic absorption of laser beam energy (10−9 s), through nanomaterial nucleation and growth (10−6–10−4 s), to particle condensation (∼ms). Figure 2 A shows a transmission electron microscope image of the PtIrCuNiCr NPs. These HEA NPs are uniformly distributed on the graphene surface with an average radius of 2 nm (Figure 2B). The inductively coupled plasma mass spectroscopy demonstrates that the actual HEA NPs loading on graphene is approximately 5 wt % (Figure S2). Energy-dispersive X-ray spectroscopy maps display Pt, Ir, Cu, Ni, and Cr elements throughout these NPs (Figure 2C). Homogeneous distribution of individual elements with no obvious segregation has been found even at the atomic scale (Figure 2D), manifesting the high-entropy character of the PtIrCuNiCr sample. Although the five compositions are thermodynamically insoluble, 18 the clear atomic lattices and positions of every atom signify that the atomic arrangement order is stabilized by the high-entropy microstructures. The corresponding line profiles (Figure 2E) show that the atomic ratio of Pt, Ir, Cu, Ni, and Cr in each projected atomic column randomly fluctuates with significantly small variation. Based on the atomic ratios, the equation calculates the configurational entropy (ΔSmix) of the PtIrCuNiCr HEA NPs to be 13.3 J/mol·K by the equation of ΔS mix  = -R ∑ i = 1 5 C i ln C i , where R is the molar gas constant and C i is the atomic ratio of the i element. The ΔS mix value is high enough to bestow the PtIrCuNiCr high-entropy feature. The valence states of the HEA components are monitored by X-ray photoelectron spectroscopy (XPS, Figure S3 and Table S2). The metallic bonding states of all five elements have been determined, implying the metallic character of HEA NPs. Notably, characteristic XPS peaks of Cu, Ni, and Cr show metallic and oxidized bonding states. This is because the NPs are exposed to air before characterization, and these 3d transition metals inevitably undergo surface oxidation. The XPS peaks of Cl element were not detected, indicating Cl was excluded in the form of chlorine during the LSA process. The XRD pattern (Figure 2F) confirms the face-centered cubic phase of the HEA NPs, distinguishing the high-entropy solid-solution structure from amorphous materials.Despite the well-maintained atomic arrangement stabilized by the high-entropy characteristic, severe lattice strain of HEAs inevitably forms due to atoms with different radii across the lattice sites. X-ray absorption fine structure analysis manifests that the metal-metal bonds in HEAs are either longer or shorter than the Pt-Pt or Ir-Ir bonds in pure foils (Figure S4 and Table S3), demonstrating the presence of severe lattice distortion. The lattice distortion of HEA can induce lattice strain. Compared with XRD characteristic peaks of Pt (Figure S5), the broadening and shifting peaks of the HEA sample clearly demonstrate the increased strain from its distorted lattices. 19 We simply calculated the strains of (111), (220), and (200) peaks using the Wilson method (Table S4). 20 The strain in HEA NPs is four to five times that in pure Pt. The severe lattice strain can induce HEAs with a thermodynamical nonequilibrium state. 13 Our previous work found that such a state contributes to the higher potential energy of catalysts, thereby contributing to lowering the energy barrier in catalytic reactions and improving performance. 21 In addition, the stacking fault (SF) number in HEAs is much higher than that in Pt. Aberration-corrected HAADF-STEM images demonstrate the presence of the SFs on the HEA surface (Figure 3 A). These SFs can lower the coordination number of surficial atoms, 22 enhancing the HEA adsorption capability. As is shown in Table S3, the coordination numbers of Pt and Ir atoms in HEAs are significantly lower than that in the pure metal foil. The unsaturated Pt and Ir sites serve as the active centers during the electrocatalysis to promote the interactions with intermediates (e.g. H∗, O∗, and OH∗) and improve OER and HER. 23 To confirm the strain types in the PtIrCuNiCr NPs, the XRD patterns of Pt and HEAs are refined and compared. In comparison to the (111), (200), and (220) planes in Pt (Figure 3B), those peaks in HEAs become broadening, weak, and shift toward a higher Bragg angle. The interplanar spacing change and the Bragg peak shift mainly result from Type I and II strains which act over a large and short distance, respectively (Figure S6b). 19 As is shown in Figure 3A, the interplanar spacings of (111) and (200) planes in HEAs are 0.215 and 0.187 nm, respectively, both of which are smaller than those in pure Pt (0.227 and 0.196 nm, respectively). The variation trends of the spacings are consistent with the XRD results (Table S4), although the values are slightly discrepant due to different detection errors of HAADF-STEM and XRD.In Figure 3A, many crystal defects such as atom dislocations and SFs have been found, contributing to the formation of Type III strain (Figure S6). Type IV strain can cause lattice expansion or compression. 19 In our case, the incorporation of 3d transition metal atoms in HEAs leads to lattice compression with small interplanar spacing (Figure 3A). To further obtain the strain statistical and spatial distributions, we further investigated the strain at the atomic scale through HAADF-STEM imaging and performed geometric phase analysis to calculate the atomic strains. In the HEA NPs shown in Figure 3C, the strain εxx is perpendicular to the (200) plane, εyy is in the (200), and εxy is the sheer strain. We take the average strain of the entire plane as a baseline (0%) for measuring the strain distribution across the plane. Broad distributions of normal strain and shear strain have been found across the HEA sample. These results are consistent with the XRD analysis (Figure 3B).The strain fields modify the electronic properties of PtIrCuNiCr by distorting the local bonding character (Figure S4), which can regulate the adsorption properties of the HEAs during electrocatalytic water splitting. The electrocatalytic HER and OER activity of HEAs in 1 M KOH was evaluated in a typical three-electrode setup. Commercial Pt/C and Ir/C, which are considered as typical HER and OER catalysts, respectively, are examined for comparison. For unit mass of Pt, the ECSA for PtIrCuNiCr/C (2.5 wt% Pt) is 178.8 m2/gPt, subsequently higher than that of Pt/C (with 20 wt % Pt, 65.75 m2/gPt) (Supplementary Inforamtion Figure S7). Figure 4 A shows the linear sweep voltammetry curves for HER. The PtIrCuNiCr catalyst exhibits remarkable catalytic activity with an onset overpotential of ∼0 mV and low overpotential of only 200 mV to drive 100 mA cm−2 (vs 274 mV for Pt/C). For OER (Figure 4B), the HEA sample requires an overpotential of only 176 mV to achieve 10 mA cm−2 which shows much higher activity than that of Ir/C (238 mV) as well as the recently reported noble metal-based catalysts (Table S5). After being normalized with the noble metal mass (Figure S8), the mass activity of PtIrCuNiCr reaches 1.57 A/mg at −0.3 V versus RHE for HER and 0.81 A/mg at 1.5 V versus RHE for OER, which are 13.0- and 9.3-fold higher than those of Pt/C (0.12 A/mg for HER) and Ir/C (0.087 A/mg for OER), respectively. In terms of such remarkable activities toward HER and OER, the PtIrCuNiCr HEA NPs were further used as both anode and cathode for overall water splitting (Figure 4C). Strikingly, it delivers a current density of 10 mA cm−2 at an overpotential of 190 mV, outperforming the Ir- or Pt-based electrocatalysts reported so far (Table S6). The HEA electrode presents excellent durability (Figure 4D), manifesting the superior stability of the HEA catalyst for overall water splitting.The appropriate adsorption energy of intermediates on the catalyst surface is crucial in improving catalytic performance. Volcano plots are often used to describe the relationship between reaction overpotentials and adsorption energy. 24 The calculated difference in binding energies of ∗O and ∗OH (ΔGO∗-ΔGOH∗) and the covalent metal-hydrogen bond absorption energy (EM-H) are verified to describe well the trend of OER and HER activities on catalyst surfaces, respectively. 25 , 26 As shown in Figures S9 and S10, ΔGO∗-ΔGOH∗ and EM-H are closely related to the radius (r) of catalysts. 27 With the catalyst radius of 2 nm, we correlated the EM-H with the exchange current density (j0) derived from the Tafel plots to obtain a volcano plot of the HER case (Figure 5 A). The ΔGO∗-OH∗ was associated with the overpotential for the current density of 1 mA cm−2 to achieve a volcano plot of OER (Figure 5B). Both the pure metals of Pt and Ir are located in the left legs of the OER volcano plots or the right legs of the HER volcano plots, manifesting strong adsorption of intermediates during the reactions. In contrast, the PtIrCuNiCr approaches to the top of the volcano, indicating that the HEA catalyst has moderate adsorption energy of intermediates than Pt and Ir. The theoretical calculation reveals that the catalyst strain plays significant role in the optimization of the adsorption energy (See details in supplemental information theoretical calculations). Unlike pure metals whose strain only depends on the radius (Figure S11), HEAs can regulate strain through entropy and radius (Figures S12 and S13). Catalyst radius can induce strain by modifying the chemical bonds of atoms at corners and edges. 27 High-entropy character of HEAs induces the inherent distortion of the lattice structure, thus releasing the local strain in materials. 28 Therefore, the catalytic behavior of HEAs differs from that of the pure metals at the same size due to the strain caused by the high-entropy character. In our case, the entropy-driven strain weakens the adsorption energy of catalytic intermediates on HEAs, facilitating the desorption of products on the HEA surface and further improving the electrocatalytic activities. Figures 5C and 5D show the dependence between the effective strain, catalyst radius, and the HER/OER performance mapping of HEAs as well as pure Pt and Ir catalysts. HEAs show larger j0 than Pt and Ir at the radius of 2 nm due to the higher strain caused by the high-entropy effect, indicating a fast reaction rate. Thus, the theoretical findings are consistent with the experimental results. To find out the optimal strain for electrocatalytic activity, the relationship between strain of HEAs and the average distance D ¯ (kcal/mol) to the volcano apex is investigated (Figure S14). It is found that when the catalyst strain is around 1.08% and 1.25%, respectively, the activity of HER and OER reaches the maximum. Thus, regulating the strain of HEAs by radius and entropy is an effective avenue to improve their electrocatalytic activity.In this work, we synthesize highly active and durable HEA electrocatalysts by a simple laser scanning ablation strategy. Applied as both anode and cathode, the HEA catalyst of PtIrCuNiCr exhibits the lowest overpotential (ca. 190 mV) to achieve a current density of 10 mA/cm2 in electrocatalytic overall water splitting. Theoretical calculation demonstrates that strain in HEAs can regulate the binding energy of intermediates during electrocatalysis by changing metal-metal bonding energy. Unlike pure metals, HEA can adjust the strain through radius and entropy to enhance electrocatalytic activity. This work offers a new trial of strain engineering to develop efficient and durable HEA electrocatalysts for overall water splitting. It will inspire a new understanding of catalytic mechanisms of HEAs and guide the search for efficient HEA catalysts in a vast materials database.Due to the short synthesis time, it is challenging to synthesize HEA catalysts with very uniform particle size by the laser scanning ablation method. Therefore, the effect of the particle size and atomic ratio of the elements on the catalysis activity was studied by the theoretical calculation instead of experiment in this work. REAGENT or RESOURCE SOURCE IDENTIFIER Chemicals, Peptides, and Recombinant Proteins H2PtCl6·6H2O Aladdin Co., Ltd 18497-13-7 H2IrCl6·6H2O Aladdin Co., Ltd 16941-92-7 CuCl2·2H2O Aladdin Co., Ltd 10125-13-010125-13-010125-13-0 NiCl2·6H2O Aladdin Co., Ltd 7791-20-0 CrCl3·6H2O Aladdin Co., Ltd 10060-12-5 Glucose Xilong Scientific Co., Ltd 14431-43-7 NH4Cl Macklin 12125-02-9 KCl Macklin 7447-40-7 NaCl Macklin 7647-14-5 Hexane Xilong Scientific Co., Ltd 110-54-3 Nafion solution DuPont 31175-20-9 KOH Macklin 1310-58-3 Commercial 20%Pt/C Johnson Matthey Cat#S128513 Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Bing Wang (bingwang@nju.edu.cn).This study did not generate new unique reagents. All chemicals were obtained from commercial resources and used as received.A typical process is as follows: 29 4 mmol of glucose, 6 mmol of NH4Cl, and 80 g of KCl and NaCl with the weight ratio of 51:49 were first treated by ball-milling, and dried at 150°C for 8 h to form brown mixture. Then the mixture was pyrolyzed at 1050 °C under N2 for 1 h with the heating rate of 35°C/min, followed by natural cooling to room temperature. Graphene forms after being ultrasonically rinsed with distilled water and ethanol several times.A pulsed fiber laser (SLT-PTM-100, Jiangsu Yanchang Sunlaite new energy Co. Ltd, China) was used during the laser scanning ablation (LSA) process. The parameters of the laser are displayed in Table S1. A Gaussian laser beam with a peak power of 20 kW was focused (Figure S1) and supplied at normal incidence to the graphene surface.Various chloride salts were mixed in ethanol with 0.01 M for each metallic element. Taking the synthesis of PtIrCuNiCr as an example, the salt precursors of H2PtCl6·6H2O, H2IrCl6·6H2O, CuCl2·2H2O, NiCl2·6H2O, CrCl3·6H2O were first mixed in ethanol. The mixed solution was then directly dropped onto graphene with a loading of ∼0.1 ml/mg, followed by ultrasonic treatment for ensuring the uniform salt load on graphene. The loaded substrates were transferred to a vacuum oven for drying at room temperature.For the synthesis of HEA nanoparticles (NPs) by the LSA method, the precursors-loaded graphene was firstly dispersed in hexane with 0.5 mg/ml by magnetic stirring. Hexane is used because the salt precursors are insoluble in it, ensuring the salts remain on substrates. Moreover, the oxygen-free structure of hexane is conducive in preventing the oxidation of the HEA NPs during LSA process. The solution was irradiated under agitation with the pulse laser for 30 min, ensuring all the substrates were irradiated by the laser beam.H2 evolution reaction (HER) and O2 evolution reaction (OER) electrocatalytic experiments were performed on a CHI 660D electrochemical workstation with a three-electrode cell system. In this system, Ag/AgCl (sat. KCl) and carbon rod electrodes were used as the reference electrode and counter electrode, respectively. To prepare the working electrode, 10 mg of the HEA NPs on graphene was dispersed in the mixture of 400 μL ethanol and 50 μL 5% Nafion solution for 20 min by ultrasonication to form homogeneous inks. 50 μL of the ink was carefully dropped onto a nickel foam (NF, 0.5×0.5 cm2), resulting in a HEA NPs/graphene loading of 4.4 mg cm−2. The electrocatalytic electrode was dried at room temperature naturally. Based on the results of inductively coupled plasma mass spectroscopy (ICP-MS), the HEA NPs loading on graphene is approximately 5 wt.% with 2.5 wt.% of Pt and 1.5 wt.% of Ir. As such, the actual HEAs NPs loading on NFs was about 0.22 mg cm−2 with Pt and Ir loading of 0.11 and 0.07 mg cm−2, respectively. Similar to the HEA catalyst, 10 mg of the commercial Pt/C (20 wt%XC-72) or Ir/C (5 wt%XC-72) was dispersed in the mixture of 400 μL ethanol and 50 μL 5% Nafion solution by ultrasonic treatment for 20 min. Then 50 μL of the ink was dropped onto a NF with the same area and dried at room temperature. The loading amount of the commercial Pt/C and Ir/C electrocatalyst on NFs was both 4.4 mg cm−2. As the mass ratio of Pt and Ir in samples is 20 wt.% and 5 wt.%, respectively. The actual Pt and Ir NPs loading on NFs was 0.88 mg cm-2 and 0.22 mg cm-2, respectively. The HER and OER electrochemical experiments were conducted in a 1.0 M KOH aqueous solution at room temperature. All potentials for OER and HER reported herein were referenced to the reversible hydrogen electrode (RHE) using the equation ERHE = EAg/AgCl + 0.197 + 0.059 × PH. The measurement was conducted under rotation to remove the produced bubbles with 90% IR correction.The overall water splitting was investigated in a two-electrode system with 1.0 M KOH electrolyte, in which PtIrCuNiCr-NF served as both anode and cathode with a loading of 4.4 mg cm−2. The durability was assessed at a constant potential of 1.5 V for 100 h.The morphology of as-prepared samples was examined by transmission electron microscopy (TEM, Tacnai G2 F20, FEI). Energy-dispersive X-ray spectroscopy (EDS, Elite T EDS System) equipped on the TEM was employed to record the element distribution of HEAs on graphene. Aberration-corrected high-angle-annular-dark-filed scanning transmission electron microscope (HAADF-STEM) analysis is characterized using Thermofisher Themis Z (FEI) with 200 kV. An EDS instrument with the SuperX detector equipping the HAADF-STEM was used to obtain the element distribution of HEAs at the atomic scale. Geometric phase analysis (GPA) was conducted with Digital Micrograph software to obtain the strain information on the surface of HEA NPs. The crystal structures of the samples were measured by a powder X-ray diffractometer (XRD, Ultima III, Rigaku Corp., Japan) using Cu-Kα radiation (λ = 1.54178 Å, 40 kV, 40 mA). The atomic ratios of HEM NPs were analyzed by PerkinElmer AVIO500 ICP-MS. The solutions were prepared by digesting the samples in aqua regia followed by dilution with 2% hydrochloric acid. The surface composition of the samples was performed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250) with the non-monochromatic Al Kα X-ray as the X-ray source. The binding energy of C1s (284.6 eV) was used to calibrate the other binding energies.The absorption energy of intermediates on the catalyst surface has been widely accepted as a descriptor for the activity of electrocatalysts. The absorption energy varies with the size of catalyst particles due to the changed coordination environment caused by the chemical bonds of the surface atoms. 30 , 31 Since metal atoms are coordinated to several neighbor atoms on the surface, we first consider the metallic bond strength. For the metallic bond strength of NPs composed of n layers of atoms, we made a hypothesis that the bond strength and radius of NPs conform to a Gaussian function: 32 (Equation 1) 1 E M − M r = 1 E M − M ∞ + ( 1 E M − M D i a t o m i c − 1 E M − M ∞ ) e − ( n E M − M r ) 2 3 σ 2 where E M − M r is the metallic bond strength for an NP with radius of r (nm), E M − M ∞ is the bond strength for bulk materials, E M − M D i a t o m i c is the covalent bond strength for a diatomic molecule, σ is a controlling constant for dimension consistency (ca. 1000 kJ mol-1). As the size of nanoparticles (NPs) decreases, metallic bonds become weaker (Figure S8). 33 Driven by the stability of the system, interaction between hydrogen/oxygen adsorbates and metal goes stronger (Figure S9). According to the covalent metal-hydrogen/oxygen bond dissociation energy and volcano plot of bulk materials, for NPs composed by n layers of atoms, we can assume: (Equation 2) 1 E M − H r = 1 E M − H O ∞ + ( 1 E M − H D i a t o m i c − 1 E M − H ∞ ) e ( n E M − H r ) 2 3 σ 2 (Equation 3) 1 Δ G O ∗ − O H ∗ r = 1 Δ G O ∗ − O H ∗ ∞ + ( 1 Δ G O ∗ − O H ∗ D i a t o m i c − 1 Δ G O ∗ − O H ∗ ∞ ) e ( n Δ G O ∗ − O H ∗ r ) 2 3 σ 2 where E M − H r is the binding energy between an NP with a radius of r (nm) and hydrogen adsorbates, E M − H ∞ is the hydrogen absorption energy for bulk materials, E M − H D i a t o m i c is the covalent metal-hydrogen bond dissociation energy, Δ G O ∗ − O H ∗ is the free energy change of the intermediate step in OER. σ is 350 kcal mol -1 and 10 eV for metal-hydrogen and metal-oxygen, respectively. We also benchmarked some data within ab initio Density Functional Theory (DFT) methods. The M-H bond dissociation energy, M-H absorption energy, and free energy change of proton desorption of Pt, Ir, Cu, Ni, Cr at atomic level and for bulk materials are displayed in Table S7. ΔGO∗-OH∗ at atomic level was carried within the GGA-PBE functional in DMol3. 34 Each atomic structure is fully relaxed until forces acting on atoms are less than 0.05 eV/Å.The DFT calculation is carried out within the GGA-PBE functional in Quantum ESPRESSO code. 35 The calculations are carried out using separable norm-conserving pseudopotentials and a plane-wave basis set with the kinetic energy cutoff of 40 Ry and Gamma k-points. Each atomic structure is fully relaxed until forces acting on atoms are less than 0.01 eV/Å. (Equation 4) ε ¯ = ∑ i ε i / N i where N is the concentration of each principle element in the HEA.Entropy is essential for the study of HEAs. The entropy of configuration is given by: (Equation 5) S c o n f = − R ∑ i = 1 n X i ln X i where X i = N i / N A , where N i is the number of i atoms, N A is Avogadro’s constant, R is a universal gas constant. The S conf dependent strain is unique in HEAs materials. For NPs with the same radius r, the overall metal distribution in HEAs is analogous to the coloring problem. 36 As shown in Figure S11, different entropy of HEAs exhibit distinct strain distribution.Thus, unlike pure metals, HEAs can adjust strain through entropy and particle radius, to optimize the electrocatalytic performance. The relationship between strain, entropy, and radius of HEA catalysts is revealed in Figure S12.The electrocatalytic activity of the catalyst is described by the average distance D ¯ (kcal/mol) to the apex in the volcano plot of HER and OER. D ¯ can be calculated by: (Equation 6) D ¯ = ∑ i ( E i r − E a p e x ) / N i where E i r is the binding energy of principle elements, E a p e x is the apex binding energy in the volcano plot. When the absolute value of D ¯ is 0, the electrocatalytic activity achieves optimum. According to the Equations 2, 3, 4, and 6, the effect of strains in HEAs on the electrocatalytic activity is disclosed in Figure S13.This project is supported by the National Key Research and Development Program of China (2021YFF0500501), Major Research Plan of the National Natural Science Foundation of China (91963206), National Natural Science Foundation of China (22279053, 52072169, 51627810, 51972164), Program for Guangdong Introducing Innovative and Enterpreneurial Teams (2019ZT08L101), Fundamental Research Funds for the Central Universities (14380180), Civil Aerospace Technology Research Project (B0108), and Foshan Xianhu Laboratory of the Advanced Energy Science and Technology Guangdong Laboratory.B.W., X.Z., Y.Y., and Z.Z. conceived the idea and designed the present work. B.W., W.L., L.H., and C.W. carried out the experiments. X.Z. and Y.L. carried out the theoretical calculations. C.W. and X.Y. performed detailed microscopic characterizations. B.W. and X.Z. drafted the paper.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.isci.2023.106326. Document S1. Figures S1–S14 and Tables S1–S7

Developing active and cost-effective bifunctional electrocatalysts for overall water splitting is challenging but mandatory for renewable energy technologies. We report a high-entropy alloy (HEA) of PtIrCuNiCr as a bifunctional electrocatalyst for overall water splitting, which shows a low overpotential of ca. 190 mV at the current density of 10 mA cm−2. Compared with pure metals, HEAs exhibit remarkable surface strain due to severe lattice distortion in their crystal structures. Theoretical calculations reveal that the strain can regulate the binding energy of intermediates on catalysts by adjusting the metal-metal bonding energy. It pushes the HEA toward the top of volcano plots to achieve superior electrocatalytic activity for both hydrogen and oxygen evolution reactions. The strain effect of HEAs on electrocatalysis can be well engineered by tuning the catalyst radius or configurational entropy. This work renders a systematic strain regulation strategy for designing a high-performance HEA catalyst for overall water splitting.

No data was used for the research described in the article.Ammonia (NH3) plays a crucial role in agriculture, pharmaceuticals, textile industry, and plastic production [1]. In addition, It is also considered an important energy storage medium and a promising carbon-free energy carrier because of its high content of hydrogen (17.6 wt%) [1]. The main industrial process for the synthesis of NH3 is carried out through the reaction of nitrogen (N2) and hydrogen (H2) at elevated temperatures (400–500 °C) and pressures (150–300 atm) using the so-called Haber-Bosch process [2]. The latter method relies on the H2 produced from fossil fuels and it requires intensive energy use leading to expensive operational costs [3]. In the past decade, the electrochemical synthesis of NH3 with renewable energy (wind or solar) input has attracted enormous interest as an alternative route [4]. Especially, the use of N2 molecule in the electrochemical reduction process became a hot-topic [4]. While interesting progress in the development of N2 reduction catalyst has been made, the low reaction rate of NH3 evolution limits its widespread application [5,6]. The reduction of N2 to NH3 also suffers from low activity and selectivity due to the high stability of the triple non-polar bond (NN) [5]. Alternatively, the moderate dissociation energy of the N = O bond (204 kJmol−1) in nitrate ion (NO3 –) makes it an attractive choice over N2 since it promises better kinetics for NH3 production [7,8]. Nitrates are common in nature in the form of metal nitrate deposits (e.g sodium nitrate (NaNO3)) and they are considered one of the major surface and groundwater pollutants that are strictly regulated by environmental agencies due to their harmful effects [9]. NO3 – source mainly comes from fertilizers, nuclear wastes, industrial wastewater, and livestock excrements with a wide range of concentrations up to ca. 2 M [4]. High levels of nitrate consumption may lead to serious immediate health problems (e.g. cancer) [10]. Because of this risk, the USEPA (U.S. Environmental Protection Agency) established that the maximum concentration of NO3 – in water should not exceed 10 mg N/L [11]. Many common technologies were used to treat water contaminated with NO3 – such as ion exchange, biological denitrification, and reverse osmosis [10]. Although, the ion exchange and the reverse osmosis are inapplicable for commercial applications because of the high cost of their additional pretreatment and posttreatment [12]. On the other hand, biological denitrification suffers from the slow potabilization system, and the risk of biological contamination since the process requires phosphorus resources and a certain amount of organic matrix, which can result in the generation of organic pollutants [12]. Using electrochemical methods to remove NO3 – contaminants from wastewater has been an important and attractive topic in the environmental research field [4]. The efficient procedure of electrogenerated NH3 from NO3 – would serve dual purposes: First, for water purification, and second, for waste NO3 – reuse to produce a highly useful chemical product (NH3) [4]. Although the electrocatalytic nitrate reduction reaction (NO3RR) is attractive, worth to mention that sometimes during NO3RR may occur side reactions like the formation of products with low oxidation states such as nitrite (NO2 –), nitrogen dioxide (NO2), nitrous oxide (N2O), nitric oxide (NO), hydrazine (NH2NH2), hydroxylamine (NH2OH), etc [4,13]. For example, NH2NH2 and NH2OH evolve preferably in acidic media, whereas NO2 – and NH3 are considered the main products in the neutral or basic environment [13]. Prior studies indicate that the transformation of NO3 – into NO2 – is the rate-determining step of NO3RR [14]. Recently, the scientific focuses were mainly on obtaining highly active and selective catalysts for NO3RR.The activity of an electrocatalyst depends on various parameters such as medium pH, applied potential, surface area, crystal planes, composition, etc [15]. Mono metallic (Cu, Ru, etc.) and bimetallic (Pd−Cu, Pt−Cu, Pd−Sn, etc.) catalysts are among the most explored materials in electrocatalytic reduction of NO3 − [16–20]. For example, Pt and Pd have been shown to exhibit a high activity toward NO3 − reduction to NH3 [20,21]. Another outstanding monometallic catalyst is the metal Ru. By using Ru clusters with internal strains, Yu et al. achieve 100 % Faradaic efficiency (FE) for NO3 − reduction. They claimed that strains in Ru clusters help the formation of hydrogen radicals, which accelerates the hydrogenation of NO3 − during the reduction process [22]. In addition to monometallic nanoparticles (NPs), a bimetallic catalyst like CuPd has been proven to exhibit excellent performance for NO3RR as well [14]. Although noble metals are efficient as catalysts, their high cost limits their widespread application. In addition to cost, an ideal catalyst for NO3RR should offer good activity and selectivity [7]. Only recently, the development of low-cost catalysts which are selective for NO3 − reduction gained interest. Zhang et al. demonstrated that oxygen vacancies in semiconductor TiO2 electrodes improve the overall FE for NO3 − reduction by weakening the N-O bond [3,23]. Using metallic Cu NPs, Shih et al. studied the impact of facets on the electrocatalytic NO3 − reduction. The authors reported that different crystal facets contribute in different ways to the kinetics and the mode of electrocatalytic NO3RR [24]. On the other hand, Sargent et al. have developed a Cu-Ni alloy catalyst with a unique electronic structure that has yielded a 6-fold increase in NO3 − reduction activity compared to regular Cu electrodes [25]. Transition metal phosphides (TMPs) have emerged as promising low-cost candidates for electrocatalytic reduction reactions [26–30]. One of the most attractive features of TMPs is the charge transfer effect, M(δ +) → P(δ −), which allows reversibly to produce adsorbed hydrogen atoms (H) on the catalyst surface [26,30,31]. Moreover, TMPs have been developed because of their good conductivity due to d‐electron configuration [32]. Recently, amorphous nickel phosphide (Ni2P) deposited on carbon cloth was demonstrated as an efficient catalyst for NO3RR [33]. Yao et al. have developed a procedure to grow Ni2P with (111) facet on Ni foam which yielded 0.056 mmol h−1 mg−1 NH3 with FE equal to 99.2 % [34]. Further, Yang et al. also introduced Ni2P catalyst for electrochemical NH3 synthesis using NO2 − as a source. Their catalyst showed a low onset potential of ∼0.2 V vs. RHE with FE exceeding 90 % [26,35].Herein, we report for the first time the NO3RR to NH3 using different phases of iron phosphide. We synthesized colloidal Fe2P and FeP NPs and deposited them on a titanium (Ti) substrate by using a spin-coating approach. To activate the iron phosphide electrodes, a short heat treatment at 450 ºC was carried out. The Fe2P catalyst showed the highest yield (0.25 mmol h−1 cm-−2) and FE (96 %) at − 0.55 V vs. RHE for NH3. Using NO3 − as a starting species, we revealed the complex reaction pathways occurring during NH3 generation. The recycling test confirmed that FeP catalysts exhibited excellent stability during the NO3RR. To get relevant information about the fundamental origins behind the better performance of Fe2P compared to FeP and to determine the free energies of intermediates, density functional theory (DFT) calculations were performed.The preparation of Fe2P and FeP was performed using a modified solvothermal synthesis procedure reported by chouki et al. [36]. Details of the synthesis procedure and the experimental measurements can be found in Supporting Information (SI).Following the activation of catalysts at 450 °C, we recorded the XRD pattern of iron phosphide samples ( Fig. 1). The diffraction patterns are assigned to different phases: hexagonal for Fe2P (P 6 ̅ 2m, PDF♯ 1008826) and orthorhombic for FeP (Pbnm, PDF♯ 9008932).The Rietveld refined crystal parameters for these phases gave: a = b = 5.910 ± 0.0070 Å, and c = 3.543 ± 0.0040 Å for Fe2P, and a = 3.105 ± 0.0006 Å, b = 197 ± 0.0004 Å, and c = 5.783 ± 0.0007 Å for FeP. These values are in good agreement with the literature [37]. The reliability of the refinement was assessed by the low values of the weighted profile (R wp) factor, which is equal to R wp = 6.34 % for Fe2P and R wp = 4.20 % for FeP. The processed data meet the established criteria (R wp < 20 %) for good refinement [38].Transmission electron microscopy (TEM) studies revealed that the FeP catalyst is composed of microsphere-like objects with typical sizes ranging from 400 to 1300 nm ( Fig. 2a). Fig. 2c,d shows the uniform distribution of Fe and P elements (at% ratio1:1) in the given agglomerate. The Fe2P sample contained three types of particles: nanospheres (NSs), nanocubes (NCs), and nanorods (NRs) (Fig. 2f). While the Fe2P NSs and Fe2P NCs show uniform average diameters (8 ± 3.2 nm and 10 ± 2.3 nm), the Fe2P NRs average sizes (7 ± 1.0 nm in diameter and 15 ± 2.7 nm in length) varies. High-resolution TEM (HR-TEM) studies indicated that the lattice spacing of 0.28 nm of the orthorhombic FeP (Fig. 2e) and 0.15 nm of the hexagonal Fe2P (Fig. 2g) correspond respectively to the (200) and (102) crystal planes.The morphology of Fe2P and FeP thin films was studied using scanning electron microscopy (SEM). As can be seen from Fig. 2h, in the FeP film that is composed of microspheres, larger voids are formed. As determined from the cross-section SEM image, the thickness of the film is around ∼26 µm (Fig. 2i). The Fe2P thin film contains large aggregates (Fig. 2j) and densely packed NPs (Fig. 2k). The thickness of the Fe2P film is about ∼10 µm (Fig. 2l).Electrocatalytic NO3 − reduction experiments using iron phosphide films were performed in a mixture of 0.2 M NaNO3 and 0.5 M sodium hydroxide (NaOH) at pH 13. The catalytic activities of Fe2P and FeP catalysts for NO3 − reduction were evaluated using the linear sweep voltammetry (LSV) method. LSV tests displayed that both catalysts have negligible electrochemical activity toward hydrogen evolution reaction (HER) in 0.5 M NaOH in the potential window from 0.0 to – 0.55 V vs RHE ( Fig. 3a). To our expectation, the current density of Fe2P and FeP cathodes were markedly increased upon adding NaNO3 into the solution, which showed an onset potential at about − 0.3 V vs. RHE for Fe2P and − 0.34 V vs. RHE for FeP. To explain the observed improved activities for NO3RR we also conducted experiments using metallic iron (Fe) thin film. Fe is widely used for catalysis [39]. Recently, it was introduced as a good catalyst for NO3 − reduction to NH3 [4]. For comparison, we also provided LSV characteristics of Fe, iron oxide, Ti, Pt, and iron phosphate thin films. The LSV test showed that the Fe catalyst is not active for HER (Fig. S2). However, it showed a good activity toward NO3 − reduction. It exhibited an onset potential at about − 0.34 V vs. RHE which is comparable to FeP and Fe2P catalysts (Fig. 3b). However, after repeated LSV cycles using Fe thin film the current greatly reduced which indicates an ongoing corrosion process (Fig. S2). Pt exhibited an onset potential at about − 0.22 V vs. RHE which is lower than FeP and Fe2P catalysts (Fig. S3). However, the high cost and scarcity of Pt limit its widespread use. On the other hand, Ti plate alone and pristine iron oxide deposited on Ti showed low current densities and high onset potentials > −0.4 V vs. RHE proving that they have negligible contributions to NO3 − reduction (Fig. S3). It is noteworthy that the pristine iron phosphate thin film produced a lower current for NO3 −reduction than the initial FeP and Fe2P thin films. In addition, during the repetitive cyclic test, the activity of iron phosphate film decreased significantly which confirms once again its unsuitability for NO3RR when is used as a stand alone film (Fig. S4). It seems that metal oxide and metal phosphate surface termination causes a synergistic effect which improves the activity of the iron phosphide films.Tafel analysis was performed as a useful metric for interpreting the polarization curves [36,40,41]. Fig. 3c shows the Tafel relationships, potential versus log|j| (logarithm of current), for the HER and the NO3RR recorded in 0.5 M NaOH and a mixture of 0.5 M NaOH / 0.2 M NaNO3 (pH 13), respectively. The HER process usually goes through three reaction pathways: the Heyrovsky reaction (e.g. desorption step: H (ads) + H3O+ + e− → H2 + H2O), where H (ads) represents a hydrogen atom adsorbed at the active site of the catalyst), the Volmer reaction (e.g. discharge step: H3O+ + e− → H (ads) + H2O, or the Tafel reaction (e.g. discharge or recombination step: H (ads) + H (ads) → H2). The Volmer reaction is considered slow since the adsorption of hydrogen on the active sites will result in a slope, higher than 116 mV dec−1 [41]. In the case of the NO3RR, the reduction of nitrates proceeds according to the proposed mechanism: NO3 − (sol) ⇌ NO3 − (ads) and NO− (ads) + H (ads) + e− → NO2 − (ads) + OH−. The produced NO2 − (ads) either desorbs from the surface of the electrode or reduces to give NH3 according to the reaction: NO2 − (ads) + 5 H (ads) + e− → NH3 + 2OH− [42]. The obtained Tafel slopes using Fe2P and FeP in 0.5 M NaOH were 184 mV dec−1 and 205 mV dec−1 respectively. The high Tafel slope values in 0.5 M NaOH suggest that the proton discharge is the rate-determining step on the surface of the catalysts [41]. However, the Tafel slopes decreased when we introduced NO3 − into the solution. The obtained values of 155 mV dec−1 for Fe2P and 158 mV dec−1 for FeP reveal that the NO3RR process follows a similar mechanism to that of Volmer-Heyrovsky where the Volmer reaction is the rate-limiting process. As expected, Ti showed the highest Tafel slope of 212 mV dec─1. Since lower Tafel slope correlates to higher catalytic activity [36], the catalysts are ranked in the following order: Fe2P > FeP > Ti.Few catalysts are available for efficient NH3 generation at low potentials. Recently, Wu et al. demonstrated the role of Fe single-atom catalyst (SAC) for NO3RR. The authors showed that at − 0.55 V vs RHE the Fe catalyst yielded a current density of 4.30 mA cm−2 which corresponds to 331 μg h−1 mgcat.−1 NH3 yield at 39 % FE [4]. Using CuO nanowire arrays, Zhang et al. demonstrated a more efficient NH3 evolution process. However, they applied a very high potential (−0.85 V vs. RHE) to get 95.8 % FE [1]. In our study, the applied potentials are under ≤ −0.55 V vs. RHE intending to suppress the competing HER process. Potentiostatic tests for NO3RR at − 0.50 V vs. RHE for three cycles revealed the stability of the electrodes (Fig. 3d). While the FeP shows decent current stability, the Fe2P suffers from the decline of current which could be a sign of an ongoing corrosion process. During operation at high current densities, factors like internal resistance losses, accessibility of catalytic surfaces to reactants (liquid-solid-gas interfaces), electron transfer rate, and bonding strength may influence the catalyst performance [43].LSVs were recorded to follow the activity of the iron phosphide films before and after the runs (Fig. S5). For the FeP catalyst, there is a slight drop in current. However, the onset potentials remained steady even after the third run. In the case of Fe2P, both current and onset potential varied. To avoid corrosion of the film, the stability tests were also performed under reduced applied potentials (−0.37 V vs. RHE). The results show that both films are stable in this case (Fig. S6).Electrochemical impedance spectroscopy (EIS) was employed to measure the charge transfer resistance (R ct) at the surface of electrocatalysts in a mixture of 0.5 M NaOH / 0.2 M NaNO3 [44,45]. Nyquist plots of Fe2P and FeP films were recorded under applied potentials in the range from − 0.1 to − 0.6 V vs. RHE ( Fig. 4a,b). The R ct values were obtained after fitting the EIS data with the relevant circuits (Fig. S7a). The low R ct values in the presence of NO3 – are consistent with fast charge-transfer kinetics. The Ti was found to be active only in the potential range above − 0.57 V vs. RHE. This result indicated that the Fe2P and FeP catalysts have fast electron transfer and promising catalytic performance for NO3 − reduction.The detection of NH3 has been identified electrochemically using FeP or Fe2P as the working electrodes (2 cm2 geometric area) following the long-term NO3RR experiment under chronoamperometric conditions in deoxygenated 0.5 M NaOH / 0.2 M NaNO3 solution. During the NO3RR experiments, we used a bipotentiostate where one of them was attached to the iron phosphide working electrode which generated the NH3 while the other was used for detection. The detection followed independent NH3 oxidation using voltammetric diagnostic experiments in a manner analogous to that described previously for the detection of the CO2 reduction products [46]. The analytical concept has been based on the previous observations postulating proportionality of the Pt-induced NH3 oxidation currents on NH3 concentration in the mM range [47]. Historically, electrooxidation of NH3 attracted broad interest with respect to wastewater treatment [48], and in NH3 fuel cells [49,50]. In this context, the concept of electrooxidation of NH3 to N2 has been widely explored despite the complexity of the mechanism for the oxidation of NH3.During NH3 detection, the second working electrode modified with Pt catalytic NPs (deposited on glassy carbon) was placed in the vicinity of the FeP, or Fe2P working electrode [51]. Fig. 4d illustrates the results of a series of blank NH3 oxidation cyclic voltammetric (CV) experiments (curves a - d) performed in 0.5 M NaOH which contain intentionally added NH3 in the concentration range from 1 to 10 mM.Single voltammetric peaks of NH3 oxidation have emerged at potentials ranging from 0.55 to 0.70 V vs. RHE. Although the peak potentials are somewhat concentration-dependent, the voltammetric peak responses are well-defined. The black line stands for the typical response of Pt NPs in NH3-free alkaline (0.5 M NaOH) medium [52]. Here, in the potential range from 0.0 to 0.4 V vs. RHE, hydrogen adsorption peaks are developed and at potentials higher than 0.7 V vs. RHE, the reversible oxidation of platinum to platinum oxides has been observed. In between the hydrogen peaks and formation of Pt oxides, platinum exists mostly in the metallic form. It is apparent from Fig. 4d (curves a – d) that the oxidation of NH3 to N2 is catalyzed by metallic platinum, rather than Pt oxides (evident from the decrease of the oxidation currents at/above 0.8 V vs. RHE). The proportionality of the peak-current densities on NH3 concentration is evident from the Fig. S7b. Since the Fe2P film gave the highest NO3RR current densities (relative to FeP) our discussion will be mainly on Fe2P (Fig. S8). The long-term (2 h) chronoamperometric reduction of NO3 − has been performed (at the Fe2P working electrode) in a two-chamber electrolytic cell (subjected to continuous saturation with argon) upon application of − 0.55 V vs. RHE (Fig. S8).By comparing the net voltammetric peak-current density (Fig. S9a) with the analogous current density values originating from blank experiments at different concentrations (working curve in the Fig. S7b), the NH3 concentration generated following the NO3RR at the Fe2P electrode upon application of −0.55 V vs. RHE has been estimated to be equal to 4.7 mM. Judging from the amount of charge (188 C cm−2) transferred during the electrolysis for 7200 s at the 2 cm2 Fe2P-electrode (Fig. S8a), and by assuming 100 % efficiency of the 8-electron reduction of NO3 − to NH3, the NH3 concentration equal to 4.9 mM has been obtained. By comparing the above concentration values 4.7 and 4.9, the FE toward the production of NH3 can be postulated to be on the level of 96 %.The appearance of a single peak in the voltammogram supports our view that NH3 is the main N2 reaction product (Fig. S9a). In particular, no NH2NH2 is expected to be formed here because its oxidation peak on platinum would appear at less positive potentials, namely starting from 0.2 V vs. RHE [53], which is not the case in this study. The fact that some current increase is observed at potentials higher than 0.8 V vs. RHE should be attributed to the system’s further oxidation, namely to the oxidation of N2-product to nitrogen oxo species [54]. This “tailing effect” observed at positive potentials does not seem to interfere with the analytical diagnosis based on the determination of the peak current density (Fig. S9a). Any formation of sizeable amounts of NO2 − and nitrogen oxides (N2O, NO, N2O3, etc.) [54] would result in the sizeable reduction peak currents at about 0.2 V vs. RHE in the reduction voltammetric scans. As demonstrated during voltammetric diagnostic experiments in solutions containing the intentionally introduced NO2 − at various concentrations (Fig. S9b). No such responses have been obtained in the analyzed solutions after electrolysis at − 0.55 V vs. RHE. Because our present results are consistent with the view that NH3, together with hydrogen, are generated at Fe2P during NO3RR, estimation of the selectivity efficiency has also been based on this assumption. Remembering that formation of H2 is the two-electron reaction, and conversion of NO3 − to NH3 is the eight-electron process, it can be rationalized from the 96 % FE (calculated for the NH3 generation) that the selectivity (molar) efficiency is equal to ca. 84 %. To validate this result, some attention has been paid to the dynamics of hydrogen evolution in the NH3-containing NaOH. Thus, we have performed an additional chronoamperometric experiment in 0.5 M NaOH containing 4.6 mM NH3, namely, to simulate hydrogen evolution in the presence of NH3 generated during NO3 − reduction in an alkaline medium. Inset b of Fig. S8 shows that the addition of NH3 tends to decrease hydrogen evolution (dashed line), relative to the performance at NH3 free conditions (solid line). Based on the comparison of the H2-evolution current (after 1000 s from the dashed line in Fig. S8b) and the current recorded after 1000 s during NO3RR electrolysis, as well as remembering that different numbers of electrons are involved in both processes, the selectivity (molar) efficiency can be estimated to be on the level 80 %. The obtained values, 80 % and 84 % are comparable, and the difference between them may reflect the uncertainty in the assumption about the hydrogen evolution efficiency in the presence of NH3 formed during the NO3RR electrolysis. At − 0.55 V vs. RHE, the Fe2P catalyst has exhibited 0.25 mmol h−1 cm−2 or 2.10 mg h−1 cm−2 reaction rates toward NH3 generation. Upon application of less negative potentials, − 0.50 and − 0.37 V vs. RHE, the yields have been lower, 1.50 mg h−1 cm−2 and 0.42 mg h−1 cm−2, respectively. While the FeP catalyst has also been characterized by the comparably high FE of 94 % for NH3 generation at − 0.55 V vs. RHE, the reaction rate has been found under such conditions to be lower (0.12 mmol h−1 cm−2 or 1.0 mg h−1 cm−2), when compared to the performance of Fe2P. Upon application of less negative potentials to FeP, − 0.50 and − 0.37 V vs. RHE, the reaction rates have been rather low, 0.71 and 0.19 mg h−1 cm−2, respectively. The obtained yields at − 0.55 V vs. RHE using the active Fe2P phase were found to be higher or comparable to what is reported in the literature (Table S1).To validate the formation of NH3 during the electrochemical NO3RR mass spectrometry (MS) analysis was used as another proof (Fig. S10). The sudden rise of ionic current at 28 s indicates the presence of fragment ions with a mass-to-charge ratio equal to m/z = 17 for NH3. In addition, the absence of H2 (m/z = 2) during NO3RR suggests that HER is suppressed during NH3 production.Fourier transform infrared (FTIR) was used to identify the characteristic vibration components present in the FeP and Fe2P samples ( Fig. 5a,b). The C-H bending vibration at 3000 cm−1 which comes from surface passivated organics is present in FeP and Fe2P samples [55]. Similarly, the intensive band at 1740 cm−1 in FeP and 1730 cm−1 in Fe2P samples are attributed to the bending vibrations of CO bonds [56,57]. Stretching vibrations of -CH3 are assigned to the bands at 1352 and 1455 cm−1 in the Fe2P sample [58]. In the case of the FeP, the CH3 band is located at around 1371 cm−1 [59,60]. The bands located at 1086 cm−1 (FeP) and 850 cm−1 (Fe2P) confirm the presence of phosphate (P-O) species [61,62]. Comparison of FTIR spectra recorded from the FeP film before and after the stability tests shows insignificant changes. However, in the case of Fe2P, the intensity of the P-O peaks dramatically decreased after the test, which confirms the masking or leaching of phosphorus during the NO3RR. The composition and the chemical state of FeP and Fe2P thin films were characterized by XPS. The XPS survey spectrum of the FeP and Fe2P electrodes before and after NO3 − reduction is shown in Fig. 5c,d. XPS narrow scans of Fe 2p, O 1 s, and P 2p regions recorded from the FeP sample (before the NO3 − reduction) are shown in Fig. 6a,c and e. The spectrum of Fe 2p displays characteristic peaks at 711.4 and 725.0 eV, corresponding to 2p3/2 and 2p1/2 levels of Fe2+ [63]. The positions of the peaks suggest that Fe appears in the form of iron oxide and iron phosphate (major phase) [64]. The peaks at 715.0 and 729.8 eV are assigned to Fe3+, indicating the coexistence of Fe3+ and Fe2+ in the FeP sample. The peaks at 719.2 and 734.3 eV are satellite peaks, which belong to Fe 2p3/2 and Fe 2p1/2, respectively [63]. The characteristic O 1 s line shows an intensive broad signal which contains several individual peaks. The peak at 530.4 eV confirms the divalent valence state of O in the FeP. The other two peaks at 531.6 and 533.4 eV are attributed to the C–O and CO bands arising from functional groups absorbed on the sample surface [65]. The three signals within the P 2p envelope of FeP are attributed to surface oxidized P species and P from the iron phosphide [64]. The two peaks at 134.0 and 133.3 eV could be from the PO4 3– or P2O5 due to the unavoidable oxidation of P species either during the synthesis or heat-treatment process. The peak at 130.8 eV reflects the binding energy of P 2p1/2 which can be assigned to P bonding to Fe. Selected regions of the XPS spectra of Fe2P recorded before NO3 − reduction are shown in Fig. 6g,i and k. The photoelectron Fe 2p signal exhibits a doublet at 711.4 eV (Fe 2p3/2) and 724.8 eV (Fe 2p1/2) due to spin-orbit splitting. These peaks can be attributed to the oxidized state of Fe formed on the Fe2P surface, demonstrating the oxidation state of Fe2+ [64,66]. A small peak at 714.8 is assigned to Fe3+, indicating the coexistence of Fe3+ and Fe2+ in the Fe2P sample [63]. The O 1 s shows a peak at 530.3 eV, confirming that the valence state of O is divalent (Fe-O, metal-containing oxygen bond). The two shoulder peaks at 531.8 and 533.6 eV are attributed to the C–O and CO bands arising from molecules absorbed on the sample surface [65]. The P 2p signal in the Fe2P sample is almost missing which can be explained by the fact that oxide-rich iron phosphate forms on the Fe2P surface and masks the signal of P 2p [67,68].To gain a better understanding of Fe2P and FeP catalyst surface chemistry, we also recorded XPS spectra after the NO3 − reduction test (Fig. 6). The results of the quantitative analysis of the XPS are shown in detail in Table S2 and Table S3. The XPS peak intensities can be converted to atomic concentrations (at%) using the sensitivity factors determined experimentally or simply calculated [69]. For the Fe2P sample, the atomic and the mass concentration (mass %) of Fe 2p, O 1 s, and Ti 2p before and after the test did not show any drastic changes. However, the analysis shows that P 2p decreased more than 3 times after the test. The at% of P 2p decreased from 4.71 % to 1.19 %. In the case of FeP, the at% of Fe 2p increased from 13.99 to 27.35. On the other side, the at% of P 2p decreased from 15.27 to 2.892. The extensive decrease of P 2p is explained by the fact that phosphorus transformed to phosphate (hidden by surface oxygen species) or consumed due to corrosion during the reduction of NO3 −.The reaction mechanism for NO3RR catalyzed by Fe2P and FeP electrocatalysts has been studied using DFT at the RPBE+D3 level of theory in an aqueous solution. Our models have been constructed based on the intensities of the XRD patterns (Fig. 1) and XPS data analysis, leading to the building of the oxidized Fe2P (111) and phosphate-coated FeP (101) surfaces (see Fig. S12). These are also characterized by the presence of a vacancy that will act as an active site where the substrates will interact with the catalytic surface along the NO3RR. The reduction of NO3 − into NH3 entails the transfer of nine protons and eight electrons, however, with the assumption that both NO3 – and its protonated form of nitric acid (HNO3) are in equilibrium, the modeling focuses on the study of the HNO3 adsorption and NH3 desorption phenomena as the initial and final steps to the global process defined by the electrochemical equation HNO3 (ac) + 8 H+ + 8 e – ⇌ NH3 (ac) + 3 H2O (l). In this context, Fig. 7 gathers the structures of each elementary reaction step and the free energies associated with each one at room temperature and pressure conditions when there is no applied potential (U = 0) and pH = 14 (basic). Interestingly, both the oxidized Fe2P (111) and phosphate-coated FeP (101) surfaces present spontaneous binding free energies for HNO3 adsorption, –0.34 and −0.51 eV, respectively. This fact is due to the presence of low-coordinated surface Fe atoms, specifically three-fold (3c), making them electrophilic and therefore expecting greater interactions with the substrates than other surfaces constituted by higher coordinated surface Fe atoms (see Fig. S13). Both, the oxidized Fe2P (111) and phosphate-coated FeP (101) surfaces present similar catalytic profiles for the first four reduction steps, that is, the formation of *NO2, *HNO2, *NO, and *NOH intermediate species (see Fig. 7, more details at Fig. S15).Both the production of NO2 and NO are spontaneous processes with –1.62 and −1.42 eV for Fe2P (111) and –1.49 and −1.14 eV for FeP (101), expected values given the great potential of these species to coordinate with metal centers. With a contrary trend, the formation of nitrous acid and nitroxyl is non-spontaneous with values of 1.17 and 1.19 eV for Fe2P (111) and 0.89 and 1.26 eV for FeP (101). This last, that is, the nitroxyl formation as a consequence of the fourth hydrogenation, *NO + H+/e – ⇌ *NOH, represents the step with the highest thermodynamics impediment, expecting a maximum overpotential of –1.19 and −1.26 V vs. CHE (computational hydrogen electrode) equivalent to the SHE, or –0.36 and −0.43 V vs. RHE (pH = 14) for Fe2P (111) and FeP (101), respectively. Interestingly, our calculations estimate values of anodic potentials very close to those observed experimentally, validating the construction of our models. Up to this point, all substrates interact with the Fe2P (111) and FeP (101) surfaces through, at least, two binding points involving the two three-coordinated surface Fe atoms. A greater distance of them in the FeP (101) makes the metal nitride species and its subsequent hydrogenated amino *NH and *NH2 intermediates less stabilized than the ones on the Fe2P (111) surface. Finally, the NH3 desorption is calculated as just 0.49 eV for the oxidized Fe2P (111) surface while for the phosphate-coated FeP (101) one a heavy value of 1.15 eV is observed, indicating a possible catalyst poisoning for this second case.In this work, we report the use of iron phosphides as highly efficient noble metal-free catalysts in NO3RR studies. A modified solvothermal synthesis procedure using triphenylphosphine precursor was used to prepare the Fe2P and FeP catalysts. Impressively, the Fe2P catalyst shows the highest FE (96 %) and yield (2.10 mg h−1 cm−2) at − 0.55 V vs. RHE for NH3 generation. For the FeP catalyst, at− 0.50 and − 0.37 V vs. RHE, the yields were found to be 0.71 mg h−1 cm−2 and 0.19 mg h−1 cm−2, respectively. The recycling test confirmed that both FeP and Fe2P catalysts exhibited excellent stability during the NO3RR at − 0.37 V vs. RHE. Herein, the reported catalytic activities also consider the presence of metal oxide and metal phosphate surface terminations, which contributes in a synergistic way to the observed enhanced activities of iron phosphide films. DFT calculations supported the experimental observations and explained the mechanism and the fundamental origins behind the better performance of Fe2P as compared to FeP. This study demonstrates the tremendous potential of iron phosphide catalysts as efficient cathodes toward NO3RR to NH3. Hence, this work could be also extended to other TMPs for selectively converting different nitrogen oxides into valuable green NH3 under benign conditions. T. Chouki: designed and conducted the experiments, analyzed the data and wrote the manuscript. M. Machreki: contributed to EIS measurements. I. A. Rutkowska and B. Rytelewska: contributed to electrochemical tests. P. J. Kulesza: contributed to electrochemical tests, data analysis, fund-raising, and Project administration. G. Tyuliev: carried out the XPS measurements. M. Harb and L. Miguel Azofra: carried out the DFT calculations. S. Emin: supervised the work, and contributed to data analysis, manuscript writing and editing, fund-raising, and Project administration. Pawel J. Kulesza: Project administration. Saim Emin: Project administration.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was financially supported by the Slovenian Research Agency under the trilateral project for scientific cooperation between the Republic of Slovenia, the Republic of Austria, and the Republic of Poland (N2-0221). T. Chouki and M. Machreki acknowledge the scholarships provided by the Public Scholarship, Development, Disability, and Maintenance Fund of the Republic of Slovenia (Ad futura program: 11011-25/2018) for Ph.D. studies at the University of Nova Gorica. S. Emin acknowledges the financial support from the Slovenian Research Agency (research core funding: P2-0412). L. M. Azofra acknowledges the KAUST Supercomputing Laboratory using the supercomputer Shaheen II for providing the computational resources. I.A. Rutkowska, B. Rytelewska, and P.J. Kulesza were supported by the National Science Center (NCN, Poland) under Opus Lap Project 2020/39/I/ST5/03385.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jece.2023.109275. Supplementary material. .

The electrochemical reduction reaction of the nitrate ion (NO3 −), a widespread water pollutant, to valuable ammonia (NH3) is a promising approach for environmental remediation and green energy conservation. The development of high-performance electrocatalysts to selectively reduce NO3 − wastes into value-added NH3 will open up a different route of NO3 − treatment, and impose both environmental and economic impacts on sustainable NH3 synthesis. Transition metal phosphides represent one of the most promising earth-abundant catalysts with impressive electrocatalytic activities. Herein, we report for the first time the electrocatalytic reduction of NO3 − using different phases of iron phosphide. Particularly, FeP and Fe2P phases were successfully demonstrated as efficient catalysts for NH3 generation. Detection of the in-situ formed product was achieved using electrooxidation of NH3 to nitrogen (N2) on a Pt electrode. The Fe2P catalyst exhibits the highest Faradaic efficiency (96 %) for NH3 generation with a yield (0.25 mmol h−1 cm-−2 or 2.10 mg h−1 cm−2) at − 0.55 V vs. reversible hydrogen electrode (RHE). The recycling tests confirmed that Fe2P and FeP catalysts exhibit excellent stability during the NO3 − reduction at − 0.37 V vs. RHE. To get relevant information about the reaction mechanisms and the fundamental origins behind the better performance of Fe2P, density functional theory (DFT) calculations were performed. These results indicate that the Fe2P phase exhibits excellent performance to be deployed as an efficient noble metal-free catalyst for NH3 generation.

The search for environmentally friendly alternative energy sources has risen to the top of the global priority list. Climate change, caused by greenhouse gas emissions associated with fossil fuels, poses a serious threat [1]. Natural gas, oil, and coal combustion are the primary sources of greenhouse gas emissions, with these three fossil fuels accounting for 20, 39, and 41% of all hydrocarbon-related CO2 emissions, respectively [2]. Furthermore, the world's supply of fossil fuels is finite and will eventually run out, especially as the demand for energy sources rises due to population growth and industrialization. As a result, the world requires clean alternative energy sources that do not pollute the environment. As an environmentally friendly energy source, hydrogen is considered one of the best solutions to the energy-environment problem [3]. Hydrogen is regarded as one of the best solutions to the energy-environment problem as an environmentally friendly energy source, particularly if produced from waste materials to promote circular economy [4]. Because hydrogen has three times the energy storage capacity per weight of the average liquid hydrocarbon, it is one of the best alternatives to fossil fuels. Hydrogen can be burned directly as a fuel, and the byproduct of its oxidation is the only pollutant-free water on the planet [5–7]. It has the highest combustion energy per unit mass of any commonly used fuel substance, and the volume of energy produced is 2.4, 2.8, and 4 times that of methane, gasoline, and coal, respectively [8]. Furthermore, hydrogen can be mixed with natural gas and used for combustion and heating in commercial and multi-family buildings, as well as increasing the power system flexibility in gas turbines [5].Furthermore, hydrogen is vital due to its role in fuel cell technology, which converts chemical energy into electricity and has numerous applications in power generation and automotive power [5,9]. However, most hydrogen applications today, whether pure or mixed, are concentrated in the industrial field, and demand for hydrogen in the industry is rapidly increasing due to the continued development of the global economy. The most common industrial uses of hydrogen are oil refining (33%), ammonia production (27%), methanol production (11%), and direct reduction of iron ore for iron and steel production (3%). Hydrogen can be produced locally from various sources, including water, oil, gas, biofuels, and so on, allowing different countries' energy needs to be met without relying on external energy suppliers [7]. In general, hydrogen production methods can be divided into three categories [10–12]. The first category includes green hydrogen production methods, in which hydrogen is produced through water electrolysis using electricity generated from renewable energy sources. Renewable energy sources include solar, wind, geothermal, hydro, and ocean thermal energy conversion. Purple hydrogen production methods based on nuclear energy belong to the second category. Nuclear fission reactors produce heat, which is then converted into steam, which is then used to power turbines, generate electricity, and electrolyze water to produce hydrogen. Furthermore, nuclear reactors' high temperatures can be used to produce hydrogen via thermochemical water splitting and steam reforming methods. The blue category of hydrogen production methods includes traditional methods based on fossil fuel-based processes.Nowadays, conventional methods account for approximately 96% of global hydrogen production, with steam reforming of methane (SRM) accounting for approximately 50%. Because methane is the primary component of natural gas, the world's methane reserves are plentiful [13]. Methane also has the highest H/C ratio of any hydrocarbon, making it the best hydrogen resource [14]. However, methane steam reforming emits approximately 830 million tonnes of CO2annually, making it one of the most significant contributors to global warming [5]. As a result, an environmentally friendly method of utilizing methane in hydrogen production is required. Catalytic decomposition of methane (CDM) is the best option for utilizing methane in hydrogen production with zero CO2 emissions [5,15,16].The only products in the CDM are gaseous hydrogen and solid carbon. The produced hydrogen does not need to be separated [5]. According to Weger et al. [17,18], using CDM instead of SMR in hydrogen production can reduce COx emissions by about 27%, potentially reducing climate change. Furthermore, CDM is a moderately endothermic reaction requiring a lower operating temperature than SRM, so it is more cost-effective [5]. Equations 1 and 2 represent SMR and CDM reactions [19]. (1) C H 4 + 2 H 2 O → 4 H 2 + C O 2 ( Δ H = 41.3 k J / m o l H 2 ) (2) C H 4 → C + 2 H 2 ( Δ H = 37.8 k J / m o l H 2 ) Aside from producing COx-free hydrogen, CDM has another valuable advantage in producing high-value-added multifunctional carbon nanomaterials, which are a valuable byproduct with a wide range of beneficial applications [20]. The primary goal of the methane decomposition process in early applications was the preparation of nano-carbon, not hydrogen [21]. In the CDM process, single and multi-walled carbon nanotubes, as well as nanofibers, are produced with distinct physical and chemical properties, which are primarily determined by the catalyst used and the experimental parameters used [14]. Carbon nanotubes produced by CDM have excellent electronic properties, high axial strength, high thermal stability, and high stiffness. Furthermore, the carbon nanofibers produced have macroporous and mesoporous structures, a high surface area, metal and semiconductor properties, high conductivity, tenacity, and mechanical strength [14]. As a result of the numerous applications of these nanocarbons (nanotubes and nanofibers), the CDM is a cost-effective process for hydrogen production from methane. Among the applications are the hydrogen storage medium for fuel cells, polymer nanocomposites, supercapacitors, water treatment, electrode material in batteries, and catalysis [6,14,22,23].A methane molecule is a highly stable inert molecule with four extremely strong C–H bonds formed by sp3 hybridization, each with an energy of about 435 kJ mol−1 [5,24]. As a result, methane decomposition is an endothermic process that requires a high reaction temperature (>1200 °C) in the absence of a catalyst to be completed with a significant product yield [5,24]. As a result, in this reaction, a suitable catalyst is always used to reduce the reaction temperature (>400 °C) by providing a pathway with lower activation energy [5,24]. Metals or carbon-based materials are the most commonly used catalysts in CDM reactions. Activated carbon, carbon black, coal chars, glassy carbon, carbon nanotubes, acetylene black, soot, graphite, diamond powder, and fullerenes are examples of carbon-based catalysts [25].However, studies have revealed that methane conversion over carbon-based catalysts is lower than that over metal-based catalysts [6]. Metal-based catalysts are classified as supported or non-supported. The most common active metals used in the CDM reaction are nickel, cobalt, and iron [5]. The catalytic activity of Ni, Co, and Fe metals in the CDM reaction is due to their non-filled 3d-orbitals, which promote methane molecule dissociation. The transfer of electrons from the catalyst containing one or more of these transition metals (Ni, Co, and Fe) to the unoccupied anti-bonding orbitals of methane molecules facilitates their dissociation [5].Herein, we present a significant contribution to research using methane in hydrogen production via COx-free methods. In the current study, non-supported pure and mixed cobalt and iron oxide catalysts with varying Co/Fe ratios were synthesized from nitrate precursors using a very simple preparation method in which water was the only solvent used. The prepared catalysts were then used as decomposers of methane into hydrogen and carbon. To the best of our knowledge, this will be the first study using non-supported Co–Fe mixed oxides to catalyze this reaction.Iron Nitrate hexahydrate (Fe(NO3)3·6H2O 433.01 g/moL; 99.99%; Aldrich), and cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 98%, Alfa Aesar) were purchased and were used as received. Ultrapure water was obtained via a Milli-Q water purification system (Millipore).The weight of an empty crucible was measured and recorded. Pure oxides were prepared by the calcination of iron nitrate hexahydrate or cobalt nitrate hexahydrate at 600 °C for 3 h at a heating rate of 10 °C/min. In the case of mixed oxides and depending on the desired composition and mass of the catalyst, calculated masses of iron nitrate hexahydrate (equivalent to an atomic ratio of 25, 50 and 75%) and that of cobalt nitrate hexahydrate (equivalent to an atomic ratio of 75, 50 and 25%) were poured inside the crucible. The mixture was ground thoroughly in the crucible to obtain a fine powder mixture. Ultrapure water was added dropwise to the ground powder mixture in the crucible to form a paste. It was well stirred, and the water was allowed to evaporate under ambient conditions overnight as a drying process. The weight of the crucible plus the sample was measured, and subsequently, that of the sample was determined after drying overnight. Thereafter, the dried sample was calcined at 600 °C for 3 h at a heating rate of 10 °C/min. The mixed oxides containing 25, 50 and 75% of Fe metal were abbreviated as 25Fe + 75Co, 50Fe + 75Co and 75Fe + 25Co, respectively.Methane decomposition experiments were carried out at a reaction temperature of 800 °C under atmospheric pressure. The reactions were performed in a packed bed reactor of stainless steel (internal diameter, 0.0091 m; height, 0.3 m). A catalyst mass of 0.30 g was carefully positioned in the reactor over a ball of glass wool. Stainless steel, sheathed K-type thermocouple positioned axially close to the catalyst bed, was used to measure the temperature during the reaction. Prior to the start of the reaction, activation of the catalysts was performed at 700 °C in an atmosphere of H2 at a flow rate of 40 ml/min. This lasted for 60 min, and the remnant H2 was purged with N2. The feed volume ratio was maintained at 3:2 for methane and nitrogen gases during the experiments, respectively. In addition, the space velocity was kept at 5.0 l/h/gcat. The reactor outlet was connected to an online gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) to analyze its composition. The methane conversion, carbon yield and hydrogen yield were thus computed according to Equations (3)–(5), respectively. (3) C H 4 c o n v e r s i o n ( % ) = C H 4 , i n − C H 4 , o u t C H 4 , i n ∗ 100 (4) C a r b o n Y i e l d % = W p − W cat W c a t ∗ 100 (5) H 2 Y i e l d % = m o l e s o f H 2 p r o d u c e d m o l e s o f C H 4 i n t h e f e e d X 2 ∗ 100 Where, CH4,in is methane in the feed, CH4,out is methane in the product, WP is the product's weight after reaction, and Wcat is the weight of the fresh catalyst.N2 adsorption-desorption isotherms of the catalysts were measured by N2 adsorption-desorption at 196 °C using a MicromeriticsTristar II 3020 surface area and porosity analyzer.The quantity of carbon deposits on the spent catalysts was measured using TGA analysis. A platinum pan was filled with 10–15 mg of the used catalysts and carefully positioned inside the device. Heating was done from room temperature up to 1000oC at a 20 °C/min−1 temperature ramp under an air atmosphere. The change in mass was continuously monitored as the heating progressed.Powder X-ray diffraction (XRD) patterns of the prepared catalysts were recorded on a Miniflex Rigakudi_ractometer that was equipped with Cu K, X-ray radiation. The device was run at 40 kV and 40 mA.In order to study the morphology of the catalyst and to elucidate carbon deposition on the used catalysts, Scanning Electron Microscopy - Energy-Dispersive X-ray spectroscopy (SEM/EDX) interpretations of the fresh and used catalyst samples were performed using a JSM-7500F (JEOL Ltd., Japan) scanning electron microscope.Transmission electron microscopy (JEOL JEM-2100F) with high resolution to give larger magnification was used to conduct the TEM measurement of both the fresh and used catalyst. The electron microscope operated at 200 kV produces the active metal nickel particle sizes and depicts the morphology of carbon deposit on the used catalyst. Before the TEM measurement, the catalysts were first dispersed ultrasonically in ethanol at room temperature. After that, the drop from the suspension was placed in a lacey carbon-coated Copper grid to produce the images.Laser Raman (NMR-4500) Spectrometer (JASCO, Japan) was used to record Raman spectra of the fresh and used catalyst samples. The wavelength of the excitation beam was set to 532 nm, and objective lens of 100 × magnification was used for the measurement. The laser intensity was adjusted to 1.6 mW. Each spectrum was received by averaging 3 exposures in 10 s. Spectra were recorded in the range 1200–3000 cm−1 (Raman shift) and processed using Spectra Manager Ver.2 software (JASCO, Japan).To confirm the crystal structure and phase purity of the fresh and used catalysts, powder X-ray diffraction (XRD) has been used. The XRD diffraction patterns of the fresh and used mixed oxide catalysts are shown in Fig. 1 . The XRD diffraction pattern of all Fe–Co mixed oxide fresh catalysts matched the reference patterns of Co3O4 (PDF- #:76–1802) and Fe2O3 (COD 1546383). The patterns of Co3O4 were detected at 2θ of 31.43, 36.8, 44.9, 55.7, 56.5, 59.6, and 65.4o, while the patterns of hematite phase (Fe2O3) were found at 2θ of 24.3, 33.7, 35.6, 41.2, 49.7, 54.4, 57.1, 62.6, and 64.2o. Furthermore, the characteristic patterns of cobalt ferrite (CoFe2O4) spinel were also found individually or overlapped with Fe and Co oxides in all catalysts. The patterns corresponding to the CoFe2O4 were located at 2θ of 18.3,30.0, 35.4, 43.1, 57.0, and 65.1o [26,27].The intensity of all diffraction peaks decreases remarkably with the increase of the Fe2O3 ratio in the fresh 50Fe + 50Co and 75Fe + 25Co catalysts, which could be attributed to a synergistic effect of Fe and Co oxides to form the CoFe2O4 spinel structure. No other phases or peak position shifts were observed in these three catalysts' XRD patterns. This suggests that the preparation method used in this study produces mixtures of cobalt and iron oxides as well as cobalt ferrite without forming an extensive solid solution. According to Abdelkader et al. [28], no solid solution is expected between Co and Fe at calcination temperature lower than 1000 °C.Considering the used catalysts, the XRD diffractogram of all used Fe–Co catalysts matched the reference patterns of a graphitic carbon diffraction peak at 2θ of 26.4° (JCPDS No. 41–1487), irrespective of the Fe/Co ratio. The intensity of the peaks corresponding to carbon nanotubes/graphene is relatively stable. The diffraction peaks corresponding to the metallic forms of Co and Fe were detected at 2θ of 44 and 51.2o in all used Fe–Co catalysts. Moreover, the intensity of the main diffraction peak at 2θ of 44o increased by increasing the ratio of Fe2O3 in the used 50Fe + 50Co and 75Fe + 25Co catalysts. This behaviour could be explained by forming Fe–Co alloy due to CoFe2O4 formation in fresh catalysts. The Fe–Co alloy is reported to have diffraction patterns at 44.80 and 65.28o [29].The presence of the Co, Fe, and Fe–Co alloy in the used catalysts is due to the reduction step prior to the reaction in addition to the expected in-situ further reduction of these catalysts by the hydrogen produced from the methane decomposition reaction.The fresh and used mixed oxide catalysts were characterized by Raman spectroscopy measurements. Fig. 2 shows the Raman spectra of the fresh and used catalysts. The Raman spectra of the three fresh catalysts show bands around 275 and 560 cm−1 attributed to hematite Fe2O3 and a band at 1092 cm−1, which is attributed to CoO [30,31]. In the case of the used catalysts, two distinct bands were observed at around 1565 cm−1 and 2660 cm−1, respectively. The band at 1565 cm−1 is called G-band and is related to the vibration of sp2 bonded carbon atoms in a 2D hexagonal lattice and represents crystalline carbon [32,33]. The band at 2660 cm−1 is called the 2D band and is common to all sp2 carbon materials (appears in the range 2500–2800 cm−1) [34]. In the used catalysts, no bands appear in the 1000–1500 cm−1 range corresponding to the D band (disorder-induced band) [35]. The D band usually indicates the amount of disorder or defects in the sample. The appearance of the G and 2D bands without the D band indicates that the carbon deposited on the used catalysts is well-ordered carbon nanotubes and/or graphene [33,35]. However, the sharp non-split appearance of the G band and the 2D band is characteristic of graphene and not graphite or carbon nanotubes [36]. In graphene, the number of graphene layers affects the shape, position and relative intensity of G and 2D Raman bands [36]. According to many studies, as the number of layers increases, the intensity of the G band increases significantly, while that of the 2D band decreases [37]. From Fig. 2, the relative intensity of G and 2D bands is quite different in the three used catalysts, which indicates the formation of different types of graphene depending on the number of layers formed. The band at 275 cm−1 in the case of the samples 25Fe + 75Co and 75Fe + 25Co and the small band at 570 cm−1 in the case of sample 50Fe + 50Co correspond to hematite Fe2O3 [31]. The bands at 2230 cm−1 in the case of the used 75Fe + 25Co sample and at 2150 cm−1 in the case of the 25Fe + 75Co sample are likely attributed to the laser scribed graphene [38].The SEM and EDX profiles of fresh and used mixed oxide catalysts are shown in Fig. 3 . All of the elements claimed in the fresh or used catalysts are seen in the EDX patterns. All the fresh catalysts contain Co and Fe with ratios relative to their expected ratios in the catalysts. At the same time, the used catalysts contain C, Co and Fe with a carbon ratio of over 70%. However, the ratios of Co and Fe are still relative to their ratios in the catalysts and the order of the catalysts according to the carbon ratio in each catalyst is 50Fe + 50Co > 75Fe + 25Co > 25Fe + 75Co. The SEM images of fresh and used mixed oxide catalysts were used to study their surface morphology and to evaluate the carbon deposition behaviour over the used catalysts. The SEM images of fresh catalysts show mixtures of two different kinds of grains with two different morphologies corresponding to the two different oxides consisting of the catalysts. The first kind of grains is the highly dense, variously sized grains corresponding to iron oxide, and the second is the irregular-sized grains with a sponge-like structure corresponding to cobalt oxide (Fig. 3A–C). The SEM images of used 50Fe + 50Co and 75Fe + 25Co catalysts show that the catalyst particles are covered with bamboo-like fibrous carbon or carbon nanotubes with a relatively higher density over the 50Fe + 50Co catalyst (Fig. 3E and F). The SEM image of the 25Fe + 75Co catalyst shows a different morphology of the produced carbon as it seems mainly like aggregates of sticked thin platelets with a few carbon nanotubes (Fig. 3D). This indicates that the carbon formed over this catalyst is mainly graphene. The presence of CNT on the surface of both 50Fe + 50Co and 75Fe + 25Co catalysts may be attributed to the dispersion and stabilization of Fe and Co oxides in the CoFe 2O4 structure. In this context, the aggregated Fe or Co oxide particles can be considered the active sites in graphene growth. The combination of Raman and SEM analysis indicates that the carbon deposited over the mixed oxide catalysts is most likely a mixture of different types of highly crystalline sp2 carbon, i.e., graphene and carbon nanotubes.El-Ahwany et al. [39] studied the methane decomposition over Fe/MgO catalysts with different iron loadings. They reported the growth of different types of carbon nanomaterials, including graphene nanoplatelets and carbon nanotubes. They found that the graphene nanoplates grow on high Fe-loaded surfaces while carbon nanotubes grow on the low iron-loaded catalyst surfaces. Fig. 4 shows the TEM image of fresh and used 50Fe–50Co catalyst in order to understand in-depth morphology. Similar to the SEM image, the TEM image of the fresh 50Fe–50Co catalyst shows the presence of two different kinds of grains with two different morphologies corresponding to the two different oxides. The first morphology is represented by large black lumps, which are likely to correspond to cobalt oxide and/or CoFe2O4 species, and the other is represented by less dark, variously sized grains with irregular shapes that correspond to iron oxide. Unlike the SEM image, no development of carbon tubes is observed in the TEM image of the used 50Fe–50Co catalyst; instead, transparent graphene nanoplatelets seem to be grown. This observation supports our suggestion that the carbon deposited over the catalysts is a mixture of graphene and carbon nanotubes.The textural properties of the fresh and used mixed oxide catalysts were studied by the N2 adsorption/desorption measurements, as shown in Fig. 5 . According to IUPAC calcification, all the isotherms in Fig. 5 belong to type IV with H3 hysteresis loops characteristic of the mesoporous materials and are usually found on solids consisting of aggregates of particles forming slit-shaped pores with non-uniform size and shape [40]. However, it can be seen from Fig. 5 that the nitrogen uptake of the fresh catalysts started at a relative pressure range of 0.7–0.85, whereas the nitrogen uptake of the spent catalysts started at the relative pressure range of 0.4–0.5. This indicates that the used catalysts exhibit a highly porous structure compared to the fresh catalysts, which can be attributed to the enhanced mesoporosity by the deposited carbon. The same result has been observed by Deyab et al. [41], who studied the preparation of nanocomposite material of Ni–Fe alloy and graphene by the chemical vapour deposition method using methane as a carbon source over Ni–Fe alloy as a substrate. They found enhancement in the mesoporosity of the composite compared to the alloy due to the improvement in the dispersion of NiFe alloy nanoparticles on the surface of graphene.The catalytic activity of pure and mixed cobalt and iron oxides towards methane decomposition is shown in Fig. 6 as methane conversion (a) and hydrogen yield (b). It is clear from Fig. 6 that the mixed oxides catalysts have higher catalytic activity than the pure oxides catalysts, and the change in Fe/Co atomic ratio plays an essential role in the performance of the mixed oxides. The significant difference in the activity between pure and mixed oxides shows the importance of mixing the two oxides and clarifies the impact of this mixing on the catalytic activity. Except for pure oxides, all catalysts were found to be highly stable until the end of the reaction time (425 min), with no drop in hydrogen yield, and it appears that they will retain their activity for long periods before deactivation. As shown in Fig. 6, the 50Fe + 50Co mixed oxide catalyst exhibits higher catalytic activity in terms of methane conversion and hydrogen yield than the other mixed oxide catalysts within all reaction periods. The hydrogen yield of 18 and 22% was obtained at the initial reaction period of 10 min using 25Fe + 75Co and 75Fe + 25Co catalysts, respectively. Following that, the hydrogen yield gradually increases with reaction time, reaching 24% at 200 min for the 25Fe + 75Co catalyst and 33% at 325 min for the 75Fe + 25Co catalyst, and these values remain unchanged until the reaction completion at 420 min (Fig. 6).On the other hand, the 50Fe + 50Co catalyst demonstrated noticeable activity during the initial stage of the reaction, yielding approximately 50% of the hydrogen at 10 min. After that, the catalytic activity decreased slightly to 44% at 20 min, then increased again to reach a steady-state value of 48% at 120 min and remained constant until 420 min of reaction time. These results indicate the presence of a synergetic effect between cobalt and iron oxides, especially in the case of 50Fe + 50Co and 75Fe + 25Co catalysts, which gives their preference over 25Fe–75Co catalyst. The low hydrogen yield at the beginning of the reaction can be attributed to the in situ consumption of H2 released from methane decomposition, which continued the reduction of unreduced Co and Fe active sites left from the pre-reduction step [32]. This in situ reduction leads to an overall increase in the number of active sites available for the reaction, which consequently increases the methane decomposition and carbon formation until reaching equilibrium [32]. However, it can be noted from Fig. 6 that the equilibrium is reached faster in the case of 50Fe + 50Co catalyst, which means less number of unreduced active sites left from the pre-reduction step and faster reduction of the remaining unreduced sites. This can be attributed to the fact that the uniformly mixing of iron and cobalt oxides improve the reducibility of both oxides [28]. Increasing the ratio of iron or cobalt oxides decreases the degree of the mutual improvement in the reducibility of the two oxides, which likely depends on an atom-to-atom mechanism, which explains the importance of the Co/Fe atomic ratio and the superiority of mixed oxides over pure oxides.Again, the higher catalytic activity of 50Fe + 50Co among the others can be explained by the presence of CoFe2O3 species, which allows more active sites for methane decomposition reaction. This suggests that the Fe/Co ratio in this catalyst is sufficient for generating a significant percentage of CoFe2O3 species, which improves metal oxide dispersion and stabilization during the reaction. In contrast, the presence of either non-interacted Fe or Co oxides may be responsible for the low activity of 75Fe + 25Co and 25Fe + 75Co mixed oxide catalysts.Furthermore, it is known that the Fe–Co mixture has higher carbon capacity compared to Co or Fe, which means better durability of catalyst towards carbon deposition [42]. This high carbon capacity that prevents the quick deactivation of the catalyst due to the formation of encapsulating carbon is another reason for the higher activity of the mixed oxides compared to pure oxides and explains the gradual deactivation of pure iron oxide. In addition, the high carbon capacity of the Co–Fe mixture results from a mutual effect between the two oxides, so that it depends on the Fe/Co molar ratio, and it is optimum in the case of a 50Fe + 50Co catalyst. Awadallah et al. [43] studied the methane decomposition over Fe–Co/MgO catalyst with a total metal content of 50 wt%, and Fe/Co atomic ratio equals one. They reported achieving more than 80% hydrogen yield and 76% carbon yield using this catalyst. They explained this high catalytic activity as it is due to the existence of large numbers of non-interacting Fe2O3 and Co3O4 oxide phases on the surface of MgO support. As a result, higher adsorption and faster solubility of the reacting methane molecules are achieved, keeping the active Fe or Co metals exposed to the reactant gas for a longer time.The used mixed oxide catalysts were analyzed by the TGA technique under an air atmosphere in order to study the thermal stability of the produced carbons. The TGA curves of the used catalysts are shown in Fig. 7 . It was observed that the TGA curves are all stable to a temperature up to about 500 °C, which confirms that the carbon deposited on these catalysts is not amorphous carbon which could decompose at a lower temperature (200–350 °C) [32]. This observation comes in accordance with the Raman analysis results, which confirm the crystalline nature of the carbon produced over the used catalysts. In addition, this relatively high degradation temperature reflects the high thermal stability of the produced carbon. The onset-end temperature ranges of the TGA curves of 25Fe + 75Co, 50Fe + 50Co and 75Fe + 25Co catalysts are 530–930 °C (wt. loss = 48%), 560–945 °C (wt. loss = 41%), and575- >1000 °C (wt. loss = >52.5%), respectively. These values of onset-end temperature ranges indicate that the carbon deposited on the 50Fe + 50Co catalyst exhibits a higher degree of graphitization and lesser defects in the structure than the carbon deposited over the other two mixed oxide catalysts [32,44]. In addition, the carbon yield over the 50Fe + 50Co and 75Fe + 25Co catalysts is much higher than that over the 25Fe + 75Co catalyst, reflecting the effectiveness of mixing the two oxides in the decomposition reaction of methane. This behaviour could be attributed to the presence of more CoFe2O4 species in these catalysts than in the third 25Fe + 75Co catalyst, as evidenced by the XRD results (Fig. 1). Furthermore, at temperatures above 500 °C and immediately before beginning the thermal degradation of the used catalysts, a slight weight gain can be observed, which increases by increasing the iron ratio in the catalyst composition. This is likely due to the oxidation of Fe in the catalysts, which explains the direct proportion between the weight gain and the percentage of iron in the sample [32]. The small weight loss peak observed in the 50Fe + 50Co sample at around 900 °C is likely due to the weight gain resulting from the oxidation of residual Fe and/or Co, which takes place after the oxidation of all carbon deposits.Carbon yield as a function of the catalyst composition of the mixed oxide catalysts is shown in Fig. 8 . The carbon yield over the catalysts 25Fe + 75Co, 50Fe + 50Co and 75Fe + 25Co is 30.4, 57.7 and 40%, respectively. These results come in accordance with the catalytic activity results shown in Fig. 6 and confirm the superiority of 50Fe + 50Co catalyst over the other two catalysts.Iron–Cobalt mixed oxide catalysts with different Co/Fe atomic ratios were prepared from nitrate precursors using a simple preparation method and water as the only solvent. According to the results of the XRD analysis, the employed method of preparation produces mixtures of cobalt and iron oxides as well as cobalt ferrite species without forming an extensive solid solution between cobalt and iron. Due to carbon deposition on their surfaces, the spent catalysts have a much more porous structure than the fresh catalysts. Moreover, the combination of Raman, SEM, and TEM analysis suggested that the carbon deposited on these catalysts is a mixture of graphene and carbon nanotubes. The Co/Fe atomic ratio significantly impacts the catalytic activity of Co–Fe mixed oxide catalysts for the conversion of methane to hydrogen and carbon. The catalyst 50Co+50Fe (Co/Fe = 1) exhibits greater activity and, as a result, produces more hydrogen and carbon than the other two catalysts. Equal mixing of the two oxides appears to optimize the mutual improvement of the reducibility of both cobalt and iron oxides, which has a positive effect on the catalytic activity of the Co–Fe catalyst mixture. The Co–Fe unsupported catalyst system is a cheap and promising candidate for the decomposition of methane into hydrogen and nanocarbons.The views and opinions expressed in this paper do not necessarily reflect those of the European Commission or the Special EU Programmes Body (SEUPB).The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia for funding this research work through the project no. (IFKSURG-2-055). Dr Ahmed I. Osman wishes to acknowledge the support of The Bryden Centre project (Project ID VA5048). The Bryden Centre project is supported by the European Union's INTERREG VA Programme, managed by the Special EU Programmes Body (SEUPB).

Herein, non-supported pure and mixed cobalt and iron oxide catalysts were synthesized from nitrate precursors using a simple, environmentally friendly preparation method in which water was the sole solvent. The prepared catalysts were then used to decompose methane into hydrogen and carbon (graphene nanosheets and carbon nanotubes). The fresh and spent catalysts were characterized by employing X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy-energy dispersive X-ray analysis (SEM/EDX), transmission electron microscopy (TEM) and N2 adsorption-desorption techniques. In addition, the spent catalysts were subjected to thermo-gravimetric analysis (TGA) in order to measure the quantity of carbon deposits on the spent catalysts. The results indicated that the carbon deposited over these catalysts is a mixture of graphene nanosheets and carbon nanotubes (CNT). The results indicated that the mixed oxide catalysts exhibit higher catalytic activity than the pure oxides and that Fe: Co atomic ratio represents the key factor in the catalytic activity of these mixed oxides. After 420 min under the reaction feed, the 50Fe + 50Co catalyst shows the highest catalytic activity towards methane conversion of about 52.6% compared to 41.6% and 31.8% for 75Fe + 25Co and 25Fe + 75Co catalysts, respectively.

Data will be made available on request.Recently, there has been a widespread development of materials and techniques to buffer the intermittent renewable energy supply via diverse applications such as supercapacitors [1–3] and lithium-ion batteries [4,5]. In addition, hydrogen production via proton exchange membrane water electrolysis cells (PEMWEs) stands in the focus of current academic and industrial research. Aside from energy storage for the grid, heat or mobility, PEMWEs can also be a key element in industrial markets that demand hydrogen, such as the ammonia chemical industry, chemical stock synthesis or fuel synthesis (power-to-x) [6]. According to the International Energy Agency (IEA), global demand for hydrogen was estimated at 87 million metric tonnes (MMT) year in 2020 and is forecasted to increase by 13 MM T/year until reaching 528 MMT in 2050 [7]. To reach a net-zero emission scenario in 2050 without further investment in fossil-fuel-based carbon capture, utilization and storage (CCUS) an extra 190 MT of hydrogen power produced by water electrolysis would be necessary to provide 2000 GW of net capacity. To put this development into perspective, by the year 2030 a target for 40 GW in electrolysis capacity should be reached according to the European Green Deal, with an associated cost of EUR 20 to 40 billion without accounting for the electricity costs [8]. While commercial alkaline electrolyzer systems are currently more economical at a lower price per kilowatt [9], PEMWEs have experienced a greater cost reduction due to R&D efforts. The interest in PEMWE development stems from their ability to provide higher current densities and to work at higher temperatures and pressure as compared to alkaline electrolyzers, which makes them a more interesting option for industrial scaling [10]. However, their most challenging limitation is the requirement of Platinum Group Metals (PGMs) such as Ir and Pt to improve the slow electrode reaction kinetics under harsh acidic conditions and high potentials. In particular, the oxygen evolution reaction (OER) occurs in a multi-step reaction that is favoured on the active sites of Ir-based oxide catalysts [11]. While Ru-based oxides have shown higher activity than IrO2 for the OER, the latter sustains the most balanced equilibrium between high activity and durability in the acidic environment [12]. Since PEMWEs should withstand periods of >50 k operating hours under high current densities and transients without showing significant degradation, the choices are further narrowed. Unfortunately, the supply of Ir and Pt is very limited and costly. Just in the first quarter of 2021, the price per ton of Iridium increased sharply by four times, which is the highest price increase registered in the last 20 years [13]. Hence, to upscale the PEMWE production the capital expenditure (CAPEX) cost including noble metal cost has to decrease. To establish Ir catalysts as a commercially viable and scalable option in PEMWEs, the catalyst loading needs to be reduced while maximizing the catalytic activity. For maintaining a high catalytic activity with low loadings, the electrochemically active surface area (ECSA) should be as high as possible. A common strategy to reduce the catalyst loading is to develop alloys with synergistic effects that increase the intrinsic activity [14] as is observed when combining Ir with Cu, Co, or Ni [15,16] or forming core-shell structures [17] that benefit from an Ir-rich surface [18–20]. Another approach is to develop completely PGM-free catalysts based on alloys of more abundant metals, i.e., Co, Ni, Fe or Mo derived from metal-organic frameworks (MOF) [21–24]. However, the most known commercial catalysts are still unsupported, Ir-based nanoparticles. Several synthesis approaches have been tested to produce nanoparticles with different characteristics [25]. It is not possible to support the nanoparticles on carbon to increase the surface area, as the carbon degrades. This has the disadvantage that the ECSA of Iridium black is small compared to Pt/C catalysts used in fuel cell systems [11,26–28]. Recently developed self-supported catalyst nanostructured catalysts present a possible solution to this problem since the catalyst is applied on the substrate (e.g., a gas diffusion electrode) directly without using binder materials. With this approach, it already has been shown that microstructure tuning and modulation of the catalysts’ electronic structure with heteroatoms can produce highly active and stable catalysts [29–32]. However, multi-step processes are often used at the lab scale to synthesize catalyst particles and coatings, which can impose limitations in the industrial scaling. On the other hand, physical vapour deposition (PVD) is a well-known technique in the industry. The plasma process yields homogenous composition in the layers and allows flexible operative conditions such as the direct deposition of oxides or control of the morphology modifying, e.g., the sputtering angle or chamber pressure [33]. In recent studies, PVD has been used to produce highly active self-supported catalysts with tunable morphologies using a co-sputtered templating metal [11,34,35]. High ECSAs are achieved by selective dissolution of the templating metal in an acid-leaching processing step, which creates an interconnected network of the active metal. Several publications concerning this method report large ECSAs and activities [11,34,35]. On the other hand, the high catalyst performance observed with traditional academic testing techniques such as the thin-film rotating disk electrode (TF-RDE) hardly ever translates to real operation conditions seen on full membrane electrode assembly (MEA) systems [36]. The step from lab testing to an industrial application is thus very wide. Testing in a liquid acidic environment under mass transport limited conditions does not describe accurately those of a Membrane-Electrode-Assembly. Furthermore, it has been discussed that the degradation trials on OER could have been systematically misinterpreted [37–40]. Due to the method limitations, the oxygen evolved during the reaction is trapped close to the surface of the catalyst causing early failure during the test, while the catalyst features remain unchanged [39]. Gas diffusion electrode (GDE) setups have been introduced as a bridging tool as they include realistic constraints (real catalyst loadings, membrane layer, gas diffusion/porous transport layers, three-phase boundary) while keeping the fast screening capabilities of the TF-RDE and retaining the ability to measure the potential drop of the anode in a three-electrode setup. While initially designed for oxygen reduction reaction (ORR) studies [41–43], an increasing number of publications with GDE setups in different configurations also prove its flexibility to explore different reactions such as the OER [25]. It is expected that this technique becomes a standard in the electrochemical community and is used more systematically to develop catalyst layers in a fast and cost-effective manner before applying MEA tests [44]. In the present study, we use a GDE setup modified to accommodate electrolysis conditions to perform an electrochemical characterization of the OER in three series of IrCo catalysts produced by PVD with different sputtering Co:Ir ratios. In particular, we aim to study the influence of the deposition parameters on the reaction performance. To that end, we use morphological and chemical characterization techniques (SEM-EDS, XAS, XRD, XPS) to follow the development of the catalyst during different steps in the material preparation (magnetron sputtering followed by acid leaching). The features observed (mesoporosity, chemical distribution, crystallinity) are further discussed alongside the electrochemical characterization of the ECSA of the catalyst by cyclic voltammetry (CV) and OER mass activity. Our findings indicate a direct relationship between the deposition parameters and the electrochemical results. Furthermore, this study underlines the interesting synergy of the PVD with the GDE method to fast-track catalyst film optimization for industrial applications.De-ionized ultrapure water (resistivity >18.2 MΩ cm, total organic carbon (TOC) < 5 ppb) from an Aquinity P −102 system (Membrapure, Germany) was used for electrolyte preparation and the cleaning of the GDE half-cell. Carbon gas diffusion layers (GDL) with a microporous layer (MPL) (Sigracet 29BCE, 325 μm thick, Fuel Cell Store) served as a substrate for the sputtering of the catalyst film. A polytetrafluoroethylene (PTFE) disk (Bola, 0.12 mm thickness), a GDL without an MPL (Freudenberg H23, 210 μm thick, Fuel Cell Store), a porous transport layer (PTL) (ANKURO Int. GmbH, 0.3 mm thickness, 50% open porosity), and a Nafion membrane (Nafion 117, 183 μm thick, Chemours, Wilmington, DE, USA) were used for the cell assembly (see Fig. 1 ). As a counter electrode (CE) a platinum wire of 0.5 mm diameter (99.99%, Junker Edelmetalle GmbH) was used, which was folded several times at one side to increase the active surface area. Another Pt wire was used to manufacture a hydrogen reference electrode (RE) using a borosilicate glass capillary of 40 mm in length and 6 mm in diameter. Additionally, self-manufactured borosilicate glass frits (6 mm internal diameter, 20 mm length) were used to hold the RE during the electrochemical measurements. Perchloric acid (70% HClO4, Suprapur, Merck) was used for electrolyte preparation. O2 (99.999%, Air Liquide) and Ar (99.999%, Air Liquide) were used for magnetron sputtering, acid leaching, and electrochemical measurements.To prepare the self-supported nanoporous catalyst film, a linear sputtering magnetron reactor (Univex 400, Leybold GmbH, Germany) was used. The process chamber was evacuated to a pressure of 1.7 · 10−5 Pa. The film substrate (GDL) was placed on a holder in a load lock at atmospheric pressure and then evacuated to a base pressure of at least 10−4 Pa. From there, a swivelling arm allowed the holder to enter the process chamber with minimal interruption. During the deposition, an Ar plasma was ignited at the magnetron electrode at a working pressure of 5 Pa and flushed through the individual magnetron sources at a flow rate of 100 sccm. For the IrxCo1-x film deposition, two magnetrons were equipped with planar targets of Co (99.95%, Evotec GmbH, Germany) and Ir (99.95%, MaTecK, Germany) of 177 x 25 × 1.5 mm located at the upper part of the chamber. The RF generators (Cito 136, COMET) operated at a driving frequency of 13.56 MHz. Further information about the sample preparation process and the reactor configuration can be found in the SI. A mask of 5 cm × 5 cm on the substrate holder limited the sputtered area during the deposition. The sample was allowed to oscillate in a linear trajectory between the two respective magnetrons. The sputtering was initiated when the sample reached the position below each magnetron. At that point, the sample holder was programmed to oscillate with an amplitude of 1 mm to increase the homogeneity of the deposition. The holder reached an acceleration of 100 mm s−2 and a maximum linear velocity of 50 mm s−1. The RF power was chosen as 225 W for Co and 50 W for Ir. The alternating sputtering process was performed for 500 cycles in all series, modifying the deposition time to achieve three different element ratios as seen in Table 1 . The average deposition time was 4–5 h. Before the measurements, a calibration of the sputtering process was performed where Ir was sputtered continuously for 20 min on a substrate. The final Ir loading was measured by mass gravimetry, and the thickness homogeneity was verified using a profilometer (Alpha Step D-600, KLA). Assuming a linear dependency of loading with the sputtering time, three series were produced with a nominal Ir loading of 0.250 mg cm2 and different Co:Ir deposition time ratios. The resulting Co:Ir ratios were determined experimentally by EDX on the as-prepared samples (Table 1).As part of the Ir–Co catalyst film preparation, the samples were leached after the deposition in 1 M HClO4 to create a nanoporous self-supported Ir structure by selectively dissolving the Co under potential-controlled conditions according to the method developed by Sievers et al. [11]. The individual steps of the acid leaching procedure are summarized in Table 2 and described more in detail within the Result and Discussion section. Once the samples were leached, they were cleaned in distilled water and left to dry in air before further manipulation.The GDE was prepared using a Nafion membrane (Nafion 117, 183 μm thick, Fuel Cell Store) hot pressed to the sputtered gas diffusion layer (GDL). In this study, the Nafion membrane was activated as described by Schröder et al. [25]. A concentric circular steel punch (BOEHM, Germany) was used to cut small disks from the GDL and the assembly material. First, a disk of Ø 3 mm was cut from the sputtered GDL. Using an in-house built hot press (Fig. S1) with a modified soldering iron and 6 kg steel weights, a Ø 10 mm Nafion membrane was hot pressed on top of the catalyst layer at 120 °C using 84 kg cm−2 for 30 s.As indicated in Fig. 1, a Ø 20 mm Gas Diffusion Layer (GDL) without a microporous layer (MPL) was placed directly over the flow field of the stainless-steel bottom cell. On top, a Ø 20 mm Teflon disk with a Ø 3 mm center hole was used as a sealant for the liquid and electrical insulator. Embedded inside, a Ø 3 mm PTL disk was positioned to allow the gas flow to contact the GDE on top and to serve as a current collector. Last, a Teflon upper cell was pressed against the assembly and secured tightly with a metal clamp. Both the Teflon upper cells and the CE and RE were cleaned before every use according to the following protocol. First, they were placed overnight in a tank with concentrated HNO3 and concentrated H2SO4 solution 1:1 in volume. Afterwards, they were rinsed and boiled in distilled water for 1 h in at least 5 cycles. The unused materials were kept in a glass vial and boiled always one last time before use. Furthermore, the Pt wire was flame annealed every time it was used to remove any organic contaminations. After every trial, all the assembly components were discarded and replaced with new ones to decrease the influence of contaminations.All the experiments were conducted with a Potentiostat (ECi-211, Nordic Electrochemistry ApS, Denmark). The Potentiostat also controlled the gas switching between humidified Ar and O2 during the experiments. An overview of the experimental protocol is presented below in Table 2.The GDE half-cell (Fig. 1) and a glass bubbler were placed inside an insulating glass chamber during the measurements (See Fig. S1 in SI). Precise temperature control (±0.1 °C) was achieved through a constant flow of distilled water recirculated in between the double glass walls with a water heating system (Lauda RC6 SC). The GDE half-cell was placed in the middle of the chamber, supported on an aluminium laboratory jack (Laborboy, Sigma Aldrich) and insulated with a PTFE plate in between. Before the start of the measurements, the system was allowed to equilibrate at a constant temperature for at least 30 min. All the temperature references correspond to the set point defined in the water heating system. To prevent any shifts in reference potential due to contaminations on the RHE electrode, the RE was protected in a glass frit manufactured by an in-house technical glassblower. In addition, the RHE electrode was calibrated before each measurement in a separate GDE cell against a Pt GDE with the same molarity and electrolyte as the testing GDE cell, i.e., 1 M HClO4 electrolyte. The H2 gas was supplied through an in-house electrolyzer, connected to the gas flow through lines of the GDE cell. The RHE offset was measured by cyclic voltammetry in a potential interval between −0.005 and 0.005 V at 100 mV s−1 for 200 cycles. The acceptable range for initial RHE values was defined as ± 0.003 VRHE. In case of a larger deviation, the RHE was remade, and the calibration procedure was repeated to avoid large iR-correction errors. Before the measurements, Ar was purged through the flow field as a conditioning step and cyclic voltammograms were recorded at a scan rate of 100 mV s−1 in a potential range between 0.025 and 1.2 VRHE until a stable cyclic voltammogram could be observed (ca. 30 cycles). The ECSA of the catalyst (Table 3 ) was determined by integrating the Hupd area in the potential window of 0.025–0.25 VRHE of the last CV acquired using a fixed conversion coefficient of 176 μC cm−2 [11] according to the following formula: (1) E C S A [ m 2 g − 1 ] = Q H u p d L I r × 176 μ C c m − 2 The OER activity was determined through a galvanostatic step protocol with increasing currents based on Schröder et al. [25] and scaled accordingly to account for the loading difference. An AC signal (5 kHz, 5 mV) was applied during the current steps to obtain an online resistance measurement between the working and reference electrode (∼10 Ω) which was used for an iR-correction of the measured potential values.The morphology of the unleached catalyst layers, i.e., after the deposition process, was characterized using secondary electron imaging (SEM), see Fig. 2 . As seen in Fig. 2a, b and c, all catalyst series featured a similarly packed globular structure. Similar morphologies have been previously observed in studies of catalyst films prepared on carbon paper substrates using comparable process conditions [33]. The size of the globular features was not substantially different between the respective series, ranging from 0.1 to 0.9 μm in diameter. However, the SEM micrographs indicate further development of nanoporous structures. That is, the surface of the globules exhibits a certain degree of roughness, which is especially distinct for the Co-rich series (Ir28Co72; in the following the notation refers to the elemental composition obtained by EDX point analysis before the acid leaching), see Fig. 2a as well Fig. S2b for a closer look. Finally, yet importantly, EDX top-down mapping of different representative areas on the catalyst films, see Fig. S3, revealed that in all cases Ir and Co were homogeneously distributed across the film. As mentioned before in the Method section, Co was removed from the sputtered films in a process that is referred to as acid leaching. As the Pourbaix diagrams show for the respective catalyst film constituents, metallic Ir is stable under the leaching conditions while Co is oxidized to soluble Co2+ ions and does not form a passive film [45,46]. Hence, Co dissolution starts spontaneously when a sputtered sample is submerged in a de-aerated 1 M HClO4 aqueous solution [11], giving the solution had a characteristic pink tone. The color of this solution has been described extensively as a result of the complexation of Co2+ complex in water to form [Co(H2O)6]2+. To confirm this, Cl− ions were added to the solution from concentrated HCl and the temperature was raised. Both effects shift the equilibrium to [CoCl4]2- as a direct consequence of Le Chatelier principle [47], which shows a distinct blue color, see Fig. S4. To attain better control of the acid leaching process and to minimize Ir oxidation before ECSA determination of the metallic surface, the samples were submitted to an electrochemical cycling protocol (Table 2) between 0.05 VRHE and 0.5 VRHE with a scan rate of 100 mV s−1 starting directly after the electrolyte was added to the upper cell compartment. The cycling continued until a stable CV was achieved. This was typically the case after 30 potential cycles. Along this process, the initial Co to Ir ratios were changed significantly. In every case, the relative amount of Co decreased to under 10% in weight according to the EDX. Using XPS for a more surface-sensitive analysis of the pre-leached and leached samples, see Fig. S5, we observed a trend in the decrease in the Co:Ir ratios after leaching following the series, albeit not proportional to the initial ratios (see Fig. S6). This discrepancy could perhaps be attributed to the drastic change in morphological differences and chemical gradients to form a more stable Ir shell with a Co core after the acid leaching [11,47,48]. The process of acid leaching has been well described for Pt-based alloys for the oxygen reduction reaction. It has been shown by low energy ion scattering (LEIS) that the exposition of PtM (M = Fe, Co, Ni, etc.) surfaces automatically leads to a full depletion of all non-noble atoms from the surface and the formation of “skeleton” or core shell surfaces [48]. In the here reported work, sparse colonies of Ir-rich dendritical structures were formed of the GDL carbon substrate, which was also left exposed over large areas. The development of this porous structure differs substantially from the preparations on glassy carbon in a former study [11], see Fig. S2. The reason for this difference might be the three-dimensional structure of the gas diffusion electrode or the hydrophobicity. The initially Co-rich sample, Ir28Co72 presents the biggest size of the dendrites and area of the exposed substrate. Both features appeared to decrease together with the Co:Ir ratio when comparing Ir28Co72 with the Ir45Co55 and Ir75Co25 series (Fig. 2d, e and f respectively). SeriesBy element wt.% wt.% norm. Unleached Leached Ir Co Ir Co Ir28Co72 27.7 ± 1.9 72.3 ± 1.9 95.0 ± 0.8 5.0 ± 0.8 Ir45Co55 44.9 ± 0.7 55.1 ± 0.7 96.2 ± 1.5 3.8 ± 1.5 Ir75Co25 75.3 ± 5.1 24.7 ± 5.1 91.9 ± 0.8 8.1 ± 0.8 SeriesBy element at.% at.% norm. Unleached Leached Ir Co Ir Co Ir28Co72 10.5 ± 0.9 89.5 ± 0.9 85.4 ± 2.1 14.6 ± 2.1 Ir45Co55 19.8 ± 0.7 80.2 ± 0.7 88.6 ± 4.1 11.4 ± 4.1 Ir75Co25 48.8 ± 6.7 51.3 ± 6.7 77.8 ± 1.9 22.2 ± 1.9 The element distribution of representative leached areas can be found in the EDX mapping of Fig. S3 of the SI. An as-sputtered XRD analysis indicated that the elements are found in a heterogeneous film with a low degree of crystallinity, as it is normal for sputtered catalysts that do not experience a heat treatment [26,49,50]. While the overall structure remains amorphous, the shift to lower theta values and narrowing of the Ir (111) Bragg peak after leaching, see Fig. S7, suggests that it might experience a slight increase in crystallinity, which has also been reported in similar studies [11,34,51]. Since the first studies on AuAg nanoporous structure formations via selective leaching, several studies have emerged to explain the behaviour of homogeneous bimetallic alloys [52–57] as well as the change in electronic properties due to the formation of core-shell nanoparticles. However, a former study using a similar magnetron-sputtering and acid-leaching process to create a self-supported Pt–CoO network revealed that no alloy was formed in the bimetallic deposition or leaching process [35]. A further look into the oxidation state and the small range structures of the Ir–Co series was conducted by ex-situ X-ray Absorption Spectroscopy (XAS) of the leached samples, see Fig. 3 and Figs. S8–10 of the SI. Data were collected at both, the Co and Ir edge, however, due to the low Co content the data quality is significantly lower for the Co edge than for the Ir edge. Therefore, we draw our conclusions mainly from the data obtained from the Ir LIII K-edge. The X-ray absorption fine structure (EXAFS) results reveal mixed metallic and oxide structures, see Fig. 3. The presence of Co–Co1 and Ir–Ir1 coordination indicates that a proportion of Co and Ir remains metallic after acid leaching and exposure to air. Furthermore, the presence of Ir–Co1 coordination shows a partial alloy character with a similar trend as observed in the Co content of the leached samples by EDX: Ir75Co25 > Ir28Co72 > Ir45Co55. In addition, Ir–O1 and Co–O1 coordination is seen indicating partial oxidation of the samples. Interestingly, the data from all series indicate a similar Ir–O1 bond length, indicating that the Co content has no measurable effect on lattice strain.Considering the mixed chemical nature of the material, we describe it as IrxCo1-x nanoclusters rather than an IrCo alloy. In this context, the self-supported structure is achieved by the dissolution of a sacrificial templating metal in a selective acid leaching process under potential conditions, coupled with surface restructuring processes in the material due to diffusive forces. In an earlier study from the same authors concerning the leaching behaviour of co-sputtered noble and non-noble metals in a Pt–Cu system, a mechanism of acid leaching process leading to self-supported nanostructured catalysts was already discussed [34]. As the non-noble metal dissolves in acid, hydrogen gas evolution starts spontaneously. Some of the gas can be trapped in interior cavities and mechanically push the material around to nucleate pores. At the same time, the catalyst-rich areas undergo a surface diffusion process due to the electrochemical and mechanical forces, which promote the redeposition of catalysts in neighbouring regions. The structures created in such a process depend on the irregularities of the morphology and porosity at the surface. Surface diffusion of catalyst particles is evidenced by an Ir enrichment and depletion of Co over the surface observed in the EDX maps (Fig. S3) and reinforced by the XPS results (Fig. S6). This process would be in agreement with the different morphologies observed in the series between the as-deposited and leached state for the different EDX Co:Ir ratios and the different initial distributions of Ir and Co-rich areas. A previous study of a very similar Ir–Co catalyst already demonstrated that Co dissolves from all areas in contact with the acid solution leaving a percolated Ir network with the same domain size as the initial deposition [11], which corresponds well with the results presented here.After sputtering and acid leaching, each series of the catalyst layers was assembled into the GDE setup for electrochemical testing. The aim of the electrochemical testing was twofold: first, the electrochemically active surface area (ECSA) of the leached Ir was determined. This was achieved by determining the Hupd area in cyclic voltammetry [58]. The leaching conditions were designed to dissolve the Co while preserving Ir in metallic state, as IrO2 does not display any Hupd area. We assume that after leaching any oxidized Ir surface would be reduced and that there is a direct relationship between metallic Ir surface before activation and ECSA after activation. The second aim was to activate the catalyst layer and determine its activity for the OER.The electrochemical characterization is exemplified in Fig. 4 a which depicts the CV and OER activity of a leached Ir-rich (Ir28Co72) nanostructured IrCo film. It is seen that after leaching, the CV displays a pronounced Hupd area indicative of metallic Ir, allowing a straight-forward ECSA determination of 52.6 ± 4.8 m2g-1, Fig. 4a. After recording the CV, the gas was switched, and oxygen was flushed through the cell at 1 sccm for 20 min to guarantee a saturated oxygen atmosphere. The OER activity was determined before and after activation and benchmarked to published data from a commercial IrO2 black powder (Alfa Aesar) [25]. It is worth mentioning that the commercial sample was prepared with a different loading (1 mg cm−2) than the samples in this study. However, it is still considered to be a useful reference since the OER activities were measured using the same protocol and setup configuration. The first set of OER activities revealed that the catalyst surface was not yet completely activated into IrOx. Yet, the OER overpotential in this state was around 40 mV lower as compared to the benchmark. Recording another set of CVs in Ar atmosphere after the first OER measurements confirmed that remainders of metallic Ir were present from a decreased but still discernible Hupd area. In addition to the reduced Hupd area, the double-layer capacity was increased (Fig. S11b). To complete the oxidation of the metallic Ir, a potentiostatic activation step was applied at 1.70 VRHE for 20 min in O2 atmosphere (Fig. S12), after which a second set of OER activities was recorded. The fact that the overpotential was reduced by an additional 10 mV as compared to before activation indicates the further formation of the active IrO2 phase. Nevertheless, recording another set of CVs in Ar atmosphere in step 2.3 of the protocol (Table 2) shows that a complete, irreversible oxidation of the surface has not yet been achieved and still some Hupd area is visible (Fig. S11c). Despite the incomplete activation, the OER mass activity at 1.50 VRHE was 101.5 Ag–1 Ir, roughly eight times higher than that of the commercial benchmark catalyst Table 3). In the third and last step of the protocol the temperature was increased from 30 to 60 °C and OER activity was determined for one last time (Fig. 4c). The raise in temperature leads to a clear decrease in overpotential of ca. 40 mV even at the lowest (20 Ag–1 Ir) current densities (Fig. S13). At this temperature, an OER activity of 117.8 Ag–1 Ir was determined at 1.46 VRHE. Interestingly, in addition to a temperature-induced kinetic activation, the raise in temperature leads to an additional activation via further oxidation. This can be seen by the fact that the relative OER mass activity increases to a threefold value. In comparison, the benchmark catalyst is mostly oxidized in its initial state. A further description of the contributions to the decreased overpotential due to temperature and activation contributions can be found in the SI. In addition to the activation, it is seen that with the temperature rise the Tafel slope decreased slightly from 62 to 49 mV dec−1. This is a small, but still, significant change, which can be explained by the temperature dependency of each rate constant for every step of the reaction according to Arrhenius’ equation [59]. The electrochemical response of the Ir45Co55 series presented in Fig. 4d–f shows a similar development as the former discussed Co-rich series. From the initial CVs after acid leaching the measured Hupd region after acid leaching was determined to be 33.7 ± 1.9 m2g-1. A possible explanation for the decreased surface area could be a smaller size of the features formed after the Co leaching (Fig. 2e) and less internal porosity. Since the measured surface area at the initial step (Fig. S11a) was only around half of the Co-rich series (Fig. 2a), the oxide formation after the activation in Step 2.3 (Table 2) also rendered a smaller oxide capacitive layer (Fig. S11d). However, at this point the Ir45Co55 samples still presented a comparable Hupd area to the Ir28Co72 series, indicating that the sample was not completely oxidized. Nevertheless, the overpotential still decreased by about 10 mV after activation as in the case of the Co-rich samples due to the oxidation of the metallic surface (Fig. 4e). As a result, the activity measured after activation at 30 °C and 1.50 VRHE increased to 57.6 Ag–1 Ir, four times greater than the value of the commercial sample (Table 3). Still, it was approximately two times lower than the Ir-rich (Ir28Co72) samples under the same conditions. OER activity measured at 60 °C was nearly doubled from the previous step, i.e., 91.8 Ag–1 Ir at 1.46 VRHE. The sharp activity increase reinforces the hypothesis that the samples only experience full activation during the protocol at high temperatures. Even at the start of this step the overpotential already decreased by 45 mV compared to the activity recorded after activation (Fig. S13). Since the reversible reduction in the overpotential due to temperature increase is approximately 25 mV, the further decrease supports the argument of a dynamic activation process. Additionally, the Tafel slope also decreased from 64 to 52 mV dec−1 between the activation and high-temperature OER respectively. The Ir-rich Ir75Co25 series exhibited the lowest values for the surface area and activity throughout the OER measurements. After acid leaching, the ECSA was determined to be 21.4 ± 2.0 m2g-1, see Table 3. This is in good agreement with the observed top-down morphology from the leached sample at the SEM (Fig. 2f), which featured the smallest clusters in all three series. The reduction in overpotential after activation was also minimal, i.e., ca. 3 mV (Fig. S13). The OER activity after activation at 30 °C was 44.3 Ag–1 Ir at 1.50 VRHE, which is approximately 30 mV lower than that of the commercial benchmark under the same conditions. As also observed for the other IrCo series, the mass activity improved at 60 °C, reaching 62.7 Ag–1 Ir at 1.46 VRHE as compared to the 10.14 Ag–1 Ir of the commercial benchmark. Interestingly, even though the mass activity results were at the lowest of the series in absolute numbers, a similar reduction in overpotential at high temperatures was observed as compared to Ir45Co55. (Fig. S13). The Tafel slope was the highest of the series and only decreased from 68 to 58 mV dec−1 between activation and high-temperature OER respectively, which was the smallest change in all series (see Table 3). Along the series, the ECSA, the Tafel and the mass activity followed the trend defined by the initial as-deposited Co content Ir28Co72 > Ir45Co55 > Ir75Co25. When combined with the catalyst morphology, this trend strongly suggests that a high initial Co content increases the catalyst utilization by increasing the ECSA in a dynamic process as the catalyst is activated. On the other hand, the specific activity was found to correlate with the XAS results and the Co content after leaching from the EDX results, which hints at a positive influence from the remaining Co in the structure. A summary of the main electrochemical results can be found in Table 3 below.Some additional factors need to be considered together with the electrochemical results. As mentioned in the methods section, the deposition time for the magnetron targets was defined between 1 s and 6 s for Co and kept constant at 3 s for Ir in each cycle. The Ir loading calibration was performed in a continuous deposition of 1200 s. In a preliminary test, it was confirmed for Ir that the loading for the continuous deposition matched the loading for the cycled deposition by mass gravimetry. However, when measuring the expected ratios by EDX they were found to be different from the nominal. While EDX is a versatile tool to determine the spatial resolution of the thin catalyst layer and the element distribution on the substrate, it is known that absolute quantification using automatic standardless EDX profiles is generally poor [60]. We found that using 15 kV for the analysis was a compromise between good surface sensitivity and exciting the higher energy lines for better elemental analysis (Co Kα = 6.924 keV, Ir Lα = 9.147 keV) to maximize the number of counts. However, in the acid-leaching process, the catalyst loading is further reduced which leads to larger errors in the elemental quantification. Hence, we assumed the initial loading was unchanged for the electrochemical mass activity results, while it is likely that both the surface area and the mass activity might be larger than what was measured. The quantification of the changes in the Ir loading during electrochemical measurements is not trivial. Unlike other PGM catalysts (Pt, Pd), iridium is known to fully dissolve only in extremely aggressive conditions requiring high temperatures, pressures, and strong acids [61,62]. Therefore, the preparation of the samples for conventional ex-situ techniques such as Inductively Coupled Plasma Mass Spectrometry (ICP-MS) that relies on the analysis of the dissolved species is non-standard and complex. However, in recent years, some approaches have been taken to quantify the Ir loading or loss during the electrochemical measurements. One of the most relevant methods is the Scanning Flow Cell (SFC) coupled with an ICP-MS system, which allows to perform time-resolved measurements of the material loss during an electrochemical protocol. Unfortunately, it also does not provide information about the remaining catalyst in the deposited layer [63]. Furthermore, there is not yet a compatible design to combine the high-current capabilities of the GDE method with the access to analytics of the SFC ICP-MS. Additionally, most techniques have been optimized so far for the study of supported catalysts with Ir nanoparticles which are known to present higher degradation rates compared to self-supported catalysts [11,34,35]. Since the purpose of this study was to assess the performance of different Ir-based catalysts under the same conditions and using a comparative approach, a quantitative study of the Ir loading loss or the formation of transient species was not performed. In addition, speculations about specific activity changes in correlation to XAS data were made with data measured at 30 °C in combination with the ECSA measurement in metallic state by Hupd. However, the increase in the double layer capacity of the CVs due to the oxidation to IrOx after the OER at higher temperatures (see Fig. 2f) would have resulted in different surface areas and thus different specific activities. Therefore the specific activity reported at 60 °C has to be taken with caution. Other in-situ methods such as the mercury underpotential deposition could have also been considered [64]. However, this was not possible, as the membrane would need to be removed to avoid poisoning, impeding further electrochemistry. For the same reason, most material characterization methods in this study have been limited to the after-leaching state. Further insight into the dynamic catalyst activation at high temperatures and its link to the morphology may be achieved with in-operando XAS methods as soon as they are developed. Nevertheless, these limitations were considered as boundary conditions to help the discussion and understanding of our results.In this study, we applied the GDE method to perform activity measurements of PVD-produced catalysts for the OER. First, three series of Ir–Co catalysts with equal 250 μg/cm2 Ir loading were sputtered on carbon substrate using different Co:Ir weight ratios (Ir28Co72, Ir45Co55 Ir75Co25). To create a self-supported nanoporous structure with increased ECSA, Co was removed in an acid-leaching step. This is rendering a distinct dendritical surface morphology with Ir-rich clusters and slight changes in crystallinity. During the process, a mixed metallic and oxide structure with local Ir–Co coordination is formed. A higher initial Co content leads to larger surface areas after leaching, outperforming the OER activity of a commercial IrOx catalyst benchmarked at 30 °C and 60 °C. Overall, the performance followed the Co:Ir series Ir28Co72 > Ir45Co55 > Ir75Co25 > IrOx, where the best-performing catalyst at 60 °C reached more than a tenth-fold increase in mass activity over the commercial sample. The performance increase as compared to the benchmark catalyst, accounting for loading and preparation differences, can be due to higher dispersion in addition to a ligand effect. The latter is supported by the specific activity trend correlation with the remaining Co after acid leaching and XAS coordination data. A strain effect, by comparison, was not supported by the XAS data. The temperature increase and dynamic surface activation due to oxidation of metallic Ir, both observed by CV and the OER activity, had a positive influence on the catalyst activity. The authors acknowledge that the complex mechanisms behind the influence of the Co content and the electrochemical performance may not be fully explained from the measurement results, but also remain beyond the scope of this study. On the other hand, it was demonstrated that the flexible and reproducible characteristics achievable from the nanostructured PVD-produced catalysts in combination with the three-electrode GDE setup can reveal further insights into the electrode evolution under more realistic conditions than traditional methods such as RDE, helping to fast-track OER catalyst experimental research. Pablo Collantes Jiménez: Methodology, Investigation, Writing – original draft. Gustav Sievers: Writing – review & editing, Supervision, Conceptualization. Antje Quade: Investigation, Methodology. Volker Brüser: Supervision, Methodology. Rebecca Katharina Pittkowski: Investigation, Methodology. Matthias Arenz: Writing – review & editing, Supervision, Conceptualization, All authors checked and approved the final version of the manuscript.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Gustav Sievers has patent #DE102016013185B4.The authors gratefully acknowledge the financial support by the German Federal Ministry of Education and Research (BMBF) in the framework of the VIP + Projekt. 03VP06451 (3DNanoMe). The authors thank Adam Clark from the SuperXAS beamline X10DA at the Paul Scherrer Institute (PSI) for measuring the XAS data via mail-in service. MA and RKP acknowledge funding from the Swiss National Science Foundation (SNSF) via project No. 200021 184742 and the Danish National Research Foundation Center for High Entropy Alloys Catalysis (CHEAC) DNRF-149.The following are the Supplementary data to this article. Fig. S1 Fig. S1 Fig. S2 Fig. S2 Fig. S3 Fig. S3 Fig. S4 Fig. S4 Fig. S5 Fig. S5 Fig. S6 Fig. S6 Fig. S7 Fig. S7 Fig. S8 Fig. S8 Fig.S9 Fig.S9 Fig. S10 Fig. S10 Fig. S11 Fig. S11 Fig. S12 Fig. S12 Fig. S13 Fig. S13 Supplementary data to this article can be found online at https://doi.org/10.1016/j.jpowsour.2023.232990.

The scarce supply of Ir used to catalyze the sluggish oxygen evolution reaction in acidic water electrolysis calls for unconventional approaches to design more active catalysts with minimal resource usage for their commercial scaling. Industrial-ready production methods and laboratory scale tests that can reflect the catalyst behaviour realistically need to be included in this process. In this work, we benchmarked three series of self-supported Ir–Co catalysts with low Ir loading produced by physical vapour deposition under relevant current densities in a gas diffusion electrode setup. It was seen that after selective acid leaching of the Co, a nanoporous structure with a high electrochemically active surface area and a mixed oxide and metallic character was formed. Depending on the initial Co:Ir deposition ratio over ten times higher oxygen evolution mass activities could be reached as compared to a commercial, unsupported IrOx nanoparticle catalyst used as a benchmark in the same setup configuration. The presented integrative catalyst design and testing strategy will help to facilitate bridging the gap between research and application for the early introduction of next-generation catalysts for water splitting.

In many cases, the aqueous streams caused by pharmaceutically contain organic pollutants such as caffeine (CAF), which is the most commonly used legal drug throughout the world in the form (beverages or combined) [1, 2], and it is toxic and poorly biodegradable. These polluting agents are also in very high concentrations, so that releasing these molecules into the water resources, affects aquatic life and ecosystems beings, adversely [3]. In these cases, it is necessary to use less conventional techniques to remove the pollutants and convert persistent chemicals into environmentally benign compounds, such as advanced oxidation processes (AOPs) like Fenton process, electro catalytic oxidation, photocatalytic oxidation and catalytic wet peroxide oxidation [4, 5, 6, 7, 8]. The oxidation process with H2O2 using a heterogeneous catalyst is commonly known as catalytic wet peroxide oxidation (CWPO). CWPO is one of the promising methods for the rate of pollutant degradation at mild temperature and pressure conditions [9, 10, 11, 12], providing that, a suitable catalytic system is used, such as catalysts based on natural and pillared clays (PILCs). These materials are porous, developed by molecular design methods, prepared by exchanging the cations located in the interlayer space of clays with large inorganic polyoxo/hydroxo cations [13, 14, 15].Clay minerals, a large family of aluminosilicates (Si4+ and Al3+) structures with a variety of chemical composition, structure and surface properties, very reactive materials due to their small particle size, high surface area and adsorption properties [16, 17]. Bentonite, with a layer structure containing a larger amount of mesoporous, has been widely used in the catalysis field. It constitutes an abundant mineral resource and an effective catalyst support because its strong metal support interaction [18, 19, 20]. Their quality depends on several parameters such as color and swelling behavior which are influenced by the crystal chemistry and mineralogical composition [21]. Bentonites consist mainly of montmorillonite, which is a dioctahedral clay of the smectite group with the 2:1 layer linkage [22, 23]. As the previous research demonstrated, most studies focus on using cheaper transition active metals, such as Ni or Cu. Both metals are widely used as catalysts for a variety of processes, their wide usage can be attributed to their several characteristics such as being very strong catalytic, optical, electrical, mechanical and antifungal/antibacterial. Moreover, Copper (Cu) has been used in catalyst based on the activated carbon (AC), nano-zerovalent copper (nZVCu) functionalized hydroxyapatite (HA), alginate [24, 25, 26] and on the perovskite (LaNiO3, NaNi0.9Cu0.1O3 and LaNi0.5Cu0.5O3) [27,28]. Thus, the objective of the present study is to evaluate the potential use of Moroccan yellow clay as a superb natural support of Copper/Nickel catalysts, in order to enhance its catalytic activity using simple impregnation method for degradation of organic pollutants in aqueous solution [29, 30, 31].The Response Surface Methodology (RSM) was widely used in many researches for the optimization of different liquid effluent treatment processes. In fact, RSM is a statistical technique applied to reduce the number of experiments, to optimize and analyze the experimental independent parameters that affect the process' efficiency, and to generate a mathematical model, which describes the processes' behavior. Central composite, Doehlert, and Box–Behnken are three classes of response surface designs. Yet, Box–Behnken design is more advantageous because it creates an experimental design with a few test runs, which makes the experiments economically feasible and beneficial [32, 33]. As far as we know, and according to the extensive literature review, CWPO of CAF onto CuNi-YC was not fully investigated. Therefore, the objective of this work is to evaluate the yellow clay extracted from the North of Morocco, exhibit its characterization as a natural eco-friendly and low-cost material and highlight its availability and usefulness when it will be modified using Nickel and Copper by impregnation method. Thus, work will allow the determination of its physicochemical properties and then the identification of its field of use as a catalyst for CWPO, which has never been studied before. In order to examine its effectiveness in oxidation after its modification, the caffeine (CAF) molecule was used in this study as a persistent and hardly removable pollutant to be removed from aqueous solution in a batch reactor. Furthermore, the optimization of degradation's efficiency is an interesting study; hence, a response surface methodology based on Box–Behnken Design (BBD) was used with a three-level factorial design, to optimize the effects of three significant factors: impregnated copper (%), temperature (20–60 °C) and H2O2 dosage (8.2∗10−2 – 24.6∗10−2 mol.L−1) which influence the CWPO process.The following chemicals were used in the catalysts’ preparation (CuNi-YC samples): Copper (II) Nitrate Hexahydrate (Cu (NO3)2.6H2O, Sigma-Aldrich, 99.99% purity), Nickel (II) Nitrate Hexahydrate (Ni (NO3)2.6H2O, Sigma-Aldrich, 99.99% purity), Hydrochloric Acid (37%, w/w), Sodium Hydroxide 97% (NaOH) and Hydrogen Peroxide (30%, w/w) Sigma-Aldrich. The Caffeine (C8H10N4O2, Sigma-Aldrich) were used as molecule models for the degradation by catalytic wet peroxide oxidation (CWPO) tests. All chemical materials were used without further purification. The deionized water has been used throughout the experiments.The catalysts support is referred to as yellow Clay (YC). The one used in this work has been taken from a natural basin of the Tidiennit massif in the North of Morocco. Fraction up to 63 μm. Cu–Ni samples were synthesized by the wet impregnation method where Cu(NO3)2•6H2O, and Ni(NO3)2•6H2O were mixed to obtain the following Cu:Ni weight ratios: 1:0, 1:1 and 0:1 and the obtained catalysts were denoted as CuNi10-YC, CuNi11-YC and CuNi01-YC, respectively. Every solution should contain as much as metal nitrate to get 10 wt% metal in the final catalyst powder. In this process, 10 wt% metal was dissolved in 50 mL of deionized water. Then, the YC was dropped into this aqueous solution with stirring speed of 200 rpm at 75 °C for 4 h, to obtain a slurry. After impregnation, the slurry was dried at 100 °C overnight, and then calcined at 500 °C for 4 h.In order to evaluate CAF mineralization using the catalysts (CuNi10-YC, CuNi11-YC and CuNi01-YC), an amount of 1 g.L−1 of each catalyst was added to 100 mL CAF solutions with a concentration of 40 mg.L−1; then, stirred to maintain a uniform suspension. Before starting the CWPO reaction, the adsorption of CAF by the catalysts was performed until reaching the equilibrium, which happen to be at 15 min. This step is a controlling experiment to compare between adsorption of CAF and the CWPO conversion; on the other hand, to insure that the decrease of CAF concentration is attributed to CWPO conversion; then, CWPO reactions were started for each catalysts by adding H2O2 to the solutions respecting Table 1 . After 120 min of reaction, the solution was centrifuged to remove particles; then, analyzed using UV–vis spectrophotometer VWR UV-6300PC at λmax of 272 nm. The degradation efficiency of CAF was evaluated in Eq. (1); Where C0 and Ct are CAF concentrations (mg.L−1) at the time of withdrawal [34, 35, 36, 37, 38]. (1) CAF Conversion  ( % ) = [ C 0 − C t C 0 ] × 100 The reaction of H2O2 with the CAF was also carried out for comparison. The total organic carbon (TOC) analyses were determined using an analyzer (TOC-VCSN, Shimadzu) at the end of each reaction to investigate the total mineralization of the CAF in the solutions. The TOC values after 2 h of CWPO reaction were calculated using Eq. (2) [4]. Both, TOC measurements and analytical determinations of CAF concentrations were performed at least twice in order to ensure reproducibility of the measurements. (2) TOC ( % ) = [ TOC i − TOC f TOC i ] × 100 The adsorption of Methylene blue dye on raw clay, CuNi10-YC, CuNi11-YC and CuNi01-YC was performed and found to be following Langmuir adsorption isotherm with a monolayers capacity of 30.40 mg.g−1, 69.12 mg.g−1, 63.54 mg.g−1, 58.06 mg.g−1 respectively. As methylene blue was reported to have flatwise adsorption from water with effective area per molecule on the surface of 130 Å2 [39,40]. Therefore, Langmuir specific surface area has been calculated instead of BET using N2, because it reflects a better interpretation of the effective surface area when the adsorption of CAF is carried out from water solution. The following equation was used (3), Where X is the monolayers capacity mentioned above for each catalyst in moles per gram; N is Avagadro number (6.019∗1023 mol−1) and A is the area of methylene blue molecule. (3) Specific surface area  ( SSA m 2 . g − 1 ) = X m . N . A Box-Behnken Design (BBD) was used for the experimental design of the CAF degradation using CuNi-YC, in order to investigate the effect of the main parameters: [H2O2], catalyst, and temperature with X1, X2 and X3 are the studied coded variables which are calculated by Eq. (4). The independent factors were studied at three different levels, low (−1), medium (0), and high (+1). The predicted response (Y) fitted by second-order polynomial equation is the most commonly used (5), where Y is the measured response, β0 is the intercept parameter; βi, βii, and βij represent the linear effects, the quadratic effects, and the interaction effects, respectively. Xi and Xj are the studied factors. K is the number of the optimized factors and ε is the random error. Hence, NemrodW software was used to process and design the experiments data Table 1 [32, 33]. (4) x i = X i − X i , 0 Δ X i ( i = 1,2,3 ) (5) Y = β 0 + ∑ i = 1 k β i X i + ∑ i = 1 k − 1 ∑ j = 2 k β i j X i X j + ∑ i = 1 k β i i X i 2 + ε The analysis test of variance (ANOVA) was applied to evaluate the results, where the determination coefficient (R2 and adjusted R2) and p-value (probability) (p < 0.05), are the main parameters used to evaluate the effectiveness, the statistical significance and the prediction capability of the model [41, 42].Plasma-Atomic Emission Spectrometry (ICP-AES) was inductively used to test the Cu and Ni contents in the prepared catalysts using a FR-T-RR-01, CURI. The phase and crystallinity of the catalysts and YC were identified by X-ray diffraction (XRD) using X'Pert Pro PANalytical diffractometer equipped with a detector operating Cu Kα radiation (λ = 1.540598 Å; 40 kV and 30 mA). The X-ray fluorescence (XRF) was used to explore the chemical composition of raw YC. The catalysts' morphology was illustrated by a scanning electron microscopy (SEM) using QUANTA 200 FEI instrument at 30 kV. BET surface area was carried out by N2-adsorption at 77 K using a Micromeritics ASAP 2010 instrument.The powder X-ray diffraction patterns of the support and synthetized catalysts are shown in Figure 1 . In the diffractogram of raw YC Figure 1a, the peaks at 2θ = 19.9°, 35.0°, 61.84° which corresponded to the d101, d107, d060 represents the characteristic reflection of montmorillonite (JCPDS card NO. 29–1499). The peaks at 2θ = 20.9°, and 26.6° of quartz (JCPDS card NO. 46–1045) and dolomite are observed at 23.43° (JCPDS card NO. 36–0426). These findings indicate that the support was a typical bentonite [43]. Chemical composition of raw (YC) which is an important mineral resource of Moroccan was given in Table 2 , indicates that the predominant oxide is silica followed by alumina Al2O3 associated with the material phases. In addition, it may be concluded referring to the highly intensive d101 features of the sample that Al is highly available in the octahedral centers of YC [44]. The high Mg and Ca contents of the raw Yellow clay (Table 2) illustrate the significant amount of Mg2+ and Ca2+ contribution from dolomite to the framework Mg and interlayer Ca cations [45]. The diffraction peaks of CuO were detected in Figure 1b around 35.43°, 38.66°, 48.7° and 61.58° corresponded to the d002, d111, d-202 and d113 respectively (JCPDS card NO. 01-089-2530) [29,31], and the characteristic peaks of NiO were detected in Figure 1c and d at 2θ = 37.2°, 43.3° and 62.87, which ascribed to the plane d111, d200, d220 of the cubic phase NiO, respectively (JCPDS card NO. 47–1049) [46] for a mixture of copper and nickel oxide are seen at 62.8° with the d220 to the crystalline structure of (Cu–Ni)O Figure 1c. The crystallite size of CuO and NiO calculated using the Scherrer formula [8] was 11.47 and 12.57 nm respectively.Some physicochemical properties of the raw YC and the other three catalysts are reported in Table 3 . As it can be seen, the samples showed an increase in the nickel loading results and a decrease in specific area using both BET and Langmuir surface area methods, relative low surface area probably associated to the micro-pore filled intrinsic impregnation procedure used, due to the deposition of the Ni and Cu hydroxyl nitrate [47, 48, 49]. Which is in a good agreement with the experimental study given CWPO reaction, because the removal efficiency of CAF with adding H2O2 to the solutions was used by the oxidation not by adsorption. The final catalysts were also analyzed by ICP in order to determine the Cu and Ni contents in the prepared catalysts. The results show that the catalysts formed are CuNi10-YC, CuNi11-YC and CuNi01-YC with Cu–Ni 87.41–0.01; 45.05–43.4 and 0.01–85.38 mg.L−1 respectively. These results are in agreement with the expected stoichiometric ratio of Cu to Ni used Table 3. Figure 2 provides a SEM micrograph, which illustrates the morphology of YC, CuNi10-YC, CuNi11-YC and CuNi01-YC catalysts. Figure 2. a shows YC structure while it was being organized into aggregated patches of various agglomeration with different sizes. The morphological appearance of the following Cu:Ni weight ratios: 1:0, 1:1 and 0:1 catalysts is illustrated in Figure 2b, c and d respectively. The samples appear to have smaller agglomerates comparing to YC alone. The most probable cause of the observed changes in the morphological appearance of the catalysts may be due to the strong immobilized CuO and NiO nanoparticles on natural yellow clay support. These obtained results are already confirmed by XRD analysis and the same results was reported by Alakhras et al [50] when the Titania (TiO2) loaded zeolite material. However, the sizes of individual particles in the agglomerate cannot be clearly seen in these micrographs.We have adopted the response surface methodology through Box–Behnken design for investigating the statistic analyze and optimizing the impact of the three factors (H2O2 dosage, impregnated copper (%) and temperature) screened concerning the degradation of CAF using the local clay as a support. The number of experiments used in this study are 17, calculated using the following formula (6), where k = 3 is the number of the studied factors and C0 = 5 is the number of central points (numbers 13–17), the (%) degradation of CAF in this study was observed in the range of 40–86 % (Table 4 ) [51]. In addition, the regression model was observed in terms of the three factors which are expressed through the following second-order polynomial Eq. (7). (6) N = 2 × k × ( k − 1 ) + C 0 (7) Y  ( % ) = 69.8 − 2.375 X 1 + 16.125 X 2 + 3.75 X 3 − 11.025 X 1 2 − 2.525 X 2 2 − 1.775 X 3 2 − 1.25 X 1 X 2 + 0.0 X 1 X 3 + 0.5 X 2 X 3 The results of ANOVA used for checking the validation of this work's model are presented in Table 5 . Certainly, the p-value corresponding to caffeine conversion is an extremely low probability (p-value less than 0.05) indicating that the model is highly significant, [33]. In addition, the determinant regression coefficient (R = 0.990 and the adjusted regression coefficient (adjusted R = 0.976) for both responses are closer to 1. Furthermore, the larger the ratio and the lower the p-value are, the more significant the corresponding parameter will be in the regression model. Therefore, these results show that the models fit well and the experimental data could be well modeled for both responses [33, 41].Besides, the obtained p-value implies the importance of each factor in obtaining an efficient removal of CAF. Therefore, it can be seen in Table 6 that all the model's terms such as linear (x1, x2 and x3) quadratic (x1 2, x2 2 and x3 2) and interactive effects (x1 x2, x2 x3 and x1 x3) are statistically significant. Hence, the results show that the catalyst has the most significant effect on the caffeine conversion. Figure 3 displays the residuals plots versus the normal plot probability of the responses residuals, which show a random distribution. This confirms the adequacy of the models. Most points relatively follow the straight-line x = y. This indication is in accordance with the previous results obtained in Table 5. Figure 4 shows the 3D response surface plots and there matching 2D contour plots corresponding to the effect of the three parameters on the removal efficiency of CAF using 1 g.L−1 of catalyst.The results show that the 3D and 2D plots related to impregnated copper and temperature effects, at 0.082 mol.L−1 of H2O2. It is shown in Figure 4A and B that the removal efficiency of CAF increased along with the increase of reaction temperature at higher amount of impregnated copper. When the temperature's reaction reached 60 °C, the conversion of CAF efficiency was the highest (>80%), which is in a good correlation with the Arrhenius theory of the temperature's positive influence on the rate constant, by enhancing the feasibility of the degradation process. However, the removal efficiency decreases if the temperature continued to rise, because the temperature was too high making the faster decomposition of H2O2, and the utilization rate of H2O2 was greatly reduced according to Eq. (8) [51, 52]. (8) 2H2O2 → 2H2O + O2 The 3D and 2D plots related to H2O2 and catalyst copper content, at 20 °C temperature. It is shown in Figure 4C and D that the maximal conversion of CAF (>80%) was achieved at higher amount of impregnated copper and medium volume of H2O2. However, at the lowest impregnated copper and an excess of H2O2, the CAF conversion reached its minimum. In addition, it was also obvious that CuNi10-YC was the most efficient catalyst and has the active phase for hydrogen peroxide as well as CAF molecules: the catalyst has the activation sites for H2O2 and caffeine. However, when nickel was incorporated in clay matrix, it led to a decrease in the CAF conversion [29]. The results could explain that the presence of nickel reduces the Langmuir surface and leads to a distortion of the clay matrix. It was more important that although the TOC value decreased until 27% in the presence of nickel.CuNi01-YC catalyst was used in the following experiments. The effect of hydrogen peroxide's reaction and temperature treatment performance of CAF wastewater were investigated. The removal efficiency of CAF with different amount of peroxide dosage was illustrated in Figure 4E and F. Hydrogen peroxide has the same behaviour as temperature on the removal efficiency of CAF. In fact, the degradation rate reached the maximum value, when peroxide dosage was ≤0.164 mol.L−1. However, once the initial concentration of H2O2 is higher than 0.164 mol.L−1, the removal efficiency drops down. This might be ascribed to the scavenging effect of excess H2O2 on the active hydroxyl radical HO• to produce HO2 • less active as illustrated by (equation: 9, 10, 11) [51,52]. (9) H2O2 → 2HO• (10) HO• + H2O2 → HO2 • + H2O (11) HO2 • + HO• → O2 + H2O The CAF conversion reached its maximal 86.16 %, when H2O2 dosage was equal to 0.15 mol.L−1, copper impregnated (10 %) and temperature value attained 60 °C Table 7 . Figure 5 shows the CAF conversion under optimum conditions after using Box-Behnken design. It was seen that CAF could be mostly (87.03%) degraded with 68.85 % as the final value of TOC mineralization after 120 min in the coexisting system of CuNi10-YC and H2O2, which is in a good agreement with the predicted value given by the Box-Behnken model. On the contrary, in the presence of CuNi10-YC or H2O2 alone, the CAF conversion after 120 min was only 14.39 % and 5.4 %, respectively. The low percentage of caffeine decomposition in the presence of the peroxide alone, can be attributed to the deficient homogenous decomposition H2O2 in hydroxyl radical [53]. In addition, the effect of both metals copper and nickel impregnated on yellow bentonite clay ameliorate the oxidation efficiency of the raw clay and as we can see in Table 3 the amount of copper (CuNi10YC) leached after 120 min of reaction, was lower than nickel (CuNi01YC). Accordingly, CuNi10YC catalyst is expected to exhibit high CAF oxidation and stability and not losing the Cu metal by leaching in the medium. When both metals (CuNi11YC) were impregnated, the CAF conversion decreases which can be explained by the decrease in the catalyst's surface area (CuNi11YC), the high amount of leached nickel leads to a distortion of the clay matrix.Additionally, to the high efficiency of the eco-friendly synthesized catalyst CuNi10-YC in degrading CAF molecule from wastewater. The stability plays an important role in checking out the best photocatalyst from others. Figure 6 shows the degradation percentages of the CAF solution (40 ppm) after 120 min of oxidation, it also shows that an important conversion is obtained (~87%) in each reuse. A slight decrease in the mineralization percentages was obtained from the TOC values during the four recycling tests. This indicates that the catalyst remains stable in successive reuses. The insignificant reduction in oxidation performance observed could be caused by the inevitable loss of the catalyst mass during washing and centrifugation processes.For germination test, populations of corn kernels were exposed to a pollutant (CAF molecule) dissolved in distilled water, its toxicity was estimated to evaluate the toxic effects, such as an inhibition of the germination rate (Figure 7 ). It was observed that in only 6 days the germination of corn kernels in distilled water (control or Blank) was totally (100 %). On the other hand, the germination rate was lower for untreated aqueous solutions containing CAF, which did not exceed 50 % (Figure 7A). However, the germination rate in the presence of catalyst (CuNi10-YC) CAF aqueous solutions reached 100 % in 6 days (Figure 7A and B). According to the caffeine conversion and TOC results, the CuNi10-YC catalyst has a significant ability to degrade CAF molecules from wastewater.The objectives of this work were successfully achieved, through incorporating copper/nickel into yellow clay prepared using wet impregnation method, and characterizing it by XRD, XRF, ICP, SEM, BET and Langmuir surface area analysis. The CWPO test for degradation of caffeine has successfully confirmed the usefulness of the modified clay as a catalyst. Thanks to Box-Behnken's design, the optimal conditions for degradation were selected. It has revealed that the highest CAF conversion was achieved was 86.16 % using CuNi10-YC catalyst. A feasible CAF conversion reaction system could be applicated as follows: 1 g.L−1 of catalyst, temperature of 60 °C, 40 mg.L−1 of CAF and time of 120 min, where CAF could be completely oxidized, and the final value of TOC mineralization more than 68.85%. This study has reported relevant and original results demonstrating that CWPO using CuNi-YC processes could be a cost-effective, stable and efficient alternative treatment for the removal of caffeine, since the germination test has given a positive index of good degradation. Another advantage of using this modified clay is its possibility of removing different type of organic compounds. Therefore, it would be interesting to continue testing on other persistent organic pollutants, and why not testing a real wastewater not only batch processes, but also column processes in a pilot scale.Ouissal Assila: Conceived and designed the experiments; Performed the experiments; Wrote the paper.Morad Zouheir, Karim Tanji: Conceived and designed the experiments; Analyzed and interpreted the data; Wrote the paper.Redouane Haounati: Contributed reagents, materials, analysis tools or data; Wrote the paper.Farid Zerrouq: Analyzed and interpreted the data; Wrote the paper.Abdelhak Kherbeche: Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.No data was used for the research described in the article.The authors declare no conflict of interest.No additional information is available for this paper.The Authors thank the general services (DRX, MEB, ICP, etc.) of the innovation centers University of Fez/Morocco (Sidi Mohammed Ben Abdellah), as well as the general research services of the University of Las Palmas de Gran Canaria (Spain).

Copper and nickel were incorporated into the prepared yellow clay (YC) using one of the most widely used methods, for the preparation of heterogeneous catalysts, which is the wet impregnation method (IPM) and its application as a heterogeneous catalyst for Caffeine (CAF). Several catalysts Cooper Nickel's Catalysts (Cu–Ni) were applied to the yellow clay with different weight ratio of Cu and Ni, in order to explore the role of both metals during the catalytic oxidation process CWPO. Furthermore, the CuNi-YC catalysts, were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), Langmuir's surface area, Brunauer Emmett Teller (BET), scanning electron microscope (SEM) and inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES), so as to get a better understanding concerning the catalytic activity's behavior of CuNi-YC catalysts. The optimization of the catalytic activity's effects on the different weight ratios of Cu and Ni, temperature and H2O2 were also examined, using Box-Behnken Response Surface Methodology RSM to enhance the CAF conversion. The analysis of variances (ANOVA) demonstrates that Box-Behnken model was valid and the CAF conversion reached 86.16%, when H2O2 dosage was equal to 0.15 mol.L−1, copper impregnated (10%) and temperature value attained 60 °C. In addition, the regeneration of catalyst's cycles under the optimum conditions, indicated the higher stability up to four cycles without a considerable reduction in its conversion performance.

Hydrogen energy is one of the most promising candidates to replace traditional fossil energy due to its cleanliness, abundant resources, and environmentally friendly features [1–6]. Among the various solid hydrogen storage materials, Mg-based materials have attracted extensive attention worldwide because of their high hydrogen storage capacity, low price, and rich natural resources [7]. However, poor hydrogenation/dehydrogenation kinetic performances and high dehydrogenation temperature are the two main drawbacks of Mg-based materials [8].We show here that the addition of catalysts such as doping with transition metals (TM) [9–16], metal oxides [17–22], or metal sulfides [23–27] is a highly effective and simple way to improve the thermodynamic and kinetic properties of Mg-based materials. Cui et al. [10] introduced the TM (Ti, Nb, V, Co, Mo, Ni) into MgH2 by coating and suggested that TM with lower electro-negativity had a better catalytic action on the hydrogenation performance of MgH2. Khatabi et al. [13] and Yu et al. [14]studied the catalytic effect of the TM on the hydrogen storage properties of MgH2 and found that the addition of TM could weaken the Mg-H bond and decrease the energy barrier for dehydrogenation from MgH2. Lu et al. [15] prepared a core-shell Mg@Pt nanocomposite and revealed that Pt transformed H-stabilized Mg3Pt, which acted as a “hydrogen pump” for the dehydrogenation of Mg and enhanced the hydrogen storage properties of Mg. Valentoniet al. [17] introduced a VNbO5 catalyst into MgH2 by ball milling and revealed that hydrogen (>5.0 wt.%) resulted in negligible degradation of a 15 wt.% VNbO5-doped sample after 70 cycles of hydrogen absorption–desorption. Zhang et al. [18] reported that the Mg-H bonds of MgH2 could be elongated and weakened under the catalytic reaction of Mn3O4; a MgH2+ 10 wt.% Mn3O4 composite could release hydrogen at 200 °C. Xie et al. [23] introduced NiS into Mg by ball milling and found that NiS reacted with Mg to form Ni, MgS, and Mg2Ni after the first absorption–desorption cycle. These multi-phase catalysts formed in situ greatly improved the hydrogenation kinetics of Mg, and the apparent activation energies of hydrogenation and dehydrogenation for NiS-doped Mg decreased to 45.45 kJ mol−1 and 64.71 kJ mol−1, respectively. However, the gradual aggregation of Mg particles after hydrogenation and dehydrogenation cycles is inevitable, leading to a decrease in hydrogen storage performance. To solve such problems, carbon materials are often compounded with TM to improve the thermodynamic and kinetic properties of Mg-based materials [28–40]. Here, 1D porous Ni@C nanostructures were adopted to enhance the dehydrogenation and hydrogenation performance of MgH2 based on An et al. [31], whose study showed that a Ni@C-doped MgH2 composite showed outstanding hydrogen storage performance. At 300 °C, the 5 wt.% Ni@C-doped MgH2 sample could release 6.4 wt.% H2 in 10 min whereas bare MgH2 could not release any hydrogen under the same condition. Liu et al. [38] synthesized a nano-V2O3@C composite and introduced it into Mg through ball milling. The dehydrogenated MgH2-V2O3@C composite could absorb hydrogen at ambient temperature and completely rehydrogenate at 150 °C after only 700 s. Theoretical studies suggested that the presence of V was responsible for the improvement in the hydrogen absorption and desorption performances of MgH2.In our previous work, carbon materials with TM or their oxides such as Y2O3@rGO [41], V2O3@rGO [42], Ni@rGO [43], and Ni-TiO2@rGO [44] were successfully synthesized using graphene oxide (GO) and TM compounds. These materials significantly improved the hydrogenation and dehydrogenation performances of Mg-based materials. This showed that catalysts based on nickel compounded with carbon materials can greatly improve the thermodynamic and kinetic properties of Mg-based materials [31,35,37]. Here, carbon-supported nickel sulfide (Ni3S2@C) composites were prepared using cheap cation exchange resins and Ni(CH3COO)2. Moreover, the impacts of Ni3S2@C on the hydrogenation and dehydrogenation kinetics and thermodynamics of MgH2 were discussed.Four different cation exchange resins (Table 1 ), AmberliteIR-120 (H) (resin-1; Alfa Aesar), Amberlite® IRN77 (H) (resin-2; Alfa Aesar), Dowex Marathon MSC (H) (resin-3; Sigma–Aldrich), and Amberlyst® 15 (H) (resin-4; Alfa Aesar), were used to synthesize Ni3S2@C composites. The preparation of Ni3S2@C is shown schematically in Fig. 1 . First, five grams of resin was dispersed in 100 ml of hydrochloric acid solution at a concentration of 15% (by weight) and magnetically stirred for five hours at ambient temperature. The resin was then washed with deionized water to remove excess acid and impurities. A nickel ion (Ni2+) solution was obtained by dissolving 2.5 g of Ni(CH3COO)2 4H2O (Analytical reagent; XILONG SCIENTIFIC) in 100 ml of deionized water followed by magnetic stirring for one hour. The as-treated resin was then added to the solution containing nickel ions. The exchange process between Ni+ ions and the resin was completed through magnetic stirring for 24 h. The exchanged resin was then cleaned and dried for 24 h in a frozen drying oven. Subsequently, the as-dried resin was heated to 500 °C for five hours under a nitrogen atmosphere. The obtained samples were milled for two hours with a ball-to-powder weight ratio of 40:1 at 500 revolutions per minute (rpm). The four milled samples corresponding to resin-1, resin-2, resin-3, and resin-4 were named Ni3S2@C-1, Ni3S2@C-2, Ni3S2@C-3, and Ni3S2@C-4, respectively. In addition, 5 g of the treated resin-4 was dried, carbonized, and milled under the same conditions, yielding carbon (C).Commercial MgH2 powder (98%; Langfang Beide Trading) was mixed with 10 wt.% of C, Ni3S2 (99.9%; Jiuding Chemical), Ni3S2@C-1, Ni3S2@C-2, Ni3S2@C-3, and Ni3S2@C-4. The resulting six mixtures were milled for five hours, and the ball-to-powder weight ratio was maintained at 40:1 at a speed of 500 rpm. The six milled composites were denoted as MgH2-C, MgH2-Ni3S2, MgH2-Ni3S2@C-1, MgH2-Ni3S2@C-2, MgH2-Ni3S2@C-3, and MgH2-Ni3S2@C-4, respectively.The phase structure of the samples was determined by X-ray diffraction (XRD; Minflex 600; Cu-Kα radiation, 40 kV, and 200 mA) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, USA; Al-Kα X-ray source). All XRD tests were performed by scanning the sample from 2θ = 10° to 2θ = 90° with a scanning speed of 5° min−1. The microstructure of the samples was determined by field-emission scanning electron microscopy (FE-SEM; SU8020, HITACHI) and transmission electron microscopy (TEM; FEI Tecnai G2, f20 s-twin, 200 kV). The distributions of Ni, S, and C in the samples were determined using an energy-dispersive X-ray spectrometer (EDS) attached to an FE–SEM. The Bruner–Emmett–Teller (BET) surface areas were measured with a Micromeritics Tristar II instrument at 77.3 K. The pressure–composition–temperature (PCT) was measured on an automatic Sievert-type device with a hydrogen pressure of 35 atm for the hydrogenation process and the lowest pressure of 0.06 atm for the dehydrogenation process. The hydrogen absorption and desorption properties of the samples were determined using a Sievert-type device. The non-isothermal hydrogenation tests were performed by heating the dehydrogenated samples from ambient temperature to 390 °C at 1 °C min−1 under 60 atm of H2. The non-isothermal dehydrogenation tests were performed by heating the rehydrogenated samples from ambient temperature to 390 °C at 0.5 °C min−1 under 0.001 atm of H2. The isothermal hydrogenation/dehydrogenation measurements were performed at various temperatures under 60 atm of H2 for hydrogen absorption and 0.001 atm of H2 for hydrogen desorption. The dehydrogenation performances of the composites were evaluated by differential scanning calorimetry (DSC, Mettler Toledo). The samples were heated from ambient temperature to 500 °C under an Ar atmosphere (flow rate: 75 ml min−1) at a heating rate of 5 °C min−1.The microstructures of the Ni3S2@C-1, Ni3S2@C-2, Ni3S2@C-3, and Ni3S2@C-4 composites examined by FE–SEM are displayed in Fig. 2 . All four Ni3S2@C samples prepared by different precursors possessed a spherical structure before ball milling as shown in Fig. 2(A–D). These spherical samples were broken into fine particles after milling for two hours (Fig. 2 (A1, B1, C1, and D1)). EDS mapping revealed that all four Ni3S2@C composites contained C, S, and Ni. The XRD patterns of the resultant Ni3S2@C samples are presented in Fig. 3 A. There were two wide peaks at about 22° and 44° from the respective (002) and (100) planes of carbon [32]. The diffraction peaks of Ni3S2@C-1, Ni3S2@C-2, Ni3S2@C-3, and Ni3S2@C-4 appeared at 21.76°, 31.10°, 37.80°, 44.35°, 49.73°, 50.11°, and 55.16° from the respective (010), (−110), (111), (020), (120), (−120), and (−121) planes of the Ni3S2 phase (JCPDS card no. 85-1802). The diffraction peak of C could not be detected in the XRD profiles of the Ni3S2@C composites because the diffraction peak intensity of C was weaker than that of Ni3S2. However, the EDS analysis results in Fig. 2 confirm that Ni3S2@C was successfully synthesized using cation exchange resin and nickel acetate as raw materials. Furthermore, the specific surface areas and pore size distributions of the Ni3S2@C composites were investigated by nitrogen adsorption and desorption isotherms at 77.3 K (Figs. 3B and C). The BET surface areas of Ni3S2@C-1, Ni3S2@C-2, Ni3S2@C-3, and Ni3S2@C-4 were calculated as 273.9 m2 g−1, 319.5 m2 g−1, 331.7 m2 g−1, and 330.3 m2 g−1, respectively. All four synthesized catalysts had a large BET surface area. The BET surface areas of the composites prepared by macroreticular resins were noticeably larger than those prepared by gel resins. Based on the desorption isotherm curves, the Barret–Joyner–Halenda (BJH) desorption average pore diameters of Ni3S2@C-1, Ni3S2@C-2, Ni3S2@C-3, and Ni3S2@C-4 were calculated as 37.3 Å, 36.0 Å, 39.5 Å, and 40.1 Å, respectively (Fig. 3C), indicating that all four catalysts were mesoporous. To further study Ni3S2@C, the Ni3S2@C-4 composite was selected as a typical representative and was investigated by the XPS method. Fig. 3(D–F) displays the high-resolution XPS spectra of Ni3S2@C-4. The peaks observed at 855.9 eV and 873.6 eV were assigned to the respective Ni 2p3/2 and Ni 2p1/2 orbitals of Ni3S2 [45,46], and the other two peaks at 861.0 eV and 879.4 eV corresponded to the accompanying satellite peaks of Ni 2p3/2 and Ni 2p1/2 (Fig. 3D). In addition, weak peaks at 853.1 eV and 870.8 eV appeared in the Ni 2p3/2 and Ni 2p1/2 orbitals of NiO [47] due to the long exposure of the sample to air during fabrication. The peaks at 163.6 eV and 164.8 eV corresponded to the respective S 2p3/2 and S 2p1/2 orbitals of Ni3S2 [48] (Fig. 3E). In the C 1 s spectrum, the peak located at 284.8 eV was assigned to the C-C bond [29,31,32]. The results indicated that the Ni3S2@C-4 composite was successfully prepared using ion exchange resin and nickel acetate as raw materials.The microtopographies of the as-synthesized Ni3S2@C and MgH2-Ni3S2@C-4 composites were further investigated by TEM, and the corresponding results are displayed in Fig. 4 . The particle size of Ni3S2 ranged between 5 nm and 20 nm (Fig. 4A and B), and the d-spacing of 0.410 nm corresponded to the (010) plane of Ni3S2 (Fig. 4C). The SAED image in the inset of Fig. 4D displays MgH2 with crystal indices of (111) and (002) planes. The MgH2-Ni3S2@C-4 composite particles were distributed dispersively as shown in Fig. 4(D, E). For the MgH2-Ni3S2@C-4 composite, the interplanar spacings of 0.207 nm and 0.219 nm were well matched with the (020) plane of Ni3S2 and the (111) plane of MgH2, respectively (Fig. 4F).To study the effect of Ni3S2@C on the hydrogen absorption and desorption kinetic performances of MgH2, hydrogenation and dehydrogenation analyses of different MgH2-Ni3S2@C composites were performed. Fig. 5 A shows the hydrogenation curves of MgH2, MgH2-C, MgH2-Ni3S2, MgH2-Ni3S2@C-1, MgH2-Ni3S2@C-2, MgH2-Ni3S2@C-3, and MgH2-Ni3S2@C-4. For the unmodified MgH2 sample, the onset hydrogenation temperature was about 120 °C, and 7.36 wt.% hydrogen was absorbed when the temperature reached ∼225 °C. The onset hydrogenation temperature of MgH2 was greatly reduced after the addition of Ni3S2@C. After the addition of Ni3S2, Ni3S2@C-1, Ni3S2@C-2, Ni3S2@C-3, and Ni3S2@C-4, the resulting MgH2 composites absorbed 4.39 wt.%, 3.74 wt.%, 2.94 wt.%, 5.15 wt.%, and 5.68 wt.% of H2 at 100 °C, respectively. The onset hydrogenation temperatures of the MgH2-Ni3S2@C-1 and MgH2-Ni3S2@C-2 composites decreased to 50 °C, and MgH2-Ni3S2, MgH2-Ni3S2@C-3, and MgH2-Ni3S2@C-4 absorbed hydrogen at ambient temperature. The addition of Ni3S2 and Ni3S2@C dramatically reduced the initial hydrogenation temperature of MgH2. The MgH2-Ni3S2@C-4 composite exhibited excellent hydrogenation performance. Fig. 5B shows that the hydrogen desorption kinetic property of MgH2 was also significantly improved when it was modified by the addition of the Ni3S2@C composites. When MgH2-Ni3S2@C-1, MgH2-Ni3S2@C-2, MgH2-Ni3S2@C-3, and MgH2-Ni3S2@C-4 were heated to 275 °C with a heating rate of 0.5 °C/min, 6.16 wt.%, 5.61 wt.%, 5.50 wt.%, and 6.35 wt.% of H2 were released, respectively. However, MgH2 could not release any hydrogen at this temperature until the temperature exceeded 300 °C. It is obvious that the catalyst containing Ni3S2 promoted the hydrogen desorption of the samples. The Ni3S2@C composite exerted an especially excellent catalytic action on the hydrogenation and dehydrogenation performances of MgH2. This may be due to the presence of C, which can make the Ni3S2 with catalytic activity disperse more evenly and inhibit the agglomeration of particles during the milling process [34]. The MgH2-Ni3S2@C-4 sample could absorb more hydrogen at 100 °C and release more hydrogen at 275 °C than any other sample, showing good hydrogen absorption and desorption performance.The Ni3S2@C composites showed excellent catalytic activity for the hydrogen storage of MgH2, and the Ni3S2@C-4 composite was selected as a representative to further study its effect on the hydrogen storage performance of MgH2. Fig. 6 presents the isothermal hydrogenation and dehydrogenation curves of the MgH2-Ni3S2@C-4 composite. For comparison, the isothermal hydrogenation/dehydrogenation curves of MgH2 are also plotted in the figure. The MgH2-Ni3S2@C-4 composite could absorb 6.08 wt.% H2 in 10 min at 150 °C and could absorb 6.0 wt.% H2 in 157 min even at 50 °C (Fig. 6A), whereas MgH2 could only absorb 0.70 wt.% H2 at 150 °C (Fig. 6C). Hence, the hydrogenation property of MgH2 was significantly improved under the catalytic effect of Ni3S2@C-4. The catalytic effect of Ni3S2@C-4 on the hydrogenation performance of MgH2 was better than that of Ni-V [12], NiS [23], Ni@rGO [43], Fe3S4 [25], and Co@C [29]. For example, MgH2-Ni@rGO and MgH2-20 wt.% Fe3S4 took 100 min and 20 min to reabsorb 5.94 wt.%[43] and 3.41 wt.% of H2 [25] at 150 °C, respectively. Furthermore, under the same hydrogenation time (10 min), Mg-Ni-V [12], Mg-5 wt.%NiS [23], and MgH2-Co@C [29] absorbed only 1.0 wt.%, 3.5 wt.%, and 2.71 wt.% of H2, respectively. The dehydrogenation kinetics of MgH2-Ni3S2@C-4 were determined at different temperatures (225 °C, 250 °C, 275 °C, 300 °C, and 325 °C) (Fig. 6B). The sample desorbed 5.61 wt.% H2 in 160 min at 250 °C. When the temperature reached 300 °C, the dehydrogenation capacity increased to 6.15 wt.% H2 in eight minutes. However, unmodified MgH2 hardly released any hydrogen at temperatures below 275 °C and took 160 min to release 4.38 wt.% H2 at 300 °C (Fig. 6D). Mg-Ni-V [12], Mg-5 wt.%NiS [23], and MgH2-Co@C [29] only released 3.0 wt.%, 3.1 wt.%, and 5.74 wt.% of H2, respectively, in 30 min at 300 °C. Hence, versus the abovementioned materials, MgH2-Ni3S2@C-4 could release more hydrogen at a faster rate, showing excellent hydrogen desorption performance.To further explore the effect of Ni3S2@C-4 on the hydriding reaction of MgH2, the Johnson–Mehl–Avrami–Kolmogorov (JMAK) and Arrhenius equations were employed to calculate the apparent activation energy (E a). The JMAK equation can be written as follows [12,18]: (1) ln [ − ln ( 1 − f ( t ) ) ] = n ln k + n ln t , where f(t) is the time-dependent function, n is the Avrami exponent, and k is an effective kinetic parameter. Based on the isothermal hydrogenation/dehydrogenation curves at different temperatures, the values of n and nlnk were obtained by fitting the JMAK plots between ln[–ln(1–f(t))] and lnt (Fig. 7 (A–D). Based on these JMAK plots, the corresponding apparent activation energies were calculated by the Arrhenius equation: (2) ln k = − E a R T + ln k 0 , where Ea, R, T, and k 0 represent the apparent activation energy, the gas constant (8.314 J K−1 mol−1), the absolute temperature, and the Arrhenius pre-exponential factor, respectively. The Arrhenius plots of MgH2-Ni3S2@C-4 and MgH2 are displayed in Fig. 7(E, F). The hydrogenation apparent activation energy of the dehydrogenated MgH2-Ni3S2@C-4 composite was calculated to be 39.6 kJ mol−1, which is lower than that of MgH2 (93.6 kJ mol−1). Table 2 presents the hydrogenation apparent activation energies of some common Mg-based composites. It is obvious that in comparison with the catalysts listed in Table 2, Ni3S2@C-4 can more effectively reduce the hydrogenation reaction potential barrier and improve the hydrogenation kinetics performance. Moreover, the dehydrogenation apparent activation energy of the rehydrogenated MgH2-Ni3S2@C-4 composite was calculated as 115.2 kJ mol−1 (lower than that of MgH2) according to the slope of the straight line shown in Fig. 7F. The hydriding reaction potential barrier for the hydrogen absorption and desorption processes of MgH2 was significantly reduced by the addition of Ni3S2@C-4. The decrease in the hydriding reaction potential barrier promoted the diffusion of hydrogen atoms in the Mg matrix and improved the hydrogenation and dehydrogenation kinetics of MgH2.DSC analysis was employed to further study the dehydrogenation performance of the MgH2-Ni3S2@C-4 composite. Fig. 8 displays the DSC curves of the MgH2-Ni3S2@C-4 composite and MgH2. The endothermic peak of MgH2 appeared at 434.3 °C; when Ni3S2@C-4 was added, the endothermic peak temperature was reduced to 296.3 °C (Fig. 8), which was 138.0 °C lower than that of MgH2. The endothermic peak temperature significantly decreased with the addition of Ni3S2@C-4, indicating an improvement in dehydrogenation performance. Fig. 9 (A, B) displays the pressure–composition–temperature (PCT) curves of the MgH2-Ni3S2@C-4 composite and MgH2. For MgH2-Ni3S2@C-4, a complete reversible hydrogenation and dehydrogenation cycle occurred at 275 °C. In contrast, only the hydrogenation process occurred for MgH2, and the dehydrogenation process did not occur at 325 °C. Based on the PCT curves in Fig. 9, the hydriding reaction thermodynamic behavior was described by the Van't Hoff equation, which can be expressed as follows [23]: (3) ln P H 2 = Δ H R T − Δ S R , where P H 2 , ∆H, and ∆S represent the plateau pressure of hydrogenation/dehydrogenation, the reaction enthalpy change, and the reaction entropy change, respectively. Generally, enthalpy change is regarded as an important parameter of the thermodynamic behavior of a reaction process. The hydrogenation/dehydrogenation enthalpy changes for the MgH2-Ni3S2@C-4 composite and MgH2 were obtained from the Van't Hoff plots shown in Fig. 9(C, D). The hydrogenation and dehydrogenation reaction enthalpy changes of MgH2 were calculated as −82.0 kJ mol−1 H2 and 89.5 kJ mol−1 H2, respectively (Fig. 9D). When MgH2 was composited with Ni3S2@C-4, the hydrogenation and dehydrogenation enthalpy changes of MgH2-Ni3S2@C-4 decreased to −74.7 kJ mol−1 and 78.5 kJ mol−1, respectively (Fig. 9C). These results further demonstrate that the addition of Ni3S2@C-4 decreased both the hydrogenation potential barrier of Mg and the stability of MgH2, thereby improving the reversible hydrogen storage property of MgH2.To study the reversible hydrogenation and dehydrogenation cycle stability of MgH2-Ni3S2@C-4, 30 cycles of hydrogen absorption–desorption were performed for MgH2-Ni3S2@C-4 at 300 °C and for pure MgH2 at 375 °C (Fig. 10 ). The maximum hydrogen storage capacities of MgH2-Ni3S2@C-4 and pure MgH2 were 6.52 wt.% and 6.92 wt.%, respectively. After 30 cycles of hydrogen absorption–desorption, the hydrogen storage capacity of the MgH2-Ni3S2@C-4 composite had no decay. The hydrogen storage capacity and the capacity retention ratio of pure MgH2 decreased to 6.45 wt.% and 93.2%, respectively, implying that the addition of the Ni3S2@C-4 composite enhanced the stability of the hydrogen absorption–desorption cycle.Therefore, the comprehensive hydrogen storage performance of MgH2 was significantly enhanced by the addition of Ni3S2@C-4. To explore the catalytic mechanism of Ni3S2@C-4, the XRD patterns of hydrogenation and dehydrogenation for both pure MgH2 and MgH2-Ni3S2@C-4 were investigated (Fig. 11 ). Fig. 11(A, B) shows that the as-prepared MgH2 was completely dehydrogenated into Mg and H2 at 380 °C, and the rehydrogenated product was MgH2. For the MgH2-Ni3S2@C-4 sample, both Mg2Ni and MgS phases appeared in the first dehydrogenation cycle. In addition, the MgH2-Ni3S2@C-4 composite was fully activated at 380 °C and then subjected to hydrogen absorption and desorption at different temperatures to obtain the diffraction spectra (Fig. 11(C, D)). Fig. 11(C) shows that the rehydrogenated MgH2-Ni3S2@C-4 sample was mainly composed of Mg2Ni, MgS, β-MgH2, and γ-MgH2 when it hydrogenated at 100 °C, 200 °C, and 300 °C, respectively. Additionally, some Mg2Ni reacted with hydrogen to form Mg2NiH4 at 300 °C (Fig. 11C). As the hydrogenation temperature increased to 380 °C, Mg2Ni and γ-MgH2 were completely converted to Mg2NiH4 and β-MgH2, respectively. In the dehydrogenation process (Fig. 11D), Mg2NiH4 was completely dehydrogenated at 250 °C. However, some β-MgH2 could not release hydrogen until the temperature reached 300 °C. The MgS phase remained unchanged during the hydrogenation and dehydrogenation process. Apparently, in the first dehydrogenation cycle, Ni3S2 reacted with Mg to generate Mg2Ni and MgS, thus forming multi-phase in situ catalysts (Mg/Mg2Ni, Mg/MgS, and Mg/C). This multi-phase interface provided more active sites and diffusion paths for hydrogen atoms to enhance the hydrogenation/dehydrogenation properties of Mg/MgH2 [50]. During the dehydrogenation process, Mg2NiH4 was dissociated into Mg2Ni and H2 at the interface of MgH2/Mg2NiH4 and caused MgH2 to break down [11]. Hence, Mg2NiH4 acted as a “hydrogen pump” to drive MgH2 to dissociate, thus reducing dehydrogenation temperatures and improving the hydrogen desorption performance of the MgH2-Ni3S2@C-4 composite. Furthermore, carbon prevented Mg, Mg2Ni, and MgS grains from agglomeration and also provided more active sites for hydrogen atoms [31]. The synthesis and catalytic mechanism of Ni3S2@C-4 during hydrogenation/dehydrogenation processes are schematically presented in Fig. 12 . MgH2 particle surfaces were covered by high-activity Ni3S2@C-4 after ball milling. Mg2Ni and MgS were formed by the reaction of Ni3S2 and Mg during the first dehydrogenation process. MgS remained unchanged during the hydrogenation and dehydrogenation reactions. Hence, the onset temperature and the apparent activation energy of Mg decreased with the formation of multi-phase catalysts. Consequently, the integrated hydrogen storage performance of MgH2 was significantly improved under the catalytic action of Ni3S2@C.Here, Ni3S2@C catalysts were successfully prepared using four different cation exchange resins: AmberliteIR-120 (H)(resin-1), Amberlite® IRN77 (H) (resin-2), Dowex Marathon MSC (H) (resin-3), and Amberlyst® 15 (H) (resin-4). The nitrogen adsorption and desorption isotherm measurement results indicated that all four types of Ni3S2@C catalysts were mesoporous materials with a large BET surface area. The comprehensive hydrogen storage performance of MgH2 was significantly improved by the addition of Ni3S2@C. The catalytic effects of the Ni3S2@C composites prepared using macroreticular resins (resin-3 and resin-4) on the hydrogen storage performance of MgH2were better than those prepared using gel resins (resin-1 and resin-2). The MgH2-Ni3S2@C-4 composite exhibited excellent comprehensive hydrogen storage performance. It absorbed hydrogen at ambient temperature, took only 10 min to absorb 6.08 wt.% H2 at 150 °C, and took eight minutes to release 6 wt.% H2 at 300 °C. The hydrogenation and dehydrogenation apparent activation energies of MgH2-Ni3S2@C-4 were 39.6 kJ mol−1 and 115.2 kJ mol−1, respectively, which were much lower than those of MgH2 (93.6 kJ mol−1 and 141.5 kJ mol−1, respectively). Ni3S2 of Ni3S2@C reacted with Mg to form Mg2Ni and MgS during the first desorption process. The multi-phase (Mg/Mg2Ni, Mg/MgS, and Mg/C) interface provided more active sites to improve the hydrogen storage performance of MgH2.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.We thank LetPub (www.letpub.com) for linguistic assistance during the preparation of this manuscript. This work was supported by the National Natural Science Foundation of China (grant number 51571065), the Natural Science Foundation of Guangxi Province (grant numbers, 2018GXNSFAA294125, 2018GXNSFAA281308, 2019GXNSFAA245050), the Innovation-Driven Development Foundation of Guangxi Province (grant number AA17204063), and the Innovation Project of Guangxi Graduate Education (grant number YCSW2020046).

Carbon materials have excellent catalytic effects on the hydrogen storage performance of MgH2. Here, carbon-supported Ni3S2 (denoted as Ni3S2@C) was synthesized by a facile chemical route using ion exchange resin and nickel acetate tetrahydrate as raw materials and then introduced to improve the hydrogen storage properties of MgH2. The results indicated the addition of 10 wt.% Ni3S2@C prepared by macroporous ion exchange resin can effectively improve the hydrogenation/dehydrogenation kinetic properties of MgH2. At 100 °C, the dehydrogenated MgH2-Ni3S2@C-4 composite could absorb 5.68 wt.% H2. Additionally, the rehydrogenated MgH2-Ni3S2@C-4 sample could release 6.35 wt.% H2 at 275 °C. The dehydrogenation/hydrogenation enthalpy changes of MgH2-Ni3S2@C-4 were calculated to be 78.5 kJ mol−1/−74.7 kJ mol−1, i.e., 11.0 kJ mol−1/7.3 kJ mol−1 lower than those of MgH2. The improvement in the kinetic properties of MgH2 was ascribed to the multi-phase catalytic action of C, Mg2Ni, and MgS, which were formed by the reaction between Ni3S2 contained in the Ni3S2@C catalyst and Mg during the first hydrogen absorption–desorption process.

Electrochemical water splitting represents a promising strategy to provide sustainable clean energy source from the conversion of water into chemicals and fuels [1–5]. It is composed of two half reactions, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), both of which are potential candidates for future clean energy sources [1,3,6–9]. The main limiting factor of these reactions is represented by their sluggish kinetics [1,10,11].The hydrogen evolution reaction (HER) is one of the most often studied electrocatalytic processes because of its industrial and technological interest for producing hydrogen gas through a limited number of reaction steps [12–14].The reaction occurs at cathode via a two-electrons reaction [15]: (1) 2 H + + 2 e − → H 2 From an industrial perspective on water electrolysis, HER is often conducted in alkaline media to achieve higher stability of the catalyst materials [14]. In alkaline medium, HER proceeds through two steps [11,12,16,17]: 1. First, the catalyst splits a H2O molecule (Volmer step) into a hydroxyl ion (OH−) and an adsorbed hydrogen atom (Hads); 2. then, a hydrogen molecule is formed via either the interaction of the Hads atom and water molecule (Heyrovsky step) or the combination of two Hads atoms (Tafel step). First, the catalyst splits a H2O molecule (Volmer step) into a hydroxyl ion (OH−) and an adsorbed hydrogen atom (Hads);then, a hydrogen molecule is formed via either the interaction of the Hads atom and water molecule (Heyrovsky step) or the combination of two Hads atoms (Tafel step).However, the kinetics of HER in alkaline medium are slow if compared with those in acid environment because of the low concentration of available protons. As a consequence, this process will require additional effort to obtain protons by water dissociation near or on electrode surface.The state-of-art HER catalyst is platinum (Pt) and its alloys, but the scarcity and cost of Pt, universally considered a critical raw material, limit its large-scale application for electrolysis [15]. In past decades, extensive research has been focused on the development of practical alternatives to Pt, as efficient and renewable energy sources [16,18]. These have resulted in the identification of a variety of promising HER catalysts free of precious metals such as sulfides, phosphides, carbides, nitrides, selenides, and borides [3,19,20].At the same time, enhancing the efficiency of noble metals utilization may also provide a realistic approach to the development of high-performance and cost-effective catalysts. While Pt is well-known to be effective for the adsorption of Hads atoms, the overall sluggish HER kinetics in alkaline solutions stems from the insufficient catalyzing capability of Pt toward the cleavage of the H−OH bond. A possible solution consists in the creation of catalysts with a combination of metal oxides and Pt, where the oxides promote the dissociation of H2O and the nearby Pt facilitates the adsorption and recombination of H ads into molecular H2 [15,18,21].The transition metal oxide NiO is considered a valuable candidate as active material for electrochemical water splitting thanks to its Earth abundance and low cost [22]. Furthermore, NiO nanostructures (interconnected networks, nanosheets, microflowers) increase electrolyte permeability through the active material, making more favorable the mass transport at the electrode-solution interface [23]. Thanks to unique catalytic properties, nanostructured NiO is often used as a high-performance OER catalyst [24–27]. Recent literature reports also evidenced that NiO is particularly interesting due to its high stability for HER in alkaline electrolytes [4].Heterostructured materials on the nanoscale have exhibited great potential in this area. These classes of catalysts, with double or multiple types of active sites on the surface, exhibit remarkable advantages for the HER in alkaline solutions. A synergistic electronic interaction between the metal and the oxide has been proposed as the reason for the enhanced HER performance [4,14,21]. In particular, Pt–NiO catalysts can be the key for designing efficient and cheap catalyst at which Pt favors H+ adsorption and NiO promotes the adsorption of OH− species [28,29].Unfortunately, there are no reports on overall water splitting using only NiO-based materials (decorated or not) as bifunctional electrocatalysts, except for two ones [1,28]. Mondal et al. tested the performance of porous hollow nanostructured NiO electrodes for overall water splitting taking advantage of their high surface areas, porous microstructures, inner hollow architectures [1]. Similarly, Bian et al. synthesized a hierarchically structured Pt/NiO/Ni/CNTs with a low loading of Pt NPs for efficient OER and HER, taking advantage of the presence of the NiO/Ni heterojunction to boost the overall water splitting performance [28].Here, we report a new strategy for overall water splitting electrodes, exploiting NiO nanostructures (microflowers, μFs) on graphene paper (GP), decorated with ultralow content of Pt nanoparticles (NPs). NiO μFs are synthesized by a chemical-based method and decorated with a colloidal solution of Pt NPs. Our hybrid metal-oxide catalyst unfolds outstanding activity toward HER, with an overpotential of 66 mV at a constant current density of 10 mA cm−2. The present electrocatalyst shows a high rate of hydrogen generation as evidenced by the remarkable turnover frequency (TOF) values despite the low amount of loaded Pt. An alkaline electrolyzer is tested using Pt–NiO μFs electrode and undecorated NiO μFs as cathode and anode, respectively. We demonstrate that this all NiO-based electrolyzer can sustain a current density of 10 mA cm−2 with a potential of 1.57 V. The present work represents a valid strategy for the development of cost-effective electrocatalysts with a very small content of noble metal for widespread water electrocatalysis application.NiO microflowers (μFs) were synthesized from a chemical solution method trough a bain-marie configuration [24]. The obtained μFs powder were dispersed in an aqueous solution of deionized water and ethanol and sonicated for 15 min at room temperature to achieve a higher dispersion of the nano-structures.Pt nanoparticles (NPs) dispersion was produced through a green chemical reduction method at room temperature with ascorbic acid (AA) as reducing agent [20]. 30 μL of 33 mM AA were dispersed in 30 mL of 0.2 mM H2PtCl6 (Sigma-Aldrich, St. Louis, MO, USA, ≥99.9%) aqueous solution. The dispersion was then stirred for 5 min and used without further purification.Graphene paper (GP) substrates (1 × 1.5 cm2, 240 nm thick, Sigma Aldrich, St. Louis, MO, USA) were rinsed with deionized water and ethanol and dried in N2 to clean the surface from any impurity. NiO μFs were deposited by drop casting by using 20 μL of NiO dispersion. The samples were then dried on a hot plate at 80 °C for 10 min. Pt NPs were dispersed onto the electrode by subsequent addition of drops with NP dispersion in order to vary the catalyst loading. The mass of NiO (0.30 mg) on GP was measured by a Mettler Toledo MX5 Microbalance (sensitivity: 0.01 mg). Decorated samples are labelled according to the number of Pt dispersion drops (e.g. 5Pt–NiO indicates NiO catalyst decorated with 5 drops of Pt NPs dispersion).Surface morphology was analyzed by using a Scanning Electron Microscope (SEM, Gemini field emission SEM Carl Zeiss SUPRA 25, Carl Zeiss Microscopy GmbH, Jena, Germany). SEM images were analyzed by using ImageJ software [30].Transmission electron microscopy (TEM) analyses of Pt decorated NiO μFs dispersed on a TEM grid were performed with a Cs-probe-corrected JEOL JEM ARM200F microscope at a primary beam energy of 200 keV operated in scanning TEM (STEM) mode and equipped with a 100 mm2 silicon drift detector for energy dispersive X-ray (EDX) spectroscopy. For EDX elemental mapping, the Pt X-rays signal was collected by scanning the same region multiple times with a dwell time of 0.5 ms. TEM images and EDX spectra were analyzed by using DigitalMicrograph® software [31].The evaluation of Pt amount on NiO μFs was analyzed by Rutherford backscattering spectrometry (RBS, 2.0 MeV He+ beam at normal incidence) with a 165° backscattering angle by using a 3.5 MV HVEE Singletron accelerator system (High Voltage Engineering Europa, Netherlands). RBS spectra were analyzed by using XRump software [32].Electrochemical measurements were carried out at room temperature by using a VersaSTAT 4 potentiostat (Princeton Applied Research, USA) and a three-electrode setup with a graphite rod as counter electrode, a saturated calomel electrode (SCE) as reference electrode, and the prepared electrodes as working electrodes. 1 M KOH (pH 14, Sigma Aldrich, St. Louis, MO, USA) was used as supporting electrolyte. Cyclic voltammetry (CV) curves were recorded at a scan rate of 10 mV s−1 in the potential range −0.7 ÷ −1.5 V vs SCE in order to stabilize the electrodes. The HER activities of decorated catalysts were investigated using linear sweep voltammetry (LSV) at scan rate of 5 mV s−1 in the same potential windows of CVs. Electrochemical impedance spectroscopy (EIS) was performed with a superimposed 10 mV sinusoidal voltage in the frequency range 104 ÷ 10−1 Hz at a potential just after the onset potential (E onset , the minimum potential at which a reaction product is formed at an electrode). Tafel plots were extrapolated from polarization curves by plotting the overpotential (η) as a function of the log of the current density. Mott–Schottky (M–S) analyses were conducted on bare and decorated samples in the potential range 0–1 V vs. SCE, at 1000 Hz frequency. Chronopotentiometry (CP) analysis was employed to study the stability of the samples in a 1 M KOH solution for 15 h at a constant current density of 10 mA cm−2.Current density was normalized to the geometrical surface area and measured potentials vs SCE were converted to the reversible hydrogen electrode (RHE) according to the equation [33]: (2) E RHE =  E SCE + 0.059 ⋅ pH  + 0.244 All measured potentials (η') were manually corrected by iR u -compensation as follows: (3) η = η ′ − i R u where i is the electrode current and R u [Ω] is the uncompensated resistance (extracted from EIS).The turnover frequency (TOF) is defined as the rate of production of oxygen molecules per active site: (4) T O F = I 2 n F where I is the measured current at a fixed overpotential, the term 2 represents the number of electrons involved in the HER, F is the Faraday constant and n is the number of moles of the active sites [34]. Once the Pt amount is known, the number of active Pt moles can be calculated as follows: (5) n Pt [ g cm − 2 ] = Dose Pt [ at cm − 2 ] N A [ at mol − 1 ] where DosePt is the RBS Dose, representing the amount of Pt atoms per cm2, and N A is the Avogadro's number.Finally, the mass activity is defined by the ratio between a fixed current density and the catalyst loading (obtained by multiplying n Pt for the atomic weight of the catalyst): (6) Mass activity = j [ A cm − 2 ] catalist loading  [ mg cm − 2 ] Fig. 1 (a) shows SEM images of NiO μFs on GP. Our catalyst totally recovers the surface of graphene electrode with an irregular thickness due to the agglomeration of μFs. Fig. 1(b) of the same figure shows a tilted view of the electrode. After decoration, Pt NPs (mean size of 2 nm) spread onto NiO μFs (bright particles on STEM Z-contrast image in Fig. 1(c)). STEM EDX elemental map in Fig. 1(d) allowed us to confirm the effective presence of Pt decorating NiO catalyst. RBS analyses (Fig. 1(e)) confirmed Pt presence and allowed us to quantify the Pt loading, by using a flat substrate covered with the same drops containing Pt NP dispersion used for the electrode fabrication. We assume that after drop casting, the measured Pt loading on a flat substrate is the same of that on NiO μFs. Moreover, this NP density is not dependent from the type of surface since it is an intrinsic quantity. The Pt loading is related to the area of Pt peak in the RBS spectrum (at around 1.8 MeV) [35]. As expected, Pt amount increases with the number of drops: 1.2 × 1016 at cm−2, 1.8 × 1016 at cm−2, 3.4 × 1016 at cm−2, for 5, 10, and 20 drops, respectively (Fig. 1(f)). Pt NP density cannot be verified by SEM analysis because of the rough surface and shadowing effect caused by NiO nanostructures. Thus, the Pt amount (D RBS , from RBS [34]) was joined with Pt NP diameter (Fig. S1) to evaluate the density N of NPs decorating NiO μFs, through the following relation [33,37]: (7) D R B S = N ρ a t V N P where ρ at is the Pt atomic bulk density (6.62 × 1022 at cm−3) and V NP is the volume of a single NP (in cm3) based on the size of NPs (measured from SEM images). Following these considerations, the NP density was found to vary from 2 × 1010 NPs cm−2 to 6.2 × 1010 NPs cm−2 (Fig. 1(f)). Finally, from eq. (5) the Pt loading can be easily calculated, confirming the extremely low content of Pt in our decorated electrodes. The obtained values for RBS Pt dose, NP density, Pt loading are reported in Table 1 .To evaluate the electrochemical performance of bare and decorated NiO μFs on HER in alkaline conditions, electrochemical analyses were performed in 1 M KOH (Fig. 2 ). Polarization curves (Fig. 2(a)) clearly show how the presence of Pt drastically reduces the activation barrier for the H2 production, confirmed by a variation in the onset potential and overpotential at a constant current density of 10 mA cm−2 from 247 to 66 mV (Table 1). Two types of behavior can be distinguished as a function of the quantity of Pt: (i) for no (or ultralow) Pt loading, both overpotential and onset potential appear at relatively high voltages, indicating that high energies are required to overcome the adsorption of H+ and subsequent production of H2 steps; (ii) by increasing the density of NPs (10Pt and 20Pt), overpotential and onset potential drastically reduce pointing out an enhanced catalytic action of Pt against HER. for no (or ultralow) Pt loading, both overpotential and onset potential appear at relatively high voltages, indicating that high energies are required to overcome the adsorption of H+ and subsequent production of H2 steps;by increasing the density of NPs (10Pt and 20Pt), overpotential and onset potential drastically reduce pointing out an enhanced catalytic action of Pt against HER.The morphology of samples after HER was compared to the pristine ones, as shown in Fig. S1, without any significant morphology variation.Tafel slopes of Pt decorated NiO μFs are reported in Fig. 2(c). Pt decoration leads to a decrease of Tafel slope to a value of 82 mV dec−1 for 20Pt–NiO sample. Tafel slope values allow a deep understanding of HER catalytic mechanism. Three possible pathways (illustrated in Fig. 2(d)) for the HER reaction in alkaline medium can be distinguished [11,12,38]: (i) electrochemical hydrogen adsorption (Volmer step, H2O + e− → Hads + OH−) at the active site of the catalyst; (ii) H2 formation through an electrochemical desorption step (Heyrovsky step, Hads + H2O + e− → H2 + OH−); (iii) H2 formation through a recombination step between two adsorbed hydrogen atoms (Tafel step: Hads + Hads → H2). electrochemical hydrogen adsorption (Volmer step, H2O + e− → Hads + OH−) at the active site of the catalyst;H2 formation through an electrochemical desorption step (Heyrovsky step, Hads + H2O + e− → H2 + OH−);H2 formation through a recombination step between two adsorbed hydrogen atoms (Tafel step: Hads + Hads → H2).Usually, the rate-determining step (RDS) can be evaluated from the value of the Tafel slope [35–37,39–41]. Our values (by considering the kinetic analysis and mechanism for HER that are based on Butler-Volmer equation [39]) suggest that the RDS for our catalysts is represented by Volmer step and by the initial hydrogen adsorption. NiO-based materials have been widely proved to be optimal catalysts for OH− adsorption [21,28,29]. The high Tafel slope value for bare semiconductor electrode clearly demonstrates that the HER proceeds much slower in NiO sample Conversely, Pt loading causes a reduction of Tafel slopes (Fig. 2(e)). This evidence suggests that HER is limited by the hydrogen adsorption, with a poor efficiency in the NiO case. Conversely, the presence of Pt in decorated catalysts reveals an enhanced adsorption of hydrogen atoms on the surface. Pt decoration not only reduces the activation barrier for the activation of the HER (evidenced by the lowest overpotential for the most decorated sample), but also favors the hydrogen adsorption at the catalyst surface.Nyquist plots from EIS analysis in Fig. 3 (a) remark the role of Pt NPs in the HER. They were acquired in the so-called turnover region, just after the onset potential of each sample in order to appreciate a good HER activity [24,42]. The experimental EIS spectra were fitted (continuous lines) by the Armstrong-Henderson equivalent circuit [43] (Fig. 3(b)) and the extracted fitting parameters are reported in Fig. 3(c and d). In the Armstrong-Henderson circuit different elements can be recognized [13,16,17,24,42,44–48]: 1. Ru is the uncompensated resistance; 2. Rct is the charge transfer resistance at the electrode-electrolyte interface; 3. Cdl is the double layer capacitance (here we used constant phase elements to take into account the non-ideal behavior of the nanostructured electrodes); 4. Rp is strictly related with the mass transfer resistance of adsorbed intermediate Hads (in particular it well describes the adsorption/desorption of intermediates at the electrode surfaces); 5. Cp is the Hads related capacitance (usually called pseudocapacitance) in the HER mechanism. Ru is the uncompensated resistance;Rct is the charge transfer resistance at the electrode-electrolyte interface;Cdl is the double layer capacitance (here we used constant phase elements to take into account the non-ideal behavior of the nanostructured electrodes);Rp is strictly related with the mass transfer resistance of adsorbed intermediate Hads (in particular it well describes the adsorption/desorption of intermediates at the electrode surfaces);Cp is the Hads related capacitance (usually called pseudocapacitance) in the HER mechanism.The adequately fitted experimental data reveal how these 5 parameters vary with Pt decoration (Fig. 3(c and d)). Ru and Cdl do not appreciably change, as expected since the Pt NPs coverage is quite limited and most of the interface among NiO μFs and electrolyte is unchanged. A clear reduction in both Rct and Rp with Pt loading indicates that Pt accelerates the electron transfer kinetics, probably enhancing the availability of electrons at surface. Cp, related to Hads adsorption, decreases as the amount of Pt increases. Pt NPs act as effective active sites for hydrogen adsorption. Consequently, the higher the number of active sites, the lower their occupancy and therefore the value of Cp.To quantitatively evaluate the effect of Pt decoration of NiO μFs we performed Mott–Schottky (M–S) analysis (SI for details). The M–S plot typically reports the inverse of squared capacitance (C −2) measured as a function of potential applied to the sample, as reported in Fig. 4 (a) [24,49–54]. By increasing E, C −2 goes to zero as the applied potential increases, indicating the presence of a capacitance at the electrode-electrolyte interface. Such behavior is typical of a p-type semiconductor, as NiO is [55,56]. The intercept with x-axis (E FB ) represents the so-called flat band potential [49–54,57–59]. For a planar semiconductor electrode, the quantity ΔE M−S  = E FB  − E OC (where E OC is the open circuit potential) represents the bending of the semiconductor energy bands [24] resulting from the alignment of the Fermi level of the electrode and the redox potential of the electrolyte (violet points in Fig. 4(b)). After the loading of Pt NPs, we observed a clear shift of EFB towards more positive potential, up to 0.454 V in 20Pt–NiO case (Table 2 ). Even if our electrodes are nanostructured semiconductors, such evidence reveals a considerable difference in energy band bending due to Pt decoration. Moreover, it is possible to correlate the effect of decoration on energy band position of NiO with catalytic properties of the electrodes by considering the value of onset potential (E onset , green points in Fig. 4(b)). Onset potential is usually considered an important indicator for the catalytic activity along with the exchange current density in electrocatalysis [34]. For a cathodic reaction, onset potential is the highest potential at which a reaction product (H2 in our case) is formed at an electrode [60]. A commonly used method for the determination of this value is the intersection point between the tangent lines of the Faradaic and non-Faradaic [60–62]. Fig. 4(b) clarifies the effect of Pt loading on ΔE M−S and E onset . As the amount of Pt NPs increases, the energy band bending grows, index of the creation of a nano Schottky junction at the metal-semiconductor interface [24]. Pt decoration increases the energy band bending, because of electron spillover effects, leading to space charge regions and localized electric field [37]. At the same time, a drastic reduction of E onset is observed in presence of Pt NPs. These two evidences confirm that surface decoration of NiO μFs is highly effective in tuning the catalytic properties of our nanostructured electrode. The increase in bending of semiconductor energy levels leads to accumulation of electrons below the semiconductor surface, considerably reducing the activation barrier for H2 production (as described in eq. (1) and demonstrated by the decreased values of E onset for 10Pt and 20Pt–NiO electrodes) and making the HER mechanism more favorable at lower overpotentials. Fig. 4(c) schematizes the effect of Pt loading on NiO band position at the electrode-electrolyte interface.TOF is a crucial parameter for evaluating the HER performance of a catalyst because it reflects the intrinsic electrocatalytic activity of the electrode [2,20,34,63]. As presented in Fig. 5 (a), Pt-decorated samples show markedly high TOF values. It is worth to note that the TOF value of 2.07 s−1 (at an overpotential of 50 mV) found for 20Pt–NiO is comparable (and even superior) to those reported in literature and confirms that the present Pt NP decorated catalyst owns extraordinary efficiency of hydrogen generation (see Table S1).The obtained results are now compared with the state-of-art. Fig. 5(b) shows the comparison of mass activity measured at 10 mA cm–2 and the overpotential for 10 mA cm–2 (based on geometric area) for our decorated electrodes with other Pt-based catalysts under alkaline conditions [2]. In our samples, an increase of the mass loading leads to a reduction of the overpotential for the HER, without a significant decrease of intrinsic activity. This comparison, together with the TOF values, makes our Pt decorated NiO μFs valuable candidates as cathode electrodes for the HER.Motivated by the excellent HER performance of catalysts and the high OER activity of our previously reported NiO μFs on GP [24], we investigated the overall water-splitting performance under alkaline condition by employing 20Pt–NiO μFs as the cathode and NiO μFs as the anode (a scheme of the Pt–NiO||NiO is reported in Fig. 6 (a)). Our as-constructed alkaline electrolytic cell requires a low potential of 1.57 V to afford a current density of 10 mA cm−2 (Fig. 6(b)) and Supplementary Video 1, which is comparable or smaller to other electrocatalysts reported in Table 3 .The following is the supplementary data related to this article: Multimedia component 2 Multimedia component 2 Supplementary video related to this article can be found at https://doi.org/10.1016/j.ijhydene.2022.08.005.In addition, the overall water-splitting durability of the two electrodes was tested using chronopotentiometry for 24 h (Fig. 6(c)) showing a good stability over prolonged times (with an increase of overpotential of 50 mV after 24 h).Our results reveal that the present Pt–NO||NiO electrodes represent highly efficient electrocatalysts.In conclusion, we developed a high-efficiency HER catalyst by synthesizing low-content Pt-NP decorated low-cost NiO microflowers (μFs) onto a graphene paper substrate. By varying the loading of Pt NPs, the role of decoration on catalytic performance of the materials was elucidated in terms of energy band bending of NiO and density of active sites. The Pt–NiO catalyst with optimized NP loading shows an overpotential of only 66 mV at current density of 10 mA cm−2 and a promisingly low Tafel slope of 82 mV dec−1. The performance of NiO μFs were also supported by high intrinsic activity, in terms of TOF of 2.07 s−1 at an overpotential of 50 mV. An alkaline all NiO-based electrolyzer was developed by using Pt–NiO as cathode and bare NiO as anode, requiring a low potential of 1.57 V to afford a current density of 10 mA cm−2 and a good long-term stability. The high activity, and low-cost of the present Pt–NiO μFs pave the way for large-scale and long-term applications of NiO-based catalysts for overall water splitting.L.B. fabricated the nanostructured electrode, acquired and analyzed data, generated the figures, and drafted the manuscript; S.B. analyzed the electrochemical data; M.S. performed STEM and EDX analysis; A.T., F.P., and S.M.: conceived the idea, contributed to data analysis and interpretation; S.M. supervised the project. All authors have given approval to the final version of the manuscript.This research was funded by the project AIM1804097─Programma Operativo Nazionale FSE −FESR “Ricerca e Innovazione 2014–2020” and was supported by the project “Programma di ricerca di ateneo UNICT 2020-22” linea 2.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors wish to thank G. Pantè and S. Tatì (CNR-IMM Catania, Italy) for technical support.The following are the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2022.08.005.

Electrochemical water splitting represents a promising alternative to conventional carbon-based energy sources. The hydrogen evolution reaction (HER) is a key process, still if conducted in alkaline media, its kinetics is slow thus requiring high amount of Pt based catalysts. Extensive research has been focused on reducing Pt utilization by pursuing careful electrode investigation. Here, a low-cost chemical methodology is reported to obtain large amount of microflowers made of interconnected NiO nanowalls (20 nm thick) wisely decorated with ultralow amounts of Pt nanoparticles. These decorated microflowers, dispersed onto graphene paper by drop casting, build a high performance HER electrode exhibiting an overpotential of only 66 mV at current density of 10 mA cm−2 under alkaline conditions. Intrinsic activity of catalyst was evaluated by measuring the Tafel plot (as low as 82 mV/dec) and turnover frequencies (2.07 s−1 for a Pt loading of 11.2 μg cm−2). The effect of Pt decoration has been modelled through energy band bending supported by electrochemical analyses. A full cell for alkaline electrochemical water splitting has been built, composed of Pt decorated NiO microflowers as cathode and bare NiO microflowers as anode, showing a low potential of 1.57 V to afford a current density of 10 mA cm−2 and a good long-term stability. The reported results pave the way towards an extensive utilization of Ni based nanostructures with ultralow Pt content for efficient electrochemical water splitting.

Converting atmospheric carbon dioxide (CO2) to reduced, value-added and energy-dense molecules is of great interest for economic and environmental reasons [1,2]. Among various approaches [3–6], the direct electrochemical reduction of CO2 (CO2RR) into high-value chemical feedstock and fuels has received high attention because it can provide a carbon-neutral energy network by connecting with electricity production from intermittent renewable sources (solar and wind, etc.) [7–9] at ambient temperatures and pressures. Efforts have been devoted to the development of metal or metal-derived catalysts for the reduction of CO2 to gaseous products [10–20], such as carbon monoxide and methane. Among the catalysts, Cu-based ones [21–29] exhibit notable catalytic properties for the conversion of CO2 into multi-carbon hydrocarbons and oxygenates by optimizing the physicochemical properties, including the morphologies [30], chemical states [31–33], alloying [34–38], and so forth. However, selective formation of valuable liquid fuel products, such as ethanol, is still found with low efficiency because of their complex multi-electrons processes.Recently, multi-component tandem catalysts have been proved to enhance the catalytic activity, selectivity and understanding of the structure-property relationships for CO2 reduction accompanied by a multi-step conversion reaction [39, 40]. Using Au nanoparticles deposited on a Cu substrate as an example, a two-step electrochemical reduction mechanism was proposed for the enhanced electrochemical reduction of CO2 to ethanol with the existence of the high concentration CO intermediate [41]. This can be seen as a model to study the tandem catalyst, however, a catalyst in reality is usually in powder and the design of such a catalyst faces several challenges. First, very few metals have been used to prepare the tandem catalysts for producing ethanol and thus the pairing of metals is tricky. Second, the combination of metals may result in the formation of alloys in experiments, suppressing the special functions provided by the individual metals. Third, the fast transfer of the CO intermediates generated on one metal to the second metal before diffusing to the electrolyte/leaking into air is critical for the selective conversion of CO2 to ethanol. A well-designed structure that can resolve the above challenges is thus expected to enhance the catalytic efficiency of the conversion of CO2 and its selectivity towards ethanol.In this work, we use the non-noble metal, carbon-supported Ni nanoparticles (namely, Ni/C), as the catalytic center for converting CO2 to CO, and carbon-supported Cu nanoparticles (namely, Cu/C) as the selective catalyst to accept CO and turn it into ethanol, which have been demonstrated separately but not have been combined for this purpose [42, 43]. The key question is how to combine them in a tandem structure without alloying them and also realize the orientated transfer of CO from Ni/C to Cu/C. We designed a one-dimensional (1D) core-shell structure as shown in Fig. 1a, wherein Ni/C is the core and Cu/C the shell. The metals are in the form of small clusters distributed in mesoporous carbon rather than pure metal wires. In this way, first, the clusters of Ni and Cu are separated by the carbon without forming alloys; second, as the core, Ni/C clusters catalyze the transformation of CO2-to-CO, after which the CO intermediates diffuse to the Cu/C clusters-formed-shell and experience the further reduction to ethanol rather than directly flowing into the electrolyte. The mesopores in the carbon skeleton provide channels for such an orientated mass transfer route, while the 1D composite fibers form a freestanding network that helps the transport of electrons and the stable electrochemical performance as a robust electrode. The material is named Ni(CNFs)@Cu(CNFs), where CNFs are carbon nanofibers. The catalytic testing shows that the ethanol formation was maximized at -1 V vs. RHE (VRHE), with a remarkable Faradaic efficiency (FE) and total current density of 18.2 % and 16 mA cm-2, respectively, and also catalytically stable for at least 100 h. By involving control samples that have the similar structure but contain only Ni/C, only Cu/C, or carbon-supported Ni-Cu alloy (namely, Ni-Cu/C) clusters, it is demonstrated that the trends in ethanol production originate from the tandem catalysis mechanism, that is, Ni/C reduces CO2 to CO near the Ni/C-Cu/C interfaces, driving a high CO coverage and facilitating the following C-C coupling reactions for the selective formation of ethanol. Furthermore, density functional theory (DFT) calculations revealed that Ni(CNFs) in core can play an important role in stabilizing COOH*, which efficiently supplies CO for the dimerization of the *CO intermediates and thus ethanol production.Nafion perfluorinated resin solution (5 wt% in lower aliphatic alcohols and water) and KHCO3 (ACS Reagent 99.7%) were purchased from Sigma Aldrich. Carbon dioxide (CO2, 99.999%), argon (Ar, 99.999%) and carbon monoxide (CO, 99.999%) were supplied by Beijing Beiwen Gases Company. Deionized water (Milli-Q Millipore 18.2 MΩ cm-1) was used throughout the experiments. All chemicals and solvents were commercially available and used as obtained without further purification.The inner precursor solution for NiAc/PAN was prepared by mixing 0.3 g polyacrylonitrile (PAN, Sigma-Aldrich, Mw = 150,000 g mol-1), 0.1 g nickel acetate (NiAc, Sigma-Aldrich, Mw = 248.84 g mol-1) and 4 mL dimethylformamide (DMF, AR, Beijing Chemical Works). After 4 h of stirring at room temperature, the NiAc/PAN inner precursor was obtained. The outer precursor solution for preparing CuAc/PAN was made by mixing 0.3 g PAN, 0.1 g copper acetate (CuAc, Sigma-Aldrich, Mw = 199.65 g mol-1) and 4 mL DMF solution. After 4 h of stirring at room temperature, the CuAc/PAN outer precursor was obtained. Both of inner and outer precursors were transferred into syringes which were equipped with a coaxial nozzle. The coaxial electrospinning setup is illustrated in Fig. S1. In addition, the feeding rates were 0.5 mL h-1 (outer precursor) and 0.4 mL h-1 (inner precursor). Subsequently, a flat Al foil covered with non-dust cloth was used as a collector and put about 15 cm away from the nozzle tip. A voltage of 18 kV was applied to the solution to start the spinning process with a high voltage source (SL50P60, Spellman High Voltage Electronics Corporation).The pristine films of NiAc/PAN@CuAc/PAN were placed in a ceramic boat, heated to 800 °C at a ramp rate of 3 °C min-1 and kept for 2 h under Ar flow. After that, the furnace was cooled down to room temperature naturally.The preparation process was same to that of Ni(CNFs)@Cu(CNFs), except that the solution of NiAc-CuAc/PAN was prepared by mixing 0.3 g of PAN, 0.1 g of NiAc, 0.1 g of CuAc and 4 mL DMF.The preparation process was same to that of Ni(CNFs)@Cu(CNFs), except that the solution of NiAc/PAN or CuAc/PAN was prepared by mixing 0.3 g of PAN, 0.1 g of NiAc or CuAc and 4 mL DMF.The preparation process was same to that of Ni(CNFs)@Cu(CNFs), except that the solution of PAN was prepared by mixing 0.3 g of PAN and 4 mL DMF.The morphology of the samples was characterized by the scanning electron microscope (Hitachi S4800), field emission transmission electron microscope and energy-dispersive X-ray spectroscopy (FEI Tecnai G2 F20 U-TWIN). A Rigaku D/MAX-TTRIII (CBO) X-ray power diffractometer was used to get powder X-ray diffraction (PXRD) patterns by using Cu Kα radiation (λ = 1.5418 Å). Raman spectra were collected using a Renishaw inVia Raman microscope with a laser wavelength of 514.5 nm. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ESCALAB250Xi apparatus with an Al Kα X-ray source. The N2 adsorption/desorption curve was carried out by Brunaue-Emmett-Teller (BET) measurements using a Micrometritics TriStar II 3020 analyzer.5 mg of catalyst was mixed with 50 µL of Nafion solution, 475 µL of ethanol and 475 µL of DI water and then placed in an ultrasonic bath for at least 30 min to achieve a homogeneous ink. Then, 50 µL of the catalyst ink was pipetted onto a carbon paper electrode (0.5 cm2). The loading mass of the catalyst was 0.5 mg cm-2. In addition, since these membranes are flexible and self-supporting, Ni(CNFs)@Cu(CNFs) membrane was also cut into the certain size or shape (for example, ∼0.5 mg) and directly used as working electrodes for bulk electrolysis of CO2RR.All the electrochemical measurements were carried out in a home-made gas-tight two compartment electrochemical cell with a proton exchange membrane (Nafion 117, Dupont) as the separator, equipped with Ag/AgCl reference electrode and platinum counter electrode. Each compartment contained 30 mL of electrolyte. Before the electrolysis, the electrolyte was pre-saturated with CO2 (99.999%, the pH value of the saturated solution was measured to be 6.8) by bubbling the gas for 30 min to remove the air in the cell. During the measurement, CO2 was continuously bubbled into the electrolyte at a flow rate of 20 mL min-1. The electrochemical tests were performed in a CHI660D electrochemical workstation (Shanghai Chenhua Instrument Co., China) at room temperature with 0.1 M KHCO3 aqueous solution as the electrolyte. All of the applied potentials were recorded against an Ag/AgCl (saturated KCl) reference electrode and then converted to those versus reversible hydrogen electrode (RHE) using E (vs. RHE) = E (vs. Ag/AgCl) + 0.197 V + 0.0591 × pH. All potentials were recorded with iR compensation. In addition, the electrocatalytic CORR in 0.1 M KHCO3 solution was performed with similar procedures.Gas products of electrocatalysis were analyzed by an online gas chromatograph (Shimadzu GC 2014) with molecular sieves C13X, Al2O3 column (50 m, 0.53 mm, 10 μm), Rt-Q-BOND PLOT column (30 m, 0.32 mm ID, 10 μm) and equipped with two flame ionization detectors (FIDs) and a thermal conductivity detector (TCD). A GC run was initiated every 15 min. High purity Ar (99.999%) was used as the carrier gas.Liquid product was quantified using 1H NMR (Bruker Advance 400 spectrometer, 400 MHz) via water suppression using a pre-saturation method. Electrolyte (700 µL) was mixed with 35 µL of 10 mM dimethyl sulfoxide and 50 mM phenol as internal standards in D2O for the 1H NMR analysis. The gaseous products were sampled and analyzed online every 15 min during the reaction, and the averaged result was used for discussion. The liquid products were collected and analyzed after the operation for 1 h.In this work, the Faradaic efficiency (FE) of gas products was calculated from the concentration determined by GC using the following equitation: (1) FE % = ppm × flow rate × ( nF P o RT ) × ( j Tot ) − 1 × 100 where ppm is the concentration of gas (CO, CH4 or H2 etc.) determined by GC, n is the electron transfer number, F is the Faradaic constant, Po is the pressure, T = 273.15 K and jTot is the total current density.The FE of liquid products was calculated as follows: (2) FE % = n × C × V × e × N A × 10 − 3 Q × 100 where n is the electron transfer number, C (mol L-1) is the concentration of liquid products in the electrolyte, V (mL) is the volume of the electrolyte, F is the Faradaic constant, e (C mol-1) = 1.6 × 10-19, NA (Avogadro Number) = 6.02 × 1023 and Q (C) is the total amount of charge passed through the system.We carried out the first-principle DFT calculations on the CO2RR activity of Ni(CNFs)@Cu(CNFs) by the projector augmented wave (PAW) method-based Vienna Ab Initio Simulation Package (VASP) [44, 45].The functional of Perdew-Burke-Ernzerhof (PBE) with generalized gradient approximation (GGA) was considered for the electron exchange-correlation [46]. The cutoff energy was set as 450 eV while the force tolerance and energy tolerance were set as less than 0.03 eV Å-1 and 10-4 eV, respectively. A Monkhorst-Pack k-point mesh of 3 × 3 × 1 grid was used to sample the Brillouin zone. A 20 Å vacuum was added along the z direction in order to avoid the interaction between periodic structures. The DFT-D3 method was employed to consider the van der Waals interaction [47].The free energies of the CO2 reduction steps (CO2RR) were calculated with the following equation [48]: (3) Δ G = Δ E DFT + Δ E ZPE − T Δ S where ΔEDFT is the DFT electronic energy difference of each step, ΔEZPE and ΔS are the correction of zero-point energy and the variation of entropy, respectively, which are obtained by vibration analysis, T is the temperature (T=300 K).The Ni(CNFs)@Cu(CNFs) was prepared by using the coaxial electrospinning approach (Fig. S1 ). This technique has the capability to afford large reactive interfaces and efficient mass transfer pathways for the carbon materials [49–55]. Briefly, the polyacrylonitrile (PAN), metal precursors (NiAc or CuAc), and solvent (dimethylformamide, DMF) were sequentially added into a glass vial. After the mixture was stirred for 4 h, the pristine NiAc/PAN@CuAc/PAN nanofibers were obtained by electrospinning, which were then transformed into the porous Ni(CNFs)@Cu(CNFs) by heating in Ar (Fig. S2). The preparation of other samples is detailed in the Material and methods. The morphology of the as-obtained catalysts was characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). Taking Ni(CNFs)@Cu(CNFs) as a representative (Fig. 1 b, c), the nanofibers have an average diameter of ∼300 nm, similar to those of CNFs, Ni(CNFs), Cu(CNFs), and Ni-Cu(CNFs) (Fig. S3). Fig. 1b shows the nanoclusters distributing in the CNFs and further characterizations using HRTEM show that the clusters are around 5 nm which have typical lattices of Ni and Cu. It is noted that the lattice of Ni is not seen as clear as that of Cu because it is in the core of the fiber (Fig. 1c). Moreover, the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image shows a clear contrast between a core and a shell, indicating the core-shell structure, which was further confirmed by the linear energy dispersive X-ray spectroscopy (EDS) scan across a single fiber (inset in Fig. 1d). The corresponding element mapping images also show the Ni/C-core and Cu/C-shell configuration and reflect the uniform distribution of Cu, Ni, N and C elements (Fig. 1d, e). Altogether, electron microscopy provides evidence for the synthesis of the porous, core-shell, and metal cluster/carbon fiber structures. The as-prepared material could probably enable the orientated mass transfer route (CO2-to-CO via Ni/C and then CO-to-ethanol via Cu/C), which is expected to be favorable for catalyzing the reduction of CO2 to ethanol selectively.The as-prepared catalysts were characterized by X-ray diffraction (XRD), nitrogen (N2) adsorption/desorption, Raman and X-ray photoelectron spectroscopy (XPS) measurements. XRD measurement was carried out to investigate the bulk and near-surface crystal structures. Fig. 2 a shows XRD patterns of the as-obtained carbon materials. A broad peak was observed for all the materials at 2θ around 23° originating from the (002) reflection of the graphitic carbon. Typical characteristic patterns of metals including Ni and/or Cu were observed in the samples of Ni(CNFs)@Cu(CNFs), Ni(CNFs) and Cu(CNFs), suggesting the crystalline metal clusters as distributed in the carbon fibers. Compared with the typical diffraction for Cu (43.5° (111) and 50.6° (200), PDF No. 004-0836) and Ni (44.5° (111) and 51.8° (200), PDF No.04-0850) [56], the Ni-Cu(CNFs) sample shows the shift of the Cu and Ni diffraction positions, indicating the presence of CuNi alloy phase [57]. It reminds us that separating the metals in core and shell is significant not only to the orientated mass transfer but also to the prevention of metal alloying.The N2 adsorption/desorption and porosity measurements were carried out to investigate the surface and bulk properties of the materials (Fig. 2c, d). All the materials present the typical type-IV N2 adsorption-desorption isotherms with a hysteresis loop, suggesting the presence of mesoporous structures. Based on the N2 sorption isotherms, the specific surface area (SBET), pore volume (VP), and pore size (DP) were calculated using BET and Barrett-Joyner-Halenda (BJH) models. The SBET of the materials are in the range of 45 to 1082 m2 g-1 as shown in Table S1. The Ni(CNFs)@Cu(CNFs) catalyst shows high surface area and pore volume, which may be due to the coexistence of two metals and the formation of the core-shell interfaces. On the one hand, metal causes the combustion of carbon during calcinations [58]. Along with the decomposition of the polymers, mesopores appeared in the carbon fibers. Compared to the alloying of Ni and Cu during the preparation of Ni-Cu(CNFs) which reduced the amount of metal species, all the Ni and Cu clusters participated in the perforation process and formed more pores in the case of Ni(CNFs)@Cu(CNFs). On the other hand, there is an intrinsic interface between Ni/C-core and Cu/C-shell during the electrospinning, such a phase separation may result in more voids/holes on the interface after heat treatment as compared to the dense fiber materials like Ni(CNFs) and Cu(CNFs). The corresponding pore size distribution curves in Fig. 2d show the hierarchical pore distributions and markedly improved pore volumes (from 0.03 cm3 g-1 for CNFs to 0.15 cm3 g-1 for Ni(CNFs)@Cu(CNFs)). The high surface area and unique porous structure provide the catalyst with abundant active sites to adsorb the reactant molecules dissolved in the electrolyte and channels to deliver the reaction species between the active sites, making the electro-catalytic process more efficient. Raman spectroscopy measurement was carried out to characterize the carbon structure (Fig. S4, ), where the intensity ratios of D band (1345 cm-1) to G band (1590 cm-1) are subject to small oscillation from 0.94 to 1, indicating the similar graphitization degree, which is in line with the broad carbon peaks as observed in the XRD patterns (Fig. 2a).The composition and oxidation states of the elements at the surfaces of all the catalysts have been investigated by XPS measurements as shown in Figs. 2e, f and S5, S6, S7. In the high-resolution Cu 2p spectrum (Figs. 2e and S5), all the samples exhibit only two main peaks associated with Cu0 and a series of satellite peaks [59, 60] from CuO, suggesting that Cu(CNFs), Ni(CNFs)@Cu(CNFs) and Ni-Cu(CNFs) also contain oxides (at least on the surface). Cu0 is generally used to catalyze the formation of multi-carbon products during CO2RR, while the oxides (i.e. Cu oxides) can assist the CORR to generate multi-carbon oxygenates and hydrocarbons due to its metastable surface features that bind CO strongly [33, 61]. In the high-resolution Ni 2p spectra (Figs. 2f and S6), the Ni 2p3/2 binding energy signals of Ni(CNFs), Ni-Cu(CNFs) and Ni(CNFs)@Cu(CNFs) were split into metallic Ni0 and low-valent Niδ+ (855.7 eV), which was reported to be due to the Ni-N bonding, as a typical active site for electroreduction of CO2 to CO [62–64]. Additionally, the Ni clusters are surrounded by N-doped carbon. Ni was reported to be efficient in stabilizing COOH* intermediate but the desorption of CO is difficult. On the contrary, carbon facilitates the escape of CO and thus the combination of Ni and carbon optimizes the stabilization of COOH*and timely desorption of CO. Carbon also helps to suppress the hydrogen evolution reaction (HER), thereby resulting in improved activity and selectivity towards CO formation [42, 65]. The N 1s XPS spectra reveal the existence of four types of nitrogen species [66], including pyridinic N (398.7 eV, Ni), pyrrolic N (400.2 eV, Nii), graphitic N (401.1 eV, Niii) and oxidized N (402.6 eV, Niv) species (Fig. S7). Quantitatively, the total N content in Ni(CNFs)@Cu(CNFs) was determined to be 7.64 at%, similar to the other catalysts. The pyridinic N generally presents higher chemical activity and tends to capture transition-metal atoms as individual atoms [62]. Therefore, the N 1s XPS spectra of the other samples are displayed (Fig. S7f) for comparison, the relative content of pyridinic N in core-shell catalysts is slightly higher than the control samples. These results further indicated that core-shell samples possess higher N contents, forming metal-N motifs, and then regulating the surface electronic structures of metal species. Especially, affording effective N-coordinated Ni (i.e. Ni-N motifs) that could facilitate the CO2 adsorption and CO desorption process boosted the conversion of CO2 into CO [62].To quantitatively analyze and compare catalytic activity, the core-shell catalyst was loaded in a gas-tight H-type cell configuration coupled with an online gas chromatography (GC). It was compared with CNFs, Ni(CNFs), Cu(CNFs) and Ni-Cu(CNFs) catalysts using the same set-up and conditions. The liquid products were analyzed after the reaction by quantitative nuclear magnetic resonance spectroscopy (NMR). The electrolysis was performed in a 0.1 M KHCO3 (pH=6.8) electrolyte saturated with CO2. The average current densities of the electrodes made of the five catalysts at potentials between -0.7 and -1.1 VRHE are summarized in Fig. 3 a. In comparison to CNFs, Ni(CNFs), Cu(CNFs) and Ni(CNFs)@Cu(CNFs), the Ni-Cu(CNFs) catalyst shows the highest current densities. This result is similar to the report [57] using Cu/Ni alloy nanoparticles embedded in a nitrogen-carbon network that demonstrated a higher intrinsic activity for the production of CO as well as current density at a low applied potential toward CO2 reduction than the individual Cu or Ni particles due to the higher CO2 adsorption capacity. The FEs of the products were calculated and the results of which are presented in Fig. 3b-f and S8a . For Ni(CNFs)@Cu(CNFs), CO with a FE of 29.3% was detected as the major gaseous C1 product accompanied by H2 as a byproduct at the positive potential. C2H5OH began to evolve at a potential of -0.8 VRHE, but CO species were still the main products. When more negative potentials were applied, the FE of C2H5OH dramatically increased to an optimal value of 18.2% at -1 VRHE and then decrease at more negative potential, which may be due to the competition from HER. It is noted that other C2 products such as C2H4 were also produced although the content was low (FE ∼1%-2%). In addition, it is observed that the FE for CO (Fig. 3e) and H2 (Fig. S8b) production decrease with increasing the FE associated with C2H5OH product, suggesting the consumption of CO during the reaction [40, 67].As the control sample, CNFs showed negligible activity for electrochemical CO2 reduction. As for Ni(CNFs) and Ni-Cu(CNFs), CO was preferentially produced throughout a broad potential range (from -0.7 to -1.1 VRHE, Fig. 3c and f). The catalytic performances of Ni(CNFs) and Ni-Cu(CNFs) are typical as the reported Ni-based electrodes [68, 69]. In the case of Cu(CNFs), CO and formate were observed as the major products at low overpotentials. It is noteworthy that CO2 reduction rates increased and CH4, C2H4 and C2H5OH were detected from the Cu(CNFs) electrode at more negative potentials, agreeing well with previous measurements on Cu-based catalysts [70]. Interestingly, although Cu(CNFs) produced formate with a maximum FE of 18%, the FE of formate on the Ni(CNFs)@Cu(CNFs) and Ni-Cu(CNFs) were less than 2%. As a key intermediate to formate, *COOH could also be reduced to other products such as CO, indicating that the binding energy of *COOH to the catalyst has been changed when there was Ni/C, which prefers the CO2-to-CO route. The above observation manifests that the formate formation pathway is less favored over the Ni(CNFs)@Cu(CNFs) catalyst.A long-term electrolysis was performed at a stationary -1 VRHE for 100 h to test the stability of Ni(CNFs)@Cu(CNFs) catalyst. Compared to the carbon paper electrode, the self-supporting character of the sample maintained a steady current density of ∼16 mA cm-2 with negligible drop throughout the stability test (Fig. 3g and S9). Note that the periodic fluctuation of the current-time curve is due to the changing of electrolyte every 12 h. Furthermore, the morphology and chemical structure of Ni(CNFs)@Cu(CNFs) after 100 hours of reaction was investigated and no noticeable changes could be observed relative to the fresh one (Fig. S10). Impressively, according to Fig. 3h, the FE of core-shell samples reached 18.2% at -1 VRHE, which outperformed many of the reported CO2 to C2H5OH reduction catalysts under similar electrolysis conditions (Table S2), that is, an H-type cell consisted of a carbon paper (the carrier electrode of as-prepared catalyst) as the working electrode, Pt as the counter electrode, and Ag/AgCl as the reference electrode in CO2-saturated 0.1 M KHCO3 solution [30, 40, 41, 70-78]. The unique structure of Ni(CNFs)@Cu(CNFs) avoids the typical powdering and binding procedures, inhibits metal particles from agglomerating and enables the orientated mass transfer between the active sites, resulting in the high current density and selectivity in reducing CO2 towards ethanol.The electrochemical testing indicates that Ni(CNFs) core and Cu(CNFs) shell act synergistically to promote the formation of ethanol. We postulate a tandem catalysis mechanism, that is, Ni/C reduces CO2 to CO near the Ni/C-Cu/C interfaces and the CO species transfer to the Cu/C sites to produce ethanol. To prove the hypothesis of the two-sites mechanism, we carried out direct CO electroreduction tests on Ni(CNFs) and Cu(CNFs) at -1 VRHE for 1h in 0.1 M KHCO3. Under pure CO feeding, the FE of C2H5OH of Cu(CNFs) increased and reached almost the same value compared to Ni(CNFs)@Cu(CNFs) catalyst, while Ni(CNFs) showed a negligible production of ethanol under the same conditions, suggesting that it could not further promote the formation of *CO intermediates, which is the necessary reaction intermediate for C2+ products generation (Fig. 4 a). Comparing the performances of Cu(CNFs), Ni(CNFs)@Cu(CNFs) and Ni-Cu(CNFs) (Fig. 3, 4a), it is clear that the core-shell structure results in the suppression of H2 evolution, increase of sufficient local CO concentration and the subsequent CO-insertion caused ethanol production. Additionally, the absolute FE for CO reduction was lower than those of CO2 reduction due to the CO is sparingly soluble in water (Henry's law [79, 80]). The insights gained from the Ni(CNFs)@Cu(CNFs) catalyst provide a new possibility for developing highly active tandem catalysts.Regarding the detailed CO2RR mechanisms of using Ni(CNFs)@Cu(CNFs) as the catalyst, DFT calculations on the free energies of the intermediates along the reaction pathway (CO2 to ethanol) were carried out. Firstly, for electrochemically reducing CO2 to CO, we considered two different graphene structures (bridge-top and top-fcc) in a carbon coated model on Ni (Fig. 4b and S11-13) [81]. For comparison, Ni(111), pristine graphene and Ni-N4 embedded graphene (Ni-N4/Gr) were used as the models, wherein Ni-N4 has been reported as an efficient Ni catalyst for CO2RR to CO [82, 83]. The generation of CO by CO2RR involves the exchanges of two electrons and two protons: (i) CO2 + * + H+ + e- → COOH*; (ii) COOH* + H+ + e- → CO* + H2O; and (iii) CO* → CO + * (* represents the active site of catalyst). As shown in Fig. 4b, Gr/Ni (111) presents a lower free energy of COOH* (0.62-0.75 eV) than Ni-N4/Gr (1.56 eV) and pristine graphene (2.17 eV), indicating that COOH* formation is more favorable, which is calculated to be the rate-determining step (RDS) for all the catalysts. Although Ni (111) shows a lowest free energy (-0.01 eV) change for COOH*, it requires a large energy penalty for CO desorption, manifesting an overall difficult CO production. Thus, Ni(CNFs) stabilizes COOH* without affecting the easy CO* desorption and improves the catalytic activity for electrochemically reducing CO2 reduction to CO. Subsequently, the preferential formation of ethanol on Cu(CNFs) was proposed as follows (Fig. 4c and S13): CO → *CO→ *CO + *CO → *CO + *CHO → *COCHO → *COHCHO → *COHCHOH → *CCHOH → *CHCHOH → *CHCH2OH → *CH2CH2OH → *CH3CH2OH. It can be seen that the formation of *CO is exergonic, indicating that the CO is prone to adsorb on the Cu(CNFs) surface. The potential-determining step (PDS) of ethanol formation is *CO + H+ + e- → *CHO and the energy barrier of which is 0.88 eV (ΔG). On the contrary, the hydrogenation is difficult with ΔG = 1.23eV, illustrating that the formation of double carbon (C2) intermediate (*CO - *COH, orange lines) is not preferable. Further, the C-C coupling by the dimerization of *CO intermediates is promoted to form the *COCHO intermediates (ΔG = 0.50 eV) and the rest hydrogenation steps to the final product ethanol are facile due to exothermic process or endothermic process consuming a small energy (blue lines), indicating that the ethanol pathway is favorable on Cu(CNFs) surface, which is in agreement with the experimental results.In summary, to realize a highly selective production of ethanol through CO2RR, an orientated mass transfer route is proposed and achieved by making a Ni(CNFs)@Cu(CNFs) membrane catalyst. It is found that the unique Ni/C-core@Cu/C-shell design synergistically resulted in the suppression of H2 production, the local CO concentration increase and a tailored transfer of the CO species from the inner Ni/C to outer Cu/C for the selective formation of ethanol. Compared with the control catalysts with individual metals or alloys, an impressive FE for ethanol of 18.2% at -1 VRHE in 0.1 M KHCO3 for at least 100 h was achieved using the Ni(CNFs)@Cu(CNFs) as the catalyst, which outperforms many of the reported CO2 to ethanol reduction catalysts under similar electrolysis conditions. The concept of tandem catalysis for selective formation of ethanol was further demonstrated by performing CO reduction on Ni(CNFs) and Cu(CNFs) catalysts. DFT calculations suggest that the combination of the Ni(CNFs) and Cu(CNFs) can enhance the stabilization of oxygenic C2 intermediate. The design and realization of the orientated mass transfer in a CO2RR could facilitate the preparation of unique hierarchical structures for tailored reactions used in catalysis or batteries.The authors declare that they have no conflicts of interest in this work.We acknowledge the financial support from the National Natural Science Foundation of China (Grants No. U20A20131 and 51425302).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.fmre.2021.08.021. Image, application 1 Image, application 2

Electrochemically reducing CO2 to ethanol is attractive but suffers from poor selectivity. Tandem catalysis that integrates the activation of CO2 to an intermediate using one active site and the subsequent formation of hydrocarbons on the other site offers a promising approach, where the control of the intermediate transfer between different catalytic sites is challenging. We propose an internally self-feeding mechanism that relies on the orientation of the mass transfer in a hierarchical structure and demonstrate it using a one-dimensional (1D) tandem core-shell catalyst. Specifically, the carbon-coated Ni-core (Ni/C) catalyzes the transformation of CO2-to-CO, after which the CO intermediates are guided to diffuse to the carbon-coated Cu-shell (Cu/C) and experience the selective reduction to ethanol, realizing the orientated key intermediate transfer. Results show that the Faradaic efficiency for ethanol was 18.2% at -1 V vs. RHE (VRHE) for up to 100 h. The following mechanism study supports the hypothesis that the CO2 reduction on Ni/C generates CO, which is further reduced to ethanol on Cu/C sites. Density functional theory calculations suggest a combined effect of the availability of CO intermediate in Ni/C core and the dimerization of key *CO intermediates, as well as the subsequent proton-electron transfer process on the Cu/C shell.

The increasingly high energy demands and the environmental issues related to energy consumption have caused great global concern over the past few decades (Li et al., 2017; Rönsch et al., 2016). Increasing global population is the main factor behind the increase in energy consumption. Natural gas has recently become a crucial resource due to its high energy density, ease of transportation and limited polluting effect. Synthetic natural gas has been produced from coal or biomass via syngas (CO ＋ H 2 ) in different ways. CO methanation is seen as an important means for transforming coal into natural gas. In fact, the production of methane from coal via syngas not only plays a highly important role in the efficient and comprehensive utilisation of coal but also provides a practical way of supplementing the shortage of natural gas reserves (Li et al., 2016; Zhang et al., 2018b).Due to its excellent activity and relatively low cost, a Ni-based catalyst has become one of the most popular catalysts for methanation. In addition to altering the surface area of the carrier, engineering the support’s morphology is considered to be a powerful way of modifying the metal–support interactions in oxide-supported catalysts, which can influence both the size and distribution of the metal nanoparticles. Silica (SiO2) has been widely applied in many methanation catalysts due to its rich pore volume and surface area (Li et al., 2019). Among the different catalyst structures, hollow-structured SiO2, which comprises a void space inside a distinct shell, has received a great deal of attention due to its intriguing physicochemical properties and huge potential (Liang et al., 2017; Yu et al., 2018). The special features of the hollow structures, such as their high surface area and high loading capacity, means that they serve as excellent platforms for catalysts in terms of improving the diffusion of the active components and offering adequate reaction sites (Yao et al., 2019). However, the common methods for preparing SiO2 hollow microspheres, namely, sol–gel, hydrothermal and microemulsion methods, involve a synthesis process that is cumbersome and time consuming. Hence, identifying a simple and quick synthesis method for producing hollow-structured SiO2 microspheres has become a major focus in recent years.In the present study, highly uniform hollow nanoflowers, SiO2 nanospheres within a range of 400 to 500 nm, were synthesised through a method known as flash nanoprecipitation (FNP). This method presents new technology that allows us to quickly prepare nanoparticles through rapidly colliding different reaction solutions in a mixed mould. The advantages of FNP technology are obvious, mainly include the following points. (1) Fast processing. (2) Simple equipment. (3) Narrow size distribution. (4) Good reproducibility. (5) The experiment can not only be carried out at laboratory scale with small amounts of solutions but also can be easily scaled up to pilot scale. Thus, it has received an increasing amount of attention in recent years (Bteich et al., 2017; Grundy et al., 2018; Morozova et al., 2019). Flash nanoprecipitation (FNP) has previously been demonstrated to produce core–shell and Janus colloids from homopolymer blends. Wang et al. (2015) obtained fluorescent nanoparticles which possessing a narrow size distribution (50 nm) with desirable fluorescence properties through Flash nanoprecipitation (FNP) method. Lorena et al. (2018) prepare colloids with internally structured geometries from blends of block copolymers and homopolymers by using FNP method.Mo–polydopamine (PDA) is a typical organic–inorganic coordination complex which can be reconstructed into hierarchical spheres (Huang et al., 2016) or hierarchical nanoflowers (Sun et al., 2017) to provide a stable template for the hydrolysis of SiO2. A hollow spherical structure with high strength and stability and a self-assembly characteristic for the generated Mo–PDA complex monomer in a two-dimensional laminar structure provides a stable framework for tetraethyl orthosilicate (TEOS) hydrolysis when applied to the process of adjusting the PH value (Cui et al., 2014; Ma et al., 2015). Chitosan (CTS), a natural cationic polysaccharide, can be introduced during the process of synthesis (Wang et al., 2018b) and, as a polymer chain, can be bound to the MoDo spherical framework to form a large nanoflower-like SiO2.In our study, a series of SiO2 methanation catalysts with different morphologies are synthesised using the FNP method. Here, highly uniformed nanospheres and nanoflowers act as carriers for the active component, Ni, during the methanation. The effect of morphology on catalytic performance is subsequently investigated. Compared to solid smooth nanospheres and hollow nanospheres, hollow nanoflowers have larger specific surface areas. This was expected to result in a higher dispersion of Ni during the impregnation process, which would result in the nanoflower catalyst demonstrating better catalytic activity than the other candidates.A new simple and generic method known as flash-nanoprecipitation (FNP) was developed to produce nanoparticles with desired particle sizes. Syringe pumps, glass syringes and multi-inlet vortex mixer are the main components of FNP technology (Scheme 1). FNP is a rapid process to prepare nanoparticles within only 1 s using a multi-inlet vortex mixer system. FNP technology has a wide range of applications in the field of drug nanoparticle preparation, but the preparation process of inorganic nanoparticles is rarely reported. In this work, different morphologies SiO2 nanospheres are synthesised within a few minutes using FNP technology which provides a fast and time-saving synthesis method for the preparation of hollow flower-like SiO2. For the synthesis of solid SiO2, 50 mL of deionised water was dripped with 1 mL of ammonia (25%) to form solution A. Then, a 2 mL solution of TEOS was added to 50 mL of ethanol and stirred well to form solution B. Two 50 mL syringes were used to extract solutions A and B before both solutions were rapidly mixed with an injection speed of 40 mL/min in a two-channel mould. The mixed solution was then collected and washed through centrifugation to obtain solid SiO2 nanospheres. Just as shown in Scheme 1(a).For the synthesis of hierarchical flower ridge-like SiO2 micro/nanostructure, 0.6 g of dopamine and 0.5 g of ammonium molybdate were each dissolved in 100 mL of deionised water, mixed with a magnetic stirrer for 5 min and labelled as solutions A and B, respectively. 6 mL of TEOS was dissolved in 400 mL of absolute ethanol and magnetically stirred for 5 min; this solution was labelled solution C. Two 50 mL syringes were used to extract 50 mL of solutions A and B, and another two 50 mL syringes were used to extract solution C. After being dispersed into a four-channel mould, the injection speed of solutions A and B was set to 40 mL/min while that for solution C was set to 80 mL/min. The precise injection speed of the syringe pump effectively controlled the alcohol–water level with a stoichiometric ratio of 1:2. The synthetic route is shown in schematic diagram 1 (b)The reaction solution was collected and 1.2 mL of 25% wt ammonia was added to adjust the pH to 9.2. The brown-coloured solution was centrifuged at 4000 rpm and washed with ethanol and deionised water before being left to dry at 120 °C. Hierarchical flower ridge-like SiO2 micro/nanostructures were obtained following calcination at 400 °C in air atmosphere and were subsequently labelled MoDoHSiO2.Large-sized hierarchical flower ridge-like SiO2 micro/nanostructure was prepared as follows. 2 g of CTS was added to 50 mL of deionised water. Then, 2 mL of glacial acetic acid was added. The solution was thoroughly stirred to dissolve CTS. When CTS completely dissolved, a further 50 mL of deionised water was added, and the solution was continuously stirred. After adding 0.5 g of ammonium molybdate, a milky solution was obtained and labelled solution A. Thereafter, 0.6 g of dopamine was dissolved in 100 mL of deionised water and labelled solution B, while 6 mL of TEOS was thoroughly dissolved in 400 mL of absolute ethanol through magnetic stirring and labelled solution C. The remaining steps were the same as those used in preparing the hierarchical flower ridge-like SiO2 micro/nanostructure. The solution was labelled CTSMoDoHSiO2 following calcination in air at 400 °C. Just as shown in Scheme 1(c)A Ni-based SiO2 catalyst was synthesised using the traditional impregnation method, with the Ni load on the catalyst set at 30 wt%. First, 0.5 g of the as-obtained SiO2 micro/nanostructure carrier was weighed and dissolved in 30 mL of deionised water. 0.75 g of Ni(NO3)2 ⋅ 6H2O was added and the mixture was stirred for 30 min. The mixed solution was evaporated to a dry state in a water bath at 80 °C, thoroughly ground and calcined in air at 400 °C (heating rate 3 °C/min) for 2 h, resulting in NiSSiO2, NiMoDoHSiO2 and NiCTSMoDoHSiO2, respectively.The crystal structure of the different materials was determined by X-ray diffractometer (XRD) with CuK α (40 kV, 40 mA, k = 1.5406 Å and 2 θ range from 10 ° –90°) radiation. The samples were subjected to X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250 Xi) by using Al K α radiation (1486.6 eV). All binding energies (BE) were calibrated using the C1s peak (BE = 284.8 eV) as a standard. Morphology and microstructure of the different samples were determined by transmission electron microscopy (TEM, Tecnai G2 F30 S-TWIN (300 KV), high resolution TEM (HRTEM) and selected area electron diffraction (Talos-F200X). The Brunauer–Emmett–Teller​ (BET) specific surface area and the Barrett–Joyner–Halenda (BJH) pore size distribution characteristics of the catalyst were measured by a nitrogen adsorption desorption analyse. The H2 reduction behaviour of the different samples was tested by a hydrogen temperature programmed reduction (H2-TPR) apparatus with a blend gas of 10% H2/Ar (30 mL.min−1) as a reducing gas and the temperature was increased to 800 °C at a heating rate of 10 °C ⋅ min−1. A stainless steel tube with a length of 75 cm and an inner diameter of 10 mm is used in the evaluation device. First, take an appropriate amount of 10 mesh quartz sand into the reaction tube to a position detectable by the thermocouple (K-type, WRNK-191), then put in an appropriate amount of quartz wool, and finally pour the required amount of catalysis. The catalyst evaluation device is a four-channel methanation fixed bed reaction platform. The reaction temperature is mainly adjusted by a temperature controller (AI-518P) and syngas is measured and fed in through a mass flow controller (S49 33/MT). The water in the product is condensed by the condenser, and further dried to remove water through a drying column. The experimental data is monitored online by gas chromatography (SHIMADZU, GC 2014), detector is a thermal conductivity detector and the column model is TDX-01. Sampling interval is 17 min. The external standard method is used to calibrate the chromatography. The CO conversion and CH4 selectivity calculation formula as fallow X CO ( % ) = ( V CO,in − V CO,out ) ∕ V CO,in × 100 % S CH4 ( % ) = V CH4,out ∕ ( V CO,in − V CO,out ) × 100 % Catalytic activity of the catalysts was measured on a fixed bed microreactor. During the test, 0.15 g of catalyst sample was introduced in a stainless steel microreactor. Firstly, the catalyst was heated to 500 °C under nitrogen with a flow of 60 mL ⋅ min−1 and then reduced with a 60 mL ⋅ min−1 H 2 flow for 2 h. Reaction was then performed with a weight hourly space velocity (WHSV) of 26,000 mL ⋅ g−1 ⋅ h−1 (H2/CO with molar ratio of 3:1 and total flow rate (STP) of 65 mL ⋅ min−1). Gas composition was evaluated by chromatography. The stability tests of different samples was carried out at 300 °C. Fig. 1a shows the XRD patterns of SiO2 carriers with different morphologies following calcination at 400 °C for 2 h. The XRD patterns of the three samples exhibited diffraction peaks only for SiO2. While (NH4)6Mo7O24 was used in the synthesis process for the MoDoHSiO2 and CTSMoDoHSiO2 samples, no diffraction peaks for MoO2 appeared following calcination. A possible reason for this is that (NH4)6Mo7O24 was coated with TEOS during the hydrolysis process and was covered with SiO2 following calcination, which cannot be detected.Field emission scanning electron microscopy (FESEM) was used to examine the morphology and microstructure of SiO2 with different morphologies. As shown in Fig. 1b, the SSiO2 exhibited uniform and unmixed slick three-dimensional monodispersed SiO2 slippery microspheres with an average diameter of approximately 500 nm. The smoother surface of the SSiO2 may result in a poor distribution of the active components during impregnation. Fig. 1c shows the MoDoSiO2 sample, which exhibited a uniform hierarchical flower-like microsphere geometry structure with an average diameter of approximately 400 nm. The nanosheets overlapped to form a flower ridge-like structure that likely has a high surface area, which will improve the impregnation efficiency. Fig. 1d shows CTSMoDoSiO2 nanospheres formed in the presence of CTS. It is clear that, in the presence of CTS, the morphology had undergone little change compared to the MoDoSiO2 sample; however, the size significantly increases to around 1 μ m (Sun et al., 2018b).The pore size distribution and specific surface area of the different SiO2 carriers following calcination were obtained from the nitrogen adsorption/desorption measurement (Zhang et al., 2018a). Fig. 1e shows that all the samples exhibited type IV isotherms except for the SSiO2, suggesting the presence of mesoporous structures (Wang et al., 2018a). The type-H 3 hysteresis loop appearing at a relative pressure of p/p0 > 0.5 indicates that slit-like pores were formed (Lu et al., 2019), which may be a result of the accumulating of flower ridge-like particles. The SSiO2 has the smallest specific surface area, only 14.9 m2/g. The analysis results of N 2 adsorption/desorption curve and pore volume and pore size distribution diagram show that the adsorption amount of SSiO2 is small. Moreover, the FESEM characterisation shows that the surface of SSiO2 is relatively smooth. Therefore, SSiO2 has a small specific surface area. The MoDoHSiO2 and CTSMoDoHSiO2 exhibited large surface areas of 244 and 194 m2/g, respectively. This can be explained by the fact that the introduction of the flower ridge-like structure enhanced the surface area. The pore size distribution curves (Fig. 1f) were acquired according to the Barret–JoynerHalenda model. In comparison, the MoDoHSiO2 exhibited small pores of around 6 nm, while CTSMoDoHSiO2 pores were over 20 nm. This may be related to the particle size of the sample itself. In general, the flower ridge-like micro/nanostructure with a large specific surface area and rich mesopores enhanced the contact area between the active component and the reaction gas, which promoted the activity of the catalyst (Das et al., 2019). All the synthesis in our work proceeded according to the self-assembly of the MoPDA complex (Dandan Wang et al., 2016), as shown in Fig. 2. Due to the slight solubility of Mo–dopamine chelates in ethanol, with the mixture of ethanol and H 2 O in the mould, an interface was formed where the hydrophobic groups of the Mo–dopamine complexes pointed towards the water and the hydrophilic groups were orientated outward (Dandan Wang et al., 2016). A rapid polymerisation of molybdate anion and dopamine occurred when the different solutions were flash mixed in the mould to produce an orange–red colour (Dandan Wang et al., 2016). With ammonia added dropwise into the solution, the self-polymerisation of the dopamine was initiated along the interface (Dandan Wang et al., 2016). At the same time, the TEOS uniformly hydrolysed on the surface of the microspheres and SiO2 with different morphologies were formed. Table 1 lists the different synthesis methods for the preparation of different morphologies and sizes of SiO2. As can be seen from the table, the traditional method of preparing hollow SiO2 was cumbersome and time-consuming. For the novel FNP synthesis technology, a variety of reaction solutions can be quickly mixed within 1 min. The entire synthesis process can be completed within 10 min. FNP technology provides a fast and time-saving synthesis method for the preparation of hollow flower-like SiO2. Fig. 3a shows the XRD patterns of the catalyst precursors formed via immersion and calcination for the SiO2 microspheres with different morphologies. The dominant diffraction peaks are well matched with the NiO, where the peaks at 2 θ = 37 .3°, 43.3°, 62.8° and 74.4° belong to the (111), (200), (220) and (311) crystalline planes of NiO, respectively (Song et al., 2019). In particular, the sharp peak of NiO in the NiSSiO2 sample indicates the formation of large-sized NiO particles (Ren et al., 2018). Meanwhile, the broader and weaker diffraction peaks of NiO in the NiCTSMoDoHSiO2 and NiMoDoHSiO2 samples can be assigned to the smaller NiO particle size (Song et al., 2019). From the XRD patterns in Fig. 3a, we can roughly infer that during the calcination process, the NiMoDoHSiO2 and NiCTSMoDoSiO2 samples had smaller NiO particle sizes and higher metal dispersion compared to the NiSSiO2 sample. To verify our inference, hydrogen pulse chemisorption (H 2 -PULSE), high-resolution transmission electron microscopy (HRTEM) and inductively coupled plasma (ICP) spectrometry were performed, with the results presented in Table 2 and Fig. 3b–f. Fig. 3b–f shows the methanation catalyst precursors formed following calcination and H 2 reduction. It is clear that Ni particles were present. There were massive Ni particles with different sizes on the smooth surface of the NiSSiO2 samples (Fig. 3b), which were extremely inhomogeneous. In terms of the NiMoDoHSiO2, as Fig. 3c and 3d show, Ni particles with a particle size of around 3 nm were uniformly supported on the flower-like surface. This was consistent with the results subsequently obtained using hydrogen temperature programmed reduction (H 2 -TPR) and H 2 -PULSE. For the NiCTSMoDoHSiO2 sample (Fig. 3e, f), Ni particles with an approximate size of 7 nm were formed on the surface, which were slightly larger than the 3 nm particles of the NiMoDoHSiO2 sample. The size of the CTSMoDoSiO2 support was around 1 μ m, which may lead to the formation of large Ni particles during impregnation. The redox properties of all the calcined samples and the interaction between the metal particles and the support were determined via H 2 -TPR (Song et al., 2019), as shown in Fig. 4. The NiSSiO2 catalyst exhibited a sharp reduction peak at 325 °C, which could have been caused by the reduction of the ‘free state’ NiO resulting in a weak interaction with the support (Song et al., 2019). This form of NiO, always characterised by a large particle size, had similar properties to the bulk NiO and could be easily reduced. The reduction peaks of the NiMoDoHSiO2 and NiCTSMoDoSiO2 samples appeared at 362 °C and 359 °C, respectively. The higher reduction temperatures can be attributed to NiO with smaller particles or to the stronger interaction and support (Saché et al., 2018). The NiMoDoHSiO2 sample had the highest reduction peak area compared to the other two catalyst samples, indicating that more NiO was reduced during the reaction. This can also be attributed to the weaker interaction between the NiO and the support (Zou et al., 2010). Fig. 4 shows that smaller NiO was present on the NiMoDoHSiO2 and NiCTSMoDoSiO2 samples following the impregnation method and that the interaction between the NiO and the support was weak.From the H 2 -PULSE and ICP results, as presented in Table 2, it is clear that NiCTSMoDoSiO2 possessed the highest metal dispersion (1.52%) and metal surface area (2.82 m2/g). The poorest metal dispersion was demonstrated by the NiSSiO2 sample, which was due to the smooth surface inhibiting the effective distribution of the active components. This result was in line with the conclusions drawn from the results shown in Fig. 3. The surface oxidation state of Ni and other different elements were studied using X-ray photoelectron spectroscopy (XPS) (Xue et al., 2019). The typical survey shown in Fig. 5a involves four distinct peaks of Ni 2p, Si 2p, C 1s, Ni 2p and O 1s. Mo 3d was not detected even though (NH4)6Mo7O24 was used in the synthesis process for the MoDoHSiO2 and CTSMoDoHSiO2 samples. This indicates that Mo was not present on the surface of the catalyst; more precisely, the Mo was covered by SiO2. The XPS results for the Ni 2p 3/2 peak are shown in Fig. 5b. In terms of the NiMoDoHSiO2 and NiSSiO2 samples, the peaks that occurred at 852.3 and 851.7 eV can be attributed to Ni0 (ref. Ni 0 = 852 ± 0.4 eV) (Shan et al., 2014). Meanwhile, the peak that appeared at 857.1 eV was attributed to Ni 2 + for the NiCTSMoDoHSiO2 sample. Compared to the peak position of Ni 2 + in the other two samples, the peak position shifted towards a higher energy band (857.1 eV) for the NiCTSMoDoHSiO2 sample. These results indicate that a stronger Ni and SiO2 interaction was obtained with the NiCTSMoDoHSiO2 sample than with the other samples, which is in line with the TPR results (Fig. 4). For the NiMoDoHSiO2 sample, a metallic Ni peak clearly appeared at 850.2 eV (ref Ni 0 = 852 . 6 eV ), signalling a shift to lower binding energies. A possible reason for this is that the small NiO particles reduced the interaction between NiO and the carrier. On comparing the satellite peaks of the three samples, the satellite peaks of the NiMoDoHSiO2 and NiCTSMoDoHSiO2 samples almost disappeared. A satellite peak originated at the long-range scattering of the structure of NiONiONiOin the lattice of a NiO nanoparticle, the surface region of which had interdigitated Ni and O atoms (Grosvenor et al., 2006; Tang et al., 2019). By checking this satellite peak, we could roughly judge whether a NiONi structure had formed on the surface region of the catalyst. The lack of a satellite peak meant that small NiO nanoclusters had formed (Akri et al., 2019) on NiMoDoHSiO2, which is consistent with the H 2 -PULSE results.The O1s XPS spectra, presented in Fig. 5c, displayed three main peaks, which were labelled O I (lattice oxygen), O II (deficient oxygen) and O III (surface oxygen) (Bao et al., 2015). From the peak area, more surface adsorbed oxygen was formed on all three catalyst samples, which may have been caused by the hydroxyl species of water molecules adsorbed on the surface. As shown in Fig. 5c, the highest percentage content of lattice oxygen (bonding of oxygen atoms and metals) was obtained for NiSSiO2, meaning this catalyst demonstrated good stability (Guo et al., 2014). The largest difference between the three catalyst samples was related to the defective oxygen. In fact, defective sites always display abundant low oxygen coordination (Zhao et al., 2018). The NiSSiO2 sample was almost without defective oxygen, whereas the NiMoDoHSiO2 and NiCTSMoDoHSiO2 samples had very clear defective oxygen peaks at 532.7 and 533.2 eV, respectively. These oxygen defects were most likely to be introduced by the flower ridge-like structures. This indicates a good relationship with the sample morphology. The peak that appeared at 102.8 eV in Fig. 5d was attributed to the bonding of SiO. Through the various characterisations, we can conclude that the NiMoDoHSiO2 catalyst demonstrated a uniform morphology with small Ni particles and high metal dispersion. Therefore, it can be concluded that the performance of the NiMoDoHSiO2 sample was the best. To verify this conclusion, we tested the CO methanation performance of the catalyst in a fixed bed reactor. The test temperature is 200 °C–500 °C, and the CO synthesis gas flow rate is 65 mL/min. We evaluate the catalyst based on the CO conversion and CH4 selectivity. It is clear from the performance test results shown in Fig. 6a and 6b that the NiMoDoHSiO2 sample demonstrated the best catalytic performance, with 100% CO conversion and 90% CH 4 selectivity at 250 °C. The by-product of the reaction is CO2 and the selectivity is 9.8%. In addition, we tested the stability of the three catalysts for 50 h at 300°C. From Fig. 6c and 6d, we can see that the activity of the NiCTSMoDoSiO2 and NiSSiO2 did not decrease following 50 h reaction. However, the previously best performing sample, the NiMoDoHSiO2 sample, exhibited a ‘cliff’ decline after 25 h of reaction with a rapid decrease from the original 100% CO conversion to a 10% conversion. While in the previous characterisation we discovered that the NiMoDoHSiO2 sample had small Ni particles and high metal dispersion, the interaction between the active component and the support was the weakest according to the H 2 -PLUSE and XPS results for the NiMoDoHSiO2 sample. The first thing that caught our attention was the agglomeration of the active component, which caused a rapid decline in performance. Therefore, we performed an HRTEM characterisation of the catalyst before and after testing to observe whether a clear agglomeration occurred after the stability test. Fig. 7a shows the XRD patterns of the catalyst sample following the 50 h stability test. Here, it was concluded that the sharper Ni peaks that appeared at 2 θ = 44 .5°, 51.8° and 76.3° belonged to the (111), (200) and (220) crystalline planes of Ni, respectively. The sharp Ni diffraction peaks mean that larger Ni particles were formed following the stability test. Fig. 7b–f shows the HRTEM images of the different samples following the stability test. After stability test, the Ni particles became larger with a clear agglomeration. For the NiMoDoHSiO2 sample (Fig. 7d), Ni particles increased from the initial 3 nm (Fig. 3d) to around 20 nm. Therefore, the rapid decline of activity during the stability test can be attributed to the aggregation of the active components (Benavidez et al., 2012; Ouyang et al., 2013). The metal–support interaction continuously existed on the surface of the SiO2 support, but the interaction force here was the weakest among all the supports. The smaller particles result in a weak interaction between the carrier and the active components, meaning agglomeration can easily occur and activity will be rapidly lost during a strong exothermic methanation reaction (Margossian et al., 2017). For the NiCTSMoDoHSiO2 sample, Ni particles with a size of around 7 nm (Fig. 3f) were formed on the surface, which is slightly larger than the 3 nm particles formed on the NiMoDoHSiO2 sample. The size of the CTSMoDoSiO2 sample was around 1 μ m, which may have caused the formation of large Ni particles during the impregnation. In fact, small-sized particles can provide more active sites during the reaction. However, small particle size does not always mean better performance. Indeed, the smaller the particle size, the greater the instability during the reaction and easier the migration and agglomeration on the surface of the carrier (Gao et al., 2015; Munnik et al., 2014). Peter Munnik (2014) reported that the most suitable particle size is 6–8 nm. The active components in this particle size range can maintain an appropriate interaction between the carriers and the active components, which results in better stability (Farmer and Campbell, 2010; Li et al., 2008). Compared to the results shown in Fig. 3f, Ni exhibited slight agglomeration in the NiCTSMoDoSiO2 sample (Fig. 7f). Therefore, the better stability of the NiCTSMoDoHSiO2 sample was attributed to the appropriate Ni particle size and the suitable interaction between the carriers and the active components (Dias and Assaf, 2003). Carbon deposition on the catalyst surface is another major factor that causes a rapid decrease in catalytic activity. To further confirm the deposited coke of the spent catalysts, the used catalysts were examined using thermogravimetric analysis (TGA), with the results presented in Fig. 8. The weight loss at 134 °C may have been caused by the evaporation of the water that had adsorbed on the catalyst surface during heating. A significant weight increase occurred at 274 °C, which was caused by Ni being calcined into NiO under air-atmosphere conditions (Liu et al., 2017). Comparing the TGA curves of the three samples, the NiMoDoHSiO2 sample clearly had the most serious carbon deposits. Therefore, it can be argued that active components with small particle sizes perform poorly in terms of resistance to carbon deposition. In terms of the NiCTSMoDoHSiO2 sample, the Ni particles with a particle size of around 7 nm not only provided suitable interaction but also performed well in terms of resistance to carbon deposition. Meanwhile, to ascertain whether graphitic and amorphous carbons had accumulated in the spent catalysts (Kopyscinski et al., 2011), XRD analysis was performed (Liu et al., 2014), with the results shown in Fig. 7a. No fresh diffraction peak at 26.55° (attributed to graphitic carbon) was observed, which indicated that the deposited carbon was amorphous (Ohtomo and Hwang, 2004) or that the amount of graphitic carbon was below the XRD detection limit.Hollow-structured SiO2 with its intriguing physicochemical properties and huge potential, has been widely applied in many methanation catalysts. In this work, highly uniform hollownanoflowers, silica (SiO2) nanospheres with different sizes, were synthesised through a rapid, time-saving method known as flash nanoprecipitation. High flexibility, reduced technological cost, and high process efficiency make FNP as an attractive method of industrial applications. Furthermore, the size and morphology of the nanoparticles can be independently controlled by adding CTS during synthesis process. Our results demonstrate that this process is highly promising for the production of structured SiO2 nanoparticles in a continuous and scalable way with independent and precise control over particle size, morphology and composition. CTSMoDoHSiO2 nanoparticles as an excellent catalyst carrier are highly sought after in methanation industrial applications due to its suitable particle size and interaction between active components.Highly dispersed and uniform flower-like SiO2 nanoparticles with different sizes were formed using the FNP method. At the same time, this method will developed a commercial solution for preparing methanation supports at lower costs and with higher flexibility than conventional processes. The size of the SiO2 was effectively enlarged in the presence of CTS. Following impregnation and calcination, the MoDoHSiO2 samples exhibited the largest specific surface area and highest metal dispersion. Ni particles with a particle size of 3 nm were successfully attached to the surface of the MoDoHSiO2 sample. The weaker interaction between the support and the active component meant that Ni significantly agglomerated during the stability test for the NiMoDoHSiO2 sample and its stability performance was poor. For the NiCTSMoDoHSiO2 sample with a particle size of 7 nm, there was no obvious agglomeration following the stability test. Overall, the catalysts with smaller particles deactivated faster and to a larger extent than those with medium sized particles.Even though FNP stands out as an one-step continuous process that operates at room temperature, consumes little energy, and has potential to scale up, the underlying microscopic mechanisms responsible for the self-assembly are still elusive, which makes the experimental study of the FNP process difficult. Furthermore, it is hard task to systematically search and screen in the experiments all relevant process parameters, such as feed ratio, feed concentration, flow rate, molecular properties. Therefore, computer simulations can provide considerably more microscopic level information than experiments, and are therefore useful tools in the study of complex mechanisms and morphologies.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by International Science and Technology Cooperation Project of Shihezi University, PR China (No. GJHZ201804), International Science and Technology Cooperation Project of Bingtuan, PR China (No. 2018BC002), Science and Technology Innovation Talents Program of Bingtuan, PR China (No. 2019CB025).

Hollow-structured SiO2, which comprises a void space inside a distinct shell with its intriguing physicochemical properties and huge potential, has been widely applied in many methanation catalysts. However, the common methods for preparing SiO2 hollow microspheres are cumbersome and time consuming. Highly uniform hollow nanoflowers, silica (SiO2) nanospheres with different sizes, were synthesised through a rapid, time-saving method known as flash nanoprecipitation. An assembling particle mechanism of the hollow structure of Mo–polydopamine complex was established and tetraethyl orthosilicate underwent uniform hydrolysis on the surface of the hierarchical structure. Spherical SiO2 samples with different morphologies were prepared as catalyst carriers, and Ni-based methanation catalysts were prepared using an impregnation method. Ni particles with size of 3 nm were successfully attached to the surface of MoDo–H–SiO2, while the particle sizes of Ni on CTS–MoDo–H–SiO2 was 7 nm. The small particles (3 nm) were found to significantly increase in size (20–50 nm), decrease by 90% in stability test with a weight hourly space velocity (WHSV) of 26,000 mL.g−1.h−1, which is detrimental to catalyst stability. However, the medium sized particles (7 nm) remained confined via a suitable interaction involving the support, thus displaying enhanced stability, with 100% CO conversion at 250 °C and no obviously decrease in stability test Although more active sites can be provided with smaller active metals, catalysts with small sized particles deactivate faster and to a larger extent than catalysts with medium sized particles. Thus, the smaller the particle size of the active component, the worse the stability.

Data will be made available on request.area/ m 2 diameter/ m diffusion coefficient/ m 2 s - 1 flow/ m 3 s - 1 equilibrium constant/ -length/ m molar mass/ kg mol - 1 flux/ mol m - 2 s - 1 pressure/ Pareaction rate/ mol m - 3 s - 1 ideal gas constant/ J mol - 1 K - 1 selectivity/ -temperature/ K volume/ m 3 mole fraction/ -conversion/ -Shell thickness/ m porosity/ -effectivness factor/ -stoichiometric coefficient/ -extent of reaction/Thiele modulus/ -Power-to-X technologies are an opportunity to store electrical energy in the form of chemical compounds [1]. For this purpose, excess renewable energy is used for hydrogen generation via electrochemical water-splitting. Subsequently, hydrogen is converted to chemicals with existing infrastructure, such as methane (Substitute Natural Gas), ammonia, or methanol. In particular, the synthesis of carbon-based products also offers the possibility for reducing carbon dioxide emissions, by consuming it as a reactant. However, the volatile supply of surplus renewable energy makes these processes technologically challenging, due to unsteady process conditions [2,3].For example, the synthesis of methane, methanol, and ammonia is often conducted heterogeneously catalyzed in wall-cooled multi-tubular fixed-bed reactors. These reactors are designed for intensive reaction heat removal, to keep the reactor temperature within desired bounds. Nevertheless, changes in process conditions can lead to uncontrollable reactor behavior, known as thermal runaway. In this case, the reaction heat release leads to an increase of reactor temperature and in consequence, to a further increase of heat release, as the reaction becomes faster. This causes a feedback loop, as reaction rate rises approximately according to the Arrhenius-law (even in presence of internal mass transport limitation), while the heat removal rate increases linearly with coolant temperature. The resulting reactor temperature rise may cause selectivity decrease, catalyst deactivation or even material damage [4,5]. Evidently, effective heat management is crucial for safe reactor operation and has been researched for decades with focus on stationary reactor operation [6]. Nevertheless, if load-flexible reactor operation is expected, controllable conditions have to be maintained at all possible steady states, and also during all dynamic transitions in between. Hence, recent works also consider safe reactor design under dynamic conditions [7–9]. Kreitz et al. [10], for example, studied the dynamic operation of micro-structured reactors. Even though such reactors exhibit a large heat transfer areas, temperature peaks of about 150 K were observed for low-frequency changes of the inlet conditions. Such studies help to identify infeasible operation conditions in advance. However, the number of possible dynamic scenarios, which can be considered is limited (e.g., due to computational power or experimental effort), and in practical applications unforeseen situations might arise, e.g., due to aging of the catalyst or fouling in the coolant system. Thus, it is indispensable to design reactors, where runaway conditions can be generally avoided.An effective opportunity is heat release control by external mass transport limitation of the reactants to the catalyst pellets, as shown in computer-based studies by Zimmermann et al. [11,12]. This can be done, e.g., by applying an inert shell onto the active catalyst pellets, resulting in so-called core–shell catalyst pellets. The inert shell is tailored, such that the mass transport through the inert shell becomes rate-determining particulary at critical reactor temperatures. In this case, the effective reaction rate and thus the heat release rate is approximately independent of temperature, as shown in Fig. 1 . Hence, a further increase of reactor temperature is prevented by the linearly increasing heat removal rate, minimizing the risk of uncontrollable conditions. Besides influencing the effective reaction rate of the catalyst pellets, the inert shell may also affect selectivity of the catalyst pellets. This can occur in principle due to different mass transport rates of reactants and products through the shell, influencing the chemical equilibrium.These potential benefits have to be distinguished from the properties of core–shell pellets prepared with zeolitic materials (a.k.a. ’membrane-encapsulated catalysts’), which arise from the component-specific permeability of zeolites, such as protection against catalyst poisons, increased sintering resistance and shape-selectivity [13]. However, if these component-specific permeabilities are not required less expensive materials can be used. In this case, the selectivity of the catalyst pellets does not depend on the material properties at all, if the diffusion rate through the inert shell is rate-determining, but rather on the properties of the reacting components. Furthermore, zeolitic core–shell materials are often manufactured at sub-millimeter scale, which is too small for application in industrial fixed-bed reactors, due to the high pressure loss involved. However, so-called ’egg-shell’ catalyst pellets are frequently used in industrial application. These consist of an inert, sometimes even non-porous core surrounded by a catalytically active shell. A typical application is the oxidation of o-xylene to phthalic anhydride [14]. By employing the ’egg-shell’ concept, internal mass transport resistances in the catalyst pellets are reduced and thus the often cost-intensive active material is used more effectively in the reactor.Capece & Dave [15] presented an approach to prepare coated catalyst pellets at lab-scale by fluidized-bed coating. In this process, pellets are fluidized in a gas stream, while a suspension is sprayed onto them in a fluidization chamber. While the liquid suspension evaporates, a solid layer forms around the substrate pellets. The procedure is subject to a complex interplay of material properties of the substrate pellets, the suspension, as well as different process conditions (e.g., spray rate, gas temperature, and velocity) and requires extensive experimental know-how. Werner et al. [16] summarize several fundamental phenomena. After coating, a calcination step is required for removing binding agents, which adds an additional challenge. The applied coating might crumble off, delaminate, or even tear the pellets apart [17]. Nevertheless, a successful procedure allows for coating pellets of various shapes and sizes with controllable coating thickness according to the demands of the catalytic process. In addition, fluidized bed-coating is established at industrial scale and has proven to be suitable to prepare the aforementioned ’egg-shell’ catalyst pellets [18].The aim of this work is twofold. First, the interplay of mass transport through the inert shell and chemical equilibrium of a multi-component multi-reaction system is investigated based on first physical principles. Based on this analysis, the prediction of the influence of an inert shell on the activity and selectivity of the catalyst pellets is possible. Due to its high exothermicity and use as a CO 2 neutral (or even negative) fuel source, CO 2 methanation is employed as case study. Second, industrial Ni / Al 2 O 3 methanation catalyst pellets are coated with an inert shell at kilogram-scale in a fluidized-bed coating apparatus. The obtained catalyst pellets are characterized via Dynamic Image Analysis, Scanning Electron Microscopy (SEM) and X-ray Computed Tomography (XCT), in order to determine the structure and integrity of the shell. In the case of hard X-ray tomography, such analysis is non-invasive and can cover large fields of view, therefore providing a representative interpretation of catalyst structure. Finally, the catalytic activity and selectivity of the catalyst pellets are investigated with respect to their dependence on temperature and compared to pellets without inert shell and to crushed catalyst pellets.To validate the model based results, spherical Ni / Al 2 O 3 catalyst pellets (SPP2080-IMRC, [19]) are coated with an inert shell, as schematically shown in Fig. 2 . For this purpose an aluminum oxide suspension is prepared. At first, polyvinyl alcohol (PVA, Mowiol(R) 8–88, Kuraray Europe GmbH) is stirred into distilled water at 343 K. After complete dissolution of PVA, pseudo-boehmite powder (Disperal P2W (R), Sasol Germany GmbH) is added and vigorously stirred for 30 min. Subsequently α -alumina powder (1.65 μ m , BA-2, xtra GmbH) is added and stirred for another 90 min. In total, the mass fractions in the suspension are 5 % pseudo-boehmite powder, 10 % α -alumina powder, 1.5 % polyvinyl alcohol and 83.5 % distilled water.As the catalysts pellets’ availability is limited, 0.15 kg thereof is diluted with 1.35 kg inert γ -alumina pellets (2.5 mm, Sasol Germany GmbH) and put into the fluidization chamber of a fluidized bed coating pilot plant. The suspension is dosed into the fluidization chamber of a pilot plant by a bottom-spray two-fluid nozzle (Mod. 940, Düsen-Schlick GmbH, Germany). A peristaltic pump conveys the solution from a tank to the nozzle. The cylindrical fluidization chamber (inner diameter 200 mm) is made of temperature-resistant borosilicate glass. Additionally, ambient air is sucked in by a pressure blower and heated up, before entering the fluidized bed chamber through a perforated disk. After passing the fluidized bed, the air enters a calming zone and overspray particles are separated via a cyclone and a filter. Every 10 min samples are taken and the coated catalyst pellets are separated from the inert pellets into ceramic dishes for calcination in a furnace. The furnace is heated from ambient conditions to 823 K with a heating rate of 1 K/min, to remove the organic binder and to calcine the applied pseudo-boehmite. The temperature was held for 3.5 h and subsequently the pellets were cooled down to ambient temperature in the closed furnace. Samples taken after 0 min (calcined catalyst pellets without coating), 10 min, 30 min, and 50 min process time were then investigated in detail.A pellet imaging system CAMSIZER®(Retsch Technology) was used to quantify the size of the coated pellets. With this equipment several parameters can be measured for an arbitrary pellet collective of d P from 20 μm to 30 mm at the same time. The principle of dynamic image analysis according to ISO-13322–1 and −2 is applied. The sample is placed on a vibrating chute via the feed hopper. In the chute they are separated and subsequently fall through a camera field to be measured (two cameras are available, basic and zoom camera). The cameras binarize the captured shadow and calculate several parameters. Depending on the pellet shape, a different pellet diameter can be used as the basis for displaying the pellet size distribution. In this work, the d area -mode was chosen (50 measurements per second with both cameras), according to which the respective apparent catalyst pellet volume is calculated.Samples for cross-sections are embedded in transparent epoxy resin, ground manually under watercooling (grit 180 to grit 2500), polished semiautomatically using polycrystalline diamond suspension (3 μ m ) and water-based lubricant for 8 min at 15 N and finished semiautomatically using alumina suspension (0.06 μ m ) for 3 min at 15 N. During preparation, the height of the samples is measured using a Nikon Digimicro MS-11C to ensure a centrical surface for SEM investigations. After preparation, samples are sputter-coated with gold to prevent charge build-up.SEM analyses are performed using a FEI Scios DualBeam (ThermoFisher Scientific, Waltham, MA, USA) microscope equipped with a TEAM Trident system (EDAX, AMETEK GmbH, Weiterstadt, Germany). Secondary electron (SE) and backscattered electron (BSE) contrast are used to image topography and microstructure. EDS is performed for integral and local analyses of the chemical composition.X-ray computed tomography (XCT) measurements were carried out using a Zeiss Xradia Versa 520 X-ray microscope (Pleasanton, United States). Selected whole catalyst pellets were scanned with a 4X objective lens in binning 2 mode using a tungsten X-ray source. Measurements were performed at 40 kV and 76 μ A using a low energy filter to optimize transmission and signal to noise ratio. The chosen setting provided an optical magnification of 3.95 and voxel size of 2.85 μ m . 2041 projections were acquired over an angular range of 0 to 360 ° with an exposure time of each 1000 ms. The total measurement time per sample was about 2 h. Tomographic reconstructions were performed with the commercial software package Zeiss XMReconstructor, using a filtered back-projection type algorithm. The tomograms were corrected for beam hardening. Image analysis of the tomography data was performed with Avizo v.9.7.0 (Thermo Fisher Scientific) as discussed in detail in SI D.2.To perform catalytic activity measurements, three catalyst pellet spheres are placed into a quartz glass tube ( d tube = 8 mm ), with each sphere separated by a quartz glass bead (2.5 mm diameter). The spheres are fixed with quartz glass wool from each side and 0.5 g silicon carbide is placed upstream of the catalyst spheres, to ensure isothermal and uniformly distributed gas flow. The silicon carbide is also kept in place by quartz glass wool. Type K thermocouples are placed before and behind the packing. The latter is considered as reaction temperature. The setup is also used with powder of the calcined SPP2080-IMRC catalyst (415 – 500 μ m sieve fraction) of crushed catalyst spheres, which is diluted with a 1:9 ratio in silicon carbide powder.After the glass tube is placed into a furnace and sealed, gases ( CO 2 3.0, H 2 5.0, He 5.0, Westfalen AG) are supplied via mass flow controllers (El-Flow®Select, Bronkhorst Deutschland Nord GmbH). The product gas is cooled down to 276 K to condense water and a constant flow of 15 Nml/min nitrogen ( N 2 5.0, Westfalen AG) is added as internal standard. Potentially remaining water is separated with a membrane, before analyzing the product gases using gas chromatography (490 Micro GC System, Agilent Technologies, Inc.).Before catalytic activity measurements, the catalyst is dried at a furnace temperature of 393 K with 120 Nml/min of a 1:1 mixture of H 2 and He. Afterward, the furnace temperature is increased to 673 K and the catalyst is reduced for eight hours at the same gas composition. Subsequently, the catalyst is aged at reaction conditions ( F CO 2 = 20 Nml/min, F H 2 = 80 Nml/min, F He = 100 Nml/min, p  = 1.2 bara) at 773 K for eight hours. Five product gas samples are taken at each furnace temperature, following a step change profile from 773 to 523 K in steps of 25 K. The temperature difference before and behind the packing was below 7 K and the furnace temperature is up to 15 K higher than the reaction temperature. The carbon balance was closed to more than 99 % in all cases.In the first section of the results, the reaction rates of catalyst pellets are derived in the presence of mass transport limitation through an inert shell. Based on this, the effect of an inert shell on the selectivity of core–shell pellets is discussed using the CO 2 methanation system as example. In the second section, the fluidized-bed coating results of industrial Ni / Al 2 O 3 methanation catalyst pellets with an alumina shell are demonstrated. A detailed characterization of the obtained alumina shell based on XCT analyses is presented in the third section. The fourth section deals with the analysis of catalytic activity measurements, which are related to the model-based predictions from the first section and the texture data from section two and three.The activity and the selectivity of a catalyst pellet is determined by calculating the fluxes across the outer pellet surface. In the following, this is done for a core–shell catalyst pellet at the limit of very fast reaction rates in the pellet cores in a simplified manner with negligible temperature gradients at steady-state. The presence of a very fast reaction rate in the context of this work is discussed in SI A. The procedure can be extended to more complex cases (e.g., non-negligible temperature gradients in the catalyst pellet, complex pellet geometries, non-negligible mass transport through the gas boundary layer) if necessary.If the shell is very thin, its curvature can be neglected and slab geometry may be assumed. Following Fick’s first law for an ideal gas, the flux of a component i ∈ 1 , . . , C through a shell of thickness δ is (1) N i = - D i RT dp i dr ≈ D i RT p i , core - p i , bulk δ . Hence, to calculate the fluxes of all components through the inert shell, the partial pressures at the interface between catalyst pellet core and shell have to be determined. In the presence of fast reaction rates, the chemical composition at the core–shell interface approaches the equilibrium composition ( p i , core ≈ p i , eq ). Consequently, the equilibrium condition holds for each linearly independent reaction j ∈ 1 , . . , R . (2) K j = ∏ i = 1 C p i , eq ν i , j The number of components is typically larger than the number of linearly independent reactions, and thus the equation system has to be supplemented by C - R equations. These are the stoichiometric relations, which express the mass conservation of chemical reactions [20–22]. Accordingly, the C fluxes given by Eq. 1 are related to R potentials ξ j , called the extent of reaction. (3) N i = ∑ j = 1 R ν i , j d ξ j dr Inserting Eq. 1 and assuming the diffusion coefficient independent of the composition (e.g., in the Knudsen diffusion regime), these can be integrated from bulk conditions to equilibrium conditions. With ξ j , bulk = 0 , the result reads (4) D i ( p i , eq - p i , bulk ) RT = ∑ j = 1 R ν i , j ξ j , eq . Evidently, the extent of reaction ξ j is defined in a similar manner as the product yield at reactor scale, but modified by the components’ diffusion coefficient. Furthermore, the extent of reaction offers a convenient opportunity to calculate the pellet reaction rates with Gauß’ theorem (5) r eff , j = ∫ V pellet r j dV V pellet = ∫ A pellet d ξ j dr dA V pellet ≈ A pellet V pellet ξ j δ , from which all other catalyst performance measures, such as activity and selectivity, can be derived.Solving the equation system analytically is only possible for simple systems, as demonstrated in SI B. In general, numerical solution techniques are required, due to the non-linear nature of the equilibrium conditions (Eq. 2). For this reason, it is illustrative to consider a specific example such as carbon dioxide methanation ( CO 2 M ) with the reverse water gas shift reaction (RWGS) as side reaction, as shown in Fig. 3 . Carbon monoxide methanation (COM) is a linear combination of CO 2 M and RWGS. Thus, two equilibrium conditions determine the system. (6) K CO 2 M ( T ) = p CH 4 , eq p H 2 O , eq 2 p CO 2 , eq p H 2 , eq 4 (7) K RWGS ( T ) = p CO , eq p H 2 O , eq p CO 2 , eq p H 2 , eq K CO 2 M and K RWGS are calculated from the equilibrium constants of the steam methane reforming reaction and the water gas shift reaction taken from literature, as given in SI C.The results for a 4:1 mixture of hydrogen and carbon dioxide at 1 bar are shown in Fig. 4 (a) at the limit of Knudsen diffusion, where diffusion coefficients differ simply by the square root of their molar masses. Furthermore, they are compared to the hypothetical case where all diffusion coefficients are the same (Fig. 4 (b)), as in this case, the mass transport through the inert shell does not shift the equilibrium partial pressures and the equilibrium state corresponds to that of the surrounding gas bulk.From this comparison it is evident, that the mass transport through the inert shell influences the equilibrium state in the active core significantly. Two limiting cases, whether CO 2 M or RWGS is preferred can be distinguished. At high temperatures, RWGS is preferred and a significant drop in carbon dioxide partial pressure is observed, whereas the hydrogen partial pressure is almost the same as in the gas bulk. The drop of hydrogen partial pressure is only about a fifth of what is expected according to the RWGS reactions’ stoichiometry, due the faster diffusion of hydrogen compared to carbon dioxide. In turn carbon monoxide and water partial pressures build up, also not according to the stoichiometry of the reaction, but according to Eq. 4 with a ratio of 1.25. Hence, the total pressure in the catalyst pellet core does not correspond to the bulk pressure. In fact, a slight overpressure is present in the catalyst pellets, which is the opposite of what might be expected for an equimolar reaction.At low temperatures CO 2 M dominates. In this case hydrogen and carbon dioxide are present in a stoichiometric ratio with respect to CO 2 M in the gas bulk, with almost complete carbon dioxide consumption in the pellet core. However, hydrogen is again present in significant amounts in the catalyst pellet core, as it diffuses much quicker through the inert shell. The present hydrogen surplus is beneficial with regard to possible coke formation, which is not expected to happen in the presence of low CO 2 / H 2 ratios, as discussed by Gao et al. [23]. Furthermore, methane and water partial pressures build up with a ratio of 0.47. In case of CO 2 M , the ratio is closer to the reactions stoichiometry, as water and methane have similar molar masses. As in the case of RWGS, a slight overpressure is present in the pellet core, which is also in opposite of what is expected from a highly mole number reducing reaction.In between these limits, mass transport shifts the chemical equilibrium in favor of CO 2 M . The reason for this is based on three synergistic effects: 1. The overstoichiometric hydrogen partial pressure in the pellet core shifts the COM and CO 2 M in favor of methane. 2. Methane is removed quicker from the pellet core than carbon monoxide, due to its higher Knudsen diffusion coefficient in the inert shell. 3. The total pressure in the catalyst pellets is elevated compared to the surrounding bulk pressure, which favors mole number reducing reactions. Therefore, as predicted by Le Chatelier’s principle, methane becomes the preferred product and carbon monoxide formation is shifted towards higher temperatures (approx. +80 K) than in the hypothetical case of equal molar masses. This is beneficial for CO 2 M , where 750 K is often the upper feasible reactor temperature. The procedure presented in this section can be readily applied to other reaction systems to determine the influence of the inert shell on the catalyst pellet behavior.The overstoichiometric hydrogen partial pressure in the pellet core shifts the COM and CO 2 M in favor of methane.Methane is removed quicker from the pellet core than carbon monoxide, due to its higher Knudsen diffusion coefficient in the inert shell.The total pressure in the catalyst pellets is elevated compared to the surrounding bulk pressure, which favors mole number reducing reactions.The applied fluidized-bed coating procedure results in very little overspray and uniform pellet growth, as shown in Fig. 5 . Starting from the pellets without coating, the Sauter mean pellet diameter d 32 increased by 0.28 mm after 50 min. The subsequent calcination step led to no determinable shrinkage of the coated catalyst pellets and they remained mechanically stable.As shown in Fig. 6 , a clear distinction between the core of the catalyst pellets and the coating is noticeable. As analyzed in ptychographic X-ray computed tomography measurements in SI D and by Weber et al. [19], the catalyst pellet core exhibits a distinct sponge-like structure, with approximately spherical macropores embedded in a mesoporous matrix. NiO is distributed as nanoparticles, and is therefore not visible at the given magnification. In the shell of the catalyst pellets, more dense particles could be observed in a less dense matrix, which might be explained by presence of α -alumina embed in pseudo-boehmite. However, clear assignment of the phases via SEM is hardly possible. Furthermore, noticeable voids are present in the shell, which are not an issue, as long as they do not directly connect the gas bulk with the pellet surface. If the latter would be the case, bypassing of the bulk gas through the shell would become possible, and the above described effects might not be present. For this reason, a more detailed analysis of the catalyst pellets is done via XCT.The XCT volume renderings of the four scanned catalyst pellets (after 0, 10, 30, and 50 min coating duration) are shown in Fig. 7 (a,d,g,j), respectively, with isotropic voxel sizes of 2.85 μ m . The obtained XCT were segmented into different labels for further quantitative image analysis. The retrieved labels in Fig. 7 (a,d,g,j) represent the core of the catalyst pellet (gray) with the core-pores (green) as well as the shell (blue) with the shell-pores (orange).The resolution of the XCT is not sufficient for a full analysis of the porosity as shown previously in [19]. However, in the present case it can provide a qualitative measure on the differences between the core and shell and their respective contribution to the overall resolved porosity. As the voxel size and measurement parameters are identical for each tomogram, this comparison is possible and observed differences can be carefully discussed. Furthermore, larger voids in the shell as indicated by the SEM images (Fig. 6) can be identified. The porosity distributions of the XCT depending on the d eq (equivalent spherical pore diameter) of the detected pores are shown in Fig. 7 (b,e,h,k) for each coating time and the obtained porosity and mean d eq values for the XCT are summarized in Table 1 .In particular, the contribution of the shell porosity to the overall resolved porosity ε tot is increasing with longer coating time, while the observed d eq , shell of the pores in the shell are not changing significantly. In the distribution of the ε weighted d eq depending on the distance to the catalyst pellet center ( d center ) the two different pore labels (core-pores and shell-pores) can be readily identified. The measured d eq for the shell-pores are showing generally larger pores in the shell compared to the core. The resolvable d eq distribution of the core is quite homogeneous and similar for all four samples. However, the resolution of the chosen XCT method is not sufficient for a complete analysis of the catalyst pore structure, which ranges from few nm up to several μ m and thus requires a combination of different imaging techniques as shown in [19] (see also SI D). It is rather sufficient to identify larger outliers of the macrcoporosity being present in the catalysts.In addition to a qualitative comparison of measurable porosity and pore diameters, XCT was used to analyze the thickness of the coated shell, and particularly to assess the presence of a closed shell. As shown in Fig. 6, it is also possible to determine the thickness of the shell from SEM images, however this only provides very local information limited to 2D. XCT allows for a 3D analysis of the shell as shown in Fig. 8 for three different coating times (10, 30, and 50 min).The cuts through the XCT volumes in Fig. 8 (a-c) illustrate the increasing shell thickness with increasing coating duration. To investigate the thickness of the shell in more detail, two surfaces where computed, one for the filled catalyst core and one for exterior of the filled shell. Cuts through the rendering of the surfaces are shown in Fig. 8 (d-f). It can be observed that after 10 min coating time a closed shell was not obtained, while for 30 min coating time the thickness increased and only few voids in the shell still remained. After 50 min coating duration, a completely closed shell with increased shell thickness could be observed. Furthermore, the shortest distance of the exterior surface of the shell to the surface of the filled core was computed for each surface point. The resulting distributions of the shell thickness ( δ shell ) are presented in Fig. 8 (g-i). The distributions clearly show the increased shell thickness with increasing coating time and allow to retrieve a mean δ shell value together with its standard deviation as summarized in Table 1. The mean δ shell increased from about 15 μ m after 10 min, over 40 μ m after 30 min and 105 μ m after 50 min coating time.In summary, the XCT results allow for a quick qualitative inspection, whether a closed shell is obtained after a certain coating duration. Furthermore, precise quantitative information about the shell thickness can be retrieved, which in combination is hardly possible with any other method. The resolution of the here applied XCT method is not sufficient for a full porosity analysis, only voids larger than about 3 μ m can be detected. A detailed study of the shell porosity is possible in future studies employing hard X-ray nanotomgraphy, which allows sub 100 nm resolution on samples that can cover the full thickness of the shell and direct retrieval of advanced pore network models for the macropores and quantitative information about the mesoporosity are available.[24].To validate the computational predictions of the influence of an inert shell on the pellet reaction rate and apparent selectivity, catalytic activity experiments have been performed in a lab-scale reactor, with results shown in Fig. 9 . At the given conditions, detectable carbon dioxide conversion are present starting at about 523 K. The calcined catalyst powder and the calcined pellets without coating ( = ̂ 0 min coating time) show slightly decreased conversion and methane selectivity, compared to their counterparts, which have not been calcined. The results of the latter are given by Weber et al. [19].The carbon dioxide conversion of the catalyst pellets increases much slower with temperature, than the carbon dioxide conversion of the catalyst powder, which indicates the presence of mass transport limitations. The limiting component is likely carbon dioxide, due to its much larger molar mass, compared to hydrogen, even though both reactants are present in a stoichiometric ratio with regard to CO 2 M . Apart from that, also the methane selectivity is shifted. As hydrogen diffuses much quicker into the catalyst pellets than carbon dioxide, its partial pressure is close to that of the surrounding gas bulk. Consequently, the CO 2 M and the COM equilibria (Fig. 3), which are present in the center of the catalyst pellets in the presence of mass transport limitations, are shifted towards the side of methane. Furthermore, as methane has a lower molar mass than carbon dioxide, it diffuses much quicker out of the catalyst pellet. Both effects combine to an increase of about 8 % in methane selectivity from 500–550 K. The coated catalyst pellets continue these trends. The carbon dioxide conversion drops and the methane selectivity rises with increasing shell thickness, which indicates that an additional mass transport limitation is induced by the inert shell. Comparing catalyst pellets without shell to the pellets with the thickest shell at 773 K, reveals an increase in methane selectivity by 30 %.Apart from that, the mass transport limitation of the inert shell does also shift the temperature dependence of the carbon dioxide consumption rate. As shown in the Arrhenius plot (Fig. 9 (c)) the logarithm of the carbon dioxide consumption rate over the inverse temperature is a straight line for the catalyst powder, as expected in absence of transport limitations. In case of the catalyst pellets without coating, the slope is equal to that of the powder at low temperatures, but starts to decrease with rising temperature, which marks the onset of mass transport limitation. At a temperature of about 700 K, the slope is about half the value of that of the powder, which indicates the presence of internal mass transport limitation.The slope in the Arrhenius plot is further decreased for the pellet sample after 10 min coating time. However, it does not approach zero, as expected from Fig. 1. This indicates, that the pellets are operating between mass transport limitation in the active core and the inert shell. The reason for this is a very thin shell, which is not completely closed, as indicated by the XCT results in Fig. 8. In contrast, the shells of the samples after 30 and 50 min coating time are (almost) completely closed. This is also reflected in the Arrhenius plot, since for these samples the slope approaches zero at high temperatures, which indicates mass transport limitation exclusively by the inert shell.Understanding and controlling mass transport phenomena at catalyst pellet scale yields favorable properties for industrial reactor operation. For instance, in the context of carbon dioxide methanation in fixed-bed reactors, it was shown by computer-based studies, that an inert shell on the catalyst pellets yields a well-controllable heat release rate, if the diffusion of the reactants through the inert shell is rate-determining [11,12]. This, e.g., minimizes the risk of reactor runaway and allows for reliable reactor heat control even at unsteady conditions.To validate these results and to demonstrate the feasibility of large-scale production, commercial Ni / Al 2 O 3 methanation catalyst pellets were coated with an inert alumina shell via fluidized-bed coating. After calcination, the obtained catalyst pellets were analyzed via Dynamic Images Analysis, REM and XCT. In particular, XCT revealed that a certain coating duration is required to obtain a closed shell, while the shells are generally quite homogenous.Catalytic activity experiments confirmed the computer-based predictions. The apparent activation energy of the pellets with fully closed shell is significantly decreased at high temperatures, which indicates the expected presence of mass transport limitation through the inert shell. For catalyst pellets without (fully closed) shell, these effects were not observed, which underlines the necessity of a defect-free shell for the mass transport through the inert shell to become rate-determining at high temperatures.Furthermore, the coated catalyst pellets with the thickest shell exhibit a methane selectivity, which is up to 30 % higher than that of the uncoated catalyst pellets at the presence of external mass transport limitation. As concluded by a model based analysis, this effect is rooted on in the interplay of the differing component mass transport rates through the inert shell, which in turn shift the equilibrium composition inside the catalyst pellets. Compared to carbon dioxide, hydrogen with the lower molar mass diffuses quicker through the inert shell into the active catalyst pellet core. Thus, hydrogen is excessively present in the pellet cores, which shifts the carbon dioxide methanation equilibrium and the carbon monoxide methanation equilibrium towards the side of methane. Additionally, the pressure is slightly elevated compared to bulk conditions and methane is removed quicker from the pellet core than the heavier side product carbon monoxide. In summary, according Le Chatelier’s principle, methane becomes the favored product.The mathematical and experimental methods used in this work can be directly applied to other reaction systems to check whether an inert shell also has a positive effect on the selectivity towards the desired product. As a rule of thumb, the formation of the product with the higher diffusion coefficient in the inert shell becomes more preferred. It is expected, that the presented core–shell catalyst pellet concept enhances reactor performance also for other challenging applications.Ronny Zimmermann, Jens Bremer, and Kai Sundmcher have patent #WO2020/234337Al pending as Inventors.This research work was conducted within the DFG Priority Program SPP2080 “Catalysts and reactors under dynamic conditions for energy storage and conversion” and was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) −406914011. (Gefördert durch die Deutsche Forschungs-gemeinschaft(DFG)-406914011.) This research work was also supported by the Center of Dynamic Systems (CDS), funded by the EU-program ERDF (European Regional Development Fund).Furthermore, this project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 731019 (EUSMI).The authors acknowledge the Paul Scherrer Institut, Villigen, Switzerland for provision of synchrotron radiation beamtime at beamline X12SA–cSAXS of the Swiss Light Source and Ana Diaz and Mirko Holler for support during beamtime. This work was partly carried out with the support of the Karlsruhe Nano Micro Facility (KNMF), a Helmholtz Research Infrastructure at Karlsruhe Institute of Technology (KIT), which provided access to FIB instruments via proposal 2020–023-028494. The authors thank Sabine Schlabach for support during FIB sample preparation.Ronny Zimmermann is also affiliated with the International Max Planck Research School (IMPRS) for Advanced Methods in Process and Systems Engineering, Magdeburg, Germany. Generous product samples by Sasol Germany GmbH are gratefully acknowledged.Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cej.2022.140921.The following are the Supplementary data to this article: Supplementary data 1

Catalyst research is concerned with synthesizing increasingly active materials, leading to safety issues at reactor scale, unless the reaction heat release is controllable. Computational studies predict that core–shell pellets with catalytically active core and inert shell are beneficial for this purpose, compared to established concepts such as catalyst pellet dilution. At high temperatures, reactant diffusion through the shell becomes rate-determining, resulting in a well-controllable heat release rate, which prevents further temperature increase. Here, industrial catalyst pellets were coated in a fluidized-bed pilot plant, demonstrating large-scale production feasibility. The obtained pellets were characterized via Dynamic Image Analysis, Scanning Electron Microscopy and X-ray Computed Tomography. Conducted CO2 methanation experiments confirm the predicted trends, if the applied shell is fully closed. Furthermore, mathematical and experimental studies demonstrate, that the inert shell shifts selectivity. Based on this work, safer and yet economical reactor operation is anticipated also for other reaction systems.

No data was used for the research described in the article. No data was used for the research described in the article.Recently, with the development of industrial industries such as petrochemical industry, coking plant, pharmaceutical and papermaking, a large number of industrial wastewater containing high salt organic pollutants was produced [1,2]. Saline organic wastewater was characterized by the biological toxicity, high color and large concentration of organic pollutants. If this kind of wastewater was discharged directly without treatment, it would cause great pollution to the water environment and endanger the health of animals, plants and human beings [3,4]. At present, the traditional methods used for the treatment of saline organic wastewater including biological methods, physical methods and chemical methods. Biological methods were widely used for wastewater treatment, however, most microorganisms were affected by salinity in the high-salt environment limiting the treatment performance. Physical methods achieved the transformation of pollutants, but failed to degrade organic pollutants completely [5,6]. Therefore, it was urgent to find an economical and efficient technology to treat saline organic wastewater. Chemical methods could completely degrade organic pollutants, among which, advanced oxidation processes (AOPs) had the characteristics of strong oxidation ability, non-selectivity, and rapid reaction [7]. Ozone oxidation was an efficient technology used in the treatment of industrial wastewater due to its strong oxidation capacity and easy operation [8]. However, selectivity of ozone oxidation and low solubility of ozone in aqueous solution led to a low utilization rate of ozone, so the ozone oxidation alone could not completely degrade refractory organic pollutants [9]. Catalytic ozonation promoted the decomposition of O3 and produced more reactive oxygen species (ROS) such as hydroxyl radical, superoxide radical, and singlet oxygen [10–12].Heterogeneous catalytic ozonation could effectively avoid or reduce the loss of active components, and improve the reusability and stability of catalysts [13,14]. A variety of heterogeneous catalytic oxidation catalysts used for the treatment of refractory pollutants in wastewater, such as activated carbon (AC) [15], metal oxides (Al2O3, TiO2 and MnO2) [16,17] and metal composite materials [18]. Among these catalysts, Al2O3 was used as the catalyst support due to its large specific surface area, low cost and stable mechanical properties [19]. Zhao et al. prepared the Mn-Cu-Ce/Al2O3 catalysts with an impregnation calcination method and the trimetal oxide catalysts were used for catalytic ozonation treatment of coal chemical wastewater (CCW). For Mn-Cu-Ce/Al2O3 catalysts, Al2O3 with the porous structure and large specific surface area were conducive to the adsorption of organic matter, and the metals were highly distributed on the surface of the Al2O3 support. In addition, the synergistic interaction of trimetallic oxides greatly enriched the catalytic active center and improved the catalytic performance. Compared with ozonation alone, the removal rate of CCW was increased by 31.6% after the addition of Mn-Cu-Ce /Al2O3 catalysts [20].Wei et al. designed and synthesized a highly efficient catalysts (CuCo/NiCAF) with the core-multi-shell structure by loading Cu-Co bimetal on Al2O3. Compared with the Al2O3 support, the treatment performance of the synthesized catalysts was significantly improved after metal loading, that was, the removal rates of total TOC were 86.7% for CuCo/NiCAF and 48.0% for γ-Al2O3 [21]. ZSM5 zeolites loaded with metallic oxides (Ce, Fe, or Mn) were used to remove the nitrobenzene from wastewater, and the load of Ce, Fe, or Mn oxides increased the catalytic performance comparing with ZSM5 zeolites alone [22]. Transition metals (such as Fe, Cu, Mo and Co) were used to modify the catalysts, however, it was inevitable that a small amount of metal ions leaching into the wastewater during the process of catalytic ozonation when the metals were directly loaded on the pure support [23,24]. Therefore, it was very important to enhance the interaction between metal and support, reduce the leaching of metal ions, and improve the stability of catalysts. Xu et al. prepared an amino functionalization catalysts (MnO2 NH2-GO) enhanced the bridging covalent bond between the oxygen groups of MnO2 and GO, preventing the uncoupling of GO and MnO2. MnO2 NH2-GO showed a higher catalytic performance and more stable performance than MnO2-GO in the process of catalytic ozonation [25]. Yang et al. prepared the CuO/SiO2 catalysts through the atomic layer deposition, which significantly improved the stability of CuO/SiO2 catalysts during the catalytic ozonation treatment of organic matter in wastewater [26].It was important to find a nontoxic and efficient metal for catalytic oxidation process. Calcium-based catalysts showed prominent advantages in the green environmental protection. Calcium was almost pollution-free to the environment and avoided the problem of secondary pollution to the water environment after ozone oxidation. Secondly, calcium as a strong base site was conducive to the decomposition of ozone into reactive oxygen species and showed a good catalytic activity to degrade organic pollutants. Liao et al. used CaO as a heterogeneous catalyst to ozonize nitrotoluene wastewater, and the results showed that CaO could promote the decomposition of ozone to produce more •OH [27]. Besides, Hsu et al. used CaO as the ozone catalysts for phenol removal and found that CaO could promote the ozonation process effectively removing phenol [28]. However, there is little reports about the combination of CaO active substance with catalysts support, and the stability of calcium-based catalysts should been attached more attention.Pectin is mainly composed of D-galacturonic acid (GalA) and extracted from plant cell walls, showing the properties of nontoxic and biocompatible. Besides, it has abundant electron-rich functional groups such as carboxyl and hydroxyl groups, and shows a strong affinity for metal ions (Affinity of metal ions to pectin is in the following order: Ca2+≈ Cu2+≈ Zn2+≈ Cr3+> Ni2+ > Pb2+ > Cd2+) [29,30]. The strong interaction between GalA in pectin and Ca2+ formed a three-dimensional network of cross-linked pectin molecules [31]. Shao et al. prepared a green adsorbent of chitosan-pectin gel pellets in alkaline solution, and used the adsorbent to remove heavy metals in water [32]. The addition of pectin not only formed a stable porous structure, but also introduced a large number of carboxyl functional groups. So it was conducive to increasing the specific surface area and active adsorption sites. Cu(II), Cd(II), Hg(II) and Pb(II) were effectively adsorbed by chitosan/pectin gel beads, and the maximum adsorption capacities were 169.4, 177.6, 208.5 and 266.5 mg/g, respectively. Wang et al. used Ca2+ as the cross-linking agent and prepared pectin-Ca microspheres through the interaction between Ca2+ and pectin [33]. By the modification of microspheres surface, a new adsorbent of pectin/poly (M-phenylenediamine) microspheres was prepared, and the adsorption performance of prepared adsorbent for Pb2+ was significantly improved. Pectin was mainly used for adsorbent preparation, but rarely used for the preparation of catalysts.In this work, the surface of Al2O3 carrier was coated with pectin, and calcium was loaded as the metal active substance. After calcination, a new green and efficient catalyst (Al2O3-PEC-CaxOy) was prepared. The introduction of pectin made Ca2+ more firmly loaded on the support, besides, a porous carbon layer on the surface of the Al2O3 carrier was formed after pectin calcination. The influence of preparation conditions such as pectin content, Ca2+ concentration, calcination temperature, and calcination time on the catalytic performance were investigated and optimized. Besides, the catalysts prepared under the optimized conditions were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and specific surface area pore analyzer (BET). The influence of operation conditions on removal rate of COD during the heterogeneous catalytic ozonation process were optimized. The generation of reactive oxygen species during the catalytic oxidation process was analyzed by the electron paramagnetic resonance technique and quenching test. Moreover, the reusability and stability of the catalysts were evaluated. Finally, the catalytic mechanism of treating saline organic wastewater by heterogeneous catalytic ozonation with Al2O3-PEC-CaxOy as catalysts was revealed.Al2O3 with a particle size of 3–5 mm and calcium chloride (analytical reagent grade (AR)) were purchased from Sinopharm Chemical Reagent Co., LTD. (Beijing, China). Pectin (CAS: 9000–65–5) was purchased from Beijing J&K Scientific Co., LTD. Quencher L-histidine (≥99%) was purchased from Shanghai Aladin Biochemical Technology Co., LTD. P-benquinone (p-BQ, 99%) and sodium bicarbonate (NaHCO3, ≥98%) were purchased from Shanghai Maclin Biochemical Technology Co., LTD. Phosphate buffer (0.2 mol/L) was purchased from Shanghai Yuanye Biotechnology Co., LTD. All the chemicals were used without any further purification and the aqueous solutions were prepared with ultrapure water producing by UK ultrapure water machine (18.2 MΩ). The detailed information about industrial wastewater before treatment in this work was shown in Table S1.Al2O3 carrier was impregnated in pectin solution and oscillated at 25 °C for 6 h to obtain the modified alumina carrier (Al2O3-PEC). The modified Al2O3-PEC was impregnated in calcium chloride solution, oscillated at 25 °C for 6 h and then kept for 12 h. Subsequently, the solid was placed in an oven and dried at 60 °C for 6 h to obtain the modified Al2O3-PEC-Ca2+. Al2O3-PEC-CaxOy catalysts were prepared by calcination of the solids at different temperatures (500–1000 °C) for 2 h under argon atmosphere with a heating rate of 5 °C/min. In conclusion, the surface of Al2O3 carrier was coated with pectin, and then the coated carrier was modified with calcium metal. The preparation process of the Al2O3-PEC-CaxOy catalysts was shown in Fig. 1 .SEM (ZEISS GeminiSEM 300, Germany) and TEM (JEOL JEM-F200, Japan) were used to analyze the surface morphology of the catalysts. The surface element composition and valence of the catalysts were determined by X-ray photoelectron spectroscopy (Thermo Scientific K-Alpha, USA). The pore size, surface area and pore volume of the catalysts were determined by an automatic specific surface and porosity analyzer (Micromeritics ASAP 2460, USA) using N2 adsorption-desorption isotherm at 77 K. The crystal structure of the catalysts was analyzed by Japanese Rigaku Ultima IV X-ray diffractometer (XRD) with a scanning angle range of 10°–80° and a scanning speed of 5°/min. The surface carbon profiles of the catalysts were characterized by Raman spectrometer (Horiba LabRAM HR Evolution, Japan). The reactive oxygen species generated during the catalytic process were analyzed by electron spin resonance spectroscopy (EPR) using Bruker EMXnano EPR spectrometer. The zero charge pH point (pHpzc) of the catalysts was detected using a Zeta potential detector (Malvern Zetasizer Nano ZS ZEN3600, UK).The catalytic ozonation system was composed of oxygen cylinder, ozone concentration detector, ozone generator, ozone catalytic reactor and absorption device of tail gas, as shown in Fig. 2 . Ozone was produced by ozone generator (Beijing Tonglin ozone) with pure oxygen. The ozone concentration at the outlet was controlled by adjusting the power of ozone generator. The catalytic ozone system was carried out in a self-made cylindrical tubular reactor, in which 50 mL wastewater was injected through the top of the reactor and filled with spherical catalysts. Ozone was injected into the solution through a microporous titanium aerator (0.22 μm) at the bottom of a self-designed cylindrical tubular reactor. The flow rate of ozone was 30 mL/min and the concentration of ozone was 4–20 mg/L. Absorption device of tail gas was carried out using KI solution (20%). The samples were filtered through a filter (0.45 μm) for quantitative analysis of COD. As comparison, the adsorption experiments of Al2O3-PEC-CaxOy catalysts were carried out with the same experimental setup in the absence of ozone. H2SO4 and NaOH solutions were used to adjust the initial pH value of wastewater from 2 to 12. Measurement of chemical oxygen demand (COD) was carried out by a UV/VIS spectrophotometer (DR5000, USA).Effects of pectin content, concentration of calcium ion, calcination temperature and calcination time on the preparation of Al2O3-PEC-CaxOy catalysts were investigated, and the optimization of preparation conditions for the Al2O3-PEC-CaxOy were shown in Fig. 3 .Effect of pectin content on the catalytic performance of the Al2O3-PEC-CaxOy was shown in Fig. 3(a). The removal rate of COD was first increased and then decreased with the increase of pectin content. The removal rate of COD was reached to 59% when the amount of pectin content was increased to 2.5% w/v, but the removal rate of COD was decreased when the pectin content was higher than 2.5% w/v. The introduction of pectin made more Ca2+ firmly loaded on the support, and a porous carbon layer on the surface of the Al2O3 carrier was formed after the calcination of pectin. So the increase of pectin content was benefit for the treatment of saline organic wastewater. However, a thick carbon layer was formed when the pectin was coated on the surface of Al2O3 carrier, so the specific surface area of the Al2O3-PEC-CaxOy catalysts was decreased with the further increase of pectin content [34]. In addition, the viscosity of solution was increased with the increase of pectin, which blocked the channel of Al2O3 carrier and eventually led to the removal rate of COD decreasing. Therefore, the pectin content of 2.5% w/v was selected as the optimal condition for Al2O3-PEC-CaxOy catalysts preparation.During the catalytic ozonation process, metal oxides were used as active sites to catalyze ozone and increase the generation of reactive oxygen species. Insufficient modification of metal ions led to the decrease of active site, while the excessive load caused the accumulation of metal oxides on the surface of Al2O3 carrier and ultimately reduced the catalytic activity [35,36]. The effect of Ca2+ concentration on the catalytic performance of the Al2O3-PEC-CaxOy catalysts was shown in Fig. 3(b). The removal rate of COD showed a trend of increase when the Ca2+ concentration was increased to 0.3 mol/L. However, the further increase of Ca2+ concentration led to the agglomeration of metal oxides on the catalysts surface and reduced the specific surface area of the catalysts, which inhibited the improvement of the catalytic performance of Al2O3-PEC-CaxOy [37]. Therefore, Ca2+ concentration of 0.3 mol/L was selected as the optimal condition for Al2O3-PEC-CaxOy preparation.Calcination temperature is an important parameter to determine the catalytic performance of catalysts. Effect of calcination temperature (500–1000 °C) on the catalysts performance was investigated (Fig. 3(c)). The catalytic performance for the prepared catalysts was lower when the calcination temperature was 500 °C. Pectin coated on the catalysts surface could not be carbonized to form the carbon layer when the calcination temperature was 500 °C, and the color of the catalysts was yellow-brown (Fig. S1). In addition, the pectin coating entered the wastewater making the increase of organic pollutants in the wastewater during the process of catalytic oxidation. The removal rate of COD was increased obviously with the increase of calcination temperature. Meanwhile, the Al2O3-PECCaxOy catalysts had the highest catalytic performance when the calcination temperature was 800 °C. The formation of carbon layer was benefit for improving the treatment performance. The removal rate of COD was decreased when the calcination temperature was further increase. The specific surface area and pores of the catalysts prepared at different calcination temperatures of 500–1000 °C were shown in the Table S1. The further increase of calcination temperature led to the decrease of the specific surface area of the catalysts and the aggregation of the active site [38,39]. In conclusion, the calcination temperature of 800 °C was selected as the optimal condition for Al2O3-PECCaxOy preparation.Calcination time is also an important factor affecting the performance of the prepared catalysts. Effect of calcination time (1–5 h) on catalytic performance of Al2O3-PECCaxOy catalysts for saline organic wastewater treatment was shown in Fig. 3(d), the generation of the carbon layer on the surface of Al2O3-PEC-CaxOy catalysts was incomplete at the calcination time of 1 h, and the calcium metal oxide was not fully formed. The removal rate of COD was increased obviously when the calcination time was extended from 1 h to 2 h, but the removal rate of COD was decreased slightly with the continuous increase of calcination time. The prolongation of calcination time increased the possibility of component agglomeration, decreasing the dispersion of metal oxides [40]. Therefore, the calcination time of 2 h was selected as the optimal condition for Al2O3-PEC-CaxOy preparation.In summary, the optimal preparation conditions were obtained and the optimal conditions for Al2O3-PEC-CaxOy preparation were pectin content of 2.5% w/v, Ca2+ concentration of 0.3 mol/L, calcination temperature of 800 °C and calcination time of 2 h.Structure and morphology of the Al2O3-PEC-CaxOy catalysts prepared under the optimal conditions were analyzed by various characterization methods. The XRD patterns of Al2O3 and Al2O3-PEC-CaxOy catalysts were observed in Fig. 4 . Peaks at 2θ = 19.5°, 37.4°, 45.7° and 66.9° were good resolution and sharp diffraction for the Al2O3-PEC-CaxOy catalysts, which were attributed to the crystal phase of Al2O3 (JCPDS 50–0741). The results showed that PEC coating and Ca2+ introduction did not damage the crystallization and structural integrity of the Al2O3 carrier. Besides, peaks at 2θ = 32.36°, 37.4°, 45.7°, 60.74° and 66.9° for the Al2O3-PEC-CaxOy catalysts were significantly enhanced, which was attributed to the characteristic peaks of CaO (JCPDS99–0070) and CaO2 (JCPDS85–0514). According to the XRD results, the Ca element was successfully deposited on the surface of Al2O3-PEC-CaxOy catalysts.The morphology and structure of Al2O3 and Al2O3-PEC-CaxOy catalysts were analyzed by the SEM. As shown in Fig. 5 , comparing with the pure Al2O3 support, the modified Al2O3-PEC-CaxOy catalysts still maintained the good surface morphology and pore structure, and there was no obvious agglomeration. The results indicated that the calcium loading on the Al2O3-PEC-CaxOy catalysts was uniform. In addition, Al2O3-PEC-CaxOy catalysts with rough and porous structure were due to the pectin coating after calcination. The rough porous structure provided active sites for the catalytic oxidation process and was conducive to the adsorption of organic pollutants on the catalyst surface, which promoted the interaction between the catalysts and organic pollutants.As shown in Fig. 6 , the morphology and structure of Al2O3 and Al2O3-PEC-CaxOy catalysts were further observed by TEM. Compared with pure Al2O3, the Al2O3-PEC-CaxOy catalysts showed rough surface after pectin modification and calcium introduction, and the results showed that PEC was successfully coated on the carrier surface. In addition, the shape of Al2O3-PEC-CaxOy catalysts was regular and there was no obvious agglomeration phenomenon, indicating that the active components were uniformly dispersed. The elements distribution of C, O, and Ca for the Al2O3-PEC-CaxOy catalysts was analyzed by elemental mapping. As shown in Fig. 7 , the elements of C, O, and Ca were evenly dispersed, indicating that Ca2+ was successfully loaded on the surface of Al2O3 and dispersed evenly. Moreover, the porous carbon layer was formed after the calcination of coated pectin, which could improve the adsorption performance and catalytic performance. The characterization of catalysts before and after calcination were carried out by Raman spectroscopy (Fig. S2), it confirmed the existence of carbon layer on the surface of the Al2O3-PEC-CaxOy.The element composition and oxidation state of the Al2O3-PEC-CaxOy catalysts were further analyzed by the XPS. As shown in Fig. 8 (a), the elements peaks of C, O and Ca were obviously observed, and the atomic concentrations of these elements were determined by the XPS and listed in Table S3. The Ca 2p spectra of Al2O3-PECCaxOy catalyst were divided into two peaks at 347.5 eV and 350.8 eV, corresponding to Ca 2p 3/2 and Ca 2p 1/2 respectively. The result confirmed the formation of Ca oxides (Fig. 8(b)). In addition, the presence of Ca-O bond at 530.8 eV was also found [41]. Therefore, the XPS analysis results confirmed that the Ca element was successfully loaded onto the Al2O3-PEC-CaxOy, which was consistent with the results of XRD characterization.As shown in Fig. 8(c), C 1 s spectra of Al2O3-PEC-CaxOy were divided into four peaks. The divided peaks of 284.6, 286.1, 288.2 and 289.8 eV were corresponding to CC/C=C, CO-C/COH, C=O and HOC=O. The formation of COH or C=O structures at the surface of Al2O3-PECCaxOy catalyst provided more catalytic sites (especial carbonyl groups) to rapidly convert O3 into free hydroxyl radical [42]. Thus, the generated carbon layer provided active sites for O3 decomposition. As shown in Fig. 8(d), O 1 s spectra of Al2O3-PECCaxOy catalyst were divided into three peaks at 530.9 eV, 532.2 eV and 533.2 eV, which were corresponding to lattice oxygen (O lat), chemisorption oxygen (O ads) and physical adsorption oxygen (O surf), respectively [43]. O lat was mainly derived from metal-oxygen bonds (Ca2+-O), which played an important role in the catalytic oxidation process.Specific surface area, pore volume and pore size of the carrier and Al2O3-PECCaxOy catalysts were shown in the Table 1 . After modification, the specific surface area of the catalysts was significantly reduced. The decrease of specific surface area of catalyst was due to the blocking of porous structure by carbon layer and metal oxide. In addition, the agglomeration of internal pores of the Al2O3-PEC-CaxOy catalyst also reduced the specific surface area of the catalysts during the high-temperature calcination. The N2 adsorption-desorption isotherm was shown in the Fig. 9 (a), isotherm of the Al2O3-PECCaxOy catalysts was the type IV and the hysteresis ring was type H3, indicating that the prepared catalysts with abundant mesoporous pores. The curve of pore size distribution was shown in the Fig. 9(b). The pore size of the Al2O3-PECCaxOy catalyst was mainly located in the mesoporous region (2 nm < pore size < 50 nm), which further confirmed the existence of mesoporous structures. The generation of mesoporous structures for the prepared catalysts was benefit for the improvement of adsorption performance.In order to verify the catalytic ozonation performance of the prepared catalysts, the systems for COD removal with ozonation, adsorption and catalytic ozonation were analyzed under the same operating conditions. As shown in Fig. 10 (a), the removal rate of COD in the ozonation system was only 33%. The removal rate of COD for saline organic wastewater was slightly increased to 37.6% in the Al2O3+O3 system. The removal rate of COD was increased to 47% in the Al2O3 CaxOy+O3 system, indicating that the calcium load played an important role in the catalytic ozonation process. The removal rate of COD was significantly increased to 49% in the Al2O3-PEC+O3 system, which indicated that the porous carbon layer formed after the calcination of coated pectin was helpful to improve the treatment performance. Meanwhile, the removal rate of COD for Al2O3-PECCaxOy catalysts was increased to 62% after further loading of Ca metal. The results indicated that the loading of Ca metal could increase the active site of the catalysts and improve the catalytic performance. The removal rate of COD was increased by 29% in the Al2O3-PEC-CaxOy+O3 system comparing with ozonation alone, indicating that the prepared catalysts had a significant performance on removing the COD from the saline organic wastewater.In order to confirm the COD removal was mainly depended on catalytic ozonation process rather than adsorption process, adsorption experiments were carried out using the prepared Al2O3-PEC-CaxOy catalysts. As shown in the Fig. 10(b), the removal rate of COD reached to 10% after five times of adsorption process, indicated that the carbon layer generated on the surface of the catalysts had a good adsorption performance. The contribution of adsorption process to the removal of COD was low when the saturation adsorption was reached, so the COD removal was mainly depended on the catalytic ozonation after five times of adsorption process.The comparison of catalyst performance between the prepared Al2O3-PEC-CaxOy catalysts in this work and those reported in the literature was shown in the Table 2 . The prepared Al2O3-PEC-CaxOy catalysts in this work showed a good catalytic performance for the treatment of saline organic wastewater. In addition, most of the literature reported that the catalysts were preparation using transition metal ion as active sites. Besides, Ca2+ was almost pollution-free to the environment and avoided the problem of secondary pollution to the water environment after ozone oxidation. So the prepared Al2O3-PEC-CaxOy catalysts showed environment friendly and high efficient performances.Influence of catalysts dosage on the treatment of saline organic wastewater was shown in the Fig. 11 (a). The removal rate of COD was increased from 41% to 59% when the dosage of Al2O3-PEC-CaxOy catalysts was increased from 100 to 400 g/L. The results showed that the increase of catalysts dosage could provide more active sites and enhance the reaction between solution phase and catalyst surface, leading to the decomposition of ozone and the generation of more reactive oxygen species [52]. However, the removal rate of COD did not further increase when the catalysts dosage was further increased to 600 g/L, indicating that the excess catalysts could not further improve the removal rate of COD when the ozone content in the system was limited. The further increase of Al2O3-PEC-CaxOy catalysts caused the reduce of the available surface area of the catalysts during the treatment of saline organic wastewater. Therefore, 400 mg/L of catalysts dosage was selected as the optimal condition for the treatment of saline organic wastewater.Ozone concentration is an important factor affecting the catalytic ozonation process. As shown in the Fig. 11(b), the removal rate of COD was increased from 46.3% to 59.0% when the ozone concentration was increased from 4 to 12 mg/L. The result indicated that the higher concentration of ozone was beneficial to the degradation of organic pollutants. The higher concentration of ozone not only increased the possibility of contact between ozone and organic pollutants, but also promoted the contact between ozone and the active site of catalysts. So the higher concentration of ozone accelerated the generation of reactive oxygen species, and improved the treatment performance of COD [53]. However, the removal rate of COD did not increase significantly when the ozone concentration was gradually increased to 20 mg/L. Due to the number of active sites on the catalysts surface was limited, so the utilization rate of ozone reached saturation and excessive ozone could not produce lots of reactive oxygen species. In addition, the production of high concentration of ozone needed more oxygen consumption and energy consumption, resulting in the increase of costs. Therefore, 12 mg/L of ozone concentration was selected as the optimal condition for the treatment of saline organic wastewater.Initial pH of solution directly affects the decomposition efficiency of ozone, degradation of organic pollutants and surface properties of catalysts, so the pH value is an important factor affecting the catalytic performance of catalysts [54]. The initial pH value of saline organic wastewater was adjusted from 2.0 to 10.0 by H2SO4 or NaOH solution. As shown in Fig. 11(c), the removal rate of COD was increased continuously when the pH value was increased from 2.0 to 7.0, and the removal rate of COD was decreased slightly under alkaline conditions. The surface charge of the prepared catalysts plays an important role in the decomposition of O3 and the formation of •OH during the process of catalytic ozonation. Most of the -OH groups on the catalysts surface were in a neutral state when the pH value of the saline organic wastewater was close to pH pzc , which was beneficial to accelerate the decomposition of O3 and generate ROS [55]. As shown in Fig. 11(d), the point of zero electric charge was 7.73. When the pH value of wastewater was close to the pH pzc =7.73 of Al2O3-PEC-CaxOy catalysts, the removal rate of COD reached to the highest value (62%). Metal oxides were easily leached from the catalysts and the active sites were reduced at the lower pH (pH = 2.0), resulting in the lower removal rate of COD. The concentration of •OH was increased and the quenching reaction between •OH was enhanced with the increase of pH value of solution, which hindered the further increase of •OH concentration. Therefore, the neutral condition of the saline organic wastewater was selected as the optimal condition for the treatment of saline organic wastewater.After the investigation of the operating parameters, 400 mg/L of catalysts dosage, 12 mg/L of ozone concentration, and neutral condition of the saline organic wastewater were selected as the optimal conditions for the treatment of saline organic wastewater.EPR technology was used to reveal the generated reactive oxygen species during the catalytic ozonation process [56]. The spin capture reagent of DMPO (5, 5-dimethylpyrrolidine N-oxide) was used in the EPR experiments to detect the •OH and •O2 −, and 2,2,6,6-tetramethylpiperidine (TEMP) was used in the EPR experiments to detect 1O2. A four-line characteristic spectral height ratio of 1:2:2:1 was observed for the DMPO-•OH, indicating the formation of •OH (Fig. 12 (a)). In addition, as shown in Fig. 12(b), six characteristic peaks of the DMPO-•O2 − was observed. A three-line characteristic spectral height ratio of 1:1:1 was observed for the DMPO-•O2 −, indicating the formation of 1O2 (Fig. 12(c)). The generation of DMPO-•OH, DMPO-•O2 −and TEMP-1O2 had no significant signal in the ozonation system without adding catalysts. In conclusion, the generation of •OH, •O2 −and 1O2 promoted the degradation of organic matter in the saline organic wastewater.Contribution of generated reactive oxygen species of •OH, •O2 − and 1O2 was further determined by the quenching experiments in the Al2O3-PECCaxOy+O3 system. Methyl orange (MO) was used as the simulated wastewater, bicarbonate anion (HCO3 −) was used as the scavenger for quenching •OH (9.7 × 108 M −1s−1), p-benzoquinone (p-BQ) was used as the scavenger for quenching •O2 − (3.5–7.8 × 108 M −1s−1) [57], and L-histidine was used as the scavenger for quenching 1O2 (3.2 × 107 M −1s−1) [30]. Radical quenching experiments were performed using the same Al2O3-PEC-CaxOy catalysts and the results were shown in Fig. 12(d). The degradation of MO was inhibited in the presence of HCO3 − (10 mM), while, the degradation of MO was relatively rapid in the presence of L-histidine in the catalytic ozonation system. Moreover, an obvious inhibition of COD removal was observed in the presence of p-BQ (10 mM).In addition, quenching experiments with different concentrations of scavengers (10 mM, 20 mM, and 50 mM) were carried out. As shown in Fig. S3, the inhibition effect on MO was significantly increased with the increasing amount of quencher. NaHCO3 had a strong affinity to Lewis acid sites on the catalysts surface, and inhibited the catalytic decomposition of O3 to •OH radical. The inhibition effect on MO removal was obvious when the concentration of p-BQ scavenger was increased to 50 mM. The reason for this phenomenon was that p-BQ not only consumed •O2 −, but also reacted with O3 in the catalytic oxidation system. Excessive p-BQ directly consumed O3 (2.5 × 103 M −1s−1) during the catalytic oxidation process [58,59], so the decomposition of ozone in the reaction system was inhibited. These results indicated that the generation of •OH, •O2 −, and 1O2 were the main reactive oxygen species attributed to the organic matter removal.Some researches reported that hydroxyl on the surface of metal-based catalysts were widely regarded as the catalytic active sites. Phosphate was a strong Lewis base that could replace the hydroxyl on the surface of catalysts and reduce the number of hydroxyl groups, which hindered the interaction of O3 with the Lewis acid site of the catalyst and ultimately hindered the generation of reactive oxygen species [60]. In order to verify the contribution of hydroxyl on the surface of the catalysts, phosphate (10 mM) was used for hydroxyl quenching. As shown in Fig. 12(d), the removal rate of MO was inhibited in the presence of phosphate during the catalytic ozonation process. The results indicated that the hydroxyl on the surface of the catalysts were acted as the catalytic active sites for O3 decomposition to generate •OH, •O2 − and 1O2.XPS analysis of the Al2O3-PECCaxOy catalysts before and after the catalytic ozonation process was shown in Fig. 13 (a), the content of C−C/C=C on the surface of the carbon layer decreased significantly (from 48.85% to 17.09%) after the catalytic ozonation process, while, the content of C=O also decreased. The C−OH or C=O structure on the surface of the carbon layer provided catalytic sites for O3 decomposition in the process of catalytic oxidation [61]. There was no change for O1s in the Al2O3-PEC-CaxOy catalysts before and after catalytic oxidation (Fig. 13(b)). O lat was the key to hydroxylation of metal oxides, and O lat as a strong Lewis base absorbed protons from water generating Ca-OH and promoting catalytic oxidation process.Oxygen vacancies (Ovs) of Al2O3 and Al2O3-PEC-CaxOy catalysts were detected by the EPR. As shown in Fig. 13(c), a signal peak was found at g = 2.0025 for Al2O3-PEC-CaxOy catalysts, but no obvious signal was found for Al2O3 catalysts. The signal peak of g ≈ 2.00 was attributed to the electrons captured by Ovs, which indicated that the carbon layer of the synthesized catalysts containing Ovs and the result was consistent with the XPS spectrum of O 1 s. O lat generated under the reaction of Ovs and continuously supplied in the catalytic oxidation system, which maintained the stability of the catalysts and further provided the active sites for the catalysts. More Ovs could improve the efficiency of electron transfer and promote the catalytic oxidation process when O lat content was remained balanced. Therefore, Ovs accelerated the electron transfer among oxygen in the process of catalytic ozone oxidation, maintaining the catalytic performance and promoting the decomposition of O3 to generate more reactive oxygen species.Mechanism diagram of treating saline organic wastewater with Al2O3-PECCaxOy catalysts was shown in Fig. 14 . During the process of non-free radical oxidation, the adsorption of O3 and organic pollutants on the surface of the catalysts was crucial. Non-radical oxidation pathways for treatment of saline organic wastewater were divided into two categories: (1) Organic matter was adsorbed on the surface of the catalysts and oxidized by O3; (2) Indirect degradation of organic pollutants with 1O2 generated by catalytic decomposition of O3 [62]. Meanwhile, radicals of •OH and •O2 −were confirmed according to the electron paramagnetic resonance and radical quenching experiments, which had a higher oxidation potential to achieve the organic matter removal.Stability and reusability of catalysts are the important factors that should be considered in the industrial applications. The used catalysts were collected from the solution and washed several times with ultrapure water for future use. As shown in Fig. 15 (a), the removal rate of COD was gradually decreased from 62% to 50.5% after five cycles. The adsorption performance of the catalysts was reduced after consecutive adsorption during the process of ozonation, which reduced the contact between the ozone and the pollutants on the catalysts surface. In addition, intermediates generated by organic pollutants oxidizing and inorganic salts in wastewater occupied the catalytic active sites, which reduced the catalytic performance after five cycles. The removal rate of COD was basically stable at about 50% after further use. In addition, Al2O3-PEC-CaxOy catalyst still maintained a good crystallinity after twenty cycles (Fig. 15(b)). The XPS characterization of Al2O3-PEC-CaxOy catalysts before and after twenty cycles was shown in Fig. 15(c, d), the peak value of Ca element was only changed slightly after twenty cycles. In this study, the losing of metal active sites was controlled through the cross-linking between pectin and Ca2+during the catalytic oxidation process. The surface of prepared Al2O3-PEC-CaxOy catalysts maintained a good morphology, and no obvious damages were observed (Fig. 15(e, f)). The results indicated that the catalytic performance of the catalysts did not decrease significantly, and the Al2O3-PEC-CaxOy catalysts had a good catalytic stability and reusability. According to the above results, the Al2O3-PEC-CaxOy catalysts showed a good stability in the long-term application.In this work, a green and efficient catalyst (Al2O3-PEC-CaxOy) was successfully synthesized. The optimal conditions for Al2O3-PEC-CaxOy preparation were investigated and summarized as pectin content of 2.5% w/v, Ca2+ concentration of 0.3 mol/L, calcination temperature of 800 °C and calcination time of 2 h. Compared with the ozone oxidation alone (removal rate of COD was 33%), the removal rate of COD was 62% in the Al2O3-PECCaxOy+O3 system under the conditions of 400 mg/L catalyst dosage, 12 mg/L ozone concentration and neutral saline organic wastewater, which was 1.9 times of that of ozonation alone. The carbon layer formed after calcination could increase the adsorption performance of Al2O3-PEC-CaxOy catalysts and contribute to the removal of organic pollutants. In addition, the structure of C−OH and C=O on the surface of the carbon layer provided catalytic sites to rapidly convert O3 to •OH, •O2 − and 1O2. At the same time, the surface -OH and calcium metal oxides also provided more catalytic active sites for Al2O3-PEC-CaxOy catalysts during the process of catalytic ozonation, which promoted the decomposition of O3 and accelerated the generation of reactive oxygen species. The Al2O3-PEC-CaxOy catalysts had a good catalytic stability and reusability after twenty cycles of continuous operation, and the interaction between pectin and calcium ions could improve the stability and catalytic performance. Therefore, the result showed the Al2O3-PEC-CaxOy catalysts with a great significance for long-term practical application with the economical and effective performances.Treatment of saline organic wastewater by heterogeneous catalytic ozonation with Al2O3-PEC-CaxOy as catalystsThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the National Natural Science Foundation of China (22125802 and 22108012), Natural Science Foundation of Beijing Municipality (2222017), and Fundamental Research Funds for the Central Universities (BUCTRC-202109). The authors gratefully acknowledge these grants.Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ceja.2023.100447. Image, application 1

Al2O3-Pectin-CaxOy (Al2O3-PEC-CaxOy) was prepared as catalysts to improve the treatment of saline organic wastewater with heterogeneous catalytic ozonation. Compared with ozonation alone (33%), the removal rate of COD (62%) was significantly increased for Al2O3-PEC-CaxOy catalysts prepared under the optimized conditions. The introduction of pectin made Ca2+ firmly loaded on the support, and avoided the loss of active sites in the process of catalytic oxidation. In addition, the formation of porous carbon layer on the surface of the Al2O3 support after pectin calcination was conducive to improving the catalytic activity of the catalyst. Electron paramagnetic resonance (EPR) and radical quenching experiments showed that hydroxyl radical (•OH), superoxide radical (•O2 −) and singlet oxygen (1O2) were the reactive oxygen species (ROS) that attributed to the organic matter removal. Both free radical and non-free radical pathways were involved in the degradation of organic pollutants during the catalytic ozonation process. The removal rate of COD was only decreased slightly after twenty times of continuous operation for Al2O3-PEC-CaxOy catalysts, indicating that Al2O3-PEC-CaxOy catalysts with the good catalytic stability and reusability. It showed a great significance for long-term practical application with the economical and effective performances.

