Datasets:

WilliamWen

/

summarization_cata

like 0

Data will be made available on request.The design of efficient electrocatalytic materials for the development of green electrochemical energy storage and conversion devices, such as unitized regenerative fuel cells or rechargeable metal-air batteries, has become a promising way to solve the global energy demand and the environmental-related problems. However, the commercial applications are severely hampered by the high cost and poor stability of the bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) [1–4].Carbon-based materials have been studied in the last decades for their use as electrocatalysts for oxygen reduction and evolution reactions due to their good electrochemical properties such as high conductivity and catalytic activity. However, their main shortcoming is related to the weak resistance to corrosion and oxidation of carbon materials during OER, which restricts their use as bifunctional oxygen catalysts. To solve this issue, advanced carbon nanostructures, such as graphene, carbon nanofibers or carbon nanotubes, have shown promising performance and stability as bifunctional catalysts [5–7]. Among them, one dimensional carbon materials like carbon nanofibers (CNFs) have shown great potential because of their graphite-like structure together with more exposed active sites [8]. Therefore, CNFs contribute to increase the electrical conductivity, improve the resistance to corrosion processes and help to the diffusion of reagents and products [9–14].One of the most relevant properties of carbon materials in increasing activity is the presence of heteroatoms at the surface, such as N, S and P. The insertion of heteroatoms into sp2-hybridised carbon structures has been proven to be an effective way of modifying their electrical properties, chemical activity and stability [15–20]. Among heteroatoms, nitrogen has been extensively investigated for fuel cells and bifunctional catalysts [21,22], including those based on carbon nanofibers [23–26], but sulfur is gaining increasing attention due to the interesting electrochemical properties generated on sulfur-doped carbons. Higgins et al. developed a sulfur-doped graphene supported Pt-based catalyst with excellent activity for the ORR [27]. They reported that the presence of sulfur led to stronger adsorptive and cohesive binding energies with Pt nanoparticles, providing both beneficial catalytic activity and stability enhancements. Pt/S-Graphene provided 139 mA mg−1Pt at 0.9 V vs. RHE, better than commercial Pt/C (121 mA mg−1Pt) and Pt/Graphene (101 mA·mg−1Pt). Hoque et al. designed Pt nanowires/sulfur doped graphene as ORR catalysts in acidic electrolyte. The amount of sulfur significantly affected the ORR kinetics of the Pt nanowires. They got 182 mA·mg−1Pt at 0.9 V vs. RHE at a content of 1.4 wt% sulfur [28,29]. Li et al. enhanced the electrocatalytic stability of Pt supported on sulfur doped pristine carbon for the ORR in 0.1 M KOH using in-situ solution plasma with a sulfur content of 4 wt% [30]. However, bifunctional electrocatalysts based on sulfur-doped carbons have been less investigated for both oxygen evolution and reduction reactions [6,31,32]. Gao et al. synthesized manganese oxide/sulfur-doped graphitized carbon as bifunctional catalyst for ORR/OER. The current density reached −3 mA cm−2 at 0.81 V vs. RHE (for ORR) and 10 mA cm−2 at 1.62 V vs RHE (for OER) in 0.1 M KOH [32]. El-Sawy et al. incorporated heterocyclic sulfur into the carbon nanotube-graphene structure by a bidoping strategy, which not only enhanced OER activity with an overpotential of 0.35 V at a current density of 10 mA cm−2, but also retained 100% of stability after 75 h. Furthermore, the sulfur-doped carbon showed high catalytic activity for the ORR [6].In previous works, we investigated a microemulsion procedure to create tantalum based catalysts [33], and carbon nanofibers as support for such tantalum-based nanoparticles, which demonstrated to be active and durable for OER [34]. Oxides from metals of groups 4 and 5 of the periodic table are known as highly durable catalysts, which combined with CNFs, represent a good opportunity towards durable and active bifunctional oxygen catalysts. In this work, we seek an enhancement of electroactivity by doping the CNF support with sulfur. We report the preparation of tantalum oxides supported on different sulfur-doped carbon nanofibers obtained by varying the temperature and duration time of the doping process. These materials were investigated as bifunctional catalysts for ORR and OER in alkaline medium. Physical and chemical properties of the supports as well as the corresponding bifunctional catalysts were characterized using several techniques. The electrocatalytic performance for ORR/OER was assessed using a rotating ring-disk electrode and the stability through time of these new materials was evaluated.The catalyst for CNF synthesis, composed by Ni–Cu–Al2O3 (Ni:Cu:Al molar ratio of 78:6:16), was prepared by coprecipitation of metal nitrates (Ni(NO3)2·6H2O, Cu(NO3)2·3H2O and Al(NO3)3·9H2O, Sigma-Aldrich, > 99%), followed by calcination (air) at 450 °C for 8 h and subsequent reduction in hydrogen (Messer, > 99.5%) at 550 °C for 1 h. The catalyst composition and preparation correspond to previous investigation showing high methane conversion and stability [35]. For the growth of CNFs, 300 mg of the Ni-based catalyst was placed into a vertical fixed bed reactor under nitrogen flow (Messer, > 99.8%) and heated up to 700 °C. Then, methane (Air Products, 99.995%) was fed to the catalyst sample for 620 min at ambient pressure. The reactor was then cooled to room temperature under inert atmosphere (N2). Finally, CNFs were washed with 0.1 M HClO4 aqueous solution at 60 °C for 15 min in order to eliminate the nickel used for the growing process, followed by thorough washing with deionized water and drying at 60 °C overnight.CNF was mixed with elemental sulfur powder (Alfa Aesar), with a mass ratio of 95:5 (carbon:sulfur), in an agate mortar and deposited in a ceramic boat. The mixture was introduced in a horizontal reactor and thermally treated under inert atmosphere (N2). Two procedures were used to dope the CNF. One of them consisted of treating the sample at 250 °C for 6 h and the other one at 400 °C for 3 h, in both cases the heating rate was 5 °C min−1. As-prepared samples were washed with carbon disulfide (99.5%, Panreac) to eliminate the non-doping sulfur in the material, rinsed with ethanol, then with water, filtered, and finally dried in an oven at 60 °C. The sulfur-doped CNFs were labeled as CNF-SX where X stands for the CNF doping temperature (250 or 400 °C).Tantalum-based oxides (general formula TaOx) were deposited on the sulfur doped CNF by a microemulsion procedure [34]. The microemulsion (ME) consisted of mixing 0.25 mL of 75 mM NaOH aqueous solution (Alfa Aesar, 99.99%) with an oil phase composed of 2.3 g of surfactant (Igepal CO-520, Aldrich), 20 mL of n-heptane (Honeywell) and 0.75 mL of ethanol (Labkem, 99.5%). Then 0.05 mL (0.3 mmol) of tantalum (V) ethoxide (Aldrich, 99.98%) was added to the ME under continuous stirring at room temperature. According to previous works, the mixture reacts producing tantalum oxide nanoparticles within 5 min [36]. Afterwards, 312 mg of the S-doped CNF was added to the suspension and stirred overnight. The two doped CNF described in the previous section were used as supports, as well as undoped CNF for comparison purposes. In the next step, the materials were washed with ethanol and then with water, followed by drying at 60 °C overnight. The final step consisted of a heat treatment in inert atmosphere (N2) for 90 min at 900 °C. Finally, the material was washed with deionized water and dried at 60 °C overnight. The catalysts were labeled as TaOx/CNF–S250 and TaOx/CNF–S400.The concentration of nickel and tantalum was determined by inductively coupled plasma atomic emission spectroscopy (ICP-OES) using a Xpectroblue-EOP-TI FMT26 (Spectro). The weight percentages of sulfur and carbon were obtained by elemental analysis in a Thermo Flash 1112 equipment.Nitrogen physisorption experiments were carried out in a Quantachrome equipment and analyzed with Quadrawin software. Adsorption-desorption isotherms were obtained at −196 °C. Brunauer-Emmet-Teller equation was used to calculate the BET specific surface area and the Barret-Joyner-Halenda model, applied to the desorption branch of the isotherms, was considered to determine the pore size distribution.X-ray photoelectron spectroscopy (XPS) was used in order to determine the concentration of species and the oxidation state of the doping sulfur and tantalum. The analyses were carried out in an ESCA+ (Omicron) and analyzed using CasaXPS software.The X-ray diffraction (XRD) analyses were obtained in a Brucker D8 Advance diffractometer with CuKα radiation of 1600 W. The diffractograms were analyzed using TOPAS and EVA software and compared to the patterns of the different phases from the International Center for Diffraction Data (ICDD).Transmission electron microscopy (TEM) pictures were taken in a Tecnai F30 (FEI) microscope. For this purpose, the electrocatalysts were dispersed in ethanol and dropped on a carbon film coated Cu grid. For each catalyst, the particle size distribution of the deposited tantalum oxides was calculated using ImageJ software on TEM images and then analyzed with the statistic tools of OriginLab software.The ratio between oxygen and tantalum was determined with a Scanning Electron Microscopy-Energy Dispersive X-ray spectroscopy (SEM-EDX) SEM-EDX Hitachi S-3400 N with EDX Röntec XFlash of Si(Li).Electrochemical analyses were carried out in a three-electrode cell at room temperature using a 0.1 M NaOH aqueous solution as electrolyte (NaOH 99.99%, Alfa Aesar), prepared with ultrapure water (Milli-Q, 18.2 MΩ cm). For ORR experiments, the NaOH solution was saturated with O2 (Messer, 99.5%). The reference electrode was a reversible hydrogen electrode (RHE) and the counter electrode a glassy carbon rod. The catalysts were placed on a rotating ring-disk electrode (RRDE) composed of a glassy carbon disk (5 mm diameter) and a Pt ring. The catalytic layer was made using 1 mg mL−1 ink obtained by sonicating the catalyst in isopropanol/water (50:50) and Nafion® (30 wt% of the catalytic layer), which acted as a binder. The required drops of ink were located onto the glassy carbon disk to get the needed mass loading. The catalyst loading on the disk was estimated to be 500 μg cm−2. The study of the ORR was performed by linear sweep voltammetry (LSV) from 1.0 to 0.3 V vs. RHE and at a scan rate of 5 mV s−1 with varying rotating speeds from 400 to 1600 rpm. The study of the OER was performed between 1.0 and 1.9 V vs. RHE and at a scan rate of 5 mV s−1 with a rotating speed of 1600 rpm to ease the diffusion of evolved oxygen.The percentage of H2O2 formation during the ORR experiments was calculated according to Equation (1) , with j r  = current at the ring (at constant potential of 1.2 V vs. RHE), j d = current at the disk and N = 0.249 (collection efficiency). (1) % H 2 O 2 = 100 · 2 · j r N · j d + j r The kinetic current density in the ORR, j k , was calculated by Equation (2) considering that the measured current density, j, can be expressed as the separate contribution from j k and the diffusion limiting current density, j lim , as follows: (2) 1 j = 1 j k + 1 j l i m The oxygen efficiency in the OER, ε, this is the percentage of current actually transformed in oxygen, was calculated according to Equation (3) , where N, j r and j d stand again for the collection efficiency, current at the ring (at 0.6 V vs. RHE) and current at the disk, respectively, whereas n ORR is the number of electrons for the reduction at the ring of the oxygen evolved from the disk (for Pt ring, n ORR  = 4). (3) ε ( % ) = 100 · 4 n O R R · j r N · j d Tafel slopes (b) for both ORR and OER were determined upon ohmic resistance correction (iR correction) of potential values. For this, the ohmic resistance was estimated from Newman's equation to be 8 Ω cm2. Equation (4) describes the correlation of current (j k for ORR and j for OER) and iR-corrected potential (E iR-free ), with Tafel slope (b), exchange current density (j 0 ) and reversible potential (E 0  = 1.23 V vs. RHE): (4) η = | E i R − f r e e − E 0 | = b · l o g ( j j 0 ) Endurance tests were done to determine the variation of the behavior of the catalysts over time. For this purpose, chronopotentiometric tests were carried out, consisting of consecutive square cycles alternating either a positive or negative current density of 1 mA cm−2 maintained for 180 s. Cut-off potentials of 1.9 V and 0.2 V vs. RHE were established. These tests were also done on a three-electrode cell with the same characteristics as those previously described, but now using a rotating disk electrode (RDE), with a glassy carbon tip of 5 mm and working at a rotating speed of 400 rpm.The chemical composition of the electrocatalysts was studied by elemental analysis, ICP-OES, SEM-EDX and XPS (see Tables 1 and 2 ). The weight percentage of sulfur for the two doped supports is similar with slightly higher concentration for the sample treated at 400 °C, as observed from elemental analysis and EDX results. The content of sulfur decreases slightly after introducing the tantalum-based catalytic particles (Table 1), most probably because of the increase of the relative content of tantalum species, which is consistent with the decrease of carbon concentration. With regard to the metallic phase, ICP analysis showed less content of Ta for S-doped CNF catalysts than for the undoped one following the same synthesis procedure (Table 2) [34]. The porous structure of supports was evaluated by nitrogen physisorption. The adsorption/desorption isotherms as well as the pore size distribution are included in the supplementary information (Fig. S1). The incorporation of sulfur species did not significantly alter the porous structure of the filaments, with similar values of both BET and external surface area (in between 55 and 70 m2 g−1, Table S1), a pore volume between 0.22 and 0.32 cm3 g−1, and a negligible content of micropores, as expected for this kind of materials [10]. The different content of Ta on supports with similar porosity indicates that the anchorage of tantalum nanoparticles is less efficient when there are sulfur species on the surface of carbon nanofibers. By comparison of the two S-doped TaOx catalysts, ICP analyses evidence a larger concentration of Ta on the support doped at 250 °C whereas XPS indicates a larger amount of Ta on the one doped at 400 °C. From this point of view, the CNF doped at 400 °C (CNF–S400) clearly favors the presence of tantalum particles on the surface compared to CNF–S250. The difference could be associated to the chemical speciation of sulfur, as discussed next. ICP results also evidenced the presence of nickel (1.8–2.6 wt%) even after the acid leaching of the samples. Interestingly, nickel was not detected in XPS analyses, most probably because it is encapsulated by carbon inside the CNF.A relevant parameter is the oxygen/tantalum atomic ratio, which is summarized in Table 2 from EDX and XPS analyses. The determination of the composition of the tantalum oxides by SEM-EDX was done by selecting areas with metal oxide nanoparticles, with the idea of minimizing the carbon and oxygen peaks from the supporting materials. The oxygen vacancies/substoichiometry has been correlated to favor the electrochemical activity for oxygen related reactions in oxides of metals from groups 4 and 5 [37]. In our case, Both EDX and XPS results do not indicate a ratio below the stoichiometric one for the most abundant phase (O/Ta = 2.5 for Ta2O5), but it must be considered that the support itself provides around 3 at.% oxygen (Table 1). Only in the case of TaOx/CNF–S400 the XPS analysis indicates O/Ta below 2.5, which points to a larger substoichiometry in this catalyst compared to the other formulations.XPS analyses also show the different speciation of sulfur in the most external surface of the doped CNFs, as represented in Fig. 1 . Regarding the supports (Fig. 1a), the S2p3/2 signal point out the presence of two main peaks at binding energies close to 164 eV and 169 eV, corresponding to two different oxidation states of sulfur: C–S–C and sulfoxide (S VI), respectively. The proportion of each functional group for each support is show in Table 3 , indicating a larger content of C–S–C in the CNF doped at higher temperature (400 °C). This difference may partially explain the higher concentration of tantalum at the surface for TaOx/CNF–S400 discussed before. Sulfur doped carbon materials are known for their capacity to adsorb metals, e.g. for decontamination purposes [38]. Since the main difference between our two S-doped supports is sulfur speciation, the enrichment of tantalum at the surface for TaOx/CNF–S400 could be related to more sulfur bonded to carbon. Interestingly, after tantalum is incorporated on the support, sulfur appears mainly as C–S–C, with 83–85% as shown in Table 3 and Fig. 1b. In the catalysts, there is also a small contribution of sulfur bonded to metal appearing at lower binding energy of 162 eV (S-M), most probably nickel sulphide as identified in XRD analyses.XPS analyses revealed a different Ta concentration at the most external surface of each catalyst. The Ta 4f signal for the catalysts is showed in Fig. 2 . The signal with a binding energy close to 27 eV, which corresponds to Ta 4f7/2, indicates the presence of Ta (V) oxidation state [39].XRD diffractograms for the S-doped and undoped catalysts are found in Fig. 3 . Different phases were detected in the different materials: carbon, nickel and nickel sulphides (Ni3S2, NiS2, NiS) from the support (see supplementary information, Fig. S2, Table S2 and Table S3), and up to three phases containing Ta oxides (Ta2O5, TaO and NaTaO3). Regarding the support-related species, the S-doped supports without Ta oxides contain both NiS2 (major) and NiS. Interestingly, the catalysts reveal that nickel sulphides have been completely converted to the nonstoichiometric Ni3S2 upon thermal treatment.With regard to the tantalum related phases, the major contribution to XRD reflections comes from Ta2O5, with a minor contribution of TaO (in particular for TaOx/CNF–S250) and even lower signal associated to NaTaO3, only for the undoped catalyst. It must be said that the presence of a mix of tantalum oxide species has been recently reported to tailor surface behavior by creating a charge transfer accumulation at their interface, caused by significant changes in the work function of the tantalum species, which results in enhanced electrocatalytic behavior [40].To delve into the crystallinity of the catalysts, XRD patterns were analyzed with Topas software (Lebail method) and the lattice parameters calculated from XRD for the CNF-supported TaOx electrocatalysts are summarized in Table 4 , whilst the crystallite sizes are gathered in Table 5 . It is interesting to mention that the lattice parameters for Ta2O5 are slightly lower compared to the reference pattern (JCPDS#89–2843) with stoichiometric formula. This is more evident in peaks (0 0 1) and (0 0 2) of Ta2O5, which appear about 0.2–0.3° shifted to higher Bragg angles (2θ) compared to the ICDD reference (JCPDS#89–2843). The contraction of the unit cell, together with chemical composition discussed before, could be associated to oxygen deficiency, as stated in previous works [33,34,41].TEM and STEM images were evaluated in order to study the morphology of S-doped CNF-supported TaOx catalysts. Some images are collected in Fig. 4 . Both electrocatalysts are composed of carbon nanofibers with tantalum-based particles on their surface. The metal oxide nanoparticles are distinguished from the filaments by darker contrast in TEM images (left side of figure) and by lighter contrast in STEM images (right side of figure).The particle size distributions of the tantalum particles are depicted in Fig. 5 for both catalytic materials from TEM images. The main difference is that TaOx/CNF–S250 shows a broader distribution and particles with bigger size than TaOx/CNF–S400. The average particle size is 24.1 ± 6 nm for TaOx/CNF–S250, and 13.6 ± 3 nm for TaOx/CNF–S400, in line with crystallite sizes reported in Table 5 for Ta2O5 from XRD results (23.5 and 17.8 nm, respectively).The electrochemical ORR activity in alkaline electrolyte for the different catalysts is depicted in Fig. 6 . The disk current density (Fig. 6a) presents a sigmoidal wave form, as typically occurs for the oxygen electroreduction with oxygen saturated in the electrolyte, reaching a limiting current density at high overpotential attributed to oxygen diffusion limitation. First, by comparing the ORR activity of CNF–S250 and CNF–S400 with the catalysts TaOx/CNF–S250 and TaOx/CNF–S400, it is clear that the addition of tantalum oxides has a positive effect on the activity, with a potential shift of about 40 mV for TaOx/CNF–S250 and 60 mV for TaOx/CNF–S400, in terms of half-wave potential. This comparative study is useful to discard the eventual effect of nickel on ORR activity over the effect of tantalum oxides. There is a clear positive effect of tantalum oxide phases on the electroactivity regardless the presence of nickel traces.On the other hand, by comparison with the undoped support (TaOx/CNF), the introduction of sulfur has also a significantly positive effect on the ORR activity of doped catalysts, with more than 50 mV enhancement. The S-doped TaOx catalysts present a small amount of Ni3S2, as discussed from XRD characterization. A certain contribution of this phase to the activity cannot be discarded, even if nickel sulfide is at the level of traces. Fig. 6b depicts the ring current at 1.2 V vs. RHE for the three TaOx catalysts, which is attributed to the oxidation of the hydrogen peroxide evolved at the disk. In the inset of Fig. 6b the evolution of H2O2 percentage with potential is shown, with about 50% average production. This indicates a number of electrons close to 3. There are no significant differences among the evaluated catalysts, which indicate a similar reaction mechanism in terms of the number of electrons. Most probably, the tantalum-based catalysts present a balanced mix of active sites towards both 2e− and 4e− (or 2x2e−) pathways, which does not change with sulfur doping of the support.The most relevant electrochemical parameters related to ORR are collected in Table 6 . The equivalent data for the experiments with only the supports are summarized in Table S4. The kinetic current density at 0.6 V vs. RHE is significantly higher (2.3 and 3.2 mA cm−2) for the two sulfur-doped CNF-supported catalysts than for the undoped TaOx/CNF (1.4 mA cm−2). In line with the previous description of results, the overpotential (ηiR-free) at −1 mA cm−2 decreases around 50 mV upon sulfur doping of the CNF support. Also the onset potential is 70 mV more positive for the doped materials. All these indicators point to the positive effect of sulfur doping in this class of catalysts. Still, despite presenting proper stability, the tantalum-based catalysts presented herein are not as active as other published catalysts in terms of activity. For example, when combining sodium tantalate (Na2Ta8O21) with tantalum oxide (Ta2O5) and tantalum nitride (Ta3N5), the ORR activity is much higher (Eonset = 0.9 V vs. RHE in 0.1 M KOH) [40]. Compared to bifunctional catalysts, titanium oxide (another group 4 metal) combined with N-doped graphene [7], or some other combinations of metal oxides (Fe, Co, Ni) with carbon nanofilaments (nanofibers, nanotubes) [23,26,42], present much better ORR activity (Eonset above 0.9 V vs. RHE in alkaline conditions), which is closer to benchmark commercial Pt/C catalyst (Eonset = 1.01 V vs. RHE [7]). In any case, the results of this work offer a new perspective of ORR improvement by means of sulfur doping of carbon supports.Tafel plots are shown in Fig. 7 . At low overpotential, TaOx/CNF–S250 showed a Tafel slope of 70 mV dec−1 whilst TaOx/CNF–S400 and TaOx/CNF a Tafel slope of 76 mV dec−1. Considering an associative mechanism for ORR in alkaline medium and the theoretical simulation of Shinagawa et al. [43], a Tafel slope close to 60 mV dec−1 indicates that the hydrolysis of adsorbed oxygen is the rate determining step, whilst values closer to 120 mV dec−1 are related to the first electron transfer to adsorbed oxygen contributing to the overall reaction rate. The latter takes place at higher overpotential values, as indicated in Fig. 7. Although S-doping affects slightly to the Tafel slope, indicating that the overall reaction mechanism is not very much influenced. However, the exchange current density (j 0 ) is positively influenced by S-doping (about two orders of magnitude higher), which explains the better behavior compared to undoped catalyst.The OER electrocatalytic activity of the investigated electrocatalysts (polarization curves) together with the ring current is reported in Fig. 8 . The rotating speed for the electrode was maintained at 1600 rpm to favor the removal of O2 bubbles from the electrode surface. There is not iR-correction applied to the OER data. By comparison of supports themselves and TaOx catalysts, the incorporation of tantalum oxides clearly contributes to the enhancement of OER activity, confirming the main conclusions derived from our previous work on undoped CNF as supports [34].On the other hand, the doping of the support with sulfur does not appear to have a positive influence in OER as it does for the ORR since a slightly lower current density is observed. The ring current in Fig. 8b accounts for the oxygen evolved at the disk, which is reduced at the Pt ring. The trend is similar to that obtained of the disk current, with oxygen efficiencies slightly lower for the S-doped catalysts. The main electrochemical parameters obtained for the OER are collected in Table 7 , while those for the supports without Ta-phases are found in Table S5. The two S-doped TaOx/CNF catalysts have less OER activity than the TaOx/CNF in terms of current at a fixed potential (j 1.65 V vs RHE ) or overpotential (η), with exception of TaOx/CNF–S250 at low current density. By comparing the two doped samples, the one treated at 250 °C presents a better OER activity as reflected by its lower overpotential and higher current density, although the oxygen efficiency is higher for the catalyst doped at 400 °C. Compared to a benchmark commercial IrO2 catalyst (η = 370 mV at 10 mA cm−2 [7]), the TaOx catalysts present between only 35 and 120 mV higher overpotential.Tafel plots were used to determine the OER rate determining step (rds) for every catalyst, as presented in Fig. 9 . Sulfur-doped catalysts exhibit a Tafel slope slightly over 100 mV dec−1, whilst TaOx/CNF has a lower Tafel slope of 72 mV dec−1. This indicates a clear change of reaction mechanism related to the doping of the support. A Tafel slope of 120 mV dec−1 appears when the surface species formed in the step just before the rate-determing step is predominant. In other cases, the Tafel slope is lower than 120 mV dec−1 for the overpotential values evaluated in Fig. 9. An intermediate value of 100 mV dec−1 is thus the result from a mixture of active species, with some of them favoring the pathway related to a Tafel slope of 60 mV dec−1 (the preferential adsorption of reaction intermediates is the rds) and some other acting with a Tafel slope of 120 mV dec−1 (the rds is the formation of hydroxide) [43].The different catalysts were investigated under a chronopotentiometric test in order to determine the electrode stability with time under ORR and OER conditions. Fig. 10 a and b show the variation of the potential with the number of cycles for OER and ORR, respectively. The test consisted of consecutive cycles of OER/ORR, implementing currents of +1 mA cm−2 and -1 mA cm−2. A duration of 3 min for sequence was programmed with cut-off potential values of 0.2 V and 1.9 V vs. RHE. In OER, the potential is quite stable with time, indicating a good stability of the set of catalysts for this reaction regardless the support used. Whereas, in ORR there is a decrease of potential in the first 10–15 cycles of approximately 50–60 mV with a much slower loss in the next ones. TaOx/CNF shows a sharp decrease of potential in ORR which is recovered after 10 cycles, indicating a reversible loss at the beginning of the experiment. Based on these results, it appears that the TaOx catalysts suffer some deactivation in ORR but good stability in OER. Upon cycling, the active sites responsible for ORR loss partial activity which does not affect OER behavior. It occurs for the three catalysts, independently of the support used. This phenomenon points to the presence of different active sites for both reactions, which is in line with other bifunctional catalysts with multiactive centers.To sum up, Fig. 10c includes the values of overpotential for both ORR and OER for the different formulations, considering both polarization curves and chronopotentiometric experiments. The sum of ORR and OER overpotentials accounts for the reversibility of catalysts defined as ΔE = EOER – EORR = ηOER + ηORR. The sulfur doping of the CNF leads to a significant improvement of ΔE from 970 mV to 905 mV in the polarization curves, and from 1100 mV to 1020 mV in chronopotentiometric experiments. This is an enhancement of up to 80 mV, i.e. considering both ORR and OER, mostly coming from the better behavior for the oxygen reduction. This enhancement is maintained upon endurance tests, with particular better results for the catalyst doped at 400 °C. This particular catalyst presents a lower particle size for Ta oxides and lower O/Ta ratio than its S-doped counterpart treated at 250 °C as main differences, which supports the hypothesis of substoichiometry and low particle size conditioning the electrocatalytic behavior. The reversibility of published bifunctional catalysts in terms of ΔE is variable and its determination depends on the electrolyte used and the criteria for calculation. For example, Luque-Centeno et al. reported ΔE of 846 mV for a catalyst based on titanium supported on N-doped graphene, at 5 mA cm−2 in 0.1 M NaOH [7]. Other transition metal-based bifuncional catalysts from the state of the art exhibit ΔE values above 800 mV (spinels or metals combined with N-doped carbons) or above 900 mV (perovskites), with nanocomposites presenting the best bifunctional behavior with ΔE in the range 700–800 mV, similar to the best noble metal combinations [44–46].To sum up, the work described herein presents an alternative strategy to improve the activity and durability of oxygen bifunctional catalysts by sulfur doping of highly resistant carbon nanofibers and using a durable active phase like tantalum oxide. More work is needed to further increase the activity towards the current state of the art noble metals.In summary, we have studied new sulfur-doped CNF-supported tantalum based catalysts with bifunctional characteristics for the ORR/OER in alkaline media. The combination of high temperature and short time (400 °C for 3 h) and lower temperature for longer time (250 °C for 6 h) have been evaluated over CNF grown at 700 °C. Sulfur doping has been successful on both supports. The combination of tantalum oxide and S-doped CNF has been tested to determine the bifunctional activity of these new materials. The support doping improved the activity of the catalysts, in particular on the oxygen reduction reaction. However, the difference in activity between the doped materials is not enough remarkable to determine the effect of each doping procedure on the overall activity of the catalyst. These new materials have interesting bifunctional activity both for the ORR and OER, being slightly better for TaOx/CNF–S250. TaOx/CNF–S new catalysts also show good stability through time, especially for the OER. Future studies will be focused on improving the OER activity preserving the interesting bifunctionality of TaOx/CNF–S electrocatalysts. Juan Carlos Ruiz-Cornejo: Methodology, Investigation, Data curation, Formal analysis, Writing – original draft. David Sebastián: Conceptualization, Methodology, Validation, Resources, Writing – review & editing, Visualization. Juan Ignacio Pardo: Validation, Writing – review & editing. María Victoria Martínez-Huerta: Supervision, Funding acquisition, Writing – review & editing. María Jesús Lázaro: 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 wish to acknowledge the grants PID2020-115848RB-C21 and PID2020-115848RB-C22 funded by MCIN/AEI/10.13039/501100011033, and to the Gobierno de Aragón (DGA) for the funding to Grupo de Conversión de Combustibles (T06_17R). J.C. Ruiz-Cornejo acknowledges also DGA for his PhD grant.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.jpowsour.2022.231988.

Highly efficient, low-cost and stable bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are highly desirable for the development of green electrochemical energy storage and conversion devices. Here, we report the synthesis of sulfur-doped carbon nanofibers (CNF) as support for tantalum oxide nanoparticles. Carbon nanofibers and tantalum oxides represent a durable choice for oxygen electrocatalysis, but an improvement of catalytic activity is required. Upon doping, sulfur is found to be mainly bonded to carbon as C–S–C species (more than 80%). The results show that the effective incorporation of sulfur in the support has a clear positive effect on the electroactivity of the tantalum oxide catalysts. It causes a decrease of OER/ORR overpotential of 80 mV with respect to the undoped counterpart, with special improvement in the ORR. The new catalysts have shown an interesting bifunctional behavior for the OER and ORR, as well as a good stability through time.

Biorefinery using lignocellulosic biomass is a sustainable technology that contributes to climate change mitigation by sustainably converting valuable organic resources originating from natural polymers, such as lignin, cellulose, and hemicellulose, into value-added products [1,2]. During a typical biorefinery process, covalently bonded holocellulose (cellulose and hemicellulose) and lignin in the lignocellulosic biomass are separated by chemical and biological depolymerization processes [3–6]. Most of the fractionation focuses mainly on providing high-quality cellulose for its subsequent conversion into biofuels and chemicals [7–11] rather than the valorization of lignin or hemicellulose to fuels, chemicals, and materials [12]. According to recent techno-economic research, aromatic monomers derived from lignin as well as polyols from hemicellulose can be utilized as chemical feedstocks or as additives for polymers [10,12–21]. For example, the production of xylan and xylose, which are the main derivatives of hemicellulose, is profitable as the dehydration of hemicellulose-derived pentoses (C5 sugars) can produce furfural and its derivatives, which are important renewable platform chemicals [22]. The conversion of lignin to its corresponding aromatic monomers and polymers is highly desirable, however, the process is limited by the irreversible condensation that occurs during the lignin depolymerization; thus, minimizing this condensation is a key factor in successful lignin valorization [6,23].Reductive catalytic fractionation (RCF) of the lignocellulosic biomass enables the extraction and conversion of the majority of lignin into soluble monomers, dimers, and oligomeric alkyl phenols, while retaining most of the holocellulose in the pulp; hence, this process is categorized as “lignin-first” in biorefinery [6,12,13,24]. Indeed, RCF yields an uncondensed low molecular weight lignin oil technically up to the theoretical yields via lignin depolymerization and stabilization [1,25–27].The major three factors affecting the RCF process are: (i) feedstock; (ii) solvent; and (iii) catalyst [1]. First, the efficiency of RCF is affected by the feedstock mass composition and structural features of lignin [28]. Lignin comprises three basic structural units: p-hydroxyphenyl (H, phenolic ring without any methoxy group), guaiacyl (G, phenolic ring with a methoxy group), and syringyl (S, phenolic ring with two methoxy groups). Understanding the structure of lignin is required for adjusting its depolymerization and the possible repolymerization of prepared phenolic monomers. For example, hardwood, which is a common feedstock used for RCF [13,23,29–32], contains approximately 18–25 wt% of lignin mainly comprising G and S units. Because S units lack a free ortho-position, the hardwood lignin cannot form 5–5 and β-5 interunit bonds by radical coupling during delignification [1,33], resulting in an abundance of easily cleaved β-O-4 moieties [1,34].Second, polar solvents can depolymerize biomass into oligomers and small amounts of phenolic monomers and dimers [1,6,18,24,35,36], even in the absence of a catalyst [12]. Particularly, methanol is highly efficient in the delignification and formation of solid fiber pulp [12,13,23,25,37]. Therefore, the RCF process can be initiated via solvolytic extraction and further proceed to partial fragmentation mainly through ether bond (β-O-4 linkages) cleavage by a catalyst [1]. Unsaturated fragments, such as G and S units, which are easily repolymerized, are produced in the initial solvolytic extraction [1,25]. The addition of organic chemicals, particularly sugar derivatives, also significantly improved the reductive depolymerization of lignin, achieving a yield of phenolic monomers as high as 83.0 % [38].Finally, heterogeneous transition-metal catalysts (e.g., Ni, Ru, Rh, Pt, Pd, and Cu) can catalyze the depolymerization of lignin oligomers to generate stable monomers while hindering unwanted repolymerization [34,36,39–41]. The combination of acid (from solvent) and metal catalyst improved RCF, contributing to hydrogenation and hydrolysis from their metal and acid components, respectively [41]. From this point of view, tungsten-based catalysts can be promising in the RCF process because the catalyst can allow hydrocracking, dehydrogenation, and alcohol dehydration at the Brønsted acid sites or oxygen vacancies on tungsten oxide [36,42,43].Despite numerous efforts to develop the economically feasible RCF process or catalyst, the efficiency of the RCF process is insufficient for industrial applications [20]. Based on these observations, the objectives of this study are (i) to adjust the major products of lignocellulose (Mongolian oak, MO) using bifunctional catalysts comprising metal and acid components for the RCF process; (ii) to establish the reaction pathway based on the roles of metal and acid components; and (iii) to optimize the reaction conditions for the efficient fractionation of lignocellulose. For these objectives, the efficient depolymerization of lignin and holocellulose to their corresponding monomers was attempted while retaining cellulose as a solid pulp. Based on their lignin depolymerization capability [23,25,34,37,44] and hydrogenation or hydrodeoxygenation activity [6,34,45,46], Ru, Pd, Ni, and Co metal catalysts were prepared as bifunctional catalysts with a WZr support for fractioning the biomass.All chemicals were used without further purification, unless mentioned otherwise. Mongolian oak (MO) provided by the Seoul National University Gwanak Arboretum (Seoul, Korea) was milled and sieved into particles (< 0.5 mm). Compositional analysis of biomass was performed in accordance with the NREL’s analytical procedure [47]: 74.9 % ± 0.2 % holocellulose, 26.1 % ± 0.1 % lignin, and 0.42 % ± 0.1 % ash were measured; these values were used to calculate the yields of products in this study. Tungstate-zirconia (WZr) powder was purchased from Luxfer MEL Technologies (Manchester, UK). WZr support was calcined at 900 °C for 6 h at a heating rate of 10 °C/min. Ruthenium(III) chloride hydrate (RuCl3∙xH2O), palladium(II) nitrate hydrate (Pd(NO3)2·xH2O), nickel(II) chloride hydrate (NiCl2·xH2O), cobalt(II) chloride hexahydrate (CoCl20.6H2O), pyridine (anhydrous, 99.8 %), and acetic anhydride (99 %) were purchased from Sigma-Aldrich (St. Louis, Missouri, USA). 1-Butanol (99 %) was procured from Junsei chemical (Tokyo, Japan). Sulfuric acid (95.0–98.0 %) was procured from J.T. Baker (Phillipsburg, New Jersey, USA). Tetrahydrofuran (> 99.9 %) was procured from Honeywell B&J (Morris Plains, New Jersey, USA). Hydrogen gas (H2, 99.999 %), nitrogen gas (N2, 99.9 %), 0.5 % O2/N2 (v/v), and 5 % H2/Ar (v/v) were procured from Shinyang Medicine (Ansung, Korea). Deionized (DI) water was prepared using an aquaMAX-Ultra 370 series water purification system (YL Instruments, Anyang, Korea).The catalysts were synthesized using the wet impregnation method. To prepare 5 wt% metal/WZr (where metal refers to Ru, Pd, Ni, and Co) catalyst, the metal precursor (i.e., ruthenium(III) chloride hydrate, palladium(II) nitrate hydrate, nickel(II) chloride hydrate, and cobalt(II) chloride hexahydrate) was dissolved in 150 mL of DI water and 15 g of calcined WZr was subsequently added to the solution. The mixture was stirred at ambient conditions for 24 h, rotary evaporated, and dried for 16 h at 105 °C. The catalyst was thermally reduced in a tube furnace at 400 °C for 2 h under 200 mL/min of 5 % H2/Ar at a heating rate of 5 °C/min. Finally, passivation was performed at room temperature with 200 mL/min of O2/N2 for 30 min. The catalyst was then ready for use.RCF was performed in a 200 mL stainless-steel batch reactor equipped with a magnetic drive stirrer. A mixture of 2.5 g of MO, 1.0 g of WZr-supported metal catalysts, and 50 mL of 65 % methanol aqueous solution (v/v) was placed in the reactor (hereafter referred to as 65 % MeOH/H2O (v/v)). After sealing the reactor, a leak test was performed with N2. Thereafter, the reactor was flushed with H2 three times and pressurized with H2 to 30 bar at room temperature. The reactants were stirred at 500 rpm and heated to 100–250 °C with a heating rate of 10 °C/min. The pressure of ~200 bar was reached at a temperature of 250 °C or above. When the reaction temperature was reached, it was maintained for 2 h before the mixture was cooled rapidly to room temperature. The reaction mixture was further depressurized to atmospheric pressure, and the products (liquid and solid phases) were collected. The residual solids (mixture of remaining biomass and the catalyst) were separated from the liquid by centrifugation (1200 rpm for 2.5 min at 4 °C), dried for 16 h at 60 °C, and exposed to ambient air. Solid residue was not observed after centrifuging, confirming the complete removal of solid from the liquid product (Fig. S1). The prepared products were classified into five fractions: (A) solid, (B) dichloromethane-extracted (DCM-extracted) monomers, (C) silylated DCM-extracted dimers, (D) DCM-extracted polymers, and (E) aqueous phase sugars ( Scheme 1). The recovered spent Ru/WZr catalyst was washed with water and methanol prior to its reuse for the catalysis reaction.The liquid products were identified using GC-MS and quantified using GC-FID. 1-Butanol (1 mL/L in liquid product) was used as an internal standard. The HP-5MS column (60 m × 250 µm × 0.25 µm) was used for both GC-MS (Agilent 7890/5795 C, Agilent, Santa Clara, California, USA) and GC-FID (YL6500GC, Young In Chromass, Anyang, Korea). For the GC measurements, 1 µL of liquid product was injected with a split ratio of 50:1 at an inlet temperature of 300 °C. The oven temperature was increased from 50 °C to 150 °C at a ramping rate of 10 °C/min, maintained at 150 °C for 2 min, increased to 250 °C at a ramping rate of 8 °C/min, maintained at 250 °C for 5 min, increased to 300 °C at a ramping rate of 10 °C/min, and maintained at 300 °C for 2 min. In addition to the monomer compounds, the dimers (mostly phenolic compounds from lignin) were silylated and observed using GC. DCM-extracted dry oil was silylated using 1 % trimethylchlorosilane dissolved in N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA). A mixture of DCM-extracted oil, pyridine, and BSTFA solution was heated at 105 °C for 2 h. The product was injected to the GC-MS (Agilent 7890/5975 C) or GC-FID (YL6500GC) with 4-phenoxy phenol as the internal standard. The HP-5MS column (60 m × 250 µm × 0.25 µm) was used for both GC-MS and GC-FID measurements. For the GC measurements, 1 µL of liquid product was injected with a split ratio of 50:1 at an inlet temperature of 280 °C. The oven temperature was increased from 50° to 150°C with a ramping rate of 10 °C/min, maintained at 150 °C for 2 min, then further increased to 300 °C with a ramping rate of 5 °C/min, and maintained at 300 °C for 18 min. A solvent delay of 8 min was used to prevent overloading of the detector with the solvent. The mass fragments and spectra were clarified through comparison with dimer structures reported in the literature [48]. Gel-permeation chromatography (GPC) was performed using an Agilent 1200 HPLC device (Santa Clara, California, USA). Two Shodex LF-804 columns (Showa Denko, Tokyo, Japan) were used for the separation of the polymeric compounds, and a UV detector (λ = 270 nm) was used to measure the molecular weight distributions of the reactants and products dissolved in the eluent flow of tetrahydrofuran (THF, 1.0 mL/min). The acetylation of lignocellulose was performed to improve the dissolution of polymeric compounds in the THF eluent [38,49,50]. The dried lignocellulose reactant (0.5 g) or the solid residue (<0.5 g, from the depolymerization product) was mixed with acetic anhydride (5 mL) and pyridine (5 mL). The mixture was stirred for 24 h under ambient conditions prior to mixing with ethanol (20 mL) and then further stirred for 30 min under ambient conditions. The solution was dried at 60 °C using a rotary evaporator and further dried at 55 °C in vacuum for 16 h to obtain the acetylated polymer. The acetylated polymer was dissolved in THF (1 g/L), filtered using a Whatman syringe filter (0.45 µm), and analyzed using GPC. The observed GPC results were converted to molecular weight distributions of the polymeric compounds and used to measure the number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI = Mw/Mn). The GPC system was calibrated using a simulated polystyrene standard ReadyCal set (250–2,000,000 g/mol, Sigma-Aldrich, Milwaukee, Wisconsin, USA). The quantity of sugar monomers was measured via HPLC analysis (Agilent 1260 Infinity equipped with an RI detector, Agilent, Santa Clara, California, USA) of the aqueous phase after extraction with DCM. The yields of aromatic and holocellulose-derived monomers or dimers were determined using the following equation:(Yield of monomers or dimers, wt%) = (weight of monomers or dimers)/(weight of reactant) × 100.The standard deviation of monomer yields was less than 0.45 % of the average yield for Ru/WZr, confirming the reliability of the observed reaction results (Table S1).After centrifuging and drying, the mass of solid residue was measured, and the yield was calculated from the following equation:(Yield of solid products, wt%) = [(weight of recovered solid residue after the reaction) - (weight of catalyst)]/(initial weight of holocellulose fraction in the biomass reactant) × 100.The X-ray diffraction (XRD) spectra of solid products were observed using D8 ADVANCE (Bruker, Billerica, Massachusetts, USA) to determine their crystal structure. The crystallinity index (%) was measured by subtracting baseline (as an amorphous fraction) at 2θ = 10–27° [51]. The 2θ range for measuring the crystallinity index was selected to contain the major diffraction peaks of cellulose and avoid the diffraction peaks of remaining catalysts. Scanning electron microscopy (SEM) images were obtained using an Inspect F50 field emission-scanning electron microscope (Thermo Fischer Scientific, Waltham, Massachusetts, USA). For SEM, the samples were coated with platinum using a Hitachi-1045 ion sputter (Hitachi, Tokyo, Japan) at 15 mA for 60 sN2 physisorption was performed using an ASAP2020 system (Micromeritics, Norcross, Georgia, USA) to measure the BET surface area (SBET) of catalysts. NH3 temperature-programmed desorption (NH3 TPD) and H2 temperature-programmed reduction (H2 TPR) analyses were performed using a BELCAT-B catalyst analyzer (MicrotracBEL Corp., Osaka, Japan) equipped with a thermal conductivity detector. The surface metal atoms were quantified by CO chemisorption, which was performed using a BELCAT-1 automated analyzer (MicrotracBEL Corp., Osaka, Japan). The acidity of WZr was measured by NaOH titration using an automatic potentiometric titrator (AT-700). WZr powder (0.1 g) was dispersed in DI water (100 mL) and stirred for 12 h. The mixture was titrated with aqueous NaOH (0.01 M) to determine the number of acid sites and the pKa was measured using the titration curve. Powder XRD (Dmax2500-PC, Rigaku, Tokyo, Japan) was performed with Cu Kα1 radiation (λ = 1.54056 Å, 40 kV, and 200 mA). X-ray photoelectron spectroscopy (XPS) was performed using a Theta Probe AR-XPS system (Thermo Fisher Scientific) with monochromated Kα excitation (hν = 1486.6 eV) operated at 15 kV and 150 W at the Korea Basic Science Institute (Busan, Korea).The fractionation of MO using WZr-supported metal catalysts was performed to produce liquid products along with solid residue (precipitated in the mixture), as depicted in Scheme 1. The observed monomer products (fraction B in Scheme 1) dissolved in the DCM phase are listed in Fig. 1 and Tables S2-S9; the yields of GC-measurable monomers were 1.15–18.6 wt%. The negligible yield of 0.15 wt% was achieved without the catalyst, which indicates that the RCF reaction requires a catalyst (Table S8). The highest yield of 18.6 wt%, containing lignin- and holocellulose-derived monomers, was achieved at 250 °C using Ru/WZr. Ni/WZr also exhibited a high yield of 16.8 wt% at 250 °C. Only a slight deactivation was observed when the spent Ru/WZr was used (Table S9), confirming that it is viable to reuse the Ru/WZr catalyst. Based on the 26.1 % lignin fraction in MO, the maximum yield (61.9 wt% from the lignin fraction) of lignin-derived aromatic monomers was obtained for Ru/WZr at 250 °C, which is one of the highest among those reported in recent literature (Table S8) [17,23,34,38,52–57]. A recent life-cycle analysis also confirmed a 16.2 wt% aromatic monomer yield (61.9 wt% lignin-based aromatic monomer yield) at a lower reaction pressure and shorter reaction time is considered a feasible strategy for reducing capital expenses [20]. The major components in fraction B were propyl- or allyl-branched syringols and guaiacols derived from lignin. Detailed product distributions were altered by the process conditions, including reaction temperature and type of catalyst.For the production of monomers (fraction B) dissolved in the liquid phase, the major lignin-derived monomers included G and S units. The formation of H units was negligible because of the native composition of hardwood MO [58]. Dimeric phenolic products were also formed, which is discussed in Section 3.6. Besides lignin-derived aromatic compounds, holocellulose-derived monomers were also formed, including ethylene glycol and furfural, which indicated significant cracking (ethylene glycol from polyol-like holocellulose) and acid-catalyzed dehydration (furfural from xylose), respectively.The metal used for catalysis affected depolymerization activity (Fig. 1). A WZr support without metal deposition, which is a solid acid catalyst, exhibited poor depolymerization activity (Table S3). The yield of G unit derivatives was 0.24 wt% at 200–250 °C and negligible at 100–150 °C, indicating a significantly lower production of G units compared with S units in the absence of metal catalysts. The depolymerization activity was significantly increased when metal particles were deposited on the WZr, indicating the beneficial roles of metals on the depolymerization [50]. Among the catalysts tested in this study, Ru/WZr exhibited the highest lignin monomer yield of 18.6 wt% at 250 °C. When using Ru/WZr, the major components of 4-propyl guaiacol and 4-propyl syringol were obtained with high selectivity (54.8–85.7 % of monomers) at all reaction temperatures. The selectivities for propyl-branched guaiacol and syringol also increased with an increase in reaction temperature [23]. Pd/WZr produced propanol-branched guaiacol and syringols as the major products with the highest yield of total monomers, 14.1 wt% at 200 °C, which increased slightly to 15.2 wt% at 250 °C (Table S4). Because a larger increase in the yield of total monomers was observed for Ru/WZr (8.24–18.6 wt%), Co/WZr (3.39–9.34 wt%), and Ni/WZr (11.5–18.3 wt%) when the temperature was increased from 200 °C to 250 °C, the smaller increase in the yield produced with Pd/WZr indicated that cracking or degradation of the aromatic monomer may have occurred at 250 °C. Ru/WZr, Co/WZr, and Ni/WZr produced propyl-branched guaiacols and syringols as the major products, which may be less reactive than the propanol-branched products produced with Pd/WZr. Allyl-branched products were obtained when Ni/WZr and Co/WZr were used, which indicated their poor hydrogenation activity. In addition, the selectivity to allyl-substituted guaiacol and syringol decreased in the order of Ni (49.7 %) >> Co (48.9 %) > Pd (~0.0 %) ≈ Ru (~0.0 %), indicating the greater hydrogenation activity of Ru/WZr and Pd/WZr [59]. Based on these observations, Ru/WZr was a selective hydrogenation catalyst that did not crack alkyl branches during RCF, but instead formed saturated alkyl branches.Focusing on the holocellulose-derived compounds, the highest yield of holocellulose-derived small molecules was observed with the acidic WZr support without metal deposition (0.39 wt%). The acid sites of WZr catalyzed hydrolysis, hydrogenolysis, or de/rehydration of holocellulose [43,60,61] to produce sugar-degraded ethylene glycol, propylene glycol, and furfural (Tables S2-S6). Compared to a WZr support without metal deposition, the deposition of Pd, Co, and Ni onto WZr decomposed holocellulose to smaller sugar derivatives at 200–250 °C (Tables S2-S6). The deposition of metals, however, did not increase the yields of holocellulose-derived small molecules, achieving yields of 0.10–0.29 wt%.Among the observed DCM-dissolved monomers (fraction B) in the liquid products, the yields of lignin-derived monomers (mostly, the derivatives of S and G units) increased at higher reaction temperatures (Fig. 1 and Tables S2-S6). For G units, propyl guaiacol and dihydroconiferyl alcohol comprising saturated propyl branches formed preferentially, indicating less cracking of alkyl branches under mild reaction conditions (30 bar H2 and 100–250 °C), although cracking to ethyl- and methyl-branched phenolic compounds has been reported to occur in the depolymerization performed under harsher reaction conditions [49].The appreciable formation of GC-detectable (distillable) holocellulose-derived monomers (fraction B) was observed at a temperature of 200 °C or higher, and the highest yield was achieved at 250 °C, indicating that the degradation of holocellulose, particularly cellulose, required a higher temperature than that required by lignin (Fig. 1). The degradation products identified in the GC results included acetic acid, diols, furans, alcohols, ketones, and esters (Tables S2-S6). The observed products are mainly obtained by the degradation of hemicellulose [62]. The production of dehydrated alcohols and aldehydes, including ethylene glycol, 1,2-propanediol, xylitol, and furfural, observed at 200–250 °C indicated the further degradation of sugars at higher reaction temperatures. Although Ru/WZr, Pd/WZr, and Co/WZr exhibited larger productions of these degradation products compared with that exhibited by a WZr support without metal deposition, no distinct increase in the production of sugars was observed at 200–250 °C. The increased formation of degradation products may indicate a greater depolymerization of holocellulose at higher reaction temperatures, while the produced sugars may undergo further conversion to the degradation products. Because the sugars could not be observed using GC, the liquid products (fraction E) dissolved in the aqueous phase were observed using HPLC after DCM extraction, and negligible amounts of glucose and xylose were observed (Table S9), indicating that the majority of sugars were converted to furans and other degradation products.The dimeric products, particularly lignin-derived varieties, were observed using GC analysis of silylated products [12,48]; the dimers composed of S and G units coupled via β-1, β-β, and β-5 linkages were identified ( Fig. 2 and S2-S9) as reported in the literature [48]. Among the catalysts, Ru/WZr produced greater quantities of dimers, with a yield of 3.87 wt% (based on the total weight of MO feed) at 200 °C, containing more β-β bonds and fewer β-1 bonds (Fig. 2). In comparison, Pd/WZr and Co/WZr produced dimers containing more β-1 bonds and fewer β-β bonds. The formation of β-5 dimers was observed only for Pd/WZr. The reaction using WZr without metal deposition exhibited the negligible formation of monomers and dimers along with the formation of low molecular weight oligomers. The further depolymerization of lignin to monomers and dimers was not observed. These observations indicate that the deposited metals are required to more efficiently depolymerize lignin polymers. For all catalysts, more significant formation of dimers composed of S-G or S-S units was observed. The formation of G-G dimers was observed for Ru/WZr, but not clearly observed for the other catalysts. β-β dimers were observed for all catalysts, exhibiting 2 wt% or higher yields. The formation of uncondensed β-β dimers with a β-β resinol structure (β-β and α-2 in Fig. 2) can be attributed to the reductive catalytic cleavage of ether bonds of the resinol structure [48]. For the S units, which are the major fractions of the hardwood used in this study, the production of β-β or β-O-4 bonds is kinetically preferred [48,63]; thus, formation of syringyl monomers occurred predominantly (Fig. 1), and led to the preferred formation of β-β dimers compared with β-5 or β-1 dimers (Fig. 2). These C-C linkages were connected through unsubstituted or -CH2OH substituted alkyl bridges [12]. These bridges were partially removed and converted to ethylene glycol at high temperatures (> 200 °C) using catalysts via Cβ-Cγ cleavage of the linked propanol side-chains during the RCF process [12]. The effects of reaction temperature on the catalysis were studied for Ru/WZr (Fig. 2(b)). The yields of uncondensed β-β resinol (β-β, α-2 of S-G) and the β-β bonds of G-G dimers increased with temperature. An increase in lignin monomer yield was observed with temperature (Fig. 1), indicating that the depolymerization activity of Ru/WZr increased with temperature.In addition to the small molecules prepared from holocellulose and lignin, the formation of cellulose-rich solid residue was observed in RCF. The yield of solid residue decreased from 82.1–88.2 wt% to 8.4–27.7 wt% with an increase in reaction temperature from 100 °C to 250 °C ( Fig. 3), indicating the facile decomposition of holocellulose at higher temperatures. This trend is consistent with the increased yields of holocellulose-derived compounds at higher reaction temperatures (Fig. 1). The pulp was not completely recovered, even at the lowest reaction temperature of 100 °C, indicating the formation of lower molecular weight holocellulose oligomers by the degradation and the loss of oligomers through their dissolution in the solvent. Although the yield of monomers was affected by the catalyst used, as depicted in Figs. 1 and 2, the yield of pulp was not significantly dependent on the deposited metal (Fig. 3). These observations indicated that MO pulping could be achieved with acidic WZr, regardless of the deposited metals.The crystal structures of cellulose-rich solid residue were analyzed using powder X-ray diffraction ( Fig. 4). Both MO feed and its recovered solid residue (fraction A) exhibited similar peaks at 2θ = 14–17° and 23°, which can be assigned as the diffractions of cellulose Iα or Iβ [51,64,65]. Because of the similarity in the powder diffraction patterns of cellulose Iα and Iβ, it was difficult to determine if the phase transitions between Iα and Iβ occurred [65]. Notably, the absence of the (1 1 ̅ 0) peak at 2θ = 12° confirmed that cellulose II, which has been reported to form through the recrystallization of dissolved cellulose [66–70], was not formed, confirming that the recovered pulp was prepared by delignification but not by extraction and recrystallization. Among the catalysts, the WZr support without metal deposition exhibited weak diffraction intensities of cellulose Iα or Iβ, indicating the poor crystallinity of pulp recovered after the reaction at 200 °C ( Table 1). Because of the lower yields of small molecule compounds (Table S3 and Fig. 1) compared to those of other metal-deposited catalysts, the formation of less crystalline solid residue on WZr indicates that the depolymerization of lignin and holocellulose was not achieved without metal-catalyzed hydrocracking or hydrogenolysis. It also indicates that cellulose was significantly decrystallized by WZr without depolymerization, exhibiting the removal of intramolecular hydrogen bonding in cellulose. The decrystallization was, however, not significant when the metal components were present, indicating that excess hydrogen adsorbed on the metal surface selectively catalyzed lignocellulose, while preserving the crystal structures of cellulose. Among the metal-deposited catalysts, Ru/WZr at the reaction temperature up to 200 °C did not cause significant modification of cellulose Iα or Iβ, but almost complete degradation of cellulose crystals was observed at 250 °C, which is consistent with the lowest yield (8.40 wt%) of solid residue (Table 1).The morphology of solid residue was further investigated using SEM ( Fig. 5). For RCF using WZr and Co/WZr, the morphology of solid residue did not significantly change except for the formation of small cracks (Fig. 5(a, c, f)). In contrast, the formation of fibrous structures was observed in RCF using Ni/WZr, Pd/WZr, and Ru/WZr (Fig. 5(b, d, e)). Reaction temperature also significantly adjusted the morphology of solid residue (Fig. 5(g, h, i, j)). For RCF using Ru/WZr, at 100 °C, minor cracks were observed (Fig. 5(g)), which became significant with increasing temperature, forming fibrous structures at 200 °C (Fig. 5(h, i)). As reaction temperature increased to 250 °C, the solid residue decomposed to form particles (Fig. 5(j)). The XRD results of the solid residue at 250 °C also confirmed the decomposition of these fibrous materials from the weak diffractions of cellulose (Fig. 4(b)).Based on the observed formation of monomers, dimers, and cellulose-rich solid residue produced by RCF, a plausible reaction pathway is proposed in Fig. 6. Although the depolymerization of holocellulose was achieved with the acid (WZr), an increase in the formation of lignin-derived aromatic monomers and dimers was observed after the addition of metals. As illustrated in the monomer and dimer analysis results, the product compositions differed because of the reaction mechanism, which, in turn, depended on the catalyst type. The major dimeric compounds prepared using Ru/WZr and Ni/WZr contained β-β and α−2 (S-G) bonds, while those obtained using Pd/WZr contained β-1 and γ-OH (S-G). This suggested the presence of a different reaction mechanism depending on the catalyst type ( Scheme 2). The cleavage of the β-O-4 bond produced unstable radical products, including phenolic radicals (1 and 2) and the γ-OH radical (3), which could be stabilized via the repolymerization, demethoxylation, dehydration, and combination of these reactions [71]. The γ-OH radicals were stabilized via the dehydration of α-OH (4) or coupling with another γ-OH to form the β-β dimer (8). The presence of the lone pair electron of phenolic radicals can lead to equilibrium between phenolic radicals and quinone methide intermediates (5 and 6). The intermediate can be further demethoxylated (7) or cross-linked with other intermediates to form β-5 linkage (9) or β-1 (10). Based on these pathways, the γ-OH radicals can be stabilized by dehydration and further hydrogenated on Pd/WZr and Co/WZr to form major products of dihydroconiferyl alcohol and dihydrosinapyl alcohol. Ru/WZr and Ni/WZr catalysts produced kinetically preferred β-β dimers [48], and the catalysts formed alkylated phenols (4-propyl guaiacol and 4-propyl syringol), indicating the further dehydration and hydrogenation of the terminal hydroxyl group. These observations indicate that Ru/WZr and Ni/WZr catalysts not only stabilized the radicals via dehydration but also catalyzed the demethoxylation or cleavage of the C-C bonds of β-1 or β-5 dimers. Hence, it can be understood that supported metals stabilize and catalyze C-C cracking to produce the aromatic monomers. WZr support provides acid sites to mainly depolymerize the lignin-holocellulose complex. However, WZr support alone cannot stabilize reactive intermediates, resulting in severe repolymerization, as indicated by the GPC results and the calculated molecular weights (Fig. S10, Tables S12 and S13). On the contrary, it was found that the addition of metals to the WZr support can successfully suppress further repolymerization via the stabilization of radicals and improved C-C cracking.The surface active sites of metals and the acidity of the WZr support were measured to understand their contributions to catalytic activity. CO chemisorption exhibited the largest metal dispersion for Ru/WZr, which was 3.9 times larger than the smallest observed for Co/WZr ( Table 2), indicating that Ru/WZr demonstrates better catalytic reduction activity. NH3 TPD results exhibited the quantity of acid sites modified by the deposited metals ( Fig. 7(a)). While the NH3 desorption peak temperature of the WZr support without metal deposition was in the broad range of 100–450 °C, those of Pd/WZr and Co/WZr slightly increased and decreased, respectively, and new peaks emerged for Ni/WZr and Ru/WZr. These observations indicate that the acidity was initially from the tungsten oxide. However, the acidic nature of tungsten oxide was affected by the deposited metals (when reduced) or metal oxides (when not reduced). NaOH titration results also confirmed that the quantity of acid sites of the catalysts varied depending on the deposited metals (Table 2). The quantity of surface active sites was in the order of Ru/WZr > Ni/WZr > Pd/WZr > WZr > Co/WZr. This catalyst-dependent acidity ultimately influenced the product distributions of the aromatic compounds (Fig. 2) and, especially, the yields of the aromatic monomers. Furthermore, for Ru/WZr and Ni/WZr, whose surface acidities are higher, the observed major dimer products were the β-β dimers, whereas the catalysts with lower acidity (Pd/WZr and Ni/WZr) exhibited β-1 or β-5 dimers. Therefore, it can be concluded that a higher surface acidity, particularly Brønsted acidity [43], improves the facile C-C bond cleavage. In addition, W 4f XPS results confirmed the adjusted electronic structures of tungsten oxides. The binding energy peak of W 4f7/2 shifted to a lower energy when the metals were deposited (Fig. S11 and Table S14), indicating the electron transfer from W to metals. Interestingly, the W 4f7/2 binding energy of WZr-supported metal catalysts decreased considerably for the catalysts containing a higher number of surface Brønsted acid sites, confirming that these are the major active sites for RCF, and that the deposited metal oxides manipulate the acidity of tungsten oxide to enable the facile production of lignocellulose-derived small molecules ( Fig. 8).The crystal structures of WZr, as described by the peak intensity ratio of tetragonal ZrO2 (PDF#80-0965, at 2θ = 30.2°) to monoclinic ZrO2 (PDF#37–1484, at 2θ = 28.2°) (hereafter, for convenience, (tetragonal)/(monoclinic) ratio will be referred to as T/M ratio), were adjusted by the deposition of metals (Table 2 and Fig. 7(b)). While the WZr support without metal deposition exhibited a T/M ratio of 6.51, it increased (9.20 for Co/WZr) or decreased (5.36 for Ni/WZr, 2.79 for Pd/WZr, and 6.43 for Ru/WZr) depending on the metal catalyst. Although the monoclinic ZrO2 has been reported to provide more (Lewis) acidic sites because of the higher Zr4+ density [72,73], the T/M ratio did not exhibit this expected linear correlation with the measured quantity of surface acid sites. These observations indicate that the quantity of acid sites was more dependent on the presence of a metal and the interaction between metal and WZr than the crystal structure of ZrO2.To illustrate the interaction between metal and acid sites, the aromatic monomer yields were plotted as a function of the ratio of the quantities of metal and acid sites (M/A ratio) (Table 2, Fig. 7(c) and S12). While the addition of metals to WZr significantly increased the monomer yields, higher yields of aromatic monomers were obtained for Ni, Ru, and Pd rather than Co. The larger Co sites did not significantly increase the yield of aromatic monomers, although the metals can enhance reactions involving hydrogen. These observations indicate that the synergy between metal and acid sites can achieve the optimal RCF performance. Catalysts composed of only acid sites (zero M/A ratio) can easily crack the C–C bonds; however, they cannot stabilize the active radical intermediates. In contrast, catalysts containing many metal sites (higher M/A ratio) cannot effectively crack C–C bonds.The activity of the metal component was further investigated using H2 TPR (Fig. 7(d)). While the metal-free WZr did not exhibit distinct reduction at temperatures lower than 400 °C, Ni, Co, and Ru were reduced at 200–400 °C. Because of the low reduction temperature (< 100 °C) of Pd [74,75], it did not exhibit the similar reduction peak at 100–400 °C. These observations confirm that Ru and Pd can excellently facilitate adsorbed-hydrogen-involved reactions, including hydrogenation, hydrocracking, and hydrogenolysis [76]. These results were in accordance with the product distribution of Ni/WZr and Co/WZr, which produced allyl aromatic monomers (Tables S5 and S6).The yield of lignin-derived monomers under different reaction conditions was investigated. The yield of phenolic monomers increased with increasing H2 pressure (measured at room temperature) ( Fig. 9(a)). A negligible yield of phenolic monomers was observed at 1-bar H2, confirming that the efficiency of RCF was highly dependent on the metal-adsorbed hydrogen-involving reactions, including hydrogenolysis and hydrocracking. Because the results at 30 bar H2 and 50 bar H2 were not significantly different, the former may be sufficient to generate the maximum yields.The weight ratio of the Ru/WZr catalyst to the MO reactant was varied from 0.2 through 0.4–0.8 w/w, and all monomer yields from substrates lignin and hollocellulose were measured. The monomer yield was the lowest (6.1 wt%) at a catalyst/MO ratio of 0.2 w/w. Increasing the amount of catalyst to catalyst/MO = 0.4 w/w increased the monomer yield to 8.2 wt%; however, a further increase in the catalyst/MO ratio to 0.8 w/w did not significantly improve the monomer yield (8.7 wt%). Therefore, among the three ratios, the catalyst/MO ratio of 0.4 w/w was sufficient to achieve the optimum monomer yield (Fig. 9(b)).The reaction temperature was also varied to examine the efficient recovery of the holocellulose pulp. As depicted in the XRD results and SEM images (Figs. 4(b) and 5(j)), RCF at 250 °C led to the decomposition of the holocellulose pulp. Based on these observations, although Ru/WZr exhibited the highest aromatic monomer yield, Ni/WZr demonstrated a more efficient recovery of the hollocellulose pulp; hence, Ni/WZr can be used to obtain less degraded hollocellulose.The RCF of MO (hardwood) was performed to obtain: (1) phenolic monomers derived from lignin fragments and holocellulose-derived monomers for platform chemicals; and (2) cellulose-rich solid products (pulp) for further commercial industrial biorefinery. High yields of low molecular weight lignin fragments (monomeric, dimeric, and short oligomeric compounds) were produced, along with a cellulose-rich solid fraction. The maximum yield of aromatic small molecules (23.6 wt% of aromatic monomer and dimer yield based on the weight of total lignocellulose) was obtained when Ru/WZr catalyst was used at 250 °C, which is one of the highest yields among those reported in recent literature, as well as a techno-economically viable yield. With an increase in temperature, the components began to decompose (150 °C), with further decomposition occurring at an even higher temperature (250 °C). Under reductive conditions (30 bar of H2 measured at room temperature) in the presence of metal catalysts, the monomeric phenols were stabilized and 4-propyl-substituted compounds were mainly generated. In the case of lignin, most of the labile ether bonds (e.g., β-O-4) were cleaved into monomeric compounds, whereas the cleavage of C-C bonds (e.g., β-β, β-1, and β-5) differed depending on the acidity of catalysts. A solid holocellulose-rich pulp was also obtained, which exhibited its cellulose crystallinity up to 200 °C. Reaction conditions were optimized to achieve an efficient RCF process, and the effects of H2 pressure and catalyst/biomass ratio were investigated. The roles of catalyst components on fractionation were discussed, concluding the occurrence of depolymerization on solid acids and stabilization on supported metals. The findings of this study provide further insight into the lignin and hollocellulose components during the RCF process and are beneficial in the future development of a feasible process for fractionating lignocellulose to obtain valuable phenolic compounds and pulp. Shinyoung Oh: Investigation, Writing – original draft. Sangseo Gu: Investigation, Writing – original draft. Jae-Wook Choi: Methodology, Investigation. Dong Jin Suh: Conceptualization. Hyunjoo Lee: Methodology, Investigation. Changsoo Kim: Methodology, Investigation. Kwang Ho Kim: Methodology, Investigation. Chun-Jae Yoo: Methodology, Investigation. Jungkyu Choi: Writing – review & editing, Supervision. 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 work 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 of Republic of Korea (NRF-2020M1A2A2079798). This work was also supported by the Technology Innovation Program (KEIT-20015401; NTIS-1415180841) funded by the Ministry of Trade, Industry & Energy (MOTIE, Republic of Korea).Shinyoung Oh: Investigation, Writing – original draft. Sangseo Gu: Investigation, Writing – original draft. Jae-Wook Choi: Methodology, Validation. Dong Jin Suh: Methodology, Resources. Hyunjoo Lee: Validation, Methodology. Changsoo Kim: Resources, Methodology. Kwang Ho Kim: Resources, Methodology. Chun-Jae Yoo: Validation, Conceptualization. Jungkyu Choi: Writing – review & editing, Data Curation. Jeong-Myeong Ha: Writing – review & editing, Funding acquisition..Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jece.2022.108085. Supplementary material .

Mongolian oak (MO), a lignocellulosic biomass feedstock comprising lignin, hemicellulose, and cellulose, was fractionated via reductive catalytic fractionation (RCF) into holocellulose-rich solid residue and lignin-derived phenol-rich liquid oil. To achieve an economically feasible RCF process, tungstate-zirconia (WZr)-supported metal catalysts, exhibiting bifunctionalities of hydrogen-adsorbing metal and acidic WZr, were used for depolymerizing and valorizing lignocellulose, and their catalytic activity was found to be highly dependent on the deposited metal. Ru/WZr exhibited excellent fractioning ability, achieving a maximum yield of 23.6 wt% of monomeric and dimeric compounds from MO and exhibiting the potential to be techno-economically viable. The superior activity of Ru/WZr can be attributed to the synergistic effects of metal and acid, which were studied by investigating the product distributions of aromatic small molecules depending on the properties of WZr-supported metal catalysts. The stabilization of reactive radical intermediates depending on the surface Brønsted acidity of acid catalysts and hydrogen-adsorbing ability of metals were also investigated. RCF reaction conditions were optimized for the maximum yield of monomeric compounds, which can be beneficial for the further development of industrial processes.