No data was used for the research described in the article.Activated CarbonAberration Corrected Scanning Transmission Electron MicroscopyAcetylacetonateAtomic Layer DepositionBorane Tert–Butylamine Complex (BTB)Black Pearls® 2000Carbon–carbon bondOrdered Mesoporous CarbonCO2 Reduction ReactionsCarbon NanoparticlesCross Polarized-Magic-Angle Spinning-Nuclear Magnetic ResonanceCovalent Triazine FrameworksChemical Vapor DepositionDensity Functional TheoryDry Reforming of MethaneDefective GrapheneDinuclear Heterogeneous CatalystDiffuse Reflectance Infrared Fourier Transform SpectroscopyActivation EnergyExfoliated Graphitic Carbon NitrideEnergy Dispersive X-ray SpectroscopyElectron Energy Loss SpectroscopyEthylene GlycolateEuropean Patent OfficeEley-RidealElectromagnetic Spin ResonanceExtended X-ray Absorption Fine StructureIron PhthalocyanineGas Chromatography-Mass SpectrometryGraphene NanosheetHigh Angle Annular Dark FieldHydroxyapatiteHydrogen Evolution ReactionsHollow N-doped Carbon SphereHigh-Resolution Transmission Electron MicroscopyInfraredIsolated Single Atomic SitesIncipient Wetness ImpregnationSolid-State Magic-Angle Spinning-Nuclear Magnetic ResonanceMolybdenum CarbideMetal–Organic FrameworkMulti-Walled NanotubesNear Ambient Pressure X-ray Photoelectron SpectroscopyNitrogen-doped CarbonNoble MetalNuclear Magnetic ResonanceNanoparticleNanotube Arrays Supported by a Ni FoamOxygen Evolution ReactionsOxygen Reduction ReactionsOperando Raman SpectroscopyPorous Nitrogen-doped CarbonPhosphomolybdic AcidPolypyrrolePreferential Oxidation ReactionTetra(4-tert-butyl-phenyl)porphyrinato PlatinumPolyvinylpyrrolidoneSingle atomSingle Atom AlloySACs Single Atom Catalyst Single Atom CatalystsStrong Metal–Support InteractionScanning Transmission MicroscopyScanning Tunneling MicroscopyTermolecular Eley-RidelTurnover FrequencyTemperature Programmed ReductionTemperature Programmed DesorptionUltra-High VacuumUnderpotential DepositionUltra VioletUltra Violet OzoneWater Gas ShiftWeight Hourly Space VelocityWorld Intellectual Property OrganizationX-ray Absorption Near Edge StructureX-ray Absorption SpectroscopyX-ray Photoelectron Spectroscopy1,3-Propanediols2-DimensionalThe first appearance of the term “catalysis” can be backdated to the early 19th Century. Jöns Jacob Berzelius, who was one of the founders of modern chemistry, had successfully discovered the existence of catalytic energy [1], whereas the first-ever documented catalyst application can be traced even earlier to the alchemical era (4th century BC) [2]. Initially, industrial catalysts were mainly used in the Deacon process (the transformation of HCl gas in oxygen to H2O and Cl), sulphuric acid production, Ostwald process (production of nitric acid from ammonia), and the Haber-Bosch process (production of ammonia via nitrogen fixation). To-date, its usage has been extended further into the majority of current industrial chemical productions [3]. Evidence has shown that the development of catalysts was one of the key factors that influence the sustainability and performance of catalytic processes, in terms of economic viability, technology feasibility, and environmental impacts [4,5]. Ever since the commercial usage of catalysts has been widespread around the globe. The subsequent innovation and revolution of catalyst development have ceaselessly progressed (Fig. 1 ). This review focuses on the emerging catalyst technologies from the last decade to state-of-the-art, atom efficiency and sustainable single atom catalysts (SACs) as well as highlighting the frontiers for future research into clean energy and growth.The term SAC refers to a catalyst where individual metal atoms are isolated and dispersed across/throughout a supporting material [13]. To retain such high atomic dispersion efficiency, strong interactions between the isolated metal atoms and the coordination sites are exerted, creating a unique tunable electronic structure. The single atom-level dispersion of metal atoms on a support is not only able to maximize the atomic efficiency by offering a greater number of active sites but at the same time generate uniform and well-engineered active sites [14]. These sites can be altered to finely tune the reactivity and selectivity of chemical reactions by manipulating coordination sites (e.g., ligand tuning) and defect designs [15]. In addition, undesirable deleterious side reactions can be mitigated if requiring more than one atomic active site, due to the high dispersion and isolation [16]. These compounded features lead to a remarkable catalytic performance which has been found to outperform conventional mono/bimetallic catalysts with bulk nanoparticles. Therefore, SACs (plural of a single atom catalyst) are currently noted as “rising stars” for the future of sustainably producing fine chemicals [17] and are often envisioned as the natural bridge between heterogeneous and homogeneous catalysis [18]. Fig. 2 presents some of the major research contributions related to the use of single atoms as catalysts in chronological order.Although single atom sites have a long history in heterogeneous catalysis spanning over five decades, the term SAC was first introduced in 2011 by Qiao et al. [19]. This work serves as an attempt to examine whether a single atom (denoted as M1 where M is the active metal used) can provide better catalytic activity, selectivity, and stability as compared to other nanometer-sized and sub-nanometer-sized particles (i.e. clusters or atom ensembles). In general, the synthesized Pt1/FeOx SAC catalysts show greater reactivity for both CO oxidation and the Preferential Oxidation Reaction (PROX) reaction (by 2–3 times) than that of bulk nanoparticle-based Au/Fe2O3 catalysts, capable of providing high reactivity for CO oxidation [20,21]. In addition, the Pt1/FeOx SAC was found to be fully regenerated even after 2 cycles of thermal treatments. These findings were no doubt a bombshell to the research community, given that the commercial Pt/Al2O3 catalysts are often less favorable than Au/Fe2O3 catalysts for both CO oxidation and PROX reaction due to their higher cost. This research proved that a high atomic efficiency level catalyst is one of the feasible directions for the future of sustainable catalysis research, especially chemical reactions that require high noble metal loadings. A year later, a study conducted by Kyriakou et al. [22] found that lower activation energy (Ea) for hydrogen dissociation can be achieved by using isolated Pd atoms across a Cu(111) surface under ultra-high vacuum conditions for hydrogenation reactions. The authors used the term Single Atom Alloy (SAA) to describe this class of material. In the same year, Knurr and Weber [23] explored the theoretical design of SACs for the solvent-driven CO2 reduction process via a first-principle calculation. Lin et al. [24], on the other hand, extended the utility of SACs to the water gas shift (WGS) reaction in 2013. Both Pt1/FeOx and Ir1/FeOx catalysts showed extraordinary catalytic performance for WGS, presenting a specific rate of around 2–3 times higher than previous Pt/MoC and Au/CeOx catalysts. Another interesting finding was reported in the work conducted by Wang et al. [25], which investigated the selective catalytic reduction of NO in the presence of H2 with an Rh1/Co3O4 catalyst. The surface of Rh1/Co3O4 had reconstructed into RhCon/Co3O4, an SAA at 220 °C. More interestingly, this re-constructed catalyst offered superior catalytic performance, when compared with its original form. In addition to the Rh–Co alloy, the catalytic activity enhancement on CO oxidation attributed to the single transition metal-atom substitution in V4O10 has also been studied based on Density Functional Theory (DFT) calculations [26]. Aside from this, an advanced fabrication technique “atomic layer deposition (ALD)” has been discovered to design and synthesize a SAC with atomic-level control on its composition and thickness [27].Thereafter, numerous works have discovered the potential of noble metal-based SACs for various reactions, which include, but are not limited to glucose oxidation using Au1/Pd nano clusters [28], the reduction of I3 - to I− using Pt1/FeOx [29], HCHO oxidation with Ag1/hollandite manganese oxide [30], syngas-to-C2 oxygenates conversion with Rh-based SACs [31], N-alkylation and α-alkylation using Ir1-doped polypyrrole [32] and Zn1/N-co-doped porous carbon [33], CH4 oxidation using Pt1/La2O3 (NM indicates noble metal) [34]. A major use of SACs and SAAs has been found for hydrogenation reactions, specifically, nitroarenes using Pt1/FeOx [35], 1,3-butadiene using both Pd1/graphene [36] and Pt1/m-Al2O3 [37]. Other reactions have been NO reduction with Pt1/FeOx [38], hydroformylation of olefins using Rh1/ZnO [39], oxygen evolution reaction (OER) using Rh1, Ru1, Ir1 and Pd1 supported on Co3O4 [40], and extensively for CO oxidation using various atomic catalysts such as Au1, Rh1, Pd1, Cu1, Ru1, Ti1/FeOx [41]. In addition, the study on the use of a SAA has also been extended for other reactions such as the Ullmann reaction of aryl halides using Au–Pd SAA [42], the dehydrogenation of propane using Cu–Pd SAA [43], the hydrogenation of C2H2 using Ag–Pd SAA [44], the oxidation of formic acid using Ag–Pt SAA [45], the selective hydrogenation of furfural using Pd atoms alloyed into a Cu nanoparticle supported on alumina [46] and the dehydrogenation of 4-hydroxypentanoic acid [47]. In addition to the reactions shown above, the use of single atoms has been effective for electrochemical applications, work can be found starting from 2015 and has been extensively explored from 2016 onwards. These applications encompass Oxygen Reduction Reactions (ORR) using Pt1/TiN [48], Pt1/zeolite-templated carbon [49] and have recently extended to a SAA which contains Pt1 and Co nanoparticles that co-encapsulated on N-doped graphitized carbon nanotubes [50]. Hydrogen Evolution Reactions (HER) have been studied using Pt1 over 2D g-C3N4 [51], CoP-based nanotubes [52], curved carbon supports [53], and TiO2 nanosheets modified with graphene [54]. Additionally, the electrochemical NO reduction was explored using Pt-CTF/CP [55], N2 reduction using Mo1/N-doped carbon [56] and Ru1/N-doped porous carbon [57]. The electrochemical CO2 reduction reaction (CO2RR) was investigated using various atom systems utilizing metals such as Pt1, Pd1, Cu1 and Ag1/graphene [58].Despite the astounding performance of noble metal-based SACs, the high unit price and the low abundance of noble metals are undoubtedly the key compromising factors that hamper the wide employment of SACs [59,60]. With the motivation for enhancing the sustainability of SACs deployment, a notable number of research works have been conducted to examine the potential of noble metal-free SACs. In 2013, Wang et al. [26] performed a systematic DFT calculation to examine the potential of four single-atom transition metals for CO oxidation, where three of them were non-noble metals (i.e. Sc1, Ti1, and Co1). To the best of the authors’ knowledge, this is the earliest reported application of non-noble metal-based SACs. The attention on non-noble metal-based SACs has grown rapidly since 2018. Their applications have been extended to non-oxidative CH4 conversion using Fe1/SiO2 [61]; HER with Co1 on N-doped graphene [59], graphitic carbon nitride [62], N-doped graphyne [63], and Ni1 on α-SiX (X = N, P, As, Sb, Bi) [64]; ethylene benzene oxidation using Co1/CN [65]; electrocatalytic ethanol oxidation using a hybrid material of Pd nanoparticles and Ni1 single atom [66]; oxidative desulphurization with Cr1/multiwalled carbon nanotubes [67]; CO oxidation using Ni1 over FeOx [68], phosphorene nanosheet [69], Sc1 and Fe1 on honeycomb borophene/Al(111) heterostructure [70], and Ti1 on MXene [71]; acetylene hydration using Zn1 with S/N co-doped defective graphene supports [72]; the electrochemical CO2RR with Ni1 on N-doped porous carbon [73], graphene nanosheets [74], carbon black [75] and other ZnN4-based SACs [76]; CO2 hydrogenation with various non-noble metal-based SAC (Mn1, Fe1, Co1, Mo1) supported on graphitic carbon nitride [77]; ORR has been carried out with Co1 on defective N-doped carbon graphene [78], Fe1 supported on N-doped porous carbon [79], hierarchically structured porous carbon [80], cellulose-derived nanocarbon [81] and phosphomolybdic acid cluster [82]; and the Oxygen Evolution Reaction (OER) has been carried out on Fe, Co and Ni-based SACs on N-doped graphene [83], N-doped biomass-derived porous carbon [84] as well as γ-graphyne monolayer [85]. Lastly, the production of H2O2 was found to be effective via hydrogenation routes Ni-based SACs [86]. In fact, based on the comparative studies conducted in some of these works, the non-noble metal-based SACs were found to be more attractive and preferable. For instance, Liang et al. [68] found that the molecular interaction between a Ni atom and an adsorbed CO gas molecule is weaker than CO binding irreversibly with Pt1 and Ir1 atoms. This further leads to a lower barrier in CO2 formation (i.e., enhances the CO oxidation process) under the use of Ni. A recent computational study also suggested that the Fe-, Co- and Ni-based SACs were capable of providing a comparable catalytic activity for CO oxidation as compared with the noble metal-based SACs (e.g., Pd-, Pt-, Ru- and Rh-based SACs) [87]. However, on the flip side, some studies focused on exploring strategies to combat the economic drawbacks of noble metal-based catalysts (e.g., Wang et al. [88] found that the use of monolayer WO3 has great potential to reduce the Pt noble metal usage on the catalyst fabrication).In addition to the aforementioned non-noble metals, in the 2020s, the use of rare earth elements (e.g. La, Y, Ce, and Sc) as SACs were proposed. It is anticipated that the multi-shell electrons of the rare earth metals will lead to strong adhesive bonding between the rare earth element and a support material [89]. Strong metal-support interactions are favorable as they have been found to reduce the chance of atoms aggregation, a common problem during the synthesis of SACs. In other words, it can promote scalable mass production of SACs. To-date, yttrium, scandium, and praseodymium-based SACs (Y1, Pr1 and Sc1) have been tested for N2 and CO2 reduction reactions [89,90]; while erbium SACs (Er1) have been tested for the effective photocatalytic CO2 reduction [91]. On the other hand, the impact of integrating a second metal species into a SAC has also started to get more attention from researchers. It is believed that such bimetallic catalysts can offer better catalytic performance than that of monometallic catalysts given the synergetic effect between the two metal species [90,92]. Evidently, the bimetallic Ir1Mo1/TiO2 SAC synthesized in a recent work [92], is capable of offering a greater selectivity (>96%) for a hydrogenation process, as opposed to the low selectivity of their monometallic forms (i.e., less than 40% for Ir1/TiO2 catalysts and negligible activity for Mo1/TiO2 catalysts). A similar strength of bimetallic SAC has also been discovered by Kaiser and co-workers [93]. The addition of Pt single atoms into the Au SAC can inhibit the sintering effect up to 800 K, which eventually leads to a 2-fold increment in catalysts' shelf life. The potential of such bimetallic SAC has been investigated for hydrogenation (e.g. Ir1Mo1/TiO2 catalysts [92]; Pt1Sn1/N-doped carbon [94]), HER (e.g. Ru1Pt1/N-doped carbon catalysts [95]; Ni1Co1/N-doped carbon catalysts [96]), Fenton reaction (e.g. hyaluronic acid-coated Fe1Co1/N-doped carbon catalysts [97]), acetylene hydrochlorination (e.g. Pt–Au-based bimetallic SAC [93]) and dichlorination (e.g. Fe1Cu1/N-doped porous carbon catalysts [98]). Meanwhile, some studies have focused on the versatile and scalable techniques for the mass production of SACs, e.g. dry ball milling [99,100], pyrolysis [60], one-pot pyrolysis [101], incipient wetness impregnation [49], anti-Ostwald method [30], photochemical strategy [102], thermal emitting strategy [103], cascade anchoring strategy [104], surface organometallic chemistry [105], and electrochemical deposition [106]. Some of those even focus on exploring ways to realize ultrahigh metal loading for SAC (e.g. 10–20 wt% Ru catalysts [107], >20 wt% Cu1-based SACs [108,109], >20 wt% Co1-based SACs [110]), where most synthesis methods are through thermal processing (e.g., pyrolysis [65,111] and annealing [108]). Fig. 3 (a) and (b) present the accumulative journal publications between 2011 and 2020, which directly relate to SAC research. Interestingly, China and the United States, the top publication hubs for SAC research, collectively account for 67.1% of the total scholarly outputs. It is then followed by Australia, Singapore, South Korea, Japan, Canada, the United Kingdom, Germany, and Spain, with a relatively gentle annual growth in publication numbers. In terms of patent applications, China has the largest patent filing numbers (>90% of the total patent filed) thus far, based on the European Patent Office (EPO), and World Intellectual Property Organization (WIPO) databases, as shown in Fig. 3(c) and (d). All the afore-presented trends have evidently shown the growing interest and potential in SAC research and development. This review paper, therefore, attempts to provide useful insights related to the development of SACs, to pave the way for potential future SAC research, especially for applications in the field of clean energy. The review is organized as follows: Section 2 outlines the synthesis and fabrication methods, specially designed for SACs. Whereas the characterization methods used in SAC research are discussed in Section 3. This is followed by current SAC state-of-the-art applications as well as the respective up-scaling challenges highlighted in Sections 4 and 5, respectively. It is then rounded up by a concluding remark in Section 6.A single atom catalyst comprises highly dispersed single atoms of a metallic species onto a support material. However, innovative synthesis methods are needed given the thermodynamically instable nature of single atoms, so that the agglomeration phenomena during the synthesis and reaction processes (as surface free energy is lower in metal cluster form as compared to single atoms form [117]) can be avoided, if not, be reduced [118]. The description of these methods is presented in the following sub-sections, where some of the respective key remarks are summarized in Table 1 .Co-precipitation is a classical method used for the synthesis of nanoparticle (NP) based catalysts by precipitating the metal in the form of hydroxide from a salt precursor with the aid of a base[119] (see Fig. 4 (a)). In 2011, Qiao et al. successfully produced the Pt SAC by dispersing the Pt atoms onto defects in FeOx [19]. This was carried out via the co-precipitation of aqueous ferric nitrate (1 mol/L) and hexachloroplatinic acid (0.076 mol/L) with sodium carbonate (1 mol/L). The co-precipitation was conducted at 50 °C and at pH 8. To mitigate the tendency of forming metal clusters, the Pt loadings were kept low (around 0.17 wt%). This, in fact, is the major limitation of this method, some practical applications physically need higher metal loadings [118]. The obtained slurry was dried and calcined at 333 °C and 473 °C respectively for 5 h each. Finally, the catalysts were reduced using H2/He at 473 °C for 0.5 h. The resultant SACs were proven to demonstrate remarkable stability and reactivity for CO oxidation. In addition to CO oxidation, the co-precipitation strategy has been applied to synthesize effective SACs for the WGS reaction [24], NO reduction [38], electrooxidation catalysis [148], CO2 activation [149] and carbon–carbon (C–C) coupling reactions [150]. However, due to the requirement of using low metal loadings (<1%) the capabilities of supported SACs are limited as metal oxide supports are not suitable for electrochemical testing [151]. There are other common pitfalls associated with the co-precipitation method, such as it being (a) time consuming, (b) not suitable for reactants that have a significant difference in precipitation rates, (c) trace impurities may be precipitated together with the desired product [152], and (d) some metal atoms are anchored in the carrier interface which makes it unable to serve as an active site for the catalytic reaction [118,122].Impregnation is a widely used and standard method of producing supported nanoparticles due to its simple execution, low waste generation, and cost effectiveness [155]. In general, this method starts with the dispersion of an aqueous solution, containing metal precursors, onto a support material. The mixture is then dried and calcined to anchor the metal atoms onto the support, be that in defect sites, onto the surface, or into the porous network of a mesoporous material, this process is depicted in Fig. 4(d). There are two iterations of the impregnation method, including wet impregnation (excess amount of solution is used; recycling is needed to mitigate waste [121]) and dry impregnation (also name as incipient wetness impregnation (IWI); which requires a lower solution volume but it is difficult to ensure uniform dispersion of metal atoms on the support [122]). For instance, Choi et al. [49], the pioneered work which synthesized Pt1/zeolite-templated carbon via a wetness impregnation protocol. In their work, the mixture of carbon and H2PtCl6·5.5H2O was dried at 353 °C under reduced pressure (0.3 bar); the products were further dried and reduced using H2 flow at 523 °C for 3 h. As a result, SACs with a relatively high Pt loading (5 wt%) were synthesized. In another attempt from Yang and co-workers [48], Pt atoms have been found to atomically disperse on TiN under a low metal loading of 0.35 wt% using the dry impregnation approach. Note that higher metal loadings could lead to the formation of NPs or clusters due to atom agglomeration. It is postulated that this could be due to a weak precursor–support interaction [49]. To-date, SACs derived from impregnation methods have been applied to oxygen reduction [48,49], formic acid and methanol oxidation [48], formaldehyde oxidation [156], and CO oxidation [157].Strong electrostatic adsorption (SEA) is a special type of wet impregnation in which the pH of the solution is adjusted to maximize the electrostatic interaction between metal precursors and the oxide surface [123] (Fig. 4(b)). It is anticipated that the oxide surface will be protonated (i.e., positively charged) under a pH condition lower than the point of zero charge (PZC; pH condition that led to zero interactions between metal precursor and support); while it is deprotonated (i.e., negatively charged) under a pH condition higher than the point of zero charge [121,158]. Taking advantage of this feature, monolayers of O−, OH, and OH2+ can be formed by controlling the pH values of the solution, which further results in the capability of anchoring varied metal precursors onto the support [159]. Note that the ligand that connects the metal precursor with the surface will be removed during calcination [160]. Using the study performed by DeRita and co-workers [153] for example, SEA is used to enhance the adsorption of Pt atoms onto the TiO2 surface. By controlling the pH at 12.2 (greater than PZC), the surface will be deprotonated and thus, attract the metal ion complexes of the precursor (i.e., [(NH3)4Pt]2 +). The ligand can then be removed through calcination to form a SAC [159]. In 2019, a US research group has attempted to synthesize a Pt1/SiO2@Al2O3 via SEA with the use of H2PtCl6 as the Pt-precursor [158]. Interestingly, given that the metal ion complexes (PtCl6 −) are carrying negative charges, the Pt atom are adsorbed at a pH lower than PZC instead. Some other successful SEA applications for SAC synthesis include Pt1/CeO2 [160,161] and Rh1/phosphotungstic acid [162] catalysts for CO oxidation; Pd1 (and Pt1)/hydroxyapatite [163]; and Ni1/hydroxyapatite for dry methane reforming [164]. Nevertheless, the metal loadings of the SAC synthesized from SEA method are usually kept low (<5wt%) [125]. Besides, the adsorption behavior is highly sensitive to the types of functional groups and the presence of defects on the support surface [124]. This is problematic since this method is heavily dependent on the electrostatic interaction to achieve atomic dispersion on the support surface.The ball-milling method, or the so-called mechanochemistry strategy, has gained interest in the synthesis of heterogeneous catalysts due to its (a) efficiency to homogeneously mix multiple precursors together [165]; and (b) is simple to scale-up [126]. In the early discovery of SACs, the ball-milling method, Fig. 4(c) was been proposed by Deng et al. [99] to synthesize FeN4/graphene nanosheets. During the ball-milling process, kinetic energy is used to break the chemical bonds of the substrates as well as the macrocyclic structure of the added metal–ligand complex, slowly at 450 rpm for 20 h [99,166], while the shearing action of the milling process is known to generate a large amount of heat which can thermally decompose organic molecules [167]. The resultant compounds such as FeN4 or CoN4 then interact with the defect sites of graphene that were formed during the ball-milling process, to form the desired SACs. Similar to other methods mentioned, the metal loadings must be kept low (∼2.7 wt%) to avoid agglomeration of the high-surface-energy metal atoms [168]. Nevertheless, such an approach possesses limited scalability that is required, this is due to (a) unique precursors which are usually expensive and not commercially available (e.g., metallophthalocyanine); and (b) specific operational conditions (e.g., under argon atmosphere) [127]. Therefore, the ball milling-assisted approach which is coupled with calcination (or other thermal treatments) approach has recently attracted more attention from research groups [100,169]. Recently, kg-scale fabrication of noble atom-based SACs has been successfully demonstrated in the work developed by He et al. [169]. In this work, the noble metal precursor Pd(acac)2 and support precursors Zn(acac)2 were mixed at a weight ratio of 1:400 and subsequently distributed into four agate grinding jars. Each jar contains 50 grinding balls of 6 mm diameter and 20 with a 10 mm diameter. The mixtures were ground for 10 h at a speed of 400 rpm. To note, the use of the acac ligand generally aids the anchoring process of the metal precursors into the bulk support during the ball-milling process [169]. The resulting milled mixtures were calcined at 400 °C for 2 h. He and co-workers [169] have successfully produced Pd1/ZnO, Ru1/ZnO, Rh1/ZnO, and Pd1/CuO via the afore-proposed procedure. Interestingly, no significant scaling-up effect was observed as the kg-scale, Pd1/ZnO exhibits almost the same catalytic performance (about 92% styrene yield via hydrogenation of phenylacetylene) as compared to the small-scale fabrication (10 g-scale) under the same conditions (10 mg of catalyst and 0.5 mmol of phenylacetylene were used, the reaction was conducted for 20 min at a temperature of 50 °C) The ball-milling method has been successfully applied to the fabrication of SACs for oxidation of benzene [99], hydrogenation of acetylene [170], phenylacetylene [169], 2-methyl-3-butyn-2-ol [171], glycerol hydrogenolysis [172], organic pollutant degradation [173], Fenton-like reaction [174], HER [175], ORR [176,177], CO oxidation [169], and photoreaction [128]. Nevertheless, (a) restricted scalability especially for co-catalyst synthesis which inevitably involves liquid-phase processes [128]; and (b) the occurrence of metallic impurities from the machinery on the catalysts are the other common weaknesses of this approach [178].Atomic Layer Deposition (ALD) is another prominent synthetic technique for the fabrication of SACs given its capability to precisely control the atomic deposition, dispersion of metal species and coating thickness [27,129]. This strength makes it a prominent approach particularly for studying the insights on SAC synthesis [122] and the influences of various parameters involved in the synthetic processes [14,122]. Generally, a complete cycle of ALD encompasses two main steps, shown in Fig. 4(e). In the first step, metal precursors such as methylcyclopentadienyl-trimethylplatinum (MeCpPtMe3) will react with oxygen atoms which were adsorbed onto the substrate's surface. The subsequent O2 exposure was oxidized into the metal precursors and thus, form a new adsorbed O2 layer on top of the metal surface (metal–oxygen (M−O) species). In a complete cycle, the surface of the support is exposed to a 1 s pulse of metal precursor, followed by 20 s pulse of N2 to purge the system and then a 5 s pulse of O2, continually. To note, the metal loading can be finely tuned by altering the number of ALD cycles, e.g., Pt loading of 2.1 wt% or 7.6 wt% on N-doped graphene nanosheets can be achieved via 50 and 100 ALD cycles, respectively [179]; 150 ALD cycles can increase the Pt loading on graphene to 10.5 wt% [27]. Thus far, ALD has been applied to generate Pt-graphene SACs for the methanol oxidation reaction [27], an N-doped graphene support for HER [179], and a CeO2 support for CO oxidation [180]. This synthetic procedure has now been extended to synthesize Pd and Fe-based SACs [36,181]. In a recent work conducted by Zhang et al. [182], a Pt–Ru bimetallic catalyst fabricated using the ALD method has proven to provide 50 times higher reactivity for HER as compared to a commercial Pt catalyst (Pt mass loadings for bimetallic catalysts and commercial catalysts are 61.2 μg cm−2 and 1.24 μg cm−2, respectively). On the other hand, the Pd1/graphene catalysts produced via the ALD method are capable of achieving 100% butene selectivity, up to 95% conversion in the selective hydrogenation of 1,3-butadiene conversion, where the attained selectivity to 1-butene (the desired product which can be used as co-monomer in polyethylene production) was more than 70% [36]. This is superior as compared with conventional Pd/C catalysts which routinely achieve ∼60% selectivity, at the same conversion. Other to-date applications of ALD include, but are not limited to Co1-modified Pt nanoparticles catalysts for ORR [183], Ru1/PtNi catalysts for methanol oxidation [184]. However, the ALD process is slow and demands greater energy consumption, as well as requiring an ultraclean surface [185]. All these compounded issues lead to a higher fabrication cost [127]. Together with the low stability of the synthesized SAC [122,131], both serve as the key factors that need to be considered under large-scale production [186].Chemical etching is a convenient and straightforward method to re-disperse nanoparticles on substrates as single atoms. Contrary to other synthesis approaches where monoatomic particles are embedded onto the substrate as a “bottom-up” approach, the chemical etching strategy is a “top-down” approach where larger clusters are first embedded on the substrate and then redistributed as single atoms [142]. As an example, Feng et al. [187] prepared nanoparticles of Ru, Rh, Pd, Ag, Ir, and Pt on an activated carbon support, respectively. Chemical etching was performed using a mixture of CO and CH3I at 240 °C for 6 h. The dispersion of Rh nanoparticles to single atom was studied, and the work found that I• radicals and CO promote the breakage of Rh–Rh bonds. Chemical etching using H2O2 on MoS2 catalyst [188] has also successfully synthesized single-atom vacancy catalysts to improve the HER performance, showing that S-vacancies provided an effective surface electronic structure for boosted electrical transport properties. Few-layer Ti3–xC2Ty nanosheets [189] are first prepared by etching Ti3AlC2 in a solution of lithium fluoride and hydrochloric acid. From this process, some Ti atoms will be ripped off, and when [PtCl6]2- complex ions are stirred with the nanosheets, Pt4+ ions will be adsorbed onto the surface, forming a SAC. Collective researchers from both Chen and Tang's laboratory [143] also demonstrated that nanoparticle graphene with doped Ni can be chemically etched by HCl solution, causing the structure of Ni nanoparticles to be inherited by the graphene. With an etching time of less than 6 h, Ni SACs have been identified via Selected Area Electron Diffraction (SAED) and High-Resolution Transmission Electron Microscopy (HRTEM).Using the deposition–precipitation strategy, a metal solution is mixed and reacted with support molecules to form a uniform suspension. The reaction temperature and pH are precisely controlled to deposit the metal species on the support [144], forming SACs. This technique was popular for oxide support, for example, Fu et al. [190] synthesized Au-ceria SAC via the dropwise addition of HAuCl4 into the suspension of ceria particles in an aqueous solution of (NH4)2CO3 at a pH of 8. The deposition–precipitation strategy was gradually extended for different SACs, such as Wang et al. [145], who studied Au/Sn–TiO2, concluding that the creation of oxygen vacancies on the TiO2 surface by single-site Sn had led to better selective activation. For this, Zhang et al. [145] also mentioned that when oxygen defects are generated via the deposition–precipitation method, the metal species precursor can be reduced to form single metal atoms. Mochizuki et al. [191] demonstrated the preparation of Au1/NiO SACs by the deposition–precipitation strategy by mixing and calcination of HAuCl4 and NiO support. Atomic-resolution HAADF-STEM was used to directly observe Au single atoms on NiO where the loading was 0.93 wt%. Yang et al. [192] reported a ∼1 wt% loading of Au atoms on titania, showing applications for SACs in WGS reactions. The work uses an Au deposition–precipitation strategy with UV irradiation of the titania support in ethanol. Excess Au loadings were also removed using sodium cyanide, leaving atomically dispersed Au on titania. This shows that the deposition–precipitation strategy is a versatile method for even when an excess of metal loading was introduced, removal is possible.The interaction between single metal atoms and the surface atoms of support materials decides the nature of the catalyst stability. The coordination site strategy assigns support sites with chemical linkers to adsorb or bind metal atoms, preventing migration and agglomeration [132]. One of the most common synthesis methods for a SAC is to use specialized N-based linkers, exploiting coordinate bonds or N-binding to implant single atoms into a larger matrix such as in a metal–organic framework. Recently, Gong et al. [193] used polypyrrole (PPy) molecules as N2-based linkers, which can fill into the 1D channel of a (non-nitrogenous) MOF (constructed by divalent Mg2+/Ni2+ ions and 2,5-dioxido-1,4-benzenedicarboxylate ligand) to form a PPy@MgNi-MOF-74 composite, illustrated in Fig. 5 (a). The composite is then annealed at 900 °C to anchor the N atoms from PPy onto a porous carbon composite, followed by the thermal decomposition of PPy (removing the carbon from MOF linkage). This ultimately forms a high-performance NiSA-N2-C electrocatalyst that has been used for the reduction of CO2 to CO [193]. For the application of an H2 evolution photocatalyst, Li et al. [51] used a liquid-phase reaction with g-C3N4 and H2PtCl6, followed by low-temperature annealing. This procedure synthesized a Pt/g-C3N4 SAC. Here, FT-EXAFS was used to deduce a coordination number of 5, which suggests that Pt single atoms were bonded on the top of the five-membered rings of the C3N4 network. The photocatalyst was able to give a high H2 generation rate of 162.8–318 μmol h−1 with a Pt loading content from 0.075 to 0.16 wt%. In general, most works [132,194] implement this strategy by ushering the single atom onto coordinated sites with a chemical linker, followed by a thermal method and other post-treatment to remove and carbonize unrequired synthetic reagents. The most critical variable for controlling the synthesis of SACs using this strategy is the concentration of the metal producing the single atoms and the annealing temperature [51]. A well-established coordination strategy known as surface organometallic coordination (SOMCs) from coordination compounds has been developed to produce single-site atom catalysts [105]. During the grafting of SOMCs, several fragments of reaction intermediates are linked to a surface-supported metal (e.g., Metal-M; M−H, M–R, M = CR2, M = O, M = NR, M–O–OR), enabling direct access of the metal atom to the surface. Copéret [195] has reported that the selective formation of isolated sites of metal via SOMF on a heterogeneous catalyst is more favorable than via wetness impregnation methods, which favor the formation of larger nanoparticles or clusters. He also demonstrated that a single-site heterogeneous catalyst can be prepared using SOMC combined with thermolytic molecular precursors (TMP). These TMP can be removed readily upon thermal treatment, giving a high flexibility SOMC/TMP procedure to engineered heterogeneous catalysts. For instance, few well-defined isolated site based SACs (e.g. Cr(III) supported on Al2O3) have been developed by the combination of SOMC and thermolytic molecular precursors (TMPs), yielding a greater catalytic activity, selectivity and stability of Al2O3 than the Cr(III)/Al2O3 bulk catalyst [196]. In 2017, a combination of SOMC and ALD approach was being developed by Liu to produce a SAC with uniform nuclearity Pt/Al2O3 [197]. By pairing the ALD strategy for mitigating agglomeration, the Pt organometallic complexes can remain monatomic even upon high-temperature thermal treatments and post ligand removal (Fig. 5(b). Notably, the Pt/Al2O3 SAC does not exhibit any thermal sintering, even at 400 °C, whereas samples without ALD overcoats exhibited significant particle agglomeration under similar conditions. Alternatively, there were also efforts to synthesize SACs for hydrogenation via less energy intensive SOMC (without subsequent thermal treatment). An ultra-low Pt single loading (∼1 wt%) was coordinated on phosphomolybdic acid (PMA)-modified active carbon [198]. The Pt SAC was synthesized by anchoring Pt on the four-fold planar geometry on PMA. The hydrogenation of N–O, CO, CC, and CC groups were studied, and the Pt-PMA/AC SAC exhibited comparable or better turnover frequencies (TOF) compared to a conventional Pt/AC catalyst. This demonstrates that single atoms have the potential to be anchored into organometallic compounds, which stabilizes the SAC structure while providing high catalytic activity.Poorly defined vacancy defects on ordered support may lead to difficulties in identifying and controlling the precise structure of the attained SAC. Therefore, a defect design strategy aims to engineer the defects on a support to stabilize the SAC surface, allowing a higher possible metal loading, providing a large number of individual active sites, as well as improve the selectivity of the desired process [133]. One of the directions followed has led to a number of works being carried out to study the effect of O2 vacancies in mixed metal oxides to stabilize active sites [200,201]. Additionally, vacancy defects on graphene or layered materials have been probed in a similar way [202,203], as well as, heteroatom defects in crystals, cation and anion vacancies [204,205]. Shen and co-authors [206] demonstrated that defects in graphene can alter the charge distribution on a carbon plane, giving evidence that graphite with more exposed edges can provide superior electrocatalytic performance. Apart from the carbon-based materials, other defect-rich oxide materials also were used as supports in the synthesis of SACs (e.g., CoO [207], TiO2 [199], and CeO2 [208]). Wang et al. [207] have employed an adsorption method to prepare an Rh-based SAC (Rh/CoO), which was stabilized in the defect sites on the surface of CoO nanosheets through the electrostatic interaction. The developed Rh/CoO SAC displayed higher selectivity toward propene hydroformylation to butyraldehyde compared to Rh/bulk-CoO. TiO2 is also another suitable support for SAC preparation that has abundant defective sites. From Fig. 5 (c), a wrap-bake-peel synthetic strategy to fabricate an enzyme-like Cu-based SAC supported on the TiO2 was proposed by Lee et al. [199]. The amorphous TiO2 was first coated on SiO2 core before the Cu atoms were adsorbed on the amorphous surface. Then, a second layer SiO2 coating was introduced to form the SiO2–TiO2–Cu1–SiO2 core−shell materials with a sandwich-like structure. Subsequent steps such as calcination, baking, and NaOH leaching were performed to ensure the single Cu atoms were located at the Ti sites of anatase TiO2. This synthetic strategy gas facilitated a valence control of co-catalyst Cu atoms and generated a reversible valence change in photoactivation that enhance photocatalytic hydrogen generation activity.It was widely known that higher thermal stability of SACs can be granted by fabricating the catalysts under higher temperatures since the precursor–support interaction will be enhanced under high temperatures [134]. Some of the state-of-art techniques reported are strong metal-support interaction (SMSI) promoted production [125], the non-defect stabilized approach [133], and the high thermal redispersion approach [209].For instance, based on a work published in 2015, the single atom active sites on a support can be formed SMSI approach during pyrolysis [60]. It is attractive due to its capability in yielding SACs with good metal dispersion although in higher metal loadings (5–20 wt%) [101,210]. Cheng et al. [210] reported the performance of the pyrolysis-assisted method in fabricating a series of atomically dispersed transition metals (M refers Ni, Co, NiCo, CoFe, and NiPt) on N-doped carbon nanotubes with metal loading of 20±4 wt%. Findings showed that the M single-atom/N-doped carbon nanotube fabricated via a pyrolysis-assisted method exhibited outstanding catalytic performance not only on ORR and HER but also in electrochemical CO2 reduction [101,211]. In general, the pyrolysis-assisted method consists of three general steps, (a) mixing of metal and support precursors, (b) pyrolysis, and (c) post-treatment, e.g., washing, drying, acid treatment (for removal of residual unencapsulated metal or unwanted ligands). Taking the deposition of Co1 on the N-doped carbon as an example (Fig. 6 (a)), the first step was to prepare the catalysts precursors by mixing urea, glucose, CoCl2·6H2O, NaSCN, and ethanol at room temperature. Then, the mixture was treated with ultrasonic treatment to form a homogeneous mixture. It was subsequently, dried at 60 °C for 8 h to remove trace ethanol residues. The resultant solids were then ball-milled to form the desired catalyst precursor. Next, the obtained materials were pyrolyzed (>600 °C), and subsequently cooled to room temperature under an inert (N2) atmosphere. This resulted in the formation of Co1/N-doped carbon SAC with a uniform distribution of Co atoms. Before the SACs can be used, it was cooled and treated in acid (0.5 M, H2SO4 at 80 °C for 8 h) to remove any non-coordinated Co atoms and precursor impurities. To note, the yielded Co1/N-doped carbon catalyst possesses the greatest catalytic activities for ORR and HER as compared to the other two conventional catalysts (Pt/C catalysts and Co/N-doped carbon catalysts), as well as excellent reusability where there was no degradation after 11,000 continuous catalytic cycles [211]. Recently, pyrolysis was also used as an efficient tool to downsize metal from the range of 100–1 nm. Based on the findings discovered by Wei et al. [134], the sintering and atomization occurred during the pyrolysis of metal-NP (i.e., Pd, Pt, Au), where atomization (i.e., the transformation of metal-NP to SA) took place at high temperature (900–1000 °C); whereas sintering was observed between 300 and 1000 °C. This represents an attractive top-down synthesis path to derive SACs from a metal NP. In fact, the pyrolysis-assisted approach appeared to be promising for the synthesis of SACs with ultra-high metal loading (>20 wt%) [65,111].Aside from this, a recent method to synthesize single-atom metals from bulk metals on the catalyst surface using high temperatures were also being developed [214]. This thermal emitting strategy provided an economically attractive advantage for producing single atom nano-catalyst from their bulk materials. Qu and co-authors [214] have used pyrolysis at 900 °C to create Zn nodes and defect sites on a pyrolyzed zeolitic imidazolate framework-8 (ZIF-8). Then, a Cu foam was reacted with NH3 gas to form Cu(NH3)x. These Cu(NH3)x species were then trapped by the defects in the pyrolyzed N-rich carbon support, which forms the Cu-single atom N-rich carbon support (Cu–SAs/N–C) catalyst. The work also showed that Co or Ni could be used to replace the Cu bulk metal, confirming its application towards a wide range of SAC syntheses. Later, Qu and co-authors [103] used this thermal emitting strategy to synthesize a high-performance Pt-SAC on defective graphene (Pt–SAs/DG) (Fig. 6(b)). For this synthesis dicyandiamide (DCD) and Pt dispersed on graphene oxide (GO) were placed under Ar flow and heated to 1100 °C. At this temperature the DCD underwent pyrolysis, generating NH3 gas. Due to the strong coordination interaction between NH3 and Pt atoms, the volatile Pt(NH3)x species was formed. The Pt entities were oxidized by O atoms based in the GO, forming Ptδ+ (0 < δ < 4) species. After that, O content from the GO was removed through thermal processing to produce graphene with defects (DG). Lastly, the isolated Pt SAs/DG catalysts were formed by trapping the Ptδ+ (0 < δ < 4) species on the DG. Notably, the prepared Pt–SAs/DG catalyst demonstrated high activity for HER and selective oxidation of various organosilanes. This demonstrates that even the surface of the catalyst can be freely changed by using the thermal emitting strategy, giving a high potential for future SAC synthesis on a large-scale production.Zhao and co-authors [104] recently proposed a cascade anchoring strategy to synthesize a SAC with a high-loading with the possibility for large-scale production. The work demonstrates that the cascade anchoring strategy could synthesize metal-Nx moieties, anchored on carbon support (M−NC) with flexibility for the metal atom. Moreover, the metal loading for the synthesized catalyst was reported up to 12.1 wt%. First, metal ions (e.g. Mn, Fe, Co, Ni, Cu, Mo, and Pt) were fixated using a chelating agent (such as glucose). The isolated metal ions are then anchored onto the O2-rich porous carbon support where the glucose can bind to the carbon support due to interaction with O-containing groups (Fig. 6(c)). Next, the compound was mixed with a nitrogen source melamine, and pyrolyzed over 600 °C to react the carbon-N species with melamine to form a SAC with M-Nx moieties. A similar anchoring strategy using immobilization and pyrolysis was also performed for the synthesis of Pt-single atoms on a commercial carbon black, Black Pearls® 2000 (PtSA@BP) [215] for the HER reaction. This work used a tetra(4-tert-butyl-phenyl)porphyrinato platinum (PtTBPP) complex as the N and Pt precursor while simultaneously providing a monodisperse purpose. The molecules achieved porphyrin adsorption equilibrium after being stirred for 12 h. After washing (by ethanol and deionized water) and drying under reduced pressure, the PtTBPP/BP hybrid underwent two-stage pyrolysis, giving a final Pt loading of 2.5 wt%.Nevertheless, SACs synthesis at temperatures >1000 °C is still challenging to be achieved via conventional strategies. To overcome this technical barrier, two ultra-high temperature-assisted strategies, namely the high-temperature shockwave strategy [212] and the high-temperature arc-discharge strategy [213] were developed. The former method utilized periodic on-off shock heating (on-state temperature can achieve up to 2727 °C; while the off-state period operates for 10x longer, leading to a lower average temperature of 127 °C) to stabilize the single metal atom on the support. This makes the strategy compatible with various materials. Yao and co-workers [212], for the first time, synthesize Pt single atoms that are deposited on the CO2-activated carbon nanofiber via a high-temperature shockwave strategy. The metal precursor, H2PtCl6 was dispersed on the carbon nanofiber and subsequently subjected to the heating shockwave. Their studies revealed that the dispersion of Pt clusters can be eliminated by increasing the heating cycles (Fig. 6(d)). Interestingly, all the weak Pt–Pt bonds and types 1–3 Pt–C bonds were transformed into stronger types 4–10 Pt–C bonds after multiple cycles of shock heating, thus, resulting in higher thermal stability.The latter strategy utilized high-temperature arc discharges to in-situ form and stabilize Pt single atoms on molybdenum carbide (MoC) support at a temperature up to 4000 °C (higher than the limit of the former strategy). To synthesize Pt1/MoC via this method, a mixture of Pt and Mo powders was first prepared and filled into the anode-side graphite tube (Fig. 6(e)). Whereas the cathode is made of a pure graphite rod. The synthesis process begins when the arc starts to be generated from the arc chamber. The entire process took tens of minutes (other methods take hours to complete the synthesis) which makes it an efficient and attractive fabrication tool. The resultant SACs exhibit excellent performance and thermal stability for the hydrogenation of quinoline. However, both the aforementioned methods are still in their infancy, and follow-up works are needed to upscale the overall synthesis process to cope with the demands for mass production of SACs.A freeze-drying method has also gained interest from the catalysis community as it enables a large specific surface area of the support, while more importantly, the mobility of the metal precursors can be mitigated simultaneously [135]. A few researchers have therefore incorporated the freeze-drying process into SACs fabrication to better design and control metal precursor deposition. Taking a Co-based SAC as an example (Fig. 7 (a)), Fei et al. [59] mixed the metal precursor (CoCl2·6H2O) and support precursor (graphene oxide) at a ratio of 1:135. The mixture was then sonicated in deionized water, and subsequently freeze-dried for 24 h to produce brownish powders. Next, the powder was annealed at 750 °C for several hours to activate the Co1/N-doped graphene. The work has proven that the obtained catalysts can work as effective HER catalysts under both acidic and alkaline environments. Thus far, the freeze-drying method has been extended to create Fe-based SACs for ORR [216] and Fenton-like reaction [217], Pd-based SACs for selective hydrogenation [218] and Zn-air batteries application [219], Ni-based SACs for CO2 methanation [220], Pt-based SACs for HER [221], Co-based SAC for lithium-sulfur batteries application [222], and bimetallic SACs (e.g., Co1–Fe1-based SACs for HER [223]). Despite the potential of having greater reactivity and stability of catalysts as compared to the conventional oven-drying approach [224], the high operational cost still appears as one of the main conundrums for the freeze-drying assisted strategy [136].Photochemical methods rely on a powerful light source (e.g. UV lights) to deposit single atoms on support via ion reduction. In years gone by the photochemical method was used for solar O2 production in metal deposition onto a semiconductor photoanode [139]. For instance, the deposition of Co on a ZnO electrode in a K3PO4–CoCl2 precursor solution under UV light emission to oxide the Co2+ ions, adhering it to the ZnO surface142. This method was recently adopted into Pt SAC preparation by Liu et al. [102]. They have successfully developed an “ultra-low” Pt loading and highly stable atomically dispersed Pd1/TiO2 SACs for catalytic hydrogenation of aldehydes (Fig. 7(b)). The single atom Pt-loaded (up to 1.5 wt% loadings) TiO2 nanosheets on ethylene glycolate (EG) were synthesized by stepwise UV-induced radical formation which causes Cl− removal by EG radicals [102]. The adsorption of photons and electronically excited states were reported to be the two key steps in the photochemical reduction process. Later, this photochemical method was used to synthesize Pd-loaded (001)-exposed anatase nanocrystals and Pd-loaded TiO2 SACs for the purpose of styrene hydrogenation and CO oxidation [226]. For solar water oxidation, an Ir dinuclear heterogeneous catalyst (DHC) on a Fe2O3 surface was prepared by ultraviolet ozone (UVO) cleaner system, providing a paradigm for SACs with multiple active sites [227]. A photochemical solid-phase synthesis method was used to adsorb PtCl6 2− onto an N-doped porous carbon and directly reduced by UV light to Pt atoms achieving 3.6 wt% loadings [228]. Another novel method to prevent atom nucleation is by using iced photochemical reduction [225]. This strategy (Fig. 7(c)) coupled with the freeze-drying technique, freezes the atom-carrying aqueous solution before UV treatment and successfully synthesizes a stable SAC with Pt atoms on various support (e.g., amorphous carbons, mesoporous carbon, graphene, multi-walled carbon nanotubes, TiO2 NP, and ZnO nanowires) for HER.The electrochemical Method is a widely accepted electrocatalyst preparation method by most researchers; this is because the metal ions can be easily deposited onto cathodes in an electrolyte solution without any complicated procedure [229]. Therefore, it is one of the attractive, scalable, facile, and cost-effective methods which is capable of synthesizing and activating single atom electrocatalysts with the aid of electrochemical potential. To-note, the size, and the respective metal loadings can be fine-tuned by altering the deposition parameters such as metal ion concentration and deposition time [140]. More importantly, the SACs derived from this method are binder-free that can be used directly in electrocatalysis [140]. Some works have proved its capability in SAC fabrication. Notably, in the work conducted by Fan et al. [230], the formulated Ni–C-based catalysts were successfully activated via the series of treatment processes, i.e., HCl leaching treatment (for removal of redundant Ni metal) and electrochemical cyclic-potential treatment (for activation). Unexpectedly, throughout the entire activation process, Ni atoms were atomically dispersed and anchored onto the carbon support, which therefore formed Ni-based SACs (Fig. 8 (a)). On the other hand, some works have attempted to in-situ grow the Pt single atoms onto Ni foam [52], single-walled carbon nanotubes [231], and a bismuth ultramicroelectrode [140]. Aside from that, Zhang et al. [232], for the first time, proposed the use of the electrochemical method to synthesize an atomic Pt and Co co-trapped carbon catalyst. Thus far, the SACs fabricated using this method have shown superior HER activity and electrocatalytic stability as compared to the conventional Pt/C (20 wt% Pt) catalysts (e.g., the catalytic activity of the PtSA-NT-NF catalysts synthesized by Zhang et al. [52] were 4 times greater than that of Pt/C catalysts). More recently, a self-terminating electrodeposition technique for SACs fabrication was proposed [141]. Generally, by controlling the electrical potential at the underpotential deposition (UPD) state, the metal-support bonding can become dominant as compared to the metal–metal bonding, which favors the formation of SACs. The continuous atom growth will be terminated once the surface-limited reaction has reached saturation. This further ensures the formation of SACs instead of metal clusters (Fig. 8(b)). More interestingly, it can be operated under ambient temperature which makes it an energy-efficient SAC fabrication method. Moreover, the two ultra-high temperature-assisted strategies (i.e. high temperature shockwave strategy and the high temperature arc-discharge strategy) presented in Section 2.2.3 can also be, arguably, categorized as electrochemical methods as the use of electric fields was involved.The mass-selected soft-landing approach is an emerging technique for SAC synthesis. It starts with the vaporization of metal atoms using a high-frequency laser, where the vaporized metal atoms are selected by a mass filter to control the deposition of the metal on the support surface [118,146] (Fig. 9 ). To-date, since this method is capable of providing exact tuning and control of the size of metal deposited on the support surface, it has been employed in a couple of SAC works for generating fundamental insights [13,233]. For example, to study the impact of Pt atom number on electrochemical catalysis, Weber and co-workers [234] have synthesized Pt n /Indium Tin Oxide (ITO) catalysts (1 = n ≤ 14) using the mass-selected soft-landing method. In their work, the Ptn + clusters (and atoms) produced from the laser vaporization were channeled through a quadrupole mass filter to generate a beam containing the desired cluster (or single atom) size which was then deposited on the ITO surface. Earlier in 2000, one of the pioneering works [235] had also attempted to verify the catalytic kinetics of Pdn/MgO(100) catalysts (1 = n ≤ 30) for the trimerization reaction using a mass-selected soft-landing method where they found the SAC (Pd1/MgO(100)) can significantly reduce the activation energy requirement. Aside from that, this approach has been applied to synthesize bimetallic SACs for OER and ORR [236]; Pt1/glassy carbon substrate catalysts for ORR [237]; Pd1/TiO2 catalysts for CO oxidation [238]; and Au1/TiO2 catalysts for CO oxidation [239]. Nonetheless, the need for ultrahigh vacuum conditions and low production yields has inevitably lowered the scalability of this method [13,118].The existence and spatial distribution of isolated single atoms are crucial for understanding the relationships between structures and the properties in SACs. However, it is quite challenging to identify the single atoms at an atomic-scale due to the requirement of high spatial resolution tools. In recent decades, techniques have been developed towards an atom-scale resolution that can be employed to the SACs at different ensemble level signals, of all responsive species, which can minimize the deceptive information and accurately reflect the real active atomic species. Hereby, in this section, we provide an overview of various advanced characterization techniques for SACs, which can be classified into several categories: (a) High-resolution electron microscopy, (b) X-ray irradiation spectroscopy, (c) in situ spectroscopy, and (d) magnetic resonance spectroscopy.With the development of advanced microscopes, the direct observation of NPs or atoms in/on catalyst supports has been realized. With the aid of different electron microcopies such as Aberration-Corrected High Angle Annular Dark Field/Scanning Transmission Electron Microscopy (AC-HAADF/STEM), the fine distributions and precise location of single atoms in the catalyst can be evaluated, differentiated using bright and dark field imaging.Transmission electron microscopy is one of the most widely used characterization methods to determine the structure and particle size of active sites in heterogeneous catalysts. It provides a clear observation of the catalyst morphology and structural change before and after interactions between metal atoms and supports [240,241]. Lately, the improvement of spatial resolution in electron microscopes has granted the possibility of identifying the active sites of SACs at a single atom level. Specifically, HAADF-STEM is one of the high-end techniques in which the high angle annular dark field imagery is coupled with a standard scanning transmission electron microscope to provide the bright/dark contrast of different elements at a sub-angstrom level resolution [242]. For HAADF-STEM, the intensity of images obtained follows the thickness of samples and the atomic number up to an exponential value of 1.4–2, but still, it is not powerful enough to identify the isolated single atoms in a SAC [243]. To improve the precision, the aberration-corrected HAADF-STEM is often used to distinguish the atoms from the support materials based on their different values and, further provide direct evidence of the existence of SACs [244,245]. For example, Qiao et al. [19] reported about Pt/FeOx catalysts and demonstrated how individual Pt atoms uniformly dispersed on a FeOx surface using HAADF-STEM (Fig. 10 (a)). Both individual Fe (Fig. 10(b)) and Ir (Fig. 10(c)) atoms can be clearly distinguished from other lighter metal atoms (highlighted by red circles) such as Si, Al, Na, C and O [246,247]. However, there is also a shortcoming where the isolated atoms in the images are not clearly shown due to the phase contrast caused by the multiple metal loadings, impurities, and the inhomogeneity of substrates (e.g. lighter elements are usually invisible when imaged together with heavier, differences in contrast). Thus, Electron Energy Loss Spectroscopy (EELS) or Energy Dispersive X-ray Spectroscopy (EDX) are often combined with HAADF-STEM to provide an in-depth insight into the catalyst [246]. Based on this method, metal atoms (e.g. Fe, Co, Ni) can be clearly distinguished from the carbon-supported SACs via STEM/EDX, where metal atoms are reported to be stabilized by the carbon surface [59,246,248] (Fig. 10(d)). However, when the atomic number of single metal atoms and supports are close, it will be challenging to obtain obvious contrast in HAADF-STEM [249]. In recent work, Guo et al. [250] prepared the Cu/Al2O3 SAC with a high loading of 8.7 wt%, but there were only a few Cu single atoms observed by HAADF-STEM owing to the weak image contrast of Cu and Al2O3. Hence, coupling of HAADF-STEM with other in-situ spatial resolution synchrotron characterization techniques is value-added to probe the structural information of the SAC (i.e, HAADF-STEM-Syn Infrared and HAADF-STEM-Syn X-ray Diffraction).Another persuasive technique for directly observing the SAC structure is by using Scanning Tunneling Microscopy (STM), in which the surface images of the conducting or semiconducting materials at the atomic level can be attained [251,252]. The typical advantage of STM is to in-situ track the reaction process on a well-defined surface and provides the opportunity to explore catalytic mechanisms in real time [253,254]. STM operates under ultra-high vacuum at the broadband temperature range from near −273 °C to around 1027 °C [255,256]. Over the decades, STM has been widely used in industries to track heterogeneous catalyst performance, as such, STM is applied to study the soft-landing deposition and atomic-layer deposition synthesized methods of SACs for the application in 1,3-butadiene hydrogenation [257], CO oxidation [258], and NO reduction [259]. For SAAs, STM can directly capture the images of metallic single atom entities (e.g., Pt [257], Ni [260], Au [261], etc.) deposited on singe crystal surfaces together with reactive hydrogen [22,257]. Fig. 11 (a and b) shows Pt atoms existing as isolated protrusions substituted onto the Cu(111) surface and H atoms spillover onto the Cu surface [257]. More recently, ultrahigh vacuum high-speed STM has been adopted to observe the real-time growth process of single Ni atoms on graphene. The catalytic effect of individual Ni atoms at the edges of a growing graphene flake was captured at the millisecond time scale by STM (Fig. 11(c and d)), providing an overall picture of the diffusion of mobile nickel atoms that catalyzes the graphene growth on the edges of SAC islands [260].X-ray Absorption Spectroscopy (XAS) is a state-of-the-art synchrotron technique to characterize the local environment of atoms in materials by measuring the variation in the absorption coefficient under the scanning of X-ray radiation in an energy range around the absorption edge [198,262]. XAS can be divided into three regions, pre-edge, near edge, and extended range (Fig. 12 (a)). When the measurement range is near the absorption edge, it stands for X-ray absorption near edge structure (XANES); while the extended X-ray absorption fine structure (EXAFS) is usually measured beyond the absorption edge in the range of 50 to >1000 eV. From both forms of characterization (which are often obtained in the same scan), detailed structural information can be provided by these techniques, including the bond lengths, the angle between chemical bonds, oxidation states, and the number of coordinating species. EXAFS is usually used to show the coordination and absence of metal–metal interactions, indicating that the supported metal atoms are individual and that particles or clusters are absent. However, if the elements of the materials are sensitive to electronic and oxidation environments, XANES will be a more suitable characterization technique. For instance, Qu et al. [214] reported the preparation of Cu single atoms on N-doped carbon (Cu SAs/N–C). EXAFS results revealed that Cu–SAs/N–C exhibits a dominant Cu–N coordination at 1.48 Å, and there was no Cu–Cu coordination since no Cu–Cu characteristic peaks were observed. As for comparison, the main peak of Cu–Cu coordination was observed at 2.24 Å on a Cu reference foil. XANES was then performed for further analysis, and it showed the intensity of the line for Cu–SAs/N–C located between those for the Cu foil and CuO, revealing its typical electronic structure (Cuδ+, 0 < δ < 2) (Fig. 12 (b)).X-ray Photoelectron Spectroscopy (XPS) is another widely used technique to characterize the chemical states and electronic structures of surfaces [39,75,265]. It has been used in many SACs to determine the chemical state and analyze their electronic environment, such as Pt/Fe–N–C [266], Pt/CeO2 [267], ZnNx/C [268]. Taking Chen and co-workers' work as an example [269], two peaks in the Pt 4f spectrum at binding energies of 75.8 and 72.4 eV, ascribing to 4f5/2 and 4f7/2 level were attained. From the XPS analysis, we can clearly distinguish that those peaks are between the Pt2+ and Pt0 states. The peak positions were between those of Pt(II) and Pt(0), suggesting that Pt atoms carry partially positive charge through electron transfer between metal and supports owing to enhanced metal−support interactions. More recently, with the rapid development of the in-situ technique, near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) was discovered to track the surface of a catalyst particle at a relatively high temperature in the gas phase (mbar pressure range). With the aid of this technique, the study of dynamic modifications at single atom-support surfaces in the vapor phase environment can be investigated, providing a sophisticated defect design of next-generation SAC catalysts [270]. Most importantly, not only the surface but also bulk-dissolved elements can be detected. The element species that may influence the chemisorption or charge delocalization of SAC atom can be analyzed, which provides a precise reaction mechanism of the catalyst in the gaseous phase [162].By comparing with X-ray spectroscopy, the transmission infrared spectroscopy or Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) uses a lower wavelength of infrared light reflection and transmission to evaluate the spatial resolution of the SACs (e.g. Acidic sites-Pyridine/NH3 as probe and Basic sites-CO2/CO as probe) or the change of organic phases of catalysts at time-resolved mode [271,272]. In addition, DRIFTS is commonly used in conjunction with EXAFS, where EXAFS provides information about the electronic and geometric structure of the SACs, while DRIFTS follows the evolutionary formation of the surface bonding between single atoms and specific acidic and basic sites, aiding the understanding of the catalyst-absorbance mechanism in operando environment [273,274]. For instance, the infrared frequency of CO adsorbed on isolated metal atoms is different from that on clusters/NPs, this is because single metal atoms generally have different chemical states due to their different coordination structures with the support [275]. Referring to Fig. 12(c), on increasing the loading of Rh on ZrO2, the surface structure and mechanism for CO molecule adsorbed can be studied via absorbance peaks (e.g., atop, bridging, and a terrace site). In short, Infrared spectroscopy can be used to provide insights into the surface adsorption mechanism with respect to the single atom loading in SACs. Taking Au–Pd SAA supported on ion exchange resins (Au–Pd/resin) for the Ullman reaction as an example [42]. DRIFTS recorded the bonding profile of CO (probe molecule) on the Au–Pd/resin SAC with different Au/Pd ratios, in which two absorption bands were observed at 1895 and 2020 cm−1 which signifies the CO bridged and on-top adsorption onto the Au–Pd (Fig. 12 (d)).Operando Raman Spectroscopy (ORS) is an in-situ technique to analyze the structures of materials at various scales from bulk to nanoscale layers through different photon energies. The application of ORS can provide unique means for a deeper fundamental understanding of layered nanomaterials and atomic-scale catalysts [276,277]. In 2019, the first attempt of investigating the atomically dispersed Rh metal on phosphotungstic acid (Rh/PMA SAC) during CO oxidation via ORS was proposed by Yan's group [278]. Based on the spectra shown in Fig. 13 (a), it shows that even after CO oxidation reaction at temperatures 573 K, the heteropoly acid structure of the Rh/PMA remains intact, suggesting that no formation of metal oxides after CO adsorption of Rh single atom. Apart from investigating the dispersion of single atoms, ORS has been used to investigate the molecular fingerprints of the MoSx species present during the electrochemical HER in an HClO4 electrolyte [279]. Based on the Raman profile (Fig. 13(b and c)), the structural evolution of MoSx films during HER can be attained. As such, the peak at 2530 cm−1 was captured at potentials relevant to H2 evolution, which corresponded to the S–H stretching vibration of MoSx–H moieties. MoSx-AE showed additional two peaks at 520 and 550 cm−1 which are not seen for MoSx-CE. These were ascribed to the ν(S–S) terminal, and ν(S–S) bridging vibrations, respectively. Hereby, ORS can be acknowledged as one of the spectroscopic techniques that are typically used to determine vibrational and rotational modes of active species bonds on the atomic metal–metal coordination.Nuclear Magnetic Resonance (NMR) for single atom detection is a solid-state spectroscopic technique used to identify the electron structures or chemical bonding of the core structure of a catalyst. The application of solid-state magic-angle spinning-nuclear magnetic resonance (MAS-NMR) has been widely reported to investigate the co-relationship of the precursor ligand bonding during the synthesis of SACs [280,281]. In 2017, Zhang and co-authors [37] have synthesized a highly active Pt-based SAC (Pt/m-Al2O3) by impregnating the Pt atom on the mesoporous Al2O3 support for selective hydrogenations and CO oxidation reaction. Based on the MAS-NMR analysis shown in Fig. 14 (a), they found that most of the Al3+ species were in tetrahedral, pentahedral, and octahedral shapes, differing from nanoparticles Pt/Al2O3 where the Al3+ pentahedral peak was not seen. This further confirmed that the significant pentahedral-coordinated Al3+ species have resulted from the calcination and reduction process, in which the Pt-atom was bound onto the Al surface via bridging O atoms. In addition, Zhang and co-authors [198] also synthesized a SAC with Pt on a phosphomolybdic acid-modified active carbon (Pt-PMA/AC) for hydrogenation reactions. A 31P MAS-NMR was used to study the electron structure of the support with and without the Pt atom loading. Through the MAS-NMR study, it revealed changes in the environment for P bonding in the Pt-loaded PMA/AC. Lately, Shao et al. [245] adopted the 13C cross-polarized-MAS NMR (CP-MAS NMR) technique to analyze carbon bonding on an Ir-based porous organic polymer with aminopyridine functionality (Ir/AP-POP). Various carbon bonding (such as CO, Py-C, Ar–C, and Py-C) were identified in the Ir/AP-POP catalyst which assisted in understanding the catalytic mechanism for converting CO2 to formate by quasi-homogeneous hydrogenation (Fig. 14(b)).Electromagnetic Spin Resonance (ESR) is another characterization technique to investigate materials with unpaired electrons. This can be used to analyze the atomic state and coordination environment of SACs. For instance, the partial reduction of Cu(II) to Cu(I) due to graphene-induced charge transfer in a mixed-valence Cu/functionalized graphene SAC (G(CN)–Cu) can be confirmed through ESR measurement [282] (Fig. 15 (a)). Based on the fresh catalyst ESR spectra, only the unpaired electron signal from isolated paramagnetic Cu(II) cations (d9) was observed, whereas the Cu(I) cations (d10) were not detected. The authors make a comparison of the ESR spectra of the G(CN)–Cu catalyst dispersed in hexane before and after adding H2O2. Notably, there was an increase in the intensity of Cu(II)-induced signal after adding the peroxide, this suggests that the Cu(I) was oxidized to Cu(II). Adding to that, ESR also can analyze the Curie behavior of the catalyst (inset in Fig. 16 (a)), such as the non-presence of magnetically interacting Cu(II) centers in the catalyst. This observation indicated that there are no antiferromagnetic interactions or any formation of bulk CuO clusters. Recently, Jin et al. [283] also reported the superior performance of the partially oxidized Ni single‐atom sites in polymeric carbon nitride for elevating photocatalytic H2 evolution. Based on these findings, the change of oxidation state in Ni has modulated the catalyst's electronic structure, leading to an optimized photocatalytic activity, where fewer unpaired electrons were observed in deeply oxidized Ni single atoms (Fig. 15(b)). ESR can be concluded as a powerful technique to identify the electronic configurations of metal atoms in SACs, specifically for unpair electron detection.Along with the state-of-art characterization techniques, there are also some supplementary techniques that also provide information on the thermal stability of SACs (Thermogravimetric analysis, TGA), Brunauer–Emmett–Teller specific surface area of SACs (N2 adsorption), single atom metal loading on the SACs (Inductively Coupled Plasma Mass Spectrometry, ICP-MS), and dispersion of the single atoms on the SACs (H2/N2O chemisorption [94,285,286]). With the aid of the supplementary characterization (Table 2 ), a more in-depth understanding of the intrinsic physicochemical properties of the catalyst can be elucidated. For instance, Zhang et al. reported that the fabricated PtCu SAA can yield a high turnover frequency that reaches up to 2.6 × 103 molglycerol·molPtCu–SAA −1 h−1 in glycerol hydrogenation, which is to our knowledge the largest value among reported heterogeneous metal catalysts [172]. Under this context, the dispersion of the Pt atoms on the SAA is highly important, and such an important piece of information only can be extracted from ICP-MS and N2O chemisorption. Another example can be derived from a study reported by Li et al., with the aid of N2 physisorption, they managed to observe that the Zn SACs are having multiple types of pores (i.e, micro, meso, and macro), which induces a large surface area of 1002 m2 g−1, that is ∼10 folds higher than the commercial zinc catalyst [287,288].As shown in Table 3 , SACs have been applied to various heterogeneous reactions (e.g. C–C coupling, oxidation, reforming reactions, and hydrogenation) for chemical and fuel production. Herein, we aim to provide a comprehensive summary of SAC usage by discussing: (a) How does the heterogeneous structure of SACs affect the catalytic activity; (b) How does the structure evolution process response to a wide variety of intrinsic and extrinsic factors and (c) What are the underlying catalytic mechanisms in different possible reactions. This section summarizes all the recent experimental activities of SACs including the operating conditions, type of reactors as well as catalyst loading that affect the yield and selectivity of the desired products. Besides that, the Computational DFT calculation which includes the binding energy, the electronic structure, and possible catalytic reaction routes of the SACs is also discussed. As known, the first-principles DFT calculations allow the investigations of the energetics of processes at the atomic-level with high precision and provide quantum mechanical-based insights into the related electronic structure of these processes and their influence on catalyst reactivity [289,290]. It is worth mentioning that not only well-understood catalyst mechanisms but also controversial hypotheses are cautiously discussed.The mechanistic fundamental questions of C–C coupling reactions on heterogeneous catalysts are still not fully understood and the metal active phases are debated [432,433]. This is because the atoms on the surface of NPs usually have different coordination numbers along with variable chemical environments (electronic effects) than those of their neighboring atoms which influences the catalytic activities [434,435]. Another relevant issue related to the use of NPs is the infeasibility of using high loadings of expensive noble metals, this reduces their attractiveness for bulk production. Thus, the synthesis of highly dispersed noble metal SACs as a catalyst is highly desirable as it could overcome this issue by maximizing the metal atomic efficiency, as well as reducing the catalyst cost [13,19]. However, single atoms are known for aggregation, leading to clusters and finally nanoparticles. In order to avoid their aggregation and induce stabilization of the single atoms, several methods have been proposed: a) the cascade anchoring strategy of the atom onto a metal oxide support [24,35]; b) a reductionist approach by alloying with other alloys to form an SAA [436,437]; or c) depositing single atoms into different oxide supports or organic frameworks for superior metal-support interactions [27,36,438]. As a whole, SACs are an attempt to bridge homogeneous and heterogeneous catalysis closer to understanding and revolutionizing the C–C coupling field.The catalytic activity of a SAC depends closely on the nature of the active isolated metal atom and the presence of functional groups, if attached by linkers, the support used can also induce a significant electronic perturbation to the atomic active site for electron charge transfer. As a result, the single dispersed atoms of metal anchored on to a support surface are expected to have a reduced number of coordination sites for reactants or intermediates in comparison with metal NPs supported on a surface due to the absence of different active sites. More importantly, once the metal is anchored on a solid support, it can be easily regenerated for the next cycle of reaction without any complex treatment [17]. Table 4 outlines the applications of SACs in the C–C coupling process.As proclaimed by Tao's group [284], the developed Pd-based SAC anchored on TiO2, Pd1/TiO2 via a deposition−precipitation method was highly selective and active for more than 10 C–C reaction cycles of phenylacetylene and iodobenzene (Fig. 16(a)). Based on XANES and EXAFS, the coordination of Pd atoms attached to TiO2 was confirmed where each Pd atom is bound to four oxygen atoms of the TiO2 surface support, forming PdO4 units, and the exposed surface of the lattice fringe of the TiO2 is (101) with an average TEM particle size of 20–25 nm (Fig. 16 (b)). To further explore the deactivation and detachment of Pd atoms from Pd1/TiO2 catalyst after the reaction (Fig. 16(c)), the authors have also performed the durability test of the spent Pd single atoms on a TiO2 support by measuring the concentrations of Ti and Pd in a solution (after centrifugation), followed by characterization using XPS (Fig. 16(d)). Surprisingly, the peak positions of Pd 3d5/2 were the same, indicating that the Pd atoms on TiO2 supports have the same chemical oxidation state before and after the reaction. Regarding the deactivation study, the authors have performed a hot filtration-leaching analysis; notably, no metals were detected in the filtered solution, indicating the strong bonding between the Pd atoms (active sites) and TiO2 (support). Moreover, computational investigations based on DFT calculation have also been carried out to identify the most thermodynamically favorable structure, which corresponds to Pd single atoms anchored to four oxygen atoms of TiO2 through Pd–O–Ti bonds.Lately, Chem and co-workers [441] reported a heterogeneous catalyst consisting of Pd single atoms anchored on exfoliated graphitic carbon nitride (Pd-ECN) for the C–C reaction of bromobenzene with phenylboronic acid pinacol ester while benchmarking with homogeneous and other bulk heterogeneous catalysts (Fig. 17 (a)). Microwave-assisted deposition was used in this study to deposit palladium on ECN, a pristine high-surface area form of graphitic carbon nitride. STEM coupled with EXAFS was used to investigate the presence of Pd single atoms, while XPS was adopted to study the electronic properties of the Pd atoms incorporated in the ECN. In addition, the DFT calculations were also carried out to understand the promising C–C coupling performance of Pd-ECN. Molecular dynamics simulations performed at different temperatures show that Pd atoms were confined within a given cavity, even though they still have some degree of freedom. This simulation evidence agreed with the experimental XPS observations, suggesting that the Pd atoms occupy two preferred positions: in the first one, Pd was located close to the surface plane, while in the second one, the metal was in between the two-top planes.In the reaction mechanism, the first step corresponds to the molecular adsorption of bromobenzene to the metal center and the consequent change of the Pd coordination number. The authors reported that the ability of Pd to change its coordination is crucial to the observed catalytic performance. In the second step, bi-hydrated potassium phenylboronic acid pinacol ester was adsorbed, and the cation from the salt occupied the nearest neighbor empty cavity. Thanks to the displacement of Br–, phenylboronic acid pinacol ester coordinates, the subsequent trans-metalation was found to be the rate-determining step of the overall reaction (Fig. 17(b and c)). After the elimination of the boronic pinacol ester, the new C–C bond was formed. The elimination of the product restores the initial coordination of the Pd atom. Even though the overall reaction mechanism catalyzed by Pd-ECN reflects that reported for Pd(PPh3)4 molecular catalyst, in the latter case the role of the ligands is crucial: the elimination of two ligands occurs prior to the reaction, and it opens the coordination sphere of Pd, allowing the coordination of the substrate. A third ligand was then released during the trans-metalation step.As mentioned above, heterogeneous alloyed SACs or known as SAA can be prepared using two metals, via isolation of the single metal atom by another metal atom. The synergistic effect between the two metals will alter the geometric and electronic structures of alloyed SACs, potentially inducing exceptional catalytic performance for various reactions [22,443]. Zhang and co-workers [42] have reported a durable and efficient Au -Pd SAA for the Ullmann reaction of aryl halides in water. The investigated Pd-based SAC exhibited an excellent Ullman coupling activity, not only of aryl bromides and iodides but also of the less reactive aryl chlorides. As known, aryl chlorides are less expensive, readily available, and more sustainable than their analogous aryl bromides and iodides. For this reason, their utilization as substrates is highly desirable. However, in the literature, only a few examples of Pd NP catalysts were reported to be worked well with aryl, but the bi-metallic SAA presented by Zhang and collaborators furnishes a valid alternative. The Au–Pd SAA was prepared with an ion exchange-NaBH4 reduction method, and the presents of Pd single atoms were isolated by the Au atoms as confirmed by the EXAFS and DRIFTS analysis. The DRIFTS result was in good agreement with the EXAFS result, indicating that the Au alloyed Pd single atom configuration was formed at Au/Pd ≥ 4. Remarkably, the Au–Pd SAA managed to convert ∼90% of aryl without deactivation over the course of eight cycles.Another class of C–C coupling reaction includes the hydroformylation of olefins to produce aldehydes, which are important intermediates to produce other chemicals. Wang et al. [207], have recently developed CoO-supported Rh single-atom catalysts (Rh/CoO) with remarkable selectivity towards propene hydroformylation. The authors investigated the yield and selectivity of butyraldehyde by increasing the Rh weight loading (e.g., 0.2, 1.0, and 4.8 wt.%). The highest turnover frequency (TOF) number of 2065 h−1 and selectivity of 94.4% for butyraldehyde were obtained when the catalyst with the lowest Rh weight loading. Furthermore, the stability of 0.2% Rh/CoO was also studied by recycling the catalyst five times. Over the cycles, the catalyst remained highly active, similar to the initial reaction, the selectivity slightly decreased to 94.0% by the final cycle. To further understand the facilitation and adsorption of propene on Rh single sites in an atmosphere containing both H2 and CO (syngas), a DFT investigation was performed. The DFT calculations showed indeed that after the adsorption of H2 and CO, Rh atoms moved from the original lattice position, leading to a reconstruction of Rh active atoms that facilitate the adsorption of propene. Furthermore, DFT calculations also showed dissociative adsorption of the H2 molecule, leading to the formation of an OH group on the CoO surface, while CO preferentially binds to the Rh active site, due to the strong interaction with Rh single atoms. DFT was also used to investigate the reaction mechanism that proceeds through three consecutive steps: a) one of the adsorbed H atoms will attack the CC bond in the adsorbed propene molecule; b) CO will insert into the opened CC bond; and c) the second adsorbed hydrogen atom will then combine with the C atom in the reactively formed terminal CO to form the final product. The remarkable activity and selectivity, and high stability of 0.2% Rh/CoO are of high importance for potential applications in industrial processes by reducing the cost and pollution efficiently. Another remarkable example from Zhang's group, thermally stable Rh-based SACs which favor the hydroformylation of olefins has been synthesized [39]. Notably, the Rh1/ZnO SAC has demonstrated a very high TON of 40,000 with 99% selectivity towards aldehyde products, which is to our knowledge the largest value among reported heterogeneous metal catalysts. This level of selectivity has not been previously reported for Rh, without the use of specific support materials such as polymers (ligand steric effects), or zeolites (confinement effects), adding great impact and benefit to the SACs. Also, the fabricated SAC has shown a high stability profile in terms of recycling, where no obvious leaching or aggregation of Rh active metals were observed after a 4th run of experiments.As shown in Table 2, all the examples mentioned imply that SACs can be a valid alternative to bridge both homogenous and heterogeneous catalysts for C–C coupling reactions. However, the development of SACs with notably improved performances relies on the contribution of both theoretical and experimental investigations. DFT methods can be applied to investigate crucial aspects that are not directly accessible by experiments, such as the nature of the active sites of different SACs and the ways substrate molecules interact with single atoms [444]. Overall, the electronic metal-support strong interactions are a critical concern to increase the catalytic performance of coupling, and thus, modulation of the charge density of anchored single metal atoms should be emphasized in future research.Selective hydrogenation represents essential processes in the organometallic chemistry process, particularly in the petrochemical and fine chemical industries. For petrochemicals, selective hydrogenation is the most common route to eliminate the impurities such as alkynes and dienes in the ethylene industry for downstream polymerization [445]. Meanwhile, in the pharmaceutical industry, alkenyl, carbonyl, and carboxyl functional groups of the feedstock are required to be selectively reduced through H2 to their corresponding alkenes, alcohols, and amine products which are key intermediates for fine chemicals production [446]. Notably, a quarter of the chemical industrial processes include at least one hydrogenation step, and therefore it is not surprising that the selective hydrogenation reaction is one of the hot topics investigated in the catalysis field [447]. However, it is a challenging task when two or multiple functional groups coexist in the substrate and also, the hydrogenation of CC bonds is much easier than that of the CO bonds, thermodynamically favored by 35 kJ/mol [448].A new generation of catalysts for selective hydrogenation reactions has been developed to tackle the challenges by introducing the “active site isolation” strategy. Atom assemblies and isolation techniques can exhibit different physicochemical properties in altering different hydrogenation mechanisms and show a better catalytic hydrogenation activity compared to NP counterparts [449,450]. Lately, SACs have also been widely applied in the selective hydrogenation of styrene, acetylene, glycerol, crotonaldehyde, and nitriles; mainly attributed to lowering the activation barriers, governing catalytic reactivity, enhancing the adsorption model, and also possessing uniform single active sites [213,451]. For example, isolated Pd atoms on a Cu surface lower the reaction barriers of both hydrogen uptake and subsequent desorption from the Cu metal surface, enhancing the selectivity of styrene hydrogenation [22]. Meanwhile, anchoring single Rh (Rh1) atoms to Mo edge vacancy sites of 2-dimensional MoS2 could also facilitate the H2 dissociation in hydrogenation [401]. There are also studies reporting that encapsulating Ni atoms with transition metals can improve covalent chemical bonding, owing to the inherent vulnerability of nickel-based SACs under acidic hydrogenation conditions [452].A comprehensive summary of the catalytic performances of different SACs for selective hydrogenation reactions is listed in Table 5 . Although the operational conditions, reactor configuration, catalyst loading, temperature, and pressure may vary greatly, it has been shown that a low Pt loading can be incorporated into graphite shells, generating ‘carbon onions’ (Pt/C) via an arc-discharge method. This was found to provide remarkable conversion and reaction selectivity, comparable with other hydrogenation SACs [453]. Notably, the Pt@C exhibits a high catalytic reactivity and stability towards the chemo-selective hydrogenation of functionalized nitroarenes under mild reaction conditions. High selectivity of p-chloroaniline at >99% was obtained using EtOH under optimized reaction conditions, 40 °C, 1.0 bar of H2 pressure, 40 mg of catalyst loading, and 1 h reaction time. On top of that, the synthesized Pt/C catalyst displays a superior reusability performance over at least 10 cycles and without any loss in hydrogenation activity and selectivity, suggesting that the graphitic shells of carbon ‘onions’ prohibited a chemical coarsening of the Pt single atoms, which alters the effective penetration channels for the transport of ions and electrons during the catalytic reaction [453]. The proposed encapsulated graphite shells of the SAC were also processed using HRTEM images (Fig. 18 (a–c)), with a well-distributed Pt(111) interplanar size of 0.255 nm. This clearly shows the effect of arc medium concentration, where Fig. 18 (a) utilizes a 0.975 mM salt which generates Pt nanoparticles, whereas an arc medium concentration of 0.0195 mM created a dispersed catalyst that was atomically resolved (Fig. 18 (b)). The x-ray diffractograms in Fig. 18 (d), clearly show that the Pt SAC does not have a (111) feature, contrary to the Pt NP catalyst.Lately, Zhang and colleagues [198] have studied the anchoring effect of mesoporous γ-Al2O3 on Pt atoms, likely on the catalyst's stability through unsaturated pentahedral Al3+ coordination for the hydrogenation of a ketone. The superior catalytic activity highlights the applicability of the catalyst for hydrogenation reactions in a small amount of Pt species loadings on Al2O3. As reported in a previous study, aromatic rings normally coordinate with multiple metal atoms before undergoing hydrogenation and remain to interact with the metal surface during stepwise hydrogenation [457]. However, the proposed mechanism is not possible for SACs as found in this study [456] where the hydrogenation ring on Pt/Al2O3 is almost fully suppressed. Alternatively, the Pt species favors CO bond adsorption and activation forming an η1(O) configuration [458,459]. In the η1(O) configuration, the acetophenone will be adsorbed on to a Pt single-atom site for reacting with H2 to form 1-phenylethanol. Then, the intermediate product transfers to the Al2O3 support where it is strongly bound before desorbing into the solution phase, suppressing the deactivation of a Pt site, and enhancing the reusability of the catalyst for the next cycle. A similar result has been obtained by Lucci et al. [257] in which the synthesized Pt/Cu(111) SAA can be reused more than six times TPD cycles with constant selectivity of butadiene to butene (∼25%) as shown in Fig. 19 (a) [257]. Further quantification of Pt atoms through CO titration after each run has highlighted the durability of Pt/Cu(111) SAC where the concentration of Pt atoms on the surface layer of the catalyst remained consistent with the number of Pt atoms present prior to each hydrogenation reaction, suggesting that the single isolated Pt atoms in Cu are capable of H2 spillover without breaking C–C bonds as well as reduce the possibility of Pt poisoning. As seen in Fig. 19(b), the addition of Pt onto Cu NPs enhanced the rate of hydrogenation, where a higher hydrogenation activity was observed in Pt0.2Cu14/Al2O3 than Pt0.1Cu14/Al2O3. Additionally, the addition of Pt single atoms to the catalyst could lower the hydrogenation reaction temperature (onset at 40 °C), which is 35 °C, i.e., lower than that of the monometallic Cu15/Al2O3 catalyst under the same conditions. In order to demonstrate the hydrogenation capability of the Pt–Cu SAC in different stressful conditions (with impurities), the authors tested the SAC in the presence of excess propylene and found that the propylene has no significant effect on the activity and selectivity of hydrogenation to butadiene as displayed in Fig. 19(c). At below 120 °C, ∼100% of butadiene was converted accompanied by a minor propylene concentration (<1%) converting to propane, implying that the Pt–Cu SAC maintains active and is stable. The use of Pt in Cu NPs (Pt1Cu20/Al2O3) has also been shown recently for the liquid phase hydrogenation of furfural, here a promoted Cu nanoparticle was found to be far more active and selective than bulk bimetallic alloys and monometallic equivalents [249]. Additionally, for the same reaction, an array PdCu SAA was created by Islam and co-workers, with decreasing Pd content to determine the atomic limit required for efficient hydrogenation [46], critically finding that 0.0067 wt% of Pd could be used to radically improve the reactivity of a Cu host nanoparticle on γ-Al2O3. Another work using a Pd–Cu SAA is by Jiang and co-workers [454]. This work exploited the change in the structure coordination during the impregnation of ultra-low Pd loading (50 ppm) in a host Cu nanoparticle. They found that the Pd1/Cu SAC is highly effective for both hydrogen spillover and selective hydrogenation. Notably, The Pd1/Cu catalysts displayed excellent catalytic performances in the semi-hydrogenation of phenylacetylene to styrene at 303 K under 0.1 MPa H2. The selectivity of ∼96% towards styrene was achieved at a conversion of 100%. This work was supported by DFT calculations which highlights the benefit of atom arrangement on the activity of the catalyst itself, globally finding the rate of reaction for Pd–Cu (111) was substantially lower than a Pd–Cu(100).There is another interesting well-dispersed single/pseudo Pt SAC catalyst impregnated on mesoporous WOx developed by Zhang's group [460]. Under the reaction conditions of 160 °C, 1 MPa H2 pressure, and a high glycerol concentration (50%), the Pt/WOx SAC processed a very high space-time yield (3.78 gPt −1 h−1) towards 1,3-propanediol (1,3 PD) as shown in Fig. 20 (a). Based on the transition of the catalyst in preparation (Fig. 20(b–d)), a well homogenously dispersed Pt atom of 2.59 wt % was impregnated on WO3 support without any formation of nanoclusters, suggesting that the isolation of Pt over WOx was successfully achieved. To further understand the physiochemical property of SACs a mechanism for how the Pt/WOx SAC behaves for the selective glycerol hydrogenation was proposed in Fig. 20(e), [460]. Firstly, the unshared‐pair of electrons in the glycerol's O atoms was trapped by the unoccupied W6+ d orbital, forming an ether‐like bond with a W atom (Step 1). The strong interaction with the W atom will weaken the bond between the O and H atoms and facilitate the oxidation of an H atom (Step 2). Then, extraction of protons from the terminal O atom of WO occurred and formed W–OH, while the W6+ species was partially reduced (Step 3). This finding was proven by Raman spectroscopy, where the terminal WO band intensity decreased significantly after contacting with glycerol. In step 4, the WOx Brønsted acid sites were consumed, catalyzing the dehydration pathway and stabilizing the formation of the secondary carbocation from glycerol. Owing to the oxophilic characteristic of W species, the H2 is assumed to be heterolytically dissociated on the interface between Pt and WOx, exhibiting both acid sites (Hδ+) and hydrogenation sites (Hδ−). Thus, rapid hydrogenation of 3-hydroxypicolinic acid can occur and gives rise to a high yield of 1,3-PD (Step 5). This study provided an in-depth synergistic mechanism between Pt and WOx species for the production of 1,3-PD. The design of the WOx supported pseudo‐single atom Pt catalyst yielded a high selectivity of 1,3-PD (45.7%) under a low H2 pressure environment.Despite many experimental works reported on the robust nature of the SACs for selective hydrogenation reactions, there is still a lack of molecular calculations and microkinetic simulations to back up the hydrogenation activity and mechanisms [463–465]. In 2018, Thirumalai and co-authors [461] reported the reactivity of a series SAAs consisting of Au, Ag, and Cu nanoparticles doped with single atoms of Pt, Pd, Ir, Rh, and Ni in the hydrogenation of acetylene to ethylene via DFT calculations. They reported that from the d-band model generated by Hammer and Nørskov, AuPd and AgPd were chosen as the potential candidate for hydrogenation. The findings indicate that the acetylene most likely adsorbs at the FCC sites of AuPd and AgPd. By comparing the binding energies of acetylene at the FCC site and atop sites, it reveals that acetylene was less stable on the atop sites by 0.111 eV for AuPd and 0.008 eV for AgPd, meanwhile the ethylene adsorption at the FCC sites and atop sites differ by 0.23 eV for AuPd and 0.019 eV for AgPd. Notably, the acetylene and ethylene prefer bonding as α-bonded complexes on pure metal surfaces. However, in the presence of a reactive Pd atom in a relatively inert host, they bind strongly with the surface through α-complexes in an atop conformation. Furthermore, the energetics of hydrogenation is more favorable for single atom alloys rather than their respective pure host metals (Fig. 20(f)), deducing that the single atom alloys are favorable for the formation of the vinyl intermediate, attributing to the strong adsorption on the Pd atom which resulted in excellent selectivity towards the formation of ethylene. The co-adsorption energy of intermediate on AuPd and AgPd are very close, suggesting that the Pd atom in the alloys is mainly responsible for driving the reaction forward. As noted, the energy barrier for ethylene desorption is almost negligible on single atom alloys, resulting in immediate desorption which prevents further hydrogenation of ethane.Moreover, a DFT study in conjunction with Scaling Relations Kinetic Monte Carlo (SRMC) simulations reported by Jørgensen and Gronbeck [462], found that the main reaction mechanisms for hydrogenation of acetylene−ethylene will be C2H2 adsorption. However, the C2H2 adsorption is exothermically stronger on Pd(111) as compared to Cu(111) and Pd/Cu(111). Meanwhile, the H2 dissociative adsorption is barrier less on the Pd-containing surfaces, but it is high for the Cu(111). The observation for the absence of an H2 dissociation barrier on Pd and a considerable barrier on Cu is consistent with the previous studies [22,466]. Based on these simulations, it can be concluded that the Pd/Cu(111) SAA has a selectivity higher than that of Pd(111), mainly due to the weak binding of ethylene on Cu as compared to Pd while a strong binding of C2H4 at the edges and corners sites hinders the ethylene desorption before further hydrogenation. In short, acetylene−ethylene hydrogenation should contain minority sites that readily dissociate hydrogen and the majority sites where ethylene is weakly adsorbed. Nonetheless, all the above findings have opened a promising avenue to the rational design of SACs for selective hydrogenation. Despite much-perceived subjectivity on the stability of SACs under high-temperature conditions for selective hydrogenation reactions, many experimental works have demonstrated that SACs can provide stable activity under many reaction cycles or reaction times for this application.SACs have emerged as a new frontier in catalysis for hydrogen production from methane reforming and water-gas shift reactions. Dry Reforming of Methane (DRM) has received much attention in the hydrogen production sector, as this sustainable process exploits two major greenhouse gases (GHG), carbon dioxide and methane to produce industrially important syngas. Table 4 shows the application of SACs for hydrogen production, specifically in DRM and the WGSR over the last five years. Numerous supported precious metal SACs (e.g., Pd, Pt, Ru, and Rh) and a few transition metals SACs (Ni and Co) have been used for DRM. Despite the high coking resistance of precious metal-based catalysts, the use of precious metals in the synthesis of catalysts is often regarded as a non-sustainable approach due to the high cost and low availability of noble metals, which are typically limited for large-scale applications [467]. On the other hand, Ni-based catalysts are generally more economical and are more abundant. Despite the low cost and high availability, Ni exhibits a high sintering tendency, poor deactivation resistance, and high affinity toward coke deposition on the active sites under a reaction temperature over 800 °C [468,469]. Ni-based catalysts are well-known for their high sintering tendency via particle migration and Ostwald ripening under a high reaction temperature and steam environment. The sintering and agglomeration of the active phase increase the particle size and reduces the dispersion of the metal atoms [469,470]. The number of atomically dispersed active species also decreases significantly and eventually leads to poor activity performance. Furthermore, severe carbon deposition on the Ni active site was also reported extensively in the literature. With such bulky NPs formed from the sintering phenomenon, a side reaction of methane pyrolysis readily takes place on the metallic NP sites due to the very high adsorption energy. The methane pyrolysis reaction could produce a layer of carbonaceous material on the metallic surface which in turn leads to catalyst deactivation after a long period of reaction time [471].As previously mentioned, the coking resistance and stabilization of an atomically dispersed active phase on a support material during a long time on stream are still regarded as one of the few major challenges encountered in the DRM and are yet to be resolved by academic and industrial practitioners. In line with the efforts in addressing the coking resistance and stabilization issue, numerous previous works have reported the considerable effect of CeO2 on the size, dispersion, stability, and deactivation resistance of active sites [203]. For the synthesis of SACs, it is summarized that the support material should possess a high affinity with the active phase leading to a high dispersion and strong metal-support interaction (SMSI) [164]. A list of SACs used for methane reforming, steam reforming, and water gas shift reactions is tabulated in Table 6 . In a recent work by Tang et al. [368], a novel bimetallic Ni/Ru SAC supported on CeO2 was synthesized and proposed for the DRM application. As shown in AP-XPS spectra (Fig. 21 (a)), the catalytic surface of Ce0.95Ni0.025Ru0.025O2 SACs consists of two sets of atomically dispersed Ni and Ru species. Also, the fraction of Ce3+ during the reaction at 550 °C was much higher than before the reaction, as evidenced by the formation of a plateau in the region of 885.2±1.5 eV, implying that no overlapping of photoemission features of Ce3+ and Ce4+ of CeO species in the catalyst. Based on the experimental findings, both active species were found to be highly active for methane reforming with a high turnover rate of 73.6H2 per site per second at 833 °C (Fig. 21(b)). The conversion of CH4 on Ce0.95Ni0.025Ru0.025O2, 91% was much higher than that of Ce0.95Ru0.05O2 and Ce0.95Ni0.05O2 at 700 °C, implying that there is a positive synergistic effect between Ni and Ru cations which enhance the DRM activity. Furthermore, computation studies also uncovered the synergetic effects and complement functions of the atomically dispersed Ni and Ru under a low concentration of 2.5 metal atomic %. Ni atoms are highly active in the adsorption of CH4, and Ru atoms have a high affinity toward CO2. From the operando studies of chemical and coordination environments, both Ni and Ru single atoms anchored on the CeO2 surface remained in a cationic form instead of a metallic state. This was found to promote the catalytic performance of the SAC significantly up to 600 °C, outperforming NP counterparts, as depicted in Fig. 21 (c).Akri et al. [164] synthesized a highly active and carbon-resistant Ni SAC supported on hydroxyapatite (HAP) using a Strong Electrostatic Adsorption (SEA) method. The 0.5 wt.% Ni SAC exhibited the highest CO2 and CH4 reaction rates of 816.5 mol/(gcat h) and 1186 mol/(gcat h), respectively, which was four and five times higher than the reaction rate of nickel NP catalysts. The 0.5 wt.% Ni SACs displayed excellent carbon deposition resistance as evidenced by the negligible weight loss of spent Ni SACs when characterized under thermal gravimetric analysis (TGA). Despite its high activity performance and carbon deposition resistance, the Ni SACs suffered from poor stabilization of the atomically dispersed Ni phase. Severe sintering and aggregation of Ni atoms were observed in the 0.5 wt.% Ni SACs, which led to its high deactivation rate after a few hours of reaction. The same research team also attempted to reinforce the stability of the previous Ni SACs by using a polyvinylpyrrolidone (PVP) assisted preparation method. A small amount of PVP was added during the co-precipitation process of the Ni SAC. It was found that the catalytic stability of Ni SACs improved significantly with very little carbon deposition on the catalyst surface. Such improvement in the catalytic behavior could be attributed to the highly dispersed Ni single atoms on the surface, hindering the inner active site from sintering phenomena during the reaction.In another study by Akri et al. [369], the stabilization effect of ceria-doped hydroxyapatite (Ce-HAP) for atomically dispersed Ni species was investigated. From the in-situ XPS and Temperature Programmed Reduction (TPR), both characterization techniques unambiguously revealed that the ceria-doped HAP stabilized the atomically dispersed Ni from sintering and aggregation under a high temperature-reducing H2 environment. Despite the high reduction temperature, the Ni(OH)x and NiO species on the Ce doped support remained unreduced and displayed high resistance behavior as compared to the undoped counterparts. In the end, the Ce species was reported to act as a stabilizing anchor for the atomically dispersed Ni rather than to suppress carbon deposition. As compared to the SAC work by Tang et al. [368], the 2 wt.% Ni SAC supported on Ce-doped HAP in Akri et al. [369] delivered similar catalytic performance and superior stability under identical reaction conditions, without using a precious metal. A low 0.5% Ni/HAP SAC (20 times less Ni loading) exhibited a comparable CH4 reforming activity to that of the 10% commercial Ni/HAP under similar reaction conditions.Despite several experimental studies reporting that Pt SAC is highly active for low temperature (120–400 °C) selective reforming and WGS, the arguments on: a) The characteristic behavior of Pt atoms in the WGS reaction at low temperature, b) The stability of Pt atoms under a reducing atmosphere and an elevated temperature, and c) The isolated Pt atoms behave only as spectators in the process [475,476]. The fundamental questions have finally been resolved by Ammal and Heyden [477] through a DFT calculation, in which positively charged single Pt atoms stabilized on a TiO2 (110) surface can be as active as Pt clusters for the WGS reaction at low and high temperatures. The calculation revealed that the interface edge Pt and single Pt2+ sites exhibited a high WGS activity at low temperatures whereas the corner Pt interface sites become active at higher temperatures. As such, the single Pt2+ sites acted as a stabilizer on an active reducible surface such as TiO2 while the oxygen vacancies in the support play a significant role in enhancing the WGS activity. A possible reaction pathway for the WGS was also proposed, containing the redox, carboxyl, and formate pathways, shown in Fig. 22 (a). As reported, the redox reaction was the dominant pathway between the temperature range of 200–400 °C, while high TOFs are possible for this active site. Meanwhile, the carboxyl pathway with redox regeneration was less favorable than the formate pathway with redox regeneration. Its rate was very close to that of the classical redox pathway at temperatures below 300 °C. In addition, the single Pt2+ sites tend to stabilize on the CeO2(110) sites with H as ligands, owing to the similar characteristics and advantages of both homogeneous and heterogeneous catalysts [478,479]. Based on the free energy profiles as displayed in Fig. 22(b), we can also clearly see that the presence of additional surface H atoms could reduce the energy barrier for the interfacial H-transfer process (TS23) by about 0.2 eV, compared to the CO-assisted redox pathway (TS18), suggesting that the associative carboxyl with redox regeneration pathway is likely the most favorable pathway.The excellent catalytic activity and stability of Pt nanoclusters reported by Ammal and Heyden [477] are in good agreement with Guo et al. [468]. In 2014, Li's group [473] synthesized stable and highly active Pt-based SACs for methanol steam reforming using the desorption-absorption method, by embedding the isolated precious metal atoms of Pt and Au onto a ZnO surface. A spin-polarized DFT calculation coupled with STEM characterization was performed to investigate the intrinsic nature of the active sites of the catalyst. The DFT calculation revealed that the corresponding formation energies of single Pt and Au atoms were 0.22 and 0.86 eV, respectively, which were much lower than the reservoirs in equilibrium with large metal counterparts. This observation indicates that the embedded Pt and Au are thermodynamically stable and resistant to segregation during the catalytic reactions and thus, providing a stronger binding toward the intermediates, as well as lowering reaction barriers. The enhancement of the catalytic activity can be seen where the TOF found in the single Pt sites embedded onto ZnO(1010) surfaces are over 1000 times higher than that of the pristine ZnO. The hypothesis was further confirmed by electron beam irradiation, where the isolated Pt single atoms were found to be relatively stable after anchoring onto ZnO(1010). All the HAADF images show no Pt or Au clusters/particles in the synthesized Pt1/Au1/ZnO SAC.In short, to cater to higher reforming rates, multi-functional SACs should be developed with a significant number of interfacial sites, resulting from the presence of individually dispersed metal atoms on the support. This could avoid the coking resistance and stabilize the atomically dispersed active phase on a support material under a long time-on-stream. Moreover, due to the involvement of multiple species and commonly complex reaction mechanisms in the catalytic reforming process, the usage of DFT for the study of reaction-free energy to unravel potential reaction pathways provides many useful insights for designing SACs from first-principles. Furthermore, although methane-based reactions are the most applicable for catalytic reforming in the industry, more effort should be carried into branching out toward other chemical species to understand the possibilities of SAC in additional applications.Extending to the energy matrices, novel SACs have been widely applied in the selective oxidation field. As such, the unique catalytic performance of SACs has demonstrated a huge prospect in various oxidation reactions such as CO oxidation or PROX, aerobic oxidation of alcohols, formaldehyde oxidation, and methane oxidation [13,480]. The sub-nanometer clusters of single metals were reported to have a better enhancement in catalytic activity or selectivity compared to larger bulk nanoparticles [481,482]. Apart from that, the utilization efficiency of the metal catalyst and selectivity, either for the adsorption or desorption activities of the active species can be modified via metal atom isolation, which directly influences the reactions kinetics [480,483]. Due to the interesting behavior found in metal SACs, these have attracted numerous researchers to have an in-depth understanding of their behavior and mechanism [102].However, a common problem faced for selective oxidation reactions is the decrease in size from a nanoparticle to a single atom, in which the surface free energy of metals increases significantly with decreasing particle size, promoting aggregation of small clusters of the catalyst [13]. This can be prevented by implementing a high surface area support material that could interrelate well with the metal atoms, and postulating an isolated metal that can accommodate the metal surfaces, metal oxides, and carbon materials in the system [484,485].According to Duprez and Cavani [486], selective oxidation is achievable using the famous Mars and van Krevelen mechanism that involves different transition metal ion oxides that display redox properties such as Cu, V, Mo, Cr, Te, Sb, Bi, and Fe [487]. Among all the metal ion oxides, the atomically dispersed Co and Cu catalysts have been reported to exhibit the highest catalytic activity in the selective oxidation of benzyl alcohol and 5-hydroxymethylfurfural (M = Fe, Cr, Co, Ni, Cu) [488]. In 2017, The first pioneering work of non-noble Co-based SAC for selective oxidation was reported by Guan's group. An atomically dispersed Co on N2-doped graphene (denoted as Co-NG) in an ammonia medium via pyrolysis technique was synthesized [489]. Notably, a high benzyl alcohol conversion (94.8%) and benzaldehyde selectivity (97.5%) were achieved over 6 h and 120 °C using a small amount of Co-NG (5 mg). However, under the absence of N2 doping, a much lower conversion of 42.5%) than Co-NG in selective oxidation of benzyl alcohol was attained. As reported previously, the single metal atom on a carbon matrix can be stabilized by introducing N atoms as an “anchor” [99]. The N2 doping does not solely strengthen the interaction between the metal atom and the support but also promotes electron transfer, which resulted in a firmly anchored, atomically dispersed metal atom on supports [490,491]. A possible catalytic reaction mechanism for the aerobic oxidation of benzyl alcohol over Co-NG was also postulated as follows: Firstly, the oxygen molecules were weakly adsorbed on the Co center, followed by electron transfer activation to form a superoxide species (Co 3d orbitals to O2 2p antibonding orbitals). Lastly, the superoxide species will react with the hydrogen bonding of benzyl alcohol to produce benzaldehyde [492,493].Furthermore, Harrath et al. [364] also studied the catalytic mechanism of M/ZrO2 SAC (M = single atom of Rh, Pd, Ir, Pt, Fe) for a one-step conversion of CH4 to CH3OH. Their work also found that Rh/ZrO2 SAC induced the dissociative adsorption of H2O2 on its surface with great binding energy (−2.87 eV), favoring the selective oxidation of CH4 pathway. Subsequent steps in the reaction pathway of Rh/ZrO2 SAC include the adsorption of methane, and formation of C–H bond (which forms a methyl radical and HOO–Rh site) to further produce CH3OH or by-product CH3OOH (Fig. 23 (a)). Apart from that, the non-noble Fe/ZrO2 SAC was also expected to give high selectivity of methanol due to the lower energy barrier for methyl radical formation (0.49 eV lower) and methanol formation (0.13 eV lower) on O–Fe/ZrO2 compared to O–Rh/ZrO2 (Fig. 23(b)). Notably, the pathway to produce the CH3COOH by-product via a Fe/ZrO2 SAC was suppressed due to a kinetically thermodynamic unfavorable energy barrier of 2.77 eV, suggesting a high selectivity for the main product CH3OH can be obtained.Inspired by the excellent results of the application of SACs in various selective oxidative reactions as shown in Table 7 , more studies further challenged the selective benzylic C–H oxidation of hydrocarbon derivatives under mild conditions. This is because most studies have reported that selective oxidation of saturated C–H bonds is difficult and aggressive conditions (>120 °C and 10 bar oxygen pressure) are usually required to obtain a high selectivity of desired products [494]. In 2019, Bakandritsos together with his co-workers disclosed a mixed-valence Cu-based SAC for oxidative homocoupling of benzylamines [282]. Fig. 24 (a) shows that the G(CN)–Cu SAC was synthesized using the coordination of Cu(II) ions to CN-functionalized graphene (cyano-graphene, G(CN)) where the graphene-induced charge-transfer reduced the Cu(II) ions anchored to G-CN into the Cu(I). Surprisingly, the G(CN)–Cu SAC was able to yield an excellent conversion (up to 98%) and selectivity (up to 99%) under mild conditions (85 °C, 1bar). In addition, the G(CN)–Cu SAC remained at a very high conversion rate (94%), even after 5 recycling steps with no change in product selectivity (98%) (Fig. 24(g)). This observation was supported by the TEM images, in which a clear detection of Cu atoms can be observed before and after the catalytic reaction (Fig. 24(b–f)), suggesting the active sites of the Cu are not prone to sintering even during a high reaction temperature. In addition to that, the XPS analysis also further confirmed the high catalytic activity of the G(CN)–Cu SAC, in which there was no change of the Cu mixed valance state from 1st to 5th cycles (Fig. 24(h)). The G(CN)Cu also displayed a high turnover frequency (TOF = 13 h−1) at low temperatures (<100 °C), proving that it has a strong electron-withdrawing character (withdraw electron from CF3-substituted benzylamine) than that of the current best performing NP catalysts in the literature (e.g., CuO nanoflakes and Cs/MnOx) [495,496]. Through the DFT analysis and EPR measurement, a possible oxidative amine coupling mechanism for the study was proposed as shown in Fig. 24(i). Firstly, the oxidative dehydrogenation of the benzylamines started with O2 reduction in the active copper enzymes through its preferential coordination with Cu(I) centers (step 1), which leads to the formation of copper-oxyl intermediate between the Cu ions (step 2). In order to yield the formation of an imine, a two-hydrogen abstraction from the neighboring amine by the reactive oxyl species was essential (step 3). In step 4, the hydroxyl radicals produced were trapped and finally, the NH3 and N-benzylidene-benzylamine were produced through the amine–imine coupling. Lastly, the catalyst was regenerated and can be reused for further oxidative coupling of benzylamines.With respect to theoretical screening works, there have been few studies investigating the fundamental mechanism of oxidation via SACs [498,499]. For instance, the fundamental insights of CO oxidation catalyzed by using single Au atoms supported on Thoria (Au/ThO2) through DFT with Hubbard-type On-site Coulomb interaction simulation (DFT + U) were reported lately [498]. From the computational study, three main steps mechanism of facilitation of the Au-doped ThO2 (111) surface for CO oxidation was analyzed: 1) Reaction between the gaseous phase CO between the lattice O2 − through Mars-van Krevelen (MvK) mechanism, 2) the adsorption process of gaseous phase of O2 − at the vacancy site to form the activated O2 −, 3) CO molecule reacts with O2 − to form the intermediate of OCCCO* which breaks down into CO2, and O* adatom. Based on the findings, the developed Au-doped ThO2 (111) showed a positive catalytic activity for CO oxidation with a lower adsorption rate and the rate-limiting step was determined to be the adsorption of O2 which takes place at the ThO2 site on the surface.Han et al. [500] investigated the Pd stripe and Pd single atom-doped Cu(111) surfaces for COOCH3 selective oxidation. Specifically, three structures of Pd monolayer, Pd4Cu8 and Pd single atom (Pd1) on Cu(111) were studied as shown in Fig. 25 (a)). For the conversion of COOCH3 to dimethyl oxalate (DMO), the strain effect decreases the activation barrier on Pd1–Cu(111), while the ligand effects caused a non-dominant increase in the activation energy barrier (Fig. 25(b)). This effect was similar for Pd monolayer but was the contrary for Pd4Cu8/Cu(111). From Fig. 25(c), it can be clearly seen that the Pd1–Cu(111) was showing an exothermic reaction energy associated with low activation energy, indicating that the oxidation reaction occurs much easier on the Pd1–Cu(111) as compared to its counterparts. In addition, two possible reaction pathways related to COOCH3 oxidation were studied through microkinetic analysis. In both pathways (Fig. 25(d and e)), the results were in good agreement that the DMC (108.8 kJ/mol) was more favorable to be produced compared to the DMO (192.6; 107.5 kJ/mol) due to a lower energy barrier of the rate-determining (Fig. 25(d)). A similar observation was also depicted in Fig. 25(e) in which the activation barrier of the Pd4Cu8/Cu(111) and Pd1–Cu(111) surfaces was 107.5 and 91.6 kJ/mol, respectively [500].Lately, another remarkable investigation of the synergistic effect of tri-metals in a Crown Jewel-Structured (IrPd)/Au SAC for selective oxidation has been revealed by Zhang and co-authors [493]. The presence of negatively charged Au and Ir atoms has elucidated two kinds of charge transfer modes, creating a synergistic effect that enhanced the catalytic activity of the (IrPd)/Au SACs to a maximum level. In order to further understand the synergistic effect, a DFT analysis was performed. The study revealed that electron transfer between O2 and anionic Au and Ir atoms possessed a hydroperoxo–like species via donating an excess electronic charge to the antibonding orbital. The adsorbed O2 molecule on the (111) face of Au of (IrPd)Au model clusters possessed the highest negative charge numbers, suggesting that this is the key factor that leads to an enhancement of synergistic catalytic activity for selective aerobic oxidation.Over the decades, photocatalysts and electrocatalysts have attracted huge attention as a method of addressing the global environmental issue and energy crisis [501,502]. Along this line, SACs have been engaged as promising candidates in the fields of photocatalysis and electrocatalysis due to their high catalytic activity, stability, and pathway selectivity [19,503]. In recent years, SACs have been utilized for a wide range of applications, such as hydrogen evolution, oxygen evolution, CO2 reduction, pollutant removal and degradation, and chemical synthesis [504–506].Photocatalyst is a unique class of materials that can accelerate chemical reactions on exposure to a specific type of light (UV, UV–Vis, or Visible). Photocatalysts offer sustainable and environment-friendly catalytic solutions by utilizing green and inexhaustible solar light to facilitate chemistry reactions compared to traditional thermal activation processes. In general, photocatalysts work using the same principle as semiconductors, when the photocatalyst is exposed to light, an electron in the valence band can absorb the energy of photons and is excited to the conduction band, leaving a hole (positive charge) in the valence band. Therefore, the electron–hole pair is produced in this process, which can provide both oxidation and reduction environments to accelerate chemical reactions [507]. Various bulk materials have shown photocatalytic capabilities, including metal oxides (TiO2, V2O5, ZnO, Al2O3, Fe2O3) [508], carbon dots [509], metal–organic-frameworks (MOFs) [510], 2D materials [511], and plasmonic metals [512]. However, current photocatalysts are facing great challenges because of fast photogenerated electron–hole recombination, limited visible-light response, and slow electron transport [513]. Along this line, SACs have emerged as capable photocatalysts that could be the answer to overcoming the typical problems that hinder conventional photocatalysts.On the other hand, electrocatalyst is a type of catalyst used to increase the rate of electrochemical reactions by facilitating the conversion between electrical and chemical energy [514]. The reaction processing in electrolysis is dominated by circuit-induced carriers, which can drive reactions far from their equilibrium potential, enabling access to difficult reaction pathways. Solid metals or oxide electrodes are usually used as heterogeneous electrocatalysts and the electrochemical processes occur at or near the liquid–solid interface. These reactions usually include multistep ion/electron coupled electron transfer with high reaction kinetics, requiring efficient catalysts to accelerate the processes [515]. Homogeneous electrocatalysts are soluble or dispersed in solutions, activating the reactions in the solutions. The processes are indirect electron transfers instead of the direct electron transfer between electrode and an electrolyte. A vast array of materials has been used as electrocatalysts, including noble metals, noble metal oxides, transition-metal-based materials, MOFs, and metal-free-carbons [516–520]. Electrocatalysts have been widely used in energy storage and conversion, metallurgy, and chemical synthesis applications. However, a bottleneck of wide spread electrocatalyst usage is the high cost of noble metals and low natural abundance, limiting the large-scale development of electrocatalysts. Noble metals are usually present in the forms of nanoparticles in conventional catalysts, however, the previous concept of efficient nanoparticles is flawed by the fact that these are in fact to be considered bulk materials due to extended terrace sites. Reducing the size of catalysts is an efficient method to expose more high-energy active sites. SACs offer promising access to address this issue due to natively possessing a maximum atom utilization. In this section, we highlight and introduce recent advances in electrocatalysis and photocatalysis using SACs, focusing on applications in CO2 conversion and hydrogen fuel cells (Table 8 ).The CO2 released in the atmosphere from both large, industrial point sources, and small, mobile sources are considered the main culprit for global warming. Its capture and utilization have received growing attention since it is a promising strategy for reducing its concentration in the atmosphere, and simultaneously obtaining valuable chemicals and fuels [528]. However, due to the very stable structure of the CO2 molecule, its conversion requires the utilization of catalysts. In this regard, electrocatalytic CO2 reduction reaction (CO2RR) holds great promise among various chemical approaches [529,530], since it can be carried out under ambient conditions with promising activity [530]. For this process to be environmentally friendly, the energy input should be obtained from a renewable and non-CO2 emitting electricity source, and combined with the utilization of ‘green’ electrocatalysts. Noble metal atoms are known for their superior activity, selectivity, and long-term stability in CO2RR, but their high cost and scarcity hinder their extensive use [531,532]. However, this issue is reduced in SACs, where the metal loading is notably decreased.The first example of electrochemically driven CO2RR over SACs was proposed in 1974 by Meshitsuka et al. [533] who showed that cobalt and nickel phthalocyanines attached to graphite electrodes are active catalysts for the electrochemical reduction of carbon dioxide. Since then, SACs have been extensively explored and several other promising SACs for CO2RR have been proposed [50,534,535]. Unlike gas-phase reactions, electrochemical reactions would have the additional requirement of high-conductivity support materials, such as carbon or doped metal oxide. The introduction of heteroatoms in the support matrix was found to be a useful strategy for modulating the electronic structures and stabilizing the metal atoms, resulting in an overall enhanced catalytic activity [536,537]. Furthermore, the process of “anchoring” the metal atom to the support involves a charge transfer among the central metal sites and the substrate [523]. This type of metal-support interaction has been extensively investigated in SACs to regulate the electronic structure of catalysts, which consequently affects the intrinsic activity of active sites toward various electrocatalytic reactions [538]. Moreover, the local environments of the metal active sites in atomically dispersed metals determined the significantly different behavior between SACs and their bulk and nanoparticle counterparts [539,540]. For instance, while the bulk and nanoparticle electrodes of Mn, Fe, Co, and Ni mainly produce H2 [540], the M−N–C (M = Mn, Fe, Co, Ni) SACs show great activity for the electrochemical CO2 conversion to CO, and the suppression of the competing HER [541].The introduction of SACs into CO2RR has yielded a high efficiency towards desired fine C1–C5 chemicals [542,543]; notably, SACs have shown promising results in boosting the catalytic conversion of CO2 up to nearly 100% in some cases [544]. Very recently, Li and collaborators [521] reported a single-Fe-atom catalyst tuned with phosphorus (Fe–N/P–C) on commercial carbon black as a robust electrocatalyst for CO2 reduction (inset in Fig. 26 (a)). The single-Fe-catalyst was synthesized by pyrolyzing a mixture of activated carbon black (ACB) with Fe3+ (Fe3+−ACB), urea, and triphenylphosphine in an argon atmosphere. Fourier transform-EXAFS sheds light on the coordination configuration of the single-Fe-atom catalysts. These findings, together with elemental composition and oxidation state analysis from in-situ XPS and AC-STEM-EDX (Fig. 26 (a) and (b)) confirm the presence of atomically dispersed Fe atoms without Fe aggregation (no Fe–Fe bonding from XAS measurements). A high mass-normalized turnover frequency of 508.8 h−1 at a low overpotential of 0.34 V, and a high Faradaic efficiency of 98% have been determined for the Fe–N/P–C SAC, that support the outstanding catalytic activity for the CO2 conversion to CO DFT calculations have been performed to further investigate the catalytic mechanism. The theoretical results have shown that the HER is largely restricted on the P-tuned Fe–N–C catalyst. Moreover, Bader charge analysis underlines a lower oxidation state of Fe which contributes to the CO2 activation and CO desorption. (Fig. 26 (c) and (d)).Even though the conversion of CO2 to CO is appealing since it is a key step in the preparation of Fischer−Tropsch synthetic fuels, extensive efforts need to be devoted to fine-tuning SAC coordination environments, to accurately change catalytic selectivity with multiple electron-reducing products which still remains a great challenge and are rarely investigated at present. The electroreduction of carbon dioxide into methanol, which involves a six electron transfer process, has been studied by Yang and collaborators [522] who prepared isolated Cu atoms decorated ‘through-hole’ carbon nanofibers (CuSAs/TCNFs), with abundant and homogeneously distributed Cu single atoms (CuSAs) for efficient electrochemical CO2RR, with high stability. CuSAs/TCNFs exhibit 44% Faradaic efficiency and −93 mA cm−2 partial current density of methanol. Moreover, the preparation of the CuSAs/TCNFs membrane fulfills the industrial production requirements. DFT calculations underline that the desorption of the adsorbed *CO on CuSAs/TCNFs model is slightly endergonic, suggesting that the CO desorption does not occur, and the *CO intermediate is further hydrogenated to methanol. In recent work, Cai and co-workers [523] proposed a carbon dot (CDs)-supported SAC, which consists of a single-Cu-atom coordinated to two N and two O sites bound to the edge of graphitic carbons (Cu-CD), as an efficient electrocatalyst for the conversion of CO2 to CH4 (eight electron transfer process). The unique structure of the synthesized Cu-CD catalyst allows for high Faradaic efficiency of 78%, superior CH4 catalytic selectivity at high negative bias, and suppression of the HER. Moreover, the limiting step for CH4 production was found to be a lower energy value than that reported in other works.Proton exchange membrane fuel cells (PEMFCs) are among the most promising devices to convert chemical energy to electrical energy [545–547]. In these devices, the cathode catalyzes the Oxygen Reduction Reaction (ORR), and the anode catalyzes the oxidation of fuels, such as hydrogen (HOR). The desired ORR is a four-electron process leading to the production of water (O2 + 4H+ + 4e− → 4H2O), that involves the cleavage of the exceptionally strong OO bond, whose bond energy is 498 kJ mol−1. Therefore, in order to overcome the slow kinetics of the ORR, efficient electrocatalysts are required. On the other hand, the anode reaction is the hydrogen oxidation reaction (H2 → 2H+ + 2e−), which is a relatively simpler reaction than ORR [514]. Platinum-based materials are the most widely used electrocatalysts for both the ORR and HOR in PEMFCs. However, its extension to large-scale and industrial applications is hindered by high costs and low reserves. Based on that, the development of non-Pt catalysts is of paramount importance. In this regard, the work of Jasinski [548] on the ORR activity of cobalt phthalocyanine paved the way for the development of atomically dispersed M–N–C materials, which exhibit strong ORR performance and great potential for substituting noble metal Pt-based catalysts. DFT was used to investigate the adsorption energy of oxygen intermediates involved in the ORR process on M–N–C, consisting of carbon nanostructures functionalized with pyridinic nitrogen atoms and transition metals [549]. The study revealed differences of up to 0.7 eV among the adsorption energies of the oxygen intermediates on different moieties, which underlined the importance of precisely determining the local site structures in M–N–C materials for understanding their reactivity.Recently, a Cu SAC was proposed by Cui and collaborators [550]. The catalyst was prepared via a pyrolysis method using Cu phthalocyanine (CuPc) as the precursor and carbon nanotubes as carriers. Aberration-corrected STEM and operando XAS techniques have been used to determine the morphology and electronic properties of the catalyst. The Cu SAC showed higher ORR stability and comparable ORR activity with respect to Pt/C in an alkaline medium, which makes this catalyst a capable non-noble ORR catalyst for fuel cell applications. Moreover, DFT calculations have been performed and suggest that the transformation process from OOH* to O* is the rate-determining step of the ORR on the Cu SAC.Characterization techniques such as XAS, along with computational methods have been used to gain insights into the dynamic evolution of active sites in operando processes, whose information is crucial for an in-depth understanding of the catalytic behavior of SACs. For instance, Han and co-workers [524] demonstrated the substrate-induced activity improvement of CuN2C2 SACs embedded into sp2-hybridized carbon graphite frameworks. Specifically, the authors state that the increase of the geometry distortion of single-atom CuN2C2 active sites, formed when going from a graphene-like material to a small-diameter carbon nanotube (CNT), leads to an improved ORR activity. Indeed, the higher strain in CNT substrates implies a more significant distortion of the CuN2C2 moieties during the ORR, which results in strengthening the Cu–O bonds of Cu and the oxygen atoms of the adsorbed species, while weakening the original Cu–N/Cu–C bonds. As a consequence, a higher electron transfer to the adsorbed O2 molecules is achieved, thus enhancing the ORR activity up to six-fold.Photocatalytic reduction of CO2 to value-added carbon-based fuels and chemicals is one of the most active research fields. In recent years, SACs have been developed and applied extensively as a new class of high-efficient catalysts for photocatalytic CO2 reduction reaction (CO2RR), using natural sunlight as an energy source. Apart from their high atom utilization, large specific surface area, and uniformity of active sites, the heterogeneous single atom photocatalysts have many other favorable features including improved energy efficiency with good excitation under visible light, and reducing recombination of photo-generated charges as well as high selectivity towards CO2 adsorption. Despite efforts being made to develop an efficient photocatalyst with high H2 generation performance, the poor visible light utilization rate, low quantum yield, severe aggregation of electron pairs caused by photogenerated electrons, and its low stability are some of the barriers and hindrances in bringing these advanced photocatalysts towards practical applications and commercialization.An example of applying SACs for photocatalytic CO2RR was by Gao et al. [551], two types of single atoms, palladium, and platinum, were supported on graphitic carbon nitride and investigated as photocatalysts for photocatalytic CO2RR. From the DFT calculations, the graphitic carbon nitride support itself offers a source of hydrogen atom (H*) from the hydrogen evolution reaction (HER). The deposition of Pd and Pt atoms onto carbon support evidently improves the visible light absorption performance, which renders them an ideal candidate for photocatalytic CO2RR. For Pd and Pt-based catalysts, the former produces more HCOOH as a product of CO2 reduction and the latter prefers to form CH4 from CO2 with a much higher rate-determining barrier of 1.16 eV, as compared to that of Pd catalysts (0.66 eV). The CO2 reduction pathways to HCOOH and CH3OH on Pd/g-C3N4 and CO2 to CH4 on Pt/g-C3N4 catalysts were illustrated in detail, as shown in Fig. 27 (a) and (b), respectively.Noble metals including Pd, Au, and Pd are commonly used as co-catalysts for the photocatalytic reduction of CO2 due to their inherent low activation energy and effective charge separation. However, such rare elements are still considerably expensive which impedes its large-scale commercial application. Recently, the incorporation of earth–abundant transition metals (e.g. Cu, Ni, Co, and Fe) as an economic alternative for photocatalytic CO2RR applications has sparked great interest among the scientific community. Chen et al. [419] designed and developed a Cu single-atom catalyst with three-dimensional ordered mesoporous TiO2 (Cu0.01/3DOM-TiO2) from a template-assisted in-situ pyrolysis method. The proposed synthesis strategy not only caters to a wider light absorption range but offers some specific active sites for the absorption and transformation of CO2 molecules via different pathways (Fig. 27(c)). From the results, the novel single atom photocatalyst exhibits a high methane selectivity of 83.3% with a formation rate of 43.5 μmol g−1 h−1 under a gas–solid system (Fig. 27(d i-ii)). Whereas under a liquid–solid system, the same single atom catalysts favored the formation of ethylene with a selectivity of 58.4% and formation rate of 6.99 μmol g−1 h−1 (Fig. 27(d iii-iv)) [551]. From the reaction mechanism study it was revealed that methane is generated from the *CHO intermediates in the gas–solid system while ethylene is produced as the main product from dimerization of *CO and *CHO in the liquid–solid system [551].Zhang et al. [526] investigated the photocatalytic performance of cobalt-based photocatalysts with Co single atoms isolated and anchored on a commercial ‘super conductive’ carbon black (Co-SA@SP-800). The as-prepared photocatalyst demonstrates a significant improvement in photoactivity, CO selectivity, and cycling stability, which is mainly due to the highly active isolated Co–N4 single atomic sites with conductive carbon support. Considering the unique electronic structure of Co SACs toward photocatalytic CO2 reduction, Di et al. [525] introduced ultra-thin Bi3O4Br nanosheets to isolated single atoms Co as an active site and form Co–Bi3O4Br catalysts for CO2 photoreduction reaction. From the results, the designed catalysts exhibited an improved selective CO formation rate of 107.1 μmol g−1 h−1, which is ∼4 and 32 times higher than that of the atomic layered Bi3O4Br and bulk Bi3O4Br nanoparticles, respectively [525].Apart from cobalt-based SACs, both nickel and iron are two low-cost earth–abundant transition metals that can be decorated as SACs for photocatalytic reactions. Zhang et al. [527] designed a highly efficient photocatalytic system by dispersing single-site iron atoms and anchoring on a porous crimped graphitic carbon nitride (g-C3N4) polymer. Surprisingly, the synergistic effect of Fe and g-C3N4 support promoted the solar-photon-driven activities [527] and led to a higher photocatalytic hydrogen generation rate of 3390 μmol h−1 g−1. Similarly, Jin et al. [283] decorated the same support with partially oxidized Ni single atoms with abundant unpaired d-electrons, which improved the absorption performance of visible light and mobility of charge carriers. As a result, the photocatalytic H2 production rate was improved by 30-fold as compared to that of the bulk g-C3N4 and other kinds of polymeric semiconductors.The unique and promising features of SACs have created a huge application potential in many areas. However, the scalability of this novel material remains a challenging barrier to mass production [14]. Several SAC production challenges need to be addressed to achieve maximum commercial value as shown in Fig. 28 .Various synthesis methods have been considered including physical and chemical methods. Despite many SACs synthesis methods having been developed, the upscaling of laboratory settings into commercial production has not been effective [552]. Physical methods require complex and expensive equipment, while chemical methods cannot be adapted to synthesize SACs containing other transition metals [553]. Besides, a larger-scale synthesis of SACs containing almost any transition metal with high metal loading, with a single synthetic strategy has proven to be elusive. He et al. [169] emphasized that the fabrication cost for SACs remains feasible for commercial production. Additionally, some experiments such as mass-selected soft-landing [554] and atomic layer deposition [555] are hindered due to expensive experimental requirements and low production efficiency. To date, the largest quantity of SACs reported in the literature was ∼1 kg under controlled laboratory environmental conditions [553]. To overcome the challenges of large-scale applications, the adoption of advanced manufacturing such as robotics and automation, not to forget nanotechnologies as well as the use of AI and machine learning is recommended. As such, 3D printing provides a convenient and precise ability to design and print geometrically complex functional SACs that integrate isolated atom, photoactive and catalytic functionalities.The performance of SACs can vary with the support material. Su et al. [556] highlighted that the limited diversity of support material for SACs has led to a narrow range of active site structures. Many works on the investigation of carbon-based support materials for SACs have been performed. The support material does not only create a strong bonding between metal atoms and the support surface, but it also affects the support for atom anchoring sites to stabilize the metal atoms [557]. Besides, the support material can improve the catalytic reaction as well. Cheng et al. [552] emphasized that a support material with a high surface area and a large number of anchoring sites can synthesize high-loading SACs. In addition, the surface material with these properties can improve the stability, selectivity, and activity of the SACs. Jing Liu et al. [479] stated that many researchers have experimentally demonstrated the application of support materials such as noble metal oxides and 2D materials to synthesize SACs. Despite the size of these metallic atoms, not all atoms are located on the surface of the catalyst and hence, are not fully accessible to the reactant. Complete accessibility can only be achieved if every single atom is well dispersed and stabilized on the support. Also, the functional groups and defects on the support surface can develop different SAC structures which can affect the stability of SAC. The selection of support material might be challenging for the mass production of SACs.To produce SACs on a commercial scale, the stability performance of SACs needs to be addressed. Cheng et al. [552] emphasized the stabilization of supported highly dispersed single atoms during catalysis can be challenging due to high surface-free energy and low coordination numbers of single atoms. With the consideration of the dynamic operation conditions, the stability of SACs can be influenced by many factors such as temperature, pressure, support material, surface condition, and reactant [558]. Mostly, in harsh reaction conditions, SACs will undergo loss of active sites under particle migration and coalescence and atomic (or Ostwald) ripening deactivation [559]. In order to prolong the lifespan and stability of the SACs, utilizing colloidal nanocrystals to independently control particle size and particle loading on the SAC is desired. However, it is very challenging to isolate a clear and precise mechanism of different species of SAC in specific reactions, due to a lack of studies in literatures. In short, further research and understanding of the stabilization of SACs under dynamic working environments is required prior to large-scale application.In the synthesis of SACs, the characterization methods are essential in determining the quality of the SAC. With the advancement in SACs, advanced characterization equipment is necessary such as STM, EXAFS, AC-STEM, and often DRIFTS [552]. These forms of characterization are in-situ methods of determining the structure of a SAC, while it undergoes its chemical reaction. Li et al. [560] highlighted that the in-situ methods can capture the reaction intermediates, identify active sites, and monitor dynamic behaviors of both geometric structure and electronic environment of catalytic sites. However, Li et al. [560] added that most of the in-situ methods are performed only for the characterization of SACs without simulating catalytic activity, simultaneously. Moreover, each characterization method has its advantage and limitation. Therefore, a detailed SAC characterization under dynamic operating conditions required the integration of in-situ characterization techniques to discover the characterization of SACs under dynamic conditions. Additionally, the cost of the characterization of SACs can be expensive before the characteristics of SACs can be fully validated for commercial use.As previously mentioned, the utilization of SACs in energy and chemical applications is still in its infancy (works are mainly at a lab-scale where pilot testing and prototyping are still pending), in which the Technology Readiness Level (TRL) is currently between 1 and 3 [561]. To date, there is no literature reporting on economic or environmental analyses of SACs in any applications. Thus, in order to provide an overview of the commercialization feasibility of SACs, this review article also provides preliminary economic and environmental analyses based on the available resources and information. The assessment portfolio was made based on the catalyst synthesis cost, production cost, CO2 emission factor, and lifespan of the catalysts (they are ordered clockwise based on increasing CO2 emissions). This representation highlights that the economic metric does not correlate with the environmental impact. Nevertheless, these findings, it is sufficient to identify the most attractive and potential catalyst, as reported in many research articles [562,563].The first step of this investigation was the selection of possible reaction which offers suitable characteristics for integration. Based on the data available, the selected reaction was dry reforming of methane (DRM, Figure S1) and six individual case studies were chosen based on the catalyst types, noted as (A) Ni-based SAC [369]; (B) Ni/Al2O3 derived from Metal–Organic Framework (MIL-53) [564]; (C) Ni/Al2O3–CeO2 [565]; (D) Ni/TNT [566]; (E) Ni/Al2O3 [565] and (F) Pure Ni [567] (note that H2 is considered as the targeted product). The calculations step and assumptions used are included in the Supplementary information (Tables S1-S8). As presented in Fig. 29 , the Ni-based SAC (Ni/HAP-Ce) was the most preferred catalyst for DRM, in terms of both being environmentally friendly and economical. According to the greater details stated in the supplementary information, the cost of preparation for Ni/HAP-Ce via co-precipitation was found to be the lowest among other counterparts, also, noteworthy to mention that, the cost was about 3.5 times lower than that of the Ni-MOF. In addition, the total production cost (USD/kg H2) of Ni-SACs was also calculated to be the lowest compared to other catalysts, which is 26.4% and 1.23% lower than pure Ni (Worst) and Ni-MOF (2nd Best), respectively. Whereas the environmental matrix is measured in terms of CO2 emissions (kg CO2/kg H2 produced). Notably, the CO2 emissions in catalytic DRM using a Ni-SAC are far lower than other catalysts, i.e., in the range of 2.3–7.5 times lower as compared to that of other catalysts. Overall, given its promising performance in the studied aspects (economic, environmental, and lifespan), SACs emerge as an attractive frontier catalyst to be exploited further too aid the commercialization purpose and strengthen other chemicals and fuel production routes. Future work on conducting rigorous “Integrated Economic, Environmental, and Energy” assessments (3Es) is required to further provide a bigger picture of the frontier of SACs, these assessments could provide decision makers in the commercialization process with feasibility data in determining the most favourable synthesis method of SACs and also suggesting the future research direction, challenges, and debottlenecking of the application of SACs in different field.The performance of SACs has gained attention in many sectors as the new frontier in catalysis science, especially in clean energy. This paper highlights the development of SAC synthesis methods from conventional (i.e., co-precipitation or sequential incipient wetness impregnation) to new synthesis methods (i.e., coordination site, defect design, photochemical and electrochemical). As the development of SAC synthesis is improved, advanced characterization methods such as high-resolution electron microscopes, x-ray irradiation spectroscopies, magnetic resonance spectroscopies, and other wavelength in-situ spectroscopies are used to provide an in-depth understanding of the characteristics of the SAC for clean energy application. On top of that, recent SAC experimental outcomes are reviewed with the consideration of operating conditions, catalyst loading, and types of reactor configurations. Based on the experimental and DFT method output, SACs appear to be an alternative to bridge both homogeneous and heterogenous catalysts for clean energy applications including coupling, oxidation, hydrogenation, and reforming reactions. Through the nano-engineering strategy, bulk nanoparticle catalysts can be downsized by using large surface area support materials, both the surface area and density of defect sites are favorable for the incorporation of single atoms of the active metal, providing some unique catalytic performance such as higher stability and flexible physiochemical properties. Besides that, studies demonstrated that SAC contributes to lowering the reaction activation barrier, enhancing adsorption pathways, processing uniform single active sites, and governing catalytic reactivity for selective routes. The commercialization and large-scale production of SACs require addressing various key challenges including the utilization of affordable support materials, stability of SACs under a mass production environment, and introducing of simple and effective in-situ characterization methods. From this review, it can be observed that the application of SACs is at the forefront of clean energy and chemicals production research, offering strategic alternatives to classical heterogeneous catalytic systems with a major impact on the economy and the environment.The review was conceptualized by ACML, SYT, MJT, and GK. Data was acquired by ACML and SYT, the overall investigation was carried out by ACML, SYT, BSH, XZ, KWC, VB, WDL, BLFC, and CLY. The review was written by all authors and was reviewed and edited by ACML, MJT, and GK.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:Dr Martin Taylor reports financial support was provided by UK Research and Innovation. Dr Kin Wai Cheah reports financial support was provided by UK Research and Innovation.MJT and KWC acknowledge funding through the THYME project (UKRI, Research England). A.C.M. Loy would also like to acknowledge the Australian Government Research Training Program for supporting this project. The research contribution from S.Y. Teng is supported by the European Union's Horizon Europe Research and Innovation Program, under Marie Skłodowska-Curie Actions grant agreement no. 101064585 (MoCEGS).The following is the Supplementary data to this article: Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.pecs.2023.101074.

The emergence of single atom sites as a frontier research area in catalysis has sparked extensive academic and industrial interest, especially for energy, environmental and chemicals production processes. Single atom catalysts (SACs) have shown remarkable performance in a variety of catalytic reactions, demonstrating high selectivity to the products of interest, long lifespan, high stability and more importantly high atomic metal utilization efficiency. In this review, we unveil in depth insights on development and achievements of SACs, including (a) Chronological progress on SACs development, (b) Recent advances in SACs synthesis, (c) Spatial and temporal SACs characterization techniques, (d) Application of SACs in different energy and chemical production, (e) Environmental and economic aspects of SACs, and (f) Current challenges, promising ideas and future prospects for SACs. On a whole, this review serves to enlighten scientists and engineers in developing fundamental catalytic understanding that can be applied into the future, both for academia or valorizing chemical processes.

Converting anthropogenic CO2 into valuable fuels (e.g. CH4) using green hydrogen generated, for instance, from water electrolysis driven by renewable electricity is key to enable the energy transition of the chemical industry [1,2]. In the conversion of carbon dioxide to methane, large quantities of heat are released due to the exothermicity of the reaction (CO2 + 4 H2 → CH4 + 2 H2O, ΔH298K = –165 kJ/mol) and, in the absence of heat removal, the adiabatic temperature rise would be rather significant (773 K) [3]. When employing this technology in large-scale Power-to-Gas (P2G) processes, the conversion of CO2 can vary significantly due to the fluctuations in the production of renewable electricity that is used to generate the hydrogen required for the process (for every mole of CO2 4 mol of H2 are needed). This results in large temperature swings as a function of time on stream (typical fluctuations are in the order of minutes) [4]. Hence, the catalyst subjected to these fluctuations undergoes accelerated aging that leads to lower metal surface area (sintering) and porosity (pore-collapsing) [4]. In order to compensate for the accelerated deactivation, one could either use an excess of catalyst, or use a fluidized reactor in which the catalyst is continuously replenished. These strategies, however, could make the process economically unattractive at high catalyst consumption rates. This challenge was highlighted before by Prof. J.D. Grunwaldt and co-workers [4]. The authors argue that coupling of thermo-/electro- catalytic processes with dynamic energy and feed supply will render additional complexities to the chemical industry as reactors are often operated within a narrow operational window for optimal performance. Clearly, new catalysts and reactor concepts are needed to facilitate the commercial take-up of renewables in the chemical industry in the near future.Supported Ru, Ni, Rh, and/or Co metals on different metal oxide supports (TiO2, Al2O3, SiO2, ZrO2, CeO2…) have been extensively studied for CO2 methanation [5,6,15–18,7–14]. Among them, Ni-based catalysts are the most researched materials, since doped or promoted Ni catalysts have shown good CO2 conversion, high selectivity to methane, and low cost compared to noble-based catalysts. In this catalyst, it has been demonstrated that the support plays a key role, not only modifiying the dispersion of the active phase and textural properties, but also its activity for CO2 activation. High-energy lattice metal oxides such as, cerium oxide and titanium, possess excellent redox properties due to their par M3+/M4+ and it exhibits high oxygen storage capacity [19–21]. As a result, ceria provides a large amount of oxygen vacancies with medium basicity, facilitating CO2 activation-dissociation and metal-support interaction [9,11,29,30,17,22–28]. Nanoshaped ceria (e.g. nanorods or nanocubes) has been synthesized to support Ni, Co or Ru to enhance its catalytic activity [13,15,31–33]. These nanoshaped ceria supports expose well-defined crystal planes that can facilitate stabilization of metal clusters for catalytic applications at elevated temperatures, which makes them suitable for CO2 methanation. In order to compare their activities, Sakpal et al. studied the influence of Ni loading, Ni cluster size, and distribution on three types of nanoshaped ceria. In this report, the authors concluded that the Ni cluster size and distribution, determined by the shape of the ceria support, was the decisive factor in the observed catalytic performance [34]. In general, nanorods-shaped ceria exhibited the highest activity compared to typical polyhedral ceria and nanocubes, mainly due to stronger metal-ceria interaction, large fraction of oxygen vacancies, and high oxygen mobility [32,33,35].In this context, ceria has been reported to help the metal dispersion and prevent deactivation due to metal sintering, which is one of the main drawbacks in CO2 methanation [12,22,24,36–38]. Despite the good stability reported in CO2 methanation on promoted Ni-ceria based catalysts, its long-term stability under fluctuating conditions remains elusive. Some stability tests have been reported, but often these studies were conducted close to the maximum equilibrium conversion where excess of catalyst can mask the catalyst deactivation. For instance, Ocampo et al. [37] have shown that it is possible to mitigate the catalyst deactivation of Ni/CexZr1-xO2 catalysts for CO2 methanation depending on the ratio of ceria and zirconia. In this study, however, the rate of deactivation was measured from the beginning at thermodynamic equilibrium regime. While significant improvements have been achieved in the past by supporting Ni catalysts on ceria-containing supports, the utilization of conversion levels close to the thermodynamic equilibrium to study the stability of these catalysts generates uncertainty on the validity of the results. [25,26,30,39].Since the appearance of hotspots and the consequent metal sintering are one the main causes for catalyst deactivation in CO2 methanation, different approaches for structuring the catalyst have been proposed in the last few years, aiming at improving heat and mass transport. Ricca et al. [38] studied the temperature profile inside the catalytic bed for 10 wt.% Ni/CeO2-ZrO2 supported on Al-foam and SiC monolith compared to the powdered catalyst. They observed that the temperature increase inside the reactor bed was reduced in the order powder > Al-foam > SiC monolith. Similarly, Frey and co-workers [40] studied the hotspots appearance and the temperature profiles on Ni/CeO2 based catalysts supported on open foams of Al, Al2O3, and SiC, showing that the highest conversion was obtained on SiC support. In this material, the higher rates per reactor volume led to the formation of hotspots according to IR thermography, which negatively affected the selectivity to methane and the catalyst stability. To mitigate these issues, the authors grew carbon nanofibers on the SiC to improve the hydrodynamic, thermal, and catalytic properties of the structured catalyst. This configuration drastically increased the heat removal, improving the catalyst performance [41]. In the same line, Fukuhara and co-workers [42,43] studied different Al-honeycomb configurations (plain, stacked, segmented, multi-stacked), combining shifted positions of the honeycomb stacks and free spaces or non-catalytic honeycomb stacks. These results showed that structuring of the Ni-Ceria catalyst improved the heat and mass transfer inside the reactor, leading to enhanced activity and stability. The authors, however, measured the stability of these materials near the equilibrium conversion, thus complicating interpretation of the results obtained.The selection of the material of the support is also important, since not only the heat transfer is a determining parameter. In addition, catalyst loading and adherence, cell density or hydrodynamic design are also important for its feasibility [44]. For instance, Schollenberger et al. [45] proposed a mixed Al-steel honeycomb to optimize the CO2 conversion level and the heat transfer. Among other metallic supports, FeCrAlloy® steel has been extensively proposed due to its good heat transfer, flexibility to create different shapes, very high cell density and ease to segregate an Al2O3 μ-layer to improve the catalyst loading showing excellent catalyst adherences [46–52]. For instance, Hernandez Lalinde et al. [46] tested a Ni/Al2O3 catalyst on FeCrAlloy plates obtaining good catalyst impregnation and homogeneous temperature profile during methanation reaction.In the present study, we show that by supporting Ni catalyst on CeO2 nanorods it is possible to prevent catalyst deactivation observed during methanation reaction when using conventional Ni supported on commercial CeO2. Our catalyst showed high selectivity to methane of c.a. 95–99 % even under fluctuating reaction conditions, where more severe deactivation is anticipated due to the large temperature swings. We demonstrate that this excellent performance is not caused by excess of catalyst as the performance of the materials was assessed far from the maximum conversion (c.a. 20 % of the equilibrium conversion). Furthermore, we show that structuring this catalyst on metallic FeCrAlloy μ-monoliths can enhance its activity and stability.Synthesis of nanorods shaped CeO2 was performed by hydrothermal process previously reported in our group [13]. In a typical synthesis, 24 g of NaOH (Sigma Aldrich) and 2.17 g of Ce(NO3)3·H2O (Sigma Aldrich) were separately dissolved in 35 mL and 5 mL of deionized H2O, respectively. Then, both solutions were slowly mixed and stirred for 30 min. The resulting slurry was transferred into a Teflon bottle (125 mL) and filled 80 % with water. The Teflon bottle was introduced in a sealed autoclave. The hydrothermal treatment was performed for 24 h at 100 °C to obtain nanorods CeO2. The resulting precipitate was separated by centrifugation (9000 rpm for 10 min) and washed with deionized water until pH 7 was reached. The sample was dried at 100 °C for 4 h, followed by calcination at 500 °C (heating rate: 5 °C/min) for 5 h in air (flow rate: 100 mL/min). On the other hand, octahedral CeO2 with an average particle size below 50 nm was obtained from commercial Sigma-Aldrich and the same calcination step at 500 °C (heating rate: 5 °C/min) for 5 h in air (flow rate: 100 mL/ min).Deposition of the desired amount of nickel on the prepared nanorods or octahedral ceria was performed by wet impregnation. Typically, 3 g of ceria was added to 60 mL of water under continuous stirring. In another flask, 0.744 g of commercial Ni(NO3)2·6H2O (Alfa Aesar) was dissolved in 20 mL H2O and slowly added to the ceria slurry under stirring. Then, the pH was adjusted to 8 by adding dropwise 0.1 M NaOH aqueous solution. The mixture was stirred at room temperature for ∼165 and ∼315 min for octahedral and nanorods shapes, respectively, in order to obtain similar Ni particle sizes [34]. Finally, the catalysts were centrifuged and dried at 100 °C for 3 h, followed by calcination at 500 °C for 5 h in air (100 mL/min) with a heating rate of 5 °C/min.On the other hand, FeCrAlloy® sheets (Fe72.8/Cr22/Al5/Y0.1/Zr0.1, GoodFellow) with 0.05 mm in thickness were used to manufacture cylindrical multichannel monoliths. As described elsewhere [53], flat and corrugated foils were co-rolled in pairs resulting in cylindrical metallic monoliths with 15.8 mm in diameter and 20 mm in height with calculated cell density of 2004 cpsi and an exposed surface of 152 cm2 (Fig. 1 ). Then, the manufactured monolith were calcined in air at 900 °C for 22 h (heating ramp of 10 °C/min) in order to form an external porous Al2O3 μ-layer by segregation from the FeCrAlloy material that facilitates the catalyst impregnation [52,53]. The calcined monolithic structure was immersed 1 min in an aqueous colloidal suspension of the desired catalyst (Ni/CeO2 oct or Ni/CeO2 rods). The channels of the monolith were gently cleaned with an airbrush to avoid obstructions. Then, the impregnated monolith was dried at 100 °C for 1 h and weighed. The impregnation process was repeated until the desired amount of catalyst was loaded on the monolithic structure. Finally, the structured catalyst was calcined at 500 °C for 5 h in air, with a slower heating rate of 2 °C/min in order to avoid fissures or fractures in the catalytic layer [54]. The typical thickness of this layer was c.a. 2 μm.To obtain homogeneous thin layers of catalyst overcoating, a stable colloidal suspension of the catalyst with optimal rheological properties was mandatory for the impregnation process. The optimization of the slurry was aimed at avoiding particles agglomeration to obtain well-controlled homogeneous thin layers over the monolith walls. This was done to prevent diffusional problems, catalyst loss, fractures, and/or peeling. The main variables to control were the particle size, viscosity, and pH of the suspension [48,55]. Particularly, the colloidal suspensions were prepared by slowly adding 20 wt.% catalyst, previously sieved below 38 μm, in deionized water. The colloidal suspensions were aged for 24 h before starting the impregnation, always under continuous stirring at room conditions.The structural analysis of the two synthesized catalysts (named Ni/CeO2 rods and Ni/CeO2 oct) and their prepared nano-shaped ceria supports (named CeO2 rods and CeO2 oct) was conducted by X-Ray Diffraction (XRD) on a Bruker D2 Phaser diffractometer with Cu Kα radiation. Ni phase of the synthesized and reduced catalysts were compared by XRD on an X’Pert Pro PANalytical instrument with Cu Kα radiation. N2-physisorption at 77 K (Micromeritics Tristar) was performed to determine textural properties of the catalysts and ceria supports. Ni loading was determined by XRF (Philips PW 1480). The surface morphology and Ni particle size and dispersion were analyzed by Scanning Electron Microscopy (SEM) in a JEOL JSM-6490 instrument, and by Transmission Electron Microscopy (TEM) micrographs recorded on a Philips CM-200 instrument equipped with energy dispersive X-ray detector (EDX).In order to analyze the reducibility of the synthesized catalyst, reductive thermogravimetric analysis (H2-TGA) was conducted using a Mettler Toledo TGA/DSC3 +. The gas flow consists of 20 mL/min Argon protective gas and 50 mL/min 90:10 H2:Argon as reactive gas. The sample was weighed in a 70 μL aluminium oxide crucible. Then, the sample was placed inside the analysis chamber and left stabilizing under the H2 environment for 30 min at 25 °C. Afterwards, the temperature was increased with 10 °C/min rate to 900 °C. The sample was kept at 900 °C under H2 environment for 10 min. Then, the reactive gas was changed from 90:10 H2:Argon to pure argon. The sample was then actively cooled to room temperature under Argon atmosphere to safely resume the measurement (i.e. avoiding explosive H2/O2 mixtures).Finally, XRD, N2-physisorption and TEM analysis were performed in the same instruments and conditions already described on the used catalyst (named “post”).CO2 methanation was carried out in a tailored-made setup at atmospheric pressure, using a cylindrical stainless-steel reactor (Hastelloy C276) of 40 cm in length, 15.8 mm of inner diameter and 2.8 mm of wall thickness, placed in the center of a cylindrical oven of 30 cm in length. Two thermocouples were used. One of them, which controlled the temperature of the oven, was placed in the center of the internal wall of the oven, in contact with the reactor. The other one, placed in the center of the reactor, was used to measure the real temperature achieved in the center of the catalyst (monolith or powder), providing the increment of temperature in the radial section. The feed consisting of a CO2:H2 mixture at the stoichiometric ratio of the reaction (10 % and 40 %, respectively) was balanced with N2 (50 %) using calibrated mass flow controllers (Brooks). Conversion curves vs temperature from 200 °C to 400 or 500 °C were performed in two different total flow rates (10 and 50 mL/min), keeping constant the feed composition. Outgoing gases were analyzed by an on-line GC (Varian CP-3800) equipped with an Agilent CP-Molsieve 5A, PoraPlot Q column and TCD detector. The catalysts (powders and monoliths) were placed in the center of the reactor using quartz wool, always loading c.a. 0.1 g of Ni/CeO2 catalyst. The powdered catalysts were sieved in the 125–250 μm range for the catalytic tests, according to the previous work carried out in our group [13]. Before catalytic tests, the catalysts were activated in situ with a heating rate of 5 °C/min in 100 mL/min of H2/N2 flow (25:75 volumetric ratio) at 400 °C for 2 h and then cooled down in N2.The stability of the catalysts in CO2 methanation in fluctuating conditions (changing the total flow rate between 10 and 50 mL/min to provide high and low conversion levels) was evaluated at 300 °C, which was found to be the temperature where the CO2 conversion rate is maximal in these operation conditions, according to the previous conversion vs temperature analysis. All the stability tests were carried out during 100 h, varying the two conditions several times.To elucidate the structural properties of the prepared materials, XRD measurements were carried out. Fig. 2 shows the diffractograms of the prepared samples once calcined. All the samples maintained the cubic fluorite type structure characteristic of CeO2 (Fm 3 ¯ m, JCPDS 34-0394). A close inspection of the diffraction line corresponding to the (111) crystallographic plane of CeO2 (Fig. 2b) indicates a small contraction of the Full Width at Half Maximum (FWHM) when the Ni was present. Such feature can be attributed to partial migration of the Ni2+ in the ceria structure [41]. It should be noted that this decrease of the lattice parameter (Table 1 ) when Ni is loaded on both ceria shapes (i.e. octahedral and nanorods), can be attributed to the smaller ionic radii of Ni2+ and Ce4+ (0.69 and 0.97 Å, respectively). Notably, the NiO phase is also recognizable (Fm 3 ¯ m, JCPDS 47-1049), particularly in the Ni/CeO2_oct sample. As plotted in Fig. 2c, the analysis of the diffractograms of calcined and activated catalysts (i.e. before and after reduction in H2/N2 flow 25:75 volumetric ratio at 400 °C for 2 h) evinces the reduction of NiO phase to Ni (Fm 3 ¯ m, JCPDS 04-0850). Table 1 also includes the CeO2 and Ni crystallite sizes estimated by the Scherrer’s Equation on the (111) crystallographic plane. Remarkably, the crystallite size of Ni and ceria on the reduced Ni/CeO2 rods reached values of 9.6 and 14.1 nm, respectively, which are significantly lower than those obtained on the Ni/CeO2 Oct (26.8 and 27.3 nm for Ni and CeO2 phases, respectively). However, it has to be pointed out that the peaks associated to Ni phase are small, decreasing the accuracy of Ni crystallite size calculation on the reduced catalysts. The textural properties obtained by N2-physisorption (Table 1), indicate that ceria nanorods exhibits a higher surface area than the octahedral samples with values of 53 and 32 m2/g, respectively. The increase in surface area was accompanied by a drop in the average pore size of the ceria support from 5 nm in the octahedral CeO2 to 2.6 nm in the nanorods, which are in line with previous reports [13,56]. Notably, the deposition of nickel catalyst on these supports did not affect the surface area as evidenced by the negligible change in BET surface area, pore sizes, and volumes.Similarly, XRD and N2-physisorption analysis of the structured samples on the monoliths were conducted in order to check the stability of the catalysts after impregnation process. As expected by the simple impregnation method used, the catalysts perfectly preserve their structural and textural properties (see supporting information, Figure S.1 and Table S.1).From the SEM images of the supports (Fig. 3 a and b) one can immediately recognize the different shapes of the commercial ceria with an octahedral-like shape and the synthesized ceria nanorods. The latter exhibited a size of c.a. 1 μm in length and only few nanometers in diameter. The SEM analysis of the Ni catalysts are identical to their respective supports, since Ni particles are undistinguishable (Fig. 3c and d).In the micrographs obtained by TEM (Fig. 4 ), the octahedral and nanorods ceria shapes are also distinguishable. Despite low contrast between Ni and Ce in TEM micrographs, identification and measurement of Ni particles have been attempted to estimate the Ni particle sizes distribution. Considering the notable dissimilarity of the Ni/CeO2 nanorods shape, the Ni particle size distribution in this sample is more reliable with 700 measurements, while only 132 measurements are available for the octahedral sample (Fig. 5 ). This analysis indicates Ni had an average particle size of c.a. 7 ± 4 nm in Ni/CeO2 nanorods. In contrast, the Ni/CeO2 octahedral catalyst had a wider particle size distribution, as evidenced by the large Ni particles of about 70 nm present in the sample, and where only over 55 % of the measured particles are in the 4–12 nm range. In this case, majority of Ni particles are averaged to c.a. 16 ± 13 nm. Detailed elemental mapping via energy dispersive X-ray spectroscopy (EDX) supported the previous observations regarding metal dispersion (Fig. 6 ). Here, it can be noted the highly heterogeneous distribution of Ni nanoparticles on the Ni/CeO2-Octahedral (Fig. 6a, Ni) as compared to the narrower distribution of Ni particles with smaller cluster size (Fig. 6b, Ni). Moreover, in the case of Ni/CeO2 nanorods, the averaged particle size (Fig. 5b and Table 2 ) is similar to that estimated by Scherrer calculation of the XRD Ni peak (see Table 1 above). However, in the case of the octahedral shaped catalyst, the Ni crystallite size detected and estimated by XRD (Table 1) is higher than that determined by TEM micrographs (Fig. 5a and Table 2). This is caused by the relatively broad particle size distribution on Ni/CeO2-Octahedral, as observed with TEM, combined with the fact that XRD is much more sensitive for larger particles. The relatively large particles therefore dominate the averaged particle size determined by line-broadening.Considering the average Ni particle size by TEM of 16 ± 13 nm and 7 ± 4 nm for Ni/CeO2 oct and Ni/CeO2 nanorods, respectively, Ni dispersion has been calculated according to the relationship between particle sizes and apparent dispersion described by Larsson [57]. Table 2 reports the estimated apparent Ni dispersion. As expected, higher dispersion was obtained for Ni on nanorods ceria shape. The Ni loadings according to XRF analysis reached values of 3.3 and 2.7 wt.% for Ni/CeO2 octahedral and nanorods, respectively. While these results indicate that both catalysts had similar metal loading, the resulting metal surface areas were different possible due to the differences in surface area and metal-support interaction [34,58–61].Conversion curves for CO2 methanation on activated Ni/CeO2 catalysts, octahedral and nanorods shapes in powders and monoliths structures, at 10 and 50 mL/min total flow rate (6 and 30 L h−1·gcat −1, respectively) are shown in Fig. 7 . The set temperature was controlled with a thermocouple inside the oven on the external wall of the reactor, while the real temperature inside the catalyst bed was ∼ 20 °C lower. This internal temperature was measured with a second thermocouple in the center of the catalytic bed or μ-monolith. Thus, the results shown in Fig. 7 indicate the temperature value inside the reactor. Here, one can note that the monolith samples showed temperatures several degrees higher than the powders and closer to the set point, even at similar conversion levels, at high values (T > 300 °C). The smaller temperature difference between the external reactor wall and the center of the catalyst bed can be associated with the enhanced heat transfer in the μ-monoliths. In addition, testing of the calcined μ-monolith without any catalyst confirmed that the metallic monolith has not catalytic activity for CO2 methanation at the reaction conditions herein employed.The conversion achieved as a function of temperature and space velocity, shown in Fig. 7, indicate that the inflection point of the conversion curve, where the variation of CO2 conversion (rate) with temperature is maximal, is around 300 °C at 6 L h−1 gcat −1, with c.a. 50–60 % of CO2 conversion (Fig. 7a). As expected, increasing the gas hourly space velocity to 30 L h−1·gcat −1 led to lower conversions (c.a. 20–30 %) (Fig. 7b).Based on these results, the stability tests were carried out at 300 °C for 100 h in order to study the catalyst behavior in the kinetic regime. Here, it is important to mention that the selectivity to methane was found in all cases to be around 90–99 %. Moreover, carbon balance was closed above 95 % in all cases during all the reaction time. Indeed, only in the tested points at 450–500 °C at 6 L h−1 gcat −1 on both samples (Fig. 7a), a small amount of CO was produced (maximal selectivity about 10 %, only found at 6 L h−1 gcat −1 in the 60–80 % range of CO2 conversion level). In addition, elemental analysis of the powdered samples carried out after stability tests indicated negligible carbon deposited even after c.a. 100 h of operation. The high selectivity of group VIII-X metals (e.g. Ni) towards methane in comparison to metals in group XI (e.g. Cu, Ag) can be rationalized in terms of the electronic structure of the metal center. Broadly speaking the as the center of the d-band of the metal is closer to the Fermi level the stronger the interaction of the adsorbates involved in the hydrogenation of carbon dioxide and carbon monoxides with the metal surface [62,63]. This results in the filling of the anti-bonding states (2p*) of the CO molecule via backdonation that weakens the internal bond of the molecule, facilitating CO bond dissociation [64,65]. In this context, metals in the group XI with fully occupied d-band weekly interact with the adsorbates as anti-bonding states between the metal atoms and the adsorbate are filled. This weak interaction in the case of Cu and Ag metals leads to the formation of η1(O)-CO bonding to the metal surface, while in the case of the Ni, Ru and Pt the η2(CO) surface species are favored [66,67]. This results in the formation of CHxO products in the case of Cu and Ag catalysts while in the case of Ni, Ru, Pt the dissociation into C* and O* species leads to methane formation in the presence of hydrogen. In the case of Ni supported on CeO2 it is believed that COx* species can be stabilized on the oxygen vacancies on the support, which favors the activation of carbon dioxide in the presence of Ni [56,68]. In this sense, it is not surprising that on both catalysts (i.e. Ni-CeO2 nanorods and octahedral) the selectivity observed was 95–99 %.In order to analyze the activity of the prepared catalysts and their stability under fluctuating conditions (i.e. varying the conversion level by only changing the total flow rate) we conducted long-term stability studies for periods of at least one week per catalyst. The complete stability tests (100 h) are reported in the supporting information (Figure S.2). Fig. 8 presents the performances with several cycles (high and low conversion) during 50 h as CO2 converted per total amount of Ni, discarding thereby the effect of slight variation on the amount of catalyst loading on each monolith. As it is shown in Fig. 7 on these catalysts the conversion of CO2 at 300 °C and 6 and 30 L h−1 gcat −1 varied in the ranges of 50–60 % and 20–30 %, respectively. Since these catalysts are operating at relatively similar levels of conversion and far from equilibrium limitations it is possible to compare their initial activity at low and high space velocities. Fig. 9 plots the activities at both space velocities to facilitate the analysis of metal oxide support (nanorods vs. octahedral ceria) and structuration (powered vs. μ-monoliths) at the beginning of the reaction, where catalyst deactivation effects are minimal.Notably, nanorods shaped catalysts showed higher activity than the octahedral catalysts on both powdered and μ-monolithic forms. This is in good agreement with the higher BET surface area obtained on nanorods and the well-dispersed Ni clusters indicated by TEM analysis. This is also supported by the disappearance of the Ni peak in the reduced XRD on Ni/CeO2 nanorods. Moreover, as it was discussed previously, some migration of Ni2+ into ceria lattice cannot be discarded, which can stabilize Ni as NiCeO3 spinel. In previous studies similar observations have been reported. Konsolakis et al. [15] showed that metal cations can be stabilized as spinel species in CeO2. For instance, Du et al. [33] studied the morphology dependence of the catalytic activity of Ni/CeO2 for CO 2 methanation. The authors observed higher activity with nanorods shaped catalysts than with nanopolyhedral structures. This higher activity was ascribed to stronger anchoring of Ni nanoparticles providing better metal dispersions. One can anticipate that higher affinity between the Ni clusters and metal oxide support should also improve the stability of the catalyst to metal sintering. In this line, our studies on the long-term stability of the catalysts indicate that the powdered octahedral shaped catalyst (blue line) suffered fast deactivation from the beginning. In sharp contrast, nanorods shaped sample (green line) showed stable activity over periods of ∼50 h of operation under fluctuating operation (Fig. 8). As mentioned earlier, CeO2 nanorods expose a large fraction of (111) facets [56,69], which are richer in defects providing a large number of oxygen vacancies with high ion mobility. This in turn can increase the metal “wettability” of the surface leading to more robust catalysts while enhancing the activity by pre-activating the CO2 molecule. In addition, the higher reducibility on nanorods-shaped ceria according to reported H2-TPR analysis [56,69] and the performed H2-TGA (see Fig. S4), supports the observed higher catalytic activity of this Ni/CeO2 nanorods sample. Here, it can be observed that CeO2 nanorods undergo a significant weight loss (c.a. 7.5 wt. %) when compared to the CeO2 octahedral (c.a. 2.5 wt. %) after reducing the catalyst at 900 °C in H2. Addition of nickel facilitated the reduction of octahedral the CeO2 leading to a weight loss of c.a. 4 wt. %, while in the case of the nanorods the extent of reduction remained constant, reaching a value of c.a. 7.4 wt. %. Similar results were observed by Gong et al. [31] during CO2 methanation. In that case, the authors assigned the higher CO2 uptake and activity of Ni supported nanorods ceria to the larger fraction and mobility of oxygen vacancies as analyzed by in situ IR and DRIFTS.Notably, structuring the octahedral Ni/CeO2 sample clearly improved stability too (orange line vs blue line). Deactivation of the powdered sample from the beginning is in good agreement with observations by Ocampo et al. [37], Zhou et al. [39] and Iglesias et al. [28]. In contrast to previous work on CO2 methanation using Ni/CeO2 catalysts, where high Ni loadings ranging from 10 to 26 wt.% yielded good stability at conversion levels close to thermodynamic equilibrium [25,26,30], our work demonstrates that Ni/CeO2 on octahedral ceria powder easily deactivates under harsh reaction environments, such those exerted during dynamic reactor operation. These results would suggest that it is possible to mitigate catalyst deactivation by supporting the catalyst on a metallic μ-monolith, thanks to the highly efficient heat diffusion inside the reactor.Ni/CeO2 nanorods not only provides higher activity due to the nano-shaped ceria, as discussed above, it also inhibits Ni sintering and deactivation [33], showing good stability under stressful and fluctuating conditions. This is supported by post-reaction TEM analysis of the powder samples (Fig. 10 and Table 3 ). Nanorods-shaped catalyst hinders the sintering compared to the octahedral sample, since the averaged Ni particle sizes increases during the stability tests from 16 nm to 23 nm in the case of Ni/CeO2 oct, but only from 7 nm to 9 nm for the nanorods-shaped catalyst. Moreover, as is shown in Fig. 10, the particle size distribution becomes flatter, increasing the relative frequency of particle sizes in 15–30 nm range.In addition, XRD analysis shown the deactivation by sintering, where the peak associated to Ni phase increased (see supporting information, Fig. S.3). The calculation of crystallite size by Scherrer equation (summarized in Table 4 ) supports the higher sintering of Ni particles in the octahedral catalyst. As it was discusses above, the XRD primarily detects large clusters. However, the increase of the Ni particles size follows the same trend. Thus, in Ni/CeO2 nanorods, Ni particles increased from 7 to 9 nm by TEM and from 9.6–13.2 by XRD (factor of 1.3–1.4), while in Ni/CeO2 octahedral, averaged Ni particles increased from 16 to 23 nm by TEM and from 26.8–50.5 by XRD (factor of 1.4–1.9). On the other hand, N2-physisorption analysis demonstrates that the catalyst keeps its textural properties during the catalytic tests (summarized in Table 4).Hence, in the case of Ni/CeO2 nanorods, structuring by deposition on the monolith does not further improve stability, as it is observed in Fig. 8, since the nanorods support already significantly hinders the catalyst deactivation by Ni sintering. However, and remarkably at higher space velocity, the activity of nanorods supported on monolith is increased compared to the powder sample, indicating that monolithic structure improves the contact between catalytic surface and reactant flow, as demonstrated by Fukuhara and co-workers [42,43]. Our estimations of the coating layer indicate that for both catalysts, Ni/CeO2 nanorods and Ni/CeO2 octahedral, the thickness of the catalyst layer is around 2 μm, which can explain the fast rates of heat and mass transport in the monoliths.Ni/CeO2 catalyst for CO2 methanation exhibits good activity and high selectivity to methane (above 95 %). However, in stressful and fluctuating conditions, it undergoes fast deactivation. Two approaches were developed in order to improve its stability, including: (1) synthesis of nanorods-shaped ceria to support the Ni and (2) catalyst structuring on metallic multichannels μ-monolith. It was observed that nanorods shaped catalysts provided higher activity, attributed to the enhancement of formation and mobility of oxygen vacancies and the increase of Ni-support interaction and dispersion. Moreover, this nanoshaped catalyst already exhibited high stability in the powdered form, indicating that nanorods can delay Ni sintering. On the other hand, supporting Ni/CeO2 octahedral powder catalyst on the monolith provided enhanced stability during fluctuating conditions, compared to the same catalyst in fixed bed operation. Moreover, catalyst structuring on the μ-monolith resulted in slightly higher catalytic activity than the powder form, indicating the relevance of efficient heat and mass transfer in the methanation reaction. Nuria García-Moncada: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Writing - original draft, Writing - review & editing. Juan Carlos Navarro: Investigation, Formal analysis, Writing - review & editing. José Antonio Odriozola: Conceptualization, Writing - review & editing. Leon Lefferts: Conceptualization, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Validation, Writing - review & editing. Jimmy A. Faria: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Validation, Visualization, Writing - original draft, Writing - review & editing.The authors report no declarations of interestThe authors acknowledge the financial support from ADEM, a green deal in energy materials program of the Ministry of Economic Affairs of The Netherlands (www.adem-innovationlab.nl). We acknowledge Ir. Ties Lubbers from University of Twente for the support in the characterization of the catalysts and relevant discussions on the physico-chemical properties of these nano-structured materials.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2021.02.014.The following is Supplementary data to this article:

Coupling inherently fluctuating renewable feedstocks to highly exothermic catalytic processes, such as CO2 methanation, is a major challenge as large thermal swings occurring during ON- and OFF- cycles can irreversible deactivate the catalyst via metal sintering and pore collapsing. Here, we report a highly stable and active Ni catalyst supported on CeO2 nanorods that can outperform the commercial CeO2 (octahedral) counterpart during CO2 methanation at variable reaction conditions in both powdered and μ-monolith configurations. The long-term stability tests were carried out in the kinetic regime, at the temperature of maximal rate (300 °C) using fluctuating gas hourly space velocities that varied between 6 and 30 L h−1·gcat −1. Detailed catalyst characterization by μ-XRF revealed that similar Ni loadings were achieved on nanorods and octahedral CeO2 (c.a. 2.7 and 3.3 wt. %, respectively). Notably, XRD, SEM, and HR-TEM-EDX analysis indicated that on CeO2 nanorods smaller Ni-Clusters with a narrow particle size distribution were obtained (∼ 7 ± 4 nm) when compared to octahedral CeO2 (∼ 16 ± 13 nm). The fast deactivation observed on Ni loaded on commercial CeO2 (octahedral) was prevented by structuring the reactor bed on μ-monoliths and supporting the Ni catalyst on CeO2 nanorods. FeCrAlloy® sheets were used to manufacture a multichannel μ-monolith of 2 cm in length and 1.58 cm in diameter, with a cell density of 2004 cpsi. Detailed catalyst testing revealed that powdered and structured Ni/CeO2 nanorods achieved the highest reaction rates, c.a. 5.5 and 6.2 mmol CO2 min−1·gNi −1 at 30 L h−1·gcat −1 and 300 °C, respectively, with negligible deactivation even after 90 h of fluctuating operation.

Electrochemical water splitting has been considered as a promising technology for sustainable production of clean and efficient energy by converting the electrical energy from intermittent-renewable resources such as solar and wind energies. Electrochemical water splitting provides simultaneous hydrogen and oxygen production according to two couple reactions. The hydrogen evolution reaction, HER (acidic medium: 2H+ + 2e− → H2 and alkaline medium: 2H2O + 2e− → H2 + 2OH−) is carried out with the involvement of two electrons at the cathode, while the OER (acidic medium: 2H2O → O2 + 4H+ + 4e− and alkaline medium: 4OH− → O2 + 2H2O + 4e−) occurs with the involvement of four electrons at the anode [1,2]. The greater overpotential in OER makes the reaction kinetically and energetically disadvantageous without an adequate catalyst. Typically, the catalysts used in the reaction contain expensive noble metals such as Ir or Ru for OER and Pt for HER, which makes the process economically not-feasible for commercial applications [3]. Recently, in the field of OER in an alkaline environment, remarkable results for the replacement of the noble metals in the catalysts by abundant, cheap, stable, and operating at low overpotential (η) transition metals have been achieved [4] and among them, Fe, Ni and Ce are promising choice [5].Generally, the mechanism of the OER reaction in an alkaline environment is considered as a step-wise process as follows: (1.1) OH− + Cat✴ → HO✴ + e− (1.2) HO✴ + OH− → O✴ + H2O + e− (1.3) O✴ + OH− → ✴OOH + e− (1.4) HOO✴ + OH− → Cat✴ + O2 + H2O + e− Where (✴) represents an active surface site of the catalyst (Cat) [6–8]. In the reactions shown above, the steps that involve a charge transfer are of crucial importance, because their mechanism is directly dependent on the applied potential. The diffusion of any species or the surface reactions are weakly dependent on the applied potential. It is interesting to note that the direct recombination between oxygen atoms to form O2 in reaction step (1.3) is not taken into consideration due to the large activation barrier that is expected from this process, leading us to think of a more probable associative mechanism to anode using the HOO✴ surface intermediate [8]. As previously anticipated, hydrogen production by electrolysis of water is, however, associated with substantial energy losses. Most of the overpotential giving rise to those losses is related to the electrochemical processes at the anode, where O2 evolution takes place [9]. So the total overpotential to conduct an OER reaction could be assimilated as the sum of three determining factors, respectively: changes in Gibbs free energy, electronic conductivity and electro-catalytic active surface area, as expressed in Equation (2); (2) ηtot = Eapp – E° = ηΔG + ηCon + ηSurf Where ηtot is the total overpotential for the catalyst to reach the given current density. Eapp is the potential actually applied, E° represents the thermodynamic equilibrium potential of water oxidation (2H2O (l) → O2 (g) + 4H+ (aq) + 4e− E° = 1.23 V), ηCon is a value strictly related to the resistance of the system and can be idealized as the potential drop between the active sites in which the reaction takes place and the external circuit. Knowing therefore the four pivotal coupled reactions of the OER and the intrinsic factors that mark the total overpotential of the catalyst, it is easy to imagine how fundamental is the optimization of the ΔG of the catalyst (and hence its intrinsic electronic structure), as the value of the overpotential ηΔG is the only one factor strictly correlated to the kinetics of the four reaction steps. The main purpose in the "evolution of a catalyst" will therefore be to regulate the ΔG of the reaction by modifying the electron transport and the active surface area to intrinsically increase the catalytic activity and decrease the overpotential.In recent years, research has been focused on the use of Fe, Ni, and Ce based catalysts for water splitting because of their lower cost and high availability. Metals like, Fe and Ni easily form NiFe alloys [10]. Besides, the Fe3+ and Ni2+ cations could be organized in NiFe2O4, spinel structure, where Ni2+ ions occupy one-eighth of the tetrahedral positions and Fe3+ ones occupy half of the octahedral ones. It was reported [11] that this NiFe oxide compound outperforms the individual Fe3O4 spinel and NiO in OER electro-catalytic activity [12]. In addition, Fe3+ and Ni2+ cations could be included in layered double hydroxides (NiFe-LDHs). They are ionic crystals composed of stratified positively charged structures and anionic counterpart [13]. This unequal structure provides an opportunity by simple regulation of the size and morphology of the crystallites to increase the intrinsic electrical conductivity and reduce the reaction barrier, which results in a decrease of the OER overpotential [14,15]. Xiang et al. observed that the surface of NiFe-LDH having an intermediate hydroxyl layer, possessed higher activity in OER than a normal NiFe-LDH. He proposed a synergistic effect between the NiFe-LDH and the hydroxyl layer, which increases the absorption of the OH intermediates [16]. Hollow-structured NiFe-LDH microsphere was prepared by Zhong et al. using a template-free synthesis [17].Ceria has noteworthy properties due to the dynamic in the cerium ions valence states and generation of oxygen defects as a result of temperature variation, size effect, doping with metal ions or application of electric potential [18,19]. The formation of oxygen defects is accompanied with the localization of the electrons on Ce 4f orbital leading to the formation of two Ce3+ ions. Although it is easy to imagine that the Ce cation closest to the location of the oxygen defect is actually the one that will reduce, in reality a computational study has shown the Ce3+ resulting from the oxygen vacancy is actually located in a location far from the neo-generated defect [20]. This process is possible thanks to the ability of the cerium atom to adjust its electronic configuration in a flexible way to environmental variations. Thus, the oxygen storage capacity of cerium oxide is closely related to the quantum effect of electrons located on the Ce 4f state, which provides excellent electronic transfer capabilities due to the reversible Ce3+-Ce4+ redox cycle [21,22].In our recent study [7] we demonstrated increase in the specific surface area and facile electron transfer by doping of CeO2 with Fe3+ ions. This effect was most pronounced for the samples with Fe:Ce molar ratio of 1:9 and 3:7. The physicochemical analyses demonstrated formation of Ce–O–Fe interface layer and stabilization of finely dispersed hematite- and ceria-like entities in its vicinity. The modification of the binary ceria-iron oxide materials with nickel [23] provided the formation of finely dispersed NiO nanoparticles nearby the Fe–O–Ce defects and the existing in their vicinity hematite-like structures. It was also reported that the presence of Ni in CeO2–Fe2O3 catalysts facilitated the formation of χ-Fe2C5 carbides during the methanol decomposition with the formation of carbon nanofibers (CNFs) which incorporate the catalyst nanoparticles.CNFs are classified among carbonaceous materials with high conductivity due to their predominantly graphitic character [6]. It has already been proven that the structural characteristics of these materials can increase the performance in many applications, in which electrical conductivity plays a role in providing them, thus crediting their use as potential elements in electrocatalytic processes [6].In the light of the above, it has been estimated that the dynamism of the class of catalysts in Fe–Ce–Ni, and species derived from them, could be very interesting for the OER reaction. In fact, this study has the double purpose of: (I) Study the evolution of the catalyst in OER in an alkaline environment (1 M KOH) and the sudden lowering of the overpotential by making structural and electro-conductive changes, starting from two different molar ratios of mesoporous catalysts of mixed oxides in Fe–Ce–Ni, passing through the same catalysts in reduced form (hence the presence of the NiFe alloy), and ending with a carbon-coating process obtained by vapor phase decomposition of methanol and the formation of metal nanoparticles encapsulated within carbon nanofibers. (II) Propose a facile and innovative possibility of carbon-coating with low-temperature methanol treatment, which with a targeted study, could allow a reuse of the mesoporous mixed metal oxide catalysts, mainly metal-carbon containing phases, into promising catalysts from electro-catalytic point of view. Such reuse would lead to greater sustainability of the hydrogen economy, as the same catalyst could be used for the production of hydrogen by two different techniques, using low-cost metals and decreasing the costs for recycling metals and manufacturing additional catalysts. Study the evolution of the catalyst in OER in an alkaline environment (1 M KOH) and the sudden lowering of the overpotential by making structural and electro-conductive changes, starting from two different molar ratios of mesoporous catalysts of mixed oxides in Fe–Ce–Ni, passing through the same catalysts in reduced form (hence the presence of the NiFe alloy), and ending with a carbon-coating process obtained by vapor phase decomposition of methanol and the formation of metal nanoparticles encapsulated within carbon nanofibers.Propose a facile and innovative possibility of carbon-coating with low-temperature methanol treatment, which with a targeted study, could allow a reuse of the mesoporous mixed metal oxide catalysts, mainly metal-carbon containing phases, into promising catalysts from electro-catalytic point of view. Such reuse would lead to greater sustainability of the hydrogen economy, as the same catalyst could be used for the production of hydrogen by two different techniques, using low-cost metals and decreasing the costs for recycling metals and manufacturing additional catalysts.Two mesoporous Fe–Ce oxides with Fe/Ce molar ratio of 5:5 and 9:1 were prepared by template-assisted hydrothermal technique as described in Ref. [7]. For this purpose, CeCl3 . 7H2O and FeCl3 . 6H2O were used as precursors of Ce and Fe, respectively, and cetyltrimethylammonium bromide (CTAB) was used as a template. An aqueous solution of the metal salts mentioned was added drop by drop to the aqueous solution of the template under magnetic stirring at room temperature, subsequently the temperature was increased up to 323 K and left to react for 30 min. Then, 40 mL of an aqueous solution of NH4OH at 12.5% (up to pH of about 10) were added drop by drop to cause the precipitation reaction of the metal hydroxides in the solution. The emulsion was kept under magnetic stirring at 323 K for the whole night. The hydrothermal treatment was carried out at 373 K for 24 h. The solid obtained was calcinated for 10 h at 573 K.2 g of the obtained mixed oxides were loaded with nickel (Fe/Ni molar ratio 2:1) by incipient wetness impregnation with 1 ml aqueous solution of Ni(NO3)2 . 6H2O. After drying for 24 h at room temperature, the samples were calcinated for 3 h at 773 K in air. The obtained composites were denoted as 5Fe5Ce_Ni and 9Fe1Ce_Ni, respectively.250 mg of the catalysts were subjected to reduction in a hydrogen flow (flow rate of 35 mL/min) at 773 K for 2 h. Thus obtained materials were denoted as xFeyCe_Ni_Alloy, where x/y was the Fe/Ce molar ratio.The encapsulation of the reduced mixed metal oxide nanoparticles inside the carbon nanofibers was obtained by decomposing the methanol on the surface of the catalysts, inside a flow type reactor using a mixture of argon as carrier gas (with a flow rate of 30 mL/min) and methanol vapor extracted by bubbling the carrier gas inside a saturator at 273 K. Typically, 200 mg of the xFeyCe_Ni_Alloy catalyst were pre-treated insitu in argon at 373 K for 20 min and the temperature was increased up to 773 K (rate of 20 K/min). Then, the methanol mixture (methanol partial pressure of 1.57 KPa) was introduced into the reactor for 2 h. The input and output flow composition was periodically analyzed on-line by a SCION 456-GC gas chromatograph, equipped with flame ionization and thermo-conductivity detectors and PORAPAC-Q column. Absolute calibration method and carbon based material balance were used for the elucidation of the conversion and products distribution during the methanol decomposition. The products selectivity was calculated as Si = Yi/X*100, where Si and Yi were the selectivity and the yield of the “i” product and X was the conversion. As for the carbon mass balance, the correction factor from the solid fraction of carbon acquired by the catalysts during the cooking process was also introduced, and determined by elemental analysis (EA). Thus obtained modifications were denoted as xFeyCe_Ni_@C. The presented above synthetic procedures are illustrated in Scheme 1 .Powder X-ray diffraction patterns were collected on a Bruker D8 Advance diffractometer with Cu Kα radiation and a LynxEye detector with constant step of 0.02° 2θ and counting time of 17.5 s per step. Mean crystallite sizes were determined by Topas-4.2 software.The Mössbauer spectra were recorded at room temperature by Wissel (Wissenschaftliche Elektronik GmbH, Germany) electromechanical spectrometer working in a constant acceleration mode. A57Co/Rh (activity ≈ 10 mCi) source and α Fe standard were used. The spectra were fitted using WinNormos software.X-ray photoelectron measurements have been carried out on the ESCALAB MkII (VG Scientific, now Thermo Scientific) electron spectrometer with a base pressure in the analysis chamber of 5 × 10−10 mbar (9 × 10−8 mbar during the measurements), equipped with twin anode MgKα/AlKα non-monochromated X-ray source that used excitation energies of 1253.6 and 1486.6 eV, respectively. The measurements are provided only with AlKα non-monochromated X-ray source (1486.6 eV). The pass energy of the hemispherical analyzer was 20 eV, because of their nature and lower signal for Ce3d, Ni2p and Fe2p, 50 eV pass energy was used. The instrumental resolution measured as the full width at a half maximum (FWHM) of the Ag3d5/2, photoelectron peak is about 1 eV. The energy scale has been calibrated by normalizing the C1s line of adventitious hydrocarbons to 285.0 eV for electrostatic sample charging. The data was analyzed by SpecsLab2 CasaXPS software (Casa Software Ltd). The processing of the measured spectra includes a subtraction of X-ray satellites and Shirley-type background. The peak positions and areas are evaluated by a symmetrical Gaussian-Lorentzian curve fitting. The relative concentrations of the different chemical species are determined based on normalization of the peak areas to their photoionization cross-sections, calculated by Scofield.Raman spectra were recorded by using Raman Microscope Senterra II (Bruker). Samples were placed onto glass (approximately 10 mg) and analyzed using the vertical 20× objective in an 180° backscattering arrangement. The Raman spectrometer parameters used to analyze the samples include: 532 nm laser wavelength and an exposure time of 100 s, resolution was 4 cm−1 for all samples, laser power was 6.5 mW.FTIR spectra were recorded on a Bruker Vector 22 spectrometer with a resolution of 1–2 cm−1, in the region 4000-400 cm−1, accumulating 128–220 scans using pellets produced from approximately 2 mg of sample diluted in 100 mg KBr.Low-temperature nitrogen physisorption was studied by a Quantachrome Instruments NOVA 1200e (USA) apparatus. The specific surface area was determined from Brunauer Emmett Teller (BET) equation, the total pore volume was obtained at a relative pressure of about 0.99, and the pore size distribution was obtained by using Non-Local Density Functional Theory (NLDFT) method and the equilibrium model for cylindrical pores.TEM analysis was performed by means of JEOL JEM 2100 high resolution transmission electron microscope at accelerating voltage 200 kV. Selected area electron diffraction (SAED) mode was applied for diffraction patterns accumulation and HRTEM imaging was used for lattice fringes registration.JEOL 2100 XEDS: Oxford Instruments, X-MAXN 80 T CCD Camera ORIUS 1000, 11 Mp, GATAN at accelerating voltage of 200 kV was applied for the elemental composition determination and mapping. Samples for TEM investigations are prepared as the suspension of the corresponding sample was dropped on standard Cu TEM grids and then dried in pure ambience.The elemental analysis (EA) was performed on Vario Macro Cube (Elementar Analyzensysteme GmbH) equipment for determination of C, H, N, S. The combustion temperature for the system was 1150 °C in an oxidizing atmosphere to form gaseous reaction products: CO2, H2O, N2, NOX, SO2 and SO3. The individual gases are transported by a carrier gas to be measured by a thermal conductivity detector (TCD).The temperature programmed oxidation (TPO) was performed in a Netzsch STA 449-F5 “Jupiter” thermos-microbalance. Around 10–15 mg of sample was loaded in the thermogravimetry (TG) instrument and heated from 30 °C to 900 °C, with an isothermal step at 400 °C for 20 min, while during the all process the temperature increase was performed with a rate of 10 °C/min under a flow of synthetic air (80 mL/min, purity 4.7 AGA).All electrochemical measurements were performed in a three-electrode system controlled by a potentiostat/galvanostat AUTOLAB PGSTAT302 N at room temperature.A rotating ring disk electrode (RRDE), with a Pt ring and a glassy carbon disk of 5 mm of diameter (0.196 cm2) was used as working electrode (WE), while reversible hydrogen electrode (RHE) installed in a Luggin capillary as a reference electrode and a glassy carbon rod for the counter electrode (CE) were used. Before the electrochemical test, the working electrode (RRDE) was polished with alumina slurry. The ink was grafted onto the RRDE maintaining the following conditions: ink concentration of 10 mg/mL, Nafion® 15 wt% in catalytic layer (CL) and total catalyst loading on WE of 1018 μg/cm2. For this purpose, the ink was prepared by mixing a quantity between 6 and 9 mg of catalyst with the respective quantity in microliters of a 5% Nafion® solution (Sigma Aldrich) and about 0.6 mL of a water/isopropanol solution (3:1). The mixed solution was sonicated for 30 min and then used as catalyst ink. Then, 20 μL catalyst ink was drip-evenly applied to the disk electrode surface of the RRDE (in 4 aliquots of 5 μL each).N2 (99.99% Air Liquide) was employed to deaerate all solutions. Before electrocatalytic studies, all composites were submitted to activation process based on 50 cyclic voltammograms (CVs) between 0.05 and 1.1 V vs. RHE, at a scan rate of 0.1 V/s in the deaerated supporting electrolyte (1 M KOH). After that, a blank voltammetry was recorded in the same conditions with a scan rate of 0.02 V/s. In order to determine the activity of the catalysts in OER, a linear sweep voltammetric curve between 0.7 and 1.8 V vs. RHE (positive going scan) was recorded at 0.005 V/s and a rotation speed of 1800 rpm. All current density values in this work were referred to the geometric area of the working electrode.XRD patterns of the initial Ni–Fe–Ce oxides and their modifications are shown in Fig. 1 A and B.The reflections at 2θ = 28.5°, 33.1°, 47.5°, 56.3°, 59.0°, 69.7°, 76.6°, 76.7° and 79.1° in the patterns of the mixed 5Fe5Ce_Ni and 9Fe1Ce_Ni oxides are associated with the lattice planes (111), (200), (220), (311), (222), (400), (331), and (420) of the fluorite-like cubic ceria, respectively (JCPDS 65–5923). The presence of reflections at 2θ = 24.1°, 33.1°, 35.6°, 40.9°, 49.4°, 54°, 57.6°, 64.2°, 72.2°, 75.3° are attributed to the crystallographic planes (012), (104), (110), (113), (024), (116), (018), (214), (1 0 10) and (220), respectively, of rhombohedral hematite (JCPDS 33–0664). The reflections at 2θ = 37.2°, 43.2° and 62.7° are assigned to the crystallographic planes (111), (200) and (220) of NiO (JCPDS 47–1049), respectively. The cerianite reflections are broader and less pronounced for 9Fe1Ce_Ni probably due to the lower content of highly dispersed ceria [7]. Besides, the increase of the intensity and the decrease of the width of the hematite reflections for this sample indicate the segregation of a significant portion of larger α-Fe2O3 nanoparticles (Table 1 , Fig, 1B). After the treatment in hydrogen atmosphere (Fig. 1A–B, samples 5Fe5Ce_Ni_Alloy and 9Fe1Ce_Ni_Alloy), the reflections of cerianite are still distinguished. The observed slight increase in the ceria unit cell parameters after the reduction (Table 1) could be due to the release of the incorporated in the ceria lattice iron ions and generation of additional oxygen vacancies in it. The reduction transformations of the parent mixed oxides are also confirmed with the appearance of additional reflections at 2θ = 43.7°, 50.8°, 74.6° and 2θ = 44.5°, 51.6°, 76.3° typical of Ni–Fe alloys (JCPDS 38–0419) and metallic Ni (JCPDS 01-071-4655), respectively.Generally, the crystallite size of the metal phases is below 35 nm (Table 1) indicating that ceria hinders their agglomeration [23].The carbon coating process, carried out on the reduced catalysts (paragraph 2.2, Fig. 1A–B in black), leads to the appearance of reflections at 2θ = 37.8°, 39.8°, 40.7°, 42.8°, 45° and 46° (in the short angle magnification in Fig. 1A) of FeC3 (JCPDS 35–0772) and reflections at 2θ = 40.9°, 43.3°, 44.2°, 45°, 46.5°, 50°, 50.3°, attributable to Fe5C2 (JCPDS 51–0997). The carbides are in coexistence with NiFe alloy and Ni0 nanoparticles, while the reflection at 2θ = 26.5° in both samples (5Fe5Ce_Ni_@C and 9Fe1Ce_Ni_@C) are associated with the (002) plane of graphite-2H (JCPDS 41–1487).In order to assess the textural changes in the specimens under investigation, N2-physissorption measurements were carried out. The absorption-desorption curves, shown in Figure S1-A and B, can be classified as type IV-a isotherms, typical of mesoporous solids (according to the IUPAC [24]), for both catalyst series (5Fe5Ce_Ni-based and 9Fe1Ce_Ni-based). The hysteresis loop, on the other hand, can be classified as type H3. The absence of sharp step-downs in the desorption curve suggests a cylinder-like pores with shrinkage-free pore-necks. As can be seen from the data obtained from the XRD analysis (Table 1), the reduction process of the metal oxides of nickel and iron leads to the formation of metal alloys and CeO2 with an overall reduction in the particle size. This process inevitably affects the BET surface area of the samples in oxidized and reduced state. The sample 9Fe1Ce_Ni_Alloy possesses higher surface area compared to its analogue before the reduction due to the larger exposed surface area given by particles of reduced nanometer size. Such effect is not observed for the 5Fe5Ce_Ni-based materials. It could be assigned to the higher amount of ceria in them, which does not take part in the reduction reaction and hinders the sintering of the metal nanoparticles. The growth of the carbon nanofibers around the nanoparticles after the treatment in methanol causes a predictable increase in the specific surface area of the samples. The significantly higher SBET for 5Fe5Ce_Ni_@C as compared to 9Fe1Ce_Ni_@C it may be due to a greater growth of the carbon component, combined with the development of a significant portion of micropores (Table 2 and Figure S2-E and F).Moessbauer spectroscopy was used to analyze in detail the energy levels of the Fe nuclei contained in the samples and to obtain precise information on the ferrous phases. In Fig. 2 , the Moessbauer spectra are shown and the parameters of the samples under study as: isomer shift (δ), quadrupole splitting (Δ), quadrupole shift (2ε), hyperfine field (Bhf), full width at half-maxima (Γexp) and the relative weight of each component (G) obtained from the least squares fitting, are summarized in Table 3 .The Moessbauer spectra of the parent mixed oxides are well-fitted with sextets and doublets. The parameters of the sextets are attributable to α- Fe2O3 with average crystallite size above 10–12 nm. The doublets possess δ = 0.35 mm/s and relatively high value of quadrupole splitting (around 0.9 mm/s), which could be assigned to Fe3+ in different environment. In accordance with the XRD data (Fig. 1, Table 1) they probably belong to ultra-finely dispersed hematite-like entities and Fe3+ ions, inserted in the ceria lattice. The sextet part in the spectra is larger for 9Fe1Ce_Ni, which corresponds to the assumption done on the base of the XRD analyses (Fig. 1, Table 1) for the segregation of bigger portion of larger hematite particles.The spectra of the reduced samples represent superimposition of two sextets (Sx1 and Sx2) and one singlet (Sn) with isomer shift about 0 mm/s. The Sx1 has a higher magnetic field and narrower lines and can be assumed as (Ni, Fe)-alloy with a body centered cubic cell of kamacite (α-(Ni, Fe) alloy, bcc) [25]. The Sx2 with a lower magnetic field and wider lines can be attributed to (Ni, Fe) alloy with a face centered cubic cell of highly distorted taenite (γ-(Ni, Fe)-alloy, fcc). The Sn could be attributed to γ-(Ni, Fe) alloy with low nickel content [26,27].The proportion of each component varies with the composition of the parent materials. Predominantly α-(Ni, Fe) alloy and negligible amount of γ-(Ni, Fe) alloy with less Ni content are registered after the reduction of 9Fe1Ce_Ni, while γ-(Ni, Fe) alloy is mainly detected when ceria is doped with lower content of Fe and Ni (5Fe5Ce_Ni). Five additional sextets appear after the treatment of the samples under the methanol decomposition medium. The two sextets (Sx3, Sx4) with isomer shift of around 0.18 mm/s and Bhf ≈ 20 T correspond to Fe3C, while the Sx5, Sx6 and Sx7 with δ ≈ 0.20 mm/s are typical of Fe5C2 carbide [28,29]. The relative part of Sx1 and Sx2, belonging to α-(Ni, Fe) and γ-(Ni, Fe) alloys, respectively, is significantly bigger for 5Fe5Ce_Ni_@C, indicating lower degree of carbides formation, as was also assumed on the base of the XRD analyses (Table 1, Fig. 1A). Note, that here small doublet peaks are also distinguished, probably due to the partial preservation of Fe3+ included into the Ce–O–Fe interface. Just the opposite, the higher amount of Fe and Ni in the sample (9Fe1Ce_Ni_@C) promotes the transformation of all iron phases predominantly to Fe5C2 carbide under the reaction medium.In order to visualize the morphology and microstructure of the samples, Transmission Electron Microscopy (TEM) was used. In Figures S3-S4-S5-S6-S7, the images at different magnifications of the samples in the mixed metal oxides are presented. For 5Fe5Ce_Ni (Figures S3-A and S8-A), aggregates, consisting of almost spherical particles with average size of 20–33 nm, which are surrounded by a multitude of smaller particles (7.5–12.4 nm) are well-distinguished. At higher magnification (Figs. S3–B) the porous texture of the clusters is detectable. The HR-TEM images (Figs. S3–C) shows presence of particles with interplanar distances of 2.7 Å, which could be assigned to the plane (200) of cubic CeO2 (PDF 96-434-3162). In addition, particles with d-spacing of 2.22 Å and 2.41 Å, attributable to the (−3 1 3) plane of Fe2O3 (PDF 96-210-8029) and (222) plane of cubic NiO (PDF 96-901-3981) are visible. The presence of CeO2 and NiO is also confirmed by the SAED analysis (Figs. S3–D) (Fig. 1).Larger nanoparticles with average crystallite size of 44 nm are well seen in the TEM images of 9Fe1Ce_Ni (Figs. S4–A and Figs. S8–B). The high resolution image (Figs. S4–C) exhibits interplanar distances of 2.08 Å and 2.68 Å, which correspond to planes (400) and (104) of NiO (PDF 96-901-3981) and hematite (PDF 96-901-5504), respectively. The presence of cubic NiO is also confirmed by SEAD pattern (Figs. S4–D).TEM images of the samples 5Fe5Ce_Ni_Alloy and 9Fe1Ce_Ni_Alloy are shown in Figs. S5 and S6. Polygonal particles with average crystallite size of 16.4 and 25.6 nm, respectively (Figure S8-C and D), are well distinguished. The SAED images of both samples are rather similar and demonstrate co-existence of ceria and metallic Ni (PDF 96-210-2273) and Fe (PDF 96-901-5259) particles. The interplanar distances (Figs. S5–C) of 2.68–2.7 Å in the HRTEM pattern of 5Fe5Ce_Ni_Alloy is attributed to the plane (200) of distorted cubic ceria lattice. In the pattern of 9Fe1Ce_Ni_Alloy, besides the interplanar distances of 3.1 Å and 1.76 Å, typical of (200) plane of CeO2 and of (200) plane of metallic Ni and Fe, respectively, the additional ones of 2.75 Å and 2.60 Å are detected. Despite that these values differ significantly from the values reported in the literature, they could be carefully associated with the planes (204) and (124) of the orthorhombic metallic Fe (PDF 96-411-3934).The high resolution images in Fig. 3-C and S7-C clarify the layering and presence of the distorted crystallographic planes of graphite-2H (3.1 Å - 3.34 Å), values confirmed for both samples by the SAED patterns shown in Fig. 3-E and S7-D. Fig. 3 -D shows d-spacing of about 2.7 Å, coming from particles adhered to the outer wall of the nanofiber, and attributable to the (200) plane of CeO2, which evidently does not seem to take an active part in the growth process of the fibers.The carbon nanofibers (CNFs) in Fig. 3-A and S7-A, can be classified, with the nomenclature "bamboo-like'-CNFs or "stacked-cups"-CNFs, due to the characteristic bamboo structure of the trunk of the fiber. The mechanism proposed by Terrones et al. [30] for the growth of CNFs from the pyrolysis of acetonitrile on Co particles, provides a mechanism that can be factored into three main phases: (I) the decomposition of the organic molecule on the active metal surface, (II) diffusion of carbon inside the metal particle, and formation of thermodynamically more stable carbides, (III) layering of graphite and encapsulation of the nanoparticle with the formation of a "carbonaceous restriction" which will act as a nucleation point for the subsequent growth of the nanotube until the formation of a new nucleation point and subsequent formation of a second segment. This "impulse growth" explains both the characteristic bamboo-shape of the nanofiber and the particular pear-like shape of the promoter particles, which always possesses a particular truncated-conical deformation in the opposite direction to the direction of growth of the CNF. The presence of thermodynamically stable carbides appears to be crucial for the growth of CNFs, in fact extensive studies conducted by Matter et al. [31] have clarified how the formation of the more stable Fe carbides increases the possibility of the formation of nanofibers compared to catalysts having Ni as the active metal, as the Ni carbides are thermodynamically less stable [32]. This last assumption in fact, would agree with the Moessbauer data (Fig. 2, Table 3) and specifically, would rationalize the total disappearance of the Sn- γ -(Ni, Fe) Alloy, with a low nickel content, due to an easier formation of iron carbides in this alloy compared to other alloys with a higher nickel content. Although the formation of carbides favors the formation of CNFs, Ni nanoparticles can also favor the growth of CNFs as exposed by Wang et al. [33] by the decomposition process of methane upon Ni-NPs. Therefore, a combined contribution of these two centers in the growth of the fibers is not excluded.The relative elemental dispersion was visualized by EDS analysis, with the TEM working in STEM mode. The analysis of a single sample 5Fe5Ce_Ni_@C is shown in Fig. S9. From the Fe and Ni samples it is shown that the metal particles are mainly located in the nanofiber tips, confirming a tip-growth mechanism. Cerium and oxygen are more dispersed in the image and almost always in marginal portions outside the larger grains. This suggests a mechanism of extrusion of Fe and Ni particles, with a progressive removal from the CeO2 support, during the formation of the larger nanofibers, while the smaller particles undergo a carbon coating process that "engulfs" the ceria in a thin carbon layer. The mapping of the oxygen in the central part of the image is almost superimposable with that of the cerium, given the presence of CeO2 in the sample. But away from the central fiber in the more marginal areas of the image it is possible to see how even the carbon, finely dispersed in the contour, probably has distinguishable oxygen functionalization.In order to clarify the amount of carbon deposits on the samples with different composition, temperature programmed oxidation-thermogravimetric analyses were performed (Fig. 4 ). The thermogravimetric profiles consist of three main features. The first one (I), starting at about 300 °C, characterizes with mass increase of 2.6% and 9% for 5Fe5Ce_Ni_@C and 9Fe1Ce_Ni_@C, respectively. It can be predominantly attributed to the oxidation of the iron and nickel species present in the system (iron carbides, Ni–Fe alloys and metallic Ni).The second feature (II), manly occurs during the isothermal step at 400 °C, as superimposition of two effects: 1) weight loss due to a principle of carbon decomposition; 2) weight increase due to the further continuation of the oxidative process. The slope of the curves in this segment of the process is different between the two specimens. The sample 9Fe1Ce_Ni_@C, being composed predominantly of Fe and Ni, during the isothermal process acquires much more mass converting into metal oxides, and at the same time decomposing part of the low crystallinity carbon, losing a total of 4% of its weight during the (II) feature. On the other hand, the sample 5Fe5Ce_Ni_@C, having a lower amount of carbides/alloys, ends the oxidative evolution during the initial phases of the isothermal process, and shows a decrease in weight of 12.6% almost exclusively due to the decomposition of carbon. The third observed feature (III), can be rationalized as the decomposition of the CNFs and it starts for both samples within 420–460 °C range. Although the catalysts underwent the same decomposition time of methanol (2 h), it can be seen that sample 5Fe5Ce_Ni_@C acquired more carbon than 9Fe1Ce_Ni_@C, with an attributed to carbon weight loss of 31 wt% and 19 wt%, respectively. However, since the total percentage of carbon loss could not to be calculated accurately, due to the feature (II) effect (oxidation/decomposition process), elemental analysis (EA) was adopted to obtain more accurate results in this respect. Table 4 shows the percentage of C, N, H, and S obtained from the samples with CNFs, confirming with good approximation the results obtained from TPO-TG analysis.Raman spectroscopy was used in order to elucidate the nature of the defects present in the samples both from the point of view of the cubic lattice of the ceria shown in Fig. 5 , and from the point of view of the defects present in the carbon nanofibers, as shown in Fig. 6 .The Raman spectra of samples 5Fe5Ce_Ni and 9Fe1Ce_Ni before and after the reduction process, shown in Fig. 5, introduces a superimposition of peaks belonging to ceria, hematite and probably bunsenite, which are difficult to resolve. Within the range of Raman shifts between 150 cm−1 and 1600 cm−1, it is possible to discern a predominant broad peak centered at ca. 435 cm−1 (especially in the sample with the higher cerium content), which can be rationalized as the resultant from the union of the vibrational contributions of the Eg vibrational modes of the hematite and the vibrational modes of the crystallographic plane (111) of the cubic fluorite-like structure of CeO2. Specifically, this is a first-order vibrational band (F2g band) associated with Ce–O bond stretching, with Ce and O respectively 8-fold and 4-fold coordinated. This broad band is generally preceded by the band of a triple degenerate vibrational mode (with F1u symmetry) positioned at ∼ 285 cm−1 [34,35].The vibrational mode D1, located at ca. 540 cm−1 is associated with the presence of defects within the fluorite CeO2 due to the introduction of low valence doping cations. The relative intensity of this band is directly proportional to the doping rate, due to oxygen vacancies generated extrinsically in the ceria to ensure the electroneutrality of the support. Taken into consideration the intensity and width of this band, the contribution made by one of the A1g modes of the hematite, usually positioned at 498 cm−1 [34], particularly distinguishable in the 9Fe1Ce_Ni sample, cannot be excluded. Another band, partially visible in the spectrum of 5Fe5Ce_Ni, is the one commonly called "Intrinsic Defect Band-D2", located at ca. 600 cm−1 and belonging in this case to defects due to changes in the oxidation state of cerium, with the formation of oxygen vacancies of the type "Ce3+-VO✴✴", this time due to a compensation of charge neutrality for reasons intrinsic to the nature of the redox couple Ce3+/Ce4+ [35].The vibrational band at 661 cm−1 (with higher relative intensity for sample 9Fe1Ce_Ni), is a longitudinal optical mode (LO), and is prohibited in Raman scattering. However, this mode can be activated by disturbances in the hematite lattice [36] and contributions from NiO cannot be excluded, given the presence of major vibrational modes in the region under consideration [37]. It is easy to see from Fig. 5 how the vibrational band, belonging to the hematite as well as part of the band belonging to the sorted ceria, considerably decrease their relative intensity after the reduction process leaving place, for the F2g and D1 peaks of the ceria and the triply degenerate vibrational bands with a broadening of the latter peaks probably due to the small particles size. Such a Raman response is in agreement with the variations in the unit cell parameters of the ceria (Table 1), and the migration-reduction process of Fe3+ and Ni2+ ions described above.The Raman spectra of samples 5Fe5Ce_Ni_@C and 9Fe1Ce_Ni_@C are shown in Fig. 6. In both spectra the G band at 1523 cm−1 and a G′ band at 2683 cm−1 are visible, both related to carbon with sp2 hybridization of graphene [38]. The disordered graphite-like edges of the graphene layers give rise to a D-band at 1333 cm−1, the ratio of the relative intensities between the D-band and the G-band is indicative of the degree of disorder of CNFs. The ID/IG ratio was 1.23 for 5Fe5Ce_Ni_@C and 1.44 for 9Fe1Ce_Ni_@C, defining a higher degree of disorder for carbon nanofibers grown on nanoparticles with a high Fe and Ni content. The latter sample also presents several peaks in the Raman shift region between about 200 cm−1 and 700 cm−1. Some of these peaks, such as the one clearly visible at 660 cm−1 and those around ca. 185, 285, 435 and 540 cm−1, can be identified as the already mentioned vibrational modes of hematite and CeO2 respectively. The broad peak at about 1000 cm−1 with reduced relative intensity can be attributed to the 2P vibrational modes of finely dispersed NiO at reduced relative intensity due to its fine dispersion [39]. The presence of metal oxides of iron and nickel in this sample can be attributed to a passivation process of the metallic Ni or Ni–Fe alloys not encapsulated within the nanofibres and therefore adhered to the surface of the nanofibers or the carbon edges of the defective portion as shown in the TEM images (Fig. 3-D and S7-B).The surface functionalization of all samples was studied by Fourier Transform Infrared Spectroscopy (FTIR), (Fig. 7 ). Although of modest intensity, the broad bands located between 3300 and 3500 cm−1, were associated with the vibrational stretching modes of the H–O–H bonds belonging to the interstitial water molecules, followed by the vibrational bending mode of the same bonds, with signal located at 1621 cm−1 [40].The FTIR spectrum of 9Fe1Ce_Ni shows two intense bands at 560 cm−1 and 475 cm−1, which have been attributed to the stretching Fe–O and bending O–Fe–O vibrations in α-Fe2O3 [41]. The same characteristic peaks were found at lower intensity for the sample with lower iron content. After the reduction process, the two peaks located at positions of 526 cm−1 and 1423 cm−1, in the sample 5Fe5Ce–Ni_Alloy, were associated with symmetric and asymmetric vibrations, respectively, of the Ce–O bond in a distorted cubic lattice of CeO2 doped with lower valence cations [42]. While the peaks at 661 cm−1 and 1542 cm−1 were attributed to stretching vibrations of the Ce–O bond in the undoped CeO2, crediting the previously introduced hypothesis that during the reduction process a part of the Fe3+ and Ni2+ ions migrates towards the surface of the support, with the formation of metal nanoparticles and a subsequent rearrangement of the cubic ceria lattice [43,44].The FTIR spectra of 9Fe1Ce_Ni_@C and 5Fe5Ce_Ni_@C show a broad band in the region between 3300 cm−1 and 3500 cm−1, which can be associated with the stretching vibrations of the O–H bond, this peak may be generated by the formation of hydroxyl groups on the surface of the nanofibers (C–OH and OC–H), or it may be due to the absorption of atmospheric water molecules [45]. The band at 1580 cm−1 and the broad band located at about 1300 cm−1 were attributed, respectively, to the stretching vibrations of the CC and C–C bonds forming part of the backbone of the fibers and the carbonaceous material around them [46].The presence of a band at about 1700 cm−1 confirms the presence of CO groups. The higher intensity of this band for the 9Fe1Ce_Ni_@C sample suggests greater presence of oxidized carbon compared to its carbonaceous counterpart [47]. In both samples a low intensity peak is visible at ca. 524 cm−1, probably due to CeO2 nanoparticles dispersed on the surface of the nanofibers (as shown in TEM images in Fig. 3-D and S7-B).X-ray Photoelectron spectroscopy (XPS) was performed to characterize the chemical state and relative abundance of elements on the surface of the catalysts under study (shown in Table 5 ). The Ce 3d region for the samples 5Fe5Ce_Ni_Alloy and 9Fe1Ce_Ni_Alloy, introduces the first surface difference between the 2 M ratios of the catalysts. Fig. 8 -B shows three 3d3/2-3d5/2 spin-orbit-split doublets, belonging to the different photoemission configurations of the final 4f state and attributable to the majority presence of Ce4+ on the surface of the catalyst [48]. On the other hand, the 3d Ce region of sample 9Fe1Ce_Ni_Alloy (Figs. S10–C), which is the region of the binding energies of the 3d electrons of Ce, shows a solvable XPS spectrum with ten Voigt-like contributions of Ce4+ and Ce3+ [48]. There the peak centered at 883 eV and the relative contributions in green (including the peak with binding energy of 918 eV), belong to the electrons of the 3d3/2 and 3d5/2 orbitals of the final state of Ce4+, while the peak centered at 880.7 eV and the related satellite peaks in blue, belong to the final state of Ce3+ [48].The Ni 2p XPS spectra, from the mixed metal oxides samples (Figs. S10–B), exhibits a main peak, with BE of 855.2 eV with a shake-up peak in the Ni 2p3/2 region, typical of the presence of finely dispersed NiO nanoparticles [49].The samples composed of reduced metal oxides (Fig. 8-C), has a XPS Ni 2p3/2 spectrum, similar to its precursor of mixed metal oxides, with the addition of a peak with a binding energy of 852.8 eV, attributable to the presence of Ni0, deriving from the H2-reduction process [50]. The Fe 2p region of the 9Fe1Ce_Ni sample (Figs. S10–E) shows only a weak peak at about 724 eV resulting from the interference due to the Auger electrons (LMM) of the nickel, while in the corresponding reduced catalyst, the main peak of Fe 2p at 711 eV is clearly visible together with the shake-up peak in the range of 716–722 eV for both molar ratios of catalysts, confirming the presence of Fe3+ [51] ions (Fig. 8-A and S10-A). The presence of metal oxides on the surface of the reduced catalysts can be rationalized with an atmospheric passivation process induced by the nano-alloys of Ni–Fe or by the nanoparticles of Ni0 (Table 1, Table 3).The encapsulation of the reduced metal oxide nanoparticles inside the carbon nanofibers further complicates the XPS analysis, due to the shielding effect that carbon exerts on the photoelectrons of the nanoparticles inside. The XPS signals reported for these catalysts can be considered as a result from photoelectrons coming from the atoms located proximal to the inner walls of the nanofibers, or proceeds from small entities, not encapsulated but adhered to the walls of the nanofibers (as shown in TEM Fig. 3-C and D).The Ni 2p3/2 spectra (Figs. S10–D) show approximately the same characteristics of the corresponding reduced metal oxides, in agreement with XRD and Moessbauer (Tables 1 and 3), while the C 1s spectrum (Fig. 8D) shows the contribution of two different peaks subdivided into four Voigt-type contributions centered at 285 eV, 285.8 eV, 287 eV, and 290.6 eV that are attributable to the binding energies of the C–C, C–O, CO bonds and to the п-п* electrons resonance of the CNFs walls respectively [52] (Table 5). The presence of binding energies belonging to oxygenated functionalization is consistent with the FTIR characterization (Fig. 7) and EDX images (Fig. S9) previously discussed.The linear sweep voltammograms of the 5Fe5Ce_Ni-based and 9Fe1Ce–Ni-based samples conducted at room temperature in a deaerated 1 M KOH aqueous solution are shown in Fig. 9 -A and 9-B, respectively. Based on the curves shown, it can be easily noticed that the reduction treatment and the subsequent encapsulation inside the carbon nanofibers on both Fe:Ce molar ratios of the mixed metal oxides, considerably increased the current density (j), produced during the OER reducing the overpotential (η) required to perform it.Specifically, the linear sweep voltammetric curves showed first of all the poor activity of the catalysts composed of mixed metal oxides with 5Fe5Ce_Ni presenting a current density less than 10 mA/cm2 at 1.8 V vs. RHE (overpotential of 570 mV), while 9Fe1Ce_Ni, with the same catalyst amount placed on the surface of the RRDE (equal to 1018 μg/cm2), presented an overpotential of 450 mV at a current density of 10 mA/cm2. The reduction treatment and consequent formation of metallic species inside the catalysts visibly improved the activity towards the OER reaction, leading the samples 5Fe5Ce_Ni_Alloy and 9Fe1Ce_Ni_Alloy to present an overpotential at 10 mA/cm2 corresponding to 340 and 390 mV, respectively. The performance of both series of catalysts took a qualitative leap forward after the partial transformation of the metal species into carbides and their subsequent encapsulation within the carbon nanofibers. In fact, the 5Fe5Ce_Ni_@C sample shows (at the same quantity of catalyst grafted onto the RRDE as the previous ones) an overpotential of 280 mV at 10 mA/cm2. The same overpotential was also reached by the catalyst 9Fe1Ce_Ni_@C. Both CNFs-containing catalysts, under the same or similar operating conditions, show lower or at least competitive values of overpotential, compared to IrO2 or Ir-based catalysts commercially in use for OER in alkaline environment. (e.g. IrO2 with η = 281 mV at 10 mA/cm2 [53,54]; black iridium with η = 295 mV at 10 mA/cm2 [55].Regarding the registered current at the ring (Fig. 9 A-B, lower panel) as an indication of the evolved oxygen (which is reduced at the ring), it is very similar for all the pairs of materials synthesized with the same method. This evolution of the ring current can only confirm that, albeit with different molar ratios of Fe/Ce and Fe/Ni, the pairs of catalysts (mixed metal oxides, reduced metal oxides, CNFs) undergo very similar surface anodic processes, considering that all the experiments were conducted with the same catalyst load on the RRDE (1018 μg/cm2). The reduction reaction on the surface of the Pt ring of the produced and solubilized oxygen was used to try to calculate the faradaic efficiency of O2 for the two CNFs [56]. In this regard, Fig. S11 shows the efficiency trend of the two samples, in an interval between 1.48 V and 1.52 V. It can be seen that sample 9Fe1Ce_Ni_@C presents an initial trend of increasing efficiency with a maximum equal to 17% and a subsequent lowering as the applied potential increases. The values of the sample 5Fe5Ce_Ni_@C, are between 20 and 26.5% at low potentials, while when the potential increases, also in this case, a decrease in efficiency is noted. The decrease of the theoretical efficiency in the two samples as the applied potential increases, may be ascribed to the formation of oxygen bubbles, which, not dissolving in the electrolyte solution, are not reduced by the Pt ring, inexorably lacking the calculation [56].In order to conceptualize the increase in activity upon reduction and nanofiber growth of the two catalysts with different molar ratios, the considerations must include both a morphological/structural aspect due to the changes made during the synthesis process, and an aspect related to the reactions that occur on the electrode-electrolyte interface during the positive scanning of the potential in the OER reaction. A first consideration can be made regarding the variation of activity between catalysts composed of mixed metal oxides and their reduced analogues. For the latter, the catalytic activity is greatly increased for both molar rations of Fe/Ce, while it is worth noting that the sample 5Fe5Ce_Ni_Alloy, has a higher activity, than the corresponding 9Fe1Ce_Ni_Alloy, despite that its precursor 5Fe5Ce_Ni is with a very low activity. Such behavior may be due to (i) the bigger surface area of 5Fe5Ce_Ni_Alloy compared to 9Fe1Ce_Ni_Alloy, and (ii) presence of less defective CeO2 support with a balanced presence of oxygen vacancies and a dispersion of metallic particles of Ni and Fe, passivated by their corresponding metal oxides. This assumption is in good consistency with the previously discussed XRD (Table 1), N2-physisorption (Table 2), Raman (Fig. 5), FTIR (Fig. 7), and XPS (Figs. 8 and S10) characterizations.The higher performance of the samples encapsulated inside the carbon nanofibers can be rationalized, bearing in mind both the superior surface area and the higher electro-conductive properties provided by the nanofibers which exhibit a free movement of п-electrons coplanar to the sp2-hybridized carbon atoms [6,57,58]. However, although the overpotential values, reached at the current density of 10 mA/cm2, are the same for both CNF-based samples (280 mV), the 9Fe1Ce_Ni_@C catalyst has a slightly higher activity (Fig. 9A–B), and this may be due to a greater presence of defects on the surface of the nanofibers (as shown by Raman spectrometry in Fig. 6). The imperfections of the fibers and functional groups (as shown in Fig. 7), allow a greater electrochemically active surface even though this sample has a much lower surface area than 5Fe5Ce_Ni_@C (Table 2).These hypotheses can be further accredited by electrochemical characterization derived from the Electrochemical Impedance Spectroscopy (EIS), carried out at 1.6 V vs RHE with 10 mV of amplitude (r.m.s.) of the catalysts. The overlapped Nyquist plots are shown in Fig. 10 A-B, while the fitting derived from the relative equivalent electrical circuit model for each catalyst under study is reported in Fig. S12 (Supplementary Informations).A full semi-circular loop was observed for each study case, the samples 5Fe5Ce_Ni and 9Fe1Ce_Ni, have a total polarization resistance (Rp), of 338 Ω and 76 Ω, respectively. Such high values are usually considered proportional to an equally high charge transfer resistance by the catalysts [59]. The reduced homologous catalysts show a huge decrease in Rp equal to 14.2 Ω and 18.4 Ω, this may be due to a fine dispersion of Ni metal nanoparticles and Ni–Fe alloys within the CeO2 which is reflected in a more efficient synergistic action in the charge transfer process. This charge transfer is slightly faster in the case of 5Fe5Ce_Ni_Alloy, which may be one of the reasons for the observed higher activity in the OER compared to 9Fe1Ce_Ni_Alloy (Fig. 9A–B). This behavior could be associated with the majority presence of Ce4+ in the sample with lower iron content, compared to the 9Fe1Ce_Ni_Alloy system, which presents ceria nanoparticles with a higher content of oxygen vacancies (as shown in Table 1 and in Fig. 8-B and S10-C). The equivalent cell series resistance (Rs), located at the point of intersection with the real axis at high frequency of the Nyquist plot, has a value included for all samples between 4.5 and 5 Ω, with the exception of the sample 5Fe5Ce_Ni (8.5 Ω). This is mainly associated to ionic conduction in the electrolyte, which is similar for all the experiments since it relays on the characterization system. In the CNF-based catalysts, the 9Fe1Ce_Ni_@C sample shows an Rp equal to 4.4 Ω, compared to the 5Fe5Ce_Ni_@C (7.8 Ω). The shown lower increase in the imaginary component of the impedance (-Z'') together with a semi-circle of smaller diameter in the low frequency zone, are an evidence of the improvement of charge- and electron-transfer properties and of the capacitive characteristics of CNFs compared to their respective precursors [60,61].Cyclic voltammetry (CV) studies have been carried out for the as-synthetized composites, at first in the potential range between 0.05 and 1.15 V vs. RHE at the scan rate of 0.02 V/s. Fig. 11 A-B shows the voltammograms of the catalysts under examination at room temperature in a de-aerated solution of 1 M KOH. The non "rectangular" shape of the voltammograms indicates an important contribution of the pseudo-capacitive nature of the process within the aforementioned potential range [62], from which, however, it can be appreciated the difference in absolute area under the CV curves (AUC), that follows positive growth following the order: Mixed Metal oxides < Reduced-Metal oxides ≪ CNFs.As it is possible to notice, the trend of capacity growth is quite similar to the trend of anodic growth in the OER reaction (Fig. 9, leading to the conclusion that, the increase in current density followed by a lowering of the overpotential is related not only to a progressive improvement of the intrinsic mechanism of charge transfer of the catalysts (as seen from the EIS analysis in Fig. 10A–B), but also due to an increase in the following characteristics: 1) electrocatalytically active surface accessible (ECSA) [63]; 2) the active sites immediately present at the electrolyte-electrode interface, and 3) possible active sites formed by activation processes that occur in the activation of carbon edges when increasing potentials [64].To better understand how the capacitive characteristics change between the reduced samples 5Fe5Ce_Ni_Alloy and 9Fe1Ce_Ni_Alloy and their counterparts encapsulated inside the carbon nanofibers, the surface area accessible to electrocatalytic reactions (EASA) was evaluated (Fig. S13 in Supplementary Informations). For this, the EASA was estimated from double layer capacitance (Cdl) via cyclic voltammetry measurements in a non-faradic region (0.95–1.15 V) in function of various scan rates (Fig. S13) and calculated using the formula EASA = Cdl/Cs, where Cs stands for specific capacitance [63,65], subsequently converted into m2/g, the values of which were reported in the summary (Table 6 ) with the name of ECSA (electrochemical catalytic surface area). In principle, it is possible to note from the summary table, the large difference in the BET surface area, compared to its electrocatalytically active component (ECSA). This is probably due to the fact that the surface area calculated for N2-physisorption includes areas, that are difficult to reach by the ions of the electrolyte, such as the inner walls of the smaller diameter micropores [66], not to mention the set of factors that can make the measurements of the double layer capacitance inaccurate, such as the chemical capacitance given by the population of trap states or forms of pseudo-capacitance due to the coordination of ions [65]. The higher content of microporous component (Table 2), would explain the lower ECSA of sample 5Fe5Ce_Ni_@C compared to 9Fe1Ce_Ni_@C and hence, the lower electro catalytic activity in OER. The ECSA values reported in Table 6, including the lowest values of the reduced metal oxides, are congruent with the benchmarking study conducted by Jung et al. [65] on similar materials and conditions, and are in agreement with the measurements and characterizations already shown in this study.To better individuate the OER rate determining step (rds), Tafel plots were examined(Fig. 12 ). The kinetics of the OER reaction on the surface of catalysts composed of mixed metal oxides and reduced metal oxides, seems to follow a similar trend. The progressive increase in the slope of the curves from ∼60 mV/dec passing through ∼135 mV/dec up to ∼290 mV/dec, can be interpreted as a progressive increase in the activation overpotential [67] used by surface active sites to overcome the energy barrier, necessary to activate the various process steps (see equations (1.1), (1.2) and (1.3), 1.4) of the OER reaction.According to the theoretical approach of Shinagawa and coworkers [68], a Tafel slope close to 120 mV/dec, indicates that the rate determining step in that determined process range can be attributed to two main conditions: I) formation of hydroxides according to the reaction M + OH− ↔ MOH + e−; (where M is the active site); II) coverage of the given surface by species such as: MOH, MO, MOOH or MOO - , at high overpotential values and immediately before the respective rds. In the case of mixed metal oxides and especially for their reduced analogues, the formation of hydroxides of Ni and Fe cannot be excluded within the range of potential under examination. Furthermore, the slope values higher than 120 mV/dec, can probably be attributed to a sum of surface mechanisms with a positive and simultaneous energy demand that occur at high potential values, while the slopes that are equal or lower than 60 mV/dec (at low current values) can be attributed to a partial coverage of the surface with oxygenated species, or to the formation of catalytically active species with low activation energy [68].The reaction of OER on the surface of the CNFs (5Fe5Ce_Ni_@C and 9Fe1Ce_Ni_@C), follows a different kinetics than the previously evaluated samples. It is possible to notice how at low current density values (until 1 mA/cm2), the Tafel slope is about 60 mV/dec with an interpretation similar to what was previously described. Remarkable is the region of current density between ∼1.5 and 20 mA/cm2, where unlike the previously evaluated samples, the CNFs produce high currents at relatively low overpotential values and with significantly lower Tafel slope (42 mV/dec), using a similar reaction mechanism for both nanofibers (given the overlapping of the curves). Such a surface reaction mechanism can be rationalized with the progressive activation and formation of the active sites through a semi-radical mechanism studied by Lin et al., of the carbon edges in the OER [64], where Tafel slope close to 60 mV/dec can be considered as an indicator of a deoxygenation/deprotonation process of the *OOH species assuming that the mechanism is single-site [69]. At higher overpotentials, the Tafel slope increases to approximately 134 mV/dec. This value, as previously discussed, could be rationalized as a super saturation of the surface with oxygenated species, or as a difference in the Gibbs free energy of the intermediates that mediate the rates of the process [68].It was previously shown through elemental analysis but also partially through TPO analysis (Table 4), as the same quantities of reduced metal oxides catalysts (200 mg), subjected to the same decomposition times of methanol (2 h), lead to a different percentage content by weight of carbon and therefore of CNFs. Specifically, 30.7%wt. of carbon acquired from the sample 9Fe1Ce_Ni_Alloy (subsequently called 9Fe1Ce_Ni_@C), was found, while a carbonaceous quantity of 49.2%wt. from the sample 5Fe5Ce_Ni_Alloy (further called 5Fe5Ce_Ni_@C) was acquired. It is therefore evident that the two catalysts composed of reduced metal oxides act differently in the decomposition reaction of methanol. In order to investigate this difference, the CNFs synthesis procedure was replicated for both catalysts, by decomposing a constant flow of methanol (flow rate of 30 ml/min), on their surface at a temperature of 773 K for a total time of 2 h. The gas phase produced was on-line analyzed by gas chromatograph every 20 min. Although both catalysts at a temperature of 773 K converted 99% of methanol, Fig. 13 A-B, the difference in the gas phase composition, which was approximately stable during the time-on-stream, was registered. Here in the histogram, to the sample 5Fe5Ce_Ni_Alloy is attributed a selectivity to CO, higher than the one found for 9Fe1Ce_Ni_Alloy, which instead shows a greater predisposition in the formation of CH4. The percentage of CO2 remains comparable between the two samples. For both samples, C2H6, C2H4, C3H8 were also detected, but with a total selectivity percentage lower than 1%.As can be seen from the histogram in Fig. 13 A-B, the decomposition process of methanol, under the conditions of temperature and pressure in question, give rise to the main reaction: CH3OH → CO + 2H2 (eq. (2.1)) with the formation of syn-gas, and with two collateral secondary reactions: CO + 3H2 ↔ CH4 + H2O (eq. (2.2); methanation reaction) and CO + H2O ↔ CO2 + H2 (eq. (2.3); reaction of water-gas shift), with the following stoichiometric overall reaction: 4CH3OH ↔ 3CH4 + 2H2O + CO2 (eq. (2.4)). Although the overall reaction does not suggest a final production of syn-gas, the reaction described by equation (2.1) is thermodynamically irreversible under the operating conditions in question, while the methanation and water-gas shift reactions are reversible and do not proceed completely [70], leading to the formation of H2, CO, CO2, CH4 as a gaseous mixture resulting from the decomposition of methanol.It is therefore evident that one of the carbonaceous molecules previously mentioned has a main role in the formation of the carbon nanofibers synthesized in this study. The synthetic literature of this type of CNFs is very vast and uses an equally vast amount of carbon sources, such as: ethylene, acetylene, propylene, or other metal-organic frameworks (MOFs) [71], but more common are the syntheses that use the decomposition of CH4 and CO [72], employing in this regard, respectively, the methane cracking reaction: CH4 → 2H2+ C(s) (eq. (2.5)) and the carbon monoxide disproportion reaction: 2CO → C(s) + CO2 (eq. (2.6)) [70].Given the complexity of the system under study and given that both CO and CH4 are formed on the surface of the catalysts, it is reasonable to assume that both molecules could contribute to the growth of nanofibers under the same operating conditions. The quantity of carbon acquired during the carbon-coating process will depend on the decomposition thermodynamics of the two molecules on the catalyst surface and also on the concentration of CO and CH4 molecules, respectively, formed during the methanol fragmentation. From this particular aspect, the metal ratio of the catalysts composition plays a fundamental role both in the decomposition of methanol even at temperatures below 773 K, and in the different mechanisms of its fragmentation, which justify the preponderant difference in selectivity of the gas phase between the two catalysts. Specifically, Fig. 14 shows the methanol conversion values obtained from a second methanol decomposition experiment, developed at increasing reaction temperature values under the same operating conditions as the synthesis process. The curves in Fig. 14 show the superior activity of 5Fe5Ce_Ni_Alloy to its counterpart, reaching a complete conversion of methanol at 648 K. This substantial difference in the degree of activity of the catalyst, as well as its high selectivity for the syn-gas formation, may be closely linked to the greater presence of CeO2 in the sample. Furthermore, the doped variant of the latter is visibly more active than pure CeO2, which accentuates the importance of the synergy between the metallic Ni and Fe–Ni alloy nanoparticles with the ceria support, in the mechanism of fragmentation of methanol and consequently in the nature and distribution of its products.The fragmentation mechanism generally described for the single-site decomposition reaction of methanol on the surface of a catalyst, consisting of metal nanoparticles supported on high oxygen storage capacity (OSC) metal oxides such as CeO2 [73], contemplates a preliminary dissociative adsorption of methanol on the surface of the ceria support, with the formation of methoxy groups, the following step involves a superficial diffusion to the proximal catalytic metal particles and therefore a translation of the aforementioned methoxy groups, with consequent dehydrogenation, formation of formyl groups that can lead to a release of CO, or to subsequent transverse reactions such as methanation (eq. (2.2)) or the water-gas shift reaction (eq. (2.3)) or to a deposition of carbon through the decomposition of CO or CH4 molecules (eqs. (2.5), (2.6)) with the formation of carbides and a subsequent growth of carbon nanofibers [30].In our previous study [23], using the insitu FTIR spectroscopy, we demonstrated the effect of molar ratio between Fe and Ce on the methanol fragmentation mechanism. The lack of formation of methoxy groups at low temperatures for the samples with high iron content as well as a different distribution of the formyl, formate and carbonate groups at high temperatures, compared to the molar ratios with lower iron content and high ceria content was established. In light of these considerations, the low CeO2 content of sample 9Fe1Ce_Ni_Alloy delays the propaedeutic fragmentation of methanol on the catalyst surface at low temperatures (Fig. 14) while its high content of Fe and Ni, widely recognized in literature as key promoters of Fisher-Tropsh reactions [74], at a temperature of 773 K favor the methanation of CO (eq. (2.2); Fig. 13), inhibiting the growth of carbon nanofibers.The progressive modification of the metal species constituting the catalysts brings about profound changes to the catalytic properties of the samples. Specifically, the metal oxides of iron and nickel within CeO2, initially not sufficiently electrocatalytically active for the reaction of OER in an alkaline environment, after being reduced, lead to the formation of finely dispersed nanoparticles of Ni–Fe alloys and Ni0, improve the qualities intrinsically linked to the electronic transport or to the pseudo-capacitive ability of the catalysts which reflected into an increased activity in the OER reaction compared to the initial catalysts. The 5Fe5Ce_Ni_Alloy catalyst obtained an electro-catalytic response exceeding expectations, an effect attributed precisely to the acquisition of better performing textural and structural properties. The previous catalytic knowledge, in terms of active sites present and the relative products formed by the decomposition reaction of methanol, on the surface of this kind of catalysts, allowed a further modification of the active metal phase. By exploiting the greater solubility of carbon in metallic iron compared to its oxides, the main products of the decomposition reaction of methanol (CO and CH4) were partially used as carbon sources, for a preparatory carburization of the Fe–Ni alloys, and a subsequent growth of carbon nanofibers through tip-growth mechanism. Although structural differences have been found between the two fibers, dictated by the different activity and selectivity of the catalysts precursors to the decomposition of methanol and its products, the electro-catalytic activities of the synthesized CNFs are almost similar, with relatively low overpotential values (280 mV at 10 mA/cm2) and the found relatively high current density values. Analyzing the results from a broader point of view, however, the samples produced in the 5Fe5Ce_Ni molar ratio (especially 5Fe5Ce_Ni_Alloy and 5Fe5Ce_Ni_@C), have triple advantage: 1) Greater selectivity for the syn-gas production from methanol (CH3OH → CO + 2H2) with the possibility of almost total conversion of methanol even at temperatures below 773 K. 2) Greater carbon acquisition for the growth of carbon nanofibers, 3) Low overpotential values combined with high relative values of current density produced in the alkaline environment OER reaction. Greater selectivity for the syn-gas production from methanol (CH3OH → CO + 2H2) with the possibility of almost total conversion of methanol even at temperatures below 773 K.Greater carbon acquisition for the growth of carbon nanofibers,Low overpotential values combined with high relative values of current density produced in the alkaline environment OER reaction.All mentioned above, suggests a potential link between the two techniques, destined for hydrogen production, and opening the possibility of using the catalysts exhausted by the techniques of decomposition of organic molecules into catalysts useful for electro-catalytic purposes. Consolato Rosmini: Conceptualization, Validation, Formal analysis, Investigation, Data curation, Writing – original draft, preparation. Tanya Tsoncheva: Supervision, Project administration, Funding acquisition, Writing – review & editing. Daniela Kovatcheva: Formal analysis, Validation. Nikolay Velinov: Formal analysis, Validation. Hristo Kolev: Formal analysis, Validation. Daniela Karashanova: Formal analysis, Validation. Momtchil Dimitrov: Formal analysis, Writing – review & editing. Boyko Tsyntsarski: Formal analysis. David Sebastián: Writing – review & editing. María Jesús Lázaro: Supervision.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research was funded by the BIKE project, which received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement no. 813748.The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2022.04.036.

Ceria-iron oxide mesoporous materials with Fe:Ce molar ratio of 5:5 and 9:1 were synthesized by hydrothermal method using CTAB as a template and subsequently modified with NiO (molar ratio Ni:Fe = 1:2) by incipient wetness impregnation technique. In order to increase the electro-capacitive properties and reduce the intrinsic impedance of the metal oxides, the samples were consecutively modified by reduction in hydrogen to obtain highly dispersed Ni–Fe alloys into ceria matrix. By exploiting the high permeability of carbon inside ferrous alloys, the metal phase has been further modified into ferrous carbides and metal alloys encapsulated within carbon nanofibers. For this purpose, a reaction, already widely studied for the production of hydrogen, was used, that is the decomposition of methanol vapors. In fact, this decomposition, in addition to producing syn-gas and methane, changes the catalysts in use through a chemical vapor deposition-carbon coating process. This fact, has been used by us to demonstrate how the newly obtained metal-carbon nanocomposites can be used for electro-catalytic purposes. The modified phases of the two molar ratios of the Fe–Ni–Ce catalysts were tested in the Oxygen Evolution Reaction (OER) in an alkaline environment (1 M KOH), showing a satisfactory and progressive increase in activity and a surprising decrease in the overpotential at 10 mA/cm2 of current density. The morphological, textural and physicochemical properties of the samples were characterized in details by XRD, N2-physisorption, TG-TPO, TEM, EDX, FTIR, XPS, Raman and Moessbauer spectroscopies.

No data was used for the research described in the article.In recent years, increasing demands in energy and the awareness of the negative environmental impact of fossil fuels have accelerated the transition to green alternative energies (such as solar, wind, and tidal energy). However, the intermittent accessibility of renewable energies by daily, seasonal, and regional factors limits their adoption to a consumption-adjusted energy supply. Thus, it is imperative to develop innovative energy conversion and storage technologies, such as water-splitting devices.In particular, understanding the mechanisms and technical challenges of the sluggish oxygen evolution reaction (OER) is indispensable to boost the electrolysis market. Unfortunately, classical electro- or physico-chemical analytic methods alone cannot provide a full picture of all processes occurring during the reaction, due to missing information on the electrochemical states of the catalysts under operation conditions. Captivatingly, the combination of electrochemistry with physicochemical characterization by operando techniques provides innovative ways to circumvent this obstacle and permits investigation of material properties, of OER intermediates or actual active sites, as well as of other processes taking place on the electrocatalyst surface during the reaction. Importantly, the field of operando analysis covers a wide range across different experimental techniques and length scales, all providing complementary information on the system under investigation.Here, we want to expand beyond the discussion of a single technique, giving a concise outline of recent advances in the understanding of OER catalysts obtained by a set of operando techniques that span the range from atomistic to systemic scales and from fundamentals to actual applied conditions. The here selected techniques aiming for a fundamental understanding are electrochemical quartz crystal microbalance (eQCM), operando X-ray spectroscopy, and inductively coupled plasma—optical emission spectrometry and mass spectrometry, respectively (ICP-OES and -MS), which are typically applied in half-cell configurations. Additionally, we set a second focus on catalyst analysis during its technical application, shedding light on further operando techniques that utilize single-cell setups to mimic technical conditions for water electrolysis.An electrochemical quartz crystal microbalance (eQCM) provides highly sensitive information on transient mass changes of catalysts. It consists of an electrochemical cell in which typically Au-coated oscillating quartz discs serve as support electrodes. For details about this technique, the reader is referred to a remarkably comprehensive eQCM guide written by Buttry and Ward [1].With eQCM, a variety of information can be obtained. In its earliest form, the QCM was used ex situ to measure the mass of electrodeposited films [2,3]. Later in the 1980s, it was used for the first time during electrochemistry to monitor adsorbates and surface reconstructions on Au surfaces [4–6]. The effect of current on Ni(OH)2 electrodeposition [7] or the redox behavior of α- and β-Nickel double hydroxides (Figure 1 I) [8] were just two further applications to follow. In the latter case, the lower relative mass changes observed in the β-phase compared to the α-phase was assigned to the shorter slab distance in the β-phase preventing water intercalation.More recently, studies on more complex processes came along, such as the investigation and identification—in terms of molar mass, kinetics, and concentration variation—of the reversibility of intercalated/deintercalated ions, such as K+ and OH−, migrating in and out of the spacing of a Ni layered double hydroxide (LDH) structure [9]. Wu et al. used nickel hydroxide Ni(OH)2 to study its ion intercalation-driven α/γ and β/β phase transformations in LiOH, NaOH, and KOH electrolytes [10]. In this work, quantitative Ni(OH)2 stoichiometry performed via eQCM revealed that displacing structural water, as depicted by the higher relative mass changes during the first cycle (K+ > Na+ > Li+) inside the LDH by intercalated cations (Figure 1II), accelerates catalyst degradation, a process which was found to be exacerbated with increasing size of the electrolyte cations (Li+ < Na+ < K+).Additionally, eQCM was used to study the effect of Fe impurities on the structural transformations between the α/γ and β/β couples on Ni-based electrocatalysts during OER via different microgravimetric characteristics of phase transition between α/γ or β/β couples. Such transformation processes are inherent to Ni(oxy)hydroxides, and via eQCM, it was concluded that Fe in NiFe-LDH inhibits the conversion of α/γ to β/β which would otherwise lead to lower activity [11]. Feng et al. reported a gradually decreasing self-healing effect of Fe impurities on the activity and stability of a Ni-based electrocatalyst due to the instability of Fe(VI)O4 2− clusters within the electrolyte, leading to Fe precipitation [12]. The authors identified an onset potential of Fe redeposition via eQCM as well as a potential gap between OER potential and Fe redeposition, which deteriorates catalyst self-healing.Another strategy to increase the performance of LDH electrocatalysts that can be investigated by eQCM is the increase of the charge transfer ability of these otherwise insulating materials. For example, modifying an LDH structure with a conductive polypyrrole (ppy) polymer was shown to enhance the adsorption and desorption of reaction intermediates and reactants (OH− and H2O) on a NiFe-LDH electrocatalyst [13]. This was performed in a setup with dissipation measurement capability (eQCM-D), which allows following the activation and the atomic rearrangement processes on the catalyst surface in real time, using a potentio-dynamic protocol. Larger observed mass changes during a potential cycle of a ppy-modified LDH catalyst were related to a layer distance increase and ion intercalation, which in turn lead to more efficient intermediate adsorption and desorption.eQCM can also shed light on the effectiveness of material alloying for enhancing activity and stability. Escudero-Escribano demonstrated the decrease of both activity and dissolution rate of Ru during OER of surface-modified RuO2 electrocatalysts by increasing the coverage by an IrOx sub-monolayer (1, 2, and 4 Å) via eQCM coupled with ICP-MS [14]. Recently, we demonstrated via eQCM the dynamic nature of IrOx structures and the manner in which OER conditions regulate their hydration degree. During potentio-dynamic conditions, a charge growth (Figure 1IIIa) was found to be accompanied by deactivation, due to the deprotonation-induced dehydration (Figure 1IIIb) of μ2-OH(H2O)x and subsurface species like sulfates etc. (species observed in the cathodic sweep with masses between 14 and 60 g mol−1) during OER [15]. Activity can be, however, fully restored upon electrochemical reduction with an accompanied mild dissolution, observed via the higher mass of the species involved (> 140 g mol−1) in the anodic sweep from 0.04 to 1.4 V (Figure 1IIIb), meaning that Ir species are dissolved. On the other hand, potentiostatic conditions irreversibly deactivate IrOx, due to accelerated growth of μ2-O species, with simultaneous decrease of μ1-O species.Although the detection of mass changes might appear of limited use at first glimpse, the proper correlation to electrochemical processes makes eQCM a highly versatile operando tool for the fundamental analysis of electrocatalysts. We believe that more interesting insights about formation-, degradation-, and sorption-processes will follow in the future when eQCM results are linked to other operando methods, such as spectroscopy techniques, of which X-ray spectroscopy is discussed in the next section.X-ray spectroscopies provide a comprehensive toolbox for the investigation of the atomic as well as electronic structures of a material in an element-specific fashion. However, its operando application on catalysts (e.g. for OER) comes along with challenges such as the design of specialized equipment, carefully prepared measurements, and cross-checks for radiation-induced damage [16–20]. Further, accurate interpretation of OER mechanisms usually requires assistance from computational approaches [21]. The proper application of operando hard X-ray spectroscopy was recently discussed in detail by King et al. [22]. Further, a protocol to apply this technique in a potentio-dynamic fashion was evaluated by Pascquini et al. [23]. For a comprehensive overview of operando X-ray techniques, the reader is referred to recent reviews and overview papers [16,18,24–28]. Here, we focus on a concise outline of the capabilities of operando X-ray spectroscopy underlined by recent publications.For the elucidation of the reaction and degradation mechanisms of an electrocatalyst, knowledge about its electronic and spatial structure under operative conditions is an important prerequisite. Such knowledge can be obtained by detecting the X-ray absorption near edge structure (XANES), which is e.g. sensitive to the oxidation state, and the extended X-ray absorption fine structure (EXAFS), providing information e.g. about atomic distances.Via combining operando XANES and EXAFS for investigating Ir oxide, Czioska et al. proposed a stronger Ir–Ir interaction as the main reason for the higher stability of IrO2 after calcination [29]. Further, they elucidated the effect of temperature, suggesting a stabilizing effect of elevated temperatures on the oxidation state of Ir oxide [30]. Regarding activity, an unusually shortened Ir–O distance at OER potential was detected by EXAFS in highly active (Ni-leached) IrNiOx nanoparticles in comparison to conventional Ir oxide, supporting the interpretation of an increase in the oxygen ligand electrophilicity as an important factor to facilitate the OER [31]. In another study, a high structural “flexibility,” measured on Li-modified amorphous IrOx, was suggested to be beneficial for high OER activity (Figure 2 I) [32].The full power of X-ray spectroscopy becomes evident when investigating multimetallic systems such as 3d transition metal compounds for alkaline OER since it enables an element-specific analysis. Several studies, on NiFe-based catalysts [33–36], suggested the changes in metal oxidation states mostly occur at the Ni sites indicated by pronounced K-edge shifts (typically related to an oxidation state change) in the Ni spectra for applied potentials, while the K-edge shift of Fe appeared to be only minor (Figure 2II). However, by dynamically tracking changes in the Ni and Fe K-edge spectra with varying potentials, a modification of the Fe coordination environment concomitantly occurring to the Ni oxidation was proposed [33].In addition to the above-mentioned hard X-ray approaches, operando spectroscopy on OER catalysts can also be performed in the soft X-ray regime, yielding intense L-edge spectra of 3d transition metals that are usually very distinct for different oxidation states and phases, therefore being interesting for the determination of material phases by comparison to simulations or for “fingerprint interpretation” [35–39]. Furthermore, in-depth analysis can be performed when applying soft X-ray spectroscopy. In an operando, soft X-ray study on MnOx, in addition to a phase transformation into δ-MnO2 for oxidative potentials, an increase of oxygen-metal-charge-transfer features in resonant inelastic X-ray scattering (Figure 2III) was revealed, associated with an increase in the hybridization of O 2p and Mn 3d states preceding the OER [40].Soft X-rays also cover O K-edge excitations, which is of particular interest for understanding the reaction mechanism. Recently, the O K-edge of oxygen evolving catalysts was extensively studied, identifying the formation of μ1-and μ2-OH species and subsequent deprotonation as an important intermediate reaction step (Figure 2IV) [35,41–45]. It is noteworthy that the formation of electrophilic oxygen species as a prerequisite to OER is proposed for both, noble metal [18,41–45] as well as 3d transition metal [35,40] oxides.The capability of X-ray spectroscopy to access the local spatial and electronic structure of different types of OER catalysts has led to important insights, still, we see terra incognita to discover. For example, when pushing reaction conditions towards harsher parameters such as elevated temperatures [30] as well as the extension towards time-resolved experiments to capture the dynamics of reaction and transformation processes on a (sub)millisecond timescale [28].To elucidate catalyst degradation rates, the application of ICP-MS or -OES during transient OER has been paramount in identifying activation/stabilization factors for catalyst development [46]. One prominent example is the work of Cherevko and co-workers on the dynamic dissolution of Au, Pt, and Ir under pre-OER and OER conditions (Figure 3 I) [47,48], in which the authors hypothesized an unstable intermediate and the transition from oxide to metallic iridium during the cathodic sweep as the main contributors to Ir dissolution. Based on these findings, alloying of Ir and Ru has been established to form catalysts of similar activity but of superior stability compared to their monometallic counterparts [14,49,50]. An example is alloying of Ru with Pt [51]. By monitoring the Ru and Pt dissolution, Yi and co-workers identified the stabilizing and activating role of Pt on synthesized and thermally treated crystalline Ru0.9Pt0.1O2 via the formation of an amorphous RuxPt1-xOy on top of the crystalline Ru0.9Pt0.1O2. This surface|bulk-combination formed via surface Pt dissolution led to enhanced activity and stability.Highlighting the necessity to find alternative catalyst supports to carbon due to its inferior stability during OER, Maillard and co-workers in 2020 were able to identify the stabilizing effect of 5% Ta on a SnO2 support for IrOx electrocatalysts, by keeping a balance between stability and activity for the catalyst material [52]. In 2020 and 2021 Over, Cherevko and Grunwaldt with co-workers used a combination of X-ray techniques and ICP-MS to study the stabilizing effect of IrO2 on highly active IrxRu1-xO2 nanoparticles as long-term operating catalysts for PEMWE, due to the enhanced Ir–Ir interactions in IrO2 during OER [29,53–55]. We want to emphasize here that these studies on the stabilizing effect caused by enhanced Ir–Ir interaction based on ICP-MS and operando X-ray techniques are a very good example regarding new insights that can be obtained when successfully combining complementary operando techniques.Online ICP spectrometry has been successfully used to identify the dissolution rates of 3d transition metal catalyst materials [46,56,57] and the dynamic nature of active sites of hydroxides [58] and perovskites, respectively [59] also, in alkaline electrolytes. Additionally, the effect of Fe impurities on activity (Figure 3II(a)) and, for the first time, the uptake of Fe impurities by a Ni-based electrocatalyst and its effect on stability (Figure 3II(b),(c)) were quantified in 1M KOH with the aid of online ICP-OES [60], highlighting the importance for better understanding the effect of electrolyte impurities in alkaline OER.In summary, the use of ICP-OES and -MS is a very powerful for fundamental investigations of catalyst stability. Most interestingly, its operando application is easily transferable from half- to single-cell configurations, as discussed in the next section, to study catalyst dissolution on a more technical scale.Electrochemical testing of newly developed catalyst materials under industrially relevant conditions is indispensable for proving their technical applicability for water electrolysis. On top of the previously discussed atomistic analysis, here we shed light on some operando techniques applied in single-cell configuration. As an example, we focus on PEM-water-electrolyzer (PEMWE). A single cell is typically composed of a membrane electrode assembly (MEA), in which the membrane is sandwiched in between liquid/gas diffusion layers (LGDLs) and catalysts, flow fields (also acting as current collectors), and end plates (Figure 4 I). However, special designs might be required when applying operando techniques, e.g., to circumvent that thick cell elements, such as end plates and flow fields inhibit access of the chosen measurement technique to the MEA. In contrast to the very controllable conditions on half cells, in single cells, the intrinsic activity of the catalyst can be misinterpreted by many other factors. Such factors, that can contribute to the overall cell performance, are i) the accessibility of water to the catalyst interface, ii) the electrical resistance between the individual components, iii) proton conductivity of the membrane and ionomers, and iv) the transport properties of oxygen or hydrogen gas. The interplay of these factors renders their analysis a complex endeavor, and various attempts of operando investigations have been undertaken to elucidate and understand the individual contributions to the overall performance of a single cell.Classical techniques such as cyclic voltammetry (CV), current-voltage (IV) measurements, and electrochemical impedance spectroscopy (EIS), are simple yet powerful methods [61]. In particular, EIS is capable to separate kinetic, ohmic, and mass transport resistances [61–63] (Figure 4II). However, an interpretation solely based on EIS needs attentive caution, since the measured resistances reflect interconnected phenomena occurring in the single-cell setup. For instance, trapped microbubbles can lead to an increased mass transport resistance but also to an apparent increase in the kinetic and ohmic resistance due to a decreased overall utilization of the catalyst.To overcome this obstacle, the combination of EIS with the operando observation of gas formation inside the single cell can add crucial information related to microkinetic and mass transport. In literature, different attempts can be found to visualize the track of bubbles through LGDLs or flow fields, namely via high-speed optical imaging [63–70], neutron imaging, and X-ray imaging [62,71–79] (Figure 4III and IV). By applying such operando imaging techniques, for instance, an increase of gas bubble volume was observed when following the flow field path from the inlet towards the outlet, leading to non-uniform gas distribution and bubbly to slug flow [64,67,71,77]. The effects of such slug flow are debated. While Majasan et al. and Wang et al. related it to a decrease in performance [67,69], Dedigama et al. suggested the tendency of the enlarged bubbles in the flowing electrolyte to combine with microbubbles on the surface of catalysts causing an increase of local current near the outlet [64]. Another observation by operando imaging was made by the F–Y. Zhang group discovered that only the catalyst material directly in contact with the LGDL is catalytically active and generates bubbles. This was related to low conductivity and high in-plane resistance in the catalyst layers located in the pore region [65]. Based on this, they developed thin and well-organized gas diffusion layers for the anode [63,70,80] and cathode [65,68] of PEMWE. Further details on the analysis of mass transport using the aforementioned operando techniques are well summarized in recent reviews [61,81].As an alternative to operando approaches in assembled cells, also studies exist on the separate investigation of individual components of single cells by scanning electrochemical microscopy (SECM). SECM uses a 4-electrode configuration (comprising two working electrodes) in which an ultramicroelectrode (UME) is brought close to the electrode substrate to enable functional operando electrochemical analysis on a micrometer scale (Figure 4V). By this, local information, e.g. of gas emission through catalyst layers or LGDLs, can be obtained. Kim et al. investigated the influence of Ir nanowire alignment in the catalyst layer on oxygen emission by detecting the frequency of bubble-collision on the UME and the designed Ir catalyst layers exhibiting an over 30-fold higher mass activity compared to conventional arrangements (Figure 4VI) [82]. Lim et al. developed an amphiphilic LGDL to enhance the mass transport of both liquid and gas in PEMWE (and also PEMFC), and proved selective gas emission through hydrophobic channels by SECM [83].Due to significantly different operation conditions in half cells and single cells, analyzing the catalyst stability in a single-cell configuration requires an adapted way of evaluation and analysis, as discussed recently for PEMWE [84]. The S-number (= generated O2/dissolved Ir) is five orders of magnitude higher in single cells compared to half cells [85–87]. Knöppel et al. figured out that the difference in pH and catalyst stabilization during long-term operation are the main factors for the discrepancy when detecting dissolution via operando ICP-MS analysis coupled with either flow cells or MEAs, respectively [87]. It is still a blue ocean of research due to the limitation of applying operando analysis to these extremely stable systems, which possess a lifetime of up to tens of thousands of hours. For an advanced grasp of affecting factors on the deterioration of technical electrolyzers, i) standardized accelerated protocols for water electrolysis [88–91] should be established in a similar manner as for fuel cells [92–94], and ii) new pathways of detecting the origins of the degradation via long-term operando techniques should be identified.Within this work, we reviewed selected techniques for the operando analysis of OER catalysts on different scales. We started with an overview of the operando application of eQCM, X-ray spectroscopy, ICP-OES and -MS for fundamental studies in half-cell reactions covering the atomistic scales of reaction and degradation mechanisms and concluded with an overview of the application of different techniques in single cells, such as high-speed optical imaging, neutron radiographs, and SECM, enabling investigations on a more systemic level of technical OER.The here summarized findings span the range from structural transformations of LDH catalysts, intercalation behavior of ions, and the formation of crucial intermediates for OER, towards the stabilizing effect of alloying on noble metal catalysts and the distribution and flow of bubbles in actual cells. We hope that by giving an insight into the range of possible operando investigations, we could point out the complexity of understanding the variety of aspects that govern catalyst performance for OER, in particular when looking at catalysts in the configuration close to technical application. While the fundamental methods are of paramount importance to elucidate reaction mechanisms and properties of the catalyst material itself, only by applying operando techniques in a more technical configuration, additional phenomena occurring in its application can be elucidated.Rational catalyst design goes far beyond pure electrochemical characterization. Many factors (fundamental and technical) along the way from developing a catalyst to its application in technical setups determine its final performance. Most importantly, these factors have to be identified and optimized by applying operando analyses.Finally, it has to be noted that the full range of operando experiments includes many more techniques than the selection presented herein. We are convinced that their rational combination to obtain complementary information, such as the interplay of different phenomena, will promote a deeper understanding of OER catalyst. Moreover, future improvements regarding spatial/time resolution and application parameters such as temperature or current density will help to further unravel both, the fundamental as well as the technical aspects that govern OER.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors are grateful for funding within the German BMBF cluster projects “DERIEL” (FKZ 03HY122I) and “PrometH2eus” (FKZ 03HY105E).

Developing high-efficiency and affordable electrocatalysts for the sluggish oxygen evolution reaction (OER) remains a crucial bottleneck on the way to practical applications of water electrolysis toward clean H2. Determining the OER mechanism and understanding the characteristics that affect OER activity and catalyst stability are of vital importance for this endeavor. In this aspect, operando characterization techniques performed under dynamic OER conditions are powerful tools to monitor key reaction intermediates, active sites, charge transfer, and material transformation coupled processes. This mini review covers noble and 3d transition metal-based OER electrocatalysts and their analysis by different operando techniques that span the range from the characterization in half cells to elucidate intrinsic properties to the analysis of phenomena occurring during their technical operation in single cells.

No data was used for the research described in the article.With increasing demands for fuel and deteriorating fossil fuel reserves, the primary concern over the last five decades have been exploring sustainable fuels and revamping energy efficient technologies. However, the research to achieve ambient temperature reduction of nitrogen to ammonia is far from perceivable. The N2 molecule being non-polarizable, highly inert and a strong triple bond, its dissociation energy of 940 kJ/mol is attainable only under high temperature and pressure [1]. This also lays forth the constant reliance on the highly energy intensive Haber-Bosch process of ammonia synthesis. Therefore, a scalable and viable synthetic route of N2 reduction at ambient conditions is an absolute necessity. The electrocatalytic nitrogen reduction reaction (eNRR) is one green approach to replace Haber–Bosch process, as this process can be actuated from renewable sources of energy and ammonia synthesis can be regulated at ambient conditions [2]. However, electrochemical N2 reduction is laced with two major challenges: a large NRR overpotential and low NH3 faradaic efficiency (FE) caused by its competing hydrogen evolution reaction (HER) [3,4].An extensive research in the recent years have been made to improve the Faradaic efficiency of NRR by implementing noble-earth metal electrocatalysts (Pd, Au, Ru, etc.), transition metal electrocatalysts (Mo, Fe, Co,V, etc.), and metal-free electrocatalysts (B-doped graphene, black phosphorus, etc.) [5–12]. Efficient NRR performance at lower overpotentials have been reported mostly on metal based electrocatalysts; in particular, NRR with FE > 20% till date has been reported on Ru single atom catalysts anchored on N-doped porous carbon (21% FE) and active Mo/MoO2 species anchored on carbon cloth (FE = 22.3% FE) [13,14]. A major advantage of such metal centers@porous carbon electrocatalyst is the synergistic utilization of buffer electrons from the two-dimensional (2D) substrates and the catalytic metal center. These metal centers route the delocalised electrons from the 2D surfaces into the antibonding orbitals of N2 molecule leading to an activation of the N-N bond, which in turn is an essential determinant to the reduction of N2. Consequently, this has prompted computationally driven studies on several noble and earth abundant transition metal centers@2D electrocatalysts for nitrogen reduction [15,16].In this respect, the main group metals owing to their electronic arrangement show only specific oxidation number and restricted orbital states fail to exhibit purported N2 reduction, except for Li and Al clusters. Li has been reported to be used directly as a catalyst for NRR or via a lithium-mediated route as metallic Li forms the only stable nitride, Li3N in ambient conditions. The Li-mediated NRR electrocatalysts are known to exhibit high NRR FE closely approaching 100% in a high-concentration imide-based lithium salt interface [17]. However, the implementation of Li for NRR becomes unsustainable due to its limited presence in the earth’s crust which correspond to only 0.002–0.006 wt%. On the other hand, Al being the most abundant metal in earth’s crust, has been less explored for ambient nitrogen reduction. Notable reports have been made on Al-based electrocatalyst for ammonia fixation include Li-aided Al doped graphene, aluminium (III) coordination complex, Al-Co3O4/NF, MoAlB single crystals, and Al-N2 battery with an Al - ionic liquid electrolyte. Huang et al implemented Al metal as a dopant on graphene as a ligating center to the NxHy intermediates generated by Li + ion aided reduction of N2 to ammonia at ambient conditions [18]. A substantial advancement in NRR performance of Al-electrocatalysts had been realized by Berben and co-workers when an aluminium (III) complex with 0.3 M Bu4NPF6 THF and DMAPH + electrolyte exhibited ammonia production at − 1.16 V (vs. SCE) with 21% FE in ambient condition [19]. However, Al as a catalytic metal center for nitrogen fixation has been reported in urchin like Al-doped Co2O3 nanospheres (Al-Co3O4/NF) with a FE of 6.25% at − 0.2 V vs RHE by Yuan et. al, and in a multicomponent boride, MoAlB wherein the layered electrocatalyst reported by Ma and co-workers showed ammonia production with a FE of 30.1% at −0.05 V vs RHE [20,21]. The highest FE of 51.2% at −0.1 V for Al-based electrocatalyst in ammonia production has been reported by Zhi and co-workers in a rechargeable Al-N2 battery composed of a graphene-supported Pd (graphene/Pd) cathode and Al anode with an ionic liquid electrolyte (AlCl/1-butyl-3-methylimidazolium chloride) [22]. The study reveals a higher feasibility of AlN (ΔG = -287 kJ/mol) formation as compared to Li3N (ΔG = -154 kJ/mol), thereby revealing a more spontaneous nitriding reaction in Al over Li. However, AlN is extremely susceptible to air unlike its lithium counterpart, and it gets easily oxidised, thereby the catalytic activity of Al gets thwarted.Fundamentally, Al being an element of boron family possess similar electronic distribution and certain similarities in electronic properties can be expected. Boron has been reported in several metal-free electrocatalyst as a dopant or catalytic center that can hold N2 and influence ammonia production via an electron “donor-acceptor” mechanism [23,24]. While the “donor-acceptor” mechanism is unlikely to occur in Al atom catalyst due to a lower electronegativity of Al (1.61) as compared to B (2.01), Al clusters have been reported to chemisorb N2 and activate the N-N bond effectively. Aguado et al. reported the chemisorption of N2 and N-N bond activation upto 1.65 Å on Al44 nanoclusters with an energy barrier of 3.4 eV [25]. The N-N bond activation barrier becomes as low as 0.65 eV in smaller Al-clusters, in particular, Al5 cluster on BN-graphene, as observed by Kumar and co-workers [26]. Henceforth, it is important to probe into these smaller Al clusters and explore the plausibility of implementing them as NRR electrocatalyst. Furthermore, the experimental realization of Al-based catalysts on graphene is not far-fetched research as synthesis of pristine Al-clusters with pulsed laser vaporization can be dated back to 2007 by Neal et. al. [27] With experimental improvements brought about by electron-beam irradiation, single atom substitution on graphene has been reported by Zagler and co-workers [28]. However, the evidence of graphene-Al clusters/nanoparticle composites is as well-known as other graphene-metal composites and the synthesis route follows the conventional chemical exfoliation or powder metallurgy technique [29–31]. In this work, we make a radical comparison of the electronic properties of NRR active Ru and Mo single atom to Al atom and Al-clusters (Aln) supported on N-doped double vacancy graphene (N4-DVG). The study focuses on modulating the electronic and catalytic properties of atomic Al catalysts by inducing changes in their shape and size.All metal atoms and clusters supported N-doped double vacancy graphene (M@N4-DVG) systems, as shown in Fig. 1 are optimized using Density Functional Theory (DFT) calculations with Vienna ab − initio Simulation Package (VASP.5.4.4) [32]. The ionic-electronic interactions on all systems are sampled with a 2 × 2 × 1 Monkhorst- Pack kpoint grid and 520 eV energy cut-off with a generalized gradient approximation Perdew-Burke-Ernzerhof (PBE) functional [33]. All M@N4-DVG systems have been relaxed with DFT-D3 corrections to incorporate long range forces till the atomic forces and energies converge to 0.005 eV/Å and 10−5 eV/atom, respectively [34]. Electronic property analysis has been carried out to evaluate the density of states and Bader charges of the M@N4-DVG systems by considering a higher kpoint grid of (9 × 9 × 1) Monkhorst-Pack grid [35]. The thermal stability of the M@N4-DVG systems analysed through Ab initio molecular dynamics (AIMD) simulations carried out in an NVT ensemble at 298 K described with a Nose–Hoover thermostat at 3 ps time step for 10 ps [36]. Furthermore, the feasibility of achieving chemically stable M@N4-DVG systems is realized via the binding energies (Eb) of atomic metal catalysts and clusters on the N4-DVG system computed using the equation, (1) E b = E M @ N 4 - D V G - ( E N 4 - D V G ) - E M where, E M @ N 4 -DVG is the total electronic energy of Ru, Mo, Al metal atom catalysts or Aln (n = 2–7) clusters supported N4-DVG systems, E N 4 -DVG is the total electronic energy of N4-DVG systems and E M is the electronic energy of Ru, Mo, Al single atom catalysts or Aln clusters. Following this, the N2 chemisorption efficacy on the M@N4-DVG catalysts is investigated via the end-on and side-on modes of N2 adsorption; and the adsorption energy, Eads is computed using the equation, (2) E ads = E M @ N 4 - D V G - N 2 - E M @ N 4 - D V G - E N 2 where, E M @ N 4 - D V G - N 2 is the total electronic energy of M@N4-DVG system after N2 adsorption, E M @ N 4 - D V G and E N 2 are the total electronic energy of M@N4-DVG systems and free N2 molecule, respectively.The free energy of the NxHy intermediates involved in the Nitrogen Reduction Reaction (NRR) is represented by the Gibbs free energy change, ΔG and the computational Standard Hydrogen Electrode model of Nørskov et al. has been implemented to calculate ΔG using the following equation [37]. ΔG = ΔE + ΔZPE − T ΔS(3) where, ΔE and ΔZPE is the change in electronic energy and zero-point energy respectively, ΔS is the change in entropy at room temperature, T is room temperature (298.15 K). The zero-point energy and entropy corrections are computed from the non-negative vibrational frequencies of the gas phase species in each intermediate. The potential rate-determining step (PDS) for the reaction is intermediate step with the highest free energy change (ΔGmax) and the limiting potential, UL is equal to –(ΔGmax)/e. For an electrocatalyst under applied potential, the free energy is calculated as, ΔGNRR = ΔE + ΔZPE − TΔS + neU + ΔGpH , where n is the number of electrons, U is the applied electrode potential equivalent to the limiting potential, UL and ΔGpH is the free energy correction to pH of the solvent. The pH correction to free energy is represented by ΔG pH = 2.303 × k B T × pH, where k B is the Boltzmann constant. The pH value is assumed to be zero as the overpotential of NRR is unaffected by the change in pH.[38–39].The electronic stability of the M@N4-DVG systems as evaluated from the binding energy calculations show the metal single atoms (Ru, Mo and Al) to be positioned in the N tetra-coordinated vacancy in the graphene plane, while the Aln clusters are anchored with one Al atom occupying the double vacant site in graphene plane (i.e., in-plane) and the remaining Al atoms bound to the in-plane Al-atom. Mo single atom (Mo@N4-DVG) has been found to possess the highest binding energy of −9.78 eV followed by Ru@N4-DVG with −8.19 eV and Al@N4-DVG with −7.81 eV. The Aln clusters supported N4-DVG systems show a gradual decrease in their binding energies as the size of the clusters increase. The Aln clusters with a planar geometry and higher coordination with the in-plane Al are more stable with binding energies ranging from −7.1 to −7.5 eV as provided in Supporting Information, Table S1. On the other hand, Al7@N4-DVG system with a nearly spherical structure and two coordinated Al atoms to the in-plane Al atom show the least binding energy of −5.82 eV. Larger clusters are thus, not considered in this study as they tend to form spherical and symmetric structures. For ambient nitrogen fixation, the thermal stability of all M@N4-DVG systems are further analysed through AIMD simulations at 298 K and small structural distortions are observed in Al4@N4-DVG, Al5@N4-DVG and Al7@N4-DVG while both Al6 clusters showed large distortions after 10 ps, see Supporting Information Figure S1. Henceforth, all the M@N4-DVG systems, except Al6a@N4-DVG and Al6b@N4-DVG are eligible candidates to be implemented as stable catalysts at room temperature and further electrocatalytic studies will be carried out on the stable catalysts.The catalytic activity of a system, being an inflection of electronic properties and charge distribution or transfer efficiency, can be primitively assessed from its work function(Φ) and p-band center. While catalysts with a lower work-function will require a smaller energy to activate the N2 molecule, a more positive p-band center will ascertain the p-orbitals of the active centers are closer to the Fermi level and possess higher carrier density. A comparative plot of work function and p-band centers of Ru, Mo, Al metal atoms and Aln clusters in Fig. 2 (a) shows the Ru single atom with most positive p-band center of −5.25 eV and work function of 4.29 eV. The p-band centers of Ru and Mo has been computed in place of d-band center to have a consistent comparison with Al which possess only p-orbitals. Ru, being the best performing metal single atom catalyst on N-doped graphene, is used as a reference for another active transition metal, Mo and our metal atom of interest Al and its clusters, Aln. Al@N4-DVG system with Al single atom shows a much lower (i.e., negative) work function than the Ru or Mo counterparts, however its p-band center is relatively more negative (-5.74 eV) thereby inferring a lower charge carrier density. Although the work function gradually increases with the increase in size of Aln clusters and a decrease in catalytic activity is expected, the p-band centers show an interesting trend with the Al5@N4-DVG system has its p-band center at −5.69 eV and more positive than Al@N4-DVG. This primitive screening of catalytic activity is ratified through the N2 adsorption strengths of the M@N4-DVG catalysts. While the presence of d-orbitals in Ru and Mo single atom catalyst allow both parallel and perpendicular modes of N2 adsorption on Ru@N4-DVG and Mo@N4-DVG, the most optimal adsorption mode and site of N2 on the Aln@N4-DVG catalysts are found to vary with the change in shape and size of the Al cluster. The most exothermic adsorption of N2 in perpendicular and parallel mode has been considered as N2 adsorption sites on Aln@N4-DVG catalysts and are shown in Table S2 of Supporting Information. From Fig. 2(b), it is evident that lower (or positive) p-band center in Ru@N4-DVG influences the exothermic adsorption of N2, while a lower work function is responsible for the same in Mo@N4-DVG. The higher activity and exothermic adsorption of N2 on Al5 cluster can also be ratified due to the shape and orientation of the Al atoms that are available for interaction with the incoming N2 molecule. As the Aln cluster size increases, one Al atom lies in-plane to the N-doped graphene sheet while the remaining Al atoms orient themselves with 3 or 4 as its coordination number. While most Aln clusters prefer to form 3-coordination leading to triangular facets, the stable Al5@N4-DVG catalyst prefer to form a rectangular facet with four Al-atoms exposed as catalytic sites from the ELF plots as shown in Table S3 of Supporting Information. Furthermore, the Bader charge analysis provided in Table S3 ensues these exposed Al atoms to be electron rich while the Al-atom ingrained to the graphene plane is positively charged or electron deficient. This further corroborates to the lower N-N activation of Al single atom as compared to electron rich Al-atoms lying above the graphene plane which can easily render electrons to N2 molecule.The N2 adsorption energies on Al single atom and its clusters are relatively lower than the transition metal counterparts; however, a similar trend of work-function influencing adsorption can be observed in Al@N4-DVG, Al2@N4-DVG and Al3@N4-DVG while the p-band center becomes accountable for Al5@N4-DVG. More interestingly, Al5@N4-DVG is the only Aln based catalyst that shows exothermic side-on N2 adsorption (-0.34 eV) and N-N bond activation (1.37 Å) brought about with the nitrogen atoms attached to different Al centers. Fig. 3 shows the overlap of Al p-orbitals (Al5@N4-DVG) and N p-orbitals (N2) in the Fermi region of PDOS and electron localization function (ELF) plot with localized electrons on the N2. Besides Al5@N4-DVG, the systems of interest that show exothermic N2 adsorption and N-N bond activation are Ru@N4-DVG, Mo@N4-DVG, Al@N4-DVG, Al2@N4-DVG and Al3@N4-DVG. The electron localization functions of adsorbed N2 on the above-mentioned M@N4-DVG catalysts are provided in Supporting Information, Figure S2. ELF plots with higher electron density concentration on the atoms will correspond to ionic bonding while contribution from covalent bonding can be accounted when the electron density is concentrated on the respective bond between two atoms. A prominent electron localization on N2 can be observed in Ru@N4-DVG and Mo@N4-DVG systems inferring an ionic bonding or stronger binding which can be interpreted as chemisorption led by electron transfer, whereas a more covalent bonding between Al atoms and N2 molecule can be observed in the Al-based catalysts inferring towards physisorption of N2. The presence of higher electron density in Aln clusters as compared to Al single atom can be a major contributor in enhancing the catalytic activity of Al metal for NRR. This is supported by the PDOS plot of N2 adsorbed Al@N4-DVG catalyst in Figure S3, Supporting Information shows minimal contribution from the Al p-orbitals. However, in aluminium cluster catalysts the contribution of Al p-orbitals is found to increase gradually along with a shift towards the Fermi level due to the conducting nature of Al. The distribution of electron density as seen from ELF plots and smaller orbital overlap between Al and N2 can be inferred as N2 physisorption on the Al-based catalysts and the N2 adsorption energies corroborate to this finding.Following this screening of N2 activation, the mechanisms of nitrogen reduction reaction (NRR) on all possible routes, Fig. 4 (a), are explored on the select M@N4-DVG catalysts that show exothermic N2 adsorption. Nitrogen reduction on Ru@N4-DVG and Mo@N4-DVG catalysts with Ru and Mo single atom center have been investigated via the distal, alternating and enzymatic route. The consecutive route has been found unfeasible as the *N-*NH2 intermediate could not be realized on the single atoms. Computational calculations on Al@N4-DVG, Al2@N4-DVG and Al3@N4-DVG catalysts that show end-on N2 adsorption have been restricted only for the distal and alternating route. Finally, Al5@N4-DVG catalyst which showed exothermic side-on N2 adsorption have been investigated for enzymatic and consecutive route of NRR, with multiple Al atoms being involved in N2 adsorption, the consecutive route becomes feasible in this catalyst. The free energy diagrams of the above-mentioned routes of NRR reaction coordinates on stable M@N4-DVG catalysts are provided in Supporting Information, Figure S4-S9. The reduction of N2 to NH3 is a multistep reaction with six elementary protonation steps and release of two NH3 molecules, the usual uphill reaction steps are the first (*N2→*N2H) and last protonation (*NH2→*NH3) steps in all routes along with the fourth protonation (*N2H3→*N2H4) step in alternating route. The uphill elementary step with the highest energy barrier becomes the potential rate determining step (PDS) of NRR and a summary of all possible routes and the PDS with ΔGmax values on all M@N4-DVG catalysts as shown in Fig. 4(b). The ΔGmax value on Ru single atom, reported as the best catalyst, has been found to be 0.53 eV in the first protonation step. However, Mo single atom which has been reported as an active NRR catalyst shows a relatively high energy barrier of 1.43 eV in the last protonation step. The ΔGmax values on the Al-based catalysts are 0.84 eV (*N2→*N2H), 1.35 eV (*NH2→*NH3) and 1.08 eV (*NH2→*NH3) in Al@N4-DVG, Al2@N4-DVG and Al3@N4-DVG respectively. While Al single atom and smaller Al-clusters show a relatively higher NRR performance than Mo single atom, the NRR performance improves to 0.78 eV in Al5@N4-DVG catalyst that showed exothermic side-on N2 adsorption. Furthermore, upon application of an external potential 0.78 eV as shown in Fig. 4(c), the elementary (*NH2→*NH3) protonation steps become exothermic, thereby inferring this catalyst can also be implemented as an electrocatalyst.A detailed electronic analysis of the Bader charges as shown in Fig. 5 (a) of Ru@N4-DVG shows a correlation between the charge transfer from Ru to the N atoms. Most importantly, the PDS fourth protonation step, *NH–*NH2 → *NH2–*NH2 step has been found to show a large difference between Ru charge and N charges, thus signifying that the electronic barrier essential to bring about ammonia production. Al@N4-DVG catalyst that show a preference of the alternating route with better stabilized *NxHy intermediates exhibit contrastingly higher Bader charge difference in the last protonation step although the PDS is the first protonation step, *N2→ *N-NH. This is in concordance to an uphill step of 0.81 eV observed in·NH3 formation as shown in the Supporting Information, Figure S6. Furthermore, in the ELF plot shown in Fig. 5(b) inset for·NH3 intermediate, a relatively higher electron density can be observed in the N atoms from the Al single atom along with Al-N covalent bond further stabilising the system. This possess a major challenge in the functionality and applicability of Al@N4-DVG catalyst, as the active Al metal site gets deactivated due to strong adsorption efficacy of NH3, which is −0.90 eV exergonic from N2 adsorption. Interestingly, Al5@N4-DVG catalyst behaves similar to Ru@N4-DVG catalyst and shows a large variation in Bader charge of Al and N only in its PDS step, i.e., *NH2→·NH3 step, Fig. 5(c). Additionally, the dissemination of electron density in the constituting Al atoms in Al5 center leads to efficient electron transfer to N atoms, leading to formation of NH3 without the manifestation of any covalent bond between Al center and N atoms of NxHy intermediates. The corresponding NH3 adsorption on Al5@N4-DVG is exoergic by −0.41 eV as compared to N2 adsorption and the possibility of catalytic center deactivation or poisoning can be reduced as 5-Al metal centers are involved. Another similarity of the Al5@N4-DVG catalyst to the Ru@N4-DVG catalyst is an exclusive NRR selectivity over the competing hydrogen evolution reaction (HER), Fig. 5(d). An interesting outlook can be accounted on the NRR performance of Al5@N4-DVG catalyst in the presence of water solvent, details of implementing solvent model and calculations are discussed in Supplementary Information. N2 molecule being non-polar, its adsorption energy in water should be endothermic as compared to its value in vacuum; while the protonated NxHy intermediates possess dipoles and water as a solvent enhances the formation of NxHy intermediates. Thereby, a lower adsorption energy of N2 and higher free energies of NxHy intermediates on Al5@N4-DVG system can be expected with solvent effects and the same has been compared with the energetics in vacuum for the consecutive route of NRR on Al5@N4-DVG, as seen in Fig. 5(e). In the presence of water, the adsorption energy of N2 on Al5@N4-DVG system reduces from −0.06 eV to −0.05 eV in perpendicular mode and −0.37 eV to −0.27 eV in parallel mode. As anticipated, the following protonation steps leading to formation of NxHy intermediates are energetically favourable with more negative ΔG values when compared to vacuum state (Figure S10). The corresponding ΔGmax reduces from 0.78 eV to 0.70 eV for the consecutive route and PDS shift from the last protonation step, *NH2→·NH3 in vacuum to fourth protonation step, *N →*NH in water solvent. This can be attributed to the solvation of the *N intermediate with open coordination sites and the water pockets hindering the transport of H+ to form the *NH intermediate. Contrastingly, the protonation of *NH2 intermediate to·NH3 intermediate which was less feasible in vacuum becomes more facile as water can enhance the transport of protons and formation of·NH3. This also substantiates that Al5@N4-DVG catalyst can be rendered for lab-scale experimentations in aqueous conditions.A comparison of the NRR performance on the Al-based catalysts has been highlighted in Table S4 of Supplementary Information. The ΔGmax of NRR on Al5@N4-DVG catalyst has been observed to be higher than several homoatomic or heteroatomic bimetallic transition metal catalysts, however in several cases of homoatomic catalysts, i.e., Ru2 @PC6, Cu2@NG, Ni4@Gr catalysts the NRR performance is on-par and higher in some cases. It can also be noted that Al5 cluster anchored on BN-doped graphene showed the lowest barrier for N-N bond activation in the study carried out by Kumar et al. and our studies concur to their findings.[26] Aluminium clusters on N-doped double vacancy graphene, despite a less attractive NRR performance than transition metal single atoms, perform as par the Ru single atom catalyst with a high selectivity for NRR and a trade-off can be achieved when researchers aim for scalable and sustainable catalyst for ammonia production.In this study, DFT investigation has been made to conform an earth-abundant metal Al to conform and exhibit similar catalytic properties to another rare earth transition metal, Ru for nitrogen reduction. Al-based catalysts have been modulated into a Ru-single atom like catalytic center by varying number of Al centers. A detailed study on the electronic and thermal stability of the model catalysts have been made via AIMD studies and the catalytic properties are primitively scoured through their inherent electronic properties. An analysis of the electron localization function and projected density of states plots shows a strong chemisorption in the transition metal, while a weak physisorption is observed in the Al-based catalysts. The change in shape and size of the atomic Al clusters reflects to a change in their corresponding catalytic properties, and Al5 supported on N-doped double vacancy graphene (N4-DVG) conform to Ru-single atom like catalyst. Bader charge analysis of the NRR reaction intermediates show a similarity in the large charge transfer to N atoms from Ru single atom and Al5 center, with respective ΔGmax of 0.53 eV and 0.78 eV in Ru@N4-DVG and Al5@N4-DVG catalysts. Despite a higher free energy change in the potential rate determining step, the high NRR selectivity of Al5@N4-DVG catalyst makes it a highly attractive catalyst for electrocatalytic ammonia production. The understanding from this work can be used to further the research on developing Al-based catalysts for nitrogen fixation and feasible ambient ammonia production with the most abundant metal, aluminium. Ashakiran Maibam: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing – original draft. Sailaja Krishnamurty: Software, Validation, Supervision, Writing – original draft. Ravichandar Babarao: Conceptualization, Software, Validation, Supervision, Writing – original draft.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.A.M. acknowledges AcSIR-RMIT for hosting the Joint-PhD Program and RMIT University for research funding. The authors gratefully acknowledge National Computing Infrastructure (NCI), and Pawsey supercomputing centre, Australia for providing the computational resources.Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc.2023.157024.The following are the Supplementary data to this article: Supplementary data 1

Density Functional Theory (DFT) investigation on the most earth-abundant Al-based catalysts, has been conducted detailing its electronic properties and catalytic efficacy for nitrogen reduction at ambient condition. The Al-based catalysts have been modulated to perform as par a highly performing, but rare, Ru-single atom catalytic center by varying number of Al atoms, shape, and size. The coalesce of band-center, work function and electronic properties in metal atom catalysts along with N-N bond activation has been demonstrated to be responsible for an efficient nitrogen reduction reaction (NRR) with ΔGmax of 0.78 eV in Al5 supported on N-doped double vacancy graphene (Al5@N4-DVG) catalyst. Electron localization function analysis has shown a weak physisorption of N2 in the Al-based catalysts. Projected Density of States (PDOS) illustrates the enhancement of aluminium electron density in Al5@N4-DVG led to enhanced orbital densities overlap of Alp and Np electrons. The Bader charge analysis and electronic analysis of the intermediates show efficient electron gain on the N atoms, leading to formation of NH3 from the NxHy intermediates in Al5@N4-DVG catalyst.

Dyes are a significant part of aquatic pollution released by the paper, tanning, cosmetic, textile and paint industries. Properties like structure uniqueness, chemical stability, different functional groups, and characteristic color of methyl orange and methylene blue make them relevant in industries [1–5]. Methyl orange is an azoic dye, and methylene blue is a thiazine dye. Wastewater containing these two compounds causes toxic effects on humans and aquatic life by getting into the food chain. The stability of dyes and their by-products makes them more hazardous due to their mutagenic and carcinogenic nature [6,7]. Due to their non-biodegradability conventional biological degradation methods are not applicable. Today scientists are extensively trying to remove or degrade these dyes from industrial waste before their release in freshwater bodies [8–11].Physical and chemical methods like adsorption and degradation, respectively, are used to remove these dyes from wastewater. Photocatalytic degradation is a widely used method for degrading toxic dyes from water bodies with nanoparticles help. NPs are directly or indirectly involved in degrading more toxic dyes into less harmful by-products [12–14]. NPs TiO2, Ag, Zn, Au, Pd, Cu, bi-metallic NPs Ag–Zn, Ag/Ni, Ag/Cu, Co/Fe, nano-composites like SnO2 decorated polystyrene, carbon-doped TiO2 and ZnO/CMC NCs are used for the catalytic degradation of dyes. Their activity in the visible region makes them photo-chemically active and efficient for dye degradation. Their small size and large surface area offer more active sites for the reaction to take place easily with less time compared to bulk material [9,15,16].Metal and non-metal NPs have ostentatious catalytic properties compared to bulk materials. High surface-to-volume ratio, surface plasmon effect, catalytic, optical, conductivity, and thermal properties make their applications in wastewater treatment by catalytic and photocatalytic reduction of organic dyes. Bimetallic nanoparticles (BMNPs) show extraordinary results in almost all fields compared to mono-metallic or non-metallic NPs in technical and scientific fields. BMNPs alter the surface plasmon band, stability, and dispersion of NPs [17–19].The number of metal-NPs comprising silver, zinc, TiO2, gold, platinum, and cobalt have been synthesized through physical, chemical, biological, and green routes. NPs prepared by chemical methods cause pollution and have adsorbed harmful chemicals on their surface, causing annoying adversarial effects in the treatment and diagnostic fields. Physical methods are expensive. They follow a top-down technique and produce NPs having a narrow morphological range [20]. Biological route uses microorganisms, enzymes and fungus for the stabilization of NPs. Green synthesis is the best way to synthesize the metallic, bimetallic NPs due to no hazardous waste, low cost, ease of characterization and bottom-up technique make one able to control their morphology, crystallinity, and size [21,22].This research work focusses on the use of Nicotiana tabacum (NT) dried leaves extract to synthesize Ag–Zn doped TiO2 nano-catalysts (NCs). Identification techniques comprises UV VIS spectroscopy, FTIR, SEM and XRD were used for the confirmation, morphology, and crystallinity of prepared NCs. These prepared nano-catalysts were used for the catalytic and photocatalytic decolorization of methylene blue and methyl orange from industrial wastewater.Analytical grades reagents including AgNO3 (≥99%), zinc nitrate hexahydrate Zn(NO3)2.6H2O (≥97%) were purchased from Fisher (UK) and TiO2 (≥97%) was purchased from Sigma Aldrich (UK). All the reaction mixtures were prepared in deionized water. pH was optimized using NaOH. Fresh leaves of N. tabacum were collected from farms at Lodhraan Uggoki Sialkot, Pakistan.Freshly collected leaves were cleaned with DI water 2–3 times to remove dust particles and dried in the oven at 40 °C. Fully dried leaves were crushed in fine powder using a grinder. 1 g of leaves powder in 200 mL of distilled water is heated on hotplate at 60 °C for 20 min greenish brown color solution is obtained which is filtered to separate leaves powder. The supernatant is stored in 4 °C for further use.Titanium oxide (TiO2), silver nitrate (AgNO3), and zinc nitrate hexa-hydrate (ZnNO3.6H2O) were used for starting precursors. In the synthesis of Ag–Zn doped TiO2 NCs, 5 mM solution of TiO2 with 5% (w/w) AgNO3 and ZnNO3.6H2O was dissolved in 100 mL of distilled water and stirred well as done previously [23]. This solution was added to leaves extract dropwise on continuous stirring at maintained pH, color changes occurred from greenish brown to white, which conformed the synthesis of NCs. The solution was further centrifuged and washed many time to remove the un-reacted ions.The NT stabilized Ag–Zn doped TiO2 NCs were characterized by recording full scan of UV-VIS spectra from 300 to 800 nm and the surface plasmon of the NPs was also observed over UV-VIS Spectrophotometer CECIL CE 7400s Aquarius. Fourier transformation infrared spectroscopy (FTIR) by ATR was adopted to determine the functional groups using (Bruker, Alpha-II, UK) in the range of 4000–500 cm−1. SEM morphologies were evaluated over (FE-SEM NOVA 450, UK). The crystalline nature was examined using an X-ray diffractometer (Bruker D-8 with Cu Kα Λ = 1.54 Å, UK).Methylene blue (MB) and methyl orange (MO) were degraded photo catalytically using synthesized Ag–Zn doped TiO2 NCs under direct sunlight. Decolorization of 0.01 mM MB and MO dyes has been studied by varying catalyst concentration (1–5 mL of 0.2 mg/L) at maintained pH and 31 °C. The reaction mixtures were prepared and stirred under room light and then placed under sunlight for decolorization. Progress of the reaction was monitored by UV-VIS spectra ranging from 300 to 800 nm in specific time. Blue and orange color of dyes appeared in case of MB and MO, respectively [24–26].MB and MO were degraded by using NaBH4 reagent at prepared NCs catalyst and its factors were also studied to find the optimum concentrations of reagents for maximum decolorization. Dyes (MO, MB) and catalyst dose effects on decolorization were observed by varying concentration from 0.004 mM to 0.025 mM and 0.02 mg/mL to 0.1 mg/mL respectively in addition with 1.8 mM NaBH4. The catalyst (Ag–Zn doped TiO2) was confirmed over UV/Vis spectrophotometer at full scan of 300–800 nm. Absorbance peak of MB at 665 nm and that of MO at 460 nm was monitored for decolorization studies.In recent studies, synthesized Ag–Zn doped TiO2 NCs using leaves extract without any external hazardous chemical stabilizing agent. In this study, obtained NCs were better in terms of size, which was improved than other reported methods [27]. Plant leaves extract itself acted for stabilization of doped NPs by controlling their size, prevented coalescence with controlled nucleation and ordered crystallinity of NPs. The methods which confirmed the formation of NCs are followed.FTIR analysis of NT stabilized Ag–Zn doped TiO2 NCs was caries out to examine which biomolecules are responsible for their reduction and stabilization. Fig. 1 and Table 1 represents the transmittance peak at 3357, 2919, 2851, 1248, and 1091 cm−1. The spectra of Ag–Zn doped TiO2 NCs peak broad peak ranging from 3650 to 3100 indicating –OH stretching due to the water of crystallization adsorbed on the NCs [28], peaks located at 2919 and 2851 cm−1 exemplifies C–H stretching and peak at 1248 cm−1 illustrated amide (III) stretching of amino groups present in leaf extract. Peaks at 1593 cm−1 indicating CC stretching [29]. Strong peak at 1091 cm−1 of C–O stretching [30]. Displacing of spectral lines from NT leave to NCs showed interconversion of functional groups for reduction and stabilization of NCs. Kumar et al. also reported the synthesis of silver NPs using NT leaf extract with peaks at 3403, 2934, 2396, 1761, 1624, 1384, 1075, cm−1 with almost similar functional groups [31].For the determination of crystalline nature of prepared Ag–Zn doped TiO2 NCs XRD analysis performed under the range of 10–80°. Fig. 2 corresponds to XRD pattern peaks appeared at 2 θ having values 69.00°, 65.50°, 64.04°, 62.75°, 56.62°, 54.31°, 44.04°, 41.23°, 39.18°, 36.08° and 27.43° represent the TiO2 (rutile) (JCPDS 75–1748), and additional peaks at 41.00°, 59.38° and 74.70° has confirmed the presence of AgZn (JCPDS 65–6585) as doped material. The presence of curve at lower angle may be due to its cubic structure and narrow peaks in the spectra are due to the nano-sized structure of the particles. High intensity peaks at 27.34° for 2 θ value is preferred orientation for synthesized nano-composites with comparatively lowest surface energy than other planes.Debye Scherer equation applied for the determination of average size calculation of synthesized Ag–Zn doped TiO2 NCs. D a v g = k λ β cos θ Where k is dimensionless crystalline factor with value of 0.96, λ represents the wavelength of Ni–Cu Kα radiations and d and θ represents the full width half maximum in radian of each peak and angle is radiant, respectively. Average crystallite size of synthesized Ag–Zn doped TiO2 NCs was found 5.66 nm.Field emission scanning electron microscopy (FE-SEM) was utilized for the morphological and structural determination of fabricated Ag–Zn doped TiO2 NCs (Fig. 3 ). It is defined that the NCs has a wide range morphology including clusters, rose petals like arrangements and nano-rods (Fig. 2a). NCs mainly formed and stabilized by the NT extract. As it is evident from the XRD analysis, the crystallite size of the Ag–Zn BMNPs was 5.66 nm. A variable shape (poly-morphological structure) was observed of Ag–Zn doped TiO2 NCs. In earlier studies, it was shown that surface area and shape play a more significant impact in the adsorption and reduction of adsorbed molecules in catalytic processes. The wide range of morphology and large size of synthesized NCs was may be due to the doping of Ag–Zn BMNPs onto the surface of TiO2 which is a well-known catalyst to enhanced the NCs catalytic properties by improving the energy bandgap and leads its vast range application in different fields [35].The UV-VIS spectroscopy was used to investigate the optical activity of Ag–Zn doped TiO2 NCs. UV-VIS measurements verified the presence of co-doped NCs. Metallic NPs have some absorption maxima in the UV-VIS region; hence UV–vis spectrophotometric measurement can be used as a quick preliminary test to validate the nanoparticles fabrication. Sudden colour change was the primary sign of Ag–Zn doped TiO2 NCs, Ag doped TiO2 and Zn doped TiO2 synthesis, and this change was then analyzed by UV–Vis spectrophotometer at wavelengths ranging 200–800 nm to find the surface plasmon resonance (SPR) of Ag NPs and lamda max of Zn NPs. Fig. 4 indicated a prominent peak at 400 nm in the UV–Vis spectra of greenly produced Ag doped TiO2 and Ag–Zn doped TiO2 NCs. Similarly, the previously results proven our results for peaks of TiO2, ZnO and Ag NPs at 276 [36], 292 [37] and 420 nm [38] respectively. The minor displacement of the spectral peaks was observed for Ag doped TiO2, Zn doped TiO2 and Ag–Zn doped TiO2 NCs, which was due to the doping and combined effect of bi-metals. These peaks were handful for the determination of bandgap.Methylene blue (MB) is the widely used dye in major industries including cosmetics, fabrics, chemical, pharmaceutical, and agriculture industries. Almost 6 tons of dyes are wasted yearly in water which effect water bodies, animals and humans. These days, scientists are producing materials to reduce these dyes by adsorption, catalytic, photocatalytic decolorization and advanced oxidation processes. In our research we mainly focused to reduce MB catalytically, photo catalytically and different factors that affect the reduction of dyes including pH, Catalyst, dye and reducing agent concentrations [2,13,39]. Catalytic decolorization means reduction of dyes using a catalyst system which provide specific area for dye and reducing agent to adsorb and complete reaction of decolorization either by reduction or breakage of the compound. It is a slurry phase reaction in which catalyst is in solid phase and reactants are in liquid phase [17,40]. Herein, Ag–Zn doped TiO2 NCs as the catalyst for the reduction and decolorization of MB dye has been done by using 0.01 mM MB, 1.8 mM NaBH4 and 0.06 mg/mL catalyst concentrations as clearly shown in Fig. 5 a. While, the decolorization also monitored in the absence of catalyst at same optimum conditions (Fig. 5b). The results revealed that after 90 min there was minimal effect of NaBH4 for the decrease in the absorbance of MB dye. Similarly, MO dye decolorization was also studied at optimum conditions, 0.1 mM MO dye, 0.06 mg/mL catalyst and 1.8 mM NaBH4 (Fig. 5c). The results were outstanding as the decolorization was completed in just 16 min and to confirm whether it is decolorization or just NaBH4 is reducing the dye molecule, another reaction was done on same conditions but without catalyst presence (Fig. 5d). The results shown no decolorization after 75 min, which proven the efficacy of NCs catalyst. Decolorization of dye in the presence of reducing agent NaBH4 and catalyst Ag–Zn doped TiO2 has been explained on the basis of Langmuir Hinshelwood (LH) mechanism. Fig. 5e explains the decolorization mechanism of both dyes. First of all, reducing agent ionizes in water and produce BH- 4 ions which get adsorb on the surface of catalyst and provide H+ on it. At the same time, dye molecules get adsorb on the surface of catalyst and reaction starts in which double bonded nitrogen atom present in the dye molecule which give characteristic color and properties to dye molecules reduces by up taking of hydrogen from catalyst surface and reduction of dye occurred in almost 8 min. Different factors including effect of catalyst, effect of dye, effect of reducing agent, pH of solution and at various temperatures the rate of decolorization of dye has been studied. Previously decolorization of dye has been monitored using NiFe2O4 NPs [41].Decolorization of MB was studies under different initial dye concentrations range from 0.001 to 0.025 mM, 0.2 mg/L NT stabilized TiO2 Ag–Zn doped TiO2 NCs dose, 27 °C temperature and contact time of 8 min (Fig. 6 a). MB colour removal was observed 73% at 0.020 mM concentration. After this concentration the decolorization decreases due the conjugation of dye molecules in solution. They face difficulty to reach at the surface of catalyst due to hyper conjugation and less molecules reach there to degrade [42]. It was previously reported that the decolorization of MB dye at Ag–Zn doped TiO2 NCs was 25% in 8 min. So it can be stated that the decolorization of dye depends upon the initial concentration of dye. It would be maximum at 0.020 mM with 1 mL of 0.2 mg/L and decreases after and before this concentration.Kinetic studies revealed the rate of the chemical reaction. Model reaction (Fig. 6b) explains that the decolorization of dye at 0.01 mM completed in just 8 min. The rate of the chemical reaction and different steps included in the complete decolorization of organic pollutant MB dye were studied (Fig. 6c). The reaction followed pseudo first order kinetics according to the equation (1) ln ( A t / A o ) = − k a p p × t It can be seen that at first there was very slow decolorization in 2 min which is induction time for the reaction to start, from 2 to 8 min' reaction speed up and completed in 8–10 min. The induction time, decolorization time and reaction completion time for different dye concentrations were different and almost increases with increase in concentration of dye. Fig. 6d and i explains the decolorization time of the catalytic reaction with same optimal conditions for MB and MO dyes’ decolorization at which the rate of decolorization was comparatively very fast which was at 0.01 mM for both dyes [43]. The more time taken by the reaction to complete at higher dye concentration is due to the limited active sites at constant amount of catalyst in all the reaction mixtures. At higher concentration of dye, molecules of dye face difficulty of come and adsorb on the surface of catalyst due to steric hindrance and take more time for completion of reaction.Decolorization of MB and MO with respected to time was observe and recorded in the form of percentage decolorization in Fig. 6e–j, which was 90% and 93% respectively. MO was completely degraded in just 16 min which is efficient as already reported decolorization time, which was about 90 to 60 min [44,45]. Kinetic studies revealed that the decolorization of MO dye was almost zero in first 5 min which could be called induction time. The next step was observed the decolorization time from 5 to 13 min and finally reaction completion time 13–16 min at different dye concentrations (Fig. 6g). Fig. 6h, corresponds to the slopes of pseudo first order catalytic decolorization kinetics of MO dye for the determination of kaap. Further, Fig. 6i, shows the MO decolorization in terms of kaap. This shows the remarkable effectiveness of NCs in the presence of NaBH4 to degrade anionic dye (MO) as well as for cationic MB dye.To study that which concentration of catalyst is most suitable for maximum decolorization of methylene blue dye, different amount of catalyst ranging from 0.02 mg/mL to 0.10 mg/mL were used against 0.02 mM MB dye. Fig. 7 a clearly shows that decolorization increases with increase in catalyst concentration. Fig. 7b shows the percentage decolorization of dye which more clearly explains that the decolorization of dye increases rapidly with increasing catalyst dose up to 1 mL and after this decolorization rate increases with very low rate. It means that 1 mL of catalyst dose if more reliable competitively for maximum decolorization of MB dye. By increasing the catalyst dose more active sites are available for catalytic decolorization but at very high concentration catalyst molecules hinders dye molecules to get adsorb on the specific active sites of the catalyst and react with the reducing agent to complete the reaction.Kinetic of the reaction revealed the best concentration at which the rate of the reaction is maximum and give better results in decolorization of pollutants in water treatment. As shown in Fig. 8 a, 0–2 min the reaction rate was almost zero at this state the molecules of the dye move towards the surface of Ag–Zn doped TiO2 NCs, after this from 2 to 8 min the rate raised exponentially due to reaction between dye molecules and reducing agent, at the end the reaction completed in 8–10 min. Fig. 8b shows the negative slope for the determination of kapp. It can be seen clearly that the rate of the reaction was very fast for 0.1 mg/mL catalyst dose due to the availability of large number of active sites for the decolorization of dye. The rate increases with increase of catalyst dose which reveals that the rate of concentration varies directly with amount of catalyst shows the efficiency of the catalyst. Fig. 8c explains the kapp at different concentration of catalyst by keeping the concentration of dye and reducing agent constant. The value of half-life and observed rate constants are shown in Table 2 [46–48].Decolorization of dyes using nano-catalysts are greatly affected by pH of solution. The effect of pH on decolorization of methylene blue (MB) and methyl orange (MO) was studied by retaining its array from 3 to 11 at 0.01 mM and 0.0045 mM primary concentration of dyes respectively, 1 mL (0.2 mg/L) Ag–Zn doped TiO2 NCs dose, 0.1% NaBH4 at room temperature for 8 min and response is exposed in Fig. 9 . Maximum MB removal was attained at highly acidic pH 3, it was 58.5% which gradually reduces to 15% at pH 11. Deceptively, the decolorization manner is acid-focused for MB. On other hand the removal of MO was maximum at highly basic pH 11, it was 67% and reduce to 0% at pH 3. Liu et al., 2017 reported that MB removal form waste water was higher at acidic medium and decreases with increase in pH of medium [49]. Table 2 gives the complete description of apparent rate constant, regression factor and Half-life of the different concentrations of MB and Ag–Zn doped TiO2 NCs dosage for decolorization of dye. It would be easy to select the optimum concentrations of reagents to get efficient decolorization of dye.The photocatalytic decolorization studies was observed on UV-VIS spectrophotometer with 300–800 nm range. The spectra were observed with initial 0.01 mM MB (Fig. 10 a) and MO dye (Fig. 10b) concentration at 1–5 mL of 0.2 mg/L. The photo-catalytic decolorization of MB and MO was studied as the function of Ag–Zn doped TiO2 NCs catalyst dose (Fig. 10c). Different conditions was adjusted including neutral pH and 180 min contact time at 26 °C room temperature. In case of MO the decolorization at 1 mL catalyst was 30% which increases to 45% at 5 mL catalyst dose after this decolorization remains constant this is due to the low absorption of light by the surface of catalyst to absorb and degrade the methyl orange. On other hand decolorization of MB increases with increase the catalyst dose. This is due to the increase the surface area for the MB molecules to be adsorbed by Ag–Zn doped TiO2 NCs. In the presence of light, the surface of catalyst absorbs the light photon to excite electron from valence to conduction band and to generate hydroxyl (OH•) free radical to decolorize dye effectively (Fig. 10d).The catalytic activity and photocatalytic activity of Ag–Zn Doped TiO2 increased due to the reduced bandgap of TiO2 after doping with Ag–Zn nano-particles. The bandgap of TiO2 material was reported to be 3.4 eV [50]. The bandgaps for Ag, Zn and Ag–Zn doped TiO2 was were calculated using UV-VIS studies (Fig. 4). In our studies the prepared nano-composites bear the bandgap of over 3 eV as shown in Fig. 11 . This was the main reason for the efficient catalytic activity of Ag–Zn doped TiO2 nano-composites. Table 3 , illustrates some previously done work for decolorization of dyes.Ag–Zn doped TiO2 NCs were effectively synthesized utilizing 5% AgNO3, ZnCl2 on TiO2 as a precursor and Nicotiana Tabacum leaves extract at 60 °C with continuous stirring. The Nicotiana Tabacum leaves extract acted as both a reducing and a stabilizing agent. The production of the Ag–Zn doped TiO2 NCs was confirmed by UV–visible spectral peaks at 274, 296 and 400 nm. Ag–Zn doped TiO2 NCs had an average crystallite size of 5.66 nm and a tetragonal geometry. The catalytic decolorization of MB-dye, NaBH4, and Ag–Zn doped TiO2NCs was then carried out using synthesized Ag–Zn doped TiO2 NCs. Detailed analysis showed that the reaction was completed in 8 min, and kinetic analyses of the data supported the pseudo-first-order process having kapp value 0.2431 min−1. Hence, Nicotiana Tabacum leaves extract can be used for synthesis of Ag–Zn doped TiO2 NCs. The authors suggested that Ag–Zn doped TiO2 NCs can be used to reduce azo-dyes, which can be a useful tool for treating wastwater from the textile industry.This research was funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2023R11), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors express their gratitude to Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2023R11), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

A facile green synthesis route was employed for the fabrication of Ag–Zn doped TiO2 nano-catalyst and utilized for the remediation of dyes. Nicotiana tabacum leaves extract was used to prepare the Ag–Zn doped TiO2 nano-catalyst by utilizing TiO2 as precursor and AgNO3, ZnNO3 as doping agents. A strong UV–Vis spectra peak confirmed the Ag–Zn doped TiO2 formation. Furior transformed infrared spectroscopy analysis revealed the role of phenolics in the N. tabacum leaves extract for the formation of Ag–Zn-doped TiO2 nano-catalyst. Moreover, Ag–Zn doped TiO2 nano-catalyst formation was confirmed by X-ray diffraction analysis, which reveals the cubic and tetragonal geometries with average size of 5.66 nm. The scanning electron microscopy analysis also confirmed the poly-morphological structure of prepared Ag–Zn doped TiO2 nano-catalyst. The nano-catalyst was used for the remediation of dyes and conditions were optimized for maximum removal of dyes using NaBH4 reducing agent. The catalytic activity results revealed that Ag–Zn doped TiO2 showed promising features versus individual counterparts, which is correlated with reduced bandgap of doped nano-catalyst. The N. tabacum leaves extract efficiently stabilized Ag–Zn doped TiO2 nano-catalyst that exhibited remarkable catalytic activity and have potential to treat the dyes in wastewater.

Steadily rising CO2 emission produced by human activities results in negative environmental consequences such as global warming and the increase of global mean sea level. It is imperative to reduce the emission of CO2. Various carbon capture and storage technologies have been developed for the reduction of CO2 emission and are employed to capture CO2 from abundant industrial sources such as fossil fuel-fired power plants. However, the availability of sufficient storage capacity is still an open question. Researchers have devoted efforts to developing more efficient approaches, which could employ CO2 to produce fuels, chemicals, and hydrocarbons [1–3]. The reverse water-gas shift (RWGS) reaction [4–7] has attracted increasing attention, especially high-temperature RWGS, which offers further CO-based process to methanol as well as long-chain hydrocarbons via Fischer-Tropsch synthesis.Various metals, including Cu, Fe, Ni, Pd, Pt, Rh, and Au, are active for the RWGS reaction. It is reported that Pd, Ni, and Cu show high catalytic activity with formate groups as an intermediate by combined in-situ FT-IR experiments and first principles [7]. Dai et al. reported that CO2 RWGS reaction catalytic activities decrease in the order Ni/CeO2 > Cu/CeO2 > Co/CeO2 > Fe/CeO2 [8]. Konsolakis et al. found that CO2 conversion followed the order: Co/CeO2 > Cu/CeO2 > CeO2 [9]. The activity can be affected by reaction condition, catalyst dispersion, particle size, surface morphology, and the nature of the oxide support [2,5,10–12], and thus there is no consensus on the activity trend of various metals. Catalysts screening of RWGS reaction under the consistent criterion is highly desired.Identify the reaction mechanism is essential to develop a more active and selective catalyst; thus, substantial efforts have been devoted to the mechanism investigation. Different reaction mechanisms have been proposed [12–15], for example, direct CO2 dissociation, COOH- and HCOO-mediated mechanism. The reaction mechanism depends on the specific catalyst and the reaction condition. DFT calculation found that direct CO2 dissociation is favorable on Rh, Ni, and Cu, while the COOH-mediated route is preferred on Pt and Pd [16]. The direct dissociate barriers can correlate to the oxygen adsorption strength, where the stronger adsorption of O provides a low CO2 dissociation barrier and results in the direct dissociation as a favorable route. CO2 dissociation can be followed by CO methanation reaction. Methane reaction is thermodynamically favored at low temperature, and high pressure [5], especially on Ru, Fe, Ni, Co, and Mo based catalyst [10]. CO methanation can either by CO-direct dissociation route or H-assisted route via HCO or COH, and it is generally accepted that the H-assisted path is more energetic favorable [17–19]. However, the results were mainly based on the analysis of DFT calculations performed at 0 K and 0 bar. It is essential to carry out a microkinetic analysis using temperature and pressure corrected free energy to identify the reaction mechanism and the rate-control steps at the realistic reaction.Microkinetic modeling based on DFT calculations is a powerful technology for the development of new or improved catalysts without intensive empirical testing. The models enable the incorporation of the fundamental catalytic surface chemistry into a kinetic model, and they can provide a fundamental understanding of reaction mechanism in addition to the prediction of activity and selectivity. Moreover, descriptor-based microkinetic modeling can correlate the activity to two simple descriptors and thus accelerate the catalyst screening. Herein, DFT calculations serve a tool to gain the adsorption energies and activation energies for the catalytic surface reaction among eight metals including Co (0001), Ru (0001), Fe (110), Ni (111), Cu (111), Rh (111), Pd (111) and Pt (111) surfaces. Microkinetic modeling of the RWGS reaction on each surface was carried out to identify the reaction pathway and rate-relevant steps. Descriptor-based microkinetic modeling on the eight metals was performed to predict the activity trend among various metals and achieve further catalyst screening, which could substantially contribute to the discovery of the RWGS catalysts.All DFT calculations were performed with the Vienna ab initio simulation package [20–22], where spin polarisation was employed for Fe, Ni and Co. The exchange-correlation functional was described by using Bayesian error estimation functional with van der Waals correlation (BEEF-vdW) [23]. The interaction between ion cores and valence electrons was described by the projected augmented wave (PAW) method [24], combined with the plane-wave expansion at a kinetic energy cut-off of 400 eV. M (111), M (110), and M (0001) surfaces were modeled by a p (3 × 3) unit cell with five layers, and a vacuum of 12 Å is set between two periodic repeated slabs. The bottom two layers were fixed at their corresponding bulk positions during the optimization. A 5 × 5 × 1 k-point grid was used to describe the Brillouin zone. The transition state was located by Dimer method [25], and the vibrational frequencies were calculated to confirm the transition states with one negative mode corresponds to the desired reaction coordinates.The adsorption energy and activation energy were calculated as E a d s = E A + s l a b − E A − E s l a b and E a = E T S − E I S , respectively, where E A is the total energy of the gas phase species, E slab is the total energy of the slab, E A+slab is the minimum total energy of molecule adsorbed on the slab. E TS is the total energy of the transition state, and E IS is the total energy of the initial state.The Gibbs free energy of each specie is calculated by using the following equation. G = E + E Z P E + Δ H o ( 0 → T ) − T S , where E refers to electric energy determined by DFT, E ZPE is zero-point energy. H, S, and T are enthalpy, entropy, and temperature, respectively. The precise calculation methods for zero-point energy, entropy, and enthalpy of adsorbed species was based on the harmonic approximation, which has been reported in the literature [26,27]. The same vibrational frequencies are employed for all the metals based on the results from Pt (111) since the variations in zero-point energies for various metal surfaces are significantly smaller compared with the adsorption energies [28]. The Gibbs free energies of the gaseous species were calculated with the Shomate equation, where the corresponding Shomate constants were reported in the NIST WebBook [29].The microkinetic modeling was carried out in Catalysis Microkinetic Analysis Package (CATMAP) [30], which can generate the catalytic trend based on the descriptor-based microkinetic modeling and is suitable for catalysts screening [31,32]. Formation energies are inputs to the model, which were calculated with the total energies of gas-phase CH4, H2O, and H2 as references. The simulation is conducted at T = 973 K and P = 1 atm with a H2/CO2 ratio of 3. High temperature chemical reactions have attracted growing attention for the next-generation energy conversion and storage processes [7]. RWGS reaction is thermodynamically favored by higher temperatures. Besides, the carbon formation by Boudouard reaction and methanation are disfavored at high temperatures. A H2/CO2 ratio of 3 was selected for stoichiometrically conversion CO2 to synthesis gas with a H2/CO ratio of 2, a typical ratio for methanol synthesis and Fischer-Tropsch synthesis. The reaction rates are generated by solving a mean-field model under the steady-state approximation. The differential equations in the microkinetic models are the following. r i = k i + ∏ j θ i j ∏ j P i j − k i − ∏ l θ i l ∏ l P i l ∂ θ i s ∂ t = ∑ s i j r j where r i is the rate of each elementary step, k i + and k i − are the forward and reverse rate constant, respectively. θ i j and θ i l are the site coverage of surface reactants and products, respectively, while P i j and P i l are the pressure of reactants and products, respectively. s i j is stoichiometry coefficients of species i in the elementary step j. ∂ θ i ∂ t equals zero at the steady-state, and the sum of site converges is constrained to 1. The pre-exponential factors of all the adsorption steps were calculated by assuming the sticking coefficient equals 1 [33].Three different reaction mechanisms were reported [16], namely, direct CO2 dissociation mechanism, COOH-mediated mechanism, and the HCOO-mediated mechanism. However, the high stability of HCOO on the surface makes it a spectator rather than a reactive intermediate [34,35] and results in high barriers for the decomposition of HCOO [36]. Therefore, we considered two reaction pathways, that is direct dissociation and COOH-mediated reaction mechanism, as shown in Scheme 1 . Methanation reaction occurs in addition to RWGS reaction, and a better catalyst should own higher RWGS activity and lower methanation activity. Thus, two methane formation pathways are taken into account, that is, CO direct dissociation and H-assisted CO to HCO and HCOH followed by dissociation to CH.The adsorption energies of various surface species on the eight metal surfaces (i.e., Co (0001), Ru (0001), Fe (110), Ni (111), Cu (111), Rh (111), Pd (111) and Pt (111) surfaces) are summarized in Table 1 . The adsorption configurations on Co(0001) have been reported in our previous publications [17,32]. C and O are firmly bound to the surfaces, indicating the high potential of carbonization and oxidation of the metals if it is not consumed efficiently in the reactions. Oxygen prefers to stick on Co (0001), Fe (110), Ru (0001), Ni (111) surfaces, while carbon more strongly binds to Fe (110), Ru (0001), Pt (111), and Rh (111). CO2, CH4, and H2O weakly adsorbed on the surfaces. Most metal surfaces display affinity to CO molecule with adsorption energies from −1.48 eV to −1.82 eV, except for Cu. The adsorption energy of CO on Cu is much lower than others, indicating that CO is more easily desorb from the copper surface rather than participates in the following methanation reactions, which may result in a high CO selectivity.The adsorption behavior of all the surface species among different metals can be roughly divided into two groups, namely carbon-based (C, CO, CH, CH2, CH3, and COOH) and oxygen-based species (O and OH), as illustrated in Fig. 1 . The adsorption energies of C, CH, CH2, and CH3 among metal surfaces follow a similar trend, where the adsorption becomes weaker in the sequence of Fe, Ru, Rh, Pt, Pd, Co, Ni, and Cu. The pattern of adsorption energies of CO and COOH slightly deviates from the CHX species. The adsorption strength of O and OH on different metals follows a similar trend. Fig. 2 illustrates that the adsorption energies of CH, CH2, CH3, CH4, CO, and COOH are correlated to the adsorption energy of C on the various surface since these carbon-based species bind with the metal surface via carbon. The adsorption energies of OH and H2O can be correlated to the adsorption energy of O. It indicates the adsorption energies can be represented by two descriptors, such as adsorption energies of C and O [37]. Table 2 summarizes the forward reaction barriers of elementary steps in the RWGS reactions on the eight metal surfaces (i.e., Co (0001), Ru (0001), Fe (110), Ni (111), Cu (111), Rh (111), Pd (111) and Pt (111) surfaces). Hydrogen gas easily dissociates on the most surfaces except for Cu. CO2 direct dissociation shows barriers smaller than 1.60 eV, indicating CO2 tends to dissociate on Co (0001), Ru (0001), Fe (110), Ni (111), Cu (111), and Rh (111), compared to the hydrogenation of CO2, as illustrated in Fig. 3 a. The direct CO2 dissociation barriers on various metal surfaces can be correlated to the adsorption energy of oxygen, as illustrated in Fig. 4 a, which is consistent with the previous report [16]. The difference between CO2 direct dissociation and CO2 hydrogenation can also be connected to the oxygen adsorption energy. It indicates the surface with higher oxygen-binding strength tends to direct dissociation rather than hydrogenation. The hydrogenation of CO2 can activate the CO–O(H) bond and decrease the CO–O(H) bond dissociation barrier on Co (0001), Ni (111), Cu (111), Rh (111), Pd (111), and Pt (111) surfaces compared with the direct dissociation. It is difficult to conclude which is the dominating reaction pathway of CO2 activation based solely on the comparison of the reaction barriers. Thus, microkinetic modeling is necessary to be carried out to figure out the reaction mechanism.The barriers of elementary steps in the two CO methanation pathways are compared in Fig. 3b. CO direct dissociation (the green column) exhibits high barriers on all the surfaces, and the barriers are higher than 3 eV for the precious metals Pd, Pt, and Cu. Similarly, CO direct dissociation barriers can be correlated to the oxygen adsorption energy, as displayed in Fig. 4a. CO hydrogenation presents lower barriers compared to the direct CO dissociation. The barrier difference between CO hydrogenation and direct dissociation can also be correlated to the oxygen adsorption energy, as illustrated in Fig. 4b. C–O bond dissociation in HCO needs a lower barrier than the direct CO dissociation, and HCO hydrogenation to HCOH can further decrease the C–O bond cleavage barrier. It seems that hydrogen-assisted CO dissociation to methane is energetically favorable on all the metal surfaces. However, the coverage of surface species also plays an essential role in the determination of reaction rates. Thus, the hypothesis needs further validation by microkinetic modeling.We performed microkinetic modeling of each metal surface to identify the reaction mechanism. The reaction rate of each elementary step is summarized in Table 3 . The reaction rate of CO2 direct dissociation is largely higher than COOH-mediated dissociation on Co, Fe, Ru, Rh, Cu, and Ni, while it is just one order of magnitude higher on Pt and Pd. Therefore, we can deduce that the direct CO2 dissociation is the dominating route on Co, Fe, Ru, Rh, Cu, and Ni, while the two pathways are competing on Pt and Pd.As we discussed in the last section, CO direct dissociation seems impossible to occur due to extremely high barriers. Surprisingly, their reaction rate of direct CO dissociation is at the same order of magnitude, or just one order of magnitude smaller than the H-assisted dissociation via HCO on Co, Fe, Pt, Pd, Ru, and Rh. It indicates the two reaction pathways for CO methanation compete to occur on these surfaces. Moreover, the HCOH dissociation to CH even owns a lower reaction rate than the direct CO dissociation on Co, Fe, Pt, Pd, Ru, and Rh, despite that the barrier of HCOH to CH is much lower, which may be due to the low coverage of hydrogen. The hydrogen-assisted pathway is the dominating route on Cu and Ni surfaces.Degree of rate control analysis is a powerful tool to identify the influential transition states, and thus a higher reaction rate can be achieved by adjusting their energies [38,39]. The degree of rate control of each intermediate and transition state to the CO2 consumption rate and the products (CO and CH4) generation rate were calculated based on the following equation. X ij  is the degree of rate control matrix, r i  is the rate of production for product i, G j  is the free energy of species j, k is Boltzmann's constant, and T is the temperature. X i j = d l o g ( r i ) d ( − G j / k T ) We summarized the most influential transition states and intermediates in Table 4 . The same rate-determining steps are observed for the CO2 conversation rate and CO production rate. It is elucidated that H–OH or CO–O is the rate-determining transition state, despite that the exact value varies on various metals. The eights metals can be divided into two groups based on the rate-determining step. CO–O bond cleavage is rate-determining on Pt, Pd, and Cu, owing to the high barrier on the surfaces, while OH binding with H to H2O is rate-determining on Co, Fe, Ru, Ni, and Rh. A negative degree of rate control of CO2 binding energy is found on Co, Fe, and Ru, while O on Ru and Ni. The negative value indicates that decrease the adsorption stability of the surface species could increase the activity.Additional rate-determining states or intermediates can affect the methane formation rate, in addition to the same rate-determining step with CO2 consumption rate. CO desorption has a negative effect on the methane formation on Co, Pt, and Cu, while O on Co, and Fe. CH3–H bond cleavage is the rate-relevant one for Co, Fe, and Ru, which is attributed to the higher barrier of the step. HC-OH bond cleavage is rate-controlling for Pt and Pd, HC-O for Rh and Cu, and CH2–H for Ni. These specific rate-relevant steps for methane is also the rate-controlling steps for methane selectivity. Tuning the energies for the particular rate-relevant transition states and intermediates can modify methane selectivity.The descriptor-based microkinetic modeling is a versatile tool to predict the catalyst activity trend and achieve catalyst screening. As we discussed, the adsorption energies of different surface species can be correlated to the carbon or oxygen binding energy. Thus, we employed the formation energy of C∗ and O∗ as two descriptors to describe the reaction kinetics of RWGS. Brønsted-Evans-Polanyi [40,41] relations were employed for transition-state scaling relations of all the steps except CO2 and CO dissociation. CO2 and CO dissociation are correlated to the final state in our setting since the barrier can be connected to the oxygen binding energy, as we discussed above.O and H are the abundant surface species at the reaction condition 973 K, as displayed in Fig. 5 . Talin et al. reported that the most abundant surface species are CO and H at 500 K [13]. The contrary is due to the modelings were performed at different temperatures. CO desorption becomes much easier compared to the CO dissociation or hydrogenation at high temperatures. Fe, Co, Ru, and Ni are covered by oxygen, indicating these surfaces are oxidized at this reaction condition, which is attributed to the strong bonding between oxygen and metal and the high barrier of oxygen hydrogenation on these surfaces.The activity of CO formation is in the sequence of Rh∼Ni > Pt∼Pd> Cu > Co > Ru > Fe, which is consistent with the experimental result from Dai et al. [8]. They reported that RWGS reaction catalytic activities are ranked as follows: Ni–CeO2 > Cu–CeO2 > Co–CeO2 > Fe–CeO2. The turnover frequency of methane formation is many orders of magnitude smaller than CO formation, and the activity trend of CH4 formation is in the sequence of Rh∼Ni > Pt∼Pd∼Co > Cu > Ru > Fe.We can find that CO selectivity is almost 100% for all the metals at high temperatures, which agrees with the experimental result performed at high temperatures [7]. They reported that CO selectivity of Pd and Cu achieve 100% CO selectivity at 973 K with H2/CO = 3, while it is slightly lower than 100% on Ni. Increasing the temperature or decreasing the H2/CO ratio can achieve 100% CO selectivity on Ni. Moreover, we calculated the ratio between the CO formation rate and methane formation rate (CO/CH4) and plotted the descriptor-based ratio mapping. CO/CH4 ratio selectivity ranks as follows: Cu∼Fe > Ru∼Pt∼Pd > Co > Ni > Rh, which is consistent with the experimental results. Chen et al. found that the trend of CO/CH4 ratio is Pt > Co > Ni. [42]. Fig. 6 demonstrated that the most active catalysts own the carbon formation energy and the oxygen formation energy around 1.64 and 0.24 eV, respectively. The interpolation concept of adsorption energy was used to search for potentially interesting bimetallic catalysts [43,44]. We performed DFT calculations to get the carbon and oxygen formation energies on hundreds of A3B type bimetal terrace surfaces, where M and N represent metals. As we found above, the methane selectivity is very low for all the metals. Thus only the energies close to the predicted optimum carbon and oxygen formation energies are interesting to us, as shown in Fig. 7 . We identified potential bimetals with high activity, that is, Cu3Ni, Ir3Sn, Pd3Co, Pt3Co, Pt3Rh, Pt3Sc, and Rh3 (Sc/Ga/Ge/In/Ir/Ni/Zn). PtCo has been reported to be highly active for the RWGS in the literature [42,45]. Cu3Ni is identified as to allow cost and highly active catalysts, which has recently been demonstrated by Xiao and coworkers [46]. The other catalysts need further experimental validation.The adsorption behavior of all the surface species on different metals can be split into two groups, carbon-based and oxygen-based species. Each group follows a similar trend among metals, which indicates that two descriptors can represent the adsorption energies of various species. It is difficult to identify the dominating route for CO2 dissociation to CO by solely comparing the reaction barriers. In contrast, hydrogen-assisted CO dissociation to methane is energetically favorable on all the metal surfaces.The microkinetic modeling suggested that the direct CO2 dissociation is the favorable pathway on Co, Fe, Ru, Rh, Cu, and Ni, while it competes with the COOH-mediated route on Pt and Pd. CO direct dissociation and H-assisted pathways for CO methanation compete to occur on Co, Fe, Pt, Pd, Ru, and Rh, despite that the high barriers of CO direct dissociation. It demonstrates that it is appropriate to identify the reaction mechanism by performing microkinetic modeling rather than exclusively comparing the reaction barrier.The degree of rate control analysis demonstrates that the rate-determining step varies on different surfaces. CO–O bond cleavage is the rate-determining on Pt, Pd, and Cu, while OH binding with H to H2O is rate-determining on Co, Fe, Ru, Ni, and Rh. Methane formation has an additional rate-controlling step, which is CH3–H bond cleavage for Co, Fe, and Ru, HC-OH for Pt and Pd, HC-O for Rh and Cu, and CH2–H for Ni. The methane selectivity can be hindered by adjusting the surface properties to increase the barrier of the rate-determining step for methanation.Two-dimensional volcano plots were constructed by coupling the scaling relations in a microkinetic model using C- and O- formation energy as descriptors. The microkinetic modeling elucidates that the activity trend of CO formation is in the sequence of Rh∼Ni > Pt∼Pd > Cu > Co > Ru > Fe, which agrees with the reported experimental results. Moreover, the model suggests that Fe, Co, Ru, and Ni tend to be oxidized at the reaction condition. We also constructed volcano plots of the ratio of CO/CH4 as a function of the two descriptors. The two-dimensional volcano plots were used to search for new alloy catalysts of high activity and low selectivity to methane, based on the interpolation concept of adsorption energy. As a result, several bimetallic catalysts were identified to be potentially interesting catalyst materials, where the Cu3Ni was screened as a candidate with a low cost and high activity.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The financial support from the Centre for Industrial Catalysis Science and Innovation (iCSI), which receives financial support from the NO-237922. The Research Council of Norway, is gratefully acknowledged.

Reverse water gas shift (RWGS) catalysis, a prominent technology for converting CO2 to CO, is emerging to meet the growing demand of global environment. However, the fundamental understanding of the reaction mechanism is hindered by the complex nature of the reaction. Herein, microkinetic modeling of RWGS on different metals (i.e., Co, Ru, Fe, Ni, Cu, Rh, Pd, and Pt) was performed based on the DFT results to provide the mechanistic insights and achieve the catalyst screening. Adsorption energies of the carbon-based species and the oxygen-based species can be correlated to the adsorption energy of carbon and oxygen, respectively. Moreover, oxygen adsorption energy is an excellent descriptor for the barrier of CO2 and CO direct dissociation and the difference in reaction barrier between CO2 (or CO) dissociation and hydrogenation. The reaction mechanism varies on various metals. Direct CO2 dissociation is the dominating route on Co, Fe, Ru, Rh, Cu, and Ni, while it competes with the COOH-mediated path on Pt and Pd surface. The eights metals can be divided into two groups based on the degree of rate control analysis for CO production, where CO–O bond cleavage is rate relevant on Pt, Pd, and Cu, and OH–H binding is rate-controlling on Co, Fe, Ru, Ni, and Rh. Both CO-direct dissociation and hydrogen-assisted route to CH4 contribute to the methane formation on Co, Fe, Pt, Pd, Ru, and Rh, despite the significant barrier difference between the two routes. Besides, the specific rate-relevant transition states and intermediates are suggested for methane formation, and thus, the selectivity can be tuned by adjusting the energy. The descriptor (C- and O- formation energy) based microkinetic modeling proposed that the activity trend is Rh~Ni > Pt~Pd > Cu > Co > Ru > Fe, where Fe, Co, Ru, and Ni tends to be oxidized. The predicted activity trend is well consistent with those obtained experimentally. The interpolation concept of adsorption energy was used to identify bimetallic materials for highly active catalysts for RWGS.