Metal Organic FrameworksCovalent Organic FrameworksOrdered Mesoporous SilicaOxygen Reduction ReactionHydrogen Evolution ReactionOxygen Evolution ReactionCatalytic Fast PyrolysisCatalytic PyrolysisSilica to Alumina RatioGlycerol to AromaticsBenzene, Toluene, XylenePolyethylenePolypropylenePolystyreneLow Density PolyethyleneHigh Density PolyethyleneCetyltrimethylammoniumCetyltrimethylammonium BromideIndexed Hierarchy FactorWaste Cooking OilPolyethylene TerephthalateTurn Over FrequencyDensity Functional TheoryFatty AcidDimethyl etherParaxyleneBicyclic Aromatic HydrocarbonsResearch Octane NumberWeight Hourly Space VelocityGlycerol Steam ReformingPalm oil fuel ash wasteCO2 reforming of methaneDry reforming of methanetungstophosphoric acidSpace-time yieldSecondary Building UnitHydrodeoxygenationAnodized Aluminium OxideDirect Methanol Fuel CellPoly (vinylpyrrolidone)Methanol Oxidation ReactionEthanol Oxidation ReactionDirect Alcohol Fuel CellsFormic Acid OxidationDirect Formic Acid Fuel CellsPoly (ethylene oxide)-b-poly (methyl methacrylate)Many efforts have been devoted to establishing alternative resources and technologies for fuel production [1–4]. Not only because the fossil-based resources have been continuously depleted, but also the fossil fuel is not environmentally friendly due to the uncontrolled carbon emission. It should be noted that the alternative resources must be renewable, environmentally benign, and easy to be converted into high calorific fuels. Biomass, carbon dioxide, and water are among the resources that fulfill those requirements [5–8]. Consequently, various chemical or electrochemical processes are needed to convert these resources into renewable fuels. Therefore, catalysts and electrocatalysts are needed to control the course of reactions kinetically, i.e., accelerate the reaction rate and shift the selectivity towards the desired products.Several heterogeneous catalysts with various functionalities such as acidic, oxides, sulfides, metallic, metal complexes, and conductive sites have been introduced to produce renewable fuels [9–16]. In general, heterogeneous catalysts rely on the number of catalytic sites per surface area, proportional to the catalytic performance. It could be achieved by down-sizing the catalyst particle into the nanoscale (less than 100 nm) and/or creating a foam-like particle with nanopore cages or nanopore channels as the interior part. The latter is known as nanoporous materials, which are classified into three types based on the nanopore size: microporous (pore size less than 2 nm), mesoporous (pore size from 2 to 50 nm), and macroporous (pore size more than 50 nm) [17]. Interestingly, the nanoporous materials do not only enhance the catalytic activity but also possess the capability to direct the selectivity based on the shape or size of the nanopores [18,19].The production of renewable fuels has benefited from nanoporous materials since they significantly improve the feed conversion and yield of desired products. Several nanoporous materials have shown the up-and-coming catalytic performance for generating renewable fuels (Fig. 1 ). For example, zeolites and ordered mesoporous silica (OMS) have been prominent in the conversion of biomass, waste, and carbon dioxide to renewable fuels [20–27]. The features of the zeolite framework provide a selective entrance for a guest molecule. Furthermore, combining the spatial confinement effect with the catalytic active sites results in a high conversion to the desired products [28]. For instance, the high selectivity of isoparaffins in gasoline-range hydrocarbon products from CO2 conversion could be boosted up to ∼70% after coupling the Fischer–Tropsch (FTs) catalyst with zeolite [29,30]. This selectivity is much higher than that obtained from classical FTs catalysts, which is limited by the Anderson-Schulz-Flory (ASF) law distribution.For biomass conversion, zeolite provides the main active sites, either Brønsted, Lewis, or Basic site for producing fuels or intermediate platform from renewable feedstock through various processes such as pyrolysis, hydrolysis, condensation, esterification, and cracking [31]. Closely related to the zeolite materials, the ordered mesoporous silica (OMS) also provides high pore volume and large surface area with an ordered mesoporous network. Furthermore, it contains a silanol-rich surface, which is favorable for functionalization to generate the catalytic active sites [32]. These properties are beneficial for catalysis, especially for renewable related technologies, e.g., the use of SBA-15 for producing biodiesel from soybean oil results in ∼83% conversion [33], and the production of hydrogen from steam pyrolysis-gasification of biomass using MCM-41 impregnated with Ni [34]. Moreover, OMS was reported as effective support for semiconductor nanostructures for hydrogen production from solar energy via water splitting [35].Metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) offer diverse nanopore sizes and shapes thanks to their reticular properties [36,37]. The ability to modify their structural properties without negatively impacting their structural integrity is their key distinctive features compared to zeolites [38,39]. MOFs and COFs have been successfully applied as heterogeneous catalysts in several reactions for producing renewable fuels. For instance, they have been investigated as the efficient photocatalysts for hydrogen production due to the suitable band gap for visible light and electron–hole recombination [40–43]. As a new kind of COFs, the covalent triazine frameworks (CTFs) are currently the cutting edge photocatalyst for hydrogen production. It can produce hydrogen 20–50 times higher compared to the graphitic photocatalyst [42,44]. Furthermore, they also have shown decent performance in CO2 reduction and hydrodeoxygenation of biomass for generating renewable fuels. Ultimately, nanoporous metals combine the interesting characteristics of nanomaterials with the mechanical strength of dense materials. They emerge as a new class of nanoporous materials showing catalytic activity in several reactions to produce renewable fuels. Yang et al. [45] reported the porous cobalt-based thin film as a bifunctional catalyst to generate hydrogen and oxygen from water electrolysis. Other nanoporous metals, such as Ni–Fe hydroxyl phosphate (NiFe-OH-PO4) supported on Ni foams' surface have also been reported for the same reaction in alkaline solution [46].Based on the above mentioned, it indicates the interest in the field of nanoporous materials and renewable fuels has grown massively in the past decade. Furthermore, It was also indicated by the exponential increase in the number of publications (Fig. 2 ). Numerous review articles have pointed out the applications of nanoporous materials for the production of renewable fuels. Nevertheless, they focused on either specific nanoporous materials or certain catalytic reactions [6,20,24,47–54]. Herein, we comprehensively review various nanoporous materials limited to the zeolite, OMS, MOFs, COFs, and nanoporous metals and their applications as heterogeneous catalysts and electrocatalysts in diverse reactions for generating renewable fuels. It should be noted that other materials, such as nanoporous carbon, are also a significant catalyst that has been widely used as supports or carriers in renewable energy-related technology, e.g., as electrocatalyst for Oxygen Reduction Reaction (ORR) and support for photocatalyst [55]. However, this material is not covered in our review. The discussion is classified by the different types of materials. Each section covers a detailed discussion of the physicochemical properties of particular nanoporous material, followed by a description of their applications in several catalytic reactions. This review is concluded with the critical remarks, opportunities, and prospects of nanoporous materials for the generation of renewable fuels in the future.Zeolites are defined as microporous (<2 nm) crystalline aluminosilicate materials composed of TO4 (T = Si, Al) tetrahedra as the primary building units [56–59]. Their frameworks bear the net negative charge because Si atom with zero formal charges undergoes an isomorphous substitution with Al atom, whose formal charge is −1 [60–63]. Therefore, zeolites also possess counter cations, commonly from the alkaline or alkaline-earth metal groups, that are present to balance the negative charge of the frameworks. Zeolites can be naturally formed due to volcanic activity or synthesized via the solvothermal (mostly hydrothermal) method [56,57,59,64,65].To date, more than 250 zeolite framework types have been reported in the database of zeolite structure provided by the International Zeolite Association (IZA) [66]. Each framework type is indicated by a three-capital-letter code, which is ordinarily derived from the names of the type materials. For example, the code LTA and MFI stand for Linde Type-A and Mordenite Framework Inverted. The zeolite porosity is originated from the uniform micropore channels and cages. The accessible pore opening is arranged by the rings comprising n TO4 units. Based on the size of pore openings, zeolites can be classified into small-pore, medium-pore, and large-pore zeolites. The small-pore zeolites have an 8-membered ring micropore with a diameter of around 3 to 4 Å. They include various framework types, including LTA, CHA, AEI, and AFX. The medium-pore zeolites (e.g., MFI, MWW, MEL, and FER) comprise pore opening arranged by 10-membered rings with a diameter of around 5 to 6 Å, whereas the large-pore zeolites possess pore openings whose diameter of about 7–9 Å, which are formed by 12 or more membered rings. FAU, MOR, ∗BEA, and EMT are included. Several structures of common zeolites are depicted in Fig. 3 .The catalytic activity of zeolites originates from their acid-active sites. In general, the counter cations of zeolites can be exchanged with ammonium ions (NH4 +), followed by calcination to release ammonia molecules (NH3), and produce proton (H+) as the Brønsted acid sites. In addition, the calcination also induces dehydration resulting in the three-coordinated Al species that act as Lewis acid sites. Despite the acidic properties, zeolite could also have the basicity, although less well-defined. Zeolite as a base catalyst usually could be represented by alkali-exchanged low-silica zeolite [67,68]. Principally, the basic site of zeolite was generated from the negative charges of the framework oxygen atom close to the tetrahedral Al atoms. The basicity of the framework oxygen atoms increases with decreasing the cation electronegativity. In this case, the order base strength of cation-exchanged-zeolite follows Li- < Na- < K- < Rb- < Cs-exchanged zeolites [69]. In addition to the activity, zeolites also display highly selective capability due to the uniform distribution of micropores. It should be noted that the size and shape restrictions play a major role in determining the accessibility for certain molecules [70–72]. The size and/or shape selectivity of zeolites results from the interaction between molecules with the well-defined pore architecture [73,74].However, there are three variants of selectivity that may overlap, i.e., selectivity based on reactant, product, and transition state molecules. The selectivity of the reactant means that only raw materials of a certain size and shape can access the zeolite pores and react at the active sites. The term “molecular sieve” is, thus, coined. On the other hand, product selectivity occurs when specific molecules with a size and shape can diffuse out of the pores. The third variant of selectivity is based on the formation of intermediate molecules in chemical reactions. The pore system merely allows certain intermediates, which are suitable within the pore system. This selectivity is favored when mono- and bimolecular rearrangements are probable. Nevertheless, it is very easy to differentiate the intermediate selectivity from the product selectivity since they may display similar effects. In practice, they may proceed simultaneously [70]. The schematic illustration of the size and shape selectivity of zeolites is depicted in Fig. 4 .Despite the traditional applications, e.g., adsorption [75], separation, and catalysis related to the petrochemical industry [76], the application of zeolite has been extended to attain sustainable goals as a primary, hybrid, or support for other materials [31,77,78]. In this part, the role of zeolite catalysts in the production of renewable fuels is discussed and summarized in Table 1 . It could be classified into the following routes: catalytic pyrolysis reaction biomass and plastic wastes and CO2 conversion reactions.Catalytic fast pyrolysis (CFP) reaction plays a major role in the production of high-quality biofuel and biomass. The catalyst also play a major role in deoxygenating bio-oil and improving its fuel properties. Several chemical reactions, i.e., deoxygenation, cracking, hydrocarbon pool mechanism, aromatization, and ketonization-aldol condensation, may coincide during fast pyrolysis catalytic reaction. Several reports have shown that zeolite provided promising results in the application for CFP reactions [1,31,79]. A highly deoxygenated, hydrocarbon-rich compound and stable pyrolysis oil product could be obtained due to the excellent capability of zeolite in promoting cracking reaction during pyrolysis. Furthermore, the tunable properties of zeolite, such as the acidity, morphology, and pore structure, make us possible to engineer the desired product of the reaction [31,79].Stefanidis et al. [80] reported that ZSM-5 catalyst presented superior catalyst performance compared to magnesium oxide (MgO) and alumina (Al2O3), nickel monoxide (NiO), zirconia (ZrO)/titania (TiO2), tetragonal zirconia, titania, and silica-alumina (SiO2/Al2O3). It displayed the highest surface area with moderate selectivity towards hydrocarbons, reducing unwanted products and yielding organic liquid products at acceptable amounts (Fig. 5 a). AbuBakar and Titiloye [81] reported the application of ZSM-5 for catalytic pyrolysis of Brunei rice husk in the fixed bed reactors. They showed that ZSM-5 increased the production of aromatic hydrocarbons and light phenols. Also, ZSM-5 increased the calorific value and water content in the bio-oil. At the same time, it decreased the viscosity, density, and acid number of the bio-oil. Thangalazhy-Gopakumar et al. [82] introduced ZSM-5 for the catalytic pyrolysis of pinewood chips under helium and hydrogen environments. The reaction was run under two methods of catalyst-bed method and the catalytic mixing method. The best condition was achieved at 1:9 biomass catalyst mixture under helium environment employing catalytic mixing method. It yielded aromatic carbon of 41.5%, which is about 51% of the theoretical yield.In addition to their superior performance for CFP, several factors affect the performance of zeolite during the reaction. Jae et al. [83] reported the influence of zeolite pore size and shape selectivity on the conversion of glucose to aromatic products. They utilized three different categories of zeolites as grouped for their different pore size and shape, i.e., small pore ZK-5, SAPO-34, medium pore Ferrierite, ZSM-23, MCM-22, SSZ-20, ZSM-11, ZSM-5, IM-5, TNU-9, and large pore SSZ-55, beta zeolite, Y zeolite. The result exhibited that the yield of aromatic products is dependent on the function of pore size of zeolite catalysts. Small pore zeolites did not yield any aromatic products whilst producing large quantities of coke. The medium pore size zeolite had the highest aromatic product and least amount of coke. The large pore zeolite results in the highest coke yield, low aromatic yield, and low oxygenated products yields. Yu et al. [84] also reported the influence of pore size and shape selectivity of zeolites in the catalytic fast pyrolysis of lignin. They utilized ZSM-5, mordenite, beta, and Y zeolites, which have various static pore sizes between 5.6 and 7.6 Å. ZSM-5 produced the highest aromatic yield among the four zeolites, followed by beta, mordenite, and Y zeolites, respectively (Fig. 5b).Moreover, Beta and Y zeolites were the most effective catalysts for the deoxygenation reaction of lignin-derived oxygenates. This result indicated that ZSM-5 is the optimal catalyst for CFP of softwood due to its ability to achieve satisfactory deoxygenation and aromatic production simultaneously. For hardwood feedstock, the beta zeolite may be a prominent candidate. In addition to the influence of pore size and shape selectivity of zeolites, the function of the framework silica-to-alumina ratio (SAR) of ZSM-5 might also define the CFP product's yield. Foster et al. [85] reported that ZSM-5 with SAR of 30 exhibited a higher aromatic yield concentration than SAR of 23, 50, and 80 (Fig. 5c). They suggested that tuning the SAR might influence the acid concentration within the zeolite framework and maximize the aromatic production of CFP reaction. Ben and Ragauskas [88] carried out softwood (SW) kraft lignin pyrolysis using various H-ZSM-5 zeolites with different SiO2/Al2O3 mole ratios from 23 to 280. The result demonstrated that H-ZSM-5 zeolite with a relatively higher SiO2/Al2O3 mole ratio was more effective at the elimination of methoxyl groups, ether bonds, aliphatic C–C bonds, and dehydration of aliphatic hydroxyl groups. However, the H-ZSM-5 zeolite with a very large SiO2/Al2O3 mole ratio, such as 280, has only limited effects on the properties of upgraded pyrolysis oil. After using zeolite, the pyrolysis oils contain some polyaromatic hydrocarbons, the content of which decreased with an increasing SiO2/Al2O3 mole ratio of zeolite.Moreover, several factors, i.e., porosity and the acidic properties of zeolite introduced to the catalytic reaction, should also be considered. Li et al. [89] investigated the effect of mesoporosity on the performance of ZSM-5 for CFP of lignocellulosic biomass. The presence of mesopores improved the diffusion property of the ZSM-5 and their catalytic activity for cracking the bulky oxygenates (e.g., syringols derived from the lignin) as schematically shown in Fig. 6 . Therefore, it produces a higher yield of aromatic hydrocarbons (26.2–30.2%) and less coke formation (39.9–41.2%). Wang et al. [90] Reported the catalytic performances of H-ZSM-5 catalyst with various porosity and acidic property in glycerol to aromatics (GTA). Both the GTA reaction and coking process were varied with the different mesoporosity of HZSM-5. Among all catalysts, the HZSM-5 catalyst with the highest mesoporosity of 0.385 (cm3 g−1) exhibited the highest BTX aromatics yield, lowest coking rate, and most extended catalyst lifetime.Furthermore, the different feedstocks of biomass also influenced the bio-oil yield produced via catalytic conversion. Huang et al. [91] reported that the catalytic conversion of several biomass feedstocks into olefins using HZSM-5 with the addition of 6 wt.% La was decreased in the order: cellulose > hemicellulose > sugarcane bagasse > rice husk > sawdust > lignin. Biomass comprising a larger amount of cellulose or hemicellulose produced higher olefins yield than feedstocks with higher lignin content. While the HZSM-5 zeolite was catalytically active, incorporating La at 2.9 and 6.0 wt.% increased the production of olefins from rice husk by 15.6% and 26.5%, respectively.In order to increase the CFP activity of zeolites, the modifications of zeolite via the impregnation of transition metals have been reported. The introduction of metals, i.e., Pb, Ni, Zn, Fe, Mo, Ga, and Co, into the zeolite framework has also improved product selectivity. Liang et al. [92] utilized ZSM-5 modified Co, Ni, and Zn for the catalytic pyrolysis of rice straw. The result exhibited that further introduction of transition metal into zeolite catalyst improved bio-oil compound selectivity. The product exhibited major contents of aldehydes/ketones and phenols with a composition of more than 50% on average of the bio-oil. Vichaphund et al. [86] reported the application of HZSM-5 promoted Co and Ni metals via liquid ion exchange for catalytic upgrading pyrolysis vapors of Jatropha (Fig. 5d). The catalytic waste was investigated using biomass to catalyst ratios of 1:0, 1:1, 1:5, and 1:10 for both types of metals (Co and Ni). The result demonstrated that both biomass to catalyst ratios and type of metals determined the aromatic hydrocarbons yields nd the oxygenated and N-containing compounds. Also, the introduction of metals, especially Ni, might inhibit the formation of coke. Therefore, it increased the catalyst lifetime.Moreover, Sun et al. [93] reported the improved performance of ZSM-5 in CFP of biomass to aromatic after introducing Fe. It was found that the Fe/ZSM-5 catalyst exhibited higher catalytic activity by increasing the yields of monocyclic aromatic hydrocarbons and hindered its further polymerization. The introduction of Cu-metals into beta zeolite was reported by Widayatno et al. [94]. The small amount loading (5%) of Cu on beta zeolite has improved the selectivity of hydrocarbons and the coking resistance. The introduction of metal on zeolite promoted the synergetic effect between the doped metal sites and the protonic sites on the zeolite structure, which may play an important role in improving catalyst performance. However, further introduction of Cu on zeolite has resulted in the formation of Cu aggregates, which blockage the zeolite pore and decrease the surface area. It also caused an increase in coke formation and decreased activity and selectivity. Recently, the modification of CFP catalysts using metal oxides exhibited a higher yield of the organic compounds in the bio-oil and lower content of undesired polyaromatic hydrocarbons and coke. The incorporation of MgO and ZnO reported by Fermoso et al. [87] increased the gas yield as high formation CO and CO2. The bio-oil products also demonstrated higher H/C and O/C ratios and larger heating values (Fig. 5e). It might be related to partial blockage of zeolite pore and decrease of the Brønsted acid site, and the increase of Lewis acid site, which was created after the deposition of both metal oxides.In addition to their application for CFP of biomass wastes, zeolites are also widely applied as catalysts for non-biomass wastes such as plastics and rubber wastes. López et al. [95] introduced ZSM-5 compared to red mud for catalytic pyrolysis of plastic wastes. Both catalysts have been examined in pyrolysis of a mixture of plastics which resembles municipal plastic wastes, at 440 and 500 °C in a 3.5 dm3 semi-batch reactor. The result exhibited better performance as its higher porosity, and strong acidity contributed to producing a greater proportion of gases and liquids with a higher aromatics content than the condition without catalyst. Santos et al. [96] reported the better performance of USY compared to ZSM-5 for the CFP of polyethylene (PE) and polypropylene (PP) wastes. USY catalyst exhibited regenerable properties, as reported by Kassargy et al. [97].Additionally, Boxiong et al. [98] explored the catalytic performance of USY and HZSM-5 in the CFP of waste tyres, and it was concluded that USY zeolites exhibited better conversion capability than HZSM-5 in the production of aromatic hydrocarbons. Recently, Wang et al. [99] reported using USY zeolites for CFP of rubber wastes. It is obtained that alkenes and aromatic hydrocarbons were the main products obtained from the CFP of rubber wastes. They showed that the USY zeolite with a low 5.3 was more effective for producing aromatic hydrocarbons, while the higher SiO2/Al2O3 mole ratio (11.5) led to greater alkenes formation. In this reaction, the pyrolysis temperature also played a vital role, in which the formation of the highest concentration of aromatics compound was achieved at 750 °C.Kassargy et al. [100] reported the catalytic degradation of PP, PE, and their mixtures to produce gasoline and diesel-like fuels using USY zeolites. The catalytic pyrolysis reaction resulted in a liquid fraction dominated by a (C5–C7) hydrocarbons fraction and the gaseous products, which are major constituents of C3 and C4. Although the synergistic effect of the plastic mixtures is still elusive, their proportion influenced the liquid fractions and the yields of the products. Both natural and synthetic zeolite (ZSM-5) for catalytic pyrolysis of polystyrene (PS), PP, PE, and their mixtures exhibited remarkable performance [101]. Interestingly, the catalytic reaction of PS plastic employed natural zeolites could result in the highest liquid oil yield of 54%. In contrast, the mixing of PS with other plastic wastes might decrease the liquid oil yield. Nevertheless, the mixture PP and PE feedstocks demonstrated a higher liquid oil yield than their individual using both catalysts.Despite the choice of catalyst and feedstock used for catalytic reaction, it is also important to choose both suitable catalyst bed temperature and the SAR of the catalysts to obtain the optimum condition of the reaction. Onwudili et al. [102] reported that the increase of catalyst bed temperature led to increased gas production, particularly C2–C4 hydrocarbons, during the catalytic reaction of simulated mix plastic feedstocks using several zeolite-based catalysts with different SAR. The catalyst with lower SAR exhibited better performance than the catalyst with higher SAR (16.4-80). It produced the highest aromatic compounds yield at both pyrolysis temperatures of 500 and 600 °C. The catalyst also generated higher hydrogen gas and higher benzene and toluene composition of 90% in the aromatic fractions. Similar behavior was also found in the hydroprocessing of thermal cracked-low density polyethylene (LDPE) plastics oil using ZSM-5 as a catalyst. The low SAR ZSM-5 also exhibited an extensive increase in the production of gaseous hydrocarbons. However, in this type of reaction (upgrading LDPE oil to fulfill the properties of fuel oil), a higher SAR zeolite demonstrated more suitable properties as its ability to attenuate the cracking activity while keeping the contribution of hydroisomerization and olefin hydrogenation. Therefore, the higher SAR catalyst exhibits a good combination of hydroisomerization and aromatization reactions along with a limited extension of end-chain cracking reactions so that it achieves high selectivity towards liquid fuels (over 95%) [103].Susastriawan et al. [104] studied the application of zeolite for the low-temperature pyrolysis of LDPE plastic waste. They also studied the influence of catalyst particle size on their performance. The results demonstrated that smaller zeolite sizes led to an increase in heat transfer rate, pyrolysis temperature, reaction rate, and oil yield. It also found that the elevated pyrolysis temperature gives rise to a higher oil yield. However, the author reported that the oil yield percentage is still relatively low compared to gas yield and remaining char. From 1000 g of LDPE plastic, the obtained oil yields of 138, 134, 126 mL from 1, 2, and 3 mm in diameter of the zeolite, respectively. Furthermore, Kadja et al. [22] examined the effect of hierarchical porosity of ZSM-5 for their catalytic activity over LDPE pyrolysis. Hierarchical ZSM-5 was developed via sequential mechanochemical treatment and recrystallization in the presence of cetyltrimethylammonium (CTA+) molecules (Fig. 7 a). Here, ZSM-5 was mechanically treated under a ball mill and sequentially recrystallized with the addition of CTA+ molecules in the autoclave at 180 °C for 10 h. In addition to its inevitable role in preventing excessive coalescence and crystal growth, CTA+ molecules also help assemble the nanosized zeolites and promote the formation of the hierarchical porous structure of the zeolite. The result of the temperature-programmed LDPE pyrolysis test exhibited that T 50, the temperature at of 0.5 or 50% LDPE conversion, shifts gradually to decrease values relative to the blank test without catalyst (476 °C) as increase amount of CTAB added (Fig. 7b). The decline of T50 was described as following order, M−the zeolite after mechanochemically treated (461 °C) > initial ZSM-5 (423 °C) > MR−mechanochemically treated and recrystallized without CTAB (419 °C) > MRCTAB0.004−mechanochemically treated and recrystallized with CTAB, Si/CTAB of 0.012 (397 °C) > MRCTAB0.012−recrystallized with CTAB, Si/CTAB of 0.012 (395 °C). Also, the observed activation energy (Eobs) calculated from Coats-Redferns plot [105] exhibited a similar trend (Fig. 7c). The E obs of LDPE without catalyst is 450 kJmol-1, while The E obs of LDPE with addition of catalysts exhibited lower values as the following order, M (341 kJ mol− 1) < initial ZSM-5 (224 kJ mol− 1) < MR (216 kJ mol− 1) < MRCTAB0.004 (146 kJ mol− 1) < MRCTAB0.012 (132 kJ mol− 1). The introduction of hierarchical zeolite for LDPE pyrolysis was also reported by Wardani et al. [21]. They introduced post alkaline treated SSZ-13 zeolite for temperature-programmed LDPE pyrolysis reaction. The relative activity of the catalyst is measured as T50. Post alkaline treated hierarchical SSZ-13 exhibited the lowest T50 (C250-AT = 460 °C) compared to blank test (T50 = 476 °C), catalysts calcined at 550 without post alkaline treatment (C550 = 468 °C), and as-synthesized catalyst with post alkaline treatment (AS-AT = 463 °C) (Fig. 7d). Also, the C250-AT catalyst exhibited the highest IHF (Indexed Hierarchy Factor) value, |ΔEobs|/nAl, and |ΔEobs|/∑ acid sites (Fig. 7e). The higher activity of hierarchical zeolite in LPDE cracking reaction was related to the presence of the additional meso- and/or macropore, which provide such a molecular highway to alleviate the diffusion constraints of bulky LPDE molecules [106].Furthermore, the zeolite-based catalyst also exhibited remarkable performance for catalytic reaction of high-density polyethylene (HDPE) waste. As reported by Hassan et al. [107], the utilization of faujasite-type zeolite for the co-pyrolysis of sugarcane baggase and HDPE increased the calorific value of product oil. It enhanced the liquid yield with the maximum bio-oil yield of 68.56 wt% and hydrocarbon yield (74.55%) and a minimum yield of oxygenated compounds (acid = 0.57% and ester = 0.67%). The suitability of ZSM-5 over catalytic cracking of HDPE was also reported by Elordi et al. [108]. HZSM-5 zeolites with SAR of 30 and 80 were employed to catalyze the polyethylene feedstock under feed flow of 1g.min−1 in 10 h to a 30 g catalyst bed. The catalyst demonstrated a very low deactivation nature and a moderate acidity that is useful to modify the product distribution. The catalyst also exhibited common behavior, where the increase of SAR ratio led to a higher yield of C2 –C4 olefins and that of the non-aromatic C5–C11 fraction, and a decrease in the yields of aromatic components and C1–C4 paraffin. Also, the rate of coke production is suppressed as the SAR is increased.The utilization of zeolite for the conversion of waste oil to fuel exhibited remarkable results. The application of zeolite for the conversion of waste cooking oil (WCO) was reported by Li et al. [109]. They have effectively carried out the catalytic conversion of WCO to liquid hydrocarbon fuels by utilizing USY zeolites as the catalyst. The catalyst exhibited higher performance in comparison to traditional base catalysts, e.g., Na2CO3 and K2CO3. Interestingly, the reaction could generate liquid hydrocarbon fuels containing C8–C9 alkanes or olefins, which is likely similar to the chemical composition of gas oil-based fuels with the high yield of liquid products was over 75% and low coke formation of 24.7%. Khowatimy et al. [110] reported the study of hydrocracking of waste lubricant into gasoline and diesel fraction using the combination of Y-Zeolite and ZnO (Y-Zeolite/ZnO). The catalytic reaction achieved a higher total conversion of 99.49 wt.% compared to the reaction without catalyst (thermal hydrocracking) with a conversion of 98.99 wt.%. The catalyst also results in the highest liquid product of 24.75 wt.% and gasoline and diesel selectivity of 25.92 and 74.08%, respectively.Moreover, AbuKhadra et al. [111] introduced alkali modified clinoptilolite for transesterification of commercial waste cooking oil into biodiesel with technical properties of EN 14214 [112] and ASTM D-6751 [113] standards. All the catalysts, i.e., K/clinoptilolite (K/Clino), Na/clinoptilolite (Na/Clino), Ca/clinoptilolite (Ca/Clino), and Mg/clinoptilolite (Mg/Clino) showed promising catalytic activities by achieving biodiesel yields of 93.6%, 95.2%, 96.4%, and 98.7%, respectively. Recently, Fan et al. [114] reported the fast catalytic co-pyrolysis of lignin and waste cooking oil for aromatics production using ZSM-5 as a catalyst. The catalyst to feedstock achieved the highest yield of aromatics at the ratio of 3:1. The further increased ratios of catalyst to feedstock enhanced the alkylation and demethoxylation of phenols.Moreover, the suitable ratio of WCO: lignin should also take into account. The optimum ratio of WCO to lignin was achieved at 1:1, which results in the highest mono-aromatic selectivity of 82.6% and a synergistic extent of 52.1%. Ding et al. [115] investigated the catalytic characteristics of modified HZSM-5 with separate NaOH/steaming treatment or integrated NaOH-steaming process in the catalytic fast pyrolysis of wasting cooking oil, and it was indicated that the integrated modified HZSM-5 exhibited higher yields of desired BTX aromatics and long-term stability due to the established micro-mesopore hierarchical system and improved acidic properties in HZSM5 zeolite. Nevertheless, most of these studies ignored the effects of the sequential order of desilication and dealumination treatment on the physicochemical property and catalytic performance of HZSM-5.The introduction of zeolites has shown remarkable results for syngas production from the gasification of plastic waste (PE, PP, and terephthalate of polyethylene (PET). Al-asadi et al. [116] reported the application of Ca, Ce, La, Mg, and Mn to promote the Ni/ZSM- 5 catalyst for syngas production. The modified catalysts enhanced the reaction rate of the pyrolysis process, resulting in high syngas in the product yields. The maximum syngas production was obtained with a La catalyst. Catalysts can also accelerate the methanization reactions and isomerize the main carbon frame. Increasing temperature and oxygen in the atmosphere led to a higher n-paraffin/n-olefin ratio and more multi-ring aromatic hydrocarbons in pyrolysis oils. The concentration of hydrocarbons containing oxygen and branched compounds was also significantly affected by catalysts.The chemical transformation of CO2 to value-added chemicals and fuels is currently a research challenge, especially in developing more efficient catalysts. Zeolite comprises unique porosity that had been widely reported for direct conversion of CO2 into fuel products. In this process, zeolite is usually coupled with metal or metal oxide catalyst, in which metals promote the CO2 activation process, and zeolites provide the selectivity role by subsequent catalysis process (Fig. 8). Several valuable products, either C1 or C2+, have been extensively produced through various processes [29,30]. Delmelle et al. [117] studied the sorption enhanced methanation of CO2 by loading Ni on zeolite 5A and 13X via wet impregnation. The result Ni/zeolite catalysts exhibited high activity and selectivity over CO2 methanation reaction. Ru is more active than Ni due to its high CH4 selectivity and low coke forming properties [118]. Ahmed [119] reported Ru/Y zeolite catalysts with loadings between 1 wt.%-5.4 wt.% Ru prepared toward ion-exchange method. The optimum loading was achieved at 2.2 wt% highest selectivities of 72% and 100% gas conversion at 170 °C. The utilization of ZSM-5 and silica MFI (S-1) as support for Rh nanoparticles catalyst also exhibited higher methane selectivity. Compared to conventional metal oxide supports, it exhibited high activity for CO2 conversion and high selectivity for CO under equivalent conditions [19]. The application of zeolite as catalyst support for metal loading catalyst was also reported for Pt. Sápi et al. [120] deposited size-controlled Pt nanoparticles on ZSM-5 supports with SAR of 30, 80, and 280. Size-controlled Pt nanoparticles significantly improved the catalytic activity of the conventional H-ZSM-5 resulted in ∼16 times higher CO2 consumption rate. Moreover, the catalytic activity increased ∼4 times higher, and CH4 selectivity at 873 K was ∼12 times higher.Bahari et al. [121] studied the effectiveness of various bimetallic on Fe-Zeolite over CO2 hydrogenation to formic acid. Several metals-loaded catalysts, i.e., Co, Cu, Pd, and Ni, had been examined for their function as the promoter for CO2 hydrogenation. The catalyst of Pd: Fe: Zeolite (0.1:1.25:2) demonstrated the highest formic acid yield of 275.91 ppm compared to other catalysts. More advanced utilization of bimetallic clusters within zeolite catalyst was reported by Sun et al. [122]. The Pd–Mn clusters encaged within S-1 zeolites prepared via ligan-protected method was employed for catalyzing CO2 hydrogenation into formate (Fig. 9 a). The obtained catalysts exhibited extraordinary catalytic activity and durability due to the formation of ultrasmall metal clusters and the synergic effect of bi-metallic components. The highest performance catalyst, PdMn0.6@S-1, exhibited the formate generation rate of 2151 molformate molPd −1 h−1 at 353 K and an initial TOF of 6860 mol H2  mol Pd−1 h−1 for CO-free fatty acid (FA) decomposition at 333 K without any additive (Fig. 9b). Moreover, the DFT calculations were employed to explain the excellent catalytic performance of Pd–Mn clusters over FA decomposition (Fig. 9c). The results demonstrated that alloying Pd with Mn led to the formation of a more compact structure. Also, the addition of Mn slightly passivated Pd active sites, preventing overly strong binding with intermediates in FA decomposition. These are key factors affecting the high performance of the catalyst.Furthermore, the metal order on bimetal-modified zeolite support has also significantly influenced the properties of the catalyst. Bacariza et al. [123] studied the effect of bimetallic order of Ni-Ci on the USY zeolite support to its performance for hydrogenation of CO2 to methane. Here, three different orders impregnation i.e., Ni before Ce (Ce/Ni), Ce before Ni (Ni/Ce), and co-impregnation (Ni–Ce) are examined to obtain the best way for obtaining the highest catalytic activity. Ce/Ni and Ni–Ce catalyst exhibited stronger Ni–Ce interaction and smaller Ni average size ot 2.5 nm, at the same time, it enhances the reducibility of the Ni species. However, these catalysts exhibited lower CO2 adsorption capacity than Ce/Ni catalyst. Among all the catalyst tested, Ni–Ce catalyst was the best ordering method as it exhibited the highest catalytic performance.In addition to the effect of particle size of metal loaded and impregnation order of the metals, the preparation, and pre-reduction of the metal loaded catalyst also played an important role in affecting the catalyst's performance. Bacariza et al. [124] reported that the drying method, the calcination temperature, and the pre-reduction temperature had influenced the catalytic performance of Ni-based zeolite catalysts for CO2 methanation. A suitable drying method and calcination temperature may result from more reducibility of Ni species and at the same time produce good structural and textural properties of support and more homogenous Ni particle size. Drying method under microwave radiation, the calcination temperature at 300 °C and pre-reduction temperature at 500 °C exhibited maximum catalytic performance of Ni-based zeolite catalyst.The effect of zeolite topology on the direct conversion of CO2 into hydrocarbon products was also studied by Ramirez et al. [125]. Both MOR and ZSM-5 are introduced separately with Fe2O3/KO2 for the direct conversion of CO2 to light olefins and aromatics. MOR and ZSM-5 exhibited different catalytic behavior as MOR directly converted CO2 into light olefins (Fig. 10 a) and ZSM-5 into aromatics (Fig. 10b). In addition, both MOR and zeolite boosted the total selectivity to desired hydrocarbon product and enabled the further conversion of undesired CO. Moreover, the remarkable difference in selectivity between the two zeolites is further rationalized by first-principles simulations, which show a difference in reactivity for crucial carbenium ion intermediates in MOR and ZSM-5 (Fig. 10c). Further study using DFT simulations exhibited the future potential of ZSM-5 to activate long alkenes via carbenium ions which may lead to higher selectivity toward the formation of aromatic products (Fig. 10d).Moreover, Bonura et al. [126] also introduced two different zeolites of FER and MOR for the catalyst support of CuZnZr. The catalyst was utilized for CO2 conversion to DME in a fixed bed reactor. The hybrid CuZnZr-FER catalyst exhibited better activity-selectivity and an interesting DME productivity, with no coke formation under the experimental condition (Fig. 11 a). The better dispersion of CuZnZr catalyst on the 2D FER framework than MOR zeolite leads to a more efficient mass transfer of MeOH from CuZnZr sites to the zeolite surface; therefore, favoring the formation of DME with higher yields. Furthermore, Bonura et al. [127] also studied the effect of catalyst acidity of CuZnZr-FER for the direct CO2 to DME reaction. The catalyst exhibited a different behavior in terms of stability which was influenced by different acidity (Fig. 11b). The acidity of the catalyst was controlled by varying the alumina amount to obtain different Si/Al molar ratios of 8, 30, and 60. Catalysts with higher SAR experienced higher metal loss during the reaction. However, all three catalysts demonstrated only a slight difference in conversion, selectivity, and product yield (Fig. 11c).Ayodele et al. [128] reported the introduction of a tandem zeolite-based catalyst for CO2 hydrogenation to methanol (MeOH). The prepared catalyst Cu/ZnO/ZSM-5 denoted CZZSM (Fig. 12 a–c), exhibited impressive CO2 conversion of 20.25% and higher selectivity of MeOH (77.7%) in comparison with other catalysts supported on Al2O3, SiO2, ZrO2 (Fig. 12d). Other tandem zeolite-based catalysts were reported by Li et al. [129]. They utilized Pd/ZnO/ZSM-5 for the hydrogenation reaction of CO2 to dimethyl ether (DME). The spatial configuration of Pd/ZnO and ZSM-5, which was adjusted by tuning the size of Pd/ZnO, defined the DME yield. The close proximity of both catalysts was unfavorable for DME production, resulting in lower selectivity due to the displacement of Brønsted acids in ZSM-5 by low-valent Zn cation. The synthesis method of the catalyst via powder mixing demonstrated the best configuration that promoted the highest CO2 conversion with the largest selectivity of DME. Compared to conventional ZSM-5 catalysts, the introduction of Pd/ZnO on ZSM-5 results in excellent long-term stability in 60 h with less coke. Another application of tandem zeolite-based catalyst was carried out by Dai et al. [130] for direct hydrogenation of CO2 to aromatics. It comprises an iron-potassium bimetal-modified alkaline Al2O3 catalyst and a phosphorus-modified ZSM-5 zeolite denoted as Fe–K/Al2O3–P/ZSM-5. The catalyst was prepared by several methods, i.e., powder-mixing, granule-mixing, dual-bed, and multi-bed techniques. Catalysts prepared by granule-mixing produce the highest aromatic products and lowest CO formation. During the reaction, P/ZSM zeolite provides acid sites which may transform lower olefin intermediates into aromatics. On the other hand, the Fe–K/a-Al2O3 served as the metal active center to hydrogenate CO2 to lower olefin intermediates. Dai et al. [130] also underlined the proximity of two components as its effect on the selectivity of the reaction.Furthermore, the appropriate loading of phosphorus on ZSM-5 (0.8 wt%) increased the amount of medium-strength acid site, resulting in higher production of aromatics and higher conversion of CO2. The high efficient conversion of CO2 to aromatic products was also obtained by utilizing a tandem Cr2O3/H-ZSM-5 catalyst [131]. The synergistic effect between two components enables aromatics selectivity to reach ∼76%, CO2 conversion of 34.5%, and there was no catalyst deactivation after 100 h, which indicates the catalyst's long-term stability. Nevertheless, the catalytic performance of the catalyst could be further optimized by adjusting the acid strength of zeolites and the mass ratio of Cr2O3/H-ZSM-5. The proximity between two catalysts was also an important factor. It played an emerging role during the direct conversion of CO2 to aromatics.Furthermore, the utilization of a zeolite-based catalyst for CO2 hydrogenation was also in the form of a bifunctional zeolite-based catalyst. This catalyst comprises two different zeolite materials which bond together. Cr2O3/H-ZSM-5@S-1 comprising a core–shell structured H-ZSM-5@S-1 zeolite capsule catalyst was found to be a promising bifunctional catalyst for CO2 hydrogenation aromatics [131]. The passivation effect of S-1 has suppressed the undesired reaction on the external site of zeolite and increased the selectivity of aromatic products. The selectivity of BTX and PX increases from 13.2% to 7.6%–43.6% and 25.3%, respectively. Noreen et al. [132] also reported utilizing bifunctional zeolite-based catalyst over CO2 hydrogenation to multibranched isoparaffins. The catalysts were configured of Na/Fe3O4 (NaFe), Zeolites SAPO-11, and ZSM-5. The zeolites have improved the long-chain-hydrocarbon selectivity by performing oligomerization on the shorter chains. The catalyst test of dual-bed reactions with SAPO-11 and ZSM-5 coupled with NaFe separately exhibited that SAPO-11 plays a pivotal role in enhancing the isomerization activities on the short-chain hydrocarbons. On the other hand, ZSM-5 boosted the isomerization activity in all aspects, especially on the long-chain hydrocracking and promoting aromatization activity. Combining triple-bed SAPO-11, ZSM-5, and SAPO-11 results in enhanced isomerization and suppressed aromatization activities. Therefore, isoparaffins' selectivity and multibranched isomers yield increased at the same time the aromatic products decreased.Ordered mesoporous silica (OMS) materials have attracted growing interest to be considered important in heterogeneous catalysis. Their large surface area highly ordered porous architecture, and the ability for metal atoms to load within the mesopores lead them to be emerging support materials for designing various catalysts. M41S was the first OMS material reported. Discovered by the scientist of Mobil corporation in 1992 [133], this catalyst exhibits well-defined hexagonal cylindrical mesopores with a relatively narrow pore size distribution. This solid possesses periodic arrangements of mesoscale porosity, but the framework pore walls are built of amorphous silica. Since the discovery of the MCM family in the early 90s, considerable progress has been achieved regarding the application of ordered mesoporous silica in various fields. The tremendous applications in the various fields have boosted the development of many other OMS materials such as the folded sheet mesoporous material-16 (FSM-16) [134], families of Santa Barbara Amorphous (SBA-n) [135,136], Fudan University Material (FDU) [137], Korea Advanced Institute of Science and Technology (KIT) [138] and anionic-surfactant-templated mesoporous silica (AMS) [139].OMS materials are generally synthesized by using a surfactant forming regularly aligned assemblies that are used as a template for the metal oxide, followed by template removal (Fig. 13 a) [140]. The flexibility of the templating methods permits the synthesis of materials with a controlled pore size and structure, controlled wall compositions, and highly interconnected surface areas, all of which allow the optimization of the material for the specific application required. Fig. 13b shows the structure of several OMS materials. SBA-15 and MCM-41 were put as examples for OMS materials that comprise two-dimensional hexagonal phases with the P6mm symmetry, composed of close-packed hexagonal arrays of cylindrical surfactant micelles [141,142]. Fm 3 ¯ m and Im 3 ¯ m phases are included in the cubic mesophases. Both symmetries are built by spherical micelles in a cubic cage structure. The OMS materials possess face-centered cubic (Fm 3 ¯ m) pore structure, such as FDU-12, is considered as close packing of spherical mesopores with every mesopore connected to 12 nearest neighboring mesopores while for body-centered cubic (Im 3 ¯ m) pore structure such as SBA-16, each mesopore is connected to 8 neighboring mesopores [143]. Other OMS materials, such as MCM-48 and KIT-16, are characterized by their bicontinuous cubic gyroid phase with the Ia 3 ¯ d symmetry. This cubic mesophase possesses networked and interconnected pores, regarded as two interwoven cylindrical channels that exhibited similar adsorption properties to 2-dimensional hexagonal materials without a pore-blocking effect. On the other hand, the mesophases with lamellar symmetry, such as MCM-50, consist of silica layers, not ordered mesoporous as others mentioned [144].OMS materials are characterized for their water-soluble, chemically, and thermally stable with mechanical strength and are toxicologically safe. These remarkable properties made OMS currently applied in countless applications. In the catalysis field, they are widely used as both catalyst and catalyst supports. They are also applied for liquid–solid or gas–solid adsorption [145–147], pollutant removal and remediation [148] as well as sensors [149] and drug delivery systems [150]. Furthermore, in energy-related devices, OMS materials have attracted much attention due to their unique properties compared to bulk materials, i.e., large surface area, tuneable pore size, and high metal loading and functionalization ability. So that, these materials are widely applied during renewable energy production, whether as electrocatalyst or photocatalysis in the renewable energy generator, i.e., hydrogen production [139,141–156] and CO2 conversion to fuels [157–171].Several works reported the utilization of OMS as a catalyst for hydrogen production. The OMS materials are introduced in several routes of hydrogen production, i.e., hydrocarbon steam and dry reforming, photo (electro)catalytic water splitting, and biomass gasification. The utilization of OMS on these three processes will be discussed in the following subsection. In addition, their catalytic performance on the renewable energy production was summarized in Table 2 .Hydrocarbon steam reforming is the production of syngas from the hydrocarbon (usually employed naphtha as feedstock) and steam. This is different to dry reforming, in which syngas was generated from the reaction between methane (CH4) and carbon dioxide (CO2). Generally, these reactions employed several noble metals (Pt, Pd, Rh, and Ru) or low-cost transition metals (Ni, Fe, Co) as the active component. During reactions, the deposition of the carbonaceous species on the active site could lead to the deactivation of the catalyst. In that sense, embedding the active sites onto suitable support, such as ordered mesoporous silica could enhance the coking and sintering resistance of the catalyst [152]. The utilization of MCM-41 and Zr incorporated MCM-41 (25Zr-MCM-41) supported Ni, or Cu catalysts was reported by Cakiryilmaz et al. [153] for steam reforming of acetic acid reaction, at 750 °C. Physicochemical characterization reveals the presence of some deformation in the ordered pore structure of MCM-41 after Zr incorporation. Furthermore, catalysts supported zirconia incorporated MCM-41 exhibited higher catalytic stability compared to the MCM-41 supported materials. The impregnation of Ni was more suitable for enhancing the catalytic performance. On the other hand, the impregnation of copper leads to the decarboxylation reaction of acetic acid. This reaction yields large quantities of methane. Recently, Liu et al. [154] loaded different compositions of LaNiO3 on MCM-41 supported as the catalyst for steam reforming of biomass in-situ tar reaction in the double fixed-bed reactor. The pure LaNiO3 was used as control catalysts. The in-situ tar was obtained from the pyrolysis of rice husk at 450 °C in the first reactor bed. Simultaneously, at second bed the LaNiO3 and XLaNiO3/MCM-41 (X = 0.025, 0.05, 0.075 and 0.1) catalysts were performed for hydrogen-rich syngas production at different reforming temperature of 500°C–900 °C and steam to carbon ratio (S/C) = 0.6-1 (w.t). The catalytic test results exhibited that 0.1LaNiO3/MCM-41 produced a higher hydrogen yield of 61.9Nm3/kg at 800 °C and S/C of 0.8. Moreover, the catalyst also experienced good stability as its ability to produce hydrogen gas composition of around 50% after five-time cycles.Furthermore, Calles et al. [155] reported other types of OMS catalysts, SBA-15, as support for Co-based catalysts in steam reforming of acetic acid. A series of Co–Cr/SBA-15 extrudates prepared by varying the binder (bentonite) content and particle size were introduced into the acetic acid steam reforming tests at 600 °C and WHSV of 30.1 h−1. The catalytic performance test of the catalysts demonstrated that the extruded particles with an effective diameter of 1.5 mm and 30 wt% of bentonite get similar conversion and hydrogen selectivity than the powder sample. Al-salihi et al. [156] introduced a series of SBA-15 supported catalysts, i.e., Co-SBA-15, Ni-SBA-15, Co–MgO-SBA-15, Ni–MgO-SBA-15, and Co–Ni-SBA-15 prepared using a one-pot hydrothermal and impregnation methods for glycerol steam reforming (GSR) reaction at reforming temperature range of 450°C–700 °C. Results from the GSR catalytic studies showed that both Co–Ni-SBA-15 prepared via impregnation and one-pot hydrothermal method were resistant to deactivation, and both yielded 100% glycerol conversion for a continuous of 40 h. Moreover, the catalyst comprises 10wt.% Co and 5wt.% of Ni, which achieved H2 selectivity of (70–78) % and (60–78) %, respectively. The effect of MgO addition was also studied, the catalyst incorporated MgO exhibited higher activity and stability.The introduction of SBA-15 prepared from renewable sources as catalyst support was reported by Abdullah et al. [157]. SBA-15 support was synthesized using extracted silica from palm oil fuel ash waste (POFA). Different impregnation techniques were used to prepare Ni/SBA-15 catalysts, i.e., the ordinary impregnation technique (Ni/SBA-15(IM)), rotary evaporator-assisted impregnation, (Ni/SBA-15 (RE)), shaker-assisted impregnation (Ni/SBA-15(SH)) and ultrasonic-assisted impregnation (Ni/SBA-15(US)). Ni/SBA-15 catalysts' performance and stability were tested in the stainless steel fixed-bed reactor setup at 800 °C for up to 24 h. The highest catalytic performance was achieved by Ni/SBA-15(US) owing to the cavitation effect of ultrasonic irradiation, resulting in better dispersion and smaller Ni particles inside the SBA-15 micelles. Both mentioned properties lead to stronger Ni–O–Si interaction, higher catalyst basicity, and suppress graphite carbon formation on Ni/SBA-15(US).Moreover, Abdullah et al. [158] also studied the effect of different amounts of Ni loaded on SBA-15(POFA) for CO2 reforming of CH4 (CRM) in a stainless steel fixed-bed reactor under a temperature of 800 °C and an atmospheric pressure of 1:1 CO2:CH4 (v:v). As observed through N2 adsorption–desorption and XRD analyses, the increase of Ni loading on SBA-15(POFA) from 1 to 5 wt.% decreased the BET surface area and crystallinity of catalyst. Moreover, the increment of Ni loading from 1 to 3% enhanced the catalytic performance of CRM. However, the catalytic performance then decreased at 5% Ni loading. This result was due to Ni's even distribution and good Ni–O–Si interaction of 3 wt.% Ni/SBA-15(POFA) as proved by TEM, FTIR, and XPS. Lowest H2/CO ratio and catalyst activity and stability of 1 wt.% Ni/SBA-15(POFA) was due to the weaker Ni–O–Si interaction and a small amount of basic sites that favor the reverse water gas shift (RWGS) reaction and carbon formation.Furthermore, Jiang et al. [159] reported the deployment of nanocomposite of mesoporous silica-supported Ni nanocrystals modified by ceria clusters (Ni–CeO2/SiO2) for CO2 reduction by CH4 to produce CO and H2 (CRM) under focused UV–visible–infrared illumination in the absence of additional heater. High production rates of CO and H2 (41.53 and 33.42 mmol min−1 g−1) and high light-to-fuel efficiency of 27.4% are obtained. The catalyst's marked CRM activity was due to highly effective light-driven thermocatalytic CRM due to good photothermal behavior of entire solar spectra of the nanocomposite. The presence of oxygen atom in ceria cluster participates in CRM reaction on Ni nanocrystals. It helps to significantly decrease the activation energies of C∗ and CH∗ the oxidation, which is CRM dominant steps. Also, the focused illumination for the experiment considerably decreases the activation energy of CRM, thus also contributing to the enhancement of the photothermocatalytic activity of the catalyst.The Ni–CeO2/SiO2 catalyst system also displayed good stability over long-term photothermocatalytic CRM reactions. The result of the photothermocatalytic durability test (Fig. 14 a and b) demonstrated that the gradual decline of catalytic performance happened during the initial 27 h of illumination time. After that, the catalyst could maintain its remarkable performance even until 100 h, as evidenced by the production reaction rates of CO and H2O of 24.61 and 27.64 mmol min−1 g−1, respectively. Moreover, in comparison with Ni/SiO2 (0.53 mmol min−1g−1), Ni–CeO2/SiO2 possesses a low carbon deposition rate of 0.034 mmol min−1g−1 (Fig. 14c). Here, it is indicated that the presence of ceria clusters on the surface of Ni nanocrystals markedly inhibits the carbon deposition rate. The carbon deposition inhibiting the ability of the ceria cluster was confirmed by HRTEM observation. HR-TEM observation demonstrated {111} facets (0.204 nm) of Ni within Ni–CeO2/SiO2 catalyst (Fig. 4d). In contrast, the surface of Ni nanocrystals in the used Ni/SiO2 catalyst is covered by graphite layers (Fig. 4e), thus resulting in its deactivation.Generally, the photocatalytic process begins with the absorption of light by photocatalyst to create electron (e−)–hole (h+) pairs, then followed by the charge separation or migration process to the surface of photocatalyst, and simultaneous hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) to overall water splitting (Fig. 15 ). Several studies showed that incorporating the photocatalyst with OMS could enhance the activity due to the benefits of the well-connected pore channel structure. For instance, the multiple light reflection in the pore channels and the enrichment of the isolated photocatalyst site in the transparent mesoporous silica matrix allow full photon exposure and utilization [160]. Therefore, the use of OMS materials in photocatalysis water-splitting has been extensively studied. Shen and Guo [161] utilized Cr incorporated, and Cr and Ti co-incorporated MCM-41 photocatalysts synthesized by hydrothermal method for photocatalytic water splitting under visible light irradiation (l > 430 nm). The catalytic examination demonstrated that the photocatalytic activities of Cr-MCM-41 and Cr–Ti-MCM-41 decreased with an increase in the amount of Cr incorporated. Compared with the Cr-MCM-41, which had the same amount of incorporated Cr, the Cr–Ti-MCM-41 exhibited much higher hydrogen evolution activities. Liu et al. [162] reported the application of CdS/M(x)-MCM-41 (M = Zr, Ti, x = molar ratio of M/Si) photocatalysts prepared via three different methods, i.e., hydrothermal, ion-exchange, and sulfidation process for photocatalytic hydrogen production. The CdS/Zr (0.005)-MCM-41 and CdS/Ti (0.02)-MCM-41 was the highest hydrogen evolution activity in triethanolamine aqueous solution under visible light (l > 430 nm) irradiation owing to the diffusion velocity of the reactants and resultants and the protection which MCM-41 provided for CdS.Furthermore, Al-doped MCM-41 (Al-MCM-41 zeolite) was introduced as catalyst support of NiCr2O4 [163]. The catalyst was synthesized via the facile sol–gel method and followed by calcination at 700 °C. According to the results of photocatalytic hydrogen evolution examination, the NiCr2O4/Al-MCM-41 hybrid demonstrated higher hydrogen generation compared with individual NiCr2O4 or Al-MCM-41. NiCr2O4/Al-MCM-41 (50%wt.) exhibited the highest hydrogen production of 8.92 mmol/g, up to 2.4 times of NiCr2O4 (3.75 mmol/g) without mesoporous silica support (Fig. 16 a and b). The excellent synergistic effect of NiCr2O4 and Al-MCM-41 zeolite is thought to be responsible for the remarkable enhancement, where the Al-MCM-41 mesoporosity increased the dispersion of NiCr2O4, thus improving the number of active sites. Also, Al-MCM-41 could enhance the charge transfer and inhibit the fast recombination of photo-generated electrons–holes during a reaction. Here, the Lewis acids and bases sites of Al-MCM- 41 zeolites act as electron acceptors and donors, respectively. Thus, in the case of NiCr2O4/Al-MCM-41, the photogenerated electrons are transferred from NiCr2O4 to Al-MCM-41 zeolite support. This electron transfer mechanism leads to the inhibition of electrons and holes recombination (Fig. 16c).In addition to MCM-41, MCM-48 was also reported as a catalyst for hydrogen production via a photocatalytic water splitting system. Zhao et al. [164] also reported the application of CdS incorporated MCM-48 prepared via the ion-exchange method for photocatalytic hydrogen evolution under visible irradiation. The SEM observation revealed that the cubic phase of MCM-48 was destroyed during the ion-exchange reaction and sulfidation process. However, the CdS/MCM-48 exhibited high photocatalytic activity for hydrogen evolution. Zhao et al. [165] also reported using Ti-modified MCM-48 (Ti-MCM-48) for photocatalytic hydrogen evolution from the methanol–water mixture under UV irradiation. The catalyst achieved high activity without platinum cocatalyst, which is usually essential to TiO2 photocatalyst for hydrogen evolution. Moreover, Peng et al. [166] introduced Ti-MCM-48 as support for CdS catalyst. The hybrid catalyst was examined for visible light-illuminated photocatalytic hydrogen production via water splitting without Pt as a co-catalyst. The highest hydrogen production rate and the most apparent quantum yield were as high as 2.726 mmol/h/gcatalyst and 36.3%, respectively.Moreover, Sanches et al. [167] introduced Ti modified SBA-15 photocatalyst synthesized using two titanium sources (Ti tetra-isopropoxide and TiO2–P25) with two Si/Ti molar ratios (20 and 40) for hydrogen production via water splitting. SBA-15 was introduced to enhance TiO2 efficiency. TiO2 prepared from isopropoxide source exhibited highly dispersed Ti-nanoclusters and Ti-coordinated species (penta-, hexa-, or octahedral) in the SBA-15 hexagonal mesoporosity. On the other hand, TiO2–P25 introduction led to Ti dispersion into the mesostructure and octahedrally coordinated into the SBA-15. In addition, the increment of Si/Ti molar ratios induced more concentrated Ti coordinated isolated species and enhanced the photoactivity of the catalyst prepared from Ti tetra-isopropoxide as Ti source. The Al-modified SBA-15 was also utilized as catalyst support toward the water-splitting process for hydrogen generation/storage under visible light irradiation [168]. Iron oxide NPs (5 %wt.) were loaded onto Al (25 %wt.)-modified SBA-15 supports via microwave-assisted and ultrasonic-assisted routes.Interestingly, the different applied loading routes result in different catalytic properties, where the ultrasonic-assisted loading technique exhibited more prevailing behavior through the photocatalytic water-splitting reaction. In addition to SBA-15, Macías-Sánchez et al. [169] introduced mesoporous silica SBA-16 as catalyst support of Cd(1-x)ZnxS solid solutions (x = 0.05–0.3) for hydrogen production from water splitting under visible light. Cd(1-x)ZnxS nanoparticles are unevenly distributed on both external surfaces and within the pore network. The increment of Zn loading up to x = 0.2 leads to an enhancement of the bandgap energy, which inhibits the enhancement of the bandgap energy, which inhibits the enhancement of the bandgap energy, which inhibits fast photorecombination. Subsequently, H2 evolution was also improved.Biomass gasification is also a promising route for renewable hydrogen production. In that sense, several reports have demonstrated the successful utilization of ordered mesoporous silica for this reaction. It acts as efficient support for various metal nanoparticles, acidic and basic sites, that provide remarkable catalytic performance [32]. Moreover, it plays a vital role in increasing the catalyst efficiency by allowing the high dispersion of metal catalyst, providing large surface areas and affinity for the formation of strong metal-support interaction, and high hydrothermal stability [170]. Wu et al. [34] reported the utilization of Ni(x)/MCM-41 (x = 5, 10, 20, and 40 wt.%) for the steam pyrolysis-gasification of wood sawdust for hydrogen production in a two-stage fixed bed reaction system. At the first stage, the wood sawdust was pyrolyzed, and at the second stage, the derived products were gasified to produce hydrogen. The 5–20 wt.% Ni/MCM-41 catalysts exhibited homogeneously dispersed NiO particles inside the pores; however, the 40 wt%. Ni/MCM-41 experienced more bulky NiO particles (up to 200 nm particle size) detected outside the pores. Moreover, the increment of Ni loading from 0.5 to 4.0 wt.% increases gas production and hydrogen production from 40.7 to 62.8 wt.% and 30.1 to 50.6 vol.%, respectively. This catalyst also exhibited low coke deposition at the range of 0.5–4.0 wt.%.Furthermore, Wu et al. [171] detailed their research to describe the effect of different location of Ni particles (inside; I-series and outside; O-series) at the pores of the supports. Interestingly, the I-series catalyst comprises 20%wt. Ni loading generated more gas and hydrogen and a lower oil fraction (21.26 mmol H2/gwood) than the O-series catalyst with 20 %wt. Ni loading (16.46 mmol H2/gwood). The better performance of the I-series catalysts is thought to be higher interaction between reactants and active Ni sites inside the pore of the supports.The modern world needs to find an alternative fuel that can replace nonrenewable fossil fuels. Being a versatile, sustainable, efficient, and clean energy carrier, hydrogen has the potential to play that role. CO2 reforming of CH4 (CRM) is a progressing technology for hydrogen production. CO2 reforming of methane is among promising renewable routes for the production of syngas. Kaydough et al. [172] employed SBA-15 contained two series of Ni (2.5–7.5 wt%) and Ce (6 wt%) for dry reforming of methane. In the Ni enriched samples, the NiO particle was formed and entrapped in the porous channels to preserve the porosities. Also, the Ce-modified samples demonstrated highly dispersed CeO2 nanoparticles. The synthesized catalysts show high activity and selectivity towards H2 and CO at atmospheric pressure with full CH4 conversion below 650 °C. Moreover, the catalyst also exhibited high stability as it demonstrated low carbon amount formation and limited sintering of the Ni nanoparticles after prolonged tests performed at 500 °C for 12 h.Furthermore, Ibrahim et al. [173] examined the effect of different catalyst promoters on the catalytic performance of 5%Ni + 1%x/MCM-41 (x = Ga, Gd, Sc, Ce, or Cs) catalysts for the production of synthetic gas. The chemical analyses demonstrated that the promoters could increase the metal-support interactions, thus improving the catalytic performance. Moreover, the introduction of Gd, Sc, Cs, or Ce-promoted catalysts yielded the lowest amounts of coke formation. However, the catalyst exhibited different CH4 and CO2 conversions. The introduction of 1% of Ga, Gd, or Ce catalyst promoters increased CH4 and CO2 conversions by 38%.On the other hand, 1% Sc or Cs decreased CH4 and CO2 conversion by 18% or 93% and 16% or 92%, respectively. Additionally, the effect of organic promoters for Ni-MCM-41 was also interesting to be discussed. Xu et al. [174] reported the utilization of alcohol-promoters Ni-MCM-41 catalyst for CRM. The alcohol-during impregnation-promotes Ni2+ species into the channels of MCM-41, thereby strengthening the metal-support interaction. Also, the introduction of alcohol decreases the particle size of Ni. It increases the surface adsorbed oxygen species over the surface of the support, thus promoting the coke resistance of the catalysts.The advantages of the structural properties of OMS have also had a positive effect on CO2 hydrogenation. It has been investigated as efficient support owing to the high surface area, which provides a high catalyst dispersion and the ordered structure that confined the metals nanoparticle catalyst, inducing a favorable reaction pathway for high selectivity to the desired product [175]. Kiatphuengporn et al. [176] carried out CO2 hydrogenation to alcohols over Cu-based MCM-41 catalyst. The series of Cu (10 wt.%) based catalysts with different pore characteristics of MCM-41 supports including unimodal (SS) and bimodal (T) pore structures, loading amount of Fe (0, 0.5, and 3 wt.%), and reaction temperature on the catalytic performance were introduced. The result showed that the activities of CO2 hydrogenation over bimodal support catalysts were higher than unimodal support catalysts. The bimodal 3 wt.% Fe–10Cu/MCM-41 catalyst exhibited the highest CO2 conversion of 20.8% (at 350 °C), alcohol selectivity of 80–99% (at 160–200 °C), and highest TOF of alcohols and CO of 1.08 × 10−25 and 5.47 × 10−25 mol surface metal atom−1 min−1, respectively. These outstanding catalytic activities could not be indispensable to the pore characteristics of the supports, where the presence of larger mesopore promote the formation of metals with larger sizes, resulting in weaker metal–support interaction of which more favorable in this reaction.Moreover, Kiatphuengporn [177] interestingly examined the effects of magnetic field orientation and magnetic flux density on activity and selectivity of ferro/ferrimagnetic xFe/MCM-41 for CO2 hydrogenation. The result demonstrated that the magnetic field promotes the reactant adsorption and surface reaction over the magnetized Fe catalysts, resulting in the lower apparent activation energy and the increase of selectivities to hydrocarbons and CH3OH. Fig. 17 a illustrates the proposed mechanism of CO2 hydrogenation without and with the magnetic field. Without the magnetic field, CO2 is converted into CH4 and CO products. On the other hand, CO2 is selectively converted to C2–C3 hydrocarbons and methanol under magnetic field-induced reaction. Moreover, the presence of an external magnetic field, especially in the north-to-south (N–S) direction, could significantly improve CO2 conversions by 1.5–1.8 times (Fig. 17b); nevertheless, the activation energy was 1.12–1.15 times lower than those without a magnetic field. The increase of CO2 conversion in the magnetized catalyst might be attributed to the advancement of CO2 adsorption of the catalyst, then the apparent activation energy of the reaction is reduced. In addition to the advancement of higher CO2 conversion, applying external magnetic field also result in higher selectivity to C2–C3 hydrocarbons and methanol (Fig. 17c). These findings indicated that the magnetic field might facilitate the chain growth probability of the reaction product to form C2–C3 hydrocarbons and methanol. Here, the optimum CO2 conversion and selectivity to CH3OH and C2–C3 hydrocarbons were achieved at the condition of the presence of 27.7 mT of an external magnetic field in N–S direction.The utilization of MCM-41 based catalyst was also reported by Taherian et al. [178]. Here, different amounts of Yttria (Y2O3) were doped with Ni over MgO-modified MCM-41 support. The study found that the addition of NiO and promoters over MCM-41 support doesn't alter its mesoporous structure. However, it may decrease the BET surface area, and pore volume as the presence of both NiO and MgO could cause partial blockage of the pores. The presence of Yttria promotes higher Ni dispersion and that of improving CO2 adsorption sites on basicity sites. The result exhibited that all catalysts which contained Yttria showed improved catalytic activity compared to those of Ni/MgO-MCM-41. Also, the addition of yttria could lessen the temperature of maximum conversion and selectivity. Here, the highest catalytic performance was owned to catalyst with 2 wt% of yttria. The catalyst possesses CO2 conversion and methane selectivity as high as 65.55% and 84.44% at 673 K, respectively, and high stability after 30 h.Recently, the utilization of MCM-41 catalyst support for CO2 hydrogenation was reported by Seker et al. [179]. The hybrid MCM-41-supported tungstophosphoric acid (TPA) catalyst with a commercial CuO–ZnO–Al2O3 catalyst was performed in the dimethyl ether (DME) synthesis by CO2 hydrogenation. Different amounts of TPA loading of 30, 40, 60, and 80 wt% lead to various TPA cluster structural distortions and the change of acid properties. A suitable amount of TPA loading could result in a high density of acid sites, thus improving catalytic performance. Here, the optimum condition was obtained at TPA loading of 60 wt% with CuO–ZnO–Al2O3:TPA/MCM-41 = 4:1 at 40,000 mL CO2 gcat −1h−1 and H2:CO2 = 3:1 at 250 °C and 45 bar. At these conditions, the DME production rate was 1551.5 gDME kgcat −1h−1. Moreover, Wang et al. [180] utilized Ni/xMg@MCM-41 (x = 0, 0.05, 0.1) catalysts synthesized via a novel in-situ one-pot method for CO2 methanation. The effect of Mg concentration was examined. Interestingly, the introduction of Mg into the support framework can significantly improve the basicity of the catalyst, which induces the adsorption and activation of CO2. So, all the synthesized catalysts demonstrated good thermal stability and catalytic activity. However, the best catalytic performance was owned to Ni/0.05 Mg@MCM-41 catalyst as it demonstrated the highest low-temperature reaction activity.Besides MCM-41, SBA-15 also demonstrated good catalytic performance for CO2 hydrogenation reactions. Lin et al. [181] reported the utilization of SBA-15 as support for three kinds hybrid catalysts of CuO–ZnO/SBA-15 (CZ/SBA-15), CuO–ZnO–MnO2/SBA-15 (CZM/SBA-15) and CuO–ZnO–ZrO2/SBA-15 (CZZ/SBA-15). The catalysts were performed for catalytic hydrogenation of CO2 to methanol on a fixed bed reactor. The results show that the introduction of metal oxide in the catalyst changes the pore size and specific surface area of the SBA-15 molecular sieve support. The utilization of CuO–ZnO–ZrO2/SBA-15 achieved the optimum methanol selectivity of 25.02%. It is 28% and 136.9% higher than CuO–ZnO/SBA-15 and CuO–ZnO–MnO2/SBA-15, respectively. Moreover, Mureddu et al. [182] utilized Cu/ZnO@SBA-15 and Cu/ZnO/ZrO2@SBA-15 nanocomposites synthesized by innovative impregnation-sol–gel auto combustion combined strategy for carbon dioxide hydrogenation to methanol. The composites comprise different active phase loading (20 and 35 wt.