Electrochemical reduction of CO2 to generate valuable chemical feedstocks and fuels is considered as one of the most promising technologies not only to reduce the rapid increasing atmospheric CO2 concentration but also to mitigate the serious fossil fuel shortage. 1–4 Furthermore, the electrochemical processes could be powered by those renewable energies that suffer from unpredictable and intermittent supply, for example solar, wind and tidal electricity. Compared with other CO2 reduction technologies, electrochemical reduction exhibits some unique advantages such as simple operation, free H source, controllable products, compact modules, etc. 5–8 The performance and economic feasibility of CO2RR can be efficiently evaluated in terms of its overpotential, activity (current density), selectivity and stability, which highly depend on the catalysts used. Therefore, most of researches focus on designing and fabricating high-performance catalysts.Traditional heterogeneous catalysts can be used for CO2RR, including metals, 9–11 metal compounds (oxides, 12 , 13 sulfides, 14–18 etc), MOFs 19–21 and their composites. 22 Although much progress has been made in the aspect of low overpotential, 21 , 23 near-unity selectivity, 24–27 large current density, 28–32 high turnover frequency (TOF) 33 , 34 and long-term stability. 22 , 35–37 The inexplicit active sites and limited atom utilization of bulk catalysts greatly hinder the in-depth understanding of the reaction process and the fabrication of electrocatalysts with high performances. Fortunately, single atom catalysts (SACs) have been developed since Pt1/FeO x was reported by Zhang et al., in 2011, 38 which rapidly attracted global attention due to their high activity and maximum atom utilization efficiency. 39 , 40 Additionally, due to the relative simple coordination configuration of SACs, their active sites for CO2RR can be clearly identified, which can be efficiently modulated through tuning coordination environment for the selective adsorption of certain reactants/intermediates. SACs could also promote the electrochemical reduction of CO2 through suppressing the competitive hydrogen evolution reaction (HER). 41–44 Fig. 1 shows the elements distribution which have been used to construct SACs for CO2RR. The color filled elements represent the center active sites, including transition metals (Mn, Fe, Co, Ni, Cu, Zn …), noble metals (Pd, Ag, Ir …) and main-group metals (In, Sn, Sb, Bi …). Although most of SACs selectively reduce CO2 to CO, other products (HCOOH, CH3OH, CH4 and C2+) can also be generated over some SACs. 5 The dominant CO2RR products are marked by different colors as shown in Fig. 1.Many previous literatures have summarized the progress and advances of SACs in the field of CO2RR. Non-precious metal based SACs could efficiently decrease the cost of catalysts, and was reviewed by Li et al. recently. 45 Xu's review article in 2021 focused on the 3d transition metal based SACs, 46 which are recognized as the most promising candidates for CO2RR. Among them, Ni SACs are known as the earliest and widest studied SACs for CO2RR, which was discussed by Yadav et al. 47 Besides central metal sites, modulating coordination environment as an efficient strategy to achieve better performance, was summarized by Wang and co-works. 48 Also, dual-atom catalysts were systematically summarized and reviewed by An et al. 49 In addition to the common CO produced by SACs, hydrocarbons and alcohols generated through multi-electron process were also discussed in detail. 50 Despite of many previous reviews on SACs for CO2RR, one that summarizes latest progress on different metal sites, coordination environments and further dual-atom catalysts is highly needed. In this review, the possible CO2RR pathways for the formation of various products were firstly summarized, followed by the recent progress of SACs for CO2RR in terms of different metal centers including transition metals, noble metals and main-group metals. Then, introducing heteroatom as the most popular coordination modulation strategy was summarized in terms of different elements (vacancies, O, S, P and halogens). The dual atom catalysts applied in CO2RR were also introduced as the case of extended SACs, which can provide additional atomic active sites for the adsorption of intermediates, realizing the formation of products beyond C1. Finally, the existing issues and possible solutions of SACs for CO2RR were discussed. Fig. 2 summarizes the main contents of this review. Table 1 lists the CO2RR performances of recent SACs.Due to the discrete active site in SACs, C1 products are preferred in CO2RR over SACs. Therefore, in this section, we mainly focus on the reaction pathways for the generation of C1 products including carbon monoxide, formic acid (formate), methanol and methane, and give a brief introduction of the formation of C2+ products.The electrochemical reduction of CO2 to CO through ∗COOH intermediate is now widely-accepted. Due to the moderate adsorption energy of ∗COOH on the active sites of SACs, CO is more preferred for most SACs. CO has important applications in both Fischer-Tropsch process and water-gas shift reaction. 51–57 The possible reaction pathways for CO is shown in Fig. 3 a. Typically, gaseous CO2 undergoes a proton-coupled electron transfer (PCET) step to directly generate ∗COOH intermediate. Afterwards, ∗COOH can further convert to ∗CO through another PCET step, simultaneously releasing one water molecule. Finally, the weak-bonded ∗CO desorbs from the catalyst surface, forming gaseous CO product. On the other hand, ∗COOH intermediate can also be generated through proton-decoupled electron transfer step. Specifically, gaseous CO2 firstly forms ∗CO2 •− through one electron transfer. Then, a protonation process takes place, where a proton from self-ionization of water attacks ∗CO2 •− to form ∗COOH. Tafel slope can be used to study the rate-determining step (RDS), which relates to the overpotential vs. partial current density for specific product. A Tafel slope around 118 mV dec−1 indicates the initial CO2 activation to from ∗CO2 •− as the RDS, while it turns to following one electron transfer step for the Tafel slope around 59 mV dec−1.Formic acid has wide applications in tanning, textile and pharmaceutical industries as well as the hydrogen carrier in fuel cell. 58–60 During the formation of formic acid, the ∗CO2 •− species is firstly generated through one electron activation of gaseous CO2. It is worth noting that bidentate mode (two oxygen atoms bond on the catalyst's surface) is not applicable for most of SACs due to the lack of adjacent active sites. Instead, ∗CO2 •− species prefers to bond with SACs through one carbon atom (monodentate mode), which undergoes a protonation process to form ∗OCHO intermediate. Similarly, gaseous CO2 can be directly converted to the same intermediate (∗OCHO) through PCET step. As an alternative process, ∗OCHO can also be generated through the attack and insertion of CO2 molecule on ∗H species. At last, ∗OCHO intermediate will convert to HCOOH or HCOO− through another PCET step.Methanol is considered as a promising fuel due to its high energy density (4.8 kWh L-1) and easy storage/transportation. Through above-mentioned CO2 activation and following PCET or proton-decoupled electron transfer step, ∗CO intermediate can be formed. If the adsorption energy of ∗CO is high enough, it will not desorb from catalyst's surface but further undergo multiple PCET steps to form CH3OH.Following the generation of methanol, ∗OCH3 can undergo an additional PCET step instead of desorbing from the surface of catalyst to form CH3OH, where the surface adsorbed oxygen atom can be quenched through protonation process. As a result, CH4 can be generated and desorbed from the catalyst's surface. Alternatively, ∗COH intermediate can transform to adsorbed carbon species (∗C) through the protonation process and simultaneous release of water. Followed by continuous four PCET steps, CH4 can also be generated.Although most C2+ products have higher market prices and more specific industrial applications, their selectivities in CO2RR are always not as high as the C1 counterparts. C–C coupling or dimerization is crucial for the formation of C2+ products, which requires adjacent active sites. However, except for some dual atom catalysts, most SACs can only provide discrete active sites, making the generation of C2+ products very difficult.As one of the most important chemical feedstocks, ethylene (CH2 CH2) can be formed through either dimerization of two neighboring ∗CH2 species or direct PCET to ethylene oxide species (∗OCHCH2). Both of the two reaction pathways need to compete with the formation of ethanol (CH3CH2OH) through the insertion of adjacent ∗CO species into catalyst-C bond of ∗CH2 intermediate or direct protonation of oxygen sites in ∗OCH2CH2 intermediate.Transition metals have shown impressive catalytic performances not only for CO2RR but also for many other important catalytic processes, 61–76 making them ideal candidates for replacing expensive noble metal catalysts that are applied in industry. For example, Huang et al. reported a general method to synthesize a series of transition metal SACs (Fe, Co, Ni) for oxygen evolution reaction (OER). The identical M-N4-C4 configuration was confirmed, which offered ideal platform for quantitatively building up the relationship between metal center and catalytic properties. The theoretical and experimental results demonstrated Ni-NHGF as a high active and stable OER catalyst. 77 The excellent performances can be attributed to their abundant d electrons and unoccupied orbitals, which make them easy to coordinate with other species. Also, the d band can be efficiently tuned through many approaches, resulting in tunable adsorption energy towards reactants, intermediates and products. As for SACs, transitional metals can coordinate with surrounding atoms through abundant configurations due to their multiple valence states. Furthermore, other coordination elements beyond N can also be introduced into such SACs, endowing more favorable electronic structure for CO2RR.Ni SAC is one of the most widely studied SAC for CO2RR. 78 Ni SAC with Ni loading of 1.53 wt% can be easily synthesized through the ion exchange between adsorbed Ni2+ and Zn2+ nodes in ZIF-8, followed by pyrolysis (Fig. 4 a). 79 Compared with Ni2+, Zn nodes are easy to evaporate during pyrolysis process, leading to defective N–C support. 80 Subsequently, neighboring Ni2+ ions tend to occupy those sites, which is protected by surrounding N atoms from aggregation and further reduced by carbon atoms. The atomically dispersed Ni atoms were confirmed by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) (Fig. 4b and c), while the dominant Ni–N coordination and absent Ni–Ni path were observed through Fourier transformed k3-weighted χ(k) function of the EXAFS (extended X-ray absorption fine structure) spectrum (Fig. 4d). The maximum Faradaic efficiency (FE) toward CO (up to 71.9%) was achieved at −0.9 V vs. RHE, while the partial CO current density could reach up to 7.37 mA cm−2 with a high TOF of 5273 h−1. Xie et al. 81 also successfully synthesized Ni–N4 sites (1.41 wt%) via topo-chemical transformation method. Due to the coating of carbon layer, Ni–N4 sties could be well-preserved from agglomeration, which showed high CO FE over 90% across a wide potential range from −0.5 to −0.9 V vs. RHE (Fig. 4e and f). In 2018, Yang et al. 82 prepared atomically dispersed monovalent Ni (I) on nitrogenated graphene (2.8 wt%) through facile pyrolysis of a mixture containing amino acid, melamine and nickel acetate in Ar atmosphere. The HAADF-STEM image of the as-prepared A-Ni-NG was presented in Fig. 4g, indicating atomically dispersed Ni sites. As shown in Fig. 4h, under a mild overpotential of 0.61 V, the A-Ni-NG displayed impressive CO2RR performance, including high specific current of 350 A gcatalyst −1, CO FE of 97% and TOF of 14800 h−1. Besides, after continuous reaction for 100 h, the Faradaic efficiency could still remain 98% of its initial value, showing excellent stability. The Ni(I)–N4 with d9 electronic configuration was demonstrated as the real active sites for the electrochemical reduction of CO2 through operando X-ray absorption spectroscopy (XAS, Fig, 4i and j) and near ambient pressure X-ray photoelectron spectroscopy (XPS, Fig, 4k and l). Under CO2-saturated electrolyte, a positive shift toward higher energy was observed in Ni K-edge XANES spectra, which can be attributed to the delocalization of unpaired 3 d x 2 − y 2 electrons and charge transfer from Ni(I) site to C 2p orbital of adsorbed CO2 molecule, forming CO 2 δ − species. This was also evidenced by the existence of CO 2 δ − species in deconvoluted O 1s spectrum of A-Ni-NG induced by CO2 adsorption. Additionally, as shown in Fig. 4l, the decreased Ni 3d density of states and down-shift of valence band (VB) edge could also be attributed to the formation of CO 2 δ − species. This work not only clearly revealed the real active sites of Ni SAC and the reaction mechanism of CO2RR, but also realized impressive catalytic performance, therefore guiding the following numerous researches targeted at Ni SACs.As displayed in Fig. 5 a, multiwalled carbon nanotubes (MWCNTs) confined Ni particles could be successfully converted to thermal-stable Ni–N3 SAC through facile pyrolysis treatment. 83 In addition to use commercial MWCNT as support, the internal residual Ni particles could serve as the Ni source and be converted to Ni single atoms during the pyrolysis process, showing the extreme simplicity of such synthesis strategy. The favorable ∗COOH formation and exothermic CO desorption were demonstrated by DFT calculations as shown in Fig. 5b, which accounted for the excellent CO2RR performance with CO selectivity over 90% and TOF of 12000 h−1. Using ZnO as the self-sacrificial hard template and Zn source, ZnO@ZIF-NiZn could be fabricated by introducing Ni(NO3)2 and 2-methylimidazole, which could be further converted to NiSA/N-CNT after carbonization treatment. The superb CO2RR performance of NiSA/N-CNT (CO FE of 98% and TOF of 9366 h−1) was attributed to the reduced energy barrier. 84 As presented in Fig. 5d, Lu et al. 85 developed an unsaturated Ni–N2/C SAC (0.45 wt%) for CO2RR through an interesting CO2-to-carbon strategy. Specifically, ethylenediamine (EDA) could form Ni-coordinated compounds through simultaneous Ni2+ chelation and CO2 adsorption, which were then added with excess Mg powder to produce metal-organic complex. By directly igniting such flammable Mg-containing complex in air, the violent combustion could reach 1000 °C for 30 s, during which single Ni atoms were anchored onto N-doped carbon matrix, forming NiN/C SAC. The impressive catalytic performance (100% CO selectivity @ 51 mA cm−2) could be explained by the formation of unpaired 3d electrons resulting from rich vacancies as shown in Fig. 5e, which effectively decreased the energy barrier for the formation of ∗COOH (Fig. 5f). Polydopamine (PDA) was also used to assist the atomic dispersion of Ni2+ through chelating effect. 86 After pyrolysis at 1000 °C under N2 atmosphere, PDA anchored Ni2+ was stabilized by N atoms and then reduced by the surrounding carbon, while g-C3N4 decomposed to form nitrogen doped carbon matrix. Considering that high temperature pyrolysis (above 900 °C) was always needed to prepare Ni SACs, He et al. 87 developed a low temperature (450 °C) strategy to synthesize Ni nanoparticle wrapped by Ni–N SA/C (Ni-NC@Ni), which could reduce the energy consumption during the preparation of SACs. NiO was firstly grown onto carbon fiber paper through hydrothermal and annealing process, which was then mixed with urea and annealed at 450 °C to finally obtain Ni-NC@Ni. Although several literatures have observed similar good performance of CO2RR to CO by nitrogen doped carbon encapsulated Ni particles, 88 Liang et al. synthesized and studied several Ni/N–C catalysts, which demonstrated the pristine high activity of single Ni sites during the conversion of CO2 to CO. In order to further elucidate the complicated CO2RR mechanism, Liu et al. 89 designed and fabricated a model Ni SAC (0.27 wt%) through the synthesis of well-defined molecular Ni-TAPc and subsequent anchoring to CNTs by C–C coupling (Fig. 5g). TEM (Fig. 5h) and HAADF-STEM (Fig. 5i) images demonstrated the morphology and atomically dispersed Ni atoms. Such Ni-CNT-CC was composed of precise NiN4 moiety, which provided a model system for the investigation of CO2RR mechanism. In Ni K-edge XANES, the negative shift (0.3 eV) of rising edge was observed under cathodic potentials (Fig. 5j and k), demonstrating the in-situ reduction of Ni2+ to Ni+, which was also confirmed by the negative shift of Ni–N vibration band in operando Raman spectra (Fig. 5l–o). The in-situ generated low-valence Ni+ could efficiently activate CO2 molecule through donating long pair of electrons, which accounted for the high activity and selectivity (near 100% CO FE and high TOF of 100179 h−1) of Ni SAC in CO2RR. In addition to experimental studies, theoretical approaches were also carried out to investigate the catalytic performances of Ni SACs. Hossain et al. predicted the onset potential, Tafel slope, FE and TOF of CO2RR over different Ni SACs (Ni–N4, Ni–N3C1 and Ni–N2C2) under different potentials using recently developed grand canonical potential kinetics formulation (GCP-K) to calculate the kinetics step by step. 90 Compared with traditional Butler-Volmer kinetics, GCP-K allowed continuously changing the transition states geometry and charge transfer during the reaction coordinates, which gave more accurate and convincing prediction of the interaction between active sites and adsorbed species as well as transition states (Fig. 6 a). It turned out that the most positive onset potential (−0.84 V vs. RHE) could be achieved on the Ni–N2C2 site, while the Tafel slope, FE for CO and TOF were calculated to be 52 mV dec−1, 98% and 3903 h−1, respectively, which agreed well with the experimental results.In addition to widely-used graphene or CNT matrix, Ni single atoms could also be anchored onto commercial carbon black with much lower price. 91 The abundant defects and oxygen-containing functional groups (–OH, –COOH …) on carbon black ensure its easy adsorption of Ni2+ ions. After annealing Ni2+-adsorbed carbon black (Ni2+-CB) and urea, gram scale NiSA-NCB could be successfully prepared (Fig. 6b). Tested in a gas-phase reactor, a high current density over 100 mA cm−2 with close to 100% CO selectivity was recorded. The large electrode with 10 × 10 cm2 as shown in Fig. 6c delivered an extremely high current above 8 A (Fig. 6d) and CO generation rate of 3.34 L h−1 (Fig. 6e), which greatly promoted the practical application of electrochemical CO2RR. Membrane electrode assembly (MEA) could be fabricated by depositing the as-prepared Ni SAC between the gas diffusion layer (GDL) and ion exchange membrane (IEM), 92 which could deliver very high current density beyond 300 mA cm−2 with nearly 100% CO selectivity. Thanks to the satisfactory mechanical strength and gas permeability, carbon paper has been widely used as the support and current collector for CO2RR, which can also be anchored with Ni single atoms to fabricate self-standing and binder-free electrode. 93 As presented in Fig. 6f, after acid activation followed by Ni2+ adsorption and pyrolysis, Ni–N3S SAC configuration with loading of 1.04 wt% could be successfully fabricated, which exhibited optimal CO selectivity of 91% at an overpotential of 660 mV (Fig. 6g).As one of the most abundant elements on earth, iron has also been widely used to synthesize SACs for CO2RR. Li et al. 94 synthesized the Fe-NC-S through the ion exchange between Fe2+ and ZIF-8, followed by pyrolysis process. Demonstrated by operando 57Fe Mössbauer spectroscopy and XAS, the in-situ generation of low-spin Fe (I) coordinated with four pyrrolic nitrogen (LS FeIN4) was confirmed (Fig. 7 a and b). Fig. 7c displays the operando XAS results. Compared with clear negative shift under cathodic potential in Ar-saturated electrolyte, the unchanged spectra in CO2-saturated electrolyte indicated electron transfer from Fe site to C 2p orbital of CO2 to form CO2 δ− species. Further DFT calculations revealed the optimal binding energy of CO2 on such LS FeIN4 site when compared with other Fe–N–C sites. On the other hand, the single-occupied 3dz2 orbital of LS Fe (I) could strongly interact with single-occupied π∗ orbital of key intermediate ∗COOH (Fig. 7d), which accounted for the excellent CO2RR activity of such Fe–N–C. It was found that the final intermediate ∗CO tended to strongly bind with Fe–N4 site, which limited the desorption of CO. Such Fe2+ doped ZIF-8 followed by pyrolysis strategy was also adopted by Gu et al. 95 Operando XAS results demonstrated the discrete Fe3+ ions in Fe–N–C, which could maintain +3 oxidation state during electrochemical reduction reaction due to electronic coupling with carbon support. The superb CO2RR performance (low overpotential of 80 mV and partial current density of 94 mA cm−2) was attributed to the fast adsorption of CO2 and favorable CO desorption. Pan et al. 96 introduced graphene oxide with abundant pores through H2O2 etching, which was then used as the support to anchor Fe single atoms (Fig. 7e). Compared to Fe–N4 sites supported on bulk graphene (FeN4-bulk), those Fe–N4 sites at the pore edges showed an obvious downshift of d-band center of Fe (Fig. 7f), therefore weakening the adsorption of ∗CO intermediate and reducing the reaction barrier (Fig. 7g). Consequently, a high CO FE of 94% and TOF of 1630 h−1 could be achieved at −0.58 V vs. RHE (Fig. 7h). The role of defective graphite was also investigated by Qin and Ni. 97 , 98 According to in-situ FTIR and DFT calculation results, Fe–N4 sites (0.71 ​wt%) supported on complete graphite layer showed very strong binding with ∗CO, and they were heavily poisoned and could not be the real active sites for CO2RR. Instead, Fe–N4 moieties anchored on defective graphite sites with nanopores showed moderate binding strength with ∗COOH and ∗CO. Ni et al. prepared NG-SAFe and DNG-SAFe through pyrolysis at different temperatures. Thanks to the coupling between carbon defects and Fe–N4 moieties, DNG-SAFe presented significantly improved CO2RR performance than NG-SAFe, which was further applied in Zn–CO2 battery with good CO selectivity of 86.5% at the current density of 5 mA cm−2. As shown in Fig. 7i, Pan et al. 99 successfully fabricated gram-scale Fe SAC (1.72 wt%) starting from commercial carbon nanotube (CNT). Highly oxidative solution containing KMnO4 and H2SO4 was used to create defects and longitudinally unzip CNT, generating carbon nanotube (CNT)@graphene nanoribbon (GNR) hieratical structure. It is worth noting that the degree of CNT unzipping could be easily controlled by tuning the ratio of CNT/KMnO4. It was figured out that the Fe–N/CNT@GNR-2 (ratio of CNT/KMnO4 equals to 2) exhibited the best properties, including high surface area and fast mass transport. The residual Fe seeds during fabrication of CNT could serve as the Fe source to form Fe–N4 moiety under 900 °C pyrolysis. Due to the high activity of Fe–N4 sites, high surface area and efficient mass transport, the Fe–N/CNT@GNR-2 achieved stable CO FE of 96% with partial current density of 22.6 mA cm−2 at −0.76 V vs. RHE.Normally, Fe tends to coordinate with four N atoms to form Fe–N4 moiety. Interestingly, as displayed in Fig. 8 a, Zhang et al. 101 fabricated unique Fe–N5 moiety (1.2 wt%) through pyrolysis of a mixture containing hemin, melamine and graphene, while Fe nanoparticle and common Fe–N4 could be synthesized by pyrolysis of hemin and the mixture of hemin and melamine, respectively. Demonstrated by Fe K-edge XANES and FT-EXAFS, the unique Fe–N5 configuration was well-verified. According to DFT calculation results (Fig. 8b), the additional axial N coordination could deplete the d electrons of Fe through electron transfer from d orbital of Fe to p x and p y orbitals of pyrrolic N, therefore weakening the Fe–CO binding strength and facilitating the desorption of CO. As a consequence, a high CO FE around 97% at a low overpotential of 0.35 V could be achieved (Fig. 8c and d). Such Fe–N5 active sites (2.68 wt%) with additional axial N coordination was also prepared by Li's group 102 through the introduction of aminated CNT. The N atom in aminated CNT could firmly anchor Fe porphyrins and prevent the agglomeration of Fe during pyrolysis at 700 °C (Fig. 8e). Through efficient control of both geometric and electronic structure of Fe–N5 site, the free energy for ∗CO desorption could be significantly decreased while the energy barrier for competitive HER was increased (Fig. 8f and g). As a consequence, high CO FE around 95.47% at −0.6 V vs. RHE could be achieved, which could maintain above 95% after 10 h of continuous reaction. A two-step annealing treatment was also developed to fabricate similar Fe–N5 sites (0.47 wt%). 103 During annealing, FeCp powder would evaporate if the annealing temperature was higher than its sublimation point, which would be trapped by neighboring ZIF-8 with abundant cavities (Fig. 8h). Following pyrolysis at 1000 °C, FeN5/N–C could be successfully prepared. The downshift of d-band center of Fe 3d orbital was verified by DFT calculation (Fig. 8i and j), which was attributed to the modulation of out-of-plane pyridinic N, leading to the decrease of CO adsorption energy (from −1.71 to −1.49 eV). The potential dependent free energy of CO2RR was also studied by Gao et al. using ab initio molecular dynamics (AIMD) simulation and constrained molecular dynamics method. 104 Compared with traditional computational hydrogen electrode model, the slope of reaction free energy vs. potential was calculated as adsorbate-specific value instead of 1 ​eV. Moreover, compared with electron transfer (ET) step during the formation of Fe–C bond, the subsequent proton transfer (PT) step forming ∗COOH is thermodynamically and kinetically more favorable, resulting in the decoupling of PT step from ET during the rate- or potential-determining calculations. Finally, the onset potential, potentials when maximum FECO and FECO=FEH2 were achieved can be semi-quantitatively reproduced.Among various metal catalysts, Cu-based catalysts are recognized as the most promising candidates for CO2RR due to their abilities to convert CO2 to C2+ products. Nevertheless, different from Cu-based catalysts, most Cu SACs can only reduce CO2 to C1 products. Zheng et al. 105 prepared Cu–N2/GN with unsaturated coordination (1.45 wt%) through a facile direct pyrolysis of graphene, chlorophyllin and dicyandiamide under inert atmosphere. The as-prepared Cu–N2/GN could deliver a maximum CO FE around 81% at −0.5 V vs. RHE, which was further used to fabricate a rechargeable Zn–CO2 battery, showing a peak power density of 0.6 mW cm−2. DFT calculations demonstrated the favorable adsorption of CO2 molecule on Cu–N2 sites and accelerated electron transfer from Cu–N2 to ∗CO2 due to shorter length of Cu–N bond. Yang et al. 106 employed Cu/ZIF-8 and poly-acrylonitrile as the precursor to electrospin polymer fiber, which was then carbonized under Ar atmosphere. The nanofiber structure is shown in Fig. 9 b with atomically dispersed Cu atoms (Fig. 9c). At laboratory conditions, the self-supported and flexible single atom Cu decorated carbon nanofibers (CuSAs/TCNFs) displayed excellent bending, twisting and tensile properties (Fig. 9a), which could be directly used as the cathode for the CO2RR to methanol with Faradaic efficiency of 44%. As demonstrated by DFT calculation, the binding energy of ∗CO on Cu–N4 sites was relatively high, enabling its further reduction to methanol. Gram-level Cu–N4/C (0.32 wt%) with outstanding CO2 catalytic performance (CO FE of 92% at −0.7 V vs. RHE) could also be prepared through pyrolysis of carbonized chitosan, KOH and CuCl2 slurry, followed by acid washing. 107 In addition to generate C1 products, some Cu SACs with well-designed configurations can also be used to reduce CO2 to C2+ products (ethylene, ethanol, acetone, etc). Zheng and coworkers 108 found that the concentration of Cu moiety and their configurations could be well-tuned by the pyrolysis temperature. Starting from the same Cu(BTC)(H2O)3 precursor, pyrolysis at 800 °C could retain more Cu active sites, producing Cu SACs with a high Cu content of 4.9 mol%, while pyrolysis at 900 °C generated Cu SACs with a lower Cu concentration of 2.4 mol% (Fig. 9e). More importantly, the distance between neighboring Cu-N x species in Cu SACs with high Cu concentration was found to be close enough for C–C coupling, resulting in the formation of C2H4. On the other hand, the more discrete Cu-N x sites in Cu SACs with low Cu concentration favored the generation of C1 products (mainly CH4). As displayed in Fig. 9f, DFT calculation confirmed that two adjacent Cu–N2 moieties could bind two ∗CO intermediates to produce C2H4, while other Cu-N x configurations, including isolated Cu–N2, isolated Cu–N4 and neighboring Cu–N4 moieties could only produce CH4. Karapinar et al. synthesized Cu–N–C catalysts through two-step method: (1) low-energy ball milling of ZIF-8, CuCl2 and phenanthroline; (2) pyrolysis at 1050 °C under Ar atmosphere. 109 Surprisingly, the synthesized Cu–N–C (1.4 wt%) could efficiently reduce CO2 to ethanol with a considerable FE of 55% at −1.2 V vs. RHE in 0.1 M CsHCO3 electrolyte. As evidenced by the operando XAS results (Fig. 9g), the appearance of Cu–Cu coordination was observed under cathodic potential, which disappeared after 10 h exposure in air. Therefore, the dynamic transformation from Cu–N4 moieties to metallic Cu nanoparticles was verified, which should be responsible for the unexpected generation of C2 product via CO2RR. It is also worth highlighting that such transformation of Cu–N4 moieties to metallic Cu nanoparticles was reversible, thus ensuring good catalytic stability of the catalyst. Xu et al. 110 also found that Cu single atomic sites could accumulate to form Cun clusters (n = 3 and 4) under CO2 reduction potential. Specifically, molten Li was used to dissolve bulk Cu, which would remain as atomic sites during the quenching of molten Li. After blending with carbon matrix (XC-72) followed by leaching process, atomically dispersed Cu could be synthesized. Owing to in-situ generation of Cun clusters evidenced by the appearance of Cu–Cu moieties with coordination number of 2 or 3 at −0.7 V vs. RHE (Fig. 9h), such Cu SAC delivered a low onsetpotential of only −0.4 V and an high Faradaic efficiency (91%) towards ethanol at −0.7 V vs. RHE. Besides ethanol, acetone could also be produced from CO2 reduction over Cu-SA/NPC (0.59 wt%) prepared by Chen's group. 111 Cu(OAc)2 was firstly encapsulated in the pores of ZIF-8, which was then carbonized at 1000 °C under inert atmosphere to prepare Cu-SA/NPC. During the electrochemical reduction of CO2, acetone was identified as the main product with FE of 36.7% and production rate of 336.1 μg h−1. DFT calculation revealed that Cu coordinated with 4 pyrrole N was the main active site, which could reduce the energy barriers for CO2 activation and C–C coupling, as well as stabilize the key intermediates for acetone formation (Fig. 9i and j).During fabrication of Zn-based single atom catalysts, increasing Zn content is difficult because of the low boiling point of Zn. Zn–N4 based SACs could be prepared through pyrolysis of Zn(OAc)2, carbon black and urea at 800–1000 °C. 112 The Zn–N4 configuration with metal loading of 0.1 ​wt% was verified by EXAFS, such configuration could lower the energy barrier of ∗COOH formation according to the DFT calculation. The highest CO FE could be recorded as 95% at −0.43 ​V vs. RHE, which showed no deterioration after 75 h continuous reaction. Low-valence Zn δ + SAC (1.08 wt%) could be fabricated by annealing Zn-BTC and dicyandiamide at 1000 °C. 113 XAS results demonstrated the existence of saturated Zn–N4 and unsaturated Zn–N3 moieties. The Zn–N3 moieties could hold electrons, leading to lower valence states of Zn sites, which could reduce the energy barrier for ∗COOH intermediate formation (Fig. 10 a). Near-unity CO selectivity at an overpotential of 0.31 V was achieved in an H-type cell, while high current density up to 1 A cm−2 along with CO selectivity over 95% was realized in a flow cell. In addition to CO, Han et al. 114 also successfully applied Zn SAC to reduce CO2 to produce CH4. A high Faradaic efficiency of 85% and partial current density of 31.8 mA cm−2 could be measured at −1.8 V vs. SCE. DFT calculations revealed high energy barrier for the formation of ∗CO, which would promote the protonation of ∗CO to form CH4.Co SACs for CO2RR could be easily fabricated through coordination interaction between CoPc and the as-prepared hollow N-doped porous carbon spheres (HNPCSs). 115 Atomically dispersed Co–N5 delivered a maximum CO FE of 99% and maintained the CO FE over 90% across a wide potential window (from −0.57 to −0.88 V vs. RHE). Both current density and CO FE kept unchanged after 10 h of electrochemical CO2 reduction reaction. The fast formation of ∗COOH intermediate and ∗CO desorption were responsible for the excellent CO2RR performance. The coordination environment of Co single atoms with loading of 0.63 wt% could also be well-regulated by changing pyrolysis temperature. 116 Through pyrolyzing ZIF at 800 and 900 °C, Co1–N4 and Co1–N4-x C x could be prepared, respectively. Based on XAFS, Co–C coordination was believed to partially replace the Co–N coordination under high temperature pyrolysis. CoSA-based 3D free-standing membrane could also be fabricated through electrospinning of ZIF-8 NPs, Co(NO3)2 and PAN. Due to the large electrochemical surface area and favorable reactant transportation, the as-fabricated CoSA/HCNFs delivered a maximum CO FE of 91% and partial current density of 67 mA cm−2 in an H-cell, while that could be further improved to 92% and 211 mA cm−2 in a flow cell.Most of today's commercial catalysts applied in industry are based on noble metals because of their high intrinsic catalytic properties. Therefore, it is of great significance to design and fabricate noble metal based SACs to maximize the noble metal utilization efficiency. Pd SAC with loading of 2.95 wt% could be prepared using a classic co-pyrolysis strategy. 117 Compared with Pd NP counterpart, such Pd SAC showed much superior CO2RR performance (373.0 vs. 28.5 mA mg−1 Pd). The obvious shift toward lower energy in in-situ XAFS (Fig. 10b) and the increased Pd–Pd bond length in FT-EXAFS (Fig. 10c) evidenced the formation of PdH species on the surface of Pd/C. In contrast, the peaks for Pd SAC (Fig. 10d and e) showed negligible changes, verifying the absence of PdH species. As shown in both in- situ XAFS and DFT calculations, the Pd–N4 served as the active sites without forming PdH species that was very crucial for bulk Pd catalysts. Recently, Ag single atom was successfully dispersed onto MnO2 substrate through the thermal-induced transformation of Ag NP and surface reconstruction of MnO2. 118 As shown in Fig. 10f, in-situ environmental transmission electron microscopy (ETEM) clearly demonstrated the transformation from Ag NP to single Ag atom. The gradual decreased Ag (111) peak in Fig. 10g again verified the reduction of Ag NP, while the dominant diffraction peak of MnO2 changed from (211) to (310), indicating the preferential lattice plane to stabilize Ag single atoms. Compared with Ag NPs located at surface and corner, Ag1/MnO2 displayed the lowest free energy of ∗COOH (0.44 ​eV). Therefore, a high CO FE up to 95.7% at −0.85 ​V vs. RHE could be achieved due to the high electronic density near Fermi level of single Ag sites (Fig. 10j). Single atom Au could be well-dispersed onto tensile-strained Pd NPs through MOF-assisted adsorption strategy. 119 The tensile-strained Pd substrate could stabilize all intermediates, while atomically dispersed Au sites could selectively destabilize CO∗, thus breaking the scaling relationship between COOH∗ and CO∗. As a result, excellent CO2RR performance to generate formic acid with FE up to 99% could be achieved at −0.25 V vs. RHE, which is much superior to Pd/C catalyst.Main group elements including Sn, Sb, Bi, Pb and In have been extensively studied in CO2RR due to their intrinsic high overpotentials for competitive HER. 120 Similar to their bulk counterparts, main group metal based SACs also favor the production of formic acid and formate in CO2RR. Kilogram Sn δ + sites could be atomically dispersed onto N-doped graphene substrate through a so-called quick freeze-vacuum drying-calcination strategy. 121 XAFS and HAADF-STEM results evidenced the positively charged Sn δ + sites, which could stabilize CO2 ·−∗ and HCOO−∗ to efficiently activate CO2 molecule and promote the subsequent protonation steps. Moreover, the introduction of N could promote desorption of formate (RDS), evidenced by the reduced desorption energy (1.01 vs. 2.16 eV) and the extended Sn–HCOO− bond. As a consequence, the reaction overpotential could be reduced to as low as 60 mV along with an impressive TOF of 11930 h−1 and long-term stability over 200 h. When anchored onto other substrates, Sn SACs could also generate reduction products beyond CO. Li et al. 122 utilized CuO with oxygen vacancy to anchor Sn single atoms. The as-prepared SnI/Vo-CuO could reduce the energy barrier for ∗COOH generation to promote the formation of ∗CO, which was then adsorbed by CuO substrate to be further reduced to methanol. Faradaic efficiency toward methanol could reach as high as 88.6% at the current density of 67 ​mA ​cm−2. Bi-N x single atomic sites could be synthesized through facile pyrolysis of Bi-MOF with assistance of dicyandiamide, which promoted the formation of ∗COOH intermediate by lowering its reaction barrier. 123 The as-prepared BiSAs/NC delivered a maximum CO FE up to 97% along with TOF around 5525 h−1. A high TOFCO above 16500 h−1 was achieved on Sb SAC, which was prepared by pyrolysis of SbCl3, activated carbon black and urea. 124 Bi–N–C SAC and corresponding Bi nanosheets were used as models to theoretically investigate the preferred product on SACs. 125 Under cathodic potentials, CO2 tends to chemisorb on Bi–N–C SAC because of charge accumulation effect. Subsequently, ∗COOH can be formed through more kinetically favorable approach when compared with the formation of ∗OCHO. In contrast, physisorption is adopted for the CO2 activation on Bi nanosheets, resulting in the selective production of formate. Additionally, it was proposed that CO2 adsorption modes can serve as solid criterion for the prediction of preferred product. On the other hand, the similar Sn δ + –N4 sites (2.86 wt%) could selectively reduce CO2 to formic acid instead of CO due to the favorable adsorption of HCOO∗. 126 Zeng et al. reported an in-situ electrochemical deposition method to atomically disperse Pb onto Cu nanosheets, forming Pb1Cu SAC. However, modulated Cu sites, instead of atomic Pb sites serve as active sites. The impressive formic acid selectivity (∼96%) at larger current density of 1 A cm−2 can be attributed to the shift from HCOO∗ pathway to COOH∗ pathway. 127 Atomic In sites on nitrogen-doped carbon (In–N–C) was prepared through the pyrolysis of In-doped ZIF-8 at 900 °C. 128 Demonstrated by DFT calculations, compared with ∗COOH formation energy of +0.84 ​eV, the mild endothermic formation of ∗OCHO (+0.19 ​eV) should be responsible for the TOF as high as 26771 ​h−1 at −0.99 V vs. RHE. Similar strategy was also reported by Li et al., 129 synthesizing isolated In δ + –N4 SAC. A high TOF up to 12500 h−1 and maximum HCOOH Faradaic efficiency of 96% can be obtained at a potential of −0.65 V vs. RHE. Nevertheless, when using mixed electrolyte containing ionic liquid and MeCN, CO instead of formic acid, was selectively produced (97.2% at 39.4 mA cm−2). The catalytic performances can be attributed to the high double-layer capacitance, CO2 adsorption capacity and low interfacial resistance. 120 Due to the low binding energy between C and metal atoms, it is difficult to anchor single metal sites on pure carbon support. In order to efficiently fix single metal sites, N-doped carbon substrates are widely used to form strong M−N bonds. In most circumstances, the metal center in SACs is coordinated with four nitrogen atoms to form stable configurations. The electronic and geometric structures of metal sites are highly related to their coordination environments, which accounts for the different adsorption free energy of intermediates and reaction pathways. Nitrogen coordination displays different electron-donating or -withdrawing features due to their different configurations. Specifically, pyrrole and quaternary N can donate electron to metal site, resulting in n-type doping, while pyridine N will delocalize electron from metal site, leading to p-type doping. The coordination environment can be efficiently adjusted by introducing vacancies or other non-metal heteroatoms, providing more complex electronic and geometric structures resulting from different atomic sizes and electronegativities. 130 Chen et al. synthesized Ni SACs with nitrogen vacancies (SA-NiNG-NV) through microwave-induced plasma treatment. 131 The coordination environment could be easily tuned by changing the duration of plasma treatment (Fig. 11 a). As compared with original SA-NiNG, SA-NiNG-NV displayed a higher ID/IG ratio in the Raman spectrum (Fig. 11b), an increased ratio of sp3 C to sp2 C (Isp3/Isp2) in the XPS spectrum (Fig. 11d) and higher spin numbers in electron paramagnetic resonance (Fig. 11c), demonstrating more unpaired electrons resulted from surface defects and vacancies. AC-HAADF-STEM images and EXAFS results confirmed the atomically dispersed Ni atoms in both SA-NiNG and SA-NiNG-NV. Via fitting the FT-EXAFS, the coordination configurations of two catalysts were determined (Fig. 11e). The original SA-NiNG composed of Ni-pyrrolic N3 moiety underwent a plasma-induced-reconstruction to form Ni-pyridinic N2 moiety by removing 2 neighboring N atoms. Such unsaturated coordination environment in SA-NiNG-NV offered sufficient space for CO2 adsorption, therefore reduced the energy barrier for the generation of key intermediates COOH∗ and CO∗. As displayed in Fig. 11f and g, thanks to the favorable thermodynamics and kinetics of CO2RR, SA-NiNG-NV exhibited outstanding catalytic performance with an impressive CO FE of 96% at the overpotential of 590 mV, a high partial CO current density of 33 mA cm−2 at the overpotential of 890 mV as well as good stability. Coordination vacancy could be introduced into Ni SAC by selective removal of O in the as-prepared Ni–N3O due to the weaker Ni–O bond. 132 According to DFT calculation, the more favorable reduction of CO2 to COOH∗ (regarded as the rate-determine step) contributed to the high CO selectivity (over 90%) and impressive TOF of 135000 h−1.M–N–O coordination structures also showed good CO2RR performances due to the efficient coordination modulation by the O atom. In order to prepare low-coordinated Cu active sites with O, the original Cu–O–C moiety in the as-prepared CuDBC was treated with high energy plasma, which was converted to low-coordinated Cu–O2–C moiety. 133 Thanks to the in-situ formed hierarchical porous matrix and low coordination environment, Cu–O2–C SAC could efficiently reduce CO2 to CH4 with a maximum FE of 75.3% and partial current density of 47.8 mA cm−2. As shown in Fig. 11h, a gas transportation strategy was adopted to synthesize atomically dispersed SnN3O sites on N-doped carbon support (Sn–NOC), where SnO2 powder and N-doped carbon were separately placed in the tube furnace serving as metal source and support, respectively. 134 Different from the dominant formate formed over classic Sn–N4 sites, Sn–NOC catalyst could reduce CO2 exclusively to CO. DFT calculations revealed that the introduction of O could promote the activation of CO2 and reduce the free energy of ∗COOH, while increase the energy barrier for HCOO∗. As presented in the in-situ surface-enhanced Raman spectra (Fig. 11i), the peaks located at 718, 1030 and 1130 cm−1 could be assigned to ∗CO2−, ∗OCO and ∗COOH, which showed similar trend (firstly became stronger then weaker with the potential changing from −0.1 to −0.9 ​V vs. RHE), agreeing well with DFT calculation results, implying favorable ∗COO− and ∗COOH reaction pathways.Microwave-induced plasma treatment could be applied to introduce alien S into unsaturated Ni–N2 moiety to tune the electronic structure by forming NiNG-S. 135 The larger ID/IG ratio (1.19 vs. 1.09) in the Raman spectra, the reduced pyridinic and pyrrolic N intensities as well as the increased binding energy of Ni 2p in the XPS spectra confirmed the successful introduction of S into the Ni–N2 moiety. In addition, by fitting FT-EXAFS in R space, the introduced S was deduced to occupy N vacancy in NiN2 species, forming NiN2–S configuration. Thanks to the S dopant, NiNG-S showed obviously superior CO2RR performance than the control NiNG catalyst, including a CO FE around 97% and CO partial current density of 40.3 mA cm−2 at −0.8 and −0.9 V vs. RHE, respectively (Fig. 11j and k). Such outstanding catalytic performance could be explained by the reduced energy barrier for CO2RR due to the favorable electronic structure of NiNG-S. Interestingly, at the potential of −0.8 V vs. RHE, a negative shift of XANES as shown in Fig. 11l indicated decreased valence state of Ni site, which was further confirmed by the loss of S according to the disappearance of Ni–S coordination in FT-EXAFS (Fig. 11m). This observation also agreed well with the absence of Ni–S bond in XPS. The unstable S dopant at high applied cathodic potentials (beyond −0.8 V vs. RHE) could result in S vacancies in Ni–N2 structure. Based on DFT calculations, although the rate-limiting steps for NiN2, NiN2–S and NiN2–Vs were different, the CO2RR overpotential for NiN2–S (0.55 eV) and NiN2–Vs (0.76 eV) were significantly lower than that for NiN2 (1.12 eV), which agreed well with the experimental results. Substitutional S could also be successfully introduced into Cu–N4/Cu x tandem catalyst through additional pyrolysis with addition of sulfur powder at 950 °C. 136 DFT calculations showed that the Cu–S coordination could effectively reduce the Gibbs free energy for ∗COOH formation by 0.61 ​eV, resulting in favorable generation of key intermediate for CO. On the other hand, the neighboring Cu x cluster could accelerate the dissociation of H2O to provide sufficient H+ to promote the protonation of ∗CO2−. Thanks to the synergistic effect of S coordination and Cu x cluster, the Faradaic efficiency for CO generation could reach up to 100% at −0.65 V vs. RHE, and it could be maintained above 90% in a wide potential window (from −0.55 to −0.75 V vs. RHE).Li and coworkers introduced P into N-doped carbon to modify the substrate electronic structure. 137 Due to the absence of Fe–P bond in XPS, P was evidenced to connect with carbon instead of Fe, which was further confirmed by FT-EXAFS. To investigate the role of P doping on the CO2RR performance, the free energy diagram of the catalysts with different P coordination was computed. The existence of P in the second coordination shell could increase the free energy of ∗COOH but significantly decrease the energy barrier for HER. Nevertheless, when P was placed in higher coordination shells (≥3), the formation of ∗COOH became more favorable along with suppressed HER. The electron localization over Fe sites induced by P doping explained the selective formation of ∗COOH intermediate to produce CO.In addition to replacing one or two N atoms with heteroatoms in SACs, the heteroatoms can also be introduced to form additional M-X bond, which shall efficiently tune the electronic structure of metal sites through charge polarization effect. Predicted by DFT calculation, when grafted with additional O atom on Ni along the axial direction, the electron pushing effect would result in significant charge polarization and lower energy barrier to form COOH∗. 138 O was chosen because of its slightly larger and moderate electronegativity than N, which could induce charge polarization effect but not destroy the pristine Ni–N4 moiety. Wang et al. then successfully synthesized Ni SAC with axial O atom supported by carbon matrix (Ni–N4–O/C) using Mn-based MOF as the host and Ni2+ as the guest ion. During the subsequent carbonization process, O was anchored onto Ni–N4 moiety as the axial atom. As demonstrated in FT-EXAFS, the dominant peak of Ni–N4–O/C was located at 1.52 Å, which is between Ni–N (1.47 Å) and Ni–O (1.68 Å) scattering paths, confirming the charging polarization effect of axial O. Thanks to the lower reaction barrier of transforming CO2∗ to COOH∗, Ni–N4–O/C exhibited excellent CO2RR performance to produce CO. The role of axial O on Ni–N4 moiety was systematically studied by Hu et al., 139 who adopted an explicit solvent model with thermodynamic integration method to build a realistic electrochemical environment. It was elucidated that axial O could facilitate the charge transfer and stabilize ∗CO2 species. Combining dynamic solvent molecules and applied cathodic potential, the energy barriers and free energies could be precisely calculated, which evidenced that axial O could activate the formation of both ∗CO and ∗COOH and simultaneously suppress the competitive HER.Through sequential pyrolysis of SnO2/PTFE and ZIF-8, Ni et al. fabricated Sn SAC with Sn–C2O2F configuration, where Sn site was not only coordinated with 2 carbon and 2 oxygen atoms in planar direction, but also axially coordinated with a F atom. 140 Compared with the predominant formate production on typical Sn–N4 sites, the unique Sn–C2O2F realized selective CO2RR to CO with a high CO FE over 90% in a potential window from −0.2 to −0.6 V vs. RHE. Comprehensive DFT calculations revealed the switching of rate-determining step (RDS) from ∗CO desorption on Sn–N4 sites to activation of ∗CO2 on Sn–C2O2F sites. The additional axial F coordination could significantly suppress the competitive HER, giving a more positive value of UL(CO2)-UL(H2). Convex inversion of carbon plane induced by firm interaction between Sn sites and CO2 molecules prohibited the possible CO2 to HCOOH conversion on Sn–C2O2F. Axial chlorine could also be coordinated to Fe–N4 sites through additional hydrochloric acid incubation method. 141 The high CO FE around 90.5% and TOF of 1566 h−1 could be attributed to the strengthened charge transfer between Fe site and the intermediate, which facilitated the desorption of ∗CO and suppressed the adsorption of ∗H.Although single atom catalysts show many advantages, the low single atom loading and discrete active sites limit their reaction activity and pathway. 142 In this aspect, dual-atom catalysts containing two atomically dispersed metal sites could offer additional benefits. 143 , 144 Different metal sites can be atomically dispersed onto carbon substrate. By adding CoCl2 and NiCl2 in the precursor, the dual-atom catalysts containing both Co-SA and Ni-SA sites could be prepared. 145 The final loading of Co and Ni could be well-tuned by changing the precursor composition. DFT calculations showed that Co-SA and Ni-SA could produce H2 and CO by stabilizing their corresponding intermediates. Therefore, syngas with controllable CO/H2 ratio could be produced with a total current density above 74 mA cm−2. Similar strategy was adopted to synthesize Zn–La dual atomic catalyst by annealing melamine sponges soaked with Zn(NO3)2 and La(NO3)2. 146 The lack of both Zn–Zn bond and La–La bond as shown in Fig. 12 a and b evidenced atomically dispersed Zn and La atoms. DFT calculations demonstrated that the electronic structures of Zn and La sites were well-tuned, which favored the CO2RR to CO and HER, respectively. Therefore, ZnLa-1/CN catalyst could produce syngas with a CO/H2 ratio of 0.5 across a wide potential range from −1.6 to 1.3 V vs. RHE.The above-mentioned dual-atom catalysts can be considered as a combination of two types of SACs with different metal centers, in which two kinds of metal sites are randomly dispersed on the substrate. As a step further, with appropriate design, two kinds of metal atomic sites can be anchored on the support next to each other, where synergistic effect between the two types of active sites and tunable electronic structure can be realized. By introducing Ni(acac)2 and Cu(OAc)2 into the synthesis of ZIF-8, atomically dispersed Ni/Cu dual sites could be prepared by pyrolysis method. 147 The adjacent atomic Ni and Cu sites were confirmed by fitting the EXAFS spectrum as displayed in Fig. 12c. By introducing neighboring Cu site, the bandgap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) was narrowed from 0.64 to 0.37 eV (Fig. 12d), which effectively improved the electronic conductivity and strengthened the bond between key intermediate ∗COOH and Ni active site. Thanks to the reduced energy barrier of RDS (Fig. 12e), the CO FE could be significantly enhanced to 99.2% at −0.79 V vs. RHE and could maintain above 95% in a wide potential range from −0.39 to −1.09 V vs. RHE (Fig. 12f). Similar mechanism of lowering the d-band center of Ni 3d orbital was achieved by constructing dual atomic Ni–Zn sites. 148 The distance between Ni–Zn was estimated to be 2.5 Å (Fig. 12g and h), which matched well with the cut-off distance of Ni or Zn atom. The region between Ni and Zn atoms showed an electron localization function (ELF) of 1/2, indicating electronic interaction between two adjacent atoms. Compared with Ni (Fig. 12k) or Zn SAC (Fig. 12l), NiZn–N6–C catalyst has 0.15 and 0.06 more electrons. In-situ FTIR measurement was carried out to investigate the possible CO2RR pathway (Fig. 12m). The appearance of ∗CO species on NiZn–N6 at a more positive potential (−0.15 V vs. RHE) verified weaker adsorption and unfavorable formation of ∗CO. DFT calculations demonstrated that the heteronuclear coordination could tune the electronic structure of Ni single atomic site, leading to reduced energy barrier in the aspect of thermodynamics and strengthened Ni–C bond in the aspect of kinetics. High CO FE over 90% could be achieved across a wide potential window from −0.5 to −1 ​V vs. RHE. Ren et al. 149 adopted a similar MOF-assisted pyrolysis method to fabricate Ni–Fe dual sites anchored on nitrogenated carbon. The excellent CO2RR performance was attributed to the structural change of adsorbed CO under CO2 atmosphere, which could further decrease the energy barrier for ∗COOH generation and favor the desorption of CO molecule. Zhang et al. 150 also found that Ni–Fe dual atom sites exhibited excellent CO2RR performance. In addition to favorable formation of ∗COOH and desorption of ∗CO as mentioned above, the competitive HER was also significantly suppressed over dual-atom catalysts.Neighboring Zn/Co monomer anchored on N-doped carbon (ZnCoNC) could also selectively reduce CO2 to CO, where CoSA moiety was considered as the active site that was modulated by the neighboring Zn site. 151 It is worth mentioning that NiSA is not always the active site in dual-atom catalysts despite of its high intrinsic catalytic activity. Xie et al. 152 prepared an integrated electrode containing NiSn atomic pair coordinated with four nitrogen atoms (N4–Ni–Sn–N4) through impregnation followed by subsequent pyrolysis. Instead of Ni–N4, Sn–N4 moiety was identified as the active site for CO2RR, resulting in the selective production of formate. Specifically, a high TOF of 4752 h−1, formate productivity of 36.7 mol h−1gSn −1 and high utilization of active sites (57.9%) could be realized, which are the highest values among reported literatures. DFT calculation demonstrated electron redistribution over the SnSA sites, thanks to the introduction of adjacent Ni–N4 moieties, which effectively decreased the energy barrier for ∗OCHO formation.Same metal atoms can also be adjacently anchored on substrate to form dual atom catalysts. Pd2 dual atom catalyst could be prepared by anion replacement deposition-precipitation (ARDP) method using binuclear Pd(II) complex as the precursor. 153 Compared with Pd1SAC, the electron transfer between neighboring dimeric Pd sites could weaken the adsorption of ∗CO, thus resulting in a lower energy barrier and outstanding CO selectivity (98.2% FE at −0.85 V vs. RHE). In Li's recent work, 154 binuclear Ag complex (Ag(NO3–O)(phtz-N)2(μ-phtz-N,N′)2) was firstly prepared by adding AgNO3 to phthalazine, followed by crystallization in a fridge, where each Ag atom was coordinated with three N atoms and two adjacent Ag atoms were weakly bonded with a length of 3.434 Å. After mixing with GO suspension, binuclear Ag complex could strongly adsorb on graphene substrate through π-π interaction. The final dual-atom Ag2-G catalyst could be fabricated after a final pyrolysis treatment of the centrifuged precipitates. Two adjacent Ag sites could interact with the C and O atom in the CO2 molecule, stabilizing ∗CO2 and lowering the reaction barrier to form ∗COOH. The CO2RR could be driven at −0.25 V vs. RHE, showing a CO FE of 93.4% with a current density of 11.87 mA cm−2. In addition to two identical adjacent metal sites, one element could also form two neighboring metal sites with different states. Jiao et al. 155 fabricated Cu atom-pair catalyst by introducing Cu(NO3)2 into the precursor solution to grow Pd10Te3 nanowires. XAFS results verified that the adjacent Cu species would form stable Cu1 0–Cu1 x ​+ pair structures, which worked together to promote the CO2RR. Specifically, Cu1 x ​+ favored the adsorption of H2O while the adjacent Cu1 0 could adsorb CO2 molecule efficiently. A high CO FE over 92% could be achieved with nearly completely suppressed undesired HER. DFT calculation attributed the reduced activation energy to the unique adsorption configuration on the Cu1 0–Cu1 x ​+ pair structure.In this review, we firstly introduced the reaction mechanism and pathways of electrochemical reduction of CO2 to various products (CO, HCCOH, CH4, CH3OH and C2+). Then recently reported single atom catalysts were systematically summarized and discussed in aspect of different active centers (transitional metals, noble metals and main group metals). Furthermore, as the most efficient coordination modulation method in SACs, strategies of introducing hetero non-metal atoms were discussed. Then, dual atom catalysts were summarized and reviewed, which could not only keep advantages of SACs, but also provide more opportunities to manipulate the coordination environments. Although many progresses of SACs have been achieved in the field of CO2RR, there still exist many challenges and problems that retard the in-depth understanding of SACs and their application in industry. 1. The catalytic activity (partial current density and turnover frequency) is highly related to the metal loading. Most of current researches on SACs can only achieve relatively low metal loadings below 5 wt% in order to prevent metal aggregation and maintain atomically dispersed active sites. Therefore, it is of great significance to further increase the metal loading amount in SACs to realize industrial-level partial current density (>300 mA cm−2). 2. As most strategies for preparation of SACs require high-temperature, the as-prepared SACs are actually composed of metal sites with very complex coordination environments. Many advanced characterization techniques including XAS and HAADF-STEM could be used to provide direct evidences of the existence of single atomic sites and their coordination environments. It is still worth mentioning that XAS can only give statistical results, demonstrating the overall information about the catalysts. On the other hand, despite of the very high resolution (0.06 nm) of aberration-corrected transmission electron microscopy (AC-TEM), it is still difficult to distinguish the coordination atoms (C, N, O, S …) surrounding the metal sites due to their similar contrast under TEM or STEM mode. Additionally, those species with the same coordination environment but at different sites (perfect sites, defect sites or edge sites) of substrate will bring even more uncertainties for precise understanding of coordination environment of SACs, which is considered as the key factor influencing the electronic structure of metal sites, adsorption energy of intermediates and final selectivity of catalysts. It is suggested that strategies adopting low-temperature preparation or immobilized molecular catalysts can be developed to establish well-designed SACs with precise coordination environment, which can serve as the platform to investigate the catalytic mechanism of SACs. 3. Due to the stable structure of CO2 molecule, the electrochemical reduction of CO2 always requires high cathodic potential when compared to HER. In this case, the chemical state of metal sites and coordination atoms are very likely to evolve under CO2RR condition, making the real active sites elusive. In-situ or time-resolved characterizations are encouraged to monitor the change of active sites during the whole reaction process, 100 , 156 which can help to figure out the real active sites catalyzing CO2RR and understand the catalytic mechanism. 4. With exception to few literatures reporting C2+ products, most predominant product over SACs in CO2RR is CO due to the lack of C–C coupling sites, which hinders the application of SACs. Fortunately, some Cu SACs have been fabricated for the catalytic production of advanced C2+ products (ethylene, ethanol, and acetone) either based on in-situ formation of Cun clusters or adjacent Cu–N2 moieties. The investigation of SACs to generate C2+ products can not only provide additional choice for the generation of high-value products, but also serve as an ideal platform to study the complex multiple protonation and electron transfer during the reduction process. 5. Widely used wet-chemistry or ball-milling methods tend to prepare powder-based catalysts, where binders or additives are needed for the preparation of electrodes and may have side effects on the catalytic performances. Therefore, the direct fabrication of monolithic SAC electrodes or dispersing single atoms onto self-supported gas diffusion substrates is very promising and can be further applied in membrane electrode assembly (MEA) electrolyzers and flow cells to overcome the solubility and diffusion limitation of CO2 molecules in electrolyte. 44 The catalytic activity (partial current density and turnover frequency) is highly related to the metal loading. Most of current researches on SACs can only achieve relatively low metal loadings below 5 wt% in order to prevent metal aggregation and maintain atomically dispersed active sites. Therefore, it is of great significance to further increase the metal loading amount in SACs to realize industrial-level partial current density (>300 mA cm−2).As most strategies for preparation of SACs require high-temperature, the as-prepared SACs are actually composed of metal sites with very complex coordination environments. Many advanced characterization techniques including XAS and HAADF-STEM could be used to provide direct evidences of the existence of single atomic sites and their coordination environments. It is still worth mentioning that XAS can only give statistical results, demonstrating the overall information about the catalysts. On the other hand, despite of the very high resolution (0.06 nm) of aberration-corrected transmission electron microscopy (AC-TEM), it is still difficult to distinguish the coordination atoms (C, N, O, S …) surrounding the metal sites due to their similar contrast under TEM or STEM mode. Additionally, those species with the same coordination environment but at different sites (perfect sites, defect sites or edge sites) of substrate will bring even more uncertainties for precise understanding of coordination environment of SACs, which is considered as the key factor influencing the electronic structure of metal sites, adsorption energy of intermediates and final selectivity of catalysts. It is suggested that strategies adopting low-temperature preparation or immobilized molecular catalysts can be developed to establish well-designed SACs with precise coordination environment, which can serve as the platform to investigate the catalytic mechanism of SACs.Due to the stable structure of CO2 molecule, the electrochemical reduction of CO2 always requires high cathodic potential when compared to HER. In this case, the chemical state of metal sites and coordination atoms are very likely to evolve under CO2RR condition, making the real active sites elusive. In-situ or time-resolved characterizations are encouraged to monitor the change of active sites during the whole reaction process, 100 , 156 which can help to figure out the real active sites catalyzing CO2RR and understand the catalytic mechanism.With exception to few literatures reporting C2+ products, most predominant product over SACs in CO2RR is CO due to the lack of C–C coupling sites, which hinders the application of SACs. Fortunately, some Cu SACs have been fabricated for the catalytic production of advanced C2+ products (ethylene, ethanol, and acetone) either based on in-situ formation of Cun clusters or adjacent Cu–N2 moieties. The investigation of SACs to generate C2+ products can not only provide additional choice for the generation of high-value products, but also serve as an ideal platform to study the complex multiple protonation and electron transfer during the reduction process.Widely used wet-chemistry or ball-milling methods tend to prepare powder-based catalysts, where binders or additives are needed for the preparation of electrodes and may have side effects on the catalytic performances. Therefore, the direct fabrication of monolithic SAC electrodes or dispersing single atoms onto self-supported gas diffusion substrates is very promising and can be further applied in membrane electrode assembly (MEA) electrolyzers and flow cells to overcome the solubility and diffusion limitation of CO2 molecules in electrolyte. 44 T. W., B. L. and S. K. conceived the topic and structure of the article. All authors reviewed and contributed to this paper.There are no conflicts of interest to declare.This work was supported by the fund from NUS Green Energy Program (WBS: A-0005323-05-00), FRC MOE T1 (WBS: A-0009184-00-00), A∗STAR LCERFI Project (Award ID: U2102d2011), Ministry of Education of Singapore (Tier 1: RG4/20, RG2/21 and Tier 2: MOET2EP10120-0002), and Agency for Science, Technology and Research (AME IRG: A20E5c0080).

Powered by electricity from renewable energies, electrochemical reduction of CO2 could not only efficiently alleviate the excess emission of CO2, but also produce many kinds of valuable chemical feedstocks. Among various catalysts, single atom catalysts (SACs) have attracted much attention due to their high atom utilization efficiency and expressive catalytic performances. Additionally, SACs serve as an ideal platform for the investigation of complex reaction pathways and mechanisms thanks to their explicit active sites. In this review, the possible reaction pathways for the generation of various products (mainly C1 products for SACs) were firstly summarized. Then, recent progress of SACs for electrochemical reduction of CO2 was discussed in aspect of different central metal sites. As the most popular and efficient coordination modulation strategy, introducing heteroatom was then reviewed. Moreover, as an extension of SACs, the development of dual atom catalysts was also briefly discussed. At last, some issues and challenges regarding the SACs for CO2 reduction reaction (CO2RR) were listed, followed by corresponding suggestions.

Hydrogen is an ideal alternative to fossil fuels for its high energy density and abundant resources [1]. However, safe, efficient and economical hydrogen storage is still a challenge for the large-scale application of hydrogen energy. [2] In the last few decades, solid-state hydrogen storage materials, including light metal hydrides [3,4] and complex hydrides [5–8], have attracted considerable attention due to their high hydrogen density and safety. Among them, MgH2, with a capacity of 7.6 wt% H2, is regarded as one of the most promising candidates owing to its high reversibility, low cost and environmental friendliness [9]. Unfortunately, it suffers from high thermodynamic stability and slow sorption kinetics [10].A variety of strategies have been developed to tackle these problems. One is to thermodynamically destabilize MgH2 by alloying Mg with other metal elements [11], such as Al [12], Ni [13], and Fe [14]. A representative intermetallic hydride is Mg2NiH4, which shows a favorable dehydrogenation enthalpy of 65 kJ mol−1 H2, [15] lowered by 10 kJ mol−1 H2 compared with that of the pristine MgH2. The reduced enthalpy change results in a low dehydrogenation temperature of 255 °C at 1 bar equilibrium H2 pressure. [15] However, a main drawback of this strategy is the inevitable capacity loss, where the theoretical hydrogen storage of Mg2NiH4 is only 3.6 wt% [16].Nanostructuring is also an effective way to modify the thermodynamics and especially kinetics of MgH2. [17–21] Theoretical calculations suggest that there is a significant decrease in thermodynamic stability when the grain size of MgH2 is reduced to less than 2.0 nm. [22] For MgH2 with a grain size of 0.9 nm, the decomposition enthalpy is only 63 kJ mol−1 H2, corresponding to a desorption temperature of only 200 °C. [23] The grain refinement also improves the hydrogen de-/sorption kinetics, owing to abundant diffusion paths and shortened diffusion distance. [24] Nevertheless, it is so far difficult to experimentally synthesize MgH2 NPs smaller than 20 nm. [25,26] In this case, attempts turn to confining MgH2 in various porous scaffolds, which makes it possible to obtain MgH2 in several nanometers according to the pore size of scaffolds. However, the scaffolds commonly take up considerable content in the confined systems, and hence the available hydrogen storage capacity is significantly lowered. For instance, a confined MgH2 system using activated carbon fibers as scaffold shows a low dehydrogenation enthalpy and a low activation energy of 63.8 ± 0.5 kJ mol−1 H2 and 1438 ± 2 kJ mol−1, respectively,  [27] indicating reduced thermal stability and improved kinetics. However, this system can only load 22 wt% MgH2, representing a theoretical hydrogen capacity of merely 1.67 wt%.Introduction of catalysts is another effective way to reduce the dehydrogenation temperature and improve the reaction kinetics of MgH2, where transition metals and their compounds are the commonly used catalysts [28–33]. Among them, Ni has attracted considerable attention due to its active role in the dissociation of hydrogen, where a vacant d-orbital of Ni first accepts electron of hydrogen and then the bind is stabilized by back-donation of electrons from the filled d-orbital to the anti-bonding orbital (σ*) of H2, thus facilitating the break of H–H bond [34,35]. The combination of H atoms to form H2 molecular is just the inverse process. Especially nanosized Ni-based catalysts are highly effective since they can provide large contact surface area and abundant active sites [36]. This helps to decrease their addition amount for effective catalysis, hence minimizing the loss of the theoretical hydrogen storage capacity. However, nanosized Ni particles are dimensionally unstable and easily agglomerate during hydrogen sorption cycling due to the high surface energy [37]. Recent studies show that the size stability and catalytic activity of Ni NPs can be further enhanced by loading them on substrates like carbon-based materials [37,38]. Moreover, carbonaceous materials are also favorable for the nucleation of Mg or/and MgH2 and provide additional channels for hydrogen diffusion, exhibiting a synergistic catalytic effect with Ni [39]. The reported carbonaceous substrates for supporting nano-sized Ni particles include graphene nanosheets [24,39,40], carbon aerogel [41], mesoporous carbon CMK-3 [42], hard carbon spheres [16,43], and so forth. Nevertheless, the Ni loading in the currently reported carbon-supported catalysts is commonly low in order to stably disperse Ni NPs and maximize their catalytic effectiveness. Challenges remain on achieving small size, high loading and uniform dispersion of nano-Ni particles on substrates simultaneously. Although the hydrogen storage properties of MgH2 are improved by these carbon-supported nickel catalysts to date, developing highly effective catalysts is still highly imperative to further improve the overall hydrogen storage properties of MgH2 at milder conditions.In the present study, a type of porous hollow carbon nanospheres (PHCNSs) that we previously reported [44] was used as the substrate to prepare the Ni-incorporated PHCNSs composites, and their catalytic effect on MgH2 was investigated. The merits of PHCNSs as the substrate are as follows. First, the PHCNSs possess high specific surface area, large pore volume and hierarchical pore structure, which can provide large amounts of dispersive sites for Ni NPs. Second, the PHCNSs have a highly amorphous structure with massive defects, brought by the pores generated during the CO2 activation process. As reported, surface defects can change the electronic structures of carbon surfaces, creating trapping centers for metal atoms [45]. Lastly, except for C, the PHCNSs also contain 18.2 at% O and minor amounts of N and H, which are originated from the surface functional groups of PHCNSs. The heteroatom-induced coordination sites in carbon substrates can anchor metal precursors via chelation [45]. It is thus believed that the abundant dispersive sites, surface defects and heteroatoms of PHCNSs favor the dispersion of metal Ni NPs. In order to optimize the catalytic effect, different amounts of Ni are incorporated to the PHCNSs substrate. Up to 90 wt% Ni NPs can be highly dispersed on to the PHCNSs without agglomeration. With an addition of only 5 wt% of the 90 wt% Ni-incorporated PHCNSs, the system shows evidently lowered dehydrogenation temperature, improved kinetics and superior cycling stability. The evolution of phase compositions and microstructures of the system during cycling is investigated for identifying the reasons for the improved hydrogen storage properties.The detailed synthesis process and structural characteristic methods of the PHCNSs were as reported in our previous work [44]. The PHCNSs have intact spherical morphology with a uniform size distribution of ca. 90 nm, and with inner cavities in size of ca. 30 nm and hence a shell thickness of 30 nm. The specific surface area and pore volume of the PHCNSs are up to 2609 m2 g − 1 and 2.275 cc g − 1, respectively. The volume of the inner cavities is not included in the measured value as the size is beyond the measurement range of the analyzer. The nanopores generated by CO2 activation in the carbon shell are mostly smaller than 4.5 nm. In addition, the PHCNSs contain 18.2 at% O and minor amounts of N and H.Ni(NO3)2•6H2O (98%, Aladdin) was used as the Ni source. Different amounts of the PHCNSs and Ni(NO3)2•6H2O corresponding to mass ratios of Ni: PHCNSs of 5: 5, 7: 3, 9: 1 and 9.5: 0.5, respectively, were added to a medium of ethanol and stirred for 30 min. The mixed suspension was then subjected to ultrasonication under dynamic vacuum for 6 h to ensure a full wetting of the Ni(NO3)2 solution to the outer and inner surface of the PHCNSs, including the pores in the shell. During this process, most of ethanol medium is removed by vacuum extraction, leaving a viscous mixture, which is then dried at 60 °C for 12 h in a vacuum drying oven to remove the residual ethanol. Thereafter, the completely dried mixture was heat-treated at 500 °C for 4 h under a flowing mixture gas of 10 vol% H2/Ar for the thermal reduction of Ni precursor to metal Ni. The final products were denoted as Ni x @PHCNSs, where x represents the mass fraction of Ni in the composites.MgH2 used in the present study was synthesized in-house by hydriding a commercial Mg powder (Macklin, purity 98%, 20 - 100 mesh) at 340 °C under 20 bar H2 for 12 h. The obtained Ni x @PHCNSs composites were introduced to MgH2 at an addition amount of 5 wt% by ball-milling under 50 bar H2 for 24 h using a planetary ball mill (QM-3SP4, Nanjing). A ball-to-sample weight ratio of 120: 1 and a rotating speed of 500 rpm were used. The mill rotated for 0.3 h in one direction, and paused for 0.1 h, and then revolved in the reverse direction for 0.3 h, to minimize the temperature increase during ball milling. The obtained systems were denoted as MgH2/Ni x @PHCNSs. In addition, the MgH2/PHCNSs system was also prepared under the same experimental procedure for comparison. All operations were conducted in a glove box (MBRAUN 200B, Germany) filled with pure Ar (H2O < 1 ppm; O2 < 1 ppm) to avoid the moisture and air contamination.The crystal structures of the samples were analyzed by X-ray diffraction (XRD, MiniFlex 600 X-ray diffractometer, Rigaku). The samples were sealed in a custom-designed container covered with a Scotch tape to prevent air and moisture contamination during operation. The Ni content of the Nix@PHCNSs composites was measured by thermogravimetry analysis (Netzsch, TG 209 F3). A NOVA-1000e automated surface area analyzer (Quantachrome, USA) was used to conduct N2 sorption measurement, and the specific surface area and pore size distribution of the samples were calculated based on the Brunner–Emmet–Teller (BET) and density functional theory (DFT) methods. Morphologies of the samples were analyzed by scanning electron microscopy (SEM, Hitachi, S-4800) and transmission electron microscopy (TEM, FEI, Tecnai G2 F20 S-TWIN). The distribution of C, Ni and Mg elements in the samples was identified with an energy-dispersive X-ray spectrometer (EDS) attached to the TEM facility. X-ray photoelectron spectroscopy (XPS, Thermo Scientific, ESCALAB 250Xi) was used to determine the chemical valence state of Ni in the samples. XPS spectra were recorded using monochromatic an Al Kα (1486.6 eV) X-ray source with a base pressure of 5 × 10 − 10 Torr of air. The adventitious C 1 s peak centered at 284.8 eV was used as a reference to calibrate the obtained XPS data, and the binding energy spectra were fitted by XPSPEK software.The temperature-dependent dehydrogenation properties of the catalyzed systems were evaluated by a custom-designed temperature-programmed desorption (TPD) instrument coupled with a mass spectrometer (MS). The TPD-MS curves at different heating rates (1, 2, 4, 8 °C min−1) were collected to evaluate the apparent dehydrogenation activation energies (E a) of the systems based on the Kissinger method [46]: d ( ln β T P 2 ) d ( 1 T P ) = − E a R where β is the heating rate, T p is the absolute temperature corresponding to the maximum desorption rate and R is the gas constant. In this work, T p is the peak temperature of the TPD-MS curves with different heating rates. Dehydrogenation and hydrogenation of the systems were quantitatively measured by a volumetric method on a custom-designed Sievert-type apparatus. For the non-isothermal testing, the systems were heated from room temperature to 400 °C at a heating rate of 2 °C min−1 under static vacuum for dehydrogenation and heated from room temperature to 250 °C at a heating rate of 1 °C min−1 under 50 bar H2 for hydrogenation. For the isothermal testing, the systems were rapidly heated to the pre-set temperature and dwelled for 1 h under static vacuum for dehydrogenation, while for hydrogenation, the systems were first heated to the pre-set temperature under static vacuum and then rapidly loaded with 50 bar H2, dwelling for 10 min. A heating rate of 10 °C min−1 was used for both isothermal dehydrogenation and hydrogenation. Cyclic performance of the optimized system was evaluated with a regime of dehydrogenation to 350 °C under static vacuum and hydrogenation to 250 °C under 50 bar H2.The preparation process of the MgH2/Ni x @PHCNSs systems is illustrated in Scheme 1 . The porous hollow carbon nanospheres are firstly fully impregnated with the ethanol solution of nickel nitrate under dynamic vacuum. After heat reduction, the Ni NPs-loaded PHCNSs composites are obtained, which are then introduced to MgH2 by ball-milling. Fig. 1 presents the XRD patterns of the Ni x @PHCNSs composites with different Ni contents. The diffraction peaks of Ni (JCPDS no. 04–0850) are clearly seen for all composites, indicating the successful formation of elemental Ni with high crystallinity. Thermogravimetric analysis (TGA) of the Ni x @PHCNSs composites (Fig. S1) conducted in air shows that there are weight gains for the composites with high Ni contents while weight losses for the composites with low Ni contents, which is ascribed to the competitive result of the oxidation of Ni to NiO and the combustion of the PHCNSs to carbon dioxide during the heating process. Based on the TGA results, the Ni contents for the composites with weight ratios of Ni: PHCNSs of 5:5, 7:3, 9:1 and 9.5:0.5 are calculated to be 54.4, 72.2, 90.7 and 96.7 wt%, respectively, very close to the designed values. Fig. 2 shows the SEM images of the Ni x @PHCNSs composites. It is seen that the spherical structure of the PHCNSs remains stable after the incorporation of Ni NPs. For the composite with Ni contents of 50 and 70 wt%, the morphology is almost the same as that of the original PHCNSs, without other evidently different morphology, suggesting that the Ni NPs are mostly deposited in the inner cavities and nanopores of the PHCNSs. With the Ni content increasing to 90 wt%, there are numerous bright dots corresponding to Ni NPs observed, which are uniformly deposited on the surface of the PHCNSs. The corresponding EDS mapping (Fig. S2) shows that Ni element is well distributed in the C substrate without obvious aggregation, demonstrating the ultrafine size and the well dispersion of Ni NPs. However, when the Ni content reaches 95 wt%, some bulk Ni aggregates with the size of several hundred nanometers are observed, as marked by white dashed circles in Fig. 2d.Further TEM observation of the Ni x @PHCNSs composites is shown in Fig. 3 , where the bright area is the PHCNSs substrate and the dark dots represent Ni NPs. The stable spherical structure of the PHCNSs is demonstrated by white dashed circles, and the Ni NPs with an average size of ca.10 nm are uniformly dispersed on the surface of the PHCNSs and also in their inner cavities. The latter is generated from the nickel nitrate solution infiltrating into the inner cavities of the PHCNSs during the vacuum impregnation process. As seen from Fig. 3a – d, with increasing Ni content from 50 to 95 wt%, the density of the dispersive Ni NPs is gradually increased. What's notable is that the Ni NPs up to a high loading of 90 wt% maintain fine size and high dispersibility, with almost no agglomeration. Although there is aggregated Ni observed at a higher loading of 95 wt%, most Ni particles are still dispersive, in consistence with the SEM observation. In addition, in the case that the samples for TEM characterization are subjected to strong ultrasonic treatment during the preparation process, the Ni NPs are still immobilized on the PHCNSs, suggesting a strong bonding between the Ni NPs and the PHCNSs substrate. Such a strong metal-substrate interaction is helpful for suppressing the aggregation of metal NPs and tailoring the geometric structures and electronic configurations of catalytic active sites [45].The nitrogen sorption isotherms and pore size distributions of the Ni x @PHCNSs composites (Fig. S3a and b) show that both specific surface area and pore volume value are all extremely lowered compared with those of the PHCNSs. Besides the observed Ni NPs that deposit at the surface and cavity of the PHCNSs, there are also some ultrafine Ni particles with the size of only a few nanometers dispersed in the pore channels of the carbon shell, leading to the reduction of porosity. The unique structure of the PHCNSs not only provides large amounts of dispersive sites for Ni NPs, but also realizes their hierarchical size distribution.Fig. 4a shows the XRD patterns of the as-milled MgH2/Ni x @PHCNSs systems as well as the pristine MgH2 and the MgH2/PHCNSs system. β-MgH2 is the main phase for all the systems. Besides, a minor amount of MgO is also identified from its main diffraction peak at 42.8° (JCPDS no. 45–0946) although its intensity is very weak, which is suggested from the chemical reaction between MgH2 and the oxygen-containing functional groups of the during ball milling. Notably that there are no diffraction peaks of Ni detected for all the MgH2/Ni x @PHCNSs systems, possibly due to its low relative content. Further XPS analysis of the Ni 2p spectrum of the MgH2/Ni90@PHCNSs system, Fig. 4 b, shows that there are two peaks at 852.9 and 870.0 eV, which are well assigned to the binding energy of Ni 2p3/2 and Ni 2p1/2, respectively, [47,48] demonstrating the existence of elemental Ni. Therefore, it is obtained that ball milling is only a physical mixing process.A representative SEM image of the as-milled MgH2/Ni90@PHCNSs system (Fig. S4a) shows that the ball-milled system is composed of irregular particles with an overall size distribution from tens to hundreds of nanometers. By contrast, the as-milled pristine MgH2 shows an obviously large size distribution, where nano-scale and micron-scale MgH2 particles coexist (Fig. S4b). The result demonstrates that the introduction of the Ni x @PHCNSs composites enhance the ball milling efficiency by serving as grinding agents. Further TEM characterization of the as-milled MgH2/Ni90@PHCNSs system is conducted, as shown in Fig. 5 a. The original spherical morphology of the Ni-incorporated PHCNSs disappears after ball milling, which is transformed into lamellar carbon with uniformly embedded Ni NPs under stress, covered homogeneously on the surface of MgH2. Although the lamellar carbon is hardly identified due to its poor contrast relative to MgH2, the superfine Ni NPs in size of ca.10 nm are vaguely seen, as marked by the circles in Fig. 5a. EDS analysis (Fig. 5b – d) further shows that Ni and C elements are uniformly distributed on the MgH2 particles without aggregation. It is thus obtained that the PHCNSs can not only disperse a high loading of Ni in the initial structure, but also preserve the high dispersibility of Ni NPs even after the high energy ball milling, which is likely due to the strong metal-substrate interaction as mentioned above. Fig. 6 a shows the temperature-dependent dehydrogenation behavior of the MgH2/Ni x @PHCNSs systems analyzed by TPD-MS measurement, and that of the pristine MgH2 and the MgH2/PHCNSs system is also shown for comparison. It is seen that with the introduction of only 5 wt% of the Ni x @PHCNSs composite, both onset and peak dehydrogenation temperatures of MgH2 are significantly reduced. By contrast, the individual PHCNSs has limited catalytic effect on the dehydrogenation of MgH2, where the onset dehydrogenation temperature is almost the same as that of the pristine MgH2 and the peak temperature is only 7 °C lower than 317 °C for the pristine MgH2, suggesting that Ni is central important for the catalysis. Additionally, it should be noted that there is an additional shoulder peak at 360 °C for the pristine MgH2, while such peak disappears in the MgH2/PHCNSs system, corresponding to an evidently reduced ending dehydrogenation temperature. As reported previously [49,50], such a shoulder peak is attributed to the nonuniform size distribution of MgH2 particle. The result illustrates that the PHCNSs act as the grinding agent during ball-milling, increasing the size homogeneity of the MgH2 particles. Moreover, with the Ni loading in the Ni x @PHCNSs composites increasing from 50 to 90 wt%, the reduction of the dehydrogenation temperatures is more significant, confirming that Ni plays the main role for the catalysis. Among them, the system introduced with Ni90@PHCNSs exhibits the lowest onset and peak dehydrogenation temperatures of 195 °C and 242 °C, respectively, which are 55 and 75 °C lower than those of the pristine MgH2. Further increasing the Ni loading to 95 wt% reverses the decreasing trend of the dehydrogenation temperatures, which is supposed due to the formation of the Ni aggregates, resulting a slightly lowered catalytic effect compared with the well dispersive Ni NPs in the Ni90@PHCNSs composite.The volumetric dehydrogenation curves of the MgH2/Ni x @PHCNSs systems as well as the pristine MgH2 and the MgH2/PHCNSs system are shown in Fig. 6b. For the MgH2/Ni x @PHCNSs systems, the main dehydrogenation temperature range is evidently down-shifted compared with the pristine MgH2, and dehydrogenation almost accomplishes at ca. 310 °C. The reduction on the dehydrogenation temperature is extremely small for the MgH2/PHCNSs system, but the system does not contain the two-step dehydrogenation process as in the pristine MgH2. The result further demonstrates that nano Ni particles play important role in improving the dehydrogenation kinetics while the PHCNSs contribute to the uniform size distribution of the MgH2 particles, which are in good agreement with the results from the TPD-MS measurement. The main non-isothermal dehydrogenation properties of the investigated systems are summarized in Table 1 . The dehydrogenation capacities of the MgH2/Ni x @PHCNSs systems with different Ni loadings upon heating to 300 °C are very close, which are 6.3, 6.4, 6.5 and 6.4 wt%, respectively, for Ni contents of 50, 70, 90 and 95 wt%. Further heating to 400 °C only results in less than 0.5 wt% H2 released. By taking overall consideration of the dehydrogenation temperature and capacity, the MgH2/Ni90@PHCNSs system shows the optimized performance among all the systems, which is performed further studies on isothermal kinetics and reversibility. Fig. 7 a shows the isothermal dehydrogenation curves of the MgH2/Ni90@PHCNSs system as well as the pristine MgH2 at 225, 250 and 275 °C. It is seen that the dehydrogenation rate of the catalyzed system is significantly increased compared with that of the pristine MgH2. There are 3.8 and 6.2 wt% H2 released for the MgH2/Ni90@PHCNSs system dwelling at 225 and 250 °C for 30 min, respectively, while for the pristine MgH2, less than 0.5 wt% H2 is released at the same condition. When the isothermal temperature is set to a slightly higher temperature of 275 °C, there is 6.4 wt% H2 already released when the temperature is just approached to 275 °C, which is almost the stable value of the dehydrogenation capacity at this temperature. By contrast, there is only 1.4 wt% H2 desorbed for the pristine MgH2 upon heating to 275 °C, and the dehydrogenation capacity is only 2.2 wt% H2 even after dwelling for 20 min. The dehydrogenation products performed at 275 °C is further used for isothermal hydrogenation property testing. The corresponding hydrogenation curves at 100, 150 and 200 °C under 50 bar H2 are shown in Fig. 7b, where hydrogen pressure is loaded only when the pre-set temperature is reached. It is seen that the MgH2/Ni90@PHCNSs system achieves a saturated hydrogen capacity of 6.3 wt% within only 100 s at 200 °C. When the isothermal temperature is decreased to 150 °C, a capacity of 6.2 wt% H2 is also achieved within 250 s. Even at a low temperature of 100 °C, there is still 5.3 wt% H2 absorbed with a dwelling period of 600 s. Whereas for the pristine MgH2, the hydrogenation capacities are only 0.6, 1.9 and 5.0 wt% at 100, 150 and 200 °C, respectively, even for a dwelling period of 600 s. It is concluded that the superfine PHCNSs-supported Ni NPs show highly bifunctional effect on both dehydrogenation and hydrogenation kinetics of MgH2.Based on the TPD-MS curves at different heating rates and the Kissinger's plots (Fig. S5a and b), the dehydrogenation apparent activation energy (E a) of the MgH2/Ni90@PHCNSs system is estimated to be 98±6 kJ mol−1, corresponding to a reduction of 30% compared with 139 kJ mol−1 for the pristine MgH2 [4], the value of which is also smaller than those of other recently reported MgH2-catalyst systems [51–57]. Therefore, a reduced dehydrogenation energy barrier is achieved with the introduction of the Ni-incorporated PHCNSs composite, which contributes to the improvement in dehydrogenation kinetics. Fig. 8 shows the selected cyclic dehydrogenation (a) and hydrogenation (b) curves of the MgH2/Ni90@PHCNSs system with a regime of dehydrogenation to 350 °C under static vacuum and hydrogenation to 250 °C under 50 bar H2. There is 6.8 wt% H2 desorbed in the first dehydrogenation. The dehydrogenation product is highly reversible in the subsequent hydrogenation. Notable that from the second cycle, the onset and ending dehydrogenation temperatures are further decreased to ca. 200 and 290 °C, respectively, and the dehydrogenation and hydrogenation curves are almost overlapped for the subsequent cycles, demonstrating a superior cyclic stability. The practical available hydrogen capacity after 50 cycles remains to be 6.4 wt%, corresponding to a capacity retention of 94.1%. In addition, the expression of cyclic de/re-hydrogenation kinetics of time versus hydrogen sorption capacities is shown in Fig. S6a and b. It is seen from that hydrogen is rapidly released during the heating period, especially from the second cycle. The main dehydrogenation occurs in 40 min from ca. 220 °C to 300 °C, corresponding to ca. 6.2 wt% H2 releasing. The dehydrogenation kinetics maintains high from the second cycle. There is ca. 6.0 wt% H2 released in 40 min from ca. 200 to 280 °C for the subsequent cycles. It terms of the hydrogenation curves, Fig. 6Sb, the curves of time versus capacity is still overlapped in a high level. There is ca. 4.0 wt% H2 absorbed in the initial 50 min, corresponding to a temperature range of ca. 50 to 100 °C, then the absorption turns to slightly lower rates, where more ca. 2.5 wt% H2 is absorbed in 150 min from 100 to 250 °C.Although the hydrogen storage properties from different laboratories cannot be quantitatively compared because of the different testing programs, it is still informative to summarize the progress reported. Table 2 lists the comparison of the hydrogen storage properties of the present MgH2/Ni90@PHCNSs system and the representative MgH2 systems added with various carbon-supported nickel catalysts. Obviously, the present system shows lower dehydrogenation temperature, better hydrogen sorption kinetics and higher reversible capacity compared with those of the reported systems. This demonstrates that the ultrafine Ni particles and their high dispersibility is highly effective in improving the reaction kinetics of MgH2. Therefore, even an addition as low as 5 wt% of Ni90@PHCNSs results in significant improvement on the hydrogen storage properties of MgH2. The abundant pores, especially the ultrafine nanopore channels, and the large surface area of the present PHCNSs offer the possibility for the high content and well dispersive distribution of ultrafine Ni NPs, which contribute to a highly active catalysis to MgH2. Fig. 9 shows the XRD patterns of the MgH2/Ni90@PHCNSs system at different dehydrogenation and hydrogenation states. After the first dehydrogenation, the diffraction peaks of MgH2 disappear and instead, peaks of Mg appear, demonstrating the complete decomposition of MgH2. Moreover, there are three new weak peaks detected at 39.8, 40.8 and 44.9°, matching well with Mg2Ni (JCPDS no. 35–1225), which suggests that the Ni NPs react with MgH2 in the initial dehydrogenation process forming Mg2Ni. The diffraction peaks of MgH2 reappear after the first hydrogenation, indicating the regeneration of MgH2, and besides, there is a minor amount of residual Mg, which may be responsible for the slight decrease of cyclic capacity. Meanwhile, the Mg2Ni phase is also hydrogenated and transformed to Mg2NiH4 (JCPDS no. 35–1225). The formation of Mg2NiH4 is likely to be the reason for the further reduction of the onset dehydrogenation temperature of the system after the first cycle (Fig. 8), as Mg2NiH4 is easier to release hydrogen than MgH2  [39] and shows better catalytic effect on MgH2 than pure Ni  [60]. The onset dehydrogenation temperature of the catalyzed system from the second cycle (ca. 200 °C, Fig. 8a) is lower than the theoretical decomposition temperature of Mg2NiH4 reported in literature (ca. 255 °C at 1 bar equilibrium H2 pressure)  [15]. It is supposed due to the low H2 pressure in the dehydrogenation process, the extra active effect of the carbon interaction and the ultrafine particle size of Mg2NiH4 for the present system. Moreover, Mg2NiH4 is still detected in the 50th hydrogenated product. It is clearly that the in-situ formed Mg2Ni/Mg2NiH4 during the first cycle exist stably in the subsequent cycles, acting as highly effective catalyst for the hydrogenation and dehydrogenation of MgH2. In addition, the minor MgO derived from the reaction between MgH2 and the oxygen-containing functional groups of the PHCNSs during ball milling also remains during the cycling, with no evidently change in the diffraction feature. Fig. 10 a shows a SEM morphology of the MgH2/Ni90@PHCNSs system at the 50th hydrogenation cycle. It is seen that the overall morphology is similar as that of its as-milled state, without obvious particle growth. In contrast, the pristine MgH2 of the same state shows severe particle agglomeration (Fig. S7). Further EDS analysis of the catalyzed system (Fig. 10b – e) shows that Ni and C elements are still homogeneously distributed in the MgH2 matrix without aggregation. The extra noises in the carbon map is originated from the carbon support film. It is thus concluded that the ultrafine size and the high dispersibility of the in-situ formed Mg2Ni/Mg2NiH4 phases maintain during hydrogen sorption cycles. Furthermore, the lamellar carbon covered on MgH2 particles act as barrier in suppressing the growth and agglomeration of MgH2 particles, both of which contribute to the superior cycling stability of the system.Ultrafine Ni NPs supported hollow carbon nanospheres (PHCNSs) are synthesized as the catalysts for promoting the hydrogen storage performance of MgH2. With a high loading of 90 wt%, the Ni NPs are well dispersed at the outer surface, in the inner cavity and also in the pore channels of the PHCNSs. Introduced to MgH2 by ball milling, the Ni NPs are uniformly distributed on the MgH2 particles with the help of the excellent dispersive role of the PHCNSs substrate, exhibiting superior bidirectional catalytic activity towards the dehydrogenation and hydrogenation of MgH2. The MgH2 system containing only 5 wt% Ni90@PHCNSs shows onset and peak dehydrogenation temperatures as low as 190 °C and 242 °C, respectively, and desorbs 6.5 wt% H2 upon heating to 300 °C. Moreover, 6.2 wt% H2 is rapidly released within 30 min at 250 °C and the dehydrogenation product can absorb almost the same amount of hydrogen within 250 s at 150 °C under 50 bar H2. Even at a low temperature of 100 °C, the system can absorb 5.3 wt% H2 in 600 s. A reversible dehydrogenation capacity of 6.4 wt% remains after 50 cycles, corresponding to a high capacity retention of 94.1%. The in-situ formed Mg2Ni/Mg2NiH4 inherit the superfine size and uniform dispersion of Ni NPs, acting as highly-active catalysts during the dehydrogenation and hydrogenation cycles of MgH2. The present work provides new ideas for developing highly effective and durable catalysts toward enhanced hydrogen storage properties of MgH2.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work is supported by the National Key Research and Development Program of the Ministry of Science and Technology of PR China (No. 2018YFB1502103), National Natural Science Foundation of PR China (Nos. 52071287, 51571175, U1601212, 51831009).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jma.2021.05.004. Appendix A. Supplementary data Image, application 1

Magnesium hydride (MgH2) is one of the most promising hydrogen storage materials for practical application due to its favorable reversibility, low cost and environmental benign; however, it suffers from high dehydrogenation temperature and slow sorption kinetics. Exploring proper catalysts with high and sustainable activity is extremely desired for substantially improving the hydrogen storage properties of MgH2. In this work, a composite catalyst with high-loading of ultrafine Ni nanoparticles (NPs) uniformly dispersed on porous hollow carbon nanospheres is developed, which shows superior catalytic activity towards the de-/hydrogenation of MgH2. With an addition of 5 wt% of the composite, which contains 90 wt% Ni NPs, the onset and peak dehydrogenation temperatures of MgH2 are lowered to 190 and 242 °C, respectively. 6.2 wt% H2 is rapidly released within 30 min at 250 °C. The amount of H2 that the dehydrogenation product can absorb at a low temperature of 150 °C in only 250 s is very close to the initial dehydrogenation value. A dehydrogenation capacity of 6.4 wt% remains after 50 cycles at a moderate cyclic regime, corresponding to a capacity retention of 94.1%. The Ni NPs are highly active, reacting with MgH2 and forming nanosized Mg2Ni/Mg2NiH4. They act as catalysts during hydrogen sorption cycling, and maintain a high dispersibility with the help of the dispersive role of the carbon substrate, leading to sustainably catalytic activity. The present work provides new insight into designing stable and highly active catalysts for promoting the (de)hydrogenation kinetics of MgH2.

Extensive use of fossil fuels is largely contributing to CO2 emissions and global warming. The current efforts of the scientific community is to develop sustainable technologies that can reduce CO2 emissions or even reach net zero or negative emission through the capture and conversion of CO2 [1]. CO2 conversion into CO, CH4, cyclic carbenes, polymers, etc can be achieved by a variety of methods [2–6]. CO2 capture and utilisation to produce fine chemicals accounts for a small percentage of the emitted CO2 levels [7]. A possible way to reach net zero emissions of CO2 is to use fuels that are derived from emitted CO2 [8,9]. However, CO2 is a stable molecule that requires a great deal of energy to activate, and therefore, efficient and low-cost conversion methods and catalysts for CO2 activation and conversion are highly sought. Current reseach in this area concern the use of Cr, Fe, Ni and Cu doped with a variety of materials (i.e. Ce, Cs, Zr or Y) or using photocatalysts in a photo-assisted revese water gas shift. Although the active phase varies quite considerably among these materials, the support media largely remains the same, usually metallic oxides like Al2O3, CeO2, ZrO2 or doped/mixed combinations.Production of higher hydrocarbon fuels through processes like the Fischer-Tropsch (F–T) synthesis and hydrogenation of CO2 to form methanol via reverse-water-gas-shift reaction (CAMERE process) are promising routes to utilise emitted CO2 [10,11]. F–T synthesis and CAMERE processes reported better efficiencies (approximately 20%) when CO generated from RWGS reaction was used as raw material [11]. Conversion of CO2 to CO through RWGS reaction is shown in (eq. 1). The reaction is endothermic and as such, is expected to demonstrate increased efficiencies at higher temperatures. There exists a major competing exothermic methanation reaction (Sabatier process), (eq. 2) that occurs at lower temperatures producing methane. A highly unwanted material where F–T or CAMERE processes are concerned. (1) CO2 + H2↔ CO + H2O; ΔH298 = +41 kJ/mol (2) CO2 + 4H2↔ CH4 + 2H2O; ΔH298= −165 kJ/mol As an additional drawbackthe Sabatier reaction consumes 4 mol of hydrogen per mol of CO2 thus imposing extra process cost. In this regard, if we aim to desing an efficient reverse water gas shift unit, it is of paramount importance to control the competition CO2-Methanation/RWGS to ensure the process is selective towards carbon monoxide. In this sense, a variety of catalytic materials has been investigated for RWGS reaction [12–15]. Noble metals, such as Au, Pt, Pd, Rh and Ru, exhibit high activity, stability and selectivity for CO2 reduction to CO. However, due to their cost and scarcity, it is desirable to replace these materials by introducing more ecominally appealing catalysts. Transition metals-based catalysts represent an economically interesting alternative [16]. For instance, copper-based catalysts were found to be more selective for CO production favouring RWGS reaction [17–24]. However, they suffered stability problems [25]. Modified Ni-based catalysts were also designed to achieve better selectivity and stability for RWGS reaction [26]. Bimetallic or metal alloy catalysts of Cu, Ni and Co exhibited activities comparable to noble metals and their alloys but, had stability issues [27]. Hence, in addition to catalyst activity and selectivity for CO, stability and sustainability of the chosen catalyst at reaction conditions are also critical.The nature of the catalyst support is known to influence the coking characteristics, stability and dispersion of catalysts. Different supports interact differently with the active catalyst [28,29]. For example, CeO2 supported Au was more active than the TiO2 supported Au catalyst due to the higher oxygen mobility of CeO2 [30]. in situ generated carbon support due to decomposition of a metal organic framework precursor showed high stability of the catalyst [31]. Mixed oxide supports of CeO2/ZrO2 or CeO2/Al2O3 supporting Ni altered the activity and selectivity of the catalyst when compared to unsupported Ni [26,32]. Reduced surface acidity of Al2O3 modified with CeO2 caused lesser extent of coking and retained catalyst activity for a longer time [33]. Specialised methods such as magnetron sputtering, atomic layer epitaxy (ALE), atomic layer/chemical vapour deposition, etc have also been employed to increase catalyst activity and stability [34,35]. Avoiding sophisticated, expensive techniques and excessive use of chemicals in catalyst preparation would make the whole process commercially more viable.Accordingly, it would be advantageous to design a sustainable synthesis of transition metal-based catalysts supported on low-cost, environmentally benign supports such as clays. Saponite is a smectite clay, having the formula NaMg6(Si7Al)O20(OH)4, with magnesium substituted 2:1 aluminosilicate layers and interlayer regions occupied by Na+ ions and water molecules. A number of transition metal based saponite catalysts has been used as catalysts in reducing gaseous compounds [36]. Change in the surface acidity of the magnesio-aluminosilicate clay layers in comparison to Al2O3 could affect the overall catalyst dispersion and stability. Recently, adamantanecarboxylates of transition metals and alkaline earth metals have been synthesized by sustainable green protocols. These adamantanecarboxylates under controlled decomposition are known to generate in situ carbon that stabilize metal nanoparticles/metal oxides/ mixed metal oxides [37].In this paper, aqueously exfoliated saponite clay layers have been used to provide better dispersibility and higher thermal stability to the active catalyst. The catalyst precursors, saponite-transition metal adamantanecarboxylates have been prepared by using metal hydroxides and 1-adamantane carboxylic acid. The resultant saponite / transition metal adamantanecarboxylates were reduced under hydrogen to achieve saponite supported carbon stabilized NiCu and NiCo metal alloy nanoparticles – our nature inspired multicomponent catalysts. These catalysts were then tested for reduction of CO2 to CO through RWGS reaction. The selectivity, activity and long-term stability of the resultant catalysts were tested and compared with that of monometallic Ni catalyst supported on saponite.Ni(OH)2, Cu(OH)2 and Co(OH)2 were used as the transition metal sources, and 1-adamantanecarboxylic acid, used as the carboxylate source (All chemicals were procured from Sigma Aldrich and used as received without further purification). Deionised water (18 MΩ.cm resistivity, Millipore water purification system) was used throughout the experiment. Na+-saponite, NaMg6(Si7Al)O20(OH)4, was synthesized hydrothermally by a procedure reported by Kawi and Yao [38].In a typical synthesis of saponite supported NiCu-adamantanecarboxylate (NiCu-Ada/Sap), 1 g of Na+-saponite was stirred in 100 ml of water for 2 days at room temperature to produce exfoliated colloidal dispersion of saponite clay. 3.79 g 1-adamantanecarboxylic acid and 0.5 g Ni(OH)2 and 0.5 g Cu(OH)2, (1-adamantanecarboxylic acid / M = 2) were added to the 100 ml exfoliated saponite suspension and stirred for 1 h at room temperature. This reaction mixture was then transferred into a teflon-lined vessel and hydrothermally treated at 150 °C for 24 h. The resultant product was washed with excess of water to remove any ionic impurities and dried at 75 °C overnight. The preparation of saponite supported NiCo-adamantanecarboxylates (NiCo-Ada/Sap) and Ni-adamantanecarboxylates (Ni-Ada/Sap) followed the same procedure, except that 0.5 g of Co (OH)2 was used instead of Cu (OH)2 and 3.88 g 1-adamantanecarboxylic acid was used for synthesis of NiCo-Ada/Sap. Ni -Ada/Sap was synthesised by using 0.5 g Ni (OH)2 with 1.75 g of 1-adamantanecarboxylic acid by following the same procedure.Saponite supported metal/metal alloys were synthesized by decomposing the catalyst precursors, NiCu-Ada/Sap, NiCo-Ada/Sap and Ni-Ada/Sap under reducing atmosphere by passing H2. In a typical decomposition experiment, about 1.0 g of the catalyst precursors was loaded into a quartz tube and subjected to decomposition at 600 °C under H2 atmosphere (50 ml/min, 10 °C/min, residence time 2 h).Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D8 Advance power diffractometer, using Ge-monochromated Cu-Kα1 radiation (λ =1.5406 Å) from a sealed tube, operating at 40 kV and 40 mA with a Lynx Eye linear detector in reflectance mode. Data were collected over 2θ angular range of 2–90° with a step size of 0.009˚. Fourier Transform Infrared spectra (FTIR) of samples were measured using Perkin Elmer spectrometer in ATR mode (4000–600 cm−1). Thermal analysis of the precursor samples was performed using a thermogravimetric analyser (TA Instruments TA 500). C and H analysis of the precursor samples was carried out by placing approximately 3 mg of sample in a tin capsule and combusting in a high oxygen environment at 950 °C using an Exeter Analytical CE-440 elemental analyser calibrated with acetanilide. Metal loading on saponite in NiCuSap, NiCoSap and NiSap were quantified by XRF using a Pananalytical Zeitium WD-XRF with a 4 kW rhodium anode tube in helium path. During XRF analysis, each sample was weighed accurately and placed in the sample cups under helium path. The cups were prepared with 50-micron prolene film.Raman spectra were recorded using Renishawin Raman Microscope with a 785 nm red laser operating WiRE® version 4.2. The data was obtained using a 10 s exposure time with 5–10% laser power.X-ray Photoelectron Spectroscpopy (XPS) was undertaken using a K-ALPHA Thermo Scientific device, utilising Al-K radiation (1486.6 eV) and a twin crystal monochromator to produce a focussed x-ray spot at 3 mA x12 kV (400 μm major axis length of the elliptical shape). Prior to the spectral acquisition, the samples were pre-reduced simulation the activation treatment. The data was then processed using the Avantage software package.N2 adsorption isotherms and textural analysis was performed on a Micrometrics 3Flex at 77 K over P/P0 0–0.99 under nitrogen. Transmission electron microscopy (TEM), high resolution TEM (HRTEM) images are acquired using a FEI Titan Themis instrument equipped with a FEI SuperX EDX detector. Surface morphology and elemental analysis of spent catalyst were done by using JEOL 7100 F scanning electron microscope (SEM) microscope with an Energy Dispersive X-Ray spectroscope (EDX) to determine both active phase dispersion and the presence and type of coking. Oxidation TGA experiments were undertaken on a Q500 V6.7, ramping from room temperature to 900 °C at 10 °C/min in air. Cation exchange capacity (CEC) was estimated by a method reported by Chapman et al. [39] by exchanging Na+ ions in Na+-saponite with NH4+ ions for three consecutive times with 1 N NH4CH3COO solution. 0.1 g of saponite was stirred with 10 ml of NH4CH3COO solution for 30 min. The supernatant was separated from saponite and collected in a volumetric flask. The exchange reaction with NH4CH3COO solution was repeated two more times and the supernatant collected as before into the same volumetric flask. The supernatant was diluted suitably to estimate for Na+ ions by comparing with a set of standard solutions using iCE 3300 AA Thermo Fischer Scientific Atomic Absorption Spectrophotometer (AAS). Na+ ions were estimated using Na- hallow cathode lamps at 589 nm under air/acetylene flame.The RWGS experiments were conducted in a tubular quartz reactor (10 mm ID) at atmospheric pressure. The catalysts were supported on a layer of quartz wool acting as a bed. The reactant gas flow used for temperature screening and stability tests contained CO2 and H2 in a ratio of 1:4 balanced with N2 to maintain a WHSV = 15 L/gcath. The gas products were analysed using an online gas analyser (ABB AO2020, ABB Ltd., Zurich, Switzerland) equipped with both an IR and TCD detectors. All catalysts were reduced pre-reaction in the reactor by flowing 100 ml min−1, 20% H2/ 80% N2 at 850 °C for 1 h. Temperature screening reactions were conducted using a temperature range of 300–850 °C at 50 °C intervals. The stability study was conducted at 500 °C for 89 h.Saponite supported metal-adamantanecarboxylates were prepared by hydrothermally treating the respective metal hydroxides with twice the number of moles of 1-adamantanecarboxylic acid in the presence of exfoliated saponite clay layers as described in the experimental section. Only the required stioichiometric amounts of metal hydroxides (metal ion source) and 1-adamantane carboxylic acid (carboxylate ion source) were used for the synthesis and no excess amounts of chemicals were used. Exfoliation of smectite clays in water is spontaneous and well known [40]. Exfoliated clay layers have been used as 2D starting materials for synthesis of various composites [41]. Using exfoliated clay layers as opposed to bulk clays enables homogeneous mixing in the catalyst precursors by accessing the interlayer regions that were generally inaccessible prior to exfoliation. Metal hydroxides and 1-adamantanecarboxylic acid under hydrothermal conditions were precipitated as NiCu/NiCo/Ni-adamantanecarboxylates over the clay sheets. 1:2 ratio of metal to 1-adamantanecarboxylic acid was taken as divalent metal cations would need two monovalent 1-adamantanecarboxylate ions for charge compensation. The elemental (C and H) analysis of the resultant catalyst precursors (Table S1, Supporting information) confirms the presence of expected amounts of 1-adamantanecaboxylates in the samples. Excess amount of hydrogen was observed due to OH groups of clay layers and adsorbed water molecules.PXRD patterns of the as synthesized catalyst precursors and pristine saponite are shown in Fig. 1 (a–d). Saponite (Fig. 1a) shows first basal (00 l) reflection at d-spacing of 12.56 Å that matches well with the values reported for sodium ion intercalated saponite [38]. Other reflections observed at higher 2θ values of about 19.6°, 28.7°, 35.5° and 60.5° correspond to the 2D reflections of saponite. The (060) reflection at 60.55° with d-value of 1.53 Å categorises saponite as a tri-octaherdral smectite clay [38]. Empirical formula for saponite deduced from XRF analysis was found to be Na0.97Mg5.96Si6.94Al0.93O24.20H5.38. CEC of saponite that evaluates the amount of exchangeable interlayer cations in the clay was estimated by a method reported by Chapman et al. [39]. CEC of Na+-saponite was found to be 123 meq/100 g which is quite high for smectite clays. High CEC values indicates spontaneous swelling and exfoliation of clays in water [42]. High degrees of exfoliation results in homogeneous composite precursors as the exfoliated 2D alumino-silicate clay layers are available for composite formation. Surface properties such as surface area, pore size and pore volume of the saponite was calculated by N2 adsorption technique as described in the experimental section. The saponite clay shows BET surface area of 147 m2/g and pore diameter and volume were 3.1 nm and 0.118 cm3/g respectively. Surface area value of saponite indicate a well stacked clay layers formed due to hydrothermal method of synthesis followed as reported [43].The PXRD patterns of Ni-Ada/Sap, NiCo-Ada/Sap and NiCu-Ada/Sap catalyst precursors shown in Fig. 1b, c and d respectively match well with Ni, Cu and Co-metal adamantanecarboxylates reported in literature [37]. Ni-Ada/Sap (Fig. 1b) shows characteristic reflections at 2θ values 6.09°, 6.29°, 6.76°, 11.10°, 12.03°, 15.97°, 16.57°, 17.77° and 24.25° corresponding to d-values of 14.49 Å, 14.03 Å, 13.06 Å, 7.96 Å, 7.35 Å, 5.54 Å, 5.35 Å, 4.98 Å and 3.66 Å respectively. Similar reflections were seen in the PXRD pattern of NiCo-Ada/Sap in Fig. 1c. PXRD pattern of NiCu-Ada/Sap catalyst precursor (Fig. 1d) have additional reflections (at 2θ values of 7.49° and 8.69° corresponding to d-values of 11.78 Å and 10.16 Å respectively) compared to the other catalyst precursors and this observation matches well with previous reports [37]. The (00 l) reflection of saponite is not observed in the catalyst precursors, due to higher intensity of the metal adamantanecarboxylates phases. However, the 2D reflections of saponite clay layers are observed in the enlarged portion of the PXRD patterns of the catalyst precursors [Fig. S1 (supporting information)]. PXRD patterns of all the catalyst precursors, thereby, indicate composite formation of metal adamantanecarboxylates with saponite.FTIR spectra of all the catalyst precursors (Fig. 2 b–d) and saponite (Fig. 2a) show a characteristic strong SiO stretching vibration at around 950 cm−1. Similarly, the hydrogen bonded OH stretching vibration is observed in all the samples at ca. 3400 cm−1. All - trans, CH stretching modes are observed at 2850 cm−1 and 2900 cm−1 in the catalyst precursors (Fig. 2b–d), attributed to the adamantane moiety. COO¯ stretching vibrations (Fig. 2b–d) observed between 1350 cm−1 and 1550 cm−1 confirm the presence of carboxylate ions in the catalyst precursors. The absence of CH and the carboxylate vibrations in Na+-saponite is evident from Fig. 2a. PXRD and FTIR analysis of the resultant precursor catalysts show the successful composite synthesis of metal adamantanecarboxylates over saponite clays sheets, as anticipated. Fig. 3 shows the thermal decomposition profile of Ni-Ada/Sap (Fig. 3b), NiCo-Ada/Sap (Fig. 3c) and NiCu-Ada/Sap (Fig. 3d) and saponite (Fig. 3a) under nitrogen gas flow as described in the experimental section. The different thermal decomposition profiles of the catalyst precursors in comparison to pristine saponite clay further indicates their composite nature. The NiCo-Ada/Sap and NiCu-Ada/Sap loses mass in three steps leaving about 30 wt% residues. Ni-Ada/Sap loses 50 wt% of mass, whereas saponite loses about 15 wt% of its mass in accordance to previous reports [38]. Lower mass loss in the case of Ni-Ada/Sap in comparison to NiCo-Ada/Sap and NiCu-Ada/Sap samples was expected due to lower amounts of metal-adamantanecarboxylate in Ni-Ada/Sap. DTG profiles of the catalyst precursors are given as supplementary information, Fig. S2. All samples show a mass loss below 200 °C that could be due to loss of water molecules. Pristine saponite sample (Fig. S2a) loses mass in two more steps at 557 °C and 749 °C due to loss of water of hydration, dehydroxylation of outer and inner hydroxyl ions as reported in literature [41]. Mass loss of Ni-Ada/Sap (Fig. S2b), NiCo-Ada/Sap (Fig. S2c) and NiCu-Ada/Sap (Fig. S2d) in between 200–550 °C could be due to the degradation of the metal-adamantanecarboxylates in the catalyst precursors.The catalyst precursors, NiCu-Ada/Sap, NiCo-Ada/Sap and Ni-Ada/Sap were reduced under hydrogen, as described in the experimental section, to obtain saponite supported NiCu, NiCo and Ni metal alloys/metal nanoparticles. PXRD patterns of the freshly prepared active catalysts NiSap, NiCuSap and NiCoSap are shown in Fig. 4 a, b and c respectively. Reflections due to the Ni metal, NiCo and NiCu alloys were observed along with those due to saponite. The reflections of Ni-metal (Fig. 4a) appear at 2θ values of 44.42°, 51.84° and 76.33° with d-values of 2.04 Å, 1.76 Å and 1.25 Å, respectively. Similarly, reflections due to NiCu-alloy (Fig. 4b) are seen at 2θ values of 43.99°, 51.26° and 75.32° with d-values of 2.06 Å, 1.78 Å and 1.26 Å respectively. While, reflections due to NiCo-alloy (Fig. 4c) are seen at 2θ values of 44.37°, 51.63° and 76.06° with d-values of 2.03 Å, 1.76 Å and 1.49 Å, respectively. Reflections due to saponite in the freshly prepared catalysts were observed at 19.67°, 28.42°, 31.12° and 35.99°. The resultant active catalyst was also characterised using FTIR as shown in supporting information, Fig. S3. All samples show vibrations due to SiO at 950 cm−1 due to saponite and CC vibration (1600 cm−1) due to residual carbon. The samples also show a broad vibration at 3400 cm-1 due to adsorbed water.The metal loadings on saponite were determined by XRF analysis. Table 1 shows the amount of Cu, Co and Ni present in various samples per gram of catalyst. The resultant catalysts were further characterized with Raman spectroscopy (Fig. S4, supporting information). The Raman spectra of the freshly prepared catalysts showed two intense bands which are attributed to vibration modes of sp2-bonded carbon atoms. The G-band observed at 1594 cm−1 is due to the sp2 carbon stretching modes in aromatic rings derived from the incomplete decomposition of 1-adamanatanecarboxylate unit. The peak at approx. 1336 cm−1 is the graphitic D-band that becomes active in the presence of structural disorders [44].The role played by the solid surface is essential in catalysis. Herein, x-ray photoelectron spectroscopy (XPS) allows us to determine the oxidation states and electronic environment of the elements in the outermost layers of the material. The Ni 2p3/2 spectra of all samples can be found in Fig. S5a, with the associated Cu 2p3/2 and Co 2p3/2 regions for the NiCu-Sap and NiCo-Sap catalysts in Fig. S5b and S5c, respectively. Table 2 contains a summary of the main peaks found in Fig. S5. Prior to this analysis, all samples were reduced under the same conditions used before a reaction (850 °C, 1 h, 20% H2:80% N2). As seen in both Fig. S5a and Table 2, there are several Ni oxidation states that exist following reduction and are seen in the deconvoluted spectra. The bands ca. 851–852 eV are characteristic of Ni0, while the bands at 852–854 eV are attributed to Ni2+ species interacting with the support [26,45,46]. The band centred at binding energy (BE) 857 eV, in the case of the NiCu-Sap catalyst, is attributed to Ni2+ as part of surface NiCu alloy species [47,48]. This assignment is corroborated when considering the Cu 2p3/2 region that details a shift towards BEs associated with NiCu alloys [49]. The remaining bands are the shake-up satellite peaks associated with the previous species. However, the BE displayed in the NiCo-Sap catalyst at 855.48 eV is indicative of Ni3+ cations present [51,52], characteristic of NiCo alloys [53].The Cu 2p3/2 region in the NiCu saponite, Fig. S5b, shows two significant bands. One band at 932 eV can be assigned to the Cu+/Cu0 species, with the higher bands at 934, 940 and 943 eV attributed to the Cu2+ species and two shake up satellites, respectively [45,54]. The main bands for Cu at 932 and 934 eV can be explained to be at higher binding energies than monometallic Cu as found in literature, due to the charge transfer from Cu to the partially empty d-band present in Ni and the oxidation of Cu [55,56]. Such electronic interaction between the two metals has been theorised to contribute to increased catalytic activity since it results in an electronically rich metal-metal interface which is ideal for reactants activation [48,49]. Furthermore, the Ni-Cu interface has been identified to be the active site for the forward and reverse water gas shift reactions by enhancing CO/CO2 adsorption and supressing methane production [50]. Additionally, it has been found that high compositional contents of Cu in a Cu-Ni alloy sufficiently increased the reducibility and mesoporosity of the structure to subsequently increase the catalytic activity [45].Finally, the Co 2p3/2 spectrum for the NiCo saponite found in Fig. S5c details two main bands at 778 and 780 eV, which are attributed as Co3+ and Co2+, respectively [53]. While the other two bands are the associated shake-up satellites [57,58]. Another key factor found in the data for this region, is that the splitting (spin-orbit coupling) energy between the Co2p1/2 and Co 2p3/2 orbitals (not shown) is approximately 15 eV, further indicating the coexistence of Co2+/Co3+ species [57].The estimated Ni/Sap atomic dispersions (Table 2) show very similar surface dispersions of the Ni over the saponite material regardless of the inclusion of Co or Cu. This is indicative of the homogeneous precipitation of the metal-adamantane carboxylates over the exfoliated clay layers during the hydrothermal synthesis of catalyst precursors. However, the slightly increased value for the Ni-Sap could indicate a slight enrichment of surface Ni on the Ni-Sap sample.The textural properties of the active catalysts were analysed using N2 adsorption. Fig. 5 shows the adsorption isotherms of all the samples and surface properties are tabulated in Table 3 . All the samples show type IV adsorption isotherm with the H4 hysteresis loop characteristic of mesoporous solids. Surface area of the catalysts calculated by the BET method and pore size and pore volume were calculated by BJH method by using the desorption branch of the isotherm. The NiSap catalyst shows 120 m2/g of surface area whereas, NiCuSap and NiCoSap show surface area of 85 and 87 m2/g, respectively, which indicates the nucleation of the second metal in the catalysts’ porous structure.Ultimately, however, beyond the onset relative pressure of the loop, the isotherms confirm the presence of narrow or slit shaped mesopores within the material that are confirmed by the BJH analysis of the material (Table 3) confirming average pore diameters between 2–50 nm.Freshly prepared catalysts were further characterised by electron microscopy and the TEM images are shown in Fig. 6 . Clay layers of saponite are clearly identified supporting the metal/metal alloy nanoparticles in the images. Fig. 6a and d show monodispersed NiCu nanoparticles in the range of 15–20 nm homogeneously distributed over the clay layers. Fig. 6b and e show the NiCo nanoparticles with many of them having sizes between 10–15 nm. A small percentage of the NiCo nanoparticles are however larger size measuring about 30–50 nm. The bigger nanoparticles in NiCo could be due to aggregation of nanoparticles on the surface of the clay sheets. Similar observation was made for NiSap catalyst (Fig. 6c and f). Different shades of the nanoparticles could indicate that they are present at varying depths in the clay matrix. The lighter shaded, smaller nanoparticles in Fig. 6 could be the ones formed due to restricted growth in the clay interlayers at greater depths. Whereas, the nanoparticles formed on the surface of the clay layers might have undergone greater extent of agglomeration resulting in a small percentage of larger particles. Excluding which, the average particle sizes of monometallic Ni-nanoparticles in the catalyst varied between 10–15 nm. It is worth noting that, larger nanoparticles are less abundant in the NiCuSap catalyst (Fig.6a and d) in comparison to the other two catalysts.NiCuSap was further characterised using STEM to map the active sites distribution on the clay support. The electronic image of the freshly prepared NiCuSap catalyst is shown in Fig. 7 a, along with the corresponding elemental maps of Ni, Cu, Mg, Al, Si and C. The NiCu-alloy nanoparticles are bright spots and the grey hazy matrix belongs to saponite clay sheets in the STEM image (Fig. 7a). The image also depicts a homogeneous dispersion of the NiCu-alloy nanoparticles in the saponite clay matrix. Complementing Ni and Cu elemental maps indicate the presence of both Ni and Cu in each of the nanoparticles. Saponite support displays coherent distribution of Mg, Al and Si accounting for homogeneously formed saponite clay layers. in situ generated carbon in the freshly prepared catalysts was found to be distributed homogeneously over the saponite clay layers. The presence of carbon in the catalysts was also indicated in the Raman analysis as discussed earlier.All saponite based catalysts were tested using a reactant ratio favourable for both the RWGS and methanation reactions and a WHSV 15 Lgcat −1 h−1 that was applied across different temperatures as mentioned in the experimental section. Fig. 8 shows the conversion and selectivity results of this testing and clearly depicting the competitive nature of both the RWGS and CO2-methanation reactions. The monometallic NiSap catalyst favours the methanation reaction, attaining 83% CH4 selectivity at 450 °C. In fact, as pointed out in the XPS section, the monometallic sample has greater exposition of Ni on the surface acting as active centres for methanation. The NiCoSap material displays an intermediate behaviour with good levels of CO2 conversion and higer selectivities to CO compared to the monometallic sample. As for the NiCuSap catalyst a highly interesting trend was obverved, specifically due to preferential formation of CO over CH4, even at lower temperatures, where typically the methanation reaction is the dominant process. This is seen in the selectivity plots, Fig. 8b and c, where there is little to no methane produced, while CO is being produced in abundance. Overall the Ni-Cu catalyst is the best material within the studied series and hence we have compared its performance with reference systems. As shown in Table 4 the NiCuSap shows either markedly improved or comparable performance as a number of transition metal and noble metal catalysts reported recently [22,26,31,59,62–64] using the same temperature window and reaction mixture (CO2:H2 1:4). Only the bimetallic Fe-Ni catalysts reported in [26] outperforms our CO2 conversion levels but the selectivy of the material is much lower than that exbited by our Ni-Cu catalysts. Hence the Ni-Cu/saponite catalyst represents an excellent balance activity/CO selectivy when compared with benchmark materials. The homogenous distribution and high dispersion of the Ni-Cu active centres shown in the STEM study, along with the Ni-Cu electronic interaction discussed in the XPS section, can explain the excellent performance of this sample. Cu suppresses the methanation activity of Ni and the Ni-Cu ensemble is an advanced active phase for the RWGS reaction, leading to high levels of CO2 conversion in the whole temperature range and remarkable selectivity levels towards CO. In fact the presence of Cu opens up the possibility to conduct the RWGS reaction via redox and/or formate mechanism as previously reported elsewhere thus favouring the CO route over the CH4 pathway [65]In any case, the superior behaviour of our catalysts compared to reference materials is indeed a very encouraging result and showcases the viability of low cost nature inspired multicomponent catalysts for the reverse water gas shift process. Due to this behaviour, the NiCuSap catalyst was selected for a stability study at 500 °C as this temperature indicated significant conversion at lower temperature for the RWGS reaction, while not reaching equilibrium.The results shown in Fig. 9 clearly illustrate this catalysts’ resistance to deactivation, maintaining considerable conversion (ca. 55% CO2 conversion) and high selectivity for CO (ca. 80%) for over 89 h of continuous reaction. Furthermore, these results are a considerabe enhancement over recently published materials.The suppression of methanation by copper containing materials has been previously reported by our team in the forward WGS reaction [60,61]. However, the potential of this alloy for the reverse water gas shift process in still under explored. The enhanced selectivity to CO at low temperatures is an encouraging result to achieve the successful coupling of RWGS with downstream processes such as Methanol synthesis and Fischer-Tropsch which typically take place at around 250–350 °C. This way we could close the cycle: CO2 conversion to fuels and chemicals in a two step-process with the RWGS as front unit and the F–T or methanol conversion reactors as second unit to produce the upgraded end products.The x-ray diffraction patterns of the spent catalysts from the temperature screening experiments are shown in Fig. 10 . All the diffractograms display peaks at around 28.2°, 31.05° and 35.5° 2θ due to saponite support. Additionally, the expected peaks for the loaded metal/metal alloys at around, 44°, 51° and 76° remain unaltered in the spent catalysts in comparison to the freshly prepared catalysts. No phase segregation of the metal alloys was observed after the screening tests and therefore the PXRD patterns of the spent (Fig. 10) and the freshly prepared catalysts are identical. While the results of the Scherrer equation from the respective PXRD patterns of both the fresh and the spent catalysts (after temperature screening experiment) displays no change, indicating almost no agglomeration of the active metal/metal alloy nanoparticles; due to the overlap of the crystal peaks, it is impossible to determine the level of sintering present for the individual components. This explains partially the continuous activity of NiCuSap catalyst for 89 h with negligible loss in activity. Presence of crystalline carbon peaks could not be found, making any carbon formation amorphous. The stability pattern of the catalyst is characteristic of the in situ generated carbon stabilized metal nanoparticle catalysts [31]. However, in this case, saponite support and the metal alloy combination has added to the conversion levels of CO2 and improved selectivity of the catalyst for RWGS reaction.Following the temperature screening experiments, the spent sample was analysed using a JEOL 7100 F Scanning Electron Microscope (SEM) with an Energy Dispersive X-Ray spectroscope (EDX) to determine both active phase dispersion and the presence and type of coking. Fig. S6, S7 and S8 present the SEM/EDX results for the post reaction NiCuSap, NiCoSap and NiSap samples, respectively. These results show clearly well dispersed active phase with some small amount of amorphous carbon present on the surface of the spent catalysts, which is in good agreement with the lack of crystalline carbon peaks in the spent materials XRD diffractograms. Fig. S8 clearly details the presence of Ni as small particles on the surface of the material, while Fig. S6 and S7 show that the Ni is highly dispersed throughout the catalyst.Combusting the spent material under air from room temperature to 900 °C at 10 °C/min revealed several zones (Fig. 11 ). Each sample underwent an initial loss between room temperature (RT) – 160 °C that is attributed to free water loss. Each sample then displayed a significant weight gain (+5-14 wt %) in between 200–450 °C for NiCuSap and NiCoSap that is attributed to the oxidation of the metals. The same weight gain zone for the NiSap occurred at the slightly higher zone of ca. 275–500 °C.The spent NiCoSap catalyst displayed a two-step weight loss totalling 3.8 wt% which is attributed to the loss of surface carbon and then engrained carbon. In a similar fashion, the spent NiCuSap catalyst details a one-step weight loss (1.5 wt %), which is also attributed to the loss of surface carbon. These conclusions are supported by the presence of an exothermic heat flow curves for the oxidation of the metals and the associated oxidation of the amorphous carbon (not shown). The weight gain of these materials being related to metallic oxidation is further supported by the curve displayed for the NiSap material, which details a much smaller increase owing to its monometallic loading. Interestingly, however, the NiSap material did not display any weight loss at higher temperatures,. In any case the TGA profiles corroborate the absence of crystalline carbon deposits in good agreement with XRD. This observation along with the lack of metallic sintering validate that these “nature inspired” catalysts developed in this work are not only highly active, but also very robust for the RWGS reaction.This work demostrates the viability of nature inspired transition metal based catalyst for gas phase CO2 upgrading via RWGS. Aqueously exfoliated saponite magnesio-aluminosilicate layers have been effectively used to support the synthesis of metal adamantanecarboxylates under hydrothermal conditions. The catalyst precursors underwent controlled decomposition under hydrogen atmosphere to produce in situ generated carbon stabilised saponite supported metal alloy catalysts. All the as prepared catalysts (mono: Ni and bimetallic: Ni-Cu and Ni-Co) display excellent activity levels in the RWGS process outperforming the activity levels exhibited by reference catalysts reported in literature. Interestingly, the undesired parallel reaction – the Sabatier process – which typically is the dominant reaction in the low temperature window can be supressed using Ni-Cu alloys as active phases. Indeed, the bimetallic Ni-Cu system is the best performing material within the studied series with an outstanding balance activity/CO selectivity in addition to be a very stable catalysts for long term runs. The electronic interaction Ni-Cu evidenced by XPS contributes to this exceptional behaviour. Indeed, such a close metal-metal contact results and electronically rich Ni-Cu interface which is ideal for CO2 activation. No signs of carbon deposition due to the reaction, nor metallic sintering were observed, explaining the enhanced stability of this material.Considering a potential application where the RWGS unit is coupled to a downstream process using syngas such as Fischer-Tropsch or methanol synthesis – the obtained results are very encouraging since our Ni-Cu catalyst is very active and selective towards CO in the low temperature range minimising the temperature gap between RWGS and the Fischer-Tropsch or methanol unit. In other words, the catalysts developed in this study may facilitate the integration of a RWGS reactor with a syngas convertor – such an integrated dual system would enable the direct conversion of CO2 to added value chemicals.The authors declare that they have no competing interests.The team at Surrey acknowledges the financial support provided by the EPSRC grant EP/R512904/1 as well as the Royal Society Research Grant RSGR1180353. This work was also partially sponsored by the CO2Chem UK through the EPSRC grant EP/P026435/1. The RCCS team at Heriot-Watt University acknowledges the financial support from EPSRC through grants EP/N024540/1 and EP/N009924/1, as well as the Buchan Chair in Sustainable Energy Engineering.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2019.118241.The following is Supplementary data to this article:

Chemical CO2 upgrading via reverse water gas shift (RWGS) represents an interesting route for gas phase CO2 conversion. Herein, nature inspired clay-based catalysts are used to design highly effective materials, which could make this route viable for practical applications. Ni and transition metal promoted Ni saponite clays has been developed as highly effective catalysts for the RWGS. Saponite supported NiCu catalyst displayed a remarkable preference for the formation of CO over CH4 across the entire temperature range compared to the saponite supported NiCo and Ni catalysts. The NiCu sample is also highly stable maintaining ∼ 55% CO2 conversion and ∼ 80% selectivity for CO for long terms runs. Very importantly, when compared with reference catalysts our materials display significantly higher levels of CO2 conversion and CO selectivity. This confirmed the suitability of these catalysts to upgrade CO2-rich streams under continuous operation conditions.

No data was used for the research described in the article. No data was used for the research described in the article. Abbreviations AIL, Acidic ionic liquid; BDC, Benzene dicarboxylic acid; BTC, Benzene tricarboxylic acid; DAIL, Dicationic acid ionic liquids; Gua, Guanidine; HKUST, Hong Kong University of Science and Technology; HPA, Heteropolyacid; IL, Ionic liquid; MBAIL, 2-Mercaptobenzimidazole IL; MIL, Matériaux de l′Institut Lavoisier; MSA, Methanesulfonic acid; PTSA, Paratoluenesulfonic acid; PTA, Phosphotungstic acid; PMA, Phosphomolybdic acid; UiO, Universitetet i Oslo; ZIF, Zeolitic imidazole framework Acidic ionic liquid;Benzene dicarboxylic acid;Benzene tricarboxylic acid;Dicationic acid ionic liquids;Guanidine;Hong Kong University of Science and Technology;Heteropolyacid;Ionic liquid;2-Mercaptobenzimidazole IL;Matériaux de l′Institut Lavoisier;Methanesulfonic acid;Paratoluenesulfonic acid;Phosphotungstic acid;Phosphomolybdic acid;Universitetet i Oslo;Zeolitic imidazole frameworkThe shift from fossil fuels to renewable biofuels is being forced by rising energy use and the related environmental challenges [1]. Biodiesel [also known as fatty acid methyl esters (FAMEs)] has been known to be a greener alternative to diesel. Biodiesel has attracted a lot of interest in recent years because of its renewability and sustainability and can be used as a substitute for fossil fuels that are becoming scarce [2]. Environmental concerns have risen as a result of the massive use of these conventional resources, prompting calls for green and alternative resources [3]. The catalysts, feedstocks, and by-products are the key differences between transesterification and esterification reactions. Long-chain triglycerides are commonly used as feedstocks for transesterification, which necessitate high-quality oils that are free of free fatty acids (FFAs), water, and contaminant. Most edible oils, such as soybean, sunflower, and rapeseed, are used for biodiesel synthesis by transesterification [4,5].The classic homogeneous and heterogeneous catalysts for biodiesel generation are rapidly being phased out in favour of innovatively designed catalysts [6,7]. Homogeneous catalysts have been exploited, however, their low recyclability and high corrosivity require proper disposal procedures. Furthermore, homogenous-catalysts for biodiesel purification are supposed to consume a lot of water [8–10]. Heterogeneous catalysts, on the other hand, are profitable due to their relative ease of separation for reusability and limiting mass transfer. To support green chemistry and sustainable biorefining, increasing the activity and efficiency of heterogeneous catalysts are critical [10–12]. Several magnetized catalysts were used in this situation for easier separation from waste resources. In this context, nanocatalysts are becoming a popular study topic as a way to speed up reactions and improve catalytic efficiency [13,14].Metal-organic frameworks (MOFs) are porous materials in which molecular building components (such as organic linkers and metal ions/inorganic clusters) are linked together by strong coordination interactions [15–17]. MOFs have shown considerable promise in sectors like as separation, storage, delivery, and catalysis due to their high function tunability, porosity, and crystallinity [18,19]. MOF structures have been reported by Tomic and others since the 1960s [20]. MOF research has blossomed since the early 1990s, notably Yaghi and his research group developed MOF-based porous materials [21,22]. MOFs are often made via a method known as modular synthesis, where porous shapes are formed by the gradual formation of crystals from a heated solution utilizing a nucleation and growth mechanism [23].Following Yaghi and co-workers’ report on MOF-5 (shown in Fig. 1 ) synthesis, the chemistry of MOF synthesis sparked a lot of attention in the material research world [24–27]. MOF-5 is a 3D cubic network made up of tetrahedral [Zn4O]6+ clusters connected by Benzene dicarboxylic acid (BDC) ligands [28]. This innovation was further developed, with ultraporous MOFs like MIL-101 (MIL stands for Material Institute Lavoisier), ultra-stable MOFs like UiO-66 (UiO stands for University of Oslo) and flexible MOFs like MIL-88 [29–32]. Synthetic strategies to bring hierarchy into secondary MOF structures, such as epitaxial growth, controlled assembly, and labilization approaches, have recently been developed in MOF growth, resulting in MOF tertiary architectures with remarkable complexity and characteristics [33,34]. The current level of hierarchy and functions of artificial framework materials, on the other hand, lags significantly behind the complex hierarchical systems seen in nature [35,36]. For the improvement of cooperative catalysis in MOFs, it will be vital to optimize the capability to control these hierarchical structures on various scales, as it needs tuning of both the activity of the catalytic core as well as the selection of porous framework [37,38]. In recent years, several noteworthy review articles on MOFs have been reported. Cong et al. [39] emphasized the mechanism of functionalized MOFs and the catalytic performance of MOF-based catalysts. Basumatary et al. [40] discussed different types of MOFs used for a variety of feedstocks to convert into biodiesel and machine learning techniques to optimize process parameters. Depending on different MOFs, the catalyst's characteristics and activity vary quite a bit. Thus, it is crucial to outline the role of MOFs to direct the catalyst synthesis employed in biodiesel production for future research. Following this idea, Ma et al. [6] tried to summarize the functions of different MOFs categorically and discussed the challenges in MOF synthesis. MOFs hold the possibility as an effective heterogeneous catalyst for the generation of biodiesel with intriguing features of huge porosity, uniform pore size, controllable functional groups, and structural tunability enabling itself as an ideal material for biodiesel production in the form of acidic, basic, bifunctional, and enzymatic catalysts [39,41,42]. To achieve this goal, a review on biodiesel production using MOF-based catalysts is required to strengthen their environmentally favourable uses [43,44]. Moreover, there is a need of addressing the challenges in the MOF synthesis and catalysis which causes deactivation and leads to less catalyst reusability. Although, several reviews have come up with promising objectives, none has given a detailed study on deactivation of MOFs and methods to overcome the poor catalytic activity of MOFs after a couple of cycles of reuse. The major objective of the present work is to review various MOF-based catalysts having primary application in biodiesel production. This review emphasised on MOF stability based on their reusability and the study of deactivation of MOFs catalyst due to various factors which recently published reviews [39,45–49] have not discussed in detail although it carries a lot of significance in the field of MOF research. Several characterization techniques also have been discussed in brief which could be used to obtain the most critical analytic results from the perspective of the future progress of MOFs.MOFs modular nature allows for an almost endless number of structures to be imagined. Statistical data analysis in Fig. 2 depicted the increasing trend of published research papers in the field of metal-organic framework. These data were collected from “Scopus Database” using the keyword “metal-organic framework”, that signifies the tremendous scope and need of MOFs in various fields of applications. MOFs, which are constructed by combining inorganic units with organic linkers, are the most promising materials among all the novel catalysts [50], with benefits such as high surface area, high pore size, structural stability, and tunable functions [51,52]. There is a lack of study on specialized MOF-based catalysts for biodiesel synthesis [53]. Numerous studies referring to MOFs for biodiesel generation have been reported in the last decade [54] and few inspections have been done on the customized applications of MOF-based catalysts in biodiesel production.Biodiesel is produced in an energy-efficient manner, which necessitates catalysts with intense activity, selectivity, and stability. The use of pristine MOFs as catalysts is limited by their poor activity due to a lack of active sites, as well as their low mechanical, thermal, and chemical stability due to the metal-linker coordinative bond's fragility, thus making them unsuitable for direct application in biodiesel refineries despite their unprecedented surface area, tunability and porosity [55,56]. Hence, based on MOFs synthesis and significant features they are broadly classified as acidic, basic, bifunctional, and enzymatic catalysts. The list of various MOFs with their metal ion and the structure of the ligand are enlisted in Table 1 that is discussed in this review.MOFs are being used as catalysts in biodiesel production and must be precisely characterized using several analytical techniques which provide a lot of ideas to predict the catalytic activity of catalysts to produce greater quality of biodiesel. Some of the significant characterization techniques have been discussed to emphasize roles of analytical techniques in MOF synthesis.Among several used characterization techniques powder X-ray diffraction (PXRD) is commonly used to investigate the degree of crystallinity and particle size of the MOF catalyst. In the XRD pattern of CoFe2O4/MIL-88B(Fe)-NH2 reported by Xie and Wang [57], the characteristic XRD peaks of CoFe2O4 and MIL-88B (Fe)-NH2 could be easily found in the XRD patterns which were in quite good agreement, indicating the successful formation of the CoFe2O4/MIL-88B(Fe)-NH2. Further Xie and Wan reported [58] another polyoxometalate- based sulfonated ionic liquid functionalized MOF AIL/HPMo/MIL-100(Fe) where the XRD peaks (Fig. 3 i) were consistent with pristine MIL-100(Fe), with no distinct XRD peaks attributable to HPMo or AIL, implying that the MIL-100(Fe) framework structure remained stable during the surface modification operations and that the active species of AIL and HPMo were well-dispersed on the MOF support. Wu et al. [59] described the confinement of Fe3O4 and ionic liquid in the NH2−MIL-88B(Fe) MOF material using XRD as the tool of analysis (Fig. 3. ii) which remained as crystalline material like the MOF with a slight shift in Bragg's position compared to the pristine. Such conformational predictions on catalysts could easily be reported using PXRD as an analytical technique.New MOF structures are being reported to have high specific surface areas, that require MOF characterization, this regard, accurate surface area measurements are crucial, as this is a key characteristic of microporous materials [60]. Generally, MOF surface areas are computed using Brunauer–Emmett–Teller (BET) theory, which obtains surface areas from gas adsorption isotherms, typically with the usage of N2 gas and other gases like CO2 and Ar.Elyazed et al. [61] reported a series of UiO-66(Zr)-structured materials with defects where BET analysis results revealed that inclusion of electron withdrawing groups changed the physical properties of those materials as the specific surface areas of UiO-66(Zr), UiO-66(Zr)-NH2 and UiO-66(Zr)-NO2 were 1115, 823 and 649 m2/g, respectively. Rafiei et al. [62] strategized to immobilize lipase (5.2 nm) inside ZIF-67 MOF, but the pore diameter of MOF came out to be 1.2 nm and surface area 1320 m2g−1 from BET analysis which allowed them to came up with another method where they incorporated a lipase solution into the initial reaction mixture forming a lipase@ZIF-67 nano bioreactor. The BET-characterization technique has a vital role in the MOF synthesis as MOFs with high surface area, porosity and stability could be better candidates for biodiesel production. Han et al. [63] synthesized MIL-100(Fe) and MIL-100(Fe)@DAILs MOFs, further BET analysis results (Fig. 4 ) of which confirmed the presence of DAILs in the nanocages of MIL-100(Fe) as the surface area and total pore volume decreased from 1183 to 170 m2g−1 and from 0.72 to 0.20 cm3g−1 for MIL-100(Fe) and MIL-100(Fe)@DAILs, respectively. Therefore, routes of synthesis like encapsulation, impregnation and immobilization can be confirmed for catalysts using BET analysis as a tool of characterization.Scanning electron microscope (SEM) and Transmission electron microscope (TEM) are one of the most extensively used instruments for the investigation of micro- and nanoparticle imaging and solid object characterization. SEM and TEM both are the powerful tools in providing invaluable topography and morphology about any material under investigation.The size, shape, composition, structure, and other physical and chemical aspects of a specimen are well known from SEM analysis [64]. Hirschle et al. [65] reported a SEM analyses on a sample that was synthesised by drying an ethanol-based dispersion of Zr-fumarate MOF nanoparticles (NPs) that revealed spherical morphology (Fig. 5 a) of the NPs. Wang et al. [66] synthesized 3D- hollow porous hierarchical Co/Ni@C microspheres from bimetallic Co/Ni-MOF as a precursor. They observed that Fig 5(a), (b) exhibited a spherical shape and constituted of several nanosheets with thickness of 2 nm, hollow structure of the MOF microspheres could clearly be observed from the SEM micrographs in Fig. 5 (c) while Fig. 5 (e) and (f) revealed morphology without structural breakdown after carbonization in N2 atmosphere, indicating the great structural stability of the 3D Co/Ni@C.One of the most common applications of TEM in MOF research is to provide direct confirmation of the crystalline structure [67]. The direct imaging of both missing-linker and missing-node defects in UiO-66(Zr) was observed by Liu et al. [68] where they used HRTEM analysis that revealed both missing-linker and missing-node defects were present corresponding to reo and scu nets (as shown in Fig. 6 ). Furthermore, HR-TEM study of isoreticular series of MOF-74 [69] also revealed the DOT (2,5-dioxidoterephthalate) link as shown in Fig. 7 displaying the expanded pore aperture.X-ray photoelectron spectroscopy (XPS) analysis is highly recognized as a measurement instrument for a wide range of organic and inorganic materials [70]. The survey spectrum is obtained using an XPS spectrometer, from which the components present can be identified. Individual spectral peaks are then investigated with a higher energy resolution to reveal chemical state information [71]. The binding energies (BEs) of ejected photoelectrons from atoms at or near the surface of a catalyst are measured using the XPS analysis technique. It enables for the detection of electronic changes as a result of varying surface penetration and material composition [72,73].Ni-MOF [74] consisting of Ni, P, O in Ni-PO ((Nickel Phosphate) displayed the high-resolution Ni 2p XPS spectrum in Ni-PO, where the Ni 2p was deconvoluted into two doublets and two satellites peaks. The asymmetric peak of P 2p. spectrum in Ni-PO was deconvoluted into two signals indicating that all P atoms were in the +5-oxidation state (shown in Fig. 8 ).In Co-BDC (Benzene dicarboxylic acid) MOF [75], the XPS spectrum displayed the peaks of C1s, O1s, and Co 2p (Fig. 9 ). Incorporation of carboxyferrocene (Fc) led to the origin of Fe 2p peak in Co-BDC-Fc MOF [76]. O 1 s in Co-BDC–Fc MOF has greater binding energy and wider peaks, showing that the inclusion of missing linkers has changed the coordination at the active centre environment. In comparison to Co-BDC (1.65 eV), the valence band maximum energy of Co-BDC–Fc shifts to the vacuum level at around 0.37 eV (shown in Fig. 10 ), implying that inserting missing linkers can effectively alter the electronic structure of MOFs.Thermogravimetric analysis (TGA) in which a sample specimen is submitted to a controlled temperature programme in a controlled atmosphere and the weight change of the material is monitored as a function of temperature or time [77]. For isothermal investigations, the weight of the sample is plotted against time, while for tests carried out at a constant heating rate, it is plotted as a function of temperature. When a loss of mass occurs due to thermal degradation or desolvation, this approach is commonly used to monitor thermal stability and the loss of volatile components [78].Saha and Deng [79] reported about the stability of MOF-177 (a framework consisting of a [Zn4O6]6+ cluster and linker 1,3,5-benzenetribenzoate) where TGA was performed under continuous oxygen flow and evacuated circumstances using MOF-177. When handled under oxygen flow, the weight loss of the MOF-177 sample was found to be very less (3.65 wt.%) from 25 °C to 330 °C (Fig. 11 ) due to desorption of gas and solvent molecules, while there was no weight loss for the evacuated sample. At temperatures ranging from 330 to 420 °C, the oxygen-treated sample lost 74.7 wt.% of its weight, while the sample in vacuum lost 55 wt.% due to the framework breakdown. From such advantageous analysis technique, the structural change in the MOF-177 was further compared with the XRD of the heated sample at 330 and 420 °C, which confirmed the structural phase change during the heat treatment.Feng et al. [80] compared the TG-plots (shown in Fig. 12 ) of UiO-66, hemilabile-UiO-66 (HI-UiO-66) and hemilabile-UiO-66 sulphate (HI-UiO-66-SO4) and reported that the average number of defects per cluster increased from 4.4 to 6.0 after treating the samples with H2SO4 solution, and greater thermal stability was observed with growing numbers of defects, i.e., UiO-66 (450 °C), Hl-UiO-66 (480 °C), and Hl-UiO-66-SO4 (515 °C). Therefore, TGA, over the years has been proved to be one of the efficient tools in finding the defects while engineering different MOFs as based on the weight loss the number of defects can be calculated [81,82].Biodiesel, mixture of FAMEs generated through a transesterification process, is a carbon-neutral and sustainable energy source that can meet the world's growing energy demand [83]. Due to the establishment of subsidiaries and tax exemptions, this green fuel has steadily become more affordable and widely used in many parts of the world. Glycerol, the main by-product of biodiesel manufacturing plants, which accounts for about 10% of the total volume, can be valorized into combustion improvers for diesel/biodiesel, such as solketal, solketalacetin, and acetins, to further strengthen the industry's economic benefits [84]. The biodiesel production process includes different generation of feedstock such as first generation (edible oils- Soybean oil, Palm oil, Mustard oil, Coconut oil, Olive oil, etc.), second generation (non-edible oils- Neem oil, Jatropha oil, Karanja oil, Rubber seed oil, etc.), third generation (Microalgae and Waste cooking oils) and fourth generation (Photobiological solar fuels and Electro-fuels) [85]. Biodiesel production is popularly carried out by transesterification and esterification process.Transesterification-It is the most widely used biodiesel synthesis technology, that is carried out in three steps. Triglyceride interacts with alcohol in the first step, producing monomolecular Fatty acid alkyl esters (FAAE) and diglyceride. The monomolecular FAAE and monoglyceride are formed when diglyceride combines with alcohol. Finally, alcohols react with monoglycerides to produce monomolecular FAAE and glycerol (shown in Fig. 13 ) [86].Esterification- The carbonyl group in carboxylic acid is first protonated using an acid catalyst during esterification. The alcohol group acts as a nucleophile and attacks on the carbonyl carbon with intermediates formation. The proton is then next transferred with the elimination of H2O, and lastly the proton is removed with the production of an ester [87] (shown in Fig. 14 ).The transesterification and esterification processes depend on several factors for a productive synthesis of biodiesel. These factors/parameters include methanol to oil ratio (MTOR), catalyst loading, reaction temperature and reaction time. (i) Alcohol to oil ratio (A:O) (i) Alcohol to oil ratio (A:O) The stoichiometric relation between alcohol and oil for the transesterification process to carry out is 3:1. However, to drive the transesterification process towards forward reaction, excess methanol is required [112]. The transesterification reaction is reversible in nature so after a threshold, excess methanol starts decreasing the conversion rate of FFA. Kataria et al. [113] observed the highest yield of 98.5% biodiesel from vegetable oil using a methanol to oil ratio as 12:1. Beyond the optimum ratio they observed depletion in yield as excess methanol produced excess glycerol which could cause blocking of active sites in the catalyst. Bhatia et al. [114] also observed similar kind of results when using A:O ratio of 25:1 obtaining a conversion of 75. 3% and any further increase in A:O would decrease the conversion rate. Furthermore, excess methanol may deactivate enzymatic catalyst and affect the overall biodiesel conversion [115]. Hence, conversion of different oil feedstocks to biodiesel requires optimization of A:O ratio to obtain maximum and successful conversion. (ii) Catalyst loading (ii) Catalyst loading Catalyst loading is one of the vital parameters which accelerates the reaction process of transesterification/esterification. After the attainment of optimum catalyst loading, sometimes excess addition of catalyst increases the viscosity of the reaction mixture and restrict the mass transfer of reactants and products to and from the catalyst reactive sites. Santya et al. [116] reported optimum catalyst loading of 1.5 wt.% for the conversion of 96.23% biodiesel from WCO. They observed that further increase of catalyst loading to 2 and 2.5 wt.% decrease the conversion rate. Mass transfer limitation was also found to be the major reason for the downfall in the conversion rate to biodiesel by Muhtaseb et al. [117] since upon increasing the catalyst loading from 4.5 wt.% (optimum) they observed a declining rate in conversion. (iii) Reaction temperature Reaction temperature is also an important factor for biodiesel production. Generally high temperature reduces the viscosity of liquid and faster the transesterification/esterification reaction. In some reported catalysts by Guan et al. [118], Laskar et al. [119], and Silva et al. [120] high biodiesel yield even have been obtained at room temperature. However, beyond optimum temperature there is decrease in the yield of biodiesel since the reaction process should be within the boiling temperature of alcohol to prevent evaporation of alcohol. Gunay et al. [121] also evidently claimed that mainly high temperature encourages saponification reaction that deaccelerates the biodiesel yield. Thus, the reaction temperature should be optimised to obtain a high biodiesel yield. (iv) Reaction time (iv) Reaction time Reaction time is one of the significant parameters which is also one of the deciding factors of turnover frequency of the catalyst. An optimum reaction time period is necessary for an effective biodiesel production because once reaching the maximum conversion the catalyst would lack active sites as a result long reaction time would push for saponification which would retard the transesterification reaction [122]. During transesterification reaction if catalyst amount is higher upon increment of time could decrease the viscosity of the reaction mixture but with long period if temperature is higher than boiling point of alcohol, there is chance of methanol evaporation too. Hence, reaction parameters are interdependent and require optimisation based on the types of feedstocks and catalyst used for the production of biodiesel [123].Low-quality oils have a high concentration of FFAs and moisture, which can drastically destabilize the alkali catalyst while also causing serious separation problems. Although basic catalyst is highly efficient for transesterification of triglyceride in vegetable oil, it cannot esterify the FFA to biodiesel. In the meantime, acidic catalyst can catalyse esterification of FFAs and transesterification of triglycerides in vegetable oil simultaneously. Hence it is desirable to use acidic catalyst for the conversion of vegetable oil with high FFA to biodiesel [14,124]. Table 2 provides the list of acid functionalized MOFs employed for the preparation of biodiesel reporting the conditions, biodiesel yield, catalyst stability in terms of number of reuses and analytical/spectroscopic techniques used to prove their stability. Liu et al. [7] have synthesized a highly stable sulfonated catalyst using MIL-100(Fe) MOF with dilute sulfonic acid. The induction of SO3H causes a degree of obstruction, which results in reduction of surface area from 1150.7 to 6.18 cm2 g −1. Furthermore, after being sulfonated by dilute sulphuric acid, the pore diameter of MIL-100(Fe) increases from 1.6 to 9.88 nm, which is favourable for esterification because the large pore is advantageous to boost the effective interaction between catalyst and reactants. The catalyst reusability test was investigated under the optimal conditions (MTOR of 10, reaction temperature of 70 °C, catalyst loading of 8 and reaction time of 2 h) and the conversion of oleic acid to biodiesel was found to reduce from 95.86% to 88.5% at fifth cycle. The stability of the recovered catalyst was assessed from FT-IR analysis. However, further decline in the conversion to 58.34% after the seventh cycle was attributed to the deactivation of catalyst due to leaching of -SO3H.Pangestu et al. [101] produced a MOF based heterogeneous catalyst from the coordination of benzene-1,3,5-tricarboxylic acid and divalent copper, Cu-BTC MOF by solvothermal method. The morphology of Cu-BTC is affected at a moderate temperature of 100–110 °C where it exhibited rod-like structure while at a higher temperature the shape altered to round shape structure. Moreover, the reusability test of catalyst is a tool of stability measurement which most importantly discusses about the catalyst activity as it proceeds to be used in repetitive cycles of reactions. Cu-BTC under optimised conditions (see Table 1) afforded biodiesel yield of 91% whereas recycled catalyst gave a yield of 86% from palm cooking oil indicating a slight decrease in its activity, attributing to blockage of active sites. Activating the catalyst through thermal treatment and series of solvent exchange can reactivate the active sites which could impart potentiality in the catalyst to provide a higher yield of biodiesel. However, no characterization data were provided to prove the recovered catalyst stability. The study of the stability of recycled catalyst is an important in heterogeneous catalysis, particularly in biodiesel synthesis since there are huge possibility of pore blocking or leaching of active sites due to the high molecular weight of reactants.Ionic liquids (ILs) incorporated MOFs have been regarded as innovative materials with tremendous potential in a variety of fields. ILs are molten salts in liquid form at low temperatures, often below 100 °C, and consist of large asymmetric organic cations and inorganic or organic anions. Wan et al. [11] proposed a new methodology to construct polyoxometalate (POM), and MIL-100 MOF composite where POM encapsulated into the cage of the MOF using direct hydrothermal method followed by sulfonation making the catalyst more acidic than the MIL-100(Fe)@DAILs catalyst reported by Han et al. [63] to obtain a higher conversion of 94.6% biodiesel from oleic acid. The phosphotungstate was evenly distributed throughout the interior of the grains and additionally the ionic liquid had no effect on the phase distribution of POM, indicating that the heteropolyanion-based IL had been successfully inserted into the MIL-100 cages. Xie and Wan [58] reported another phosphomolybdenum-sulfonated IL functionalized MIL-100(Fe) MOF which was developed using a heterogeneous MOF microreactor to combine the benefits of Lewis and Brönsted acids by immobilising IL on the MIL-100(Fe) host matrix, which led to a substantial rise in surface acidities. The recovered catalyst after first cycle was used further under the optimised conditions of MTOR of 30:1, catalyst loading of 9 wt.%, reaction temperature of 65 °C and reaction time of 5 h exhibiting conversion of 90.3% without a significant loss attributing to the ion exchange effect that electrostatically linked the imidazolium cations to the POM anions. Thus, the active species was prevented from leaching off the MIL-100(Fe)support. Additionally, Inductively Coupled Plasma Optical Emission spectroscopy (ICP-OES) confirmed leaching of 1.8 ppm of molybdenum in the biodiesel thus reasoning out the decrease in catalytic activity in fifth cycle. Further Xie and Wan [90] synthesised Keggin-type 12-tungstophosphoric heteropolyacid (HPW) encapsulated in UiO66–2COOH, prepared via the in-situ synthesis method with the incorporation of AIL (acidic ionic liquid) [SO3H- (CH2)3 HIM][HSO4], resulting in biodiesel conversion of 95.8% from soybean oil. Over the years hierarchically porous Zr based MOFs have been exploited to a great extent. Ye et al. [89] followed a strategy of approximate ligand substitution where task specific IL (TSIL) was introduced into the porous network of Zr MOF. After ligands and metal clusters were replaced with propionic acid, UiO-66 was etched to form hierarchical porous UiO-66 (H-UiO-66). The numerical optimisation of catalytic activity done using response surface methodology (RSM) approach provided optimum reaction parameters (MTOR of 10.39:1, catalyst loading of 6.28 wt.%, reaction temperature of 80 °C and reaction time of 5 h) for biodiesel yield of 93.82%.Wu et al. [59] reported IL based MOF catalyst by with the incorporation of 1,4-butanediyl-3,3′-bis-(3-sulfopropyl) imidazolium dihydrogensulfate in amino-functionalized magnetic MOF composite, DAIL-Fe3O4@NH2−MIL-88B(Fe) optimized through RSM with fine catalytic performance of 93.2% conversion rate of oleic acid. Xie and Wang [57] reported an IL based acidic MOF catalyst where the reaction of pyridine with 1,3-propane sultone, followed by ion exchange with POM acids, produced POM-based sulfonated ILs containing Brönsted-Lewis acid sites, (indicated as (Py-Ps) PMo) which was then injected onto the CoFe2O4/MIL-88B(Fe)-NH2 framework. The functionalized catalyst was novel in that it could simultaneously catalysed the transesterification of soybean oil and the esterification of FFAs. Additionally, the catalyst exhibited a conversion of 82.5% even after five cycles which is not far from the initial activity of 95.6% conversion. This minor loss in activity could be attributed to the partial deactivation of the catalyst due to the possible leaching of IL and blockage of active pore sites. However, these assumptions were not proved with convincing evidence.Recently Youssef et al. have reported MOF-5 [102], synthesized from BDC linker and zinc nitrate hexahydrate which could convert WCO and Jatropha Curcas oil (JCO) without any further functionalization. The transesterification reaction of WCO using the MOF-5 resulted in a decrease of FAME conversion from 82.0 to 41.2% when investigated for reusability. There might be a loss of active sites or a deposition of intermediates in the pore of the catalyst, which could be the probable reasons for such decrease in catalytic activity of MOF-5. Catalyst stability issue should be addressed to understand the deactivation mechanism. Copper-based MOF efficiently oxidized alcohols, transforming them into corresponding products, making it a viable catalyst for improved and stable biodiesel generation [130]. Jamil et al. [40] synthesized a Cu-Ca-MOF where the Cu-MOF showed the biodiesel yield of 78.3% while Ca-MOF recorded 78% and Cu-MOF + Ca-MOF showcased 85% of biodiesel yield. Also, the Cu-MOF + Ca-MOF showed a specific gravity of 0.88 (in the range of ASTM standards, 0.86–0.9).Microwave-assisted technology is a promising technique that has transformed chemical synthesis and research [131] [132]. Salam et al. [97] synthesized Mg3(bdc)3(H2O)2 via microwave (MW) irradiation (Fig. 15 ), then in turn used microwave for the production of biodiesel from oleic acid. A high conversion of 97% was obtained with the reaction conditions of methanol to oil molar ratio of 15:1, catalyst loading of 0.15 wt.%, power of 150 W, and reaction time of 8 min. Because of the MOF's hygroscopic nature, methanol molecules were completely replaced by water molecules after heating at 220 °C under vacuum conditions and subsequently exposing to the surrounding environment.Heteropolycids have some drawbacks of low surface area and high solubility in the reaction medium which limits their industrial applications while to overcome this immobilization of HPA come up as new strategy. Nikseresht et al. [126] reported a heterogeneous catalyst that fabricated by heteropolyacid and Fe(III) based MOF under ultrasound irradiation at ambient temperature and pressure. In reusability test of the PTA@MOF, it was recovered 5 times during oleic acid conversion and showed upto 80% conversion in the last cycle. ICP analyses showed 5 wt.% PTA leaching during the recovery process which could be due to clinging of PTA ions to the network of the MIL-53. The cages of the PTA-MIL-53 were occupied by phosphotungestic acid (PTA) molecules, so the weight loss of bare MOF was the greatest (confirmed from TGA-analysis Fig. 16 ), and the weight loss of prepared PTA-MOF was the least.Liu et al. [125] reported UiO-66-supported sulfonic acid catalysts (MSA-UiO-66) which successfully converted palmitic acid into biodiesel using impregnation method of synthesis. After ten runs, elemental analysis revealed that the MSA concentration was 209.3 mg g −1 in the solid catalyst supporting the hypothesis that low alcohol polarity prevented organosulfonic acid leaching during the esterification reaction process. Another stable catalyst MOF-801 was reported by Shaik et al. [103] a unique microporous Zr-based structure consisting of Zr6 nodes connected by fumarate linkers converted used vegetable oil (UVO) to biodiesel confirmed from 1H NMR of biodiesel and the morphology analysed by SEM provided a dense and defect-free morphology.Rodríguez et al. [100] synthesized Co based acidic MOF by the inclusion of two organic linkers 1,2-di-(4-pyridyl)-ethylene and 5-nitroisophthalic with the inorganic node of Co(NO3)2·6H2O resulting in the yield of Erythrina Mexicana oil into biodiesel by 83.7%. The biodiesel mainly consists of certain alkyl esters (as confirmed from gas chromatography) that included methyl linoleate (21.3%), methyl palmitate (15.24%), methyl oleate (4.8%), methyl stearate (37.2%) and methyl arachidate (0.98%).Although basic catalysts can cause saponification with low-quality raw oils, but the base-catalysed system is relatively favourable because of the mild reaction conditions i.e., lower temperature and shorter time (enlisted in Table 3 ). Xie and Wan [104] developed a core–shell structured Fe3O4@HKUST-1 composite by using layer by layer assembly method with the encapsulation of basic ionic liquid (Fig. 17 ). The amino groups in the ABILs molecule can act as plausible coordination sites to the coordinative unsaturated metal sites (Cu2+) in the HKUST-1 framework and the ABILs are thought to be attached to the Fe3O4@HKUST-1 support via a coordination mode.Furthermore Xie and Wan [105] reported another basic MOF ZIF-90-Gua where the guanidine base anchored onto the porous support by covalent bonds to form the hybrid solid catalyst. Due to the firmly covalent binding of guanidine to the support, the such-formed solid catalyst was expected to possess long-terms catalytic activities. Notably the acidity and basicity were assessed by the Temperature programmed desorption (TPD) peak integration from CO2 and NH3 profiles of the ZIF-90 and ZIF-90-Gua catalyst. When Gua loading was increased in the support from 2.26 to 4.02 mmolg−1, the basicity, as well as the corresponding soybean oil conversion over the solid catalyst were both increased from 1.05 to 1.56 mmolg−1 and 81.38 to 95.42%, respectively.Saeedi et al. [106] synthesized basic solid catalyst out of Zeolitic imidazole framework doped with potassium, which was used without any prior thermal pre-treatment at high temperatures, which is commonly used to activate heterogeneous catalysts for the activation. The maximum biodiesel conversion of 98% was achieved with 0.08% of potassium loading. ICP (Inductively coupled plasma) analysis was used to analyse the leached metals in the biodiesel phase after the catalyst was removed. The authors analysed the potential leaching of potassium and sodium in the produced biodiesel and have observed a leaching of 7 and 3 ppm respectively, indicating that very little amount of metal dissolved in the reaction medium during the transesterification process.MOFs can be a better choice if they can be utilized as a support to restrict leaching of metal oxides. Li et al. [133] have developed a strontium oxide supported by MIL-100(Fe) derivate for transesterification of palm oil using a novel magnetic catalyst generated by using MIL-100(Fe) as a carrier to support strontium carbonate and calcining it in an inert atmosphere producing SrO-MIL-100(Fe) derivative. The highest conversion of 96.19% was accomplished at a methanol/oil molar ratio of 12 at 65 °C after 30 min. Further, the catalyst showed a conversion of 82.49% after 3 cycles.Yang et al. [134] developed a basic catalyst MgO@Zn-MOF synthesised by thermally decomposing Mg precursor particles contained within Zn-MOF to MgO without disrupting the Zn-MOF structure using different roué as shown in Fig. 18 . MgO nanoparticles were found to be inside the Zn-MOF particles and the shape of the particles grew more irregular and the relative intensity of Mg to Zn elements rose as concentration of Mg was raised from 12.1 to 24.1 wt.%. A substantial amount of MgO nanoparticles were deposited at the outer surface of the Zn-MOF particle when the concentration of Mg was 24.1 wt.%. The catalytic activity of the catalyst towards transesterification was investigated under the optimised conditions (MTOR of 3:1, catalyst loading of 1 wt.%, reaction temperature of 210 °C and reaction time of 2 h) to obtain a biodiesel yield of 73.3 wt.%. Further, towards the recyclability the catalyst activity dropped to 5.9% but the MOF structure was not found to be much affected, and the crystallite size remained unchanged as evident from the XRD of spent catalyst (Fig. 19 ).ZIFs (Zeolite Imidazolate Frameworks) have received a lot of attention since they combine the benefits of both zeolites and regular MOFs. Another basic catalyst Fazaeli and Aliyan [135] reported using ZIF-8 MOF where graphene oxide (GO) nanoparticles was encapsulated into the large sodalite cavity structure of it and further ZIF-8@GO doped with Sodium and Potassium. The KNa/ZIF-8@GO exhibited much rougher surfaces than the aggregated graphene nanosheets in a dried state which might be attributed to the adsorption of cubic ZIF-8 nanoparticles on the surface of graphene sheets. The solid basic catalyst was found to contain K loading of 0.05%, resulting in a 98% soybean oil conversion. It was observed that with the increase of catalytic run during recyclability test (Fig. 20 ) there was decrease in the biodiesel yield which was attributed to the development of multilayer on the support due to blockage of active sites.Porous magnetic materials are increasingly employed as support for preparing heterogeneous catalysts. Not only for its abundant pore structure, but also it allows for quick and simple separation by using extra magnetic field compared with the conventional catalyst support. Li et al. [136] synthesized magnetic mesoporous Fe@C support SrO heterogeneous catalyst where MIL-Fe (100) was initially employed as precursor to obtain the mesoporous magnetic support (Fe@C) through carbonization under nitrogen atmosphere followed by loading of SrO. The morphologies of Fe@C and Fe@C-Sr depicted highly uniform virgulate shaped particles in comparison with Fe@C, Fe@C-Sr showed an obvious rise in size with cross linked rod structure and more rugged surface revealing SrO was loaded on porous Fe@C. Moreover, the saturation magnetization of Fe@C-Sr used in the fourth cycle (displayed in Fig. 21 ) was 87.95 emu g-1, where the corresponding magnetization curve almost coincided with fresh Fe@C-Sr indicating the magnetic substance was stable during transesterification.Table 3 summarizes the use of base functionalized and MOF-derived basic catalysts for biodiesel production along with the yield of biodiesel and evidence for catalyst stability through number of reuses.Bifunctional MOF catalysts with both acidic and basic active sites have been considered as a suitable candidate for esterification/transesterification for low-quality raw oils. Table 4 provides the experimental conditions, biodiesel yield, number of reuses and stability evidence for these MOFs. Jeon et al. [108] reported a bifunctional catalyst where in the HZN (HPA functionalised ZIF-8 nanoparticles) show a bi-functionality resulting from the acidity of HPA and basicity of the imidazolate in ZIF-8. The total basicity of 22.82 mmol g −1 from the pure ZIF-8 is approximately half compared to the total acidity of 41.95 mmol g −1 from the pure HPA, along with a small acidity from pure ZIF-8 (4.26 mmol g −1). The catalyst converted rapeseed oil into 98% biodiesel under the optimized conditions of MTOR 10, catalyst amount of 4 wt.%, reaction temperature of 200 ºC and reaction time of 2 h.Recently another bifunctional catalyst was reported by Ahmed et al. [61], where a series of UiO-66(Zr)- structured materials with defects were used as solid catalysts for the esterification reaction of oleic acid. They observed that the catalytic activity of UiO-66-(Zr) -NH2 > UiO-66-(Zr) -NO2 > UiO-66(Zr) prior to the bifunctionality and biodiesel conversion. The electron donating groups like -NH2 group are inductively pushing electron toward zirconium sites through benzene ring and decrease the acidity of zirconium centers to increase lewis basicity of Zr-sites and Brønsted basicity of Zr-OH, Zr-O-Zr. The BET surface areas of UiO-66(Zr), UiO-66(Zr)-NH2, and UiO-66(Zr)-NO2 were 1115, 823, and 649 m2g−1 respectively, indicating that the addition of electron withdrawing and donating groups to the BDC ligand can significantly alter the physical properties of UiO-66(Zr). The basic heterogeneous catalyst UiO-66-NH2 converted oleic acid into 97.3% biodiesel under the optimized conditions of MTOR 39, catalyst amount of 6 wt.%, reaction temperature of 60 ºC and reaction time of 4 h.Hasan et al. [138] successfully synthesized Zr (IV)-Sal Schiff base complex incorporated into amino-functionalized MIL-101(Cr) framework by salicylaldehyde condensing to amino group and coordinating Zr (IV) ion (Fig. 22 ). The specific surface area amounts to 1691 m2g−1 for pristine NH2−MIL-101(Cr) and decreases to 571 m2g−1 and 437 m2g−1 for NH2−MIL-101(Cr)-Sal-Zr and NH2−MIL-101(Cr)-Sal-Zr respectively, while the pore volume decreases from 1.18 cm3g−1 to 0.48 and 0.35 m2g−1 for the same materials. The catalyst successfully converted oleic acid to 74.1% of methyl oleate under the optimized conditions of MTOR of 10:1, catalyst amount of 4, reaction temperature of 60 and reaction time of 4 h. The stability of the recovered catalyst was for six cycles and there were no significant changes in the conversion.The alternative way to address enzyme stability and recyclability difficulties in biodiesel synthesis is to immobilize enzymes using porous materials as supports. In order to improve recycling stability, enzymes can be covalently bonded to MOF surfaces. Normally, the free amino groups on the enzyme or MOF surface bind to the carboxylate groups on the enzyme or MOF surface to form peptide linkages [139]. The cage inclusion process involves the diffusion-mediated encapsulation of small enzymes within the cages of mesoporous MOFs. Even in hostile environments or unnatural conditions, the stability of enzymes may be considerably improved by encapsulation. It also creates a protective environment that reduces the impact of denaturation [140]. Li et al. [53] reported an immobilized lipase in metal-organic based frameworks constructed by biomimetic mineralization where zinc acetate was used as an inorganic node and adenine as an organic linker (Fig. 23 ). The temperature-tolerance assay performed at 70 °C, 40% (approx.) of the initial activity of lipase@Bio-MOF was detected after the incubation for 100 min, while free lipase was completely inactivated. Moreover, lipase@Bio-MOF could still retain approximately 32% of the enzymatic activity after storage at room temperature for 4 weeks, but free lipase lost almost all the enzymatic activity which suggests that the enzyme possessed enhanced tolerance against high temperatures and long-term storage after the immobilization in MOFs, which is useful for executing the catalytic reaction under harsh conditions.Adnan et al. [109] reported a zeolitic imidazolate framework (ZIF -sub class of MOF) acting as a carrier for lipase to generate a hierarchical ZIF-8 towards immobilizing Burkholderiacepacia Lipase (BCL-ZIF-8, as shown in Fig. 24 ). Immobilization efficiency improved continuously between 20 and 45 ºC and despite the high temperature, activity recovery increased at the same time, reaching a maximum of 1196% at 25 °C. Higher operational stability was demonstrated by the immobilized BCL in the mesoporous ZIF-8. The BCL-ZIF-8 was damaged by the excess alcohol and the glycerol byproduct that formed many layers around it. for the decline in biodiesel generation as the number of cycles increased up to 8, because of the mechanical stress generated by the ongoing reaction, a portion of the carrier was damaged, causing enzyme leakage and a decline in the immobilized enzyme's catalytic activity.Enzyme immobilization onto/into appropriate carriers may improve bio-catalytic industrial biodiesel production by increasing enzyme stability, allowing for continuous reuse, and allowing easy separation [141,142]. Rafiei et al. [62] synthesized a heterogeneous biocatalyst by encapsulating lipase into the microporous zeolite imidazolate framework, ZIF-67. The free lipase retains just 43.7% of its initial activity at 50 °C, whereas the encapsulated lipase retains 72.6%. These findings showed that encapsulating enzymes in MOFs can prevent them from changing conformation at high temperatures, hence improving their thermal stability. Moreover, the stiff scaffold of the ZIF-67 improves the pH and heat stability of the embedded enzyme, preventing it from deactivation and allowing for up to 8 cycles of reusability in biodiesel production from soybean oil.Adnan et al. [110] also reported a one-step encapsulation method of synthesizing X-shaped ZIF-8 (as shown in Fig. 25 ) and immobilizing Rhizomucor miehei lipase. They observed a 26-fold of increase in the activity recovery of the enzyme because of the encapsulation method as it inhibits direct contact with the substrate. Secondly, ZIF-8, an immobilization carrier, has a major impact on enzyme activity by establishing microenvironments that are favourable for enzyme catalysis for a limited time in mild biocompatible conditions, allowing enzymatic activity to be preserved. Investigation into the reusability of RML@ZIF-8 in an isooctane medium revealed that after a continuous run of 10 cycles, the encapsulation of ZIF-8 to RML still maintained an 84.7%. The additional ethanol and glycerol by-products that were adsorbing to the surface of RML@ZIF-8 were thought to be the cause of the declining of biodiesel yield as the number of cycles rose. Aspergillus niger lipase (ANL) was employed by Hu et al. [94] with a hydrophobic UiO-66, which was modified by using facile polydimethylsiloxane (PDMS)-coating using chemical vapour deposition (CVD) treatment. The contact angle of a water droplet on UiO-66 is 111°, which increases to 121°, 144° and 157° after PDMS coating by CVD (2 h, 6 h, 10 h) indicating that the PDMS coating significantly enhances the surface hydrophobicity of UiO-66. Another hydrophobic MOF was reported by Zhong et al. [143] where hydrophobic ZIF-L coated with polydimethylsiloxane (PDMS) was prepared by CVD and used to immobilize lipase from Aspergillus oryzae (AOL) for biodiesel production (Fig. 26 ). The maximum activity obtained when lipase concentration was at 0.24 mg/mL but decreased with further increase of lipase concentration due to steric hindrance effect of the enzyme molecules and the diffusional limitations. PDMS hydrophobic modification on the surface of MOF improved the activity of immobilized lipase and the strong hydrophobic interaction between the lipase and the PDMS coating stabilizes the confirmation of the lipase [144,145]. Table 5 provides the summary of enzymatic MOFs used for biodiesel synthesis with their experimental conditions, yield, number of reuses and stability evidence. These data indicate that less attentions are given to characterize the spent catalyst. This is one of the important issues that should be given higher priority to understand the reasons for activity decay.A major issue for heterogeneous catalysis is the leaching of active species into the liquid phase, which ultimately causes a significant deactivation of solid catalysts. ILs are the anionic-cationic salts with the ability to dissolve a wide range of compounds, carry some features of being non-volatile and possess excellent thermal, chemical, and electrochemical stability [146,147]. Acidic and basic ILs are being used for the functionalization of MOFs which none the less show high conversion of oil to biodiesel conversion. However, leaching of ILs have been a concern as it deactivates the activity of the catalyst. Wan et al. [11] reported hteropolyanion-based IL within the framework of MIL-100(Fe) where they observed instability of MOF due to leaching of Cu2+ and HPW in presence of acetic acid. Leaching test of DAIL-Fe3O4@NH2−MIL-88B(Fe) was performed by Wu et al. [59] by running the reaction with catalyst for 2 h and then further without catalyst for 3 observed that there was slight conversion into biodiesel without catalyst possibly due to leaching of ionic liquid DAIL (Fig. 27 ) caused by ion exchanges supposed to take place during catalytic process.AILs/UiO-66–2COOH [90] composite, has the synergistic effect between the Bronsted acid sites from AILs and the Lewis acid sites from the HPW that boost the catalytic activity of the catalyst. The strong interaction between the sulfonic acid groups and the HPW molecules appeared to be effective in preventing the active component loss from MOF supports. However, it was observed that there was significant reduction in the solid catalyst's catalytic activity in the second cycle of oil to biodiesel conversion. Furthermore, the major leaching of MBIAILs from the channels of MOFs was noticed by Han et al. [50] which could be leaching of ILs on the surface of MIL-101(Cr) frameworks due to weak physical adsorption. However, to enhance the recyclability Liu et al. [125] suggested separation of methanol firstly to reduce leaching of organosulfonic acids from UiO-66 which could amplify solid catalyst recovery effect. In most cases leaching is due to weak interaction between precursors, Abdelmigeed et al. [107] synthesised NaOH/ Magnetised ZIF-8 catalyst where they reported that the interaction between NaOH and magnetised ZIF-8 was enhanced by calcining at 200 ℃ under inert atmosphere.During catalytic processes, Pd nanoparticles (Pd-NPs) supported on porous materials are prone to significant leaching or aggregation, resulting in a loss of catalytic activity and cyclic stability [148]. In ZIF-8 supported carbon-stabilized Pd nanoparticles (C@Pd/ZIF-8) MOF [149] based catalyst, TEM analysis (Fig. 28 ) confirmed that Pd particles supported by pure ZIF-8 suffered substantial aggregation or leaching during the reactions, which accounted for the catalyst deactivation.Enzyme immobilization technique with the use of MOF to solve the enzyme solubility and leaching issues is one of the strategies recently adopted for the synthesis of enzymatic based MOF catalyst. However, still challenges related to leaching prevails due to weak interaction between MOF and enzymes [150]. MOF based catalysts like MIL-100(Fe)/PPL (PPL-Porcine pancreatic lipase) and HKUST-1/PPL [151] were used for enzymatic esterification of cinnamic acid. ICP-AES studies of both the catalyst suggested 0.8 wt.% of Fe from MIL-100(Fe)/PPL and 10 wt.% of Cu from HKUST-1/PPL were leached after 12 catalytic cycles but significant amount of leaching was from PPL (Fig. 29 a) as 53% and 34% of activity (Fig. 29 b) were retained after 12 cycles of test respectively. Recently, strategies like entrapment, crosslinking [94,152] have been adapted to prevent enzyme leaching. Chen et al. [153] developed a new method to encapsulate enzymes in hollow MOF. They encapsulated catalase inside ZIF-67 and made ZIF-8 to overgrow on it followed by removing ZIF-67 cores by means of modest hollowing process [154] which in turn was confirmed by SEM and TEM micrographs displaying confined enzymes in the MOF core (Fig. 30 a–c) and enzymes in the hollow MOF cavity (Fig. 28 d–f) without affecting the morphology of the MOF.MOFs structural deformation can also lead to the deactivation of catalyst. Excess amounts of modulators used in the synthetic process cause deformation in the MOF structure which deactivates the catalytic activity during the catalytic run. Recently, Conley and Gates [155] reported about the deactivation stages of UiO-66 (Zr) MOF with various proportion of acetic acid (aa) modulator and provided a quantitative approach towards methanol dehydration. The life cycle of the MOF catalyst included an activation stage (blue), during which the conversion reached its maximum peak, subsequently deactivation stage (yellow) and finally the catalyst almost completely lost (red) (as shown in Fig. 31 . a). The MOF loses its crystallinity and leads to MOF unzipping when reaction of methanol proceeded with the node linkers to form methyl esters, where the inference was confirmed by XRD (Fig. 31. b) which showed missing of characteristic peaks of UiO-66 at 2θ = 7.45° and 8.6° when allowed for overnight reaction. Furthermore, Yang et al. [156] also observed the deactivation in UiO-66 type MOFs where the carboxylate groups of the linkers reacted with ethanol to create esters, thereby unzipping the MOFs.Opanasenko et al. [157] studied the deactivation pathways for the catalytic activity of Fe-MOFs in condensation reactions. The study was based on monitoring of byproducts which could deactivate the activity of the MOF by deteriorating its lattice. Li et al. [137] also evaluated such deactivation of catalyst and considered it as a cause of organics blockage since XRD of the MOF- derived catalyst after reuse gave similar peaks as that of the fresh catalyst. Formation of strongly adsorbed byproducts formed during the reaction interacts with the MOF lattice and cause structural disintegration in the MOF. The loss of active sites cause the catalyst to deactivate as a result of pore obstruction since intermediates or products such as diglyceride, monoglyceride, glycerol, and biodiesel masque the catalysts [158]. Thus, the problem before the MOF research is to control the disintegration of MOF structure. In this run so far, Yunan et al. [159] provided striking results with the usage of custom-built reactor developed at the Christian- Albrechts University (Kiel, Germany) virtue of which any crystallographic changes with respect to chemical reactions could be estimated efficiently. Their observation in probing of Pd(II)@MIL-101-NH2 during Heck coupling was quite successful. They reported that deactivation of catalyst was neither due to Pd leaching nor MOF decomposition rather chemical deactivation caused due to catalyst poisoning by Cl−ions that masked the surface of Pd clusters and obstructed the access of starting materials from reacting with the active sites. As solutions arrive from knowing the cause of problem, the use of different precursors of Pd for encapsulation or carrying out reactions under continuous flow would solve the catalyst poisoning and deactivation of catalyst [160].The review summarises different types of MOFs as MOF derived, and novel precursor used as heterogeneous catalyst for biodiesel production. MOFs can be utilized as a template of support for metal oxides, an immobilising material for enzymes or whole new framework of sophisticated 3-D material with various organic linkers. MOFs emerging as a new class of materials with unimaginable tunability, porosity and unprecedented surface area which increases its potential to be used in industrial scale to meet the requirement of selectivity for biodiesel production. This review consists of some relevant characterization techniques like XRD, BET, SEM and TEM, XPS and TGA for MOF catalysts where important findings have been mentioned from several literatures which include the hollow MOF structure, missing linker detection, thermal stability, and crystallinity of MOFs from morphological, elemental, thermal and structural characteristic. The aim of this review is to provide a deep insight to readers about the extended coordinated network, facile tunability of different class of MOFs, and factors (leaching, blocking of active sites, unzipping of MOF structure) causing deactivation of MOF catalysts which will enhance the understanding of immense possibilities and opportunities carried by MOFs. Though this review discussed catalytic activity of MOF in biodiesel but still there are some major challenges in MOF synthesis and needs special consideration. The stability of MOFs is one of the major issues in the synthesis of acid/base functionalized MOFs as it affects the pristine structures. Therefore, different design strategies need to be adopted during the synthesis process to keep the highly coordinated network intact. More stable MOF could be an ideal candidate to immobilize types of enzymes to promote eco-friendly production of biodiesel.There are very few bifunctional MOFs which have been used to produce biodiesel, although bifunctional MOFs are strategically more efficient to show synergistic effect of both acidic and basic nature, there are still less reported number of bifunctional MOFs. Thus, bifunctional based MOFs need to be synthesised for the rapid transesterification reaction process. Designing MOFs by inserting heteroatoms in the interstitial voids of MOF in appropriate proportions by controlling its electronic dynamics and environment is still a great challenge to tackle. Engineering defects in MOFs enhance the reactivity of MOFs and such underexplored field in MOF requires ample attention from its future catalytic activity perspective in the field of biodiesel.The reticular synthesis of MOFs is also a field of synthetic strategy confined with the issues of solubility and stability which needs to be explored more in near future. We believe in the vast scope of MOF chemistry and that may be get uncovered in the coming years which not only would pave the way for well stabilised gigantic architectural framework but also would find practical applications in the present world.CaO works well as a catalyst for producing biodiesel. It can achieve conversion of more than 95% oil to biodiesel. Moreover, it can resist more than 5 cycles with good efficiency. Natural waste products like chicken eggs, snail shells, and animal bones can be used as a source of generating CaO. However, leaching of CaO is an issue which is difficult to achieve using ion exchange resin. On the other, MOF derived catalysts possess the potential to restrict leaching of CaO where MOFs can act as a support to CaO, boost up the surface area and stability of the catalyst to achieve high biodiesel yield. Above all MOF catalysts deactivation due to catalyst poisoning, leaching or self-inactivation require a detailed experimental study in order to improve the catalyst reusability that supposed to be the backbone of the heterogeneous catalyst.This research received no specific grant from any funding agency.Authors declare no conflict of interest.

Nowadays, as there is a rapid depletion of fossil fuels, the need for alternative resources has become inevitable. Biodiesel is such an alternative that is environmentally friendly and sustainable. Over the years there has been a significant amount of research done for biofuel production by converting various feedstocks such as edible and non-edible vegetable oil, animal fats, waste cooking oil, and microalgae as the feedstock which requires a highly workable and efficient catalyst for the transesterification process. Several heterogeneous catalysts have been used for the transesterification of biodiesel feedstock to biodiesel, among which metal-organic framework (MOF) has gained popularity owing to its high surface area, high pore volume, and facile tunability of the active sites. This review focuses on different types of MOFs, characterization techniques used to identify vital structurally invoked changes, and deactivation of MOFs due to leaching of various active species, blocking of active sites, and unzipping of MOFs by covering the literature from the year 2000 to till date. Finally, a brief conclusion and the author's perspective depicting several challenges and scope for future research needs in the MOF study have been provided.

The authors do not have permission to share data.Human society is confronted with many difficulties such as climate change, environment protection and energy security issues arising from the depletion and the over-reliance on fossil fuels [1–8]. With the increasing demand for clean energy and medicine, it is imperative to adjust petroleum-based resources to renewable biomass for the industrial production of bulk chemicals [9–14]. As a promising intermediate, lignocellulose-derived furfural (FF) can be potentially utilized for many downstream organic synthesis [15–17]. Among these, 2-methylfuran (2-MF) has been the dominant alternative deriving from FF selective hydrogenation of CO bond [18,19]. It has been broadly used in bio-refinery and chemical manufacture [20]. In particular, 2-MF is a pivotal compound to synthesis chloroquine phosphate, which is vital for ultimate production of the 2019-nCov commercially [21].The selective hydrogenation reaction (HDO) of FF provides a route to prepare 2-MF without breaking the furanic “O” and the ring double bonds [22]. Actually, CuCr catalyst has been used for the industrial production of 2-MF for decades [23,24]. However, it is essential to concern at the consequent environmental pollution and potential human health threat caused by toxicity chromium in the catalyst. In recent studies, given that the natural scarcity and expensive price of noble metals might impede the large-scale commercialization, transition metals (Cu, Co and Ni) are adopted and showed high selective hydrogenation activity for 2-MF fabrication [25–28]. Unfortunately, most of these studies are performed with FF instead of xylose as initial feedstock. The tandem conversion [29] of xylose hydrolysis to produce FF and subsequent HDO of FF is still a huge challenge within the scale utilization of biomass.Followed this cascade strategy, Lessard et al. has employed a continuous two-liquid-phase (aqueous-toluene) plug-flow reactor with (H+) mordenite and Cu/Fe as catalysts respectively and obtain 96% yield of 2-MF from the xylose [30]. Cui et al. designed a process for the production of 2-MF on a continuous fixed-bed reactor with butyrolactone/water as solvent [31]. After the xylose was dehydrated by Hβ zeolite, the obtained FF was selectively hydrogenated to furfuryl alcohol (FA) or 2-MF by changing the hydrogenation temperature over the Cu/ZnO/Al2O3 catalyst. The highest 2-MF yield of 86.8% was achieved at 190 °C. In recent research, Rafael F. et al. converted xylose to furfuryl alcohol over Zr-SBA-15 at 130 °C, 30 bar N2 for 6 h and about 30% xylose conversion and 45% selectivity to 2-MF were obtained [32]. Deng et al. completed one-pot cascade conversion of xylose to furfuryl alcohol over a bifunctional Cu/SBA-15-SO3H catalyst at 140 °C and 4 MPa for 6 h [33]. 93.7% xylose conversion and 62.6% 2-MF yield were achieved.As a novel contribution to this challenge, xylose was employed as the staring material to produce FF and 2-MF via tandem conversion in this work, where Hβ zeolite and NiCu/C were used as dehydration and HDO catalyst in isopropyl alcohol/water solution. Subsequently, the stability of NiCu/C and the effects of reaction conditions on FF conversion were evaluated comprehensively. At last, the tandem conversion of xylose was investigated with different catalyst combination. 95.1% 2-MF yield based on xylose was achieved over Hβ zeolite and 0.5NiCu/C catalyst.D-xylose, nickel (II) nitrate hexahydrate (Ni(NO3)2∙6H2O), copper (II) nitrate hydrate (Cu(NO3)2∙3H2O), citric acid (C6H8O7), 2-methylfuran (2-MF), furfural (FF), furfuryl alcohol (FA) and 2-methyltetrahydrofuran (2-MTHF) were purchased from Shanghai Macklin Chemistry Co., Ltd. in analytical grade. HY, Hβ, USY and MCM-41 zeolites were purchased from Nankai University Catalyst Co., Ltd. Isopropanol and tetrahydrofurfuryl alcohol (THFA) were applied by Aladdin Chemistry Co., Ltd. All chemicals were used as received without further purification.The bimetallic catalysts were prepared according to the following procedures. 5.8 g citric acid (CA), 7.3 g Cu(NO3)2∙3H2O and a designated amount of Ni(NO3)2∙6H2O (the weight ratio of Ni/Cu = 0, 0.25, 0.5, 1, 2.5, 5) were added to 10 ml deionized water. According to Ni/Cu weight ratio, different bimetallic catalysts were denoted as xNiCu/C. After being kept at 90 °C for 3 h with vigorously stirred, the gelatinous mixture was transferred into a 100 °C drying oven over night. The obtained complex was denoted as CA-Ni-Cu. Finally, the bimetallic catalysts were collected by annealing the spongy compound CA-Ni-Cu at a temperature range of 500 to 800 °C for 3 h in N2.The synthesis of Cu/C catalyst was conducted according to our previous literature [34].The Brunauer-Emmett-Teller (BET) surface areas of the prepared catalysts were determined by the nitrogen physisorption at −196 °C on a Tristar II 3020 volumetric adsorption analyzer. Prior to nitrogen physisorption, the samples were pretreated at 200 °C for 12 h under the high vacuum. X-ray diffraction (XRD) were carried out in the 2θ range from 10 to 80 o at the scanning rate of 0.02 degree−1 by an X'Pert Pro MPD equipped with CuKa radiation. The X-ray photoelectron spectroscopy (XPS) analysis were taken on a Kratos AXIS ULTRA DLD spectrometer equipped with a monochromatic Al-Ka radiation source (h v  = 1486.6 eV). The C 1 s peak (284.8 eV) was applied as the reference for binding energy calibration of other elements. HRTEM was performed using a JEOL-2100F microscope operated at the accelerating voltage of 200 kV. The surface acidity of catalyst was conducted by NH3-temperature programmed desorption (NH3-TPD) using a Tp-5080 China Xianquan machine (TCD detector).All typical FF hydrogenation experiments were performed in a 50 ml stainless steel micro autoclave (MS-50-316 L, Anhui Kemi Machinery Technology Co., Ltd., Hefei, China). 0.1 g catalyst, 5 ml deionized water and 30 ml isopropyl alcohol were loaded to the autoclave. Afterward, the reactor was sealed and purged with hydrogen for 6 times. H2 pressure was kept at certain figure, i.e., 1, 2, 3, 4 or 5 MPa. Finally, the reactor was maintained at specific temperature for 0–10 h under mechanical stirring (800 rpm). After completion, the autoclave was cooled down to room temperature naturally and the solid catalyst was separated from the residual liquid through the centrifugation. For the recycling test, the collected catalyst was washed with deionized water after each cycle and dried at −48 °C under vacuum before the next run.For the typical tandem conversion, 1.0 g xylose and 0.3 g Hβ zeolite were added to react firstly at 140 °C for 5 h and the obtained liquid product could be separated by distillation. Subsequently, NiCu/C catalyst was added for FF hydrogenation experiments at 220 °C for 5 h. As a contrast, Hβ zeolite and NiCu/C catalysts were respectively used for the conversion at the segmented optimal temperatures. For one pot conversion of xylose, Hβ zeolite and NiCu/C at different temperatures catalyzed the reaction for 10 h. Two-step reaction was completed at 140 °C and 220 °C for 5 h on the basis of one pot conversion.The liquid products were further analyzed using a gas chromatograph (Agilgent, USA, CP-Wax 58 capillary column). The product identification was analyzed using a GC–MS (Agilgent, USA, FID, INNOWAX column) equipped with a flame ionization detector. The calibration curve was established by an external standard method. The conversion of substrate and the product yields were calculated using the following equation: (1) Conversion % = n c − n s n c (2) Yield % = n i n c where n c represents the moles of FF or xylose before the reaction; n s represents the moles of remained substrate after the reaction and n i is the mole amount of product i.The impact of Ni content on the hydrogenation activity of xNiCu/C catalysts at 220 °C under 4 MPa H2 is shown in Table 1 . Except for the targeted 2-MF, side products including FA, 2-methyltetrahydrofurfuryl alcohol (2-MTHF) and tetrahydrofurfuryl alcohol (THFA) were also detected. For single metal catalyst Cu/C, 84.1% FF was converted and the medium yield of 2-MF was 61.5%. With the introduction of Ni, the feedstock was consumed completely and the highest yield of 2-MF (97.5%) was achieved over 0.5NiCu/C catalyst. This could be assigned that the recommendation of Ni facilitated the quantitative growth of reactive sites, leading to the enhancement of the selectivity for 2-MF. However, superfluous Ni is not always advantageous because it is stimulative for the formation of side products. Especially, the yield of 2-MTHF also varies with the increase of Ni proportion, suggesting that the introduced Ni of xNiCu/C catalyst breaks CC bonds adequately.In order to investigate the relationship between the structure and activity of the xNiCu/C catalyst, the physicochemical properties of xNiCu/C were characterized. As shown in the Table 2 , the specific surface area (S BET) of Cu/C sample is 48.8 m2/g, the pore volume (V p) is 0.08 m3/g and the average pore diameter (D p) is 6.7 nm. Minor but significant changes in S BET, V p and D p of the xNiCu/C catalysts are observed with the introduction of Ni metal. When the Ni loading is below 1.0%, the S BET is gradually ascending, indicating that, to a great extent, the slight nickel can promote the increase of the specific surface area of the catalyst. Otherwise, once too much nickel (>1%) diffuses to the catalyst pore, the S BET decreases apparently, thus reducing the specific surface area and average pore diameter of xNiCu/C. Because of the above variations, the decrease of catalyst activity ultimately affects the conversion of FF and the yield of 2-MF.The crystalline structures of xNiCu/C catalysts were identified by XRD (Fig. 1 ). All the samples show the characteristic diffraction peaks at 2θ = 43.3o, 50.4o and 74.1o, corresponding to the (110), (200), and (220) crystal planes of Cu, respectively. The XRD results suggest that the Cu2+ is reduced to metallic Cu during the fabrication of xNiCu/C. It is worth noting that the typical peaks of Ni or NiO have not been detected in xNiCu/C samples. It might be explained by the low loading of Ni or the coverage of the characteristic diffraction peak of Cu particle. In addition, the diffraction peak of copper does not shift significantly when the content of Ni increases, verifying that the Ni or CuNi alloy has little effect on the crystal form of copper.The HRTEM images of Cu/C and 0.5NiCu/C are presented in Fig. 2 . Virtually, it is feasible to observe that the particle size and morphology of copper and nickel particle are distinguishable. The (110), (200) and (220) crystal planes of copper can be found from the HRTEM pattern of Cu/C catalyst (Fig. 2a), as confirmed by the results of XRD. In Fig. 2b, the Ni particles with sizes of 2–5 nm are uniformly dispersed on the 0.5NiCu/C catalyst and the presence of nickel has a negligible effect on the morphology of copper.The valence states of metal phases in xNiCu/C with different nickel loading were determined by XPS (Fig. 3 ). For Ni 2p doublets, there is an absence of the characteristic peaks of Ni0 and Ni2+ when the Ni loading is below 1.0%. However, with the increase of Ni amount, the Ni 2p spectra are successfully fitted to Ni0 and Ni2+. Ni 2p3/2 (∼855.4 eV) with its associated satellite (∼861.3 eV) as well as Ni 2p1/2 (873.1 eV) and the related satellite (∼879.3 eV) are observed for NiO. The doublets at around 852.3 eV (2p3/2) and 868.4 eV (2p1/2) are ascribed to Ni0 species. Additionally, the intensity of the typical peaks of Ni 2p also enhance with the Ni amount. This suggests that more Ni amount cannot be completely reduced. Similar to the XPS spectra of Ni, the Cu 2p spectra are fitted into four peaks, representative of Cu0 (932.5 eV, 2p3/2; 952.4 eV, 2p1/2) and Cu2+ (934.9 eV, 2p3/2; 953.4 eV, 2p1/2). Contrary to the CuO characteristic peak, the intensity of the Cu0 peak has gotten an elevation with the increase of nickel loading, demonstrating the promotion of nickel in the reduction of Cu2+. According to the above results, the existence of Ni0 and Cu0 phase accounts for an efficient reduction of xNiCu/C during the reduction‑carbonation process.Because of the surface acidity of catalyst as a crucial factor for the hydrogenation of FF, these directional parameters of the xNiCu/C were determined by the TPD method based on the various desorption temperatures of NH3 on different acid sites. From the entire curves of NH3-TPD profiles (Fig. 4 ), three NH3 desorption peaks are observed within the ranges of 150–250 °C, 300–500 °C and 600–800 °C respectively, which implies that all catalysts have similar acid sites [35]. The three peaks are assigned to NH3 desorbed from weak, medium and strong acid sites [36]. In comparison to the other catalysts, the highest total acid content (0.971 mmol/g) is the Ni free catalyst (Cu/C). According to the literature, Clemens et al. studied a larger number of copper loadings and the first two peaks was assigned to different unspecified types of Cu sites on this basis. As the increase of Ni, the contents of weak and medium sites decrease strikingly. Lezcano-Gonzalez et al. attributed the intermediate peak to NH3 adsorbed over Cu2+ sites [35]. For strong acid sites, its peak intensity remains constant or even slightly decreased, further proving that the acid sites of the xNiCu/C catalyst mainly originates from CuO [37].When the deionized water and isopropyl alcohol were chosen as the reaction media, FF hydrogenation experiments to 2-MF over 0.5NiCu/C were conducted at diverse reaction parameters for optimization.The effects of initial H2 pressure on FF conversion are shown in Fig. 5a (220 °C, 5 h). When H2 pressure is at 1 MPa, the conversion of FF lies at around 56% and merely 40% 2-MF could be collected eventually. The highest 2-MF yield is 97.5% at 4 MPa H2 pressure with 100% FF conversion. The result data confirms that reducing H2 pressure properly is beneficial to modulate the hydrogenation capability without causing the appearance of other by-products. When H2 pressure is 5 MPa, over‑hydrogenation process leads to the formation of 2-MTHF. Therefore, there is a decline about the 2-MF yield.The influence of reaction time (220 °C, 4 MPa) during the conversion was also investigated (Fig. 5b). In the range of 5 h, FF conversion and 2-MF yield grow linearly with the reaction time and the hydrogenation about FF is further developed. However, when the reaction time is prolonged persistently, 2-MF is gradually converted to 2-MTHF. The preference toward 2-MTHF is discovered, thus leading to a descending trend of 2-MF yield.As shown in Fig. 5c (5 h, 4 MPa), the hydrogenation reaction of FF is also sensitive to the reaction temperature. Around 70% of FF was converted at 140 °C with 44.1% 2-MF obtained. Meanwhile, the other liquid products include FA (18.9%), THFA (6.8%), 2-MTHF (1.1%). The conversion of FF is rapidly increased when the reaction temperature is elevated from 140 °C to 180 °C. In this case, the temperature of 220 °C is appropriate for the formation of 2-MF, resulting in the maximum yield (97.5%). However, the trend of 2-MF yield is in the opposite direction during the temperature range of 220 °C to 260 °C. The typical FF hydrogenation is an exothermic reaction and its thermodynamically unfavorable properties promotes the hydrogenation of CC bonds in the furan ring at high reaction temperature, thus generating the by-product 2-MTHF. And the yield of 2-MTHF elevates from 1.9% (220 °C) to 25.9% (260 °C).The recyclability experiments of the 0.5NiCu/C was carried out (Fig. 6 ). A slightly decline in the conversion of FF and the yield of 2-MF is observed after multiple circulation. In the fifth reaction, the yield of 2-MF is still maintained at the higher level of 85%. The structure and acid properties of the used catalyst were analyzed by XRD, XPS, HRTEM and NH3-TPD. As shown in Fig. 7a, compared with the fresh catalyst, the location of (110), (200) and (220) crystal planes of Cu remain invariant but the intensity of different diffraction peaks further strengthens after reaction. No significant change is found in Cu 2p spectra of fresh and used catalysts, showing the valence stability of surface copper species (Fig. 7b). Noticeably, the result of NH3-TPD (Fig. 7c) reveals that the total acid sites amount of the catalyst after hydrogenation apparently drop, which might be responsible for the decrease of 2-MF yield. Besides, according to the HRTEM images (Fig. 7d), the retention of immutable nickel and copper species reasonably explain the high catalytic activity of NiCu/C.The tandem conversion from xylose to 2-MF over 0.5NiCu/C catalyst and Hβ zeolite was tested and the consequences were listed in Table 3 . For one pot conversion of xylose, using Hβ zeolite and 0.5NiCu/C as catalyst, the yield of 2-MF is only 45.7% with 64.2% xylose conversion at 140 °C (Entry 1, Table 3). Elevating the reaction temperature to 220 °C, more xylose is converted (100%). But the selectivity of 2-MF decreases and its yield is only 24.7% (Entry 2, Table 3). This verifies that xylose dehydration and FF hydrogenation correspond to different optimum temperatures and no single temperature is suitable for the whole catalytic process. Therefore, the conversion process following the cascade strategy is divided into two stages at their respective optimized temperature for 5 h (Entry 3, Table 3). In comparison to single temperature, the yield of 2-MF is elevated to 67.2%. The problem about rate-limiting step under different reaction temperature is solved. Then, the process is changed to two-step tandem reaction. Xylose is converted to FF with a yield of 99% over Hβ zeolite, and the obtained liquid product can be separated by distillation. Tandem method applied xylose conversion improves the 2-MF yield rate by 41.5% and 95.1% 2-MF yield is obtained eventually (Entry 4, Table 3). To understand the catalysis of Hβ zeolite and 0.5NiCu/C, single catalyst experiments were performed (Entry 5 and 6, Table 3). Using Hβ zeolite or 0.5NiCu/C as catalyst alone, the yield of 2-MF is relatively low (4.2% and 1.4%). It can be inferred from these results that Hβ zeolite has more acid sites compared to 0.5NiCu/C catalyst, which is beneficial for xylose conversion. But Hβ zeolite lacks the hydrogenation active sites and the generated FF fails to convert to 2-MF over Hβ zeolite. For 0.5NiCu/C, the opposite is true. Hence, we can conclude that, in the whole tandem conversion, xylose is converted to FF under Hβ zeolite at 140 °C for 5 h firstly, followed by FF hydrogenation to FA over 0.5NiCu/C at 220 °C for 5 h. Hβ zeolite and 0.5NiCu/C realize the sequential catalysis of the corresponding process through tandem strategy, bringing the obvious enhancement of the ultimate increase in 2-MF yield.A tandem strategy that combined acid dehydration catalyst and HDO catalyst to convert xylose to 2-MF was developed in a continuous reaction process. Effective HDO of furfural via NiCu/C under isopropyl alcohol/water was achieved. Compared with NiCu/C catalyst, Hβ zeolite provides sufficient acid sites used for xylose hydrolysis. Therefore, 0.5NiCu/C catalysts assisted with Hβ zeolite promotes tandem conversion of xylose. 2-MF yield from tandem utilization increases by 41.5% as compared with those from two-step reaction. In this work, the exploitation of tandem dehydration and HDO provides new ideas for one-step conversion of monosaccharide to 2-MF. Hao Li: Methodology, Investigation, Writing – original draft, Writing – review & editing. Huimin Liu: Methodology, Validation. Chiliu Cai: Investigation, Validation. Haiyong Wang: Conceptualization, Methodology, Investigation, Funding acquisition, Writing – review & editing. Youwang Huang: Formal analysis, Data curation. Song Li: Resources. Bin Yang: Project administration. Chenguang Wang: Resources, Supervision. Yuhe Liao: Supervision, Validation, Formal analysis. Longlong Ma: Supervision, Project administration, Writing – review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work is financially supported by the National Natural Science Foundation of China (52006225, 52236010, 52006228, 52206288), and R&D Plan of Key Fields in Guangdong Province (2020B1111570001), Natural Science Foundation of Guangdong Province (2018A030310135). Supplementary material Image 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2023.106625.

Nowadays, 2-methylfuran is urgently needed especially in pharmaceutical manufacture. However, the direct synthesis of 2-methylfuran via the efficient dehydration and hydrogenation of xylose usually requires a multistep reaction. Herein, a route of tandem conversion xylose to 2-methylfuran was developed. The xylose was firstly converted into furfural by Hβ zeolite, followed by the catalysis of furfural to 2-methylfuran over 0.5NiCu/C. Compared with two-step reaction, cascade strategy applied xylose conversion improved the 2-methylfuran yield rate by 41.5%. In general, this work expanded the application of tandem method for the enhancement of xylose effective conversion and provided a new biomass utilization technology.

With the depletion of conventional energy sources and the growing need for energy in society, coupled with the challenge to ameliorate climate change, the development and improvement of safe, renewable, and low-cost clean energy technologies are necessary. The development of fuel cells has attracted significant attention recently owing to their promising potential to provide a clean and possibly sustainable energy generation [1,2]. Currently, the Proton Exchange Membrane (PEM) fuel cell is highly developed, however, there are many inherent safety and practical problems such as the production, storage, and distribution of hydrogen that still present strong limitations for industrial-scale applications [3–6]. In recent years, the direct hydrazine fuel cell (DHFC) has gained significant research interest because of the high power density and therefore has promising application for the automotive industry [7–9]. DHFC is a carbon-free process and has the potential to output a power generation performance of 0.5 W cm−2 and a cell voltage of 1.56 V, which is comparable to the PEM fuel cell [10,11]. However, its full implementation is hindered by the lack of highly efficient, stable and inexpensive catalysts. Other suggestions for using hydrazine towards an environmentally friendly economy include its potential as a hydrogen storage material, via its decomposition pathway: N 2 H 4 → 2 H 2 + N 2 This is a competitive pathway with the decomposition to ammonia – with the route depending upon whether the NN or NH bonds of hydrazine are broken first [12].Ni-based catalysts - non-noble metal alternatives to platinum have been shown to be active for each of these processes. Hydrazine decomposition has been shown on Ni-nanofibres when supported on carbon nanotubes by Ding, Lin, and Guo - achieving 100% selectivity towards hydrogen [13]. Researchers at the Dihastu Motor Company have also shown that carbon supported-Ni was active for the DHFC, showing good selectivity and low amounts of ammonia production [14]. Unfortunately, Ni-nanoparticles suffer from atom run-off, hence dramatically lowering the activity of the monometallic catalysts [15]. Bimetallic catalysts can be formed to overcome these issues, as Ni has an innate ability to produce alloyed systems with other transition metals [16]. Ni-based bimetallic heterogeneous catalysts often show new and improved activity and stability towards their respective reactions - such as Ni-Rh [17], Ni-Ir, [18,19] or Ni-Pt [20–22] - which show marked improved activity for hydrazine decomposition. However, avoiding expensive and less abundant noble metals is necessary if we are to achieve widespread, sustainable commercialisation of DHFC technology. Promising non-noble bimetallic catalysts for DHFC’s include alloys with other first row transition metals, such as Ni-Cu, [23] Ni-Fe, [24] and particularly Ni-Zn [25,26]. Recently, Feng et al. [27] synthesised an Ni-Zn catalyst combined with a reduced graphene oxide layer, finding this gave almost 100% selectivity towards hydrazine electro-oxidation in the DHFC. The catalyst also showed good stability over time, hence improving substantially on the monometallic Ni-catalyst. Atanassov et al. [11] have also found NiZn catalysts to be active for DHFC’s when supported on Ketjenblack, a carbon-based support material. As a result, the surface area of the catalyst was increased, leading to higher contact times with hydrazine, thereby increasing the activity.Hydrazine adsorption onto the bimetallic Ni-Zn catalyst surface would precede any reactions that take place inside the DHFC or any decomposition routes, as such, understanding of the strength and features of adsorption of hydrazine onto the catalyst surface is essential for the successful development of efficient bimetallic Ni-Zn catalysts in these areas. The fundamental aspects of hydrazine adsorption, including the initial adsorption geometries, adsorption energies, structural parameters, and electronic properties, are deemed vital for the rational design of improved Ni-Zn catalysts. Detailed information is, however, difficult to obtain directly from experiments and the underlying physical driving forces that control the reactivity of hydrazine with the bimetallic Ni-Zn surfaces remain not fully understood. First-principles density functional theory (DFT) calculations provide an alternative way to gain fundamental insight, as it is capable of accurately predicting lowest-energy adsorption geometries and identifying charge transfer and further electronic effects [28–31]. DFT-based calculations have been employed extensively to predict the adsorption geometries of hydrazine on metallic surfaces and offers good insight into catalyst activity [24,32,33]. Previous DFT work has been done on the Ni-Zn alloy, although the specific factors behind how hydrazine adsorbs are yet unknown [33].In the present study, dispersion-corrected DFT-D3 calculations are employed to comprehensively investigate the adsorption properties of hydrazine on the bimetallic β1-NiZn alloy catalyst (100), (110), and (111) surfaces. Insights into the synergistic beneficial effects of Ni-Zn alloying was derived by drawing a comparison between the hydrazine adsorption energetics and mechanisms on the bimetallic NiZn surfaces to the monometallic Ni(111) and Zn(001) surfaces. The energetics and structural parameters of the lowest-energy adsorption configurations of the hydrazine are presented and a d-band model was developed to gain insight into the differences in reactivity of the bimetallic catalyst compared to the monometallic counterparts. Differential charge density iso-surface contour and projected density of states analyses were carried out to gain further atomic-level insights into the hydrazine adsorption mechanism.The optimized surface and adsorption structures were determined using the plane-wave-based DFT method, implemented in the Vienna Ab-Initio Simulation Package (VASP) [34–36]. The interactions between the valence electrons and the ionic core were described with the projected augmented wave (PAW) method [37,38]. The electronic exchange–correlation potential was treated using the Perdew-Burke-Ernzerhof (PBE) functional [39]. A high energy cut-off of 600 eV was used for the plane-wave basis sets, with a convergence criterion set to 10−6 eV between two ionic steps for the self-consistency process. The Brillouin zone was sampled with a Monkhorst-Pack [40] k-point grid of 5 × 5 × 5 for bulk Ni, Zn, and NiZn, while for geometry optimisation of cleaved surfaces, a k-point grid of 3 × 3 × 1 was used.The low miller index surfaces of β1-NiZn were created from the relaxed bulk material using the METADISE code [41], which ensures the creation of surfaces with zero dipole moment perpendicular to the surface plane. The surfaces were modelled using the slab model and for each surface a slab thickness of at least 10 Å was increased until convergence of the surface energy was achieved within 1 meV per cell. In each simulation cell, a vacuum region of 15 Å was tested to be sufficient to avoid interactions between periodic slabs in the z-direction. From the full geometry relaxation of each surface we have calculated the surface energy (γ), which is the energy required to cleave an infinite crystal in two along a given crystallographic plane using the relation: γ = E surface - n E bulk 2 A where E surface is the energy of the naked surface, n is the number of repeating unit cells in the z-direction, E bulk is the energy of the bulk system, A is the surface area of the relaxed system - where the factor of 2 reflects the fact that there are two surfaces for each slab with identical atomic ordering at the bottom and top layers.The hydrazine adsorption calculations were carried out on a 3 × 3 supercell of the bimetallic NiZn (111), (110) and (100) surfaces, which are large enough to minimize the lateral interactions between the hydrazine molecules in neighbouring image cells. The structural optimizations of Ni-Zn systems were carried out without any symmetry constraint and the hydrazine molecule was free to move away laterally and vertically from the initial binding site or reorient itself to find the minimum energy adsorption structure. To determine the optimum adsorption sites and geometries, the hydrazine molecule and the topmost three layers of each surface slab are allowed to relax unconstrainedly until residual forces on all atoms had reached 0.03 eV Å−1. Van der Waals dispersion forces were accounted for by utilising the Grimme DFT-D3 functional [42], which adds a semi-empirical dispersion correction to the conventional Kohn-Sham DFT method. This is important because standard DFT calculations fail to provide an accurate description of the asymptotic decreasing behaviour of the long-range vdW interactions that are ubiquitous in hybrid inorganic/organic systems [43–45]. Previous studies have shown that the inclusion the dispersion correction have led to proper description of the hydrazine adsorption structures and energetics on metallic Ni and Cu surfaces [32,46,47]. To quantify the hydrazine adsorption strength on the NiZn surfaces, the adsorption energy (Eads) was calculated using the following equation: E ads = E System PBE + D 3 - ( E Surface PBE + D 3 + E Adsorbate PBE + D 3 ) where E System PBE + D 3 is the energy of the adsorbed hydrazine to the catalyst slab, E Surface PBE + D 3 is the energy of the naked surface and E Adsorbate PBE + D 3 is the energy of the adsorbate in the gas phase. Therefore, a negative E ads value indicates an exothermic, favourable adsorption, whereas a positive value indicates an endothermic, and less favourable adsorption.The d-band centre (E d) was calculated for each surface using the relation: E d = ∫ - ∞ ∞ E · D E d E ∫ - ∞ ∞ D E d E where Ed is the d-band centre and D(E) are the density of states (DOS) for the surface atoms. The d-band centre is a useful descriptor as the Fermi level of transition metals originates primarily from the d-orbitals. Hybridisation between surface orbitals and adsorbates forms bonding and anti-bonding states. A higher d-band centre is therefore associated with higher energy anti-bonding states and hence, stronger adsorption.The bulk β1-NiZn was modelled in the tetragonal crystal structure, as shown in Fig. 1 a. The fully optimized lattice parameters are predicted at a = b = 2.686 Å, and c = 3.262 Å in good agreement with previous experimental results of a = b = 2.75 Å and c = 3.21 Å [48]. The partial density of states (PDOS) shown in Fig. 1b reveals the metallic conductivity of the alloy, with the Ni-d orbitals dominating states surrounding the fermi level, in agreement with earlier DFT results [33]. From the optimized bulk NiZn material, the (100), (110), and (111) surfaces were created and fully relaxed in order to determine their relative stabilities (Fig. 2 ). The surface energies of the (100), (110), (111) surfaces are calculated at 2.06, 1.75, and 2.53 J m−2 respectively, which indicates that the order of decreasing stability is (110) < (100) < (111). The differences in the stabilities can be attributed to differences in the surface terminations and the coordination numbers of the topmost surface atoms. The (110) surface is terminated by Ni:Zn in a 1:1 ratio, with the Ni and Zn atoms in an 8-fold coordination number (CN). The (111) surface is also terminated by Ni and Zn, but in a 1:2 ratio, with the surface Ni atoms having coordination number of 6. The (100) surface on the other hand is terminated by a Zn-ad-atom having 5-fold coordination, with accessible Ni-sites below with higher coordination. Based on the calculated surface energies, we have simulated the equilibrium crystal morphology of the NiZn nanoparticle using Wulff construction [49–51]. As shown in Fig. 3 , all three surfaces are expressed in the NiZn nanoparticle with the (110) covering the largest area, in line with it being the most thermodynamically stable.Prior to investigation the adsorption of hydrazine on the bimetallic NiZn surfaces, the adsorption process has been systematically characterized on the monometallic Ni(111) and Zn(001) surfaces for comparison. Hydrazine in the gas phase has been found to adopt the gauche formation [52]. This is due to the hyper conjugate mechanism, which minimises repulsion between the lone pairs of each nitrogen by rotation about the NN axis [53]. Consequently, the relative energies of the eclipsed and trans geometries are higher than gauche in the gas phase [54].Shown in Fig. 4 a-c are the lowest-energy gauche, trans, and eclipsed adsorption configurations of hydrazine on the monometallic Ni(111) surface, with the characteristic binding energies and structural parameters given in Table 1 . The gauche, trans, and eclipsed binding configurations released adsorption energies of −1.62, −1.65, and −1.97 eV, respectively. This indicates that the eclipsed binding mode, wherein both N atoms interact with adjacent Ni sites, is the most stable binding geometry on the Ni(111) surface. The surface Ni-N bond distances in the most stable eclipsed geometry are calculated at 1.996 and 2.011 Å, with hydrazine’s NN bond converged at 1.440 Å. For the monodentate trans and gauche adsorption configurations, the interacting Ni-N bond distance is calculated at 1.965 and 2.012 Å, respectively, whereas the NN bonds are converged at 1.457 and 1.441 Å. Bader population analysis revealed that the hydrazine molecule is oxidised to only a small extent, characterized by loss of charge to the interacting surface species. The hydrazine molecule lost 0.108, 0.171, and 0.178 e− to the surface when adsorbed in the gauche, trans, and eclipsed configurations, respectively. Although small, the loss in electron density resulted in structural modification (internal rotation) of the hydrazine molecule with the dihedral angle reduced to 37.1° in the eclipsed adsorption geometry, compared to the gas phase gauche conformer dihedral angle of 91.0°. This disturbance of the hyper conjugate mechanism has also been observed on several other metallic surfaces, such as hydrazine adsorption to Pt(111) [53]. The NN distance of 1.440 Å also shows a minute change from the gas phase species, 1.441 Å, hence presenting suitability towards the DHFC and decomposition mechanisms by reducing ammonia formation through the NN bond cleavage mechanism and favouring N2 production [18,55].Compared to the Ni(111) surface, the gauche, trans and eclipsed adsorption configurations of hydrazine on the monometallic Zn(001) surface (Fig. 4d-f) released lower adsorption energies of −1.15, −0.94, and −0.49 eV, respectively, indicating that the Ni(111) surface is more reactive towards hydrazine adsorption than the Zn(001) surface. In the most stable gauche geometry, the Zn-N adsorbate–surface bonds and NN hydrazine bonds are predicted at 2.196 and 1.428 Å, respectively. In the trans and eclipsed adsorption configurations the interacting Zn-N bond distance is predicted at 2.142 and 2.267 Å, respectively, whereas the NN bond length is converged to 1.461 and 1.453 Å. Similar to the Ni(111), the hydrazine is only slightly oxidized upon adsorption on the Zn(001) surface: the hydrazine molecule lost a charge of 0.047, 0.062, and 0.086 e− to the interacting surface species when adsorbed in the gauche, trans, and eclipsed configurations, respectively. The dihedral angle is predicted at 98.8, 179.9, and 26.3° for the gauche, trans and eclipsed adsorption configurations at the Zn(001) surface. The smaller changes in the dihedral angles, along with the extremely small Bader charge transfers indicate only minor disruption in the hyper conjugate mechanism. Considering that stronger adsorption is correlated with high activity [33], the weaker hydrazine adsorption on the Zn(001) compared the Ni(111) surface suggest that the monometallic Zn(001) may be inappropriate for the DHFC applications. This is consistent with the work of Wang et al. [25], who found Zn-film to be inactive for the DHFC.As for the monometallic Ni and Zn catalysts, hydrazine has been adsorbed onto the bimetallic NiZn (100), (110), and (111) surfaces in the gauche, trans, and eclipsed conformations. The lowest-energy adsorption geometries are displayed in Fig. 5 and the optimized structural parameters are summarized in Table 2 . For the NiZn(111)-hydrazine interactions, the eclipsed geometry (Fig. 5a) has been found to be the most stable, with a highly exothermic adsorption energy of −2.71 eV. In the eclipsed structure, hydrazine is bound to adjacent Ni and Zn sites, with the NiN and ZnN bond distances calculated at 2.032 and 2.136 Å, respectively. As shown by the dihedral angle, the hydrazine NH2 units are not fully aligned to 0°, as for gas-phase eclipsed, but to 35.0°, indicating that hydrazine has not been fully oxidised and some repulsion between the two N-lone pairs remains. Consistent with this, our Bader population analysis shows a lesser extent of oxidation (Δq(N2H4) = 0.128 e−) compared to the eclipsed geometry on the monometallic Ni(111) surface (Δq(N2H4) = 0.178 e−). Although these differences are small, it is enough to favour the eclipsed formation. The high binding energy is attributed to the low-coordinated Ni atoms present on the surface (CN = 6), strengthening the surface-hydrazine interactions. The second most stable hydrazine binding geometry on the NiZn(111) surface is a monodentate trans configuration (Fig. 5b), also releasing a high adsorption energy of −2.25 eV. In the adsorbed trans geometry, the dihedral angle calculated at 173.4° is relatively close to that of the gas phase trans configuration (180.0°). The NN bond is somewhat activated, as reflected in the small increase in the NN distance from 1.468 to 1.491 Å. The gauche adsorption configuration on the NiZn(111) surface (Fig. 5c) released the lowest adsorption energy, calculated at −1.99 eV. In the gauche configuration, hydrazine adsorbs atop a single Ni-site with the NiN, NN bond lengths, and dihedral angle predicted at 1.960 Å, 1.450 Å, and 106.4°, respectively. The generally stronger binding of hydrazine to the bimetallic NiZn(111) surface compared to the monometallic Ni(111) and Zn(100) surfaces indicates that the Ni-Zn alloy presents a more active site for hydrazine activation, which results from the synergistic effects between the two metals.For the NiZn(110)-hydrazine interactions (Fig. 5d-f), the preferred adsorption configuration is predicted to be the eclipsed geometry, wherein the hydrazine binds via both N atoms at adjacent Ni and Zn sites (Fig. 5d), releasing an adsorption energy of −2.08 eV. The NiN, ZnN, and NN distances are calculated at 2.016, 2.179, and 1.472 Å, respectively. The dihedral angle is predicted to be closer to zero (θ = 8.7°) than atop the NiZn(111) surface, where the dihedral angle is predicted at 35.0°. This may partly be attributed to the differences in the surface structure as the NiZn(110) surface has a flat topology, with the NN axis contorted to allow adsorption to adjacent surface sites. However, the coordination number of the surface Ni and Zn both stand at 8 on NiZn(110), compared to 6 and 7, respectively, on the NiZn(111) surface. Hence, a lower adsorption energy is observed for NiZn(110) as shown from bond order conservation theory [56]. Compared to the most stable bidentate eclipsed geometry, the monodentate trans and gauche configurations at Ni top sites on the NiZn(110) surface released adsorption energies of −1.89 and −1.71 eV, respectively. The NiN and NN distances are calculated at 2.000 and 1.464 Å, for the trans geometry and at 2.022 and 1.448 Å the for gauche geometry. Minor oxidation occurs in hydrazine for both the trans and gauche conformations and leads to an adsorption structure similar to that of the respective gas-phase species. This is shown by the dihedral angle of 177.1 and 104.0° for the trans and gauche systems.The bimetallic NiZn(100) surface is terminated in Zn ad-atoms and hence exhibits adsorption trends akin to the monometallic Zn(001) surface. Low-coordinated Zn sites dominate the NiZn(100) surface (CN = 5) and as a result the gauche adsorption has been found to be the most stable (Fig. 5g). The bidentate gauche adsorption geometry released an adsorption energy of −2.11 eV, with the Ni-N and Zn-N distances converged at 2.024 and 2.342 Å, respectively. Although the gauche adsorption is favoured here, as for Zn(001), the adsorption energy and interaction is much stronger, owing to the synergistic effect between Ni and Zn in the alloy. The geometry has been ascribed to be gauche, however, the dihedral angle of 63.4°, shows it lies between an eclipsed and gauche formation. The top-most NH2 unit has rotated in order to align its lone pair with the low-coordinated Zn atoms, hence stabilising the system. This is consistent with the inability of Zn sites to bind to hydrazine efficiently. The trans (Fig. 5h) and eclipsed (Fig. 5i) configurations released adsorption energies of −2.10 and −2.00 eV, respectively. In both geometries, the hydrazine molecule is only marginally oxidized as reflected in the Bader charges reported in Table 2. The less stable hydrazine adsorption structures and energetics at Zn-sites on the (111) and (110) NiZn surfaces are shown in Supporting Information Fig. S1 and Table S1.Modification of the d-band centre resulting from the synergistic effects between the two metals is another origin of the differences in reactivity of the bimetallic NiZn surfaces compared to the monometallic. Since the adsorbate interaction to the metal surface occurs via the N-lone pair (N-p) and surface metal d-states, the bonding interaction creates a set of bonding and anti-bonding states. The energetic level of these is then defined by the d-band centre as the anti-bonding states are less occupied when the d-band centre is closer to the Fermi level due to their resulting higher energy [57]. To probe the electronic effects of NiZn surface reactivity, the d-band centres (E d ) for each surface Ni and Zn sites have been analysed [56–58]. A plot of the calculated E d values for the Ni(111), Zn(001), NiZn(111), (110) and (100) surfaces is shown schematically in Fig. 6 a. The d-band centre projected on Ni-atoms of Ni(111) and Zn-atoms of Zn(001) surface is predicted at −1.65 eV and −7.14 eV, respectively (Fig. 6b and c). The weaker binding of hydrazine on Zn(001) compared to Ni(111) can thus be attributed to the higher d-band centre and formation of higher energy antibonding orbitals for Ni(111), causing them to be less filled [59]. Compared to the monometallic surfaces, Ed of Ni and Zn atoms of the bimetallic NiZn surfaces has shifted closer to the Fermi level from each respective monometallic counterpart (Fig. 6d-f). The Ni d-band shifts from −1.65 eV for Ni(111) to −0.91, 1.13, and −0.99 eV for NiZn (111), (110) and (100), respectively. Similarly, the Zn d-band centre shifts from −7.14 eV of the Zn(001) to −6.51, −6.74 and −6.60 eV for the NiZn (111), (110) and (100) surfaces, respectively. The shift in the d-band centre closer to the Fermi level in the bimetallic NiZn surfaces is consistent with the stronger binding energy observed on the NiZn surfaces compared to the monometallic surfaces [60].These results suggest that the combination of two weakly active metals (Ni and Zn) gives a highly active bimetallic NiZn catalyst for hydrazine adsorption and activation towards direct hydrazine fuel cell applications. The improved activity of the bimetallic NiZn catalyst may, therefore, be attributed to the beneficial synergistic effects derived from the composition and electronic structure modulation. Further insights into the hydrazine adsorption process on the bimetallic NiZn catalyst was ascertained through the projected density of states (PDOS) and differential charge density isosurface analyses. As shown in Fig. 7 a-c, the chemisorption of hydrazine on the bimetallic NiZn catalyst is found to be characterized by strong hybridization between the d-orbitals of interacting surface Ni and Zn sites and on the N p-orbitals of hydrazine. Consistent with chemisorption, we observed electron density accumulation within the Ni-N and Zn-N bonding regions as shown in Fig. 7d-f. Considering that the charge transfers between hydrazine and the surface is small and that there is no clear trend between charge transfers and the calculated adsorption energies, the differences in the hydrazine adsorption strength to the different surfaces and may also be attributed to the differences in the coordination numbers of the surface Ni and Zn atoms. This invariably dictates adsorption environment at each active site, whereby the low-coordinated Ni sites on the NiZn (111) surface enables the strongest hydrazine adsorption.In summary, we have performed a comprehensive first-principles dispersion-corrected DFT investigation of hydrazine adsorption on bimetallic NiZn (111), (110) (100) surfaces and compared the results to the monometallic Ni(111) and Zn(100) surfaces. The synergistic beneficial effects derived from surface composition and electronic structure modification with Ni and Zn alloying gave rise to more reactive surface sites that bind hydrazine more strongly than the single-component nickel and zinc metal surfaces. It is found that the Ni-terminated (111) and (110) NiZn surfaces preferentially bind the hydrazine molecule in a bidentate eclipsed geometry, compared to the Zn-terminated (100) surface where binding a bidentate gauche adsorption geometry is favoured. The stronger adsorption of hydrazine on the bimetallic NiZn nanocatalyst than the monometallic is shown to be characterised by stronger hybridisation between the d-orbitals of the interacting surface sites and the N p-orbitals of the hydrazine, which is corroborated by the observed shift in the d-band centre closer to the Fermi level. These results should provide new possibilities for the design and development of Ni-Zn alloy catalysts with improved activity and selectivity of hydrazine electro-oxidation in the DHFC.R.W.C is responsible for data curation, formal analysis, and writing of original draft. S.R.R performed writing - review and editing. N.Y.D is responsible for funding acquisition, project conceptualization, administration, supervision, and writing - review and editing of manuscript.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research was funded by the UK’s Engineering and Physical Sciences Research Council (EPSRC), grant number EP/S001395/1. R.W.C. acknowledges the College of Physical Sciences and Engineering, Cardiff University for studentship. We also acknowledge the used of computational facilities of the Advanced Research Computing at Cardiff (ARCCA) Division, Cardiff University, and HPC Wales. Information on the data that underpins the results presented here, including how to access them, can be found in the Cardiff University data catalogue at http://doi.org/10.17035/d.2020.0115779666.Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc.2020.147648.The following are the Supplementary data to this article: Supplementary data 1

We present a systematic first-principles density functional theory study with dispersion corrections (DFT-D3) of hydrazine adsorption on the experimentally observed (111), (110) and (100) surfaces of the binary β1-NiZn alloy. A direct comparison has been drawn between the bimetallic and monometallic Ni and Zn counterparts to understand the synergistic effect of alloy formation. The hydrazine adsorption mechanism has been characterised through adsorption energies, Bader charges, the d-band centre model, and the coordination number of the active site - which is found to dictate the strength of the adsorbate–surface interaction. The bimetallic β1-NiZn nanocatalyst is found to exhibit higher activity towards adsorption and activation of hydrazine compared to the monometallic Ni and Zn counterparts. The Ni-sites of the bimetallic NiZn surfaces are found to be generally more reactive than Zn sites, which is suggested to be due to the higher d-band centre of −0.13 eV (closer to the Fermi level), forming higher energy anti-bonding states through NiN interactions. The observed synergistic effects derived from surface composition and electronic structure modification from Ni and Zn alloying should provide new possibilities for the rational design and development of low-cost bimetallic Ni-Zn alloy catalysts for direct hydrazine fuel cell (DHFC) applications.

With the wide use of fossil fuels, a substantial amount of CO2 emissions is emitted into the atmosphere, leading to severe environmental issues [1], such as sea level rise [2], ocean acidification [3] and extreme weather [4]. In order to reduce industrial carbon emissions, CO2 capture is one of the most promising techniques and has attached a lot of attention [5–8]. Currently, the most mature CO2 capture technologies are solvents adsorption (i.e. MEA) [9] and calcium looping [10–13], which are both operated by swinging temperature to regenerate the adsorbents. The obtained high concentration of CO2 could be stored by deep-sea injection or mineralisation [14], which is known as carbon capture and storage (CCS). However, the CCS requests a high capital investment and may not be a long-term sustainable solution due to the risk of the second release of stored CO2 [15]. Therefore, there is an increasing interest in the direct utilisation of the captured CO2, which is known as integrated CO2 capture and utilisation (ICCU).The ICCU process can eliminate the CO2 enrichment, storage and transportation steps by in-situ converting the captured CO2 [16] and isothermally producing valuable products (e.g. CH4 or CO [17–20]). In a typical ICCU process, CO2 is first adsorbed on adsorbents (i.e. CaO), and then a reducing agent (i.e. H2) is introduced to react with the captured CO2 with the assistance of active catalytic sites. Among the final products of the ICCU process, carbon monoxide (CO) is considered to be one of the most valuable chemicals to be used as the feedstock for the mature Fischer-Tropsch process to further produce liquid products [21–23]. The commonly used CO2 reduction technologies include thermal-catalysis, photo-catalysis, electro-catalysis, plasma-catalysis and etc. [24]. Thermal-catalyzed CO2 reduction is a relatively compromised choice in terms of high process efficiency and low input cost (e.g. equipment investment and materials costs). The reverse water–gas shift (RWGS) reaction, as shown in Eq. (1), is a promising process for thermal catalytic conversion of CO2 that uses renewable H2 to reduce carbon emissions and produce syngas. (1) CO 2 + H 2 = C O + H 2 O The bifunctional combined materials (BCMs), containing CO2 adsorbent and active catalytic sites, have been proven effective for the ICCU process [20,23,25–27]. Specifically, CaO is widely used as high temperature CO2 adsorbent [28,29], and Ni can act as catalytic sites for RWGS [30,31]. The previous study [32] has concluded that the physical mixing is a superior material preparation method by avoiding catalytic sites coverage due to CaO sintering. The supports of Ni-catalysts are widely believed to play key roles in the catalytic process owing to the metal dispersion, metal-support interaction, etc. [33–35]. However, as a novel process, ICCU provide CO2 in the form of carbonates, which possesses different chemical environment than traditional RWGS. Therefore, there is a gap on understanding of the effects of supports in ICCU, including the effects on CO2 adsorption and catalytic conversion.In this work, to investigate the support effects on ICCU-RWGS, two active supports (CeO2 and TiO2) and inert materials (ZrO2 and Al2O3) supported Ni are synthesised as catalysts, and a sol–gel prepared CaO as the adsorbent to prepare the BCMs. The sol–gel CaO has been proven a stable and excellent CO2 adsorbent in carbon capture [36]. The CeO2 and TiO2 have been widely applied to catalytic processes, including RWGS, CO2 methanation, dry reforming etc. [37,38]. Furthermore, the active TiO2 and inert Al2O3 could form spinel with Ni (strong metal-support interaction) [39,40], which provide valuable benchmarks for identifying active sites.ZrO2 (Sigma-Aldrich, 99%), TiO2 (Sigma-Aldrich, 99.5%) and Al2O3 (Sigma-Aldrich, 99.5%) were dried at 120 °C before the impregnation process. CeO2 was prepared by a hydrothermal method as reported in previous work [17,19]. Briefly, 5.21 g Ce(NO3)3·6H2O (Sigma-Aldrich, 99%) was dissolved in deionised water (30 ml) to prepare a Ce source solution, followed by the dissolution of 57.6 g NaOH (Sigma-Aldrich, 99%) in deionised water (210 ml) to prepare the precipitant. The Ce source was mixed with the precipitant dropwise for 30 mins at room temperature to obtain a slurry. The slurry was transferred into a stainless-steel autoclave and kept at 100 °C for 24 h. The precipitate was washed and separated by vacuum filtration using distilled water and ethanol to neutrality and dried at 120 °C overnight, to produce a yellow powder, labeled as CeO2. Ni-based catalysts were prepared by the wet impregnation method, using Ni(NO3)2·6H2O (Sigma-Aldrich, 97%) as the metal precursor. Typically, 3.0 g support material was added into 30 ml Ni(NO3)2 (0.15 mol L-1) aqueous solution, stirred at room temperature for 2 h and evaporated under stirring to produce a sample paste. The sample paste was dried at 120 °C overnight, and calcined at 800 °C for 5 h with a heating rate of 5 °C min−1, to produce NiO/ZrO2, NiO/TiO2, NiO/CeO2 and NiO/Al2O3, respectively. The NiO/supports were reduced at 550 °C for 2 h with a heating rate of 5 °C min−1 in 5% H2/N2, and labeled as Ni/ZrO2, Ni/TiO2, Ni/CeO2 and Ni/Al2O3, respectively.The sol–gel derived CaO was prepared as reported in previous work [20,32]. Briefly, 23.6 g Ca(NO3)2·4H2O (Sigma-Aldrich, 99%) and 19.2 g citric acid monohydrate (Sigma-Aldrich, 99.5%) were dissolved into 72 ml distilled water, stirred at room temperature at 80 °C, and dried at 120 °C overnight. The sample was ground and calcined at 850 °C for 5 h with a heating rate of 5 °C min−1, labeled as sol–gel CaO. The bifunctional combined materials were prepared by physically mixing the Ni/support catalysts and the sol–gel CaO with a mass ratio of 1:2, labeled as Ni/ZrO2-CaO, Ni/TiO2-CaO, Ni/CeO2-CaO and Ni/Al2O3-CaO, respectively.The loadings of Ni on various supports were measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES). The samples were digested in nitric acid and then analysed using a Perkin Elmer PE2400 CHNS. X-ray diffraction (XRD) patterns (2 Theta: 5°-75°) were measured by a PANalytical Empyrean Series 2 diffractometer with a Cu Ka X-ray source. The surface area and pore structure of the Ni/supports catalysts were characterised by an ASAP 3000 analyser at 77 K. The Brunauer-Emmett-Teller (BET) method and the desorption isotherm branch were applied to calculate the surface area and the pore size distribution, respectively. The X-ray photoelectron spectrum (XPS) analysis was performed on a Thermo Fisher Scientific NEXSA spectrometer. H2 temperature-programmed reduction (H2-TPR) of the NiO/support catalysts was tested on a Hi-Res TGA 2950 thermogravimetric analyser. Typically, the samples were pre-treated under N2 at 300 °C for 30 mins to remove the adsorbed H2O and gases, equilibrated to 50 °C and temperature programmed reduced in 100 ml min−1 5% H2/N2 (10 °C/min to 800 °C). The CO2 temperature-programmed desorption (CO2-TPD) patterns of the Ni/support catalysts were measured by a Micromeritics Autochem II 2920 analyser. The Ni/support catalysts were in-situ reduced at 550 °C in H2 for 1 h and then cooled down to 30 °C in He. After adsorbing CO2 at 30 °C in 10% CO2/He, the temperature was increased to 800 °C in He with a heating rate of 10 °C min−1. Scanning electron microscopy coupled with an energy dispersive X-ray spectrometer (SEM-EDX, FEI Quanta FEG) was used to characterise the morphology and element dispersion. Transmission electron microscopy (TEM, FEI Titan3 Themis 300) and high-angle annular dark-field transmission electron microscopy (HAADF-TEM) were utilised to observe the morphology and Ni particle size of Ni/support catalysts. The Ni particle size distribution was calculated from TEM observation.The ICCU-RWGS evaluations of Ni/support-CaO were carried out in a fixed-bed reactor and monitored by an online gas analyser (Kane Autoplus 5). The stainless-steel reaction tube (length: 500 mm, inner diameter: 64 mm) was placed in the middle of the tube furnace (Elite TSH-2416CG). Briefly, 0.30 g Ni/support-CaO bifunctional combined materials (BCMs) were placed in the middle of the reaction tube and fixed in place by quartz wool. Two thermocouples were placed in the reaction tube and inside the tube furnace to monitor the temperature of the BCMs and tube furnace, respectively.In a typical evaluation test, the BCMs were reduced at 550 °C in 5% H2/N2 for 2 h, and then the gas was switched to 100 ml min−1 20% CO2/N2 for ∼28 mins. 100 ml min−1 5% H2/N2 was then introduced for ∼28 mins for the RWGS. Then the flowing gas was switched to N2 and equilibrated the temperature for the following test. The baseline of carbonation and hydrogenation steps was monitored using 0.3 g SiO2 to eliminate the analyser signal delay. The CO2 conversion, CO yield and selectivity were calculated by integrating the real-time data collected from the online gas analyser from 0 s to 1700 s, referring to the equations below. (2) C C O 2 = ∫ 0 1700 ( C O + CH 4 ) ∫ 0 1700 ( C O + CH 4 + CO 2 ) ∗ % (3) S CO = ∫ 0 1700 C O ∫ 0 1700 ( C O + CH 4 ) ∗ % (4) Y CO = ∫ 0 1700 C O ( % ∗ s ) ∗ 1.667 ( m l / s ) 22.4 ( m l / m m o l ) ∗ 0.30 ( g ) CCO2, SCO and YCO represent to CO2 conversion (%), CO selectivity (%) and CO yield (mmol gmaterial -1).The elemental analysis of the Ni/support catalysts using inductively coupled plasma (ICP) showed that the Ni loadings on the various supports were controlled at 8 ± 1.5 wt% (Table 1 ). N2 isothermal adsorption–desorption was carried out to characterise the surface area and pore structure of the Ni/support materials (Fig. 1 ). As summarised in Table 1, the Ni/ZrO2 and Ni/TiO2 showed poor porosity with low BET surface areas (< 5 m2 g−1). In contrast, the Ni/CeO2 and Ni/Al2O3 were more porous and possessed typical type IV isotherms with distinct hysteresis loops. As shown in Fig. 1, the Ni/CeO2 and Ni/Al2O3 exhibited type H3 and H4 hysteresis loops with various pore size distribution. The pore size of Ni/CeO2 and Ni/Al2O3 are ∼10–40 nm and 3–10 nm, respectively. It can also be demonstrated from TEM observation (Fig. 2 i) that Ni/CeO2 could accumulate slit-shaped pores with regular rod-like morphology. The higher surface area and porous structure could contribute to the dispersion of metals and the diffusion of reactants in catalytic reactions.High-angle annular dark-field transmission electron microscopy (HAADF-TEM) was carried out to observe the dispersion of Ni species and the morphologies of the Ni/support catalysts. As shown in Fig. 2, all the Ni/support catalysts possessed gaussian distribution of Ni species size, following the decreasing order: Ni/TiO2 > Ni/ZrO2 > Ni/Al2O3 > Ni/CeO2. The Ni species size distribution and the average Ni size are presented in Fig. 2d, h, l, p. The CeO2 and Al2O3 can disperse Ni better than other supports, which is attributed to the morphology and porosity of supports. Specifically, the Ni/CeO2 possesses the optimal Ni dispersion with a ∼14.1 nm Ni species size.The X-ray diffraction patterns of the Ni/support materials are presented in Fig. 3 . Except for the amorphous Al2O3 supported Ni, all the other three samples showed distinct metallic Ni peaks (PDF#87-0712) and strong diffraction peaks which belong to the support materials, referring to ZrO2 (PDF#86-1451), TiO2 (PDF#75-1753) and CeO2 (PDF#78-0694). In addition, the Ni/TiO2 and Ni/Al2O3 catalysts exhibited NiTiO3 (PDF#76-0334) and NiAl2O4 (PDF#77-1877) spinel peaks, respectively. There were no distinct NiO peaks, indicating that NiO was fully reduced at 550 °C during the reduction process. The reducibility of spinel is much poorer than NiO species [41,42], which could be verified by the presence of significant spinel peaks after reduction.XPS was carried out, as shown in Fig. 4 , to further confirm the valence state and chemical environment of surface nickel species of the reduced Ni/support catalysts. The Ni 2p3/2 XPS profiles of Ni/supports exhibit common peaks at ∼852.1 eV and ∼855.8 eV, which are attributed to the metallic Ni and its satellite peak, respectively [43]. The air contact could quickly oxidise surface metallic Ni, resulting in the existence Ni2+ peaks on the reduced Ni/support catalysts [44]. The multi-split peaks at 853.5–855.5 eV and 857–861 eV could be assigned as Ni2+ signals from NiO and their satellite peaks. Notably, there is no distinct NiO peaks on XRD characterisations (Fig. 3), indicating the oxidation only limitedly occurs on the surface of Ni. It is believed that the support interacted Ni would possess higher binding energy [45], which can be located at 856.5 eV and 862.3 eV. Notably, the Ni/Al2O3 possessed the most abundant metal-support interaction, indicating that the NiAl2O4 dominates on the surface of Ni/Al2O3 [39]. As a comparison, the surface NiTiO3 could be effectively reduced and exhibited ∼34% Ni0 even after air oxidation. It is noted that NiTiO3 still dominate in bulk (Fig. 3), indicating that the Ni might parse out from NiTiO3 spinel and act as catalytic sites in ICCU. The Ni/CeO2 exhibits lower metallic Ni fraction (18%) on the surface, attributed to the highly dispersed Ni, which could be easily reacted with O2 in air.The reducibility of the Ni species over the prepared NiO/support catalysts was further investigated by H2-TPR, as shown in Fig. 5 a. There are two reduction peaks over Ni/ZrO2 and Ni/CeO2 at 380–430 °C and 450–480 °C, which could be assigned as weakly and strongly interacted NiO species, respectively [41,46]. It is noted that the NiO/CeO2 possessed the lowest reduction temperature, which is attributed to the smallest Ni particle size [47]. The spinel species (i.e. NiAl2O4 and NiTiO3) formed during the high temperature calcination are more difficult to reduce, which is consistent with the XRD analysis. Specifically, the NiTiO3 spinel can be reduced from ∼600 °C and showed only one reduction peak at ∼680 °C [41]. And there was no distinct reduction of Ni/Al2O3 in the TPR procedure [42]. The formation of spinel is believed to improve Ni dispersion [23],[48]; however, only the reducible Ni species are active catalytic sites.CO2-TPD profiles are presented in Fig. 5b to investigate the basicity of the Ni/support catalysts. Only Ni/CeO2 and Ni/Al2O3 possessed CO2 desorption peaks, which are attributed to their surface basic sites and superior porosity. The two desorption peaks of Ni/CeO2 and Ni/Al2O3 at temperatures of < 100 °C and between 150 and 250 °C are attributed to weak and intermediate CO2 chemisorption, respectively [49]. Ni/CeO2 showed a higher CO2 desorption temperature indicating that CeO2 could provide stronger basic sites, which are beneficial for CO2 adsorption and activation.The bifunctional combined materials (BCMs) for integrated CO2 capture and reverse water–gas shift reaction (ICCU-RWGS) in this work were prepared by physically mixing the Ni/support catalysts and the sol–gel prepared CaO. As shown in Fig. 6 a, 6b and 6c, the images of SEM and TEM were applied to exhibit the morphologies of sol–gel CaO and fresh Ni/CeO2-CaO BCM. The sol–gel CaO adsorbent possessed a sponge-like structure with abundant large pores, consistent with the BET characterisation (Fig. 1 and Table 1). The porous structure could improve the CaO accessibility and prevent the severe sintering of CaO due to volume expansion during the carbonation process [36]. As shown in Fig. 6b, 6c and 6d, the physical mixing method could disperse Ni/CeO2 into the sponge structure CaO, providing active sites near the adsorbent. In the previous research [32], the physical mixing method has been demonstrated to avoid the coverage of the catalytic sites caused by the volume expansion of CaO during the carbonation-hydrogenation ICCU process.The ICCU-RWGS performance using Ni/support-CaO bifunctional combined materials (BCMs) with various supports (e.g. ZrO2, TiO2, CeO2 and Al2O3) are shown in Fig. 7 . Here Ni particles were used as active catalytic sites, and CaO played as the CO2 adsorbent. A typical ICCU-RWGS process mainly included two steps, one was CO2 adsorption from a diluted CO2 source (e.g. 20% CO2/N2), and another was hydrogenation of the captured CO2. The above two steps were isothermally operated by switching the inlet gas between 20% CO2/N2 and 5% H2/N2. The SiO2/CaO was used as a blank benchmark material to demonstrate the ICCU-RWGS performance with bare inert support compared to active Ni metal over active supports. To reveal the optimal support for the ICCU-RWGS process, various Ni/support-CaO bifunctional combined materials were evaluated at 550–750 °C.As shown in Fig. 7a and 7c, the CO2 conversions and CO yields of various Ni/support-CaO BCMs followed the decreasing order: Ni/CeO2-CaO > Ni/TiO2-CaO > Ni/ZrO2-CaO > Ni/Al2O3-CaO. All of the Ni/support-CaO BCMs exhibited excellent CO selectivity (∼100%) over the investigated temperature range. The Ni/Al2O3-CaO showed only a comparable performance with SiO2-CaO in terms of CO2 conversion and CO yield, indicating the poor catalytic activities of nonreducible NiAl2O4 spinel (Fig. 5a). However, Ni/TiO2-CaO with NiTiO3 spinel achieved much high CO2 conversion compared to Ni/Al2O3-CaO owing to the better spinel reducibility. Although there was no spinel formation, Ni/CeO2-CaO outperforms Ni/ZrO2-CaO in relation to CO2 conversion and CO yield. For example, Ni/CeO2-CaO and Ni/ZrO2-CaO achieved 56.1% and 34.0% for CO2 conversion and 2.7 and 1.1 mmol g−1 for CO yield at 650 °C, respectively.The performances of ICCU-RWGS were also significantly affected by the reaction temperature. For example, 87.4% and 25.6% of CO2 conversions were obtained over Ni/CeO2-CaO at 550 °C and 750 °C, respectively. As an endothermic reaction, RWGS favors a higher reaction temperature which is consistent with the decomposition of CaCO3. However, the fast decomposition of CaCO3 could increase the CO2 concentration near the catalytic sites resulting in a decrease in CO2 conversion.It is necessary to monitor the real-time ICCU-RWGS process to evaluate the CO2 adsorption performance and catalytic activity. The promotion effect of support of Ni could be clearly demonstrated in the CO2 adsorption stage, presenting as the enhanced CO2 capture rate compared to the benchmark (SiO2 line in Fig. 8 a). Specifically, Ni/CeO2-CaO and Ni/Al2O3-CaO exhibited superior CO2 capture rate and capacity (∼9.58 and 9.31 mmol g-1 at 650 °C for ∼28 min), which was attributed to the abundant basicity of Ni/CeO2 and Ni/Al2O3, as indicated in Fig. 5b.The various Ni/support-CaO BCMs also exhibited distinct CO generation rates and real-time CO2 conversion during the hydrogenation step. As shown in Fig. 8b and 8c, the Ni/CeO2-CaO BCM could achieve an optimal real-time CO2 conversion (∼60%) and CO generation rate (∼1.7 μmol s-1g−1) at 650 °C. The excellent Ni dispersion and stronger basicity of Ni/CeO2 might contribute to the superior ICCU-RWGS performance. It is worth noting that Ni/TiO2-CaO also exhibited outstanding real-time CO2 conversion (∼57%), which might be attributed to the slowly released Ni from easily reducible NiTiO3 spinel species (Figs. 3 and 4) [50]. As a comparison, Ni/Al2O3 showed poorer catalytic activities, indicating that the nonreducible spinel (Figs. 3 and 4a) played poor catalytic performance in ICCU.In this work, we focused on the initial 1500 s of CO2 conversion to evaluate the real-time gas production in ICCU-RWGS. The CO2 desorption is relatively fast in the initial stage of the hydrogenation step (0–500 s), especially under higher temperature (e.g. 700 °C), which directly limits the CO2 conversion rate at this stage (as shown in Fig. 8e and 8f). Since only 5% H2/N2 was used in this work, excessive CO2 release would decrease the ratio of H2:CO2 and affect the equilibrium of RWGS reaction. After the rapid decomposition of the surface layer of CaCO3, the release of CO2 and the performance of RWGS is gradually stabilised until the carbonates are thoroughly consumed.The scaled-up preparation of Ni/supports-CaO BCM is critical for deploying the ICCU-RWGS in an industrial scale. The adsorbents (CaO) are expected to be obtained from limestones, representing a low-cost mineral (<30 dollars per ton (DPT)). The Ni/supports are comprised of earth-rich elements, such as CeO2 (∼1600 DPT), TiO2 (∼3300 DPT), Al2O3 (∼350 DPT), ZrO2 (∼1000 DPT) and Ni precursor (Ni(NO3)2*xH2O, ∼4000 DPT). By impregnating Ni onto a support and physically mixing catalysts and CaO, the BCMs could be obtained with an expected cost of ∼300–1000 DPT. Notably, although H2 can be obtained from renewable energy, the cost would dominate in the ICCU-RWGS process. The development of low-cost hydrogen production will also be the key to the low-cost deployment of ICCU.The stability of Ni/CeO2-CaO bifunctional combined material was presented by carrying out 20 cycles of ICCU-RWGS at 650 °C (Fig. 9 ). The Ni/CeO2-CaO possessed a < 5% decrease for CO and C1 yield (CO2 + CO + CH4) and > 99% CO selectivity after 20 cycles, indicating the excellent and stable ICCU-RWGS performance. Notably, the overall CO2 conversion slightly increased from 56.07% in the 1st cycle to 62.03% in the 20th cycle, which outperforms the state-of-art ICCU-RWGS performance using similar conditions (Table 2 ). It is believed that the effective reactant spillover onto catalytic sites is critical in the catalytic process [51]. As shown in Fig. 6e, Ni/CeO2 and CaO exhibited closer contact in spent Ni/CeO2-CaO, indicating a shorter CO2 spillover distance. The self-optimisation of Ni/CeO2-CaO bifunctional combined material in ICCU-RWGS might be attributed to the volume expansion–shrinkage effect of the sol–gel CaO in cyclic carbonation-hydrogenation, which partially embedded Ni/CeO2 onto the surface layer of CaO.In this work, several Ni/support-CaO bifunctional combined materials (BCMs) were prepared by physically mixing sol–gel CaO and Ni catalysts with various supports (ZrO2, TiO2, CeO2 and Al2O3). The CO2 adsorption and catalytic performance of Ni/support-CaO BCMs were evaluated via integrated CO2 capture and reverse water–gas shift (ICCU-RWGS) process at different temperatures from 550 to 750 °C. The Ni/CeO2-CaO outperformed the other Ni/support-CaO (support = ZrO2, TiO2 or Al2O3) over ICCU-RWGS performance (CO2 adsorption and catalytic activity). The enhanced CO2 adsorption performance was attributed to the stronger basicity of Ni/Al2O3 and Ni/CeO2. And the improved catalytic performance (CO2 conversion, CO yield and CO generate rate) was related to the excellent Ni dispersion and reducibility. The spinel formation contributed to the Ni species dispersion by forming strong interaction with support; however, only the easy-reducible spinel (NiTiO3) was active in the ICCU process. Furthermore, the Ni/CeO2-CaO exhibited excellent stability in 20 cycles of ICCU and showed a self-optimising trend in CO2 conversion (56.07% and 62.03% for the 1st and 20th cycles, respectively) due to the gradually close distance between CaO and Ni/CeO2 owing to the volume expansion and shrinkage of CaO during carbonation-hydrogenation cycles. Shuzhuang Sun: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing – original draft. Chen Zhang: Methodology, Investigation, Formal analysis. Shaoliang Guan: Formal analysis, Investigation, Resources. Shaojun Xu: Formal analysis, Resources, Writing – review & editing, Supervision. Paul T. Williams: Formal analysis, Resources, Writing – review & editing, Supervision. Chunfei Wu: Conceptualization, Writing – review & editing, Supervision, Project administration, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors gratefully acknowledge financial support from the China Scholarship Council (reference number: 201906450023). This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 823745. The UK Catalysis Hub is kindly thanked for the resources and support provided via our membership of the UK Catalysis Hub Consortium and funded by EPSRC grant: EP/R026939/1, EP/R026815/1, EP/R026645/1, EP/R027129/1 or EP/M013219/1(biocatalysis). The XPS data collection was performed at the EPSRC National Facility for XPS (‘HarwellXPS’), operated by Cardiff University and UCL, under contract No. PR16195.

Integrated CO2 capture and utilisation (ICCU) is a promising strategy for restricting carbon emissions and achieving carbon neutrality. Bifunctional combined materials (BCMs), containing adsorbents and active catalysts, are widely applied in this process. Producing syngas via reverse water–gas shift reaction (RWGS) and integrating with Fischer-Tropsch (F-T) synthesis is an attractive and valuable CO2 utilisation route. This work investigated a series of Ni/support-CaO BCMs (supports = ZrO2, TiO2, CeO2 and Al2O3) for the integrated CO2 capture and RWGS (ICCU-RWGS) process. The Ni/support-CaO BCMs were prepared by physically mixing various metal oxide supports loaded Ni with sol–gel derived CaO. The ICCU-RWGS performance (CO2 conversion, CO yield and CO generation rate) of these BCMs followed the order during tested conditions (550–750 °C): Ni/CeO2-CaO > Ni/TiO2-CaO > Ni/ZrO2-CaO > Ni/Al2O3-CaO. A comprehensive characterisation of Ni/support materials showed that Ni/CeO2 had the characteristics of stronger basicity, optimal Ni dispersion and improved NiO reducibility, which led to the outperforming ICCU-RWGS activity over Ni/CeO2-CaO (e.g. 56.1% CO2 conversion, 2.68 mmol g−1 CO yield and ∼100% CO selectivity at 650 °C). Furthermore, the Ni/CeO2-CaO BCM showed a stable, yet, self-optimising catalytic performance during the cyclic ICCU-RWGS reaction over 20 cycles. The TEM characterisation suggested that was ascribed to the volume expansion and shrinkage of CaO in the cyclic adsorption–desorption altering the distance between the adsorbent and Ni/CeO2, resulting in an enhanced CO2 conversion during the cycle.

No data was used for the research described in the article.Rhenium is sometimes described as a ‘chameleon’ among the elements due to the large variety of oxidation states it can adopt and interchange between [1]. This protean nature makes it an interesting candidate as a potential catalytically active component for myriad reactions [2,3]. The main industrial catalytic application of Re is in the form of bimetallic Re–Pt/Al2O3 as a catalysts for the catalytic reforming of hydrocarbons in the petrochemical value chain, where Re benefits the catalyst stability compared to the monometallic Pt catalyst [4,5]. Re is also investigated as a promoter for Fischer-Tropsch synthesis [6,7] and can catalyze olefin metathesis reactions [8,9].In recent years, alongside generally increasing research interest into biomass utilization, a growing number of research articles reporting Re-based or Re-modified catalysts for a variety of biomass-related catalytic reactions has been published (Fig. 1 ). As previously outlined, e.g. by Sadaba et al. [10], two aspects are characteristic for biomass utilization: (1) The non-volatile nature of many biomass-derived compounds demands reactions to be conducted in liquid phase and (2) reactions are preferably conducted over solid catalysts since a heterogeneously catalyzed process makes recovery of the catalyst more facile. Solid Re catalysts were successfully applied in all types of heterogeneously catalyzed biomass upgrading reactions that rely on active metal centers. As outlined, e.g. by Alonso et al. [11] these are: (1) aqueous-phase reforming (APR), i.e. the production of H2 typically from carbohydrates or glycerol; (2) hydrogenation (HG), i.e. the saturation of double bonds such as C=C bonds as well as C=O bonds resulting in a selective reduction e.g. of carboxylic acids or ketones to alcohols; (3) hydrogenolysis (HL), i.e. the cleavage of bonds involving H2, which is in particular applied to glycerol and other polyols; (4) hydrodeoxygenation (HDO), i.e. a subtype of HL where C–O bonds are cleaved, which can be employed to upgrade e.g. lignin-derived molecules such as guaiacol or anisole. In addition, just recently (5) deoxydehydration (DODH) was successfully implemented as a heterogeneously catalyzed process. For this last reaction, which completely removes oxygen from vicinal diol groups by converting them into unsaturated C=C bonds, Re-based catalysts are of particular importance [12,13]. A common feature of all five reaction types is the importance of redox reactions. Moreover, a common aspect of reaction types 2–5 in the aim of removing oxygen from organic molecules, mostly relying on H2 as reducing agent.The general importance of Re-containing catalysts for biomass transformation is evident from many recent review articles that put the performance of these catalysts into perspective [2,12–48]. Most notably, Re is very versatile and can fulfill several different functions. Perhaps the most common effects ascribed to ReOx species as a catalyst promoter are based on their acidity and oxophilicity, which facilitate bifunctional reaction mechanisms and additional interactions with functional groups of adsorbed substrate molecules [49–58]. Similarly, the presence of oxygen vacancies on low-valent Re oxide species has been suggested to be of crucial importance [59–62]. Furthermore, electron transfer effects are discussed [63] as well as effects on stabilization of the active phase [64,65], enhanced reducibility of non-noble metals in the presence of Re [66], and H2 spillover from a second metal to Re [67,68]. In other cases, metallic alloys between Re and a second metal were found to be beneficial [53,64,69]. While for DODH high-valent Re species are considered crucial for catalytic activity [13,70,71], HG of carboxylic acids was found to require low-valent Re species including metallic Re [72–75]. This influence of the Re oxidation state is lucidly described by Tomishige et al. [26].While considerable effort has been dedicated to identifying the role Re plays in enhancing catalytic activity and how to modify catalysts to improve their efficiency, the stability of Re-containing solid catalysts has been investigated to a much lower degree. Even though many recent studies include catalyst reuse experiments, they are typically performed only for the best-performing catalyst, which can hardly reveal which factors are relevant to catalyst deactivation. On the other hand, a few studies have been dedicated to the issue of catalyst stability and phenomena like (Re) metal leaching in recent years [76,77]. The importance of this topic, however, is apparent from several reports of Re catalysts showing strong deactivation over several recycling runs or over longer time on stream [52,53,55,58,78–87].The purpose of this review article is to shed a light on the stability of Re-containing solid catalysts applied in various heterogeneously catalyzed reactions conducted in liquid phase that are relevant for biomass conversion and upgrading. Based on the >200 research articles identified to match these criteria, about half of them were found to contain information on catalyst stability and are included in the analysis here. Different deactivation mechanisms will be discussed and strategies to restore catalytic activity, which is paramount to establishing sustainable catalytic processes.The main focus is placed on leaching of Re, which is a phenomenon characteristic for liquid-phase reactions and can lead to irreversible loss of the valuable and rare element Re [10]. Therefore, a variety of different catalyst properties as well as reaction conditions will be analyzed regarding their role in Re leaching throughout the lifetime of the catalyst. In particular, the influence of the Re oxidation state is analyzed. Furthermore, Re leaching-related issues like catalytic activity of dissolved species and strategies to reliably detect leaching are discussed as well as approaches to turn the tide and use of leaching of Re to one's advantage.Systematic studies of catalytic deactivation are scarce in the context of liquid-phase reactions of biomass-derived compounds, probably because catalyst stability only attracts attention when it poses an apparent problem. However, many studies include tests regarding the catalyst deactivation to prove that a well-performing new catalyst can also withstand the reactions conditions over an extended period of time without significant decrease in catalytic performance, i.e. in catalytic activity and/or selectivity. Usually only if problems with catalyst stability are apparent, their causes are investigated more thoroughly. In case of experiments conducted in a continuous-flow mode, deactivation is tested by observing changes in catalytic behavior over a certain time-on-stream (in certain cases up to several weeks [83,88]). For batch experiments the typical procedure is reuse/recycling of the catalyst for one or more consecutive experiments, where fresh reactants are supplied. In certain cases, this was repeated up to 25 times [89], but one to three recycling runs are most common. Excellent insights into the methodology of catalyst stability tests are provided by Hammond [90] and Miceli et al. [91] give an overview of conventional as well as advanced catalyst recovery methods.Assessing reusability (or performance over time-on-stream) alone is an unspecific method to investigate catalyst stability. It will only provide an overlaid overall picture of all occurring types of catalyst deactivation. Additional information can be gained from changing the treatment of the catalyst in between different recycling runs. Often, thermal treatment(s) under oxidizing (i.e. calcination) and/or reducing atmosphere are conducted to remove deposits or adsorbates on the catalyst accumulated during reaction or to (re)adjust the oxidation state of the metal phase. While these types of catalyst deactivation are reversible, i.e. catalytic performance after treatment is comparable to the fresh catalyst, others like e.g. irreversible catalyst poisoning, sintering, or metal leaching are not. Thus, limited insights into the nature of the deactivation are possible. However, reusability experiments rather serve as a general assessment whether or not catalyst deactivation takes place and to what degree it may be recoverable by certain treatment procedures. Deeper understanding of the underlying deactivation process requires additional experiments, in particular the extensive characterization of fresh and spent catalysts.A plethora of thermal, chemical, and mechanical deactivation processes have been described for heterogeneously catalyzed reactions. In one way or another, the physical and/or chemical properties of the solid catalyst are altered during the reaction, which results in a detrimental effect on the catalytic behavior, i.e. decreasing catalytic activity and/or selectivity over time. Previous articles [10,92–96] provide classifications of different deactivation types; however, an unambiguous assignment can be difficult because deactivation mechanisms are often not entirely mechanical or thermal but intertwined chemical contributions play a role.In general, the deactivation of catalysts in the liquid phase follows similar mechanisms as in the gas phase. However, especially the presence of a liquid solvent leads to different ways of chemical deactivation and can be detrimental to catalyst stability. The solvent itself can react with the catalyst, and most importantly, partial dissolution of the solid catalyst and subsequent loss of the active metal phase can be encountered. Therefore, leaching-related phenomena are characteristic for liquid-phase reactions [10,94]. The use of liquid medium is often necessitated by the instability and/or the low volatility of the highly functionalized reactants. On the other hand, many biomass-based feedstocks, in particular when biotechnological processing steps are involved, are already in the form diluted aqueous solutions [97]. Therefore, hydrothermal conditions are very typical for the catalytic conversions of biomass-derived compounds. Another important aspect when dealing with such feedstocks is the biogenic impurities they may contain either from the original feedstock or from previous processing steps. These can detrimentally effect the heterogeneously catalyzed reaction and, in the worst case, irreversibly poison the catalyst [97].The relevant deactivation types will be outlined in more detail in the subsequent sections with a focus on examples of liquid-phase biomass valorization over Re catalysts. An overview of all Re-related studies and experiments included in this review is given in Table 1 (continuous flow) and Table 2 (batch) summarizing catalyst and reaction conditions used as well as the observations of quantitative descriptors of catalyst deactivation and leaching. Furthermore, other (suspected) types of catalyst deactivation are listed as well as information on how deactivation and leaching was experimentally assessed.Mechanical alterations can be induced to a solid catalyst by mechanical, thermal, and/or chemical stress. Typical processes are crushing of shaped catalysts, attrition, and erosion and lead to breaking of catalyst particles, decrease in particle size, and formation of ‘fines’ [92]. In case of continuously operated reactors, this affects the permeability of the catalyst bed and can lead to a continuous loss of catalyst material due to the transport of fine particles. In case of batch reactions, too fine particles may not be recyclable causing lower activity in subsequent runs. According to Sadaba et al. [10], attrition is the main pathway of mechanical catalyst degradation for heterogeneously catalyzed reactions in the liquid phase and especially problematic for fluidized-bed and slurry reactors.In many studies evaluating the reusability of a solid catalyst, the amount of catalyst that can be recovered is lower than the initial amount. Losses during separation are almost inevitable due to the small scale of catalytic experiments in most studies (typically 10–103 mg). Whether there is a distinct contribution of mechanical degradation is usually not considered. However, to compensate for the expected incomparability among catalyst reuse experiments with significantly different catalyst amounts, two adjustments are commonly applied: either the lost amount of catalyst is compensated with fresh catalyst (e.g. [79]) or the batch size is adjusted to the amount of catalyst (e.g. the study by Liu et al.[139]and Liu et al.[140]). In general, the influence of mechanical processes resulting in catalyst deactivation are of considerably lower research interest compared to all other deactivation pathways. Several factors may play a role, e.g. the comparably low number of experiments operated under continuous flow, the predominant use of powder catalysts as well as the low relevance of mechanical stability at the current stage of catalyst development for liquid-phase biomass conversion.The vast majority of Re-containing catalysts for liquid-phase biomass applications (Table 1 and Table 2) are supported catalysts with Re loadings below 10 wt.-%. Thus, their mechanical stability will be primarily governed by the respective support material. Besides the chemical nature of the support material, a variety of parameters can influence the resistance of different materials toward mechanical deactivation, such as particle shape, modification, and porosity. Moreover, the preparation method of oxidic supports can have significant influence. These aspects are discussed in detail e.g. by Argyle and Bartholomew [93].A catalyst poison is an inhibitory substance that can very strongly and selectively bind to the active sites of a catalyst. A decrease in the catalytic activity occurs already in the presence of small quantities of the poison and the effect is often irreversible under process conditions [173]. Besides the poison molecule physically occupying the catalytically active center by being chemisorbed to it, its effect on the catalytic behavior of the catalyst can also arise from e.g. electronic interactions with adjacent active sites, inhibit surface diffusion processes, or lead to surface restructuring [92]. Catalyst poisons can either be formed during the reactions, typically as undesired side products, or already be present in the feedstock. The latter is often the case for biomass-based feedstocks which can contain several types of impurities, many of which are biogenic [95,97]. These types of impurities range from inorganic compounds to functionalized small molecules, e.g. carboxylic acids, amino acids, sugars, sugar-derived molecules, to larger structures such as proteins and cell (fragments), all of which can interact with the solid catalyst. Therefore, catalyst poisoning is often characteristic for studies using raw biomass sources or hardly purified products from biomass processing such as fermentation broths rather than using pure model substrates. Due to the highly selective interaction between poison and catalytically active sites, there are different types of poisons and correspondingly different poisoning mechanisms depending on the catalyst. A typical experiment to assess the deactivation caused by a suspected poison is the deliberate addition of this compound to the feed of the reaction and the variation of its concentration. Especially insightful are studies conducted under continuous flow, which allow switching between poisoned and un-poisoned feed since the possible recovery of the catalytic activity can be investigated. On the other hand, the direct poisoning of the catalyst, e.g. by impregnation before the reaction, is another option. Moreover, adsorption studies can be used to quantify the interactions of the catalyst with different poisons and reveal competitive adsorption between reactants and other molecules.Brønsted acid sites have been found to be susceptible to poisoning by (earth) alkali metal cations. This deactivation process was described to take place in the manner of an ion exchange process [10]. Poisoning by metal cations has been primarily reported for zeolite-based catalysts, e.g. by Metkar et al. [174], but similar deactivation is possible for Re-containing catalyst where ReOx species are acting as Brønsted acid sites. This was suggested by Falcone et al. [57] for a supported Pt–Re catalyst, which showed significant deactivation in the HG of crotonaldehyde after adding NaOH or NaCl to the feed. Another study by Liu et al. [132] showed similar results for an Ir-ReOx/SiO2 catalyst applied in the heterogeneously catalyzed valorization of xylan. An increasing amount in Na+ ions had detrimental effect on the catalytic behavior probably due to ion exchange of the proton associated with the Brønsted acidity of the Re species (Fig. 2 ). It was also shown that the addition of acid could restore the activity due to the formation of new acid sites [132]. Besides their influence on acid sites, cations can also poison metal centers as described in a study by Mortensen et al. [175] on Ni-based HG catalysts.Commonly observed is also catalyst poisoning due to heteroatom-containing organic molecules, in particular amino acids. Very instructive studies were published study by Schwartz et al. [176], Zhang et al. [177], and Harth et al. [178] investigating the influence of additives comprising different biologically relevant molecules as well as structural motifs on a heterogeneously catalyzed HG reaction in liquid phase. The poisoning strength was shown to be in a direct correlation with the adsorption energy of the heteroatom-containing molecules to the Ni, Pd, or Pt center [176]. While sulfur-containing molecules like cysteine showed a highly selective and very strong poisoning effect, nitrogen-containing ones deactivated the catalysts to a lesser degree. This is in accordance with several other studies that identified sulfur-containing amino acids as irreversible catalyst poisons [173,177,179,180]. Non-sulfur containing amino acids are less problematic and were found to inhibit the supported metal catalysts either reversibly [176,177] or hardly at all [173]. The presence of other organic acids (acetic acid, pyruvic acid) on the HG of succinic acid over a Ru–Re catalyst was investigated by Di et al. [67,68]. These acids, which can be present in fermentation-derived succinic acid feedstock, resulted in a slight decrease in catalytic activity. Preferable adsorption of the ‘impurity acids’ to the catalyst's active sites was suggested, which temporarily poisons the catalyst until the acids are hydrogenated and release the active sites for succinic acid [67].Unfortunately, the number of studies conducted with realistic, biomass-derived feedstocks is very limited. Among the Re-based catalysts included in this review article, this was studied for a carbon-supported Re–Pd catalyst that was used for the aqueous-phase HG of pure as well as fermentation-derived succinic acid purified by crystallization [181]. As depicted in Fig. 3 , the fermentation-derived succinic acid was converted significantly slower while the selectivity was not affected. Which impurities in the feed cause the decrease in catalytic activity was not investigated. Studies by Binczarski et al. [180] and Zhang et al. [177] show how different purification techniques can be employed and how gradual the removal of impurities affects the HG of fermentation-derived lactic acid. In particular, when irreversible poisoning occurs, so that the catalyst cannot or only with significant effort be reactivated, it is of particular importance to remove the poisons from the feed before the reaction.On the other hand, the catalyst poison may be formed during the reaction. The most common example for this is poisoning by CO, which can be formed by the decomposition of organic compounds. This was observed when formic acid was added as a renewable hydrogen donor in HG reactions. Di et al. [67,68] found that Re-containing bimetallic catalysts for the HG of succinic acid could not tolerate the presence of formic acid and were probably poisoned by CO from its decomposition. Similar findings are reported for other studies using formic acid as an in-situ hydrogen donor [173,182,183]. Due to the fact that the catalytic activity is recovered upon the decomposition of formic acid, it is also possible that adsorbed formic acid species cause the poisoning effect [173,184].Fouling describes a type of catalyst deactivation that arises from depositions accumulating on the surface of the catalyst that prevent reactants from accessing the active sites. Lange [95] distinguishes between two types of fouling based on the origin of the deposited species. First, the depositions can already be present in the feed, e.g. impurities like large molecules, proteins, cells, particles, and so on, which are physically depositing on the catalyst during the reaction. In this case, the deactivation is of mechanical nature and similar to membrane fouling [185]. Such impurities are typical for direct biomass-derived feedstock and their impact can be influenced by feed purification. Among the studies included in this article, typically model feedstock was used. Therefore, no example with a Re catalyst can be mentioned for this particular type of fouling. One study by Zhang et al. [177] of a supported Ru catalyst in the aqueous-phase HG of lactic acid conducted in continuous flow showed that proteins deposited in the pores of the carbon support. This was evident from drastically deteriorating textural properties of the catalyst after exposure to protein-enriched feed (loss of specific surface area from 780 to 80 m2/g). The blockage of reactant molecules from accessing some of the catalyst's active sites was suggested to be partially responsible for a considerable decrease in catalytic activity (conversion decreased from 58 to 35%) [177].The other possible origin of physical deposits on the catalyst is their formation during the reaction by undesired side reactions. This phenomenon is a well-known problem from gas-phase reactions such as reforming of methane or naphtha, fluid catalytic cracking, or others, where it is often referred to as catalyst coking as well as deposition of carbon or carbonaceous species [186,187]. In the context of biomass valorization in liquid phase, organic depositions often originate from condensation reactions of reactants, intermediates, or products that form oligomeric species [95]. In case of solid carbonaceous species formed from sugar or hydroxymethylfurfural feedstocks, the term humins is used frequently [188]. Depending on the respective reaction system and catalyst, the underlying reactions as well as the chemical nature of the deposited species can vary strongly and it is also not unusual that the deposits interact with the catalyst in ways (e.g. by acting as catalyst poisons) beyond the mere physical blockage of active sites. In general, fouling due to the formation of carbonaceous species is (unfortunately) a very common type of deactivation when converting renewable feedstocks [95].Due to the versatile nature of Re catalysts, they can be applied in several types of reactions with different feedstock. As can be seen from Table 1 and Table 2, however, carbonaceous deposits resulting in significant catalyst deactivation was reported in many studies regardless of the type of catalyst, the feedstock or the reaction conditions. Typically, the proof of deactivation by carbon deposition is based on the combination of two experiments. On the one hand, the presence (and the amount) of carbon-containing species deposited during the reaction is investigated e.g. by thermogravimetric analysis or total organic carbon analysis of the spent catalyst. On the other hand, the influence of the deposits on the catalyst activity is assessed. Since the carbonaceous deposits can often be removed by comparatively simple thermal treatment, a calcination and/or reduction step is conducted to reactivate the catalyst by uncovering its surface. The catalytic behavior of the thus reactivated catalyst compared to the fresh catalyst as well as the non-activated spent catalyst reveals the influence of deactivation caused by fouling by carbonaceous species. It should be noted, however, that during the reactivation procedure also other properties of the catalyst can be altered, e.g. oxidation states of active species, and that processes like sintering can take place. Additional characterization (e.g. by infrared (IR) spectroscopy, elemental analysis, imaging techniques, etc.) of the spent catalyst to investigate the detailed chemical nature of the deposited species and to elucidate the formation of these species from the feedstock may provide additional insights. They are, however, beyond the scope of this review and more detailed information regarding the characterization of carbonaceous deposits is available, e.g. in a review by Ochoa et al. [189].A representative example was reported by Jin and Choi [69], who investigated the deoxygenation of palm oil to paraffin over a carbon nanotube-supported bimetallic Re–Pt catalyst. The results of recycling experiments are shown in Fig. 4 (b). When the recycling experiments were conducted without calcination, catalyst deactivation was pronounced very strongly. Paraffin yield dropped by nearly 50% in the second run and down to 20% of the initial yield in the third run. Thermogravimetric analysis of the spent catalyst (Fig. 4(b)) revealed the presence of ca. 14 wt.-% of solid deposits formed during the reaction, probably by the undesired polymerization of reactants and/or products. Introducing catalyst calcination as a reactivation method in between consecutive catalyst recycling experiments proved that the deactivation of the catalyst is indeed mainly due to the carbon deposition. Only a slight decrease (ca. 10%) in paraffin yield was observed over 5 reuse runs, which could have been caused by Re leaching or by metal sintering [69].Many similar examples for deactivation by fouling can be found in Table 1 and Table 2. Often, the deactivation by carbon deposition is to a large degree reversible, which is evident from the comparison of experiments with and without the removal of the deposit [58,79,80,85,128,132,170,171]. In some cases [55,86], the overall catalyst deactivation could only slightly be mitigated by thermal reactivation, which is indicative of severe deactivation by other causes. The wide range of feedstock and reaction types as well as the different catalysts for which deactivation by fouling by carbonaceous species formed during the reaction was observed proves that it is a widespread but manageable type of deactivation. An important aspect that must not be overlooked is that even though carbonaceous deposits are formed on the catalyst this does not necessarily mean that the catalytic activity is (negatively) influenced by that. Two examples [101,118] of Re-based catalysts were reported where despite significant deposit formation (5.5 and 6.8 wt.-% of the catalyst after reaction) catalytic behavior was not significantly changed. One aspect that might explain some of these differences is that while in the ideal sense fouling is an entirely physical deactivation mechanism, in reality deposited species can additionally react with the catalyst or act as catalyst poisons. Therefore, the chemical composition and the interaction with the catalyst are of crucial importance as well.Thermal deactivation describes the decline of catalytic activity due to the changes in material properties of the catalyst upon exposure to elevated temperatures. Considering that myriad deactivation processes require thermal activation, this term may include almost all types of catalyst deactivation. On the other hand, a narrow definition of ‘purely’ thermal deactivation would exclude all pathways including chemical reactions with other compounds of the reaction system and is therefore limited to sintering of the support or the active metal phase of a catalyst as well phase transformations, thermal decomposition, and other restructuring processes.Sintering is defined as the temperature-induced coalescence and densification of porous solid particles below the melting points of their major components [190]. It is driven by the thermodynamically favored reduction of surface area and typically includes migration or diffusion processes resulting in crystal growth [191,192]. Looking at the catalyst support, sintering leads to a densification of the bulk material and a collapse of pores, which reduces the specific surface area of the catalyst. In most instances, however, sintering occurs in the form of crystal growth of the dispersed metal, which is typically the active phase. Therefore, the catalyst deactivation by metal sintering is caused by a reduction in active metal surface area. Typical techniques to investigate metal sintering are chemisorption experiments with probe molecules such as CO or H2 to quantify the amount of active metal centers as well as XRD and microscopy techniques to determine the size of the metal particles. Comparing fresh and spent catalysts reveals to what degree metal surface area is reduced during the reaction. However, for structure-sensitive reactions, not all surface metal atoms contribute to catalytic activity in the same way. Furthermore, in case of bimetallic catalysts, metal segregation has been observed, which reduces the interface between the two metals [55,83]. So far, these effects have rarely been identified as crucial mechanisms for the catalyst deactivation in liquid phase and require further research attention.In solid-gas reaction systems with supported metal catalysts, sintering occurs at high reaction temperatures (from 500 °C) [92]. The conversion of biomass-derived compounds, however, is conducted mostly in the liquid phase and at lower temperatures (mostly between 100 and 250 °C, rarely >300 °C), as the examples in Table 1 and Table 2 show, which makes classical sintering less likely. Especially, the presence of liquid phase has an immense influence on the stability of supported catalysts, which will be discussed in a subsequent separate section. Unfortunately, there is hardly any information available, how sintering of the metal phase as a catalyst deactivation mechanism of supported metal catalysts is influenced by solvents in general and water in specific. From liquid-phase sintering, as a materials preparation technique, it is evident that the solvent plays a crucial role since soluble species are involved that drastically accelerate sintering processes [193]. It is certainly conceivable that dissolution and redeposition of metal species, which are mobile in the solvent, can play a role, similar to the contribution of volatile species to sintering in solid-gas reaction systems. Regardless of the underlying mechanisms, the term sintering is generally used when the growth of metal particles is observed during the reaction.Among the studies on solid Re-based catalysts selected for this article, metal sintering is claimed to contribute to the overall catalyst deactivation in several cases. In all cases, this is evidenced by proving the growth of metal particles during the reaction. One instance of sintering of a monometallic Re catalyst is reported in the study by Godina et al. [100]. A carbon-supported Re catalyst was employed for the APR of xylitol, operated under continuous flow at 225 °C for 30 h. Re particle size was <0.5 nm before the reaction but increased slightly to 0.7 nm during the time-on-stream. At the same time, the catalyst completely lost its activity for hydrogen formation. Unfortunately, the reason for deactivation was not investigated further and the contribution of other types of deactivation is therefore not known. It appears unlikely, however, that the observed difference in metal particle size can explain the full deactivation of the catalyst. In another study [60], the sintering of Re/SiO2 during catalytic HDO of guaiacol in n-dodecane at 300 °C lead to an increase in average Re particle size from 2.8 to 3.8 nm. Jeong et al. [119] investigated different supported Re catalysts for the HDO of guaiacol in n-heptane at 280 °C. After 1 h, sintering was observed for Re/SiO2 (Re particle size increase from 0.5 to 1.1 nm), while the changes for Re/TiO2 were not significant. The corresponding transmission electron microscopy images are shown in Fig. 5 . For these catalysts, the effect of sintering as well as leaching, which was also detected in considerable amounts, on the catalytic activity was not investigated.Looking at supported bimetallic or trimetallic Re catalysts, a common finding is that the Re species are highly dispersed while the other metals form comparatively larger particles. (The factors governing Re dispersion are investigated e.g. in the study by Mine et al. [194], while more general aspects on this topic are summarized in recent review articles [195,196].) In many cases of supported catalysts with high Re dispersion, the particle size of Re species is below the detection limit in the catalyst's XRD pattern [85,113,131,139]. Besides high dispersion this could, on the other hand, also be the result of Re being present as a variety of species or as amorphous particles. In practice, this means that sintering is typically identified only for the other metal. In several examples for all kinds of reactions, the growth of metal particles has been observed, as indicated in Table 1 and Table 2. The gradual sintering of Ir on a supported Ir-ReOx catalyst was e.g. shown by Luo et al. [83]. Ir particle size increased from 2.1 nm on the freshly reduced catalyst to 3.1 nm after 150 h and to 3.2 nm after 500 h on stream in the aqueous-phase HL of glycerol. Similar increases are reported for Re-based bimetallic catalysts with Ir [85,113,132,133,140], Pt [69,84], Pd [86], Rh-Ir [52], Pt-Ir [107] as well as Ni [142]. While it is typically assumed that sintering takes place during the actual reaction time, Liu et al. [131] suspect the calcination treatment in between reuse runs to be responsible. When metal particle growth was identified its effect on the catalytic activity was considered to be rather small compared to e.g. fouling due to carbon deposit formation and never was a clear correlation between the decrease of metal surface area proven in the literature.As mentioned before, for bimetallic and multimetallic catalysts, the interface between different metals is considered of particular importance, which complicates matters. Often it is assumed that the effect of combining different metals relies on their direct interaction, which requires special contact of the metals. Therefore, the interface between metal species is considered to be most relevant for catalytic activity. Migration and sintering of metal species can lead to different effects on such catalysts. In some studies metal segregation was suggested to cause catalyst deactivation e.g. for Pt–Re APR catalysts [99,101] or other bimetallic catalysts [55,104]. Interestingly, in certain instances, the opposite effect, i.e. the accumulation of Re species on Ir particles during the reaction, was considered to be detrimental to catalytic activity [139,140]. Either way, the thermally induced changes in metal arrangement resulted in a decrease in accessible bimetallic sites. A more detailed insight into such migration processes should be possible e.g. by X-ray absorption or photoelectron spectroscopy.Thermal deactivation is not only relevant under the actual reaction conditions but can also play an important role in thermal pretreatment or recovery processes such as catalyst calcination or reduction. While sintering can also occur under these conditions, there are other types of catalyst degradation that are relevant. First, carbon supports often cannot withstand typical calcination conditions and undergo decomposition or combustion. This means that often carbonaceous deposits cannot completely be removed without damaging the catalyst support. The carbon nanotube-supported catalyst in Fig. 4, however, is comparatively thermally stable, which is why the catalyst could be successfully reactivated. In other studies [170,171], this was not the case. Moreover, Jang et al. [172] showed that thermal treatment under H2 atmosphere can be a suitable approach to remove organic deposits while preventing carbon support decomposition for a Pt-ReOx/C catalyst applied in mucic acid DODH.A related thermally induced type of catalyst deactivation is the loss of Re from the catalyst due to vaporization of volatile species. It is most commonly observed during the thermal treatment of Re-containing catalysts rather than during the catalytic reaction itself. E.g., the oxidative treatment of Re species can result in the formation of high-valent, volatile species, and subsequent removal of Re from the catalyst. She et al. [197] observed a decrease in Re content of a silica-supported ReOx catalyst from 21.5 to 18 wt.-% after calcination in a flow of dry air at 400 °C for only 1 h. This was also observed by Liu et al. [139] for bimetallic Re–Ir catalysts with high Re loading. In general, the sublimation of Re2O7 at elevated temperature is a common phenomenon, which can be influenced by the choice of support material [198].One instance of a similar phenomenon occurring during a liquid-phase was reported by Canale et al. [199]. During the DODH reaction of neat glycerol over unsupported Re2(CO)10 at 170 °C, it was found that the catalyst underwent (partial) sublimation. Unfortunately, neither the amount of Re loss not the effect on catalytic activity was reported. It should be noted that this reaction was conducted in a semi-continuous setup under constant purging of the liquid phase with a flow of air or H2, which is a crucial factor since this leads to the transport of Re species out of the reactor. Interestingly, under more reducing reaction conditions (100 bar H2, 160 °C), another study showed that Re2(CO)10 decomposes into Re nanoparticles [200], which indicates that for HG and HDO reactions, volatile Re species are not likely to be present and, thus, the loss of Re due to vaporization is a rather seldom phenomenon.Since most heterogeneously catalyzed transformation processes of biomass-derived feedstocks are conducted in the liquid phase, the role of the solvent is of central importance. In the context of catalyst stability and deactivation, two processes are particularly affected: Leaching of the active (metal) phase, which will be discussed in detail as the main focus of this article, and degradation of the support material in the liquid reaction medium. Especially the use of water and other polar reaction media can induce severe structural changes to solid catalysts. Being the natural process medium for biological processes and considered as a green solvent [201,202], water is the most commonly used reaction medium for biomass conversion and upgrading reactions, which is also the case for Re-based catalysts (Table 1 and Table 2). A series of heterogeneously catalyzed reactions can be conducted in sub-critical liquid water (Fig. 6 , left). The issue of hydrothermal stability has attracted considerable attention and a comprehensive review article was published by Xiong et al. [203].As depicted in Fig. 6 (right), silica- and alumina-based as well as zeolites show a comparatively low stability in hot liquid water. In case of zeolites, this behavior has been thoroughly investigated in recent years [204–208]. Depending on the reaction conditions, in particular pH, the ordered framework structure of zeolites disintegrates via dealumination or desilication, i.e. hydrolysis of the zeolite lattice starting preferably from defect sites. During treatment in hot liquid water, the characteristic zeolite framework disintegrates and the support is gradually converted into a non-microporous, amorphous solid. However, the degree of disintegration and structural collapse also depends on the framework type and the modulus. Similar to high-surface-area alumina or silica [203,209], the drastic loss of specific surface, which is caused by the hydrothermal transformation of the support, can have severe detrimental effects on the catalytic activity of the supported metal catalyst. Loss of crystallinity is apparent from X-ray diffraction patterns of the catalyst and the loss of porosity can be quantified by physisorption of nitrogen or other gases. Structural changes can also be investigated by IR spectroscopy and solid-state nuclear magnetic resonance (NMR) spectroscopy. Besides the physical loss of surface area, changes e.g. in acidic properties are crucial for bifunctional catalysts involving acid-catalyzed reactions. This can be studied by temperature-programmed desorption of probe molecules in combination with IR spectroscopy.Zeolites are rarely used in combination with Re, perhaps because Re itself is often suspected to provide Brønsted sites to enable bifunctional reaction mechanisms [51,56,57]. In general, none of the studies in Table 1 or Table 2 determined support degradation to be the main deactivation pathway which may, in part, be due to a lack of interest in this particular phenomenon. It is, however, noteworthy that especially for the hydrothermally demanding APR processes exclusively activated carbon or TiO2 were used as support materials [50,54,82,88,98–100], which are among the most hydrothermally stable materials. Unfortunately, this does not necessarily guarantee thermal stability as well. In particular, the use of carbon-based support can be limited by thermal decomposition when thermal reactivation of the catalyst is required (see above). Thermodynamic considerations by Lange [95] further prove that TiO2 and ZrO2 are the most hydrothermally stable among commonly used oxide supports. Furthermore, high-surface area modifications, like e.g. mesoporous silica materials, are metastable phases, which make them more prone to recrystallization.There are myriad ways in which supported metal catalysts can undergo chemical reactions. Many of such reactions play a role for previously described types of catalyst deactivation, such as thermal deactivation by combustion of carbon supports, hydrothermal support degradation by hydrolysis of oxide materials, or chemical reactions resulting in the formation of carbon deposits or poisons, the latter of which cause deactivation by chemisorption to the catalytically active centers of the catalyst. Besides theses effects, that lead to reduced numbers of accessible active metal sites for supported metal catalysts, there is also the possibility of chemical transformations of the active metal species converting them into a non-active phase. In particular, the contact of supported metal catalysts with reducing or oxidizing atmosphere either inside or outside the reactor can change the oxidation state of the metal, which can have severe effects on the catalytic behavior. As mentioned in the introduction, Re catalysts in particular are very versatile in the context of heterogeneously catalyzed biomass valorization due to their characteristic redox chemistry [26]. On the other hand, this makes them susceptible to deactivation by changes in oxidation state.Of particular importance is the actual Re oxidation state under reaction conditions. Conventional characterization techniques include temperature-programmed reduction and/or oxidation experiments to reveal the redox-behavior of the catalyst under different gas atmospheres as well as X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (e.g. XANES) to give insight into the Re oxidation states(s). Furthermore, Raman spectroscopy has been used to identify different Re species [50,71,210,211]. These ex-situ methods, however, cannot account for the actual reactions conditions. In particular, the influence of the solvent, which can strongly influence Re reduction [74], and of high pressure can only be investigated by more complex in-situ methods [212].Several studies thoroughly investigated the oxidation state of Re and other metals. It is not uncommon to find the oxidation state change during the course of a catalytic reaction but it is rarely considered a factor responsible for catalyst deactivation. Such examples are also highlighted in Table 1 and Table 2 distinguishing between reduction and oxidation of the Re species during the reaction. In case of HG reactions, e.g. the catalytic reduction of carboxyl or carbonyl functionalities, the catalytic activity strongly depends on the oxidation state of Re for both monometallic [72,87,213] and bimetallic Re catalysts [74,150,151]. In the latter case, the ratio between different oxidation states was found to be a crucial factor governing catalytic behavior. Any changes to the Re oxidation state can therefore shift the delicate equilibrium to a less active position, i.e. the catalyst shows deactivation.Staying with the example of HG reactions, Takeda et al. [74] found a clear correlation between the initial ratio of metallic and oxidized species and the catalytic performance of a Pd–Re/SiO2 catalyst in the HG of succinic acid. Despite the catalyst being handled under N2 atmosphere during recovery in between reuse experiments, the catalytic activity decreased by ca. 60% over four consecutive runs. Metal leaching was contributing to catalyst deactivation but it was suggested that in addition changes to the active sites must have occurred. After one catalytic run, it was found that the average oxidation state of Re, determined by XANES analysis, changed from +1.6 to +1.2 [74]. While other effects like changes in spatial arrangements may also play a role, it appears likely that the overall reduction of Re species during the reaction is a crucial factor for the observed catalyst deactivation.For the DODH of vicinal diols it is typically suggested, e.g. by Tomishige et al. [26], that highly oxidized Re species are required. They proposed that during the catalytic cycle the oxidation state of Re switches between +6 and + 4. Due to the presence of a reducing agent, often H2, the overreduction of Re is a possible deactivation mechanism. Ota et al. [71] suggested that the benefit of a CeO2 support compared to SiO2 was to maintain a higher oxidation state of Re (+5.0 vs. +3.2; determined by XPS) throughout the reaction.Furthermore, the oxidation of Re species can also lead to catalyst deactivation. For the HDO of isoeugenol over bimetallic Ir–Re catalysts, Alda-Onggar et al. [115] suggested that the presence of Re4+ (ca. 12% of all Re species) corresponds to high catalytic activity. During the course of the reaction, however, the Re species were slightly oxidized and no Re4+ was found on the spent catalyst (Fig. 7 ). The fact that the catalyst's activity could not be fully restored for a second run despite reactivation was suggested to be a result of the lack of Re4+ [115].Considering the crucial importance of the Re oxidation state for the catalytic behavior of Re-containing catalysts in redox reactions, it is imperative to control the conditions that the catalyst is exposed to at all stages of its lifetime, i.e. during pretreatment, reaction, recovery as well as reactivation. The pronounced susceptibility of Re catalysts to changes in oxidation state was also reported e.g. by Ly et al. [214] for a TiO2-supported Re catalyst, which impressively shows the complexity of the Re redox chemistry. XPS analysis of the catalyst (Fig. 8 ) after (a) gas-phase reduction at 450 °C, followed by (b) exposure to air, and (c) subsequent second reduction revealed the overall reduction and oxidation of different Re species as well as the ratios of different oxidation states. While after the first reduction, Re is primarily in the metallic state (89%) and some in oxidation state +3, contact with air rapidly oxidizes the Re species to a mixture of higher oxidation states (17% + 4, 44% + 6, 39% + 7). A second reduction step can only restore parts of the initial metallic Re (15%) while most Re is +3 [214].This influence is even more important considering that to reactivate the catalyst after other types of deactivation occurred (e.g. fouling by carbonaceous deposits or poisoning), it often has to be calcined or treated and/or undergo thermal treatment in reductive atmosphere. The example showed that despite nominally similar treatment (reduction at 450 °C), the obtained distribution of Re species was not comparable. On the other hand, often the recovery procedure in case of carbonaceous depositions is analogous to the initial preparation of the catalyst. Therefore, the influence of Re oxidation or reduction may often be overlooked.Deactivation by reduction can also be a result of catalyst reactivation, as a recent study showed [172]. Attempting to mildly remove carbon deposits from a Pt-ReOx/C catalyst in between recycling experiments, thermal treatment at 230 °C under H2 atmosphere was conducted. This resulted in a significant reduction of the Re species from an overall oxidation state of +6.1 (primarily +7, only 1% metallic Re; determined by XPS) in the fresh catalyst to +1.82 after reduction (Fig. 9 ). This caused a ca. 10-fold decrease in catalytic activity in the DODH of mucic acid. An additional oxidative treatment at 120 °C brought the Re species back to the approximately the initial state (+5.7). This resulted in a nearly complete recovery in the catalytic activity despite 9% of Re remained as metallic Re. Interestingly, after 6 h of reaction, the initial catalyst was found to be already significantly reduced to an average Re oxidation state of +3.4. However, the contribution of this change to the observed lower activity of the untreated spent catalyst (ca. 35%) was not assessed in the study [172], even though this could further underline the previously discussed findings by Tomishige et al. [26]. It should also be noted that besides Re also the oxidation state of Pt was slightly changed; however, at least ca. 80% were present as metallic Pt in all catalysts, which does hint at a decisive role for catalyst deactivation.Besides the immediate effect of changes in oxidation state described so far, the solubility of Re species strongly depends on their oxidation state as will be discussed later in detail. In particular, in water as well as in polar, oxygenated solvents high-valent Re oxides are considerably more soluble than more reduced Re species [3,215]. Therefore, the oxidation of supported Re catalyst before, during, or after the reaction can also result in various leaching phenomena and corresponding effects on catalytic activity.According to Sadaba et al. [10], leaching of the active phase is a characteristic feature of heterogeneously catalyzed reactions in the liquid phase. Leaching describes an extraction process, which results in the transfer of a compound from the solid catalyst into the liquid reaction medium [216]. In the context of supported metal catalysts (and in this review article), this typically refers to the selective loss of active (metal) phase of the catalyst but the previously described hydrothermal destruction of catalyst supports may also include leaching processes. In the simplest case, a soluble species exists on the catalyst and is dissolved when the solid catalyst comes into contact with the liquid reaction medium. On the other hand, the soluble species can also be formed during the reaction, which means that the leaching process includes e.g. a redox reaction. The details on mechanistic aspects of leaching can be found, e.g. in the study by Eremin et al. [217]. Sadaba et al. [10] distinguish between two types of leaching: direct solubilization and leaching involving chemical transformations. Moreover, the solubility of a compound can be significantly increased in the presence of complexing agents, which facilitates metal leaching. Besides the leaching of single atoms or ions, there is also the possibility of multi-atom nanoparticles being solubilized or being formed in the solution [217].Among the articles included in this review article, which all contain Re, there are several bimetallic or trimetallic catalysts that can give an indication regarding the susceptibility of Re to leaching compared to other metals. A drastic example is e.g. the study by Zhang et al. [82] on APR of glycerol over bimetallic supported Pt–Re. While after one week of continuously operated reaction at 225 °C ca. 50% of Re was found to be leached from the catalyst its Pt content was not changed significantly. Similarly, a leaching test of another Pt–Re catalysts revealed leaching of ca. 10% of Re without any Pt leaching after treatment in hot liquid water. For Ir-ReOx catalysts, for glycerol HL leaching of Ir was in some studies higher than Re (ca. 2% vs. 1%) [58,126,127], but in most similar studies, Ir leaching was negligible while slight Re leaching was observed, e.g. 0.9% Re leaching without detectable Ir leaching [132]. Bimetallic supported Pd–Re catalysts also rather show Re leaching than Pd leaching, e.g. 3% [152] or 3.5% [74] compared to no detectable Pd leaching in both HG studies, and Ru leaching was at least 50% lower than Re leaching in a different study [150]. Similar results were obtained for different trimetallic Re-containing catalysts, where among Ir, Rh, Pt, and Re, only the latter was found to leach into the solution in quantifiable amounts (0.8–2.8%) [52,107]. Overall, it becomes apparent that Re has a stronger tendency to be leached from solid catalysts than commonly used noble metals like Au, Pt, Pd, Ir, Ru, or Rh barring a few exceptions mentioned here.The combination of Re with non-noble is not as common as with noble metals. Therefore, not much data are available of the leaching behavior of such catalysts. However, two studies indicate that when combining Re with non-noble transition metals, the latter are either similarly or more prone to leaching than Re. In the first study [112], Re–Ni/ZrO2 was investigated in the aqueous-phase HG of lignin model compounds. After 1 h at 300 °C ca., 10% of Ni was leached while no Re leaching could be detected. In a recent study, similar amounts (ca. 10%) of Re and Ni were leached during lignin hydrotreatment [142]. In a recent study on levulinic acid HG in 1,4-dioxane at 180 °C over bimetallic Co–Re/TiO2, the amounts of relative metal leaching for both metals were comparable and below 0.2% [159].Among the studies included in this article, metal leaching and, in particular, Re leaching are the most commonly found type of catalyst deactivation. Furthermore, when Re leaching occurs, a strong deactivation is typically observed (Fig. 10 ). Even though the graph is a raw plot not taking into account other types of deactivation as well as ignoring the bimetallic nature of many catalysts, it becomes apparent that there are several drastic examples of catalyst deactivation where Re leaching strongly contributes to loss of catalytic activity. In addition, it shows that severe leaching of Re even beyond 10% is not uncommon.Leaching is not only problematic from the viewpoint of lowering catalytic activity; if the leached species remains in the liquid phase, the compound is irreversibly lost once the solid and liquid phase are separated. Furthermore, leached metal species can remain in and contaminate the product and/or the waste of the catalytic process, which is particularly problematic in case of toxic metals. While the latter is not the case for Re, it is on the other hand, economically fatal to lose significant amounts of such a precious element due to leaching. In particular, this non-recoverability makes Re leaching highly problematic and its prevention imperative for a successful catalytic process. Therefore, the different catalyst properties and reaction conditions will be discussed regarding their influence on Re leaching.A common approach to influence both catalytic activity as well as the stability of the catalyst is the use of different support materials. Therefore, several studies investigated support effects on leaching and/or deactivation of supported Re catalysts. This is in particular the case for the DODH reaction of vicinal diols, which was first discovered as a homogeneously catalyzed process and recent efforts have been dedicated to immobilizing Re species on different support materials to enable a heterogeneously catalyzed process. To prove the heterogeneous nature of the reaction significant effort is dedicated to identifying Re leaching and especially the contribution of dissolved species to the overall catalytic activity. This homogeneous contribution is typically investigated by removing the solid catalyst from the reaction medium (preferably at reaction temperature) and observing whether the reaction continues even without the solid catalyst, which is a strong indication for dissolved Re species. More details on the methodology of such ‘(hot) filtration tests’ will be discussed in a later chapter. In early studies [77,78,81] of supported monometallic Re catalysts for DODH reactions, both a clear contribution of dissolved species to catalytic activity and a significant loss in catalytic activity when reusing spent catalysts were observed. Therefore, Re leaching has from early on been considered a critical challenge for this reaction.Sandbrink et al. [77] tested NH4ReO4 on different supports and found a clear trend regarding the stability of the catalysts in the DODH of 1,2-hexanediol in 3-octanol at 170 °C. While Re/SiO2 was already deactivated completely in the second recycling run, the stability of other catalysts, which were not fully deactivated in the seventh consecutive run, increased in the following order: (Re/SiO2) < Re/ZrO2 < Re/C < Re/TiO2. After the catalysts were exposed to a reductive pretreatment at 300 °C, the influence of which will be discussed in a subsequent section, the trends of Re leaching observed during the reaction were similar. Reduced Re/TiO2 was most resistant to catalyst deactivation and showed no significant loss of catalytic activity in recycling runs. However, a hot filtration test confirmed the presence of leached Re species during the reaction even for this most stable catalyst. Interestingly, from the third consecutive catalytic run on leaching could not be observed anymore [77].In a similar study, Sharkey et al. [78] investigated a range of different support materials on which NH4RO7 was deposited followed by calcination. While dissolved Re species were not quantified in the reaction solution after the DODH of 1,2-decanediol, hot filtration tests indicated that Re leaching was increasing in the following order: ReOx/CeO2 ≈ ReOx/Al2O3 < ReOx/Fe2O3 < ReOx/SiO2. Furthermore, catalyst recycling was investigated with ReOx/Fe2O3. Product yield decreased by ca. 50% in the third run, which was suggested to be most likely due to Re leaching [78].A subsequent study by Sharkey and Jentoft [76] primarily targeted understanding the phenomenon of Re leaching observed previously. A series of supported ReOx was prepared and applied to the DODH of 1,2-dodecanediol in toluene at 150 °C. Subsequent recycling experiments (Fig. 11 , a) showed a drastic loss of catalytic activity already in the first reuse run. This was most significant for the most active catalyst Re/SiO2 (decrease by ca. 80%). Moreover, ReOx/TiO2 (which was the most stable catalyst in Sandbrink et al.‘s study [77]) and ReOx/C showed considerable deactivation while ReOx/ZrO2, ReOx/Al2O3 and ReOx/Fe2O3 were considerably less active but also deactivated to a considerably lower degree. These observations were in excellent agreement with elemental analysis of the reaction medium proving Re leaching. Cumulative Re leaching (Fig. 11, b) was clearly responsible for the low recyclability of the catalysts. In particular, 95% of Re was leached from ReOx/SiO2 during the initial reaction, followed by ReOx/TiO2, which lost 60% of Re in the first run and over all five runs >80%. From the other oxide supports ‘only’ 40–50% was leached overall. The authors concluded that due to strong interactions on some types of support materials, e.g. ZrO2, Re species are less easily soluble compared to others like SiO2. In particular, a reducible support was suggested to benefit leaching resistance. This was further proven by investigating leaching at different temperatures. While Re leaching was comparable at 25 °C and 150 °C for ReOx/TiO2 indicating facile dissolution, Re leaching from ReOx/ZrO2 was negligible at 25 °C. Interactions between Re and support were suggested to be stronger in the latter case, which requires elevated temperatures to induce leaching.Recently, Meiners et al. [168] investigated different zeolites as supports for the Re-catalyzed DODH of 1,2-hexanediol in 3-octanol. Stability was assessed for two (NH4)ReO4/H-ZSM-5 catalysts with different nSi:nAl ratio by recycling experiments. While the catalyst with higher Al amount (and lower nSi:nAl of 30 compared to 400) was initially twice as active, both were completely deactivated even in the first recycling run. At the same time, severe Re leaching was occurring with the high-Al catalyst losing 65% and the low-Al catalyst 85% of its initial Re loading (ca. 4 wt.-%) after 0.5 h reaction at 170 °C. Including additional characterization data, this was suggested to be linked considerably stronger interactions of the Re species with Al sites than Si sites, which also leads to Re being preferably located at Al sites.A very commonly used support material for DODH catalysts is CeO2, in particular, in studies by the Tomishige group [79,161,166,169–171]. While other factors may also play a role (e.g. H2 atmosphere or the presence of a second metal on the catalyst, factors that will be discussed later), none of these CeO2-based catalysts is as susceptible to leaching as some of the previously described catalysts were, as is apparent from comparing the DODH catalysts in Table 2. Even though no direct comparison of quantitative leaching for Re on different supports was conducted, it was concluded that Re leaching is more pronounced on carbon than on CeO2 [171], which is in agreement with Sharkey et al. [78]. In another study [170], it was found that high-valent Re species were dissolved during the initial time in the reactor and migrated from a carbon support to a CeO2 support, which can serve as an additional indication that ReOx species are more stable on CeO2 than on carbon supports.Re leaching is not as well investigated for other catalytic reactions as it is for DODH. However, there is one study proving the susceptibility of SiO2-supported Re to leaching beyond DODH. Jeong et al. [119] investigated monometallic Re catalysts for the HDO of guaiacol in heptane at 280 °C. While 20% of Re were lost from Re/SiO2 during the reaction Re/TiO2 was considerably more stable (0.5%).In general, the support material is a crucial factor determining the stability of supported Re catalysts towards Re leaching. In particular, the redox activity of the support appears a crucial criterion and strong interactions of Re with the support enhance its stability. At the same time, catalyst activity and selectivity also strongly depend on the support material. Unfortunately, higher stability toward Re leaching was at least in some cases found to correlate with a lower catalytic activity [76,119].Sharkey and Jentoft [76] also investigated the influence of Re content of supported catalysts on the leaching occurring during the DODH reaction. Both TiO2 and ZrO2 were selected and the Re content varied from 1 to 4 wt.-%. Independent of the support material leaching was lower for lower Re loading; however, there was hardly any difference between 1 and 2 wt.-%. For the TiO2-supported catalysts, which are more prone to leaching, ca. 60% of Re were lost in the first batch DODH reaction in toluene at 150 °C when the Re loading was 4 wt.-%. Using 2 wt.-% Re or less, only around 20–25% of Re were leached. As observed for different supports, there was an unfortunate correlation between leaching and catalytic activity. With increasing stability toward leaching, catalytic activity normalized to Re content of the solid catalysts decreased. Despite >50% of Re remaining on the catalyst, catalysts with 2 wt.-% or lower initial Re loading showed almost no catalytic activity in their third run. It should be noted that, for these cases, there was a strong contribution of homogeneously catalyzed reactions by dissolved Re species to the overall activity, which also affects the observed catalytic activity in catalyst recycling experiments. However, there is also the possibility of other types of catalyst deactivation limiting the reusability of supported Re catalysts in this case [76]. Regardless its eventual impact on the catalytic activity, catalysts with higher Re loadings are more prone to Re leaching.Besides this detailed study, it is very rare to encounter information on Re leaching for similar catalysts with different loadings in the literature. Even though the variation of metal loading as well as the ratio of two metals in case of bimetallic catalysts is often investigated to enhance catalytic activity and to gain insight into the way the catalyst functions, the issue of catalyst deactivation is typically only investigated for a single selected catalysts. In a study by Liu et al. [139], therefore, different earlier studies from the same research group [58,124,126] on similar SiO2-supported Re–Ir catalysts were in included in the assessment of the influence of metal loading on Re (and Ir) leaching. Despite differences in reaction conditions, it could be concluded that, in this case, a catalyst with higher loadings (ca. 20 wt.-% Ir, 6.8 wt.-% Re) was slightly more stable toward metal leaching during the aqueous-phase hydrolysis of glycerol than one with lower metal contents (ca. 3.9 wt.-% Ir, 3.2 wt.-% Re). While for the high-loading catalyst leaching of Re was below the detection limit (<0.25%), the low-loading catalyst showed detectable leaching (up to ca. 1%). Ir leaching behaved analogously. Considering only Re loading, this appears to be a contradiction to the study by Sharkey and Jentoft [76] presented before on monometallic DODH catalysts. Leaving other aspects like the different reaction system aside, there are at least two major aspects that need to be taken into account. First, the catalysts studied by Tomishige et al. are bimetallic. The (often inhibitory) influence of a second metal on Re leaching will be discussed in the subsequent chapter. Second, when the metal loadings were varied in Liu et al.‘s study [139], the Re/Ir ratio was not kept constant, but at higher loading, the Re/Ir was significantly lower (0.34 vs. 0.83). Therefore, it is likely that, in this case, the Re/Ir ratio is a deciding factor and the effect Re loading per se may not be visible.In a study by Ly et al. [157], supported Re–Pd catalysts were investigated for the aqueous-phase HG of succinic acid. Very significant leaching was detected during the heat-up phase (to 160 °C) in the reactor, which was conducted under inert Ar atmosphere. A catalyst with a Re loading of 3.4 wt.-% lost ca. 60% of its initial Re content due to leaching into the reaction medium while 100% of Re was leached from a similar catalyst with 0.8 wt.-% Re content. The Pd content was 2 wt.-% in both cases; however, the preparation method was different (sequential impregnation vs. reductive deposition). As will be discussed in Section 3.2.4, it is likely that, in particular, the differences in Re oxidation state (confirmed in an earlier study on the same catalysts [214]) arising from the different preparation methods is mainly responsible for the different leaching behavior of the two catalysts. Furthermore, the fact that one of the materials was exposed to air could play a role, which is also mentioned by the authors [157]. Besides the initially discussed study, which varied only the parameter of Re content and therefore allows for an unambiguous assessment of the influence of Re loading, all other studies varied other parameters at the same time as Re loading. Thus, in these latter examples, it is not clear to what degree Re loading plays a role, if any.When considering the possible influence of Re content on Re leaching, differences may actually arise from increasing Re particle sizes. Therefore, for this review article also correlations between Re particle size and Re leaching as well as catalyst deactivation were investigated based on the collected literature (Table 1 and Table 2). While it is difficult to draw universally applicable conclusions from such an inhomogeneous set of data, there appeared to be no indication that larger or smaller Re particles are more prone to leaching. Thus, while Re loading on supported catalysts can have considerable effect on its leaching, other factors are likely more influential.Another factor that can affect the leaching of Re from supported catalysts during their use in catalytic applications in the liquid phase is the presence (or absence) of additional metals. Unfortunately, there is no direct comparison available in the literature of how quantitative leaching of Re catalysts behaves in the presence of a second metal even though the effect of different Re-metal combinations are well-investigated regarding their effects on catalytic activity and selectivity. Furthermore, monometallic Re catalysts are often used for DODH reactions under inert or oxidizing atmosphere whereas bimetallic catalysts are typically used under reducing atmosphere and involve HL and HG reactions, which makes comparisons of different studies problematic. In addition, different support materials are used throughout the literature, which is a strong influencing factor as discussed in chapter 3.1.1.When Re and a second metal are combined on a support material, this usually results in changes in the redox properties of the metal phase, which can e.g. be observed by temperature-programmed reduction. A typical finding is that the combination of Re with noble metals, such as Pt [54,218], Pd [71,75,219], Ir [52,126,134], Rh [51,52], Ru [116,211,220], or Au [161,162,170], result in a lower reduction temperature for (alloyed) Re species. Di et al. [68] showed by XPS analysis that Pt stabilizes the metallic state of Re and lowers Re re-oxidation when exposed to air. Daniel et al. [221] observed that while monometallic Re/C could not be reduced in H2 at 200 °C, the presence of Pt on a comparable bimetallic Pt–Re catalyst facilitated Re reduction to partially reduced species. Moreover, Wang et al. [170] concluded that the presence of metallic Au particles can provide H2 for ReOx reduction, which prevents leaching compared to monometallic catalysts.There are further indications that bimetallic interactions can influence the leaching behavior of Re. For instance, Liu et al. [139,140] found that during reuse experiments of SiO2-supported Ir–Re catalysts in the HL of glycerol Re species accumulate on and cover Ir particles. It was suggested that some high-valent Re species are leached from the support and redeposit on the metal particles. This finding suggests that Re species are stabilized in the proximity of Ir metal particles. A similar phenomenon was observed by Ly et al. [157], who investigated the deposition of dissolved Re species on a Pd/TiO2 catalyst under HG conditions. Re was found to be primarily deposited in the vicinity of Pd, which was suggested to enable chemical reduction of Re by H2 activation. This further proves that, at least under reducing conditions, deposited Re species are stabilized in the presence of a noble metal. Re redeposition on Pd was also suggested in a different study [114]. Similarly, Pieck et al. [222] found that Pt catalyzes and thereby enables the reductive deposition of Re on Pt/Al2O3 under H2 atmosphere. Importantly, under the same conditions, deposition on Al2O3 was more than five times lower, indicating the importance on Pt.Wei et al. [50] investigated supported Pt–Re APR catalysts. They identified two different Re species on their bimetallic catalysts: Re associated with Pt and terminal Re–O. Only the latter was found to be leached from the catalyst while interaction with Pt apparently stabilized the other species against leaching. A similar behavior was also suggested in other studies [69,150]. During subsequent reuse experiments with bimetallic catalysts, stabilization was observed, which was explained by a non-stabilized Re species being completely leached in the initial stages whereas another metal-stabilized Re species remained deposited on the catalyst.Even though there is no unambiguous proof that the presence of noble metals can by reduce the tendency of supported Re species toward leaching, several individual findings strongly indicate a positive effect. Most likely, this is linked to a redox interaction that promotes the reduction of Re. All the presented examples of catalysts were tested under reducing H2 atmosphere, therefore it could be speculated whether this is a requirement for this stabilizing effect to occur. Interestingly, it was found that Pt on a bimetallic Pt–Re/C catalyst can also promote the oxidation of Re species [82]. The influence of the gas atmosphere during the reaction will be discussed in a subsequent chapter. It should be noted that, even though trimetallic and multimetallic catalysts were not discussed here due to the low number of studies on such catalysts, similar stabilization against Re leaching can be expected for those materials. It remains an open question, however, which metals are particularly suited to stabilize Re. Furthermore, for combinations of Re with non-noble transition metals, there was not sufficient data available to analyze the effect of those metals on Re leaching.The preparation method of supported Re catalysts can influence the properties of the catalyst in myriad ways. Depending on the Re precursor and the process chosen to load the support material e.g. the Re dispersion or the oxidation state of Re can vary, which in turn affects catalytic activity and stability. Among the most commonly used preparations methods are wet impregnation, incipient-wetness impregnation and reductive deposition. For the case of bimetallic or multimetallic catalysts, an important aspect is the order in which the different metals are introduced and what other treatment steps may be included in the preparation procedure. Regardless the preparation method, catalysts may be subjected to various pretreatment procedures, the influence of which is discussed in the next chapter.Regarding the influence of the preparation method on the leaching of Re during catalytic application, there is only limited information available. In the previously mentioned study by Ly et al. [157], bimetallic TiO2-supported bimetallic Pd–Re catalysts were investigated. On the one hand, one catalyst was prepared by successive impregnation, where Re was introduced onto a monometallic Pd/TiO2 (2 wt.-%) catalyst by wet impregnation. On the other hand, a (catalytic) reductive deposition procedure was used where the monometallic Pd catalyst was first activated in H2 at 300 °C. Afterward, an aqueous NH4ReO4 solution (like in the case of wet impregnation) was added and H2 gas was led through the dispersion. Both catalysts were also reduced in H2 gas at 450 °C. Interestingly, Re leaching was only detected during the heat-up phase of the catalytic experiment, which was conducted with aqueous succinic acid at 160 °C. Under Ar atmosphere, the catalyst prepared by successive impregnation lost 60% of its 3.2 wt.-% Re content due to leaching. The Re species on the catalyst prepared by reductive deposition, however, completely dissolved. Characterization revealed that chemical reduction allows for a more controlled deposition of Re at the interface between Pd and TiO2 [214]. Another major difference was the Re oxidation state for the two catalysts. Two Re species were identified by XPS, metallic Re0 and Re3+. The latter was the only Re species on the catalyst prepared by reductive deposition while successive impregnation (and subsequent reduction and passivation) resulted in a mixture of both states with a majority (70%) being Re3+. As previously discussed, higher-valent species are known to be considerably more soluble in the reaction medium. Therefore, the difference in Re oxidation state is likely the reason for the different leaching behavior. It is, however, not unambiguously clear to what degree the Re oxidation states are directly related to the preparation method itself or to differences in the catalyst pretreatment. While the catalyst from successive impregnation was passivated after reduction and stored under Ar, the catalyst prepared by reductive deposition, which was subjected to the same gas-phase reduction, was not passivated but exposed to and stored under air atmosphere. As will be discussed further in Section 3.2.2, air exposure can strongly influence the catalytic properties. In the study, it is also shown that in-situ preparation methods, which completely avoid the exposure to air and do not necessitate passivation, were a successful approach to prevent Re leaching. Unfortunately, no other studies on Re catalysts include the effect of different preparation methods on catalyst stability or Re leaching.Before a supported metal catalyst is applied in a reaction, it may be subjected to a variety of treatment procedures influencing the nature of the metal species in order to generate a specific one desired for the catalytic application. Besides an initial preparation method, like e.g. different impregnation techniques or reductive deposition to load the metal(s) on the support material, the preliminary material is in many cases thermally treated under defined gas atmospheres to generate the catalyst that is used in the reaction. Oxidative treatments using O2 or air atmosphere, aiming at decomposing metal precursors and forming a defined metal oxide species, are referred to as ‘calcination’, while reductive treatments to generate low-valent and often metallic species using (diluted) H2 gas are called ‘reductions’. In some cases, after reduction, the catalyst is passivated, i.e. mildly oxidized, to protect it against uncontrolled oxidation upon contact with air. The latter can be avoided by reduction directly in the reactor, in certain cases even in the presence of the reaction medium, directly prior to the catalytic phase of the experiment. Therefore, it can be distinguished between such ‘in-situ’ treatment compared to the for liquid-phase batch reactions more typical case of ‘ex-situ’ treatment. Using this variety of available catalyst pretreatment procedures is a common approach to tune the properties of supported metal catalysts. As discussed above, the catalytic behavior strongly depends on the oxidation state of the metal and changes during the reactions can be a reason for catalyst deactivation. Furthermore, different studies found that also the Re leaching is influenced by the pretreatment conditions a catalyst is subjected to. However, literature is mostly limited to the influence of reductive pretreatments.In the previously mentioned study by Sandbrink et al. [77], a series of different monometallic Re catalysts was investigated for the DODH of 1,2-hexanediol. Four differently supported Re materials were both directly used after the deposition of (NH4)ReO4 on the support and each catalyst was subsequently reduced in H2 atmosphere at 300 °C. In catalyst recycling tests, all reduced catalysts were considerably more stable than their unreduced counterparts (Fig. 12 ). The unreduced silica-supported Re catalyst was completely deactivated after the second run while the initially reduced catalyst still showed some activity in the fifth consecutive experiment. Similarly, unreduced Re/TiO2 (Re oxidation state most likely +7 as in NH4ReO4) showed a decreased activity to ca. 40% of its initial activity in the seventh run while the reduced analog (Re oxidation state ca. 4.5 calculated from XAF spectroscopy) did not show signs of catalyst deactivation. Unfortunately, this study does not include quantitative information on Re leaching. Considering similar studies [76,78], it is, however, very likely that Re leaching is the main cause of catalyst deactivation under the applied reaction conditions. Due to the lower solubility of reduced Re species compared to Re+7, reduction of the catalyst was suggested to increase stability toward Re leaching [77].The influence of reductive pretreatment on quantitative Re leaching was investigated in more detail by Chia et al. [51,53], who developed bimetallic supported Re–Rh catalysts for polyol HL. Catalysts were reduced at different temperatures under H2 atmosphere (and in one case not at all) to investigate the influence of this pretreatment step on the catalytic activity and stability. A clear trend could be observed indicating that reduction at higher temperatures is beneficial to prevent Re leaching during the aqueous-phase HL of 2-(hydroxymethyl)tetrahydropyran at 120 °C. The unreduced catalyst Rh–Re/C lost ca. 2% of its Re content due to leaching after the 4-h reaction. Prior reduction of the catalyst at 120 °C slightly reduced the leached amount to 1.2% and when the temperature was 250 or 450 °C, no Re leaching could be detected. Despite the drastic differences regarding both reaction conditions and the fact that a bimetallic catalyst was used compared to the previously discussed study, the effect of reductive catalyst pretreatment is the same. The reduction of Re-containing catalysts mitigates leaching most likely by lowering the amount of particularly water-soluble high-valent species.However, it has to be considered that the oxidation state is crucial in governing catalytic activity, as e.g. outlined by Tomishige et al. [26]. This aspect was already discussed in Chapter 2.7 since changes in Re oxidation state during the reaction can be one type of catalyst deactivation. Manipulating the Re oxidation state by pretreatment procedures to increase stability, therefore, can have the negative side-effect of lowering catalytic activity. This was observed by Chia et al. [51,53]. While reduction at 250 °C compared to 120 °C completely prevented Re leaching catalytic activity decreased by ca. 40%. Thus, in this case as well as in other reactions where high-valent Re species are required, there is a trade-off between catalytic activity and stability toward Re leaching. Interestingly, Sandbrink et al. [77] encountered findings (gradual loss in DODH activity of ReOx/TiO2 over multiple recycling runs when changing reduction temperature from 300 °C to 400 or 500 °C) that can also be interpreted in this way.Another important aspect is that while pretreatment procedures can be used to control the initial state of a supported metal catalyst, it may change strongly under reaction conditions. During reactions under reducing or oxidizing atmosphere, the Re oxidation state can be altered almost immediately. In particular, for HG or HL reactions, it was found that already during the heat-up phase of the reaction the catalyst was reduced [119]. In some cases, this was even a deliberate choice and considered an (additional) in-situ catalyst reduction [74,131,139,140]. In many other cases, this effect is neglected even though, as shown in Chapter 2.7, changes in Re oxidation state in spent catalysts compared to the fresh catalyst are not a rare finding.Another type of unintentional ‘pretreatment’ is the exposure of supported metal catalyst to air. This is particularly relevant when transferring the freshly reduced catalyst from ex-situ reduction to the reactor. Reduced rhenium species are known to be sensitive to air due to their oxophilicity and prone to oxidation [51,57,68,135,151,158,214,223]. Therefore, handling a reduced Re catalyst in air prior to the catalytic experiment as well as before subjecting it to a characterization method can be problematic due to the alterations in the catalysts’ properties that can occur.Considering the beneficial effect of catalyst reduction before conducting a catalytic experiment, the influence of re-oxidation due to air exposure can be expected to have a negative influence on Re leaching. In the previously mentioned study by Ly et al. [157], one catalyst was exposed to air and showed complete leaching of all its Re during the heat-up phase of the catalytic experiment under Ar atmosphere. A second catalyst that was passivated and kept under inert atmosphere without uncontrolled air exposure showed less pronounced Re leaching (70%). For these examples, however, also Re loading and catalyst preparation were different and offer possible explanations for the differences in Re leaching. In the same study, an in-situ preparation method was developed, where Re was introduced directly in the reactor, which makes catalyst transfer (and possible air exposure) completely obsolete. In contrast to the partially oxidized catalysts, the in-situ prepared catalyst showed no significant Re leaching under inert atmosphere in aqueous medium. Even though the Re oxidation state was not determined for the latter catalyst, it is highly likely that the protection from oxygen prevented the oxidation of Re to more water-soluble high-valent species.Exposure of catalysts to air is also relevant for the recovery procedure in recycling experiments. Wang et al. [171] found that Re leaching could only be detected during the recovery of ReOx/C and ReOx-Au/CeO2 catalysts (0.25% leaching) when the catalysts were separated from the reaction medium in air. In the absence of air, leaching was <0.01% of the overall Re amount on the catalysts. This can be explained by the oxidation of Re to soluble species. A more drastic example was reported by Sadier et al. [137] investigating Rh-ReOx/ZrO2 catalysts for aqueous-phase HL reactions. While Re leaching was insignificant during the reaction, as analysis of the reaction medium showed, considerably leaching occurred during separation and washing of the catalyst. Analysis of the solid catalyst revealed that Re loading decreased from 3.2 wt.-% to 2.5 wt.-% during the recovery procedure.There are several other studies on Re-containing catalysts that found that exposure of the catalyst to air during recovery has detrimental effects to the catalysts' recoverability. However, the role of Re leaching is not always clear, even though the previously presented studies indicate that leaching is likely to play a decisive role. One example is studies by Liu et al. [139,140] on supported Re–Ir catalysts for polyol HL. Catalysts only retained their catalytic activity if the recovery procedure was conducted in ways that prevented the catalysts from coming into contact with air. For catalysts with low Re loading this even required degassing of water used to wash the catalyst. Similar to the study by Sadier et al. [137], the reaction medium did not contain significant amounts of leached Re that could explain catalyst deactivation. It could be speculated whether also in the case of the catalysts by Liu et al.‘ [139,140] leaching primarily occurred during the recovery procedure itself. However, other possibilities that were discussed is a migration of Re species (probably via dissolved species) that leads to an accumulation of Re on top of Ir particles that is detrimental to catalytic activity [139] or that the oxidation of Re itself is the cause of catalyst deactivation [140]. Furthermore, an earlier study by Takeda et al. [151] showed that also for Re–Pd/SiO2 catalysts for stearic acid HG, the catalyst recovery procedure has to be conducted without exposure to air to avoid catalyst deactivation.In agreement with the previous studies, Haus et al. [224] found that their Pt–Re/TiO2 HG catalyst suffered severe deactivation due to Re leaching during catalyst recycling. Also in this case, Re oxidation upon air exposure was suggested as the main cause for the catalyst's susceptibility toward leaching. In particular, a washing step with water during the recycling procedure was highly detrimental and its omission mitigated catalyst deactivation.Overall, the examples presented here conclusively show that air exposure at different stages of the catalyst lifetime can lead to severe Re leaching and subsequent catalyst deactivation. Consequently, attention should be paid to ensure suitable handling of the catalysts at all times.Like for many other chemical processes, Re leaching from solid catalysts can be expected to be strongly influenced by the temperature of the process. First, the solubility of Re species is temperature dependent. It can therefore be expected that leaching is more pronounced at higher reaction temperatures in the case that solubility is limiting the leaching process. Consequently, when the reaction mixture is allowed to cool down before separation of the solid catalyst, this can result in the redeposition of the dissolved species [76,78,81]. This also has implications for the limitations to detect Re leaching and will be discussed in more detail in a subsequent chapter. Sharkey and Jentoft [76] also found that whether or not temperature-dependent redeposition occurs is also dependent on the other parameters of the reaction system, in their particular case the concentration of diol molecules that was depending on the reaction progress.Besides the temperature effect on Re species solubility, there is also the possibility that the dissolution of the Re species is a thermally activated process. This aspect was investigated by Sharkey and Jentoft [76] for two oxide-supported ReOx catalysts (Fig. 13 ). As previously discussed, the differences between the leaching behaviors of the two different catalysts arise from the different support-metal interactions. In case of the TiO2 support, Re leaching is essentially temperature-independent in the investigated range. On the other hand, ReOx/ZrO2 shows a typical behavior for thermally activated Re leaching. While at room temperature negligible Re was dissolved, Re leaching is clearly apparent at 150 °C. In general, it can be assumed that thermally activated leaching can be observed in cases of strong interactions of Re species with the support material, a second metal or a promoter. When the Re oxidation state is affected as a result of these interactions, it can also be expected to play a role in governing the temperature dependence of the Re leaching behavior.Both previously discussed effects result in increased Re leaching at higher temperature. However, in the presence of a reducing atmosphere, there is also the possibility to observe reductive deposition of dissolved Re species at elevated temperature. Ly et al. [157] found that dissolved Re7+ species could be rapidly and almost completely (>95%) deposited in-situ onto a Pd/TiO2 catalyst under H2 atmosphere at 160–180 °C. While in this case the reductive deposition is a deliberate method of catalyst preparation, the previously discussed (section 3.2.1.) in-situ reductive pretreatment of supported catalysts is a comparable process and is a measure to mitigate leaching by lowering the amount of oxidized Re species [119]. Regarding the temperature-dependence of the Re reduction as well as the deposition process, no data are reported. It can, however, be expected that, similar to gas-phase reduction processes, there is a threshold temperature required that is characteristic for different Re species. Moreover, it can be influenced by the catalyst composition and depends on the process conditions.Therefore, process temperature has influence on Re leaching and deposition processes by governing solubility, via activated leaching processes as well as by influencing temperature-dependent redox reactions between different Re species. Among the studies included in this article, there is no clear overall trend regarding the influence of reaction temperature influence on Re leaching and deactivation, which is, on the one hand, a consequence of the described different underlying processes. On the other hand, there are other parameters that play a more pronounced role in governing Re leaching behavior. While it may appear reasonable to assume that at higher reaction temperatures leaching is more prevalent, this is not supported by the compiled data. Even though DODH processes are typically conducted at comparatively mild reaction temperature between 100 and 200 °C and often <150 °C, Re leaching is very common and severe due to other leaching-promoting factors. Overall, the effect of reaction temperature has to be considered in the context of the reaction system and its characteristic process parameters.Due to the nature of the leaching process, which is a transfer of the leached species from the solid phase of the catalyst into the liquid reaction medium, the reaction medium must be expected to play a crucial role in governing the leaching process. As mentioned previously, high-valent Re oxides are known to be most prone to leaching in water and polar oxygenated solvents [3,215]. Due to the ionic nature of these species, e.g. the [ReO4]- ion, their solubility is higher in polar solvents. Reviewing leaching of various metals from solid during biomass conversion, Sadaba et al. [10] concluded that polar media can be considered more aggressive and therefore leaching is, in general, more pronounced in polar reaction media than in less polar ones.Catalyst stability is, however, rarely a main priority when selecting a suitable solvent for a catalytic reaction. For the catalytic conversion of many polar, biomass-derived compounds like, e.g. organic acids, polyols, or sugars, water is often the solvent of choice and the significance of aqueous-phase process is apparent from Table 1 and Table 2. Typical catalytic processes in aqueous phase are reforming, HL or HG of biomass-derived molecules. On the other hand, less polar molecules like fatty acids or lignin-derived aromatics are typically dissolved in non-polar hydrocarbons. In general, due to the use of H2 for most of the reactions catalyzed by supported Re-containing catalysts, the use of oxygen-containing solvents, with the exception of 1,4-dioxane, is rare. Only in case of DODH, alcohols are commonly used as solvents.DODH is also the only Re-catalyzed reaction for which the influence of the solvent on Re leaching was systematically investigated. Sharkey and Jentoft [76] studied leaching of a TiO2-supported monometallic Re catalyst for the DODH of vicinal diols in different solvents (Fig. 14 ). The results of detected Re leaching into the liquid phase after 15 min in the respective pure solvent at 150 °C confirms the outlined assumptions regarding the influence of solvent polarity. Re leaching was insignificant for non-polar toluene due to the low solubility of the Re species but slightly increased for more polar solvents like secondary alcohols. In pure water, however, the concentration of leached Re was more than one order of magnitude higher than in all other solvents. Therefore, Re leaching follows the trend of solubility of high-valent Re species in pure solvents. Interestingly, in case of the DODH reaction, the presence of especially the reactant can drastically enhance leaching in less polar solvents by the complexation of Re, which will be outlined in the next section.Even though water is the solvent that has the highest potential to cause Re leaching, there are myriad examples of catalysts stable in aqueous-phase reactions (see Table 1 and Table 2). In particular under H2 atmosphere and in combination with noble metals, Re-containing catalysts rarely show leaching >1%. Therefore, on the one hand, it appears that polar solvents and especially water can strongly enhance Re leaching for catalysts that, due to other factors, contain considerable amounts of easily leached Re species. On the other hand, there is no clear indication that polar solvents have a corroding effect that can convert deposited Re into more leachable species. Regardless, it should be noted that due to a lack of targeted investigations into the leaching phenomenon (besides the discussed study on DODH in different solvents), no definitive conclusions can be drawn.For the case of heterogeneous catalytic DODH over Re catalysts, an investigation by Sharkey and Jentoft [76] revealed that the diol reactant can play decisive role in causing Re leaching from supported Re catalysts such as ReOx/TiO2. As shown in Fig. 14, in non-polar solvents, Re leaching is enhanced drastically in the presence of diol molecules. In case of the least polar solvent, toluene, this effect is most pronounced, where without diol, no significant leaching is observed, but in its presence, the concentration of leached Re is greater than the amount dissolved water. Interestingly, Re leaching was also promoted in non-polar solvents when the reaction product decene was present, but by a factor of ca. 2–3 lower than for the corresponding reactant 1,2-decanediol. During the course of the catalytic DODH reaction, the concentrations of both compounds change, which also has implications for Re leaching. When the DODH reaction was stopped at around 50% conversion, Re leaching was very severe and the recycled catalyst was ca. 90% less active than the fresh catalyst. However, running the reaction to full diol conversion drastically reduced Re leaching and the catalyst deactivation was significantly lower. It was concluded that primarily the diol enhances Re solubility, and after its consumption, Re redeposition onto the catalyst takes place.The key to this phenomenon lies in the interaction of the Re species with the reactant molecules. Re is suggested to be present as high-valent species on the ReOx/support catalysts, which suggests that the catalytic cycle is similar to the case of homogeneous reactions catalyzed by Re+7-based catalysts like methyltrioxorhenium or similar [225–227]. It is known that the diol reactant binds to the (partially reduced) ReOx species as a bidentate chelating ligand. In case of diols with a hydrophobic alkyl chain, such as the investigated 1,2-decanediol and 1,2-hexanediol, the corresponding chelate complex can be expected to have a considerably higher solubility in non-polar reaction media, e.g. toluene, than the oxidic Re species alone. As mentioned, also the presence of alkene product leads to enhanced leaching due since the reverse reaction with ReOx species also leads to the formation of a chelate complex, however, to a lower degree. Additional tests with a primary alcohol and 1,4-butanediol confirmed that the vicinal 1,2-diol motif is crucial for this leaching mechanism, which further suggests a bidentate chelate structure of the leached Re species [76].Further evidence for the Re-diol-complex was provided by UV–Vis spectroscopy [76,172]. As shown in Fig. 15 , spectra of the leached Re species from solid Re catalysts are similar to complexes found during the homogeneously catalyzed reaction for the case of the model compound (R,R)-(+)-hydrobenzoin. Moreover, the band at 476 nm is not a characteristic of dissolved (ReO4)- ions. While additional characterization of the leached Re complexes by EPR [76] was inconclusive, 1H NMR spectroscopy [172] further indicated that a Re-diolate complex is the leached species formed.Leaching of Re via the chelate complex mechanism is characteristic for the DODH over supported Re catalysts, where it was suggested or confirmed in several studies [71,76,81,172]. It requires specific conditions, i.e. non-polar solvent, vicinal diol, and probably also high-valent Re species. When water is used as a solvent, which is common for other Re-catalyzed reactions, this phenomenon can be neglected (Fig. 14).The gas atmosphere, under which a reaction is performed, can have significant effects on the oxidation state of supported Re catalysts as described in Section 2.7, and consequently, also influences Re leaching. In general, under reducing, i.e. H2-containing, atmosphere, the easily leached high-valent ReOx species can be reduced to more stable, low-valent, or metallic species, an effect that is occasionally utilized as an in-situ reduction [74,131,139,140] of the catalyst. On the other hand, under oxidizing conditions, the reverse process is possible resulting in overall increase in Re oxidation state. Oxidation processes can be further enhanced under hydrothermal conditions [82,106]. Unfortunately, there is not a single study available where the influence of the gas phase composition on the Re leaching behavior is systematically investigated. This is mainly due to the fact that many types of reactions, e.g. HG, HL or HDO, demand a H2 atmosphere. It is, however, noteworthy, that among the plethora of studies on these reactions, the number of severe cases of Re leaching in heterogeneous catalytic reactions is very small (compare data in Table 1 and Table 2).In contrast to that, Re-based catalysts for DODH reactions, which are typically conducted under N2 or air, are often reported to suffer pronounced Re leaching. While many other parameters need to be considered (often no reductive catalyst pretreatment, leaching via Re-reactant complexes possible, often monometallic catalysts), the non-reducing atmosphere is one of the major characteristics of the reaction and is likely playing a crucial role. At the same time, alcohol solvents, that can act as reducing agents, and/or molecular reducing agents are present, whose influence is also largely unknown. When DODH is coupled with HG and H2 is used, Re leaching also appears less relevant than in pure DODH processes (compare data in Table 2). However, considering the lack of direct evidence, the influence of gas phase composition on Re leaching remains ambiguous.Additional insights can be gained by looking at the reverse process, i.e. deposition of dissolved Re species. Ly et al. [157] found that dissolved Re7+ species could be reductively deposited onto a Pd/TiO2 catalyst in-situ (Fig. 16 ). Under non-reducing Ar atmosphere, about 10% of the dissolved Re was found to deposit on the catalyst at 160 °C. This was explained by the sorption of (ReO4)- on Pd/TiO2 similar to comparable studies on Re sorption on a Pd/C catalyst [228,229]. Another possibility is that a small amount of hydrogen was adsorbed on the Pd/C catalyst, which lead to the observed amount of reductive deposition of Re, as observed by Pieck et al. [222], for a Pt/Al2O3 catalyst under inert atmosphere. When H2 gas (150 bar) was added to the initially described Re deposition experiment onto Pd/TiO2, the amount of Re in the solution was rapidly decreased to < 5% of the initial value (Fig. 16). This clearly indicates that the deposition of Re is governed by a redox reaction and the composition of the gas phase is crucial for the process. However, the study by Pieck et al. [222] also showed that for the reductive Re deposition, the presence of Pt was crucial to facilitate Re reduction.In this context, it should also be considered that exposure of Re catalysts to air in between catalytic experiments was found to have detrimental effect on catalyst recyclability. As outlined above (Section 3.2.2.), this is in most cases probably due to Re oxidation when in contact with air and subsequent leaching of the more water-soluble high-valent species during washing steps or in the reuse experiment. Overall, there are strong indications that reducing atmosphere can mitigate Re leaching by preventing Re oxidation.The addition of mineral acids, typically H2SO4, or solid acids, such as zeolites or ion exchange resins, as co-catalysts in the catalytic HL of biomass-derived molecules over bimetallic Re-containing catalysts goes back to investigation by the Tomishige group [124,126,127]. In the aqueous-phase HL of glycerol over Ir-ReOx/SiO2, the presence of H2SO4 (pH = 3) as a co-catalyst strongly influences catalytic activity and selectivity. Moreover, the catalyst was only reusable without significant loss in catalytic activity when H2SO4 was present. This is most likely due to the reduced Re leaching of <0.3% (compared to 2%) in the acidified reaction solution [124,126]. Further investigations, including density functional theory calculations, showed that H2SO4 stabilizes protonated Re-OH sites on the catalyst [138,139]. While this can explain the differences in catalytic activity, in remains unclear how it mitigates Re leaching. The beneficial effect of H2SO4 was also proven for a supported Rh-ReOx for the aqueous-phase HG of erythritol [135]. Re leaching decreased from 6% to 2% upon acid addition, which indicates similar behavior compared to the initially discussed Ir-based catalysts.Besides aqueous mineral acids, solid acids can have a similar stabilizing effect as shown by Nakagawa et al. [58]. Both an ion exchange resin (Amberlyst 70) and different zeolites were studied. Re leaching was only investigated for an H-ZSM-5 co-catalyst in combination with Ir-ReOx/SiO2. The amount of Re leaching (0.4%) was considerably lower than without co-catalyst (2%) but higher than with H2SO4 (<0.3%). Most notably, pH was also not as low in the presence of the zeolite (4.4) compared with H2SO4 (2.8, compared to 5.7 without co-catalyst). It remains ambiguous to what extent the H-ZSM-5 co-catalyst also mitigates the deactivation of the Re-based catalyst during recycling experiments. When calcination treatments were applied between the runs, both with and without the zeolite catalytic HL activity declined by ca. 12% over 3 runs. However, the presence of the zeolite enhances activity by ca. 75% compared to only Ir-ReOx/SiO2. It is noteworthy that the ion exchange resin had an even stronger promoting effect but its influence on the catalyst deactivation was not analyzed in detail since the catalyst recovery procedure required calcination. It was suggested that the addition of solid acids results in enhanced amounts of Re-OH on the supported Ir-ReOx catalyst [58,128], similar to H2SO4.Overall, the stabilizing effect of acids is remarkable but it is still not understood. Moreover, it is unclear, whether it is limited to the specific, bimetallic catalysts and/or the HL reaction. One speculative explanation could be that Re leaching is suppressed by the protonation of ReOx species due to alterations of the redox properties, possibly facilitating Re reduction or stabilizing (partially) reduced oxidation states or due to changes in surface charge. Those were found to play a role, e.g. during reductive Re deposition [228,230]. In general, the pH of the reaction medium can be an important factor impacting metal leaching from solid catalysts [10], but its effect has not yet been systematically studied for Re-based catalysts.The vast majority of Re-catalyzed reactions in the context of liquid-phase biomass upgrading are conducted as batch experiments, as Table 1 and Table 2 show. However, a few examples of mainly APR and HL reactions have also been conducted under continuous flow operation. In principle, continuous operation allows for additional insight into the time-dependent leaching behavior, similar to the observation of catalyst deactivation over time-on-stream that can be followed without additional effort and over comparatively long periods of time. Tracking Re leaching continuously may, however, be analytically challenging in case of low amounts of metal leaching and high liquid flow rates due to low concentrations. To the best of our knowledge, time-dependent Re leaching has not yet been studied even though it could be a promising tool to understanding Re leaching by revealing kinetic information of the processes. In particular, it could reveal whether leaching takes place continuously or only during specific phases of the reaction. While in some cases catalytic activity stabilizes overtime-on-stream [51,101], this is not the case for others [53,83,103]. Here, the data on the corresponding leaching behavior would provide further insights into how and Re leaching contributes to catalyst deactivation. However, in current literature on continuous, heterogeneously catalyzed reactions the investigation of Re leaching typically relies on elemental analysis of the catalyst after the reaction, from which only the overall amount of leached metal is available.Only one study with supported Re catalysts investigated differences between batch and continuous operation mode. Chia et al. [53] compared the stability of a RhRe/C catalyst for the aqueous-phase HL of 2-(hydroxymethyl)tetrahydropyran for both operation modes. It was observed that over 100 h time-on-stream under continuous operation at 120 °C, the catalyst strongly and continuously deactivated to ca. 60% of its initial activity, which was explained by severe Re leaching. In contrast, in a batch experiment (4 h) under comparable conditions, only 1.2% of Re were leached from the catalyst (same pretreatment conditions). It was, therefore, suggested that leaching is enhanced under continuous flow operation. It remains unclear, however, whether this is merely an effect of the longer exposure time or an inherent effect of the operation mode. Moreover, there was no check for other types of deactivation. On the other hand, the fact that differently pretreated catalysts showed no detectable Re leaching in the batch experiments also did not deactivate under continuous flow can be seen as further evidence that the flow operation can exacerbate the leaching process.In cases where leaching is governed by the solubility of the Re species or by the concentration of a complexing agent, an equilibrium is reached that limits the total leaching amount during batch reactions. In contrast to that, under continuous flow the leached species is constantly removed from the reactor and leaching can continue. This is particular problematic in cases where Re species are leached at an early stage but can later redeposit on the support material [76,78,81,157]. The same phenomenon is known, e.g. in the better known case of the Pd-catalyzed Heck reaction [217,231,232], and is a critical challenge to the viability of continuous processes.In the previous sections, it was shown how different individual catalyst preparation and pretreatment conditions as well as process parameters can influence Re leaching and thereby cause catalyst deactivation. A common aspect of many of them is that they exert their influence via redox reactions. This is mainly due to the fact that the oxidation state of Re plays a crucial role for many of them due to the significantly higher (water) solubility of high-valent Re species (in particular Re7+) compared to low-valent or metallic Re [215,233].Most prominently, exposing Re-based catalysts to reducing or oxidizing conditions was shown to have often pronounced effects on Re leaching and catalyst stability. Reductive pretreatment of solid Re catalysts was shown to mitigate Re leaching and initially leached Re species were found in some cases to redeposit under reducing reaction conditions. On the other hand, exposure of catalysts to air in several cases leads to enhanced Re leaching. Besides, there are indications that e.g. the presence of an additional noble metal on bimetallic Re catalysts can stabilize deposited Re species by catalyzing their reduction.For a number of Re catalysts included in this study, the initial average Re oxidation state is known or can be estimated from the characterization data provided. In Fig. 17 the respective amount of Re leaching is plotted against initial average oxidation state. As expected, Re leaching appears to be considerably more likely when Re is predominantly in a high oxidation state, in particular +7. At the same time, there is an indication that Re leaching is also more pronounced under non-reducing, i.e. N2 or air, atmosphere whereas H2 appears to have a mitigating effect. However, it should be noted that several other factors, e.g. support-metal interactions, metal-metal interactions, structure of the catalyst, reaction conditions, and solvent, can significantly influence Re leaching as well. Thus, Fig. 17 cannot be as conclusive as a dedicate study into these effects. A good example for this is the huge variation in Re leaching found in the study by Sharkey and Jentoft [76] even though the Re oxidation states of the catalysts is given, the Re precursor and the calcination pretreatment very likely comparable around +7.Overall, however, it is apparent that Re oxidation state is arguably the most important material property governing Re leaching. Therefore, controlling the Re oxidation state and preventing the formation of more leachable, high-valent Re species is an important parameter in mitigating the deactivation of solid Re catalysts in liquid media by Re leaching. It is noteworthy that the Re oxidation state was also suggested to be the crucial property in governing the catalytic activity and selectivity of Re-containing catalysts [26]. Another conclusion from the correlations in Fig. 17 is that due to the typically applied reductive catalyst pretreatment and the reducing atmosphere in HG, HDO, HL, and also APR processes, solid Re catalysts applied in these processes are far less subject to extensive Re leaching than in the particularly challenging DODH reaction. Importantly, in the latter reaction type, high-valent Re species are necessary to obtain catalytically active Re materials [26], and it should be highlighted again that the change in oxidation state itself is a type of catalyst deactivation that can significantly influence catalytic activity and selectivity (see also Section 2.7).One of the key insights from the systematic analysis of all available studies on Re leaching during biomass-related liquid-phase reactions it that both leaching and redeposition can happen at various stages of the reaction. Moreover, also typically neglected phases during the lifetime of supported Re catalysts, such as the transfer between pretreatment and reactor set-up or heat-up stages during the reaction, can have pronounced effects on leaching behavior. Therefore, a summarizing overview of possible leaching and redeposition effects and different influencing factors throughout the catalyst lifetime is provided in Fig. 18 .The initial steps of catalyst preparation and ex-situ pretreatment steps, e.g. calcination or reduction, are highly relevant in regards to Re leaching since they determine the initial physical and chemical properties of the catalyst, in particular the initial oxidation state of the catalyst. Similarly, catalyst regeneration procedures can be used to regenerate the initial Re oxidation state. Even though liquid-phase Re leaching is not directly occurring during the catalyst preparation, pretreatment or regeneration, two related effects have are taking place. Many typical impregnation and deposition procedures have similarities to Re leaching and deposition processes during reactions since they are governed by the same parameters, e.g. the solubility of Re species and redox reactions. A detailed look into the preparation processes, however, is beyond the scope of this article and an overview of different preparation techniques can be found, e.g. in the study by Gothe et al. [2]. Second, during calcination pretreatment of Re-based catalysts, Re can be lost via sublimation of volatile Re7+ oxides. In some cases [139,197], this has led to significant decrease in Re loadings before the catalyst was even applied in the liquid-phase reaction. Re leaching observed during the reaction phase depends strongly on the initial Re oxidation state (Section 3.4.), which is a result of all preparative and pretreatment steps. Moreover, it has been shown that also the transfer process between catalyst pre- or regeneration treatment can cause significant leaching when this leads to exposure of a reduced catalyst to air due to the facile oxidation of Re species (Section 3.2.2.). Consequently, these processes need to be considered to ensure that Re leaching is avoided as best as possible during the reactions itself. Particular attention should also be paid to filtration or washing steps in between subsequent catalytic reuse experiments. Cases have been reported where conducting these processes under air atmosphere resulted in severe leaching and the reused catalyst contained significantly lower amounts of Re [137].As soon the fresh or recycled catalyst is exposed to the reaction medium, Re leaching can occur. However, it needs to be considered that during different stages of the reaction the conditions change, which can lead to a variety of possible leaching and redeposition processes. Depending on the catalyst, Re leaching may occur directly when the catalyst is exposed to the reaction solvent even at room temperature, as was shown for the case of a ReOx/TiO2 catalyst [76]. In this case, Re is leached before the catalytic reaction is beginning to taking place. In case of Pd-ReOx/TiO2 catalysts investigated by Ly et al. [157], initially up to 100% of Re was leached into the aqueous reaction medium during the heat-up phase under inert atmosphere. However, nearly all of it was redeposited via reductive deposition as soon as the HG reaction was started by pressurizing the reactor with H2, and throughout the following reaction time, the concentration of leached Re species in the reaction medium remained low. Therefore, in this particular procedure, Re leaching is only problematic in the initial heat-up phase. On the other hand, in other studies, a reducing atmosphere of H2 is applied from the very beginning of the reaction to allow for in situ reduction of Re species and to prevent Re leaching [74,119,131,139,140].To understand the mechanism of the processes involved in Re leaching, it is required to understand the time-dependent behavior. In the previously described examples, it can be typically assumed that leaching or redeposition happen very fast, even though evidence is rarely provided, e.g. for the reductive (re-)deposition of Re on Pd/TiO2 as soon as H2 is applied [157]. There is, however, also the possibility of gradual leaching over reaction time, which may also be related to a slow oxidation of the catalyst. Due to the lack of time-dependent leaching data in the available literature, no such case has been identified so far. Especially, under continuous flow operation when catalytic activity was found to decline gradually this could be a reasonable explanation (Section 3.3.6.).A very interesting conversion-dependent Re leaching behavior has been observed during DODH reactions [76]. As discussed in detail in Section 3.3.4, the complexation of Re species with the diol reactant results in pronounced Re leaching. As the diol concentration decreases toward complete conversion, Re is found to redeposit on the support material. In this example, the understanding of the leaching mechanism and the corresponding conversion- and time-dependent leaching behavior is crucial to mitigating the loss of Re during recycling experiments.Finally, redeposition of leached Re species after the main reaction phase can also be a results of cooling down the reactor [78,81]. While Re leaching was considerably during the reaction in these studies, the recovered catalysts could be reused without significant loss in Re content as well as with comparable catalytic activity due to the nearly complete redeposition. In general, it could be worthwhile to consider deliberate aftertreatment procedures that promote the deposition of Re species after the actual reaction period is completed. This could significantly enhance catalyst recyclability if Re leaching cannot be avoided.Overall, Re leaching can occur at many stages during the lifetime of a catalyst, inside and outside of the reactor. Moreover, the conditions the catalyst is exposed to at any stage can have, in certain cases severe, effects on catalyst stability. Knowing the underlying processes and relevant parameters governing Re leaching is crucial in avoiding or at least mitigating Re leaching.Investigations into catalyst stability and metal leaching are typically driven by the initial observation of catalyst deactivation over time-on-stream or in recycling experiments. These experiments reveal whether or not a catalyst is stable, however, alone they do not allow to distinguish which types of deactivation are present (as outlined in Chapter 2.1). Consequently, the observation of catalyst deactivation will not reveal directly to what degree leaching may play a role in causing the decrease in catalytic performance. On the other hand, the information is crucial for assessing the consequences of Re leaching when it occurs, in particular when the contribution of other deactivation processes is known. In this context, it should be mentioned that while many other types of deactivation are reversible and the catalyst can (partially) be regenerated, this is often not the case for Re leaching.In addition to experiments revealing catalyst deactivation, other approaches are required to directly identify and quantify Re leaching (Fig. 19 ). Elemental analysis of both the recovered catalyst and the reaction medium after the catalytic experiment allow for a quantitative assessment of the amount of Re leached from the solid catalyst to the liquid. Techniques that are commonly applied are primarily atomic emission spectroscopy or mass spectrometry with inductively coupled plasma (ICP-AES, ICP-MS) but also atomic absorption spectroscopy or X-ray fluorescence spectroscopy are used. The detection of the, in many cases very low, concentrations of leached metal as well as the precise determination of small differences in metal loading on a supported catalyst before and after the catalyst is challenging. Depending on the detection limit of the respective technique, the reliable detection of Re leaching is only possible to a certain limit, which can be in the range of several percent in case of supported catalysts as highlighted by Sadaba et al. [10]. Nevertheless, only elemental analysis can unambiguously and quantitatively proof Re leaching.To detect metal leaching, probably the most important aspect to consider is the time in the catalyst lifecycle at which the analysis is conducted and the corresponding conditions the catalyst is exposed to. As discussed in Section 3.5, Re leaching can differ widely during different stages of the lifetime. The typically applied procedure of separating catalyst and reaction ex situ after the reaction gives valuable insight whether irreversible leaching of Re from the catalyst is occurring. However, depending on the conditions of the separation process it is possible that the Re leaching detected by this procedure is not happening during the reaction itself but, e.g. after subsequent contact with air [137]. This can be avoided by preventing direct exposure of the catalyst to oxidizing atmosphere to enable an assessment of how much Re is lost during the catalytic application.Besides, the possible redeposition of Re species needs to be considered that can occur at the end of the reaction either due decreasing concentration of a reactant using as a complexing agent [76] or due to the lower solubility of Re species at lower temperatures [78,81]. While this does not result in an overall loss of Re metal, revealing Re leaching during the reaction is crucial to understanding the behavior of the catalyst and the mechanism of the reaction. The only possibility to reveal this hidden leaching is by separating the catalyst from the reaction medium under reaction conditions. Similar to sampling during the course of a batch reaction to gain information on the reaction kinetics, analyzing immediately separated reaction medium at different stages can reveal the time-dependent leaching behavior.Uncovering the presence of leached Re species is particularly important in revealing whether homogeneous catalysis contributes the observed overall catalytic activity. While a lower concentration of leached Re in reuse experiments in combination with decreased catalytic activity can be an indication that there is a contribution of dissolved Re species to the catalytic activity, this can and should be tested directly. This is possible by separating catalyst and reaction medium at an intermediate stage and observing the progress of the reaction of the liquid reaction medium alone under reaction conditions. Following Sheldon et al. [234], three cases can be distinguished. In the cases where either no Re leaching occurs (1) or the leached Re species are not catalytically active (2), no further reaction progress can be observed (or only conversion comparable to the control experiment without catalyst). On the other hand, when the catalytic reaction still occurs after complete separation of the solid catalyst, this proves the catalytic activity of leached metal species (3) and allows for a quantitative assessment of the homogeneous contribution to the overall reaction. The observation of catalytic activity of homogeneous species also serves as an indirect proof of Re leaching.Due to the previously discussed problem of possible redeposition, it is crucial to separate the catalyst from the reaction medium under conditions as close to the actual reaction conditions as possible. In particular, due to the lower solubility of Re species at lower temperature, this requires separation at reaction temperature, which is typically referred to as ‘hot filtration’. Therefore, ‘hot filtration test’ is a common phrase for this type of experiment to detect possible homogeneous catalytic contributions to a process that is supposed to or thought to be heterogeneously catalyzed. As indicated in Table 2, in a small number of studies, hot filtration tests were performed; in a few additional ones, separation was conducted at room temperature before heating up for the activity test of homogeneous species. Hot filtration tests have comparatively often been used in studies on the DODH reaction, probably due to increased awareness of catalytic activity of leached Re species given the origin of the reaction being a homogeneously catalyzed process. Moreover, the hot filtration tests have impressively shown that in some cases almost all catalytic activity originates from homogenous catalysis [76,78].Overall, the phenomena of Re leaching, the deactivation of solid catalysts and catalytic activity of dissolved species are interlinked, however, each should be verified directly since the observation of one does not necessarily allow conclusions on the others. E.g. when the hot filtration test indicates the absence of catalytically active dissolved species and the solid catalyst does not show signs of deactivation, this does not mean that no leaching is occurring. Still, non-active Re species can be leached that are also not active as homogeneous catalysts, as found in the study by Wei et al. [50]. Another example is reported in the study by Li et al. [141], where severe Re leaching did not affect catalyst recyclability. Therefore, Re leaching should always be confirmed by elemental analysis.Finally, additional analytical methods can be applied to gain additional insight into the nature of the leached Re compounds. As discussed in Section 3.3.3, this is particularly interesting when a complexing agent is involved in the leaching mechanism. Leached Re species were successfully detected and characterized using UV–Vis spectroscopy [76,172], an example is shown in Fig. 15. Moreover, NMR [172] and EPR [76] spectroscopy can be applied to gain further information on the chemical composition and the structure of the leached Re complex. In these cases, identification of the leached species was also crucially important to reveal information on the reaction mechanism.In the previous sections, Re leaching was primarily viewed as an undesired process resulting in catalyst deactivation and/or loss of precious Re metal. There are, however, also opportunities arising from Re leaching when it can be applied in a controlled manner. One such strategy relying on Re leaching is the in-situ preparation of Re-containing catalysts by reductive deposition. Ly et al. [157] observed that in the initial heat-up phase of the catalytic experiment under inert atmosphere >50% of Re was leached from their Pd-ReOx/TiO2 catalyst before it was redeposited upon the addition of H2. Instead of trying to prevent leaching from occurring, an in-situ deposition method was developed based on reductive deposition. This method relies on the metallic Pd sites of the catalyst, where the reduction and deposition of dissolved high-valent Re species occurs, and high H2 pressure (150 bar) at elevated temperature (160 °C). The in-situ prepared bimetallic catalysts were suitable for the HG of succinic acid and comparison with a catalyst prepared by conventional (ex-situ) reductive deposition revealed only slightly lower selectivity for the desired product 1,4-butanediol (18% compared to 23%). Unfortunately the influence on catalytic activity was not reported. Overall, the study showed that the catalyst preparation steps related to Re introduction can be omitted and a more efficient in-situ method can be applied.Re leaching was also utilized for a different type of catalyst system consisting of a physical mixture of two different solids by Tomishige et al. [170,171]. In these investigations, Re-based catalysts were developed for the catalytic conversion of 1,4-anhydroerythritol to 1,4-butanediol including an initial DODH reaction as well as HG and ether hydrolysis steps. In the initial heat-up phase of the reaction conducted in aqueous phase under H2 atmosphere, partial Re leaching from one material and redeposition on the second solid occurs. In the simplest case of ReOx/C and CeO2, the migration of Re species from the carbon to the CeO2 support was observed, as shown in Fig. 20 . The process results in the formation of high-valent ReOx species on CeO2, which are considered the main active site for the DODH reaction. Importantly, two separate processes are occurring in parallel. On the one hand, high-valent Re species are leached from the carbon support and redeposited on the CeO2, where they are considerably more stable against being leached again (see also Section 3.2.1.). On the other hand, the reducing reaction conditions result in the reduction of Re species to low-valent or metallic species, which are considerably less soluble than high-valent Re. Therefore, reduction mitigates the migration of Re species from the carbon support. Since the Re species on CeO2 are not as easy to reduce as the ones on carbon, they remain in a high-valent oxidation state required for DODH. Overall, this is an example for an innovative in-situ preparation method utilizing Re migration between two materials via leaching. It was also shown that the overall migration process depends on the redox properties of the catalysts and promoting ReOx reduction, which can be achieved in the presence of noble metals, mitigates the amount of Re leached from the carbon support. It should, however, be noted that these catalyst systems were not fully recyclable and, in particular, fresh ReOx/C had to be supplied.Besides the potential benefits of the purposeful in-situ catalyst preparation methods shown above, similar Re leaching-related processes might be occurring during the heat-up or the reaction phase of catalytic experiments of many other studies without being detected. As a consequence, the chemical and/or structural composition of the catalyst can be significantly altered from the initial ex-situ state of the catalyst. This highlights the importance of in-situ characterization methods, such as in-situ liquid phase XANES [212], to characterize the properties of the catalyst as it exists under reaction conditions.A second possibility to employ Re leaching in a controlled and advantageous manner has been suggested as ‘release and catch’ catalysis by Sharkey and Jentoft [76] for the case of Re-catalyzed DODH. The concept is based on the dependence of Re leaching on the concentration of the diol reactant, which decreases during the course of the reaction (details in Section 2.3.3.). While initially large amounts of Re are leached from supported Re catalysts, it later redeposits to a large degree when the diol is completely consumed. Thus, while the reaction is occurring Re species are dissolved and act as homogeneous catalysts, which were found to significantly contribute the overall catalytic DODH activity. Afterward, the solid catalyst with the redeposited Re species can be separated and reused. It should be noted, however, that in the example discussed here catalytic activity declined in recycling experiments, and it was suspected that unreactive Re species can be formed. Since the leaching of Re from DODH catalysts is very difficult to prevent due to a combination of unfavorable conditions, ‘release and catch’ concepts could be an interesting alternative, in particular when considering that often stability toward Re leaching comes at the cost of lower catalytic activity [76,77]. Thus, the concept allows for combining the advantages of homogeneous (high catalytic activity) with heterogeneous catalysis (facile recyclability). It is also worth mentioning that ‘release and catch’ concepts have also been employed for other catalyst systems [217].Due to its versatile chemical nature, Re has been used as a catalytically active metal in many different applications, in recent years also increasingly in the context of biomass utilization. While much attention is paid to the activity and selectivity of catalysts, catalyst stability is often neglected. The overview of stability data accumulated in this article on solid Re-containing catalysts applied in liquid-phase reactions in the context of biomass valorization reveals that several different types of catalyst deactivation need to be considered. While many severe causes of catalyst deactivation, e.g. fouling due to carbon deposition or catalyst poisoning, can be to a large degree reversible by suitable regeneration procedures such as calcination and/or reduction, Re leaching is particularly undesirably due to the irreversible loss of this precious and rare element. The plethora of studies reporting severe deactivation by Re leaching, especially compared to noble metals, indicates that its oxophilic nature makes Re particularly susceptible.A thorough look at possible influencing factors revealed that the oxidation state of Re species on the catalyst plays arguably the main role in governing leaching behavior since high-valent Re species are considerably more prone to leaching than low-valent or metallic Re. While this property is related to the composition of the catalyst (support material, promoters, bimetallic catalyst), considerable attention needs to be paid to the conditions the Re-containing catalyst is exposed to. Even short contact of a reduced Re catalyst with air can result in partial catalyst oxidation, and also during the catalytic reaction the oxidation state of Re is often changed, which can then result in severe leaching. One of the primary conclusion is, therefore, that the Re oxidation state needs to be controlled at all times to prevent Re leaching. In this context, it is important to consider that Re leaching can not only occur during the actual phase of the reaction but throughout the lifetime of the catalyst and many factors can have significant impacts on the catalyst stability.Of particular importance is the gas atmosphere of the reaction and reducing conditions in the presence of H2 are arguably the most effective way to mitigate Re leaching. This is also the reason why there is a large discrepancy between different reaction types and Re leaching occurring under the corresponding reaction conditions. Most reactions in the context of biomass valorization involve H2, which mitigates Re leaching even though they are often conducted in aqueous-phase and Re species are considerably more soluble in water than in less polar solvents. Moreover, often bimetallic catalysts are used and a noble metals can provide H2 to promote the reduction of Re species and stabilize low oxidation states. On the other hand, DODH reactions require high-valent Re species and are conducted under inert or oxidizing atmosphere relying on organic molecules as reducing agents. While this make DODH catalysts exceptionally susceptible to Re leaching, there is also the phenomenon of chelate complex formation between Re species and the reactant diols to be considered which was found to considerably enhance Re leaching. This explains the drastic examples of catalyst deactivation for this particular type of reaction.The particular requirements of different reactions regarding the chemical and physical properties of the catalyst as well as the reaction conditions makes the development of strategies to mitigate Re leaching particularly challenging. In general, different strategies to improve catalyst stability have been reported [10,96,235]. For the case of DODH reactions, a promising strategy could be the combination with an additional HG step to obtain saturated products. Studies on this combined process indicate that Re leaching is considerably less pronounced than in pure DODH processes, probably due to the combination of different factors.While the dark side of Re leaching, i.e. the role it plays in catalyst deactivation, is often in the focus of reports on Re leaching, it can also be applied in a controlled fashion to one's benefit. First, Re leaching and redeposition can be used for in-situ preparations as well as the modifications of catalyst systems. Second, the concept of ‘release and catch’ catalysis utilizes that in some cases, the superior catalytic activity of dissolved Re species acting as homogeneous catalysts and combines it with the recyclability of a solid catalyst. The crucial aspect is the controlled deposition of initially leached Re species after the completion of the reaction.Whether or not Re leaching is desired, its detection is crucial to fully understanding the catalytic behavior including the deactivation processes of a solid catalyst. Due to the complexity of Re leaching and, in particular, its occurrence at different stages of the catalyst lifetime as well as the possibility of Re deposition, this is not trivial. While elemental analysis of the reaction medium, if possible at different stages, unambiguously and quantitatively detects Re leaching, care must be taken to ensure that no dissolved Re can be redeposited before solid catalyst and reaction medium are separated, e.g. by filtration under reaction conditions. An indirect way to detect Re reaching is through the catalytic activity of leached species, which is preferably conducted as a hot filtration test. However, in cases of catalytically non-active or predominant homogeneous catalytic activity, the extent of Re leaching can be over- or underestimated. In general, the phenomenon of Re leaching and its effects on catalytic activity can be interlinked in different ways, and, therefore, both aspects should be investigated independently. Even though not commonly applied in conventional studies, there is considerable benefit in experimentally tracking the time-dependent leaching behavior to gain insight into the mechanism and kinetics of Re leaching. Moreover, additional methods like, e.g. UV–Vis or NMR spectroscopy, are invaluable to identify the chemical nature of leached Re species. These techniques resulted in the discovery of the crucial role of diols as chelating agents that promote Re leaching during DODH reactions in non-polar solvent.Overall, the phenomenon of Re leaching during liquid-phase biomass valorization reactions is widespread and often complex. Given the importance of catalyst stability for industrial applications, in particular since the rare element Re is involved, it is unfortunate that little attention is paid to this side of catalyst behavior in many studies. Understanding the leaching behavior of Re (and metal leaching in general) is a necessity to be able to design stable catalysts and sustainable processes.Finally, it should be noted that the findings in this article are not limited to Re but of similar relevance also for similar types of catalysts. The deactivation mechanisms outlined in Chapter 2 are of general applicability, even though the relevance of each mechanism may vary from case to case. Similarly, the lessons learned from understanding Re leaching are transferable to metal leaching from other supported metal catalysts. Nevertheless, Re is an outstanding example due to its versatile and sensitive redox behavior, the role of redox reactions in governing catalyst deactivation is particularly prominent and the leaching phenomena considerably more pronounced than e.g. for most noble-metal catalysts.Florian M. Harth: Writing - Original Draft, Investigation, Visualization, Conceptualization; Blaž Likozar: Conceptualization, Project administration, Miha Grilc: Supervision, Writing - Reviewing and Editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research was funded by the Slovenian Research Agency (basic J1-3020 research project and research core funding P2–0152). The work was partially carried out within the RDI project Cel. Cycle: »Potential of biomass for development of advanced materials and bio-based products«, which is co-financed by the Republic of Slovenia, Ministry of Education, Science and Sport of the Republic of Slovenia, and the European Union through the European Regional Development Fund, 2016–2020. The authors acknowledge Dr. Brigita Hočevar and Dr. Vili Resnik, pioneers on Re-catalyzed valorization of aldaric acid, for their kind support. The authors also acknowledge Mr. Jaša Bukovec for preparing the graphical abstract.

Rhenium is a versatile element and increasingly used in solid catalysts for the conversion of biomass, where it can fulfill different roles in providing or improving catalytic activity. On the other hand, this also makes Re-based catalysts susceptible to various types of catalyst deactivation. Deactivation mechanisms, detection methods, and coping strategies are discussed for each type of deactivation using the collected literature on Re-containing catalysts for biomass utilization in liquid phase. Particular focus is placed on the correlation between catalyst deactivation and Re leaching, a commonly observed problem when using Re-containing catalyst in liquid-phase reactions that can lead to severe and irreversible loss of catalytic activity. Material properties, reaction conditions, and other factors influencing Re leaching are systematically assessed and leaching mechanisms discussed, which also opens possibilities how the problem can be mitigated. In particular, the role of the Re oxidation state is identified as a key material property influencing Re leaching and a variety of processes and parameters that alter the Re oxidation state during the lifetime of the catalyst are analyzed. Moreover, insights into the intricate procedure of detecting Re leaching and identifying the correlation between leaching and catalyst deactivation are presented. Finally, strategies to purposefully use Re leaching are shown.

Data will be made available on request.Oxygen evolution reaction (OER) is an enabling step for most of the key electrochemical applications such as hydrogen production, CO2 electrolysis, and energy storage [1]. The interface between catalyst and electrolyte (or membrane) is at the heart of the OER catalysis, and plays a pivotal role in determining the activity, selectivity, and stability of the electrochemical systems [2]. However, these catalysts are usually unstable under oxidation potentials, and are subject to dynamic surface restructuring during the reactions [3,4]. This operando surface restructuring process has a profound impact on the overall reaction system [5–7].Recent studies are sought to take advantage of such surface reconstruction phenomena to boost the OER activity by tuning the compositional chemistry of metal oxides [8]. The most common strategy is to incorporate electrolyte-soluble ions into the metal oxide lattice, such as alkali (e.g., Li+) [9], alkaline-earth (e.g., Sr2+ and Ba2+) [10,11], Al3+ cations [12], and halide anions (e.g., Cl−) [13]. Under oxidation potentials, the dissolution of these cations or anions from the catalysts causes significant surface reconstruction to form an OER-active surface layer consists of catalytically-active phases (e.g., cobalt oxyhydroxide [3] in cobalt oxides) and lattice vacancies [14]. However, there are also reports raising concerns of the cation dissolution and anode restructuring that could also cause rapid cell failure in the application of CO2 electrolysis [15], where the local reaction environment (e.g., (bi)carbonate cross-over from the cathode to the anolyte) is drastically different from water electrolysis. This discrepancy originates from the negligence of the electrolyte’s role in determining the overall reaction reactivity and stability [16]. The catalytic process and surface restructuring process should depend both on the catalyst properties and potentials and their local reaction environment, such as electrolyte compositions and local pH, which remain poorly understood by far [17].Hence, this work seeks to study the roles of electrolyte ions and catalyst compositions in the catalyst ion leaching and surface restructuring process during OER catalysis in a non-acidic medium, mostly in a pH-neutral electrolyte. We chose SrCoO3 perovskite (SC) as the model anode catalyst and phosphate buffer solutions (PBS) as the main model electrolytes. The perovskites are emerging cost-effective alternatives to precious metals for the OER catalysis [18], while the PBS-electrolytes with a neutral pH have the potential to minimize the potential corrosions in the electrolyzers [19,20]. We chose PBS as the model electrolyte mainly because of the phosphate anions are different from hydroxide ions [21,22] and allow us to explore the impacts of anions on the catalyst surface restructuring and reactivity. Our experimental results confirm that the catalyst surface restructuring process during OER in varied electrolytes takes place involving ion leaching, electrolyte cation backfilling and anion incorporation. This restructuring process is closely related to the solubility of the cations of the metal oxides, the sizes of the ions of catalyst and electrolyte, and buffering capacity of the electrolyte anions. Consequently, an amorphous surface shell structure can be formed covering the Sr-containing perovskite core in the presence of Na+-PBS electrolytes after anodic conditioning. The shell structure contains more oxygen vacancies that strengthen binding with oxygen intermediates and phosphate ions that promote proton transfer, so as to exhibit significantly enhanced OER activity in pH-neutral electrolyte and in alkaline medium.The solid-state preparation method was applied to synthesize the catalysts, including SrCoO3-δ (SC), BaCoO3-δ (BC), LaCoO3-δ (BC), La0.5Sr0.5Co0.8Fe0.2O3-δ (LSCF), Ba0.5Sr0.5Co0.8Fe0.2O3-δ (LSCF), SrNb0.1Ta0.1Co0.8O3-δ (SNTC) and SrSc0.175Nb0.025Co0.8O3-δ (SSNC). Stoichiometric mixtures of Co3O4 (Aldrich, ≥99.5 %), SrCO3 (Aldrich, ≥99.9 %), BaCoO3 (Aldrich, ≥99.98 %), La2O3 (Aldrich, ≥99.9 %), Nb2O5 (Aldrich, ≥99.99 %) and Ta2O5 (Alfa Aesar, ≥99.0 %) were weighed and ball-milled at 260 rpm for 20 h. Then the samples were dry-pressed in a die under 90 MPa and sintered at 1200 °C for 20 h. Finally, the sintered tablets were crushed into powders through ball-milling at 350 rpm for 8 h. As the SC powders were prepared via solid-state method and ground through ball-milling, their sizes may vary widely. Thus, the as-prepared SC powders were dispersed in ethanol via ultrasonication for 1 h, and centrifuged at the rotation speed of 1000 rpm. The supernatant was collected and dried under vacuum overnight to obtain the SC-origin with monodispersed particle size.La0.6Sr0.4CoO3-δ (LSC) and La0.6Sr0.4MnO3-δ (LSM) were purchased from Fuel Cell Materials, whose particles size is 0.4–0.8 μm and 0.4–1.0 μm, respectively.To prepare the 1.0 M sodium phosphate buffer solution (Na+-PBS), 15.6 g sodium phosphate monobasic dehydrate (NaH2PO4·2H2O, Aldrich, ≥99.0 %) was dissolved in 100.0 mL deionized water, and denoted as solution A. Meanwhile, 28.4 g sodium phosphate dibasic heptahydrate (Na2HPO4·7H2O, Aldrich, ≥98.0 %) was dissolved in 200.0 mL deionized water, and denoted as solution B. Then 97.5 mL of solution A was mixed with 152.5 mL of solution B to obtain the 250 mL of Na+-PBS. Its pH was tested to be 6.65.To prepare the 1.0 M potassium phosphate buffer solution (K+-PBS), 13.6 g potassium phosphate monobasic (KH2PO4, Aldrich, ≥99.0 %) was dissolved in 100.0 mL deionized water, and denoted as solution A. Meanwhile, 24.2 g potassium phosphate dibasic heptahydrate (K2HPO4·3H2O, Aldrich, ≥98.0 %) was dissolved in 200.0 mL deionized water, and denoted as solution B. Then 97.5 mL of solution A was mixed with 152.5 mL of solution B to obtain the 250 mL of K+-PBS. Its pH was tested to be 6.62. To prepare the 1.0 M sodium sulfate solution (Na2SO4), 35.51 g anhydrous Na2SO4 was dissolved in 250 mL deionized water. Its pH was tested to be 7.18.10.0 mg active catalyst and 10.0 mg carbon black were dispersed in 1.0 mL ethanol with 100 µL 5 wt% Nafion solution through ultrasonication for 30 min. Then 400 µL of the obtained ink was loaded on the Ni foam (1 cm × 2 cm) to achieve the loading amount of 1.67 mg cm−2, and dried under vacuum overnight. To carry out the catalyst reconstruction process, the continuous potentiometry V-t treatment was employed under a constant current density of 3.0 A g−1 for 40000 s.The reconstructed catalysts were stripped from the Ni foam through ultrasonication with ethanol for 20 min and then dried under vacuum overnight for characterization. High-resolution transmission electron microscope (HR-TEM, Tecnai F20), with energy dispersive spectrometer (EDS) mapping details of Sr, Co, and O elements, were applied to study the morphologies of the materials at a voltage of 200 kV. The line scan spectra of as-prepared SC and reconstructed SC were collected on the HF5000 Cs-TEM at the accelerating voltage of 80 kV. X-ray diffraction (XRD) patterns (2θ, 10–70°) were recorded on a Bruker D8-Advanced X-ray diffractometer with the nickel-filtered Cu-Kα radiation. The different electrodes were immersed in the various electrolytes, including Na+-PBS (1.0 M), K+-PBS (1.0 M), Na2CO3/NaHCO3 (1.0 M), and Na2SO4 (1.0 M), and the anodic conditioning was then conducted for a certain period. Subsequently, the electrolytes were collected and sent for ICP analyses. The concentrations of their ions were analyzed with a Varian Vista Pro ICP-OES instrument. Co K-edge XAS spectra of all samples were recorded on at BL14W1 station in Shanghai Synchrotron Radiation Facility (SSRF). The beam energy was 3.0 GeV and the maximum beam current was 400 mA. The FTIR spectra were obtained by a PerkinElmer Spectrum 100 FT-IR spectrometer.In 1.0 M Na+-PBS solution, high-resolution transmission electron micrographs (HR-TEM) of SC manifest that a surface restructuring process took place during the electrochemical anodic conditioning at 3.0 A/g for 10 min, 40 min, 6 h, and 12 h (Fig. 1 a–1e), respectively. The bulk core of SC could sustain its high crystallinity after conditioning treatment in neutral solution, which is confirmed by the distinguishable lattice fringes in HR-TEM images with a lattice spacing of 0.277 nm that corresponds to (0 1 1) lattice of cubic perovskite phase of SC [23,24]. The sustained structural integrity of SC is also confirmed by its X-ray diffraction (XRD) patterns before and after conditioning (Fig. S1) [25,26]. In contrast, the amorphous surface shell of the SC becomes thicker when the anodic conditioning duration increases, with an average thickness of the amorphous region increasing from 5.0 ± 0.3 nm for 10 min treatment to about 35.0 ± 1.0 nm for 12 h treatment. The thickness of the shell structure increases quickly at the first 6 h treatment and gradually slows down in the next 6 h (Fig. 1f). The slowing down of restructuring should be ascribed to the steric hindrance of the thick shell that prevents further penetration of electrolyte to the SC core when the shell structure is thick.Analysis of energy-dispersive X-ray spectroscopy (EDS) line scans over SC oxides before and after 12-h treatment suggests a notable decrease of Sr/Co atomic ratio but an increase of phosphorus (P) content near the surface shell (Fig. S2), meeting well with the X-ray photoelectron spectroscopy (XPS) data (Fig. S3). X-ray absorption spectroscopy (XAS) was applied to investigate the relatively thick shell structure to reveal the chemistry of the catalyst restructuring process. Cobalt K-edge of the X-ray absorption near edge structure (XANES) spectrum of the 12 h-treated SC oxide shifts to a higher energy by nearly 1.0 eV as compared to the original SC (Fig. 1g), indicating that the cobalt ions would be oxidized during treatment [22,27]. When fitting the Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectra against SrCoO3 model [28], we find that the coordination number is reduced by 4.0 for the first cobalt-strontium shell and by 0.5 for the first cobalt-oxygen shell in the SC lattice (Fig. 1h and Table S1). The reduced coordination number indicates loss of strontium atoms in the lattice after the anodic conditioning, which is consistent with the EDS line analysis result (Fig S2). We also observe the formation of cobalt-phosphate bonds in SC oxides after 12 h treatment from the k3 -weighted EXAFS spectra particularly at k = 5.0 Å−1, 8.5 Å−1, and 11.5 Å−1 (where k represents photoelectron wavenumber, Fig. 1i) [29]. The formation of second cobalt-oxygen shell and cobalt-phosphate shell as observed from the EXAFS analysis further confirm the incorporation of phosphate into the SC lattice from the electrolyte solution. Moreover, FTIR results reveal a stronger broad characteristic peak at 1230 cm−1 for the typical vibrational and bending modes of phosphate when the anodic treatment duration increases (Fig. S4) [20]. The amorphous surface shell should be the region where the restructuring takes place, involving cobalt oxidation, loss of strontium cations, and phosphate incorporation.The surface restructuring significantly improves the OER activity of SC in 1.0 M Na+-PBS. The linear sweep voltammograms presented in Fig. 2 a show that the mass-specific OER current densities increase with the conditioning durations. Specifically, the current densities at 1.75 V versus reversible hydrogen electrode (vs RHE) increased rapidly in the first 40 min treatment and then further to 31.4 A g−1 after 12 h treatment (Fig. 2b and Fig. S5). From the TEM-EDX mapping images, the homogeneous distribution of the ions in SC-Re could be confirmed (Fig. S6). We further carried out the in situ EIS test to study the charge-transfer ability of SC during the anodic conditioning. From Fig. S7 it can be found that charge-transfer ability of SC can be improved with the treatment duration. The enhanced charge-transfer ability of SC-Re should also contribute to the highly improved OER activities of SC-Re. The 40000-s stability test over pristine SC catalyst also reveals that the required potential decreases from 1.764 V to 1.675 V vs RHE to drive a current density of 6.0 A g−1, meaning that the OER activity of SC was greatly enhanced under anodic restructuring (Fig. 2c).Interestingly, we also studied the surface change of Co3O4, BaCoO3-δ (BC) and LaCoO3-δ (LC) catalysts during the anodic conditioning in 1.0 M Na+-PBS, and found that they experienced no significant surface restructuring (Figs. S8–S10). Their TEM-EDX mapping images have also been provided (Figs. S11 and S12). Consequently, negligible OER improvements could be observed over Co3O4, LC, and BC catalysts after anodic conditioning treatment (Fig. 2d, 2e). Meanwhile, the SC exhibits no significant activity enhancement after being treated in 1.0 M K+-PBS electrolyte (Fig. 2f), because the anodic conditioning in K+-PBS electrolyte could not initiate the surface restructuring process of SC (Fig. 2g and Fig. S13).Noticeably, the SC-Re exhibits a remarkably high activity towards OER in 1.0 M Na+-PBS, reaching a current density as high as 31.4 A g−1 at 1.75 V vs RHE. This performance is 2.1 times higher than that of IrO2 (13.5 A g−1), and 9.2 times that of IrO2 after 12 h anodic treatment under the same conditions (2.2 A g−1) (Fig. S14). We also compared the performance (i.e., overpotentials at 5.0 A g−1 and current densities at 1.75 V vs RHE) of our catalyst against the recently reported catalysts in neutral electrolytes, such as Ni0.1Co0.9P [30], RuIrCaOx [31], and 1-D CoHCF [32] (Fig. 2h and Table S2), and the OER activity of SC-Re is among the best-reported values. In addition, the performance of SC-Re in 1.0 M KOH is also comparable to the benchmark catalysts such as NiCo2S4 NW/NF [33] and CoSx/Ni3S2@NF (Table S3) [34]. Specifically, SC-Re could achieve the current density of 65.3 A/g at 1.65 V vs RHE, 3.0 times of SC-origin (21.7 A/g), and 1.5 times of IrO2 (43.0 A/g).To unveil the surface restructuring mechanism of SC, we compared the electrolyte compositions before and after anodic conditioning in 1.0 M Na+-PBS and K+-PBS electrolyte solutions at 3.0 A g−1 for 12 h. The ICP-OES results reveal that there are significant amounts of Sr2+ ions in both electrolytes (i.e. 0.44 mM in used Na+-PBS and 0.37 mM in used K+-PBS) after the treatment (Fig. 3 a), and the Sr2+ ion concentration in 1.0 M Na+-PBS electrolyte is consistent with the loss amount of Sr in the SC surface as observed in EDS results (Fig. S2). The relatively high solubility of Sr in the PBS (solubility product (Ksp) of SrHPO4 is 1.072 × 10−7) should be the main driving force for the Sr leaching to the electrolyte [35].More interestingly, it is found that the Na+ ion concentration in Na+-PBS electrolyte decreases significantly during anode conditioning, meaning that Na+ ions could be backfilled into the SC (Fig. 3b). In contrast, the loss of K+ ion is negligible in K+-PBS after the anodic conditioning (Fig. S15), but the concentration of Co species in used K+-PBS (ca. 0.054 mM Co) was clearly higher than that in Na+-PBS (ca. 0.018 mM Co) (Fig. 3c). It indicates that the Sr2+ leaching could lead to the slow decomposition of the reconstructed surface of SC in K+-PBS, exposing the unchanged inner SC crystal to the electrolyte. Therefore, the TEM image of SC after anodic conditioning in K+-PBS shows no amorphous outer layer (Fig. S13). Instead, the observed backfilling of Na+ in SC could help stabilize the reconstructed SC framework and restrain Co dissolution into the electrolyte and thus lead to the formation of an extended amorphous structure on the SC surface.We postulate that the easier backfilling of Na+ in SC than K+ should be attributed to the smaller ionic size of Na+. Compared to the host Sr2+ in SC with an ionic radius of 1.44 Å, Na+ shows a relatively smaller radius (1.39 Å) while K+ has an obviously larger ionic radius of 1.64 Å [36]. To better understand the roles of Na+ and K+ during the reconstruction of SC, we performed density functional theory (DFT) simulations to mimic the process of Na and K atoms passing a simple and respective channel caused by the leaching of Sr atom (Fig. 3d) [37,38]. We consider the passage of an atom through the [O4] neck structure from one Sr-vacancy to another as a 5-step process with six defined states (Fig. 3e): (0) reference state with the isolated atom and the supercell; (1) entering the Sr-vacancy; (2) approaching the [O4] neck structure; (3) locating at the center of the [O4] neck structure; (4) moving out of the ring; (5) entering another Sr-vacancy [39]. The DFT calculations show a substantial energy decrease when the Na atom enters the channel, indicating that the introduction of the Na atom is favorable for the stability of the reconstructed structure. At the same position, the Na atom is more likely to pass through the channel spontaneously, clearly different from the case with the K atom. Specifically, Fig. 3e shows positive relative energy for the K atom at State (3), which indicates that it is difficult for the K atom to pass through the [O4] neck structure. The high selectivity of Na+ over K+ can be also observed in the experimental measurements based on an artificial sodium-selective ionic device with sub-nanometer pores, which is attributed to the size effect and molecular recognition effect [40]. To gain insight into the influence of the Na atom on the Co-O bond, we further explored the electronic structures of a unit cell of SC as shown in Fig. 3f, and compared the initial cubic structure (Case 1) to the case when the Sr-vacancy (Case 2) is occupied by the Na atom (Case 3). Charge density differences of the [O4] neck structure for Case 1 and Case 3 are calculated by subtracting the charge density of Case 2 and corresponding atom from that of Case 1 and Case 2, respectively, as the sectional diagrams shown in Fig. 3g. We confirm that the stability of the Na-backfilled structure is much higher than that of the Sr-vacant structure, while the Co-O bond weakens (i.e., benefiting the formation of oxygen vacancies) when the Sr atom is replaced by the Na atom. The Na-backfilling is likely the reason for the formation of the amorphous shell at the SC core during the restructuring process. The weakened Co-O bond could also contribute to the observed reduced Co-O coordination number from the EXAFS results (Fig. 1i).We also studied the SC surface restructuring process at the same anodic conditioning treatment in other electrolytes with different anions such as 1.0 M Na2CO3/NaHCO3 and 1.0 M Na2SO4. We noticed that these anions failed to maintain a relatively neutral local pH close to the catalyst surface during 12 h anodic conditioning (which releases protons as product), and led to the significant dissolution of the SC catalyst and even nickel support into the electrolytes.We further studied the role of A-site cations in the surface restructuring processes over Co3O4 and BC catalysts during the anodic conditioning (Fig. 3b). The absence of the Sr in the Co3O4 led to negligible surface restructuring after 12 h anodic conditioning in Na+-PBS (Fig. S8), and no Co species was detected in the used electrolyte. In addition, the negligible surface restructuring on BC (Fig. S9) should be attributed to the low solubility of barium phosphate (Ksp = 3.40 × 10−23) that limits the dissolution of the A-site cations. The limited Ba2+ dissolution can be confirmed by the ICP-OES results that there is a much lower concentration of Ba2+ (ca. 0.11 mM) as compared to Sr2+ for SC (ca. 0.44 mM) in the Na+-PBS. This result indicates that the Sr2+ dissolution is one of the main drivers for the surface restructuring and causes the Co2+ leaching in SC. After 12 h anode conditioning, the content of Na+ remained almost unchanged for both Co3O4 and BC samples, further confirming that Sr dissolution is essential for the Na+ backfilling in SC.The electrochemical conditions should also play a role in facilitating the SC surface restructuring. We treated the SC catalysts in Na+-PBS electrolyte using three different current densities while maintaining the same passage quantity of total charge. There was no clear trend spotted from HR-TEM images of the structural evolution over the SC particles (Fig. S16). Similarly, there are no obvious correlations between the treatment conditions and dissolution of the Sr and Co species in the electrolyte, suggesting that the Sr dissolution is not initiated by the electrochemical conditioning (Table S4). Instead, the Sr dissolution is likely driven by the Sr concentration gradient across the catalyst-electrolyte interfaces. However, the concentrations of Na+ in the electrolyte are similar for the three current densities, meaning that the Na+ backfilling is correlated to the total charges transferred in the reactions (Table S4). We also found that immersing SC in the Na+-PBS with no charge transferred could not induce noticeable surface restructuring (Fig. S17).To examine the role of the surface reconstruction on the OER intrinsic activity of the active sites, we compared the activity of SC before and after surface reconstructions in 0.0316 M, 0.1 M, 0.316 M, and 1.0 M Na+-PBS electrolytes. Fig. 4 a–4c presents obvious dependency of the OER activity over the Na+-PBS concentration, confirming the contribution of Na+-PBS to the OER catalysis over both original and restructured SC oxides. Interestingly, evidenced by its smaller slope, the SC-Re should have a lower OER dependency over Na+-PBS concentration than the original analogue, meaning that the effect of the Na+-PBS concentration is weakened after the development of the core–shell structure. We believe the phenomenon is related to the aforementioned steric hindrance of the thick amorphous shell that limits further electrolyte penetration and SC/electrolyte interfacial interactions. The lower OER dependency on Na+-PBS could be attributed to the enhanced proton transfer process by the incorporation of phosphate, which was previously reported for (La, Sr)CoO3 with the surface-modified with phosphate [41]. Our results of the density-functional theory (DFT) calculation further confirm that phosphate could also lower the activation energy by 0.19 eV by accelerating the proton removal from H2O and HO* intermediate, where * represents the adsorption site at SC surface (Figs. S18 and S19) [42,43].Furthermore, we observe a stronger pH dependency of OER activities over SC-Re than over the SC-origin in KOH electrolytes (Fig. 4d-4f). A high pH dependency indicates the participation of the lattice oxygen in the OER catalysis, where lattice oxygen vacancy is the essential ingredient [18,44]. The surface reconstruction process can create surface oxygen vacancies to strengthen intermediate adsorption and tether the surface with phosphate to accelerate proton transfer, jointly enhancing the overall OER reactivity [45]. This proposed controllable catalyst/electrolyte cations matching-induced restructuring strategy can be a more effective pathway to achieve a high electrochemical activity compared with the conventional surface restructuring in alkaline solution. After reconstruction in 1.0 M Na+-PBS, the SC-Re even shows a remarkably higher OER activity compared with the SC-origin in 1.0 M KOH (Fig. 4g). To achieve the mass current density of 100.0 A/g, SC-Re needs only an overpotential of 385 mV, clearly lower than that of SC-origin (494 mV, Fig. 4f). In contrast, the reconstructed SC sample in 1.0 M KOH and 1.0 M NaOH can only induce minor activity enhancement, which needs 491 mV and 450 mV, respectively, to achieve 100 A/g (Fig. 4h).We tested the single-chamber full cell water electrolysis in pH-neutral 1.0 M Na+-PBS electrolyte by applying the SC-Re as the anode catalyst and Pt-loaded carbon black (Pt/C) as the catalyst to evolve hydrogen (Fig. S20a). The SC-re-based cell achieved a current density of 28.8 A g−1 at an overall cell voltage of 2.2 V, outperforming the RuO2-based equivalent (4.1 A g−1 at 2.2 V) by almost seven-folds (Fig. S20b, c). We also assembled a 5.0 cm2 SC-Re electrode into an AEMWE, where the Pt/C catalyst deposited on Ti foam serves as the cathode and the Sustainion X37-50 Grade T as the anion exchange membrane (Fig. S20d). This cell can achieve a current density of 90.7 A g−1 at an overall cell voltage of 2.2 V (Fig. S20e), and the gas chromatograph results as shown in Fig. S21 confirmed that the products of the cell are H2 and O2. No CO and CO2 were detected (precision: ppm). This cell can produce 1.8 ± 0.2 mL min−1 O2 gas and 3.9 ± 0.2 mL min−1 (Fig. S20f), and therefore faradaic efficiency for OER and HER are both ∼100 %. The stability test of SC-Re ‖ Pt/C at 85.0 A g−1 shows negligible degradation for 24 h (Fig. S22). These results demonstrated the potential of the SC-Re to be applied in a large-scale water electrolyzer.Based on the results of the controlling experiment, we could safely conclude that the surface restructuring of SC during anodic conditioning involves the dissolution of A-site cations and backfilling of electrolyte cations (Fig. 5 a). This restructuring process mainly arises from (1) the solubility of A-site cations in the electrolyte, (2) the size of electrolyte ions, and (3) the charge transfer process. The solubility of the catalyst cation determines the concentration gradient across the catalyst-electrolyte interface, which drives the catalyst dissolution into the electrolyte. The catalyst dissolution together with anodic conditioning charge transfer enables the backfilling of Na+ in the electrolyte to the vacant Sr site and subsequently stabilizes the B-site cobalt structures at the SC surface. The slightly higher cobalt oxidation states as observed from the XAS results should be attributed to the Sr-Na swap on the SC surface that causes the reduction of A-site cation valence and the electrochemical oxidation during the anodic treatment (Fig. 1g, h). Meanwhile, the exchange between Sr and Na could be associated with the formation of oxygen vacancies (as evidenced by the reduced Co-O coordination numbers) and phosphate incorporation (as evidenced by the featured XAFS spectra of Co-phosphate (Fig. 1i) and FTIR characteristic peak for phosphate (Fig. S3). The induced charge imbalance, like other perovskite metal oxides [46], could also create oxygen vacancies with each carrying two positive charges, as depicted by the equation (inset in Fig. 5a). Therefore, the negatively charged oxygen ions from phosphate could interact with the positively charged oxygen vacancies on the surface, resulting in the adsorption of phosphate in the surface shell lattice.The surface restructuring process in Na+-PBS can be a general restructuring pathway for the ABO3 materials that meet the rules described in Fig. 5a, and could effectively enhance the OER activity. To demonstrate its generality, we prepared a few Sr-containing cubic perovskites, including La0.6Sr0.4CoO3-δ (LSC), La0.6Sr0.4MnO3-δ (LSM), La0.5Sr0.5Co0.8Fe0.2O3-δ (LSCF), Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), SrSc0.175Nb0.025Co0.8O3-δ (SSNC), and SrNb0.1Ta0.1Co0.8O3-δ (SNTC) catalysts, and compared the OER reactivity in 1.0 M Na+-PBS before and after anodic treatment for 12 h at 3.0 A g−1. Their XRD patterns are shown in Fig. 5b. All these materials show discernible enhancement of the OER activity after the anodic treatment. The overpotential to achieve a current density of 3.0 A g−1 is reduced by 70 mV for LSCF, 125 mV for BSCF, 50 mV for SSNC, 68 mV for SNTC, 130 mV for LSM, and 86 mV for LSC. (Fig. 5c–5 h). This result further confirms the important role of pairing cathode cations (Sr2+) with electrolyte cations (Na+) in improving OER activity via surface restructuring.To conclude, we report a novel operando surface restructuring pathway, highlighting the important role of pairing cations in catalyst and electrolyte in the electrochemical surface restructuring process. In our study, we use the SrCoO3-δ perovskite as the model catalyst to evolve oxygen from water in the Na+-PBS electrolyte and study the effect of surface restructuring in enhancing OER catalytic activity. We find that the surface restructuring process requires the dissolution of soluble A-site cation (Sr) to the electrolyte, backfilling of small electrolyte cations (Na+) to large A-site vacancy in the catalyst lattice, and anion (phosphate) incorporation. Through both experimental and theoretical studies, we confirm that the A-site cation dissolution is driven by the concentration gradient across the catalyst-electrolyte interface, and this dissolution process together with the anodic polarization initiates electrolyte ion backfilling and incorporation. Consequently, the surface restructuring leads to the formation of an amorphous shell with a thickness of ten of nanometers at the catalyst/electrolyte interfaces. This shell structure is highly active in OER catalysis due to the created lattice oxygen vacancies that strengthen intermediate adsorption and incorporate phosphate that accelerate proton transfer. Overall, this work highlights the important roles of the cations in both the catalyst and electrolyte in determining the electrode–electrolyte interactions during electrocatalysis. We anticipate this work to offer alternative strategies to advance electrochemical applications such as water and CO2 electrolysis via optimizing the catalyst compositional chemistry with properties of electrolyte.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.L. Z. and Z. L. contributed equally to this work. Z. Z. likes to thank the financial support from Australian Research Council Discovery Projects (DP190101782) and (DP200101397). The authors thank the valuable advice from Prof. Honglai Liu about the DFT calculation. The authors also thank the Shanghai Synchrotron Radiation Facility (BL14W1, SSRF) for XAS equipment access.Supplementary data to this article can be found online at https://doi.org/10.1016/j.cej.2022.140071.The following are the Supplementary data to this article: Supplementary data 1

The highly efficient and stable electrolysis needs the rational control of the catalytically active interface during the reactions. Here we report a new operando surface restructuring pathway activated by pairing catalyst and electrolyte ions. Using SrCoO3-δ-based perovskites as model catalysts, we unveil the critical role of matching the catalyst properties with the electrolyte conditions in modulating catalyst ion leaching and steering surface restructuring processes toward efficient oxygen evolution reaction catalysis in both pH-neutral and alkaline electrolytes. Our results regarding multiple perovskites show that the catalyst ion leaching is controlled by catalyst ion solubility and anions of the electrolyte. Only when the electrolyte cations are smaller than catalyst's leaching cations, the formation of an outer amorphous shell can be triggered via backfilling electrolyte cations into the cationic vacancy at the catalyst surface under electrochemical polarization. Consequently, the current density of reconstructed SrCoO3-δ is increased by 21 folds compared to the pristine SrCoO3-δ at 1.75 V vs reversible hydrogen electrode and outperforms the benchmark IrO2 by 2.1 folds and most state-of-the-art electrocatalysts in the pH-neutral electrolyte. Our work could be a starting point to rationally control the electrocatalyst surface restructuring via matching the compositional chemistry of the catalyst with the electrolyte properties.

Data will be made available on request.Fossil fuels are a finite resource and cannot be replenished once they are used up. This implies that eventually, fossil fuels will run out and we will need to find new sources of energy. As per the reports about 934 million tons of diesel are consumed globally each year and approximately 97.6% of all oil resources used in transportation are derived from fossil fuels [1,2]. The consequences of all these factors contribute to the potential depletion of petroleum reserves as well as the increase in fuel prices and global warming [3–6]. Hence Renewable energy sources are becoming increasingly important as we strive to reduce our reliance on fossil fuels and address the negative environmental impacts associated with their use. Biodiesel, being a renewable fuel, adapts as a great alternative to fossil fuels because it is more sustainable and environmentally friendly [7–9]. And it possesses desirable characteristics like biodegradability, a high cetane number (which reduces ignition delay time and provides smoother engine running), a high flash point (which allows safer handling and storage), non-toxicity, renewability, a reduced sulphur and aromatic content, great lubricity, and about 10–12% oxygen content in the molecular structure (which reduces CO, HC, smoke, and life cycle net carbon dioxide emissions) [10–13]. The use of biodiesel as fuel improves air quality, create new jobs and stimulate economic growth. Additionally, without requiring any modification, it can be used straight in diesel engines [14,15]. Overall, the transition to renewable energy sources is an important step towards a more sustainable and resilient energy system.Biodiesel is produced from vegetable oils or animal fats have a chemical resemblance to that conventional diesel. To produce biodiesel (methyl or ethyl ester), a chemical reaction involves known as transesterification in which triglycerides of oil or fat with an alcohol (usually methanol or ethanol) in the presence of either homogeneous or heterogeneous catalysts [16–19]. The use of homogeneous catalysts has the disadvantage of being chemically manufactured, increases production cost, generates a lot of effluents, affects biodiesel yield, and limits catalyst re-use. To overcome these issues the heterogeneous catalyst plays crucial role in biodiesel production as they shows high catalytic activity could speeds up the reaction, making it occur quickly, efficiently and improve the product yield and shows reusability. For producing biodiesel there are two basic requirements for feedstocks: low manufacturing costs and large production scales [20]. Over 350 oil-producing crops have been identified as potential feedstocks for biodiesel production. Considering these factors soybean oil is a popular feedstock for biodiesel production because it is widely available, relatively inexpensive, has good cold weather performance. Soybeans are crops that can be grow each year, unlike petroleum, which is a non renewable source. Additionally, soybean oil has a lower carbon footprint than petroleum based diesel fuel because it is derived from a renewable resource and produces fewer greenhouse emissions during its production and use. Hence soybean oil is versatile and sustainable choice as feedstock for biodiesel production.There is increasing interest in using ash-based heterogeneous catalysts obtained from biomass waste for biodiesel synthesis, as they are renewable and sustainable catalysts often derived from non renewable sources, they can be produced using relatively low cost methods, eliminates the need for toxic chemicals, and help to reduce waste generation. It is also suggested that biomass-derived ash catalysts shows good catalytic activity in reaction process with low leaching and great recyclability [21] this is may be the presence of alkali and alkaline earth metals in their chemical composition. Biomass derived ash catalysts that can be used to produce biodiesel have been evaluated in numerous studies. Such as the use of coconut-husk ash [22], ripe plantain fruit peel ash [23], M. acuminata peel ash [24], snail shell ash [25], banana peel ash [26], B. nigra plant ash [27], sugarcane leaves ash [28], pineapple leaves ash [29], walnut shell ash [30], waste ginger leaves ash [31], wheat bran ash [32], moringa leaves ash [33], acai seed ash [34], and hazelnut shell ash [35]. The catalytic performance of these ash catalysts are summarized in Table 8. Owing to all of that research, the use of biomass ash catalysts with high catalytic activity represents an exciting, relatively novel approach and promising development in the field of renewable energy. Consequently, all these factors make ash-based catalysts more useful and efficient than conventional catalysts due to their abundance, simplicity of collection, and reusability. These biomass ash catalysts are typically produced by drying, burning and calcination and can be directly utilized as catalysts without any modification and showed potential catalytic activities due to their basic nature. As a result, in the current work, simple combustion was used to obtain ash from discarded karanja seed shells.Karanja seed shell (KSS) ash is a waste material that is produced from the combustion of karanja shells, which are the outer coverings of karanja seed. Karanja, also known as pongamia pinnata, abundantly available, can easily grow on the edges of roadways, rivers, and agricultural boundaries with no maintenance. From the himalayan foothills to kanyakumari, it can be found in India and many other regions [36]. The karanja tree plant has a variety of uses notably it is used in the production of soap, lamp fuel, finishing and tanning of leather, veterinary medication, and other products that are used to cure humans and animals. The oil of karanja seed is utilized as a feedstock in the production of biodiesel. However in this research we provide novel information of the use of discarded karanja seed shells as catalyst which are elliptical, 2–3 cm broad, 3–6 cm long have a thick walls and contain a single seed [36] because they presents high amount of potassium, calcium and magnesium, which are known to have catalytic properties. Thus it can be used as a low cost and environmentally friendly catalyst for the transesterification to produce biodiesel.As a consequence, the feasibility of karanja seed shells (KSS) ash as a green heterogeneous catalyst for biodiesel production utilizing soybean oil has been investigated in the present work. The catalyst was analyzed to illustrate how the structure, elemental composition, and morphology impacted the production of biodiesel. Investigations were conducted into how reaction conditions affected the transesterification reaction. In addition, four reuse cycles were performed to assess the catalyst's reusability. It is therefore a promising catalyst for biodiesel production since it is a renewable resource, rich in alkaline elements and offers a viable, long-lasting precursor.For the catalyst preparation, Karanja seed shells were collected from the Jiwaji University campus in Gwalior, Madhya Pradesh, India. A market in Gwalior, Madhya Pradesh, India, was visited to purchase soybean oil to test the proposed catalyst's catalytic performance. The methanol was acquired from Merck and was of HPLC quality (purity of 99%). Rankem's distilled water was used throughout the research. All chemicals were utilized without any purification.The Karanja seed shells were collected and sun-dried for 10 days after being rinsed many times with distilled water to remove impurities. The Karanja seed shells were then crushed and burned in the air to produce ash catalyst. Following that, the catalyst was calcined for 4 h at a range of 200 °C–800 °C in a muffle furnace. To prevent the catalyst from coming into touch with the air, it was placed in a desiccator.KSS ash catalyst was studied using XRD, WD-XRF, SEM; FT-IR; BET; and TGA techniques, respectively. XRD is a non-destructive technique for examining a sample's atomic structure and determining the qualities and characteristics of the atoms' chemical bonds. X-ray diffraction patterns of a KSS ash catalyst were obtained on a 5th generation Rigaku X-ray powder diffractometer, (Model No - Mini Flex 600). Analysis of the catalyst elemental composition was carried out using X-ray fluorescence (WD-XRF, PANalytical spectrometer, AxiosMAX, The Netherlands). Scanning Electron Microscopy analysis was used to study the catalyst surface texture (SEM, Carl Zeiss Ultra Plus model). The functional groups of the KSS ash catalyst were discovered using FT-IR spectroscopy. Measurements in the 400-4000 cm−1 range were made with an FT-IR spectrometer (PerkinElmer, serial number 105627). Surface area, porosity, and pore diameter were determined using the BET method under N2 gas using the BELSORP max equipment. TGA (Shimadzu TGA50 series) instrument was used to determine the thermal decomposition of the catalyst (TGA). An Auto deluxe digital pH meter was used to measure the pH of the catalyst and the Hammet indicator titration method was used to determine the KSS ash catalyst basicity using benzoic acid [18].Transesterification reaction of soybean oil with methanol to produce biodiesel was performed in 250 mL three-necked round-bottom flasks with refluxing condensers and temperature-regulated magnetic stirrer. At room temperature, the magnetic stirrer was used to swirl the round bottom flask for 10 min to ensure that the prepared ash catalyst with 2 wt% amount and methanol were homogeneously mix. Then the soybean oil was poured into flask and reaction takes 60 min to complete at 65 °C. once the reaction is complete, the mixture is allowed to cool and separate into two layers - a top layer of biodiesel and a bottom layer of glycerol. The prepared biodiesel then washed to remove any residual catalyst or impurities and allow drying to eliminate any water content before storing or using. To learn about the KSS ash catalyst's effectiveness in soybean oil transesterification to biodiesel, calcinations temperatures ranging from 200 °C to 800 °C, catalyst amounts (1, 2, 3, and 4 wt% of oil) and the methanol to oil ratio (6:1, 10:1, 12:1, and 15:1) under the optimal reaction conditions were tested. The produced biodiesel was characterized using GC-MS, FT-IR, and ASTM standards. To check the conversion of triglycerides of oil into fatty acid methyl esters (FAME), gas chromatography-mass spectrometry (GC-MS) (Clarus*680 GC, Clarus*SQ8C MS) was used. Functional groups were studied using PerkinElmer's FT-IR spectrophotometer (Serial No. 105627). The prepared biodiesel physico-chemical properties such as density, kinematic viscosity, flash point, fire point, cloud point, pour point, cetane index and oxidation stability were determined by ASTM standards.X-ray diffraction spectrum of uncalcined and calcined KSS ash catalyst is given in Fig. 1 . The crystalline phases of the prepared ash catalyst are inspected from the XRD results by comparing the 2θ values with JCPDS data (joint committee on powder diffraction standard, ICDD2003) and reported literature. The catalytic activity of the catalyst was observed to be mediated by a number of potassium carbonates, chlorides, and oxides, as well as a few other metal oxide compounds. According to the XRD results, the peaks at 2θ values of 28.298, 40.427, 50.088, 58.601, 66.297, and 73.594 are attributable to KCl (JCPDS file no 41–1476). Vadery et al. and Nath et al. found the similar 2θ value for KCl in ash based catalysts derived from coconut husk, B. nigra, and sesamum indicum [22,27,37]. K2CO3 (JCPDS file no 87–0730) is attributed to the peaks at 26.278, 29.734, 31.27, and 41.722, whereas K2O (JCPDS file no 77–2176) is assigned to the peaks at 27.87, 38.792, 46.78, and 48.10. Nath et al. revealed the comparable 2θ value for K2CO3 and K2O in B. nigra and Sesamum indicum catalysts [27,37]. And Gohain et al. also reported similar 2θ values for K2CO3 in M. balbisiana peel catalysts, corroborating this study's findings [38]. The presence of CaO was identified at 2θ = 32.670, 37.109, and 54.05 (JCPDS file no 82–1691) these results are consistent with the results reported by Laskar et al. and Zhao et al. [25,39] and the peaks at 21.283 and 43.296 confirm the existence of SiO2 in the catalyst (JCPDS file no 81–0069). Hence, according to the XRD results, the catalyst contains a number of basic oxides and carbonates of K, Ca, and Si and potassium was observed to be the main element of the ash catalyst in the forms of KCl, K2CO3, and K2O, all of which were essential in the converting oil into biodiesel.The presence of inorganic elements in the calcined KSS ash catalyst was assessed using WD-XRF analysis, and the findings are shown in Table 1 . Many elements notably “K, Ca, Mg, Na, Al, Si, P, S, Cl”, and other elements were identified as producing the phases that gave catalysts their catalytic activity for biodiesel production. Some transition metal oxides also coexisted with these metal oxides in trace amounts, as can be seen in Table 1. From the results we can conclude that “K, in the form of KCl, K2CO3, and K2O components”, is the primary basic metal accountable for catalytic activity in transesterification reaction to produce biodiesel [24], as proven by XRD analysis.As depicted in Fig. 2 , the calcined KSS ash catalyst shows various adsorption bands of functional groups. OH groups are attributed to IR peak at 3139 cm−1, whereas peaks at 1646 and 1373 cm−1 are ascribed to CO stretching frequencies and peak at 1117 cm−1 evident to CO bending frequency in the form of K2CO3, which confirmed that the metal carbonates in the form of K2CO3 was present. These results are good agreement with the FT-IR results of the reported ash based catalysts [23,27,39–42]. The peak at 844 cm−1 may likely show the presence of the CO3 2− group [43]. Si–O–Si stretching band of SiO2 is represented by peak at 1041 cm−1, while OH bending vibrations of water molecules adsorbing on catalyst are represented by peak at 616 cm−1. The 532 cm−1 signal is caused by bond stretching vibrations of K–O and CaO, indicating the presence of these components in the catalyst. All of the peaks found in the calcined KSS ash catalyst are good agreement with the ash catalyst of banana peel [40], B. nigra [27], cocoa pod husk [41], ripe plantain peel [23], and C. papaya stem [42]. Hence as demonstrated in this study, the presence of carbonates and metal oxides in the prepared ash catalyst can improve their catalytic activity in reaction process and these results are consistent with XRD findings. Fig. 3 (A) reveals surface structure and morphology of the prepared KSS catalyst at different magnifications. These micrographs display the calcined ash catalysts have porous surface morphology, large aggregation of particles, fibrous and spongy texture which are in support of the characters of porosity of the materials. Oxygenated catalyst matter, such as metal oxides, could explain the bright particles seen in the SEM images [44]. Hence the prepared ash catalyst shows better catalytic activity as the highly porous catalyst achieves higher efficiency in the process of biodiesel production [42,45].Thermal gravimetric analysis (TGA) was used to investigate the weight loss percentage of the calcined KSS ash catalyst. Fig. 4 depicts the relationship between weight loss percentage and temperature as a whole. The presence of two weight loss is observed showing similar profile to walnut shell ash catalyst [30]. First loss observed in the range of 25 °C–200 °C is about 3.1%, which was attributed to the removal of moisture and low molecular weight compounds [34]. The main weight loss happened in range 200 °C–800 °C with a 22%, which may be related to degradation of carbonaceous materials, carbonates, releasing CO and CO2 [28,30]. It is reported that most of the carbonaceous materials decomposed after the calcination process [30,46]. Therefore this work uses 650 °C as the calcined temperature to enable the transition from carbonaceous materials to the materials that composed of mostly of metal oxides which is comparable calcined temperature reported by Etim et al. and Gohain et al. [23,38].Surface area and pore structure had a great impact on catalytic activity in the transesterification reaction process. KSS has a total specific surface area of 4.2 m2 g-1, pore volume 0.006 cm3g-1 and mean pore diameter 5.3 nm which are in range of mesoporous structure. The prepared ash catalyst's N2 adsorption-desorption isotherm exhibited the Type-IV isotherm, that is particularly closely followed by mesoporous material as shown in Fig. 5 . Mesoporous catalyst assisted biodiesel synthesis reactions have been reported with good catalytic properties and this is due to their ability to accommodate the basic sites that enhance the catalytic activity [47]. A mesoporous material can increase the rate of reaction by dispersing the reactants throughout its pores. On the other hand, microporous materials have a lower reaction rate than mesoporous materials because reaction occurs at the pores entrances [48,49]. As a conclusion, this mesoporous calcined KSS ash catalyst has the potential to considerably improve reaction rate while biodiesel synthesis. Relatively low surface area of various ash catalysts such as Gasified straw slag, B. nigra, M. Acuminata, Tucuma peels are reported of 1.26, 1.45, 3.66, and 1.0 respectively with high catalytic activity, shown in Table 2 [24,27,50,51]. This is due the highly basic nature of the materials that arises because of the dominant quantity of alkali and alkaline earth metal carbonates and oxide s that facilitate the strong basic sites on the surface of the catalyst to carry out the reaction [47,52].The pH of an aqueous solution of calcined KSS ash catalyst was determined by diluting KSS catalyst ash in distilled water in the following ratios, in Fig. 6 , the ratios of (w/v) and pH change were shown at 1:5, 1:10, 1:15, 1:20, 1:30, and 1:40. The catalyst was observed to be a strong base, with a pH of 11.46, which could indicate the presence of high potassium amount, as demonstrated by WD-XRF, and XRD analysis. High concentrations of alkali metals, particularly potassium are suggested to be behind the catalyst's increased pH. The catalytic activity of M. paradisiacal plant ash [52], Sesamum indicum plant ash [38], B. nigra ash [24], and Eichhornia crassipes ash [53] catalysts in biodiesel synthesis was considered inadequate compared to the current calcined KSS ash catalysts, with pH values of 11.30, 12.8, 11.76 and 9.6 respectively, presented in Table 3 .Transesterification can be carried out using KSS ash catalyst due to its high basic strength. The Hammett indicator test is used to determine its basicity and the results are based on color variations [54]. The tests shows the basic strength of the prepared catalyst in the range of 11.5 <H_ <15 which is comparable to walnut shell ash [30] and hazenut shell ash [35] catalysts, presented in Table 4 . The calcined KSS ash resembles these shell ash catalysts in terms of constituents, with minor differences in element percentages [30]. It is reported that high basic strength of the catalysts was due to the presence of metal oxides in the ash [35,55]. Hence, the presence of several strong basic sites in ash composition is an important to improve biodiesel conversion [56].In order to evaluate the calcination effect of KSS ash catalyst, the calcination temperature range of 200 °C–800 °C were used in transesterification of soybean oil with reaction conditions of methanol to oil ratio of 10:1, and catalyst amount of 2 wt% at 65 °C. The results can be seen in Fig. 7 . The calcination of the ash catalyst at 650 °C led to the resulted in a high 96% biodiesel output. Catalytic activity increases when an uncalcined catalyst is activated at 650 °C, but it decreases when it is further calcined at 800 °C. This is due to a decrease in catalyst carbonate (CO3 2−) concentration and a corresponding decline in reaction activity when the calcination temperature was raised to 800 °C. It could be linked to the partial or complete breakdown of K2CO3 to K2O at higher temperatures (800 °C). Compared to K2O, K2CO3 is more basic because of its weak acid and strong base nature, as well as the decrease in carbonate at higher calcinations temperatures [57]. As a result, the high concentration of K2CO3, a crucial element of the catalyst's high basic nature, leads to the selection of 650 °C as the appropriate calcination temperature to activate the catalyst. This outcome is consistent with the XRD and FT-IR predictions.Transesterification reaction rate and biodiesel yields can be affected by catalyst amount. Fig. 8 shows the results of the catalyst amounts (1, 2, 3, and 4 wt%) in a biodiesel production process using a calcined KSS catalyst with a 10:1 methanol to oil ratio at 65 °C. According to the findings, the KSS ash catalyst amount of 2 wt% resulted in the best results, yielding 96% biodiesel in a reaction time of 60 min compared to the other catalyst amounts. When the catalyst amount was increased from 1 wt% to 2 wt% the marginal rise in biodiesel yield from 75% to 96% observed. Moreover, despite increasing the catalyst loading by 4 wt% while maintaining the same reaction conditions, no noticeable improvement in biodiesel yield and reaction time was observed. This could be due to the initiation of a side-product or backward reaction. The literature has reported that increasing the amount of catalyst increases the yield of biodiesel up to a specific concentration of catalyst, after which the yield of the product either does not increase or remains intact [27,38]. It is also possible that a further increase in catalyst loading may make the three-phase solution more viscous, restricting mass transfer between the phases and saponification side reactions can be produced, lowering oil conversion for base-catalyzed reactions. Accordingly, in this work KSS catalyst amount of 2 wt % were found to be the best experimental conditions for soybean oil transesterification to biodiesel [57].Effect of different methanol to oil ratios on biodiesel production was also investigated with soybean oil at 65 °C with an optimized reaction time 60 min and catalyst amount 2 wt%. Fig. 9 demonstrates that raising the methanol-to-oil ratio from 6:1 to 10:1 increases biodiesel output from 60% to 96%. Farooq et al. observed that increasing methonaol produced more biodiese by increasing methoxy groups on the catalyst surface [58]. But when the methanol to oil ratio is further increased to 15:1, it does not show a considerable increase in biodiesel yield. This could be because transesterification is a reversible process that can be catalyzed both forward and backward by a base; however, due to high concentrations of methanol in the reaction mixture, reversible reactions occur, formulating monoglycerides and diglycerides, resulting in low oil to biodiesel conversion [24]. The works of Nath et al. and Basumatary et al. also displayed a similar pattern of experimental results, and in their reports, it was indicated that high levels of methanol to oil ratio beyond the optimized reaction condition dilute the reaction mixtures, swamping the catalyst's active sites, which in turn reduces their interactions for effective reaction, and as a result, the rate of reaction and the product yield fall [27,38,52].Catalyst reusability research is required to establish the efficiency of utilizing a catalyst to minimize production costs. The KSS ash catalyst was tested for reusability under ideal conditions, which included a methanol to oil ratio of 10:1, catalysts amount 2 wt%, and a reaction time of 60 min at 65 °C temperatures. After the transesterification reaction product was centrifuged, cleaned, and dried overnight in an oven at 100 °C. After 4 h of calcination at 650 °C, the catalyst was reactivated. Then, using fresh reactants, a similar reaction was carried out. The experimental results, shown in Fig. 10 , demonstrated that the yield dropped to about 70% after 4th run. As a result, biodiesel yields were found to have decreased significantly, possibly due to triglyceride contamination and pores clogged by glycerol and triglycerides on the catalyst surface. The catalyst amount was changed after each usage to achieve optimal reaction conditions. However, because of leaching during the washing phase, there was some catalyst loss in each cycle. This could explain the steady decline in yield after each catalytic cycle [59]. The SEM image of recycled catalyst revealed morphological alterations, as well as the collapse and emergence of agglomerated particles, Fig. 3 (B). These results coincide with the reusability findings reported by Gohain et al. and Basumatary et al. [42,52].Analysis of soybean oil biodiesel's chemical composition utilizing GC-MS testing is depicted in Fig. 11 , and in Table 5 , we summarize the composition of prepared biodiesel based on retention time which appears to contain a total of five main different fatty methyl esters (FAME). As a result soybean oil biodiesel consists primarily of methyl linoleate (Rt-21.93 min, C18:2, 43.23%) as the main FAME followed by methyl oleate (Rt-20.38 min, C18:1, 7.57%), methyl palmitoleate (Rt-22.03 min, C16:0, 6.05%), methyl stearate (Rt-20.84 min, C18:0, 5.87%), and methyl nonadecan (Rt-23.18 min C20:0, 3.72%). Fig. 12 represents the FT-IR spectral analysis of prepared soybean oil and soybean oil biodiesel. Soybean oil's CO stretching vibration at 1744 cm−1 is replaced by the methyl esters in biodiesel's 1739 cm−1 peak, indicating that the transesterification process has gone through an intermediate step, this results is agree with results reported by Nath et al. [27]. The result of C–H stretching vibrations, soybean oil has peaks at 2923 cm−1 and 2854 cm−1, while biodiesel has peaks at 2924 cm−1 and 2854 cm−1. At 1463 cm−1 and 1377 cm−1 in soybean oil; and at 1456 cm−1 and 1436 cm−1 in biodiesel, the CH3 bending vibrations are observed. The IR peaks at 1196 cm−1 and 1170 cm−1 for biodiesel and 1160 cm−1 and 1098 cm−1 for soybean oil demonstrate triglyceride and ester molecule C–O stretching bands, respectively. Methyl ester molecules emitted an infrared signal at 722 cm−1 and 723 cm−1 due to the rocking of fatty acid chains. All of the above-noted peaks are consistent with the FT-IR results reported by Nath et al. [27].Soybean oil biodiesel physicochemical properties using the KSS ash catalyst have been studied using ASTM standards, as shown in Table 6 and comparison of physicochemical properties of produced soybean oil biodiesel with reported soybean oil biodiesel by different authors illustrated in Table 7 . The results show that prepared biodiesel is almost within ASTM limits. In Table 6, the density of produced soybean oil biodiesel is 0.879 g/cm−3, well within the ASTM limit and in line with similar reported biodiesels [25,34]. It has been reported by Moser et al. that kinematic viscosity is the key element in selecting biodiesel as an alternative fuel compared to pure vegetable oil [60]. There is a slight increase in kinematic viscosity over the ASTM limit, but it is comparable to what Laskar et al. has reported [25]. This could be due to saturation and un-saturation, un-reacted molecules in the product, or manual errors in the separation of glycerol and biodiesel. Among other properties the produced soybean oil biodiesel exhibited flashpoint and fire points of 170 °C and 184 °C, respectively, to allow safer storage and transportation. And the cloud point and pour point were determined to be (+) 1 and (−) 2, respectively, which were found to be comparable to those of the previously reported soybean oil biodiesels. The cetane index of the produced soybean oil biodiesel was determined to be 48.5, which is lower than the cetane index of soybean biodiesel observed by Nath et al. [27].In this experiment, the KSS ash biomass-based heterogeneous catalyst was revealed to be exceptionally basic and make a significant contribution to catalytic activity. This outcome is a result of the potassium content, which is found in the forms of KCl, K2CO3 and K2O as the active components, having the highest level as determined by WD-XRF, XRD and SEM analysis. Studying the best conditions for biodiesel production with KSS ash catalysts revealed that they were 2 wt% catalyst amounts, 10:1 methanol to oil ratio, and 65 °C reaction temperatures. Under these conditions, calcined KSS ash catalysts demonstrated better catalytic activities and 96% of the product yield. Table 7 compares calcined KSS ash catalyst to other biomass-derived ash heterogeneous catalysts for biodiesel production [22–35]. In their study, ash was calcined in order to create a catalyst and with comparable reaction conditions high catalytic activities in the transesterification reaction were discovered. Table 7 illustrates that biodiesel yields of greater than 95% were achieved in all studies that used biomass ash based heterogeneous catalyst for biodiesel production without any chemical modification. Thereby, the findings from the present research support the utility of using KSS ash as a basic heterogeneous catalyst in the production of biodiesel and exhibit a pattern that is consistent with that previously reported.To conduct a cost analysis of soybean oil biodiesel production using KSS ash as a catalyst, we need to consider various factors such as raw material cost, catalyst cost, equipment cost, utility cost, and transportation cost etc. Soybean oil's price as a raw material can change based on market prices and the quantity needed to produce biodiesel. In general, it is envisioned that using KSS ash as a catalyst will reduce the cost of producing soybean oil biodiesel compared to using traditional catalysts. This is because KSS ash is a rich in alkaline earth metals, low in cost, and readily available discarded material in many areas, and it can be used in a simple and relatively low cost process. In the present work, the produced KSS ash catalyst exhibited adequate catalytic activity under optimum reaction conditions including catalyst amounts (wt%) and methanol to oil ratios when compared to those reported literatures and it showed reusability up to a fourth cycle. However, other aspects like transportation costs or the requirement for more tools may outweigh the cost savings. To ascertain its economic viability, a thorough cost analysis would need to be done.We have concluded in this paper, the karanja seed shells (KSS) which is a widely available agricultural waste, is a potential green heterogeneous catalyst for the synthesis of biodiesel. The use of KSS ash catalyst offers several benefits over non heterogeneous catalysts including cost effectiveness, environmental friendliness, high catalytic activity, reusability and easy separation. - Studies demonstrated that calcined KSS ash catalyst exhibits high activity and reusability. This is because it contains high amounts of potassium and calcium which can effectively catalyze the transesterification reaction and produce high yield of soybean oil biodiesel. - Based on the results the KSS ash catalyst presented a biodiesel yield of 96% from soybean oil in 60 min at 65 °C with catalyst amount of 2 wt % and 10:1 methanol to oil ratio. The catalyst was reused up to the 4th cycle which yielded a drop to 70% after the 4th run. - Catalyst characterization was done using XRD, WD-XRF, SEM, FT-IR, BET, and TGA techniques, and results were compared to those of previously reported biomass-based ash catalysts. The produced biodiesel is confirmed by GC-MS, FTIR, and ASTM testing. Studies demonstrated that calcined KSS ash catalyst exhibits high activity and reusability. This is because it contains high amounts of potassium and calcium which can effectively catalyze the transesterification reaction and produce high yield of soybean oil biodiesel.Based on the results the KSS ash catalyst presented a biodiesel yield of 96% from soybean oil in 60 min at 65 °C with catalyst amount of 2 wt % and 10:1 methanol to oil ratio. The catalyst was reused up to the 4th cycle which yielded a drop to 70% after the 4th run.Catalyst characterization was done using XRD, WD-XRF, SEM, FT-IR, BET, and TGA techniques, and results were compared to those of previously reported biomass-based ash catalysts. The produced biodiesel is confirmed by GC-MS, FTIR, and ASTM testing.Consequently, utilizing KSS catalyst will result in simple reaction procedures, low-cost catalyst preparation, and minimal waste problems for biodiesel synthesis. In addition, there is scope for future studies to investigate the feasibility and economic viability of KSS ash as catalyst in large scale biodiesel production. Pooja Prajapati: Conceptualization, Methodology, Investigation, Experiments, Formal analysis, Writing – original draft, review & editing. Sakshi Shrivastava: Conceptualization, Investigation, Collaborator, Formal analysis, Writing – review & editing. Varsha Sharma: Investigation, Writing – review & editing. Priyanka Srivastava: Conceptualization, Writing – review & editing, Resources. Virendra Shankhwar: Formal analysis, Writing – review & editing, Resources. Arun Sharma: Formal analysis, Writing – review & editing, Resources. S.K. Srivastava: Conceptualization, Supervision, Writing – review & editing. D. D. Agarwal: Conceptualization, Investigation, Supervision, Writing – review & editing, Resources.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors gratefully acknowledge the CIF, Jiwaji University, Gwalior (M.P.), India for providing XRD, FT-IR, and TGA facilities; the Innovation Centre, Bundelkhand University (U.P.), India for GC-MS analysis; the SAIF, IIT Patna, India for the 1H NMR facility; the CSIF, University of Calicut, India for providing BET facility; IIT Roorkee, India for SEM analysis; SICART Anand, India for WD-XRF facility; and Hindustan Laboratories Regd., New Delhi, India for testing biodiesel properties. KSS Karanja seed shell XRD X-ray diffraction WD-XRF Wavelength dispersive-X ray fluorescence FT-IR Fourier transforms infrared spectroscopy SEM Scanning electron microscopy TGA Thermogravimetric analysis BET Brunauer – Emmett - Teller GC-MS Gas chromatography-mass spectroscopy ASTM American standards testing methods Karanja seed shellX-ray diffractionWavelength dispersive-X ray fluorescenceFourier transforms infrared spectroscopyScanning electron microscopyThermogravimetric analysisBrunauer – Emmett - TellerGas chromatography-mass spectroscopyAmerican standards testing methods

In the present study, the ash from discarded Karanja seed shell (KSS), obtained after burning dried seed shells has been identified as one of the most cost-effective and efficient green heterogeneous catalysts for biodiesel production. Soybean oil methanolsis was used to study the catalytic activity of karanja seed shell ash as solid base heterogeneous catalysts in the biodiesel production. To characterize the catalyst, XRD, WD-XRF, SEM, FT-IR, BET and TGA techniques were utilized. Soybean oil methyl ester was converted under the following experimental conditions, as determined by GC-MS, and FT-IR: a catalyst amount of 2 wt%, a methanol to oil molar ratio of 10:1, a reaction temperature of 65 °C, a reaction time of 60 min. The synthesized heterogeneous catalyst is an efficient alternative that may be utilized as an eco-friendly catalyst because it is benign, reusable, sustainable, and widely available.

No data was used for the research described in the article.Functional amines constituting primary, secondary, and tertiary amines, as well as imines are key precursors and central intermediates in the synthesis of bulk and fine chemicals and have a wide range of applications in biology [1], medicine [2], energy [3], materials, and environment [4,5]. Generally, the primary benzylic and aliphatic amines can be easily functionalized to further form highly valuable chemicals, and pharmaceuticals [6,7]. Secondary and tertiary amines are considered as precursors for life science and material applications. Their structural motifs are widely found in a large number of biological molecules, pharmaceuticals, and agrochemicals as well as natural products [8–10]. On this basis, great efforts for the synthesis of functional amines are developed, including but not limited to the reductive amination of carbonyl compounds [11,12], the alkylation of ammonia or amines [13,14], and the hydrogenation of nitrile [15]. The catalytic reductive aminations of aldehydes in the presence of molecular hydrogen are found to be a convenient and cost-economic process for the preparation of different kinds of amines. However, reductive amination reactions often are non-selective and easily to be over‑hydrogenated to form corresponding alcohols or aminated to generate secondary/tertiary amines. The development of suitable catalysts is thus crucial for selective controlling in the reductive amination of heterocycle aldehyde towards specific amines with high activity and selectivity.Regarding the potential hydrogenation and amination catalysts for the synthesis of amines, transition metal-based (Ru [16,17], Ir [18], Pt [19,20], Rh [21], etc.) heterogeneous ones are frequently applied. Gu et al. [22] reported a strategy for the direct reductive amination of aldehydes by the unsupported ultra-thin Pt-nanowires catalyst, which exhibited excellent catalytic performance towards dibenzylamines with a high yield of 96% rather than primary amines under mild reaction conditions. Moreover, the Pt-nanowires were stable and recycled 6 times without any metal leaching or decrease in catalytic activity, and 9 examples of corresponding aromatic secondary amines were obtained in 70–96.9% yield (Fig. 1a). In the meantime, Kawanami et al. [23] chose Rh/Al2O3 catalyst for the reductive amination of furfural to furfurylamine by using aqueous ammonia solution and molecular hydrogen with very high selectivity of furfurylamine (∼92%) under the reaction time of 2 h at 80 °C. Interestingly, benzaldehyde was converted to benzylamine and dibenzylamine with a selectivity of 77.1 and 22%, respectively (Fig. 1b). In 2017, Beller and co-workers [24] developed the MOF-derived cobalt nanoparticles as an expedient and advantageous catalyst for the reductive amination of carbonyl compounds (aldehydes, ketones) with ammonia, amines or nitro compounds and molecular hydrogen to produce corresponding primary, secondary, tertiary and N-methylamines (Fig. 1c). Shortly afterward, their study group [25] immobilized Ni-tartaric acid complex on silica by solvothermal synthesis and then applied pyrolysis (400–1000 °C). For the one pyrolyzed at 600 °C (Ni-TA@SiO2–600), 2-bromobenzylamine was obtained in up to 60% yields. Some secondary amines could be also observed. However, the Ni-based catalyst obtained at 800 °C exhibited significant activities and produce 2-bromobenzylamine in 96% yield (Fig. 1d). This work provides a simple and obvious method to precisely control catalyst nanostructures for better selectivity in reductive amination process. Similarly, several groups also demonstrated that suitable support could provide interaction for the reactants, so the intrinsic catalyst structure can be responsible for the diversity of hydrogenation and ammonifying process, and thus affecting the catalytic activity and selectivity in reduction hydrogenation reaction.Very recently, covalent organic frameworks (COFs) had been used as sustainable support candidates for catalysis applications [26–29]. In contrast to other support to immobilize active metal nanoparticles, the COFs own diverse chemical structures, which could precisely tune the surrounding coordination environment and electronic interaction between metal nanoparticles and support materials [30,31]. Tremendous results in previous study demonstrate that the uniform porous structure in COFs can provide abundant metal active sites, thus improving the catalytic activity [32,33]. Based on this, it would be natural to expect that immobilizing the different kinds of active transition metals on the porous COFs support would be a feasible and referable route for the selectivity control-oriented modulation in the catalytic reductive aminations reactions. However, this application of precise hydrogenation and amination selectivity control has seldom been explored.In this study, we designed and synthesized COF support via a facile hydrothermal synthesis method, and noble metal (Pt, Pd, and Rh) was introduced to COF host matrix, respectively. Moreover, the influence of reaction conditions towards the reductive aminations activity and selectivity performance of three kinds of COFs supported metals catalysts were investigated. Among them, Pd and Pt based COF catalysts were more active in catalytic reductive amination towards secondary amines, while the Rh based COF catalyst was effective for the selective reductive amination of benzaldehyde to secondary imines and a primary amine, respectively. This kind of selectivity performance-responsive can be employed for the development of smart catalyst systems to rationally control the catalytic activities and functions on demand shortly.The COF support was synthesized similarly to our previous work with slight modified [34]. In brief, 0.2 mmol of 1,2,4,5-tetrakis(4-formylphenyl) benzene (TFPB) and 0.4 mmol of p-phenylenediamine (PD) were added to the mixture solution containing 5 mL of 1,4-dioxane, and then 0.3 mL of citric acid and 5 mL of 1,3,5- trimethyl-benzene were subsequently added under vigorous stirring. 1, 4-dioxane was used as a polar solvent. Then the system was purged with nitrogen gas for 5 min and transferred in an autoclave at 120 °C for 72 h. After cooling to room temperature, the obtained precipitates were washed with N, N-dimethylformamide, tetrahydrofuran, and acetone, respectively. Finally, the crystals were collected by drying under a high vacuum at 80 °C for 12 h and denoted as COF.Typically, COF supported metal catalysts were prepared by a vacuum-assisted impregnation method in this experiment. To prepare Pt/COF composite, 4 mg Pt precursor (1 g H2PtCl6·6H2O dissolved in 250 mL ethanol) was added drop by drop into the slurry of COF and followed under vacuum pressure. After the solvent was evaporated, sodium borohydride methanol solutions with a volume of 10 mL were quickly mixed as a reducing agent and stirred for 2 h. Subsequently, the obtained powder was washed with methanol several times to remove the excess solvent and finally dried at 60 °C for 12 h. In the same process, Pd/COF and Rh/COF catalysts were synthesized by using PdCl2·2H2O as the Pd source and RhCl3 as the Rh source, respectively. The theoretical amount of noble metal loading amount was 4 wt%.The crystalline structure of COF and COF supported metal catalysts was studied by the X-ray powder diffraction (XRD) patterns. As shown in Fig. 2a, for the COF sample, a narrow and sharp peak at 4.9 o and a broad diffraction peak at 19 o were observed, revealing a good crystallinity. When introducing metal into the COF, the diffraction peaks ascribed to the COF were still present; however, the intensity was decreased, indicating the lower crystallinity of COF after reduction with borohydride sodium borohydride. For the Pt/COF sample, another peak at around 39.7 o was detected, associated with the (111) plane of Pt [35]. In addition, the peak represented to Pd or Rh was observed in neither Pt/COF nor Rh/COF samples, possibly because of the small particle size and high dispersion of palladium and rhodium nanoparticles [36,37]. Fig. 2b showed the FTIR spectra of COF and corresponding COF supported noble metals catalysts. In the spectrum of COF, the absorption broad band at 1606 cm−1 representing the stretching vibration of CN was a result of schiff base formation in the COF synthesis. Moreover, the characteristic behavior corresponded to aromatic structures (vCC and vCC) were observed in the range from 1650 cm−1 to 1450 cm−1 , the peaks located at around 1276 cm−1 were a counterpart of CN stretching vibration. For Pd/COF and Rh/COF catalysts, the band at 1276 cm−1 assigned to CN stretching vibration became broad, indicating that the metal center was successfully coordinated with the nitrogen of COF outer layer. In addition, the elemental analysis results in Table S2 shown that the amount of carbon, hydrogen and nitrogen in the COF was 86.79%, 4.39% and 8.82% in terms of the weight, respectively. To investigate the thermal stability of COF and the corresponding COF supported metal catalyst, TGA was performed with a heating rate of 10°C/min under a nitrogen atmosphere from ambient temperature to 800°C (Fig. S5). The results indicated that the COF support has thermal stability up to 300°C.As displayed in Fig. 3 , the COF exhibited spherical morphology with an average diameter of 1.2 μm. The COF sphere consisted of different amorphous nanoflakes, revealing a highly ordered porous structure. To observe the metal nanoparticles and confirm their particle sizes on the COF, TEM images for the Pt/COF, Pd/COF, and Rh/COF TEM were shown in Fig. 4 . The Pt NPs size in Pt/COF (average particle size: 10 nm) was mainly in the range of 3–4 Fig. 4(b-c). Especially, some relatively larger sizes reaching around 5 nm could be detected. For the Pd/COF sample, small Pd nanoparticles (about 3–5 nm) were densely and uniformly assembled on the surface of COF, as shown in Fig. 4(d-f). The Rh NP was highly dispersed and its average size in the Rh/COF was approximately 2–3 nm (Fig. 4i). This result suggests that the use of well-dispersed Rh particles with a smaller size exposed more catalytically active metal sites, which could be favorable to activity performances. Moreover, elemental mapping using scanning transmission electron microscopy (STEM) confirms the homogeneous distribution of metal elements throughout the whole COF matrix in Fig.S1-S3, indicating the suitable procedure during the preparation process.We further explored the chemical state's identification and quantification of the synthesized COF and M/COFs. In the C 1 s spectra of COF in Fig. 5 , two prominent peaks were observed, where the one at around 284.8 eV was in accordance with the –C–(C, H)/C=C bonds in the graphitic structures, and the one located at 286.8 eV was attributed to the –C–N bonds. For the Pt/COF catalyst, an obvious shift of –C–N– XPS peak towards higher binding energy compared to that for COF was observed, suggesting that the C atoms are probably coordinated with Pt ions at the hybrid interface. Furthermore, the peak for –C–N– of the Rh/COF composite showed a lower energy shift, which was attributed to the interaction between the C and Rh species. The N 1 s spectra of COF could be deconvoluted into sp2 hybridized nitrogen in the form of C-N=C appeared at 398.9 eV and the C − N bond at 401.4 eV. Meanwhile, a similar shift of C − N 1 s XPS peaks towards higher binding energy in Pt/COF catalyst and a shift towards lower binding energy in Rh/COF catalyst could be detected as compared to that of pristine COF. This result implies a strong chemical bond forming between noble metal and N. On the other hand, the high resolution Pt 4f7/2 XPS spectra showed two characteristic peaks at 71. 7 eV and 74.2 eV, which were ascribed to the Pt0 and Pt2+ groups, respectively. Notably, the Pt 4f7/2 characteristic peaks at 71. 7 eV were slightly higher than the typical Pt0 (71.3 eV) groups, revealing partial electron migration from Pt to contiguous C or N. The Pd 3d5/2 spectrum was split into two peaks located at 335.83, and 337.63 eV (Fig. 5e), which can be attributed to Pd0 and Pd2+, respectively. The binding energies observed in Fig. 5f showed the appearance of Rh0 3d5/2 and Rh3+ 3d5/2, with their characteristic peaks at 307.4 and 309.2 eV. The Rh3+ characteristic peak at 309.2 eV was also slightly higher than the typical Rh3+3d5/2 (308.6 eV) groups, corresponding to partial electron migration between metal and surrounding C or N element.The N2 adsorption-desorption isotherms and corresponding pore size distribution of the prepared catalysts were displayed in Fig. 6 . It can be seen that the COF exhibited a typical type IV isotherm, revealing the mesoporous structures with a small number of micropores. The corresponding BET surface area and pore size were calculated to be 297 m2/g and 4.5*10−1 cm3/g, respectively (Table 1 ). The Pt/COF and Pd/COF had a relatively lower BET surface area (98.7 m2*g−1 and 68.7 m2*g−1, respectively) than that of COF, which implies that the introduction of Pt and Pd species slightly blocked the pore structure. In addition, the BET surface area of Rh/COF was calculated to be 185.1m2/g, while the corresponding total pore volumes was 3.2 *10−1 cm3/g. In comparison, the slightly higher BET surface area of Rh/COF was detected after metal incorporation, suggesting that the void space in the pores is not occupied by the Rh particles and the small particle size of Rh dispersion on COF support, as confirmed by the TEM images. Additionally, the average pore diameter of the COF supported Pt and Pd samples all increased to more than 10 nm, indicating the partial blocking of the pore under 10 nm after metal incorporation. Additionally, the Py-IR of COF and COF supported metal catalyst at 35 °C was shown in Fig. S4, the peaks near 1550 and 1450 cm−1 were attributed to the pyridinium ions adsorbed on Brφnsted and Lewis acid sites, respectively, indicating the acidic sites were mainly composed by weak acidity. Since Lewis acid catalysts have been demonstrated as the profitable catalyst for reductive amination reactions.We investigated the catalytic performance for the reductive amination of benzaldehyde (BD) and active metal-dependent amines selectivity over the COF supported metal catalysts along with commercial Pt/C, Pd/C, and Rh/C catalysts as a comparison in the presence of ammonia solution and H2 gas. As shown in Fig. 7a, among them, the Pt/COF and Pd/COF exhibited excellent selectivity in the production of dibenzylamine (DA) with a yield of 81% and 91%, respectively, while the Rh-based COF tended to selectively convert benzaldehyde to benzenemethanamine (BN) with approaching 90% yield under mild reaction conditions (90 °C, 2 MPa H2) in a 1.2:1 M ratio of BD to ammonia. It could be found that the metal Pt/COF and Pd/COF catalysts were more active in catalytic reductive amination towards DA when compared to the metal Rh/COF catalyst. It seems that the main influence on the activity and selectivity in tandem reductive amination reactions is not only the Lewis acid active sites within the COF but also the hydrogenation activity of active metal nanoparticles. This phenomenon was also detected in commercial noble catalysts, in which the Pt/C owned a relatively high DA yield of 72% and Pd/C possessed a DA yield of 61%. Additionally, as proved in previous literature, the adsorption of a high concentration of ammonia improves primary amine selectivity. We further evaluate the amine selectivity of Rh/COF with an enhanced molar ratio of ammonia to BD from 1/1.2 to 60/1.2 in Fig. 7b. When the ratio of ammonia to aldehyde increased to 60:1.2, the yield of BN decreased from 90% to 2%, while the yield of primary amine BM was indeed grown from 0 to 76%. It could be concluded that not only the active catalyst but also the concentration of NH3 is critical to the amines' selectivity. It seems that the Rh/COF catalyst had high selectivity for secondary imine with a 1.2/1 M ratio of BD to ammonia, and was more active in the production of a primary amine with a 1.2/60 M ratio of BD to ammonia. Moreover, the reuse experiment under identical conditions in successive runs for the reductive amination reaction of benzaldehyde by using COF supported catalysts was added in Fig. S6–8, the heterogeneous catalyst remained stable after three cyclic runs. A trifling drop in the yield was obtained, which might be attributed to the inevitable loss during the recovery process and active sites leaching under the high-temperature reaction conditions. Moreover, the concentration of the metal leaching test was also added by ICP-AES. As detected in Table S1, the tiny loss rate of ∼0.2 wt% in the content of Pd or Rh was observed in Pd/COF or Rh/COF catalysts after three runs.To test this, we further explored the substrate scope of the Rh/COF in the reductive amination of heterocycle aldehyde for the selective synthesis of target amines by adjusting the mole ratio of heterocycle aldehyde to ammonia. As listed in Table 2 , the reaction was carried out under the same conditions optimized for benzaldehyde. In all five cases, approximately 68–92% yields of the corresponding secondary imines were obtained from each benzaldehyde under the 1.2/1 mol ratio of heterocycle aldehyde to ammonia. Furthermore, the Rh/COF catalyst afforded the corresponding primary amines with preferable yields (42–78%) under the 1.2/60 mol ratio of heterocycle aldehyde to ammonia. Accordingly, it was found that Rh/COF could selective transformation of heterocycle aldehydes towards corresponding target heterocycle secondary and primary amines via reductive amination reaction, which can easily be functionalized further and thus was diversity-oriented synthesis for fine chemicals or pharmaceutical drugs.According to our studies and controlled experiments, we expected the reaction pathways for the reductive amination of benzaldehyde with NH3 in the presence of H2 gas in Fig. 8 . Firstly, the BD reacted with ammonia to spontaneously form the intermediate product [38]. In the following step, the HH bond was activated on the surface of metal based COF catalyst, and subsequently reacted with an intermediate product to yield BN. The secondary imine BM was unstable and it rapidly hydrogenated to reduce to DA under the catalysis of active metals such as Pt and Pd with strong hydrogenolysis ability. On the other hand, the BM could be also transformed to primary amine BM over the active catalyst with relatively weak hydrogenolysis but strong ammonolysis capacity. Meanwhile, the intermediate product could be directly heated up to generate tertiary amine. Most notably when hydrogen was excessive, the BA could be obtained as the main product. Based on the above results, we might considered that a suitable amount ratio of ammonia to hydrogen, hydrogenolysis, and ammonolysis capacity of active catalyst were responsible for the high selectivity production of target amines.In summary, we successfully designed and synthesized the Pt-based, Pd-based, and Rh-based COFs by a convenient and efficient post-treatment method and realized a selective and high-yield synthesis of target primary or secondary amines from heterocycle aldehyde via reductive amination reaction. The optimized Pd/COF catalyst displayed better catalytic performance for the reductive amination of benzaldehyde towards secondary amines with 91% yield under a 1.2:1 M ratio of aldehyde and ammonia under mild reaction conditions (2 MPa of H2 and 90 °C, 15 h), while the Rh/COF catalyst exhibited relatively high selectivity for secondary imines with 90% yield. More importantly, the Rh/COF catalyst had a relatively high selectivity towards corresponding primary amine from benzaldehyde derivatives when adjusting the molar ratio to 1.2:60. Thus, we anticipate that the present study will provide the feasible preparation of efficient active metal based catalysts for selective control in the reductive amination of heterocycle aldehyde towards functional amines.J.L. and L.M designed and supervised this research. N.W. performed the experiments data, analysis and wrote the original manuscript. J.L. reviewed and corrected the manuscript. All authors talked over the results and helped during manuscript preparation.Correspondence and requests for materials should be addressed to J. G. L. or L. L. M.The authors declare no competing financial interests.This work was supported financially by National Natural Science Foundation of China (52236010, 51976225), Science and Technology Program of Guangzhou, China (202201010014). Supplementary material Image 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2023.106620.

Since selective control in catalytic reductive aminations of aldehyde for the synthesis of functional amines was highly valued, we developed three kinds of COFs supported catalysts (Pt/COF, Pd/COF, and Rh/COF) for selectively catalytic reductive amination of benzaldehyde. Results indicated that Pd/COF and Pt/COF exhibited good selectivity towards secondary amines with a 1.2/1 M proportion of aldehyde and ammonia, while Rh/COF catalyst can selectively convert benzaldehyde to secondary imine with an excellent yield of 90%. Furthermore, an improved yield of primary amine could be obtained over Rh/COF catalyst with a 1.2/60 M proportion of aldehyde and ammonia.

Data will be made available on request.There is a constantly increasing interest in improving electrochemical devices for the production of energy in order to satisfy its currently high demand and sustainability requirements. However, these devices traditionally employ noble metals catalysts [1–4], since they have excellent properties mainly related to activity and stability, such as (i) avoiding detachment of particles, (ii) minimizing loss of electrical contact, and (iii) improved durability due to their less susceptible to degradation [5]. In the search for innovative catalysts, noble metals have been usually incorporated into different materials that act as supports and facilitate the availability of these active sites. Furthermore, even more recently, some innovative materials are being developed to provide physicochemical properties, such as porosity and high surface area, to the noble metal catalyst themselves in order to improve their performance avoiding the use of supports [6,7]. Among these new materials the use of noble metals for the manufacture of aerogels has been of great interest in recent years, however, most of the metallic aerogels investigated recently are made with an organic compound and noble metals, such as Pt [8–10], Au [11,12], Pd [13–16], Ag [17,18] and their combinations [7,19–23], as well as aerogels of metal oxides and noble metals [16,24,25]. These unique materials have been applied as efficient electrocatalysts for the electrooxidation of organic fuels such as ethanol, formic acid, and oxygen. These materials are highly novel due to their unique morphology of assembled nanoparticles; recent work shows that assembling sheets considerably improves the catalytic activity of aerogels towards oxidation reactions [49–52].The synthesis methods of aerogels have changed over the years [6,13], although literature shows that slight modifications of the synthesis steps will be traduced in notable differences in their final properties [26]. In 1846, J.J. Ebelmen, synthesized the first silica aerogel using the sol-gel method [27]. This methodology involves a gelation process, where a solution loses its fluidity and takes the appearance of an elastic solid, followed by curing and drying steps. The drying step is usually performed under supercritical conditions (i.e. temperatures higher than 300 °C and up to 219 atm) [28]. However, other drying methods have been proposed in the last decades, such as lyophilization, which avoids such as extreme operating conditions [7,29].Microwave heating can be used for chemical synthesis as a heating source besides to favour interactions between reactants [30,31]. Their dipolar polarization and conduction of ions in liquid solutions cause the collision of ions and molecules to produce uniform heating in the precursor mixture, promoting the reactions. This technology is very effective as the energy is transferred directly to the reactants, minimizing gradients and energy losses. As a consequence, synthesis time is considerably reduced. Hence has been previously used for sol-gel synthesis of carbon [32,33] and silica gels [34,35]. Moreover, Martínez-Lázaro et al. used microwave heating to produce Pd aerogels with high yield by using a rapid and controlled synthesis process [29].Pd has been successfully used as electrocatalyst due to its high speed of electron transfer [21,36,37], and is considered a replacement for Pt as electrocatalyst in the ethanol oxidation reaction due to the higher density current in alkaline media [13,19–21,38]. On the other hand, there is an increasing interest in developing new and efficient electrocatalysts but avoiding the use of noble metals due to their high cost and low availability [46–48]. Therefore, in this work, Pd was partially substituted by transition metals (TM). Specifically Fe, Co, and Ni were combined with Pd to produce a cost- and electrocatalytic-effective metallic aerogel by means of microwave heating. The innovative procedure and final aerogels obtained are presented as an interesting step forward in the development of innovative and efficient electrocatalysts for energy generation devices.PdCl2 (99%, Sigma-Aldrich ReagenPlus®, anhydrous powder, St Louis, MO, USA), CoCl2 ∗ 6H2O (98%, Sigma-Aldrich ®, ACS reagent, St Louis, MO, USA), NiCl2 ∗ 6H2O (99.9%, Sigma-Aldrich®, St Louis, MO, USA), FeCl2 (98%, Sigma-Aldrich ®, St Louis, MO, USA), Na2CO3 anhydrous (99.5%, JT. Baker R®), Glyoxilic acid (98% Sigma-Aldrich). Water was provided with the UPT-IV-10L system (18.3 Mꭥ cm).The synthesis procedure is based on previous work on Pd-aerogels [29] with some modifications. Aerogels [Pd:TM] were prepared in proportion [4:1] for each metal salt (Co, Fe, Ni). The concentration of the precursor salt solution mixture was 2 mg/ml20 ml of the mixture was taken to react with sodium carbonate and glyoxylic acid monohydrate solution (ratio 6:1) [14] in deionized water. The heating source consisted in a microwave heating (MW) at 67 °C controlled by introducing a thermocouple into the precursor mixture. All process was carried out during 7 h: the first 2 h takes place the formation of the colloids and the last 5 h the gelation and crosslinking processes are carried out. Subsequently, the samples were cooled to room temperature for washing before drying.The drying process was performed with a residual volume of deionized water. The mixture was then frozen with liquid N2 and the solvent eliminated by lyophilization (HyperCOOL, model: HC3110) at −110 °C for 24 h. A scheme of the whole synthesis process is presented in Fig. 1 .The morphology of Pd-TM aerogels was characterized using a scanning electron microscope (SEM, Quanta FEG 650 microscopes from FEI). The crystal structures were evaluated by X-ray diffraction (XRD; D8-advance diffractometer Bruker) equipped with CuKα X-ray source (λ = 0.1541 nm, 40 kV, 40 mA) using a step size of 0.02° 2ϴ and a scan step time of 5 s. The calculation of the lattice constant ( d h k l ) was made from the following expressions: (1) d s p a c i n g = λ 2 sin θ (2) d h k l = d s p a c i n g h 2 + k 2 + l 2 where d s p a c i n g is the interplanar spacing and h, k and l the miller index. The position of the 2ϴ peaks and the width at the half height of the peak ( β h k l )sample were also determined. The instrumental correction was carried out with a standard pattern and the error of the instrument and β T o t a l was adjusted using the following expression: (3) β T o t a l = β E x p e r i m e n t a l + β s a m p l e where β E x p e r i m e n t a l is the error of the instrument and β s a m p l e the sample values. From the data collected in the XRD, the crystallite size and the microstrain of the crystal lattice were determined. First, the crystallite size ( D 111 ) was calculated using the Scherrer equation: (4) D 111 = K λ β 111 cos Ɵ where K is Scherrer constant corresponding to cubic symmetry, and ( β 111 ) is the line broadening of width at half maximum.The specific surface area was estimated by nitrogen adsorption-desorption isotherms at −196 °C (Micromeritics ASAP 2020), after outgassing the samples overnight at 120 °C. The chemical surface composition and the oxidation state of elements were measured by X-ray photoelectron spectroscopy (XPS, Analyzer Phoibos 100, SPECS, Germany). A monochromatic AlKα X-ray source operating at 14 kV was employed to perform the analysis.The electrochemical evaluation of the Pd-TM aerogels was carried out in a conventional three-electrode cell in alkaline media at 50 mV/s scan rate. A glassy carbon electrode (3 mm of diameter), an Ag/AgCl electrode, and a Pt wire were used as working, reference and counter electrodes, respectively. The electrocatalyst ink was prepared using each sample in a mixture of deionized water (500 μL) and Nafion® (5%, 50 μL) per milligram of aerogel.The ink was sonicated during 15 min and then 10 μL were deposited over the working electrode surface. The electrolyte was bubbled with N2 for 40 min before the electrochemical measurements. For each sample, the electrochemical profile was obtained by cyclic voltammetry (CV) experiments in 0.5 M KOH within a potential range of −0.25 to 1.6 V vs RHE. The electrochemical active surface area (ECSA) of each aerogel was estimated from the cyclic voltammograms by using the reduction charge of Pd (II) oxide according to the following equation: (5) E C S A = Q m m P d e d m where, Q m denotes coulombic charge (Q per μC cm-2) for the reduction of Pd (II) oxide achieved by integrating the charges related to the reduction of Pd (II) oxide for the different samples; mPd is the mass amount of Pd loaded (g cm-2) on the glassy carbon electrode surface and ed m is a constant (424 μC/cm), which corresponds to the reduction of a Pd (II) oxide monolayer.The electrocatalytic activity of the Pd-TM aerogels towards the electrooxidation of ethanol was tested by cyclic voltammetry (CV) in 0.5 M KOH + 0.5 M ethanol solution within a potential range between −0.25 and 1.6 V vs. RHE. This solution was bubbled with N2 for 40 min before each electrochemical measurement.The stability performance was carried out by chronoamperometry in the three-electrode system at 0.6 V vs. RHE for 90 h in 0.5 M KOH +0.5 M ethanol as electrolyte.XRD spectra of the synthesized aerogels are shown in Fig. 2 a. In the XRD pattern of the Pd and Pd-TM aerogels were observed four major diffraction peaks appear at the 2ϴ values. The peak with the highest intensity was used for the calculation of crystallite size. For Pd aerogel, the peaks were 40.1°, 46.6°, 68.1° and 82.1° which are ascribed to the (111), (200), (220) and (311) reflection planes of Pd (JCPDS #46–1043), respectively. These peaks are according with a face-centered cubic (FCC) crystal structure. Despite the resulting aerogels revealed characteristic peaks of Pd, a shift in the position of the Pd-peaks was observed for Pd-TM aerogels. These show a change to the left suggesting the presence of alloys. The PdCo aerogel presents the peaks in 2ϴ were 40.1°, 46.2° 68.1° and 81.8°, comparing with the reference JCPDS #65–6075 corresponding to PdCo, which has a peak with high intensity in 2ϴ with the value of 41.7° corresponding to reflection plane (111), being further away from the one obtained and coinciding even more with the JCPDS #46–1043 of Pd [39]. In the case of PdNi aerogel, peaks were located at 39.9°, 46.6°, 67.9° and 81.9, presenting a more significant displacement compared to the JCPDS #46–1043 of Pd, JCPDS #72–9071 corresponding to PdNi [40] with peaks in 40.2°, 47°, 68.8°, 82.9° and 87.3° which do not correspond to those obtained, again indicating more excellent proximity to the JCPDS of the Pd. Finally, PdFe aerogel with peaks in 35.5°, 40.1°, 46.9°, 57.1°, 62.9°, 68°, 81.9° and 86.6, presenting again the trend observed with the previously mentioned aerogels where a shift to the left is shown. Although the closeness of the peaks is observed, corresponding to the JCPDS of the Pd, it is taken into account presents peaks at 35.5° and 86.6° that correspond to reflection planes of FeO(OH) (JCPDS #74–3080) [41].The d s p a c i n g for Pd, PdCo, PdNi and PdFe aerogels calculated at (111), (200), (220) and (311) were 2.24 Å, 1.95 Å, 1.37 Å and 1.17 Å, respectively. These values were employed to calculate the lattice constants ( d h k l ) and evaluate if the addition of TM to the Pd structure makes any difference. Thus, the calculated d 111 values were 3.89, 3.90, 3.89 and 3.88 for Pd, PdCo, PdNi and PdFe, respectively (the error is ≤ 0.3 of the standard JCPDS #46–1043 with a value of 3.89).The crystallite sizes obtained using the 2ϴ value of 40.1°, with the Scherrer equation (Eq. (4)) were 9.46, 3.70, 3.96 and 11.04 nm for Pd, PdCo, PdNi and PdFe, respectively. There is a great difference between materials, detecting a notable decrease of crystallite size for PdCo and PdNi aerogels and a slight increase for PdFe aerogel compared to that obtained for Pd aerogel.Calculation of microstrain (ε) with XRD data was also performed to evaluate if it was the cause of the broadening of the peaks, indicating dislocations within the crystal structure. For this evaluation the following equation was used: (6) ε = β T o t a l 4 tan θ The data obtained could indicate a decrease in microstrain as the angle increases and the crystallite size should be consistent in a range of two theta values. However, the microstrain values obtained do not show a decreasing trend and the crystallite size decreases as the angle increases. This result clearly indicates that the crystallite size also contributes to peak broadening. Therefore, the Williamson Hall method (W–H) was employed using the following expression: (7) β T o t a l cos θ h k l = ε ( 4 s e n θ h k l ) + K λ D The first four reflections (111), (200), (220) and (311) were used to construct the plot of 4senϴhkl vs βTotalcosϴhkl. The crystallite size and the microstrain are obtained from the intercept and slope of the fitted line, respectively. The values obtained for crystallite size were 5.5 nm and 2.53 nm for PdNi and PdFe (Fig. 2b). According to the results, an increasing trend is observed, implying the presence of defects (i.e. distortions, dislocations, faulting and relaxation of the grain surface). In addition, the fitting line does not cross the ordinate axe through the origin, indicating a significant size effect. The microstrain obtained in both samples is very close, as shown by the slope of the adjustment. For PdNi a linear adjustment was obtained without taking into account the point that is too deviated (1.6, 0.041). In the case of PdCo, the W–H method was not able to be applied, since the values of 4 s e n θ h k l do not vary with βTotalcosϴhkl. This is due to the fact that the stress in the crystal lattice since the strain is not related to the broadening of the diffraction peaks. In the case of PdNi and PdFe assuming that the broadening of the peaks is due to the contribution of microstrain.The specific surface area of the Pd-TM aerogels was determined by the Brunauer-Emmett-Teller (BET) method applied to the nitrogen adsorption-desorption isotherms (Fig. 3 ) yielding values of 77, 66, 47 and 4 m2/g for Pd, PdCo, PdNi, and PdFe respectively. This indicates the presence of certain microporosity in these samples, especially for Pd, PdCo and PdNi aerogels. The helium densities of these samples are 1.4, 1.2 and 2.5 g/cm3 for PdCo, PdNi and PdFe, respectively. These values indicate a clear difference in the total porosity of the samples, being the Pd–Ni the sample less dense, and therefore, with higher total porosity. By applying 2D-NLDFT to the adsorption isotherms, a pore size distribution is obtained (Fig. 3b). Although it cannot be provided a mean pore size for each aerogel obtained due to the wide pore size distribution, it can be said that there is a clear trend to increase in the pore size from PdFe, PdNi, to PdCo. The lowest helium density of PdNi aerogel probably indicates the presence of macroporosity in higher extent than in PdCo and for sure in much higher extent than PdFe. This latter aerogel is the less porous of the series.BET surface areas of Pd aerogels should be in the range of 32–162 m2/g, the density in the range of 0.10–12 g/cm3 and the pore size <50 nm according to the bibliography [6,19,20]. In fact, the surface area of the Pd aerogel is referenced as 50 m2/g and the pore size of about 11 nm [20]. Comparing these values with the values obtained in this work, except in the case of PdFe aerogel, in general, the porous properties are preserved. This is a very relevant feature because is a key factor for promoting mass transport through the 3D porous network.Further, the morphology of the aerogels was characterized by Field Emission Scanning Electron Microscopy (FESEM) and High Resolution Transmission Electron Microscopy (HRTEM). The micrographs are presented in Fig. 4 .The aerogels present a foam-like morphology, with a 3-dimensional open porous network. The FESEM images show a more open porosity structure in the case of PdCo (Fig. 4a), and a more fused and less porous structure in the case of the PdFe aerogel (Fig. 4c). The aerogel PdNi (Fig. 4b) presents an intermediated porous morphology. These observations are in agreement with the results previously discussed from helium density and N2 adsorption-desorption isotherms. HR-TEM analysis reveals an interconnected distribution of particles in all samples obtained (Fig. 4 d-f). Images at different magnifications show chains of particles defining pores of different sizes between them, typically characteristic for aerogels. PdCo aerogel shows particle sizes between 2 and 4 nm, similar to the sample PdFe (between 1 and 3 nm). It seems that Co and Fe metal transition incorporation allows obtaining smaller sizes than that obtained for PdNi particles (5–8 nm). At higher magnification, it can be seen that the interplanar distances are slightly higher for PdNi aerogel (0.225 nm), which can be attributed to the presence of alloy between the two metals used in the synthesis. The interplanar distance for Pd–Co and PdFe is 0.22 and 0.223 nm respectively.By means of X-ray photoelectronic spectroscopy (XPS), the analysis of the Pd–Ni, Pd–Fe, and PdCo aerogels was carried out to know their chemical composition and the oxidation state (see Fig. 5 ). The general spectrum of the samples (Fig. 5a) indicates the presence of C, Pd, Co, O, Fe, and Ni, as expected. The spectra of Pd 3d are shown in Fig. 5b, where a comparison is made of the oxidation states present in Pd–Fe, Pd–Ni, and Pd–Co samples. Each spectrum was deconvoluted into the orbitals Pd 3d5/2 and Pd 3d3/2, which peaks are centered at 335.1 eV and 342.2 eV corresponding to Pd2+, 336.2 eV and 342.5 corresponding to Pd4+ while those located in 335.1 eV and 340.7 eV refer to Pd0 [22,39,40]. A higher concentration of Pd2+ is observed in the Pd–Fe and Pd–Ni samples, meanwhile, Pd4+ is present in a higher concentration in the Pd–Fe and Pd–Co samples. The high-resolution scan of Fe 2p is presented in Fig. 5e. The spectra of Fe 2p3/2 and Fe 2p1/2 were deconvoluted, giving the peaks 709.8 eV and 724.09 eV that refer to Fe0, signals in 712.5 eV and 724.2 eV that corresponds to Fe2+ and those located in 715.0 eV and 732.5 eV attributed to Fe3+. These results suggest the presence of the Fe2O4 structure as already reported in the literature [41]. The quantitative evaluation of each peak was also made from the XPS results by dividing the area of the peak, which was calculated from the cross-sections and the mean depth of the exhaust electrons [42]. XPS data were interpreted using the Avantage Thermo software, with angular resolution (ARXPS) and the adjustment of the peaks by the Analyzer software. Fe0.94 Pd0.1 was the molar proportion obtained for the PdFe aerogel. Furthermore, the chemical composition was also determined by calculating the percentage of the relative weight of the sample. PdFe aerogel presents a higher percentage of Pd2+ (41.8%) in comparison with Pd4+ (11.7%), the percentage of the iron species in its metallic state (Fe0) is 40.2%, while the percentage of the oxidized species Fe2+ and Fe3+ is 38.5% and11.9%, respectively. These results suggest that PdFe aerogels can be oxidized, resulting in the oxidation of Fe0 to Fe3+. However, the oxidation is not complete, as can be seen in Fig. 5e, which is consistent with the results of XRD. The spectrum of Ni 2p is observed in Fig. 5c, having deconvolution orbitals in Ni 2p3/2 and Ni 2p1/2, with the peaks of 855.1 eV and 872.5 eV corresponding to Ni2+, while those located at 856.6 eV and 874.0 eV refer to Ni3+ [43,44]. The molar proportion obtained for PdNi aerogel is Ni 0.64Pd0.1. From the calculation of the percentage of the relative weight of the sample, it can be inferred that PdNi presents a higher percentage of Ni2+ (45.8%) than Ni3+ (28.7%).The high-resolution scanning of Co 2p is presented in Fig. 5d. The spectra of Co 2p3/2 and Co 2p1/2 were deconvoluted, giving the peaks 784.9 eV and 797.2 that correspond to Co2+ and those located in 780.2 eV and 796.72 referred to Co3+, which correspond to the Co3O4 structure [45]. The presence of the PdCo alloy on the surface can be appreciated, having the two spectra in their oxidation state. The quantitative evaluation of each peak was obtained by dividing the area of the peak, which was calculated from the cross-sections, and the mean depth of the escape electrons. The molar proportion for PdCo aerogels was Pd0.1Co0.72. In addition, the percentage of the relative weight of the sample indicates that there is a higher percentage of Pd0 (40.3%) than the oxidized species Pd2+ (10.5%) and a higher proportion of the oxidized species Co2+ (38.5%) in comparison to Co3 (18.6%). This shows that PdCo aerogels can be oxidized, resulting in a larger proportion of Co2+ than Co3+, which is consistent with the results of XRD.The elemental composition, the oxidation states, and the relationship between their concentrations evaluated by the XPS analysis will be related to the following electrochemical analysis.Aerogels were evaluated by cyclic voltammetry (CV) to determine their electrocatalytic activity for ethanol oxidation reaction (EOR) in a potential range between −0.25 and 1.6 V vs RHE. Electrochemical profiles were obtained in 0.5 M KOH as electrolyte at ambient conditions at 50 mV/s scan rate (Fig. 6 a). A Pd aerogel sample obtained by the same synthesis method was also characterized for comparative purposes. The experimental CVs show the peaks attributed to Pd activity: (i) the hydrogen desorption in the range 0.1–0.2 V vs RHE, (ii) hydrogen adsorption at 0.3 V vs RHE, (iii) reduction of Pd (II) oxide from 0.55 to 0.8 V vs RHE and (iv) formation of Pd (II) oxide at 1–1.4 V vs RHE. All these phenomena are present in the cyclic voltammograms of all Pd aerogels. However, samples containing transition metals show peaks that are attributed to the oxidation and reduction of each metal species (i.e. Co, Ni and Fe). The redox processes of the noble metal species can be observed at different potentials (Fig. 6a), for PdNi at 1.2 V vs RHE, marked with the legend 3 [Ni]; for PdCo oxidation and reduction processes are observed between 1.1 and 1.3 V vs RHE, which can be seen with the legend 2 [Co]; while Fe redox processes occur between 0.3 and 0.6 V vs RHE, which are labeled 1 [Fe] and 1 [Fe]'. ECSA values obtained were 3.2, 5.7, 9.4 and 26.5 m2/g for Pd, PdNi, PdFe, and PdCo respectively. The Pd-TM aerogels show a significant improvement in the electrochemical performance in comparison to bare Pd and using low loading of noble metals. The ECSA values obtained in the PdTM improve the diffusion of the reagents, in this case, ethanol, through the electrode. In addition, XRD analysis shows that the formation of crystals between Pd and transition metals is clearly an advantage for electron transfer in comparison to bare Pd.In order to compare the electrochemical performance of the Pd-TM electrocatalysts, the aerogels were evaluated for the ethanol oxidation reaction (EOR). It was performed by CV experiments in an electrolyte containing 0.5 M C2H6O + 0.5 M KOH. EOR was carried out in the same range of potential than the CV tests (i.e., from −0.25 to 1.6 V vs RHE). The electrochemical profiles for EOR demonstrated that Pd-TM aerogels show high performance at room temperature in the range of 0.7–1.1 V vs RHE, Fig. 6b.It is observed that the current activity of the Pd-TM aerogels is improved, in comparison to bare Pd aerogel with a value of 48 mA/mg, following the trend PdNi > PdCo > PdFe, with 117, 96.1, and 73.35 mA/mg, respectively. PdNi aerogel shows the best electroactivity towards EOR (117 mA/mg). However, the thermodynamic reaction potential is slightly favored by the PdCo aerogel, whose highest peak current can be observed at (0.86 V vs. RHE, while thePdNi peak appears at 0.91 V vs. RHE. Since the difference in reaction potential towards the OER is very small, it can be assumed that the best catalyst is PdNi. These two values are close to the potential obtained with the Pd bare aerogel (i.e. 0.88 V vs RHE). In the case of the PdFe aerogel, this sample shows its maximum peak at 0.93 V vs RHE. The suggested mechanism for C2H5OH in an alkaline environment on electrode surfaces with Pd-TM aerogels is the next: (8) Pd + OH− → Pd-OHads + e− (9) Pd–CH3CH2OH → Pd(TM)-(CH3CH2OH)ads (10) Pd-(CH3CH2OH)ads + 3OH− → Pd-(CH3CO)ads + 3H2O + 3e− (11) Pd-(CH3CO)ads + Pd-OHads + OH− → CH3COO− + 2Pd + H2O Ethanol oxidation mechanism with these Pd-TM aerogels occurs in two pairs of reactions (Eqs. (8)–(11)) [51–53]. In the first two steps, the OH− present in the solution reach the surface of the electrode and it absorbs OH− efficiently since the transition metals accelerate this process [58]. On the other hand, the CH3CO (Eq. (3)) generated in this reaction are adsorbed on the surface of Pd catalysts, this combination between Pd-TM considerably accelerates the EOR (Fig. 6b). In addition, the combination of the reaction of Pd-TM with hydroxyl groups and the CH3CO adsorbed on the catalyst surface are the rate-determining step of the EOR (Eq. (4)). The transition metals (Ni, Fe, Co) provide an opportunity to improve the adsorption of OHads onto the surface of the catalyst and thus activate the catalyst surface and finally help to enhance the ethanol oxidation process [59]. On the other hand, the C–C bond in the C2H5OH molecule is very difficult to break, despite the fact that Pd-TM aerogels work efficiently, the interaction between C2H5OH and the catalyst ends up generating subproducts. Pd-TM catalysts promote the oxidation of the fuel following the trend: Co > Ni > Fe. These aerogels are highly active catalysts for EOR, and their performance is shown in Fig. 6b, where peaks between 0.7 and 1.1 V vs RHE for all aerogels represent the EOR reaction and CO2 formation, and CO2 desorption is observed in the potential range between 0.55 and 0.68 V vs RHE [38,53,54]. Comparison of ECSA, potential peak value and current density for EOR for obtained catalysts in this work with electrodes already reported is shown in Table 1 . The results demonstrated that the electrocatalysts applied in this study have relative advantages, in terms of the potential value and current density.The maximum electrochemical activity value among all Pd-TM aerogels evaluated was detected in the range from 0.5 to 1.0 V vs RHE. Therefore, the stability test was carried out at 0.6 V vs RHE. Chronoamperometry was performed for 90 h as shown in Fig. 7 . Apparently, PdNi retains 25 mA mg-1 at 10 h, and is higher than the rest of the catalysts, except PdCo, which retains 23 mAmg-1 in the same time interval. These two catalysts show the highest antitoxic capacity of the intermediate products and the high electrochemical stability [56] due to their constant performance. Although the Pd and PdFe samples also turn out to be highly stable and resistant to the corrosion of the medium, the PdNi and PdCo samples are superior. The retention capacity of the samples can be attributed to the number of active sites and their BET surface area mainly [56–58]. At the end of the 90-h test, the PdNi and PdCo samples showed only a 10% decrease in their charge retention. Some analysis of the materials after these electrochemical tests reveals that the degradation of PdNi and PdCo aerogels are negligible (see Supplementary Material) showing that they are very stable and effective electrocatalysts.Pd-TM aerogels were successfully synthesized using a quick and simple synthesis procedure based on microwave heating and freeze-drying. The nanostructured aerogels obtained present high specific surface areas and electrochemical surface areas, favoring the diffusion of reactants and their activity versus electrochemical reactions. On the other hand, the chemical characterization of these Pd-TM aerogels may suggest the formation of alloys, although the crystalline size and morphology are maintained.These physicochemical properties are traduced in a notable improvement of the electrochemical behavior of these aerogels. In fact, in the ethanol oxidation reaction, the Pd-TM aerogels result in 117, 96.1, and 73.35 mA/mg for PdNi, PdCo and PdFe, respectively. These values are clearly higher than the 48 mA/mg corresponding to the bare Pd aerogel.The incorporation of these transition metals clearly improves the ethanol oxidation reaction compared to Pd due to their excellent combination of physicochemical properties. This opens the possibility towards the development of highly active catalysts with a low loading of noble metals, reducing dependence and costs in electrochemical conversion energy systems as fuel cells and water electrolyzers. A.Martínez-Lázaro: Writing-original draft, Methodology. M.H. Rodriguez-Barajas: Writing-original draft, Methodology. N. Rey-Raap: Investigation process, visualization, supervision. F.I. Espinosa: Data curation. J. Ledesma-García: Resources, Project administration, supervision. A. Arenillas: Supervision, Resources. L.G. Arriaga: Writing –Review and editing, Project administration, supervision.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: L. G. Arriaga reports financial support was provided by Mexican Council for Science and Technology.Authors thank to Consejo Nacional de Ciencia y Tecnología CONACYT (Mexico) for funding through the Ciencia de Frontera 2020–845132, 2019-39569 and LN-321116. Also to Ministerio de Ciencia e Innovacion (Spain), the European Union Next Generation EU/PRTR and Science, Technology and Innovation Plan 2018-2022 of the Principado de Asturias with the projects PCI2020-112039 MCIN/AEI-10.13039/501100011033, PID2020-113001RB-I00 MCIN/AEI-10.13039/501100011033 and IDI/2021/50921. N.R.-R is grateful to the Horizon-MSCA-2021-PF-01-01 call for financial support through the Metgel project 101059852. Finally, thanks to Anabel de la Cruz and Cesar Leyva for TEM-analysis.

In the present research work, unsupported Pd-TM aerogel catalysts are prepared by an ultrafast synthesis by means of a combination of microwave heating and lyophilization. These novel aerogels are synthesized to explore the effect of transition metals on a Pd aerogel matrix in order to reduce the dependence on noble metals and increase its electrocatalytic properties for different electrochemical reactions. Physicochemical characterization of Pd-TM aerogels reveals a successful combination of high specific surface area, electrochemical surface area, and specific oxidation states. The combination of these properties in Pd-TM aerogels enhances the electrocatalytic activity towards ethanol electrooxidation compared to bare Pd aerogels. Among all the Pd-TM, PdNi aerogel presents the highest current per unit mass with 117 mA/mg, being a clear improvement compared to the Pd aerogel (48 mA/mg).

No data was used for the research described in the article.CO2 is one of the most important greenhouse gases that cause global warming, and its emissions keep increasing in recent years. Thus, it is urgent to reduce CO2 emissions to mitigate global warming. CO2 capture and utilization (CUU) enable the synthesis of a wide range of value-added products such as methane, long-chain hydrocarbons, olefins, methanol, and higher alcohols, presenting a promising way to achieve carbon neutralization. Recently, higher alcohols have drawn increasing interest because they can be used as an alternative fuel, fuel additive, solvent, and raw material. Most of the studies on higher alcohol synthesis by CO2 hydrogenation focus on analyzing the thermodynamics[1,2], designing highly efficient catalysts[3–6], as well as unraveling the reaction mechanism[3–6].As far as the catalyst design is concerned, four categories of catalysts, including Rh, Cu, Mo, and Co-based catalysts have been widely studied. Amongst, Co-based materials have been intensively investigated due to Co being non-noble metal and possessing relatively high CO2 conversion. Metallic Co generally catalyzes the conversion of a CO2/H2 mixture yielding alkanes. However, it is found that the ability of Co to break the C-O bond can be turned down by forming metal alloys or interacting with oxide support[7–9], which is crucial for the formation of higher alcohols. As presented in Table 1, Co-based catalysts such as reduced CoAlOx, Na-Co/SiO2, and Pt/Co3O4 have been studied in a fixed bed or tank reactor for higher alcohol synthesis. Accordingly, CO2 conversion ranges from 4.6% to 67.2%, and selectivity to higher alcohols ranges from 0.05% to 92.1%, with a higher alcohol yield of 0.01–2.16 mmol gcat −1 h−1 are obtained.Generally, higher alcohols formation over Co-based catalysts involves the following steps ( Fig. 1): (i) activation of CO2 and H2 forming C1 intermediates, including CHx*, CO*, and HCOO*; (ii) formation of C-C bond through CHx*-CO* or CHx*-HCOO*coupling; (iii) hydrogenation of C2 intermediates to form ethanol. Besides, the C2 intermediates can also participate in forming higher alcohols with a longer carbon chain.In a recent review by Tang et al., they suggested that metallic Co (Co0), ionic cobalt (Coδ+), cobalt oxide (CoO), and cobalt carbide (Co2C) are catalytically active species for CO2 conversion[18]. Even though the exact nature of the active Co site has not yet reached a consensus due to the interconversion of these species under reaction conditions, and more importantly the intermediates such as CHx*, CO*, and HCOO* are suggested to form on different surfaces of these species or the modified ones. Thus, the synergy between the active species on which CHx* and CO*/HCOO* occur respectively is of great importance for the coupling of C1 intermediates and higher alcohol formation. Moreover, many hetero-site Co-based catalysts such as Co0-CoO, Co0-Coδ+, and Co2C-NaCo2C have been proved efficient for higher alcohols synthesis by CO2 hydrogenation; however, a review focusing on the catalytic activity of various Co species as well as their synergy in higher alcohols synthesis by CO2 hydrogenation is still absent.Thus, in this review, we first introduce the catalytic activity of Co0, CoO, Coδ+, and Co2C in CO2 hydrogenation, as well as the strategies to tailor their structure. Then, the formation of hetero-site Co catalysts and the synergy of these hetero sites in promoting higher alcohols synthesis are discussed. Finally, we propose new strategies to further enhance the synergy of hetero sites in Co-based catalysts for boosting higher alcohols synthesis by CO2 hydrogenation. Hence, we hope that this review will be beneficial to those working on Co-based catalysts for higher alcohols synthesis by CO2 hydrogenation.Co0, CoO, Co2C, and Coδ+ are active for CO2 hydrogenation forming various products including CO, CH4, C2+ hydrocarbons, and alcohols. The catalytic activity of Co species depends on their structure which can be tailored by rational catalysts design. In Sections 2 to 4, we discuss the roles of Co0, CoO, Co2C, and Coδ+ in CO2 hydrogenation as well as the strategies to tailor their structure.Even though both Co0 and CoO catalyze CO2 hydrogenation, the activity and selectivity vary over catalysts prepared by different synthesis methods under various conditions. For example, Have et al. prepared SiO2, Al2O3, CeO2, and TiO2 supported Co0 and CoO catalysts by controlling the reduction temperature by H2 (i.e., 450 and 250 °C for Co0 and CoO, respectively) and studied their catalytic performance in CO2 hydrogenation (in a fixed bed reactor, 250 °C, 20 bar, H2/CO2 = 3)[19]. Accordingly, SiO2, Al2O3, and CeO2 supported Co0, as well as TiO2 supported CoO, possessed a higher Co-time yield than their counterparts ( Fig. 2a). Regarding product selectivity, Co0/Al2O3 mainly produces CO while all other catalysts produce mainly methane with a minor amount of CO (Fig. 2b). The differences in the catalytic performance of these catalysts are ascribed to the differences in the oxidation state of Co and the Co-metal oxide support interaction, indicating the complex nature of Co-based catalysts in CO2 hydrogenation. More specifically, in Co0 catalysts, the incomplete reduction, the Co-metal oxide support interaction, the oxidation by CO2/H2O during the reaction, as well as the passivation during catalyst synthesis and transport may lead to the presence of CoO/Coδ+ species, while in CoO catalysts, the reduction by H2 during the reaction may result in the formation Co0. Hence, all these factors make it difficult to unravel the exact nature of the catalytically active species.However, it is generally believed that Co0 is more active to dissociate H2 offering active H[20,21]. Besides, Co0 possesses a high electron density near the Fermi level ( Fig. 3), offering excited electrons for the hydrogenation steps[22]. Accordingly, the active H formation together with the excited electrons provided by Co0 enhances CO2 hydrogenation. On the other hand, the onset of the edge of the CoO valence band is 1.15 eV below the Fermi level (Fig. 3), so it is difficult to thermally excite sufficient electron-hole pairs for catalytic reaction at low temperatures (e.g., <350 °C)[22]. Thus, Co-based catalysts with more Co0 species generally possess higher CO2 conversion and CH4 selectivity (or CH4 production rate)[22–24].Moreover, Co0 and CoO offer different surfaces for CO2 adsorption and activation. Density functional theory (DFT) calculation is used to study the adsorption of CO2 on Co0 (110) and CoO (100) facets. It is found that the O–C–O bond angle deforms more and the C–O bond elongates more on Co (110) facet than on the CoO (100) facet, suggesting a stronger activation of CO2 by Co0 (110) surface[19]. Zhao et al. compared the CO2 adsorption on (Co)0.5(CoO)0.5 and (Co)0.2(CoO)0.8 catalysts using CO2 temperature programmed desorption (CO2-TPD) experiments[22]. They found that CO2 binds stronger over (Co)0.5(CoO)0.5, suggesting the presence of a more basic surface probably due to a higher concentration of Co0. Furthermore, Yin et al. proposed that the defective unsaturated CoO can effectively accelerate CO2 adsorption and activation forming carboxylate intermediates at the Co-CoO interface. The carboxylate intermediates can be further hydrogenated to CH4 or decomposed to form CO at higher temperatures[25]. Besides, the CO2 may also directly dissociate on Co0 surface[21], which will be further discussed (next paragraphs in this section, Fig. 4).Co0 and CoO possess different activities for carbon chain propagation. Co0 has been recognized as the active species for Fischer-Tropsch synthesis (FTS) and is active for carbon chain growth[26–30]. Consistently, in CO2 hydrogenation, Zhao et al. found that C2H6 generated through carbon chain growth is preferably produced on catalysts with higher Co0 concentration. This suggests that Co0 rather than the CoO is responsible for the C-C coupling in CO2 hydrogenation[22]. However, Ten Have et al. observed that the CoO catalysts produce more olefinic C2 and C3 products than Co0, indicating that the lower active H availability and less favorable hydrogenation on CoO, which may promote C-C coupling[19].Notably, the CO2 hydrogenation reaction mechanisms over Co0 and CoO are different. Ten Have et al. studied the reaction mechanism over SiO2, Al2O3, CeO2, and TiO2 supported Co0 and CoO by time-resolved infrared spectroscopy (IR)[19]. And it was found that CO* species only occur on Co0 catalysts irrespective of the support material employed, indicating a direct CO2 dissociation mechanism over the Co0 sites (Fig. 4). However, formyl, formate, and carbonate species instead of CO* species are observed over CoO-based catalysts, suggesting that CoO catalysts follow the H-assisted formate mechanism over CoO (Fig. 4)[31]. It is also proved that the direct CO2 dissociation pathway occurs at a higher rate than the H-assisted pathway. However, the H-assisted pathway is more favorable for C2+ hydrocarbons formation[31]. Besides, Wang et al. proposed the carboxylate pathway over Co0, in which the carboxylate is subsequently dissociated into *CO and then further hydrogenated to CH4, due to the stronger CO and H2 adsorption and activation on Co0 sites (Fig. 4)[31]. The carboxylate intermediate also forms on the oxygen vacancies presenting in CoO followed by hydrogenation or decomposition to form CH4 and CO respectively[23].Interestingly, there are also differences in the CO2 hydrogenation over hexagonal close-packed (hcp) and face-centered cubic (fcc) Co0 sites. DFT calculations indicate that the dissociation of CO2 into chemisorbed CO* and O* occurs on both hcp-Co0 and fcc-Co0 ( Fig. 5). Subsequently, over hcp-Co0 (10−10) facet, the energy barrier of CO*+H*→HCO* is lower than CO* desorption, leading to high CH4 selectivity. In the contrast, over fcc-Co (111) facet, CO* is easier to desorb to form CO product due to a lower CO* desorption energy (Fig. 5) [32].Controlling the reduction of Co3O4 is a simple way to tailor the content of both CoO and Co0 species in the catalyst. Generally, the reduction of Co3O4 proceeds through two steps, i.e. Co3O4 → CoO and the subsequent CoO → Co. Lv et al. studied the reduction of spherical Co3O4 nanoparticles by temperature programmed reduction (TPR) using a H2/N2 mixture (10 vol% H2)[33]. Accordingly, two distinct reduction peaks at 276 °C and 350 °C are observed. Besides, the amount of H2 consumed by the low-temperature reduction step is about 1/3 of that of the high-temperature reduction step, confirming the reduction through Co3O4 → CoO → Co. Moreover, Zhao et al. used a microbalance to track the weight loss during the reduction of Co3O4 in a H2/He mixture (40 v% H2)[22]. As presented in Fig. 6, the mass loss indicates that unsupported Co3O4 starts to be reduced at around 200 °C and completely transformed to CoO at 300 °C. The complete reduction to Co0 occurs at 370 °C. Thus, it is feasible to tailor the percentage of CoO and Co0 in the catalyst by controlling the reduction temperature and the reduction duration. In situ X-ray diffraction (XRD) study on the reduction of unsupported Co3O4 also proves a two-step reduction process; however, the reduction temperature differs due to different properties of the Co3O4 precursor used as well as different reduction conditions employed[34]. It is found that Co3O4 is first reduced to CoO, followed by complete transformation to hcp-Co0 at 250 °C in the H2 atmosphere. Then, hcp-Co0 transforms to fcc-Co0 at 350 °C, and at 450 °C only fcc-Co0 is detected. Increasing the temperature to 500 °C increases the crystallinity of the fcc-Co0. However, reducing unsupported Co3O4 first at low temperatures (e.g., 250–300 °C) for 3–5 h before increasing the temperature enables the production of hcp/fcc-Co0mixture up to 450–500 °C[34].By controlling the reduction temperature, a series of Cox(CoO)1−x catalytic systems were obtained for CO2 hydrogenation[22]. At 1 bar and 180/200/220 °C, the CO2 conversion increases with increasing content of Co0, reaching the maximum when x = 0.2 ( Fig. 7). Then, CO2 conversion decreases with further increasing Co0 content. Moreover, methane is the main product, whose selectivity and production rate possess a similar volcano trend.The support/promoter plays a key role in tailoring the reducibility as well as the dispersion of the Co species and in turn, affects the catalytic performance. Jacobs et al. found that the interaction between Co surface species and the support is stronger for Al2O3 and TiO2 supported catalysts than the SiO2 supported one[35]. The strong interaction on one hand decreases the reducibility of Co, and on the other hand, leads to the formation of smaller clusters. Al2O3 supported catalyst is more difficult to be reduced, but it possesses a smaller cluster size and thus resulting in higher availability of surface metal sites after reduction than the SiO2 and TiO2 supported ones. Further incorporation of noble metals (i.e., Ru, Pt, Re) increased the degree of reduction but shows a negligible change in the cluster size[35]. More specifically, Ru and Pt were found to facilitate the reduction of both Co oxides and Co species which strongly interacted with the support, while Re mainly promotes the reduction of the Co species which strongly interacted with the support. Accordingly, on the catalysts possessing weak Co clusters-support interaction (i.e., Co/SiO2), the noble metal promoter was found to only slightly increased the number of surface Co metal. However, over the catalysts with strong interaction (i.e., Co/Al2O3 and Co/TiO2), the number of active sites increases significantly. Besides, the metal oxide (i.e., B, La, K, Zr) promoter decreases the reducibility of the catalyst, thereby reducing the number of surface Co atoms[35].The change in reducibility and dispersion further influences CO2 hydrogenation activity. As a support/promoter, metal oxide shows significant influences. Wang et al. investigated the effects of silica on the property and catalytic activity of Co-based catalysts[36]. Without silica, the Co is completely reduced to Co0, which possesses a higher hydrogenation activity and produces mainly methane in CO2 hydrogenation. Incorporating silica inhibits the complete reduction of Co, forming Co2+ which weakens the hydrogenation ability. And, a balance between the Co0 and Co2+ species formed favors methanol synthesis by CO2 hydrogenation ( Fig. 8). Besides, Janlamool et al. found that incorporating Ti in mesoporous silica support facilitates the reduction of Co oxides which are strongly interacted with the support, enhancing CO2 hydrogenation and methane formation[37]. Moreover, Co supported on various oxides and reduced at 450 °C in H2 shows different activity for CO2 hydrogenation to methane. The methane yield is in the order of Co/CeO2 (∼96%) > Co/ZnO (∼54%) > Co/Gd2O3 (∼53%) ∼Co/ZrO2 (∼53%)[38] with CO as the side product. The high catalytic performance of Co/CeO2 is mainly due to improved reducibility caused by Co-ceria interaction. Co may also react with the support/promoter forming a new species possessing different reducibility. For example, reducing Co/KIT-6 at high a reduction temperature leads to the formation of Co2SiO4 and/or Co-O-Si species. Thus, the formation of Co0 is inhibited, resulting in poor CO2 adsorption and activation. Accordingly, the high reduction temperature for Co/KIT-6 leads to decreasing activity and CH4 selectivity for CO2 hydrogenation[39].( Fig. 9).Noble metal also presents an efficient promotional effect to tailor the reducibility and the CO2 hydrogenation activity. Beaumont et al. found that Pt nanoparticles (NPs) can promote the reduction of Co, due to the transportation of H atoms dissociated on Pt NPs to the Co NPs via H atom spillover[40]. This probably also occurs under reaction conditions and facilitates the removal of surface Co oxide formed during the reaction, generating more catalytically active sites and therefore significantly promoting CO2 hydrogenation to CH4 [40]. Alkali metals such as K can enhance metal-support interaction, inhibiting the catalyst's reducibility and shifting the reduction to higher temperatures[32]. Besides, the K promoter improves the surface basicity of the catalyst and meanwhile increases the particle size. All these contribute partly to the formation of C2+ hydrocarbons by CO2 hydrogenation which will be further discussed.In some cases, the interface between Co0/CoO and the support/promoter is also catalytically active for CO2 hydrogenation. Melaet et al. found that CoO/TiO2 outperforms Co0/TiO2 in CO2 hydrogenation and is more selective to methane, but Co0/SiO2 is more active than CoO/SiO2 [41]. The high catalytic performance of CoO/TiO2 is ascribed to the unique interface formed between CoO and TiO2. Besides, a Co/Mn-oxide hybrid catalyst possesses high activity for CO2 hydrogenation with high selectivity even at high temperatures and ambient pressure which are thermodynamically unfavorable for CO2 hydrogenation[42]. In this catalyst, a unique Co0/Mn2+ interface is formed. At the interface, Mn2+ activates CO2 through bridge bonded formate, while Co0 could deliver H atoms to rupture the C-O bond in the formate intermediate for methane formation. Moreover, a Co-Mn hybrid oxide catalyst is found to be active for methanol synthesis by CO2 hydrogenation[43]. The MnO/CoO interface is suggested to facilitate the CO2 conversion as well as the formation of methanol. Furthermore, based on in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments, Yue et al. proposed that the reduced-Co3O4/ZnO interfaces are the active sites and facilitate the formation of HCOO* and CH3O* intermediates during CO2 hydrogenation, promoting the conversion to C1 products[44].Furthermore, the support/promoter may influence CO2 hydrogenation via tailoring the reaction route. For example, K as a promoter enhances CO2 hydrogenation over Co0 due to the K-Co0 interaction which causes increased electron density around Co0 and strengthened CO2 adsorption[32]. It is further suggested that two reaction routes occur over the K-Co0 catalysts. The major one follows the direct CO2 dissociation to CO*, which then desorbs to form CO as a product ( Fig. 10). The minor one follows the H-assisted CO2 activation through HCOO* intermediate. Subsequently, the reduction of HCOO* and C-C coupling steps results in the formation of C2+ products (Fig. 10). Besides, the CO2 hydrogenation performance and reaction route over Co-based catalysts supported on anatase (Co/a-TiO2) and rutile (Co/r-TiO2) differ[45]. CO is the main product over Co/a-TiO2, while Co/r-TiO2 selectively produces CH4. In-situ DRIFTS measurements suggest that the reaction on Co/a-TiO2 follows direct CO2 dissociation to CO*, forming gas-phase CO instead of subsequent hydrogenation. However, over Co/r-TiO2, CO2 hydrogenation proceeds via formate species, followed by hydrogenation to CH4.Further addition of K, Zr, and Cs improves the CO2, CO, and H2 adsorption capacity and strength over Co/r-TiO2 and Co/a-TiO2 [45]. Adding Zr promoter to Co/a-TiO2 enables CO2 hydrogenation via formate intermediate, which is subsequently hydrogenated to CH4. Moreover, the promoters influence the surface C/H ratio significantly, following the order of unpromoted < Zr-promoted < K-Zr-promoted ∼ Cs-Zr-promoted. The high C/H ratio benefits C2 + hydrocarbons formation. Accordingly, K-Zr-Co/a-TiO2 achieves the highest C2 + hydrocarbons selectivity of 17% with 70% CO2 conversion ( Fig. 11).The particle size is another important factor influencing the properties and catalytic activity of Co0/CoO catalysts. Generally, the bigger Co cluster resulting from the higher Co loading decreases the Co-support interaction and increases the reducibility of Co[35]. Besides, the more Co-oxide interface as a result of low Co loading leads to more positively charged Co due to the strongly perturbated structural and electronic properties at the interface[24]. As a result, over Co/CeO2 catalysts, the high Co loading catalysts (i.e., 2 and 4 wt% Co/CeO2) with a larger particle size as well as a higher percentage of Co0, while the low Co loading catalyst (1 wt% Co/CeO2) is smaller with a significant amount of CoOx after reduction. Accordingly, 1 wt% Co/CeO2 is less active for H2 activation and thus less efficient in CO2 hydrogenation. Moreover, in all cases, the direct dissociation of CO2 to CO* with subsequent hydrogenation is the main route for CH4 formation. However, when carbonyl production is faster than its consumption, part of it will desorb into CO gas ( Fig. 12). In the minor reaction route (i.e., CO2 associative mechanism), the HCOO* intermediate forms at the Ce3+. The subsequent hydrogenation of HCOO* to CH4 and CO is favored over 2 and 4 wt% Co/CeO2 rather than over the 1 wt% Co/CeO2, due to the insufficient H atoms migrating from Co0 particles to adjacent ceria over 1 wt% Co/CeO2 (Fig. 12).K as a promoter can also influence the Co particle size. For example, over reduced 15 wt% Co/Al2O3 catalysts, the particle size of Co increases with increasing K loading[46]. Moreover, the Co particle size directly correlates with CO2 conversion and product selectivity. Generally, very small particles favor CO formation, and methane formation increases with particle size. However, K addition also enhances Co-support interaction and decreases the reducibility, inhibiting the reduction of the catalyst. Thus, a higher K loading suppresses methane formation, increasing the selectivity to C2+ hydrocarbons.The carbon chain growth is also particle size dependent. Somorjai group found that the CO2 hydrogenation turnover frequency increases with increasing Co particle size (reduced in H2 at 450 °C)[47,48]. CH4 which is most favorable and CO are the main products for all the catalysts with different particle sizes (i.e., 3–10 nm). At low temperatures (i.e., below 250 °C), the FTS reaction shows slight changes in the product selectivity for all particle sizes, but at high temperatures, the carbon chain growth is enhanced with increasing Co particle size. Besides, the crystal size of the precursor may influence the reduction process. A crystal size smaller than 200 Å favors fcc-Co0 formation, while hcp-Co0 is obtained when the crystal size is larger than 400 Å[34], accordingly the different crystal phase may show different catalytic performance for CO2 hydrogenation as discussed in Section 2.1.Partially oxidized Co sites (Coδ+) are known to be active for CO2 hydrogenation/reduction. It is generally believed Coδ+ favors the formation of *CO intermediate, which may insert into *CHx intermediate promoting higher alcohols synthesis[10,49]. Jimenez et al. also found that SiO2 supported single atom Co (Co2+) as an active species for CO2 hydrogenation, reaching a CO2 conversion of 7%, with 95% selectivity to CO[50]. Besides, Co single atoms supported on carbon material in the form of Co–C2N2 moieties are active sites for electrochemical reduction of CO2 to CO[51]. Furthermore, DFT calculation demonstrated that the ligand is important to tailor the catalytic activity of single Co site. For single atom Co sites supported on N doped carbon material, the adsorption strength of the CO* intermediate becomes weaker as the coordination number of the N ligand increases from 1 to 4, providing an opportunity to tailor the catalytic activity of Co site for higher alcohols synthesis by CO2 hydrogenation[52].Coδ+ site generally exists at the Co-support/promoter interface. The interaction between Co and support/promoter as well as the charge transfer from Co to metal oxide at the interface is the main reason for the formation of Coδ+. For example, Coδ+ forms at the Co-CeO2 interface[53,54]. Moreover, in a Ga-doped Co-Al spinel-derived catalyst, the transfer of electrons to Ga3+ leads to the formation of Coδ+ sites[49]. Rodrigues et al. also identified Coδ+ species over Co/SiO2 [55]. The formation of silicates and/or Co hydrosilicate that are formed by the reaction between silanol groups on the surface of silica and Co ions is the main reason for the presence of Coδ+.Single-atom Co site anchor on the surface of carbon, metal oxide, and other materials also provide Coδ+. The electronic properties of the supported Co single atoms depend on the host materials and the employed preparation methods[56]. Co atoms supported on N-doped carbon nanosheet derived from cocoon silk are positively charged Coδ+ (2 < δ < 3)[57]. Besides, Jang et al. found that the valence state of Co single atoms on N-doped porous carbon nanotubes is between Co2+ and metallic Co0 [58]. Details on the effect of synthesis methods and electronic properties of single atom Co catalysts can be found in the review by Kaiser et al.[56].The formation of Co carbide has traditionally been considered a possible reason for the deactivation of Co-based FTS catalysts[59–62]. However, more recently, van Ravenhorst et al. found that when Co carbide formation over Co/TiO2 is detected, the product formation does not noticeably change[63], indicating that Co carbide formation is not a major reason for deactivation. Besides, for higher alcohols synthesis by CO hydrogenation, Co carbide acts as an active site for CO insertion during the formation of higher alcohols[64–68]. Moreover, Co carbide nanoprisms are promising catalysts for lower olefin synthesis by CO hydrogenation[69], and the facet geometry and preferential exposure of Co carbide play a key role[59,61,67,69–71]. Thus, the knowledge of CO hydrogenation over Co carbide is an important input to rationalize the design of catalysts for CO2 hydrogenation[72].Co carbide is also active for CO2 hydrogenation, mainly leading to the formation of methane. Yu et al. compared the CO2 hydrogenation activity of Co oxide (CoO/γ-Al2O3) and Co carbide (Co2C/γ-Al2O3)[73]. Accordingly, Co2C/γ-Al2O3 was found to be highly active for methanation, reaching a CO2 conversion up to 89% with nearly 100% selectivity to CH4. However, CoO/γ-Al2O3 possesses much lower activity, producing both CH4 and significant amounts of CO. Besides, Khangale et al. investigated the deactivation of 15%Co-6%K/Al2O3 catalyst during the hydrogenation of CO2 to long-chain hydrocarbons[74]. CO2 conversion, as well as the selectivity to long-chain hydrocarbons, decreased with increasing reaction time, while the selectivity to methane increased. And, the formation of Co2C in the spent catalyst is supposed to account for the formation of methane.Furthermore, Lin et al. studied the effects of CO2 on the structure and catalytic performance of Na-promoted CoC2, which is active in the conversion of syngas to olefins[75]. In CO hydrogenation, the main products were olefins and oxygenates (88.4% selectivity) with methane as a minor product (3.1% selectivity). However, CO2 hydrogenation mainly produces paraffins. Furthermore, increasing the CO2 content in the syngas feed reduces the activity, olefin selectivity, and olefin/paraffin formation, meanwhile the carbon chain growth is inhibited ( Fig. 13a-d). Interestingly, the Na-promoted Co2C nanoprisms remained stable in syngas; however, in the presence of CO2 in the feed, competitive adsorption between CO and CO2 occurs on the catalyst’s surface, reducing the CO coverage and creating a higher H2/CO surface ratio. Accordingly, Co2C nanoprisms with (101) and (020) facets change to nanospheres with (111) facets and even reduce to Co0, changing the structure of the active sites as well as their catalytic behavior (Fig. 13e).Co carbide forms under FTS conditions and has traditionally been considered a possible cause for the deactivation of Co-based FTS catalysts[59–62]. However, Claeys et al. proposed that the formation of Co carbide seems to be kinetically inhibited[76]. Generally, only a small percentage (e.g., 5–10%) of Co may be transformed into Co2C under extreme reaction conditions such as low H2/CO ratios. However, in the H2 free syngas (i.e., only CO in the feed), a significant amount of bulk Co2C may form over an extended duration.Co0 and CoO can be also transformed to Co2C under FTS conditions. For CoO → Co2C process, CoO transforms directly to Co2C, and no Co0 forms in the whole process. Thermodynamic analysis indicates that the Gibbs free energy change of the CoO → Co2C process is much smaller than that of the Co0 → Co2C process, suggesting that the CoO process is thermodynamically more favorable[77].However, Paterson et al. suggested that Co0 is the intermediate for the conversion of CoO to Co2C[78]. In this study, Co3O4 spinel was used as a precursor for the synthesis of Co2C. The reduction of Co3O4 to CoO in CO occurs rapidly at 200 °C, and the transition to Co2C occurs at 250 °C. Co0 is likely formed and then quickly consumed to produce the Co carbide. Furthermore, Paterson et al. treated Co0 with CO at 200 °C to synthesize Co carbide with a particle size of about 9 nm[78]. A direct relation between the CO partial pressure with the rate of Co2C formation was observed. The carbide formation rate in 5% CO feed at 10 barg and 50% CO feed at 1 bara is similar ( Fig. 14). Besides, the crystal structure of Co0 influences the carbonization process. Under the FTS conditions, the hcp-Co0 transforms into Co2C quickly, while a fraction of the fcc-Co0 remains metallic[79], consistent with in situ XRD studies[80].Alkali metal promoter also influences the carbonization process. Pei et al. used a reduced physical mixture of Co3O4 and alkali metals (i.e., Li2O, Na2O, and K2O) as the precursor and CO as the carbonization source[81]. They found that adding Li2O accelerates Co2C formation and shortens the carbonization time significantly. The promoting effect of Li can be related to the adsorption of H2 on the reduced precursor, enhancing the precursor’s ability to react with CO.It is generally believed that Co2C is unstable[75], and its decomposition into Co0 is observed under FTS conditions[61]. Kwak et al. found that the hydrogenation of Co2C starts at ∼160 °C in H2. Gnanamani et al. also observed the conversion of Co2C to Co0 for the sample without an alkali metal and adding alkali metal significantly stabilizes Co2C[82]. Furthermore, The Co carbide is stable in air up to 90 °C; however, it decomposes to Co0 in H2 at 120 °C. During the decomposition, fcc/hcp mixed phases form after initial reduction but finally transform to an hcp phase after the carbides are removed[78].Co0, CoO, Co2C, and Co can activate CO2 and form C1 intermediates such CHx*, CO*, and HCOO*, which are also the intermediates for higher alcohols synthesis. Thus, designing hetero-site Co catalysts and tailoring their structure and catalytic activity by the above-mentioned methods to enable CHx*-CO*/HCOO* coupling may promote higher alcohols synthesis by CO2 hydrogenation. The hetero-site Co catalysts can be created by tailoring the reduction process, incorporating alkali metals, or interacting with support/promoter. We discuss these strategies in Sections 5 to 7.Co0 possesses a strong hydrogenation ability and favors CH4 formation, while CoO favors CO formation. Thus, designing Co0-CoO hetero site catalysts and tailoring the synergy between them may facilitate CHx*-CO* coupling, thus facilitating ethanol formation. Controlling the reduction process is a facile way to create the Co0-CoO hetero sites.Wang et al. reduced a layered double hydroxide (LDH) derived CoAlOx precursor in H2 at various temperatures to create Co0-CoO hetero sites and tailor their structure[15]. At a reduction temperature of 300 °C, only Co3O4 is obtained, and Co0 is almost undetectable. At 400 °C, Co3O4 is reduced to CoO, while Co0 and CoO coexist after reduction at 600 °C. Further increasing the reduction temperature decreased the CoO content and the Co0 content increased ( Fig. 15a-d). Compared with the catalysts reduced at other temperatures, the 600 °C reduced catalyst achieved a balanced content between Co0 and CoO, promoting their synergy. Accordingly, it possesses the best performance in CO2 hydrogenation (Fig. 15e), reaching the highest ethanol selectivity (92.1%) and yield (0.444 mmol g−1 h−1). In-situ Fourier-transform infrared spectroscopy (FT-IR) is further used to unravel the mechanism for the enhanced ethanol formation over the hetero site Co0-CoO catalyst[15]. It is suggested that ethanol formation proceeds via CO2 adsorption and activation, formation of HCOO* and CHx* intermediate, formation of acetate through CHx*-HCOO* coupling, and subsequent hydrogenation to ethanol. The enhanced ethanol formation is ascribed to the coexistence of Co0 and CoO after the reduction treatment at 600 °C, which promotes the formation of CHx* for converting HCOO* into acetate by insertion, an important intermediate for synthesizing ethanol. A poorly reduced catalyst (i.e., reduced at 400 °C) possesses lower activity and ethanol selectivity but increased methanol selectivity. The reason is ascribed to the weak hydrogenation activity due to the low reducibility, inhibiting the formation of acetate intermediate.It is further suggested that CHx* formation is the rate-determining step for ethanol synthesis over reduced CoAlOx catalysts. Hence, to enhance the formation of CHx*, a Co0Ni0 alloy-CoO hetero site catalyst is further prepared by reducing Co0.52Ni0.48AlOx at 600 °C[12]. The incorporated Ni facilitates the reduction of Co. More importantly, Ni0 possesses a stronger hydrogenation ability, enhancing the formation of CHx*. Furthermore, the CHx* is involved in the formation of CH3COO* and C2H5O*, which are subsequently hydrogenated to form ethanol. As a result, an ethanol yield of 15.8 mmol·gcat −1 with an ethanol selectivity of 85.7% was obtained. However, the reduced Co-free NiAlOx produces mainly methanol, CH4, and CO, again indicating the importance of CoO as well as the synergy between Co0Ni0 alloy and CoO for ethanol synthesis.The morphology of the Co precursor is another important factor influencing the reduction of Co3O4 and the formation Co0-CoO hetero site. Mesoporous Co3O4 and Co3O4 nanoparticles have shown different reduction behavior as well as different catalytic performance in higher alcohols synthesis by CO2 hydrogenation[11]. Even though mesoporous Co3O4 reduced at 300 °C (Co3O4-m-300) and Co3O4 nanoparticle reduced at 300 °C (Co3O4-np-300) possess similar CO2 conversion, the selectivity to higher alcohols over Co3O4-m-300 was found to be significantly higher. It was observed that Co3O4-np-300 comprises hcp-Co0 and fcc-Co0, while Co3O4-m-300 consists of CoO, hcp-Co0, and fcc-Co0. Co0 provides dissociative H, while O vacancy on the surface of partially reduced Co3O4 enhances CO2 dissociation over Co3O4-m-300. The synergy between the Co0 and CoO sites is partially responsible for the high performance of higher alcohol synthesis. Besides, Co3O4-np-250 and Co3O4-m-300 are composed of similar phases; however, their space-time yield (STY) for higher alcohols differs. Interestingly, the STY of higher alcohols for Co3O4-m-300 is 40 times that over Co3O4-np-250 (i.e., 1.6 and 0.04 mmol gcat −1 h−1, respectively). This is probably due to the confinement effect of the mesoporous structure.Besides, Ouyang et al. prepared Pt-promoted Co3O4 rods (Pt/Co3O4-r) and nano-plates (Pt/Co3O4-p) for CO2 hydrogenation to higher alcohols[13]. The incorporation of Pt promotes the reduction of Co3O4. Thus, after reduction at a low temperature (i.e., 200 °C), some of Co3O4 in Pt/Co3O4-r is reduced to CoO, while in Pt/Co3O4-p, CoO and Co0 coexisted. Accordingly, the synergy between the Pt0/Co0 nanoparticles and the O vacancy over the partially reduced Co3O4 facilitates H2 and CO2 adsorption, resulting in a C2+OH yield of 0.69 mmol gcat −1 h−1, which is higher than 0.62 mmol gcat −1 h−1 over Pt/Co3O4-r.Alkali metals can donate electrons to the Co site and have been widely used in promoting higher alcohol synthesis by CO hydrogenation. Also, they are effective promoters in creating hetero Co sites for higher alcohol synthesis by CO2 hydrogenation. Gnanamani et al. studied the hydrogenation of CO2 over a Na/Co–SiO2 catalyst[16]. Reducing Na-Co/SiO2 in H2 at 350 °C results in the formation of Co0, which mainly hydrogenates CO2 to methane and C2–4 hydrocarbons. However, reduction of Na-Co/SiO2 in pure H2 or syngas at 250 °C leads to the formation of CoO, suppressing the high hydrogenation ability and decreasing the selectivity to methane. Moreover, activating Na-Co/SiO2 in CO forms CoO and Co carbide species, significantly decreasing methane selectivity to 15.3% and reaching 73.2% selectivity for alcohols. In contrast, the Na-free Co/SiO2 catalysts pretreated in CO mainly produce methane. Furthermore, it was found that Co carbide which forms during CO activation converts to Co0 after the reaction. This suggests that the Na promoter and carbide phase formation are important to reduce methane selectivity and enhance oxygenate formation. The Co0-CoO/Co2C hetero sites, which are stabilized/promoted by Na seem to play a key role in the CO2 hydrogenation to alcohols.Besides, Zhang et al. studied the CO2 hydrogenation over a Na-promoted Co2C catalyst derived by treating a Na-Co complex in CO using both theoretical and experimental methods[83]. By DFT calculation, they found that Na-free Co2C favors the formation of CHx* intermediates. Incorporating Na as a promoter inhibits the dissociation of CO* intermediate with higher energy barriers and favors non-dissociative CO* adsorption ( Fig. 16), increasing the CO*/CHx* ratio on the surface of the catalyst. Hence, the synergy between NaCo2C and Co2C is of great importance. At the interface, CO* at Na-Co sites insert into the adjacent CHx* on Co atoms to form ethanol. Thus, CO*-CHx* coupling could be tailored by controlling the interaction between Na and Co2C. Introducing 2 wt% Na results in moderate interaction reaching an ethanol STY of 1.1 mmol g−1 h−1, which is 10 times higher than the catalyst without Na. However, higher Na contents (e.g., 5 wt% Na) weaken the strength of CO adsorption, which is unfavorable to CO coupling and increases CO selectivity.Besides, Witoon et al. studied K-Co/In2O3 catalysts for CO2 hydrogenation to higher alcohols[84]. Over Co/In2O3 catalysts, a mixture of Co0 and CoO forms after reduction in H2. CO2 is converted to CO at the surface O defects of In2O3. The Co0 site takes part in the dissociative adsorption of C-O and C-C bond formation, as well as the hydrogenation of adsorbed carbon to form CxHy*. Then, the CO* formed on the surface of CoO migrates and inserts into the adjacent CxHy* species at Co0 sites, forming C2+OH. However, they observed a higher selectivity to hydrocarbons than the oxygenated products over Co/In2O3. The reason is ascribed to the faster hydrogenation of CxHy* species than the CO* insertion due to the presence of weakly adsorbed H*. After adding K promoter, K-O-Co species is created, which considerably reduces the weakly adsorbed H* and strengthens H* adsorption, suppressing the hydrogenation of alkyl intermediates to form hydrocarbons. Accordingly, CO* insertion and C-C bond formation are promoted, enhancing the formation of higher alcohols.Co hetero site catalysts can also be created through tailoring the Co-support/promoter interaction. Zheng et al. prepared a Co/La2O3-La4Ga2O9 catalyst by reducing the LaCo1−xGaxO3 perovskite for CO2 hydrogenation to ethanol[10]. The interaction between Co and Ga leads to the existence of Coδ+ at the interface after reduction. The synergy between Co0 and Coδ+ moderately weakens the hydrogenation ability of Co0 and inhibits the hydrogenation of CO2 to CH4, promoting the formation of ethanol. Accordingly, the optimized Co/Ga ratio of 7:3 leads to a CO2 conversion of 9.8%, an alcohol selectivity of 74.7% with an ethanol content of 88.1% in the alcohols mixture.Besides, An et al. prepared a series of Co-based catalysts for CO2 hydrogenation to ethanol by reducing SiO2-supported CoGaxAl2−xO4 precursor[49]. The reverse H spillover ability of Ga inhibits excessive reduction of Co, and the electron donation from Co0 to Ga3+ leads to the formation of Co0-Coδ+ active pairs. Furthermore, Co-Ga interaction stronger than the Co-Al enables tailoring the Co0/Coδ+ ratio by changing the Ga/Al ratio. It is suggested that CO2 hydrogenation over the reduced CoGaxAl2−xO4 proceeds through a reverse water gas shift reaction followed by CO hydrogenation to higher alcohols. Associative CO adsorption (forming CO*) and dissociative CO adsorption (forming CH4*) occur on Coδ+ and Co0 sites, respectively. The synergy between Co0 and Coδ+ with an optimized Co0/Coδ+ ratio promotes CHx*-CO* coupling for ethanol synthesis ( Fig. 17). On the other hand, the selectivity to ethanol reaches 20.1% over the reduced CoGa1.0Al1.0O4/SiO2 catalyst.Na-promoted Co catalysts supported on different metal oxides (Al2O3, ZnO, AC, TiO2, SiO2, and Si3N4) were also studied for CO2 hydrogenation to higher alcohols[17]. After a reduction in pure CO, Co2C was formed; however, during the reaction, Co2C only remained intact on the SiO2 and Si3N4 supports rather than on other supports. Hence, due to the strong Co–support interaction through Si-O-Co bond formation, CO* as a reactive intermediate could regenerate and reconstruct the decomposed Co2C on the surface. Moreover, CO* produced over the Co2C sites can be inserted into CHx* intermediates to form ethanol. Thus, the coexistence of Co2C and Co0 seems to play a key role in enhancing ethanol synthesis by CO2 hydrogenation. Accordingly, the SiO2 and Si3N4-supported catalysts are efficient, possessing 18% CO2 conversion and 62% selectivity in the alcohol distribution, while CH4 is found to be the main product over other supported catalysts.Synthesis of higher alcohols by CO2 hydrogenation presents a promising way to reduce CO2 emission and meanwhile produce value-added fuels and chemicals. Earth-abundant and economic Co-based catalysts, which possess relatively high activity and selectivity have been intensively studied for higher alcohols synthesis by CO2 hydrogenation. Amongst, hetero Co sites and the synergy between them are reported as important sites for CO2 hydrogenation to higher alcohols. Besides, Co0, CoO, Coδ+, and Co2C are the main catalytic species for CO2 hydrogenation. Until now, various strategies have been developed to tailor the properties and structure of the active species, which further influence their catalytic performance in CO2 hydrogenation. Moreover, by controlling the reduction process, incorporating alkaline metal as a promoter, and tailoring the Co-support/promoter interaction, hetero Co sites such as Co0-CoO, Co0-Coδ+, and Co2C-NaCo2C could be obtained for higher alcohol synthesis; however, the CO2 conversion and selectivity to higher alcohols are still low, hindering their commercial application. Taking commercial methanol synthesis by syngas conversion as a reference, we believe that catalysts with a single-pass conversion higher than 5%[85], a C2+OH selectivity higher than 95%[86], as well as a space-time yield higher than 0.5 kg L−1 cat h−1 [87] would be of interest for industrial application. Thus, strategies to further tailor the structure and properties of the active Co sites as well as promote their synergy are indispensable. These strategies include: (1) Designing new hetero sites Only a few hetero-site Co catalysts such as Co0-CoO, Co0Ni0-CoO, Co2C-NaCo2C, and Co0- Coδ+/metal oxide are investigated until now. Hence, other hetero-site catalysts including Co0-Coδ+/N doped carbon material, Co2C-CoO, Co2C-Coδ+/metal oxide, Co2C-Coδ+/N doped carbon material, etc. can be considered as potential candidates. We, therefore, believe that due to their different properties, these hetero sites potentially possess different catalytic performances compared with the already reported hetero Co-sites for the synthesis of higher alcohols by CO2 hydrogenation. The synthesis of these hetero-site structures is the main challenge. Co2C-CoO sites can be obtained by carbonization of CoO in CO. By carefully controlling the carbonization atmosphere, temperature, and duration, the nature and amount of Co2C-CoO sites can be rationally tailored. We propose two synthesis routes to prepare Co2C/Co0-Coδ+ hetero sites. In the first one, during the synthesis of single-atom catalysts, both unstable metal species (i.e., metal clusters/nanoparticles) and stable single-atom metal form on the surface of N doped carbon/metal oxide support. The unstable metal species can be removed by acid leaching, thus their amount can be controlled. By subsequent treatment in CO or H2, the Co2C/Co0-Coδ+ hetero sites can be obtained. The second synthesis route to synthesize Co2C/Co0-Coδ+ hetero sites is based on the diffusion and aggregation of single metal atoms by treating in H2 at high temperature. By controlling the treating conditions such as H2 concentration, the content of CO in the atmosphere, temperature, and duration, the amount and nature of the Co2C/Co0 and Coδ+ sites can be rationally tailored. (2) Tailoring the structure and property of the hetero sites Tailoring the adsorption strength and surface abundance of CHx* and CO*/HCOO* on Co sites as well as controlling the ratio and proximity between the hetero sites is of great importance in promoting their synergy, which further enhances CHx* and CO/*HCOO* coupling and higher alcohols formation. The formation of CHx* and CO*/HCOO* intermediates depends on the amount and structure/properties of the Co0, CoO, Co2C, and Coδ+ sites. The key factors influencing the structure and properties of the active sites include the predominant exposure of the crystal facets, degree of reduction, particle size, employment of promoter (K, Zr, Ce, Pt, Ni, Ru, Re, B, La, etc.) and support (SiO2, Al2O3, CeO2, TiO2, ZnO, Gd2O3), and coordination environment of Co, offering various methods to modify the structure and properties of the hetero sites. Besides, tailoring the proximity between the hetero sites by surface and interface control may favor the coupling between C1 species, promoting the formation of higher alcohols. Moreover, understanding the nature of the hetero Co sites using advanced characterization techniques, including in situ DRIFTS, in situ X-ray absorption spectroscopy (XAS), and near ambient pressure X-ray photoelectron spectroscopy (XPS), will facilitate illustrating the structure-activity relationships and rationalizing the catalyst design. (3) Synthesizing Co-based alloy followed by surface segregation By forming a Co-based alloy such as CoCu, CoPt, and CuNi[7,12,88], the structure of the active sites on the catalysts’ surface can be tailored to favor the formation of higher alcohols. Moreover, it is recently found that treating the CoCu alloy with CO leads to surface segregation and accordingly influences the surface composition of the catalysts[7]. Moderate capacity for C-O bond cleavage due to a moderately CO-induced CoCu surface results in a high ethanol selectivity. Thus, by controlling the CO treatment process, the Co/Cu molar ratio on the surface of the catalysts can be rationally tailored to promote the formation of higher alcohols. Extending CO-induced surface segregation of alloy to tailor the surface structure of other Co-based alloys deserves further study. (4) Confining the hetero sites in a nano volume Confining the hetero sites in a nano volume can inhibit CHx* and CO*/HCOO* intermediates from fast removal, increasing their concentration on the catalyst’s surface. Accordingly, the formation of CO and methane by the fast desorption of CHx* and CO* can be reduced to a limited degree, which will further incorporate in higher alcohol synthesis. The active species can be encapsulated in inorganic nanoshells (e.g., SiO2, TiO2, CeO2, and C shells) or nanopores (e.g., zeolites, MOFs, and COFs) through various synthesis methods such as sol-gel coating, coating by hydrolysis of metal ions, impregnation, templated synthesis, etc.[89] This enables tailoring the dimension and volume of the nanoreactor. Accordingly, the residence of reactants, intermediates, and products on the surface of the catalysts can be controlled to enhance the formation of higher alcohols. Designing new hetero sites Only a few hetero-site Co catalysts such as Co0-CoO, Co0Ni0-CoO, Co2C-NaCo2C, and Co0- Coδ+/metal oxide are investigated until now. Hence, other hetero-site catalysts including Co0-Coδ+/N doped carbon material, Co2C-CoO, Co2C-Coδ+/metal oxide, Co2C-Coδ+/N doped carbon material, etc. can be considered as potential candidates. We, therefore, believe that due to their different properties, these hetero sites potentially possess different catalytic performances compared with the already reported hetero Co-sites for the synthesis of higher alcohols by CO2 hydrogenation. The synthesis of these hetero-site structures is the main challenge. Co2C-CoO sites can be obtained by carbonization of CoO in CO. By carefully controlling the carbonization atmosphere, temperature, and duration, the nature and amount of Co2C-CoO sites can be rationally tailored. We propose two synthesis routes to prepare Co2C/Co0-Coδ+ hetero sites. In the first one, during the synthesis of single-atom catalysts, both unstable metal species (i.e., metal clusters/nanoparticles) and stable single-atom metal form on the surface of N doped carbon/metal oxide support. The unstable metal species can be removed by acid leaching, thus their amount can be controlled. By subsequent treatment in CO or H2, the Co2C/Co0-Coδ+ hetero sites can be obtained. The second synthesis route to synthesize Co2C/Co0-Coδ+ hetero sites is based on the diffusion and aggregation of single metal atoms by treating in H2 at high temperature. By controlling the treating conditions such as H2 concentration, the content of CO in the atmosphere, temperature, and duration, the amount and nature of the Co2C/Co0 and Coδ+ sites can be rationally tailored. Tailoring the structure and property of the hetero sites Tailoring the adsorption strength and surface abundance of CHx* and CO*/HCOO* on Co sites as well as controlling the ratio and proximity between the hetero sites is of great importance in promoting their synergy, which further enhances CHx* and CO/*HCOO* coupling and higher alcohols formation. The formation of CHx* and CO*/HCOO* intermediates depends on the amount and structure/properties of the Co0, CoO, Co2C, and Coδ+ sites. The key factors influencing the structure and properties of the active sites include the predominant exposure of the crystal facets, degree of reduction, particle size, employment of promoter (K, Zr, Ce, Pt, Ni, Ru, Re, B, La, etc.) and support (SiO2, Al2O3, CeO2, TiO2, ZnO, Gd2O3), and coordination environment of Co, offering various methods to modify the structure and properties of the hetero sites. Besides, tailoring the proximity between the hetero sites by surface and interface control may favor the coupling between C1 species, promoting the formation of higher alcohols. Moreover, understanding the nature of the hetero Co sites using advanced characterization techniques, including in situ DRIFTS, in situ X-ray absorption spectroscopy (XAS), and near ambient pressure X-ray photoelectron spectroscopy (XPS), will facilitate illustrating the structure-activity relationships and rationalizing the catalyst design. Synthesizing Co-based alloy followed by surface segregation By forming a Co-based alloy such as CoCu, CoPt, and CuNi[7,12,88], the structure of the active sites on the catalysts’ surface can be tailored to favor the formation of higher alcohols. Moreover, it is recently found that treating the CoCu alloy with CO leads to surface segregation and accordingly influences the surface composition of the catalysts[7]. Moderate capacity for C-O bond cleavage due to a moderately CO-induced CoCu surface results in a high ethanol selectivity. Thus, by controlling the CO treatment process, the Co/Cu molar ratio on the surface of the catalysts can be rationally tailored to promote the formation of higher alcohols. Extending CO-induced surface segregation of alloy to tailor the surface structure of other Co-based alloys deserves further study. Confining the hetero sites in a nano volume Confining the hetero sites in a nano volume can inhibit CHx* and CO*/HCOO* intermediates from fast removal, increasing their concentration on the catalyst’s surface. Accordingly, the formation of CO and methane by the fast desorption of CHx* and CO* can be reduced to a limited degree, which will further incorporate in higher alcohol synthesis. The active species can be encapsulated in inorganic nanoshells (e.g., SiO2, TiO2, CeO2, and C shells) or nanopores (e.g., zeolites, MOFs, and COFs) through various synthesis methods such as sol-gel coating, coating by hydrolysis of metal ions, impregnation, templated synthesis, etc.[89] This enables tailoring the dimension and volume of the nanoreactor. Accordingly, the residence of reactants, intermediates, and products on the surface of the catalysts can be controlled to enhance the formation of higher alcohols.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors acknowledge the financial support from National Natural Science Foundation of China (22208143), Nanjing Tech University (39801170), and the State Key Laboratory of Materials-Oriented Chemical Engineering (38901218). Longfei Liao gratefully acknowledges the financial support received from the Foundation of National Key Laboratory of Human Factors Engineering, China Astronaut, Research and Training Center (Grant number: 6142222210601).

Synthesis of higher alcohols by CO2 hydrogenation is a promising way to mitigate CO2 emissions, meanwhile producing value-added fuels and chemicals. However, CO2 hydrogenation to higher alcohols is kinetically hindered and due to the absence of highly efficient catalysts, its industrial implementation is still limited. Among the catalysts designed for this reaction, Co-based catalysts are widely investigated due to earth-abundance and economic, possessing also relatively high activity and selectivity for this reaction. Considering the nature of the active site, the hetero sites of Co-based catalysts such as Co0-CoO, Co0-Coδ+, and Co2C-NaCo2C are critical for higher alcohol synthesis by CO2 hydrogenation. Thus, in this review, we first introduce the roles of Co0, CoO, Coδ+, and Co2C, as well as strategies to tailor their structure which influences the performance in CO2 hydrogenation. Then, we discuss the strategies to create highly efficient hetero-site Co-based catalysts. Finally, emerging methodologies yet to be explored and future directions to achieve highly efficient hetero-site Co catalysts for CO2 hydrogenation to higher alcohols are discussed.

Molar flow rate of gaseous products (i = H2, CO2, CO and CH4), mol i ·min−1 Molar flow rate of glycerol, mol G ·min−1 Purity of H2, %Selectivity towards carbon-containing gaseous products (i = CO2, CO and CH4), %Selectivity towards H2, %Glycerol conversion into carbon-containing gaseous products, %Total glycerol conversion, %Yield of gaseous products (i = H2, CO2, CO and CH4), m o l i · m o l G , i n − 1 Annular bright fieldBrunauer, Emmet and TellerCitrate sol-gelFlame ionization detectorGas chromatographGlycerol steam reformingHigh-angle annular dark fieldHydrogen evolution reactionHigh performance liquid chromatographyInductively coupled plasma optical emission spectroscopyIncipient wetness impregnationMass spectrometerOxygen evolution reactionSteam-to-carbon molar ratioSteam methane reformingThermal conductivity detectorTransmission electron microscopy/energy-dispersive X-rayTime-on-streamTemperature-programmed oxidationTemperature-programmed reductionWater-to-glycerol molar feed ratioWater-gas shiftWeight hourly space velocityX-ray diffractionglycerolreactor inlet conditionsreactor outlet conditionsThe environmental consequences of increased fossil fuel consumption due to energy demands, such as greenhouse gas emissions, have compelled the world to find carbon-neutral, cost-effective and especially renewable eco-friendly alternatives for a sustainable future [1]. As an alternative and eco-friendly fuel, biomass-based hydrogen has been receiving special attention worldwide. However, around 95% of the hydrogen produced nowadays comes from fossil fuels, particularly via steam methane reforming (SMR), which is a non-renewable process in terms of feedstock point of view [2]. As a result, it is urgent to start producing hydrogen from renewable sources.The biodiesel production has been on a increasing trend over the last few decades [3]. Nevertheless, the increase in the biodiesel demand has resulted in an undesirable oversupply of crude glycerol – the main by-product [4]. In 2016, it was reported that the biodiesel manufacture resulted in 3.28 megatons of crude glycerol and it is expected to increase in the near future [5]. This surplus has led to a collapse in terms of glycerol price over the last years, which jeopardizes biodiesel competitivity. The impurities present in crude glycerol limit its purification to be used in pharmaceutical or food industries [6]. However, among many potential applications for the use of crude glycerol, the steam reforming technique is emerging as one of the most suitable for solving the glycerol utilization problem, while at the same time providing a new and potentially green source of fuel [2,7,8]. In addition, the use of crude glycerol not only would promote both commercialization and further development of biodiesel production – and therefore improve the economics of this biofuel production –, as it would also reduce the dependence on non-renewable sources [9].The technological advances in the fuel cell industry are expected to considerably increase in the near future, which also implies the increase in hydrogen demand. Apart from environmental advantages that glycerol steam reforming (GSR) provides, there are also processual advantages – while the SMR leads to production of only 4 mol of H2 per mole of reformed CH4, the GSR allows the formation of 7 mol of H2 per mole of reacted glycerol (Table 1 – Eq. (1)). The global GSR reaction is a combination of glycerol decomposition (Eq. (2)) into syngas (CO and H2) and the water-gas shift (WGS) reaction (Eq. (3)). In addition, the methanation of CO (Eq. (4)) and CO2 (Eq. (5)) and the dry reforming of methane (Eq. (6)) are considered as the main secondary reactions. The formation of coke is also very common to occur during GSR [10–14] – carbon-containing gaseous products (CO, CH4 and CO2) can be coke precursors according to Eqs. (7)-(10) [15]. Moreover, some intermediates and by-products that are formed during the reaction may also react, further complicating the process [11,16]. In fact, Pompeo et al. [14] noticed the formation of heavy compounds (2-methyl-2-cyclopentanone, phenol and 5-hydroxyl-2-methyl-1,3-dioxane), which were assumed that could have been formed by hydrogenolysis, dehydration and condensation reactions; these compounds are known to possibly be coke precursors.In order to achieve a good performance in the GSR process, new catalysts must be developed. Ideally, a GSR catalyst should promote the cleavage of C–C, O–H and C–H bonds, while at the same time eliminating the metal-passivating carbon monoxide formation via reverse WGS reaction (Eq. (3)). This means that the catalyst should be able to inhibit C–O cleavage and the CO2 or CO hydrogenation, which can lead to the formation of methane and/or more polar compounds [17–19]. In the last years, most of the works related to GSR have focused on the development of Ni- and Co-based catalysts on a variety of supports because they provide excellent intrinsic activity when well dispersed over the support, are easily available and cheaper comparatively to noble metal-based catalysts [3,20]. In the last years, the use of Co-based catalysts for GSR has been proven to be efficient since cobalt has the capacity to break C–C bonds and suppress the formation of coke [21].The nature of the catalyst support also has a huge influence, affecting especially the catalyst selectivity. Several supports have been investigated in steam reforming reactions – among all, alumina is known as the best choice due to its high specific surface area and thermal stability. Nevertheless, alumina supports are also prone to suffer deactivation due to carbon deposition and metal particle sintering. Therefore, the addition of proper elements to alumina supports – e.g., zinc oxide (ZnO2), magnesium oxide (MgO), cerium oxide (CeO2) or lanthanum oxide (La2O3) – can be beneficial due to an increase in the catalyst basicity and, therefore, stability [3]. Kraleva et al. [22–24] studied Ni- and Co-based catalysts supported on AlZnOx for hydrogen production from bio-ethanol partial oxidation and observed a good catalytic activity of those materials, as well as a high resistance to coke formation. In another work by Goicoechea et al. [25], Ni- and Co-promoted catalysts supported on single La2O3 and mixed AlLaOx were prepared and tested for hydrogen production from steam reforming of acetic acid. All catalysts were observed to be active, stable and highly selective towards the production of H2 during the screening experiments.Cobalt-based catalysts have also been extensively used to produce molecular H2 from water splitting [26–29]. The water splitting reaction can be viewed as a combination of two reactions: the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Several reports demonstrated that Co-based catalysts are highly active electrocatalysts towards the HER under either basic, neutral or acidic media, apart from being easily coupled with the most active catalysts that are available towards the OER, which plays a huge role in the overall water splitting reaction [26].Based on this background, cobalt-based catalysts supported on La2O3, AlZnOx and AlLaOx were prepared to be used for hydrogen production from GSR for the first time to the best of our knowledge. The materials were prepared using the incipient wetness impregnation (IWI) and citrate sol-gel (CSG) methods and physico-chemically characterized in terms of inductively coupled plasma optical emission spectroscopy (ICP-OES), physical sorption-desorption of N2 at −196 °C, X-ray diffraction (XRD), transmission electron microscopy/energy-dispersive X-ray (TEM/EDX) and temperature-programmed reduction (TPR). The materials were screened in terms of catalytic activity with the aim of identifying the effect of temperature on the total conversion of glycerol and on the conversion of glycerol into gaseous products, as well as on the yield towards gaseous products (H2, CH4, CO2 and CO). Additionally, time-on-stream stability experiments were conducted to assess the deactivation of the materials. For comparative purposes, an additional NiAlLaOx catalyst was prepared to be compared with the best Co-based sample: CoAlLaOx. In conclusion, the main goal was to prepare and find promising materials having high catalytic performances (i.e., high H2 yields) and stability (i.e., low coke formation and/or low sintering) during the GSR process.Cobalt nitrate hexahydrate (Co(NO3)2·6H2O) (Merck, ≥ 99.0%) and nickel nitrate hexahydrate (Ni(NO3)2·6H2O) (Merck, ≥ 99.0%) were used as Co and Ni precursors, respectively. Single oxide support La2O3 was obtained from the calcination of lanthanum nitrate hexahydrate (La(NO3)3·6H2O) (Alfa Aesar, ≥ 99.9%), whereas AlLaOx and AlZnOx mixed oxide supports were obtained from lanthanum nitrate hexahydrate (La(NO3)3·6H2O) (Alfa Aesar, ≥ 99.9%) for AlLaOx, zinc nitrate hexahydrate (Zn(NO3)2·6H2O) (Merck, ≥ 98.0%) for AlZnOx, and from aluminum nitrate nonahydrate (Al(NO3)3·9H2O) (Merck, ≥ 98.0%) for both AlLaOx and AlZnOx. Anhydrous citric acid (ACS, ≥ 99.5%) was used for catalysts prepared by the CSG method.Nitrogen (Linde, ≥ 99.999%) was used as dilution gas in the catalytic experiments and makeup gas in the gas chromatograph (GC) for the flame ionization detector (FID); argon (Linde, ≥ 99.999%) was used as carrier, reference and makeup gas in the GC for the thermal conductivity detector (TCD); hydrogen (Linde, ≥ 99.999%) was used for catalysts activation and as fuel gas in the GC-FID; and reconstituted air (20 vol% of O2/N2) was used in the GC-FID and to regenerate the catalysts (as detailed in Section Catalytic experiments).An aqueous solution of glycerol (C3H8O3) (VWR Chemicals BDH, ≥ 99.6%) with a water-to-glycerol molar feed ratio (WGFR) of 15 – corresponding to 25.4 wt% of glycerol and a steam to carbon (S/C) ratio of 5 – was used in all reaction tests.Inert silicon carbide (SiC) (Alfa Aesar) was used as catalyst bed diluting agent during all experiments.Co/La2O3 single supported catalyst was synthesized by the incipient wetness impregnation (IWI) method. The single oxide support (La2O3) was obtained from the calcination of lanthanum nitrate hexahydrate at 610 °C for 2 h, as suggested by Mentus et al. [30]. The support was thereafter impregnated with an aqueous solution containing the Co active metal precursor. The metal hydrate nitrate solution was stirred for 30 min at room temperature to obtain a homogeneous solution prior to the addition of the support. The required amount of Co metal hexahydrate nitrate was used to yield the catalyst with a Co loading of 10 wt% in comparison to La2O3 (90 wt%). Afterwards, the metal was impregnated by adding the pre-calcined powdered single oxide support; the solution was mixed continuously at 60 °C for 60 min and then concentrated in a rotary evaporator at 80 °C under vacuum. At last, the samples were dried at 120 °C for 12 h and calcined in air at 700 °C for 2 h.CoAlZnOx, CoAlLaOx and NiAlLaOx mixed oxide catalysts were synthesized by the citrate sol-gel (CSG) method. The CSG method is a successful technique to produce homogeneous mixed oxides having high specific surface area, good stability, and high porosity in the mesoporous range [31]. The required amounts of hydrate nitrates (Al, La or Zn and Ni or Co) were weighted and mixed at the same time with 15 mL of water per 5 g of catalyst in order to yield Ni or Co metal loadings of 10 wt% and theoretical Al2O3/La2O3 or Al2O3/ZnO molar ratios of 1. Subsequently, citric acid was added to an aqueous solution containing the metal salts in a proportion equal to the amount of metal cations and the solution was stirred at 60 °C for 120 min. The formed wet gel was thereafter concentrated by slowly evaporating the water in a rotary evaporator at 75 °C under vacuum until a viscous clear gel was obtained. The final gel obtained was dried at 120 °C for 12 h and calcined in air at 700 °C for 2 h. The high calcination temperature of 700 °C was selected for all prepared catalysts to ensure structure stability of the materials in accordance to Barroso et al. [32]. Detailed information on the prepared catalyst can be found in Table 2 .The IWI method is by far the most attractive and widely used to prepare heterogeneous catalysts due to its technical simplicity, limited amount of waste and low cost. In general, the IWI method allows the impregnation of a simple support with a precursor-containing solution [33]. On the other hand, the CSG method offers significantly better control of stoichiometry and produces samples with high homogeneity, in particular in multi-component materials, since no washing or filtering steps are needed [34]. For this reason, the Co/La2O3 catalyst was prepared by IWI, while the CoAlZnOx, CoAlLaOx and NiAlLaOx catalysts, which are considered as multi-component materials, were prepared by the CSG method.The catalysts and supports elemental composition were determined by inductively coupled plasma optical emission spectroscopy (IPC-OES) on a Varian 715-ES spectrometer.The specific surface areas of the catalysts were determined through N2 sorption-desorption isotherms recorded at −196 °C using a BELSORP-mini II apparatus (BEL Japan, Inc.). Samples were degassed under vacuum at 250 °C for 2 h before analysis. The specific surface area was determined using the Brunauer, Emmett and Teller (BET) method for the N2 relative pressure range of 0.05 < p/p0 < 0.30.The powder X-ray diffraction (XRD) measurements were carried out using a STADI P automated transmission diffractometer (STOE, Darmstadt) with CuKα1 radiation and a Ge monochromator. The XRD patterns were collected in the 2θ range of 5–60° in 0.5° steps with the dwell time of 100 s and recorded with a STOE position sensitive detector. The phase analysis was performed using the program suite WINXPow (STOE&CIE) with inclusion of the Powder Diffraction File PDF-2 of the International Center of Diffraction Data (ICDD).A high-resolution transmission electron microscopy (TEM) study was done with an aberration-corrected JEM-ARM200F (JEOL, Corrector: CEOS) 200 kV transmission electron microscope. The microscope was equipped with a JED-2300 energy-dispersive X-ray (EDX) spectrometer for chemical analysis. The aberration corrected STEM imaging high-angle annular dark field (HAADF) and annular bright field (ABF) were performed with a spot size of approximately 0.13 nm, a convergence angle of 30–60°, and collection semi-angles for HAADF and ABF of 90–170 mrad, respectively. For the TEM analysis, the samples were deposited without any pretreatment onto a holey carbon supported Cu-grid (mesh 300) and then transferred into the microscope.Temperature-programmed reduction (TPR) experiments were carried out on 40–50 mg of each sample. The samples were loaded into a fixed-bed continuous flow quartz reactor and heated to 900 °C at 10 °C/min in a 5 vol% H2/Ar gas mixture (total flow 7 mLN/min). The hydrogen consumption and water formation were monitored with a quadrupole mass spectrometer (MS, Pfeiffer OmniStar). The total amount of hydrogen consumed by each sample was determined by integration of the profile of the MS signal.In situ temperature-programmed oxidation (TPO) was performed prior to the stability tests for all catalysts. After performing the GSR at different temperatures, the reactor was flushed with N2 for 1 h, and the reactor was cooled down to 600 °C (since the reactor was at 700 °C). Then 100 mLN∙min−1 of reconstituted air passed through the catalyst bed until no CO2 nor CO were observed at the reactor outlet. The CO2 and CO vol% concentrations in the outlet stream were continuously monitored by an infrared-based CO2/CO analyzer (Servomex, model 4210).The catalytic tests were carried out in an experimental setup as described in Fig. 1 . The catalysts were placed in a stainless steel packed-bed reactor (120 mm of height and 7.2 mm of inner diameter), which was placed inside a tubular oven (model Split from Termolab, Fornos Elétricos, Lda.) divided in a three-zone PID programmable temperature controller (model MR13 from Shimaden), connected to three type-K thermocouples in contact with the wall of the reactor. In addition, for the continuous monitoring of the bed temperature, two type-K thermocouples were inserted laterally and radially centered (40 and 80 mm of the column length) in direct contact with the catalyst bed.Nitrogen was fed to the reactor to serve as dilution gas by using a mass flow controller (model F201 from Bronkhorst High-Tech) while the glycerol aqueous solution was fed by an HPLC pump (Eldex, 1LMP model) and forced to pass through an evaporator/mixing zone at 315 °C before entering the reactor. The pressure in the setup was monitored by means of two pressure transducers (both model PMP 4010 from Druck) placed before and after the reactor. A system of two Peltier condensers, located after the reactor (cf. Fig. 1), was used to collect the condensable products produced during the reaction. In addition, a coalescence filter and a filter were used between the reactor and the analysis system in order to further retain any condensable species and eventually catalyst/inert particles, respectively. The tube between the reactor outlet and the first Peltier condenser was kept at 120 °C to avoid water/glycerol condensation.During the GSR experiments, small samples of the dry outlet gas stream containing N2, H2, CO, CO2 and CH4 were analyzed using a gas chromatograph (GC, Agilent 7820A) – equipped with a thermal conductivity detector (TCD – using argon as makeup and reference gas), a flame ionization detector (FID – using nitrogen as makeup gas, reconstituted air as oxidizing gas and hydrogen as fuel gas) and a CO/CO2 methanizer at 350 °C fed with hydrogen – used to analyze and determine the concentration of these gas phase species produced during the GSR experiments. The GC is equipped with two capillary columns: Plot 5A (30 m × 0.32 mm) and Plot Q (30 m × 0.32 mm). The H2 that was formed during the reaction was analyzed by TCD (with argon as carrier gas to achieve better response owing to the higher difference in thermal conductivity). On the other hand, CO2 and CO were detected in the FID detector in the methane form through the utilization of the methanizer.The condensed species were collected periodically and analyzed in terms of glycerol concentration by high performance liquid chromatography (HPLC, Elite LaChrom HITACHI apparatus) equipped with a refractive index detector. An Alltech OA-1000 ion exclusion column (300 × 6.5 mm) – using 0.005 mol L−1 H2SO4 solution as mobile phase at a flow rate of 0.5 mL min−1 – was used to separate the liquid products collected. The quantity of glycerol was determined based on the calibration curve of the standard compound.For each experiment, a stainless steel reactor, closed at both ends with stainless steel mesh discs (10–15 μm), was filled with inert SiC (241–559 μm) at both ends and the middle was filled with 200 mg (315–500 μm) of catalyst homogenously diluted with SiC. The dilution of catalyst with inert SiC is known to enhance the heat transfer and minimize temperature gradients, which was confirmed through the measurement of the bed temperature throughout the experiments (it remained nearly constant, i.e., ±1 °C). The temperature profile along the reactor length under inert atmosphere was observed to be uniform (differences <2 °C).Prior to the GSR experiments, the catalysts were activated at 600 °C for 1 h under a 20 vol% H2/N2 mixture (total flow rate of 100 mLN min−1) taking into account the H2-TPR results (see Section 3.1.5. H2-TPR). The setup was then purged under N2 atmosphere (100 mLN min−1) for 30 min and the temperature of the reactor was lowered to 400 °C. Subsequently, the glycerol aqueous solution (WGFR of 15), which was kept under continuous stirring at room temperature, was fed into the evaporator at a constant flow rate of 0.1 mL min−1, corresponding to a weight hourly space velocity (WHSV) of 8.05 h−1 (defined as the ratio between the mass flow rate of glycerol and the mass of catalyst; similar to other works [13,35,36]). The vaporized solution was then mixed with N2 flowing at 25 mLN min−1. The screening tests were carried out in the temperature range of 400–700 °C and atmospheric pressure.Prior to the catalytic screening, a blank test – herein referred to as case study #1 – was conducted at various temperatures for a time-on-stream (TOS) of approximately 5.5 h, with the reactor filled with only SiC (bed diluting agent/inert). Following that, two distinct catalytic experimental runs were performed. The case study #2 was performed for all catalysts and aimed at assessing the catalytic activity and selectivity towards the reaction products for approximately 5.5 h (TOS) at temperatures ranging from 400 to 700 °C; prior to this experiment, catalysts were reduced/activated under an H2 atmosphere. The final experiment (case study #3) was carried out for all catalysts using the samples employed during case study #2; however, prior to this, a TPO experiment was performed at 600 °C using a reconstituted air stream to regenerate the catalysts as stated by Silva et al. [13], followed by a new reduction/activation step. Case study #3 consisted of a stability test performed at 625 °C that was run until the glycerol conversion into gaseous products reached approximately 10%. These experiments are summarized in Table 3 .The performance of the catalysts used in this work was measured in terms of products yield, selectivity towards H2 and other gases (CO2, CO and CH4) and conversion of glycerol.The glycerol conversion obtained during GSR was determined in two different ways, namely in terms of glycerol conversion into carbon-containing gaseous products (XG,gas) and total glycerol conversion (XG,total). The glycerol conversion into carbon-containing gaseous products was calculated as follows: (11) X G , g a s ( % ) = F C O 2 o u t + F C O o u t + F C H 4 o u t 3 × F G i n × 100 in which F C O 2 o u t , F C O o u t and F C H 4 o u t are the molar flow rates of CO2, CO and CH4 at the reactor outlet, respectively, and F G i n is the molar flow rate of glycerol fed to the reactor. The total glycerol conversion was calculated based on Eq. (12). (12) X G , t o t a l % = F G i n − F G o u t F G i n × 100 In this equation, F G o u t is the molar flow rate of unreacted glycerol at the reactor outlet. The yield of the gaseous products (Yi) and the selectivity to the gas products denoted as S C O 2 , SCO and S C H 4 (Sc,i) were defined as follows: (13) Y i = F i o u t F G i n (14) S C , i ( % ) = F C , i o u t F C O 2 o u t + F C O o u t + F C H 4 o u t × 100 where F i o u t is the molar flow rate of species i at the reactor exit, in which i corresponds to H2, CO2, CO or CH4 in Eq. (13) and F C , i o u t is the molar flow rate of gas product i at the reactor outlet, wherein i corresponds to CO2, CO or CH4 in Eq. (14). Regarding the selectivity to H2 comparatively to carbon-containing gaseous products ( S H 2 ), it was calculated according to the following equation: (15) S H 2 ( % ) = F H 2 o u t F C O 2 o u t + F C O o u t + F C H 4 o u t × 1 R R × 100 in which F H 2 o u t is the molar flow rate of H2 at the reactor outlet. The RR is the H2/CO2 reforming molar ratio and is related to the stoichiometry of the global GSR reaction (Eq. (1)) in which 7 mol of H2 and 3 mol of CO2 are ideally produced from each mole of reacted glycerol. Finally, the purity of the produced H2 ( P u r H 2 ) was calculated as follows: (16) P u r H 2 ( % ) = F H 2 o u t F H 2 o u t + F C O 2 o u t + F C O o u t + F C H 4 o u t × 100 These definitions are in line with several reported works in the literature on GSR [13,37,38].Besides assessing the catalytic performance of the materials, this study aimed also to correlate physicochemical features of the materials with the catalytic behavior. Samples herein prepared were physicochemically characterized through several techniques, namely ICP-OES, physical sorption-desorption of N2 at −196 °C, XRD, TEM/EDX and H2-TPR, as detailed below.The results regarding the elemental composition of catalysts and mixed supports measured through Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) are shown in Table 4 . As it can be observed, the obtained elemental compositions of the samples are close to the theoretical ones. However, the amount of Co present in the calcined single supported catalyst (Co/La2O3) was only of 6.8 wt%, while for the mixed supported catalysts the Co amount was superior and of 7.9 wt% and 8.4 wt% for the CoAlZnOx and CoAlLaOx catalysts, respectively. This might indicate that the mixed oxide supported catalysts – prepared by the CSG method – are able to incorporate more quantities of Co into their structure. The Al/Zn and Al/La weight ratios of the mixed oxide supported catalysts were of around 0.88 and 0.18, respectively, which are close to the intended theoretical values – these correspond approximately to the expected Al2O3/ZnO and Al2O3/La2O3 molar ratios of 1 (as described in section Catalysts preparation), therefore corresponding approximately to Al/Zn and Al/La molar ratios of 2 and 1, respectively, as can be observed in Table 4.The BET specific surface area was determined for all samples through physical sorption-desorption of N2 at −196 °C (Table 5 ). Although the specific surface areas of the materials are quite small, it can be clearly seen that the type of support influences the BET specific surface area following the order La2O3 > AlLaOx > AlZnOx. On the other hand, the addition of Co precursors resulted in an increase in the specific surface area for the CoAlZnOx and CoAlLaOx catalysts. However, this increase by Co addition was not observed for the single oxide supported catalyst, maybe due to the precision of the measurement method. This indicates that catalysts prepared by the CSG method (mixed oxide supported catalysts) are able to achieve higher specific surface areas, suggesting a high degree of incorporation of Co precursors into the structure of the mixed oxides. Although the AlZnOx support possesses the lowest BET specific surface area, it did not hinder the deposition of high amounts of Co on the surface.The X-ray diffraction patterns of the catalysts calcined at 700 °C are shown in Fig. 2 . The diffractograms of the single oxide supported catalyst (Co/La2O3) and pure La2O3 support (Fig. 2 (a)) show an identified crystal phase that corresponds to lanthanum hydroxide (La(OH)3, 2θ = 15.7°, 27.3°, 27.9°, 31.7°, 35.9°, 39.5°, 48.5°, 49.9° and 55.2°), which was expected due to the hygroscopic nature of La2O3. The diffractogram of Co/La2O3 shows also four peaks that correspond to lanthanum cobalt oxide perovskite (LaCoO3, 2θ = 32.9°, 33.3°, 47.4° and 58.6°) and two peaks of Co3O4 spinel phase (2θ = 31.7° and 36.4°).In the diffractogram of AlZnOx mixed support and CoAlZnOx catalyst (Fig. 2 (b)), intense reflections corresponding to a zinc aluminate spinel phase (ZnAl2O4, 2θ = 31.2°, 36.7°, 44.7°, 49.0°, 55.5° and 59.2°) were observed. However, reflections of a ZnO phase were not found in the diffractogram of CoAlZnOx mainly because the Zn atoms were totally combined with Al to form the ZnAl2O4 spinel phase. Even though Co oxide phases, essentially in the form of Co3O4, were not observed in the CoAlZnOx diffractogram, the active metal should be present in the catalyst as confirmed by ICP, as well as through TEM/EDX analysis (see Section TEM/EDX). The reason for the non-appearance of Co must be because either its phase might be amorphous, and/or because the particle size is below the limit of detection of the XRD technique (4 nm).Finally, the diffractograms of AlLaOx mixed support and CoAlLaOx catalyst are represented in Fig. 2 (c). Pure AlLaOx shows a lanthanum aluminate perovskite structure (LaAlO3, 2θ = 23.4°, 33.4°, 41.2°, 47.9°, 54.0° and 59.6°), being also present in the diffractogram of CoAlLaOx. Reflections that correspond to Co3O4 (2θ = 31.0° and 36.7°) were also found in the CoAlLaOx sample. The presence of mixed phases between Co and La2O3 or Al2O3 were excluded (i.e., CoAl2O4) since reflections of pure single oxides were not identified.As a physicochemical complementary analytical technique, the single supported catalyst (Co/La2O3) and the catalyst supported on AlZnOx were examined by TEM/EDX to obtain information regarding the phase composition of the samples as well as the Co distribution on both supports. The Co/La2O3 catalyst (Fig. 3 (a)) contains particles having different sizes and shapes, but in general it has round particles with diameters between 20 nm and 100 nm as well as some particles up to 150 nm. Resorting to EDX measurements, it was possible to observe that the biggest particles are composed mainly by LaCoO3 perovskite, being also observed long needle-like particles (size of around 100 nm) corresponding to LaO. The images having superior resolution (located on the right side of Fig. 3 (a)) demonstrate the lattice planes of areas where perovskite and LaO were detected. The perovskite high resolution image displays an electron diffraction pattern in which the characteristic rings of the reflections of the nanometric polycrystalline material can be observed.Regarding the TEM images of CoAlZnOx (Fig. 3 (b)), it is shown that the morphology of the sample is very similar to that of Co/La2O3 having round particles but having smaller diameters (between 20 nm and 30 nm); however, for the CoAlZnOx material, no needle-like particles were observed. The EDX chemical mapping (micrograph image placed on the fourth position in right in Fig. 3 (b)) showed that Co (red), Al (green) and Zn (blue) were uniformly distributed in the sample. However, in some locations are observed characteristic crystallite rings of Co3O4 (red) and ZnAl2O4 (green) smaller than 20 nm. In addition, in the EDX chemical mapping is observed that the Co3O4 crystallite size is below of around 4 nm; this might be the reason for the non-appearance of Co in the diffractogram of the CoAlZnOx catalyst, which is below the detection limit of the XRD technique (4 nm).To establish appropriate pre-treatment temperatures for the activation (reduction) of the catalysts prior to the GSR experiments, and access their reducibility, samples were submitted to temperature-programmed reduction under H2 (H2-TPR).The H2-TRP profiles of the Co-based single oxide and mixed oxide catalysts are shown in Fig. 4 . The reducibility of Co/La2O3 depends on the potential of the cobalt present in the Co3O4 spinel structure and LaCoO3 perovskite to be reduced since they were the identified phases present in this catalyst at room temperature, as concluded by XRD. However, the reduction process of LaCoO3 perovskite is still not well defined in the literature. The H2-TPR profile of Co/La2O3 demonstrates the first event at 322 °C, being also observed a small shoulder prior to this, which might be attributed to the reduction of Co3+ species in the LaCoO3 perovskite [25]. Depending on the oxygen defects in the Co/La2O3 perovskite lattice, the Co3+ species may have different natures inside the structure, therefore resulting in different reducibilities and, consequently, in the broad H2 consumption event registered in the TPR profile. This might also be the explanation for the appearance of the shoulder before the peak at 322 °C. The H2 consumption before 322 °C might also be due to the reduction of Co3+ species to Co2+ in the Co3O4 spinel phase [25]. Subsequently, three peaks registered at 417 °C, 468 °C and 614 °C and a small plateau between 505 and 529 °C were observed, which can be attributed to the reduction of Co2+ species to metallic Co0 present in the Co3O4 spinel phase [32].In relation to the pattern of the CoAlZnOx catalyst, it can be observed multiple H2 uptake events, which might be associated to Co species, such as surface cobalt oxides having different strength in the interactions with the support. At first, a small peak is observed at 369 °C which can be attributed to the reduction of Co3+ to Co2+ in the Co3O4 species weakly attached to the support [22,24]. These species are present in low quantities based on the peak size. The second and broad peak, displayed from approximately 445 °C–702 °C has an asymmetric shape suggesting that it might also be due to the reduction of Co3+ to Co2+ in the Co3O4 species having different interaction strength with the support [22,24]. The major and last peak registered at 775 °C and having a pronounced tailing on the high temperature side might be associated to the reduction of Co2+ to metallic Co0 as well as to the reduction of lattice-intercalated Co, as is the case of CoAl2O4 [22,24].Finally, as is observed from the H2-TPR pattern, the Co species present in the CoAlLaOx catalyst were more easily reduced than the Co species in the other two catalysts. Multiple major events registered at 338 °C, 565 °C and 676 °C were observed in the pattern of the CoAlLaOx catalyst. The first event was due to the reduction of Co3+ to Co2+ in the Co3O4 weakly attached to the support [25]. The second event, registered as a broad peak at 565 °C, suggests the simultaneous reduction of Co3+ to Co2+ species in the Co3O4 particles having stronger interactions with the support, as well as the reduction of Co2+ species to metallic Co0 [25]. The final event is associated to the total reduction of Co2+ species to metallic Co0 [25].Taking into account the H2-TPR results, catalysts were reduced/activated at 600 °C for 1 h under a 20 vol% H2/N2 mixture (total flow rate of 100 mLN min−1). The selected temperature is enough to reduce both Co/La2O3 and CoAlLaOx catalysts as it was observed in Fig. 4. On the other hand, although the used temperature may seem low to completely reduce the CoAlZnOx sample, it should be emphasized that a high H2 percentage of volume was used (20 vol% instead of only 5 vol% used during the H2-TPR experiments) to ensure a complete activation of catalysts. In fact, it was verified for the CoAlZnOx catalyst that the use of a reduction temperature of 780 °C did not demonstrate any significant changes on its performance in terms of glycerol conversion into gaseous products and H2 yield (data not shown).Prior to the catalysts screening, a blank test (case study #1) was performed in the reactor filled with only SiC particles (bed diluting agent/inert) at temperatures ranging from 400 °C to 700 °C. Additionally, the yields of H2, CO2, CO and CH4, as well as the total glycerol conversion and glycerol conversion into carbon-containing gaseous products in the thermodynamic equilibrium were determined using the equations listed in section Reaction performance indicators. All thermodynamic equilibrium values were obtained using the Aspen Plus V9 software under the same operating conditions – i.e., nitrogen was used as dilution gas and the species considered for the simulation were C3H8O3, H2O, H2, CO2, CO, CH4 and C (coke), based on the compounds formed during the catalytic experiments. Details on the methodology used for the simulations can be found elsewhere [39,40]. Results obtained are shown in Table 6 .The maximum theoretical yield of H2 – considering a complete glycerol conversion and assuming that no secondary reactions occurred, in accordance to Eq. (1) shown in Table 1, is of 7 mol of H2 per mole of glycerol fed. An excess of H2O promotes the GSR, according to the thermodynamics, shifting the equilibrium of the WGS reaction (Eq. (3) – Table 1) in order to produce more H2, while inhibiting the methanation of CO (Eq. (4) – Table 1) and CO2 (Eq. (5) – Table 1). However, it has been reported that for very high WGFRs (i.e., >15), the production of H2 increases moderately. For this reason, and taking into account that an excess of H2O was used in the simulations (WGFR of 15), the H2 yield obtained at the thermodynamic equilibrium for temperatures above 550 °C was close to the maximum theoretical of 7 m o l H 2 ⋅ m o l G , i n − 1 ; for excessive temperatures such yield decreases due to the exothermal nature of the WGS reaction that is therefore inhibited. In addition, the use of an excess of H2O has also the advantage of minimizing coke formation (see Eqs. (9) and (10) – Table 1). On the other hand, in Table 6 is observed that the use of only inert does not catalyze the GSR as observed from the low glycerol conversion into gaseous products and, consequently, the low production of gaseous products. On the contrary, at high temperatures (>550 °C), the total glycerol conversion was not low, which can be ascribed to the thermal decomposition of glycerol. Nevertheless, taking into consideration that the glycerol conversion into gaseous products was residual – low formation of H2, CO2, CO and CH4 –, it means that almost none of the converted glycerol and/or condensable products formed were steam reformed. Instead, it might be suggested that the majority of glycerol fed was converted into condensable products.To better comprehend how the catalysts perform under the GSR process, two sets of experiments were carried out. The case study #2 (see Table 3) aimed to assess the catalytic performance in terms of activity and selectivity towards the reaction products at temperatures ranging from 400 °C to 700 °C, while case study #3 (see Table 3) consisted on determining the stability of materials at 625 °C.The catalytic performance of the Co-based materials in terms of both total glycerol conversion and glycerol conversion into gaseous products at different temperatures is shown in Fig. 5 . The increase in temperature favors the total glycerol conversion as well as the glycerol conversion into gaseous products for all catalysts, as expected, due to the endothermic nature and kinetics of the reforming process, which are favored with the increase in temperature. On the other hand, according to the activity results, it can be concluded that the type of support plays an important role. At 700 °C, the total glycerol conversion obtained for Co/La2O3 and CoAlLaOx catalysts was of around 90% and the glycerol conversion into gaseous products was of 59% and 65%, respectively (values being referred to the first point obtained at that temperature). This constitutes a significant enhancement in comparison to the blank test with only inert, in which only approximately 11% of glycerol was converted into gaseous products (Table 6). In addition, it is observed that at low temperatures (400–550 °C) the difference between total glycerol conversion and glycerol conversion into gaseous products is superior than for higher temperatures (625 °C and 700 °C), suggesting that at low temperatures a preferential conversion of glycerol into condensable products occurs. In fact, at low temperatures – particularly at 400 °C, 475 °C and 550 °C – was observed a change in color (see Fig. S1 in Supplementary Information) between the fed aqueous glycerol solution and the liquid effluent collected in the Peltier condensers from transparent to yellow/light brown, respectively, contrarily to what happens at higher temperatures, meaning that conversion of glycerol into condensable products occurred mostly at lower temperatures. As found in literature, such conversion of glycerol into condensable products occurs mainly through thermal decomposition of glycerol [41,42]; also, in accordance to some works [43,44], a temperature as low as 400 °C favors substantially the formation of such condensable products comparatively to higher temperatures. Even though a detailed analysis of such compounds was not performed, condensable products such as acetaldehyde, acrolein, propanal, acetone, acetic acid, methanol, ethanol, allyl alcohol and acetol might have been formed during GSR [36,38,42–45]. Additionally, it is observed that the promotion of Al2O3 support with La2O3 is better in terms of catalytic activity when compared to the promotion of Al2O3 support with ZnO (cf. Fig. 5). On the other hand, by comparing the results of both Co/La2O3 and CoAlLaOx catalysts, it is observed that the addition of Al2O3 to La2O3 support increases the glycerol conversion into gaseous products within all temperature range, in particular at 550 °C and 625 °C.The influence of reaction temperature on the yields of gaseous products is represented in Fig. 6 . For all samples, the yields of products followed, in general, the same trend as total glycerol conversion and conversion of glycerol into gaseous products as the temperature increased. Among all gaseous products, the H2 yield was the highest, followed by CO2 and CO, suggesting that these catalysts present capacity to convert CO to H2 and CO2 through the WGS reaction. As can be observed, in general, the CoAlLaOx catalyst demonstrates a higher H2 yield in the whole range of temperature in comparison to the other two materials. A maximum H2 yield of 3.85 m o l H 2 ⋅ m o l G , i n − 1 was obtained for both Co/La2O3 and CoAlLaOx at 700 °C. On the other hand, residual contents of CH4 were observed for all catalysts at low temperatures (400 °C and 475 °C), while for higher temperatures (above 475 °C) a slight increase was observed, reaching a plateau between 625 °C and 700 °C. The increase in the production of CO and CH4 might be associated with the thermal decomposition of acetaldehyde ( C H 3 C H O → C H 4 + C O ) – favored at high temperatures –, which might have been formed as an intermediate product through glycerol dehydrogenation by a mechanism of radical decomposition [43]. Although the yield of CH4 increased with temperature, it is worth mentioning that only small amounts of this compound were formed within the range of temperatures studied and, in almost all the cases, was even below the thermodynamic equilibrium, which means that the methanation reactions (Eqs. (4) and (5) – Table 1) occurred in a low extent.Furthermore, the decrease observed in the total glycerol conversion (cf. Fig. 5), glycerol conversion into gaseous products (cf. Fig. 5) and yield of gaseous products (cf. Fig. 6) with the TOS at each temperature might be attributed to the deactivation of the catalysts provoked by the deposition of carbonaceous deposits on their surfaces. This issue is further detailed in Section Catalytic stability.The influence of reaction temperature on the H2 selectivity and purity is represented in Fig. 7 . Regarding the H2 selectivity (Fig. 7 (a)), the increase in temperature provides in general its increase for all catalysts. The CoAlLaOx sample provides higher hydrogen selectivities when compared to the other materials at temperatures ranging from 400 °C to 550 °C. Over 550 °C, the Co/La2O3 catalyst becomes superior in terms of hydrogen selectivity. On the other hand, the purity of hydrogen is a very important criterion when evaluating the efficiency of steam reforming processes. In Fig. 7 (b) is observed that the purity of the H2 produced slightly increases with temperature for all Co-based catalysts from around 62-66% to 68%. This indicates that although the production of all gaseous products increased with temperature, the increase in H2 production was slightly superior.Catalyst deactivation is one of the main concerns in industrial reforming processes as it seems to be unavoidable, affecting the performance of the entire process over time, and for this reason, it is a subject that should be carefully addressed. Co-based catalysts have demonstrated good catalytic performance to produce H2 through GSR, being suggested as appropriate materials to be applied in catalytic systems. However, Co-based catalysts are also prone to suffer from deactivation as reported in the literature [46–48]. It has been suggested that the deactivation of such catalysts can be attributed to three main reasons: (i) oxidation and sintering of the metal particles, (ii) transformation of the solid state that involves the diffusion of Co into the support, therefore allowing the formation of irreducible Co support compounds (i.e., silicates and aluminates) and (iii) formation of carbonaceous species, namely through Boudouard (see Eq. (7) – Table 1) and methane cracking (see Eq. (8) – Table 1) reactions that lead to a blockage of Co active sites on the catalyst surface [49].As mentioned in Section 2.5, stability experiments were proceeded until approximately 10% of the glyerol conversion into gaseous products was reached. It should be emphasized that prior to the stability experiments, all catalysts were regenerated in situ using a TPO step (for coke gasification) followed by a new reduction/activation step, as described before. It should be mentioned that this step was carried out since the catalyst samples used during case study #3 were the same as the ones used during case study #2. The evolution of total glycerol conversion and glycerol conversion into gaseous products over time at 625 °C is represented in Fig. 8 for the Co-based catalysts. For all catalysts, it is observed that both total glycerol conversion and conversion of glycerol into gaseous products decrease drastically after a few hours, indicating a loss of catalysts’ capability to break C–C bonds of the glycerol molecules. During the overall time of reaction with each catalyst, XG,total decreased from 89% to 75%, 80%–61% and 91%–72%, while XG,gas decreased from 53% to 15%, 42%–12% and 65%–14% for Co/La2O3, CoAlZnOx and CoAlLaOx catalysts, respectively. This might suggest that both the formation of coke (and possibly also metal particle sintering) and the conversion of glycerol into secondary condensable products increased over time, which was visually noticed in the periodically collected liquid samples that became darker with time. After 2.9 h of the experiment at 625 °C, XG,gas decreased 71%, 65% and 44% for Co/La2O3, CoAlZnOx and CoAlLaOx catalysts, respectively, which indicates that the Co/La2O3 catalyst, followed by CoAlZnOx, is more prone to suffer from deactivation due to coke deposition (and possibly also from metal particle sintering) and also from the increase in the conversion of glycerol into secondary condensable products. This can also be observed in Fig. 8 since the decline in XG,total and XG,gas over time for the CoAlLaOx catalyst was smoother comparatively to the other two. The difference in the time-dependence for the conversion of glycerol might be directly related to differences in the amount of carbon deposits formed by secondary reactions that involve polymerization and/or breakage of C–H bonds of intermediate species. Additionally, as can be observed in Table 7 , both XG,total and XG,gas increase after regeneration of the catalysts by coke gasification (through TPO) followed by Co reduction (case study #3), which indicates that during case study #2, carbon deposits were formed, therefore decreasing the catalytic activity. While the deactivation of catalysts due to carbon deposition can be reversed by performing a coke gasification with air (O2), the metal particle sintering, on the other hand, is an irreversible phenomenon that occurs due to the exposure to high temperature leading to agglomeration of crystals on the surface of the support; as a consequence, it causes an irreversible loss of catalyst activity. In this sense, the fact that all catalysts were able to recover activity after regeneration by coke oxidation, followed by a new catalyst reduction, might suggest that metal particle sintering did not occur during case study #2.The yields of H2, CO2, CO and CH4 over time for all Co-based catalysts are depicted in Fig. 9 . All catalysts demonstrate not being stable in terms of H2 production during the stability experiment. The main products were H2, CO2 and CO since low contents of CH4 were observed during the experiments for all catalysts. On the other hand, it was observed that the CO2 selectivity decreases with the TOS, while the CO selectivity increases (Fig. S2 in the Supplementary Information). At the beginning of the reaction, the CO2 selectivity was superior to CO; however, as the reaction proceeded, the selectivities of both CO and CO2 became closer. Although both CO2 and CO yields decreased over time, as was observed in Fig. 9 (b) and (c), the decrease in the CO production was less affected in comparison to CO2 production, which might suggest that the WGS reaction (Eq. (3) – Table 1) was the most affected over time – this might also be confirmed by the decrease in the H2 selectivity over time (see Fig. 10 (a)). On the other hand, although the CH4 yield decreased over time, the CH4 selectivity slightly increased (specially for the CoAlZnOx catalyst), meaning that the methanation reaction (Eq. (4) – Table 1) was probably less affected by the deactivation of the catalyst. In addition, the reason for the increase in selectivity of both CO and CH4 might also be associated to the thermal decomposition of acetaldehyde, which may have been less affected by catalyst deactivation due to coke deposition. It should be stressed that during the stability experiments at 625 °C and due to the formation of solid compounds in the catalysts’ surface, the reactor was being plugged over time, which slightly increased the pressure in the system. Although the activity of the catalyst was being lost due to formation of coke, the increase in pressure favors thermodynamically the forward CH4 methanation reaction, which might be the reason for the methanation reaction be less affected overtime, therefore leading to an increase in the CH4 selectivity.The selectivity towards H2 (Fig. 10 (a)) suffers a reduction over time for Co/La2O3 and CoAlZnOx catalysts. The H2 selectivity reduction was accompanied by a decrease in the CO2 selectivity and an increase in the CO selectivity, as observed above, being suggested the weakening of the WGS reaction with time-on-stream (Eq. (3) – Table 1). Contrarily, for the CoAlLaOx catalyst, during the first hours, the selectivity towards H2 remained almost constant at around 80%, decreasing to 66% in the last 2 h of experiment. Consequently, for all catalysts, the H2/CO molar ratios (Fig. S3 (a) in Supplementary Information) decreased drastically, while the CO/CO2 molar ratios (Fig. S3 (b) in Supplementary Information) increased. On the other hand, the purity of H2 (Fig. 10 (b)) remained almost constant during all experiments for both Co/La2O3 and CoAlZnOx catalysts, while for the CoAlLaOx catalyst a slight decrease was observed in the last 2 h of the experiment.In Table 8 is shown the stability performances of different Co-based catalysts used for GSR found in the literature. However, it should be emphasized that although being difficult to make a detailed comparison between catalysts tested in different works – as reaction conditions and catalyst pre-treatment may differ from work to work and are considered as crucial factors for catalytic efficiency – the comparison was made between the results herein obtained and the results obtained in other works found in the literature. Papageridis et al. [48] observed that the conversion of glycerol into gaseous products obtained for the 8 wt% Co/Al2O3 catalyst decreased from around 64%–41%, while the H2 yield decreased from 2.5 to 1.9 m o l H 2 ⋅ m o l G , i n − 1 in approximately 3.0 h – see Table 8. Comparing these results with the results obtained in this work for the 10 wt% Co/La2O3 catalyst (decrease of XG,gas from 53% to 15% and H2 yield from 3.23 to 0.82 m o l H 2 ⋅ m o l G , i n − 1 in 2.6 h), it can be concluded that the Co/Al catalyst suffered a less aggressive catalyst deactivation. However, in the work of Papageridis et al. [48], a higher WGFR was used, and it is well known that the higher the WGFR, the lower the probability of forming carbon deposits. On the other hand, the metal loadings of Co were also distinct. In another work, Menezes et al. [46] observed that for the 20 wt% CoNbAl catalyst, the glycerol conversion into gaseous products decreased from 96% to 89%, while the H2 yield decreased from 66% to 64% in 3.0 h of experiment. Once again, it is observed that the CoNbAl catalyst suffered from a less severe deactivation in comparison to the CoAlLaOx catalyst prepared in this work, which resulted in a lower loss in the catalytic activity; however, the operating conditions used by Menezes et al. [46] were slightly different from the ones used in this work.The catalysts used in case study #2 (section Catalytic activity) were regenerated resorting to a temperature-programmed oxidation (TPO) with air, as previously described, and were thereafter used to carry out case study #3 (section Catalytic stability). Regenerative oxidation was performed at 600 °C as this temperature is known to be sufficient to fully remove both amorphous and graphitic carbon species [50]. The CO2 and CO vol% concentrations obtained from the burning of carbon deposits were continuously monitored during the regeneration step (Fig. S4 in Supplementary Information). The total amount of carbonaceous deposits was determined by integration of the respective TPO curves of each catalyst – see results in Table 9 . It can be observed that the Co/La2O3 catalyst formed the highest quantity of carbon deposits during case study #2 (25.2 mgC·gcat −1), followed by the CoAlZnOx catalyst, which produced 20.0 mgC·gcat −1 of carbonaceous species. On the other hand, on the surface of the CoAlLaOx catalyst were deposited 13.2 mgC·gcat −1 of coke. It is known that the rate of carbon removal during combustion with air depends on the type of catalyst, and from the TPO profiles it is possible to observe some particular differences. The Co/La2O3 catalyst (Fig. S4 (a)), which produces a higher quantity of carbon deposits in comparison to the other two materials prepared, required more time to be fully regenerated. On the other hand, the CoAlLaOx catalyst (Fig. S4 (c)), which formed the lower quantity of coke, was the fastest catalyst to be fully regenerated. These results are in line with the deactivation of catalysts observed during the stability experiment. As can be seen in Fig. 11 , the higher the quantity of carbon deposits formed during case study #2, the higher the percentage of loss in XG,gas and in H2 yield obtained during case study #3. In other words, the loss in catalytic activity over time may be attributed to the formation of carbonaceous species on the catalyst surface, therefore decreasing the performance of the GSR process, namely in terms of hydrogen production.The catalyst deactivation, mostly due to coke deposition, is still one of the main concerns for the application of GSR in industrial scale. Catalysts based on noble metals, specially Ru- and Pt-based, are normally more stable and active for GSR comparatively to Co-based catalysts; however, these materials are known to be much more expensive [3]. In this sense, it is desirable to develop efficient, reliable and economic methods to regenerate catalysts. Fortunately, the catalyst deactivation due to the formation of coke deposits can easily be reverted by resorting to TPO with air as this method burns the carbon deposits formed on the surface of the catalyst, therefore allowing to recover its catalytic activity. However, hot spots can be originated during this process due to the exothermicity of coke burning, which can damage the catalyst and affect its catalytic performance [3]. Additionally, to remove all carbon deposits (i.e., all amorphous and graphitic coke), high temperatures are mandatory ( ≥ 600 °C), therefore implying high operating costs. However, if the use of high temperatures is not an option to regenerate the catalyst (due to high operating costs, for example), it should be mentioned that the use of lower temperatures (350–450 °C [51]) is efficient to remove at least the amorphous coke as this is considered to be more detrimental to the performance of the catalyst in comparison to graphitic carbon [50]. There are other methods to regenerate catalysts with minor consequences and that allow the removal of coke at low temperatures, as is the case of solvent extraction methods, supercritical fluid extraction and the use of ozone (O3) [50]. Other alternatives that meet the concept of “green carbon science”, such as the catalysts regeneration combined with coke gasification (resorting to CO2 and H2O), are also efficient to revert catalyst deactivation with the advantage of producing synthesis gas, instead of CO2 [49]. Nevertheless, it should be stressed that for traditional reactors, if a catalyst regeneration is necessary, a shut-down of the entire process is required, which will affect the continuous H2 production (otherwise parallel reactors are required). In addition, for most of the catalysts, after an oxidative regeneration a new activation/reduction is mandatory. With this being said, it is always preferable to have active and stable catalysts that are resistant to deactivation either due to coke deposition – therefore not being implied the shut-down of the GSR process – and/or to sintering.The catalytic behavior of Co-based catalysts supported on La2O3, AlZnOx and AlLaOx was studied for the GSR. It was concluded that the type of support is essential for both catalytic activity and stability. In addition, the findings from the catalysts physicochemical characterization contributed to better comprehend the results obtained during the GSR experiments.The CoAlLaOx mixed oxide catalyst was observed to provide the highest catalytic activity and H2 yield during GSR, which might be attributed to different physicochemical properties. This catalyst was also observed to be the most stable. By comparing the results between the Co/La2O3 and CoAlLaOx catalysts, it was observed that the addition of Al2O3 to the La2O3 support was responsible for the increase in the specific surface area of the CoAlLaOx catalyst. In addition, among the catalysts prepared, it was observed by ICP-OES that the CoAlLaOx catalyst incorporated the highest Co content. These factors might have been responsible for the good performance of this material in comparison to the others. On the other hand, although the CoAlZnOx catalyst demonstrated to have a higher specific surface area in comparison to CoAlLaOx, the Co-based catalyst supported on AlZnOx incorporated a lower Co content into its structure, which might have been responsible for the lower catalytic activity. Although the addition of ZnO to Al2O3 and La2O3 to Al2O3 lead to a decrease in the specific surface area, as observed by Kraleva et al. [24] and Charisiou et al. [17], respectively, it also lowered the acidity of the support in comparison to pristine Al2O3, therefore increasing the long-term stability of catalysts. The alumina acid sites are known to be responsible for the formation of coke as carbon deposits are preferably formed on those sites.In the materials prepared were identified perovskite structures, as well as some spinel phases. From XRD and TPR analyzes, a LaCoO3 perovskite structure was observed in the structure of the Co/La2O3 catalyst, which was the active metal promoter. Regarding the mixed oxide CoAlZnOx catalyst, it was observed through XRD that the active phase was mainly a ZnAl2O4 spinel. In relation to the CoAlLaOx catalyst, it was observed through XRD the formation of a LaAlO3 perovskite structure, which is known to be very stable even at high temperatures [25]. Taking into account the results obtained from the GSR experiments, it might be concluded that the ZnAl2O4 spinel phase present in the CoAlZnOx catalyst was less active in terms of glycerol conversion into carbon-containing gaseous products and H2 yield. On the other hand, the Co/La2O3 catalyst suffered from a more severe deactivation with time-on-stream, followed by CoAlZnOx. This might suggest that the LaAlO3 perovskite structure can prevent more efficiently the coke formation, while on the contrary, the LaCoO3 perovskite structure is not so efficient in preventing the deposition of carbon deposits.Additionally, in relation to the CoAlLaOx catalyst, it might be suggested that the presence of lanthanum can facilitate the dispersion of active species, strength the interactions between Co species and support, as well as increase the number of basic sites and redistribution of the acid ones in terms of strength and density, providing a catalyst with improved performance for GSR in terms of H2 production and stability, as lanthanum is known to hinder the formation of olefins and oxygenates, considered as coke precursors [17]. Nevertheless, although the addition of basic promoters to alumina supports might improve the catalyst activity, it has also been demonstrated that the use of basic materials does not necessarily ensures the stability of catalysts during GSR.The Co-based catalysts prepared in this work were concluded to not being stable during the catalytic stability experiments (cf. Section 3.2.2.2). In this sense, and taking into account that the CoAlLaOx catalyst was observed to be the most active and stable towards the GSR, comparatively to the other Co/La2O3 and CoAlZnOx catalysts, a NiAlLaOx catalyst was additionally prepared. The NiAlLaOx catalyst was evaluated in terms of catalytic stability during GSR and compared with CoAlLaOx – results are represented in Fig. 12 . Although both catalytic performances at the beginning of the experiment are similar, the NiAlLaOx catalyst demonstrated an outstanding improved stability comparatively to CoAlLaOx in terms of both glycerol conversion into gaseous products and H2 yield, making it a suitable candidate to be applied in a continuous GSR process. Additional experiments on the NiAlLaOx would be interesting; detailed analysis on this catalyst will be done in a subsequent work in conjunction with other Ni-based catalysts.Co-based catalysts supported on La2O3, AlZnOx and AlLaOx were prepared with the aim of assessing the effect of the support on the catalytic activity – at different reaction temperatures – and stability of the materials for the valorization of glycerol through steam reforming for H2 production.For all catalysts, the increase in temperature favored both total glycerol conversion and conversion of glycerol into gaseous products, and subsequently, favored the H2 production. As for the CH4 yield, although it increased with temperature, residual amounts of this compound were formed within the temperature range studied. The CoAlLaOx catalyst demonstrated higher catalytic activity than the other catalysts prepared, producing at 700 °C a H2 yield of 3.85 m o l H 2 ⋅ m o l G , i n − 1 and a glycerol conversion into gaseous products of 65%.On the other hand, it was observed that the Co-based catalysts were not stable with time-on-stream for the GSR at 625 °C, demonstrating a drastic decrease of the catalytic activity after a few hours of reaction, suggesting a loss of catalysts’ capability to break C–C bonds of the glycerol molecules. The production of carbon-containing gaseous products was more affected over time than the total glycerol conversion, being this attributed to the increase of the production of liquid products and to the formation of carbon deposits on the surface of the catalysts. The CoAlLaOx catalyst demonstrated to be more carbon-resistant – followed by CoAlZnOx –, being observed a clear relationship between the quantity of coke formed and the loss in H2 yield and glycerol conversion into gaseous products, i.e., the more the quantity of carbonaceous species formed, the more the loss in H2 yield and glycerol conversion into gaseous products.The combination of GSR with oxidative regeneration of catalysts is a possible way to revert the catalyst deactivation. However, this step involves the shut-down of the entire process when traditional reactors are being used (unless parallel devices are employed), and as a consequence, the H2 production decreases. In this sense, active and stable catalysts that are resistant to deactivation due to coke formation should be preferably selected.An additional NiAlLaOx catalyst was prepared – taking into account that the CoAlLaOx catalyst was the one that showed the best catalytic performance comparatively to the other catalysts prepared – and evaluated in terms of catalytic stability for GSR. The NiAlLaOx catalyst showed a remarkable stable behavior throughout all experiment, both in terms of glycerol conversion into gaseous products and H2 yield. Detailed analysis on this catalyst will be done in a subsequent work.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was financially supported by i) Base Funding – LA/P/0045/2020 of the Associate Laboratory in Chemical Engineering (ALiCE) and UIDB/00511/2020 – UIDP/00511/2020 of the Laboratory for Process Engineering, Environment, Biotechnology and Energy (LEPABE) – funded by national funds through the FCT/MCTES (PIDDAC) and by ii) the German Federal Ministry of Economics and Energy (BMWi), Project KF2031911ZG2 and the Leibniz Society.M. Salomé Macedo is grateful to the Portuguese Foundation for Science and Technology (FCT) for her Ph.D. grant (SFRH/BD/137106/2018), with financing from national funds of the Ministry of Science, Technology and Higher Education and the European Social Fund (ESF) through the Human Capital Operational Programme (POCH). M. A. Soria also thanks the FCT for the financial support of his work contract through the Scientific Employment Support Program (Norma Transitória DL 57/2017).The authors are indebted to Lucília Ribeiro from LSRE-LCM for the glycerol conversion measurements.The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2022.07.236.

A comparative study of 10 wt% Co-based catalysts supported on La2O3, AlZnOx and AlLaOx was performed for glycerol steam reforming (GSR). The catalysts physicochemical characterization was done through several techniques. All catalysts were screened in terms of catalytic activity and time-on-stream stability for GSR. The catalytic activity experiments aimed to assess the effect of temperature (400–700 °C) on the glycerol conversion and yield of gaseous products (H2, CO2, CO and CH4). Additionally, catalytic stability experiments were conducted at 625 °C to investigate deactivation of the catalysts, in which a drop in the activity was observed, especially for Co/La2O3. The glycerol conversion into gaseous products as a function of the time-on-stream was more affected for all catalysts in comparison to total glycerol conversion, being this effect assigned to the increase in the formation of liquid products and to the formation of coke. CoAlLaOx was observed to be more carbon-resistant, followed by CoAlZnOx, through the measurement of the quantity of carbonaceous species formed during the GSR experiments. A NiAlLaOx catalyst was also prepared and assessed in terms of catalytic stability for GSR; a stable behavior was observed throughout all experiment in relation to glycerol conversion into gaseous products and H2 yield.

The development of innovative heterogeneous catalysts with enhanced performance and prolonged lifetimes takes a prominent place to tackle global environmental issues whilst meeting increasing demands for commodity chemicals [1,2]. Significant advances have been made in engineering well–defined materials with versatile architectures at the nanoscale [3,4], thereby overcoming structural nonuniformity that hinders the identification of active sites and the correlation of relationships at the molecular level [5]. Among several examples, single–atom catalysts, containing spatially isolated metal atoms on appropriate hosts, have attracted significant attention in recent years [6–9]. These systems have proven effective for the derivation of structure–performance trends in various chemical transformations [10,11], such as hydrochlorination [12,13] or alkynes semi-hydrogenation [14,15]. Specifically, isolated atoms displayed distinct characteristics with respect to the conventional supported nanoparticles [16,17]. Therefore, a systematic investigation of the nanostructure using a platform of catalysts ranging from single atoms with defined environments up to nanoparticles with controlled size, on catalytic performance constitutes an important step for the rational design of promising systems [18].An application of potential practical relevance is the halogen–mediated natural gas upgrading to chemicals and fuels through hydrodehalogenation of dihalomethanes (CH2X2, X  = Cl, Br) [19,20]. Selectively reforming these polyhalogenated compounds is required since they contribute to halogen and carbon–losses in the downstream halomethanes (CH3X) upgrading step [21,22]. Various nanoparticle–based metal catalysts (Fe, Co, Ni, Cu, Ru, Rh, Ag, Ir, Pt) deposited on SiO2 were studied in CH2Br2 hydrodebromination (HDB), revealing an outstanding CH3Br selectivity over ruthenium (≤96%), great propensity to CH4 over iridium and platinum (>50%), intermediate selectivity performance of nickel and rhodium (<60%), and inactive behavior of the other metals (Fe, Co, Cu, and Ag) [23]. Despite its selective character, Ru/SiO2 deactivated rapidly due to coking and sintering, whereas the iridium and platinum nanoparticles displayed the highest activity and stability. A global performance descriptor based on the adsorption strength of the halogen/carbon fragment on the active metal phase was presented to rationalize the observed reactivity patterns [23]. Building on these results, a recent study systematically investigated nuclearity– and host effects, using a platform of activated– (AC) and nitrogen–doped (NC) carbon–supported platinum nanostructures, from single atoms to nanoparticles of ca. 4 nm, in CH2Br2 hydrodebromination. The exceptional CH3Br selectivity over the NC–supported single atoms (≤98%) was disclosed, outperforming AC–supported analogues and the reference catalyst Ru/SiO2. The performance was explained by the geometric effects of the single atom and the participation of nitrogen sites in the reaction by storage of H–atoms [24].In contrast to HDB, only a single study targeted selective CH2Cl2 hydrodechlorination (HDC) to CH3Cl [25]. Therein, SiO2–supported ruthenium, platinum, and iridium nanoparticles were investigated, showing a low selectivity to CH3Cl (≤38% with CH4 as main product, achieved over Ir/SiO2). Attempts to increase the selectivity by supporting Ir nanoparticles (0.8–1.6 nm) on ZrO2, Al2O3, CeO2, anatase TiO2, and MgO did not provide the desired improvements. On the other hand, epitaxially directed iridium nanostructures on rutile TiO2 showed unprecedented activity and CH3Cl selectivity (≤95%), though limited lifetime due to poisoning by chlorination. Other relevant iridium nanostructures, such as single atoms, were not evaluated which hampers the formulation of robust structure–performance relationships. Furthermore, current CH2X2 HDH studies were confined to a single halogen, leaving ample room for further investigations [23].To systematically address the catalytic search, we synthesized a platform of Ir/NC catalysts with distinct nanostructures, ranging from single atoms to size–controlled nanoparticles of ca. 3.5 nm, and assessed their performance in both HDC and HDB. By extending the scope to other NC–supported metals (Pt, Ru, and Ni) prepared as single atoms and nanoparticles (Fig. 1 ), we consistently investigate active phase size– and halogen effects with the aim to advance the design of promising hydrodehalogenation catalysts.Commercially available AC (Norit ROX 0.8) was used for evaluating the catalytic response of this metal–free support. NC was synthesized following the protocol reported by Kaiser et al. [12] Prior to its use as carrier for metal species, the NC was ground and sieved into particles of 0.4–0.6 mm. The metal precursors, IrCl3·xH2O (abcr, 99.9%), H2PtCl6 (abcr, 99.9%), RuCl3·xH2O (abcr, 99.9%), and Ni(NO3)2·6H2O (Strem Chemicals, 99.9%) were dispersed on the support via incipient wetness impregnation. Appropriate amounts of the precursors required to obtain a nominal metal loading of 1 wt% were fully dissolved in a volume of deionized water (Pt and Ni) or aqua regia (Ir and Ru) equal to the pore volume of the carrier. The precursor solution was added dropwise to the support, and the resulting mixture was magnetically stirred for 2 h at room temperature. The impregnated solids were dried at 473 K for 16 h in static air (heating rate = 5 K min−1). Subsequently, all samples were thermally activated (T act = 473–1073 K) for 16 h in N2 atmosphere (heating rate = 5 K min−1). The catalysts were referred to as M/NC–T act, where M denotes the metal (Ir, Pt, Ru, and Ni). The Ir/NC-1073, Pt/NC–1073, and Ni/NC–1073 catalysts underwent an additional reductive treatment in 20 vol% H2/He (PanGas, purity 5.0) flow for 3 h at elevated temperatures (T red = 773 or 873 K, heating rate = 10 K min−1) and are denoted as Ir/NC(773), Pt/NC(873), and Ni/NC(773). Fig. 1 provides a guideline for the catalysts developed in this study.Powder X–ray diffraction (XRD) was measured in a PANalytical X’Pert PRO–MPD diffractometer with Bragg-Brentano geometry by applying Ni–filtered Cu Kα radiation (λ = 1.54060 Å). The data were recorded in the 10–70° 2θ range with an angular step size of 0.017° and a counting time of 0.26 s per step. N2 sorption at 77 K was measured in a Micromeritics TriStar II analyzer. The samples (ca. 0.10 g) were degassed to 50 mbar at 423 K for 12 h prior to the measurement. The Brunauer–Emmett–Teller (BET) method was applied to calculate the total surface area, S BET. The pore volume, V pore, was determined from the amount of N2 adsorbed at a relative pressure of p/p 0 = 0.98. The metal content in the catalysts was determined by inductively coupled plasma optical emission spectroscopy (ICP–OES) using a Horiba Ultima 2 instrument equipped with photomultiplier tube detection. The solids were dissolved in a HNO3:H2O2 = 3:1 mixture under sonication until the absence of visible solids. CO pulse chemisorption was performed on a Thermo TPDRO 1100 set-up equipped with a thermal conductivity detector. Prior to the analyses, the samples (ca. 0.15 g) were pretreated at 423 K under flowing He (20 cm3 STP min−1) for 30 min, and reduced at 623 K under flowing 5 vol% H2/He (20 cm3 STP min−1) for 30 min. Thereafter, 0.344 cm3 of 1 vol% CO/He were pulsed over the catalyst bed every 4 min at 308 K until the area of the pulses remained constant. To avoid desorption of CO, the interval between successive pulses was minimized. Scanning transmission electron micrographs with a high-angle annular dark-field detector (HAADF–STEM) were acquired on FEI Talos and Hitachi HD2700CS microscopes operated at 200 kV. All samples were dispersed onto lacey carbon coated copper or nickel grids. The size distribution of the metal nanostructures was obtained by examining over 100 nanoparticles. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Physical Electronics Quanterna SXM X–ray instrument using monochromatic Al Kα radiation, generated from an electron beam operated at 15 kV, and equipped with a hemispherical capacitor electron-energy analyzer. The samples were analyzed at constant analyzer pass energy of 55.00 eV. The spectrometer was calibrated for the Au 4f 7/2 signal at 84.0 ± 0.1 eV. The envelopes were fitted by mixed Gaussian–Lorentzian component profiles after Shirley background subtraction. The selected peak positions of the different species were based on literature reported data [26]. X-ray absorption fine structure (XAFS) measurements at the (Ir, Pt) L 2 and L 3- and (Ru) K–edge were carried out at the SuperXAS beamline. The incident photon beam provided by a 2.9 T superbend magnet was selected by a Si(111) channel-cut Quick-EXAFS monochromator. The rejection of higher harmonics and focusing were achieved with rhodium-coated collimating and toroidal mirrors, respectively, at 2.5 mrad. The area of sample illuminated by the X-ray beam was 0.5 mm × 0.2 mm. All spectra were recorded in transmission mode at room temperature. The extended X-ray absorption fine structure (EXAFS) spectra were acquired with a 1 Hz frequency (0.5 s per spectrum) and then averaged over 15 min. The procedures for analysis and fitting of the EXAFS spectra are reported elsewhere [12,27].The hydrodechlorination of CH2Cl2 (HDC) and hydrodebromination of CH2Br2 (HDB) were conducted at ambient pressure in a home–made continuous–flow fixed–bed reactor set-up. H2 (PanGas, purity 5.0), He (carrier gas, PanGas, purity 5.0), and Ar (internal standard, PanGas, purity 5.0) were dosed by a set of digital mass flow controllers (Bronkhorst) and the liquids, CH2Br2 (Acros Organics, 99%) or CH2Cl2 (Sigma Aldrich, >99.9%), were fed by a syringe pump (Fusion 100, Chemyx) equipped with a water-cooled syringe to a vaporizer unit operated at 393 (CH2Br2) or 353 K (CH2Cl2). A quartz reactor (internal diameter, d i = 12 mm) was loaded with the catalyst or metal–free carrier (catalyst/carrier weight, W cat = 0.05–1 g, particle size, d p = 0.4–0.6 mm) and heated to the reaction temperature (T = 523 K) in an electrical oven under He flow. The catalyst bed was allowed to stabilize for at least 10 min before the reaction mixture was fed at a total volumetric flow of F T = 30 and 50 cm3 STP min−1 for HDB and HDC, respectively, and with a composition of CH2X2:H2:Ar:He = 6:24:5:65 (vol%, X  = Cl, Br). The kinetic tests were performed with a variable feed composition of CH2X2:H2:Ar:He = 6:6–72:5:17–83. Downstream linings were heated at 393 K to prevent the condensation of unconverted reactants and/or products. Carbon-containing compounds (CH2X2, CH3X, CH4) and Ar were quantified online via a gas chromatograph equipped with a GS–Carbon PLOT column coupled to a mass spectrometer (GC–MS, Agilent GC 6890, Agilent MSD 5973 N). After GC–MS analysis, the gas stream was passed through two impinging bottles in a series containing an aqueous solution of NaOH (1 M) for neutralization prior to its release in the ventilation system.The conversion of the reactant, X(CH2X2), was calculated using Eq. (1) , (1) X ( CH 2 X 2 ) = n ( CH 2 X 2 ) in - n ( CH 2 X 2 ) out n ( CH 2 X 2 ) in × 100 , % where n(CH2X2)in and n(CH2X2)out are the molar flows of CH2Br2 or CH2Cl2 at the reactor inlet and outlet, respectively. The selectivity, S(j), to product j (j: CH3X, CH4) was calculated according to Eq. (2) , (2) S ( j ) = n ( j ) out n ( CH 2 X 2 ) in - n ( CH 2 X 2 ) out × 100 , % where n(j)out is the molar flow of product j at the reactor outlet. The selectivity to coke, S coke, in all tests was calculated according to eq. (3) , which is based on a generally applied carbon balance that determines the accumulation of carbon–containing species in the catalyst. (3) S coke = n ( CH 2 X 2 ) in - n ( CH 2 X 2 ) out - n ( j ) out n ( CH 2 X 2 ) in × 100 , % Therein, n(CH2X2)in, n(CH2X2)out, and n(j)out stand for the molar in– and outlet flows of gaseous reactants and products. The reaction rate, r, based on the metal loading and expressed with respect to the consumption of CH2X2, was calculated using eq. (4), (4) r = n ( CH 2 X 2 ) in × X ( CH 2 X 2 ) W cat × ω M , mol CH 2 X 2 h - 1 mol M - 1 where W cat is the weight of the catalyst and ωM is the metal loading determined by ICP–OES analysis (Table 1 ). The turnover frequency, TOF, was calculated using eq. (5) , (5) TOF = n ( CH 2 X 2 ) in × X ( CH 2 X 2 ) W cat × ω M × D M , h - 1 where DM is the metal dispersion, determined by CO pulse chemisorption. A metal dispersion of 100% was used for the single atom–based catalysts. After the tests, the reactor was quenched to room temperature in He flow, and the catalyst was retrieved for further characterization analyses. Evaluation of the dimensionless moduli based on the criteria of Carberry, Mears, and Weisz–Prater [28,29] indicated that the catalytic tests were performed in the absence of mass and heat transfer limitations.Density Functional Theory (DFT) on models of the nanoparticles and single atoms representing the different catalytic systems was employed as implemented in the Vienna ab–initio Simulation Package (VASP 5.4.4) [30,31]. Generalized Gradient Approximation with the Perdew–Burke–Ernzerhof functional (GGA–PBE) [32] was used to obtain the exchange–correlation energies with dispersion contributions introduced via Grimme’s DFT–D3 approach [33]. Projector Augmented Wave (PAW) [34,35] and plane waves with a cut-off energy of 450 eV, with spin polarization allowed when needed, were chosen to represent the inner electrons and the valence monoelectronic states, respectively. For simulations of the single atoms, a one-layer (6 × 6) slab of graphitic carbon separated by 19 Å of vacuum was used and sampled through a gamma–centred grid of 3 × 3 × 1 k–point grid. Carbon materials are ill–defined for several reasons: (i) the graphene layers constituting them can be stacked in different arrangements due to the van der Waals interactions; (ii) they store different types of structural and particularly point defects; (iii) depending on the atmosphere under which they are prepared they can have a variable (almost continuous) stoichiometry with different types of functionalities; (iv) overall if doped or created by different precursors a wider chemical versatile (and defect types) renders an almost continuous of chemical environments. This wide spread cannot be addressed efficiently by present characterization techniques to set structure–activity relationships as, for instance, EXAFS is not able to distinguish C/N/O coordination and vibrational patterns can hint on the nature of N-cavities. The computational solution to this conundrum is to devise a set of models compatible with the stoichiometry and nature of the most-abundant cavities according to the available characterization. In line with this, the NC support was represented by a set of three defects containing various nitrogen functionalities, including pyrrolic and/or pyridinic moieties, and different coordination structures; (i) non-planar 3 N and square-planar 4 N arrangements, labelled 3 × N5 (tri–pyrrolic), (ii) 4 × N6 (tetra-pyridinic), (iii) and 2 × N5 + 2 × N6 (tetra-pyrrolic/pyridinic), whereas AC was modelled by adding an epoxide on the carbon matrix, all adopted from previous studies [12]. Single atom catalysts were modelled by placing the metal atom in the centre of each cavity. For nanoparticles, systems were modelled as a four–layer p(3x3)–(111) fcc (Pt, Ir, and Ni), or a p(3x3)–(0001) hcp slab (Ru) interspaced along the z–direction by a vacuum space of 15 Å, and k-point sampling of 5 × 5 × 1 (Gamma centered). The two top layers and the adsorbates were allowed to relax while the bottom two were fixed to the bulk lattice. The arising dipole was corrected in all slab models [36]. Gas-phase molecules were optimized in a box of 14.0 × 14.5 × 15.0 Å3. For all investigated systems, structures were relaxed using convergence criteria of 10–4 eV and 10–5 eV for the ionic and electronic steps, respectively.To assess the stability of the single atoms in the NC cavities, formation energies were estimated using the metal species and the scaffold as reference states. Binding energies of the halogen to the halogenated single atoms were calculated using the non-halogenated single atom and Br2/Cl2 as reference states. For the Gibbs free energy on the reaction network, CH2X2, H2, and metal surfaces or pristine single atoms (defined as the isolated and non–chlorinated metal species) were utilized, and the vibrational, rotational, and translational entropic contributions from gas–phase reactant molecules were included.The Climbing Image Elastic Band (CI–NEB) method [37,38], improved dimer method [39,40] and quasi–Newton algorithms were employed to locate the transition states (TS) in the reaction profiles, where the TS were further verified by their single imaginary frequency character.All the structures presented in this work have been uploaded to the ioChem-BD database [41] https://iochem-bd.iciq.es/browse/review-collection/100/29816/440d6583bf1645c23bc615b4 [42].To build a fully consistent platform on NC supports and since iridium has been targeted as a potential HDX active phase our work starts with the preparation of NC–supported iridium catalysts, applying the reported procedures, which include dry impregnation of the chloride precursor on NC followed by thermal activation to obtain the final catalyst, designated Ir/NC–T act (T act, 473–1073 K). This procedure follows our recent synthetic strategies presenting structures of platinum and ruthenium (1 wt% metal basis) on NC, from (chlorinated) single atoms to nanoparticles [24,27].In the low– and medium–temperature catalysts, Ir/NC–473, 673, and 873, the metal was mostly atomically dispersed as evident from the HAADF–STEM images (Fig. 2 ) and corroborated by the absence of iridium diffraction peaks in the XRD patterns ( Fig. S1 ). The high–temperature system, Ir/NC–1073, contains nanoparticles with an average size of 1.3 nm, although single atoms still make up a considerable fraction of the nanostructures. Aimed at further sintering the active phase, an additional reduction step in an H2–rich atmosphere at 773 K was applied to Ir/NC–1073. The resulting Ir/NC(773) exhibited a narrow nanoparticle size distribution with an average size of 2.1 nm (Fig. 2), in agreement with its XRD pattern that shows diffraction peaks compatible with the metallic phase. Although nanoparticles are dominant in Ir/NC(773), the presence of single atoms cannot be totally discarded. Further analysis of the Ir/NC catalysts by N2–sorption revealed the close similarity of the specific surface areas (S BET, 310–392 m2 g−1) and pore volumes (V pore, 0.23–0.32 cm3 g−1), whereas ICP–OES confirmed that the metal content was approximately the targeted 1 wt% (Table 1). The speciation of the nitrogen content was preserved over the whole temperature range, as indicated by N 1 s XPS analysis (Table S1). Fitting the Ir 4f spectra of the Ir/NC–473, 673, and 873 samples revealed dominant contributions at binding energies (BEs) of ca. 62.4 and 63.0 eV commonly assigned to oxidized species (Fig. 3 , Fig. S2 ) [25,43], corroborating the atomic dispersion of iridium as visualized by HAADF–STEM. In contrast, the main feature of the Ir/NC–1073 and Ir/NC(773) samples was centered at ca. 61.0 eV, compatible with the metallic phase [43]. Notably, analysis of the Cl 2p XPS suggests that the nature of the iridium site in the single–atom based catalysts is directly affected by the activation temperature ( Fig. S2 ). In particular, Ir/NC–873 shows no peaks that could be ascribed to Cl–species, suggesting that the iridium single atom is only coordinated to N/O–related cavities in the scaffold, whereas the low–temperate catalyst reveal contributions at 198.0 and 200.2 eV, indicative of the presence chlorinated species [44]. This evolution is in line with previous reports that documented the gradual change of the coordination environment of platinum single atom from predominant Cl– to N/O–neighboring atoms at higher activation temperatures [12,24]. As anticipated, chlorine was not detected in the nanoparticle–based (NP–based) catalysts, Ir/NC–1073 and Ir/NC(773).The catalytic performance of the derived iridium nanostructures was evaluated in HDC and HDB, which were conducted at constant reaction temperature (523 K), feed composition (CH2X2:H2:Ar:He = 6:24:5:65), and atmospheric pressure. The initial HDC activity, expressed per surface iridium atom (TOF), decreases in the following order (Fig. 4 a): Ir/NC(773) ≫ Ir/NC–1073 > Ir/NC–873 > Ir/NC–673 ≈ Ir/NC–473. Nanoparticles display a higher activity than their chlorinated SA–based analogues (>5 times), showing comparable performance to benchmark rutile TiO2–supported iridium (Ir/r–TiO2, Table S2). Moreover, single atoms with a N/O coordination (Ir/NC–873) exhibit up to 2.5–fold higher TOF than their chlorinated counterparts, possibly due to the ability of the metal center to activate the reactants. Assessment of the product distribution at ca. 20% CH2Cl2 conversion (Fig. 4 a), revealed that Ir/NC–873 yields a high CH3Cl selectivity (≤95%), matching that of Ir/r–TiO2. A key feature that governs this performance is the absence of coordinating chlorine atoms, which mainly promote coking pathways ( Fig. S3 ).Upon increasing the active phase size from single atoms to nanoparticles of 2.1 nm, the selectivity to CH3Cl decreases to ca. 48% at the expense of the generation of CH4. The Ir/NC–1073 system, with an average nanoparticle size of 1.3 nm, exhibits a relatively high CH3Cl selectivity (ca. 70%), which is due to the considerable number of single atoms still present. The activity– and selectivity trends were complemented with stability tests, revealing that all systems deactivate over time. The stability decreases in the following order: Ir/NC(773) > Ir/NC-1073 > Ir/NC–873 > Ir/NC–673 > Ir/NC–473 ( Fig. S4 ), thus showing that NP–based systems display improved stability compared to their SA–based counterparts (up to ca. 4 times higher activity after 10 h, Table S2). Despite these results, Ir/r–TiO2 remains the best performing catalyst, showing unparalleled reactivity and stability in HDC (Table S2). Nevertheless, the fixed structure directed by the epitaxial growth of iridium on rutile–type carriers does not allow investigations on active phase size effects, which is one of the main aims of this study.Further, the NC–supported iridium nanostructures were also tested in HDB to determine possible halogen effects (Fig. 4 b). Notably, the activity was comparable to that in HDC, indicating that the type of halogen plays a minimal role. The selectivity to CH3Br shows a comparable volcano shape as to that of CH3Cl, with the N/O–coordinated single atom system presenting the highest selectivity (>90%) regardless of the halogen. The increasing propensity to coke and CH4 in chlorinated single atoms (<43%) and NP–based systems (<25%), respectively, are also observed, albeit less pronounced than in HDC ( Fig. S3 ). To provide a complete overview of halogen effects, HDB stability tests were conducted. The deactivation patterns were comparable to that in HDC ( Fig. S4 ), with nanoparticles preserving their initial activity better than single atoms.Briefly, the results identify that the initial HDH reactivity of iridium catalysts is predominantly governed by their active phase nanostructure: irrespective of the choice of halogen, nanoparticles display the highest activity and stability, and chlorine–free single atoms exhibit superior selectivity to CH3X compared to their Cl–coordinated counterparts and nanoparticles, which favor coke and CH4 production, respectively.This study was further expanded with nanostructures of platinum and ruthenium (Fig. 1), which were chosen as representative metals based on previous HDH studies and prepared following established synthesis procedures [12,27]. Furthermore, nickel–based catalysts were also included in the evaluation, despite that their HDH performance is considered poor [23]. Previous investigations revealed that moderate changes in the adsorption energies of the CH/Br fragments could lead to a dramatic increase in the HDB selectivity of nickel, rendering it an attractive candidate for studying nuclearity– and halogen effects [23].To achieve the targeted metal speciation (one system based on single atoms and one on nanoparticles, Fig. 1), each system was prepared applying a thermal treatment step (under N2 atmosphere) at a specific temperature (T act), indicated as M/NC–T act. Systems that underwent an additional reduction step under H2 at elevated temperatures (T red) with the aim to induce sintering of the metal were labelled M/NC(Tred). The resulting six catalysts were denoted Pt/NC–1073, Pt/NC(873), Ru/NC–473, Ru/NC–1073, Ni/NC–1073, and Ni/NC(773) (for an overview, please consult Table 1).The data revealed that the porous properties of the catalysts are comparable with the iridium–based systems, exhibiting S BET and V pore in the range of 324–545 m2 g−1 and 0.27–0.43 cm3 g−1, respectively; whereas ICP–OES confirmed that the actual metal content was close to the nominal value of 1 wt% (Table 1). The HAADF–STEM images clearly visualize the attainment of single atoms in Ru/NC–473 and Pt/NC–1073 (Fig. 5 ), and corroborated by XRD analysis, where reflections assigned to the metallic phases are not observed ( Fig. S5 ). The micrographs further show that nanoparticles are the dominating nanostructure in the high–temperature catalysts, Ru/NC–1073 and Pt/NC(873), with average particle size of 1.6 and 2.9 nm, respectively (Fig. 5). Similar to platinum, the high–temperature nickel catalyst (Ni/NC–1073) still exhibits single atoms as main species ( Fig. S6 ), thus requiring an additional high-temperature reduction step for the evolution of nickel into nanoparticles. The micrographs of the resulting catalyst, Ni/NC(773), indicate that active phase sintering occurred, leading to an average nanoparticles size of ca. 11.3 nm, in agreement with the XRD patterns that show a sharp reflection of metallic nickel. Interestingly, the necessary synthesis conditions to obtain nanoparticles are different across the metals: at 1073 K, iridium and ruthenium have an average nanoparticle size of 1.3 and 1.6 nm, respectively. At that temperature, platinum and nickel are still atomically dispersed, suggesting that these metals are highly stabilized in the cavities of the host as demonstrated by modeling, see below.To gain insights on the chemical state of the metals, XPS analysis was conducted. In accordance with the micrographs, a metallic phase was absent in the SA-based ruthenium and platinum catalysts, Ru/NC–473 and Pt/NC–1073 (Fig. 6 ). The Ru 3d spectrum of Ru/NC–473 revealed a contribution at a binding energy of ca. 464.1 eV (Fig. 6 a), commonly assigned to RuCl3 [27,45], suggesting that these single atoms are coordinated to chlorine. On the other hand, the dominant feature of the Pt 4f spectrum of Pt/NC–1073 is centred at ca. 73.0 eV, indicating the oxidized character of the platinum atoms (Fig. 6 b) [26,46]. In line with the XPS results, the EXAFS analysis shows pronounced Cl– and N/O–coordination for the ruthenium and platinum single atoms, respectively (Fig. 7 ). However, a clear N/O–Ru signal is also present, implying that a fraction of ruthenium single atoms are also N/O–coordinated.The performance of the platinum–, ruthenium–, and nickel–based catalysts was evaluated in HDC as well as in HDB at 573 K. For comparative purposes, representative iridium SA– and NP–based catalysts were selected (Ir/NC–873 and Ir/NC(773), respectively) and included in Fig. 8 . Analysis of the HDC results revealed the following trend of decreasing metal activity with the speciation of the dominating species indicated between brackets (Fig. 8 a): Ir (NP) ≈ Pt (NP) > Ir (SA) ≈ Pt (SA) > Ru (NP) > Ru (SA) > Ni (NP) > Ni (SA), with the nickel–based systems being virtually inactive. Over each metal, nanoparticles provide higher activity than the single atom analogues, which is in line with the trend previously distinguished over iridium–based catalysts (Fig. 4). A similar pattern was observed in HDB (Fig. 8 b), implying that halogen effects on catalytic activity are minimal, irrespective of the metal.In contrast, the selective behavior of the catalyst depends on both nuclearity and the type of halogen. In HDC, SA–based systems display a CH3Cl selectivity of ca. 95% (Ir), 80% (Pt), and 70% (Ru), higher than the NP–based catalysts (Fig. 8 a). Nanoparticles provided a lower selectivity (≤50%), favoring CH4 (Ir and Pt, up to 46%) and coke (Ru and particularly Ni, up to 92%).These results are comparable with the selectivity patterns obtained in HDB (Fig. 8 b), although iridium and platinum nanoparticles are less prone to over hydrogenation (CH4 selectivity up to ca. 30%). In stark contrast, ruthenium nanoparticles display a higher propensity to CH3Br (≤94%) than their single atom counterparts (≤71%). In addition, whereas ruthenium nanoparticles coke significantly in HDC (up to 60%), this side reaction does not occur in HDB as evidenced by the only two products CH3Br and CH4. The performance of ruthenium breaks with the trend that single atoms are more selective to the monohalogenated product, as seen in HDC.Stability tests were also conducted to gain a complete overview of the catalytic performance. The depletion of activity was expressed with the constant k D to enable a direct comparison, indicating the activity loss per hour derived via linear regression of the data in the time–on–stream (tos) range of 0.25–10 h ( Fig. S7 ). The active phase nanostructure clearly affects catalyst lifetime, with nanoparticles preserving their initial activity better than single atoms in HDC and HDB (Fig. 8 c, d). The stability decreases in the following order for both reactions: Pt (NP) > Ir (NP) > Ir (SA) ≈ Pt (SA) ≈ Ru (NP) > Ru (SA). Evolution of the products are presented in Fig. S7 , revealing that SA–based systems, except for ruthenium which shows an opposite trend, display enhanced propensity to CH4 over time. This suggests that sintering of the active phase into nanoparticles occurred during exposure to the reaction conditions.In summary, NC-supported metal (Ir, Pt, Ru, and Ni) catalysts with single atoms or nanoparticles as dominating nanostructure were prepared, characterized, and tested in HDC and HDB. It was revealed that hydrodehalogenation activity and stability are enhanced over nanoparticles when compared to single atoms, whereas the latter limit over hydrogenation and coking pathways, leading to outstanding CH3X selectivity. Ruthenium displays an inverse trend due to halogen effects, showing a higher selectivity to CH3Br over nanoparticles. Nevertheless, all catalysts deactivate over time and further investigation of the used systems is required to gain insight on deactivation mechanisms.Kinetic experiments reveal significant differences in the partial order of H2 over the catalysts, which can be grouped as systems (i) selective to CH3X, showing a p(H2) in the range of 0.41–0.58, (ii) that display significant selectivity to CH4 with p(H2) of 0.78–0.91, and (iii) that predominantly produce coke (Ru nanoparticles in HDC), showing a partial order of 0.67 (Fig. 9 ). Particularly single atoms are in the first group, whereas platinum and iridium nanoparticles are in the second. Notably, all nanoparticles have a higher p(H2) than their single atom analogues in both HDC and HDB except for ruthenium in HDB, which exhibits a p(H2) of 0.55 for single atoms and 0.43 for nanoparticles. These fingerprints suggest that the reaction mechanism may differ over the nanostructures and depends on the type of halogen. The results are likely a direct consequence of the ability to activate H2 and store H–atoms that can react with surface species, which may depend on the geometry of the active phase and participation of the basic sites of the carrier in the reaction, as was found in previous HDB studies [24].To examine the development of the iridium–based catalysts during exposure to HDC and HDB conditions, selected systems were characterized after 10 h on–stream using N2–sorption, XRD, HAADF STEM, and XPS, revealing three main deactivation mechanisms: (i) fouling due to coking, (ii) metal sintering, and (iii) poisoning by halogenation. HAADF–STEM micrographs of the used systems display the sintering of single atoms into nanoparticles with an average size in the range of 2.1–2.7 nm, regardless of the halogen type (Fig. 2). Even though the nanoparticle size of these catalysts is comparable after 10 h on–stream, their performance is dissimilar due to the contributions of coking and surface halogenation. Analysis of the structural properties point at the remarkable instability of the NC carrier (Table 1), with the specific surface areas (S BET) and pore volumes (V pore) strongly decreasing after exposure to the reaction conditions (up to 90 and 70% lower than the original values, respectively). This suggests that deposition of carbonaceous species on the active sites contributes significantly to activity losses over all catalysts, likely more pronounced over catalysts that generate coke as main product, such as Ir/NC–473, Ir/NC–673 ( Fig. S3 ). On the other hand, the NP–based iridium catalyst, (Ir/NC(773)), was less prone to active phase agglomeration with an increase of the particle size from 2.1 to 3.5 and 2.6 nm in HDC and HDB, respectively. These results were corroborated with XRD analysis ( Fig. S1 ), showing reflections compatible with metallic iridium, and by the XPS spectra that display contributions assigned to the metallic phase (Fig. 3, Fig. S2 ). Furthermore, the peaks at BEs of 67.4 and 70.0 eV reveal the poisoning of the surface via bromination in HDB (Fig. 3) [24], whereas chlorination occurs in HDC as evidenced by the contributions at 197.9 and 200.8 eV ( Fig. S2 ). This indicates that all iridium–based catalysts, in addition to fouling due to the deposition of carbonaceous species and active phase sintering, also suffer from halogenation.Whereas all active systems, regardless of the metal, suffer from the poor stability of the carrier, used ruthenium and platinum catalysts were characterized to assess the extent of metal sintering and halogenation on their lifetime. Similar to their iridium counterparts, HAADF–STEM microscopy reveals that ruthenium– and platinum single atoms undergo pronounced sintering, increasing up to 2.7 and 3.6 nm, respectively, whereas nanoparticles remain relatively stable (Fig. 5). The weak reflections assigned to the metallic phase in the XRD spectra of the used platinum catalysts and Ru/NC–1073 suggest that larger nanoparticles were formed. This observation was confirmed by XPS analysis, distinguishing contributions at BEs of 461.8 (Ru/NC–473) and 72.1 eV (Pt/NC–1073), representing metallic ruthenium and platinum, respectively (Fig. 6). Further analysis of the spectra indicates that surface chlorination is limited over both metals, displaying a relatively unaltered fraction of oxidized species in used ruthenium–based systems compared to the fresh one, whereas platinum catalysts are largely metallic. On the other hand, the catalysts used in HDB display contributions assigned to Br–species (Fig. 6, Fig. S8 ), suggesting bromination of the surface. However, as platinum is mostly in the zero-oxidation state, it suffers the least from halogenation, which implies that mainly the carrier was brominated.In short, the loss of catalytic performance over time–on–stream is caused by: (i) fouling by coking, which is expected to occur over all systems due to carrier metastability, (ii) active phase sintering, more pronounced for single atoms, and (iii) halogenation. While chlorination mainly occurs on iridium–based systems, bromine poisons iridium, ruthenium, and platinum catalysts, albeit the latter to a lesser extent. Overall, these results underline that deactivation mechanisms are complex and intertwined, governed by three modes that depend on the type of halogen, the metal, and its nuclearity, thereby highlighting the need for individual optimization strategies to develop stable systems.Whereas Ir/NC activated at 1073 K results in an average nanoparticle size of 1.3 nm (Fig. 2), the platinum– and nickel–based systems that underwent a similar thermal treatment (Pt/NC–1073 and Ni/NC–1073) have single atoms as predominant species (Fig. 5 , Fig. S6 ). Furthermore, the platinum single atoms are non–chlorinated, in contrast to the single atoms in Ru/NC–473, Ir/NC–473, and Ir/NC–673 (Fig. 5, Fig. 6), which are coordinated to Cl. These results suggest that the speciation, as nanoparticle or as (chlorinated) single atom, depends on various factors, including the anchoring sites in the NC carrier, the metal, and the activation temperature. Therefore, to complement the characterization of the materials, a molecular–level understanding of the active phase speciation can help establishing more robust structure–performance relationships. For this purpose, Density Functional Theory (DFT) studies were conducted. Therein, the NC support was represented by a set of three defects; (i) non-planar 3 N and square-planar 4 N arrangements, labelled 3 × N5 (tri-pyrrolic), (ii) 4 × N6 (tetra-pyridinic), (iii) and 2 × N5 + 2 × N6 (tetra-pyrrolic/pyridinic) [12]. Formation energies were evaluated to shed light on the interaction of the single atom with the host, using metal chloride precursors MCl (M = Ir, Pt, Ru, or Ni), the pristine metal species (M), and the NC support as reference states.The results of the speciation analysis reveal two general trends related to the Cl–ligands and cavities in the NC–carrier (Fig. 10 , Table S3): (i) starting from the pristine single atom, iridium and ruthenium display the highest affinity toward chlorination, with binding energies ranging from –1.13 to –2.44 eV, explaining why these systems remain chlorinated at elevated temperatures, and (ii) single atoms in 3 × N5 sites are expected to be more chlorinated than those in the 4 × N6 and 2 × N5 + 2 × N6 cavities due to the square–planar arrangement in 4 N-defects, which is responsible for the superior atom stabilization. This prevents the sintering of single atoms into nanoparticles during the removal of Cl at increasingly higher activation temperatures. starting from the pristine single atom, iridium and ruthenium display the highest affinity toward chlorination, with binding energies ranging from –1.13 to –2.44 eV, explaining why these systems remain chlorinated at elevated temperatures, andsingle atoms in 3 × N5 sites are expected to be more chlorinated than those in the 4 × N6 and 2 × N5 + 2 × N6 cavities due to the square–planar arrangement in 4 N-defects, which is responsible for the superior atom stabilization. This prevents the sintering of single atoms into nanoparticles during the removal of Cl at increasingly higher activation temperatures.To further understand the speciation trends, cohesive energies were computed. Ruthenium and iridium present the strongest propensity to leaching, with cohesive energies of −7.3 and –8.0 eV/atom, respectively (Fig. 10), whereas nickel (–5.3 eV/atom) and platinum (-6.2 eV/atom) display lower values. These results are in agreement with the experimentally observed metal speciation, which showed that nickel and platinum remained as single atoms while iridium and ruthenium were mainly nanoparticles at 1073 K (Figs. 2 and 5, Fig. S6 ). Consequently, the stabilization of iridium and ruthenium single atoms in N–containing defects is not sufficient to prevent their sintering at elevated temperatures, while nickel and platinum single atoms exhibit superior stability in most cavities. To further assess the effect of N–functionalities on the atomic dispersion on the carrier, adsorption energies of single atoms on a N–free carrier were evaluated. Previous investigations disclosed the synthesis of platinum single atoms on activated carbon (AC) [12,24]. Nonetheless, the significantly smaller adsorption energy of the atoms in AC cavities (–3.15 eV, Table S3) relative to the anchoring sites in NC (<-5.44) indicate that the latter provide better stability. Comparable values were found for nickel (–3.97 and < –7.12 eV on AC and NC, respectively). In stark contrast, the atomic dispersion of iridium and ruthenium on AC is not favored due to the full coordination and high cohesive energy of these metals. This leads to insufficient atom anchoring on AC, thereby highlighting the positive impact of N–moieties on single atom stability. The potential catalytic activity of single atoms in each defect was studied by exploring the adsorption energy of the substrates, CH2Cl2 and CH2Br2. Metal single atoms in the tri–pyrrolic (3 × N5) cavity displayed superior substrate activation (Table S4), in line with previous studies [24] and in agreement with the presence of pyrrolic sites as shown in the characterization data (Table S1). Hence, a single atom in the 3 × N5 cavity was retained as the most representative for single atom–based catalysts.The reaction network leading to CH3Br, CH4 and C is described (Table S5) and the associated thermodynamic and kinetic parameters were calculated (Tables S6-S8). The Gibbs free energies were computed including entropic contributions from the molecules, using CH2X2, H2, and the corresponding catalysts as reference systems operating at 523 K ( Fig. S9 , Fig. S10 ).At first, the effect of a full coordination of the single atom with Cl ligands was assessed. The network starts by the dissociative adsorption of CH2X2 on the active ensembles, leading to CH2X*. The presence of Cl in the active phase hinders the activation of CH2X2 over the single atoms, with barriers ranging from 0.9 to 1.8 eV. Furthermore, subsequent adsorption of H2 on CH2X* is virtually prohibitive as the valence of the metal atom is full. Consequently, chlorinated sites are prone to coke, which is consistent with the observed selectivity performance of chlorinated iridium single atoms ( Fig. S3 ). In contrast to the chlorinated systems, the dissociation of CH2X2 on the pristine (non–chlorinated) single atoms is quite exothermic and instantly leads to CH2X* fragments. Nickel–based systems form an exception to that rule. The challenging activation of CH2X2 on the nickel single atoms (∼2 eV) can be rationalized in terms of the low affinity toward both halogens, being ca. 1 eV lower compared to the iridium, platinum, and ruthenium single atoms (Table S9), resulting in poor catalytic activity in both HDC and HDB (Fig. 8).Once the substrate is dissociated into CH2X*, one of the following steps take place: (i) heterolytic H2 dissociation promoted by the basicity of the cavity [24] or (ii) halogen elimination to form CH2*, thereby filling the valence of the metal atom in an octahedral configuration, which poisons the active site by impeding the incorporation of H2, thus promoting coke formation. Although this step is energetically feasible (<0.9 eV in all cases, except for Ni), the greater stability of the CH2X* compared to the dissociated CH2*+X* inevitably lowers the population of CH2*X* intermediates, which promotes H2 incorporation on CH2X*. Upon the heterolytic dissociation of H2, HX* and CH3X* are formed on the single atom, thereby re–establishing the active site after their desorption.Among the three active single atom catalysts (Ir, Ru and Pt, Fig. 8), ruthenium displays the lowest activity and a significant selectivity to coke (∼30%), owing to the presence of chlorinated sites in the active phase, thus following comparable performance patterns as the chlorinated iridium single atoms. In contrast, single atoms of iridium and platinum display superior activity and CH3X selectivity performance due to their facile hydrogen dissociation (<1.10 eV, Fig. 11 ), and favourable CH2X2 adsorption (<0.6 eV).The basic N–sites of the host can participate in the reaction by storing H–atoms that can be transferred to other moieties. This leaves the single atom free for coordination, thereby enhancing catalytic activity [24]. To further assess the contribution of N–functionalities to the reaction, dissociation energies of H2 over single atoms on the carbon supports were computed. Hydrogen undergoes barrierless homolytic dissociation over platinum and nickel on AC (Table S10). In contrast, even though the reaction is energetically feasible, activation of H2 over AC–supported iridium and ruthenium is not possible, as these single atoms cannot be stabilized (Table S3). To gain further insights, experimental evaluation of bare NC and AC in CH2X2 hydrodehalogenation (reaction conditions specified in the caption of Fig. 2) was conducted, revealing the inactivity of AC. On the other hand, NC displayed low activity (Table S2), suggesting that N–species can act as catalytically active sites. Nevertheless, the metal–free carrier deactivates rapidly, showing deactivation constants more than 6–fold higher than Pt/NC–1073 in both reactions. These results indicate that the catalytic response is mainly determined by the metal species in the cavity.Still, the contribution of the host is crucial in enabling the adsorption of the substrates and the full mechanism. Particularly, the evolution of CH2X* moieties depends on the cooperation of the support with the single atom species, since it enables the formation of CH3X by providing a sufficient number of protons and prevents the generation of CH2*, which likely leads to coke. High CH3X selectivity is ensured over iridium and platinum due to the participation of the NC carrier. In addition to promoting the heterolytic dissociation of H2, the diffusion of carbonaceous species and halogen fragments over the surface of the scaffold is hindered due to the non-continuous structural morphology of the host. Consequently, CH4 is not generated and only coke is formed as a side product when H2 cannot be incorporated to CH2X*. The active phase sintering occurs due to the formation of mobile fragments during the course of the reaction. Such moieties are obtained when substrates are adsorbed on single atoms with non-planar configurations. The binding energy of the single atom to the scaffold is lowered and the resulting metal complex is susceptible to diffusion.The facile homolytic dissociation of H2 over the surface of metal nanoparticles results in higher hydrogen coverages when compared to single atoms, explaining the superior activity of nanoparticle-based catalysts in the hydrogenation paths (Table S4). The reaction starts similar to that over their single atom counterparts, but the system is more prone to form CH2*, facilitated by the diffusion of halogen and methylene moieties over the surface [24], which in general (except for ruthenium in HDB) leads to CH4 and C. However, if H* is incorporated on CH2X*, CH3X* is generated and subsequently desorbed. After the formation of CH2*, one of the following steps takes place: (i) CH2* dissociation to CH* and H* promoted by the diffusion of H* and the affinity of the metal toward CH* or (ii) H* incorporation to form CH3* and subsequently, CH4.Whereas metal surfaces of the nanoparticles are continuous, multiple adsorption/reaction events take place simultaneously. Particularly, the HDC and HDB reactions lead to CH2* with almost no barrier for cleaving the second CX bond. Over hydrogenation occurs over iridium and platinum sites due to the greatly exothermic formation of H* intermediates (Fig. 11), which leads to the formation of CH4. On the other hand, the high affinity to carbonaceous fragments such as CH2*, CH*, and C* results in coke over nickel nanoparticles. Even the penetration of C into the metal lattice is exothermic (-0.8 eV, Table S11), which can contribute to rapid catalyst deactivation in both HDC and HDB.Among the nanoparticles, ruthenium shows a notable performance trend in HDB, where nanoparticles are more selective to CH3Br than the single atom analogues. In HDC, the controlled H2 coverage, due to a less favourable H2 dissociation, leads to the formation of coke. However, in HDB, the binding strength of CH2Br2 on the metal surface is optimal (–0.13 eV, 0.94 eV for CH2Cl2), resulting in the exceptional performance of the nanoparticles. This defines energetic regions where HDC and HDB take place with improved selectivity (Fig. 11). Optimal H2 adsorption and CH2X2 binding energies were found to be between 0.25 and 1.25, and lower than 0.6 eV, respectively.Altogether, the systematic study of the synthetic platform for two closely related reactions allowed us to widen the conceptual framework encompassing synthesis, characterization and understanding allowing us to establish robust structure (species)-performance patterns.In this study, a strategy combining catalytic evaluation complemented with extensive characterization of a platform of NC–supported metal nanostructures (Ni, Ru, Ir, and Pt), from single atoms to nanoparticles of ca. 3 nm, coupled with kinetic analysis and density functional theory was adopted to systematically investigate to effects of the active phase and the halogen on activity, selectivity and stability in CH2X2 hydrodehalogenation. Substantial activity differences were observed over the nanostructures, attaining the highest reaction rates over NP–based systems, ranking as Ir ≈ Pt > Ru ≫ Ni independent of the halogen. Moreover, these systems preserve their initial activity better than their single atom analogues, which mainly suffer from sintering. The catalytic tests further revealed a marked impact of nuclearity, single atom coordination environment, and halogen type on the product distribution. In hydrodechlorination, CH3Cl is the main reaction product over single atoms, whereas nanoparticles exhibited significant selectivity to CH4 or coke. Among the metals, iridium–based single atoms exhibit exceptional CH3Cl (≤95%) selectivity, in stark contrast to their chlorinated analogues which favored the formation of coke (<90%). Comparable performance patterns were observed in hydrodebromination with the exception for ruthenium, which displayed an inverted selectivity–structure trend with improved CH3Br selectivity over nanoparticles (≤96%) compared to the single atoms (≤72%). Kinetic and mechanistic studies correlate these results with the ability of the active phase to activate CH2X2 and H2, and to store H–atoms. Furthermore, the intrinsic stability of the single atoms in the cavities and the potential catalytic response were computed, setting a basis for understanding the effects of synthetic protocols on speciation and coordination. The findings reported in this work are directed at elucidating hydrodehalogenation performance patterns, highlighting the impact of nanostructuring and the halogen type to advance future catalyst design.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by ETH research grant ETH-43 18-1 and NCCR Catalysis, a National Centre of Competence in Research funded by the Swiss National Science Foundation. We thank BSC–RES for providing generous computational resources. The authors thank the Scientific Center for Optical and Electron Microscopy, ScopeM, the Paul Scherrer Institute, PSI, and the Swiss Federal Laboratories for Materials Science and Technology, EMPA, for access to their facilities. The authors thank Dr. Frank Krumeich for performing some of the microscopic analyses.Supplementary information associated with this article, containing additional characterization and catalytic data, can be found in the online version. The computed structures have been added to the ioChem-BD database ref. [42]. Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2021.10.008.The following are the Supplementary data to this article: Supplementary data 1

Nanostructuring metal catalysts has been demonstrated as an attractive strategy to enable selective hydrodehalogenation of CH2X2 (X = Cl, Br) to CH3X, but active phase size effects of promising metals and the role of the halogen are still poorly understood. Herein, the impact of these parameters on performance (activity, selectivity, and stability) is systematically assessed by employing a platform of N–doped carbon–supported metal nanostructures (Ir, Pt, Ru, and Ni), ranging from single atoms (SA) with defined coordination environment to nanoparticles (NP) of ca. 3.0 nm. Catalytic tests reveal that when compared to single atoms, highest reaction rates are attained over NP–based systems, which also exhibit improved stability ranking as Ir ≈ Pt > Ru ≫ Ni, independent of the halogen. The product distribution was markedly affected by the nanostructure and speciation of the active center as well as the dihalomethane type. Specifically, CH3Cl is the main reaction product over SA in hydrodechlorination, achieving an exceptional selectivity over Ir (up to 95%). In contrast, NP mainly generated CH4 or coke. Comparable patterns were observed in hydrodebromination, except over Ru, which exhibited an inverse structure–selectivity trend. Density Functional Theory simulations shed light on the speciation of the active phase and identified the adsorption and dissociation energies of CH2X2 and H2 as descriptors for catalytic reactivity. These findings elucidate hydrodehalogenation performance patterns, highlighting the impact of nanostructuring and the halogen type to advance future catalyst design.

Climate change has greatly threatened the sustainable development of mankind. (Nong et al., 2021; Nielsen et al., 2021) In this regard, the net-zero technologies are of great practical importance, which can convert carbon dioxide (CO2) into reusable chemical feedstocks (e.g., CH4, CO, etc.). (Vo et al., 2021; Teo et al., 2022; Cai et al., 2023) Among various approaches, the feasibility of CO2 methanation has been demonstrated by many methods, like electrocatalysis, (Chen et al., 2021a; Zhao et al., 2021) photocatalysis, (Shi et al., 2022; Cabrero-Antonino et al., 2022) and thermal catalysis. (Lee et al., 2021; Galadima and Muraza, 2019) Due to the multiple-electron transfers occurring throughout the reaction as well as the thermodynamic equilibrium between CO2, its derivatives and by-products (such as hydroxide and bicarbonate), conducting effective hydrogenation of CO2 with long-term stability remains difficult in electrocatalysis and photocatalysis. (Ma et al., 2020; Ding et al., 2020) In comparison, thermal catalysis shows its advantages in efficiency and stability. (Tu et al., 2014; Wang et al., 2019) However, due to the carbon deposition during the thermal CO2 methanation process, the catalysts could be deactivated in long-term reactions. It remains a challenge to rationally design a catalyst that can efficiently suppress the carbon deposition.Recently, immobilizing metal nanoparticles (NPs), on supporting materials has emerged as an attractive strategy to realize efficient CO2 methanation processes, for example, Ni (Wang et al., 2021a) and Ru. (Chen et al., 2021b) Nickel-based catalysts are traditional for methanation. (Wang et al., 2022a) However, carbon deposition is more prone to occur on Ni-based catalysts. In contrast, platinum group metal NPs (Ru, etc.) have received great attention since they are more active than those made of Ni-based catalysts. (Zhang et al., 2012) However, the high price of platinum group metals greatly limits their applications. (Wang et al., 2022b) Therefore, developing catalysts with less platinum group metal usage is regarded as a promising method to make the CO2 methanation closer to industrialization. (Yu et al., 2021) In this regard, constructing bimetallic catalysts is considered as one of the most promising approaches, benefiting from the synergistic effects of the two metallic components. (Tahir et al., 2017; Zhang et al., 2021) Besides platinum group metal, metallic Cu is also widely studied due to its outstanding performance and affordability in CO2 methanation. (Zhang et al., 2021) Moreover, Cu NPs can also be employed as a promoter to improve catalysts’ structure and surface properties, reduce deactivation, and even boost activity and selectivity during CO2 reduction reactions. (Dias and Perez-Lopez, 2020) However, the current study on Cu/Pt-group bimetallic catalyst's application in CO2 methanation is still in its infancy, which calls for more investigation.Besides the catalytic active sites, the supports also play a vital role during the methanation process. Currently, most metal NPs are deposited on oxide supports, such as silica (SiO2), (Wang et al., 2021a) alumina (Al2O3), (Quindimil et al., 2021) titania (TiO2), (Wang et al., 2022c) zirconia (ZrO2), (Gao et al., 2022) and ceria (CeO2). (Xie et al., 2022) However, exothermicity of CH4 methanation reactions (ΔH=−165 kJ mol−1) remains a challenge for all investigated systems, (Hervy et al., 2021) since the high surface temperatures may displace the thermodynamic equilibrium and favor competing reactions (reverse water-gas shift and steam reforming) to produce CO instead of CH4. (Fatsikostas and Verykios, 2004; Baudouin et al., 2013) In this regard, carbon-based supports provide us with an alternative due to their high thermal conductivity that can reduce the local temperature of the catalyst. (Sun et al., 2021) Additionally, porous carbon has also shown its boosted adsorption capability towards H2 and CO2 due to its relatively large surface area. (Lee et al., 2021) But in terms of its application in thermocatalytic methanation, it remains veiled.Herein, we developed a novel RuCu bimetallic catalyst on nitrogen-doped mesoporous carbon nanospheres (NMCN) for thermocatalytic CO2 methanation. Cu and Ru are supported onto NMCN to create bimetallic active sites for thermocatalytic CO2 methanation with different Ru/Cu ratios. In our study, it is demonstrated that, due to the formational of RuCu bimetallic catalysts, the thermocatalytic activity of RuCu-x/NMCN (x = 1, 2, 3) is obviously higher than that of pure NMCN and NMCN-support Cu NPs (Cu/NMCN). Moreover, the synergistic effect between Ru and Cu nanoparticles is found, which could efficiently prevent the carbon deposition processes. As a result, the optimal RuCu-3/NMCN catalyst exhibits remarkable thermocatalytic stability in 1380 min.3-Aminophenol (99 wt%, Sigma-Aldrich Co.), formaldehyde (36 wt% in water, Sigma-Aldrich Co.), sodium hydroxide (98 wt%, Sigma-Aldrich Co.), Pluronic F127 (Mw=12,600, PEO106PPO70PEO106, Sigma-Aldrich Co.), L-cysteine (98 wt%, Sigma-Aldrich Co.), Copper (II) chloride (99 wt%, Sigma-Aldrich Co.), Ruthenium (III) chloride hydrate (99 wt%, Sigma-Aldrich Co.), ethanol (99 wt%, Sigma-Aldrich Co.) were used as received without any further purification.NMCN is synthesized according to the reported literature. (Yang et al., 2014) At a temperature of 25 °C, 80 mL of ethanol and 200 mL of distilled water were mixed to obtain an aqueous-alcoholic solution. Then, while the mixture was continuously stirred, 1 g of Pluronic F127 (Mw = 12,600, PEO106PPO70PEO106), 1.3 g of CTAB, and 2 g of cysteine were added. Then, 2 g of 3-aminophenol was added, and it was thoroughly dissolved while being agitated. After 24 h of stirring, 2.85 mL of 36 wt% formaldehyde was added. The mixture was then placed in an autoclave and kept at 100 °C for 24 h. After three rounds of washing with water and ethanol, a brown-red powder was obtained. The powder was calcined in N2 flow with a heating rate of 1 °C min−1 up to 350 °C, dwelled for 1 h, and resumed a heating rate at 1 °C/min up to 800 °C and dwelled for 2 h to produce NMCN.Cu/NMCN was prepared by a reduction method. The NMCN was impregnated with CuCl2·3H2O solution. Then dried at 110 °C in an oven overnight before calcination in H2/Ar at 400 °C for 2 h. The as-prepared catalysts were denoted as Cu/NMCN.The RuCu-x/NMCN (x = 1, 2, 3) catalysts were synthesized by a reduction method. The NMCN was impregnated with a mixture of RuCl3 and CuCl2·3H2O solution with varied Ru/Cu molar ratios. All samples were subsequently dried at 110 °C overnight before calcinated in H2 /Ar at 400 °C for 2 h. The as-prepared catalysts were denoted as RuCu/NMCN.A scanning electron microscope (SEM, Zeiss Auriga) was used to capture the morphology of nanospheres. The high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission microscopy (HAADF-STEM) images of the samples were acquired from on FEI Themis Z equipped with double spherical aberration corrector operating at 300 kV. X-ray diffraction (XRD) measurements were recorded on a Rigaku D max-3C diffractometer using Cu Kα radiation (40 kV, 20 mA, λ = 0.15408 nm). X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo Fisher ESCALAB 250Xi spectrometer with a focus monochromatic Al Kα-rays (1486.6 eV) source. The samples were tested under a vacuum below 5.0 × 10−10 Mbar, energy resolution spectra were collected using a pass energy of 20 eV. Peak fitting of the high-resolution data was carried out by Thermo Avantage 5.9925 surface chemical analysis software. N2 sorption isotherms were recorded on an Autsorb iQ gas sorption system at 77 K. The Brunauer-Emmett-Teller (BET) method was used to calculate to measure the specific surface areas (SBET) using adsorption at a relative pressure of P/P0 = 0.05–0.30 and the Barrett Joyner Halenda (BJT) method was used to estimate the total pore volume and pore size distribution. The Cu and Ru content were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) on an R4 Perkin Elmer ICP-OES 8300DV. Before analysis, the powder samples were digested by aqua regia and dissolved in 70% HNO3 and diluted by deionized water.Catalytic experiments were carried out under atmospheric pressure in a continuous quartz glass flow reactor, loaded with 100 mg variable of catalyst powder. Gaseous mixtures of CO2 and H2 diluted with nitrogen were fed into the reactor at the gas hourly space velocity (GHSV) of 28,350 ml gcat −1 h−1 and H2/CO2 mass ratio of 4/1. The temperature was varied step by step between 300 °C and 800 °C.The pre-reduction of catalysts at 400 °C was adopted to reduce the metal oxides on the catalyst surface into metallic state. The procedure included heating the catalysts in a gas flowing of 20% H2 at 60 ml/min for 30 min at 400 °C. After termination of reduction, the catalyst was cooled in flowing H2 to room temperature.The reaction products were analyzed by Agilent 7890B gas chromatograph equipped with a ShinCarbon column and a thermal conductivity detector (TCD). The activity of the catalysts was indicated by the CO2 conversion and CH4 selectivity.NMCN is prepared based on a previous procedure utilizing a dual-soft templating approach. (Yang et al., 2014) Herein, 3-aminophenol and formaldehyde in ethanol aqueous are mixed with L-cysteine to generate the resin spheres. Then, the obtained precursors are carbonized at 800 °C to get the NMCN samples, as depicted in Scheme 1 . The RuCu/NMCN with different molar ratios of Ru/Cu (denoted as RuCu-x/NMCN) is synthesized according to the procedure detailed in the experimental section. For comparison, the reference Cu supported on NMCN nanoparticles (denoted as Cu/NMCN) is prepared without adding the Ru-precursor. Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis shows that the molar ratios of Ru/Cu in RuCu-1/NMCN, RuCu-2/NMCN and RuCu-3/NMCN are 1:128, 1:27 and 1:10, respectively (Table S1).The scanning electron microscopy (SEM) images of as-prepared Cu/MCN and RuCu-x/NMCN (x = 1, 2, 3) catalysts are depicted in Fig. 1 a and Supplementary Fig. S1, respectively. The morphology of RuCu-x/NMCN remains spherical. The obtained Cu/MCN and RuCu-x/NMCN (x = 1, 2, 3) show a narrow average size distribution of 40–50 nm. Taking the RuCu-3/NMCN as an example, with the loading of Cu and Ru metals, the presence of well-dispersed Cu and Ru can be observed in the elemental mapping images (Supplementary Fig. S2). The uniform spherical morphology of the catalyst is further validated by the transmission electron microscopy (TEM) images (Fig. 1b and Supplementary Fig. S3a). The randomly dispersed nanoparticles on the carbon nanospheres could be evidenced with a metal distribution diameter of 2.16±0.60 nm (Supplementary Fig. S3b and 1c). HAADF-STEM energy dispersive X-ray spectroscopy (HAADF-STEM-EDS) is adopted to further identify the Ru nanoparticles from the Cu nanoparticles. The corresponding elemental mapping images of RuCu-3/NMCN catalysts (Supplementary Fig. S4a-e) indicate the homogeneous distribution of each elements. Moreover, the line scanning profile (Supplementary Fig. S4f) also shows a uniform distribution of Ru and Cu on the particles. Additionally, high-resolution transmission electron microscopy (HR-TEM) is also used to analyze the detailed structure of the metal particles. As shown in Fig. 1d, the clear lattice fringes with an interplanar distance of approximately 2.3 Å can be assigned to the (100) facets of the hexagonal phase of Ru-Cu alloy. (Wang et al., 2022d; Zheng et al., 2021)To further confirm the composition of the RuCu-3/NMCN, Fig. 2 shows the X-ray diffraction (XRD) patterns of RuCu-x/NMCN (x = 1, 2, 3), Cu/NMCN, and NMCN catalysts. For all catalysts, diffraction peaks are observed at 2θ values of 25° and 43°, corresponding to the (002) and (100) planes of graphite, respectively, and the broadness of these peaks indicated the amorphous nature of the carbon-based catalysts. (Jaleel et al., 2022) Moreover, compared with the standard Cu (PDF#04–0836), the diffraction peak of Cu/NMCN, RuCu-1/NMCN and RuCu-2/NMCN catalysts could match well at 2θ values of 43.6 and 50.7 representing the (111) and (200) planes of Cu. The absence of the associated Ru diffraction peaks may due to the small size of the metal particles. In addition, the XRD pattern of the RuCu-3/NMCN catalyst reveals that the characteristic peak of Cu is positively shifted compared to the standard pattern of Cu for the formation of the RuCu alloy. (Wang et al., 2022d) Combined with the HR-TEM image (Fig. 1d), it can be concluded that RuCu alloy is formed in RuCu-3/NMCN.The chemical state information of the above catalysts is investigated through X-ray photoelectron spectroscopy (XPS). XPS survey spectra for all catalysts confirm the presence of Cu, Ru, O, N, and C, shown inFig. S5. For Cu/NMCN, RuCu-1/NMCN and RuCu-2/NMCN catalysts, the Cu 2p core-level spectra had two sets of peaks, one set had Cu 2p3/2 and Cu 2p1/2 peaks at 931.7 and 951.6 eV, respectively, indicating the existence of metallic Cu (Fig. 3 a). Meanwhile, the peaks of Cu 2p3/2 at 934.66 eV and Cu 2p1/2 at 955.89 eV in combination with the satellite peaks are the typical characteristics of Cu2+. (Wang et al., 2022e) From the Ru 3p doublet spectra (Fig. 3b), the Ru 3p3/2 and Ru 3p1/2 peaks at 461.7 and 438.8 eV comfirm the presence of metallic Ru. Meanwhile, the Ru 3p3/2 peak at 463.8 eV and Ru2p1/2 at 485.6 eV are the typical characteristic peaks of Ru4+. (Kim et al., 2022) More importantly, the Ru 3p binding energy of the RuCu-3/NMCN catalyst shifted to a higher binding energy position, while the Cu 2p binding energy shifted to the lower energy position, which also indicates that the presence of RuCu alloy in RuCu-3/NMCN catalyst. The positive shift of the binding energy of metallic Ru reveals the electron transfer from Cu to Ru due to the higher electronegativity of Ru than that of Cu. (Wu et al., 2022) Compared to the Cu/NMCN catalyst, no obvious shifts of the binding energies of Cu 2p spectra can be observed for RuCu-1/NMCN and RuCu-2/NMCN catalysts, which could be attributed to the absence of metal alloyed phase. (Salazar et al., 2014)Nitrogen-sorption analysis is used to describe the porosity of catalysts. Fig. 4 displays the nitrogen adsorption and desorption isotherms for the RuCu-1/NMCN, RuCu-2/NMCN, RuCu-3/NMCN, Cu/MCN, and NMCN catalysts together with the related pore size distribution curves. The BET surface area, total pore volume and average pore diameter are shown in Table 1. All the nitrogen adsorption-desorption isotherms of RuCu-1/NMCN, RuCu-2/NMCN, RuCu-3/NMCN, Cu/MCN and NMCN exhibit pseudo-type IV curves. The average pore diameters of RuCu-1/NMCN, RuCu-2/NMCN, and RuCu-3/NMCN are similar to each other, and are calculated to be in the range from 2 to 3 nm based on the Barrett-Joyner-Halenda (BJH) method. The pure NMCN exhibits the highest surface area (282 m2 g−1) and pore volume (0.30 m3 g−1). After loading metal particles, both the surface area and pore volume of the catalysts decrease slightly. All these results indicate the successful incorporation of the metal particles within the support. Moreover, the change in pore volume is slight, indicating that the loading of metal NPs will not block the pore channels of NMCN.The CO2 conversion performance at different reaction temperatures is summarized in Fig. 5 . The results are the average values of five GC measurements at each temperature. Fig. 5a demonstrates that a very small amount of CO2 conversion is observed at 300 °C. The CO2 conversion rate significantly increases from 400 to 800 °C, reaching its maximum at 800 °C. For NMCN (Fig. 5b-d), the conversion of CO2 is extremely low. The low conversion of NMCN is due to the absence of metal sites, which is crucial for H2 and CO2 dissociation. (Sun et al., 2020) In the absence of Ru, the highest conversion of CO2 reached around 15% with 100% selectivity for CO of Cu/NMCN at 600 °C. It could be concluded that the existence of Ru had a promoting effect on the catalytic activity and selectivity. Compared to the poor catalytic performance of NMCN and Cu/NMCN, both the conversion and selectivity of CH4 are obviously enhanced when RuCu-x/NMCN (x = 1, 2, 3) is used as the catalyst, indicating that the synergy between Ru and Cu can significantly promote the catalytic CO2 methanation. In the presence of Ru, the RuCu-1/NMCN catalysts still keep a relatively low conversion when compared with Cu/NMCN catalysts, due to the minimal loading of Ru. RuCu-2/NMCN catalysts of CO2 conversion increase 2-fold to RuCu-1/NMCN catalysts at 500 °C with 100% selectivity for CH4. The increase in Ru loading leads to a higher CO2 conversion level, since more active sites are present on the RuCu-3/MCN catalysts. The CO2 conversion of RuCu-3/NMCN catalysts increases 18 folds and 9 folds to RuCu-1/NMCN and RuCu-2/NMCN catalysts at 500 °C, relatively. The conversion of RuCu-3/MCN catalysts in CO2 methanation is 53% and the major product is CH4 with a small amount of CO at 500 °C. It should be noted that, in such a readction condition,the CH4 selectivity achieves 88%, as shown in Fig. 5c. At 500  °C, the catalysts' selectivity for CH4 is in the following trends: RuCu-3/MCN > RuCu-2/MCN > RuCu-1/MCN. As for the RuCu-2/NMCN catalysts, the CO2 conversion decreases as the reaction temperature increases. The Cu/NMCN catalysts have a similar phenomenon at 700  °C, starting to decrease due to reverse water gas shift reaction (rWGS) becoming the most favored reaction at high temperatures. Due to the low conversion below 300 °C, the selectivity behavior in this range is not conclusive.The long-term stability of RuCu-3/NMCN has been investigated by adopting a realistic feed composition of 15% CO2, 60% H2 and 25% N2. The space velocity (28,350 mL gcat –1 h–1) and reaction temperature (500 °C) employed are chosen so as to keep the conversion of CO2 around 52%. The results obtained are presented in Fig. 6 . The stability of the RuCu-3/NMCN catalysts is evaluated over a 1380 min successive run at atmospheric pressure. The RuCu-3/NMCN catalyst appears to be quite stable, retaining nearly 99% of its initial activity. Furthermore, the RuCu-3/NMCN can also retain a CH4 selectivity above 85% after the 1380 min reaction. Therefore, it could be concluded that RuCu-3/NMCN is a promising candidate for CO2 methanation with H2-rich gas.XRD analysis of the RuCu-3/NMCN catalysts before and after the reaction suggests that the catalyst is stable over an extended time (Supplementary Fig. S6). According to the Brunauer-Emmett-Teller (BET) method, the specific surface areas (SBET) are calculated to be 259 m2 g−1 for the used RuCu-3/NMCN. The total pore volumes and pore diameter are 0.20 cm3 g−1 and 2–4 nm for the used RuCu-3/NMCN, respectively. Therefore, it could be confirmed that the texture properties of the used catalysts show no significant change. TEM image of the used catalysts is shown in Supplementary Fig. S8a, it could be seen that the particle size of RuCu-3/NMCN remains unchanged, and the metal alloy particles are away from sintering during the reaction. To further prove the component stability of the used catalyst, the bimetallic catalyst after reactions is analyzed by HADDF-STEM and EDS (Supplementary Fig. S8). The HADDF-STEM image (Supplementary Fig. S8b) further confirms the unchanged spherical morphology of the used alloy catalyst and the well dispersion of the metal particles on the NMCN nanospheres. More important, it should be pointed out that no carbon deposition could be evidenced on the catalyst's surface. From the EDS mapping images (Supplementary Fig. S8c-f), the uniform distribution of Cu and Ru could be evidenced. These findings solidly support the component stability of the used RuCu-3/NMCN catalyst. High-resolution XPS scans show the chemical states ofRuCu-3/NMCN after recycling. In the Cu 2p spectra (Fig. S9a), the peaks at around 931.5 and 951.6 eV are referred to Cu0 for Cu 2p3/2 and Cu 2p1/2, respectively. The satellite peaks observed at around 934.7 eV and 955.9 eV indicate the presence of Cu in its oxidation state. Ru on the surface exists in an oxidized state as Ru4+ (the Ru 3p3/2 peak at 463.8 eV and Ru 3p1/2 at 485.6 eV). And the surface layer revealed the existance of metallic Ru0 in Ru 3p spectra (461.7 and 438.8 eV). These results suggest that the Cu and Ru chemical states of RuCu-3/NMCN remain unchanged compared with the fresh catalyst.The results of this experiments enabled a possible thermal catalytic CO2 methanation reaction mechanism over the RuCu-3/NMCN alloy catalysts (Scheme 2 ). CO2 and H2 are adsorbed on the catalysts at the beginning of the reaction. Then, CO2 prefers to form key initial intermediates (CO2 δ−, bicarbonate) on Ru and Cu surface, (Kapiamba et al., 2022; López-Rodríguez et al., 2021) which are then reacted with H2 dissociatively adsorbed on Ru sites to generate the H* species at the RuCu alloy. As generated H* could react with the CO adsorbed on the RuCu alloy, which is stepwise hydrogenated to CH4. The proposed reaction sequence is: (1) C O 2 + H 2 → CO + H 2 O , facilitated by sites at the bimetal surface (2) CO + 2 H 2 → C H 4 , facilitated by Ru sites It is widely acknowledged that the product selectivity can be affected by the binding strength of the generated CO on the surface of the metal nanoparticles. (Kim et al., 2021; Wang et al., 2021b) While weak interactions between CO and metal nanoparticles result in either desorption of CO as the end product or the production of alcohols and aldehydes. Strong interactions between CO and nanoparticles cause C-O bond dissociation, which leads to the formation of CH4. Cu nanoparticles undergo CO2 reduction, resulting in the formation of CO by a non-dissociative C-O bond mechanism. In contrast to Cu, Ru tends to generate CH4 due to C-O bond dissociation followed by C-H bond creation. (Wang et al., 2011; Ciobica et al., 2003; Patra et al., 2020)As for the reaction process, a hypothesized reaction mechanism is proposed. Due to the existence of Ru species, the formate (HCOO*) species will act as a spectator. (Xu et al., 2021; Eckle et al., 2011) The CO2 adsorbed on the RuCu alloy sites is firstly dissociated into CO* and O* to meet the goals of CO2 activation and CO production. In addition, the first step in the CH4 production pathway is the formation of the HCOO* species, which is then hydrogenated in stages to produce HCOOH* and H2COOH*. Due to the dissociation of H2COOH* and the breakage of the C-OH bond, H2CO* and OH* would be generated. Then the C-O bond in H3CO* is subsequently broken and releases H3C* and O*, which ultimately forms CH4 and H2O. (Yan et al., 2018) In our work, it is evidenced that Ru has exceptional CH4 selectivity in thermocatalytic CO2 methanation. (Zhuang and Simakov, 2021) However, the high cost of Ru greatly limits its practical application. In this regard, partially substituting Ru with low price Cu, which could lead to a synergistic effect to hinder the carbon deposition process, turns to be an ideal choice, (Liu et al., 2013) as Cu NPs can be utilized as promoters to enhance the activity of the applied catalysts. (Ndolomingo et al., 2020) Therefore, the optimal RuCu-3/NMCN catalyst shows remarkably enhanced product selectivity with a balanced Ru/Cu ratio.In this work, bimetallic RuCu/NMCN is successfully synthesized for thermal catalytic CO2 methanation. As a result, the bimetallic NPs achieve remarkably enhanced CO2 methanation performance and stability. It is demonstrated that the thermocatalytic activity of RuCu-x/NMCN (x = 1, 2, 3) is substantially higher than that of pure NMCN and Cu/NMCN. Especially for the RuCu-3/NMCN catalyst, the carbon deposition side reaction is suppressed, leading to a good thermal stability at 500 °C. Moreover, at this temperature, the CO2 conversion and CH4 selectivity of RuCu-3/NMCN remain at 52% and 85%, respectively, for at least 1380 min. This work demonstrates the potential for the NMCN-supported bimetallic catalysts to mitigate the anthropogenic greenhouse gas emissions from the fossil fuel combustion and encourage sustainability.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors acknowledge the Australian Research Council (ARC) under the Discovery Projects funding scheme (DP 220102851) and the Sydney Nano Grand Challenge, the University of Sydney.Dedicated to Professor Jianzhong Chen on the occasion of his 70th birthday.Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ccst.2023.100100. Image, application 1

CO2 methanation draws great attention since it could relieve the global climate change while providing important chemical feedstock simultaneously. In this work, we designed a nitrogen-doped mesoporous carbon spheres supported RuCu bimetallic nanoparticles (RuCu/NMCN) for high-performance thermal catalytic CO2 methanation. It is found that, due to its high thermal conductivity, and excellent capacity to store H2 and CO2, NMCN is an ideal supporting material. Moreover, the relative Ru/Cu ratio of the RuCu bimetallic catalysts strongly influences the reaction performance, among which the optimal reaction performance is evidenced with a Ru/Cu ratio of 1/10. More interestingly, it is proved that the presence of Cu is more conducive to inhibiting carbon deposition. At 500 °C, the optimised RuCu/NMCN-3 exhibited the highest catalytic activity for thermocatalytic CO2 methanation (53%) with 88% CH4 selectivity. This work provides insight into the rational application of bimetallic catalyst for CO2 methanation processes.

Currently, the depletion of oil reserves due to the rapid growth of its global consumption as the industry develops intensively and against the background of stricter requirements for the environmental friendliness of technology have contributed to the search for alternative sources of raw materials. One of the sources of raw materials is bioethanol, obtained by processing biomass, which can replace oil in the chemical industry, and is also used in the production of fuel. The use of biomass helps to reduce the "greenhouse effect" by reducing carbon dioxide emissions into the atmosphere (Bridges et al., 2015; Galadima and Muraza, 2015; West et al., 2009). Promising is the further catalytic processing of bioethanol as a feedstock for the production of various valuable chemicals, such as hydrogen (Rossetti et al., 2020), propylene (Xue et al., 2019), ethylene (Dugkhuntod and Wattanakit, 2020; Sun and Wang, 2014), butanol (Dai and Zhang, 2019; Wu et al., 2018), 1,3-butadiene (Zhao et al., 2020), acetaldehyde (Carotenuto et al., 2013), aromatic compounds (Liu et al., 2020; Phung et al., 2015), etc. Among the above products obtained from bioethanol, aromatic hydrocarbons are of great importance in the chemical and petrochemical industries as a component of motor fuels, solvents and intermediate products, and also have a wide range of applications in many other industries (Kianfar, 2019; Kianfar et al., 2018; Lu et al., 2018; Nezam et al., 2021).It should be noted that a large amount of aromatic hydrocarbons is produced mainly during oil refining and catalytic reforming of naphtha (F. Wang et al., 2016; X. Wang et al., 2016; Wang et al., 2021). This process is carried out at high temperatures (up to 800°С) and pressures (10–85 bar), as well as the use of mineral resources, the high cost of the process, and the low selectivity of aromatic hydrocarbons indicate the inefficiency of the process (Rodríguez and Ancheyta, 2011). A non-petroleum method for producing aromatic hydrocarbons from methanol is known in the literature. The main disadvantages of this method include the rapid deactivation of the catalyst and the need for sequential purification of the target products (Fu et al., 2021; Li et al., 2014). In addition, in the main products of this process, alkanes and alkenes are formed first, and not light aromatic hydrocarbons. Methanol is a toxic substance.Thus, one of the effective sustainable methods for obtaining aromatic hydrocarbons is the catalytic conversion of ethanol. Many advantages, such as low cost of raw materials, high yield of target products, and reduced dependence on oil, make this method the most promising way to obtain aromatic hydrocarbons from renewable sources (Liu et al., 2016; Xiang et al., 2022). Zeolites HZSM-5 and ZSM-5 are mainly active in the production of aromatic light hydrocarbons due to their large surface area, adsorption capacity, and controlled acid properties (Migliori et al., 2017; Wannapakdee et al., 2019; Zeng et al., 2022; Said et al., 2020; Zhang et al., 2011; Zhang et al., 2013; Ramesh et al., 2010). However, the strong acidic properties of these zeolites contribute to the occurrence of undesirable reactions, limiting the selectivity towards aromatic compounds, and the catalyst undergoes coke formation (Almutairi et al., 2012; Wang et al., 2020). To eliminate these shortcomings, various metals/metal oxides were included in the composition of zeolites, such as Ni (Liu et al., 2020; Niziolek et al., 2016), Fe (Calsavara et al., 2008), Zn-Ga (Hodala et al., 2016), Mo (Barthos et al., 2006), P (Lu and Liu, 2011), Ag (Hsieh et al., 2017), Ce (Bi et al., 2011), etc. Among them, the zinc-based catalyst has been intensively studied due to its relatively low cost, availability, and low toxicity. The best results in the process of ethanol conversion are provided by mixed systems for the preparation of characteristic acid-base catalysts (Vlasenko et al., 2019.; Cheng et al., 2014.; Chistyakov et al., 2014). Compared to the ZnO/ZSM-5 monometallic catalyst, the modified catalysts (Zn-ZrO2/ZSM-5) show resistance to carbonization and increased catalytic activity (Ohayon Dahan et al., 2021).In a number of works (Ramesh et al., 2009; Song et al., 2010), it is reported that the addition of phosphorus to the ZSM-5 increases the catalytic stability of the catalyst and its resistance to coke formation.The main disadvantage of ZSM-5 zeolite is its high cost; therefore, in our work, cheaper molecular sieves of KA modified with zinc and phosphorus oxides were studied as a catalyst. In terms of pore size, KA is close to ZSM-5 zeolite, the pore size of KA is approximately 4–5 Ǻ; for ZSM-5 zeolite, the pore size is 5.4–5.6 Ǻ (Baerlocher et al., 2007). An analysis of the literature shows that KA as a support for zinc catalysts has not been previously studied.Thus, the purpose of this work is to study the activity of catalysts based on KA modified with zinc and phosphorus oxides and the effect of the method of catalyst preparation on its activity in the conversion of ethanol to aromatic hydrocarbons.The catalysts were synthesized by capillary impregnation of the support and by the "solution combustion" method. The choice of synthesis methods as capillary impregnation and "solution combustion" is justified by the fact that these methods have a number of advantages compared to other methods (sol-gel, deep impregnation, etc.): relative simplicity, less hazardous waste and more efficient use of low-percentage active component, there is no loss of the impregnating solution, which is especially important in the manufacture of expensive catalysts (Yergaziyeva et al., 2021).Materials used in the preparation of catalysts and catalytic reaction: molecular sieve KA (Shanghai JiuZhou Chemicals Co. Ltd, China), Zn(NO3)2 (Sigma Aldrich, USA 98%), phosphoric acid H3PO4 (GOST 6552–80, 99%), helium (LLP «IkhsanTechnoGas», 99%), argon (LLP «IkhsanTechnoGas», 99%), dispersing agent: urea (Sigma Aldrich, USA 99,5%), distilled water. ZnО/KA was obtained by the method of capillary impregnation according to the moisture capacity of the KA carrier to aqueous solutions of Zn(NO3)2·6H2O (GOST TU 5106–77). The content of zinc oxide in the catalyst was 1 wt.%.The ZnО-P2О5/KA (CI) catalyst was also obtained by capillary impregnation according to the moisture capacity of the KA carrier with an aqueous solution of Zn(NO3)2·6H2O salt (GOST 5106–77) and H3PO4 (GOST 6552–80, 99%).The catalyst ZnO-P2O5/KA (SC) was obtained by the "solution combustion" method with the addition of a dispersing agent-urea, ratio of ZnO/urea = 1:0.5. The content of oxides in the bimetallic samples was 1 wt.% ZnO and 1 wt.% P2O5.Heat treatment of all samples was carried out in air at 300 оС for 2 h, then at 500 оС for 3 h.The activity of synthesized catalysts in ethanol conversion was tested in a stainless steel reactor with an internal diameter of 1.7 cm in a flow mode at a temperature of 200–400 °C, pressure P = 0.1 MPa, with a volumetric ethanol flow rate of 1 h −1. A catalyst in the amount of 2 ml was placed in the reactor between thin layers of glass wool. Raw material - ethanol (95%) was supplied with the help of a high-pressure pump. The reaction products were identified online on a CHROMOS GCH-1000 (GCH-1000 LLC "Chromos" Russia) device.Separation of the components was carried out on two columns (length 2 m, inner diameter 3 mm) filled with NaX zeolite and porapak-T, carrier gas - helium and argon. Ethanol conversion was calculated according to the following equation: (1) Ethano l conversion ( % ) = Ethano l in − Ethano l out Ethano l in · 100 % The physicochemical characteristics of the catalysts were studied by SEM, TEM, TPD-NH3, and TPR-H2.The surface morphology of the catalysts was studied by scanning and transmission electron microscopy (Quanta 200i 3D, FEI Company, USA). SEM micrographs of the catalysts (ZnО/KA, ZnО-P2О5/KA (CI) and ZnО-P2О5/KA (SC)) are shown in Fig. 1 .The results of SEM analysis showed that the catalysts contained round particles with sizes from 300 to 2500 nm. It can be seen from the images that the preparation of catalysts by means of capillary impregnation and "solution-combustion" does not show any significant morphological difference, having the same particle size distribution. Similar results were observed in (Chen et al., 2015). SEM images show that the zeolite structure did not collapse during synthesis and morphology of zeolite crystals. The ZnO-P2O5/KA (CI) and ZnO-P2O5/KA (SC) catalysts were also studied by TEM (FEI Tecnai) (Fig. 2 ). The results of TEM images show that there is a difference in the distribution of active phases on the catalysts.On the ZnO-P2O5/КА (SC) catalyst, the particle distribution is more uniform than on the ZnO-P2O5/КА (CI) catalyst. The ZnO-P2O5/КА (CI) catalyst contains nanoparticles ranging in size from 51 to 30 nm (Fig. 2a). Whereas the preparation of 1 wt.% ZnO-1 wt.% P2O5/KA catalyst by the "solution combustion" method (Fig. 2b) leads to a decrease in the size of the catalyst particles, nanoparticles with sizes from 2 nm are observed. An increase in the dispersion of particles of the catalyst ZnO-P2O5/KA (SC) is also confirmed by the TPR-H2 (USGA-101, Russia) method.From the results of TPR-H2 shown in Fig. 3 , it can be seen that the ZnO-P2O5/КА (CI) and ZnO-P2O5/KA (SC) catalysts have 3–4 reduction zones in the range of 250–850 °С.On the TPR profile of the ZnO-P2O5/KA (SC) catalyst, three reduction zones are observed with T1 max = 606 оС, the amount of absorbed hydrogen A1 = 47 μmol/g, T2 max = 661 оС, A2 = 62 μmol/g, T3 max = 770 оС, A3 = 30 µmol/g. On the TPR-H2 profile of the ZnO-P2O5/KA (CI) catalyst, there are 4 reduction zones with T1 max = 500 оС, A1 = 32 µmol/g, T2 max = 579 оС, A2 = 114 µmol/g, T3 max = 720 оС, A3 = 328 µmol/g and T4 max = 760 оС, A4 = 100 µmol/g. Peaks in the region of 500–720 °C refer to the reduction of zinc oxide. It is known that zinc oxide has several reduction zones in the region of 100–1000 °C (Ebrahimi et al., 2022). The presence of several zones is associated with zinc oxide with different dispersity or with different forces of interaction with the carrier. The peaks T4 max = 760 оС and T3 max = 770 оС can be attributed to the restoration of the P–O bond (Huang et al., 2021).The results obtained show that the preparation method affects the reduction characteristics of the catalyst. The preparation of the catalyst by the “solution burning” method leads to a decrease in the amount of hydrogen consumed for reduction, as well as in the reduction temperature of zinc oxide from 720 to 661 °C, which indicates an increase in the dispersion of catalyst particles. (Ganiyu et al., al.,2017; Van et al., 1990). Table 1 .Acidity is one of the important factors in the catalytic performance of a zinc-based catalyst. NH3-TPD (USGA-101, Russia) analysis was performed to determine the acidity of the ZnO-P2O5/КА (SC) and ZnO-P2O5/КА (CI) catalysts. The TPD profiles of the carrier KA and catalysts are shown in Fig. 4 , TPD data are presented in table 2 .According to the literature data (Chen et al., 2015; Niu et al., 2014), ammonia desorption can be divided into three temperature ranges: low temperature (120–200 °C), medium temperature (200–300 °C), and high temperature (300 °C<), corresponding to weak, medium, and strong acid sites.It can be seen from the figure that the KA carrier has acid sites, and ammonia desorption occurs in two temperature ranges. A peak in the temperature range of 50–365 оС with a maximum of Т1 max = 173 оС may indicate the presence of weak acid centers, and a peak in the range of 585–780 оС with Т2 max = 627 оС may indicate the presence of strong acid centers. The amount of desorbed ammonia is 171 and 95 µmol/g, respectively (Table 2).With the application of zinc oxide to the KA, the intensity of the peaks increases, a new acid center of medium strength appears with the participation of zinc oxide (Т2 max = 223 оС, the amount of desorbed NH3 is 558 μmol/g). The amount of desorbed ammonia from strong acid sites increases from 95 to 292 µmol/g compared to the initial KA (table 2). Modification of the ZnO/KA catalyst with phosphorus leads to an increase in the intensity of ammonia peaks related to weak and strong acid sites. The amount of ammonia desorbed from weak acid sites increases from 136 to 550 µmol/g, and from strong acid sites from 292 to 351 µmol/g, respectively (Table 2). The preparation of ZnO-P2O5/KA by the "solution combustion" method leads to an increase in the total acidity of the catalyst. Compared to ZnO-P2O5/KA (CI), the amount of ammonia desorbed from weak acid sites increases from 550 to 783 µmol/g, from strong acid sites from 351 to 423 µmol/g.The results of ammonia TPD showed that zinc-phosphorus-containing catalysts ZnO-P2O5/KA (CI) and ZnO-P2O5/KA (SC) synthesized by various methods have both weak and strong acid sites.It is known that oligomerization strongly depends on Brønsted acid sites (Zhang et al., 2022). According to the literature (Kamyar et al., 2020), the low-temperature peak (120–200 °C) is attributed to weakly adsorbed NH3 on weak Lewis acid sites, the medium-temperature peak (200–300 °C) to ammonia adsorbed on strong Lewis acid sites, and the high-temperature peak (300 °C<) of NH3 desorbed from the Bronsted acid site. Therefore, an increase in Bronsted acid sites is favorable for further ethylene oligomerization into aromatic hydrocarbons. The highest concentration (23 vol.%) of aromatic hydrocarbons is observed on the ZnO-P2O5/KA (SC) catalyst. The catalyst prepared by the "solution combustion" method has a higher acidity than the catalyst prepared by the impregnation of the carrier in terms of moisture capacity.The thermal catalytic conversion of ethanol on a KA and a ZnO-P2O5/KA catalyst includes the parallel formation of gaseous products CH4, CO2, CO, H2, ethylene, and aromatic hydrocarbons through decomposition, dehydration of ethanol, and oligomerization of ethylene, respectively (Tretyakov et al., 2010; Dosumov et al., 2014) (Scheme 1 ).The application of zinc oxide leads to an increase in the decomposition products of ethanol (CH4, CO2, CO, H2), possibly due to an increase in the average acid sites. Modification of ZnO/КA with phosphorus oxide leads to a decrease in the concentration of methane decomposition products and an increase in ethylene, a dehydration product (Table 3 ).The concentration of ethylene in the reaction products increases symbatically with an increase in the weak acid sites of the catalysts. These data are confirmed by the data in (Kamsuwan et al., 2020; Xin et al., 2014), where it is indicated that catalysts with increased weak acidity provide a higher yield of ethylene. The highest concentration of aromatic hydrocarbons is observed on ZnO-P2O5/KA (SC), which has a high total acidity. For this ZnO-P2O5/KA (SC) catalyst, the highest ethanol conversion of 85% is observed. Therefore, an increase in acid sites is favorable for the adsorption of ethanol and its activation.In this work, we studied the activity of catalysts based on KA modified with zinc and phosphorus oxides and the effect of the method of catalyst preparation on its activity in the conversion of ethanol to aromatic hydrocarbons.It has been shown for the first time that KA-based catalysts can be used to produce aromatic hydrocarbons from ethanol. According to SEM, TEM, and TPR-H2 data, the method of preparation affects the dispersity of catalyst particles and its reduction characteristics. The preparation of the ZnO-P2O5/KA catalyst by the "solution combustion" method, in comparison with capillary impregnation, leads to an increase in the dispersion of particles. In the composition of the ZnO-P2O5/KA (SC) catalyst, nanoparticles with sizes from 2 nm are observed. The results of TPD-ammonia showed that catalysts based on KA modified with zinc oxides have weak, medium and strong acid sites. Modification of ZnO-P2O5/КA with phosphorus oxide leads to an increase in the number of weak and strong acid sites. The highest ethanol conversion of 85% and the concentration of aromatic hydrocarbons (23 vol.%) was obtained for the ZnO-P2O5/KA (SC) catalyst. According to physicochemical methods, the particle size of the ZnO-P2O5/KA (SC) catalyst decreases to 2 nm, their uniform distribution on the catalyst surface is observed, the total number of acid sites reaches up to 1206 μmol/g, which positively affects the activity of the catalyst in conversion ethanol to aromatic hydrocarbons.The results obtained indicate that the cheaper molecular sieve KA can be used as a carrier for zinc-containing catalysts for the production of aromatic hydrocarbons from ethanol as an alternative to the expensive ZSM-5. The preparation method can control the acidity of the catalyst and the dispersion of its particles.The authors declare no conflict of interests.This research has been funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan (Grant No. AP08855936).

The activity of catalysts based on KA modified with zinc and phosphorus oxides and the effect of the method of catalyst preparation on its activity in the non-oxidative thermocatalytic conversion of ethanol to aromatic hydrocarbons was studied in this work. The ZnO-P2O5/KA catalyst was prepared by capillary impregnation and solution-combustion methods and characterized by SEM, TEM, TPD-NH3, and TPR-H2. The results showed that the method of preparation affects both the reducibility, dispersity, as well as the acidity of the catalyst. The preparation of the ZnO-P2O5/КА catalyst by the "solution combustion" method, in comparison with capillary impregnation, leads to an increase in the dispersion of particles. In the composition of the ZnO-P2O5/КА (SC) catalyst, nanoparticles with sizes from 2 nm are observed. The results of TPD-ammonia showed that catalysts based on KA modified with zinc oxides have weak and strong Lewis acid sites, and Bronsted acid sites. Modification of ZnO-P2O5/КА with phosphorus oxide leads to an increase in the number of weak and strong acid sites. The highest ethanol conversion (85%) and the concentration of aromatic hydrocarbons (23 vol.%) was obtained for the ZnO-P2O5/КА (SC) catalyst, which has the highest acidity of 1206 µmol/g.