%) and Cu/Zn molar ratio (1.0–2.5 mol mol−1). According to the result of characterization techniques, the active phase of the catalyst was highly dispersed into/over the well-ordered mesoporous channels, especially at low loading and low Cu/Zn molar ratio. Thus, this result corroborated the catalytic results, where the catalyst with the lowest Cu/Zn molar ratio (1.0 mol mol−1) exhibits the best catalytic performance with a STY of methanol of 376 mgCH3OH·h−1·gcat −1. This obtained STY value was much higher than the unsupported catalyst (10 mgCH3OH·h−1·gcat −1).Furthermore, Li et al. [183] reported utilizing monometallic Pd or Ni/SBA-15 and bimetallic Ni–Pd/SBA-15 alloy catalysts with different ratios of Ni/Pd as a catalyst for CO2 methanation. The catalytic examination demonstrated that bimetallic Ni–Pd/SBA-15 catalysts owned higher catalytic activity than monometallic Pd- or Ni/SBA-15. The bimetallic catalyst exhibited the highest catalytic performance with Ni:Pd atom ratio of 3:1, which yielded 0.93 mol CH4 per mol CO2 at 430 °C. This outstanding performance was due to Ni and Pd's synergistic effect with the high dispersion of active sites. Another Pd-based SBA-15 supported catalyst was reported by Jiang et al. [184]. In their work, a series of Pd/In2O3/SBA-15 catalysts prepared by the citric acid method was performed toward CO2 methanation. The presence of Pd species helps to facilitate H2 dissociation. On the other hand, the introduction of In2O3 induced CO2 activation, resulting in the promotion of high-efficiency conversion of CO2 to methanol with maximum methanol selectivity of 83.9%, CO2 conversion of 12.6% corresponding to a space-time-yield (STY) of 1.1 × 10−2 mol h−1·gcat −1 under reaction conditions of 260 °C, 5 Mpa and 15,000 cm3 h−1·gcat −1. In addition to an ordinary rod-like one, the fibrous type of SBA-15 was also reported as catalyst support for CO2 methanation [185]. The fibrous type of SBA-15 (F-SBA-15) was obtained by transforming the rod-like SBA-15. The obtained Ni/F-SBA-15 catalyst exhibited higher superior catalytic performance with CO2 conversion of 99.7%, and CH4 yield of 98.2% compared to those of rod-like Ni/SBA-15 with CO2 conversion of 91.1%, and CH4 yield of 87.5%. Also, Ni/F-SBA-15 exhibited higher catalytic stability and coke resistance than Ni/SBA-15. The distinctive SBA-15 morphology led to a higher homogeneity of finer Ni, which reinforced the Ni–F-SBA-15 interaction and increased the amount of moderate basic sites.Photoreduction of CO2 to hydrocarbons is one of sustainable energy technology which not only mitigates emissions but also provides alternative fuels. However, one of the largest challenges is to increase the overall CO2 photo-conversion efficiency when water is used as the reducing reagent. The use of ordered mesoporous silica to construct the isolated photoactive centers in porous matrices or to provide the high surface area for nanoparticles photocatalyst loading is an effective strategy to develop a highly active photocatalyst [186]. It is clear that the activity and selectivity of photocatalysts for different products strongly depend on the chemical nature of the supports. In brief, highly dispersed titanium dioxide in mesoporous silica materials (KIT-6, FSM-16, SBA-15, and TUD-1) leads to relatively high yields of CH4 or/and CH3OH, which makes the OMS-supported photocatalysts a promising candidate for CO2 photoreduction.A number of works have been reported the utilization OMS as support for photocatalyst for CO2 reduction. Li et al. [187] utilized mesoporous silica-supported Cu/TiO2 nanocomposites obtained through a one-pot sol–gel method for the photoreduction reaction of CO2 (Fig. 18 a and b). The catalyst test was conducted in a continuous-flow reactor using CO2 and water vapor as the reactants under the irradiation of a Xe lamp. The presence of high surface area OMS support (>300 m2/g) greatly improved CO2 photoreduction by improving TiO2 dispersion and increasing adsorption of CO2 and H2O on the catalyst (Fig. 18c). The addition of Cu species significantly improved the overall CO2 conversion efficiency and the selectivity to CH4 compared to TiO2–SiO2 catalysts without Cu whose CO is the primary product of CO2 reduction. The optimum production rates of CO and CH4 achieved by using 0.5%Cu/TiO2–SiO2 as their produced CO and CH4 with production rate of 60 and 10 mol gcat−1 h−1, respectively; the peak quantum yield was calculated to be 1.41% (Fig. 18d). Moreover, Sasirekha et al. [188] and Yang et al. [189] employed the metal-doped TiO2 with mesoporous silica for CO2 photoreduction. Ordered mesoporous silica leads to the enhancement of the reaction rate because of the highly dispersed photocatalyst over the support and the improvement CO2 and H2O adsorption on the support surface.The improved surface area and better dispersion of the active sites were also demonstrated for the introduction of SBA-15 support to Ce–TiO2 photocatalyst for CO2 photoreduction reactions [190]. It shows that Ce–TiO2 dispersion on the silica matrix (Ti:Si = 1:4) is responsible for the enhanced textural properties of the catalyst compared to pristine TiO2. In addition, the unique mesoporous structure was one of the contributing factors for highly localized CO2 concentration near the TiO2 surface, thus leading to higher photocatalytic activity. Moreover, Tasbihi et al. [186] reported the utilization of Pt/TiO2–COK-12 photocatalysts prepared by a deposition–precipitation method for the photocatalytic reduction of CO2 under UV light in a continuous flow gas-phase photoreactor. Pt/TiO2 was used as a comparison. Carbon monoxide is the major product obtained over TiO2 photocatalyst regardless of the presence of COK-12 as mesoporous support, while the addition of an appropriate amount of Pt to the catalysts leads to CO2 reduction towards CH4 formation, with selectivity as high as 100% in optimum loading (Fig. 19 a). Fig. 19b exhibited the mechanism of CO2 photoreduction reaction over TiO2 and Pt/TiO2. Here, the total reduction of CO2 into CH4 was promoted by the strong chemisorption of CO over Pt active site. Also, the phenomena could prevent CO from being the main product as it is the case for bare TiO2, either unsupported or supported onto mesoporous silica. Moreover, introducing COK-12 support on Pt/TiO2 catalysts maintains the selectivity of the reaction towards CH4 and further improves the overall activity of the Pt/TiO2 materials, as observed in the photocatalytic tests.More recently, Fu et al. [191] developed new spherical hybrid materials cobalt oxide-coated spherical mesoporous silica for visible-light-driven photocatalytic reduction of CO2 to CO (Fig. 20 a-o). Among all the catalysts tested, CoO/s-SBA-15 exhibited the best performance with an average generation rate of CO of up to 25,626 mol h−1 g−1 with a selectivity of 83.0% after 2.5 h illumination time under the white LED lamp (Fig. 20n). It achieved maximum production of CO as high as 71.69 μmol and selectivity of 84.8% when the 3% wt. In comparison with Co3O4 catalysts, the intrinsic properties of CoO catalyst towards CO2 reduction to CO were also examined using DFT calculation (Fig. 20o). The result exhibited that the conversion of OCOH∗, a chemical species generated after the transfer of hydrogen atom to O atom of elongated CO bond in CO2, to CO∗ and OH∗ using CoO catalyst exhibited a lower energy barrier of 1.51 compared to 3.76 eV for Co3O4. Moreover, the total energy change for Co3O4 was 1.52 eV larger than that of 0.24 eV for CoO. This result corroborated the result of the catalytic test, in which the reaction pathway is favored in the case of using CoO as the catalyst. In this work, the role of the support is also examined. This study shows that the presence of interaction between the Si–OH functional groups of the support and reaction substrate molecules around the catalytically active center leads to the enhancement of CO2 reduction to CO.Metal–organic frameworks (MOFs) are a class of nanoporous materials consisting of metal ions or clusters coordinated to organic ligands to form two- or three-dimensional crystal structures [192,193]. Analog to that of zeolite, the porosity of MOF is dictated by its secondary building units (SBUs) which are the basic structure of the metal ions or clusters. The SBUs are linked in an infinite lattice by the organic ligands, often referred to as linkers. Fig. 21 a shows a schematic representation of the formation of MOF from its building blocks. By designing the SBUs and the linkers, the structure of MOF can be tuned easily. In fact, the designability of MOF has been emerging a new branch of chemistry, i.e., reticular chemistry [194]. Numerous researchers have been pursuing MOF development in the last decades, as shown by steadily increased publications on the topic. Furthermore, approximately 70,000 crystal structures of MOF have been stored in the Cambridge Structural Database [195]. Fig. 21b shows the crystal structure of the commercially available MOF— HKUST-1.The wide variety of structures with tuneable physicochemical properties leads MOFs to be applicable in various fields. With a pre-designable porous size and shape, MOF is an impeccable choice of material for separations and selective catalysts. The surface area of MOF could be tailored up to >14,000 m2 g−1 theoretically [196], which is much higher than those of zeolites and active carbons. This feature may tackle the problems among gas and energy storage materials. The synergy between the metal ions and organic ligands is also a great interest in sensor development. In the field of bio-related materials, the potential of MOF as drug delivery [197] and a carrier of genetic material [198] also has been investigated with quite promising results. However, some may be concerning the toxicity of the metal moiety [199]. Furthermore, the current progress shows the development of MOF with anisotropic crystal structures, enabling it to possess sequences of unique chemical properties. A comprehensive review by Xu et al. on anisotropic reticular chemistry is available elsewhere [200].In catalysis, MOF has gone through a continued enhancement since firstly studied in the 1990s [201]. Thermal and chemical stability were the main issues in developing MOF as a catalyst [202]. MOFs suffered a structural collapse at a high temperature and in the presence of certain functional groups, organic solvents, or moisture. Nevertheless, the development of MOF catalysts was continuously pursued due to their irreplaceable unique properties. The combination of designable pores and intrinsic Lewis acidity out of the metal ions is a promising feature for catalysts.Moreover, additional catalytic active sites could be generated as well by introducing metal nanoparticles (NPs) into the pores. The insights on the MOF/NPs catalysts have been reviewed by D. Astruc's group recently [43]. Nowadays, most MOF structures can endure severe conditions—to certain extents—during the catalytic process. In the following sub-sections, we will highlight the remarkable success of MOFs as catalysts, particularly for the generation of renewable fuels.In addition to MOF, the rapidly growing research on reticular chemistry also brought up a new type of material, namely covalent organic frameworks (COFs). COF is often considered the “cousin” of MOF due to its similarity in designability, porosity, and high surface area. Unlike MOFs, however, COFs are built up out of entirely organic molecules as the building blocks. Fig. 21c and d shows the building up of the COF structure from its components and a COF structure with hexagonal pores, respectively. The topology of COF is governed by the origin of the building blocks and their arrangement [203]. For instance, a combination of building blocks with C3 symmetries resulting in a hexagonal skeleton, a combination of C2 and C4 symmetries giving in a tetragonal skeleton, a combination of C2 and C6 symmetries giving in a trigonal skeleton, and so on. Various structures and pores can be built up extensively by varying the symmetries, sequences, orientations, and length of the building block. Commonly, the skeletons of COFs are constructed in a two-dimensional lattice due to planar organic molecules. The 2D skeletons are then assembled in 3D construction material via π–π stacking [204].COFs are potentially applied in many areas such as energy storage, electronic devices, biomaterials, and catalysis [204]. In the field of electronic devices, COF showed a promising result and unique characteristics as a light-emitting diode (LED) [205] and organic semiconductor [206], thanks to their highly crystalline and porous structure. Note that conventional OLEDs and organic semiconductors are typically fabricated using semi-crystalline polymers. As a biomaterial, COFs are gaining more attention than MOFs due to the absence of metal moieties. Although toxicity is not always related to the presence of metals and some organic molecules that can be highly toxic due to the ability to penetrate cell membranes, COFs are considered as a potential candidate for drug delivery [207] and other bio-related materials [208]. In catalysis, COFs are commonly combined with metal nanoparticles (NPs) [209]. Pores in COFs are selective to some substrates because of the size and the nature of functional groups in the building blocks. In typical reactions requiring both acid and base catalytic sites, metal nanoparticles provide the Lewis acid sites, and the COF framework acts as the base sites. Therefore COF/NPs can be a powerful catalyst for the generation of renewable fuels.Even though there are many similarities between MOFs and COFs, the chemistry is quite different. MOFs are based on metal coordination chemistry which the type of metal orbitals and the type donor ligands are essential [193]. On the other hand, COFs are based on polymer chemistry, in which C–C bonds and π–π stacking play a crucial role [204]. The differences certainly affect the techniques of synthesis and characterization of each material. Reviews on the synthesis techniques of MOFs [210] and COFs [211] have been available and are beyond the scope of this review. In the following sub-sections, the use of MOFs and COFs as catalysts in generating renewable fuels is elaborated alternately.The role of MOF and COF catalysts in the production of renewable fuels could be classified into the following routes: hydrogenation of CO2 or CO, hydrodeoxygenation of biomass, hydrogen evolution reaction (HER), and photocatalysis. The latter might also include hydrogenation and HER, but we would separate the discussion for photocatalysis since the mechanism is quite different. In summary, we have tabulated the role of MOF and COF on the production of renewable energy in Table 3 .Production of hydrocarbons, alcohols, or carboxylic acids through hydrogenation of CO2 using MOF catalysts has been extensively studied. The high affinity of MOFs in capturing CO2 has been demonstrated in numerous studies [212–214], thanks to the remarkably high surface area and porosity. In the successful hydrogenation of CO2, precious metals and/or other transition metals are typically assembled into the MOF catalysts. Xu et al. [215] reported a selective conversion of CO2 to CH4 using Ru/Zr-MOF catalyst in a plasma-assisted system. Under the optimized condition and molar ratio of CO2:H2 = 4:1, a yield of 39.1% with 94.6% selectivity to methane could be achieved. The MOF catalyst also showed good stability under a plasma environment, as indicated by preserved crystallinity and morphology after catalytic reactions. Zhao et al. [216] developed another Zr-based MOF (UiO-66) embedded with Ni nanoparticles. The Ni@UiO-66 catalyst showed excellent activity with 57.6% conversion of CO2 and 100% selectivity toward methane. The absence of precious metals in this catalyst is certainly favored in the economic viewpoints. The catalyst was stable up until 100 h under a reaction temperature of 300 °C. Introducing Ni nanoparticles into another MOF, MIL-101, showed a remarkable performance in selective methanation of CO2 [217]. Zhen et al. optimized the reaction conditions, studied the thermodynamic, and proposed a reaction mechanism in this work. The 20Ni@MIL-101 catalyst could achieve 100% CO2 methanation under a temperature of 300–340 °C with 100% selectivity. According to the DFT calculation, the key to the outstanding performance is that the MIL-101 framework enables Ni particles to be exposed on the crystal plane of 111, which has a lower potential energy barrier (10 kcal/mol) for CO2 dissociation into COads and Oads compared to those of other planes. The potential of COF catalysts for selective CO2 reduction has also been studied in recent years. Wang et al. [218] investigated the catalytic performance of triazine-based COF for selective methanation of CO2. Using an electroreduction system, a perfluorinated covalent triazine framework (FN-CTF-400) catalyst converted CO2 to CH4 with a faraday efficiency of 99.3%. The DFT study suggests that fluorine doping is essential to improve electrocatalytic efficiency since it regulates the activity of N and enhances the conductivity of CH4 production.To generate liquid fuels, CO2 can be directly converted into methanol. Yin et al. [219] developed a PdZn alloy on ZnO (PZ8-400) catalyst out of direct pyrolysis of palladium at zeolitic imidazole framework-8 (Pd@ZIF-8) for methanol synthesis. The ZIF-8 structure enables the growth of Pd particles at the sub-nano level, which then facilitates the formation of small-sized PdZn particles well-dispersed on the surface of oxygen defect ZnO. The best conversion of CO2 to methanol over the PZ8-400 catalyst was 15.1%, with a selectivity of 54.2%. A fundamental study on the kinetic and mechanistic of CO2 to methanol conversion over Pt nanoparticles encapsulated in Zr-based MOF, Pt@UiO-67, was investigated by Gutterød et al. [220]. Based on the spectroscopy analysis and DFT modeling, it was revealed that methanol formation is taken place at the interface between Pt nanoparticles and defect Zr nodes through the formation of formate species on Zr nodes. Fig. 22 a shows the 3D representation of the reaction pathways. Under total pressure of 8 bar and temperature of 170 °C, approximately 1% conversion of CO2 led to the formation of methanol (≈20%) and methane (≈80%). Conversion of CO2 to methanol over another Zr-based MOF, UiO-66, was studied by Stawowy et al. [221]. After an exchange of less than 50% Zr content in the UiO-66 framework with Ce, the selectivity toward methanol production was enhanced from 3.5% to 28.7%. Further improvement of selectivity up to 59% was achieved by introducing Cu nanoparticles onto the UiO-66 (Ce/Zr) framework. Zeng et al. [222] reported an interesting technique to activate the Cu center at Ru-UiO MOF for excellent and controllable catalytic performance. Using light as a trigger, Cu0@Ru-UiO—which is selective (99%) to methanol production—can be converted to CuI with 99% selectivity to ethanol production. Fig. 22b shows the controllable catalytic selectivity of Cu@MOF catalysts. An alternative product of CO2 conversion into liquid fuels other than methanol is formic acid (HCOOH). Notwithstanding, in recent years, most of the study on MOF catalysts was still focusing on DFT calculation and simulation [223,224].Another route to produce renewable fuels is through Fischer–Tropsch synthesis (conversion of syngas—, i.e., CO and H2—to hydrocarbons). Typically, MOFs were employed as the precursor material to develop highly dispersed fine metal nanoparticles such as Co, Fe, and Ni to catalyze Fischer–Tropsch synthesis. Sun et al. [225] developed a highly loaded Co on silica (Co@SiO2) using ZIF-67 as the hard template. The Co@SiO2 catalyst prepared using MOF precursor could achieve CO conversion of >15% with >90% selectivity toward C5+ products. Qiu et al. [226] further investigate the use of MOF as a precursor by comparing nitrogen-rich (ZIF-67) and nitrogen-free MOF (Co-MOF-74) to prepare Co-based catalysts. The catalyst prepared using nitrogen-rich MOF (Co@NC) exhibited 10% CO conversion with 31% selectivity to C5+ products. On the other hand, the nitrogen-free MOF (Co@C) exhibited 30% CO conversion with 65% selectivity to C5+ products. Ping et al. used Ni/UiO-66 to prepare the fine structure of Ni/ZrO catalyst through impregnation followed by calcination [227]. The Ni/ZrO catalyst was able to carry out selective production of CH4 from CO reactant less than 10 ppm, which was considered as a non-poisonous concentration of CO, with a selectivity of >50%. Recently, Panda et al. [228] incorporated Pt to Co nanoparticles catalyst using MOF as the phase controller. The phase of Co nanoparticles (FCC or HCP) on the Pt@Co/C catalyst derived from Co2(bdc)2 (dabco) could be controlled by adjusting synthesis conditions. CO conversion through the catalyst was at around 35% with high selectivity (≈70%) toward C1 product.One of the most effective ways to convert biomass into fuels is through a hydrodeoxygenation (HDO) reaction. Typical biomass such as lignin is a big organic polymer containing a high amount of oxygen. Thus, depolymerization through transfer hydrogenation and deoxygenation is required to decrease the oxygen content so that the fuel quality can be improved. HDO of biomass can generate hydrocarbons with comparable quality to that of fossil fuels. In recent years, MOF catalysts were employed to realize this reaction with high efficiency. Zang et al. [229] developed palladium nanoparticles well dispersed onto amine-functionalized UiO-66 MOF catalyst (Pd@NH-UiO-66) for selective conversion of vanillin—a typical model of lignin—into 2-methoxy-4-methyl phenol. A 100% conversion of vanillin was achieved using the catalyst under mild conditions. The catalyst was stable, at least for the 6-cycle of stability test. The authors pointed out that the remarkable performance is attributed to the synergic of well-dispersed Pd nanoparticles and the presence of the amino group. Bakuru et al. studied the same reaction—selective conversion of vanillin into 2-methoxy-4-methyl phenol—using a similar Pd@UiO-66(Hf) catalyst without amine functionalization (Fig. 23 ai) [230]. Note that the UiO-66(Hf) contains μ3-OH groups. The catalyst also showed excellent performance with vanillin conversion of >99% and >99% selectivity (Fig. 23aii). The synergic between Pd nanoparticles and the Brønsted acid sites are responsible for the remarkably good performance (Fig. 23aiii). Phan et al. [231] demonstrated a similar effect on the synergetic of metal nanoparticles and Brønsted acid sites for converting fatty acid into heptadecane (Fig. 23bi). Using a phosphoric acid-enhanced Pt-encapsulated MOF catalyst (Pt/P@MIL101-Cr), 95% of fatty acid conversion could be achieved with 75.5% selectivity (Fig. 23bii). The potential of COF in HDO of biomass is, unfortunately, not widely explored yet. In COF, the Brønsted acid could be designed as the intrinsic structure of the lattice instead of through post-functionalization. Thus, COF catalysts may show outstanding performance as well in HDO of biomass.As an alternative to synthetic hydrocarbons, hydrogen energy has gained much attention since it promises a cleaner combustion system. MOF-derived materials are recently extensively studied for preparing electrodes in an electrocatalytic reactor for hydrogen production through HER. MOF-derived materials can substitute noble metals as electrodes and generate unique porous structures that increase the efficiency of hydrogen adsorption during hydrogen evolution. Nivetha et al. [232] prepared a Meso-Cu-BTC MOF catalyst to perform hydrogen evolution in 1M NaOH solution. Compared to those of conventional Pt electrode (onset potential = 0.002 V, overpotential = 79.0 mV, Tafel slope = 33.41 mV dec−1), the electrocatalyst developed in this study showed a faster kinetic in producing hydrogen (onset potential = 0.025 V, overpotential = 89.32 mV, Tafel slope = 29.0 mV dec−1). Xu et al. [233] developed another method by mixing Ni–Co MOF, ammonium molybdate, and thiourea to prepare a Ni0.15Co0.85S2@MoS2 electrocatalyst via hydrothermal synthesis. Using 1M KOH as the reactant, ultrahigh HER was performed with an overpotential and Tafel slope of 79 mV and 52 mV dec−1, respectively. Incorporating Ni and Co from the MOF precursor successfully enhanced the electrocatalytic performance of MoS2—a potential non-precious metal electrocatalyst that often-suffered low conductivity. Another recent study on MOF-modified molybdenum sulfide electrocatalyst for HER was reported by Do et al. [234]. Using a facial solvothermal method, MoSx was anchored on the surface of rod-like Co-MOF-74 particles, resulting in MoSx/Co-MOF-74 composite as an electrocatalyst. The catalyst could carry out a high HER performance at the optimized conditions with a low onset potential of −147 mV and a Tafel slope of 68 mV dec−1. An XPS analysis showed the formation of CoMoS species, which may correspond to the decreasing electron transfer resistance of Co-MOF-74.The potential of COFs and their derived materials as an electrocatalyst for HER also has been investigated in recent years. Siebels et al. [235] studied the electrocatalytic performance of Rh nanoparticles supported on covalent triazine framework-1 (Rh@CTF-1) and compared it to commercial Pt/C (Fig. 24 a). The Rh@CTF-1 electrocatalyst showed overpotential and onset potential of −58 mV and −31 mV, respectively, while the commercial Pt/C exhibited an overpotential of −77 mV with an onset potential of −38 mV. The results demonstrated the great potential of precious metals/COF electrocatalyst to be applied to HER. As an alternative to using precious metals, Qiao et al. [236] developed CTF@MoS2 electrocatalysts to proceed HER. The catalyst demonstrated excellent performance with an overpotential of 93 mV and a Tafel slope of 43 mVdec−1. The study also suggests that the inherent π-conjugated crystal channels in the CTF support mass diffusion and electron transmission during the HER process. The further interesting potential of COFs was demonstrated by Zheng et al. [237] as the CTF-based material could be electrocatalytic active without any metal content. A nitrogen-doped hollow carbon nanoflowers (N-HCNFs) were prepared using CTF and melamine-cyanuric acid (MCA) as the precursors (Fig. 24b). In carrying out HER in acidic media, the N–HCNF electrocatalyst exhibited an overpotential of 243 mV with a Tafel slope of 111 mVdec−1. Metal-free catalysts are certainly a great interest in the future.Besides solar cell technology, photocatalysis is the key to harvesting unlimited solar energy and turning it into renewable fuels. MOFs and COFs provide unique properties that potentially tackle the main issues in conventional photocatalysts, e.g., the bandgap is barely suitable for visible light and electron–hole recombination problems. In MOF photocatalysts, the organic linker acts as the semiconductor absorbing photons and generating electrons (light harvester) [43]. Subsequently, the electrons can be transferred into the adjacent metal center through ligand to metal charge transfer (LMCT) and eventually shift to the doped metal NPs to carry out the desired catalysis reactions. Likewise, COFs act as organic semiconductors, and with the addition of metal nanoparticles, can be an efficient photocatalyst [204]. Such a combination of organic semiconductor and metal mimics the natural system of chlorophyll which arguably has the highest quantum efficiency under sunlight exposure.In the effort to generate renewable fuels, photocatalysts are mainly employed to carry out the production of H2 through water splitting or HER and the production of hydrocarbons through CO2 reduction. Bai et al. [238] fabricated noble-metal-free g–C3N4–MIL-53(Fe) composite by a simple grinding method as a photocatalyst for H2 production from water splitting. Under simulated sunlight, hydrogen evolution was performed with a rate of 0.9054 mmol g−1 h−1. The synergy between graphitic carbon nitride (g-C3N4) and MOF intimates interfacial contact and increases active sites on the photocatalyst, which can substitute the role of noble metals. Another noble-metal-free photocatalyst was developed by Tian et al. [239] by introducing annealed Ti3C2Tx MXenes to Zr-MOFs (UiO-66-NH2) precursors via hydrothermal process to form Ti3C2/TiO2/UiO-66-NH2. The hydrogen production rate under simulated sunlight was 1980 μmol h−1 g−1, significantly higher than its precursors. The synergistic effects of Schttoky junctions for Ti3C2/TiO2/UiO-66-NH2, Ti3C2/TiO2, and Ti3C2/UiO-66-NH2 interfaces was the key to the enhanced photocatalytic performance. A schematic illustration of the photocatalytic mechanism involving the charge transfer is shown in Fig. 25 a. Wang et al. [240] developed a different approach using a porphyrin-based MOF with the Ti-oxo cluster as the metal center. With the small addition of Pt (3 wt%) as the co-catalyst, the rate of H2 production could reach as high as 8.52 mmol g−1 h−1 under visible light irradiation up to 700 nm. The outstanding photocatalytic performance was attributed to the feasible LMCT from the porphyrin (photon harvester) to the Ti-oxo, then to the Pt NPs. A schematic illustration of the proposed photocatalytic mechanism is shown in Fig. 25b.The synergy of Pt co-catalyst, TiO2, and COF was investigated by Chen et al. [241] using 2,2′-bipyridine-5,5′-diamine (Bp-COP). The light-harvesting properties of the COF led the catalyst suitable under visible light up to more than 600 nm. The presence of TiO2 as a charge transfer media from the Bp-COP to Pt NPs significantly enhanced the H2 production rate to 1333 μmol h−1 g−1. Biswan et al. [242] developed a noble-metal-free photocatalytic system by combining thiazolo [5,4-d]thiazole-linked COF (TpDTz) as the photon-harvester, Ni-thiolate cluster as the co-catalyst, and triethanolamine (TEoA) as the sacrificing agent. The H2 production rate over this photocatalyst reached 931 μmol h−1 g−1 for more than 70 h. A kinetics study over the photocatalyst reveals that an outer-sphere electron transfer from the photo absorber to the catalyst is the rate-limiting step (Fig. 25c). The further potential of COF photocatalyst was demonstrated by Li et al. [243] in the production of H2 directly from seawater, which has been a challenging problem because it contains various salts. Using a thioether-functionalized covalent organic framework (TTR-COF), selective adsorption of Au over other metal ions was demonstrated; thus, Au@TTR-COF photocatalyst could endure decomposition under seawater. The H2 production of over the photocatalyst using pure water was 501 μmol h−1 g−1 and only slightly decreased in the presence of dissolved salts. DFT calculation shows that the chelation of TTR-COF and the metal ion in seawater such as Mg2+ is energetically not favored (Fig. 25d).Photocatalytic reduction of CO2 into fuels over MOF and COF catalysts was also continuously studied in recent years. Ma et al. [244] developed a hierarchically porous TiO2/UiO-66 photocatalyst via a simple solvothermal and assembly method. The deposition of ultrafine TiO2 NPs on the surface of UiO-66 provides interlaced spacing owing to electrostatic repulsions. Conversion of CO2 to CH4 over the photocatalyst composite gave a production rate of 17.9 μmol g−1 h−1 with a selectivity of 90.4%, even when the CO2 concentration was decreased to ≤2%. Wang et al. performed conversion of CO2 to methanol over porous Cu–Zn oxide derived from Cu/Zn-bimetal MOF [245]. Under simulated sunlight, fast production of methanol (3.71 mmol−1 g−1 h−1) over the photocatalyst could be achieved. The synergy between CuO and ZnO on a large specific surface area with the unique mesoporous structure of the catalyst dictated the photocatalytic activity. Chowdhury et al. [246] designed a sheet-like nanoporous covalent organic framework (TFP-DM COF) catalyst to convert CO2 into HCOOH and HCHO. The rapid production rate for both HCOOH (19.2 mol g−1 h−1) and HCHO (0.54 mol g−1 h−1) was obtained over the photocatalyst under a white light LED irradiation. The photocatalyst showed good stability over a 5-cycle reusability test.During the past several years, nanoporous metals have also emerged as one of the most studied catalytic materials due to their many promising opportunities. This class of materials was reminiscent of the great success of Raney Nikel by Muray Raney in the early of 20th century. It was described as the spongy and porous state of highly active unsupported Ni materials, which could be easily produced by leaching the Si or Al from Ni–Si or Ni–Al alloys in aqueous NaOH. It has been widely used especially in the hydrogenation process. Also, it has been utilized as powerful catalyst for conversion of lignocellulosic biomass feedstock in recent years. Its emergence was significant for the development of nanoporous materials, in which people finally realized that active nanoporous metal catalysts could be prepared through a simple alloy corrosion method [247–250].In general, the vast majority of the applications were emphasized where the exploitation of the nanoporous metal's unique porous structures and their large specific surface area is much needed [251]. In most cases, full control in structure and pore features would allow one to significantly enhance the performance of such materials in various applications, such as in catalysis [252–254], actuation [255–257], sensing devices [258–260], energy storage (batteries) [261–263], and many more. It is primarily due to the porous structures' unique ability to amplify the effectiveness of several local processes at the material's surface or its interface with bulk, such as mass transport, electric and thermal conductivity, light scattering, etc. Nevertheless, the conflicting requirement of material's porous structures for certain application types has been a significant challenge in nanoporous metals. For instance, in the application of nanoporous metals as sensing devices, fabrication of nanoporous metals with high surface area and thus having small pores are required to provide many surface-active areas. Simultaneously, large pores are also desired to facilitate efficient and fast mass and ion transport. Thereby, controlling the materials' structural hierarchy has been one of the most potential approaches for reconciling these conflict requirements between small and large pores.Recently, tremendous efforts have been made to develop strategies for efficiently fabricating nanoporous metals with the desired structural hierarchy. In general, three main synthetic methods (Fig. 26 ) can be used to manufacture nanoporous metals with hierarchical and multimodal structures, i.e., (i) dealloying-based, (ii) templated-based, and (iii) assembly method [251,264]. In the dealloying method, porous structures of metals are generally obtained via corrosion processes. This synthetic method has been proven reliable for the fabrication of various types of nanoporous metals, such as single and binary or multi-component noble nanoporous metal alloys and 3D nanoporous structures. Depending upon its corrosion process, the dealloying method can be categorized into chemical and electrochemical dealloying. Typically, the fabrication of nanoporous metals via chemical dealloying can be carried out using either acid or base solutions, depending on the type of metals and the desired porous structures. For example, a well-defined crystal structure of nanoporous PdPt alloy with a typical pore size of about 5 nm was successfully prepared from the chemical corrosion of ternary alloy of PdPtAl in 0.5 M of NaOH [265]. Meanwhile, Xu and co-workers [266] have also reported that a very similar nanoporous PdPt alloy could also be obtained from chemical corrosion of Pd16Pt4Al80 in acid conditions using HCl or H2SO4 solution. Based on the result, it was found that chemical corrosion in an acid condition of the precursor was able to form a similar uniform sponge-like nanostructure with a bicontinuous 3D network structure. Fig. 27 a shows the structure comparison of nanoporous PdPt alloy prepared from chemical corrosion using acid and base solutions.In literature, other noble mesoporous metals and/or alloys with unique 3D structures, such as PtAu, AuAg, and PdAg have also been successfully prepared using this chemical dealloying method using both acid and base solutions [267–271]. However, recent efforts have been shifted towards the fabrication of nanoporous non-noble metals and alloys driven by the need for economic and natural abundance consideration. It is reported that low-cost and naturally abundant metals, such as Cu, Ni, or Ti, could efficiently be used to reduce the high loading of nanoporous Pt metal catalysts and significantly enhance their catalytic activities. For instance, Qiu and co-workers have successfully prepared various kinds of nanoporous PtCu alloys with different structures, including wire-like structure, core–shell porous structure, and aligned 3D structure, using chemical dealloying method from PtCuAl alloy precursors [272–274]. Furthermore, chemical dealloying at basic condition using NaOH have also been reported to be able to successfully prepare various kinds of low cost and earth-abundant nanoporous binary metal alloys, such as PdNi, PdFe, PdCu, PdCr, PdCe, and PdZr, from the corresponding PdNiAl, PdFeAl, PdCuAl, PdCrAl, PdCeAl, and PdZrAl alloy precursors, respectively [257,259,275–278]. Recently, the fabrication of multicomponent quaternary metal alloys with 3D nanoporous structures has received plenty of attention due to their unique physicochemical properties, and they are proven to exhibit excellent performance in various applications. For example, nanoporous PdAuCu metals with unique well-aligned 3D bicontinuous ligaments and pores have successfully synthesized and showed excellent electrocatalytic activity towards oxygen oxidation reaction (ORR) [279].In the dealloying-based method, nanoporous metals can also be prepared via electrochemical dealloying. In several cases, the electrochemical method is preferred due to the ability to control the chemical composition of the resulting nanoporous metals, and it is considered cheaper and easier to perform [264]. According to literature, both structural and physicochemical properties of the resulting nanoporous metals prepared using electrochemical dealloying are highly dependent on two key factors, i.e. (i) the parting limit, which is the concentration of the alloy precursors; and (ii) critical potential, which refers to the applied potential threshold during the electrochemical desolation [280]. For instance, electrochemical dealloying has been successfully employed to synthesize nanoporous PdAu alloys from PdAuNi ternary precursors in a 0.5M H2SO4 solution [281]. Here, the structural morphology and Pd/Au ratio of the as-prepared nanoporous metals could easily be controlled by tuning the appropriate critical potentials. Fig. 27b shows the micrographic images of the nanoporous PdAu alloys prepared at various Pd/Au ratios. A similar approach was also used to fabricate nanoporous PtAu alloys with open 3D nanoporous network structures [282]. Based on the report, the nanoporous was fabricated using electrochemical dealloying by selectively etching Cu from the Pt10Au10Cu80 ternary alloy precursor. In another report, Zhang and co-workers [283] have also successfully fabricated ferromagnetic nanoporous PtFe by dealloying an amorphous FePtB alloy precursor. The result revealed that the nanoporous PtFe was composed of a single face-centered cubic phase and exhibited excellent performance as an electrocatalyst for methanol oxidation.The second synthetic method that can be used to fabricate nanoporous metal is the templated-based approach. In this approach, the structure of nanoporous is typically obtained within several steps of synthetic pathways, such as (i) the preparation of the original template; (ii) impregnation of the metal precursor into template's void spaces; (iii) metallization via reduction of precursors by either chemical or electrochemical methods; (iv) crystallization at desired temperature; and (v) removal of the template. Depending on the template, the synthesis of nanoporous metals via template-based method can generally be divided into two types, i.e., hard template-based and soft template-based synthetic methods [284,285]. In general, rigid natural and artificial minerals or biological molecules are commonly used in the hard template-based method. For example, Qiu et al. [286] have successfully used SiO2 nanospheres as the hard template to prepare hollow porous PdPt alloy nanospheres, proving to be efficient for methanol electro-oxidation (MOR). According to the result, it was revealed that the SiO2 template played a critical role in controlling PdPt alloys' thickness due to the presence of its strong electrostatic interaction with charged metal precursors. In another report, 3D nanoporous of PdNi alloys were also successfully prepared using nanoporous alumina as the hard template [287]. Meanwhile, Nguyen and co-workers [288] were also able to efficiently synthesize and control the porous structure of PdCo thin films using anodized aluminum oxide (AAO) as the template. Recently, Fang et al. successfully used several types of mesoporous silica, such as KIT-6, SBA-15, and EP-FDU-12, as hard templates for synthesizing mesoporous noble metal networks using chemical reduction process [289]. Fig. 28 a shows the micrographic images of several mesoporous noble nanoporous metal structures prepared using silica templates.On the other hand, the soft template can be in the form of both biological and artificial self-assembled structures such as surfactant micelles or reverse micelles, microemulsions, polymers, or gas bubbles [264]. For instance, a study reported by Kang and co-workers [290] revealed that mesoporous PtCu nanostructures were successfully synthesized using self-assembled block copolymer micelle as a soft template and ascorbic acid as a reducing agent. Based on the result, it is reported that the composition of the as-prepared mesoporous alloys could easily be controlled by tuning the ratio of metal precursors. Fig. 28b presents the schematic illustration and micrographic images of mesoporous PtCu alloys prepared using diblock copolymer, i.e., poly (ethylene oxide)-b-poly (methyl methacrylate) (PEO-b-PMMA) as the soft template. Furthermore, a honeycomb-like AuPt nanoporous alloy structure was also successfully prepared via electroreduction process in the presence of in-situ hydrogen gas bubbles as a soft template [291]. According to the result, the as-prepared AuPt alloys exhibited excellent sensitivity and selectivity for non-enzymatic glucose sensing applications due to their large surface area and the homogenous spread of Au and Pt throughout the surface.Finally, nanoporous metals could also be prepared using the assembly method. However, this method is less common than the previously discussed dealloying- and templated-based method. It is primarily because the metal precursor's assembly process is considerably hard to control and susceptible to various external factors, such as temperature, solvent, and the type and concentration of metal precursors. In this method, the porous structure is typically growing randomly during the assembly process, leading to less-ordered and low-intensity nanostructure formation. Nevertheless, several types of nanoporous metal alloys have been successfully prepared using this method. In literature, solvothermal and/or hydrothermal processes are considered one of the most common techniques in facilitating the precursors' self-assembly to form nanomaterials with nanoporous features. For instance, Li and co-workers [292] have successfully developed a simple one-pot solvothermal method to prepare porous PdCu alloys with a unique nano frame structure. Their report revealed that such a unique nano frame structure was achieved by simultaneous co-reduction of Pd and Cu precursors in the presence of oleylamine and NH3. In another report, a series of nanoporous Pd-M alloys (PdCd, PdPb, PdIr, and PdPt) have been successfully prepared using a robust and straightforward hydrothermal method [293]. According to the report, the as-prepared nanoporous alloys showed excellent performance as electrocatalysts for formic acid oxidation due to their small dendritic and random array structures.Nanoporous metals have been widely considered one of the most efficient catalysts for generating renewable fuels, especially in designing high-performance fuel cells. This is not only due to their exceptional catalytic activity as the consequences of the large surface area resulted from their unique porous structures but also their high electric conductivity, thermal and chemical stability, and excellent optical properties. One of the most common applications of nanoporous metals-based catalysts is in the non-spontaneous water-splitting reaction. In general, nanoporous metals are introduced to suppress the high overpotential required to make the reaction occur. Additionally, nanoporous metals could also be used to overcome one of the major fundamental challenges in water-splitting reaction: the low electrical conductivity of conventional electrocatalyst due to the resistance overpotential. For example, Lei and co-workers [46] have successfully prepared, and utilized nanoporous Ni–Fe hydroxyl phosphate (NiFe-OH-PO4) supported on Ni foams' surface as a bifunctional electrocatalyst for whole-cell water electrolysis in alkaline solution. Based on the result, it was found that the system was able to generate a current density of 20 and 800 mA/cm2 at oxygen evolution overpotential of 240 and 326 mV, respectively, and a current density of 20 and 300 mA/cm2 at hydrogen evolution overpotential of 135 and 208 mV, respectively. Interestingly, the as-prepared electrocatalyst was also found to exhibit exceptional prolonged stability under continuous and intermittent electrolysis reactions.In another report, Detsi and co-workers [294] have also prepared ultrafine nanoporous NiFeMn alloys using the dealloying method and used them as electrocatalysts in water splitting reaction. The report revealed that such material could facilitate efficient water oxidation with a current density of 500 mA/cm2 at 360 mV overpotential in 1 M KOH solution. It is believed that such excellent catalytic activity was primarily due to the small size of the ligaments and pores of the nanoporous, which was proven by the high BET surface area of 43 m2/g. Additionally, the as-prepared nanoporous NiFeMn alloys' high electrical conductivity was also responsible for effective current flow. Another strategy was also developed by Dong et al. [295] by selective dealloying of Al97¬NixFe3-x to form high-performance porous NiFe nanowire network alloy as water splitting electrocatalyst. Based on the result, it was found that the as-prepared electrocatalyst exhibited excellent performance to oxidize water with only ∼244 mV overpotential in 1 M of KOH solution. Further suppression in water oxidation overpotential was also achieved by partial dealloying removal of Cu from Cu-rich NiFeCu ternary alloys [296]. According to the report, the as-prepared porous electrocatalyst required only about ∼180 mA/cm2 for water oxidation in an alkaline solution.Aside from water oxidation reaction, nanoporous metal's performance as a catalyst has also been widely investigated for the generation of renewable fuels from other types of sources, such as methanol, ethanol, and formic acid. For example, Zhang and co-workers [297] reported that nanoporous PdPt alloys were able to exhibit exceptional performance in facilitating methanol oxidation reaction (MOR) in a direct methanol fuel cell (DMFC) system. According to the report, the high stability and catalytic activity of the as-prepared nanoporous PdPt alloys were primarily caused by the high density of twinned and ultrathin ligaments, which creates large curvatures between concave and convex region as well as low as forming low-coordination surface atomic steps and kinks. Consequently, these features render the appearance of many active low-coordination atoms sites essential for catalytic activity. A similar phenomenon was reported elsewhere when dendrite-like nanoporous PtAu was used as the catalyst [298]. Here, the catalyst was prepared using a soft-template method where l-histidine and PVP were used as the template and as structure-directing and dispersing agents. Based on the result, it was reported that the as-prepared dendrite-like nanoporous PtAu alloys showed a superior mass activity (MA) and specific activity (SA) in MOR than that of Pt black. Fig. 29 presents the micrographic images and catalytic performance of the as-prepared dendrite-like nanoporous PtAu alloys in MOR.Furthermore, various types of palladium-based of ternary nanoporous alloys have also been developed and used in MOR. It is reported that introducing a different metal into binary alloy would allow more abilities to tailor the overall catalytic properties of the material. For example, Li and co-workers have successfully prepared tri-metallic mesoporous PdPtAu alloys, demonstrating excellent MOR performance [299]. According to the report, the electrocatalytic system efficiently facilitated MOR with mass activity (MA) of 1010 mA/mg. It was believed that such excellent catalytic activity was primarily caused by the synergistic effect between the mesoporous structure and the ternary metal alloy. Recently, tremendous efforts have been made to develop non-noble metal-based nanoporous alloys due to their low-cost, non-toxicity, and high abundance properties with comparable catalytic activity with that of noble metal-based alloys. In literature, various noble/non-noble nanoporous metal alloys, such as PtCu, PdCu, PdNiO, PdZr, PtCo, PdCo, PtFe, etc., have been successfully prepared and could potentially be used in MOR. Table 4 summarizes the performances of several types of nanoporous metals-based electrocatalyst in MOR.Nanoporous metals and alloys have also been widely investigated for ethanol oxidation reaction (EOR) in direct alcohol fuel cells (DACFs). In general, ethanol is preferred in renewable energy generation over methanol due to its higher energy density, lower toxicity, less volatility, and easier storage and transport [308]. Moreover, ethanol can also be produced from renewable sources, such as biomass like cellulose or starch. Nevertheless, most conventional electrocatalysts, such as bulk and nanosized mono-noble metal catalysts (Pd, Pt, and Au), are easily poisoned by the adsorbed CO during the electrocatalysis, leading to the significant reduction in electric conductivity and catalytic activity [264]. Therefore, the unique porous structure of nanoporous metals and alloys has been investigated and considered as the potential solution for such issues. For example, Chen et al. [281] have successfully prepared a typical nanoporous bimetallic PdAu alloy with a tunable metal ratio for EOR. Here, the nanoporous PdAu at various PdAu ratios were synthesized by electrochemically dealloying ternary metal alloy precursor of PdAuNi, which led to geometrically formation controllable nanoporous structure. As a result, the resulting catalyst exhibited superior catalytic performance in EOR compared to the conventional Pt/C, Pd nanoparticles, and non-porous PdAu nanoparticle alloy.In another report, mesoporous PdPt alloys have also been efficiently utilized as the electrocatalyst in EOR [306]. The report revealed that the formation of unique porous and hollow structures of the materials was obtained due to the presence of halide ions (Br- and I- ions) during the facile one-pot hydrothermal process. Furthermore, the result also demonstrated that the as-prepared mesoporous PdPt alloys showed much higher specific and mass activities than the commercial Pt black and Pt/C electrode. A similar excellent catalytic performance in EOR was also observed when nanoporous bimetallic PdAg alloy was used as the electrocatalyst [269]. This exceptional catalytic activity was believed to be originated from the synergistic effect of bimetallic alloy and the unique porous structure of the material, which was obtained by dealloying melt-spun AlPdAg ribbon in 10wt% of H3PO4 solution containing polyvinylpyrrolidone (PVP). Table 5 lists the comparison of different types of nanoporous metal and alloy-based electrocatalysts in facilitating EOR.Another potential application of nanoporous metal-based catalysts in renewable fuels is the formic acid oxidation (FAO) reaction in direct formic acid fuel cells (DFAFC). Recently, renewable fuel generation via the DFAFC process has gained much attention due to its high-power output and proton exchange's low membrane permeation rate [311,312]. In DFAFC, the FAO process can typically proceed via either a direct or indirect pathway. In the earlier pathway, CO2 is formed directly by the hydrogenation reaction of formic acid. Meanwhile, the latter pathway suggests that the CO2 product is obtained via the formation of intermediate CO molecules due to the dehydration reaction of formic acid. Therefore, there is a chance for such intermediate molecules to poison and inhibit the active site of electrocatalyst, which was also often observed in methanol and ethanol oxidation. During the past several years, various collections of nanoporous metal-based catalysts have been employed as anode materials to efficiently facilitate the FAO process due to their ability to prevent CO poisoning during electro-oxidation. For example, Xu and co-workers [266] reported that nanoporous PdPt alloy with uniform pore size prepared by chemical dealloying could efficiently facilitate FAO reaction. Based on the result, it was found that the as-prepared nanoporous PdPt demonstrated superior catalytic activity than the conventional Pd/C electrode and the corresponding mono-metal analogs to facilitate not only the oxidation of formic acid but also methanol and ethanol. Fig. 30 a and b presents the as-prepared nanoporous PdPt alloy's micrographic images and its electrocatalytic performance in FAO reaction (Fig. 30c). Assaud and co-workers also observed a similar result when the 3D-nanoarchitecture PdNi alloy catalyst was employed as the anode material [287]. According to the result obtained from cyclic voltammetry, it was found that the oxidation of formic acid proceeded via a direct pathway instead of an indirect route. Recently, other types of nanoporous metal alloys, such as PtGa, PdNi, PdCu, PdAg, AuPt, and PdM (M: Pb, Cd, and Ir), have also been proven to exhibit excellent catalytic activity in FAO reaction [270,293,313–316].Nanoporous metals and alloys have also been investigated for their application to generate renewable fuel via carbon dioxide (CO2) reduction. During the past several years, this research direction has been attracting much attention due to the ability to simultaneously generate clean energy and reduce the effect of global warming due to the large accumulation of anthropogenic CO2 emissions [53]. In general, nanoporous metals and alloys were employed to facilitate the catalytic conversion of CO2 into several types of reusable carbon fuels such as CO, CH3OH, C2H5OH, C2H5, or CH4. For example, Selective and efficient conversion of CO2 to CO has been reported by Lu and co-workers [317] using nanoporous gold prepared by electrochemical dealloying. Based on the result, it is reported that the porous structures of the metal were responsible for the formation of many high-density steps or kink sites, exposing the essential high-index facets inside the curve of the metal's structure (Fig. 31 a). As a result, the as-prepared nanoporous gold exhibited excellent catalytic performance in CO2 to CO conversion with Faradaic efficiency (FE) of 98% at an overpotential of 390 mV (Fig. 31b). Furthermore, the result of the long-term stability test exhibited that the reduction of surface step/kink sites and the deposition of metal impurities are responsible for catalysis decay. However, by applying potential cycling, the catalytic performance of the deactivated electrode can be recovered. The reactivation of catalysts was caused due to the rejuvenation of the reduced step/kink sites and the removal of surface metal impurities (Fig. 31c).Meanwhile, Hong et al. [318] have also reported that nanoporous copper could be efficiently used to convert CO2 to various types of renewable fuels. Here, the unique porous structure of Cu film was obtained by highly-controlled electrodeposition using 3,5-diamino-1,2,4-triazole (DAT), which was responsible for directing the crystal's growth with high exposure to catalytic sites. According to the report, the as-prepared electrocatalyst exhibited an auspicious catalytic performance for electroreduction of CO2 to C2H5 and C2H5OH with FE of 40% and 20% at −0.5 V vs. RHE, respectively. Furthermore, it was also found that the overall mass activity for the CO2 reduction was ∼700 A/g at −0.7 V vs. RHE.Furthermore, selective electroreduction of CO2 to C2H4 was also carried out using nanoporous copper film electrodes [319]. In this report, Peng and co-workers [319] used the chemical dealloying technique to fabricate the nanoporous Cu surface structure from the Cu–Zn surface alloy obtained from electrodeposition and thermal treatment processes (Fig. 32 a). Based on the result, the as-prepared electrocatalysts were found to suppress the FE of CO2 conversion to methane down to 1% while keeping selective reduction of CO2 to C2H4 with FE of 35% in an aqueous solution of 0.1 M KHCO3 at −1.3 V vs. RHE (Fig. 32b). The high selectivity of C2H4 products could be attributed to several synergic factors, including the exposed (100) facets, along the possible existence of step and edge atoms (Fig. 32c).In another report, the influence of ligament size of nanoporous Ag network was also evaluated for the application in CO2 reduction [320]. Here, two nanoporous Ag networks with the average ligament sizes of 21 nm and 87 nm were fabricated by dealloying binary Mg80A20 alloy ribbon in the presence of citric acid and phosphoric acid (Fig. 33 a). According to the report, it was found that the ligament size of the as-prepared Ag network was significantly influencing the catalytic performance of the catalyst in facilitating CO2 conversion to CO (Fig. 33b). Results demonstrated that Ag network with smaller ligament exhibited a superior catalytic performance (FE of 85% at −0.8 V vs. RHE) than that of larger ligament size (FE of 41.2% at −0.8 V vs. RHE) (Fig. 33c). Recently, Lu et al. [321] have also successfully prepared and used nanoporous AuSn alloy to enhance and selective electroreduction of CO2 to CO. Compared to the analogous bulk nanoporous Au, the as-prepared nanoporous AuSn alloy exhibited a significantly higher catalytic activity for CO2 conversion with FE of 92% at −0.85 V vs. RHE. It is believed that such exceptional activity was primarily due to the presence of a trace amount of Sn solute in Au lattice, which results in a more pronounced tensile strain on the surface of 3D nanoporous. As a result, this would allow the metal surface's d-band center to be shifted and ultimately lead to the enhancement for the adsorption of key intermediate species of ∗COOH during the electroreduction process.Renewable fuels are urgently needed as alternative energy sources to the increasingly depleted fossil-based resources. These fuels are expected to tackle the depletion issue and realize an environmentally benign energy cycle. In this sense, a catalyst is inevitable to assist the conversion of renewable sources into fuels. Nanoporous materials have served as efficient heterogeneous catalysts that possess high catalytic activity and control product selectivity. This review summarizes recent advances in nanoporous materials, i.e., zeolites, ordered mesoporous silica (OMS), metal- and covalent organic frameworks (MOFs and COFs), and nanoporous metals, and their use in diverse chemical processes as a catalyst for producing renewable fuels.Zeolites have exhibited superior catalysis performance in producing renewable fuels through fast catalytic pyrolysis (CFP) and CO2 conversion. In the former case, zeolites simultaneously reduce undesirable products and yield organic liquid products at an acceptable amount. Zeolites also increase aromatic hydrocarbons and light phenols while decreasing the bio-oils viscosity, density, and acid number. On the other hand, zeolites are up-and-coming candidates in CO2 conversion into renewable fuels, which exhibited remarkable performance when combined with CO2 conversion into renewable fuels, which were remarkably successful when combined with a metallic catalyst. In both applications, several parameters need to be considered. The first is the type of zeolite frameworks. Note that each framework possesses a unique pore size and shape, which could govern the product selectivity. The second is the framework silica-to-alumina ratio (SAR) of zeolites, an essential factor since it determines the number of acid sites within the zeolite frameworks. The third is the incorporation of metallic sites. In the catalytic fast pyrolysis, introducing transition metals such as Pb, Ni, Zn, Fe, Mo, Ga, and Co into zeolite framework has improved the bio-oil yield and lowered the content of undesired polyaromatic hydrocarbons and cokes. Furthermore, metallic (e.g., Pd, Pt, and Ni) sites are indispensable as hydrogenation sites for converting CO2 into various renewable fuels. In addition, the use of bimetallic or trimetallic sites is intriguing owing to the synergistic effect, which might increase the overall catalytic performance.The performance of zeolite catalysts can be enhanced by introducing larger porosity (mesopores and/or macropores), so-called hierarchical porosity. Nevertheless, it should be emphasized that the larger pores should be interconnected with the micropores. The inlet flow of reactants must first go through the larger pores prior to the micropores. These requirements should be met to maximize the utilization of acid sites and overcome the diffusion issue within purely microporous zeolites. Moreover, metal size, shape, and facet engineering should be pursued since many molecules exhibit preferences over particular crystal facets. The ability to control the physicochemical properties of zeolites and metallic sites will enable the highly active, selective, and efficient catalyst system to produce desired products.OMS has demonstrated unmistakable performance in the hydrogen production reaction and the conversion of carbon dioxide to fuels and chemicals, respectively. The well-defined mesoporosity of this material has played a major role in improving the catalyst's performance in both types of reactions, where the pores in the OMS play a pivotal role in providing an active site for the reactants during the reaction. In addition, the pores in OMS can increase the dispersion of metal loading, thereby reducing the presence of catalyst particle agglomeration, which can cause pore blocking. In addition, a larger mesopore promotes the formation of metals with larger sizes, resulting in weaker interaction of metal–support which is more favorable for both reactions.Several types of OMSs reported as catalysts in the hydrogen production and carbon dioxide conversion reactions include MCM-41, MCM-48, SBA-15, SBA-16. MCM-41 is the most widely used type. The use of the MCM-41 as support is owing to its interpenetrating 3D pores and high stability. MCM-48 is claimed to be a better candidate than MCM-41. This OMS is characterized by the interwoven and continuous three-dimension pore system. Mesoporous silica SBA-15 has a hexagonal structure (p6mm) similar to the MCM-41, but SBA-15 presents greater hydrothermal stability due to the thicker silica walls and has larger pores when compared to the MCM-41 mesostructure. In addition to SBA-15, which has a 2D hexagonal porosity, SBA-16 possesses cage-like mesopores organized in a three-dimensional cubic body-centered Im3m symmetry. The structure of SBA-16 can be described by a triply periodic minimal surface of I-WP (body-centered, wrapped package). The mesophase might also be a triply periodic minimal surface. In comparison with SBA-15, the synthesis of the cubic SBA-16 material is more challenging. This factor makes the use of SBA-16 support relatively rare.OMS materials are generally used to support several types of metal or metal oxide catalysts. The addition of these catalysts can improve the performance of CSO support. Several types of catalysts supported on the OMS surface include metal, both monometallic and bimetallic, such as Ni, Fe, Mg, Cu, Pd, Pt, Ni–Pd; metal oxides such as metal oxides as CeO2, CdS, CuO, ZnO, ZrO2, NiCr2O4, Fe2O3, Al2O3. The results show that the introduction of metal oxide in the catalyst changes the pore size and specific surface area of the support. Among several types of metal and metal oxide catalysts added to the catalyst's surface, based on the literature that has been described, the addition of Ni is the most widely used because of the significant impact on improving the catalyst's performance. The incorporation of Ni could enhance catalyst coke resistance. However, adding Ni with high concentrations can also cause a decrease in the catalyst's performance. Therefore, attention to proper concentration is necessary.Furthermore, several things should be noted and worthy of being developed in the search for a reaction catalyst for hydrogen production and CO2 conversion, as follows. (i) The addition of a catalyst to the promoter. The addition of catalyst promoters could increase the metal-support interactions, decrease metal and/or metal oxide particles, thus improving the catalytic performance. In addition, the addition of a promoter can also reduce coke formation. The type of promoter used can be derived from metal particles such as Ga, Gd, Ce; organic compounds, such as alcohol; or metal oxide compounds such as Yttria (Y2O3). (ii) The incorporation of the acid or basic sites. The addition of the acid sites can increase the incorporation of metal catalysts on the support. Meanwhile, the addition of the base, such as magnesium atoms, could induce the adsorption and activation of CO2 in the CO2 conversion reaction. (iii) The morphology of the catalyst used. The different types of morphology in OMS support can determine the performance of the OMS catalyst. As a comparison of the performance of rod-like SBA-15 and fibrous type-SBA-15. The fibrous-type SBA-15 has higher catalyst performance than a rod-like one. Also, fibrous-type SBA-15 exhibited higher catalytic stability and coke resistance. In addition, the fibrous-type morphology led to higher homogeneity of a finer metal catalyst, which subsequently reinforced the metal-SBA-15 interaction and increased the amount of moderate basic sites.Despite the excellent catalytic activities for renewable energy production, some might still be concerning the stability of MOF and COF catalysts since the main frameworks are organic compounds. Compared to the other nanoporous materials, e.g., zeolites and porous metals, MOFs' thermal and mechanical stability are generally somewhat lower. Likewise, COFs with their fully organic contents need significant enhancement in thermal stability. Nevertheless, the potential of COFs in photocatalytic reactions is remarkably good. Manipulating the band gab in COFs (and MOFs) with their designability feature is certainly more feasible than inorganic crystals. The generation of renewable energy, either hydrogen or synthetic hydrocarbons, using photocatalysts suitable for visible light, would be a breakthrough to achieve a clean and renewable fuels supply.From the perspective of material science, we should note that the number of MOF and COF materials is extremely big, increasing in the coming years. To deal with such types of big data, incorporating machine learning can be useful. Quantitatively, machine learning with appropriate algorithms can reveal the structure–properties relation of MOFs and COFs. However, the number of studies incorporating a machine learning approach for reticular chemistry is still limited. Moghadam et al., for instance, reported the structure–mechanical stability relationships for 3385 MOF materials with 41 distinct topologies using a combination of molecular mechanics calculations, machine learning, and molecular dynamics simulations [322]. Once computational chemistry is combined with a machine learning algorithm, the synthesis of new MOF and COF structures can be completely guided by the insight of numerous previous data concerning the designated applications.Another potential issue with MOF and COF catalysts is, in general, their reliance on the use of precious metals as active catalytic sites. Although there has been a continuous effort in recovering precious metals out of the end-of-life products [323,324], the use of common metals or even metal-free catalysts would be favored in the economic and environmental viewpoints. The concern of metal scarcity should be taken into account in mining sectors and in fine chemicals industries, including catalysts. In recent years, the trend shows the concern for developing the precious metal-free catalyst [325]. Continuing this path would be of great interest in the realization of sustainable development goals.Moreover, it is no secret that the current trend on utilizing nanoporous metals holds great potential for generating renewable fuels. Additionally, the ability to fabricate earth-abundant non-noble nanoporous metals and metal alloys-based catalysts has also enabled researchers to expand the utilization of such materials for large-scale applications. This would significantly reduce production and waste handling costs, making such technology one of the most attractive options for replacing fossil fuel-based energy generation. However, there are still plenty of challenges that need to be addressed regarding applying nanoporous metals and alloys in the generation of renewable fuels. For instance, albeit the ultra-small porous structure is essential for high surface area in the water oxidation reaction, the resulting O2 gas is often trapped inside the nanoporous bulk structure and could not easily escape from the electrode.Consequently, this O2 gas build-up would cause an increase in the system's internal resistance, which ultimately leads to the rise in the reaction overpotential. A similar phenomenon is often observed in other renewable energy generation reactions, such as methanol/ethanol/formic acid oxidations and CO2 reduction. One of the most promising solutions for such an issue is fabricating the catalysts with bimodal hierarchical porosity where macroporous and meso-/nano-porous features coexist. In such hierarchical-based material, it is believed that the meso-/nano-porous feature would still provide the large surface area needed for the reaction. At the same time, macropores would allow the escape of the reaction products and prevent the increased internal resistance.Furthermore, large-scale industrial application of nanoporous metal for renewable energy is also hindered by the fabrication method of the material itself. As previously discussed, most nanoporous metal-based catalysts that have been proven to exhibit exceptional ability in facilitating the generation of renewable fuels such as water/methanol/ethanol/formic acid oxidation and CO2 reduction are mostly prepared via the dealloying method. It is reported that the utilization of certain types of expensive binary and ternary metal alloys as a precursor is often considered to be one of the significant drawbacks of the dealloying technique. Besides, it is also known that the most convenient way to employ nanoporous metals as a catalyst in renewable fuels generation is to have them supported on conductive substrates such as carbon-based support. Table 6 summarizes all the advantages, drawbacks, and future potential developments of the nanocatalysts discussed in this article. It is, thus, clear that although possessing several superiorities owing to physicochemical properties, however, based on the information on the table above, significant effort must still be devoted in the upcoming years to rationally design a better candidate of nanoporous catalyst for renewable fuel production. Ultimately, a further extensive investigation should be carried out to realize the nanoporous catalyst with high activity, low cost, and applicable for the fabrication on a large scale. Here, the role of computer-assisted catalytic experimentations should also be considered. In addition to the well-established methods, such as DFT and molecular dynamics, which has been discussed above in several catalytic reactions [122,125,191,220,223,224,243], the application of underdeveloping methods such as machine learning (ML) might also be put on the table in order to accelerate new heterogeneous catalyst discovery for renewable energy [326,327]. Several works have been devoted in order to employ machine learning (ML) for enabling catalyst discovery even also predicting its catalytic behavior [328–330]. Thus, it does not only accelerate the discovery of novel catalysts but also provides a powerful tool to explore a deeper understanding of relationships between the properties of catalyst materials and their catalytic performance, i.e., activities, selectivities, and stabilities. Therefore by this knowledge, we could better design the catalysts and enhance their efficiencies [331]. Furthermore, apart from the catalyst perspective, a more comprehend the fundamental reaction mechanisms need to be also developed as a strong foundation to improve the catalytic performance. Also, understanding surface chemistry at the molecular level is crucial for designing a catalyst system with the desired nanoarchitecture that is expected to exhibit high activity and selectivity.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 Hibah P2MI (Research, community services, and innovation) – Institut Teknologi Bandung 2021, and the Ministry of Research and Technology (Kemenristek)/National Agency for Research and Innovation (BRIN) of the Republic of Indonesia, and Indonesia Endowment Fund for Education (LPDP) under the Program of Prioritas Riset Nasional No. 78/E1/PRN/2020.