No data was used for the research described in the article.As per a report published by the Joint Monitoring Program of the World Health Organization and the United Nations International Children’s Emergency Fund, 1 out of 3 people worldwide lack clean sources of drinking water (Progress on household drinking water, sanitation and hygiene I. 2000-2017, 2019). Furthermore, access to clean drinking water is expected to get consistently more difficult as pollutants continue to be released into water (Tijani et al., 2013). For the past few decades, emerging contaminants have caught the attention of scientists and government organizations alike (Montes-Grajales et al., 2017a; Tijani et al., 2013, 2016). According to the United States Environmental Protection Agency (US EPA), these emerging micropollutants (MPs) are compounds that are, at present, unregulated, and the environmental impacts of which are not very well understood (Montes-Grajales et al., 2017a). Similarly, the US Geological Society defines MPs as chemicals, microorganisms, or metabolites that may cause adverse effects to the environment or human health, but the release of which into the environment is not monitored (Tijani et al., 2016).The environmental impact of MPs was first described in Rachel Carson’s 1962 book The Silent Spring (Tijani et al., 2016). While MPs have existed in the environment for decades, they have only recently been able to be detected (Richardson and Kimura, 2017). This is due to the development of highly sensitive analytical techniques and on-site sensing devices that allow quantification of MPs, which are usually present in concentrations in the order of ng L−1 to μ g L−1 (Richardson and Kimura, 2017; Vodyanitskii and Yakovlev, 2016). Even at these extremely low concentrations, MPs can pose a significant threat to aquatic life and human health. Hazards include bioaccumulation (Vodyanitskii and Yakovlev, 2016), toxicity to humans and aquatic life, increased risk of cancer, increase in antibiotic-resistant bacteria, reproductive health problems (Gogoi et al., 2018a), brain damage, development of cardiovascular disease, liver damage, pulmonary defects, and feminization of fish and other aquatic life (Tijani et al., 2013).Recently, there has been great interest in using MOF composite-based catalysts for degradation of MPs in wastewater. Like the aforementioned composites, activity of conventional catalysts used for this application shows significant increase when they are paired with MOFs (Wang et al., 2020c). This review describes the different MP degradation techniques currently under study, with special focus on MOF-catalyst composites. It also discusses synthesis methods of MOF-catalyst composites and the photodegradation mechanism of MPs using these materials. Finally, current trends in research, the associated cost, and environmental considerations of these composites are presented.The remainder of the paper is organized as follows. In Section 2, the different types of micropollutants ranging from pharmaceuticals and personal care products to dyes are examined along with their global presence. In Section 3, the commonly used degradation techniques are discussed. Next, in Section 4, MOF-composite-based catalysts are introduced along with their potential applications. We also examine the synthesis methods of MOF-composite-based catalysts and discuss their associated photodegradation mechanism. We then present the current research on the use of MOF-composite-based catalysts for MPs degradation, their associated costs along with the health and environmental considerations. Lastly, Section 5 concludes the paper and proposes potential topics for future research.Anthropogenic activities are the main cause of MPs in potable water (Noguera-Oviedo and Aga, 2016). Most of the MPs come from municipal wastewater (Gogoi et al., 2018b; Noguera-Oviedo and Aga, 2016), while industrial waste (Tkaczyk et al., 2020), mining (Tijani et al., 2016), aquaculture (Noguera-Oviedo and Aga, 2016), and agricultural practices (Richardson and Kimura, 2017; Tijani et al., 2013) also contribute to MPs in drinking water. Given that the harmful effects of MPs, being recently identified and studied, current wastewater treatment practices are ineffective towards MPs (Luo et al., 2014). Therefore, MPs pass through wastewater treatment plants (WWTPs) unaltered and are released into the environment or drinking water supplies (Luo et al., 2014).Before MP degradation methods can be developed, the nature of MPs present in WWTP effluents must be identified. The most common types of MPs are human and veterinary pharmaceuticals, personal care products, endocrine disruptor compounds (Bolong et al., 2009; Gogoi et al., 2018b; Luo et al., 2014; Montes-Grajales et al., 2017a; Tijani et al., 2013), and dyes (Tkaczyk et al., 2020). Furthermore, these MPs can react with each other in wastewater, or transform during wastewater treatment process, to give transformation products that may be more harmful than the original materials (Richardson and Kimura, 2017). As such, it is imperative to understand the nature and activities of these MPs, and develop and implement efficient techniques to remove them from WWTP effluents. Table 1 lists the maximum allowable concentrations of some MPs in surface waters in addition to their structures and applications. We notice that these concentrations are significantly low. For example, the maximum allowable concentration of ibuprofen is 0.011 μ g L−1 and Atenolol is 150 μ g L−1. This demonstrates the need for early detection and appropriate treatment of MPs even at very low concentrations. Pharmaceuticals are biologically active compounds that can enter the ecosystem in a number of ways, including disposal of unwanted medicines and wastes from pharmaceutical industries, research labs, hospitals, and animal husbandry (Tiwari et al., 2017). Due to their aversion to degradation, pharmaceuticals can accumulate in the environment and have the potential to cause long-term detrimental effects (Tiwari et al., 2017). At the same time, because they are designed to be effective at very low concentrations, they can even induce toxicity in aquatic environments at concentrations in the order of ng L−1 (He et al., 2016). For example, the concentration of estrogen, which is known to cause the expression of feminine traits in male fish, was found to be ranging from 0.3 to 12.6 ng L−1 in municipal WWTP effluents (Tiwari et al., 2017). Chronic exposure to estrogen has toxic effects on fish populations (Tiwari et al., 2017). Other drugs such as antidepressants which are known to increase serotonin levels were found to be present in the aquatic environment in concentrations ranging from 0.3 μ g L−1 to 100,000 μ g L−1 (Brodin et al., 2014). This in turn was found to reduce the aggression levels in several fish species such as the coral reef fish and the Siamese fighting fish making them vulnerable to attacks from other species. They also cause disorders in tadpole development (Tiwari et al., 2017). Moreover, antiepileptic drugs were found to be present in wastewaters at a concentration of 6100 μ g L−1 (Argaluza et al., 2021).Further, a study revealed that consumption of diclofenac, an anti-inflammatory drug, caused renal failure in vultures, and resulted in a decrease in population (Tiwari et al., 2017). Moreover, large amounts of antibiotics consumed pass through the body unmetabolized and enter municipal wastewater. For instance, up to 62% of the administered dose of ciprofloxacin, a broad-spectrum antibiotic, is excreted, which can lower the number of microbes used in WWTPs and decrease efficiency (Girardi et al., 2011). This could also give rise to antibiotic resistance in bacteria populations (Girardi et al., 2011).Nevertheless, some metabolites of drugs can be more toxic than the starting compounds, therefore, these must be identified and regulated as well (Tiwari et al., 2017). Some techniques, like UV irradiation and ozonation, can be used to degrade pharmaceuticals in WWTP, but their applications are limited by high capital and running costs (He et al., 2016). Hence, there is a need for more cost-effective and efficient ways to remove pharmaceuticals from wastewater as their presence is harmful to aquatic life and the overall environment.The World Health Organization and the European Union Commission define Endocrine Disruptors (EDs) as external chemicals that cause disruptions in an organism’s endocrine system, and as a result, have negative impacts on the organism or its progeny (Kuckelkorn et al., 2018; Varjani and Sudha, 2020). Several studies, conducted all over the world, have found EDs in drinking water, which is a major source of exposure for humans (Kuckelkorn et al., 2018). EDs are known to be carcinogens and cause a number of endocrine-related health issues, including, pituitary and thyroid gland malfunction at concentrations of less than 1 μ g L−1 (Kuckelkorn et al., 2018; Petrovic et al., 2002; Varjani and Sudha, 2020). For instance, wastewaters in Greece were found to contain different EDs with concentrations ranging from 64.7 to 15,320 ng L−1 (Pironti et al., 2021). Whereas the EDs found present in the drinking water in Serbia were at concentrations ranging from 0.4 to 6.6 ng L−1 (Pironti et al., 2021).EDs act by either imitating natural hormones produced by the body in order to bring about physiological changes, or by competitively binding to hormone receptors and inhibiting natural bodily functions (Varjani and Sudha, 2020). Furthermore, it is difficult to predict which synthetic chemicals will exhibit endocrine-disrupting effects (Varjani and Sudha, 2020), as EDs comprise numerous categories of chemical, from pesticides and pharmaceuticals to plastics (Kedzierski et al., 2018; Petrovic et al., 2002; Pironti et al., 2021; Varjani and Sudha, 2020).Plastics, in particular, can be dangerous to aquatic life and human health as, upon breakdown in seawater, they do not only tend to release estrogenic EDs, but also microplastics (Kedzierski et al., 2018). These microplastics ( < 5 mm in size) can adsorb EDs like polycyclic aromatic hydrocarbons (PAHs) and can transport them around the world through ocean currents (Kedzierski et al., 2018). PAHs are known to desorb faster in the guts of aquatic animals than in seawater and can easily enter the food chain (Kedzierski et al., 2018).Personal Care Products (PCPs) include toiletries, cosmetics, perfumes etc., and they typically get released into the ecosystem through WWTPs (Montes-Grajales et al., 2017a). This constitutes a global problem. A study, conducted in several countries, showed evidence of the presence of PCPs in municipal wastewater, soil, and drinking water supplies with Spain and the United States reporting the largest numbers of PCPs with 42 and 36 compounds, respectively (Hao et al., 2019; Montes-Grajales et al., 2017b). Table 2 summarizes the maximum and minimum concentrations of different PCPs found in the surface waters of various countries. As can be seen Spain reports the presence of most of the PCPs mentioned. Moreover, it is clear that even the maximum concentration of each PCP is still considered to be low as it is in a nanoscale. While several PCPs are short-lived, their consistent release into the environment can pose serious risks to human and aquatic life (Hao et al., 2019), due to their bioactive nature and endocrine disrupting effects (Montes-Grajales et al., 2017a). For instance, compounds found in sunscreens that filter UV radiation are known to act similar to estrogen, and those are some of the most common PCP contaminants found in water (Gogoi et al., 2018b). This suggests that even at low concentrations PCPs can pose serious risks to human health and the environment. Moreover, PCPs such as galaxolide (HHCB) were found in wastewater effluents in 10 countries with concentrations ranging from 0.14 to 108,000 ng L−1 (Montes-Grajales et al., 2017b). HHCBs at high concentrations tend to pose toxicological risks to green algae, fish and daphnids (Montes-Grajales et al., 2017b). Tonalide (AHTN) is another PCP found in the wastewater effluents of 16 countries at concentrations ranging from 0.05 to 7555 ng L−1 (Montes-Grajales et al., 2017b). It also poses health risks to aquatic life. Dyes are complex aromatic compounds that are used in numerous industries like, paint manufacturing, printing paper, textiles, cosmetics, pharmaceutical, food, etc. (Chiong et al., 2016). Since natural dyes tend to degrade quickly, synthetic dyes have replaced them in almost all instances (Chiong et al., 2016). Of these, azo dyes (Chiong et al., 2016) and sulfur dyes (Nguyen et al., 2016) are among the most common while sulfur dyes are also very cheap and are thus used extensively (Nguyen et al., 2016). In addition, Table 3 mentions the types of dyes along with their industrial applications. It is obvious that dyes are mainly used in textile industry as shown in Table 3. The wide use of dyes; however, is associated with environmental problems. The textile industry is a major source of dyes released into the environment. In fact, it is responsible for 20% of all industrial water pollution (Tkaczyk et al., 2020). Not only are synthetic dyes resistant to breakdown in WWTPs, but they are also known to be carcinogens (Rani et al., 2017), as are their transformation products (Chiong et al., 2016). Furthermore, dyes change the color of water, thereby, affecting the amount of sunlight that can penetrate the water’s surface (Chiong et al., 2016). This reduces the photosynthetic ability of underwater flora, decreasing the amount of oxygen dissolved in water, which adversely affects aquatic life (Chiong et al., 2016). Fig. 1 depicts the way in which MPs may enter drinking water supplies from wastewater. Occurrence of emerging MPs in WWTP effluents is a global problem. Though, the concentrations and variety of MPs are specific to each place and dependent on the annual consumption trends of MPs by the population. A review study on the presence of MPs in 14 countries, from Europe, North America, and Far East Asia, reported that the most common source of MPs in potable water is treated sewage water that is released into surface water from WWTPs (Jiang et al., 2013). Furthermore, a study conducted on the Yangtze River Estuary in China identified high concentrations of pharmaceutical compounds in the area where a sewage treatment plant discharge treated water, indicating inefficiency of the plant at removing pharmaceuticals (Yang et al., 2011). Another study examined the presence of EDs in Anzali Wetland in Iran, and found high concentrations of 4-nonylphenol, octylphenol, and bisphenol A (BPA) in the area (Mortazavi et al., 2012). The investigators attributed the presence of these EDs to industrial waste discharge, and WWTP effluents (Mortazavi et al., 2012). Similarly, a survey conducted in Canada by the Ontario Ministry of the Environment over 16 months concluded that several of the MPs considered, including pharmaceuticals and EDs, were present in drinking water, with carbamazepine, gemfibrozil, ibuprofen, and BPA being detected most frequently (Kleywegt et al., 2011). Furthermore, a comprehensive review of emerging MPs in India reported extremely high concentrations of pharmaceuticals in wastewater. The concentrations of fluconazole (a fungicide) and ciprofloxacin were noted to be 236,950 μ g L−1 and 31,000 μ g L−1, respectively (Philip et al., 2018). In addition, samples of drinking water from Indian rivers contained antibiotic resistant genes (Philip et al., 2018). According to the aforementioned study, inefficiency of sewage treatment plants for processing pharmaceutical waste was a major reason for the high concentrations of pharmaceuticals in drinking water (Philip et al., 2018).Studies analyzing the concentrations of MPs in WWTP influents and effluents in two different regions of Spain noted the inefficiency of WWTPs for removing MPs after observing a slight difference in the concentrations of MPs in and out of WWTPs (Fernández-López et al., 2016; Rodil et al., 2012). Another study, which focused on surface waters near agricultural lands in North-East Denmark found pharmaceuticals, PCPs, and EDs with concentrations of up to 1476 ng/L (Matamoros et al., 2012). Similar to the aforementioned study, concentrations of MPs in Central Greece (Papageorgiou et al., 2016) and Berlin, Germany (Pal et al., 2014) were also found to be high due to the inefficiency of WWTPs to remove them.In contrast, low, yet significant (ng L−1), concentrations of MPs (pharmaceutical and PCPs) were detected in drinking water in Milan (Riva et al., 2018). As per the study, the low concentrations are the result of drinking water being sourced from groundwater present deep within the earth (Riva et al., 2018). Similar to Milan, drinking water in Singapore is largely protected from MPs, as WWTP effluents are not discharged into surface water (Xu et al., 2011; You et al., 2015). Even so, MPs have been detected in urban surface waters in Singapore, albeit in low concentrations (Xu et al., 2011; You et al., 2015). Nevertheless, You et al. found that BPA concentration exceeded its Predicted No-Effect Concentration (You et al., 2015). Moreover, it is clear that current treatment plants are not efficient in the removal of certain micropollutants such as pharmaceuticals, EDs, etc. as seen in Fig. 2 (Joseph et al., 2019). This is mostly due to low concentrations which makes this treatment a low priority. Another reason could be the sudden emergence of such micropollutants. Therefore, conventional treatment plants are not equipped to treat and remove the micropollutants. However, as is evident, MPs are ubiquitous. As such, there is an urgent need for regulations on the concentrations of MPs that can be discharged into surface water, as well as, for development of techniques to efficiently remove MPs in WWTPs. For this reason, the EU Water Framework Directive has recently updated its Watch List, which already contained some emerging MPs, to include several pharmaceutical compounds (European Commission, 2020). Additionally, the US EPA added various pharmaceutical compounds and EDs to its Drinking Water Contaminant Candidate List 4, published in 2016 (Drinking Water Contaminants Candidate List 4, 2016). While these lists do not pose legal ramifications for release of MPs into surface waters, they help to emphasize the importance of controlling these substances in WWTP effluents.As demonstrated previously, current WWTPs lack the ability to remove emerging MPs efficiently from wastewater. Hence, new techniques are under development that can be incorporated into the wastewater treatment process in order to improve WWTPs’ removal efficiency. The main techniques currently under study are discussed in the subsequent discussion. The efficiencies of some MP removal techniques applied in WWTPs are summarized in Table 4. It can be observed that the efficiency of the removal technique varies depending on the micropollutant. For example, in the removal of Ciprofloxacin, membrane bioreactor has a removal efficiency of 92.1% whereas primary and secondary activated sludge treatment has an efficiency of 42.4%. Because there are so many different types of MPs that need to be considered, the removal method that is applicable to a broad range of MPs would be the most suitable. Moreover, the removal technique must not require high capital and operating costs and needs to be easily integrated into current WWTPs. Furthermore, health and safety concerns must be considered before employing these techniques. Each of the techniques described in this review will be evaluated based on the aforementioned parameters.Membranes selectively restrict the flow of chemical species, and in doing so, they filter out unwanted compounds. Here, filtration is based on: (1) the pore size of the membrane, (2) the adsorption onto the surface of the membrane, or (3) the repulsion from a charged membrane (Luo et al., 2014; Silva et al., 2017). Because MPs are generally much smaller than conventional membrane pore sizes, they cannot be filtered by currently available techniques of microfiltration and ultrafiltration (Luo et al., 2014). This is because the pore size of the membrane is much bigger than the size of the MPs. Furthermore, the diversity in physicochemical properties of MPs does not allow for a single type of membrane to remove multiple MPs. For this reason, there has lately been much emphasis on combining membrane technology with other MP removal techniques in an integrated system, which, reportedly, are more efficient than the individual removal methods (Silva et al., 2017). Two of these systems are discussed below.Membrane bioreactors (MBRs) have gained traction as alternatives to conventional activated sludge treatment over the last couple of decades, with plants set up in China, the USA, and the EU (Park et al., 2017). MBRs can be used to remove various MPs but are most widely employed for pharmaceuticals and PCPs. They act as physical barriers for MPs, as well as sites for degradation of MPs through photo transformation or biodegradation (Besha et al., 2017; Goswami et al., 2018). The main advantages of MBRs over other MP removal methods are their capability to remove a wide variety of MPs from wastewater more efficiently than other biological treatment techniques and their scalability which enables them to be adapted to the size of the WWTP (Rodriguez-Narvaez et al., 2017). The ability of MBRs to degrade the microplastics makes it much easier for removal and can even reduce the toxicity of the MPs. However, it has been reported that MBRs are not well-suited for eliminating MPs that exhibit low biodegradability (Rodriguez-Narvaez et al., 2017), possess high degree of branching or saturation, and contain sulfate and halogen groups (Bui et al., 2016). For instance, in addition, the high energy requirement of MBR drives up its operating cost, which is much higher than the cost of existing wastewater treatment technologies (Besha et al., 2017; Goswami et al., 2018), even though the cost of labor associated with MBR is low due to the process being highly automated (Besha et al., 2017). Therefore, in using MBR there seems to be a tradeoff between efficiency and operating cost.As noted previously, MBRs are not adept at removing chlorinated MPs due to their low biodegradability. Therefore, other techniques are needed to eliminate these as chlorinated MPs are known to be highly toxic and have strong likelihood of environmental persistence and bioaccumulation (Nieto-Sandoval et al., 2019). For such species, degradation is generally achieved through oxidation using ozone or other strong oxidizing species, like hydroxyl ( • OH) or sulfate radicals (Lee et al., 2019). Due to the highly reactive nature of the catalysts being involved, it is important to use materials possessing high chemical and physical stability to construct the membranes, instead of the usual polymers (Lee et al., 2019; Wang et al., 2020a). For this reason, ceramic membranes are preferred as catalyst support, as they are able to withstand high mechanical strain and oxidation (Lee et al., 2019), which allows them to have a long lifetime (Wang et al., 2020a). Additionally, due to the catalysts embedded within its pores, CCMs are less susceptible to membrane fouling (Lee et al., 2019). This makes CCMs particularly advantageous over MBRs, as the latter process has a very high degree of membrane fouling especially in the presence of highly reactive species such as the hydroxyl radical. In fact, about 85% of the total energy required for MBR is used to reduce fouling (Bui et al., 2016).Despite the tremendous advancement in the membrane filtration technology for water reuse, yet the commercially available state-of-the-art membrane technologies including polyamide reverse osmosis (RO) and nanofiltration (NF) membranes suffer a critical deficit due to their insufficient rejection (below 50%) of some toxic organic micropollutants (OMPs) (Guo et al., 2022) Consequently, several studies focused on tailoring and modifying the chemistry and structure of polyamide membranes for enhanced removal and rejection of OMPs. Various parameters have been reported to influence the removal of OMPs such as operating conditions, membrane fouling, and characteristics of OMPs, media/solute and membrane (Ojajuni et al., 2015; Wu et al., 2022a,b). The most important factor in achieving an optimal removal/rejection efficiency of OMPs is through member selectivity enhancement which could be attained by one of these techniques: (1) surface modification, (2) membrane nanoarchitecture, and (3) alternative membrane chemistry. Membrane surface plays a significant role in the rejection of OMPs, therefore membrane surface modification through coating and grafting is among one of the most promising techniques to enhance OMPs rejection. Huang et al. (2021) investigated the use of hydrophilic polydopamine (PDA) as a model coating of NF90 membrane for the transmission of 34 OMPs. The PDA-coated NF90 membrane has shown a reduced transmission of MOPs up to > 70%. Another study by Zhu et al. (2022) demonstrated the in situ grafting of ferric ion and tannic networks onto the polyamide membrane. Furthermore, the grafting layer was implemented into commercial NF 270 membrane. It was found that the proposed grafted membrane has achieved much superior chlorine resistance (ratio of salt rejection decline 7.4%) than pristine membrane (ratio of salt rejection decline 26.9%). In addition, the grafted membrane has shown enhanced hydrophilicity, smaller pores size and decreased negative charge. Compared to the modification performed on the surface of the membrane, controlling the interfacial polyamide (IP) reaction conditions could directly control the properties of the membrane nanoarchitecture and thus affect its structural and physiochemical properties including roughness, pore size and distribution, and interior voids/channels (Guo et al., 2022; Liu et al., 2022). For example, vaporization of the organic solvent during the exothermic interfacial polymerization process has contributed to the formation of nanovoids which in turn let to large size polyamide thin film voids and higher membrane water permeability (Peng et al., 2021). Moreover, the addition of secondary monomers such as zwitterions, bipiperidine (BP), 3,5-diaminobenzoic acid (BA), or melamine could also influence the IP reaction and enhance the OMPs rejections. For example, Guo et al. (2020) prepared a grafted zwitterionic membrane with efficient separation ability toward monovalent salt/antibiotics. The grafted zwitterionic membrane exhibited optimal permeability of 14.6 L m−2 h−1. Bar −1 and high rejection of organics. Lastly, due to the advancement in nanotechnology and development of novel materials for water treatment an alternative strategy to tailor and enhance membranes performance is through synthesis of next generation of high-performance membranes, by incorporating novel materials including carbon nanotube (CNT), graphene oxide (GO), covalent organic frameworks (COFs) and metal organic frameworks (MOFs), while replacing current polyamide-based membranes (Guo et al., 2022). In a recent study, Zhu et al. (2022) reported high performance GO-based membrane modified with the phytic acid which exhibited a high-water flux of 6.31 L m−2 h−1 bar−1 under ultralow pressure nanofiltration condition. In addition, it was found that the new GO-based membrane is also capable of rejecting difference charged dye with rejection rate higher than 99.88%. In addition, Liu et al. (2019) successfully modified a commercial cellulose acetate membrane support by integrating nanocomposite MOF-HKUST-1 reduced graphene oxide (GO) incorporating polydopamine (PDA) on the surface. The prepared PDA/RGO/HKUST-1 membrane was tested for the removal of methylene blue and Congo red with removal rate of 99.8% and 89.2%, respectively. Furthermore, the PDA/RGO/HKUST-1 membrane exhibited superior performance of 33-fold increase in the dye flux compared to PDA/RGO membrane. Despite the potential reward of the aforementioned studies considering enhance selectivity and removal of OMPs, the lack of a deep understanding of the rejection mechanism guarantees continued research in this area.Advanced oxidation processes and MBRs have significant drawbacks in the form of harmful oxidation by-products and high energy demand (Sher et al., 2021). For this reason, other methods of MP removal are being studied. One such technique is adsorption of MPs using activated carbon (AC), which is widely considered to be a highly efficient method (Ruiz-Rosas et al., 2019; Sher et al., 2021). AC has significant advantages such as high tunability of physical and chemical properties, which allows for adsorption of a wide variety of MPs, fewer by-products, and low cost (Rodriguez-Narvaez et al., 2017; Ruiz-Rosas et al., 2019; Sher et al., 2021). However, because AC is a category of highly porous carbon-based materials, the production of AC was originally heavily dependent on coal, which makes the process expensive and unsustainable (Ouyang et al., 2020). In addition, AC usually requires to be regenerated which can add to the overall cost of the process. As a result, much emphasis is currently put on generating AC using biomass from agricultural and municipal waste (Chen et al., 2020a; Ouyang et al., 2020; Ruiz-Rosas et al., 2019). This is known as biochar. While biochar is considered a cheap and renewable material that can be used to efficiently remove MPs from wastewater (Ruiz-Rosas et al., 2019), its production is energy-intensive as it requires biomass to be heated at high temperatures for a long time (Chen et al., 2020a; Ouyang et al., 2020). Another disadvantage of using AC is that it increases turbidity of WWTP effluent, so additional measures (i.e., membrane filtration, sedimentation, etc.) must be taken to remove AC from water (He et al., 2016; Kumar et al., 2016). Such incorporation of other techniques would decrease the turbidity of WWTPs and increase the overall cost. In addition, membrane filtration can be used to remove MPs. Therefore, the use of AC is not feasible in terms of cost and efficiency.Recently, catalysts have garnered much interest as viable materials for the degradation of MPs. Particular attention is made on catalyst-assisted photodegradation, as photocatalysts are known to make the natural photolysis of MPs and their by-products by sunlight more efficient (He et al., 2016; Kumar et al., 2016). Furthermore, among the numerous photocatalysts that have been studied for this application, titanium dioxide (TiO2) constitutes one of the most promising candidates because it is cheap, highly efficient, physically and chemically stable, and is non-corrosive (Dong et al., 2015). It can also be used to degrade a wide range of MPs (Dong et al., 2015). Nevertheless, commercialization of TiO2 is limited by its poor absorption of the full spectrum of sunlight. TiO2 absorbs light in the UV region, which makes up less than 5% of sunlight (Dong et al., 2015). Therefore, TiO2 would need to be manipulated further before it can be used to degrade MPs efficiently under sunlight. Table 5 shows some examples of photocatalysts along with their structure and band gap. NiFe2O 4 possesses the smallest band gap which implies higher intrinsic conduction making it the most efficient photocatalyst mentioned.Extensive research has been conducted on enzymes for degradation of MPs. One clear advantage of these over other catalysts is that enzymes are biological materials, and hence pose little to no danger to the environment (Shakerian et al., 2020). Moreover, enzymes require ambient conditions of temperature and pressure to work efficiently, which would make the process highly energy efficient (Shakerian et al., 2020). Two enzymes that have been heavily researched are laccase and horseradish peroxidase, as these can be easily extracted and used to degrade pharmaceuticals, PCPs, Eds and dyes (Bilal et al., 2016; Kadam et al., 2018; Kashefi et al., 2019; Li et al., 2017b; Nadaroglu et al., 2019; Zhou et al., 2021). However, most enzymes target specific chemical species only, therefore, a significant work needs to be done on identifying other enzymes that are as versatile as laccase and horseradish peroxidase (Stadlmair et al., 2018). Another limitation of enzymes is that they need to be immobilized on a substrate as the activity of free enzymes is significantly lower, comparatively (Bilal et al., 2016; Kadam et al., 2018; Kashefi et al., 2019; Li et al., 2017b; Nadaroglu et al., 2019; Zhou et al., 2021). Table 6 compares the different MP removal methods by listing some advantages and disadvantages of each method. Biochar is a carbon rich material produced by the pyrolysis of biomass (Santos et al., 2019). Biochar can be used in the removal of contaminants from water due to its outstanding properties such as high surface area, high porosity, presence of functional surface groups, excellent ion exchange ability, and high stability (Qiu et al., 2021). As a result, the main mechanism for removal is biosorption. Biochar can be incorporated with other materials to enhance adsorption capacity and visible light absorption, thus increasing the removal efficiency of materials (Qiu et al., 2021). For instance, through adsorption and photocatalytic reduction heavy metals, are removed from water using biochar-based materials (Qiu et al., 2021). Moreover, biochar-based materials can also remove organic pollutants through the same mechanism (Qiu et al., 2021).Biochar can also be used as a catalyst for the removal of micropollutants. For instance, Song et al. (2017) used wheat straw as a catalyst for the dechlorination of hexachlorobenzene (Song et al., 2017). The presence of chlorine in many compounds is the main contributor to its toxicity. The more chlorine present, the more toxic a compound is likely to be. Therefore, the dichlorination of hexachlorobenzene will result in a less toxic compound. The study by Song et al. (2017) showed a removal efficiency up to 56% and the main driving force was carbon centered plug flow reactors. This low removal efficiency suggests that the use of biochar, specifically wheat straw, or the driving force is inappropriate. Another study conducted by Yavari et al. (2019) used rice husk for the degradation of imazapic and imazapyr herbicides. The results demonstrated that the use of rice husk has enabled to decrease the half-life of imazapic from 40.7 days to 25.6 days and that of imazapyr decreased from 46.2 days to 26.5 days (Yavari et al., 2019). This demonstrates the effectiveness of rice husk in reducing the overall time over which the herbicides remain present in the environment by about 50%. On the other hand, the extensive use of biochar in water applications is limited due to the association of toxic elements such as heavy metals, metalloids, and polycyclic aromatic hydrocarbons with biochar (Tan et al., 2015).MXenes belong to a family of two-dimensional nanomaterials that have the formula M n+1 X n T x (n = 1–3) (Yu et al., 2022). M denotes transition metals such as Ti, Zr, Hf, X denotes carbon and/or nitrogen, and T denotes surface terminated groups such as F, OH, O (Rafieerad et al., 2021). Titanium based MXenes are the most promising for environmental applications due to the abundance of the element and the production on non-toxic byproducts during degradation (Chen et al., 2021; Hermawan et al., 2021). Specifically, Ti3C2T x constitutes a good candidate because of its intrinsic properties such as abundant functional groups and large surface area, outstanding metallic conductivity, and the reactivity of the terminal metal sites (Yu et al., 2022). Moreover, MXenes can be used in the removal of heavy metals as it can effectively capture copper, lead, mercury, and chromium. This is achieved through the reaction of the heavy metal with the surface groups of the MXenes (Yu et al., 2022). MXenes can also remove organic contaminants present in water such as dyes, aromatic compounds, and pharmaceuticals through photocatalytic degradation (Kim et al., 2021).A study by Tu et al. (2022) investigated the efficiency of Carbon nitride coupled with Ti3C2-Mxene derived amorphous titanium (Ti)-peroxo heterojunction in the degradation of RhB and tetracycline (Cao et al., 2020a). The results revealed that the degradation efficiency of RhB reached 97.2% while the degradation efficiency of tetracycline was 86.3% under visible light within 60 min (Cao et al., 2020a). Moreover, Shahzad et al. (2018) studied another Maxine based catalyst (TiO2/Ti3C2T x ) for the degradation of carbamazepine (Tu et al., 2022). The catalyst demonstrated a high removal efficiency of 98.7% under UV light. The main radicals associated with the degradation of carbamazepine were found to be • HO and • O2 (Shahzad et al., 2018). These results indicated that MXene-based-catalysts are quite promising in the degradation of micropollutants. However, MXenes can produce TiO2 in water which presents a challenge in water purification. In addition, oxidized MXenes can release adsorbed contaminants causing secondary pollution (Yu et al., 2022).Nanoscale zero valent iron consists of an iron (0) core and iron oxide layers (Li et al., 2021). Due to its structure and composition, it possesses the ability to adsorb contaminants or transform them via oxidation or reduction (Li et al., 2021). For instance, NZVI was able to remove organic contaminants, nitro-aromatic compounds, inorganic contaminants, heavy metal ions, and radionuclides by adsorption or reduction techniques (Choi and Lee, 2012; Gu et al., 2010; Khalil et al., 2016; Li et al., 2016a, 2019b; Qiu et al., 2018). Gao et al. (2020) synthesized a sulfided nanoscale zero-valent iron catalyst supported by biochar for the removal of ciprofloxacin (CIP) (Gao et al., 2020). The study showed that the catalyst had a removal efficiency of 89.8%. Furthermore, the radicals • HO and SO 4 − • played a major role in the degradation of CIP (Gao et al., 2020). Jia et al. (2019) developed a graphene-like sheet supported nZVI for the degradation of atrazine (Jiang et al., 2021). The results demonstrated that the catalyst rapidly degraded atracine and achieved a removal efficiency of 97.2% within 2 min (Jiang et al., 2021). The main degradation pathways were found to be dichlorination, dealkylanation, and alkyl oxidation. SO 4 − • was also observed to play a major role in the degradation. These studies showed that when nZVI is combined with other materials such as biochar and graphene, it can achieve higher removal efficiencies. In addition, it seems that SO 4 − • plays a major role in both studies and this indicates that this radical may be a common amongst nZVI catalysts. Regardless of its promising applications in the removal of contaminants from water, NZVI can be toxic to aquatic organisms and can harm human cells (Laurier et al., 2013). Therefore, extreme care must be taken when using NZVI in the remediation of water in order to prevent human and animal exposure.MOFs are highly porous frameworks made up of inorganic clusters linked together through organic species (Jiang et al., 2018). Furthermore, their chemical properties can be easily tailored to the application requirements by selecting the organic and inorganic species most suited for the application (Jiang et al., 2018). This outstanding property of MOFs is further illustrated in Table 6 and Fig. 3 where several MOFs are compared based on their metallic cluster, organic linker and surface area. For example, UiO-67 and NU-100 MOFs are two water stable Zirconium-based metal organic frameworks, yet they possess different structures and surface areas due to the difference in the organic linker. In addition, Table 7 shows that MOFs possess very high surface areas as the highest surface area mention is Cr-MIL-101 (3360 m2 g−1) and the lowest surface area mentioned is HKUST-1 (1800 m2 g−1). Owing to this versatility, there has lately been great interest in developing MOFs that can be used for photocatalysis of MPs (Dias and Petit, 2016). Because MOFs can be specialized for an application, they can be made to absorb light over a large range of wavelengths, thus ensuring greater performance compared to other catalysts, such as TiO2 (Dias and Petit, 2016). Nevertheless, some leakage of the inorganic and organic species from MOFs has been noted in research conducted on MOFs for wastewater treatment (Jiang et al., 2018). Since MOFs constitute a relatively new class of materials, they need to be studied further to assess their potential environmental impact before they can be commercialized (Dias and Petit, 2016). We summarize in Table 8 the main advantages and disadvantages of MOFs for photocatalytic degradation. Another class of MOF catalysts is biomimetic MOFs which are inspired by photosynthesis of microorganisms and plants (Wu et al., 2022b). Biomimetic MOFs can be used to degrade micropollutants in water. For instance, Wu et al. (2022a, b) developed a three-dimensional coral zirconium-based metal organic framework (Zr-TCPP-bpydc) via a double-ligand strategy for the degradation of tetracycline (TC) and ofloxacin (OFX) (Wu et al., 2022b). The catalyst was synthesized using a solvothermal method. Characterization studies showed that Zr-TCPP-bpydc has a pore volume of 0.307 cm3 g−1 and a surface area of 228 m2 g−1 which indicates the presence of mesopores and micropores (Wu et al., 2022b). Moreover, Zr-TCPP-bpydc showed high adsorption under ultraviolet and visible light range. Regarding the degradation of TC and OFX, the catalyst demonstrated an almost complete degradation of both pollutants after 120 min in the dark. This resulted in a removal efficiency of 98% for TC and 97% for OFX. The main radicals associated with the degradation of TC and OFX are • HO and • O 2 − (Wu et al., 2022b). We report in Table 9 a set of biomimetic MOFs used for degradation of micropollutants along with their main characteristics. Enzymes can be used as a catalyst to convert micropollutants present in water to fewer toxic substances (Zdarta et al., 2022). Properties such as high activity and selectivity as well as quick catalytic reaction make enzyme a viable option for biological treatment. Enzymes can be used as free enzymes or as immobilized biocatalysts (Zdarta et al., 2022). Metal organic framework composites can be used as carriers in the immobilization of enzymes as it shows high efficiency on enzyme binding and high stability. However, the binding of enzymes to MOFs reduces its enzyme activity (Zdarta et al., 2022). Li et al. (2020) conducted a study on the removal of dyes using immobilized enzymes on MOFs (Li et al., 2020). ZIF-8 was selected as the carrier for horseradish peroxidase (HRP). However, due to the micropore range of the metal organic framework, a single layer microporous (SOM-ZIF-8) was synthesized and used. SOM-ZIF-8 enhanced the mass diffusion, stability, and recyclability of the composite. Furthermore, the composite was formed by encapsulating the enzyme in SOM-ZIF-8 using post-synthetic immobilization. Characterization results revealed that the surface area of the composite was 1350 m2 g−1 while the pore volume was 0.90 cm3 g−1 (Li et al., 2020). The composite displayed a detection limit of 0.48 μ M indicating that the composite can be used as a one-step indicator. Regarding dye removal, HRP@SOM-ZIF-8 was able to rapidly degrade Congo red and rhodamine blue after 2 min resulting in high removal efficiencies (Li et al., 2020).Another study by Jia et al. (2019) investigated enzyme immobilized onto MIL-53(Al) for the catalytic conversion of triclosan (TCA) (Jia et al., 2019). The enzyme used in the composite is laccase which was encapsulated within MIL-53(Al). Mesoporous MIL-53 was specifically used due to its high water and chemical stability. The surface area of Lac-MIL-53(Al) was found to be 1030 m2 g−1 with a pore size of 4 nm (Jia et al., 2019). Moreover, the tests, conducted to determine the efficiency of the composite in the removal of TCA, revealed a removal efficiency of 99.24% within 120 min. Of interest, the main mechanism for the removal of TCA by the composite was adsorption combined with oxidation as the surface area decreased from 1030 m2 g−1 to 268 m2 g−1 (Jia et al., 2019). Table 10 reports other Enzyme immobilized on MOFs used in various applications related to micropollutants degradation and removal. Table 10 shows that enzyme-immobilized MOFs can achieve up to 100% removal efficiency and can even enhance the properties of MOFs such as stability. The Fenton reaction is an efficient oxidation method as it produces highly reactive hydroxyl radicals that can degrade most organic compounds present in wastewater (Gao et al., 2017a). However, Fe2＋ and Fe3＋ are hard to recover from the system. A solution to this problem is to immobilize them in porous materials such as MOFs. The typical Fenton reaction is as follows where reaction (3) shows the decomposition of the organic pollutant via hydrogen abstraction and reaction (4) shows the decomposition of the organic pollutant via hydroxyl addition (Cheng et al., 2018): (1) Fe 2 + + H 2 O 2 ⟹ Fe 3 + + HO − + • HO (2) Fe 3 + + H 2 O 2 ⇆ Fe 2 + + H + + • HO 2 (3) RH + • HO ⟹ H 2 O + R • ⟹ further oxidation (4) R + • HO ⟹ • ROH ⟹ further oxidation For instance, Gao et al. (2017a, b) synthesized MIL-88B-Fe for the degradation of organic pollutants. The catalyst consists of open iron sites which are filled by non-bridging ligands (Gao et al., 2017a). H 2 O2 can then displace these ligands as it is absorbed into the catalyst. Furthermore, characterization tests of MIL-88B-Fe were conducted and showed that the surface area is 165.4 m2 g−1 and the pore volume is 0.2 cm3 g−1 (Gao et al., 2017a). TGA measurements revealed good thermal stability of the catalyst up to 350 °C. Next, the efficiency of phenol removal by the catalyst was tested by first employing an H 2 O2 oxidation process without MIL-88B-Fe. The results indicated that phenol barely degraded. However, adding MIL-88B-Fe and H 2 O2 showed 99% removal efficiency (Gao et al., 2017a). Increasing the dosage of the catalyst from 0.1 g L−1 to 0.2 g L−1 reduced the removal efficiency to 98% due to the agglomeration of the catalyst. Lastly, • HO was found to be the main radical involved in the oxidation of phenol (Gao et al., 2017a).A study conducted by Wang et al. (2020a, b, c, d) investigated the enhancement of Fenton catalytic performance by the addition of NH2-MIL88B(Fe) for the degradation of acetamiprid (ACTM) (Wang et al., 2020d). In this study, the MOF was enhanced to achieve photo-induced electron and Fenton-generated radicals. NH2-MIL88B(Fe) consists of various ligand defects; therefore, benzoic acid (Bac), pyrrole (Py), pyrrole-2-carboxylic acid (Pca) were in-situ engineered into the MOF (Wang et al., 2020d). The effect of each ligand and MOF was then tested. Characterization tests showed that the surface area and pore volume of the MOFs increased in the following order: MIL88(Fe) < Bac-MIL88(Fe) < Py-MIL88(Fe) < Pca-MIL88(Fe) (Wang et al., 2020d). Regarding ACTM adsorption, Pca-MIL88(Fe) showed the highest uptake. Pca-MIL88(Fe) also demonstrated the highest catalytic activity as it completely degraded ACTM within 40 min. • HO is the main radical involved in the oxidation of ACTM for all MOFs (Wang et al., 2020d). Table 11 summarizes other Fenton-like MOFs used in various applications and reports their main capabilities in terms of removal efficiencies and other characteristics. MOFs lend themselves very well to catalytic applications due to their large surface areas and uniform porous structures (Guo et al., 2017; Xiang et al., 2017). In order to take advantage of these features, other chemical species may be added to MOFs to improve their chemical and thermal stability and enhance some physicochemical properties like electrical conductivity and magnetism (Chen et al., 2019a; Xiang et al., 2017). At the same time, MOFs may help stabilize a catalyst and protect it from harsh environments (Zhong et al., 2019), promote even distribution of chemical species due to their uniform crystal structure, and provide many active sites for catalytic activity (Xiang et al., 2017). As a result, MOF-composite-based catalysts often exhibit higher efficiency than free catalysts. Liu et al. (2022) reported the use of some MOF composites in the adsorption of heavy metals. For example, Zr-DMBD and UiO-66-(SH)2 are employed in the adsorption of Hg 2 + and were found to have a removal efficiency of 99% (Liu et al., 2022). Moreover, they were also used in the removal of dyes from wastewater. To elaborate, MIL-101 was found to remove reactive black 5 with an efficiency of 99.9%, while Co-MOF removed Congo red with a removal efficiency of 99.4% (Davoodi et al., 2021; Karmakar et al., 2019). Additionally, MOF composites exhibit high antibacterial removal efficiency. In particular, PCN-124-stu (Cu) was used in the removal of NOR with a removal efficiency of 99.8% and HpZIF-8-10 in the removal of TH with a removal efficiency of 98.6% (Chen et al., 2019b; Jin et al., 2017). These studies demonstrate that MOFs are highly efficient adsorption materials which is mainly due to their surface area and porous structure.Moreover, Zhong et al. (2019) found that immobilizing the enzyme α -glucosidase in a Cu-MOF, to be used for screening antidiabetic drugs, not only helped to stabilize the enzyme, but also improved selectivity, sensitivity, and thereby, efficiency by about 4.58 times (Zhong et al., 2019). Similarly, Zhang et al. (2018) observed that Hemin-Au@MOF has higher affinity for alpha-fetoprotein – a biomarker present in very low levels in human body fluids during early stages of cancer – than free hemin catalyst (Zhang et al., 2018). This has potential application in early diagnosis and treatment of cancer (Zhang et al., 2018). As previously discussed, enzymes immobilized on MOFs are highly efficient in the degradation of micropollutants as well as sensors in the human body. Moreover, immobilizing the enzyme on MOFs does not only stabilize the MOF but stabilizes the enzyme as well.Another study examined the role of Mn-MOF-74 grown on carbon nanotubes (Mn-MOF-74@CNT) as Li- O 2 battery electrode material in limiting side reactions during operation, and improving its performance (Zhang et al., 2019a). Furthermore, Chen et al. (2017) showed that electrodes made of Cu-based MOF-199 and graphene oxide composite material can be used to detect the hazardous chemicals, catechol and hydroquinone in the environment, at concentrations as low as 1 μ M (Chen et al., 2017). Moreover, Zhu et al. (2018) successfully synthesized a composite of Cu-based MOF, MoS2/rGO-MOF, to facilitate a hydrogen evolution reaction (Zhu et al., 2018). The authors attributed the effectiveness of the material to its large surface area and the combined effect of MoS2 and GO-MOF on the reaction (Zhu et al., 2018). The outstanding properties of MOFs make them suitable for a variety of applications and enable them to achieve extremely high efficiencies.One study conducted on desulfurization of oil used a polyoxometalate (POM) catalyst encapsulated in MOF-199, which was constructed in the pores of another MOF, MCM-41, to form the composite: POM@MOF-199@MCM-41 (Li et al., 2016b). The researchers found that the composite material was more effective at desulfurization than its constituents on their own (Li et al., 2016b). This is due to the fact that MOF composites incorporate the properties of the isolated materials and enhances the overall properties. Lastly, several studies have reported the effectiveness of composite materials made of quantum dots and MOFs, as well as TiO2 and MOFs, in applications ranging from hydrogen evolution, carbon dioxide reduction, environmental remediation, etc. (Aguilera-Sigalat and Bradshaw, 2016; Wang et al., 2018, 2020c; Wu et al., 2020).CuBTC@NH2 composite was synthesized by Samuel et al. (2022) for the removal of ibuprofen and acetaminophen (Samuel et al., 2022). First, they found that the composite had an increased surface area from 47 m2 g−1 to 64 m2 g−1 while other properties such as pore volume, average pore diameter, and pore size decreased compared to CuBTC (Samuel et al., 2022). Next, they observed that the degradation efficiency of both pharmaceuticals was around 96% after 300 min with a maximum adsorption capacity of 187.97 mg g−1 for ibuprofen and 125.45 mg g−1 for acetaminophen. They also conducted a secondary test to determine the presence of any leaching and found that CuBTC@NH2 composite can be used several times in a treatment process as there was no evidence of further pollution (Samuel et al., 2022). Nikou et al. (2021) synthesized GO/ZIF-8 composite using the solvothermal method for the simultaneous removal of two pesticides diazinon and chlorpyrifos (Nikou et al., 2021). They found that the adsorption of the pesticides was higher when using GO/ZIF-8 in comparison to using ZIF-8 as seen in Fig. 4. This is due to the enhanced interactions of the carboxylic and hydroxyl graphene oxide groups present in GO/ZIF-8 with the functional groups of the pesticides (Nikou et al., 2021). In addition, the optimum removal efficiency (83% of diazinon and 73% of chlorpyrifos) were found at a pH of 7, adsorbent dosage of 24 mg, and a contact time of 24 min. The composite also displayed high reusability and can be easily regenerated (Nikou et al., 2021). ZIF-8/ZnO@ESM an MOF biocomposite was studied by Shahmoradi Ghaheh et al. (2021). The composite was fabricated using a green method in which water is used as a solvent instead of methanol. The green method is highly advantageous as it reduces the carbon emissions which is usually associated with the synthesis of MOFs or MOF based composites using solvents other than water. A Brunauer–Emmett–Teller (BET) test was then conducted to determine the surface area of the composite which showed a decrease in the surface area from 1493.5 m2 g−1 ZnO/ZIF-8 to 1428.1 m2 g−1 (Shahmoradi Ghaheh et al., 2021). A similar trend was also seen in the pore volume and mean pore diameter. This decrease in surface area, pore volume, and mean pore diameter is due to the presence of ESM in the pores of the MOF. In comparing the removal efficiency of methyl green by ESM and by the composite, it is observed that the removal efficiency increased from 10.66% to 52.11% with a contact time of 90 min (Shahmoradi Ghaheh et al., 2021). ZIF-8/ZnO@ESM was also used in the removal of tetracyline and found a removal efficiency of 50%. Ghorbani-Choghamarani et al. (2021) considered the use of Fe3O4@GlcA@Ni-MOF composites as a green catalyst for the synthesis of Rhodanine (Ghorbani-Choghamarani et al., 2021). The BET surface area and pore volume were found to be 97 m2 g−1 and 22.25 cm3 g−1, respectively. The composite further exhibited high catalytic activity in the purification of the desired product and demonstrated a high degree of reusability making it an optimal catalyst in the synthesis of Rhodanine (Ghorbani-Choghamarani et al., 2021). Furthermore, when comparing Fe3O4@GlcA@Ni-MOF to other catalysts such as TiO2 nanoparticles and MCM-41, it has been demonstrated that Fe3O4@GlcA@Ni-MOF is helpful from different aspects including the low cost of synthesis, the non-toxicity and stability of the catalyst, and the easy separation of the catalyst from the products (Ghorbani-Choghamarani et al., 2021).Depending on the nature of precursors, MOFs, catalysts, and reaction conditions, there are a few methods that can be applied to obtain MOF-catalyst-based composites. The synthesis technique must be carefully selected, as this can have an effect on the structure of the composite (Chen et al., 2020c) and activity of the catalyst (Xiang et al., 2017). We display in Fig. 5 a schematic depiction of all the synthesis methods mentioned below.The “Ship-in-a-bottle” method involves depositing catalyst nanoparticles, or catalyst precursors into the pores of an already formed MOF (Aguilera-Sigalat and Bradshaw, 2016; Chen et al., 2020c; Wu et al., 2020; Xiang et al., 2017) as seen in Fig. 5(a). The nanoparticles may be deposited into the MOF cavities through solution deposition, vapor deposition, (Wu et al., 2020; Xiang et al., 2017), or mechanochemistry (grinding) (Xiang et al., 2017), after which the precursors may undergo transformations inside the pores to form catalysts under heat, light, or chemical stimulation (Chen et al., 2020c). Zheng. et al., demonstrated this approach by synthesizing Pt@DUT-5 (Zhang et al., 2016). It was found that the effectiveness of this method depends on the selection of MOF material. Specifically, MOFs that have big size cavities but small pores such as MIL-101 work best with this method. The main challenge of this approach, however, is to control the deposition of the nanoparticles such that they do not attach to the external surface of the MOF instead of being embedded within the pores (Chen et al., 2020c). Other aspects of interest are the stability of MOF under the conditions required to form the catalyst, the control over where the nanoparticles are deposited within the MOF, and the ability to deposit a sufficient number of nanoparticles (Aguilera-Sigalat and Bradshaw, 2016).The “bottle-around-ship” approach is also referred to as the template synthesis method (Aguilera-Sigalat and Bradshaw, 2016). In this approach and as observed in Fig. 5(b), the catalyst is synthesized first and stabilized in a solvent using surfactants or other materials (Xiang et al., 2017). The role of the surfactant is to prevent agglomeration of catalyst nanoparticles (Wu et al., 2020). MOF precursors are then added to this dispersion, which leads to the MOF being constructed around the nanoparticles (Aguilera-Sigalat and Bradshaw, 2016; Chen et al., 2020c; Xiang et al., 2017). This approach not only ensures encapsulation of catalysts within MOFs instead of on their external surfaces (Xiang et al., 2017), but also avoids any limitations associated with diffusion of nanoparticles deep within the framework (Wu et al., 2020). This method also prevents the nanoparticles from leaching into the water when it is used in certain applications. Additionally, because the catalyst nanoparticles are formed before encapsulation, there is greater control on their size and structure, which can be tailored as per the application’s requirements (Wu et al., 2020; Xiang et al., 2017). Furthermore, Xiao et al. (2016) demonstrated the difference between this approach and the “ship-in-a-bottle” approach by synthesizing Pt/UiO-66-NH2 and Pt@UiO-66-NH2, respectively (Xiao et al., 2016). It was observed that Pt@UiO-66-NH2 shortens the electron transport distance which in turn enhances the catalytic activity compared to Pt/UiO-66-NH2. It was also found that Pt@UiO-66-NH 2 does not undergo aggregation and thus leading to better catalytic recyclability (Xiao et al., 2016). As a result, it can be concluded that while the “bottle-around-ship” approach has its own advantages, “ship-in-a-bottle” enables superior performance due to better confinement of the nanoparticles.The self-sacrificed template method is similar to the “bottle-around-ship” approach in that the MOF forms around the nanoparticle species. However, Fig. 5(c) shows that in this case the nanoparticles serve as the source of metal ions for the MOF (Chen et al., 2020c). Organic linkers and metal-containing nanoparticles are mixed together, and the metal ions form coordination bonds with organic linkers at the same rate as they are released from the nanoparticles (Chen et al., 2020c). This method allows a high degree of control over the structure and morphology of MOFs, but it limits the choice of nanoparticles that can be encapsulated into the MOFs (Chen et al., 2020c). Table 12 shows the different MOF-catalyst composites synthesized using self-sacrificed template approach along with their respective applications and properties. Table 12 indicates that MOFs synthesized via self-sacrificial templates can be applied in a variety of applications as it produces MOF based composites with different advantageous properties. Table 12 also implies that the application of the MOF based composite relies on its properties Nanoparticles may be sandwiched between two layers of MOFs as a solution to the problem of adherence of nanoparticles to MOF surface as seen in Fig. 5(d). This can be thought of as employing “ship-in-a-bottle” and “bottle-around-ship” approaches sequentially. First, nanoparticles are loaded onto MOF, and then a thin layer of MOF is constructed around the MOF-nanoparticle composite (Chen et al., 2020c). This approach was reported to improve selectivity of the catalyst, but it is very important that the outer layer of MOF be thin enough to allow diffusion of reactants and products in order to not adversely affect catalytic activity (Chen et al., 2020c). In addition, Liu et al. (2017) synthesized TiO2 nanosheets with MIL-100 using this approach and found an increased absorption ability due to an increase in the surface area. This in turn demonstrates the ability of the approach in enhancing the overall photocatalytic performance (Liu et al., 2017).In the one-pot synthesis method, the precursors of nanoparticles and MOF are mixed together for simultaneous synthesis of the two similar to Fig. 5(e), followed by assembly of the composite (Chen et al., 2020c; Xiang et al., 2017). This one-step synthesis strategy is simpler than the ones mentioned previously, nevertheless, the rates of nanoparticle, MOF and composite formation must be carefully balanced in order to achieve the desired product (Chen et al., 2020c; Xiang et al., 2017). For this reason, the choice of reactants and solvents that can be used is limited, and this approach is not applicable to all MOF-catalyst composites (Chen et al., 2020c; Xiang et al., 2017). Chen et al. (2014) used this approach in the synthesis of Pd@MOF composites without the addition of any stabilizing agents. It was observed that the catalysts were stable, reusable, and its performance did not decrease with the amount of time it was used (Chen et al., 2014). Furthermore, this method seems very promising as it reduces production cost and eases scale up by reducing the synthesis method to one step. Molecular orbitals in semiconductors are arranged in the valence band and conduction band, which are separated by a band gap (Lopes Colpani et al., 2019). Electrons (e − ) in the valence band can be promoted to the conduction band via light energy if the energy is greater than or equal to the band gap; at the same time, holes (h + ) are generated in the valence band (Candia-Onfray et al., 2021; Lopes Colpani et al., 2019; Ni and Khan, 2021). Similarly, organic species have the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) that act as the valence band and conduction band, respectively (Ni and Khan, 2021; Zhang et al., 2020a). In MOFs, the organic linkers absorb light energy, and e − are promoted from the HOMO to the LUMO and are then transferred to the metal cluster (Zhang et al., 2020a). In this way, the organic linkers in a MOF can be thought of as the valence band and the metallic clusters as the conduction band (Ni and Khan, 2021). The mechanism is similar for MOF-catalyst composites, with the addition of valence and conduction bands of the photocatalyst (Xue et al., 2018).The mechanism can be further elaborated by considering the photocatalyst TiO2. In the presence of light, TiO2 produces electrons in the conduction band ( e cb − ) and electron holes in the valence band ( h vb + ), as illustrated in Eq. (5). The generated holes then reach the surface of TiO2 and react with either absorbed hydroxyl groups or water to produce HO • radical, as shown in Eq. (6). The generated HO • radical then desorbs from the surface into the bulk of the medium to form free HO • . If electron donors are present, then the reaction follows Eqs. (7) and (8), and if electron acceptors are present then the reaction follows Eqs. (9), (10), (11), and (8) (Georgaki et al., 2014). A schematic illustration of the overall mechanism can be seen in Fig. 6. (5) TiO 2 + h ν → h vb + + e cb − (6) h vb + + H 2 O → H + + • HO (7) Oxidation site : h vb + + Red org → Ox org (8) • HO + Red org → Ox org (9) Reduction site : e cb − + O 2ads → • O 2 − (10) • O 2 − + e cb − ( + 2 H + ) → H 2 O 2 (11) • O 2 − + H 2 O 2 → • HO + OH − + O 2 (12) H 2 O 2 + h ν → 2 • HO Several research groups have investigated the applicability of MOF composite-based catalysts for degradation of MPs. For instance, Chang et al. (2017) successfully synthesized TiO2@MIL-101 through the ship-in-bottle method and studied its ability to degrade methyl orange in solution (Chang et al., 2017). The composite was synthesized from TiO2 precursor, and MIL-101 (Chang et al., 2017). The BET surface area of MIL-101 reduced from 1918.8 m2 g−1 to 1033 m2 g−1 (for TiO2@MIL-101), indicating that TiO2 was deposited in the pores of the MOF. The authors noted that TiO2 also formed a 100 nm thick shell around the MOF (Chang et al., 2017). The removal mechanism of MO was both adsorption and photocatalytic degradation. In terms of adsorption the MOF based catalyst was able to adsorb 19.23 mg g−1 (Chang et al., 2017). If adsorption was not employed, the photocatalytic degradation of MO reached 99% which is much higher than that of isolated TiO2. This is due to the fact that the MOF adsorbed the MO which increased its concentration in TiO2 allowing a greater portion to be degraded. Another study examined the efficiency of NH2-MIL-125(Ti)/BiOCl for the removal of tetracycline hydrochloride (TC) and bisphenol A (BPA) (Hu, 2019). BiOCl was coated on the outer surface of the MOF through the solvothermal method as seen in Fig. 7 (Hu, 2019). It was found that the composite removed 78% of TC in 2 h, and 65% of BPA in 4 h, which is much higher than the degradation efficiency of BiOCl on its own which is found equal to 48% (Hu, 2019). The improved activity was attributed to the incorporation of MOF, which provided a greater surface area for MP adsorption (Hu, 2019). In addition, it was noted that the main reactive species responsible for photodegradation were h + and • O 2 − , which were detected via radical trapping experiments and the electron spin resonance technique (Hu, 2019). Chen et al. (2019a, b, c) studied the effectiveness of the composite of a titanium based MOF, CdS/g- C 3 N4/MOF, in degrading Rhodamine B (RhB) dye under visible light (Chen et al., 2019c). The possible photodegradation mechanism is illustrated in Fig. 8. When exposed to visible light CdS, g- C 3 N4, and RhB produce electrons. The produced electrons then reduce Ti 4 + in MIL-125 to Ti 3 + and O2 from the atmosphere is absorbed on the surface of the MOF and reduced to • O 2 − . Lastly, • O 2 − . Along with the electron holes in the valence band of CdS and g- C 3 N4 oxidize RhB directly (Chen et al., 2019c). Moreover, the CdS and g- C 3 N4 were deposited on the outer surface of a previously synthesized MOF, and the final BET surface area for the composite was found to be 283.43 m2 g−1 (Chen et al., 2019c). The research study showed that while CdS on its own could degrade 40% of the initial concentration of RhB in solution in 60 min, the composite degraded RhB by about 90% in the same amount of time (Chen et al., 2019c). The authors also reported that the composite could be reused for three cycles with minimal decline in photocatalytic activity (Chen et al., 2019c). The composite’s success was achieved due to the enhanced visible light absorption, and decrease in recombination rate of e − and h + , which allowed • O 2 − and h + to degrade RhB (Chen et al., 2019c). In another study, Abdelhameed et al. (2018) reduced the band gap of NH2-MIL-125(Ti) from 2.51 eV to 2.39 eV by coating Ag3PO4 nanoparticles on its outer surface (Abdelhameed et al., 2018). The authors claimed that this is a significant contributing factor towards Ag3PO4@NH2-MIL-125 being up to 39 times more effective at photodegradation of methylene blue and RhB than P25 TiO2, a well-known photocatalyst for the two MPs (Abdelhameed et al., 2018). Furthermore, the composite exhibited greater catalytic activity and reusability than Ag3PO4 on its own (Abdelhameed et al., 2018). Adding scavengers for ROS revealed that • O 2 − and h + radicals are involved in the catalytic mechanism, while • OH radicals are not (Abdelhameed et al., 2018).Another study investigated the loading of carbon quantum dots (CQDs) on NH2-MIL-125 for photodegradation of RhB (Wang et al., 2018). A summary of the synthesis method and reduction conditions is demonstrated in Fig. 9. Furthermore, according to the findings of the study, CQDs can accept electrons from the MOF and allow for charge separation in the composite (Wang et al., 2018). CQDs may also convert NIR radiation into visible light, thereby expanding the range of wavelengths over which NH2-MIL-125 can function as a photocatalyst (Wang et al., 2018). CQDs/NH2-MIL-125 with 1% CQD loading proved to be a highly efficient catalyst under full spectrum of light, visible light, and NIR, as it was able to remove RhB by almost 10 0%, with the smallest amount of time taken for 100% removal (125 min) under full spectrum (Wang et al., 2018). Furthermore, this composite was reusable for 7 cycles, and showed negligible decrease in photocatalytic activity (Wang et al., 2018). The main ROS involved here were • O 2 − , h + and • OH (Wang et al., 2018). Han et al.’s (2019) work on photodegradation of methyl orange under visible light using RhB/MIL-125 yielded promising results, with methyl orange being degraded by more than 90% in just 60 min (Han et al., 2019). RhB acted as a sensitizer and helped broaden the range of visible light absorbed by MIL-125(Ti) as seen in Fig. 10 as the removal efficiency of MO increased when using RhB/Mil-125 in comparison to MIL-125 alone (Han et al., 2019). The main reactive species were found to be • O 2 − , h + , and the composite was tested for reusability for three cycles and showed minimal loss of activity (Han et al., 2019). Li et al. (2016a, b, c) attached 2-anthraquinone sulfonate (AQS) to NH2-MIL-101(Fe) to enhance the MOF’s ability to degrade bisphenol A (BPA) through persulfate activation as observed in Fig. 11 (Li et al., 2017a). The resulting composite, AQS-NH-MIL-101(Fe), was able to remove more than 97.7% of BPA from aqueous solution in 180 min, which was much higher than removal by NH2-MIL-101(Fe), and NH2-MIL-101(Fe) in the presence of free AQS (Li et al., 2017a). This increased catalytic activity was attributed to improved electron transfer between Fe(III) and Fe(II) in the MOF, and AQS (Li et al., 2017a). Moreover, when persulfate was added to the reaction mixture, degradation of BPA became more rapid (Li et al., 2017a). The main reactive species were • SO 4 − , while • OH and • O 2 − also contributed to the reaction (Li et al., 2017a). Lastly, the composite was tested for three cycles, and did not show any significant losses in activity (Li et al., 2017a). Huo et al. (2019) demonstrated the ability of α -Fe2O3/MIL-101(Cr) to degrade carbamazepine (CBZ) under visible light (Huo et al., 2019). The composite removed 100% of CBZ in 180 min, and was successfully reused, with the removal efficiency in the fourth cycle being 91% (Huo et al., 2019). Fig. 12 also demonstrates that the photodegradation efficiency of different combinations of α -Fe2O3/MIL-101(Cr) is higher than that of α -Fe2O 3 and MIL-101(Cr) alone. In addition, It was found that • O 2 − , h + , • OH were responsible for degradation of CBZ, with • OH controlling the photocatalytic activity (Huo et al., 2019). Mehrabadi and Faghihian (2018) compared the ability of TiO2 supported on clinoptilolite nanoparticles (TiO2/NCP) and TiO2 supported on salicylaldehyde-NH2-MIL-101(Cr) (TiO2/SN-MIL-101(Cr)) to degrade atenolol under UV irradiation and visible light (Mehrabadi and Faghihian, 2018). The conducted tests showed that TiO 2 had a band gap of 3.20 eV, TiO2/NCP had a band gap of 2.70 eV, and TiO2/SN-MIL-101(Cr) had a band gap of 2.15 eV (Mehrabadi and Faghihian, 2018). While under UV TiO2/NCP performed better (75% removal), the TiO2/SN-MIL-101(Cr) degraded 82% of atenolol under visible light in just 60 min (Mehrabadi and Faghihian, 2018). The authors attributed this high degradation efficiency to low energy band gap in TiO2/SN-MIL-101(Cr), and its large surface area, which allowed greater interaction between atenolol and reactive species (h + , • OH, e − ) (Mehrabadi and Faghihian, 2018). This indicates that the degradation efficiency of TiO2/SN-MIL-101(Cr) is much higher than that of TiO2 due to the reduction in band gap as its removal efficiency was 65% in 180 min. Several TiO2 doses were evaluated, and the composite with 3.4% TiO2 loading was found to perform most efficiently (Mehrabadi and Faghihian, 2018). It was suggested that in higher doses, TiO2 particles agglomerated, increasing the turbidity of the reaction mixture, and thereby decreasing penetration of light (Mehrabadi and Faghihian, 2018). Additionally, the reusability of TiO2/SN-MIL-101(Cr) was considered, and its degradation efficiency was observed to decrease steadily with each cycle, with only 35% of atenolol being removed in the sixth cycle (Mehrabadi and Faghihian, 2018). Emam et al. (2019), compared the effect of silver vanadate (Ag3VO4) and silver tungstate (Ag2WO4) on the ability of MIL-125-NH2 to degrade methylene blue and RhB under UV and visible light (Emam et al., 2019). All of the experiments were conducted over 60 min, and while both Ag3VO4@MIL-125-NH2 and Ag2WO4@MIL-125-NH2 performed well, the former was more efficient for both dyes under UV and visible light, as shown in Table 13 (Emam et al., 2019). Ag3VO4@MIL-125-NH 2 achieved a removal efficiency of around 100% in 60 min while Ag2WO4@MIL-125-NH 2 achieved a removal efficiency of around 95%. The higher removal efficiency achieved for composites, in comparison to that of the MOF (75%) on its own as well as Ag3VO 4 (80%) and Ag2WO4 (70%) was due to lower band gaps of the composites (Emam et al., 2019). Adding Ag3VO4 decreased the band gap of MIL-125-NH2 from 2.65 eV to 2.27 eV, while the band gap of Ag2WO4@MIL-125-NH2 was evaluated to be 2.56 eV (Emam et al., 2019). The lower band gap of Ag3VO4@MIL-125-NH2 was also the reason for performing better than Ag2WO4@MIL-125-NH2 (Emam et al., 2019). Upon checking for reusability under UV light for a total of five cycles, it was found that the degradation efficiency of Ag3VO4@MIL-125-NH2 decreased from 99.9% to 74% and 68% for methylene blue and RhB, respectively, while that of Ag2WO4@MIL-125-NH2 decreased from 96.2% to 72% for methylene blue and 84.2% to 45% for RhB (Emam et al., 2019). Lastly, it was found that • OH radical was majorly responsible for the photodegradation of the dyes, and h + had also some effect (Emam et al., 2019).Another research compared the photodegradation efficiencies of uncoated MIL-53(Al), MIL-53(Al)@TiO2 and MIL-53(Al)@ZnO, specifically in the depredation of Naproxen, Ibuprofen, and Methyl Orange in both single and binary systems (Murtaza et al., 2022). It was found that uncoated MIL-53 was efficient in the removal of all three micropollutants and was the most efficient in the removal of naproxen (89.5%) and ibuprofen (76.1%) when compared to MIL-53(Al)@TiO2 (80%), TiO2 (55%), MIL-53(Al)@ZnO (72%), and ZnO (67%), as shown in Figs. 13 and 14 (Murtaza et al., 2022). Whereas MIL-53(Al)@ZnO was the only photocatalyst able to degrade MO in both systems and in fact was able to completely degrade it under UV light in 30 min (Murtaza et al., 2022). Moreover, • OH plays a crucial role in the photodegradation for all the catalysts. A different research by Roshdy et al. (2021), studied the adsorption and removal efficiency of 4-nitrophenol by P/W@ZIF-8 and P/W@UiO-66-NH2 (Roshdy et al., 2021). Both composites were synthesized by ‘in-situ’ growth of the MOFs in the tungsten NPs. Characterization studies demonstrated that the surface area of P/W increased from 236.51 m2 g−1 to 424.54 m2 g − 1 for P/W@ZIF-8 and increased to 823.01 m2 g−1 for P/W@UiO-66-NH2 (Roshdy et al., 2021). Whereas the pore volume and pore size decreased in comparison to P/W. In regard to the photocatalytic degradation of 4-nitrophenol, P/W@ZIF-8 had an efficiency of 89.8% whereas P/W@UiO-66-NH2 had an efficiency of 100% after a time of 180 min under visible light and P/W had a removal efficiency of 50% as seen in Fig. 15. Moreover, the main reactive species involved in the photodegradation were found to be • OH and • O 2 − . Both composites also demonstrated incredible stability and reusability (Roshdy et al., 2021). Several studies reporting the utilization of MOF-catalyst composites for the degradation of numerous micropollutants, including dyes and pharmaceutical compounds, are summarized and their main characteristics are compared in Table 13. These include the removal efficiency, reactive species and catalysis driving force. As is evident, much research is currently being carried out on different combinations of MOFs and catalysts that would be useful in removing/degrading emerging MPs from wastewater. Some of the most promising MOF-catalyst composites are stated in Table 13 along with their removal efficiency. Of interest, some of the MOF-catalyst composites can remove 100% of a micropollutant such as Ag3PO4@NH2-MI in the removal of Methylene blue. However, certain driving forces must be present in order to achieve such high removal efficiency of a micropollutant such as visible light and UV. One factor that is likely to hinder the large-scale commercialization of MOF-based technologies is the high cost of the associated materials. Due to the raw materials such as the salts and ligands, the operating conditions such as the temperature, pressure, and solvent, as well as the energy-intensive synthesis methods, the cost of MOFs can be as high as 27,500 USD per kg (Kumar et al., 2019a). Moreover, the majority of the cost comes from the organic ligand and the solvent used. Larger organic ligands are much more expensive than the smaller ones and these large organic ligands are typically used for the synthesis of MOF (Witman et al., 2017). An example of the ligands used are 2,5-dioxido-1,4-benzenedicarboxylate (DOBDC) and benzenedicarboxylic acid (BDC) which cost 3268 USD per kg and 394 USD per kg, respectively (2, 5-Dihydrox y − 1 , 4 − b enzoquinone 98 615 − 94 − 1 , 2022; Terephthalic acid 98 100-21-0, 2022). Table 14 presents a list of MOFs available through Sigma-Aldrich and their respective costs. Table 14 indicates the high cost associated with the use of MOFs for micropollutants degradation. For example, the Basolite C300 costs approximately 28,516 USD per kg. Preparing MOF composite-based catalysts that hold the high removal efficiency while reducing the raw material cost could be the future for implementing MOFs in large scale applications. Moreover, when it comes to the process of synthesizing MOF catalysts, it can also be expensive depending on the approach being used: whether it is “Ship-in-a-bottle”, “Bottle-around-ship”, Self-sacrificed template, Sandwich-like heterostructure, or One-pot synthesis. While it is difficult to determine the direct cost of each process, the reduction conditions provide enough information to approximate whether the process would be costly or not. Table 15 provides examples of different synthesis processes along with the catalyst produced and its reduction conditions/agents. Table 15 shows the different synthesis methods along with the associated enthalpy of reaction. In general, the smaller (more negative) the enthalpy of reaction is, the less costly it is as it does not require the addition of a large amount of energy as the process is exothermic. As shown in Table 15, the one-pot synthesis method is the most costly as it is the most endothermic reaction meaning that it requires the addition of a large amount of energy for the MOF composite-based catalyst to be produced. On the other hand, the Pd@MOF-5 formation is considered as the least costly as it has the smallest enthalpy of reaction. In addition to their cost, another limitation that may affect the progress of MOF composite-based catalysts in the application of wastewater treatment is that most MOFs exhibit poor stability in the presence of water (Kumar et al., 2019a). In addition, the presence of chemicals such as hydroxides, amines, and alkoxides displace the organic ligand in the MOF resulting in the disintegration of the MOF in the water (Chen et al., 2020c). Therefore, there is a danger for metal ions and organic components to leach into water, which could lead to the presence of toxic chemicals in water such as iron, cadmium, terephthalic acid (TPA), and 4,4-bipyridine (BP) (Kumar et al., 2019b).Furthermore, the synthesis of MOFs and MOF catalysts employ solvents such as DMF and Dimethyl sulfoxide (DMSO) which can be toxic to humans. Since MOFs are porous materials, there is a possibility that these solvents are trapped within the structure during the synthesis procedure. As a result, once the MOF is placed in the water, it releases the solvent toxifying the water. The impact of these toxic solvents varies from mild to severe. For example, exposure to DMF, which is one of the most commonly used solvent in the synthesis of MOF, can lead to health issues such as nausea, vomiting, rashes, and even liver damage (Kumar et al., 2019b).The size of a material is also an important parameter when determining its toxicity. As the size of any material decreases, it becomes more reactive due to the high surface to volume ratio. Therefore, MOFs tend to be more toxic when their size decreases to the nanoscale (Kumar et al., 2019b; Sajid, 2016). Another problem with nanoscale MOF is that due to their small size they can penetrate the blood–brain barrier and the cell membrane of living organisms causing adverse effects (Grande et al., 2017).Regarding the environmental impact of MOFs, the degree of the impact depends on the synthesis route such as the operating conditions and the used solvent. For instance, the synthesis of ZIF-8 and MOF-74 can be done using DMF as a solvent and using water as a solvent. Research studies demonstrated that in the synthesis of the two aforementioned MOFs, the use of water has less environmental impact when compared to the use of DMF. The environmental impact of using water in the synthesis of ZIF-8 is quantified as 86.60 kg CO2 eq. whereas using DMF it is increased 1571.16 kg CO2 eq. (Grande et al., 2017). In the case of MOF-74, the use of water leads to 12.3 kg CO2 eq. On the other hand, it increases to 1136.2 kg CO2 eq. when using DMF (Ntouros et al., 2021).With a rise in micropollutant concentrations in different water bodies the accessibility to clean, drinkable water is becoming more strenuous. As a result, this serious issue has recently caught the attention of various organizations such as the United States Environmental Protection Agency (Montes-Grajales et al., 2017a). While their impact and presence are not fully understood yet, it is clear that MPs pose a hazard to the environment and human health, even when present at very low concentrations in the order of ng L−1 and μ g L−1. Some of these hazards include bioaccumulation (Vodyanitskii and Yakovlev, 2016), toxicity to human and animal health, increased risk in cancer, increase in antibiotic-resistant bacteria, and reproductive health problems (Gogoi et al., 2018b). This review discussed the use of MOF composite-based catalysts for the degradation of MPs ranging from the synthesis of MOF catalysts to their photodegradation mechanism, as well as current trends in research, associated cost, and environmental considerations.First, different emerging micropollutants were discussed with an emphasis on their source, their nature, and their impact. Since little attention has been given to MPs in the past, current WWTPs seem to be quite inefficient in their removal. Several studies conducted in various regions in the world from Europe such as Spain, North America, and Far East Asia such as China, revealed that the most common source of MPs in potable water was from sewage water released into surface water from WWTPs (Jiang et al., 2013). As a result, the EU Water Framework Directive and the US EPA both updated their watch list in order to emphasize the importance of controlling these compounds in WWTP effluents.Next, Metal Organic Frameworks were introduced and reviewed. Their properties can be easily tailored to their specific application through the selection of the inorganic linkers and organic species. Indeed, they are very advantageous in many applications such as the photodegradation of MPs. To compare the promising effect of MOF composite-based catalysts in the degradation of WWTPs, current methods were first discussed. Membrane technology for instance must be used alongside another removal technique in an integrated system as the diversity in the MPs properties make it difficult to remove them all using a single membrane type (Silva et al., 2017). Membrane bioreactors seem very promising in the removal of various micropollutants; however, it was found that it is not well-suited for the removal of MPs exhibiting low biodegradability, process high degree of branching and saturation, and contain sulfate and halogen group. In addition, to the high energy requirement which leads to a high operating cost. Another option is the use of CCM for advanced oxidation processes. This method was used for the removal of chlorinated MPs that have high toxicity and a strong likelihood to persist in the environment using a ceramic membrane making this method more efficient compared to a membrane bioreactor as it also has a less chance of fouling (Goswami et al., 2018). Other methods used the application of activated carbon and biochar. An advantage to this method is that it does not produce harmful by-products and does not require high amounts of energy (Sher et al., 2021). On the other hand, the production of biochar is extremely expensive as it is energy intensive (Chen et al., 2020a; Ouyang et al., 2020) and AC increases the turbidity of WWTP effluents which would require additional measures to be taken to remove it from the water (Sher et al., 2021). Recently, catalysts have gained a lot of attention as a material in the photodegradation of MPs. Specifically on catalyst assisted photodegradation in which photocatalysts are used to make the natural photolysis of MPs and their by-products more efficient using sunlight (He et al., 2016). Examples of photocatalysts include TiO2 which is considered the most promising as it is cheap, highly efficient, physically, and chemically stable, and non-corrosive (Dong et al., 2015). However, it is limited by its poor absorption of the full spectrum of sunlight (Dong et al., 2015). Enzymes have been also studied as photocatalysts and particularly two types of enzymes have been found to be particularly efficient laccase and horseradish peroxidase. A disadvantage of using enzymes is that they can only remove specific individual MPs therefore more studies should be conducted on other enzymes and the type of MPs that they can target (Stadlmair et al., 2018).Another promising option of photocatalysts is MOF catalyst composites which can be synthesized using 5 different approaches. Regardless, of the synthesis method, the photodegradation mechanism remains unchanged. Organic linkers absorb sunlight causing the electron to go from the HOMO to the LOMO and then transferred to the metallic cluster.In addition, the present review highlighted the current research being conducted using different MOF catalyst composites and their respective results. It was observed that MOF catalyst composites are quite effective in the degradation of various micropollutants, and the efficiency of MOF catalyst composites increased in comparison to free catalysts. Lastly, the cost and environmental impacts were also considered in order to determine the applicability of the MOF catalyst composites on a large scale. It was demonstrated that free MOFs are quite expensive and can reach up to 28,516 USD per kg. However, this high capital cost is balanced out by a low maintenance and operating cost. Furthermore, the synthesis of MOF catalyst composites can also increase the overall capital cost depending on the synthesis method. Another limitation to their large-scale application is the MOFs instability in water causing it to degrade leaving traces of metals and organic compounds in the water which can in turn harm humans and the environment. The synthesis method also plays a role in the environmental impact. Although MOF-composite-based catalysts have proven to be a promising hybrid catalyst with versatile applications, yet their investigation is still in its infancy and many challenges should be addressed before they are employed in real-life industrial applications. One of the main issues that should be further studied is the MOF-composite stability in aqueous solutions. It is also very crucial to develop MOF-composites that can possess high chemical and physical stable even after multiple cycles of photodegradation. Therefore, recyclability is another important property that future research should investigate in more depth. Furthermore, it is necessary to develop mechanically stable MOF-composite and be able to shape it into pellets or beads for its the final industrial applications. In addition, the mechanism of photodegradation of micropollutants still lacks in depth understanding of active species involved in the degradation steps and deactivation procedure. This aspect can be considered in future research studies. Sana Z.M. Murtaza: Data curation, Writing – original draft. Hind Tariq Alqassem: Data curation, Writing – original draft. Rana Sabouni: Conceptualization, Investigation, Methodology, Project administration, Resources, Supervision, Writing – review & editing. Mehdi Ghommem: Funding acquisition, Project administration, Resources, Supervision, Writing – review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The study presented in the paper was financially supported via the faculty research grant from the American University of Sharjah, United Arab Emirates (FRG21-M-E63 and FRG-S22-E05) and Sharjah Research Academy (SRA 223150). The work in this paper was supported, in part, by the Open Access Program from the American University of Sharjah (OAP23-CEN-033) This paper represents the opinions of the author(s) and does not mean to represent the position or opinions of the American University of Sharjah.

The presence of various micropollutants in different water sources has become a major problem due to their significant impact on both humans and the environment. This review highlights the different types of micropollutants present at the global scale and the methods applied to reduce and possibly eliminate them. These methods include membranes, adsorption and photocatalysis. While membrane filtration is extremely effective, one membrane can eliminate only a few micropollutants and its deployment remains expensive. On the other hand, adsorption constitutes a very efficient and cost-effective method, but the production of adsorbents is extremely energy intensive. Lastly, the photocatalysis method is considered to be the most promising as it avoids the problems associated with the aforementioned methods. Specifically, photocatalysts make use of direct sunlight in order to degrade micropollutants. Several types of photocatalysts, including biochar, Mxenes, nanoscaled zero valent iron, and MOFs, are discussed. Unlike the first four aforementioned types, MOFs can be combined with different materials to enhance the overall property of the composite and its efficiency in the degradation of micropollutants. The MOF-catalysts discussed in this paper include biomimetic MOFs, enzyme MOFs, and Fenton-like MOFs. The obtained system is referred to as MOF-composite-based catalysts. MOFs can be synthesized by combining an appropriate organic linker with a metallic cluster that would provide the material with the required properties for photodegradation. Several metal–organic framework catalyst composites synthesis approaches are reviewed and discussed. The selection of the approach depends on the requirements associated with the application of interest. To date, extensive research has been conducted on the performance analysis of metal–organic framework composites to investigate their efficiency in the removal of micropollutants. Several studies demonstrated their great removal capability which may reach up to 99 %. Finally, cost, health and environmental considerations are discussed with the view of the industrial applicability of MOF-composite-based catalysts. This comprehensive review presents the current state of the art and proposed promising research directions for the implementation and advancement of MOF-composite-based catalysts for micropollutants degradation.

Any additional information required to reanalyze the data reported in this paper is available from the lead contact on reasonable request.The development of our society is full of rapid industrialization progress, but this progress comes with many serious environmental issues. In particular, we are facing global warming caused by excessive greenhouse gas (GHG) emissions, as well as increasing resource constraints, such as clean water.The amount of carbon dioxide (CO2) as one of the primary GHG emissions accelerates with growing industrial and farming activities. 1 , 2 The concentration of CO2 in our atmosphere had already surpassed 400 ppm in 2016 and is predicted to increase to 550 ppm by roughly the end of this century, which threatens ecosystem and human health. 3 As an emerging solution, the CO2 could be electrochemically converted into value-added chemicals. 4 However, sustainable efforts that consider an entire system, including catalyst preparation, CO2 conversion, and downstream product separation, are still rare despite the raised demand. 5 Water contaminated with heavy metal ions and organic micropollutants from minerals mining and industrial processes poses severe risks to aquatic ecosystems and human health. 6 , 7 Adsorption by porous composites is a commonly used strategy to remove heavy metal ions because of high adsorption efficiency with desired porous structure and surface functional groups. However, the desorption process is always required to regenerate adsorbents, and the effluent-containing metal has to be treated as hazardous waste. 8 Catalyst-based oxidation processes have been recognized as a techno-economic option to remove the organic micropollutants in wastewater. 9 However, extra energy and chemicals are necessary for catalyst recycling and treatment after its use.Although CO2 emissions and water quality issues, representing two major environmental challenges, have been tackled from standalone perspectives, a sustainable strategy is urgently required to solve these environmental issues effectively and synergistically. Herein, we propose a systematic strategy to address the environmental issues based on the upcycling of post-consumer waste to green chemicals and clean water (Figure 1 ). Every year, industrial waste from seafood, such as crab, shrimp, and lobster, could reach 6–8 million tonnes globally, resulting in a non-negligible burden on the environment. 10 Chitosan (CS), a biopolymer that could be derived from these seafood residues, has become an emerging solution to treat metal-contaminated water owing to its remarkable ability of metal ion chelation. 11 Instead of being disposed of as hazardous waste as usual, here we directly convert post-consumer metal/CS, containing Cu, Pd, Cd, Mn, Zn, Ni, Ag, and Cr ions as a mixture, from simulated wastewater to metal-doped laser-induced graphene (M-LIG) under air via rapid laser scribing. The metal/CS composite with individual ion metal represents the post-consumer after treating wastewater with those ions as the dominant contaminant, for example, Cu in rinsing tank wastewater from an electroplating plant. 12 Thus, these seafood residues with wasted metal ions could be upcycled into valuable M-LIG catalysts. Most importantly, the upcycled Cu-LIG catalysts here are successfully utilized for the electroreduction of CO2 (CO2ER) to high-valued chemicals. The degradation of organic micropollutants in wastewater is also demonstrated via a Cu-based Fenton-like reaction with high efficiencies and recyclability of catalysts. A life-cycle assessment (LCA) is also used to systematically evaluate the carbon footprint of the entire system from catalyst preparation to product separation after CO2ER and organic micropollutants degradation. Therefore, producing green chemicals and clean water from industrial waste without introducing external environment and energy concerns may be realized by such an upcycling strategy.CS, as a biopolymer obtained from alkaline N-deacetylation of chitin, derived from waste seafood shells, contains abundant free amino groups. As a result, CS shows an effective adsorption ability of heavy metal ions, owing to the electrostatic attraction between them and its protonated amino groups, as well as hydroxyl groups. 13 In brief, CS solution was prepared by dissolving short-chain CS powder in deionized (DI) water with glycerol that could improve the flexibility of CS film by forming hydrogen bonding within CS chain as plasticizer. 14 After drying, the CS film was immersed in 2 wt % NaOH solution for precipitating and washed in the DI water until neutral. Finally, the CS film was immersed in diverse heavy metal ions solution for chelation (Figure 2 A). For adsorption of Cu2+, the field emission scanning electron microscope (SEM)-energy dispersive X-ray spectroscopy (SEM-EDS) shows a homogeneous distribution of Cu2+ chelated in CS film (Figure S1). The interaction of Cu2+ is also evidenced by Fourier transform infrared spectroscopy (FTIR) analysis (Figure S2).In our strategy, the post-consumer metal/CS film could be directly converted to M-LIG at ambient conditions under a commercial 10.6-μm CO2 infrared laser. Eight heavy metal ions (Zn2+, Mn2+, Ni2+, Cu2+, Cd2+, Pd2+, Ag+, and Cr6+, respectively) and a simulated industry wastewater containing Cr6+ and Cu2+ were tested (see experimental procedures). With computer control, the M-LIG pattern could be customized under laser scribing (Figure 2B). The resulted graphene composite contains macropores and nanopores simultaneously (Figures S3 and S4A). The nanopore sizes are distributed between 1.3 and 1.8 nm, suggesting that the M-LIG has a porous structure at multiple scale that could enlarge accessible surface areas and facilitate the mass transfer (Figure S5). The high-resolution transmission electron microscopy (HR-TEM) image also shows the average lattice space (about 3.4 Å) between two neighboring planes (002) as multilayer graphene, agreeing with the intense X-ray diffraction (XRD) peak at 2θ = 25.9° (Figures S4B and S6). The D peak (∼1,350 cm−1) of graphene presents in the Raman spectra (Figure 2C), reflecting the doping effect of heteroatoms (especially for N atoms introduced by CS) or the bent sp2-carbon bonds. The 2D peak at 2,700 cm−1 is originated from the second-order zone-boundary phonons, 15 and the I2D/IG ratio demonstrates the M-LIG has a multilayer structure.It is noted in the X-ray photoelectron spectroscopy (XPS) that heavy metal ions are reduced during the lasing process. For instance, Cu2+ is reduced into Cu+/Cu0 after lasing, which could enable more potential as catalysts (Figures S7E and S7F). Meanwhile, the core-shell structure is also formed in laser printing with the Cu wrapped by graphene layer, shown in HR-TEM (Figure S8). CS provides not only the essential carbon but also the abundant nitrogen (N), which could be doped in graphene as the heteroatom to tune its electronic properties. Four types of N atom are doped in the graphene structure. Specifically, the pyridinic N (398.9 eV) is more dominant, compared with oxidized N (403.5 eV), graphic N (401.6 eV), and pyrrolic N (400.6 eV) in N 1s peaks (Figure S7C).Laser-induced graphitization of biopolymers is challenging. To our best knowledge, lignin from lignocellulosic biomass as a complex of phenolic compounds is the only biopolymer converted to LIG via one-step lasing scribing in air atmosphere reported so far, owing to its aromatic ring structures. 16 Polysaccharide chains, such as cellulose and CS, could be easily decomposed into volatile compounds under the lasing process with the intensive local heat (Figure 2D). 17 Alternatively, the multi-step carbonization, 18 reduced gas protection, 19 and flame-retardant 20 methods have been applied together with the lasing process to obtain graphene from non-phenolic biopolymers. As a result, additional operations steps, chemical input, and energy consumption are essential, compromising the sustainability of lasing technology. Here, we successfully demonstrated the direct conversion of CS to LIG in the presence of wasted metal ions.These metals are expected to play important roles during the lasing process. 21 Take Cu-doped CS film as an example. The thermal gravimetry (TG) and derivative thermal gravimetry (DTG) curves show that the temperature corresponding to the maximum weight loss is higher for Cu-doped CS film than CS film, revealing improved thermal stability of CS film in the presence of copper ions. At 900°C, 7.6% of Cu-doped CS film remains eventually, compared with CS film that is almost decomposed (Figure S9; Table S1). This reservation of solid char could provide the essential carbon as graphene precursors. The finite element analysis (FEA) was applied to simulate the temperature increase on the surface of CS film and Cu-doped CS composite when they were exposed to laser irradiation (Figures 2D, S10, and S11). An ultra-high temperature of over 1,500 K is expected on the surface for Cu-doped CS film with laser power of 2.5 W at 1 ms, which is high enough to melt reduced Cu nanoparticles into their liquid form. This endothermic phase change of copper potentially happened during the process and echoes the simulation results that a lower surface temperature was obtained for Cu-doped CS film (5,880 K) compared with CS film (6,340 K) after lasing for 30 ms. In addition, the liquid copper could be an ideal catalytic component for the rapid orientation of graphene structure, caused by its relatively low carbon solubility and quasi-atomic surface. 22 In detail, the thermal motion of liquid Cu atom together with the surface tension collection minimized the surface energy, leading to the formation of a non-defect and smooth liquid Cu surface. This surface enables the rapid migration of carbon atoms and subsequent alignment of graphene structure. 23 Moreover, because of the higher temperature induced by laser irradiation, the surface tension of liquid Cu tends to decrease further, leading to more rapid transport and diffusion of carbon atoms. 24 Different from copper, other liquid metals have higher carbon solubilities, resulting in the extra carbon atom precipitated from the liquid metal surface to form the multilayer graphene structure. 25 Notably, because of the inertness of Cr for catalyzed graphene formation, 22 more graphene defects were observed in Cr-containing LIG (Figures 2C and S12). After cooling at room temperature, the M-LIG was successfully formed (Figure 2E). Moreover, the characteristics of M-LIG could be tuned by controlling the lasing parameter and the concentration of chelated metal ion in CS film, revealing its potential as high-valued M-LIG catalysts for a wide range of applications (Figures S13–S15). As a demonstration, we first employed the Cu-LIG as the catalyst for CO2ER and investigated its sustainability in turning multiple waste streams into green chemicals.First, the CO2ER was performed in the H-type reaction cell. A series of electrochemical tests had been explored to demonstrate the CO2ER activity of Cu-LIG catalysts with different initial Cu contents (2, 5, and 10 mg mL−1), termed Cu-LIG-x (x = 2, 5, and 10, respectively) (Figures S16–S20). The results demonstrate that the Cu-LIG-5 with a lower Tafel slope of 270 mV dec−1 is more beneficial for CO2ER than its counterparts. The Faraday efficiency (FE) of ethanol reaches up to about 20%, and the selectivity of ethanol exceeds 50% in the liquid products at −0.6 V (versus RHE) for Cu-LIG-5, whereas the selectivity of formic acid (FA) is over 90% in the liquid products at −1.2 V (versus RHE) (Figure 3 A). When the contents of Cu are decreased (Cu-LIG-2) or increased (Cu-LIG-10), the primary liquid product changes to FA under all overpotentials, but the FE of total carbon products is still maintained above 60% for both catalysts at even higher overpotentials. The semi-in situ XPS was employed to investigate the CO2ER process of the three Cu-LIG catalysts (Figure S21). For the Cu-LIG-5 catalyst, the Cu+ and Cu0 could be simultaneously observed at a lower overpotential of −0.6 V (versus RHE), whereas almost all the Cu+ is reduced to Cu0 at a higher overpotential of −1.2 V (versus RHE) (Figures 3B and S22). For the Cu-LIG-2 and Cu-LIG-10 catalysts, in contrast, the Cu+ can be barely observed at even lower overpotentials (Figure S23). This indicates that the concurrence of Cu+ and Cu0 in the Cu-LIG-5 at lower overpotentials is the key to the C–C coupling and the formation of C2 products (ethanol), but the Cu0 mainly leads the production of C1 product (FA), which shows a good consistency with many other reports 26 , 27 , 28 and can also be supported by density functional theory (DFT) calculations (see Figures S25–S29 and the supplemental information for more DFT calculation details).Moreover, in order to satisfy the higher current density required on an industrial scale, we also performed the CO2ER in the flow cell with the optimal Cu-LIG-5 catalyst. The detailed setup of flow cell is shown in Figure 3C. The linear sweep voltammetry (LSV) curves demonstrate that the CO2ER effectively occurs in the flow cell (Figure 3D). The CO2ER results indicate that the FE of total carbon products is higher in low current density (almost 85% at −50 mA cm−2), because of the better suppression of hydrogen evolution reaction (HER) (Figure 3E). The gas products are more dominant (above 70%) in the flow cell due to the rapid diffusion of reactants and intermediates, in which FE of ethylene (C2 product) is around 25% at −100 mA cm−2. Besides, the CO2ER could be stably conducted for at least 420 min in the flow cell, where the FE of total carbon products gradually decreases to 60%, but the C2+ products still preserve above FE of 20% (Figure 3F). Similarly, the production pathway of ethylene as the main C2 product was also calculated with DFT models, where the Cu+ + Cu0 shows a lower change of free energy (1.42 eV) in the C–C coupling (∗CHOH-CO formation) and is more conducive to produce the ethylene, compared with Cu0 and Cu+ models (Figures 3G–3I, S30, and S31). 28 Moreover, pyridinic N is conducive to enriching CO2 molecules into the carbon layer around Cu sites, and the increase of local concentration of CO2 around the Cu sites provides a favorable condition for promoting efficient CO2ER (Figure S32; Table S3).Apart from heavy metal ions, the organic micropollutants in the wastewater also greatly negatively affect human health and our ecosystem. In the proposed strategy, we chose the representative six kinds of organic micropollutants (2,4-dichlorophenol [2,4-DCP], 2-naphthol [2-NO], bisphenol A [BPA], bisphenol S [BPS], methylene blue [MB], and ethinyl estradiol) as the degradation models to demonstrate an application of M-LIG in producing clean water (Figure 4 A). As a demonstration, the Cu-LIG film is prepared and assembled in an in-house device containing H2O2, where the self-standing membrane form could be very attractive for instant applications (Figure 4B). For the Cu-based Fenton-like reaction, the parameters, such as H2O2 concentration and catalyst dosage, were first optimized with the BPA as the model pollutant (Figures S33 and S34). Compared with Cu powder catalysts, the consumption of membrane-based catalysts is much less (Table S7). Neutral reaction condition (pH 7.0) was chosen because of the favorability of practical application. Under the optimized conditions, the degradation efficiencies within 180 min for BPA, 2-NO, 2,4-DCP, and ethinyl estradiol were more than 90%, whereas those for BPS and MB are 80% and 75%, respectively (Figure 4C). Their total organic carbon (TOC) removal efficiencies were about 55% for MB, BPA, BPS, and 2-NO, whereas the efficiencies were 30% and 20% for 2,4-DCP and ethinyl estradiol, respectively, indicating reasonably good mineralization percentages. Furthermore, the continuous batch-type degradation of BPA was tested for self-standing membrane catalysts. The removal efficiency is up to 93.2%, achieved in the first cycle and gradually stable at 70% after the fourth cycle. According to the inductively coupled plasma mass spectrometer (ICP-MS) results, 0.45 ppm copper was leached after the first cycle, which could reduce the copper active sites, weakening the catalytic activity (Figure 4D). However, the leached Cu concentration is well below the control limit for drinking water (1.3 ppm). 9 The stable regeneration ability of Cu-LIG could be attributed to its core-shell structure formed during lasing printing. For the degradation mechanism, the electron paramagnetic resonance (EPR) results confirm the generation of ·OH radical in the presence of Cu-LIG catalyst with H2O2 reaction (Figure 4E). The process of ·OH generation could be explained in Figure 4F, Figure S35, and Note S5 (Supplemental experimental procedures). The possible degradation pathway of BPA is also proposed in Table S8 and Figure S36. Overall, the ·OH free radicals attack organic micropollutants and finally mineralize them to nontoxic CO2 and H2O. Combined with heavy metal ions removal, the wastewater could be synthetically purified into clean water via natural seafood waste.As emerging technology being developed in the lab, prospective LCA based on experiments and assumptions is a method to give environmental guidance to identify sustainable technologies in the early stage. 29 , 30 , 31 , 32 Herein, the carbon footprint of our proposed CO2ER was assessed according to the system boundary as defined in Figure 5 A. The baseline case (CO2ER) was constructed based on in-house experimental results and a state-of-art membrane electrode assembly (MEA) design using a cation-exchange membrane (CEM) coupled with a permeable CO2 regeneration layer (PCRL). This design enables over 85.0% single-pass CO2 conversion efficiency and results in FA product, instead of formate. 33 The produced gas products (ethylene, H2, and CO), as syngas after recycling CO2 were converted to olefins via the methanol (MTO) process and treated as by-products replacing the conventional fuels (see supplemental experimental procedures for more details). Global warming potential value (GWP), also as known as carbon footprint, illustrates that electricity and heat for FA distillation are the significant contributors in the baseline case reflecting the FE achieved in the lab, resulting in an overall GWP of 14.77 kg CO2 e kg−1 FA (Figure 5B). To incorporate future improvement, we simulated a forward-looking case with over 80% FE for FA, whose GWP could be reduced to 8.62 kg CO2 e kg−1 FA, as a result of a reduction in electricity consumption. For both cases, the electrolyte was assumed to circulate until the FA concentration accumulated to 9.6 wt % before being concentrated to 85 wt % via conventional distillation. 34 As an alternative to this energy-intensive product separation, a hybrid extraction-distillation (HED) using 2-methyltetrahydrofuran 35 was incorporated in the scenario analysis whose GWP reaches 3.85 kg CO2 e kg−1 FA, close to the lower bound of the reported range (3.10–5.30 kg CO2 e kg−1 FA) from other CO2ER studies. 36 This value is comparable with the conventional FA produced via the methyl formate route (2.24 kg CO2 e kg−1 FA) 37 and much lower than that via decarboxylative cyclization of adipic acid (7.34 kg CO2 e kg−1 FA). 38 Furthermore, decarbonized electricity supplies were also incorporated from a low-carbon future perspective. According to the projection in energy supplies for the national grid, along with China’s 3060 ambition, the GWP of our system could be further reduced to be 3.17 kg CO2 e kg−1 FA (S2), 1.38 kg CO2 e kg−1 FA (S3), and 1.30 kg CO2 e kg−1 FA (S4), respectively. 39 Under the most optimistic conditions, our system could achieve carbon negative (−0.55 kg CO2 e kg−1 FA) if both electricity and heat could be provided by renewable sources (i.e., solar and biomass). Overall, these results reveal that the upcycling of post-consumer waste to catalyst for CO2ER has the promising potential to convert captured CO2 to FA with a comparable or even lower GWP. Alternative to FA, if the FE is improved in the prospective scenario toward producing ethylene, a lower GWP would be expected because of the elimination of liquid product separation. To be conservative, emission credits from avoided disposal of post-consumer waste Cu-doped CS film were not included in the analysis because of its minor contribution to GHG emissions compared with energy use. However, other environmental benefits such as water- or human health-related categories from the avoided disposal could be substantial because of the appearance of heavy metal ions in the waste. 40 Nevertheless, our prospective LCA incorporating joint efforts in reactor design and products upgrading provides a holistic view on future directions to improve the sustainability of CO2ER. From the economics reviewed by Somoza-Tornos et al., 36 carbon monoxide and FA are the two products that are closest to being cost competitive, with electricity consumption, capital cost, and CO2 feedstock as major contributors to the cost of CO2ER. However, because of variabilities in process parameters assumption and capital cost estimate, future techno-economic assessment (TEA) is encouraged to be validated by pilot plant or industrial process. Therefore, similar conclusions from TEA are expected for our study, and further work is proposed to combine efforts in industrial development in the future.With regard to organic pollutants degradation, Table S7 summarized GWPs per ppm BPA degraded, mainly caused by the consumption of H2O2. Results represent that GWP of BPA degradation demonstrated in our study is at the lower end of the range owing to relatively higher degradation efficiency. However, it is notable that, to be conservative, credits as a result of the significantly smaller loading of catalyst in the process were not considered.In this work, we present an upcycling strategy to tackle environmental challenges caused by multiple waste streams. After removing the heavy metal ions in wastewater, the post-consumer metal/CS film is successfully upcycled into M-LIG via one-step laser scribing in ambient air. As a demonstration, the obtained Cu-LIG with controllable contents of metal ions and N sites in the porous graphene is utilized as a catalyst for CO2ER. The FE of total carbon products in the flow cell exceeds 80%, in which the FE of ethylene as a C2 product is approximately 25% at 100 mA cm−2. Together with DFT calculation results, it is confirmed that the concurrence of Cu+ and Cu0 in the Cu-LIG catalyst is the key to promoting the C–C coupling at low overpotentials. LCA results from the evaluation of a system design, which integrates joint efforts in reactor design and upgrading downstream products, reveal that CO2ER has the promising potential to convert captured CO2 to FA and olefins with a considerably low GWP in the decarbonized future. Furthermore, the organic micropollutants in wastewater could also be effectively degraded by a Fenton-like reaction using the self-standing Cu-LIG film. The estimated GWP per unit of BPA degraded demonstrated in our study is at the lower end of the range owing to the relatively higher degradation efficiency. Overall, our proposed strategy could be an inspiring and synergetic solution to upcycle multiple waste streams to green fuels and clean water as we strive for a sustainable future.Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Lei Wang (wang_lei@westlake.edu.cn).This study did not generate new unique reagents.Short-chain CS (degree of deacetylation >90%, viscosity 45 mPa·s for 1% (w/v) solution, molecular weight: ≈ 10 kPa) was purchased from Jinhu Company, China. Glycerol, copper nitrate trihydrate (Cu(NO3)2·3H2O), manganese sulfate monohydrate (MnSO4·H2O), zinc acetate dihydrate (Zn(CH3COO)2·2H2O), nickel chloride hexahydrate (NiCl2·6H2O), cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O), lead nitrate (Pb(NO3)2), silver nitrate (AgNO3), and potassium dichromate (K2Cr2O7) were of A.R. grade and purchased from Sigma-Aldrich. Pollutant model compounds were obtained from commercial sources and used as received.The glycerol (0.4 g) was dissolved in the DI water (98.0 g), before mixing with CS powder (2.0 g). Then, the CS solution was stirred at 900 rpm for 24 h until the solution became totally transparent. After that, the solution was poured into a Petri dish and dried at 40°C for 48 h. As control, each dried CS film was cut into 0.37 g, followed by immersion in NaOH solution (2 wt %) for 24 h and rinsing with DI water until neutral. The prepared CS film was preserved in DI water before using.The prepared CS film was sunk in heavy metal ion solution with chelation process for 24 h. In detail, eight heavy metal ions (Cu2+, Pd2+, Cd2+, Mn2+, Zn2+, Ni2+, Ag+, and Cr6+, respectively) were commonly found in wastewater and chosen with initially gradient concentrations (2, 5, and 10 mg mL−1, respectively). Meanwhile, to mimic the real application, we tested simulated industry wastewater from an electroplating process that contains Cr6+ (42 mg mL−1) and Cu2+ (3 mg mL−1). 12 After removal, the metal/CS film was dried at room temperature for 24 h and put in a desiccator for transforming to M-LIG.Laser printing was performed on conversion of metal/CS into M-LIG using a CO2 laser system (Universal laser cutter platform, 10.6 μm, 50-W laser) at a scan rate of 3% (measured to be ∼3.6 mm s−1), 1,000 pulses per inch, and laser powers ranging from 2.2 to 2.6 W with increments of 0.1 W. The image density was set at 4, which means the spacing between raster lines during lasing. The laser beam was focused at a z distance of 6.4 mm, to partially defocus the laser (∼1.4 mm).In the FEA, the photothermal model of laser scribing was suitable for demonstrating the temperature change during the laser processing. A few of the simulations have been successfully performed, based on the carbonized silk and polyimide film. 41 , 42 Followed with similar steps, we built two simple photothermal models to better understand the generation and difference of temperature with or without Cu doping in CS film.Combined with Gaussian beam used as the laser source and Beer-Lambert law, the heat source density per unit volume at a position (r, z) was (Equation 1) q ( r , z ) = α ( 1 − R ) 2 P π w 0 2 exp [ − 2 r 2 w ( z ) 2 ] exp [ − α z ] (Equation 2) w z = w 0 1 + ( z − E o F z R ) 2 (Equation 3) z R = π w 0 2 λ 0 , where r is the radius distance away from the center spot of laser beam and z is the vertical distance from CS film surface, P is the laser power, and w 0 is the incident laser beam radius at focus. The extent-of-focus (EoF) is the focus distance, which could be 0 (on focus plane), negative (over focus plane), and positive (under focus plane). α and R are the optical adsorption coefficient and reflectivity of material, respectively. For simplicity, α and R could be calculated according to the complex refractive index: 41 n + i k = 2.49 + 0.015 i for carbon materials because of the rapid carbonization of biopolymer.With the time t evolution and spatial (r, z) distribution of the temperature T (r, z, t), the heat transfer equation during the entire process is as follows: (Equation 4) ρ C p ∂ T ∂ t − ∇ ⋅ k ∇ T = q ( r , z ) . Corresponding to the initial condition, (Equation 5) T ( r , z ; t = 0 ) = T e x t = 293 K . The boundary condition was: (Equation 6) n ⋅ q = h 1 ( T e x t − T ) for surface and (Equation 7) n ⋅ q = 0 for edges , where ρ, Cp, and k are, respectively, the density, specific heat, and thermal conductivity of CS film. These parameters were measured with pure CS film and Cu-doped CS film at room temperature (Table S2). The photothermal model built through Equations 1–7 was finally solved by FEA method with commercial COMSOL Multiphysics. To be mentioned, the laser process is really complicated to accurately simulate the energy transfer, caused by chemical reaction, pyrolysis, and material phase transition. The models still need to be optimized.The Cu-LIG powders (5.0 mg) were scraped from Cu-LIG film surface using a razor blade and poured into mixture solution with 400 μL isopropanol and 75 μL DI water. After bath sonication for 10 min, the 25 μL Nafion solution (5 wt %) was added into the above prepared suspension solution, followed by sonicating for another 60 min until the ink was homogeneous. For each electrode, a 100 μL mixture solution was repeatedly dropped onto a piece of carbon fiber paper and dried in the oven for 6 h at 80°C. Note that the carbon fiber paper was dried in a vacuum oven for 120 min at 100°C before using. The prepared electrode was directly exposed to the electrolyte with the fixed geometric area (1 cm2).The morphology of samples was observed with Zeiss Gemini 500 instrument field emission SEM. TEM and HR-TEM images were operated using a Thermo Fisher Scientific Talos F200X G2 system. FTIR spectra were recorded using a Thermo Fisher Scientific Nicolet iS50 spectrophotometer using an attenuated total reflection (ATR) mode over the range 400–4,000 cm−1. Semi-in situ XPS data were obtained on a Thermo Fisher ESCALAB Xi + microprobe using monochromatic Cu-Kα radiation (hv = 1,486.6 eV) as the excitation source. The electrode used in CO2ER was protected in the N2 atmosphere to avoid oxidation of the catalytic surface. Powder XRD measurement was carried out on a Bruker D8 Advance with Cu Kα radiation (λ = 1.5406 Å). A Raman microscope using 532-nm laser excitation at room temperature with a laser power of 2 mW was employed to obtain Raman spectrum. The surface area of Cu-LIG was measured with a Micromeritics 3FLEX Brunauer-Emmett-Teller (BET) surface analyzer.Similar with Xu et al.’s work, 43 the semi-in situ XPS was performed as follows: the CO2ER was conducted in a glove box that was filled with nitrogen for 6 h in advance to ensure the catalyst is protected from oxidation. After CO2ER, the electrode was taken out and moved to a gas-tight chamber of XPS in the glove box. The gas-tight chamber was then sealed and passed to the XPS system for testing immediately (Figure S21).The electrochemical experiments were carried out at ambient temperature and pressure in the two-compartment electrochemical (H-type) cell, separated by Nafion 117 cation exchange membrane. The measurement used a prepared carbon fiber paper with catalyst loaded as the working electrode operated by a CHI660E electrochemical workstation (Shanghai Chenhua, China). The counterelectrode was platinum, and the reference electrode was Ag/AgCl (3 M KCl), calibrated against hydrogen reference electrode (RHE; HydroFlex; Gaskatel). Before CO2ER tests, 0.1 M KHCO3 as electrolyte was saturated by CO2 (99.999%) through bubbling the gas for at least 30 min and calibrated by mass flow controller at a constant rate of 30 sccm. The gas continuously was bubbled into the electrolyte during electrolysis at a flow rate of 10 sccm. The LSV and cyclic voltammetry (CV) at a scan rate of 10 mV s−1 were performed at different overpotentials. The ohmic drop between the working electrode and the reference electrode was determined using potentiostatic electrochemical impedance spectroscopy at −0.6 V versus Ag/AgCl between 106 and 1 Hz with an amplitude of 10 mV. The current density was calculated on the basis of the total current divided by the geometric area of Cu-doped graphene. All overpotentials (V) were converted to the RHE scale using the following formula: (Equation 8) V R H E = V A g / A g C l + ( 0.199 V ) + ( 0.0592 V ) × p H . For durability analysis of the CO2ER process, the electrolyte was taken out of the electrochemical cell every 2 h to quantify the product by nuclear magnetic resonance (NMR) measurement.For the electrochemical tests in gas diffusion flow cell, three compartments, including gas diffusion chamber, cathode chamber, and anode chamber, were separated by working electrode and anion exchange membrane. The electrolyte (1.0 M KOH) with 10 mL min−1 is continuously pumped into the cathode and anode chambers, respectively, while CO2 gas was introduced into the gas diffusion chamber at the rate of 20 mL min−1. The electrochemical test process is consistent with those in the H-type electrolytic cell.The gas-phase products were collected and analyzed at 20-min intervals by gas chromatography (GC), which is equipped with thermal conductivity detector (TCD), flame ionization detector (FID), and methanizer. High-purity Ar was used as the carrier gas of TCD, and high-purity N2 was used as the carrier gas of FID. The peak areas of the products (H2, CO, C2H4, and CH4) were converted to gas volumes using calibration curves that were obtained, using a standard gas diluted to different concentrations. The liquid products were quantified using 600 MHz Solution NMR (Bruker BioSpin, AVANCE NEO) spectrometer. After electrolysis, 450 μL electrolyte was mixed with 50 μL D2O (99.9%; Sigma-Aldrich) for the 1H NMR spectroscopy analysis with water suppression. Standard curves for each product were prepared by the relative peak area ratio between product and internal standard.FE calculation, based on the definition of FE, is 44 (Equation 9) F E i = Q i Q t o t a l , where i represents different products, such as H2, CO, C2H4, CH4, HCOOH, and ethanol, and Qi and Qtotal are the number of charges transferred to the product and the total number of charges passed into the solution, respectively. FE for formation of gas products, such as C2H4, CH4, H2, and CO, was calculated as follows: (Equation 10) F E = n F V v P 0 R T i × 100 % where n is the number of electrons required for products, V (vol %) is the volume concentration of gas product from the electrochemical cell, ν (ml min−1 at room temperature and ambient pressure) is the gas flow rate, i (mA) is the steady-state cell current, p = 1.01 × 105 Pa, T = 273.15 K, F = 96,485 C mol−1, and R = 8.314 J mol−1 K−1.The FE for formation of liquid products, such as ethanol and FA, was calculated as follows: (Equation 11) F E = α n F Q × 100 % , where α is the number of electrons required for each product, n is the mole number of products (mol), F is the Faradaic constant (96,485 C mol−1), and Q is the total charge passed during the overall run.The partial current density of each product was calculated as follows: (Equation 12) j ( p a r t i a l ) = F E ( p a r t i a l ) 100 % × j ( t o t a l ) , where FE (partial) is the FE of product and j (total) is the total current density of steady-state cell.First, three models (Cu0, Cu2O (Cu+), and Cu0 + Cu2O (Cu0 + Cu+), respectively) were built, followed with the results of semi-in situ XPS. We employed the Vienna Ab Initio Package (VASP) 45 , 46 to perform all the DFT calculations within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) formulation. 47 We chose the projected augmented wave (PAW) pseudopotentials to describe the ionic cores and take valence electrons into account, using a plane wave basis and setting a kinetic energy cutoff of 400 eV. 48 , 49 Partial occupancies of the Kohn-Sham orbitals were allowed to use the Gaussian smearing method and a width of 0.05 eV. The electronic energy was taken self-consistently when the energy change was smaller than 10−4 eV. A geometry optimization was considered convergently when the force change was smaller than 0.05 eV Å−1. The vacuum spacing perpendicular to the plane of the structure was 18 Å. The Brillouin zone integral used the surfaces structures of 2 × 2 × 1 Monkhorst-Pack K point sampling.In a typical vial experiment, two pieces of prepared catalyst films (effective area: 0.5 cm × 1 cm for each piece) were set up in 10 mL of 23 ppm pollutants solution (specifically, 2 ppm for ethinyl estradiol). After establishment of adsorption/desorption equilibrium for 30 min, 10 mM H2O2 was added to the pollutant suspension under stirring at 150 rpm throughout the experiment. At time intervals, 500 μL of suspension was collected and centrifuged at 10,000 rpm for 20 min, then 200 μL of the supernatant was sampled and analyzed immediately. For regeneration analysis of catalyst film, the fresh BPA solution was added into the vial with the recycled catalyst film, followed by reaching balance of adsorption and desorption with 30 min. Then, 10 mM H2O2 solution was added, and removal efficiency was measured with a specific time interval.The samples were analyzed by ultra-performance liquid chromatography (UPLC) equipped with PDA detector (H-Class; Waters). The compounds were separated by C18 column (1.7 μm, 2.1 × 50 mm, Acquity UPLC@BEH) with gradient elution. Mobile phases of H2O (0.05% FA) (A) and acetonitrile (0.05% FA) (B) were used. The flow rate was kept at 0.4 mL min−1. The initial gradient of 10% B was held for 0.5 min and increased to 40% B within 1 min and held for 2 min. The gradient was further increased to 100% B and held for 3 min. The chromatogram was acquired with a wavelength of 276 nm. TOC was determined by a Shimadzu TOC-L-CPH analyzer. The leached Cu during Fenton-like catalytic reaction was determined by ICP-MS (iCAP RQ; ThermoFisher). The EPR spectra were recorded on Bruker EMXPLUS EPR spectrometer at room temperature, and DMPO was used as the spin trap agent for ·OH free radical. The intermediates during the degradation process were analyzed by GC-mass spectrometry (GC-MS; Trace1300-ISQ7000; ThermoFisher) with DB-5 MS capillary column. The GC oven temperature program was set as follows: initial 60°C was held for 2 min followed by linear temperature gradient to 280°C at 6°C min−1, held for 5 min. For liquid samples for GC-MS analysis, the catalytic film was separated with pollutant suspension after 180-min reaction, followed by freeze-drying for 2 days. Then both residues were dissolved in 2 mL of dichloromethane. After dehydration by anhydrous sodium sulfate, 0.2 mL of N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) was added during stirring at 150 rpm for 2 h, and precipitates were separated by centrifugation until chromatographic analysis.A prospective LCA based on lab experiments and assumptions reflecting the recent progress in reactor and process designs was conducted in this study. GWP is for CO2ER system where products replace traditional fuels and chemicals. Contribution analysis was applied to reveal major contributors to LCA results, while scenarios analysis is supplemented to incorporate future improvements in technologies. A schematic diagram of “cradle-to-gate” system for CO2ER is defined in following the ISO 14040 series. 50 The objectives of LCA studies are (1) to assess environmental performances of CO2ER using an innovative catalyst, and (2) to compare the new systems with conventional products. The system boundary thus includes energy and materials flows associated with CO2 capture, electrochemical CO2 conversion, CO2 recycling, products separation, and the conversion of syngas to olefins. The functional unit for CO2ER system was defined as 1 kg of main product FA (>85.0 wt %) for the ease of comparison with other studies. To deal with by-products such as O2 and olefins produced from syngas, environmental burdens from their productions are assumed to be avoided as a result of replacing the conventional, following the system expansion principle. More information about technology description, assumptions, and life-cycle inventory can be found in the supplemental experimental procedures.This work was supported by the Natural Science Foundation of China (grant 52003225), the Westlake Multidisciplinary Research Initiative Center (MRIC), the Research Centre for Industries of the Future (RCIF), and the foundation of Westlake University. The authors thank the Westlake Center for Micro/Nano Fabrication, the Instrumentation and Service Center for Molecular Sciences, and the Instrumentation and Service Center for Physical Sciences (ISCPS), Westlake University.Conceptualization: Z.L., X.H., B.Z., L. Wen, and L. Wang; methodology: Z.L., X.H., and L. Wang; investigation: Z.L., X.H., S.Z., Y.C., L. Wang, and X.D.; visualization: Z.L.; funding acquisition: B.Z., L. Wen, and L. Wang; project administration: L. Wen and L. Wang; supervision: L. Wen and L. Wang; writing – original draft: Z.L. and X.H.; writing – review & editing: L. Wen and L. Wang.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.xcrp.2023.101256. Document S1. Supplemental experimental procedures, Figures S1–S36, Tables S1–S8, and Notes S1–S5 Data S1. Life-cycle assessment results Document S2. Article plus supplemental information

Our ecosystem is endangered by an increasing demand for resources, emission of pollutants, and wastes generated from industrial processes. Herein, we propose a sustainable upcycling strategy to tackle excessive carbon dioxide emissions and water quality issues synergistically. The crux of the strategy is to use seafood waste, easily converted to chitosan, to chelate heavy metals in contaminated water and produce metal-doped graphene composites. Graphene composites are prepared by scalable one-step laser scribing with controllable metal ion and nitrogen sites. The catalysts enable the efficient electroreduction of carbon dioxide to carbon products with over 80% Faradaic efficiency at 100 mA cm−2 in a flow cell reactor and can also catalyze the degradation of organic micropollutants. A prospective life-cycle assessment demonstrates a lower global warming potential, compared with conventional systems, for this system being used to produce formic acid and olefins in the future. Our sustainable upcycling strategy is expected to inspire practical techniques for producing green chemicals and clean water.

In recent years, researches on renewable and sustainable fuels have been highly prioritized around the world to create alternatives to fossil fuels. In this way, different types of biofuels have gained evidence due to their biodegradable, non-toxic, and physical–chemical properties, which allow the total or partial replacement of diesel (Li et al., 2014). Biofuels also are interesting under economic viewpoint since they can be synthesized from vegetable oils, animal fats, and raw materials rich in free fatty acids, such as residual oils (Lam et al., 2010).The improper disposal of the residual oils can cause environmental problems since each liter of oil poured into the drain can pollute about 20 thousand liters of water (Georgogianni et al., 2009; Al-Hamamre and Yamin, 2014; Baskar et al., 2018). Indeed, it is possible to find in the literature several works (Widayat et al., 2019; Dai et al., 2017; Corro et al., 2016; Gan et al., 2010, Ashok et al., 2019) which the cook oils are used to synthesize biodiesel, and it is undoubtedly an eco-friendly and sustainable strategy for the next generations (Aghbashlo and Demirbas, 2016).Biodiesel is synthesized via transesterification or esterification chemical reactions. In some cases, depending on the origin of the raw material (such as the presence of triglycerides and free fatty acids), the reaction kinetics can be directed by simultaneous transesterification and esterification. On an industrial scale, biodiesel can be produced by both homogeneous and heterogeneous catalysis. Homogeneous catalysis has several disadvantages, such as the formation of soaps and their by-products, corrosion of the reactors, in addition to requiring several purification steps during the production process (Lee et al., 2014; Avhad and Marchetti, 2015; Mardhiah et al., 2017). On the other hand, the synthesis of biodiesel via heterogeneously catalyzed reactions has been the subject of promising studies being a viable solution to replace homogeneous catalysis, since it is possible to significantly reduce the number of purification steps and the possibility of separating and reusing the catalyst (Correia et al., 2014; Rashtizadeh et al., 2014; Paiva et al., 2015; Kim et al., 2016).In the heterogeneous catalysis, several types of catalysts stand out; however, ceramic compounds in the form of oxides (Baskar et al., 2018; Xie and Zhao, 2014; Gurunathan and Ravi, 2015; Sun et al., 2015; Sulaiman et al., 2019), have been extensively investigated in recent years for their high catalytic activity, excellent thermal and chemical stability, high corrosion resistance and environmentally optimized properties (Pradhan and Parida, 2012), and can be recovered and reused without significant loss efficiency in synthesis (Dantas et al., 2013).Several chemical synthesis techniques can be used to obtain ceramic oxide catalysts, among them stand out the sol–gel route (Kesavamoorthi and Raja, 2016), co-precipitation (Zaharieva et al., 2015), Pechini method (Gerasimov et al., 2015) and combustion reaction (Dantas et al., 2020). The combustion reaction, which is the chemical synthesis techniques used in this work, has stood out for being a simple, effective, economical method (it uses low-cost reagents, less reaction time), it allows control of stoichiometry and morphology, besides promoting the obtaining of high crystallinity ceramic powders (Costa and Kiminami, 2012). Due to its various advantages, numerous studies have reported the use of the combustion reaction in the production of materials applied in several areas, such as photocatalysis (Das et al., 2019; Hermosilla et al., 2020), electronic materials (Vieira et al., 2014; Shanmugavani et al., 2015; Tholkappiyan et al., 2015; Diniz et al., 2017), heterogeneous catalysis (Manikandan et al., 2014; Alaei et al., 2018; Dantas et al., 2020; Kombaiah et al., 2019; Mapossa et al., 2020), and biomaterials (Araújo et al., 2018; Khot et al., 2013; Kombaiah et al., 2018; Leal et al., 2018).Among the oxides already reported in the literature with catalytic potential, the hematite (α-Fe2O3) with binary structure type AnXp is widely used in several catalysis reactions because it is stable in ambient conditions and easy to process by different methods (Aghbashlo and Demirbas, 2016; Gurunathan and Ravi, 2015; Tholkappiyan et al., 2015; Kombaiah et al., 2019; Widayat et al., 2019). Widayat et al. (2019) used hematite (α-Fe2O3) synthesized by chemical co-precipitation in residual oil esterification/transesterification reactions and obtained 87.88% conversions in methyl esters. Studies conducted by Shi et al. (2017) showed the efficiency of hematite (Fe2O3) also as the support of oxides (CaO) in the production of biodiesel, in transesterification reactions of soybean oil and methanol, showing conversion to esters of 98.80%.Iron-based catalysts, such as ternary oxides of the type (AB2X4), have also attracted attention from the scientific community due to their properties and new technological applications, especially when the particle size approaches the nanoscale, which allows the control of properties such as magnetic characteristic and anisotropy (Dantas et al., 2020, Mapossa et al., 2020; Dai et al., 2017). In heterogeneous catalysis (Dantas et al., 2013, Dantas et al., 2017), the Ni0.5Zn0.5Fe2O4 and Ni0.7Zn0.3Fe2O4 ferrites synthesized by combustion reaction and tested the catalytic behavior in transesterification and esterification using methyl and ethyl routes, obtaining conversions in esters above 94%.Another heterogeneous catalyst that has a consolidated catalytic activity in the literature is zinc oxide (ZnO). According to Lamba et al. (2019), which synthesized the ZnO by combustion reaction and tested catalytically against methanol and madhuca oil, obtaining about 80% in conversion into esters. Baskar et al. (2018) also revealed the efficiency of the ZnO phase as support for biodiesel production, presenting conversions of 95.20% in methyl esters.In this work, the combustion reaction was used to synthesize a new magnetic catalyst with composition equal to ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3. The material was synthesized in a pilot-scale (Costa and Kiminami, 2012) and characterized in terms of its structure, morphology, magnetic and catalytic properties. Also, its catalytic capacity was investigated on the synthesis (TES reaction) of biodiesel from residual oil.In this research, the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst was synthesized via a combustion reaction from the following chemical reagents, nickel nitrate hexahydrate (Ni(NO3)26H2O), hexahydrate zinc nitrate (Zn(NO3)2·6H2O), iron (III) nitrate nonahydrate (Fe(NO3)3·9H2O) and urea. All chemical reagents used were purchased on the Dinâmica (Brazil) with purities between 98 and 99%. The performance of the catalyst was evaluated on the conversion of residual oil into biodiesel via simultaneous transesterification and esterification reactions (TES). The residual oil used was collected in pastry shops in the city of Campina Grande, located in Paraíba state - Brazil. The physicochemical parameters of the residual oil were accomplished in agreement with AOCS Cd 3d-63 standard, and the result showed a value of 14.8 ± 0.005 mg of KOH/g of sample, methyl alcohol (CH3OH)-purity 99.8% (Dynamic) and ethyl alcohol (CH3CH2OH) - purity 99.5% (Dynamic).The combustion reactions were accomplished in a pilot plant, which was built in the agreement of the patent BR 10 2012 002181–3 (Costa and Kiminami, 2012), see Fig. 1 . The pilot-plant is constituted of the stainless-steel container, which is connected to a conical reactor with a capacity of 200 g/batch. The system reaches a maximum temperature equal to 350 °C after 60 min.Before the synthesis of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst, the initial composition of the precursor solution was calculated based on the total valence of oxidizing agents and reducing reagents using the propellants and explosives chemistry concepts (Jain et al., 1981). The auto-ignition (combustion) of a stoichiometric mixture of metallic nitrates and urea allocated in a stainless-steel container in a conical reactor with a production capacity of 200 g/batch (see Fig. 1). The temperature of the combustion reaction was measured every 5 s with the aid of an infrared pyrometer (Raytek, model RAYR3I ± 2 °C).The performance of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst was evaluated in the synthesis of biodiesel from residual oil via TES reaction. Before biodiesel synthesis, the residual oil was filtered (filter paper ₵15,00 ' ± 0,15 cm) to remove the suspended particulate matter. The catalytic tests were conducted in duplicates and a pressurized stainless-steel reactor equipped with a pressure gauge, a thermocouple inlet duct, a borosilicate glass (80 mL). The conditions of the experiment were 30 g oil mass, time 1 h, and alcohol/oil ratio (15:1), see Table 1 . The heating and agitation of the system were carried out with the aid of a plate model IKA C-MAG HS 7, external electrical resistance, and a magnetic bar of approximately 2.5 cm. After the reactions, the products of the catalytic tests were centrifuged to separate the catalyst, purified, and dried in an oven at 110 °C for 20 min with manual stirring at 5-minute intervals.For the analysis and optimization of the biodiesel synthesis from residual oil, a 23 factorial experimental design was drawn up in which it was analyzed the response surface and Pareto graph, evaluated using the Statistic 7.0 program. Table 1 describes the input levels and variables for the proposed planning.The temperature, catalyst concentration, and alcoholic route were the factors considered in the 23 factorial experimental design. The three levels for the selected factors were determined from preliminary experiments and literature published elsewhere (Dantas et al., 2020) (Table 1). The conversion of residual oil into biodiesel was performed as the answer to determine the optimized parameters. The effect of the independent factors on dependent factors was analyzed according to Eq. (1): (1) Y = a 0 + a 1 X 1 + a 2 X 2 + a 3 X 3 + a 12 X 1 X 2 + a 13 X 1 X 3 + a 23 X 2 X 3 + e , where Y is the answer (biodiesel conversion, %), a0 is the compensated term; a1, a2 e a3 are linear coefficients; a12, a13 , and a23 are the interaction coefficients; and e is the error. X1, X2 e X3 are input variables: temperature, the quantity of catalyst, and alcoholic route, respectively.The reuse tests were accomplished under the best reaction conditions established by the experiments and experimental planning. Before each reuse step, the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst was removed from the synthesis products and clean with water 70 °C and hexane 99% (C6H14). After biodiesel synthesis, the catalyst was removed from the reaction medium using the following experimental procedure: application of an external magnetic field (magnet), washing with hot distilled water (~60 °C), washing the hexane solvent, centrifugation for 15 min, and oven drying at 110 °C for 24 h. This experimental procedure was adapted from the work published by Dantas et al. (2020). Finally, the reuse tests were performed under the best reaction conditions established by the experiments and experimental planning with the tested catalyst.The ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst synthesized was characterized by X-ray diffraction (XRD) using a BRUKER X-ray diffractometer (model D2 PHASER, Cu-Kα radiation), operating with 30 kV and 10 mA. The angular step and counting time used were 0.016° and 44 min, respectively. The crystallite size was calculated with the aid of the Scherrer equation (Klug and Alexander, 1974), and from the peak of the most intense basal reflection, spinel d(311). The identification of the main crystalline phases was performed with the DiffracPlus Suite Eva software and Joint Committee on Powder Diffraction Standards (JCPDS). The quantification of each main crystalline phase was carryout by the Rietveld refinement (Rietveld, 1967; Moulton, 2019) with the aid of Diffrac. Topas software. The residual error of the Rietveld refinement was calculated from Eq. (2), which Wi = 1/Iobs and Iobs e Icalc are the observed and calculated intensities for each step, respectively. (2) Sy = ∑ i W i I Obs - I Calc 2 The surface of the catalyst was characterized using the nitrogen gas adsorption and desorption technique. All experiments were carried out in a Quantachorme model NOVA 3200 equipment. The surface area and pore diameter were calculated using the Brunauer, Emmett, and Teller (BET) and by Brunauer, Joyner, and Halenda (BJH) methods, respectively.The morphological aspects of the catalyst sample were acquired by scanning electron microscopy (SEM), brand Tescan, model Vega3. The Laser diffraction technique was used to measure the particle size distribution using a nanoparticle analyzer SZ-100 series (HORIBA Scientific).Hysteresis plots were measured at room temperature using a vibrating sample magnetometer (VSM, Lake Shore model 7404), with a maximum applied magnetic field of 13,700 G. Saturation magnetization (Ms), remaining magnetization (Mr), and coercive field (Hc) were the properties obtained from this experiment.The acidity of the catalyst was determined through desorption analysis at the programmed ammonia temperature (TPD-NH3) in the SAMP3 multipurpose analysis system. Approximately 100 mg of sample was pretreated at 400 °C under helium atmosphere (30 mL.min−1). Then, the temperature was reduced to 100 °C, and the sample was subjected to ammonia current, for chemical adsorption, for 45 min. In the final step of the adsorption process, NH3 molecules were removed at 100 °C for 1 h and helium flow rate 30 mL.min−1. The thermograms were obtained on heating (from 100 °C to 800 °C), at 10 °C.min-1, and under a helium flow rate (30 mL.min−1).Thermogravimetric analysis (TG/DTG) was performed using Perkin Elmer STA 6000 TG-DTA equipment in N2 atmosphere with the flow of 20 mL.min−1 and heating rate of 10 °C.min−1, using 10 mg of sample in an alumina crucible, and a temperature range from 30 to 850 °C;The percentages of methyl or ethyl esters were determined via gas chromatography, using a chromatograph (VARIAN 450c) instrument with a flame ionization detector and a capillary column as the stationary phase (Varian Ultimetal “Select Biodiesel Glycerides RG”; dimensions: 15 m × 0.32 mm × 0.45 mm). The initial injection temperature was 100 °C, the oven temperature was 180 °C, and the detector operated at a temperature of 380 °C.The acidity index (official AOCS method, Cd 3d-63) was used to characterize both the residual oil and the products resulting from the catalytic tests. It was possible to quantify the mass yield of synthesized biodiesel, considering the initial mass of the residual oil in the TES reaction, assuming that the complete reaction of a specified amount (x) of residual oil leads to the achievement of 100% yield mass (X) of biodiesel. Therefore, the percentages of mass yields were defined and calculated as the values that express the masses of the final products of the reactions after the purification processes. Fig. 2 shows the combustion reaction behavior measured during the synthesis of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst. In summary, it was possible to identify three stages, where stage 1 was characterized by an oscillation in temperature that favored the evaporation of moisture followed by liquefaction of the reagents. In stage 2, the formation of the “mushroom” was observed (due to an increase in viscosity), followed by an excessive gas release. The ignition of the reagents combustion occurred in the final part of stage 2 (~1500 s). Stage 3 was instantaneous (~10 s) and reached a maximum temperature of 316 °C. In this last stage, there was the formation of an orange flame with a continuous and intense gas release. Still in step 3, it was possible to see a reaction explosion with flaking of the reaction product. The yield of the synthesis of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst was 82.93%, more precisely 165.8 g of catalyst per batch.As the maximum temperature reached during the synthesis was relatively low (<500 °C), the materials synthesized have a high surface area, and a very pronounced nanometric characteristic, therefore, is suitable for its use as catalysts. This constitutes ease and versatility of the combustion reaction technique because, due to the control of the synthesis temperature, it becomes possible the morphological and structural control of the material, which is required for a given application (Dantas et al., 2017). Fig. 3 shows X-ray diffraction obtained from the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst synthesized via a combustion reaction. The following crystalline phases were identified, inverse spinel of Ni-Zn ferrite (JCPDS 52-0278), hematite (JCPDS 89-0599), and zinc oxide (JCPDS 36-1451). The total crystallinity of the synthesized material was estimated at 43%, and the average crystallite size (calculated by Scherrer equation (Klug and Alexander, 1974; Avila et al., 2019) was equal to 25 nm. The estimated low crystallinity presented probably is related to the low-temperature of synthesis of the catalyst via combustion reaction (316 °C, see Fig. 2). This result is in agreement with other works that synthesized materials via combustion reaction and with a chemical composition similar to the one studied in this work (Dantas et al., 2020; Mapossa et al., 2020). Fig. 4 shows the Rietveld refinement accomplished on the X-ray diffractogram obtained from a ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst. From this analysis, it was possible to see that the hematite was the major crystalline phase (55.87%); the inverse spinel of Ni-Zn ferrite was the second most abundant crystalline phase (36.96%), and zinc oxide was the crystalline phase with the lowest percentage (7.16%). Table 2 summarizes the crystalline system, percentage of the crystalline phases, and the space groups calculated from the Rietveld refinement. In general, it is observed that the calculated parameters were very close to the theoretical values, and the values of the GOF, Rwp, and Rexp were 2.87, 0.99, and 0.35, respectively. Similar results were related by Mapossa et al. (2020). Fig. 5 shows the N2 adsorption/desorption isotherms measured from the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst. The isotherm obtained is of type III; which is an indication that the adsorption process is characteristic of non-porous or macroporous materials (Alothman, 2012). Also, the isotherms showed an inflection at a relative pressure (P/P0) of approximately 0.2 cm3/g, which is also indicative of the presence of micropores (Alothman, 2012).In agreement with IUPAC, solids containing pores diameter greater than 50 nm are called macroporous, between 2 and 50 nm are mesoporous, and those with pores smaller than 2 nm are called of microporous. The measured pore diameter from the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst was equal to 3.33 nm. Thus, the pore diameter and the isotherm profile corroborate with the indication that the synthesized catalyst has a mixed surface, that is, non-porous regions and other regions that have mesoporous or microporous.The specific surface area values (SBET) measured from the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 catalyst was 52.9 m2g−1. This value is considered relatively high and is a consequence of the method used to synthesize the catalyst (combustion reaction at temperatures below 500 °C). The fact of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 has a relatively high specific surface area and nanometric characteristics make it an excellent candidate to be used as a catalyst. Some studies report that the synthesis temperature is a significant factor in obtaining materials with high surface area and nanometric characteristics. Materials synthesized at high temperatures (>1000 °C) have surface changes that are more pronounced, and in some cases, these modifications considerably reduce the surface area and active sites of catalysts, which negatively affect their catalytic activity (Tang et al., 2012). Fig. 6 a–b shows SEM images obtained from the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst. It is possible to observe several clusters of different sizes (Fig. 6a). This characteristic is more evident in Fig. 6b, where it was possible to detect agglomerates with high porosity and dimensions between 20 μm and 10 μm, respectively. These results are in line with the discussion as mentioned earlier about the specific surface area analysis when it is indicated that the catalyst obtained has disordered surface characteristics with non-porous regions and other regions that have mesopores or micropores with different types of shapes and sizes. The high porosity is due to the release of large quantities of gas during the synthesis process by combustion reaction (see step 3 in Fig. 2). Still in Fig. 6a–b, both indicate a surface with a certain roughness, it is also possible to infer that the particles are weakly connected in an interparticular way. Similar results were observed by Tatarchuk et al. (2020) when studying the morphology of zinc spinel type ferrites. Fig. 7 shows the cumulative curve of the distribution range of the agglomerates (“S” shape) and histogram of the frequency of the distribution of agglomerate populations with the same diameter (first derivative of the distribution curve) measured from the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst. The distribution range of the particle diameter was between 20 nm and 100 nm, with an average diameter of 39.2 nm.From the distribution of the clusters, it was possible to observe that all samples showed a symmetrical and monomodal distribution of clusters, indicating samples with most of the total number of their clusters, as well as a finer particle size between them (values < 100 nm). Such a result can be associated with the characteristics of particle size; smaller particle diameters necessarily imply a more remarkable ability to agglomerate by electrostatic forces.The structure, shape, and reactivity of the catalyst surface have a strong interaction with nature, the number and the intensity of the active sites available for the reaction. Thus, the reactivity of the catalyst surface is one of the inherent characteristics and its processing method. Therefore, to obtain a better precision of the surface reaction and to verify if this material is promising for catalysis, one must understand the acidity and alkalinity of the catalyst. (Dantas et al., 2017). In this way, the active acid sites of ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 were determined via TPD-NH3 analysis, see Fig. 8 .Still in Fig. 8, it is possible to identify three NH3 desorption peaks. The first peak presented greater intensity, occurred at 208 °C, and is related to weak to moderate acidic sites. The second and third peaks occurred at 493 °C and 595 °C, respectively. These peaks are related to the strong acidic sites. The temperatures and intensity of the peaks observed in this work are in agreement with Dantas et al. (2017) and Dantas et al. (2020). Table 3 lists the results obtained from the TPD analysis. The acidity of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 catalyst was calculated from the integration of the Gaussian curves observed in the TPD-NH3 analysis. The result indicated the existence of two types of NH3 desorption sites, which the first peak related to weak and moderate acid sites, represented by the temperature range between 100 and 350 °C, while the strong acidity sites are in the range temperature between 450 and 650 °C. Similar results were also reported by Masiero et al. (2009).Therefore, from TPD-NH3 analysis, the desorption events present in the samples showed concentrations corresponding to weak, moderate, strong acidic sites, and the calculated values were 169, 73, and 14 μmol/g of NH3, respectively. The sample had a total acidity of 256 μmol/g of NH3. From the highlighted results, it is possible to conclude that the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst has a strong acid character. Also, the results shown in this work corroborate with studies published by Dantas et al. (2020) and Mapossa et al. (2020), which investigated acidic sites by means TPD-NH3 analysis, and confirmed the presence of weak, moderate, and strong total acidic sites for spinel-type ferrites with composition chemistry, structure, and morphology similar to those synthesized in this work. Fig. 9 shows the dependence of magnetization (M) as a function of the applied magnetic field (H) for the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst. From the hysteresis curve, it is possible to conclude that the studied catalyst has characteristics of soft magnetic materials. The low values of remaining magnetization (Mr = 6.12 emu/g) and coercivity (Hc = 5.3 G) support this information, since the magnetic hysteresis cycle is shown is narrow. Also, the well-defined S shape of the hysteresis curve is indicative that the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst has ferrimagnetic properties (Wang et al., 2012; Nihore et al., 2019).Also, in Fig. 9, the low value of the remaining magnetization (Mr) observed can be explained in terms of the composition of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst, since there is coexistence between crystalline ferromagnetic phases (55% Fe2O3), (36.96% Ni0.5Zn0.5Fe2O4) (Dantas et al., 2020, Shi et al., 2017), and diamagnetic phase (7.16% ZnO) (Franco et al., 2017). Similar results have been reported in the literature (Diniz et al., 2017) for the Ni-Zn system by microwave energy, where its magnetic characteristics are of a ferrimagnetic material (Hajalilou et al., 2015). Also obtained Ni-Zn ferrites synthesized by high-energy grinding and found that their magnetic hysteresis characteristics were presented in an “S” format with a unique coercive field. The results of this work corroborate those reported in the literature, emphasizing that the material is magnetic and its application in obtaining biodiesel because under the incidence of an external magnetic field, the catalyst will be easily removed from the reaction medium and thus reused.The fact that the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 catalyst has magnetic properties can help minimize the cost of biodiesel production since the catalyst can be easily removed from the reaction medium by applying an external magnetic field (magnet). Thus, magnetic separation is a relevant alternative to filtration and/or centrifugation since it contributes to reducing the loss of the catalyst and increases the reuse capacity, making the cost-benefit of the catalysts quite promising for industrial applications (Vieira et al., 2014). Fig. 10 shows the conversion of residual oil into esters and the mass yield obtained from the TES reaction through the ethyl and methyl routes catalyzed by ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3. All experiments were accomplished in agreement with the experimental planning indicated in Table 1. In general, it was possible to observe (Fig. 10) that the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 catalyst was active and satisfactory conversions were obtained in esters of fatty acids, in 96.1% ethanolysis, and 92.5% methanolysis. The best catalytic activity was obtained for the ethyl route, which is beneficial for the process since the alcohol used is less polluting and comes from the culture of sugarcane (Shikida and Bacha, 1998). The efficiency of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst may be associated with the presence of acidic and basic sites (see the results obtained in Section 3.6), which gives the system great versatility, such as the possibility of conducting simultaneous esterification and transesterification reactions.The experimental data shown in Fig. 10 also made it possible to observe that the profile of mass yield in biodiesel corroborates the profile of conversion into ethyl esters obtained from the tested reaction condition that showed the best catalytic activity, i.e., 30 g oil mass, time 1 h, and alcohol/oil ratio 15/1, 5% by weight of catalyst and 200 °C.Besides, regard to the percentage of acidity index reduction (Fig. 10), it was possible to verify that in all reactions, there were still unreacted free fatty acids. However, this occurs with greater emphasis on milder conditions of the percentage of catalyst and temperature. For example, using 3% catalyst and 180 °C, there was a reduction in the acidity of biodiesel by an average of only 44%, on the other hand, when elevated conditions reactions of the percentage of catalyst (5%) or temperature (200 °C), occurs a greater consumption of the fatty material available in the reaction, causing percentages of acidity reduction and conversions in higher esters, respectively, in the methyl route (88.4% and 92.5%) and the ethyl route (84.4% and 96.2%) using the so-called optimal conditions. Oprime et al. (2017) emphasize in its research the importance of developing and using a formulation well resolved by process optimization methods through experimental planning, thus reporting the gain in time and amount of experiments as well as total process costs. In this context, in the present work, the statistical study was carried out, and Table 4 describes the planning matrix used to analyze the statistical data on biodiesel production from reaction TES, using the magnetic catalyst and residual oil through the routes methanolic and ethanolic.Based on Table 4, it was possible to infer that the best catalytic condition for biodiesel’s synthesis from residual oil was verified in experiment 7 since this favored a higher conversion into esters (96.16 ± 0.08). Still, it was possible to verify that was obtained using ethyl alcohol, which is beneficial because besides being considered less toxic, it is produced directly from sugarcane. It is relevant to highlight that Brazil in the world ranking, is among the largest producers of sugarcane, trailing only Colombia, Australia, China, and the USA.The responses of the statistical analysis carried out for the synthesis of biodiesel by TES from residual oil were evaluated using the Pareto graph, see Fig. 11 .Analyzing the Pareto Graph (Fig. 11), it is possible to observe that the statistical analysis indicates 95% reliability (p < 0.05) (Gan et al., 2010), showing as significant variables: quantity of catalyst (%) and temperature, as well as the effects of secondary interaction, the quantity of catalyst × temperature (1by2), the quantity of catalyst × alcoholic route (1by3), and the interaction between temperature × alcoholic route (2by3). From this analysis, it was possible to conclude that the variable quantity of catalyst (%) and the secondary interactions (1by2) and (1by3) had a positive influence. In contrast, the variable temperature and the secondary interaction (2by3) had an influence negative. These observations are confirmed through the data in Table 4 and Fig. 10, suggesting that a mass increase in the quantity of catalyst significantly increases the conversion of residual oil into biodiesel through TES reaction. Fig. 12 (a) illustrates the level curves obtained as a statistical response of the independent input variables: quantity of catalyst (%) and temperature. The reliability of the analysis was p < 0.05, see Fig. 11. Fig. 12(b) illustrates the results obtained from the linear regression model (Eq. (3)), which dependence between a dependent variable or response (the content of converted esters) and a series of values (predicted results) of the independent variables describes the experimental data versus the predicted ones.The effect of secondary interaction between the percentage of catalyst and reaction temperature was better evaluated from the level curve (Fig. 12a). It was possible to observe that for high levels (+1) of temperature (200 °C), and quantity of catalyst (5%), the conversion of biodiesel was maximum (close to 100%), corroborating what was observed in the Pareto graph (Fig. 11). However, maintaining the upper level (+1) of the percentage of catalysts, at both temperatures studied, the conversion into esters remained at or above 85%. Therefore, it was possible to infer that the catalyst variable is the most significant and shows an excellent catalytic behavior of the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic composite.In the investigated region, the response surface is described satisfactorily by the linear mathematical model given by Eq. (3), which presented an R2 of 94% and which defines the plane represented in perspective on the contour line (Fig. 12a), from according to the experimental planning carried out for the TES reaction of the residual oil, which best represents the data collected, analyzed and adjusting to the data in Table 4. (3) Y = 82.08375 + 7.07625 ∗ x - 2.395 ∗ y + 7.0475 ∗ x ∗ y + 4 , 2575 ∗ 0 . ∗ x - , 16625 ∗ 0 . ∗ y + 0 . The linear regression model (Fig. 12b) specifies the linear relationship between a dependent variable (or response) and a series of the predicted independent variables. This linear model governed by Eq. (3), represents a good description of the experimental data related to the content of the converted esters. It is possible to see in Fig. 12b that the results obtained experimentally are close to the values predicted by the model, considering that the modeling shows a correlation factor (R2) equal to 0.94. This figure shows that the model represents a relatively good description of the experimental data related to the methyl/ethyl ester content at 1 h reaction time and the alcohol-oil ratio of 1/15. The modeling results showed that the most significant effects were the linear effects of the quantity of catalyst, temperature, and the combined effects between temperature, catalyst concentration, and alcoholic route. The other effects showed less significance.The suitability of the linear model was also tested by analysis of variance (ANOVA) according to Table 5 .The ANOVA results (Table 5) for biodiesel production showed the Fcal/Ftab value of 6.54, which indicates that the model was statistically significant. Therefore, the regression model is given in Eq. (3) was a reasonable prediction of the experimental results, and the factors affected were real at a 95% confidence level, as already observed in Fig. 11. Based on all the statistical planning, we can conclude that the planning gives the optimized condition at a 95% confidence level. The maximum conversions in esters would be observed: upper level (+1) for the catalyst quantity variables (5%) and temperature (200 °C) and lower level (−1) for the alcoholic route variable (methyl route).The recovery of magnetic particles for reuse in catalytic processes, in the most varied applications, has been highly reflected in the literature (Baskar et al., 2018; Dai et al., 2017; Guldhe et al., 2017). In the field of biodiesel production, some authors have already started to report excellent performances; for example, it is mentioned in (Ashok et al., 2019) with the use of nanoparticles of the ZnFe2O4Mn magnetic catalyst. Saxena et al. (2019), using magnetic Fe III nanocatalysts doped with ZnO, obtained high catalytic activity for the production of biodiesel, around 90 ± 2%, and exhibited excellent transesterification capacity after its reuse. About the reuse of heterogeneous catalysts, this is one of the advantages highlighted in the literature and in this case, those catalysts with intrinsic magnetism are considered an advantageous resource, as they can promote different results in terms of recovery and reuse (Dantas et al., 2020; Farias et al., 2020).In this context, the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst was reused in the TES reaction using the optimized conditions: 30 g oil mass, time 1 h, and 15/1 alcohol/oil ratio, 5% by weight of catalyst and 200 °C. The conversion results obtained on reuse are illustrated in Fig. 13 .Based on Fig. 13, it is possible to see that after 2 reuses, a loss of about 19.23% in efficiency in the catalytic activity was found. However, it was found that the catalyst showed an average conversion of 90.29 ± 0.44%. Therefore, the magnetic catalyst sample is economically viable for practical industrial applications.To assess possible structural modification on the ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 catalyst during the TES reaction, thermogravimetry analyses were performed before the first catalytic test, and XRD analyses were accomplished before and after the catalytic tests.As shown in Fig. 14 (a), there was no significant structural change in the catalyst after evaluating its useful life, when comparing the two diffractograms before and after the TES reaction. Also, it was possible to observe that the structural parameters remained unchanged, i. e., crystallinity and crystallite size showed a value of 41.2% (before 43%) and 26 nm (before 25 nm). This characteristic was confirmed with the aid of thermogravimetric analysis (Fig. 14(b)), where it was possible to observe that in the range up to 200 °C (maximum temperature used in TES reaction), it refers to the mass loss corresponding to humidity. According to Farias et al. (2020), subsequent mass loss events are attributed to decomposition processes and do not interfere with the TES reaction, since that the temperatures (180 and 200 °C) used in this work were lower. It is also possible to verify that the catalyst shows up to 800 °C, a total loss of mass of only 4.66%, where stabilization is verified. This statement is consolidated in the literature (Silva et al., 2019; Dantas et al., 2020; Farias et al., 2020) that indicates the thermal stability of spinel-type ferrites, starting at 800 °C. Based on the above, it is evident that there was no structural modification in the catalyst produced after the TES reactions, and the thermal stability of the same was also proven.These data suggest that the decrease in catalytic capacity of the catalyst after the second cycle of reuse may be related to the residual presence of triglycerides, and/or unconverted fatty acids and/or impurities arising from the frying process in the residual oil that were possibly adsorbed on the surface of the catalyst, preventing the participation of the active sites available for the reaction. Therefore, it becomes clear the need to optimize the cleaning process of the residual starting oil and the catalyst after TES reaction, for greater efficiency in subsequent reactions.The Fe2O3-Ni0.5Zn0.5Fe2O4-ZnO magnetic catalyst was synthesized on a pilot-scale using combustion reactions. The pilot-scale production was safe, reproducible, and efficient. The catalyst synthesized is ferrimagnetic (6.12 emu/g), polyphasic (Fe2O3-Ni0.5Zn0.5Fe2O4 - ZnO), nanometric (24 nm), and with high surface area (SBET = 52.9 m2g−1). Before, the use of factorial design made it possible to evaluate the process in a multivariate manner, leading to the identification of variables that significantly influenced the response variable (conversion of residual oil into esters). The factorial design allowed to identify the influence of the variables (percentage of catalyst, alcoholic route, and temperature) on the TES reaction, which, according to the statistical study, the quantity of catalyst and temperature followed by secondary interactions between all input variables (percentage of catalyst, temperature, and alcoholic route), were the ones that most affected the value of the response variable, with a significance level of 95%. The catalyst was effective in all conditions tested with conversions from 58% to 96%, with significantly promising results in the ethyl route. From the results obtained, it can be concluded that the studied catalyst can be successfully applied in the production of biodiesel, as the advantages have surpassed traditional methods since this polyphasic catalyst is a new product with innovative, magnetically active, and sustainable characteristics, and had a catalytically active useful life for two reuse cycles.The authors thank CAPES/CNPq for their financial support.

A magnetic catalyst with composition ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 was synthesized by a combustion reaction on a pilot-scale and applied in the conversion of residual oil into biodiesel by simultaneous transesterification and esterification reactions (TES). For that, statistical analysis of the factors that influence the process (catalyst concentration, alcoholic route, and temperature) was evaluated by 23 factorial experimental design. The ZnO-Ni0.5Zn0.5Fe2O4-Fe2O3 magnetic catalyst was characterized in terms of the structure, morphology, magnetic, TPD-NH3 acidity analysis and catalytic properties. The results indicate the formation of a catalyst with a surface area of 52.9 m2g−1, and density of the sample was 4.8 g/cm3 which is consisted of a mixture of the phases containing 55.87% Fe2O3, 36.96% Ni0.5Zn0.5Fe2O4, and 7.16% ZnO. The magnetic characterization indicated that the synthesized catalyst is ferromagnetic with magnetization 6.12 emu/g and coercive field of 5.3 G. In the TES reactions, the residual oil was active showing conversion to 96.16% ethyl esters and with a long useful life maintaining sustained activity after two consecutive reuse cycles with the conversion of 95.27%, 93.07% and 76.93%, respectively. The experimental design was significant and presented a 95% reliability level. The statistical analysis identified (+1) and (−1) as higher and lower level variables, respectively. The amount of catalyst used was equal to 5%, at 200 °C in methyl alcohol (alcoholic route). In summary, a new catalyst composed of a mixture of magnetically active phases was developed and successfully applied in biodiesel’s synthesis from residual oil. Undoubtedly these results have a positive and significant impact on the environment and to society as a whole.

With accelerating climate change and a dependence on dwindling supplies of non-renewable carbonaceous energy resources, much research is focused on lowering greenhouse gas emissions as well as switching from a carbon-based to a hydrogen-based economy (Oliveira et al., 2021). Currently the majority of hydrogen is produced from steam-methane reforming (SMR), a non-sustainable and polluting process (Bauer et al., 2022). While attempts are being made to improve sustainability through the sequestration of the produced CO2 to provide so-called blue hydrogen, realistically, this is not sufficiently sustainable (Bauer et al., 2022). Another alternative which has been gaining increasing attention (Jang et al., 2019) as a potentially more environmentally friendly method of production, is the dry reforming of methane (DRM) (Lavoie, 2014). The DRM process replaces the water used in SMR with second greenhouse gas, carbon dioxide (Eq. (1)). (1) C H 4 + C O 2 → 2 CO + 2 H 2 Δ H o = + 247.3 kJ mo l − 1 CO2 and CH4 are the chief contributors to the greenhouse effect and as such, methods that convert both of these gases into other more environmentally benign products are key to reducing global greenhouse gas emissions. As a highly endothermic reaction, typical operating temperatures for DRM of over 600 °C are implemented to boost conversion, though catalysts are still required to achieve commercially viable reaction rates. Noble metal catalysts have shown high DRM activity with good stability (Edwards and Maitra, 1995), however their high cost prohibits their implementation as the sole active catalytic species (Pakhare and Spivey, 2014). More recently, focus has shifted towards cheaper, more abundant, transition-metal-based catalysts in an attempt to achieve a cost-effective process. Of the metals investigated thus far, nickel-based catalysts, supported on inorganic oxides, represent one of the most industrially relevant choices, due to their low cost, good activity, and the relatively high abundance of nickel (Kawi et al., 2015).Despite their high activity for DRM, nickel catalysts still present issues that must be overcome before they are utilised more widely. The main issue at present is rapid catalyst deactivation through metal sintering and degradation through coking (Kim et al., 2007). Coking refers to the deposition of carbon on the surface of the catalyst, mainly through unwanted side reactions such as the disproportionation of the carbon monoxide product, the Boudouard reaction (Eq. (2)), and the cracking of methane (Eq. (3)). The formation of coke through these reactions is more serious for DRM than SMR due to the higher CH4:CO2 ratio and lack of the oxidising steam component which helps to prevent carbon build-up. (2) 2 CO → C + C O 2 Δ H o = + 75 kJ mo l − 1 (3) C H 4 → C + 2 H 2 Δ H o = − 172 kJ mo l − 1 The formation of carbon can block catalyst pores and coat its active sites, resulting in a decrease in catalytic activity. The type and quantity of coke formed is dependant on the reaction conditions, such as the ratio of reactants, the temperature and the nature of the catalyst employed. At the relatively high temperatures employed in DRM, the formation of coke through the cracking of methane (Eq. (3)) is reported to be the main reaction responsible for coke formation (Liu et al., 2011).As aforementioned, a typical supported catalyst used for this transformation is composed of metallic nickel supported on an inorganic oxide, such as alumina. Catalysts of this nature have thus far, however, proven too prone to coke formation (Liu et al., 2011; He et al., 2021; Swaan et al., 1994). Much current work focuses on the optimisation of this catalyst system to reduce coke formation while retaining the desired high activity (Wang et al., 2018). The addition of promoters such as noble metals and rare earth elements has proven successful in reducing the extent of coking during the DRM (Laosiripojana et al., 2005; Tsyganok et al., 2005). The support used can also play an important role in producing catalysts with improved resistance to coke deposition (Luisetto et al., 2012; Gadalla and Sommer, 1988; Erdogan et al., 2018). In addition to the composition of the utilised catalyst, the preparation method has also been demonstrated to have an effect on the nature of coke formation(Abdollahifar et al., 2016).An important area of research is now the use of bimetallic catalysts for reduced carbon deposition (Sasson Bitters et al., 2022; Bian et al., 2017; Yentekakis et al., 2021). The chemical and physical properties of bimetallic catalysts differ from the properties of either of the metals when used in isolation and as such the combination can offer systems with higher catalyst activities and less coking than is achievable with monometallic catalysts. Co-metals studied in conjunction with nickel include cobalt, copper, iron, chromium, and bismuth, amongst many others (Sasson Bitters et al., 2022; Han et al., 2021; Sharifi et al., 2014; Li et al., 2019; Rouibah et al., 2017; Sutthiumporn et al., 2012). Of these, cobalt has generated some of the most significant interest as a potential co-metal for both improved catalyst activity and resistance to coking (Sasson Bitters et al., 2022).The addition of a small amount of cobalt to a nickel system is sufficient to enhance both the catalytic performance and improve resistance to coking, however, due to cobalt's lower activity relative to nickel, the addition of too much can be detrimental to performance. As such, the ratio of cobalt to nickel in a system can have a significant effect on catalytic activity (Sengupta et al., 2014; Li et al., 2022). Understanding how this nickel-to-cobalt ratio effects the composition and quantity of the coke formed is less explored, although in general it has been observed that higher quantities of cobalt result in less carbon formation (Takanabe et al., 2005). Despite a higher resistance to coking being observed in nickel-cobalt bimetallic catalysts, carbon formation still occurs and is unlikely to be eliminated as an issue entirely. Therefore, to develop catalyst systems that are stable for the long durations required for industrial applications, a better understanding of coke formation in this process is required to aid appropriate catalyst design.Characterisation of coke, understanding its formation and its effect on catalyst degradation is still a challenging process since carbon formation and its consequences on catalyst performance and durability occur across multiple time and length scales. Several characterisation techniques have proven vital in understanding the bulk properties of the coke formed, such as Raman spectroscopy and thermogravimetric analysis (TGA) (Sasson Bitters et al., 2022). While these techniques can provide important information about the bulk properties and quantity of carbon formed, they provide little insight into how this carbon formation affects individual catalyst particles and no spatial information as to where coke formation occurs.More recently, tomographic studies of catalytic systems have begun to be reported and can provide information about catalyst structure, performance and degradation that is not available through bulk techniques (Beale et al., 2014; Meirer and Weckhuysen, 2018). Studies have shown that these tomographic techniques are useful for understanding the porosity in fluidised catalytic cracking (FCC) catalysts (Meirer et al., 2015; Dasilva et al., 2015; Bare et al., 2014) as well as their degradation after use, including structural changes and the redistribution of elements (Meirer et al., 2015; Gambino et al., 2020). A limited number of studies have investigated coke formation on FCC catalysts with a focus on its position and effect on porosity (Veselý et al., 2021; Zhang et al., 2020). Similar techniques have also been applied to other catalyst systems such as those used for the Fischer-Tropsch process (Cats et al., 2016; Price et al., 2017) and in automotive applications (Schmidt et al., 2017). Recently hard X-ray ptychographic computed tomography was conducted using synchrotron radiation to investigate the structure of Ni/Al2O3 DRM catalysts (Weber et al., 2020) and the deposition of coke (Weber et al., 2021). Using this technique, Weber and co-workers were able to visualise the three-dimensional position of the formed coke throughout a single catalyst particle.Since catalyst properties influence the quantity, type and location of coke and have long-term implications on the catalyst performance and structural stability, we report here an investigation into the effect of the nickel-to-cobalt ratio on coke formation in Ni/Co/Al2O3 catalysts. The properties and quantities of coke formed are reported alongside easily accessible, lab-based X-ray nano-computed tomography data which gives spatially resolved information about coke formation on individual catalyst particles and new insights into catalyst degradation pathways after extensive coking, as a function of the nickel-to-cobalt ratio.A range of catalysts containing various ratios of nickel and cobalt were prepared using incipient wetness impregnation. Table 1 shows the nominal nickel, cobalt and alumina content of the prepared catalysts based on fully reduced, metallic nickel and cobalt. EDX measurements were conducted to verify these values which showed broad agreement with the trend expected from calculated loadings. Particle-to-particle variation was observed; the discrepancy between nominal and measured values is thought to be due to the small number of particles available for measuring using this approach. Various quantities of aqueous solutions of Ni(NO3)2•6H2O (99.98% metal basis, Sigma Aldrich) and Co(NO3)2•6H2O (≥98%, Sigma Aldrich) were slowly added to γ-alumina, 50–200 µm, 60 Å pore size (Acros Organics). The quantity of each metal precursor was calculated to give the values shown in Table 1 when fully reduced. The mixtures of alumina and metal precursors were dried in an oven at 100 °C for 4 h. The resulting dried powder was then calcined in static air at 550 °C for 16 h and allowed to slowly cool to room temperature inside a baffle furnace.Powder X-ray diffraction (pXRD) of the as-calcined, as-reduced, and coked samples was performed on a SmartLab diffractometer (Rigaku, Japan) fitted with a Mo Kα (λ = 0.71 Å) source. Samples were scanned from 2ϴ = 5° to 80° with a step size of 0.01° and a step time of 0.6°.min−1. Peak identification and data analysis were conducted using SmartLab Studio II Software v4.1.0.182 (Rigaku, Japan) and the Crystallography Open Database (Gražulis et al., 2012).The carbon deposited on each sample was analysed using a PerkinElmer Pyris 1 (PerkinElmer, US) thermogravimetric analyser. Approximately 4–8 mg of each coked catalyst was placed in a platinum sample crucible and heated to 900 °C with a ramp rate of 10 °C.min−1 under an air flow rate of 20 mL.min−1.Raman spectra of the as-reduced and coked samples was acquired using a Renishaw Invia Raman (Renishaw, UK) microscope with a RL532C class 3B continuous wave, diode-pumped solid-state laser which operates at 532 nm. Data was obtained over a period of 30 s at low laser power (< 15 mW), averaging over five acquisitions taken between 1000 and 1800 cm−1. Data was collected at five separate points on different catalyst particles to ensure the results were representative of the bulk sample. Data fitting was conducted using OriginPro 2021b data analysis and graphing software (OriginLab Corporation, U.S.A.) according to the method reported by Sadezky et al. (2005).Transmission electron microscopy (TEM) was conducted using a JEOL 2100 LaB6 transmission electron microscope (JEOL, Japan). Samples were prepared by sonicating in ethanol before dropping the suspension onto carbon-coated copper discs.Powder scanning electron microscopy (pSEM) of the as-calcined, as-reduced, and coked samples was conducted on a Zeiss SEM EVO MA10 (Carl Zeiss, Germany) by loading the respective powders on conventional sticky carbon tabs atop aluminium stubs and coating with nanometre-thick layers of gold using a Quorum SC7620 Mini Sputter Coater (Quorum, UK) to avoid charging. Micrographs were taken at various magnifications to capture general morphology, cracking extent, and the carbon formed on coked samples.Cross-sectional scanning electron microscopy (xsSEM) and energy-dispersive X-ray spectroscopy (EDX) were performed on the same instrument as pSEM with the use of an INCA x-act SDD detector and INCA suite V5.05 software (Oxford Instruments, UK). For the 20Ni, 10Ni10Co and 20Co samples, as calcined powders were encased in an epoxy puck by overnight vacuum-curing of a mixture of epoxy resin and hardener (EpoFix kit, Struers, Denmark) before grinding with increasingly fine silicon carbide papers (Buehler, US). The epoxy pucks were subsequently carbon coated to prevent charging. This allowed for mapping of the metal distribution within the catalyst particles.Lab-based X-ray nano-computed tomography (nano-CT) was performed for both the reduced and coked variants of the 20Ni, 10Ni10Co, and 20Co samples, using a Zeiss Xradia 810 Ultra (Carl Zeiss) with a fixed X-ray energy of 5.4 keV and field-of-view of 65 × 65 µm. Imaging was performed with either binning 1 (voxel dimension of approximately 63 nm) or binning 2 (voxel dimension of approximately 126 nm) in both absorption and phase contrast modes. The full set of parameters are given in Table 2 .Reconstruction of all scans was performed in Zeiss XMReconstructor software utilising a standard filtered back-projection algorithm. Absorption and phase contrast tomograms were combined for all but the 20Ni coked sample (due to imperfect registration of the two tomograms) using the Zeiss DSCoVer tool (Version 16.1) before data processing and visualisation were carried out in Avizo 2022.1 (Thermo Fisher Scientific, US). All tomograms were Gaussian-filtered (3D, standard deviation of 1.1) and imported into open-source, machine-learning segmentation software (Ilastik 1.3.3 (Berg et al., 2019)) whereupon manual training was provided for the central slice and an initial segmentation prediction was generated. This approach (shown in Fig. S1) was taken as traditional supervised segmentation approaches in Avizo, such as watershed segmentation, did not appear to generate a reliable result. From the overlay of this prediction on the raw data, manual corrections were made by training two more slices before the final segmentation output was generated. Raw and segmented orthoslices and volume renderings were output from Avizo and equally sized sub-volumes (of ca. 6600 µm3) were extracted from each segmented tomogram before applying voxel counting to assess the phase fractions for solids and pores. The spatial resolution in each tomogram was estimated by using a sharp-edge fitting (Fig. S2). This involved drawing line plots across sharp feature boundaries and the number of voxels required to move from 10% to 90% of the voxel intensity difference between the feature and the background was combined with the nominal voxel size to provide an estimate of the true spatial resolution in each acquired tomogram (See Table 3 for estimated spatial resolution). Small particles, meso‑ and micropores below these resolution limits will not be detected and only features at or above will appear detectable in the acquired images.Catalyst pre-treatment and accelerated coking tests were conducted in a tube furnace (Lenton, UK). Catalysts were held within a quartz tube with gas flow of nitrogen, hydrogen and methane controlled using EL-Flow mass flow controllers (Bronkhorst, Germany). For catalyst pre-treatment, the sample under test was heated at 20 °C.min−1 to 700 °C under a flow of 100 sccm N2. Once at temperature, a flow of 100 sccm 4% H2 in N2 was introduced and maintained for 2 h. To understand the long-term effects of coking on catalysts for CO2 reforming of methane, accelerated coking tests were conducted. These tests involved inducing high levels of coking by using only methane as a feed gas as is commonly done for in situ experiments (Weber et al., 2021; Mutz et al., 2018). The catalyst samples under test were heated to 600 °C under a flow of 100 sccm N2. Once at temperature, a mixture of 5% CH4 in N2 was introduced at 100 sccm. The accelerated coking tests were conducted over a period of 4 h and once complete the gas flow was switched back to 100 sccm N2 and the sample allowed to cool naturally.To study the effect of nickel-to-cobalt ratio on the quantity and properties of the carbon formed during accelerated coking tests, a range of catalysts were prepared according to the procedure outlined in Section 2.1 and hereafter will be referred to by the nomenclature shown in Table 1. Representative SEM micrographs of each catalyst composition in the as-calcined state are shown in Fig. 1 (a-f). Each of the catalysts were observed to be similar in morphology with particle sizes remaining relatively consistent across all compositions. Some surface roughness was observed in each of the samples, potentially related to the nickel and/or cobalt metal oxide loaded onto the support. Some evidence of support particle cracking was also observed and, although minimal and difficult to observe in the 20Ni and 20Co samples, this was more clearly observed in the catalysts containing mixtures of the two metals. An SEM image of 10Ni10Co at higher magnification is shown in Fig. 1(f); here a small number of cracks appeared to be running parallel to one another, suggesting a slight delamination in a particular crystallographic direction. pXRD analysis performed on the as-calcined catalysts, displayed in Fig. 1(g), shows that the monometallic samples (20Ni and 20Co) consisted primarily of their respective oxide and the alumina support. A small unidentified peak (2θ = 17.1°), potentially indicating a minor impurity, is present in all samples. The fact that this is observed in an XRD pattern of the as-received alumina source (see Fig. S3) suggests that this is the source; since it is present in all samples it is unlikely to affect the results of these studies. For the bimetallic cases (15Ni5Co, 10Ni10Co and 5Ni15Co), all three phases, γ-Al2O3, NiO, and Co3O4 are evident, with no indication of any residual nitrates. Despite overlapping peaks, there is some evidence of the formation of some NiAl2O4, which is common for catalysts calcined at these temperatures (Siang et al., 2018). There is some evidence that this phase can improve nickel dispersion and reduce nanoparticle size, improving catalytic activity (He et al., 2021). The presence of this spinel structure is important since this phase has been shown to be more tolerant to coking and less likely to sinter (Salhi et al., 2011). Based on the XRD patterns, there is still a significant quantity of NiO present in the sample which will be more readily reduced and will likely contribute more significantly to the coking behaviour of the catalysts.The majority of metals present in the catalyst are expected to be present as nanoparticles throughout the internal microporous structure of the alumina support. To investigate the distribution of nickel and cobalt in the alumina support particle, xsSEM/EDX measurements were taken on 20Ni, 10Ni10Co, and 20Co. These tests involved encasing the catalyst powder in an epoxy resin before grinding with silicon carbide abrasives allowing for SEM and EDX measurements to be conducted on a smooth cross-section of each of the particles. The results from these tests are displayed in Fig. 2 . The 20Ni and 20Co particles investigated showed a uniform distribution of each of the metals throughout the particle, indicating thorough impregnation of the precursor salts throughout the alumina support. The bimetallic 10Ni10Co sample showed a similar, even distribution throughout the particle, again indicating appropriate impregnation and effective mixing of cobalt and nickel with no major agglomeration of either metal in any area.Prior to use, the catalysts analysed in Section 3.1 were first reduced to form metallic nickel and cobalt. This reduction step was achieved by heating the as-calcined samples at 700 °C and flowing 4% H2 in N2 over each sample for 2 h. Since this reduced form is the active phase of the catalyst, analysis of these resulting powders was also conducted to determine any changes that may have occurred during this short, high-temperature treatment. Representative SEM micrographs of each of the reduced catalysts are shown in Fig. 3 . No significant change in catalyst morphology was observed, with the individual catalyst particles remaining similar in both size and morphology to the as-calcined samples shown in Fig. 1. No significant change in cracking or crack sizes was observed, indicating that short-term exposure to a high temperature reducing environment does not result in significant catalyst particle cracking.Higher magnification images of selected catalysts are shown in Fig. 3 (f-i), where slight changes in morphology of some of the smaller particles on the catalyst surface were observed, suggesting that while the majority of the metals are well dispersed throughout the support particle, some small micron-scale deposits may be present on the outer surface of each particle, somewhat agglomerated by the reduction procedure.After reduction at 700 °C for 2 h in an atmosphere of 4% H2 in N2, pXRD analysis was performed on the complete set of as-reduced catalysts, X-ray diffraction patterns of which are shown in Figs. S4-8. As expected, a characteristic new Ni0 peak appeared at 2θ = 23.2° in the reduced 20Ni sample and equally, characteristic NiO peaks at 2θ = 19.6°, 27.8° were no longer present, consistent with the full reduction of any nickel oxides present. In the reduced 20Co case, a characteristic new Co0 peak appeared at 2θ = 23.1°, however, some characteristic Co3O4 peaks at 2θ = 16.7°, 28.7° persisted, albeit at a lower intensity. This indicated that some residual oxide remained present after the reduction step, although it is not clear if this residual oxide is present due to incomplete reduction of the nickel oxide, or due to a small degree of re-oxidation after reduction, prior to XRD measurements. In the bimetallic cases (15Ni5Co, 10Ni10Co and 5Ni15Co), there was evidence of γ-Al2O3 as well as characteristic peaks for both metals (Ni and Co), and as with the pure Co sample, there was still some evidence of small quantities of residual Co3O4. Testing showed that while small quantities of residual Co3O4 remain at this temperature (700 °C), the majority of the oxides are reduced.Industrial catalysts are subjected to use online for thousands of hours to make processes cost effective. As such, for the bimetallic Ni/Co catalysts to be developed commercially, thorough understanding of the effects of long-term coking are required. Most papers that cover the coking of Ni/Co catalysts tend to only test catalysts for hours to occasionally tens of hours (Sasson Bitters et al., 2022). This is not sufficient to understand the long-term catalyst degradation that occurs after extended coking. To simulate the effect that long-term exposure to carbonaceous gases would have on varying nickel-to-cobalt ratios, accelerated coking tests were carried out. To maximise the amount of coke-induced degradation in a reasonable time, a relatively low, but still applicable, temperature of 600 °C was used. Since the presence of CO2 in the stream can oxidise and aid coke removal, the accelerated coking tests were carried out in 5% CH4 in N2. Accelerated coking tests such as these are commonly applied when running in-situ experiments (Weber et al., 2021; Mutz et al., 2018). After four hours on stream under these accelerated coking conditions, each of the catalysts was analysed in order to gain a deeper understanding of how the nickel-to-cobalt ratio affected the type and quantity of the coke formed and the influence these factors had at the catalyst-particle scale.After accelerated coking tests, the catalyst samples all appeared black, although their consistency and the apparent tap density of the spent catalyst powders appeared to vary. TGA was performed on each of the catalyst samples to determine the quantity of carbon and gain information about its type and how this varied with nickel-to-cobalt ratio. The results from the TGA testing are shown in Fig. 4 . Fig. 4(a) shows the percentage weight change against the temperature for each of the samples. Based on these results it was possible to infer the quantity of carbon formed on each of the samples (see insert Fig. 4(a)). As expected from previous studies (Li et al., 2021), the 20Ni sample contains the largest quantity of carbon with a weight loss of 35 wt%. Due to the higher quantity of nickel present in this sample it is likely that more of the NiAl2O4 spinel phase is present. Despite this, the significant quantities of metallic nickel present in the sample from the reduced NiO results in significant quantities of coke formation. Alteration of the preparation methods to produce more of the spinel phase would likely improve the resistance of this catalyst to coking. The addition of Co to give the 15Ni5Co sample had a significant impact on the quantity of carbon formed with the carbon content dropping to 15 wt%. Addition of further cobalt to give the 10Ni10Co sample further improved the coke resistance of the catalyst with carbon content dropping to 7 wt% and the 5Ni15Co sample carbon content was found to be less at ca. 6 wt%, however by this point it appears that the improvements were diminishing. Interestingly, when the nickel is removed to give the 20Co sample, there was an increase in coke formation (ca. 12 wt%), illustrating the benefits of bimetallic catalysts, whereby the synergistic effects of combining two metals results in properties not observed for either of the metals alone.The derivative weight change is shown in Fig. 4(b), the positions of the peaks in this plot indicate the temperatures at which the majority of the carbon is removed from the bulk catalyst. If the type and composition of the carbon were consistent across all samples, the temperature at which the carbon burnt off would be expected to be the same, barring any small catalytic effects of the metals present. Based on the results shown in Fig. 4(b), the type of carbon formed was dependant upon the nickel-to-cobalt ratio present in each sample.For the 20Ni sample, most carbon was removed at ca. 700 °C, higher than the other samples, and this relatively high temperature could be interpreted as there being more graphitic carbon present in this sample, since graphitic carbon tends to be more thermally stable than amorphous carbon. With the addition of small quantities of cobalt to produce the 15Ni5Co sample, the peak position moved to a lower temperature of ca. 600 °C, indicating that the type of carbon is different in this sample relative to the 20Ni, as the coke formed is less thermally stable. Addition of more cobalt to form the 10Ni10Co and 5Ni10Co samples further changed the peak position and shape, although these two samples did appear similar to one another. For these two catalysts, two small peaks were present at ca. 700 °C, which indicate that there was a small quantity of carbon present with a similar thermal stability to that seen in the 20Ni sample, but the majority was composed of carbon that decomposes at ca. 550 °C.Finally, the 20Co sample showed peaks similar to the high-cobalt-containing mixed catalysts with peaks present at both 700 °C, similar to 20Ni, but the 20Co also had a major peak at just below 550 °C, indicating the majority of the carbon present was in the less thermally stable form. These results show the significant effect that the nickel-to-cobalt ratio has on the quantity and type of carbon formed during operation.Raman spectroscopy was performed on each of the coked catalyst samples to provide bulk characterisation of the formed carbon. Two major peaks associated with the D and G band of carbon were observed in the 1000–1800 cm−1 range. The Raman spectra obtained in this wavenumber range for each of the catalysts is shown in Fig. 4(c) – (g). According to work by Sadezky et al., the first-order region of the Raman spectra can be deconvoluted and fitted to five bands; four Lorentzian-shaped bands at 1580, 1350, 1620 and 1200 cm−1 referred to G, D1, D2 and D4, respectively, and one Gaussian-shaped band at 1500 cm−1, referred to as D3 (Sadezky et al., 2005). The G band is commonly referred to as the graphitic band and corresponds to an ideal graphitic lattice vibration mode. As such, the ratio of this peak to the D1 peak is often used to give an indication of how disordered or graphitic a carbonaceous material is. While these two bands are regularly used, it is less common to investigate the D2, D3 and D4 bands. The D3 band is likely related to the amorphous sp2-bonded forms of carbon such as those present in polycyclic aromatic compounds or other organic molecules and fragments. The D4 band can likely be attributed to sp2-sp (Jang et al., 2019) bonds or CC and C = C stretching vibrations of polyene-like structure (Sadezky et al., 2005).From the results shown in Fig. 4(c) - (g), it may appear that all catalyst samples give similar Raman spectra, however, when deconvoluted to give the five bands discussed, information on the differing nature of the carbon can be obtained. The ratio of the intensity of the D1 band to G band gives an indication of the ratio of the disordered carbon to the carbon that is graphitic in nature. Based on the acquired data, the 20Ni catalyst had the highest quantity of disordered carbon present. This apparent discrepancy with the TGA data is addressed in the next section. Addition of a small quantity of cobalt resulted in a decrease in the ID1/IG ratio, suggesting more graphitic carbon was formed for this sample. (Fig. 4(h)) Further addition of cobalt to form the 10Ni10Co and 5Ni15Co samples both resulted in a decrease in ID1/IG, suggesting that the more cobalt that there is in the system, the higher the degree of graphitic carbon formation. While the addition of increasing quantities of cobalt caused ID1/IG to continuously decrease, when nickel was removed completely (20Co sample), the ID1/IG value increased. This again illustrates the beneficial nature of bimetallic catalysts possessing properties that neither metal possesses alone.For all nickel-to-cobalt ratios, bands representing graphitic carbon (G) and defects/heteroatoms present in graphitic lattices (D1 and D2) were observed, however there were no D3 and D4 bands present. The D3 and D4 bands are related to the sp (Bauer et al., 2022) and sp (Jang et al., 2019) hybridised carbons found in typical carbon chains, e.g., polyene CC and C=C type bonds, suggesting that there were few of these forms of carbon present within the coked catalyst samples, implying that the carbon formed was almost purely graphitic in nature, but with varying amounts of defects present.XRD of the coked samples (Figs. S9–13) indicated that after accelerated coking studies there was little change to the crystallographic nature of the catalysts observable using this technique. The majority of the metals remained present in their reduced form with small amounts of NiO reformed in the high nickel containing catalysts and a small additional peak ascribed to the formed coke was observed. The exception to this was the 20Ni system where larger NiO peaks were observed in addition to the metallic nickel peaks, it is not clear why this sample shows oxidation while others do not, all samples were treated in the same manner, but it is possible that this oxidation may be related to the significant morphological changes that occurred to this sample which are discussed in more detail below.To understand how the nickel-to-cobalt ratio influences the extent and type of coking of these catalyst systems, methods other than bulk analysis techniques are required. The ratio of metals not only affects the quantity and properties of the coke formed as analysed in Section 3.4.1 but can also have a significant effect on the catalyst-support particle as whole. SEM micrographs of the coked samples are shown in Fig. 5 (a) – (f). Comparison of these images with those of the reduced catalysts shown in Fig. 3 show that a significant change in morphology has occurred during the accelerated coking procedure. In addition to this, while the reduced catalyst particles all appeared relatively similar, there was a significant difference between the coked samples based on the composition of the catalyst, indicating that nickel-to-cobalt ratio has a significant influence on the resulting morphology.The 20Ni catalyst was distinctly different from its reduced form, with individual catalyst particles no longer visible - these were replaced with long elongated forms with a texture significantly different to that of the original catalyst particles. Based on the image shown in Fig. 5(a), it appears as though the particles were entirely coated or consisted almost entirely of fibrous carbon. A similar, albeit less drastic, effect was observed for the 15Ni5Co catalyst sample shown in Fig. 5(b). Here the elongated forms did not appear as long and some of the rough catalyst-support particle shape observed in the reduced sample was retained. This image appears to show plates of the support connected by fibrous carbon, consistent with carbon having formed within particles, cracking them apart. Interestingly, all cracks appeared parallel as if delamination had occurred along the same crystallographic plane. The 10Ni10Co and 5Ni15Co samples shown in Fig. 5(c) and (d) most closely resembled that of the reduced catalysts prior to coking. The catalyst particles were clearly visible with little change in shape or size, although they did appear to have some fibrous, carbonaceous species formed on the surface.The 20Co sample was distinctly different to both the 20Ni and the bimetallic catalysts (Fig. 5(e) and (f) (magnified)). The formation of fibrous carbon across the surface of catalyst particles was observed and while the particles retained their rough shape, significant cracks were evident. These cracked particles, however, appeared significantly different from the cracks observed for the other systems. Fig. 5(g) - (j) shows TEM micrographs of the coked catalysts. TEM analysis confirms the presence of filamentous carbon. This form of carbon is thought to form through the following mechanism. First, carbon is absorbed on the surface of metal particles giving Ca. Most of this carbon is gasified, first methane dissociates on the surface of metallic nickel producing reactive carbon species, often referred to as alpha carbon, Ca. Most of this form of carbon is gasified but a small proportion is converted into the beta form, Cb, a less reactive form which either begins to encapsulate the active nickel or dissolves into the nickel itself (Trimm, 1997). Dissolution of carbon into the nickel structure can result in the formation of carbon whiskers. The dissolved carbon tends to dissolve through the nickel particle to the rear of the crystallite, where the carbon begins to precipitate, such that continuous build-up results in the formation of carbon whiskers which break the nickel particle from the surface of the support (Trimm, 1997). While this type of carbon formation is often seen as less severe than other forms of coke which coat the active surface and deactivate the catalyst, excessive formation of this type of carbon has been shown to destroy catalyst particles or block the reactor (Liu et al., 2011). The dark particles observed in Fig. 5(g) – (j) are thought to be metal particles and their presence in all samples supports this universal mechanism of filament/ whisker formation. The formation of carbon whiskers in this manner has previously been deemed responsible for the fragmentation of catalyst particles (Ochoa et al., 2020), as is clearly observed in the case of 20Ni, 15Ni5Co and 20Co samples. The extent of cracking appears to be related to the quantity of carbon formed since the 20Ni sample, which had the largest amount of carbon present after testing, resulted in the most significant particle breakdown, followed by the 15Ni5Co catalyst which has the second highest quantity of carbon present.Despite all systems showing the presence of this filamentous carbon, there are differences between the samples, with the nickel-to-cobalt ratio appearing to have an influence and effect on the morphology. One of the main differences between the 20Ni and the other samples is the presence of small particles of support intertwined with the carbon filaments. This suggests a more significant break-up of the support in this sample than all of those containing cobalt. In addition to the presence of small support fragments, there are significant differences in the thickness of the carbon filaments. For the 20Ni sample, it mainly consisted of relatively thick, multi-walled, carbon filaments, predominantly ca. 40 nm in diameter with a wall thickness of ca. 17 nm. For the 15Ni5Co sample, there were still some of these larger thicker-walled filaments but in addition there were also numerous thinner filaments with a diameter of ca. 20 nm and wall thickness of ca. 5 nm. When the cobalt loading is increased (10Ni10Co), no larger, thicker filaments are present with most filaments measuring ca. 15 nm in width, with wall thickness of ca. 5 nm. The 5Ni15Co sample consisted almost entirely of filaments in the 15–20 nm range with a wall thickness of approximately 5 nm.The TEM micrographs support the Raman spectroscopy and TGA data presented in Fig. 4. As the cobalt content is increased, the ID1/IG ratio decreases, suggesting an increase in the graphitic nature of the carbon present in the sample. Consequently, it would be expected that the temperature at which the carbon is thermally degraded and removed would increase, however for these samples the opposite is true. Bimetallic samples with increasing cobalt content combust at lower temperatures, which would typically be expected for samples with higher ID/IG ratios. This apparent contradiction is explained by the carbon morphology. Studies have shown that carbon nanotubes (CNTs) with larger diameters and thicker walls will thermally decompose at higher temperatures than narrower CNTs. (Singh et al., 2010) In addition to increased thermal stability, a more prominent d-band is observed in their Raman spectra due to the more defective nature of thick, multi-walled CNTs, resulting in higher ID1/IG ratios for larger, thicker-walled CNTs (Singh et al., 2010). There are two main peaks present in the derivate weight change TGA plot (Fig. 4(b)), one at ca. 700 °C and another at ca. 550 °C. It is likely that the peak at 700 °C relates to the thicker-walled, 40-nm-diameter filaments, whereas the peak at ca. 550 °C corresponds to the narrower CNTs observed with diameters in the range 15–20 nm. This would be consistent with the fact that the 20Ni sample has almost exclusively thick CNTs, and has a prominent peak at 700 °C, whereas the bimetallic catalysts have peaks covering both types. The 10Ni10Co and 5Ni15Co samples, which are composed almost exclusively of the thinner type of filament possess only a very small peak at 700 °C, with a far more prominent peak present at 550 °C. The decreasing ID/IG ratio in the bimetallic catalysts with increasing cobalt supports the formation of thinner filaments.Accessible, lab-based X-ray nano-CT provided three-dimensional information about the catalyst samples both post-reduction and post-coking. Raw ortho-slices, segmented ortho-slices, full volume renderings, and volume renderings of “internal non-support” phase of sub-volumes are shown for the as-reduced samples in Fig. 6 , and similarly for the samples post-coking in Fig. 7 . It should be noted that “internal non-support” phase relates to the darker grayscale regions, internal to the catalyst support particles which are either macropores (in the case of as-reduced samples) or a combination of macropores and carbonaceous material (in the case of as-coked samples), to a greater or less degree. The carbonaceous filaments are not sufficiently large or X-ray attenuating to be detected here.Figs. S14-19 show several slices from various sections of each sample. It is clear from Fig. 6(a) – (c) that there were interparticle voids present within all as-reduced samples, prior to exposure to CH4 at elevated temperatures, as well as macroporosity within individual support particles; the tomograms showed that the samples prior to coking were nominally the same in this regard. It is worth noting that since the spatial resolution of this technique has been estimated to be between 320 and 740 nm, the meso‑ and micropores known to be present within the Al2O3 support particles are not resolved in this study, although the largest of these may be resolvable by synchrotron X-ray ptychographic techniques (Weber et al., 2022). Some ‘excess’ large metal particles were observed on the support particle surfaces but this is not thought to majorly influence the behaviour of samples in this study given that the size of these particles (indeed, observable by X-ray CT) is significantly larger than the catalytically active particles contained within the catalyst support particles. This demonstrates the power of lab-based X-ray CT for identification of large metal particles, something that is not easily achieved with the majority of other techniques normally employed for heterogenous catalyst characterisation. None of these larger metal particles are observed within the support particle. Based on these observations it might be expected that there may be a gradient from the centre of the support particle to the surface in terms of metal concentration, however based on the cross-sectional SEM-EDX shown in Fig. 2, this gradient is not significant. Fig. 6(d) – (f) represent robust segmentations of air (black), yellow (supported catalyst), blue (internal non-support) and purple (residual metal) using a machine-learning-based approach, whilst the volume renderings in Fig. 6(g) – (i) display the similar morphology of each of the samples examined, constituting multiple individual support particles adjoined to one another. Fig. 6(j) – (l) illustrate a similar quantity of internal non-support phase associated with a sub-volume (ca. 6600 µm3) extracted from within each sample. A semi-quantitative approach has been taken to characterise the change in morphology associated with the extent of coking observed for each sample. The volume of “internal non-support phase”, which encompasses interparticle voids and macropores, as well as undetectable carbonaceous material, has been estimated by voxel counting after segmentation. It should be reiterated that features below the resolution limit will not be detected and therefore are excluded from this analysis. Table 4 illustrates that this phase constituted approximately 0.4–1.4% across the as-reduced samples, supporting the assertion that these samples were nominally the same in terms of interparticle voids and macropores.As observed from the two-dimensional SEM imaging, the morphology is significantly changed following the accelerated coking procedure, and vast differences between samples are apparent, as seen in Fig. 7. Firstly, the most significant morphological change can be seen to occur in the 20Ni sample, as observed in 2D in the SEM micrograph of Fig. 5(a). At first viewing, the foreground material appears counterintuitively to be unconnected and therefore floating in the air, however this apparent discrepancy is explained by the low X-ray attenuation coefficient of the carbonaceous material that surrounds the broken shards of the support. It is supposed that coking has occurred to such a significant degree that the bulk of the imaged sample is in fact carbon, but the contrast versus air is insufficient to detect all three phases. It is evident that the original support structure is no longer present, and the vast majority of the remaining sample comprises carbon material that is not directly observed using the X-ray CT technique, both due to the technique's resolution and the low density of the carbon material. Fig. 6(a) – (c) shows that there are some parallel cracks within particles, as inferred from the SEM micrographs shown in Fig. 1 (highlighted by red arrows). It is thought that these represent weak points that help explain the morphology of the coked samples wherein parallel shards of alumina support appear interspersed within carbonaceous filament networks. A sub-volume within the sample, delimited by the visible fragments, was extracted and the blue volume shown in Fig. 7(j) represents “internal non-support phase”, comprising both previous and newly formed voids but mostly filamentous carbon, all ascribed to this one composite phase as the carbon phase is not distinguishable. Clearly, a vast reduction in the volume percentage attributable to supported catalyst and excess metal was observed, which is quantified in Table 4. The next most significant transformation occurred in the pure cobalt sample (20Co). Fig. 7(f) clearly displays that coking has led to significant breakdown of the individual support particles and their cracks can be seen to align with one another, potentially along a single crystallographic direction. Although the support particle structure has degraded, the overall morphology is intact, highlighting that the behaviour of 20Co differs from that of 20Ni, and the lesser degree of coking (corroborated by TGA) leads to less support particle degradation in the former. This is also consistent with the observations by SEM of parallel cracking within catalyst support particles (Fig. 5(f)).However, the morphological change in the 10Ni10Co catalyst is clearly less dramatic still. Although not observable from two-dimensional SEM, some minor internal cracking between previously adjoined catalyst support particles was evident from the X-ray tomograms of this sample (see Fig. 7(b)). On the other hand, far fewer cracks were present within each individual catalyst-support particle, indicating a different degradation pathway from that followed by 20Co (and indeed 20Ni). Consistent with the lesser extent and different nature of the coking indicated by TGA and Raman spectroscopic analysis, the bimetallic catalyst (10Ni10Co) appeared more resistant to the accelerated coking procedure used in this study than the individual metals, exhibiting greater morphological durability and therefore suitability for this important industrial process. For the 10Ni10Co and 20Co samples, wherein the support particle morphology is broadly retained, there is a significant increase in the presence of large metal particles on the exterior surface of the catalyst-support particles. When compared to the as-reduced samples in Fig. 6, the significant extent of this agglomeration can be seen. This shows the power of X-ray CT for not only tracking the breakdown of catalyst particle structures but also for the study of the spatial redistribution of metals during coking, something that is not possible with the techniques regularly used for catalyst characterisation. This metal redistribution is significant since the formation of these large metal particles must result in the loss of the smaller, high-activity particles, below the resolution of the X-ray CT, meaning a loss in catalyst performance. It is thought that this phenomenon is not only related to the high temperatures that the catalysts have been subjected to but also due to the atmosphere the catalyst is exposed to. All as-reduced catalysts shown in Fig. 6 were subjected to higher temperatures (750 °C) than those during the accelerated coking (600 °C), as such the agglomeration behaviour is likely related to the presence of methane. Further time-resolved 4D tomography is planned to investigate this effect.By extraction of a similarly sized sub-volume from each of the six samples investigated, simple voxel counting of the phases attributed to solids (supported catalyst and excess metal) and internal non-support phase (i.e., voids and filamentous carbon) was performed and the results are shown in Table 4. The quantification is consistent with the above qualitative analysis: the largest “internal non-support phase” increase, thought to be mostly carbon, was observed to occur for the 20Ni sample, and least for the bimetallic case (10Ni10Co), whilst the pure cobalt (20Co) case displays intermediate behaviour. Volume renderings of the segmented phase that does not contain support or catalyst are shown in Fig. 7 (j-l), with the internal non-support phase for the 10Ni10Co case shown in black, indicating the lower degree of intra-particle cracking (blue) present in this sample, versus the 20Co sample.This sort of particle degradation can have a significant impact on catalyst performance, potentially leading to reactor blockage but just as significantly, it may impede catalyst reactivation. Coking is a common occurrence in hetero-catalytic processes where carbon is involved. In many cases, catalysts can be reactivated relatively easily by oxidising carbon deposits, removing them in the form of carbon dioxide and re-exposing the coated catalyst surfaces. Due to the lower thermal stability of the filamentous carbon, it is often seen as a less serious form of coking since it can be more easily removed at lower temperatures and is less prone to coating active layers. Here, however, the serious implications of the filamentous carbon formation are demonstrated. Should the 20Ni catalyst be reactivated by the removal of carbon the remaining support particles are so deleteriously affects that the regenerated catalyst would retain none of its mechanical strength and cause serious issues with reactor performance. The introduction of cobalt to the system (e.g., 10Ni10Co) aids the reduction of coke formation resulting in a catalyst that would retain some mechanical strength after regeneration or would have to be regenerated significantly less often, improving process efficiency.The X-ray nano-CT results shown here clearly corroborate the findings from the other characterisation performed as part of this study, but also give rise to extra insights with respect to the types and extent of cracking occurring in the samples with different nickel-to-cobalt ratios. Moreover, X-ray nano-CT has been shown to be a robust tool for quantifying the extent of support degradation by quantifying the “porosity” changes as a function of the nickel-to-cobalt ratio, indicating once more than the synergy between Ni and Co gives rise to a more robust supported catalyst than either of the monometallic analogues. In addition to these important insights into support particle behaviour, the technique has been used to clearly show agglomeration issues that are occurring during the accelerated coking tests that would have significant impact on the catalyst performance. The spatially resolved nature of X-ray CT means that this technique can give vital information to understand catalyst deactivation that is not clearly obtained from any of the other techniques employed.The use of bimetallic catalysts has been shown to have a significant impact on the performance of catalyst systems for the dry reforming of methane, whereby dual metal systems result in behaviour not observed in either of the single metal systems. The nickel-to-cobalt ratio in the bimetallic system has a considerable influence on both the quantity and properties of carbon formed during accelerated coking.The system containing only nickel was observed to result in the largest quantity of carbon formation with the introduction of cobalt significantly reducing the extent of coking. For the bimetallic systems, a nickel-to-cobalt ratio of 10Ni10Co or 5Ni10Co was observed to be optimal in terms of coking resistance. An increased quantity of cobalt also resulted in the formation of less thermally stable carbon fibres.Lab-based X-ray nano-computed tomography was used effectively to show that it is not only the quantity and bulk properties of the deposited carbon that are important. To gain a deeper understanding of coke formation and ensure long catalyst lifetimes, it is important to consider phenomena occurring at the scale of the supported catalyst particles. Both the nickel- and cobalt-only samples showed significant and detrimental supported catalyst particle cracking that is not observed in the mixed 10Ni10Co system. Interestingly, the technique also provides insights into agglomeration of active metal particles in to large, likely inactive, particles during coking. Further studies are planned to gain a better understanding of this phenomenon since the effect, if any, of nickel-to-cobalt ratio on agglomeration is not clear.The ease and relatively cheap nature of lab-based X-ray computed tomography measurements opens the possibility of the specially resolved tomographic techniques becoming used in the design and formulation of future catalysts. The nature of the information that can be obtained using these methods is something that is not currently easily available for catalyst designers but, as shown in this work, can give information key to understanding catalyst degradation and methods for overcoming these factors.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the Qatar National Research Fund (QNRF) from National Priority Research Program (NPRP9–313–2–135) and funding from the Faraday Institution (EP/S003053.1, grant numbers FIRG014 and FIRG015). Use of X-ray CT instruments was supported by the EPSRC (EP/K005030/1 and EP/P009050/1). The Royal Academy of Engineering is acknowledged for funding the Research Chairs of Shearing and Brett (including the National Physical Laboratory and HORIBA MIRA). Steve Hudziak and the CDT-ACM are gratefully acknowledged for their help with the Raman spectroscopy measurements.Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ccst.2022.100068. Image, application 1

The switch from a carbon-based to a hydrogen-based economy requires environmentally friendly methods for hydrogen production. CO2-reforming of methane promises to be a greener alternative to steam-methane reforming, which accounts for the majority of hydrogen production today. For this dry process to become industrially competitive, challenges such as catalyst deactivation and degradation through coke formation must be better understood and ultimately overcome. While bulk characterisation methods provide a wealth of useful information about the carbon formed during coking, spatially resolved techniques are required to understand the type and extent of degradation of supported catalyst particles themselves under coking conditions. Here, lab-based X-ray nano-computed tomography, in conjunction with a range of complementary techniques, is utilised to understand the effects of the nickel-to-cobalt ratio on the degradation of individual supported catalyst particles. Findings suggest that a bimetallic system greatly outperforms monometallic catalysts, with the ratio between nickel and cobalt having a significant impact on the type and quantity of the carbon formed and on the extent of supported catalyst breakdown.

Data will be made available on request. Data will be made available on request.Volatile organic compounds (VOCs) were considered organic compounds with high vapor pressures that evaporate easily at normal temperatures. There are many sources of VOCs emissions, such as the chemical processing industry, household activities and vehicles [1–4]. The emission control of VOCs is an urgent technology due to its highly toxic. Among the various technologies, catalytic oxidation is the most ideal method to eliminate large amounts of VOCs [5,6]. Therefore, the research on VOC combustion has become a key project [7].At present, metal catalysts were widely studied as highly efficient catalysts. Noble metal-based catalysts were often used in the catalytic oxidation of toluene because of their superior catalytic performance [8,9]. However, its practical application was limited. Transition metal oxides have become alternative catalysts owing to low cost, abundant resources and excellent catalytic performance [10]. Manganese-based catalysts have been used for catalytic oxidation of VOCs [11–13]. It is well known that the chemical valence state and oxygen species of manganese species are the key factors affecting its performance. Guo et al. found that it could achieve completed oxidation and removal of benzene at 210 °C [14]. Liu et al. confirmed that T-MnO2 manifested the best degradation of toluene [15]. Therefore, the MnO2 catalysts were widely investigated in the field.Generally, the catalytic efficiency of manganese-based catalysts was related to the electron transfer between manganese ions of different valence states [16]. The researchers found that the amount of manganese ions could be changed by the adjusting the proportional content of the Mn precursors. Li et al. found that better crystallinity and the abundance of Mn4+ was related to the ratio of precursor drugs, which could improve the catalytic oxidation of toluene over MnO2-based catalyst [17]. Dong et al. proved that MnO2 with the different the ratio of Cu/Mn exhibited the superior catalytic performance due to the highest content of Mn3+ [18]. Qu et al. demonstrated that Ag-Mn/SBA-15 possessed the excellent catalytic activity because of the highest ratio of Mn4+/Mn3+ [1].Another important factor affecting toluene removal was the oxygen vacancy content, which could contribute to the superior catalytic performance. Zhang et al. adjusted the molar ratio of Mn(NO3)2/KMnO4 to prepared a series of manganese oxides. MnO x exhibited the optimal catalytic performance with higher oxygen vacancies when the ratio was 3:7 [19]. Yang et al. changed the ratio of MnSO4 to KMnO4 to prepare a series of MnO2-USY [20]. The catalyst with a content of 3% exhibited the best activity because of more surface-active substances. Huang et al. explored the effect of different Ni/Mn ratios on the catalytic activity of toluene, and reported that the Ni-Mn catalyst presented the best outstanding catalytic activity due to the abundance of oxygen vacancies while the Ni loading was 10 wt.% [21]. Hence, the valence states and oxygen vacancy content of MnO x -based catalyst depended on the content of the precursor, which was closely related to the catalytic performance.In the present study, a series of δ-MnO2 catalysts for catalytic oxidation of toluene were synthesized by hydrothermal method. The effects of different MnSO4 contents on the generation of oxygen vacancies and valence states were investigated. The physico-chemical properties of the obtained catalysts were investigated by characterization techniques such as SEM, XRD, XPS, BET, Raman and In situ DRIFT.A series of MnO2 were prepared by hydrothermal method. Dissolved 0.1g MnSO4 in 64 mL of water, dissolved fully through magnetic stirring, then added 1.2g KMnO4, and form a homogeneous mixture after 10 min, and conducted hydrothermal reaction at 160 °C for 24 h, which was marked as Cat-1. Subsequently, the content of MnSO4 was changed to 0.2g, 0.4g, 0.8g and 1.2g, respectively, and the catalyst was prepared by the same preparation method. And the samples were named as Cat-2, Cat-3, Cat-4 and Cat-5, respectively. After the hydrothermal reaction, the reaction product was collected, washed and dried with distilled water, and then placed in a muffle furnace to heat up to 300 °C at a rate of 3 °C/min from room temperature. After holding for two hours, the calcination temperature continued to increase to 550 °C at a heating rate of 3 °C/min, keeping for three hours and cooling to room temperature to obtained a series of MnO2 catalysts. As a comparison, Cat-0 was prepared by pure KMnO4 with the same preparation method.Power X-ray diffraction patterns (XRD) of the samples were recorded Bruker/AXS D8 Advance diffractometer with a radiation source of Cu Kα (λ=0.15406 nm) and operated at 35 kV and 35 mA. Diffraction data were collected with the 2θ ranged from 10° to 80° with a scanning rate of 8°/min.The Brunauer–Emmett–Teller (BET) specific surface area and pore characteristics of the catalyst was measured via nitrogen adsorption and desorption isotherms at -196 °C on an SSA-6000 adsorption analyzer with the samples being performed under vacuum at 200 °C for 150 min before testing.Temperature-programmed reduction by hydrogen (H2-TPR) was carried out in a chemisorption analyzer(PCA-1200). 200 mg of catalyst was pretreated under hydrogen steam at 200 °C for 30 min and cooled down to room temperature. Then the samples were measured continuously by a thermal conductivity detector (TCD) from 100 to 900 °C with 10°C min-1 under 5% H2/Ar flow.XPS were obtained on a ESCALab 250Xi electron spectrometer using Al Kα as an excitation source. The experimental condition was constant at 15kV. The binding energies were calibrated via using the adventitious carbon at 284.6 eV as an internal standard.O2-TPD was conducted using the same reactor as the H2-TPR. The samples (100 mg) were first degassed at 200 °C for 60 min in highly pure He and cooled down to 50 °C. When the temperature dropped below 50 °C, O2 was introduced to absorb oxygen for 1 hour, and then He was introduced again, and the temperature was raised from 50 °C to 900 °C with a heating rate of 10°C min-1.In situ DRIFT spectra were recorded of the toluene oxidation on a Nicolet IS 10 FT-IR spectrometer, equipped with a MCT detector. The catalysts were pretreated at 350 °C for 45 min in a flow of N2, and then cooled down to room temperature naturally. Adsorption was carried out at 260 °C (flow of toluene/N2), followed by desorption or oxidation (flow of O2/N2) which were recorded at different times.The catalytic performance was analyzed and detected in a fixed-bed quartz tube reactor, a gas chromatograph with FID flame ion was connected to the reactor, and the toluene concentration was tested by FID. The sieved catalyst (40-60 mesh) was weighed to 0.1 g and placed in a quartz tube, and 500 ppm of toluene (100 mL/min) diluted with N2 was used as the gas inlet. The reaction temperature range was 180-300 °C using a thermocouple detector. Before the test, the pretreatment of the mixed gas stable gas system was carried out, and the heating experiment was carried out after 15 min of treatment.The conversion of toluene ( C t ) was calculated by the following formulas:The conversion of toluene ( C t ): (1) C t = ( t o l u e n e ) i n − ( t o l u e n e ) o u t ( t o l u e n e ) i n * 100 % The CO2 selectivity ( C C O 2 ): (2) C C O 2 = ( C C O 2 ) o u t 7 ( t o l u e n e ) i n * 100 % The crystal structures of different samples were investigated by XRD. As could be observed in Fig. 1 , all the samples showed typical MnO2 structures (PDF#44-0141). The peaks at 12.5°, 17.9°, 28.8°, 37.4°, 41.9°, 52.8°, 56.3° and 60.2° corresponded to the (110), (200), (310), (211), (301), (440), (600) and (521) planes, respectively. More crystalline phases appear after the addition of MnSO4. A large number of diffraction peaks indicated a high degree of structural disorder in the crystal structure of the MnSO4-containing catalyst, and then the catalytic activity [22]. The Mn3+ ion has a strong Jahn-Teller effect, which leads to the stretching of the Mn-O bond length [23]. Therefore, a large amount of Mn3+ could be readily dissociated and activated the surrounding oxygen atoms and promoted their catalytic properties. Furthermore, Cat-1, Cat-2, Cat-3, Cat-4 and Cat-5 were all weak and broad peaks, which indicated structural ordering reduction. This phenomenon represented that different MnSO4 contents could change the crystal size of the catalyst, which improved the catalytic activity. The grain size of the catalyst was calculated by Scheler's formula, and the order was ranked as: Cat-0 (36.1 nm) > Cat-1 (20.2 nm) > Cat-2 (19.5 nm) > Cat-3 (18.9 nm) > Cat- 5 (18.7 nm) > Cat-4 (18.3 nm). The smaller grain size facilitated the adsorption of toluene and accelerated the redox reaction [23]. Obviously, Cat-4 possessed a weaker peak shape and the smallest grain size, which could enhance the catalytic activity [24].The morphology of catalysts with different MnSO4 contents was studied by SEM technique. Fig. 2 was the SEM images of Cat-0, Cat-1, Cat-2, Cat-3, Cat-4 and Cat-5. All catalysts exhibited nanorod-like structures. In Fig. 2A, the MnO2 by hydrothermal treatment of pure KMnO4 was a bulk nanorod aggregate. With the addition of MnSO4, small aggregates of MnO2 nanorods were formed in Fig. 2B, Fig. 2C and Fig. 2D, respectively. However, no agglomerated structure was detected in Fig. 2E. Interestingly, agglomeration was observed in Fig. 2F. The addition of MnSO4 gradually dispersed the MnO2 nanorods, but the addition of excessive MnSO4 would cause to the re-agglomeration of MnO2. The agglomerated structure was not observed in Cat-4, which made it easier for toluene to adhere to its surface, speeding up the reaction rate. Fig. 3 showed the N2 adsorption-desorption isotherm and pore size distributions of catalysts with different contents of MnSO4. The catalysts all showed IV-type isotherms and H3-type isotherms according to IUPAC classification, which proved that the catalysts were all mesoporous materials. Table 1 showed the specific surface area (SBET), pore volume (Vp) and pore size (Dp). The average pore size of Cat-4 was 6.8 nm. It was well known that the diameter of toluene was 0.583 nm, which was much smaller than the pore size of the prepared samples. Hence, toluene could be easily adsorbed in the pore channel of the catalyst during the degradation process of toluene catalytic combustion. The SBET of Cat-0, Cat-1, Cat-2, Cat-3, Cat-4 and Cat-5 was 20, 19, 24, 20, 22 and 24 m2/g, respectively, which showed that the content of MnSO4 had no significant effect on the specific surface area of the catalyst.The atomic states of the outermost layer of catalysts with different contents of MnSO4 were detected by XPS test. From Fig. 4 a, there were two obvious main peaks for Mn 2p3/2, which was further decomposed into four peaks. The peaks at 642.7-643.8 eV and 654.4-655.9 eV were assigned to Mn4+. The peaks at 641.9-642.4 eV and 653.5-653.8 eV corresponded to Mn3+. In general, the amount of Mn3+ would determine the catalytic efficiency of MnO2. The ratio of Mn3+/(Mn3++Mn4+) of Cat-0, Cat-1, Cat-2, Cat-3, Cat-4 and Cat-5 was 0.53, 0.55, 0.56, 0.57, 0.59 and 0.58 in Table 2 , respectively, which was consistent with their catalytic activity. More Mn3+ meant more oxygen vacancies, which provided more active sites for MnO2 [26]. Zhang et al. suggested that Mn-120 exhibited a high proportion of Mn3+/Mn4+, and enhanced the catalytic efficiency [28]. Wang et al. reported that the valence state transition between manganese ions facilitated the migration of oxygen species [26]. Santos et al. demonstrated that the lower binding capacity between Mn3+ and O resulted in more oxygen vacancies [29]. Cat-4 exhibited the highest proportion of Mn3+, which enhanced the catalytic ability. Fig. 4b was the O 1s energy spectrum of the catalyst. 529.4-529.8 eV was adsorbed oxygen (O ads ) and 531.2-531.5 eV was lattice oxygen (O latt ) peaks [27]. The transport transformation between oxygen species was the key to catalytic reaction generation. Table 2 listed the ratios of O ads /(O ads +O latt ) in Cat-0, Cat-1, Cat-2, Cat-3, Cat-4 and Cat-5, which was 0.60, 0.38, 0.53, 0.62, 0.69, 0.67, respectively. Obviously, Cat-4 exhibited the highest O ads content, which accelerated the catalytic process [30]. Zhang et al. found through LaMnO x perovskite catalyst owned the abundance of adsorbing oxygen, which improved the catalytic performance [31]. Huang et al. demonstrated that a large amount of O ads could be easily volatilized at lower temperatures, which could be contributed to the superior catalytic performance [32].Raman testing was used to test the catalyst microstructure. Fig. 5 was the Raman spectrum of the catalyst. The peak positions of Cat-1, Cat-2, Cat-3, Cat-4 and Cat-5 all appeared at 600 cm-1, which was the stretching between Mn3+ and O-connected Mn-O bonds in the MnO6 octahedron vibration [33]. According to literature reports, the weakening of Raman spectral peaks was related to the formation of structural defects or the formation of solid solutions [34]. The formation of structural defects could be induced a large number of oxygen vacancies, accelerating the oxygen migration rate. Compared with other catalysts, Cat-4 exhibited the weakest and broadest peaks, which indicated massive oxygen vacancy defects in Cat-4. The presence of oxygen vacancy defects accelerated oxygen species migration and transformation. The appearance of a peak at 400 cm-1 in the spectrum could be observed, which was attributed to the asymmetric stretching vibration of the oxygen species. Cat-0 showed a sharp peak at 1200 cm-1, which proved that it processed less oxygen vacancy defects. Through the analysis of Raman spectrum, the proportion of precursor content would affect the content of oxygen vacancy defects in the catalyst.The redox performance of the catalysts was characterized using temperature-programmed studies. The reducibility of catalyst was demonstrated using H2-TPR. Two peaks appeared for all catalysts, the first was attributed to MnO2 reduction to Mn2O3, and the second was assigned to Mn2O3 reduction to MnO. Compared with other samples, Cat-4 exhibited the lowest initial reduction temperature (265 °C). Generally, a lower reduction temperature represented higher content of surface-active sites and oxygen species, which enhanced the reduction ability of the catalyst [26]. The, Cat-4 showed the largest peak area for the second reduction peak, which represented the most Mn3+. The presence of a large number of Mn3+ and more active sites improved the degradation efficiency of toluene. The hydrogen consumption of the sample was calculated by integrating all the main peaks, compared with Cat-0 (1.27 mmol/g), Cat-1 (1.41 mmol/g), Cat-2 (1.43 mmol/g), Cat-3 (1.48 mmol/g) and Cat-5 (1.58 mmol/g), H2 consumption of Cat-4 (1.64 mmol/g) was the largest, which proved that Cat-4 exhibited more reducible oxygen content. Fig. 6 b showed the hydrogen consumption rate, and Cat-4 exhibited the highest H2 consumption rate at the same temperature. Thereby, Cat-4 possessed the excellent redox ability, which was responsible for the superior catalytic activity.In order to understand the effect of different MnSO4 contents on oxygen vacancies, O2-TPD was used for characterization analysis in Fig. 7 . All catalysts showed two main peaks. Generally, the peak below 400 °C corresponded to surface adsorption oxygen desorption, the peak between 400-700 °C was surface lattice oxygen desorption, and the peak above 700 °C was bulk lattice oxygen desorption [35]. Typically, the peak area represented the amount of oxygen desorbed. Compared with other catalysts, Cat-4 desorbed the highest and widest surface oxygen peaks in the range of 400-700 °C, which indicated that Cat-4 exhibited the best oxygen mobility. According to previous XPS studies, Cat-4 owned the most Mn3+. In the Raman spectrum, the binding force of Mn3+-O was the weakest and more oxygen species were released, which was the reason for the best oxygen mobility of Cat-4 catalyst.The toluene catalytic oxidation performance of all the catalysts with different MnSO4 loading content were evaluated, and the results were shown in Fig. 8 (a). All samples showed a positive correlation between toluene conversion and temperature. The T50 (50% toluene conversion) of Cat-0, Cat-1, Cat-2, Cat-3, Cat-4, and Cat-5 was 308, 291, 282, 268, 251, and 257 °C, respectively. The temperature order of T90 (90% toluene conversion) followed: Cat-4 (260 °C) < Cat-5 (263 °C) < Cat-3 (273 °C) < Cat-2 (291 °C) < Cat-1 (298 °C) < Cat-0 (326 °C). The combined results of T50 and T90 showed that all catalysts with MnSO4 addition outperformed the original catalysts, indicating that the interaction of MnSO4 with KMnO4 promoted the catalytic activity. In the four comparative catalysts with MnSO4 addition, Cat-4 showed the most significant improvement in catalytic activity, the T90 decreased by a full 56 °C, indicating that the appropriate amount of MnSO4 doping contributed to the improvement of the catalytic performance of toluene. In addition, Fig. 8(a) and Fig. 8(b) illustrated that the carbon dioxide selectivity of the catalyst was essentially the same as the catalytic activity, indicating that few intermediates were produced during the degradation of toluene.The stable and continuous use of the catalyst was more critical in industrial applications, so the stability of the catalyst was important. The stability of Cat-4 catalyst was investigated. The catalytic reaction was continued for 100 hours at 260 °C. Cat-4 exhibited the excellent catalytic stability, and the toluene conversion rate was maintained at 90%. Moreover, the catalyst showed the superior catalytic performance for dry toluene gas. However, the emission of VOCs was often mixed with the existence of water vapor, so the water resistance of the sample was worth exploring. After water vapor was introduced, the conversion rate of toluene gradually decreased to about 81% over Cat-4, and the conversion rate of toluene was maintained at 80% after continuous introduction of water vapor for 10 hours. After the water vapor was stopped, the conversion rate of toluene quickly rose to 90%. The results showed that the Cat-4 catalyst was tolerant to water vapor, and there was reversibility between the water vapor and the sample. Noteworthy, the conversion rate of toluene was slightly higher than 90% after stopping the introduction of water vapor because that the presence of hydroxyl groups enhanced the surface-active oxygen species and increased the catalytic activity [36] (Fig. 9 ).To detected the intermediates in toluene combustion, in situ DRIFTS experiments was conducted to further study the reaction mechanism on Cat-4. Fig. 10 showed the DRIFTS spectra over Cat-4. The adsorption bands at 1601 and 1487 cm-1 were ascribed to typically skeletal vibration of the aromatic ring, manifesting that toluene was adsorbed on the surface of catalyst [37–40]. Notably, the peaks at 1067, 1117 and 1338 cm-1 were recognized as the alkoxide species (C-O stretching vibration), indicating that the adsorbed toluene could form benzyl alcohol (C6H5-CH2O) by smashing the C-H bond of the methyl (-CH3) [40–42,35]. Moreover, the peaks at 1473 and 1646 cm-1 corresponded to the C-O-H and ʋ(C=O) stretching vibrations, declaring the formation of benzaldehyde (C6H5-CHO) [43,44]. The peaks at 1413, 1542 and 1582 cm-1 were belonged to typical bands carboxylate species, demonstrating the formation of benzoate species [38–40,45,46]. Benzoate species were present almost throughout the entire time period, suggesting that benzoic acid species were key intermediates in the oxidation of toluene. The detected peaks near 1734, 1807 and 1865 cm-1 was belonged to the acid anhydride species, manifesting the formation of maleic anhydride [40,41,47,48], which was an important intermediate for the ring opening of benzoic acid. For the Cat-4, in situ DRIFT spectrum, the formation of maleic anhydride did not increase with extension of time for adsorption during the oxidation of toluene molecules. On the contrary, the characteristic band of benzoic acid increased monotonically with the increased of reaction time, confirming that maleic anhydride was prior transformed to CO2 and H2O.To sum up, according on the in situ DRIFTS results, the proposed oxidation reaction mechanism of toluene on Cat-4 catalyst followed the MVK mechanism. Firstly, toluene was adsorbed over the Cat-4 catalyst and then reacted with lattice oxygen species to form benzyl alcohol species through one-step dehydrogenation [44]. Simultaneously, the resulting oxygen vacancies could be replenished by gas-phase O2 and reactive oxygen species (O2→O2 −→O2 2−→O−→O2−). The benzyl alcohol would be oxidized to benzaldehyde, which was then oxidized to benzoic acid. Benzoate was cleaved by ring opening to form maleic anhydride, which was further converted to CO2 and H2O. It was important not to mention that oxygen vacancies played an important role in the catalytic combustion process, because oxygen vacancies could quickly adsorb and activate O2 molecules, thus accelerating the degradation of reactants. There was the highest Mn3+/(Mn3++Mn4+) over Cat-4 catalyst, which accelerated the catalytic oxidation efficiency of toluene.XRD analysis showed that with the increase of MnSO4 content in the precursor, the crystal size of the catalyst gradually decreased. The addition of excess MnSO4 increased the catalyst particle size again. Liu et al. reported that the catalyst with small particle size exhibited excellent catalytic activity, and the smaller particle size was beneficial to enhance the oxygen transport ability of the catalyst [49]. Zhu et al. demonstrated that the small particle size of catalyst showed abundant adsorption sites, which was favorable for the adsorption and activation of toluene [25]. Therefore, Cat-4 with the smallest particle size could accelerate the adsorption of toluene and the redox capacity, leading to the best catalytic performance.To further explore the specific factors affecting the catalytic activity, the XPS results and catalytic activities were shown in Fig. 11 . The change trend of the catalytic activity at 260 °C was related to Mn3+/(Mn3++Mn4+), which proved that the presence of Mn3+ could be promoted the catalytic reaction. It was generally believed that because of the existence of the Mn4+–O2−-Mn4+ → Mn3+-□-Mn3++1/2O2 cycle, more Mn3+ concentration was favorable for the formation of oxygen vacancies to balance the charges over MnO x [50]. In addition, higher Mn3+ concentration caused lattice defects for MnO x , which promoted the formation of oxygen vacancies [51]. Genuino et al. proposed that Mn3+ could be induced a large number of active sites, thereby promoting the catalytic oxidation of toluene [52]. The change of MnSO4 content affected the content of Mn3+ in the catalyst, which in turn changed the content of oxygen vacancies. In addition, with the increased of MnSO4 content, the content of Mn3+ increased first and then decreased, and Cat-4 possessed the highest content of low-valence Mn3+, consequently it had the best catalytic activity. O ads / (O ads  + O latt ) and Mn3+/Mn4+ showed the same trend. As MnSO4 increased from 0 to 0.8, the Oads concentration concurrently increased Cat-4 possessed the most amounts of O ads species. With the further increase of MnSO4, the O ads content decreased, which was consistent with the trend of catalytic activity of the catalyst. The phenomenon implied that surface adsorbed oxygen played a key role in the catalytic combustion process of toluene. The ratio of O ads was considered as an indicator of oxygen vacancy concentration because that gas-phase O2 could be adsorbed on oxygen vacancies and activated as electrophilic reactive oxygen species (ads), which participated in the catalytic oxidation process of toluene [53]. Li et al. demonstrated that O ads accelerated the migration of O latt , which in turn enhanced the catalytic oxidation performance [54]. Qu et al. reported that a great quantity of O ads facilitated the circulation of oxygen species, which promoted the catalytic combustion reaction of toluene [55]. The addition of appropriate amount of MnSO4 could be increased the content of Mn3+ and O ads , which played a vital role in the formation of oxygen vacancies. The abundant oxygen vacancies improved the adsorption of oxygen in the gas phase and promoted the conversion of lattice oxygen, which greatly improved the oxygen mobility [56]. The H2-TPR results proved that the reduction performance of MnO x was improved by changing the content of MnSO4. Furthermore, Cat-4 showed more O ads , which led to the best catalytic activity.Therefore, combined with the analysis of XPS, H2-TPR, O2-TPD and XRD, the content of MnSO4 improved the reducibility of MnO x and the content of oxygen vacancies, which increased the catalytic activity. More amounts of Mn3+ and the oxygen vacancy content could be provided more adsorption sites for O ads and then accelerated the migration rate of oxygen species. The relationships among Mn3+, O ads and toluene catalytic activity were shown in Fig. 11. Evidently, the concentration of Mn3+ and O ads had a linear relationship with the catalyst activity. The superior activity of Cat-4 should be attributed to the increased of oxygen vacancies. As shown in Fig. 11, the particle size of the catalyst was negatively correlated with the catalytic activity. Obviously, the smaller grain size provided more toluene adsorption sites of toluene, which improved the catalytic activity. Hence, Cat-4 with the smallest particle size possessed the best catalytic performance.In this work, the toluene oxidation reaction by adjusting the content of MnSO4 in the precursor samples was investigated. Compared with the single KMnO4, the catalyst with the addition of MnSO4 showed better catalytic activity. When the MnSO4 content increased from 0 to 0.8, the catalytic activity simultaneously increased. With the further increased of MnSO4, the activity started to decreased, which indicated that the content of MnSO4 had a significant effect on the performance of the catalyst. Among them, Cat-4 showed the best catalytic performance and reaches 90 % conversion of 260 °C. According to the characterization of Cat-4 samples, the improved catalytic activity, better stability and water resistance could be attributed to better low-temperature reduction, more oxygen desorption, more oxygen vacancies, higher content of Mn3+/(Mn3++Mn4+) and more surface adsorbed oxygen content. In addition, Cat-4 showed excellent stability and water resistance. Furthermore, the oxidation process of toluene was inferred from the in situ DRIFT result: toluene → benzyl alcohol → benzaldehyde → benzoic acid → maleic anhydride → CO2 and H2O. Therefore, different MnSO4 contents changed the microscopic properties of the catalysts as well as the redox capacity of the catalysts. Cat-4 exhibited the best catalytic activity because it showed the most structural defects, excellent oxidative ability and strongest oxygen transport ability.We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. There is no professional or other personal interest of any nature or kind in any product, service and/or company. Zhongxian Song: Writing – review & editing, Funding acquisition. Haiyang Li: Writing – original draft. Xuejun Zhang: Writing – review & editing, Project administration. Zhuofu Zhang: Investigation. Yanli Mao: Formal analysis. Wei Liu: Formal analysis. Zepeng Liu: Resources. Dujuan Mo: Software. Xinfeng Zhu: Methodology. Zhenzhen Huang: Writing – review & editing.This work is supported by the National Natural Science Foundation of China (No.21872096), Natural Science Youth Fund of Henan Province (No. 202300410034); Young Teacher Foundation of Henan University of Urban Construction (No. YCJQNGGJS201903), Academic Leader of Henan Institute of Urban Construction (No. YCJXSJSDTR202204), Science and technology major special of Pingdingshan, (No. 2021ZD03) and Doctoral Research Start-up Project of Henan University of Urban Construction (No. 990/Q2017011).

A series of MnO x catalysts with different content of MnSO4 were prepared by hydrothermal method and used for the catalytic oxidation of toluene. The MnO x (Cat-4) showed the best catalytic activity when the MnSO4 content was 0.8 g, and the toluene conversion rate reached 90% at 260°C. Cat-4 catalyst showed more Mn3+ and O ads concentrations, which provided more oxygen vacancies and accelerated the migration rate of oxygen species, and then enhanced the redox performance, resulting in the improvement of the catalytic activity, which depended on the content of MnSO4 over Cat-4. The 100 h stability test showed that the Cat-4 catalyst presented superb stability and water resistance. Furthermore, the oxidation process of toluene was inferred from the in situ DRIFT result: toluene → benzyl alcohol → benzaldehyde → benzoic acid → maleic anhydride → CO2 and H2O.

Data will be made available on request.Recently, hydrogen ( H 2 ) as a fuel has been the focus for a great research attention due to its importance in the development of clean energy technologies [1,2]. Proton exchange membrane fuel cells (PEMFCs), with their have high efficiency, low operating temperatures, and low emissions, have been considered promising to utilize H 2 or any other proton-containing fuel to directly generate electricity in electrochemical galvanic reactors [3,4].In this regard, the direct formic acid (FA) fuel cells (DFAFCs) owned a high (1750 kW h L − 1 ) energy density [5], large theoretical open-circuit potential ( ∼ 1.40 V) [6,7], and non-toxic fuel that has a lower crossover through the Nafion® membrane compared to other liquid fuels [8]. Although platinum (Pt) was identified as the most frequently used catalyst for FA electro-oxidation reaction (FAOR) [9], its scarcity and high cost along with the lack of a long-term durability prohibited the large-scale commercialization of DFAFCs. At Pt surfaces, FAOR adopts a dual pathway mechanism, i.e., dehydrogenation to CO2 and dehydration to CO that further blocks the Pt active sites and therefore retards the catalyst activity [10]. To overcome such a bad impact of a catalyst’s poisoning with CO, Pt was modified previously with Pd [9], Ag [11], Au [3], Bi [12], and Ni [13] that could switch the mechanism (completely or to a great extent) toward the direct dehydrogenation pathway. We, herein, report on the fabrication of a simultaneously co-electrodeposited PtPd binary catalyst that was assembled onto a GC surface for enhanced FAOR. A combination of electrochemical and materials measurement techniques assisted in the evaluation of the morphology, activity, and stability of the proposed catalyst.The chemicals used in this investigation were of high purity and used as received from trusted suppliers as Sigma Aldrich and Alfa Aesar. The surface morphology of the proposed catalysts was investigated using the field emission scanning electron microscopy (FE-SEM, Quattro S, Thermo Fisher Scientific USA equipped with AMETEK USA Element Detector). The electrochemical experiments were performed using a Bio-Logic SAS (model SP-150) potentiostat operated with EC-Lab software. All electrochemical experiments were performed using a conventional three-electrode system including a GC working electrode (5 mm in diameter with 0.196 cm 2 geometric area), spiral Pt wire as an auxiliary electrode, and an Ag/AgCl/NaCl (3M) reference electrode. All these electrodes were purchased from ALS Japan. The PtPd catalyst was fabricated using the simultaneous co-electrodeposition technique as previously explained [3]. Briefly, this involved the potentiostatic electrodeposition of Pt and Pd onto the GC surface at −0.2 V permitting the passage of only 10 mC in 0.1 M Na2SO4 aqueous solutions containing 2.0 mM H 2 PtCl6. xH2O and 2.0 mM Pd (CH3COO)2.The surface composition of the catalyst’s ingredients was explored using the cyclic voltammetry (CV) experiments. Fig. 1 represents the CVs measured in aqueous solution of 0.5 M H 2 SO4 for (a) Pt/GC and (b) PtPd/GC catalysts in a potential range between −0.2 and ＋1.2 V at a potential scan rate of 100 mV s − 1 . Fig. 1a (Pt/GC catalyst) shows the characteristic performance of a poly-Pt electrode in an acidic conditions [14]. This displayed the Pt oxidation (Pt → PtO) which extended over a wide potential range between ca. 0.6 and 1.2 V and coupled with the subsequent (PtO → Pt) reduction peak at ca. 0.5 V. Moreover, the peaks that appeared in the potential range between 0 and −0.2 V were assigned to the hydrogen adsorption/desorption ( H ads/des ). The PtPd/GC catalyst (Fig. 1b) retained such behavior with an increasing in the currents of the Pt → PtO, PtO → Pt, and H ads/des peaks due to the overlapping of Pd with Pt peaks [14]. Fig. 2 shows FE-SEM micrographs of the Pt/GC (Fig. 2A) and PtPd/GC (Fig. 2B) catalysts. Fig. 2A displayed the Pt electrodeposition onto the GC surface in a spherical well-dispersed structures with an average particle size of ca. 220 nm with some intensive aggregations reaching ca. 600 nm. On the other hand, the PtPd/GC catalyst (Fig. 2B) retained the same spherical shaped structure that has an average particle size a little bit larger (ca. 238 nm) with very little aggregations (reaching ca. 250 nm) compared with the Pt/GC catalyst. Fig. 3 displays the CVs of FAOR at the (a) Pt/GC and (b) PtPd/GC catalysts in an aqueous solution of 0.3 M FA (pH ∼ 3.5) at a potential scan rate of 100 mV s − 1 . Commonly, the mechanism of FAOR on Pt-based electro-catalysts proceeds in two different routes [15]. The direct one involves the dehydrogenation of FA to CO2. This direct route takes place at a low potential domain and this will shift the actual voltage of DFAFCs closer to its theoretical value. That is why this route is the preferred route for FAOR. In Fig. 3a (Pt/GC catalyst), two oxidation peaks were observed at 0.35 and 0.82 V in the anodic-going scan. The first one (at ca. 0.35 V) was assigned to the direct (dehydrogenation pathway) oxidation of FA to CO2. The current density of this peak will be abbreviated as I p d . The second one (at ca. 0.82 V) was assigned to the oxidation of the pre-adsorbed CO (CO ads ) to CO2 after the Pt surface hydroxylation (Pt-OH) at ca. 0.7 V. The current density of this peak will be abbreviated as I p ind . In fact, the main challenge of proposing Pt-based catalyst toward FAOR is related to the adsorption of the poisoning CO which takes place spontaneously from the non-faradaic dissociation of FA at open circuit potentials. This deactivates the Pt surface and prompts a potential poisoning for a significant number of Pt active sites, which, in turns, impede the direct “preferred” pathway of FAOR [4,15–19]. Herein, several significant parameters ( I p d , I p ind , I p b and E onset ) were calculated to quantify the degree of catalytic enhancement toward FAOR and the reduction in CO poisoning for both (Pt/GC and PtPd/GC) modified catalysts. The relative ratios of I p d / I p ind (that evaluates the enhancement in the catalytic activity in the favorable direct oxidation pathway) and I p d / I p b (that estimates the catalytic tolerance of the catalyst for poisoning CO species) at the Pt/GC catalyst (Fig. 3a) were 0.59 and 0.16, respectively with E onset (that reflects the capability of the catalyst to overcome unnecessary overpotentials (particularly of charge transfer) that normally detracts the voltage output of the cell) of ca. 96 mV measured at 0.4 mA cm − 2 . Interestingly, as obviously observed from Fig. 3b (PtPd/GC catalyst), these values reached 7.33 (i.e., ca. 12.4 times higher) and 0.32 (i.e., ca. 2 times higher) compared with the Pt/GC catalyst. This was observed concurrently with ca. -91 mV shift in the E onset . This indicates the superiority of the modified PtPd/GC catalyst toward FAOR.Besides the catalytic activity enhancement, it is also very important to improve the catalytic stability [7]. Herein, the stability of the Pt/GC (Fig. 4a) and PtPd/GC (Fig. 4b) catalysts were measured by recording the current transients ( i − t ) curves, under continuous electrolysis for 3600 s in FA solution at 0.2 V. These measured data came consistent and as expected with the data of Fig. 3, the PtPd/GC catalyst acquired a lower current decay with time which means a prolonged stability during continuous electrolysis. As obviously seen from Fig. 4 after 3600 s of continuous electrolysis, the current density of the PtPd/GC catalyst was ca. 2.5 times higher than that observed at the Pt/GC catalyst. This represented an additional value for Pd in boosting the catalytic tolerance of the PtPd/GC catalyst against CO poisoning during FAOR.A PtPd/GC binary catalyst prepared by the simultaneous co-electrodeposition method was endorsed for efficient FAOR. The catalyst retained the highest catalytic activity (with up to ca. 12.4 times increase in the I p d / I p ind index, 2 times increase in the I p d / I p b index and −91 mV shift in E onset ) toward FAOR compared to the conventional Pt/GC catalyst. This associated a critical improvement in the catalytic stability that appeared in maintaining a higher (by ca. 2.5 times) current density after prolonged electrolysis for 3600 s at 0.2 V. It was thought that minimizing the CO adsorption at the Pt surface was mostly behind the observed enhancement.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supoorted from the British University in Egypt and the Faculty of Science - Cairo University .

This investigation displayed a superb catalytic performance toward the formic acid electro-oxidation reaction (FAOR) in an alkaline medium at a binary catalyst composed of Pt and Pd that were simultaneously electrodeposited onto a glassy carbon (GC) surface. Interestingly, the proposed PtPd/GC catalyst displayed a significant enhancement in the catalytic activity (by ca. 12 times higher direct ( I p d ) to indirect ( I p ind ) current ratio concurrently with ca. -91 mV shift in the onset potential ( E onset ) and stability (ca. 2.5 times higher current density after 3600 s of continuous electrolysis) toward FAOR if compared to the Pt/GC catalyst. The catalytic enhancement was thought to arise mostly from minimizing the CO adsorption, i.e., third body effect, at the Pt surface during FAOR.

Data will be made available on request.The combustion of fossil fuels for energy production releases into the atmosphere a large amount of CO2 which is claimed as a main cause of the greenhouse effect [1–4]. The commitment of the International governments to mitigate the inherent issues has recently driven 195 Parties setting new targets during the 27th UN Climate Change Conference (COP27, Sharm el-Sheikh 2022) to reach zero emissions by 2050 and to keep global warming below 1.5 °C.Furthermore, the energy crisis in Central and Eastern Europe poses the need for searching safe alternatives to energy supply to give immediate and practical answers to the contingent needs. From this point of view, the technologies related to Carbon Capture and Utilization (CCU) are arousing a strong interest, for the possibility to close the carbon loop by an effective recycling of CO2 captured either directly from the air or from industrial power plants, then reusing it in presence of renewable hydrogen for the production of value-added products [5–8].Dimethyl ether (DME) has been getting growing attention as an alternative diesel fuel and also as a feedstock for producing versatile chemicals and fuels, such as light olefins, methyl acetate, dimethoxyethane, etc [9–13]. Conventionally DME is synthesized starting from syngas in two steps, involving first the formation of methanol over a multi-metallic catalyst and then the dehydration of methanol to DME over solid acid systems [14–20]. The final productivity is controlled by the rate of methanol synthesis, in turn limited by thermodynamic constraints, feed composition, extent of the recycling stream [21,23]. The direct synthesis of DME taking place in a single reactor from either CO/CO2/H2 or CO2/H2 mixtures can overcome these limitations, leading to higher DME productivity owing to a more favourable equilibrium conversion prompted by the continuous consumption of methanol initially formed [20–22,24–26]. However, this one-step process has not reached an industrial maturity yet, being still performed on a lab-scale in presence of a hybrid metal-oxide-acid catalyst.Typically, copper-based ternary systems, such as CuO-ZnO-Al2O3 or CuO-ZnO-ZrO2, are integrated with solid acidic phases, like γ-Al2O3 or zeolite materials [23–31], active in the dehydration of methanol. The extent of the interface area among different phases leading to an effective interaction, the proximity of catalytic sites of different nature preventing a measurable mass diffusion control, the sample reproducibility overcoming the complexity in controlling several variables during preparation, the need for a stable lifetime under the adopted experimental conditions, represent all challenging factors requiring further R&D for an industrial process scalability.Among variously shaped solid catalysts (e.g., powders, beads, granules, pellets, scaffolds), matrix-like structures with different geometry of channels and typically prepared by impregnation or washcoating have been demonstrated to be promising systems for various catalytic processes [32–38], offering many benefits in terms of control of the sample architecture, increase of mass and heat transfer, decrease of pressure drops and related costs of process management. Nevertheless, their specific utilization in CCU applications results to be scarcely documented yet, now receiving a decisive boost by the recent progresses in the three-dimensional (3D) printing techniques, expected to revolutionize in the short term all the sectors of research and industry and to have implications on the concepts of production and work too, with economic and ethical consequences [39,40]. 3D Printing, also referred as “additive manufacturing” (AM), designates a technology suitable to build a material directly from a virtual 3D model by overlapping layers of the same material. In general, to produce a piece by 3D printing is sufficient the choice of a 3D software, a 3D model and a starting material. Consequently, it is possible to generate parts with arbitrary geometries without the need to adopt the usual productive processes bound to mass production [41,42]. The pieces thus created are ready for use, not requiring other finishing treatments, as well as the manufacture of semi-finished components results to be economically profitable. The starting material in the process is typically used in the form of a powder, paste, ink, suspension or solid in an optimized phase for layered deposition.Among the AM methods more and more importantly impacting the sector of the 3D printing, the direct-ink-writing (DIW), or robocasting, offers the superior potential not only for the realization of purely ceramic [43,44], but also of metallic or hybrid materials [45–50]. Its strong point is the relatively low cost of the machine, which normally uses an ink paste with specific rheological and viscosity features due to the addition of binders or additives in the parent material. This technology uses an extrusion process through a nozzle which generally varies from 0.1 to few millimeters, from which the material comes out in the form of a continuous filament which is deposited in superimposed layers through the control of a system robot, following a path generated starting from a suitably designed 3D model. Therefore, in a robocasting procedure, a 3D model is layered similar to other additive manufacturing techniques, but the nozzle position is controlled, extrapolating the shape of each layer by a CAD model. The first part of a product made by robocasting is obtained by extruding the “ink” threads onto the first layer. Subsequently, the working area is moved down or the formation hole rises and the next layer is applied to the required position. This is repeated until the product is completed.In this work, the effectiveness of structured catalysts, prepared via robocasting in the form of matrix-like cylinders, was evaluated as viable alternative to conventional powdered catalysts for the development of a scalable CO2 hydrogenation technology for the direct synthesis of DME. The physico-chemical and catalytic properties come out upon the robocasting procedure were compared with their powdered counterparts, in order to understand the key aspects behind a 3D-printing technique in the preparation of effective materials for CCU applications.The composition of the ink-pastes to be micro-extruded by 3D printing was based on a 85 wt% dry content basis of a previously optimized hybrid CuO-ZnO-ZrO2/zeolite formulation, well diluted (ca. 15 wt%) by an inorganic silica-based binder. In particular, the hybrid paste was preliminarily prepared via slurry coprecipitation of nitrate metal precursors, in a relative atomic ratio Cu/Zn/Zr of 60/30/10, by adding a suitable amount of oxalic acid to an ethanolic slurry solution containing a calculated amount of a commercial MFI-type zeolite (Alfa Aesar, Si/Al=25 mol/mol) so to get a final CuO-ZnO-ZrO2:zeolite weight ratio of 1:1. The hybrid coprecipitated catalyst was then dried overnight, calcined at 500 °C for 4 h, before undergoing the mixing with the binder for printing. The micro-extrusion process was carried out through a customized LUTUM® 3D clay printer, by setting specific printing parameters in relation to the ink paste viscosity, with nozzles diameters of few hundreds of micron and stack layers in controlled patterns according to the desired architecture. After printing, the cylinders were dried in a humidity chamber at 25 °C for two days until conferring a firm structure. Afterwards, the dried monoliths were calcined by applying a slow heating rate of 1 °C/min until 500 °C under a helium atmosphere (100 STP mL min−1).A powdered hybrid CuO-ZnO-ZrO2/MFI catalyst (Hyb-pwd), prepared by conventional coprecipitation with a Cu/Zn/Zr ratio of 60/30/10 at/at and a metal-oxide(s)-to-zeolite ratio of 1:1 wt/wt, was taken as a reference [20].The elemental composition of catalysts was determined by X-ray fluorescence analysis, using a S8 TIGER spectrometer (Bruker AXS, Germany), equipped with a rhodium anode tube (power 4 kW and 75 µm Be window and LiF 220 crystal analyze). The samples were analyzed as loose powders, considering the emission transitions of copper, zinc and zirconium (Cu-Kα1, Zn-Kα1, Zr-Kα1).The crystallinity of the prepared samples was analyzed upon crushing by a D8 Advance diffractometer (Bruker AXS, Germany), operating with a Ni b-filtered Cu-Kα radiation (λ = 1.5406 Å) in the 2θ range 5–80° at 40 kV and 40 mA and a scan step of 0.03° s−1.The measurements of reducibility under hydrogen atmosphere (TPR) were performed in a linear quartz micro-reactor (i.d., 4 mm) fed with a 5.6 vol% H2/Ar mixture at the flow rate of 60 STP mL/min. The experiments were carried out in the range 0–800 °C with a heating rate of 12 °C/min. The hydrogen consumption was monitored by a thermal conductivity detector, calibrated by the peak area of a known amount of CuO. TPR data resulted very reproducible in terms both of maximum position ( ± 3 °C) and extent of H2 consumption ( ± 3%).The copper surface area (S Cu) was obtained by “single-pulse” N2O-titration measurements at 90 °C. Preliminarily the samples were reduced in situ at 300 °C in flowing H2 (100 STP mL/min) for 1 h, then “flushed” at 310 °C in nitrogen carrier flow (15 min) and further cooled down at 90 °C. The values of metallic area were calculated assuming a Cu:N2O= 2:1 titration stoichiometry and a surface atomic density of 1.46 × 1019 Cuat/m2.A stereomicroscope Nikon® SMZ1500, with a zoom ratio of 15–1 accounting for a total magnifying capability of 3.75x up to 540x, was coupled to a Coolpix 5400 digital camera to carry out the high-resolution study of the printed structure of the samples.Measurements of temperature-programmed desorption of carbon dioxide (CO2-TPD) and ammonia (NH3-TPD) were performed in the experimental setup used for TPR to determine the surface concentrations of base and acidic sites respectively. Before TPD experiments, the catalyst samples (∼100–200 mg) were pre-reduced in a linear quartz micro-reactor (l., 200 mm; i.d., 4 mm) at atmospheric pressure, by flowing hydrogen (100 STP mL/min) from room temperature to 300 °C (heating rate of 10 °C/min). After an isothermal step of 60 min at 300 °C under hydrogen flow, followed by purging with helium, the samples were saturated for 60 min at 200 °C in a gas mixture (flow rate of 50 mL/min) either of 20 vol% CO2/He or 5 vol% NH3/He. Then, the samples were cooled down to 100 °C in He flow until a constant baseline level was maintained. The desorption measurements were carried out in a range from 100° to 600°C, at a heating rate of 12 °C/min, using helium as the carrier flow (50 STP mL/min). CO2 (m/z, 44) or NH3 (m/z, 17) desorption process was monitored by a quadrupole mass spectrometer (Thermo Star) equipped with a heated (150 °C) fast-response inlet capillary system, quantitatively calibrated by known pulses of CO2 or NH3.In order to operate under a similar residence time without any control due to possible mass and heat diffusion resistances, two differently sized fixed bed stainless steel reactors were adopted either for the catalytic measurements with the 3D sample (i.d., 12.8 mm; l., 400 mm) or the powdered sample (i.d., 6.4 mm; l., 400 mm) respectively, being jacketed within a stainless steel rod to maintain an effective control of temperature during the run. The 3D monoliths were reduced in situ at 300 °C for 1 h under a “pure” hydrogen flow at atmospheric pressure. The catalytic data were achieved at 30 bar, in a range of temperature between 200 and 260 °C, under CO2-to-DME hydrogenation conditions by feeding a mixture of CO2/H2/N2 at a volumetric ratio of 3/9/1, operated at a space velocity of 1000 NL/kgcat/h. The reaction stream was analyzed by a GC equipped with a two-column separation system connected to a flame ionized detector (FID) and a thermal conductivity detector (TCD), respectively. Both internal standard and mass-balance methods were adopted for the calculation of conversion-selectivity data, with an accuracy of ± 3% and carbon balance close to 100%. Fig. 1-A) displays the digital image of the micro-extruded matrix-like sample. It should be noted that the calcined cylinder typically presented an external diameter of 12.5 mm suitable to fit the inner diameter of the reactor. In particular, the top-view image in Fig. 1-B) reveals the uniform square channel cross-section of the structures with the wall thickness of ∼0.65 mm and channel width of ∼0.4 mm.Both the XRD patterns of the printed and powdered samples, after the reduction treatment, are shown in Fig. 2. As it can be observed, not only the reflections of the MFI framework of the HZSM-5 structure (JCPDS 38–0246), but also the crystallinity of the metallic phase of copper at 43° (JCPDS 01–089–2838), were retained after the three dimensional method.Regarding other main catalyst features, Table 1 reports a comparison of the main physico-chemical properties determined for the printed (Hyb-3D) and the powdered sample (Hyb-pwd).As it is possible to observe, the printing procedure really allows a perfect control of the catalyst properties, considering that in terms of composition (from XRF analysis), texture (values of surface area) and metallic properties (TPR and N2O chemisorption) the Hyb-3D sample practically mirrors the features of the Hyb-pwd sample prepared by conventional coprecipitation.The only differences are visibly associated to the surface properties, as the result of a relatively lower acid-base capacity exhibited by the printed sample. Despite similar desorption profiles of the two samples both in terms of surface sites and temperatures of maximum desorption (not shown for the sake of brevity), the quantitative data reported in Table 1 show that the Hyb-3D sample exhibits not only a smaller CO2 uptake (0.089 mmol/gcat), but also a comparably smaller NH3 uptake (0.362 mmol/gcat), both values resulting ca. 50% than in the counterpart (0.178 mmol/gcat and 0.721 mmol/gcat for CO2 uptake and NH3 uptake respectively). Even without any distinction between Brønsted or Lewis sites, however the desorption profiles clearly suggest, on one hand, the presence of sites of same nature (although quantitatively different) and, on the other hand, how in presence of a multi-site surface the acid-base capacity is significantly dependent on the preparation procedure. Evidently, the intrinsic thermofluidic properties of the catalytic ink-paste prepared for the process of micro-extrusion (as related to the use of binders and additives for proper rheological features [44–46]), significantly controls the surface affinity of the chemisorption sites, namely basic CO2 activation sites at the metal-oxide(s) surface and acidic dehydration sites at the zeolite surface [23,26,46,47]. Anyhow, the CO2/NH3 uptake ratio, taken as an index of relative concentration of the acid/base population, results to be practically identical on both samples (246.1–247.2), suggesting the same balance of acid-base population directly affecting the CO2 activation process as well as the final step of methanol dehydration into DME.Regarding the catalytic behavior, in Table 2 the catalytic results obtained in the direct hydrogenation of CO2 to DME are reported, in terms of CO2 conversion (X CO2, %), selectivity to the various compounds (Si, %) and yield to DME (Y DME, %), in the temperature range 220–260 °C, 30 bar and space velocity of 1000 NL/kgcat/h.As a rule, irrespective of the sample considered, the CO2 conversion progressively increases with temperature, the highest values (22.8–23.6%) being attained at 260 °C, as leveled in proximity of the thermodynamic equilibrium [15]. At lower temperature, however, the sample Hyb-pwd exhibits a relatively higher activity as much higher as more determinant is operation under a pure kinetic regime (200–220 °C). In terms of product distribution, on each catalyst the DME selectivity regularly decreases with temperature, the trend resulting more marked on the 3D sample (51.6→36.0%) compared to the trend exhibited by the powdered catalyst (51.2→42.6%). On the other hand, despite a progressive increase on both samples, the CO selectivity more steeply rises on the Hyb-3D catalyst from 29.6% (at 200 °C) up to 53.1% (at 260 °C), against a less limited increase (36.3→43.7) exhibited by the Hyb-pwd sample in the range of temperature considered. Regardless of the reaction temperature, the MeOH selectivity remains almost stable and comparable (11.0–13.7%) on both systems, apart from a maximum value of 18.8% recorded for the printed sample at 200 °C.It is clear that, with similar composition, texture and metallic features, the observed differences in the activity-selectivity pattern of the investigated samples have been primarily associated to their different surface properties as induced by the preparation method. Accordingly, to confirm the control of surface availability of acid-base adsorption sites on the catalytic behaviour, the rate of CO2 conversion was normalized for each catalyst at a temperature as low as 200 °C (wherein the low activity allows to rule out any control exerted by possible diffusional phenomena), both with respect to the number of basic sites (from CO2 uptake) and with respect to the number of acid sites (from NH3 uptake), so determining turnover frequency values associated to the CO2 conversion (TOF CO2) and DME formation (TOF DME), respectively. As it is possible to argue from Fig. 3, similar values of TOF CO2 and TOF DME allows a rational overview of the peculiar reactivity of the 3D-printed hybrid CuO-ZnO-ZrO2/zeolite system, clearly pointing to the chemisorption capacity as the critical factor controlling the specific activity of the investigated sample. On the whole, this finding matches the evidence that the reactivity of the 3D sample basically depends on the surface availability of the C-containing surface intermediates, in turn proportional both to CO2 desorbed from basic CO2 activation sites at the metal-oxide(s) interface [18,19,21] and to ammonia desorbed mainly from Brønsted acidic sites, considered of primary catalytic importance in the MeOH-to-DME dehydration reaction [26,29]. Although necessary in the step of H2 activation, less decisive appears the role of metallic Cu0 sites on the catalytic reactivity, considering not only a similar copper surface exposure exhibited by the two differently prepared investigated samples (19.7–20.3 m2/g, see Table 1), but also a stoichiometrically larger surface availability of activated hydrogen species in respect of the other intermediate species, prompted by the volumetric H2/CO2 feed ratio of 3–1.Finally, in Fig. 4 the stability pattern over the 3D-printed hybrid catalyst is reported in terms of CO2 conversion and DME yield as a function of the time on stream, evidencing an almost steady state during 5 days of experimental run. Considering that, under the adopted conditions, no coke or metal sintering have been ever detected over the “used” catalysts [51], this meaningful result reflects the effectiveness of matrix-like materials on the management of the water formed during reaction, which is considered the main factor affecting the catalyst lifetime in powdered catalysts [28,51].Once found the proper combination among ink-paste composition, 3D model and sintering treatments, the robocasting technique shows all its effectiveness, offering an alternative, cost-effective and facile approach to fabricate structured catalysts with tunable structural, chemical and morphological properties, comprehensively mirroring the features of conventional powdered catalysts used in CO2 utilization technologies.Probing the reactivity of a 3D-printed hybrid CuO-ZnO-ZrO2/zeolite catalyst in the direct hydrogenation of CO2 to DME, the activity-selectivity pattern was put in relation with the surface availability of acid-base adsorption sites, in turn controlled by the thermofluidic properties of the catalytic ink-paste prepared for the process of micro-extrusion. Considering the multi-site nature of the process considered, the specific functionality of 3D catalysts perfectly matches the behavior of the corresponding powdered counterparts only if a suitable exposure both of basic CO2 activation sites at the metal-oxide(s) interface and acidic dehydration sites at the zeolite surface can be ensured, together determining the rate of formation/transformation of the C-containing surface intermediates.This new knowledge ultimately informs process and equipment design for extrusion-based 3D-printing using not only ceramics but also hybridized catalytic ink-pastes, suggesting the need for an optimized setup capable of realizing structured systems with tailored chemico-physical properties. Giuseppe Bonura: Writing – review & editing, Funding acquisition, Supervision, Project administration. Serena Todaro: Investigation (XRF, XRD, BET, TPR, TPD) and Writing – original draft preparation; Vesna Middelkoop: Conceptualization, Funding acquisition, Project administration; Yoran de Vos: Formal analysis; Hendrikus C.L. Abbenhuis: Supervision; Gijsbert Gerritsen: Methodology; Arjan J.J. Koekkoek: Data curation; Catia Cannilla: Investigation (SEM measurements), Visualization; Francesco Frusteri: Writing – review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work is part of the CO2Fokus project which is supported by the European Union’s Horizon 2020 Research and Innovation programme under Grant Agreement No. 838061. The authors would like to thank the EU Horizon 2020 Programme for this opportunity. This document reflects only the authors’ view and the Innovation and Networks Executive Agency (INEA) and the European Commission are not responsible for any use that may be made of the information it contains.

This work highlights the effectiveness of an unconventional synthesis of hybrid systems for the direct hydrogenation of carbon dioxide into dimethyl ether (DME), based on micro-extrusion of a ink-like catalytic paste by a robocasting procedure. Due to the possibility to exert a fine control over the structure, surface and geometric architecture, the adopted printing technique really ensures a superior management of heat and mass constraints in respect of the conventional powdered catalysts, the catalyst functionality resulting to be tightly dependent on the cooperation between metal-oxide and acidic phase. Additionally, the accessibility both of the CO2 activation and methanol (MeOH) dehydration sites over the hybrid micro-extruded catalyst most importantly affects the catalytic performance, as suggested by the values of turnover frequency of CO2 conversion and DME formation pointing out the need for a favorable exposure of chemisorption sites of different nature to enhance the specific reactivity.

Data will be made available on request. Data will be made available on request.Since the Industrial Revolution, one of the main concerns of industries has been to provide human beings with the energy necessary for daily activities. Nowadays, petroleum and its derivatives are the main primary energy sources worldwide. According to a recent statistics study, approximately 37% of primary energy consumed around the globe comes are derived from petroleum. The sectors that depend energetically on petroleum-derived fuels are extensive; the most critical is transportation, which obtains 93% of its energy consumption from fossil fuels, and industrial activities, which obtain around 40% of its energy from these resources [1].Additionally, oil provides a large number of raw materials for other industrial processes, such as chemicals, petrochemicals, plastics, fibers, and pharmaceuticals. Specifically, the refining industry faces two serious problems today: on the one hand, environmental regulations are increasingly demanding lower levels in the content of polluting compounds in gasoline, and on the other, the crude oil extracted worldwide is increasingly more viscous and difficult to refine; these facts have an impulse that an important part of the current research in refining processes has focused his efforts on the preparation of more effective catalysts for oil hydrotreatment, specifically for the refining of heavy crudes.In recent years, numerous studies have demonstrated the attractiveness of using porous silica materials as catalytic supports. They have specifically highlighted research focused on using mesoporous silica materials that have shown remarkable results in terms of their catalytic performance derived from the high surface area offered by these materials and the physicochemical properties of silica [2–9]. However, due to the small pore size, these materials have disadvantages related to the diffusion of viscous fluids (such as heavy crude oils) through the mesoporous structure. Which negatively impacts their catalytic performance and, in consequence, the application of these materials in the synthesis of industrial catalysts; therefore, it is necessary to look for alternatives of catalytic supports that combine the textural properties of mesoporous materials while presenting lower resistance to mass transfer processes involved in hydrotreating reactions.Materials with hierarchical porosity are those which contain a network of interconnected pores in the range of micro (< 2 nm), meso (2 – 50 nm), and macropores (> 50 nm); due to their unique properties. They have generated substantial interest in materials science research in recent years, and numerous studies have reported their convenience in applications such as biomaterials, semiconductors, chemical separation, and catalysis [10–13]. These materials combine the properties of high surface area and chemical selectivity provided by the microporous and mesoporous materials and the low mass transfer limitations attributed to macroporous materials; this fact makes them especially useful in applications such as catalysis and separation of chemical compounds where both features are significant [14]. Hierarchical porous structures appear naturally in numerous biological materials such as corals, shells, rocks, and even bones, which have inspired the development of synthetic porous materials with the properties and structures of these materials.The first material with hierarchical porosity was reported in 1993 [12,15], but only a few years ago, the catalytic applications of these materials began to be considered. A lot of research has been developed in recent years related to the catalytic applications of hierarchically structured materials in areas such as photocatalysis [16,17], organic oxidation reactions [18–20], hydrogenation [21], alkylation [22], and isomerization [23,24]. They show that the hierarchical porous structure promotes greater accessibility to active sites and lower diffusive impediments, which results in improved catalytic activity. It has also been reported the use of silica materials with hierarchical structures as catalytic supports for biodiesel production reactions, finding that the activity depends on the degree of macroporosity of the material, which seems to demonstrate the importance of said structure for reactions with complex and viscous systems [25,26].Regarding using materials with hierarchical porosity as catalytic supports within the petrochemical and refining industry, many studies have reported the convenience of using silica materials with meso‑macroporous structure in the Fischer-Tropsch reactions [27–29], showing improved activities and selectivity concerning only mesoporous and microporous catalysts. The application of modified zeolites with pores arranged in the micro‑meso-macroporous range in the catalytic cracking reaction of aromatic petroleum derivatives showing higher activities than commercial zeolites has also been reported [30–33].In the case of hydrodesulfurization (HDS) reactions, several studies have reported using alumina and carbon materials as catalytic support. Han et al. have used a meso‑macroporous alumina material impregnated with the Mo-Co system, synthesized by molding with PMMA and Pluronic F127®, which have exhibited an improved activity compared with mesoporous alumina in the HDS reaction of dibenzothiophene [34]. Liu et al. have used alumina with disordered hierarchical porosity as support for the Co-Mo system, tested in the hydrodesulfurization of 4,6-dimethyl-dibenzothiophene, which provides an activity comparable to that of commercial mesoporous alumina catalysts [35]. Additionally, Huang et al. prepared a catalyst with the Co-Mo-Ni system supported on meso‑macroporous alumina with little ordering, which is more active in the HDS of thiophene compared to its mesoporous counterpart; the increase in activity is attributable to the reduction of diffusion limitations provided by the macroporous structure [36]. Regarding using coal with hierarchical porosity as catalytic support, Hussain et al. have synthesized catalysts with the Co-Mo system, which have shown a greater catalytic activity when compared to mesoporous carbons and activated carbons used for the same purpose [37].Finally, for the use of hierarchical porous silica materials, Zhang et al. have synthesized micro-mesoporous silica materials used in the catalysis of HDS reactions of dibenzothiophene by the Ni-Mo system, which have given better results than type-SBA mesoporous silicas materials and commercial alumina-supported catalysts. The improved catalytic performance is attributed to a better diffusion of the reagents involved and to a more significant number of acid sites promoted by the hierarchical porous structure [38,39]. Despite the fact that in recent years numerous studies have been carried out on the synthesis of materials with hierarchical porosity and their application in catalysis, few have been applied to the catalysis of reactions involved in oil refining; specifically, there are no studies reported to date that use silicas with a hierarchical structure in the macropore to micropore range that have been applied to hydrodesulfurization reactions of complex organic molecules present in petroleum fractions. Previous studies have shown the relevance of the use of mesoporous silica materials as catalytic supports, so the incorporation of a quasi-ordered hierarchical porous structure can result in a decrease in the resistance to diffusion of reactants and products and, with it, improve the catalytic activity in hydrodesulfurization reactions.Silica materials with hierarchical porosity were synthesized using the sol-gel method in combination with a dual soft-hard templating route that has already been applied in the synthesis of carbon and aluminosilicate materials with hierarchical structure [10,15,40,41]. Macroporous structure was templated by a hard polymer stencil, while the micro-mesoporous structure was obtained by a soft colloidal frame.The polymer template was prepared from polystyrene(PS)-HEMA spheres obtained by emulsion polymerization by the "emulsifier-free" method, which has proved to be suitable for obtaining polymer spheres with uniform and monodisperse sizes [41,42]. Styrene has been used as the main monomer, 2-hydroxyethyl methacrylate (HEMA) as co-monomer and ammonium persulfate as initiator. Before the polymerization, the monomers were purified to remove inhibitor traces in the commercial reagent. Styrene was purified by washing with sodium hydroxide (NaOH) in 0.1 M aqueous solutions, using a volume equal to one-third of the volume of the monomer to be washed and dried with anhydrous calcium chloride for 12 h; 2-hydroxyethyl methacrylate was distilled under vacuum at 35 °C and 50 mmHgCo-polymerization reaction was performed in a batch glass reactor equipped with a condenser, paddle stirrer, nitrogen bubbling system, and thermometer. The reaction medium was deionized water (1000 mL) previously bubbled with nitrogen to remove the dissolved oxygen. 50 mg of sodium persulfate was added as a polymerization initiator and stirred until complete dissolution. Subsequently, the co-monomers were added in a weight ratio of 100:2 (Styrene:HEMA). The reaction was carried out at 70 °C and 300 rpm for eight hours. The obtained PS-HEMA copolymer emulsion was discharged and stored in polypropylene containers at 4 °C.The PS-HEMA copolymer spheres were packed by centrifugation at 2000 rpm for 2 h, the liquid phase was decanted and obtained solid phase was washed and dried at 60 °C for 4 h. The dry solid was used as a hard template for the synthesis of the silica material.Silica support monoliths with hierarchical porous structures were synthesized by the sol-gel method combined with a dual hard-soft templating route. For the synthesis, 4 g of hard polymer template, synthesized according to the method described in the previous section, was used. The soft template was prepared by an emulsion of amphoteric polymeric surfactant type PPO-PEO-PPO (Pluronic P-123® and Pluronic F-127®) according to the methodology previously described by Zhang and Cooper [43].Silica precursor solution was obtained using tetraethyl orthosilicate (TEOS, Sigma-Aldrich, 99%) as silica precursor and HNO3 1.0 M as acid catalyst. The appropriate amounts of deionized water, TEOS, and surfactant were used to obtain a molar ratio of 1:208:6.21:0.0017 (TEOS:H2O:HNO3:surfactant). The solution was prepared by initially dissolving the necessary surfactant mass in deionized water and nitric acid solution until total dissolution at 35 °C and 300 rpm. Once a homogeneous mixture was obtained, the TEOS was added dropwise, and it was left in agitation for 15 min before being transferred into polypropylene tubes containing hard template until it was completely covered. The tubes were sealed and placed in an oven at 60 °C for 72 h to consolidate the silica structure. Synthesized monoliths were washed and dried at room temperature for seven days. Dried monoliths were pre-calcined at 110 °C for 18 h with a 1 °C per minute temperature ramp and finally calcined at 550 °C for 15 h with the same temperature ramp in order to remove the template. For comparison purposes, silica monoliths with only mesoporous structure were synthesized by a similar method to that previously described, by the remotion of the hard template. The synthesized silica materials were identified by nomenclature shown in Table 1 . As-synthesized materials are shown in Fig. 1 .Co-Mo-W trimetallic oxide-state catalysts were prepared by simultaneous impregnation via immersion method. Each support was loaded with fixed equal amounts of molybdenum (5.75 wt% as MoO3), tungsten (10.92 wt% as WO3), and cobalt (3.05 wt% as CoO. Impregnation aqueous solution containing ammonium heptamolybdate tetrahydrate [(NH4)6Mo7O24·4H2O, assay: 81–83%, Sigma–Aldrich], ammonium metatungstate hydrate[(NH4)6H2W12O40xH2O, assay: 99%, Sigma–Aldrich] and cobalt nitrate hexahydrate [Co(NO3)2·6H2O, assay: 98%, Sigma–Aldrich] was used. The concentration of each transition-metal precursor was calculated to achieve a Mo(W)/(Mo+W) atomic ratio of 0.5 and a Co/(Mo+W) atomic ratio of 0.43. Monoliths was inmmersed for an hour in the impregnation solution with stirring.The impregnated monoliths were dried at room temperature for 24 h and then at 110 °C for 16 h in a static air muffle furnace with a 2 °C/min temperature ramp. Finally, they were calcined at 500 °C for 4 h in a static air muffle with the same temperature ramp. It is important to mention that the aqueous solution containing the precursors of the three transition metals is stable during the preparation of the catalysts.Fresh sulfided catalysts were prepared by sulfidation of the oxide-state catalysts. The sulfidation reaction was carried out in a glass tubular reactor; oxide-state catalysts were triturated and sieved between ASTM No. 100 and 120 meshes, the material retained by the latter was sulfided (particle size between 0.125 and 0.149 mm). Meshed catalysts were charged into the reactor and heated until 400 °C in N2 atmosphere at 1 °C/min temperature ramp; when the reaction temperature was raised, the sulfidation reaction was carried out at constant temperature for 4 h using a stream of 15 (v/v)% of H2S in H2 with a flow of 26 cm3 per minute. Once the sulfidation time was completed, sulfided catalysts were cooled to room temperature and charged directly to the HDS reactor in an inert atmosphere in order to avoid oxidation of the transition metal sulfides.The morphology of the supports and catalysts particles was studied through high-resolution scanning electron microscopy. The images of the silica monoliths and catalysts were obtained in a Hitachi SU-8230 Cold Field Emission Scanning Electron Microscope (FESEM) which was operated with an acceleration voltage of 1 kV, with ultra-high vacuum and using secondary and backscattered electron detectors. Images were obtained with amplifications of 5000X, 10000X, 20000X, 50000X and 100000X. The samples were sprayed prior to analysis and deposited on copper supports with graphite conductive ink. The analysis was carried out without coating the sample.Images for the hard PS-HEMA template were obtained in a JEOL JSM-6060LV Scanning Electron Microscope (SEM). The analysis was carried out under high vacuum conditions, with an acceleration voltage of 20 kV, using a secondary electron detector. Images were obtained at 2500X, 5000X, and 10000X. The samples were prepared by depositing the polymer material on copper supports with graphite conductive tape and coated with a gold film.The chemical analysis of the oxide-state and fresh sulfided catalysts (wt%) was determined by energy-dispersive X-ray spectroscopy (EDS); samples were analyzed with a Bruker X Flash 6/60 system reporting the average of five measurements in different points. Elemental distribution maps of fresh sulfided catalysts were recorded in the same system making a complete scanning of the samples. Samples were measured as granulated powder.Catalysts composition of the oxide-state and sulfided samples was also determined by energy dispersive X-ray fluorescence spectrometry (ED-XRF). A Bruker PUMA S2 X-ray fluorescence spectrometer was used, equipped with a silver X-ray source and a X-Flash® Standard detector with a resolution of 135 eV for the Kα line of the Mn and 100,000 cps. The samples were measured in powder form. The analysis was carried out in air atmosphere at 20, 40 and 50 KV.Support and catalysts textural properties (type of porosity, surface area, and pore diameter), in the ranges of mesopores and micropores were studied by nitrogen physisorption at 77 K. The analysis was carried out in an Autosorb Quantachrome 1MP. Before the analysis, the samples were milled to a fine powder and degassed under vacuum conditions at 150 torr and 270 °C for 24 h. The volume of adsorbed nitrogen was normalized to standard temperature and pressure. The specific surface area was calculated through the BET method (Brunauer-Emmett-Teller), taking the data in the range of relative pressures (P/Po) from 0.05 to 0.30. The distribution of pore diameters was calculated by means of two models: the BJH method (Barret-Joyner-Halenda) using the data of the desorption branch of the isotherms and the DFT method (Density Functional Theory), making a mechanical-statistical analysis of all the points of the isotherms. The pore accumulated volume was obtained from the nitrogen adsorption-desorption isotherms at 77 K for a relative pressure (P/Po) of 0.99.Macroporous structure of silica supports were determined by mercury intrusion porosimetry (MIP). The analysis was performed in a Micromeritics AutoPore Series IV 9500 porosimeter. Prior to the analysis, the samples were milled to a fine powder and evacuated to an initial measurement of 5 psi/min (1.8 mmHg/min), with a suction limit of 500 μm Hg and a maximum vacuum of 50 μm Hg. The pressure applied for the test was between 0.0026 MPa and 220.08 MPa, with 0.222 MPa being the dividing point between the high and low pressures. The decreasing pressure for the determination of the extrusion branch of the curve has been applied between 220.08 and 0.0634 MPa. The following parameters for mercury have been considered: density 13.5335 g/mL, surface tension of 485 dynes/cm and a contact angle of 139°. Total surface area and pore diameter were calculated by the Washburn method, assuming the presence of cylindrical and spherical pores.The ordered mesoporous structure of the monolithic supports was investigated by small-angle X-ray diffraction in the 0.05–5° range for 2θ. Samples were analyzed on a Rigaku Ultima IV diffractometer, using Ni-filtered Cu-Kα radiation (λ = 1.54˚A), operated at 40 kV and 30 mA and with a step size of 0.5° per minute and scanning every 0.02 s, with rotation of the specimen at 30 rpm. Samples were analized as fine powders.The presence of crystalline phases of oxides and sulfides of transition metals in the synthesized catalysts was determined by X-ray diffraction in the 5–80° range for 2θ; diffractograms were recorded in a Rigaku Ultima IV diffractometer, working with Co-filtered Cu-Kα radiation (λ = 1.54˚A), with a step size of 5° per minute and sampling every 0.02 s and rotating the specimen at 30 rpm. The crystalline phases and indexing were determined using the MDI -Jade® V 5.0.37 software.The structure of the crystalline phases of transition metal oxides in the oxide-state catalysts was established by Micro Raman spectroscopy. The samples were analyzed at room temperature in a Horiba-Xplora Plus micro spectrometer, using a 532 nm laser beam operated at a power of 25 mW. A holographic filter of 1800 gr/mm was used. The catalysts were analyzed as fine powders without additional preparation, in the range of 200 – 2000 cm−1.Diffuse reflectance spectroscopy in the UV–visible range was aimed to determine the coordination environment of transition metal atoms in oxide-state precursors. Spectra were recorded in the 200–800 nm range at room temperature using a Varian Cary 5000UV–vis spectrometer equipped with an integration sphere. Spectra were determined using an internal MgO reference material and using the MonoSBA-15 material as blank in order to avoid the appearance of the electronic transitions corresponding to the siliceous material. Samples were analyzed as monoliths.The XPS spectra of the samples were recorded using a SPECS® spectrometer with a PHOIBOS® 150 WAL hemispherical energy analyzer with angular resolution (< 0.5°), equipped with an XR 50 X-Ray Al-X-ray and μ-FOCUS 500 X-ray monochromator (Al excitation line) sources. The binding energies (BE) were referenced to the C 1 s peak (284.8 eV) to account for the charging effects. Gaussian/Lorentzian functions were employed to fit the spectra after background subtraction according to the Shirley equation.Hydrodesulfurization (HDS) reaction of dibenzothiophene (DBT) was carried out in a Parr model 4842 high-pressure batch reactor at 350 °C and 33.8 bar, for 5 h and using an excess of hydrogen. Samples were thoroughly ground in a mortar to a fine powder and meshed for use materials with particle sizes between 106 and 125 μm.For the experiment, 0.5 g of sulfided catalyst was introduced into the batch reactor containing a solution of DBT in decalin with a concentration of 195 mmol/L at room temperature. The reactor was then pressurized to 33.8 bar with hydrogen and heated up to 350 °C at a rate of 10 °C per minute. The stirring rate of the reaction mixture was 900 rpm. Once the working temperature was reached, samples of the reaction medium were extracted every 30 min until reaction time was reached. Concentracion of DBT and HDS products in the samples was measured by gas chromatography to determine conversion versus time dependence and products distribution; the analysis was done in an HP 4890 gas chromatograph provided with a 10-ft packed column. This column contains 3% OV-17 as a separating phase on Chromosorb WAW 80/100. A commercial CoMo/Al2O3 catalyst (KF-752) was used as a reference.Catalytic performance was measured by taking into account two parameters: the apparent rate constant (related to the rate of reaction) and selectivity (related to the capability of the catalyst to promote the formation of desirable products). The apparent rate constant is a good approximation to the reaction rate in the sense that it involves the effect of the concentration of reactants and products, the effect of temperature, and the negative effect of the presence of the H2S in the batch reactor. Previous studies have shown that HDS reactions of simple sulfur compounds follow pseudo-first-order kinetics [44–49]. Therefore the apparent rate constants (kapp) were calculated by this model (Eq. (1), where rHDS is the reaction rate for the global HDS reaction, kapp is the apparent rate constant and CDBT is the DBT concentration along the time). We assumed pseudo-zero order respect to hydrogen and excess of hydrogen. Hydrogen partial pressure could be considered as constant during the overall reaction time. Apparent reaction constants were calculated using an exponential fit of the DBT concentration data and a linearization of this fitting according to Eq. (2), where CDBT0 represents the initial concentration of DBT and XDBT is the DBT conversion at overall reaction time. (1) r H D S , a p p = d C D B T d t = − k a p p C D B T (2) C D B T 0 ( 1 − X D B T ) = k a p p t Hydrodesulfurization of dibenzothiophene can occur by two parallel pathways shown in Fig. 2 : direct desulfurization (DDS) and hydrogenation (HYD); the first leads to the formation of biphenyl (BP) via hydrogenolysis, this is, the sulfur atom is removed from the molecule by the breaking of the carbon-sulfur bonds, while the hydrogenation of one or two aromatic rings produces cyclohexylbenzene (CHB) and bicyclohexyl (BCH), respectively. The ability of the catalyst to promote one route preferably is known as selectivity, and it was determined as the ratio between hydrogenation and direct desulfurization products at 30% conversion of dibenzothiophene according to the Eqs. (3)–(5), where the factors in brackets are the molar concentrations of the HDS products. (3) D D S = [ B P ] × 100 [ C H B ] + [ B C H ] + [ B P ] (4) H Y D = ( [ C H B ] + [ B C H ] ) × 100 [ C H B ] + [ B C H ] + [ B P ] (5) S e l e c t i v i t y = H Y D D D S For comparative purposes, in the reaction experiments were used equal catalyst mass, with similar particle size (100–125 μm) and with the same metal charge (added during catalysts synthesis); in addition, the HDS performance of a commercial CoMo/Al2O3 catalyst was measured as a comparative model (Mo = 14.2%; Co = 3.8%; P = 0.83%; SBET = 223 m2/g; average pore diameter = 6.6 nm).The morphology of the synthesized materials was analyzed by high-resolution scanning electron microscopy, which allowed us to obtain images of the materials in the micrometer ranges up to nanometers, being able to study the porosity in the range of macropores and mesopores. Fig. 3 shows the images for the styrene-HEMA copolymer hard template. Image A in the Figure corresponds to the obtained copolymer spheres in the emulsion polymerization procedure; it is possible to see the formation of spheres of copolymer with a well-defined spherical shape and with little dispersion in size, with an average diameter of about 800 nm, as planned in the synthesis methodology. The image in B shows the structure of the synthesized hard template obtained after centrifugation and dying of the copolymer emulsion, a regular and uniform arrangement of the spheres was obtained, with a cubic-like packing of solid spheres, which is convenient for the synthesis of hierarchical porous silicas.Scanning electron microscopy images obtained for mesoporous silica monoliths are shown in Fig. 4 . MonoSBA-15 material (A and B) is constituted for worm-like aggregated in roller-like clusters of several micrometers of length, which have been widely documented previously [4,50–53]; in Fig. 5 is possible to see that mesoporous structure is constituted for cylindrical channels of few nanometers (5-10 nm) arranged in a hexagonal array as expected in the proposed method. Likewise, it is possible to notice the presence of interstices between the sintered particles whose dimensions are both in the mesoporous and macroporous range, which produces surface porosity, confirmed from the nitrogen physisorption and small-angle X-ray diffraction analyses presented in the following sections. On the other hand, images for MonoSBA-16 (shown in Fig. 4C-D) reveal the presence of particles with quasi-spherical morphology, which are sintered to form large clusters (30 μm approximately); this is in agreement with previous reports of the morphology of materials SBA-16 [9,52,53], additionally is possible to appreciate the existence of interstices between the particles that generate surface porosity. At higher amplifications, the material shows the presence of quasi-spherical pores in the walls with a certain degree of ordering and whose dimensions are in the mesoporous range, with an approximate diameter of 5-10 nm.Concerning hierarchically porous silica monoliths, scanning electron microscopy images are shown in Fig. 5. Images for HOPSHM material display the presence of a macroporous interconnected network with a certain degree of order and an average pore diameter of 626 nm (measured by statistical analysis on pore sizes). This pore size is slightly smaller than the diameter of the styrene-HEMA copolymer spheres shown in Fig. 3, because, during the calcining process of the silica material, the spheres suffer from reduction due to the high temperature; this effect has already been previously reported [15,41]. An analysis at larger amplifications shows that particles of mesoporous silica material constitute the walls of the macropores with similar morphology to SBA-15 particles. The Figure also reveals that macropores walls possess interstices between sintered mesoporous particles, whose dimensions are in the range of 20–100 nm, originating porosity in the material. Regarding HOPSCM material, scanning electron microscopy images clearly showed a macroporous structure similar to that of the HOPSHM sample, with a similar average pore diameter (621 nm). At higher amplifications, it was possible to appreciate that particles of silica constitute the walls of the material with morphology similar to SBA-16 with pores in the mesoporous range (4–5 nm); surface porosity is also appreciable in the walls of macropores in the range of 20–100 nm.It must be emphasized that information obtained from scanning electron microscopy images is consistent with the results found by nitrogen physisorption, mercury intrusion porosimetry, and small-angle X-ray diffraction techniques described in the following sections.The nitrogen adsorption-desorption isotherms at 77 K for the supports are shown in Fig. 6 . According to the IUPAC classification, all the supports show Type IV nitrogen adsorption-desorption isotherms [54–57], which are characteristic of mesoporous materials which present a hysteresis cycle related to the capillary condensation and evaporation within the mesopores. For the MonoSBA-15 material, a well-defined type H1 hysteresis cycle is observed in the range of relative pressures from 0.5 to 0.8, with adsorption and desorption isotherms nearly parallel, which is indicative of the presence of pores with regular morphology, as expected for the characteristic cylindrical channels in SBA-15-type materials that have been widely reported in previous studies of this material [4,41,52,58–60].With respect to the MonoSBA-16 sample, it shows a type H2 hysteresis cycle in the range of relative pressures from 0.4 to 0.7 in which the desorption branch is wider and vertical than the adsorption one, which means that mesopores do not have uniform morphology. This is because this material is characterized by the presence of mesopores in the form of interconnected spheres, which generates differences between the input diameter and inside the pore. This isotherm has also been widely documented in previous studies [9,52,58,60].On the other hand, HOPSHM material presents an isotherm with a bimodal hysteresis cycle, enclosed to the H1 type in the range of relative pressures of 0.4 to 0.8, which may be indicative of the presence of mesoporous with regular morphology like those of the MonoSBA15 material but with two different pore sizes [41,54]. Regarding the HOPSCM material, it presents a type H2 hysteresis cycle type in the range of relative pressures between 0.4 and 0.7, similar to that of MonoSBA-16. This is indicative of the presence of bottle-ink pores, which have been widely reported for the SBA-16 material. It is also possible to see an increase in the amount of nitrogen adsorbed in all the synthesized samples at relative pressures above 0.95. This is indicative of the presence of larger pores in the range of 30 to 50 nm which may be due to the superficial porosity that forms in the walls of the macropores by the sintering of individual particles of mesoporous silica.The fact that the amount of adsorbed nitrogen below the occurrence of hysteresis in all isotherms is greater than the increase due to capillary condensation suggests that not only does multilayer adsorption occur on the surface of the mesopores. Also, there is the filling of the micropores present in the samples, which comes from the insertion of the PEO block terminations in the silica walls. As well as from the mesopores entrances if these are in the range of micropores; this is consistent with previous findings related to the fact that silica networks formed with oligomers or block copolymers tend to exhibit microporosity, which holes originally occupied by PEO constitutes blocks occluded in the silica matrix [60,61]. Fig. 7 shows pore diameter distributions for the synthesized supports. In all cases, it can be observed that closed distributions are generated, which points to the presence of pores of uniform sizes in all the synthesized materials. In the case of the MonoSBA-15 material, the distribution is centered at 6.57 nm, while the MonoSBA-16 material shows a bimodal distribution, in which one of the tails is wide and centered at 3.89 nm while the second is very narrow and centered at 5.81 nm, this suggests the existence of pores with two different diameters that can be attributed to pore and interconnection diameters. With respect to materials with hierarchical porosity, the HOPSHM material shows a bimodal distribution in which the two loops are closed and centered at 3.92 nm and 4.98 nm, respectively, which may also be due to the synergistic effects between the methods of colloidal molding and hard templates [15,54]; On the other hand, the HOPSCM sample shows a distribution of wide pore diameters in the range of 2 to 3.8 nm, centered at 2.82 nm.The nitrogen adsorption-desorption isotherms and pore diameter distributions for the oxide-state and sulfided catalysts (not shown here) have the same tendency and shape that those for the supports. Being indicative that the porous morphology of the materials is preserved through the synthesis process, uniquely low displacements to lower relative pressures are observed, related to the decreasing of the pore diameter associated with the deposition of the precursors and active phases over the silicas surface onto the pores. Table 2 summarizes the textural properties of the porous silica supports. It is possible to observe that the MonoSBA-15 and MonoSBA-16 materials have large surface areas, suitable for the use of these as catalytic supports. While the materials with hierarchical porosity (HOPSHM and HOPSCM) showed slightly smaller surface areas as previously reported for similar materials [15,41,54]. In those reports the samples showed a loss of surface area due to the sintering of the individual particles of mesoporous silica around the hard template in order to form the walls of the macroporous structure, which may be occluding part of the mesoporous structure. In the same table, it can be appreciated that a drastic reduction in surface area is obtained after oxide-state catalysts synthesis (between 50–60%) related to the deposition and formation of oxide precursors of the active phases; after sulfidation, the reduction is almost 70–80% respect to support surface area; similar tendency is seen in the reduction of pore diameter and pore volume, which is indicative of the high dispersion of the active phases onto the support Fig. 8 shows the mercury intrusion-extrusion curves for the synthesized silica materials with hierarchical porosity. Both samples present typical curves for highly porous materials, which exhibit a considerable increase in the volume of intruded mercury in the range of 100 – 5000 psi, which is indicative of the presence of large pores in the range of 500 – 600 nm, fact that is consistent with the used experimental method. Also, a moderate increase in the volume of intruded mercury is observed in the range of high pressures above 10,000 psi, corresponding to pore diameters between 50 and 200 nm, corresponding to the appearance of surface porosity that is formed by the sintering of individual mesoporous silica particles around the macroporous template during the synthesis. Finally, a slight increase in the amount of mercury intruded at pressures above 100,000 psi is observed, which corresponds to pores in the mesoporous range. However, the analytical technique is not conclusive concerning this range of porosity because of the limitations of the technique.The fact that intrusion and extrusion curves do not coincide, giving rise to a hysteresis region, indicates the entrapment of mercury inside the pores during the extrusion process. This is justified by the presence of interconnected ink-bottle macropores, as expected from the use of spherical polymer particles as a template so that lower pressures are required to empty the pores during the mercury extrusion until pressure decays to that corresponding to the interconnection diameters. The connectivity and the pore size determine the extrusion that occurs when the pressure decreases. Therefore a deep analysis of the differences between both curves allows determining the dimensions of macropore diameter and the interconnection channels between them.Pore diameter distribution for the materials can be seen in Fig. 8. It is evident the predominance of pores in the macroporous range, with an average diameter of 613 nm for the HOPSHM samples and 621 nm for the HOPSCM sample. This is consistent with the synthesis method in which polystyrene-HEMA copolymer spheres with average diameters of 650 nm were used as a hard template, with a slightly smaller diameter derived from the contraction of the spheres that have already been reported in previous studies [10,15,20]. Likewise, the presence of pores in the boundary between the macropores and mesopores (50–200 nm) can be observed, related to the appearance of interparticle porosity generated by the sintering of SBA-type particles around the polymer spheres during the synthesis process, as previously described [10,15]. Textural properties obtained by mercury porosimetry are summarized in Table 3 , it can be seen that both materials show a surface area smaller than that measured by the nitrogen physisorption technique; also, it is observed that the main part of the porosity of the material is in the macroporous range (approximately 80% for both samples). However, this technique is not accurate for analysis of the mesoporous fraction of the supports, so it is expected that the mesoporous fraction could be more significant.Small-angle X-ray diffractograms obtained for porous silica supports are shown in Fig. 9 , in all of them is noticed the existence of defined peaks which evidence the existence of well-structured mesoporous phases within the synthesized materials. MonoSBA-15 and HOPSHM materials show peaks corresponding to (100), (110), and (200) reflections of a 2D hexagonal array of cylindrical mesopores with spatial group p6mm. They have already been widely reported for materials SBA-15-type [4,21,41,51,59,60]. On the other hand, MonoSBA-16 and HOPSCM show peaks corresponding to the (110), (211), and (220) reflections of a cubic structure Im3m, which are also already widely characterized in the literature [9,51,52,60].All synthesized materials exhibit a very wide peak of high intensity between 0.1° and 0.5°. This peak can be attributed to the presence of pores in the range of 10 – 30 nm, corresponding to the surface porosity which appears from the sintering of individual silica particles around the macroporous template, the analysis of nitrogen physisorption and mercury intrusion porosimetry also described the presence of these phases.Concerning the oxide-state and sulfided catalysts, small-angle diffractograms (not shown) still exhibit the presence of these characteristic peaks, indicating that the mesoporous structure is maintained after impregnation with the active phases and their subsequent oxidation and sulfidation, solely a shift of the peak at 0.9–1.0° at slightly higher angles is observed, demonstrating the reduction of the mesoporous diameter of the catalysts, an effect that had already been observed in the nitrogen physisorption results presented previously.Wide-angle diffractograms obtained for porous silica supports (not shown) reveal, in all the cases, a very broad peak centered around 24° corresponding to amorphous silica, which has been widely reported in previous studies [4,14,41,51,52,59,60], this peak still appears in the oxide-state and fresh sulfided catalysts as shown in the Figs. 10 and 11 . Fig. 10 shows diffractograms for the oxide-state supported catalysts, in all of them the presence of low intensity peaks at angles 2θ of 19.5, 25, 28, 32.5, 38, 43, 28, 57 and 59.5° corresponding to the β-CoMo(W)O4 (JCPDS-ICDD 21–0868, 15–0867). The presence of this phase has already been previously reported in numerous studies of hydrodesulfurization catalysts based on active species of cobalt, molybdenum, and tungsten, and that has been documented as a precursor phase of active sites for catalysis [4–8, 34,36,62,63]. Low intensity peaks have also been identified corresponding to the isolated molybdenum trioxides (MoO3, JCPDS-ICDD 76–1003) and tungsten trioxides (WO3, JCPDS-ICDD 85–2460), with an orthorhombic and hexagonal structure, respectively; which agrees with previous results presented by Huirache et al. [4–8, 62–63]. Finally, diffractograms also show the presence of dispersed species of molybdenum and tungsten polyoxides with the general formula MoxW1-xO3 type (JCPDS-ICDD 28–0668), as well as polymolybdates and polytungstates (not shown in the Figure) corresponding to compounds containing a large number of molybdenum and tungsten atoms, in which the metal/oxygen ratio is 2.75 to 2.9, such is the case of compounds such as Mo4O11, Mo17O47, Mo9O26, W24O68, W17O47, W20O58, W19O55, which present coordination structures similar to those of Mo(W)O3. The low intensity diffraction peaks in all samples are indicative that most of the supported species are widely dispersed on the surface of the supports and the presence of not well-defined peaks is due to the supported species are amorphous or have crystallite sizes below the detection limit of the technique (<4 nm).Fresh sulfided diffractograms are shown in Fig. 11; for all the synthesized materials, characteristic peaks corresponding to hexagonal molybdenum and tungsten disulfides (MoS2, JCPDS-ICDD 75–1539; WS2, JCPDS-ICDD 08–0237) were observed which have been widely documented as hydrodesulfurization catalytic species [4–8, 36,62–66]; showing differences in the intensity of the same, due to the degree of dispersion of the sulfides on the surface of the support. The presence of wide peaks in all the samples is indicative of the presence of crystalline domains of different sizes supported on the porous silica support.The presence of cobalt sulfide as Co9S8 form (JCPDS-ICDD 02–1459) has also been identified and documented as an active phase in previous studies of hydrodesulfurization catalysts containing molybdenum and cobalt [62], hierarchical porous silica supports exhibit very intense peaks for this phase, exposing the strong influence of the support structure over the type of formed supported sulfur species in catalysts. Finally, the presence of peaks corresponding to Co1.62Mo6S8 (JCPDS-ICDD 30–0450) has also been identified. Samples exhibited low intensity peaks in the range of 20 – 30 ° due to the presence of oxidized transition metal species because of an incomplete sulfidation of oxide state precursor or for oxidation of sulfided catalysts before the analysis.Micro Raman spectra obtained for the oxide-state catalysts are shown in Fig. 12 ; all the samples show a very intense band in the range of 900–1000 cm−1, which due to its amplitude and position, indicates the presence of various tungsten and molybdenum species with different molecular symmetries. Decomposition of such band reveals an intense peak at 940–960 cm−1, which is usually attributed to the symmetric stretching vibration of the terminal bond Mo(W)=O in various types of polymolybdates and polytungstates with octahedral metal coordination, such as Mo7O24 6− and W7O24 6−, whose intensity is enriched by the contribution of the Si-O stretch of the silanol groups of the silica. Additionally, a shoulder is present at 980–985 cm−1, and a band of low intensity around 860 cm−1 that has been reported as corresponding to the stretches of the Mo-O-Mo bonds in irregularly shaped polymolybdates [67,69,70]. The presence of polytungstates in the catalysts is confirmed by the appearance of low intensity bands at 510 cm−1 and 200–300 cm−1, corresponding to the stretching and angular deformation of the W-O-W bonds, respectively [68,71,72]. In the same region, contributions of tetrahedral isolated dioxo molybdenum and tungsten compounds are found. These appear as an intense band at 970–975 cm−1 related to the asymmetric stretch of the bond O=Mo(W)= O [67,68,71,72].The presence of supported isolated CoMo(W)O4, with tetrahedral coordination, is confirmed by the presence of an intense band between 935 and 945 cm−1 (attributed to the symmetrical stretching of the W=O of the tungsten in WO4 2−), accompanied by a low intensity peak at 730–740 cm−1 related with the asymmetric vibration of O-W-O bonds [68,71,73]. The displacement of this band is indicative of distortion in the tetrahedral structure. Isolated species of molybdenum in tetrahedral coordination is assumed by the presence of the bands in 890–900 cm−1, corresponding to the stretch Mo-O-Co mode, in 830–840 cm−1, associated with the asymmetric stretch Mo-O and 317 cm−1 related to the angular bend of O-Mo-O [67,74].Bands in 990–995 and 815–820 cm−1 appear within the two regions of more intense bands of the spectra for the whole catalysts, in addition to small peaks in 708, 666, 417, 377, 338, 290, 248, 217, 198 and 160 cm−1, indicating the occurrence of MoO3 supported on silica as previously reported [70]. Low intensity peaks are observed in all spectra at 715 cm−1 and 435 cm−1 associated with WO3 [68,71].Finally, all the spectra exhibit bands in 990, 970, and 910 cm−1, which appear as shoulders in the main band, in addition to peaks of low intensity in 635, 252, and 220 cm−1, which have been previously reported as corresponding to silico-molybdic anion [SiMo12O40]4−. In addition, around 1020 cm−1, very low intensity peaks are observed associated with stretching the Mo=O bond in mono-oxo molybdenum species directly linked to the silica network (SiO2−Mo=O) [70].Values for the ratio between terminal links, Mo(W)=O, and bulk links, Mo(W)-O-Mo (W), were obtained from the areas of the deconvolution peaks of the bands found at 950–960 cm−1 and 980–985 cm−1 associated with such vibrations. This value is of paramount importance in the catalysis of HDS reactions since previous studies have shown the influence of the presence of metal-oxygen terminal bonds in the formation of catalytic sites [75]. Results are shown and discussed in the Catalytic Performance section.Chemical analysis of the catalysts was made by energy dispersive spectroscopy (EDS) and X-ray fluorescence spectrometry. EDS spectra for oxide-state and sulfided catalyst (not presented) have shown the presence of characteristic signals corresponding to the electronic transitions of the expected elements in the samples (O, Si, Mo, Co, W and S); the absence of the signal corresponding to carbon is indicative of complete removal of the polymeric porogen agents (surfactant and styrene-HEMA copolymer spheres). By this technique, semiquantitative elemental compositions of the catalysts were determined which are reported in Table 4 . It can be noted that the values determined experimentally are close to those established theoretically in the synthesis. This fact is evidence of the homogeneity in the distribution of the metals transition on the supports and the effectiveness of the impregnation method used for the synthesis of catalysts. The table also reveals that the chemical composition is similar for all the catalysts, and the differences between samples are in the range of experimental error. Hence it can be assumed a similar metal charge in all the catalysts.In the other hand, energy dispersive X-ray fluorescence spectra (not shown) reveal the presence of characteristic bands corresponding to the lines Kα1 and Kβ1 of silicon, Kα1 and Kβ1 of cobalt, Kα1 of molybdenum and Lα1 and Lβ1 of tungsten, indicating the presence of such metals in the catalysts, these results are consistent with EDS results. The semiquantitative composition of the catalysts, determined as oxides, is presented in Table 4. It is possible to appreciate that all the catalysts exhibit similar quantities of each of the oxides of transition metals, which are close to the expected percentages according to the atomic proportions proposed in the experimental procedure, differences are in the level of experimental error and not significant difference is seen between samples.Oxide-state catalysts DRS-UV–Visible spectra are shown in Fig. 13 , as well as the peaks deconvolution. All spectra exhibit a very intense band between 200 and 350 nm that, when deconvolved in peaks, reveals the presence of three different bands. The band between 220 and 240 nm corresponds to the transition of ligant-metal charge transfer related to the presence of Mo6+ and W6+ ions with tetrahedral coordination, as in the isolated species of WO4 2− and MoO4 2−, whose presence in HDS catalysts supported on silica has been previously reported [4,5,36,73]. The band at 290-300 nm has been attributed by Jeziorowski et al. to transitions of binder-metal charge transfer in Mo-O-Mo groups present in octahedral polymolybdates and polytungstates [76,77]. While that at 320-340 nm has been reported as corresponding to the ligant-metal charge transfer transition of O2− to Mo6+ or W6+ in compounds with octahedral coordination of the transition metal, such as polymolybdates and polytungstates [36,70,71,78]. Both types of compounds were identified in the X-ray diffraction analysis, and these results are consistent with the fact that the structure of the supported species is governed by the acid-base interactions between the transition metals and the acid silica surface. The presence of wide bands indicates that metals are present in aggregates of different sizes, in all the samples, a similar quantity of molybdenum and tungsten species with octahedral and tetrahedral coordination is observed. However, the electronic transition of the octahedral species is less probable, so the fact that it can be seen in the spectra indicates that a high concentration of those structures is present.Concerning the band at 500–520 nm, this has been previously reported as corresponding to the transitions of charge in complexes of Co2+ with octahedral coordination [4,5,79–81]. The appearance of the band between 565 and 580 nm has been attributed to d-d electronic transitions (4T2g to 4A2g and 4T2g to 4T1g) in octahedral cobalt complexes of high spin, present in the β-CoMoO4. In which the cobalt interacts with the molybdenum and whose presence has already been identified by X-ray diffraction. Octahedral cobalt ions are important in HDS catalysis because of their easy sulfidation [4,5,81]; since the transitions of these octahedral ions are not very probable, intensity refers that they are present in a high concentration. In the same range (500–520 nm), appears the band adscript to charge transitions for Co2+ species with tetrahedral coordination that has also been reported [79–81], indicating the presence of cobalt ions interacting directly with the support in the form of Co2SiO4, hence there could be small amounts of these species supported on the catalyst.The catalytic activity of the synthesized materials in the hydrodesulfurization reaction (HDS) of dibenzothiophene (DBT) was measured using the apparent rate constant calculated by the method descript in the Experimental section. In Fig. 14 the dibenzothiophene conversion profiles are shown, as was experimentally determined; it can be seen that all catalysts follow a quasi-linear behavior, as is expected for a pseudo-first-order chemical reaction in which kinetics is the controlling reaction step; linearity is an evidence of mass transfer limitations absence in the reaction media [44–48]. At high reaction times, catalysts supported on mesoporous silica monoliths, CoMoW-MonoSBA15 and CoMoW-MonoSBA-16, exhibit less linear behavior compared to those supported in hierarchically porous silicas (CoMoW-HOPSHM and CoMoW-HOPSCM) indicating that the diffusive and mass transfer effects in the reaction system and the presence of reversible collateral reactions become important for long reaction times.In the Fig. 14 it is possible to note that the highest conversion is obtained for catalysts supported on hierarchically structured porous silicas, even greater than that of the commercial catalyst used as reference, nearly 20–30% of extra conversion is obtained by the use of hierarchical porous support in comparison with an only-mesoporous one. Catalysts supported on hierarchical porosity silicas present a conversion between 75 and 85% higher than that obtained for the commercial catalyst used for comparative purposes.The values found for the apparent rate constant (kapp) for the tested catalysts are shown in Table 5 . These values were obtained from the time versus conversion data using linear regression, calculated regression coefficient (R2) values were between 0.982 and 0.999 for the different catalysts and the standard error for the values of apparent rate constants were between 3 and 5%. The fact that an equal mass of catalyst was used for every experiment and that all of them have similar metal charges (as determined by the EDS and ED-XRF results) allows using this parameter as a comparative parameter of the catalytic activity of the synthesized materials. Catalysts supported on hierarchically structured silica monoliths exhibit higher values for the apparent reaction constant than those for the catalysts supported on an only-mesoporous media and the commertial catalyst, indicating better performance in terms of conversion of dibenzothiophene and, thus, sulfur remotion. The difference between the values (15 – 20%) is higher than the experimental error (5%), indicating that the use of hierarchically porous silica as support is advantageous for preparing catalysts based on metal transition sulfides for hydrodesulfurization of dibenzothiophene.The observed behavior in catalytic activity is related to a higher density of molybdenum and tungsten sulfides deposited on the surface of the catalysts. Which is higher for those supported on hierarchical silicas, as established by the results of X-ray diffraction, where more intense peaks are obtained for these catalysts. This tendency is confirmed by the diffuse reflectance UV–Visible spectroscopy results, in which more active catalysts exhibited more intense bands related with the tetrahedral and octahedral molybdenum and tungsten ions, representing a higher concentration of polymolybdates and polytungstates which are appropriate oxide-state precursors for HDS active phases. It has also been found in all samples the presence of oxide precursors with cobalt ions with octahedral coordination, which are advantageous in the formation of active sites for hydrodesulfurization catalysis [44–47, 75].Additionally, Micro Raman spectra reveal the presence of molybdenum and tungsten species in octahedral coordination in all samples, showing the catalysts supported on hierarchical porous silica monoliths a greater intensity of the bands related to this species. This is related to the fact that those catalysts contain the highest number of Mo(W)=O terminal bonds, which has been reported as responsible of the catalytic activity in the hydrodesulfurization of dibenzothiophene [75]. The relation between terminal and bulk bonds (obtained from Micro Raman spectra deconvolution) and apparent reaction rate constant (kapp) is shown in Fig. 15 , indicating that activity increases with a higher value of this ratio (corresponding to a higher quantity of easy-to-sulfide terminal bonds and a higher quantity of Mo(W)S2 edge sites). On the other hand, it is known that the sulfidation of isolated β-CoMo(W)O4 produces the segregation of crystals of Co9S8 and curved structures of Mo(W)S2 doped with cobalt, therefore the catalytic activity is the result of a “joint effect” derived from the presence of metal centers with octahedral and tetrahedral coordination, hence more active catalysts are those with higher concentrations of both kind of chemical species.Another parameter determined for the catalytic performance was selectivity. Two parallel pathways are typically described in literature in which HDS of DBT may occur: direct desulfurization (DDS) and hydrogenation (HYD). The first one leads to the formation of biphenyl (BP) via hydrogenolysis, while the hydrogenation of one or two aromatic rings produces cyclohexylbenzene (CHB) and bicyclohexyl (BCH), respectively. Selectivity results for synthesized catalysts are shown in Table 5, it can be noted that catalysts supported on mesoporous silica monoliths exhibit low selectivity values related with a strong tendency of those catalysts to promote the direct desulfurization route (DDS), as have seen in previous studies of catalysts supported in mesoporous silicas [4–9]. On the other hand, catalysts supported in hierarchically structured porous silicas show higher selectivity values demonstrating a better promotion of both catalytic routes, despite a tendency to DDS is still observed, just similar to the commercial catalyst used as a comparative model. Hierarchically structured HDS catalysts include a network of large and small interconnected pores which reduce diffusion limitations and allow more facile access of the active sites for DBT conversion compared with their counterparts (SBA-15 mesoporous support). Selectivity results suggest that the use of silicas with hierarchical porosity seems to improve not only the apparent reaction rate but also the selectivity, promoting almost simultaneously both reaction routes, probably because the hierarchical porous structure could promote the formation of Mo(W)S2 edge active sites for direct desulfurization (Type I and II) as well as Brim bulk sites necessary for the hydrogenation route.The XPS analysis of the CoMoW-HOPSCM and CoMoW-MonoSBA-15 are presented in oxide and sulfide states to compare the surface of the highest-lowest activity catalysts. In Fig. 16 is possible to observe the Co 2p (Fig. 16A), Mo 3d (Fig. 16B), W 4f (Fig. 16C), and S 2p (Fig. 16D) core emission-line regions for the mentioned catalysts. Spectra related to the oxide state of CoMoW-HOPSCM and CoMoW-MonoSBA-15 samples were labeled with a and b in the plots of Fig. 16, while sulfided catalysts with c and d. The electrons arising from the Co 2p3/2 spin-orbit in the oxide state spectra resulted at 782.15 eV. The sulfided spectra presented a shift in the electrons from the Co 2p3/2 to lower BE (778.5 eV), indicating the change in the Co environment. A similar observation was registered for the Mo 3d core level as the oxide spectra presented the main peak of the characteristic doublet Mo 3d5/2 at 233.3 eV (Δ = 3.13 eV) for both oxide samples. In the sulfided samples, this peak was observed at 228.3 eVIn the same way, the oxide samples presented the peak of W 4f7/2 at 36.4 eV and the sulfided samples at 32.1 eV. In the case of the S 2p sulfided samples, they presented only one peak centered at 161.7 eV. No shoulder related to the presence of sulfates species at 168 eV were observed, indicating that the experimental procedure for the sulfidation and sample transfer to the spectrometer chamber was efficient in avoiding air contact. The general surface quantification for the samples in both states is presented in Table 6 . The atomic% for Co, Mo, and W resulted in 0.35, 1.45, and 1.38 in the sulfided CoMoW-HOPSCM and 0.37, 1.53, and 1.45 in the sulfided CoMoW-MonoSBA-15. Both samples presented a global promotional ratio Co/(Co+Mo+W) equal to 0.11; this value is closer to the intended nominal ratio. The average at.% values for the O and Si atoms at the surface resulted in 57% and 37.5% for both sulfided CoMoW-HOPSCM and CoMoW-MonoSBA-15 samples. Finally, the sulfur at.% resulted in 2.91 for the CoMoW-HOPSCM and 1.67 at.% for the CoMoW-MonoSBA-15 catalysts. This difference in the sulfur content could be related to better transforming the surface oxide species into sulfide species in the CoMoW-HOPSCM catalyst. Usually, a better sulfidation degree impacts beneficially, confirming the trend observed in activity presented in Section 3.5. Additionally, a careful deconvolution process was performed in the Co 2p3/2, Mo 3d, and W 4f core emission regions (see Fig. 17 ) to get light on the species on the surface of the catalyst.As seen in Fig. 17A in the sulfided samples, Gaussian-Lorentizian curves related to Co oxide, CoMoS, and Co9S8 species were used to fit the spectra with BE at 781.2 eV, 778.8 eV, and 777.5 eV, respectively [82,83]. For the Mo region presented in Fig. 17B, the MoO3, MoOxSy, and MoS2 species at 230.7, 229.4, and 228.4 eV were used to form a perfect envelope [82,83]. In this region, the peaks of S2− and S2 2− were also used to perform the fit at 225.8 and 227.8 eV In the case of the W 4f region observed in Fig. 17C, the WO3, WOxSy, and WS2 species with BE at 36.1, 32.6, and 31.7 eV were used to deconvolute the spectra as reported in [84]. The results of the relative proportions of species in the mentioned core levels are presented in Table 7 .As seen in Table 7, the CoOx species represent 25% in the CoMoW-HOPSCM; meanwhile, in the CoMoW-MonoSBA-15, the contribution resulted in 37.4%. For the CoMoS species related directly with the promotion and the catalytic activity, the% resulted in 64.2 in the CoMoW-HOPSCM and 53.4% for the CoMoW-MonoSBA-15. The third species related to the segregate cobalt sulfide (Co9S8) was around 10% for both sulfided catalysts. In the region of Mo 3d, the MoO3 resulted in 18% lower in the CoMoW-HOPSCM than in the CoMoW-MonoSBA-15. Inversely the MoOxSy species displayed a value 39% higher in the CoMoW-HOPSCM than in the CoMoW-MonoSBA-15. In the meantime, the active phase MoS2 resulted in approximately the same (4%±) in both catalysts.Likewise, the W species presented only minor differences between catalysts not larger than 10%. From the chart, it is possible to observe that the tungsten and molybdenum species containing sulfur are approximately the same in both regions and catalysts, i.e., the sum of MoOxSy and MoS2 is 90.3% in the CoMoW-HOPSCM and 88.4% in the CoMoW-MonoSBA-15. Meanwhile, the sum of WOxSy and WS2 species is 75.2% and 74.3% for CoMoW-HOPSCM and CoMoW-MonoSBA-15, respectively. Nevertheless, Mo oxide species are better sulfided than their W counterpart under the same sulfidation conditions. It has to be considered that MoOxSy and WOxSy species undergo higher sulfidation under reaction conditions, helping to continuously transform these phases into more active MoS2 and WS2 phases. This effect could explain the activity trend observed for the catalysts tested.Hierarchical structured porous silicas have shown advantages when used as support for trimetallic hydrodesulfurization catalysts, based on CoMoW sulfide system, compared with only-mesoporous materials. Improvement in catalytic performance can be attributed to the synergetic effect caused by the presence of pores in different lengths of scale, and this fact has a positive effect on the diffusion processes of the oxide-state precursors for the sulfided active phases and over the mass transfer limitations of reactants, which result in a higher catalytic activity.The presence of pores of different dimensions also improves the sulfidation processes of the oxide-state precursors; hence a greater quantity of sulfided phases are obtained in the catalysts supported on silicas with hierarchical porosity. In addition, hierarchical porous structure promotes the formation of terminal metal-oxygen bonds, which enhances the formation of Mo(W)S2 edge bonds that are more active in the hydrodesulfurization reactions. The diversity of porous sizes in the support structure also favors the appearance of cobalt species in octahedral coordination, which is known to be of great importance in the catalysis of hydrodesulfurization reactions, therefore an increase in the number of octahedral cobalt results in an improvement of the promotion effect, which is reflected in greater catalytic activity.The synergistic effect obtained by the presence of pores of different scales promotes the formation of more active sites for the two parallel reactions that occur in the hydrodesulfurization of dibenzothiophene: type I and II edge bonds related to direct desulfurization and Brim hydrogenation sites. This results in more significant catalytic activity and selectivity for hydrodesulfurization of the dibenzothiophene reaction.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to thank CONACYT for the financial support of this project. Dr. Antonio Gómez Cortés (IF-UNAM) by the assistance in nitrogen physisorption analysis, Dr. Damián Compéan (IPICYT) by the help with Micro-Raman Spectroscopy characterization, David Dominguez for the XPS acquisition and M. Ing. Q. Alicia del Real López for her assistance in the HR-SEM and EDS studies. J.N. Díaz de León wants to recognize the financial support of DGAPA-PAPIIT project IN104122. Dr. R. Huirache-Acuña thanks to CIC UMSNH 2022 and ICTI PICIR 2022–23 project support. The authors acknowledge the support and facilities of the Laboratorio Nacional de Caracterización de Materiales (LaNCaM).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ceja.2023.100454. Image, application 1

In this work, monolithic silica materials with hierarchical porosity were synthesized by the sol-gel method combined with a dual hard-soft templating route. Silica materials were used for the synthesis of hydrodesulfurization CoMoW-S catalysts by the immersion technique using transition metal salts as precursors, followed by oxidation and sulfidation in H2S/H2 mixture. Styrene-HEMA copolymer hard template presented homogeneous well-defined spherical shape with an average diameter of about 800 nm. Samples prepared over the hard template presented similar morphology. The surface areas of all supports prepared resulted in around 800 m2.g−1 and decreased to 220 m2.g−1 on average after the sulfidation process. Small-angle X-ray diffraction confirmed the presence of the 2D hexagonal or Im3m array of mesopores in all samples. The CoMoW oxide state catalysts presented low intensity peaks assigned to the b-CoMo(W)O4 phase and minor peaks related to MoO3 and polyoxides with the general formula MoxW1-xO3. The high conversion was obtained for catalysts supported on hierarchically structured porous silicas, even greater than that of the commercial catalyst used as reference (>30–50%). XPS results revealed that the degree of sulfidation and CoMoWS active species resulted higher in the CoMoW-HOPSCM catalyst compared to the CoMoW-MoNoSBA-15 sample, which in turn coincides with the catalytic activity results.

Currently, the industry of ethylene derivatives presents two opposing trends. On the negative side, the storage and disposal of plastics (among which those derived from ethylene are highlighted) is environmentally complicated. On the other hand, the future limitation of oil derivatives and natural gas for their use in the automotive industry and as fuels suggests a great availability of oil and natural gas for conversion into light olefins in the mid-term. Unlike other chemicals, ethylene production has continuously increased even in the economic recession of 2008. In fact, before the corona outbreak, the expected ethylene consumption worldwide for 2021 was higher than 190 million metric tons [1,2], which is ca. 25% more than the consumption in 2011.Currently, most ethylene is produced via steam cracking of ethane, LPG and naphtha. This process presents several problems [2–5] among which the high energy consumption stands out. To save these drawbacks related to the energy consumption, the oxidative dehydrogenation (ODH) of ethane is a clean alternative worthy to be studied. Moreover, the ODH of ethane requires fewer separation units than steam cracking, uses catalysts which hardly deactivate (due to the in-situ regeneration by the oxygen consumed), and shows negligible coke formation (which reduces the number of by-products) [1,5,6].The two most promising catalytic systems for the ODH of ethane are multicomponent Mo-V-Te-Nb-O catalysts [7–9] and promoted or supported NiO catalysts [10–16].Bulk NiO is a p-type semiconductor [17–19] which, when working as a catalyst in the presence of ethane or olefins and oxygen, tends predominantly to the formation of carbon oxides [18,19]. However, by incorporating an appropriate promoter to the NiO (such as Nb5+ [11,20–22]) or supporting it with a suitable material such as Al2O3 [10,23–26], the ethylene formation can be drastically enhanced. The nature of the support and/or the promoter must be carefully selected, otherwise the amount of ethylene formed can be even lower than that using unpromoted NiO, as this is the case of potassium as promoter [21]. Then, it has been widely reported that the excess of the electrophilic oxygens in undoped NiO can be decreased by using different cations with high valences (such as W6+, Nb5+, Sn4+, Zr4+, Ti4+) and this positively affects the formation of ethylene [17–21,27–32]. Promoters with acidic characteristics [17–21,27–32] or supports like TiO2 [23], Al2O3 [10,34,35] or siliceous porous clay heterostructure [36] favour the ethylene formation. However, the presence of strong Lewis acid sites provokes the decomposition of ethylene resulting in a low selectivity to olefin [1,6,34,37,38]. Therefore, an excess of promoter usually leads to a decrease in the olefin formation. In this way, it has been demonstrated that in promoted NiO catalysts a loss in the conductivity takes place in the catalysts with an enhanced ethylene formation [18,19,39], which could be related to a decrease in the non-stoichiometric oxygen.On the other hand, the crystallite size of NiO is also a factor that has been shown as crucial in promoted NiO catalysts, those with lower size presenting the highest ethylene formation [11,28,30–32]. This is especially notable in the case of supported NiO catalysts, in which the desired Ni-support interaction involves a decrease in the mean NiO crystallite size [34–36], whereas low Ni-support interaction leads to large NiO crystallites [40,41].In most of these studies dealing with promoted NiO catalysts, the catalytic results have been explained in terms of the characteristics of NiO and the way the cation employed influences the physicochemical properties of the nickel oxide (NiO crystallite size, amount of non-stoichiometric oxygen, morphology, lattice parameters, conductivity…). However, it seems that most times the possible direct role of the promoter has been underestimated. Then, in order to properly assign the catalytic properties to the nickel species it would be desirable to study pure NiO catalysts instead of supported or promoted NiO catalysts. For example, in Nb-promoted NiO catalysts the formation of a Ni–Nb-O solid solution was identified as the selective sites in the ethane ODH, and maybe the amount of non-stoichiometric oxygen is not so tightly related to the selectivity to ethylene [19]. However, this behaviour can change if promoters, especially Nb5+, are absent (if niobium is not present in the catalyst formulation).Recently, Zhao et al. [41] studied undoped NiO showing that it is possible to tune the concentration of non-stoichiometric oxygen with low changes in morphology. In that work, using NiO samples calcined at different temperatures the amount of non-stoichiometric oxygen could be modified and, unlike that proposed in promoted NiO catalysts, it was observed that the selectivity to ethylene increased as the amount of that excess of oxygen increased. On the other hand, the particle size has also been proposed as an important parameter in the performance in the ODH of ethane. In this way, mesostructured NiO catalysts seem to present higher selectivity to ethylene (at isoconversion conditions) than the corresponding nanostructured NiO [42].At the present work, we have synthesized, characterized and tested in the oxidative dehydrogenation of ethane a set of pure NiO catalysts modifying both the concentration of oxalic acid in the synthesis gel and the final calcination temperature. Moreover, we have studied the influence of different parameters such as the nature of Ni-species, the amount of non-stoichiometric oxygen, the NiO crystallite size or the charge carriers´ density of the catalysts on the catalytic performance.Nickel oxides catalysts were prepared through the evaporation at 90 °C of aqueous solutions of nickel nitrate (> 99.99% Sigma-Aldrich) with different amount of oxalic acid (> 99.5% Acros Organics) in the synthesis gel. The solids were dried overnight in a furnace at 120 °C and, finally, they were calcined in static air for 2 h at 350 or 500 °C with a heating ramp of 5 °C/min from room temperature to the desired final calcination temperature. The catalysts will be named as Ni-x-Y, where x is the oxalic acid/nickel ratio in the synthesis gel (i.e. Oxalic acid/Ni molar ratio of 0, 0.8, 1.0, 1.5 or 3), and Y is the calcination temperature (350 or 500 °C). Nomenclature and physicochemical properties of these catalysts are shown in Table 1 .X-ray diffraction patterns were collected in an Enraf Nonius FR590 diffractometer with a monochromatic CuKα1 source operated at 40 keV and 30 mA. The size of the NiO domains has been calculated through XRD technique by the Scherrer’ equation, D = k·λ/w·cosθ, where D is the crystallite size, k is a shape factor (0.9), λ is the X-ray wavelength (0.15406 nm), w is the full width at half maximum intensity (FWHM, in radians) and ϴ is the Bragg angle [43].The surface area of catalysts were determined by multi-point N2 adsorption at −196 °C. Estimations of surface areas were made in accordance with the BET method.Raman spectra were obtained in an inVia Renishaw spectrometer, equipped with an Olympus microscope, using a wavelength of 514 nm (visible Raman) or 325 nm (UV-Raman), generated with a Renishaw HPNIR laser with a power of approximately 15 mW.UV–vis diffuse reflectance spectroscopy (DRS) measurements of the solids were carried out within the 200–800 nm range using a Varian spectrometer model Cary 5000.Temperature-programmed reduction experiments (H2-TPR) were carried out in an Autochem 2910 (Micromeritics) equipped with a TCD detector, using 0.10 g of catalyst and a reducing gas of 10% H2 in Ar (total flow rate of 50 mL min−1). The samples were heated from room temperature to 800 °C, with a heating rate of 10 °C/min.X-ray photoelectron spectroscopy (XPS) measurements were performed on a SPECS spectrometer equipped with a Phoibos 150 MCD-9 detector using a monochromatic Al Kα (1486.6 eV) X-ray source. Spectra were recorded using an analyzer pass energy of 50 eV, an X-ray power of 100 W, and an operating pressure of 10−9 mbar. Spectra treatment was performed using CASA software. Binding energies (BE) were referenced to C 1 s at 284.5 eV.TEM (transmission electron microscopy), HRTEM (high resolution transmission electron microscopy) and SAED (Selected area electron diffraction) were conducted using a FEI Field Emission gun Tecnai G2 F20 S-TWIN microscope working at 200 kV. Structural and morphological characterizations were obtained from the TEM and HRTEM images. The lattice parameters were determined via Fourier transformation from HRTEM images. The preparation of the samples for the microscopy analyses involves the sonication in ethanol of the sample and a further deposition of the solution over a holey‑carbon film supported on a Cu grid where it is finally dried.Electrochemical Impedance Spectroscopy measurements (EIS) were performed at a frequency of 5000 Hz in a 0.1 M Na2SO4 electrolyte. The potential was scanned from 1 VAg/AgCl in the negative direction using an amplitude signal of 0.01 V at 0.05 V/s. For this purpose, a three-electrode electrochemical cell was used. The catalyst was the working electrode (deposited on fluorine tin oxide (FTO)), a platinum tip the counter electrode and an Ag/AgCl (3 M KCl) the reference electrode. An area of 0.5 cm2 of the catalysts was exposed to the electrolyte. From EIS results, Mott-Schottky plots were carried out to evaluate the semiconductor behaviour of the catalysts and to determine their density of dopants.The catalytic tests in the oxidative dehydrogenation of ethane were carried out at atmospheric pressure in a tubular isothermal flow quartz reactor (15 mm inner diameter) at 300–340 °C, with a mixture consisting of C2H6/O2/He with a molar ratio of 3/1/29. Ethane (99.95% purity, Carburos Metálicos) and O2 (99.995% purity, Carburos metálicos) were used in these experiments. Typical reaction conditions used were 0.1-0.5 g of catalyst and 100 mL min−1, although both the total flow and/or the catalyst amounts were changed in order to achieve different ethane conversions. Catalysts were introduced in the reactor diluted with silicon carbide in order to keep a constant volume in the catalytic bed. Reactants and products were analyzed by gas chromatography using two packed columns: (i) molecular sieve 5 Å (2.5 m); and (ii) Porapak Q (3 m). Blank runs were undertaken without catalyst until 450 °C and no conversion was observed in all cases [28].NiO catalysts heat-treated in air at different temperatures (350 or 500 °C) with or without the addition of oxalic acid in the synthesis gel have been prepared, tested in the oxidative dehydrogenation of ethane and characterized through several physicochemical techniques. Selected physicochemical characteristics of the prepared samples are included in Table 1.Representative catalytic results of the differently synthesized NiO catalysts in the oxidative dehydrogenation of ethane are shown in Table 2 . The reaction temperature used in this study has been fixed at 300 and 340 °C in order to have a better comparison of the catalytic performance of the catalysts, minimizing the effect of the reaction temperature in the selectivity to ethylene vs ethane conversion plot. Moreover, by not exceeding 350 °C in the reaction temperature, the modification of the catalysts heat-treated at the lowest calcination temperature is minimized.In all cases, the only reaction products detected have been ethylene and CO2. No carbon monoxide, acetic acid nor another O-containing product have been identified. An accurate carbon balance in the 98–102% range has been obtained in all the experiments.Overall, the most active catalysts were those calcined at 350 °C with a catalytic activity ca. 3–4 times higher than that of the catalysts calcined at 500 °C. Moreover, a clear positive effect of the addition of oxalic acid has been observed regardless of the calcination temperature. Then, the addition of oxalic acid increases the reaction rate for ethane conversion by a factor of 2 (Fig. 1a). This increase is observed regardless of the amount of oxalic acid employed, so that the key factor of the catalytic activity is the presence or the absence of oxalic acid rather than its amount. As it can be seen in Table 2, the specific activity (activity normalized per surface area) is very similar for all catalysts regardless of the presence of oxalic acid or the calcination temperature used. Therefore, the surface area (i.e. the available active sites) seems to be the governing factor determining the catalytic activity.Whenever there is not a drastic difference in reactivity, the key factor to be optimized in the catalytic performance for the oxidative dehydrogenation of ethane is the selectivity to ethylene. Then, Fig. 1b shows the values of the selectivity to ethylene at isoconversion conditions (10%). As it can be seen, the presence of oxalic acid in the synthesis gel leads to a remarkable increase in the selectivity to ethylene regardless of the calcination temperature. Then, the catalysts prepared in absence of oxalic acid present a selectivity of ca. 45–48% whereas that achieved on catalysts prepared in the presence of oxalic acid ranges between 63 and 75%. According to these results, the optimal catalysts were the ones prepared with an oxalic acid/nickel molar ratio of 1.0. Additionally, for a fixed oxalic acid/nickel molar ratio, the lower the calcination temperature the higher the selectivity to ethylene is.Since the reactivity of the catalysts is different, we employed different contact times in order to obtain a comparative selectivity to ethylene vs. ethane conversion curve. Fig. 2 shows the variation of the selectivity to ethylene with the ethane conversion for selected catalysts. Noteworthy, in the range of conversions studied (until 20%) the fall in the selectivity to ethylene is not drastic. Therefore, it can be seen that the Ni-1-350 catalyst presents the highest selectivity to ethylene, whereas Ni-0-500 is the least selective.Although there are many factors that define the catalytic performance of NiO based catalysts, the crystallite size has shown to have certain effect on both the catalytic activity and the selectivity [11,21,27–32,41]. Fig. 3 shows the XRD patterns of nickel oxide catalysts calcined at 350 (Fig. 3a) or 500 °C (Fig. 3b). In all cases, the main peaks appear at 2θ: ~37.2, 43.2, 62.8, 75.3 and 79.3º which correspond to the (111), (200), (220), (311) and (222) planes of cubic NiO crystalline phase (JCPDS: 78–0643), respectively. The presence of partly (Ni2O3) or totally reduced (Ni0) phases have not been detected. As it can be observed, all the samples present five intense peaks, regardless of the addition of oxalic acid and the calcination temperature, which have been considered to apply the Scherrer’ Eq. [43].However, interesting differences in terms of crystallinity are observed (Fig. S1, supporting information). Samples heat treated at 500 °C present narrow Bragg peaks which means that relatively large crystallites have been formed. Then, according to the Scherrer equation, a mean NiO crystallite size that varies from ca. 22 nm in the catalyst prepared without oxalic acid in the synthesis gel to ca. 17 nm in the catalysts prepared with oxalic acid in the synthesis gel has been determined (Table 1 and Fig. S1). As expected, the calcination at 350 °C leads to a widening of the Bragg peaks, which is related to material with lower crystallinity with tinier NiO crystallites. Then, average NiO crystallite size of ca. 12 nm in the catalyst prepared without oxalic acid decreased to 7–8 nm for those catalysts with moderated use of oxalic acid (oxalic acid/Ni ratio of 0.8 to 1.5). Finally, the use of higher oxalic acid loadings resulted in a further decrease of the average size since Ni-3-350 presents an average size for NiO crystallites of ca. 5 nm. The determined crystallite sizes of the catalysts follow the trend expected considering their surface areas.We must inform that the as-synthesized samples (before calcination) have been also characterized by XRD and Thermogravimetric analysis (Figs. S2 and S3, respectively, in the Supporting Information), in order to understand better the changes achieved in calcined samples, depending on the amount of oxalic acid in the synthesis gel. Thus, the XRD pattern of the as-synthesized samples indicates de presence of Nickel(II) nitrate hexahydrate (Ni(NO3)2 · 6 H2O, in the sample prepared without oxalic acid in the synthesis gel), whereas Nickel(II) oxalate dihydrate (NiC2O4·2 H2O) is mainly observed in samples prepared with oxalic acid in the synthesis gel (Fig. S2; Supporting information). This is also confirmed by Thermogravimetric analysis (Fig. S3; Supporting Information), in which the decomposition of Ni-nitrate (broad peak, not very intense) and Ni-oxalate (narrow peak, very intense) occurs at ca. 350 °C in TG.The samples have been characterized by Visible Raman (using an excitation wavelength of 514 nm, Fig. S4) and UV Raman (using an excitation wavelength of 325 nm, Fig. 4 ), in order to differentiate the crystallinity and crystal sizes of catalysts.Raman spectra of catalysts, using a 514 nm laser (Fig. S4), present main bands at 462 and 497 cm−1, in addition to small broad bands at 710, 930 and 1081 cm−1. Bands at 462 and 497 cm−1 have been assigned to the NiO stretching mode related to a rhombohedral deformation of the structure or to non-stoichiometric NiO [19,20,44]. The other bands have been related to overtones of the first ones or their combinations [20]. A small band at 1060 cm−1 is also observed in some cases, which can be related to the ν1 vibration mode of carbonate groups [20,45,46]. These results agree with previous ones on samples calcined at temperatures higher than 400 °C [19,20,44–46].In addition to these, it can be also seen a shoulder at 410 cm−1 (especially in samples prepared with an oxalate/Ni ratio of 3.0), which could be related to the non-stoichiometry of catalysts and/or a higher nickel vacancy concentration [19,20]. On the other hand, the appearance of 2-magnon band (at ca. 1402 cm−1) in samples calcined at 500 °C (Fig. 4a), which is absent in samples calcined at 350 °C, can be attributed to the transition from ferromagnetic to antiferromagnetic characteristics, in NiO materials calcined at temperatures higher than 400 °C [47,48]. Fig. 4 shows the UV Raman spectra of nickel oxide catalysts calcined at 350 or 500 °C, which can help to follow the spin-phonon interaction in these catalysts [48–50]. In general, it can be distinguished the presence of two main bands at ca. 571 and 1128 cm−1 (1145 cm−1 in samples calcined at 500 °C), which can be related to the one-phonon (1P) longitudinal optical (LO) mode and two-phonon (2P) longitudinal optical (2LO) mode, respectively, in NiO crystals [48,49]. In addition, two small bands are also observed at ca. 724 and 901 cm−1 related to two-phonon (2TO-transverse optical) and (TO) modes.As it can be seen, the intensities of bands at ca. 571 and 1128 cm−1 change depending on the catalyst preparation procedure and/or calcination temperature. In this way, it has been proposed that the intensity of these most characteristic bands (1P LO band at ~571 cm−1 and 2P 2LO band at ~1120 cm−1) could be related to the specific structural and chemical features of the catalyst. Thus, an increase of the intensity of 2P 2LO band (I1120) higher than that of 1P LO band (I571) is observed for catalysts calcined at 500 °C and for samples prepared without oxalic acid in the synthesis gel, suggesting that they present the higher NiO crystal size and/or the lower concentration of defects [51]. In an opposite trend, samples prepared in the presence of oxalic acid in the synthesis gel and calcined at 350 °C present the highest values of I571 and the lowest I1120/I571 ratios. Accordingly, low I1120/I571 ratios in the UV Raman spectra should correspond to samples presenting NiO particles with the lowest crystal size and/or the highest concentration of defects [50], being these aspects mainly related to the catalyst preparation (presence of oxalic acid in the synthesis gel) and/or lower calcination temperature. In the present work, a clear correlation between the relative intensity of the bands at 571 and 1120 cm−1 (ILO/I2LO) and the NiO crystallite size determined by XRD has been observed (Fig. S5). Therefore, an estimation of the concentration of over-stoichiometric oxygen in these catalysts is not straightforward to reach according to UV Raman spectroscopy.The catalysts were also characterized by Diffuse Reflectance UV–vis spectroscopy (DRS). The DRS spectra of catalysts calcined at 350 or 500 °C are shown in Fig. 5 . No distinguishable differences were observed in the UV area of the spectra, but some features can be observed in the visible area.Several absorption bands have been observed (at 380, 416, 450, 650 and 722 nm) which are typical of bulk NiO [10,35,52,53] with Ni2+ in its octahedral coordination. In fact, it is well known that bulk NiO shows bands at 715, 420 and 377 nm, which can be related to the presence of octahedrally coordinated Ni2+ species in the cubic (rock-salt) NiO lattice [10,51,52]. In addition, a band at ca. 510 nm could be also related to charge transfer in NiO crystals [53,54]. On the other hand, and although there is not a clear trend in these spectra, it can be observed a higher intensity of the absorption in the 400–600 nm range for the catalysts calcined at 350 °C (Fig. 5A) than those at 500 °C (Fig. 5B). This is especially notorious in the sample prepared in the absence of oxalic acid in the synthesis gel (i.e. Ni-0-350). A high background absorbance for this area has been linked with a high concentration of non-stoichiometric oxygen. Therefore, it seems that those samples calcined at 350 °C, and especially Ni-0-350, present the highest amount of non-stoichiometric oxygen.A detailed study of these catalysts was also undertaken through TEM and High Resolution TEM (Fig. 6 ). Catalysts calcined at 350 °C show drastic differences when adding oxalic acid in the synthesis. The basic Ni-0-350 catalyst presents large agglomerations of several micrometres consisting of highly porous poorly defined particles of variable size, ranging from large (50 nm) to small (5 nm) particles. The Ni-1-350 sample presents well defined particles with sizes between 5 and 50 nm. Interestingly, small cavities in the range of 1–2 nm are located within these particles. In the inset of Fig. 6b, it is observed by HR-TEM some ca. 10–15 nm particles with tiny cavities. Finally, the sample prepared with the highest concentration of oxalic acid (sample Ni-3-350) is formed by tiny particles with a rather uniform size (ca. 4 nm) and aspect. In the inset of Fig. 6c, small 3 to 6 nm particles are clearly observed in close contact with each other. Fig. 6 shows also selected TEM/HRTEM images of NiO catalysts calcined at 500 °C. Overall, the effect of the oxalic acid is similar to that observed for catalysts calcined at 350 °C but at 500 °C the porosity and the presence of cavities is much lower. This fits with the decreased surface area observed in the latter. Ni-0-500 catalyst has large agglomerations of rather big particles without apparent porosity and a few small nanoparticles of 10 nm and less (Fig. 6d). Ni-1-500 presents well defined particles most of them between 10 and 30 nm of diameter without apparent internal cavities (Fig. 6e). Finally, Ni-3-500 shows well defined particles and a similar morphology but with slightly smaller particles than that sample with lower oxalic acid content (Fig. 6f).SAED patterns (Fig. S6) demonstrate that all NiO catalysts are formed by crystalline Ni-containing nanoparticles, which can be indexed unambiguously to cubic NiO. No metallic Ni nor Ni2O3 were identified. A further analysis indicates that the lattice parameter ranges from 0.419 nm to 0.416 nm (see Table 1) and a clear effect of the preparation procedure on this value has not been observed. However, by taking separately both calcination temperatures, a decrease of the lattice parameter has been observed in the samples treated with oxalic acid. In this way, the decreased particle size could partially explain the decrease lattice parameter observed.The reducibility of the NiO catalysts has been also studied by temperature programmed reduction (Fig. 7 ) as it can influence the catalytic performance of this type of catalysts [11,19–21,27–33,54–57]. Thus, the oxidative dehydrogenation of ethane takes place by a redox mechanism [57], being the reduction part considered as the limiting step at moderate and high temperatures. Then, the catalytic activity in this reaction could be related to the reducibility of the sample. Moreover, it has been proposed for many NiO based catalysts [11,19–21,27–33,54–57] that the most selective sites are the ones that present the lowest reducibility.Overall, the reduction profile of these catalysts is rather similar with a broad reduction peak centred between 254 and 351 °C. This broad peak, which can contain a shoulder, has been related to the reduction of bulk NiO (lattice oxygen) [55– 57]. Interestingly, the onset temperature for this band takes place at similar temperatures regardless of the catalyst, but the maximum shifts towards lower temperatures when the amount of oxalic acid used increases. It is noteworthy the presence of a low intensity reduction band at ca. 200 °C.This band at 200 °C has been reported to be related to the reduction of over-stoichiometric oxygen, i.e. Ni3+ species or even Ni2O3 [29,56]. An enlargement of the area around 200 °C (see Fig. 7) shows that the low temperature band is especially intense in the samples calcined at 350 °C and, above all, in the reference catalyst (sample Ni-0-350). However, the use of oxalic acid in the synthesis gel leads to a decrease in its height. This reduction peak, with low intensity, can be also observed in the reference sample calcined at 500 °C (sample Ni-0-500) but not in the other catalysts.The hydrogen consumption observed during the TPR experiments slightly exceeds the hydrogen necessary to completely reduce Ni2+O into metallic Ni (102–105% of the theoretical NiO + H2 ➔ Ni + H2O reaction). However, a defined trend regarding calcination temperature or presence of oxalic acid has not been detected. Nevertheless, this higher hydrogen consumption could indicate that these catalysts present an excess of oxygen over the stoichiometric NiO.The surface of the nickel oxide catalysts has been characterized by XPS of samples calcined at 350 or 500 °C. Fig. 8 shows the Ni 2p 3/2 core level spectra for the catalysts calcined at 350 °C (Fig. 8a) and 500 °C (Fig. 8b), respectively. In all cases, the spectra show a wide band centered at ca. 854.5 eV. This band presents two maxima. The first one (usually referred as Main Peak) at binding energy ca. 853 eV is attributed to structural Ni2+ species. The second maximum corresponds to a satellite (usually referred as Sat I) that appears at 855 eV and it is related to the presence of multiple defects in the structure (i.e. Ni2+ vacancies, Ni3+ and/or Ni2+-OH species) but also to nickel atoms not coming from lattice oxygen‑nickel bound but from octahedral NiO6 neighbour cluster units [57,58]. A second wide satellite (Sat II) with its maximum at 860.2 eV is associated to ligand-metal charge transfer [28,36,57,58].The relative intensity of the Sat I compared to the Main peak (Sat I/Main peak ratio) has been determined by the deconvolution of the wide band into two bands (centered at 853 and 855 eV). This ratio has been roughly related to the presence of nickel defects [27]. As it can be seen, and in accordance with the XRD results, lower calcination temperatures lead to worst crystallization of the NiO regardless of the oxalic acid amount, and therefore, higher presence of nickel defects, as it can be stated for the higher Sat I / Main Peak relationship of their relative intensity values (see Table 3 ). However, the Sat I / Main Peak relationship for the catalysts treated at 500 °C leads to fairly lower values, suggesting that a severe thermal treatment favors better crystallization of the catalysts. Regrettably, due to the uncertainty of the assignment of Ni bands accurate conclusions cannot be drawn.On the other hand, Fig. 8 shows also the O 1 s core level XPS spectra of the NiO catalysts calcined at 350 °C (Fig. 8c) and 500 °C (Fig. 8d). In our case, it can be differentiated three types of signals [41,59,60]: the first one (OI) at binding energy of ca. 528.6 eV is the majority and it is related to structural nucleophilic lattice oxygen species; the second one (OII) appears at 530.5 eV, and it is attributed to OH− species; and finally, the third signal (OIII) at 532.5 eV is ascribed to electrophilic (O2 − and/or O-) species on the surface of the catalysts. Table 3 shows the relative proportion of these oxygen species.According to these results, those catalysts calcined at 350 °C present a similar concentration of nucleophilic species (51–60%), those catalysts with an oxalic acid/Ni ratio of 1 presenting the highest concentration. In addition, it is clear that the signal corresponding to OH−/defects (OII) is more intense in the catalysts calcined at 350 °C to the detriment of the electrophilic oxygen (OIII) signal (Table 3), which are shown to be responsible for the deep oxidation of the paraffin [62].These results are in good agreement with all the characterization reported above and suggests that a lower presence of electrophilic species enhance the selectivity to the partial oxidation product.In order to get more information about the number of vacancies we have decided to evaluate the semiconductor behaviour of the catalysts through Mott-Schottky plots (see conditions in Experimental section). Mott-Schottky plots represent the reciprocal of the square capacitance vs the applied potential. In particular, the Mott-Schottky equation for a p-type semiconductor (such as NiO [63]) is the following: (1) 1 C 2 = 1 C H − 2 ε r · ε 0 · e · N A E − E FB − kT e where C is the value of total interfacial capacitance calculated from EIS, CH is the capacitance of the Helmholtz layer, εr is the dielectric constant of the semiconductor used as a catalyst (~12 for nickel oxide) [64], ε0 is vacuum permittivity (8.85·10−14 F cm−1) and e is the electron charge (1.60·10−19C), NA is the density of acceptors in the semiconductor, E is the applied potential, EEF is the flat-band potential, k is the Boltzmann constant (1.38·10−23 J/K) and T is the absolute temperature. From the slope of the linear region of the Mott-Schottky plots, NA values for the different catalysts can be calculated.Mott-Schottky plots for the catalysts with and without oxalic acid in the synthesis gel are presented in Fig. 9 . It is observed that, for all the catalysts, a linear region with a negative slope is displayed in the Mott-Schottky plots, which is characteristic of p-type semiconductors with an excess of cationic vacancies [64–67], the predominant defect type in NiO [41]. Fig. 9 also shows the value of the acceptor densities calculated for catalysts calcined at 350 or 500 °C, synthesized with or without oxalic acid in the synthesis gel. Independently of the oxalic acid content in the synthesis gel, NA values are in general higher for catalysts calcined at 350 °C but the presence of oxalic acid clearly diminished the number of NA with respect to the base NiO catalyst (i.e. Ni-0-350), regardless of the calcination temperature (Table 1). This confirms that catalysts prepared with oxalic acid in the synthesis gel present less cationic vacancies, which are related with the excess of non-stoichiometric oxygen, i.e. mainly electrophilic oxygens [11,41,57].These electrochemical results are in good agreement with the DRS and H2-TPR measurements, where the highest amount of non-stoichiometric oxygen species correspond to the catalysts synthesized without oxalic acid in the gel solution and calcined at 350 °C. Additionally, acceptor densities are consistent with XRD and XPS results, since catalysts obtained at low calcination temperatures presented the highest concentration of nickel defects.In the present article we have demonstrated that by controlling the calcination temperature and adding oxalic acid in the synthesis gel, a rather unselective material as unsupported and unpromoted NiO can turn into a selective catalyst for the oxidative dehydrogenation of ethane. A 73% selectivity to ethylene at 10% of ethane conversion as well as a highly stable behaviour has been reached with the optimal catalyst.Several factors have been shown to influence the catalytic performance (and more importantly, the selectivity to ethylene) of NiO-based catalysts. Tiny NiO crystallites seem to be desirable to obtain high selectivity to ethylene in promoted NiO catalysts [19–21,27–32] although in supported catalysts this trend is not clear [33–36]. In those cases, the nature of the support and the NiO-support interaction play an important role. Fig. 10 shows the influence of the average NiO crystallite size over the selectivity to ethylene. It can be observed that, among those samples calcined at a given temperature, the catalysts with the lowest size are the most selective ones. However, it is not a general trend.For example, the catalyst prepared in the absence of oxalic acid calcined at 350 °C presents a selectivity lower than 50% with a mean crystallite size of ca. 12 nm and those prepared with oxalic acid at 500 °C can reach 66% selectivity with a size of ca. 18 nm. Therefore, other factors are involved in the enhanced performance of the catalysts synthesized in the presence of oxalic acid.Recently, Zhao et al. [41] studied pure NiO catalysts heat-treated at different temperatures between 400 and 1000 °C and observed a direct correlation between the amount of non-stoichiometric oxygen and the selectivity to the olefin. This observation contrasts with that reported previously in different articles. Relationships between nucleophilic oxygen and the formation of partial oxidation and dehydrogenation products, as well as electrophilic oxygen and formation of total oxidation products, have been often proposed [11,21,61,68,69]. In the case of electrophilic oxidation, it proceeds through the activation of the molecular oxygen fed favouring the formation of cracking products and carbon oxides (total oxidation is highly favoured). Conversely, nucleophilic oxygen tends to attack CH bonds in a previously activated organic molecule, maintaining the same size of the hydrocarbon. In this sense, non-stoichiometric oxygen is more electrophilic than lattice oxygen, so that one could think that catalysts with high concentration of oxygen in excess tends preferentially to transform ethane into carbon oxides. In fact, several authors have found in promoted NiO catalysts that the decrease in the amount of non-stoichiometric oxygen leads to an enhanced selectivity to ethylene [19–21,27–31,70]. Similarly, it was demonstrated an inverse relationship between the selectivity to the olefin in promoted NiO catalysts with the p-conductivity [19,39,70]. Accordingly, NiO doped with high valence promoters (+4 and above) have been reported to be the most selective catalysts for the olefin formation, but they also provoke a decrease of the mean oxidation state of Ni, consequently decreasing the amount of non-stoichiometric oxygen [11,19–21,27–32]. In the same way, it has been shown for supported NiO catalysts that the selectivity to ethylene increases concomitantly when the number of Ni neighbours in the first (NiO) and the second coordination shell (NiNi) decreases [33]. The elimination of surface electrophilic species (which is somewhat related to the density of over stoichiometric oxygen) has been also linked to a higher ethylene formation in supported or promoted Nb- and Ti- doped NiO catalysts [41].Most of these studies have been focused in promoted or supported NiO since undoped bulk NiO usually presents a poor performance with a massive formation of carbon dioxide. Maybe, in those cases, the role of the promoter and/or the support has been underestimated. In the present article several pure NiO catalysts have been tested, being the catalysts calcined at 350 °C the ones that present the highest concentration of non-stoichiometric oxygen. As expected, the amount of non-stoichiometric oxygen decreased when the calcination temperature increases [71]. However, the most selective catalysts are those prepared with oxalic acid and, although the calcination temperature plays a role, it is upset by the effect of the use of oxalic acid. The presence of oxalic acid in the synthesis gel also leads to a decrease in the concentration of overstoichiometric oxygen species. This is due to the reductant behaviour of the oxalic acid, which favors the Ni3+ to Ni2+ transition. However, this elimination of non-stoichiometric oxygen when using oxalic acid takes place to a lesser extent than by increasing the calcination temperature. Therefore, no relationship has been observed between the amount of non-stoichiometric oxygen and the selectivity to ethylene. Fig. 11 shows the evolution of the ethane conversion and the selectivity to ethylene with the time on line for Ni-1-350 and Ni-1-500 catalysts together with the DR-UV–Vis. spectra of these catalysts before and after use. It can be seen that, after 9 h of use the catalytic performance kept invariable, maintaining both the same conversion and selectivity.Interestingly, the UV–Vis. spectra of the used catalysts changed with respect to the fresh catalysts since the absorbance in the 450–600 nm range, which is qualitatively related to non-stoichiometric oxygen, is remarkably lower. Then, after a moderate use in an atmosphere with ethane and oxygen (9 h with an ethane/O2 ratio = 3 M), the catalysts seem to lose part of the over-stoichiometric oxygen. This is in agreement with the decrease in the oxygen excess observed in Nb-doped NiO catalysts after their use in the oxidative dehydrogenation of ethane [29,72]. In our case, the loss of oxygen in excess has not meant a drop of either catalytic activity or selectivity to ethylene.The apparent contradiction observed in different works about the need for the presence or absence of non-stoichiometric oxygen can be due to the fact that electrophilic oxygen species have been reported to be the ones that activate the ethane molecule [73]. Thus, these electrophilic species would be necessary for the activation of the ethane, but they should be isolated (and, therefore, be present in low concentration) to avoid the transformation of ethane into carbon dioxide. Accordingly, surface area should also play a role as higher surface areas would allow a higher dispersion of electrophilic species. Perhaps, due to the need for electrophilic species to activate the ethane molecule, NiO based catalysts have not been described in the literature to present consistent selectivity to ethylene above 95%. Hence, a general correlation between the concentration of electrophilic oxygen and the selectivity to ethylene is not direct and will depend on other factors such as the surface area, the presence of promoters or supports or even to the different morphologies and exposed planes.As mentioned above, it has been proposed in several articles that in supported or promoted NiO based catalysts [11,19–21,27–33,54–57] the most selective sites are those with the lowest reducibility. However, in the present work, the study of unpromoted bulk NiO does not show a clear correlation although the least selective catalysts (the ones without oxalic acid in the synthesis gel) are those that show the highest reduction maxima temperature, according to our H2-TPR assays.On the other hand, considering that the reduction band at 200 °C of the TPR experiments is related to non-stoichiometric oxygen (Ni3+-like species), the extent of isolation of electrophilic species could be roughly estimated by dividing that area by the specific surface area of each catalyst. Then, for a given calcination temperature the most selective catalysts are those in which non-stoichiometric oxygen concentration (normalized per surface area) is lower, i.e., non-stoichiometric oxygens are more isolated (Fig. 12a). In any case, samples calcined at 350 °C present higher selectivity than samples calcined at 500 °C.As mentioned in the Results section, cationic vacancies have been estimated through an electrochemical assay determining the NA value (acceptor density). This NA value turned out to be higher for catalysts calcined at 350 °C; although the presence of oxalic acid in the synthesis gel also leads to a decrease of the NA value. Thus, Fig. 12b shows the relationship between the selectivity to ethylene with the acceptor density values for the three set of catalysts, i.e. catalysts prepared with an oxalic acid/Ni molar ratio of 0, 1.0 or 3.0, and calcined at 350 or 500 °C. It can be clearly observed that the highest selectivity to ethylene is related to lower acceptor densities (in Fig. 12b). In particular, the catalysts synthesized with oxalic acid show lower NA values, that is, the lowest concentration of cationic vacancies. Hence, catalysts prepared with oxalic acid have less concentration of non-stochiometric oxygen which, in turn, could overoxidize ethane to non-desired carbon compounds, such as CO2.The main achievement of the present article is that the simultaneous use of low calcination temperatures and a certain amount of oxalic acid leads to the formation of highly selective undoped NiO catalysts which additionally are very stable.The characterization of as-synthesized samples (before calcination) suggests important changes depending on the absence or presence of oxalic acid in the synthesis gel. Thus, Ni-nitrate was observed in the sample prepared without oxalic acid in the synthesis gel, whereas Ni-oxalate is observed in the precursors of catalysts prepared with oxalic acid in the synthesis (Fig. S2, in the Supporting information). These differences, which determine also a different behaviour during the thermal decomposition of precursors (Fig. S2, in the Supporting information) should have an important influence on the physicochemical characteristics, and then, on the catalytic properties of the calcined catalysts [74,75], especially in samples calcined at 350 °C.In addition, these catalysts also present high catalytic reactivity, making it possible for them to operate at low reaction temperatures (ca. 300 °C) with a negligible loss of activity. This high stability contrasts with that reported in several articles in which a certain deactivation is observed in the ODH of ethane using NiO based catalysts. Deactivation has been related in former articles to formation of mixed inactive phases such as NiWO4 [72] or NiNb2O6 [20], the reduction of NiO overstoichiometric oxygen [20] and the decrease in the surface area [73]. In our catalysts, no mixed phases can be formed since the only chemical element present in these catalysts, apart from oxygen, is nickel. Interestingly, we have observed an apparent decrease in the amount of non-stoichiometric oxygen after using the optimal NiO catalyst, which, on the contrary, has had no effect on catalytic activity and ethylene selectivity.A non-promoted and non-supported NiO catalyst active and with a reasonably high selectivity to ethylene in the oxidative dehydrogenation of ethane has been prepared. The joint use of low calcination temperature (350 °C) and the inclusion of an appropriate oxalic acid loading in the synthesis gel during the preparation procedure has led to notable selectivity to ethylene (ca. 73%). Interestingly, a stable catalytic performance with the time on line has been observed in the optimal catalysts. This high stability can be related to the low reaction temperature required to undertake the reaction. The use of oxalic acid in the synthesis gel has been shown to highly improve the catalytic performance as an increase in the selectivity to ethylene by ca. 25 points and the reactivity by a factor of 1.5–2.0 compared to the reference sample have been obtained. Moreover, the calcination temperature has been shown as a determining factor in a way that catalysts calcined at 350 °C are more active and more selective to ethylene than analogous catalysts calcined at 500 °C. The enhanced catalytic performance of catalysts prepared in the presence of oxalic acid has not been only related to the NiO crystallite size and, more interestingly, the amount of electrophilic oxygen does not seem to play alone a determining role in the selectivity to ethylene. However, although a precise correlation has not been obtained, the most selective catalysts present high extent of isolation of non-stoichiometric oxygen and low p-type semiconductor character. Yousra Abdelbaki: Investigation, Formal analysis. Agustín de Arriba: Investigation, Formal analysis. Rachid Issaadi: Methodology, Supervision. Rita Sánchez-Tovar: Methodology, Investigation, Formal analysis. Benjamín Solsona: Conceptualization, Supervision, Writing – review & editing. José M. López Nieto: Conceptualization, Project administration, Writing – review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Authors would like to acknowledge the Ministerio de Ciencia, Innovación y Universidades in Spain through projects CRTl2018-099668-B-C21 and MAT2017-84118-C2-1-R. A.A. acknowledges Severo Ochoa Excellence Program for his fellowship (BES-2017-080329). Y.A. and R.I. thank the Ministry of Higher Education and Scientific Research of Algeria for the National Exceptional Program for the fellowships. Supplementary material Image 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.fuproc.2022.107182.

Highly stable and selective bulk NiO catalysts have been synthesized for the oxidative dehydrogenation (ODH) of ethane to ethylene. Interestingly, by optimizing synthesis parameters such as the amount of oxalic acid in the synthesis gel and the calcination temperature, undoped NiO catalysts have shown a consistent selectivity to ethylene of ca. 75%. The optimal catalyst requires the presence of a certain amount of oxalic acid in the synthesis gel and a final calcination at low temperatures (i.e. 350 °C). These catalysts have been deeply characterized by means of XRD, TPR, HRTEM, Raman and UV–vis diffuse reflectance spectroscopies, XPS and Electrochemical Impedance Spectroscopy measurements and tested in the ethane ODH. A novel electrochemical study has been undertaken, showing the p-character of all the NiO catalysts synthesized but differing in their capacitance values and density of cationic vacancies. The catalytic performance of NiO catalysts has been explained in terms of the different physicochemical properties (including changes in the number of vacancies) of the samples and the isolation of electrophilic oxygen species.

phosphorphosphor-doped titaniaphosphor-doped iron on titaniaphosphor-doped copper on titaniaphosphor-doped cobalt on titanianickel phosphide on titaniaalkly-methoxyphenolThe growing demand for clean energy with depleting fossil resources and climate change because of increased carbon dioxide (CO2) emissions have motivated renewable energy development. Lignocellulosic biomass is an abundant renewable energy source that can replace crude oil to produce commodity chemicals and liquid transportation fuels. The biological and chemical processing of biomass can produce various chemicals and fuels [1]. However, biomass pyrolysis oil, referred to as bio-oil, has emerged as an essential feedstock to supplement petroleum oil because it can completely utilize organic components of the biomass. Moreover, replacing petroleum-based fuels with bio-oil-derived liquids reduces carbon footprints. A previous study suggested that producing petroleum-like fuels using bio-oils derived from several lignocellulose feedstocks could reduce the amount of greenhouse gas by 53–72% [2]. Although bio-oil exhibits physical properties similar to those of crude petroleum oil, its poor chemical properties (low heating value, high acidity, and high water content) limits its application as an energy source [3]. Furthermore, bio-oil contains labile chemical components with a high degree of oxygen functionalities and therefore requires chemical processes, including hydrotreatment [4] and water-soluble phase separation [5], before converting into chemicals and fuels.Hydrodeoxygenation (HDO) has been performed to valorize the bio-oil into a petroleum-like liquid. Noble metal catalysts, such as ruthenium (Ru) [6] and rhodium (Rh) [7], have been frequently used for hydrodeoxygenation. However, many transition metal catalysts, such as nickel (Ni) [8], molybdenum (Mo) [9], and Ni-Ru bimetals [10], modified using various promoters and supports have been developed for the inexpensive HDO reactions of bio-oil. Amongst the base transition metals, supported nickel (Ni) catalysts have potential as HDO catalysts because of the high hydrogenation activity of Ni [8]. However, the low affinity of Ni toward oxygen functionalities of lignin-derived oxygenates (phenolic monomers) suppresses the complete deoxygenation on Ni catalysts. Therefore, titania (TiO2)-supported Ni catalysts have been suggested for improving the poor deoxygenation activity of Ni by providing strong metal support interactions on the Ni–titanium (Ti) interfacial sites [11].Supported phosphor (P)-modified transition metal, including Ni, molybdenum (Mo), tungsten (W), and iron (Fe) [12], catalysts have displayed high activity in HDO reactions of biomass-derived oxygenates [13] and bio-oil [14] because of their high thermal stability and synergistic effects between Brønsted and Lewis acid sites [15]. However, HDO using supported phosphor-modified transition metal catalysts has been limited to the conversion of simple model compounds, such as alkyl esters [16], furfural [17], phenol [18], and guaiacol [19] (Table 1 ), and their application in the HDO of bio-oil has not been well discussed. In addition, it is challenging to extend the results of such model compound studies to the complex mixture of actual bio-oil. Therefore, to use phosphor-modified transition metal catalysts practically for upgrading bio-oil, it is necessary to use catalysts for the HDO of actual bio-oil and analyze the process information for determining optimum conditions.The objectives of this study are: (i) to successfully operate the HDO of bio-oil and its model compounds using phosphor-modified transition metal catalysts and (ii) to determine the optimum conditions for catalyst preparation and process operation. The lignin-derived model compounds and actual bio-oil were converted using several TiO2-supported phosphor-modified transition metal catalysts, and the optimum catalysts were selected based on their catalytic activity. Hydrothermal methods were employed for improving the interaction between transition metals and the TiO2 support. Metal nitrate salts of Ni, cobalt (Co), copper (Cu), and Fe were used as precursors of transition metals, which were mixed with a phosphor source of diammonium hydrogen phosphate ((NH4)2HPO4). The effects of metal types and phosphor doping on the HDO activity were investigated. The active sites of the catalysts were elucidated to understand the HDO of lignin-derived oxygenates and actual bio-oil. Therefore, the successful HDO of complex bio-oil using phosphor-modified transition metal catalysts can replace the HDO processes using noble metals and help reduce the production cost of biomass to fuels.Catalyst preparation, reaction procedure, chemical analysis, and catalyst characterization techniques used in this study have been described to provide insights to the preparation of deoxygenated fuels.All chemicals were used without further purification. Nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O, 99.999%), copper(II) nitrate hemipentahydrate (Cu(NO3)2·2.5H2O, 99.99%), cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O, 99.999%), iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, ≥99.95%), titanium(IV) oxide (TiO2, P25, ≥99.95%), 2-methoxy phenol (guaiacol, C7H8O2, 99%), 3-methyl phenol (m-cresol, C7H8O, 98%), 2-methoxy-4-ethylphenol (ethyl guaiacol, C9H12O2, 98%), 2-methoxy-4-propylphenol (propylguaiacol, C10H14O2, 99%), 2-methoxy-4-(2-propenyl)phenol (eugenol, C10H12O2, 98%), n-decane (C10H22), and n-dodecane (C12H26) were purchased from Sigma-Aldrich (St. Louis, Missouri, USA). Diammonium hydrogen phosphate ((NH4)2HPO4, 99%) and ammonium bicarbonate (NH4HCO3) were purchased from Daejung Chemicals and Metals (Siheung, Gyeonggi-do, Korea). 2-Methoxy-4-methylphenol (methyl guaiacol, C8H10O2, 98%) was purchased from Alfa Aesar (Haverhill, Massachusetts, USA). Hydrogen gas (H2, 99.999%), nitrogen gas (N2, 99.9%), H2 mixed with argon (5% v/v H2/Ar), oxygen gas mixed with N2 (0.5% v/v O2/N2), and carbon monoxide mixed with helium (10% v/v CO/He) were purchased from Shinyang Medicine (Seoul, Korea). The bio-oil, which was prepared as a liquid condensate of the fast pyrolysis of the pellet residue from pinewood, was purchased from Biomass Technology Group (BTG) BV (Enschede, Overijssel, The Netherlands). Deionized (DI) water was obtained from a purification system (EXL® 7S Analysis Water, Vivagen Co., Ltd., Seongnam, Gyeonggi-do, Korea) equipped with a 0.22 µm filter.A hydrothermal method was used to prepare TiO2-supported phosphor-modified transition metal catalysts. For the preparation of 20 wt% Ni and 5 wt% P on a TiO2 support, denoted as Ni–P/TiO2, Ni(NO3)2·6H2O (1.65 g) and (NH4)2HPO4 (0.19 g) were dissolved in DI water (20 mL) and placed in a 100 mL Teflon-lined chamber. Subsequently, TiO2 (P25, 1.50 g) was added to the prepared solution, which was ultrasonicated for 30 min to obtain a well-dispersed suspension. An aqueous solution of NH4HCO3 (1 M, 30 mL) was added dropwise to the solution and simultaneously stirred at room temperature. The Teflon-lined chamber was closed and heated to 150 °C for 6 h. The mixture was cooled to room temperature, and the prepared powder was recovered through vacuum filtration. The powder was washed with DI water to reach a neutral pH and further washed three times with ethanol (50 mL each). The washed powder was calcined in a N2 flow at 500 °C for 2 h and reduced in a H2/Ar flow (5% v/v) at 450 °C for 4 h. Additionally, Cu–P/TiO2, Co–P/TiO2, and Fe–P/TiO2, containing 20 wt% of the corresponding transition metal and 5 wt% P, were prepared using a procedure similar to that used to prepare Ni–P/TiO2. Cu(NO3)2·2.5H2O, Co(NO3)2·6H2O, and Fe(NO3)3·9H2O were used as precursors of the corresponding transition metal catalysts.X-ray diffraction (XRD) results of the catalysts were obtained using a Dmax2500/PC diffractometer (Rigaku, Tokyo, Japan) equipped with a scintillation counter with a graphite monochromatic detector and an average Cu Kαave radiation generated at 40 kV and 200 mA. The particle size (d XRD) was calculated using the Scherrer equation (Equation (1)). (1) d X R D = K λ B c o s θ where d XRD (nm) is the crystal domain size obtained using selected diffraction (hkl); K is the Scherrer constant (0.94 for spherical crystals with a cubic symmetry); λ is the wavelength (0.15418 nm for Cu Kαave); B is the modified full width at a half maximum (FWHM) of the diffraction peak calculated as (FWHM)2 − (FWHM of the bulk crystal, 0.2° in this study); θ is the Bragg angle of the selected diffraction (hkl). The morphology of the catalysts was observed through field-emission scanning electron microscopy (FE-SEM) with energy dispersive X-ray spectroscopy (FE-SEM/EDS, Teneo VS, Thermo Fisher Scientific, Waltham, Massachusetts, USA). The atomic structure and elemental distribution of the catalysts were investigated using high-resolution transmission electron microscopy (TEM, Talos F200X, Thermo Fisher Scientific, Waltham, Massachusetts, USA). The electronic structures of the catalysts were observed through high-performance X-ray photoelectron spectroscopy (XPS, ESCALAB 250 spectrometer, VG Scientific, Thermo Fisher Scientific, Waltham, Massachusetts, USA).The HDO activity of the catalysts was measured using a custom-built SUS 316 batch reactor [21]. For a typical HDO reaction with a model compound, catalyst (0.12 g), n-decane solvent (50 mL), and a reactant mixture of phenolic monomers (1.2 g) were placed into the reactor. The inner reaction system was purged three times with N2 and filled with 4 MPa H2 at room temperature. The HDO reaction was performed under three different conditions. After the reaction was completed, the reactor system was cooled to 50 °C or lower temperature using a cold-water coolant. The liquid product was the mixed with n-dodecane, an internal standard, and further diluted with methanol for gas chromatography (GC) measurements. The catalytic activity was illustrated by calculating conversion (Xfeed, %), product yield (Yproduct, %), and selectivity (Sproduct, %). The parameters were determined as follows: (2) X f e e d % = 1 - n f e e d n f e e d 0 × 100 (3) Y p r o d u c t % = n p r o d u c t n f e e d 0 × 100 (4) S p r o d u c t % = n p r o d u c t ∑ n p r o d u c t × 100 where nfeed is the moles of the remaining reactant; n0 feed is the initial moles of reactant; nproduct is the moles of product; Σnproduc t is the moles of all identified products. A two-step reaction was performed for the HDO of bio-oil reactant. In the first step, bio-oil (2 g) and n-decane (50 mL) were placed in the reactor without catalyst and heated to 300 °C under H2 atmosphere at 4 MPa (measured at room temperature) for 3 h. The product was collected, and the liquid phase was vacuum filtered from the solid residue (heavy oil). The obtained liquid filtrate is denoted as BO-S1. BO-S1 was mixed with a fresh catalyst powder (0.5 g) in the reactor and heated at 300 °C and a pressure of 4 MPa (measured at room temperature) under H2 atmosphere for 3 h. After the reaction was completed, the liquid product was vacuum filtered. The obtained liquid filtrate is denoted as BO-S2. The liquid products were identified through GC–mass spectrometry (GC–MS, Agilent 7890A with 5975C inert MS XLD with triple axis-detector, Agilent Technologies, Santa Clara, California, USA) equipped with an HP–5MS capillary column (60 m × 0.25 µm × 0.25 mm). The liquid products were also quantified using a GC-flame ionization detector (FID) (Hewlett 5890 Packard Series II) equipped with an HP-5MS capillary column (60 m × 0.25 µm × 0.25 mm). The spent catalyst powder was collected from the remaining liquid product via vacuum filtration and washed three times using acetone (25 mL). The obtained solid product was dried in air at 105 °C for 16 h. The dry solid product is referred as the spent catalyst.The observed results are described in this section and further discussed to demonstrate the effect of Ni–P/TiO2 for improved HDO.According to the manufacturer, the commercially available bio-oil from BTG contained a high oxygen content (O/C = 0.74 atom/atom and H/C = 1.8 atom/atom), leading to high acidity (pH = 2.42) as reported in the literature [22]. The organic compounds present in the bio-oil were detected using GC–MS. A considerable number of compounds, including carboxylic acids, esters, aldehydes, ketones, alcohols, and phenols, were observed along with sugars and minuscule amounts of other hydrocarbons (Table 2 and Fig. 1 ). Acetic acid, acetic acid methyl ester, 1-hydroxyl-2-propanone, ethanol, guaiacol, 2-methoxy-4-methyl-phenol (4-methyl guaiacol), and D-allose were the most abundant compounds in the bio-oil based on the GC–MS peak areas. Because the presence of molecules with carbonyls can produce high molecular weight polymers by condensing small molecules [23], the stabilization or promotion of bio-oil via hydrotreatment to reduce carbonyls can produce the highly stable bio-oil; however, complete deoxygenation was not easily achieved [1,8,10].The surface structures of the catalysts can highly manipulate the catalytic activity. Therefore, the crystal structures of the prepared TiO2-supported phosphor-modified transition metal catalysts (Ni–P/TiO2, Co–P/TiO2, Cu–P/TiO2, and Fe–P/TiO2) were observed through powder XRD to elucidate the modification of metal structures by adding phosphor (Fig. 2 ). For all catalysts, a mixture of anatase and rutile TiO2 phases was observed for the TiO2 support, exhibiting strong diffraction peaks at 2θ = 25.2° and 27.4°, respectively. The addition of P (without transition metal) to TiO2 did not significantly alter the TiO2 crystal structure (Fig. 2(i-v)). The mixtures of metals and metal oxides were observed for phosphor-added Ni, Co, Cu, and Fe. However, weak diffractions of metal phosphide were observed only for Ni. Distinct diffraction peaks of Ni phosphide (Ni3P) were observed at 2θ = 41.7°, 43.6°, and 46.6°, corresponding to the (231), (112), and (141) planes of Ni3P (PDF#34–0501), respectively, when Ni precursor was mixed with the phosphor precursor and TiO2 (Fig. 2(A-i and B)). The reaction between NiO and PH3, which was prepared via the thermal decomposition of (NH4)2HPO4, formed Ni3P [24]. Phosphides of Co and Cu were not observed on Co–P/TiO2 and Cu–P/TiO2, respectively, indicating the poor reaction of Cu and Co with PH3. The XRD results also confirmed the higher dispersion of metal particles for Ni–P/TiO2 and Co–P/TiO2. Based on the crystal size (d XRD) of Ni and Co metal particles (excluding their oxides and phosphides) calculated using the Scherrer equation, Ni–P/TiO2 and Co–P/TiO2 demonstrated small metal particles with a size of 17.0 and 21.1 nm, respectively. However, Cu–P/TiO2 exhibited sharp XRD peaks of Cu, indicating agglomeration of Cu particles during the high-temperature preparation (Table 3 ). Notably, the calculations using Scherrer equation considering the broad Ni3P(112) diffraction peak also suggested the existence of small particles with a diameter of 11.4 nm, indicating the formation of small particles in the range of 11.4–17.0 nm because of the formation of Ni metal and Ni3P domains.The structures of the catalysts were investigated through TEM (Fig. 3 ). For Ni–P/TiO2 (Fig. 3(A)), Ni nanoparticles with an average diameter of 20 nm (Fig. 3(A1)) and an irregular shape were dispersed on the TiO2 support. As observed in the XRD results, Ni–P/TiO2 exhibited the co-existence of Ni and P (Fig. 3(A1–A3)). The boundaries between the Ni phosphide particles and the TiO2 support were also observed, confirming the formation of isolated Ni phosphide particles. The high-resolution TEM images demonstrated d-spacings of 0.210, 0.195, and 0.242 nm, which were attributed to the Ni3P phase (Fig. 3(A5–A7). Significantly, Ni/TiO2 without phosphor formed isolated Ni particles on TiO2 (Fig. 3(D)). However, other catalysts prepared in this study did not form the corresponding metal phosphides compared with Ni–P/TiO2. Highly dispersed non-metallic Co and P were observed for Co–P/TiO2 in the EDS results, suggesting the less agglomeration of Co and P on the TiO2 support. Furthermore, Co, P, and Ti were highly dispersed without clear segregation, indicating the possible formation of the Co–P–Ti mixed oxide, although its formation was not confirmed from the XRD results. Large Cu particles were observed on the TiO2 support for Cu–P/TiO2 (Fig. 3(C)), and P was concentrated on the Cu particles. The P atoms isolated on the surface of Cu particles indicated the segregation of Cu and P.The surface chemical oxidation states of catalysts were observed through XPS, and the electron transfer from Ni to P that formed slightly cationic Ni0 was elucidated (Fig. 4 ). The wide survey of XPS exhibited binding energy peaks of P 2s, P 2p, C 1s, Ti 2p, and O 1s (Fig. 4(A)). In addition, binding energy peaks were observed at 705–950 eV, confirming the presence of transition metals (Fig. 4(i–iv)). The peaks of Ni0 and Ni2+ were observed for Ni–P/TiO2 at 853.3 and 857.0 eV, respectively. The Ni0 peak of Ni–P/TiO2 shifted to the higher binding energy by 0.5 eV compared with that of Ni/TiO2 (852.8 eV Ni 2p for Ni/TiO2, Fig. 4(B)(iv)). The negatively charged Pδ− correlated with the slight cationic shift of the Ni0 peak, which was attributed to the electron transfer from Ni to P, confirming the formation of Ni3P on silica [25] or without support [26]. The formation of slightly cationic Ni0 led to the Lewis acidity of Ni0, improving the interaction between Ni0 and oxygen atoms of reactants. The peaks of Co0 and Co2+ were observed for Co–P/TiO2 at 778 and 782 eV, respectively. Both metal and metal oxide peaks were observed for Ni–P/TiO2 and Co–P/TiO2. Furthermore, the satellite peaks of Co 2p and Ni 2p were observed at 785 and 864 eV for Co–P/TiO2 and Ni–P/TiO2, respectively, exhibiting their multiple oxidation states. In contrast, Cu–P/TiO2 and Fe–P/TiO2 exhibited less complex oxidation states. Reduced metallic Cu0 species were observed for Cu–P/TiO2 with a strong peak at 932 eV, confirming the formation of large Cu particles observed in the TEM image (Fig. 3(C)). Furthermore, the absence of Cu2+, Cu+, and satellite peaks suggested the absence of Cu oxides and other Cu species. Although the peaks of Fe2+ and Fe3+ were observed for Fe–P/TiO2, the Fe0 peak was absent. Further examination of phosphor at P 2p exhibited peaks at 137–131 eV for the P–O bonds and approximately 129 eV for the negatively charged Pδ− for NiP catalysts on mesoporous silica [27] or without support [26]. The strong peaks for Pδ− species observed for Ni–P/TiO2 at 129.1 eV confirmed a strong electron transfer from Ni0 to P. Additionally, a weak peak of Pδ− was observed for Co–P/TiO2. However, Pδ− was not observed for Fe–P/TiO2, Cu–P/TiO2, and P/TiO2 (Fig. 4(C)).The HDO of lignin-derived phenolic monomers as model compounds was investigated using TiO2-supported phosphor-modified transition metal catalysts. The catalytic activity was measured, and the process for upgrading bio-oil was suggested. Mixtures of alkyl-methoxyphenols (AMPs) containing different amounts of guaiacol, m-cresol, methyl guaiacol, ethyl guaiacol, propyl guaiacol, and eugenol were used as reactants. As illustrated in Fig. 5 (A), the conversion of AMPs was negligible at 300 °C under an initial H2 pressure of 4 MPa for 2 h in the absence of P/TiO2. The products obtained without catalysts and using metal-free P/TiO2 were almost identical with the mixture of reactants (Fig. 5(A-i, ii, and iii)), indicating their negligible HDO activity. Before conducting HDO using metal phosphide catalysts, HDO using Ni/TiO2 and commercially available Ru/C catalysts was performed. For Ni/TiO2, the conversion of phenolic compounds yielded saturated hydrocarbons, including cyclohexyl ketones or alcohols (42%) and cycloalkanes (57%), suggesting the hydrogenation of aromatic compounds via the HDO reaction using Ni/TiO2. However, the conversion of phenolic compounds was incomplete for Ru/C, producing cyclic ketones and cyclic alcohols (43% combined yield). Additionally, several unknown compounds were observed.Based on the observed complete conversion of phenolic compounds on Ni/TiO2, the phosphor-modification of metals on TiO2 was attempted. Although HDO using Fe–P/TiO2 formed methoxybenzenes and diols, significant hydrogenation did not occur (Fig. 5(B-i and D) and 6(A)). The corresponding products are illustrated in Fig. 5(C-i, ii, and iii) and 6(A) for the reaction using Ni–P/TiO2, Co–P/TiO2, and Cu–P/TiO2 catalysts. The complete HDO of the phenolic mixture was observed for Ni–P/TiO2, which selectively produced cyclic alkanes (87% yield). These observations indicate that multifunctional catalysts containing both metals and acid sites are essential for improving HDO with hydrogenation and deoxygenation [7]. Notably, Cu–P/TiO2 and Co–P/TiO2 exhibited lower AMP conversions (40.5 and 69.5%, respectively) and cycloalkane yields (3.8 and 30.0%, respectively) compared to those of Ni–P/TiO2 (100 and 87% AMP conversion and cycloalkane yield, respectively).The catalytic activity of Ni–P/TiO2 was adjusted by manipulating the reaction temperature (Fig. 6 (C)). Both AMP conversion and cycloalkane yields increased from 90 to 100% and 45 to 87%, respectively. The product distribution at 300 °C included a small number of light compounds, including CH4, indicating that the cracking occurred at 300 °C or higher temperatures. Based on these observations, HDO at 300 °C is optimum to suppress further cracking at a higher temperature.The stability of Ni–P/TiO2 was observed for different reaction times from 0 to 3 h (Fig. 6(B)). The yield of cycloalkanes increased to reach the plateau of 87% when the reaction time was increased up to 3 h. The recyclability of Ni–P/TiO2 was observed by repeatedly performing the HDO of AMP at 300 °C and 4 MPa H2 (measured at room temperature) for 2 h (Fig. 6(D)). The AMP conversion and cycloalkane yield gradually decreased, which could be attributed to the deactivation of the catalyst.The high HDO activity of Ni–P/TiO2 can be attributed to the highly dispersed Ni particles of Ni–P/TiO2. Moreover, modifying Ni particles with phosphor to form Ni3P particles can improve HDO activity. The negatively charged Pδ− affects the slightly cationic Ni0, which can be an active site for the complete HDO of phenolic compounds. Furthermore, the slightly cationic Ni0 increases the Lewis acidity of the catalyst surface, which improves the hydrodeoxygenation at the catalyst surface, as observed in the previous study (Fig. 7 ) [7].Based on the catalyst characterization results and optimized conditions of Ni–P/TiO2, the HDO of AMP was suggested to proceed by the adsorption of aromatic rings on the Ni surface and was promoted by the presence of cationic Niδ+ The HDO of the AMP molecule may have proceeded by the dehydration reaction because of the phosphor sites adjacent to Ni, which act as Brønsted acid sites to remove oxygen atoms in the AMP compounds.The HDO of actual bio-oil was performed using Ni–P/TiO2 at 300 °C and 4 MPa H2 (measured at room temperature) based on the observed HDO results of phenolic mixtures. A two-step HDO was performed (each step for 3 h) to achieve better HDO results because of the complex nature of the bio-oil feedstock. The first step was the thermal pretreatment of the bio-oil dissolved in n-decane without catalyst, while the second step was the HDO of bio-oil using Ni–P/TiO2. A transparent liquid was obtained from the two-step HDO reaction: gasoline (C6–C9: n-hexane (10)), methyl cyclopentane, cyclohexane, methyl cyclohexane, ethyl cyclohexane (11), propyl cyclohexane (12), kerosene-like (C10–C13: 1,1′-methylenebiscyclohexane (13)), and diesel-fractions (C14–C17: 1,1′-ethylenebiscyclohexane (14) and 1-(cyclohexylmethyl)-4-isopropylcyclohexane (15)) were observed in the liquid product (Fig. 8(A-i) and Table 4 ). The formation of dimeric compounds (13, 14, and 15) using Ni–P/TiO2 was distinct for this process, indicating the successful production of high carbon number hydrocarbon fuels via condensation between phenolic compounds observed in BO-S1 (Fig. 8 (A-iii)). For Ni–P/TiO2, the oil yield was 85%, leading to 37.4% of total cycloalkane yield containing 37 and 7% monocycloalkane and dicycloalkane selectivities, respectively (Fig. 8(C)).Compounds with lower molecular weights, including carboxylic acids, esters, aldehydes, ketones, and alcohols derived from cellulose and hemicellulose, were not significantly observed after the HDO. This observation can be attributed to the conversion of the small molecules to deoxygenated light hydrocarbons. These light hydrocarbons may not be condensed at the condenser of the reactor and thereby are not observed in the GC results of the liquid products.Compared to the products obtained using Ni–P/TiO2, the liquid product after the first step without catalyst contained (holo)cellulose-derived small oxygenates (acetone (1) and ethyl acetate (2)) and lignin-derived phenolic monomers (guaiacol (4), methyl guaiacol (5), ethyl guaiacol (6), 4-allyl-guaiacol (7), isoeugenol (8), and 4-allyl-syringol (9)) (Fig. 8(A-ii)). These observations indicate that the first step reaction removed or cracked the high carbon number products present in the bio-oil (Fig. 1). Compounds with high molecular weights, including the GC-undetectable molecules, can be cracked during the first step reaction, thereby improving the HDO activity at the second step. Because the compounds described in Fig. 8(A-ii) are close to the lignin-derived phenolic molecules, further HDO at the second step using Ni–P/TiO2 is essential. A close examination of the GC peak at 11.30–11.40 min indicated that the predominant guaiacol monomer disappeared when the two-step HDO using Ni–P/TiO2 was performed (Fig. 8(B-iii)), while the first step reaction with or without the catalyst resulted in the conversion of guaiacol (Fig. 8(B-ii and iii)).The HDO of phenolic monomers as a model mixture of lignin-derived compounds was performed and the successful deoxygenation and hydrogenation were achieved using Ni–P/TiO2. Based on these observations, the HDO of the bio-oil diluted in n-decane was also attempted. Complete deoxygenation was observed with a distinct production of high carbon number hydrocarbons formed by condensing phenolic monomers. The titania-supported nickel phosphide catalyst produced completely saturated deoxygenated compounds with an 87% yield of cycloalkanes at 300 °C and 4 MPa H2 (measured at room temperature) for a reaction time of 2 h. Upgrading bio-oil using titania-supported nickel phosphide catalyst afforded a 37.4% yield of deoxygenated hydrocarbons. Although transition metals supported on TiO2 exhibited high catalytic hydrogenation activity, adding phosphor to Ni/TiO2 improved the HDO activity and induced the production of deoxygenated cyclic alkanes. The formation of Ni nanoparticles modified with highly dispersed phosphor atoms was also observed. In addition, the electron transfer from Ni0 to P formed slightly cationic Ni0. The strong interaction between the cationic Ni0 and oxygen atoms of the reactant could improve HDO. Therefore, the findings of this study provide insights for promoting the efficient conversion of lignocellulose into chemicals and fuels using non-precious transition metal catalysts. Rizki Insyani: Investigation, Writing – original draft. Jae-Wook Choi: Methodology, Investigation. Chun-Jae Yoo: Methodology, Investigation. Dong Jin Suh: Conceptualization. Hyunjoo Lee: Methodology, Investigation. Kyeongsu Kim: Data curation. Chang Soo Kim: Methodology, Investigation. Kwang Ho Kim: Methodology, Investigation. Jeong-Myeong Ha: Conceptualization, Writing – review & editing, Supervision.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research was supported by the Technology Development Program to Solve Climate Change of the National Research Foundation (NRF) funded by the Ministry of Science and ICT (NRF-2020M1A2A2079798).

Biomass pyrolysis oil is a potentially essential renewable energy source that can serve as an alternative to petroleum-based fuels and chemicals. In this study, biomass pyrolysis oil was converted into petroleum-like deoxygenated hydrocarbons via catalytic hydrodeoxygenation using a titania-supported nickel phosphide catalyst. The phosphor precursor was added to several transition metals, including nickel, cobalt, copper, and iron, supported on titania. The formation of isolated nickel phosphide particles, which were active for complete hydrodeoxygenation, was confirmed by the characterization of prepared catalysts. As a model reactant of biomass pyrolysis oil, a mixture of alkyl-methoxyphenol compounds was hydrodeoxygenated to produce completely deoxygenated compounds, generating an 87% yield of cycloalkanes at 300 °C and 4 MPa H2 for a reaction time of 2 h. The hydrodeoxygenation of biomass pyrolysis oil also generated a 37.4% yield of hydrocarbon fuels. The high hydrodeoxygenation activity can be attributed to the synergy between the hydrogenating metals and the acid sites, which can be improved by electron transfer from a slightly cationic nickel to a slightly anionic phosphor. Furthermore, the addition of phosphor improved the formation of highly dispersed nickel particles, increasing the quantity of hydrogen-adsorbing surface metals. The observations in this study indicate that the efficient conversion of lignocellulose-derivatives into chemicals and fuels can be achieved using modified non-precious transition metal catalysts.

All data supporting this study are available in the article and supplemental information. Any additional requests for data will be handled by the lead contact upon reasonable request.As an important chemical, hydrogen peroxide (H2O2) has been widely applied in a series of processes including disinfection, wastewater purification, and oxidation reaction. 1 , 2 It has been demonstrated that H2O2 serves as an effective and environmentally benign oxidant in propylene epoxidation 3 and cyclohexanone ammoximation because water serves as the only byproduct. 4 In addition, in situ generation of H2O2 in tandem reactions for selective oxidation has recently attracted increasing attention in several challenging reactions, for example methane conversion to methanol 5 ; hydroxylation of benzene to phenol 6 , 7 ; selective oxidation of benzyl alcohol 8 ; selective oxidation of cyclohexane 9 ; and oxidation of propylene to propane oxide. 10 , 11 The current industrial-scale production of H2O2 proceeds primarily through the anthraquinone process, which involves sequential hydrogenation and oxidation. 12 This process is energy intensive and environmentally unfriendly due to the substantial content of waste chemicals. Therefore, the direct H2O2 synthesis from H2 and O2 is highly desirable and has attracted considerable attention given the advantages of environmental benignity and atomic economy. 13 , 14 , 15 , 16 However, it remains a challenge due to several critical issues, especially the H2O2 degradation, in the direct H2O2 synthesis from H2 and O2. 1 , 17 , 18 Hutchings and co-workers 19 first reported that the incorporation of Pd into a supported gold (Au) catalyst could improve the direct H2O2 synthesis from H2 and O2 and confirmed the detrimental effect of H2O2 degradation on the synthesis process. It is thus crucial to improve H2O2 productivity and selectivity by inhibiting the reactivity of H2O2 degradation. Several strategies, for example optimization of reaction conditions 20 , 21 ; introduction of halogen ions and acids 22 , 23 ; modulation of metal-support interactions 24 ; development of bimetallic 2 , 25 , 26 (e.g., Pd-Au, Pd-Ag, and Pd-Zn) and multi-metallic 18 , 27 (e.g., Pt-Pd-Au) components; and modification of supports, 28 , 29 have been explored. A supported Au-Pd catalyst prepared by Edwards et al. via the acid pretreatment on carbon support could prevent the H2O2 hydrogenation due to a size decrease of alloy nanoparticles (NPs), giving high H2O2 productivity of 175 mol kgcat −1 h−1 with H2O2 selectivity of higher than 95%. 28 Choudhary and co-workers found that adding chloride or bromide anions to an acidic aqueous medium could enhance the H2O2 selectivity and yield as a result of the significant decrease in H2O2 hydrogenation and decomposition activities. 30 Considering the higher safety and easier availability of CO compared with H2, the direct H2O2 synthesis from CO, O2, and H2O may turn out to be highly attractive and economic. However, this process is found to be much lower in productivity compared with the direct H2O2 synthesis from H2 and O2. Brill et al. reported for the first time that the H2O2 productivity over Pd/CaCO3 and Ru/graphite catalysts was only 0.1 molH2O2 kgcat −1 h−1 toward the H2O2 synthesis from CO, O2, and H2O, 31 , 32 which is about 3 orders of magnitude lower than that of the H2O2 synthesis from H2 and O2 on Pd-based catalysts. 24 Other catalysts, like Ni-La-B/Al2O3, 31 , 33 Cu/A12O3, 34 and Ni-P-B/A12O3, 35 have been explored to improve the H2O2 productivity to 0.326 molH2O2 kgcat −1 h−1. Recently, Cao and co-workers reported a highly efficient H2O2 productivity of ∼74.6 molH2O2 kgcat −1 h−1 (9,097 mmolH2O2 gAu −1 h−1) employing anatase supported an Au catalyst in the solvent consisting of H2O and acetone (1:1). 36 This impressive work, together with the results of Ma and co-workers, 37 , 38 demonstrated that supported Au catalysts can potentially achieve high productivity of H2O2 from CO, O2, and H2O. However, phosphorous acid or acetone was used with H2O as co-solvent in these studies, 31 , 36 which may lead to serious separation problems.Therefore, it is imperative to further improve H2O2 productivity by designing efficient catalysts and circumventing the use of environmentally unfriendly co-solvents in H2O2 synthesis from CO, O2, and H2O. In this work, we report a tetraethyl orthosilicate (TEOS)-modified ZnO supported Au catalyst (Au/ZnO-TEOS) in which Au NPs are partially encapsulated by an SiO2 overlayer. This catalyst is highly efficient for H2O2 synthesis from CO, O2, and H2O, and H2O2 productivity of 168 molH2O2 kgcat −1 h−1 is achieved only using H2O as the reaction solvent. Detailed characterizations including aberration-corrected scanning transmission electron microscope with high-angle annular dark-field detector (AC-HAADF-STEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FT-IR) revealed the importance of structural confinement of the SiO2 overlayer on the Au/ZnO-TEOS catalyst to restrict the agglomeration of Au NPs, inhibit the degradation of H2O2, and improve the hydrophilicity of the catalyst surface in the Au/ZnO-TEOS catalyst, thus significantly enhancing the catalytic performance. A facile reaction mechanism is also suggested according to the isotopic labeling experiments and theoretical calculations. This work not only provides an efficient approach to construct a highly active supported Au catalyst toward the direct H2O2 synthesis from CO, O2, and H2O but also highlights the importance of the confinement effect in supported catalysts leading to several concurrent positive effects.The catalysts of Au/ZnO (without TEOS modification) and Au/ZnO-TEOS (with TEOS modification) series were prepared by a co-precipitation method and calcined at 200°C, 400°C, and 600°C, respectively, and which are designated as Au/ZnO-T or Au/ZnO-TEOS-T (T is the calcination temperature). The activity of direct H2O2 synthesis from CO, O2, and H2O was evaluated under the optimized conditions of 60°C, 5 mg catalyst, 10 mL H2O, 4 MPa with 75% O2:5% CO:20% He feed gas, 5 min reaction time, and 1,200 RPM stirring speed. It should be noted that both H2O2 productivity and selectivity usually need to be reported to evaluate the performance of the direct H2O2 synthesis, while in this work, the CO conversion was too low (<3%) to be accurately measured by conventional gas chromatography, leading to the poor reproducibility in H2O2 selectivity (see supplemental experimental procedures 2.3 for the associated details and discussion on H2O2 selectivity measurement). 31 , 34 , 35 As a result, only the data of H2O2 productivity are reported and compared in the batch autoclave reactor with 10 mL H2O. ZnO support itself was inactive in the direct H2O2 synthesis. Interestingly, it is evident from Figure 1 A that the H2O2 productivity of Au/ZnO-TEOS catalysts is remarkably higher than that of Au/ZnO catalysts under the same calcination temperature. Notably, the H2O2 productivity of 168 mol kgcat −1 h−1 on Au/ZnO-TEOS-200 is achieved in the additive-free conditions, which is more than twice the current highest value obtained using the Au/anatase catalyst under the conditions of 1:1H2O and acetone as solvents by Cao and co-workers in the direct H2O2 synthesis from CO, O2, and H2O. 36 In addition, Figure 1A shows that H2O2 productivity of the Au/ZnO series significantly decreases from 84 to 17 mol kgcat −1 h−1 as the calcination temperature increases from 200°C to 600°C. Meanwhile, Au/ZnO-TEOS series after TEOS modification bears resemblance to the Au/ZnO series in the decline tendency of the catalytic performance with calcination temperature. This can be attributed to the size variation of Au NPs with calcination temperature. 39 , 40 , 41 , 42 AC-HAADF-STEM was performed to analyze the Au NP size distribution of the Au/ZnO and Au/ZnO-TEOS series (Figures 1B–1G). The mean sizes of Au NPs in Au/ZnO-200 and Au/ZnO-TEOS-200 catalysts are nearly identical at 1.8 and 1.7 nm, respectively. This indicates that TEOS modification imposes a negligible effect on the mean size of the Au NPs after calcination at 200°C (Figures 1B and 1E). As the calcination temperature elevates from 200°C to 600°C, the mean size of Au NPs in both Au/ZnO and Au/ZnO-TEOS series gradually increases. Combined with the aforementioned catalytic performance, AC-HAADF-STEM characterizations reveal obvious particle size effects of Au/ZnO and Au/ZnO-TEOS series, i.e., the larger the Au NP size, the poorer the H2O2 productivity. It is worth mentioning that the mean size of Au NPs of the Au/ZnO-TEOS series catalysts only increases to 4.7 nm, much smaller than that of the Au/ZnO series (∼12.9 nm) after calcination at 600°C, demonstrating that the TEOS modification could effectively restrain the agglomeration of Au NPs.Comparing the performance of the catalysts with similar Au particle sizes, such as Au/ZnO-200 and Au/ZnO-TEOS-200 (1.8 versus 1.7 nm) as well as Au/ZnO-400 and Au/ZnO-TEOS-600 (4.2 versus 4.7 nm), the H2O2 productivity of the TEOS-modified catalyst is about twice that of the non-TEOS-modified catalyst. Meanwhile, the Au loadings of Au/ZnO-200 and Au/ZnO-TEOS-200 catalysts are also close (1.11 versus 1 wt %) as determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) (Table S1). As a result, the difference in the H2O2 productivity between them cannot be rationalized solely by the effects of Au particle size and loading content. Other critical roles of TEOS modification could be expected.To further clarify the role of TEOS modification on the improvement of the H2O2 productivity, we comprehensively compared the microstructures of Au/ZnO-200 and Au/ZnO-TEOS-200 catalysts using energy-dispersive X-ray (EDX), FT-IR, and AC-HAADF-STEM. EDX measurements (Figure S1) reveal a highly dispersed Si species on the surface of the Au/ZnO-TEOS-200 catalyst. IR measurements verify the existence of SiO2 on the Au/ZnO-TEOS-200 catalyst as a result of the detected peak at 1,088 cm−1, which is assigned to the antisymmetric stretching vibrations of Si-O-Si (Figure S2). 43 , 44 The presence of defined edge and lattice fringes of Au NPs on the Au/ZnO-200 catalyst was testified to by AC-HAADF-STEM characterizations (Figure 2 A). In contrast, the lattice fringes of Au NPs on the Au/ZnO-TEOS-200 catalyst become fuzzy (Figures 2B–2D). Despite the presence of fully encapsulated Au NPs by the SiO2 overlayer (Figure 2B), partially encapsulated Au NPs by the SiO2 overlayer can be identified on the Au/ZnO-TEOS-200 catalyst (Figures 2C and 2D; more AC-HAADF-STEM images of the Au/ZnO-TEOS-200 catalyst are presented in Figure S3). AC-HAADF-STEM characterizations clearly demonstrate that TEOS modification results in Au NPs encapsulated by an SiO2 overlayer, diminishing the exposure of the Au-ZnO interface.It is widely recognized that H2O2 degradation can significantly affect H2O2 productivity. 13 , 17 , 24 , 28 , 45 H2O2 degradation experiments were thus performed to examine whether the TEOS modification influences H2O2 degradation activity of the supported Au catalysts. As shown in Figure 2E, the H2O2 degradation rate is 8.6 mol kgcat −1 h−1 on the Au/ZnO-TEOS-200 catalyst, which is only about 1/9 of the value of the Au/ZnO-200 catalyst (78.9 mol kgcat −1 h−1), unambiguously demonstrating that the H2O2 degradation rate was remarkably suppressed by TEOS modification. This suggests that the inhibition of H2O2 degradation activity is indeed critical to enhance the H2O2 productivity on the Au/ZnO-TEOS-200 catalyst. Considering the structural characteristics of the Au/ZnO-TEOS-200 catalyst, it is reasonable to speculate that the Au-ZnO interface and/or the exposed Au NPs may be the active site for H2O2 degradation. In order to identify the origin of the suppression activity of H2O2 degradation over the Au/ZnO-TEOS-200 catalyst, two typical models, i.e., stepped Au(211) surface and Au/ZnO(110) interface, were taken to address the degradation as calculated using periodic density functional theory (DFT) calculations. Similar to the previous correction on the energetics of aqueous H2O2 solution, 46 it is obvious that the adsorption of H2O2 is endothermic on the Au(211) surface, while it is exothermic at the Au/ZnO(110) interface (see Figure 2F). The subsequent decomposition of the adsorbed H2O2 proceeds readily into two OH∗ on both sites. It is noted that the decomposition of H2O2 into an O2 molecule is less favored. As a result, the difference in the adsorption behavior of H2O2 on the Au surface and the Au/ZnO interface rationalizes the experimental results; the Au-ZnO interface is thus the dominating active site for the H2O2 degradation, and the degradation activity is indeed weakened on Au/ZnO-TEOS-200 catalysts with a less-exposed Au-ZnO interface.Water acts as both reactant and solvent in this reaction, and its adsorption and activation on the catalyst surface would be crucial for the reaction. Figure 3 A displays the XPS O 1s spectra of Au/ZnO-200 and Au/ZnO-TEOS-200 catalysts, which deconvolute into three peaks (shown in Figure S4). The peaks at 530.1, 532.2, and 533.4 eV can be assigned to lattice oxygen (OI), hydroxyl groups (OII), and adsorbed molecular water (OIII), respectively. 47 , 48 It can be observed that the percentage of surface hydroxyl groups (OII) on the Au/ZnO-200 catalyst is about 30%. In contrast, it shoots up to approximately 70% on the Au/ZnO-TEOS-200 catalyst (Figure 3B), revealing that TEOS modification can also modulate the surface structure of the catalyst. FT-IR characterization was employed to clarify the change of surface properties after TEOS modification as well. The hydroxyl region of IR spectra of the Au/ZnO-200 and Au/ZnO-TEOS-200 catalysts is shown in Figure 3C, and the specific group assignment of each peak is listed in Table S2. 49 , 50 The peaks at 3,743 and 3,654 cm−1, which can be attributed, respectively, to isolated Si-OH and vicinal Si-OH, are observed on the Au/ZnO-TEOS-200 catalyst. This result agrees well with the percentage variation of surface oxygen species from the XPS measurements. Furthermore, the hydroxyl groups on the surface alter the hydrophilicity of the Au/ZnO-200 and Au/ZnO-TEOS-200 catalysts as confirmed by water droplet contact-angle tests (91° versus 68°; Figure 3D).To further investigate the effect of the catalyst hydrophilicity on H2O2 productivity in the direct H2O2 synthesis from CO, O2, and H2O, we prepared three amorphous SiO2 supported Au catalysts by varying the calcination temperature of support, i.e., Au/200-SiO2, Au/500-SiO2, and Au/800-SiO2. It can be seen from Figure S5 that the H2O2 productivity of these model catalysts toward the direct synthesis of H2O2 monotonously decreases (107, 88, and 78 mol kgcat −1 h−1), while the size of the Au NPs and the Au loading are almost identical in all three catalysts (Figure S6; Table S3). Additional effects other than the size and loading content of Au NPs may lead to such difference in the activity. 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) analyses (Figure S7; Table S4) show that the proportion of surface hydroxyl groups (Q3 configuration) on the supports gradually decreases from 24.2% to 15.8% when the pretreatment temperature of the supports elevates from 200°C to 800°C. The contact angles of Au/200-SiO2, Au/500-SiO2, and Au/800-SiO2 catalysts are 12°, 32°, and 38°, respectively (Figure S8), which may be the consequence of the decrease of surface hydroxyl groups. It is therefore more reasonable to deduce that the difference of the H2O2 productivity between these amorphous SiO2 supported Au catalysts comes from the decrease in the hydrophilic ability of the catalyst surface. Likewise, the Au/ZnO-TEOS-200 catalyst is more hydrophilic compared with the Au/ZnO-200 catalyst (see Figure 3D), which means that the intensification of hydrophilicity induced by TEOS modification may also benefit the enhancement of H2O2 productivity of the Au/ZnO-TEOS-200 catalyst.The stability of the Au/ZnO-200 and Au/ZnO-TEOS-400 catalysts toward the direct H2O2 synthesis from CO, O2 and H2O was tested through multiple catalyst use and recovery cycles. An apparent deactivation was observed for the Au/ZnO-200 catalyst only after two runs (see Figure 4 A), and H2O2 productivity decreases from 86 to 58 mol kgcat −1 h−1. The loss of H2O2 productivity was frustratedly close to 80% after six runs. AC-HAADF-STEM displays an obvious agglomeration of Au NPs on Au/ZnO-200 catalysts with an average size increase from 1.76 to 3.55 nm and a broader particle size distribution after six runs (Figure S9). In addition, the loading of Au apparently decreases to 0.13 wt % as well (Table S5). The agglomeration and the loss of Au NPs are presumably the predominant effects upon the deactivation of the Au/ZnO-200 catalyst. In contrast, the decay of H2O2 productivity of the Au/ZnO-TEOS-400 catalyst is negligible and holds approximately 150 mol kgcat −1 h−1 after 12 runs (see Figure 4B). H2O2 productivity only decreases by 22% after 24 runs, and the average size of Au NPs only increases from 1.89 to 1.91 nm (see Figure S10), which can be attributed to the confinement effect of the SiO2 overlayer. These results manifest that TEOS modifications prominently improve the stability of ZnO supported Au catalysts.To gain insights into the underlying reaction mechanism, the direct H2O2 synthesis from CO, O2 and H2 18O was first performed. The oxygen isotope distribution in the generated H2O2 was detected using highly sensitive synchrotron-based vacuum UV photoionization mass spectrometry (SVUV-PIMS). Only a signal of mass/charge (m/z) = 34 assigned to H2 16O16O was observed in the effluents at a photon energy of 12 eV, and no signal of H2 16O18O or H2 18O18O appears (see Figure S11). This indicates that oxygen in molecular O2, rather than H2O, is more likely to serve as the exclusive oxygen source for the synthesis of H2O2. Furthermore, periodic DFT calculations were carried out to elucidate the detailed reaction mechanism. Two possible reaction pathways were proposed according to the origin of two oxygen atoms of H2O2 (see Figure 5 and Table S6). 51 , 52 , 53 The stepped Au(211) was employed as the active surface for the reaction. In path 1, H2O2 comes from the consecutive hydrogenation of an O2 molecule, while in path 2, the coupling of two OH molecules results in the formation of H2O2. More specifically, both pathways start from the decomposition of H2O assisted by O2 to form OH∗ and OOH∗ species (TS1, 0.42 eV of the free energy barrier at 60°C). The direct decomposition of H2O without O2 assistance is energy demanding (1.91 eV). Starting from the formed intermediates including CO∗, OH∗, and OOH∗, the following pathways produce H2O2 and then bifurcate. The coupling between CO∗ and OH∗ (TS2, 0.69 eV) is involved in path 1, and the formed OCOH∗ reacts with OOH∗ to form CO2 and H2O2 (TS3, 0.32 eV), while in path 2, the coupling between CO∗ and OOH∗ results in the formation of peroxycarboxylic species OCOOH∗ (TS4, 0.08 eV), and the latter could be easily decomposed into CO2 and OH∗ (TS5, 0.19 eV). However, the formed two OH∗ species are particularly stable, and the coupling between them (TS6) needs to overcome a free-energy barrier as high as 2.12 eV at 60°C. As indicated by the calculated Gibbs free-energy profile (see Figure 5A), path 1 is kinetically favorable over path 2, and the coupling between CO∗ and OH∗ (TS2) is rate determining in the reaction. In addition to OCOH∗, it should be noted that H2O can also proffer the proton to OOH∗ for subsequent hydrogenation to H2O2. CO thus acts as the eliminator of surface OH∗ species because the direct coupling of OH∗ is kinetically demanding (see Figure 5B).In summary, the TEOS-modified ZnO supported Au catalyst, in which the Au NPs are encapsulated by an SiO2 overlayer, was precisely prepared. The catalyst shows excellent catalytic activity and high stability for the direct H2O2 synthesis from CO, O2, and H2O under mild and additive-free conditions. An H2O2 productivity of 168 mol kgcat −1 h−1 on the Au/ZnO-TEOS-200 catalyst has been achieved, which is two times higher than the Au/ZnO-200 catalyst without SiO2 encapsulation and, to the best of our knowledge, is also the highest value reported so far. Detailed analyses based on AC-HAADF-STEM, XPS, FT-IR, and EDX characterizations and DFT calculations reveal that such enhancement of H2O2 productivity of Au/ZnO-TEOS catalysts can be attributed to the functional confinement of the moderate SiO2 overlayer. This confinement leads to the inhibition of the Au NP agglomeration, the suppression of the H2O2 degradation at the Au/ZnO interface, and the improvement of the catalyst surface hydrophilicity. Oxygen isotope labeling experiments and DFT calculations further confirm that both oxygen atoms in the H2O2 product exclusively come from molecular O2 and that the reaction proceeds via the consecutive hydrogenation of O2. This work proposes an efficient strategy to construct highly active catalysts utilizing the confinement effect of an overlayer for the direct synthesis of H2O2 from CO, O2, and H2O and further broadens the concept of confinement to surface systems.Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Chuanming Wang (wangcm.sshy@sinopec.com).The materials generated in this study will be made available on request to the lead contact.We gratefully acknowledge financial support from the National Natural Science Foundation of China (U22B6011 and 92045303) and the China Postdoctoral Science Foundation (2020M681444).J.L. performed the catalyst preparation, characterization, and catalytic test. W.H. and C.W. performed the DFT calculations. J.L. and W.H. contributed equally to this work. Yu Wang performed the electron microscopy characterization. W.W., C.L., and Y.P. conducted the SVUV-PIMS experiment. S.L. and J.D. helped with the catalytic performance tests and discussed the results. J.L., Yu Wang, C.W., and W.H. wrote the manuscript. Yangdong Wang and Z.X. designed this study, analyzed the data, and supervised the project.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.xcrp.2022.101236. Document S1. Figures S1–S14, Tables S1–S6, and supplemental experimental procedures Document S2. Article plus supplemental information

Due to the intensive energy consumption and environmental unfriendliness of the current industrial anthraquinone process for producing hydrogen peroxide, it is of interest to develop a clean and efficient alternative. Herein, we report that a zinc-oxide-supported gold catalyst encapsulated by a silica overlayer efficiently catalyzes the direct synthesis of hydrogen peroxide from carbon monoxide, oxygen, and water. Detailed characterizations demonstrate that the confinement effect of the silica overlayer may enhance hydrogen peroxide productivity by limiting the agglomeration of gold nanoparticles, inhibiting the hydrogen peroxide degradation activity, and improving the hydrophilicity of the catalyst surface. Isotope-labeling experiments and theoretical calculations reveal that both oxygen atoms in hydrogen peroxide come from molecular oxygen, and that a consecutive hydrogenation process is followed. This work poses a facile strategy to construct highly active catalysts for the direct synthesis of hydrogen peroxide, employing the confinement effects of an overlayer.

Due to the increasing urgent concern about energy scarcity and environmental pollution, the use of alternatives to traditional fossil fuels has drawn increasingly attractive in recent years (Liang et al., 2022; Cipolletta et al., 2022). Biodiesel, as a sustainable, biodegradable, and clean fuel, can be produced from conventional lipid feedstocks (Ennaceri et al., 2022; Costa and Oliveira, 2022). Hermetia illucens L. is a non-pest, which is found in warm temperate regions worldwide (Rehman et al., 2017). Commonly, insect larvae can feed on various kinds of organic matter in the biowastes (Lalander et al., 2019). Importantly, insect larvae have high levels of lipids after decomposing this organic waste (Deng et al., 2022). The sum of saturated and unsaturated fatty acids accounted for more than 90% of the total insect lipids. In recent years, the city of Wuhan in China has been producing roughly 365,000 tons of kitchen waste annually (Zheng et al., 2012). Meanwhile, approximately 3,000 tons of insect lipids will be produced. Due to society’s environmental awareness and the considerably high production of insect lipids, the energy insect larvae as a renewable resource are expected to have great potential for biodiesel production (Li et al., 2015; Feng et al., 2019).Many investigations have been reported on biodiesel production by using solid sulfonated carbon-based biomass catalysts (Mansir et al., 2017; Ibrahim et al., 2020). Recently, various biomass wastes have been converted into sulfonated carbon supports for biodiesel production. In this study, in continuation of our interest in the development of renewable methodologies, the waste biomass carbon was derived from the shell of the energy insect (Hermetia illucens L.) as above. The life cycle of the insect consists of four stages: egg, larva, pupa, and adult (Proc et al., 2020). The process of pupation is the transformation from a pupa into a fly. The adult fly will crawl out from the part of the shell and then fly away (Raksasat et al., 2020). This process is similar to the cocooning process of butterflies. A large number of insect shells as residues were generated during pupation. Notably, the insect shell has a unique composition (called chitin) and can be used as a bio-waste carbon feedstock with cost-free and renewable (Guo et al., 2021). The utilization of the energy insect shell as a good candidate feedstock is expected to be highly promising for synthesizing sulfonated activated biochar.In addition, it should be noted that the metal oxide elements as promoters have been introduced into sulfonated carbon-based biomass catalysts to induce and accelerate the conversion of oils. Among these, zinc oxide (ZnO) is extensively used as a solid base catalyst because of its distinctive features and low cost (Gurunathan and Ravi, 2015; Liu and Zhang, 2011). Note that ZnO as a base metal oxide is a reactive metal promoter in the catalytic process of biodiesel. Baskar et al. (Baskar et al. 2018) conducted a conversion reaction on castor oil with high free fatty acid over a Ni-doped ZnO nanocatalyst under optimum conditions (biodiesel yield = 95.2%). As ZnO is a base promoter, a bifunctional acid-base catalyst may be synthesized by incorporating ZnO material on the surface of the support with acid sites. Furthermore, the presence of excellent catalytic activity of these catalysts with many strong acid-base sites is useful for promoting simultaneous esterification and transesterification.Until now, the sulfonated biochar-based heterogeneous catalyst prepared from the insect shell for biodiesel production has been rarely reported. Here, a surfactant methodology was adopted to meditate the mesopores of sulfonated biochar support from the outset, which ensures spatial compartmentalization of chemically distinct active sites. The molecular weight, polarity, and hydrophilicity of the introduction of polymer matters in the catalyst synthesis process influence the specific surface area and catalytic activity of the catalyst and result in composite catalyst with different morphologies and catalytic capacities (Du et al., 2019; Jeon et al., 2013). Based on the present studies, polyvinyl pyrrolidone (PVP) as a support mediator was introduced to improve the textural properties and catalytic capacity of the catalyst for the conversion reaction. This paper focused on the utilization of the insect shell for the development of a novel bifunctional acid-base catalyst (ZnO/PVPmediate-BC-S) for the conversion of the insect lipid into biodiesel. The synergistic catalysis of both active acidic and basic sites will be an efficient and reusable approach for the production of biodiesel from renewable insect lipid to biodiesel with environmentally friendly. The physiochemical characterization of the synthesized composite catalyst was investigated. The influences of the composite catalyst preparation conditions and the catalytic reaction conditions on the biodiesel yield were also investigated. More importantly, the possible catalytic mechanism of the prepared catalyst was comprehensively described. Moreover, the reusability of the prepared catalyst during five reaction cycles was demonstrated and studied to evaluate its stability. Finally, the physicochemical properties of biodiesel were also studied.The energy insect shells were collected from the Wuhan Institute of Technology (Fig. 1 ). The lipids extraction process is similar to that reported in our previous work (Feng et al., 2019). The fatty acid compositions of insect lipids were listed in Table 1 . Besides, Zn(Ac)2, NaOH, PVP, sulfuric acid, and methanol were analytical reagents and purchased from a local supplier in China.Briefly, the insect shell was first washed and dried in an oven at 80 °C overnight. Then, the shell was milled and sieved to obtain uniform-sized particles (80 mesh screen, average pore size≈0.18 mm). Subsequently, appropriate masses of resultant particles and PVP (5–45% by weight, occupy the weight of insect particles) were introduced into the distilled H2O. The resulting mixture was continuously stirred at 60 °C for 3 h. After treatment, the precipitate was separated and then calcined at different temperatures (300–700 °C）in a muffle furnace for 6 h. The carbonized product (denoted as PVPmediate-BC) was later further treated with sulphuric acid at 90 °C for 3 h. Immediately after that, the activated biochar was washed and dried at 100 °C overnight (denoted as PVPmediate-BC-S).Typically, the required amount of Zn(Ac)2 (Zn(Ac)2 concentration = 0.05 to 0.7 mol/L) and NaOH were dissolved in distilled water. Then the support (PVPmediate-BC-S) was immersed in the mixture solution. Furthermore, the mixture was heated at 90 °C for 3 h. Finally, the resulting solid precipitate (ZnO/PVPmediate-BC-S) was isolated from the solution and then dried at 80 °C for 12 h.The prepared solid catalyst was used to evaluate its catalytic performance in converting insect lipid into biodiesel. An appropriate amount of methanol (molar ratio of methanol/lipid = 3:1–15:1), catalyst (2–10% by weight), and 10 g of insect lipid are poured sequentially into a 250 mL flask. Then, the mixture was heated at 65 °C for 4 h. The reaction mixture was centrifuged to separate the solid catalyst after the reaction. The biodiesel yield was calculated based on a formula (1) (Zhao et al., 2018): (1) Biodiesel yield % = Weight of obtain biodiesel g Weight of insect lipid g × 100 % The morphologies of the prepared catalyst were investigated using scanning electron microscopy (SEM, JSM-7500, Japan). Transmission electron microscopy (TEM) images were recorded with a Tecnai G2 F20 S-TWIN microscope instrument. The structure and crystalline phase of the synthesized catalyst were determined using X-ray diffraction technique (XRD, Rigaku, Japan). The XRD patterns were recorded from 2θ of 10 to 60°. The XPS analyses were performed on an ultra-high-vacuum VG channel detector, using Al Kα radiation (1486.7 eV). Surface functional groups of the catalyst sample were tested by Fourier transform infrared spectroscopy (FTIR, American Thermo Electron) with spectrum of 400–4000 cm−1. The surface area of the prepared catalyst was evaluated by multi-point Brunauer-Emmett-Teller (BET) analysis with a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). The pore volume and pore diameter were determined by the Barrett-Joyner-Halenda (BJH) method. 1H NMR spectrum of insect lipid and biodiesel were recorded with CDCl3 in FT-NMR spectrometer (Bruker Avance II). The basicity of the prepared catalyst was measured using Hammett indicators.The catalytic reaction procedures were followed in duplicate. The experiment results were processed and analyzed by using a traditional editing software (origin pro 9.1, USA). Fig. 2 depicts the mesoscopic morphological characteristics of raw biochar support, PVPmediate-BC-S support, and ZnO/PVPmediate-BC-S catalysts. The presence of a few pores on the surface of raw biochar support is shown in Fig. 2a. Meanwhile, it is interesting to find that there are some discrete pores of various sizes on the surface of PVPmediate-BC-S support when PVP was used as a support mediator (Fig. 2b). The results of this study indicated that high molecular polymer mediation was effective in improving the porous properties of biochar support and was also a prominent technique for realizing hierarchical pore structure in support. However, it is clear that the pores on the support surface were blocked due to the dispersion of the ZnO materials from Fig. 2c. It can thus be concluded that ZnO materials have been successfully dispersed in the biochar-based support.The textural properties of the prepared catalyst were further investigated via N2 adsorption–desorption isotherms (Fig. 3 ).The prepared catalyst shows H3-hysteresis and a typical IV isotherm, indicating the formation of a predominantly mesoporous structure (Fig. 3). A significant increase was observed at a relatively high P/P0 (0.8–1.0), indicating the presence of capillary structures (Fig. 3a). Furthermore, based on these observations, it is speculated that the synthesized catalyst had uniform pores. The primary requirement for an ideal conversion reaction in the presence of solid catalysts is the pore structure. The pore diameter fractions of raw biochar (BC-S) are 5–10 nm, according to BJH desorption isotherms (Fig. 3b). The average pore diameter and pore volume of raw BC-S were 8.79 nm and 0.0821 cm3/g, respectively (Table 2 ). Clearly, the pore diameter fractions and surface area are increased after PVP mediation. The pore size distribution revealed that the PVPmediate-BC-S support had more mesopores. The SEM micrographs also revealed these changes. Furthermore, after loading ZnO material on the support, there is a slight decrease in pore volume and pore diameter. This could be attributed to the fact that the surface of catalyst support was covered upon ZnO active components and the pores of the catalyst were blocked, which in turn caused the decrement in pore properties.The typical XRD pattern obtained from ZnO/PVPmediate-BC-S is displayed in Fig. 4 . The peaks at 2θ = 31.5°, 34.2°, 36.3°, 47.35°, and 56.3°, indicate (100), (002), (101), (102), and (110) crystalline phases in the ZnO structure (JCPDS 36-1451), respectively (Lokhande et al., 2009). The appearance of peaks associated with the synthesized biochar at 2θ = 10–30° and 2θ = 40–50° can be readily indexed to (002) and (101) crystalline phases, respectively (Wang et al., 2018). According to Aziz et al. (Aziz et al., 2015), the broad peaks (2θ = 10°–30°) can be well assigned to those of amorphous carbon. Zhao et al. (Zhao et al., 2009) mentioned that the COC cleavage bonds of the biochar precursor were disrupted by the synergistic interaction between the carbonization process at high temperature and the sulfonation process. The sulfonated carbon became more rigid and amorphous due to the high degree of carbon disorder during this process. Hence, this structural modification of the sulfonated carbon catalyst is an important step that is responsible for the efficiency of the catalytic reaction in this study.The structural information of the synthesized catalyst has been further examined by TEM observation, and some representative images are presented in Fig. 5 . According to Fig. 5a, the polymer functionalization approach used in this study can introduce a significant number of mesopores with worm-holes into BC support. In addition, the lattice fringes on the surface region correspond to the (110) plane of ZnO. This finding suggests that ZnO material is well loaded on the surface of the biochar-based support (Fig. 5b) (Malhotra and Ali, 2019; Kazmi et al., 2020).The FT-IR analysis of the functional group bond on the support surface was presented in Fig. 6 . The broad absorption band at around 1426.89 cm−1 can generally be assigned to the aromatic ring CC stretching mode of carbonate species (Hsiao et al., 2011). Moreover, the bands at 1120 cm−1 are related to the symmetric stretch of the SO3H bond, present in the structure of the sulfonic groups (Zong et al., 2007). It should be pointed out that the sulfoxide group still exists on the synthesized carbon support, although the support was washed three times during the end of the pretreatment process. This finding suggests the successful grafting of -SO3H groups onto the carbon support after the sulfonation process for the application in catalytic reaction (Sangar et al., 2019). In addition, two characteristic peaks at 601.99 and 670.74 cm−1 were attributed to the stretching vibrations of ZnO (Zhang et al., 2009.The surface chemical composition of the prepared sample was further investigated by using XPS analysis (Fig. 7 ). The peak at 169 eV is ascribed to the sulfur of -SO3H groups (Song et al., 2020). These indicate that the SH, SO3H groups are present in the prepared composite catalyst. The two peaks in the spectrum of C 1s (Fig. 7) at 285.01 eV and 288.2 eV represent the binding energies of carbon of carbonates present in the catalyst (Pan et al., 2020). The predominant chemical states in ZnO materials are oxygen (O 1s) and Zinc (Zn 2p). There is one peak with a binding energy of 531.11 eV (O 1s) associated with the elemental oxygen of oxides present in the catalyst (Medeiros et al., 2020). Furthermore, the peak at 1022.1 eV (Zn 2p) is associated with Zn in the zinc oxide (Feng et al., 2016). These binding energies fit well with the chemical bonding structure of the composite catalyst.The synthesized catalyst was applied to the catalytic reaction of the insect lipid. Here, a series of experiments were performed to test the effects of different conditions on catalytic performance.The carbonization temperature was investigated to optimize the synthesis of catalysts for higher biodiesel yield. As seen in Fig. 8 a, the biodiesel yield increases with the increase in carbonization temperature from 300 to 600 °C (from 57.14% to 94.36%). The carbonization at higher temperature led to the desorption of CO2 from the insect shell support, producing more pores that catalyzed the transesterification of insect lipids. Additionally, the surface area of the catalyst could be affected by carbonization temperature, which consequently affects the efficiency of the catalyst. Moreover, it is also found that the catalytic capacity of the catalyst (86.68%) decreased with the further increase in the carbonization temperature. This can contribute to both the high sintering rate of the catalyst and the high energy consumption (Obadiah et al., 2012).A series of experiments were carried out by altering Zn(Ac)2 concentrations to investigate the influence of ZnO loading on the catalytic process (Fig. 8b). Remarkably, with this increasing Zn(Ac)2 concentration (0.05 to 0.7 mol/L), the biodiesel yield gradually increased from 52.66% to 94.36%. However, there was no noteworthy increase in conversion yield with Zn(AC)2 concentration beyond 0.3 mol/L. The reason for this result can be deduced from the crystallization or agglomeration of ZnO materials which resulted in poor dispersion of active components on the surface of the support (Tantirungrotechai et al., 2013). Meanwhile, according to the Hammett indicator method results, the catalyst with 0.3 mol/L Zn(Ac)2 concentration had the strongest basicity. With the catalytic activity of samples, it could be concluded that high basicity could contribute to a higher biodiesel yield.It is evident from this work as well as from literature that the polymer amounts can have a pronounced effect on pore textural properties of the prepared catalysts, and therefore play a key role in the catalytic performance. We can see that the pore size of the catalyst is primarily distributed at 10–19 nm for the synthesized catalyst with PVP as a structural meditation agent (Table 3 ).Since the pore size of the synthesized catalyst is larger than that of the triglyceride molecule (58 Å), the triglyceride molecule would react with the active component on the surface or in the channel of support (Li et al., 2019). The biochar support by PVP-mediated had a significant influence on the catalyst, whereas the catalyst without PVP had a smaller specific surface area and pore volume (Fig. 9 ). Based on these observations, this is presumed to be due to the introduction of appropriate PVP intensifying the dispersion of active components on the surface of the catalyst (Margellou et al., 2018). The other possible reason is that the organic polymer with free radical structures and organic groups such as Lewis acids could withdraw or donate electrons (Figueiredo et al., 2007). It has been reported in the literature that the carbonyl/quinone groups on the surface of carbon are the strong active sites for oxidative dehydrogenation reactions (Pereira et al., 1999). Following a further increase in PVP amount (beyond 35 wt%), no appreciable improvement in biodiesel yield was seen. One possible explanation for this phenomenon is that the excessive organic calcination residues on the surfaces of support may lead to a significant decrease in the pore volume and pore size of support (Qu et al., 2021). This phenomenon in this study is similar to that reported in other previous literature (Guisnet and Magnoux, 2001; Moreno-Castilla et al., 2001). Nevertheless, the mechanism by which the organic polymer assisting improves catalytic performance remains unclear. Thus, further investigations are required to uncover this mechanism, and this is the next step in our future studies.The catalytic reactions were carried out by varying the loading of the synthesized catalyst (Fig. 10 a). It is interesting to notice that the conversion yield can reach 65.37% even at lower catalyst loading. The biodiesel yield increased progressively with increasing in the catalyst loading. Thus, the biodiesel yield of 94.36% could be achieved when the catalyst loading was 6 wt%. The increase in biodiesel yield with increasing loading of the catalyst was due to the increase of active sites of the catalyst participated in the conversion reaction. Moreover, the biodiesel yield decreased with increased catalyst loading by more than 6 wt%. There are two possible reasons caused the decrease in biodiesel yield at a higher catalyst loading: (1) The higher catalyst loading may lead to the increase of the viscosity in the mixture reaction system, which has influenced the mass transfer between the reactants and the catalyst (Negm et al., 2017; Hwa et al., 2015). (2) ZnO catalyst is an amphoteric composite, the saponification side reaction might have induced resulting in decreasing biodiesel yield (Santana et al., 2012).The catalytic reactions of the insect lipid were conducted at methanol to lipid molar ratio of 3:1–15:1 (Fig. 10b). The lowest conversion of lipid into biodiesel (34.22%) was obtained when the molar ratio of methanol to lipid was 3:1. It is evident from Fig. 10b that the biodiesel yield increases linearly with methanol amount. The biodiesel yield of lipid was 94.36% at a methanol/lipid molar ratio of 9:1. However, the biodiesel yield decreases slightly when the molar ratio of methanol/lipid exceeded 9:1. But at a higher molar concentration of methanol, there were two possibilities for reducing biodiesel yield: (1) Due to the reversible nature of the trans-esterification reaction, the equilibrium of reaction has shifted toward the backward reaction (Bhatia et al., 2020; Mutreja et al., 2014). (2) Additionally, further excess deluges the active sites of the catalyst that may hinder the protonation of fatty acid at the active sites, resulting in a decrease in biodiesel yield (Khan et al., 2020; Roy et al., 2020).To evaluate the effect of the catalytic reaction temperature, the catalytic reaction was conducted at different ranges of temperature (Fig. 10c). It is thus apparent that the biodiesel yield increased with increasing the reaction temperature. The reason for this result can be deduced from the mass transfer principle considering that the high temperature condition is helpful to increase the probability of effective collisions between the catalyst and the reactant molecules, resulting in a faster rate of mass transfer, hence resulting in increased conversion yield of biodiesel (Sahani et al., 2019). At the higher temperature (65 °C), a conversion yield of 94.36% was obtained. However, a slight decrease in the biodiesel yield was observed when the reaction temperature exceeded 65 °C under the same reaction conditions. This phenomenon occurred because of the vaporization of the excess methanol at the optimal temperature (Saravanan et al., 2015).The effect of alcohol types in the conversion reaction is shown in Fig. 10d. Using the prepared catalyst, the biodiesel yield decreases with increasing alcohol molecular weight. The order of the conversion capacity is methanol, ethanol, propanol, isopropanol, and tert-butanol. The reason was ascribed to the fact that the larger alcohol chains have inferior nucleophilicity. By comparing the decomposition capacity of other alcohol, methanol with more nucleophilicity of methoxide ion decomposes more quickly (Basumatary et al., 2021). In addition, larger chains have a greater sterile hindrance, which prevents fatty acids from interacting with the catalyst (Araujo et al., 2019).As regards the homogeneous catalyst, the most intriguing feature of this heterogeneous catalyst is that it still had high catalytic activity and stability after successive reaction cycles. Therefore, further recycling experiments were performed using the synthesized catalyst under the defined conditions to evaluate the reusability of the catalyst (Fig. 11 ). Upon completion of each reaction, the solid was collected and directly reused in subsequent cycles. The second catalytic reaction was carried out under the previous same conditions and a conversion yield of 88.66% was obtained. This procedure was repeated five times. It is noticeable that there appears to be no significant decrease in biodiesel yield for each repeated experiment. There are three possible reasons caused the decrease in the activity of the catalyst. (1) The loading of catalyst used in the next cycle was lower than the initial cycle, which might partly be responsible for reducing biodiesel yield during the recycling experiments. (2) The produced by-product glycerol covered the surface of the catalyst and blocked the active sites of the catalyst (Gohain et al., 2020). (3) The leaching of active components of the catalyst would lead to the decrease of catalytic activity during the recycling experiments (Khan et al., 2020).To further explore the feasibility of the synthesized catalyst, the characterizations of SEM and TEM were carried out for the fresh and reused catalyst (five cycles). The SEM (Fig. 12 a) micrograph shows that the synthesized catalyst retains its bulk-like nanostructure. Compared with Fig. 2(c) and Fig. 5(a), it is likely that the surface and porous structure of the synthesized catalyst changed slightly. The change in the surface and porous structure of the synthesized catalyst was due to the catalyst being poisoned by the irreversible adsorption of the free fatty acids on the active sites, which destroys the pore structure and increases the mass transfer resistance (Kwong and Yung, 2016; Li et al., 2022).Meanwhile, combining Fig. 13 (a) with Fig. 13(b), there was no obvious apparent difference in the structure and the physicochemical properties of the reused catalyst after five cycles. However, there was a significant decrease in the peak intensity of ZnO after the reused reaction, implying the loss of active components during each reaction. Besides, the newly detected functional groups (2920 and 2850 cm−1, CH stretching vibration of glycerol molecule) on the reused catalysts, may block the active sites and result in reduced activity on the reused catalyst (Yusuff et al., 2021). Consequently, the biodiesel yield decreased as the reaction cycle increased. Further work is ongoing to optimize the composite catalyst to limit the decrease in the yield of biodiesel in the subsequent cycles of use.In comparing the catalytic activity of those previously reported catalysts, it is clear that the synthesized catalyst has better stability and reusability for biodiesel production in the current work (Table 4 ). For instance, biodiesel yield reduced from 94.36% to 47.52% after the fifth cycle when corncob biomass waste-based acid catalyst was utilized as an efficient catalyst in the transesterification of palm fatty acid (Tang et al., 2020). Macawile et al. reported that biodiesel yield decreased from 93.10% to 58.09% after the third cycle of reuse of the novel modified solid acid catalyst (Macawile et al., 2020).The ZnO materials were loaded on the sulfonated biochar-based support in this study. As both acid and base groups are present on the surface of the biochar support, both acid-base mechanisms are likely to occur. Fig. 14 depicts the proposed mechanism simplified for the catalytic reaction.Acid catalysis: The first step involves protonation of the carbonyl group of insect lipids by the protons generated from the acidic sites. The sulfonate groups (-SO3H) on the framework of the prepared catalyst have labile hydrogen protons that can be easily migrated to attack the most nucleophilic centers of lipids (CO) to form the protonated form of lipids (Macawile et al., 2020). Then, the electron-rich hydroxyl groups of methanol would nucleophilic attack the cationic centers on protonated lipid (Betiha et al., 2020). The unstable protonated lipid-methanol ether complex formed undergoes two successive stabilization steps. The first includes the loss of water molecules to produce the protonated lipid-methanol ester intermediate. The second involves the conversion of the protonated lipid-methanol ester intermediate into the corresponding lipid-methanol ester by losing protons, which are combined with the ionized acidic catalysts to retain their chemical structures (Khan et al., 2021).Base catalysis: The two types of esterification and transesterification simultaneously took place on the surface of the biochar-based catalyst. ZnO as a Brønsted base was loaded on the support during the catalysis process. A part of methanol and triglyceride reactants may be adsorbed at different Zn active sites on the ZnO catalyst surface. Thus, it was reasonable that the interaction of the ZnO catalyst and triglyceride would form the bond of Zn and -O-CH2. The presence of ZnO in the chemical structure of the catalyst has a high influence on its alkalinity (Kouzu et al., 2009; Putra et al., 2018). The fatty acid methyl ester is then produced and formed diglyceride. The diglyceride further reacted with methanol to produce the object product (fatty acid methyl ester) and intermediate (monoglyceride). In the meantime, the monoglyceride is further involved in the reaction to form fatty acid methyl ester and by-product (glycerol). Finally, the by-product glycerol may be desorbed from the active sites of the biochar-based catalyst. In this way, the catalyst can be reused in subsequent reactions. Fig. 15 a shows the FTIR spectra of insect lipids and biodiesel. The absorption band at 3400 cm−1 for insect lipids, which indicates the occurrence of (OH) stretching vibration of carboxylic acid, alcohol, or phenol (Ayoob and Fadhil, 2019). Interestingly, the observed absorption band disappears in the FTIR spectra of biodiesel. Its absence in biodiesel indicates that esterification has been highly effective. As shown in Fig. 14a, the biodiesel spectra have two sharp and broad peaks at 2920 cm−1 and 2851 cm−1, respectively. The bands at 2920–2851 cm−1 are due to symmetric and CH asymmetric stretching vibrations (Falowo et al., 2020; Falowo and Betiku, 2022). The CO stretch vibration at 1745 cm−1 of insect lipid shifts to 1742 cm−1 in biodiesel, which represents the conversion reaction (Ayoob and Fadhil, 2020). The peak at 1461 cm−1 corresponds to the bending of CH2 (Li et al., 2015). A peak at 720 cm−1 was observed and assigned to the vibration of the CH2 bond from the long fatty acid chain (Li et al., 2019). The existence of major absorption bands proved that biodiesel was produced in this work. The results obtained in this study were in good agreement with those reported in the previous study (Tsanaktsidis et al., 2013).The synthesis of biodiesel from insect lipids was characterized by the 1H NMR spectra (Fig. 15b). The major differences in signals indicated the conversion of insect lipids to biodiesel. In Fig. 13b, the peaks present at 4.28 ppm and 5.33 ppm correspond to the glyceridic protons and olefinic protons (CHCH) in insect lipids, respectively (Fadhil et al., 2019). A new singlet observed on the spectra around 3.57 ppm indicates the appearance of methoxy protons (COOCH3) (Li et al., 2015). Instead, the glyceridic peaks near 4.28 ppm completely disappeared. The results observed here illustrated that the insect lipid was successfully converted to biodiesel. The signals at 2.30 ppm in the 1H NMR spectrum of insect lipid and 2.22 ppm in the 1H NMR spectrum of biodiesel are due to α-CH2 protons to the ester (CH2 CO2R) groups present in the insect lipid and biodiesel, respectively (Macina et al., 2019). The 1H NMR signals of the methylene (CH2)n protons of the fatty acid chain in the insect lipid and biodiesel are observed at 1.25 ppm and 1.18 ppm, respectively (Nath et al., 2019). The terminal methyl protons (CH3) of the triglycerides and biodiesel are indicated by the 1H NMR signals at 0.87 ppm and 0.81 ppm, respectively.The physicochemical properties of the biodiesel sample obtained from the conversion of the insect lipid over the prepared catalyst were determined and listed in Table 5 . It can be noted that most of the fuel properties of biodiesel produced in this paper meet the specifications of the ASTM standard. Biodiesel with low viscosity is preferable for better combustibility in the engine. The viscosity reported in this study was well below the maximum limits of the ASTM standard. It is interesting to note that measurements of other fuel properties (acid value, density, and moisture content) from insect lipids derived from biodiesel were similar to the previously reported results.In summary, a novel biochar-based heterogeneous catalyst (ZnO/PVPmediate-BC-S) with acid-base bifunctional catalytic capacity was successfully synthesized by using PVP as a support mediator for the conversion of the insect lipid into biodiesel. The results of these experiments suggested that the biodiesel yield was 94.36% at the defined condition. As a mediator, PVP has enhanced the interaction between active components and support in the catalyst synthesis process. The appropriate PVP amount will be beneficial for the catalyst to have greater structure and crystallography of support, to maintain better catalytic activity. Meanwhile, the biodiesel yield did not decrease significantly after five cycles of reuse, indicating excellent stability and reusability of the prepared catalyst. Most of the fuel properties of the prepared biodiesel in this paper meet the ASTM standard specifications. This work demonstrates that biodiesel production with high yield catalyzed by inexpensive and facilely fabricated biochar support from the waste insect shell has tremendous potential for large-scale production and practical applications.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was financially supported by the Open Project of Beijing Key Laboratory for Enze Biomass and Fine Chemicals, Chongqing Key Laboratory of Soft-Matter Material Chemistry and Function Manufacturing (No. 20210005), Hubei Key Laboratory of Novel Reactor and Green Chemical Technology (No. NRGC202209).

This paper focused on the utilization of the waste insect shell for the development of a novel biochar-based heterogeneous catalyst (ZnO/PVPmediate-BC-S) with a highly acid-base bifunctional catalytic capacity for the conversion of the insect lipid into biodiesel. The introduction of polyvinyl pyrrolidone (PVP) as a support mediator was believed to improve the textural properties of support and catalytic activity of the catalyst for the conversion reaction. Meanwhile, the physicochemical properties of the synthesized composite catalyst were characterized with XRD, SEM, TEM, XPS, BET, and FT-IR analysis. The high biodiesel yield (94.36%) was obtained at the defined condition (carbonization temperature = 600 °C, Zn(Ac)2 concentration = 0.3 mol/L, PVP amount = 35 wt%, reaction temperature = 65 °C, catalyst loading = 6 wt%, methanol/lipid molar ratio = 9:1). Moreover, the possible catalytic mechanism of the prepared catalyst was comprehensively described. In addition, the stability and reusability of the prepared catalyst during five reaction cycles were also demonstrated. Finally, the physicochemical properties of the biodiesel studied were well comparable with the ASTM standard as well as with the reported literature.

The progress of human society depends on the development of industry, and the latter has increasingly resulted in the production of several harmful and poisonous pollutants that are difficult to degrade in sewage. In recent years, pharmaceuticals, personal care products [1,2], and endocrine disruptors [3] that adversely affect human health and ecological environment have attracted increased research attention. Therefore, efficient methods for wastewater treatment are necessary to achieve the goal of clean production and promote the sustainable development of human society.Several organic pollutants are difficult to remove effectively using conventional water treatment technologies [4–6], and the average removal of many organic pollutants, including atrazine, in sewage treatment plants is less than 50% [7]. To resolve this problem, advanced oxidation technologies are emerging with the rapid development of wastewater treatment technologies. Advanced oxidation processes are powerful and efficient methods to degrade the pollutants in water. Among these methods, sulphate radical-based advanced oxidation processes have attracted considerable research interest owing to their high redox potential and selectivity for oxidation [8,9]. The activation of peroxymonosulfate (PMS) can be accomplished using techniques, such as thermal activation, photoactivation [10], ultrasonic irradiation, electrochemical methods, homogeneous metal-ion catalysis, and heterogeneous catalysis [11,12]. In recent years, heterogeneous catalysis has been widely studied owing to its high efficiency and less secondary pollution [8]. Currently, semiconductors, transition metals, and metal-free materials are widely used to activate PMS [13,14]. In addition, the development of magnetic heterogeneous catalytic materials resolves the problem of material separation in aqueous solutions and improves the possibility of practical use [15–17].The development of heterogeneous catalytic oxidation materials encounters several problems: The use of precious metals makes the materials expensive; the catalysts are difficult to separate from the aqueous environment [18], and the recycling effect is limited [19]. In general, the development of materials is a trade-off between their cost and efficiency. Although the adsorption process is simple and economical, it does not resolve the fundamental problem of pollution in wastewater treatment. In contrast, the advanced oxidation technologies combined with the adsorption process could be more economical and efficient techniques that further promote clean and sustainable development. Several studies have investigated the application aspects of the adsorption−catalytic oxidation process. For example, Wang et al. [20] observed that the adsorption−degradation cycle was conducive to the removal of the bisphenols. Peng et al. [21] demonstrated that the synergistic effect of the adsorption and catalysis on Fe/Fe3C@NG achieved an efficient removal of norfloxacin (Nor).Metal–organic frameworks (MOFs) were selected as the potential adsorbents and heterogeneous catalytic materials owing to their large specific surface area and variable reaction sites [22,23]. MOFs are three-dimensional ordered porous materials formed by metal ions and organic ligands [24]. MOFs are also called porous coordination polymers (PCPs), and are widely used in gas storage [25], catalysis [26], adsorption [27], chemical sensing [28], drug transport [29], semiconductors [30], and biomedical imaging [31]. Moreover, many researchers have used MOFs as templates or precursors to synthesize carbonaceous materials or metal composites [32–35] to investigate their applications. MOFs-based carbon composites that are a combination of metal composites and carbon, exhibit superior potential in adsorption and heterogeneous catalysis [36,37].However, stability is an important factor for all heterogeneous catalysts. Therefore, the practical applications of MOFs are controlled by their recycling performance and stability. Among all the reported MOFs, MOF-5 is one of the most typically used materials that exhibits open-skeleton structure, large pore surface area, and good thermal stability [38]. However, MOFs comprising divalent metal centers and multi-carboxylate ligands, such as MOF-5, are sensitive to water and can collapse in aqueous environment [39], making them less competitive in wastewater treatment. Considering that the ligands bind to nickel ions in a more stable manner than to zinc ions, the doping of MOF-5 with nickel ions can improve its stability in aqueous environment. Thus, the nickel-doped MOF-5 can be used in wastewater treatment [40]. Moreover, the addition of nickel to MOF-5 and its subsequent calcination yields a magnetic composite that facilitates the solid–liquid separation and its subsequent regeneration, as well as resolves some of the problems encountered in the development of heterogeneous materials.We prepared a magnetic heterogeneous catalyst, denoted as ZN-CS, via a previously reported hydrothermal synthesis method [41]. The removal of rhodamine B (RhB) was selected as the model process to investigate the proposed mechanism and the coupling effects. Furthermore, the removal of different target pollutants (acid orange 7 (AO7), methylene blue (MB), tetracycline hydrochloride (TC), and Nor), and the factors affecting the degradation of RhB were studied. Finally, the analysis results of scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) analysis, powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and electron paramagnetic resonance (EPR) analysis and the quenching experiments demonstrated that the degradation of the absorbed pollutants enabled the regeneration of the active sites, contributing to a high recycling performance. Compared with the systems used in some previous studies, this system did not use any precious metals. Moreover, this system employed the adsorption–degradation process to achieve a balance between the economic and treatment effect. Additionally, the synthesized catalyst exhibits magnetic properties, recyclability, stable structure, and good removal efficiency for a variety of organic matter. The adsorption−interpretation coupling process provides a new approach for the development of catalytic materials with adequate adsorption performance.Ethylene glycol, zinc nitrate hexahydrate (Zn(NO3)2·6H2O), N,N-dimethylformamide (DMF), methanol, tert-butanol (TBA), ethanol, nickel nitrate hexahydrate (Ni(NO3)2·6H2O), RhB, anhydrous sodium sulphate, and potassium hydrogen phosphate (K2HPO4) were obtained from Chengdu Kelong Chemical Reagent Co., Ltd. (China). p-Phthalic acid (H2BDC), AO7, Oxone (PMS), and TC were obtained from Aladdin Chemistry Co., Ltd. (China). Nor and MB trihydrate were obtained from TCI (Shanghai) Development Co., Ltd. and Sinopharm Chemical Reagent Co., Ltd. (China), respectively. Ultrapure water was used to prepare all the aqueous solutions. All chemicals used in the experiments were of analytical grade.The core–shell ZN-CS nanocomposite was prepared using a previously reported method [41] with some modifications. First, 0.75 g each of Zn(NO3)2·6H2O and Ni(NO3)2·6H2O were added to the solvent mixture (75 mL ethylene glycol and 120 mL DMF). The resulting sample was stirred under magnetic stirring till the solids dissolved completely. Subsequently, 0.45 g of H2BDC was dissolved in the prepared solution. The solution was placed in a Teflon-lined stainless-steel autoclave at 150 °C for 6 h. The contents were collected through centrifugation, purified with ethanol and DMF, and subsequently dried in a blast drying oven at 100 °C overnight. The sample thus obtained was calcined at 450 °C in a tube furnace under a nitrogen atmosphere for 20 min, washed with deionized water. and finally dried to obtain ZnO@Ni3ZnC0.7. The high structural stability of the synthesized catalyst (denoted as ZN-CS) was confirmed using XRD and XPS analysis.The RhB concentration was analyzed using a spectrophotometer (MAPADA UV-1800PC, China) with maximum absorption wavelength of 554 nm. The N2 adsorption/desorption isotherms were obtained using a QuadraSorb Station 2 at −196 °C. The zeta potential of the ZN-CS surface was determined using a zeta potential analyzer (Nicomp Z3000, USA). The surface morphologies and atomic composition of the newly prepared and used catalysts were analyzed using a JSM-5900LV scanning electron microscope (JEOL. Ltd. Akishima, Japan) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. The XRD patterns were obtained using an X’ Pert Pro MPD DY129 X-ray diffractometer. Infrared spectra were obtained using FT-IR (Nicolet 6700, Thermo Scientific, USA).The adsorption performance of the ZN-CS towards RhB was studied by an extra batch-adsorption experiment in a glass beaker at 20 °C. The catalysts were withdrawn at the pre-determined intervals and immediately separated by Whatman GF/F glass-fiber membranes to measure the residual RhB concentration. To evaluate the activation ability of the ZN-CS towards PMS, catalytic experiments were conducted with the pristine ZN-CS in a 500 mL glass beaker at room temperature. Because ZnO has been widely studied as a semiconductor photocatalyst [42], we conducted a control experiment under dark conditions to eliminate the effect of light. The results thus obtained exhibited no significant difference (Fig. S1 in Appendix A). Therefore, the subsequent experiments were conducted under indoor light conditions. Before the addition of PMS, different dosages of the catalysts were dispersed in a 200 mL RhB solution, which was stirred for approximately 15 min to achieve the adsorption equilibrium. The degradation reaction was triggered by adding the desired amount of PMS. The samples were withdrawn, and filtered at certain time intervals to determine the residual pollutant concentration. The blank test without the catalyst was conducted under the same conditions. PMS was the principal source of hydroxyl and sulphate radicals that are essential for the degradation process. Therefore, to investigate the effect of the initial PMS concentration, experiments were carried out using PMS concentrations in the range 100–400 mg·L−1. The experimental results indicated a noticeable increase in the RhB removal with 200 mg·L−1 PMS. Therefore, the subsequent experiments were performed using the PMS concentration of 200 mg·L−1. The effect of the catalyst dosage was evaluated at 25, 50, 100, and 150 mg·L−1. Additionally, the effect of the initial RhB concentration was investigated.To study the contribution of the reactive species, methanol and TBA were used as the radical scavengers. To observe the effect of the reactive sites, dipotassium phosphate was used to mask them. The used catalysts were washed with ultrapure water and dried at 100 °C overnight. The recycling experiments were carried out at [RhB]0 = 3.40 mg·L−1, which was equal to the concentration of RhB after adsorption by the pristine ZN-CS at [RHB]0’ = 7.6 mg·L−1; all other steps remained the same. All the experiments were carried out twice or thrice, and the average data with their standard deviations were presented.The ZN-CS exhibited a strong adsorption affinity for RhB before the addition of PMS, with over 50% removal of RhB in 15 min (Fig. 1 ). The pH change of the solution during the removal process and all the kinetic results are shown in Fig. S2, Text S1 and Tables S1 separately. Additionally, the adsorption rate of RhB increased gradually, probably owing to both the decreasing RhB concentration in the aqueous phase and the gradual exhaustion of the adsorption sites. PMS was added to the solution to initiate the reaction when the adsorption equilibrium was reached. After 30 min, approximately 90% RhB was eliminated in the ZN-CS/PMS system, while only 8% RhB was removed in the PMS system. Moreover, the ZN-CS exhibited the best removal efficiency among the precursor and the catalyst with single metal (Appendix A Fig. S3). Additionally, the PMS concentration decreased rapidly in the beginning, and the decrease became gradual with time (Appendix A Fig. S4). The rapid consumption of PMS at the beginning was probably owing to the adsorption or some binding effects with the catalyst. Subsequently, the activation of PMS became gradual because of the saturation and depletion of the active sites. Thus, the material can adsorb RhB, and activate PMS for further degradation of the substrate. The coupling effect of adsorption−degradation presents certain practical application potential (Fig. S5). In the following analysis, the elimination process of RhB could be separated into two stages, adsorption and degradation. The possible mechanisms of both the stages were proposed.To confirm the crystallographic structure, phase purity, and structural stability of the ZN-CS, XRD patterns of the pristine and used samples were recorded (Fig. 2 (a)). The results demonstrated that the catalyst comprised ZnO (Joint Committee on Powder Diffraction File (JCPDF) #89-0510) and Ni3ZnC0.7 (JCPDF #28-0713). The distributions of ZnO and Ni3ZnC0.7 in the shell and core were approximately uniform (Appendix A Fig. S6 and Table S2). This indicated that the sample was of high purity, and no other crystalline impurities were detected. Additionally, the phase of the used sample was confirmed by XRD analysis. The phase of the obtained catalyst remained unchanged during the process. As can be confirmed from the wide XPS spectrum (Fig. 2(b)), the ZN-CS comprised four elements—Zn, Ni, C, and O. This result was consistent with those obtained from XRD analysis. The high-resolution Zn 2p spectrum (Fig. 2(c)) revealed two components: ZnO with binding energies of 1024.3 and 1047.6 eV, as well as Zn−Ni with two peaks positioned at 1021.7 and 1043.5 eV. The high-resolution Ni 2p spectrum (Fig. 2(d)) revealed two components: Ni(0) at 852.3 and 869.5 eV, and Ni2+ at 854.8 and 872.2 eV. Two shake-up satellite peaks at 859.6 and 879.1 eV were also observed. In general, the two forms of metals corresponded to the two main components— ZnO and Ni3ZnC0.7— in the XRD analysis. The formation of Ni2+ occurred possibly because of the surface oxidation of Ni. In addition, the relative content of ZnO slightly increased from 20% to 30% after the degradation process, indicating that Zn was partially oxidised and thus acted as an electron donor. There was no remarkable change in the valence ratio of Ni. This indicated that the contribution of metal gain and loss electrons to degradation was not significant.There are some classical explanations for the adsorption mechanism, including physical and chemical adsorption. Physical adsorption mainly involves the van der Waals forces and electrostatic attraction. In contrast, chemical adsorption involves the formation of chemical bonds, either by transfer or sharing of electrons, between the adsorbent molecules and the atoms or molecules on the solid surface of absorbent [43–45]. To determine the adsorption mechanisms, several experiments and theoretical calculations were conducted. (1) Physical adsorption capacity To determine the physical adsorption capacity of the samples, we examined their surface morphologies and atomic composition using SEM-EDS, and calculated their specific surface area and the average pore diameter by the nitrogen adsorption/desorption experiment. The synthesized catalyst exhibited a sphere-like morphology with a core–shell structure. Fig. 3 (a) presents an image of the pristine catalyst. The catalyst surface was loose and porous, with the external shape similar to that of Hydrangea macrophylla. Fig. 3(b) illustrates the electron micrograph of the catalyst magnified to 2000 times. The particle size of the catalyst was uniform, and the shell structures of few particles were damaged. Agglomeration in the range of approximately 2−4 μm within the particles was observed. After the adsorption, the pore channels were filled, with further aggregation of the particles (Figs. 3(c) and (d)). The catalyst shape did not change remarkably, and the core–shell structure remained stable (Figs. 3(e) and (f)) after the degradation. This was consistent with the results of XRD analysis. Owing to the continuous deposition of the surface materials, the surface pores were filled, and the particle surface was gradually passivated. This can be expected to result in a decrease in the adsorption capacity. with subsequent release of the active sites in the degradation process. Thus, the adsorption ability was regenerated for reuse. The bars in Figs. 3(d) and (f) were considered to be the impurities introduced in the recycling process. Additionally, the results of EDS analysis (Table 1 ) indicated changes in the oxygen-containing functional groups, which will be explained in the collaborative analysis of the FT-IR characterization later.As illustrated in Fig. 4 , the N2 adsorption–desorption isotherms were identified as type II with a type H3 hysteresis loop [46]. This was owing to the presence of large pores formed by the accumulation of flaky particles, and was consistent with the morphology of the precursors. The specific surface area of the sample calculated using BET analysis was 55.311 m2·g−1. As can be observed from the pore-size distribution diagram, the average pore diameter of the ZN-CS was less than 20 nm. The large specific surface area and narrow pores may also contribute to the enrichment of RhB and potentially provide enough active sites for the heterogeneous reaction process. (2) Electrostatic attraction The electrostatic factor may also play an important role in the adsorption process [47] as discussed here. We measured the zeta potential of the catalyst to determine its charge properties at different pH levels. The pH value at the point of zero charge (pHPZC) of the catalyst in the reaction system measured by zeta potential analyzer was approximately 7.5 (Appendix A Fig. S7). This result could be discussed from the following two aspects. First, the catalyst surface was negatively charged, and the negative charge increased with the pH value at pH > 7.5 for the ZN-CS. Moreover, when the pH was less than 7.5, the surface became positively charged, and the positive charge increased as the pH value decreased. The pK a of RhB is 3.0 and its K OW is 190 [48]. For pH > 7.5, 90% of the carboxylic acid molecules on RhB dissociated, and the number of the amphoteric ions (those containing the carboxylate ion and quaternary ammonium cation) of RhB increased with the pH value. For pH < 7.5, the carboxylic acid dissociation of RhB decreased with decrease in the pH value.Thus, an increase in the pH value was conducive to the improvement in the electrostatic attraction between the catalyst and quaternary ammonium cation of RhB. In addition, the electrostatic repulsion between the catalyst and carboxylate ion on RhB increased with the increase in the pH value. As can be observed from Appendix A Fig. S8 an improved adsorption ability was obtained with the pH value of 3.02 or 8.96, both at severe conditions. The possible reason is that the greater charge on the catalyst led to stronger electrostatic attractions under the abovementioned conditions.In addition to the physical adsorption between RhB and the ZN-CS, the surface complexation that involved chemical bonding and contributed to the adsorption process is also discussed here. As the functional groups played a vital role in the chemical bonding between the absorbent and adsorbate, FT-IR analysis of the catalyst was conducted to determine the main functional groups involved in the adsorption process. The samples were dried at 100 °C overnight to decrease the interference of the bound water with the absorption peak. The band at 750 cm−1 was assigned to the bending vibration of O−H (γ O−H) (Fig. 5 ). The broad band observed at approximately 3425 cm−1 was attributed to the stretching mode of O−H (ν O−H) owing to the presence of hydroxyl [18], and the decline in the band intensity might be attributed to the consumption and regeneration of the surface hydroxyl groups. Furthermore, when the catalyst was used to adsorb RhB, the abovementioned peak underwent a blue-shift of 5 cm−1 (from 3425 to 3430 cm−1), which indicated that RhB bonded with the catalyst by replacing the O−H groups on the surface of the oxide [49,50]. Additionally, because of the vibration of the aromatic rings [51,52], a new peak at 1178 cm−1 was observed for the RhB-adsorbed sample. These results indicated that the adsorption mechanism involved the surface complexation between RhB and the ZN-CS. To substantiate the role of chemical adsorption, we used phosphate to mask the hydroxyl groups on the surface of the ZN-CS, as phosphate exhibits stronger affinity for this adsorption site [53]. The results revealed that the adsorption capacity decreased by approximately 10% in presence of the masking agent (Appendix A Fig. S9). This indicated that the hydroxyl groups were involved in the chemisorption.The study of adsorption kinetics is essential to elucidate the adsorption mechanism. Therefore, we calculated the kinetic data of the adsorption process using pseudo-first-order [54] and pseudo-second-order [55,56] simulations (Appendix A Text S2). The calculated kinetic data (Table 2 ) revealed that the adsorption process was better described with the second-order kinetics, indicating that the chemisorption was the rate-determining step [56].In addition, the fitting results of different adsorption models demonstrated that the adsorption process can be best described with the Freundlich model and the sorption of RhBon the ZN-CS surface was essentially chemical (Appendix A Text S3 and Tables S3). The values of thermodynamic parameters (ΔG, ΔS, and ΔH) revealed that the adsorption of RhB on the ZN-CS surface was spontaneous, feasible, and exothermic (Appendix A Fig. S10, Text S4 and Tables S4). In conclusion, the adsorption process was mainly determined by the van der Waals forces, electrostatic attraction, and the surface complexation of the hydroxyl groups, with chemisorption being the rate-determining step.To determine the reactive species involved, different quenchers were used, and their contribution to the RhB degradation was investigated (Fig. 6 (a)). Methanol and TBA were used to quench the S O 4 - ∙ and H O ∙ radicals [57–59]. However, stronger inhibition effect was observed after the addition of TBA (Fig. 6(a)), which was contrary to what was expected. This might have resulted from the high viscosity of TBA [60]. Therefore, additional experiments were required to be conducted to identify the active species.EPR analysis was carried out to determine the responsible radical species using dimethyl pyridine N-oxide (DMPO) and 2,2,6,6-tetramethylpiperidine (TEMP) as the spin-trap agent. The characteristic peaks corresponding to the DMPO-OH adducts and feeble signals corresponding to the DMPO-SO4 adducts were observed in the PMS/ZN-CS system (Fig. 6(b)). As the capture of the trace sulphate radical in the actual detection was difficult, the signal corresponding to the DMPO-SO4 adducts was weak, and had the same height as that of the noise signal. Moreover, no signal corresponding to O 2 ∙ - was detected. These results were consistent with some previous observations [61–63]. Considering the production of the singlet oxygen during the self-decomposition of PMS [64], we conducted a controlled experiment using TEMP as the capture agent to determine if the catalyst could promote the production of the singlet oxygen. Consequently, the signal strength of the PMS system was similar to that of the PMS/ZN-CS system within the error range. This indicated that the production of the singlet oxygen cannot be facilitated in the PMS/ZN-CS system. Moreover, the self-decomposition of S O 5 · - radicals can readily proceed owing to its high reaction rate ( ≈ 2 × 108 M−1·s−1) and low activation energy (E a = 7.4 ± 2.4 kcal·mol−1), resulting in the fast generation of 1O2(Appendix A T e x t S 5 , E q s . ( S 14 ) a n d ( S 15 ) ) [65–68]. Thus, it can be concluded that radicals (mainly H O ∙ ) were generated in the ZN-CS/PMS system and that these radicals played an important role in the degradation of RhB. In the radical pathway, Zn and Ni were involved in the direct redox process of PMS; the possible reaction is given in T e x t S 6 , Eqs. (S1 6 - S 19 ) in Appendix A [69–71]. However, the effect on the degradation rate was small, owing to the masking of the S O 4 - ∙ and H O ∙ radicals by methanol. This suggests a more dominant mechanism of degradation.Ding et al. [72] summarized the methods for the estimation of the contribution rates of the radical and non-radical processes (Appendix A Text S7, Eqs. (S20−S22)). The results revealed that the contribution rate of the radical process was approximately 34.1%, and that of the non-radical process was approximately 65.9%. This indicates that the non-radical process played an important role in this system.In recent years, the mechanism of the indirect oxidation of pollutants by oxidants has been proposed. Increased attention has been paid to the direct electron transfer between the pollutants and high-potential intermediates formed by the carbon materials and oxidants. Ren et al. [73] suggested that peroxydisulfate (PDS) can be catalyzed by carbon nanotubes (CNTs) to form a high-redox potential composite to degrade organic compounds directly. Based on his research, we used the catalyst as electrodes to confirm the formation of the high-potential intermediates (Appendix A Text S8), and monitored the open-circuit potential by chronopotentiometry analysis. The open-circuit potential increased remarkably after the addition of PMS (Fig. 6(c)), indicating that the catalyst and PMS combined to form the high-potential intermediate (denoted as ZN-CS*PMS). The gradual decrease in the potential can be attributed to the consumption of highly potential-active substances. Subsequent supplementation with PMS can aid in the recovery of the potential (Fig. 6(d)). This indicates the potential for the direct oxidation ability of ZN-CS*PMS.Additionally, it is essential to determine the active sites on the catalysts to elucidate the mechanism. To determine the active sites that activate PMS, changes in the functional groups of the catalysts after degradation were analysed. Fig. 5 illustrates the FT-IR spectra of the ZN-CS in the range 4000−400 cm−1. Five distinct adsorption bands were identified at approximately 460, 750, 1137, 1383, 1570, and 3425 cm−1. As mentioned earlier, owing to the presence of the hydroxyl [18], the increased intensity of the broad band at approximately 3425 cm−1 indicated the regeneration of the O−H in the degradation process. The red shift of this band after the addition of PMS indicated that the complexation between RhB and the ZN-CS was destroyed, and that the RhB adsorbed on the catalyst surface was partially degraded. After the reaction, the decrease in the absorption band at approximately 460 cm−1 (corresponding to the stretching of Zn−O bond) [74–77], indicated that ZnO was either consumed or leached. Finally, the remaining three spectral lines were almost the same, confirming the stability of the ZN-CS. To confirm the role of the surface hydroxyl groups, a masking experiment was conducted using a phosphate with stronger affinity [53]. The RhB degradation exhibited a remarkable inhibition (Fig. 6(a)), confirming the role of the surface hydroxyl groups in the PMS activation. ZnO is a semiconductor that contains numerous mobile electrons and exhibits good capacitance characteristics [78]. It can transfer and store electrons, and is conducive to the electron transfer and conduction between the ZN-CS*PMS intermediates and pollutants. In addition, Ni3ZnC0.7 exhibits good electrical conductivity [79] and electron-transfer ability. Thus, it can be assumed that Zn and Ni play an important role in the electron transfer between the ZN-CS*PMS and organic contaminants in the non-radical pathway. Therefore, Zn and Ni may act as electron donors and carriers in the free radical process, whereas in the non-radical process, both of them mainly contribute to the electron conduction. Thus, a possible degradation mechanism with the effect of organic moieties of the ZN-CS can be expressed in terms of the following equations (Eqs. (1–7)). (1) e - + H S O 5 - → S O 4 2 - + H O ∙ (2) e - + H S O 5 - → O H - + S O 4 - ∙ (3) e - + H S O 5 - → S O 5 - ∙ + H + (4) O H - + S O 4 - ∙ → S O 4 2 - + H O ∙ (5) SO 4 2 - + HO ∙ → SO 4 - ∙ + OH - (6) ZN - C S - O H + P M S → Z N - C S ∗ P M S (7) ZN - C S ∗ P M S + P o l l u tan t s → C O 2 + H 2 O In the recycling experiments, the removal of RhB was divided into two stages. To simplify the regeneration and reuse, we exclusively used deionized water to clean and dry the catalyst without taking special measures for the catalyst desorption. In the recycling experiments, the catalyst maintained the removal rate of over 90% as shown in Fig. 7 . The regenerated catalyst exhibited better degradation effect on RhB. The reasons for the better recycling performance of the catalyst are as follows. First, the recycled samples exhibited a certain adsorption capacity towards RhB in the recycling experiments even without desorption. This was because some of the originally adsorbed RhB had been degraded in the batch experiments, and the recycled samples could recover a certain adsorption capacity. The second factor is that the generated free radicals or the ZN-CS*PMS mainly attacked the adsorbed dyes on the surface. The free RhB molecules in the solution were rarely attacked, resulting in a low decolorization rate in the solution during the degradation stage. In contrast, the pre-adsorption step was omitted in the recycling experiments, and the free radicals generated in the ZN-CS/PMS system attacked many free RhB molecules in the solution, thereby improving the removal efficiency. The third factor is that the main active sites in the adsorption and degradation stages were all surface hydroxyl groups. In the recycling experiments, only a fraction of the surface hydroxyl groups was occupied by the dye molecules, thereby leading to more available active sites and improved removal efficiency. This result indicated that the adsorption and degradation processes exhibited a coupling effect, and the ZN-CS maintained adequate performance in such a continuous process. The recyclability of the catalyst is conducive to promoting cleaner production techniques.Based on the phenomenon and analysis mentioned above, we can summarize the mechanism of the entire process (Fig. 8 ). First, because of the van der Waals forces, electrostatic attraction, and hydrogen bond complexation, several RhB molecules were adsorbed on the catalyst surface leading to their partial removal from the solution. Simultaneously, the RhB molecules were transformed into two forms: the adsorbed form and the free form (free in the aqueous phase). After the addition of PMS, a radical and a non-radical pathway of the degradation were observed; these pathways simultaneously attacked the RhB molecules in both the forms. Subsequently, the free RhB in the solution was almost completely removed, and so was the adsorbed RhB. This resulted in the partial regeneration of the adsorption capacity of the ZN-CS. The catalyst was now ready for reuse. Furthermore, the surface hydroxyl groups were the main active sites for both the adsorption and degradation processes. Therefore, the degradation of the adsorbed RhB was conducive to the regeneration of the active sites that promotes the degradation process. This may be a reason for the improvement in the regeneration performance. Finally, the magnetic ZN-CS could be easily separated from the solution that had been degraded.To check the wide suitability of the ZN-CS, elimination experiments on different target contaminants (AO7, MB, Nor, and TC) were carried out using the ZN-CS/PMS system. Fig. 9 and Fig. S11 illustrate the experimental results. The basic information about the target pollutants and experimental conditions are given in Appendix A Table S5. MB is a typical cationic dye, AO7 is a typical anionic dye, and TC and Nor are representatives of pharmaceuticals and personal care products (PCPs), respectively, in water. They exhibit different sizes and structures, different electronegativities in water, and different hydrogen bond receptors and donors, leading to their possibly different removal effects. As illustrated in Fig. 9, the ZN-CS/PMS system exhibits a removal efficiency of more than 70% for the AO7 removal, while in the PMS system, the removal effects could be ignored. The increase in the AO7 concentration in the solution phase at the fifth minute may be because of the addition of PMS that leads to the desorption of the partially adsorbed AO7. For MB, the ZN-CS/PMS system demonstrates over 90% removal efficiency, while the removal efficiency under the PMS system is less than 20%. For Nor, the system exhibits a removal efficiency of more than 50%, while the efficiency is approximately 20% in the PMS system. For TC, the removal efficiency of the system can reach approximately 80%, while the removal efficiency of the PMS system is approximately 40%. Fig. S9 and Text S9 illustrate the effect of several vital parameters. Fig. S12 illustrate the elimination of RhB in real water sample. In brief, the ZN-CS offers good adsorption and degradation efficiency towards various pollutants that exhibit different electric properties and sizes. Thus, the ZN-CS system presents a wide range of application prospects.In summary, the magnetic composite ZnO/Ni3ZnC0.7 was successfully synthesized and developed as an effective adsorbent and a heterogeneous catalyst for the PMS oxidation to eliminate a variety of organic compounds. The magnetic properties of this nanocomposite led to a rapid and easy separation from the solutions. This study proposes a probable mechanism of the adsorption process that relates to the electrostatic factor and hydrogen bonding. The mechanism of the degradation process indicated that the organic compounds were mainly oxidized by the high-potential intermediate, ZN-CS*PMS, and the hydroxyl radicals generated by PMS, which were primarily activated by the surface hydroxyl groups. The adsorption capacity of the ZN-CS is regenerated owing to the maximum degradation of the adsorbed substrate, achieving the coupling effect. Compared with the systems used in some previous studies, this system used no precious metals. Moreover, this system employed the adsorption–degradation process to achieve a balance between the economic and treatment effect. Furthermore, the synthesized catalyst exhibits magnetic properties, recyclability, stable structure, and good removal efficiency for a variety of organic matter. Our work provides an insight into the development of highly efficient magnetic MOF-based materials for wastewater treatment, and has potential application prospects in the treatment of printing and dyeing wastewater or medical wastewater.This work was supported by the National Natural Science Foundation of China (51878357), the National Science Foundation of Tianjin (18JCYBJC23200), the Innovation Spark Project of Sichuan University (2019SCUH0009), and the Foundation of Science & Technology Department of Sichuan Province (2020YJ0061).Youwen Shuai, Xue Huang, Benyin Zhang, Lu Xiang, Hao Xu, Qian Ye, Jinfeng Lu, and Jing Zhang declare that they have no conflicts of interest or financial conflicts to disclose.

The heterogeneous catalytic activation of peroxymonosulfate for wastewater treatment is attracting increased research interest. Therefore, it is essential to find a sustainable, economical, and effective activated material for wastewater treatment. In this study, metal–organic frameworks (MOF)-5 was used as the precursor, and a stable and recyclable material ZnO@Ni3ZnC0.7 that exhibited good adsorption and catalytic properties, was obtained by the addition of nickel and subsequent calcination. To investigate and optimize the practical application conditions, the elimination of rhodamine B (RhB) in water was selected as the model process. This study demonstrated that the degradation of organic matter in the system involved a coupling of the adsorption and degradation processes. Based on this, the mechanism of the entire process was proposed. The results of scanning electron microscopy, infrared spectrum analysis, and theoretical analysis confirmed that the van der Waals forces, electrostatic attraction, and hydrogen bonding influenced the adsorption process. Electron paramagnetic resonance analysis, masking experiments, and electrochemical tests conducted during the oxidative degradation process confirmed that the degradation mechanism of RhB included both radical and non-free radical pathways, and that the surface hydroxyl group was the key active site. The degradation of the adsorbed substrates enabled the regeneration of the active sites. The material regenerated using a simple method exhibited good efficiency for the removal of organic compounds in four-cycle tests. Moreover, this material can effectively remove a variety of organic pollutants, and can be easily recovered owing to its magnetic properties. The results demonstrated that the use of heterogeneous catalytic materials with good adsorption capacity could be an economical and beneficial strategy.

Nowadays, governments around the world are all committed to achieving carbon neutral goals. Under this background, many countries are developing new energy technology and searching for alternative fuel [1–3]. Hydrogen is a kind of non-carbon energy. It is helpful for solving energy depletion and environmental pollution problems [4]. Water electrolysis is a common way for hydrogen production [5,6]. The most efficient catalysts for water electrolysis are Platinum group materials [7]. But they are always rare and expensive [8]. It is particularly important to develop highly active non-noble metal catalyst. Nickel-based coatings are excellent electrode materials for HER. Various Ni-based electrode materials, typically metallic oxide composite material, have been intensively investigated as promising alternative catalysts for the HER. Ren B et. al. [9] prepared Ni-MoO2 composite electrodes for HER. The combination of Ni and MoO2 species increased the Ni-H bond strength, which accelerated the formation of Hads on Ni. The catalyst activity was enhanced by the synergistic effect between Ni and MoO2. Wang N et. al. [10] made Ni-ZnO electrode. The ZnO nanowires on Ni substrates provided high electron mobility to facilitate hydrogen evolution. Kullaiah R et. al. [11] deposited Ni-TiO2 electrode by composite electroplating method. The addition of TiO2 facilitated the formation of tiny nickel grains and increased the number of active site. These composite electrodes exhibited excellent HER activity in alkaline solutions. However, the addition of metallic oxide might reduce the electrical conductivity for electrodes, which greatly limited the further increase of electrocatalytic performance [12]. The carbon materials were always used to modify metallic oxide for their high specific surface and good electronic conductivity [13–15]. Shibli et. al. [16] used RGO to modify Fe2O3-TiO2-NiP coating. The synergistic effect between Fe2O3-TiO2 and RGO enhanced HER performance. Sasidharan et. al. [17] studied the effect of GO on the activity of Fe2O3-TiO2-NiCoP coatings for HER. The results showed that the Fe2O3-TiO2-GO-NiCoP exhibited much better catalytic activity than Fe2O3-TiO2-NiCoP. The addition of GO greatly decreased the electrical resistance of composite coatings. It had been proved in our pervious study that SnO2 was a good catalyst for HER [18]. However, its low conductivity limited the further increase of the catalyst activity. In this study, SnO2 combining with XC-72 carbon were used as composite particles to improve the hydrogen evolution activity. We first synthesized C-SnO2 composite particles by hydrolysis method. And then, these C-SnO2 composite particles were suspended in nickel plating solution. The Ni/C-SnO2 composite coating was prepared by composite electrodeposited method. Multiple physical and electrochemical tests were employed to investigate the effect of C-SnO2 particles embed in Ni coating on the activity of HER.Preparation of C-SnO2: The 200 mg carbon powder was dispersed in 25 mL ethylene glycol. The mixture was stirred for 3 h at room temperature. The powder of 64 mg anhydrous SnCl2 was dissolved in 25 mL ethylene glycol. The acquired solution was dispersed by ultrasonication for 30 min and then stirred for 30 min at room temperature. The above two solutions were mixed evenly and then moved into three-necked flask (100 mL). The mixture was heated and refluxed at 196 ℃ for 8 h. After cooling to room temperature naturally, the product was separated from reaction mixture by suction filtration. The products were washed in sequence with deionized water and absolute ethanol, and then dried at 80 ℃ for 12 h in vacuum.Preparation of Ni/C-SnO2 composite coating: The Ni/C-SnO2 composite coating was fabricated by a composite electrolytic deposition method. The composition of the bath was 350 g·L−1 Ni(NH2SO3)2·4 H2O, 10 g·L−1 NiCl2·6 H2O, 30 g·L−1 NH4Cl and different content of C-SnO2 nanoparticles. The composite electrolytic deposition process was carried out at a current density of 3 A/dm2 for 30 min at 35 ℃ with a magnetic stirring at 850 rpm. The pH value was 3.8. These obtained composite coatings were labeled as Ni/C-SnO2-0.25, Ni/C-SnO2-0.5, Ni/C-SnO2-0.75 and Ni/C-SnO2-1, which represented 0.25 g·L−1, 0.5 g·L−1, 0.75 g·L−1 and 1 g·L−1 C-SnO2 nanoparticles applied in above nickel plating bath, respectively.The morphology of the Ni/C-SnO2 coatings was studied using SEM on FEI Quanta 400 equipment operating at 20 kV acceleration voltage. The composition of coatings were analyzed from quantitative EDX coupled with the SEM. The crystal structure of Ni/C-SnO2 coatings were determined by XRD (D8 Advance, Bruker). Fourier transform infrared (FT-IR) spectra were performed on a Nicolet iS10 with the wave number from 4000 to 400 cm−1.All the electrochemical measurements were performed in a standard three-electrode cell with 1 M NaOH solution by the PARSTAT 4000 electrochemical workstation. The composite electrodes were used as the working electrode. A platinum foil (1 cm × 1 cm) was used as the counter electrode and an Hg/HgO electrode was used as the reference electrode. The scan rate of steady-state polarization test was 5 mV·s−1. The EIS measurements were carried out at a potential of − 0.125 V (vs. SCE) in frequency range frm 100 kHz to 0.01 Hz with an AC voltage amplitude of 0.005 V. The chronopotential curves were continuous measured at − 100 mA·cm−2 for 10 h. Fig. 1 is the FTIR spectra of homemade C-SnO2 nanoparticles. Before the spectra, the C-SnO2 particles are dehydrated at 200 °C under vacuum condition. The band at 500–750 cm−1 is related to Sn-O stretching vibration. The bands at 1600 cm−1 and 3434 cm−1 are due to O-H bending vibration and O-H stretching vibration [19]. As shown in Fig. 1, there are abundant OHads species on the surface of C-SnO2 nanoparticles. These OHads promote the formation of Hads and improve the activity of HER [20].The XRD pattern of C-SnO2 particles is shown in Fig. 2. The SnO2 has a cassiterite phase structure, showing the major diffraction peaks of (110), (101), (210), (211), (301), (321), and so on. The first diffraction peaks for C-SnO2 at 2θ about 26° can be attributed to the hexagonal graphite structures (002) of the XC-72 carbon black [21]. The SnO2 (211) peak is chosen to calculate the mean particle size of SnO2 according to Debye–Scherrer formula [22]. The calculated particle size of C-SnO2 is about 150 nm. Fig. 3 shows the XRD patterns of Ni and Ni/C-SnO2-0.5 coatings. It can be seen that the diffraction peaks of Ni and Ni/C-SnO2-0.5 at 44.5°, 51.8° and 76.4° are indexed to (111), (200) and (220) crystal planes of standard Ni(JCPDS:03–1051). The peaks of Ni/C-SnO2-0.5 at 26.6°, 33.8° and 38.9° are assigned to SnO2 (JCPDS:41–1445). The C-SnO2 composite particles are nicely co-deposited in Ni coating. The addition of C-SnO2 composite particles increases the diffraction intensity of Ni (111) and decreases that of Ni (200). The preferred orientation has been changed to Ni (111) in composite coatings. The catalytic activity of Ni (111) is higher than Ni (200) for HER [23]. The half width values of Ni (111) for Ni and Ni/C-SnO2-0.5 are 0.194° and 0.223°, respectively. According to Debye-Scherrer formula, the larger value of FWHM represents the smaller crystal size [24,25]. Therefore, the grain size of Ni/C-SnO2-0.5 is smaller than Ni. The tiny Ni grains of composite electrode support more active sites for HER. Fig. 4 shows EDX analysis of Ni/C-SnO2-0.5 composite electrodes. The presences of all elements are confirmed, and the quality percentage of C-SnO2 in the composite coatings is about 15.94%. The content of C-SnO2 in the coatings analyzed by EDX are shown in Fig. 5. The amount of C-SnO2 in composite electrode tend to rise with the increase of the C-SnO2 concentration in the bath. Fig. 6 is the SEM images of different electrodes with low and high-magnification. As shown in Fig. 6(a), the pure Ni electrode appears a typical block shape. Fig. 6(b) shows that the spherical C-SnO2 particles are deposited uniformly on the surface of composite electrode. The particle size of C-SnO2 is about 150 nm. This result is consistent with the above XRD. The addition of C-SnO2 composite particles refine the crystallization of coating, which enlarge the active surface area of composite coatings. Fig. 6(c)-(j) shows that, too much amount of C-SnO2 makes the coating compact and nonporous.The effect of C-SnO2 on catalytic activity of composite coatings for HER are investigated by several electrochemical tests. The relevant parameters of these electrochemical measurements are listed in Table 1. Here η 10 is the cathode overpotential at 10 mA·cm−2. j 0 is the exchange current density. b is the Tafel slope. The cathode polarization curves of different coatings are shown in Fig. 7(a). Fig. 7(a) reveals that the C-SnO2 addition enhance the HER performance. Too much C-SnO2 in electrodes are bad for HER. The Ni/C-SnO2-0.5 electrode shows the best catalytic activity for HER. Ni/C-SnO2-0.5 composite coating has the lowest overpotential of 304 mV to reach current densities of 10 mA·cm−2.The Tafel curves of different electrodes are shown in Fig. 7(b). As shown in Table 1, the composite electrodes exhibit higher j 0 and lower η 10 than pure Ni. The j 0 values firstly enhance and then decline with the increase of C-SnO2 content in coatings. The Ni/C-SnO2-0.5 composite coating presents the highest j 0 value of 57.44 uA·cm−2, which is 87.96 times higher than that of the Ni coating. The Tafel slope b is a crucial parameter. b can reflect the dominant mechanism of HER. There are three principal steps in HER in alkaline media, including Volmer, Heyrovsky and Tafel. When Volmer step dominates the reaction, the corresponding b values is about 120 mV·dec−1 [26]. As shown in Table 1, the b values of the Ni and those Ni/C-SnO2 coatings are 122, 144, 136, 140 and 156 mV·dec−1 respectively. The rate determined step of HER is Volmer reaction. The facilitation of Hads formation can accelerate HER. The abundant OHads on C-SnO2 surface can accelerate the water molecular decomposition and Hads formation. The addition of C-SnO2 enhances the catalytic activity of HER [18,20]. Fig. 7(c) displays the Nyquist plots of electrodes, and the inset shows the equivalent circuit. The fitting results are listed in Table 1. R s is the solution resistance. R ct is the charge transfer resistance, and C dl is the double layer capacitance. It can be seen that the R ct value of Ni/C-SnO2-0.5 is the lowest. It means that the performance of HER on Ni/C-SnO2-0.5 is the highest. The R S value of the composite electrodes are smaller than that of Ni. The carbon in C-SnO2 increases the conductivity of composite electrode. As shown in Table 1, the C dl value of Ni/C-SnO2-0.5 is the highest. The value of C dl is proportional to the quantity of active sites [27]. The large surface area is conducive to raise the hydrogen evolution activity of composite coating [28–30]. The results are conform with the analysis of LSV and Tafel. Fig. 8 exhibits the volume of H2 produced on different electrodes for 300 s at a cathode current density of 300 mA·cm−2. The Ni/C-SnO2-0.5 composite electrode produces the largest amount of H2 (19.2 mL), The Faraday efficiency values of the different electrodes Ni, Ni/C-SnO2-0.25, Ni/C-SnO2-0.5, Ni/C-SnO2-0.75, and Ni/C-SnO2-1 are calculated to be 92.18%, 95.24%, 96.46%, 95.75%, and 94.65% according to the formula (1) [31], which verifies the best activity of Ni/C-SnO2-0.5 for HER. (1) F E = 2 ⋅ N A ⋅ P ⋅ V H 2 ⋅ e i ⋅ t ⋅ R ⋅ T where V is the volume of H2, i is the current, t is the reaction time, P is the standard atmospheric pressure, T is the reaction temperature. Fig. 9(a) shows the chronopotentiometry curves of different coatings in 1 M NaOH. The analysis results of the stability test are shown in Table 2. φ 0 and φ 10 are the hydrogen evolution potentials of the electrodes before and after the 10 h stability test. Δφ is the potential difference between φ 0 and φ 10 . After stability test, the Δφ of Ni/C-SnO2-0.5 is the lowest and the decrease of potential is less than 2%. The polarization curves of Ni/C-SnO2-0.5 before and after the stability test are shown in Fig. 9(b). The Ni/C-SnO2-0.5 composite electrode shows excellent stability in alkaline solution.The Ni/C-SnO2 composite coating are compared with some composite electrode materials reported in literatures. The kinetic parameters of HER on these electrode materials are listed in Table 3. The Ni/C-SnO2 composite coating exhibits more excellent activity of HER.In this work, the C-SnO2 composite particles are synthesized by high temperature hydrolysis method. The Ni/C-SnO2 electrodes with high HER catalytic activity have been prepared by composite electrolytic deposition technology. Compared to the Ni coating, the Ni/C-SnO2 catalyst shows higher exchange current density and lower hydrogen evolution overpotential for HER. Moreover, the activity of Ni/C-SnO2 catalyst is better than other reported electrocatalysts for HER. The co-deposition of C-SnO2 refines the crystallization of Ni, which enlarge the active surface area of coating. The co-deposition of C-SnO2 changes the preferred orientation of Ni to (111) crystal plane which is more conducive to HER. The addition of C-SnO2 enhances the conductivity of composite coating and accelerates the rate of HER. Meanwhile, the OHads on C-SnO2 surface promotes the decomposition of water and the formation of adsorbed Hads. These results indicate that the Ni/C-SnO2 catalyst is a high-activity, low-cost and stable electrocatalysts for HER.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the Natural Science Foundation Project of Heilongjiang Provincial (No. LH2022B023, JJ2022LH0472), the Basic Scientific Research Program of Heilongjiang (No. 1452ZD008), the Youth Backbone Project of Mudanjiang Normal University (No. QN2022007), the Ideological and Political High-quality Construction Project of Mudanjiang Normal University, the Innovation Project of University Students (202210233001).

The high performance Ni/C-SnO2 composite electrodes were successfully prepared for hydrogen evolution reaction (HER) in alkaline media by an easy and low cost composite electrolytic deposition method. The Ni/C-SnO2 composite coating was physically examined by using various techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive analysis of X-ray (EDX). The XRD and EDX results showed that C-SnO2 particles prepared by high temperature hydrolysis method were successfully incorporated in Ni coatings. The preferred orientation of Ni particle in composited coatings had been altered to (111) crystal plane which was more benefit for HER. The SEM and XRD results indicated that the addition of C-SnO2 particles refined Ni crystallization in composite coatings. The catalytic activity and stability of the composite coatings for hydrogen evolution reaction were examined by steady-state polarization, Tafel plots, electrochemical impedance spectroscopy (EIS) and chronopotentiometry technique. Compared to Ni coating, the Ni/C-SnO2 composite coatings exhibited higher hydrogen evolution performance. The abundant OHads on C-SnO2 surface accelerated the water molecular decomposition and Hads formation which determined the rate of HER (Volmer step). The carbon in C-SnO2 improved the conductivity of composite electrode, which was also helpful for HER.

Much attention was paid to the fluid catalytic cracking (FCC) of heavy oil due to its greatly enhanced processing difficulty because of the large molecules (Fu et al., 2006; Puente et al., 2007; Talmadge et al., 2014; Corma et al., 2017). It was well known that FCC of large molecules of heavy oil would be carried out in a successive way: (1) the large heavy oil molecules are precracked firstly in the macropores of the matrix, such as kaolin; (2) the products obtained in step (1) cracked further in the mesopores; (3) the intermediate products of step (2) cracked more selectively to valuable products, such as gasoline, diesel, and other chemical products (Ji et al., 2018). In addition to this, the improvement of FCC catalysts in the face of new challenges (such as new feeds and less polluting products) would be extremely urgent (Valle et al., 2019; Palos et al., 2021). Therefore, it is of vital importance to implant mesoporosity into FCC catalysts of heavy oil.To date, many approaches to introducing mesoporosity into FCC catalysts have been developed. A typical way is the so-called “top-to-down” method, which involves the removal of Al or Si species of zeolites (Verboekend et al., 2013; Jia et al., 2019). Unfortunately, this process suffers from zeolitic loss due to the destructive route. Another procedure “down-to-up” is a convenient approach to generating mesoporosity directly (Saxena et al., 2014; Kerstens et al., 2020). However, the low hydrothermal stability and high synthesis cost of the obtained materials still hinder its industrial application in severe conditions of FCC. Therefore, how to obtain hydrothermally stable MAs synthesized with low synthesis cost is still a great challenge in both FCC process and materials science.Our group (Liu et al., 2013a; Cao et al., 2014; Jin et al., 2014; Mi et al., 2018a; Chen et al., 2021; Li et al., 2022) has obtained MAs with excellent hydrothermal stability by a novel strategy that introduces zeolite Y precursors (the primary and secondary structural unit of zeolite Y) into the walls of mesoporosity. Just in this idea, our group obtained MAs with hydrothermal stability comparable to that of USY (Liu et al., 2014a, 2021; Mi et al., 2017a, 2017b, 2018b). For example, the BET surface area of MAs decreased from 595.4 m2 g−1 to 153.9 m2 g−1 after a severe hydrothermal treatment in 100% water vapor at 800 °C for 12 h. Moreover, the materials showed excellent catalytic properties when it was employed in heavy oil FCC in the MAT unit (Liu et al., 2013b, 2014b, 2014c; Deng et al., 2022). Although the significant progress in synthesis and application of hydrothermally stable MAs, the commercial manufacturing of materials, preparation of industrial catalysts, and industrial application in pilot FCC unit are still a great challenge for the material scientists.Herein, we report the most recent progress in commercial manufacturing and catalytic cracking performance in the FCC unit of 1.2-million tons in a refinery. To the best of our knowledge, the research progress represents the most advanced stage of commercial manufacturing and industrial application in FCC of MAs.Industrial reagent of Pluronic P123 triblock copolymer (EO20PO70EO20) was obtained from Henan Ruiyi Chemical Company. Water glass (containing 27.78 wt% SiO2 and 8.98 wt% Na2O) was purchased from Tangshan Shihe Sodium Silicate Company.The mixtures of Na2SiO3, Al2(SO4)3·18H2O, and NaOH solution with a molar ratio of Al2O3/SiO2/Na2O/H2O = 1/16–19/15–20/300–320 were prepared. After rapidly stirring for 30–80 min, the prepared mixtures undergo an ageing with stirring at 80–100 °C for 4–20 h. The obtained sticky solution was denoted as “zeolite Y precursors”. (1) P123 and water were added to the reactor. Zeolite Y precursors and H2SO4 (6 M) were slowly added to the above reactor to keep the pH of the system at about 1.0–3.0. The obtained solution was stirred at 30–60 °C for 10–40 h. (2) The liquid product of step (1) were transferred into crystallization autoclaves of 10 m3 for crystallization under 100–140 °C for 24 h. The as-synthesized products were processed by the subsequent filtering, washing, and drying at 120 °C for 2 h. Then, the resultant solid was calcined at 550 °C for 6 h in order to remove the organic template, the obtained product was denoted as “IS” (3) IS was hydrothermally aged under a severe condition (800 °C, and 100% water vapor) for 4 h and the product was named as “HIS”. P123 and water were added to the reactor. Zeolite Y precursors and H2SO4 (6 M) were slowly added to the above reactor to keep the pH of the system at about 1.0–3.0. The obtained solution was stirred at 30–60 °C for 10–40 h.The liquid product of step (1) were transferred into crystallization autoclaves of 10 m3 for crystallization under 100–140 °C for 24 h. The as-synthesized products were processed by the subsequent filtering, washing, and drying at 120 °C for 2 h. Then, the resultant solid was calcined at 550 °C for 6 h in order to remove the organic template, the obtained product was denoted as “IS”IS was hydrothermally aged under a severe condition (800 °C, and 100% water vapor) for 4 h and the product was named as “HIS”.Diffractometer Rigaku D/Max 2500VB2+/PC equipped with Cu Kα radiation was used for the study of X-ray diffraction (XRD) patterns for the obtained samples. JEM 100CX instrument with 200 kV acceleration voltage was used to study the TEM images. A Micromeritics ASAP 2405N system was used to investigate the N2 ad-desorption isotherms using liquid nitrogen at 77 K. Moreover, The curves of pore-size distribution of materials was obtained by the traditional Barrett-Joyner-Halenda (BJH) method.The amount of acid and the type of acid are calculated using the following formula: C p y − B = 1.88 I A B R 2 W ; 1.88 = π ε B , ε B = 1.67 ± 0.1 c m / μm o l C p y − L = 1.42 I A L R 2 W ; 1.42 = π ε L , ε L = 2.22 ± 0.1 c m / μm o l where C py-B and C py-L represent the concentration of Brønsted and Lewis acids, respectively. I A(B) and I A(L) represent the integrated absorbance of Brønsted and Lewis acids, respectively. R is the radius of the wafer (cm). W is weight of wafers (mg).The catalytic properties of FCC catalysts were evaluated at Lanzhou Petrochemical Research Center, using the ACE unit Model R+MM from Kayser Technology. The fluidization of the reactor was achieved by a stream of nitrogen. 9 g catalyst was fluidized and stabilized at catalyst/oil ratio 5 and the reaction temperature 530 °C.The feeding heavy oil was characterized with viscosity 10.38–14.35 mm2/s, residual carbon 3.4–4.2 wt%, and density 900 kg/m3.∼25 tons of MAs were manufactured at a commercial zeolite manufacturing corporation by using the existing production equipments. This is the first time that MAs are manufactured at an industrial scale. Characterization results (Fig. 1 and Fig. 2 ) showed that the industrial product had similar physicochemical properties with those obtained at the laboratory. With a BET surface area of 769 m2 g−1, the industrial product has a narrow mesopores centered around 3.5–4.5 nm (Fig. 3 ). After the hydrothermal deactivation at 800 °C for 10 h under 100% water vapor, the total surface area decreased from 769 m2 g−1 to 154 m2 g−1, the total pore volume decreased from 0.77 cm3/g to 0.32 cm3/g, while the mesopore volume dropped from 0.57 cm3/g to 0.25 cm3/g (Table 1 ). Interestingly, the diameter and the size distribution of mesopores became wider (Fig. 3). All these results demonstrated the excellent hydrothermal stability of industrial products. TEM images (Fig. 4 ) showed that IS after the severe hydrothermal ageing still exhibited mesoporous structure which is typically wormlike, suggesting the high hydrothermal stability of IS.The MAs was milled and then mixed with kaolin, rare earth salts, pseudo-boehmite, and alumina sol. The mixtures are spray-dried into 429 tons of FCC catalysts microspheres with the diameter of 75 μm. The final catalyst was named “LPC-65”. The incumbent catalyst applied in the industrial FCC unit LDO-70 was used to compare with LPC-65.To study the acidity characteristics of LDO-70 and LPC-65, Brønsted acid sites (BAS) and Lewis acid sites (LAS) were calculated from Py-FTIR spectra in the temperature range of 473–623 K, and the corresponding results were depicted in Fig. 5 , Fig. 6 and Table 2 . From Table 2, we could see that both the total acid sites and the BAS of LPC-65 are much higher than those of LDO-70. In this sense, it could be reasonably deduced that the conversion of heavy oil would increase greatly.The final catalyst LPC-65 exhibited a total surface area of 258 m2 g−1, including micropores area of 173 m2 g−1 and mesopores area of 85 m2 g−1 (Table 3 ). As for comparison, the incumbent industrial catalyst LDO-70 has a total surface area of 239 m2 g−1, including micropores area of 169 m2 g−1 and mesoporosity surface area of 70 m2 g−1.LPC-65 and LDO-70 were hydrothermally deactivated at 800 °C for 4 h under 100% water vapor to simulate the catalytic properties of equilibrium catalysts. The mesoporous surface area of steamed LPC-65 is 195 m2 g−1, in which the mesoporous surface area is 64 m2 g−1, much higher than that of the reference catalyst (49 m2 g−1), indicating that the remaining ratio of mesoporosity is high even after severe hydrothermal treatment. ACE test of the two deactivated catalysts using the same heavy oil feed suggested the clear advantage of LPC-65 in selectivity, mainly in increased conversion of 3.36%, reduced heavy oil yield of 1.39% and increased total liquid yield of 0.67% (Table 4 ). The lower coke factor of LPC-65 compared with that of LDO-70 exhibited much enhanced selectivity, which was the most important factor for FCC catalysts. These interesting results could be reasonably ascribed to the presence of enhanced hydrothermally stable mesoporosity.It is well known that vanadium and nickel will cause damage to zeolites in FCC conditions (Trujillo et al., 1997; Xu et al., 2002; Hagiwara et al., 2003; Cerqueira et al., 2008). The ACE test of the contaminated LPC-65 and LDO-70 (deactivated with 3000 ppm Ni and 5000 ppm V) suggested the clear advantage of contaminated LPC-65 in selectivity, mainly in increased conversion of 4.48%, reduced heavy oil yield of 2.82% and increased total liquid yield of 2.14% (Table 5 ). These results demonstrated that the obtained MAs could withstand the severe conditions in industrial FCC units. To the best of our knowledge, this is the first time that a mesoporous aluminosilicate demonstrated excellent hydrothermal stability in FCC units. Moreover, it is the first time that a mesoporous aluminosilicate is employed in FCC catalysts with good activity and selectivity.LPC-65 was added to 1.2-million tons equipment at the same addition rate with that of the incumbent catalyst. As a result of this, a constant catalyst inventory of 429 tons was maintained in the industrial unit. The change-over from the incumbent catalyst to LPC-65 resulted in an 83.37% inventory ratio at the end of 68 days trial. Equilibrium catalyst samples in different inventory ratios were collected and characterized periodically. Interestingly, the surface area of trial equilibrium catalysts (30% inventory ratio) increased from 110 m2 g−1 to 120 m2 g−1, consistent with the higher surface area of fresh LPC-65. Surprisingly, a significant increase in the mesoporous surface area of trial equilibrium catalysts (30% inventory ratio) from 33 m2 g−1 to 40 m2 g−1 (22% increase), indicating the high hydrothermal stability of the mesoporosity in this industrial unit. Table 6 summarized the industrial results of equilibrium catalysts of the incumbent and LPC-65. Compared with LDO-70, the equilibrium catalyst that contain 80% LPC-65 yields significantly lower heavy oil (0.23%) and higher total liquids (0.53%). These results are very close to those obtained from laboratory ACE testing. Generally, the activity of equilibrium catalyst could be considered as the average activity of all the catalyst in FCC unit, which included fresh catalyst and deactivated catalyst. Therefore, it could be suggested that LPC-65 was an ideal FCC catalyst.Mechanical resistance is another important quality of FCC catalyst, because it reduces the flow of catalyst that must be purged. Interestingly, the abrasion index of LPC-65 is 1.6, much lower than the prerequisite value 3.0. These results indicated that LPC-65 developed in this paper had comprehensive advantages compared with the commercial catalysts.For the first time, mesoporous aluminosilicates with excellent hydrothermal stability were manufactured at the commercial scale by a unique process. FCC catalysts obtained from the MAs exhibited high stability in an industrial FCC unit. The catalyst showed improved product selectivity compared with the incumbent catalyst, at a high inventory ratio of 80%. Gasoline oil yield with 80% LPC-65 equalized catalyst enhanced by 1.22% and the total liquid yield enhanced by 0.53%. The results of synthesis and application represent the most advanced development of MAs in heavy oil FCC to date, which bring a ray of hope for the industry-scale application of MAs in heavy oil FCC.The authors acknowledge PetroChina Co. Ltd. for financial support through the research programs (Grant Nos. DQZX-KY-21-008, KYWX-21-023, and KYWX-21-022).

Well-ordered aluminosilicates (MAs) were prepared by in-situ assembly of pre-crystallized units of zeolite Y precursors at a commercial scale, and applied in an industrial fluid catalytic cracking unit for the first time. Compared with incumbent equilibrium catalyst, the surface area of trial equilibrium catalysts (30% inventory ratio) increased from 110 m2 g−1 to 120 m2 g−1. Moreover, a significant increase of the mesoporous surface area of trial equilibrium catalysts (30% inventory ratio) from 33 m2 g−1 to 40 m2 g−1 (22% increase). Furthermore, the equilibrium catalyst that contain 80% LPC-65 yields significantly lower heavy oil (0.23%) and higher total liquids (0.53%) compared with LDO-70. The industrial results demonstrated excellent hydrothermal stability and superior catalytic cracking properties, showing the promising future in the industrial units.

Among the primary fossil energies, the natural gas composed of mainly methane is believed to be a cleaner energy carrier than coal, which has already been widely used in the industrial and transportation sectors. However, the worldwide reservoir, distribution, and the market demanding of the natural gas are greatly unbalanced. In this case, the coal to natural gas via the syngas route emerges as a viable and important technology for the region or country such as China, where characterizes in a high market demanding but greatly insufficient supply of the natural gas and relatively rich coal reserves (Kopyscinski et al., 2010a; Razzaq et al., 2013; Rönsch et al., 2016). Thus, the production of synthetic natural gas (SNG) from coal via the CO methanation reaction has drawn intensive attention in recent years.In essence, the CO methanation reaction is a part of the Fischer-Tropsch synthesis, of which the carbon-containing products are limited to methane, i.e., CO + 3H2 = CH4 + H2O, ΔH 298K = −206.1 kJ mol−1 (Kopyscinski et al., 2010a). Considering the highly exothermic and entropy-decreased nature of the methanation reaction, the industrial process is commonly composed of a series of adiabatic fixed-bed reactors, which are operated in wide ranges of temperatures from ~200–350 °C to 650–700 °C (Nguyen et al., 2013). To improve the process efficiency, contradictory requests are imposed on the catalyst, i.e., sufficiently active at lower temperatures vs. highly stable at higher temperatures resistant to the sintering. Thus, the development of an efficient catalyst pertinent to the opposing requirements is practically important for a more efficient CO methanation process although it has already been industrialized. Moreover, the high activity and anti-sintering property are commonly challengeable issues for designing and developing high-performance metal-supported catalysts.Generally, the supported group VIIIB metals including Pt, Ru, Rh, Fe, Ni, and Co are active catalysts for the CO methanation reaction. Among these metals, Ni is concentrated for the industrial application in considering its relatively richer reservoir and the reasonably higher activity. In the case of the support, alumina or silica are commonly employed. Thus, Ni/alumina and Ni/silica catalysts are extensively studied for the CO methanation reaction (Liu et al., 2016; Tao et al., 2016). To achieve a higher activity at lower temperatures, small Ni particles with a high dispersion are generally targeted. On the contrary, bigger Ni particles are relatively resistant to the sintering at higher temperatures although their activity is lower (Munnik et al., 2014). More importantly, the deactivation of the Ni-supported catalysts induced from either the coke deposition, the sintering of Ni particles and/or the support is a chronic process for the CO methanation reaction (Barrientos et al., 2014). Thus, great efforts have been devoted to mitigate the sintering of Ni particles and to suppress the coke deposition as summarized in our previous work (Xiao et al., 2020).As an efficient method to retard the sintering of Ni particles, the embedment or encapsulation of Ni particles within the pore wall of oxide supports such as ordered mesoporous alumina (OMA) has been quantitatively practiced in recent years (Tian et al., 2015b). Indeed, as a result of the strong Ni-support interactions, both the sintering of Ni particles and the coke deposition over the embedded or encapsulated Ni catalysts such as Ni-OMA are greatly suppressed, leading to a stable catalyst for the CO or CO2 methanation reactions at higher reaction temperatures (Aljishi et al., 2018; Tian et al., 2015a). Unfortunately, the activity of these catalysts is low due to the lower reduction extent of Ni, which is still originated from the strong Ni-support interactions (Aljishi et al., 2018; Liu et al., 2016). Very recently, we demonstrate that a high-performance Ni/Ni-OMA catalyst for the CO methanation can be obtained by balancing the free Ni via the impregnation route and the confined Ni within OMA, which shows a high space-time yield of methane and long-term stability under severe conditions (Xiao et al., 2020).Alternatively, the introduction of the second metal into the Ni-based catalysts is also practiced as an effective method to meet the contrary requirements of the CO methanation reaction. According to the generally observed orders for the specific activity of Ru > Fe > Ni > Co > Rh > Pd > Pt > Ir and the CH4 selectivity of Pd > Pt > Ir > Ni > Rh > Co > Fe > Ru, different configurations of bimetals including Ni-Co, Ni-Fe, Ni-Ru are applied for the CO methanation reaction, and an enhanced catalytic performance was commonly observed (Chen et al., 2010; Liu et al., 2020, 2017). Among these bimetal catalysts, Ni-Co was concentrated as a result of the relatively higher performance for the titled reaction, which is normally explained as the synergetic effects between Ni and Co metals or the formation of the Ni-Co alloy (Liu et al., 2020).In fact, the intrinsic kinetics has long been practiced as an effective aid for developing high-performance catalysts and understanding the reaction mechanism (Zhang et al., 2020). If the references on the CO methanation reaction are analyzed, the catalyst development and the interpretation of the reaction results are overwhelmingly dependent on the correlation between the characterization data and the catalytic results. In contrary, the kinetics study on the CO methanation is scare. Moreover, two different reaction mechanisms over Ni- and Co-based catalysts, namely “direct CO dissociation” and“hydrogen-assisted CO dissociation” are proposed for the methanation reaction (Chen et al., 2017; Lim et al., 2016). However, contributions from the kinetics results on understanding the reaction mechanism and explaining the catalytic performance are very limited. Thus, more deep understanding on the CO methanation reaction is reasonably expected if the catalyst characterizations and the intrinsic kinetics studies are integrated.From these analyses and understandings, in this work, a series of Co/Ni-OMA catalysts with varied Co/Ni ratios was synthesized by impregnating Co over Ni-OMA. We found that the Co/Ni ratio had a strong effect on the CO conversion, CH4 selectivity, and the long-term stability of the catalyst for the CO methanation. Combining the characterization results of the catalysts with the intrinsic kinetics, it is revealed that Ni confined within OMA undertake the dominant active sites for catalyzing the CO methanation, while the post-impregnated Co promotes the H-assisted CO dissociation step, resulting in an enhanced low-temperature activity over the optimal 8Co/15Ni-OMA catalyst. These findings are important for further optimizing or designing a high-performance bimetallic catalyst for the methanation reactions of CO or CO2.All chemicals with analytical grade were directly employed as received without further purifications. Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), nitric acid (HNO3, 67 wt%), hydrochloric acid (HCl, 37 wt%) and anhydrous ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Aluminum isopropoxide (Al(OPr i )3) and (EO)20(PO)70(EO)20 triblock copolymer (Pluronic P123, Mn = 5800) were bought from Sigma-Aldrich.The Ni-OMA catalyst was fabricated via the one-pot evaporation induced self-assembly (EISA) process with some modifications according to reported reference (Morris et al., 2008; Yuan et al., 2008). Typically, 2.5 g Pluronic P123 was dissolved in 40 mL anhydrous ethanol at room temperature. Next, 3.2 mL nitric acid, 4.08 g Al(OPr i )3, and 0.70 g Ni(NO3)2·6H2O were successively introduced to the above ethanol solution under a vigorous stirring. After stirring vigorously for 6 h, the solvent evaporation process was conducted in a drying oven at 60 °C for 48 h to form a green xerogel. Finally, the xerogel was calcined at 600 °C for 4 h with a temperature ramp of 1 °C min−1. The thus reaped solid was denoted as 15Ni-OMA, where 15 represented the weight percentage of NiO. For the sake of comparison, the pristine OMA was synthesized without introducing Ni(NO3)2·6H2O, and the 15Co-OMA counterpart was prepared by adding Co(NO3)2·6H2O.The xCo/15Ni-OMA catalysts with varied content of CoO were synthesized by incipiently impregnating Ni-OMA with the aqueous solution of Co(NO3)2·6H2O, where x indicates the weight percentage of CoO, i.e., 1, 3, 5, 8, 13 wt%, respectively, and the content of NiO is always fixed at 15 wt%. During preparation, the exact dosage of Co(NO3)2·6H2O and Ni(NO3)2·6H2O required were summarized in Tab. S1. After impregnation, the samples were dried at 120 °C for 12 h, and then calcined in air at 600 °C for 2 h with a temperature rate of 2 °C min−1. For comparison, 15Co/OMA catalyst was synthesized via impregnating OMA with the aqueous solution of Co(NO3)2·6H2O.The exact NiO and CoO content over the calcined catalysts was determined by inductively coupled plasma mass spectroscopy (ICP-MS, M90, Bruker). Before each measurement, the sample was digested in a mixed solution of concentrated nitric acid (67 wt%) and hydrochloric acid (37 wt%). The calculated NiO and CoO contents over different catalysts are summarized in Tab. S2 in Supplementary materials.The N2 physisorption isotherms of the different samples were measured on a Micromeritics ASAP 2020 instrument at −196 °C for calculating textual data including the BET surface area, pore volume, average pore size, and pore size distribution (PSD) curves. Prior to the measurement, all the samples were pretreated under vacuum condition at 90 °C for 1 h and then at 300 °C for 8 h.Powder X-ray diffraction (XRD) patterns of the samples were conducted on a Bruker D8 Advance X-ray diffractometer using the monochromatized Cu/Kα radiation at 40 kV and 40 mA in the 2θ ranging from 0.5° to 6° and 10° to 90°. The scanning rate of the sample was 1° and 6° per minute with a step size of 0.02° (2θ) for small-angle and wide-angle region, respectively.The hydrogen temperature-programmed reduction (H2-TPR), O2 titration, and H2 pulse chemisorption experiments were performed on a Micromeritics AutoChem 2920 device. For H2-TPR, all the samples were pretreated in a high-purity Ar flow at 450 °C for 1 h and then cooled down to 50 °C before measurement. Afterward, H2-TPR was performed until 1000 °C with a heating rate of 10 °C min−1 in a 10 vol% H2/Ar flow. The O2 titration experiment was used to estimate the degree of reduction for the different samples. Firstly, the sample was reduced in a high-purity H2 flow at 700 °C for 1 h, and then purged with a high-purity Ar for 0.5 h at 700 °C. After cooling down to 600 °C in the Ar flow, 3 vol% O2/Ar was pulsed consecutively. The extent of reduction was calculated from the total oxygen consumption, the amount of which is measured by a pre-calibrated thermal conductivity detector (TCD). The H2 pulse chemisorption experiment was used to calculate the dispersion of the metallic Ni and Co. Firstly, the fresh catalyst was reduced at the temperature of 700 °C for 1 h in a high-purity H2 flow, and then cooled down from 700 °C to 35 °C in an Ar flow. Finally, 0.5 mL 10% H2/Ar was introduced by the consecutive pulse-dosing until a constant area of the TCD peak.X-ray photoelectron spectra (XPS) were collected in a high vacuum environment (approximately 5 × 10−9 torrs) on an X-ray photoelectron spectrometer (Kratos Analytical Ltd.) with an Al Kα radiation (1486.6 eV) at the room temperature. The C1 s peak at 284.8 eV was applied to calibrate the binding energy.To investigate the location of metallic Ni and Co species as well as their particle size distributions, transmission electron microscope (TEM) and high-angular annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were carried out on a FEI Tecnai G2 F20 transmission electron microscope with the accelerating voltage of 200 kV.The CO methanation reactions were performed under 0.1 MPa in a quartz-lined stainless-steel fixed bed reactor with an inner diameter of 8 mm. Prior to the evaluation, 100.0 ± 0.1 mg catalyst (40–60 mesh) diluted with 4.0 g quartz sands (40–60 mesh) was reduced under the conditions of T = 700 °C, reducing time = 1 h, and pure H2 flow rate = 100 mL min−1. After the temperature had been cooled to the desired temperature, the syngas with a molar feed ratio of H2/CO/N2 = 3/1/1 was purged into the fixed-bed reactor. The low-temperature tests of all the catalysts for the CO methanation were assessed at the temperature region of 300–450 °C with the gas hourly space velocity (GHSV) of 60,000 mL g− 1 h−1. A harsh reaction condition comprised of 40 h running at 180,000 mL g− 1 h−1 and high temperature, i.e., 400 °C for 10 h, 700 °C for 20 h and 400 °C for 10 h again, was conducted to evaluate the high-temperature stability of the representative catalysts. The long-terms durability experiments were carried out under 600 °C with GHSV of 180000 mL g− 1h−1. After remove of water with the ice-water trap, the products were analyzed by an on-line GC (GC-9560, Shanghai Huaai chromatographic analysis Co., Ltd.) equipped with a 5A molecular sieve column and a Porapak Q column and a TCD. The CO conversion (X CO), selectivity of methane ( S C H 4 ), and the yield of methane ( Y C H 4 ) are defined as follows: X CO % = F CO, in - F CO , out / F CO , in × 100 S C H 4 ( % ) = F C H 4 , o u t / ( F C O , i n - F C O , o u t ) × 100 Y C H 4 ( % ) = F C H 4 , o u t / F C O , i n × 100 where F i, in and F i, out are the flow rates of CO or CH4 at the inlet and outlet of the reactor, respectively.The kinetics measurements of CO methanation were conducted at 0.1 MPa and 300 °C with different gas partial pressures of CO and H2 (5–30 kPa for CO, 10–60 kPa for H2, total pressure of 100 kPa balanced with N2). The formation rate of CH4 was used to represent the total reaction rate. To regulate the experimental conversion level (<10%) far below the thermodynamic equilibrium conversion of CO methanation, the relatively lower temperature range from 260 to 290 °C cooperating with varied GHSV of 60,000 and 120,000 mL g− 1 h−1 were employed to assess the apparent activation energy (E a) according to the standard Arrhenius equation. In addition, the potential mass transport and heat transfer limitations were estimated via Weisz-Prater and Mears criterions. The kinetics models were derived by quasi-equilibrium approximations and Langmuir-Hinshelwood mechanism. The unknown kinetics parameters involved in the derived kinetics expressions based on different rate-determining steps were fitted by the nonlinear least-square method according to the experimental data. The minimization problems arisen in the least square-curve fitting were solved by the trust-region-reflective algorithm. The lower bounds of the unknown parameters were set as zero to ensure that the fitted values are positive. The coefficient of determination (r 2) is used to measure how well the experimental data replicated by different kinetics models. r 2 = 1 - ∑ i r i E x p . - r i F i t . 2 ∑ i r i E x p . - r ¯ i E x p . 2 The ordered mesoporous structure and crystalline phase of as-synthesized catalysts were characterized by XRD, as shown in Fig. 1 . From the small-angle XRD patterns (Fig. 1A), 15Ni-OMA and 15Co-OMA showed the prominent peak at 2θ = 1.0° and the weak peak at 2θ = 1.6°, indicative of the (100) and (110) plane of the p6mm hexagonal symmetry, respectively (Morris et al., 2008; Yuan et al., 2008). In the case of the Co-impregnated catalysts, i.e., xCo/15Ni-OMA and 15Co/OMA, the loss of characteristic peak (110) and the broadened characteristic peak (100) were observed, indicating the preserve of the ordered mesoporous structure with slightly decreased orderliness. Moreover, 15Ni-OMA and 15Co-OMA showed a type IV isotherm with a sharply steep H1 hysteresis loop at the relatively pressure of 0.7–0.9 (Fig. S1A), indicating the typical feature of mesoporous materials because of the capillary condensation of N2 in uniform mesochannels (Yuan et al., 2008). When the Co-impregnated catalysts were concerned, both total volume of absorbed N2 and the relative pressure range of hysteresis loop decreased, indicating the decreased pore volume and average pore size (Tab. S3), as illustrated in pore size distribution (PSD) curves (Fig. S1B).For the wide-angle XRD patterns (Fig. 1B), since no diffraction peaks of NiO and γ-Al2O3 were observed in the case of 15Ni-OMA, it is determined that Ni species were highly dispersed within pore wall of amorphous alumina matrix (Liu et al., 2016). This conclusion is also the same with 15Co-OMA catalyst. However, in the cases of Co-impregnated catalysts, those characteristic diffraction peaks corresponding to γ-Al2O3 phase were observed, which was probably attributed to solvent-induced the crystalline transformation during the post-impregnation process, as reported in our previous works (Xiao et al., 2020). More importantly, the peak intensity of γ-Al2O3 phase decreased with increasing the cobalt content from 1 to 15 wt%, especially for 13Co/15-OMA and 15Co/OMA, which is due to the enhanced interactions between cobalt species and γ-Al2O3. Given diffraction peaks overlapping of NiAl2O4, CoAl2O4 and γ-Al2O3 (Gonçalves et al., 2018), these spinel phases cannot be excluded tentatively from xCo/15Ni-OMA catalysts. When cobalt content was increased from 1 to 15 wt%, a set of diffraction peaks assigning to the crystalline phase of Co3O4 were observed, and continuously intensified with increasing cobalt content. Taking account of post-impregnated 15Co/OMA and 13Co/15Ni-OMA catalysts with similar cobalt content, the diffraction peaks of Co3O4 over 15Co/OMA are weaker than that of 13Co/15Ni-OMA, from which it is proposed that (I) there is strong interaction between CoO and OMA, and (II) Ni species embedded within pore wall have steric hindrance for post-impregnated cobalt. After reduction (Fig. 1C), three new diffraction peaks at 44.6°, 51.8°, and 76.6° corresponding to (111), (200) and (220) lattice planes of metallic Ni and/or Co were observed for all the reduced catalysts. Comparing 15Co-OMA with 15Co/OMA, it is found that the diffraction peak at 2θ = 44.6° over the former is much lower than that over the latter, indicating that the post-impregnated Co species are relatively easier to be reduced.HAADF-STEM and TEM images of all the catalysts after reduction were shown in Fig. 2 and Fig. S2, respectively. Obviously, these catalysts clearly represented a highly ordered cylindrical pore aligned along the [110] direction, indicating the preserve of the highly ordered mesoporous structure even after impregnating as high as 15 wt% CoO and with pretreatment of high-temperature calcination and reduction. This is a strong evidence for high thermal stability. Moreover, the metal mean size distributions were determined via counting overall particles from the HAADF-STEM images, as shown in insets in Fig. 2 and Table 1 . Specifically, in the case of 15Ni-OMA, Ni particles with small size (4.2 ± 1.2 nm) were uniformly distributed into mesochannels. The similar observation can be obtained for 15Co-OMA with mean size of 4.8 ± 1.0 nm. The 15Co/OMA catalyst synthesized via post-impregnation delivered a relatively larger size (11.5 ± 3.7 nm). However, for the xCo/15Ni-OMA catalysts, the metallic mean size also gradually increased (from 4.9 ± 1.4 to 9.3 ± 4.1 nm) and size distribution became widened, while a few large metal particles were partially distributed outside mesochannels, but all of them are better than 15Co/OMA. Therefore, it is assumed that the post-impregnated Co species prefers to aggregate on the outer surface of the OMA compared with Ni species confined within pore wall of OMA. Fig. 3 showed the H2-TPR profiles of all catalysts. The 15Ni-OMA catalyst exhibited only one broad hydrogen-consumption peak centered at around 654 °C, indicating the presence of Ni particles with varied sizes, in accordance with the HAADF-STEM images (Fig. 2). In the case of 15Co-OMA, two reduction peaks at ~810 °C and ~915 °C were observed, which were attributed to the reductions of Co species confined within pore wall of OMA and non-stoichiometric amorphous CoAl2O4, respectively. This result points out that Co species are more difficult to be reduced than Ni species if both are separately confined within pore wall of OMA (Xu et al., 2016). Moreover, there is no reduction peak at temperature blow 500 °C for 15Ni-OMA and 15Co-OMA, indicating the absence of free bulk NiO and Co3O4 (Ma et al., 2016). However, 15Co/OMA synthesized via post-impregnation showed a very broad peaks centered at around 574 °C, 763 °C, and 934 °C (Wang et al., 2018). The maximum reduction temperature ( T max = 574 °C) is different with that of 15Co-OMA ( T max = 810 °C), which can be attributed to the medium strong interactions between the impregnated Co species and OMA. Based on the above insights into reduction behaviors of monometallic catalysts, the assignment of the reduction peaks of xCo/15Ni-OMA can be roughly identified. Specifically, the peaks at temperature below 500 °C and at the range of 800–1000 °C ascribed to the reduction of surface free bulk Co3O4 and CoAl2O4, respectively (Wang et al., 2018; Xu et al., 2016). The T max shifted toward higher temperature (~690 °C), which was indicative of enhanced metal-support interactions. These broad and unresolved reduction peaks were further deconvoluted into five reduction peaks to differentiate the interaction difference, as illustrated in Table. 2 and Fig. S3. The first reduction at 340–460 °C (α) is attributed to the reduction of bulk Co3O4 with large sizes via the CoO intermediate. The second reduction at around 500–590 °C (β) is ascribed to the reduction of NiO and/or CoOx weakly interacted with OMA (Ma et al., 2016). The third reduction at around 650–710 °C (γ) is referred to the reduction of those Ni species confined within pore wall of OMA, and/or the non-stoichiometric amorphous NiAl2O4, and/or the CoOx moderately strong interacted with OMA (Ma et al., 2016). The fourth reduction at around 740–810 °C (θ) is associated with the reduction of the NiAl2O4 spinel bearing with very strong metal-support interactions, and/or those Co species confined within OMA framework, and/or the non-stoichiometric amorphous CoAl2O4 compound (Tao et al., 2013). The final reduction at around 850–940 °C (δ) is due to the reduction of CoAl2O4 spinel bearing with very strong metal-support interactions (Huang et al., 2017; Xu et al., 2016).In general, the stronger interaction between metal and OMA will lead to a worse reduction degree of the Ni and Co oxide species, vice versa. The reduction degree of Ni and Co oxide species was determined by the O2 titration, as shown in Table 1. As expected, the reduction degree of 15Ni-OMA (81.2%) is higher than that of 15Co-OMA (42.6%), in line with H2-TPR profiles. In the cases of xCo/15Ni-OMA catalysts, an increased reduction degree was observed in comparison with pristine 15Ni-OMA except 13Co/15Ni-OMA, indicating the addition of CoO could improve the reduction of Ni oxide species in the OMA. In view of the similar reduction extent between 15Co/OMA (73.7%) and 13Co/15Ni-OMA (78.9%), it is supposed that 13Co/15Ni-OMA with low surface area and hydrogen diffusion limitation probably results in a relatively lower reduction degree than other xCo/15Ni-OMA catalysts. In addition, the metal dispersion degree was measured by H2 pulse chemisorption (Table 1). Both 15Co-OMA and 15Co/OMA showed poor dispersion, but it is contrary for 15Ni-OMA. As increasing the content of Co oxide, the dispersion gradually decreased from 11.3% to 2.8%, which can be explained by the hydrogen diffusion limitation of Ni and Co oxide generated from the confinement of OMA (Wang et al., 2018; Xiao et al., 2020).To further study the intermetallic interaction of xCo/15Ni-OMA catalysts, XPS analyses of fresh and reduced catalysts were conducted, and the deconvoluted XPS spectra of Ni 2p and Co 2p were shown in Fig. S4. In the case of fresh catalysts, two characteristic spin-orbit splitting of Ni 2p3/2 peak and Ni 2p1/2 peak at binding energy (BE) of 856.2 and 873.5 eV were observed over 15Ni-OMA, corresponding to those unreduced Ni2+ species (Fig. S4A). A set of Co 2p3/2 peaks and Co 2p1/2 peaks at BE of 780.6 and 782.2 eV, 796.0 and 797.6 eV were observed over 15Co/OMA catalyst (Fig. S4B), of which BE values located at 780.6 and 796.0 eV were assigned to the presence of Co3O4 with two oxidation states (Co2+ and Co3+), and BE at 782.2 and 797.6 eV was attributed to CoAl2O4 compound (Horlyck et al., 2018; Ji et al., 2000). For bimetallic xCo/15Ni-OMA catalysts, it is shown that there is a positive shift of binding energy (0.3–0.4 eV) for the peaks containing Co2+ and Co3+ species, implying that post-impregnated cobalt oxide might donate electrons to unreduced Ni2+ species and/or OMA support resulting in decreased outer electron density. However, no significant alteration in the BE values of CoAl2O4 compound and unreduced Ni2+ species, which is probably because the effect of electron transfer to unreduced Ni2+ species is shielded by strong metal-support interaction or formation of NiAl2O4 compound, as evidenced by H2-TPR. After reduction, the BE values centered at 852.5 and 869.7 eV, 778.2 and 793.5 eV are attributed to metallic Ni0 and Co0 (Fig. S4C and D), respectively. As compared with pristine 15Ni-OMA and 15Co/OMA, it is noted that BE values of Co0 increases from 778.2 to 779.0 eV, and BE values of Ni0 increases from 852.5 to 853.2 eV in the cases of bimetallic xCo/15Ni-OMA catalysts. Likewise, there is still no any significant shift in the BE values of 856.2 and 782.2 eV corresponding to those compounds in oxidation state that are very difficult to be reduced (e.g., NiAl2O4 and CoAl2O4). In principle, an increase in BE values is characteristic of decreased outer electron density of atomic nucleus. Together with the difference of electronegativity ( χ C o  = 1.88, χ N i = 1.91), it is thus assumed that metallic Co0 might donate electrons to metallic Ni0, but a reversed shift in BE values of metallic Ni0 occurs probably because those nickel oxides embedded inside OMA framework are partially reduced, resulting in the formation of Co-Ni-NiAl2O4 intermetallic phase and subsequent electron transfer to unreduced compounds with strong metal-support interaction. In short, it is thus confirmed that the intermetallic interaction occurs via cobalt donating electron to nickel species, and subsequent electron transfer to OMA support or hard-to-reduce metal-support compounds, which would contribute an effect on catalytic behavior of CO methanation.The as-synthesized catalysts for CO methanation were initially tested at low temperature region (300–450 °C), and the resultant CO conversion, CH4, CO2 selectivity and CH4 yield as function of reaction temperature were plotted in Fig. S5. In the cases of 15Ni-OMA and xCo/15Ni-OMA catalysts, CO conversion and CH4 yield (Fig. S5a and d) increased gradually as reaction temperature rising, and eventually reached to ca.99% and ca.85%, respectively. In addition, it is shown that CH4 selectivity initially decreases as reaction temperature increasing, but it increases if reaction temperature increases further (Fig. S5b). However, the lesser CO2 selectivity is observed at a low temperature (300–320 °C) and it increases rapidly when reaction temperature is higher than 320 °C, but it decreases if temperature increases continuously (Fig. S5c). Based on total Gibbs free energy minimization method and the feed gas with a molar ratio of H2/CO/N2 = 3/1/1 in absence of catalyst, two parallel side-reactions, e.g., water-gas shift reaction (WGSR) and CO disproportionation reaction (Boudouard reaction) were involved during CO methanation, and the thermodynamic equilibrium composition were calculated via HSC Chemistry software. The resultant conversion, selectivity and equilibrium constant K values are shown in Fig. S6. It is found that the variation trend of methane selectivity obtained over xCo/15Ni-OMA catalysts agrees with our thermodynamic calculation (Fig. S6a) and the early reports (Liu et al., 2016, 2017; Gao et al., 2012). The equilibrium constant value calculated (Fig. S6b) of CO methanation (K M) is higher than that of water-gas shift reaction (K W), and almost same with that of Boudouard reaction (K B), revealing CO methanation and Boudouard reaction occurs easier than water-gas shift reaction at reaction temperature of 300–450 °C, accounting for the formation of CO2 and carbon deposition (Rönsch et al., 2016). However, the carbon selectivity (Fig. S6a) obtained from thermodynamic calculation is negligible when reaction temperature is lower than 450 °C, which is in keeping with our experimental result. Interestingly, the variation trend of CO2 selectivity obtained from experimental results seems to be not in line with thermodynamic calculation, which is probably because those side-reactions involving CO2 are controlled simultaneously by both thermodynamics and kinetics in the presence of xCo/15Ni-OMA catalysts. Fig. 4 exhibited the effect of Co loading on the reaction rate under the different reaction temperatures. Clearly, on the condition of reaction temperature of 300–340 °C, a set of volcanic curves were observed when cobalt loading increased from 0 to 13 wt% as shown in Fig. 4a. However, when reaction temperature was operated at 360 °C, the consuming rate of CO remained constant and then decreased sharply as cobalt loading was beyond 8 wt%. The maximum consuming rate of CO (0.14 mol kgcat − 1 s−1) was obtained at 340 °C over 8Co/15Ni-OMA. Moreover, the similar observations for the formation rate of CH4 as a function of cobalt content were obtained under different reaction temperatures as indicated in Fig. 4b. However, both catalyst counterparts of 15Co-OMA and 15Co/OMA showed poor activity (Fig. S5) even if a higher reduction temperature of 750 °C was applied (Fig. S7) to reduce those Co oxide species confined within pore wall of OMA. Thus, it can be concluded that incorporating an appropriate amount of cobalt into 15Ni-OMA matrix favors the enhancement of CO methanation reactivity at low-temperature.Since the industrial CO methanation over Ni-based catalysts are usually operated at a wide temperature window (typically from 300 to 700 °C), the high-temperature stability of catalyst is an important index for practical application (Rönsch et al., 2016). In this regard, a harsh reaction condition comprised of 40 h running at 3-fold GHSV (180,000 mL g− 1 h−1) and high temperature, i.e., 400 °C for 10 h, 700 °C for 20 h and 400 °C for 10 h again, was conducted to evaluate the high-temperature stability of the representative catalysts (8Co/15Ni-OMA and 15Ni-OMA) (Fig. S8). When CO methanation reaction was operated at 700 °C, CO conversion and CH4 selectivity over 8Co/15Ni-OMA and 15Ni-OMA were almost same and close to the thermodynamic equilibrium. After running at 700 °C for 20 h, 8Co/15Ni-OMA regained the original CO conversion level (97.7%) at 400 °C, but 15Ni-OMA showed a ca.10% loss of CO conversion. This is a clear evidence that post-impregnating a certain amount of cobalt into 15Ni-OMA matrix could facilitate the improvement of high-temperature stability. Moreover, the long-term durability of 8Co/15Ni-OMA were assessed at 600 °C and with GHSV of 180,000 mL g− 1 h−1, and the results were shown in Fig. 5 . Obviously, a very slight change on CO conversion was observed over 8Co/15Ni-OMA for TOS of 200 h, of which the initial and final CO conversions were about 69.0% and 67.9%, respectively (Fig. 5a), and the selectivity (Fig. 5b) and yield (Fig. 5c) of CH4 also remained stable. As a consequence, both stability and long-term durability of 8Co/15Ni-OMA at high temperature are reasonably good.Prior to the kinetics analysis, the potential effects of mass and heat transfer limitations were checked by Weisz-Prater criterion and Mears’ criterion (Mears, 1971; Weisz and Prater, 1954). Specifically, the internal diffusion and external diffusion limitations were evaluated via Eqs. (1) and (2), respectively. The extent of the interphase heat transfer was evaluated via Eq. (3), whereas the interparticle, intraparticle and the axial conduction are negligible in the present reaction condition, as reported in our previous work (Xiao et al., 2020). All related parameters are listed in Tables 3 and 4 . C W P = - r o b s ρ c R p 2 D e C s < 1 (1) C M M = - r o b s ρ b R p n k c C A b < 0.15 (2) C M H = | - Δ H r ( - r A ) ρ b R E h t T b 2 R g | < 0.15 (3)After substitution of corresponding values into C WP and C MM equations, results in C WP = 2.80 × 10− 5 < 1 and C MM = 1.82 × 10− 4 < 0.15. Therefore, it can be determined that both internal and the external diffusion limitation were negligible under the conditions applied in this work. Likewise, according to C MH equation, leads to C MH = 0.03 < 0.15, indicating that the resistance of the interphase heat transfer is negligible. Moreover, according to the previous work (Mears, 1971; Xiao et al., 2020), the temperature difference between the fluid phase and catalyst particle calculated is 4.9 K when the CO methanation is carried out under reaction condition of 653 K, 0.1 MPa, and 60,000 mL g− 1 h−1. Following the same method, the values of Weisz-Prater and Mears’ criterions under different reaction temperatures can be obtained and collected in Tab. S4. All in all, the effect of mass transport under the conditions applied in the present work is negligible. The interphase heat transfer resistance is also insignificant based on within 5% deviation (Mears, 1971), leading to the maximum temperature difference of less than 4.9 K. Therefore, it is reasonable to conduct a kinetics analysis on CO methanation over the different catalysts.To better understand the cooperative effect of bimetallic xCo/15Ni-OMA catalysts on CO methanation at low temperature, the apparent activation energies (E a) of different catalysts for CO methanation were determined according to Arrhenius equation. Specifically, the relationships between CO consuming rate and reaction temperature, as well as CH4 formation rate and reaction temperature were plotted, respectively (Fig. 6 ). Obviously, the E a values obtained showed same variation trend over different catalysts no matter which reaction rate was used. In addition, E a values derived from CH4 formation rate are higher than that from CO consuming rate due to in presence of side-reactions during CO conversion process. Herein, taking CH4 formation rate for example because of methanation purpose, it can be determined that E a values obtained over 15Ni-OMA, 15Co-OMA, and 15Co/OMA are 124.0, 131.9, and 117.2 kJ mol−1, respectively. However, when an appropriate amount of cobalt was introduced into 15Ni-OMA matrix, E a values firstly decreased with increasing cobalt content, but it increased again for 13Co/15Ni-OMA. As expected, 8Co/15Ni-OMA showed the lowest E a value of 100.2 kJ mol−1, accounting for high activity at low temperature.The effects of CO and H2 partial pressure on reaction rate of CH4 formation were first investigated and showed in Fig. 7 . Under the present methanation condition, the main products were CH4 and H2O, almost in absence of CO2 and C2 + products. With the exception of 8Co/15Ni-OMA, other catalysts showed that the CH4 formation rate decreased with increasing CO partial pressure. On the contrary, the CH4 formation rate was positively related to H2 partial pressure, in keeping with literatures (Chen et al., 2017; Yang et al., 2013). For 8Co/15Ni-OMA, a volcanic curve of reaction rate as function of CO partial pressure was observed, where the maximum CH4 formation rate was obtained at CO partial pressure of 15 kPa. This result is an indicative of different kinetics behavior proceeding on 8Co/15Ni-OMA and other catalysts.In view of the reaction mechanism of CO methanation, the hydrogen-assisted dissociation mechanism as compared with direct dissociation pathway is so far well-recognized, where the oxygenated intermediate species of COH is involved (Kopyscinski et al., 2010b; Lim et al., 2016; Miao et al., 2016; Pham et al., 2014; Yang et al., 2013). In addition, our experimental results revealed that methanation rate increased as function of H2 partial pressure increasing (Fig. 7b) when CO partial pressure was fixed, which provided a favorable evidence supporting the domination of hydrogen-assisted CO dissociation mechanism regarding the driving force of hydrogen. Therefore, a couple of potential micro-kinetics models consisted of nine elementary steps are proposed based on hydrogen-assisted dissociation and Langmuir-Hinshelwood mechanism (Scheme 1 ), where all kinds of surface adsorbed species, i.e., H*, CO*, COH*, C*, CH*, CH2*, CH3* and OH* are fully considered. Moreover, given that different effects of CO and H2 partial pressure on reaction rate, H-assisted CO dissociation, the first-step hydrogenation of surface carbon species (C*) and last-step hydrogenation of surface CH3* species are assumed as rate-determining steps (RDS), respectively, whereas other steps are counted as quasi-equilibria approximation.Model I: The H-assisted CO dissociation (Step 3) is RDS. The resultant reaction rate expression is indicated as the Eq. (I-1). r = k 3 + θ C O θ H - k 3 - θ C O H θ ∗ (I-1)Based on quasi-equilibria approximation for each of elementary steps, the coverage of various surface species can be determined as function of measurable experiment variables: θ H = θ ∗ K 1 1 2 P H 2 1 2 (I-2) θ C O = θ ∗ K 2 P C O (I-3) θ C = P C H 4 θ ∗ K 1 2 K 5 K 6 K 7 K 8 P H 2 2 (I-4) θ C H = P C H 4 θ ∗ K 1 3 2 K 6 K 7 K 8 P H 2 3 2 (I-5) θ C H 2 = P C H 4 θ ∗ K 1 K 7 K 8 P H 2 (I-6) θ C H 3 = P C H 4 θ ∗ K 8 K 1 1 2 P H 2 1 2 (I-7) θ O H = P H 2 O θ ∗ K 9 K 1 1 2 P H 2 1 2 (I-8) θ C O H = P H 2 O P C H 4 θ ∗ K 1 5 2 K 4 K 5 K 6 K 7 K 8 K 9 P H 2 5 2 (I-9)Because of coverage normalization of all surface species (Eq. (I-10)), θ ∗ + θ H + θ C O + θ C + θ C H + θ C H 2 + θ C H 3 + θ O H + θ C O H = 1 (I-10)The coverage of empty active site θ ∗ can be formulated as function of known variables (Eq. (I-11)): θ ∗ = 1 1 + K 1 1 2 P H 2 1 2 + K 2 P C O + P C H 4 K 1 2 K 5 K 6 K 7 K 8 P H 2 2 + P C H 4 K 1 3 2 K 6 K 7 K 8 P H 2 3 2 + P C H 4 K 1 K 7 K 8 P H 2 + P C H 4 K 8 K 1 1 2 P H 2 1 2 + P H 2 O K 9 K 1 1 2 P H 2 1 2 + P H 2 O P C H 4 K 1 5 2 K 4 K 5 K 6 K 7 K 8 K 9 P H 2 5 2 (I-11)After substitution of corresponding values into Eq. (I-1) and grouping the constants for convenience, the kinetics model I is derived as Eq. (I-12): r = K ' P C O P H 2 3 - P C H 4 P H 2 O K ' ' P H 2 5 2 1 + K 1 1 2 P H 2 1 2 + K 2 P C O + P C H 4 K 1 2 K 5 K 6 K 7 K 8 P H 2 2 + P C H 4 K 1 3 2 K 6 K 7 K 8 P H 2 3 2 + P C H 4 K 1 K 7 K 8 P H 2 + P C H 4 K 8 K 1 1 2 P H 2 1 2 + P H 2 O K 9 K 1 1 2 P H 2 1 2 + P H 2 O P C H 4 K 1 5 2 K 4 K 5 K 6 K 7 K 8 K 9 P H 2 5 2 2 K ' = k 3 + K 1 3 K 2 K 4 K 5 K 6 K 7 K 8 K 9 k 3 - = K 1 3 K 2 K 3 K 4 K 5 K 6 K 7 K 8 K 9 , K ' ' = K 1 5 2 K 4 K 5 K 6 K 7 K 8 K 9 k 3 - (I-12)Model II: The first-step hydrogenation of surface carbon species (Step 5) is RDS. The resultant rate expression is written as the Eq. (II-1). (II-1) r = k 5 + θ C θ H - k 5 - θ C H θ ∗ Following the same method as above, the coverage of various surface species can be determined as the Equation SII-1 to SII-8 (Supplementary materials). Finally, the resultant kinetics model II is formulated as Eq. (II-2): r = K ' P C O P H 2 3 - P C H 4 P H 2 O K ' ' P H 2 3 2 P H 2 O 1 + K 1 1 2 P H 2 1 2 + K 2 P C O + K 1 1 2 K 2 K 3 P C O P H 2 1 2 + K 1 K 2 K 3 K 4 K 9 P C O P H 2 P H 2 O + P C H 4 K 1 3 2 K 6 K 7 K 8 P H 2 3 2 + P C H 4 K 1 K 7 K 8 P H 2 + P C H 4 K 8 K 1 1 2 P H 2 1 2 + P H 2 O K 9 K 1 1 2 P H 2 1 2 2 (II-2) K ' = k 5 + K 1 3 K 2 K 3 K 4 K 6 K 7 K 8 K 9 k 5 - = K 1 3 K 2 K 3 K 4 K 5 K 6 K 7 K 8 K 9 , K ' ' = K 1 3 2 K 6 K 7 K 8 k 5 - Model III: The last-step hydrogenation of surface CH3* species (Step 8) is RDS. The resultant rate expression is indicated as the Eq. (III-1). (III-1) r = k 8 + θ C H 3 θ H - k 8 - P C H 4 θ ∗ 2 Following the same method as above, the coverage of various surface species can be determined as the Equation SIII-1 to SIII-8 (Supplementary materials). Finally, the resultant kinetics model II is formulated as Eq. (III-2): r = K ' P C O P H 2 3 - P C H 4 P H 2 O K ' ' P H 2 O 1 + K 1 1 2 P H 2 1 2 + K 2 P C O + K 1 1 2 K 2 K 3 P C O P H 2 1 2 + K 1 K 2 K 3 K 4 K 9 P C O P H 2 P H 2 O + K 1 3 2 K 2 K 3 K 4 K 5 K 9 P C O P H 2 3 2 P H 2 O + K 1 2 K 2 K 3 K 4 K 5 K 6 K 9 P C O P H 2 2 P H 2 O + K 1 5 2 K 2 K 3 K 4 K 5 K 6 K 7 K 9 P C O P H 2 5 2 P H 2 O + P H 2 O K 9 K 1 1 2 P H 2 1 2 2 (III-2) K ' = k 8 + K 1 3 K 2 K 3 K 4 K 5 K 6 K 7 K 9 k 8 - = K 1 3 K 2 K 3 K 4 K 5 K 6 K 7 K 8 K 9 , K ' ' = 1 k 8 - Herein, a group of representative catalysts, including bimetallic and monometallic catalysts, i.e, 8Co/15Ni-OMA, 15Ni-OMA and 15Co/OMA, were selected to match with three kinetics models, respectively. The results of experimental rate fitted with estimated rate using different kinetics models were showed in Fig. 8 . It is obvious that not all the rate expressions given agree with the experimental results. In the case of bimetallic 8Co/15Ni-OMA catalyst, the experimental rate measured matched very well with the predicted reaction rate by all of the kinetics models, as reflected by aligning on the diagonal line with a slope of unity, along with a determination coefficient of 0.927, 0.930 and 0.958 for model I, II and III, respectively. Likewise, three kinetics models were fitted well with the experimental results measured over monometallic 15Ni-OMA catalyst, as evidenced in good coefficient of determination (r 2 = 0.904, 0.920, 0.964). Thus, it is assumed that the kinetics behavior occurring on 8Co/15Ni-OMA and 15Ni-OMA is probably same. For monometallic 15Co/OMA catalyst, however, the reaction rate were predicted very well by kinetics model II and III (r 2 = 0.929 and 0.989), whereas kinetics model I could be rejected according to the dataset with poor coefficient of determination (r 2 = 0.809), indicating the reaction rate difference of stepwise hydrogenation of surface species over Co-based active sites might be very little, and namely the overall reaction rate of lumped hydrogenation steps could be considered as rate-determining step.As demonstrated in catalyst characterization (Table 2), 15Ni-OMA and 15Co/OMA showed a comparable degree of reduction (81.2% versus 73.7%), but the latter dispersion is poor mainly due to the very low surface area (Tab. S3) and hydrogen diffusion limitation of Co oxide generated from the confinement of OMA. Taking into the catalytic assessment results account (Fig. S5), it was obviously observed that the catalytic performance obtained over 15Ni-OMA and 15Co/OMA was totally different at low temperature range (300–450 °C), and the latter only delivered less than 10% CO conversion and CH4 yield. On the other hand, 15Ni-OMA and xCo/15Ni-OMA catalysts showed parallel trends of CO conversion and CH4 yield as function of reaction temperature, and the corresponding activity are much higher than that of 15Co/OMA. Moreover, 13Co/15Ni-OMA also exhibited higher catalytic performance than 15Co/OMA even though both of them represent comparable physicochemical and textural properties (Table 2 and Tab. S3). More importantly, the kinetics behavior observed over 15Ni-OMA and 8Co/15Ni-OMA catalysts are well suited to all the kinetics models, indicating to follow the same elementary sequence and rate-determining step. Thus, it is reasonably proposed that Ni species confined within mesochannels of OMA undertake the dominant active sites for catalyzing CO methanation, whereas the post-impregnated Co species might serve a promotion effect. Considering the kinetics model discrimination over 15Co/OMA, it is assumed that H-assisted CO dissociation rate is faster than the rate of stepwise hydrogenation of surface species because the approving kinetics models II and III are developed under the assumption of the first-step and last-step hydrogenation as the rate-determining step, respectively. As a result, it is supposed that Co-promoted H-assisted CO dissociation results in an enhanced catalytic activity at low-temperature over the bimetallic 8Co/15Ni-OMA catalyst.In summary, we successfully synthesized a set of Co-Ni bimetallic catalysts via post-impregnating cobalt-precursor within the mesochannel of Ni-OMA. Among all of the catalysts, 8Co/15Ni-OMA showed the highest activity at a temperature of as low as 300 °C and long-term stability (TOS of 200 h) under the conditions of 600 °C and an extremely high GHSV of 180,000 mL g−1 h−1 for the CO methanation reaction. Without the potential limitation of the mass transport and the heat transfer, the apparent activation energy of 8Co/15Ni-OMA for the CO methanation was clearly lower than those of 15Ni-OMA and 15Co-OMA. Based on hydrogen-assisted CO dissociation and Langmuir-Hinshelwood mechanism, three micro-kinetics models were developed by assuming the H-assisted CO dissociation, the hydrogenation of surface carbon species (C*) and surface CH3* species as a rate-determining step, respectively. Moreover, the kinetics data over 15Ni-OMA and 8Co/15Ni-OMA were well fitted with all of the kinetics models. The CH4 formation rate over 15Co/OMA catalyst was satisfactorily fitted with the calculated rate from kinetics model II and III, whereas kinetics model I could be ruled out according to the dataset. Based on the kinetics model discrimination, Ni species confined within mesochannels of OMA was revealed to be dominant active sites for catalyzing the CO methanation, while the post-impregnated Co undertook a promotion effect in the H-assisted CO dissociation step, leading to an enhanced activity at low-temperature and lower apparent activation energy of 100.2 kJ mol−1 over the bimetallic 8Co/15Ni-OMA catalyst.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The financial supports from the National Natural Science Foundation of China (U1862116 and 21706155), the National Key Research and Development Program of China (2018YFB0604600-04), and the Fundamental Research Funds for the Central Universities (GK201901001) are highly appreciated.Supplementary data to this article can be found online at https://doi.org/10.1016/j.cesx.2021.100094.The following are the Supplementary data to this article: Supplementary data 1

The severe requirement of a higher activity at lower temperatures and a longer stability at higher temperatures evokes a great challenge for the development of an industrially viable catalyst for the CO methanation reaction. In this work, the Co-Ni bimetallic catalysts were synthesized via post-impregnating the cobalt precursor within the mesoporous channel of Ni-embedded ordered mesoporous alumina (Ni-OMA). The low-temperature activity and high-temperature stability of Co/Ni-OMA for the CO methanation were significantly regulated by easily tuning the ratio of free Co and confined Ni species. The optimal catalyst of 8Co/15Ni-OMA showed a high activity with the CH4 formation rate of 126 mol kgcat − 1 h−1 at a temperature of as low as 300 °C and a long-term durability for a time-on-stream of 200 h without an observable deactivation under the conditions of 600 °C and an extremely high GHSV of 180000 mL g−1 h−1. Kinetics results reveal that the apparent activation energy of the CO methanation over 8Co/15Ni-OMA (100.2 kJ mol−1) was clearly lower than that over 15Ni-OMA (124.0 kJ mol−1) or 15Co-OMA (131.8 kJ mol−1). In the absence of mass transport and heat transfer limitations, three microkinetics models were developed following the H-assisted CO dissociation and Langmuir-Hinshelwood mechanism, which the H-assisted CO dissociation, the hydrogenation of surface carbon species (C*) or surface CH3* species are proposed as the rate-determining step, respectively. The kinetics behaviors over 15Ni-OMA and 8Co/15Ni-OMA are matched well with all of the kinetics models, indicating the same elementary sequence and rate-determining step. In the case of 15Co/OMA, the CH4 formation rate was predicted very well by the kinetics models derived from the stepwise hydrogenation of surface carbon species as the rate-determining step, and the kinetics model based on the H-assisted CO dissociation as the rate-determining step could be ruled out, indicating that the rate for the H-assisted CO dissociation rate is faster than that of the following stepwise hydrogenation. Based on the discrimination of different kinetics models, Ni species confined within OMA matrix were proposed as the dominant active sites for catalyzing the CO methanation, while the post-impregnated Co was acted as a promoter for the H-assisted CO dissociation. As a result, an enhanced low-temperature activity was achieved over the optimal 8Co/15Ni-OMA catalyst.

Carbon dioxide is the most abundant greenhouse gas and is mainly responsible for the observed global warming. It is the main product of total combustion in power plants and its concentration in the atmosphere has risen considerably since the beginning of the industrial era. In fact, CO2 emissions related to energy grew by 1.4% in 2017, with a record level of 32.5 gigatonnes (Gt) according to the International Energy Agency [1]. Today, CO2 recycling is under increased scrutiny as an alternative to the carbon capture and storage strategy to help CO2 mitigation [2,3]. Carbon dioxide contributes to the synthesis of various higher value chemicals through organic carboxylation reactions leading to chemicals like urea, carboxylic acids, and isocyanates among others [2]. Another route to valuable chemicals is through the formation of syngas followed by further conversion processes such as the Fischer-Tropsch synthesis (FTS) that produces a variety of hydrocarbon fractions. The direct synthesis of methanol from syngas requires a H2/CO ratio of about 2 [4], where the hydroformylation process needs a H2/CO ratio of 1. On the other hand, the FTS usually uses a H2/CO syngas ratio of 2 but would benefit from a higher selectivity towards long-chain hydrocarbons with lower H2/CO ratios [5]. The most commonly used technology to produce syngas is the Steam Reforming of Methane (SRM, Eq. 1) producing a hydrogen rich syngas with a H2/CO ratio of about 3. The dry reforming of methane (DRM, Eq. 2) on the other hand, uses CO2 as oxidant and leads to a syngas H2/CO mixture ratio of a maximum of 1. DRM has the advantage of utilising two of the most abundant greenhouse gases and hence has been increasingly investigated as a CO2 recycling strategy [6,7]. Because the syngas produced by DRM is too poor in H2 to be fed to a FT unit, bi-reforming of methane (BRM, Eq. 3) combining SRM and DRM is proposed to tune the syngas composition. (1) SRM: CH4 + H2O ⇌ CO + 3H2 (2) DRM: CO2 + CH4 ⇌ 2CO + 2H2 (3) BRM: 3CH4 + CO2+ 2H2O ⇌ 4CO + 8H2 BRM is also advantageous in terms of biogas upgrading [8]. The composition of biogas produced through anaerobic digestion varies depending of the source and type of waste used, but consists mainly of CH4, CO2, O2, H2O and impurities which can, after purification treatments, be used in a BRM unit.Reforming processes require high reaction temperatures to reach full reactant conversions but when exposed to such temperatures, typical metal supported on oxides catalysts are subject to deactivation due to sintering of the active phase [9,10]. Coke formation is also a major cause of deactivation due to several side reactions producing carbon such as the Boudouard reaction, CH4 decomposition, CO reduction and CO2 reduction [11–13]. Noble metals such as Rh, Ru, Pd and Pt have shown great catalytic activity and coke resistance, however for applications in large scale industrial processes low cost transition metals are preferred. Extensive research has been conducted in the recent years using Ni catalysts. They are low cost and exhibit good performance for reforming but suffer from severe deactivation. Stabilising Ni is essential to prevent sintering and at the same time to reducing carbon formation by preserving small Ni particles. The use of materials such as hexaaluminates, fluorites, perovskites and pyrochlores have been investigated for this purpose in reforming reactions [14–19]. Pyrochlores are mixed oxides of general formula A2B2O7. The A-site represents a large trivalent cation, typically a rare-earth metal such as La and the B-site is occupied by a tetravalent cation of smaller diameter, typically a transition metal such as Zr [20]. They are benefit from high thermal stability and high oxygen mobility which makes them suitable candidates for high temperature operations and coke resistance [17]. For this reason, pyrochlores have been previously investigated, in particular in the steam reforming reactions. Ma et al. demonstrated that Ni supported on La2Zr2O7 had superior activity to Ni supported on La2Sn2O7 or γ-Al2O3 due to the large amount of La2O2CO3 formed, effectively suppressing coke formation [18]. Zhang et al. supported Ni on various Ln2Zr2O7 supports with different degrees of order, from pyrochlore to defective fluorites. The amount of oxygen vacancies and therefore mobility was key to mitigate carbon deposition [17]. Substitution of Ni in the B site of a pyrochlore has also shown promising activity in reforming reactions [21–24]. Previous work in our group showed that the substitution of 10 wt.% Ni on the B site of a La2Zr2O7 pyrochlore led to a very active, stable and carbon resistant catalyst for DRM [25,26]. However the syngas obtained through DRM had a H2/CO ratio of maximum 0.8 which limits its applicability for further chemical upgrading. With the purpose of tuning the H2/CO ratio of the syngas produced by a stable DRM pyrochlore catalyst, a 10 wt.% Ni doped La2Zr2O7 catalyst was tested under different sets of conditions, including BRM and compared to a supported 10 wt.% Ni on La2Zr2O7 catalyst. The effect of temperature, space velocity and water content in the feed stream were studied as well as the catalyst stability and coke resistance. The studies show promising results for flexible syngas production.The pyrochlore based materials were prepared using a modified citrate method described elsewhere [25]. Lanthanum nitrate [La(NO3)3·6H2O], nickel nitrate [Ni(NO3)2·6H2O], and zirconium nitrate [ZrO(NO3)2·6H2O] provided by Sigma-Aldrich were used as precursors. The necessary amount of each precursor was dissolved in deionized water and then mixed with a citric acid (CA) solution using a CA:metal molar ratio of 0.6:1. The solution was stirred and concentrated in a rotary evaporator. The resulting mixture was dried for 12 h at 100 °C prior combustion at 200 °C. The final powders were calcined at 1000 °C for 8 h to insure phase transition to pyrochlore. Ni was impregnated on the prepared un-doped pyrochlore using an incipient wetness method. [Ni(NO3)2·6H2O] was dissolved in ethanol and mixed to the support. The solvent was removed in a rotary evaporated and the resulting powder was dried for 12 h at 100 °C before calcination at 500 °C for 4 h. The doped catalyst will be referred as LNZ10 and the supported catalyst as Ni/LZ.The textural properties of the material were determined by nitrogen adsorption-desorption measurements at −196 °C in an AUTOSORB-6 fully automated manometric equipment. The sample was degassed under vacuum at 250 °C for 4 h before each measurement. The BET equation was applied to estimate the specific surface area whilst pore-size distributions were determined using the Barett–Joyner–Halenda (BJH) method.X-ray diffraction (XRD) analysis was conducted on fresh, reduced and used catalysts using an X’Pert Pro Powder Diffractometer by PANalytical. The 2θ angle was increased by 0.05° every 240 s over a range of 20–80 °. Diffraction patterns were recorded at 30 mA and 40 kV, using Cu Kα radiation (λ =0.154 nm).Temperature programmed reduction with hydrogen (TPR) analysis was carried out on the calcined catalyst in a U-shaped quartz reactor. A 50 mg sample was heated to 900 °C at a rate of 10 °C min−1 in a flow of 50 mL min−1 of 5% H2 in Ar. A CO2-ethanol trap was used to condense the gaseous products, mostly water, before the on stream thermal conductivity detector (TCD). The H2 uptake was quantified by comparison with the hydrogen consumption of a CuO reference sample.Temperature programmed oxidation (TPO) was conducted in a U-shaped quartz reactor coupled to a PFEIFFER Vacuum PrismaPlus mass spectrometer. Samples were heated up to 900 °C at a rate of 10 °C min−1 in a flow of 50 mL min−1 (5% O2, 95% He).Raman spectroscopy measurements were performed on a Thermo Scientific DXR Raman Microscope using a green laser (λ =532 nm, maximum power 10 mW) with a spot diameter of 0.7 μm and a pinhole aperture of 50 μm. A diffraction grating of 900 grooves mm−1, a CCD detector and a 50× objective were used.Catalytic activity tests were performed in a computerised commercial Microactivity Reference catalytic reactor (PID Eng&Tech), employing a tubular quartz reactor of 9 mm internal diameter. The catalyst was sieved and the 100–200 μm fraction was used for testing, diluted with quartz to achieve a catalytic bed of 0.32 cm3. Water was injected into the system by an HPLC pump (Gilson) before being vaporized and mixed with the gas stream before entering the reactor. The composition of the outlet of the reactor was followed by on-line gas chromatography using a MicroGC (Varian 4900) equipped with Porapak Q and MS-5A columns. Prior to reaction, the catalyst was reduced for 1 h at 650 °C in H2 (10%, v/v in N2). The gas composition was set to CH4/CO2/H2O/N2: 1/1/1/1 to achieve Weight Hourly Space Velocity (WHSV) from 20 to 60 L.g−1. h−1.The effect of water partial pressure variation on the catalytic activity was also studied at 700 °C. In these experiments, the total flow was kept constant using N2 to maintain the WHSV at 60 L.g−1. h−1. The feed composition was 25% CH4 and 25% CO2 while the water concentration was modified taking values of 15%, 25% and 35% (v/v).ChemStations’ ChemCad software package was used to calculate the thermodynamic equilibrium fractions for both DRM and BRM reactions over a range of temperatures. The Soave-Redlich-Kwong equation of state was used in a Gibbs reactor. Material flows into the reactor are identical to those intended to be used for experimentation.The XRD profiles of the freshly prepared catalysts are shown in Fig. 1 . The LNZ10 sample presents the characteristic diffraction features of two different phases. First, a La2Zr2O7 pyrochlore phase (JCPDS Card No. 01-73-0444) was identified, the superstructure peaks (331) and (551) at 36.2° and 43.5° respectively, indicates that the phase transition between fluorite and pyrochlore was achieved [27–29]. These two diffraction peaks, low in intensity, correspond to the ordering of the cations (and anions) in the pyrochlore structure. This was confirmed by Raman analysis (Figure S2) where 5 peaks attributed the pyrochlore phase can be observed. Indeed the group theory predicts six Raman active modes (A1g + Eg + 4 F2g) for the pyrochlore structure (Fd3m) and only one Raman mode (F2g) for the fluorite structure (Fm3m). Here, five peaks corresponding to the pyrochlore-type structured lanthanum zirconate are visible in agreement with the XRD results. The intense Raman peak at 280 cm−1 is the Eg mode associated to O-Zr-O bending vibrations and two F2g modes at 492 and 391 cm−1 are associated to Zr-O and La-O bond stretching with bending vibrations. The A1g mode at 530 cm−1 corresponds to Zr-O6 bending vibrations and the peak at 680 cm−1 is assigned to the dopant –O6 symmetrical stretch in the pyrochlore phase [30]. Second, two diffraction peaks at 31.5 and 45.1° indicate the presence of a La2NiZrO6 rhombohedral double perovskite oxide phase (JCPDS Card No. 00-044-0624). The Ni loading used here is above of the maximum substitution limit of the pyrochlore structure in agreement with Haynes et al. findings [31], leading to the formation of this additional phase. No characteristic diffraction peaks of individual La2O3 or ZrO2 oxides are observed suggesting a complete incorporation of La2O3 and ZrO2 into the pyrochlore and double perovskite structures. Since no peaks attributed to Ni or NiOx species are detected, Ni is either fully incorporated into the mixed structures or some individual Ni particles are formed outside of the bulk inorganic lattice but are sufficiently small and well dispersed not to be detected by XRD. The Ni/LZ sample on the other hand presents the typical diffraction peaks of NiO additionally to the La2Zr2O7 pyrochlore phase. The La2NiZrO6 rhombohedral double perovskite oxide phase was not formed on the supported catalyst since this phase requires calcination temperatures ≥ 800 °C to be formed [31].In order to obtain information about the reduction behaviour and interactions among the active species of the as-prepared catalysts, temperature-programmed reduction treatment were performed and the resulting H2 consumption profile are shown in Fig. 2 . The La2Zr2O7 pyrochlore alone is not reducible [18,26] therefore NiOx species are responsible for any H2 consumption in the catalysts [17,18]. Four reduction processes can be distinguished in the doped catalyst. First, easily accessible NiOx particles located on the outer layer of the catalyst are reduced at 330 °C. Their weak interactions with the bulk of the catalyst facilitate their reducibility [17,31]. The second reduction process at 370 °C is attributed to NiOx exsolved from the pyrochlore structure. Those particles are interacting with the La2Zr2O7 pyrochlore and are therefore reduced at higher temperature [26]. The temperature peak at 485 °C corresponds to most of the H2 uptake and is probably due to the reduction of La2NiZrO6 as observed by Haynes et al. [31]. The fourth reduction process at 600 °C could correspond to Ni exsolved from the pyrochlore structure and still strongly interacting with the pyrochlore. Overall the hydrogen uptake of the doped catalyst was 1.49 mmol/gcat which corresponds to the reduction of 69% of the Ni content of the catalyst. This suggests that some Ni2+ remains under the double perovskite phase and inside the pyrochlore structure in fair agreement with the X-Ray diffraction results. Indeed, the XRD profile of the reduced catalyst shown in Fig. 7 still presents the characteristic pattern of the double perovskite phase. The supported catalyst on the other hand only presents two reduction processes. The peak at 345 °C corresponds to the reduction of large NiO particles in loose contact with the support and the second process at 400 °C is attributed to the reduction of NiO clusters in intimate contact with the pyrochlore [17].The performance of the catalysts were tested under DRM and BRM conditions at 700 °C for a period of 24 h. The CH4 and CO2 conversions under both reaction conditions are shown in Fig. 3 . Under DRM conditions, the doped pyrochlore exhibits excellent catalytic activity with CH4 and CO2 conversions of 87% and 90% respectively, reaching thermodynamics equilibrium. However, when 25% steam is introduced into the system, the conversions decrease to 54% for CH4 and 39% for CO2. The latter is a consequence of the thermodynamic constraints when DRM and BRM are coupled and also reflects the increased competition of both reactants with the new reactant (water) to reach the active sites of the catalysts. Thermodynamics predict a methane conversion of 92% and a carbon dioxide conversion of 47%. The lower performance of the doped catalyst, in particular in terms of methane conversion may be attributed to the reduced activation of H2O on the pyrochlore. In both scenarios, the doped catalyst stabilises very rapidly and shows no deactivation over the time frame of the experiments. On the other hand, the supported catalyst deactivates rapidly, emphasizing the Ni stabilisation induced by the doping strategy. In dry conditions the conversion of CO2 is slightly larger than CH4 likely due to the occurrence of the reverse water gas shift reaction (RWGS), consuming some of the carbon dioxide as reported elsewhere [32]. On the other hand, under BRM conditions, CH4 conversion is largely above the one of CO2. When steam is introduced, RWGS is no longer favoured and in turn SMR and forward WGS occur, therefore consuming more methane and increasing the H2/CO ratio.The Ni-doped pyrochlore catalyst showed great performance in terms of activity and stability for both DRM and BRM. In order to tune the H2/CO ratio for downstream processes, the effect of steam addition in the feed stream was studied. The catalytic activity of the catalyst in terms of CH4 and CO2 conversions and H2/CO ratio as a function of water content is shown in Fig. 4 . The performance of the catalyst was tested at relatively high space velocity (60 L.g−1. h−1) due to equipment limitations. As expected, the H2/CO ratio of the products increases greatly as the water content increases. DRM produces a syngas of H2/CO = 0.7 but, by introducing 35% steam, this ratio can be increased to 2.5. For an FT unit or methanol production, a quantity of 30% water would be necessary to obtain a H2 rich syngas or metgas of H2/CO = 2. It seems however that the improvement in selectivity is made at the expense of conversion. Indeed, as more water is introduced the reactant conversion decreases. Water may promote the SMR reaction but overall, the catalytic activity of the pyrochlore catalyst decreases. Thermodynamically, the addition of water should lead to larger CH4 conversion but a reduced CO2 conversion. The observed decrease in both conversions is possibly due to a change in the kinetics of the reaction induced by water introduction. To the best of our knowledge, no kinetic or mechanistic study has been conducted on BRM to date using a comparable reactants mixture. However, SMR kinetics have been studied. Various studies in the literature have claimed a negative order of steam for SMR [33,34]. The dependence of steam on the rate of reaction can be due to the competition between CH4 and H2O on the catalyst active sites as previously reported elsewhere and in good agreement with our trends [35].Space velocity is a major parameter to consider for scaling up. It determines the volume of the reforming unit and the amount of catalyst needed. The space velocity effect was investigated under BRM conditions with 25% of steam and the results are shown in Fig. 5 . Overall conversions of CH4 and CO2 decrease by increasing the space velocity, although the selectivity remains unchanged. At high space velocity, conversions are far from equilibrium values. However, when the space velocity is decreased to 20 L.g−1. h−1, CO2 conversion nearly reaches the thermodynamic value. CH4 on the other hand seems to be more affected by WHSV as a more significant decrease in conversion is observed when WHSV is increased. This observation actually reflects the fact that methane activation is the rate limiting step for this reaction [33] and therefore the conversion of this reactant is very sensitive to the operation conditions and the catalysts choice. In any case the fact that our catalyst can maintain a H2/CO ratio of over 1.5 (very close to the equilibrium limit) is a commendable achievement for the pyrochlore-perovskite material which reflects the potential of this advanced catalyst for hydrogen-rich syngas production.The effect of temperature in BRM conditions was studied using a feed containing 25% of water and results are shown in Fig. 6 . An increase in conversion is observed as the temperature increases as the thermodynamics predicts. At low temperature methane conversion is low and far away from equilibrium but as the temperature increases it gets closer to the equilibrium values and reaches a conversion of 80% at 750 °C. The low methane conversion at low temperature can be related to the high activation energy of CH4. Methane needs high temperature to overcome the energy barrier necessary for its activation. Similarly to methane, CO2 conversion is lower than the equilibrium at low temperature but gets closer to it as the temperature increases. At 750 °C, CO2 conversion is only 2% below equilibrium reaching 51% conversion. In terms of selectivity, the H2/CO ratio follows the equilibrium trend and decreases slightly with the temperature.The development of stable catalysts is one of the bottleneck for the implementation of combined reforming in commercial CO2 conversion units. In this scenario, post reaction analysis is necessary to ascertain the robustness of our multicomponent catalyst under the studied reaction conditions. XRD was performed on the catalyst after reduction pre-treatment and after reaction at 700 °C under DRM and BRM conditions to detect any structural changes induced to the catalyst. The resulting profiles are shown in Fig. 7. No structural changes were detected between the fresh catalyst (Fig. 1) and the reduced catalyst. The characteristic diffraction features of La2Zr2O7 are still present attesting of the thermal stability of this material. No trace of metallic Ni was detected showing that either the reduced Ni particles are small and well dispersed or that Ni remains in the pyrochlore and double perovskite phases. Moreover the double perovskite phase La2NiZrO6 is still present and did not completely reduce to Ni, La2O3 and ZrO2. After DRM and BRM reactions, a shoulder is detected at 44.4° and a small peak appears at 51.7°, corresponding to the main diffraction peaks of metallic Ni. The appearance of these peaks could be due to a certain degree of Ni sintering but also to the exsolution of Ni from the pyrochlore structure during reaction [26]. In view of the excellent performance with no deactivation observed during 24 h of continuous run the exsolution of Ni is plausible in agreement with previous reports [36]. In fact, the low surface area of the catalyst (Table S1, Supporting information) supports this claim as Ni is very small (around 21 nm after reaction, according to the Scherrer equation) and therefore must be well dispersed on the surface and possibly released from the structure as the reaction takes place. Traces of La2O3 (JCPDS Card No. 01-074-2430) are detected after reaction, resulting either from the partial decomposition of the pyrochlore or from the partial reduction of the double perovskite.Carbon is a side product of CO2-reforming reactions and is the main cause of catalyst deactivation. Carbonaceous species potentially formed on the catalyst were quantified and identified by temperature programmed oxidation. The CO2 production profiles of samples that have undergone DRM and BRM with different amount of steam are shown in Fig. 8 . This analysis shows that no carbon was formed on the catalyst when water was introduced into the system. Indeed under BRM conditions, carbon formation is minimised due to the reverse CO reduction reaction (C(s) + H2O → CO + H2). The presence of steam prevents the deactivation of the catalyst by carbon formation. However under DRM conditions (i.e. 0% water) a significant amount of carbon was formed (0.01gC/gcat). Three different oxidation processes can be distinguished corresponding to different carbonaceous species. The low temperature peak at 270 °C is attributed to the gasification of Cα amorphous carbon. Cα are believed to be active species in reforming, originating from nickel carbide produced during methane decomposition [37]. The second peak at 445 °C corresponds to Cβ filament carbon. This type of carbon can be eliminated at relatively low temperature [38]. Finally, the third peak, at 705 °C, corresponds to the oxidation of more graphitic carbon, inert and requiring high temperature to remove. The later indicates that coking will be a factor to consider if our pyrochlore-perovskite catalysts are going to be used in a DRM unit. Nonetheless, the addition of water (BRM mode) heavily mitigates the impact of carbon deposition resulting in a stable catalyst to produce H2-rich syngas streams. The Raman analysis presented in Figure S2 supports this claim. Experiments were conducted on the samples after reaction. No evidence of carbon species were found on the samples after BRM reaction. However the typical D and G bands of multiwall carbon nanotubes were observed at 1342 and 1576 cm-1 respectively on the sample after DRM.This work provides evidence of the excellent performance of a Nickel-doped pyrochlore catalyst for chemical CO2 recycling via DRM and BRM. Structural analysis revealed the presence of the pyrochlore and a secondary double perovskite phase which constitutes the basis of this novel catalyst. After the reaction, small Ni clusters are present on the surface of the catalyst as suggested by XRD and TPR. In fact, it is very likely that active Ni clusters are exsolved from the pyrochlore during BRM and DRM leading to highly dispersed active ensembles which account for the high activity and stability of the catalyst during both reactions. Very importantly, the H2/CO ratio produced by the catalyst can be fine-tuned by introducing steam into the system, enabling a flexible syngas production for a variety of applications. Our engineered catalyst also allows adjustment of the syngas ratio under different reactions conditions such as temperature and space time, thus making it very versatile when process integration is considered. As an additional advantage, carbon deposition over the pyrochlore-perovskite catalyst is fully eliminated when steam is added to the reforming mixtures. Overall, this work showcases a strategy to design highly effective heterogeneous catalysts for gas-phase CO2 valorisation – the stabilisation of Ni particles on a complex mixed oxide structure resulting in a powerful dry and bi-reforming catalyst able to deliver customised syngas for chemical synthesis.This work was supported by the Department of Chemical and Process Engineering at the University of Surrey and the EPSRC grant EP/R512904/1 as well as the Royal Society Research Grant RSGR1180353. This work was also partially sponsored by the CO2Chem through the EPSRC grant EP/P026435/1. The Ministerio de Economía, Industria y Competitividad of Spain (Project ENE2015-66975-C3-2-R) co-financed by FEDER funds from the European Union supported the work done in Spain. Finally, E. le Saché would like to acknowledge the Armourers & Brasiers Gauntlet Trust for their travel grant award.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2019.05.039.The following is Supplementary data to this article:

The bi-reforming of methane (BRM) has the advantage of utilising greenhouse gases and producing H2 rich syngas. In this work Ni stabilised in a pyrochlore-double perovskite structure is reported as a viable catalyst for both Dry Reforming of Methane (DRM) and BRM. A 10 wt.% Ni-doped La2Zr2O7 pyrochlore catalyst was synthesised, characterised and tested under both reaction conditions and its performance was compared to a supported Ni/La2Zr2O7. In particular the effect of steam addition is investigated revealing that steam increases the H2 content in the syngas but limits reactants conversions. The effect of temperature, space velocity and time on stream was studied under BRM conditions and brought out the performance of the material in terms of activity and stability. No deactivation was observed, in fact the addition of steam helped to mitigate carbon deposition. Small and well dispersed Ni clusters, possibly resulting from the progressive exsolution of Ni from the mixed oxide structure could explain the enhanced performance of the catalyst.

Oxygen-involved electrocatalytic reactions, including 4e−/2e−oxygen reduction reaction (ORR), 4e−oxygen evolution reaction (OER), and 2e−water oxidation reaction (WOR), are key reactions for new-generation energy technologies utilizing renewable clean fuels [1–3]. Up to now, tremendous effort has been devoted to the discovery and design of advanced electrocatalysts, such as transition metal/alloys [4], sulfides [5], phosphides [6], oxides [7,8], (Oxy)hydroxides [9], and carbon-based materials [10]. Among them, precious metal materials, mainly platinum-group metals [11,12], are the only commercially available catalysts, but their high costs and scarcity severely inhibit their large-scale commercial applications. Designing non-precious metal catalysts with high oxygen electrocatalytic performance to replace noble-metal-based catalysts is essential for practical applications.Single-atom catalysts (SACs) can be obtained by anchoring the single transition metal (TM) atom on nitrogen-doped carbon materials [13–17], and have been employed in oxygen-involving reactions. The active centers of SACs are mainly tetracoordinate planar TM-N4 moieties, and some SACs (e.g., Mn, Fe, Co, Ni, and Cu) can efficiently catalyze oxygen reactions [18–23]. For example, Fe-based SACs favor ORR, while Co-based SACs tend to catalyze O2 into H2O2 [24,25]. Nevertheless, other SACs, especially those that originated from early TMs, generally exhibit extremely poor performance due to their relatively excessive adsorption strengths [26].In this work, to effectively reduce the adsorption strength of single-atom catalysts, we tune strong axial coordination (-OH, =O, and ≡N) to SACs for tailoring the adsorption ability of TM-derived SACs. Using density functional theory (DFT) calculations, we systematically investigated the role of axial coordination in regulating O2 adsorption and catalytic performance on various SACs. Our results revealed that many SACs, not reported yet, exhibited exceptionally good activity for 4e−/2e− oxygen reactions, including V-SAC-OH with an overpotential of 0.61 V for 4e−ORR, Mo-SAC-OH with an overpotential of 0.05 V for 2e−ORR, Mo-SAC-O with an overpotential of 0.52 V for 4e−OER, and Nb-SAC-OH with an overpotential of 0.14 V for 2e−WOR. Herein, we demonstrated that weakening the adsorption ability of SACs towards oxygenated species (∗OOH, ∗OH, and ∗O) can promote the catalytic activity toward different oxygen-involved reactions. Furthermore, we proposed a theoretical framework that integrates the SAC configuration, TM species, and TM charges to describe the catalytic ability of SACs. Our results built a full profile to understand the catalytic behavior of SACs and provided a new approach for developing highly active SACs in oxygen-involved reactions.All spin-polarized [27] DFT calculations were conducted with the Vienna Ab initio simulation package (VASP) [28,29]. The exchange-correlation functional was described by the popular Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) [30]. The frozen-core projector-augmented wave (PAW) method with a cutoff energy of 520 eV was used to describe the interaction between core electrons and valence electrons [31]. In addition, Grimme's DFT-D3 scheme was used to describe the long-range vdW interactions [32]. A 6 × 6 × 1 supercell of graphene layer embedded with The TM-N4 moiety was embedded in a 6 × 6 × 1 supercell of graphene layer to simulate the TM-SAC catalyst. A 15 Å vacuum layer was set to eliminate the interactions with the periodic images along the Z axial direction. Γ-centred Monkhorst–Pack k-point mesh grid of 3 × 3 × 1 was employed for all supercells [33]. Moreover, the criteria of energy and force convergence were set to 1.0 × 10−5 eV per atom and 0.02 eV Å−1 for geometry optimization, respectively. Bader charge analysis was used to study the atomic charge changes. The VESTA program [34] was employed to construct all models and to plot charge density differences.The elementary steps for 4e−ORR in an acidic medium are shown in Eqs (1)–(4), whereas the elementary steps for 2e−ORR to produce H2O2 are the combination of Eqs (1) and (5). (1) ∗ + O 2 + H + + e - → ∗ O O H (2) ∗ O O H + H + + e - → ∗ O + H 2 O (3) ∗ O + H + + e - → ∗ O H (4) ∗ O H + H + + e - → ∗ + H 2 O (5) ∗ O O H + H + + e - → ∗ + H 2 O 2 The 4e−OER pathway is shown in Eqs (6)–(9), and the 2e−WOR pathway to form H2O2 is the combination of Eqs (6) and (10). (6) ∗ + H 2 O → ∗ O H + H + + e - (7) ∗ O H → ∗ O + H + + e - (8) ∗ O + H 2 O → ∗ O O H + H + + e - (9) ∗ O O H → ∗ + O 2 + H + + e - (10) ∗ O H + H 2 O → ∗ + H 2 O 2 + H + + e - where ∗ denote adsorbed sites on SACs, adsorbed intermediates ∗OOH, ∗O, and ∗OH are adsorbed intermediates, respectively.For each elemental step, the Gibbs free energy Δ G i can be calculated using Eq (11). (11) Δ G = Δ E + Δ E Z P E - T Δ S + Δ G p H + Δ G U where Δ E is the total energy of reactions obtained from DFT calculations, Δ E Z P E and Δ S represent the changes of zero-point energy and entropy, respectively. T denotes the temperature (298.15 K). The zero-point energy and entropy are calculated using the vibrational frequencies of the oxygenated intermediate species based on the harmonic normal mode approximation while fixing the catalyst slab. ΔG pH = −k B Tln[H+] = k B × pH × ln10 is the contribution of H+ concentration change for Gibbs free energy during the ORR process, where k B is the Boltzmann constant, and the value of pH is assumed to be zero. Δ G U = - n e U is the contribution of applied electrode potential, where n is the number of electrons transferred in each elemental reaction and U is the applied electrode potential. Besides, according to the computational hydrogen electrode (CHE) model suggested by Nørskov et al. [35,36], the chemical potential of a proton/electron pair is equal to half of the energy of H2. Due to the difficulties in the DFT calculations of open-shell triplet O2, the free energy of the O2(g) molecule is calculated by G O 2 ( g ) = 2 G H 2 O t 2 G H 2 + 4.92 eV [35].Based on the free energies of elemental steps, the thermodynamic overpotential of ORR/OER/WOR on SACs can be obtained via Eqs (12)–(15). The elementary step with the maximum overpotential is considered the potential-determining step (PDS), which limits the ORR/OER/WOR processes. (12) η 4 e − O R R = m a x { Δ G 1 , Δ G 2 , Δ G 3 , Δ G 4 } / e + 1.23 V (13) η 2 e − O R R = m a x { Δ G 1 , Δ G 5 } / e + 0.68 V (14) η 4 e − O E R = m a x { Δ G 6 , Δ G 7 , Δ G 8 , Δ G 9 } / e − 1.23 V (15) η 2 e − W O R = m a x { Δ G 6 , Δ G 10 } / e - 1.78 V Eqs (16)–(18) are the reactions for the formations of key intermediates on SACs. (16) ∗ + 2 H 2 O → ∗ O O H + 3 / 2 H 2 (17) ∗ + H 2 O → ∗ O + H 2 (18) ∗ + H 2 O → ∗ O H + 1 / 2 H 2 The calculated free energy of formation of each key ORR-intermediate can be obtained by using Eqs (19)–(21): (19) Δ G ∗ O O H = G ∗ O O H + 3 / 2 G H 2 − G ∗ − 2 G H 2 O (20) Δ G ∗ O = G ∗ O + G H 2 - G ∗ - G H 2 O (21) Δ G ∗ O H = G ∗ O H + 1 / 2 G H 2 - G ∗ - G H 2 O Experimentally, TMs can be embedded into the bulk vacancies such as single vacancy, double vacancies, and Stone-Wales defects [37], to form single-atomic active sites. It is observed that most of the metals are trapped at the nitrogenated double vacancies based on the advanced characterization including transmission electron microscopy and X-ray absorption near edge structure [23,38], displayed in Fig. 1 (a), and it is the SAC model in this study. A total of 18 metals, including all 3d TMs except Sc, six 4d TMs, and three 5d TMs can be anchored on N-doped graphene via chemically binding with four pyridinic-nitrogen atoms (Figs. S1–2). Due to their lone-pair orbitals, we choose axial ligands (e.g., –OH, =O, and ≡N) to strongly bound to the central metals of these SACs (Fig. S3) via single-, double-, and triple-bonds, respectively, in Fig. 1(b). Increasing the axially coordinated orbitals with d orbitals can gradually increase the interaction with central metals of SACs. In addition, the dissolution potential in Fig. S4 shows that SACs with axial ligands exhibit higher stability in an electro-chemical environment. Finally, 72 SACs were screened for further investigation.Theoretically, O2 adsorption/desorption is a key process for ORR/OER, reflecting the reactivity of SACs. We thus estimated the O2 adsorption ability on SACs through O2 adsorption energy and configuration. Fig. 1(c) demonstrated four adsorption configurations of O2 molecules, including dissociation adsorption O 2ad (1), side-on adsorption O 2ad (2), top-on adsorption O 2ad (3), and physisorption O 2ad (4). These configurations of O2 adsorption highly correlate with the interaction between oxygen molecules and active sites [39], revealing the change in the adsorption ability of SACs.We then optimized O2 adsorption on SACs. Our computations in Fig. 1(d) demonstrated that SACs derived from early TMs strongly adsorb O2 and form the O 2ad (2) configuration with stretched O2 bonding length. This indicates that these early TMs delivered a high adsorption ability towards O2. SACs derived from late TMs show relatively weak adsorption ability with the O 2ad (3) or O 2ad (4) configuration. With the axial ligands being stronger, the O2 adsorption configuration gradually changes from O 2ad (2) to O 2ad (4), especially for Ti, V, Nb, and Mo. Besides, the corresponding O–O binding length and adsorption energy decrease with stronger ligand bonding as displayed in Figs. S5 and S6. Hence, axial coordination was confirmed to effectively regulate the adsorption of SACs for further investigation in oxygen reactions.The intrinsic catalytic activity of SACs with axial ligands towards oxygen reaction was further investigated. As exhibited in Fig. 2 (a), the 4-electron ORR can be divided into four steps based on the number of electron transfers. Depending on the O2 adsorption configuration, the adsorbed O2 interacted with the first H+/e− pair and then generate three different species 1, including ∗O+∗OH, ∗O∗OH, and ∗OOH. Besides, all these species 1 can expose two oxygen sites for the subsequent hydrogenation. The second H+/e− pair can accordingly attack either of the oxygen atoms of species 1 to form species 2, including the intermediates (∗OH+∗OH or ∗O) or the production of H2O2. The generation of H2O2 implies the completion of 2e−ORR and the start of the next reaction cycle [2]. If the third electron/proton transfer occurs, the above intermediates can only convert into species 3, namely ∗OH, and then reduced to H2O, followed by desorption. With 4-electron transferred, O2 can be reduced to H2O, which finalizes the 4e−ORR process [40]. Along the reverse direction of electron transfer, the H2O molecule couples two or four electrons to form H2O2 and O2, corresponding to 2e−WOR and 4e−OER, respectively [8,41].Within the theoretical frame of such four oxygen-involving reactions, we calculated the free energy change for each elementary reaction in Eqs (1)–(10) (Tables S3–4). Based on Eqs (12)–(15), we estimated the catalytic activity of SACs (with/without axial coordination) by using the DFT calculated overpotential. Fig. 2(b-e) summarized the heatmaps of the overpotentials on SACs. Taking the 4e−ORR overpotential of 0.43 V on Pt (111) as a golden standard [35], a few SACs even without axial coordination exhibit high catalytic performances such as Cr- and Mn-SAC for 2e−ORR, Co- and Rh-SAC for 4e−ORR, 2e−ORR, and 4e−OER, Ni-, Cu-, Ag-, and Au-SAC for 2e−WOR, Zn-SAC for 4e−OER. None of the SACs derived from the other 11 TMs, especially the early TMs, exhibit low overpotentials for the oxygen-involving reactions, revealing that the late TM metal-derived SACs have a superior catalytic activity compared to the early TM-derived SAC in catalyzing O2/H2O conversion [42]. With axial coordination, a large number of newly active SACs arise including 6 SACs for 4e−ORR, 16 SACs for 2e−ORR, and 5 SACs for 4e−OER, and 20 SACs for 2e−WOR. Meanwhile, some SACs possess higher or comparable catalytic performance to the conventional tetra-coordinated SACs: 4 SACs for 4e−ORR, 10 SACs for 2e−ORR, and 6 SAC for 2e−WOR. Especially, Mo-SAC-OH possesses the lowest overpotential of 0.05 V towards 2e−ORR, revealing that the Mo-SAC-OH is a potential excellent catalyst for 2e−ORR. Nb-SAC-OH exhibited the best catalytic activity towards 2e−WOR with the lowest overpotential of 0.01 V. Therefore, it proved again that moderate axial coordination can be an efficient strategy to regulate the catalytic activity of SACs. Furthermore, it is worth noting that early TM-derived SACs display great potential in the development of new catalysts for catalyzing oxygen reactions via axial coordination.The above activity data motivated us to find an underlying mechanism to shed light on the regulation rule for SAC systems. First, we constructed the scaling relation between the adsorption energy of three key intermediates (species 1, 2, and 3) to describe the adsorption behavior of SACs. Apart from ∗O+∗OH on Ti-SAC, V-SAC, Nb-SAC, and Mo-SAC, the adsorption energy of other species 1 (∗O∗OH and ∗OOH) linearly correlated with the adsorption energy of ∗OH with R-square of 0.93 in Fig. 3 (a).The linear relation between ΔG ∗OOH and ΔG ∗OH is ΔG ∗OOH = 0.87ΔG ∗OH + 3.2. This is similar to the scaling relation: ΔG ∗OOH = ΔG ∗OH + 3.2 in metal oxides [43]. Meanwhile, due to the different bonding patterns in ∗O/∗OH (double-bond vs single-bond) and the high sensitivity of double-bonded ∗O to the adsorption site, the linear relationship for ∗O/∗OH (R-square = 0.84) is not as good as that for ∗OH/∗OOH (R-square = 0.93), and the fitted linear relationship for ∗O/∗OH is: ΔG ∗O = 1.57ΔG ∗OH + 0.9. Accordingly, these linear fitting showed that the adsorption energy of ∗OH can be employed as a descriptor to describe the influence of the change of adsorption in the catalytic activity. Furthermore, Fig. 3(b) exhibits a volcano-shaped relationship between free energy change along oxygen reactions and the adsorption energy of ∗OH. The commercial Pt/C has an overpotential of 0.43 V for ORR, and we have used this value to screen the electrocatalysts. It is worth noting in Fig. S7 that there are four volcano peaks corresponding to 4e−ORR [44], 4e−OER [45], 2e−ORR [46], and 2e−WOR [41], respectively. The optimal ΔG ∗OH is corresponding to 0.92, 1.10, 1.20, and 1.93 eV. It indicates that the high-performance 4e−ORR catalysts require relatively strong adsorption, whereas 4e−OER, 2e−ORR, and 2e−WOR require weak ΔG ∗OH. Fig. S8 demonstrates that weakening ∗OH adsorption can gradually increase the catalytic activity toward 4e−ORR, 4e−OER, 2e−ORR, and 2e−WOR. Fig. 3(c) presents the ΔG ∗OH values of conventional SACs, and Ti-, V-, Cr-, Mo-, Nb-, Ru-, and Os-SACs possess strong adsorption with ΔG ∗OH below or close to 0 V and are located in area I in Fig. 3(b). This indicated that they have poor catalytic activity towards all oxygen reactions, which is consistent with the previous discussion. Mn-, Fe-, Co-, Ni-, and Rh-SACs exhibited a feasible ΔG ∗OH near 1 eV in area II. These SACs deliver good ORR performance [21]. Among them, with the larger value of the ΔG ∗OH, the SACs tend to be closer to area II, implying an improved activity of 2e−ORR, and 2e−WOR. With high ΔG ∗OH above 1.90 eV, Ni-, Cu-, Zn-, Pd-, Pt-SACs exhibit weak adsorption in area IV, which indicates the high activity of 2e−WOR. Besides, it can be found that axial coordination can decrease ΔG ∗OH. Furthermore, early TMs exhibit a sharp decrease of ΔG ∗OH compared with late TMs and a moderate ΔG ∗OH approaching or slightly exceeding 1 eV. This breaks the limitation of SACs derived from early TMs to catalyze oxygen reactions. Consequently, as shown in Fig. 3(d), there are a growing number of SACs with axial coordination exhibiting high catalytic activity towards either 4e−ORR, 4e−OER, 2e−ORR, or 2e−WOR. Therefore, the axial coordination strategy can effectively tune the catalytic activity of SACs towards oxygen-involved reactions and extend the SACs to the early TMs.In this section, we discuss the correlation between the reduced adsorption energy of SACs and the improved catalytic activity towards oxygen-involved reactions. Electron transfer processes on ∗OOH, ∗OH, and ∗O adsorption on Mo-SAC with/without axial coordination were analyzed, and displayed in Fig. 4 (a-d). Mo-SAC displays strong adsorption of ∗OOH (ΔG ∗OOH = −1.55 eV), ∗OH (ΔG ∗OH = −1.52 eV), and ∗O (ΔG ∗O = −2.56 eV), phenomenally corresponding to the prolonged O–O the bond length of 2.63 Å in ∗OOH, short Mo–O bond lengths of 1.86 and 1.69 Å in ∗OH and ∗O. Comparably, with the additional –OH ligand in an axial direction, the adsorption ability is weakened (e.g., ΔG ∗OOH = 4.18 eV, ΔG ∗OH = 1.04 eV, and ΔG ∗O = 0.70 eV), which attributed to a shorter O–O the bond length of 1.46 Å. Additionally, the Mo–O bond lengths in ∗OH and ∗O were stretched to1.91 and 1.72 Å, respectively.As shown in Fig. 4(b) and (d), the presence of axial coordination can reduce the electrons transferring from the central Mo site into oxygenated intermediates, which is in good agreement with the observation of the change of adsorption. In addition, considering the effect of potential by using the potential-fixed method, the axial coordination still promotes the catalytic activity (Fig. S11) [47]. Therefore, weakening the electron-donating ability of a central metal atom in SACs can tailor the adsorption ability. The Bader charge of central metals is shown in Fig. 4(e), and the plot of ΔG ∗OH vs Bader charge is given in Fig. 5 , which demonstrates that most conventional SAC configuration without axial coordination locates at the lower bound of the triangle framework. This certifies SAC configuration plays a significant role in tailoring the ∗OH adsorption energy of SACs. Besides, increasing the Bader charge of the TM center via axial coordination boosts the ∗OH adsorption energy of SACs. Increasing axial bond endows TM the highest positive charge, accordingly resulting in the weaker ∗OH adsorption. Furthermore, it can be observed that the SACs, located farther to the left in the periodic table, exhibit a higher charge state, indicating the role of TM species in influencing adsorption. Therefore, SAC configuration, charge state, and transition metal species were integrated into the theoretical triangular framework (SI) to describe the catalytic behavior of SACs.To explore the role of TM species in tailoring the adsorption of SACs, we further investigated the frontier d-orbital distribution. As displayed in Fig. S12, the coefficients of determination R-squares between spin-up (UP) d-band center and spin-down (DW) d-band center, including the projected d y z − , d x z − , d x y − , d x 2 − y 2 − , d z 2 − band and the full d-band center, are 0.78, 0.88, 0.88, 0.84, 0.88, and 0.88, respectively. Besides, their corresponding slopes are close to 1, indicating the high correlation between the spin-up (UP) d-band center and the spin-dw (DW) d-band center. It is thus reasonable to apply the single spin-up (UP) d-band center to describe the variation of the total d-band center. Furthermore, the linear fitting of d y z − , d x z − , d x y − , d x 2 − y 2 − , d z 2 − band center (UP) exhibited in Fig. S13 showed their R-squares of 0.88, 0.91,0.93, 0.81, and 0.94, respectively, and their corresponding formulas are shown in Table 1 . These fitting results manifest that the energy level of all d-band centers (UP) can be expressed as a function of the energy level of d z 2 − band center (UP). As can be seen in Fig. 6 (a), with the increase of the d z 2 − band center (UP), the order of the projected d-band is changing. Based on the points of intersection listed in Table 1, we can divide Fig. 6(a) into four areas, and each area corresponds to a d-orbital splitting shown in Fig. 6(b).As shown in Fig. 6(a) and (b), with the increase of the d z 2 − band center (UP), the relative order of d z 2 − orbitals downshift, while d x z − and d y z − orbitals upshift to a relatively higher order. As reported previously, decreasing d-electrons can induce the upshift of the d-band center [48–50]. Such upshift will lead to the reordering of the dz2-orbital (downshift) and d x z − and d y z − orbitals (upshift) in Fig. 6(b). It drives the d z 2 − orbital approaching the Fermi level and becoming a good electron donor, whereas the d x z − , d y z − orbitals becoming good electron acceptors. Such change can strengthen the interaction between SACs and the adsorbates since the three d-orbitals are mainly involved in the bonding with adsorbates [51]. Therefore, the early TM-derived SACs, with fewer electrons in d-orbitals, have the weak adsorption ability of SACs. We then divided the framework in Fig. 6(c) into 4 areas. There are various cases of 3d-orbital splitting in each area. In conventional SACs, Cu, Ag, and Au belong to area I, and most Mn, Fe-, Co-, Ni-, Ru-, Pd-, and Pt-derived SACs are in area II, while Ti-, V– Cr-, Nb-, Mo-, Rh-, and Os-derived SACs located at area III. It is worth mentioning that part of SACs, such as Ni, exists in two areas because axial coordination can regulate not only the charge state of metal sites but also the energy level distribution of 3d-orbitals through orbital interaction [52–54]. Therefore, the number of d-electrons in the 3d-orbital of SACs, determining the 3d-orbital splitting, further affects the adsorption behavior of SACs. Therefore, a theoretical framework plotted in Fig. 6, combining TM species, charge states, and SAC configurations can be employed to describe the adsorption of SACs and catalytic activity toward oxygen-involved reaction.In this work, we investigated the catalytic activity of a single atomic catalyst (SAC) with axial coordination towards oxygen-involving reaction, including 4e−/2e−ORR, 4e−OER, and 2e−WOR by density functional theory calculations. With the presence of axial coordination (-OH, =O, and ≡N), SACs, even early TMs-derived SACs, can exhibit high catalytic activity towards four oxygen-involved reactions. The accurate scaling relation confirmed weakening the adsorption ability can improve the catalytic activity towards different oxygen reactions. More importantly, a theoretical framework of SAC configuration, transition metal (TM) species, and TM charge states have been established to describe the adsorption ability of SACs. This work offers an intrinsic landscape to explore the catalytic activity of SACs, providing rational guidelines for designing high-performance SACs. Zhang Chengyi: Conceptualization, Software, Writing- Original draft preparation. Dai Yuhang: Conceptualization. Sun Qi: Supervision, Data Curation. Ye Chumei: Visualization. Lu Ruihu: Conceptualization, Software, Formal analysis. Zhou Yazhou: Validation. Zhao Yan: Funding acquisition, Project administration, SupervisionThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The work in this paper was supported in part by the Foshan Xianhu Laboratory of the Advanced Energy Science and Technology Guangdong Laboratory (XHT2020-003) and by the Key R&D program of Hubei (2021BAA173).The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtadv.2022.100280.

Single-atom catalysts (SACs) are promising for 4e−oxygen reduction reaction (4e−ORR). However, they are rarely utilized in other oxygen-involved reactions, e.g. 2e−ORR to produce H2O2, 4e−oxygen evolution reaction (4e−OER), and 2e−water oxidation reaction (2e−WOR). Herein, we applied density functional theory (DFT) calculations to investigate the applicabilities of SACs, including Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Mo, Ru, Rh, Pd, Pt, and Au with axial coordination (e,g, –OH, =O, and ≡N) for all 4e−/2e− oxygen reactions. With axial coordination, SACs derived from early transition metals exhibit high catalytic performance, including V-SAC-OH with an overpotential of 0.61 V for 4e−ORR, Mo-SAC-OH with an overpotential of 0.05 V for 2e−ORR, Mo-SAC-O with an overpotential of 0.52 V for 4e−OER, and Nb-SAC-OH with an overpotential of 0.14 V for 2e−WOR. Among them, most SACs deliver a trend of adsorption-energy decreasing with the increase of axial bond, which successively meets various adsorption requirements of all 4e−/2e− oxygen reactions. This finding has led to the discovery of highly active SACs adapted to different oxygen reactions. Importantly, an intrinsic framework that combines SAC configuration, transition metal (TM) species, and TM charges was established to describe the adsorption ability of SACs. This work offers an intrinsic landscape to understand the correlation of the adsorption ability of SACs with the tendentiousness of oxygen-involved reactions and guides the rational design of SACs.

Global warming and environmental protection concerns triggered by fossil fuel combustion have accelerated our demand for sustainable and clean energy resources [1,2]. As one of the perfect clean energy sources, hydrogen is oxidized into water and release high energy, which has no pollution to the environment and has a promising application prospect [3,4]. So far, many methods for producing sustainable hydrogen have been developed [5]. Considering the safety and stability of commercial products, the conversion of electric energy to hydrogen energy by electrolysis of water to hydrogen has caused wide attention. And hydrogen evolution reaction (HER) is the fundamental step in the process of water electrolysis [6,7]. Currently, precious metal Pt is believed to be the most effective and stable catalyst for HER, and Pt containing catalysts have been used to catalyze the decomposition of water to make hydrogen [8,9]. Unfortunately, scarce reserves and high prices of Pt have greatly hindered the practical application of Pt-based catalysts.As economical and efficient replacements to Pt-based catalysts, nickel-based electrocatalysts have showed potential electrocatalytic activity and kept stable for HER, such as nitride, alloys, phosphides, chalcogenides, metal organic frameworks (MOFs) [10–14]. Despite the low price and good conductivity, Ni-based electrocatalysts have much higher overpotential than that of Pt-based, so Ni-based electrocatalysts still face challenges [15,16]. Hence, in order to further improve HER performance of Ni-based catalysts, one of the strategy is to synthesize multi-metal catalysts containing Ni. For example, Alinezhad et al. achieved the goal of improving HER activity by growing Pt islands on branched Ni nanoparticles [17]. Moreover, Pd and Pt belong to the same family in the Periodic Table and show very similar catalytic properties in many cases. Pd-based catalysts could be the ideal substitute for Pt-based catalysts in multiple applications. Furthermore, Pd/Ni bimetallic catalyst may improve electrocatalytic effect. It has been reported that ternary Pd-Ni-P nanoparticles are superior in HER performance [18,19]. Due to the influence of Ni and P, the hydrogen adsorption energy of Pd is weakened and hydrogen is more easily released [20]. Therefore, the combination of Pd with Ni will be a promising method for the outstanding electrocatalytic performance.However, bare transition metal alloy catalysts are not sufficiently stable, especially when the durability test is carried out in the electrolyte solution. In recent years, metals@carbon composites are recognized as effective catalysts for HER, due to the protection of carbon shell structure. This kind of materials has good corrosion resistance and facilitate electron transfer [21–24]. To implement this kind of structure, MOFs are considered as appropriate precursors, which are synthesized by automatic combination of metal ions and organic ligands. The conversion from MOFs to carbon shell usually only requires a simple carbonization process. The carbon shell derived from MOFs can achieve huge surface and significantly improve the dispersity of the active ingredients, while protecting the transition metal by means of fending off direct contact with electrolytes [25–28]. In addition, N contained in the organic component of MOFs will create more reactive sites at the interfaces to activate transition metal after calcination. In summary, MOFs containing N element are appropriate precursors for the construction of N-doped porous carbon containing metal nanoparticles [29].Although Pd/Ni binary materials could be used as potential efficient catalysts for HER, little research has been done in this field [30]. Herein, we synthesized Pd/Ni bimetallic nitrogenous carbon (Pd/Ni-NC) material with a unique structure by calcination for high efficient HER, which showed better performance than Pt/C in electrocatalytic hydrogen evolution. In this work, the precursor was obtained in two steps, including the synthesis of Ni-MOF and the realization of cation exchange between Pd and Ni. The product was calcined into Pd/Ni-NC with a setaria-shaped morphology. Pd and Ni in the material have a good synergistic effect. Moreover, the precursor contains a large of N element, which can be used to prepare nitrogen-containing carbon materials without additional doping of nitrogen, thus improving the electronic conductivity and electrocatalytic activity [31,32].All chemicals used in the experiment were not further purified. Nickel(II) acetate tetrahydrate (Ni(CH3COO)2•4H2O) and palladium(II) chloride (PdCl2) were purchased from Sinopharm Chemical Reagent Co., Ltd. Dimethlylglyoxime (DMG) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Hydrochloric acid was purchased from Chinasun Specialty Products Co., Ltd.Dimethylglyoxime (DMG, 0.1161 g) was dissolved in ethanol (25 mL) at 65°C. Then, 25 mL of ethanol dissolved Ni(CH3COO)2•4H2O (0.1244g) was added to the above DMG solution. The mixture was allowed to react under stirring for 3 h. The red precipitation (Ni-MOF) was collected by centrifugation and ultrasonic washing for several times. Pd-MOF was prepared through the same method except replacing Ni(CH3COO)2•4H2O with PdCl2.Ni-MOF (0.03 g) was added to an ethanol-water mixture (15 mL, VEtOH: VH2O= 2:1), followed by addition of 5 mL of PdCl2 solution (5 mg/mL). After 20 min, 0.5 mL dilute hydrochloric acid was added to the above mixture under stirring at 65°C and keep it for 6 h. The black precipitation (Pd/Ni-MOF) was collected by centrifugation and ultrasonic washing for several times.In order to highlight the advantages of the above two-step method for synthesis of Pd/Ni-MOF, one-step method was adopted to prepare MOFs containing Pd and Ni ions named as Pd-Ni-MOF. The procedure of one-step method is the same as synthesis of Ni-MOF only except replacing Ni(CH3COO)2•4H2O with the mixture of different Pd/Ni proportions. The obtained precursor is marked as Pd-Ni-MOF-x (x= 1, 2, 3, 4 and 5, which is corresponding to the molar ratios of Pd/Ni are 4:1, 2:1, 1:1, 0.5:1 and 0.25:1, respectively).Ni-MOF, Pd-MOF, Pd/Ni-MOF and Pd-Ni-MOF-x were sintered from room temperature to 450°C at a heating rate of 2°C/min. The samples were kept warm in Ar atmosphere to form Ni-NC, Pd-NC, Pd/Ni-NC and Pd-Ni-NC-x, respectively.Scanning electron microscopy (SEM Hitachi, S-4700), transmission electron microscopy (TEM, TecnaiG220, FEI) and high-resolution TEM (Tecani G2 F20 S-TWIN) were used to characterize the morphologies. X-ray diffraction (XRD, X'Pert-Pro MPD diffractometer, Cu Kα radiation of 1.540598 Å) and X-ray photoelectron spectroscopy (XPS, Escalab250Xi) measured the composition and structure. Inductively coupled plasma (ICP, Varian 710-ES) analysed the elements.A traditional 3-electrode system (CHI660E workstation, CH Instruments, China) was used to conduct the electrocatalytic properties of the materials at room temperature. Glassy carbon (GC) disk electrode (5 mm in diameter), Platinum electrode and Ag/AgCl (KCl saturated) electrode were used as working electrode, counter electrode and reference electrode respectively. The catalyst suspension was prepared by dispersing 5 mg sample and 5 mg carbon powder in 1 mL of solution containing 970 μL isopropanol and 30 μL 0.5 wt. % Nafion solution. Then, the combination was processed by ultrasonication for 0.5 h. At the last, 21 μL reagent were dropped on the working electrode in 7 times. After calculation, the area of the working electrode was 0.19635 cm−2 and the loading of catalyst on the working electrode was 0.53476 mg/cm−2. According to the potential conversion equation, all potential values were referenced to the reversible hydrogen electrode (RHE) [33]. All electrochemical-related tests were performed in 1 M KOH solution. Linear sweep voltammetry (LSV) was measured with a scan rate of 10 mV•s−1. The differences of current density (ΔJ) in different scan rates of cyclic voltammetry (CV) further determined the double layer capacitance (Cdl) values. Cdl is equal to the liner slopes of curves of ΔJ/2 vs. scan rate. Most electrochemical tests were modified with IR correction except for the chronopotentiometry. Electrochemical impedance spectra (EIS) experiments were performed in a frequency range from 10−2 to 105 Hz with 5 mV amplitude. Scheme 1 demonstrates the complete synthesis process of Pd/Ni-NC. Firstly, Nickel ions in solution react with DMG to form a rod-like material (Ni-MOF). Afterwards, under weak acidic conditions, Ni2+ will slowly dissociate with DMG, while Pd2+ in solution will quickly react with the dissociated DMG to generate particles on the surface of Ni-MOF to form the Pd-doped Ni-MOF (Pd/Ni-MOF) subsequently. Finally, Pd/Ni-NC is obtained by calcining Pd/Ni-MOF in argon atmosphere.In this work, the obtained Ni-MOF exhibits a cuboid shaped microrod (Fig. 1 a and 1d) with a common diameter of about 1 μm and the size of about 10 µm (Fig. S1a and S1b). In the process of synthesizing the Pd-doped Ni-MOF precursor, the acid will slowly dissociate part of the coordination between Ni2+ and DMG, in the meantime, Pd2+ will rapidly coordinate with the dissociated DMG. The coordination field theory gives the explanation that the energy level difference (Δ0) of d orbital of Pd2+ caused by coordination field is much larger than that of Ni2+, and the corresponding coordination bond strength formed by Pd2+ with DMG must be stronger than that by Ni2+ with DMG [34]. This inference has been confirmed by the fact that the stretching vibration constant of Pd-N (2.84 * 10−8 N/Å) is greater than that of Ni-N (1.88 * 10−8 N/Å) [35]. Resulting from Pd ions displacing Ni ions on the surface of Ni-MOF, Pd complexes form particles, and the resulting Pd/Ni-MOF are showed in Fig. 1b and 1e. Particles of uneven size are distributed on Ni-MOF and there is no great change for the microrod. After subsequent calcination, the obtained Pd/Ni-NC catalyst has a porous interior with particles on the surface and needle-like carbon tubes growing also on it, as shown in Fig. 1c and 1f. The tubes are extremely thin, just a few nanometers (Fig. 1g and 1h). This special structure with nanotubes on the surface is like the shape of setaria which is considered a serious weed of crops. The unique morphology of Pd/Ni-NC will provide large contact areas with electrolytes and abundant active sites for rapid generation and release of hydrogen [36,37]. Furthermore, the amount atomic ratio of C, N, O, Ni elements in Ni-MOF showed by energy dispersive X-ray spectroscopy (EDS) is 51.11: 18.35: 25.94: 4.6 (Fig. S2a) and that of C, N, O, Pd, Ni elements in Pd/Ni-MOF is 50.82: 17.20: 22.83: 4.37: 4.79 (Fig. S2b). After calcination by Ar, Pd/Ni-NC is composed of C, N, O, Pd, Ni elements with the atomic percentages of 59.25, 10.71, 7.68, 10.96, 11.41 (Fig. S2c). By comparing the element content, we find that carbonization greatly reduced the amount of O element and makes Pd/Ni-NC stable. N is also partially lost during carbonization, which is beneficial to produce more defects. The content of Pd and Ni in Pd/Ni-NC is approximately equal. The content of Pd and Ni content in Pd/Ni-NC was 37.30 wt.% and 20.56 wt.%, respectively, which is consistent with EDS results (Table. S1).In order to evaluate the superiority of Pd/Ni-NC, a series of contrast materials were synthesized. Firstly, Ni-MOF and Pd-MOF were annealed into Ni-NC and Pd-NC as single metal catalyst, respectively. The surface morphology and internal structure of Ni-NC and Pd-NC are showed in Fig. S3a - S3d. Both Ni-NC and Pd-NC have similar rod-like structures and rough porous surfaces. Then, the one-step method was used to mix Pd2+ and Ni2+ in different proportions and react with DMG simultaneously to synthesize the corresponding materials, which were labelled as Pd-Ni-MOF-x (x= 1, 2, 3, 4 and 5). These precursors were calcined to obtain five kinds of Pd-Ni-nitrogenous carbon materials labelled with Pd-Ni-NC-x (x= 1, 2, 3, 4 and 5), respectively. The metal contents of Pd-Ni-NC-x measured by ICP are listed in Table S2. Fig. S4a - S4j show SEM and TEM morphologies of the above materials and the amount ratios of Pd to Ni. The metal ratios of Pd to Ni in Pd-Ni-NC-x decrease with the decrease of the feed ratio, but do not equal to the feed ratio. There are no Pd particles and carbon tubes on the surface of Pd-Ni-NC-x, which indicates that carbon tubes are formed only in the case of calcining Pd/Ni-MOF.XRD analysis depicted in Fig. 2 a shows the characteristic peaks of metal elemental Pd and Ni in both Pd/Ni-NC and Ni-NC. Diffraction peaks appear at 40.23°, 46.77° and 68.36°, which correspond to (111), (200) and (220) planes of elemental Pd (JCPDS#87-0639), respectively. The x-ray diffraction signals at 44.35°, 51.74° and 76.05° correspond to the diffraction peaks of (111), (200) and (220) planes of elemental Ni (JCPDS#89-7128). For Pd-Ni-NC-x, their X-ray powder diffraction peaks are between the corresponding diffraction peaks of elemental Pd and elemental Ni, and approach the peak of Pd with the increase of the relative content of Pd to Ni (Fig. S5). The surface compositions and chemical states of Pd/Ni-NC was analyzed by XPS. It can be seen from the survey spectrum that C, N, Ni and Pd elements exist in Pd/Ni-C (Fig. 2b). In Fig. 2c, Ni 2p has two main peaks at 854.6 eV and 871.98 eV, as well as a satellite peak of each, matching with Ni 2p3/2 and Ni 2p1/2 of elemental Ni [38]. In addition, Ni 2p also have peaks at around 852.1 eV and 869.27 eV. These peaks correspond to the characteristic of Ni2+ 2p3/2 and Ni2+ 2p1/2, indicating the presence of oxidized Ni in the sample (Fig. 2c) [39,40]. The existence of such peaks is related to the incomplete reduction from Pd/Ni-MOF calcination and the oxidation of the metal Ni in the air. Fig. 2d shows the two characteristic peaks of the XPS spectra of N 1s at 398.07 eV and 399.05 eV, which are attributed to pyridinic-type N and pyrrolic-type N, respectively [41,42]. For C 1s, sp3C, C=N, C-N bonds can be used to explain peaks at 284.21 eV, 285.08 eV and 287.97 eV, respectively (Fig. 2e) [41,43]. These results indicate that N atoms partly remain in the carbon materials and perform an important function in promoting electrical conductivity and reactivity. Concerning the XPS spectrum of Pd 3d (Fig. 2f), the peaks of metallic Pd 3d5/2 and Pd 3d3/2 are positioned at 334.75 eV and 340.75 eV [44]. Meanwhile, there are still existing weak Pd metal oxidation peaks due to air exposure [45]. Similarly, after calcination, the elements contained in Pd-Ni-NC-x remain unchanged (Fig. S6a) and the metal elements are reduced to the zero valence stats. In the Ni 3p spectrum (Fig. S6b), the peaks located at 66.17 and 68 eV are assigned to Ni 3p3/2 and Ni 3p1/2. It is worth noting that the peaks of Ni 3p weaken with the decrease of Ni content. The XPS spectrum of Pd 3d can be assigned to two peaks shown in Fig. S6c. There is also a very small amount of oxidized Pd in Pd-Ni-NC-x. Fig S7 shows the element distribution of Pd/Ni-MOF (the precursor of Pd/Ni-NC), which proves that Pd replaces Ni and forms many Pd particles on the surface of Ni rods. From the element distribution, Fig. 3 a-3f show that an amount of Ni and Pd uniformly disperse in the materials and heterostructured Pd particles are scatted on the surface. Regarding the mechanism of carbon tube formation, the Ni compounds may be influenced by Pd particles to release the carbon of ligands at high temperature, and then elemental Ni play a role of catalyst for the growth of carbon tubes. Meanwhile, the elemental Pd will be loaded on the carbon tube during the growth process. This can be verified by the following tests. On the one hand, Fig. 3g shows the HRTEM image of the carbon tubes on the surface of Pd/Ni-NC. Good crystallinity is further demonstrated by the wide and clear rings in selected area electron diffraction (SAED) shown in Fig. 3h. After calculation, these diffraction fringes are caused by the (111), (200), (220) lattice planes of Pd, respectively. On the other hand, the lattice fringes of the carbon tubes have been photographed (Fig. 3i). The unique lattice spacing of about 0.22 nm on carbon tube is corresponding to the (111) plane of metallic Pd. Any other lattice spacing could not find. It proves the formation of carbon tubes and the loading of Pd on carbon tubes.The electrocatalytic hydrogen evolution properties of different materials were analyzed by liner sweep voltammetry (LSV). In general, the catalytic capacity of electrode materials is determined by comparing the magnitude of overpotential under a certain density. At the first, Fig. 4 a shows the polarization curves (I-V plots) of Ni-MOF, Ni-NC, Pd/Ni-MOF, Pd/Ni-NC and Pd-NC, respectively. The comparison of corresponding overpotential (η) vs. RHE at 10 mA cm−2 was exhibited in Fig. 4b. It is found that Pd/Ni-NC only needs a low overpotential of 16 mV to drive current density of 10 mA cm−2, which is the lowest η value among these materials. Ni-NC and Pd-NC as single metal catalysts need 84 mV and 61 mV, respectively. One important reason why the catalytic activity of Pd/Ni-NC is higher than that of Ni-NC or Pd-NC is that Ni atoms speed up the water dissociation by means of adsorbing OH− and promote the formation of H2 by the recombination of H+ at Pd sites [46–48]. At the same time, as shown in the Fig. 4e and Fig. 4f, the η value of Pd-Ni-NC-x are 84.5, 82.6, 104.5, 276.6 and 296.6 mV, respectively. The HER performance of Pd/Ni-NC is evidently still the best. These results explain the advantages of Pd/Ni-NC with setaria-shaped structure. Compared with previous literatures, such HER activity of Pd/Ni-NC catalyst is superior to that of most Pd/Ni-based catalysts (Table S2). To understand the whole catalytic process, the dynamic process is usually analyzed by calculating Tafel slope [48,49]. Fig. 4c shows the Tafel slope of Pd/Ni-NC is 130 mV dec−1, which is lower than that of Ni-MOF (144.9 mV dec−1), Ni-NC (155.35 mV dec−1), Pd/Ni-MOF (205.04 mV dec−1), Pd-NC (136 mV dec−1). This further proves that a synergistic impact of Pd and Ni exists in Pd/Ni-NC, which explains the better HER activity of Pd/Ni-NC than that of Ni-NC or Pd-NC. In an alkaline medium, HER occurs at the cathode as a multistep reaction, including the first step for Volmer reaction, the second step for Heyrovsky reaction or Tafel reaction [50,51]. According to the Tafel slope values, all materials follow a Volmer-Heyrovsky pathway, involving a relatively slow hydrogen adsorption (Volmer step) and a fast electrochemical desorption process (Heyrovsky step) [52–54]. This means the Volmer step is the rate-limiting step. In order to further evaluate its electrocatalytic performance, the effective sites of Pd/Ni-NC were evaluated by electrochemically active surface area (ECSA). And ECSA is linearly proportional to double layer capacitance (Cdl), so the active sites can also be evaluated by comparing the Cdl values calculated at different scanning speeds [55,56]. As shown in Fig. S8, CV curves at various scanning speeds (10-50 mV s−1) were tested in the region of 0.88-0.98 V (vs. RHE). In Fig. 4d, the Cdl value of Pd/Ni-NC is 16.02 mF cm−2, which is greater than that of Ni-NC (12.53 mF cm−2) but lower than that of Pd-NC (18.9 mF cm−2). This result directly proves that the introduction of metal Pd into Ni carbon material can increase ECSA and also directly indicates that Pd/Ni-NC has a larger surface roughness than that of Ni-NC and exposes more active sites, which is conducive to the adsorption of water and electron conduction. Notably, even if the Cdl of Pd/Ni-NC is not as great as that of pure Pd-NC, HER performance of Pd/Ni-NC is the best. This result indicates that Pd/Ni-NC can better utilize the synergistic effect of Pd and Ni and highlight the importance of the setaria-shaped structure.To further investigate the kinetic mechanism of hydrogen evolution, the electrochemical impedance spectroscopy (EIS) of above materials was tested in alkaline solution. The upper right corner of Fig. 5 a is the equivalent circuit diagram of the fitting curve, where R1 represents solution resistant (Rs) and R2 represents charge transfer resistance (Rct). After fitting, the R1 value of each material are close to each other, which indicates that the difference in activity caused by the electrode manufacturing process can be ignored. Fig. 5a shows that the Rct of Pd/Ni-NC (10.69 Ω) is smaller than that of Pd-NC (14.7 Ω), Ni-NC (29.93 Ω) and much less than Pd/Ni-MOF (244.6 Ω) and Ni-MOF (647.4 Ω). This means that the transfer rate of electrons in the electrode-electrolyte interface is the fastest, when Pd/Ni-NC is used as electrocatalyst. The reason could be related to the composition and the setaria-shaped structure of Pd/Ni-NC. Fig. S9 shows the EIS Nyquist plots of Pd-Ni-NC-x. The Rct values of Pd-Ni-NC-x (x=1, 2, 3, 4 and 5) are 1.538, 4.298, 3.396, 3.355 and 3.917 Ω, respectively. Although the impedance values of Pd-Ni-NC-x are smaller than that of Pd/Ni-NC, their electrochemical catalytic effects are still lower. This proves that the setaria-shaped structure of Pd/Ni-NC has the great promotion to the catalytic performance. Moreover, stability is also an important index for a high efficient catalyst, which was investigated by testing overpotential at constant current density. Fig. 5b shows the overpotential under the catalytic function of Pd/Ni-NC at 10 mA cm−2, and it could be stabled well even over 25 h. Then polarization curves before and after 25h constant current test is shown in Fig. S10, which further highlights the excellent stability and practical applications potential.According to the above characterization results and HER performance for the Pd/Ni-NC catalyst, the good HER activity and long-term stability can be ascribed to the cooperative effect from the following facts: (1) Ni atoms are widely regarded as excellent hydrolytic off-center, while Pd atoms have excellent adsorption properties to hydrogen. The corresponding overpotential of Pd/Ni-NC at 10 mA cm−2 is 68 mV and 45 mV less than that of Ni-NC and Pd-NC, respectively. The synergistic effect of Pd and Ni in the Pd/Ni-NC makes the materials exhibit excellent electrocatalytic activity, which is much better than their respective single metal catalysts. (2) The Pd/Ni-NC material derived from the corresponding MOF has a fluffy, porous and special structure with carbon tubes on the surface. This bimetallic carbon material offers abundant active sites. In addition, the carbon tubes on the surface of Pd/Ni-NC increase the contact area with electrolyte, which is more favorable for hydrogen release and mass transport. (3) As the reactant of MOF precursors, one molecule of DMG ligand has 2 nitrogen atoms. The presence of rich nitrogen causes more defects in the calcination process of Pd/Ni-MOF, exposing more active sites. Meanwhile, the nitrogenous carbon structure of Pd/Ni-NC can be conductive to optimize the interfacial electronic structure, so as to improve the overall electrocatalytic activities.In conclusion, we presented a setaria-shaped micron rod with carbon tubes and Pd particles grown on the surface as the electrocatalyst. With the addition of metal Pd, the nitrogenous Pd/Ni-NC catalyst has many more active sites and better synergies among its components. This catalyst exhibits an excellent activity for the HER with small overpotential of 16 mV, low resistive impedance and high stability. Although the Tafel slope of Pd/Ni-NC is 130 mV dec−1, it is lower than that of the corresponding single metal catalysts, Ni-NC or Pd-NC. Volmer step is the rate-limiting step for the catalyzed HER in alkaline medium. This work provides an idea for the synthesis of Pd/Ni bimetallic nitrogenous carbon material with a special structure. And the catalyst exhibits Pt-like catalytic properties, which maybe act as an alternative non-platinum electrocatalyst.There are no conflicts to declare.H.W.G. and X.Q.C acknowledges financial support from the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the project of Scientific and Technologic Infrastructure of Suzhou (SZS201708) and the Research Fund Program of Key Laboratory of Rare Mineral, MNR (No. KLRM-KF202004).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ceja.2021.100101. Image, application 1

Developing stable, efficient and economical electrocatalytic materials is still challenging for hydrogen evolution reaction (HER). Hence, we develop a Pd/Ni bimetallic carbon electrocatalyst (Pd/Ni-NC) with outstanding electrocatalytic performance. The catalyst derived from Pd-doped Ni-MOF (Pd/Ni-MOF) has particles and needle-like carbon tubes on its surface and is similar in shape to setaria. Benefiting from the composition and the unique structure, Pd/Ni-NC shows excellent HER catalytic performance with 16 mV at 10 mA cm−2, superior to Pd or Ni single metal-carbon catalyst. Furthermore, it maintains stable catalytic activity under constant current for 25h. These results show the strategy that obtaining Pd and Ni bimetallic MOF by cation exchange and its corresponding bimetallic carbon material with setaria-shaped structure by calcination is powerful for high efficient HER performance.

Excessive carbon dioxide (CO2) emissions caused by human activities, for example, the consumption of fossil fuels, deforestation, and forest degradation, have been considered the major culprit of climate change and ocean acidification [1–3]. In the past decade, shale gas has set off a global energy revolution. Especially in the United States, shale gas has become the most important source of natural gas. With CO2 as a soft oxidant, ethane (C2H6), the second most abundant component (approximately 10% of the content) of shale gas, can be converted into important raw materials [4]. Generally, the reaction between C2H6 and CO2 occurs via two distinct pathways: ① dry reforming of ethane (DRE) into syngas through cleavage of the C–C bond (C2H6 + 2CO2 → 4CO + 3H2); and ② oxidative dehydrogenation of ethane (ODHE) into ethylene (C2H4) by blocking cleavage of the C–C bond (C2H6 + CO2 → C2H4 + CO + H2O) [5–7]. Syngas, a mixture of hydrogen (H2) and carbon monoxide (CO), is an important feedstock for fuels and chemicals and is conventionally produced by steam reforming or partial oxidation of natural gas, liquefied gas, naphtha, and so on [8–10] or dry reforming of methane (DRM) [11–14]. However, the above processes are highly endothermic, with high energy consumption, and most DRM catalysts suffer deactivation due to coke formation and active site sintering at high operation temperatures above 1000 K [6,15–17]. A reaction temperature at least 100 K lower for DRE enables the production of syngas and the reduction of catalyst deactivation under milder conditions [5,6].Ni-based catalysts, especially supported Ni catalysts, are widely used in DRM because of their high catalytic activity [12,18–22]. However, Ni-based catalysts suffer from deactivation due to poor coke resistance and particle sintering. Thus, alloying Ni with transition metals (Co, Ru, Pd, Pt) [21,23–25], developing advanced supports [26–28], and using alkali cations as promoters [29–31] have been investigated to overcome the drawbacks of traditional Ni-based catalysts. Because of the broad application prospects in producing chemicals and fuels at operation temperatures below 900 K, DRE has drawn much attention, and a series of catalysts have been developed, including trimetallic perovskites [32,33], supported Pt-based bimetallic catalysts [5,6,17], and supported Ni composite catalysts [34,35]. The strong metal–support interaction (SMSI) effect has been proven to have a significant impact on the catalytic performance of Ni-based catalysts for DRM and DRE. Ceria (CeO2) supports with more oxygen vacancies show a stronger SMSI effect, which not only improves the dispersion of Ni species but also enhances the bonding between Ni species and the CeO2 supports [36,37]. Liu et al. [38] reported the SMSI effect between small Ni nanoparticles (NPs) and partially reduced CeO2. CO2 adsorbs and dissociates at oxygen vacancies to generate CO and active oxygen. The synergy between Ni and active oxygen reduces the activation barrier of CH4 bond dissociation and generates CH x (x = 2, 3) species on the surface of the Ni/CeO2 catalyst, making the dissociation temperature of CH4 as low as 700 K. Lustemberg et al. [39] further proved the important role of Ce3+ in the dissociation of C–H bonds. Smaller Ni particles on the CeO2 support experience larger electronic perturbations, resulting in a more significant binding energy and a lower activation barrier for the first C–H bond cleavage. Recently, Xie et al. [40] investigated the effects of oxide supports for DRE over both reducible and irreducible oxide-supported Pt–Ni bimetallic catalysts. The DRE performance over the PtNi/CeO2 catalyst was greatly improved because the reducible CeO2 effectively activated CO2 and promoted the dissociation of C2H6 through a bifunctional Mars–van Krevelen redox mechanism.Moreover, the dry reforming process is always accomplished by the reverse water–gas shift (RWGS) reaction with a low activation barrier [13,17,40,41]. It has been proven that the introduction of Al into CeO2 promotes the formation of oxygen vacancies, which inhibits the RWGS reaction because of the powerful activation and dissociation capacity of CO2 [13,42]. FeO x has also been reported to be a promoter of supported Ni catalysts for both the RWGS reaction [43] and DRM [44] due to the enhancement of Ni dispersion and the formation of Ni-rich NiFe alloys. Recently, Yan et al. [45] reported the adjustment of active interfacial sites by changing the composition of FeNi active components, which was shown to greatly influence the selectivity toward ODHE or DRE. Herein, we consider an FeNi/CeO2 catalyst with high ethylene selectivity as the initial system and adjust the distribution of surface active components by introducing Al into the CeO2 support to obtain high-performance catalysts for DRE.In this article, we synthesized a series of FeNi/Al–Ce–O catalysts with an enhanced SMSI effect via a facile sol–gel and impregnation method and investigated the catalytic properties for DRE over supported FeNi catalysts under steady-state reaction conditions. The enhanced SMSI effect between the surface active components and Al–Ce–O supports and its influences were confirmed via X-ray photoelectron spectroscopy (XPS), H2-temperature programmed reduction (TPR), and energy dispersive X-ray spectroscopy (EDS) element mapping. This work also proposes a possible reaction mechanism and the corresponding adsorbed intermediates over supported FeNi catalysts via in situ Fourier transform infrared (FTIR) spectra under the reaction conditions. We established a relationship between catalytic properties and surface structure over supported FeNi catalysts, which will greatly contribute to the research on related catalytic systems.Pluronic® F127 (EO106PO70EO106, M w = 12600) was purchased from Sigma-Aldrich Chemical Inc. (USA). Ce(NO3)3·6H2O (analytical reagent (AR), 99.0%), Al(NO3)3·9H2O (AR, 99.0%), Ni(NO3)2·6H2O (AR, 98.0%), quartz sand (25–50 mesh, AR, 95.0%), and absolute ethanol (AR, 99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Fe(NO3)3·9H2O (AR, 98.5%) was purchased from Nanjing Chemical Reagent Co., Ltd. (China). All the chemicals were used as received.Al–Ce–O supports were synthesized via a facile sol–gel process combined with evaporation-induced self-assembly (EISA) in ethanol using F127 as the template, which provides a large specific surface area and superior catalytic performance [13,46–49]. In a typical synthesis, 1.6 g of F127 was dissolved in 40 mL of ethanol at room temperature (RT). A total of 10.0 mmol of metal precursors (Ce(NO3)3·6H2O and Al(NO3)3·9H2O) with an Al molar ratio between 10% and 90% (10% increment of each sample) were added into the above solution with vigorous stirring. The mixture was covered with polyethylene (PE) film and stirred for at least 5 h at RT. The homogeneous sol was then transferred into an oven and underwent solvent evaporation. After aging at 313 and 333 K for 24 h successively, the gel product was dried in an oven at 373 K for another 24 h. Calcination was performed by slowly increasing the temperature from RT to 923 K (2 K·min−1 ramping rate) and then heating at 923 K for 4 h in air. The CeO2 and Al2O3 supports were synthesized via the same procedure except the mixed metal precursors were replaced with 10.0 mmol of Ce(NO3)3·6H2O (4.34 g) or Al(NO3)3·9H2O (3.75 g), respectively. The yellow or white products were then ground into a powder.Supported FeNi bimetallic catalysts were synthesized via an incipient wetness impregnation method over as-synthesized CeO2, Al2O3, and Al–Ce–O supports [6,40,45]. In a typical synthesis, a bimetallic co-impregnation procedure was used to maximize the interaction between the two metals. Precursor solutions were prepared by dissolving 101 mg of Fe(NO3)3·9H2O and 83 mg of Ni(NO3)2·6H2O in a specific amount of deionized water sufficient to fill the pores of 0.981 g of corresponding metal oxide support, the pore volume of which was determined by means of nitrogen adsorption measurements. The solution was added dropwise to the support with thorough stirring. The loadings of bimetallic active components were 1.15 wt% for Fe and 0.40 wt% for Ni to obtain a 3:1 molar ratio of Fe/Ni. The catalyst was then dried at 353 K for 12 h and calcined at 723 K for 4 h with a ramping rate of 2 K·min−1 from RT to 723 K. Since the FeNi/CeO2 catalyst with an Fe/Ni molar ratio of 3:1 shows high ethylene selectivity due to its special Ni–FeO x interfacial sites [45], such active metal components would better reflect the enhanced SMSI effect of Al–Ce–O supports on the catalytic properties.Powder X-ray diffraction (XRD) analysis was performed on a Bruker D8 Advance SS diffractometer (Bruker Corporation, USA) operated at 40 kV and 40 mA with a slit of 0.5° at a 2θ scanning speed of 2°·min−1 under a Cu-Kα source (0.15432 nm). Nitrogen adsorption–desorption isotherms were measured at 77 K using a Micromeritics ASAP 2010 analyzer (Micromeritics Instrument Corporation, USA). The Brunauer–Emmett–Teller (BET) specific surface area was measured after degassing the samples at 373 and 623 K for 3 h successively under vacuum. The elemental composition of the supported FeNi catalysts was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES; Varian Vista AX, Varian Inc., USA). 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was conducted with a Varian VNMRS-400WB nuclear magnetic resonance instrument (Varian Inc., USA) with a frequency of 104.18 MHz, a spinning speed of 10 000 Hz, and a relaxation delay of 4 s. Chemical shift values are reported with respect to KAl(SO4)2·12H2O as the standard. XPS was performed on a Thermo ESCALAB 250 spectrometer (Thermo Fisher Scientific Inc., USA) with a monochromatic Al-Kα X-ray source (1486.6 eV, 1 eV = 1.602176 × 10−19 J) and an analyzer pass energy of 20 eV. The C 1s line at 284.6 eV was used to calibrate the binding energies (BEs) of the measured elements.H2-TPR experiments were conducted on a Micromeritics AutoChem II 2920 chemisorption analyzer (Micromeritics Instrument Corporation, USA). In a typical experiment, 50 mg of the as-synthesized catalyst was put into a U-shaped quartz tube and pretreated in a 50 mL·min−1 He flow at RT. Then, the TPR experiment was performed under a 10 vol% H2/Ar mixture at a space velocity of 50 mL·min−1 with a ramping rate of 10 K·min−1 from RT to 1173 K. Pulse CO chemisorption experiments were also performed on a Micromeritics AutoChem II 2920 chemisorption analyzer to measure the CO uptake value of the catalyst. Approximately 150 mg of the catalyst was first reduced at 873 K in a 10 vol% H2/Ar flow for 30 min. Then, the reduced catalyst was purged in a He flow until the temperature was decreased to 313 K. The loop gas of 10% CO/He (590 μL) was pulsed with a He stream over the catalyst until the peak area of CO became constant. The CO uptake values of the catalysts could provide an approach to estimate the turnover frequency (TOF), as reported previously [17,33,45].Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and EDS element mapping were conducted on an FEI Talos F200X TEM (Thermo Fisher Scientific Inc., USA) with a probe aberration corrector operating at 300 kV. The TEM samples were prepared by drying a drop of the sample dispersion in ethanol on carbon-coated copper grids. In situ FTIR spectra were obtained on a Bruker Vertex 70 V FTIR spectrometer (Bruker Corporation, USA) with a stainless steel high-vacuum transmission infrared cell. The samples were pressed on a tungsten mesh support and heated to 623–673 K at a ramping rate of 10 K·min−1 under vacuum for 2 h to remove the surface adsorbed water. The background spectra were collected after the tungsten mesh support was cooled to RT. In a typical in situ infrared (IR) experiment under the reaction conditions, 1.0 mbar (1 mbar = 100 Pa) of C2H6 and 1.0 mbar of CO2 were introduced into the cell, and IR spectra of each sample were collected in the temperature range of 373–873 K. In situ CO adsorption IR spectra of the FeNi/Ce–Al0.5 catalyst were collected under a CO pressure of 5.0 mbar from 373 to 573 K and 1.0 mbar from 673 to 873 K.The catalytic performance of supported FeNi bimetallic catalysts was evaluated in a continuous-flow fixed-bed quartz tabular reactor (7.5 mm inner diameter) under atmospheric pressure, utilizing a mixture of 100 mg of catalyst (20–40 mesh) and 100 mg of quartz sand loading between two quartz wool plugs at the center of the reactor. The catalyst was pretreated in situ with 40 mL·min−1 H2 at 673 K for 1 h and then heated to 873 K at a ramping rate of 5 K·min−1 with a constant total flow rate of C2H6, CO2, and N2 of 40 mL·min−1. The volume ratio of the C2H6, CO2, and N2 mixed gas was 1:1:2. The temperature of the catalyst bed was held at 873 K for 8 h, and the outlet stream was analyzed online using a gas chromatograph (Agilent 6820B, Agilent Technologies, Inc., USA) with a thermal conductivity detector (TCD). Water within the outlet stream was removed by a condenser. N2 was used as an internal standard to account for the volume effects due to the high temperature during the reaction. Blank experiments without supported FeNi catalysts were conducted at 873 K to evaluate the contribution of the gas-phase reaction and system, and the results showed a negligible effect on DRE performance. In this article, the steady-state conversion (X), TOF, C2H6-based selectivity (S), and yield (Y) of species i were defined using the following equations: (1) X i = F i , i n - F i , o u t F i , i n × 100 % (2) T O F i = F i , i n ∙ X i U C O ∙ m c a t (3) S i = F i , o u t F C 2 H 6 , i n - F C 2 H 6 , o u t ∙ N i , C N C 2 H 6 , C × 100 % ( i ≠ C O ) (4) S C O = 1 - ∑ i ≠ C O S i × 100 % (5) Y i = X C 2 H 6 ∙ S i where F in and F out are the inlet and outlet flow rates of the reactant (mol·min−1), U CO is the CO uptake value (mol·g−1), m cat is the mass of the catalyst (g), and N C is the carbon atom number of the products.Powder XRD patterns of the as-synthesized catalysts with standard Joint Committee on Powder Diffraction Standards (JCPDS) cards are shown in Fig. 1 . The diffraction peaks of FeNi/Ce–Al x (x ≤ 70%) and FeNi/CeO2 confirm the fluorite cubic structure of CeO2 (Fm 3 ¯ m, JCPDS 75-0120), while the pattern of FeNi/Al2O3 is consistent with the structure of η-Al2O3 (Fd 3 ¯ mS, JCPDS 77-0396). FeNi/Ce–Al0.9 shows significant phase separation into CeO2, η-Al2O3, and Al(OH)3 (P1(1), JCPDS 24–0006). No peaks attributed to Fe and Ni are observed in these patterns. As shown in Table 1 , the average crystallite size of the catalysts calculated by the Scherrer equation [50] significantly decreases with increasing Al content, which indicates that Al improves the sintering resistance of the CeO2 supports [47]. The size distributions of supported FeNi catalysts were further estimated by the size statistics of NPs from TEM images. The TEM and HRTEM images of supported FeNi catalysts are shown in Figs. S1 and S2 in Appendix A. The size distributions of the FeNi/CeO2 and FeNi/Ce–Al x (10% ≤ x ≤ 50%) catalysts in Table 1 are close to the corresponding average crystallite sizes. As shown in Table 1 and Fig. S3 in Appendix A, the BET specific surface area of supported FeNi catalysts is positively related to the Al content. The average crystallite size of the FeNi/Ce–Al x (70% ≤ x ≤ 90%) catalysts becomes even smaller upon formation of a mesoporous structure, as shown in Fig. S2, which reveals the relationship between the size distribution and BET specific surface area of supported FeNi catalysts. In Figs. S1 and S2, lattice spacings of 3.12, 2.71, 1.91, and 2.38 Å correspond to the (111), (200), and (220) facets of CeO2 and (311) facet of η-Al2O3, respectively. The lattice spacing of FeNi/Ce–Al0.9 is indistinguishable owing to the poor crystallinity and severe phase separation.To further discuss the microstructure of the catalysts, the microstrains and lattice parameters were calculated, and the results are listed in Table 1. The microstrain in the lattice (lattice strain) of the samples was estimated via the single line method for analysis of XRD line broadening using a pseudo-Voigt profile function [51,52]. As shown in Fig. 1, for the FeNi/CeO2 and FeNi/Ce–Al x (10% ≤ x ≤ 70%) catalysts with fluorite cubic structures, the microstrain in the crystal lattice of the oxide supports increases as a function of Al content to maintain the original crystal structure. The lattice distortion is relieved by a decrease in the microstrain and phase transition after the introduction of a high content of Al. The lattice parameters determined via Bragg’s law from the (111) diffraction peak of CeO2 (Fm 3 ¯ m, JCPDS 75-0120) for FeNi/CeO2 and FeNi/Ce–Al x (10% ≤ x ≤ 70%) as well as the (440) peak of η-Al2O3 (Fd 3 ¯ mS, JCPDS 77-0396) for FeNi/Ce-Al0.9 and FeNi/Al2O3 are also listed in Table 1. Fig. S4 in Appendix A shows the 27Al MAS NMR spectra of FeNi/Ce–Al x (10% ≤ x ≤ 50%) and of FeNi/Al2O3. The peaks at approximately 8 and 66 ppm are assigned to octahedrally (Aloct) and tetrahedrally (Altet) coordinated Al3+, while the Al species with a chemical shift of 38 ppm is a CeO2 lattice occupied by Al3+ [53]. The increased peak intensity at 38 ppm for FeNi/Ce–Al0.1 and FeNi/Ce–Al0.3 is a result of Al3+ ions present in the CeO2 lattice. The similarity of the spectra of FeNi/Ce–Al0.5 and FeNi/Al2O3 indicates a stable octahedral coordination of Al3+ species with a high content of Al in FeNi/Ce–Al x (50% ≤ x ≤ 90%). Regardless of the phase transition, the change in the lattice parameter of the supports is mainly related to two factors: ① the formation of oxygen vacancies by replacing Ce4+ with Al3+, which leads to crystal lattice shrinkage, and ② the transition from Ce4+ to Ce3+ with a corresponding reduction in the ionic radius, which is essential to balance the electric charge of the unit cell. The change in the lattice parameter is thought to be a result of the synergistic effect of the two factors: the formation of surface oxygen vacancies and Ce3+ species.The steady-state catalytic performance of the FeNi/Al–Ce–O catalysts at 873 K is shown in Figs. 2 and 3 , where the FeNi/CeO2 and FeNi/Al2O3 catalyst data were also plotted as a reference. The experimental data indicate that the composition of oxide supports plays an important role in the catalytic properties for DRE over supported FeNi catalysts. As shown in Figs. 2(a) and (b), C2H6 and CO2 conversion is positively correlated with the Al content (0 ≤ x ≤ 50%), whereas completely opposite trends are observed in Figs. 3(a) and (b) when the Al content is above 50%. In Fig. S5 in Appendix A, the CO selectivity from C2H6 also significantly increases with increasing Al content (0 ≤ x ≤ 30%), while the ethylene selectivity decreases correspondingly. As the Al content changes between 30% and 90%, the CO selectivity of the FeNi/Al–Ce–O catalysts remains stable at 96%–98%. In Figs. 2(c) and 3(c), the CO yield from C2H6 over the FeNi/Al–Ce–O catalysts follows the same trend of first increasing and then decreasing with increasing Al content. The FeNi/Ce–Al0.5 catalyst provides the best DRE performance, with the highest C2H6 and CO2 conversions and CO selectivity and yield. The introduction of Al into CeO2 possibly enhances the interaction between the surface active components and the Al–Ce–O support, which further affects the catalytic properties over supported FeNi catalysts.The average catalytic performance data of the supported FeNi catalysts between 420 and 480 min are summarized in Table 2 . After several hours of steady-state reaction, the C2H6 and CO2 conversion, CO selectivity, and CO yield over the supported FeNi catalysts maintain the same relative order. The FeNi/Ce–Al0.5 catalyst exhibits the best DRE performance with the highest C2H6 conversion (11.7%), CO2 conversion (33.1%), and CO yield (11.5%). The TOF values based on CO uptake also indicate the outstanding catalytic activity of the FeNi/Ce–Al0.5 catalyst for both C2H6 (47.1 min−1) and CO2 (133.1 min−1). As a comparison, the catalytic performance data of recently reported DRE catalysts are listed in Table S1 in Appendix A. The FeNi/Ce–Al0.5 catalyst shows high TOFs and CO selectivity similar to other high-performance DRE catalysts, whereas the conversions are possibly restricted by the low loading of bimetallic active components. Catalysts with Al contents above 50% show lower TOF values for both C2H6 and CO2 in this reaction, which demonstrates that the enhancement of DRE performance over the FeNi/Ce–Al0.5 catalyst should be attributed not only to the increasing Al content but also to the interaction between surface active components and the Al–Ce–O support. H2/CO molar ratios lower than 0.75 due to the side reaction of the RWGS are also shown in Table 2 [33]. The highest H2/CO ratio of the FeNi/Ce–Al0.5 catalyst indicates that the SMSI effect enhanced by the introduction of Al partially inhibits the RWGS process in this reaction, which is also related to the surface oxygen vacancy over the FeNi/Al–Ce–O catalysts.The elementary composition and chemical valence on the surface of hydrogen-reduced catalysts were detected via XPS. Typical Ce 3d and Al 2p core level spectra of the FeNi/CeO2, FeNi/Al–Ce–O, and FeNi/Al2O3 catalysts are shown in Appendix A Fig. S6. During typical data processing, the complex spectra of the samples in the Ce 3d region were deconvoluted into ten components via the generally accepted approach of extracting the ratio of Ce3+ and Ce4+ [54,55]. The ten peaks contained five spin-orbital split pairs of Ce 3d5/2 (vi : v 0, v, v′, v″, and v‴) and Ce 3d3/2 (ui : u 0, u, u′, u″, and u‴), of which the area intensities, the full widths at half-maximum (FWHM), and the position distances were fixed as constants during the deconvolution. Herein, the peak positions are marked in Fig. S6(a); the relative contents of Ce3+ to the total Ce content (c Ce)in the samples were calculated via the following equation, and the results are listed in Table 3 : (6) c C e 3 + c C e = I v 0 + I v ′ + I u 0 + I u ′ ∑ i ( I v i + I u i ) × 100 % where c Ce3+ is the content of Ce3+, and I is the area intensity of the given component. As shown in Table 3, the surface relative content of Ce3+ increases significantly with Al content. Shyu et al. [56] reported that the area under the u‴ peak in the total Ce 3d region could be used to describe the relative content of Ce4+ in the samples. In Table 3, it is observed that the area under the u‴ peak shows a negative relationship with the Al content, which also confirms the correlation above. Herein, the theoretical effective surface area (TESA, S eff) of supported FeNi catalysts is defined by the following equation: (7) S e f f = S B E T ∙ P r , C e 3 + ∙ P C e where S BET is the BET specific surface area of the catalyst, P r , C e 3 + is the relative surface content of Ce3+, and P Ce is the total surface content of Ce of the catalyst. According to the data in Table 3, as the Al content increases, the TESA shows the same trend as the conversions and TOFs of C2H6 and CO2, indicating that the reactivity of C2H6 with CO2 is closely related to the content of surface Ce3+ species over supported FeNi catalysts. Moreover, the dispersion of surface active components should be another important factor to be discussed later. In Fig. S6, the binding energy of the Ce 3d core level of the FeNi/Al–Ce–O catalysts decreases slightly with increasing Al content compared with that of FeNi/CeO2. In addition, the peaks of the Al 2p core level of the FeNi/Al–Ce–O catalysts shift to lower binding energies than that of FeNi/Al2O3 with increasing Ce content, which demonstrates electron transfer between Ce or Al and adjacent atoms. Fig. S7 in Appendix A shows the O 1s and Fe 2p XPS spectra of the reduced FeNi catalysts. In Fig. S7(a), the binding energies at approximately 529.2 and 531.7 eV are ascribed to the lattice oxygen of Ce-based oxides (OI) and the adsorbed oxygen or hydroxyl groups (OII) on the surface, respectively [41,57,58]. The O 1s core level binding energy of Al2O3 is located at 530.9 eV [57]. The OII/OI ratios in Table 3 increase gradually with increasing Al content, which indicates an increase in surface oxygen vacancies over the FeNi/Al–Ce–O catalysts [13,41]. In Fig. S7(b), the Fe 2p core level binding energies of the reduced FeNi catalysts at approximately 710.9 and 724.0 eV are ascribed to Fe2O3, which means that the surface Fe species should be highly oxidized during the reaction [57]. Raman spectroscopy was also conducted to determine the surface oxygen vacancy of the catalysts. Fig. S8 in Appendix A shows the Raman spectra of the supported FeNi catalysts excited by a 532 nm laser. The strong band at approximately 462 cm−1 is ascribed to the F2g vibration mode of the Ce–8O vibrational unit of the fluorite structure, while the weak bands at approximately 254 and 596 cm−1 are attributed to the second-order transverse acoustic (2TA) mode and the defect-induced (D) mode of oxygen vacancies, respectively. The relative intensity ratio I D/I F2g reflects the content of oxygen vacancies [59,60]. As seen in Fig. S8, the intensity ratio of I D/I F2g increases slightly with Al content, which also indicates that the introduction of Al improves the content of surface oxygen vacancies and the inhibition of the RWGS reaction over the FeNi/Al–Ce–O catalysts. According to the results above, the introduction of Al into the CeO2 support leads to a higher density of surface Ce3+ species and oxygen vacancies, which further improves the catalytic performance for DRE over FeNi/Al–Ce–O catalysts.The H2-TPR profiles of the as-synthesized FeNi catalysts are shown in Fig. 4 . To assign the peaks in the pattern, profiles of the as-synthesized Ce–Al0.5 supported monometallic catalysts and pure CeO2 support are also shown in Fig. S9 in Appendix A as a comparison. As seen in Fig. S9, CeO2 reduction can be divided into two stages. The first short and wide peak located between 600 and 800 K is assigned to the reduction of surface active oxygen of CeO2, which leads to the formation of surface oxygen vacancies and nonstoichiometric CeO x , and the second peak above 800 K is attributed to bulk CeO2 reduction [61–63]. In Fig. 4, after adding surface active components to the support, a strong peak at approximately 550 K appears for the FeNi/CeO2 catalyst. This peak can be attributed to the reduction of both surface active components and the CeO2 support by surface hydrogen spillover [64]. Compared with the patterns of monometallic catalysts in Fig. S9, for the Ni/Ce–Al0.5 catalyst, the peak below 673 K shows a lower reduction cutoff temperature than that of Fe/Ce–Al0.5, which indicates that surface Ni species are much easier to reduce than Fe on Ce-based composite oxides. For the FeNi/Al–Ce–O catalysts, compared with the pattern for FeNi/CeO2 in Fig. 4, the peak broadening and cutoff temperature rise also indicate that the introduction of Al significantly enhances the SMSI effect between surface active components and oxide supports.To identify the dispersion of surface active components on the composite support, EDS elemental mapping measurements were conducted on three representative samples: FeNi/Ce–Al0.1, FeNi/Ce–Al0.5, and FeNi/Ce–Al0.9. The element mapping images in this article are representatively chosen from many different regions of the samples. As shown in the EDS mapping images of Ce and Al in Figs. 5–7 , Ce and Al are well distributed over the FeNi/Al–Ce–O catalysts. Nevertheless, the elemental distributions of Fe and Ni are quite different. Small bimetallic FeNi NPs are observed on the surface of FeNi/Ce–Al0.1, as confirmed by the EDS mapping images shown in Fig. 5. Bimetallic FeNi NPs with similar structures have been proven to have high selectivity for ethylene [45]. As demonstrated by the gradual FeNi distribution changes shown in Figs. 6 and 7, as the Al content increases, the surface Fe and Ni species become well dispersed on the Al–Ce–O supports. The introduction of Al greatly increases the interaction between the surface active components and the composite support. The surface Fe and Ni species are dispersed randomly and independently throughout the support because of the enhanced SMSI effect, leading to peak broadening and an increase in the reaction cutoff temperature of H2-TPR over the supported FeNi catalysts, as shown in Fig. 4.To further investigate the surface active species during the DRE reaction, in situ IR spectroscopy studies were carried out at temperatures ranging from 373 to 873 K and a total pressure of 2.0 mbar (C2H6:CO2 = 1:1). In situ IR spectra of the FeNi/Ce–Al0.1, FeNi/Ce–Al0.5, and FeNi/Ce–Al0.9 catalysts are shown in Figs. S10–S12 in Appendix A. All the spectra were normalized by subtracting the corresponding IR spectrum under vacuum at RT. A typical IR spectrum is divided into three different characteristic vibrational regions that will be discussed individually.The in situ IR spectra in the region of 3900–3500 cm−1 in Fig. 8 provide information on surface hydroxyl and carbonate species. The wide band at 3770–3790 cm−1 and the strong band at approximately 3706 cm−1 are ascribed to the monocoordinated OH groups (Type I OH) of Al and Ce, respectively [65–68]. The band at approximately 3732 cm−1 is mainly ascribed to the Type II-A OH species of Al (hydroxyl groups bibridged across Al–Al ion pairs) with a possible contribution of terminal OH groups bound to surface Ce4+ cations [65]. The band located at approximately 3625 cm−1 is thought to be a combination of two bands: ① the Type II-B OH species of Ce with adjacent oxygen vacancies (O–Ce–OH–Ce–□) at 3630 cm−1; and ② the surface bicarbonate (HCO3 −) species at 3619 cm−1, as confirmed by the delay of the band at approximately 3706 cm−1 [67,68]. The presence of Type II-B OH species and the absence of Type II-A OH species of Ce indicate that the surface of the FeNi/Al–Ce–O catalysts is highly active with oxygen vacancies under the reaction atmosphere. The band at approximately 3598 cm−1 can also be separated into two bands: the tribridged OH species (Type III OH) of Ce at approximately 3600 cm−1 and surface protonated carboxylate species (–COOH) at approximately 3593 cm−1 [67–70]. The formation of surface carbonate species will be discussed in detail below. As seen in Figs. 2(d) and 3(d), as the reaction temperature increases, the decreased intensity of the OH band over the FeNi/Ce–Al0.5 catalyst implies a reduction in H2O production, which further leads to lower RWGS activity and a higher H2/CO ratio than those of other supported FeNi catalysts.The bands in the region of 3150–2750 cm−1 correspond to the CH stretching bands of adsorbed species. The strong wide bands at approximately 3005 and 2931 cm−1 in Fig. 9 can be ascribed to the antisymmetrical (νas) and symmetric (νs) CH stretching vibrations of a series of methyl species in the gas phase [71]. The CH vibration bands in this region indicate the presence of adsorbed ethyl ( ν as,C H 3 at 2970 cm−1, ν as,C H 2 at 2931 cm−1, and ν s,C H 3 at 2880 cm−1) [72,73] and ethanol ( ν as,C H 3 at 2977 cm−1, ν as,C H 2 at 2933 cm−1, and ν s,C H 3 at 2878 cm−1) [73–75]. The sharp band at 2953 cm−1 is ascribed to the CH vibration band of bridged formate species, while the other sharp band at 2895 cm−1 is attributed to the CH stretching band of bidentate formate [67,76,77]. Little difference in the spectra of the three catalysts is observed in the temperature range from 373 to 873 K in the region of 3150–2750 cm−1, which means that the FeNi/Al–Ce–O catalysts have the same kind of surface CH-containing species during the reaction, independent of the Al content and reaction temperature. Fig. S13 in Appendix A shows the in situ IR spectra in the region of 1800–1000 cm−1, where the peaks are mainly ascribed to the carbonate-like (OCO) species adsorbed on the samples [67,68,77–79]. The complex band assignments of different carbonate, carboxylate, and formate species adsorbed on the supported FeNi catalysts are summarized in Table S2 in Appendix A. The peaks attributed to the corresponding carbonate-like species are marked in the original spectra of FeNi/Al–Ce–O in Fig. 10 . The band distribution in the IR spectra over the FeNi/Ce–Al0.1 catalyst in Fig. 10(a) is quite similar to those of the other two samples in Figs. 10(b) and (c), except for the bands at 1430–1425, 1236–1217, and 1057–1050 cm−1. The CO adsorption IR spectra of the FeNi/Ce–Al0.5 catalysts are shown in Fig. 10(d) for comparison. The missing bands at 1430–1425 cm−1 in both Figs. 10(a) and (d) are attributed to the intermediate adsorbed species of C2H6. Since it has been reported that the bands at 1580 cm−1 (νas,OCO), 1429 cm−1 (νs,OCO), 1306 cm−1 ( δ C H 3 ), and 1026 cm−1 ( ρ C H 3 ) are the characteristic peaks of acetate species, an oxidation product of C2H6 on CeO2 at 355 K [74], the increased IR spectral intensity for the FeNi/Ce–Al0.5 and FeNi/Ce–Al0.9 catalysts at 1430–1425 cm−1 can be attributed to the formation of surface acetate species. The bands at 1660–1640 cm−1 and approximately 1230 cm−1 arising from the olefinic C=C and CH stretching vibrations imply the formation and adsorption of ethylene on the FeNi/Ce–Al0.1 catalyst, rather than surface acetate species [80]. Thus, it can be inferred that the different product selectivity of the FeNi/Al–Ce–O catalysts results from the changes in the surface adsorbed species. Moreover, the bands at 1057–1050 cm−1 in Fig. 10(a) below 673 K are attributed to the CO stretching vibration of bidentate ethoxide and methoxide species [74,75]. Since the formation of the *C2H y O intermediate has proven to be essential for C–C bond cleavage and syngas production [7], the reduced band intensity at high reaction temperatures indicates that the weak adsorption of surface bidentate ethoxide species on the FeNi/Ce–Al0.1 catalyst is beneficial to the formation of ethylene.On the basis of the discussion above, a typical catalytic cycle of CO2 over FeNi/Al–Ce–O catalysts involves the following process, as shown in Fig. 11 . CO2 in the gas phase first adsorbs onto the surface hydroxyl species or oxygen vacancies and generates adsorbed carbonate-like species, such as bicarbonate and carboxylate. In addition, C2H6 adsorbs on metallic or oxidized FeNi active sites and dissociates into ethyl or ethoxy groups and a hydrogen atom. As a result of surface hydrogen spillover, adsorbed bicarbonate or carboxylate species are reduced to carboxylic acid or formate species, which further decompose into surface hydroxyl and carbonyl species through a possible formyl transition intermediate [77]. Surface hydroxyl species or oxygen vacancies regenerate after the release of CO to the gas phase. Nevertheless, the oxidation of C2H6 involves two different paths determined by the dispersion of FeNi active components. The impregnation of Fe and Ni precursors on pure CeO2 tends to generate bimetallic FeNi NPs, which prevents the excessive oxidation of adsorbed ethyl or ethoxy species and improves the selectivity of ethylene [45]. The introduction of Al into the lattice of CeO2 greatly improves not only the content of surface Ce3+ species and oxygen vacancies but also the dispersion of surface active components through the enhanced SMSI effect. The strong interaction between FeNi and the Al–Ce–O support stabilizes the adsorbed ethoxy moiety and its further oxidation products, which are essential for C–C bond cleavage and syngas generation.FeNi/Al–Ce–O catalysts synthesized via a facile sol–gel and impregnation method exhibit a composition-induced SMSI effect for DRE. The Al content in the Al–Ce–O supports significantly influences the metal–support interface structure of the catalysts and further determines the catalytic properties during the reaction. As the Al content increases, the C2H6 and CO2 conversion, CO selectivity and yield, and TOF first increase and then decrease according to the same trend as the TESA. The FeNi/Ce–Al0.5 catalyst exhibits the best DRE performance with the highest C2H6 conversion (11.7%), CO2 conversion (33.1%), and CO yield (11.5%). The increased surface oxygen vacancy partially inhibits the RWGS reaction over FeNi/Ce–Al0.5 catalysts, which leads to a higher H2/CO ratio than that of other FeNi/Al–Ce–O catalysts. The selectivity over the supported FeNi catalysts is determined by the dispersion of the surface active components. As the Al content in the Al–Ce–O supports increases, the dispersion of surface active components is promoted by the enhanced SMSI effect over the supported FeNi catalysts. The enhanced SMSI effect stabilizes the adsorbed *C2H y O intermediate and produces excessive oxidation products, leading to C–C bond cleavage and syngas generation. In summary, the introduction of Al into the CeO2 support not only increases the content of surface Ce3+ and oxygen vacancies but also promotes the dispersion of surface active components, which further adjusts the catalytic properties for DRE over supported FeNi catalysts.The authors gratefully acknowledge the support from the National Key Research and Development Program of China (2017YFB0702800), the China Petrochemical Corporation (Sinopec Group), and the National Natural Science Foundation of China (91434102 and U1663221).Tao Zhang, Zhi-Cheng Liu, Ying-Chun Ye, Yu Wang, He-Qin Yang, Huan-Xin Gao, and Wei-Min Yang declare that they have no conflicts of interest or financial conflicts to disclose.Supplementary data to this article can be found online at https://doi.org/10.1016/j.eng.2021.11.027.The following are the Supplementary data to this article: Supplementary data 1

Dry reforming of ethane (DRE) has received significant attention because of its potential to produce chemical raw materials and reduce carbon emissions. Herein, a composition-induced strong metal–support interaction (SMSI) effect over FeNi/Al–Ce–O catalysts is revealed via X-ray photoelectron spectroscopy (XPS), H2-temperature programmed reduction (TPR), and energy dispersive X-ray spectroscopy (EDS) elemental mapping. The introduction of Al into Al–Ce–O supports significantly influences the dispersion of surface active components and improves the catalytic performance for DRE over supported FeNi catalysts due to enhancement of the SMSI effect. The catalytic properties, for example, C2H6 and CO2 conversion, CO selectivity and yield, and turnover frequencies (TOFs), of supported FeNi catalysts first increase and then decrease with increasing Al content, following the same trend as the theoretical effective surface area (TESA) of the corresponding catalysts. The FeNi/Ce–Al0.5 catalyst, with 50% Al content, exhibits the best DRE performance under steady-state conditions at 873 K. As observed by with in situ Fourier transform infrared spectroscopy (FTIR) analysis, the introduction of Al not only increases the content of surface Ce3+ and oxygen vacancies but also promotes the dispersion of surface active components, which further alters the catalytic properties for DRE over supported FeNi catalysts.

No data was used for the research described in the article.With the rapid development of the world economy and industry, CO2 produced by human activities has had a significant impact on the ecological environment of the earth, such as the sea level rise, glacier melting, and earth temperature rise, so it is urgent to reduce CO2 emissions [1,2]. 70% of global CO2 emissions comes from the burning of fossil fuels in which converts them into energy needed for human life [3–5]. Under the current conditions of aggressively developing renewable energy, it is not possible to limit the use of fossil fuels in order to reduce CO2 emissions in a short period of time. Therefore, the most beneficial method to reduce CO2 emissions is CO2 capture and utilization (CCU) technology, which attracts more and more attention from all walks of life [6]. CCU not only has great environmental and economic benefits, but also has a great impact on the future energy structure of the world [7]. Pre-combustion capture, post-combustion capture, and oxygen-rich combustion are the three basic types of carbon capture [3,8]. The method of removing carbon from fuel before it is consumed is known as pre-combustion capture [9]. Post-combustion capture is the capture of fuel after combustion, such as coal-fired power plants in the process of flue gas emissions set up adsorption devices to capture CO2 [10]. Oxygen-rich combustion refers to combustion in a medium with a higher oxygen content than the air, so that the fuel is fully burned so as to generate a high concentration of CO2 for compression and storage [11,12]. CO2 is a low-cost, non-toxic, plentiful carbon-one material that can be utilized in food packaging, carbonated drinks, as a refrigerant to make dry ice, and injected into geological formations to enhance oil recovery ( Fig. 1) and more chemical conversion utilization routes [13]. There are two methods for CO2 chemical conversion: reduction and non-reduction [14,15]. Under certain catalysts and other circumstances, reduction conversion is the conversion of CO2 to carbon monoxide, methane, methanol, formic acid, and so on. A non-reduction reaction is the reaction of CO2 and other molecules under certain conditions to form esters, urea, carboxylic acids, and so on. From the perspective of inorganic chemistry, the standard heat of formation of CO2 is 394 kJ/mol and the standard Gibbs free energy is 395 kJ/mol. Because of its great thermodynamic stability, the conversion of CO2 demands a lot of energy, whereas the non-reduction process requires less. So, the non-reduction reaction conversion is easier than the reduction reaction conversion.In the non-reduction reaction conversion, as early as 1969, Inoue et al. discovered the copolymerization of CO2 and epoxide, which made the non-reducing transformation of CO2 develop rapidly [16]. CO2 and epoxides react to generate cyclic carbonate, this route is atom economy reaction process by 100%. Compared with the traditional toxic phosgene to participate in the preparation of cyclic carbonate, CO2 and epoxide bonus to prepare cyclic carbonates is environmentally friendly, which is in line with the concept of modern green chemistry, therefore become one of the high-profile CO2 conversion path [17]. Cyclic carbonates is widely used in medicine and the chemical industry as it is shown in Fig. 2. It is not only a good organic solvent, but also in the preparation of other chemical materials and intermediates, such as CO2 and ethylene oxide cycloaddition after ethylene glycol synthesis, ethylene carbonate reproduction; additionally, cyclic carbonates have a wide range of applications in the production of lithium batteries, etc [18–20]. Similarly, CO2 and epoxides can be copolymerized to produce polycarbonate, which also has a wide range of uses. Charlotte Williams et al. have reported a number of studies on this route in recent years [21,22].Cyclic Carbonates are synthesized when CO2 combines with epoxides. Due to its high kinetic stability, catalysts and solvents are frequently utilized to speed up the chemical conversion of CO2, as indicated in Fig. 3(a). Epoxides are ethers containing oxygen ternary rings, but they are more reactive than other ethers, especially with nucleophiles. The epoxides selected as substrates are generally propylene oxide (PO), styrene oxide (SO), epichlorohydrin (ECH), etc. The reaction generates ring carbonic acid esters such as propylene carbonate (PC), chloropropyl carbonate (CPC), and so on [23].As illustrated in Fig. 3(b), the overall mechanism of the cycloaddition reaction between CO2 and epoxide may be separated into three parts. The ring-opening reaction of the epoxide is the first step [24]. First, lewis acids activate epoxides. Generally, transition metals (such as Cu2+ and Zn2+) or hydrogen bond donor groups (such as OH-, COOH-, and NH2 -) are regarded as the active components of lewis acids [25]. They are bonded with oxygen atoms of epoxides through hydrogen bonds to achieve epoxide activation. The ring is subsequently opened by the X- molecule's nucleophilic assault on the epoxy carbon atom, which has a low steric barrier. Following the opening of the epoxide, CO2 combines with the oxygen anion on the ring to generate the carbonate intermediate. Here, the adsorption and activation of CO2 by the catalyst is particularly important. The final step is intramolecular cyclization to obtain cyclic carbonates and regenerate the catalyst simultaneously. Catalysts serve a crucial role in decreasing reaction time and decreasing reaction pressure during the initial activation of epoxides, ring-opening, and carbonate intermediate stabilization [26].In this review, we focus on the reaction path of cyclic carbonates through the reaction of CO2 and epoxide. The importance of CO2 activation in the reaction process is emphasized, and typical modes of CO2 activation are summarized and compared. Then, the research progress in various typical catalysts in recent years are reviewed, and the catalytic properties and principles of the promising catalysts are introduced in detail. Finally, the development trend and characteristics of this pathway are summarized and prospected. It is hoped that this review can provide a reference for future researchers in the synthesis of cyclic carbonates from CO2 and epoxides.Among the many CO2 chemical conversion routes, the activation of CO2 has always been a crucial step or first step in the conversion reaction, and the conversion of CO2 to cyclic carbonates is also the case. In 2018, Moya et al. [27] synthesized aprotic heterocyclic anion ionic liquids (AHA-ILs) to convert cyclic carbonates from CO2. This ILs first activated CO2 to form IL-CO2. The epoxide is then attacked to open the ring for intramolecular cyclization, and finally the cyclic carbonate is formed. They performed DFT calculations and operando FTIR analysis of the conversion process and found that the reason why AHA-IL was better at converting CO2 to cyclic carbonates than other catalysts was that the ILs could first activate CO2 molecules to form intermediates, which was beneficial for the whole reaction process [27]. Therefore, it is important to consider the activation of CO2 by various catalysts. The key to CO2 activation is its high stability, which is closely related to its structure. From the standpoint of CO2 structure, CO2 is a normal linear molecule with zero dipole moment [28]. The carbon atom in the CO2 molecule forms a bond with oxygen in the form of a sp hybrid orbital. The remaining 2 Py and 2 Pz orbitals of carbon and oxygen atoms and their electrons form two perpendicular three-center four-electron delocalized bonds. Compared with the carbon-oxygen double bond and the carbon-oxygen triple bond, the C-O bond in the CO2 molecule has the characteristics of shorter bond length and higher bond energy [29]. Therefore, the CO2 molecule is relatively stable, so in order to achieve large-scale transformation of CO2, its activation is a key link. Generally speaking, the electronegativity of oxygen in the CO2 molecule is 3.44, and that of carbon is 2.55. The electron cloud obviously favors oxygen, making carbon more positively energetic [30]. From a coordination chemistry point of view, CO2 usually complexes with some transition metals and organic molecules in a variety of ways. A CO2 molecule has two active sites, one of which is its lowest unoccupied molecular orbital (LUMO), that is, its carbon atom, which has lewis acidity and usually acts as an electrophilic. The other active site is its highest occupied molecular orbital (HOMO), which contains its two oxygen atoms, which have a weak lewis base and usually act as nucleophiles [31]. In most cases, the chemical transformation of CO2 requires at least one form of coordination activation of CO2, that is, nucleophilic coordination with carbon, or electrophilic coordination with oxygen, or both [32]. Seven major CO2 activation methods are amine activation, frustrated Lweis pairs (FLPs) activation, ILs activation, N-heterocyclic carbenes (NHCs) activation, transition metal coordination activation, photocatalytic reduction activation and electrocatalytic reduction activation. They can form complexes with CO2, which react with substrates such as epoxides activated by organic or metal-based catalysts to form cyclic carbonates for conversion purposes.The basic group can react acid-base with CO2 to generate acid-base adducts, which can then react with epoxides as key intermediates. Woolee Cho et al. [33] exploited tertiary amines as green organocatalysts to activate CO2 and produce cyclic carbonates. Fig. 4 illustrates the possible mechanism of cycloaddition reaction catalyzed by tertiary amines. The combination of amine and CO2 induces the CO2 activation, producing carbamate salt. The reaction of amine and epoxide generates quaternary ammonium salt and therefore achieve epoxide activation. The carbamate salt and quaternary ammonium salt are considered are pivotal intermediates corresponding to Cycle I and Cycle II.One of the most common and effective CO2 activation methods is the coordination reaction of CO2 and transition metal to form transition metal CO2 complexes, which reduces the activation energy required for further conversion reactions and makes CO2 conversion reactions easier. Mascetti et al. [34] summarized four binding modes of coordination between CO2 and transition metals ( Fig. 5). The η1(C) and η2(C, O) coordination modes are both affected by electrostatic and orbital overlap. Under the η1(C) coordination mode, there is a strong electron transfer phenomenon between the dz 2 orbital of transition metal space and the π * orbital of CO2. In the η2(C, O) coordination mode, the dz 2 orbital of the transition metal space forms σ bonds with the π orbital of CO2, while the occupied dxy orbital of the transition metal forms feedback bonds with the π * orbital of CO2. The repulsive electrostatic interaction is decreased when the metal is in a low oxidation state, making it simpler to adopt a η1(C) coordination mode, such as [Rh(Diars)2(Cl)], [Co(Salen)], and so on. dπ with high-energy orbitals more inclined to eta η2(C, O) coordination, such as [Ni(PR3)2], [Mo(PMe3)4], [Fe(PMe3)4], [Cp2Mo], etc. Under certain conditions, CO2 can be inserted into the M-C, M-H, M-O, M-N, and other chemical bonds of transition metal complexes to form carboxylic esters and carboxylic acid complexes containing new M-C, M-H bonds [28,35].Michael North et al. [36] reviewed six-class heterogeneous catalysts used in the cycloaddition reaction of epoxide and CO2, and pointed out that some catalysts have little metal loading or do not require cocatalysts, but it is difficult to obtain conversion rates comparable to those of metal catalysts. CO2 coordinates with the metal center to form a complex, which is η2-O, C-side coordination to the metal center. There are two possible forms of CO2 bonding, one is CO2 binding to metal strong π-donation, η2(O, C) [1], and the other is reduction to a η2(O, C) [2] when the bond between carbon and oxygen in CO2 is formed simultaneously by metal center π-back and M-C, M-O bonds.According to the classical lewis acid-base theory, the reaction of lewis base (LB) and lewis acid (LA) generates a classic lewis adduct (CLA). The essence of the acid-base reaction is the formation of coordination covalent bonds between electron docking receptors and electron pair donors [37]. However, not all LA and LB could generate stable CLA. It was not until 2006 that Douglas W.S. Tephan proposed a definition of "frustrated Lweis pairs" (FLPs), which refers to electron donor and electron acceptor pairs that do not bind to stable CLA due to steric inhibitors or that can be dissociated by CLA [38]. Lewis acids and lewis bases, which cannot form coordination bonds due to steric hindrance, form an active region in which the bonding orbitals of reactant molecules interact with the vacant orbitals of lewis acids. At the same time, the antibonding orbital of lewis base interacts with the nonbonding orbital of lewis group, the electrons in the molecular bond orbital move to the unoccupied orbital of lewis acid, while the lone pair electrons of lewis base transfer to the antibonding orbital of the molecule, leading to the polarization of the reactant molecules, the elongation of molecular bonds, and finally the isomerization. Therefore, FLPs have the characteristics of an acid-base double active center, which makes them have an important application in CO2 transformation [39]. Through the interactions between Lewis basic sites/C and Lewis acidic sites/one O atom of CO2, FLPs can activate CO2. Zhang et al. [40] addressed interfacial FLPs constructed on defect-enriched CeO2 (110). The oxygen vacancy clusters and isolated Ce3+ ions are generated when two adjacent oxygen atoms are removed from CeO2 (110). At a proper distance, adjacent Lewis acidic Ce3+ ions in surface and Lewis basic lattice O2– produce the interfacial FLPs. Although the defective CeO2 can achieve the activation of CO2, FLPs constructed on defective CeO2 can enhance furtherly CO2 adsorption. The C atom and two O atoms of CO2 are bound at a Lewis basic lattice O2– and two Lewis acidic Ce3+ ions, respectively. The isolated Ce3+ ions can catalyze olefin epoxidation and interfacial FLPs can effectively activate CO2. A catalytically tandem conversion of olefins and CO2 into cyclic carbonates is proposed, as shown in Fig. 6. Increasing the surface defects could create more FLPs and weaker interactions between epoxide and catalyst. The more FLPs can benefit CO2 activation, while the subdued interactions can improve the selectivity of cyclic carbonates.Ionic liquids (ILs) have attracted a lot of attention in the last decade. Compared with traditional organic solvents, ILs have the characteristics of low volatility, low flammability, excellent thermal stability, and strong solubility [41,42]. The ILs contains large organic cations and anions with different structures, and its melting point is less than 100 °C [43]. Generally, the structural design of cations and anions in ILs is carried out to fine-tune the physical and chemical properties of ILs to meet specific needs [44]. Therefore, ILs are generally known as "design solvents" [45]. The most common cations in ILs include imidazole, pyridine, piperidine, quinoline, morpholine, pyrrolidine and its monoalkyl or polyalkyl derivatives, as well as tetraalkyl ammonium or phosphonate and trialkyl sulfonic acid [46]. Most cations are formed by proton or alkyl substitution of heteroatoms in the molecular structure of ILs. By adding R+ and H+ groups to the cations, "aprotic" and "proton" ILs were created. Protic ILs vary from aprotic ILs in which they have proton acceptors and donor atoms, as well as the ability to build large hydrogen bond networks. Functionalized side chain cations, such as polar, fluorinated, or chiral cations, have sparked a lot of attention in recent years, and they are typically created for specific uses. "Mission-specific ILs" is a term used to describe certain ILs. Fig. 7 depicts the various cations and anions that are often employed in the synthesis of ILs.ILs are widely used in the conversion of CO2 into products such as methanol, formic acid, and cycloaddition reactions to form cyclic carbonates and other more complex organic compounds [48]. The functional design of ILs can effectively reduce the energy required for the conversion reaction and plays an important role in the activation of CO2 and epoxides [49]. Lian et al. [50] introduced amino-based ILs, imidazolium-based ILs, pyrazolium-based ILs, others ILs and ILs-modified catalyst. ILs and ILs-modified catalysts have inherently high affinity to activate CO2. CO2 molecules can be easily activated and the cycloaddition reaction can occur at mild conditions in the presence of catalysts to obtain cyclic carbonates. The mechanism of CO2 activation can be ascribed to the hydrogen bond in ILs. ILs have biological toxicity, which could not be ignored [51]. Indeed, ILs have greatly promoted the conversion of CO2 and will have far-reaching effects.Generally speaking, under certain conditions, the catalytic material is irradiated with light of appropriate energy [52]. Driven by the incident light energy, the electrons in the valence band (VB) are excited, and the excited electrons jump across the band gap to the conduction band (CB) with higher energy [53]. When an electron leaves the valence band (VB), an equal number of holes will be created simultaneously in the valence band, forming an electron-hole pair [54]. The electron-hole pair then moves together to the active site on the surface of the catalytic material to participate in the reaction [55]. The lifetime of the excited electron-hole pair is only a few nanoseconds, but this is enough to facilitate redox reactions [56]. Common reduction products are carbon monoxide, methane, methanol, ethanol, acetic acid, and so on [57]. The main reason for the different products is the different numbers of electrons involved in the reduction reaction [58]. One problem is that electrons and holes may recombine during the journey of the electron-hole pair to the surface of the catalytic material. Catalytic materials are a key factor in the process of photoactivation reduction of CO2, and there are many classical semiconductor materials such as TiO2, CdS, G-C3N4, ZnO, BiVO4, etc [59–63]. However, the band gap of traditional semiconductor materials is generally wide, resulting in a high electron-hole pair recombination rate, and the activity is greatly affected by the wavelength of incident light. For example, TiO2 only has Ultraviolet (UV) light activity (wavelength less than 380 nm), its adsorption capacity for CO2 is weak, and its structural tunability is not satisfactory [64]. With the development of energy band engineering, many materials have been designed, such as core shells, egg yolk shells, multi-shells, hollow structures, etc [65–67]. Not only do they have a band gap that matches the thermodynamic reduction capacity requirements, but also they have a structure that facilitates electron transport [68]. Anjan et al. [69] reported the successful activation of CO2 and reaction with epoxides to synthesize cyclic carbonate by using covalent organic framework (COF) as a photocatalytic material under atmospheric pressure and visible light. The experiment shows excellent yield under visible light irradiation and CO2 (1 atm), and the reaction can be easily controlled by changing the illumination.Electroactivation of CO2 is also a promising method of activation, highlighting many advantages such as controlled reaction rates, mild reaction conditions, and product selectivity through the potential [70]. Electrochemical activation of CO2 is achieved either directly at the electrode or indirectly by heterogeneous or homogeneous catalysis [71]. It has become a focus to search for catalysts that can reduce the relatively high evidence and improve the selectivity of the reduction process. Many transition metal complexes have been shown to be effective in the electrolytic reduction of CO2 [72]. For example, Fisher and Eisenberg [73] reported for the first time the catalytic activity of the Ni tetraaza macroring for CO2 reduction and found that it reduced the CO2 reduction potential by about 0.5 V in non-aqueous media. Cycloaddition reactions of CO2 and cycloalkyl oxides with cyclocarbonates have been studied by some groups under the electrolysis of transition metal complexes. Electrocatalysis is actually a superposition of electrochemistry and catalysis. Electrochemistry converts electrical energy into chemical energy to catalyze the reaction [74]. Khoshro et al. [75] reported that a nickel complex, 2,4,10,12-tetramethyl-1,5,9,13-(14-nitrobenzene) tetra-cyclopentalkyl (2-) nickel (II), exhibited good electrocatalytic activity in acetonitrile (ACN) solution at room temperature. The intermediate is then oxidized by a nickel (II) complex to obtain a cyclopropane product. However, although electrocatalysis has been developing in recent decades, research groups around the world have not yet been able to design efficient electrostatic CO2 reduction solutions for industrial applications [76].N-heterocyclic carbenes (NHCs) are heterocyclic compounds containing carbon with at least one nitrogen atom in the ring structure, including a large number of substituents [77]. At the same time, it is a new strong σ coordination ligand, especially with the formation of excessive metal complexes, which has important applications in catalysis, materials, and other aspects [78]. In 1968, Ofele and Wanzlick et al. [79] synthesized the metal complex of NHCs for the first time, but they did not separate the free NHCs. In 1991, Arduengo et al. [80] successfully separated the free carbines. Common NHCs are shown in Fig. 8. These free carbines have strong electron-giving ability, show strong nucleophile and lewis alkalinity, and form NHC-CO2 admixtures with CO2 molecules, thereby activating CO2 [81]. It can also directly coordinate with many transition metals to form compounds with specific structures and functions [82,83]. Table 1 summarizes the research progress and characteristics of the seven activation methods. Among them, amines, transition metals, and FLPs have been widely studied in the activation of CO2, which are often used in catalysts such as metallic, ILs, Metal Organic Frameworks (MOFs), and NHCs. ILs are very promising catalysts, because their functions can be designed, and different types of ILs can activate CO2 in different ways. The key to photocatalytic CO2 activation is the selection of materials and the control of the process. There are few reports on the non-reductive transformation of CO2. In recent years, NHCs is one of the most promising organic catalysts, which is often used as an efficient nucleophile to activate CO2. The synergistic catalytic activation of CO2 by NHCs and other catalysts is a hot research topic.Catalysts and epoxides have different effects on the rate and product of the CO2 cycloaddition reaction [84]. Specifically, the catalyst to reaction rate adjustments to the appropriate range, different epoxy compound cycloadditions of CO2 have different reactivity, and the result of CO2 and epoxide reaction selectivity (i.e., cyclic carbonates and polycarbonates) at the same time under the influence of catalyst, epoxide, and reaction conditions [85]. As a result, the major subject in the field of CO2 conversion to cyclic carbonate is the design and development of the entire catalytic system [86]. At present, the trend of catalytic system development is under mild conditions, requiring both high efficiency and selectivity, but also good stability and recyclability, and the catalytic process does not need cocatalyst and volatile organic solvents for the catalyst itself to be cheap and easy to obtain and easy to synthesize [87–90].Over the past few decades, many catalytic systems have been developed for coupling reactions of CO2 and epoxy compounds. Büttner et al. [91] reviewed recent catalysts, which can be classified into three types: organic catalysts, special metal complex catalysts, and miscellaneous catalysts based on transition metals and main group elements. Ammonium, phosphonate, imidazolium, amide-based catalysts, and carbenyl catalysts were introduced as organic catalysts. Among transition metal and main group element-based catalysts, alkali metal and alkali earth metal base catalysts, boron and carbon base catalysts, and transition metal catalytic systems are introduced. Catalysts containing halogen salts are poisonous, but halogen-free catalysts exhibit great attraction. Zhang et al. [19] reviewed the research progress of halogen-free catalysts, including metal catalysts, metal-free catalysts, and other catalysts. Metal oxides, metal complexes, metal salts, molecular sieves, MOFs, zeolitic imidazolate frameworks, and metal-porous materials were introduced in metal catalyst materials. Metal free catalysts include nitrogen-rich catalysts, CO2 adducts, and HBDs catalysts (e.g., alkyl amines, salophens, amino acids, biological acids). There are other catalysts, such as ammonium salts, organic bases, and so on. Liang et al. [92] reviewed many porous catalytic materials, such as MOFs, COF, nanoporous ionic organic networks (NION) and amorphous porous organic polymers. ILs, metal complexes, nitrogenous polymers, organic catalysts, and metal-Salen complexes, metal-organic frameworks, and molecular sieves are examples of homogeneous catalysts. The homogeneous catalyst has the advantage of high efficiency and high activity. For example, the earliest dual-functional or binary catalytic systems composed of homogeneous metal complexes (such as Al, Zn, Mg, etc.) usually convert CO2 into cyclic carbonate at room temperature and pressure [93–96]. However, such catalysts are generally poorly selective, expensive to use on a large scale, and even toxic [97]. The shortcomings of these metal complex catalysts have promoted the development of metal-free organic catalysts such as the organic halide [98,99]. In recent years, several functional organic catalysts have been reported to be as competitive as binary or ternary catalysts [100]. Some metal-free catalysts have also performed well in combination with organic halides [101]. At present, such metal-free organic catalysts are not as active as metal complex catalysts, but they have obvious advantages such as low price, simple synthesis, and non-toxicity. Recently, some researchers have synthesized some metal polyphase catalysts by combining metal with MOFs and Porous Organic Polymers (POPs). Such catalysts not only increase the active center of the catalyst and improve the catalytic activity, but also significantly improve the inherent difficulty in dissolution and separation of metal catalysts. Therefore, heterogeneous catalysts are more suitable for future chemical industry applications [102].Metallic catalyst, ILs, MOFs, NHC catalyst for CO2 or epoxide activation effect is better, which has a higher catalytic production rate, and provide the future development direction of CO2 conversion catalyst of cyclic carbonate.Catalysts with metallic elements are continuously developed. The crust contains a lot of metal elements, so the raw materials for the synthesis of metal catalysts are easy to obtain [103]. Metal catalysts involved in the catalytic conversion of CO2 and epoxides into cyclic carbonate can be divided into transition metals, main group metals and Rare-Earth (RE) metals. Büttner et al. [104] found that the binary catalytic system composed of FeCl3 and [Oct4P] had the best catalytic effect after screening a variety of combinations of ferric salts and phosphine salts on the premise of ensuring non-toxicity of the catalyst. Metal-amide combined catalysts are also commonly used to catalyze the reaction of CO2 with epoxides. Rios Yepes et al. [105] prepared a series of single, double, and trimethylamide-aluminum complexes that exhibit high catalytic activity with the help of tetrabutylammonium iodide (TBAI) as a catalyst. The activity of this type of aluminum complex catalyst is closely related to the coordination mode, showing the great potential. RE-based complex catalysts always have strict requirements on reaction conditions, usually 10–20 bar CO2 or high temperature [106]. Xin et al. [107] synthesized for the first time a lanthanum complex catalyst stabilized by a polydentate N-methylethylenediamine bridged triphenol ligand. Under the conditions of room temperature, 1 bar CO2, TBAI as cocatalyst and 1,2-epoxyhexane as substrate, the cyclic carbonate yield reached 99% and TOF reached 18.3 h-1, and the catalyst reacted with 1,2-epoxyhexane and CO2 for 5 consecutive cycles, and the yield was maintained at 96%. For RE based catalysts, their reported results are unprecedented. At present, NHCs linked RE metal catalysts are still in the development stage, which is also very exciting [108]. Table 2 shows the metal catalysts reported in the last decade. These metals combine with porphyrins, such as Entry 1, 2 and 3 to form catalysts with multiple active centers, and halogen ions also appear in the porphyrins framework. Or with molecules or group ligands form binary bifunctional catalysts. Adding cocatalyst is also a good strategy. Ema et al. [109] developed a catalytic system by combining metal porphyrins with nucleophiles, and examined the effects of Mg, Co, Ni, Cu, Zn and Tetraphenylphosphonium Bromide (TPPB), TetrabutylAmmonium Bromide (TBAB), phenyltrimethylammonium Tribromide (PTAT), and 4-dimethylaminopyridine (DMAP). The combination of Mg porphyrin and TBAB was found to have the best catalytic effect by comparison.Catalytic mechanism of metal catalyst is roughly same, usually by metal center as a lewis acid, activate the epoxide, formation of intermediates, add the nucleophilic reagent, attack epoxide steric lower side, promote and realize the epoxide ring opening, this step is largely affect the reaction rate and time. The nucleophilic reagent is commonly halogen ions. They are present in porphyrin frameworks, in ligands or provided by cocatalysts. The catalyst is renewed when the ring-opening epoxides react with CO2 molecules to create cyclic carbonates [119].Compared with traditional ILs, ILs containing hydroxyl, carboxyl and amino functional groups show very good advantages [120]. Qu et al. [121] reported the synthesis of new AAILs ([HTMG] [AA] [X], AA=(Histidine [His]2-, Lysine [Lys]−), X = Cl, Br, or I) consisting of superbase 1,1,3,3-tetramethylguanidine ([HTMG]+), which has the ability to well catalyze CO2 and epoxide reactions at room temperature. In particular, the reaction of CO2 and propylene oxide at room temperature and atmospheric pressure for 20 h can obtain 99% of the yield of propylene carbonate. In previous reports of amino acid ILs being difficult to reuse under the reaction conditions of ideal, Yue et al. [122] reported the synthesis of dual amino functional imidazole ILs under the conditions of solvent-free helpless catalyst as CO2 and epoxide catalyst for the synthesis of cyclic carbonate reaction. After repeated use for five cycles, the production rate did not decline. Excitingly, the reaction of CO2 with epachlorohydrin for 13 h at 105 °C and 0.5MPa resulted in a 98.3% yield of allyl chloride carbonate. Yang et al. [123] have synthesized multifunctional monocomponent zinc halide-based ionic liquids, amidinothiourea-ZnI2 (ATUI) and N-phenylthiourea-ZnI2 (NTUI), which are used as homogeneous catalysts for the conversion of CO2 with epoxides to cyclic carbonates. Owing to the synergistic effects of multifunctional components in ATUI, ATUI has the better catalytic activity and more superior reusability for CO2 cycloaddition than NTUI. Under 110 °C, 1.0 MPa, 4.0 h or even mild conditions, the yield of propylene carbonate can reach 95%. Only a slight decrease in the yield is caused by the loss of ATUI during the reuse process. As shown in Fig. 9(a), Dai et al. [124] produced a variety of new functionalized phosphonium-based ionic liquids (FPBILs) constituted of hydroxyl, carboxyl, and amino functionalized phosphonium-based ILs. For the first time, it was utilized as a catalyst in the cycloaddition reaction of CO2 with epoxides to produce cyclic carbonate. By comparison, [Ph3PC2H4COOH]Br with carboxyl functional group had a high catalytic activity with a TOF of 64.9 h-1. In the absence of solvent or cocatalyst, the reaction with 7 epoxides showed a conversion rate of more than 80%, among which the conversion rate of ethylene oxide as the reaction substrate was up to 100% within 1 h. The advantages of FPBILs lie in the synergistic effect of hydrogen bonding between functional groups and epoxides and the nucleophilic attack of halide anions on epoxides [124]. The hydrogen bond connection between the functional group H atom and the epoxide O atom may lead to the polarization of the C-O bond of the epoxide and the generation of intermediate I. Br then performs a less steric impeding nucleophilic assault on the carbon atoms of the epoxide complex, encouraging the opening of the epoxide ring and the production of intermediate II. After CO2 is inserted into intermediate II, halogenated carbonate III is formed, which is converted to cyclic carbonate by intermolecular displacement, as shown in Fig. 9(b).Hydrogen bonding has been known to efficiently activate epoxide or CO2 and significantly promote cycloaddition reactions. Therefore, many hydroxyl-functionalized ILs have been developed and designed [125]. Peng et al. [126] reported a series of novel polyhydroxyl bisquaternary ammonium ILs and designed six novel bifocal polyhydroxyl ILs as shown in the Fig. 10. The design principle is that halide ions act as lewis bases to carry out nucleophilic attacks, and hydroxyl groups act as lewis acids to activate epoxides within a molecule. At 120 °C, 2 MPa, 3 h, 99% yield of propylene carbonate can be obtained.The application of supported ILs in the fixation of CO2 was first proposed by Xiao et al. [127] in 2006. They used immobilized ILs-zinc chloride polyphase catalysts to chemically immobilize CO2 to cyclocarbonate effectively under mild conditions at high TOF without any additional cosolvent, and they maintained selectivity above 98%, which can be reused twice. Liu et al. [128] reported that at 80 °C atmospheric pressure, a novel polystyrene supported ILs (PS-IMPCOOHTMGBR) demonstrated high catalytic activity for the cycloaddition reaction of CO2 and epichlorohydrin (ECH), with a product yield of 97.2%. The optimal reaction conditions were determined as follows: 0.1 MPa CO2 pressure, 3 mol% catalyst dose, 80 °C temperature, 5 h reaction duration, and it may be reused nine times. It has several active centers and a good capacity to absorb CO2 when compared to other PS loaded ILs. Fig. 11 depicts a putative reaction process. CO2 is initially adsorbed by the –COO- group in PS-IMPCOOHTMGBr in S1. The hydrogen atoms in the imidazole ring work as electrophiles to activate ECH, which is then polarized in the nucleophilic area by the anion Br-. Electrophilic and nucleophilic activation work together to open the ring and produce a complex (I), while hydrogen bonds form between ILs and ECH. The collected CO2 is transferred to ECH to generate complex (II) beginning with complex (II) (III). Finally, ring closure converts the intermediate (III) to chloropropylene carbonate (CPC). PS-IMPCOOHTMGBr retains a strong electrophilic activity center and a nucleophilic activity center after trapping CO2, which is another major explanation for the high activity of PS-IMPCOOHTMGBr. The proton in the [TMGH+] cation is attracted to the –COO- group in Scheme 2. The TMG group then adsorbs the CO2, and the next stages differ from scheme one in that hydrogen is utilized as an electrophile. The proton transfer from the [TMGH+] cation to the –COO- group, on the other hand, is difficult. And because the TMG group will quit after many runs, the chance of a response based on S2 is extremely low. PS-IMPCOOHTMGBr has a lower adsorption capability than porous materials. However, it plays a vital role in improving catalytic performance [129].Organic amines or alcohols (e.g., urea, ethylene glycol) combined with metal halides in appropriate molar ratios can form eutectic ILs, such as urea and zinc chloride, simply mixed in a molar ratio of 3:1, heated to about 100 °C, and stirred in air until a transparent, homogeneous liquid is formed. The formation and structure of eutectic ILs are shown in Fig. 12(a). This ILs has the advantages of low synthesis cost, environmental friendliness, simple operation, and excellent performance [130]. It was not until 2016 that Liu et al. [131] reported the [urea-Zn]I2 eutectic based ILs, which catalyzed the reaction of CO2 with propylene oxide through a cationic structure and halogen anion to generate propylene carbonate (PC). It has a 95% yield, 98% selectivity, and does not require cocatalysts and solvents. This avoids the need for additional cocatalysts and nucleophiles in the catalytic cycle.The mechanism is the oxygen coordination of NH2 groups and lewis acid zinc sites with epoxides. Polarization of the C-O bond in epoxides occurs through hydrogen bonding and the formation of zinc adducts to epoxides (Step 1) [132]. Then, during the ring-opening process of the epoxide, the I- ion nucleophilic attacks the carbon atoms in the epoxide with low steric resistance, forming the oxygen anion intermediate (Step 2). It is stabilized by synergy between cations and I- anions. Thereafter, the oxy-anion intermediate further interacts with the active CO2 via the amino group of the catalyst, leaving a catalyst behind (Step 3). Finally, I- is released simultaneously during intramolecular cyclization to generate cyclic carbonate products for catalyst regeneration (Step 4) as shown in Fig. 12(b). Thus, eutectic ILs provides lewis acid-base bifunctional groups that activate both CO2 and epoxides in the catalytic cycle [133].In summary, as shown in Table 3, ILs can be divided into three categories: functionalized, supported, and common. Functionalized ILs contain a variety of functional groups, which are key to the activation of CO2 or epoxide and accelerate the reaction. Supported ILs are grafted onto solid materials, and the activation sites on these materials can assist ILs to activate CO2 and epoxide, and this kind of ILs is also convenient for separation. Common ILs consists of basic anions and cations with nucleophilic ability, and halogen ions are usually involved in the catalytic reaction of CO2 and epoxide.MOFs have received a lot of attentions since they were first reported in 1995. MOFs used as heterogeneous catalysts have unique properties such as high specific surface area, high stability, open channels, and permanent porosity, giving MOFs an advantage over other adsorbents or catalysts in CO2 chemistry [134–137]. In recent years, it has become a promising stationary CO2 heterogeneous catalyst [138]. In the past, the addition of lewis acid active sites to MOFs, such as coordination unsaturated metals, has been very beneficial in catalyzing cycloaddition reactions of CO2 and epoxides [139]. In recent years, some researchers have prepared catalysts by combining one or two metals with inorganic nodes in MOFs, as shown in Table 4.The reaction conditions are relatively mild, even at room temperature, but also have a high yield. Compared with mono-metal organic complexes, catalysts with MOFs generally have the advantage of easy separation and recovery [144]. Gao et al. [142] innovatively introduced zinc into Mg-MOF-74 inorganic nodes and added catalytic sites in MOFs to enhance catalytic performance. They used a facile One-Pot method to synthesize and characterize catalytic examples with different Zn-Mg ratios, as shown in Fig. 13(a).The reaction mechanism is shown in Fig. 13(b). Firstly, unsaturated coordination of Zn and Mg serves as lewis acid centers to cooperatively activate oxygen atoms in epoxides to form an admixture of metal epoxides. At the same time, metal oxygen pairs (Zn-O, Mg-O) adsorbed and activated CO2 as lewis alkaline sites, which enhances the charge of oxygen atoms in CO2 and the nucleophilic ability. Then Br- in Bu4NBr acts as a nucleophile to attack the β-C atom with low steric hindrance in the epoxide, realizing the ring-opening of the epoxide and obtaining an oxygen anion intermediate. This oxygen anion intermediate reacts with CO2 to form an alkyl carbonate intermediate. Finally, the inner ring is closed to form cyclic carbonate, and the catalyst is regenerated ( Fig. 14).It is of great significance to graft ILs onto the carrier for the development of an efficient heterogeneous CO2 conversion catalyst [145]. Xu et al. [146] recently reported that MIL-101-N(Bnme2)Br functionalized by quaternary ammonium salts was employed as a bifocal catalyst for the cycloaddition of CO2 and epoxides for the synthesis of cyclic carbonate at 1.4 MPa CO2 pressure, 17 mmol epoxide, 100 °C, 5 h. At 0.93 mol percent catalyst, the yield and selectivity of PC were 93% and 99%, respectively. Fig. 13 (a) shows the preparation process and structural model of MIL-101-NH2. As illustrated in Fig. 13 (b), MIL-101-N(Bnme2)Br polyphase catalyst was produced by an aldehyde-amine condensation process employing MIL-101-N(Bnme2)Br ILs (N(Bnme2)Br). Fig. 15 describes the activation mechanism. The epoxide is first activated by the interaction of the chromium in the lewis acid core of MIL-101-N(Bnme2)Br with the oxygen atom of propylene oxide (PO). At the same time, the β-carbon atom of the epoxides with moderate steric hindrance is attacked by the nucleophilic bromide ions of the ILs, promoting ring opening of the epoxides. Subsequently, CO2 enters the ring-opening intermediate and eventually forms the corresponding ring carbonate, which is also released by the intramolecular Br- sealing to regenerate the catalyst [147].Puthiaraj et al. [148] employed Friedel-Crafts polymer to manufacture porous ZnBr2 grafted with N-heterocyclic carbene-based aromatics for CO2 adsorption and conversion to cyclic carbonate. Using 1, 3-dibenzylbenzimidazolium bromide (DBBIBr), triphenylbenzene, formaldehyde dimethyl acetal, and 1,2-dichloroethane as raw ingredients, a porous N-heterocyclic carbinyl crosslinked aromatic ionic polymer (NHC-CAP-1) was produced at 80 °C for 18 h. Following that, NHC-CAP-1 was suspended in a DMF: tetrahydrofuran mixture, tert-butyl potassium was added and agitated at 50 °C under nitrogen blowing conditions, ZnBr2 was added, and the solution was heated to 80 °C for 12 h. To remove and metallize the solid, it is filtered and washed with water, resulting in strong NHC-Zn linkages in the polymer network. Fig. 16 (a) shows the synthesis process. When selecting different epoxides as substrates for cycloaddition processes, this catalyst exhibits outstanding performance and a high TOF, and the reaction condition was 100 °C, 2 MPa, and no cocatalyst and solvent was required in particular, the yield of propylene carbonate was 95%, and the TOF was as high as 2202 h-1 as illustrated in Fig. 16(b).Multiphase and multifunctional charged polymers have many advantages, such as graded pore structure, high thermal stability, abundant active metal sites, and high specific surface area, which have broad application prospects in the formation of cyclic carbonate from CO2 and epoxide. Bai et al. [149] synthesized zinc(II)porphyrin-based porous ionic polymers [PIP-ZnTIPP/DVB (1:20)], which had high yields in catalyzing the reaction of CO2 with a variety of epoxides. Surprisingly, at 120 °C, 1 MPa of CO2, 6 h, 99% yield of propylene carbonate was obtained, the selectivity was greater than 99%, and TOF reached 759 h-1.Zeolitic imidazolate frameworks (ZIFs) are highly adsorbable and selective for CO2, and have been used to catalyze the formation of cyclic carbonates from CO2 and epoxide [150]. Li et al. [151] reported a new type of zeolitic tetrazolate−imidazolate frameworks (ZTIFs), which can be regarded as N-functionalized ZIF-8, called ZTIF-8. In fact, ZTIF-8 contains a basic N atom active site that is the key to CO2 conversion [152]. The yield of chloropropene carbonate catalyzed by ZTIF-8 was more than 99% (the first cycle) and 81.2% (the third cycle) at room temperature of 1 atm for 48 h with cocatalyst tetra-n-tert-butylammonium bromide (TBAB). Therefore, the recovery and recycling of catalysts still needs to be improved.Directly reducing the concentration of CO2 in the atmosphere is an effective way to improve the greenhouse effect, and the conversion and utilization of CO2 resources has been a huge and promising industry. There are many ways to convert CO2 in the past, but all of them are insufficient and do not conform to the development strategy of green chemistry. The reaction of CO2 with epoxides to form cyclic carbonates is a promising conversion method, and many types of catalysts have been reported with good results for these reactions. All kinds of catalysts, which still adopt effective classical methods for the activation of CO2 or epoxide, this paper summarizes seven CO2 activation methods, explains their respective research progress, and points out that the commonly used methods for the activation of CO2 by various catalysts, these other reviews are not described in such detail. At present, metal-based catalysts ILs and MOFs have obvious advantages in CO2 conversion reactions. The yield of some products can reach more than 95%, and the catalyst can also be reused. In addition, catalysts such as MOFs also have a good effect on CO2 adsorption. Dozens of organic catalysts have been reported, and NHC is a very promising one. There are few experiments related to NHC catalysts, but it also has a good application prospect. The design trend of future catalysts is easy synthesis, cheap, high yield, multiple active sites, designable performance, mild reaction conditions, and environment-friendly. However, more researches are needed to be made to achieve industrialization.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to thank the support from Shanghai Non-carbon energy conversion and utilization institute.