The rapid and continuous depletion of fossil-based resources has boosted extensive research on alternative energy from renewable resources. In this sense, heterogeneous catalysts play an inevitable role in converting renewable resources into fuels. The performance of heterogeneous catalysts strictly depends on their structures and physicochemical properties. As a rule of thumb, heterogeneous catalysts with large specific surface areas possess more catalytic sites to enhance the overall catalytic performance. Nanoporous materials have emerged as highly active heterogeneous catalysts due to their large internal surface area, enabling a high density of active catalytic sites. The presence of nanopores also allows the selectivity towards the desired products. Herein, we provide a comprehensive review of recent advances of several typical nanoporous catalysts, i.e., zeolites, ordered mesoporous silica (OMS), metal- and covalent organic frameworks (MOFs and COFs), and nanoporous metals. Each nanoporous catalyst's characteristics and synthesis strategies are elaborated in detail, followed by discussions on their applications in various chemical processes to produce renewable fuels. Finally, challenges and opportunities for future improvement are provided.

Solid propellant is widely used in aerospace and military fields, one of the most effective methods to improve the combustion performance of solid propellant is to advance the thermal decomposition process of energetic components (such as ammonium perchlorate (AP) and 1,3,5-trinitroperhydro-1,3,5-triazine (RDX)) by adding a few combustion catalysts (3%–5%) [1–4]. Nowadays, several combustion catalysts, such as metals [5–8], metal oxides [9–13], bimetals [14,15], and GO-based composition [3,16,17], have been proved to present compelling advantage in reducing the pressure exponent and forming platform combustion effect.The copper ferrite (CuFe2O4) nanoparticle, as a typical magnetic semiconductor with narrow band gap, has been extensively applied as gas-sensing materials [18], supercapacitor electrodes [19], and efficient catalysts [20–23] owing to its magnetic properties, excellent electronic conductivity and high thermal stability. Therefore, its application in solid propellent has been concerned as well. Zhang et al. prepared hollow CuFe2O4 nanosphere by hydrothermal method and confirmed its higher catalytic activity on RDX and FOX-7 than that of CuO and Fe2O3 [24]; Liu et al. further introduced graphene oxide (GO) to synthesize CuFe2O4/GO composite by self-assembly method and demonstrated that it could reduce the apparent activation energy of RDX by 98.71 kJ/mol [25]. However, CuFe2O4 particles are inevitably to aggregate into clusters because of its nanometers size effect, which is detrimental for combustion of solid propellant [26,27]. The most common strategy for inhibiting aggregation and improving the dispersion of CuFe2O4 nanoparticles is to introduce appropriate nanoparticles carriers. Besides, pursuing the carrier that could display certain catalytic synergy properties with CuFe2O4, improve the combustion performance and anti-electrostatic discharge sensitivity of energetic components are more preferably.Many researchers have devoted themselves in seeking for layer carbon materials with two-dimension (such as GO) as right catalyst carrier, whereas the high-price, complex processing technology and waste acid contamination limit its engineering application [2,3]. But for silicon-based composites, holding abundant resources, low prices and facile preparation process, have been widely studied in solid propellent [28–30]. Kline et al. investigated the influences of thermally conducting (graphite) and insulating (SiO2) particles on film propagation rate of solid propellent, and demonstrated that the latter has a higher burning rate with low mass percentage additives [31]; Wang et al. demonstrated that Al/AP/SiO2 (7 wt%) composite particles was more reactive and that is approximately 7 times higher than Al/AP particles [32]; Chen et al. successfully prepared AP/RDX/SiO2 nanocomposite via the sol-gel method and confirmed that SiO2 could accelerate the decomposition of AP and reduce the sensitivity of energetic materials [33]. Therefore, using SiO2 as combustion catalyst carrier will produce many positive effects in thermal decomposition, combustion and safety properties.For CuFe2O4/SiO2 composite combining silica and copper ferrite through in situ growth or sol-gel method, has been widely applied in fields of peroxidase and visual biosensing [34], magnetic materials [35], chemical looping gasification [36] and coupled with Li2O as anode material [37], yet there isn’t any report of CuFe2O4/SiO2 applied in solid propellent so far. On the basis of above analysis, we speculated that CuFe2O4/SiO2 composite would be an excellent candidate material as combustion catalysts for AP and RDX.In this work, CuFe2O4/SiO2 binary nanocomposite was successfully prepared via the solvothermal method. This material was further used as combustion catalyst and its effect on thermal decomposition of AP and RDX was studied. Moreover, the combustion and safety properties of RDX over CuFe2O4/SiO2 nanocomposite were investigated. All results indicated that CuFe2O4/SiO2 nanocomposite could produce excellent catalytic effect on energetic components of solid propellent.All chemical reagents are of analytical grade and were used without further purification. FeCl3·6H2O and CuCl2·2H2O were purchased from Sinopharm Chemical Reagent Co., Ltd. Anhydrous CH3COONa and polyvinylpyrrolidone (PVP) were sourced from Damao Chemical Reagent Co., Ltd. Ethylene glycol was purchased from Kelong Chemical Reagent Co., Ltd. SiO2 (micrometer grade) was obtained from Macklin Biochemical technology Co., Ltd. The energetic materials (AP and RDX) were provided by Xi’an Modern Chemistry Research Institute.CuFe2O4/SiO2 composite was synthesized through a versatile solvothermal method [3], and the preparation process is shown in Fig. 1 . Firstly, 10 mmol FeCl3·6H2O and 5 mmol CuCl2·2H2O were separately dissolved in ethylene glycol (35 mL) accompanied by magnetic stirring. Secondly, different proportions SiO2 (1 wt%, 3 wt%, 5 wt%, 7 wt%) were dissolved in FeCl3·6H2O solutions, respectively. Meanwhile, 1 g PVP was dissolved in CuCl2·2H2O solution as a dispersant [38]. Those solutions were dispersed by ultrasonication under ultrasonic powder of 600 W for 1 h. Thirdly, the Fe3+ solution was mixed with the Cu2+ solution under continuous magnetic stirring until the complete mixture was achieved. Then, 2.46 g CH3COONa, used as a sedimentation agent to promote the formation of the final product, was added into the mixture accompanying with vigorous stirring. Subsequently, the obtained homogenous reaction solution was sealed in a 100 mL Teflon-lined stainless-steel autoclave and maintained at 180 °C for 12 h. The final product was rinsed by ethanol and deionized water for three times, respectively, the suspension was centrifuged and dried at 60 °C for 12 h. In contrast, pure CuFe2O4 nanoparticles were prepared under the same condition without adding SiO2 carrier.The phase structures of as-obtained samples were investigated by X-ray diffraction (Rigaku, Smart LAB SE); The surface morphology, size and element component were recorded by Scanning electron microscopy (Zeiss SIGMA) coupled with energy dispersive spectrometry (Oxford 51-XMX); Functional groups on the surface of samples were investigated by Fourier transform infrared (Shimadzu, IRAffinity-1S); Element composition and chemical valence state were determined via X-ray photoelectron spectra (ThermoFisher, NEXSA); Zeta potential analysis (Anton Paar, Litesizer™ 500) of SiO2 carrier was determined by Dynamic Light Scattering method; The specific surface areas and pore size distributions were measured by Nitrogen adsorption/desorption (Micromeritics, ASAP 2460) through the Brunauer-Emmett-Teller method.The catalytic decomposition behavior of energetic materials was studied by Differential scanning calorimetry (Netzsch, DSC-200F3), the N2 flow rate is 100 mL/min, the heating rate is 10 °C/min and temperature ranges are 30–460 °C and 30–310 °C for AP and RDX, respectively; The ignition characteristics were recorded by a CO2 laser ignition system (SLC 110) at air atmosphere. The stacking density of sample in crucible is around 0.76 g/cm3; The electrostatic discharge sensitivity (EDS) was analyzed by a JGY-50(III) electrostatic test apparatus [39–41]. The test energy was determined by the energy calculation equation: E 50 = 1/2CV2, in which C is the capacitance of the capacitor, V is the charge voltage in volts [42]. The charge capacitance was set as 10,000 pF, and the electrode gap length was set as 0.12 mm. In the above three types of characterizations, the catalyst was physically mixed with energetic materials at a mass ratio of 1:4.SEM was used to observe the loading of CuFe2O4 on the surface of SiO2 from apparent morphology, as shown in Fig. 2 (a)–2(d). Fig. 2(a) shows the SiO2 substrate with a micrometer sheet structure, while the CuFe2O4 nanoparticles in Fig. 2(b) are spherical with an average particle size of 160–170 nm. Through further observation of CuFe2O4/SiO2-3% composite in Fig. 2(d), CuFe2O4 nanoparticles are uniformly distributed on the surface of SiO2 substrate. In comparison with CuFe2O4/SiO2-1% composite in Fig. 2(c), the agglomeration phenomenon of CuFe2O4 nanoparticles is effectively suppressed. Besides, the elemental mapping images in Fig. 2(d-1)–2(d-4) reveal that the elements of Fe, Cu, Si, and O are distributed on the surface of samples, which further demonstrates the uniform loading of CuFe2O4 particles on the surface of SiO2 substrate.XRD patterns and FT-IR spectra are often applied to study the composition and chemical bonding information of materials. The crystal structures of CuFe2O4, SiO2 and CuFe2O4/SiO2 composite were verified via XRD, as shown in Fig. 2(e). Feature peaks at 18.50°, 30.17°, 35.64°, 43.04°, 57.05° and 62.77° can be connected well with (111), (220), (311), (400), (511) and (440) panels of CuFe2O4, matching well with the standard ICDD card NO.025-0283 [26,27,43,44]. This proves that high purity CuFe2O4 was successful prepared. Meanwhile, feature peaks at 20.88°, 26.58°, 50.08°, 60.02° and 67.86° are correspond to (100), (101), (112), (211) and (212) panels of SiO2, which are well compatible with the standard ICDD card NO. 001–0649 [43]. It is essential that the intensity of feature peak at 26.58° presents a gradually increasing tendency with larger SiO2 content in CuFe2O4/SiO2 composite. These typical feature peaks of CuFe2O4 and SiO2 co-exist in CuFe2O4/SiO2 binary composite, indicating that CuFe2O4 nanoparticles are successfully coupled with SiO2 substrate. Furthermore, the FT-IR spectrums of as-obtained samples are shown in Fig. 2(f), the bands around 1662 cm−1 and 3400 cm−1 belong to stretching vibrations of absorbed water molecules and hydroxyl group, respectively. The strong absorption bands at 468 cm−1, 798 cm−1 and 1060 cm−1 are assigned to oscillatory vibration, symmetric stretching vibration and asymmetric stretching vibration of Si–O–Si bond, respectively [20,37,45,46]. The Si–O–Si bond at 451 cm−1 exists in the tetrahedral position is overlapped with metal oxides vibration [37]. The absorb peak at 543 cm−1 is attributed to metal-oxygen stretching [26,27,37]. XRD and FT-IR results demonstrate the successful synthesis of CuFe2O4/SiO2 composite.In the survey spectrum of CuFe2O4/SiO2-3% composite in Fig. 3 (a), the Fe, Cu, Si and O elements can be seen clearly, there is no extra element in comparison with the survey spectrum of CuFe2O4 (Fig. 3(d)). The broad and asymmetric peak of O1s (Fig. 3(b)) spectrum implies that there can be more than one chemical state for O element, the two peaks centered at 530.16 eV and 532.91 eV are assigned to lattice oxygen in CuFe2O4 and nonmetal oxides (Si–O), respectively [27]. Whereas, the peak centered at 531.55 eV belongs to the surface hydroxyl group (-OH) [43]. As shown in Fig. 3(c), the peaks centered at 103.38 eV and 100.30 eV indicate that Si element is presented in the form of SiO2 [30]. The Cu2p spectrum (Fig. 3(e)) exhibits four peaks located at 933.92 eV for Cu2p3/2, 953.40 eV for Cu2p1/2, 941.58 eV for the satellite feature of Cu2p3/2 and 962.56 eV for the satellite feature of Cu2p1/2 [43]. Meanwhile, the Fe2p spectrum (Fig. 3(f)) can be fitted into three contributions (FeO, Fe2O3 and Fe3O4), the peaks at 711.08 and 724.13 eV are assigned to the binding energies of 2p3/2 and 2p1/2 of Fe3+, 714.73 and 726.93 eV are attributed to the binding energies of 2p3/2 and 2p1/2 of Fe2+. In addition, the peak locating at 718.63 eV indicates that Fe3+ and Fe2+ coexist in the CuFe2O4/SiO2-3% composite [27,35]. XPS results further confirm the successful preparation of CuFe2O4/SiO2 composite.To illustrate the combination state between CuFe2O4 nanoparticles and SiO2 substrate, Zeta potential measurement of SiO2 particles was carried out at room temperature (25 °C) and laser wavelength of 660 nm. SiO2 particles were firstly suspended in ethylene glycol solution and then sonicated for 2 h to form a homogeneous dispersion. The measurement was calculated for three times and results are shown in Fig. 4 (a) and 4(b). The large zeta potential value (−54.0 mV) of SiO2 particles indicates its good dispersion stability in ethylene glycol solution, and suggests that particles repel each other and do not undergo flocculation [47]. The negatively charged surface of SiO2 supplies active sites to absorb cations of Fe3+ and Cu2+ through electrostatic attraction. Results demonstrate that SiO2 can be an effective carrier for CuFe2O4 nanoparticles.The specific surface areas of as-obtained catalysts were investigated by a nitrogen adsorption/desorption instrument and calculated by the multipoint Brunauer-Emmett-Teller (BET) method, nitrogen adsorption-desorption isotherms curves and pore size distributions are shown in Fig. 4(c) and (d), respectively. Pure CuFe2O4 has a relatively small specific surface area (∼42.6125 m2/g), but the specific surface areas of CuFe2O4/SiO2 composites progressively increases to 49.0999 m2/g, 70.7566 m2/g, 56.7653 m2/g and 53.6471 m2/g with SiO2 content increasing from 1 to 7 wt%. Moreover, the maximum value (70.7566 m2/g) achieved at 3 wt% content of addition suggests that 3 wt% content of SiO2 carrier possess enough capacity for dispersing CuFe2O4 nanoparticles. The results further suggest that the SiO2 carrier can inhibit the aggregation of CuFe2O4 nanoparticles, which is consistent with SEM image in Fig. 2(d). Additionally, the adsorption-desorption isotherms curve in Fig. 4(c) present typically type IV hysteresis loops when P/P 0 range is 0.4–1.0 [34,44]. The maximum pore size can reach 148 nm (Fig. 4(d)), indicating that the introduction of SiO2 carrier increases the pore content, pore size and specific surface area of catalysts, thus greatly improving the catalytic activity. Large specific surface area of CuFe2O4/SiO2 could offer more adsorption and reaction sites, which consequently result in better catalytic activity on energy components [43].The catalytic abilities of CuFe2O4/SiO2 composites on AP and RDX were investigated and shown in Fig. 5 (a) and (b). DSC curves in Fig. 5(a) show a distinct endothermic peak of AP at 245 °C assigning to transformation from orthorhombic phase to cubic phase [26]. The pure SiO2 advances the high temperature decomposition (HTD) and the low temperature decomposition (LTD) of AP from 403.8°C to 309.2 °C–389.8 °C and 300.6 °C, respectively, presenting a weaker catalytic decomposition on AP. Comparatively, the two exothermic peaks merge into one broad peak under the action of CuFe2O4 nanoparticles, and the peak temperature is advanced to 334.9 °C. It can be seen clearly in Fig. 5(a) that there exists a remarkably synergistic catalytic effect after CuFe2O4 and SiO2 combining, the exothermic peak temperatures of AP decrease to 315.8 °C, 310.7 °C, 313.3 °C and 318.6 °C with SiO2 content increasing from 1 to 7 wt%. In comparison with pure CuFe2O4 (334.9 °C) and SiO2 (389.8 °C), the 3 wt% content of SiO2 in CuFe2O4/SiO2 composites can achieve the optimum catalytic effect and the exothermic peak temperatures of AP is advanced by 24.2 and 79.1 °C, respectively.Taking good catalytic effect of CuFe2O4/SiO2 composites on AP thermal decomposing into consideration, its catalytic activity on RDX was further investigated, as shown in Fig. 5(b). The peak temperature of RDX exothermic decomposing is advanced to 241.1 °C and 239.8 °C under the catalytic effects of CuFe2O4 and SiO2, respectively, and the exothermic peak temperature of RDX decreases to 238.3 °C, 234.9 °C, 236.4 °C and 237.4 °C, with SiO2 content increasing from 1 to 7 wt%. Interestingly, the variation tendency of catalytic effect of CuFe2O4/SiO2 composites on RDX is consistent well with that of AP, in which the peak temperatures of them all decrease firstly and then increase accompanied with the increasing of SiO2 content in CuFe2O4/SiO2 composites.The synergistic effect of CuFe2O4 and SiO2 can be explained as: (1) The uniform dispersion of CuFe2O4 nanoparticles on the surface of SiO2 by electrostatic interaction and the effective inhibition of aggregation of nanoparticles proved by SEM analysis in subsection 3.1, both RDX (234.9 °C) and AP (310.7 °C) realized the lowest thermal decomposition peak temperature under the catalytic effect of CuFe2O4/SiO2 (3 wt%) composites; (2) The specific surface area of CuFe2O4/SiO2 composite achieves the maximum at the carrier content of 3 wt%, as described in subsection 3.1, which generates more active sites and higher reaction activity, resulting in better catalytic ability; (3) CuFe2O4 nanoparticles show more obvious catalytic effect than pure SiO2 carrier for thermal decomposition of AP and RDX, hence higher content of SiO2 (5 wt% and 7 wt%) would weaken the synergistic catalytic effect.In addition, the apparent activation energy (Eα) is an essential indicator reflecting the difficulty of decomposition process of energetic materials, which is of great significance for the study of catalytic decomposition performance [48,49]. In order to contrast the catalytic effect of CuFe2O4/SiO2 on AP and RDX, the Eα values of AP + CuFe2O4/SiO2 (3 wt%) and RDX + CuFe2O4/SiO2 (3 wt%) were calculated from DSC data recorded at different heating rates (β = 5.0, 10.0, 15.0 and 20.0 °C/min) using Kissinger method [50] and Flynn-Wall-Ozawa method [51]. As shown in Fig. 5(c) and (d), the peak shape remains unchanged while the decomposition peaks advance to high temperature successively with heating rate increasing. With the addition of CuFe2O4/SiO2 catalyst, the Eα values of AP and RDX decrease from 161.0 kJ/mol and 239.6 kJ/mol to 139.3 kJ/mol and 113.6 kJ/mol for Kissinger method, 141.7 kJ/mol and 116.1 kJ/mol for Flynn-Wall-Ozawa method, respectively. Comparing with previous research [26], CuFe2O4/SiO2 (3 wt%) decreases the Eα value of AP and RDX by 21.7 kJ/mol and 126 kJ/mol, respectively. It can be seen in Table 1 that fewer catalyst carrier (3 wt%) but higher catalytic activity is achieved, which illustrates the prominent catalyst effect of CuFe2O4/SiO2 composites on AP and RDX, and further confirms that SiO2 can be used as an effective carrier for CuFe2O4 nanoparticles.Multiple catalysts have been used to improve the thermal decomposition properties of AP and RDX, results all confirmed the catalytic effects on advancing the high exothermal peak temperature, reducing the apparent activation energy, and even transforming the slow two-stage decomposition process into a rapid one-step process. For comparison, the catalytic capacities of several catalysts or the combination of them are listed in Table 2 , which further verifies the excellent synergistic catalytic effect of the CuFe2O4/SiO2 composites on both AP and RDX.Both ignition delay time and flame propagation velocity are important parameters to investigate the ignition and combustion performance of energetic materials [56], in which the former is defined as the time interval between the triggering of laser and the appearance of igniting flame [39,40], and the latter is related to the amount and rate of gas production.As shown in Fig. 6 (a), the ignition delay time of all samples decreases sharply with the increase of laser power density. CuFe2O4 nanoparticles can shorten ignition delay time of RDX in lower power density of 109.3 W/cm2 and improve the ignition properties of RDX, revealing that CuFe2O4 nanoparticles tend to be more sensitive to laser radiation. However, the ignition delay time of RDX increases after introducing SiO2 and higher SiO2 content is corresponding to longer ignition delay time. Moreover, the ignition delay time values of all composites tend to be close to each other after the power density exceeding 155.3 W/cm2. In Fig. 6(b), the flame propagation velocity of RDX + CuFe2O4 is investigated, which exhibits a lower flame propagation velocity than that of pure RDX but the longest combustion time (272 ms). Besides, the flame propagation velocity of CuFe2O4/SiO2(3 wt%) is 2.73 times faster than pure RDX. Considering the thermally insulating property of SiO2 carrier, the abnormal phenomenon may be explained as (1) The poor thermal conductivity of SiO2 makes it a barrier for thermal conduction, therefore the temperature at the vicinity of SiO2 particles increases rapidly and results in multiple ignition points for combustion, thereby promoting the combustion progress [57,58]; (2) Once ignited, particulate product is ejected along with high-pressure gas to increase the size of the flame, facilitating laser feedback and heat transfer [31,32]; (3) It is worthy to note that higher content (5 wt%) of SiO2 may decrease the energy density and reactivity because of heat-sink effect [32].To illustrate the combustion performance of CuFe2O4/SiO2 composites, the burning snapshotting of RDX, RDX + CuFe2O4 and RDX + CuFe2O4/SiO2 (1 wt%, 3 wt%, 5 wt%) composites were recorded under the same condition for comparison. As shown in Fig. 7 (a), the combustion flame of pure RDX is dark red and weakly propagated, while the flame of RDX + CuFe2O4 is more luminous and lasts for 272 ms in Fig. 7(b), proving that CuFe2O4 could promote burning process of RDX. This might be attributed to the good thermal diffusivity of CuFe2O4 nanoparticle which facilitates heat transfer and accelerates combustion progress. Yet the combustion time decreases to 213 ms (Fig. 7(c)) after the introduction of SiO2 carrier (1 wt%). As shown in the 5 ms picture (Fig. 7(d)) of RDX + CuFe2O4/SiO2 (3 wt%), the mixture immediately forms an accelerated flame propagation fronter at the highest velocity and the combustion process completes in the shortest time (103 ms). The reaction intensity in Fig. 7(e) becomes weaker and flame propagation velocity declines sharply, which might be due to the high content of SiO2 in composite. All results reveal that the influence of reaction front area is greater than that of thermal diffusion at low mass percentage (3 wt%) additives in solid propellent [32]. Therefore, it is necessary to control the appropriate content of SiO2 carrier to achieve the expected effect in practical application.The production, transportation and storage process of energetic material has high requirements due to its high energy density, therefore the sensitivity regulation is particularly important [59]. The electrostatic discharge sensitivities (EDS) of samples were investigated by apparatus JGY-50(III) introduced in subsection 2.4. As shown in Fig. 8 , the introduction of CuFe2O4 nanoparticles produces a lower EDS value of RDX, which may easily produce security problems. However, E 50 values of RDX + CuFe2O4/SiO2 composites increase to 7.32 ± 0.05 mJ as the SiO2 content rises to 7 wt%, indicating that the EDS value decreases by 189% and 230% when compared it with pure RDX (3.87 ± 0.05 mJ) and RDX + CuFe2O4 (3.20 ± 0.05 mJ), respectively. Additionally, the value of electrostatic discharge sensitivity of RDX + SiO2 (100 wt%) decreases by 253% compared with that of pure RDX, which might be attributed to the micron-size structure of SiO2 which facilitates the dissipation of energy when electrostatic forces act on the composites [33,41], and the charge accumulation caused by aggregation of CuFe2O4 nanoparticles is eliminated to a great extent after introducing the SiO2. Therefore, in carrier content range of 3–5 wt%, the CuFe2O4/SiO2 catalyst not only possess excellent catalytic and combustion properties, but also present stable safety characteristics.In this work, CuFe2O4/SiO2 composite was successfully synthesized through solvothermal method, proving by phase structure, morphology and chemical bond. The effect of SiO2 content on the catalytic property of CuFe2O4/SiO2 composite was explored, and it is found that CuFe2O4/SiO2 (3 wt%) reduced the exothermic decomposition temperature of AP and RDX by 93.1 °C and 7.4 °C, respectively. In addition, the promotion effect of CuFe2O4/SiO2 composite on the combustion and safety performance of RDX were investigated and the results indicate that SiO2 also plays a significant role in accelerating the flame propagate and enhancing the anti-electrostatic ability of RDX, in which the flame propagate velocity increases from 0.705 ± 0.005 to 1.923 ± 0.025 m/s after introduction of SiO2 carrier (3 wt%) and the E 50 value rises from 3.87 ± 0.05 to 7.32 ± 0.05 mJ after introduction of SiO2 carrier (7 wt%). Therefore, SiO2 is confirmed to be an excellent carrier that can synergistic with CuFe2O4 to form combustion catalysts and be used in solid propellants. Furthermore, the range of SiO2 carrier content in which the catalyst exhibits remarkably catalytic performance, certain combustion promotion effect, and controllable electrostatic discharge sensitivity is found to be 3 wt% to 5 wt%.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 investigation received financial assistance from the National Nature Science Foundation of China (Grant Nos. 21673178, 22105160), the Natural Science Foundation of Shaanxi Province (Grant No. 2023-JC-ZD-07), the Foundation of Key Laboratory of Defense Science and technology (Grant No. 6142603032213) and the Key Science and Technology Innovation Team of Shaanxi Province (Grant No. 2022TD-33).

To enhance the catalytic activity of copper ferrite (CuFe2O4) nanoparticle and promote its application as combustion catalyst, a low-cost silicon dioxide (SiO2) carriers was employed to construct a novel CuFe2O4/SiO2 binary composites via solvothermal method. The phase structure, morphology and catalytic activity of CuFe2O4/SiO2 composites were studied firstly, and thermal decomposition, combustion and safety performance of ammonium perchlorate (AP) and 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) with it affecting were then systematically analyzed. The results show that CuFe2O4/SiO2 composite can remarkably either advance the decomposition peak temperature of AP and RDX, or reduce the apparent activation energy at their main decomposition zone. Moreover, the flame propagation rate of RDX was promoted by about 2.73 times with SiO2 content of 3 wt%, and safety property of energetic component was also improved greatly, in which depressing the electrostatic discharge sensitivity of pure RDX by about 1.89 times. In addition, the effective range of SiO2 carrier content in the binary catalyst is found to be 3 to 5 wt%. Therefore, SiO2 opens a new insight on the design of combustion catalyst carrier and will promote the application of CuFe2O4 catalyst in solid propellant.

Hydroformylation, also known as oxo-synthesis reaction, is one of the most important homogeneously catalyzed industrial processes for the production of aldehydes from alkenes and syngas with 100% atom economy, which was found by Otto Roelen as early as 1938. 1 , 2 , 3 , 4 , 5 Up to now, the production of chemical produces through this hydroformylation has exceeded 12 million tons every year, which is one of the most significant commercial uses of soluble homogeneous metal catalyst in the chemical industry. 6 , 7 The obtained aldehyde products for hydroformylation can be converted to the valuable and stable products (alcohols, ketones, acetals, amines products, etc.) through oxidation, hydrogenation, or reductive amination, which are extensively utilized in the synthesis of fine compounds such as insecticides, spices, food additives, and plasticizers. 8 , 9 The typical catalysts for hydroformylation of olefins are homogeneous complexes of the type [HM(CO)xLy], where L can be further CO or an organic ligand. A generally accepted series of the activities of the unmodified metal is as follows: 10 Rh >> Co > Ir > Ru > Pd > Mn > Fe > Ni >> Re. To date, only cobalt and rhodium catalysts can be used in industrial application; other metals only stay in the stage of academic research. Rhodium-phosphine complex catalyst has the advantages of mild reaction conditions, well catalytic performance, and low energy consumption. It has gradually become the mainstream in the industrial hydroformylation instead of cobalt carbonyl catalyst. 11 The uniform distribution of active sites, excellent catalytic activity, and superior chem/regioselectivity of homogeneous catalysts are only a few of their many benefits. However, the issue of catalyst separation leads to the loss of active metal and phosphine ligand, which is not conducive to large-scale application in industrial production. In contrast, heterogeneous catalysts can overcome catalyst separate deficiencies. Due to the surface properties of the support, the interaction between metal and support, and the microenvironment of the catalytic sites, heterogeneous catalysts can demonstrate excellent performance. Therefore, the development of heterogeneous catalysts with high activity and high stability for hydroformylation has important theoretical and practical significance. 12 The term “heterogeneous catalyst” describes a catalyst that immobilizes the active metal or metal complex on a solid support. Molecular sieve, 13 , 14 , 15 carbon materials, 5 , 16 , 17 inorganic oxides, 18 , 19 , 20 magnetic nanoparticles, 21 and organic polymers 22 , 23 , 24 are the examples of supports.In contrast to traditional heterogeneous catalysts, single-atomic catalysts (SACs) are a recently emerging class of catalytic material featured with unique single-atom dispersion and maximum atomic utilization of active metal. 25 , 26 The atomically dispersed metal anchored on support brings similar catalytic behavior to homogeneous catalyst. In addition, the heterogenous property of SACs makes them easy to be separated from the liquid-phase reaction mixture and achieve convenient recovery as well as recycling. Combining the advantages of homogeneous catalysts and heterogeneous catalysts, SACs exhibit high catalytic activity and selectivity in hydroformylation. 27 , 28 , 29 , 30 , 31 , 32 , 33 However, up to now, very few reviews of SACs in hydroformylation have been reported. In this paper, we summarize recent advances of SACs for hydroformylation. The effects of microstructure of SACs on the reactivity and chem/regioselectivity of hydroformylation are discussed. The support effect, ligand effect, and electron effect on the performance of SACs in hydroformylation are proposed. The mechanism of SACs in hydroformylation is elaborated. Finally, we summarize the current problems and challenges in this field, and propose the design and research ideas of SACs for hydroformylation (Figure 1 ).The application of SACs in hydroformylation is still in the early stage. According to the current results, SACs have great potential to achieve high activity and selectivity of hydroformylation (Figure 2 ) since they have extremely high metal dispersion, low coordination environment in the metal center, and the strong interaction between metal atoms and support. Herein, we focus on the recent development of SACs in the field of hydroformylation.Homogenous phosphines-modified Rh catalysts have shown remarkable performance in the hydroformylation process before the application of SACs. In the 1950s, Union Carbide applied RhCl(PPh3)3 to the industry. The “low-pressure oxo-progress” has much higher stability and milder conditions than Co-based catalysts. 47 Later, Rhone-Poulenc Company and Ruhrchemie Company jointly developed RCH/RP process to achieve a new two-phase (organic/water) catalytic system. In this process, the water-soluble Rh-P complex was dissolved in the water phase; the products were dissolved in the oil phase. 48 The effective separation of the products and catalyst can be achieved by simple static layering and decanting operation. Compared with Co-based catalysts, Rh-based system possessed higher catalytic activity, selectivity and stability, and the milder conditions. 49 , 50 Therefore, Rh SACs are the potential supported catalysts for hydroformylation under mild conditions.In 2016, Zhang et al. 42 synthesized Rh SACs by the impregnation method to adsorb Rh3+ onto ZnO nanowires (Rh1/ZnO-nw) for the hydroformylation process (Figure 3 A). Compared to the typical Wilkinson’s catalyst RhCl(PPh3)3 (Turnover number (TON) = 19000), the Rh1/ZnO-nw showed excellent activity (TON = 40000), and can be recycled and reused for four times without significant loss of reactivity and selectivity. However, the ratio of linear to branched aldehyde (L/B) was only 1.0. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images clearly distinguished that isolated Rh atoms were distinguishable from the ZnO nanowires (Figure 3B). In the characterization of in situ diffuse reflectance infrared Fourier transform spectroscopy study of CO adsorption (CO-DRIFTS) (Figure 3C), the absence of a Rh0-CO bridge adsorption peak implied that Rh was dispersed on ZnO support in the form of single atoms, which was compatible with the characterization results of HADDF-STEM. 54 , 55 , 56 In the same year, Wang et al. 51 reported CoO-supported Rh SACs (Rh/CoO) for the hydroformylation of propylene. The selectivity of butyraldehyde was as high as 94.4% and the turnover frequency (TOF) can reach 2065 h-1. After five cycles, the activity and selectivity of the catalyst remained at high levels (Figure 3D). Extended X-ray absorption fine structure (EXAFS) revealed that Rh atoms were atomically dispersed for the sample of 0.2% Rh/CoO. As shown in Figure 3E, the peak at ca. 2.0 Å was attributed to Rh-O shell, and no other peaks for Rh-Rh contribution were observed. Additional mechanistic studies revealed that the structural reconstruction of the Rh SACs took place during the catalytic process, which facilitated in the adsorption and activation of the reactants.Gao et al. 52 synthesized a phosphorus coordinated Rh SACs (Rh1/PNP-ND) through the metal-ligand coordination approach (Figures 3F and 3G), and applied it to the hydroformylation of styrene to achieve high conversion (>99%) and high selectivity (>90%) under mild conditions. The abundant carboxyl groups loaded on the surface of nanodiamond (ND) selectively react with the amino groups of pincer ligands (PNP) to obtain PNP-ND, which provided a large number of sites for the highly dispersed anchoring of Rh. The 31P solid-state NMR spectra showed that the chemical shift of P atoms migrated to the low-field region after the Rh species were anchored by PNP-ND. This provided direct evidences for the successful anchoring of Rh species in PNP-ND. In order to further demonstrate the adaptability to different substrates, the Rh1/PNP-ND was applied to the hydroformylation of a series of styrene derivatives, which demonstrated exceptional selectivity and activity, comparable to homogeneous catalysts.In 2020, Li et al. 45 synthesized 0.5% Rh/CeO2 SACs by electrostatic adsorption method innovatively, and coupled hydroformylation with low-temperature water-gas shift reactions. Without using any ligand, the catalytic system not only avoided the hydrogenation of styrene and phenylpropyl aldehyde occurred as the side reaction but also achieved high selectivity to obtain linear aldehyde in the hydroformylation of styrene and its derivatives (L/B = 3). Due to the high Ce vacancy density in CeO2 support, high loading of active Rh sited can be achieved. HAADF-STEM images and CO-DRIFTS proved that Rh existed in the form of single atom in 0.5% Rh/CeO2 catalyst. To further comprehend the new reaction route, a number of comparative tests were conducted. In contrast to the conventional reaction with styrene, CO, and H2 as substrates, the authors found that the higher linear/branched aldehyde ratio obtained was related to the reactants of CO and H2O. When secondary alcohol dehydrogenation was coupled with hydroformylation reaction, the selectivity of linear products was still higher than that of branched products, although the activity was lower. Based on the above facts, the author proposes a six-membered transition formed by the combination of C=C unsaturated double bond and formic acid. According to Marcovnikov's rule, the active H addition the end of C=C bond. This intermediate not only facilitates the insertion of carbonyl groups into the terminal C=C bond to form the linear aldehydes but also prevents the formation of phenyl Rh species, which ultimately inhibits the formation of linear aldehydes. 57 Zhao et al. 46 successfully encapsulated Rh within porous monophosphine polymers (POPs) by one-pot method to prepare Rh@POP-PTBA-HA-50. 58 , 59 , 60 According to the characterization of HAADF-STEM and EXAFS (Figure 3H), it is proved that Rh species were encapsulated as a single-atom in the POPs skeleton. Fourier transform infrared spectroscopy (FT-IR) spectrum of Rh@POP-PTBA-HA-50 showed that a strong C=N stretch at 1623 cm-1, and the peaks at 1700 and 3345 cm-1 attributed to aldehyde group were obviously weakened compared to 4,4′,4’’-phosphanetriyltribenzaldehyde (PTBA) and N2H4H2O (HA). In addition, the 13C magic angle spinning NMR peak of Rh@POP-PTBA-HA-50 at 162 ppm matched to the carbon atom of the C=N bond. Both of them indicated the formation of imine bonds. Compared to Rh(CO)2(acac)-PTBA, Rh@POP-PTBA-HA-50 showed a significant improvement in regioselectivity (linear aldehydes) from 62% to 92% in hydroformylation of 1-octene (Figure 3I). Due to the robust coordination of dispersed phosphine ligands with metal active species, the catalyst demonstrated remarkable catalytic activity (TON = 60000) and good thermal stability.The obtained aldehyde products from hydroformylation can be further converted into high-valuable chemicals like amines, carboxylic acids, and alcohols through additional oxidation, reduction, and hydrogenation. 61 , 62 , 63 , 64 , 65 Hydroformylation followed by other reactions through one-pot method has been extensively explored, such as “hydroformylation-hydrogenation”, “hydroformylation-acetalization”, “hydroformylation-aldol condensation”, “hydroformylation and reductive amination”, and so on. Following the atom economy and low energy consumption in green chemistry, combining SACs with tandem hydroformylation have become a powerful and promising synthetic method. Li et al. 53 successfully prepared hydroxyapatite (HAP)-supported single-atom Rh catalyst (Rh1/HAP) for the tandem hydroaminomethylation of olefins. (Figure 3J). HAADF-STEM and CO-DRIFTS results revealed that Rh atom was atomically dispersed on the HAP support. 1-hexene was almost entirely converted over 0.5Rh1/HAP under moderate reaction conditions, and the selectivity was 93.2%. The hydroformylation, condensation, and hydrogenation reactions are all parts of the overall hydrocarbamoylation reaction. According to the mechanistic study, the hydrocarbamoylation process is a speed-regulating step. Through separate evaluation of hydroformylation reaction, 0.5Rh1/HAP guaranteed high activity of hydroformylation reaction, thus ensuring the excellent catalytic activity of the tandem reaction.Co is another metal catalyst applied for hydroformylation industry. The catalytic activity of Rh is 103–104 times than that of Co. 66 , 67 However, the shortage and the high price of precious Rh limit its development and application in hydroformylation. Co continues to have a long-term role, since its effective antitoxic performance and the weak requirement for olefin’s purity. As early as 1952, the carbonyl cobalt catalysts with HCo(CO)4 as the active ingredient were first applied in the oxo-synthesis of propylene in 1952. Later, cobalt carbonyl modified by phosphines could decrease pressure to 5–10 MPa in the 1950s and the CO was replaced by PR3, P(OR), etc. Compared to CO, the phosphines possessed stronger σ-electron-donating ability and weaker π-receptor-accepting ability. The selectivity of linear aldehydes is significantly increased in phosphines modified Co system. 68 , 69 However, the hydrogenation of olefins to alkanes occurred, which reduced the activity relatively. In recent years, supported cobalt-based catalysts have attracted a lot of attention. Basic researches have been done in the laboratory, but there is still a significant gap between these efforts and industrial manufacturing.Recently, Cong et al. 34 developed the ultrasound-assisted impregnation method to design Co SACs supported by zirconium phosphate (CoZrP-2.0). The tight coordination of Co atom with phosphate group of ZrP prevented the leaching of Co, and enhanced the activity and stability of catalyst (Figures 4A and 4B). In CoZrP-2.0-catalyzed hydroformylation, the conversion of 1-octence was about 100%, and the selectivity of C9 aldehyde was 91.3%. After six cycles, the activity and selectivity of the catalyst remained at high levels. Based on the pyridine adsorption FT-IR spectrum and inductively coupled plasma-atomic emission spectrometry (ICP-AES) analysis of CoZrP-X catalyst (Figure 4C), it was clearly observed that the leaching of Co was closely related to the loss of Brønsted acid. On the CoZrP-2.0, the higher the loss of B-acid sites (CoZrP-2.0: 174.7 mmolg-1), the lower leaching of Co species. With the increase of P/Zr ratio, more Co atoms combined with phosphate groups by replacing the protons of the B-acid sites, which promoted the formation of ionic Co atoms. The coordination structure and chemical surroundings Co cites were examined in depth using EXAFS structural characterization (Figures 4D and 4E). Similar peaks at 1.5 Å and 2.6 Å corresponded to the initial coordination shells of Co-O and Co-Co, respectively, were presented in samples of CoZrP-0.5, CoZrP-1.0, and CoZrP-1.5. Only the Co-O bond (1.5 Å) was observed on CoZrP-2.0, revealing the single atom characteristic. With the increase of Brønsted acid site and BET surface area with the increase of P/Zr (from 0.5 to 2.0), Co atoms are more evenly dispersed on the support. According to the EXAFS characterization results, when P/Zr = 2.0, the Co atoms are dispersed as a single atom on the support. By comparing the catalytic performance of different P/Zr catalysts for 1-octene hydroformylation, it can be found that with the increase of P/Zr from 0.5 to 2.0, the conversion rate of the catalyst decreased slightly, but the selectivity for aldehydes increased from 59.4% to 89.6%, and the leaching rate of Co decreased sharply from 29.1% to 0.5%. Co SACs mainly take aldehydes as the main product, which has better activity, selectivity, and stability.Current industrial production of hydroformylation mainly employs Rh-based catalyst. The expensive price of rhodium has promoted the research of other alternative transition metal catalysts in hydroformylation. A pioneering work on ruthenium catalysts for homogeneous hydroformylation in 1965 by Wilkinson et al 70 is worth noting that Ru can significantly promote the catalytic activity of cobalt catalyst in hydroformylation reaction. Masanobu Hidai et al. 71 studied the synergistic effect of bimetallic catalysts in the hydroformylation of olefin. The initial reaction rate of hydroformylation of cyclohexene catalyzed by Co2(CO)8/Ru3(CO)12 was 19 times higher than that of Co2(CO)8, and this bimetallic catalyst has a wide range of substrate applicability. The addition of 1 wt% Ru to 10 wt% Co/AC increased the conversion of 1-hexene by nearly 60%, inhibited the side reaction of alkene isomerism, and improved the selectivity of aldehydes. Zhang et al. 16 believed that the addition of Ru can greatly improve the reducibility, provided more cobalt metal centers for the reaction.Escobar-Bedia et al. 35 developed a novel Ru-based catalyst (Ru@NC) containing isolated single atoms and disordered clusters in nitrogen-doped carbon matrix, that applied to hydroformylation of 1-hexene with good activity and selectivity (Figures 5A and 5B). The strong interaction between Ru and N atoms can improve the dispersion of metal and change the electronic properties of Ru atoms on the surface, thus affecting the stability and activity of catalyst. According to scanning electron microscope energy dispersive spectrometer findings, N and Ru atoms embedded in the carbon substrate were distributed uniformly on the support and their signals overlapped. Moreover, the potential connection between surface N and Ru atoms was confirmed by a prominent Ru-N bond at 460 cm-1 on the Raman spectra. According to the X-ray absorption near-edge region (XANES, left) spectral and EXAFS spectral (right) analysis (Figure 5C), Ru mainly existed in the form of single atoms in Ru@NC. However, Ru-Ru scattering intensity increased with the increase of the metal loading. Ru-Ru is detected to exist in a highly disordered state in the highly loaded catalyst, indicating the formation of small, dispersed, and highly disordered Ru clusters. In order to further investigate the impact of N atoms on the performance of catalyst, 0.2Ru@NaC and 0.2Ru@NC were conducted as the comparison experiments under the same circumstances (Figure 5D). The results showed that the rate of 0.2Ru@NaC decreased significantly with the prolongation of reaction time; the leaching of Ru in solution increased significantly. Moreover, the regioselectivity of 0.2Ru@NaC in hydroformylation was lower than that of 0.2Ru@NC. The interaction of surface N atom with Ru atom can stabilize and change the electronic properties of Ru atom.Various transition metals including Rh, Co, Ir, Ru, and Fe have been proven to be efficient catalysts for hydroformylation. Although Au is conventionally considered inactive for hydroformylation, numerous studies have shown that Au exhibits high olefin activity, 72 H2 dissociation, 73 , 74 and CO bonding capabilities. 75 At same time, Au was applied for CO oxidation, 75 water gas conversion reaction, 76 and methanol synthesis. 77 , 78 Wei et al. 40 encapsulated dispersed Au into purely siliceous zeolite to prepare Au(0.2%)@S-1 (Figure 6 A). The Au SACs showed high activity noticeable stability after 5 cycles in the hydroformylation of propylene, which was one order of magnitude greater than Au nanoparticle catalysts (Au(0.8%)@S-1 and Au(0.2%)/S-1) (Figures 6B and 6C). In addition, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS) provided evidence that Au was coated in the molecular sieve and the morphology of the molecular sieve was unaffected by Au species. The utilization of Cs-corrected HAADF-STEM (Figure 6D), XANES spectra (Figure 6E), and EXAFS spectra (Figure 6F) confirmed that the oxygen bridge bond of molecular sieve enclosed the atomically scattered Au to create the Au-O-SiOx structure, which maximizes the active site’s density and structural stability.The supports of heterogeneous catalysts often employed inorganic oxides, POPs, metal-organic frameworks (MOFs), carbon materials, etc. The chem/regioselectivity of reactions increased through the modification of organic phosphine ligands or inorganic materials, and MOF domain limitation. However, the deactivation of catalysts and the loss of active components are still needed to be explored. Table 1 summarizes the catalytic performance of hydroformylation on SACs.POPs are a new material composed of C, N, O, and H atoms with high specific surface area, low skeleton density, controllable pore structure, and excellent stability. 79 , 80 , 81 , 82 , 83 It provides a new class of polymer support for the preparation of SACs that possess the advantages of both homogeneous and heterogeneous catalysts. The utilization of POPs was conducive to the diversification of ligand modification, due to the immobilization of phosphines in polymer chains by covalent bonds. Further coordination of Rh atoms with P atoms can realize high loading of active metal. The high concentration of ligand can stabilize the metal atoms and prolong the life of the catalyst. In addition, POPs are typically insoluble in the majority of solvents, which prevents catalyst loss via dissolution. 84 However, the poor mechanical strength, poor thermal conductivity, complicated synthesis steps, and strict preparation conditions limit the large-scale production of POPs. For the first time, POL-PPh3 was synthesized via solvothermal polymerization in 2014 by the Xiao team and Ding team. 58 N2 adsorption/desorption curves showed high specific surface area of POL-PPh3 (1086 m2/g), and the concentrated pore size distribution at 0.7, 1.5, and 3–70 nm. The synthesized POL-PPh3 with graded porosity is conducive to the uniform dispersion of active centers. Jiang et al. 85 used POL-PPh3 as a support to synthesize Rh SACs (Rh/POL-PPh3), which exhibited outstanding activity in the hydroformylation of ethylene in a fixed-bed reactor. After long-term stability test for more than 1000 h, the conversion of ethylene maintained at a stable level, and the loss of Rh catalyst was only 0.0046%. HAADF-STEM image clearly shows that the isolated Rh atoms are uniformly dispersed on the POL-PPh3 with porous structure. In addition, no sintering or aggregation was seen in the Rh species after long-term stability tests. Only Rh-P and Rh-C bonds were found according to EXAFS spectroscopy, which demonstrated that the strong coordination of Rh atoms with the exposed P atoms in the POL-PPh3 framework prevented the loss of active metal during the reaction.Metal oxides (ZnO, CoO, CeO2, and Al2O3) are frequently employed as support due to their outstanding chemical, thermal, and mechanical stability. The interactions between metal and support, the hydrogen overflow effects, and synergistic effects influence the catalytic activity of oxide-supported metal catalysts. 86 , 87 However, the interaction between the active metal and the oxide support may lead to the migration of surface particles, and finally the inert oxide will coat the active metal particles, thus deactivating the catalyst. 88 Amsler et al. 66 investigated the activity and stability of Rh SACs loaded with different oxides (MgO, CeO2, and ZnO) in the hydroformylation of olefin by combining theoretical calculation and experimental study. Through calculating the free energy of supported catalysts in comparison to the complex HRh(CO)4, the atomically dispersed Rh/MgO was determined to be the most stable. HRh(CO)4 on flat oxide surfaces (CeO2 (111)) has catalytic activity comparable to that of molecular complexes. However, for the step edge on the MgO (301) surface, the calculation shows that the catalytic activity was significantly reduced. EXAFS characterization showed Rh atoms on MgO in higher coordination environments and higher degrees of confinement. The strong contact between Rh and the support, which interfered with the recovery of active species and the product’s desorption, resulted in the low activity.Molecular sieves have been applied in numerous catalytic fields because of their special shape selectivity, adjustable acidity, and high water/thermal stability. In addition, molecular sieves can also be used as carriers to support, coat, disperse, and stabilize metal-active species (nanoparticles, clusters, single atoms, and isolated ions), achieving excellent activity and stability in heterogeneous process. The main problem of using molecular sieve as support is that the pore size of the support itself will affect the activity and selectivity of the catalyst. Moreover, there are abundant proton acid sites on the surface of molecular sieve, which may promote side reactions such as aldol condensation. 89 Shang et al. 39 prepared Rh SACs (Rh@Y) utilizing an in situ guiding agent. In hydroformylation of olefins, Rh@Y showed significant catalytic activity, cycle stability, and substrate suitability under relatively mild conditions. Under the same experiment conditions, compared to other kind of catalysts (Rh/S-1, Rh/USY, Rh/ZSM-5, Rh/Beta, Rh/Mor, Rh/Y, and Rh-Y), Rh@Y showed higher catalytic efficiency. The characterization of MAS NMR, XPS, XAS, and HAADF-STEM images revealed that the introduced Rh species had no effect on the structural stability of zeolite. The isolated Rh&+ (& = 2.5) was successfully confined to the molecular sieve structure. The researchers explored the reaction mechanism of hydroformylation on the catalyst through density functional theory (DFT) calculation. The results show that the combination of α-C atom on 1-hexene and C atom on CO is the rate-determining step (RDS) of the reaction. The energy barrier of RDS for straight chain aldehydes is obviously lower than that of RDS for branched chain aldehydes, which is consistent with the experimental results. In addition, the calculation results further revealed the dynamic change information of the active Rh site in the reaction, which confirmed the space limitation of the molecular sieve and the stabilizing effect of the skeleton oxygen atom on the active Rh site in the catalytic system.Carbon materials have adjustable physical and chemical characteristics, adequate pore size distribution, and appropriate pH, which can improve the high dispersion of active components and accelerate the diffusion of reactants and products. Therefore, carbon materials, of which activated carbon is the most common, offer enormous potential in catalytic reactions. 90 Ligands play a major decisive role in the reactivity of Rh metal, especially in the selectivity. Due to the limitations of carbon material itself, it is difficult to build a stable bond structure between carbon material and organic phosphine ligand, and it is easy to lose phosphine ligand in the reaction, which will lead to the reduction of selectivity and the loss of Rh atoms, and ultimately affect the life of the catalyst. Feng et al. 91 constructed Rh SACs (Rh1/AC) on activated carbon for the carbonylation of methanol. The activity of Rh1/AC was three times than that of homogeneous catalyst ([Rh(CO)2I2]-) and 30 times than that of carbon-supported Rh nanoparticle catalyst. The HAADF-STEM diagram of Rh1/AC showed that isolated Rh atoms were dispersed on the surface of support before or after the reaction. DFT calculation and differential charge density calculation showed that the carbonyl group with electron donor properties were the optimal anchoring sites of Rh atoms, and the coordination bonds enhanced the electron density of the central Rh atoms and reduced the energy barrier of the speed control step.MOFs are crystals with adjustable pore structure, large specific surface, high porosity, and stable multi-dimensional network structure generated by the coordination and hybridization of multi-dentate organic ligands with transition metal ions. Selecting an appropriate metal precursor and confining it to the MOF pore through the nanoconfinement effect is the efficient method to achieve atomic-level dispersion of metal. The main problem of using MOFs as support is that the pore size of the support itself will affect the activity and selectivity of the catalyst, just like molecular sieves. 92 , 93 , 94 So far, Rh-based nanocluster catalysts combined with MOFs have been applied in the hydroformylation of alkenes, such as Rh@MIL-101, 95 Rh@IRMOF-3, 96 Rh@ZIF-8, 97 and Rh/MnMOF. 98 This paved the way for the continued development of atomically distributed catalysts based on MOFs.Phosphines-modified transition metals are widely employed in the hydroformylation of olefin, due to the high activity and selectivity under mild conditions. When coordinating with transition metals, P atoms can provide lone pair electrons to the empty tracks of transition metal to form σ-bond; at the same time, they also accept the filled d-orbital feedback electronic of metal atoms to form feedback π-bond. The σ-donor and π-acceptor properties of phosphines can regulate the performance of transition metal catalysts in organic reaction. 99 Compared to ligands composed of other elements of the same main group, phosphine ligands have the highest catalytic activity in hydroformylation (activity sequence: PPh3 > NPh3 > NP3 > AsPh3, SbPh3 > BiPh3).There are two or more P atoms in bi-/multi-dentate phosphines to coordinate and chelate with the transition metal to form a more stable catalyst active intermediate and avoid the deactivation. A bi-dentate phosphine ligand vinyl-biphephos functionalized by vinyl was copolymerized with vinyl monomer through solvothermal synthesis method reported by Li et al. 43 The prepared corresponding Rh catalyst Rh/CPOL-1bp&10P showed higher regioselectivity (L/B > 24) and activity (TOF > 1200 h-1) in propylene hydroformylation of fixed-bed reactor in comparison to Rh/POL-PPh3, Rh/POL-dppe, Rh/CPOL-bp&DVB, Rh/CPOL-bp&dppe, and Rh-biphephos/SiO2 (Figures 7A and 7B). The metal Rh atomically dispersing on the porous polymer support achieved high reactivity of catalysts according to results of EXAFS and HAADF-STEM. Moreover, the same group synthesized Xantphos-doped Rh/POPs-PPh3 via the copolymerization of vinyl-Xantphos and vinyl-PPh3 (Figure 7C). 44 Although the lower conversion of 1-octene was obtained in Xantphos-doped Rh/POPs-PPh3 system, the selectivity and regioselectivity of the target product were significantly superior to that of Rh/POPs-PPh3.In addition to phosphine ligands, some other ligands can also be used to regulate the hydroformylation of olefins in SACs. Yuan et al. 100 synthesized the hydrophilic catalyst (Rh1/PIPs) through alkalization, polymerization, impregnation, and other steps. When CO feed contained 1000 ppm H2S, the hydrocarboxylation of olefin was facilitated unexpectedly. The characterization of HAADF-STEM (Figures 7D and 7E) and EXAFS demonstrated that Rh existed as single atom in the Rh1/PIPs. Ex situ EXAFS and in situ DRIFTS revealed a ternary cycle mechanism of olefin hydrocarboxylation reactions (Figure 7F). The authors used CO and CO-H2S (1000 ppm H2S) as probe molecules, and added mixed liquids (cyclohexene, water, iodomethane, and other reactants and auxiliaries) in the form of bubbles to perform in situ DRIFTS (Figure 7G). The findings demonstrated that, in contrast to Rh-H bonds in CO systems, Rh-H bonds in CO- H2S systems exhibit a red-shift, which is attributable to the coordination of the strong electron ligand S species with Rh atoms. DFT calculation confirmed that the energy barrier of each step can be reduced with the addition of H2S, including the speed control step. This work offered a sulfur-resistant strategy for the carbonylation reactions, and advanced the theory of SACs in heterogeneous catalysis.In the catalytic process, the formation and breakage of chemical bonds of substrates, intermediates, or products on the catalyst surface are the results of the interactions between reactant molecules and metal atomic orbitals. Besides the properties of metal elements, the electronic structure of active metals is also influenced by metal size, supports, and the coordination environment of surface atoms. Wei et al. 41 developed a high-performance Rh SACs (Rh-Co-Pi/ZnO) by adding heteroatoms to regulate the microenvironment of active metals (Figure 8 A). In addition, the HAADF-STEM image (Figure 8B) of Rh-Co-Pi/ZnO clearly showed that the introduction of Pi greatly significantly increased the dispersion degree of Rh atoms. According to the characterization of CO-DRIFTs (Figure 8C) and XPS, the presence of Co atoms reduced the electron density around Rh and impaired the interaction of Rh-CO. Compared to Rh/ZnO system, the selectivity for linear aldehydes increased from 32.1% to 54.9%, the L/B ratio increased from 0.7 to 2.1, in Rh-Co-Pi/ZnO-catalyzed hydroformylation of 1-decene (Figure 8D). Inductively coupled plasma optical emission spectrometer showed that 94.1% of Rh, 96.4% of Co, and 95.9% of P remained in the recycled Rh-Co-Pi/ZnO. The well thermally stability and recyclability of Rh-Co-Pi/ZnO was reused for five cycles without noticeably decline of catalytic activity.In previous studies, ionic liquids (ILs) have been shown to be effective in protecting and stabilizing nano- and homogeneous catalysts. Ding et al. 101 found that ILs can increase the activation energy of single atoms aggregation and adjust the oxidation valence state of metal atoms, and first proposed a simple and universal strategy of stabilizing SACs. In 2021, the team extended this strategy to Rh SACs to investigate the effect of ILs on the stability of styrene hydroformylation on Rh1/TiO2 (Figure 8E). After five cycles of reaction, the TOF of unmodified Rh1/TiO2 decreased from 1250 h-1 to only 10 h-1, and the loading of Rh decreased from 0.1% to 0.05%. The initial TOF of the IL-stabilized catalyst was lower than that of Rh1/TiO2, but its stability was significantly increased. In particular, the TOF value of 1-(2-hydroxyethyl)-3-methylimidazolium bis (trifluoromethanesulfonyl) imide ([OHEmim][Tf2N])-stabilized Rh1/TiO2 only decreased from 878 to 800 h-1, and the loading of remained at about 0.1%. DFT calculation showed that ILs can increase the binding energy from 0.69 to 1.19 eV by acting as the linker between Rh atoms and TiO2, thus improving its anti-leaching performance.There are few studies on the mechanism of hydroformylation catalyzed by SACs; many models and inferences need to be further explored for verification. Lee et al. 36 studied the influence of ReOx promoter and the mechanism of hydroformylation of ethylene catalyzed by atomically dispersed Rh-ReOx/γ-Al2O3 (Figure 9 A). The synergistic effect of atomically dispersed Rh-ReOx with γ-Al2O3 was revealed using DFT calculations and microkinetic modeling. In contrast to the typical Wilkinson’s catalyst, the stable Rh(CO)2 precursor exhibited a 16-electron square planar structure by coordinating with two oxygen atoms on the surface. The RDS of hydroformylation depended on the local surroundings of Rh atoms. In the absence of ReOx, the rate was controlled by the CO insertion step; in the presence of ReOx, the Rh-CO coordination bond weakened, and CO coordination was the rate-controlling step. Meanwhile, ReOx improved the selectivity of propanal by blocking the main hydrogenation pathway through steric effects.Rh/POL-PPh3 synthesized by Jiang et al. 85 showed excellent performance and ultra-high stability in fixed bed of olefins hydroformylation, which was due to the similar catalytic function to that of homogeneous catalyst HRh(CO)(PPh3)3 (Figure 9B). Ma et al. 102 revealed the dual role of the polymer 3V-PPh3 monomer as both support and ligand in Rh/3V-PPh3 catalyzing hydroformylation of ethylene through quantum chemistry method. Compared with PPh3 as support, the adsorption energy of Rh atoms on 3V-PPh3 increased, indicating that the introduction of vinyl increased the Rh-P bond interaction. Secondly, the high density of P atoms (Rh: P = 1:3) exposed on supports helped to improve the dispersion degree of Rh atoms, to increase the energy barrier formed by Rh-Rh bond, finally to improve the stability of catalyst. In this paper, the dual action mechanism of 3V-PPh3 as support and ligand in Rh/3V-PPh3 is studied, and the reason why Rh atoms are not easy to lose is explained from a microscopic perspective, which is helpful to understand the relationship between microstructure and electronic effect, and provides theoretical guidance for the development and design of efficient heterogeneous catalyst.Gao et al. 52 used DFT calculation to explore the regioselective mechanism of Rh1/PNP-ND-catalyzed hydroformylation of styrene (Figure 9C). The Rh1/PNP-ND model was established based on the coordination of one Rh atom with two P atoms on the similar single-layer graphene. After optimization, a square planar structure was obtained under reaction conditions, with the coordination of one Rh, two P, one H, and one CO. It is generally recognized that the step of olefin insertion is crucial for determining the regioselectivity. The reaction barrier of the formation of branched aldehydes (0.69 eV) was significantly lower than that of linear aldehydes (0.74 eV). After calculation, the ratio of the relative rates of branched aldehydes and linear aldehydes was 5.75. The ratio of two products was predicted to be 85:15, which was consistent with the experimental data. Further thermodynamic analysis revealed that, starting from the same alkene coordination state, the ΔG of the branched alkyl complex and linear alkyl complex is -3.36 and -1.21 kcal/mol, respectively. In conclusion, the coordination environment of Rh atoms in Rh1PNP-ND was favorable for the formation of branched chain products in both thermodynamics and dynamics.Wang et al. 51 thoroughly investigated the significant performance of 0.2% Rh/CoO in hydroformylation of propylene combining DFT calculation with characterization data (Figure 9D). The DRIFT spectrum of 0.2% Rh/CoO demonstrated that the adsorption of propylene was greatly enhanced when exposed to H2 and CO atmosphere. The binding energy of Rh 3d in the mixed atmosphere (H2, CO, and propylene) was 1.3 eV lower than that without any gas treatment, which was obviously larger than the bias generated by the catalyst exposed to H2 or CO atmosphere, according to the XPS spectrum of 0.2% Rh/CoO. It can be inferred that Rh atoms in Rh/CoO undergo structural reconstruction during the catalytic process, which promoted the adsorption and activation of reactants. Based on the calculation of DFT, four stable co-adsorption configurations were proposed, designated as configurations Ⅰ, Ⅱ, Ⅲ, and IV. The relative positions of the H atoms and adjacent unsaturated C atoms helped to conclude that configurations Ⅰ and Ⅲ tend to form linear products, whereas configurations Ⅱ and IV tend to form branched products. Further investigation of the paths of configurations Ⅰ and Ⅱ showed that the last step of product formation was the rate-limiting step with the highest energy barrier. The energy barrier of the rate-limiting step in configuration I is 0.063 eV lower than that in configuration II, indicating the favorable formation of linear products.Wei et al. 37 created an effective and stable Co SACs (Co/β-Mo2C) by utilizing the potent electronic metal–support interaction (EMSI) effects between Co atoms and β-Mo2C. This catalyst outperformed all previously reported heterogenetic Co-based catalyst in hydroformylation of propylene (TOF up to 749 h-1). After repeated use for five times, the activity of catalyst did not decline noticeably. Compared to the bulk Co particles, Co/β-Mo2C with single Co atom has better activity for hydroformylation of olefin, and the TOF value was increased by 8.7 times. Based on the characterization of XPS, Baber charge, charge density, and Co density of states, it can be seen that the EMSI effect between single Co atoms and the support tailored the electronic properties of metal to be positively charged Co2+ and reduced the electronic density of Co. According to DFT calculations, the electron deficient property of the Co atoms contributed to CO insertion, thereby increasing the activity of hydroformylation of olefins (Figure 9E). For Co-based hydroformylation, the significant EMSI interaction between single-atom Co and the support in Co/β-Mo2C was crucial in optimizing the charge density, lowering the reaction potential energy, and stabilizing the active site.Insoo Ro et al. 38 reported heterogeneous Rh-WO x pair site catalysts for ethylene hydroformylation. Two active sites (Rh atoms and WO x species deposited on Al2O3) worked together to catalyze various stages of the reaction. The structure of catalyst can be altered by varying the loading of WO x on the support, which regulated the catalytic activity of ethylene hydroformylation in turn (Figure 9F). According to HAADF-STEM (Figure 9G) and CO-Fourier transform infrared spectrometer characterization, Rh and WO x were located in Rh/0.7W as independent sites and were also forming Rh-W pair sites. Due to the synergistic interaction between the active sites on Rh and WO x , Rh/0.7W possessed the highest activity (0.1 gpropanal cm-3h-1) and selectivity ( > 95%). A bifunctional mechanism was proposed based on the experimental kinetics and First-principles microdynamics simulations (Figure 9H). Rh assisted WO x reduction, which binded ethylene molecules; ethylene was transferred from WO x to Rh; H2 dissociated at the Rh-WO x interface to form two hydrogen atoms, one of which binded to the Rh-WO x interface. The bi-functional catalyst also depended on the geometry of the Rh-WO x interface, the energetics of reconfiguring the coordination of the pair site during the reaction, and the capacity to transfer molecules between the active centers of the pair site.In SACs, active metal is loaded on the surface of the support in the form of a single atom, and the requirements for characterization methods of SACs have also reached unprecedented atomic-level accuracy. In recent years, the rapid development of electron microscopy and spectroscopy technology has provided support for analyzing the spatial distribution, electronic structure, and coordination environment of metal centers, and provided reliable evidence for exploring the catalytic performance, structure-activity relationship, and catalytic mechanism. The characterization techniques applicable to SACs include HAADF-STEM, XAS, and CO-DRIFT.HAADF-STEM improves the measurement accuracy to atomic level, and can clearly observe isolated metal atoms and their spatial distribution on the support, becoming the most direct means to characterize SACs. 103 It provides strong evidence for further understanding the mechanism of atomic catalytic reaction and identifying the coordination structure of metal center on the support. The brightness of the atoms in the image is proportional to the square of the atomic number, so as to distinguish between heavy atoms (such as Pd, Pt, Ru, Rh, Co, etc.) and light atoms (such as N, O, C, etc.). 104 , 105 In the prepared Rh1/HAP, Li et al., 53 using HAADF-STEM, can clearly observe that the isolated Rh atoms are evenly distributed on the HAP (Figure 10 A). Shang et al. 39 observed atomically dispersed Rh species distributed in molecular sieve using Cs-HADDF-STEM. Due to the difference in atomic contrast (Si = 14, O = 8, Al = 13, and Rh = 45), the brightest spot in the image can be identified as the Rh atoms (Figure 10B).X-ray absorption spectroscopy (XAS) is used to measure the structure of X-ray absorption coefficient varying with energy. The sample excites its core electrons to transition to the empty orbit by absorbing X-ray (XANES) or transition to continuous state to form wave dry radiation with surrounding atoms (EXAFS). The chemical valence state and electronic structure of elements can be obtained from XANES, and the two-dimensional local structure information of adjacent atoms can be obtained from EXAFS. Therefore, XAS is widely used to study the structural model of active sites and explore the mechanism of monatomic catalysis. 27 Ding et al. 101 used XAS to characterize the synthesized atomically dispersed Rh1/TiO2. The K-edge XANES of Rh was studied with Rh foil and Rh2O3 as reference materials. Compared with the standard samples, the energy absorption curve of Rh1/TiO2 was very close to that of Rh2O3, indicating that the average oxidation state of Rh was close to 3+ (Figure 10C). According to EXAFS, Rh displayed a dominant peak at around 1.6 Å, which was assigned to the Rh-O first shell. There was no obvious peak at 2.3 Å, which was attributable to Rh-Rh scattering (Figure 10D).The probe molecule infrared spectroscopy shows different vibration frequencies for metal atoms in different chemical environments, which is an effective mean to characterize the dispersion state and electronic state of metal particles in supported catalysts. In the CO-DRIFT spectrum, the adsorption form of CO on metal can be judged according to the position of CO adsorption peak, and then the dispersion state of metal particles can be determined. 45 , 106 , 107 , 108 Li et al. 45 used CO-DRIFT technology to identify the existence of Rh loaded on CeO2. For Rh1/CeO2, the positions of infrared absorption peaks of CO were 2010 and 2052 cm-1, which are attributed to the symmetric and asymmetric vibration of gem-dicarbonyl doublet CO on positively charged Rh atoms (Figure 10E). A peak centered at 2052 cm-1 was also observed which corresponds to the linear CO adsorption on Rh atoms. For NP-Rh/CeO2 and 5Rh/CeO2, the positions of the infrared adsorption peaks of CO were 1860 or 1800 cm-1, and 2046 and 1960 cm-1, which respectively correspond to the bridge adsorption between two Pt atoms, the linear adsorption of CO molecules on the surface of Rh atoms and the adsorption at the interface (Figure 10F).DFT calculation is often combined with relevant experiments to further explore the reaction mechanism by studying the properties of catalytic materials (such as bond length, adsorption energy, etc.). DFT calculation in catalyst research mainly starts from the following four aspects: structural stability judgment, reaction free energy calculation, electronic structure analysis, and molecular diffusion/adsorption dynamics simulation. It is helpful to predict catalyst structure and stability, evaluate catalyst performance, innovate catalyst design strategies, and finally achieve SACs with high activity, high selectivity, and strong stability. 109 , 110 , 111 Wei et al. 40 clarified the reaction mechanism of propylene hydroformylation on the catalyst through DFT calculation. Firstly, the most reasonable model of Au(0.2%)@S-1 is determined, that is, a single Au atom replaces the Si atom at the T8 site on the crystal S-1 zeolite. Secondly, the adsorption energy of H, CO, and propylene is calculated on the Au(0.2%)@S-1 model. Among them, the adsorption capacity of H is the strongest (2.19 eV), followed by propylene (1.18 eV) and CO (1.27 eV), indicating that the adsorption capacity of propylene on the Au(0.2%)@S-1 model is moderate, which is conducive to the adsorption and desorption of reactants on the active center. It is also calculated that the adsorption energies of CH3CH2CH2 ∗ and CH3CH2CH2CO∗ are much more negative than those of propylene, indicating that the olefin insertion and CO insertion reactions are thermodynamically favorable. Based on the above analysis, a possible mechanism of propylene hydroformylation on Au(0.2%)@S-1 is proposed (Figure 10G).It is difficult to separate and detect the free radicals and intermediates in the chemical reaction process, which makes it difficult to speculate the reaction mechanism. In situ Raman, in situ XPS, isotope labeling, and other technologies can monitor the dynamic evolution of catalysts and reaction intermediates in real time under experimental conditions, which helps to accurately understand the structure of catalysts and build theoretical models, making outstanding contributions to the design of various effective catalysts.In this paper, the catalytic application and reaction mechanism of SACs in hydroformylation of olefins are summarized. The effects of microstructure regulation on catalytic activity, chemical/regioselectivity, and stability are discussed. The strategies of support effect, ligand effect, and electronic effect are proposed to adjust the performance of SACs. Advanced characterization techniques HADDF-STEM, XAFs, and DFT calculations are used to further study the mechanism. Although the application of SACs in hydroformylation is still in its infancy, SACs have already shown excellent performance. Existing research demonstrates that the SACs, notably the Rh SACs, have distinctive electronic/coordination structure, high atom utilization, unsaturated active center coordination, and tunable central metal electronic structure; the above characteristics make its catalytic activity equal to or even better than that of homogeneous catalyst. More importantly, the strong coordination between active metals and supports can effectively avoid the loss of Rh, which provides a new direction for the development of heterogeneous hydroformylation.Despite the significant development, there are still a dearth of pertinent studies and numerous pressing issues that need to be resolved. (1) In contrast to nanocatalyst and cluster catalyst, the active metal in SACs is atomic dispersion on the support, and the metal surface energy in SACs increases sharply. In the process of preparation and reaction, metal atoms are easy to migrate and agglomerate, which lead to the instability and deactivation of the catalyst. Therefore, the key to the synthesis of catalyst is to select an appropriate support. The coordination between the defect sites on the support surface and the single metal atom to prevent the agglomeration phenomenon can not only stabilize the single metal atom but also expose the active sites of the metal. The atomic dispersion of metal precursors can be achieved by means of space limitation, defect capture, and ligand anchoring, which can effectively limit the migration and aggregation of monodisperse metal atoms on the support. In addition, SACs cannot provide multiple adjacent metal sites, and its metallicity is often regulated by the support. Therefore, when multiple metal active sites are required to be activated cooperatively and active metals are required to have strong metallicity for catalytic reactions, SACs are difficult to achieve efficient catalytic activity. Fully exposed cluster catalysts (FECCs) can not only provide adjacent metal active sites but also partially maintain its metallicity on the basis of 100% metal dispersion. Metals in FECCs are mainly composed of very small clusters, and all atoms in the clusters are in the state of coordination unsaturated. FECCs have been widely used in alkane dehydrogenation, toluene hydrogenation, CO2 reduction, LGWS, and other reactions, and become an important field in heterogeneous catalysis. FECCs, as an extension of the concept of SACs, can well solve the problem of single active site in SACs, which makes it possible to efficiently carry out multi-step and complex catalytic reaction systems. As a conceptual extension of SACs, FECCs can solve the problem of single active sites in SACs, and provide a new way to design efficient catalysts. (2) In order to fill the defect that SACs have a single metal center and low loading, a second metal is introduced to synthesize dual-atom-site catalysts (DASCs) and nano-single-atom-site catalysts (NSASCs). As a further extension of the concept of SACs, DACs/NSASACs achieve low-cost, high selectivity, high stability, and antitoxicity catalysts. They retain the advantages of SACs, and introduce a variety of interactions, such as synergistic effect, geometric effect, and electronic effect. With the diversity of metal atoms in DACs/NSASACs, it is of great significance to save precious metal resources, reduce production costs, and realize industrial applications. So far, the reported DACs/NSASACs have been successfully applied to hydrogen evolution reaction, O2 reduction/evolution reaction, N2 reduction reaction, CO oxidation reaction, and other catalytic fields. However, how to control the structure of diatomic sites, improve the density of catalytic sites, and reveal the synergistic effect between atomic sites and the structure-activity relationship of catalysts through accurate structural characterization or theoretical calculation is still a major challenge. (3) Most of the reported catalyst supports with remarkable performance are limited to metal oxides and porous polymers. The synthesis of SACs using metal oxide as the support has the advantages of simple synthesis process and straightforward catalyst model, which is conducive to exploring the reaction mechanism of olefin hydroformylation. In addition, the absence of phosphine ligand is extremely valuable for environmental preservation. However, the surface modification of this catalyst is limited, and the poor regioselectivity is difficult to reach the level of homogeneous catalyst. Due to the diversity of synthesis methods of porous polymers, mono/multidentate phosphine ligands can be modified into porous polymer materials. The SACs supported by this method have excellent catalytic activity and selectivity for hydroformylation. The high density of phosphine ligand can avoid the loss of Rh and improve the stability of catalyst. However, the complex synthesis of porous polymer materials, the expensive ligand, and the poor mechanical strength seriously limit the mass production of catalysts. (4) As the products of hydroformylation, aldehydes are high value-added intermediates that can be converted into amines, alcohols, or acetals by further reactions. The one-pot tandem hydroformylation-hydrogenation reaction, hydroformylation-adol condensation reaction, hydroformylation-acetalization reaction, and hydroformylation-reductive amination reaction are economical methods to obtain above productions. At present, SACs or even supported catalysts are rarely reported in this area, which calls for more investigation. (5) Hydroformylation with synthesis gas as raw material has become the mainstream of modern chemical industry for its mature process, low cost, and suitability. However, due to the high toxicity and explosiveness of syngas, researchers are committed to studying green and efficient alternatives, such as HCHO, CO2, HCOOH, aldehydes, etc. Among them, the hydroformylation of HCHO as raw material has achieved good progress in reactivity and regioselectivity, and HCHO is cheap and easy to obtain, convenient for storage, transportation, and atmospheric pressure application. Therefore, HCHO is a promising substitute for synthesis of gas. CO2 is a clean, low-cost, and abundant raw material. However, the inert carbon-oxygen bond in CO2 makes it difficult to add metal-activated species, resulting in the poor selectivity of target products. The utilization of CO2 in hydroformylation is still in the laboratory research and development stage. The hydroformylation of olefins using these syngas substitutes often requires the modification of precious metal catalysts with complex phosphine ligands, which leads to high production costs, so it is still a long way for the industrial application. (6) The prepared SACs are still in the early stages of fundamental research, with the defects such as poor thermal stability, high metal surface energy, and low active metal loading. Therefore, there are still great challenges in industrial production. The development of high stability and applicability of SACs is crucial for meeting the demands of industrial applications. The macroscopic preparation of SACs is the long-term pursuit and the most challenging ultimate goal in hydroformylation. (7) Further research is still required to fully understand the catalytic mechanism of SACs in hydroformylation, including the catalytic active species, reaction mechanism, and inactivation process. The synthesis and regulation of particular catalysts from the atomic scale can be realized by the means of in situ electron microscopy, in situ synchrotron radiation, and other contemporary characterization technologies. Combined with DFT calculations, the catalyst structure and reaction pathway can be simulated. Above methods provide a crucial scientific foundation for explaining the structure-activity relationship of SACs. In contrast to nanocatalyst and cluster catalyst, the active metal in SACs is atomic dispersion on the support, and the metal surface energy in SACs increases sharply. In the process of preparation and reaction, metal atoms are easy to migrate and agglomerate, which lead to the instability and deactivation of the catalyst. Therefore, the key to the synthesis of catalyst is to select an appropriate support. The coordination between the defect sites on the support surface and the single metal atom to prevent the agglomeration phenomenon can not only stabilize the single metal atom but also expose the active sites of the metal. The atomic dispersion of metal precursors can be achieved by means of space limitation, defect capture, and ligand anchoring, which can effectively limit the migration and aggregation of monodisperse metal atoms on the support. In addition, SACs cannot provide multiple adjacent metal sites, and its metallicity is often regulated by the support. Therefore, when multiple metal active sites are required to be activated cooperatively and active metals are required to have strong metallicity for catalytic reactions, SACs are difficult to achieve efficient catalytic activity. Fully exposed cluster catalysts (FECCs) can not only provide adjacent metal active sites but also partially maintain its metallicity on the basis of 100% metal dispersion. Metals in FECCs are mainly composed of very small clusters, and all atoms in the clusters are in the state of coordination unsaturated. FECCs have been widely used in alkane dehydrogenation, toluene hydrogenation, CO2 reduction, LGWS, and other reactions, and become an important field in heterogeneous catalysis. FECCs, as an extension of the concept of SACs, can well solve the problem of single active site in SACs, which makes it possible to efficiently carry out multi-step and complex catalytic reaction systems. As a conceptual extension of SACs, FECCs can solve the problem of single active sites in SACs, and provide a new way to design efficient catalysts.In order to fill the defect that SACs have a single metal center and low loading, a second metal is introduced to synthesize dual-atom-site catalysts (DASCs) and nano-single-atom-site catalysts (NSASCs). As a further extension of the concept of SACs, DACs/NSASACs achieve low-cost, high selectivity, high stability, and antitoxicity catalysts. They retain the advantages of SACs, and introduce a variety of interactions, such as synergistic effect, geometric effect, and electronic effect. With the diversity of metal atoms in DACs/NSASACs, it is of great significance to save precious metal resources, reduce production costs, and realize industrial applications. So far, the reported DACs/NSASACs have been successfully applied to hydrogen evolution reaction, O2 reduction/evolution reaction, N2 reduction reaction, CO oxidation reaction, and other catalytic fields. However, how to control the structure of diatomic sites, improve the density of catalytic sites, and reveal the synergistic effect between atomic sites and the structure-activity relationship of catalysts through accurate structural characterization or theoretical calculation is still a major challenge.Most of the reported catalyst supports with remarkable performance are limited to metal oxides and porous polymers. The synthesis of SACs using metal oxide as the support has the advantages of simple synthesis process and straightforward catalyst model, which is conducive to exploring the reaction mechanism of olefin hydroformylation. In addition, the absence of phosphine ligand is extremely valuable for environmental preservation. However, the surface modification of this catalyst is limited, and the poor regioselectivity is difficult to reach the level of homogeneous catalyst. Due to the diversity of synthesis methods of porous polymers, mono/multidentate phosphine ligands can be modified into porous polymer materials. The SACs supported by this method have excellent catalytic activity and selectivity for hydroformylation. The high density of phosphine ligand can avoid the loss of Rh and improve the stability of catalyst. However, the complex synthesis of porous polymer materials, the expensive ligand, and the poor mechanical strength seriously limit the mass production of catalysts.As the products of hydroformylation, aldehydes are high value-added intermediates that can be converted into amines, alcohols, or acetals by further reactions. The one-pot tandem hydroformylation-hydrogenation reaction, hydroformylation-adol condensation reaction, hydroformylation-acetalization reaction, and hydroformylation-reductive amination reaction are economical methods to obtain above productions. At present, SACs or even supported catalysts are rarely reported in this area, which calls for more investigation.Hydroformylation with synthesis gas as raw material has become the mainstream of modern chemical industry for its mature process, low cost, and suitability. However, due to the high toxicity and explosiveness of syngas, researchers are committed to studying green and efficient alternatives, such as HCHO, CO2, HCOOH, aldehydes, etc. Among them, the hydroformylation of HCHO as raw material has achieved good progress in reactivity and regioselectivity, and HCHO is cheap and easy to obtain, convenient for storage, transportation, and atmospheric pressure application. Therefore, HCHO is a promising substitute for synthesis of gas. CO2 is a clean, low-cost, and abundant raw material. However, the inert carbon-oxygen bond in CO2 makes it difficult to add metal-activated species, resulting in the poor selectivity of target products. The utilization of CO2 in hydroformylation is still in the laboratory research and development stage. The hydroformylation of olefins using these syngas substitutes often requires the modification of precious metal catalysts with complex phosphine ligands, which leads to high production costs, so it is still a long way for the industrial application.The prepared SACs are still in the early stages of fundamental research, with the defects such as poor thermal stability, high metal surface energy, and low active metal loading. Therefore, there are still great challenges in industrial production. The development of high stability and applicability of SACs is crucial for meeting the demands of industrial applications. The macroscopic preparation of SACs is the long-term pursuit and the most challenging ultimate goal in hydroformylation.Further research is still required to fully understand the catalytic mechanism of SACs in hydroformylation, including the catalytic active species, reaction mechanism, and inactivation process. The synthesis and regulation of particular catalysts from the atomic scale can be realized by the means of in situ electron microscopy, in situ synchrotron radiation, and other contemporary characterization technologies. Combined with DFT calculations, the catalyst structure and reaction pathway can be simulated. Above methods provide a crucial scientific foundation for explaining the structure-activity relationship of SACs.This work was supported by National Natural Science Foundation of China (Nos. 22108306, 22102214), Taishan Scholars Program of Shandong Province (No. tsqn201909065), Shandong Provincial Natural Science Foundation (Nos. ZR2021YQ15, ZR2020QB174), and the Fundamental Research Funds for the Central Universities (No. 22CX07009A).Writing - Original Draft, Conceptualization, S.T. and D.Y.; Review & Editing, M.W., G.X., W.W., and Y.Z.; Writing - Review & Editing, Supervision, Funding acquisition, Y.P.The authors declare no competing interests.

Hydroformylation is one of the most significant homogeneous reactions. Compared with homogeneous catalysts, heterogeneous catalysts are easy to be separated from the system. However, heterogeneous catalysis faces the problems of low activity and poor chemical/regional selectivity. Therefore, there are theoretical and practical significance to develop efficient heterogeneous catalysts. SACs can be widely applied in hydroformylation in the future, due to the high atom utilization efficiency, stable active sites, easy separation, and recovery. In this review, the recent advances of SACs for hydroformylation are summarized. The regulation of microstructure affected on the reactivity, stability of SACs, and chem/regioselectivity of SACs for hydroformylation are discussed. The support effect, ligand effect, and electron effect on the performance of SACs are proposed, and the catalytic mechanism of SACs is elaborated. Finally, we summarize the current challenges in this field, and propose the design and research ideas of SACs for hydroformylation of olefins.

Polyurethanes are one of the most versatile polymer materials in the world and can be used for a wide range of end-user applications, such as furniture, coatings, adhesives, building materials, fibers, cushions, paints, elastomers and synthetic leathers [1,2]. Methylene diphenyl diisocyanate (MDI) is one of the main raw materials to produce polyurethane. MDI is prepared by phosgenation of methylene diphenyl diamine (MDA). MDA is synthesized by the reaction of aniline and formaldehyde in the presence of hydrochloric acid catalyst. The purity of aniline determines the quality of MDI and affect polyurethane production [3]. Therefore, aniline synthesis is an inevitable part of the ecosystem of polyurethane industry. Nitrobenzene reduction is a widely used process for aniline production. Mostly, various oxides (silicates, alumina) and carbon-supported (carbon nanotubes, graphene, carbon black) catalysts are used with palladium, platinum, or rhodium metals in nitrobenzene reduction [4–10]. Other special carbon containing systems including different carbide composites (e.g. MXene, Ti3C2(OHxF1-x)2), defect-rich nitrogen-doped reduced graphene oxide (RGO) and its chitosan combined hydrogel form are also promising catalyst supports for the hydrogenation of nitro-compounds, such as 2-nitrophenol, 4-nitrophenol [11–14].Recently, magnetic nanocatalysts have received a lot of attention and are widely used in catalytic processes due to their many advantages (high specific surface area, good dispersibility, efficient separation due to their magnetic properties, reusability) [15–18]. Several research groups have synthesized ferrite nanoparticles as catalyst supports and have effectively used them for the reduction of nitroaromatic compounds [19–21]. Ferrites are spinel transition metal oxides with the general formula of MFe2O4 (M is a transition metal such as Fe, Ni, Mn, and Zn). Several methods have been successfully applied in the synthesis of ferrite nanoparticles such as co-precipitation [22], sol–gel [23], microemulsion [24], hydrothermal [25], thermolysis [26], and mechanical alloying [27]. A relatively new technique for the synthesis of ferrite nanoparticles is based on sonochemical treatment, which includes the exposure of the reaction mixture to intense ultrasonic irradiation [28–30]. Sound waves entering the liquid medium create high as well as low pressure cycles, depending on the frequency. During the low–pressure cycle, high-intensity ultrasonic waves create small vacuum bubbles, or rather cavities in the liquid. When the bubble reaches a volume that is no longer able to absorb energy, it collapses during the high–pressure cycle, consequently “breaking” the solid particles in the liquid. This phenomenon is called acoustic cavitation, during which the released energy can cover the needs of certain chemical reactions [31].In the current work, ferrite supported Pd catalysts were prepared by combining sonochemical and combustion methods. The prepared catalysts are in active form after the final step and do not require post-treatment, and thus, a simplified catalyst preparation is achieved. Due to their magnetic properties, the produced spinel ferrite catalysts can be easily recovered from the liquid phase by magnetic separation using a magnetic field.To synthesize the spinel catalyst supports, zinc(II) nitrate hexahydrate (Zn(NO3)2 ∙ 6H2O, ThermoFisher GmbH, 76870 Kandel, Germany), nickel(II) nitrate hexahydrate (Ni(NO3)2 ∙ 6H2O, ThermoFisher GmbH, 76870 Kandel, Germany), and iron(III) nitrate nonahydrate (Fe(NO3)3 ∙ 9H2O, VWR Int. LtD., B-3001 Leuven, Belgium) were applied. Polyethylene glycol (PEG 400, Mw: ∼400 g mol−1, Molar Chemicals Ltd., Budapest, Hungary) was used as reducing agent and dispersion media of the metal precursors. During the catalyst preparation step, palladium(II) nitrate dehydrate (Pd(NO3)2 ∙ 2H2O, Merck Ltd., Darmstadt, Germany) was applied as precursor of the catalytic active metal, and patosolv, a mixture of aliphatic alcohols (90 vol% ethanol and 10 vol% isopropanol, Molar Cemicals Ltd., Budapest, Hungary) was used as reducing agent to carry out the conversion of Pd(II) ions to elemental Pd.To prepare the ferrite samples and decompose the palladium nanoparticles on the surface of the ferrite supports, Hielscher UIP100 Hdt. tip homogenizer (1000W, 20 kHz) was applied. Bs4d22 ultrasonic block sonotrode (D = 22 mm) was used to initiate the formation of metal hydroxides in polyethylene glycol dispersion. The spinel nanoparticles were examined by high-resolution transmission electron microscopy (HRTEM, FEI Technai G2 electron microscope, 200 kV) and their morphology has been characterized. The samples were prepared by dropping aqueous suspension of the nanoparticles on 300 mesh copper grids (Ted Pella Inc.). X-ray diffraction (XRD) measurements were used by Rietveld analysis to identify and quantitatively characterize the different oxide phases in the samples. Bruker D8 Advance diffractometer (Cu-Kα source, 40 kV and 40 mA) in parallel beam geometry (Göbel mirror) with Vantec detector was applied. Average crystallite size of the domains was calculated by the mean column length calibrated method by using of full width at half maximum (FWHM) and the width of the Lorentzian component of the fitted profiles. The quantity of the deposited palladium in the catalysts have been analyzed by Varian 720 ES inductively coupled optical emission spectrometer (ICP-OES). For the ICP-OES measurements, the samples have been solved in aqua regia. The specific surface area (SSA) measurements of the samples were carried out by nitrogen adsorption–desorption method at 77 K. For this, the Micromeritics ASAP 2020 equipment was used, and the evaluation was carried based on the Brauner-Emmett-Teller (BET) method. The ferrite samples were examined by applying a Vario Macro CHNS element analyzer to quantify the carbon content. Certified phenanthrene (C: 93.538%, H: 5.629%, N: 0.179%, S: 0.453%; from Carlo Erba Inc.) was used as standard. The carrier gas was helium (99.9990%) while oxygen (99.995%) was used for oxidation of the carbon content. The quantitative analysis of the samples after hydrogenation tests was carried out by Agilent 7890A gas chromatograph coupled with Agilent 5975C Mass Selective detector with RTX-624 column (60 m × 0.25 mm x 1.4 μm). The injected sample volume was 1 μL at 200:1 split ratio, while the inlet temperature was set to 473 K. Helium was the carrier gas with constant flow (2.28 mL/min), and the oven temperature was set to 323 K for 3 min and after it was heated up to 523K with a heating rate of 10 K/min and kept there for another 3 min. The analytical standards of the main product, the by-products, and intermediates purchased from Sigma Aldrich and Dr. Ehrenstorfer Ltd.Zinc and nickel containing ferrite spinel nanopowders were synthesized by using a two-step process including ultrasonic cavitation and combustion and thus, nickel ferrite, zinc ferrite, and zinc doped nickel ferrite nanoparticles were achieved. In the first step, iron(III) nitrate nonahydrate and one of the precursors (Table 1 ) were dissolved in 20 g polyethylene glycol (PEG 400). The solutions were sonicated by using a Hielscher UIP1000 Hdt tip homogenizer for 5 min (130 W, 23 kHz). Due to the high energy caused by the ultrasonic cavitation, in the presence of polyol, a brownish red, high viscosity colloid system was formed which contained hydroxide and oxide nanoparticles. Thereafter, the total polyol content of the system was burned, and the oxides and hydroxides of the transition metals converted to ferrite nanoparticles with water outlet.The prepared nickel ferrite, zinc ferrite, and zinc doped nickel ferrite (4.00 g) supports were dispersed in ethanolic solution of palladium nitrate dihydrate (0.50 g). The dispersions were sonicated by using the high energy ultrasound homogenizer (130 W) for 4 min. During this process, the Pd(II) ions were reduced to elemental palladium nanoparticles due to the released energy caused by the cavitation, and Pd nanoparticles were deposited onto the surface of the magnetic spinel supports which led to the formation of the final catalysts (Pd/NiFe2O4, Pd/ZnFe2O4, and Pd/NiZnFeO4). Then, the catalyst samples were removed from the dispersion with a Nd magnet, washed with patosolv, and dried at 105 °C overnight. The final palladium content of the magnetic catalysts was determined by ICP-OES measurements.The catalysts (0.10 g) were tested in nitrobenzene hydrogenation. To carry out the test, a methanolic solution of nitrobenzene (c = 0.25 mol dm−3) was used in a Büchi Uster Picoclave reactor (200 ml volume) (Fig. 1 ). The pressure of hydrogenation was constant (20 bar) during the tests, and the reactions were carried out at four different reaction temperatures (283 K, 293 K, 303 K, and 323 K). Rotational speed of agitation was 1000 rpm. Sampling took place after the beginning of the reaction at 0, 5, 10, 15, 20, 30, 40, 60, 80, 120, 180, and 240 min.The catalytic activity of the palladium decorated ferrite catalysts was determined by calculating nitrobenzene conversion (X%) based on the following equation (Eq.(1)): (1) X % = c o n s u m e d n n i t r o b e n z e n e i n i t i a l n n i t r o b e n z e n e × 100 Further, the yield (Y%) of aniline was calculated as follows (Eq. (2)): (2) Y % = n a n i l i n e n n i t r o b e n z e n e × 100 where n aniline and n nitrobenzene are the corresponding chemical amounts of the compounds.To determine the catalytic efficiency selectivity (S%) is very important parameter, which was calculated as follows (Eq. (3)): (3) S % = n a n i l i n e Σ n p r o d u c t s × 100 The phase composition of the synthetized ferrites was examined with XRD measurements by using the Rietveld analysis (Fig. 2 ). Next to the ferrite phases, oxide forms of the transition metals were identified. On the diffractogram of the nickel ferrite sample, there are peaks at 30.3°, 35.7°, 43.4°, 53.8°, 57.4°, and 62.9° two Theta degrees, and these can be associated with the (220), (311), (400), (422), (511), and (440) reflections of the NiFe2O4 phase (PDF 10–0325) which was the target product (Fig. 2 A). In the nanopowder, NiO was also found next to the NiFe2O4, indicated by the peaks at 37.3° (111), 43.4° (200), two theta degrees (PDF 47–1049). Moreover, awaruite (FeNi3) was also identified in the nickel ferrite sample as peaks at 44.1° (111), and 51.3° (200) 2Θ degrees (PDF 38–419) were located.In case of the zinc ferrite support, reflections which belongs to the ZnFe2O4 phase were identified at 18.2° (111), 29.9° (220), 35.3° (311), 36.9° (222), 42.8° (400), 53.1° (422), 56.6° (511), and 62.2° (440) two Theta degrees (Fig. 2 B), (PDF 22–1012). Furthermore, a zinc(II) oxide phase was also identified at 31.8° (100), 34.4° (002), and 36.3° (101) 2Θ degrees (PDF 36–1451).The zinc doped nickel ferrite support was also characterized and reflections of the NiZnFeO4 phase were located at 30.0° (220), 35.5° (311), 43.1° (400) 53.4° (422), 56.9° (511), and 62.5° (400) two Theta degrees (Fig. 2C). During the formation of the zinc doped nickel ferrite spinel, by-products, ZnO, NiO, and FeNi3, were also formed, which were identified on the diffractograms of the other two support, the nickel ferrite and zinc ferrite. The reflections of these by-products are visible on the deconvoluted diffractogram of NiZnFeO4.Quantitative analysis of the different phases in the three ferrite samples was carried (Table 1). In case of the zinc ferrite support, the content of the spinel phases was the highest 94.7 wt% compared to the other two ferrite samples. In this case, ZnO was also formed in relatively low quantities (5.3 wt%). Presence of the non-magnetic phases, as zinc oxide, nickel oxide may cause a problem in the magnetic separation of the catalyst unless it adsorbs well on the magnetic particles or forms a stable aggregate with it. On other hand, these oxides can also play a role in the catalytic behavior of the spinel samples. Since the production of the samples includes a combustion step, it was assumed that the burning of polyethylene glycol was not complete, and thus, carbon forms can also remain in the samples. To verify this, CHNS element analysis was carried out, which confirmed that the samples also contain carbon in low quantities (<1 wt%) (Table 2 ). It has to be noted, that minimizing carbon content is also important, because the remaining carbon can cover the spinel particles, and thus, the supports’ promoter effect cannot fully prevail during the catalytic hydrogenation processes.The size of the particles in the support samples was also determined by applying the Scherrer equation and using the full width at half maximum (FWHM) intensity of the reflexion peaks (Table 2). The mean particle sizes of the different nanoparticles are between 10 and 25 nm.The small size of the particles can be explained by the applied PEG400 dispersant, which prevented aggregation of the particles during the sonochemical step. To get a more detailed picture about the formation of the nanoparticles, additional samples were prepared for which the synthetic procedure of the supports was not carried out completely. After the ultrasonic treatment the processes were stopped, and the formed nanoparticles were washed with distilled water and were separated by centrifugation. The drying of the solid phases was carried by lyophilization (at 213 K and 1.0 mbar vacuum). XRD measurements were carried out on the separated nanoparticles to examine the solid phases after the sonication step. Metal oxide and oxyhydroxide structures such as nickel oxide hydroxide (Ni3O2(OH)4, PDF 06–0144), franklinite (ZnFe2O4, PDF 22–1012), zincite (ZnO, PDF 36–1451), maghemite (Fe2O3, PDF 39–1346), and trevorite (NiFe2O4, PDF 10–0325) were identified which formed from the nitrate salts of the precursors (SI Fig. 1). Bunsenite (NiO) and FeNi3 was not identified after in samples and thus, these phases are formed only after the combustion step (Fig. 2 A and C). Furthermore, the results shows that the nickel oxide hydroxide phase was eliminated by the combustion step, possible by water elimination due to the high temperature, which reacted with the zincite or maghemite and formed the spinel phases.The spinel particles were examined by transmission electron microscopy (Fig. 3 ). On the HRTEM images carbon layers are visible, which cover the surface of the metal oxide nanoparticles. To further analyze the carbon layers and identify the surface functional groups, Fourier transform infrared (FTIR) spectroscopy measurements were carried.On the FTIR spectrum of the nickel ferrite sample two main broad bands at 424 cm−1 and 601 cm−1 wavenumbers were identified which can be associated with the νFe-O stretching vibration of the tetrahedral metal–oxygen bond and the metal–oxygen vibrations in the octahedral sites (Fig. 4 ). A weak vibration band was located around 1062 cm−1 and can be assigned to the νC−O, which originates from the carbon content remained after the combustion. Although the carbon content only 0.6 wt% (Table 2) in this case, it is also detectable by the symmetric and asymmetric stretching vibration modes of the –CH2 bonds, which absorbs at 2852 cm−1 and 2924 cm−1. The presence of hydroxyl groups was also verified as a band was identified at 1386 cm−1. The hydroxyl functional groups may belong to the carbon or to the surface of the metal oxide nanoparticles. The presence of hydroxyl functional groups on the surface of the developed catalytic ferrites improves their polar feature and thus, their wettability and dispersibility in polar solvents, such as methanol (SI Fig. 2). Two bands of the hydroxyl groups are located at 3438 cm−1 and 1640 cm−1, which can be attributed to the stretching modes of the H–O–H stretching and bending vibrations of free or absorbed water. There are only slight differences in the spectra of the other two support samples, ZnFe2O4 and NiZnFeO4, compared to NiFe2O4.Specific surface areas (SSA) of the ferrite supported palladium catalysts have been measured and it was found that the SSA of the Pd/NiFe2O4 is almost three times higher (64.0 m2 g−1) than in case of the Pd/ZnFe2O4 and Pd/NiZNFeO4 catalysts where it is only 22.6 m2 g−1 and 22.2 m2 g−1, respectively.On the XRD pattern of the palladium decorated spinel catalysts, reflections at 40.0°, 46.6°, and 68.2° two Theta degrees were located which can be associated with Pd(111), Pd (200), and Pd (220) (ICDD card number 046–1043), respectively (Fig. 5 A, B and C). This indicates that the palladium is in the elemental state in the catalytic systems. The efficient ultrasonic treatment of the palladium(II) nitrate precursor which was carried out in alcoholic phase led to formation of elemental palladium particles. On the Rietveld refined diffractograms palladium oxide is not detectable, which shows that the reduction step was successful, and the total amount of palladium ions converted to elemental Pd. The palladium content of the ferrite supported catalysts were measured by ICP-OES analysis. Based on the spectroscopic results, the palladium content of the catalysts was the highest, 4.20 wt% in case of Pd/ZnFe2O4, which was followed by Pd/NiFe2O4 (4.10 wt%), and NiZnFe2O4 (3.82 wt%).The particle size of palladium was calculated based on the XRD results. The mean particle size of the Pd nanoparticles in the prepared catalysts were small, 5±2 nm for Pd/NiFe2O4, and 5±1 nm for Pd/ZnFe2O4, while a slight increase is experienced (6 ± 2 nm) in case of Pd/NiZnFeO4. The particle size of the phases presents in the catalysts (e.g. supports) were not changed significantly after the deposition of palladium nanoparticles (Table 3 and SI Table 1).The individual particles cannot be distinguished from each other on the HRTEM images of the palladium decorated ferrite catalysts. The palladium nanoparticles are not identifiable next to the other metal oxide particles (SI Fig. 3). On the TEM images, significant change in the particle morphology cannot be detected, and aggregates or new structures cannot be identified in the samples.The magnetic catalyst supports were tested also in nitrobenzene hydrogenation. After 3 h only 23%, 29.2%, and 22.8% nitrobenzene conversions were achieved by using the NiFe2O4, ZnFe2O4, and NiZnFeO4 supports, respectively (SI Fig. 4). The aniline yields were also low (18.8 n/n%, 21.2 n/n% and 21.6 n/n%) after 3 h. Thus, the activity of the ferrite nanoparticles is not adequate for aniline synthesis. The catalysts were also tested, and their activity was compared. Pd/NiFe2O4 showed the lowest catalytic activity, after 3 h 92.8 n/n% nitrobenzene conversion was achieved at 323 K (Fig. 6 A). In case of the Pd/ZnFe2O4 and Pd/NiZnFeO4, 99.8 n/n% conversion was reached after only 2 h, and 99.0 n/n% after only 80 min hydrogenation at 323 K. Thus, by the presence of Zn in the support, the activity of the catalysts increased.Aniline yield (Y%) and selectivity (S%) were calculated in case of each catalyst applied at different temperatures (Fig. 7 A and B). After 3 h of hydrogenation, the highest yield (99.2 n/n%) was achieved by the Pd/ZnFe2O4 catalyst at 323 K, while the corresponding selectivity was also high, 98.8 n/n%. In general, all three catalysts showed high selectivity for aniline formation at all temperatures (Fig. 7 B), but the yield was lower when the Pd/NiFe2O4 catalyst was applied.During the catalytic tests, several intermediates such as nitrosobenzene (NOB), azoxybenzene (AOB), and azobenzene (AZB) have been identified in each case (Fig. 8 ). However, these species converted to aniline at end of the reaction, so the yield was not affected by them. Furthermore, by-products were also formed, but only in very small quantities, and thus, the selectivity of aniline was not significantly impaired.By applying Pd/NiFe2O4 and Pd/NiZnFeO4, N-methylaniline (NMA) formed as a by-product, but only in concentration <2 mmol dm−3. NMA can be formed directly from aniline with methylation in the presence of methanol (solvent). By using Pd/ZnFe2O4 as catalyst, two more by-products (<1 mmol dm−3) were observed, dicyclohexylamine (DCHA) and N-methyl-1-phenylethanimine (NMPE). It is interesting to note that, all catalyst contained the same catalytically active metal (palladium), nonetheless by using the Pd/ZnFe2O4 sample additional by-products were formed. This phenomenon cannot be explained by only the effect of palladium. Moreover, the catalysts with Zn in their support, also contained a ZnO phase (5 wt%), but DCHA and NMPE were formed only when Pd/ZnFe2O4 was applied. Thus, the presence of zinc oxide does not justify the emergence of new molecules either. In this sense, it can be assumed, that the zinc ferrite opened a new reaction pathway towards the formation of the imine and DCHA.The highest aniline yield and selectivity was achieved by using the Pd/ZnFe2O4 catalyst. Thus, this catalyst was tested in reuse tests and at 323 K 4 cycles of nitrobenzene hydrogenation was carried out. After 3 h reaction time the aniline yield and selectivity values were calculated in each cycle. Significant decrease in the yield was not experienced during the first three cycles. However, in the 4th cycle, the aniline yield dropped to 67% (SI Fig. 5 B). In contrast, the selectivity remained similar during the reuse cycles and were between 98 and 99 n/n%. The palladium content of the Pd/ZnFe2O4 catalyst was determined after the 4th cycle, and it was found that the initial palladium content decreased from 4.20 wt% to 1.8 wt%. This palladium loss led to a decrease in the catalytic activity during the reuse tests. Based in this, it will be necessary to further improve the stability of the catalyst, to create an stronger interaction between the noble metal and the support.All in all, three highly selective, easy to use, magnetic catalysts (Pd/NiFe2O4, Pd/ZnFe2O4, and Pd/NiZnFeO4) were successfully prepared, and their applicability was tested in nitrobenzene hydrogenation. Based on the results, Pd/ZnFe2O4 catalyst was found to be the most efficient for aniline synthesis. First, the nickel ferrite, zinc ferrite and zinc doped nickel ferrite spinel nanoparticles were synthesized. These nanoparticles show high dispersibility, due to their surface functional groups (-OH) and their small mean particle sizes (21 ± 5 nm, 17 ± 4 nm, and 20 ± 5 nm, for NiFe2O4, ZnFe2O4, and NiZnFeO4, respectively). Therefore, they are excellent support materials and were applied in catalyst preparation. Palladium nanoparticles were deposited onto the surface of the magnetic supports, by using ultrasonic cavitation which assisted the adsorption and reduction of the Pd. After the process, the catalyst is ready to use, and further activation step is not necessary. In this sense, the applied catalyst preparation method is fast and simple. By using the Pd/ZnFe2O4 and Pd/NiZnFeO4 catalyst, high aniline selectivity (99.8 and 99.9 n/n%) and yield (99.2 and 92.8 n/n%) were achieved, while with the nickel ferrite supported catalyst, lower selectivity and yield was reached. Reuse tests were further verified the applicability of the prepared catalyst in the hydrogenation of nitro compounds, but the improvement its stability is necessary.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 European Union and the Hungarian State, co-financed by the European Regional Development Fund in the framework of the GINOP-2.3.4-15-2016-00004 project, aimed to promote the cooperation between the higher education and the industry. Prepared with the professional support of the Doctoral Student Scholarship Program of the Co-operative Doctoral Program of the Ministry of Innovation and Technology financed from the National Research, Development and Innovation Fund. Further financial support has been provided by the National Research, Development and Innovation Fund (Hungary) within the TKP2021-NVA-14 project.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.jmrt.2022.06.113.

Aniline is one of the most important chemical in the polyurethane industry and it is produced by the catalytic hydrogenation of nitrobenzene. The development of novel, multifunctional catalysts, which are easily recoverable from the reaction mixture is therefore of paramount importance. Transition metal-containing ferrites decorated with palladium were prepared by using a combination of sonochemical and combustion steps. First of all, magnetic ferrites were produced to be used in catalyst preparation as supports for palladium nanoparticles. On the surface of the ferrite particles palladium nanoparticles were deposited by applying ultrahigh sonication in an alcoholic phase. All in all, three magnetic catalysts, Pd/NiFe2O4, Pd/ZnFe2O4, and Pd/NiZnFeO4 have been created. The catalysts have been tested and their activity have been compared in nitrobenzene hydrogenation to synthesize aniline at four different temperatures, and 20 bar pressure. The most active catalysts were the Pd/ZnFe2O4 and Pd/NiZnFeO4 systems with which aniline yields of 99.2 and 92.8 n/n% were achieved after 3 h of hydrogenation, respectively. In contrast, by applying the Pd/NiFe2O4 catalyst, a significantly lower aniline yield was achieved. It was proved that, due to their magnetic properties, the prepared catalysts are easily removable from the reaction medium by using a magnetic field. Thus, catalysts with excellent properties have been successfully developed and tested in nitrobenzene hydrogenation.

Data will be made available on request.According to the low reactivity and high corrosion resistance of platinum, the global demand for platinum is increasing. Platinum has catalytic properties and hydrogen adsorption and is used in the manufacture of catalysts for the refining, petrochemical and automotive industries. Industrial wastes including platinum are considered secondary sources of this metal. (Diederen, 2009; Morcali, 2020 [1,2]; Yakoumis et al., 2020). Despite the use of platinum in catalysts, especially automobile catalysts, high amounts of soil and water contamination with platinum group metals have been observed in the tests of soil and running water along the roads. Also, the recovery of platinum reduces hazardous wastes and proper disposal of residues [3–5]. Based on the use of platinum in high technologies, this metal is in the near-critical category [6,7]; Henckens et al., 2014; Sverdrup et al., 2017). South Africa and Russia are the world's largest mining suppliers of platinum. According to the report of the platinum group metals market by Johnson-Matthew; the supply of platinum from recycling is about one-fifth of primary sources. The supply and demand of platinum are shown (Fig. 1 ).The main uses of platinum are in the chemical, electrical, glass industry, petroleum refining, medical/biomedical and other industries. Platinum is mostly used in the production of chemicals such as industrial catalysts. Platinum catalysts have different applications in industries including increasing the octane number in reforming units, producing hydrocarbons from synthesis gas, reducing toxic gases in the automotive industry, oxidizing ammonia and converting it to nitrogen oxide or nitric acid, catalytic reforming, etc. [8]. Consumption rates in these industries over the past years are compared (Fig. 2 ).South Africa, as the major mining supplier of platinum, has always insisted on producing this metal from the mine and primary sources. But in consuming countries, the recovery of this metal from secondary sources is important. Therefore companies like Johnson Matthey (UK and USA), BASF (USA), and Umicore (Belgium), tend to recover platinum from secondary sources (Saguru et al., 2018). Concentrations of platinum in the secondary sources are higher than natural ores. Platinum from natural ores requires 18, 860–254, 860 MJ per kg of metal and 100,000–1,200,000 m3 of water per ton of metal extracted, while recycled platinum needs 1400–3400 MJ and 3000–6000 m3, respectively. So platinum recovery is cost-effective (Comisión Europea, 2018). One of the largest sources of platinum extraction from secondary sources is industrial spent catalysts. Based on the value of platinum and its amount in the waste of spent catalysts, its extraction and recovery have been considered. [9–11]; Zanjani & Baghalha, 2009). To separate and purify platinum from spent catalysts, the catalysts need to be crushed and prepared. Pyrometallurgical and hydrometallurgical methods are used to separate platinum. The important issue in the latest research is attention to the economic aspect (such as the amount of energy consumption), environmental constraints and standards. The innovation of this research is that proposes the optimal parameters for the pre-treatment and leaching stages in the hydrometallurgical method for platinum recovery from spent catalyst.A catalyst increases the rate of reaction but remains unchanged at the end of the reaction. Catalysts make processes more economical and environmentally friendly [12]; Trimm, 2008). The selectivity and activity of catalysts decrease after a period of cycle life and are so-called deactivated. Deactivated catalysts are regenerated and returned to the process. If it is not possible to regenerate the catalyst, it will be unusable and become hazardous industrial waste. Spent catalysts in the refining, petrochemical and automotive industries contain precious metals: platinum, aluminum, iron, nickel and several other (Table 1 ).One of the most important goals in selecting catalysts is their cycle life. Catalyst consumption and spent catalyst production are reduced with longer cycle life. Catalysts are deactivated for a variety of reasons which is categorized into six intrinsic mechanisms in the following cases: 1. Poisoning of active sites, 2. Fouling, coking, and carbon deposition, 3. Thermal degradation and sintering of the catalyst, 4. Vapor compound formation and/or leaching accompanied by transport from the catalyst surface or particle, 5. Vapor–solid and/or solid-solid reactions, and 6. Attrition/crushing [12,17–19]; Marafi et al., 2017; Zhou et al., 2020). Various catalysts are used in petrochemical industries, the main metals in these catalysts are cobalt, molybdenum, nickel, platinum and tungsten. The surface of this group of catalysts is deactivated for three main reasons, which are: coking, sintering and poisoning [18]. In Table 2 , the amount of contamination of each catalyst is given according to the deactivation factors. In Platinum spent catalysts, the coking deposit has the highest rate of catalyst deactivation factor. Sintering is reduced due to the presence of chloride in the catalyst. But the moisture in the reactant stream can wash away the chloride in the catalyst and increase sintering. For this reason, the inlet stream must be completely free of moisture [12].According to the importance of supplying platinum from secondary sources, the recovery and extraction of this metal from spent catalysts in oil, petrochemical, automotive, and pharmaceutical are considered [20]; Naghavi et al., 2016; Shams & Goodarzi, 2006; Yoo, 1998). The following steps are required for metal recovery are: recyclability of material composition, availability of related compounds, affordable, safe waste collection system, complete the recovery chain steps, use optimal set-up for recovery chain and build enough capacity to the end of the chain [21]. Pyrometallurgical and hydrometallurgical methods are mainly used to recover precious metals from primary/secondary sources. The pretreatment steps such as heating and crushing are necessary to remove excess compounds from the industrial spent catalyst. Finally, the platinum is separated and purified. The choice of method for processing of metal recovery depends on the purity and final value of the product [10,11,22–24]; Yakoumis et al., 2021). State-of-the-art, recovery methods focus not only on maximum recovery but also on economic and environmental priorities (Yakoumis et al., 2021). The use of renewables and safer solvents, less waste generation, lower energy, hazardous chemicals, space, time consumption need to be considered to achieve economic and environmental goals. The recovery method is selected based on the number and type of metals in the spent catalyst, the concentration of metals, the basic nature of the catalyst, the conditions and materials and equipment required. Due to high energy consumption, production of polluting gases and low purity products in the pyrometallurgical method, this study will explain the hydrometallurgical method for the recovery of platinum. Pre-treatment is required before the hydrometallurgical method that increases the leaching and recovery efficiency. Therefore, the pre-treatment will also be explained. The general schematic of the aforementioned methods for platinum recovery is shown in Fig. 3 .In pyrometallurgy, there are problems such as the production of polluting gases and high energy consumption. According to new environmental regulations and restrictions, the use of new and alternative methods of pyrometallurgy has been considered. Hydrometallurgy techniques will fill the gap in the industry [8]. The pyrometallurgical method requires industrial equipment to supply high temperatures, which may not be suitable for some small-scale recovery units. BASF (New Jersey, USA), Johnson Matthey (2 refineries in England, 1 in USA and 1 in China), Multi Metco (Alabama, USA), Umicore (Hoboken, Belgium), Stillwater (Colorado, USA), and NonX21 (Quebec, Canada) are some pyrometallurgical refineries in the world (Saguru et al., 2018). The pyrometallurgical process for the recovery of platinum from the spent catalyst involves three methods: smelting, vaporization, and sintering (Peng et al., 2017). Crushed spent catalysts are mixed with various collectors such as lead, copper, iron, matte, or PCB (printed circuit board) for helping platinum recovery. Then, this mixture melted at a furnace (plasma furnace, electric arc furnace, or inductive furnace) at a temperature above 1000 ⸰C to become low-viscosity liquids (Morcali, 2020; Fernández et al., 2021; Peng et al., 2017; Yakoumis et al., 2021). After the smelting process, the metal vaporizes and purifies. Platinum chloro-complexes are formed with the chlorination process. Separation of chloro-complexes is performed by volatilization or adsorption on an activated carbon bed [25]. Sintering is based on the chemical-physical properties of materials. In this method, with the help of plasma, the amount of oxide of platinum compounds is reduced at high temperatures (>1200 ⸰C). Sintering and volatilization are more limited methods than smelting (Peng et al., 2017). The most important factor for recovering platinum from spent catalysts is temperature and the retention time on process temperature [23]. A summary of the three methods is given in Table 3 .In the overview of the pyrometallurgical process, spent catalysts are mixed with fluxes and collectors. This mixture enters the furnace according to the aforementioned methods. The outlet of the furnace is purified to extract platinum. An overview of the pyrometallurgical process is shown in Fig. 4 .Pre-treatment is used before the hydrometallurgical process to remove surface contaminants and increase leaching efficiency by improving chemical attack (Saguru et al., 2018; Yakoumis et al., 2021). Crushing and particle size smoothing of the spent catalyst is also required to improve the leaching process (Yakoumis et al., 2020). Preheating or thermal pretreatment is one of the pretreatment methods (Yuliusman et al., 2020). This method uses heating in an environmentally friendly atmosphere such as air, oxygen, nitrogen, and hydrogen [26]. Of course, it depends on the type of contamination on the spent catalysts [8]. According to the high oxidation potential of platinum (Marinho et al., 2010), its dissolution in the acidic leaching process is difficult. The use of various agents and the formation of chloro-complexes reduce the oxidation potential. Calcination and crushing of spent catalysts into micron-sized particles are also pre-treatment methods [16,27]; Zanjani & Baghalha, 2009). Fig. 5 shows an overview of the pretreatment process.In most studies, calcination and crushing and homogenization of spent catalysts have been considered as a pretreatment method to the metallurgical process. However, in some cases, heating and calcination have not been performed (Yakoumis et al., 2020). The spent catalysts are oxidized in a furnace at 500 °C for 5h (Marinho et al., 2010), crushing into 10–100 μm particles and calcination at 800 °C [16], drying at 900 °C for 3 hour and broking down into 300 μm (Yuliusman et al., 2020), calcination for 30 min at 600 °C and crushing into of 500 μm [27], for increasing the efficiency of the platinum leaching. Pretreatment at high temperature for increasing the efficiency of the platinum leaching is given in Table 4 .Reduce energy consumption with drying in an oven at 110 °C and crushed into 106 μm (Zanjani & Baghalha, 2009), crushing to 200 mesh and drying at 120 °C for 24 hour [28], crushing into 100-μm and drying in an oven [29], crushing into 0.3 mm and drying by 8% H2, at 100 °C for 20 hour [26], crushing to 200 mesh [30], crushing to 0.16 mm (Lanaridi et al., 2021), crushing to 2 μm (Atia et al., 2021), just crushing (Hasani et al., 2015), using formic acid to improvement of leaching (Equation (1)) (Upadhyay et al., 2013), that has more applications in industries. Pretreatment at minimum energy consumption for increasing the efficiency of leaching is given in Table 5 . (1) P t O ( S ) + H C O O H → P t + H 2 O + C O 2 The hydrometallurgical process has practical and economic advantages compared to the pyrometallurgical process. These advantages include lower energy consumption, achievement of higher purity materials, lower process temperature, easier process control(Saguru et al., 2018), applicable on large and small scales, and less toxic gas wastes [29]; Lanaridi et al., 2021). The steps of the hydrometallurgical process are explained in Fig. 6 . In the process of hydrometallurgy, first the metal is separated and leached in different ways and purified based on the required purity of the product [30]. After the leaching step with acid, amine family solvents were used to extract selectively and purify platinum group metals. Iron is always present in solutions obtained from leaching of the platinum group metals, but iron and aluminum do not interfere in the extraction of platinum with solutions of amines [24]; Sun & Lee, 2011). According to the dissolution of aluminum in the leaching process, the least dissolution of aluminum is always the optimal method for leaching (Méndez et al., 2021). On the laboratory scale, precious metals are separated from leaching solutions by precipitation, adsorption of activated carbon, ion exchange, and solvent extraction. But solvent extraction has also been used for industrial scales. So far, some ionic liquids have been used in solvent extraction for laboratory scales (Xing & Lee, 2019). In the present work, leaching and purification steps are considered as a subset of the hydrometallurgical process. Leaching, microwave leaching, and bio-leaching are mentioned in detail. The purification steps also include precipitation, solvent extraction, ion exchange, and ionic liquid extraction, which are described at the same time as the leaching steps. Precipitation and solvent extraction methods have been the most widely used in recent research. The use of ionic liquids or any other solvent on industrial scale requires economic estimation (Fig. 6).Spent catalysts leached after pretreatment. At this stage, platinum is extracted from the waste catalyst and then purified. The recovery rate of Platinum has increased by increasing acid concentration and temperature. The level of acidity in the acidic leaching process is always high, which causes environmental hazards and the consumption of large amounts of acidic agents. In the latest studies, non-volatile chloride salts are used instead of HCl in acid leaching that increases the dissolution of platinum and decreases the dissolution of catalyst base. Aluminum chloride is a good choice for oxidizing agents due to its 3 chloride ions per molecule. Increasing H2O2 accelerates the reaction by reducing the potential required for the reaction. The effect of different operating parameters such as temperature, solid to liquid ratio, time was investigated and optimized. Homogeneous spent catalyst particles have been used in direct and single-step leaching with 3 M HCl, 4.5 M NaCl, 1% v/v H2O2, at 70 °C, S/L of 0.7, for 2 hr. 100% platinum is recovered in this leaching with low acidity (Yakoumis et al., 2020). At a low concentration of HCL of about 0.66 M, 75% of platinum is recovered in I2 30 g/l, L/S of 20 g/g, at 75 ⸰C and for 60 min (Zanjani & Baghalha, 2009). For reducing the hazardous effect of HCl, aluminum chloride with a low concentration of nitric acid was used as the oxidizing agent. Aluminum chloride is a good choice for oxidizing agents due to its 3 chloride ions per molecule. Platinum is recovered at 96.8–98.8% at 103 °C, for 1 hr, 2 M aluminum chloride, and 1 M nitric acid [28]. Aqua regia (HCl + HNO3, 3:1 v/v) was used for leaching of oxidized spent catalyst at 75 °C, for 20–25 min. 99 wt% of platinum is purified with 15 vol% Aliquat 336 in kerosene at 25 °C (Marinho et al., 2010). 98.1% of platinum is recovered by using the response surface methodology when leaching experiments were performed with HCl of 1.45 M, NaCl of 4.55 M, 10% H2O2/spent catalysts of 0.66 mL/g, and L/S of 4.85:1 at 90 °C for 2 h [16]. More than 80% of platinum is recovered by 11.6 M HCl leaching solution with 1% vol of H2O2, at 60 ⸰C and 1 hr. Using 8 M HCl/2 M CaCl2 combination instead of HCl under the same conditions has similar responses (Méndez et al., 2021). The leaching of catalyst particles is done with 15 mL HCl 37% and 5 mL HNO3 65%, L/S of 8, and heated at 109 °C for 3h. In this process, platinum is completely recovered [29]. In leaching with 1 M oxalic acid, 5.58% of platinum is extracted at 60 °C for 10 hr. 19.72% of Platinum is recovered under similar conditions to the UOP method 896–930 and leaching with aqua regia (HCl: HNO3 3: 1) at S/L of 20, at 300 °C and time depends on the rate of evaporation and boiling of solution (Yuliusman et al., 2020). The use of HCl/H2O2 in the leaching process is environmentally friendly. 96% of platinum is separated with 0.8 vol% H2O2, and 9.0 M HCl at 60 °C for 2.5 hour [26]. In leaching with sulfuric acid, the base of the catalyst also dissolves. Dissolution in sulfuric acid for the Catalyst based on γ-Al2O3 (such as R-134) is higher than the Catalyst based on a mixture of γ-Al2O3 and α-Al2O3 (such as AR-405). According to the dissolution of the base of the catalyst, the separation of the target metal becomes more difficult. Catalyst particles are dissolved in 6 M H2SO4, at 100 ⸰C for 4 hours to leach alumina. 90% of aluminum sulfate crystals are formed after adding distilled water to the leaching solution. 52% of Platinum in AR-405 and 83% of Platinum in R-134 are dissolved in solution [27]. Leaching of spent catalysts in 60% H2SO4, 0.1 M NaCl, S/L of 1/100, at 135 °C after 2 hour has been investigated to avoid the use of aqua regia that platinum recovery was 95% [30]. The reaction of hydrochloric acid and nitric acid with platinum is given in equations 2–(4. (2) P t + C l 3 ( a q ) − + C l ( a q ) − → P t C l 4 ( a q ) 2 − (3) P t C l 4 ( a q ) 2 − + C l 2 ( a q ) → P t C l 6 ( a q ) 2 − (4) 3 P t + 4 H N O 3 + 18 H C l → 3 H 2 P t C l 6 + 4 N O + 8 H 2 O To avoid energy consumption and large volumes of liquid waste production in hydrometallurgical and pyrometallurgical processes, we need processes with high efficiency and environmentally friendly achievements. Deep eutectic solvent ionic liquids are always available, inexpensive, and chemically stable and can be used in the extraction and recovery processes of precious metals [22,31,32]; Olga Lanaridi et al., 2022). Aliquat-336 has been used for selective extraction of precious metals from a mixture under optimal conditions. The results show that the use of this solvent is suitable for the separation of high purity metal from the mixtures [24]; Wei et al., 2016). Polymerized ionic liquid has high efficiency for separating platinum from spent materials. The complete recovery of platinum occurs by leaching with 1% H2O2 in 8 M HCl, poly SILPs 20%, in S/L of 1:5, at 65 °C for 3 hour (Lanaridi et al., 2021). Aluminum and iron are separated by adding sodium phosphate to the acid leaching solution. The solution of 0.01 M Aliquat 336 in kerosene is used to extract platinum from the residues. After loading the organic phase, 0.5 M HCl and thiourea were used and 99.91% of platinum was obtained from stripping (Raju et al., 2012). After achieving the best conditions in the leaching stage, platinum is separated from the leach liquor by resins. These resins are formed from Merrifield beads (M) with triethylenetetramine (TETA), ethane-1,2-dithiol (SS) and bis((1H-benzimidazol-2-yl)methyl)sulfide (NSN) to form M-TETA, M − SS and M-NSN, respectively [33]. Platinum recovery efficiency in acidic leaching is high. It is recommended to replace the acidic agent with salt or metal oxide to reduce the effects of acid on equipment and the environment.According to the results in Table 6 , the efficiency of acidic leaching based on temperature and energy consumption is shown in Fig. 7 . In the process of leaching with hydrochloric acid and hydrogen peroxide or iodine, platinum can be completely recovered at an optimum temperature of 65–70 °C.Using the microwave saves the time and energy required for the pretreatment of spent catalysts. The use of microwaves in the absence of oxidants has also led to an increase in platinum recovery (Trinh et al., 2020). In the leaching method with aqua regia at L/S of 5, at 109 ⸰C and for 2.5 hr, 96.5% of platinum with a purity of 94.2% was recovered. In the second leaching method with aqua regia at L/S of 2, at 400 ⸰C, and for 5 min, microwave radiation was used for heating and 98.3% of platinum with a purity of 98.9% was obtained [15]. The microwave is used for leaching in two stages, 500 W and 900 W. Two leaching samples were performed with a concentration of 6 M HCl in the presence and without 10% vol. H2O2 at 150 °C for 25 min. As H2O2 increases, the recovery efficiency of platinum slightly increases. With the addition of 10% vol. H2O2, the recovery efficiency of platinum has been increased from 90% to 96% (Atia et al., 2021). The combination of the spent catalyst, nickel matte and sodium salts with microwaves, reduces the viscosity and melting temperature, which results in 98.59% of the platinum being recovered at temperature of 1250 °C, time of 2 h, and N2 atmosphere. High efficiency in a short time is the result of using microwaves (Huimin Tang et al., 2021).In leaching and metal extracting, acidic and chemical substances are widely used. This consumption is not only costly but also poses risks to the environment. Therefore, replacing bio and nano solvents can be an alternative method. Bioleaching is done in two groups of direct and indirect processes. The direct method occurs in the presence of microorganisms, but in the indirect method, biometabolite is first produced by microorganisms and the leaching process is performed with this solution in the absence of microorganisms. The second method is more applicable on an industrial scale [20,34]. According to the biological methods and with the help of desferrioxamine B, about 30% of platinum has been obtained from the spent catalyst (H [13]. The refinery reforming catalyst was tested to recover platinum by the bioleaching method. The percentage of platinum recovery by oxalic acid in the bioleaching process increases from 13% to 30%. Comparing the bioleaching process with chemical leaching, it is concluded that oxalic acid has an effective role in both processes. The recovery of platinum in chemical leaching is slightly higher than in the bioleaching process [14]. About 93% platinum in the spent car exhaust catalyst was transferred to the solution phase of hydrochloric acid 12 M and nitric acid 9.5 M. Magnetite nanoparticles with silicate coating have been used to recover platinum from acidic solution (Hasani et al., 2015). The Gemini process (a liquid-solid resin ion exchange system) is used to recover platinum from a spent platinum-rhenium catalyst based on the alumina of the reforming unit. This process involves the steps of catalyst base dissolution, platinum separation, and purification. 91.71% of platinum is recovered in leaching with sodium borohydride at 60 °C for 15 min (Soltan mohammadzadeh et al., 2003). Selective recovery of platinum from automotive spent catalysts by anionic platinum chloride and cationic biocarbon adsorbents has been compared. The adsorbent Tu–N–SCG–C–A for selective recovery of precious metals is a priority not only economically but also environmentally [35].The reduction of time with the high efficiency of platinum recovery in the microwave leaching method is significant. But how this method is used on an industrial scale needs further investigation. The conditions for using this method on a laboratory scale are summarized in Table 7 . The bioleaching process is both environmentally and economically beneficial but needs to be further explored for use on an industrial scale due to its low efficiency and longtime requirement. The factors influencing each research are presented in Table 7.The efficiency of platinum recovery in microwave and bioleaching leaching processes increases with increasing temperature. But in the same conditions of microwave leaching process, with increasing temperature from 109 to 400 °C, the efficiency increases from 96.5 to 98.9. It is not economical to use energy for increasing 2% of efficiency, so the suitable temperature is about 110–100 °C for acquiring an efficiency of about 96% is very good. Also in the bioleaching process, increasing the temperature from 60 to 90 °C only increases the 1% in efficiency. Therefore, a temperature of 60 °C seems appropriate (Fig. 8 ).The importance of supplying platinum for various industries and removing environmental pollution from industrial wastes have led to the recovery of this metal from secondary sources. The general methods of extracting and separating platinum from spent catalysts include pyrometallurgy and hydrometallurgy. In this article, the hydrometallurgy method is described as an optimal method and its steps are explained in detail. The spent catalyst as feed is prepared. In the pre-treatment stage, heating removes pollutants from the surface of the spent catalyst, but it is suggested to use a method similar to washing and avoid energy consumption. Investigating, studying and comparing factors such as the use of economic and environmentally friendly solvents, lower temperature, lower solid-to-liquid ratio, shorter required time, reducing the chemical potential of the reaction and accelerating the separation of the metal from the base of spent catalyst, will improve the leaching process. It is suggested that experiment design software and artificial neural network; be used to predict the optimal values of leaching parameters and recovery percentage.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.

According to the technological importance of platinum in the modern industries, demand for platinum has been increasing. On the other hand, the supply of platinum from mineral resources had environmental and economic challenges. Therefore, recovery of platinum from secondary sources such as spent catalysts is inevitable and important as a valuable solution for waste management and supply of required platinum. The general methods of extracting and separating platinum from spent catalysts include pyrometallurgy and hydrometallurgy. Choosing and optimizing the method of platinum recovery includes advantages such as: lower energy consumption, production of more platinum and less gas pollutants, less production of industrial waste, more safety and less investment. In this article, the hydrometallurgical method was chosen by examining various references, which includes pretreatment and leaching steps. Also, the effect of parameters such as: amount of crushing and temperature in the pre-treatment stage, type of solvents, temperature, solid-to-liquid ratio and time in the leaching stage has been investigated on the efficiency of platinum recycling. Consequently, crushing to 100 μm and calcination at about 500 °C or oven drying at about 100 °C are suggested for pretreatment. In the leaching step, the replacement of metal salts with acid, microwave heating speed and the use of bio/nano solutions help the environment. Washing efficiency increases with increasing temperature up to 100 °C, ratio of acidic agent to solid catalyst and washing time. Attention to economic and environmental issues is suggested for future works.

Nowadays, one of the main concerns of global society is public health and environmental safety. Industrial and agricultural activities have released various organic toxic compounds, which can contaminate surface and groundwaters, thus threatening access to fresh drinking water [1]. One of the various contaminants coming from anthropogenic activities is p-nitrophenol (PNP), widely known as a toxic and carcinogenic nitroaromatic chemical compound. The primary source of industrial activities facilitating the PNP contamination is the production of rubbers, pesticides, textile, dyes, and pharmaceuticals [1–3]. It can easily intoxicate living animals and humans and lead to serious health problems such as confusion, skin and eye irritation, loss of consciousness, and even potential carcinoma [2]. Moreover, exposure to the PNP might cause a negative effect on blood cells and damage the central nervous system and other human organs [4].Various treatment technologies, including advanced oxidation processes, thermal degradation, photodegradation, electro-coagulation, biological treatment, adsorption, and others have been used to efficiently remove PNP [1,5,6]. Although biological treatment can efficiently degrade PNP, it has several disadvantages, such as a slow start-up time and decreased efficiency at low temperatures and high PNP concentrations [7]. At the same time, purification techniques, such as electro-coagulation, photodegradation, and adsorption, also have several drawbacks, including high cost, long operation time, and reduced efficiency [6]. Moreover, there are conventional methods available for the reduction of PNP to p-aminophenol (PAP), such as the use of hazardous Sn/HCl, or Fe/HCl, catalytic transfer hydrogenation (CTH), or molecular hydrogen (H2) [8]. However, those methods have several disadvantages since they demand complex experimental design, high pressure, and temperature [8].Recently, metal nanoparticles such as Au, Ag, Ni, Pt, Co, and Pd have attracted attention for the catalytic reduction of PNP due to their good initial activity [9–14]. However, the agglomeration of the nanoparticles resulting in decreased removal efficiency has been reported as a fatal defect [15]. Various immobilization techniques have been developed to prevent the agglomeration of nanoparticles using support materials such as graphene hydrogel, polystyrene beads, graphene oxide, magnetite, etc. [10,15–17]. However, no significant study has been conducted to use nanoscale zerovalent iron (NZVI) as a support material for the immobilization of metal nanoparticles to degrade PNP efficiently. In recent years, the well-known advantages of NZVI, such as high reductive capacity and economical synthesis method, made it one of the most widely studied and used environmental materials for the treatment of various surface and groundwater pollutants found in the industrial and agricultural sectors [2,7,18]. For example, NZVI has been proven to effectively remove diverse halogenated organic compounds, oxy-onions, and heavy metals [19]. However, NZVI has several disadvantages, such as rapid oxidation of its surface to Fe oxides, which decreases the activity of NZVI acting as a reductant. Another serious drawback of NZVI is its tendency to agglomerate due to magnetic forces, which also decreases the reactive surface and reduces the reductive efficiency of the material [1]. On the other hand, the magnetic property of NZVI allows the easy collection of NZVI-supported catalysts from the suspension system after catalytic reaction [7,18]. Hence, NZVI could be a promising support material with a great potential for the enhanced reduction of PNP.Previous studies showed that a variety of metallic catalysts with the promoter and noble metals on the surface of NZVI were successfully applied to remove nitrate, trichloroethylene, and tetrabromobisphenol [18,20–24]. The type of the promoter metal highly affects the degradation kinetics of the nitrate removal [25]. Hence, promoter metals including Cu, Sn, In, and Zn on the surface of various supports were extensively tested and evaluated for the degradation of nitrates in combination with noble metals such as Pd, Pt, and Au, where Pd was the most widely and successfully used noble metal [7,25–28]. However, the combination of metal catalytic components deposited on the NZVI surface have not been used and investigated for the reductive degradation of PNP.The present work aimed to investigate the reduction of PNP by NZVI-supported metal catalysts. First, different types of promoter metals have been tested for the enhancement of the rate of reduction of PNP. Then, more suitable promoter metals (Cu, In, Ni, Zn, Sn) have been tested along with a noble metal (Pd). Finally, the effect of significant factors such as catalyst loading, nature of chemical promoter, and noble metal loading were investigated. Based on the present results, the reaction mechanism was suggested.NZVI was synthesized by a well-established method [18]. 50 mL of NaBH4 solution (0.9 M) was first prepared using deaerated and deionized water (DDIW). An exact concentration of FeCl3 .6H2O (0.11 M) was prepared in ethanol and DDIW (1:8 v/v) and the NaBH4 solution was added dropwise into FeCl3 .6H2O under constant mixing for >15 min to remove the remaining H2 gas. The suspension was sonicated for 2 min and washed with DDIW three times. The resulted suspension was used for the synthesis of bimetallic catalysts. Precursors for the promoter and noble metals were prepared by dissolving an appropriate amount of the relevant metal salt in DDIW, respectively. The solution was then added dropwise into the NZVI suspension under vigorous stirring. After addition of precursors, the suspension was stirred for 3 min to ensure reduction of metals by NZVI, and then washed with DDIW three times. The resultant slurry was used for the batch catalytic experiments.A morphological analysis of the catalyst was conducted using Scanning Electron Microscopy (SEM) with Energy-dispersive X-ray spectroscopy (EDX, Hitachi S-4700). Dried catalysts were placed onto metal sample holders and covered with a gold film. Catalytic activity experiments were conducted in a batch reactor (20 mL amber vial), and details are provided in the ESI.SEM/EDX analysis was conducted to investigate the morphological characteristics of NZVI and the dispersion of Pd particles on its surface. Fig. S1a-b (Electronic Supporting Information, ESI) illustrates the SEM images of 1.5%Pd/NZVI particle surface with magnifications of 20 k and 100 k, respectively. Fig. S1a shows that plate-shaped NZVI particles were synthesized. During the synthesis of NZVI, an ultrasonication process was applied [29], and thereby, round-shaped NZVI particles (~50 nm) can also be seen in Fig. S1b. The results indicate a successful synthesis of nano-sized iron particles. In addition, EDX mapping of surface elements of the catalyst was carried out to investigate Pd distribution on the NZVI surface. Fig. S1c-d shows e EDX mapping images of Pd and Fe, respectively, indicating that the chemical elements were well-mixed. It is also shown that Pd particles were uniformly dispersed on the surface of NZVI support. These results suggest that the applied synthesis method of Pd/NZVI provides proper dispersion and distribution of metal catalysts on the surface of NZVI support.Kinetic experiments in a batch reactor mode were conducted to evaluate the catalytic reduction of PNP by the bimetallic 4%Zn-1.5%Pd/NZVI, and the monometallic 1.5%Pd/NZVI and 4%ZnNZVI catalysts (Fig. 1 ). The reduction kinetics of PNP by bare-NZVI is also shown in Fig. 1 and compared to that obtained by the other catalysts. The control test (absence of catalyst) showed no removal of PNP throughout the experiment, indicating that no adsorption of PNP on the reactor's wall and no reduction by photolysis in the amber vial (reactor) occurred during the reaction. The reduction of PNP by bare NZVI reached 93.7% in 5 min, while a monometallic catalyst (4%Zn/NZVI) can completely degrade PNP in 3 min. The presence of promoter metal (Zn) could facilitate an electron transfer from the NZVI surface compared to the relatively slow direct electron transfer from the bare-NZVI surface, resulting in the accelerated catalytic reduction kinetics of PNP [7]. The complete reduction of PNP by 4%Zn-1.5%Pd/NZVI occurred in 1 min, and its pseudo-first-order kinetic rate constant k1 (0.0954 s−1, R2 = 0.979) was found to be 3.6 and 11.8 times higher than that of 4%Zn/NZVI and bare-NZVI, respectively. Much faster reduction kinetics of PNP by the 4%Zn-1.5%Pd/NZVI solid could be originated from the additional formation of activated hydrogen on the surface of noble metal (Pd) during the facilitated electron transfer at the Zn/NZVI interface. This can rapidly and strongly degrade PNP on the Pd surface inducing much higher catalytic activity for the enhanced PNP reduction [4,30,31]. Hence, the addition of promoter and noble metal to the bare-NZVI can increase the catalytic reduction rate of PNP by facilitating electron transfer and subsequent hydrogenation [32]. In contrast, 1.5%Pd/NZVI showed the fastest reduction kinetics of PNP (k1 = 0.248 s−1, R2 = 1), of which the kinetic rate constant k1 is 4.1 times higher than that of 4%Zn-1.5%Pd/NZVI. It indicates how the hydrogenation occurred on the Pd surface could overwhelmingly contribute to the enhanced reductive catalysis of PNP with the fastest reduction kinetics. We show here the superiority of NZI-supported mono noble metal (Pd) catalyst over the bimetallic one for the enhanced reduction of PNP.The batch kinetic experimental results for the removal of PNP by the 1.5%Pd/NZVI catalytic system were compared to those obtained by other catalysts recently reported. Table S1 summarizes the kinetic rate constant for the removal of PNP by each of the catalysts under diverse experimental conditions. It can be seen that the 1.5%Pd/NZVI has the highest catalytic activity for the PNP reduction among the catalysts reported to date. Most of the previously reported catalysts for the PNP removal used passive support materials that cannot donate electrons and facilitate the electron transfer from the support, while NZVI-supported catalysts can actively donate electrons to the promoter metal or directly to the contaminant [1,15,33–35]. For instance, Chen et al. [35] investigated the performance of Au/Pd bimetallic catalyst deposited on the surface of graphene nanosheets, which did not possess any reductive capacity, and they were simply used to prevent the agglomeration of the nanoparticles. Here, NZVI-supported mono- and bimetallic catalysts showed high activity for the enhanced PNP removal. It can be concluded that the synthesized Pd/NZVI catalyst appears as one of the most promising nanocatalysts for the enhanced PNP removal.NZVI-supported monometallic catalysts with different promoter metals including Cu, Sn, Zn, Ni, and In were tested for the reduction of PNP. 4%Zn/NZVI showed the fastest reduction kinetics; hence, it was selected for further experimental studies. An increase in the catalyst loading resulted in the saturation point of its catalytic reactivity at a concentration of 500 mg/L. Monometallic Pd/NZVI catalyst showed a faster reduction kinetics than bimetallic Zn-Pd/NZVI since Zn particles could block available reactive surface sites of Pd. However, an increase in Pd loading of Pd/NZVI catalyst led to a decreased reduction kinetics of the catalyst. The details of the section are provided in the ESI. Fig. 2 shows the variation of UV–Vis spectra during the reduction of PNP by Pd/NZVI. Once PNP was added to a weak basic aqueous solution (pH ~7.5), it could be easily deprotonated to form p-nitrophenolate. The catalytic reduction is initiated by the addition of Pd/NZVI. As the catalytic reaction proceeds, the peak at 400 nm corresponding to p-nitrophenolate is decreased, while a peak at 300 nm corresponding to p-aminophenol (PAP) is increased [2]. It indicates that the main reduction product of the catalytic reduction of PNP by Pd/NZVI is PAP.The catalytic reduction of PNP to PAP on the surface of Pd/NZVI can be explained via two main reduction pathways: (i) direct reduction of PNP to PAP via electron transfer from the reactive NZVI support in the form of Fe(II) and Fe(0) (Eqs. (1), (2)) and (ii) indirect reduction (hydrogenation) via reactive Hads generated on the Pd surface (Eqs. (2)–(4)) [4,7,31]. (1) Fe 2 + → Fe 3 + + e − (2) Fe 0 → Fe 2 + + 2 e − (3) 2 H + + 2 e − → 2 H ads → H 2 (4) Pd 0 + H 2 → Pd − 2 H ads The surface of NZVI could be oxidized to Fe(II) oxides with the generation of electrons until its surface reached complete passivation by Fe(III) oxides. They could further react with aqueous H+ forming the reactive Hads adsorbed species on the Pd surface that is the main overwhelming driving force to vigorously reduce PNP to PAP in the monometallic system (Eq. (3)). Pd particles were able to continuously activate H2 to the reactive Hads species on their surface (Eq. (4)), leading to the enhanced PNP reduction kinetics by the continuous catalytic reduction system of PNP. Lai et al. [31] demonstrated that the generation of Hads was the main reducing power in the reductive degradation of PNP by Fe/Cu catalyst. It could not completely reduce the PNP under high pH conditions since low H+ concentration limited the generation of Hads [31]. Moreover, since the addition of promoter metal and its loading increase have deteriorated the catalytic reduction kinetics of PNP, we can conclude that the indirect reduction of PNP via hydrogenation pathway with the reactive Hads species played the main role in the reaction mechanism of the catalytic PNP reduction.The study provided insights on the proper synthesis of NZVI-supported metal catalysts for the enhanced catalytic reduction of PNP. The effect of significant factors such as catalyst loading, promoter type and loading, and noble metal loading on the performance of catalytic PNP reduction were evaluated for the optimal operation of the batch catalytic system. Monometallic catalyst with a noble metal (1.5%Pd/NZVI) showed the fastest PNP reduction kinetics (k1 = 0.248 s−1, R2 = 1) among the catalysts reported to date, while bimetallic catalyst (4%Zn-1.5%Pd/NZVI) has shown much faster PNP reduction kinetics (k1 = 0.095 s−1, R2 = 0.979) than the Pd monometallic catalysts with different promoters. The optimal catalyst loading was observed at 500 mg/L for the enhanced catalytic reduction of PNP. Indirect reductive transformation of PNP to PAP via hydrogenation with reactive Hads on Pd surface was suggested as the main reduction pathway since 1.5%Pd/NZVI has shown the highest rate for the catalytic reduction of PNP to PAP. The type and content of noble metal influencing the catalytic activity for an application to practical water treatment systems need to be carefully selected and evaluated by considering its role and behavior in the catalytic reduction of PNP. The limitations of this study are the absence of activity tests under different pHs of the suspensions and the absence of stability test of the catalyst during repeated cycles, which will be both our near-future research tasks.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 Research Grants of Nazarbayev University (091019CRP2106 and 021220FD1051) and the Ministry of Education and Science of the Republic of Kazakhstan (APO9260229). The authors would like to thank Prof. Sungjun Bae of Konkuk University for basic environmental monitoring training. The authors also would like to extend their gratitude to the anonymous reviewers that helped significantly to improve the quality of the paper. Supplementary material Image 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2021.106337.

Nano-sized zerovalent iron (NZVI) - supported metal catalysts were synthesized to characterize their reactivity for the reductive degradation of p-nitrophenol (PNP). Among the tested monometallic catalysts using metal promoters, Zn/NZVI showed the highest reactivity with complete reduction of PNP in 5 min (k = 0.0263 s−1). The addition of Pd accelerated the degradation kinetics of PNP with complete reduction in 1 min (k = 0.095 s−1) but promoter's presence on bimetallic catalyst surfaces simply decreased their reactivity. A proper Pd amount (1.5 wt% Pd/NZVI) showed the highest degradation rate (k = 0.248 s−1), while after its content increased to 10 wt% the rate was reduced by 5.8 times.