With the continuous emission of greenhouse gases, the rational transformation and utilization of CO2 is particularly important. Cyclic carbonates are a kind of versatile compounds and have wide applications in Li-ion batteries, pharmaceutical manufacturing and many fine chemicals. Cycloaddition of CO2 and epoxide to synthesize cyclic carbonates is considered one of the most promising CO2 conversion routes because of its 100% atomic economy, non-toxicity, as well as a more economic technical route for the utilization of CO2. In this paper, this review surveys the synthesis of cyclic carbonates employing CO2 as a building block. The mechanisms of CO2 activation have been described in detail due to the thermodynamic stability of CO2 molecule. The reaction mechanism of CO2 and epoxide is expounded, and seven CO2 activation methods are summarized and compared, deeply analyzing the research progress of recent years. To reduce the activation energy of the CO2 conversion, the utilization of catalysts is very crucial. Various types of catalysts suitable for the synthesis of cyclic carbonates derived from CO2 have been expounded in depth. Finally, the development trend of catalysts is prospected. The development of improved catalysts is strongly demanded for successful commercialization of CO2 transformation technologies. This review enables researchers to timely seize the current advancements and thus may provide some rewarding insights for future investigations on the synthesis of cyclic carbonates employing CO2 as the feedstock. It will provide a good reference and guide for scholars to achieve the better improvements.

Perovskite catalyst general formula ABO3; typically the A elements are rare earth alkaline (Ce, La, Pr etc.), alkaline earth metals (Ca, Cs, Sr, Ba etc.) and the B sites are usually occupied by transition metals (Fe, Co, Cu, Mn, Ni, and Cr). Perovskite shows high activity for CO oxidation and high thermal resistance [1]. The performance of perovskite catalysts in CO oxidation processes can be increased by fractional replacement of metal in position A and/or position B with metal cations varying in their valence number [2]. The catalytic activity of perovskite catalysts can be enhanced by the incomplete substitution of metal in position A or B with cations of noble metals like Ag, Au, Pt and Pd etc [3]. When deposited of noble metals into perovskite is main factor in the catalyst's activity, which, however, is also highly effected by the type of perovskite was used. The activity of perovskite catalyst is strongly influenced by their preparation methods [4]. The main benefit of perovskite catalysts lies in the fact that they are posses' higher activity and thermal stability compared to pure oxides [5]. The addition of noble metals into perovskite reduces sintering and reduction in mass as a consequence of volatilization at a high temperature in oxidizing conditions [6].Increasing the number of vehicles on roads, CO concentration has reached an alarming level in urban areas. In the CO oxidation process, the oxygen is first adsorbed on the perovskite catalyst surface with the energy of activation [7]. By substituting the A and B cations, one can control the total amount of substitution and apply for suitable cations that will get important structural changes, such as lattice distortions, stabilization of multiple oxidation states or generation of cationic and anionic vacancies, all have a direct effect vary in catalytic activity [8,9]. The dispersion of perovskites on a support, one can select the most excellent matrix to contain the oxide particles and expose the major amount of active sites in order to get the better catalytic activity for CO oxidation [10]. Perovskite-type (ABO3) catalysts can be well modified by the partial substitution of atoms at A and/or B-sites, producing iso-structural, which may stabilize unusual oxidation states of B component, induce structural distortions and create cationic or anionic vacancies [11,12]. The best catalyst for CO oxidation could able to maintain the oxide structure throughout the process. The fast changes in temperature of wash coat and active layer on the support may break due to their thermal extension [13,14]. Partial substitution of lanthanum perovskite increases with increases of noble metals into the catalyst. It also influenced by the time and temperature of perovskite calcination and increase with the rise in calcination temperature [15]. The size of perovskites has a smaller influence on the activity of catalysts. This catalyst exhibits good adhesion to the monolithic metal support. In geometric factors, the perovskites shows that lanthanum, which is major lanthanide ion in the series, leads to the most steady perovskite structure [16]. The substituting cations increase the activity and stability of perovskite oxide structure. Manganite and cobaltate perovskite catalysts have been reported to be highly active for CO oxidations [17,18]. In comparison to manganese-based perovskite, the cobalt-based oxides are difficult to support directly on alumina because cobalt ions easily diffuse into the bulk of support to form cobalt aluminum perovskite structure [19]. The catalytic activity of various perovskite catalysts having different compositions in CO oxidation reactions involving at various temperatures has been discussed in this review paper.Perovskite-catalyst (ABO3) can crystallize in cubic structure in space group Pmm or indistinct rhombohedral, tetragonal, orthorhombic and triclinic symmetry as represent in Fig. 1 . The presence of oxygen and vacancy can be change depending on the composition due to a great stability range of structure [20]. The larger A-site cation is frequently rare earth, alkaline earth or an alkali metal cation coordinated to 12 oxygen anions. The B-site cation is usually a minor transition metal cation covering octahedral interstices in oxygen structure. Several combinations of A and B site cations can form a stable perovskite-like structure. These cations with oxygen anions can be partially substituted by other suitable elements. Electronic structure descriptions in Fig. 2 are sum of active quantities used to generate qualitative correlations for a wide range of properties. In particular, the oxygen p-band center has been used to direct material finding and basic considerate of a range of perovskite compounds for utilize in catalyzing the oxygen reduction and advancement reactions [21,22].Partial chemical substitutions of A and/or B sites for ABO3 type perovskite and structures drive structural and electronic transformations, foremost to functional properties such as large magneto-resistance and high-temperature superconductivity [23]. A couple of various metal ions covering equal crystallographic sites create spatial ordering of atoms, crystallizing in ordered perovskite structures with chemical formulae such as A2BB'O6, AA'B2O6 and AA'3B4O12 [24]. Combinations of A, A′ and B site ions provide surprising properties, some of which are functional. The charge-disproportionate or transfer transitions are abruptly switched by bond strains on rare-earth metals. Crystal structure analysis shown in Fig. 2 suggests that metal oxygen bonds make neighboring adsorbates close enough to interact, probably facilitate two active site reaction mechanisms [25]. The conventional single-active-site reactions for plain perovskite catalysts and expected to keep away from rate-determining steps of usual mechanisms. Perovskites may be arranged in layers, with the ABO3 structure separated by thin sheets of interfere material [26]. The various forms of intrusions, based on the chemical structure of groups are defined as: • Aurivillius phase: The major layer is contained a [Bi2O2]2+ ion, covering every nABO3 layers, foremost to an generally chemical formula of [Bi2O2]-A(n−1)B2O7. • Dion−Jacobson phase: The main layer is collected of an alkali metal (M) each nABO3 layers, giving the in general formula as M+A(n−1)B n O(3n+1). • Ruddlesden-Popper phase: The major layer occurs between everyone (n = 1) or two (n = 2) layers of the ABO3 lattice. Aurivillius phase: The major layer is contained a [Bi2O2]2+ ion, covering every nABO3 layers, foremost to an generally chemical formula of [Bi2O2]-A(n−1)B2O7. Dion−Jacobson phase: The main layer is collected of an alkali metal (M) each nABO3 layers, giving the in general formula as M+A(n−1)B n O(3n+1). Ruddlesden-Popper phase: The major layer occurs between everyone (n = 1) or two (n = 2) layers of the ABO3 lattice.In the cubic unit cell, the ‘A' atom sits at cube corner positions (0 0 0), type ‘B' atom sits at body-center position (½ ½ ½) and oxygen atoms meet at face-centered positions (½ ½ 0). The comparative ion size for steadiness of cubic structure are moderately rigid, so small buckling and warp can generate many lower-symmetry indistinguishable versions, in which the coordination numbers of A cations and B cations [27,28]. The most active oxygen reaction catalysis for quadruple perovskite oxides containing of earth-abundant elements exposed that exploitation of ultra-high-pressure preparations facilitates the increasing of novel functional materials. A large amount of perovskites compounds prepared in high pressure also shows best candidates for functional materials. Several crystal structures are closely related to perovskite structure is called hexagonal perovskites. The perovskite structure (shown in Fig. 3 ) contains two A-site cations with robustly various sizes are used, further complication increases from ordering of A-sites and oxygen vacancies as in the double perovskite (AA’B2O5+δ) [29]. The natural of cation in ABX3 act as a major role in the formation of structure of perovskites structure moreover a large outcome on stability and electronic property of the materials. The sharing of various cation and anions in different perovskite catalysts is shown in the Fig. 3.A cation exchange should be based on BX6 octahedral allocation with respect to a Goldschmidt tolerance factor. A cation substitution is intentional to get more-stable and suitable dynamic position of transmission band of perovskite film. The octahedral deformation increases by increase in an ionic radius of organic cation [30]. The traditional position of perovskite lattice is discussed in Table 1 . It consists of small B cations within oxygen octahedral and larger A cations which are XII fold coordinated by oxygen. The A3+B3+O3 perovskites are most symmetric structure observed in rhombohedra structure. It involves a rotation of BO6 octahedral with respect to cubic structure [31]. The A cations are present in corners of cube and B cation in center with oxygen ions in the face-centered positions. The decrease of A cation size will be reach where the cations will be very small to stay behind in make contact with anions in the cubic structure. The lowest lattice energy was recognized for all compounds so that an energy value was assigned to all the composition.The A cation is huge and B cation is lower as a result of the lowest energy structure is rhombohedral in nature. As the A cation radius decreases and B cation radius increases, the lower energy structure changes to be orthorhombic. The reducing of A cation radius and raising in B cation radius result in the creation of hexagonal structure. The lattice or internal energy does not change significantly with the changes in crystallographic structure [32]. The structures of perovskites compounds have been studied by many workers. The actual perovskite compounds with few binary oxides have simple cubic in structure as shown in Fig. 4 at room temperature and this structure maintained at higher temperatures. The X-ray patterns of many compounds can be indexed on the basis of distortion of perovskite structure [33]. In addition to various types of disturbances that involve a multiplication of pseudo cubic cell resultant in tetragonal, orthorhombic and rhombohedral symmetries. The most interests study of ferroelectric forms of perovskite structure, especially in two groups of mixed oxides A+2B+4O3 and A+3B+3O3. A classification of perovskite-type structures was done on the basis of radii of their metallic ions [34].Perovskites showed excellent catalytic activity and high chemical stability; therefore, they were studied in a wide range in the catalysis of different reactions. Perovskites can be described as a model of active sites and as an oxidation or oxygen-activated catalyst. The stability of the perovskite structure allowed the compounds preparation from elements with unusual valence states or a high extent of oxygen deficiency. In Fig. 5 shows a unit cell of perovskite structure. Perovskites exhibited high catalytic activity, which is partially associated with the high surface activity to oxygen reduction ratio or oxygen activation that resulted from the large number of oxygen vacancies. Perovskites can act as automobile exhaust gas catalyst, intelligent automobile catalyst and cleaning catalyst, etc., for various catalytic environmental reactions. It was reported in the literature that perovskites containing Cu, Co, Mn, or Fe showed excellent catalytic activity toward the direct decomposition of NO at high temperature, which is considered one of the difficult reactions in the catalysis (2NO → N2+O2). Perovskites showed superior activity for this reaction at high temperatures because of the presence of oxygen deficiency and the simple elimination of the surface oxygen in the form of a reaction product. NO decomposition activity was enhanced upon doping. Also, under an atmosphere that is rich with oxygen up to 5%, Ba(La)Mn(Mg)O3 perovskite exhibited superior activity toward the decomposition of NO.Perovskite showed a great impact as an automobile catalyst; intelligent catalyst. Pd–Rh–Pt catalysts was utilized as an effective catalyst for the removal of NO, CO and uncombusted hydrocarbons. There is another catalyst that consists of fine particles, with high surface-to-volume ratio, and can be utilized to reduce the amount of precious metals used. However, these fine particles exhibited very bad stability under the operation conditions leading to catalyst deactivation. Therefore, the perovskite oxides can be used showing redox properties to preserve a great dispersion state. The crystalline structure of various perovskite catalysts and their formation is shown in the Fig. 6 . Upon oxidation, Pd is oxidized in the form of LaFe0.57Co0.38Pd0.05O3 and upon reduction; fine metallic particles of Pd were produced with radius of 1–3 nm. This cycle resulted in partial replacement of Pd into and sedimentation from the framework of the perovskite under oxidizing and reducing conditions, respectively, displaying a great dispersion state of Pd. Also, this cycle improved the excellent long-term stability of Pd during the pollutants removal from the exhaust gas. Exposing the catalyst to oxidizing and reducing atmosphere resulted in the recovery of the high dispersion state of Pd. This catalyst is known as intelligent catalyst because of the great dispersion state of Pd and the excellent stability of the perovskite structure.One of the important characteristic of perovskites is ferroelectric behavior, which is obvious in BaTiO3, PdZrO3, and their doped compounds. The ferroelectric behavior of BaTiO3 was strongly related to its crystal structure. BaTiO3 was subjected to three phase transitions; as the temperature increases, it was converted from monoclinic to tetragonal then to cubic. One of the major properties of perovskites is superconductivity. The halide perovskite catalyst crystalline structure is shows in the Fig. 7 . Cu-based perovskites act as high-temperature superconductors, and La–Ba–Cu–O perovskite was first reported. The presence of Cu in B-site is essential for the superconductivity and various superconducting oxides can be manufactured with different A-site ions. Furthermore, some perovskites exhibited great electronic conductivity similar to that of metals like Cu. LaCoO3 and LaMnO3 are examples of perovskites exhibiting high electronic conductivity, and therefore they are utilized as cathodes in solid oxide fuel cells displaying superior hole conductivity of 100 S/cm. The electronic conductivity of the perovskites can be improved by doping the A-site with another cation, which resulted in increasing the amount of the mobile charge carriers created by the reparations of charges.In the ABO3 form, B is a transition metal ion with small radius, larger A ion is an alkali earth metals or lanthanides with larger radius, and O is the oxygen ion with the ratio of 1:1:3. In the cubic unit cell of ABO3 perovskite, atom A is located at the body center, atom B is located at the cube corner position, and oxygen atoms are located at face-centered positions. The 6-fold coordination of B cation (octahedron) and the 12-fold coordination of the A cation resulted in the stabilization of the perovskite structure. The perfect perovskite structure was a corner linked BO6 octahedra with interstitial A cations. Some distortions may exist in the ideal cubic form of perovskite resulted in orthorhombic, rhombohedral, hexagonal and tetragonal forms. In general, all the perovskite distortions maintaining the A and B site oxygen coordination was achieved by the tilting of the BO6 octahedra and an associated displacement of the A cation. The different perovskite catalyst unit cell structure is shown in the Fig. 8 .Goldschmidt presented much of the early work on the synthetic perovskites and developed the principle of the tolerance factor t, which is applicable to the empirical ionic radii at room temperature. Goldschmidt presented much of the early work on the synthetic perovskites and developed the principle of the tolerance factor t, which is applicable to the empirical ionic radii at room temperature. Where rA is the radius of the A-site cation, rB is the radius of the B-site cation, and rO is the radius of oxygen ion O2−. The tolerance factor can be used to estimate the suitability of the combination of cations for the perovskite structure. It is a real measure of the degree of distortion of perovskite from the ideal cubic structure so that the value of t tends to unity as the structure approaches the cubic form. From the equation, the tolerance factor will decrease when rA decreases and/or rB increases. Based on the analysis of tolerance factor value, Hines et al. solely suggested that the perovskite structure can be estimated. For 1.00 < t < 1.13, 0.9 < t < 1.0, and 0.75 < t < 0.9, the perovskite structure is hexagonal, cubic and orthorhombic, respectively. For t < 0.75, the structure was adopted to hexagonal ilmenite structure (FeTiO3). t = ( r A + r o ) [ √ 2 ( r B + r o ) ] BB ₁ Electro neutrality; the perovskite formula must have neutral balanced charge therefore the product of the addition of the charges of A and B ions should be equivalent to the whole charge of the oxygen ions. An appropriate charge distribution should be attained in the forms of A1+B5+O3, A4+B2+O3 or A3+B3+O3. Ionic radii requirements; r A > 0.090 nm and r B > 0.051 nm, and the tolerance factor must have values within the range 0.8 < t < 1.0. Perovskite exhibited a variety of fascinating properties like ferro electricity as in case of BaTiO3 and super conductivity as in case of Ba2YCu3O7. They exhibited good electrical conductivity close to metals, ionic conductivity and mixed ionic and electronic conductivity. In addition, several perovskites exhibited high catalytic activity toward various reactions. There are some properties inherent to dielectric materials like ferroelectricity, piezoelectricity, electrostriction and pyroelectricity.In solid-state reactions, the raw materials and the final products are in the solid-state therefore nitrates, carbonates, oxides and others can be mixed with the stoichiometric ratios. Perovskites can be synthesized via solid-state reactions by mixing carbonates or oxides of the A- and B-site metal ions corresponding to the perovskite formula ABO3 in the required proportion to obtain the final product with the desired composition. They are ball milling effectively in an appropriate milling media of acetone or isopropanol. Then the obtained product is dried at 100 °C and calcined in air at 600 °C for 4–8 h under heating/cooling rates of 2 °C/min. After that, the calcined samples are grinded well and sieved. Then it was calcined again at 1300–1600 °C for 5–15 h under the heating/cooling rate of 2 °C/min to confirm the formation of single phase of perovskite. Again grinding and sieving was carried out for the calcined samples. The synthesis of BaCeO3-based proton conductor perovskites and BaCe0.95Yb0.05O3− δ was achieved through the previous methodology using BaCO3, CeO2 and Yb2O3 as the starting materials and isopropanol as the milling media.These methods included the sol-gel preparation, co-precipitation of metal ions using precipitating agents like cyanide, oxalate, carbonate, citrate, hydroxide ions, etc., and thermal treatment, which resulted in a single-phase material with large surface area and high homogeneity. These methods presented good advantages such as lower temperature compared to the solid-state reactions, better homogeneity, greater flexibility in forming thin films, improved reactivity and new compositions and better control of stoichiometry, particle size, and purity. Therefore, they opened new directions for molecular architecture in the synthesis of perovskites. Solution methods were classified based on the means used for solvent removal. Two classes were identified: (i) precipitation followed by filtration, centrifugation, etc., for the separation of the solid and liquid phases and (ii) thermal treatment such as evaporation, sublimation, combustion, etc., for solvent removal. There are several factors must be taken in consideration in solution methods like solubility, solvent compatibility, cost, purity, toxicity, and choice of presumably inert anions.This method is built on the assimilation of oxalic acid with carbonates, hydroxides, or oxides producing metal oxalates, water and carbon dioxide as products. The solubility problem is minimized as the pH of the resulting solution is close to 7. An oxidizing atmosphere like oxygen was used during calcination to avoid the formation of carbide and carbon residues. It utilized an aqueous chloride solution with oxalic acid to obtain unique and novel complex compound of BaTiO(C2O4)2.4H2O as a precursor for the preparation of finely divided and stoichiometric BaTiO3.This method is often used due to its low solubility and the possible variety of precipitation schemes. The sol-gel process can be used to produce a wide range of new materials and improve their properties. It presented some advantages over the other traditional methods like chemical homogeneity, low calcination temperature, room temperature deposition, and controlled hydrolysis for thin film formation. BaZrO3 powders in its pure crystalline form can be prepared by the precipitation in aqueous solution of high basicite. LaCoO3 was prepared by the simultaneous oxidation and coprecipitation of a mixture containing equimolar amounts of La(III) and Co(II) nitrates producing a gel containing hydroxide then calcination at 600 °C.Different perovskites were prepared by mixing acetate ions alone or together with nitrate ions with the metal ions salts. La1-xSrxCoO3 with x = 0, 0.2, 0.4, 0.6 was prepared using acetate precursors then calcination at 1123 K in air for 5 h La1- xSrxCo1-yFeyO3 was prepared using iron nitrate and strontium, cobalt and lanthanum acetates then calcination at 1123 K in air between 5 and 10 h.Citrate precursors can be used and undergo several decomposition steps in the synthesis of perovskite. These steps included the decomposition of citrate complexes and removal of CO3 2− and NO3 ¯ ions. LaCo0.4Fe0.6O3 can be prepared by this method, and the mechanism was investigated by thermo-gravimetry, XRD, and IR spectroscopy.The freeze-drying method can be achieved through the following steps: (i) dissolution of the starting salts in the suitable solvent, water in most cases; (ii) freezing the solution very fast to keep its chemical homogeneity; (iii) freeze-drying the frozen solution to get the dehydrated salts without passing through the liquid phase; and (iv) decomposition of the dehydrated salts to give the desired perovskite powder. The rate of heat loss from the solution is the most important characteristic for the freezing step. This rate should be as high as possible to decrease the segregation of ice-salt. Also, in case of multi-component solutions, the heat loss rate should be high to prevent the large-scale segregation of the cation components.This method was applicable to various precursors, including gaseous, liquid, and solid materials. It was applied for the preparation of various ceramic, electronic, and catalytic materials. It presented many advantages in terms of economy, purity, particle size distribution, and reactivity. This method was achieved through two steps: (i) injection of the reactants and (ii) generation and interaction of the molten droplets (with substrate or with the previously generated droplets). The thick film of YBa2Cu3Ox covering large areas was prepared via this approach, and the optimum superconducting oxide phase was obtained by varying the preparation conditions like plasma parameters, substrate temperatures, and film post deposition treatment.A redox reaction, which is thermally induced, occurs between the oxidant and fuel. A homogenous, highly reactive, and nanosized powder was prepared by this method. When compared with the other traditional methods, a single-phase perovskite powder can be obtained at lower calcination temperatures or shorter reaction times. One of the most popular solution combustion methods is citrate/nitrate combustion, where citric acid is the fuel and metal nitrates are used as the source of metal and oxidant. It is similar to the Pechini process “sol-gel combustion method” to a large extent, but in citrate/nitrate combustion, ethylene glycol or other polyhydroxy alcohols are not used. In addition, in citrate/nitrate combustion, the nitrates are not eliminated in the form of NOx, but they remain in the mixture with the metal-citrate complex facilitating the auto-combustion. Iron, cobalt, and cerium-perovskite can be prepared via citrate/nitrate combustion synthesis. In addition, uniform nanopowder of La0.6Sr0.4CoO3− δ was prepared by the combined citrate–EDTA method, where the precursor solution was made of metal nitrates, citric acid and EDTA under controlled pH with ammonia. La0.8Sr0.2Co0.2Fe0.8O3− δ and Sr or Ce-doped La1− xMxCrO3 catalysts were prepared by citrate/nitrate combustion method. Furthermore, the Pechini “citrate gel” process includes two stages: (i) a complex was formed between the metal ions and citric acid, then (ii) the produced complex was polyesterified with ethylene glycol to maintain the metal salt solution in a gel at a homogenous state. This approach presented some advantages like high purity, minimized segregation and good monitoring of the resulting perovskite composition. LaMnO3, LaCoO3, and LaNiO3 were prepared by citric acid gel process producing nanophasic thin films.The microwave irradiation process (MIP), evolving from microwave sintering, was applied widely in food drying, inorganic/organic synthesis, plasma chemistry, and microwave-induced catalysis. MIP showed fascinating advantages: (i) fast reaction rate, (ii) regular heating, and (iii) efficient and clean energy. The microwave preparations were achieved in domestic microwave oven at frequency of 2.45 GHz with 1 kW as the maximum output power. Dielectric materials absorbed microwave energy converted directly into heat energy through the polarization and dielectric loss in the interior of materials. The energy efficiency reached 80–90% which is much higher than the conventional routes. MIP was recently utilized to prepare perovskites nanomaterials reducing both the high temperature of calcination (higher than 700 °C) and long time (greater than 3 h) required for pretreatment or sintering. GaAlO3 and LaCrO3 perovskites with ferroelectric, superconductive, high-temperature ionic conductive and magnetic ordering properties, faster lattice diffusion, and grain size with smaller size were prepared in MIP. The CaTiO3 powders prepared in MIP presented a fast structural ordering than powders dealt in ordinary furnace. Hydrothermal conventional and dielectric heating were utilized to prepare La–Ce–Mn–O catalysts. Hydrothermal MIP leads to formation of La1− xCexMnO3+ εCeO2 (x + ε = 0.2) with enhanced catalytic activity while using the conventional heating methods lead to formation of LaMnO3 + CeO2. Moreover, nanosized single-phase perovskite-type LaFeO3, SmFeO3, NdFeO3, GdFeO3, barium iron niobate powders, KNbO3, PbWO4, CaMoO4 and MWO4 (M: Ca, Ni), strontium hexaferrite and SrRuO3 were prepared in MIP showing finer particles, higher specific surface areas and shorter time for synthesis of single crystalline powders.Perovskites showed a good catalytic activity, which is moderately associated with more surface activity to oxygen decline ratio or oxygen activation that creates from huge amount of oxygen vacancies. It can act like a catalytic converter and cleaning catalyst, etc., for different catalytic environmental reactions [35]. Perovskites containing Cu, Co, Mn or Fe showed excellent catalytic activity toward direct oxidation of CO at high temperature. The LaCoO3, LaMnO3 and BaCuO3 perovskite catalysts showed great catalytic activity for CO oxidation at higher temperatures. The perovskites represents best activity for reaction at high temperatures because the presence of oxygen shortage and easy removal of surface oxygen in the form of reaction product. The addition of smaller amounts of element in perovskite catalyst improved their performances. The Cu0.15Ce(La)0.85Ox catalyst synthesized by wet impregnation method showed that the best activity toward CO oxidation [36]. It fine particles with high surface-to-volume ratio be capable of utilized to decrease the amount of noble metals used. However, the fine particles bad stability in operation conditions mostly to catalyst deactivation. So that it can be used to the showing redox properties to maintain a more dispersion state [37]. Lanthanum (La) is oxidized in the form of various La oxide catalysts with fine metallic particles of La were produced in a radius of 1–3 nm. The LnCoO3 catalyst is known as an intelligent catalyst because of great dispersion state of Ln and excellent stability of perovskite structure. The different properties of perovskites and their catalytic activity are highly affected by the method of preparation, calcinations conditions and A- and/or B-site substitutions [38,39].The doping in perovskite catalysts the catalytic activity, ionic radius, electronic conductivity, physical and chemical properties can be changed for exploitation in different applications. Different cations with various sizes and charges can be hosted in perovskites; thus, many studies can be performed to utilize doped perovskites in CO oxidation [40,41]. The chemisorptions of CO and CH4 over perovskite catalysts are shown in the Fig. 9 . The material characteristics of perovskite oxides mainly related with structural characters were very much affected by structural changes from perfect cubic structure of perovskite catalysts. The synergism effect between the crystal lattice of perovskite and metal ions dissolved in lattice upon doping [42]. It results in an improved redox reaction and best catalytic activity of synthesized perovskite was obtained. A remarkable modifies in transportation and magnetic properties of ABO3 perovskite can be done by doping in the B-site due to an ionic valence effect and/or anionic size effect. The doping in B-site of ABO3 perovskites with transition metals mainly noble metals, the strength of perovskite was enhanced and catalytic activity was improved considerably [43]. In LaMnO3 + CeO2 perovskites with a low surface area (<15 m2/g) the Ce4+ replacement into La3+ sites reduces both cell parameters of rhombohedral unit cell and crystalline domain sizes, since the ionic radius of Ce4+ is lesser than La3+. The selective of CO oxidation in which CO and O2 were totally converted into CO2 and presence of cerium affected the reaction kinetics shifting CO conversion to higher temperatures [44]. The Ce4+ distorted some Co3+ to Co2+ to maintain the charge neutrality within the LaxCe1-XCoO3 structure, as a result decreasing the amount of active Co3+ sites on the LaCoO3 surface and declining the activity for CO oxidation. The charge neutrality would stabilize the total Co3+/O2 on the surface, ensuring high CO2 selectivity for cerium substituted perovskites [45]. The effect of strontium insertion into La0.5Sr0.5CoO3-d on the catalytic performance of CO oxidation was discussed in Table 2 (see Fig. 10).Differently, from Ceria the Sr2+ as a cationic dopant is probable to raise cobalt oxidation state and/or produce oxygen vacancies inside the crystal lattice, it was the oxygen mobility and supply lattice oxygen for CO oxidation on the surface [46]. The complete Sr2+ substitution into the rhombohedral crystalline structure of perovskite which led to declined and extension of unit cell volume, since the ionic radius of Sr2+ (0.132 nm) is superior than La3+ ion. In LaFeO3 molecular oxygen chemisorbs on Fe+ cations as an O2− anion, dissociating to form atomic oxygen (O−) on the iron sites. The CO adsorbs on the surface oxide ions formed a labile species that interacts with adsorbed atomic oxygen, producing carbonates which decompose towards CO2 and oxygen [47]. The manganese promoted La0.7Sr0.3Mn1-XCoXO3 perovskites were investigated as a catalyst in the CO oxidation reactions. Increasing the amount of Mn atoms on the La0.7Sr0.3Mn1-XCoXO3 catalyst surfaces affecting the catalytic behavior: the greater Coo + Mn0 exposition. The higher extension of La0.7Sr0.3Mn1-XCoXO3 phase derived from the perovskite structure, higher the activity and stability of the catalysts. In Zn1−XNiXMnO3 catalyst the complete conversion of CO was obtained at 300 °C [48].This catalyst is highest resistance to carbon deposition among all the catalysts. The rhombohedral structure of perovskite towards metallic Ni0 and hexagonal Mn2O3 phases (for high Zn content, ZnO and NiXMnO3 phases also emerged). The fractional replacement of Ba by Zn in La0.9Ba0.1CoO3 raising the oxidation temperature of perovskite, signifying a more constant structure in reaction conditions which might be stay away from Ba sintering [49]. In LaMn1-xCuxO3+§ perovskites the Cu replacement could enhance the amount of chemisorbed oxygen species over the perovskite, improving the catalytic activity for CO oxidation. The addition of iron (Fe) in LaFe0.8Co0.2O3 lattice the iron valence changed from Fe3+ to Fe4+ improving the catalytic performance [50]. The A series of B site replacements over LaCoO3 perovskite showed that Mn2+, Fe2+, Ni2+ and Cu2+ dopants could get better CO conversion. To modify the characteristics of supported metal catalysts obtained from precursor perovskite under oxidation conditions. The important catalytic reactions made to better comprehend the role of active sites on the perovskite-type oxides [51].The efficiency of perovskite catalysts for reactions with CO molecules is strongly depending upon the chemisorptions process. The discrete reaction mechanisms are steady with the observed kinetics [51,52]. A better device for measuring the activity of perovskite catalysts for CO oxidation is reported the activation energy of the process. Early study represented that the catalyst starting oxidized CO before its oxidized by air, and this is an investigation of a Mars-van Krevelen-type mechanism which has consequently found support [53]. The perovskite oxides frequently exhibit strong electronic and/or magnetic correlations, band gaps and bending, which may affect the mechanism. Various synthesis methods have been presented in Table 2 intended at increasing the surface area mainly mixed oxide and fast synthesis; still the surface areas between 5 and 50 m2/g at most are achieved [54,55]. The macro-porous perovskite catalysts illustrate better catalytic activities for CO oxidation than consequent nanometric sample. Calcination temperature highly affects the crystallization and particle size of perovskite catalyst [56].In the calcination of perovskite at higher temperature raise the crystalline and particle size. Carbon monoxide can be adsorbed either in a linear or bridged form covering respectively over perovskite catalysts. Which structure is formed depends on the chemisorptions conditions and nature of support. The CO adsorbed on perovskite could react with oxygen held by these species. This catalyst able to absorb oxygen at low temperature suggests that the CO oxidation should be done at low temperatures. The activation of surface oxygen vacancy in the perovskite catalysts performance for CO oxidation is properly represents in the Fig. 11 .However, the catalytic activity of perovskite catalyst is quite low in spite of fact that catalysts contain adsorption sites both for CO and O2 adsorption. It causes a result of no dissociative adsorption of oxygen. The reaction mechanism of perovskite catalysts is represents in the Fig. 12 . In stoichiometry of CO oxidation reaction needs the dissociation of oxygen molecules followed by reaction between adsorbed oxygen atom and CO to CO2 is one of the accepted mechanisms for CO oxidation. In this condition, the reaction rate is limited by the dissociation of O2. The molecular adsorption of CO occurs at higher temperatures, which ensures that the appearance of reactive oxygen forms [57,58].The oxygen adsorption occurs mainly in the form of O2 −, while above the calcination temperature of 350 °C the O− species is predominate. The O− ions are highly active and reactivity of superoxide is also high, though much lower as compared to O−. In oxygen species, the CO molecules from gas phase can be directly oxidized [59]. The marsvan krevelen mechanism for conversion of CO over perovskite catalysts is shown in the Fig. 13 . The conversion of CO by the Mars-van Krevelen mechanism would give details the relationship between easiness of catalyst activity and reducibility. Different mechanisms have been suggested for the oxidation of CO over metals and metal oxides. The CO oxidation over metals is thought to follow a Langmuir-Hinshelwood mechanism [60]. The CO2 produced is poorly adsorbed and does not influence the rate substantially, since it's rapidly desorbed to the gas phase. The rate of reaction will be proportional to the total coverage of Oads and COads [61]. (1) O 2 ( g ) → O 2 ( a d ) - → 2 O ( a d ) - (2) C O ( g ) → C O ( a d ) (3) C O ( a d ) + 2 O ( a d ) - → C O 3 ( a d ) 2 - (4) C O 3 ( a d ) 2 - → C O 2 ( a d ) + O ( a d ) 2 - → C O 2 ( g ) + O 2 ( a d ) - (5) O2 + 2∗→2Oads (6) CO +∗ → COads (7) COads + Oads → CO2 + 2∗ The procedure of CO oxidation does not take place as long as the adsorbed molecules of O2 change to the reactive form of oxygen. The variation of activity and the binding energy of perovskite catalysts as a function of tolerance factor for the series of catalysts. The high spin state of perovskite catalysts at the surface may be favorable for the strong chemisorptions of oxygen which accounts for increased activity [62]. As far as catalyst development is concerned, it is critical to discover the structure–activity correlation of catalysts. A Langmuir-hinshelwood mechanism predicts the reactivity of perovskite catalysts in CO oxidation. Low lattice oxygen mobility and kinetic effect of O2 rule out the MvK redox mechanism [63]. Under reaction conditions, the rate was proportional to the O2 pressure and independent of CO pressure. The rate of CO oxidation was done by following either rate of formation of CO2 or, when the CO was intent and rate of losing of CO [64]. The mechanism for CO oxidation over perovskite catalysts shows in the Fig. 14 . The reaction rate was found to be reduced sharply when CO was introduced into the gas phase during the oxidation. In the presence of CO, the reaction was first order in CO and zero order in O2. The CO molecule retained its integrity during the oxidation reaction [65].The number of CO2 molecules adsorbed corresponded to the number of oxygen atoms pre-adsorbed on the surfaces of catalyst. The equivalent concentration of oxygen atoms in the gas phase and on the surface, therefore, heterogeneous exchange reaction was taking places. The mechanisms of CO oxidation on the surfaces of catalysts are a top tactic in nature, therefore the reversible failure and uptake of huge oxygen or for the production and destruction of vacancies reported these systems as attractive oxidation catalysts. The inconsistent oxygen in the perovskite structure is accountable for the unusual performance of these materials. The removal of oxygen from frame works of perovskite structure and possibility of deriving different structural in ideal perovskites catalysts [67–70]. The various mechanisms of CO oxidation and formation of CO2 over perovskite catalysts are shown in the Fig. 14.The mixing of the pollutants gases are constantly measured by an oxygen sensor and the air-to-fuel ratio is tuned consequently by the fuel-control reaction conditions. The kinetics study of CO oxidation over perovskite catalysts at the adsorption and desorption cycle is shown in the Fig. 15 . The performance of perovskite-type oxides convincingly increases with increasing the concentration of available active phase. Therefore the higher performance was obtained on extruded and layered on the structured catalysts containing a more adding of energetic component [71,72].The activity and selectivity of perovskites catalysts in catalytic converter are crucial for CO oxidation reaction. The catalyst deactivation can be divided into six different types: (i) poisoning, (ii) thermal degradation, (iii) fouling, (iv) vapor compound formation (v) vapor-solid reactions and (vi) crushing/abrasion. The lead, sulphur poisoning, carbon formation and sintering is the main cause of catalyst deactivation. The dispersion of active phase rapidly decreases, which is one of the main reason for catalyst deactivation. The catalytic activity of metal support (La2O3) is susceptible to sulphur poisoning, which is one of the most contaminants in catalytic converter exhaust emissions. The substitutions of materials in perovskite catalysts should not influence their activity in reforming reaction; but the changes in structure should remain their resistance to carbon deposition as well as to sulphur poisoning [66]. The promising stability of this catalyst could be attributed to the high mobility of oxygen on the interface between the MnCeOx solid solution and MnOx, which is critical for removing the Cl species produced during CB decomposition. The Ce–Pr mixed oxides, specifically Ce0.5Pr0.5O2, have been reported to exhibit higher stability for the catalytic combustion of 1,2-dichloroethane. Conspicuous catalytic deactivation was, however, induced through the formation of by-products such as C–C coupling products, higher chlorinated compounds and cracking compounds. The sulphur poisoning over perovskite catalysts are shown in the Fig. 16 . The LaMnO3 perovskite oxide catalyst synthesized by co-precipitation was found to exhibit significant activity for the catalytic oxidation of CO emissions. Moreover, its activity was enhanced by A or B site substitution. Because a promising catalyst for industrial applications should present not only high catalytic activity, but also good stability and durability, further study relative to stability and deactivation issues for LaMnO3 is now of the utmost urgency and significance [73,74].The stability of perovskite catalyst could be well recognized to the high mobility of oxygen on the interface of mixed oxides. The dispersion of active phase rapidly decreases, which is one of the main reasons for catalyst deactivation. Chemical poisoning and coke formation are one of the main reasons for catalytic deactivation. The deactivation of perovskite catalyst reduced the surface area of catalysts if available to the surroundings [75,76].The poisoning is due to strong adsorption of feed impurities; therefore, the poisoned catalysts are generally difficult to regenerate. Catalyst restoration is the least desirable approach to defeat catalyst deactivation and restore their activity and selectivity. The catalyst regeneration and reforming processes are mostly classified into three types: semi-regenerative, cyclic and constant regenerative process. Catalyst regeneration is mainly to recover activity defeat due to fast coking with failure of active metal diffusion. The regeneration of perovskite catalysts by various processes is shown in the Fig. 17 . The small amounts of noble metals added in perovskite due to their regenerating mechanism. The Pd in LaFe0.95Pd0.05O3 exists as a solid solution dispersed all over perovskite lattice. In the perovskite catalyst, the oxidizing/reducing cycle maintains the catalytic activity by regenerating the nano-particles and preventing metal nano-particle growth [77,78].Perovskite oxides are versatile materials due to their wide variety of compositions offering promising catalytic properties, especially in oxidation reactions. Perovskites ABO3 are exciting materials for oxidation catalysis as they provide considerable flexibility regarding their compositions and the possibility to implement oxygen vacancies with a selective modification of the cationic sublattice An interesting property of perovskite nanocrystals is their ability to undergo reversible exchange of halide ions (I, Br and Cl). While this property is useful in preparing nanocrystals of different halide composition, it also hinders the use of different nanocrystals or films. Upon contact, two different nanocrystals (e.g., CsPbBr3 and CsPbI3) form mixed halide perovskites. Another interesting approach is to couple perovskite nanocrystals with a different semiconductor to create a hetero structure with Type I or Type II configurations, based on their bandgap alignment. Furthermore, the perovskite structure is tolerant to the formation of anionic and cationic vacancies, which can tune the catalytic properties of the materials. The oxygen activation and dissociation capabilities at perovskite surfaces are strongly correlated to the composition and number of oxygen vacancies. These vacancies can promote the formation of monoatomic oxygen (O−), which would act as the primary type of oxygen in the system. One straightforward approach to determine catalytic activity and oxygen activation capability is the CO oxidation reaction as a prototypical reaction for heterogeneous processes. The reaction only has a single gaseous product, which interacts with metal oxides either strongly or weakly. For Co3O4, no adsorption of CO2 on the surface was found, whereas an adsorption capacity has been reported for Al2O3. Furthermore, this reaction pathway is involved in the total oxidation mechanism of hydrocarbons and oxygenated molecules, which leads to a decrease in selectivity towards valuable intermediates. The effect of Co incorporation into oxides and LaFeO3 perovskite on CO oxidation catalysis has also attracted attention. For example, including only 1% Co has been shown to increase the CO oxidation activity of NiO significantly, but no steady conversion increase with Co incorporation has been observed. On Sr and Co-doped LaFeO3, highly at intermediate Co level were observed for transition metal surface content, oxygen storage capacity, reducibility and methanol oxidation activity. In our upcoming research work, we will perform further mechanistic studies on the CO oxidation on different perovskites catalysts and also test the performances in different oxidation reactions.In perovskite catalysts the partial substitution of cations, which stabilizes unusual oxidation states of metal components and creates anionic or cationic vacancies within the perovskite lattice. The partial substitution of cations can increase the reducibility and metal dispersion of catalyst. The support perovskites on porous materials like a monolith or a usual high surface area material to raising the amount of uncovered perovskite active sites. The reaction intermediates and a mechanistic condition are paramount for fundamental insight into the origin of activity and product selectivity. Structure-function relationships are crucial concept to develop basic guidelines for the design of more active catalysts after the mechanism is sufficiently understood. The surface area of perovskite oxides falls behind into simple metals. In addition, organometallic halide perovskites exhibited efficient intrinsic properties to be utilized as a photovoltaic solar cell with good stability and high efficiency. The reducing of particle size, making perovskites with hierarchical porosity is a promising approach to enhance mass activity and control applications. Nano-perovskites have been utilized as catalysts in oxygen reduction and hydrogen evolution reactions exhibiting high electro-catalytic activity, lower activation energy and high electron transfer kinetics. In addition, some perovskites are promising candidates for the development of effective anodic catalysts for direct fuel cells showing better catalytic performance. The relative ease of preparation, thermal and chemical stability and good catalytic activities of perovskites catalysts offer good performances for environmental pollution.This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.The statements in the paper are properly cited in the manuscript and no additional data is available.The authors declare no conflict of interest.The authors are thankful for the support from all the faculty members and lab in charges of Environmental Engineering Department, Rajiv Gandhi Proudyogiki Vishwavidyalaya, Bhopal, India.

Automobile exhaust contributes the largest sources of carbon monoxide (CO) into the environment. To control this CO pollution, the catalytic converters have been discovered. The catalytic converters have been invented for regulating the CO discharge. There are many types of catalysts have been investigated for CO emission control purposes. Inorganic perovskite-type oxides are fascinating nanomaterials for wide applications in catalysis, fuel cells, and electrochemical sensing. Perovskites prepared in the nanoscale have recently received more attention due to their catalytic nature when used as electrode modifiers. Perovskite catalysts show great potential for CO oxidation catalyst in a catalytic converter for their low cost, high thermal stability and tailoring flexibility. It is active for CO oxidation at a lower temperature. The catalytic activity of these oxides is higher than that of many transition metals compounds and even some precious metal oxides. They represents attractive physical and chemical characteristics such as electronic conductivity, electrically active structure, the oxide ions mobility through the crystal lattice, variations on the content of the oxygen, thermal and chemical stability, and supermagnetic, photocatalytic, thermoelectric and dielectric properties. The surface sites and lattice oxygen species present in perovskite catalysts play an important role in chemical transformations. The partial replacement of cations A and B by different elements, which changes the atomic distance, causes unit cell disturbances, stabilizes various oxidation states or added cationic or anionic vacancies inside the lattice. The novel things disturb the solid reactivity by varying the reaction mechanism on the catalyst surface. Thus, the better cations replacement may represent more activity. There are lots of papers available to CO oxidation over perovskite catalysts but no review paper available in the literature that is represented to CO oxidation.

Currently most energy sources used by our society are based on fossil fuels. Their combustion (coal, oil, and gas), together with large-scale deforestation, is causing massive emissions of greenhouse gases. Given the destructive environmental impact of these gases, effort has focused on the production, storage and transport of renewable energy (wind or sunlight) [1]. A promising technology to address this issue uses renewable energy to produce chemical energy through the splitting of water into hydrogen and oxygen (water electrolysis) [2]. However, the efficiency of the electrolysis process is hampered by the sluggish kinetics of water oxidation to O2, also known as oxygen evolution reaction (OER). This reaction has been described as the bottleneck of the water splitting and understanding its mechanism at the atomic scale could be a first step in addressing this challenge [2]. Many catalysts have been proposed to reduce the overpotential losses for OER and investigated in different pH conditions [3], from acidic (2H2O → 4H+ + O2 + 4e−) to alkaline (4OH− → 2H2O + O2 + 4e−) media. In acidic media, noble metals such as Ru or Ir show promising OER stability and activity. However, due to their limited availability and high price many researchers are seeking alternative catalysts based on earth-abundant elements [3,4,5,6,7,8]. In an alkaline environment, oxides and hydroxides of late first-row transition metals (Mn, Fe, Co, Ni) have been found to have comparable performances to noble metals [3]. In particular, NiFe-based (oxy)hydroxide catalysts are reported to show the lowest overpotential for OER in alkaline conditions (pH 13 and 14) [9], but the synergistic role of Fe and Ni is still under debate.Comparing OER catalysts is complicated by many underlying factors, including differences in electrochemically active surface area, catalyst electrical conductivity, surface chemical stability, surface composition, and reaction mechanism. In this work, we describe our efforts to circumvent these issues by using a combined surface science/electrochemistry approach to develop an atomically controlled model system for the OER on FeNi-based catalysts. Having previously solved the surface structural model for the Fe3O4(001) surface [10] and learned how to judiciously alter this surface by doping with nickel atoms [11], we have prepared well-defined Ni-modified Fe3O4(001) surfaces in ultra-high-vacuum (UHV) with different Fe:Ni ratios and, after characterization with surface science techniques, we have studied their electrochemical performances towards OER using cyclic voltammetry and electrochemical impedance spectroscopy. A significant increase in the OER activity is observed as the Ni content increases, and the optimum composition has an iron fraction among the cations in the top surface layer in the range of 20-40%. These results are in good agreement with literature for the best OER powder catalysts [9]. Furthermore, based on the analysis of the surface morphological changes before and after reaction, together with adsorption capacitance measurements, we propose that the active sites responsible for the formation of the OER precursor are the same on the clean and on the Ni-modified magnetite. Nevertheless, the presence of the Ni on the surface shifts the formation of this precursor to lower overpotential.Our study provides a well-defined model catalyst that is at the same time simple, highly active, and stable under operation conditions, and therefore ideal to be used as model system to gain atomic-scale insights into the complicated OER mechanism.The experiments were performed on a natural Fe3O4(001) single crystal (SurfaceNet GmbH) prepared in UHV by cycles of 1 keV Ar+ sputtering and 900 K annealing. Every other annealing cycle was performed in an O2 environment ( p O 2  = 5 × 10−7 mbar, 20 min) to maintain the stoichiometry of the crystal selvedge. Surface analysis was performed in a UHV system with a base pressure <10−10 mbar, furnished with a commercial Omicron SPECTALEED rear-view optics and an Omicron UHV STM-1. XPS data were acquired using non-monochromated Al Kα x-rays and a SPECS PHOIBOS 100 electron analyser at grazing emission (70° from the surface normal). The same analyser was used to carry out the low-energy He+ ion scattering (LEIS) experiments (1.225 keV He+, scattering angle 137°), an exquisitely surface-sensitive technique. For quantification of LEIS data, we assumed that the concentrations are proportional to the peak areas, which is justified due to the very similar cross sections, electronic structure, and work function of these metals. Ni was deposited using a Focus electron-beam evaporator, for which the deposition rates were calibrated using a temperature-stabilized quartz crystal microbalances (QCM). One monolayer (ML) is defined as one atom per (√2 × √2)R45° unit cell, which corresponds to 1.42 × 1014 atom/cm2. Ni depositions higher than 2 ML were prepared by first depositing 2 ML Ni on the surface at room temperature, followed by mild annealing at 200 °C for 10 min. This causes a transition from Ni being present as 2-fold coordinated adatoms to 6-fold coordinated “incorporated” cations [11], see Fig. 1 ; the procedure was then repeated as many times as necessary to reach the desired coverage.After UHV-preparation and characterization as well as after the electrochemical measurements, the samples were brought to air and imaged using an Agilent 5500 ambient AFM in intermittent contact mode with Si tips on Si cantilevers.Cyclic voltammetry and impedance spectroscopy were performed using a Metrohm-Autolab PGSTAT32 potentiostat and a custom-made electrochemical flow cell (made from perfluoroalkoxy alkane, PFA), mounted to the vacuum chamber. Prior to experiments, the chamber was filled with Ar (99.999%, Air Liquide, additionally purified with Micro-Torr point-of-use purifiers, SAES MC50−902 FV) to ambient pressure. The contact between sample and flow cell was sealed with Kalrez O-rings. Prior to measurements, the electrolyte reservoir was evacuated and ultrasonicated to remove dissolved CO2. The flow cell was filled with electrolyte by increasing the pressure in the electrolyte compartment with Ar to slight overpressure. A glassy carbon counter electrode and a leak-free Ag/AgCl reference electrode (Innovative Instruments Inc.) were used. For impedance measurements, the latter was coupled to a glassy carbon quasi-reference electrode through a 100 nF capacitor. All electrochemical data were corrected for iR u drop; the uncompensated solution resistance R u was determined from impedance Nyquist plots by extrapolating the minimum total impedance in the linear regime between 10 kHz and 100 kHz. All electrochemical potentials are referred to either the measured Ag/AgCl reference electrode E Ag/AgCl or given as the overpotential η, which was determined via the equation η = E Ag/AgCl+E RHE−1.229 V −iR u. E RHE is the potential of the reversible hydrogen electrode (RHE) vs a Ag/AgCl electrode. The potential of the RHE (Hydroflex) was measured before and after the electrochemical measurements to improve consistency of the results. The electrolyte was prepared from level-1 water (Merck Milli-Q, ρ= 18.2 MΩ cm, 3 ppb total organic carbon), and reagent-grade NaOH (50 mass % in water, Sigma-Aldrich). Prior to use, all glassware and PFA parts where cleaned by boiling in 20% nitric acid and copious rinsing with Milli-Q water. Fig. 1a shows a schematic model of the UHV-prepared Fe3O4(001) surface. The surface is oxidized with respect to the bulk Fe3O4 and is not a simple bulk truncation. Specifically, an interstitial tetrahedrally coordinated iron in the second layer (Fetet, light blue in the model) replaces two octahedrally coordinated iron atoms (Feoct, dark blue) in the third layer [10], giving rise to a (√2 × √2)R45° periodicity. All surface Fe is in the 3+ state in the so-called subsurface cation vacancy (SCV) reconstruction, and it is the most stable termination of Fe3O4(001) over the range of oxygen chemical potentials encountered in UHV-based experiments [10].In the lower part of Fig. 1a, a typical STM image of the UHV-prepared Fe3O4(001) surface is shown. Undulating rows of surface Fe atoms appearing as protrusions run in the [110] direction. It is common to observe surface hydroxyl groups OsH (i.e. hydrogen atoms bonding to surface oxygen atoms, which are themselves not imaged) as bright protrusions on the Fe rows. This occurs because the hydroxyl modifies the density of states of the nearby Fe cations, causing them to appear brighter in empty-states STM images [12,13]. Fig. 1a also displays other common defects visible on the clean surface, such as antiphase domain boundaries, which are imaged as meandering line defects, and unreconstructed unit cells, which appear similar to two neighboring hydroxyl groups. These are caused by two additional Fe atoms in the subsurface layer (instead of one interstitial Fe), which again modifies the density of states of the surface atoms [12,14]. It is not possible to image the surface oxygen atoms in STM as they have no density of states in the vicinity of the Fermi level. However, their positions are exactly known from density functional theory calculations and quantitative low-energy electron diffraction (red in model in Fig. 1a) [10].The surface reconstruction makes it possible to progressively modify the magnetite surface and accommodate foreign metal atoms (such as nickel) in specific positions. [11] Following Ni evaporation under the appropriate temperature conditions, it is possible to obtain two different Ni geometries: Ni adatoms 2-fold coordinated to surface oxygen atoms (model in Fig. 1b, green) and incorporated Ni occupying octahedrally coordinated sites below the surface (model in Fig. 1c) [11,15]. Ni deposition at room temperature leads to Ni adatoms in the 2-fold coordination, which are imaged in STM as isolated, bright protrusions appearing between the Fe rows (light blue circles in Fig. 1b). The transition from 2-fold to 6-fold coordination is achieved by annealing the surface at 200 °C for 10 minutes. As the incorporated Ni atoms are in the subsurface, they cannot be imaged directly in STM, but they modify the electronic structure of the nearby Fe cations, making them to appear brighter in empty-state images (red circles in Fig. 1c) [11,15]. Their appearance is similar to the unreconstructed cell discussed earlier (Fig. 1a). Furthermore, the STM image in Fig. 1c shows additional protrusions within the Fe rows (highlighted with yellow circles), which we previously assigned to Ni replacing Fe atoms in the 5-fold-coordinated position in the top surface layer [15].The incorporation of Ni in the vacant subsurface octahedral site is only possible if the interstitial Fetet moves back into the other subsurface octahedral site of the unit cell. The resulting cation rearrangement closely resembles a bulk-truncated Fe3O4(001) surface [11,16], and a (1 × 1) periodicity is observed in LEED. It is possible to recover the clean (√2 × √2)R45° reconstructed surface by annealing to high temperatures, which causes the Ni atoms to diffuse into deep bulk layers.Hereafter, we deal exclusively with the incorporated Ni-doped magnetite shown in Fig. 1c, which resembles the structure of mixed spinel ferrite, i.e., a NixFe3-xO4-like system, suggested to be one of the most active phases in OER [17,18].The XPS spectra in Fig. 2 a shows the Ni 2p region for different coverages after Ni was deposited onto the Fe3O4(001) surface at room temperature and annealed at 200 °C. Five different total Ni depositions are considered: 1 ML (green), 10 ML (purple), 50 ML (blue), 120 ML (pink), and 180 ML (light blue). Corresponding fits for the Ni 2p peaks are shown in Figure S1 in the supporting information.After deposition of 1 ML, a small signal is observed in XPS at 855.5 eV, corresponding to the Ni 2p 3/2 peak [19,9]. This is a higher binding energy than metallic Ni [19], which, together with the strong satellite at ≈862 eV, indicates that the nickel is oxidized. Earlier DFT calculations predicted that incorporated Ni atoms are Ni(II) [11], as in NiFe2O4.As the Ni deposition increases to 10 ML, the Ni 2p3/2 at 855.5 eV increases in intensity, together with the 861.9 eV satellite and the 2p1/2 peak at 873 eV, which are harder to see at lower Ni coverage. These features increase in intensity as the Ni deposition increases up to 50 ML. At even higher Ni load (120 ML), two new signals at 853.1 eV and 870.2 eV emerge, indicating that metallic Ni is present on the surface [19]. At 180 ML Ni doping, , the XPS spectrum changes shape to a peak with only two main features at 853.1 eV and 870.2 eV, indicating that the surface is fully covered with metallic Ni.We imaged the Fe3O4(001) surface before and after Ni-doping using ambient AFM right after removing the crystal from the UHV chamber (Fig. 3 a-d). The corresponding LEED patterns acquired in UHV are shown as insets in each AFM image.The clean Fe3O4(001) surface appears overall flat in ambient AFM, with micrometer-wide terraces separated by step bunches [20] (Fig. 3a). The corresponding LEED pattern exhibits the (√2 × √2)R45° periodicity of the SCV reconstruction [10] (yellow square in the inset). Fig. 3b shows the AFM image of a magnetite surface doped with 50 ML Ni. The large terraces as well as the step bunches observed earlier [20] on the clean magnetite remain visible, suggesting that the doping did not affect the overall surface morphology. Isolated (white) features 0.4-0.6 nm high are visible on the surface. Based on the corresponding line profile (Fig. 3a´´, blue), which shows step heights similar to what is observed in Fig. 3a, we suspect these to be residues originating from dust or carbonaceous species. The LEED pattern in the inset shows that the reconstruction spots are now absent and a (1 × 1) symmetry is observed (blue square), which is known to occur above 1 ML Ni atoms incorporated in the subsurface [11]. Fig. 3c-d show AFM images of magnetite surfaces following doping with 120 ML and 180 ML Ni, respectively. The surface in (c) exhibits a rougher morphology than observed in (a) and (b), with a corrugation of ≈0.5 nm (Fig. 3c´´, lilac). Accordingly, the corresponding LEED pattern shows weaker (1 × 1) spots. Following higher Ni doping, the surface morphology changes considerably (d). Although the step bunches are still visible underneath, the surface appears covered in round features having height of ~2nm (Fig. 3d´´, lilac). Based on the XPS data showed in Fig. 2a, we assign these features to metallic Ni clusters. The corresponding LEED pattern shows very weak (1 × 1) spots with a high background, indicating an increasing fraction of the surface covered by structures with no well-defined crystallographic relationship to the substrate, in agreement with the presence of metallic agglomerates on the surface.A quantitative measurement of the surface composition, given as the Fe:Ni ratio for each Ni modified surface can be obtained with LEIS measurements (Fig. 2b-e). The clean surface exhibits a LEIS peak centered at 910 eV (Fig. 2b), corresponding to the surface Fe atoms. Following 10 ML Ni doping, the LEIS signal is broader and shifts to higher kinetic energy KE (Fig. 2c, purple). This peak can be well fitted by a (slightly shifted) peak from the surface Fe and an additional component at 931 eV corresponding to the Ni (Fig. 2c, green and blue respectively). By comparing the area of the Fe and Ni contributions we can estimate an Fe:Ni top surface ratio on the 10 ML Ni-doped surface of 55:45. Similarly, we calculate that the surfaces following 50 ML and 120 ML Ni-doping show Fe:Ni ratios of 40:60 and 15:85, respectively. At higher Ni-doping (180 ML) the whole surface is covered in metallic Ni particles, which makes it difficult to use LEIS to quantify the Fe:Ni surface ratio. Therefore, we restrict ourselves to the coverage regime prior to the formation of metallic Ni clusters. Fig. 2b-e also shows how the surface oxygen peak (centered at ~470 eV) evolves as a function of the Ni doping. The intensity of the surface oxygen peak seems to remain constant as the Fe:Ni ratio decreases down to 40:60. Differently, a clear decrease in the oxygen intensity is observed for the surface with lower Fe:Ni ratio (15:85). We can speculate that this behavior correlates with the presence of some metallic Ni on top, as observed in the XPS in Fig. 2a, pink.Importantly, no systematic change in consecutive scans was observed, which rules out substantial damage to the surface by He+ sputtering during LEIS measurements. In what follows, we will use the LEIS-determined Fe:Ni ratio to refer to our model catalysts.The electrochemical performance of the clean and Ni-doped Fe3O4(001) surfaces was investigated using cyclic voltammetry. The overpotential required to reach a given current density is a key catalytic parameter to compare several catalysts and to estimate the energetic efficiency of integrated (photo-) electrochemical water splitting devices [3]. The cyclic voltammograms (Fig. 4 a) were acquired in 1 M NaOH under Ar with a scan rate of 10 mV s−1 after cycling the electrode until a stable OER current could be observed on two subsequent CVs. Data corresponding to the surfaces imaged in Fig. 3a-d are shown, as well as for surfaces with an Fe:Ni ratio of 98:2 and 55:45. Furthermore, CVs collected before and after electrochemical impedance spectroscopy (EIS) measurements - described later in section 3.4 - up to 1mAcm−2 (see Figure S2) showed that our catalysts are stable over the time range of our experiments (typically 5-9 hours).The clean Fe3O4(001) surface shows an overpotential of 597 mV at a current density of 5 mAcm−2 (Fig. 4a, black), and the surface with an Fe:Ni ratio of 98:2 (green) exhibits similar performance. As the Ni content in the subsurface increases, higher activity towards OER is observed. The OER overpotential decreases by ~110 mV when the Fe:Ni ratio is 55:45 (purple), and reaches ~340 mV vs RHE when the Fe:Ni ratio is 40:60. A higher Ni load (Fe:Ni = 15:85, pink) results in a similar activity as the surface with Fe:Ni ratio of 40:60. Additionally, the surface with an Fe:Ni ratio of 15:85 exhibits a pair of anodic and cathodic peaks at 1.369 and 1.311 V vs RHE respectively (pink, inset in Fig. 4a), consistent with the reversible oxidation of Ni(II) to a higher oxidation state (III), as it is reported for the case of the nickel hydroxide/oxyhydroxide couple (Ni(OH)2/NiOOH) [21]. It can also be observed that the charge (peak area) of this peak increases with cycling, indicating the growth of a thicker Ni oxide film on top of the Fe3O4(001) surface. These observations suggest a change in the Fe:Ni ratio at the surface following electrode cycling. When only metallic Ni is present on the as-prepared sample, an increase in the overpotential of ~88 mV is observed (Fig. 4a, light blue). A corresponding increase in the charge of the Ni(OH)2/NiOOH peak is observed, as well as anodic shifts of 170 mV and 130 mV for the anodic and cathodic peaks respectively. A similar anodic shift of the Ni peak has been observed with increasing Fe:Ni ratio in the NiOOH phase either by co-deposition of Fe during the film synthesis [22,23] or by incorporation of Fe impurities from the electrolyte into NiOOH electrodes [24]. Moreover, the charge of the Ni(OH)2/NiOOH peak remains constant with cycling, indicating a saturation of the surface with nickel (oxy)hydroxide.As a comparative metric of activity, Tafel plots are also shown (Fig. 4b). The determination of Tafel slopes can help elucidating the rate-limiting step of a mechanism, but their analysis is particularly difficult in the case of multiple electron-proton transfer reactions such as OER [3]. The clean and low Ni-doped (Fe:Ni = 98:2) surfaces display values of 92 mV/dec and 88 mV/dec respectively, whereas the Ni-doped Fe3O4(001) surfaces with a Ni load of 50-85% all show similar values in the range 50-61 mV/dec.In Fig. 4c we plot the overpotential values and the Tafel slopes showed in Fig. 4a-b as a function of the surface Fe:Ni ratios. Interestingly, the lowest OER overpotential values are obtained for the catalysts with a surface Fe:Ni ratio between 15-40 %, in agreement with what is reported in literature for the best OER powder catalysts [9]. Furthermore, the Tafel slopes fall in the same range as observed for NiFe (oxy)hydroxide catalysts, which typically vary between 25 and 60 mV/dec [9], which could point towards a similar OER reaction mechanism [21].To check whether catalyst aging in electrolyte affects the activity, we performed cyclic voltammetry on the same surfaces after leaving the Ni-doped electrodes for three days in electrolyte. Fig. 5 a-b show CVs of the surfaces with an Fe:Ni ratio of 40:60 (blue), 15:85 (pink), and a sample with metallic Ni clusters (light blue); the dashed curves show the performance after aging. The aged samples show a decrease of the OER overpotential by ~20-100 mV, in good agreement with what has been observed for powder catalysts prepared by wet chemistry [9,25]. Interestingly, the surface with an Fe:Ni ratio of 15:85 is similarly active to the one with Fe:Ni ratio of 40:60 when freshly prepared, but shows a much lower onset of the overpotential after aging. This observation indicates a profound structural difference in the two catalysts, despite the similar performance at first. Fig. 5b shows a magnification of the capacitive regions of the CVs. The Ni-doped magnetite with metallic Ni at the surface (light blue) shows an anodic oxidation peak before OER onset and subsequent cathodic reduction in the backward scan direction. On the surface with an Fe:Ni ratio of 15:85 (pink), these peaks evolve upon cycling and aging, both in terms of charge as well as shift in overpotential. However, this effect is not so marked in the case where the whole surface is covered with metallic Ni clusters, where only a (slight) shift in potential is observed (blue). The interpretation of the redox behavior is in general very difficult due to possible formation of electrically disconnected domains upon cycling because of the different conductivity of the oxidized and reduced phase [26]. Fig. 5c shows the comparison of the Tafel plots for the surfaces in (a). The aged surfaces show Tafel slopes values in the range 43-62 mV/dec range, similarly to the freshly prepared catalysts (Fig. 4b). Fig. 3a´-d´ shows the AFM characterization of the surfaces imaged in Fig. 3a-d after OER and three days aging in electrolyte. Before imaging, each surface was rinsed in milli-Q water several times, for several minutes and blow-dried using a gentle Ar flow to minimize the presence of salt residue from the electrolyte.The morphology of the clean Fe3O4(001) remains unchanged after OER (a´), and shows a smooth appearance with the wide terraces and step bunches still visible, in agreement with earlier stability tests[20]. The presence of small particles (white) is associated with residue from the electrolyte. Fig. 3b´ shows the AFM image of the surface imaged with initially 40:60 Fe:Ni (Fig. 3b) after resting in electrolyte for three days, followed by cycling the electrode until a stable current was observed (Fig. 5). The terraces and step bunches remain visible underneath, but white features of irregular shape and height between 1-2 nm are now common on the surface (blue line profile in Fig. 3b´´). Fig. 3c´-d´ show AFM images of the 15:85 and metallic Ni surfaces, respectively, after the electrode has been exposed to the electrolyte for three days and cycled until a stable OER current was observed (Fig. 5). Their morphologies appear similar in AFM. Due to the appearance of protrusions with 3-7 nm (line profile in Figure 3 c´´, blue) and 4-8 nm high (line profile in panel d´´, blue), it is almost impossible to discern remainders of the original surface morphology consisting of flat and wide terraces. Since the density and height of the protrusions increases with Ni content, they likely consist of a Ni-(oxy)-hydroxide phase, grown from pre-existent metallic Ni upon electrochemical cycling [25], in agreement with equilibrium potential−pH diagrams (i.e. Pourbaix diagrams) that show NiOOH as the predominant species in neutral-to-basic aqueous solutions at OER potentials [25].Electrochemical impedance spectroscopy (EIS) measurements were performed on the Ni-doped model catalysts electrochemically investigated in Fig. 4. In the OER region the EIS Nyquist plots (Figure S3a) exhibit two relaxation processes characterized by two semi-circles that can be assigned to two capacitances while the phase in Bode plots (Figure S3b) exhibits two maxima eventually merging into a broad peak. This impedance behavior is consistent with previous measurements on metal transition oxides and perovskites during the OER. [27,28,29] The EIS response can be modelled by the equivalent circuit (EC) shown as an inset in Fig. 6 a with a double-layer capacitance (Cdl) in parallel with the combination of a polarization resistance (Rp) and an adsorption pseudo-capacitance (Cads) in parallel with a resistor Rs. The Cdl element accounts for the charging of the electrified interface. Cads models the accumulation of an adsorbed intermediate involved in the rate-limiting step of the OER. The sum of the resistive elements Rs and Rp bear a physical meaning as the zero-frequency electron transfer resistance defined as Rf = Rp + Rs, i.e., the slope of the steady-state polarization curve after Ohmic-drop compensation. RΩ represents the electrolyte resistance. It has to be noted that both capacitors were modeled as constant phase elements (CPEs), defined as Z = C n = 1 − 1 ( j ω ) − n , where C n = 1 − 1 is the impedance of the capacitor without frequency dispersion, i.e., if the coefficient n = 1 which is the case for a perfect capacitor. The interpretation of the CPEs dispersion coefficient n is varied and complicated; its origin has been attributed to surface roughness, inhomogeneities, or inhomogeneous adsorption of ions [30]. In the double-layer region, prior to the onset of the OER, we will show in a separate work that the impedance response of the single-crystal magnetite electrode has to be modified by adding a Warburg element in series with Cads corresponding to a diffusion impedance that we attribute to electrolyte cations intercalating into the oxide (Figure S4a). Of interest in this work is the impedance response in the OER region.All the surfaces investigated in this work, with the exception of the one fully covered by metallic Ni clusters (light blue), show a roughly constant double layer capacitance values in the 10-25 μFcm−2 range prior to the OER onset (Fig. 6a). The exponent of the CPE element used for the fitting was equal to 1 in the double-layer region (Figure S4d) and diverged from 1 at high current densities or when Ni is exposed such that Ni(OH)2 is oxidized to NiOOH. These values are comparable to a Cdl observed on metallic single crystals, suggesting that our catalysts have a perfect capacitor-like behavior. Fig. 6c shows the value of this capacitance as a function of the Ni content: Cdl slightly increases from 10 to 15 μF cm−2 as the Ni loading increases, but a higher value is observed in the case of the surface fully covered with Ni metallic clusters (180 ML). The higher Cdl values observed for this surface may be explained by the formation of an irregular Ni(OH)2 layer upon oxidation of the metallic Ni by contact with the electrolyte. In this way, more active surface area is exposed to the electrolyte and polarized, leading to a higher Cdl.The adsorption capacitance plot in Fig. 6c shows that the Ni-doped Fe3O4(001) surfaces display a peak with similar Cads values independent of the Ni doping level, which however shifts to lower overpotential as the Ni load increases (Fig. 6d). The surface fully covered with metallic Ni clusters appears to develop two additional capacitance peaks (Fig. 6c). The overlay of the corresponding CV and Cads in Figure S3e, shows that the additional initial (pre-)peak is observed at the same potential as the Ni(OH)2 oxidation peak.The group of Bandarenka [31,32] associated the observation of peaks or increase in Cads to the adsorption of OER reaction intermediates and reported them for various transition metal oxides. These observations suggest that the formation of the intermediate species before the onset of the OER involves similar mechanisms for pure and Ni-modified magnetite. This is also supported by the fact that value of Cads retains similar values at the maximum of the peak. From the capacitance data in Fig. 6a and c we can draw the following conclusions: (i) given that the initial Cdl values hardly vary with Ni loading, there is no significant increase in electrochemically active surface area, and the catalytic effect of Ni shown in Fig. 4 cannot be ascribed to an effective enhancement of the surface area; (ii) the fact that a similar peak in Cads is observed for all surfaces, also the one where Fe is expected to be the only active site (98:2), would be in agreement with the commonly held view that Fe is the active site in NiFe catalysts, but that it becomes more active in an Ni environment. The presence of two peaks in the EIS of the metallic Ni-decorated surface if not a noise effect can be interpreted as two types of adsorbates on Ni (and perhaps Fe) sites that are accessible due to the porosity and layered structure of Ni films, providing access to active sites down to 5 nm in depth [33].The experimental data acquired on clean and Ni-doped Fe3O4(001) surfaces show that Ni doping enhances the OER activity of magnetite. Electrochemical voltammetric responses, in combination with surface sensitive techniques, suggest a strong dependence of the OER activity on the atomic structure of the surface exposed to electrolyte. In particular, LEIS measurements indicate that the catalyst with the best OER performances, with an overpotential of 340 mV vs RHE at 5 mAcm−2, exhibits a surface Fe:Ni ratio of 40:60.In order to shed light on how the presence of Ni affects the magnetite atomic surface structure-activity relationship, the following observations have to be considered:We have previously shown that following 1 ML Ni-doping and subsequent mild annealing at 200°C, the Ni atoms fill all the vacant sites in the Fe3O4(001) subsurface, resulting in neighboring Fe and Ni in the second surface layer [8]. The voltammetric response of this surface (green, Fig. 4a) shows no improvement in the OER activity compared to the clean magnetite. Corresponding LEIS measurements (see supplemental material, Figure S6) suggest that this surface exhibits a surface Fe:Ni ratio of 98:2, confirming that almost no Ni is present in the outermost surface layer. These results suggest that the presence of subsurface Ni is insufficient to improve the OER activity. Based on our XPS and LEED data, we propose that modification of the Fe3O4(001) surface with a Ni load > 1 ML leads to the formation of a multilayer mixed ferrite spinel oxide with a structure similar to NixFe3-xO4-like systems. Now the model catalyst exposes both, Fe and Ni atoms in the outermost surface layer as seen from LEIS. In these conditions, the OER activity increases, reaching a maximum when the surface exposes an optimum Ni content of 60-85%. XPS and LEED suggest that the structure of this surface stays characteristic of mixed spinel up to a Ni doping corresponding to a surface Fe:Ni ratio of 40:60. Higher Ni doping results in the formation of metallic Ni clusters, which compromise the spinel long-range order, leading to a loss of atomic control without substantial further enhancement of the activity and, eventually, a decrease in the OER activity when Fe is no longer accessible.Our AFM results suggest that the surface prepared with an Fe:Ni ratio of 40:60 appears stable after OER, albeit with some new features, 1-2 nm high, scattered all over the surface. In contrast, the surfaces with higher Ni loads show the growth of a new phase, which increases in volume and roughness (effective surface area) as the metallic Ni concentration increases. This suggests the growth of a new phase on top of the doped magnetite surface. On the basis of our XPS results, as well as earlier studies [25,34,35], we interpret this phase as the growth of Ni-(oxy)-hydroxide. Similar phases have been also observed on powder Fe-Ni based catalysts, and have reported in literature to affect the catalytic activity towards OER [9]. In particular, Burke et al. [25] observed that electrochemical cycling leads to a transformation from nano-crystalline NiOx films to a layered (oxy)-hydroxide that correlates with an increase in OER activity. Similarly, Deng et al. [35] monitored the dynamic changes of single layered Ni(OH)2 using in situ electrochemical-AFM, and observed dramatic morphology changes already after one linear voltammetry sweep, as well as a direct relation between increase in OER activity and increase in volume and redox capacity of the layered oxy-hydroxide phase. Our results are, however, are not entirely in agreement with these observations. The increase in volume and surface area of the hydroxide phase does not correlate directly with our catalysts’ activity: the surface with the highest amount of the Ni-(oxy)-hydroxide phase and redox capacity is ≈ 200 mV less active than the (almost) flat surface with Fe:Ni ratio 40:60. At this Ni surface concentration, we do not observe in AFM (Figure 3b´) the growth of the new phase covering a large fraction of the surface, but only some scattered features. Nevertheless, the activity of this surface (expressed by the overpotential, Fig. 5a) is close to the optimum. This clearly indicates that the layered Ni-(oxy)-hydroxide is not the active phase in our catalyst.It is also important to mention that the activity exhibited by the surface prepared with a Fe:Ni ratio of 40:60, with an overpotential of 340 mV vs RHE is comparable to values reported for OER on (Fe)Ni based catalysts [34-40]. For comparison, the overview in Table 1 shows a selection of some of the best OER catalysts based on Ni-Fe oxides reported in literature. The lowest overpotential values measured on these catalysts at 5 mAcm–2 vary typically in the 210 - 347 mV vs RHE range (in 1 M KOH or NaOH electrolyte). Similar overpotential values were also obtained from our surface prepared with a higher Ni load (Fe:Ni = 15:85).When comparing the latter surface with the one having an Fe:Ni ratio of 40:60 after electrochemical cycling and subsequent aging for three days in electrolyte, a different activity trend is observed (dashed lines in Fig. 5a). On the one hand, both surfaces show a significant increase in activity following voltammetric cycling and aging, in agreement with previous studies [3,35]. On the other hand, their activity does not increase in the same way. Surprisingly, the surface with metallic Ni shows a much lower onset of the OER overpotential than the one with an Fe:Ni of 40:60, despite the similar performances when freshly prepared. This surface is by far the most active with an overpotential of 247 mV vs RHE. However, it has to be taken into account that this surface, being characterized by the presence of a large fraction of metallic Ni in the as-prepared state, shows neither a well-defined spinel structure nor any other ordered structure over most of the surface and, therefore, cannot serve as a model system. Since one of the scopes of this work is to propose a working model system for the understanding of the OER mechanism, a compromise between activity and the ability to preserve atomic control has to be made. In this regard, the surface with a Fe:Ni ratio of 40:60, very well defined, stable and highly active, fits the criteria to be used as model catalyst.Finally, the analysis of the Tafel plots and adsorption capacitance measurements can help extracting information to identify the OER active sites. Our Ni-modified magnetite surfaces show similar absolute Tafel slopes values (Figs. 5b and 6c) in the 43-62 mV/dec range, independent of the degree of Ni doping for Fe:Ni ratios down to 15:85. Furthermore, the clean and the Ni-modified surfaces show similar maximum values of the adsorption capacitance before the OER onset. These values are associated to the appearance of the OER precursors [31,32] and the shift to lower overpotentials as the Ni doping increases and finally reaches a steady value with the optimal Ni content (Ni content ≈ 60-80%).To explain these observations, we propose the following scenario: the intermediate species that forms on the surface before the onset of the OER might be the same on the clean surface as well as on the Ni- modified one, indicating Fe as the active sites. Accordingly, the right amount of Ni in the spinel surface does not cause the formation of intermediates but facilitates it. Similar conclusions have been proposed by Bell and co-workers who used DFT to compare the OER activity of pure and Fe-doped γ-NiOOH and of pure and Ni-doped γ-FeOOH catalysts [43]. They showed that pure γ-NiOOH adsorbs the OER intermediates too weakly and pure γ-FeOOH too strongly. They found a considerable increase in activity for Fe sites that are surrounded by Ni next-nearest neighbours in both γ-NiOOH and γ-FeOOH. Similar results have also been obtained by Klaus et al. who, on the basis of turnover frequency (TOF) calculations, proposed Fe atoms as the OER active sites in Fe-doped NiOOH catalysts [24].Nevertheless, despite the fact that the OER mechanism on NiFe-based catalysts is still unclear together with several fundamental open questions such as the clear identification of the rate limiting step, our results tend to confirm that the OER intermediates are located on Fe sites, the surrounding Ni having a promoting effect on the latter. Furthermore, a deep understanding of the observed electrode aging effect on the OER activity, following long exposure of the material to the electrolyte, remains open, but reveals the importance of the nature of the electrolyte and its interaction with the material.Moreover, it should be pointed out that the use of a single crystal enables an accurate determination of the electrochemically active surface area (ECSA) of these materials and provides reference values for the double-layer capacitance and adsorption capacitance on Fe-Ni based catalysts. The double-layer capacitance values are slightly affected by the Fe:Ni ratio and this should be taken into consideration for further determination of the ECSA of such electrodes [31,32]. Additionally, our results point out that, beyond the Fe:Ni ratio, the nature of the interface (spinel or separated NiOOH/Fe-Ni spinel) significantly affects the capacitance of the interface and its use as a reference for ECSA determination could be compromised.The high intrinsic OER activity of mixed Fe-Ni oxides motivated our efforts to make further steps in the understanding of the fundamental roles of Fe and Ni in OER catalysis.In this work, we show a combined surface science/electrochemistry approach for the preparation of well-defined Ni-modified Fe3O4(001) surfaces and the investigation of their electrochemical performances with respect to OER. We have found that the surface prepared with an Fe:Ni ratio of 40:60 shows a performance comparable to those of the best powder catalysts reported in literature, and still maintains a well-defined structure. Being at the same time simple, highly active, and stable under operation conditions, this surface is an ideal candidate to serve as a working model system to gain atomic-scale insights into the complicated OER mechanism. Whereas a Ni-based phase, probably a Ni (oxy)hydroxide covers all of the surface at high Ni coverage, the highest activity is observed when the Ni-modified Fe3O4(001) surface is still accessible, indicating that this surface is essential for the reaction. Electrochemical impedance spectroscopy suggests that on our Fe-Ni catalyst, the active site for the OER is located on Fe atoms at the surface regardless of the Ni:Fe ratio in the structure, suggesting that the Ni does not cause the formation of intermediates but facilitates it.Putting our results in the context of future perspective, a well-defined model system such as the Ni-modified Fe3O4(001) presented in this work is desirable to address the fundamental aspects that are still controversial. With a limited variety of possible adsorption sites and being accessible to methods benefitting from on single-crystal surfaces, this model surface could thus be used for further investigations on the exact nature of the adsorbates involved in the rate limiting step, using in-situ surface science techniques, to shed more light on key parameters to improve the stability and activity of amorphous catalysts used in water splitting devices. We also believe that the good agreement of our results with what is reported in the literature for powder or amorphous catalyst makes our model surface worthwhile to be used as a model to guide future computational studies.Francesca Mirabella: Conceptualization, Methodology, Data Curation, Writing- Original draft preparation; Matthias Müllner: Conceptualization, Methodology, Data Curation; Thomas Touzalin: Conceptualization, Methodology, Data Curation; Michele Riva: Reviewing and Editing; Zdenek Jakub: Reviewing and Editing; Florian Kraushofer: Reviewing and Editing; Michael Schmid: Reviewing and Editing; Marc T.M. Koper: Supervision, Reviewing and Editing; Gareth S. Parkinson: Supervision, Reviewing and Editing, Ulrike Diebold: Supervision, Reviewing and Editing.None.This work was supported by the European Union under the A-LEAF project (732840-A-LEAF), by the Austrian Science Fund FWF (Project ‘Wittgenstein Prize, Z250-N27), and by the European Research Council (ERC) under the European Union's HORIZON2020 Research and Innovation program (ERC Grant Agreement No. [864628]).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2021.138638. Image, application 1

Electrochemical water splitting is an environmentally friendly technology to store renewable energy in the form of chemical fuels. Among the earth-abundant first-row transition metal-based catalysts, mixed Ni-Fe oxides have shown promising performance for effective and low-cost catalysis of the oxygen evolution reaction (OER) in alkaline media, but the synergistic roles of Fe and Ni cations in the OER mechanism remain unclear. In this work, we report how addition of Ni changes the reactivity of a model iron oxide catalyst, based on Ni deposited on and incorporated in a magnetite Fe3O4(001) single crystal, using a combination of surface science techniques in ultra-high vacuum such as low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS), and scanning tunneling microscopy (STM), as well as atomic force microscopy (AFM) in air, and electrochemical methods such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in alkaline media. A significant improvement in the OER activity is observed when the top surface presents an iron fraction among the cations in the range of 20-40%, which is in good agreement with what has been observed for powder catalysts. Furthermore, a decrease in the OER overpotential is observed following surface aging in electrolyte for three days. At higher Ni load, AFM shows the growth of a new phase attributed to an (oxy)-hydroxide phase which, according to CV measurements, does not seem to correlate with the surface activity towards OER. EIS suggests that the OER precursor species observed on the clean and Ni-modified surfaces are similar and Fe-centered, but form at lower overpotentials when the surface Fe:Ni ratio is optimized. We propose that the well-defined Fe3O4(001) surface can serve as a model system for understanding the OER mechanism and establishing the structure-reactivity relation on mixed Fe-Ni oxides.

Recently, the study of transition metal oxide (TMO) nanostructures, specifically ZnO nanostructures on solid metallic substrates has taken center stage due to advantages, such as high electrical conductivity, better adsorption of reactant on substrates, reusability, and easy retrievability [1–3]. Various solid substrates such as silicon [4,5], glass [6], polyethylene terephthalate-indium tin oxide (PET-ITO) [7,8], brass [9], aluminum [10], and stainless-steel [11] mesh have been utilized for the growth of ZnO nanostructures by different techniques. Graphene has also been used for the precipitation of TiO2 -based photocatalyst [12]. Moreover, there has been a recent reports of growth on tubular substrate having higher surface-area-to-volume ratio, making it suitable for various applications [13,14]. The broad band gap of ZnO (3.2 eV) makes it efficient only in a narrow range of solar spectrum; however, the narrow-band-gap semiconductors like CuO (2.0 eV) demonstrate superior photo response in a wide solar spectrum. However, their photocatalytic efficiency is hampered by the recombination of electron-hole pairs on the semiconductor. Hence, in the past several years, coupling of a broad band gap semiconductor like ZnO with narrow band gap semiconductors, like CuO, Cu2O, Fe2O3, WO3 has taken center stage [15–17].Although these nanocomposite structures have shown great potential in the field of photocatalytic degradation as well as antibacterial activity, it has been a challenge to stabilize these nanostructures on metallic substrates. Therefore, the development of composite metal oxide nanostructures anchored over thin metallic films is strongly desired. Besides, the corrosion and wear resistance of metallic films along with their utilities in electronic industries, pipeline, heat conductors, and heat exchangers make them useful substrates in tubular form.The mixed oxide system with n-type ZnO and p-type CuO offers some interesting properties. It has been developed through various fabrication processes, such as hydrothermal-thermal oxidation [18], hydrothermal/sonochemical [19], plasma assisted synthesis [20], SILAR (successive ionic layer adsorption and reaction) [21,22], and chemical bath deposition [23,24]. Hydrothermal technique has also been utilized to manufacture other heterostructures, such as ZnO-TiO2 [25,26] and novel composite, of BiSbO4 and BiOBr [27]. However, most of these processes require high processing or annealing temperatures and repeated cycles. In contrast, electrodeposition is garnering attention for the synthesis of metal oxide thin films because of its ability to engineer the size and morphology of the nanostructures through control of its parameters like electrolyte concentration, current density, electrode distance [28] etc. In addition, electrodeposition as a low-cost, simple and low-temperature process makes large scale deposition possible on a range of substrates. The electrochemical route has also been reported to enhance the acceptor level in nanostructures because of the externally applied potential [29]. A great number of studies have involved electrochemical deposition for the development of unary oxides such as TiO2, ZnO, WO3,Cu2O etc. [30–32]. However, little literature is available for the co-electrodeposition of binary oxides on metal substrates, though substrate-based growth of oxide nanostructures addresses the challenge of retrieving photocatalyst existing in powdered-form from the water medium. Moreover, template or substrate-based growth renders larger surface area for catalytic reaction and enhances the dispersion of metal-oxide nanostructures. Metal-based substrates like Ni are particularly useful owing to their magnetic and anti-corrosion properties. Some recent research have developed ZnO/CuO nanostructures on Cu substrates through a two step process [33,34]. Jung etal fabricated ZnO-CuO nanowires on a stainless steel mesh, but the mesh structure posses the drawback of transmitting light thereby resulting in a loss of net photon conversion for a given intensity of light [35]. Also, the photodegradation time for various dyes has been invariably long with the use of these substrates [36,37]. Therefore, the present research has presented the fabrication of ZnO/CuO nanocomposite both on nickel and copper tubular thin film substrates through the economic and environment-friendly technique of electrodeposition. Moreover, metal oxide nanoparticles such as CuO and ZnO have piqued the interest of researchers due to their potential use in antibacterial applications. Some evident benefits include low costs and low toxicity, high extraction performance, long-acting antibacterial capabilities, and quick adsorption rates for contaminants [38,39]. Both co-electrodeposition of ZnO and CuO and step-by-step deposition of ZnO decorated CuO nanostructures have been undertaken for a comparison of their morphologies, properties, photocatalytic and anti-bacterial performances. The challenges of improvement in surface area for the catalytic substrates and reduction in the degradation time are addressed. Furthermore, the developed tubular photocatalytic substrates are characterized and utilized for dye degradation and reduction in bacterial growth.Nickel sulphate (NiSO4·6H2O),Copper sulphate (CuSO4·5H2O), Nickel chloride (NiCl2·6H2O), Boric acid (H3BO3), zinc nitrate hexahydrate (Zn (NO3)2·6H2O), hexamethylenetetramine (HMTA, (CH2)6N4), potassium hydroxide (KOH), sodium nitrate (NaNO3) and acetone are among the chemical reagents employed to fabricate the metallic tubular films and the subsequent development of nanostructures. The above-mentioned chemicals were obtained from Central Drug House (CDH), Delhi, India while Isopropyl alcohol (IPA) was purchased from Merck Limited, Mumbai. The compounds were utilized without additional purification. All the aqueous solutions were made using deionized water. For dispersion and solution preparation, ultrasonication and mechanical stirring were utilised. The experimental work employed a hot plate with a temperature control unit and a magnetic stirrer (IKA RCT Basic model, range: 0/50–1700 rpm, 50–380 °C). The electroforming process for the manufacturing of thin walled tubes as well as the pulse deposition of ZnO nanostructures employed a pulse generator (Scientific SM5035, range: 20 mHz- 20 MHz). The following steps were followed for the preparation of metallic film and ZnO nanostructures.The fabrication of the nickel and copper tubes was materialized through electroforming using a set-up designed and developed by retrofitting of a micro-drilling equipment. The novel arrangement made the micro-fabrication of hollow tubular structures possible by providing controlled rotation to the cathodic mandrel. Being an additive manufacturing technique, electroforming involves the building up of material atom by atom on a pre-shaped cathode, that is a negative replica of the component to be manufactured. Ni and Cu wires were used as anodes and aluminum mandrels of diameter 3 mm as the cathodic mandrels. The deposition process for nickel tubes was carried out in an electrolyte cell containing 600 ml of Watts bath having a composition of 300 g/L of NiSO4·6H2O, 30 g/L of NiCl2 ·6H2O and 35 g/L of H3BO3. The bath temperature and pH value were kept at 4.5 and 54 °C, respectively. The hot plate was utilized to both regulate the temperature and agitate the bath. The results of a previous set of experiment [40] were used to optimize and choose the parameters (duty cycle of pulse current waveform and deposition time) that were most appropriate for the fabrication of the tubular films. Similarly for the fabrication of copper electroformed microtubes, an electrolyte containing 0.5 M CuSO4·5H2O and 0.5 M H2SO4 was utilized. Following the deposition of Ni and Cu upon the mandrel, the elctroformed section was chemically etched from aluminum in a 2 M solution of KOH.Bare ZnO as well as CuO/ZnO nanorods were synthesized on the electroformed nickel substrates using co-electrodeposition technique. The fabrication of bare ZnO nanorods (NRs) was materialized using the electrolyte containing 5 mM of zinc nitrate hexahydrate (Zn (NO3)2·6H2O) and 5 mM hexamethylenetetramine (HMTA, (CH2)6N4). The growth of Cu doped ZnO NRs were carried out by incorporating copper sulphate pentahydrate (CuSO4·5H2O), into the electrolyte in different concentrations (25 μM, 50 μM). The nickel tubular substrates were cleansed in running water followed by acetone and ethanol before deposition. The electrodeposition was performed in a three-electrode electrochemical cell (Metrohm Autolab PGSTAT302N). The nickel tube of surface area 3.77 cm2 was treated as the working electrode while Pt and Ag/AgCl electrodes were taken as the counter and reference electrode respectively. The electrodes were set at a distance of 6 cm from each other and a potential of −1.1 V was applied with respect to the Ag/AgCl electrode. A pulse current waveform with 40% duty cycle was used to carry out the electrodeposition. The temperature was maintained at 80 °C and deposition carried out for 3600 s. The deposited samples were rinsed with deionized water afterwards and dried.The fabricated Cu tubes in size of 2 cm × ø3 mm were used for the synthesis of CuO nanorods. The thermal oxidation route was utilized by keeping the substrates at 400 °C for 3 h.The samples were then cooled inside the furnace. ZnO electrodeposition was then carried out by utilizing Cu tubes with CuO nanorods on them as cathodes. The electrolyte contained 5 mM of zinc nitrate hexahydrate (Zn (NO3)2·6H2O) and 5 mM hexamethylenetetramine (HMTA, (CH2)6N4) with 4 h of deposition time. The temperature was maintained at 80 °C with constant stirring rate of 100 rpm.The surface morphologies and composition analysis of bare and CuO/ZnO NRs were examined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) using a ZEISS MERLIN compact) instrument. The crystalline phases and compound formation in the samples were analysed by X-ray diffraction (XRD) (PANalytical X’Pert ProMPD) with a monochromatizer Cu K α irradiation (l 1/4 1.5406 Å).For the calculation of band gap energy of the various photocatalytic substrates the UV–vis spectra in diffuse reflectance mode has been recorded using Shimadzu UV-2450 (Japan) spectrophotometer. The photocatalytic degradation experiments for Methylene Blue (MB) dye were carried out in the presence of three tubular nickel and copper tubular films (2 cm length and 3 mm diameter) immobilized with ZnO or CuO/ZnO photocatalyst. The photo-reactor containing 60 mL of aqueous MB dye solution was illuminated in sunlight (Month of July, 2021, Bhubaneswar, Odisha, India). The concentration of dye solution was varied to study their effect on the degradation efficiency with pH remaining constant at 10. Before the experiments, the solution along with the catalysts were magnetically stirred for 30 min to achieve the adsorption/desorption equilibrium. The dye samples of about 1.5 mL were taken out at regular intervals and recorded using UV spectrophotometer. The adsorption intensity peaks (664 nm) were monitored at certain time intervals. All the degradation experiments were continued till complete decoloration of the dye solution have been achieved. The degradation efficiency ( η ) was calculated using the following formula: (1) η = ( 1 - C C 0 ) × 100 where C is the equilibrium concentration of MB dye after exposure to sunlight while C 0 is the initial concentration of the dye in the solution. The antibacterial activity of ZnO/CuO nanocomposite immobilized metallic tubular structures were assessed by evaluating their performance against gram negative E.Coli bacteria. The bacteria E.coli were grown initially in LB medium and the culture was cultivated at 37 °C for 8–12 h. The growth of bacteria was confirmed by turbidity visualization. 50 μL of the isolated bacteria were then added to each media containing different samples of ZnO/CuO nanocomposite. The antibacterial activity of these nanocomposite grown substrates were supported by measurement of optical density at 600 nm using UV–visible spectrophotometer (Hitachi U-2900).Nickel tubes were fabricated at an optimum parameter of 50% duty cycle and 6 h of deposition time. Only peaks pertaining to nickel were detected indicating the purity of deposition. The narrow peak width and intensity as shown in Fig. 1 (a) confirm high crystallinity and fine grain size of the deposition. The high intensity of the (111) plane as compared to the other peaks indicate it to be the preferred orientation of the microcrystalline. The crystallite size was calculated from the well-known Scherrer formula [41] and found to be 55 nm.The tube films were characterized using FESEM for their structural integrity. The thicknesses of the tubes were found to be around 90 μm as shown in Fig. 1. The above-mentioned process conditions resulted in manufactured tubes with a satisfactory hardness value of 158 HV and a surface roughness of 2.02 μ m . As hardness values do not play any role in the current application of nanostructure growth, the parameters were chosen to fabricate rigid tubes having minimum wall thickness in order to render higher surface area films. The copper tube shown in Fig. 2 (a) was fabricated at 50% duty cycle and 4 h of deposition time to further reduce the thickness and thereby enhance the exposed area within a limited space for growth of photocatalyst. The Fig. 2b demonstrates the EDS analysis of the tube with strong presence of Cu peaks. This confirms the fabrication of high purity Cu microtubes through the process of electroforming.The electrochemical synthesis of ZnO and CuO-ZnO nanocomposite underwent with the electro-reduction of nitrate ions generating hydroxide ions. HMTA was used as a chemical agent to control the growth direction and morphology of the NRs when nitrate precursor is utilized. HMTA, being water soluble decomposes into ammonia and formaldehyde. Thereafter the ammonia acts as an additional source of hydroxide ions [42]. HMTA also contributes towards regulating the pH of the electrolyte. The NaNO3 as a supporting electrolyte plays the role of supplying the OH− ions through reduction of nitrate ions. The high solubility of NaNO3 makes it possible to maintain the nitrate concentration in excess of the Zn+2 concentration. The step-by-step reaction mechanism can be summarized in the following equations: (2) Zn ( NO 3 ) 2 · 6 H 2 O → Zn + 2 + 2 NO 3 + 6 H 2 O (3) C 6 H 12 N 4 + 6 H 2 O → 6 HCHO + 4 NH 3 (4) C 6 H 12 N 4 + 4 H 2 O → ( CH 2 ) 6 ( NH ) 4 + 4 OH - (5) NH 3 + H 2 O → OH - + NH 4 + (6) Zn + 2 + 2 OH - → Zn ( OH 2 ) (7) Zn ( OH 2 ) → ZnO + H 2 O (8) Zn + 2 + 2 OH - → ZnO + H 2 O While the involvement of an intermediary phase (Zn (OH)2) has been established by literature [43,44], a direct synthesis of ZnO from Zn + 2 and OH - ions as given in the step of eqn. (8) has been supported by the investigation carried out by Mcpeak etal . [45]. The ZnO formation, although not a Faradic process, can be termed as an electrochemically activated precipitation process that can be controlled by experimental parameters like potential, concentration and pH. The precipitation reaction of CuO is similar to that of ZnO on the electrode surface through the process of co-electrodeposition (Eqn. (9)). (9) Cu + 2 + 2 OH - → Cu ( OH ) 2 → CuO + H 2 O The present electrolyte containing Zn(NO3) and NaNO3 as supporting electrolyte facilitates the deposition of CuO instead of Cu2O as the nitrate system has a more positive potential (E0 = 0.93 V vs. SHE) compared to Cu(II)/Cu(I) redox couple (E0 = 0.16 V vs. SHE) [46].Based on the above reaction steps, it is clear that the formation of Zn + 2 ions come from the source material, hence constant and predictable. But the source of OH− ions are the hydrolysis of HMTA as well as reduction of nitrate ions. The reduction of nitrate ions is catalyzed by metal ions present in the solution like Zn + 2 though it gets consumed subsequently [44]. Hence, it can be hypothesized that the presence of Cu + 2 ions play the same role as that of Zn + 2 . This is supported by the cyclic voltammetry curves obtained in the growth solutions without or with copper sulphate in Fig. 3 .In the CV studies, it can be noticed that the cathodic current begins at −0.9 V (vs. Ag/AgCl electrode) and a steep increase of current up to 6 mA can be seen at a potential of −1.1 V corresponding to deposition of ZnO film. As the potential becomes more negative, the current increases because of increased rate of nitrate reduction. Hence a more negative potential leads to faster growth of nanorods. The hydrothermal process of growth is dependent on parameters like growth temperature and concentration. But the electrodeposition process has a significant parameter like applied potential as well. The effect is enhanced with an increase in the current density as Cu + 2 is added to the electrolyte. The cathodic current also becomes slightly positive as the concentration of copper ions becomes higher and higher. Hence, the overpotential decreases leading to ready electrochemical reaction. Two successive cathodic waves can be noticed which can be attributed to the presence of copper, but further investigation is necessary.The preparation process of the ZnO decorated CuO nanorods on Cu tubular film substrate materialized in a two step process as demonstrated in Fig. 4 . In the first step, the CuO nanorods were developed as a result of the accumulation and relaxation of the stress during the thermal heating process. The Cu tubes were annealed at 400 °C for 180 min. During the process of annealing, a compressive stress is induced because of an increase in molecular volume (Cu < Cu2O < CuO). The protrusion on the oxide surface is brought on to release the stress. The diffusion of atoms is driven by this stress besides the thermal diffusion [47]. Meanwhile, accumulating larger stress needs higher temperature and long time. But, if the growth temperature was too high, like in the case of 600 °C or the growth time too long, CuO crystallites formed rather than nanorods owing to surface diffusion. To develop the ZnO shell on the CuO core, electrodeposition was utilized with the electrolyte mentioned above. Hence the mechanism for deposition of ZnO was similar to the development of ZnO on Ni substrate.SEM micrographs were utilized to analyze the surface morphologies of the ZnO as well as CuO/ZnO nanorod composite developed on the Ni tubular thin film. The undoped ZnO nanorods in Fig. 5 (a) show flower shaped arrangement of hexagonal ZnO nanorods and some porous film in the background. The agglomeration of nanorods and nanocrystals may have led to this film formation. The undoped ZnO nanorods have a diameter in the range of 250–300 nm and length of 1 μM. Likewise, the CuO-doped ZnO nanorods in Figs. 5(b–c) have been aligned in the form of flower. The CuO/ZnO nanorods with 25 μmol l - 1 Cu + 2 concentration (Ni_ZnO/CuO_25) in the electrolyte have an average diameter of 100–150 nm while the average length is 2.5 μm. The incorporation of CuO has led to the fragmentation of ZnO nanorods as well as its reduction of size. These rods have pencil like sharp tip instead of the blunt tips of undoped ZnO. When the Cu concentration was further increased to 50 μ mol l - 1 (Ni_ZnO/CuO_50), a further reduction in the diameter of nanorods was observed, as shown in Fig. 6 . The nanorods possessed an average diameter of 90 nm. Higher concentration of copper has led to the arrangement of nanorods in the form of petals which was also noticed by Wei etal . [48].The atomic Cu/Zn ratios for the composite films fabricated with different concentrations of Cu + 2 in the electrolyte (0, 25 μmol l - 1 , and 50 μmol l - 1 ) were calculated from the EDS element distribution given in the Fig. S1 and S2 of the Supplementary information. Those were found to be 0, 1.11, and 1.52 respectively. This indicates that the concentration variation in the electrodeposition process can easily engineer the composite film. However, the Cu/Zn atomic ratios in the CuO/ZnO composite film is much higher than the actual concentration ratio in the electrolyte solution (0.005 and 0.01 corresponding to 25 μmol l - 1 and 50 μmol l - 1 , respectively).The reason behind this can be attributed to the difference in solubility of Cu(OH)2 and Zn(OH)2 as both act as precursors for the reduction of CuO and ZnO, respectively. While several literature have reported the formation of flakes and grains at higher Cu/Zn atomic ratios ( > 1.1), the current research demonstrates an economical fabrication technique with superior nanorod morphology, rendering higher surface area. Moreover, the strong peaks of Cu, Zn and O reveal the composition of the composite film, without the presence of any impurity element.SEM images of the CuO nanorods grown on the Cu tubes through thermal oxidation as well as the ZnO decorated CuO nanostructures were studied to reveal information regarding the external morphology and topography. It can be observed clearly in Fig. 7 (a & b) that the surface is covered with CuO nanorods along with some nanoparticles. The nanorods have an average size of 250 nm.At a temperature of 600 °C the samples prepared demonstrated agglomerated CuO plates, thereby not enhancing the surface area of the composite photocatalyst for performance improvement.The higher temperature may have led to fusion in the crystal growth phase. The EDS analysis in the Supplementary Information (Fig. S3) presents the composition of the nanorods and the atomic ratio of Cu and O confirms the synthesis of CuO. Fig. 7 (c & d) depicts the ZnO decorated CuO nano-structure arrangement with ZnO being wrapped around the CuO nanorods in the form of nanobulges. The diameter and morphology of the CuO/ZnO nanorods are different from that of CuO nanorods. The deposition of ZnO on the CuO nanorods has resulted in an average diameter increase from 250 nm to 700 nm. The step like formation on the nanorods has led to a further improvement in the active surface area for the nanocomposite films. Elemental mapping and distribution of atoms in the formation of CuO/ZnO heterostructured nanorods are illustrated in Fig. — of the Supplementary information. The figure indicates the distribution and interfacial contact between the two oxides. It can be noticed that there is uniform distribution of Cu and Zn all over the surface with abundance of oxygen. Fig. 8 (a) shows the XRD spectra for ZnO and ZnO/CuO composite nanostructures on Ni tubular substrate while Fig. 8(b) shows the spectra for CuO and ZnO decorated CuO nanostructures on the Cu tubular substrate. For comparison the XRD pattern of Ni and Cu tubes were also taken. The diffraction peaks of all samples are well-defined revealing the crystalline structure of the nanocomposites.In the XRD data of Cu_CuO sample, the diffraction peaks clearly indicate the monoclinic CuO phase, which is consistent with the published findings (JCPDS Card. No. 89–5899) [49]. The hexagonal wurtzite structure of ZnO has also appeared in the sample Ni_ZnO (JCPDS card No.36–1451) similar to data reported earlier [50]. The XRD pattern of the Cu_CuO/ZnO sample shows diffraction peaks of both ZnO and CuO. The nickel tubular samples having ZnO/CuO composite nanorods show peaks of ZnO and CuO owing to their co-electrodeposition. While, in the case of nanocomposite grown on the Cu tubular substrates, the CuO peaks remain dominant. The uncontrolled growth during thermal oxidation can also lead to larger crystallite as observed in the crystallite size calculation. The observed lattice constant values were found to be a  = 4.653 Å, b  = 3.410 Å, c  = 5.108 Åfor CuO and a  = 3.22 Å, c  = 5.20 Åfor ZnO respectively. The average crystallite sizes were calculated using the Debye–Scherrer equation which is expressed as d = 0.9 λ /FWHM cos θ where d is the average crystallite size, λ is the wavelength of incident X-ray beam (1.541 Å), FWHM is the full width at half maxima of the most dominant peak and θ is the Bragg’s diffraction angle. The estimated crystallite size for ZnO at 14.15 nm and CuO at 15.3 nm were minimum for the sample Ni_ZnO/CuO_25. The Cu tubular substrate having nanocomposite (Cu_CuO/ZnO) has slightly larger crystallite size at 18.9 nm for ZnO and 15.58 nm for CuO. The low intensity CuO peaks can be due to the ZnO decorated CuO nanorods. Hence, the XRD patterns confirmed that the synthesis has been successful, and the separate peaks indicate that ZnO and CuO exist as two individual phases instead of forming an alloy.The FTIR study of the electrodeposited nanocomposite structures were carried out to determine the functional groups present. The FTIR spectra of CuO, ZnO and ZnO/CuO heterostructures have been depicted in Fig. 9 in the range of 400–4000 cm - 1 . The absorption bands of metal-oxides owing to the inter-atomic vibrations lie in the fingerprint region, i.e, below 1000 cm - 1 . The strong bands in the range of 400–600 cm - 1 in the current study is attributed to the combined presence of ZnO and CuO. It has been reported that ZnO demonstrates absorption band around 464 cm - 1 [51] but depends on the particle morphology and chemical composition. The spectra for electrodeposited ZnO shows a peak at 466 cm - 1 while the CuO sample demonstrates a peak at 473 cm - 1 . The ZnO decorated CuO nanocomposite from the Cu tube shows additional peaks at 475 cm - 1 and 492 cm - 1 signifying the inclusion of CuO which represents Cu-O stretching. CuO absorption peaks around 500 cm - 1 have also been noticed for samples having CuO/ZnO heterostructures. The broad peaks with higher intensity in case of Ni_ZnO/CuO_25 and Ni_ZnO/CuO_50 confirms good presence of CuO. The absorption peak at 1380 cm - 1 can be assigned to the stretching vibration of left over nitrate ions. The peaks are prominent for ZnO/CuO samples deposited on the Ni tubes. The absorption peak at 1635 cm - 1 and the broad absorption band centered at 3435 cm - 1 corresponds to the O - H bending and stretching vibration respectively. The FTIR results are in accordance with those obtained from EDS analysis.The optical properties of the functional substrates with ZnO nanorods and modulation in their properties with CuO addition were estimated through the reflectance spectra obtained with respect to wavelength. The range of wavelength considered was 300–800 nm. The UV–vis spectra in Fig. 10 (a) demonstrates a trailing edge to the spectrum in the UV region followed by a sharp increase for the sample Ni_ZnO while the spectrum slightly shifts towards higher wavelength for the Ni_ZnO/CuO_25 sample. Higher reflectance in the visible region was noticed for Ni_ZnO/CuO_25 as well as Cu_CuO/ZnO sample as shown in Fig. 10 (c).The band gap energies ( E g ) of the bare semiconductor nanorods and their composites were determined using the theory proposed by P. Kubelka and F. Munk in 1931 [52]. The theory proposed a method for transformation of reflectance spectra to absorption spectra using the Kubelka–Munk function F(R) which is given by the following equation. (10) F ( R ) = K S = ( 1 - R ) 2 2 R The reflectance is denoted by R, while the absorption and scattering coefficients are K and S, respectively. The approach developed by Tauc [53] for determination of band gap using absorption spectraF(R) can then be employed and F(R) can be used in the place of α . The resulting equation is given by: (11) ( F ( R ) h ν ) 1 / γ = B ( h ν - E g ) where h is the Planck constant; ν is the photon’s frequency; E g is the band gap energy, and B is a constant. The value of γ depends the nature of semiconductor band transition and is 1/2 and 2 for direct and indirect band gap transitions respectively. Fig. 10 (b) and (d) demonstrates the Tauc plot for ZnO/CuO nanocomposite with varying percentages of CuO on Ni and Cu tubular film substrates respectively. The x-axis linear interpolation of the Tauc plot gives the band gap energy. For CuO semiconductor oxide, it has been reported to have direct band gap [54,55]; therefore, both ZnO and CuO have been considered as direct band gap semiconductor oxides. The band gap energy of CuO nanorods on the Cu tubes was found to be 1.38 eV which is in good agreement with previously reported literature [56]. The bare ZnO nanorods on the Ni substrate has revealed a band gap energy of 2.87 eV. The lower E g can be attributed to the higher surface area of nanostructured morphology. Further narrowing of E g to 2.48 eV has been observed for Ni_ZnO/CuO_25 samples due to the incorporation of CuO. This can be attributed to the interfacial contact between the ZnO and CuO nanorods. But further increase in CuO incorporation in Ni_ZnO/CuO_50 sample has led to bare CuO like behavior with band gap energy of 1.25 eV. The improvement in optical properties because of the ZnO/CuO heterostructure has been lost here which is reflected in the photocatalytic study as well. In the literature, the band gap of ZnO/CuO, developed through solution-based co-precipitation method has been observed to be in the range of 2.34–3.25 eV [57,58]. The optical band gap of Ni_ZnO/CuO_25 and Cu_CuO/ZnO nanocomposite, developed in the current study is 2.48 eV and 2.95 eV, respectively. The optimum band gap suggests that electron transfer is inevitable. Moreover, the variation in band gap energy arises because of the synergistic effect of ZnO and CuO nanostructures. The lower band gap energy of CuO is the reason behind a decrease in the band gap of the ZnO/CuO composite.The photocatalytic activity of CuO, ZnO and CuO/ZnO heterostructure fabricated both on Ni and Cu tubes under solar irradiation were examined till complete decoloration. Significant decomposition of MB dye was observed for all the catalyst substrates used with varying efficiency and degradation time. Initially, a 50 ppm aqueous solution of MB dye was utilized for Ni_ZnO, Ni_ZnO/CuO_25 and Ni_ZnO/CuO_50 samples in order to distinguish clearly the degradation time and efficiency. Afterwards 20 ppm aqueous solution of MB dye was used to demonstrate a comparison between different CuO/ZnO nanostructures on Ni and Cu substrates. Fig. 11 (a) and (b) demonstrates the diminishing peak for characteristic peak at 664 nm with gradual increment in time. The complete decoloration of the dye samples were achieved within 105 min and 95 min of time for Ni_ZnO and Ni_ZnO/CuO_25 respectively. The complete decoloration thereby the decomposition of the dye sample was not achieved in the presence of Ni_ZnO/CuO_50 samples. The Cu content in the solution and the morphology of the nanocomposite structure play a significant role in the degradation process. Higher amount of CuO results in the coverage of active sites on ZnO surface thereby reducing the photocatalytic efficiency. The Ni_ZnO/CuO_50 samples presented an efficiency of 74% while the most significant efficiency of 95.3% was demonstrated by Ni_ZnO/CuO_25 samples. The Ni_ZnO samples also rendered good efficiency of 94% though it consumed higher amount of time for complete decoloration. Fig. 11 (c) shows the comparison between degradation results of different samples. The improved activity of the mixed nanocomposite at an optimum concentration is due to the formation of a heterojunction system between the p-CuO and n-ZnO which prevents the recombination of charge carriers. The appropriate position of the band edges provides sufficient and irreversible charge transfer thereby reducing the photocatalytic degradation time. Besides, the hierarchical structure and morphology play a significant role in the superior photodegradation efficacy. The petal like morphology in sample Ni_ZnO/CuO_50 has reduced its exposed area for degradation thereby restricting the activity. Other literature have also reported such optimal concentration of CuO for superior photodegradation of various dyes [59–61]. The data obtained from the degradation experiments were further examined to find out whether they follow the Langmuir–Hinshelwood kinetics model as given below: (12) ln ( C 0 C ) = kt + constant The plots between ln (C0/C) and irradiation time were constructed for the samples that are depicted in Fig. 11 (d). The rate constants for Ni_ZnO, Ni_ZnO/CuO_25 and Ni_ZnO/CuO_50 are 0.024 min−1, 0.026 min−1 and 0.009 min−1 respectively. Thus, it is clearly evident from the rate constants that the Ni_ZnO/CuO_25 samples possess the highest degradation rate, hence the most suitable for MB dye degradation. Fig. 12 (a) and (b) demonstrates the gradual decoloration and thereby the reduction in the characteristic peak absorbance at 664 nm for Cu_CuO and Cu_CuO/ZnO samples respectively. The degradation experiments were performed on a 20 ppm aqueous solution of MB dye and the ZnO decorated CuO nanostructures on Cu tubes (Cu_CuO/ZnO) exhibited superior performance by decomposing the dye into non-toxic products within 60 min of time. The Cu_CuO substrates took longer time with lower efficiency of 87%, but it has been found to be superior compared to other literature [34,62,63].This can be attributed to the nanorod like morphology and substrate based growth which has been rarely reported in literature. The interface between the metal substrate and the semiconductor oxide nanostructures is also expected to separate the charge and enhance the photocatalytic activity. The enhanced efficacy of the Cu_CuO/ZnO samples is due to the presence of CuO as an efficient photocatalyst in the visible range of sunlight along with the broad band gap of ZnO preventing the recombination of electrons and holes. Fig. 12 (c) and (d) show the degradation efficiency and rate constants for samples on Cu tubular substrates. The rate constants for Cu_CuO and Cu_CuO/ZnO samples are 0.029 min−1 and 0.035 min−1 respectively.Here, an interesting aspect of the research work is to compare the samples prepared by two different techniques. While the first process involved the fabrication CuO/ZnO heterostructure by a one step electrodeposition method, the second process involved both thermal annealing and electrodeposition. Hence, the later samples have resulted in ZnO decorated CuO nanostructures. Therefore, for comparison, the photocatalytic degradation of 20 ppm MB dye solution was carried out using Ni_ZnO/CuO_25 samples. The rapid decoloration can be noticed in Fig. 13 (a) with the decreasing trend in characteristic peak at 664 nm. In merely 40 min, the characteristic absorbance spectrum of MB dye ranging from 550 nm to 725 nm nearly vanished with 93.57% efficiency. It is evident from Fig. 13 (b) that an efficiency of 91.58% has been achieved within 20 min of time. A higher rate constant of 0.074 min−1 which is more than double that of the degradation rate constant in the presence of Cu_CuO/ZnO validates the superior photocatalytic performance of Ni_ZnO/CuO_25 samples under solar irradiation. The activity followed the trend Ni_ZnO/CuO_25 > Ni_ZnO > Cu_CuO/ZnO > Cu_CuO > Ni_ZnO/CuO_50. The following Table 1 shows a comparison of the photocatalytic results of present work with earlier reported literature.The reusability of the prepared photocatalytic samples were examined through repeated use of the Ni_ZnO/CuO_25 samples for the degradation of MB dye. The photocatalytic substrates retained their photocatalytic degradation capability even after 3rd cycle with an efficiency of 91.5%. Fig. 14 shows the degradation kinetics and photocatalytic efficiency for repeated cycles. (see Fig. 15 ).The enhancement of photocatalytic activity of ZnO/CuO nanocomposite is attributed to two main factors that are the separation of electrons and holes and the utilization of a large portion of sunlight, i.e, the visible light because of the incorporation of CuO. The special flower like morphological feature of the nanorods has enhanced the adsorption of the dye molecules on the semiconductor oxide nanocomposite structures. The use of organic coloured dye has facilitated the process as they get excited by the impingement of sunlight and may transfer electron to the conduction band (CB) of CuO [74]. And the nanojunctions formed at the interfaces of the n-type-ZnO and p-type-CuO makes the migration of those electrons from the CB of ZnO to CB of CuO possible. The holes also migrate from the valence band (VB) of CuO to VB of ZnO as that is thermodynamically advantageous. While the electrons lead to the generation of superoxide radical anions( · O 2 - ), the holes at the surfaces are captured by the H2O molecules to produce the hydroxyl radicals (·OH). These play a dominant role in the redox degradation of the organic dye compound. Moreover, the coloured dye used in the present research acts as a medium for the transfer of electrons from the excited dye to the electron acceptors (e.g O 2 ). The excited dye molecules absorbed on the semiconductor oxide surface inject the electrons to the conduction band of CuO. The possible decomposition mechanism has been depicted by the following equations: (13) MB + h ν → MB ∗ (14) MB ∗ + CuO → MB + · + CuO ( e - ) (15) { p - CuO } { n - ZnO } + h ν → { p - CuO } { n - ZnO } ( e - ( C . B ) + h + ( V . B ) ) (16) ZnO ( e - ) + O 2 → · O 2 (17) CuO ( h + ) + H 2 O → H + + · OH (18) · O 2 / · OH + Dye → Degradationproducts It has been observed in the present work that the photocatalytic activity increases with incorporation of CuO but then decreases with higher concentration of CuO. The CuO crystallites facilitate the movement of photo-generated electrons and holes to the surface, but a higher growth can make the process time-consuming. This can lead to recombination and absorption of the charge carriers within the crystalline resulting in a delayed and incomplete degradation as in the case of Ni_ZnO/CuO_50. Besides, the shadowing effect of CuO on ZnO can lead to reduction in number of photo-generated electrons and holes thereby diminishing the absorption capacity of ZnO. The band gap energy values of different samples present a similar picture with lowest band gap energy of 2.48 eV for Ni_ZnO/CuO_25. It implies that the concentration ratio of ZnO/CuO as well as their morphology greatly affects their photocatalytic performance.The antibacterial activities of Cu and Ni tubular substrates carrying ZnO/CuO nanocomposites were determined by assessing their resistance towards bacteria E.coli. The optical densities measured at 600 nm were analyzed and Fig. 16 (a) shows the ability of various samples to inhibit bacterial growth with respect to the positive control.It has been found that the sample Ni_ZnO/CuO_25 demonstrates highest inhibition towards the growth of E.coli with 92% reduction in bacterial growth. The Ni_ZnO substrate also showed good activity against E.coli with 88% reduction. The Cu tubes with ZnO decorated CuO nanostructures and Ni_ZnO/CuO_50 sample were able to inhibit the growth only up to 55% of the positive control. It has been shown in previous research that the cell wall of E.coli possesses negative charge, hence has an electrostatic attraction towards positively charged metal ions [75]. As higher percentage of metal ions are released from nanocomposites, the ZnO/CuO nanostructures have performed well as compared to other samples. It can be said that the Zn + 2 and Cu + 2 ions were able to interact with the cell wall of the bacteria thus causing surface irregularities and destabilization [76]. The antibacterial activity of metal oxide nanostructures depends on the detachment of metal ions, nanostructure morphology and size and generation of reactive oxygen species (ROS). The good morphology with nanorod diameter of around 100 nm in case of Ni_ZnO and Ni_ZnO/CuO_25 samples may have contributed towards their superior antibacterial activity. Besides, direct contact between the nanorods and bacterial cells obstruct the electron transport to and from the bacterium cell thereby triggering cell death. A similar reduction in bacterial growth of approximately 90% has been achieved in literature, through a multi-step process of layer-by-layer casting [38]. Also it was demonstrated in the photocatalytic studies that the Ni_ZnO/CuO_25 samples posses the capability of producing higher ROS (Hydroxyl, Superoxide radical), thus leading to faster degradation of MB dye. The same can be attributed to its ability of preventing bacterial growth. The presence of Ni tube as substrate has prevented the agglomeration of the nanostructures thereby enhancing their reactive surface area and retaining their morphology.In summary, ZnO/CuO heterostructures possessing different morphology and composition were successfully anchored over electroformed Ni tubular thin films, through pulse electrodeposition. ZnO decorated CuO nanostructures were developed on electroformed Cu tubular substrates by means of a two step process of thermal oxidation and pulse electrodeposition. The FE-SEM analysis and FTIR study confirmed the mixed structures based on simple oxide constituents. The CuO content was optimized for enhanced photocatalytic and antibacterial performance of the ZnO/CuO nanocomposite immobilized catalytic tubes. ZnO/CuO nanocomposite structures on Ni tubes having 25 μM Cu + 2 concentration in electrolyte demonstrated superior photocatalytic and antibacterial activity.The catalytic tubes decolorized high concentration of MB dye within 40 min of time with a degradation efficiency of 95.3%. This could be attributed to the high surface area of ZnO/CuO nanostructures attached to the tubular substrates, efficient utilization of the visible light, and enhanced separation of electron-hole pairs. The results were corroborated by the band gap energy study as well. Moreover, it was economical and easy to retrieve, recycle, and reuse the metallic catalytic tubes. The same sample was found to have prevented bacterial growth by 92% with respect to positive control. The Ni tubes being corrosion resistant could be retrieved and reused with good photocatalytic efficiency. Therefore, the Ni_ZnO/CuO_25 samples fabricated through electrodeposition route with optimum concentration of CuO demonstrated superior photocatalytic and antibacterial performance. Hence, this study introduced a new route of fabricating highly efficient metallic-tube-based photocatalysts having antibacterial activity.HJB performed the experimental work; HJB, KM, and AG analyzed the results; HJB wrote the manuscript; and AG, KM, NPS and PRV did the review and editing. Hrudaya Jyoti Biswal and Ankur Gupta: Conceptualization, Methodology, Writing. Pandu R. Vundavilli, Kunal Mondal and Nagraraj Shetty: Reviewing and Editing. Ankur Gupta: Supervision, Resources, Funding Acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Ankur Gupta wishes to acknowledge the affiliating institute (IIT Jodhpur) for providing the research seed grant (I/SEED/AKG/20190022), which was instrumental in completing the work. Kunal Mondal gratefully acknowledges Department of Energy and Environment Science and Technology at the Idaho National Laboratory, USA, for their support.

In this work, we report the fabrication of Ni and Cu tubular substrates and the synthesis of ZnO/CuO nanocomposite on them through the process of pulse electrodeposition. The systematic variation in CuO incorporation in the ZnO matrix and the processing technique were noticed to affect the structural, optical, photocatalytic, and anti-bacterial properties, which are well in accordance with the Field Emission-Scanning Electron Microscope, X-ray Diffraction, Fourier transform Infrared Spectroscopy and UV-Differential reflectance spectroscopy results. The remediation capabilities of the photocatalytic substrates were assessed through the degradation of methylene blue (MB) dye under solar irradiation. Optimized CuO incorporation within the ZnO nanorods resulted in the degradation of a 20 ppm of MB dye solution within 40 min and a higher concentration of 50 ppm within 95 min. The Ni and Cu electroformed tubes as substrates provided not only a reusable supporting frame but also a large surface-area for the growth of ZnO/CuO nanocomposite. The current study also dealt with the anti-bacterial efficacy of the above-mentioned substrates against E.coli. Hence, the Ni and Cu tubular thin film substrates with nanorods of ZnO/CuO composite were explored for the removal of organic as well as biological contaminants from waste water.

Greenhouse gas emissions need to be reduced drastically to meet the Paris Agreement's climate objectives of limiting global average temperature increase below 2 °C and pursuing efforts limiting it to 1.5 °C [1,2]. This will require energy systems that differ much from today. Since industrial practices will depend on non-renewable sources for a relatively long time before there is a drastic shift to renewable energy, capture of produced CO2 emerges as transit solution up to that date [3]. Once the CO2 is captured, it can be stored (CCS) or used (CCU). At this point, creating a sustainable market demanding recovered CO2 may provide a better option than CCS, while also helping the economy [4,5]. Specifically, research is shifting towards CO2 capture and utilization (CCU). So that, the use of CO2 in the synthesis of value-added products is increasing the attention of several industrial companies [6].Recently, to avoid energy penalties associated with the regeneration and compression steps required for transportation and storage prior to conversion, researchers have attempted to integrate the CO2 capture and utilization (ICCU) [7]. In this context, the CO2 is captured and converted at the same place using dual function materials (DFMs). The DFMs consist of CO2 adsorbents and catalytic phases. First, DFMs capture CO2 from flue gas (4–14 vol% CO2) to effectively reduce carbon emissions. When the carbon capture process is completed, the feed gas is switched to a reducing renewable agent for the conversion of the adsorbed CO2 to synthetic fuels. An interesting option is the conversion of the adsorbed CO2 into CH4 through the Sabatier reaction (Eq. 1). (1) CO2 + 4 H2⇆CH4 + 2 H2O Duyar et al. [8] in 2015 published the first work of the operation in cycles of CO2 adsorption and hydrogenation to CH4. The authors used a DFM based on CaO as adsorbent, Ru as metal phase and Al2O3 as support. They demonstrated the possibility of producing CH4 from CO2 adsorbed in an earlier step. From then, publications on cycles of CO2 adsorption and hydrogenation to CH4 is growing exponentially [7,9–11]. In general, DFMs are a combination of a compound based on Na [12,13], Ca [8,14], Mg [15,16] or K [15,17], as adsorbent, and a Ru- [12,18], Ni- [17,19] or Rh-based [20] as catalytic phase. Both phases are commonly supported on a high surface area carrier. Specifically, γ-Al2O3 is proposed as the best support [21].One of the problems of the CO2 methanation reaction is its highly exothermal character [22]. Therefore, it leads to a demand for highly thermostable catalysts to resist deactivation phenomena caused by hotspot formation in industrial fixed-bed application [23]. This problem is highly relevant in methanation with continuous feeding of CO2 and H2. In the operation in cycles of CO2 adsorption and hydrogenation to CH4, the temperature control is easier. Nevertheless, the main deactivation phenomenon in the dual operation is the presence of oxygen and steam in the feed stream of the adsorption period, which influence has been analysed by several authors [21,24–26]. The presence of O2 partially oxidizes the metal phase, which is reduced again during the hydrogenation period. On the other hand, the presence of H2O reduces the CO2 adsorption capacity due to competitive adsorption of both compounds over the same basic sites.Another way to analyse the resistance of the DFMs to the presence of O2 and steam is to age the DFMs at high temperature in the presence of O2 and steam. This strategy is commonly used for NSR or SCR catalysts for NOx removal in diesel vehicle engines [27–29]. In this way, the state of the catalyst at the end of the life of the vehicle can be simulated at a laboratory scale. In order to simulate long periods of DFM operation in cycles, with the presence of oxygen and steam during the adsorption period, the analysis of aged DFMs can be of great interest. The evaluation of the activity of the DFMs after being subjected to the aging protocol, as well as their physicochemical properties, will provide valuable information on their resistance to aging. However, to the best of our knowledge, the study of the resistance to hydrothermal aging in the presence of oxygen of DFMs has not yet been reported.In this work, the effect of hydrothermal aging in the presence of oxygen on DFMs is analysed. Changes that are caused in their physicochemical properties are studied, as well as its influence in the activity in cycles of CO2 adsorption and hydrogenation to CH4. To have a broad view of the effect of aging, DFMs with different formulations are studied. With that aim, DFMs based on ruthenium, nickel or both as metals and on sodium, calcium or both as adsorbents are selected.Five DFMs have been selected based on ruthenium, nickel or the combination of both as active metals, whereas sodium, calcium or the combination of both are used as adsorbents. Table 1 lists the complete formulation of DFMs prepared and their nomenclature used in this work, also classified into Ru-DFMs or Ni-DFMs group. Ru-DFMs with single-Na and single-Ca have been chosen, both with comparable activity [12]. In addition, a Ru-DFM with both adsorbents jointly (Na and Ca) is also studied, as it has recently been shown that this combination improves activity [30]. On the other hand, in the case of Ni-DFMs only Na is used as adsorbent, due to its significant higher activity as Ni-DFMs with Ca [14].The DFMs were prepared by wetness impregnation. Firstly, appropriated amount of Ca(NO3)2.4 H2O (Merk) and/or Na2CO3 (Riedel de-Haën) was impregnated over γ-Al2O3 (Saint Gobain). The impregnated powder was dried at 120 °C overnight. Then the powder was calcined at 400 °C (Ru-DFMs) or 550 °C (Ni-DFMs) for 4 h (1 °C min−1). Afterwards, Ru(NO)(NO3)2 (Sigma Aldrich) and/or Ni(NO3)2.6 H2O (Fluka) was impregnated over the previous calcined powder. After drying at 120 °C, the samples were stabilized by calcining again at the same conditions.The calcined DFMs were placed in their granulated form (0.3–0.5 mm) in a quartz tube reactor and were heated rom RT to 400 °C (Ru-DFMs) or 500 °C (Ni-DFMs) at 10 °C min−1 during 1 h under 10% H2/Ar (50 cm3 min−1).For hydrothermal aging studies in the presence of oxygen, the DFMs were placed in their granulated form (0.3–0.5 mm) in a quartz tube reactor placed in a horizontal furnace. The DFMs were aged under 5% H2O and 5% O2 in Ar for 3 h, at a total flow rate of 550 ml min−1. The temperature for the aging procedure was 400 °C for Ru-DFMs and 550 °C for Ni-DFMs.X-ray diffraction spectra were obtained in a Philips PW1710 diffractometer. The DFMs were finely ground and were subjected to Cu Kα radiation in a continuous scan mode from 5° to 70° 2θ with 0.02 per second sampling interval.Textural properties of the DFMs were determined from N2 adsorption-desorption isotherms measured at − 196 °C using a Micromeritics TRISTAR II 3020 instrument. Pore volumes were calculated by t-plot method while pore size distribution of mesoporous solids was determined using BJH method. The samples were pre-purged with nitrogen for 10 h at 300 °C using SmartPrep degas system (Micromeritics).The dispersion of active metal sites was measured by H2 chemisorption using a Micromeritics ASAP 2020 instrument. Prior to the experiments, DFMs were reduced with pure H2 for 1 h at 400 °C (Ru-DFMs) or 500 °C (Ni-DFMs) in order to obtain a material with similar reduction degree than in the catalytic activity test. After that, the samples were degasified at the same temperature for 90 min. For both groups of DFMs, the adsorption isotherms were recorded at 35 °C varying the pressure between 50 and 450 mmHg. Adsorption stoichiometries of Ni/H = 1 and Ru/H = 1 were assumed [31].The morphology of the DFMs was analysed by transmission electron microscopy (TEM) in a JEM-1400 Plus instrument using a voltage of 100 kV. The reduced DFMs were dispersed in distilled water ultrasonically, and the solutions were then dropped on copper grids coated with lacey carbon film.The CO2-TPD experiments were carried out on a Micromeritics AutoChem 2920 instrument coupled to a HIDEN ANALYTICAL HPR-20 EGA mass spectrometer. The DFMs (0.1 g) were pre-reduced at 400 °C (Ru-DFMs) or 500 °C (Ni-DFMs) under 5% H2/Ar flow (1 h) and then cooled down to 50 °C. The adsorption of CO2 was performed at 50 °C in a flow of 5% CO2/He (50 cm3 min−1) for 60 min. After CO2 adsorption, the samples were treated with He for 90 min and heated at 10 °C min−1 up to 1000 °C in flowing He (50 cm3 min−1).The catalytic activity, of the synthesized DFMs, in the cyclic operation of CO2 adsorption and hydrogenation to CH4 was evaluated in a vertical tubular stainless steel reactor. In each experiment, the reactor was loaded with 1 g of DFM with a particle size between 0.3 and 0.5 mm. Prior to the analysis, the DFMs were reduced with a stream composed of 10% H2/Ar, progressively increasing the temperature from RT to 400 °C (Ru-DFMs) or 500 °C (Ni-DFMs) and maintaining the final temperature for 60 min. During the adsorption period, a stream composed of 10% CO2/Ar was fed for 1 min, followed by a purge with Ar for 2 min to remove weakly adsorbed CO2. Next, during the hydrogenation period, a stream consisting of 10% H2/Ar was fed for 2 min, followed by an Ar purge for 1 min before starting the adsorption period again. The total flow rate was 1200 cm3 min−1, which corresponds to a space velocity of 45000 h−1. The flue gas composition was continuously measured using the MultiGas 2030 FT-IR analyzer for quantitative analysis of CO2, CH4, CO and H2O. The experiments were carried out in the 280–400 °C (Ru-DFMs) or 280–520 °C (Ni-DFMs) temperature ranges. At this point, it is important to note that nickel has a lower intrinsic activity than ruthenium [32]. Therefore to favour kinetics Ni-DFMs operate at higher temperatures compared to those based on ruthenium [12,14]. Hence, as detailed in the aging procedure section, the temperatures at which DFMs were aged, were also different.The CH4 and CO productions were calculated by the following expressions: (2) Y CH 4 ( μ mol g − 1 ) = 1 W ∫ 0 t F CH 4 out ( t ) d t (3) Y CO ( μ mol g − 1 ) = 1 W ∫ 0 t F CO out ( t ) d t were W is the catalyst weight loaded in the reactor. On the other hand, CH4 selectivity is determined by relating the CH4 and CO productions since they were the only detected carbon based products: (4) S CH 4 ( % ) = Y CH 4 Y CH 4 + Y CO × 100 The error in the carbon balance was deduced by the following expression: (5) s CB ( % ) = ( Y CH 4 + Y CO s t o r e d C O 2 − 1 ) × 100 where the amount of CO2 stored was calculated from Eq. (6). For that, the amount that leaves the reactor was subtracted from the amount fed. To determine the amount of CO2 fed, the stream from the feed system was led directly to the analyser. This profile corresponds to the actual CO2 input that was fed to the reactor. (6) s t o r e d C O 2 ( μ m o l g − 1 ) = 1 W ∫ 0 t [ F C O 2 i n ( t ) − F C O 2 o u t ( t ) ] d t Fig. 1 shows the X-ray diffraction spectra of the DFMs after the calcination step, the reduction protocol and the aging process. In general, in all spectra a background belonging to alumina can be seen on which different peaks stand out. Fig. 1a shows the spectra of the calcined DFMs. In all the Ru-DFMs there are three peaks at 28.0, 35.1 and 54.2° 2θ belonging to RuO2 and in the Ni-DFMs another three peaks at 37.3, 43.4 and 63.0° 2θ belonging to NiO. Furthermore, a peak belonging to NaNO3 appears at 31.9° 2θ (marked with “o”) for Na-based DFMs, whereas for the 4Ru-16Ca two peaks belonging to Ca6Al2O6(NO3)6·xH2O appear at 11.1 and 18.9° 2θ (marked with “+”). The appearance of NaNO3 peak evidences that this is an intermediate formed from nitrates coming from Ru, Ni and Ca precursors (nitrates) during the calcination step, which in the subsequent reduction step is finally reduced into the Na2O active sites for adsorption. The detection of peaks assignable to nitrogenous species indicates the presence of residual nitrates that have not been completely decomposed during the calcination step. In general, the intensity of the peaks of the nitrogen species is higher for the Ru-DFMs compared to the Ni-DFMs. Note that calcination temperatures are different for Ru- (400 °C) and Ni-DFMs (550 °C). At this point, a higher calcination temperature achieves a deeper decomposition of the nitrates. Echegoyen et al. [33] obtained similar conclusions in their study of the calcination temperature with Ni-Al and Ni-Cu-Al catalysts. Fig. 1b shows the diffraction spectra of the reduced DFMs. As expected, there is only one peak at 44.0° 2θ, belonging to metallic ruthenium, in the Ru-DFMs and two peaks at 44.6 and 51.8° 2θ, belonging to metallic nickel, in the Ni-DFMs. No nitrogenous compound or any peak assignable to the adsorbent phases is detected. Therefore, it can be concluded that after the calcination step and the reduction protocol all elements of the DFMs are in the desired oxidation state. In previous work [19], we concluded that, as the calcination temperature increases, the nitrates decompose largely. However, too high calcination temperature, despite achieving complete decomposition of nitrates, penalizes notably the physicochemical properties of DFMs and consequently their activity. In addition, as demonstrated by XRD, even though the calcination step does not decomposed nitrates completely, the reduction protocol does.The aging process does not modify significantly the X-ray diffraction spectra (Fig. 1c). In all cases the spectra are very similar to those of the reduced DFMs, with the only exception of two peaks belonging to glass wool (17.0 and 26.5° 2θ), which cannot be completely removed after the aging process, since it is used to fix the catalytic bed in the reactor. Table 2 summarises the values of specific surface area, pore diameter and pore volume for the different DFMs after the calcination step, the reduction protocol and the aging process. Ru-DFMs increase their specific surface after reduction pretreatment (104.5–110.6 vs. 124.6–130.6 m2 g−1) while Ni-DFMs do not (115.2–115.6 vs. 113.0–118.7 m2 g−1). This fact is assigned to the different calcination temperatures of the DFMs. A higher calcination temperature decomposes nitrates to a greater extent, as deduced from XRD. In this context, in the calcined Ni-DFMs there is a lower proportion of residual nitrates that are partially or totally blocking the smaller pores. After the reduction protocol, the nitrates decompose completely. At this point, comparing the two families, the reduced Ru-DFMs present a greater specific surface area (121.6–130.6 m2 g−1) compared to the reduced Ni-DFMs (113.0–118.7 m2 g−1). This fact is assigned to a minor proportion of alumina in the Ni-DFMs formulation (73–74 vs. 80%) and also due to a certain pore blockage by larger nickel particles, as will be verified in the next section. On the other hand, the pore diameter and pore volume increase in reduced DFMs, confirming the decomposition of residual nitrates observed by XRD. Specifically, the increase in pore volume is significantly greater in the Ru-DFMs, confirming the presence of a greater quantity of residual nitrates.The aging process causes a reduction in specific surface area and pore volume and an increase in pore size (Table 2). It is suggested that continued exposure of DFMs to temperature in the presence of O2 and H2O leads to sintering of the catalytic phase and agglomeration of the adsorbent, which causes irreversible blocking of smaller pores. In order to confirm this aspect, Fig. 2 shows the pore size distributions for the different DFMs after the calcination step (black line), the reduction protocol (red line) and the aging process (blue line). All distributions are unimodal centered around 80–120 Å. As can be observed, the distribution shifts towards higher values with the reduction pretreatment. This fact is ascribed to the elimination of the residual nitrates which partially block the pores. On the other hand, with the aging process, only the beginning of the curve moves towards higher values, while the declines are almost coincident. This fact confirms the total blocking of the small-size pores. Burger et al. [23] observed a progressive decrease in the specific surface area of NiAl2Ox and NiFeAl2Ox catalysts as time increased in the operation with continuous feeding of CO2 and H2. The authors assigned the decrease to the growth of Ni particles and to sintering of the mixed oxide phase. De-La-Torre et al. [27] also observed a reduction in the specific surface area after hydrothermal aging for Pt-Ba/Al2O3 and Pt-Ce-Ba/Al2O3 NSR catalysts. The authors assigned the decrease to the formation of barium aluminate and the blocking of the pores of the alumina by platinum and cerium.The dispersion of the active phase/s in the DFMs is determined by H2 chemisorption considering a stoichiometry H/X = 1 (X = Ru or Ni) [31]. Table 3 shows the dispersion values (D m) of the reduced DFMs. Very different dispersion values are obtained, comprised in the range 2.2–24.8%. The choice of DFMs with such disparate dispersion values allows us deeping into the influence of the aging process on DFMs. In general, different dispersions are obtained depending on whether the DFMs are based on Ru or Ni and depending on the adsorption phases. The discussion about the different dispersion values can be found in previous works [12,14,19,30].The aging process causes a drastic reduction in the metallic dispersion of DFMs (Table 3). The dispersion values of aged DFMs are comprised in the range 0.9–11.3%, which corresponded to a reduction of 54.1–64.4% compared to the values of the reduced DFMs. Based on these results, it can be confirmed that the continued exposure of DFMs to high temperatures in the presence of O2 and H2O leads to a sintering of the catalytic phase.Transmission electron microscopy (TEM) is used to corroborate the sintering of metallic particles after the aging process. Fig. 3 shows the TEM micrographs of the reduced (left column) and aged (right column) DFMs. The darkest areas with circular-shaped in the micrographs correspond to metallic particles. In the case of DFM 1Ru/10Ni-16Na they can correspond to metallic or bimetallic particles of Ni and Ru [19,34,35]. At this point, the average particle size of the metallic or bimetallic particles is estimated by measuring at least 100 particles and the results are collected in Table 3. The histograms of the distribution of the metallic particles of the reduced and aged DFMs are been shown in the supplementary material (Fig. S1). In general, the particle sizes obtained are similar to that determined by H2-chemisorption. De-La-Torre et al. [27] also observed a considerable reduction in platinum dispersion after hydrothermal aging process for Pt-Ba/Al2O3 and Pt-Ce-Ba/Al2O3 NSR catalysts. The reduction in dispersion was assigned to the sintering of the metallic phase.The basicity of DFMs is determined by CO2 desorption experiments at programmed temperature. Samples were first saturated with a 5% CO2/Ar mixture, and then purged in an inert atmosphere, and finally a temperature controlled ramp was applied in He. During the temperature ramp, the intensity of signal 44 is monitored with a mass spectrometer. Fig. 4 shows the evolution of the CO2 signal as a function of temperature during the CO2-TPD experiments for the reduced (solid line) and aged (dotted line) DFMs. Depending on the desorption temperature, weak, medium and strong basic sites are distinguished. Weak basic sites are unstable and easily decompose below 250 °C. Medium strength basic sites decompose between 250 and 700 °C and strong basic sites are highly stable and do not decompose until 700 °C. All DFMs studied show weak and medium basicity, while only DFM 4Ru-16Ca shows strong basicity. This fact indicates that the strength of calcium carbonates is higher compared to sodium carbonates. A more in-depth analysis of the different types of basicity depending on the phase or phases of the adsorbent can be found in own previous works [12,14,19,30].If the profiles of the reduced samples and the aged samples are compared, both follow a similar evolution. On the one hand, the CO2 desorption for aged Ru-DFMs shifts towards lower temperatures compared to reduced counterparts. On the other hand, the desorption profiles of aged Ni-DFMs present a lower intensity than the reduced counterparts. At this point, it must be taken into account that the aging temperature is different, 400 °C for Ru-DFMs and 550 °C for Ni-DFMs. Furthermore, also the metallic contents are different, 4% for the Ru-DFMs and 10–11% for the Ni-DFMs. The aging process for the Ni-DFMs causes greater coverage of the adsorbent by larger nickel loading. On the other hand, given the lower metallic loading of Ru a lower proportion of the adsorbent phases are covered. Therefore, aging causes agglomeration of the adsorbent and consequently desorption of CO2 at lower temperatures.The catalytic activity is evaluated in cycles of CO2 adsorption and hydrogenation to CH4. Fig. 5 shows a complete cycle for the reduced and aged DFM 4Ru-8Na/8Ca at 400 °C. The cycles have a total duration of six minutes. First, it begins with the adsorption period by introducing a stream of 10% CO2/Ar for one minute, followed by a two-minute purge. Then, the hydrogenation period begins by introducing a stream of 10% H2/Ar for two minutes. Finally, the global cycle ends with an additional one-minute purge. The detailed description of the temporal evolution of reagents and products, as well as the mechanism, can be found in own previous works [12,14]. Table 4 summarizes the chemical reactions proposed in each period for DFMs based on sodium, calcium or both.In the adsorption period, the CO2 is adsorbed, forming carbonates. CO2 can be adsorbed on oxide sites (Eq. 7 and Eq. 8) or on hydroxide sites (Eq. 9 and Eq. 10). On the other hand, in the hydrogenation period, the carbonates decompose due to the presence of hydrogen (Eq. 11 and Eq. 12), the desorbed CO2 is hydrogenated to CH4 (Eq. 1) and part of the produced water remains adsorbed, forming hydroxides (Eq. 13 and Eq. 14). If the evolution of the reagents and products between the reduced and aged DFM is compared, it can be seen that both follow a very similar evolution with small differences in intensity. Therefore, it is concluded that the aging process does not modify the previously proposed mechanism. Fig. S2 shows a complete cycle for the reduced and aged DFM 10Ni-16Na at 400 °C. The previously proposed mechanism is also valid for reduced and aged Ni-DFMs.The CO2 adsorption and hydrogenation cycles to CH4 have been carried out at different temperature ranges for Ru-DFMs and Ni-DFMs. Note again that, given the lower intrinsic activity of nickel compared to ruthenium, Ni-DFMs commonly operate at higher temperatures. Therefore, the samples have been aged at different temperatures, 400 °C (Ru-DFMs) and 550 °C (Ni-DFMs). Hence, the influence of the aging protocol is studied independently for Ru- and Ni-DFMs in the following sections. Fig. 6 shows the evolution of CH4 concentrations profiles during the hydrogenation period for the reduced (solid line) and aged (dotted line) Ru-DFMs in the temperature range 280–400 °C. In all cases, CH4 production begins at minute 3 of the cycle that is immediately after the H2 admission, in correlation with cycle timings shown in Fig. 5. Consequently, the decomposition of carbonates (Eq. 11 and Eq. 12) and their subsequent hydrogenation (Eq. 1) is an instantaneous process with the change of the feed to 10% H2/Ar. Comparing the DFMs with each other, different evolutions and different trends are observed with the increase in operating temperature. In general, the DFMs 4Ru-16Na (Fig. 6a-e) and 4Ru-8Na/8Ca (Fig. 6f-j) show little variability in CH4 concentration evolution with increasing operating temperature. On the other hand, DFM 4Ru-16Ca (Fig. 6k-o) clearly modifies its CH4 concentration evolution with operating temperature. These results are consistent with the different strength of basic sites. The DFM 4Ru-16Ca is the only formulation that presents strong basic sites (Fig. 4), therefore more quantity of CO2 is available to hydrogenate at high temperature.Another important aspect to take into account is the CH4 formation rate. The faster CH4 forms, the greater the utilization of the H2 fed. In own previous works [36,37], the CO2 adsorption and hydrogenation to CH4 operation has been modeled, simulated and optimized. It was concluded that adsorption times close to DFM saturation and moderate hydrogenation times are optimal, in which there is a compromise between the amount of CH4 produced and the conversion of H2 fed. At this point, among the reduced DFMs, the maximum CH4 concentration for DFM 4Ru-16Na is 4700 ppm at 340 °C, while for DFM 4Ru-16Ca is 7300 ppm at 400 °C. Furthermore, the maximum production for DFM 4Ru-16Ca is detected at earlier hydrogenation times. Consequently, Ca-based DFMs has more favourable CH4 formation rate than Na-base one for cyclic operation. On the other hand, DFM 4Ru-8Na/8Ca reaches a concentration of 8000 ppm, also in the first moments of the hydrogenation period. At this point, the simultaneous presence of both adsorbents increases the CH4 formation rate which contributes to improve the compromise between the amount of CH4 produced and the H2 conversion.Comparing the evolutions of the reduced and aged DFMs, in general, they follow the same trend, the concentration of CH4 for the aged DFMs being slightly lower. However, for DFM 4Ru-16Ca when operating at low temperatures (280–340 °C) the CH4 concentration of the aged DFM is higher in the early seconds of the hydrogenation period. This fact is due to the adsorbent agglomeration caused by the aging process. As observed in CO2-TPD experiments (Fig. 4) this favours the desorption of higher amount of CO2 at lower temperature for aged 4Ru-16Ca. Fig. 7a shows the evolution of CH4 production for the reduced and aged Ru-DFMs. The CH4 productions are obtained from the direct integration of the profiles shown in Fig. 6 aplying Eq. (2). To check the reliability of the data, the error, with which the carbon balance is closed is determined (Eq. 5). In all cases, it is possible to close the carbon balance with an error below 5%. The DFM 4Ru-16Ca shows an upward trend with operating temperature and the DFMs 4Ru-16Na and 4Ru-8Na/8Ca show less variability. At this point, the DFM composed of both adsorbents (4Ru-8Na/8Ca) presents the highest CH4 production in the entire temperature range studied. In agreement with that reported in a previous work [30], the modification of the Na2CO3/CaO ratio modulates the basicity of DFM (Fig. 4) and improves the dispersion of the metallic phase (Table 3). Consequently, these aspects promote the CO2 adsorption and hydrogenation to CH4.The CH4 productions of the aged DFMs (hollow symbols linked by dotted lines) decrease with respect to reduced DFMs. Therefore, it is confirmed that exposure to temperature with a stream composed of O2 and H2O limits the activity of DFMs. This limitation, as mentioned above, is caused by the decrease in dispersion due to sintering of the active phase (Table 3) and the agglomeration of the adsorbent. Even so, DFMs composed of a single adsorbent, 4Ru-16Na and 4Ru-16Ca, produce 256 µmol g−1 at 340 °C and 271 µmol g−1 at 400 °C, respectively. On the other hand, DFM containing both adsorbents (4Ru-8Na/8Ca) produces 286 µmol g−1 at 400 °C and in the temperature range 340–400 °C the production does not fall below 275 µmol g−1. At this point, DFM 4Ru-8Na/8Ca is proposed as the most active after the hydrothermal aging in the presence of oxygen. However, the higher activity is not due to a better resistance to aging, but to the higher activity of the reduced DFM. The three Ru-DFMs present a similar aging resistance. For an easier interpretation. Fig. 7b shows the percentages of decrease in the CH4 production of the aged Ru-DFMs with respect to the reduced at the different operating temperatures. Remarkably, no decrease of methane production higher than 25% is observed. In general, the reduction values are between 17% and 25%, with the exception of DFM 4Ru-16Ca at 280 °C which reduction in only 7%. As previously suggested, the agglomeration of the adsorbent leads to desorption of CO2 at lower temperatures, which contributes to maintain the production at similar level for this DFM. Fig. 7c shows the evolution of CO production for reduced and aged Ru-DFMs. In all cases, an upward trend is obtained with temperature and the aging process does not modify the quantity produced. All Ru-DFMs are highly selective to CH4, in all cases, the selectivity is above 90% (Eq. 4). Furthermore, in DFMs with Ca, the selectivity is above 95%. Previous studies carried out by other authors [38,39], or by ourselves [12,14], reported that the presence of Ca favours the selectivity to CH4 while the presence of Na favours the formation of CO.Based on the results of Ru-DFMs, it can be concluded that although the aging process limits the textural properties; considerably high CH4 productions are still obtained. The fact that all Ru-DFMs studied are highly resistant to aging indicates the possibility of operating for long periods. Fig. 8 shows the evolutions of the CH4 concentrations of the reduced and aged Ni-DFMs at different operating temperatures. For both reduced DFMs a strong dependence on the operating temperature is appreciated. At low temperatures (280 °C) the maximum CH4 concentration is limited. Subsequently, it increases markedly at moderate temperatures (360–440 °C), and the maximum CH4 concentration decreases again at higher temperatures (520 °C). On the other hand, comparing the DFMs with each other, it is clearly observed that the promotion of Ni-DFM with small amount of Ru (1% wt.) boosts the CH4 production. Furthermore, Ru-promoted Ni-DFM (1Ru/10Ni-16Na) exhibits significantly faster CH4 formation rate. In the previous sections of characterization, the Ru-promotion of a Ni-DFM boosted the metallic dispersion. In agreement with higher melting point of Ru relative to Ni, it is suggested that ruthenium acts as a shell protecting from sintering nickel in the nucleus during the calcination step [34]. Besides, the strong interaction between Ni and Ru also prevents nickel nanoparticles from sintering [19,40]. Tsiotsias et al. [41] in their review analysed bimetallic Ni-Based catalysts for CO2 methanation. They conclude that the insufficient low-temperature activity, low dispersion and reducibility, as well as nanoparticle sintering of Ni-based catalysts can be partly overcome via the introduction of a second transition metal (e.g., Fe, Co) or a noble metal (e.g., Ru, Rh, Pt, Pd and Re). Through Ni-M alloy formation, or the intricate synergy between two adjacent metallic phases, new high-performing and low-cost methanation catalysts can be obtained. Renda et al. [42] and Zeng et al. [43] in their studies also obtained similar conclusions.Regarding to aged samples, a noticeable decrease in CH4 concentration can be observed with respect to reduced sample. This limitation is accentuated in the second minute of hydrogenation (minute 4 of the cycle). It is proposed that with the higher metallic loading (10–11%) the aging process modifies the proximity between the adsorbed carbonates and the available metallic sites. In fact, a significant decrease in the dispersion of the metallic phase and the coverage of the adsorbent phase by the metal has been observed for aged DFMs by characterization techniques. Consequently, a significant proportion of carbonates do not have nearby metal sites available for decomposition and hydrogenation.For a more in-depth interpretation, the profiles in Fig. 8 were integrated (Eq. 2) and the evolution of CH4 production per cycle with reaction temperature is shown in Fig. 9a. In all cases, it is possible to close the carbon balance with an error below 5% (Eq. 5). Both DFMs present a similar trend with a maximum of CH4 production at intermediate temperatures (400 °C) as has also been observed in Fig. 8. The reduced DFMs 10Ni-16Na and 1Ru/10Ni-16Na yield 172 and 250 µmol g−1, respectively. On the other hand, analysing the productions of the aged DFMs, a significant decrease is clearly appreciated. For an easier interpretation, Fig. 9b shows the percentages of decrease in the CH4 production of Ni-DFMs at the different operating temperatures. On this occasion, compared to the Ru-DFMs (Fig. 7b) the decrease is significantly greater. In fact, production is reduced by up to 60% for the DFM 1Ru/10Ni-16Na operating at 280 °C. At this point, keep in mind that nickel-based catalysts or DFMs commonly operate at higher temperatures than ruthenium-based ones. Therefore, to emulate a long period of operation at a higher temperature, the aging process of Ni-DFMs has been carried out at 550 °C compared to 400 °C of Ru-DFMs. Consequently, the aging process is carried out under more severe conditions in the case of Ni-DFMs.Comparing both Ni-DFMs with each other, 1Ru/10Ni-16Na shows a greater decrease in CH4 production at all operating temperatures (Fig. 9b). It is suggested that improvements in the textural properties due to synergistic effects between ruthenium and nickel in the reduced DFM are limited after the aging process. Even so, in general, 1Ru/10Ni-16Na presents a higher CH4 production in the studied operating temperature range. A production of 137 µmol g−1 is obtained at 360 °C for DFM 1Ru/10Ni-16Na and 108 µmol g−1 at 400 °C for DFM 10Ni-16Na. Fig. 9c shows the evolution of CO production of reduced and aged Ni-DFMs. Again, an upward trend is obtained with temperature, however, this time the aging process also decreases the amount of CO produced. DFM 10Ni-16Na exhibits low selectivity to CH4. In fact, at 400 °C, being the temperature at which the CH4 production and its selectivity is maximized, both for the reduced and for the aged DFMs, the selectivity is 88%. On the other hand, 1Ru/10Ni-16Na presents a selectivity above 95% in the temperature range 320–400 °C, both for reduced and aged DFM.The ability to produce CH4 from DFM 1Ru/10Ni-16Na with high selectivity after the aging process stands out. Despite the fact that the textural properties are remarkably diminished and the CH4 production is remarkably decrease, it is possible to produce 137 µmol g−1 at 360 °C with fast CH4 formation rate. Fig. 10 shows the accumulated methane production with the duration of the hydrogenation period. At 0.7 min (3.7 min of the complete cycle), 100 µmol g−1 was produced, confirming the adequate CH4 formation rate of the aged DFM for the dual process of CO2 adsorption and hydrogenation to CH4.Based on the results of the Ni-DFMs, it can be concluded that aged DFM 1Ru/10Ni-16Na still exhibits acceptable CH4 production with high selectivity and fast CH4 formation rate, allowing proper use of hydrogen. However, the DFM 10Ni-16Na has poor catalytic activity after the hydrothermal aging in the presence of oxygen and is therefore not suitable for long periods of operation.Ru/Ni-Na/Ca-Al2O3 DFMs with different formulation have been aged, characterised and evaluated in cycles of CO2 adsorption and hydrogenation to CH4. The aging process causes a decrease in the textural properties of all DFMs. Furthermore, the dispersion of the metallic phase is also reduced. The continuous exposure of DFMs to temperature in the presence of oxygen and steam causes the sintering of the metallic phase, the agglomeration of the adsorbent phase and the blocking of smaller alumina pores. In Ru-DFMs, adsorbent agglomeration shifts CO2 desorption at lower temperatures. However, in Ni-DFMs CO2 desorption is decreased after the aging process. This difference is assigned to the greater coverage of the adsorbent phase due to a higher nickel loading (10–11 vs. 4%) and to the higher temperature of the aging process for the Ni-DFMs (550 vs. 400 °C).In the activity in cycles of CO2 adsorption and hydrogenation to CH4, all the aged DFMs show, in the entire temperature range, a lower CH4 production compared to the reduced DFMs. Consequently, continued exposure of DFMs to temperature in the presence of oxygen and steam also limits activity. However, the decrease in the CH4 production does not exceed the 25% for Ru-DFMs subjected to aging process. Therefore, despite the fact that the aging process limits the physicochemical properties of Ru-DFMs, considerable CH4 productions are obtained with adequate CH4 formation rate. From that it can be concluded, that all Ru-DFMs studied are suitable for long operation periods. Specifically, the aged DFM 4Ru-8Na/8Ca produces 286 µmol g−1 at 400 °C. Furthermore, in the range 340–400 °C, the production is above 275 µmol g−1 with a selectivity greater than 95%. As the decrease in CH4 production after the aging process is comparable for all Ru-DFMs, 4Ru-8Na/Ca is proposed as the best alternative given the higher activity of the reduced DFM.The catalytic activity in Ni-DFMs is considerably reduced after the aging process. CH4 production is limited, especially in the second minute of the hydrogenation period. It is proposed that given the higher metallic loading (10–11%), the aging process limits the proximity between the carbonates and the metallic sites. The aged DFM 1Ru/10Ni-16Na produces 137 µmol g−1 at 360 °C with a selectivity greater than 95%. Furthermore, in the first 0.7 min of the hydrogenation period, 100 µmol g−1 are produced. Therefore, it can be concluded that despite the fact that the aging process considerably decreases the physicochemical properties and the activity, the DFM 1Ru/10Ni-16Na continues to present acceptable CH4 production with adequate CH4 formation rate for the dual operation of CO2 adsorption and hydrogenation to CH4. The higher CH4 production of the aged DFM is due to the higher activity of the reduced DFM. Nevertheless, 10Ni-16Na DFM is not suitable for long periods of operation due to its poor catalytic activity after the hydrothermal aging in the presence of oxygen. Alejandro Bermejo-López: Validation, Methodology, Investigation, Writing – original draft. Beñat Pereda-Ayo: Conceptualization, Methodology, Visualization, Writing – review & editing. Jon A. Onrubia-Calvo: Methodology, Visualization, Writing – review & editing. José A. González- Marcos: Methodology, Data curation, Supervision, Funding acquisition. Juan R. González-Velasco: Conceptualization, Supervision, Project administration, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The financial support from the Science and Innovation Spanish Ministry (PID2019-105960RB-C21) and the Basque Government (IT1297-19) is acknowledged. The authors thank for technical and human support provided by SGIker (UPV/EHU Advanced Research Facilities/ ERDF, EU). One of the authors (JAOC) acknowledges the post-doctoral research grant (DOCREC20/49) provided by the University of the Basque Country.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jece.2022.107951. Supplementary material .

Integrated CO2 capture and utilization (ICCU) is a promising alternative to revalue CO2. In this work, the influence of the aging process on dual function materials (DFMs) Ru/Ni-Na/Ca-Al2O3, for the conversion of CO2 into CH4 is studied. DFMs are characterized by N2 adsorption-desorption, XRD, H2 chemisorption, TEM and CO2-TPD. The catalytic behavior of the prepared DFMs is analyzed in consecutive cycles of CO2 adsorption and hydrogenation to CH4. The aging process notably limits the physicochemical properties, especially the metallic dispersion. However, the CH4 production decrease is less than 25% for aged Ru-DFMs, which makes them suitable for long-term operation. The aged DFM 4Ru-8Na2CO3/8CaO-Al2O3, presents a CH4 production greater than 275 µmol g−1 with high selectivity in the range 340–400 °C. On the other hand, the aging process is more noticeable for Ni-DFMs; in fact, it limits the CH4 production to half compared to reduced Ni-DFMs.

The world consumption of vast quantities of fossil fuels and its influence on climate change and environmental issues have sparked a lot of interest in developing clean, environmentally friendly, and renewable energy systems. Due to its greater specific gravity density than other fuel cells, hydrogen (H2) is an excellent and clean energy carrier with zero carbon dioxide emissions and a higher gravimetric energy density than other fuels that may be utilized in fuel cells. 1 H2 has also been employed as a reducing agent in a variety of sectors, including ammonia synthesis, hydrocarbons hydrogenation, and metal manufacturing. Unfortunately, coal steam gasification or methane reforming accounts for more than 95% of the hydrogen manufacturing business, which uses non-renewable fossil fuels and generates significant volumes of carbon dioxide. As a result, it is critical to produce green hydrogen using non-polluting and eco-friendly methods. Electrochemical water splitting is a crucial energy conversion reaction that converts plentiful and simple H2O molecule into valuable H2 and O2 molecules. 2 Specifically, alkaline electrochemical water splitting is one of the most exciting approaches for generating hydrogen via cathodic hydrogen evolution reaction (HER) with high conversion efficiency and a broad potential range. 3–7 It plays a great role to treat the excreted alkaline media such as chlor-alkali and water-alkali electrolyzers. 8 Theoretically, the thermodynamic equilibrium potential for the overall water electrolysis is approximately 1.23 V at standard conditions. 9 However, experimentally, the operating potential is far beyond the thermodynamic equilibrium potential due to the sluggish kinetics, especially for the alkaline HER, where the difference value is called overpotential (η) applied to overcome the reaction energy barrier. 10–12 In this context, it is crucial to design electrocatalysts that can give high performance and long-term stability at lower overpotential and minor parasitic reactions. 13–15 Even though platinum-based catalysts are the most effective for HER, their widespread utilization is hampered by their high cost and insufficient reserve. 16–20 Consequently, it is highly appealing to explore alternative electrocatalysts based on transition-abundant elements, such as metal sulfides, oxides, phosphides, borides and carbides to replace the noble metal-based catalytic materials. 21–23 Besides, there are many strategies to improve the catalyst performance for HER, such as exposing high-active facets, 24 , 25 constructing nanostructures, 26 modifying electronic structures 27 and doping with other elements. 20 Nonetheless, low activity still plagues non-precious metal-based catalysts. Furthermore, the chemical composition and density of the active sites impact the catalytic activity of HER. 28–30 Single-atom catalysts (SACs) have been demonstrated to be indispensable materials in electrochemical energy conversion and storage applications. 31–33 They are well-defined mononuclear active sites in which all active metal species are isolated and maintained by the support of or alloying with another material. Compared with nanoparticles, SACs do not have metal-metal bonds and have positive charges. The reduction of the metal atoms generates plenty of unsaturated coordination centers which stimulate high surface energy of the metal components resulting from the high chemical interaction affinity of the metal center with the support and adsorbates. The significant electronic and geometric properties of SACs have displayed a potential change in the reaction pathways. 34 , 35 The simplicity and homogeneity of SACs facilitate the identification of active sites and correlation of the structure-activity relationships. Moreover, SACs are considered the bridge between the homogenous and heterogeneous catalysts. 36 SAC structures range from single metal atoms supported on a mesoporous oxide surface to those supported on single-layer materials to isolated surface metal atoms as part of an alloy, and beyond. 37 There are generally two strategies to improve the SAC performance, namely tuning the intrinsic properties of SACs 38 and increasing the metal amount on the support to increase the active site concentration. 39 More importantly, SACs allow the use of a tiny amount of noble metals, resulting in a decrease in cost while achieving increased mass activity for HER, which is highly promising for practical applications.In this review, we provide a timely overview of the very recent development of SACs in the area of alkaline HER. While several review papers have covered the topic of SACs for HER, 40–46 a comprehensive review dedicated especially to the alkaline HER application of SACs is still lacking in the literature and is in need of time given the large interest of researchers. We begin by introducing the fundamentals of alkaline HER and basics of SACs. A special focus is given on the rational design of SACs categorized into four aspects, including regulating the inherent element properties, adjusting the coordination environment, tuning the SAC morphology, and increasing the mass loading of single atoms on the substrate surface. Finally, major challenges and prospects for further research on SACs for alkaline HER are highlighted. With the present review, we hope to feature the outlines of designing efficient SACs to be used for hydrogen production in alkaline media.HER is a two-electron transfer and a multi-step electrochemical reaction that occurs on the electrode surface. It takes place in a wide range of pH; acidic, neutral, and alkaline media. Proton or molecule H2O adsorption, quick proton union (combination), and rapid electron transfer to the active centers all play a role in this process. Volmer-Heyrovsky and Volmer-Tafel pathways are the two mechanisms that govern HER. More accessible protons created by a simple reduction of the hydronium ion promote HER production in acidic environments. In alkaline media, on the other hand, it takes more energy to break the covalent link H–O–H of H2O molecules to form protons, which governs the total reaction kinetics. 47 , 48 Alkaline HER is regulated by three stages, according to theoretical and experimental studies: water adsorption, water dissociation, and adsorbed hydrogen intermediate. 45 In alkaline electrolytes: ∗ indicates the single-atom catalyst surface.Adsorption of water molecules on catalyst surface 45 H 2 O ​ + ∗ →  ​ ​ ​H 2 O ∗ i) Proton adsorption in Volmer reaction: Coupling of H2O molecule and an electron to form adsorbed hydrogen intermediate on the electrode surface (H∗).  ​ ​H 2 O ​ + e − → H ∗ + OH − Volmer ​reaction ​ ​ ​ ( 120  ​ ​mV ​dec − 1 ) T S 1 ,  V = 2.3 R T α F where T s is the Tafel slope, R is the universal gas constant, T is the absolute temperature, α is the symmetry coefficient (0.5), and F is the Faraday constant. ii) Electrochemical desorption: H∗ combines with a water molecule. H 2 O ​ + e − +  ​H ∗ → H 2 + OH − Heyrovsky ​reaction ​ ​ ​ ( 40  ​ ​mV ​dec − 1 ) T S 2 ,  H = 2.3 R T ( 1 + α ) F iii) Chemical desorption: two H∗ couple together on the electrode surface to form H2 molecule. H ∗ + H ∗ → H 2 Tafel ​reaction ​ ​ ( 30  ​ ​mV ​dec − 1 ) T S 2 , ​T = 2.3 R T 2 F Proton adsorption in Volmer reaction: Coupling of H2O molecule and an electron to form adsorbed hydrogen intermediate on the electrode surface (H∗).Electrochemical desorption: H∗ combines with a water molecule.Chemical desorption: two H∗ couple together on the electrode surface to form H2 molecule.Experimentally, Tafel analysis is a great method for gaining experimental insight into the mechanism of HER happening at the catalyst surface. In Tafel analysis, the logarithm of current or current density is plotted against the (over)potential, the linear portion of which is fitted to the Tafel equation to extract the Tafel slope. According to Butler-Volmer kinetics, the theoretical values for the Tafel slope are 120, 40, and 30 mV dec−1, corresponding to the Volmer, Heyrovsky, and Tafel reaction, respectively, being the rate-determining step. Theoretically, a promising HER catalyst should have a d band that can extend across the Fermi level and have a strong coupling to hydrogen. 45 Hydrogen adsorption Gibbs free energy (ΔG H) is considered the major descriptor to evaluate the catalyst performance. Additionally, overpotential, Tafel slope value, exchange current density, stability, Faradaic efficiency and turnover frequency are also good measures of the catalytic HER activity.There is a great effort to develop HER catalysts in alkaline system to substitute the acidic system to overcome the dissolution of some catalysts and the difficulties of finding inexpensive anodes stable in acidic solutions in addition to the cost and safety concerns. 49 , 50 Due to the large activation energy of the water dissociation, the benchmark Pt-based catalyst performance for alkaline HER is roughly 2–3 orders of magnitude lower than in acidic media. As a result, the poor hydrogen kinetics associated with H2 generation in alkaline media is a significant obstacle. 51–53 The features of binding hydrogen species and dissociating water are required for the rational design of electrocatalysts with excellent alkaline HER performance.Atomically dispersed active sites have attracted great attention as a new frontier in the catalysis field. 54 , 55 Supported metal nanoparticles maximize the efficiency of atom utilization and provide the opportunity to alter the reaction pathway. With the supported nanometal catalysts being mostly surface atoms, SACs display an isolated atom from each other and high tunability owing to the strong interaction between the metal center and the substrate. While the intrinsic activity of nanoparticles is determined by the accessibility of the exposed edges, defects or corners and the interfaces between two phases, the down-scaling of the catalysts to single atoms could extremely improve the activity and durability. SACs could maximize the atom-utilization efficiency, leading to cost-effectiveness, in particular for the noble metal-based catalysts such as Pt, Ru, Ir and Pd. 56–58 Surprisingly, SACs not only have homogenous active centers for the reactions due to the unsaturated coordination atoms as homogenous catalysts but also possess the advantages of reusability and stability similar to the heterogeneous catalysts. 31 Moreover, the potential metal-support interactions could modulate the electronic structure of the metal atoms because of the electron transfer between the substrate and metal centers. 16 Theoretically, SACs have a unique HOMO (highest occupied molecular orbital)-LOMO (lowest occupied molecular orbital) gap due to the quantum size effect reflecting distinct energy level distribution. 49 , 59 The hybridization with atoms could generate asymmetrical spin and charge density. 55 As a result, synthesizing SACs and sustaining atomic dispersion of single metal atoms in the face of particle agglomeration under realistic synthesis and reaction circumstances are significant challenges. Currently, various synthetic methodologies have been employed to prepare SACs aiming to obtain high SACs capacity on the support surface and enhance the stability of dispersed atoms on the host framework. The high-temperature pyrolysis technique, wet-chemistry approach, and physical and chemical deposition method are the most prevalent synthesis procedures for SACs. 53–56 The structure and properties of SACs, as well as the chemical state of the metal center and metal-support interactions, are often studied and verified using advanced characterization techniques such as atomic-resolution aberration-corrected scanning transmission electron microscopy (STEM) and synchrotron-based X-ray absorption spectroscopy (XAS). 60–62 Furthermore, because SACs have essentially homogenous single dispersed active sites, the catalytic process may be identified using rational design and computation techniques like density functional theory (DFT).The performance of HER catalysts is related to the chemical nature and density of active centers. Furthermore, the development of alkaline HER electrocatalysts is critical to addressing the reaction kinetics and stability difficulties. Tuning the surface chemistry by altering the electronic structure, composition, morphology, and porosity/active surface area can solve these problems. Downscaling the particle size results in a greater volume-to-surface-area ratio of SACs, which allows them to tune the atomic distribution and electronic structure. For alkaline HER catalysis, noble metal-based and non-noble metal-based catalysts (e.g., Co, Ni, Fe, V) have recently been explored. Pt-based catalysts are the most efficient electrocatalysts for HER, with higher mass activity than nanoparticle catalysts. Non-noble metal-based SACs, on the other hand, have been widely adopted due to their significantly reduced cost. 44–50 SACs are different from cluster catalysts and nanoparticle catalysts. 31 , 32 The surface of catalysts including certain atoms featuring unsaturated coordination, such as the atoms at defect sites, edges, and vertices influence the catalytic reactions. Thus, downsizing the nanoparticles and more undercoordinated surface atoms on the surface have been used to increase the catalyst efficiency. This smaller particle often has size, structural effect as well as coordination environment effect that grant the promising physicochemical property of the catalysts. SACs have gained momentum since Zhang et al. originally introduced them in 2011, 63 particularly in the electrocatalytic sector. Numerous SACs have demonstrated exceptional catalytic properties for electrochemical water splitting. According to the different types of support materials, SACs can be divided into many different types, such as alloy-based SACs, carbon-based SACs, and SACs supported on other compounds. 42 The excellent atom utilization of SACs and one-of-a-kind size quantum impact have aroused interest in catalysis and chemical transformation applications. The development of a flexible and simple synthesis approach to modify the interaction between the metal centers and supports can be used to tune the inherent features of SACs. 64 , 65 In the alkaline HER electrocatalysis, key challenges for SACs include the exact control over the local structure of single-atom sites and the increase of the active-site density. The intrinsic activity of atomic structures is determined by their rational design, which affects the activation and adsorption of reactants across single sites. 49 , 66 Increasing the metal loading of SACs, on the other hand, would greatly increase the density of active sites and the related mass activity.It is known that a SAC made of a certain element should have its unique chemical properties. Tailoring the electronic properties and structures can effectively control the adsorption behaviour during the HER and consequently the catalytic activities. 67 , 68 By employing DFT calculations, Chen et al. performed a systematic investigation on the HER performance of more than 20 different single transition-metal (TM) atoms implanted in phosphorus carbide monolayer (α-PC) (denoted TM@α-PC). 69 It was found that all the TM doped α-PC monolayers are energetically stable. The Gibbs free energy for hydrogen adsorption, which concerns adsorption sites of either TM sites or reversed P site, was found to show a volcano-shaped relationship with respect to the HER activity (characterized by the overpotential) (Fig. 1 a). 69 The absolute adsorption energies of Ir-αPC (TM site), Fe-αPC (TM site), Cu-αPC (P site), and Rh-αPC (P site) are all lower than that of Pt (111), with Ir-αPC (TM site) attaining the ideal value of 0.008 eV. Computational screenings have also been conducted on other SACs, such as TM atoms supported on a C9N4 monolayer (TM@C9N4) and on a graphdiyne monolayer (TM@GDY) (Fig. 1b and c). 49 , 70 In a similar fashion, volcano relationships were obtained between the Gibbs free energy for hydrogen binding and the HER activity (characterized by exchange current), from which optimal SACs can be determined. These theoretical studies may provide useful guidelines for the experimental development of advanced SACs toward the alkaline HER.While the catalytic efficacy of SACs can be modified by selecting a proper active center, the simplicity of a single-atom center sometimes makes it difficult for further materials tuning to achieve improved catalytic performance. Incorporating a secondary metal atom to fabricate a metal-metal dual atom site (single-atom dimer: SAD) has the potential to further alter the electronic structure of SACs and increase their intrinsic activity, which is likely due to the distinctive atomic interface and synergistic effect of the dual-metal site. 71–73 Inspired by this approach, Kumar et al. theoretically and experimentally reported the synergistic interaction between Ni–Co at the atomic level in the SAD configuration for the alkaline HER (Fig. 2 ). 74 Firstly, different transition metal-based SADs (TM-SADs) anchored on N-doped carbon (NC) were studied by DFT calculations for the HER catalysis in alkaline media (Fig. 2a). The d-band center of TM-SADs was found to correlate linearly with the kinetic barrier for water dissociation (Fig. 2b). Of note, the d-band centers of Co and Ni atoms in the NiCo-SAD-N6C sample were the nearest to the Fermi level (−0.87 eV), showcasing its excellent capability to bring fast water dissociation and favorable proton adsorption, both beneficial to the alkaline HER kinetics. 75 Following this theoretical understanding, the authors experimentally prepared NiCo-SAD supported on NC (NiCo-SAD-NC) by trapping Ni/Co ions in the polydopamine spheres followed by annealing. Combined aberration-corrected high-angle annular dark-field STEM (HAADF-STEM) and electron energy loss spectroscopy (EELS) studies verified the emergence of Ni–Co dual sites in the NiCo-SAD-NC sample with an average dimer distance of 0.241 nm (Fig. 2c–f). Due to the strong electronic coupling between the Ni–Co dual sites at the atomic level, the NiCo-SAD-NC catalyst exhibited outstanding HER activity in 1 M KOH (overpotential of only 61 mV at −10 mA cm−2), much superior to the monoatomic Ni-SAC or Co-SAC, the NiCo nanoparticle counterparts, and the commercial Pt/C benchmark (Fig. 2g and h). This work thus offers a viable approach to leverage the dual-metal atom synergism for the design of highly efficient SAC-based HER electrocatalysts. Considering the ratio of dimer structure in NiCo-SAD-NC to be about 78%, developing methodologies that can achieve pure SADs may further improve the HER performance.The coordination environment is another factor with a profound effect on the catalytic performance of SACs. 76 It is clear that depending on the coordination environment the single atom is embedded in, the corresponding SAC should exhibit different HER activities. Very often, the great diversity of SACs and supports can present a challenge to determine an optimum coordination scenario. Ma and colleagues employed DFT simulations to screen a series of TM single atoms (from Ti to Zn and from Zr to Cd) embedded at the different vacancy sites of the MoSSe monolayer as HER electrocatalysts. 77 The stability of the formed SACs was first assessed by calculating the binding energy (E b) of the 18 different TM atoms anchored on Mo, S, and Se vacancy (TM@MovaSe, TM@MoSvaSe, TM@MoSSeva, respectively) (Fig. 3 a and b). The negative value of E b means strong bonding between the TM atoms and the defective MoSSe substrate, while the positive value indicates difficulty in the adsorption of TM atoms. Based on the results of E b, 23 SAC structures could be excluded due to the positive E b values. In addition, a general repeated trend for E b of TM@MovaSSe in the same row of the periodic table was found, namely, E b increases as elementary metallicity decreases. The hydrogen adsorption process on various TM@MoSSe surfaces was further studied (Fig. 3c and d). Three candidates, i.e., Zn@MoSvaSe, Cd@MoSvaSe and Co@MovaSSe were found with near-zero Gibbs free energy for HER, with values of around −0.095, −0.098, and −0.050 eV, respectively, comparable to the ideal Pt-SACs (about −0.070 eV). These theoretical investigations highlight the importance of anchoring single-atoms to the appropriate site of a support material in bringing about optimized HER performance.As mentioned, surface defects on the supports can be used to stabilize the single atoms, 77 sometimes via an increased charge–transfer process, preventing the isolated atoms from aggregating. 59 In addition, the defect-induced stabilization effect could also originate from the intimate interaction of the resulting SAC structure. 78 Recently, Zhang et al. rationally constructed a single atom ruthenium SAC anchored on defective NiFe layered double hydroxide nanosheets (Ru1/D-NiFe LDH) for use in the alkaline HER (Fig. 4 ). 79 Ru1/D-NiFe LDH was fabricated using a simple electrodeposition and etching method (Fig. 4a and b). The combined aberration-corrected TEM and X-ray absorption spectroscopy measurements including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) confirmed the existence of Ru single atoms with good homogeneous dispersion on the surface of defective NiFe LDH (Fig. 4c–e). The presence of the defects and Ru single atoms was found to have a synergistic effect in increasing the active site density and promoting the charge transfer process. As a consequence, Ru1/D-NiFe LDH displayed exceptional HER performance in alkaline solutions. The optimum HER activity for the Ru1/D-NiFe LDH system was observed on the sample with a moderate Ru loading of 1.2 wt%, requiring an ultralow overpotential of 18 mV to reach a HER current density of 10 mA cm−2 in a 1 M KOH electrolyte while also giving a stable operation for 100 h at current densities of about 10 and 100 mA cm−2 (Fig. 4f–g). The quantity of surface defects present is likely to limit the loading amount of single atoms and eventually the HER activity of the SACs. It is intriguing to study the influence of defect concentration/amount on the stabilization and HER performance of SACs.The homogeneous atomic coordination environment of SACs makes them a suitable and simplified model system for studying catalytic processes both mechanistically and experimentally. Rodriguez and colleagues proposed the electronic metal-support interaction (EMSI) as one of the ways to improve the electronic characteristics and to regulate the interaction between the single atom and the support. 80 Due to orbital rehybridization and charge transfer for the metal-support interface, the EMSI can result in the formation of new chemical bonds and the realignment of molecular energy levels. 81 , 82 Shi et al. recently found that the EMSI can be leveraged to realize the fine tailoring of the oxidation state of single-atom Pt. 17 By applying a site-specific electrodeposition method, 83 they loaded Pt single atoms onto four types of different 2D transition metal dichalcogenides (TMDs) (MoS2, WS2, MoSe2, and WSe2) (Fig. 5 a). The core anchoring chalcogen (S, Se) and the neighboring transition metal (Mo, W) were found to synergistically regulate the electronic structure of SAMC through EMSI, giving rise to Pt SACs with fine control over the Pt oxidation state ranging from +1.24 to +2.61 (Fig. 5b). Notably, the Pt SACs supported on MoSe2 (Pt–SAs/MoSe2) with a suitable Pt oxidation state (+2.11) exhibited the optimal HER activity in 1 ​M KOH (Fig. 5c), due to its neither too weak catalyst–OH interaction for water dissociation nor too strong catalyst–H interaction for hydrogen release. As schematically illustrated in Fig. 5d, the kinetic rate of water dissociation increases with the increase in oxidation state of single-atom Pt while the kinetic rate of H2 desorption decreases, hence optimal performance is achieved at a moderate Pt oxidation state. This work highlights how engineering the interaction between single atoms and supports by modifying the nature of support materials can result in different electronic and catalytic effects of the SACs. One question remains open as to if the Pt oxidation state can be tailored to a near-zero value and its effect on the alkaline HER catalysis.The potential cohesive energy between the atoms induces aggregation when the metal is split into single atoms. 84 Anchoring the single atom on the supporting molecule via a strong chemical interaction is an excellent technique for addressing the agglomeration challenge. This interaction not only affects the stability of metal atoms, but also influences the catalytic activity by modulating the electronic structure of single atoms. 85–89 The support has a similar role as the ligand in homogenous catalysts to stabilize the metal site. The type of support influences the coordination number, strain environment, as well as the chemical interaction of metal sites that is reflected in obtaining different electronic and geometric structures of SACs. 90 Metals, metal (hydro)oxides/nitrides/carbides, and carbon-based nanomaterials have all been employed as supports. 63 , 87 , 91 , 92 However, the conventional support materials may not be sufficient for use in the alkaline HER which involves adsorption toward both OH∗ and H∗ species. Interestingly, Zhou et al. reported the adoption of a 2D NiO/Ni heterostructure as a novel support for Pt single-atoms (PtSA-NiO/Ni), which could provide dual active sites to independently regulate the binding strength of OH∗ and H∗ (Fig. 6 ). 16 By using a facile electrodeposition method, Pt single-atoms were successfully immobilized in NiO/Ni nanosheets, as evidenced by HAADF-STEM image (Fig. 6a). The as-obtained PtSA-NiO/Ni catalyst exhibited excellent performance for hydrogen production in alkaline media, showing a mass activity much greater than Pt single-atoms supported on NiO or Ni alone (Fig. 6b–c). DFT computations suggest that the dual active sites comprising metallic Ni sites and oxygen vacancies-modified NiO sites adjacent to the interfaces of the NiO/Ni heterostructure are responsible for the high alkaline HER activity (Fig. 6d–g). The former efficiently promotes water adsorption, reaching a barrier-free water dissociation step with a lower energy barrier of 0.31 eV in the Volmer step compared with that of PtSA-Ni (0.47 eV) and PtSA-NiO (1.42 eV), while the latter offers more suitable hydrogen binding (−0.07 eV) than that of PtSA-Ni (−0.38 eV) and PtSA-NiO (0.74 eV), together accelerating the overall alkaline HER kinetics. This work paves the way for the advancement of alkaline HER SACs by coordinating single atoms with heterostructure supports. Introducing a silver nanowire network into the 2D PtSA-NiO/Ni resulted in a seamlessly conductive 3D nanostructure, which brought further enhancement of the alkaline HER performance. 16 The geometric structure of electrocatalysts plays an important role in the distribution of active sites that also controls their adsorption nature and catalytic performance. Theoretically, the utilization of SAC active sites could be 100%, however, the practical efficiency of the active sites is still less than 15% with mass loading >2 wt% due to the aggregation within the support and/or encapsulation of the active sites. 93 , 94 Finding strategies to optimize the SAC structure by constructing more accessible surfaces or interfaces is of high importance. Fei et al. 95 used a two-step technique to build a single cobalt atom on nitrogen-doped graphene architecture (Co-NG) via sonication, followed by freeze-drying and pyrolysis in ammonia atmosphere. Co-NG exhibited similar morphologic features to graphene, showing nanosheet-like structures with ripples observed on the surface (Fig. 7 a and b). Thanks to the single-atom nature and the favorable morphological character, the Co-NG catalyst showed appreciable catalytic activity in alkaline solutions (Fig. 7c). Jiang and coworkers employed nanoporous MoS2 with a bicontinous structure as a template to fabricate a single ruthenium atom catalyst (Ru/np-MoS2) (Fig. 7d). 96 Changing the ligament size of the nanoporous MoS2 can be used to precisely tailor the strain of the catalyst as induced by the sample's curvature. As shown in Fig. 7e, such a bending strain could effectively regulate the electronic structure of single-atom Ru, hence effectively catalyzing the water dissociation and H–H coupling. As a result, Ru/np-MoS2 demonstrated a low overpotential of 30 mV to afford a current density of 10 mA cm−2 and a Tafel slope of 31 mV dec−1 toward the HER in 1 M KOH. While effective, morphology tuning in the design of SACs for alkaline HER is relatively less explored. It is suggested that more design strategies should be developed to tune the morphology of SACs for the HER in basic media.SACs are a rising star in catalysis due to the unique features of combining the merits of both heterogeneous and homogeneous catalysts. Although tailoring the morphology of SACs influences the available active sites for the reaction, the number of active sites is also important to the catalysis, which is mainly dependent on the SAC mass loading on the support surface. In other words, a higher mass loading of SAC would give more exposed active sites for the reactions of interest that could enhance the catalytic activity. Li and coworkers reported Pt single atoms incorporated in a nitrogen-doped porous carbon (Pt1/NPC) with Pt loading of up to 3.8 wt% relative to the carbon. 97 Because of the large specific surface energy of SACs, the primary hurdles to obtaining the high mass loading of SACs are migration and agglomeration tendency of the active atoms during synthesis or applications. 64 , 94 , 97–104 When a large number of metal atoms are fixed to a support surface, they invariably aggregate into nanocluster particles rather than disperse as single metal atoms. In this regard, effective synthesis strategies for constructing SACs with high mass loading content are critical. Wei and coworkers developed an iced-photochemical reduction approach to synthesize Pt SACs on different substrates including carbon nanotubes, graphene, mesoporous carbon, zinc oxide nanowires and titanium dioxide nanoparticles. 105 The atomically dispersed Pt SACs were formed via the exposure of the frozen chloroplatinic acid solution to UV light to prevent agglomeration and obtain high mass loading (approximately 13%).More recently, Zhang et al. reported that electrochemical deposition (either cathodic or anodic deposition) can be utilized as a generic strategy to fabricate SACs (Fig. 8 a), as demonstrated by the successful synthesis of SACs consisting of a wide variety of metals (metal: Ru, Rh, Pd, Ag, Pt, and Au) and supports (support: MnO2, MoS2, Co0.8Fe0.2Se2, and NC). 106 Importantly, the loading amount of single atoms can be facilely controlled by varying the concentration of metal precursors, the number of scanning cycles, or the scanning rate. For example, during the cathodic deposition of Ir in 1 M KOH electrolytes, the mass loading of Ir constantly increased with the increasing Ir concentration. At an Ir concentration of 150 μM, Ir single atoms were still obtained with a mass loading of 3.5% while Ir clusters emerged with a mass loading of 4.7% when Ir concentration further increased to 200 μM, suggesting that for the formation of SACs, there exists an upper limit of mass loading between 3.5% and 4.7% (Fig. 8b). Notably, SACs obtained from cathodic deposition show great potential for catalyzing the alkaline HER, with Ir single atoms on Co0.8Fe0.2Se2 nanosheets delivering the lowest overpotential of 8 mV to achieve a current density of 10 mA cm−2 in 1 M KOH (Fig. 8c). Similarly, Liu et al. established an electrochemical pulse voltammetry method for the preparation of U single atoms on MoS2 nanosheets (U/MoS2) from radioactive wastewater (Fig. 8d–e). 107 The mass loading of U single atoms was controlled by increasing the pulse cycle, and the sample obtained at a pulse cycle of 100 had an appropriate U loading of 5.2% giving the optimum HER activity in 1 M KOH (overpotential of 72 mV at 10 mA cm−2) (Fig. 8f).To summarize, SACs with the lowest size, highest volume-surface ratio, and optimal atom utilization efficiency of any metal-based catalysts provide exciting opportunities for the alkaline HER application including high activity and stability. During the last few years, a wide range of metal-based SACs (e.g., Pt, Pd, Ru, Fe, Co, Ni, Mo, and W) have been systematically explored for the alkaline HER. Table 1 gives a performance summary of the state-of-the-art SACs for the HER in alkaline media. Despite the enormous progress made in the field of SACs for electrochemical HER, there are still obstacles to overcome in this fascinating field of research. For example, the structure-activity relationship of SACs and their catalytic mechanism at the atomic scale remain elusive. To address this, well-defined SAC model systems should be established by advanced synthesis methodologies. In addition, the combined use of operando characterization tools and theoretical calculations (e.g., DFT) should be applied. Due to their intrinsic activity advantage, SACs are very appealing for industrial electrochemical hydrogen generation. One major difficulty, however, lies in the unsatisfactory stability of SACs, especially at high single atomic metal loadings. Furthermore, catalyst evaluation in commercial applications usually involves testing under more harsh conditions (e.g., high temperature and high-concentration electrolytes) than what is currently seen in fundamental lab-based research. Therefore, developing SACs for real water splitting devices represents another research topic worthy to be investigated. With efforts from both research and industry communities, we anticipate to see breakthroughs in SACs for HER catalysis in both fundamental understanding and practical applications.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.F.A. and X.X. contributed equally to this work. This work was supported by the Australian Research Council Discovery Projects (Grant Nos. ARC DP200103332 and ARC DP200103315) F.A. acknowledges the Egyptian Ministry of Higher Education and Scientific Research, Cultural Affairs and Missions Sector, Egypt, for a PhD scholarship.

Electrochemical water splitting powered by renewables-generated electricity represents a promising approach for green hydrogen production. However, the sluggish kinetics for the hydrogen evolution reaction (HER) under an alkaline medium causes a massive amount of energy losses, hindering large-scale production. Exploring efficient and low-cost catalyst candidates for large-scale H2 generation becomes a crucial demand. Single-atom catalysts (SACs) demonstrate great promise for enabling efficient alkaline HER catalysis at maximum atom utilization efficiency. In this review, we provide a comprehensive overview of the recent progress in SACs for the HER application in alkaline environments. The fundamentals of alkaline HER are first introduced, followed by a justification of the need to develop SACs. The rational design of the SACs including the inherent element property, coordination environment, SAC morphology, and SAC mass loading are highlighted. To facilitate the development of SACs for alkaline HER, we further propose the remaining challenges and perspectives in this research field.

CNT fiber has made it possible to significantly translate the properties of individual CNTs, trapped in nano-length scale, to macro-length scale. This has enabled CNT fiber to obtain extraordinary properties like tensile strength values of above 9 GPa, making it one of the strongest synthetic fibers, elapsing conventional materials by a huge margin [1]. CNT fiber is also a good conductor of electricity with achievable electrical conductivity of 6.7 × 104 S cm−1 [2]. Among the methods available for the synthesis of CNT fiber, floating catalyst chemical vapor deposition (FC-CVD) has produced the fiber with the best mechanical properties [3]. The FC-CVD process also has the added advantage of being continuous and scalable. One of the major problems in the synthesis of CNT fiber by the FC-CVD method is its low carbon conversion that hinders in commercial deployment of this process. The carbon conversion reported by most of the researchers is in the range of 3–5% [4]. In the case of traditional CNT powder synthesis conversion above 50% is achieved by many researchers [4, 5]. Also, the catalyst utilization in CNT fiber synthesis is less than 1% [4, 6]. This restricts the production rate of CNT fiber to be in few grams per day for most research reactors. Researchers have so far employed five strategies to improve the conversion of CNT fibers, namely, improving catalyst carbon diffusivity, reducing catalyst agglomeration, removing the amorphous coating from the catalyst, improving hydrocarbon cracking rate, and reuse of precursor. The diffusivity of carbon in the catalyst has been improved by using Group 16 elements and also by using bi and trimetallic catalysts. The most commonly used group 16 element is S and it is used by almost all the researchers. Other than S, CNT fiber from Se and Te has been spun by Mas et al. [7]. Bi-metallic catalyst (Fe–Ni) was first used by Moon et al. [8] and trimetallic catalyst (Fe–Ni–Co) was first used by Karaeva et al. [9] for enhancing the diffusivity of C in the catalyst. Reducing catalyst agglomeration was done either by the use of S or by deep injection method. S forms FeS with Fe catalyst, which reduces agglomeration due to its low surface energy [10]. The agglomeration of catalyst can also be reduced by deep injection by introducing the catalyst deep into the reactor, away from a vortex at the inlet, where the catalyst is susceptible to getting trapped and getting agglomerated [11]. The removal of amorphous carbon coating from the catalyst is done mainly by the use of hydrogen or water. Hydrogen gas is used as the carrier gas by most of the researchers. Hydrogen can etch the amorphous coating from the catalyst [12, 13]. Water can also be used for the removal of amorphous coating as it can oxidize the amorphous carbon coating [14]. Improving hydrocarbon cracking is generally not done by many researchers as it could lead to the formation of amorphous soot. Rodiles et al. [15]. has improved carbon conversion by enhancing hydrocarbon cracking by using mullite reactor tube. The reuse of precursor has been carried out by Zhang et al. [16] were the tail gas from the reactor was reused by sending into another reactor as the precursor for CNT fiber synthesis. An improvised method of FC-CVD called blown aerosol FC-CVD was used by Zhang et al. [17] to produce transparent CNT film with higher conversion. However, this method has been utilized for the production of film. Apart from the discussed strategies, researchers have also used the statistical method like design of experiments to obtain optimized process conditions for improved conversion [18].Flow pattern in a CNT fiber reactor was first determined by Conroy et al. [19] in a vertical reactor by computation fluid dynamics (CFD) and found a re-circulation vortex in the inlet. Hou et al. [20] were the first to determine the flow pattern in a horizontal CNT fiber reactor and found recirculation vortex in both the inlet and outlet. The recirculation vortex in the outlet was shown to produce a 50-fold increase in the velocity which helps in CNT assembly and alignment. Lee et al. [11, 21] have shown that the recirculation in the inlet can lead to a reduction in the yield due to catalyst agglomeration. Lee et al. [11, 21] have overcome this by deep injection method as discussed earlier. The attempt of reducing recirculation in the inlet was carried out by Oh et al. [22] by lowering the Grashof number by optimizing the tube diameter and tube material. Oh et al. [22] was able to reduce the recirculation at the inlet by reducing tube diameter and using an alumina tube. The reduction in recirculation in the inlet resulted in an increase in specific strength. In the present work based on the analysis of the flow pattern in the reactor, we came up with the idea of bi-directional injection of catalyst into the CVD reactor for enhancing the carbon conversion. So far, nobody has reported such a technique in FC-CVD method for CNT fiber synthesis. We also propose a mechanism based on the yield and structural characterization.The synthesis of CNT fiber was carried out in a CVD furnace, fabricated in-house, with an attached glove box. The furnace retort was made of alumina tube of 45 mm inner diameter and 1 m length. It was heated to 1200 °C by resistance heating of SiC rods. The synthesis of CNT aerogel was achieved with a precursor composition consisting of 0.5–4.5 wt% ferrocene (catalyst and source of Fe, purity ≥99.98%, Sigma Aldrich, USA), thiophene (promoter and source of S, purity ≥99.98%, Sigma Aldrich, USA adjusted to maintain a Fe/S molar ratio of 2.65), and remaining ethanol (hydrocarbon and source of C, purity ≥99.9%, Hayman Ltd, England). The precursor mixture was injected at a flow rate of 0.2 ml/min using a syringe pump (Model – Legato ® 270 / 270P Syringe Pump, KD Scientific™, USA). Ferrocene beyond 2 wt% was sent into the reactor by sublimating in a preheater as ferrocene start precipitating beyond 2%. The flow rate of the ferrocene was controlled based on Eq (1-3) [4]. (1) ln ( P v ) = 273.6 R − , 815 , 35 . , 7 R , . T − 29.6 R ln ( T 298.15 ) (2) n ˙ i n = P v P o T o T V ˙ i n 0.0224 (3) m ˙ i n = n ˙ i n M m Where, Pv  = Parital pressure of ferrocene (Pa) R = 8.3145 J/mol K (Gas Constant) T= Temperature (K)Po = Standard atmospheric pressure (101,325 Pa)To = Standard temperature (298.15 K)ṅin = Molar flow rate (mol/s) V ˙ in = Volumetric flow rateMm = Molar mass (g)ṁin = Mass flow rate (g/s)Argon and hydrogen gases (1 lpm each) were used as the carrier gases and were introduced into the system through mass flow controllers (Model - MC-10SLPM-TFT, ALICAT, USA). Argon gas provided an inert condition in the reactor. Hydrogen controlled the cracking of hydrocarbon and maintained a reducing atmosphere in the reactor. The aerogel produced in the reactor was taken out of the outlet and was passed through a water bath inside the glove box to condensate and form fiber (see supporting video SV1). A rotating drum was used to collect the fiber in a continuous manner by synchronizing the drum speed with the formation rate of aerogel. Unidirectional injection of all the precursors (hydrocarbon, catalyst and promoter) through inlet is the conventional method of FC-CVD, which has been followed by almost all researchers in literature. The bi-directional catalyst injection was achieved by sublimating ferrocene separately in a preheater and introducing the ferrocene vapor at the outlet which was taken into the heating zone by the backflow gas. Additional details and snapshot are given in section 3.1.3. The mass flow rate of the ferrocene was varied using Eq (1-3) as discussed earlier. Schematic of bi-directional catalyst injection for CNT fiber synthesis are shown in Fig. 1 .The Raman spectra of the CNT fiber were recorded using an alpha 300R confocal Raman spectrometer (WITec GmbH, Germany). 514 nm diode laser with a max power of 80 mW was used as the light source for recording spectra in a CCD-based ultra-high-throughput efficiency spectrometer which was cooled to −60 °C. Thermogravimetric analysis (TG) was carried out on CNT fiber samples using Labsys EVO, SETRAM, France, in an oxygen atmosphere with a gas flow rate of 20 ml/min and a heating rate of 10 °C /min up to 1000 °C. The microstructural imaging of the samples was carried out using GEMINI SEM 300 field emission scanning electron microscope (Carl Zeiss, Germany). Imaging was carried out at an acceleration potential of 2 kV and a working distance of 4.6 mm. The electrical conductivity of CNT fibers was measured using a four point probe technique on the Ossila Four-Point Probe System (Ossilla Ltd., United Kingdom).Conventional unidirectional catalyst injection in FC-CVD was carried out to obtain a benchmark or reference for comparing it with the bi-directional catalyst injection. The conversion of carbon into CNT fiber in unidirectional injection for different catalyst concentrations was calculated Eq (4). (4) C a r b o n C o n v e r s i o n ( % ) = M a s s o f C N T f i b e r ( a f t e r a c i d p u r i f i c a t i o n ) M a s s o f c a r b o n i n p r e c u r s o r × 100 The carbon conversion values of CNT fiber were based on CNT fiber weight after acid purification for 10 h in concentrated HCl to prevent error to weight by iron followed by partial oxidation at 400 °C to remove the amorphous carbon. Fig. 2 (a) depicts the variation in carbon conversion with ferrocene wt% in unidirectional injection mode. The major chemical reaction involved in the formation of CNT is given in Eq (5-9) [23–25]. (5) Fe ( C 5 H 5 ) 2 → Fe + H 2 + C H 4 + C 5 H 6 … ( other hydrocarbon ) (6) C 4 H 4 S → S + HCC − CH = C H 2 (7) C H 3 C H 2 OH → C H 4 + CO + H 2 (8) 2 CO → C O 2 + C (9) C H 4 → C + 2 H 2 In the CNT formation, initially ferrocene starts decomposing to iron Eq (5)) followed by the decomposition of thiophene to S (Eq (6)). The S prevents the agglomeration of iron and also forms FeS, FexS, Ls as per the temperature and S concentration according to phase diagram as shown in Fig. 2 (b). Lee et al. [10] has shown FeS formed on catalyst surface acts as nucleation site for CNT formation and Weller et al. [4] has shown S rich liquid (Ls) phase acts as nucleation site. The ethanol cracks to carbon (Eq (7)-(9) at higher temperatures after the formation of catalyst and diffuses into catalyst to form CNT. The catalyst also lowers the barrier for hydrocarbon cracking and makes cracking on catalyst preferable [26].Unidirectional catalyst injection was carried out for different catalyst precursor concentrations from 0.5 wt% to 4.5 wt%. Experiments were not continued below 0.5 wt% due to excess amorphous carbon (soot) generation. Ferrocene beyond 4.5 wt% could not produce spinnable CNT aerogel as higher ferrocene causes an decrease in C/Fe ratio which leads to the inability of the CNT to form sufficient bundles for aerogel formation. Weller et al. [4] have reported a minimum C/Fe requirement of 300 for the production of spinnable CNT aerogel. In the present study ferrocene beyond 4.5 wt% causes C/Fe to go below 160 which is much lower the threshold proposed by Weller et al. [4]. The conversion of CNT fiber is extremely low in comparison to conventional CNT powder where the conversion is above 60% [4]. This low conversion is a result of extremely dilute carbon concentration in the reactor along with the presence of hydrogen, which lowers the cracking rate. Amorphous soot formation is highly susceptible in CNT fiber synthesis as the process is carried out at a high temperature in comparison to conventional powder synthesis. Hence, increasing the carbon concentration in the reactor is not a good option but increasing catalyst concentration is possible which can result in increased CNT nucleation resulting in enhanced conversion. The unconverted hydrocarbon leaves the reactor along with carrier gas as lower hydrocarbon [27] and also as carbonaceous particles [6]. In literature, it can also be seen that catalyst utilization has been reported less than 1%. More than 99% of the catalysts do not take part in CNT formation and get either absorbed in aerogel as impurity [6] or deposit at reactor wall [4]. Catalyst particles also leave the reactor along with the carrier gas as particulate [6]. The low utilization also suggests that increased catalyst concentration may be required. But as observed in the present study, for unidirectional injection, beyond 1 wt% of catalyst precursor, the conversion starts to drop. It can be seen from Fig. 2 (c and d) that catalyst particles tend to form large agglomerate in a higher concentration of ferrocene (encircled in Fig. 2d), which is incapable of nucleating CNT. Asli et al. [28] have observed a similar drop in conversion at higher concentrations of catalyst which was attributed to the catalyst agglomeration. The formation of catalyst particles and their agglomeration occur at two places in the reactor as per Hoecker et al. [29]. Initially it occurs towards inlet side before the highest temperature zone and also towards the outlet after the highest temp zone, in between the catalysts undergo evaporation. The schematic of the agglomeration process is depicted in Fig. 3 (a). Low catalyst concentration results in low conversion due to a smaller number of interactions between the catalyst and the carbon particles. Increased catalyst concentration results in an increase in conversion due to increase in total numbers of interaction between catalyst and carbon. But beyond a certain concentration catalyst also tends to agglomerate resulting in reduced conversion.The increase in catalyst size with catalyst concentration can be mathematically determined by Eq (10) [30]. (10) d p = ( 3 β V e f f m ˙ ρ π Q 2 ) 1 3 Where, dp =particle size (m) β = coagulation kernel (m3 s−1) Veff =effective volume (m3) ṁ = Flow rate (kg/s) ρ = density (kg/m3) Q 2 = Volumetric flow rate (m3/s)Feng et al. [30] have utilized the above Eq (10) for determining the Au nanoparticle size in a flow system accurately. It can be seen that the catalyst size is directly proportional to the catalyst flow rate (ṁ). This direct relation shows the reason for agglomeration coupled with a loss of conversion in higher catalyst precursor flow. Hence, it can be inferred that increased concentration of catalyst without agglomeration is required for improved conversion. Even though the effect of temperature on catalyst agglomeration is not given directly in Eq (10) it affects the coagulation kernel term (β). The coagulation kernel increases with temperature due increase in Brownian diffusion and thermophoretic convection [31]. The coagulation kernel for the Fe–S catalyst system as a function of temperature is not available in the literature. The approximate trend of catalyst size along the tube is calculated by replacing Veff with A*L where A is the cross-section area and L is the distance along the tube. Coagulation kernel of pure metal at room temperature condition given by Feng et al. [30] was used for calculation and the calculated catalyst size as a function of distance along reactor tube is given in Fig. 3 (b). The effect of temperature on carbon conversion is shown in Fig. 3 (c). It can be seen that carbon conversion increases with temperature up to 1200 °C which can be attributed to increase in reaction rates and diffusion. At temperatures beyond 1200 °C the carbon conversion starts dropping which can be due to increase in non-catalytic cracking of hydrocarbon and also increase in agglomeration.In order to achieve a higher concentration of catalysts without agglomeration leading to higher carbon conversion, bi-directional catalyst injection was thought of based on CFD results.In bi-directional catalyst injection, catalyst precursor was also introduced through the outlet of the reactor tube along with the inlet. To know whether the catalyst precursor introduced in the outlet can reach up to the heating zone of the reactor, CFD studies were carried out. COMSOL multi-physics software was used for carrying out CFD analysis. A 3D model comprising both the reactor tube and glove box was generated as flow from the glove box could enter the reactor tube. In addition, the glove box was maintained at a slight positive pressure with argon purging to prevent ingress of atmospheric gases which could also alter the flow behavior. The flow was determined by coupling a weakly compressive fluid flow condition model with heat transfer. Radiation in the model was incorporated by the Hemicube method in-build in COMSOL software. The flow of catalyst and CNT in the reactor were neglected and only the flow of carrier gas was analyzed. The governing equations utilized in the model are given in Eq(11–14): (11) d ρ d t + ∇ . ( ρ u ) = 0 (12) ρ d u d t + ρ u . ∇ u = − ∇ p + ∇ . ( μ ( ∇ u + ( ∇ u ) T − 2 3 μ ( ∇ . u ) I ) + ρ g (13) ρ C p u . ∇ T + ∇ . q h = Q h (14) q h = − k ∇ T Where, ρ = Density (kg/m3) μ = Dynamic viscosity (Pa·s) u = Velocity vector (m/s) p = Pressure (Pa) g = Gravitation acceleration (m2/s) Cp  = Specific heat capacity at constant pressure (J/(kg·K)) T = Absolute temperature (K) q h = Heat flux vector (W/m2) k = thermal conductivity (W/(m⋅K)) Qh  = heat sources (W/m3)The buoyancy which is essential for generating backflow in the reactor is taken into account by the addition of a body force ρg in the Navier–Stokes equation (Eq (12)). The density (ρ) is given as a function of temperature (from the COMSOL database) and gravitational force is given with its directional as per the horizontal configuration of the furnace. The change in density with temperature generates a force in the gravitational direction and generates buoyancy.The temperature profile and the flow pattern in the FC-CVD system are shown in Fig. 4 (a & b). It can be observed that backflow from the bottom side of the reactor tube in the outlet is possible. The gas from the outlet is capable of reaching high temperature region of the reactor. The maximum temperature that backflow gas can attain is above 1000 °C, which is a sufficiently high temperature for CNT nucleation.The temperature distribution along the centerline of the reactor is given in Fig. 5 (a). The maximum distance traveled by the backflow gases can be determined by finding the distance where the axial component of velocity becomes negative to positive which signifies a direction change and hence the end of the backflow. The axial component of velocity is given in Fig. 5 (b). It can be seen that the backflow gases can traverse 320 mm into the reactor which enables the gases to reach temperature above 1000 °C. As the minimum nucleation temperature of CNTs is 600 °C which is achieved at 276 mm [32], the backflow gases can nucleate CNTs for 88 mm (combining inward and backward travel of gases). The flow of gases has a residence time of ∼1 s in the CNT nucleation temperature zone. The tendency of a system to generate backflow can be quantified by the density variation of gasses in the system. In the present study, a maximum density variation (Fig. 5 (c)) of 0.682 kg/m3 was seen.Many researchers use vertical FC-CVD for the production of CNT fibers, so we have explored the possibility of bi-directional catalyst injection in a vertical FC-CVD reactor as well. CFD of a vertical reactor was carried out with the same carrier flow and temperature as the present study. The CFD results (Fig. 6 ) indicate that the backflow from the glove box to the heating zone is absent which makes bi-directional catalyst injection impossible. The temperature gradient and the resulting buoyancy are only able to generate a recirculation near the inlet of the reactor in a vertical reactor. Presence of convective backflow is not necessary for the formation of CNT aerogel, as the CFD analysis indicates the absence of convective backflow in a vertical reactor and many researchers were able to successfully produce CNT fiber in a vertical reactor [21, 33]. The formation of CNT aerogel primarily depends on the CNT length, the number density of CNT, and the impurity level. The CNT does not stick to the reactor wall due to thermophoresis which causes the migration of particles from the hot reactor wall to the gas stream [19].The photograph of the reactor outlet shown in Fig. 7 (a) indicates that the aerogel comes out only from the top region of the reactor tube when the aerogel is left undisturbed (without fiber production). The bottom half of the reactor is empty and has a clear path for the backflow gas to reach the heating zone without interference from aerogel. The snapshots of CNT fiber synthesis by unidirectional catalyst injections and bi-directional catalyst injections are shown in Fig. 7 (b). The snapshots show that the CNT aerogel is darker and the resulting fiber is thicker for bi-directional catalyst injection compared to unidirectional injection, which indicates higher conversion of carbon in the former case. The bulk densities of the aerogel are ∼2.3 kg/m3 and ∼3.4 kg/m3 for uni and bi-directional catalyst injections, respectively. The video of CNT fiber synthesis by bi-directional catalyst injection is given in the supplementary video (SV1). The maximum conversion in bi-directional catalyst injection in FC-CVD is shown in Fig. 7 (c). It can be observed that a 56% increase (corresponding to value 6.49) in conversion could be achieved when bi-directional injection was employed. The carbon conversion in bi-directional catalyst injection for different ferrocene flow rates at the outlet is given in Fig. 7 (d). The aerogel comes out of the reactor as a hollow tube filled with hot carrier gas which makes the aerogel go upwards by buoyancy. This upward movement of aerogel gives us some gap at the outlet for inserting the ferrocene vapor injection tube as shown in Fig. 7 (b). The gap can be increased by reducing the winding speed.The mechanism of improvement of conversion in bi-directional catalyst injection is proposed schematically in Fig. 8 . In the case of unidirectional injection, the additional catalyst beyond 1% from inlet resulted in a drop in carbon conversion, whereas, for bi-directional injection additional catalyst from the outlet side resulted in improvement in carbon conversion. In the previous section, it was concluded that additional catalyst concentration makes CNT nucleation even worsen by increasing catalyst agglomeration for unidirectional injection. This indicates that catalysts injected from outlet nucleate CNTs independently without getting affected by the agglomeration of catalyst from the inlet. The catalyst vapor can reach the reaction zone with the help of the backflow from the outlet as predicted in the CFD studies. After reaching the region in the reactor with a temperature more than required, the catalyst particles nucleate CNTs. This enables the bi-directional catalyst injection to have two separate CNT nucleation zones enabling higher conversion in the reactor in comparison to unidirectional catalyst injection which has only one CNT nucleation zone.Raman spectra of the CNT fibers (with highest conversion) synthesized by unidirectional and bi-directional catalyst injection are shown in Fig. 9 (a). The Raman spectrum of the CNT fiber from unidirectional catalyst injection shows the D, G, and 2D peaks, whereas, the Raman spectrum of the CNT fiber from bi-directional catalyst injection shows additional radial breathing mode (RBM) peak. The RBM peak is the signature of SWCNT and this peak is absent in MWCNT as the large number of walls restrict the radial vibration [34]. The G peak is due to the vibration of sp2 hybridized structure, D peak is due to disorder in the sp2 hybridized structure, and 2D is the overtone of D peak. The CNTs from the unidirectional catalyst injection are multi-walled CNT (MWCNT) that can be inferred from the lack of RBM peak. For the bi-directional injection, the CNTs are a mixture of MWCNT and SWCNT. The presence of RBM peak and splitting of G peak into G − and G + confirms the presence of SWCNTs [35]. The Fit of G − peak can determine whether the SWCNTs are metallic or semi-conducting. A Lorentz Fit was observed for G- peak which indicates the semi-conducting nature of SWCNTs [35, 36]. The distribution of SWCNTs and MWCNTS in the CNT fiber was obtained by performing Raman mapping and is shown in Fig. 9 (b & c)). This indicates that SWCNTs are formed from the catalysts injected from the outlet that back flowed into the reactor. The CFD derived flow pattern (Fig. 4 b) indicates that backflow gas from the outlet only penetrated the heating zone for a short distance hence spending less time in the reaction zone. The SWCNT is produced when small patches of S-rich liquid is formed on the catalyst surface which enables nucleation of SWCNT [4]. Generally, a low concentration of S causes SWCNT since at a high S concentration S rich liquid is high enough to cover entire catalyst promoting MWCNT. The amount of S-rich liquid depends on the temperature and S concentration according to the phase diagram (Fig. 3(a)). The low residence can reduce the catalyst agglomeration and also S intake into the catalyst which can reduce S rich liquid on the catalyst, hence promoting SWCNT formation. The distribution of ID/IG ratios for the unidirectional and the bi-directional injections is shown in Fig. 9 (d) as comparative histograms. ID/IG ratios of 0.4 ± 0.2 and 0.4 ± 0.3 were obtained for unidirectional and bi-directional flows, respectively. Bi-directional catalyst flow has a larger standard deviation of ID/IG ratios due to multiple types of CNTs. Fig. 10 (a) shows the mass loss of CNT fiber samples performed by TG under oxygen atmosphere. The oxidation behavior of various structures of carbon can be utilized for determining the phase composition of CNT fiber. Amorphous carbon generally oxidizes below 400 °C and CNT oxidize at a higher temperature [37]. Fe does not convert in gas after oxidation hence the mass remaining after TG analysis is considered as the Fe2O3 which can be used to calculate the fraction of Fe [38]. The mass fraction of amorphous carbon and CNT can be determined by finding the Differential TG (DTG). The DTG of the samples from unidirectional and bi-directional catalyst injection are shown in Fig. 10 (b & c). The area under the peak of DTG plots below 400 °C gives the amorphous carbon fraction and peaks at higher temperature corresponds to CNT mass fraction [39]. CNT shows multiple peaks in DTG. The peaks above 600 °C are considered highly crystalline and peaks at lower temperatures have lower crystallinity [40]. The unidirectional catalyst injection sample has one CNT peak in the 500 °C range and another peak in the 600 °C range, whereas the bi-directional catalyst injection sample has two peaks in the 500 °C range and 2 peaks in the 600 °C range. More amount of highly crystalline CNTs in the case of bi-directional injection is because of the presence of SWCNT in this sample. The calculated phase fraction of various phases in CNT fiber is given in Fig. 10 (d). It can be seen that bi-directional catalyst injection results in lower amorphous content. The residual iron in the CNT fiber is also higher for bi-directional catalyst injection. These can be attributed to the higher catalyst utilization in the bi-directional injection.The SEM images (Fig. 11 (a-b)) were taken for the CNT aerogel samples (unidirectional and bi-directional) that were not condensed to make fiber. This was done as the condensation in the bath could change the macro-structure of the CNT aerogel making capture of the structural changes due to catalyst injection difficult. SEM micrographs indicate that the content of iron was more for the sample obtained by bi-directional injection though the CNT bundle diameter was similar to that of unidirectional injection. Bundle diameters of 41.75±10.67 nm and 44.04±11.01 nm were obtained for unidirectional and bi-directional flow respectively and their distribution is shown in Fig. 11 (c) (ImageJ software was utilized for determining bundle size from SEM image [41, 42]). The high resolution (HR) TEM images of the CNT aerogel samples (unidirectional and bi-directional) are shown in Fig. 11 (d-e). The HRTEM shows the multi-walled nature of the synthesized CNT aerogel. SWCNT was additionally detected (inset Fig. 11(e)) for bi-directional injection. Table 1 shows the electrical conductivities of CNT fibers grown by unidirectional and bi-directional catalyst injections. It can be seen that the electrical conductivities of the fibers from unidirectional catalyst injection was slightly higher than that of the fibers from bi-directional injection. The electrical conductivity of CNT fiber is generally controlled by the impurities, alignment of CNT, and conductivity of individual CNT [43]. The changes in conductivity for the present case comes from the type of CNTs. The CNT fiber produced by bi-directional catalyst injection consists of MWCNTs and SWCNTs having semi-conductive nature. On the other hand, unidirectional catalyst injection produces only MWCNTs which are exclusively metallic making CNT fiber produced by unidirectional catalyst injection more conductive.The SEM micrographs of the CNT fiber produced by bi-directional catalyst injection after purification are shown in Fig. 12 (a and b). The SEM micrograph (Fig. 12 (a and b)) indicates the absence of catalyst particles and amorphous particles after purification. The CNTs are also aligned in the rolling / gas flow direction which can enhance the electrical and mechanical properties. Issman et al. [44] reported improvement in the electrical conductivity and tensile strength with improvement in CNT alignment.CNT in the form of powder was recovered when produced by only adding catalyst in the outlet. The SEM and Raman spectra of CNT powder formed are given in Fig. 12 (c and d). CNT produced contained a significantly large amount of amorphous carbon and was single-walled in nature. Even though some CNT powder could be recovered from the reactor tube, aerogel formation did not occur as the hydrocarbon introduced from the inlet would crack and form soot in absence of the catalyst in the inlet region. The excess soot formed in the inlet region would mix with the limited CNT formed in the outlet making self-assembly into fiber impossible. The soot thus formed can also deactivate the catalysts introduced in the outlet by coating them. Hence, due to the soot interference, it is not possible to make fiber by sending catalyst only from the outlet.A novel concept of bi-directional injection of catalysts in FC-CVD for the synthesis of CNT fiber demonstrates a 56% improvement in carbon conversion with respect to unidirectional injection. This method does not require any changes in the hardware of the synthesis system. CFD analysis predicts that the convection vortex naturally present in the system can be utilized to implement bi-directional injection. CNT fiber synthesized by bi-directional catalyst injection has lower amorphous carbon content and is composed of both SWCNT and MWCNT. The ID/IG ratio and the CNT dimensions are similar for both unidirectional and bi-directional catalyst injection. The electrical conductivity for the CNT fiber obtained by bi-directional injection drops slightly due to semiconducting nature of the SWCNTs present in the fiber.The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing studyThis research was funded by Bhabha Atomic Research centre, Mumbai, IndiaThere are no conflicts to declareSupplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cartre.2022.100211. Image, video 1

Carbon nanotube fiber (CNT fiber) synthesized through floating catalyst chemical vapor deposition (FC-CVD) is one of the strongest man-made fibers ever synthesized. The poor carbon conversion in the FC-CVD process is one of the major hurdles in its commercial deployment. In this work, we have employed a novel method of bi-directional catalyst injection where catalysts were injected from both inlet and outlet sides of the reactor. The injection of the catalyst from the outlet into the reactor reaction zone was possible by a backflow caused by the convection vortex as predicted by the computational fluid dynamics (CFD) analysis. Bi-directional catalyst injection was able to enhance the carbon conversion by 56% compared to conventional unidirectional injection. CNT fibers obtained in bi-directional catalyst injection are a mixture of multi-walled (MW) and single-walled (SW) CNTs whereas unidirectional catalyst injection resulted in MWCNTs only. The average CNT bundle diameter dimensions were similar in both unidirectional and bi-directional catalyst injection. The amorphous carbon content was lower for bi-directional catalyst injection. A mechanism for the improvement of carbon conversion in bi-direction catalyst injection has been proposed.

The substantial reserves of gas resources worldwide necessitate technological advances to obtain cost-effective clean energy and chemical feedstocks [1]. Methane, the most abundant component of natural gas, typically produces energy by forming CO2 or by itself as a greenhouse gas. Therefore, methane requires careful management from an environmental perspective. The synthesis of transportable liquids can effectively control the methane reserves diverging in depopulated areas. A key challenge is to determine a conversion process that can decrease the stability of methane and cleave its C–H σ bond, which requires high energy (438.3 kJ mol−1). Therefore, for several decades, the industry has adopted methane reforming-based multistage processes that co-feed steam or oxygen for producing hydrogen and chemicals such as methanol and fertilizers [2]. Contrary to expectations, only 7.8% of methane (in the United States of America) is currently used as a non-combustion source in the industry, because the existing process requires a substantial capital investment owing to its low energy efficiency (up to 60%) [3,4]. Therefore, energy-efficient conversion processes must be developed to use methane as an alternative to petroleum [5,6].Non-oxidative conversion of methane is the desirable chemical route because it potentially enhances the carbon efficiency through a simplified process for hydrocarbon commodity chemicals [7,8]. Methane exhibits high selectivity toward olefins and aromatics through catalytic conversion owing to the functionality of metallic [9] and metal–acid [10] sites for selective C–H activation and consecutive C–C coupling. However, solely optimizing the catalytic active sites does not ensure an increase in hydrocarbon yields, because this approach is thermodynamically unfavorable. Engineering approaches for improving the aerodynamics [11,12] or equilibrium yields of hydrocarbons [13,14] can increase the economic viability of the process. The efficient control of methyl radicals, with a lifetime of microseconds during the reaction, is a driving force that determines the productivity [15]. Excessive carbon formation is inevitable unless the partial pressure of unstable methyl radicals is lowered during short reaction times [15]. Metallic nanoclusters catalyze methane conversion; however, coke is formed from the radicals on less sophisticated surfaces [16].Guo et al. [17] were the first to demonstrate rigorously designed lattice-confined single iron sites that can non-oxidatively convert methane to olefins, aromatics, and hydrogen (MTOAH). In their study, the catalytic reactor was stable at > 950 °C, which thermodynamically favored coke formation; an ethylene selectivity of 48.4% was achieved at a maximum methane conversion of 48.1% at 1090 °C [17]. This unprecedented selective behavior for C–H activation signifies the importance of site isolation and neutralization of surface defects [17,18]. Based on techno-economic analysis, the conversion of MTOAH is economically viable if the coke formation is less than 20% and the minimum conversion of the product is 25% [19]. Šot et al. [20] used high-surface-area silica with Fe2+ single sites for the MTOAH process at 1000 °C and found that iron sites selectively inhibit the surface reactivity and enhance the hydrocarbon selectivity. According to detailed theoretical analyses of the surface reaction mechanism, the Fe2+ single sites and adjacent carbon sites can be sensitively involved in C2 formation through the activation of methane, and can subsequently serve as dual sites for C–C coupling and hydrogen transfer [21–23]. The design of In- [24–26], Pb- [27], Pt- [28–32], Pd- [33,34], and Ni-based [35,36] active sites has been further studied to increase the hydrocarbon selectivity while decreasing the coke selectivity. Li et al. [32] designed atomically thin Pt nanolayers anchored on two-dimensional molybdenum titanium carbide (MXene) and found that the altered adsorption properties of these Pt active sites promote methane coupling to ethane/ethylene with 98% selectivity at 750 °C.Subsequent studies were conducted to demonstrate that the modulation of gaseous free-radical chemistry for iron-based catalytic surfaces is a prerequisite for increasing the hydrocarbon yield. Hydrogen removal from hydrogen-permeable tubular membrane reactors during the MTOAH process can favorably drive the gas-phase reaction and consequently increase methane conversion [37–39]. The millisecond catalytic wall reactor provides an appropriate Fe/SiO2 [40] or Fe/SiC [41,42] surface on the reactor wall to effectively activate methane in short contact times and enhance gas-phase reactions within the reactor compartment. The methane activation rate can be uniformly improved by hydrogen radicals, which are typically formed in the presence of hydrocarbons that can generate H-radicals in the gas-phase reaction [41]. Dong et al. [43] recently designed a Joule-heating-based programmable heating and quenching system involving rapid switching between low (815 °C) and high temperatures (2000 °C). Consequently, the temperature-dependent reaction pathway between gas-phase reactants and the adsorbed surface species could be precisely controlled. A C2 product selectivity exceeding 75% was obtained at a methane conversion of approximately 13%. Postma and Lefferts [44] proposed a practical approach to separate the catalytic and gas-phase reaction zones of a tubular reactor, wherein the axial temperature profile and residence times upstream and downstream of the catalyst bed were varied to increase the methane performance. However, the difference in reactivity for complex catalysts and gas-phase reactions in the MTOAH process has not been experimentally verified and elucidated thus far.In this study, we aimed to determine the effect of the catalyst surface on the selective production of hydrocarbons from methane based on systematically controlled parametric studies of the MTOAH process. We further investigated the selective conversion of C2 species (ethane, ethylene, and acetylene) from 500 to 1020 °C to elucidate the secondary reactivity of the primary product of methane conversion. Finally, we attempted to maximize the reactivity by optimizing the ratio of catalytic to gas-phase reactions in the reactor.Fe(NO3)3·9H2O (Sigma Aldrich, 216828) and SiC (Alpha Aesar, A14470) were used to prepare SiC catalysts doped with 0.17, 0.26, 0.32, 0.59, and 1.25 wt% Fe via wet impregnation; the samples are denoted 0.1Fe, 0.2Fe, 0.3Fe, 0.5Fe, and 1Fe, respectively. Fe(NO3)3·9H2O was dissolved in de-ionized water and mixed with SiC, whose amount was 50 times lower than that of the aqueous Fe solution. The mixture was stirred at 60 °C for 6 h at 120 rpm and evaporated using a rotary evaporator (RV 10 digital V, IKA). The final solid was dried overnight at 110 °C and calcined at 550 °C for 4 h at a ramping rate of 4 °C min−1. The actual Fe loading in the catalyst was analyzed through inductively coupled plasma–optical emission spectroscopy (ICP-OES; iCAP 6300 Duo, Thermo Fisher Scientific), and more than four batches of catalyst were prepared to confirm reproducibility.Solid density was determined using an AccuPyc II 1340 pycnometer (Micromeritics) with He as a gas displacement medium. Void space in the catalytic reactor was calculated by subtracting the apparent density of the sample from the reactor space in the heating zone of the furnace.All samples were subjected to nitrogen physisorption at − 196 °C in an ASAP-2420 system (Micromeritics) for a relative pressure (P/P 0) range of 0.01–0.30, and the Brunauer–Emmett–Teller (BET) equation was used to determine the specific surface areas (S BET). Before measurement, each sample was degassed at 90 °C for 30 min and subsequently heated at 150 °C for 6 h under a vacuum. The catalyst amounts were adjusted; consequently, the relative error was minimized when constant C in the BET equation exceeded 100.To analyze the crystallinity of each sample, powder X-ray diffraction (XRD) patterns were obtained using an Ultima IV diffractometer (Rigaku) with Cu K α radiation (λ = 0.154 nm) at 40 kV and 40 mA. The crystal structures were assigned according to the Inorganic Crystal Structure Database (ICSD).To confirm the surface species of each sample, X-ray photoelectron spectroscopy (XPS) was performed using an AXIS Nova (Kratos) instrument equipped with a monochromatic Al-K α X-ray source. The acceleration voltage and the pass energy were 15 keV and 40 eV, respectively. The binding energies were calibrated to the C1s peak at 284.8 eV.The coke deposition on the spent samples was determined via thermogravimetric (TG) analysis and differential scanning calorimetry (DSC) using an SDT Q600 instrument (TA Instruments) in the temperature range of 30–900 °C at a heating rate of 5 °C min−1 under a constant airflow of 100 mL min−1. The total weight loss after 200 °C was regarded as the amount of coke.The morphologies of the spent catalysts were investigated through transmission electron microscopy (TEM) using a FEG S/TEM instrument (Talos F200S) operated at 200 kV. Scanning transmission electron microscopy (STEM) was performed using a TalosTM F200S device (FEI) containing a 200 kV field-emission gun. For energy-dispersive X-ray spectroscopy (EDS) analysis, a high-angle annular dark-field (HAADF) detector (Super-X EDS) with 0.16 nm beam resolution was selected.To analyze the nature of coke in the spent catalysts, Raman spectroscopy was performed (DXR3 Raman microscope, Thermo Fisher Scientific) under 30 °C with 532 nm laser excitation. For all measurements, the excitation power and exposure time were 2 mW and 120 s, respectively. Measurements from at least five runs were averaged at different positions.Reaction measurements of methane were performed in a vertical fixed-bed quartz reactor (inner diameter = 4 mm) at ambient pressure (1 bar). To ensure plug flow behavior in the reactor, sieved catalyst particles (425–850 μm) were charged into a reaction zone supported by a minimal amount of quartz wool. Three R-type thermocouples were placed in direct contact with the outer surface of the reactor to ensure a uniform temperature profile in the catalytic reactor in the furnace. The void space between the catalyst beds was varied to modify the gas-phase reaction. To minimize the secondary reaction, a quartz rod (inner diameter = 3 mm) was placed at the bottom of the reactor. The downstream lines were heated to 150 °C to prevent partial condensation of hydrocarbons. The catalytic reactor was ramped up to the reaction temperatures at a rate of 6 °C min−1 in a He flow. Methane gas containing 10% Ar was subsequently allowed to flow into the reactor, and mass flow controllers (5850E, Brooks Instrument) were used to achieve the desired gas hourly space velocity (GHSV). Non-oxidative conditions were established for the system, with all gases except C2 (ethane, ethylene, and acetylene) passing through oxygen/moisture traps (OT3-4, Agilent). Reaction measurement was performed at 1020 °C for 10.2 h. The reaction rate was measured as a function of temperature from 965 to 1020 °C at a rate of 0.10 °C min−1 after the catalyst was pretreated with 90% CH4/10% Ar at 1020 °C for 1 h to stabilize the catalytic surface. Similarly, the reaction measurement of C2 (ethane, ethylene, acetylene) was performed from 500 to 1020 °C at a rate of 0.70 °C min−1 after the catalyst was pretreated with 90% CH4/10% Ar at 1020 °C for 1 h. The reactants used in the study were: 5% C2H6/5% H2/10% Ar in He balance, 5% C2H4/5% H2/10% Ar in He balance, and 1% C2H2/1% H2/4% Ar in He balance. Additionally, the reactor configuration was modified to optimize the catalytic reactor. The gas effluent from the reactor was analyzed through online gas chromatography (GC, 6500GC, Youngin) using a thermal conductivity detector coupled with a ShinCarbon ST column (Restek Corp., Catalog No. 80486–800) and two flame ionization detectors coupled with an RT-Alumina BOND column (Restek Corp., Catalog No. 19756) and an Rtx-VMS column (Restek Corp., Catalog No. 49915) column. Detailed online GC analysis conditions and calculation formulae to obtain the conversion, product selectivity, and product distribution based on moles of carbon have been described in our previous report [18]. The product yield was calculated by multiplying the methane conversion by the product selectivity and dividing by 100. Herein, products of C2 or higher are classified as hydrocarbons.The catalytic activities of Fe/SiC catalysts are shown in Fig. 1 . The reactivities of the blank reactor (B) and pure SiC were also measured at the same GHSV (637 h−1) for comparison. In this experiment, the particles were packed in all the reactor spaces in the heating zone of the furnace. We measured the amount of Fe in the catalyst-packed reactor to distinguish the surface reaction between SiC and Fe species. SiC, 0.1Fe, 0.2Fe, 0.3Fe, 0.5Fe, and 1Fe resulted in the presence of 0, 97, 151, 186, 327, and 695 µmol of Fe in the reactor, respectively. The internal surface of the quartz reactor could function as a catalytic surface; therefore, we minimized its reactivity by adjusting the reaction parameters. As shown in Fig. 2 , the spent catalysts maintain the high crystallinity of SiC (ICSD #015325). The Fe species include Fe3C (ICSD #044354), which is weakly confirmed at 37.6°; the formation of Fe3C is thermodynamically favorable under the reaction conditions considered in the present study (Fig. S1). The catalysts contain crystalline SiO2 (ICSD #039830) as an impurity. SiC has a low S BET of 0.72 m2 g−1, which approximately doubles with increasing Fe loading (Table 1 ). These changes in physical properties are possibly induced by the Fe particles adhering to the SiC surface. The Fe species are dispersed on the external surface of SiC, minimizing the chemical interactions (metal–support interaction) between them.The Fe/SiC catalysts are stabilized in the reaction condition before 0.2 h, whereupon CO and CO2 are observed. These gases can be generated during the carbonization of metal oxides. According to theoretical calculations using HSC Chemistry 9 (Fig. S2), 57.8% of methane can be converted to C2, benzene, and naphthalene at 1020 °C. These values are considerably higher than those presented in Fig. 1, implying that the reaction in the catalytic reactor proceeds selectively in a nonequilibrium state. Over 10.2 h, the blank reactor achieves methane conversions in the range of 2.4%–3.0%, which are lower than those of the catalytic reactors (Fig. 1a). The increase in the methane conversion primarily depends on the amount of Fe in the reactor, which predominantly induces coke formation in the initial 0.2 h of the reaction (Fig. 1b). The blank reactor, SiC, 0.1Fe, and 0.2Fe result in hydrocarbons with minimal coke formation during the initial 0.2 h of the reaction, wherein the highest hydrocarbon yield of 4.4% is obtained using SiC. The activities of the Fe/SiC catalysts are stabilized after 0.2 h, and the results are averaged over 1.2–10.2 h, except for catalysts wherein the amount of Fe in the reactor exceeds 186 µmol (0.3 Fe, 0.5 Fe, and 1 Fe), for which the results of the last 3 h are averaged (Fig. 1c). At this stage, each catalyst can begin to minimize the coke yield. The hydrocarbon yield is proportional to the coke yield, and is higher than the coke yield at a steady state. Compared with the results at 0.2 h, the hydrocarbon yield is increased by 12% and 16% with coke formation in the blank reactor and on SiC, respectively. The hydrocarbon yield is maximized when the amount of Fe in the reactor is 327 µmol, and this value slightly decreases when the Fe content is increased by 2.1 times. Based on the hydrocarbon product distribution (Fig. 1d), compared with the blank reactor, the SiC surface increases the selectivity of acetylene and ethylene. Therefore, the initial activation of methane on the surface is accompanied by C2 formation. The catalytically improved hydrocarbon formation through the decreased selectivity of C3–C5, benzene, toluene, naphthalene, and alkyl-aromatics is accompanied by coke formation.The Arrhenius plot (Fig. 3 a) and apparent activation energies (E a) (Fig. 3b) for methane consumption between 965 and 1020 °C indicate that E a is decreased owing to the higher reactivities of the SiC surfaces compared with that of the blank reactor. The blank reactor exhibits an E a of 388.5 kJ mol−1, which is similar to the values for the thermal decomposition of methane (362–422 kJ mol−1) [45]. Packing the blank reactor with pure SiC results in a 29.1% decrease in Ea , which further decreases when SiC is doped with Fe. The pre-exponential factor (A) of the blank reactor (Fig. 3b) is comparable to that of the gas-phase unimolecular reaction (1013 s−1) [46]. The value of A decreases in the presence of the SiC surface, and further decreases with the increasing amount of Fe in the reactor. The experimentally obtained A values are strongly influenced by surface elementary steps including adsorption, surface diffusion, surface reactions, or desorption [46]. The gradual decrease in E a and A along the surface of Fe/SiC catalysts indicates that gas–solid reactions substantially contribute to the reactivity of methane. However, this decrease entails coke formation via higher-order surface reactions (Fig. 1), and a catalyst surface that can maximize the ratio of hydrocarbon to coke is essential. Therefore, selective hydrocarbon production requires optimization of the catalyst surface for bimolecular reactions in the gas phase. Although SiC alone can induce the surface reaction, the Fe species must be loaded to improve the surface reactivity within the reactor. Fig. 4 shows the effect of the GHSV on the product distribution in the catalytic reactor at 1020 °C, based on the methane conversion. More methane is converted by the SiC surfaces than by the blank reactor at GHSV values exceeding 319 h−1 (Fig. 4a). In this region, the methane conversion further increases as additional Fe (above 151 µmol) is included in the reactor. The effectiveness of catalytic surfaces compared with the gas phase is observed when the methane conversion is below 15%. Above this value, the reactor used in this study exhibits minimal differences in activity with and without a catalyst. Both the blank and catalyst-packed reactors exhibit similar trends in product selectivity as a function of hydrocarbon yield (Fig. 4b–f). The ethane selectivity decreases, whereas the acetylene selectivity reaches the maximum value with increasing hydrocarbon yield. In this region, the C3–C5 selectivity gradually decreases. With the decreasing selectivity of C2 (ethane, ethylene, and acetylene) and C3–C5, the aromatic (benzene, toluene, naphthalene, and alkyl-aromatics) selectivity increases when the hydrocarbon yield is increased further. This phenomenon is consistent with the typical methane pyrolysis mechanism comprising a series of reactions involving dehydrogenation and C–C coupling. In this study, ethane is the primary product of the conversion of methyl radicals that are produced during the rate-determining step of methane conversion. If ethylene and acetylene are sequentially produced from ethane through endothermic reactions, aromatics and coke can be produced spontaneously through exothermic reactions. According to Puente-Urbina et al. [47], C2–C5 radical species, including propargyl radicals, are produced under non-oxidative conditions at 945–1,400 °C during the chain growth of hydrocarbons in methane conversion, thus leading to aromatic formation. In the present study, a hydrocarbon yield of approximately 7% is considered a marginal range for stabilizing C2 species. The formation of C2–C5 radical species appears to be minimized in this range, whereas ethane, ethylene, and acetylene are stably balanced. At this stage, the acetylene selectivity is maximized in the blank and catalytic reactors. At similar hydrocarbon yields, the catalytic surfaces demonstrate higher C2 selectivity but lower aromatic selectivity than that of the blank reactor, implying that the aromatics are partially converted to coke at the catalytic surfaces. At similar C2 yields, the catalytic surfaces induce more ethane and ethylene, but less acetylene than that induced by the blank reactor (Fig. 4g–i). The low selectivity of acetylene and aromatics in hydrocarbons in the catalysts indicates that they are further subjected to C–C coupling. These results suggest that the catalyst surfaces contribute to changes in the composition of C2 species and their conversion.The methyl radicals initially couple to form ethane, which undergoes a series of dehydrogenation reactions to form ethylene and acetylene, which can further compete with each other as reactants for subsequent reactions. To elucidate the C2 reactivity on the catalytic surfaces, reactions were performed at 500–1020 °C, wherein each C2 species was used as a model compound (Fig. 5 ). The C2 conversion was measured by regarding ethane, ethylene, and acetylene present in the stream as lumped reactants. For comparison, the equilibrium of C2 species with temperature for each feedstock composition was calculated using HSC Chemistry 9. At temperatures above 800 °C, the C2 conversion in the blank reactor increases in the order of acetylene < ethylene < ethane feed but increases in the order of ethane < ethylene < acetylene feed at the catalytic surface (Fig. 5a–c). When ethane is added to the reactor with hydrogen, the catalytic surfaces are less reactive than the blank reactor. This is probably because homolytic cleavage of the C–C bond of ethane is selectively promoted on the catalyst surfaces, resulting in methane formation [48,49]. In contrast, for ethylene and acetylene, the reactivity on catalytic surfaces tends to improve compared with that in the blank reactor after 850 °C. The C2 conversion tends to increase with the increasing Fe content in the reactor, and the maximum increase is observed for acetylene conversion. Furthermore, considering the acetylene conversion, the blank reactor exhibits a different trend for C2 conversion starting at 800 °C. However, at the same point, the catalytic reactor does not demonstrate a change in the trend, indicating that it further promotes the C–C coupling reaction of acetylene.The lumped C2 species can be balanced with respect to temperature through dehydrogenation and hydrogenation at non-equilibrium conditions (Fig. 5d–f). With increasing temperature, the blank reactor exhibits higher ratios of acetylene to ethylene, when ethylene is used as a reactant instead of ethane. Under the non-equilibrium conditions, acetylene is partially converted to ethylene through hydrogenation in the blank reactor. For ethane and ethylene reactions, the catalytic surfaces shift the ratio of acetylene to ethylene toward equilibrium with increasing temperature, which allows additional ethylene to be present in the stream. Although this tendency increases with increasing Fe content, the difference is not as noticeable as that observed between the ratios for the blank reactor and the reactor containing SiC. However, for the acetylene reaction occurring below 717 °C, the ratios of acetylene to ethylene are higher for the catalytic surfaces than for the blank reactor. This difference decreases with increasing temperature and decreasing Fe content. Therefore, in contrast to the gas-phase reaction, the Fe catalyst favors the C–C coupling of acetylene over selective hydrogenation. Using excess hydrogen as a reactant may accelerate the conversion of acetylene to ethylene at the catalyst surface.The lumped C2 is subsequently converted to aromatics through C–C coupling reactions, and the maximum selectivity at 933 °C in the blank reactor increases in the order of acetylene < ethane < ethylene feed (Fig. 5g–i). When acetylene is used as a reactant, the aromatic selectivity is at least 2.7 times higher than that of ethane and ethylene below 803 °C. Here, the catalytic surfaces decrease the aromatic selectivity irrespective of the C2 feedstock, depending on the amount of Fe in the reactor. Aromatics are found to be sensitive to partial pressure [10], and they are converted to coke above a certain partial pressure; coke formation is further promoted by the Fe/SiC surface. Compared with ethane and ethylene, acetylene in the reactor favors the conversion of aromatics to heavier hydrocarbon products (coke) via hydrogen abstraction–acetylene addition routes [47]. This is consistent with the finding that the coke yield increases simultaneously with the hydrocarbon yield as the amount of Fe is increased in the reactor (Fig. 1). Thus, the catalytic surface is suitable for the C–H activation of methane. However, optimization with the gas phase is necessary to decrease coke formation. Fig. 6 shows the results of methane conversion according to the ratio of void space to 0.2Fe catalyst-packed space in the quartz tube reactor. Here, only the post-catalyst zone is considered for the void space, which is controlled using a quartz rod with an outer diameter of 3 mm (Fig. 6a). Here, the surface reaction is considered to occur in the catalyst-packed space, whereas the gas-phase reaction is considered to occur in the void space in the blank reactor or post-catalyst zone. A gas-phase reaction can occur in the interparticle space; however, it has not been considered in this study because the interparticle space is substantially smaller than the void space. Moreover, we defined the interfacial space as the space between the reactor and the quartz rod. The void space includes the interfacial space. If the inner space of the reactor is completely occupied by the quartz rod, the interfacial space is 0.89 mL; additionally, the methane conversion, which is the sum of hydrocarbon (1.0%) and coke (0.2%) yields in this space, is as low as 1.2% (Fig. 6b). The methane conversion in the interfacial space is at least twice as low as that in the blank reactor (Fig. 1). However, when comparing the methane reactivity on a per-unit volume basis, the values obtained for the interfacial space and the blank reactor are almost identical (1.3% mL−1). The yields of hydrocarbons and coke increase rapidly as the catalyst space increases from 0 to 0.63 mL; the yields steadily increase as the space is further increased to 1.88 mL (Fig. 6b). When the ratio of interfacial space to catalyst space is less than 0.95, the ratio of hydrocarbon to coke yields converges between 4.1 and 5.4. When the catalyst space exceeds the interfacial space, a hydrocarbon yield of approximately 6% is achieved with a coke yield of 1.1%–1.4%. The molar carbon selectivity values for C2, C3–C5, and aromatics converge when the ratio of interfacial space to catalyst space is less than 0.95 (Fig. 6c). The selectivity of the C2 species is in the range of 52.0%–66.9%, which is higher than that of C3–C5 and aromatics (11.0%–22.6% and 13.0%–20.1%). The C3–C5 selectivity is higher than the aromatic selectivity in the catalyst space ranging from 0 to 0.12 mL, wherein coke formation is minimized. With the decreasing ratio of hydrocarbon to coke yields, the preceding trend is reversed. Therefore, the catalytic surfaces are probably involved in improving the methane conversion and inducing the C–C coupling of aromatics, leading to coke formation. In the present study, increasing the catalyst space favors the formation of aromatics and coke from methane, indicating that additional catalytic surfaces serve as active sites for the C–C coupling reaction in the axial direction of the reactor. The C–C coupling and coke formation reactions are kinetically faster than methane activation even in autocatalysis [50]; therefore, the possibility of excessive catalytic surface reactions should be reduced.Increasing the void space in the post-catalyst zone increases the hydrocarbon yield relative to coke over catalyst spaces of 0.37 and 0.63 mL (Fig. 6d). The post-catalyst zone can induce gas-phase reactions, and a 2.1-fold increase in this space increases the hydrocarbon yield of 0.37 mL and 0.63 mL catalyst-packed reactors by 1.6 and 1.8 times, respectively. A catalyst space of 0.63 mL converges to a hydrocarbon yield of 7.1% depending on the void space, whereas a catalyst space of 0.37 mL converges to a hydrocarbon yield of 6.6%. The coke selectivity at the maximum hydrocarbon yield is less than 2% in both catalytic reactors. Compared with the catalyst surface, the void space in the post-catalyst zone is less reactive to C–C coupling reactions. Similar results were observed by Van Der Zwet et al. [51], who investigated the effect of surface area on methane conversion at 1125 °C; the high surface area was found to considerably affect the termination of chain reactions involving free radicals, and graphitic coke and hydrogen were primarily produced. The 0.63 mL catalyst-packed reactor is more active than the 0.37 mL catalyst-packed reactor in a similar post-catalyst zone. Similar trends are observed for the selectivity of C2, C3–C5, and aromatics in the two catalytic reactors. However, at similar hydrocarbon yields, the 0.67 mL catalyst-packed reactor demonstrates higher aromatic selectivity and lower C3–C5 selectivity than that of the 0.37 mL catalyst-packed reactor (Fig. 6e).In Fig. 6, the 0.2Fe catalyst (0.63 mL) indicates that 50.3 µmol of Fe is present in the reactor; this Fe amount is 6.5 times lower than that (327 µmol) in the reactor exhibiting the maximum hydrocarbon yield in Fig. 1. Although less catalyst surface (6.5 times lower) is used in the reactor containing the 0.2Fe catalyst, the hydrocarbon yield is 0.45% higher than that in the reactor containing 327 µmol of Fe, and the coke selectivity is also 24.5% lower. Considering methane conversion, the gas-phase reaction in the void space is less active than that on the 0.2Fe catalyst surface, with the same volume (Fig. 1). The significant increase in coke selectivity implies that hydrocarbons stably present in the gas phase are converted to coke through adsorption on the 0.2Fe surface.If the catalyst selectively converts methane to hydrocarbons under non-oxidative conditions, these hydrocarbons can act as radical donors for further methane conversion in the void space located below the catalyst [49]. Postma and Lefferts [52] reported similar results, confirming that methane activation in the gas phase could be promoted by co-feeding ethane or ethylene with methane to the reactor. In the present study, when an additional 0.63 mL of void space is added, the hydrocarbon yield increases by 1.6 times, compared with that when the interfacial space below the 0.2Fe catalyst is only 0.60 mL (Fig. 6b and d). Simultaneously, the aromatic selectivity sharply increases, compared with the C2 selectivity in the void space (Fig. 6e). In gas-phase reactions, aromatics are expected to considerably influence the methane conversion. Hao et al. [41] experimentally demonstrated that the methane conversion could be enhanced by co-feeding aromatics such as 1,2,3,4-tetrahydronaphthalene or benzene into a Fe-coated catalytic quartz reactor. Furthermore, they [42] used the H-atom Rydberg tagging time-of-flight technique to demonstrate the formation of hydrogen radicals that were only decomposed from the aromatic structure during the MTOAH reaction over a catalytic quartz wall reactor containing embedded iron species. These results imply that aromatics, which are hydrogen radical donors, can promote methyl radical formation in the gas-phase reaction even under the reaction conditions considered in the present study.In contrast to the results depicted in Fig. 4, if the reactivities of aromatics and acetylene on the catalyst surface are minimized by controlling the space velocity of the catalyst, acetylene can be stably present in the post-catalyst space. Similarly, Toraman et al. [23] and Postma and Lefferts [44] reported that reactor configurations with sufficient void space in the post-catalyst zone after catalyst overhead packing could decrease coke formation. In this study, an optimal hydrocarbon range provided by the catalyst exerts a synergistic effect on hydrocarbon formation in the post-catalyst space; the effect is more pronounced when the product contains more aromatics than C3–C4 in the catalyst space (Fig. 6c and e). This is also considered the reason for the minimized coke yield when the catalyst space is less than the void space, in contrast to the maximum hydrocarbon yield that is obtained when the catalyst is completely packed in the reactor space. These results suggest that increasing the ratio of hydrocarbon to coke while increasing the methane conversion requires a balance between the catalyst surface and the gas-phase reaction. Toraman et al. [23] presented a first-principles-based microkinetic model consisting of catalytic and gas-phase reactions over iron atoms anchored on silica, and the results revealed that increasing the influence of the gas-phase reaction on the overall reaction increases the selectivity of aromatics rather than that of ethylene. Nevertheless, the balance between the catalyst and the gas-phase reaction is crucial for increasing the hydrocarbon yield. The present study reveals that the hydrocarbon selectivity-to-methane conversion trend is unaffected by the rate-determining or catalyst initiation steps, whereas a given level of methane conversion can be attained rapidly with minimal coke formation. Fig. 7 shows the XPS profiles of the spent SiC and Fe/SiC catalysts. In the C 1s spectrum (Fig. 7a), the spent SiC surface is characterized by deconvoluted peaks with binding energies of 282.5, 284.4, 285.3, and 286.5 eV associated with C–Si, C–C, C–O, and C–N bonds, respectively, implying a typical SiC surface [53]. The Fe loading on SiC increases the ratio of C–C to C–Si bonds in the spent catalysts, indicating that carbon is developed on the catalytic surface during the reaction. Note that solid carbon in the interparticle space, which is physically separated from the catalyst, appears to be included in the above results. In the Fe 2p spectrum (Fig. 7b), the spent Fe/SiC catalysts exhibit weak broad peaks in the range of 705 and 740 eV, associated with Fe carbides (707.7 eV), Fe3+ (711.4 eV), and Fe 2p 1/2 (724.9 eV) [54]. A substantial proportion of the Fe particles is possibly obscured from the surface by carbon layers. For methane decomposition with iron ores, Zhou et al. [55] report that graphite layers are developed on the Fe particle surface to form carbon nano onions, which are responsible for encapsulation. Therefore, the difference in Fe species exposed to the surface between each catalyst is negligible. As the Fe and carbon species are not directly comparable when considering the low intensities and high heterogeneity, respectively, it can be seen that the spent catalysts exist together with the deposited carbon. Fig. 8 shows the TG and DSC profiles of the spent SiC and Fe/SiC catalysts. In the temperature range of 400–800 °C, the amount of carbon deposited in the spent SiC is 1.5 wt%, which is increased to 8.5 wt% with increasing Fe loading on SiC (Fig. 8a). The spent SiC has a broad exothermic peak, which gradually becomes stronger as the Fe loading on SiC increases (Fig. 8b). In contrast to the 5.7-fold increase in the amount of carbon deposited, the DSC peak position shows a maximum of 712 °C at 0.3Fe and then decreases to 683 °C at 1Fe (Fig. 8c). This trend is attributed to the nature of carbon and the thickness of the carbon layer [56]. Similarly, we observed an increase in the amount of carbon deposited in the spent 0.2Fe catalyst as the catalyst space increased, probably because the partial pressure of C2 + hydrocarbons produced from methane in the stream increases along the axial direction of the reactor (Fig. S3). There was less than 1.5 wt% of coke in 0.63 mL of the catalyst space, most likely formed during the initial activation stage of the 0.2Fe catalyst. This observation regarding the carbon deposited on the spent catalysts is consistent with the trend depicted in Fig. 6, wherein only a small amount of catalyst surface is required at the top of the reactor to minimize the coke selectivity in the MTOAH reaction. Consequently, the rate of the C–C coupling reaction accelerates, and carbon formation on the catalyst surface increases with the increase in the Fe content. Fig. 9 shows the morphological results that can be used to qualitatively analyze carbon deposition on the spent 0.2Fe catalyst. The spent 0.2Fe catalyst has non-uniform Fe particle sizes due to low metal–support interaction between the Fe species and SiC. The results of STEM with EDS analysis (Fig. 9a) show that carbon is deposited differently on the Fe particles and SiC in the 0.2Fe catalyst. Fig. S4 depicts the lattice structure with a lattice parameter of 0.26 nm, which is identical to that of β-SiC. A thin carbon layer (1.5 nm) is observed on the SiC surface (Fig. 9b and S4), whereas the Fe particles are covered by a thick carbon layer (2.3–114.6 nm), as shown in the TEM image (Fig. 9c). Upon measuring the carbon layers surrounding the Fe particles, the thickness of the graphitic layer is found to increase with the size of Fe particles on the SiC surface for the spent 0.2Fe catalyst (Fig. 9d–e). Here, we note that the Fe particle size of the spent catalysts with different Fe content differs slightly (Fig. S5), indicating that the carbon deposition depends on the concentration of Fe species on SiC. Catalysts with different Fe loadings undergo particle stabilization (sintering), and the reaction proceeds under the MTOAH reaction condition of 1020 °C; this reaction temperature is higher than the Tamman temperature, which is the absolute melting temperature of Fe nanoparticles (Fe = 631 °C, FeO = 585 °C, and Fe3O4 = 699 °C). If the Fe particles are considered spherical, the volume of the graphite layer can converge to become equal to that of the Fe particles. This statement implies that the amount of carbon deposited over the Fe particles is more than four times smaller than the Fe content, which is lower than the amount of bulk carbon based on TG profiles in Fig. 8. The physical desorption of carbon occurs during the reaction, and it appears to proceed after carbon sufficiently grows from the surface [55]. The predominant presence of solid carbon in the interparticle space is probably derived from the active Fe species, and carbon layers on the Fe particles do not considerably decrease the activity. Because the hydrocarbons can permeate the internal structure of the crystalline carbon layer, the surface reaction of the catalyst is still considered to be dominated by the Fe species. This finding is consistent with the results in Fig. 1, indicating that Fe species on the SiC surface can enhance the methane reactivity but further induce coke formation through consecutive C–C coupling reactions. Fig. 10 shows the Raman spectra, which can be used to distinguish the type of carbon in the spent catalysts. The Raman spectra of each catalyst exhibit two distinct peaks, which can be deconvoluted into five Lorentz peaks, as reported by Sadezky et al. [57]. The spent catalysts in Fig. 10(a) exhibit the morphology of typical graphite carbons based on a subdivision of peaks: the range 1570–1590 cm−1 represents the sp 2-hybridized carbon bond of the ideal graphitic lattice (G); 1346–1353 cm−1 represents vibrations of disordered carbon such as graphene layer edges (D); 1602–1619 cm−1 represents disordered aromatic structures such as surface graphene layers (D’); 1492–1521 cm−1 represents vibrations of carbon defects such as amorphous carbon (D’’), and 1171–1235 cm−1 represents C–H vibrations of disordered graphitic lattice (I) [57–59]. Here, we obtained the characteristic coke fraction as the area fraction of deconvoluted peaks (Fig. 10b). In characteristic fractions, the ideal graphitic lattice-induced (G)-band becomes stronger as the Fe content on SiC increases. These structural differences are likely attributed to the increased probability that hydrocarbons with different carbon numbers expose the catalytic surface.We predicted the crystallite dimension of carbon materials according to the equation established by Tuinstra and Koenig [60]: I D/I G = (2.4 × 10−10)λ 4/L α, where λ represents the wavelength of the laser, and L a represents the nano-sized graphite crystallite. Here, the crystallite size of the carbon is inversely proportional to the area ratio of the D and G bands (I D/I G). Ishii et al. [61] characterized typical non-graphitizable and graphitizable carbons and found that the crystallite size of carbon was inversely related to the number of carbon edge sites. We measured the distribution of graphite crystallite by analyzing at least five points, as shown in Fig. 10(b). The crystallite size of carbon is almost constant until the Fe content on SiC is 0.26 wt%, but further increase leads to its non-uniformity. The minimum values of the crystal size of carbon in the spent catalysts range from 8.7 to 10.0 nm, which are similar to the results obtained after increasing the space of the 0.2Fe catalyst along the axial direction of the reactor (Fig. S6). The 0.2Fe catalyst space in the reactor is directly proportional to the amount of carbon deposited in the spent 0.2Fe catalyst. However, the characteristic fraction of carbon differs only slightly during the methane conversion from 1.2 to 7.2% (Fig. 6 and S6). However, the spent 0.3Fe, 0.5Fe, and 1Fe catalysts include carbon deposits with high crystallinity ranging from 13.6 to 29.0 nm, which is attributed to the Fe particles.Compared to pure SiC, adding up to 97 µmol of Fe to the reactor (0.1Fe) induces a smaller crystallite size of carbon, which improves the methane reactivity by minimizing the coke selectivity (Fig. 1a and Fig. 10b). However, providing excess Fe sites up to 186 µmol (0.3Fe) appears to increase the surface reactivity of aromatics, resulting in more disordered carbon, which is probably responsible for the increase in the coke yield. When more than 327 µmol of Fe is present in the reactor (0.5Fe), the smaller carbon crystallites partially accompany the formation of larger carbon crystallites. At this time, high carbon deposits reduce the interparticle space in the catalyst zone, which appears to be the reason for the slight decrease in the reaction activity, as shown in Fig. 1(c). The carbon deposition in this range appears to be governed by graphite formation from Fe particles [55]. Fe particles appear to induce different types of coke precursors (i.e., acetylene, benzene, and polyaromatic compounds) with methane activation depending on their concentration and size. These coke precursors may terminate in solid carbon on the catalyst, with different properties [62]. Under our experimental conditions, the catalyst appears to provide sufficient surface area to randomly act as a radical terminator for coke precursors.Solid carbons with different structures, such as activated carbon, carbon black, mesoporous carbon, and carbon nanofiber, have different reactivities in methane decomposition at 900 °C; the activity is affected by the surface area [64]. This interpretation and the results presented in Fig. 10 suggest that the carbon deposited with smaller crystallite sizes on the catalyst may provide additional carbon edge sites, which can participate in the reaction as additional active sites. According to Muradov et al. [65], who used varied surfaces including carbon materials at 850 °C for the catalytic decomposition of methane, the hydrocarbon formation rate increased in the order: methane < ethylene < acetylene < benzene as the crystallite size of carbon materials decreased. Solid carbon is considered to behave similarly to SiC surfaces by promoting the desorption of hydrocarbons rather than Fe species. However, further studies are required to obtain insights into more atomically precise active sites.With reference to catalysis, one possibility to consider is that the proximity between the Fe particles on the SiC surface affects methane reactivity. In the thermal decomposition of methane, Ea for carbon nuclei formation was determined to be considerably higher (316.8 kJ mol−1) than that required for carbon crystallite growth (227.1 kJ mol−1) [63]. This range of values is similar to that of the experimentally obtained Ea between SiC and 0.3Fe catalysts (Fig. 3b), emphasizing the effect of the surface Fe concentration in reducing coke selectivity during the MTOAH reaction. To confirm this observation, we performed a reaction by physically mixing the catalyst with different Fe contents and SiC so that 97 µmol of Fe was present in the reactor, and the results of hydrocarbon and coke yield are shown in Fig. 11 . The proximity of Fe particles on the SiC surface increases with the Fe loading of each catalyst (Fig. S5). The hydrocarbon and coke yields increase along with the Fe content in the Fe/SiC catalyst. Coke formation is favored over hydrocarbon formation with the increase in the proximity of Fe particles. The proximity of the Fe particles increases the amount of carbon surrounding the Fe particles and that is present in the interparticle space. This indirectly indicates that the catalytic surfaces are active as radical terminators to increase the coke selectivity. The solid carbon in the interparticle space is probably derived from the active Fe species, and carbon layers on the Fe particles do not appreciably decrease the activity. Consequently, the coke selectivity increases through consecutive C–C coupling reactions, implying that the presence of highly concentrated Fe on the SiC surface does not favorably decrease the coke selectivity in the product. This finding is consistent with that reported by Han et al. [18], who confirmed through electronic structure calculations on methane activation that Fe3C clusters favor coke formation, whereas confined Fe sites favor methyl radical formation. To maximize hydrocarbon yield margins while minimizing coke selectivity, further studies should be conducted to optimize the surface dispersion and nature of metal cations, thus preventing carbon formation on the surface or ensuring crystalline coke formation with extremely few defects. Fig. 12 shows the space–time yield (STY) optimized by the reactant flow rate in enlarged reactors packed with 0.63 mL of the 0.2Fe catalyst on top. In this case, only the post-catalyst zone was considered for the void space, and a tube with an inner diameter of 7 mm was connected so that the ratio of the void space to the catalyst space exceeded 2. A quartz rod (inner diameter = 3 mm) was used to minimize the effect of the overall reactivity of the unwanted sections of the reactor and the void space of this part was also included in the total void space. As the ratio of void space to catalyst space increases by 3.1 times, the space velocity in the total volume of the reactor decreases by 0.4 times (Fig. 12a). For the same space velocity in the catalyst (20 mL min−1), as the ratio of void space to catalyst space increases, the STY of coke increases, whereas that of C2, C3–C5, and aromatics decreases. The STY of methane evidently does not differ with the reactor configuration, indicating that hydrocarbons favor being converted to coke in the excess void space. However, each catalytic reactor has an appropriate methane flow rate, which reduces the coke selectivity. In this case, the STY of methane is almost unchanged. In a previous study, we used artificial intelligence to optimize the reaction parameters of a gas-phase-dominated reactor for methane conversion and found that partial pressure was the most important factor affecting C2 and coke selectivity [66]. This indicates that the stability of the hydrocarbons (such as acetylene and aromatics) in the gas phase depends on the partial pressure. Thus, the role of the 0.2Fe catalyst is possible to produce hydrocarbons from methane without coke formation, and the hydrocarbons promote the formation of methyl radicals in the gas phase of the post-catalyst zone, thus improving the reactivity.When the ratio of void space to catalyst space is 6.13, the 0.2Fe catalytic reactor exhibits a stable STY of hydrocarbons with minimal coke formation during 40.2 h of the reaction at 1020 °C (Fig. 12b). The 0.2Fe catalytic reactor converts methane to 43.8% C2, 5.2% C3–C5, and 51.0% aromatics on average, comparable to the selectivity of Fe©SiO2, as reported by Guo et. al. [17]. From a practical engineering perspective, catalytic reactor optimization is considered to minimize coke selectivity, thereby potentially increasing the chemical process stability. To further increase the STY of hydrocarbons in a packed bed reactor, a reactor may need to be designed with a gas-phase zone that facilitates local control of the partial pressure of the product. In addition, based on Fig. 5, when a surface reaction is induced in the thermal gradient reactor, acetylene can be more selectively hydrogenated to ethylene at a lower temperature than that required for methane activation. In the future, the design of catalysts that selectively produce radical donor molecules capable of reversibly donating CH3– and H-radicals will be instrumental in obtaining higher olefin yields.In this study, the effect of the catalytic surface in a quartz tube reactor on the MTOAH process was investigated using SiC catalysts impregnated with 0–1.25 wt% Fe. The SiC surface decreased E a and A for methane consumption, compared with the values obtained for the blank reactor. The presence of 97–695 µmol of Fe in the reactor further decreased E a and A, thus increasing hydrocarbon yield, which reached a maximum of 6.7% when the amount of Fe in the reactor was 327 µmol at 1020 °C. The hydrocarbon formation from methane in the Fe/SiC packed reactor was 2.7–8.4 times higher than the accompanying coke formation, for each catalyst. The Fe/SiC surfaces induced more ethane and ethylene than the blank reactor, but less acetylene at similar C2 yields. According to selective C2 conversion studies of ethane, ethylene, and acetylene mixed with hydrogen, an excess amount of Fe in the reactor favors the C–C coupling reaction over the selective hydrogenation of acetylene, resulting in coke formation. The closer contact between Fe particles with increasing Fe loading on SiC at 97 µmol of Fe in the reactor increased the coke yield. By optimizing the ratio of the void space of the post-catalyst zone to the 0.2Fe catalyst-packed space, the obtained hydrocarbon yield was 7.1% with a coke selectivity of less than 2% at a catalyst space of 0.63 mL and void space of 1.26 mL. Although the ratio of void space to catalyst space was further increased to 6.13, no significant difference was observed in the STY of hydrocarbons in the 0.2Fe catalytic reactor. Moreover, at this ratio, the STY of the hydrocarbons in the 0.2Fe catalytic reactor was maintained at 1784 µmol C mL−1h−1 during 40.2 h of the reaction. Therefore, these findings can provide guidelines to optimize the design of catalytic reactors, thereby facilitating the scale-up from the laboratory to the commercial scale.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research was supported by the C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017M3D3A1A01037001). This research was supported by the Ministry of Trade, Industry and Energy (MOTIE), Korea Institute for Advancement of Technology (KIAT) through the Virtual Engineering Platform Program (P0022334).Supplementary data to this article can be found online at https://doi.org/10.1016/j.jechem.2023.03.019.The following are the Supplementary data to this article: Supplementary data 1

The conversion of methane to olefins, aromatics, and hydrogen (MTOAH) can be used to stably obtain hydrocarbons when the effect of the catalytic surface is optimized from the reaction engineering perspective. In this study, Fe/SiC catalysts were packed into a quartz tube reactor. The catalytic surfaces of SiC and the impregnated Fe species decreased the apparent activation energies (E a) of methane consumption in the blank reactor between 965 and 1020 °C. Consequently, the hydrocarbon yield increased by 2.4 times at 1020 °C. Based on the model reactions of ethane, ethylene, and acetylene mixed with hydrogen in the range of 500–1020 °C, an excess amount of Fe in the reactor favored the C–C coupling reaction over the selective hydrogenation of acetylene; consequently, coke formation was favored over the hydrogenation reaction. The gas-phase reactions and catalyst properties were optimized to increase hydrocarbon yields while reducing coke selectivity. The 0.2Fe catalyst-packed reactor (0.26 wt% Fe) resulted in a hydrocarbon yield of 7.1% and a coke selectivity of < 2% when the ratio of the void space of the post-catalyst zone to the catalyst space was adjusted to be ≥ 2. Based on these findings, the facile approach of decoupling the reaction zone between the catalyst surface and the gas-phase reaction can provide insights into catalytic reactor design, thereby facilitating the scale-up from the laboratory to the commercial scale.

1,2-propanediol (1,2-PDO), also called propylene glycol (PG) is widely known as valuable chemicals used as monomer or additive in production of pharmaceuticals, cosmetics, solvent in food, as engine coolant, de-icing agent, and raw material for polyester resins (Gallegos-Suarez et al., 2015; Mauriello et al., 2015). Therefore, it has been regarded as a major commodity chemical with an estimated global production of about 1.4 million tons yearly at a 4% of annual market growth rate (Vasiliadou et al., 2011). The conventional production of 1,2-PDO is from petroleum derivatives via hydration process of hazardous propylene oxide (Bagheri et al., 2015; Rajkhowa et al., 2017). However, due to concern of petroleum shortage in the long-term, as well as the environment pollution issue, it is highly desirable to produce 1,2-PDO from a renewable source which may also substantially alters the price of 1,2-PDO. The surplus of glycerol as by-product from the rapid development of biodiesel (1 kg glycerol for every 9 kg biodiesel produced) could serve as an advantage and ideal solution for converting it into 1,2-PDO (Pandhare et al., 2016; Zhao et al., 2020). Due to above, the conversion of glycerol into 1,2-PDO via catalytic hydrogenolysis reaction has generated research interest. Generally, hydrogenolysis reaction require molecule bond dissociation and insertion of hydrogen into generated fragments which involve the cleavage of C-O bond of glycerol molecule while the C-C bond cleavage is undesired as it would lead to side products (Zheng et al., 2015). The evolution of 1,2-PDO from glycerol hydrogenolysis was described to proceed via dehydration of glycerol molecule to form acetol on acid site and further hydrogenation of acetol intermediate to 1,2-PDO on metal site (Balaraju et al., 2009, Mallesham et al., 2016, and Gandarias et al., 2012). The general reaction route for 1,2-PDO production is shown in Scheme 1 .Various heterogeneous catalysts have been well studied in glycerol hydrogenolysis, yielding different product compositions. In particular, the use of noble and lanthanide metals such as Pd, Pt, Ru and Ce have been reported with high selectivity to 1,2-PDO and high conversion of glycerol (Soares et al., 2016, Xia et al., 2011, and Yu et al., 2010). Alternatively, the use of transition metal-based catalysts such as Cr, Co, Ni, Cu, Zn and Zr, have also been associated with high catalytic activity. The transition metal-based catalysts were often preferred as a choice due to their efficiency towards C-O bond cleavage in contrast to C-C bond cleavage reaction (Freitas et al., 2018, Putrakumar et al., 2015, Zhao et al., 2020, and Mauriello et al., 2015). In addition, the lower price of transition metals in comparison to noble metals, emerge as the promising cost-effective substitute for noble metal as catalysts for hydrogenolysis of glycerol. However, the use of transition metal alone is of a big concern since metal leaching and sintering commonly occur. This is because the metal particles tend to aggregate under elevated reaction temperature due to a weak interaction among the metal species and thus easier to deactivate especially for a long reaction time thereby decreasing the catalytic performance (Wen et al., 2013). The presence of a support is highly desired in order to raise the activity and stability of the metal catalyst. The support behaves as a reservoir of spill over hydrogen that helps to hydrogenate surface species in which the available hydrogen from the support surface can pass to the surface metal and generate interfacial active reaction sites on metal-support surface and thus promote higher catalytic activity.Substantial studies have been performed to probe the relationship between the catalytic reaction performance and the interaction of supports with catalyst metals of different nature. It is generally accepted that a combination of support and metal catalyst exhibit more attractive properties such as chemical and thermal strength, metallic phase stability, high metal dispersion and high metal reducibility. All these properties promote higher glycerol conversion and 1,2-PDO selectivity. Specifically, during glycerol hydrogenolysis, a catalyst support is supposed to be defined as a good reduction agent by the interaction of support and metal oxide. In this way, the reduction property of oxide species is enhanced. It has been reported that when a metal oxide is supported, the electrons from the support were directly transferred to the metal oxide species which then promote the formation of metallic species acting as active reaction sites for hydrogenation of C-O bond (Gallegos-Suarez et al., 2015). Due to the concern of good electronic properties, a metal catalyst is suggested to preferably interact with the oxide supports than the non-oxide supports such as carbon and polymeric resin.In view of the above facts, thus supporting metal on a good support will no doubt improve catalyst efficiency in glycerol hydrogenolysis. In this study, dolomite with a mixture of mainly calcium carbonate (CaCO3) and magnesium carbonate (MgCO3) with several appreciable amount of SiO2, Fe2O3, Al2O3 at (<5%) has been selected to be used as support in this work. Apart from its lower price, this support material has gained attention due to its acidic characteristics which are important in glycerol hydrogenolysis but the characteristic is rarely reported in literature. The presence of calcium and magnesium in dolomite may help in reducing metal oxides into metallic species since both metals have been identified as good reducing agents in electrochemical series. Incidentally, dolomite is abundantly available in Perlis, Malaysia and therefore can easily be accessible. Therefore, the objective of this research is to develop bifunctional supported metal catalysts comprising of acid and metal sites that possess good metal-support interaction species for high hydrogenolysis efficiency.In this study, the dolomite used as catalyst support was supplied by quarries in Chuping, Perlis Dolomite Industries, Malaysia. The metal precursors of copper nitrate hexahydrate (Cu(NO3)2·6H2O) (≥99%), nickel nitrate hexahydrate (Ni(NO3)2·6H2O) (≥99%) and cobalt nitrate hexahydrate (Co(NO3)2·6H2O) (≥99%) were purchased from R&M Chemical Company, Malaysia. For iron nitrate nanohydrate (Fe(NO3)3·9H2O) (99%) and zinc nitrate hexahydrate (Zn(NO3)2·6H2O) (98%), the precursors were supplied from Bendosen Company. Glycerol (≥99.5%) was acquired from Sigma-Aldrich. All chemicals in this study were used as provided.Impregnation method was used in the production of all catalysts with a metal loading of 20 wt%. In a typical synthesis, 3.8 g of metal nitrate precursor was separately dissolved in 10 ml distilled water and was then poured into 4 g dolomite powder and was referred as supported metal catalyst denoted as M*/Dol (M = Cu/Ni/Co/Fe/Zn). The mixture was then stirred using magnetic stirrer at 300 rpm and further dried for 3 h with heating at 90 °C on a hot plate. The dried mixture was then aged in drying oven for 24 h at 120 °C. After that, the synthesized catalyst was charged for calcination at 500 °C for 3 h in a tube furnace under static air with ramping of 10 °C/min in order to remove all the nitrate salt present in the catalysts. The calcined catalysts were then reduced by 5% H2/Ar at 600 °C for 3 h at a heating rate of 2 °C/min in the same tube furnace. All the synthesized catalysts were used as such for glycerol hydrogenolysis reaction.The textural properties of catalysts were determined from the adsorption–desorption isotherms of nitrogen using Gemini apparatus (Micromeritics 2010 Instrument Corporation). Prior to measurements, the catalyst sample was degassed at 150 °C for 24 h in order to remove all the moisture and foreign gases deposited on the catalyst surface. Then the adsorption and desorption processes of N2 was then analyzed in a vacuum chamber at −196 °C. The catalyst surface area was determined by Brunauer-Emmett-Teller (BET) method while the pore size distribution was calculated using the method of Barrett, Joyner and Halenda (BJH).The X-ray diffraction (XRD) analysis was performed in order to analyse the phase composition structure of the crystalline catalysts and its crystallite size. It was conducted using a Shimadzu diffractometer model XRD-6000 by employing CuKα radiation source with wavelength of λ = 0.1541 nm, generator current of 30 mA and voltage of 40 kV. The finely ground samples were scanned at a speed of 2°/min using a Siemens D-500 diffractometer and the corresponding diffractogram data were collected from scattering angles at range 2θ = 10–80° while phase identification was determined by matching experimental patterns with the JCPDS diffraction file. The crystallite size (nm) of the catalyst particle was calculated using Debye-Scherrer equation corresponding to full width of half maximum (FWHM) of respective peak.The characteristic of metal reducibility was measured by temperature-programmed reduction (H2-TPR), using Thermo-Finnigan TPD/R/O 1100 SERIES equipped with a TCD (thermal conductivity detector). In a typical experiment, the amount of hydrogen consumption was initially calibrated using known amount of CuO powder as reference standard by pulse chemisorption technique in order to ensure the sensitivity of thermal conductivity detector (TCD) signal. The H2 consumption generated from the calibration of CuO powder was calculated and the value was set as a calibration factor to calculate the H2 uptake for the next analysis. Prior to sample analysis, catalyst sample (~0.05 g) was pre-treated for removing moisture content using N2 flow at a heating of 120 °C for 30 mins (at a rate of 20 cm3/min) before cooling down to room temperature. After the catalyst pretreatment, an in-situ H2 chemisorption analysis was performed from 50 to 1000 °C for 1 h (10 °C/min) in 5% H2/Ar (25 cm3/min). Thereafter, the data from reduction of chemisorbed sample was measured from the generated peak area of hydrogen consumption.The acid sites distribution and total acidity amount of catalyst were studied by temperature programmed desorption of ammonia (NH3-TPD) (Thermo-Finnigan TPD/R/O 1100 SERIES). Before sample analysis, the TCD signal was initially calibrated using known amount of CuO powder as reference standard. The generated NH3 concentration was then referred as calibration factor value for the next sample analysis. As for sample analysis, catalyst sample (~0.05 g) was initially carried out with ammonia adsorption in ammonia flow at room temperature for 1 h. Thereafter, the adsorbed ammonia was desorbed at 50–1000 °C in helium flow (30 cm3/min) and with heating rate of 10 °C/min. The total acidity amount of the catalyst was determined by the integration of peak area (area under graph) of the analyzed sample.The morphological characteristic of all the catalysts were acquired using scanning electron microscopy (SEM) using an apparatus from Rayny EDX-720. During the analysis, the surface images of a catalyst were spotted through LEO 1455 VP electron microscope in a high-vacuum condition at 20 kV.The catalytic tests were conducted in a 150 ml stainless steel autoclave reactor (SS316L series) equipped with Teflon lining cup, an electrical heating jacket and a magnetic stirrer. In a typical experiment, the autoclave reactor was charged with 4 g glycerol solution, 16 g distilled water, and 1 g synthesized catalyst. The reactor then was purged and pressurized with H2 to the desired pressure. Afterwards, the reactor was heated in a defined reaction time for hydrogenolysis reaction. During the catalytic reaction, the reactor was set at maximum H2 pressure, temperature and time of 4 MPa, 200 °C and 10 h, respectively. For all catalytic reactions, the reactor was left stirred at 400 rpm. The reaction starting time was defined once the reactor temperature reached the desired reaction temperature. After completion of the reaction, the reactor was cooled down to room temperature, and the obtained liquid product was collected and separated from the catalyst by centrifugation process at 3000 rpm for 15 min. For comparison study, a blank reaction (reaction being conducted without the presence of any catalyst powder and/or support) was also performed under similar reaction parameters.The obtained liquid product from glycerol hydrogenolysis reaction was analyzed using gas chromatography-flame ionization detector (GC-FID) equipped with HP-5 capillary column (length: 30 m ⨯ inner diameter: 0.32 mm ⨯ film thickness: 0.25 µm). It was operated at 300 °C with splitless inlet mode. Prior to analysis, the liquid product was extracted using ethyl acetate in a 1:1 ratio. The extraction was carried out three times. Subsequently, the product solution was dried in oven at 70 °C for 15 mins in order to concentrate the solution. Lastly, a derivatization process was charged to the liquid sample before it is being analyzed by GC analysis. Typically, N-O-bis(trimethylsilyl)trifluroacetamide (BSTFA) was used as the silyl agent and was mixed with pyridine (C5H5N) as binding solvent in a 1:1 ratio and was then left dried in oven for 20 mins at 60–70 °C so as to achieve complete silylation process. 1 µL amount of the derivatized product was directly injected to GC. The initial temperature was determined at 40 °C and held for 6 min with rate of 7 °C min−1 towards reaching the final temperature of 270 °C. The temperature for injection was set at 250 °C. The glycerol conversion and the selectivity of product were acquired by comparing the retention time of standard with the obtained experimental-based products on GC chromatogram peak. The equations for calculation of glycerol conversion and 1,2-PDO selectivity are depicted in Equation (1.1) and Equation (1.2), respectively. (1.1) Glycerol conversion , % = C glycero l , i n - C glycero l , o u t ∑ C glycerol , i n × 100 % (1.2) 1 , 2 - P D O s e l e c t i v i t y , % = C 1 , 2 - P D O C Total × 100 % Where, Cglycerol,in is described as the initial concentration of glycerol and Cglycerol,out as the final concentration of glycerol. And Ctotal is the sum of the product detected in the liquid product. (All peaks regarded to the product in this study were confirmed by the peak of standard solution)The textural properties of all catalysts derived from N2 adsorption–desorption isotherms are presented in Table 1 . The BET specific surface area of dolomite, Cu/Dol, Ni/Dol, Co/Dol, Fe/Dol and Zn/Dol, were found to be 13.3, 9.7, 3.5, 7.8, 2.1 and 2.9 m2g1, respectively. The decreasing surface area of supported metal oxide samples as opposed to dolomite was due to the filling of metal oxide in the support pores. This finding was similar to Thirupathi et al. (2012), who stated that the reduction in the surface area of Mn–Ni(0.4)/TiO2 catalyst was due to the blocking effect of the loaded nickel oxide on the support material. The decrease of catalyst’s surface area was consistent with the catalyst pore volume which shows decrease from 0.276 cm3g−1 (dolomite) to 0.096, 0.071, 0.145, 0.037, 0.073 cm3g−1 for Cu/Dol, Zn/Dol, Co/Dol, Fe/Dol and Ni/Dol catalyst, respectively. This behavior was due to the blockage and destruction of the catalyst structure similar to the report of Zhao et al. (2013).Similarly, different supported metal catalysts exhibited different pore diameter ranging from 19.04 Å to 156.34 Å. Cu/Dol, Fe/Dol and Zn/Dol catalysts showed smaller pore diameter in the range 19.07–27.82 Å compared to that of dolomite support (152.02 Å) while Ni/Dol and Co/Dol catalyst presented bigger pore diameter of 155.90 Å and 156.34 Å, respectively. It is worth mentioning that the Cu/Dol, Fe/Dol and Zn/Dol catalysts exhibited both smaller in pore diameter and pore volume of (0.098 cm3g−1 and 19.07 Å), (0.037 cm3g−1 and 19.04 Å) and (0.071 cm3g−1 and 27.82 Å), respectively than dolomite support (0.276 cm3g−1 and 152.02 Å). The small pore diameter and pore volume of those catalysts could be related to the occurrence of new active sites (new pore) formed on the catalyst surface. This characteristic is somehow an advantage because it would reduce the metal species from being easily leached. This consequently may lead to the stronger adsorption–desorption of the active sites during catalytic reaction. Also, the presence of active sites inside the small pore was assumed to help in the reusability and stability for the next reaction cycle.Meanwhile, the N2 adsorption–desorption isotherms and pore size distribution curves of all catalysts are compiled in supplementary material (Fig. S-1 and Fig. S-2). It shows the type III isotherm for dolomite support and all supported metal catalysts. This isotherm was assigned to the weak interaction characteristic of multilayer adsorption of typically clustered catalyst material. Furthermore, the similar isotherm of dolomite support and supported metal catalysts suggesting that the dolomite structure was not significantly modified even with addition of metals. The formation of hysteresis p/p° > 0.8 was observed, which showed a characteristic of typical non-rigid aggregates of plate-like particles (slit pore shape) with non-uniform size of the catalyst (Luna et al., 2018). Meanwhile, the isotherms for dolomite support exhibited lower N2 adsorbed volume (~1 cm3g−1) than supported metal catalysts, indicating the macroporous characteristic in the catalyst material. The higher N2 adsorbed volume exhibited by supported metal catalysts could be corresponded to the presence of some mesopores. The distribution of pore size curve of all catalysts obtained by Barrett, Joyner, and Halenda (BJH) method shown in Fig. S-2 revealed that the pore size distributions of dolomite, Cu/Dol and Ni/Dol catalysts were in range 2 to 50 nm. On contrary to the Co/Dol, Fe/Dol and Zn/Dol catalysts, the pore sizes were in range 1–90 nm.The XRD diffractograms of calcined and reduced samples are presented in Fig. 1 (A) and Fig. 1(B), respectively. The XRD pattern of dolomite support was observed with mixed crystalline phases. The diffraction peaks at 2θ = 18.1°, 28.3°, and 33.8° were assigned to CaMg2 (JCPDS; 01–1070). The peaks at 2θ = 37.51°, 50.76°, and 62.20° were due to dolomite phase (JCPDS; 02–0767) while two peaks at 2θ = 44.2° and 47.4° were denoted as MgCO3 phase ((JCPDS; 02–0871). The presence of CaO phase shows peak at 2θ = 32.4° and 54.2° (JCPDS; 01–1160) and the less intense peak for MgAl2O4 phase was detected at 2θ = 78.7° (JCPDS; 03–1160). These phases are also present in all supported metal catalysts but with reduced peak intensities as a consequence of the embedment of metal oxides in the dolomite matrix. In return new phases corresponded to the respective metal oxide were seen such as CuO at 2θ = 35.5° and 38.8° (JCPDS; 44–0706), NiO at 2θ = 37.2° (JCPDS; 01–1239), Fe2O3 at 2θ = 23.7°, 34.1°, 50.1° and 58.2° (JCPDS; 02–0915) and ZnO at 2θ = 32.5°, 36.5°, 56.8°, 68.3° and 69.7° (JCPDS; 03–0888).Nevertheless, in the case of Co/Dol catalyst, no characteristic peak of CoO phase was detected, instead cobalt carbonate (CoCO3) was formed at 2θ = 34.1° (JCPDS; 01–1020). Apart from that, an alloy phases were also detected with the presence of MgNiO3 spinel (JCPDS; 03–0999) at 2θ = 75.2°, Ca2Fe2O5 spinel (JCPDS; 02–0936) at 2θ = 24.8°, 34°, 45.5° and 60.1°, MgZn (JCPDS; 08–0206) and CaZn3 (JCPDS; 35–1159) at 2θ = 50.2° and 2θ = 53.5°, respectively. On the other hand, it was noted that there was no characteristic peak related to any metallic species was observed in all calcined samples.The XRD patterns of the reduced catalysts in Fig. 1(B) shows that the intensity of diffraction peak at 2θ = 43.5° corresponded to MgCO3 phase of all supported metal catalysts became more intense and higher upon the addition of respective metal to dolomite support. This is due to the interaction of metal species with dolomite support. For reduced dolomite, apart from peaks presented in calcined dolomite, new peaks were also detected such as CaCO3 at 2θ = 29.3° and 72.2° (JCPDS; 01–1032). For supported metal catalysts, it was displayed that upon reduction by H2 at 600 °C, the diffraction peaks of CuO (2θ = 35.5°, 38.8°) and NiO (2θ = 37.2°) of Cu/Dol and Ni/Dol catalysts disappeared, while the characteristic peaks attributed to metallic Cu species (2θ = 43.5° and 74.1°) (JCPDS; 085–1326) and metallic Ni species (2θ = 44.2°, 52.1° and 76.1°) (JCPDS; 001–1260) were emerged. Similar diffraction peaks of metallic copper and nickel was also reported by Wen et al. (2013) and Srivastava et al. (2017). Additionally, it was found that no characteristic peak attributed to any Cu2O and Ni2O phases was detected in Cu/Dol and Ni/Dol catalysts, indicating the reduction of Cu2+ and Ni2+ species was complete (Zhu et al., 2013, Zhao et al., 2013and Gandarias et al., 2012). The presence of metallic Cu and Ni species are regarded as active reaction site for the catalytic reaction and thus could increase the glycerol hydrogenolysis reaction.The presence of Cu0 and Ni0 species was attributed to their high reduction ability from metal oxides-dolomite interaction. It could be suggested that the migration of electron (oxidation and reduction) happened on metal oxide-support surface via electrons lone pair would cause the destabilization of metal oxide bond and thus promote the reducibility of oxides (Nagaraja et al. 2007). In this present work, the CaO, CaMg(CO3)2 and MgCO3 species were suggested to be the one involved for the copper and nickel oxide reduction since calcium and magnesium has been identified as good reducing agent (Tasyurek et al., 2018).Accordingly, it has been revealed that metal oxide species was prone to generate spinel when it was supported with clay or limestone material containing Mg and Ca (Kovanda et al., 2001; Pardeshi et al., 2010). Apparently, the Cu2MgO3 (2θ = 35.3°, 37.5°, 38.2° and 48°), MgNiO2 (2θ = 75.3° and 79.2°) and (Ca2Fe2O5) (2θ = 23°, 24°, 32°, 33°, 34°, 44°, 47° and 49°) phases were detected for Cu/Dol, Ni/Dol and Fe/Dol catalysts, respectively, thereby confirming the formation of metal species in spinel. For Co/Dol and Zn/Dol catalysts, alloy phases of Co2Mg and (CaZn3, MgZn) were detected. Notably, no characteristic peak of any metallic Co, Fe, and Zn species was observed in Co/Dol, Fe/Dol, and Zn/Dol catalysts possibly due to the incomplete H2-reduction of the catalysts. In particular for Zn/Dol catalyst, ZnO phases was obviously seen, indicating higher reduction temperature is required to transform the oxide phase into metallic species.The dolomite’s crystallite size was estimated from the XRD peak by choosing 2θ = 62.45 and the results are summarized in Table 1. The dolomite’s crystallite size was found increases when Ni and Cu were supported on it. The trend of crystallite size ranks as Cu/Dol > Ni/Dol > Dol > Co/Dol ≈ Zn/Dol > Fe/Dol. The larger crystallite size of Cu/Dol and Ni/Dol than dolomite probably attributed to the metal species which occupied in the interstitial support bulk thus increased the catalyst crystal size. This could also be correlated to the non shifted peak of MgCO3 phase at 2θ = 43.5°, with respect to dolomite support. The non shifted peak reflected to the presumption that metal promoter was incorporated well on the support (Asikin et al., 2017). However in the case of Co/Dol, Fe/Dol and Zn/Dol catalysts, the dolomite’s crystallite size were decreased from 27.4 nm to 22.9 nm, 19.6 nm, and 22.9 nm, respectively, indicating Co, Fe and Zn species were prone to dissolve in the support lattice as substitutional metal rather than occupied in the interstitial support lattice (Liu et al., 2014). The presence of substitutional metal could be also corresponded to the shifted peak of MgCO3 phase at 2θ = 43.5° to slightly lower degree than dolomite peak.In the case of Fe/Dol and Zn/Dol catalysts, a higher reduction temperature of 900 °C was applied to reduce both oxide species into their metallic form and the XRD patterns are depicted in Fig. 2 . The results obtained were compared with the previous catalysts reduced at 600 °C. It can be seen that the characteristic peak of metallic Fe (JCPDS; 01–1267) was clearly appeared at 2θ = 44.8° indicating that the reduction temperature of 900 °C successfully reduced iron oxide to its metallic species. Meanwhile the formation of Ca2Fe2O5 spinel was also noticeable. Nevertheless, it was observed that the presence of Ca2Fe2O5 spinel became gradually invisible as compared to 600 °C reduced sample which suggested that the spinel species was also reduced at higher temperature.In the case of Zn/Dol catalyst, no characteristic peak attributed to metallic Zn species was detected even after reduction at 900 °C, rather the presence of alloy phase (CaZn3 and MgZn). This indicates that reduction at 900 °C was still not able to transform Zn oxide into its metallic form. However, it was noticed that the diffraction peaks of ZnO phase at 2θ = 32.5°, 36.5°, 56.8°, 68.3° and 69.7° disappeared, while CaZn3 and MgZn phases at (2θ = 32.5° and 54.5°) and (2θ = 68.5° and 75.5°), respectively became more intense peak. This finding is in good agreement with the work of Consonni et al. (1999), who investigated the reduction property of Pt/ZnO catalyst and found that the reduced ZnO catalyst had resulted to the formation of PtZn alloy instead of metallic Zn species.The H2-TPR profiles of dolomite and supported metal catalysts (Cu/Dol, Co/Dol, Zn/Dol, Ni/Dol and Fe/Dol) are depicted in Fig. 3 (A) while the corresponding hydrogen consumption data is tabulated in Table 2 . From TPR profiles, it was discovered that Cu/Dol and Co/Dol catalysts gave a lower reduction peaks as opposed to dolomite support at 689 °C. The reduction of Cu/Dol and Co/Dol was assigned at (291, 455 and 630 °C) and (435 and 638 °C), respectively. Apart from that, it is worthy to note that, the reduction peak of all supported metal catalysts was found to emerge broader and higher than dolomite due to the species reduction from metal alloy phases thereby consumed higher hydrogen adsorption and hence enlarged the reduction peak (Li et al., 2009). Similar behavior was outlined by Soares et al. (2016a,b), the authors indicated that the broader reduction range was detected after addition of Cu to Ru/ZrO2 catalyst which caused interphase hydrogen adsorption of the metals (slow adsorption) due to metal cluster formation from Ru and Cu alloys.According to Zhao et al. (2017), the reduction of dispersed copper oxide species to metallic copper (Cu0) was effective at < 250 °C. Smaller catalyst particles reduce faster when compared with that of CuO in bulk (Zhu et al., 2013). Correspondingly, the reduction of bulk CuO phase took place at temperature higher than 250 °C (Wen et al., 2013). According to Vargas-Hernandez et al. (2014), reduction at > 400 °C was due to the metal-support species or copper in spinel phase. Tanasoi et al. (2009) reported that the Cu-containing mixed oxide reduced at range 400–750 °C due to the presence of complex copper phases of CuAl2O4 and CuxMgxAl2O4. Therefore, in this study, the first two reduction profiles of Cu/Dol catalyst were assigned for reduction of CuO to metallic Cu. Peak at 291 °C corresponded to the reduction of small and big clusters of CuO to metallic copper (Cu0) while peak at 455 °C attributed to the reduction of copper oxide in interstitial defects in dolomite crystalline phase since Cu2MgO3 was previously detected by XRD peak. Reduction of CuO corresponded to two reduction steps of Cu2+ ions to Cu+ ions (CuO → Cu2O), followed by reduction of Cu+ ions to metallic copper (Cu2O → Cu0). Peak maximum at 630 °C was ascribed to reduction of dolomite because the peak profile was close to that of bulk dolomite (639 °C).As for Co/Dol catalyst, it was reported that the reduction temperature of CoO to Co was occurred below 400 °C (Yan et al., 2011). Thus, peak at 435 °C could be referred to reduction of cobalt species from CoCO3 phase as presented in XRD profile in Fig. 1(B). The presence of metallic cobalt species was not noticed in XRD profile probably due to well dispersed metallic Co species or with minor proportion, rather the formation of CoMg2 phase was observed. Apart from that, the formation of CoCO3 peak was still detected in the reduced catalyst. This shows that cobalt species in the form of carbonate was not easily reduced at 600 °C. As stated in literature, the reduction cobalt oxide depends on the cobalt particle size and the properties of the support used (Yan et al., 2011). The presence of broad peak at 638 °C could be due to the reduction of cobalt species which strongly interacted with support. From this study, the high metal reducibility and lower reduction temperature of Cu/Dol and Co/Dol catalysts could be due to the good electronic interaction of Cu and Co oxide with calcium and magnesium species from dolomite.For Ni/Dol, Fe/Dol and Zn/Dol catalysts, higher reduction temperature was observed at (690 and 962 °C), (646 and 946 °C) and 700 °C, respectively. In the case of Ni/Dol catalyst, the broader and higher peaks at 690 °C and 962 °C than that of dolomite peak could be attributed to the reduction of nickel and dolomite species which had stronger metal-support interaction or attributed to the reduction MgNiO2 phase. Similar results were proposed by Srivastava et al. (2017), who stated that the broad peak and high reduction temperature of Ni/Al2O3 catalyst was due to the reduction of NiO species which was in intimate contact with Al2O3 support and/or attributed to the reduction of NiAl2O4 phase. For Fe/Dol catalyst, peak at 646 °C was attributed to the reduction of dolomite. Peak at 946 °C was ascribed to the reduction of iron species in Ca2Fe2O5 spinel phase which strongly interacted with dolomite. In the case of Zn/Dol catalyst, peak at 700 °C could most likely be related to the reduction of dolomite with zinc species from CaZn3 and MgZn alloys. On a general note, Cu/Dol catalyst could be proposed to predominantly exhibit higher metal reducibility than Co/Dol due to its lower reduction temperature. The presence of metallic Cu in XRD peak agreed well with its high reduction character. This observation provided the bases for conducting the hydrogenolysis of glycerol reaction at 200 °C since the presence of active reaction sites (metallic copper species) would be preserved and thus stable during the catalytic reaction.From Table 2, it was observed that the total hydrogen consumption of all supported metal catalysts was higher than that of dolomite following this trend Co/Dol > Ni/Dol > Fe/Dol > Cu/Dol > Zn/Dol > Dol. This finding could be correlated to a study reported by Zhao et al. (2019) who stated that the total amount of H2 consumed for CuO/CeO2 catalyst was far exceeded than that necessary for the complete reduction of pure CuO, specifying that some ceria support would be involved during the reduction process. In this study, the addition of respective metal to dolomite support influenced catalyst reducibility due to higher species exposure area and thus elevates the hydrogen consumption amount. Mallesham et al. (2016) and Gandarias et al. (2012) proposed that when a support was promoted by reducible metal oxide, the availability of hydrogen to be consumed was enhanced as well as the catalytic hydrogenation of alcohol. The presence of CaCO3, MgCO3, CaO and MgAl2O4 phases from dolomite could be considered to provide a source of chemisorbed hydrogen atoms and increased the amount of hydrogen uptake.Additional analysis of metal oxides reduction was carried out as shown in Fig. 3(B) while the corresponding hydrogen consumption data is given in Table 2. It can be seen that copper oxide shows the lowest reduction temperature at 338 and 428 °C, while the reduction peak for nickel oxide was appeared at 392 °C. The peak for cobalt oxide and iron oxide was at 498 °C and (416 °C and 821 °C), respectively. In the case of zinc oxide, the reduction profile was rather flat and the blow-up image shows peaks at 411 °C and 702 °C. Comparing the reduction profiles of both Cu and Co oxides with that of the supported metal catalysts, the later gave a lower reduction peaks. This confirms that the reducibility of those oxide species in supported catalysts was enhanced which due to the promotional effect of metal oxide-dolomite interaction. On the other hand, the reduction of nickel oxide, iron oxide and zinc oxide was noticed far from the reduction region of their supported metal catalysts. This confirms the poor metal reducibility of their oxides. From Table 2, it was demonstrated that the H2 consumption needed for the complete reduction of metal oxides was in range 47–929 µmol/g which was obviously far less than the amount required for supported metal catalysts (14955–73962 µmol/g). Thus, it was suggested that the capability for hydrogen consumption was enhanced in the case of metal oxide supported on dolomite.The available acid sites of dolomite and all supported metal catalysts were performed via NH3-TPD. The classification of acid strength was interpreted as weak (<250 °C), medium (250–500 °C) and strong (>500 °C) (Srivastava et al., 2017). The desorption profiles of all catalysts are indicated in Fig. 4 while the corresponding acidity (amount of ammonia uptakes) is tabulated in Table 3 . All catalysts exhibited desorption peaks above 500 °C, indicating the presence of strong acid sites on the catalyst surface. In all supported metal catalysts, the presence of desorption peaks showed rather smaller and lower desorption profile than dolomite support. Cu/Dol catalyst appeared with higher desorption temperature (at 948 °C) than dolomite (at 874 °C) and other supported catalysts. Dolomite showed two desorption peaks at 805 °C and a shoulder at 874 °C. For Cu/Dol and Co/Dol catalysts, two desorption peaks emerged at (718 and 948 °C) and (712 °C and 815 °C), respectively. In the case of Ni/Dol, Fe/Dol and Zn/Dol, only one desorption peak appeared which at 722 °C, 672 °C and 755 °C, respectively.Notably, the desorption peak of supported metal catalysts with the exception of Cu/Dol shifted towards lower desorption temperature than dolomite from 805 to 874 °C (dolomite) to 672–815 °C (supported metal catalysts). In the case of Cu/Dol, the second desorption peak was shifted to 948 °C (Cu/Dol), indicating the presence of much stronger acid sites in the catalysts. This attributable to the strong interaction of copper on the dolomite support and a sign that copper was well incorporated and dispersed over dolomite surface than other catalysts. The acidity data in Table 3 shows that the addition of respective metal promoter to dolomite support contributed to different acid amount. Incorporation of nickel, cobalt and iron, gave a decrease in the acidity from 16149 µmol/g (dolomite) to 14305, 11172, 6075 µmol/g for Ni/Dol, Co/Dol and Zn/Dol catalyst, respectively. This could be assigned to the coverage of the metal species (Ni, Co and Fe) on dolomite surface, therefore allowed limited accessibility of NH3 gas to be bonded with the catalyst pore. This finding was in agreement with the work of Priya et al. (2017), who proposed the decrease of acid amount in metal-promoted mordenite catalyst was attributed to the blockage caused by the metal species. In another study reported by Vasilidiaou et al. (2009), it claimed that the decrease in catalyst acidity of Ru-supported catalyst was due to the Ru species was possibly reside (occupied on the alumina support surface).On the contrary, when copper and zinc was added to dolomite support, the acidity was increased from 16149 µmol/g (dolomite) to 19528 µmol/g (Cu/Dol) and 17085 µmol/g (Zn/Dol), respectively. The similar behavior was also reported by Thirupathi et al. (2012), where the addition of nickel species on Mn/TiO2 catalyst improved and broadened the acid site distribution of the catalyst. The order of catalyst acidity ranks as Cu/Dol > Zn/Dol > Dol > Ni/Dol > Co/Dol > Fe/Dol. It should be noted that the high acid sites of Cu/Dol and Zn/Dol catalysts could act as active reaction sites which contribute to high reactivity of C-O bond cleavage of glycerol molecule during dehydration step and consequently enhance the catalytic reaction of glycerol hydrogenolysis to a higher level. The presence of carbonate phases (CaCO3, MgCO3) in dolomite could be the one responsible to provide a source of chemisorbed NH3 gas and increase catalyst acidity of Cu/Dol and Zn/Dol catalysts. Fig. 5 shows the surface morphology of all catalysts. As seen in the figures, all samples present agglomerated structure with an irregular shape of an average size of 10 mm (from scale bar), emphasizing the formation of a macroporous solid in a cluster of closely spaced crystals.The glycerol hydrogenolysis reaction was carried out and the results of catalytic activity are summarized in Table 4 .A reaction without the presence of catalyst and/or support was also conducted and referred to as blank experiment. For the blank experiment, a very low glycerol conversion (8.7%) and no selectivity towards 1,2-PDO were observed. When dolomite was added, a little increase of glycerol conversion of 10.6% was observed but still no selectivity to the desired product (1,2-PDO). These results indicate that the support by itself cannot catalyze the hydrogenolysis of glycerol. A significant catalytic activity was observed on supported metal dolomite samples. Cu/Dol exhibited the highest activity in both glycerol conversion (78.5%) and 1,2-PDO selectivity (79%) among all supported metal catalysts. In contrast, Ni/Dol, Co/Dol, Fe/Dol and Zn/Dol catalysts exhibited lower glycerol conversion and 1,2-PDO selectivity of (69.5%, 52.7%), (60.9%, 58.1%), (44.8%, 5%) and (20.4%, 2.7%), respectively.Notably, Zn/Dol showed the lowest activity of both glycerol conversion (20.4%) and 1,2-PDO selectivity (2.7%) while Ni/Dol and Co/Dol catalysts presented moderate activity in both glycerol conversion and 1,2-PDO selectivity. The trend of catalytic activity ranks as Cu/Dol > Ni/Dol ≈ Co/Dol > Fe/Dol > Zn/Dol > dolomite. A high glycerol conversion over Cu/Dol catalyst was due to its high surface acid sites. The reaction was initiated by the adsorption of glycerol on the acid sites, dehydrated and then converted to give acetol as intermediate product, and consequently yielded to 1,2-PDO. Therefore, a higher acidity provides a greater number of acid sites, hence a greater number of glycerol molecules can be adsorbed on the catalyst surface (Cu/Dol) than other catalysts. This result is consistent with the literature reported that the acid site was a great influencing factor in hydrogenolysis reaction (Putrakumar et al., 2015; Yuan et al., 2009).Apart from that, metallic site is also important for the hydrogenation of acetol intermediate to 1,2-PDO. In the case of Cu/Dol catalyst, based on the H2-TPR profile presented in Fig. 3(A), it showed that copper species of Cu/Dol catalyst was essentially reduced at the lowest reduction temperature (291 °C) than other supported metal catalysts. According to that, the catalytic reaction conducted at 200 °C in this study is within the reduction temperature region of copper species (≥~200 °C). While for Ni/Dol, Co/Dol, Fe/Dol and Zn/Dol, the presence of respective metallic species was poor and incomplete since the reduction temperature of the catalysts occurs at higher temperature of 690 °C, 435 °C, 646 °C, and 700 °C, respectively and thus corresponded to the lower 1,2-PDO selectivity obtained by those catalysts.For Fe/Dol and Zn/Dol catalysts, the absence of the metallic species when catalysts were reduced at 600 °C (Fig. 2(B)) was consistent with the poor activity attained by the catalysts. However, when both catalysts were reduced at 900 °C, improved activity result was obtained. This was attributed to their metal reduction behavior, of which iron and zinc oxide species was low reducible compared to copper, nickel and cobalt oxides. Apart from the formation of high 1,2-PDO product, another essential character for a good hydrogenolysis catalyst is the ability to promote high dehydration of glycerol to form acetol intermediate product and in return suppress the excess hydrogenation reaction towards side product, methanol (C-C bond cleavage). In this study, it was revealed that no selectivity towards acetol intermediate was detected for Fe/Dol and Zn/Dol catalysts. Thus, the poor selectivity towards 1,2-PDO obtained by both catalysts could be related to their incapability for producing acetol as intermediate product which subsequently hydrogenate to desired 1,2-PDO product. In addition, both catalysts also prone to catalyse C–C cleavage reaction as the selectivity towards methanol were significantly higher with 95% and 97.2%, respectively than other supported metal catalysts. Cu/Dol catalyst exhibited the lowest selectivity towards methanol (2.1%), indicating the addition of Cu to dolomite support seemed to hinder C-C cleavage reaction.In this study, various metals supported on dolomite (Cu/Dol, Ni/Dol, Co/Dol, Fe/Dol and Zn/Dol) were synthesized for glycerol hydrogenolysis reaction and Cu/Dol catalyst was found to exhibit a promising activity when compared to other catalysts with highest glycerol conversion and 1,2-PDO selectivity of 78.5% and 79%, respectively at 200 °C of reaction temperature, 4 MPa of reaction pressure and 10 h of reaction time. The performance was attributed to its high acidity and high metal reducibility. Additionally, the reduction profile of Cu/Dol which occurred within the range of catalytic reaction temperature (at 200 °C) was important to preserve the stability of metallic Cu during catalytic reaction. The finding from this study is a valuable step in search of precious metal free and environmentally benign catalysts for the development of biomass valorization.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors thank the Universiti Putra Malaysia for the Research Grant under Geran Inisiatif Putra Siswazah, GP-IPS/2018/9619500 in support of the project.All authors contributed to the success of this paper from the conception to the methodology, analysis of the results, writing proofreading, and review of the paper.Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2021.103047.The following are the Supplementary data to this article: Supplementary Data 1

Hydrogenolysis of biomass-derived glycerol is an alternative route to produce 1,2-propanediol. A series of transition metals supported on dolomite catalysts (Cu/Dol, Ni/Dol, Co/Dol, Fe/Dol, Zn/Dol) were prepared via impregnation, calcined at 500 °C and reduced at 600 °C. The synthesized catalysts were then characterized by BET, BJH, XRD, H2-TPR, NH3–TPD, and SEM, and subsequently evaluated in glycerol hydrogenolysis reaction to produce 1,2-propanediol (1,2-PDO). The nature of transition metals was found to influence the activation of the catalysts. Among the tested catalysts, copper supported on dolomite (Cu/Dol) exhibited appreciable hydrogenolysis performance due to the mutual interaction between the copper species and the dolomite. The findings from the various characterization results showed the addition of copper to dolomite ameliorates the chemical and reduction of the catalyst. It appears that the copper species were essentially enriched on the grain surfaces of the dolomite, the reduction properties, and the acidity of the catalyst enhanced. All the features of Cu/Dol catalyst contributed to the high glycerol conversion (78.5%) and high 1,2-PDO selectivity (79%) with low methanol production as the by-product at 200 °C of reaction temperature, 4 MPa of reaction pressure and 10 h of reaction time.