Most of non-renewable energy sources, such as coal, oil, and natural gas contribute to the increase in the CO2 emission [1,2]. Turning CO2 into high-value fuels or chemicals becomes an alternative strategy to replace fossil fuels with renewable energy and reduce the emissions of CO2 in the atmosphere [3,4]. Converting of CO2 to single carbon (C1) products, such as formic acid, carbon monoxide, methane, and methanol through direct hydrogen reduction or hydrothermal-chemical reduction in water have gained many interests of researchers. Methane (CH4) produced from hydrogenation of CO2 or CO2 methanation becomes one of the renewable energy carriers. Extensive research works in CO2 methanation to CH4 have been reported [2,5–7]. CO2 methanation is considered to be an effective strategy to reduce the CO2 emission by generating CH4 in places where H2 is produced from renewable energy sources and afterwards to use it everywhere [2,5–7]. CO2 methanation, also called the Sabatier reaction, is exothermic reaction and limited by the equilibrium at high temperatures. It usually conducts at temperatures of 250-500 °C and pressures of 1–80 bar in the presence of hydrogen and heterogeneous catalysts [8–10]. Since this equilibrium limitations, a superior catalyst with high activity and selectivity towards CH4 at moderate temperatures and pressures condition is needed to be developed [5].Nickel-based catalysts are known as the most studied for CO2 methanation due to their low cost and high natural abundance [5,10,11]. The catalytic performance of the Ni-based catalysts depends on various parameters such as the Ni content, type of promoters and supports, preparation method, and reaction conditions [12]. Many studies have been reported in order to improve their catalytic performance at low temperature and stability at higher temperature [10,13,14].Modification of physicochemical properties by adding the promoters, such as transition metals [14], alkali and alkaline earth metals [15], and rare-earth [16] was reported as an alternative way to improve the catalytic performance of Ni-based catalysts. The transition metal additives, such as V, Cr, Mn, Ce, Fe, Co, Y and Zr was investigated as efficient promoters in CO2 methanation [17–21]. For example, Y and Zr are the most metal used as dopants to modify the properties of support. Zr is used to stabilize the CeO2 structure and enhance its oxygen vacancies population [17], while Y can produce oxygen vacancies in ZrO2-based supports [18,19]. Xu, et al. reported the incorporation of rare earth (La, Ce, Sm, and Pr) increased the surface basicity and electron properties of the Ni-based catalysts which contributed to the activation of the CO2. The Ni species were dispersed well and formed the strong metal framework interaction. The thermal sintering of the metallic Ni nanoparticles during CO2 methanation conditions was minimized and there was no obvious deactivation was observed after 50 h stability test. Thus, Ni-promoted catalyst could be considered as promising catalyst candidates in low-temperature CO2 methanation [22].The introduction of a second metal, such as Fe, Co or noble metals which are Ru, Rh, Pt, Pd and Re as a dopant was reported to influence the properties of Ni, increase the dispersion, stability, and reducibility. Thus, it enhanced the CO2 methanation activity of nickel-based catalyst by creating bimetallic and multi-metallic catalysts system [14,23,24]. For example, NiFe alloys were reported as an active and stable catalysts for dry reforming of methane due to the addition of Fe promoted carbon gasification and minimized coking [25]. Ru and Ni mostly form monometallic heterostructures that rely on the synergistic effect between the two separate metallic phases, while Pt and Pd mostly lead to the creation of NiPt and NiPd alloys. It has been shown that small addition of noble metal (e.g., 0.5% or 1%) enhanced the reducibility and low-temperature activity of Ni-based catalysts [26].The catalyst support also plays an important role on the morphology of the active phase, adsorption ability and catalytic properties [27]. The use of metal oxides as supports for nickel, such as γ-Al2O3, SiO2, CeO2, ZrO2, and different combinations of mixed oxides CeO2-Al2O3, CeO2-ZrO2, Y2O3-ZrO2 was reported [5,9,11,13,21,28]. Among the supports, alumina is commonly used due to its high specific surface area and strong interaction with active metal [29]. The modification of the catalyst support becomes an alternative solution to increase the catalytic activity and reducibility. Different support modifiers, such as ZrO2, SiO2, MgO, La2O3, CeO2, and TiO2 showed better conversion, higher redox property, higher thermal stability and resistance against sintering due to their excellent properties [30].Yttria-stabilized zirconia(YSZ) as known as SOEC material has been utilized for catalytic applications due to their properties [31]. YSZ as an oxygen ion conductor is a ceramic material combines different functionalities such as good thermal stability, selective bulk oxygen mobility and high surface oxygen vacancy concentration. YSZ has been considered as a promising support for metallic nanoparticles or as a catalyst itself [19,32]. Nickel and yttria-stabilized zirconia (Ni–YSZ) cermet has been commonly employed as cathode material of SOEC due to its high catalytic activity, excellent electronics and conductivity, low cost, sufficient thermal expansion coefficient, and mechanical-chemical compatibility with other components [33]. Ni plays the main role of catalyst for oxidation of the fuel, and YSZ acts as a support to hold the porous structure and prevent Ni coarsening [13]. However, few researches have been done regarding to methanation over YSZ as catalyst support so far. Kosaka, et al. prepared Ni-yttria-stabilized zirconia (Ni-YSZ) tubular catalysts with different NiO contents ranging from 25 to 100 wt% and investigated the effect of Ni content on the CO2 methanation performance. The results showed that catalysts with Ni contents >75 wt% produced CH4 yields >91% and high CH4 selectivities (>99%) [5]. Watanabe, et al. developed an yttria-stabilized zirconia catalyst-supported nickel (Ni/YSZ) with high tolerance to coke deposition during methane steam reforming (MSR). Ni/YSZ prepared by an electroless plating method exhibited better stability during the MSR than prepared by a conventional impregnation method [34]. Fakeeha, et al. studied the use of yttria stabilized zirconia support with different loadings (5, 10, 15 and 20 wt%) of yttria. The results showed that Y2O3 stabilized ZrO2 supported catalysts produced higher conversions of CH4 and CO2 and higher stability compared with unstabilized ZrO2 supported catalysts at 700 °C [35].In this research, 60wt%NiO-40wt%YSZ and 60wt%NiO + 40wt%YSZ catalyst were investigated their catalytic activity for CO2 methanation. The ratio of NiO and YSZ which are 60 wt% and 40 wt%, respectively, was the optimum ratio for NiO-YSZ as a SOEC cell stack. The effect of feed ratio, catalyst reduction temperature, reaction temperature, and support to the activity and selectivity of the catalyst were investigated. The common methanation catalyst, 10wt%Ni/Al2O3 was compared as a benchmark. Furthermore, the stability test was also investigated through long-term tests.Catalyst precursors used are NiO (Sumitomo), YSZ (Tosoh), Ni(NO3)2.6H2O (Wako) and KHO-12 (spherical alumina (1–2 mm) Sumitomo Chemicals). Reactant gases used are hydrogen, argon, carbon dioxide, and carbon monoxide. Each of their purity are 99.999%, 99.995%, 99.95%, and 99.95%, respectively.Three type of catalysts used in this research are 60wt%NiO-40wt%YSZ, 60wt%NiO + 40wt%YSZ, and 10wt%Ni/Al2O3 catalyst. 60wt%NiO-40wt%YSZ was prepared in Chubu Centre of the National Institute of Advanced Industrial, Science and Technology (AIST). The powders of NiO and YSZ were calcined at 1400 °C, and reduced at 700 °C at 2 h by hydrogen flowing. Then, the bulks were crushed to small powder. While, 60wt%NiO + 40wt%YSZ was prepared in Department of Applied Chemistry and Biochemical Engineering, Shizuoka University. As of 60 wt% NiO and 40wt%YSZ were mixed physically by using mortar until homogeneous. After that, the mixture was reduced in situ with 10 ml·min−1 H2 flow at various temperature in the range of 300 °C until 500 °C. 10wt%Ni/Al2O3 catalyst was prepared by impregnation method from Ni(NO3)2.6H2O and KHO-12 (spherical alumina (1–2 mm)). The Ni/Al2O3 was calcined at 500 °C for 5 h, and reduced in a reactor with 10 ml·min−1 H2 flow at 500 °C for 5 h before reaction.Powder X-ray diffraction patterns were collected using a Rigaku RINT 2000 equipped with a Cu Kα (λ = 1.5418 Å) source and the Brag-Brentano θ–θ configuration in the 10–90° 2θ range, with 0.05° step size and 2 s acquisition time. The crystallite size (d) of the catalyst samples was determined using Scherrer formula [30]: d = 0.9 λ βcosϴ Where λ is the x-ray wavelength, β is the full-width for the half maximum intensity peak, and θ is the diffraction angle.The surface area of the catalysts was measured by N2 adsorption desorption isotherms at 77 K using a Micromeritics ASAP 2010 apparatus. Before measurement, the catalysts were degassed at 300 °C in N2 for 5 h. The surface area was calculated by the Brunauer-Emmet-Teller (BET) method in the equilibrium pressure range 0.05 < P/P° < 0.3.H2-temperature programmed reduction (H2-TPR) of the samples were performed using BEL MULTI-TASK-TPD. Analysis was carried out on 300 mg of sample, heating from 50 to 500 °C at a heating rate of 5 °C·min−1 with 10%H2 flow rate of 5 ml·min−1 and holding at the final temperature for 5 h. The H2 consumption was measured by a mass spectrometer (MS).CO2-temperature programmed desorption (CO2-TPD) of the samples were performed using BEL MULTI-TASK-TPD. Analysis was carried out on 1000 mg of sample, heating from room temperature to 500 °C at a heating rate of 5 °C·min−1 with helium gas flow rate of 50 ml·min−1 for 30 min. After the cleaning with He gas, the samples were cooled to 50 °C, switched in CO2 and saturated down at 50 °C. Then, the samples were purged again with He. The CO2 consumption of it was measured by an MS detector with heating from 50 °C to 500 °C.The performance of the catalysts was evaluated for CO2 methanation. The reactions were performed in a fixed bed reactor operating at atmospheric pressure by means of gaseous mixtures of H2/ CO2/Ar with different volumetric ratios in a temperature range 160–440 °C and GHSV 18,750 h−1. Prior to the activity tests, 1000 mg of catalyst was placed inside a stainless steel fixed bed reactor (inner diameter: 9.0 mm), with quartz wool at both ends, and reduced in situ with 10 ml/min H2 flow, increasing the temperature from room temperature up to 700 °C and isothermally kept at this temperature for 60 min. Different reduction temperatures were employed to determine the effect of reduction temperature. Afterwards, the feed mixture was flowed through the reactor. The products were analyzed by two online gas chromatographs (Shimadzu GC-14B) consisted of two detectors, thermal conductivity detector (TCD) and flame ionization detector (FID) in series. Each of GCs have a Molecular Sieve 5A and a Porapak T columns, for analyses of gases and liquids including CO2, respectively.The specific surface area of the catalyst samples was calculated from their respective N2 adsorption isotherms. YSZ showed higher specific surface area compared to 60wt%NiO-40wt%YSZ and 60%NiO + 40%YSZ. It indicated that the deposition of Ni0 changed the textural properties of the YSZ support as shown by BET surface areas (6.93–7.70 m2. g−1) that are lower than YSZ support (13.7 m2. g−1). There was no significant difference observed between the specific surface area of the 60%NiO + 40%YSZ reduced at range temperature of 300-500 °C. The specific surface area of 60%NiO + 40%YSZ catalysts after reduced at range temperature of 300-500 °C tend to remain stable in the range of 6.93–7.70 m2. g−1. 60wt%NiO-40wt%YSZ catalyst reduced at 700 °C also exhibited the similar value of specific surface area. It indicated that YSZ as support contributed to maintain the specific surface area and minimize the sintering effect of NiO particles in 60wt%NiO-40wt%YSZ at higher reduction temperature.The XRD analysis was carried out to find out the presence of the metallic states of active metals. Fig. 1 shows the XRD patterns of the catalysts before (fresh) and after reaction (used) at various reduction temperatures. Both of fresh and used 60wt%NiO-40wt%YSZ and 60wt%NiO + 40wt%YSZ catalysts showed the existence of cubic fluorite structure of YSZ support at 2θ = 30o, 34o, 59o, and 73o (JCPDS 81–1550). There were significant differences observed between fresh and used catalysts. The 60wt%NiO-40wt%YSZ fresh showed the characteristic peaks of cubic nickel oxide (NiO) at 2θ = 37.3o, 43.3o and 62.9o (JCPDS 73–1519). Whereas, either 60wt%NiO-40wt%YSZ or 60wt%NiO + 40wt%YSZ used catalysts, all of NiO phase peaks were disappeared and metallic fcc-Ni0 phase existed in all the reduced catalysts at 2θ = 44.5o, 51.8o, and 76.5o (JCPDS 04–850). It suggested that catalyst reduction was succeeded to reduce the Ni2+ into Ni0. As can be seen from the XRD patterns that the characteristic peak of YSZ as support shifted to the lower 2θ. It indicated that addition of NiO into YSZ decreased the crystallinity and crystallite size of YSZ. The crystallite sizes of YSZ, d (nm), calculated by Scherrer's equation decreased from 20.45 nm to 17.92, 19.04, and 18.79 nm for 60wt%NiO + 40wt%YSZ 350 °C-red-5 h, 60wt%NiO + 40wt%YSZ 400 °C-red-5 h, and 60wt%NiO + 40wt%YSZ 500 °C-red-5 h catalysts, respectively.The reducibility of the NiO, YSZ, 60wt%NiO-40wt%YSZ and 60wt%NiO + 40wt%YSZ catalysts were studied by H2-TPR as shown in Fig. 2 . NiO, 60wt%NiO-40wt%YSZ and 60wt%NiO + 40wt%YSZ reduced samples showed that there were two main peaks observed for hydrogen consumption and the reduction started at temperature about 200 °C. The first peak at lower temperature (200-300 °C) is associated with larger NiO particles that are of similar nature to pure bulk NiO or weakly interact with the YSZ support. These particles can be reduced at low temperatures. Whereas, the second peak at higher temperature (342-485 °C) is attributed to the greater dispersion of metallic oxide that strongly interact with the YSZ support. The absence of peaks for YSZ indicates that YSZ was not a highly reducible support material.The CO2-TPD patterns were obtained as shown in Fig. 3 . All the catalysts had only weak low-temperature TPD peak implying that there were only weak basic sites for CO2 chemisorption. 60wt%NiO-40wt%YSZ and 60wt%NiO + 40wt%YSZ exhibited two peaks which first peak was at 55-144 °C and second peak was at 152-335 °C. The first peak of 60wt%NiO-40wt%YSZ is higher than 60wt%NiO + 40wt%YSZ indicated the higher CO2 methanation activity of 60wt%NiO-40wt%YSZ at lower reaction temperature.To study the effect of feed ratios, several test runs were performed at reaction temperatures of 160 to 440 °C with a total feed flow rate of 100 ml/min by varying only the volume percentages of the reactant gases CO2 and H2 while the volume percentage of inert gas Ar was kept constantly. CO2 conversion, CH4 selectivity and CH4 production rate results are displayed in Fig. S1. The excess amount of H2 enhanced CO2 conversion and CH4 selectivity. However, an increase in the CO2 reactant amount favored CO formation, since a lower CH4 selectivity was produced in the case of 1/1 (H2/CO2) ratio. These results suggested that the probability of CO2 reacting with the other reactant H2 and converting into CH4 was higher in the presence of an excess amount of hydrogen. As shown in Fig. S1, the highest CH4 production rate of those tested was obtained at a H2/CO2 ratio = 4/1, this ratio was used for the next catalytic activity tests.Different reduction temperatures were chosen to determine the effect on the performance of catalysts for CO2 hydrogenation reaction at the optimum of feed gas ratio H2: CO2: Ar = 72:18:10 ml·min−1. The higher reduction temperature of 60wt%NiO + 40wt%YSZ catalysts improved the performance of catalysts on methanation reaction. 60wt%NiO + 40wt%YSZ catalysts reduced at 300–500 °C produced CO2 conversion more than 60% and CH4 selectivity is almost near 100% as shown in Fig. 4 . This result was better than previous research using Ni/YSZ catalysts that performed lower CO2 methanation activity. CO2 conversion and CH4 selectivity reported were 60% and 75%, respectively [19]. Fig. 5 shows CH4 production rates of nickel supported on YSZ at various reduction temperatures. For 60wt%NiO + 40wt%YSZ catalyst, the CH4 production rates tend to increase by the increase of reduction temperature in the following order: 350 °C < 400 °C < 500 °C. The 60wt%NiO + 40wt%YSZ catalyst reduced at 500 °C for 5 h produced the highest CH4 production rates of 31.81 mmol·gcat −1·h−1 at reaction temperature of 400 °C. It was little bit higher compared to 60wt%NiO-40wt%YSZ catalyst reduced at 700 °C for 1 h which produced CH4 production rates of 30.79 mmol·gcat −1·h−1 at 400 °C.Methanation activity of nickel supported on YSZ was better than those on YSZ. 60wt%NiO-40wt%YSZ reduced at 700 °C for 1 h performed higher CH4 production rate per catalyst weight compared with YSZ catalyst in all reaction temperatures carried on. YSZ as the support performed poor methanation activity at all reaction temperatures conducted. It suggested that the existence of YSZ minimized the sintering of nickel metal particle at higher reaction temperature and contributed to the catalytic activity of 60wt%NiO-40wt%YSZ catalyst for methanation at higher reaction temperature.Ni/Al2O3 catalysts are well-known as common catalysts for methanation due to the high specific surface area of Al2O3. The specific surface area of 10wt%Ni/Al2O3 catalyst prepared was 210 m2/g. However, one big drawback of Ni/Al2O3 catalyst is these catalysts are easily deactivated due to sintering of Ni particles and coke deposition during the exothermic methanation [6,9]. Fig. 6 shows the turnover frequency (TOF) for methane (methane produced per nickel site per second) production by CO2 methanation on nickel supported on YSZ compared with nickel supported on Al2O3, nickel unsupported, and YSZ. The TOF for methane production by CO2 methanation on 60wt%NiO-40wt%YSZ was higher than 10wt%Ni/Al2O3 catalyst due to the existence of YSZ support as described in Fig. 6. At lower temperature, the TOF of 60wt%NiO-40wt%YSZ was lower and increased continuously by the increase of reaction temperature. It remained stable about 15 × 103-16 × 103 h−1 after 360 °C.The activation energy produced by 60wt%NiO-40wt%YSZ catalyst is 3.46 kJ/mol and it is lower than other Ni-YSZ published with activation energy of 93.6 kJ/mol [5]. This lower value of activation energy is apparent activation energy by reaction data. However, 60wt%NiO-40wt%YSZ catalyst produced activity and selectivity as high as published.As shown in Fig. 7 , the 60wt%NiO-40wt%YSZ catalyst was stable during 220 h of CO2 methanation. There was no distinguishable difference observed in the activity of the catalyst. A long-term reaction at 400 °C revealed the remarkably stable performance of the 60wt%NiO-40wt%YSZ catalyst under CO2 methanation conditions with 29.56 mmol·gcat −1·h−1 of CH4 production rate and 1.52 mmol·gcat −1·h−1 of CO production rate. This result is similar as previous reported that Ni-based catalysts could maintain a very good methanation activity over a reaction time of nearly 100 h with high CH4 selectivity (about 100%) [5].The CO2 methanation activity was studied over nickel supported on YSZ catalyst. 60wt%NiO-40wt%YSZ catalyst exhibited high CO2 conversion close to the equilibrium one from 360 °C and above with CH4 selectivity of 100% under a GHSV of 18,750 h−1 at H2/CO2 = 4. It was confirmed from XRD and TPR results that the catalytic pretreatment in H2 completely reduced the NiO to Ni0. CO2-TPD demonstrated the amount of CO2 adsorbed on 60wt%NiO-40wt%YSZ was much larger than that on 60wt%NiO + 40wt%YSZ. 60wt%NiO-40wt%YSZ catalyst produced higher TOF for methane production than 10wt%Ni/Al2O3 catalyst due to the existence of YSZ support and stable during more than 220 h. Anis Kristiani: Experimental work, Data analysis, Writing- Original draft preparation, Writing- Review and Editing. Kaoru Takeishi: Conceptualization, Methodology, Supervision, Funding acquisition. All authors have read and agreed to the published version of the 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.The authors would like to acknowledge funding from Japan Science and Technology Agency – Core Research for Evolutional Science and Technology (JST-CREST) through a project entitled “Development of Innovative Technology for Energy-Carrier Synthesis using Novel Solid Oxide Electrolysis Cell (JPMJCR1343)”. Supplementary mateial Image 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2022.106435.

The CO2 methanation becomes promising solution to mitigate global warming and energy issues. Yttria-stabilized zirconia (YSZ) has been utilized for solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) applications. However, few researches about the use of YSZ as nickel-based catalyst support for CO2 methanation. Herein, we investigated the catalytic performance of 60wt%NiO-40wt%YSZ. The results showed that 60wt%NiO-40wt%YSZ produced methane with almost 100% selectivity and stable during more than 220 h. It confirms the excellent performance of 60wt%NiO-40wt%YSZ catalyst.

Data will be made available on request.The high energy demand due to the rapid growth of modernisation has accelerated the predicted depletion of conventional fossil-based resources such as coal, petroleum and natural gas. Fossil fuels are not eco-friendly as they are related to poisonous gas exhausts from automobile and industrial internal combustion engines, which eventually lead to global warming, acid rain and ozone depletion [1–3]. Our carbon demand for chemicals and energies along with the vast amount of greenhouse gas (GHG) emission attracts the development of renewable, non-polluting fuels and cleaner energy production methods [4,5]. In the EU, 80–95% GHG reduction is aimed to achieve in 2050 which is portrayed to be achievable with the use of renewable energy resources [6]. To promote the use of renewable energy resources as circular economy approach, political enactments such as the renewable portfolio standard (RPS), renewable fuel standard (RFS) and feed-in tariffs (FiTs) have been made in both developed and emerging economies countries [7–9].The use of renewable biofuel is the most promising alternative source of energy considering its biodegradability and low emission of carbon dioxide, free sulphur and non-toxic nature and possesses all the physicochemical properties of traditional fossil fuels such as liquid, solid and natural gas in terms of improved cetane number and high flash point [10,11]. Biofuels are biobased products, in solid, liquid or gaseous form which are produced from organic material such as plants, and their residues, agricultural crops, and by-products that can be an adequate substitute for petroleum-derived fuel [12–14]. The types of biofuels can be classified based on the feedstock used for production [13]. Waste cooking oil (WCO) is one of the most prudent sources of renewable energy feedstock because of its restrictions on reusability as food, freely available and its suitable properties for fuel production. Their valorisation can concurrently solve the WCO mismanagement issue and substantially reduce the cost of biofuel as feedstock constitutes 75–90% [15] of the overall cost which impeded the transition to the greener fuel option. WCO which originated from a plant (palm, soybean, rapeseed, etc.), consist of triglycerides with a long hydrocarbon chain ranging from C16 and C18, which has a similar molecular network to hydrocarbon fossil fuel [16]. During the repeated high-temperature exposure during frying, three main chemical reactions took place (thermolytic, oxidative and hydrolytic reaction) [17], which increases the oxidation variations (acidic compound and polymerised materials), and water content [18,19]. The difference between the fatty acid composition in fresh cooking oil and waste cooking oil has been reported in depth previously by Thawatchai et al. [20].Two routes that are mostly studied for WCO conversion to biofuel are the transesterification process and the pyrolysis process [21]. Biodiesel consists of alkyl esters produced by transesterification and contributes to most of the biofuel production [21,22]. Feedstock pre-treatment which requires an extra step in the production process is unavoidable for the sample with high moisture content, high impurities and FFA values such as WCO [5,23–25]. Moreover, a significant amount of glycerol is produced as a by-product of the conventional transesterification method [26]. Green diesel on the other hand contains deoxygenated hydrocarbon which may be obtained from the catalytic deoxygenation reaction [27]. Pyrolysis reaction is a rapid thermochemical conversion method that breaks the chemical bonds of the feedstock in an oxygen-free environment at a temperature range of 300–500 °C into valuable products for biofuel or as chemical feedstock [16,28,29]. The product obtained from this pyrolysis reaction includes the organic liquid product, gaseous product, coke and water. In terms of the conversion process, the catalytic deoxygenation process has many advantages especially its great flexibility in the choice of raw materials, rapid conversion gradient, straightforward process, and industrial upscaling possibility [30,31]. The deoxygenated product (green diesel) possesses a higher heating value, higher energy density, higher cetane number and lower NOx emissions as compared to FAME biodiesel [27].Pyrolysis, however, is deemed to be cost-ineffective due to the high-temperature requirement compared to the product yield. Catalytic cracking with selective heterogeneous catalysts is a potential candidate for renewable-process-based industrialization to increase the pyrolysis yield and hence decrease the cost of liquid fuels [21]. These catalysts can be easily regenerated, recycled and environmentally friendly [31–33]. The development of heterogeneous catalysts is aimed to overcome the technical shortcomings associated with conventional non-recyclable homogeneous catalysts such as KOH and H2SO4 for biodiesel production. Up to date, a great deal of research on the development of a solid base heterogeneous catalyst for catalytic pyrolysis has been conducted such as biochar derived from chicken manure [34], Mg/Activated carbon [35], and K2O/Ba-MCM-41 [30]. On the other hand, only several solid acid heterogeneous catalysts have been proposed as catalytic material for biodiesel production such as composite zeolites [36], ZrO2 [37], Bentonite [38] and etc.With the conventional optimization method, it is time-consuming and requires a large number of experiments to determine optimum levels, which may be unreliable as the yield of organic liquid product (OLP), gas, water and coke are influenced by various factors such as pyrolytic temperature, residence time, heating rate, nitrogen flow rate [35,37,39]. By using the Taguchi method, the limitations of the conventional optimization method can be eliminated by optimising all the process parameters collectively using statistical experimental design. The most important feature of the Taguchi method is the use of an orthogonal array that can stipulate the way of conducting the minimal number of experiments which could give the full information of all the factors that affect the performance parameter [40,41]. The use of an adequate experimental design such as the Taguchi method for WCO pyrolysis is particularly important. To the best of our knowledge, no other work has been reported on the utilization of this heterogeneous acid, sulphated-ferric (II) oxide/alumina oxide catalyst via catalytic deoxygenation of WCO. Herein, in this study, the Taguchi method was used to investigate the optimal conditions for catalytic deoxygenation of WCO using sulphated-ferric (II) oxide/alumina oxide as a deoxygenation catalyst. This paper demonstrates the usefulness of using Taguchi coupled with product pyrolysis with GC-MS to predict pyrolysis yields with a great reduction in the number of experiments. Table 1 represents the nomenclature of this paper.Alumina-supported-sulphated-ferric oxide (II) (SO4 2--Fe2O3/Al2O3) catalyst (patented commercial catalyst Patent File No.: PI 2017702072 [42]) with purity >90% was provided by Catarim Sdn Bhd, Malaysia. The physicochemical properties of these heterogeneous catalysts are summarised in Table 2 . The specific surface area and pore distribution of the synthesized catalysts were evaluated using a Brunauer-Emmett-Teller (BET) method using Thermo-Finnigan Sorpmatic 1990 series with an N2 adsorption/desorption analyzer. The particle size distribution of the synthesized catalyst was measured by laser diffraction (Malvern Mastersizer Hydro 2000S, Malvern Instruments Ltd., UK). While the acidity of the synthesized SO4 2--Fe2O3/Al2O3 catalysts were investigated using temperature-programmed desorption with NH3 as probe molecules. The NH3-TPD analysis was carried out by using Thermo Finnigan TPD/R/O 1100 instrument equipped with a thermal conductivity detector (TCD. Based on Table 2, Brunauer-Emmett-Teller (BET) analysis showed that SO4 2--Fe2O3/Al2O3 catalyst is classified as a macroporous structured catalyst with an average pore diameter >50 nm (67.77 nm). This heterogeneous catalyst has a small pore volume of around 0.19 cm3/g which will limit the rapid access of reactant into the porosity of the catalyst and the small amount of coke will be expected to form after the reaction. Besides that, through its relatively high surface area of 109.87 mg2/g, this catalyst will provide sufficient contact area between the WCO substrate and active sites for the reaction to take place. The particle size of the SO4 2--Fe2O3/Al2O3 catalyst is smaller than 28–29 nm. The smaller the catalyst particle size, the larger the surface area for a given mass of particles as shown in Table 2. Based on a previous study conducted by Ng et al. [43], SO4 2--Fe2O3/Al2O3 catalyst has composed of mild weak acid sites and a majority of strong acid sites with 1.03 × 1021 atom/g. As reported by Nur Azreena et al. [44], weak or medium acid sites played an important role in the removal of oxygenating species, while strong acidic sites possessed the ability to catalyse the cracking reaction and dehydrogenation. This justified that SO4 2--Fe2O3/Al2O3 catalyst has the potential to be used as a deoxygenation heterogeneous acid catalyst in green diesel production via deoxygenation of WCONitrogen gas (N2) with 99% purity was supplied by Smart Biogas Sdn Bhd. GC-MS analysis using n-hexane with purity >98% from Merck was utilized. The feedstock i.e. palm oil waste cooking oil (WCO) was collected from a local restaurant in Serdang, Selangor. The WCO was filtered, centrifuged at 3000 rpm for 30 min and heated to 100 °C prior to experimental work. To eliminate the differences due to feedstock differences, all the pyrolysis tests used the WCO from the same batch. The major composition of WCO consists of high fatty acids such as oleic acid (43.2%), linoleic acid (30.1%) and palmitic acid (19.4%) as summarised in Table 3 . As reported by Hafriz et al. [45], these fatty acid structures were still maintained even after deep frying due to high boiling points and these fatty acid contents are the major indicators of the properties of biofuels produced via catalytic deoxygenation.Catalytic deoxygenation processes were conducted at temperatures ranging from 350 ± 5 to 450 ± 5 °C in a three-neck round bottom flask along with varied reaction parameters. Fig. 1 [16] displays the apparatus setup of the experiment. Both the WCO feedstock (150 g) and SO4 2--Fe2O3/Al2O3 catalyst (according to the experimental design) were placed in the triple neck glass reactor, stirred at 400 rpm continuously and heated at the rate of 20 °C/min with a flow of 10–20 cm3/min of N2. The thermocouple was calibrated to control the temperature of the reactor and the tip was inserted into the WCO feedstock. In the catalytic deoxygenation process, the WCO cracked and vaporized when the reaction temperature was reached. The vapour left the reactor through the graham coil rectification column and condensed in the second graham coil condenser unit. The condensed liquid products were collected in the receiving flask (round bottom flask) and the residue (catalyst and coke) was left in the reaction flask. The experiment was allowed to settle and cooled for approximately 30 min for each experiment, and the liquid product from the condensation of the oil was analysed using the GC-MS analysis. All the products and glassware were weighed before and after the experiment for mass balance calculation.In the present work, the Taguchi method (Minitab 16 software) was used to design optimized WCO cracking experiments with the temperature (°C), catalyst loading (wt.%), residence time (min) and N2 flow rate (cm3/min) as the independent variables, while the selected output response was the yield (%) of the biodiesel fractions. The Taguchi method suggested a set of 9 experiments for optimization which is a significant reduction from the original 81 experiments without the use of the Taguchi method. Table 4 represents the suggested L9 orthogonal array for the waste cooking oil cracking experiments.Gas Chromatography-Mass Spectrometry (GC-MS) was used to analyse the composition of cracked liquid oil. The samples of pyrolysis oil (PO) were diluted with GC-grade n-hexane to 100 ppm. The obtained oil was analysed qualitatively and quantitatively in a non-polar ZB-5MS model column (30 m × 0.25 mm I. D x 0.25 μm film thickness) in a split mode. The oven temperature was programmed to hold at 40 °C for 3 min, ramp at 7 °C/min to 300 °C and hold at 300 °C for 5 min. The injector temperature was set at 250 °C and the flow rate of the He carrier gas was 3.0 cm3/min. A different class of compound present in the liquid oil were identified using the National Institute of Standards and Testing (NIST) library. As reported by Hafriz et al. [46,47], the identification of the feedstock and the major products of pyrolysis oil using GC-MS analysis was based on the probability match between 95% and 100%. The WCO, product yield of pyrolysis oil and hydrocarbon selectivity can be determined by comparing the peak areas of the chromatogram as it is proportional to the relative content of the products as shown in Eq. (1) and Eq. (2), respectively [48,49]. (1) Y i e l d o f h y d r o c a r b o n ( % ) = T o t a l a r e a o f C 8 − C 24 ∑ a r e a o f t o t a l p r o d u c t (2) H y d r o c a r b o n s e l e c t i v i t y ( % ) = D e s i r e d h y d r o c a r b o n f r a c t i o n Σ a r e a o f h y d r o c a r b o n x 100 % The reaction conditions and the activity of synthesized sulphated-ferric (II) oxide/alumina oxide catalyst (patented solid acid catalyst) in the catalytic deoxygenation of waste cooking oil (WCO) were investigated based on the design given by the Taguchi method. In the receiving flask, two phases of liquid were detected in the flask, which was the pyrolysis oil (PO) on the top and the acid phase at the bottom layer. The liquids were separated by using the decantation technique. From Table 5 , the PO yield ranged from 37.88% to 75.86%, varied based on the parameter, mostly affected by the temperature. In addition, the presence of SO4 2--Fe2O3/Al2O3 catalyst has improved the yield of PO due to the strong acidic sites of the catalyst which possessed the ability to catalyse the cracking reaction and dehydrogenation.No liquid product was obtained at 350 °C (Runs 2, 4 and 9) as the vapour was unable to vaporise through the column of the condenser. Hence, from this point onwards, all the runs from 350 °C (Run 2, 4 and 9) had been opt-out from the figures. It is speculated that below 400 °C, the conversion of acetic acid was reduced due to catalyst deactivation, and the deoxygenation process was incomplete. At low temperatures or atmospheric pressure, deactivation of the catalyst might occur and faster carbon deposition occurred on the catalyst surface which affected the selectivity and the quality of the gasoline phase [50]. The same observation was reported by Lam et al. [51] in which no liquid product was obtained after 1 h of catalytic deoxygenation reaction at 350 °C. However, other works obtained pyrolysis oil at 300 °C with Co3O4–La2O3/ACnano catalyst [52] and at 350 °C with NiO–5CaO/SiO2–Al2O3 catalyst [33], in which the type of catalyst used might be played a better role for the deoxygenation reaction or the feedstock load was higher than in this present work.It was observed that all reactions at a higher temperature (450 °C) gave the highest PO yield (75.86% in Run 8), which shows that temperature was the most influencing parameter for obtaining a higher amount of PO. The high PO yield was obtained accompanied by the low amount of coke and slightly higher amount of gaseous product compared to other temperatures. The PO yield amount was only slightly lower than that obtained by Wako et al. [37] (83% PO yield) with zirconium oxide as another type of acid catalyst at 475 °C used in the catalytic deoxygenation reaction. However, the PO obtained at 450 °C appeared to be darker than the product at a lower temperature as displayed in Fig. 2 . The colour changes were observed when the temperature rose from 430 to 450 °C during the experiment. As reported by Makcharoen et al. [53], the colour of the liquid product was changed with increasing reaction temperature and the colour of pyrolysis oil did not significantly change when the reaction temperature was below 400 °C. However, the samples appeared distinctively darker at 420 °C possibly due to the contribution of the thermal cracking reaction that occurred in the deoxygenation of crude palm kernel oil (CKPO). According to Shurong et al. [50], the dark colouration was due to the high amount of oxygenates in the product, which is undesirable in biofuel production. Wako et al. [37] also reported the unsuitability of using higher temperatures for biofuel production as it leads to secondary cracking that shall favour gaseous products. Fig. 2 clearly showed that the catalytic reaction at a temperature of 400 °C produced a lighter yellow colour of PO despite the variations in the reaction parameters. The highest PO yield observed at 400 °C was 42.66% at Run 6. Although results at 400 °C were the most promising in terms of PO obtained, a high amount of coke was also obtained at 400 °C.In the present work, 1 wt% of catalyst loading gave the highest yield of product across different temperatures and other parameter variations. Increasing the catalyst loading in some cases increases the yield of biodiesel but overloading beyond the optimum amount may lead to non-proper mixing and over-saturate the catalyst's active sites. Besides the parameter investigated in this study, Shurong et al. [50] showed that a pressurised system could also promote oil phase yield, 32.2% of PO at 3 MPa compared to 10.8% of PO at 0 MPa.The presence of high oxygenates in the brownish-coloured oil obtained at 450 °C was confirmed by the GC-MS analysis, presented in Fig. 3 a. From Fig. 3a, the oxygenates were high in carboxylic acid and alcohol content, while a minor quantity of ketone was detected in most of the runs except in Run 6. The highest amount of carboxylic acid (mainly C12 lauric acid) was found in Run 7, while traces of other compounds including aldehydes and esters found in three runs with the sequence of Run 7 > Run 8 > Run 1. A high carboxylic acid value indicates a high acidity value of the pyrolysis oil. Hence it can be deduced that Run 7 has the highest acidity value and vice versa for Run 6. The high amount of carboxylic acid obtained in the present work might be affected by the heterogeneous acid catalyst used [16], but as shown in Fig. 3a, it can be minimised by parameter control.The hydrocarbon composition is dominated by aliphatic hydrocarbons which were alkene and alkane, followed by cyclic hydrocarbons; cycloalkane and cycloalkene as depicted in Fig. 3b Catalytic deoxygenation reaction may take place through decarbonylation (Eq. (2)) or via decarboxylation (Eq. (3)), which is usually influenced by the catalyst type. The high amount of alkane in most of the results shows domination by a decarboxylation reaction. In their work, Hafriz et al. [16] and Ali et al. [54] revealed that alkene is the main product from the WCO deoxygenation reaction with Ni-dolomite catalyst which they concluded that deoxygenation via decarbonylation dominates the conversion reaction. (3) Decarbonylation: R–COOH → Cn-H2n(Alkenes) + CO + H2O (acid phase) (4) Decarboxylation: R–COOH →Cn-H2n+2 (Alkanes) + CO2 No traces of aromatic compounds were detected in the runs except for Run 1 (0.73%) and Run 7 (0.71%) as depicted in Fig. 3b. Aromatic compounds detected in the pyrolysis oil were from benzene, naphthalene, xylene and their derivatives (butylbenzene, pentylbenzene, ethylbenzene). Aromatics are essential in gasoline or petrol fuel as they gave them high octane rating and have higher knocking resistance. A small amount of aromatic compound in a form of xylene and toluene in aviation fuel will prevent fuel from freezing at elevated temperatures. In biomass-source diesel, low aromatic compounds are desirable as aromatics are carcinogenic and posed health-threat to humans [55].The result in Fig. 3b shows high hydrocarbon content in the PO, but also accompanied by a high amount of coke as given in Table 4. In the deoxygenation of WCO, coke may be produced by two solid phase reactions: aromatic hydrocarbon polymerization (Eq. (5)) or condensation of WCO (Eq. (6)) [16]. (5) Polymerization: Cn-Hn (Aromatics) → Carbon(s) (6) Condensation: WCO → Carbon(s) It was suggested that the acidity of the catalyst might enhance both polymerization and cyclization [33]. The two runs with the highest coke amount which were Run 5 and Run 6 also had a higher yield of cycloalkane as in Fig. 3b, which verifies the claim [33]. According to Asikin-Mijan et al. [33], acidic and transition metal catalysts have high deoxygenation activity but at the same time, are more susceptible to coke formation compared to basic catalysts. Fig. 4 shows the selectivity of the hydrocarbon chain from C8 to C24 of the PO and the corresponding hydrocarbon liquid fraction to the biofuel range (gasoline, kerosene and diesel) based on the GC-MS analysis. Typical palm-oil-based WCO contains carboxylic acids in the range of C16 and C18 which reflects a high amount of palmitic acid (C16) and oleic acid (C18) [16,33]. From the decarboxylation/decarbonylation reaction (Eq. (1) and Eq. (2)), the product should contain liquid hydrocarbon in the range of C15–C17. From Fig. 4, the highest amount of liquid hydrocarbon detected in the range of C11–C17, mixtures of the long and shorter hydrocarbon chain, indicates the presence of deoxygenation and catalytic cracking reaction. The experiment runs at 450 °C (Run 1, 3 and 7) had a shorter hydrocarbon chain (<C11), indicating catalytic cracking was favoured at higher temperatures as opposed to deoxygenation reaction. Shimada et al. [56] suggested that the after deoxygenation reaction where carbon was lost in a form of CO (decarbonylation) or CO2 (decarboxylation), intermediate hydrocarbon was formed and catalytic cracking of these intermediate products produced light hydrocarbons.The liquid hydrocarbon from the GC-MS analysis could be further classified into a range of alkenes C10–C19, alkanes C10–C22, cycloalkenes C10–C13, and cycloalkanes C11–C15. Liquid hydrocarbon obtained was in accordance with the petroleum fractions [57]; where a range of C5–C10 indicates the presence of gasoline, while C11–C12 is in the range of kerosene-like fractions, C13–C17 is in the range of light diesel-like fractions, and C18–C25 is in range of heavy diesel-like fractions. Fig. 4 strikingly shows that the diesel fractions were the highest in the various experimental runs conducted, followed by kerosene and gasoline. Run 6 at the reaction temperature of 400 °C, 90 min of reaction time, 20 cm3/min N2 flow and 1 wt% catalyst loading had the highest yield of 49.66% of green diesel fractions. This was followed by Run 5 and Run 3 with 33.93% and 39.98% respectively and the remainder of runs (1,7 and 8) falls within the ranges of 20%–26%.ANOVA was used in the Taguchi method to assess the result from catalytic deoxygenation and to determine the influence of every factor on the product yield [58]. ANOVA is just like the regression analysis, used to access the relationship among the response variables [59]. Fig. 5 shows an analysis of variance (ANOVA) results on the parameter contributions to produce biodiesel, tabulated from Table 3. The temperature was the most significant attribute with a per cent contribution of 86.62%, while the catalyst contributed the least, which influenced only 1.53%. As the experimental study did not compare with a blank experiment (i.e. without a catalyst), resulted in a less apparent influence of the catalyst on the diesel yield. Due to this, Taguchi's analysis of the influence of time and nitrogen flow rate had a greater impact than the catalyst tested. Table 6 illustrates the ANOVA analysis obtained from the Taguchi method, using Minitab 16 software. The effect of temperature on the diesel fraction yield was significant, the model showed that the F-value of 17.61 implies the model was significant and Prob > F less than 0.0500 indicates the model terms were significant. If Prob > F values are greater than 0.100, the system indicated the model as insignificant for the regression model.From Table 7 , the fit of the model can be confirmed by R-squared values. The Predicted R-Squared of 0.7279 was in reasonable agreement with the Adjusted R-Squared of 0.8925. This verifies that this model was statistically significant. Additionally, adequate precision estimates the signal-to-noise ratio, this helps in determining the validity of the model. It is desirable when the ratio is greater than 4. The ratio of 11.265 indicates an adequate signal. Therefore, it is convinced that the model was significant. Therefore, the above explanation of the ANOVA results deduced that this model could be used for deoxygenation and catalytic cracking reaction in this work, hence can be used to operate in the design space in terms of green diesel production [58]. Table 9 below shows the suggested parameter by Taguchi which predicts the diesel fraction yield of 44.78%. Experimental work according to the suggested conditions was carried out to validate the optimized suggested parameter. The targeted result obtained 38.96% of the diesel fraction. This showed a reasonable agreement between the experimental and predicted results under the conditions given to be optimized. Fig. 7 shows the selectivity comparison between the targeted result from Taguchi and Run 6 which shows the similar height of C14 peaks, the lower peak of C11 and C17, but a broader range of C8–C10 fraction. It is speculated that the gap between the experimental and predicted result may result from a more catalytic deoxygenation reaction favoured with the slightly altered parameter, which in this case, with lower N2 flowrate, from 20 cm3/min in Run 6–10 cm3/min of N2.As a comparison study, the effect of individual parameters is investigated while maintaining other process parameters constant at unspecified levels. Wako et al. [37] reported the optimum condition achieved with 83 wt% of pyrolysis oil obtained with 4 wt% ZrO2 as a catalyst, reaction temperature of 475 °C, 20 min of resident time, and 10 °C/min heating rate [37]. In the work by Wako et al. [37], the effect of N2 flow was not studied. It was proved in this study that a high temperature of reaction gave the highest yield of pyrolysis oil (>70 wt%). On the other hand, with a continuous feed system, Jungjaroenpanit and Vitidsant [35] reported optimum conditions at 430 °C, 154.20 mL/h flow rate of raw material (WCO), 102.73 cm3/min N2 flow rate, and 60 wt% catalyst loading resulted in the production of 57.07% diesel fraction. Only Chen et al. [39] and Ahmad et al. [59] reported optimization work for pyrolysis by the Taguchi method. With castor meal as the feedstock, Chen et al. [39] reported that the effective order of pyrolytic parameters in their work was nitrogen flow rate > heating rate > pyrolytic temperature > residence time. Whereas, Ahmad et al. [59] stated that all three parameters investigated gave a significant impact on the bio-gasoline yield in the following order; temperature > time > catalyst loading. As compared to this study, reaction temperatures gave a significant impact on the deoxygenation of WCO using SO4 2--Fe2O3/Al2O3 catalyst followed by reaction time > nitrogen flow > catalyst loading.Waste cooking oil could be catalytically deoxygenated and cracked using a solid acid catalyst (sulphated-ferric (II) oxide/alumina oxide catalyst) into bio-gasoline, bio-kerosene and green diesel fractions at different reaction conditions and optimized with L9 orthogonal array by Taguchi method. The temperature of 400 °C gave a better-quality product with a PO yield of 42.66% whereby the liquid hydrocarbon yield was as high as 78.45% and less amount of oxygenated compound of 21.55%. The high liquid hydrocarbon gave rise to high biodiesel fractions of 49.66% and gasoline 28.79% whereas a temperature of 450 °C awarded the highest yield of 75.86% with a high oxygenated compound of 63.89% and less liquid hydrocarbon yield of 36.11%. As no result was obtained at 350 °C, it is safe to deduce that the temperature was too low for the reaction to occur with the catalyst used. Optimization of the catalytic deoxygenation was determined using the Taguchi method with the optimum conditions, temperature 400 °C, catalyst loading 1 wt%, time 90 min and the N2 flow rate 20 cm3/min. It can be concluded that the catalytic deoxygenation process of WCO was achieved and the DoE and optimization of operating conditions are realistic with fewer numbers an experiment. Finally, the predicted optimized values and the actual yield of pyrolysis oil data obtained were in close agreement. Shafihi U: Conceptualization, Investigation, Methodology, Software, Formal analysis, Validation, Writing - original draft. R.S.R.M. Hafriz: Conceptualization, Investigation, Data curation, Methodology, Visualization, Writing – review & editing. N.A. Arifin: Conceptualization, Investigation, Validation, Writing – review & editing. Nor Shafizah I: Conceptualization, Investigation, Validation, Writing – review & editing. Idris A: Project administration, Resource. A. Salmiaton: Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Supervision, Validation, Writing – review & editing. N.M Razali: Project administration, Resource.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 the Fundamental Research Grant Scheme (FRGS/11/TK/UPM/02), the AAIBE Chair of Renewable Energy Grant No. 201801KETTHA and 202102 KETTHA for funding this research publication.

This work investigates the optimization of reaction parameters with the Taguchi method for catalytic deoxygenation of waste cooking oil (WCO) as an alternative renewable fuel process. Commercial sulphated-ferric (II) oxide/alumina oxide catalyst has the potential as a deoxygenation catalyst due to its good physicochemical properties which enhance the removal of oxygenated species. The obtained pyrolysis oil analysed by GC-MS revealed the selectivity of the pyrolysis oil mostly in the range of light diesel and kerosene fraction. From an analysis of variance (ANOVA), temperature awarded the most significant impact (86.62%) in this catalytic deoxygenation as compared to three other parameters followed by reaction time > N2 flow > catalyst loading. From the GC-MS analysis, the maximum renewable diesel fraction of 49.66% was obtained at 400 °C, 1 wt% of catalyst, 90 min of reaction time and 20 cm3/min N2 flow. The predicted model by Taguchi in the present study validated by the experimental work shows a promising application in optimising the catalytic pyrolysis process for future use.

Energy utilization has become a necessity for the normal functioning of human life, growth, and survival. The primary energy consumption across the globe is growing day by day due to the increasing energy usage in industries, buildings, and transportation sectors (Balajii and Niju, 2019). Presently, most of the energy comes from fossil fuel that is finite. The combustion of fossil fuels emits various greenhouse gasses like CO2, CO, SO2, NOx, etc. which have adverse effects on the environment and are the primary factors for global warming in the 21st century. To protect nature and human beings, sustainable and suitable alternative energy sources to fossil fuels are in utmost need. Utilization of bioethanol and biodiesel as alternative to fossil fuels has attracted global attention due to their renewability, carbon-neutral character, and good combustion efficiency (Dehkhoda et al., 2010; Ezebor et al., 2014). Chemically, biodiesel is known as the fatty acid methyl esters (FAME) commonly produced via catalytic esterification (Scheme 1) and transesterification (Scheme 2) reactions from biological sources like animal fats, non-edible oils, edible oils, and fatty acids (Brahma et al., 2022; Basumatary et al., 2022).Biodiesel is widely endorsed as an alternative to fossil fuels because of its better performance and eco-friendly nature. However, the main obstacle for biodiesel to be used as an alternative source of fossil fuels is due to its higher price characteristics compared to diesel (Knothe, 2002). Therefore, recent research aimed at reducing the production cost of biodiesel which is mainly caused by the high price of feedstocks (such as soybean oil, sunflower oil. etc.) used for biodiesel production and catalyst used for its production (Konwar et al., 2014a). In search of viable feedstocks of biodiesel, several oil sources including edible and non-edible oils for biodiesel synthesis have been reported by several researchers. Since the non-edible feedstocks are low valued, locally available throughout the year, and do not create the food vs. fuel impingement, they are consistently targeted as a viable feedstocks for the preparation of biodiesel.Catalyst can increase the rate of a reaction and plays a very important role in biodiesel synthesis in terms of efficiency and production cost. The catalysts which are applied in the synthesis of biodiesel are classified into three classes viz. homogeneous, enzyme and heterogeneous catalysts. Homogeneous catalysts are of two types, they are homogeneous acid and homogeneous base. Homogeneous acid catalysts are not commonly employed in the reaction for the synthesis of biodiesel due to the requirement of the high ratio of alcohol to oil, high pressure, and high reaction temperature, and also due to the problems linked to the reusability of catalyst (Canacki and Van Gerpen, 1999). Though the transesterification reaction time is shorter for homogeneous base-catalyzed reaction, it possesses difficulty due to soap formation during conversion producing a low yield of biodiesel and the catalyst cannot be separated easily (Basumatary et al., 2018). Because of higher costs and time-consuming reactions, enzyme catalysts are not feasible for broad application in biodiesel production (Sandoval et al., 2017; Kalita et al., 2022).Heterogeneous catalysts are also divided into two categories viz. solid acid and base catalysts. Due to the simpler and fast reaction process even at low temperatures, easy separation process and reusability for several cycles of reactions, heterogeneous base catalyst can reduce the overall production cost of biodiesel, and hence, it has been gaining importance in recent years (Kondamudi et al., 2011; Konwar et al., 2014a). Nowadays, researchers prefer heterogeneous catalysts prepared from renewable natural sources because of their renewability, catalytic efficiency, and eco-friendliness. Few researchers reported the biodiesel production using various biomass-based heterogeneous catalysts such as palm kernel fronds (Ameen et al., 2013), Musa paradisiacal peel (Betiku and Ajala, 2014), Musa balbisiana peels and underground stem (Gohain et al., 2017; Sarma et al., 2014; Aslam et al., 2014), banana peels, and cocoa pod husk (Odude et al., 2019), Musa acuminata peel (Pathak et al., 2018), coconut waste (Sulaiman and Ruslan, 2017), wood ash (Sharma et al., 2012), Lemna perpusilla (Chouhan and Sarma, 2013), rubber seed shell (Onoji et al., 2017), camphor tree (Li et al., 2018), rice husk ash (Chen et al., 2013; Zeng et al., 2014), Musa paradisiaca  (Basumatary et al., 2021a), sugarcane bagasse (Basumatary et al., 2021b), Heteropanax fragrans  (Basumatary et al., 2021c), Sesamum indicum plant (Nath et al., 2020), Brassica nigra plant (Nath et al., 2019), etc.Several researchers also reported various heterogeneous base catalysts which include oxides of metals such as CaO (Refaat, 2011; Verma and Sharma, 2016), MgO (Refaat, 2011; Vyas et al., 2010), SrO (Refaat, 2011), and transition metal oxides and derivatives such as ZrO2 (Zabeti et al., 2009; Mahdavi et al., 2015), TiO2, ZnO2, zeolite (Lee et al., 2009) and basic hydrotalcite (Silva et al., 2010). However, heterogeneous base catalysts also have some insufficiencies like leaching of catalyst and inappropriateness of the feedstocks due to high free fatty acids (FFAs) contents (Tangy et al., 2016; Wilson and Lee, 2012). Heterogeneous acid catalysts are developed for biodiesel synthesis from the feedstocks with FFAs contamination, and also for the simultaneous reactions of esterification and transesterification (Patel and Narkhede, 2012). Nonetheless, the heterogeneous acid catalyst has also some disadvantages like it requires higher reaction time and temperature in comparison to heterogeneous base catalysts. To resolve these problems and to develop a more efficient catalyst for biodiesel synthesis, researchers are now investing their time in the preparation of nanocatalysts due to their high surface areas, higher catalytic activity and longer stability. A nanocatalyst is a nanoparticle with size ranging between 1 and 100 nm. The nanocatalyst can be synthesized by different methods such as hydrothermal method, wet impregnation method, co-precipitation method, precipitation method, sol-gel method, combustion method, and self-polymerization based grafting technique. Due to the small size of the particle, the surface-to-volume ratio increases which leads to the accumulation of a higher number of atoms on the surface of the catalyst (Zambre et al., 2012). The present work is aimed to review the recent applications of the different types of nanocatalysts employed in biodiesel synthesis which include metal oxide nanocatalysts (CaO, ZnO, MgO, TiO2, CuO, and ZrO2), magnetic nanocatalyst, bifunctional nanocatalyst, nano-zeolite, nano-hydrotalcite, etc. The catalytic performances of these nanocatalysts are highlighted and discussed herein.Biodiesel is a renewable diesel fuel that contains short chain esters (methyl or ethyl) made by transesterification process in the presence of a suitable catalyst. Heterogeneous catalysts are widely used due to easy recovery of the catalysts by simple gravity filtration which has a great contribution in reducing the overall biodiesel production cost. The metal oxide catalyst gives a high yield of biodiesel and requires less time for the transesterification reaction (Vasić et al., 2020). The higher catalytic activity of metal oxide is attributed to its high surface property (Gawande et al., 2012). Besides alkaline earth, the metal oxides, transition metal oxides such as zinc oxide, iron oxide, tin oxide, and zirconium oxide are widely used as a catalyst.The use of CaO nanocatalyst deals with the effective, economic, and eco-friendly conversion of vegetable oil and animal fat into biodiesel. The catalyst is subjected to different calcination temperatures and their effects on complete transesterification of different oils at different concentrations are discussed. Many researchers found naturally available CaO from waste materials such as dolomite, snail shell, waste eggshell, waste mussel shell, chicken bone (Nakatani et al., 2009; Diaz and Borges, 2012; Niu et al., 2014; Moradi and Mohammadi, 2014). The performances of CaO nanocatalysts derived from several renewable sources in biodiesel synthesis from different feedstocks are represented in Table 1 . In this table, the catalyst preparation methods and conditions along with the particle size (nm) and surface area (m2 g−1) of the prepared nanocatalysts were mentioned. It was found that various parameters such as reaction time, temperature, and catalyst loading/amount for various feedstocks and various catalysts influenced the FAME yield. Reddy et al. (2006) investigated the catalytic activity of nanocrystalline CaO, a powder having a crystallite size of 20 nm with the specific surface area of 90 m2 g−1, in the reaction of poultry fat and soybean oil at room temperature.The nanocrystalline material CaO showed much superior activity than the laboratory-grade CaO and it was due to higher surface area of the nanocrystalline material. It was observed that when pure CaO is exposed to the atmosphere its surface basic sites get poisoned because it absorbed CO2 and H2O which were converted to CaCO3 and Ca(OH)2. To improve basic strength and to remove poisoned species, CaO was calcined using ammonium carbonate solution at higher temperatures (>750 °C) (Kesic et al., 2012), and above 850 °C, calcium carbonate decomposes into calcium oxide and carbon dioxide. Zik et al. (2020) investigated the coconut residue and chicken bone-derived catalyst of nano-crystal cellulose (NCC) and CaO for biodiesel synthesis from waste cooking oil (WCO). They prepared the catalyst by calcining the dried, crushed, and blended bone at 700 °C, 800 °C, and 900 °C for 4, 5, and 6 h, respectively. They carried out the transesterification reaction in a packed bed reactor in the presence of CaO/NCC/PVA (PVA–polyvinyl alcohol). They found the highest biodiesel yield of 98.4% under the optimum reaction conditions (ORCs) of 65 °C of temperature, 6:1 of methanol to oil molar ratio (MTOMR), and 0.5 wt.% of catalyst loading. This study also reported that the catalyst was reusable up to the 4th cycle with a product yield of above 90%. Krishnamurthy et al. (2020) demonstrated the synthesis of CaO nanocatalyst from a snail shell for biodiesel production from dairy scum and Hydnocarpus wightiana oil. They prepared the catalyst by hydrothermal method, in which the clean and dry snail shells were finely crushed using a blender, washed with nitric acid for 3–4 min, the snail shell was meshed and rinsed with distilled water and then dried and calcined at 900 °C for 4 h. The average crystallite size of nano CaO catalyst was found to be 40 nm. In the BET analysis, the surface area was found to be 9.37 m2 g−1, and the average pore diameter and pore volume were 2.29 nm and 0.0538 cm3 g−1, respectively. They found that the maximum biodiesel yield for scum oil and Hydnocarpus wightiana oil was 96.92% and 98.93% at the ORCs of MTOMR of 12.7:1 and 12.4:1, catalyst dosage of 0.866 wt% and 0.892 wt%, reaction temperatures of 58.56 °C and 61.6 °C, and the reaction time of 2 h and 2.42 h, respectively. They also found that the catalyst can be reused up to 5th catalytic cycle though the product yield decreased slightly and after the 5th cycle, the yield declined to a greater extent due to leaching which results in a decrease in surface area, pore volume, and total basicity of the catalyst. Kaur and Ali (2011) investigated lithium-ion impregnated calcium oxide as a nanocatalyst for biodiesel production from karanja and jatropha oils. They prepared the Li-CaO nano-catalyst by wet impregnation method in which CaO was suspended in deionized water, and an aqueous solution of LiNO3 was added, then stirred for 2 h and evaporated to dryness and heated at 120 °C for 24 h. The CaO impregnated with 1.75 wt% of lithium was used as a solid catalyst for the transesterification of karanja and jatropha oils which contains 3.4 and 8.3 wt% of free fatty acids, respectively. They found that the surface area, pore-volume, and pore size of the catalyst were 1.7 m2 g−1, 0.004 cm3 g−1, and 95.02 Å, respectively. The ORCs for the transesterification of karanja and jatropha oils were achieved in 1 and 2 h, respectively, MTOMR of 12:1, 5 wt% of catalyst, and a reaction temperature of 65 °C resulting in >99% conversion of oils to FAME. Zhao et al. (2013) reported the transesterification of canola oil catalyzed by nanopowder CaO. They reported that the nano-CaO catalyst with the surface area of 89.25 m2 g−1 displayed a faster chemical reaction and adsorption due to a larger surface area and stronger basicity. They found that the ORCs for production of 99.85% of biodiesel yield was obtained at 2 h when 3 wt% of the catalyst was used with 9:1 MTOMR at 65 °C. They also investigated the reusability and lifetime of the nano-CaO catalyst under ORCs for 15 catalytic cycles although a slight decrease in yield was observed after the 10th cycle. After the 15th cycle, the yield dropped by around 70%. This decreased in biodiesel production was due to the loss of catalyst during the recovery process. Borah et al. (2019a) synthesized the Zn substituted waste eggshell-derived CaO nanocatalyst for biodiesel production from WCO. They found that the maximum FAME conversion of 96.74% was obtained under the reaction conditions of 20:1 MTOMR, 5 wt% catalyst loading, 65 °C of reaction temperature, and 4 h of reaction time. In this experiment, they found that the catalyst was reusable for 5 consecutive cycles under ORCs. Seffati et al. (2019) reported biodiesel production from chicken fat using CaO/CuFe2O4 nanocatalyst. The results of their study showed that a maximum biodiesel yield of 94.52% was obtained at the MTOMR of 15:1, a reaction time of 4 h, a reaction temperature of 70 °C, and a catalyst amount of 3%. Bharti et al. (2019) reported biodiesel production from soybean oil by the use of CaO nano-catalyst. From the BET analysis, they found that the average surface area and the pore diameter of the CaO nanocatalyst were 67.781 m2 g−1 and 3.302 nm, respectively. The average particle size of the catalyst found from the TEM image was ranging from 5.68 to 8.33 nm. They reported that the maximum biodiesel yield of 97.61% was found at 3.675 wt% of catalyst loading, 11:1 MTOMR at 60 °C within 2 h of reaction time. Ahmad et al. (2020) demonstrated the synthesis of nano-CaO catalyst for biodiesel production from algal biomass (Chlorella pyrenoidosa). The catalyst was prepared by the use of calcination-hydration-dehydration method in which finely crushed powder of waste eggshells was calcined at 900 °C for 3 h in a muffle furnace. Characterization of catalyst showed that the average particle size was 23.65 nm, the surface area was 64.51 m2 g−1, and the average pore size was 9.28 nm. The catalyst at the operating conditions of 2.06 wt% of catalyst amount, 30:1 MTOMR, 60 °C of reaction temperature, and 180 min reaction time provided maximum biodiesel production of 93.44%. They also reported that the catalyst could be reused up to the 6th time and after that, the biodiesel production decreases rapidly due to the decrease in the number of active sites which was blocked by the byproduct. Erchamo et al. (2021) investigated biodiesel production from WCO using eggshell-derived CaO nanocatalyst. Since the use of ethanol in the transesterification reaction arises with various problems such as emulsification and difficulty in the separation process, they used a mixture of methanol-ethanol. Ethanol has better solvability than methanol and hence, it can easily mix the oil, alcohol, and catalyst, while methanol can minimize the emulsification effect of ethanol. After considering various optimization parameters like catalyst amount, mixed methanol-ethanol (8:4) to oil ratio, reaction time, and reaction temperature as 2.5 wt%, 12:1, 120 min, and 60 °C, respectively, a biodiesel yield of 92% was obtained. Gupta and Agarwal (2016) investigated biodiesel production from soybean oil using calcium nitrate (CaO/CaN) and snail shell (CaO/SS) derived CaO nanocatalyst. In the study, it was that CaO/SS catalyst was more basic than CaO/CaN that enhanced the catalytic activity. They reported that the maximum respective biodiesel yields of 96% and 93% were found at 8 wt% of catalyst loading, 12:1 MTOMR at 65 °C within 6 h of reaction time. They also reported that these catalysts were reusable up to the 5th transesterification cycle. Degirmenbasi et al. (2015) reported biodiesel production from canola oil using CaO nanocatalyst. In the study, CaO nanocatalyst was prepared using the incipient-wetness impregnation process in which CaO was taken in a flask, and a vacuum was applied. Then K2CO3 solution was added to the CaO nanoparticles and dried at 393 K. Finally, the impregnated CaO particles were calcined at 773 K for 3 h. From the BET analysis, they found that the surface area of the catalyst was in the range of 10.24–14.65 m2 g − 1. They reported that the pore size and particle size of the catalyst were between 2 and 300 nm and 20–160 nm, respectively. They found that the maximum biodiesel of 97.67% was obtained at the ORCs of 9:1 MTOMR, 3 wt% of catalyst amount, a reaction time of 8 h, and a reaction temperature of 65 °C. They also reported that the catalyst could be reused up to five successive times.The catalytic performances of MgO nanocatalysts in biodiesel synthesis from different feedstocks are shown in Table 2 . In this table, the methods of catalyst preparation along with the particle size (nm) and surface area (m2 g−1) of the prepared nanocatalysts were also mentioned. Amirthavalli and Warrier (2019) reported the production of biodiesel from WCO using MgO nanocatalyst. They prepared the nanocatalyst using the sol-gel method in which magnesium acetate tetrahydrate was dissolved in absolute ethanol under constant stirring. Oxalic acid was added to maintain the pH of the solution. The mixture was continuously stirred until they form a thick white gel. Then it was dried in an oven at 100 °C for 15 h. It was powdered using the mortar and calcined at 600 °C for 2 h. They carried out the transesterification in the presence of MgO nanocatalyst and found the highest yield of 80% biodiesel was produced at the ORCs of 60 °C, 10:1 MTOMR, and 2 wt% of catalyst loading. Vahid and Haghighi (2017) investigated biodiesel production from sunflower oil over MgO/MgAl2O4 nanocatalyst. They prepared the MgO/MgAl2O4 catalyst by the combustion method. The ORCs for the transesterification were achieved in 3 h, MTOMR of 12:1, and a reaction temperature of 110 °C, which resulted in 95.7% conversion of oils to biodiesel. They also found that the catalyst was reusable up to six successive rounds of reaction though the yield decreased slightly. After the 6th cycle, the yield declined to a greater extent due to poisoning of catalyst active sites by adsorption which also resulted in the decrease of surface area, pore volume, and basicity of the catalyst. Ashok et al. (2018) prepared a nanostructured MgO catalyst following the co-precipitation method and produced biodiesel from WCO. They found that the maximum biodiesel yield of 93.3% was achieved using 2 wt% of nanocatalyst, MTOMR of 24:1, reaction temperature of 65 °C, and reaction time 1 h. They also found that the catalyst was reusable at least 5 times with a decrease in activity which may be due to the change in the structural form of the MgO catalyst transforming to Mg(OH)2 (Boro et al., 2011). Feyzi et al. (2017) prepared MgO-La2O3 nanocatalyst and used it in the production of sunflower oil biodiesel. The maximum biodiesel yield of 97.7% was found using 3 wt.% of catalyst loading, 18:1 MTOMR at 65 °C within 5 h of reaction time. They also reported that the catalyst could be reused up to the 4th cycle without significant loss in catalytic activity. Rasouli and Esmaeili (2019) demonstrated the production of biodiesel from goat fat by the use of MgO nanocatalyst. They found that the specific surface area, total pore volume, average diameter, and volume of pores were 40.44 m2 g−1, 9.29 cm3 g−1, 36.69 nm, and 0.371 cm3 g−1, respectively. They reported that the catalyst was mesoporous because the average pore diameter was less than 50 nm. They found that the average particle size of the catalyst was 5.5 nm. Under the ORCs of 1 wt% of catalyst amount, 12:1 of MTOMR, 3 h of reaction time, and reaction temperature of 70 °C, the yield of biodiesel was 93.12%. Esmaeili et al. (2019) studied biodiesel production from Moringa oleifera oil by the use of MgO nanocatalyst. The different physicochemical properties of the catalyst were determined by using SEM, TEM, EDX, and BET techniques. The specific surface area and volume of pores of the catalyst were 14.19 m2 g−1 and 0.045 cm3 g−1, respectively. They reported that the maximum biodiesel yield of 93.69% was found at 1 wt.% of catalyst loading and 12:1 MTOMR at 45 °C within 4 h of reaction time. Rafati et al. (2019) demonstrated the synthesis of MgONaOH nanocatalyst for the production of biodiesel from WCO by electrolysis method. They prepared the catalyst by adding NaOH to the solution of magnesium nitrate hexahydrate and ammonia solution with constant stirring. The material was dried and calcined at 400 °C. They reported that the catalyst was spherical in shape and the average size of the particle ranged from 40 to 80 nm, but the actual particle size was determined by XRD analysis and was found to be 66.77 nm. They found that the ORCs for transesterification reaction were catalyst amount of 3 wt.%, MTOMR of 6:1, a reaction time of 6 h, reaction temperature of 50 °C, and the yield of biodiesel reached above 98%.Nanocatalyst plays a fateful role and resolves the problems associated with the transesterification reaction which leads to the reduction in the biodiesel production cost. Nanocatalysts have high selectivity and catalytic activity due to their non-dimensional pores present on the surface of the catalyst (Baskar and Aiswarya, 2016). The ZnO nanocatalysts are well known for their non-toxic and biodegradable property. Due to its hexagonal wurtzite structure, ZnO has high transparency and oxygen vacancy and possesses a higher affinity for the polar substrate (Dantas et al., 2020). Nowadays, researchers are concentrating on the doping of transition metals like Mn, Co, Cu, Ni, and Fe which have a variety of applications in the field of semiconductor devices, drug carrier molecules, and biodiesel production. The performances of ZnO nanocatalysts derived from several sources in biodiesel synthesis from different feedstocks are summarized in Table 3 . Baskar et al. (2018) studied biodiesel production from castor oil by the use of heterogeneous Ni-doped ZnO nanocatalyst. They prepared the catalyst with the help of the co-precipitation method in which Ni acetate solution was mixed with Zn acetate solution and stirred constantly and then ammonia solution was added to the mixture. Then NaOH was added to the mixture dropwise and then the mixture was filtered and dried at 80 °C for 3 h. and then calcined for 3 h. Ni-doped ZnO nano-composite showed an average particle size of 35.1 nm. They found that a maximum biodiesel yield of 95.20% was obtained under the reaction conditions of 55 °C of reaction temperature, 60 min of reaction time, 8:1 MTOMR, and 11 wt% of catalyst amount. They also found that the catalytic activity was maintained up to three cycles and the yield of biodiesel decreased slowly from 95.2% to 91.5% in the fourth cycle, and in the fifth cycle, it was found to be 85%. The decrease in biodiesel production was mainly due to the accumulation of organic matter on the surface of the catalyst. Raj et al. (2019) investigated biodiesel production from microalgae (Nannochloropsis oculata) oil using heterogeneous polyethylene glycol (PEG) encapsulated ZnOMn2+ nanocatalyst. They reported that the particle size of the catalyst ranged from 20 to 42 nm. They also reported that the maximum biodiesel yield of 87.5% was achieved using 3.5 wt% of catalyst loading, and 15:1 MTOMR at 60 °C within 4 h of reaction time. They found that the catalytic activity was maintained up to four cycles and reported that the yield of biodiesel decreased gradually to 85.8% in the fifth cycle and in the sixth cycle, it decreased to 73.5%. This decrease was mainly due to the loss of PEG capping on ZnO on the surface of the catalyst. Baskar and Aiswarya (2015) demonstrated the synthesis of Cu doped ZnO nanocomposite and used it as a heterogeneous catalyst for biodiesel production from WCO. They found that the average size of the nanocatalyst was 80 nm. After considering various optimization parameters, they reported 97.71% of the yield of biodiesel at catalyst amount of 12 wt%, MTOMR of 8:1, and reaction time of 50 min at 55 °C. They also found that the catalytic activity was maintained up to five cycles and after the 5th cycle, the biodiesel yield decreased by 10%. Baskar et al. (2016) reported the production of biodiesel production from mahua oil using Mn-doped ZnO nanocatalyst containing an average particle size of 24.18 nm. The catalyst was prepared by co-precipitation method followed by calcination at 600 °C for 2 h. They found a 97% yield of biodiesel under the reaction conditions of 50 °C of reaction temperature, 50 of min reaction time, 7:1 MTOMR, and 8 wt% of catalyst loading. They concluded that the catalyst could be reused up to the 5th cycle and after that, the biodiesel yield decreased sharply due to the deactivation of the active sites of the catalyst. Baskar and Soumiya (2016) studied the production of biodiesel from castor oil using Fe (II) doped ZnO nanocatalyst. They obtained a maximum yield of 91% biodiesel in 50 min at 55 °C with 14 wt% catalyst loading and 12:1 MTOMR ratio. They also reported that the catalyst was reusable up to the 4th reaction cycle and the major drop of the biodiesel yield was observed from the 4th cycle (87%) which was due to the deactivation of active sites. Thangaraj and Piraman (2016) demonstrated the production of biodiesel from Madhuca indica oil with the use of a heteropoly acid-coated ZnO nanocatalyst. The particle size of the catalyst was within the range of 5–29 nm. They reported that the ORCs for the maximum biodiesel yield of 95% was found at 0.6 wt.% of catalyst loading, 6:1 MTOMR at 55 °C within 5 h of reaction time. Gurunathan and Ravi (2015) investigated Cu doped ZnO as a catalyst in the production of neem oil biodiesel. Under the ORCs of catalyst amount of 10 wt%, MTOMR of 10:1, and a reaction time of 60 min at 55 °C, the yield of biodiesel could reach above 97.18%. They also reported that the catalyst was reusable up to six consecutive cycles beyond which yield decreased sharply due to the deposition of organic materials on the surface of the catalyst. Nagaraju et al. (2017) utilized Ag-doped ZnO material as a nanocatalyst in biodiesel synthesis from simarouba oil. This catalyst could produce a maximum yield of 84.5% biodiesel under the ORCs of 64 °C of temperature, 9:1 of MTOMR, and 2 h of reaction time with 1.5 wt% catalyst loading. A magnetic ZnO/BiFeO3 nanocatalyst was used in the synthesis of canola oil biodiesel by Salimi and Hosseini (2019). They reported that the average crystallite size and the particle size of the catalyst were 31.27 nm and 20–60 nm, respectively. The investigation yielded 95.43% of biodiesel at the best ORCs of 65 °C, 6 h of reaction time, 4 wt% catalyst loading, and 15:1 MTOMR. They also reported that the catalyst after the 5th cycle could yield 92.08% of biodiesel and this decrease might be due to a decrease in the number of basic sites on the catalyst. Borah et al., 2019b studied the synthesis of Co-doped ZnO and used it as a nanocatalyst for the reaction of Mesua ferrea oil. They reported that a maximum yield of 98.03% biodiesel was obtained under the ORCs of 2.5 wt% of catalyst loading, 3 h of reaction time, 9:1 MTOMR at 60 °C. They also reported that at the end of the 4th cycle, the biodiesel yield decreased to 43.13%.It has earlier been mentioned that a heterogeneous catalyst is preferred over homogeneous catalyst for biodiesel synthesis. Heterogeneous solid base catalysts are widely applicable for biodiesel production due to their high catalytic activity, low-temperature requirement, easy separation, and reusability, which could potentially reduce the biodiesel production cost. However, it has some disadvantages too such as catalyst leaching (Wilson and Lee, 2012), unsuitability for feedstocks containing high FFAs (Borges and Díaz, 2012), and dissolution of catalyst in the reaction medium (Mbaraka et al., 2006), and some catalysts have low activity and low porosity with low surface area (Taufiq-Yapa et al., 2011). Furthermore, solid base catalysts are widely reported for the conversion of edible oils to biodiesel, which tends to create a competition between food and fuel (Lin et al., 2011; Qiu et al., 2011). Due to strong acid sites, high activity and sensitivity, and low-cost properties, solid acid catalysts were also studied for biodiesel production (Sharma and Singh, 2011). Many researchers reported the studies of biodiesel production using solid acid catalysts such as heteropolyacid impregnated on different supports (silica, alumina, zirconia, and activated carbon), WO3–ZrO2, SO4–ZrO2 (Kulkarni et al., 2006; Laosiripojana et al., 2010), etc. The synthesis of biodiesel from different feedstocks using ZrO2 nanocatalysts is mentioned in Table 4 . Takase et al. (2014) studied biodiesel synthesis from Silybum marianum oil using ZrO2 modified with KOH as a nanocatalyst. The surface area and pore volume of pure ZrO2 were found as 7.02 m2 g−1 and 0.01 cm3 g−1, whereas the nanocatalyst showed 3.05 m2 g−1 and 0.01 cm3 g−1, respectively. A biodiesel yield of 90.8% was obtained at ORCs of 6 wt% of catalyst amount, 15:1 of MTOMR, and 2 h reaction time at 60 °C. They reported that the catalyst could be reused up to five times after washing with methanol and re-calcination at 530 °C and after the 5th cycle, the biodiesel yield decreased to 82.4%. Qiu et al. (2011) investigated the reaction of soybean oil using ZrO2 coupled C4H4O6HK heterogeneous solid base nanocatalyst. The catalyst preparation was done by the incipient wetness impregnation method. They reported that when the reaction was carried out with MTOMR of 16:1, a reaction temperature of 60 °C, a reaction time of 2 h, and a catalyst amount of 6 wt%, the highest biodiesel yield reached 98.03%. They also reported the catalyst was reused up to the 5th cycle of reaction and in the 5th cycle, the biodiesel yield decreased from 98.03% to 89.65% which was basically due to the leaching of metal. Mahdavi et al. (2015) utilized oleic acid as feedstock for biodiesel production using ZrO2/Al2O3 as the catalyst. They found the particle size and surface area of the catalyst in the range of 20.59–29.86 nm and 253–283 m2 g−1, respectively. The nanocatalyst at the ORCs of 1 wt% of catalyst amount, 8:1 of MTOMR, 67 °C of reaction temperature, and 2 h reaction time could provide 90.47% of biodiesel. They also investigated the reusability of the catalyst and found it reusable up to the 4th cycle. Booramurthy et al. (2021) studied the transesterification of animal fat using ferric-manganese doped sulfated zirconia (Fe-Mn-SO4/ZrO2) as the catalyst. It was reported that using an optimized catalyst amount of 6 wt% and alcohol to oil molar ratio of 12:1, biodiesel yield was found to be 96.6% at the reaction temperature of 65 °C in 5 h. They recycled and reused the catalyst several times and easily activated the catalyst by washing it with hexane and methanol followed by being dried at 120 °C for 8 h. They reported that the catalyst could be reused up to the 5th cycle and after that, the biodiesel yield decreased because of the loss of acid sites from the catalyst surface due to the weak bonding strength and aggregation of the catalyst particles. Saravanan et al. (2016) demonstrated the application of sulfated zirconia as the nanocatalyst in the reaction of palmitic acid. They reported that the ORCs for the production of 90% of biodiesel from palmitic acid were found as 6 wt.% of catalyst loading, 20:1 of MTOMR, 60 °C of reaction temperature, and 5–7 h of reaction time. They also reported that the catalyst was reusable up to five times and at the end of the 5th cycle, the yield decreased to 59%. Faria et al. (2009) studied the synthesis of SiO2/ZrO2 nanocatalyst and utilized it in biodiesel synthesis from soybean oil. They found that the surface area of the catalyst was 135 m2 g−1 and particle diameter was 200 nm. They achieved 96.2% of biodiesel under the ORCs of 0.5 g of catalyst amount, and 10:1.5 of MTOMR at 50 °C in 3 h. The catalyst was reused up to six times. Helmiyati et al. (2021) reported the synthesis of biodiesel from lauric acid using cellulose@hematite-zirconia as the catalyst. For the preparation of nanocatalyst, they isolated the cellulose from rice straw and converted it into nanocellulose with the help of H2SO4 and then filtered out the precipitate followed by washing it with water and dried. The nano-α-Fe2O3 was prepared by mixing the solutions of FeCl2⋅4H2O and FeCl3⋅6H2O and then NH4OH was added to the above mixture with constant stirring. The precipitate was filtered and washed with water and ethanol, and calcined at 600 °C for 1 h. For the preparation of nano-ZrO2, NaOH solution was added to ZrOCl2⋅8H2O solution with constant stirring. Then the white precipitate was filtered out and wash with water and acetone, and calcined at 700 °C for 1 h. α-Fe2O3-ZrO2 composites were prepared by adding α-Fe2O3 into a solution containing H2O, ethanol, and ammonia. Then ZrOCl2⋅8H2O was added to the above mixture with constant stirring. The precipitate was separated followed by washing with ethanol and water and dried at 60 °C for 12 h. Then nanocellulose was mixed with an aqueous solution of NaOH and urea and then mixed with α-Fe2O3-ZrO2 in aqueous NaOH with constant stirring. The product was then separated followed by washing with ethanol and water and dried. The nanocatalyst contained a surface area of 852 m2 g−1, pore volume of 0.85 cm3 g−1, the pore size of 13 nm, and average particle size of 42.5 nm. At the ORCs of 2 wt% catalyst amount, and 12:1 MTOMR at 60 °C in 3 h, the biodiesel yield of 92.50% was obtained. They found that the catalyst was reusable and at the end of the fifth cycle, biodiesel yield decreased to 80% which was mainly due to the deactivation of catalyst by absorbing unreacted lauric acid and by-product species.Metal oxide-based solid catalysts are being conventionally used in biodiesel production. Due to the large surface area, acid-base properties, strong metal-support interactions, and chemical stability, titanium dioxide (TiO2) nanoparticles are widely used in the transesterification reaction for biodiesel production (Carlucci et al., 2019; Li and Wang, 2012). Nowadays, binary metal oxides, for example, TiO2–ZnO nano mixed metal oxide, etc. are receiving interest due to their high surface acidity for biodiesel production (Li and Wang, 2012; Gurusamya et al., 2019). The performances of TiO2 nanocatalysts in biodiesel synthesis from different feedstocks are summarized in Table 5 . Gurusamya et al. (2019) reported the synthesis of biodiesel from Ulva lactuca seaweed using TiO2-ZnO nanocomposite catalyst. They found 82.8% yield of biodiesel at the ORCs of MTOMR of 6:1, catalyst dosage of 4 wt%, reaction temperature of 60 °C, and reaction time of 4 h. They also reported that the catalyst was reusable up to the 5th cycle. Madhuvilakku and Piraman (2013) prepared TiO2–ZnO mixed oxide nanocatalyst for synthesis of palm oil biodiesel. They also compared the TiO2–ZnO nanocatalyst with the ZnO nanocatalyst. They reported that TiO2–ZnO mixed oxide catalyst showed 92.2% yield in 5 h, whereas ZnO nanocatalyst showed only 83.2% of biodiesel yield in 5 h under the ORCs of 6:1 MTOMR at 60 °C with 200 mg of nanocatalyst. Zulfiqar et al. (2021) prepared lipase-PDA-TiO2 (PDA–polydopamine) nanoparticles using the hydrothermal method and self-polymerization-based grafting technique and utilized as the nanocatalyst for the synthesis of jatropha oil biodiesel. The ORCs for the transesterification to obtain the maximum yield of 92% biodiesel were 10 wt.% of catalyst loading, 6:1 MTOMR at 37 °C in 30 h of reaction time. They also reported that the catalyst could be reused up to the 4th reaction cycle with the decrease in the catalytic activity. Chen et al. (2018) demonstrated the synthesis of biodiesel from Jatropha curcas oil with the help of a nano-sized SO4 2−/TiO2 catalyst. 85.3% yield of biodiesel was reported at the ORCs of 4 wt% of catalyst amount, 9:1 of MTOMR, 24 h of reaction time, and 140 °C of reaction temperature. They also reported that the catalyst was reusable up to the 3rd cycle and at the 3rd cycle, the biodiesel yield decreased to 25.3%. This decrease was due to the aggregation of cokes on the surface of the catalyst which leads to the decrease in the catalytic activity. A nanocatalyst, Ti(SO4)O, was utilized by Gardy et al. (2016) for the preparation of biodiesel from WCO. They reported that the surface area, mean pore size, and total pore volume of the nanocatalyst were 44.4563 m2 g−1, 22.7347 nm, and 0.312459 cm3 g−1, respectively, and the average particle size of the catalyst was 45 nm. They reported that the ORCs for production of 97.1% of biodiesel were 1.5 wt% of catalyst amount, 9:1 of MTOMR, 3 h of reaction time, and 75 °C of reaction temperature. The nanocatalyst was reused up to the 8th cycle and the biodiesel yield decreased to 85.91%. Biodiesel was synthesized from WCO by Gardy et al. (2017) using TiO2/PrSO3H as the catalyst. TEM analysis showed that the average particle size of the catalyst was 23.1 nm. The BET analysis revealed a surface area of 38.59 m2 g−1, pore volume of 0.192 cm3 g−1, and mean pore size of 24.55 nm. The ORCs of MTOMR of 15:1, 4.5% of catalyst amount, 60 °C of reaction temperature, and reaction time of 9 h resulted in 98.3% of biodiesel yield. The reusability of nanocatalyst was investigated and at the 4th cycle, the biodiesel yield was 94.16% and after the 6th cycle, it was 20.64%. The decrease in yield was due to the blockage of the active site of the catalyst by the organic or carbonaceous material. Mihankhah et al. (2018) also studied biodiesel synthesis from waste olive oil with the help of a TiO2 nanocatalyst. They reported that the surface area and average particle size of the catalyst were 238 m2 g−1 and ∼30 nm, respectively. They also reported that conversion of 91.2% was obtained at an ORCs of 30:1 of MTOMR, 200 mg of catalyst amount, 120 °C of reaction temperature, and 4 h of reaction time. The catalyst at the 3rd cycle of reaction yielded 88% of biodiesel.The utilizations of different CuO nanocatalysts in the synthesis of biodiesel and their results reviewed are shown in Table 6 . Santha et al. (2021) investigated biodiesel synthesis from WCO using CuO nanoparticles as the heterogeneous catalyst. They found that the CuO nanocatalyst at the operating conditions of 2 wt% catalyst amount, 4:1 MTOMR at 60 °C reaction temperature, and 2.5 h reaction time provided a maximum yield of 88.64% biodiesel. Varghese and Prabu (2017) reported the synthesis of a needle-shaped CuO nanomaterial for biodiesel production. They reported that the biodiesel yield of 86.56% was found at relatively low catalyst loading (0.75 wt%), 3.5:1 MTOMR within 2 h of reaction time. Varghese et al. (2017) demonstrated the synthesis of Mg-CuO heterogeneous nanocatalyst for the synthesis of sunflower oil biodiesel. They reported that a biodiesel yield of only 71.78% was obtained at the reaction conditions of 0.25 wt% catalyst amount, 6:1 MTOMR, and 30 min of reaction time at 60 °C. Suresh et al. (2021) studied the preparation of biodiesel from pig tallow using CuO nanocatalyst. The CuO nanocatalyst was synthesized using C. tamala leaves. They reported that the average particle size of the nanocatalyst was 19.01 nm. The CuO catalyzed biodiesel preparation yielded 97.82% of the product under the ORCs of 60 °C of temperature, 29.87:1 MTOMR, and 2.07 wt% of catalyst loading.Commercialization and application of biodiesel to replace fossil fuels are hindered by the outrageous cost of production which is mainly due to the cost of raw materials. The cost can be reduced up to 77% of the total cost by the use of non-edible vegetable oil feedstocks (Skarlis et al., 2012). In biodiesel production, the use of acid or alkaline homogeneous catalysts is linked to some kind of problem which hinders commercial production (Seffati et al., 2019). Nowadays, nanocatalysts are receiving more attention for the synthesis of biodiesel due to their high recovery factor, large surface area, high energy consumption recovery, and requirement of low reaction temperature (Shahid and Jamal, 2011). Magnetic nanoparticles are the most popular materials due to their high surface to volume ratio, lower mass transfer resistance for reacting with substrates, and easy way of separation from the reaction mixture by an external magnetic field, and hence reducing the loss of catalyst and increasing the reusability (Verma et al., 2015; Rajkumari et al., 2017). This makes the catalyst more profitable for industrial applications. The catalytic performances of different magnetic nanocatalysts in biodiesel synthesis are represented in Table 7 . Changmai et al. (2021) utilized Citrus sinensis peel ash (CSPA) coated magnetic material (CSPA@Fe3O4) as the nanocatalyst for biodiesel synthesis from WCO. They prepared the catalyst by burning the dried orange (C. sinensis) peel in the air for 30 min to form ash and mixed with water and stirred for 2 h at 80 °C to extract the basic components present in CSPA. Fe3O4 nanoparticles were synthesized via the traditional co-precipitation method. Then CSPA extract was added dropwise to the mixture, stirred, and allowed to settle followed by decantation of the solution and collected the solid part, which was then washed with deionized water. At last, the catalyst was collected by evaporation of the solid portion. The chemical composition of the catalyst was investigated by X-ray photoelectron spectroscopic technique (Fig. 1 ), and the major basic elements obtained were K (8.64%) and Ca (4.46%). The prepared catalyst was also characterized using SEM, XRD, FT-IR, BET and TEM (Fig. 2 ). They found that the average particle size of the nanoparticle was ̴ 12–13 nm, the surface area was 15.55 m2 g − 1, and the pore diameter was found to be 2.45 nm. The CSPA@Fe3O4 material catalyzed transesterification produced a maximum yield of 98% biodiesel under the ORCs of 65 °C, 6:1 and 6 wt.% of temperature, MTOMR, and catalyst loading, respectively. They also reported that the catalyst was reusable for up to 9 consecutive cycles. Dantas et al. (2020) demonstrated the synthesis of Ni0.5Zn0.5Fe2O4 magnetic material and utilized it as nanocatalyst for the production of biodiesel. They found that the average particle size of the Ni0.5Zn0.5Fe2O4 catalyst was 31.1–42.6 nm. BET surface area of magnetic nano-catalyst was found to be 50.94 m2 g −1, and the average pore diameter and pore volume were 48.042 Å and 0.171 cm3 g−1, respectively. In their experiment, they found that the ORCs for yielding 99.54% of soybean oil biodiesel was 12:1 MTOMR, 2 wt% of catalyst loading, 180 °C of reaction temperature, and 1 h of reaction time. They recovered the catalyst with the help of an external magnet and reused it for up to 3 cycles. Ali et al. (2017) reported the synthesis of biodiesel from date palm oil using magnetic nanocatalyst, CaO-Fe3O4. The catalyst was prepared by the chemical precipitation method. They found that a biodiesel yield of 69.7% under the conditions of 65 °C reaction temperature, 300 min reaction time, 20:1 MTOMR, and 10 wt.% of catalyst amount. Feyzi and Norouzi (2016) prepared magnetic nanocatalyst, Ca/Fe3O4@SiO2 following sol-gel and impregnation methods, and utilized it in biodiesel synthesis. The surface area of the nanocatalyst was found to be 189.2 m2 g−1, and the average pore diameter and pore volume were 2.4 Å and 0.238 cm3 g−1, respectively. The nanocatalyst at the operating conditions of 8 wt% of catalyst amount, 15:1 MTOMR, 65 °C of reaction temperature, and 5 h of reaction time yielded 97% biodiesel. The catalyst could be recovered simply by using an external magnetic field and reused several times without appreciable loss of its catalytic activity. Ambat et al. (2019) prepared nano-magnetic K impregnated ceria and demonstrated it in biodiesel synthesis. The BET surface area, pore volume, and pore size were found to be 72.84 m2 g−1, 0.18 cm3 g−1, and 9.99 nm, respectively. A biodiesel yield of 96.13% could be achieved under the reaction conditions of 4.5 wt% catalyst amount, 7:1 MTOMR, and 120 min of reaction time at 65 °C. They observed that the catalyst was stable up to five cycles without considerable loss of activity. Liu et al. (2017) reported biodiesel synthesis from soybean oil using a nano-magnetic solid catalyst (K/ZrO2/γ-Fe2O3). They found that the yield of biodiesel was above 93.6% at ORCs of 5 wt% of catalyst amount, and MTOMR of 10:1 at 65 °C in 3 h. They also found that the catalytic activity was maintained up to six cycles and after that, the yield of biodiesel decreased due to the loss of nano-powder and alkaline sites in the recycling process of the catalyst. Hazmi et al. (2021) reported the preparation of bifunctional magnetic nano-catalyst (RHC/K2O-20%/Ni-5%) from rice husk char (RHC) for the production of biodiesel. It was revealed that the surface area, pore diameter, and pore volume of the catalyst were 32.40 m2 g−1, 5.8355 nm, and 0.0966 cm3 g−1, respectively. They reported 98.2% of biodiesel at 4 wt.% of catalyst loading and 12:1 MTOMR at 65 °C within 2 h of reaction time. They also reported that the catalyst could be reused up to the 5th reaction cycle and it was noticed that the major drop of the biodiesel yield was observed from the 4th (81.8%), 5th (71.0%), and 6th (45.9%) cycles. Hazmi et al. (2020) also studied the synthesis of nano-bifunctional super magnetic material (RHC/K2O/Fe) from rice husk and application as heterogeneous nanocatalyst for biodiesel synthesis from WCO. The surface area, average pore diameter, and total pore volume of the catalyst were 57.89 m2 g−1, 4.70 nm, and 0.0588 cm3 g−1, respectively. They achieved 98.6% of biodiesel under the ORCs of 4 wt% of catalyst amount, and 12:1 of MTOMR in 4 h at 75 °C. They also reported that the catalytic performance of the nano-catalyst was maintained for five consecutive cycles.On investigating a promising alternative to the homogeneous catalyst, some chemically modified natural materials and rocks were assessed for the synthesis of low-cost, abundantly available, and ecofriendly heterogeneous catalysts for the transesterification reaction (Rabie et al., 2019; Abukhadra and Mostafa, 2019). Zeolite has wide applications in biodiesel synthesis due to its microporous structure, high surface area, high stability, high mechanical strength, and high cation exchange capacity (Liu et al., 2018; You et al., 2017). By controlled functionalization of the surfaces of natural and synthetic zeolites by an acidic or basic group, biodiesel yield can be enhanced (Manique et al., 2017; Du et al., 2018). Several studies demonstrated that the activation of natural zeolite by alkali metal ions can enhance the catalytic property due to an increase in their basicity, which is a vital factor for the transesterification process (Ballotin et al., 2016; Abukhadra and Sayed, 2018). Faujasite zeolite (NaX) was reported to be beneficial for biodiesel synthesis due to its high surface area and a huge amount of basic sites, which is attributed to its aluminum content (Davis, 2003). The catalytic performance of nano-zeolite catalysts in biodiesel synthesis is shown in Table 8 . Dehghani and Haghighi (2019) reported biodiesel production from WCO by the use of cerium-doped MCM-41 as a catalyst. They prepared the catalyst by hydrothermal method in which cetyl trimethyl ammonium bromide, tetraethyl orthosilicate, and cerium nitrate were mixed in distilled water. To this mixture, NaOH solution was added with continuous stirring. The suspension was put into an autoclave, dried and the sample was filtered, washed, and dried at 110 °C. The sample was irradiated with the solution of magnesium nitrate and support solution and sonicated. The mixture was filtered and dried, and finally, the power was calcined at 600 °C for 3 h. The particle size of the catalyst was about 17.3 nm. They observed that the surface area of the catalyst was 1200 m2 g−1. They reported 94.3% of biodiesel yield using 5 wt% of catalyst loading, 9:1 MTOMR at 70 °C within 6 h of reaction time. The catalyst could be reused up to 7 times and after the end of the 7th cycle, the biodiesel yield was found to be 88.7%. Alkali trapped zeolite composite was prepared by AbuKhadra et al. (2020) and utilized as the basic catalyst for biodiesel preparation from WCO. They prepared four types of alkali-modified clinoptilolite (K, Na, Ca, and Mg) which were extracted from green tea. The SEM images of the studied samples (Fig. 3 ) showed different morphological characteristics, and varied elemental compositions were revealed from the EDX investigations (Fig. 4 ). They found that the average pore size of the clinoptilolite, K/clinoptilolite, Na/clinoptilolite, Ca/clinoptilolite and Mg/clinoptilolite nanoparticles were 18.3 nm, 17.6 nm, 17.3 nm, 15.4 nm, 19.6 nm, and the surface areas were 258 m2 g−1, 263 m2 g−1, 312.7 m2 g−1, 252.4 m2 g−1, and 342.5 m2 g−1, respectively. The biodiesel yields achieved with the modified catalyst were 93.6%, 95.2%, 96.4%, and 98.7% for K/clinoptilolite, Na/clinoptilolite, Ca/clinoptilolite, and Mg/clinoptilolite, respectively under the ORCs of 70 °C of temperature, 16:1 MTOMR, and 4 wt.% of catalyst loading. The reaction time taken was 120 min, 120 min, 180 min, and 150 min for K/clinoptilolite, Na/clinoptilolite, Ca/clinoptilolite, and Mg/clinoptilolite catalysts, respectively. They also reported that the reusability of catalyst for up to five times and after the 5th cycle of reaction, the catalytic activity decreased due to the coating of the byproducts on the active's sites of the catalysts. Luz Martinez et al. (2011) demonstrated the preparation of CaO nanoparticles/NaX zeolite for the transesterification of sunflower oil. They reported a 93.5% yield of biodiesel using this catalyst under the ORCs of 16 wt% of catalyst amount and 6:1 MTOMR at 60 °C in 6 h. Saeedi et al. (2016) prepared KNa/ZIF-8 (Zeolite imidazolate framework, ZIF-8 doped with K) material following sol-gel method and investigated as a catalyst for biodiesel production from soybean oil. They reported that the surface area, pore volume and pore diameter of the catalyst was 1195 m2 g−1, 0.527 cm3 g−1, and 1.21 nm, respectively. A biodiesel yield of 98% was obtained under the ORCs of 0.0125 wt% of catalyst loading and 10:1 MTOMR at 100 °C within 3.5 h of reaction time. Dehghani and Haghighi (2020) studied the preparation of sono-enhanced CaO-dispersed over Zr-doped MCM-41 nanocatalyst for the synthesis of WCO biodiesel. From the BET analysis, it was revealed that the surface area and the pore size of the nanocatalyst were 350 m2 g−1 and 5 nm, respectively. From FESEM analyses (Fig. 5 ) and size distribution histogram (Fig. 6 ), they found that the average particle size of the nanocatalyst was 15.9 nm for Ca/ZM-U (Si/Zr = 10) sample, and the size was found in the range of 20–80 nm in the case of non-sonicated sample Ca/ZM-I (Si/Zr = 10). The EDX analyses of the studied nanocatalysts are displayed in Fig. 7 . They reported a yield of 88.5% biodiesel under ORCs of 5 wt% of catalyst amount, 9:1 of MTOMR, and reaction time of 6 h at 70 °C. They also reported that the catalyst was reused up to the fifth reaction cycle and after that, the catalytic activity decreased which was mainly due to the blockage of the active site on the catalyst.Hydrotalcites are anionic clays that can be prepared by coprecipitation method and they have a common notation of [M2+ 1-XM3+(OH)2] X +[A n −]X/N.yH2O, where M2+ and M3+ are representing divalent and trivalent metals, and An−(CO3−, SO4 2−, Cl−, NO3−) is an n- valent anion (Helwani et al., 2009; Endalew et al., 2011). These are considered heterogeneous base catalysts and their basic strength depend on the ratio of Mg/Al. Nano-hydrotalcites are used as catalysts in biodiesel synthesis and their catalytic performances are shown in Table 9 . Deng et al. (2011) investigated the preparation of biodiesel from jatropha oil with the help of nanocatalyst derived from hydrotalcite with Mg/Al following the coprecipitation method. The physical-chemical properties such as Mg/Al molar ratio, surface area, pore volume, and pore diameter for the catalyst were found as 2.78, 218 m2 g−1, 0.17 cm3 g−1, and 3.9 nm, respectively. They reported that 95.2% yield of biodiesel was found at 1 wt% of catalyst loading and 4:1 MTOMR at 45 °C within 1.5 h of reaction time. They also reported that the catalyst was reusable up to 8 times after removing the glycerol. They found that the catalyst could yield 89.1% of biodiesel at the 8th cycle and the 9th cycle, the yield decreased to 43.7%, and this was due to the blocking of active sites of the catalyst by the glycerol. Dias et al. (2012) demonstrated soybean oil biodiesel synthesis using Ce modified Mg-Al hydrotalcite. They found that 90.2% biodiesel yield could be achieved under the ORCs of 5 wt% of catalyst loading and 9:1 MTOMR at 67 °C within 4 h of reaction time. Nano-hydrotalcite (Mg-Al) was prepared by Obadiah et al. (2012) and applied as the catalyst for biodiesel synthesis from pongamia oil. Under the ORCs of 5 wt% of catalyst and 6:1 of MTOMR at the reaction temperature of 65 °C in 4 h, the yield of biodiesel could reach above 90.8%. Gao et al. (2010) prepared biodiesel from palm oil using KF/Ca-Mg-Al hydrotalcite base catalyst. High biodiesel of 99.6% could be achieved at ORCs of 12:1 MTOMR, 5 wt% of catalyst, a reaction time of 5 min, and a reaction temperature of 65 °C. They reported a decrease in the biodiesel yield during reusability of catalyst and it was due to the absorption of by-products on the surface of the catalyst. Chelladurai and Rajamanickam (2014) demonstrated neem oil biodiesel synthesis using a nano-Zn-Mg-Al hydrotalcite catalyst. This catalyst yielded 92.5% of biodiesel at 7.5 g of catalyst loading, and 10:1 MTOMR at 65 °C within 4 h of reaction time.The catalytic performances of some other nano-catalysts used in the synthesis of biodiesel are presented in Table 10 . Abdullah et al. (2022) demonstrated the synthesis of biodiesel from WCO using activated carbon as a catalyst prepared from empty fruit bunch. They prepared the nano-catalyst following the hydrothermal technique wherein carbonization was performed at 600 °C for 3 h. They reported that the BET surface area, pore volume, and pore diameter were 4056.17 m2 g−1, 0.827 cm3 g−1, and 5.42 nm, respectively. The particle size of the catalyst was 58 nm. They found a high yield of 97.1% biodiesel at ORCs of 12:1 MTOMR, 5 wt.% catalyst amount, a reaction time of 2 h, and a reaction temperature of 70 °C. They also concluded that the nano-catalyst was reusable up to 5th reaction cycles. In the 5th cycle, the yield was 85% and at the end of the 6th cycle, the biodiesel yield decreased to merely 61.7%, which was due to the decrease in the number of activated components during the calcination process. Abdullah et al. (2020) demonstrated the synthesis of bifunctional nanocatalyst from waste palm kernel shell for the preparation of WCO biodiesel. The surface area, pore volume, and pore diameter of the catalyst were 438.08 m2 g−1, 0.3674 mm3 g−1, and 3.8 nm, respectively. Under ORCs of 5 wt% of catalyst amount, 12:1 of MTOMR, 4 h of reaction time, and reaction temperature of 80 °C, a 95% biodiesel yield could be found. Abdullah et al. (2021) also studied the synthesis of bifunctional nanocatalyst from palm kernel shell by carbonization technique and applied the material in biodiesel production from WCO. In this study, the surface area, pore volume, and pore diameter of the nanocatalyst were found to be 3368.60 m2 g−1, 2.36 mm3 g−1, and 5.17 nm, respectively. The FESEM images (Fig. 8 ) revealed the successful impregnation of K2CO3 and CuO active components showing the formation of irregular shaped nanomaterials. The EDX mapping of the impregnated nanomaterial is shown in Fig. 9 . They reported that the yield of biodiesel could reach about 95.36% under ORCs 4 wt% of catalyst amount and MTOMR of 12:1 within 2 h at 70 °C. They also reported that the catalyst could be reused up to the 5th reaction cycle. At the end of the 6th cycle, yield decreased to 57.5% which was due to poisoning by the by-products such as unreacted oil and glycerol. Kuniyil et al. (2021) reported the application of ZnCuO/N-doped graphene (NDG) as a catalyst for biodiesel synthesis from WCO. The structure of the prepared nanocomposite catalyst was studied by HRTEM technique (Fig. 10 ), and this revealed the formation of nanoparticles (ZnCuO) deposited on N-doped graphene sheets. The size of the particles was found in the range of 12–18 nm. They found 97.1% of biodiesel at ORCs of 15:1 MTOMR, 10 wt% of catalyst amount, a reaction time of 8 h, and a reaction temperature of 180 °C. They also reported that the catalyst was successfully reused up to six cycles. Ibrahim et al. (2022) recently prepared magnetic bifunctional nanocatalysts from fruit bunch and employed in WCO biodiesel synthesis. The preparation of catalyst and biodiesel synthesis is shown in Fig. 11 . The surface morphological structures and metal oxide distribution on the surface of the catalyst are displayed in FESEM images (Fig. 12 ). The catalyst, CaO (10%)-Fe2O3 (10%)/AC (AC-activated carbon), could yield 98.3% of biodiesel under the reaction conditions of 18:1 of MTOMR and 3 wt% of catalyst loading at 65 °C in 3 h of reaction. The reusability studies showed a good catalytic activity (biodiesel yield > 80%) even at the 6th consecutive cycle. However, after the 6th catalytic cycle of the reaction, the catalyst showed leaching of active sites and changes in the surface morphological characters as revealed by FESEM analysis (Fig. 13 ). Bet-Moushoul et al. (2016) prepared Ag/bauxite nanocatalyst and utilized it in sunflower oil biodiesel synthesis. The SEM analyses of uncalcined bauxite, calcined bauxite (850 °C), bauxite/Ag nanocomposite, and recycled bauxite/Ag nanocomposite are displayed in Fig. 14 . This study revealed the formation and a nice dispersion of Ag nanoparticles on the bauxite surface. They reported a yield of 94% biodiesel under the conditions of 3 h of reaction time, 67 °C of reaction temperature, 9:1 of MTOMR, and 0.3 wt% catalyst loading. They also reported that the catalyst was successfully reused up to the 8th cycle and the biodiesel yield decreased to 71.79%, which might be due to the leaching of catalyst's active sites, Fig. 14(D). Rashtizadeh et al. (2014) demonstrated the synthesis of Sr3Al2O6 nanocatalyst and applied it in soybean oil biodiesel synthesis. The catalyst could produce 95.7% of biodiesel at the ORCs of 1.3 wt% of catalyst and 25:1of MTOMR at 60 °C in 1 h. They reported that the catalyst was successfully reused up to 4th time. Foroutan et al. (2022) also reported the reaction of sunflower oil to biodiesel using waste chalk/CoFe2O4/K2CO3 as the nanocatalyst. The nanocatalyst was prepared following the co-precipitation chemical method. They found that the surface area, pore volume, and pore size of the catalyst were 5.839 m2 g−1, 0.0118 cm3 g−1, and 8.08 nm, respectively. A maximum of 99.27% biodiesel yield was obtained at 2.95 h of reaction time, 80 °C of reaction temperature, 15:2 of MTOMR, and 2.65 wt% of catalyst loading. They also reported that the catalyst was successfully reused for 6 consecutive cycles.Due to eco-friendly nature and comparable combustion properties with fossil-based fuel, biodiesel can act as a clean and substitute to fossil fuel. It is found that biodiesel is sometimes auspicious compared to fossil fuel due to its properties such as better lubricity, high cetane number, smaller carbon footprint, and easy biodegradability. Though it is eco-friendly nature, it has to satisfy the term and conditions (Table 11 ) established by the United States Environmental Protection Agency (EPA) and the American Society of Testing and Materials (ASTM) (ASTM, 2003). According to ASTM standard D6751, the maximum water content is 0.05% volume because high water contents can cause cruces such as microbial growth in fuel handling, storage, and transportation equipment (Van Gerpen, 2005). The density is also an important property of fuel and according to ASTM, the specific density of fuel at 40 °C should lie between 0.82–0.90 g cm−3 because high density can cause difficulties such as imperfect combustion and particulate matter emission (Blangino et al., 2008).Viscosity indicates the ability of a material to flow and according to ASTM (ASTM, 2003), it should lie within the range of 1.9–6 mm2 s−1. Fuel with higher viscosity can form large droplets in the injection and as a result, high energy is required to pump fuel, and poor combustion leads to the emission of greenhouse gasses. The acid value is the quantity of KOH in milligram required to nullify one gram of oil (ASTM, 2003). Since the oil contains FFAs, the acid value can be related to the number of carboxylic groups present in the oil. According to ASTM (ASTM, 2003), the acid value of fuel should be less than 0.5 mg KOH g−1 because a higher acid value can erode the engine and fuel tank. Similarly, cetane number is also an important property for fuel since it measures the delay of the ignition of diesel fuel in a compression ignition engine, and the cetane number of biodiesel fuel should be greater than 47 (ASTM, 2003). Similarly, there are various properties such as boiling point, flashpoint, cloud point, pour point, ash content, and sulfur content. These are also important for fuel, and according to EPA and ASTM, they also have specified values (Table 11).Biodiesel is the FAME produced via catalytic esterification and transesterification processes from biological sources like edible oil, non-edible oil, animal fats, algae oil, etc. Though biodiesel is ecofriendly, it is not extensively commercialized till today due to its high price. The higher price of biodiesel is mainly due to the cost of raw-feedstocks and devices utilized in the preparation process (Skarlis et al., 2012). Using cheaper oil feedstocks, the production cost can be reduced but these feedstocks have some disadvantages such as high FFAs which lead to the formation of soap (Abdullah et al., 2017). By developing a suitable technology and enhancing the catalytic activity with an increased yield of biodiesel, the production cost can be reduced.It is known that the application of homogeneous catalyst is not feasible as it causes soap formation and complication during separation (Basumatary et al., 2018). The heterogeneous catalyst has more advantages because of easy separation and reusable property which has a great contribution in decreasing the cost involved in biodiesel production (Konwar et al., 2014a; Konwar et al. 2014b; Kondamudi et al., 2011). In this 21st century, the nanocatalysts have attracted everyone's attention due to their large surface area, efficient catalytic activity, easy preparation process, and reusable properties. To reduce the biodiesel cost, researchers nowadays use different non-edible feedstocks such as dairy scum, WCO, animal fat, and tea kernel oil which are some of the cheapest feedstocks. The nanocatalyst preparation methods are also simple and easy. Nanocatalysts can be prepared from wastes and cheap materials such as eggshell, snail shell, coconut shell, scallop waste shell and waste biomass (Tables 1,7,10). The least cost and the reusable properties of these catalysts would bring economic advantages for the production of biodiesel.In conclusion, compared to exhaustible fossil fuels, biodiesel is more reliable due to its non-toxic, renewable, and biodegradable properties. In this review, the meticulous discussion on biodiesel production via transesterification reaction with the help of different nanocatalysts has been systematically presented. Homogeneous, heterogeneous, and enzyme catalysts are employed in the transesterification for the synthesis of biodiesel. Each catalyst has its own merits and demerits, nonetheless the main focus of biodiesel synthesis is the reusability of the catalyst, easy separation and purification process, and minimal waste production, which makes the whole biodiesel production process economically worthwhile. Many researchers reported that heterogeneous catalyst satisfies the above-mentioned requirements but it needs some modification to improve its performance such as providing better selectivity and generating a highly active reaction site. In this review, nanocatalyst types such as metal oxides, nano-hydrotalcite, magnetic nanocatalyst, and nano-zeolite, and other nanocatalysts were comprehensively discussed. These nanocatalysts are considered as better and more efficient for yielding of higher biodiesel and selectivity compared to homogeneous and enzyme catalysts.The applications of magnetic materials as catalysts prepared from the waste-biomass sources have favourable potentials among the present trends of catalysts for biodiesel synthesis. This is due to their better catalytic performance, environment friendliness, rapid and easy separation using external magnetic field, and significant reusability for several cycles of reaction. These characters can reduce the overall time required in processing and energy consumption, and eventually decreases the overall cost of produced biodiesel. Bifunctional nanocatalyst and magnetic bifunctional catalyst from the waste biomass also showed significant efficacy and reusability characters for several reaction cycles, and have high prospective for the ecologically benign and economical synthesis of biodiesel from the low-cost WCO having high FFA content. The most important common factors influencing the biodiesel synthesis in terms of efficiency and economy are the type of oil feedstock, catalyst, and the reactor used. Besides these, other factors such as reaction time and temperature, MTOMR, and catalyst dosage have impacts on the synthesis of biodiesel leading to the overall production cost. In a nutshell, the usage of nanocatalyst in biodiesel synthesis can provide a cheap and clean renewable energy and thus it will become a strong contender for the global industry in the future.In the days to come, low-cost feedstocks such non-edible oil, WCO, waste animal fat, and waste municipal/sewage sludge should be considered for the synthesis of biodiesel by developing a potential and cost-effective catalyst that can simultaneously perform both esterification and transesterification reactions. Utilization of mixed oil feedstocks (hybrid oils) along with the effective reactor and machine learning techniques will certainly help in overall cost reduction of biodiesel. A chain system for continuous-supply of raw oil feedstocks for biodiesel synthesis needs to be strategized to meet the demand of large-scale production. Shamim Islam: Conceptualization, Investigation, Writing – original draft. Bidangshri Basumatary: Writing – review & editing. Samuel Lalthazuala Rokhum: Validation, Writing – review & editing. Prince Kumar Mochahari: Writing – review & editing. Sanjay Basumatary: Conceptualization, 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 authors are thankful to the authority of Bodoland University, Kokrajhar, India for the facilities in carrying out this study.

Energy consumption is increasing day by day, thereby depleting the fossil fuel reserve at an alarming rate. The fossil-based fuels have many adverse effects on the environment and cause global warming due to emission of greenhouse gases. Biodiesel produced via the transesterification process is an alternative, eco-friendly, and renewable fuel. Transesterification is carried out using homogeneous, enzyme, and heterogeneous catalysts. Heterogeneous catalysts can resolve the issues faced by the homogeneous and enzyme catalysts during biodiesel synthesis. At the same time, heterogeneous nanocatalysts have much more potential due to their higher surface area, more selectivity, and stronger catalytic activity. In this review, various nanocatalysts such as metal oxides (CaO, MgO, ZnO, Ti2O, CuO, and ZrO2), magnetic nanocatalyst, nano-zeolite catalyst, and nano-hydrotalcite catalysts were studied. In addition, catalyst preparation methods, physical properties of catalyst along with various reaction parameters such as reaction temperature and time, methanol to oil molar ratio (MTOMR), catalyst loading, and biodiesel yield were highlighted and discussed. In short, biodiesel synthesis using nanocatalyst can provide a cheap and clean energy and thus the nanocatalyst can be further developed as a strong candidate for the global energy industry in the future.

Environmental pollution, resource shortages, and increased energy demand are becoming increasingly serious problems [1,2]. As potential solutions, fuel cells, water electrolyzers, and metal–air batteries convert and store renewable clean energy through related electrochemical reactions, including the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) [3–6]. Currently, precious metal catalysts are acknowledged as superior catalysts—for example, Pt for the ORR [7,8] or HER [9,10], IrO2 and RuO2 for the OER [11,12], and so forth. However, precious metals are, by definition, rare and expensive, restricting their large-scale commercial application.In recent years, single-atom catalysts (SACs) with low metal support have improved the utilization of atoms and been used in various reactions due to their high activity and selectivity, and relatively low cost [13,14]. Transition metal-embedded nitrogen-doped graphene (MNx-G) SACs are considered as promising non-precious metal catalysts for electrochemical reactions [15]. Some theoretical studies have been undertaken [16–18] to improve the activity of MNx-G SACs successfully prepared in experiments [19–22]. However, most of those studies are about Fe/Co/NiN4-G or N2-G, with other metals and coordination largely ignored. Variances in method, model size, and so forthothers also make it difficult to compare results from different studies, even for the same MNx-G. For example, the combined theoretical and experimental works [23,24] showed that FeN2-G was more active than FeN4-G for the ORR, which agreed with the findings from a previous experiment [19]. However, according to the theoretical work by Chen et al. [25], the adsorption of O2 on edge FeN2-G was too strong, leading to a low activity. The calculation from Kattel et al. suggested that FeN4-G had higher stability and performance than FeN2-G [26]. Recently, Zhang et al. found that the ORR activity of CoN2-G was better than that of CoN4-G [27], while Yang and co-workers reported that CoN4-G was the best candidate for the ORR and OER [28]. Hence, a computational study that applies uniform standards is much needed to establish agreement in this area.Notably, Xu et al. [29] recently investigated the performance of MN4-G SACs, and Yang's team studied the activity of CoNx-G (x ​= ​1–4) [28], but there is still a lack of information about coordination between other metals and nitrogen. Although Lin et al. [30] applied a machine learning (ML) model to predict the activity of MNxCy-G SACs, the unreliable training data (see Supporting Information Note 1 and Table S1) makes the ML model unconvincing. For example, they failed with respect to ZnNx-G's outstanding ORR activity [31,32], NiN4-G's poor HER activity [33], as well as Fe/TcC3-G's HER activity [34]. All these divergences arose from ignorance of the basic principles of quantum mechanics for spin states. A comprehensive study of MNx-G with different metals, nitrogen coordination, and reasonable spin states is therefore necessary to gain an overview of these materials and develop a consensus on them.In this work based on density functional theory (DFT) calculations, the electrocatalytic performance of 3d transition metal single atoms with various forms of nitrogen coordination is systematically explored using unified calculation standards and reaction mechanisms. The adsorption free energies of reaction intermediates, the scaling relationships between them, and the volcano curves are calculated. The activity for the ORR, OER, and HER is investigated to find the optimal SAC configuration. Furthermore, ML is used to unveil possible factors affecting activity, thereby providing solid guidance for improving catalytic performance.All spin-polarized theory calculations were performed using the Vienna Ab-initio Simulation Package (VASP) with the projector augmented wave (PAW) pseudopotentials [35–37]. The cutoff energy for the plane-wave basis set was 520 ​eV, and the Perdew-Burke-Ernzerhof (PBE) functional was used to describe the exchange-correlation interaction [38]. The MNx-G was modeled on the p(6 ​× ​6) graphene unit cell. Geometry optimization was not accomplished until the maximum force per atom was less than 0.02 ​eV ​Å–1 and the energy difference between two adjacent electronic steps was less than 10–5 ​eV. The 5 ​× ​5 ​× ​1 Gamma-centered k-point grid was employed to sample the Brillouin zone, and a 15 ​Å vacuum layer was adopted to avoid interaction between surfaces due to periodicity. The Gibbs reaction free energy change ( Δ G ) is estimated by the relation: (1) Δ G = Δ E + Δ E ZPE − T Δ S where Δ E is the energy difference of products and reactants, T is room temperature (298.15 ​K), and Δ E ZPE and Δ S are the changes in the zero-point energy and the entropy, respectively. The latter were calculated from the vibrational frequencies for adsorbed species. For gas-phase molecules such as H2, entropy was obtained from the NIST database [39]. The recommended PAW potentials in the VASP manual were applied (Table S2).To reduce the influence of different spin configurations on energy, several spin states of each model were considered, and the lowest energy was used for the free energy calculations. The CoN3-G SAC is shown in Table S3 as a demonstration. This treatment obeys the fundamental law of Pauli's Exclusion Principle, guaranteeing the reasonableness of the data from the calculations with respect to physics (Supporting Information Note 1).The ML model based on the eXtreme Gradient Boosting (XGBoost) algorithm [40] was achieved using the scikit-learn package [41]. The training data was a subset of the whole data; 80% of the whole data was randomly selected as the training set, while the rest was used as the test set. Three indices were employed to estimate the prediction accuracy: coefficient of determination (R 2), mean squared error (MSE), and Pearson correlation coefficient (r). The impact of each feature was investigated using the Gini importance method [42]. The specific details about free energy calculation and machine learning analysis are presented in the Supporting Information.In this work, a 3d transition metal single atom (Sc to Zn) is located in various nitrogen coordination structures: double vacancy surrounded by one nitrogen atom (MN1-G in Fig. 1 a), by two nitrogen atoms (MN2-G in Fig. 1b), by three nitrogen atoms (MN3-G in Fig. 1c), and by four nitrogen atoms (MN4-G in Fig. 1d). The MN2-G structure was initially considered to contain three configurations: MN21-G, MN22-G, and MN23-G (Fig. S1). For most metals, MN21-G has a lower formation energy and is more stable than the MN22-G and MN23-G structures (Table S4). Although the MN22-G structures of Sc, Ti, Cr, and Zn are somewhat stable, it is noteworthy that the energy differences between the MN21-G and MN22-G structures for Sc, Ti, and Cr are tiny. Therefore, to simplify the discussion, this study focuses on the MN21-G structure (Fig. 1b) for MN2-G.The binding energies ( E b ) of single metal atoms on a defective graphene support, the cohesive energies ( E c ) of metal atoms in crystals, and the energy differences Δ E b − c = E b − E c are calculated to determine the stability of the SACs. E b and E c are obtained according to the following equations: (2) E b = E M + substrate − E substrate − E M (3) E c = E M − bulk / N − E M where E M + substrate is the total energy of the metal atom at the support, E substrate is the energy of nitrogen-doped defective graphene, E M − bulk is the energy of the metal crystal, N is the number of metal atoms in the bulk cell, and E M is the energy of the isolated metal atom. Negative values for E b and Δ E b − c indicate that the binding between a single metal atom and the substrate is thermodynamically more favorable than the aggregation of metal. All 40 SACs meet the above two conditions (Table S5), which suggests MNx-G is stable and matches the observations in experimental and other theoretical works [22,34].First, we study the catalytic performance for the ORR, OER, and HER under acidic conditions (pH ​= ​0). For the possible reaction pathway of the ORR, there are mainly two mechanisms: dissociation and association. Since the former, O2 dissociation into two separate O atoms, needs to overcome higher barrier than the latter, O2 hydrogenation to form OOH [43], the more favorable path is chosen: (4) ∗ +  ​O 2 g + H + + e − ↔ ∗ OOH (5) ∗OOH+H + + e − ↔ ∗ O + H 2 O l (6) ∗ O + H + + e − ↔ ∗ OH (7) ∗ OH + H + + e − ↔ ∗ + H 2 O l where ∗ indicates the active sites on the catalyst surface, and (g) and (l) denote gas and liquid phases, respectively. The adsorption of reaction intermediates such as ∗OOH, ∗O, and ∗OH is studied, and their adsorption free energies ( Δ G ∗OOH , Δ G ∗O , and Δ G ∗OH ) are obtained with H2O and H2: ∗ + 2 H 2 O ↔ ∗ OOH + 3 / 2 H 2 , ∗ +  ​H 2 O ↔ ∗ O + H 2 and ∗ +  ​H 2 O ↔ ∗ OH + 1 / 2 H 2 . The zero-point energy and entropy of the gas molecules are given in Table S6. Taking the relaxed structures of CoN3-G (Figs. 1e–h) as an example, the adsorption structures for the other metal atoms and coordination are similar to them.Generally, the potential-determining step—namely, the step with the largest reaction free energy change—has the highest overpotential, and it thus may also be the rate-determining step (RDS). To identify the RDS in the reaction pathway, the reaction free energy changes of Equations (4)–(7) are calculated using the following formulae, and the results are shown in Tables S7–S10. (8) Δ G 1 = Δ G ∗OOH − 4.92 (9) Δ G 2 = Δ G ∗O − Δ G ∗OOH (10) Δ G 3 = Δ G ∗OH − Δ G ∗O (11) Δ G 4 = − Δ G ∗ OH As the reverse reaction process of the ORR, the OER possesses the same reaction path, which is the opposite of Equations (4)–(7). The corresponding reaction free energies ( Δ G 5 to Δ G 8 , Tables S7–S10) are calculated as follows: (12) Δ G 5 = Δ G ∗OH (13) Δ G 6 = Δ G ∗O − Δ G ∗OH (14) Δ G 7 = Δ G ∗OOH − Δ G ∗O (15) Δ G 8 = 4.92 − Δ G ∗OOH Fig. 2 shows the free energy diagrams of the ORR and OER on MNx-G structures. Overall, along the selected ORR path, most of the metal reaction processes are exothermic and proceed in a favorable direction. Compared to the MN1-G system (3 downhill), more SACs in MN2-G (6 downhill), MN3-G (5 downhill), and MN4-G (4 downhill) present a downhill situation in the ORR, suggesting single N coordinated cases may be not good for the ORR. In the free energy diagram of the ORR, it can be observed that the last step of ∗OH reduction to the second H2O molecule is uphill and holds the maximum free energy change in terms of most systems, implying that it is the RDS. For individual metals, the process of O2 hydrogenation to ∗OOH is also the RDS. Unlike the ORR, the OER process is endothermic, requiring external energy to promote the reaction. The OER is mainly determined by the process of ∗O becoming ∗OOH or the transformation of ∗OH to ∗O.We also studied the HER on MNx-G catalysts and the free energy diagram of the HER is shown in Fig. 3 . When the HER takes place in an acidic environment (pH ​= ​0), first protons and electrons interact to form adsorbed H, and then this desorbs into H2 molecules. Based on previous research [33], the following path is taken: (16) ∗ +  ​H + + e − → ∗ H (17) ∗H +  ​H + + e − → ∗ +  ​H 2 g The HER activity depends on the adsorption free energy of H (Table S11). The Δ G ∗H value of the ideal catalyst for the HER should be close to zero to balance the processes of hydrogen adsorption and desorption. In Fig. 3a, CoN1-G has the minimum Δ G ∗H value of –0.51 ​eV, and ScN1-G has the maximum Δ G ∗H value of 1.05 ​eV. In Fig. 3b, VN2-G has the lower Δ G ∗H value of –0.17 ​eV, and CuN2-G has the higher Δ G ∗H value of 1.39 ​eV. In Fig. 3c, the Δ G ∗H values vary from –0.63 ​eV for TiN3-G to 0.89 ​eV for ZnN3-G. In Fig. 3d, the Δ G ∗H value of TiN4-G is –0.58 ​eV, and the Δ G ∗H value of CuN4-G is 1.75 ​eV. The maximum Δ G ∗H value of MN2/N4-G is more positive than that of MN1/N3-G, which hinders the adsorption process and reduces the HER activity. Taking | Δ G ∗H | < 0.5 ​eV as the reference range, we also calculate the variance ( S 2 ) of the adsorption free energy of H on MNx-G SACs relative to the expected value of 0 ​eV. The S 2 values of the SACs are 0.044 for MN1-G, 0.045 for MN2-G, 0.027 for MN3-G, and 0.086 for MN4-G. The small S 2 value for the MN3-G configuration means there are more metal systems around Δ G ∗H = 0 ​eV, displaying better HER catalytic activity.Since the RDS is related to the adsorption free energy of the reaction intermediates, we further explore the relationship between Δ G ∗OOH , Δ G ∗O , and Δ G ∗OH . Taking Δ G ∗OH as an independent variable, Δ G ∗OOH and Δ G ∗O are plotted versus Δ G ∗OH for all the investigated SACs. By linearly fitting these points, the relationship between oxygenated intermediates is determined to be as follows: (18) Δ G ∗ OOH = 0.85 × Δ G ∗OH + 3.28 (19) Δ G ∗O = 1.27 × Δ G ∗OH + 0.98 In Fig. 4 a, the data points of Δ G ∗OOH vs. Δ G ∗OH are well fitted. The slope and intercept of the curve are close to those in previous studies ( Δ G ∗OOH = Δ G ∗OH + 3.2 [11]). Although the linear fit for the data points of Δ G ∗O vs. Δ G ∗ OH displays a weak correlation in Fig. 4a, a scaling relationship with a slope of about 1 has also been proposed in other studies [44,45]. The weak linear relationship between Δ G ∗O and Δ G ∗OH may be related to the RDS of individual SACs. The transformation of ∗O to ∗OOH or of ∗OH to ∗O is the main RDS of the OER. But for individual systems such as Sc/TiN2-G, their RDS is the process from ∗OOH to O2. After removing these points, as shown in Fig. S2b, the linear relationship between Δ G ∗O and Δ G ∗OH is improved, with R 2 changing from 0.74 to 0.81. Given that Δ G ∗O , Δ G ∗OOH , and Δ G ∗OH are related to each other, we can simplify the description of ORR/OER/HER activity by using the adsorption free energy of only one intermediate. The catalytic performance of MNx-G SACs is evaluated using the overpotential ( η ): (20) η ORR = max { Δ G 1 0 , Δ G 2 0 , Δ G 3 0 , Δ G 4 0 } / e + 1.23 V (21) η OER = max { Δ G 5 0 , Δ G 6 0 , Δ G 7 0 , Δ G 8 0 } / e − 1.23 V (22) η HER = − | Δ G ∗H | / e For the ideal ORR or OER catalyst, the equilibrium potential U 0 is equal to 1.23 ​V (4.92 ​V/4 ​= ​1.23 ​V) versus the standard hydrogen electrode (SHE). Therefore, the more the overpotential tends to zero, the better the catalytic activity is.Here, Δ G ∗OH is used as an activity indicator for the ORR, and η ORR as a function of Δ G ∗OH is plotted in Fig. 4b. The calculated results are compared with the activity of Pt (111). On the left side of the volcano curve, as the adsorption of ∗OH weakens, η ORR gradually decreases and the ORR activity is promoted. However, on the right side, η ORR decreases as the adsorption of ∗OH strengthens, which indicates that if ∗OH adsorption is too strong or too weak, the ORR activity will be reduced. When the value of Δ G ∗OH is about 1 ​eV, the volcano top has a minimum η ORR value, and the catalytic performance is optimal for the ORR. The η ORR of the MNx-G systems (Table S12) gradually increases in the following order: CoN3-G (0.84 ​eV, 0.39 ​V) ​< ​CoN4-G (1.07 ​eV, 0.42 ​V) ​< ​ZnN2-G (0.84 ​eV, 0.47 ​V) ​< ​ZnN3-G (0.85 ​eV, 0.49 ​V) ​< ​ZnN4-G (0.89 ​eV, 0.54 ​V) ​< ​CoN2-G (0.67 ​eV, 0.56 ​V) ​< ​FeN4-G (0.65 ​eV, 0.58 ​V). With a Δ G ∗OH of 0.84 ​eV and an η ORR of 0.39 ​V, the CoN3-G catalyst exhibits optimal ORR activity. The η ORR value of some systems approaches or even exceeds 0.43 ​V of Pt (111) surface, suggesting that these structures may be promising for replacing precious metal materials as SACs for the ORR. In our study, CoN4-G (FeN4-G) has a lower overpotential than CoN2-G (FeN2-G), which is consistent with the results of Yang's group [28]. Unlike previous reports [29,30], ZnNx-G displays excellent ORR activity in our work (Fig. 4b and Table S12), agreeing with recent experimental observations on the activity of ZnNx-G catalysts [31,32].The catalytic activity for the OER is described by Δ G ∗O and Δ G ∗OH . The volcano relationship between η OER and ( Δ G ∗O − Δ G ∗OH ) is shown in Fig. 4c. The points deviating from the trend lines are caused by Sc and Ti metal atoms (Fig. S2c). As already mentioned, the RDS of Sc and Ti metal atoms is different from the RDS of most other SACs for the OER. After we remove the points with different RDSs, as shown in Fig. S2d, the volcanic activity curve of OER is significantly enhanced. When the difference between Δ G ∗O and Δ G ∗OH is around 1.5 ​eV, the η OER reaches the minimum value, meaning the highest OER activity. The η OER of the MNx-G systems (Table S13) gradually increases in the following order: CoN4-G (1.56 ​eV, 0.33 ​V) ​< ​NiN2-G (1.71 ​eV, 0.51 ​V) ​< ​CoN3-G (1.05 ​eV, 0.79 ​V) ​< ​NiN3-G (2.05 ​eV, 0.82 ​V) ​< ​CoN2-G (1.11 ​eV, 0.86 ​V). Among these, CoN4-G, with an overpotential of 0.33 ​V, shows outstanding catalytic performance for the OER, comparable to the activity of IrO2 (0.65 ​V). According to the activity analysis for the ORR and OER, more of the metals in MN4-G have an overpotential of less than 1 ​V than in other configurations, indicating that N4 coordination seems to be more active than the others. Moreover, CoN4-G possesses high activity in the ORR and OER, suggesting the CoN4-G system is a potential bifunctional SAC for the ORR and OER. An experimental study by Lv et al. [46] also showed the high ORR/OER catalytic performance of the CoN4 site.Similarly, the data points for η HER vs. Δ G ∗H present the volcano curve relationship shown in Fig. 4d. When Δ G ∗H is about 0 ​eV, the catalytic performance for the HER peaks, which is consistent with the theoretical analysis. MNx-G systems with excellent HER performance are listed in Table S14; their activity gradually decreases in the following order: NiN3-G (–0.02 ​V) ≈ CuN3-G (–0.02 ​V) ​> ​CoN3-G (–0.04 ​V) ​> ​CrN1-G (–0.05 ​V) ≈ CoN2-G (–0.05 ​V) ​> ​TiN1-G (–0.07 ​V). The HER activity of individual structures, particularly Ni/CuN3-G, is similar to that of Pt(111), with an η HER of 0.09 ​V. Notably, NiN4-G exhibits poor HER activity in our work, with a Δ G ∗H value of 1.68 ​eV, which is close to previous experimental [33] ( Δ G ∗H  ​= ​1.62 ​eV) and theoretical studies ( Δ G ∗H  ​= ​1.20 ​eV [47] and 1.35 ​eV [48]). The trend of nitrogen coordination affecting the catalytic performance of the HER is in line with a recent study by Song et al. [49].The above discussion has revealed the optimal SAC configurations for the ORR, OER, and HER. The favorable adsorption strength of the reaction intermediates is significant for catalytic performance. We therefore go on to study the intrinsic factors affecting the binding strength between the intermediates and metal atoms.The ML method is applied to find the possible internal factors that affect the activity of MNx-G SACs. By continuously adjusting the ML model and algorithm, we screen out a series of important features to describe Δ G ∗OH (for the ORR), Δ G ∗O − Δ G ∗OH (for the OER), and Δ G ∗H (for the HER); these features include the valence electron occupancy of the d orbitals of metal atoms ( d Ve ), covalent radius ( r M ), the electronegativity of metal atoms ( χ M ), the coordination number of nearest-neighbor N and C atoms for metal atoms ( n N / C ), and the bond length between metal atoms and intermediates ( d M − O / H ). Comparison of the Δ G ∗OH , ( Δ G ∗O − Δ G ∗OH ), and Δ G ∗H values predicted by the XGBoost algorithm with those calculated by DFT are shown in Figs. 5 a, 5c, and 5e, respectively. The trend predicted by the XGBoost algorithm is in good agreement with the value calculated using DFT. A high R 2 (r) and a low MSE indicate that the ML model is effectively trained for prediction. The Gini importance method is used to explore the impact of each intrinsic feature, as shown in Figs. 5b, 5d, and 5f.The catalytic activity of the MNx-G structures displays varying degrees of correlation with 6 features, where d Ve plays the most important role in the catalytic activity for the ORR, OER, and HER. The linear relationship between the number of metal valence electrons and the adsorption energy, as revealed in the work of Su et al. [50], also proves the importance of d Ve . Apart from electronic properties, geometric structures such as the bond length d M − O / H also affect the activity. In addition, Bader charge analysis shows that more electrons of metal atoms are taken away as the number of N atoms increases (Table S15), which affects the adsorption strength and catalytic performance of the MNx-G. As stated in Section 3, the adsorption of an optimal catalyst to intermediates should be moderate. One may modulate the electronic states and adsorption strength of the SACs by adjusting the coordination number of N atoms to improve the catalytic activity, and a recent study on FeN5-G proves this idea [15]. The selected features are simple and easily available physical variables, which is beneficial for screening to find other efficient MNx-G SACs. We consider four elementary reactions, and the RDS of the ORR is usually different to that of the OER. The RDS of the ORR is the last step of ∗OH reduction to H2O or the process of O2 hydrogenation to ∗OOH, while the OER is mainly determined by the process of ∗O to ∗OOH or the transformation of ∗OH to ∗O. As a result, the importance of the key features affecting the ORR/OER activity is dissimilar, though the OER is apparently reversible with the ORR.Inspired by the excellent ORR activity of Co and Zn metal atoms in the above 3d study, we calculate for Rh and Cd, which have the same valence electrons as Co and Zn, to demonstrate the conclusion of the ML method (Table 1 ). As expected, the RhN3-G, RhN4-G, and CdN1-G structures are also potential ORR catalysts with a low overpotential, and RhN3-G may be more active than CoN3-G.The catalytic activity of SACs based on MNx-G (M ​= ​Sc to Zn, x ​= ​1–4) for the ORR, OER, and HER has been systematically investigated. Different spin configurations of each model were considered, and the lowest energy was adopted to calculate subsequent reaction free energy. When the corresponding volcano curves were also considered, the optimal SACs for the studied electrochemical reactions were revealed to be CoN3-G for the ORR, CoN4-G for the OER, and Ni/CuN3-G for the HER. In addition to the commonly acknowledged Fe/Co/NiN4 or N2 moiety, other metals or nitrogen-coordinated MNx-G systems, including ZnNx-G and CoN3-G, also displayed outstanding performance. Catalysts with N2, N3, and N4 coordination were found to be more active for the ORR and OER than those with N1 coordination. More metals in the MN3-G configuration ​with Δ G ∗H appoaching to zero have superior HER activity. This trend suggests that controlling the concentration of the N component may be more flexible when preparing ORR/OER catalysts for the four coordination cases, while it is better to use as much control as possible with the MN3-G structure when preparing HER catalysts. Furthermore, the ML method revealed that the high catalytic activity of the MNx-G structures can be ascribed to the valence electron occupancy of the d orbitals, the covalent radius, the electronegativity, the coordination number, and the adsorption bond length of the metal atoms. Among these features, the valence electron occupancy of the d orbitals has the greatest influence on the activity for the ORR, OER, and HER. This research paves the way for the development and design of high-performance non-precious metal SACs.Z.P. Hu proposed the concept. C.Y. Zheng performed the density functional theory calculations. X. Zhang and Z. Zhou performed the machine learning analysis. C.Y. Zheng, Z.P. Hu, X. Zhang, and Z. Zhou co-wrote the manuscript. All authors participated in data analysis and manuscript discussion.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 National Natural Science Foundation of China (No. 21933006, 21773124), the Fundamental Research Funds for the Central Universities Nankai University (No. 63213042) and the Supercomputing Center of Nankai University (NKSC). We thank L.F. Zhang and Q. Gao for the fruitful discussions.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.2022.02.009.

Electrochemical reactions are essential in the processes of energy storage and conversion, and performance is tightly dependent on the electrocatalysts. Herein, we systematically investigate the activity of 3d transition metal embedded nitrogen-doped graphene (MNx-G) for single-atom catalysts (SACs) in the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The calculated volcano curves reveal the optimal SAC configuration for each reaction to be CoN3-G for the ORR, CoN4-G for the OER, and Ni/CuN3-G for the HER. Analysis based on the machine learning method suggests that high catalytic performance is dominated by the number of valence electrons occupying the d orbitals, the covalent radius, the electronegativity, the ratio of nearest-neighbor N and C atoms for the metal atoms, and the bond length between metal atoms and adsorbates. This work may shed some light on further studies of the ORR, OER, and HER with non-precious metal SACs.

With the increasing global energy demand, non-renewable fossil fuels play an important role. But the overuse of fossil fuels has worsened the environment. Hence, developing clean energy is a significant direction for sustainable development [1]. Hydrogen is considered as a green energy carrier to eliminate our dependence on fossil fuels [2]. However, hydrogen almost does not exist in nature, the actual use of hydrogen is prepared by artificial reaction. Almost all hydrogen comes from industrial steam reforming of natural gas, which consumes a large amount of energy and emits a lot of carbon dioxide. At the same time, hydrogen produced by this method contains a trace amount of carbon monoxide, which can poison platinum catalysts in fuel cells. In contrast, electrolysis of water can produce clean hydrogen [3]. Although platinum-based catalysts still have the best hydrogen evolution performance in the field of electrolytic water, precious metal-based catalysts are not only too expensive, but also have very limited reserves on the earth. It is difficult to be applied to the actual industrial production of hydrogen on a large scale [4]. Hence, seeking non-noble metal catalysts with excellent hydrogen evolution performance plays an important role in entering the hydrogen energy society [5]. Recently, many reports have shown that oxide [6], carbide [7] and chalcogenide [8] possess a good hydrogen evolution properties. In the study of non-noble metal electrocatalysts for hydrogen evolution reaction (HER), transition metal sulfides have attracted attention because of their rich natural reserves and high electrocatalytic activity for hydrogen evolution. Nickel sulfide has excellent hydrogen evolution performance because of its similar structure to the hydrogenase activity site [9]. Compared with oxygen, sulfur has lower electronegativity and can form different nickel sulfide phases (NiS, NiS2, Ni3S2) after coupling with nickel [10]. Moreover, nickel sulfides have metal-like band structure and excellent electrical conductivity [11]. It has a great potential in the field of electrolytic water. Compared with carbon materials such as carbon cloth, carbon nanotubes and graphene, carbon spheres can be fabricated by a facile hydrothermal method, which is an ideal electrocatalyst carrier material.Herein, by using a simple hydrothermal method, we use carbon spheres as the support and grow Ni3S2 and NiS on its surface as the catalytic active materials. And then we analyze the crystal structure, microstructure and electrochemical hydrogen evolution properties of the composites. Besides, hydrogen evolution reaction is the key half reaction of water splitting, which is a clean but high energy-consuming technology. Hence, developing electrocatalysts with good catalytic activity to reduce energy consumption is essential.Glucose monohydrate (C6H12O6·H2O) was bought from Macklin Co., Ltd. Cetyl Trimethyl Ammonium Bromide (CTAB) and Nickel sulfate hexahydrate (NiSO4·6H2O) were furnished by Keshi Co., Ltd. Thioacetamide (C2H5NS) was supplied by Bidepharm Co., Ltd.XRD-6100 was used to study the crystal structure of the catalysts. The scanning rate was 4°·min−1 and 2θ were ranging from 10°-80°. Using a step-scanning mode with a step size of 0.02°, the data for the Rietveld method (by Fullprof software) was obtained. By using SEM, the surface morphology of the carbon spheres, nickel sulfides and nickel sulfides supported by carbon spheres were gained. The surface element composition was probed by EDS. The chemical state of the catalysts was analyzed by XPS. Adsorption isotherms and pore size distribution were obtained by N2 on the Autosorb-3B automatic physical adsorption instrument in 77 K.The hydrothermal method was used to prepare carbon spheres. 3.5 g of glucose monohydrate (C6H12O6·H2O) and 0.5 g of Cetyl Trimethyl Ammonium Bromide (CTAB) were blended with 65 ml deionized water and stirred to get a homogeneous solution. Then the solution was poured into the autoclave. The temperature was set up to 180 °C and the reaction time was 6 h. The samples were cleaned several times with deionized water and ethanol. The products were put into the oven to dry for use.The carbon spheres (0.025 g) were adding into a beaker containing 30 ml deionized water and 30 ml DMF. With the assistance of ultrasonication, the solution was mixed evenly. Nickel sulfate hexahydrate (NiSO4·6H2O, 0.263 g) and thioacetamide (C2H5NS, 0.075 g) were then added into the solution under stirring. The solution was poured into a 100 ml autoclave. Besides, the temperature was set up to 200 °C and the reaction time was 24 h. The as-obtained product was cleaned by deionized water and ethanol to wash away impurity ions and dried in the oven for use (denoted as C25-M). C-M materials were also prepared by altering the relative ratios of carbon spheres (0.010 g and 0.040 g for C10-M and C40-M, respectively). Ni3S2 and NiS were prepared under the same condition without adding carbon spheres into the beaker (denoted as NixSy).By using a three-electrode system (the glassy carbon electrode coated with catalysts as the working electrode, Ag/AgCl electrode as the reference electrode and graphite rod as the counter electrode), the hydrogen evolution performance of the products were researched. In the experiments, we used a magnetic stirring device for to remove bubbles produced on the electrode surface during the catalytic process. By using a magnetic stirring device during the test, the solution is fully stirred to make the electrode surface concentration basically the same as the bulk concentration. In this case, the effect of mass transport could be minimized.The potential values were switched to the reversible hydrogen electrodes (RHE) according to the Nernst equation [12]: E V v s . R H E = E V v s . A g / A g C l + 0.197 + 0.059 ∗ p H The working electrode prepared by this procedure are as follows: 1) Taking 5 mg of catalyst and mixng with 500 μL deionized water and 500 μL ethanol with ultrasonication for 30 min; 2) Dropping 5 μL of the mixed ink on the electrode; 3) Dropping 2 μL of 0.5% Nafion solution on the electrode. The electrolyte was degassed by nitrogen for 30 min. Linear sweep voltammograms (LSV) were gained in N2-saturated 0.5 M H2SO4 solution. The potential range was set up to 0 to − 0.9 V vs. RHE at a scanning speed of 5 mV·s−1. Tafel slopes could been derived from the resulting polarization curves using the Tafel formula. To evaluate the durability, i-t curve (i-t) was recorded over C25-M under a working potential of −0.4 V vs. RHE for 18 h. The double layer capacitance (Cdl) and electrochemical impedance spectra (EIS) were measured to assess the electrochemical activity. All the data were gained without IR compensation.From the XRD scheme (Fig. 1 ), carbon spheres merely possess a wide diffraction peak at 22°, which is coincided with the former literature [13]. The peaks at 21.8°, 31.1°, 37.8°, 44.3°, 49.7°, 50.1° and 55.2° are corresponding to (101), (110), (003), (202), (113), (211) and (122) lattice planes of Ni3S2 (JCPDF#44–1418). Other diffraction peaks at 18.4°, 30.3°, 32.2°, 35.7°, 40.5°, 48.8°, 52.6°, 57.4° and 59.7° are from the (110), (101), (300), (021), (211), (131), (401), (330) and (012) crystallographic planes of NiS (JCPDF#12–0041). The peaks illustrated that Ni3S2 and NiS possess good crystallinity and high purity. By using Rietveld calculation, we conclude that the weight fraction of NiS is 37.47% and the weight fraction of Ni3S2 is 62.53%. Using the Scherrer formula, the calculated crystal size of C25-M is 26.8 nm. Fig. 2 a exhibits the microstructure of nickel sulfides. From Fig. 2b, it can be illustrated that the even diameter of carbon spheres is between 300 and 400 nm. As depicted in Fig. 2c, Ni3S2 and NiS generated on the carbon spheres and distributed evenly. The diameter of composite increased owing to the growth of nickel sulfides. After introducing the carbon spheres, the Ni3S2 and NiS particles are more refined, which helps to prevent aggregation, migration and structural destruction of nickel sulfides during the catalytic reaction process. The elemental distribution of C25-M is revealed by EDS mapping images (Fig. 3 a-e). From the scheme, Ni and S atoms distribute homogenously over carbon spheres and other impurities are not detected. In Fig. 4 , adsorption isotherms and pore size distribution were provided. The surface area of C25-M is 67.12 m2/g, which is lager than carbon spheres and NixSy. It can also be seen that the pore sizes of C25-M mainly distributed at 1.867 nm.Furthermore, Fig. 5 shows X-ray photo-electron spectroscopy (XPS) of C25-M. In the Ni 2p region of the composite (Fig. 5a), the peaks situated at 872.98 and 855.18 eV could be ascribed to 2p1/2 and 2p3/2 of Ni2+ and peaks situated at 875.18 and 857.48 eV are bound up with 2p1/2 and 2p3/2 for Ni3+. The peaks of Ni2+ are contributed by NiS and Ni3S2 and peaks of Ni3+ are related to Ni3S2. There are two peaks at 860.78 and 879.18 eV, which are from the satellite peaks of Ni 2p3/2 and Ni 2p1/2 [14]. From S 2p diagram, the peaks at 169.18 and 170.28 eV are bound up with the sulfur in nickel sulfides [15]. The analysis mentioned above illustrates that the nickel sulfides emerge in the products. Fig. 6 exhibits the electrochemical data. In the LSV curve, overpotential is an important parameter to appraise the hydrogen evolution performance. From Fig. 6a, carbon spheres exhibits a very poor catalytic activity. In contrast, all C-M samples show lesser overpotential than nickel sulfides, which demonstrates the advantage of growing Ni3S2 and NiS on the surface of carbon spheres. To explore the optimal proportion of carbon spheres, we used 10, 25 and 40 mg carbon spheres as the support and grow nickel sulfides on them, respectively. Among all the C-M samples, C25-M displays the optimal hydrogen evolution activity. It is strange that with more carbon spheres adding into the autoclave, the catalytic capability of the composite is not always improving. Although C40-M has the largest amount of carbon spheres, its overpotential is larger than C25-M, which is probably caused by excessive carbon spheres resulting in a abatement of active sites on the carbon spheres. In addition, we conduct linear sweep voltammograms performance test on the commercial Pt/C catalyst. The overpotential of commercial Pt/C at a current density of 10 mA·cm−2 is 57 mV. And the overpotential of C25-M is negatively shifted by 259 mV compared to the Pt/C catalyst.To analyze the mechanism of the reaction, Tafel slopes are obtained by plotting overpotential against the logarithm of current density. As depicted in Fig. 6b, Tafel slopes of carbon spheres, nickel sulfides and C25-M are 156 mV/dec, 120 mV/dec and 82 mV/dec, respectively. It can be illustrated that in virtue of carbon spheres, the Tafel slope of the composite is evidently reduced. This signifies that the C25-M can obtain a higher catalytic current at the same increment of overpotential, thus improving its catalytic activity. In the acid solution, hydrogen evolution reaction involves two pathways [16]. The first step is that the proton in the solution binds to the electron to adsorb and form the adsorbent hydrogen atoms (Hads) at the active site of the catalyst surface (Volmer step). The second step is that the active hydrogen atoms (Hads) desorb to form hydrogen. This step may have two reactions due to the reaction dynamics and nature of different catalysts: adsorbent hydrogen atoms (Hads) at two adjacent catalytic sites combine to generate hydrogen (Tafel step) or adsorbent hydrogen atoms (Hads) bind to a proton in the solution to produce hydrogen (Heyrovsky step). Volmer step: H+ + e- → Hads Heyrovsky step: Hads + H+ + e- → H2 Tafel step: Hads + Hads → H2 Each reaction step has an important impact on the hydrogen evolution process. The reaction mechanism is determined by the Tafel slope. C25-M displays a smaller Tafel slope (82 mV/dec) and the Tafel slope of this electrocatalyst is between the theoretical values of Volmer step (120 mV/dec) and Heyrovsky step (40 mV/dec), so the reaction mechanism of C25-M is Volmer-Heyrovsky mechanism [17]. In Table 1 , the Tafel slopes of similar sulfur-rich HER catalysts are listed. Compared with similar sulfur-rich HER catalysts, the tafel slope of C25-M is lower than others, which means that proton adsorption is easier than others. Using the Scherrer formula, the calculated crystal size of NiS in reference [9] is 28.6 nm, which is larger than that of C25-M (26.8 nm). This means that a smaller crystal size can expose more active sites, thus to improve the hydrogen evolution reaction performance. The electrocatalytic performance of Mo-Ni3S2/NF [18] and Co-Ni3S2 [19] is enhanced by element doping. And the hydrogen evolution performance of NiSx-3 [20] catalyst is improved by changing the molar ratio of NiS and Ni3S4. We use carbon spheres as support to enable the substantial and effective exposure of the catalytic active material (nickel sulfides). Moreover, the good stability and high electron conduction properties of the carbon spheres promote the hydrogen evolution process, thus to reduce the Tafel slope of C25-M.To better understand the reasons why each electrode material has different catalytic properties, the electrochemical active surface area (ECSA) is appraised by testing the double-layer capacitance (Cdl) of the C25-M electrode and other contrast electrode materials in the non-Faradaic voltage region (−0.043 ~ 0.057 V vs. RHE). The cyclic voltammetry curves at different scanning speeds (20, 40, 60, 80, 100 and 120 mV/s) of C25-M and other contrast data was achieved by applying CV technique. As depicted in Fig. 6c and d, C25-M has the largest electrochemical double-layer capacitance value, which is exceeding the values of carbon spheres and Nickel Sulfides.Electrochemical impedance spectroscopy (EIS) can further reflect the electrode kinetic characteristics of electrocatalysts during catalytic hydrogen evolution. Fig. 6e reveals the Nyquist diagram of different catalytic active materials and the corresponding equivalent fitting circuit. Compared with carbon spheres and Nickel Sulfides, C25-M exhibits a smaller semicircle diameter, indicating that compositing carbon spheres and sulfides could decrease charge transfer resistance. This result illustrates that combing carbon spheres and sulfides together is propitious to the electron conduction between the active sites.For an ideal electrocatalyst, it needs not only a small HER overpotential, but also a good catalytic hydrogen evolution stability. Consequently, we further explore the durability of C25-M (Fig. 6f). The durability of C25-M is tested at a voltage of −0.4 V vs. RHE. After 18 h, the composite material still maintains high hydrogen evolution capacity. Further, we conduct stability tests on the Pt/C catalyst and C25-M (Fig. 7 ). The Chronoamperometric experiment of C25-M and Pt/C is tested at a voltage of −0.04 V vs. RHE. With time increases, the current of Pt/C catalyst and C25-M both decreases. After 18 h, the current of Pt/C drops more than that of C25-M. This means that the C25-M catalyst still maintains higher hydrogen evolution capacity than the commercial catalyst Pt/C. Accordingly, the synthesized catalyst C25-M has a long-range electrochemical catalytic stability.In summary, Ni3S2 and NiS supported on carbon spheres were successfully fabricated via a facile hydrothermal method. The composite has been testified to actualize a current density of 10 mA·cm−2 at a lesser overpotential compared with Nickel Sulfides. The outstanding hydrogen evolution capacity of this composite is ascribed to the well-distributed dispersion of Ni3S2 and NiS nanoparticles by carbon spheres, which effectively promotes the exposure of hydrogen active sites. Moreover, carbon spheres support and immobilize Ni3S2 and NiS nanoparticles, which effectively alleviates the migration and aggregation of active substances during hydrogen evolution. These results demonstrate its potential as an electrocatalyst for hydrogen evolution and we are convinced of the fact that this experimentation will open up a way to rationally design electrocatalyst in the future. Tong Gao: Writing - original draft. Ming Nie: Writing - review & editing. Jin Luo: Writing - review & editing. Zhi Huang: Data curation. Hai Sun: Conceptualization. Peitao Guo: Data curation. Zhenhong Xue: Visualization. Jianming Liao: Investigation. Qing Li: Supervision, Validation. Liumei Teng: Data curation.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 study is supported by Chongqing Municipal Training Program of Innovation and Entrepreneurship for Undergraduates (Project No: S202010635145), Zeng Sumin Scientific Research Program (Project No: zsm20190629), Fundamental Research Funds for the Central Universities (XDJK2020B004), Chongqing Key Laboratory for Advanced Materials & Technologies of Clean Energies (JJNY202002), Chongqing Graduate Student Research Innovation Project (CYS19106) and Chongqing University Key Laboratory of Micro/Nano Materials Engineering and Technology (KFJJ2015, KFJJ2009).