With the increasing demand for energy, it is expected that the world's total energy consumption will increase from 30% to 50% in the next 20 years. The use of traditional fossil fuels will accelerate the increase in greenhouse gases and change the climate environment. As shown in Fig. 1 , the total apparent CO2 emissions in China have continued to increase over the past few decades. At the General Debate of the 75th United Nations General Assembly, China made a solemn commitment to achieve a "carbon peak" by 2030 and a "carbon neutral" by 2060, which also shows the determination of mankind to reduce carbon dioxide content. This has accelerated the development of a range of technologies for the degradation/reduction of CO2, including chemical, photocatalytic, electrocatalytic and photoelectrochemical methods (Chang et al., 2019; Guan et al., 2021; Prabhu et al., 2020; Zhang et al., 2020c; Zhang et al., 2019b), in the hope of converting CO2 into useful chemical products/fuels in dynamic equilibrium. Among these technologies, the electrocatalytic reduction of CO2 (CO2RR) to CO and other chemical products using renewable electricity serves the twofold purpose of CO2 storage and clean energy conversion, the possible reaction mechanism of which is indicated in Fig. 1. The CO2RR is appealing for both industry and academia with the following advantages: (1) it is easy to combine with other renewable energy sources, such as solar energy and wind energy, and will not generate additional CO2; (2) it has a low cost, safe operating conditions and mild reaction conditions, i.e., room temperature and normal pressure; (3) the reaction pathway can be controlled by electrochemical parameters; (4) by optimizing the electrocatalyst, CO2 reduction byproducts can be minimized; (5) the electrolyte can be reused to reduce its overall chemical consumption; and (6) the electrochemical system can be applied in practice, with the characteristics of compact, modular and on-demand expansion (Clark et al., 2018; Feng et al., 2021; Koshy et al., 2021; Lee et al., 2021; Morimoto et al., 2018). Therefore, the electrochemical reduction of CO2 has received extensive attention and has become an effective method with potential.However, there are still many problems and challenges in the electrocatalytic CO2RR, including the following: (1) As CO2 with linear chemical bonds is a chemically inert molecule, it is difficult to activate. The electrocatalytic process of converting CO2 into desired products is challenging. (2) Electrocatalytic reduction of CO2 often faces problems such as high reaction potential, poor product selectivity and inability to maintain high catalytic activity for a long time. (3) The side reaction hydrogen evolution reaction (HER) must be suppressed. Although H2 is a good clean energy source, it has the disadvantage of low energy density, as well as H2 transportation, storage and safety issues. That is why our goal is to suppress H2 production while maximizing the production of energy-intensive carbon-based fuels. Therefore, it is very important to select suitable electrocatalysts to improve the selectivity stability and reduce the overpotential.As the only known monometal that can reduce CO2 to a number of hydrocarbons, aldehydes, and alcohols, metallic Cu has been extensively studied (Peterson et al., 2010; Schouten et al., 2012; Zhang et al., 2022c). This has also led to an upsurge in CO2RR research on different metal and alloy catalysts. Second-phase metals were introduced to provide active sites (Andrews et al., 2018; Barasa et al., 2019; Li et al., 2021d; Zhang et al., 2018c), thereby improving the activity and selectivity of the reaction. As a representative of traditional electrocatalysts, metal-based electrocatalysts have an irreplaceable position in the field of electrocatalysis. However, metal-based electrocatalysts themselves are always expensive pure noble metals (Ag, Au, Pt, etc.) or need to be doped with noble metals (Ag, Au, etc.), which leads to an increase in the cost and disturbs the HER side-reaction. Porphyrin-based frameworks, as a special class of metal-organic frameworks and covalent organic frameworks, have also been widely used in the field of energy conversion. Their unique and tunable structures greatly reduce the design difficulty for highly selective electrocatalysts (Liang et al., 2021). On the other hand, with the increasing demand for the environment, nonmetallic nitrogen-doped carbon electrocatalysts using biomass as carbon precursors have received extensive attention. Such biomass-derived catalysts could achieve comparable activity to the first three types of catalysts without the introduction of expensive noble metals and exhibit a lower Faradaic efficiency for the HER, making them inexpensive and environmentally friendly electrocatalysts. We therefore systematically reviewed the recent important progress of CO2RR electrocatalysts, including metal-based electrocatalysts, single-atom catalysts (SACs), porphyrin-based framework complexes and metal-free nitrogen-doped carbon electrocatalysts, in Section 2. Challenges and future research directions are also proposed.The reaction thermodynamics and kinetics determine the activity and selectivity of electrocatalysts (Fig. 2 ). It is important to focus on the thermodynamic and kinetic details of various CO2RR catalysts. Thermodynamically, reactants can be driven only if the free energy of the system is reduced after the reaction. For the electrocatalytic reaction, the energy of electrons can be tuned by the biased potential and thus launch the reaction. Generally, the thermodynamics of the electrocatalytic reactions are relatively simple to obtain with the computational hydrogen electrode (CHE) model (Norskov et al., 2004). Dynamically, reaction rates depend exponentially on the reaction energy barrier, i.e., r ∼ exp (-Ea/kT). To calculate the energy barrier, the transition state calculation is usually performed by DFT NEB (Kildgaard et al., 2020) or the Brønsted-Evans-Polanyi (BEP) relation ( E a = α Δ G + β ) (Darby et al., 2018; Liu et al., 2019a). Although the numerical relationship between the CO2RR thermodynamics and the kinetics has not been established, we still know that the reactive thermodynamics determines the energy barrier. As in the Sabatier principle, a volcano relationship could be obtained when correlating the electrocatalyst activity with the adsorption energies of key intermediates, and the optical performance comes with a moderate adsorption strength that is neither too strong nor too weak. (Valdes et al., 2012) The most widely used thermodynamic model, the CHE model, the improved models involving kinetics such as the implicit/explicit solvation model, the H-shuttling model, the water-solvated model, and the possibility of thermokinetic models are reviewed and discussed in Section 3.In terms of the current development of CO2RR, catalyst design and theoretical investigation are equally important and mutually promoted. Obtaining a highly effective catalyst is important, and the structure-dependent mechanism and the reactive thermodynamics and kinetics behind the performance are even more important. We will review the preparation, performance, and reactive mechanism as well as the theoretical insights of four groups of efficient CO2RR catalysts in recent years. Aside from the traditional theoretical techniques either thermodynamically or kinetically, a new concept of thermodynamic-kinetic synergy is also highlighted, which could probably facilitate the design and selection of CO2RR electrocatalysts. Finally, the issues of current catalyst and theory as well as the outlooks for future work are offered.Metal-based catalysts are one of the most popular electrocatalytic materials for the CO2RR, which not only provide abundant binding sites for reaction intermediates but also produce different valuable chemical products, such as carbon monoxide, methane, formic acid, methanol, and ethylene. In addition, it is interesting that unstable metal-based compounds can also be used as catalysts, which would be in situ reduced to the metallic state during the CO2RR, leading to the reconstruction of surface structures with higher CO2RR activity (He et al., 2021). The early work of pure metal catalysts was performed by Hori in the 1980s, where various metal electrodes were tested for the CO2 electroreduction reaction (Hori et al., 1994; Ju et al., 2019). This work opened the door for research on CO2RR. Based on multiple reaction routes and products, as shown in Fig. 3 , pure metals can be divided into three main groups (Hori et al., 1985; Wu et al., 2020a): (1) transition metals generating CO as the main product, such as Ag, Au and Pd, etc. (2) metals generating HCOOH as the predominant product, such as Sn, Pd and Bi, etc. (3) Cu is the only metal that is capable of producing a considerable yield of hydrocarbons and multi-carbon products (Weng et al., 2018). The representative pure metals and metal alloys developed recently as CO2RR electrocatalysts are reviewed as follows, focusing on their preparation, performance and reaction mechanism. Considering the existence of a mass of reviews focused on metal-based catalysts in recent years (Hoang et al., 2020; Li et al., 2021b; Wu et al., 2020b), Nanostructured Cu catalysts. Cu, as a kind of metal catalyst, often exhibits limited selectivity and activity toward a specific product, leading to low productivity and substantial postreaction purification. In recent year, nanostructured Cu have attracted much attention for the CO2RR (Jeon et al., 2017; Kim et al., 2017a; Zhang et al., 2022b). For example, Cu nanoparticles, nanocubes and nanoclusters have shown the Faradaic efficiency (FE) of the C2 product (mainly ethanol and acetic acid) to be 60%, while plasma-activated Cu has shown 60% FE toward ethylene (Xu et al., 2020; Yin et al., 2019; Zhu et al., 2019b). Nanostructured Cu can be easily prepared via appropriate synthetic processes, allowing for a deep understanding of catalyst performance through precise control of the active sites. For example, Cu nanoparticles. Dong and his coworkers developed a strategy to improve the selectivity of CH4 by increasing the adsorption energy difference between CO* and CHO* intermediates (Dong et al., 2018). They prepared Cu79 NPs and compared it with Cu (211) and Cu (111) for the reduction of CO2 to CH4. According to the first principles calculations, Cu79 NPs exhibited a higher negative onset potential for the formation of CH4 than Cu (211), which induced better selectivity toward CH4 for Cu79 NPs. Dongare et al. synthesized a highly stable metallic copper nanoparticles (Cu NPs) (Dongare et al., 2021). The total faradic efficiency for the liquid products reached to 58% at -0.8 V vs. RHE using prepared Cu NPs as electrocatalysts and the maximum FE of formic acid is over 45% (Fig. 4a ). Nanocrystals Cu may have a better catalytic performance than pure bulk Cu. Pranit Iyengar at el. synthesized octahedral Cu nanocrystals (Oh-NCs) in the range of 75-310 nm as a promising platform to study the electrochemical performance of CO2 to CH4 conversion (Iyengar et al., 2019). The best performance was achieved by the 75 nm Oh-NCs with 77% FE towards the CO2RR and 55% FE for CH4 at -1.25 V vs. RHE (Fig. 4b). Compared with the bulk Cu electrodes, Cu nanowires (NWs) possess advantages, including larger surface-to-mass ratio, more low-coordinated adsorption sites, and capability of resisting much higher elastic strains. Vijayakumar et al. synthesized an aligned copper nanowires catalysts with tunable selectivity for producing CO or formate in aqueous media (Vijayakumar et al., 2021). All these Cu nanowires were demonstrated with excellent catalytic activity regarding the total faradic efficiency of carbonaceous products (FEC, sum of FECO and FEHCOO-) over 70% from -0.5 V to -0.9 V vs. RHE (Fig. 4c), peaked at -0.8 V vs. RHE and Cu-8 showed the highest EFC of 88%. In addition, for nanoporous Cu, Yang et al. constructed a vanadium oxide integrated on hierarchically nanoporous Cu electrocatalysts (Yang et al., 2021a). As a CO2RR catalyst, the nanoporous copper integrated with vanadium oxide reached a 30.1% faradic efficiency for CO2 to ethanol production and an ethanol partial current density of -16 mA/cm2 at -0.62 V vs. RHE (Fig. 4d). Other Cu catalysts. External factors also greatly impact Cu catalysts (Chen et al., 2022c). First, functional groups can be used to tune the selectivity and activity of Cu for the CO2RR. Chang and his coworkers investigated the role of functional groups (-COOH and -CF2) on the CO2RR of Cu catalysts (Chang et al., 2022). As seen from the reaction pathways in Fig. 5a , the formation of *HCOOH was the rate-determining step (RDS), the formation energy of which on COOH-Cu (111) was more energetically favorable than that on Cu (111) and CF-Cu (111). This work revealed that functional groups influenced the binding energies of key intermediates involved in both the CO2RR and the competing hydrogen evolution reaction. Second, the electrochemical pulse also influences the selectivity. Tang used pulsed overpotential to improve the selectivity of ethylene on Cu (100) foil (Fig. 5b) (Tang et al., 2021b). C2H4 was the major product with 50% ∼ 67% selectivity, while the selectivity for CH4, H2, and CO was less than 20%, 15% and 10%, respectively. The enhancement of C2 (ethanol and ethylene) selectivity was attributed to the improved CO dimerization kinetics on the Cu (100) surface resulting from the reduced hydrogen adsorption coverage during the pulsed process. Third, engineering the surface strain is a powerful method to improve the electrocatalytic performance. For example, tensile and compressive strains arising from the two-way shape memory effect of a NiTi substrate were used to control the activity and selectivity of CO2RR on Cu nanofilms (Du et al., 2021a). It was found that tensile strain could improve the CO2RR activity of the 32 nm-thick Cu nanofilm and favored CH4 generation from 42.76% to 50.64% (Fig. 5c). The strain effects on the C2 products were relatively weak, with a little FE increase. Compared with the pristine Cu, tensile strain contributed to a total CO2RR faradaic efficiency from 65.02% to 76.48% at -1.2 V, while compressive strain had an opposite effect. According to DFT calculations and the derived positive correlation between the thermodynamic free energy change and the kinetic energy barrier, the mechanism of strain-controlled CO2RR performance was revealed. An upshifting of the d-band center of Cu and a larger adsorption strength of key intermediates induced by tensile strain were proved. As a result, the reduced free energy change in the potential-limiting step and the corresponding positive-correlated smaller energy barrier contributed to a higher CO2RR performance for the tensile-strained Cu nanofilm. Cu-Ag alloy. Compared with pure Cu, Cu-based alloys have advantages including low coordination (Andrews et al., 2018; Qin et al., 2022), reduced overpotential (Lu et al., 2018), and long-term durability (Barasa et al., 2019). Due to the excellent CO formation ability of Ag, coupling Ag with Cu was considered to be an efficient way to improve the selectivity toward C2 products (Zhang et al., 2021a). Dutta et al. synthesized bimetallic Cu-Ag metal foams by means of an additive-assisted electrodeposition process using the dynamic hydrogen bubble template approach (Dutta et al., 2020). They denoted this Cu-Ag alloy as oxide-derived Ag15Cu85, which showed high selectivity toward alcohol formation. The FE of C2H5OH was 33.7% at -1.0 V vs. RHE, and the FE of n-propanol was 6.9% at -0.9 V vs. RHE (Fig. 6a ). The reason for the high selectivity of alcohol was that the oxide-derived bimetallic catalyst exhibited the ability to effectively suppress the C1 hydrocarbon reaction. In the above-mentioned report, the ratio of Cu to Ag was fixed at 15 to 85. The ratio was changed by Xu et al., who used E-beam evaporation to synthesize a series of Cu-Ag films with uniform distribution and controllable stoichiometry (Xu et al., 2022). They confirmed that when the Ag dopant was 20%, the Cu1-xAgx (x=0.05-0.2) alloy showed an apparent suppression of HCOOH, and the Faradic efficiency of the C2 product (mainly C2H5OH) increased (Fig. 6b). In addition, the change of core-shell structures also affects the C2 product selectivity. Zhang, et al., synthesized a series of Cu@Ag core-shell nanoparticles by tunning different silver layer thicknesses to improve the selectivity of C2 products (Zhang et al., 2021a). Notably, Cu@Ag-2 (core-shell with a thickness of 11.2 nm) exhibited excellent selectivity and activity for C2 products (Fig. 6c-d). Specifically, the total FE of C2 products for Cu@Ag-2 reached a maximum of 67.7% at -1.1 V vs. RHE, and the C2 partial current density for Cu@Ag-2 presented the highest value of -22.7 mA/cm2 at -1.1 V vs. RHE. Au-based catalysts. Au is one of the most widely studied noble metal catalysts for electrochemical conversion of CO2 to CO with a high selectivity at a low overpotential. In order to improve the electrocatalytic performance, Au-based materials with specific structure are developed. As it has been proved that Au-based nano-catalysts showed nanostructure dependent performance for CO2RR, indicating that we can enhance the electrocatalytic properties by controlled the synthesis of structure. In earlier research, a variety of Au-based nanostructure, such as nanoparticles (Andrews et al., 2015; Feng et al., 2015), nanoclusters (Kauffman et al., 2012), nanowires (Cho et al., 2019), nanoporous structure (Kwok et al., 2019) and so on, has been proposed to improve catalytic performance. There is no end to the quest to improve the catalytic performance of Au-based catalysts.Doping is a facile way to improve the property for Au-based catalysts. Sun et al. synthesized an Mo-doped Au nanoparticles (MDA NPs) (Sun et al., 2020a). MDA NPs was reported that effective CO2 reduction by the synergies between electronic and geomatic effects. A 97.5% CO faradic efficiency and 75-fold higher current density than pure Au nanoparticles were achieved at -0.4 V vs. RHE for MDA NPs with durability of over 50 h (Fig. 7a ). DFT calculation results revealed that the increased electron density of Mo on Au surface could effectively enhance CO2 activation and *COOH may be further stabilized by the local Mo atom through additional Mo-O binding to decrease the energy barrier. Additionally, the electrocatalytic performance of Au-based may improve because of the formation of some special active interface. Chen et al. successfully synthesized heterogeneous Ag2S-Au nanoparticles (NPs) as effective catalysts for CO production (Chen et al., 2022a). The Ag2S-Au showed a high selectivity for CO2 to CO (FE=94.5%) and an appreciable CO particle current density of 9.17 mA/cm2. Moreover, the Ag2S-Au exhibited good stability of over 30 h. DFT result revealed the formation of the heterogeneous Ag2S-Au interface was beneficial to the generation of COOH* intermediate, and the charge density proved a good number of electrons was concentrated on the Ag2S-Au interface (Fig. 7b), indicating the interface was the active site for CO2RR. Besides, designing bimetallic electrocatalysts is also an attractive strategy to enhance the catalytic performance of Au-based material. An AuNi bimetallic catalyst was prepared, which supported on electrospum carbon nanofibers (NCFs) (Hao et al., 2021a). The AuNi bimetallic catalyst exhibited high CO selectivity with CO faradic efficiency of 92% at -0.92 V vs. RHE and good durability of over 16 h. DFT results indicated that the incorporation of Ni into Au made the d-band center more positive (Fig. 7c) and reduced the free energy barrier of *COOH. In addition, whether the presence of the support may affect the catalytic property of Au-based material? Zhang et al. reported carbon nitride (C3N4) supported Au nanoparticles (Au/C3N4) exhibited a better electrocatalytic performance (Zhang et al., 2018b). Notably, compared with Au/C, Au/C3N4 exhibited a higher current density and FECO (Fig. 7d). Au/C3N4 reached FECO of over 90% at a wide potential window of -0.45 V to -0.85 V vs. RHE, demonstrating C3N4 could significantly enhance the CO2RR activity. The key of the excellent catalytic performance was Au-C3N4 interaction induces the formation of negatively charged Au surface, which could stabilize *COOH intermediate. Last but not least, strain is a new facile strategy to easily enhance the catalytic property for Au-based material. Zhang et al. synthesized gold nanoparticles (Au NPs) with rich compressive strain (Au-LAL) for electrocatalytic CO2 reduction (Zhang et al., 2022a). Au-LAL achieved a CO partial current density of 24.9 mA/cm2 and a maximum CO faradic efficiency of 97% at -0.9 V vs. RHE, which demonstrated that the rich compressive strain could greatly promote the CO2RR performance. The presence of the compressive strain could induce a unique electronic structure change in Au NPs, significantly up-shifting the d-band center of Au, and greatly enhance the adsorption strength of Au NPs toward the key *COOH intermediate (Fig. 7e). Ag-based catalysts. Compared with Au, Ag catalysts are more popular due to the lower price and high selectivity. Ag is a candidate catalyst for converting CO2 to CO, the faradaic efficiency of which still needs to increase at a relatively lower overpotential (Kuhl et al., 2014). Changing the morphology of Ag nanoparticles is a facile strategy. Liu et al. prepared triangular Ag nanoplates (Tri-Ag-NPs) (Liu et al., 2017). Compared with similar sized Ag nanoparticles (SS-Ag-NPs) and bulk Ag, Tri-Ag-NPs exhibited excellent CO2RR performance (Fig. 8a ). The maximum current density was over 5.5 mA/cm2, and the maximum FE of CO was 96.8% at a lower overpotential of 0.746 V. Liu, et al., further explored Ag nanocubes (NCs) with lengths below 25 nm and 70 nm (L25- and L70-Ag-NCs), respectively (Liu et al., 2020b). The L25-Ag-NCs exhibited a larger current density, a significant FECO and a better stability of 18 h compared to L70-Ag-NCs, Ag bulk and Ag nanoparticles (Fig. 8b). Dutta et al. synthesized Ag-foam catalysts based on a concerted additive- and template-assisted metal-deposited process (Dutta et al., 2018). Such Ag foams showed high activity, selectivity and stability toward CO production at both low and moderate overpotentials. The FE for CO never fell 90% from -0.3 V to -1.2 V vs. RHE, and the stability was more than 70 h. They proposed the possible mechanistic pathway of CO2 conversion on the Ag-foam catalyst (Fig. 8c); CO was the only and final CO2RR product. In addition to the morphology engineering of Ag, size control is also feasible. Liu et al. synthesized 5-fold twinned Ag nanowires (NWs) with diameters less than 25 nm (D-25) and 100 nm (D-100) (Fig. 8d) by a facile bromide-mediate polyol method (Liu et al., 2018). Compared with D-100 nm Ag NWs and Ag nanoparticles, D-25 Ag NWs had markedly enhanced current density, together with significant Faradic efficiency. The maximum values of the current density and FECO are 6.65 mA/cm2 and 99.3%, respectively. In addition, the low onset overpotential and the stability at 24 h further verified the superior performance of D-25 Ag NWs for the CO2RR.In view of the poor absorption capacity of CO2 on Ag, significant efforts have been dedicated to preparing Ag-based alloys to create new binding sites and improve the CO2RR activity. For example, an Au-Ag bimetallic alloy was synthesized by the thermal evaporation method (Li et al., 2021a). It showed the best performance with a CO FE of 89% at -0.7 V vs. RHE with good stability (Fig. 9a ). The well-dispersed Ag atoms at the grain boundaries were inferred to be the contribution of such good performance. Interestingly, it was found that composition changes of Ag-based alloys could break the inherent scaling relationship of the binding energies of various intermediates (Zeng et al., 2019). A series of Pd1-xAgx bimetallic alloys were prepared, and the optimal Pd0.75Ag0.25/C provided a higher CO FE of 95.3% at -0.6 V vs. RHE. This was attributed to the Pd0.75Ag0.25 alloy gaining obviously weakened *CO and *H bindings but retaining the binding with *COOH well, thus facilitating the kinetics toward CO generation (Fig. 9b). An Ag-Zn bimetallic alloy was designed by pulse deposition of Zn dendrites onto Ag foams (PD-Zn/Ag foam) (Low et al., 2019). The nanostructure PD-Zn/Ag foam reduced CO2 to methanol with an FE of 8.1% and a current density of -2.1 mA/cm2 at -1.38 V vs. RHE (Fig. 9c). An Ag-Co surface alloy electrocatalyst was obtained by the cold H2-plasma activation method (Fig. 9d) (Zhang et al., 2020d). It exhibited high activity for the CO2RR to ethanol with an FE of ethanol (72.3%) and a current density of 7.4 mA/cm2 at -0.80 V vs. RHE. The formation of the (Zhao et al.) Ag-Co surface alloy was believed to distort the Ag lattice and reduce the energy barrier for *CO2 δ− formation. Sn-based catalyst. Sn is abundant, nontoxic and quite suitable for large-scale applications but also has a low cost compared to other noble metals. More importantly, Sn exhibits high selectivity and catalytic activity for C1 (mainly formate) products (Fig. 10a ). Therefore, Sn has attracted much attention in the field of CO2RR. Zhong, et al., developed ultrasmall Sn nanoparticles inlaid on N/P-doped carbon (Warnan et al.) using Sn electroplating sludge (Zhong et al., 2022). Sn@NPC exhibited excellent selectivity and activity for HCOOH (Fig. 10b). The FE of HCOOH reached 87.93% together with a stable j HCOOH of -8.05 mA/cm2 at -1.05 V vs. RHE. Moreover, the Sn@NPC electrode achieved excellent long-term stability up to 105 h. In addition, Li, et al., reported a unique Sn-doped Bi2O3 nanosheet (NS) electrocatalyst by a simple solvothermal method for the highly efficient electrochemical reduction of CO2 to formate (Li et al., 2021c). By synthesizing different atomic percentages of Sn-doped Bi2O3 NS (1.2%, 2.5%, 3.8%), they found that the 2.5% Sn-doped Bi2O3 NS exhibited the highest FE of 93.4% with a current density of 24.3 mA/cm2 for formate at -0.97 V vs. RHE and achieved long-term stability over 8 h with formate FE maintained at 90%. DFT calculations revealed the strong synergistic effect between Sn and Bi contributing to the larger adsorption capacity of the OCHO* intermediate, thus improving the activity toward formate generation (Fig. 10c). Rahman Daiyan et al. obtained Sn electrode (referred to as An-Sn) by anodizing Sn foil in organic solvents (Daiyan et al., 2017). This as-prepared An-Sn electrode reduced CO2 to formate with a maximum FEHCOO − of 77.40% and a stable current density of 4.80 mA/cm2 at -1.09 V vs. RHE (Fig. 10d). Ivan Merino-Garcia et al. synthesized high surface area SnO2 nanoparticles (NPs) for continuous formate generation at high current density in a flowing electrolyzer (Merino-Garcia et al., 2021). The manufactured SnO2-based gas diffusion electrodes (SnO2-GDEs) exhibited a maximum formate generation concentration of 27 g/L with 44.9% FE at 300 mA/cm2 and could be sustained for up to 10 h (Fig. 10e).Sn-based alloys, in contrast to pure Sn, accurately control the surface electronic state as well as the binding energy of intermediates via the addition of foreign atoms (Shao et al., 2019). Thus, various Sn-based alloy systems have been reported (He et al., 2017; Ren et al., 2016). For example, a bimetallic Sn-Sb alloy film was obtained by electrodeposition on different substrates (Lucas and Lima, 2020). Compared with the pure Sn film, the Sn-Sb alloy film exhibited excellent electrocatalytic performance, and the faradic efficiency reached 96.2% at -1.25 V vs. RHE (Fig. 11a ). This was ascribed to the balance between Sn atoms and the induced morphologic effect brought by Sb, generating cube-shape crystallites. These crystallites have a large number of undercoordinated surface atoms and grain boundaries to generate more reactive Sn atoms as CO2RR active sites, increasing the overall activity and FE for formate production. In addition, Sn-Ni alloy was also proven to be an efficient catalyst. Xie et al. reported an efficient NiSn atomic pair electrocatalyst (NiSn-APC) on a hierarchical integrated carbon nanosheet array electrode, which boosted the activity and selectivity of formate (Xie et al., 2021b). As seen from Fig. 11b-c, the maximum current density was -43.7 mA/cm2, and the maximum FE of formate was 86.1%. The electron redistribution of Sn imposed by adjacent Ni was ascribed to the activity improvement, which reduced the energy barrier of the *OCHO intermediate and made the potential-limiting step thermodynamically spontaneous. Moreover, a bimetallic Sn-Bi aerogel with a 3D porous structure was reported (Wu et al., 2021b). Compared with Sn, Bi and bulk Sn-Bi, Sn-Bi, the Sn-Bi aerogel exhibited better catalytic activity and higher selectivity for formate (Fig. 11d-e). The Sn-Bi aerogel exposed more active sites and had favorable mass transfer properties, endowing it with a high FEHCOOH of 93.9%. Sn-Bi also possessed good stability for up to 10 h when 90% FEHCOOH was maintained. DFT results revealed that the coexistence of Sn and Bi degraded the energy for the production of HCOOH, thereby improving the catalytic activity (Fig. 11f). Bi-based catalyst. Due to it possesses some advantages, such as eco-friendly, cost-effective, inhibit H2 production and highly active for CO2 reduction (Guan et al., 2021; Jiang et al., 2021a), Bi-based is a promising catalyst for CO2RR. For example, it is reported that Bi-based catalyst can selectively reduce CO2 to formic acid in aqueous solution (Duan et al., 2020). As it has proved that the CO2RR is more attractive in aqueous solution, most of the existing studies focused on improving the formate production efficiency on Bi-based catalysts. The scholar has found the selectivity and activity to formate production on Bi-based catalysts were greatly improved by tunning the catalyst structure. Bi-based catalysts can be flexibly synthesized into a variety of different structures, such as dendrites, nanosheets, nanoparticles, etc. by various techniques including Electrodeposition, in situ electrochemical transformation of Bi precursors, wet chemical reduction and so on.Electrodeposition is one of the various techniques, which is a facile approach to tune the nanostructure to improve the catalytic efficiency of Bi-based catalysts (Wang et al., 2019b). Guo et al. synthesized oxide-containing Bi (Bi-PMo) nanosheets by electrodeposition in the presence of phosphomolybdic acid. (Guo et al., 2019). These Bi nanosheets catalyzed CO2 to formate with a faradic efficiency of 93±2% at -0.86 V vs. RHE with a formate particle current density as high as 30 mA/cm2 and the stability over 10 h (Fig. 12a ). In addition, in situ electrochemical transformation of Bi-precursors like oxide, oxychloride and so on were widely used to prepare Bi-based catalysts. Wei et al. dispersed Bi dendrites on 3D carbon cloth and then used in situ chemically oxidized Bi dendrites to Bi nanoparticles (Bi NPs/CC) (Wei et al., 2022). The Bi NPs/CC exhibited a current density of 6.8 mA/cm2 at -0.87 V vs. RHE with a CO2-to-formate faradic efficiency of 97.4% and excellent durability of 72 h. DFT calculation revealed that the exposed specific facet was the key to stabilize the OCHO* intermediate contributed to high activity, selectivity and durability of Bi NPs/CC (Fig. 12b). Liu et al. obtained a 2D Bi nanosheets electrocatalyst via in situ transformations from optimized thickness and sizes of the bismuth oxychloride precursors (Liu et al., 2021a). The 2D Bi nanosheets (EG/H2O,1:1) showed high selectivity of formate at -0.9 V vs. RHE and high stability of 15 h with a current density of 10.5 mA/cm2 (Fig. 12c). Wet chemical reduction is also used to prepare nanostructure of Bi-based catalysts. Yang et al. synthesized stable free-standing hexagonal Bi-based nanosheets catalyst (Bi NSs) with different thickness via wet chemical reduction and demonstrated its high electrocatalytic performance for formate formation from CO2RR (Yang et al., 2020a). The prepared 0.65 nm Bi-based catalyst exhibited high CO2RR electrocatalytic activity (Fig. 12d), which offered a superhigh FECHOO − of 99% at -0.58 V vs. RHE and durability of over 75 h. The reason was the structures-sensitivity of the CO2RR over Bi-based nanosheets, leading the unique compressive strain to have a high selectivity of formate. Use the same preparation way, Xie et al. synthesized a single-crystalline Bi-based rhombic dodecahedrons (Bi RDs) exposed with (104) and (110) facets via wet chemical reduction (Xie et al., 2021a). The Bi RDs reached high selectivity for formate production of over 92.2% at a low overpotential and an excellent electrocatalytic active (partial current density range from 9.8 to 290.1 mA/cm2). The significantly reduced overpotential was caused by the enhanced adsorption of *OCHO on the Bi RDs. The key of the high selectivity of formate was ascribed to the topological surface states and the trivial surface states opening small gaps in the bulk gap on Bi RDs. Due to this change, the adsorption of *OCHO had been strengthened and stabilized, while the competing adsorption of *H had been mitigated (Fig. 12e).In this section, the representative metal-based CO2RR electrocatalysts including Cu and Cu-Ag alloys for high-vale hydrocarbons, Au-based and Ag-based catalysts for CO product, Sn-based and Bi-based catalysts for formic acid/formate products were overviewed.For Au-based electrocatalysts, we summarized the CO partial current density of different Au-based electrocatalysts via different strategies in Section 2.1.2. As seen from Fig. 13a , MDA NPs show the best activity among the all Au-based electrocatalysts. The activities of Au/C3N4 and Au-LAL are similar and second-best. It is obvious that the active of Au film is the worst. For Ag-based electrocatalysts, the shape and morphology effects are further discussed based on the comparison of CO partial current density (jCO) among the abovementioned Ag catalysts in Section 2.1.2. As seen from Fig. 13b, Ag foam exhibits the best activity with the largest jCO of -17.658 mA/cm2, which is obviously superior to other catalysts. However, the largest jCO values for Tri-Ag-NPs, L25-Ag-NCs and D25 Ag NWs were slightly different, indicating weak morphology effects on them. It is also interesting to find that jCO of bulk Ag, Tri-Ag-NPs, Ag foil and Ag foam all first increased and then decreased along with the biased potential negatively swept. For Sn and Sn-based alloy electrocatalysts, as indicated in Fig. 13c, the HCOO − partial current densities (jHCOO −) for all modified Sn-based electrocatalysts are larger than that for Sn foil. Among the modified Sn-based electrocatalysts, Sn-Bi2O3 exhibited the highest activity for formate generation.The comparison of Bi-based catalysts is listed in Table 1 . Several strategies have been proven to be effective for the improvement of formate selectivity, for example, wet chemical reduction, in situ electrochemical transformations, and so on. It is clear that the electrocatalytic performance of Bi-based catalyst for formate generation has been improved a lot. Most of the reports show a quite high FE (over 90%) toward formate, but the long-term stability of CO2RR is still insufficient (less than 100 h), indicating these catalysts to be far from the industrial application requirements.The comparison of Cu and Cu-based catalysts is listed in Table 2 , the product selectivity of which varied from sample to sample. Several strategies have been proven to be effective for the improvement of C1 or C2 product selectivity, e.g., alloying, facet tuning, morphology engineering, defect engineering, strain effects, etc. However, the active sites, reaction pathway and reactive thermodynamics and kinetics are still not clearly understood. To date, most of the mechanistic insights have been obtained by DFT calculations. Profound theoretical insights and operando characterizations are highly necessary to decode the dynamic CO2RR process and reveal the composition/structure-performance relationship of electrocatalysts. It is noteworthy that the long-term stability of CO2RR catalysts and cells has been far from the industrial application requirement. Most of the reports show a very limited test time, usually less than 100 h. For reference, industrial water electrolyzers have demonstrated stable performance over 80,000 h (Kibria et al., 2019).Single-atom catalysts (SACs) are defined as catalysts consisting of only uniformly separated isolated single active sites on the surface of the substrate (Qiao et al., 2011). Individual active sites of SACs consist of an isolated metal atom bonded to adjacent atoms in the host material (Sun et al., 2021). In principle, SACs can expose all metal atoms on the surface, thus achieving 100% atomic utilization, which is particularly attractive for reducing the cost of precious-metal-based catalytic materials. In addition, SACs also feature uniformly distributed uncoordinated active sites, providing a bridge to combine the advantages of heterogeneous catalysts and homogeneous catalysts for efficient chemical conversion and energy conversion (Chen et al., 2022b; Wang et al., 2018a). Moreover, compared with nanoparticles or clusters, SACs have a greater number of active sites per mass and prominent size effects (Chen et al., 2020c; Liu et al., 2022), giving rise to distinctive behaviors of reactants/intermediates and outstanding performance. To distinguish these catalysts from other single-atom catalysts, we denote single-atom catalysts used in the electrocatalysis field as single-atom electrocatalysts (SAECs), whose active sites are usually transition metals in cationic or metallic states. In the territory of electrocatalysis, the footprint of SAECs has gradually extended to the hydrogen evolution reaction (HER), oxygen evolution reaction (Wu et al.), CO2 reduction reaction (CO2RR) and N2 reduction (N2RR) from the initial focus on the oxygen reduction reaction (ORR). In this review, the latest and representative work of SAECs in CO2RR are summarized.M-N-C-type SAECs, as emerging metal-nitrogen-doped carbon materials wherein dispersive metal atoms are coordinated to nitrogen atoms doped in carbon nanomaterials, have presented a high expectation to be substitutes for metallic electrodes. Because only the single atom position can be used for intermediate adsorption, it can stabilize carbonaceous reaction intermediates (e.g., CO2 ·− ) and restrict the configurations of the adsorbate, giving rise to enhanced catalytic activity and selectivity (Yang et al., 2013). In addition, M-N-C-type SAECs have an appealing nitrogen-doped carbon (NC) substrate (Shi et al., 2020). The NC substrate possesses prominent advantages, including good mechanical properties, large specific surface area, excellent electronic conductivity, large specific surface area, excellent electronic conductivity, structural flexibility beyond the atomic scale, low cost, and ideal stability under acidic/alkaline conditions. There are three main reasons (Jin et al., 2019) why M-N-C-type SAECs can be widely used: 1) the carbon-based structure, such as graphene, shows superior ductility and electroconductivity; 2) N shows better coordination ability with metals; and 3) the exposed metal atoms, as the extra active sites, enhance their electrocatalytic performance. However, M-N-C SEACs also have some problems, including (Wang et al., 2020b): 1) the synthesis is complex, which makes industrial production difficult to realize; 2) the carbon substrate and the heteroatomic dopants are usually active for the HER, reducing the FE of CO2 reduction. How to adjust the atomic structure of the atomic metal center is the key for solving the above problems. Next, we will further discuss the M-N-C type of SAECs for the CO2RR based on the different metals involved as the active site to provide suggestions for the design of efficient M-N-C SAECs.Fe-based SAECs are embedding nonnoble metal Fe in a nitrogen-doped carbon support electrocatalyst, efficiently converting CO2 to CO in aqueous solution. As early as 1985, Fe metal electrodes were studied as CO2RR catalysts but were found to mainly produce hydrogen, with FEH2 being > 95%. Follow-up research by the same period confirmed the inactive nature of metallic Fe in CO2 reduction, so Fe as an electrocatalyst for the CO2RR was stagnated. In 2015, Strasser and coworkers demonstrated the structure of Feδ+−Nx centers being active for the CO2RR (Varela et al., 2015), and the research status of Fe-based electrocatalysts changed. They found that single atoms of Fe coordinated on N-doped carbon (Fe−N−C, Fig. 14a ) could achieve >80% FECO at -0.5 V vs. RHE, which is comparable to the CO2RR activity of Au (FECO is 87%), one of the most active metal catalysts for the electroreduction of CO2 to CO. Moreover, compared to Au, the onset potential of Fe-N-C was also found to be reduced by 100 mV. In Fe-N-C-type SAECs, the structure of Fe-N4 is proposed. Huan and his coworkers found that the structure of Fe-N4 was the key catalytic substance for CO2 to CO by studying a series of Fe-based catalyst pyrolyzes (Huan et al., 2017). Materials containing only Fe-N4 sites are able to selectively reduce CO2 to CO in an aqueous solution with a Faraday efficiency yield of over 90% and at low overpotential (Fig. 14b). Although the structure of Fe-N4 is the critical catalytic substance, we do not know who acts as the catalytic active site. To explore this question, Tour and his coworkers dispersed single Fe atoms on N-doped graphene to prepare Fe/NG (Zhang et al., 2018a). The oxidation status of Fe/NG was analyzed by XPS (Fig. 14c), and they identified a +2-oxidation state for Fe within Fe-N4, which was believed to be the active site for the CO2RR. The +2 oxidation state for Fe can be the active sites for the CO2RR in Fe-N-C, and the +3 oxidation state for Fe can also be the active sites for the CO2RR in Fe-N-C. Gu and his coworkers reported a catalyst of dispersed single-atom Fe sites that produces CO at an overpotential as low as 80 mV (Gu et al., 2019). X-ray absorption spectroscopy revealed the active sites to be discrete Fe3+ ions (Fig. 14d), and their electrochemical data suggested that the superior activity resulted from faster CO2 adsorption and weaker CO absorption on Fe3+ sites. The difference in oxidation state was suggested to be caused by different ligand environments, particularly with regard to the N atoms. The spectroscopic data indicated that Fe3+-N-C comprises pyrrolic N ligands, whereas Fe2+-N-C comprises pyridinic N ligands. The above study discusses the CO2RR activity of Fe-N4, and some scholars proposed a new Feδ+-Nx group, namely, Fe-N5. They think the catalytic activity of Fe-N5 surpasses that of Fe-N4. Zhang and his coworkers reported a novel synthesis approach involving thermal pyrolysis of hemin and melamine molecules on graphene to prepare Fe-N5-C SAECs (Zhang et al., 2019a). These SAECs exhibited a high Faradaic efficiency (∼97.0%) and CO production at a low overpotential of 0.35 V, outperforming all the Fe-N-C-based catalysts. The DFT calculations revealed that the axial pyrrolic-nitrogen ligand of the FeN5 site further depleted the electron density of the Fe 3d orbitals and thus reduced the Fe-CO π back-donation (Fig. 14e), which was responsible for the rapid desorption of CO with a high selectivity toward CO production.Co-based SAECs have also attracted much attention for their CO2RR activity in the past decade. In early studies, it was proposed that cobalt phthalocyanine and cobalt porphyrin are active for the CO2RR. For instance, cobalt porphyrin was used as a building unit to prepare a heterogeneous catalyst for the aqueous electrochemical reduction of CO2 to CO. These atomic Co-N4 sites exhibit a high FE (90%) at an overpotential of 0.55 V Lin et al., 2015). Similarly, carbon nanotube-supported cobalt phthalocyanine with four Co-N bonds has been reported to fulfill CO2-to-CO conversion with 90% CO selectivity in bicarbonate electrolyte (Zheng et al., 2018b). Zhang and his coworkers used metal phthalocyanines (MePcs) as a model to synthesize FePc, NiPc and CoPc catalysts (Zhang et al., 2018e). All these catalysts had a clear metal-N4 coordination structure similar to the structure of the M-N-C SAECs active site. They established the linear relations of the reaction energies of COOH* formation and CO* desorption as functions of the *CO adsorption energy, as shown in Fig. 15a . This figure reveals an inverted volcano curve in the activity trend of MePcs for CO2 reduction to CO, and CoPc is located at the position closest to the volcano peak. This means that the activity of CoPc is best. CoPc and Co-N4 moieties have similar coordination structures, but their catalytic activities are different, which provides more opportunities for the study of Co SAECs in the electrocatalysis of CO2 reduction reactions. In addition, the coordination number also affects the electronic structures of the metal centers in SAECs. To strengthen the molecular understanding of the reaction intermediates and the reactive sites, a series of atomically dispersed Co catalysts with different N coordination numbers were prepared, and their catalytic performance toward CO2 reduction was studied. Based on the fact that the coordination number of single Co atoms was controlled by varying the pyrolysis temperature, Wang et al. prepared different coordination numbers Co-N-C SAECs by pyrolyzing bimetallic Co/Zn zeolitic imidazolate frameworks (ZIFs) (Wang et al., 2018c). These electrocatalysts were Co-N4, Co-N3 and Co-N2, respectively. To compare their electrocatalytic activity, they also prepared Co NPs. As a result, they found that the catalytic activity for CO production followed the trend Co-N2> Co-N3> Co-N4. The central current density of Co-N2 moieties reached 18.1 mA·cm−2, and FECO achieved 94%. They investigated the relationship between the coordination number of Co centers and the activity of the CO2RR. It was generally regarded that the first electron transfer largely determined the overall reduction process rate, during which the adsorbed CO2 would be reduced into a CO2 •− intermediate (Lei et al., 2016). Electrochemical impedance spectroscopy (EIS) revealed that Co-N2 exhibited the lowest charge transfer resistance from the catalyst surface to the reactant. This meant that Co-N2 was faster to transfer to the CO2 •− intermediate. For further understanding, they employed OH− to evaluate the binding affinity of CO2 •− through oxidative LSV scans in N2-saturated 0.5 M NaOH electrolyte (Fig. 15b). As a result, the stronger adsorption of CO2 •− on Co-N2 relative to the Co-N4 surface benefits the overall CO2 reduction. Moreover, compared with Co-N4, DFT showed a lower energy required to form CO2 •−* on Co-N2, thus explaining the higher CO2 electroreduction catalytic activity. This result demonstrated that a lower coordination number facilitates the activation of CO2 into the CO2 •− intermediate and hence enhances CO2 electroreduction activity. In addition to Co-N2 moieties, Co-N3 moieties, Co-N4 moieties and Co-N5 moieties are also active sites. Pan and his coworkers prepared a type of SAECs with an atomically dispersed Co-N5 site anchored on polymer-derived hollow N-doped porous carbon spheres (Pan et al., 2018a). For CO2 to CO conversion, Co-N5 was an excellent active center, exhibiting high selectivity for CO with a Faradaic efficiency above 90% over a wide potential range from -0.57 V to -0.88 V. The catalyst exhibited a higher FECO of -0.73 V and -0.79 V, 99.2% and 99.4%, respectively, which was equivalent to a 15.5-fold increase in cobalt phthalocyanine activity. The CO2RR pathway was investigated via a computational hydrogen electrode model (Fig. 15c, left), which revealed that the free energy difference for Co-based SAECs from CO2 to COOH* was close to zero (0.02 eV), lower than other catalysts (Fig. 15c, right), indicating a higher CO2RR activity.The single atoms of Ni coordinated on the N-doped carbon substrate material to form Ni-N-C-type SAECs. This SAECs shows high selectivity for CO, and its EFCO can achieve over 90%, making it a highly efficient and durable electric catalyst for CO2 reduction. To explore highly efficient Ni single-atom catalysts, He and coworkers designed Ni single-atom catalysts that consisted of isolated Ni single atoms anchored on nitrogen-doped winged carbon nanofibers (NiSA-NWC, Fig. 16a ) (He et al., 2020). This catalyst exhibited high intrinsic selectivity for CO2 to CO. The single-atom Ni catalyst possessed a maximum CO FE of over 95% in 0.1 M NaHCO3 solution, -1.6 V vs. Ag/AgCl. Similar to Fe-Nx moieties and Co-Nx moieties, Ni-Nx moieties are the most widely studied and are generally considered to be the active sites for the CO2RR. Mou and his coworkers obtained a Ni single-atom catalyst with 2.6 wt% Ni loading, denoted NiSA-NGA, through one-step pyrolysis of graphene oxide aerogel (Mou et al., 2019). This catalyst showed excellent electrochemical reduction of CO2 to CO. They found that the high selectivity of CO was caused by coordinatively unsaturated Ni-Nx sites (Fig. 16b). In addition, they simulated the Gibbs free energy of the reaction pathway of Ni-Nx sites and found the reaction mechanism of NiSA-NGA for CO formation. Among the various coordination structures of Ni-Nx, Ni-N4 moieties are widely considered active sites for the CO2RR. Li and his coworkers used a topochemical transformation strategy to synthesize Ni-N4-type SAECs (Li et al., 2017c). This strategy successfully ensured preservation of the Ni-N4 structure to the maximum extent and avoided the agglomeration of Ni atoms to particles, providing abundant active sites for the catalytic reaction. The Ni-N4 structure exhibited excellent activity for the electrochemical reduction of CO2 with particularly high selectivity for CO, achieving a high faradaic efficiency over 90% for CO in the potential range from -0.5 V to -0.9 V and giving a maximum faradaic efficiency of 99% at -0.81 V with a current density of 28.6 mA/cm2. To explain the high selectivity of Ni-N4 moieties, they performed density functional theory calculations. Comparing Ni-N4-C with N-C, the reduction free energy is indicated in the left subfigure of Fig. 16c. The formation of the adsorbed intermediate COOH* was the potential limiting step for both Ni-N4-C and N-C. From a thermodynamic point of view, the reaction free energy can be linked to the reaction energy barrier, so the trend of free energy can be associated with the activity of CO2 reduction. It can be clearly seen from the left subfigure of Fig. 16c that the introduction of Ni-N4 sites lowered the formation energy of COOH* compared with that for N-C, facilitating the subsequent formation of CO and thus showing higher activity. As seen in the right subfigure of Fig. 16c, Ni-N4-C showed a significantly more positive value for UL(CO2)-UL(H2) (the difference between thermodynamic limiting potentials for CO2 reduction and H2 evolution denoted as UL(CO2)-UL(H2), which can reflect the selectivity in CO2 reduction) than that of N-C. This meant that Ni-N4-C possessed high selectivity for CO2 reduction to CO. Ni-N4-C showed highly efficient activity for the CO2RR, as did Ni-N3-C. Zhang and his coworkers used a postsynthetic metal substitution (PSMS) strategy to prepare single-atom Ni catalysts with different N coordination numbers (denoted Ni-Nx-C, Fig. 16d) on predesigned N-doped carbon derived from metal-organic frameworks (Zhang et al., 2021b). At -0.65 V, the current density (JCO) was 6.64 mA/cm2, and the obtained Ni-N3-C catalyst achieved a CO Faradaic efficiency up to 95.6%, much higher than those of Ni-N4-C and N-C. To explain this, DFT calculations were performed. The lower Ni coordination number in Ni-N3-C greatly reduced the formation energy of the rate determining step, thereby promoting the CO2 reduction process. In addition, a framework other than MOFs was used to build Ni-based SAECs. Su et al. prepared covalent triazine frameworks (CTFs) modified with coordinatively unsaturated 3D Ni metal atoms (Su et al., 2018). Such Ni-CTF catalysts effectively reduced CO2 to CO at -0.5 V vs. RHE, and the Faradaic efficiency reached 90% at -0.8 V vs. RHE (Fig. 16e).Due to its unique capability of catalyzing C-C coupling, Cu is unique as the only metal known for the production of hydrocarbons from the electroreduction of CO2 and is also the most studied metal for CO2 reduction. It can be used to produce value-added hydrocarbons, such as ethylene, acetate, ethanol, etc. Many studies have reported different mechanisms, but it is generally accepted that the C-C coupling pathway requires a high coverage of CO* intermediates on continuous Cu surfaces, which suggests the unlikely formation of C2+ hydrocarbons on single-atom Cu sites (Jiao et al., 2019; Liu et al., 2019b). Therefore, research on Cu SAECs has enhanced the selectivity of C1 hydrocarbons, such as methanol and methane. In addition to C1 hydrocarbons, the effect of Cu-Nx on CO2 reduction to CO was also studied. Zheng and his coworkers prepared a highly efficient CO2 electrocatalyst (Cu-N2/GN) composed of unsaturated single-atom copper coordinated with nitrogen sites anchored into a graphene matrix (Zheng et al., 2019). Benefitting from the unsaturated coordination environment and the atomic dispersion, Cu-N2/GN exhibited a high CO2RR activity and selectivity for CO production with an onset potential of -0.33 V and a maximum Faradaic efficiency of 81% at a low potential of -0.50 V (Fig. 17a ). For comparison, they also prepared Cu-N4/GN. Compared with the activity of Cu-N4 sites, the current density and Faradaic efficiency of Cu-N2 sites were better. DFT calculations revealed that the catalytic activity of Cu-N2 surpassed that of Cu-N4, and Cu-N2 was the true active site. The effect of Cu-N3 active sites can not be ignored. Chen and his coworkers prepared a Cu SAECs with Cu-N3 coordination, which achieved a high CO faradic efficiency of 98% at -0.67 V (vs RHE) as well as an excellent stability over 20 h of successive electrolysis (Fig. 17b) (Chen et al., 2021).For comparison, they prepared Cu-N4 active sites. DFT showed the catalytic activity of Cu-N3 was better than Cu-N4. However, not all the activity of Cu-N4 sites is weak. Yang and his coworkers prepared a Cu single atom (Cu SAs/NC) with exceptional CO production performances (Yang et al., 2020b). The as-prepared Cu SAs/NC catalyst delivered a high CO faradaic efficiency of 92% at -0.7 V vs RHE as well as the excellent durability over 30 h of successive electrolysis. DFT calculations revealed that Cu-N4 was the true activity site, which gave rise to the high selectivity and the high production of CO (Fig. 17c). Cheng and his coworkers prepared a Cu-N4-C SAECs through facile one step thermal activation (Cheng et al., 2021). The as-prepared Cu-N4-C catalyst exhibited high CO faradic efficiency with a maximum value of 98% at -0.9 V vs RHE (Fig. 17d) and an excellent durability over 40 h. DFT results revealed that Cu-N4 active sites substantially lowered the energy barrier for the information of COOH*, thus enhancing catalytic performance. In addition, Cu+ can act as active sites to affect the reduction of CO2 to CO. Zhang and his coworkers designed a strategy of single-atom Sn anchored on Cu2O nanosheets to stabilize the key Cu+ species for electroreduction of CO2(Zhang et al., 2020e). Infrared spectroscopy suggested that the survival of Cu+ species on the catalyst surface promotes the adsorption of CO* during the CO2RR, leading to the obvious improvement of CO2-to-CO conversion. As a result, the catalyst possesses 30% remarkably stable Cu+ species during the reaction and was able to selectively convert CO2 to CO with a CO Faradaic efficiency of 87.9% at -1.3 V (Fig. 17e). In CO2RR research, Cu SAECs could also turn CO2 into other hydrocarbons. Guan and his coworkers reported that the CO2RR products depend on the distance between neighboring Cu-Nx moieties (Guan et al., 2020). The Cu doping concentrations and Cu-Nx configurations were well-tuned by the pyrolysis temperature. All the prepared Cu-Nx configurations had efficient catalytic activity to reduce CO2 to hydrocarbons (methane and ethylene). At a high Cu concentration of 4.9%mol, the distance between neighboring Cu-Nx species was close enough to enable C-C coupling and produce C2H4. In contrast, at Cu concentrations lower than 2.4%mol, the distance between Cu-Nx species was large so that the electrocatalysis favored the formation of CH4 as C1 products. DFT calculations confirmed the capability of producing C2H4 by two CO intermediates binding to two adjacent Cu-N2 sites, while the isolated Cu-N4, the neighboring Cu-N4, and the isolated Cu-N2 sites led to the formation of CH4 (Fig. 17f). It is also possible to reduce CO2 to methane without other byproducts. Cai and his coworkers reported a carbon dot-based SAC with unique CuN2O2 sites for the first time (Cai et al., 2021b). The catalyst exhibited extraordinary selectivity (99% ECR) for the electrochemical reduction of CO2 to CH4 over a wide potential range from -1.14 V to -1.64 V vs. RHE. In addition, for methanol generation, Yang and his coworkers synthesized isolated Cu-decorated through-hole carbon nanofibers (CuSAs/TCNFs,) on a large scale by a facile strategy (Yang et al., 2019). These CuSAs/TCNFs could generate nearly pure methanol with 44% Faradaic efficiency in the liquid phase at a current density of 93 mA/cm2 (Fig. 17g). DFT calculations indicated that Cu single atoms possessed a relatively higher binding energy for the CO* intermediate. Therefore, CO* could be further reduced to products such as methanol instead of being easily released from the catalyst surface as a CO product. Moreover, synergistic effects between the carbon substrate and the Cu single atoms were claimed to be the key for the reduction of CO2 to methanol. The same CO2RR products, but different preparation. Yang and his coworkers synthesized C3N4 supported Cu-N4 SAC via the thermal pyrolysis of melamine for low temperature CO2 hydrogenation (Yang et al., 2021b). The Cu-N4 catalyst favored CO2 hydrogenation to from CH3OH, the productivity and selectivity of CH3OH reached 4.2 mmol/(g•h) and 95% (Fig. 17h), respectively. It is demonstrated that the first H easily preferred to bond with C atom of CO2 to from *HCOO instead of forming the *HOCO intermediate, and thus generated CH3OH.We discussed the M-Nx-C type of SAECs for the CO2RR based on the different metals involved as the active site (Table 3 ). As seen from Table 3, when at the same overpotential, these SAECs exhibit excellent catalytic activity and selectivity, and their FECO could be over 80%. The rate-determining step of these SAECs is the 1st step as CO2 +*+H++e−→COOH*, and the reaction energy of this uphill step varies from 0.25 eV to 1.78 eV. Some problems indeed exist for the development of SAECs. For instance, the stability. In all M-Nx-C-type SAECs, the stabilities were weak, the majority of which could not be sustained for 50 h. In addition, a comprehensive understanding of the thermodynamic adsorption energies, surface electronic configuration, reactive-interface structure, and reaction kinetics of M-Nx-C-type SAECs in real electrolytes during the reaction is urgently needed. In the Sabatier principle, a volcano relationship is reached when correlating the electrocatalyst performance with the adsorption energies of intermediates. First-principles quantum chemical calculations, a typical DFT formalism, enabled calculation of the adsorption energies of the presumed intermediates. However, the DFT method can only provide a qualitative result to account for the experimental trend. An improved model or a new theoretical method with higher precision is highly desired, which will be discussed in detail in the next section.Porphyrin is a highly heterocyclic macromolecule with π-electron conjugation. As a planar macrocyclic molecule, porphyrin consists of four pyrroles connected in a ring fashion through four methine carbons at their α-positions (Jiang and Sun, 2019). Due to their chemical properties, porphyrin can exist in the form of derivatives of the basic molecules, the peripheral eight pyrrole β-carbon atoms and the four meso-carbon centers of which can be replaced. This leads to its extensive natural existence in plants and animals and plays an important role in life activities. For example, chlorophyll and heme (or hemoglobin) are important participants in photosynthesis and oxygen transport. Owing to its special structures, porphyrin has many advantages, such as strong visible absorption, excellent emission intensity, convenient structural functionalization, good redox performance and relatively high stability. Due to these features, different kinds of porphyrin-based complexes have been synthesized as optical catalysis and electrochemical catalysts.Metalloporphyrin, as one of the various porphyrin-based complexes, is widely used to activate and reduce small molecules, particularly O2 and CO2 (Zhang and Warren, 2021). The porphyrin ring has a vacant site at its center, which is suited for a metal atom to cooperate with it to form a metalloporphyrin complex. Because of the structure of metalloporphyrin, the common coordination number of the central metal ion of metalloporphyrin is four, and the number of coordinates can change through ligation of suitable moieties, either neutral or anionic. The advantages of metalloporphyrin as a CO2RR electrocatalyst are as follows (Liang et al., 2021). First, porphyrin can provide a stable and rigid coordination environment for the incorporated metal ion. This ensures that the obtained metalloporphyrin is stable in acidic and alkaline solutions. Second, porphyrins are redox-active, which can enhance the redox chemistry of the obtained metalloporphyrin, benefiting multielectron catalytic processes. Third, the meso- and β-positions of porphyrin can be modified by different functional groups, endowing metalloporphyrin with different properties. Fourth, catalytic efficiency can be improved by tuning the second coordinate spheres of metalloporphyrin in a stable coordinate environment (Nam et al., 2020).Iron porphyrin and cobalt porphyrin are the most extensively studied metalloporphyrins. Two types of iron-porphyrins (Davethu and de Visser, 2019)were used to explore why iron metalloporphyrin is active for the CO2RR: tetraphenyl porphyrin (TPP) and meso-(ortho-2-amide-phenyl) (triphenyl)porphyrin ligands. Their calculations showed that CO2, as an [ F e III ( C O 2 2 − ) ( T P P − • ) ] 2 − complex, existed stably in the electron transfer processes during the CO2 reduction cycle (Fig. 18a ). Step III in the CO2 reduction cycle should be considered as a proton-coupled electron transfer, while the second proton transfer does not change the electronic configuration of the metal complex. In addition, electron transfer mechanisms and second-coordination sphere effects on the reaction mechanism have also been studied. Iron porphyrins are efficient CO2 reducing systems that can reduce CO2 into CO efficiently. Modifying the constituents or the structure, the electrocatalytic performance of iron porphyrins will be further improved. For example, an iron hangman porphyrin interacting with phenol, guanidinium and sulfonic acid proton donor groups on the iron porphyrin platform, as shown in Fig. 18b (Margarit et al., 2018), could reduce CO2 to CO at above 93% Faraday efficiency. Fe-1, Fe-2, and Fe-3 porphyrins (Liu et al., 2020a) were synthesized by simple aminophenyl substitution. Compared with Fe-2 and Fe-3, Fe-1 porphyrin exhibited an improved turn over frequency and high CO selectivity (88% FE, Fig. 18c), confirming that amino groups in the secondary sphere of iron could enhance the catalytic activity of the CO2RR.Compared with iron porphyrin complexes, the activity and selectivity of cobalt porphyrin complexes for CO2RR can be easily adjusted by structure. For example, a bimetallic central electrocatalyst denoted CoCoPCP/CNT (Wang et al., 2021) exhibited high CO2RR activity and CO selectivity in aqueous solution benefiting from the regulation of the central site of the cobalt porphyrin conjugated polymer (PCP) (Fig. 19a ). The FECO of this catalyst reached 94% at an extremely low overpotential of 0.44 V(Valero-Romero et al.). Enlarging the π-conjugation of porphyrin by appending more aromatic subunits on the periphery of 5,10,15,20-(tetraphenyl) porphyrin (TPP), the 5,10,15,20-tetrakis(4-(pyren-1-yl) phenyl) porphyrin (TPyPP) molecule was generated, which coordinated with cobalt ions and noncovalently immobilized on carbon nanotubes (CNTs) as CO2RR electrocatalysts (denoted CoTPyPP/CNT) (Dou et al., 2021). CoTPyPP/CNT exhibited a large CO2 reduction current density and high Faraday efficiency (Fig. 19b), indicating that a larger π-conjugation on the porphyrin could lead to higher electrocatalytic CO2RR activity and selectivity. In addition, a grafted catalyst (denoted as CoPP@CNT) was synthesized by protoporphyrin IX cobalt chloride (CoPPCI) and hydroxyl-functionalized carbon nanotubes (Fig. 19c) (Zhu et al., 2019a). The TOF for CO formation of the catalyst was significantly improved. The Faradic efficiency toward CO generation reached 98.3%, and the current density was 25.1 mA/cm2 at an overpotential of 490 mV with excellent stability, indicating that covalent grafting is an effective way to improve CO2RR electrocatalysis.Metalloporphyrin, as a homogeneous molecular catalyst, exhibits high activity and selectivity for the CO2RR but is difficult to use in electrocatalytic devices. First, it suffers from poor electrical conductivity and weak interactions with electrodes, leading to low electron transfer efficiency (Hu et al., 2019). Second, it is difficult to recycle and recover molecular catalysts in homogeneous catalysis Sun et al., 2020b). Third, in the electrocatalytic process, only molecules near the electrode surface can be reduced or oxidized, while other molecules are not active (Sun et al., 2020b). To solve these problems, some strategies must be implemented. It was found that porphyrin could be integrated into frameworks to avoid these problems, including metal organic frameworks (Kung et al., 2017; Wang et al., 2017; Yi et al., 2021) and covalent organic frameworks (Huang et al., 2020b; Huang et al., 2019b; Yao et al., 2018). The superiorities of porphyrin-based frameworks are as follows (Liang et al., 2021). First, porphyrin backbones can be directly used to design and synthesize porphyrin-based frameworks. Second, porphyrin backbones can provide many sites to install functional groups. These functional groups can not only improve catalysis efficiency but also control the structure and morphology of the framework. Third, in porphyrin-based frameworks, porphyrins act as catalytic sites and structural units. Bimetallic and polymetallic porphyrin-based frameworks can be readily constructed for synergistic catalysis. Therefore, the application of porphyrin-based frameworks builds a bridge from homogeneous molecular catalysts to multiphase electrocatalysts and provides a larger and more attractive platform for electrocatalysis.Metal organic frameworks (MOFs) are a class of crystalline porous materials with periodic network structures that consist of metal ions or clusters and organic linkers (Khan et al., 2022; Lu et al., 2020a). Due to their unique structure, MOFs have some special and appealing features. First, a regular and adjustable pore size (Lei et al., 2018) is suitable for capturing small molecules such as CO2. The frame structure facilitates the adsorption, diffusion and reaction of carbon dioxide. Second, controllable active metal sites (Hinogami et al., 2012) and organic ligands (Kirchon et al., 2018). Due to the presence of nanopores, the metal sites in MOFs have very limited space. The metal sites can be controlled to enhance the reactive selectivity. Third, the large specific surface area (Yang et al., 2017) benefits gas adsorption. These features make MOFs an ideal electrocatalytic platform and provide desired catalytic environments for the electrocatalytic of CO2. A Co metal-porphyrin MOF nanofilm was prepared (Fig. 20a ) (Kornienko et al., 2015) and showed efficient and selective reduction of CO2 to CO in aqueous electrolytes. Implanted polypyrrole in Co metal-porphyrin MOF, denoted PPy@MOF-545-Co (Xin et al., 2021). It exhibited better CO2RR performance, with 98% FECO at -0.8 V vs. RHE and the largest current density of 32 mA cm−2 at -1.1 V vs. RHE (Fig. 20b). In addition, implanting polyoxometalates in MOFs, denoted as M-PMOF (M=Fe, Co, Ni, Zn)(Xie et al., 2018a), resulted in the best performance. The Faradic efficiency remained larger than 94% over a wide potential range (-0.8 V to -1.0 V), and the stability was also impressive (>36 h). The largest Faradic efficiency was 99%, and the turnover frequency reached 1656 h−1. DFT calculations indicated that the favorable active site is the cobalt in Co-TCPP instead of POM and the efficient synergistic electron modulation between POM and the porphyrin metal center (Fig. 20c). In addition to reducing CO2 to CO, other reduction products are possible for porphyrin MOFs. Take a copper(Valero-Romero et al.) paddle wheel cluster-based porphyrinic MOF nanosheet as an example (Wu et al., 2019b). It exhibited activity for formate and acetate production (Fig. 20d). The highest faradaic efficiencies of formate and acetate were 68.4% and 16.8%, respectively, at -1.55 V vs. Ag/Ag+. The maximum current densities for formate and acetate were 3.5 mA cm−2 and 1.0 mA cm−2 at -1.6 V vs. Ag/Ag+, respectively.Covalent organic frameworks (COFs) are porous materials consisting of organic molecules through covalent bonds with ordered crystal and periodic structures (Kandambeth et al., 2019). COFs have some features, such as large specific surface areas and permanent porosity. These features could improve the local CO2 concentration near the active sites and provide efficient transport channels for carriers (Duan et al., 2019; Ozdemir et al., 2019; Veldhuizen et al., 2019). Compared with MOFs, COFs have three additional features (Abednatanzi et al., 2022). First, COFs have a variety of structural units, and these units are all organic small molecules. Second, the periodic network structure of COFs is formed by covalent bonds (Fig. 21a ). Strong covalent bonds are more stable than the coordinate bonds of MOFs. Third, the density of COFs is low since they are composed of light elements, such as C, H, O, and N. COFs have become a promising platform for fabricating efficient electrocatalysts for the CO2RR. In 2015, Co porphyrin-based COF (Lin et al., 2015) showed high catalytic activity for the CO2RR in water, denoted COF-366-Co (Fig. 21b). It exhibited high Faradaic efficiency for CO (90%) and turnover numbers (up to 290,000, with an initial turnover frequency of 9400 h−1) at pH 7 under an overpotential of -0.55 V. Furthermore, a Co porphyrin-based COF containing donor-acceptor (D-A) heterojunctions, termed TT-Por (Co)-COF, was prepared (Fig. 21c) (Wu et al., 2021a). It was able to selectively convert CO2 to CO with a high FECO of 91.4% at -0.6 V vs. RHE and exhibited a large partial current density of 7.28 mA/cm2 at -0.7 V vs. RHE in aqueous solution. In addition, there was a Co porphyrin-based COF based on amino-functionalized carbon nanotubes (Lu et al., 2020b) capable of an efficient electrocatalytic CO2 reduction reaction, denoted COF-366-(OMe)2-Co@CNT. This Co porphyrin-based COF exhibited high activity and selectivity for the CO2RR, exhibiting the highest FE of up to 93.6% at -0.68 V and delivered a total current density up to 40 mA/cm2 at -1.05 V (Fig. 21d).Recently, reported electrocatalytic CO2RR performances of different porphyrin-based complexes were compared. As shown in Table 4 , porphyrin-based complexes exhibit outstanding catalytic performance and higher selectivity toward CO. At the same bias potential, the FECO of most porphyrin-based complexes is over 85%. The rate-determining step of porphyrin-based complexes is CO2 +*+H++e−→COOH*, the reaction energy of which varies from 0.21 eV to 1.86 eV. Compared with SAECs (Table 1), the stabilities of porphyrin-based complexes are better, the majority of which could be sustained longer than 10 h. However, 10 h is still obviously far from the industrial requirements. The phase transition of porphyrin building units, the decomposition of frameworks and the corrosion of byproducts need to be suppressed to further improve the stability of porphyrin-based complexes. For example, Cu and Co were introduced into COF-367. Such COF-367-Co (1%) showed superior activity compared to COF-367-Co (10%), COF-367-Co and COF-367-Cu, along with a highly stable operation for 136 h (Lin et al., 2015). In addition, a comprehensive understanding of the correlation between the thermodynamic adsorption energetics and the reaction kinetics of porphyrin-based complexes in a real electrolyte during the reaction is urgently needed, which will be discussed in detail in the next section.Biomass is a broad concept that includes the lignocellulosic material starch/oilseed/sugar aquaculture and bioderived waste. From an environmental point of view, the use of biomass for catalysis is very friendly and inexpensive, so biomass-derived catalysts have received extensive attention. However, biomass-derived materials contain a large number of unwanted compounds, making it difficult to control the morphology, porosity and surface chemistry. Thus, their application in industry is restricted (Rodríguez‐Padrón et al., 2018). Appropriate synthesis for controlling the chemical and physical properties of biomass-derived materials is important for the development and application of biomass-derived catalysts.Carbon is often used as the substrate of metal catalysts (Pt, Ir, Ru, etc.) to improve the specific active area and conductivity of the catalyst (Wang et al., 2020a). However, these catalysts often suffer from high cost and sensitivity to CO poisoning (Paul et al., 2019). Incorporating heteroatoms into carbon, the physicochemical and electronic properties of the catalyst can be specifically tuned, a strategy that has become quite popular. At present, modified carbon-based materials with different heteroatoms (B, P, N, S, and F) have been reported (Hu et al., 2020; Jiang et al., 2021b; Li et al., 2019; Sui et al., 2018; Tang et al., 2021a; Xie et al., 2018b; Yu et al., 2018). Among them, N-doped carbon-based nonmetallic catalysts exhibit high electrocatalytic activity, mainly attributed to three configurations of N (graphite-type N, pyridine-type N, and pyrrolic-type N) (Hao et al., 2021b; Zhang et al., 2020b). It is generally believed that the pyridine nitrogen is the main active site for the electroreduction of CO2 because the pyridine nitrogen retains a lone pair of electrons that can bind CO2 and could greatly reduce the free energy required for *COOH intermediate formation (Sharma et al., 2015). The large electronegativity difference between N and C also leads to a lower work function of graphitic carbon, high surface energy, increased n-type carrier concentration and tunable polarization of graphitic carbons. In addition, since the dopant atoms are often covalently bound to the C atoms in carbon materials, they exhibit extremely strong durability, even comparable to noble metal and transition metal-based catalysts.In this section, the development and application of biomass-derived metal-free nitrogen-doped carbon electrocatalysts reported in recent years are summarized.Wood exhibits a porous hierarchical structure and good electrical conductivity, the efficient gas and ion transport capacities of which have been demonstrated in lithium-CO2 or lithium-O2 batteries (Song et al., 2018; Xu et al., 2018; Zhu et al., 2018). Wood-derived electrocatalytic materials have become one of the important branches of novel electrocatalysts (Chen and Hu, 2018; Huang et al., 2019a). There are two categories of wood-derived electrocatalytic materials: Wood-derived materials used as the catalysts support, where the electrocatalysts were loaded in (Huang et al., 2020c; Min et al., 2020; Sekhon et al., 2022). The other is the modified wood, which is doped with N, S, P, Fe and other elements to improve the electrocatalytic performance (Chen et al., 2020a; Meng et al., 2019; Zhang et al., 2020b). As we all know, catalytic performance is closely related to the structure, especially the specific surface area. Thus, most of these catalysts are produced by hydrothermal carbonization (HTC) (Lei et al., 2021; Zhang et al., 2020f), a process that could increase solubility and the speed of carbonaceous structure formation (Hu et al., 2010).N-doped carbon materials are effective for the reduction of CO2 to CO, HCOO− and CxHyOz (Wanninayake et al., 2020), and N atoms can change the charge and spin density of carbon atoms and serve as the active sites (Yao et al., 2019). Nicolas Brun et al. (Brun et al., 2014) reported for the first time in 2014 a microporous nitrogen-doped carbon material (N-doped carboHIPE) (Fig. 22 ), which used the dehydration product of N-acetylglucosamine and hexose, 5-hydroxymethyl-2-furaldehyde and phloroglucinol to design functional carbon-based monomers. The conductivity of the resulting monolithic N-doped carbon-based foam reached 300 S m−1, and the specific surface area and porosity were 568 m2 g−1 and 0.26 cm−3 g−1, respectively. It was believed that the continuous monolithic structure provides higher electrical conductivity (avoiding the electrical resistance associated with particle contact) and better mechanical integrity compared to powder. Treatment of nitrogen- or oxidation-modified wood-based activated carbon with melamine showed good CO2 reduction activity (Li et al., 2017b), with 40% FE for CO and 1.2% FE for CH4. The positively charged carbon close to the pyridine nitrogen could stabilize the C O 2 • − intermediate. In addition, N-oxide (C-N+-O−) is also the active site of the CO2RR. The contents of pyridine-N and FE in the process of CO2 production are linearly correlated. These results showed that these substances play a leading role in the CO2RR. The graphitization degree of the porous structure and N doping type of wood-derived carbon can be controlled by the ratio of the initial raw materials (Hao et al., 2021b). What is exciting is that the activated wood also exhibits favorable CO2RR activity and stability (Fig. 23 ). Huang et al. (Huang et al., 2019a) The poplar diameter section was heated at 240 °C for 6 h, placed in an Ar atmosphere for carbonization at 1000 °C for 6 h, and finally heated at 750 °C in a CO2 atmosphere for 6 h to obtain the activated wood. The highest formic acid Faradaic efficiency was 70.8% at -1.8 V, and the high current density in aqueous solution was 53.8 mA/cm2, with a minimum of 24 h electrolytic activity. Moreover, it was also applied to the electrode matrix for the electroanalysis and detection of drugs for the first time. Compared with the glass carbon electrode (GCE) and its derivative-modified electrodes, it has a wider linear detection range and a lower detection limit.Metal-CO2 fuel cells are capable of converting CO2 into valuable chemicals and generating electricity at the same time. However, metal-CO2 fuel cells are currently in the preliminary stage, and the development of more efficient cathode catalysts is particularly important. The use of woods’ excellent ion and gas transport capacity to improve the efficiency of metal-CO2 cathode catalysts is of significance. Xu et al.(Xu et al., 2018) reported a high-capacity, mechanically flexible and highly rechargeable lithium-carbon dioxide battery. The flexible cathode of the battery utilized the natural structure of wood, and the battery exhibited a stable cycle period of more than 200, with low overpotential and ultrahigh discharge capacity (11 mAh cm−2). Cedar was used as the biomass carbon precursor (Hao et al., 2021b), together with melamine as the nitrogen source and FeCl3 (activator) for the preparation of the Zn-CO2 battery. The cedar biomass-derived N-doped graphitized carbon (CB-NGC)-2) exhibited a three-dimensional (3D) structure, the specific surface area of which was as high as 1673.6 m2 g−1. It selectively converted CO2 to CO at a high Faraday efficiency of 91% at 0.56 V (vs. RHE).Mesoporous carbon can be directly synthesized from various biomass polysaccharides, such as wheat flour, sodium alginate, and chitosan. This family of carbonaceous materials, also known as Starbons (Rodríguez‐Padrón et al., 2018), have achieved remarkable texture characteristics and adjustable surface function, making them good candidates for catalysts and catalyst supports. These materials can be designed and prepared by a simple method, primarily hydrogel formation followed by solvent exchange and thermal carbonation (Fig. 24 ). Li et al. (Li et al., 2017a) used wheat flour to obtain a high specific surface area and hierarchical porous nitrogen-doped carbon material by one-step thermal carbonization. The nitrogen content and functional species were controlled by the pyrolysis temperature. The catalyst had a high Faraday efficiency FECO of 83.7%, the current density of which reached 6.6 mA/cm2 with an overpotential of 0.71 However, the stability was only 72 h, far from the requirement of industrial application. Chen et al. (Chen et al., 2020b) started from silk cocoon, an animal N-rich product, to synthesize intrinsic defect-rich biocarbon by the activation of the pre-carbonized precursor by mixing with ZnCl2 and then pyrolyzing (Fig. 25a ). The activation produced vacancy defects by removing part of the N-containing part of the graphitic carbon substrate, which greatly improved the electrocatalytic efficiency. Therefore, at the optimal pre-carbonization and carbonization temperatures (350 °C and 1000 °C, respectively), the catalyst exhibited the highest current density jCO ≈ 1.3 mA cm−2, FECO ≈ 89%, and maintained good selectivity for 10 days (Fig. 25. b-e). The rate-determining step was considered to be a direct electron transfer step to CO2 rather than proton coupling electron transfer. Theoretical calculations also showed that the inherent defects (especially pentagonal defects) of the carbon matrix were the main active sites for direct electron transfer during CO2 reduction (Fig. 25.f-g).The performance of biomass-derived nitrogen-doped carbon electrocatalysts is listed in Table 5 . Compared with Bi-based metallic electrocatalysts (Table 1), Cu-based metallic electrocatalysts (Table 2), SAECs (Table 3) and porphyrin-based complexes (Table 4), biomass-derived nitrogen-doped carbons show low activity for the HER owing to the pyrrolic nitrogen (Wu et al., 2016). Nitrogen plays an important role in the electrocatalytic performance; both pyridine N atoms and graphite N atoms show high activity for CO2 reduction and H2O reduction and greatly reduce the free energy difference of the rate-determination step. Adjusting N atoms in nonmetallic carbon-based materials greatly improved the Faraday efficiency of the catalysts, comparable to that of metallic catalysts. However, the current density of biomass-derived nonmetallic nitrogen-doped carbon is generally smaller than that of metallic catalysts. It should also be stressed that increasing the ratio of pyrrolic N in nitrogen-doped carbon is an effective way to improve CO2 reduction selectivity. Although it was demonstrated that pyridine N could be converted to pyrrolic N via thermal nitro-nitro rearrangement (Cui et al., 2017), controlling the structure after nitrogen doping remains a challenge.The partial current densities toward the CO2RR (jCO2RR) as the kinetic descriptor along with the thermodynamic free energy changes of the rate-determining step (ΔGRDS) for the four groups of electrocatalysts are shown in Fig. 26 . It is interesting to see a negative correlation between the reactive thermodynamics and kinetics for all these electrocatalysts, i.e., a larger partial current density is obtained by reducing the ΔGRDS of the uphill rate-determining step. It should be emphasized here that the ΔGRDS and jCO2RR of the metal-free nitrogen-doped carbon catalyst in Fig. 26 is not strictly negatively correlated, mainly because the thermodynamic data of such a catalyst is calculated for a specific form of N, and therefore does not show a typical negative correlation mode. However, from the overall perspective of Fig. 26, ΔGRDS and jCO2RR of catalysts still show a negative correlation. Accordingly, a positive correlation between the thermodynamic ΔGRDS and the kinetic activation energy could be inferred. Such correlation is largely neglected and underestimated by electrocatalyst researchers. The thermodynamic-kinetic synergistic relationship was proposed by Liu et al. in structural metallic materials in 2020 (Huang et al., 2020a).To the best of our knowledge, there is no review focusing on the reactive thermodynamics, kinetics and the correlation between the two, as well as their application in the design and screening of CO2RR catalysts. Thus, the electrochemical thermodynamic framework, kinetics, and thermodynamic-kinetic correlation in the CO2RR are discussed in detail in this section.The design of selective and efficient catalysts for CO2 electroreduction requires deep insights into the relationship between the composition, structure and catalytic performance. Reactive thermodynamics determines the energy barrier of a chemical reaction. In 1920, Sabatier firstly proposed the Sabatier principle (van Santen et al., 2010) to sketchily describe general correlation. In the Sabatier principle, Sabatier assumes that a ‘‘volcano relationship’’ exists between the binding energy of key intermediates and catalytic activity. The best catalyst is located on the top of the volcano, which indicates adsorbates binding on the catalyst neither too strongly nor too weakly. Why does the volcano relationship exist, and how can such a relationship be established theoretically? Nørskov and coworkers proposed the famous electrochemical thermodynamic framework (ETF) to provide an answer (Norskov et al., 2004). ETF is also called the computational hydrogen electrode (CHE) method, which relies on the calculation of the adsorption energy and the H2 gas reference to account for the free energy of the proton-electron pair, μ H + + μ ( e − ) = 1 2 μ ( H 2 ) − e U , where U is the potential of the relative reversible hydrogen electrode (RHE). In this way, the explicit handling of solvated protons and electrons in each proton-coupled electron transfer (PCET) process is elegantly avoided, greatly reducing the computational cost. In other words, the CHE model can be used to generate the thermodynamic free energy diagram (without explicit kinetic barriers) of a series of PCET steps under an applied potential and avoid the explicit treatment of solvated protons and electrons in the reaction (Alfonso et al., 2018).With ETF, the Sabatier principle can be quantified rather than a conceptual statement. First, for the horizontal axis in volcano plots, linear correlations exist between any two of the absorbents, which is called the “scaling relationship”. The scaling relationship is used to analyze and predict catalyst reactivity and efficiency since it is easy to obtain other absorbents’ absorption energy in the case of a known adsorbent's adsorption energy. Second, for the vertical axis in volcano plots, due to the low proton transfer barriers, the kinetic aspects in the proton transfer can be ignored. The ETF considers the maximum standard Gibbs free energy differences ( Δ G m a x 0 ) among all the elemental steps to be the potential-determining step (PDS). The overpotential is generally calculated by η = U 0 − U l i m = U 0 − Δ G m a x 0 / e , where U 0 is the equilibrium potential and U l i m is the limiting potential of the reaction. A positive overpotential is required for the oxidation process, whereas a negative overpotential is necessary for the reduction reaction.The CHE model and the thermodynamic overpotential η are by far the most popular way to evaluate the electrocatalytic activity. The application of the electrochemical thermodynamic framework in the research of SAECs toward CO2 electroreduction reactions, as an example, will be summarized subsequently. For the M-N-C type of SAECs, ETF has been applied to understand the reaction mechanism of M-Nx (M=Fe, Co, Ni, Cu) moieties embedded in graphene (N-doped). The generation of CO on Fe-N-C SAECs, Co-N-C SAECs, Ni-N-C SAECs and Cu-N-C SAECs consisted of the following electron/proton transfer steps, C O 2 + * + 2 H + + 2 e − → C O O H * + H + + e − → C O * + H 2 O → C O + * + H 2 O . The calculated free energy profiles are shown in Fig. 27 . The rate-determining step for the four M-N-C types of SAECs is C O 2 + H + + e − → C O O H * . The adsorption of CO2 molecules initially occurred on the Fe-N4 catalytic site with concerted protonation and electron transfer, leading to the formation of COOH* with an uphill energy of 0.63 eV (Fig. 27a). With nitrogen doping in graphene, the energy difference for the generation of *COOH intermediates decreased from 0.63 eV (only Fe–N4 motif in graphene) to 0.29 eV (Fe–N4 motif with two graphitic N), indicating that nitrogen doping into the graphene matrix could facilitate the catalytic pathway (Zhang et al., 2018a). Wang et al. calculated the Gibbs free energy of the Co-Nx moiety from CO2 to CO and found that the formation of *CO2 •− was key for the high electrocatalytic activity (Wang et al., 2018c). The less endergonic formation of *CO2 •− on the Co-Nx moiety, the more beneficial it is for CO2 to form COOH* and CO (Fig. 27b). Mou et al. calculated the Gibbs free energy of the Ni-Nx moiety from CO2 to CO, and the free energy differences of the rate-determining steps on Ni-N2, Ni-N3, and Ni-N4 were 0.17 eV, 0.84 eV, and 1.34 eV, respectively (Fig. 27c). They suggested that the high CO2RR activity originated from coordinatively unsaturated Ni-N sites (Mou et al., 2019). To identify the high activity of Cu-Nx for CO generation, Zheng et al. calculated the free energy profiles of the whole process according to ETF (Zheng et al., 2019). The free energy difference between CO2* and COOH* was 1.599 eV in the Cu-N4 moiety, larger than that in the Cu-N2 moiety (0.96 eV), as shown in Fig. 27d. It was suggested that the length of the Cu-N bond in Cu-N2 (1.825 Å) was shorter than that in Cu-N4 (1.939 Å), benefiting electron transfer to CO2* and thus boosting COOH* generation.Although the CHE model could generate the thermodynamic energy landscape of a series of PCET steps and identify the PDS, it should be emphasized that the CHE model ignores activation energies for all PCET steps and is therefore unreliable for kinetic analysis. The main reason is that CHE model does not take into account the effect of solvation on reaction kinetics. Therefore, a series of models have been developed to explain the effects of solvent molecules on the kinetics of catalytic reactions. In this section, we first summarized the influence of electrolyte on reaction kinetics, and then summarized the various theoretical models involved in kinetics. Influence of electrolyte. Electrolyte composition and pH are important factors in studying CO2RR performance. In the current research process, KHCO3 and KCl solutions are the most common aqueous electrolyte solutions. The biggest difference between KHCO3 and KCl is that KHCO3 is a buffer solution, which can compensate for the hydroxide ions (OH−) generated by CO2RR and HER during the reaction process, so that the pH value of the electrode surface does not change much, so as to maximize the reduce the loss caused by electrode polarization. Adam Z. Weber et al. (Hashiba et al., 2018) found that the methane yield of polycrystalline copper increased significantly after replacing 0.5M KCl with 0.5M KHCO3. Therefore, the effect of buffer capacity of electrolyte on CO2RR was investigated. Through experiments and the establishment of a 1D model, the limiting reaction rate and limiting current density (Jlim) of CO2 were obtained under different CO2 partial pressure and electrolyte conditions. It was finally found that KHCO3 provided a higher CO2 flux than KCl and resulted in a slower local pH increase and a slower uniform consumption of CO2 by OH−. This is important under high current density conditions where large amounts of OH− are generated. The partial pressure of CO2 was changed under the condition of constant current density, and it was found that under low pressure conditions (<∼2.5 atm), increasing the concentration of bicarbonate was beneficial to improve CO2 transport, and under high pressure conditions (>∼2.5 atm), with the increase of CO2 partial pressure, the CO2 transport is improved faster.Besides the influence of the buffer capacity of the electrolyte, the cations and anions in the electrolyte also have an important influence on the selectivity of the electrode. The addition of halides to the bicarbonate electrolyte can stabilize the carboxyl intermediate to further enhance CO2RR. Larger alkali metal cations in the electrolyte lead to improved formation of C2+ products on copper foil, Cu (1 0 0) and (1 1 1) surfaces (Perez-Gallent et al., 2017; Singh et al., 2016). The dependence of the product on cation type is mainly due to its obvious specific adsorption, preferential hydrolysis or electrostatic interactions between solvated cations and adsorbed species in the outer Helmholtz plane (OHP) (Gao et al., 2018; Perez-Gallent et al., 2017; Singh et al., 2016). Gao et al. (Gao et al., 2018)studied the effect of cation size on the activity and selectivity of copper oxide catalysts and found that with the increase of cation size, the selectivity for C2+ products increased significantly (FE ∼ 69%), and the addition of I− to CsHCO3 further increased the formation of C2+, which may be due to the formation of a large amount of species with Cu+ during the reaction. The adsorbed cations, calculated by DFT, promoted the formation of C2+ intermediates (*CO, *OCCO, *OCCOH), of which Cs showed the greatest promotion. Besides cations, anions have also received extensive attention to influence catalyst selectivity by adsorbing on surfaces and affecting active sites. Huang et al. found that Cu (1 0 0) and Cu (1 1 1) surfaces undergo CO2RR in non-buffered solution electrolytes (KClO4, KCl, KBr, and KI), the yields of ethylene and ethanol increase sequentially with ClO4 −→Cl−→Br−→I−, which is attributed to the anion promoting the adsorption of more *CO, thereby promoting C—C coupling into the C2 product.In addition, pH plays an important role in the CO2RR mechanism, from a computational point of view, successive proton-coupled electron transfer steps are usually assumed at each step, so that the CHE model can be used. Note that this model cannot capture pH effects, as the adsorption energies of all intermediates shift proportionally. Therefore, it's necessary to study and explain the influence of electrolyte pH value. It is currently known that pH has a critical effect on the selectivity of copper-based catalysts (Rendon-Calle et al., 2018; Varela, 2020). For example, the formation of methane is pH-sensitive, while the formation of ethylene is not pH-sensitive, which is mainly related to the synthesis pathway of these hydrocarbons (Fig. 28 ). The green path shows a pH-dependent path that primarily produces methane and forms ethylene by dimerization of intermediates, and the orange path shows a pH-independent path that produces ethylene via the formation of a CO dimer intermediate. It should be noted that the pH value here refers to the local pH rather than the overall pH. This is because during the reaction process, the associated proton consumption of CO2RR and HER or the release of OH− can lead to a concentration gradient change of OH−, resulting in a gradient change of pH. This effect of local pH can be critical when using high-performance electrocatalysts, as the local pH and the pH of the bulk solution system can vary greatly. According to this feature, the selectivity of the catalyst can be regulated by adjusting the local pH. For example, the yield of hydrogen and methane increases with the increase of bicarbonate concentration, but the yield of ethylene is not affected. This can improve the selectivity of ethylene by diluting the buffer capacity of bicarbonate and allowing alkaline local pH to inhibit CH4 formation (Hashiba et al., 2018).Therefore, we know that the electrolyte plays an important role in the selectivity of CO2RR products, that is, plays a decisive role in the kinetic energy barrier of the reaction. In order to fully consider the role of these solvated molecules in the kinetic process, a series of kinetic models have been derived to explain, which will be introduced in detail below. Explicit Solvation Model. Few layers of water molecules are placed above the catalyst surface to reproduce the dielectric response of the liquid environment. The presence of explicit solvent molecules enables the location of the transition states as well as the calculation of kinetic activation barriers. Nie et al. proposed a water solvation model and H-shuttling model to explore the kinetic barriers of CO2 electroreduction on various copper facets (Luo et al., 2016; Nie et al., 2013; Nie et al., 2014). In these two models, an adsorbed H* on the electrode was used as the proton-electron pair instead of the hydrate proton. In the water-solvated model, surface protons are added to the adsorbed intermediate directly, whereas in the H-shuttling model, H* is transferred to neighboring water, and then the hydrogen of the water reacts with the intermediate. The calculated activation energy is recorded as E a c t 0 ( U 0 ) , and then according to the Butler-Volmer equation, E a c t ( U ) = E a c t 0 ( U 0 ) + β ( U − U 0 ) , the activation energy at any potential can be calculated, where β is considered to be 0.5. Nie et al. (Nie et al., 2014) found a completely different route from that obtained by Peterson et al.(Peterson et al., 2010) in the reduction of CO2 to CH4 on the Cu(111) surface (Fig. 29a ). They believe that the formation of COH* via the H-shuttling mechanism was kinetically easier than the formation of CHO*, due to its lower activation barrier (0.21 eV vs 0.39 eV). Subsequently, COH* was reduced to CHx*(x=0-3) and eventually converted to methane or ethylene (Fig. 29b, c). In addition, other metal surfaces covered with explicit water molecules have been explored, such as Pt (Hussain et al., 2016), Pb (Zhao et al., 2017) and a series of transition metals (Akhade et al., 2016), indicating the validity of the explicit solvation model. Implicit Solvation Model. The implicit solvation model represents a solvent as a polarizable medium represented by a dielectric constant (ɛ), thereby reducing the degrees of freedom and computational cost brought by solvent molecules and ions. The charge distribution in the solvent appears as an electric field that is polarized by the solute and responds to the presence of the solute. Therefore, one foundation of the implicit solvation model is the coarse-grained electrolyte. The discussion of electrolytes should start with a fully ab initio quantum mechanical treatment. Considering the mobility of molecules in the liquid phase, the evaluation of the equilibrium state requires averaging or sampling of the nuclear degrees of freedom, generally using ab initio molecular mechanics (AIMD) accomplish (Hassanali et al., 2014). Hybrid DFT with advanced dispersion corrections (Ambrosio et al., 2016), strongly constrained and appropriately normed (SCAN) meta-GGA functional(Pestana et al., 2018; Zheng et al., 2018a) and the revised version of the Perdew-Burke-Ernzerhof (RPBE) functional(Grimme et al., 2016) are generally recommended for use. Chan et al. proposed an approach to relate the constant-charge result to the constant-potential condition by the equation E 2 ( φ 1 ) − E 1 ( φ 1 ) = E 2 ( φ 2 ) − E 1 ( φ 1 ) + ( q 2 − q 1 ) ( φ 2 − φ 1 ) / 2 , where φ 1 is the work function and q is estimated from Bader charge analysis. Thus, the kinetic energy barrier at constant φ 1 could be obtained. JDFT Method. Another approach to handling solvent effects is joint DFT (JDFT), which is an ab initio description of an electronic system in equilibrium with a liquid environment. In particular, the CANDLE implicit solvation model is powerful with the consideration of solvation and external potential effects, which are implemented within the framework of joint DFT (Sundararaman and Goddard, 2015). Xiao et al. studied the initial steps of pH-dependent C1/C2 selectivity on Cu(111) surfaces and determined the rate-determining steps of the whole path (Xiao et al., 2016). Hossain et al. applied the grand canonical potential kinetics (GCP-K) formulation of quantum mechanics to predict the kinetics as a function of applied potential and determined the faradic efficiency, TOF, and Tafel slope for CO2 electrochemical reduction to CO on graphene-supported Ni-single atom catalysts (Hossain et al., 2020). Microkinetic Model. The microkinetic model makes a direct obtain of reactive thermodynamics and kinetics through experimental C-V curves. Although this method is generally used for multiple input parameters, it means that the model and the data are consistent. The idea of this method is as follows: First, each step in the whole reaction process is determined, and then the rate constant from intermediate a to intermediate b is calculated using transition state theory: k a → b = ( k b T / h ) × exp ( − Δ G ≠ / k b T ) , where k a → b is the rate constant of a→b and Δ G ≠ is the energy barrier (Motagamwala and Dumesic, 2021; Singh et al., 2017). The reaction rate is r a → b = k a → b × θ a , where θ a is the surface coverage of intermediate species a. Finally, all reaction rates are coupled to obtain the total reaction rate J. The details of the solution steps can be found in this article (Motagamwala and Dumesic, 2021). For the CO2RR, the energy barrier Δ G ≠ depends on the applied potential U, so the predicted J fits the U-dependent function. The experimentally measured C-V curves for J(U) can then be compared to verify the rate-limiting step and corresponding activation energy with a function of U. However, the microkinetic model can only represent the intrinsic rate of the multistep reaction and does not consider factors such as the spatial variation of the concentration, e.g., the local pH change on the electrode surface and the local CO2 concentration changes under the condition of limited transport. This affects the comparison between theoretical predictions and measured values. To this end, Singh et al. (Singh et al., 2017) combined a microscopic kinetic model with a continuum model describing mass transport in electrochemical cells. The model significantly improves the comparison with the experimental C-V curves after accounting for local pH and CO2 concentration changes at the electrode surface. Such fully coupled multiscale simulations of electrochemical interfaces are of great significance and prospects for evaluating the mechanism of CO2RR. Marcus theory-based methods. Akhade et al. reported a simple and transferable DFT approach to estimate the potential-dependent activation energy (Akhade et al., 2017). The challenge of finding the transition state for an electrochemical reaction step A*+H++e−→AH* was met by using an equivalent analogous non-electrochemical reaction of A*+H*→AH*. The transition state of such non-electrochemical reaction step was referenced to an equilibrium potential U0, and the analogous non-electrochemical state μ(H*) was considered to be in equilibrium with its equivalent electrochemical state μ(H++e−), allowing for the kinetic barrier to be referenced to the chemical potential of the ion in the bulk electrolyte. Then, the potential-dependence could be incorporated by extrapolating the activation energy using Marcus theory. Later, a kinetic model based on Marcus theory was developed to calculate the potential-dependent reaction barrier of the elementary concerted proton-electron transfer (CPET) steps of the CO2RR (Gao et al., 2020). The rate-determining steps for CO and CH4 formation, the influence of binding energy, electrode potential and the reorganization energy on the reaction barrier were also discussed.It is emphasized that the activity and product selectivity of an electrochemical reaction are determined by the cooperation and competition of reactive thermodynamic and kinetic factors. For the CO2RR, the thermodynamic controlling product is formed by a pathway with the lowest onset potential (the least-negative potential at which the pathway to each product becomes exergonic), while the kinetic controlling product is formed by a pathway with the smallest energy barrier. One question is raised here: is the impact of thermodynamic and kinetic factors on one electrochemical reaction consistent? The answer could be sought by revealing the synergy of the thermodynamics and kinetics of the electrochemical/electrocatalytic reactions.It is not uncommon to link material properties by revealing thermodynamic and kinetic correlations. Our team has previously proposed thermodynamic-kinetic synergies in the phase transition and processing of structural materials. In these articles (Du et al., 2020; Gou et al., 2021; Huang et al., 2020a; Liu et al., 2016; Song et al., 2021; Wang et al., 2018b; Wang et al., 2019a), we have shown how to design high-performance structural materials based on thermodynamic-kinetic synergies, which may inspire researchers in other areas. The negative correlation between the reactive thermodynamics and kinetics for the four groups of CO2RR catalysts, as indicated in Fig. 26, enlightens the similar synergies in the electrochemical/electrocatalytic reactions.The thermodynamic-kinetic correlation in electrochemistry could be firstly traced back to the Brønsted-Evans-Polanyi (BEP) relationship, which states a linear correlation of the kinetic activation energy/transition state energy with the reaction free energy for essential bond breaking and forming reactions, including C-H, C-C, N-H, O-H, C-O, C-N,N-O, O-O, and N‒N (Bligaard et al., 2004; Cheng et al., 2008). The linear factor β is the so-called BEP coefficient (0<β<1). Recently, extended to the broader category, the thermodynamic-kinetic correlation for a single electron transfer reaction and an electrocatalytic reaction were determined by our team (Du et al., 2021a). The correlation between the free energy change ΔG and energy barrier Q for a single electron-transferred oxidation reaction is derived to be Δ G = Q 1 − α − Δ G 0 c ≠ − α 1 − α Δ G 0 a ≠ , and similarly, for a single electron-transferred reduction reaction, Δ G = Q α − Δ G 0 a ≠ + ( 1 − 1 α ) Δ G 0 c ≠ , where α is a transfer coefficient (0 ≤ α ≤ 1) that represents the symmetry of the energy profiles and Δ G 0 c ≠ and Δ G 0 a ≠ are cathodic and anodic activation energies at the equilibrium potential E 0 . The free energy change ΔG and energy barrier Q for a proton-coupled electron-transferred electrocatalytic reaction are also proven to be linearly and positively correlated. Based on this correlation, a tensile-strained Cu catalyst with improved CO2RR activity and CH4 selectivity was designed. As shown in Fig. 30-a , b, tensile strain (TS) contributed to a higher faradaic efficiency for the CO2RR of 76.48% at -1.2 V together with a 38% enhancement in the partial current density toward CH4 generation (iCH4) compared to that of pristine unstrained Cu. Based on the derived positive correlation between the free energy change and energy barrier of the electrocatalytic reaction, the reaction mechanism and the strain effects were also revealed (Fig. 30c). Tensile strain lowered the initial state free energy of CO*+H2O from the IS of the pristine Cu to the IS’ of the TS Cu and moved the final state free energy of CHO* from FS to the lower FS’. The free energy change ΔG expressed by FS-IS for pristine Cu is shown in black, and the corresponding ΔG’ for tensile-strained Cu is illustrated in red. Since CHO* was found to be strain sensitive and CO* was strain insensitive, ΔG’ was smaller than ΔG. Q was positively correlated with ΔG, thus resulting in a smaller Q’ for TS Cu. As indicated by iCH4 as the kinetic descriptor, iCH4 and ΔG were negatively correlated, which was reasonable since I and Q were in a negative exponential relationship according to the Butler-Volmer equation.The above linear thermodynamic-kinetic correlation seems to directly contradict the Marcus theory of electron transfer, since Marcus states a quadratic relationship as E a = λ 4 ( 1 + Δ G λ ) 2 , where λ is the reorganization energy, Ea is the activation energy and ΔG is the Gibbs free energy change. However, this is a direct consequence of how these theories modeled the free-energy landscape: while BEP and Du et al. assumed the free energy to be a linear function of the reaction coordinate, Marcus assumed it to be quadratic. It is stressed that the application of these correlations depends on the validity of these presumptions and should ideally be tested on a system-by-system basis. Generally, the discrepancy between the two is less apparent near equilibrium, and the predictions diverge considerably when the overpotential is increased. The most iconic example is the Marcus inverted region, where the activation energy starts to increase if the overpotential increases beyond a certain value (Hammes-Schiffer, 2009).Now back to the question raised at the beginning of Section 3.3, our answer is as follows: Generally, the impact of thermodynamic and kinetic factors on the electrochemical reaction should be consistent according to the positive correlation between the thermodynamic free energy change and kinetic energy barrier, i.e., the larger the driving force is, the lower the reaction barrier is, and the higher the activity is. In addition, the Sabatier principle and its thermodynamic interpretation have been successfully applied for the design and screening of catalysts. The Sabatier principle is actually based on thermodynamics and leaves the kinetics out of consideration, rendering a discrepancy between the theoretical framework and experimental performance of real catalysts. The establishment of thermodynamic-kinetic correlation makes up for the deficiency of the Sabatier principle and may play a significant role in the development of electrocatalysts. Taking the design of a highly active catalyst as an example, the thermodynamic factors and correlated coefficient could be designed or modulated to obtain a kinetic energy barrier as small as possible.Last but not least, the thermodynamic-kinetic correlation is useful to predict and design the intrinsic activity of the catalyst, but it needs to be noted that the intrinsic activity of the material may not be the limiting factor of the catalysis in some cases. Mass transport, the electrolyte (particularly the pH of the electrolyte) and the electrochemical active surface area (ECSA) are all possible limiting factors for catalytic performance. For example, Seifitokaldani et al. reported hydronium(H3O+)-induced switching between CO2 electroreduction pathways, where the product selectivity of a silver catalyst switched from entirely CO under neutral conditions to over 50% formate in the alkaline environment (Seifitokaldani et al., 2018). This study provides new insights into the role of hydronium in CO2 electroreduction processes and the ability for electrolyte manipulation to influence transition state kinetics, altering favored CO2 reaction pathways. The catalytic performance is suggested to be considered less of an intrinsic catalytic property and rather a combined result of the catalyst and reaction environment.Using electrocatalytic technology to convert CO2 into multi-carbon products with high energy value is one of the most ideal solutions to alleviate global environmental problems and energy shortages. In this review, the recent developments of the most popular CER catalysts, including metal-based catalysts, single-atom catalysts (SAECs), porphyrin-based complexes and biomass-derived nitrogen-doped carbon catalysts, are summarized. Focusing on the activity, selectivity and stability of catalysts, metal-based electrocatalysts have achieved tremendous achievements in the past few years, the performance of which has been largely improved. SAECs exhibit excellent catalytic activity and selectivity, and their FECO could be over 80%. Porphyrin-based complexes exhibit outstanding catalytic performance and selectivity toward CO (>90% FECO). Biomass-derived metal-free nitrogen-doped carbon catalysts, as emerging and environmentally friendly catalysts, exhibit excellent activity and selectivity, particularly with ultralow HER activity. However, the current density of nitrogen-doped carbon catalysts during the reaction process is generally smaller than that of other catalysts. There is a common problem for the current CER catalysts, the long-term stability. Most of the reported data show a very limited test time, usually less than 100 h, which is definitely far from the industrial application requirement. For reference, industrial water electrolyzers have demonstrated stable performance over 80,000 h.The design of electrocatalysts can be facilitated by accurate computational simulations and theoretical understanding of the mechanism. Thus, the development and application of the electrochemical thermodynamic framework (CHE model) and the improved models involved with reactive kinetics are reviewed. The CHE model simply and directly generates the thermodynamic energy landscape of a series of PCET steps under an applied potential. Although the CHE model and the thermodynamic overpotential η are by far the most popular way to evaluate electrocatalytic activity, they ignore activation energies for all PCET steps and are therefore unreliable for kinetic analysis. The improved explicit solvation model, implicit solvation model, JDFT model, microkinetic model and Marcus theory-based method involving reactive kinetics, together with the electrolyte influence on the kinetics are all summarized. Furthermore, inspired by the negative correlation between the thermodynamic free energy difference (ΔGRDS) and the kinetic partial current density for the CO2RR (jCO2RR) of various electrocatalysts (Fig. 26), the thermodynamic-kinetic correlation and the thermodynamic-kinetic synergy during the electrocatalytic reactions are discussed. The linear or quadratic thermodynamic-kinetic correlations are both reasonable causes of the difference in how the theories model the free-energy landscape. The consistency between thermodynamic and kinetic factors on the electrochemical reaction could be predicted according to the positive thermodynamic-kinetic correlation. More importantly, the thermodynamic-kinetic synergy may play a significant role in the design and screening of the electrocatalyst. In other words, aiming at the larger thermodynamic driving force and smaller kinetic energy barrier bridged by a more suitable correlated coefficient, it is more efficient to modulate or design the catalysts’ composition, structure, strain state, defects, etc.Identifying catalytic mechanisms of CO2 electroreduction could accelerate design of highly active and selective catalysts. In recent decades, numerous theoretical studies have contributed a lot for understanding reaction pathways, identifying rate-limiting steps, and revealing reactive thermodynamics and kinetics. In addition, the rapid development of in situ/operando characterization techniques, e.g., X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), Raman, infrared (IR) spectroscopy, have proved to be quite powerful in tracking the structure reconstruction of the heterogeneous catalysts, identifying real active sites and recording intermediates formed during the reaction.(Cao et al., 2021; Handoko et al., 2018)There is a trend of combining theoretical calculations with in situ/operando experimental analysis to provide a plausible mechanism for CO2RR. Profound theoretical insights and operando characterizations with higher resolution and higher signal-to-noise ratio are highly required. The efficient electrocatalysts combined with advanced electrochemical flow reactors, the facile and clean recycling of carbon resources for renewable fuels and high-value chemicals is expected to be realized in the future. Feihan Yu: Conceptualization, Data curation, Formal analysis, Writing – review & editing. Kang Deng: Conceptualization, Data curation, Funding acquisition, Writing – review & editing. Minshu Du: Writing – original draft, Supervision, Project administration. Wenxuan Wang: Funding acquisition. Feng Liu: Writing – original draft, Supervision, Project administration. Daxin Liang: Writing – original draft, Supervision, Project administration.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research was supported by the National Natural Science Foundation of China (52130110). We also appreciate the support of the Fundamental Research Funds for the Central Universities (2572021BB02) and the Natural Science Foundation of Chongqing (cstc2021jcyj-msxmX1162).

The carbon dioxide electroreduction reaction (CO2RR) can convert CO2 into value-added fuels or chemicals, thus becoming a promising approach for balancing the carbon cycle and realizing a low-carbon economy. Here, we describe in detail the preparation, performance, reaction mechanism and theoretical research progress of four representative CO2RR catalysts, including metal-based electrocatalysts, single-atom electrocatalysts, porphyrin-based complexes and biomass-derived nonmetallic carbon-based materials. In particular, the electrochemical thermodynamic framework, kinetics, and thermodynamic-kinetic correlation in the CO2RR are discussed. The concept of thermodynamic-kinetic synergy and some perspectives on the future design of electrocatalysts are also presented, with the aim of facilitating research and development in this area.

Data will be made available on request.Palladium-catalyzed cross-coupling reactions (PCCCRs) are one of the greatest milestones in organic chemistry. Particularly the procedures described by Suzuki-Miyaura, Sonogashira, Stille or Heck are good examples of this paradigm [1,2], although they all follow different reaction mechanisms. Rapid progress in heterogeneous Pd catalysts for cross-coupling reactions have been realized for the synthesis of compounds for pharmaceutical and chemical industries [3]. The three pillars of heterogeneous catalysis are activity, selectivity and stability. Consequently, it is well established that an ideal heterogeneous catalyst should has high geometric surface area that allow incorporation of high active catalytic sites, mechanical robustness, thermal stability, control loading, negligible leaching, recyclability and any loss of activity by poisoning or deactivation. All these limitations often result in a significant decrease in catalytic activity. Efficient catalyst will clearly require the combination of diverse strategies considering different aspects, such as thermal, mechanical and chemical related with the stability and morphology of the catalyst.Monolithic catalysts are structures comprising functional interconnected microchannels with a regular three-dimensional structure. They can replace conventional catalysts (homogeneous catalysts) and chemical reactors as well as helping to overcome different problems posed by traditional systems. Monolith reactors were initially developed in mid 1970s for the automotive industry to remove NO, CO and hydrocarbons through gas-solid reactions in engine emission converters. In solution phase chemistry, the monoliths have many advantages over heterogeneous powdery catalytic systems (polymeric reagents, nanoparticles, in general, supported reagents) and traditional packed-bed reactors, such as high transport rates of heat and mass per unit pressure drop, small transverse temperature gradients ease of scale-up and work-up (avoiding filtration processes) [4,5]. When a monolithic catalyst is considered as an alternative to a fixed-bed reactor packed with commercially available catalyst particles, a straightforward prototyping and development program is needed to produce the monolithic catalyst.Beyond the importance of the catalyst itself in organic reactions, microwave heating has several applications in almost every field of chemistry, due to the advantages that this technology offers compared to traditional heating methods. Microwave assisted organic synthesis (MAOS) has rapidly gained acceptance as a valuable tool for accelerating drug discovery and development processes [6]. Advantages of microwave heating over conventional heating stem mainly from its ability to interact with the material at the molecular level. Heat losses (conductive and convective) associated with traditional heating methods are negligible with microwave heating [7,8]. However, this synthetic methodology is not without risks. In the case of catalysts with metal loadings, microwave-metal discharge could trigger hot-spot formation, local microplasmas and arcing at metal sites, generating hazardous conditions in the presence of flammable solvents and/or gas [9]. These phenomena can cause sparkling or flame inside the microwave reactor. Explosions could occur, for example, in the case of homogeneous PCCCRs using common palladium reagents. These processes are particularly critical at points in the reactor where metal palladium deposits become embedded in the reactor wall in the absence of solvent and therefore under the direct action of microwave radiation [10]. The sparkling phenomena in metal-solvent mixtures has been reviewed by Kappe [11] and coworkers supporting the data reported by Hulshof [12] who observes that arcing phenomena is basically linked to large metal particles. Consequently, an effective immobilization of palladium species on a monolithic catalyst must be taken to avoid metal leaching, hot-spots formation, sparkling and electric discharges in PCCCRs under MAOS.Cermet is a composite material made up of ceramic materials and metals. Cermets are used to combine the high temperature and abrasion tolerance qualities of ceramics with the malleability of metals. The combination of a heterogeneous cermet-type catalyst with the microwave heating tool has many advantages. Most of the solid catalysts highly absorb microwave irradiation thus they can be considered as an internal heat source. In fact, a way in which microwaves can be selective is by heating a certain part of the catalyst better than the rest. For instance, low-dielectric loss materials such as alumina loaded with microwave active metal (Fe, Pt, Mo or Pd). The microwave active material will heat to very high temperatures leading to selective overheating of metallic sites that cause increased reaction rates [13]. For Mars van Krevelen mechanism [14], where the catalyst surface itself is an active form of the reaction, forming a thin layer of metal-reactant (metal-oxide, metal-sulfide etc.) on the surface, microwave heating causes selective acceleration of primary reaction step, resulting in increased overall rate of reaction [15].Most cermet manufacturing processes are based on powder metallurgy techniques. Metal and ceramic powders are mixed and ground together in a ball mill or an attrition mill. A lubricant or humectant is often added to facilitate shaping operations. In many cases, after grinding, a suspension is prepared with the raw materials, which is atomized to obtain fine, homogeneous and spherical particles. The pieces are formed by compacting the powder by cold pressing, cold isostatic pressing, or hot isostatic pressing. Except in the latter case, the pieces already formed are thermally processed for sintering at high temperatures in continuous or discontinuous furnaces, with or without controlled atmospheres, depending on the case. Numerous studies have been performed using a support material where the active metal phase is incorporated by impregnation [16–18], absorption [19], deposition-precipitation [20], ion exchange [21] and encapsulation [22,23] process. Some of these methods are usually followed by a thermal or chemical treatment to activate the metal. However, these strategies have their own limitations in terms of design, loading and mechanical/thermal stability.A highly promising technique for the fabrication of structured catalyst is the 3D printing, an additive manufacturing technique [24,25]. It has been widely used to generate complex-shaped structures with controlled composition and architecture. This approach is based on the extrusion of ink through a nozzle, which can be deposited over a substrate in a layer-by-layer sequence. Owing to the spatial resolution that can be achieved, this technique can be extended to a broad range of technological applications ranging from optoelectronics [26–29], catalysis [30–33], to biomaterials [34–36]. These functional structures can be fabricated using different materials such as metals, ceramics, metal oxides, hydrogels, and composites. The ability to fabricate 3D structures with micrometer resolutions at both meso- and micro-scale depends basically on the rheological behaviour inks and printing parameters. In this respect, the control over these features is fundamental to allow the development of 3D structures with controlled composition, architecture, and specific properties.In previous works of our research group, we have carried out the manufacture of monoliths using 3D-printing technology, following different strategies such as robocasting and sintering for the manufacture of composite materials based on alumina and copper oxide [30]. In other works, the surface functionalization of SiO2 monoliths was carried out by silanization and metalation [37], coating with polyimide-palladium composite on the surface [38] as well as incorporation of metallic catalytic species on the surface using the strong electrostatic adsorption technique [39]. To our knowledge, there are no examples of monolithic cermet-type catalysts, manufactured by 3D-printing, tailored to a microwave reactor-vessel. In this work, we present the design and fabrication of the first Pd0/Al2O3 cermet monolithic catalyst [40] specifically designed to fit in a microwave reactor. The catalyst can be rapidly prepared via combination of 3D printing (direct ink writing technique) and subsequent thermal treatment, without any other type of surface treatment after sintering. The catalyst, with 3D structured morphology and long-term stability, was used as an efficient, extremely robust and safe catalyst for Suzuki, Stille, Sonogashira and Heck palladium catalyzed cross-coupling reactions assisted by microwave heating.The ink is synthetized using a protocol similar to that previously described [30]. Briefly, 5 g of PdCl2 (on a stoichiometric basics, 99.9%, Alfa Aesar) was dissolved in 14 mL of deionized (DI) water. Then 50 g of Al2O3 powder (mean particle size 0.5 μm and real density of 3.96 g/mL, Almatis GmbH, Germany) was added into the PdCl2 solution, and mixing in a planetary mixer (ARE-250, Thinky, USA) at 2000 rpm for 2 min. After, 0.065 g of (hydroxypropyl)methyl cellulose (HPMC, viscosity 2600–5600 cP, Sigma-Aldrich) was added to the suspension, followed by through mixing at 2000 rpm for 2 min. After 1 h equilibrium, the resulting suspension was gelled by adding 0.13 mL of polyethylenimine (PEI, Mw = 2000, Sigma-Aldrich), and was again homogenized in the planetary mixer at 2000 rpm for 2 min. This mixing process is repeated until to obtain the desired homogeneity. The final printable ink was composed of 5.6 wt% of Pd (relative to the ceramic content) and a ceramic concentration of about 45 vol%.A robotic deposition apparatus (Model A3200, Aerotech Inc., USA) was used to fabricate the catalyst sample. The ink was loaded into the syringe (3 mL, Nordson EFD Inc., Japan), which is attached by a nozzle tip (410 μm diameter size, Nordson EFD Inc.). An air dispenser (Performus VII attached to HP7x, Nordson EFD Inc.) is used to control the ink flow rate. The ink was extruded (Direct Ink Writing) under pressures ranging from of 7–10 bar at a speed of 3–5 mm/s. Catalytic structures were designed using CAD software (Robocad 3.4, 3D Inks, USA), with cylindrical shape (diameter 10 mm and height 5 mm), and open square pores of 590 × 590 μm. After drying at room temperature, the samples were sintered in air at 1500 °C for 2 h with a ramp rate of 10 °C/min.The surface morphology and microstructure of the samples were characterized using scanning electron microscopy (SEM, JEOL 6400, JEOL Corporation, Japan) and stereomicroscope (OlympusSZX12, Olympus, Japan). The surface elemental analysis of sintered samples was measured using energy dispersive X-ray spectrometer (EDS, AZTEC/Xact, Oxford, UK). The crystal structure of samples was monitored by a Siemens D5000 diffractometer (Siemens, Germany) with CuKα radiation (λ = 0.15418 nm). Data was collected in the range of 20–90° (2θ) with a step size of 0.05°. The palladium content in the monolith catalyst was determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Varian Liberty 200). Temperature programmed reduction (TPR): The reducible species formed during the calcination step were determined by this technique using an Autosorb 1C-TCD equipped with a thermal conductivity detector. The monolith was loaded in U shaped quartz tube and heated from room temperature to 473 K for 1 h in Ar stream (30 mLN/min). The sample was then cooled down to 313 K and the Ar was replaced by a 5 vol % H2/Ar gas stream (45 mLN/min). The sample was heated from 313 K to 673 K, at a ramp rate of 10 K/min. CO chemisorption (CO–C): The metal dispersion was measured by CO pulse chemisorption using an AutoChem II 2920 apparatus equipped with a TCD detector. The analysis started by heating the catalysts sample from room temperature to 673 K at 10 K/min under 40 mLN/min of 5 vol % H2/Ar flow. Then, the sample was kept under 50 mL/min of helium for 30 min and cooled down to 308 K. Finally, when the detection baseline was stable, CO pulse chemisorption started. The dosage was repeated every 2 min until equal peaks were detected or 20 dosages were carried out. Metal dispersion was determined by assuming a stoichiometric ratio of Pd/CO = 1. For surface analysis, X-ray photoelectron spectroscopy (XPS) was carried out using a Physical Electronics PHI 5700 spectrometer with nonmonochromatic Mg Kα radiation (300 W, 15 kV, 1253.6 eV) as the excitation source. High-resolution XPS spectra were recorded at a given take-off angle of 45° by a concentric hemispherical energy electron analyzer, operating in the constant pass energy mode at 29.35 eV, using a 720 μm diameter analysis area. Adventitious carbon at 284.8 eV has been used for charge referencing. Images of Pd nanoparticles dispersed in the ceramic matrix were acquired using a Gemini-500 Field-Emission Scanning Electron Microscope (FESEM) operating at 20 kV using a back-scattering AsB detector with a size resolution of ±0.5 nm. Selected images were analyzed by counting more than 100 particles using ImageJ software.All reactions were performed in a CEM/DISCOVER SP-D-Closed Vessel Microwave apparatus. Iodoarenes, 5-haloisatins, alkenes, alkynes, boronic acids and organotin reagents were purchased from Aldrich and Alfa Aesar [(4-methoxicarbonyl)phenylboronic acid)]. All reactions were monitored by TLC with 2.5 mm Merck silica gel GF 254 strips and the purified compounds showed a single spot. Detection of compounds was performed by UV light and/or iodine vapor. Purification of isolated products was carried out by preparative TLC using silica gel plates. The synthesized compounds were characterized by spectroscopic and analytical data. The NMR spectra were recorded on Bruker AM 300 MHz (1H) and XM500 spectrometers. Chemical shifts are given as δ values against tetramethylsilane as internal standard and J values are given in Hz. Proton and carbon nuclear magnetic resonance spectra (1H NMR) were recorded in CDCl3. Melting points were determined on a Gallenkamp melting point apparatus and are uncorrected. Mass spectra were obtained on a Varian MAT-711 instrument. High resolution mass spectra (HR-MS) were obtained on an Autospec Micromass spectrometer.General procedures for PCCCRs: All reactions were performed using the 3D printed Pd0/Al2O3 cermet monolithic catalyst [total content of Pd on monolithic surface: 1.1 mg; it means 1.1% mmol Pd on the reaction]. No magnetic bar was used in these experiments although they are compatible in these procedures.In a capped microwave reactor vessel (13 mm internal diameter), provided with the 3D-printed Pd0/Al2O3 cermet catalyst were dissolved the corresponding haloarene (0.98 mmol), Na2CO3 (2.94 mmol) and the boronic acid (1.07 mmol) in a mixture of iPrOH/H2O (2:1 ratio, 5 mL). The mixture was heated at 120 °C under microwave irradiation (200 W) for 20 min. Once the reaction finished, the catalyst was removed from the vial, sonicated, washed with water (5 mL), MeOH (5 mL) and acetone (5 mL) and dried under vacuum for 30 min for reuse. The mixture was washed with water and extracted in AcOEt. The organic phase was dried with anhydrous Na2SO4 and evaporated at room temperature on a rotary evaporator. The resulting solid was recrystalized by isopropanol to give the final products compounds 2a-e (entries 2a-d)].In a microwave reactor vessel (13 mm internal diameter), provided with the 3D-printed Pd0/Al2O3 cermet catalyst were dissolved the corresponding iodoarene (0.98 mmol), TEA (2.94 mmol) and the alkyne (1.07 mmol) in iPrOH (5 mL). The mixture was heated at 120 °C in a microwave reactor (200 W) for 30 min. Once the reaction was finalized, the catalyst was removed from the vial, sonicated, washed with water (5 mL), MeOH (5 mL) and acetone (5 mL) and dried under vacuum for 30 min for reuse. The mixture was washed with water and extracted in AcOEt. The organic phase was dried with anhydrous Na2SO4 and evaporated at room temperature on a rotary evaporator. The mixture was purified by preparative TLC (AcOEt/Hexane) to give compounds 2f-i.In a microwave reactor vessel (13 mm internal diameter), provided with the 3D-printed Pd0/Al2O3 cermet catalyst were dissolved the corresponding iodoarene (0.98 mmol), TEA (2.94 mmol) and the alkyne (1.07 mmol) in MeCN (5 mL). The mixture was heated at 120 °C in a microwave reactor (300 W) for 20 min. Once the reaction was finalized, the catalyst was removed from the vial, sonicated, washed with water (5 mL), MeOH (5 mL) and acetone (5 mL) and dried under vacuum for 30 min for reuse. The mixture was washed with water and extracted in AcOEt. The organic phase was dried with anhydrous Na2SO4 and evaporated at room temperature on a rotary evaporator. The mixture was purified by preparative TLC (AcOEt/Hexane) to give compounds 2j, d, k, l.In a microwave reactor vessel (13 mm internal diameter), provided with the 3D-printed Pd0/Al2O3 cermet catalyst were dissolved the corresponding iodoarene (0.98 mmol) and the corresponding organostannane (1.07 mmol), in MeCN (5 mL). The mixture was heated at 120 °C in a microwave reactor (200 W) for 20 min. Once the reaction was finalized, the catalyst was removed from the vial, sonicated, washed with water (5 mL), MeOH (5 mL) and acetone (5 mL) and dried under vacuum for 30 min for reuse. The mixture was washed with water and extracted in AcOEt. The organic phase was dried with anhydrous Na2SO4 and evaporated at room temperature on a rotary evaporator. The mixture was purified by preparative TLC (AcOEt/Hexane) to give compounds 2a, e, f, m, n.Activity, selectivity and stability of a monolithic catalyst could be positively affected during microwave-assisted heterogeneous catalysis. Consequently, in this work we set as specific objectives: the design of a cermet type monolith catalyst provided with an appropriate design (presence of interconnected channels) (1), simple and direct manufacturing (2), catalytic efficiency (3), chemical and mechanical robustness (4), as well as almost unlimited reusability and safety in MAOS (5). We present here a direct manufacturing process based on 3D-printing (direct ink writing, DIW) of catalytic inks based on α-alumina and palladium species and subsequent sintering of the monolith, to obtain a cermet type monolith Pd0/Al2O3.Monolithic catalyst design: The shape and size of the monolith, to adapt it to a certain reactor, can be modulated by 3D printing, so this technology is ideal in prototyping processes. As can be seen in Figs. 1b,d and 3b,c , the shape of the catalyst is adjusted to the shape and dimensions of the microwave vessel. For this reason, a slightly flattened cylindrical shape structure was designed, adapted to the bottom of the vessel.Regarding the composition of the final product, it is important to highlight that the material described here contains metallic species both inside the filaments of the structure and on the surface as Pd0 and not as PdO (oxide). Therefore, the material obtained in this work is a true cermet: metal (0)-ceramic composite, so this monolith is particularly efficient in palladium-catalyzed cross-coupling reactions (Suzuki, Stille, Sonogashira, Heck).Alumina-based ceramics (Al2O3) have excellent physical and chemical properties. In addition, they have good mechanical resistance and thermal stability [41]. However, their high Young's modulus values make the applications somewhat limited due to their high brittleness, as they are not easily deformed. Consequently, alumina ceramics are very sensitive to minimal defects in their microstructure, which acts as a crack initiation point [42,43]. Nevertheless, ceramic materials can improve their fracture toughness by homogeneous incorporation of fine particles of ductile metals in the matrix. Different reinforcement metal amounts of Al2O3/Al [44], Al2O3/Cr [45], Al2O3/Cu [46], Al2O3/Ni [47], Al2O3/Mo [48], Al2O3/Ti aluminide [49] and Al2O3/Ni3Al [50] have been reported. Many of these composites are synthesized using powder techniques. These techniques start from a mixture of powders from a high-energy mechanical grinding, and later they are subjected to a pressing process, and finally they are sintered at a certain time and temperature, giving rise to the composite material [51]. Therefore, the robustness of the monolithic material improves with the incorporation of the metallic component in the composition of the internal matrix of the filaments. It is well known that cermet experiences a decrease in their hardness and elasticity module in comparison with the material base. However, his fracture toughness increases. Consequently, these factors contribute to the new composite better tolerating the generation of cracks when the material is working under conditions of high loads and friction [51].The synthesis of the monolithic catalyst is very direct and simple since it is a process of 3D-printing and sintering, which is carried out without any other type of surface treatment. Specifically, we first developed an aqueous colloidal Pd/Al2O3 ink with tailored rheological properties for 3D printing that contains moderate Pd loading (5.6 wt%). Al2O3 has been used as matrix due to its excellent thermal and mechanical properties [52], which leads to obtain a catalyst with remarkably stability. After an appropriate ink synthesis, a 3D structure with high geometric surface area and open square pores was fabricated, followed by thermal annealing at 1500 °C for 2 h to obtain a catalyst with thermal/mechanical integrity. A detailed physico-chemical characterization for the catalyst was carried out by optical, scanning electron microscopy (SEM), EDS, and X-ray diffraction (XRD) techniques, and it was evaluated as catalyst in Suzuki, copper-free Sonogashira, Stille and Heck cross-coupling reactions under microwave heating conditions. The proposed strategy can be extended to other metals, opening new opportunities in the design and synthesis of efficient metal catalyst. Fig. 1 shows the physical parameters and different images of the 3D Pd0/Al2O3 catalyst. This structure was prepared using colloidal Pd/Al2O3 ink, where the stabilization of Pd in the sample combines chemical and physical strategies. In the first step, concentrated colloidal ink was prepared by using a Pd+2 precursor (PdCl2) that was absorbed into the Al2O3 particles through electrostatic interactions in an aqueous solution under stirring. Content of Pd could be adjusted in this step by varying the amount of the metal precursor. Particularly, we created ink with a weight percentage of 5.6 wt% Pd to achieve sufficient reinforcement in the internal structure, as well as a minimum palladium content on the surface, necessary for catalysis to occur. Then, a non-ionic (hydroxypropyl)methyl cellulose (HPMC) and a cationic polyethylenimine (PEI) were added to impart the desired rheological properties to the ink. Subsequently, the obtained ink was used to fabricate the Pd/Al2O3 catalyst by 3D printing technique. This catalyst was designed with well-controlled morphology using CAD software, where the size and shape is proportional to the microwave reaction vessel. Finally, as discussed above, the resulting sample was sintered at 1500 °C for 2 h to form a catalyst with remarkable mechanical and structural properties, which are strongly related with the catalytic activity. Note that the thermal treatment was selected, in order to achieve a catalyst with excellent mechanical properties and a specific oxidation state (0) of Pd. At a lower temperature, the catalyst could be easily cracked when the reactions will be carried out under shaking and microwave heating conditions. During the sintering process, no reducing atmosphere was used to get Pd0 in the cermet. As shown in Fig. 1b–i, the sintered sample present a uniform shape, network structure, and a homogeneous surface without cracks. Fig. 1c shows the sintered structure with interconnected square pores. The cross-sectional image in Fig. 1e confirms the multilayer and interconnected morphology of the final catalyst. The advantages of choosing this morphology are the formation of high surface area to volume ratio, which enhances the number of active sites for catalytic reactions. Noticeably, the sintered structure is mechanically robust to tolerate long-term and repeated reaction cycles. As expected, the colour of catalyst changed after sintering from light-grey (dried sample) to more black (sintered sample), suggesting an efficient reduction of Pd+2 into Pd0, probably as a consequence of the presence of polyethylenimine (PEI), in the catalytic ink, acting as a palladium reducing agent. In addition, moderate diffusion out of the Pd metal through the substrate was observed after sintering, suggesting a significant metal immobilization in the sample. Particularly, the results of the chemical analysis obtained by ICP-OES indicate that metallic content (Pd wt%) is around 1.66 wt% for the sample after sintering. Furthermore, the optimum sintering temperature results in a catalyst with a geometric surface area (surface area-to-volume ratio) of 33.2 cm2/cm3, according to the dimensions the external dimensions of structure.A smooth surface was further confirmed by the magnified SEM image (Fig. 1g and h). The cross section of the monolith (view of the inner cross section of the metal-ceramic composite) is shown in Figs. 1f, i and 2a, where the two materials (metal and ceramic) are observed. In addition, from this image it cannot be observed any segregation or precipitation of the Pd at the grain boundary region, demonstrating that Pd metal was well integrated within the Al2O3 matrix.The qualitative and quantitative distribution of palladium on the monolith surface were experimentally confirmed by the SEM, EDS and mapping, XRD, TPR and CO–C. Fig. 2a shows the surface SEM image of the Pd 0 /Al2O3 filament, and Fig. 2b represents its spectrum in two different regions. The results, together with the EDS-mapping results (Fig. 2c), reveal that the Pd element is evenly distributed throughout the monolith structure. This outer Pd content is available for the catalytic reactions. The presence of Pd is confirmed in the two regions (inner and outer sections) by the EDS.These results demonstrated that Pd could be efficiently loaded on the ceramic network with an excellent confinement into the Al2O3 matrix after the sintering process. In addition, FESEM images at higher resolution allow to observe that Pd is dispersed along the ceramic matrix surface but also inside their pores in the form of nanoparticles. These appear in the form of aggregates/clusters with a mean size of ca. 185 ± 50 nm (Fig. 2f–h). This agglomeration can result from the sintering of adjacent Pd ions distributed along the matrix during the thermal reduction process due to the absence of any stabilizer to allow control of nucleation and growth. In addition, the clusters seem to be formed by particles/crystallites of lower size. Inspection of selected areas in the acquired images provides approximate values of the crystallites composing the particle agglomerates of ca. 40 ± 5 nm.X-ray diffraction analysis was performed to verify the presence of the Pd on the sample (Fig. 2d). The XRD pattern of the sample shows characteristic peaks of two different phases corresponding to α-Al2O3 (JCPDS No. 05–0712) and Pd (JCPDS No. 05–0681). In particular, the peaks observed at 2θ values of 40.11°, 46.64°, 68.19°, 82.05° and 86.4° correspond to (111), (200), (220), (311) and (222) planes of the Pd metal, respectively. These peaks are sharp and well defined, which indicates that the metallic Pd has a high degree of crystallinity. Clearly, there are not obvious diffraction peaks that could be associated to crystalline palladium oxide species or Pd–Al alloys, indicating that the introduction of Pd into the Al2O3 not generate Pd complexes during the chosen thermal treatment. Finally, the reduction properties of the monolith were measured by TPR. The obtained TPR profile was flat suggesting that most of the palladium species incorporated was already reduced. Results of the CO chemisorption showed a metal (Pd) dispersion of 0.192% on the surface, which is sufficient and beneficial to carry out MAOS safely. To evaluate the oxidation state of the palladium in the Pd/Al2O3 catalyst, XPS was utilized. The high-resolution XPS spectrum (Fig. 2e) reveals the existence of Pd0, where it showed double peaks with bending energies at 335.5 and 340.8 eV, which correspond to Pd0, Pd 3d5/2 and Pd 3d3/2, respectively. These results combined with the XRD spectra (Fig. 2d) confirm the presence of Pd0 on the outer surface of the catalyst.These four reactions follow different mechanisms for the formation of carbon-carbon bonds and have been extensively used in organic chemistry. However, the reaction conditions of homogeneous catalysis are not always extrapolated to heterogeneous catalysis. This fact makes it necessary to carry out an exhaustive screening of the reaction conditions for a new catalytic material. The first studies of the catalytic activity of the monolithic catalyst adapted to the microwave vial (Fig. 3b and c) focused on the following issues: performing PCCCRs in secure MAOS, obtaining selectivity and high yields for the four transformations studied (Suzuki, Sonogashira, Stille and Heck); check for possible palladium leaching on the monolith surface under different reaction conditions (alkenes, alkynes, boronic acids, stannanes, bases and solvents) at high temperature and the reusability of the monolithic catalyst in MAOS. The results of catalytic activity for the four protocols are shown in Table 1 . In all reactions, taking into account the total amount of palladium detected on the monolith surface [(0.192% Pd on surface, it means 1,1 mg on the surface (standard monolith weight: 600 mg)], the reactions worked well using 0.98 mmol of starting substrates [4-iodobenzene (1a), 4-iodotoluene (1b), 4-iodoanisole (1c) as well as 5-haloisatins such as 1-benzyl-5-iodoindoline-2,3-dione (1d) and 1-benzyl-5-bromoindoline-2,3-dione (1e)] and the corresponding coupling partner (boronic acid, alkyne, alkene or stannane) in four different protocols. For the Suzuki reaction, standard conditions were explored using inorganic bases such as sodium carbonate in hydroalcoholic mixtures. The best results were achieved using 3 equiv of base (Na2CO3) and iPrOH/H2O (2:1 ratio) as a solvent mixture, under microwave heating, 200 W at 120 °C, for 20 min, rendering almost quantitative yields. No collateral products were detected during the reactions. Therefore, the selectivity towards the desired products 2a-e was excellent. As can be observed in Fig. 3d, reaction products frequently crystallize in the microwave vial when the mixture has cooled. The monolith is easily removable from the microwave reactor for reuse after washing and sonication. It is important to point out that during the optimization process of the Suzuki reaction conditions, the presence of two phases is a critical issue for the efficiency of the catalyst. The correct dissolution of all the reagents in a suitable hydroalcoholic phase is required. In this sense, the iPrOH/H2O mixture proved to be effective for these transformations. iPrOH is stable under normal conditions of use and is completely soluble in water.The TOF values (Table 1) were calculated using the formula: TOF = mol product / [time (min) × mol catalyst] TOF values range from 100 to 450h-1, suitable for a sintered catalyst and comparable to other types of palladium-based powdery catalysts. The reactivity of the CERMET catalyst under conventional heating conditions is quite similar, although the final yield of the reactions decreases somewhat, especially the bromo-derivatives. Reaction times are longer too. Other "pseudo heterogeneous” powdery catalysts such as palladium on charcoal give a similar result under conventional heating conditions but are effective for a limited number of cycles. Therefore, under microwave heating conditions, the cermet behaved with good performance and, what is not less important: safe reaction conditions, without risk of explosion due to the presence of palladium inside the reactor. Therefore, this catalyst could be used in even more challenging reactions in which MAOS is especially required.In Sonogashira reactions, the best conditions for the copper-free coupling between iodoarene or haloisatin derivatives with simple terminal alkynes were studied. The best results were obtained using triethylamine (TEA) as base and iPrOH/H2O as solvents (200 W, 120 °C). As shown in Table 1, the reactions were performed in 20 min and with high yields. Finally, Heck and Stille reactions were carried out using acetonitrile (MeCN) as a suitable solvent for these transformations, with short reaction times, at 120 °C. Organostannanes react efficiently using standard conditions (20W, 120 °C), in 10–30 min. In the case of Heck reactions, an increase in power (300W) was necessary to complete the reactions. The catalyst can carry out every reaction described here without the need to incorporate the magnetic stirrer into the microwave reactor. In this way, the reactants flow through the catalyst channels through the simple action of microwave energy (Fig. 3b). The work-up is very simple since no filtration process is necessary. The monolithic catalysts prepared and used in this work were reused, indistinctly in the four procedures, for hundreds of times without appreciable loss of catalytic activity. In addition, the catalyst was completely safe under microwave heating conditions. No sparkling or arcing phenomena were detected during these experiments. Fig. 3a shows the recyclability diagram for the first six cycles for each transformation, taking as model reactions those aimed at obtaining compounds 2a (Suzuki or Stille), 2e (Sonogashira), or 2h (Heck). This fact, together with the strong mechanical resistance offered by the catalyst (without fractures, scratches, breaks or surface poisoning) define this cermet system as a true “long-life catalytic material”.To determine if the monolithic catalyst works through a true heterogeneous catalysis, several studies were carried out. On the one hand, the Inductive Coupling Plasma (ICP/OES) results, measured in the reaction crudes (Suzuki, Sonogashira, Heck or Stille), showed that the Pd concentration in the reaction solution was less than the detection limit (i.e., 50 ppb) which corresponds to less than 0.02% of the starting Pd-amount (see S4 supplementary material). Secondly, the overwhelming reusability discussed above (each monolithic catalyst can work for hundreds of times in different reaction solvents) shown by the catalyst demonstrates the excellent possible applicability of this device in parallel drug synthesis (in which the presence of traces of metal is absolutely avoided). Third, the graph corresponding to the CO chemisorption made after carrying out numerous catalysis experiments was identical to the initial one (see S3 supplementary information), so it can be considered that the dispersion of the metal on the surface remains constant after numerous reaction cycles. Finally, hot filtration tests (HFT) (see S4, supplementary material) were performed for the four methods. Every experiment showed the absence of catalysis when the monolith is removed from the reaction mixture. The results of these tests indicated that the metal leaching is negligible or undetectable under the applied reaction conditions.In addition to the factors related to chemical resistance discussed above, it is worth highlighting the great mechanical robustness, demonstrated by the total absence of fractures on the surface after 200 reaction cycles (see tables S3, supplementary material). It is important to point out that, although the exact determination of the mechanical properties of the monolith is not an objective for this work, we have compared the robustness of the Pd0/Al2O3 cermet catalyst with other previously reported catalysts38-40 that contain palladium on the monolith surface but do not present palladium content throughout its internal structure. This comparison has been made based on the presence of fractures on the catalyst surface and the maximum number of times each monolith can be reused in solution phase cross coupling reactions. These studies indicate that cermet is much more resistant, robust and durable than catalysts that do not have internal metal content, due to a much lower degree of internal crystallinity than monoliths internally composed of pure Al2O3 or SiO2. (see table S3 in supplementary material).In summary, we have developed a strategy for the fabrication of an efficient and robust Pd0/Al2O3 cermet catalyst for use in safe Suzuki, Sonogashira, Stille and Heck protocols in microwave assisted cross-coupling reactions. Combining 3D printing technology and thermal annealing, it is possible to obtain a monolithic catalyst with open square pores, high geometric surface area of 33.2 cm2/cm3, uniform Pd loading after sintering of 1.66 wt% and increased robustness related to pure ceramic monoliths. EDS and mapping analysis confirmed the homogeneous distribution of Pd metal on the structure surface. Due to the synergistic effects of the metal-oxide interfaces, the monolithic catalyst is highly efficient and extremely robust in organic mixtures with aqueous solvents and high temperatures and Pd leaching was completely undetected by HFT or ICP/OES. The versatility of our fabrication approach provides an efficient strategy for the development of safe microwave-assisted cross-coupling reactions in the presence of a metal catalyst. In addition, the immobilized Pd catalyst could be easily washed, reused and applied safely as a “long-life catalytic device” in 200 different cross coupling reaction experiments without arcing phenomena, catalyst deactivation, surface poisoning or structural damage.The corresponding author is responsible for ensuring that the descriptions are accurate and agreed by all authors.Carmen R. Tubio: Investigation, Writing- Original draft preparation. Camilla Malatini: Investigation. Laura Barrio: Investigation. Christian F. Masaguer: Supervision. Manuel Amorín: Data Curation Management. Walid Nabgan: Data Curation Management. Pablo Taboada: Resources, Data Curation Management. Francisco Guitián: Funding acquisition, Resources. Alvaro Gil: Conceptualization Ideas, Supervision. Alberto Coelho: Conceptualization, Management and coordination responsibility for the research activity planning and execution, Writing- Original draft preparation, Writing- Reviewing and Editing, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was financially supported by the Consellería de Cultura, Educación e Ordenación Universitaria of the Galician Government: EM2014/022 to A.C., ED431B2016/028 to F.G. The Strategic Grouping AEMAT grant No. ED431E2018/08 and the Spanish Ministry of Science, Innovation and Universities with grant No: MAT2017-90100-C2-1-P "MA thanks Xunta de Galicia and the ERDF (ED431C 2021/21)".The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtchem.2022.101355.

A straightforward manufacture strategy is proposed to obtain an efficient and robust palladium-alumina (Pd0/Al2O3) cermet monolithic catalyst, specifically designed to perform safe microwave assisted organic synthesis (MAOS). In this approach, a cermet catalyst with high surface area, controlled composition and adapted shape and dimensions to a microwave reactor vessel is generated via 3D printing technology and sintering. The resulting catalyst has been explored in heterogeneous Suzuki, Sonogashira, Stille and Heck cross-coupling reactions, in MAOS. The Pd0 catalyst is permanently active, stable, without leaching and can be recycled and reused at least 200 reaction cycles. The generation of hot spots, sparking or hazardous discharges is controlled by the effective immobilization of the palladium in the monolithic structure during the reaction. The palladium content is forming part of both the internal and external structure, providing greater mechanical resistance and catalytic activity with respect to the basic ceramic material (alumina).

Niobia (Nb2O5) has attracted a great deal of attention for catalytic applications in aqueous media owing to its water-tolerant Lewis acidity (Barrios et al., 2017; Brayner and Bozon-Verduraz, 2003; Chan et al., 2017; Chary et al., 2003; Francisco et al., 2004; Graça et al., 2013; Guan et al., 2017; Herval et al., 2015; Holtzberg et al., 1957; Jasik et al., 2005; Ko et al., 1984; Lopes et al., 2014; Nakajima et al., 2013; Rojas et al., 2013; Solcova et al., 1993; Tanabe and Okazaki, 1995; Valencia-Balvín et al., 2014; Wojcieszak et al., 2006). Generally, amorphous Nb2O5 shows high surface acidity, which is related to its high specific surface area and number of surface defects (do Prado and Oliveira, 2017; Ziolek and Sobczak, 2017). However, amorphous Nb2O5 is a fragile material, susceptible to changes by temperature and pressure (Pinto et al., 2017; Wojcieszak et al., 2006). Considering Nb2O5 crystalline phases, they are formed by distorted octahedra (NbO6), connected by edges and corners. The distortion degree of NbO6 octahedra depends on the polymorph structure (Nico et al., 2016; Pinto et al., 2017; Valencia-Balvín et al., 2014). This distortion leads to varied textural and structural stabilities as well as different surface acid properties and, therefore, impacts on catalytic properties. H-Nb2O5 (monoclinic structure) and T-Nb2O5 (orthorhombic structure) are the most common crystalline phases, whereas the TT-Nb2O5 (pseudohexagonal structure) is the least thermodynamically stable phase and is often considered as a less ordered form of the T-phase. By increasing the temperature and pressure in hydrothermal synthesis, the conversion of Nb2O5 phases takes place following the sequence: amorphous Nb2O5 → TT-Nb2O5 → T-Nb2O5 → H-Nb2O5 (Nowak and Ziolek, 1999; Pinto et al., 2017; Valencia-Balvín et al., 2014). Nb2O5 phase transitions are typically followed by a progressive decrease in the surface area, porosity, and acidity (Ali et al., 2017; Graça et al., 2013; Kreissl et al., 2017; Pinto et al., 2017; Raba et al., 2016; Valencia-Balvín et al., 2014). Among the crystalline phases, the TT-Nb2O5 phase is the one presenting the highest number of oxygen vacancies in the structure, and so the greatest degree of polyhedral distortion (Pinto et al., 2017; Rani et al., 2014). TT-Nb2O5 is characterized by the presence of distorted octahedra and pentagonal and hexagonal bipyramids, i.e., NbO6, NbO7, and NbO8 polyhedra, which are the structural units also present in amorphous Nb2O5 (Nakajima et al., 2011; Nico et al., 2016). Notably, TT-Nb2O5 structural features translate into a highly polarized and disordered surface with high levels of Lewis and Brønsted acid sites, which are essential to the high performance of hydrodeoxygenation (HDO) catalysts.Metal-based (mainly Pt, Pd, Ru, Ni) catalysts supported on acidic materials have been widely examined in the HDO of lignin model compounds (Cui et al., 2017; Shao et al., 2017; Teles et al., 2018; Wang and Rinaldi, 2016; Zhao et al., 2009). For the HDO of lignin model compounds and lignin streams, niobium oxides have been studied as supports for noble metals (Shao et al., 2017). Studies on multifunctional Fe3O4/Nb2O5/Co/Re catalysts have also been reported (Parvulescu et al., 2017). Pd catalysts supported on niobia revealed promising results for the dehydroxylation of phenol to benzene, presenting a reaction rate 90-fold higher than that observed for a Pd/SiO2 catalyst (Barrios et al., 2017). Importantly, Pt/Nb2O5-Al2O3 has been reported as an active catalyst for the hydrotreating of diphenyl ether, showing stability higher than that of Pt/Al2O3 owing to the water-tolerant nature of niobium(V) Lewis acid sites (Jeon et al., 2018). Subjecting lignin-derived dimers to a Ni0.92Nb0.08 catalyst resulted in full conversion of the substrates into liquid alkanes at 200°C after 2 h, demonstrating the outstanding ability of this material for C–O cleavage and HDO (Jin et al., 2017). For the selective production of arenes from lignin, it was reported that Ru-Nb2O5 catalysts present unique catalytic properties, compared with Ru supported on traditional oxide supports (Shao et al., 2017).In an approach for lignin-to-liquid fuels, one of the challenges is to design inexpensive catalysts with high activity, selectivity, and stability under process conditions. Since the hydrotreating of lignin streams releases water, the solid catalyst must be stable in the presence of water under high-severity conditions. Commercial niobia (Nb2O5⋅xH2O) is a bulk amorphous material that lacks stability under hydrothermal conditions, thus losing surface area and leading to the sintering of supported metallic particles (Pham et al., 2011). To overcome the poor structural stability of commercial Nb2O5, various synthesis methods have been a subject of research in producing highly stable nanostructured materials (Zhao et al., 2012b). Nb2O5 nanoparticles with no defined shape can be obtained by precipitation and sol-gel synthesis methods followed by calcination. These routes have extensively been studied in the preparation of the Nb2O5 supports applied to the HDO of lignin and lignin-derived molecules with good results (Shao et al., 2017). Nb2O5 crystallization under low-severity solvothermal conditions constitutes another progress in this field. This synthetic route produces single TT-Nb2O5 nanorods with controlled size and morphology, high surface area, and improved acid properties (Ali et al., 2017; Leite et al., 2006; Zhou et al., 2008). TT-Nb2O5 nanorods exhibit shape-dependent acidic sites (Zhao et al., 2012a). On (001) TT-Nb2O5 surface of the nanorods, Lewis acid sites are much stronger than those of spherical Nb2O5 particles. Despite the interesting acidic properties, the production of Nb2O5 nanorods employs oleic acid and trioctylamine as structure-directing agents in the solvothermal synthesis. Especially for catalytic applications, the use of such structure-directing agents surfactants in the synthesis of Nb2O5 presents disadvantages owing to their high costs, low volume of material production limitation, and the need to remove the agents via calcination, which may modify the morphology, particle size, and surface chemistry of Nb2O5 (Ali et al., 2017; Zhao et al., 2012a).Hydrothermal synthesis of TT-Nb2O5 nanorods in the presence of H2O2 represents a route receiving far less attention, but with the most promising results regarding the textural and acidic properties of niobia (Leal et al., 2019). Despite the improved chemical and physical properties, such Nb2O5 nanorods have not yet been explored in the chemistry of lignin hydrotreating. Therefore, this knowledge gap brought us to the study of nickel supported on hydrothermally stable TT-Nb2O5 nanorods as a potential catalyst for HDO of lignin streams. As about 80% of the primary interunit linkages of lignin are ether bonds (Rinaldi et al., 2016), and a considerable number of other oxygenated functionalities are present in lignin-derived phenolics, a highly stable and highly acidic niobia could well hold the key to produce efficient catalysts for lignin-to-liquid fuels, owing to an expected synergism between metal phase and support toward lignin depolymerization and acid-catalyzed deoxygenation of intermediates formed throughout the HDO course (Cao et al., 2018; Wang and Rinaldi, 2016, 2013).In this report, we examine the catalytic properties of Ni-supported on TT-Nb2O5 nanorods for the hydrotreating of a model compound (diphenyl ether) and lignin oil produced by a lignin-first biorefining process based on H-transfer reductive processes, the so-called catalytic upstream biorefining (CUB), which is also denoted as ‘reductive catalytic fractionation’ (RCF) by several research groups. CUB constitutes a class of methods for deconstruction of lignocellulose that renders high-quality pulps together with depolymerized and passivated lignin streams (Ferrini and Rinaldi, 2014; Galkin and Samec, 2016; Graça et al., 2018; Renders et al., 2017; Rinaldi, 2017; Schutyser et al., 2018; Sultan et al., 2019; Rinaldi et al., 2019). TT-Nb2O5 nanorods were prepared via hydrothermal synthesis by employing ammonium niobium oxalate and H2O2 as the structure-directing agent (Leal et al., 2019; Leite et al., 2006; Pavia et al., 2010). TT-Nb2O5 nanorods were then loaded with several Ni contents. In this report, the results and discussion are organized as follows. First, the characterizations of the as-synthesized TT-Nb2O5 nanorods and Ni/Nb2O5 catalysts are briefly presented. Ni/Nb2O5 catalysts are subsequently applied to the HDO of diphenyl ether at 160°C and 200°C under 4 MPa H2. The catalyst performance and stability in the HDO of diphenyl ether at 200°C under 4 MPa H2 were assessed. Finally, under more severe conditions (300°C and 7 MPa H2), the 15%Ni/Nb2O5 catalyst was applied to the hydrotreating of the lignin oil. Figure 1 shows the X-ray diffraction (XRD) patterns obtained from both the hydrothermally as-synthesized Nb2O5 after calcination at 380°C and Ni/Nb2O5 catalysts reduced at 320°C. XRD pattern of the Nb2O5 support exhibits peaks characteristic of the pseudohexagonal TT-Nb2O5 phase (Ko and Weissman, 1990). Notably, a high-intensity signal is observed at 22.8°, which is associated with (001) reflection of TT-Nb2O5. In addition, a low-intensity and broad signal related to the (100) plane appears at 28.0°. As next confirmed by scanning transmission electron microscopic (STEM) imaging, a preferred growth of TT-Nb2O5 along the (001) direction creates the preferential orientation feature in the XRD pattern, indicating the formation of TT-Nb2O5 as nanorods (Ali et al., 2017). For the Ni/Nb2O5 materials, the XRD patterns demonstrated that the structural features of the TT-Nb2O5 phase were preserved after the reduction procedure. Hence, for simplicity, when referring to the materials produced in this study, the TT-Nb2O5 phase will be denoted as “Nb2O5” henceforth. As expected, Ni(111) and Ni (200) reflections are observed at 44.6° and 52.1°. These reflections become more intense and sharper with an increase in Ni content from 5 to 25 wt %. Considering the Ni(111) reflection, the average Ni crystallite sizes estimated by the Scherrer equation grow from 7 to 15 nm with the rise in Ni content from 5 to 25 wt % (Table 1 ).To verify whether the synthesis rendered Nb2O5 nanorods, the Nb2O5 material was examined by using high-angle annular dark-field (HAADF)-STEM (Figure 2 ). The HAADF-STEM images show that the hydrothermal synthesis produced Nb2O5 nanorods with approximately 8–25 nm length and 3–4 nm width (Leal et al., 2019). The nanorod dimensions are in line with those of previous studies showing that the crystal growth in the hydrothermal method follows an oriented attachment mechanism (Leite et al., 2006), producing nanorods smaller than those synthesized in the presence of a surfactant as a shape-directing agent. In fact, surfactant-based syntheses yield large particles, owing to a decrease in the rate of crystal growth (Zhao et al., 2012a). In turn, longer (200–500 nm) and thinner (5–20 nm) TT-Nb2O2 nanorods were produced by a synthesis employing oleic acid as a structure-directing agent and ammonium niobium oxalate hydrate as the starting material (Zhao et al., 2012b). Table 1 summarizes the textural properties of the support and Ni/Nb2O5 materials. N2 adsorption-desorption isotherms are presented in Figure S1. Nb2O5 and Ni/Nb2O5 materials exhibit a type II isotherm with an H3 hysteresis (Thommes et al., 2015), corroborating the non-structural porosity created by the packing of Nb2O5 nanorods. Niobium oxide nanoparticles can be obtained by various synthesis methods, which leads to the preparation of materials of different shapes with Brunauer-Emmett-Teller (BET) specific surface areas that can range from about 20 m2 g−1 to 530 m2 g−1(Luisa Marin et al., 2014; Morais et al., 2017; Shao et al., 2017). Table 1 shows the as-synthesized Nb2O5 support to possess a relatively high specific surface area (196 m2 g−1). Notably, no significant decrease in the surface area of Ni/Nb2O5 materials, with 5–15 wt % Ni loading on Nb2O5 was observed. Likewise, as the porosity of the Nb2O5 support is non-structural, the deposition of Ni phase onto the nest of nanorods does not significantly decrease the specific surface area. However, for 25%Ni/Nb2O5, the specific surface area slightly decreased (from 196 m2 g−1 to 141 m2 g−1).Temperature-programmed reduction (TPR) profiles of the Nb2O5 nanorods and Ni/Nb2O5 precursors (catalysts before reduction) are shown in Figure 3 . The TPR profiles of Ni/Nb2O5 precursors exhibit two reduction events. The first occurs at around 335°C. This event is assigned to the reduction of Ni(II) to Ni(0). The reduction temperature of the Ni(II) species supported on Nb2O5 nanorods is much lower than that of bulk NiO (450°C) (Graça et al., 2014). Nevertheless, the range of Ni reduction temperatures between 320°C and 345°C is in line with previous studies on Ni/Nb2O5 materials containing high nickel loadings (Janković et al., 2008; Liu et al., 2016). Interestingly, and contrary to what has been previously observed by other research groups (Chary et al., 2003; Wojcieszak et al., 2006), no shift of the Ni reduction peak to higher temperatures with an increase in Ni loading was observed. As will be presented later, in the 15%Ni/Nb2O5 material, the Ni nanoparticles are embedded in a nest formed by Nb2O5 nanorods (Figure 11). Thereby, such an entanglement of metal phase and oxidic support may result in few (but strong) connecting points between these phases so that an effect of the increase in the Ni loading on the reduction temperature of NiO species is not apparently observed. However, in the second reduction (at about 800°C), which is related to the partial reduction of Nb2O5 to NbO2 (Wojcieszak et al., 2006), the temperature required for the reduction of Nb2O5 progressively decreases (from 870°C to 816°C) with increasing Ni content (Table S1). This observation indicates that there is an interaction between Ni and NbO5 phases in which the hydrogen spillover appears to be more prevalent for the samples containing higher Ni loadings.Nb2O5 polymorph crystals are formed by distorted octahedra (NbO6) connected by edges and corners, the degree of distortion depending on the polymorph structure (Nico et al., 2016; Pinto et al., 2017; Valencia-Balvín et al., 2014). In the TT-Nb2O phase, the highly distorted octahedra (NbO6) units exhibit Nb=O bonds, enabling the Nb(V) center to act as a Lewis acid site. In turn, the slighted distorted NbO6, as well as NbO7 and NbO8 groups, only present Nb-O bonds, which provide scaffolding for the [Nb(V)---OH2 2+] Brønsted acid sites (Chan et al., 2017). In this study, we found the quantity of Lewis acid sites on Nb2O5 (210 μmol g−1) to be higher than that of Brønsted acid sites (143 μmol g−1, Figure S2 and Table S2).To assess the nature of the acidic sites of Nb2O5 support and Ni/Nb2O5 catalysts (after the reduction procedure), attenuated total reflection (ATR)-Fourier transform infrared (FTIR) spectra of adsorbed pyridine (Py) were collected (Figure 4 ). We chose to assess the nature of acid sites by ATR technique because of the dark color of the activated Ni/Nb2O5, which hinders FTIR transmission experiments (as those performed on the Nb2O5 support). Pyridine adsorbed on the materials exhibits infrared (IR) bands at around 1,446 cm−1 and 1,606 cm−1. These bands are related to the Py coordinated to Lewis acid sites. In addition, the IR spectra show bands at 1,639 cm−1 and 1,540 cm−1, which are assigned to the formation of the pyridinium ion (PyH+) on Brønsted acid sites (Figure 4) (Datka, 1992; Dollish et al., 1974; Iizuka et al., 1983; Parry, 1963). An IR band of adsorbed Py common to both Lewis and Brønsted acid sites is also visible at 1,489 cm−1. These observations indicate that both Lewis and Brønsted acid sites are present in the Nb2O5 nanorods. From the relative intensities of Py adsorbed on Lewis and Brønsted acid sites, it can be inferred that the Brønsted acidity decreases as the Ni loading increases, as indicated by the reduction in the intensities of the bands at 1,639 cm−1 and 1,540 cm−1. This finding is explained by the ion exchange of Brønsted acid sites by the positively charged Ni species. However, in the 25%Ni/Nb2O5 catalyst, the support still presents some residual Brønsted acidity. Notably, the Lewis acidity (bands at around 1,606 cm−1 and 1,446 cm−1) appears to be mostly preserved even at such a high loading of Ni on Nb2O5 nanorods.Acid supports are active in the dehydration of cyclohexanol to cyclohexene and, therefore, are vital to the hydrotreating of lignin to alkanes and arenes (Wang and Rinaldi, 2016; Zhao et al., 2010, 2009). Hence, to further assess the effect of Ni loading on the acidic properties of the catalysts, cyclohexanol dehydration was carried out at 200°C. As will be presented in the next section, this reaction is key to produce cyclohexane from the conversion of diphenyl ether, as well as to obtain cycloalkanes from lignin. Table 2 summarizes the cyclohexanol conversion values obtained after 30 min of reaction at 200°C.As expected, for the Nb2O5 nanorods, cyclohexanol conversion is considerably high. However, cyclohexanol conversion significantly decreases in the presence of 5%Ni/Ni2O5 catalyst. By increasing Ni loadings, cyclohexanol conversion continually drops, plateauing at 31% for 25%Ni/Nb2O5. These results show that the activity of the catalysts is partially affected by Ni deposition. Overall, the results presented in Table 2 indicate that the decrease in Brønsted acidity is detrimental to the dehydration performance. These results confirm that, at promoting the dehydration of cyclohexanol, Brønsted acid sites are more active for alcohol dehydration than the Lewis acid sites (Foo et al., 2014).The catalytic performance of the Ni/Nb2O5 catalysts was evaluated for the conversion of diphenyl ether as a model reaction. The cleavage of diphenyl ether serves as a model reaction for the breakdown of 4-O-5 ether linkages occurring in lignins. Owing to its high bond dissociation enthalpy (BDE: 330 kJ mol−1), the 4-O-5 linkages are resistant against cleavage via non-catalytic thermal processes, compared with α-O-4 and β-O-4 ether linkages occurring both in native and technical lignins (BDE: 215 kJ mol−1 for α-O-4 in phenylcoumaran subunits, and 290–305 kJ mol−1 for β-O-4 in lignin's aryl alkyl ether-bonding motifs) (Dorrestijn et al., 2000; Parthasarathi et al., 2011; Rinaldi et al., 2016; Wang and Rinaldi, 2012; Younker et al., 2011). Therefore, the ability of a Ni catalyst for hydrogenolysis can be evaluated with little contribution of thermolysis to the overall reaction results. In this instance, diphenyl ether is also a useful model compound for another reason. It allows for the evaluation of the activity of the Ni phase toward hydrogenation of phenol and benzene, the intermediates formed by the hydrogenolysis of diphenyl ether. In the presence of acid sites, the intermediate mixture is ultimately funneled to cyclohexane, as schematically represented by the reaction network presented in Figure 5 .To investigate the different catalyst functionalities, the hydrotreating of diphenyl ether was carried out at two temperatures, 160°C and 200°C. These two conditions were chosen because dehydration of alcohols has significant enthalpic barriers for the formation of carbocations (Liu et al., 2017), meaning that relatively high temperatures are required for the alcohol dehydration. By this choice, the hydrogenolysis and hydrogenation extents can be better discerned in the experiments carried out at 160°C, whereas the performance for the full HDO of diphenyl ether is better addressed by the experiments performed at 200°C.When targeting cycloalkanes, the results of a model compound reaction can be more conveniently compared by computing the HDO extent and degree of deoxygenation (DOD), as given by Equations 1 and 2, respectively (Rinaldi, 2015). (Equation 1) H D O e x t e n t = H 2 i n c o r p o r a t e d i n t h e p r o d u c t s H 2 f o r c o m p l e t e c o n v e r s i o n t o c y c l o h e x a n e × 100 (Equation 2) D O D = ( 1 − w t % O i n p r o d u c t w t % O i n f e e d ) × 100 Figure 6A compares the performance of the Ni/Nb2O5 by evaluating the HDO extent achieved by the reaction network as a function of time for the conversion of diphenyl ether at 160°C. As expected, an increase in HDO extent with time for all tested catalysts was observed. By analyzing the results obtained at 180 min (Figure 6B), the HDO extent increased linearly from 28% to 46% with the rise in the Ni content (from 5 to 25 wt %). On the other hand, as expected at this temperature, low DOD was obtained for all catalysts at 180 min, with a decrease being in general noticed with the increase in Ni content, owing to the decline in the Brønsted acidity, as previously discussed. In these experiments, conversions of diphenyl ether in the range of 55%–98% at 180 min were achieved (Figure 6B). A blank test and a catalytic run with the pure Nb2O5 were also carried out (Table S3). By stark contrast, in these control experiments, only very low conversion of diphenyl ether (8% and 15%, respectively) was achieved at 180 min, with no selectivity to a specific product. Furthermore, to verify whether there is a contribution of the leached species to the reactions, the catalyst 15%Ni/Nb2O5 was contacted with the solvent under the same conditions of the reaction. After this, the catalyst was separated from the liquid product, and then diphenyl ether and the internal standard were added to the reaction media for reaction run. The results were similar to those of the blank reaction, confirming that the catalytic process is exclusively taking place on the catalyst surface.To examine in more detail the results of the experiments conducted at 160°C, the product distribution at a similar conversion level of about 50%–60% was analyzed (Table 3 ). Two critical ratios of products' groups were considered. The ratio Σ(4–7)/Σ(2,3) was used to define the selectivity to monocyclic products produced by the cleavage of the C–O ether bond. The ratio Σ(5,7)/Σ(4,6) indicates the selectivity to HDO after ether bond cleavage, which reflects the ability of the catalyst to execute the following reaction sequence: phenol → cyclohexanol → cyclohexene → cyclohexane. Evolution of the product selectivity with time at 160°C is given in Figure S3. Table 3 shows that diphenyl ether was converted into three main products: cyclohexyl phenyl ether (22%–25%), cyclohexanol (29%–36%), and cyclohexane (27%–38%). Small quantities of dicyclohexyl ether, phenol, and benzene were also found in the reaction mixture (individual selectivity values lower than 11%). Σ(4–7)/Σ(2,3) ratio higher than 1 was observed for all the Ni/Nb2O5 catalysts. This observation indicates the formation of monocyclic products to prevail over the partial or full saturation of diphenyl ether. The latter renders the bicyclic products 2 and 3, respectively. With the rise in Ni content in Ni/Nb2O5 catalysts (from 5 to 25 wt %), a gradual reduction in the Σ(4–7)/Σ(2,3) ratio (from 2.57 to 2.03) was observed. Taking the results from the experiment carried out in the presence of 25 wt % Ni/Nb2O5 catalyst into account, the reduction in the Σ(4–7)/Σ(2,3) ratio is related to the accumulation of dicyclohexyl ether in the reaction mixture. As previously reported, dialkyl ethers are not prone to undergo hydrogenolysis in the presence of Ni catalysts under relatively mild reaction conditions (Wang and Rinaldi, 2016; Zhao et al., 2012a). Should a dialkyl ether be cleaved, the reaction pathway would begin with a hydrolysis step instead (Figure 5). However, in this study, the formation of dicyclohexyl ether constitutes a dead end, as its conversion was not observed. Confirming this, we could successfully employ n-dibutyl ether in the reaction mixtures as an internal standard for gas chromatography (GC) analysis. Likewise, no decomposition of the internal standard was detected.The results in Table 3 also shows a rise in the selectivity to cyclohexanol (from 6% to 11%) for the experiment carried out in the presence of 25 wt % Ni/Nb2O5 catalyst. This outcome agrees with the decrease in Brønsted acidity at a high Ni content supported on Nb2O5, as verified by ATR-IR spectra of pyridine adsorbed on the reduced Ni/Nb2O5 catalysts (Figure 4) and model reaction experiments (dehydration of cyclohexanol, Table 2). Therefore, the increase in Ni loading on Nb2O5 has implications for both the accumulation of dicyclohexyl ether (i.e., raises the likelihood of full saturation of diphenyl ether to dicyclohexyl ether) and of cyclohexanol (i.e., lessens the extent of dehydration of cyclohexanol).The product distributions in Table 3 show similar values of selectivity to cyclohexanol and cyclohexane, revealing that Nb2O5 plays a marginal role in the HDO extent at 160°C. Under these conditions, low DOD values (5%–12%) were achieved. The catalyst's ability to dehydrate cyclohexanol significantly reduces with the rise in Ni content, as indicated by the decrease in the Σ(5,7)/Σ(4,6) ratio from 1.40 to 0.81. The decrease in the dehydration capability is correlated with the decrease in the number of Brønsted acid sites with the increase in Ni content, as discussed in the previous section.According to the results from Table 3, 10%Ni/Nb2O5 and 15%Ni/Nb2O5 catalysts present the best balance between HDO extent and selectivity to monocyclic deoxygenated products. These catalysts were thus chosen for the conversion of diphenyl ether carried out at 200°C. Figure 7 shows the monitoring of the reaction mixture components over time. For both experiments, full conversion was achieved at 180 min. Cyclohexane was the main product obtained, with selectivity values of 81% and 88% for the 10%Ni/Nb2O5 and 15%Ni/Nb2O catalysts, respectively. These results confirm that the dehydration of cyclohexanol is encouraged at 200°C. At 180 min, the HDO extent and DOD were both greater for 15%Ni/Nb2O5 (HDO extent: 91%; DOD: 85%) than for the 10%Ni/Nb2O5 catalyst (HDO extent: 82%; DOD: 72%). Based on these results, 15%Ni/Nb2O5 catalyst was considered as the most efficient. Thus the 15%Ni/Nb2O5 catalyst was selected for the recycling experiments and studies on the conversion of lignin oil.The catalytic performance after five reaction cycles was investigated for the HDO of diphenyl ether at 200°C for 240 min using the 15%Ni/Nb2O5 catalyst. Again, diphenyl ether serves as a model compound because, when targeting the full HDO of lignin streams, the accumulation of cyclohexanol (derived from hydrogenation of phenol) indicates a decay of the initial acidic properties of a bifunctional catalyst. After each reaction run, the catalyst was washed with solvent and reused in the following reaction run. Figure 8 displays the conversion and product distribution after each reaction run. Figure 8 shows that the catalyst presents a sustained performance, still producing a 91% yield of cyclohexane after five reaction runs. A slight decrease in the cyclohexane selectivity is, however, observed from the second to fourth reaction runs, with the formation of cyclohexanol and dicyclohexyl ether (around 4%–5% each) from the third cycle on. These results translate into a slight decrease in both HDO extent (from 100% to 92%) and DOD (from 100% to 94%) throughout the recycling experiments.To examine surface, structural, and morphological alterations occurring in the 15%Ni/Nb2O5 catalyst, the fresh and spent catalysts were analyzed by using a set of techniques (pyridine adsorption, XRD, and HAADF-STEM). Figure 9 shows the ATR spectra of adsorbed pyridine on the fresh catalysts and spent samples after five reaction runs. The data indicates that the population of Brønsted acid sites dramatically decreased after five successive reuses of the catalyst. This means that even though water is generated during the reaction, no regeneration of Brønsted acidity takes place in the process. In this context, the accumulation of cyclohexanol appears to be related to a decrease in the population of Brønsted acid sites. On the other hand, Lewis acidity is preserved, which explains the sustained high selectivity to cyclohexane at 200°C, demonstrating that Nb2O5 Lewis acid sites are stable under the reaction conditions. Moreover, a modest decrease in BET surface area was observed (fresh catalyst: 180 m2 g−1 versus spent catalyst: 144 m2 g−1). The XRD pattern of the used catalyst (Figure 10 ) also shows that the crystalline structure of the catalyst is maintained after five reaction runs. Also, no significant change in the Ni average crystallite size occurred after five reaction runs (fresh catalyst: 14 nm versus spent catalyst: 15 nm). Finally, the comparison of STEM-HAADF images (Figure 11 ) indicates that the spent catalyst after the fifth reaction run maintains the original features of the fresh catalyst, that is, the arrangement and size distribution of Ni nanoparticles entangled in the Nb2O5 nanorod nest remained, to a great extent, unaltered. Overall, these observations together with the sustained catalytic performance of the 15%Ni/Nb2O5 indicated that this material holds potential as a robust and active catalyst for the conversion of phenolic streams derived from lignin.To explore the potential of 15%Ni/Nb2O5 catalyst in the conversion of lignin oil, lignin oil was subjected to hydrotreatment under an H2 pressure of 7 MPa (measured at room temperature) at 300°C for 16 h. We chose to increase the reaction temperature from 200°C (as for the model compound experiments) to 300°C to encourage extensive HDO of lignin oil to cycloalkanes, leading to full conversion of lignin into products soluble in n-pentane (reaction solvent), thus avoiding the accumulation of lignin residues throughout the catalyst recycling experiments (Wang and Rinaldi, 2012). However, even under harsh conditions, an appreciable amount of a residue insoluble in n-pentane or even methanol (a good solvent for lignin oil species) was formed and, thus, accumulated with the catalyst. Thereby, in this study, the “conversion of lignin oil” is estimated as a “net conversion,” which takes into account the weight of residue insoluble in either n-pentane or methanol formed in each reaction run (in conjunction with the initial weight of fresh catalyst) and the amount of lignin oil added in each reaction run. For the catalyst recycling experiments, the spent catalyst was washed with methanol to extract soluble residue species. The spent catalyst containing lignin-derived residues insoluble in methanol was then recovered by filtration and dried at 40°C in a vacuum oven. The liquid products and the fraction of lignin residues soluble in methanol were characterized by elemental analysis, gas chromatography (GC)-flame ionization detector (FID)/mass spectrometry (MS), and gel permeation chromatography (GPC).Throughout the catalyst recycling experiment, which processed in total ca. 6.0 g of lignin oil, the amount of lignin-derived residue increased from ca. 0.23 g (first run) to 0.30–0.31 g (second or third run, Table 4 ). Logically, the accumulation of the lignin-derived residue impedes the precise determination of the initial quantity of substrate present in the second and third reaction runs, as it is not possible to discern whether a part of the lignin-derived residue was also consumed throughout the recycling experiment and replaced with a fresh, more oxygenated carbonaceous residue derived from the fresh substrate. Further exploration of the data listed in Table 4, that is, the determination of weight ratio of liquid-product-to-residue, should be carried out with caution. A mass ratio of liquid-product-to-residue is only meaningful if both liquid product and residue present similar values of O/C and H/C ratios, which is not the case when the catalyst loses part of its performance in the recycling experiments. Figure 12 summarizes in a van Krevelen diagram the results obtained from the control experiment and catalyst recycling in the hydrotreating of lignin oil. In the absence of the catalyst, a 68% conversion of the lignin oil was achieved by thermal processes (Table 4), increasing the H/C ratio from 1.51 ± 0.01, for the lignin oil, to 1.74 ± 0.02 for the liquid fraction. Conversely, the O/C ratio decreased from 0.46 ± 0.01, for the lignin oil, to 0.23 ± 0.01 for the produced liquid fraction. This decrease in O/C ratio is associated with the elimination of the γ-OH group of p-dihydrolignols, among other thermal processes, leading to deoxygenation (Table S4). The solid residue exhibited an H/C ratio of 1.45 ± 0.03 and an O/C ratio of 0.34 ± 0.01. These results indicate that the residue no longer corresponds to the initial lignin stream.In the catalytic experiments, an 89% conversion of lignin in the first reaction run was achieved. For the liquid product obtained from the first reaction run, a substantial increase in the H/C ratio from 1.51 ± 0.01, for the lignin oil, to 1.80 ± 0.01 was achieved. In parallel, the O/C molar ratio decreased from 0.46 ± 0.01 to 0.006 ± 0.004 for the liquid product. These results demonstrate the extensive removal of oxygen and incorporation of hydrogen in the liquid product. In the subsequent catalyst reuse, the net conversion of lignin oil slightly decreased from 89% to 85%, for both the second and third reaction runs (Table 4). For the liquid products, H/C ratios of 1.76 ± 0.01 and 1.75 ± 0.03 for the second and third reaction runs, respectively, were obtained. These values are slightly lower than those of the liquid products from the first reaction run (H/C:1.80 ± 0.01). On the other hand, O/C ratios substantially increased from 0.006 ± 0.004, for the first reaction run, to 0.14 ± 0.01 and 0.16 ± 0.01, for the second and third reactions runs, respectively. For the residue fraction soluble in MeOH, which corresponds to ca. 10% of the lignin-derived residues, the H/C ratio decreased from 1.73 ± 0.01 (first reaction run) to 1.62 ± 0.01 and 1.62 ± 0.06, for the second and third reaction runs, respectively. In parallel, O/C ratios rose from 0.29 ± 0.04 (first reaction run) to 0.33 ± 0.01 and 0.36 ± 0.01 for the second and third reaction runs, respectively. Altogether, the O/C and H/C ratios found for the liquid products and residues indicate that the catalyst's hydrogenation activity deteriorated to an extent lesser than that of the deoxygenation ability.To gain an in-depth insight into the composition of the volatile fraction of the liquid products, GC-FID/MS analysis was carried out (Figure 13 ). In the volatile fraction of the lignin oil (corresponding to 28% at an injector temperature of 300°C), the main components were p-dihydrolignols [4-(3-hydroxypropyl)-2-methoxyphenol and 4-(3-hydroxypropyl)-2,6-dimethoxyphenol, Table S4] followed by other alkylphenol compounds. In the control experiment, thermolytic processes on the p-dihydrolignols caused the elimination of γ-OH group, rendering 4-propylguaiacol and 4-propylsyringol. Other products from the cracking of the propyl side chain were formed (Table S4). At a much lesser extent, cyclohexanols (6.6%) and cycloalkanes (1.3%) were also formed. In the presence of 15%Ni/Nb2O5 catalyst, the primary volatile products were cycloalkanes (47%) and cyclohexanols (2%). Half of the cycloalkanes' fraction content corresponded to bicyclic aliphatic compounds. In the catalyst recycling, the content of cycloalkanes in the liquid product significantly decreased from 47%, for the fresh catalyst, to 8% and 5%, for the second and third reaction runs, respectively. As a result, the dominant species in the liquid products became cyclohexanols (31%–35%). The high content of monophenolic species (18–19%) reveals that the catalyst's hydrogenation ability was also impaired after the first use of the catalyst. However, the catalyst hydrogenation ability was affected to an extent lesser than that for the dehydration of cyclohexanol intermediates.To assess the extent of decrease in the acidity of the Nb2O5 support, ATR-IR measurements of pyridine adsorbed on the spent 15%Ni/Nb2O5 catalyst were performed. After the third reaction run, Figure 14 reveals that the spent catalyst no longer presents either Brønsted or Lewis acid sites accessible to pyridine adsorption. These results demonstrate the decrease in the deoxygenation activity of 15%Ni/Nb2O5 to be caused by the blocking of acid sites on the Nb2O5 support. Surprisingly, despite the loss of acidity, the structural properties of the Nb2O5 nanorods were not affected upon recycling (as shown by XRD pattern features, Figure S4). By stark contrast, the size of the Ni particles increased from 14 to 80 nm after three reaction runs (Figure S4). Thereby, the decrease in the hydrogenating activity of the catalyst appears to be related to the decrease in metal surface area due to Ni particle growth.From the data presented in Figure 13, the sum of compounds visible by the GC technique corresponds to approximately half of the content of species occurring in the liquid products. To expand our analysis toward the heavy species, GPC was performed on the hydrotreated liquid products and residue fractions soluble in methanol. Noteworthy, when applied to product mixtures obtained from lignin, direct information regarding the content of species cannot be retrieved from an ultraviolet-visible (UV-vis) detector (in this study, a photodiode array [PDA] detector), as the detector response is not universal. Furthermore, in samples containing aliphatic hydrocarbons, these compounds will be invisible to the UV-vis detector. Despite these limitations, the GPC technique coupled with UV-vis spectroscopy provides useful information on the apparent distribution of M w and spectral signature of the eluting species. Figure 15 displays the chromatogram traces at a wavelength of 280 nm. Figure 15 shows that the lignin oil substrate encompasses species of apparent M w from 100 to 66,000 Da. In the absence of the 15%Ni/Nb2O5 catalyst (control experiment), thermal processes on lignin generate soluble species of M w lower than 1,200 Da for both the product oil and solid residues, at the expense of the heavy species. In the presence of the fresh 15%Ni/Nb2O5 catalyst, both the liquid product and the residue fraction soluble in methanol still contain UV-absorbing species heavier and of much broader apparent M w distributions, compared with those from the control experiment. Surprisingly, the subsequent reaction runs yielded liquid products and residues of an apparent M w distribution comparable to the apparent M w range of products formed in the control experiment.To gain further information about the chemical nature of the UV-absorbing species, the spectral data collected by the PDA detector in the GPC analysis was examined in detail (Figure 16 ). In the samples from the first reaction run, a key feature distinguishing the PDA images is the presence of species absorbing at wavelengths higher than 300 nm for the residue fraction soluble in methanol (indicated in Figure 16 by a yellow-coded dotted line). In lignin chemistry, this spectral feature is often related to the presence of quinone methide intermediates, stilbene species, and other conjugated unsaturated species associated with lignin condensation processes (Lin, 1992; Schmidt, 2010). Overall these observations suggest that, in the presence of the fresh 15%Ni/Nb2O5 catalyst, the condensation of lignin species could not entirely be suppressed by the reductive processes, as the former process appears to take place at a rate faster than the latter. Interestingly, similar PDA images are found for both liquid products and the residues soluble in methanol from the second and third reaction runs. These images show no strong absorption spot at wavelengths higher than 300 nm. Altogether, these observations support the hypothesis that Lewis acid sites of Nb2O5 play a role in the condensation of lignin species. As these sites become largely blocked in the first reaction run, the condensation of lignin species should occur to a lesser extent in the subsequent reaction runs. This hypothesis appears to be plausible also considering that the weight of lignin residue accumulated with the catalyst plateaued after the second reaction run (Table 4).This study provided a beginning-to-end analysis of the multifaceted picture of the design of water-tolerant catalysts for the hydrotreating of lignin streams. From the observations of this study, the following conclusions and recommendations for future research are given: 1. In the design of bifunctional Ni/Nb2O5, the incorporation of the Ni phase reduces the population of Brønsted acid sites. However, the population of Lewis acid sites remained almost unaltered. The dehydration of cyclohexanol over Brønsted acid sites takes place at temperatures lower than those required for the reaction catalyzed by Lewis acid sites. By employing the hydrotreating of diphenyl ether to cyclohexane as a model reaction, it was possible to find a compromise between hydrogenation and dehydration catalyst's capabilities, thus taking the benefit from the catalyst Lewis acidity for the hydrotreatment. The 15%Ni/Nb2O5 catalyst showed sustained results in the recycling experiments. As a result of the high stability of the water-resistant Lewis acid sites, a 91% yield of cyclohexane could be achieved even after five reaction runs. 2. Despite the promising results achieved in the hydrotreatment of diphenyl ether, the 15%Ni/Nb2O5 catalyst lost its activity toward dehydration of the cyclohexanol species already after the first reaction run performed on the lignin oil stream. This disappointing outcome is associated with the blocking of the Lewis acid sites. 3. In the current literature on lignin hydroprocessing, little attention has been given to the fact that the acid sites, needed for the dehydration of cyclohexanol species, can also catalyze the condensation of lignin oligomeric species. In this study, we demonstrated that lignin condensation occurs even under reductive conditions and when beginning the process with passivated streams from the lignin-first biorefining based on reductive processes. 4. The condensation of lignin catalyzed by Nb2O5 nanorods' Lewis acid sites appears to be a chemical process faster than the saturation or HDO of lignin species. As a result, in the presence of Ni/Nb2O5 catalysts, lignin condensation is not entirely suppressed by reductive processes. Consequently, carbonaceous matter is formed, blocking the Lewis acid sites. 5. Previous studies on hydrotreating of lignin oils in the presence of phosphided Ni/SiO2 catalysts demonstrated that recyclable hydrotreating catalysts could be produced (Cao et al., 2018; Samec, 2018). Confronting those results with the current ones, it is concluded that the control of the surface acidity is mandatory for the success of lignin oil hydrotreating. Further research is required to define the type of acidity and a threshold of acidity required for the hydrotreating of lignin while not encouraging acid-catalyzed condensation processes on the lignin oligomeric species. Surprisingly, such a research line has not been receiving much attention from the community. Indeed, often studied model compounds (e.g., diphenyl ether, benzyl phenyl ether, (alkyl)guaiacols, and several others) cannot undergo condensation reactions. Therefore, the crucial role of lignin condensation in hydrotreating processes cannot be mimicked by the current set of model compounds employed in this research field. This fact clearly limits the translation of technologies designed for the HDO of model compounds to the hydrotreating of real-world lignin streams. 6. The balance of dehydration and hydrogenation abilities of a heterogeneous catalyst becomes a very complicated issue when considering the significant impact of lignin condensation throughout the hydrotreating process. Considering this, a tentative solution could be the utilization of two solid catalysts, one for hydrogenation and another one for dehydration, so that the balance of these specific tasks could be then more easily adjusted by the weight ratio of each catalyst component in the mixture of catalysts. This idea has been already exploited with success (Wang and Rinaldi, 2013). However, the conversion of a batch reaction process to a continuous flow process based on a mixture of catalysts constitutes a challenging task. A more practical approach appears to be the combination of flow-through reactors operating in series but at different temperatures. This approach could circumvent catalyst deactivation by the formation of coke via lignin condensation, by gradually saturating the lignin stream under conditions of gradual increase in process severity. A similar approach was demonstrated to be very fruitful for the hydrotreatment of pyrolysis oil in the presence of Ni-Cu catalysts (Yin et al., 2016). In the design of bifunctional Ni/Nb2O5, the incorporation of the Ni phase reduces the population of Brønsted acid sites. However, the population of Lewis acid sites remained almost unaltered. The dehydration of cyclohexanol over Brønsted acid sites takes place at temperatures lower than those required for the reaction catalyzed by Lewis acid sites. By employing the hydrotreating of diphenyl ether to cyclohexane as a model reaction, it was possible to find a compromise between hydrogenation and dehydration catalyst's capabilities, thus taking the benefit from the catalyst Lewis acidity for the hydrotreatment. The 15%Ni/Nb2O5 catalyst showed sustained results in the recycling experiments. As a result of the high stability of the water-resistant Lewis acid sites, a 91% yield of cyclohexane could be achieved even after five reaction runs.Despite the promising results achieved in the hydrotreatment of diphenyl ether, the 15%Ni/Nb2O5 catalyst lost its activity toward dehydration of the cyclohexanol species already after the first reaction run performed on the lignin oil stream. This disappointing outcome is associated with the blocking of the Lewis acid sites.In the current literature on lignin hydroprocessing, little attention has been given to the fact that the acid sites, needed for the dehydration of cyclohexanol species, can also catalyze the condensation of lignin oligomeric species. In this study, we demonstrated that lignin condensation occurs even under reductive conditions and when beginning the process with passivated streams from the lignin-first biorefining based on reductive processes.The condensation of lignin catalyzed by Nb2O5 nanorods' Lewis acid sites appears to be a chemical process faster than the saturation or HDO of lignin species. As a result, in the presence of Ni/Nb2O5 catalysts, lignin condensation is not entirely suppressed by reductive processes. Consequently, carbonaceous matter is formed, blocking the Lewis acid sites.Previous studies on hydrotreating of lignin oils in the presence of phosphided Ni/SiO2 catalysts demonstrated that recyclable hydrotreating catalysts could be produced (Cao et al., 2018; Samec, 2018). Confronting those results with the current ones, it is concluded that the control of the surface acidity is mandatory for the success of lignin oil hydrotreating. Further research is required to define the type of acidity and a threshold of acidity required for the hydrotreating of lignin while not encouraging acid-catalyzed condensation processes on the lignin oligomeric species. Surprisingly, such a research line has not been receiving much attention from the community. Indeed, often studied model compounds (e.g., diphenyl ether, benzyl phenyl ether, (alkyl)guaiacols, and several others) cannot undergo condensation reactions. Therefore, the crucial role of lignin condensation in hydrotreating processes cannot be mimicked by the current set of model compounds employed in this research field. This fact clearly limits the translation of technologies designed for the HDO of model compounds to the hydrotreating of real-world lignin streams.The balance of dehydration and hydrogenation abilities of a heterogeneous catalyst becomes a very complicated issue when considering the significant impact of lignin condensation throughout the hydrotreating process. Considering this, a tentative solution could be the utilization of two solid catalysts, one for hydrogenation and another one for dehydration, so that the balance of these specific tasks could be then more easily adjusted by the weight ratio of each catalyst component in the mixture of catalysts. This idea has been already exploited with success (Wang and Rinaldi, 2013). However, the conversion of a batch reaction process to a continuous flow process based on a mixture of catalysts constitutes a challenging task. A more practical approach appears to be the combination of flow-through reactors operating in series but at different temperatures. This approach could circumvent catalyst deactivation by the formation of coke via lignin condensation, by gradually saturating the lignin stream under conditions of gradual increase in process severity. A similar approach was demonstrated to be very fruitful for the hydrotreatment of pyrolysis oil in the presence of Ni-Cu catalysts (Yin et al., 2016).In a broader context, this work provides substantial evidence that the use of model reactions has severe limitations for the design of catalysts for the hydroprocessing of lignin streams. Accordingly, the catalyst screening carried out on real-world lignin streams is a more productive enterprise to pursue, regardless of the complexity of the product mixtures obtained. In this quest, the evaluation of H/C and O/C ratios as the response variables (either for the catalyst discovery or in recycling experiments) constitutes a strategy effective in the simplification of characterization procedures applied to the lignin products. Such a strategy should become a gold standard in the high-throughput screening catalysts for the hydroprocessing of lignin streams to produce drop-in lignin biofuels, as it allows for the direct comparison of catalyst performance without the need of scrutinizing the lignin product compositions at an early stage of technology-readiness levels (TRL), thus contributing to accelerating catalyst discovery.The hydrothermal synthesis of Nb2O5 is based on the decomposition of niobium peroxo species formed by the reaction of ammonium niobium(V) oxalate and hydrogen peroxide. CAUTION: As the hydrothermal synthesis is performed in a closed stainless-steel vessel, it is mandatory to check if the vessel is rated to operate under the pressure built by the decomposition of the full content of hydrogen peroxide employed in the synthesis.All methods can be found in the accompanying Transparent Methods supplemental file.R.R. acknowledges the financial support provided by the ERC Consolidator Grant LIGNINFIRST (Project Number: 725762). R.R. and A.A.S.C. thank FAPESP for the support provided (Process Number: 2016/50423-3). The authors are grateful to LNLS/CNPEM for the infrastructure (XPD beamline and chemistry laboratory), LNNano for the STEM infrastructure, the GPMMM laboratory (IQ-UNICAMP) for the quantitative FTIR of adsorbed pyridine analysis, CNPq for the PhD scholarship (Process Number: 165106/2014-0), and CAPES for the PDSE scholarship (Process Number: 88881.132245/2016-01). This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001. Finally, the authors are thankful to CBMM for the ammonium niobium oxalate hydrate samples.Conceptualization, G.F.L, A.A.S.C., and R.R.; Methodology, G.F.L. and R.R.; Investigation, G.F.L., I.G., S.L., H.C., D.H.B., A.A.S.C., C.B.R., E.T.-N., and R.R.; Writing – Original Draft, G.F.L.; Writing – Review & Editing, G.F.L., C.B.R, A.A.S.C., and R.R.; Funding Acquisition, C.B.R., A.A.S.C., and R.R.; Resources, C.B.R., A.A.S.C., and R.R; Supervision, C.B.R., A.A.S.C., and R.R.The authors have no conflict of interests to declare.Supplemental Information can be found online at https://doi.org/10.1016/j.isci.2019.05.007. Document S1. Transparent Methods, Figures S1–S4, and Tables S1–S4

In biomass conversion, Nb2O5 has attracted increasing attention as a catalyst support presenting water-tolerant Lewis acid sites. Herein, we address the design of Ni/Nb2O5 catalysts for hydrotreating of lignin to hydrocarbons. To optimize the balance between acidic and hydrogenating properties, the catalysts were first evaluated in the hydrotreating of diphenyl ether. The best catalyst candidate was further explored in the conversion of lignin oil obtained by catalytic upstream biorefining of poplar. As primary products, cycloalkanes were obtained, demonstrating the potential of Ni/Nb2O5 catalysts for the lignin-to-fuels route. However, the Lewis acidity of Nb2O5 also catalyzes coke formation via lignin species condensation. Thereby, an acidity threshold should be found so that dehydration reactions essential to the hydrotreatment are not affected, but the condensation of lignin species prevented. This article provides a critical “beginning-to-end” analysis of aspects crucial to the catalyst design to produce lignin biofuels.

The increasing estimates of CO2 emissions, at a rate of 33 GT/year, a concentration forecast of 570 ppm by the end of the 21st century, and the serious consequences of climate change, as numerous natural disasters (heat waves, hurricanes, wildfires, droughts, sea level rise), are some of the most pressing problems for humanity. In this scenario, a deep transition period towards a zero-emissions energy model, based on the increasing utilization of renewable energy sources, may be expected. The taxes to the countries for CO2 emissions [1] and the economic consequences of climate change (valued at a loss of 31 billion-dollar in 2017 [2]) are also an incentive to take measures aimed at reducing the net emissions of CO2.The technologies for CO2 capture and storage/sequestration (CCS) have received extensive attention [3]. The physical absorption (where CO2 is scrubbed from the flue gas) is common for high CO2 partial pressure. It is carried out at low temperature, with low energy requirement and it is favored using commercial solvents. Using chemical absorption, CO2 present in low concentration can be separated reacting with alkanolamines or dissolved alkaline salts. Among the former, MEA (monoethanolamine), DEA (diethanolamine), TEA (triethanolamine), MDEA (methyldiethanolamine), DIPA (diisopropanolamine) and DGA (diglycolamine) are used. KOH is commonly used as alkaline reactant. Separating CO2 using membranes requires lower capital cost and the equipment occupies smaller space. The membranes used are prepared with different materials: zeolites, carbon nanotubes (CNT), polyamides, polyether sulfone or polydimethyl phenylene oxide, among others [4].Adsorption is effective for low CO2 concentrations using zeolites, silica based-materials (microporous as SAPO-34 or mesoporous as MCM-41 or SBA-15), activated carbon, graphene, metal-organic frameworks (MOFs), lithium orthosilizate (Li4SiO4), lithium zirconate (Li2ZrO3) and other porous materials as adsorbents. For chemical adsorption, materials (mainly carbons) functionalized by polymeric amines (polyethylenimine, polypropylenimine, polyallylamine, polyaniline, amino dendrimers, and hyperbranched polyamines) are used [5,6]. In general, the capture capacity is higher for adsorption than for absorption (88–176 kg of CO2 per kg of adsorbent, and 0.4–1.2 kg of CO2 per kg of absorbent, respectively). Cryogenic distillation produces high purity liquid CO2, and is an interesting method to treat gas at high pressure. However, the cost of this technology is high, due to the energy requirement for refrigeration. Electrochemical technology is another emerging alternative for CO2 capture from mediums of different concentration. The characteristics of this technology and the development state have been explained by Sharifian et al. [7]. Using the “pH swing” concept, CO2 can be captured and recovered, which facilitates its subsequent online valorization. Given the higher cost of this technology over others commonly used (and in particular with respect to absorption with amines), its economic viability requires using renewable energies and developing low-cost membranes.CCS technologies for stationary sources contemplate the [8]: i) Direct air capture (DAC) for CO2 removal from small sources and from the transport sector, responsible of 1/3 to 1/2 of total emissions, and; ii) moving to remote sites for large-scale CO2 sequestration. To finance the expensive investments required by these technologies, it is essential to promote CO2 upgrading generating an economic benefit. Among the CO2 utilization technologies [9–24], two objectives are distinguished, the direct use (pure or in solution), and its use as feedstock for the production of chemicals and fuels (use after transformation). The direct use of CO2 for carbonated drinks is associated to the origin of the commercialization of soft drinks. It is also used as fire extinguisher, refrigerant, anesthetic gas, dry ice, solvent, process fluid and welding medium. Other routes directly using CO2 on a larger scale comprise methods for the extraction of mineral sources: EOR (enhanced oil recovery), ECBM (enhanced coal-bed methane) recovery, and EGR (enhanced gas recovery). The use of CO2 in micro-algae cultivation along with free sunlight has the advantage of operating at mild conditions, but requires controlling the pH (in the 6.6–10.5 range) and a sealed reactor.The transformation of CO2 into chemicals and fuels is difficult, given the thermodynamic stability of the molecule due to its structure, constituted by a carbon atom with its four electrons bonded to oxygen atoms through covalent double bonds (O=C=O). Moreover, the Gibbs free energy of CO2 (∆G0 = −394 kJ mol−1) is much lower than that of the possible products of its transformation. Consequently, the challenges of the processes for this transformation are very demanding. Among them [11]: i) Great energy supply from renewable and carbon-neutral sources; ii) the use of high temperature and/or pressure, or; iii) the intervention of catalysts active sites, organisms or biological species capable for activating the reactions involved. In Fig. 1 different routes for the transformation of CO2 are gathered.The processes for CO2 transformation through chemical and electrochemical reactions have multiple technological alternatives. As to the electrochemical reduction regards, two possible routes are distinguished [25], with CO2 as intermediate to produce formic acid, or CO and hydrocarbons (mainly methane). Jiang et al. [26] have summarized the recent advances in understanding the reaction mechanism and exploring cathode materials. The external energy source in these processes can be thermal, electrocatalytic or photocatalytic, providing the opportunity to these processes to be integrated with renewable energy production (solar, wind and marine). In the artificial photosynthesis strategy, semiconductor catalysts convert CO2 into hydrocarbons with solar energy through a multielectron transfer mechanism. In this mechanism, TiO2 (commonly used as catalyst) absorbs light upon illumination and generates a pair of photo-excited electrons and holes. These initiators interact with H2O and CO2 molecules to produce methane and other products by selecting appropriate catalyst (usually prepared by doping TiO2) and conditions [27].However, the chemical reactions occur at a high rate and are carried out in an easier-to-scale reactor. Some authors classify the chemical transformation pathways according to their energy requirements [13]. Kamkeng et al. [11] make a comparison of the CO2 transformation routes according to different criteria (technological maturity, cost considerations, market analysis and amount of CO2 used). Taking into account the technological readiness level (TRL) tool (Fig. 2 ), synthesis of methane and methanol have high TRL values (7–9). The main advantages of hydrogenation processes focus on the market interest of CO2-derived fuels and raw materials (gasoline, methanol, DME, methane, olefins, aromatics), and on the amount of CO2 used in their production (2.6 t fuel/t CO2 in Fischer-Tropsch synthesis). Nonetheless, in terms of cost per ton of product, the interest of these processes is conditioned by the price of H2.The different catalytic and electrocatalytic processes for CO2 conversion into fuels and chemical products have been reviewed several times [10,13,28–31], and these are schematized in Fig. 3 . It can be observed that some products are, at the same time, raw materials for other processes. That is, oxygenates (methanol and DME) with interest as fuels, are converted into olefins (MTO and DTO processes, respectively) [32,33], into hydrocarbons in the gasoline range (MTG and DTG processes, respectively) [34,35], or in BTX aromatics [36]. These reactions proceed according to the dual cycle mechanism, with arenes and olefins as intermediates [37], and the product distribution is dependent on the acidity and shape selectivity of the catalyst (based on SAPO-34 in the MTO process and based on HZSM-5 zeolite in the other processes). Besides, methanol and DME are hydrogen vectors (through reforming) [38,39]. Methanol (MeOH) can also be selectively dehydrogenated towards formaldehyde [40], which will be used in polymers and resin production.Furthermore, CO2 allows for the production of synthesis gas (H2/CO) through the reverse Water-Gas-Shift (rWGS) reaction (where CO2 takes the role of H2 acceptor) [41] or by dry reforming of methane, hydrocarbons or oxygenates (where CO2 acts as oxidant agent) [42]. In addition, synthesis gas or CO2 directly can be converted into a hydrocarbons mixture, either through the Fischer-Tropsch (FT) route [43] or with MeOH/DME as intermediates, over bifunctional catalysts [44,45]. These reactions can be controlled by choosing selective acidic functions for the production of C2+ alcohols, isoparaffinic gasoline or aromatics. From the energy requirement point of view, the reactions in which the second reactant has a higher Gibbs free energy have lower energy requirement and so, are more favorable. However, CO2 hydrogenation reactions require a large amount of external energy and the use of a catalyst to overcome the activation barrier. According to this classification, already suggested by De et al. [46], the characteristics of the CO2 conversion processes are described in the next sections, distinguishing those not requiring H2 as reactant (Section 2.1) and hydrogenation processes (Section 2.2).These reactions are of greater interest in an energy transition state like the current one, prior to the availability of H2 produced from sustainable sources and using renewable energies.The direct conversion of methane into ethane (Eq. (1)) or into ethylene (Eq. (2)), through oxidative coupling (OCM) forming C‐C bonds, has a growing interest in valorizing burgeoning natural gas reserves, in which CO2 content may reach 10%. (1) 2 CH 4 + CO 2 → CH 3 CH 3 + CO + H 2 O (2) 2 CH 4 + 2 CO 2 → CH 2 CH 2 + 2 CO + 2 H 2 O These reactions occur through the following mechanism [47]: 1) Cleavage of methane C‐H bonds in the active sites of the catalyst, forming CH3* and CH2* radicals; 2) dissociation of CO2 towards CO and O* active oxygen; 3) coupling of these radicals; 4) recombination of CH3* and CH2* radicals; 5) dehydrogenation, either oxidative or radical, of ethane to ethylene. The catalysts must be selective, avoiding the formation of syngas by dry reforming. The strong basic metallic oxide catalysts used can be grouped into [19,48]: 1) Pure oxides of the lanthanide series, of which La2O3 shows the greatest performance; 2) basic oxides loaded with Group 1 or 2 cations (Li/MgO, Ba/MgO, and Sr/La2O3); 3) transition metal oxides containing Group 1 cations, and; 4) redox catalysts, like CeO2 modified by Group 1 and 2 cations. Over ZrO2/TiO2 catalysts acetic acid is formed by the insertion of the adsorbed CO2 into the CH3* species, followed by the hydrogenation with H* in the adsorption of methane [49].The production of light olefins through oxidative dehydrogenation of their corresponding paraffins (ODP) (Eq. (3)) is an upgrade. In this manner, raw materials are obtained for the production of polyolefins and, at the same time, the high-energy requirement of steam cracking, as well as the rapid deactivation of the catalyst due to coke deposition (attenuated by the gasification capacity of CO2) are avoided. (3) C n H 2 n + 2 + CO 2 → C n H 2 n + CO + H 2 O The most studied catalysts for ODP are based on redox properties, principally MoO3, Cr2O3 and V2O5. CeO2 (with well-established redox properties), ZrO2, TiO2, SiO2 and zeolites (HZSM-5, MCM-41) have been used as supports, since the mesoporosity of the latter is known to favor the dispersion of metallic oxides [50]. The basic character of these catalysts favors CO2 adsorption and olefins desorption, while paraffins dehydrogenation is activated by the presence of the acidic sites.ODP mechanism [51] considers the rWGS (Eq. (5)) reaction, where H2, product of the dehydrogenation, is oxidized by CO2. Furthermore, paraffin dry reforming (Eq. (6)) and coke deposits oxidation through reverse Boudouard reaction (Eq. (7)) are considered. (4) C n H 2 n + 2 ⇌ C n H 2 n + H 2 (5) H 2 + CO 2 ⇌ CO + H 2 O (6) C n H 2 n + 2 + nCO 2 → 2 nCO + n + 1 H 2 (7) CO 2 + C ⇌ 2 CO The thermodynamic analysis of the CO2 assisted dehydrogenation of ethane (ODE) shows the need for reaction temperatures above 550 °C for a good compromise between the extent of the hydrogenation and rWGS reactions according to Najari et al. [52]. These authors review the advances in this reaction, comparing the behavior of the most used catalysts. The catalysts are based on Ni, Ni‐Fe, Cr2O3, Ga2O3, or CoOx, and use acidic supports as γ-Al2O3, SiO2, CeO2, ZrO2, TiO2, SBA and zeolites (being HZSM-5 the most common).Jiang et al. [53] classify the catalysts for CO2 assisted dehydrogenation of propane (ODP) according to the nature of their metallic function, distinguishing: i) Redox-type catalysts (those based on CrOx are the most used ones). The redox cycle is described in Eqs. (8)–(10) [54]; ii) non-redox type catalysts (Ga2O3 polymorphs, Ga2O3-Al2O3 solid solutions and mixed GaO2-ZrO2), and iii) other transition metal catalysts (Fe2O3, Fe‐Ni, Mo2C). As supports, the afore-mentioned ones for ODE, mesoporous zeolites (such as MCM-41) and activate carbons have been tested. (8) C 3 H 8 + CrO x ⇌ CrO x − 1 + C 3 H 6 + H 2 O (9) CO 2 + CrO x − 1 ⇌ CO + CrO x (10) H 2 + CrO x ⇌ Cr x − 1 + H 2 O It should be pointed out that the dehydrogenation of C5+ paraffins is not viable due to the fast catalyst deactivation by coke deposition.The oxidative dehydrogenation of ethylbenzene (ODE) to styrene is of great interest to avoid selectivity limitations and catalyst deactivation by coke in the conventional industrial process without oxidant agent, which require an excess of vapor. ODE with CO2 as dehydrogenating agent, with the steps described in Eqs. (11), (12) and (5), results in a styrene selectivity of 97% and its energy demand is of approximately a tenth of that of the conventional process. Therefore, it offers an attractive option for satisfying the growing demand of styrene (yearly production of 14.6 Mt) in the production of synthetic rubber, polystyrene and styrene-acrylonitrile copolymers. (11) C 6 H 5 CH 2 CH 3 + CO 2 → C 6 H 5 CH = CH 2 + H 2 O + CO (12) C 6 H 5 CH 2 CH 3 → C 6 H 5 CH = CH 2 + H 2 Fe2O3/CeO2 catalyst has a high activity attributable to the redox activity of the Ce sites (changing Ce4+ and Ce3+), promoted by Fe3+ and whose presence improves the oxygen storage capacity of Ce [55]. The relevance of both the redox efficiency and the mesoporous structure of the support has been proven by Burri et al. [56] using CeO2-ZrO2 supported on SBA-15. VOx, MoOx, WOx, CrOx-based catalysts have also been studied, either supported on SiO2, mesoporous zeolite (MCM-41) or active carbon incorporated in hydrotalcite (Mg-V-Al structures) [57]. With the latter, Sakurai et al. [58] obtained an ethylbenzene (EB) conversion of 67.1% and styrene selectivity of 80%. Different mechanisms have been proposed for ODE from EB, considering the three step mechanism the most favorable thermodynamically [59]: (13) C 8 H 10 + os ⇌ C 8 H 10 − os (14) C 8 H 10 − os + rs → C 8 H 8 ∙ + 2 H − os (15) H − os + H − os → H 2 + 2 os (16) CO 2 g + rs ⇌ CO g + O − rs (17) H 2 g + O − rs ⇌ H 2 O g + rs (18) C 8 H 8 − rs → C 8 H 8 + rs where “os” refers to the oxidizing sites and “rs” to the reducing sites.Beyond CO2 transformation, the lower energy requirement of dry reforming than that of steam reforming is a remarkable advantage, although H2 yield and the resulting H2/CO ratio are lower. Its application has extended to the conversion of fossil sources (methane) and sources derived from biomass (as ethanol, glycerol and bio-oil).Methane dry reforming (MDR, Eq. (19)) is the principal route for the current production of H2. Although CH4 is a fossil source, the process has good future prospects for biogas feedstocks (with CH4 and CO2 as major components) derived from the anaerobic digestion of organic waste materials [60]. (19) C H 4 + C O 2 → 2 H 2 + 2 CO The reaction steps of MDR on the catalyst surface involve: 1. Methane adsorption and abstraction of hydrogen: (20) C H 4 → CH 4 ∗ → CH 4 − x ∗ + χ H ∗ 2. CO2 adsorption and abstraction of an oxygen atom: (21) C O 2 → CO 2 ∗ → CO ∗ + O ∗ 3. The formation of CO and hydrogen on the surface: (22) CH ∗ + O ∗ → CO ∗ + H ∗ 4. The formation of H2O: (23) O + H ∗ → OH ∗ + H ∗ → H 2 O ∗ → H 2 O 5. The recombination of hydrogen on the surface and desorption: (24) H ∗ + H ∗ → H 2 ∗ → H 2 Methane adsorption and abstraction of hydrogen:CO2 adsorption and abstraction of an oxygen atom:The formation of CO and hydrogen on the surface:The formation of H2O:The recombination of hydrogen on the surface and desorption:In addition, the WGS reaction (Eq. (5)), the coke formation reactions by decomposition of CH4 (Eq. (25)) and formation/gasification of coke by the Boudouard reaction (Eq. (7)) take place. (25) CH 4 → C + 2H 2 The main limitations of MDR are the high-energy requirement (even being lower than for steam reforming, MSR) (heats of 247 kJ mol−1 and 228 kJ mol−1, respectively), since temperature above 800 °C is required; and catalyst stability, affected by sintering and coke formation. The energy demand is reduced and coke formation is attenuated by combining MDR with MSR and POM (partial oxidation of methane). For that purpose, according to the tri-reforming concept, methane is co-fed with H2O and O2 [61]. Li et al. [62] have made a review on the advances on the technologies for heat supply, alternative to fossil fuels, including photochemical and electrochemical, plasma-assisted, solar energy, operating in solid oxide fuel cells, coupled with inorganic membranes and chemical looping reforming.Noble metal and transition metal based-catalysts have been exhaustively studied [63–67]. According to activity they can be ordered as follows [68]: Ru ≈ Rh > Ni ≈ Ir > Pt > Pd > Co. Ni catalysts are generally used regarding their high activity and low cost. Anyhow, sintering and coke formation is quite fast for these catalysts. A great deal of effort has been addressed to improve the stability of Ni catalysts in MDR. Thus, various strategies have been used for attenuating sintering through strengthening the metal-support interactions: forming bimetallic catalysts, where metal is dispersed in nanoparticles [69], incorporated within perovskites [68] or with spinel and core-shell configurations [62]. The stability of Ni catalysts has also been upgraded incorporating in the support (Al2O3, SiO2) basic promoters as alkaline metals (Li, Na, K), rare earth metal oxides (La2O3, CeO2, Y2O3, Sm2O3) and reducible transition metal oxides (ZrO2, TiO2, MnO2, MoO3). These materials promote Ni dispersion, metal-support interaction, oxygen mobility and CO2 and H2O adsorption, attenuating coke formation [70–72]. A strategy to avoid catalyst deactivation by coke in the dry reforming of methane is to carry out the reaction without catalyst, with acetylene as intermediate. However, very high temperature is required for this approach (1400–1800 °C) [73].Although the stoichiometry of ethanol dry reforming (EDR) corresponds to Eq. (26), in parallel, the steam reforming (ESR) reaction will also take place, because the H2O content in the ethanol (bio-ethanol) obtained from hydrolysis/fermentation of biomass is remarkable. This coexistence of dry and steam reforming also occurs for other bio-alcohols (such as butanol) and biomass-derived oxygenates, such as glycerol and oxygenates in bio-oil (product of the fast pyrolysis of biomass), whose stoichiometry of dry reforming ideally corresponds to Eqs. (27) and (28) respectively. Furthermore, all these bio‑oxygenates undergo decomposition and dehydrogenation reactions, which require a catalyst and suitable reaction conditions to reform the by-products (CH4, olefins and aldehydes) and to minimize the formation of coke. (26) C 2 H 5 OH + CO 2 → 3 CO + 3 H 2 (27) C 3 H 8 O 3 + CO 2 → 4 CO + 3 H 2 + H 2 O (28) C x H y O z + CO 2 → CO + H 2 These reactions have received less attention than bio‑oxygenates steam reforming and the main challenge has been achieving catalyst stability [74]. In EDR catalysts based on noble and transition metals have been studied. Da Silva et al. [75] propose a mechanism for the Rh/CeO2 catalyst involving the role of oxygen vacancies in the CeO2. As to attenuate coke deactivation high values of temperature (around 1073 K) and an ethanol/CO2 ratio (around 3) are required [76]. The combination of SBA-15 (with high specific surface) with CeO2 (redox capacity) in the support improves the activity of the catalyst [77]. Comparing different supports, Drif et al. [78] determined the following activity: Rh/NiO-Al2O3 > > Rh/Al2O3 ≈ Rh/MgO-Al2O3 ≈ Rh/CeO2-Al2O3 > Rh/ZrO2-Al2O3 ≈ Rh/La2O3-Al2O3 at 1073 K. The high activity of Rh/NiO-Al2O3 was attributed to the smaller Rh particle size and to the presence of NiAl2O4 spinel phase, which limited the migration of Rh in Al2O3. Ir/CeO2 catalyst has also shown a good behavior in the EDR reaction at 973 K, with the complete elimination of coke formation on the catalyst [79].Ni-based catalysts are also very active, according to CO2 conversions following the order: Ni/CeO2 ≈ Ni/Al2O3 > Ni/MgO ≈ Ni/ZrO2. To attenuate sintering and coke deactivation, the interest of incorporating Co and promoters with redox capacity has been assessed [80,81]. The activity of Cu and Co as primary catalysts and the effect of promoters with redox capacity for enhancing their stability has also been studied [82,83].The studies on glycerol dry reforming (GDR) are focused on Ni-based catalysts, with particular emphasis on the influence of types of supports and promoters. As CO2 and glycerol are adsorbed at different sites of the bifunctional catalyst, the reaction is controlled by the glycerol adsorption step. The complex mechanism of glycerol conversion explains the fast deactivation by coke, whose precursors are the by-products of the reaction (CO, CH4, aldehydes, hydrocarbons). To attenuate coke deactivation, limiting the acidity of the support is essential. Thus, γ-Al2O3 catalyst is very active, but undergoes fast deactivation mainly due to the deposition of whisker type of carbon on the catalyst surface [84]. The deposition of La2O3 on the Al2O3 support prior to Ni, increases Ni dispersion and attenuates coke formation [85]. Several attempts have been made to optimize the Ni-based catalysts for higher activity and stability. Among these the use of CaO [86] or SiO2 [87] or ternary oxides (Al2O3–ZrO2–TiO2) [74] as supports, the addition of Re to the catalyst [88] or Ag as promoter [87].Precious metal (Rh, Ru, Ir, Pd and Pt) catalysts with MgO stabilized Al2O3 as support were also tested for their activity towards GDR by Tavanarad et al. [89]. It should be noted, that after the fast initial deactivation due to whisker carbon, these catalysts maintain a pseudosteady state.Acetic acid production is an example of an opportunity to valorize low cost reactants like CO2 and CH4. The production of benzoic acid from CO2 and benzene is equally interesting. Furthermore, acrylic acid production through the direct carboxylation of ethylene with CO2 on Ni catalysts (Eq. (29)) is of great interest. This reaction is particularly interesting for valorizing CO2 generated in the ethylene production units by steam cracking of naphthas [90]. (29) Here, CO2 is a raw material for the production of linear and cyclic carbonates. Among the first ones, dimethyl carbonate (DMC) (CH3O)2CO, with low toxicity, is used as solvent, gasoline additive and reactant in alkylation and acylation reactions. It is produced by reacting with methanol (Eq. (30)). Several catalysts have been reported for this reaction (based on Cu and Cu‐Ni, and on CeO2) [91–96]. (30) Cyclic carbonates (of ethylene, propylene, cyclohexane, styrene and others) are produced by the addition of CO2 to an epoxy (Eq. (31)). They are used as solvents, electrolytes and raw material in the production of poly‑carbonates, other polymeric materials and fine chemicals (dialkyl carbonates, glycols, carbamates, pyrimidines, etc.). The formation reactions are catalyzed by alkali metal halides, metal oxides, zeolites and organic bases [97]. (31) Acetylsalicylic synthesis (CH3COOC6H4COOH) is an example of the insertion capacity of CO2 in the C‐H bonds of alkenes, aromatics or olefins. The products of greatest interest are carbonic acids, esters, lactones, and heterocyclic; in other words, compounds with functional groups potentially applicable as solvents, plasticizers, detergents, antioxidants, sun-protection agents, etc. [98].CO2 is valorized in the NH3 production industry itself for the synthesis of urea (carbamide, (NH2)2CO). This consists of the carbamate (H2N-COONH4) (Eq. (32)) formation reaction and further dehydration towards urea (Eq. (33)). Xiang et al. [99] reach a CO2 conversion up to 82.16% at atmospheric pressure and 20 °C. According to the stoichiometry, to obtain 1 t of urea 0.75 t of CO2 are required. Nevertheless, urea is principally used as fertilizer, with the role of releasing NH3 (adsorbed by plants) and CO2. Therefore, this route would not diminish CO2 emissions. Urea production at room temperature has been studied by means of electrochemical synthesis by coupling CO2 and N2 in H2O using PdCu/TiO2 electrocatalyst [100]. (32) 2 NH 3 + CO 2 ⇌ H 2 N − COO NH 4 (33) H 2 N − COO NH 4 ⇌ NH 2 2 CO + H 2 O Other polymers, like aliphatic polycarbonates, are produced by the reaction of CO2 with epoxides or through transesterification of diols with DMC. They are substitutes of polyethers for the fabrication of polyurethane (formed by urethane bonds, −N − (C = O) − O−) [101]. In the same way, by the reaction of CO2 with epoxides, aromatic polycarbonates based on bisphenol can be synthesized. Polyoxymethylene is another polycondensation polymer that can be produced from CO2 and 1,3,5-trioxane (in this case with formic acid as intermediate). Although polyoxymethylene incurs a higher cost than poly- ethylene and propylene, it provides a higher mechanical resistance. Moreover, using another intermediate (such as methanol) CO2 can be applied in the large scale production of polymethyl-methacrylate (PMMA).In different reviews the main advances conducted in these routes are collected [102–106]. The scheme in Fig. 4 (reproduced from [106], adapted from [107,108]) includes the main routes, which according to the products can be classified as: routes with C1 compounds as products (methane, carbon monoxide, methanol, formaldehyde); and those that form compounds with 2 or more carbon atoms (hydrocarbons and oxygenates). The mechanisms for these routes are significantly different, and, consequently, have been studied under different process conditions and with different catalysts.As aforementioned, the hydrogenation routes in Fig. 4 require external energy supply and the use of catalysts, due to unfavorable thermodynamics. In Table 1 the standard enthalpy and Gibbs free energies values of different CO2 hydrogenation reactions are listed (values taken from [46,109]). The role of the conditions (pressure, temperature, H2/CO2 ratio) on thermodynamics is important to achieve an acceptable extent of the reaction and adequate products distribution, but the use of active, selective and stable catalysts is also necessary.Even if alternative routes for CO2 methanation are studied, including photosynthesis and photocatalysis [110], electrochemical reduction [111] and biological conversion [112], the main attention is focused on the thermal catalytic process [113,114]. It proceeds with the following stoichiometry: (34) CO 2 + 4 H 2 → CH 4 + 2 H 2 O Additionally, the CO formed through the rWGS reaction (Eq. (35)) also leads to CH4 formation: (35) CO + 3 H 2 → CH 4 + H 2 O In addition, the side reactions of methane dry reforming (MDR), (Eq. (19)), Boudouard (reverse of Eq. (7)), decomposition of CH4 (Eq. (25)) and gasification of the coke formed by the two previous reactions (Eq. (7)) also take place.According to thermodynamics, CO2 conversion and CH4 selectivity are favored at high pressure and low temperature [115,116], and the results are good (almost complete conversion and selectivity close to 100%) with the appropriate catalyst even at atmospheric pressure if temperature is low enough (< 350 °C). Catalysts based on noble and non-noble metals are used [117], according to activity ordered as: Ru > Fe > Ni > Co > Rh > Pd > Pt > Ir, and according to selectivity: Pd > Pt > Ir > Ni > Rh > Co > Fe > Ru. The general use of Ni catalysts (due to the good compromise between their performance and cost), instead of Ru-based catalysts, requires working at temperatures for which catalyst stability problems arise, especially due to the formation of coke.The improvements of Ni catalysts are aimed at increasing surface defects, to facilitate the generation of surface-dissociated hydrogen, active for the removal of surface nickel carbonyls [118]. The role of the supports, aside from increasing surface defects, is to improve the dispersion of the metal and facilitate the storage and release of oxygen (redox properties). For these purposes, Al2O3, SiO2, ZrO2, TiO2, CeO2, perovskite, structured metal oxides, carbon materials and zeolites have been used as supports. Among the interesting properties of the supports, the following are to be mentioned, providing: i) Mechanical resistance; ii) metallic sites dispersion capacity (minimizing their aggregation); iii) hydrophilicity (the presence of H2O favors the sintering of the metallic sites); iv) thermal conductivity (avoiding the generation of “hot spots”), and: v) reduced presence of acidic sites capable for coke formation. Some of these properties are improved incorporating promoters, including ZrO2, CeO2, La2O3, Mn2O3, MgO and alkali metals [113,114].CO2 methanation mechanism takes place with three pathways, the relative importance of which depends on the catalyst and reaction conditions [116]: i) Direct CO2 dissociation and hydrogenation of CO (intermediate) to CH4; ii) through the reaction of formate (HCOO−) (intermediate formed from the adsorption of CO2) with chemisorbed hydrogen, and; iii) with formyl species as intermediates. These species result from the reaction of adsorbed CO (product of CO2 dissociation) with atomic hydrogen. Miguel et al. [116] have compared the LHHW kinetic equations of these mechanisms for a commercial Ni catalyst, proving that the best fit to their experimental results corresponds to the kinetic model for the pathway with formil species as intermediates, developed by Koschany et al. [119], assuming hydroxylic groups as the most abundant species.From the operational point of view, it is important to highlight the relevance of separating the H2O from the reaction medium to favor the extent of the reaction. This objective has led to the proposal using reactors with hydrophilic, steam-selective sodalite membranes [120,121] to replace conventional packed or fluidized bed reactors.CO is more reactive than CO2 and a key intermediate for the production of methane, methanol, DME and hydrocarbons from CO2, which explains why synthesis gas is used as feedstock in commercial processes for the production of these compounds. However, these reactions are carried out under unfavorable conditions for CO production. The conversion of CO2 by the rWGS (Eq. (5)) is an endothermic reaction, and temperatures above 700 °C are required in order to obtain considerable CO2 conversion. Under these conditions, CO2 and CO methanation (Eqs. (34) and (35), respectively) and Boudouard (Eq. (7)) side reactions also take place.The reaction mechanisms for the rWGS reaction is a topic of intensive debate [122], being redox and dissociative mechanisms the most widely accepted. In the redox mechanism, H2 does not participate as reactant, but reduces the surface of the catalyst. Metallic crystals are the active sites for CO2 dissociation, and the oxidized metallic sites are reduced releasing H2O and being therefore the metallic sites regenerated. Thus, the redox stages for Cu catalysts are: (36) CO 2 g + 2 Cu s 0 → CO g + Cu 2 O s (37) H 2 g + Cu 2 O s → H 2 O g + Cu s 0 In the dissociative mechanism H2 reacts with CO2, leading to the subsequent formation of formate species (HCO2-M), which will release CO right away. These formate species are formed by the attack of OH− groups on M-CO species and MO2H species, formed through intermediates CO2-metal protonation. According to this mechanism, the significant effect of the presence of surface hydroxyl groups to facilitate CO2 adsorption and hydrogenation has been verified [123].The activity of the catalysts for rWGS is associated with the presence of oxygen vacancies and the capability for adsorbing CO2 and generating formate active species. These are formed in the vicinity of the H supply (metal-support interface) [124]. However, the selection of the catalyst is conditioned by stability and selectivity requirements, due to the high reaction temperature. A key property for CO selectivity is to achieve a weak binding energy of CO. Cu catalysts (with low CO adsorption energy) are commercially used for the WGS reaction with CuO-ZnO/Al2O3 (CZA) configuration, but undergo notable sintering in the rWGS reaction. The stability of the Cu sites improves using different supports (β-Mo2C, In2O3 [125,126]); with Cu‐Al spinel [122]; or generating particular configurations as Cu/CeO2 hollow spheres [127] or an inverse metal-oxide/metal structure of CeOx/CuOx [128].Promising CO selectivity has also been achieved with other non-noble metal catalysts using carbide structures prepared with Ti, V or W [129,130] and with bimetallic catalysts (Ni‐Fe, Ni‐Co) [131]. Although noble metals have high CO adsorption energy, high CO selectivity is achieved with strategies such as the preparation of bimetallic catalysts (Pd‐Ni) [132] and the atomic dispersion of Rh or Ru nanoparticles on the support [133].Olah [134] reflected the relevance of the “methanol economy” as a complement to the established “oil economy”. Fulfilling his forecasts, the production of methanol is a key reaction in the development of the GTL (Gas to Liquid) concept, with synthesis gas (produced from biomass, carbon or natural gas) as feedstock (Fig. 5 ). Methanol is an energy vector according to its utilization as fuel, whether pure or mixed with gasoline and the production of H2 by reforming. Additionally, it is an important raw material for the production of other fuels, solvents and base-chemical products, such as light olefins (MTO process), BTX aromatics, formaldehyde, acetic acid, methyl methacrylate, dimethyl terephthalate, methylamines, chloromethane, dimethyl carbonate, methyl tertbutyl ether (MTBE) and others.Albeit methanol production is carried out from synthesis gas (with a small concentration of CO2) (Eq. (38)), its potential capacity for valorizing CO2 on a large scale led Goeppert et al. [136] to highlight the strategic interest of the reaction for this objective. The plant in Reykjavik (Iceland), with an annual capacity of 4000 metric tons and valorization of 5600 tons of CO2, is the main industrial reference for renewable methanol synthesis from CO2 and H2 using geothermal energy [137]. (38) CO + 2 H 2 ⇌ CH 3 OH The exothermic synthesis of methanol from CO2 (Eq. (39)) requires 3 H2 molecules per CO2 molecule. Thermodynamically, low temperature and high pressure are required to facilitate the extent of the reaction. However, given the low reactivity of CO2, temperature above 240 °C is necessary to achieve an acceptable reaction rate. Thus, under the reaction conditions, the side reactions of rWGS (Eq. (5)) and synthesis from CO (Eq. (38)) take place. The rWGS generates a high content of H2O, which limits the equilibrium conversion of CO2, attenuates the activity of the catalysts and favors deactivation. (39) C O 2 + 3 H 2 ⇌ CH 3 OH + H 2 O To overcome the limitations of the reaction, the action routes are focused on developing new, active, selective and stable catalysts [138–141], reactors and operating strategies [142]. The knowledge of the mechanism for the conversion of CO2 into methanol is necessary to progress in the improvement of catalysts, and so, has received great attention due to its relevance in the synthesis of methanol from syngas, where CO2, of greater apparent reactivity than CO at low conversion conditions [143] is co-fed in a concentration within the 2–8% range. In Fig. 6 , the three routes proposed for CO2 conversion are outlined [142], that is, with formate and hydrocarboxyl species as intermediates or through the rWGS reaction.Cu/ZnO/Al2O3 is the most commonly used catalyst for the synthesis of methanol from CO2, given its commercial use for the same purpose from synthesis gas feedstock, based on the proposal of Imperial Chemical Industries in 1960. In this catalyst, Al2O3 acts as structural promoter favoring the distribution of Cu and providing surface area and mechanical resistance to the catalyst. ZnO also acts as a structural promoter, separating the Cu crystals, and modulates the electronic properties owing to the metal/support interactions between Cu and ZnO. The presence of ZnO reduces the sintering of Cu [139]. The use of ZrO2 in Cu/ZrO2 or Cu/ZnO/ZrO2 catalysts leads to good results, due to the lower hydrophilicity of ZrO2 with respect to Al2O3. Furthermore, the presence of Lewis acidic sites, non-active for the conversion of methanol into hydrocarbons, contributes to attenuate the formation of coke [144]. The incorporation of metallic oxides (SiO2, MgO, Ga2O3, La2O3, TiO2, Y2O3) and noble metals (Pd, Au) as promoters favors Cu dispersion and modifies acid-base and redox properties of the catalyst, improving the selectivity and stability of the catalyst [138]. As an alternative to Cu catalysts, more stable Pd and PdZn alloys on different supports have been proposed, including metal oxides (ZnO, CeO2, In2O3) mesoporous silica (SBA-15, MCM-41) and carbon materials [140].Alternatively to the direct synthesis of methanol from CO2, a two-step process (rWGS-syngas hydrogenation) has been adopted. The advantages over the direct methanol synthesis process rely on the ease for removing the H2O generated in the rWGS. With this approach, its entry to the hydrogenation reactor is avoided and the temperature in each reactor can be optimized. With this technology the Korea Institute of Science and Technology installed the CAMERE (carbon dioxide hydrogenation to methanol via reverse water gas shift) process on a pilot plant scale, with a capacity of 100 kg of methanol per day [142].The mechanism for ethanol synthesis from CO2 (Eq. (40)) is more complex than that for methanol, because comprises more elementary reactions involving C‐C coupling and accurate stages of carbon chain growth and termination. The most accepted mechanism is the so-called CO2-Fischer Tropsch (CO2-FTS). CO generated through the rWGS reaction inserts into *CH3 or *CH3-(CH2)n species produced by CO-FTS to form ethanol or superiors alcohols (C3+OH) [108]. (40) 2 C O 2 + 6 H 2 → C 2 H 5 OH + 3 H 2 O The selection of the composition of the selective multifunctional catalyst is also complex. So far, good results have been obtained with Rh-based catalysts with SiO2 and TiO2 as supports and Fe, Li and Se as promoters [145]. Other catalysts also selective towards ethanol production are prepared with Pt, Au, Mo, Co, and Cu as metallic function [140].The direct production of hydrocarbons from CO2 is a paradigm of catalytic processes integration, with the attraction of lowering equipment cost. However, this route implies important challenges to select the catalyst and establish the appropriate reaction conditions for a good compliance between the thermodynamic requirements and the mechanism of the involved reaction stages [105]. The reaction is carried out in tandem catalysts in the same reactor, through two alternative routes [104,146]: i) Modified Fischer-Tropsch synthesis (MFTS), incorporating a zeolite together with the FTS catalyst. In this manner, hydrocarbons are formed according to the Anderson-Schulz-Flory mechanism [43] and selectively converted on the zeolite, and; ii) with methanol/DME as intermediates (Eq. (41)), using OX/ZEO (metal oxide/zeolite) catalysts, suitable for the reactions of methanol/DME synthesis and the in situ conversion of these oxygenates into hydrocarbons [44]. (41) CO + C O 2 + H 2 ⇒ CH 3 OH / DME + H 2 O ⇒ Light olefins ⇒ Light paraffins The development of the MFTS route has been carried out mainly using Fe-based catalysts. CO2 hydrogenation proceeds through a mechanism with two stages. The formation of CO by the rWGS reaction followed by the chain growth in FT reactions. The selection of the zeolite allows the selective formation of light olefins, aromatics or isoparaffinic gasoline (Fig. 7 ) [147,148]. The addition of other metals (Co, Cu or Ni) to Fe, modifies the adsorption of H2 and CO, improving conversion and selectivity. Thus, with Fe‐Cu the selectivity of C2-C7 hydrocarbons is four times that obtained with Fe, decreasing the formation of CH4 [149]. In this case, as Fe support γ-Al2O3 (followed by SiO2 and TiO2) shows a better behavior than other supports to avoid sintering, thanks to the good dispersion of Fe obtained, based on the strong metal-support interaction [150].In the route with methanol/DME as intermediates, the limitations of the Anderson-Schulz-Flory mechanism are avoided, and as a result, achieving higher selectivities of a family of hydrocarbons is feasible. Carrying out the second reaction (oxygenates conversion) in the same reactor displaces the thermodynamic equilibrium of methanol/DME synthesis, favoring the further conversion of CO2 and CO. Consequently, the reaction can be performed at lower pressure and lower H2/CO2 ratio than for methanol/DME synthesis, easing the supply of H2 from commercial PEM electrolyzers, which supply hydrogen at 15–30 bar [151]. The reaction conditions must be intermediate to those suitable for the two reaction steps. Thus, the conversion of methanol/DME into hydrocarbons occurs through the dual cycle mechanism (Fig. 8 ) [152], requiring temperature above 325 °C for a significant extent [153]. However, this temperature is excessive for the synthesis of methanol/DME, which occurs through a mechanism with formate ions as intermediates [154].The presence of oxygen vacancies in the metallic function is a key feature for the adsorption of CO2 [139]. In addition, this function must have a limited capacity for over‑hydrogenating the double C=C bonds, as to avoid the formation of methane [155]. Besides, the distribution of hydrocarbons depends on the acidic strength and pore size of the zeolite [156]. According to these conditions, In2O3-ZrO2/SAPO-34 tandem catalyst shows good prospects for the selective production of light olefins from CO2 [157], given the capacity of the superficial oxygen vacancies of the In2O3-ZrO2 system for CO2 adsorption and the high light-olefin selectivity achieved in the conversion of methanol/DME over SAPO-34 (CHA topology). Similarly, the use of HZSM-5 zeolites (MFI topology) together with the ZnO/ZrO2 system allows obtaining high aromatics selectivity [152].Wang et al. [158,159] obtained high gasoline yield with a Fe/Zn/Zr@HZSM-5 core-shell catalyst, with isoalkanes as main components and with low aromatics concentration. However, as a drawback, CO selectivity of 40% resulted from the RWGS reaction. This reaction was later suppressed by treating the Fe/Zn/Zr catalyst with tetrapropylammonium bromide (TPAR) [160]. These authors also determine that the treatment affects the hydrocarbon formation mechanism, which proceeds through the two routes (FT and oxygenates as intermediates) with the Fe/Zn/Zr catalyst and mainly with oxygenates as intermediates with Fe/Zn/Zr-Treated catalyst, due to the enhanced adsorption strength of the HCOO* species and desorption rate of CH3O* species. The Fe/Zn/Zr-Treated@HZSM-5 core-shell catalyst is stable for 120 h on stream, with 76% hydrocarbons selectivity and C5+ isoalkane content of 93% in the gasoline, with a CO selectivity of 24% and a CO2 conversion of 18%.The interest in the production of DME is based on its usefulness as fuel and intermediate raw material for the production of hydrocarbon fuels and chemicals, and on the capacity of the process for valorizing synthesis gas derived from renewable sources (biomass) and CO2. The cost and energy- and exergy- efficiencies of DME production from syngas depend on the syngas source and the reactants used in gasification or reforming. These factors determine the H2/CO ratio of the resulting syngas. The interest of valorizing low rank coal to DME via gasification has received continued attention [161] and this attention has extended to the valorization of natural gas and biomass [162]. The urgency for mitigating the effects of climate change by reducing CO2 emission rates has reoriented DME production technologies to make CO2 co-feeding together with syngas or CO2 hydrogenation feasible. The joint valorization of CO and CO2 as carbon sources is an initiative applicable to different industrial emissions and to bio-gas (product of the anaerobic fermentation of biomass composed of CH4 (50–70%) and CO2 (30–50%) [163]). In this line, recycling in the synthesis of DME the CO2 used as biomass gasifying agent reduces up to 20% the environmental impact of the process [164].Dieterich et al. [165] gather the pathways for transforming renewable energy into sustainable energy vectors (DME, methanol and hydrocarbons) in the diagram in Fig. 9 .DME (CH3-O-CH3) is an environmentally benign, non-toxic, non-teratogenic, and non-carcinogenic species, with a slight ethereal odour, which has multiple applications due to its properties (Table 2 ) [166–168]. Among others, it is used as aerosol, propellant (substituting chlorofluorocarbons), pesticide and ecological refrigerant [169]. It is of great interest also as organic solvent, due to the low dielectric constant of liquid DME (5.34 at 30.5 °C and 6.3 MPa), medium polarity, partial miscibility with water, no reactivity, chemical inertness, and affinity for oily compounds (given its capacity for developing one-way hydrogen bonds with hydrogen bonding solutes). These properties along with the easy removal by pressure reduction make it suitable for the extraction of products in food and pharmaceutical industry (lipids, essential oil, flavonoids), of contaminants (as phenols) from mixtures with water [170,171] and in solvent injection processes for heavy oil recovery [172].The large-scale implementation of DME production is based on its properties as fuel, either for domestic use, in the automotive industry or for electrical energy generation. According to Semelsberger et al. [166] a transition from petroleum to DME to hydrocarbons is more cost-effective than a direct change to hydrogen, considered as the “end-game” fuel, since the existing LPG and NG transport and storage infrastructure can be used. The main advantages as fuel are: [173]: i) High oxygen content, lack of C‐C bonds, N, and S compounds, reasons for the soot-, SOx- and NOx-free combustion; ii) low boiling point (−24.9 °C) and consequently, small energy requirement for vaporization, which facilitates its use as fuel gas, alone or blended with liquefied petroleum gases (LPG: propane and butane) given its similar vapor pressure and the same storage and transport characteristics [174]. In addition to domestic use, gas DME is used as fuel in homogeneous charge compression ignition (HCCI) engines, in mixtures with natural gas and hydrogen [175]. iii) High cetane number (> 55) that results in very low auto-ignition temperature. In spite of its low heating value (LHV) of 27.6 MJ/kg, inferior to that of diesel fuel (42.5 MJ/kg), the high cetane number and the short delay-time in the injection, make DME suitable for compression ignition (CI) engines. Using the existing technology, the well-to-wheel efficiency is DME > LPG > Gasoline > CNG (compressed natural gas) and the associated greenhouse gas emissions are significantly lower (DME < CNG < LPG < Gasoline) [167]. Tomatis et al. [164] estimate that replacing diesel by pure DME results in a decrease in greenhouse gases (GHG) of 72%, while limiting the emission of particulates (diesel soot). This emissions decrease has an impact on human health and ecosystem of 55% and 68%, respectively. However, due its high vapor pressure, very low boiling point, high compressibility, low density, low viscosity and the capacity of dissolving some elastomers and plastics, different modifications in diesel engines and in the selection of the materials are required for using DME. The main modifications consist of incorporating a pressurized DME tank, and a high-pressure fuel pump.The evolution towards a DME economy is based not only on its use as fuel, but also on its future as intermediate sustainable raw material. Thus, DTO (dimethyl ether-to-olefins) process may replace or complement MTO (methanol-to-olefins) process, developed by UOP/Mobil and successively improved [32]; and MTP, developed by Lurgi (to selectively obtain propylene) [176]. The implementation of both processes is growing as to satisfy the burgeoning demand of light olefins, which is currently covered through naphtha steam cracking [177] and fluid catalytic cracking (FCC) [178] processes with high energy requirements and high CO2 emissions. The DTO process offers advantages over MTO: i) DME is more reactive than methanol, which allows carrying out the reaction at lower temperature [179]; ii) the lower reaction heat favors temperature control. The DTO process has been mainly studied using SAPO-34 [180,181] and HZSM-5 zeolites [182] as catalysts. For the selective production of olefins, the use of HZSM-5 zeolites of moderate acidity (SiO2/Al2O3 ratio around 180) is suitable. Indeed, the rate of coke deposition is also reduced. Whereas higher acidity (SiO2/Al2O3 of 30) boosts (C5-C11) gasoline yield [35]. Using pseudo-boehmite as a binder, HZSM-5 zeolite is embedded in a mesoporous matrix of γ-Al2O3, providing mechanical resistance to the catalyst particles and attenuating the blockage of the micropores of the zeolite by coke [183,184].DME conversion into hydrocarbons proceeds, like methanol conversion, through the dual cycle mechanism [37], with polyalkylbenzenes as intermediates for the formation of light olefins as primary products. The mechanism occurs along with different side reactions (isomerization, cyclization and hydrogen transfer) forming together with olefins: light paraffins, BTX aromatics, C5 + aliphatics and coke. On the basis of this mechanism, kinetic models for HZSM-5 based catalysts have been established, which allow quantifying the evolution of products distribution with time on stream [185]. With these models, evaluating the effect of various operating strategies on the deactivation and on products distribution is possible, such as the effect of co-feeding H2O or feedstock dilution. Indeed, the models have been used in the design of alternative reactors (packed bed, captive fluidized and fluidized with catalyst circulation) and of a reactor-regenerator system with circulation of the catalyst between both units, based on the technology implemented for the MTO process [186].Another application for DME with development potential is as H2 vector, because its characteristics (high hydrogen content, absence of C‐C bonds and low toxicity) facilitates the reforming at low temperature (< 300 °C) and results in high H2 yield. This can be applied for proton exchange membrane fuel cells (PEMFC) [190] and solid oxide fuel cells (SOFC) [191], as well as to cover, on a large-scale, the growing demand of H2 in the petrochemical industry. Catizzone et al. [39] propose DME as a good candidate for energy storage through the cycle comprising the synthesis of DME from CO2 (exothermic) and the reforming of DME to H2 (endothermic). In this way, the energy demand of the reforming is covered by the energy generated intermittently from renewable sources.Steam reforming takes place on bifunctional catalysts, through DME hydrolysis in the acidic function (Eq. (42)) followed by methanol reforming in the metallic function (reverse of Eq. (39)). Additionally, the secondary reactions (rWGS (Eq. (5)), DME partial decomposition, methanation (Eqs. (34) and (35)), Boudouard (reverse of Eq. (7)) and hydrocarbons formation) contribute to products distribution, (42) CH 3 O CH 3 + H 2 O ⇌ 2 CH 3 OH The most used catalysts in a lab-scale have been prepared with CuO-ZnO-Al2O3 (CZA) metallic function, based on the commercial catalyst for methanol synthesis and methane reforming. The main innovations have mainly consisted of the utilization of CuM2O4 spinels (M = Fe, Mn, Cr, Ga, Al, etc). Among these, CuFe2O4 spinel has received a great attention due to its thermal stability [192,193], which recovers its activity in reaction-regeneration cycles [194,195]. γ-Al2O3 has been the most used acid function for DME hydroxylation [196,197], but has been progressively substituted by HZSM-5 (more active). HZSM-5 needs to be adequately treated (as desilicated by alkaline treatment) in order to avoid the formation of hydrocarbons and the consequent formation of coke [198,199]. Oar-Arteta et al. [194,200] have improved the properties of γ-Al2O3, obtaining it by calcination of pseudo-boehmite. This treatment provides the catalyst with high mechanical resistance (a deficiency of the CuFe2O4 spinel) and also with moderate acidity, limiting the formation of hydrocarbons. Therefore, it allows for stably operating in reaction-regeneration cycles at 350 °C achieving a yield of 82%. Filling the gap in the kinetic modeling for oxygenates reforming, Oar-Arteta et al. [195] have proposed a kinetic model based on LHHW expressions for each step, establishing as optimal reforming conditions: 360–380 °C and a steam/DME ratio of around 6. The use of microreactors with ceramic channels eases H2 generation for portable fuel cell applications [201].Zhan et al. [202] have conducted a review of the studies of ethanol production from DME through carbonylation. This reaction is a key stage in the valorization of synthesis gas. The reaction, as the formation of methyl acetate (MA), takes place through the Koch-type CO insertion into DME, with zeolites (typically HMOR and HZSM-5) as catalysts. The MA is later converted into ethanol on Cu-based catalysts.DME production (10 Million tons per year) is carried out in a two step process, in separate units (indirect synthesis) using syngas feedstocks [203]. Methanol is synthesized in the first unit (under reaction conditions described in Section 2.3.3) and dehydrated towards DME in the second unit (MTD process). Methanol dehydration is a reversible exothermic reaction on acid catalysts, whose thermodynamics is not favored increasing pressure, but rather decreasing temperature. The process has been reoriented towards valorizing CO2. In Fig. 10 the routes for CO2 upgrading to DME are plotted [165]. Michailos et al. [204] estimate within the 1.83–2.32 € kg−1 range the cost of DME production from captured CO2. Schemme et al. [205] determine that the production of DME (equaling its technical maturity to that of methanol synthesis) is a cheaper route for valorizing CO2 than the production of alcohols (methanol, ethanol, butanol, octanol), polyoxy dimethyl ether, and hydrocarbons (synthetic gasoline, paraffinic diesel, and paraffinic kerosene), emphasizing the relevance of H2 production costs (58–83% of the total manufacturing costs). Uddin et al. [206] make a techno-economic analysis of the two stage DME synthesis via the birreforming of landfill gas (with steam and CO2 from an ammonia plant). These authors estimate a price of 0.87–0.91 $ gal−1, competitive with the price of diesel fuel. Furthermore, using landfill gas sourced CO2, the process achieved negative emissions.The industrial process has different licenses and an extensive implementation in asiatic countries since the beginning of the 21st century with carbon as raw material [174]. It is performed under moderate pressure (below 20 bar) and within 150–300 °C temperature range. γ-Al2O3, of low manufacturing cost, is generally used as catalyst [207–209]. The weakly acidic nature of the Lewis sites of γ-Al2O3 is appropriate to achieve a high DME selectivity, inhibiting the formation of hydrocarbons as by-products. Nonetheless, its activity is moderate and temperatures above 250 °C are required, besides the activity may be improved by modifying γ-Al2O3 with P, Ti, Nb, B, etc. [210]. In addition, due to its hydrophilic character, it has a great capacity for adsorbing H2O (product of dehydration), reducing thereby its activity and causing dealumination, particularly when aqueous methanol is fed [211]. Catalysts with higher acidity than γ-Al2O3 have also been studied, which allows the reaction to take place at lower temperature, avoiding the formation of hydrocarbons. For this purpose, the optimal performance of heteropolyacids (HPAs) (more active than HZSM-5 catalyst) has been proven, and enhanced by incorporating W and P [212] and supporting HPAs on TiO2 [213].The greatest research effort in the design of catalysts for methanol dehydration has focused on zeolites, whose performance (activity, DME selectivity and stability) is influenced by the configuration of the channels of their crystalline structures and the quantity and strength of the acidic sites [214]. HZSM-5 zeolite (MFI topology), which is less hydrophilic than γ-Al2O3 has received a special attention. In particular, for the valorization of CO2 together with syngas, in order to avoid the separation of the high content of H2O in the aqueous methanol produced in the first stage. This zeolite contains pores with moderate severity of shape selectivity and the acidity is dependent on the SiO2/Al2O3 ratio, with sites of moderate acidic strength mainly. Besides, the behavior of hybrid catalysts composed of HZSM-5 zeolite impregnated with γ-Al2O3 is also of interest, being it more active and selective than each separate catalyst, due to the dilution of the strong sites of the zeolite [215,216]. The desilication of HZSM-5 by means of an aqueous solution of NaOH is effective to attenuate the deactivation by coke, because the treatment decreases the acidic strength of the sites. In addition, the coke is deposited in the generated mesopores, reducing the blockage of the micropores of the zeolite [217].Catizzone et al. [218] have proposed ferrierite (FE) as ideal catalyst, since its crystalline structure with two dimension channels make it highly selective and, additionally, coke deposition is reduced. This zeolite, prepared with a high Al content, allows achieving DME selectivity close to 100% at 200 °C and high methanol conversion (up to 82%), in contrast to γ-Al2O3 (conversion of 25%). Moreover, methanol conversion and DME selectivity of FE can be improved by increasing the density of Lewis sites and reducing the crystal size [219]. Comparing the features of FMI and FE zeolites, Catizzone et al. [220] achieve similar DME selectivity with nano-sized MFI and FER, whereas for the former higher reaction rate and lower coke deposition are reported.Methanol dehydration to DME (reverse of Eq. (42)) proceeds through two competitive reaction pathways: Associative (or direct) and dissociative (or sequential) (Fig. 11 ). In the first, two methanol molecules are adsorbed on an acidic site and react to form DME and H2O. The reaction can occur by splitting of protonated methanol dimer into the methyl carboxonium ion and carbenium ion at the same time, or into two methyl carboxonium ions, which are further combined to form DME molecule [221]. In the second, one adsorbed methanol molecule reacts to form H2O and a CH3 species bound to the deprotonated zeolite, and then, a second methanol molecule adsorbs to react with the CH3 group to form DME. Park et al. [222] highlight the discrepancies in the literature on the predominant mechanism, which depends on the catalyst and the operating conditions. These authors, using computational chemistry and microkinetic modeling, determine that the dissociative pathway is the dominant for the reaction with an H-zeolite, being DME formation reaction the rate-controlling step. However, these theoretical results differ from those obtained by Trypolskyi et al. [223]. Adjusting the experimental results of methanol dehydration on a HZSM-5 zeolite these authors propose methanol adsorption as the rate-limiting stage; being equally valid the kinetic expressions of LHHW deduced for the associative and dissociative pathways to adjust the experimental results.The reactions involved in the process are:Methanol synthesis (Eqs. (38) and (39)); Reverse Water Gas Shift (rWGS) (Eq. (5)); Methanol dehydration towards DME (Eq. (43)): (43) 2 CH 3 OH ⇌ CH 3 O CH 3 + H 2 O ΔH 0 = − 23.4 kJ mol − 1 ΔG 0 = − 16.8 kJ m o l − 1 and paraffins formation secondary reaction (mainly methane): (44) nCO + 2 n + 1 H 2 ⇌ C n H 2 n + 2 + nH 2 O n = 1 − 3 The interest in the direct route for DME synthesis is based on different factors: i) Thermodynamic advantages. Conducting methanol dehydration (Eq. (43)) in situ in the same reactor displaces the equilibrium of methanol formation reactions (Eqs. (38) and (39)). ii) lower cost of production in comparison to the synthesis of DME in two steps and to the synthesis of methanol [224]. Thus, the energy efficiency is around 64–68% for a 2500 equivalent t/day, higher than methanol synthesis, with an energy requirement 5% lower and a lower capital cost (8% lower) [225,226]; iii) possibility of using synthesis gas generated from various hydrocarbonated raw materials as carbon, natural gas, biomass or residues of the consumer society (Fig. 12 ), and from a steel-making plant (mixture of coke oven gas and tail gas) [227]; iv) boost of gasification and anaerobic digestion of biomass [228] in order to contribute to neutral carbon balance. A comparative exergo-economic analysis of the indirect and direct routes for DME synthesis, based on air-steam biomass gasification with CO2, has evidenced the lower cost of DME production through the direct route (1.66 $ kg−1, whilst 2.26 $ Kg−1 for the indirect route), and also, the lower energy consumption and net CO2 emission [229]. In addition, given the higher price of the product, the gasification-DME process from biomass was approximately 7% more economically feasible than the gasification-MeOH process [230]; v) opportunity to maximize the natural gas operating profit, integrating its valorization with DME synthesis.Taking into account these advantages, Olah et al. [231] considered the one step synthesis of DME (Fig. 13 ) a key route for the catalytic valorization of CO2 on a large-scale. Furthermore, these authors have placed great emphasis on the sustainability of the process when CO2 is co-fed with synthesis gas produced from lignocellulosic biomass.In the literature regarding methanol synthesis thermodynamics [232–234] and one step DME synthesis [235–237], synthesis gas has been studied as feestock, whereas little attention has been given to CO2 conversion capacity, whose role has been restricted to secondary product of the reaction. The interest in CO2 conversion processes on a large-scale requires new studies regarding the thermodynamics and kinetics, aimed at establishing the appropriate conditions and the reactor design. Chen et al. [238] have compared the DME synthesis thermodynamics in two steps and in a single step, co-feeding CO2 with synthesis gas. The results support that with both strategies CO2 co-feeding decreases DME yield, and also that the direct synthesis of DME has lower thermodynamic limitations and allows achieving higher CO2 conversion.Ateka et al. [153] have compared in depth the thermodynamics of both methanol synthesis (MS) and the direct synthesis of DME (DS), from the perspective of the capacity of these processes for valorizing CO2. The effect of the reaction conditions (temperature, pressure and feed composition) in regard to CO2 conversion, oxygenates yield and selectivity (MeOH and DME) and heat generated in each process were determined. Being CO and CO2 hydrogenation exothermic reactions with reduction of mole number, oxygenates production is favored with increasing reaction pressure, while penalized upon increasing temperature. The study ascertained that valorizing CO2 is feasible in MS and DS processes for CO2 rich feedstocks (CO2/COx > 50%) at 250–300 °C (suitable range to obtain good catalytic performance [239] and avoid sintering [240]) (Fig. 14 ). Nonetheless, higher CO2 conversion values can be achieved in DS than in MS (for CO2/COx > 75%), greater upon further increasing CO2 concentration in the feedstock (Fig. 15 ).The study of Ateka et al. [153] highlighted the relevance of the CO2 content in the feedstock, and that the DS is more thermodynamically favorable than MS for oxygenates production under suitable operating conditions. For its interest for simplifying reactor design, the possibility for operating at thermo-neutral conditions was tested, combining the aforementioned exothermic nature of CO and CO2 hydrogenation reactions and the endothermic nature of the involved rWGS reaction (of special relevance for CO2 containing feedstocks). Clearly, CO2 co-feeding positively contributes to reduce the heat released in the reaction and helps avoiding hot spot formation (Fig. 15). Heat production diminishes from 80 to 45 kJ mol−1 for MS and from 90 to 60 kJ mol−1 for DS for CO2/COX = 0.5 feedstocks. Anyhow, the study reveals the impossibility of working with Cu based traditional catalysts at thermo neutral conditions, since temperatures above 340 °C are required for this purpose in any case and Cu catalysts undergo sintering at temperatures above ~300 °C.Furthermore, it should be noted that the effect of the reaction conditions on DME yield is opposite to the effect on CO2 conversion and so, that optimizing of each of these objectives requires different reaction conditions. Thus, CO2/COx ratios below 0.25 are suitable for enhancing DME production, whereas ratios above 0.5 improve the conversion of CO2. Consequently, to combine the economic objective associated with the production of DME and the economic/environmental target of reducing CO2 emission rates, intermediate conditions are necessary.For this process, bifunctional catalysts comprising metallic catalysts for methanol synthesis (as introduced in Section 2.2.3) and acidic catalysts for methanol dehydration into DME are required. In addition, by feeding CO2 various differences from the syngas-to-DME process arouse. One the one hand, as introduced in Section 3.3, according to thermodynamics, lower DME yield is obtained. On the other, the role of the rWGS (Eq. (5)) reaction is more relevant, giving way to higher H2O content in the medium. H2O inhibits the production of methanol (reduces the reaction rates of methanol formation by CO and CO2 hydrogenation and of WGS reactions) since H2O molecules tend to strongly adsorb on the surface active sites of the catalyst [241–243]. Moreover, deactivation problem assumes greater relevance [244]. Thus, the higher CO2 and H2O concentrations in the reaction medium favor CuO oxidation and its sintering, which is an important feature due to its irreversibility. However, these unfavorable effects should not fade the main advantage, that is, the attenuation of coke deposition due to the aforementioned role of H2O in the reaction medium for controlling the concentration of superficial methoxy species, as well as the ability of H2O to diffuse coke precursors [245]. This said, within the research works to improve the catalyst, two pathways can be distinguished: 1) focused on improving each function of the catalyst, and; 2) oriented towards optimizing the contact between both functions of the catalyst by changing the structure of the bifunctional catalyst particle. Besides, given its importance in the viability of the process, the deactivation of the catalyst is also worth of study. These features of the bifunctional catalysts for the direct synthesis of DME from CO2 are studied separately in the following sections.In the 1960s Imperial Chemical Industries proposed CuO-ZnO-Al2O3 (CZA) metallic function a suitable option for methanol synthesis under mild conditions and has been widely used since [246]. Cu (Cu0 and Cu+) is the active species for CO and CO2 hydrogenation, whereas ZnO is used as geometric spacer for enhancing its dispersion and for stabilizing it [247,248], helping to hider sintering and poisoning. Nevertheless, it has been substituted by La2O3 [249], MgO [250], Fe2O3 and CeO2 [251,252] for promoting CuO dispersion, catalyst stability and COx conversion.Al2O3 in the CZA catalyst has also been replaced, partially or totally, by other metal and non-metal materials. Among others, MnO has been reported to enhance CuO and ZnO dispersion and reduce the temperature required for CuO reduction, giving way to a larger specific surface area of active Cu0 [253,254], and so, boosting DME yield. Moreover, the Cu‐Mn spinel formed resulted very active in the WGS reaction [253,254]. Likewise, the addition of ZrO2 is widely reported [255,256] to improve the performance of the catalysts as a result of the stabilization of the Cuδ+ sites under reducing and oxidizing conditions [257] and higher H2O tolerance [258–264]. On the one hand, the weak hydrophilicity of ZrO2 hinders the adsorption of H2O (competing with the adsorption of the reactants), and on the other, its basicity favors CO2 adsorption, improving therefore methanol production. Given the promising results of Cu/Zn/Zr catalysts, various authors have deepened in broadening the knowledge on their activity. As to tailoring the catalyst, Sánchez-Contador et al. [144] have further studied the effect of ZrO2 loading into the CuO-ZnO metallic function, synthesizing MeOH from CO2/CO/H2 mixtures under the reaction conditions required for the direct synthesis of DME. Cu/Zn/Zr = 2:1:1 was determined to be the most suitable ratio for achieving an optimal agreement between COx conversion (8.14%), methanol yield and selectivity (over 98%) and catalyst stability. Singh et al. [265] attribute the high activity of the Cu/Zn/Zr catalysts to the interactions between Cu and ZnO and ZrO2 oxides, generating oxygen vacancies and stabilizing the methoxy species intermediates in the formation of methanol. Moreover, ZrO2 tunes the acidity of the bifunctional Cu/ZnO/ZrO2, adapting it to the selective production of DME. Through steam-treatment of Cu/Zn/Zr catalysts using tetrapropylammonium bromide (TPABr) Chen et al. [266] manage to suppress the formation of CO via the RWGS reaction, in addition to increasing the activity, selectivity and stability of the catalysts, due to the increase in the concentration of oxygen vacancies. The same goal is achieved by ultrasonic-assisted impregnation of TPABr to stabilize the CuBr phase on the catalyst surface [267].As to the reaction mechanism regards, Frusteri et al. [268] hypothesize that ZrO2 could also have the capability for activating the adsorbed CO2 giving way to CO2* species. These CO2* species are assumed to react with H2* species to give intermediate species (formate, dioxomethylene, methoxy), which will further evolve to methanol. According to Witoon et al. [269,270] bicarbonate species formed from CO2* are considered to be the ones reacting with H2* to give way to methanol. Both CuO-ZnO-MnO and CuO-ZnO-ZrO2 catalysts outperform the results obtained with CuO-ZnO-Al2O3 in a similar manner for H2 + CO + CO2 feedstocks. The cost of the former is lower, and so its use for CO/CO2 mixtures hydrogenation is suitable, while for pure CO2 hydrogenation the latter outstands [255]. Li and Chen [271] studied in detail the synergyes induced by ZrO2 (Fig. 16 ) and summarized the approaches to improve the catalytic performance of ZrO2-containing catalysts for CO2 hydrogenation to methanol.Ga2O3 promoter (with lower capability for adsorbing H2O than ZrO2) [272] has been reported to facilitate the reducibility of the catalyst [273–275], improve Cu stability [276,277] and dispersion [278]. Moreover, enhances ZnO conductivity and favors the creation of redox-active defect sites as structural promoters [273]. Also, high methanol yields have been achieved by the addition of Ga2O3 to Cu-ZrO2 catalysts [279,280]. Furthermore, in this line, quaternary catalysts have also been proposed, like Cu-ZnO-ZrO2-TiO2 [259] given the addition of TiO2 leads to the creation of oxygen vacancies for the adsorption of CO2 [281], and Cu-ZnO-Al2O3-CeO2 [282,283].Pursuing the increase of methanol formation reaction rate by favoring the adsorption of the reactants (H2 and CO2), the addition of small amounts of noble metals to Cu-ZnO based catalysts has been suggested [284]. The promoting effects of this addition have been mainly attributed to the hydrogen spill-over mechanism [285]. Among these metals: Au [286–288], Pd [289–291], Pt, Rh [292].As an alternative approach, the use of SBA as support for the confinement of Cu-ZnO actives sites within its mesoporous structure has been studied by Prieto et al. [293]. This configuration enhanced the contact of the active sites with the reactants, resulting in higher activity and thus, methanol production. Carbon nanotubes [294], graphene oxides [281], and carbonaceous coordination polymers have also been reported as supports to boost the activity and stability of Cu-ZnO catalysts. These supports reduce the size of the active sites and favor distribution, facilitating the reduction, and hampering the strong adsorption of H2O, giving way to more stable and active catalysts for methanol production.Nevertheless, as to overcome the limitations of Cu based catalysts (sintering, low CO2 activation capacity) non Cu-based oxide catalysts are being tested for methanol production, especially seeking for stable catalysts for CO2 hydrogenation. In this regard, Wang et al. [295] studied binary ZnO-ZrO2 catalyst obtaining high per-pass CO2 conversion and resistance to poisoning by SO2 and H2S. The -Zn-O region for dissociating H2 is also the active site for the direct hydrogenation of CO2 to methanol with HCOO, H2COO and H2CO as intermediates. These authors reported outstanding stability during 500 h TOS, and Wang et al. [296] doubled (1000 h TOS) the stability with In2O3 catalyst. In these catalysts, defective oxygen vacancies are considered the active sites for the direct hydrogenation of CO2 to methanol with HCOO, H2COO and H2CO as intermediates [297–299]. With this catalyst the rWGS reaction is inhibited [300], thus, the CO2-CO-methanol pathway of Cu based catalysts is avoided. The addition of ZrO2 as structural promoter prevents In2O3 sintering and, considering that In and Zr metals have different valence number, within the In2O3 structure additional surface oxygen vacancies are created due to the replacement of In by Zr atoms [301], helping CO2 adsorption [299,302] and so, the selective formation of methanol [299,303]. Similar effect has been demonstrated for Ga insertion into the In2O3 lattice [304], and in both cases, controlling the ratio between the metals is a key feature to be optimized for maximizing the performance of the catalyst.Co containing catalysts have also exhibited high activity for selectively producing methanol from CO2, inhibiting the rWGS reaction [305]. With Mn‐Co catalysts a synergy between the metals results in increasing surface basicity and improving methanol selectivity [305,306]. According to Wang et al. [307], for Co based catalysts, the addition of SiO2 leads to the formation of Co-O-Si species, favoring the formation of methanol by increasing *CH3O species reactivity and hydrogenation over methane production by C‐O dissociation.For their excellent stability and resistance to poisoning, noble-metal based catalysts such as Pd/ZnO [308], Pd/In2O3 [309] and Au/ZrO2 [310], with different supports (i.e. Ga2O3 [311], CeO2 [312] or In2O3 [309], MOF, SBA-15, CNT, SiC…), promoters (e.g. K2O, MgO, CaO) and preparation methods are also being tested with good results despite its higher cost. Ca-promoted Pd nanoparticles (2–6 nm) over mesoporous CeO2 are active for metanol synthesis and dehydration to DME [313]. To be highlighted, the stability of Pd0 nanoparticles, the induction of structural defects by Ca in CeO2 that favor the absorption of CO2 and the balance between the amount of basic and acidic sites. It is claimed that Pd‐Zn alloys stabilize the formate intermediates and ease the direct formation of methanol from CO2, inhibiting the CO formation by the rWGS reaction [314,315].For Au-based catalysts, the relevance of the support on the overall activity and product selectivity is highlighted [142]. For these catalysts, Hartadi et al. [316] explain the selectivity order: Au/ZnO > Au/ZrO2 > Au/TiO2 > Au/Al2O3 by the larger size of Au particles, although it is accompanied by a decrease in activity. These authors determine for the Au/ZnO catalyst that CO2 is directly hydrogenated to methanol and that this reaction proceeds via an independent reaction pathway (presumably with adsorbed formate and methoxy species as intermediates) [317]. This independence of the mechanisms explains the shift in the main carbon source for methanol from CO2 to CO as the temperature increases from 240 to 300 °C [318]. Wu et al. [310] confirmed the higher activity and selectivity of Au/ZrO2 catalysts prepared with sub-nanometric particles (1.6 nm) was due to the appropriate coupling between the Au and the support.For methanol dehydration to DME solid-acid catalysts are required. Desirably hydrophobic, stable, active and selective under the required reaction conditions. In the vast majority of studies, γ-Al2O3 is used, given its reported high selectivity within the temperature range required in the process (200–300 °C) and relatively low manufacturing cost [208,319]. Ghorbanpour et al. [320] made a computational assessment of the reaction mechanism and determined that depending on the reaction conditions (temperature and pressure) methanol dehydration could proceed through: i) A dissociative route, that is, methanol adsorbed in an acidic site would lose a water molecule and transfer into a surface methoxy group to react to with another methanol molecule leading to the formation of DME; or ii) an associative route, where two methanol molecules co-adsorb on an acidic site to give DME. Nonetheless, given the hydrophilic nature of γ-Al2O3, its activity decays significantly due to the ability for adsorbing the H2O formed in the process leading to dealumination. Moreover, H2O has multiple roles in the conversion of methanol to DME: i) shifts the thermodynamic equilibrium of methanol dehydration to DME; ii) decreases the acidity of the catalyst by adsorbing on the acid sites (competing with methanol [321,322]), and iii) inhibits the formation of methoxy ions by shifting the equilibrium [245]: (45) Al − OH + CH 3 OH ⇌ Al − O − CH 3 + H 2 O This feature is way more relevant for the direct CO2-to-DME process, where hydrothermal conditions are more severe than with syngas as reactant. Therefore, the research on the acid catalysts has focused on mitigating the activity decay due to H2O adsorption by progressively diminishing hydrophilicity and facilitating its desorption from the acid sites, bearing in mind the acid catalyst for the process requires limited acid strength, as to avoid the formation of hydrocarbons [323]. MCM-41 supported tungstophosphoric acid (TPA) has also been used, based on the high turnover frequencies for methanol dehydration to DME [324]. On the basis of the above premises, besides modifications of γ-Al2O3 [210,325], various alternatives have aroused among which zeolites (framework types as BEA, EUO, FER, MOR, MTW, TON [326,327]) and in a wider extent MFI type (HZSM-5 [328,329] and silicoaluminophosphates (SAPO-11, −18, −34)) outstand [330]. Catizzone et al. conducted a screening among different framework type zeolites for methanol to DME dehydration and studied the effect of crystal size, Si/Al ratio and acidity. These authors claimed the better performance of FER- and MFI-type zeolites among others, especially in terms of selectivity, stability and limited formation of carbon species [326,327]. In the literature HZSM-5 is the most studied zeolite since it exhibits good hydrothermal stability and activity due to its topology and acidic properties. Anyhow, the strong Brönsted nature of the sites makes it prone to coke deposition [331]. To overcome this a great deal of effort has been placed on tailoring HZSM-5 [332] and numerous modifications have been widely studied [333–335], most of them oriented towards the passivation of the acid strength, to attenuate coke deposition [218,336]. Zeng et al. [216] determined that with the partial desilication and dealumination of ZSM-5 the strength of the surface acidic sites diminishes and the mesoporous presence increases. As a consequence, not only the catalytic performance, but also the hydrothermal stability and deactivation resistance improved. According to Ordomsky et al. [337] silication also resulted effective for stabilizing the HZSM-5 based catalyst, minimizing the progress of the hydrocarbon pool mechanism, while Wei et al. [338] used alkaline treatment passivation and partial activation for the same purpose. Aboul-Fotouh et al. [339] tuned the acidity (more active catalysts achieved) by chlorination or fluorination methods. Aloise et al. [217] reported that the increase of mesopore diameter, obtained by desilication, allows the formation of larger amount of accessible acidic sites, minimizing therefore the formation of coke deposits and upgrading DME production. Krim et al. [340] attained a DME selectivity of 74% with hollow nano-HZSM-5 with mesoporous shell synthesized by alkaline treatment.Sanchez-Contador et al. [330] compared the performance of HZSM-5 zeolite with SiO2/Al2O3 ratios of 80 and 280, subjected to thermal and dry steaming treatments for acidity passivation, and SAPO-18 and -11 [330]. This study claims that under the conditions required for the CO2-to-DME process (250–325 °C, ~20 bar), the performance of SAPO-11 is slightly better than that of the thermally treated HZSM-5(280) zeolite, and this, better than for SAPO-18 [255]. The better behavior of SAPO-11 molecular sieve is attributed to the properties of the acidic sites (high density of weak strength acidic sites) and the AEL topology of its porous structure) [341,342]. These properties minimize the adsorption and retention of hydrocarbon molecules, as well as their condensation to form polyaromatic components of coke [330]. Chen et al. [342] demonstrated that the acidity of SAPO-11 could be diminished and specific surface and mesoporosity increased by synthesizing nano-sized particles (~200 nm), resulting in a better activity for methanol dehydration. On the other hand, even if high methanol conversion and DME selectivity is accomplished with SAPO-34, given the large channels and narrow openings of its structure, suffers severe deactivation since large hydrocarbon molecules are retained blocking the pores [343,344].To a lesser extent, other materials have also been tested. For example, HY zeolites or HMCM-22, Witoon et al. studied the use of sulfated zirconia, Frusteri et al. [345] and Catizzone et al. [214,326] justified the optimal performance of ferrierite by its porous structure and moderate acidic strength.For the preparation of the catalyst, the metallic functions presented in Section 4.1 and the acid functions presented in Section 4.2 have to be combined. The typical strategy is to provide an excess of acid function. In this way, the displacement of methanol synthesis equilibrium is ensured (see Section 3.3) and the overall reaction is controlled by methanol formation, which is the slowest step. Given the relevance of the intimacy of the contact between the metallic and the acidic functions on the overall performance of the catalyst, the configuration of the catalytic bed has been largely addressed. Yao et al. [346] ascertain that with a close contact between the functions DME could be generated through a shortcut methoxy-DME pathway, with no need for methanol formation as intermediate (typical methoxy-methanol-DME route), resulting in a more efficient production of DME. In the literature the following arrangements are studied: 1) Dual bed configuration, placing first the metallic function for CO2 hydrogenation to methanol, and subsequently the acidic function for its dehydration to DME; 2) physical mixture of metallic function and acidic function particles; 3) hybrid configuration, the most common configuration where both functions are mixed conforming bifunctional catalyst particles; 4) core-shell configuration, where one function is encapsulated by the other, and; 5) structured catalyst. Regarding thermodynamic basis, in the first strategy a two-set process would be taking place, at the same reaction conditions. Therefore, the lower activity of this system over other configurations reported by several authors is to be expected [258,346–349].Ateka et al. [347] conducted the comparison of the strategies 1–3 for the combination of CuO-ZnO-MnO (CZMn) metallic and SAPO-18 acidic functions, for valorizing CO2 co-fed with synthesis gas, emphasizing the low cost of CZMn metallic catalyst among other options [153,255]. In all cases, both functions where mixed at the optimal 2/1 mass ratio (metallic function/acid function). In the dual bed strategy (strategy 1), DME selectivity did not surpass 85%, evidencing the suitability of combining the proposed functions. The conversion of the CO2 + CO mixture fed (50% each) with the dual bed strategy resulted 50% lower than when particles of both functions where mixed conforming a single catalytic bed (strategy 2). Moreover, combining CZMn and SAPO-18 in a single hybrid catalyst particle (strategy 3), the closer contact between the functions led to improve DME selectivity (~95%) and boost CO2 + CO conversion, doubling that obtained in the dual bed strategy (22% vs 10%). Yao et al. [346] performed a similar study for the combination of Cu-In-Zr-O (CIZO) and SAPO-34. They reported that the adjacency of both functions facilitates the migration of intermediate methoxy ions from CIZO to SAPO-34, so that DME could form directly. That is, CO2 conversion improved from <3% to ~4.5% when changing from the dual bed strategy to hybrid catalyst, whereas DME selectivity remained around 60% in all cases. In other cases, like for Bonura et al. [348], the performance of the catalyst is lower for the hybrid catalyst configuration than for the catalytic bed composed of pre-pelletized individual functions (strategy 2) of Cu-ZnO-ZrO2 (CZZr) and HZSM-5 zeolite in a 1/1 mass ratio [348]. This decay is related to the blockage of the zeolite pores inlet by the metallic function on the mortar treatment and pelletizing steps. Later, these authors studied the influence of the precipitating agent on the generation of the metallic function directly in a solution containing the zeolite (HZSM-5 [350], MOR or FER [349]) as to “englobe” the latter. The procedure improved the activity of the system, presumably by the enhanced hydrogenation functionality related to the “multisite” reaction path; primary adsorption of H2 on the metallic sites reacting with the CO2 adsorbed on the strong basic sites to form methanol, and the subsequent dehydration on the acidic sites of the zeolite.The core-shell structure (strategy 4) is being explored as an alternative to hybrid catalysts [342,351]. Unlike the hybrid catalysts prepared by extrusion of the metallic and acidic functions configuring each catalyst particle, the core-shell structure consists of depositing the one function on a previously prepared nucleus of the other. Typically, the acid function covering the metallic nucleus (Fig. 17 ). This structure can be prepared by either hydrothermal synthesis, single-crystal crystallization, dual-layer method or physically adhesive method. The general objective of the core-shell structure in catalytic processes is to preserve the catalyst from poisons adsorption, attenuating the sintering of the metallic particles and controlling DME selectivity by space confining the reactions. Thus, in multiple step reactions (cascade reactions), a more favorable reaction medium is achieved for each step. There are contributions in the literature for this initiative in the direct synthesis of DME, with core-shell catalysts prepared with the conventional CuO-ZnO-Al2O3 metallic function and using as acidic function HZSM-5 zeolite [352], γ-Al2O3 [353,354], SiO2-Al2O3 [355] or SAPO-11 [356]. Guffanti et al. have conducted model analyses for evaluating the effect of the active phase distribution [357] and of the kinetics, adsorption capacity and mass and heat transfer [354] in the performance of hybrid, mechanically mixed and acidic-function@metallic-function and metallic-function@acidic-function structured core-shell catalysts. These works highlight the influence of the internal diffusion on productivity, pointing out metallic-function@acidic-function as the most suitable configuration, and that the small particle diameters and limited contact between phases avoids hot spots generation, favoring DME formation.Sánchez-Contador et al. [144,330,351] have prepared a CuO-ZnO-ZrO2@SAPO-11 core-shell catalyst by physical adhesive methodology (in a mass ratio of 1/2) with SiO2 solution as adhesive [356,358]. With this configuration, methanol synthesis occurs in the CuO-ZnO-ZrO2 core and diffuses for later being dehydrated in the surrounding SAPO-11 acidic shell. These authors have corroborated that the preparation method of core-shell particles prevents the partial blockage of SAPO-11 mesopores by CuO-ZnO-ZrO2 particles in the pelletizing step used for preparing hybrid catalysts. For CO2 + CO mixture hydrogenation (50% each) a DME yield of 8.7% and selectivity of 81% are achieved with this core-shell catalyst, whereas 7% and 77%, respectively, for the hybrid system (325 °C, 30 bar, 7.6 gcat h molC −1). Fig. 18 compares the COX conversion and products yields obtained with the core-shell configuration with those obtained with the conventional hybrid configuration. Moreover, the core-shell configuration prevents catalysts deactivation. After 24 h TOS, ~ 37% of DME yield decrease has been reported for conventional hybrid catalysts (from 7.4 to 4.7%), whereas the lessening is contained (to 21%) for the core-shell configuration (from 8.67 to 6.8%) [351].Among the causes for the better performance of the core-shell over hybrid catalysts, the above mentioned works emphasize the creation of a favorable reaction medium by separating the methanol synthesis and its dehydration reactions in different regions, providing a higher availability of acidic sites on the catalyst particle surroundings for the conversion of the methanol formed in the nucleus. On this manner, limiting the presence of H2O in the metallic nucleus leads to a greater resistance towards sintering of the Cu species in the nucleus [359]. Moreover, with a core-shell structure the adverse effects derived from the interaction between phases can be minimized. Thus, Nie et al. [360] have highlighted the advantage of the confinement of Cu species in the nucleus, avoiding their migration towards the acidic function. García-Trenco and Martínez [361] have proven through XPS analysis and 27 Al MAS-NMR spectra the migration of Al3+ species from HZSM-5 zeolite towards the CuO-ZnO-Al2O3 metallic function, resulting in catalyst deactivation by Cu sintering.An important challenge for the scale-up of the CO2-derived DME synthesis is to prepare catalysts with appropriate particle size and mechanical strength for industrial fixed-bed reactors. This requires addressing the agglomeration (using binders) of the catalysts configured with optimal structure according to laboratory scale results (as shown in this section) to build catalysts of several mm of particle size, high mechanical resistance and minimal performance loss (activity and selectivity) due to limitations of mass and heat transport. An overall view of the stages to progress towards the scale-up in the preparation of catalysts has been described in the literature [362,363].To overcome the heat transfer limitations of the commonly used packed bed reactors with catalyst particles, the use of monolithic reactors (strategy 5) has been proposed and experimentally studied for syngas feedstocks [364,365]. For such configuration, the conductivity of the materials, cell density of corrugated monoliths and tortuosity of open cell foams are relevant parameters. Magzob et al. [364] compared the performance of HZSM-5 powder and monolith-structured (HZSM-5 and HZSM-5@SAPO-34) catalysts within 180–320 °C temperature range. With the HZSM-5 monolith configuration, a reduction on Brönsted acidic sites (and increase of Lewis acidic site density) and improvement of mesoporosity was reported. With this characteristics, better catalytic performance than for the powder zeolite was achieved, thus, methanol conversion ~70%, with high DME selectivity (96%) yet at 180 °C. Pérez-Miqueo et al. [365] investigated the use of metallic structured reactors for the direct DME synthesis process. These authors prepared the monoliths by wash coating the substrates with CZA and HZSM-5, and concluded that working at almost isothermal conditions is feasible with a volumetric productivity up to 0.20 LDME h−1 m−3 at 300 °C and 4 MPa, with a catalyst hold-up of 0.33 gcat cm−3 in a brass monolith (for syngas feedstocks).Given its importance in the viability of the process, the attenuation of catalyst deactivation is a priority challenge. Understanding the problem is hampered by the coexistence of different causes and by the synergy between the deactivation mechanisms of the metallic and acid functions. The main causes of deactivation are [366]: i) partial blockage of the metallic sites by coke (being considered as the fastest step in the deactivation); ii) coke deposition on the micro and mesopores of the acid function; iii) sintering of the metallic function; and iv) the detrimental interactions between the metallic and the acidic sites.Coke characterization studies through Temperature Programmed Oxidation (TPO) have determined its presence both on the metallic and acidic sites, as well as on the interphase between them (corresponding to the inert Al2O3 in the CuO-ZnO-Al2O3/γ-Al2O3 catalyst [254,367–369]). However, coke is present on the metallic function since the initial stages of the reaction, achieving a limit value in a short period of time. This dynamic can be explained because the hydrogenation of coke precursors slows down its evolution [370,371]. The amount of coke deposited on the acidic function increases with time on stream, tending to a maximum value, resulting from the equilibrium between its formation and its diffusion to the exterior of the catalyst particles. Consequently, the properties of the acidic function are also important both for attenuating coke formation and for favoring the circulation of the intermediates towards the exterior of the catalyst particles.It is worth mentioning the contribution of promoters like MgO [250,372], CeO2 [252], and ZrO2 [373] for preventing the sintering of CuO-ZnO metallic functions. The incorporation of these promoters pursues enhancing CuO crystallites dispersion and stabilizing its interaction with the support.The presence of H2O in the reaction medium (higher in the conversion of CO2 than of syngas) has different effects on the activity of the catalyst. In first place, decreases the initial activity of the catalyst due to the competitive adsorption with the reactants in the metallic and acidic sites of the catalyst. The effect is very important for γ-Al2O3, due to the affinity for H2O of its Lewis sites [211,370]. Furthermore, it favors the sintering of the metallic function, which has been proven for Cu catalysts as their oxidation is favored [337,374,375] and generates the disruption of the Cu‐Zn synergy [240]. Fan et al. [376] have verified the increased stability of a Cu-ZnO-ZrO2-Al2O3 catalyst used together with HZSM-5 catalyst, when modified with Fe, which is attributed to oxygen spillover between deficient iron oxide and Cu, mitigating oxidation (by CO2 and H2O) and Cu sintering.On the other hand, it is well established that the presence of H2O decreases the rate of coke formation [328]. This effect has been explained by the key role of methoxy ions as coke precursors on the metallic and acidic sites, whose formation is thermodynamically limited with the increase of H2O concentration [245]. In addition, H2O is competitively adsorbed with coke-forming intermediates, which are identified as monocyclic arenes, and whose formation takes place from hydrocarbons formed from methanol and DME [35]. Besides, the acidity and porous structure of the acid function have a great effect on the rate of coke deposition and on its nature and deactivating effect. Thus, Brønsted sites with high acidic strength are active in the reactions of coke precursors condensation towards polyaromatic structures and, their confinement is favored in acid functions with cavities in the porous structure [366].Fan et al. [377] compare the individual deactivation of the two catalysts, CuO-ZnO-ZrO2-Al2O3 (CZZA) and HZSM-5 zeolite, when mixed or separated in cascade (first CZZA and zeolite in line). Among the conclusions, the convenience of the proximity of both catalysts stands out, but avoiding an excessive concentration of H2O on the surface of the CZZA catalyst (to attenuate the sintering of Cu) and also the excessive concentration of methanol (precursor of coke deposition in the HZSM-5 zeolite).The configuration of the catalyst particle receives great attention for avoiding deactivation due to the close contact between the metallic and acid functions. García-Trenco and Martínez [361] have verified the migration of extra-framework Al3+species of the HZSM-5 zeolite to the metallic function (CuO-ZnO-Al2O3) through a mechanism assisted by H2O, causing the disruption of the Cu‐Zn synergy, and facilitating the sintering of Cu. Likewise, the migration of Cu2+ ions is facilitated by the presence of H2O and hydroxyls (Brønsted) sites [337,378,379]. These problems advise avoiding intimate contact between the metallic and acid functions in the preparation of the catalyst, being the pre-pelletization of each function separately more suitable than the joint pelletization of a fine powder of both functions in this case [380].Ateka et al. [254] have studied the regeneration of a CuO-ZnO-MnO/SAPO-18 hybrid catalyst, on which coke deposition is reported to be the main responsible for deactivation. Working at reaction-regeneration cycles, these authors have determined that it is possible to regenerate the bifunctional catalyst by coke combustion with air at 300 °C for 48 h. Even if at these conditions the catalyst undergoes a slight sintering of Cu in the first cycle, in the succeeding cycles it demonstrated to reach a pseudo-steady state, completely recovering the activity. Being therefore coke deactivation reversible, this study pointed out sintering as the limiting factor for using these type of catalysts. The small activity loss observed in the first reaction-regeneration cycle was attributed to the sintering of a certain fraction of unstable metallic sites either due to the high water content in the reaction medium or by the generation of hot spots in the regeneration step [254]. Consequently, enhancing the stability of the metallic function also favors the regeneration of the catalyst by allowing to perform coke combustion at higher temperature. In addition, the porous structure and acidity of the catalyst, besides being important for the attenuation of coke condensation [366], are also relevant factors to facilitate its combustion.The interest of the direct synthesis of DME for valorizing CO2 on a large scale is based on the capacity for the conversion of CO2 and syngas and on the good prospects of the applications of DME as “green” fuel and as raw material for the sustainable production of chemicals and H2.Carrying out the methanol dehydration reaction in situ, in the same reactor as methanol synthesis, shifts the thermodynamic equilibrium, upgrading oxygenates formation. Moreover, with this strategy co-feeding of CO2 together with syngas is more favorable than in the synthesis of methanol, which is interesting to valorize (via gasification) lignocellulosic biomass and wastes from the consumer society (as plastics and used tires). The conversion of CO2 attained in the direct synthesis of DME is higher than that in the synthesis of methanol and in the conventional production of DME in two stages.The reaction conditions (pressure and temperature) in the direct synthesis of DME are different to the optimal conditions for each of the individual reactions. Furthermore, CO2 is less reactive than CO and its hydrogenation generates a higher concentration of H2O. These differences in the operating conditions and concentration have required studying the suitable composition and properties of the metallic and acid functions of the catalyst. As consequence, a reasonable understanding of the performance of some suitable compositions has been reached, in particular for conventional configurations (hybrid catalysts prepared by mixing and pelletizing/extrusion of both functions). As in most catalytic processes, the main challenges correspond to the attenuation of the deactivation of the catalyst, being the sintering of the metallic function and coke deposition on both functions the main causes.It is well established that the contact of the metallic and acid functions favors deactivation, due to the development of species (as Cu2+ and Al3+) transport mechanisms, and also that favors the synergy of coke formation mechanisms in both functions. This knowledge has opened a wide research field pursuing to establish the ideal core-shell configuration to minimize the negative effects derived from the contact between the two functions of the catalyst, and in particular, to achieve the stability of the catalyst.The level of knowledge achieved in the fundamental aspects (collected in this review) allows considering that the CO2 to DME synthesis process can effectively contribute to the mitigation of climate change. Achieving the necessary challenges for this objective requires a multidisciplinary work at different scales (catalyst, kinetic modeling, reactor design and scaling).The scaling-up of the CO2-derived DME synthesis process requires catalysts prepared based on the important advances carried out in the design of catalysts for the reactions of CO2-to-methanol and methanol dehydration to DME. To meet this objective, the advances must be adapted to the different conditions and the different composition of the reaction medium of the integrated process. In this sense, the co-feeding of CO2 together with syngas has good perspectives to favor the viability of the process, but requires adequate catalysts, and the resolution of the unknowns regarding the different mechanism for the formation of methanol from CO and CO2 and the synergy between both mechanisms. Likewise, the stability of the catalyst is a challenge requiring more attention.The adaptation of catalysts optimized at nanometric scale to the needs of the industrial reactors is an important challenge. This requires studying composites with the appropriate size and with high mechanical resistance, without deterioration of the performance of the catalyst particles.The viability of the process on an industrial scale also requires adapting the design of the catalysts to the innovations in the design of the reactors, which, like for the hydrophilic membrane reactor, require increasing the per pass conversion. With a different composition in the reaction medium, a different thermodynamic situation is created in these reactors. Accordingly, an adaptation of the catalysts to the optimal conditions and composition in these reactors will also be required.Furthermore, the important development of CO2 valorization initiatives to mitigate climate change, advise expanding the field of study of the CO2-derived DME synthesis process, also considering it as preceding stage to the subsequent synthesis (online stage, or in an integrated process) of fuels and chemicals (olefins or aromatics). In the latter case, the direct DME synthesis catalyst will be used in a tandem catalyst together with an acid catalyst for the selective conversion of DME.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work has been carried out with the financial support of the Ministry of Science, Innovation and Universities of the Spanish Government (PID2019-108448RB-100); the Basque Government (Project IT1645-22); the European Regional Development Funds (ERDF); and the European Commission (HORIZON H2020-MSCA RISE-2018. Contract No. 823745).

The direct synthesis of dimethyl ether (DME) on bifunctional catalysts is highly attractive for valorizing CO2 and syngas derived from biomass gasification and is a key process to reduce greenhouse gas emissions. DME economy (conventionally based on its use as fuel) arouses growing interest, in parallel with the development of different routes for its conversion into hydrocarbons (fuels and chemicals) and H2 production. This review, after analyzing different routes and catalytic processes for the valorization of CO2, focuses on studies regarding the thermodynamics of the direct synthesis of DME and the advances in the development of new catalysts. Compared to the synthesis of methanol and the synthesis of DME in two stages, carrying out the reactions of methanol synthesis and its dehydration to DME in the same reactor favors the formation of DME from CO2 and from CO2 co-fed with syngas. Starting from the experience for syngas feedstocks, numerous catalysts have been studied. The first catalysts were physical mixtures or composites prepared by extrusion of methanol synthesis catalysts (CuO-ZnO with different carriers and promoters) and dehydration catalysts (mainly γ-Al2O3 and HZSM-5 zeolite). The performance of the catalysts has been progressively improved with different modifications of the composition and properties of the components to upturn the activity (lower for the hydrogenation of CO2 than for CO) and selectivity, and to minimize the deactivation by coke and by sintering of the metallic function. The core-shell configuration of the bifunctional catalyst allows physically separating the environments of the reactions of methanol synthesis and its conversion into DME. The confinement facilitates the extent of both reactions and improves the stability of the catalyst, since the synergies of the deactivation mechanisms are eliminated.

Hydrogen sulfide (H2S) is an acidic, corrosive, toxic, and harmful waste gas pollutant with a rotten egg smell [1–3]. It is commonly found as an impurity in raw natural gas and as a byproduct gas pollutant in crude oil processing, the coal industry, iron, and steel smelting, etc., with the presence of sulfur-containing. The combustion of H2S or the gas−containing streams will produce highly toxic and corrosive byproducts (such as SO2, CS2, COS, H2SO4, etc.), which will again be harmful to cause a threat to the health of human beings and the ecological balance of the environment if they are freely discharged [4,5]. In addition, it is easy to cause the deactivation upon many metal catalysts such as Ni, Fe, and Pt due to the high toxicity of H2S [6,7]. Therefore, it is urgent to selectively remove H2S in industrial processes, which has derived great focus on both academic and practical perspectives [8]. More importantly, there has been an increase in interest in how to use the highly purified H2S separated from absorption separation.Ionic liquids (ILs), as a novel type of green solvent, have attracted great attention because of their unique properties, such as negligible vapor pressure, structural designability, high thermal stability, good affinity to gas, etc [9–14]. In terms of the good affinity for acidic gases by ILs, it has also been well explored in H2S capture with good selectivity and high absorption capacity [15–17]. Jou and Mather first investigated H2S absorption in 1-N-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and observed that H2S was only physical dissolution in the IL [18]. Pomelli and coworkers extended this investigation with a variable ion pair to demonstrate ILs with extremely high H2S absorption capacities [19]. Wu et al. reported that cost-effective protic ionic liquids (PILs) derived from alkanolamines MDEA and formic acid or acetic acid were contributing to the absorption separation of H2S from CO2 [20]. As for the functionalized ILs, our group first developed 1-alkyl-3-methylimidazolium carboxylates for the highly efficient absorption of H2S [21]. Whereafter, tertiary amine and carboxylate group as two functionalized sites were incorporated simultaneously into triethylbutylammonium-based ILs toward selectively separating of H2S from CO2 [22,23]. We also synthesized a class of hydrophobic tertiary amine functionalized half protonated PILs paired with Tf2N for highly selective absorption of H2S from CO2 [24]. To achieve both high H2S uptake capacity and superior H2S/CH4 and/or H2S/CO2 selectivity, we prepared strongly basic azole-based PILs to separate H2S with the highest capture capacity up to 6.81 mol kg−1 [25,26].The aim of H2S separation is to make it profitable. In the aspect of H2S conversion into an inorganic sulfur chemical, our group developed a highly efficient ILs-mediated Claus reaction under mild conditions [27,28]. Yu et al. reviewed metal-based ILs for the efficient H2S absorption and oxidative conversion into Sulphur [29]. Wu et al. disclosed a liquid-phase Claus reaction mediated in lactate-based ILs and ethylene glycol (EG) or H2O mixture [30]. It should be noted that the deep processing utilization of H2S discussed above is low-value-added sulfur. The conversion of H2S into value-added organic chemicals is another option that makes more sense.Recently, our group has made progress on the conversion of H2S into a value-added organosulfur compound mediated in ILs. For example, a series of task-specific hydrophobic PILs catalyzed H2S conversion with unsaturated acids into value-added sulfydryl acids was pioneered [31]. To prepare sulfydryl alcohols, two different pathway-the reaction between H2S and epoxides or unsaturated alcohols catalyzed by hydrophilic PILs-was developed [32,33]. In addition, Sen's group used the trihexyltetradecylphosphonium chloride (THTDPC) as the phase transfer catalyst for converting H2S into bis(2-phenylethyl) sulfide (PES) with a high product selectivity [34]. Although the above-mentioned ways have the advantages of solvent-free, high conversion, good recoverability of catalyst, and excellent substrate universality, the separation of reactant and product requires water addition to split them into two phases. The removal of water is highly energy-intensive because of the large latent heat required for the subsequent recovery of catalyst or purification of products [35]. Generally, the addition of a third component for the extraction operation is typically necessary to separate the catalyst from the product in a homogenous reaction. Can a direct phase separation procedure or methodology be designed once the reaction is finished for the resource utilization of H2S without involving the third component?Herein, we develop a novel H2S resource way through the reaction with α,β−unsaturated carboxylate esters to prepare thiols mediated by functionalized carboxylate-based ILs because of their recognition for H2S. Importantly, the self-separation was realized to facilitate the progress of reactions and make the convenient separation of catalysts and products possible. The product can be controlled by regulating the molar ratio of H2S to α,β−unsaturated carboxylate esters. The reaction was systematically investigated by means of condition optimization, product selectivity control, substrate expansion, etc. The gaussian calculation was performed to explore the reaction mechanism. It is anticipated that this self-separation method mediated by task specific ILs provides a potential strategy for utilizing H2S.The specifications and sources of the chemicals used in this work were summarized in Table S1. All reagents were used without further purification. In addition to the ILs which were directly purchased, the three ILs were prepared by one-step neutralization reaction, including tetraethylammonium acetate ([N2222][Ac]), tetraethylammonium 2−hydroxypropanoate ([N2222][LAc]), and tetraethylammonium formate ([N2222][For]). In brief, taking the synthesis of [N2222][Ac] as an example, the equimolar acetic acid (HAc) was added into tetraethylammonium hydroxide 25 wt.% aqueous solution ([N2222][OH]), followed by stirring for 12.0 h at ambient temperature to form a transparent liquid. Then, the final product was purified on a Schlenk line at 60 °C and 0.15 mbar for 4.0 h until no bubbles evolved to ensure the [N2222][Ac] free of water.Nuclear magnetic resonance (NMR) spectra were obtained at ambient temperature on a Bruker DPX spectrometer with CDCl3 as the internal standard solvent at 400 MHz for 1H and 101 MHz for 13C NMR, respectively. Conversion and selectivity were determined by GC analysis (Shimadzu, GC-2014C). FT−IR spectra (Tensor II, Bruker) were carried out with the spectral resolution and the number of scans of 4 cm−1 and 32, respectively. ESI-MS (Thermo Fisher) was used to characterize the molecular weight of the product at m/z. For the separation of the mixture after the reaction, the pure product was obtained by thin−layer chromatography (TLC) with petroleum ether (PE) and ethyl acetate (EA) as eluent (3:1). The structures of thiols and thioethers were verified by 1H and 13C NMR spectra with CDCl3 as solvent.For typical experiments on the reaction of α,β-unsaturated carboxylate esters with H2S, butyl acrylate (1.0 g, 8 mmol) was added into a reaction vessel. The amount of catalyst is 10 mol% of butyl acrylate, and the reaction condition was 30 °C and 2.0 h. In order to ensure the equimolar reaction between H2S and substrate, the molar amount of H2S was calculated from its density (mol·L−1) according to our previous work [25,26,36,37]. Generally, the required H2S molar is ensured by adjusting the pressure drop (ΔP) of the large tank. Since the substrate (butyl acrylate) can be accurately weighed, the required H2S can be determined by its mass (Eq. S1), so as to obtain the partial pressure of H2S at the corresponding temperature through the NIST database (DOI: https://doi.org/10.18434/T4D303, Eq. S2). The detailed description can be found in ESI.The characterizations of the catalyst ([Emim][Ac]) are presented in the ESI (Figs. S1–S2). Taking butyl acrylate (1a) as a probe substrate of α,β-unsaturated carboxylate esters, we explored its reaction effect under different catalyst conditions (Table 1 ). The substrate butyl acrylate has no reaction activity with H2S without the addition of ILs catalyst in a blank experiment, indicating that the autocatalysis is negligible (Entry 1, Figs. S3–S4). We also performed the investigations on the reaction of H2S and butyl acrylate with conventional [Emim][BF4] and [Emim][PF6] as catalysts (Entries 2–3). It is found that the conversion of the substrate is significantly restricted with a catalyst loading of 10 mol% under reaction conditions. This is because the alkalinity of [Emim][BF4] and [Emim][PF6] is too weak to activate H2S [33,38]. [Hmim][Cl] was also fed into the system to explore this reaction process (Entry 4). It is found that the conversion of butyl acrylate is only 6%, indicating that this catalyst has a poor activation effect on H2S. The main reason may be the fact that [Hmim][Cl] brings a relative acidic environment to the entire reaction system so that H2S can hardly be activated [19].When the [Emim][Ac] with weak alkalinity was fed to the system, a quantitative conversion of substrate was realized, indicating that the presence of carboxylate-based ILs is favorable for the conversion of butyl acrylate. As the reaction proceeds, the system comes to turbid and the [Emim][Ac] is split out in the lower phase because [Emim][Ac] is almost insoluble in the product mixture (Fig. 1 a, see Fig. S5 for an enlarged view). No signal of ILs was detected from the upper phase except for the mixed components of the two products, which can be assigned to butyl 3-sulfanylpropanoate (1b) and dibutyl 3,3′-thiodipropionate (1c), respectively (Fig. 1b, see Fig. S6 for the corresponding 1H NMR). To investigate the self-separation process, the FT-IR spectra of pure butyl acrylate, butyl 3-sulfanylpropanoate, dibutyl 3,3′-thiodipropionate, and upper phase have been carried out. As demonstrated in Fig. S7, it is found that the FT-IR of upper phase are almost consistent with the dibutyl 3,3′-thiodipropionate and butyl 3-sulfanylpropanoate, demonstrating the formation of products. In addition, the signal of [Emim][Ac] cannot be detected from the upper phase after reaction, which further verifies the reliability of self-separation of catalyst and products. The conversion of butyl acrylate (1a) was as high as 99%, and the selectivity of 1b and 1c were 7% and 93%, respectively (Entry 5, Table 1) at 30 °C and 2.0 h. The two products can be separated easily by TLC (PE/EA = 3:1). The structure characterization and molecular weight of 1 b and 1c were verified by NMR spectra and ESI−MS, respectively (Fig. S8−S13).To evaluate the effect of ILs with different cations on the conversion of butyl acrylate, we carried out the reaction with other carboxylated-based ILs by increasing the chain length and changing the configuration of cations, including [Bmim][Ac], [Hmim][Ac], [Omim][Ac], [N2222][Ac], [Pyr12][Ac], and [Epy][Ac]. Clearly, the conversion and selectivity keep almost unchanged, indicating the effective catalytic role of the carboxylate (Entries 6–10), which means the cations have almost no effect on the conversion of butyl acrylate. To evaluate the influence of various carboxylate-based anion, [N2222][Ac], [N2222][Lac], and [N2222][For] were employed to be as catalysts for the conversion of butyl acrylate (Entries 11−13). It is found that the conversion of butyl acrylate is up to 99% with different catalysts, and the selectivity of 1 b and 1c is around 10% and 90%, respectively, further indicating that the carboxylate has a good activation to H2S.It is well known that almost all industrial gases are accompanied by humidity. In order to study the effect of water on the conversion of butyl acrylate, [Emim][Ac] containing 3 wt% H2O was used as the reaction medium (Entry 14). It is shown that the conversion of butyl acrylate was as high as 99% within 2.0 h, which was almost the same as those under anhydrous conditions. Therefore, the effect of H2O on the reaction of H2S and butyl acrylate can be negligible. We also prepared the potassium acetate (KAc) into a 4 mol L−1 aqueous solution as the representative inorganic salt to catalyze the reaction at 30 °C for 2.0 h (Entry 15). In contrast, no conversion of butyl acrylate is discovered. This may be due to the obvious interface between KAc aqueous solution and butyl acrylate, which separates the two substances from each other, so the chemical mass transfer between them is greatly limited. Besides, the pH of KAc aq. (4 mol L−1) was measured to be 8.3, which means its alkalinity is too weak to activate H2S. The carboxylate−activated H2S cannot be transferred to the butyl acrylate side, resulting in an ineffective reaction process. To confirm the above assumption, the solubility of H2S in KAc aq. (4 mol L−1) has also been measured at the temperature of 30 °C and pressures up to 1.0 bar. As is shown in Fig. S14, the absorption isotherm of H2S in KAc (4 mol L−1 aq.) demonstrates an ideal linear type with increasing pressure, indicating the entire absorption process follows a physical behavior. Moreover, the solubility of H2S in KAc (4 mol L−1 aq.) is only 0.75 mol kg−1, significantly lower than those of our prepared ILs [26], implying that the KAc (4 mol L−1 aq.) cannot activate H2S.It is well known that the selective preparation of intermediate products in cascade reactions is not easy to realize. Cascade reaction, described as two-step reaction (A → B → C) [39], one of which accelerated from A to B and the other accelerated the reaction from B to C (Fig. 2 ). As a kind of value-added product, butyl 3-sulfanylpropanoate has been widely applied in pharmaceutical science and chemical engineering [40–44]. To control the selectivity of butyl 3-sulfanylpropanoate, the experiments of different molar ratio of H2S to butyl acrylate were carried out with a 10 mol% loading of [Emim][Ac] at 30 °C and 2.0 h. As demonstrated in Fig. 3 , the selectivity of butyl 3-sulfanylpropanoate increases up to 96% with increasing molar ratio of H2S to butyl butyrate from 0.25 to 28. When the molar ratios of H2S to butyl butyrate are 0.25 and 0.5, the corresponding conversion rates of butyl acrylate are 48% and 96%, respectively. In these points case, the selectivity of dibutyl 3,3′-thiodipropionate was achieved as high as 100%. The maximum selectivity of butyl 3-sulfanylpropanoate is as high as 96% when the molar ratio of H2S to butyl acrylate is 28, indicating that the intermediate product can be effectively prepared through optimizing the molar ratio of H2S to butyl acrylate. It is believed that the selectivity of butyl 3-sulfanylpropanoate is affected by the concentration of H2S, which would reach up to 99% if we continue to improve the molar ratio of H2S to butyl acrylate.To investigate whether the generation of butyl 3-sulfanylpropanoate is related to the feeding mode, an experimental process of intermittent feeding was carried out herein. It is known that every kilogram (kg) of [Emim][Ac] can absorb 2.8 and 5.0 mol of H2S at 1.0 and 3.0 bar, respectively [21]. In the process of intermittent feeding operation, H2S is first captured by excessive ILs to enrich the concentration of SH−, and then the butyl acrylate is directly added to the H2S-saturated [Emim][Ac] to participate in the reaction. The schematic diagram of the reaction device is shown in Fig. S15 and the detailed operation steps can be found in Supporting Information. It is found that the selectivity of butyl 3-sulfanylpropanoate and dibutyl 3,3′-thiodipropionate were up to 77% and 23% (Table 1, Entry 16), respectively, suggesting that the H2S concentration dissolved in the liquid played a decisive role in the control of product selectivity during the reaction.The cyclic catalytical performance of ILs catalyst is of great practical significance for the conversion of H2S. The reutilization of catalyst in this system is quite convenient to realize. For example, butyl acrylate (1.3 g, 10 mmol) together with 60 mol% [Emim][Ac] loading (1.0 g) was added for reaction in the H2S atmosphere at 30 °C for 2.0 h. After the reaction was completed, the self-phase separation mixture composed of the product and [Emim][Ac] was exhibited, which was consistent with that shown in Fig. 1a. Then, the upper phase was removed and the lower phase was retained for the next reaction with the addition of fresh butyl acrylate. It should be noted that the reused [Emim][Ac] catalyst, without any activation or regeneration steps, was investigated in the next addition reaction of H2S and butyl acrylate. As demonstrated in entries 17−18 (Table 1), after the fifth and tenth catalyst recycling, the conversion of butyl acrylate and the selectivity of butyl 3,3′-thiodipropionate show no significant decrease, suggesting that [Emim][Ac] has good durability, robustness, and recoverability after ten catalytic cycles.To better understand the process of the consecutive H2S conversion, Fig. 4 shows an example of the pressure−time kinetic curve of butyl acrylate conversion in [Emim][Ac] within 6500 s [27]. During the entire reaction, three stages can be distinguished: preparation (keep vacuum), reaction, and equilibrium stage. It is impressive that after the introduction of H2S in a relatively short period of time, the pressure in the reaction tank decayed rapidly to a quite low level, indicating that H2S was quickly trapped in the liquid phase and completely reacted with butyl acrylate. The intersection point of the tangents of the next two parts was taken as a reference, and the whole reaction was almost completed within 460 s. According to the data in Table 1 (Entry 5), it can be considered that it has been completely converted in the equilibrium stage, suggesting that the reaction kinetics of the H2S conversion are at a rapid rate, showing a good industrial prospect.With the data aforementioned, a plausible mechanism for the H2S conversion by butyl acrylate mediated in [Emim][Ac] was proposed in Fig. 5 a. Firstly, the carboxylate group on [Emim][Ac] interacts with free H2S, which will promote the release of nucleophilic SH− (1). In the effect of induction effect by the electron-withdrawing ester group, the secondary carbon in the CC double bond shows electropositivity (δ+) [45], indicating it is easier to be attacked by the nucleophilic SH− group [46]. At the same time, the hydrogen protons on H2S activated by carboxylate of [Emim][Ac] are transferred to the tertiary carbon on the other side of the CC double bond to generate the target product 3-sulfanylpropionate because (2) and (3) can reach equilibrium at a lower barrier [47]. Once there is adequate butyl acrylate, it will continue to react with the 3-sulfanylpropionate in the presence of basic [Emim][Ac] to generate dibutyl 3,3′-thiodipropionate. Similar to the previous part, the carboxylate group on [Emim][Ac] interacts with the sulfhydryl group on 3-sulfanylpropionate to release nucleophilic SR− to attack the secondary carbon on butyl acrylate, and the reaction formula was demonstrated in (4). The hydrogen proton attracted by the carboxylate can be transferred to the tertiary carbon. The ultimate dibutyl 3,3′-thiodipropionate can be obtained since a chemical equilibrium of the processes (5) and (6) is realized at a lower barrier [48]. The catalyst [Emim][Ac] can be easily separated from the product mixture for the catalytic circulation because it is almost insoluble in the binary mixture system of 3,3′-thiodipropionate and 3-sulfanylpropionate. As shown in Fig. 5b–d, the red and blue colors on the molecular surface indicate regions with more negative and positive electrostatic potentials (ESPs), respectively. It is obvious that the side of the sulfydryl group chain in [Emim][Ac]-H2S adducts and butyl 3-sulfanylpropylene exhibits increased electronegativity, which visually supports the idea that the sulfydryl group will attack the CC double bond on butyl acrylate.Immediately, the catalytic mechanism between [Emim][Ac] and butyl acrylate (1a) was discussed through DFT calculations. The bond length and transition state of the structures were further examined to further determine the reaction mechanism of the formation of thiols between H2S and butyl acrylate by using DFT-based theoretical calculations at the B3LYP/6-311G (d, p) level (more details can be seen in Supporting Information). All calculations were performed using the Gaussian 09 program, and the results are shown in Fig. 6 . First, the binding between H2S and the carboxylate anion of the catalyst is slightly exothermic (−6.7 kcal mol−1), it is found that the bond length of H–S of free H2S molecule was obviously elongated due to its affinity with carboxylate anion (Int1, 1.35 and 1.35 Å vs. 1.35 and 2.09 Å). And then, Int2 is formed after the addition of butyl acrylate with a corresponding energy change of 3.1 kcal mol−1. The activated H2S can easily attack the CC double bond with a low barrier of 12.8 kcal mol−1 via transition state 1 (TS1). The intermediate product, 3-sulfanylpropionate, is produced by the nucleophilic attack of the SH group in the TS1 (2.25 vs. 1.85 Å, Pro1b). As the reaction we investigated is a cascade reaction, the reactant butyl acrylate will continue to react with the reaction product (1b) in the first step. The Int3 is formed with the addition of 1a. At this time, two different pathways of transition state were discovered. In terms of configuration, the interaction of carboxylate anions on [Emim][Ac] to H on the sulfhydryl group of 3-sulfanylpropionate can be almost ignored during the formation of transition state 0 (TS0, bond length of H−OIL−anion: 2.53 Å). Different from TS0, the configuration of TS2 is more moderate. The carboxylate anion has an intense interaction with H on the sulfhydryl group of 3-sulfanylpropionate (bond length of H−OIL−anion: 1.02 Å), which can promote the release of nucleophilic SR− to attack the secondary carbon on butyl acrylate, which matches the proposed reaction mechanism in Fig. 5. The energy barrier of TS2 is only 10.8 kcal mol−1, which is roughly a 4/5 reduction compared to TS0, considerably decreasing the reaction energy and making this catalytic reaction more plausible, which is reasonable for the reaction to proceed readily under such mild conditions [49]. The bond length TS2 was tightened (2.14 Å vs. 1.83 Å) to obtain the Pro1c, making the structure more stable. According to calculation results, the energy barriers of TS1 and TS2 are 12.8 and 10.8 kcal mol−1, respectively, indicating that it is easier to approach the formation of thioether-based compounds, which is in good agreement with the experimental results under such mild conditions.The optimization procedure enabled us to efficiently examine substrate scope and limitations. Herein, we continue to study the range of substrates for such α,β-unsaturated carboxylate esters catalyzed by [Emim][Ac] (10 mol%) at 30 °C and 1.0 MPa H2S for 2.0 h (Fig. 7 ). It was found that seven substrates were investigated, and good to excellent conversion was achieved in all cases. The selectivity of products is affected by the chain length of the substrates (1a−4a) because of their different miscibility of corresponding products with [Emim][Ac]. Surprisingly, it is found that the structures of the products are affected by the steric hindrance of the groups connected on the CC bond (5a−7a), indicating that the increase of steric hindrance on the CC double bond inhibited the formation of thioether-based products. The conversion of 7a is as high as 99% at 70 °C and 6.0 h because of its high steric hindrance (>CC−) [31,33], while the other six substrates achieve quantitative conversions with the Anti−Markovnikov addition at 2.0 h. For the whole substrate expansion, TLC was utilized to separate the products with EA/PE as eluent. NMR and ESI−MS spectra of all products are presented in ESI (Figs. S16–S42).In summary, a self-separation carboxylate-based ILs catalyst was developed for the effective conversion of H2S by α,β−unsaturated carboxylate esters. These ILs are immiscible with ether products, making the recovery and catalytic reuse of catalyst very convenient. It is found that the IL catalyst [Emim][Ac] can be reused for ten cycles without activity loss, revealing the highly efficient catalytic performance and excellent catalyst durability. This cascade reaction may effectively produce the sulfydryl based intermediate, butyl 3-sulfanylpropionate, whose selectivity is as high as 96% when the molar ratio of H2S to butyl acrylate is 28. The pressure–time kinetic behavior shows that the conversion of butyl acrylate can be achieved almost quantitatively within 460 s with the catalyst loading of 10 mol% at 30 °C, suggesting extremely fast reaction kinetics. Besides, several α,β-unsaturated carboxylate esters were successfully converted into the corresponding products with high quantitative conversion. Furthermore, a new reaction mechanism for the H2S conversion by task-specific ILs [Emim][Ac] was proposed, in which the formation of thioether-based products between α,β−unsaturated carboxylate esters and H2S takes place with a low energy barrier of 10.8 kcal mol−1. It is believed that this green, efficient, and simple strategy would present great potential for industrial application.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was sponsored by the National Natural Science Foundation of China (Nos. 22208140 and 22078145).The following is the Supplementary data to this article. Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.gee.2023.03.001.

The deep-processing utility of pure hydrogen sulfide (H2S) is a significant direction in natural gas chemical industry. Herein, a brand-new strategy of H2S conversion by α,β-unsaturated carboxylate esters into thiols or thioethers using task-specific carboxylate ionic liquids (ILs) as catalyst has been developed, firstly accomplishing the phase separation of product and catalyst without introducing the third component. It can be considered as a cascade reaction in which the product selectivity can be controlled by adjusting the molar ratio of H2S to α,β-unsaturated carboxylate esters. Also, the effects of ILs with different anions and cations, intermittent feeding operations, as well as pressure−time kinetic behaviors on cascade reaction were investigated. Furthermore, the proposed interaction mechanism of H2S conversion using butyl acrylate catalyzed by [Emim][Ac] was revealed by DFT-based theoretical calculation. The approach enables the self−phase separation promotion of catalyst and product and achieves 99% quantitative conversion under mild conditions in the absence of solvent, making the entire process ecologically benign. High-efficiency reaction activity can still be maintained after ten cycles of the catalyst. Therefore, the good results, combined with its simplicity of operation and the high recyclability of the catalyst, make this green method environmentally friendly and cost−effective. It is anticipated that this self-separation method mediated by task-specific ILs will provide a feasible strategy for H2S utilization, which will guide its application on an industrial scale.

Data will be made available on request.Lower olefins (C2–C4) are crucial chemical building blocks in the chemical industry as they are used to produce the necessary basic chemicals, such as propylene oxide, as well as a wide variety of polymers [1,2]. Considering the increasing demand for polyethylene and polypropylene and the transition towards fossil-free chemical building blocks, new ways to produce light olefins are needed [1]. The Methanol-to-Hydrocarbons (MTH) process offers an alternative route to produce lower olefins, such as ethylene and propylene, in which methanol and/or dimethyl ether is converted to olefins and gasoline-range products [3–6].The MTH reaction is an acid-catalyzed reaction in which various zeolites and zeotypes have been studied as catalyst materials during the past decades [3]. ZSM-5 and SAPO-34 zeolites have proven to be the most promising solid catalysts. Even though both of these catalysts are great for the MTH process, they have striking differences in their catalytic performance [3,5,7–9]. Zeolite framework structures with small-sized pores, e.g., SAPO-34 (with CHA framework structure), exhibit high selectivity towards light olefins, while they deactivate relatively fast. In contrast, zeolite framework structures with medium-sized pores, such as ZSM-5 (with MFI framework structure), are more stable and less prone to deactivation, while also having a much larger product distribution (i.e., the formation of light olefins and aromatics) [3,10,11].There are many strategies to enhance the selectivity towards ethylene and propylene and both acidity and porosity play a crucial role. Firstly, a facile way to increase olefin yields is to regulate the reaction conditions, such as low pressures and high reaction temperatures. Altering the strength and the density of acid sites by modifying the Si/Al ratio has been another approach to increase the production of lower olefins [12,13]. Post-synthetic modifications, such as dealumination and/or desilication, can influence selectivity and the lifetime of the catalyst [7,9,14–23]. Metal doping is proven to help enhance the efficiency of the MTH process. Alkaline earth metal (e.g., Mg, Ca, Sr, and Ba), rare earth metals (e.g., La), transition metals (e.g., Co, Ni, Fe, and Zr), and other chemical elements, such B and P, are, among others, used to achieve higher selectivity to light olefins [20,24–37].Chen et al. studied the effect of Mg modification on ZSM-5 zeolites for the methanol conversion. Mg addition reduced the number of strong acid sites, while new medium strength acid sites were created. Regarding the catalytic performance, Mg-modified ZSM-5 zeolites exhibited enhanced propylene selectivity and possessed increased lifetime [38]. Similar results were found in a study of Bakare et al., in which the effect of alkaline earth metals, such as Ca, Mg, and Ba, was investigated. Their research also revealed the formation of extra Lewis Acid Sites (LAS) [25]. Goetze et al. have investigated and compared the coke formation of H-ZSM-5 and Mg-ZSM-5 zeolites using operando UV–Vis spectroscopy. It was found that Mg-ZSM-5 showed a prolonged lifetime due to the slower progression of the coke front along the catalyst bed. The latter can be attributed to decreased Brønsted acidity, which hinders the formation of secondary coke [24]. Yarulina et al. confirmed these findings as it was revealed that propylene selectivity is firmly related to the isolation of Brønsted Acid Sites (BAS), while the formation of LAS upon addition of Mg or Ca inhibits reactions, involving aromatic moieties and, thus, prevents coke formation [30].It is important to mention here that the majority of the studies in the literature are based on zeolite powder samples, and only a few studies shed light on shaped multi-component catalyst bodies, which are often the real catalyst material used in an industrial reactor. Pérez-Ramírez et al. investigated the impact of various binders on the performance of zeolite ZSM-5-based technical catalyst materials in the MTH reaction. Attapulgite (which is a magnesium aluminum phyllosilicate clay) has been shown to prolong the lifetime of the catalyst material and increased the selectivity towards light olefins. The authors attributed these observations to mobile Mg species migrating from the attapulgite binder to the zeolite [39]. De Jong's group has underlined the importance of nanoscale intimacy in a bifunctional zeolite-binder catalyst for the conversion of hydrocarbons. They showed that the control over Pt location on the zeolite or binder significantly influenced the catalytic performance [40–43].Nonetheless, the influence of Mg in zeolite-based catalyst materials on the MTH reaction has not been investigated thoroughly. To the best of our knowledge, the effect of Mg location in zeolite-based extrudates on the methanol conversion has not yet been studied. Here, by tuning the Mg location in zeolite alumina-bound shaped catalyst bodies, and, thus, the interaction between Mg and zeolite and/or binder has been investigated. To accomplish this goal, three different approaches for Mg addition have been followed; namely before, during, and after the extrusion process. Altering the step in which Mg was added (i.e., pre-, during, and post-extrusion) was crucial for the physicochemical properties of the technical catalyst bodies and their performance in the MTH reaction. The pre-extrusion modification resulted in a prolonged lifetime and large increase in the yield of light olefins due to a better Mg-zeolite interaction. A clear and distinct link was established between acidity, molecular transport, and deactivation during the MTH process.The following chemicals, gases, and materials were used: methanol (Acros, HPLC grade, 99.99% pure), N2 (Linde, 99.998%), He (Linde, >99%), Ar (Linde, 99.998%), methylcellulose (Sigma Aldrich, 4000 CP), acetic acid (Sigma Aldrich, 99.5%), boehmite (CATAPAL D, SASOL), Zeolite HZSM-5 (Zeolyst, 3024E), magnesium nitrate (Mg(NO3)2 6H2O, Sigma Aldrich, 99%), and magnesium oxide (MgO, Sigma Aldrich, 99%).Unmodified or modified powder H-ZSM-5 and boehmite were mixed in a 50-50 wt% zeolite-to-binder ratio. Methylcellulose was added and further mixed to acquire a homogeneous solid mixture. Acetic acid diluted in ultra-pure water was then added to form a paste. A Mini-Screw Extruder (Caleva) equipped with a 2-mm-diameter die plate with a cylindrical shape was used to extrude the paste. The formed catalyst extrudates were dried in air overnight, followed by a calcination step in a tubular oven at 600 °C for 6 h (with a ramp of 1 °C/min) in a flow of air. In the case of Mg addition during the extrusion process, the proper amount of MgO powder, to achieve a loading of 0.5 and 1 wt%, was added at the same moment together with the unmodified powder H-ZSM-5 and boehmite.Mg modification of ZSM-5 and boehmite powder, as well as zeolite-alumina catalyst extrudates, was done by wet impregnation. In brief, the proper amount of magnesium nitrate hexahydrate (aiming for 0.5 and 1 wt% of Mg in the final samples) was dissolved in 15 ml of ultra-pure water and added with the support in a round-bottom flask. The mixture was left to mix for 10 min to ensure homogeneity. The round-bottom flask was attached to a rotary evaporator, and vacuum was applied until the water was evaporated and the support was dry. The samples were calcined at 550 °C for 4 h (with a ramp of 1 °C/min). This synthesis method was used to prepare the individual Mg-modified components, such as ZSM-5 and boehmite (further denoted as the pre-extrusion modification). Furthermore, zeolite-alumina catalyst extrudates were impregnated in the same way (further termed as the post-extrusion modification).N2 physisorption was performed using a Micromeritics TriStar 3000 instrument operating at liquid N2 temperature. Before the measurements, a drying step was applied at 300 °C under an N2 flow for 15 h. Ammonia Temperature-Programmed Desorption (NH3-TPD) was measured on a Micromeritics AutoChemII 2920 instrument. X-ray diffraction (XRD) was measured on a Bruker D2 X-ray powder diffractometer with a Co Kα X-ray tube (λ = 1.7902 Å) as the source. The imaging of the spent catalyst samples after the MTH reaction was done using Confocal Fluorescence Microscopy (CFM), and the procedure followed is described in detail elsewhere [44].The performance of the various catalyst samples under study for the MTH reaction was tested in an operando UV–Vis Diffuse Reflectance Spectroscopy (DRS) set-up. ∼ 69 mg of catalyst was placed in a fixed-bed reactor operating at a Weight Hourly Space Velocity (WHSV) of 6 h−1 at 400 °C. Methanol and the products formed during the reaction were detected using an Interscience Compact Gas Chromatograph (GC). More details of the set-up can be found elsewhere [11,24].In this study, we investigated the effect of the location of Mg in zeolite-based catalyst extrudates. Three different synthesis approaches were chosen to modify the zeolite-alumina shaped catalyst bodies with Mg. These methods are summarized in Scheme 1 and Table S1. In these methods, Mg was introduced as both Mg2+ and MgO and is likely present as a mix of both Mg2+ ions and MgO in the final catalysts. When we refer to Mg we are referring to all potential Mg species present and not suggesting that Mg is present in the metallic form.Firstly, Mg was added to the individual components, such as zeolite and/or binder, before extrusion, in the so-called pre-extrusion-addition. In this case, Mg was impregnated in the ZSM-5 zeolite (further denoted as MgZ) and/or the alumina precursor binder (further denoted as MgA). Then, the modified samples were used to make the shaped catalyst bodies. The samples that belong in this category are: a sample which consists of Mg impregnated zeolite (MgZ) and unmodified alumina binder (A) (further denoted as Ext. MgZ/A), a sample with unmodified zeolite (Z), and Mg impregnated alumina (MgA) (further denoted as Ext. Z/MgA), and, finally, a sample with Mg impregnated zeolite (MgZ) and Mg impregnated alumina (MgA) (further denoted as Ext. MgZ/MgA). Secondly, the addition of Mg was done during the extrusion process, in the so-called during-extrusion-addition. Mg was added in the form of MgO, and it was mixed with the unmodified zeolite material and binder powders during the extrusion process. The samples are denoted as Ext. Z/A/0.5 Mg and Ext. Z/A/1 Mg where 0.5 and 1 represent the wt% of Mg in the sample. Thirdly, Mg modification was performed after the extrusion process, in the so-called post-extrusion-addition. In this case, unmodified zeolite and binder powder samples were used to make technical catalyst bodies, and then Mg was impregnated in the extrudates. The samples are further denoted as x/yMg/Ext. Z/A, where x and y represent the amount of Mg in the final sample.We used Scanning Electron Microscopy with Energy-dispersive X-ray spectroscopy (SEM-EDX) to validate the synthesis method and the location of Mg on selected samples. The results are shown in Figs. S1–3. Regarding the pre-extrusion modification, point spectra in zeolite and/or alumina binder–rich areas proved the presence of Mg only in the zeolite for the Ext. MgZ/A sample and only in the binder for the Ext. Z/MgA. SEM-EDX for the Ext. Z/A/1 Mg sample shows larger agglomerates of Mg and no presence of Mg in zeolite and/or the alumina binder. Lastly, Mg appears in the zeolite and/or the alumina binder for the 1Mg/Ext. Z/A sample. Fig. 1 provides an overview of the textural properties of the shaped catalyst bodies before and after the addition of Mg as a function of the three ways of preparation. The reference catalyst extrudate sample has a bimodal pore size distribution. A first small peak is observed at 1 nm which appears to be the tail of a peak, suggesting the presence of pores <1 nm. This is attributed to the presence of micropores, associated with the zeolite material. The second peak centered at ∼8 nm is attributed to the presence of mesopores, most probably from the alumina binder and interparticle domains formed upon extrusion, such as zeolite-binder and/or binder-binder interactions. Regarding the pre-extrusion-addition, Fig. 1a and Fig. S4 indicate that when only the zeolite was impregnated prior to extrusion (Ext. MgZ/A, pink color), a lower amount of micropores of ∼ 1–2 nm is observed, resulting in a subsequent slight decrease in surface area (Fig. 1d). When Mg is added before extrusion with the alumina binder, the second peak is shifted to pores of smaller sizes. Regarding the Ext. Z/MgA and the Ext. MgZ/MgA materials, the boehmite, used as alumina precursor, was impregnated with Mg and then calcined before using it for extrusion. Thus, it can be expected that strong interaction between Mg-boehmite can be achieved and Mg will fill the porous structure of Al2O3 and decrease its porosity. Similar results have been found in literature when Mg–Al2O3 has been evaluated as catalytic support in a hydrodesulphurization reaction [45]. The authors claim that when Mg is introduced, a mixed Mg–Al oxide is formed which fills the surface of the alumina support material. An alternative explanation could be that Mg addition before extrusion prevented the formation of interparticle pore space between zeolite-binder or binder-binder materials. Moving to the addition of Mg during the extrusion process, the Ext. Z/A/0.5 Mg sample shows an extra third peak centered at ∼10.5 nm. An increase of the Mg amount to 1 wt% shows a decrease of the extra third peak and further decrease in the pore size as the peak at ∼8 nm shifts to smaller pore size (∼6 nm).However, as shown in Fig. 1d, the addition of Mg during the extrusion process showed no significant effect on the surface area. All the findings mentioned above imply that Mg mainly interacts with the alumina binder in this approach. Lastly, Mg impregnated in the extrudate samples (i.e., the post-extrusion-addition samples) shows a minor decrease in the amount of micropores (Fig. 1c) as well as in the total surface area (Fig. 1d), suggesting blockage of the zeolite pores during the impregnation process.All the extrudate samples, i.e., both the unmodified and Mg-modified ones, show a crystal phase characteristic of an MFI topology, as shown in Fig. 2 , as assessed by X-ray Diffraction (XRD) [38]. No significant framework change on the samples can be noticed upon adding Mg. Further inspection of the XRD patterns shows a lack of additional peaks related to the presence of Mg species, which can be justified by the low content of Mg (0.5–1 wt%) and/or the high level of Mg dispersion. Regarding the samples in which Mg was added before extrusion (Fig. 2a and Fig. S5), the Ext. MgZ/A and Ext. MgZ/MgA samples showed a decrease in the overall XRD intensity. The impregnation of metals in zeolites ZSM-5 is known to decrease the overall crystallinity due to the formation of defects and dealumination [46].However, when only the binder is impregnated with Mg (Ext. Z/MgA, orange color), no decrease in XRD intensity is observed compared to the reference sample. The observations mentioned above are implicit of the interaction between Mg and zeolite materials. On the other hand, when Mg is added during the extrusion process (Fig. 2b and Fig. S5), no major differences can be found compared to the reference sample, which implies no or poor interaction between Mg and the zeolite. Furthermore, as illustrated in Fig. 2c and Fig. S5, post-extrusion modification of extrudates with Mg also shows a decrease in the XRD peak intensities correlated to an increasing amount of the Mg loading.The Ammonia Temperature-Programmed Desorption (NH3-TPD) experiments, of which the results are summarized in Fig. 3 , were used to measure the total amount of acid sites as well as their strength. Furthermore, the NH3-TPD curves were deconvoluted and used to quantify the amount of weak, intermediate and strong acid sites, which is illustrated in Fig. S6. The reference sample shows two major TPD peaks, centered at ∼ 220 °C and ∼410 °C. According to literature, the first peak is attributed to the presence of weak acid sites, while the second is due to the presence of strong acid sites [47–50]. In previous reports, zeolite modification using Mg affected the overall acidity of the samples [24,30,38]. Firstly, regarding the pre-extrusion modification (Fig. 3a), the sample when only the zeolite is impregnated with Mg (Ext. MgZ/A, pink color) shows a significant decrease in the number of strong acid sites. This is likely due to Brønsted acid sites being exchanged for Mg2+ cations. The addition of Mg resulted in an increase in the number of weak acid sites as well as in the formation of extra intermediate strong acid sites. Similar results were noticed for the sample where both components were impregnated with Mg before extrusion (Ext. MgZ/MgA, red color). Unexpectedly, impregnating only the binder material resulted in an increased strong acidity, but also in the formation of weak acid sites. Even though the increase of the number of weak acid sites could be expected, as Mg species acts as LAS [24,30,38], the formation of strong acid sites are clearly not. However, the latter observation could be explained by the migration of Al species from the binder material to the zeolite framework, which is also reported in the literature [51,52]. Mg modification of the samples prior to the extrusion process causes an increase in the total amount of acid sites, as shown in Fig. 3d. At the same time, the relative ratio between weak, medium, and strong acid sites vastly differs among the samples under study. To further assess the changes in acidity resulting from the addition of Mg during the extrusion process (Fig. 3b), our NH3-TPD measurements show that there is an increase in weak and medium acidity, while there is a minor decrease in the number of strong acid sites. The latter implies no significant Mg-zeolite interaction. It can only be assumed that the increase of weak and medium acidity is due to the presence of Mg species.On the other hand, the decrease in the number of strong acid sites is due to the interaction of Mg species with the acid site of the external zeolite surface or the migration of Mg species in the zeolite pores. The post-extrusion modification showed a decrease in the overall acidity (Fig. 3d), especially of the weak and the strong acid sites, and the formation of intermediate acid sites (Fig. 3c). This could arise from a combination of Mg exchanging with BAS in the zeolite, as well as blocking pores and lowering the overall accessibility to acid sites. Based on the results as mentioned earlier, the pre-extrusion modification with Mg shows the highest level of interaction, as shown from the XRD and NH3-TPD measurements performed. This can be concluded from the decreased overall relative intensity in the XRD pattern when Mg is impregnated in the zeolite. At the same time, NH3-TPD experiments showed a significant loss of the number of strong acid sites and the formation of new medium-strong acid sites. Weak interaction between Mg and the zeolite material was achieved in the post-extrusion impregnation. Zeolite-based extrudate catalyst materials are known to suffer from pore blockage phenomena, which makes diffusion of Mg precursor more difficult and causes further pore narrowing [51,53]. On the other hand, Mg-ions added to the technical catalyst bodies during the extrusion lead to a minimum interaction between Mg and the zeolite.The samples were tested on the activity and selectivity in the MTH reaction in order to investigate the effect of Mg modification in the zeolite-based alumina-bound shaped catalyst bodies on their catalytic performance. Fig. 4 illustrates the methanol conversion and the yield of the ethylene and propylene formed during the MTH reaction operating at a Weight Hourly Space Velocity (WHSV) of 6 h−1 and 400 °C for 35 h Time-on-Stream (TOS). The reference sample (Ext. Z/A, black color), as Fig. 4a illustrates, exhibits a high level of conversion at ∼94%. However, the methanol conversion decreased fast and reached a value of ∼84% within the first 20 h TOS. The same trend is observed for the sample in which Mg is impregnated only in the alumina binder, which implies that Mg species present in the binder and the slight increase in acidity have no effect on the methanol conversion. Ext. MgZ/A (pink color) and Ext. MgZ/MgA (red color) samples can achieve a high level of conversion ∼ 92 and ∼89%, respectively. Although this is slightly lower than the reference sample and Ext. Z/MgA sample, the decrease of strong acid sites can explain this due to Mg addition. Impregnating the zeolite with Mg before extrusion shows a prolonged lifetime as both samples are less prone to deactivation as they preserve a higher level of conversion than the reference sample. This is in line with findings in literature that show Mg addition reduces strong acidity, increasing the catalyst's stability [24,30,38].Regarding the samples in which Mg is added during extrusion (Ext. Z/A/0.5 Mg, dark green color, and Ext. Z/A/1 Mg, light green color), it can be seen that they achieve a lower level of conversion compared to the reference material. This can be explained by the decrease in surface area, as well as the minor decrease in the strong acidity, as shown in Figs. 1b and 3b, respectively. Further decrease in the conversion was observed for the samples in which Mg was impregnated after the extrusion process, with the sample containing 1 wt% exhibiting ∼70% of methanol conversion.Nonetheless, the reference sample showed similar trends in ethylene and propylene yield. On the contrary, the Ext. Z/MgA (Mg impregnated only in the binder) sample exhibited the same yield of propylene at the same time as decreased ethylene yield underlying the major effect of the properties of the binder. As in this case, Ext. Z/MgA (orange color) sample showed pores of smaller size, while the Mg addition foresees the basicity of the binder. These changes promote the olefin cycle over the aromatic cycle. Mg modification of only the zeolite before extrusion (Ext. MgZ/A, pink color) resulted in a ∼133% increase in propylene yield and a simultaneous decrease in ethylene yield. Impregnating both components prior to extrusion (Ext. MgZ/MgA, red color) caused an increase in propylene yield (∼166%). These findings also point out that Mg present in the binder holds a crucial role in the selectivity towards light olefins. Even though Mg and zeolite have little interaction when Mg is added during the extrusion process, based on the physicochemical characterization and the results mentioned above, it can be seen that Ext. Z/A/0.5 Mg and Ext. Z/A/1 Mg samples favored the propylene formation. Post-extrusion modification produced lower yields of ethylene and propylene in total due to the lower conversion levels recorded in these sets of samples. However, it can be seen that propylene formation is favored over ethylene due to the presence of intermediate strong acid sites. Last but not least, Fig. S7 illustrates the selectivities towards ethylene (black), propylene (red), C4 olefins (blue), C5 olefins (pink), and paraffins (sum of methane, ethane, propane, C4, and C5) (green) of all samples under study versus TOS for the MTH reaction. The latter was done in order to rule out any possibility to mislead due to the fact that the yield could be affected by the different conversion levels. Similar trends were noted for ethylene and propylene selectivities as those described above for the yields. Moreover, the paraffins selectivity draws great interest. We note that the reference sample (Ext. Z/A) exhibits the higher selectivity towards paraffins which can be attributed to the high content of strong acid sites. Relatively high selectivity towards paraffins can also be noted for sample Ext. MgZ/A in which Mg is added only in the zeolite before extrusion. In parallel, it can be seen that in all the other samples in which Mg is added either in the binder prior to extrusion, during, and/or after extrusion, the selectivity towards paraffins is considerably lower compared to Ext. Z/A and Ext. MgZ/A samples. A representative example is Ext. Z/MgA (in which only the binder is modified with Mg) and it can be noted that it exhibits ∼ 10% less selectivity towards paraffins compared to the reference sample (Ext. Z/A). Hydrogen transfer reactions could take place in acid sites located in the alumina binder that could lead to the formation of aromatics and alkanes. The above mentioned observation could imply that modification of the binder with Mg can inhibit these type of successive reactions leading to lower selectivity towards paraffins. This consideration underlines the importance of the presence of the binder as well as its properties in the physicochemical properties and the catalytic performance in the MTH reaction. The zeolite powder material and Mg-modified zeolite powder was also tested and compared for the MTH reaction, as shown in Fig. S8. The latter experiments confirmed the beneficial effect of Mg in the catalytic activity and the increase towards propylene.To further understand the effect of Mg, Thermogravimetric Analysis (TGA) measurements were performed on a sample modified by each approach (pre-extrusion, during extrusion, and post-extrusion) with 0.5 wt% of Mg and the reference sample after 35 h time on stream. As illustrated in Fig. 5 a, the TGA curves for every sample showed a low-temperature weight loss at below 150 °C and a high-temperature weight loss at above 150 °C, which are attributed to removal of water and coke, respectively. The reference sample, the sample in which Mg was impregnated prior to extrusion, and the one in which Mg was added during extrusion showed a similar amount of weight loss. On the contrary, the 0.5Mg/Ext. Z/A shows lower weight loss.To further evaluate the differences in the deactivation of these samples, the individual percentages of the two types of weight losses (low- and high-temperature) are presented in Fig. 5b. Focusing on the coke content, it is clear that the Mg addition reduces the formation of coke deposits. Post-extrusion modification showed less formation of coke, which can be ascribed to the lower methanol conversion levels. Comparing Ext. MgZ/A and Ext. Z/A/0.5 Mg samples, it is obvious that higher interaction reduces further the coke formation. The CO2 fragment signal is plotted against the temperature and shown in Fig. 5c to gain more insights into the type of coke species formed during the reaction. The reference sample shows different coke species that can be separated into two categories, the “soft” and “hard” coke. “Soft” coke can consist of smaller aromatic compounds, such as alkylated benzenes and naphthalene, which need a lower temperature to burn off, while “hard” coke can consist of larger polyaromatic compounds and even graphite-like coke which need higher temperatures to be removed. Comparing the reference sample with the one pre-extrusion modified, it is clear that Mg-zeolite interaction strongly reduces the formation of “hard” coke. The reduction of strong acidity and the formation of moderate strong acid sites play a significant role in the coke species formed. However, the samples 0.5Mg/Ext. Z/A (dark blue) and Ext. Z/A/0.5 Mg (dark green) both show the presence of “soft” and “hard” coke with “hard” coke dominating.Confocal Fluorescence Microscopy (CFM) was used to visualize the nature of the coke species and their spatiotemporal distribution throughout the catalyst extrudate in the spent unmodified and modified samples after 15 and 75 min TOS in the MTH reaction. Various reaction products can be formed during the MTH process. These products can be separated into “less conjugated” species, such as alkylated benzenes and naphthalene, and “more conjugated”, such as alkylated phenanthracenes (PH), pyrenes (PY), and (LPAs). Two lasers (i.e., 488 and 642 nm) were used to detect and separate the two types of reaction products. Green fluorescence can be emitted from the “less conjugated” species, while red fluorescence originates from the “more conjugated” species.The top-view 3D CFM images of the reference sample, as shown in Fig. 6 a, show the presence of two different color areas, which translates into different types of coke deposits. After 15 min TOS, a green/yellow fluorescent near-edge region exists in the catalyst extrudates, while the core of the catalyst extrudates shows an orange fluorescence. The green/yellow color in the near-edge region indicates the presence of “less conjugated” hydrocarbon species, while the orange core region indicates the presence of “more conjugated” hydrocarbon species. According to the literature, there is a molecular transport boundary towards the core of the catalyst extrudate [44,54]. Thus, “less conjugated” species produced in the core of the catalyst extrudate in their way to diffuse out can fall into secondary oligomerization reactions to form larger and more conjugated species, which are trapped in the core of the catalyst extrudate, explaining the orange core of the extrudate. The existence of less conjugated species in the near-edge region (bearing a green/yellow fluorescent color) could be explained by the cracking reaction of larger species to form smaller aromatic species. At 75 min TOS, it is evident that the two coke regions are still present, and their colors are more red due to the larger conjugated species formed.Upon adding Mg during the extrusion process, as illustrated in Fig. 6c, no major change in the nature and the distribution of coke species is observed in the 15 min spent samples. After 75 min TOS, the near-edge and the core regions of the catalyst extrudate show less red color, implying the formation of smaller aromatic species upon addition of Mg. Regarding the Ext. Z/A/1 Mg sample, it is clear that the near-edge region is thicker. N2 physisorption results and pore volume distribution showed a decrease in mesoporosity, which could explain the thickening of the near-edge region. The latter is in line with the literature as Whiting et al. reported that a decrease in porosity and accessibility resulted in the trapping of larger molecules in the core of the zeolite-containing catalyst extrudates [44]. Similar results regarding the thickness of the near-edge region were observed for the post-extrusion modified samples. After 75 min TOS, both samples, 0.5Mg/Ext. Z/A and 1Mg/Ext. Z/A, appear to have an even darker red core region compared to the reference sample. Even though Mg impregnation on the zeolite-based catalyst extrudates decreases both weak and strong acidity, we observe a higher formation of larger hydrocarbon species in the core region of these catalyst extrudates. This could be explained by the pore narrowing and decrease in porosity, as explained previously in Fig. 1c and d.Interestingly, pre-extrusion modification of the samples with Mg drastically changed the nature and molecular distribution of coke deposits throughout the catalyst extrudate, as demonstrated in Fig. 6b. Regarding the samples in which Mg was added only in the binder material, it still shows two areas, namely near-edge and core of the catalyst extrudates. However, the Ext. Z/MgA sample is characterized by a thicker yellow near-edge region and a red core region. The thickness of the yellow near-edge region could be explained by the smaller pore size after Mg modification, as shown in Fig. 1. The darker and/or more red hue of the two regions of interest could be attributed initially to the pore narrowing as entrapment of small molecules would be more profound. Furthermore, NH3-TPD analysis showed an increased acidity (for both the weak and strong acid sites) upon impregnating Mg in the alumina binder, which could increase secondary reactions of aromatic moieties and, thus, the genesis of the red fluorescence. Regarding Mg addition before extrusion in the zeolite (Ext. MgZ/A and Ext. MgZ/MgA), CFM images on the 15 min spent samples show a uniform coke formation of “less conjugated” hydrocarbon species. After 75 min TOS, conjugation into larger species is noticed, as shown from the red fluorescence emitted. The coke formation in these modified samples is vastly different from the reference sample, where two distinct areas of small and large aromatic species are formed. This is attributed to the different acidic properties of the samples. According to the literature, the formation of LAS induced by Ca or Mg modification prevents cyclic hydrocarbon pool intermediate species from participating in reactions involving aromatic moieties [30]. This could justify the existence of mainly “less conjugated” species, as the Ext. IZ/A and Ext. IZ/IA samples mainly contain weak and intermediates acid sites.About the post-extrusion modification, as shown in Fig. 6d, a similar deactivation pattern was observed compared to the reference sample and the samples in which Mg was added during extrusion. It seems also that in this case the near-edge region of the catalyst extrudates is enlarged for both 15 and 75 min TOS. Both samples show a more red color in the core region of the catalyst extrudates, implying the dominant presence of “large conjugated” hydrocarbon species. The latter is in line with the results from the TGA measurements, which are shown in Fig. 5c. This observation can be explained by the pore narrowing which would explain the entrapment of aromatic compounds and their further oligomerization to larger polyaromatic hydrocarbon.Whiting et al. proved the existence of a molecular transport boundary has been proven [44]. They showed that tailoring the level of accessibility and porosity is strictly related to molecular transport. Even though the effect of the pore architecture on the molecular transport and the deactivation was established, there was no correlation on the effect of acidity. Here, our findings underlying the importance of the type of acid sites and the location of Mg in zeolite-alumina catalyst extrudates in their performance during the MTH reaction. Acidity and pore architecture are both important contributing factors in catalyst deactivation. However, acidity appears to be the most determining factor as weak and intermediate acid sites, as formed by the addition of Mg, inhibit the formation of larger polyaromatic moieties.As shown in Fig. 7 a–b, the ethylene and propylene selectivities were correlated to the changes in acidity upon Mg modification (pre-, during, and post-extrusion). The NH3-TPD analysis were used to calculate the amount of weak, intermediate and strong acid sites, as previously explained. Then, the concentration of each type of acid sites were calculated based on the amount of acid sites divided by the SSA of the catalyst shaped bodies, to normalize for the changes in textural properties upon Mg modification. The above mentioned approach clearly shows a linear correlation between concentration of the strong acid sites (SAS) and ethylene selectivity as well as the concentration of the weak and intermediate acid sites (WAS) and propylene selectivity. Regarding ethylene selectivity, we show that at high concentration of strong acid sites, a slight increase in the ethylene selectivity can be observed. The latter observation can be explained by the presence of strong acid sites in close proximity and to the consecutive reactions which could enhance the aromatic cycle. Simultaneously, high concentration of weak acid sites resulted in high propylene selectivity. Incorporation of Mg induced lower strong acid sites while increased the concentration of weak and intermediate acid sites, as shown in Fig. 3. The latter could lead to inhibit aromatization and coke formation, promoting the alkene cycle, and thus, propylene selectivity. Similar approach has been followed from Yarulina et al. to describe the structure-performance descriptors and to underline the role of LAS for the MTO reaction [30]. The latter research study confirms our findings.To further understand the effect of Mg in the deactivation of zeolite-based shaped catalyst bodies, the concentration of strong acid sites was correlated to the deactivation rate calculated for each sample by the conversion over TOS. As illustrated in Fig. 7c, it is clear that there is a linear correlation between the strong acidity and the deactivation rate. This phenomenon can be attributed to the higher formation rate of methylated aromatic species, and thus, faster coke formation due to the high amount of strong acid sites. Last but not least, the relationship between the coke content and the ratio of the sum of the concentration of weak and intermediate acid sites is shown in Fig. 7d. The reference sample, Ext. Z/A (black), with a relative low ratio of weak and intermediate to strong acid sites exhibits a high coke formation. Moving to the sample in which Mg was added during extrusion, it is obvious that coke content is reduced. Furthermore, the status is completely reversed when maximum Mg-zeolite intimacy succeeded by addition of Mg before extrusion (Ext. MgZ/A, pink). In the case of Ext. MgZ/A (pink) we see a high ratio of weak and intermediate to strong acid sites resulted relatively low coke content. We conclude that the ratio of acid sites is crucial for the coke formation and catalyst deactivation.In this study, we have investigated the influence of the location of magnesium in zeolite-based shaped catalyst bodies on their physicochemical properties and catalytic performance in the Methanol-to-Hydrocarbons (MTH) reaction. Magnesium has been introduced in different steps of the extrusion process, namely before, during, and after the extrusion process. Physicochemical characterization of the different samples prepared proved that the pre-extrusion modification of the samples resulted in a decrease in strong acid sites, while new weak and intermediate acid sites were formed due to strong magnesium-zeolite interaction. At the same time, magnesium added prior to the extrusion process showed a significant increase in the propylene yields and the lifetime of the catalyst material prepared. A clear correlation between magnesium location, molecular transport, and catalyst deactivation during the MTH reaction of zeolite-based catalyst extrudates was made. We anticipate that this approach may contribute to designing a better catalyst material for the MTH process as well as applying this knowledge to other zeolite-based catalytic systems and acid-catalyzed chemical reactions. Nikolaos Nikolopoulos: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Luke A. Parker: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Maurits W. Vuijk: Formal analysis, Data curation. Bert M. Weckhuysen: Writing – review & editing, Writing – original draft, Supervision, Resources, Project administration, Funding acquisition, Conceptualization.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research has received funding from the European Union's EU Framework Program for Research and Innovation Horizon 2020 under Grant Agreement No. 721385 (MSCA-ETN SOCRATES - https://etn-socrates.eu/) and from the US Army Research Office (ARO, with reference number W911NF-18-1-0284). Joren Dorresteijn (Utrecht University, UU), Sebastian Haben (UU), and Silvia Zanoni (UU) are acknowledged for performing the N2 physisorption measurements. We would like to thank Dennie Wezendonk (UU) for the TGA measurements. The authors would like to thank Christia R. Jabbour (UU) for her contribution and help during the revision process.The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.micromeso.2023.112553.

One of the main challenges for the chemical industries is finding new ways to produce lower olefins, such as propylene and ethylene, to satisfy the increase in demand for e.g., polymers, namely polypropylene and polyethylene. The Methanol-to-Hydrocarbons (MTH) process is an alternative manufacturing process that can help to address this increasing demand for these important chemical building blocks. It has been proposed that the addition of magnesium to zeolites, in the form of powdered catalyst materials, enhances the selectivity towards light olefins. In this work, the impact of the location of magnesium (present as Mg2+ and MgO) in zeolite-based shaped catalyst bodies on their physicochemical properties and catalytic performance in the MTH reaction has been studied. By adjusting one of the preparation steps of the overall extrusion process in which magnesium is added tuning the location of magnesium, higher interaction between magnesium and the zeolite material could be achieved. Pre-extrusion modification showed the most favorable results in terms of physicochemical properties and catalytic activity. We found that the magnesium location could be crucial for altering molecular transport, coke formation, and catalyst deactivation during the MTH reaction due to its pronounced effects on the acidity as well as porosity of the shaped catalyst bodies. These new insights can be applied to other zeolite-based extrudate materials and other acid-catalyzed reactions as it can be crucial for the design of better and more efficient catalyst materials in their industrially shaped form.