Ni3S2 and NiS supported on carbon spheres are successfully synthesized by a facile hydrothermal method. And then a series of physical characterizations included XRD (X-ray diffraction), EDS (energy dispersive spectroscopy), FESEM (field emission scanning electron microscopy) and XPS (X-ray photo-electron spectroscopy) were used to analyze the samples. XRD was used to confirm that Ni3S2 and NiS were successfully fabricated. FESEM indicated that Ni3S2 and NiS disperse well on carbon spheres. Electrochemical tests showed that nickel sulfides supported by carbon spheres exhibited excellent hydrogen evolution performance. The excellent catalytic activity is attributed to the synergistic effect of carbon spheres and transition metal sulfides, of which the carbon spheres act to enhance the electrical conductivity and the dispersion of Ni3S2 and NiS, thus providing more active sites for the hydrogen evolution reaction.

No data was used for the research described in the article.In order to understand the processes occurring on the surface of adsorbents and catalysts and thus be able to modify and optimize them, we need to study their surface properties. In the case of supported metal catalysts, the metal surface must be exposed to these processes, in other words the metal must be dispersed in order to be accessible. This process can be performed during the early stages of the material preparation method [1,2]. Therefore, being able to analyze and characterize a metal that is dispersed in small particles on a surface is of vital importance. In general, the surface structure of a metallic crystal varies greatly depending on the size of the particles and, especially, on how they are found on the surface of a support and their size distribution. Likewise, the surface of the support may affect the properties of the particles, making them different to those of isolated particles. Similarly, the acid–base properties of catalytic supports and catalysts are also important as they may affect catalyst preparation [1,2], and play a role in the mechanism of the reactions [1]. In order to understand the catalytic and adsorption behavior, evaluating the properties of such materials is essential.The dispersion of a supported metal is defined as the fraction of metal atoms found on its surface and can be related to the number of metal centers accessible to reactants and products [1]. Understanding this dispersion is crucial to interpret the kinetic data of a catalytic reaction and to compare catalysts of the same family; the Turn Over Frequency (TOF), defined as the number of molecules reacting per active site per second or the number of molecules reacting per surface metal atom per second, is key for this purpose. As such, several techniques have been proposed and used to measure the TOF, including transmission electron microscopy and X-ray diffraction; the most widely used, however, is surface and selective gas adsorption [2–5]. Chemisorption methods are particularly important for highly dispersed catalysts given the difficulty in estimating their dispersion using other techniques, such as X-ray diffraction or electron microscopy. The most frequently used gases in chemisorption are H2, CO, O2 and even N2O. Other gases are used in specific cases, such as NO, H2S, CS2, C6H6, etc. Several organizations (e.g., American Society for Testing and Materials - ASTM; International Union of Pure and Applied Chemistry-IUPAC) have provided guidance for analyzing the surface of metals, generally recommending hydrogen as an adsorbent [6]. Hydrogen has the advantage of being mainly chemisorbed on the metallic part of the surface, and the amount retained on the non-metallic part is relatively small and weak in many cases. Hydrogen is physically adsorbed on metallic and non-metallic parts, but when measuring at room temperature and pressure, the contribution of the physically adsorbed layer can be neglected due to the very small adsorption enthalpy of hydrogen (less than 8 kJ/mol). However, an exception may be necessary for activated carbon and MOFs as supports, especially when this material has a high specific surface area. Indeed, these materials, which can have a surface area of 5000 m2/g and sometimes more, have been used in H2 storage [7]. These values can be corrected by measuring the physical adsorption of hydrogen on the metal-free support separately. CO adsorption also allows the determination of metallic surfaces, since there is a specific reaction between the gas molecule and the metal. The main drawback of CO chemisorption is determining the CO/metal stoichiometry, since the CO molecule can form various types of bonds with the metal, as well as polynuclear complexes [8]. Transmission electron microscopy, in turn, has the great advantage of providing a direct view of the particles to be analyzed. This analysis allows the size distribution and all its characteristic parameters to be obtained, and shows whether the particles formed are large, what shape they have, and it is even possible to determine their crystal structure. In the case of X-ray diffraction, measurement of the size of metal crystallites is based on the presence of diffraction lines provided the particles are sufficiently small. Quantitatively, it can be determined using the Scherrer equation, which relates the average diameter of the crystals to the broadening of the diffraction peaks. The disadvantage of this technique is that it can only be applied to samples presenting a diffraction line, which means that it cannot be used for catalysts with a very low metal loading (less than 1 % by weight, although this value depends on the metal). Crystalline supports or supports presenting diffraction lines can interfere with the determination of the diffraction lines. In general, glass particles with a size of between 5 and 50 nm can be determined. In conclusion, most authors/researchers indicate that selective gas chemisorption is the best method for characterizing/determining the active surface of a metal catalyst, due to several reasons, especially to its easy accessibility, however, other techniques may be necessary to corroborate the presence of very large crystal materials.Acid catalysts are very important in alkylation, dealkylation, cracking, hydrocracking, isomerization and reforming reactions [1], all of which are used in petroleum refinery processes. Two types of acid centers can be distinguished: Lewis and Brönsted. Lewis centers accept a pair of electrons from the adsorbed species to form a coordination bond between the adsorbed molecule and the solid surface. Brönsted centers however, provide a proton to the adsorbed molecule to form an ion-dipole interaction between the adsorbed species and the solid surface. Both types of centers are found in alumina, for example [1], where aluminum acts as the Lewis acid center and the OH groups on the surface as the Brönsted centers. Characterization of surface acidity is normally performed using techniques such as infrared spectroscopy, NMR spectroscopy, thermal analysis and titration with basic molecules such as ammonia or pyridine (C5H5N), although numerous studies have rather focused on temperature-programmed desorption (TPD). Base catalysis is less widespread than acid catalysis—as it provides lower yields—although it is more selective than the latter. Lewis bases act as an electron donor and can be related to the surface lattice oxygen, O2-. Pyrrole (C4H5N), deuterated chloroform (CDCl3), H2S and CO2 have been suggested as probe molecules [9,10]. Other applications in which the surface nature of the materials may also be important—such as the use of silicates as anticorrosive agents, preventing the deterioration of washing machines, zeolitic materials as ion exchangers, activated carbons with a hydrophilic character applied as adsorbents—are also worth mentioning.A several of factors that can affect gas chemisorption measurements have been reported, for instance the presence of impurities in the catalysts, surface reconstruction due to sintering during adsorption, the nature of metal/support interactions, spillover of the H2 molecule, and even the contamination of gases used in chemisorption [11,12]. In catalytic supports such as TiO2, V2O3, CeO2—which exhibit a high degree of reactivity with metals—supported metal particles and other metal species originating from the “strong metal–support interaction” (SMSI) can occur; this phenomenon reduces the adsorption capacity of the metal [13]. The SMSI state enables the metallic particles to present reversible characteristics, that is, reduction of the particles at low temperatures allows them to be in their metallic state, while the same process at high temperature favors the SMSI state. This state causes the support to develop semiconductor and even metallic properties that differ from those presented initially. Several models have been proposed to explain the SMSI phenomenon. One of these is the formation of metallic alloys, and another proposes that the reduced species on the support can present high mobility and are capable of coating the metallic particles, thereby blocking their adsorption capacity [13]. In the case of the spillover phenomenon, this involves the transport of active species adsorbed or formed in a first phase (structure) to another in which they are not generated directly under the same conditions. The most common example is hydrogen adsorbed from the gas phase onto a metal (Pt, Pd, Ni, etc.), where it dissociates into atomic hydrogen. The dissociated hydrogen can subsequently be transported to the support. This phenomenon causes more hydrogen to be adsorbed than is necessary in the chemisorption process, thereby interfering with monolayer volume determinations. Thus, concludes into erroneous dispersion results.The techniques and procedures presented below are often routine in many laboratories, since they allow the evaluation and determination of the surface properties of materials through chemisorption processes. The aim of this work is to review them and include the updates published by several researchers, who mostly aim to explain the results of bifunctional metallic and acid–base catalytic behavior.Two phenomena can be observed in the adsorption process: physisorption and chemisorption [14]. In general, differentiating between these two processes is not easy, especially since intermediate behaviors can occur. Interactions between the adsorbent surface and the adsorbate are generally relatively weak via coulombic and dispersion forces, although defection at the atomic level or atoms with the availability to form bonds may be present on the surface. In such a case, chemical bonds can be formed and the process is known as chemisorption. This process often occurs at temperatures higher than the critical temperature of the adsorbate. Chemical adsorption is often irreversible, at least under mild conditions, and is characterized by large interaction potentials that lead to high adsorption heats, although this factor is not the only aspect that differentiates physisorption from chemisorption.Physical and chemical adsorption are usually characterized by the following properties:In physical adsorption, the gas molecules interact with the solid surface via van der Waals-type forces. This type of interaction determines the characteristics of the adsorption: – physical adsorption involves a weak interaction between gas molecules and the surface of the solid. As such, no surface modifications occur during adsorption measurements; – physical adsorption is an exothermic process: the interaction forces are attractive, and the heat released is similar to the enthalpies of condensation of the adsorbed substance (20–40 kJ/mol). As this process is exothermic, physisorption increases with decreasing the adsorbent temperature or increasing the adsorbate pressure; – the physisorbed molecule maintains its identity, since the energy is insufficient to break the bond, although its geometry can be distorted. – physisorption is a non-specific process, since the forces involved are not specific either. Molecules do not usually interact with specific adsorption centers. – physisorption occurs in multilayers, meaning that another layer can be adsorbed on top of a layer of adsorbed molecules. The first adsorbed layer is formed by direct interaction with the surface, while the successive ones are interactions between molecules, like the condensation process. However, the difference between these two processes is not so clear and we often find intermediate situations, especially when the chemisorption process is weak. physical adsorption involves a weak interaction between gas molecules and the surface of the solid. As such, no surface modifications occur during adsorption measurements;physical adsorption is an exothermic process: the interaction forces are attractive, and the heat released is similar to the enthalpies of condensation of the adsorbed substance (20–40 kJ/mol). As this process is exothermic, physisorption increases with decreasing the adsorbent temperature or increasing the adsorbate pressure;the physisorbed molecule maintains its identity, since the energy is insufficient to break the bond, although its geometry can be distorted.physisorption is a non-specific process, since the forces involved are not specific either. Molecules do not usually interact with specific adsorption centers.physisorption occurs in multilayers, meaning that another layer can be adsorbed on top of a layer of adsorbed molecules. The first adsorbed layer is formed by direct interaction with the surface, while the successive ones are interactions between molecules, like the condensation process. However, the difference between these two processes is not so clear and we often find intermediate situations, especially when the chemisorption process is weak.In chemical adsorption, the gas molecules interact with the solid surface through chemical bonds. Similarly, to physical adsorption, this type of strong interaction conditions the characteristics of the adsorption: – in chemical adsorption, the interaction forces are attractive, and the heats released are similar to the enthalpies of formation of a chemical bond (100–500 kJ/mol). In chemisorption, both bond formation and bond breakage can occur, so the values of these enthalpies can be both positive and negative. – as there is a strong interaction—bond formation—between the molecule and the adsorption center, chemical adsorption is only defined in a monolayer. In the rest of the layers, physical adsorption may occur. – if a chemical bond is formed, the chemisorbed molecule does not maintain the same structure as in the gas phase. – chemisorption is specific. There are certain centers on the surface of the solid at which interaction occurs whereas at others it does not. in chemical adsorption, the interaction forces are attractive, and the heats released are similar to the enthalpies of formation of a chemical bond (100–500 kJ/mol). In chemisorption, both bond formation and bond breakage can occur, so the values of these enthalpies can be both positive and negative.as there is a strong interaction—bond formation—between the molecule and the adsorption center, chemical adsorption is only defined in a monolayer. In the rest of the layers, physical adsorption may occur.if a chemical bond is formed, the chemisorbed molecule does not maintain the same structure as in the gas phase.chemisorption is specific. There are certain centers on the surface of the solid at which interaction occurs whereas at others it does not.The two adsorption processes (physical and chemical) can be illustrated by representing the evolution of the potential energy of a gaseous diatomic molecule in the vicinity of a surface, where attractive and repulsive forces may appear ( Fig. 1) [15]. This figure includes the option of diatomic adsorption or bond cleavage and atomic adsorption. If adsorption occurs, the potential energy decreases, thus implying that the concentration of the gas will be higher on the surface than inside the gas, due to the adsorption phenomenon. In this situation, if the gas molecule is very close to the surface of the adsorbent, the potential increases again because of the repulsion effect. The figure illustrates how molecule B2 approaches the surface of a material at a distance r. The first interaction process is physical adsorption of the molecule on the surface of the solid. The equilibrium situation is represented by the potential minimum or adsorption potential well, which is characterized by a negative energy value (exothermic process). Below is an endothermic process in which an energy E must be overcome. If this energy value is exceeded, a new equilibrium situation can be reached but, in this situation, dissociation of the diatomic molecule occurs. Each of these situations depends on the adsorbate/adsorbent system and the temperature of the adsorption process. Three situations are represented in the figure. In the first (a), the molecule is more strongly adsorbed (its equilibrium state has a lower energy) than in the dissociated state. This the preferred form of adsorption and could represent physical adsorption. In the second case (b), the dissociated situation has a higher adsorption energy than the diatomic molecule, but there is an energy barrier to overcome. If this barrier is high enough, we would have the first case. Finally, the third case (c) is similar to the previous one, but with a very low energy barrier, so dissociated adsorption normally occurs.The bond between the chemisorbed molecule and the adsorption center is often very energetic, even though the net heat of adsorption may be low. The requirement to overcome an activation energy in chemisorption explains the low heat of adsorption and also why such a phenomenon can be relatively slow. Since chemisorption is often an activated process, the net heat of adsorption is small at low temperatures and large at high temperatures. This situation means that physisorption predominates at low temperatures and chemisorption at higher temperatures.In contrast to measurement of the specific surface area, the surface area of a catalyst component, usually the metal surface, can be measured using selective adsorption (chemisorption). The principle of selective surface area measurement by chemisorption is similar to specific surface area measurement by physisorption. As such, it will be necessary to make a series of assumptions: the metallic surface is free from other adsorbates such as carbon or other poisons that prevent or affect the gas–solid interaction; metal atoms must be in its normal metal state (normally zero) that allows interaction; and the stoichiometry of the interaction must be known and be independent of the size of the metal crystal [11,12]. As such, preparation and pretreatment of the sample have to be more rigorous than when characterization is carried out by physisorption. Intrinsic to each of the techniques to be used, the kinetics and strength of the adsorption are important aspects that must be evaluated. The techniques that allow the chemical adsorption process to be analyzed and, therefore, the active sites (acid-basic and metallic adsorption centers) to be characterized, can be divided into three categories: volumetric static, gravimetric, and dynamic flow methods (isothermal or programmed temperature) [11,12].Several studies have been performed by static volumetric and the unit descriptions have been published by various authors [11,12]. The materials to be characterized can present a wide distribution of active centers, either in terms of acid–base strength or metal particle sizes. These characteristics call for a technique that allows this analysis, thus meaning that the adsorbate gas must be added in small quantities in a controlled manner. Under these conditions, a technique such as the static volumetric method can guarantee low-pressure dosing of adsorbate gas, thus, could identify the different adsorption layers. The chemisorption isotherm is described as the variation in the amount of gas adsorbed as a function of pressure at equilibrium while maintaining the sample at a constant temperature. In a previous step, the surface of the sample must be cleaned with a vacuum; in many cases, pre-treatment with a cleaning gas current is preferable or even—if the chemisorption is to be conducted on a metal catalyst—reduction of the oxides so that it is in the form of a metal as this is the sensitive phase for the adsorption of gases such as H2, CO, etc. Chemisorption isotherms are expressed in terms of amount adsorbed at normal conditions (NTP) versus absolute pressure, rather than amount adsorbed versus relative pressure, as in the case of physical adsorption. The static volumetric technique generally produces an experimental adsorption isotherm similar to that shown in Fig. 2, which involves a combination of physisorption, spillover and chemisorption. Hence, it is not a purely type-I isotherm with an adsorption plateau (constant amount adsorbed) as pressure increases. To differentiate the contribution of chemisorption from that of physisorption, the sample is evacuated after completion of the initial run, thus removing only reversibly adsorbed gas. The analysis is then repeated under the same conditions as the original analysis, except that during the second analysis, the active area of the sample is already saturated with chemisorbed molecules. Some authors have criticized the application of this second isotherm given that a greater amount of gas can be desorbed than the purely thermodynamic one; therefore, identical vacuum conditions to those used in the first isotherm and treatment time of up to 30 min [16]. The adsorbed volume data for the first adsorption isotherm A are a combination of physical and chemical adsorption (reversible and irreversible, respectively). Isotherm B is the result of repeated analysis, where only reversible physisorption occurs. The isotherm represented by the dashed line C is generated mathematically by subtracting the adsorbed volume data for isotherm B from that for isotherm A. The result is the amount of active gas irreversibly absorbed by the sample.As in physisorption, the adsorption isotherm allows qualitative characterization of the material. For quantitative characterization, the volume of the monolayer (V m ) chemisorbed on an active surface is determined. One way to determine this volume is by extending a line tangential to the plateau of the initial adsorption isotherm to the zero pressure axis. This is the procedure proposed in the ASTM D 3908–88 method to determine the amount of H2 adsorbed on a Pt catalyst supported on alumina previously reduced at 450 °C and the adsorption capacity evaluated at 25 °C [6]. The pressure range for the adjustment is between 100 and 300 torr. It has also been proposed to subtract the (reversible) physisorption isotherm from the combined isotherm as described above, and then extend a line tangential to the plateau of that isotherm to the zero pressure axis. Both methods should give approximately the same results, as long as the same analysis conditions are maintained. This value gives the amount adsorbed by weight of adsorbate. In the case of NH3 adsorption, the ASTM D 4824–93 method proposes the adsorbed volume as that obtained at a pressure of 150 torr and a temperature of 175 °C [17] to minimize physisorption of ammonia. Additionally, repeated measurements at various temperatures can be used to calculate heats of adsorption (see next sections for details).In the case of the previous procedure, the surface of the solid to be analyzed must initially be free from any type of substance, that is, an initial heat treatment must be applied to clean the surface. Next, consecutive small amounts of adsorbate are added to allow the adsorption isotherm to be built, which means that a system that allows for the dosed volumes to be measured, as function of the increasing pressure. All this is performed under equilibrium conditions. Another possible procedure involves the solid sample being subjected to a stream of an inert gas to clean the surface and, subsequently, a known volume of the adsorbate gas being injected into this inert stream. This procedure has the following advantages: the measurements are fast compared to volumetric measurements; the weak bonds between adsorbent and adsorbate are not detected; the dead-volume need not to be measured; and the measurement can be easily tuned for small amount of samples. Thus, the adsorption centers can retain the adsorbate gas until the surface is completely covered, that is, until it is saturated (see Fig. 3) [18]. If the adsorption isotherm is previously constructed, then, and, after calibration of the signal, the amount adsorbed will be obtained (V m , see Eq. 1). This case requires a system for detecting adsorbate gas in a gas stream, which is normally achieved by using a thermal conductivity detector (TCD). (1) V m = ∑ i = 1 i = n ( h saturation − h injected ) h saturation · V injected where V m is the volume of the chemisorbed monolayer, expressed in cm3 at standard temperature and pressure (STP), V injected corresponds to the loop volume previously calibrated and its volume is continuously monitored by the system for any temperature and pressure change in order to deliver a corrected number of moles at each injection, h injected is the peak area corresponding to the injected volume. h saturation corresponds to the injected volume that produce same peak area, and indicate saturation or end of the analysis is reached. Some practical advice can be suggested for this method: the relation between the amount of adsorbate gas injected and the sample mass should be adjusted to ensure at least one the injected dose to be completely adsorbed by the sample, the interval of time between the pulses should be constant and long enough to allow for the TCD signal to return to base line, and consecutive pulses should be injected until no increase of the signal area for consecutive pulses can be detected.In the two previous procedures, the working temperature remains constant. However, there is the possibility of repeating the analyses under other temperature conditions, which may allow additional information regarding the heat of adsorption to be obtained, or a method that enables the temperature to be increased to obtain information about the strength of adsorption to be used. This would be the case for the temperature-programmed desorption (TPD) procedure. In this case, upon sample saturation with a specific adsorbate, desorption can be carried under a specific ramping rate. If this analysis is repeated and desorbed at a different ramping rate, say (3, 5, 10, 15 and 20 ℃/min), thus would yield information about the strength of the adsorption centers ( Fig. 4). This is the case for the adsorption of bases such as NH3 or other amine molecules, as well as CO2 [10]. As a gas stream that is in continuous contact with the solid is required, the detector used could also be a TCD. If no re-adsorption of gas takes place during desorption, and provided the molecules are adsorbed on a homogeneous surface without mutual interactions, the maximum temperature peak (T m ) can be related to the activation energy of desorption (E d ), see Equation 2 [19]: (2) 2 ln T m − ln β = E d R · T m + ln E d · V m R · k d where β is the rate of linear temperature increase, V m is the amount adsorbed at saturation, and k d is the pre-exponential factor in the expression for the desorption rate. If the kinetics of desorption are first order, it is possible to calculate E d . In the case of the presence of surface heterogeneities (large surface areas and microporosity), deviations could be found.The acidic or basic nature of the centers cannot be determined by this method, although it is possible to calculate the change in desorption activation energy with surface coating. If it is not possible to measure the amount of base adsorbed, or the amount that remains after desorption, the method can only give qualitative or semi-quantitative information (which can be obtained from the TPD profile).As a chemical bond forms between the adsorbate molecule and a specific center on the material surface, the number of sites can be determined by measuring the amount of chemisorbed gas. Although this may appear to be an easy and simple process, it should be noted that, depending on the nature of the metals and gases concerned and the operating conditions (temperature, pressure, measurement method), chemisorption could be partially reversible. The terms reversibility and irreversibility only have an operational meaning and are more important in the case of dynamic methods. In metallic catalysts, the active center is often a metal atom, with examples of this including nickel and platinum for the hydrogenation of unsaturated carbon-carbon bonds [1]. However, several important metal oxides and other non-metal catalysts must also be considered. As an example, we can cite the case of iron, to which other promoters are added to favor the synthesis of ammonia. The metallic atoms are found forming islands or clusters, rather than being distributed individually, on an inert porous material that acts as a support and favors their dispersion and stability. In several cases, this situation is not clear. The size of these islands and clusters depends on the nature of the metal and the support, as well as the method used to deposit it (preparation method). In such a case, the exposed active centers can be determined by the gas adsorption method. For supported metal oxides, the same gases used in selective chemisorption on metals (H2, CO, O2 and N2O) are not compatible, since they adsorb weakly on these surfaces: CO is only weakly adsorbed on metal oxides, and all exposed surface sites cannot be evaluated; H2 adsorption involves reaction with the surface and subsurface lattice oxygen; and O2/N2O are not adsorbed on oxidized surfaces [8]. Adsorption of H2 and O2 at sub-ambient temperatures has been attempted to avoid the participation of subsurface lattice oxygen and lattice oxygen vacancies, respectively, but was unsuccessful in avoiding the participation of these species [20]. However, small alcohols are adsorbed on dehydrated and/or evacuated oxides and allow the number of active surface sites to be quantitatively and selectively assessed. Thus, methanol is a highly reactive molecule that has been reported to be chemically adsorbed on oxides and allows quantitative determination of the number of surface active (Ns) centers. It has been observed that methanol follows several routes of chemisorption in oxides [21], depending on the nature of the metal oxide, and some of these reactions can occur and allow the quantification of adsorption centers: C H 3 OH + M − OH → M − OC H 3 + H 2 O (M is a metal cation site) C H 3 OH + M − O − S → M − OC H 3 + S − OH (S is the oxide support cation site) by breaking open hetero-bonds C H 3 OH + * → C H 3 OH − * ([*] is a coordinatively unsaturated Lewis acid site).It has been reported that the typical number of active surface sites on oxides is about 0.7 × 1015 sites/cm2, which is about half the value for metals (1.2 ×1015 sites/cm2), because the surface density of sites on oxides is less than on metals. The number of active surface sites on MoO3, V2O5 and ZnO is significantly lower (0.1 ×1015/cm2) due to the presence of much less active exposed surface planes due to the presence of coordinatively saturated sites [22]. Stoichiometry: Knowing the relationship (stoichiometry) between an exposed metal atom and an adsorbate gas molecule is an important factor in this type of determination as many polyatomic gas molecules do not adsorb to a single active site. This is the case, for example, for the hydrogen molecule (H2). It has been reported that hydrogen adsorbs dissociatively, that is, it separates into two atoms, each of which reacts with a single metal atom. Thus, a gas molecule has bound to two metal atoms (this is the case of Pt, Pd, Rh, Ru, Ir and Ni). As such, the stoichiometry is said to be two (2) for this surface reaction. Similarly, a molecule of adsorbate gas could associate with more than one metal atom without dissociating. This is the case for carbon monoxide (CO), which is normally expected to bind in a one-to-one ratio (Me-CO) but could form a bridge between two metal atoms (Me-(CO)-Me). This situation would also result in a stoichiometry of two. Cases in which an excess of adsorption would result in a stoichiometry of less than one are not implausible. This is the case for the formation of hydrides (for hydrogen) and carbonyls (for carbon monoxide). These latter situations should be controlled and avoided in whatever way possible. In the case of O2 the O-Metal stoichiometry is 1.0, although the possible formation of metal oxides and bulk metal oxides may modify this relation.The value of the stoichiometric factor X m can be determined, for example, by chemisorption measurements using metal powders with known specific surface areas. In general, the number of atoms per unit area for polycrystalline metal surfaces is not known. For hydrogen chemisorption up to full coverage, X m , the average number of surface metal atoms associated with the adsorption of an adsorbed hydrogen molecule is assumed to be 2. However, some uncertainty also exists in this regard. Fundamental studies on hydrogen chemisorption on Ni yield solid evidence that strongly chemisorbed hydrogen atoms are attached to, or just below, so-called C8 sites, which are the holes formed by a cluster of three densely packed Ni atoms above an octahedral interstice. The number of C8 sites is equal to the number of Ni atoms in the (111) plane and thus, for the (111) plane of free metals, X m = 2 is a realistic choice. The total adsorbate uptake, n m , is also subject to uncertainties. In many group VIII metals (Ni, Pt, Pd, Ru, Rh), the H2 chemisorption isotherm has the form shown in Fig. 2. The highest amount of hydrogen is adsorbed (strong chemisorption) at a pressure of less than 133.22 Pa (1 torr). Above that pressure, weakly chemisorbed hydrogen adsorption occurs, mostly of the order of 20–25 % of the strongly retained monolayer. The difficulty is that the transition pressure between strongly chemisorbed hydrogen and weakly chemisorbed hydrogen is not clearly defined. As such, the X m value of 2 refers to strongly chemisorbed hydrogen only.Several factors affect the accuracy of chemisorption methods. These include factors associated with the stoichiometric factor, the crystallographic heterogeneity of the surface, the presence of a support that theoretically does not chemisorb, the possible absorption or dissolution of the adsorbate gas in the metal, reconstruction of the surface atoms during the process of chemisorption, as well as contaminants adsorbed on the surface. The stoichiometric factor is usually not a problem when H2 is used as the adsorbate, since it generally dissociates by adsorbing on catalytically important transition metals and chemisorbs with a stoichiometric factor of 2 (based on the H2 molecule). For the other gases mentioned, obtaining an exact and constant stoichiometric factor may be difficult as adsorption of such molecules will highly depend on the surface of the adsorbent. Thus, for example, in the adsorption of CO on Pd/SiO2, if analyzed by IR, two adsorption bands that correspond to the Pd-(CO)-Pd bridge and to the linear form PdCO are observed. For particle sizes less than 10 nm, the geometry of the surface and, therefore, the stoichiometric factor depend on the size of the particle. Thus, in supported Pt catalysts, for small particles, the stoichiometric factor for CO adsorption can vary between 1 and 2. For metal particles larger than 10 nm, this effect disappears, and it can be considered constant.From an experimental point of view, the amount of gas adsorbed is measured. Therefore, it is essential to establish the stoichiometry involved, knowing the nature of the adsorbate gas and the active site. This information can be obtained from the literature on catalysts or by direct measurement (see Table 1) [1]. Monolayer coverage: Once the amount of gas adsorbed by the sample (the adsorption isotherm) has been determined, the number of active centers can be calculated from the capacity of the monolayer, V m . A number of graphical and numerical methods can be applied for that purpose, and the most widely used are described below. In the case of the volumetric dynamic procedure, the adsorbed volume (V m ) would be obtained directly (see Eq. 1). Extrapolation. This method involves plotting points on the adsorption isotherm until the plateau is reached (e.g, ASTM method D 3908–88 for H2 adsorption on Pt/Al2O3 catalyst) [6]. In this region, the surface has become saturated with the adsorbate and monolayer formation has been ensured. If the pressure and the amount of gas dosed are increased, only additional physical adsorption occurs. The contribution of this physisorption can be explained by assuming that it is zero at zero pressure. If the line joining the points of the plateau is extrapolated to the value of zero pressure (intercept with the OY axis, the value of the monolayer is obtained. This value of V m represents the total amount of chemisorbed gas irrespective of the exact nature of the bonding type (strong or weak; see Fig. 2). Irreversible isotherm: Some applications require that only strong chemisorption centers be determined and physisorption or weaker chemisorption centers excluded. In these cases, it is necessary to obtain a second adsorption isotherm. After acquisition of the first isotherm, the sample is evacuated at the analysis temperature to desorb loosely bound gas molecules. Strongly adsorbed molecules remain bound to active centers on the sample surface. A second adsorption analysis is repeated to produce a second isotherm that would provide information on weak chemisorption and physisorption and is obtained in the same way as the first. The difference between the two isotherms at any given pressure represents the amount of chemisorbed gas. Alternatively, the plateau of the irreversible isotherm can be extrapolated to zero pressure to determine V m graphically (see Fig. 2).The above methods try to describe a simple (or pure) chemisorption process, although in some cases the interference of the catalytic support can be considered as it may have its own adsorption centers that can interfere with the process. This may be the case, for example, for the so-called spillover process in which the hydrogen that is dissociatively adsorbed on the metal (normally Pt) migrates to the surface and the bulk of the support. In cases where there is spillover (or at least there may be), two isotherms must be measured to determine the adsorption capacity: one for the supported metallic catalyst and the other for the support only (normally is known as blank), without the active metallic phase. The first isotherm yields adsorption data consisting of strong chemisorption at the active sites, weaker chemisorption, physisorption at the active sites and on the exposed support surface, plus active site spillover. The second isotherm simply consists of physisorption on the support. The net amount of chemisorption, including spillover, can be easily calculated by subtracting the second data set from the first.If the stoichiometric factor of chemisorption is known, it is possible to calculate the accessible number of surface atoms (N S ) of the component (generally metal) from the amount of adsorbed gas using Eq. 3: (3) N s = V m · N A · X m V mol where V m is the volume of the chemisorbed monolayer, expressed in cm3 at standard temperature and pressure (STP); V mol is the molar volume of adsorbate (22414 cm3 occupied by one mol of gas at STP); N A is Avogadro’s number (6.022 ×1023); and X m is the average stoichiometric factor. X m indicates the number of surface atoms of the component that are covered by an adsorbate molecule after chemisorption.In many cases, the small metallic crystallites are firmly attached to the support via chemical bonds. As a result, the distribution of the crystallographic planes on the surface is, in most cases, different to the equilibrium distribution that would correspond to a free particle. Therefore, the value of N S is strongly affected by support/particle interactions. The presence of the SMSI (strong metal/support interaction) effect can even completely suppress any form of hydrogen chemisorption. In this case, there would be no metallic species on the surface sensitive to chemisorption and therefore this cannot be evaluated.The specific metallic surface area, A m , is determined as the product of the number of exposed metal atoms, N S , by the cross-sectional area of each atom (see Table 2), A X , and per unit mass, W (see Eq. 4): (4) A m = N s · A X W It can also be expressed per gram of metal in the catalyst if the experimentally determined metal content (%) is included (see Eq. 5). (5) A m = N s · A X W · % metal 100 Another factor to consider when calculating the metallic surface (m2 of metal/g) from chemisorption measurements is a lack of information about the heterogeneity of the crystallographic surface of the dispersed metal particles. In such a case, the number of accessible metal atoms on the surface can be calculated using Eq. 3. However, the calculation of the metallic surface requires information about the number of atoms per unit surface. This value is clearly defined in the ideal plane of a single crystal, but not for the case of metallic particles with surfaces exposing several crystallographic planes. To avoid this difficulty, the three most prominent planes—(111), (100) and (110) for cubic face-centered and (110), (100) and (211) for cubic body-centered—are generally considered to be present in equal numbers. As such, the number of atoms per m2 of surface for face-centered metals (Ni, Pd) is 1.91 × 1018/ a 2 and (Fe, W) is 1.35 × 1018/ a 2 for body-centered metals, where a is the lattice constant. The specific metal surface of a supported metal catalyst can be calculated using Eqs. 4 or 5, where N s is the number of accessible atoms on the metal surface per gram of catalyst.In the case of supported metal catalysts, it is important to know what fraction of the active metal atoms is exposed and available to catalyze a reaction. This is a surface phenomenon as the atoms inside the metal particles do not participate in surface reactions. Hence, these atoms must be dispersed as widely as possible. Dispersion is defined as the percentage of all metal atoms in the sample that are exposed at the surface. As the total amount of metal in the sample can be determined by chemical analysis of the sample, if the weight of the metal in the catalyst is known, the degree of dispersion D(%), that is, the ratio of atoms on the surface (N S ) with respect to the total number of atoms (N T , atoms on the surface and in volume), of the metal can be calculated (see Eq. 6). (6) D % = N s N T = V m · X m · M á tomoMetal V mol · % metal 100 Logically, if gas-adsorption techniques are used, the atoms on the surface will be those that can be evaluated by chemisorption, and it is precisely those atoms that can participate in gas-solid reactions. This property is important since it can affect both the selectivity and catalytic performance in supported metal catalysts.If both the mass of metal in the catalyst and its density are known, the volume of metal can be estimated. If the metallic surface area (A m ) is already known, the equivalent particle diameter, d, can be estimated by assuming a shape factor for the particle (see Eq. 7). (7) d = 6 A m · ρ metal · ( % r eduction ) This diameter is assumed to correspond to a hemisphere in contact with the surface of the catalytic support. The geometric factor (in this case 6) is identical if it is a totally spherical geometry. These two geometries have been reported as the most frequent for supported metal catalysts.Metal–support interactions and metal particle shape play an important role in determining particle size by gas chemisorption. A hemispherical shape is usually assumed, but can give misleading results of up to one order of magnitude. In such a case, the metal particle sizes are underestimated when the metal strongly interacts with the support and overestimated when there is a weak metal–support interaction. The assumption of spherical shapes always underestimates the size of the particles, with this error being considerably smaller with regular geometries than that associated with the effect of the metal–support interaction due to its effect on the shape of the particle. Therefore, some authors have introduced a particle–support interaction factor when determining particle size by chemisorption.As indicated in the Introduction, clusters and particles have unique chemical and physical properties that depend largely on their size. In the case of heterogeneous catalysis, a relationship between the size of the metal particle and its performance and selectivity for multiple systems is acknowledged, and the particle size can even determine whether or not a system is active.High-resolution transmission electron microscopy (TEM) provides qualitative and semi-quantitative information on the size and shape distribution of metal particles, as well as their dispersion in the support. In this technique, the contrast depends on the ratio of the atomic numbers of the metal and the support, with small particles having a lower contrast than large ones. Particles with diameters smaller than 1–1.5 nm are considerably more difficult to detect, thereby limiting accurate quantification of the particle-size distribution. Although these instrumental limitations have been resolved in recent years to be able to quantify particle sizes on the sub-nanometric scale, this technique is still not commonly used because of its low availably. It is also possible to obtain information using X-ray diffraction (XRD), in this case regarding the crystal size from the broadening of the diffraction line. As in the case of electron microscopy, limitations appear for the smallest particles and for those that do not exhibit crystallinity. Gas chemisorption, typically using H2 and CO as probe molecules, is widely used in combination with TEM and XRD to quantify the particle-size distribution, or alone to estimate the metallic surface area accessible to the molecule probe. As reported previously, this technique consists of measuring the number of probe molecules adsorbed on the metallic surface of a material. Knowledge of the stoichiometric factor for the number of adsorbed probe molecules per metal surface atom allows the metal surface area, mean particle size and metal dispersion to be calculated. It is widely accepted that one of the main limitations of gas chemisorption as a particle-size determination technique is the precise determination of the aforementioned stoichiometric factor, which largely depends on the arrangement of the surface atoms. Indeed, the probe molecule can form linear, double or triple adduct bridges, therefore its value ranges between 0.5 and 2 for a given metal. It has been reported that the effect of the interaction of the metal and a support (the contact angle between the two) on the determination of the resulting average particle size may be greater than the effect of the stoichiometric factor due to the conventional assumption of the hemispherical shape of the particle.A well-accepted fact in the field of heterogeneous catalysis is that the method of metal deposition affects not only the resulting particle size and distribution, but also the metal–support interaction. For example, the deposition-precipitation method generally produces hemispherical metal particles in which the flat planes of the metal are attached to the support, while impregnation methods produce spherical particles with very weak interactions with the support. The type of metal–support interaction (strong or weak) can have a key effect on the catalytic behavior. It has also been possible to demonstrate, by means of high-angle annular dark field (HAADF) images taken in a STEM, that when interaction with the support is very strong, the morphology of the particles can be more similar to two-dimensional plates rather than three-dimensional particles. Thus, the conventional assumption of metal particles with a hemispherical geometry for the calculation of average metal sizes by gas adsorption characterization can give misleading results if the metal particle is not hemispherical in shape. In fact, the metal–support interaction and, consequently, the resulting metal–support contact angle must be taken into account for an accurate estimate of the mean metal size. Particle sizes are slightly overestimated when their contact angle is > 90 ° (low interaction with the support); however, particle sizes are greatly underestimated when their contact angle is < 90° (high interaction with the support) (see Fig. 5) [23].When selecting the adsorbent gas to be used when using chemisorption measurements as part of the experimental method, it should be taken into account that the stoichiometric relationship that allows the quantity of metallic atoms on the surface to be determined should be known, thereby preventing the support from being able to adsorb or interact with the adsorbent gas [3,5]. Therefore, an initial study, including the operating conditions, is required for each metal to be analyzed to determine the most suitable conditions and adsorbents in order to determine the metallic atoms on the surface. Pt catalysts: supported platinum catalysts, and how their dispersion is measured, are perhaps the most widely studied systems due to their widespread applications. The adsorption of hydrogen on Pt has been studied by several authors, who found that it is dissociative, that is, the H2 molecule breaks and each atom binds to a different Pt atom. Adsorption is normally carried out at temperatures of between 0 and 35 °C. To try to clarify how H2 is retained on the surface of Pt catalysts, these authors have conducted studies of hydrogen desorption at programmed temperature and found the presence of up to four states: a) hydrogen weakly adsorbed in a non-dissociative manner (–73 °C); b) hydrogen atoms adsorbed on the surface Pt atoms (130 °C); c) reversibly adsorbed hydrogen (180 °C); d) hydrogen spillover (480 °C; see Fig. 6) [11]. Of these states, it appears that option b) may have the highest possibility of being related to chemisorption. The possible contributions of the other states would cause errors in the determination of the amounts adsorbed. The stoichiometry accepted by most authors working with Pt catalysts is H2:Pt = 1:2, although deviations from this stoichiometry may exist in the case of highly dispersed catalysts.CO chemisorption has also been used in the characterization of Pt catalysts [24–26]. The main problems in this case are: a) the possibility of CO chemisorption in a linear (Pt-CO) or bridged (Pt-CO-Pt) form and, b) the possibility of formation of volatile carbonyls, and even other forms of triple bonds and dissociated molecules have been described [27–29]. The fact that one form or another predominates can cause the stoichiometry to be 1 or 2. The problem worsens because the relative proportion of these two forms depends on the particle size (the linear form predominates in high dispersions and the dotted form for particle sizes above 5 nm [30]). In general, it is considered that the two forms predominate, therefore a CO:Pt = 1:1.15 ratio is normally used. This situation is more common in the case of metallic catalysts containing Ni, Co, Ru, Mo, W, etc.If the results obtained upon the adsorption of CO and H2 on Pt are compared, the additional H2 consumption observed can be explained by a spillover effect, which increases at high dispersions in which the metal–support interfaces increase. There may also be differences between the two measurements if the Pt is not fully reduced, in which case CO is adsorbed rather than H2.One alternative that has been proposed to increase the sensitivity to H2 adsorption is H2-O2 titration reactions [31]. This method was proposed based on the chemisorption of H2 and O2 on Pt atoms on the surface, as well as on the reaction of H2 with oxygen chemisorbed on Pt, and on the reaction of O2 with hydrogen chemisorbed on Pt [32,33]. All these reactions are carried out at room temperature: Pt + 1 2 H 2 → Pt − H , hydrogen chemisorption ( HC ) Pt + 1 2 O 2 → Pt − O , oxygen chemisorption ( OC ) Pt − O + 3 2 H 2 → Pt − H + H 2 O , hydrogen titration of oxygen covered surface ⁢ ( HT ) 2 Pt − H + 3 2 O 2 → 2 Pt − O + H 2 O , oxygen titration of hydrogen covered surface ( OT ) An HC:OC:HT:OT stoichiometry of 1:1:3:3 was initially proposed and the sensitivity of H2-O2 titration was found to be three times greater than for direct H2 or O2 chemisorption. More recently, some authors indicated that the results of the titration depend on pretreatment of the catalyst and on the titration procedure [34,35]. Pd catalysts: As in the case of Pt catalysts, CO adsorption can be used to characterize the metal surface of these catalysts [24–26]. The linear bond usually predominates, although it is necessary to control the conditions to ensure that this is the case [36–38]. Nevertheless, an average value of close to 2 was found for any support with dispersed Pd, although the measurements were performed using a pulse flow technique [39]. If hydrogen chemisorption is used to measure Pd dispersion, hydrogen absorption must be avoided. Thus, for example, exposure of supported Pd to a hydrogen atmosphere at room temperature results in the formation of β-Pd-Hx, where x decreases as Pd dispersion increases [40,41]. Starting from a 30 % dispersion, and heating above 70 °C, the absorption of hydrogen decreases considerably. Despite the absorption of hydrogen, the H:Pd ratio is considered to be 1:1 [42–45].In catalysts of this family, the H2/O2 (or O2/H2) titration sequence has also been used as this technique has the main advantage that it allows the amount of adsorbed gas to be increased in catalysts with low dispersion. The reactions in this case would be [35]: 2 Pd − H + 1.5 O 2 → 2 Pd − O + H 2 O Pd − O + 1.5 H 2 → Pd − H + H 2 O Rh catalysts: in catalysts of this type, it has been reported that the CO:Rh stoichiometry can be 2:1, 1:1 and 1:2 [46], and even 1:3. In the case of supported Rh catalysts, 1:1 or 1:2 is proposed. If H2 adsorption is used, it has been proposed that there is a 1:1 ratio, which is confirmed for low dispersions. The stoichiometry is 2:1 in the case of high dispersions [47–51]. Ni catalysts: the first drawback that can occur in this family of catalysts is that Ni is not completely reduced to the metal [52]. Although it can be assumed for the previous catalysts that all the metal is reduced, in the case of Ni catalysts this may not be the case [53]. A non-reduced phase can be found between the support and the reduced metal particle, therefore this effect should be taken into account when calculating the metal dispersion [54]. In the case of some supports, such as alumina, this phase can be incorporated into the support by the formation of a spinel [55].The formation of up to four Ni(CO)x complexes has been described, with the stoichiometry depending on the degree of dispersion and the adsorption temperature; therefore, the use of CO is not recommended when characterizing Ni catalysts [53]. The best method to characterize Ni-containing catalysts is the chemisorption of H2 at temperatures of between 0 and 35 °C and at a pressure of up to 10–20 kPa. The stoichiometric factor in this case is 2 [53,56,57]. Cu catalysts: for this type of catalyst, H2 chemisorption is not a good option due to its low sensitivity at low temperature. CO chemisorption cannot be used either, since it can be confused with physical adsorption. Alternatively, the adsorption of O2 at −136 °C has been proposed. Under these conditions, the process is not activated and the stoichiometric factor is 4. However, the main drawback involves reaching the adsorption temperature. As an alternative, the adsorptive decomposition of N2O at 90 °C is proposed: N 2 O ( gas ) + 2 Cu → Cu 2 O + N 2 ( gas ) + ( E x c e s s o f N 2 O t h a t ⁢ h a s ⁢ t o ⁢ b e ⁢ t r a p p e d ) As the pressure remains constant during the process, nitrogen can be measured by assuming one N2 molecule per two Cu atoms on the surface [58,59]. This method is also proposed for Ag and Ru. Although this method is rather difficult to be determined by the dynamic technique due to the fact that the TCD is not capable to differentiate between the peak of N2 produced by the surface oxidation of Cu by N2O and the excess of N2O that does not react, Alternatively, a cold trap at − 80 ℃ is recommended to trap the excess of N2O before reaching the TCD and allows the N2 peak to pass on. Another useful alternative is to adapt a separation column that enables the separation of the N2 peak from those corresponding to N2O, thus the N2 peak will arrive and be detected by the TCD before the delayed peak of N2O reaches the TCD. In this case, the method becomes available to properly compute the amount of N2 and to be related to the amount of Cu on the surface. This phenomenon of adversity can be easily resolved if a mass spectrometer is connected at the exhaust of the instrument. Example of this analysis is shown in Fig. 7. Bimetallic catalysts: the presence of two metals makes it more complex to characterize the superficial metallic centers, specifically to know the stoichiometric relationship between the adsorbate gas and the metal. In these cases it is normally necessary to use other characterization techniques such as DRX or TEM. The simplest case for the use of chemisorption in bimetallic systems is when only one of the system components chemisorbs the adsorbate gas. For example, Ru-Cu and Os-Cu systems can be analyzed, since copper atoms do not adsorb hydrogen [60]. In Pt(Re,Ir,Ru) systems, selective chemisorption is performed by means of O2/H2 titration as it allows Pt and Re on the surface to be determined. The chemisorbed oxygen in Pt can be reduced by hydrogen at 25 °C, and a second titration with oxygen allows the Re atoms to be estimated by difference. This procedure can be used if the formation of alloys between metals does not occur. In the case of the Pt-Ru system, a titration using O2 and CO is used following the same previous strategy [61].In the case of acid centers, the nature of the surface must be taken into account, as well as the strength and number of centers [62]. First of all, it should be possible to differentiate between Brönsted- and Lewis-type acidity. In the former, a proton is brought into play as a Brönsted acid center is one capable of transferring a proton from the solid surface to an adsorbed molecule. This type of acidity can be generated when a trivalent ion is present in tetrahedral coordination with oxygen, with the most common example being aluminum [63]. When all the tetrahedral oxoanions are shared with two cations, a negative charge is created on cations with a charge of less than 4. This is the case, for example, in aluminosilicates [1]: Table Image 1 When the excess of negative charge is compensated with protons, silanol groups are formed, which can be presented as: Table Image 2 This is also a Brönsted center. In this case, the oxygen does not have a trigonal structure and is only represented as such to indicate that both Si and Al retain their tetrahedral coordination. This center is best detected by treatment with a basic molecule (e.g., an olefin) and subsequently observing the equilibrium: Table Image 3 Depending on the strength of the Brönsted center, this balance can be displaced. The acidic surface is therefore dynamic and depends on both the chemical nature of the adsorbed base and the solid.In Lewis-type acidity, the surface accepts an electron pair from the adsorbed molecule, forming a coordinate bond. In the case of silica-alumina, this could be represented as [1]: Table Image 4 In the particular case of clays in which a dehydration point has not been reached and the exchange centers are occupied by cations such as Na+, Ca2+, Mg2+, etc., the main Lewis centers are due to Fe(III) in the structure and the octahedral Al(IV) located on the edges of the particles [64]. Interactions between Brönsted and Lewis centers may also occur. Thus, for example, in a clay at 300 °C, the structural OH begins to be eliminated, forming trigonal Al(III) and H2O. Dehydration processes accompany the formation of Lewis centers. Synergistic interactions between the Brönsted and Lewis centers may also occur. For example, an electron-deficient Al(III) (when in tetrahedral coordination) exerts an inducing effect on a neighboring silanol group, thus favoring H+ mobility.Depending on the nature of the surface, all materials, have an acid type and strength. The most representative acidic materials include alumina, silica-alumina, and zeolites, amongst others [65]. However, given the importance of this property, a series of solids known as superacids have been developed in recent years [66]. Treatment of activated carbons with acids (H2SO4, HNO3) or other oxidants (H2O2, Cr3O7 2-, MnO4 -) also creates acidic surface groups. Given the hydrophobic nature of the surface, these new centers will increase their hydrophilic character. The groups that have been proposed to exist on the surface of oxidized carbons are shown in Fig. 8 [67].A solid of an acidic nature will not usually have a single class of acidity and will normally present a large distribution of acid centers. This may be due to a heterogeneity in the composition of the solid or the existence of a small range of interactions or surface structures. Both Brönsted and Lewis centers are often found in the solid at the same time. As such, it will be necessary to use methods that allows to differentiate and characterize a surface in terms of the nature, number, and strength of acid centers.The titration of acid/basic centers using dynamic methods is carried out by injecting pulses of ammonia/carbon dioxide into a gas stream that allows the ammonia/carbon dioxide to pass through the adsorbent or catalyst bed at atmospheric pressure. This procedure has already been described above and allows the amount adsorbed, which is related to the capacity of the monolayer, to be determined. Similarly, it is possible to work in TPD mode. This method is the most commonly used in solid acid catalyst due to its simplicity and low cost and its ability to determine both the number of acid sites and their strength [68,69]. However, the use of ammonia presents limitations. Thus, ammonia is a small molecule that is able to penetrate the smallest pores of the material. However, these very small pores make a very small contribution to the catalytic behavior, therefore their contribution to the acidity of the material can be neglected. Additionally, it should be noted that ammonia is a base that can react with relatively weak acid centers and does not contribute decisively to the overall catalytic behavior. Typically, ammonia-TPD curves show two peaks (see Fig. 4), which may be related to the existence of at least two types of acid sites. The first peak (A) is related to desorption of weakly bound ammonia and was found to be of no catalytic relevance (it is relevant for gas-sensing applications); the other peak (B) reflects the desorption of ammonia probably from the Brönsted acid sites, which determines, for example, the acidic properties of zeolites. A classification related to weak, medium and strong acidic sites, related to the temperature of desorption peaks centered in the ranges 25–200, 200–400 and over 400 °C is also proposed [68], although there are currently no standardized criteria. Other larger molecules, such as pyridine or tert-butyl amine, isopropylamine, etc, are preferred because they only penetrate the largest pores and these are the ones that contribute most to the catalytic behavior observed. However, these molecules present operational problems since they can condense under operating conditions. The TPD of amines has recently been reported as technique for measuring Brönsted acid site concentrations. This method is based on the formation of alkylammonium ions from the adsorbed alkyl amines that are protonated by Brönsted sites, which decompose into ammonia and olefins in a range of temperatures. Typically, amine-TPD curves show two peaks (see also Fig. 4). The first peak is related to desorption of weakly bound amine and the other peak reflects the decomposition of amines at the Brönsted acid sites. In the case of isopropyl amine, propylene and ammonia would be obtained. The CO2-TPD method allows analysis of the nature of basic sites. As in the case of ammonia, the strength of basic sites may be classified according to their different CO2 desorption temperatures. In this case, the temperatures of desorption peaks below 400 °C, between 400 and 600 °C, and over 600 °C are related to weakly, medium and strongly basic sites. Adsorption of the probe molecule and analysis by IR spectroscopy. There are numerous studies on the interaction of surfaces and basic molecules such as pyridine by IR spectroscopy [70,71]. Pyridine (C5H5N) is the preferred molecule to study Brönsted and Lewis acidity separately as these interactions can be easily distinguished from the IR spectra [72–74]: Table Image 5 Other proposed molecular probes include acetonitrile (CH3CN), benzonitrile (C6H5CN), CO, H2 and NO. Direct measurement of the intensity of the frequencies of the OH groups does not provide information on the acid strength of the Brönsted centers and shifts in the frequencies of these vibrations by interaction via hydrogen bonds with adsorbed molecules provide more information. This interaction can be quantified as [75]: (8) ∆ γ = 3 qE 4 r ( 2 μ ) 1 / 2 D 1 / 2 where ∆γ is the frequency shift of the hydroxyl group involved in the hydrogen bond interaction, q is the dipole charge, E is the electric field across the O-H axis, μ is the reduced mass, and D is the dissociation energy of the O-H bond. The values of ∆γ can be estimated, thus giving the Brönsted-type acid strength. The strength of the acid centers can also be studied from the evolution of these bands under different conditions of temperature and vacuum.Pyridine is the most widely used molecule, since it is a weak Brönsted base (pkb = 9) that only interacts with the strong protonic centers, that is, with the interesting ones from a catalytic point of view. The absorption bands of adsorbed pyridine are fine and allow Brönsted centers to be distinguished from Lewis centers. Information can be obtained on: – The types of acid centers, as identified by the characteristic absorption frequencies. – Their strength, from the variations in intensity of the bands upon desorption at increasing temperature. – The reactivity of the OH groups with respect to the base, as seen from both the variation of intensities of the absorption maxima υ(OH) during adsorption and desorption, and by the positions of the reagent bands. – The density of acid centers, from a plot of normalized absorbance (IR) against the amount of pyridine adsorbed. The types of acid centers, as identified by the characteristic absorption frequencies.Their strength, from the variations in intensity of the bands upon desorption at increasing temperature.The reactivity of the OH groups with respect to the base, as seen from both the variation of intensities of the absorption maxima υ(OH) during adsorption and desorption, and by the positions of the reagent bands.The density of acid centers, from a plot of normalized absorbance (IR) against the amount of pyridine adsorbed.In this case, information about the nature, density, and strength of the acid positions on the surface can be obtained. The nature of the interaction can be determined by assigning the frequencies of the physisorbed and chemisorbed pyridine bands in the 1400–1700 cm−1 region of the IR spectrum (see Table 3). The strength of these acid positions can be evaluated by exposing the sample to vacuum treatments and at several temperatures. The reference spectrum (baseline) corresponds to the sample prior to contact with pyridine. To evaluate the density of acid centers, a magnitude known as the normalized absorbance of the intensities must first be defined. absorbance · n º of wavelength · ( mm 2 IR beam section ) ( g of absorbent ) Normalized absorbance represents an acid number that reflects the number of adsorbed species per unit area. It must be assumed that each acid center retains one adsorbed molecule. The sample is prepared in the form of a pellet, which is placed in a cell equipped with NaCl windows in which the sample can be degassed under vacuum and at a temperature of between 400 and 500 °C (depending on the previous treatment to which the catalyst has been subjected). After a desorption time, the sample is cooled to room temperature before being brought it into contact with pyridine for a short period of time. The sample is desorbed under vacuum at room temperature for 30 min to remove the physisorbed pyridine. Subsequently, it is subjected to vacuum and at several temperatures. At the end of each treatment, the IR spectra are recorded in the range 1300–4000 cm−1. The spectra obtained upon subtracting the spectrum of the sample before pyridine adsorption and after each desorption are analyzed in the region from 3200 to 3700 cm−1 (O–H vibration) and in the region from 1400 to 1700 cm−1 (vibration of adsorbed pyridine). Various types of OH groups can be observed in the O–H vibration region (e.g., in PILC: structural hydroxyls and those related to pillared species) [64].The frequencies assigned in Table 3 suggest that the adsorption bands located around 1620, 1575, 1490 and 1450 cm−1 are associated with coordinated pyridine (PyL), thus characterizing the Lewis-type acidity. In contrast, the bands at 1640, 1540 and 1490 cm−1 are due to the presence of pyridinium ions formed by the interaction with the protonic positions (Brönsted acidity). The band at 1545 cm−1 is most characteristic of the Brönsted-type acidity. The band around 1450–1455 cm−1 corresponds to a Lewis-type acidity if the sample has been previously degassed, since physisorbed pyridine exhibits a characteristic band at 1440–1445 cm−1. The bands at around 1490 and 1620 cm−1 contain a contribution from both types of acidity. In addition, the band at 1620 cm−1 provides information on the strength of the Lewis-type positions: a shift towards higher frequencies (even above 1626 cm−1) indicates the presence of a high Lewis-type acidity, whereas if the band moves towards lower frequencies (below 1615 cm−1), the acidity of the centers is weaker.It is also well known that CO can reach the Brönsted and Lewis acid sites of microporous zeolites due to its small size [76,77]. This molecule allows determination of the oxidation state and environment of the metal cations on the surface and the amount and strength of Brönsted and Lewis acid sites. Quantitative analysis: from the ratio of the absorbances of the bands due to pyridine adsorbed at a Lewis-type acid position and a band corresponding to pyridine adsorbed at a Brönsted position, the ratio of the Lewis and Brönsted-type acid positions multiplied by a constant K can be obtained ( L B ·K ).This expression comes from application of the integrated form of Beer’s law [1]: (9) B = C · L · ∫ γ ∈ γ a · d γ where B is the peak area (absorbance/cm), C is the concentration of the adsorbed species (mol/g), L is the tablet thickness (g/cm), γ is the wavenumber (cm−1), and ∫ γ ∈ γ a ·d γ , is the integrated apparent extinction coefficient (cm/mol).The concentration of species at an IR absorbance maximum can be calculated assuming that the integrated apparent extinction coefficient is known. It can be determined if [1]: (10) ∫ γ ∈ γ a L ∫ γ ∈ γ a B = 2 B L T 1 − B L T 2 B B T 2 − B B T 1 where the subscripts L and B refer to a specific IR band for pyridine (e.g., 1450 and 1550 cm−1) and T 1 and T 2 to two treatment temperatures for the catalyst. The concentration relationship between the Lewis and Brönsted positions will be [1]: (11) L B = B L B B ∫ γ ∈ γ a L ∫ γ ∈ γ a B In the case described initially [1]: (12) L B = L B · K , si L ≡ B L y B ≡ B B ; K ≡ ∫ γ ∈ γ a L ∫ γ ∈ γ a B The best bands are 1450 cm−1 (19b vibrations of the coordinated pyridine) and 1344 cm−1, although it must be considered the proximity of the band to 1447 cm−1 due to the presence of pyridine linked via a hydrogen bond that may affect the validity of the measurement. The band at 1610 cm−1, which is also assigned to coordinated pyridine, can also be used, but in this case, there is likely to be a contribution of the band at 1639 cm−1 due to the pyridinium ion. Hence, in general, only the relationship between the sum of the Lewis positions (plus the protonic H positions due to the OH on the surface) and the Brönsted acid positions can be determined.Other alternative methods, such as thermogravimetry and pyridine thermo-desorption, have been proposed to quantify the number of acid centers, depending on their strength. Thus, the evolution of the band at 1445 cm−1 can be evaluated as a function of the desorption temperature and quantified by representing the amount of pyridine adsorbed per mass of solid as a function of the absorbance per mass of solid. Applications. A generic description of pyridine adsorption and its use in the characterization of acid centers in adsorbents and catalysts is difficult, so its study usually involves specific examples. Hence, herein it has been decided to use intercalated/pillared clays as study materials.The acidity and nature of the acid centers (Brönsted and Lewis) depend on the cations exchanged, the method of preparation, and the nature of the clay [64,78–82]. In the case of aluminum-intercalated clays, Lewis-type acidity is related to two types of acid centers, both of which arise due to the aluminum present in the tetrahedral layer of the clay (LPy, 1641 cm−1) and to the aluminum in the pillars (LPy, 1621 cm−1) [83]. This latter center is the one that is usually related in the literature to Lewis acidity. In contrast, the origin of the Brönsted acid centers in the intercalated clays is not clear. These centers have been related to the structural hydroxyl groups of the clay layer, which in turn are related to the exchange centers, in other words, the protons of the oligomeric cations that form the pillars after heating, and to a synergistic phenomenon between the Si layer of the clay and the pillars [78,80–87].When characterizing a hectorite intercalated with pillars of Al, ZrAl and Zr, Occelli observed that, after adsorption of pyridine and being subjected to vacuum (10−6 torr) at 300 °C, the natural hectorite presents both Brönsted and Lewis centers [84]. The pillars introduced affect the acidity observed in the initial clay. Thus, with only Al pillars, the characteristic PyH+ bands (1638, 1547 and 1490 cm−1) practically disappear or are significantly reduced in intensity, whereas with Zr and mixed ZrAl pillars, the pyridine is retained at both Brönsted and Lewis centers, even after degassing under vacuum and at 400 °C. At 300 °C and under vacuum in an intercalation with Al2O3, the pyridine is first removed from the Brönsted centers. In contrast, pyridine adsorbed at Lewis centers remains practically unchanged above 400 °C. The presence of Zr increases the Brönsted acidity in the intercalated hectorite. It is clear that the absolute intensities of the intercalated hectorite bands increase due to an increase in surface area.In a study on montmorillonites intercalated with aluminum, the same author observed that, after being subjected to a vacuum at 400 °C, the pyridine continues to be found as PyH+ and PyL. Proton acidity must be responsible for the instability of inorganic pillars at high temperature. Thus, when the pillars are formed by dehydration of the interlayer polymeric cation, protons are generated: 2 Al 13 O 4 ( OH ) 24 ( H 2 O ) 12 7 + → Δ 13 Al 2 O 3 + 14 H + + 41 H 2 O At high temperature, these protons are able to react with the aluminum in the pillars in the same way that acids extract the aluminum from the zeolite structure. Table Image 6 When this reaction occurs, the pillars decrease in size, and if Al3+ extraction continues, collapse occurs.Fripiat et al. [88] reported that the most important acidity of the montmorillonite surface is due to hydrated H2O molecules, which means that progressive dehydration can occur. Hence, comparing the pyridine and IR adsorption data obtained, these authors considered that both the Brönsted centers and the derivative of acid centers in intercalated montmorillonites decrease rapidly at 200 °C.Despite the above, the actual nature of the acid centers in the pillars remains unknown. Assuming that the intercalated species is Al13 7+, currently there are no information about the nature of its transformation after thermal activation at 300 °C, although the formation of a bayerite or gibbsite-type structure has been proposed [64]. In any case, protons from different sources must be the source of the acidity of pillared clays. These authors conclude by proposing that more than 90 % of the acid centers in both intercalated montmorillonites and calcined intercalated beidellites are of the Brönsted type, which are able to protonate pyridine to PyH+ (band at 1640 cm−1). Some examples of the adsorption and desorption spectra of pyridine adsorbed on different samples (montmorillonite, alumina, silica, and aluminum-intercalated montmorillonite) are shown in Fig. 9 [73]. Adsorption isotherm of the probe molecules. Acidic and basic centers can also be characterized using the static volumetric procedure described in Section 3.1. The amount of adsorbed gas (probe molecule) is obtained as a function of the equilibrium pressure at a constant adsorption temperature (see Fig. 10). The probe molecules used are those that characterize the acidic or basic properties of the adsorbent/catalyst listed above, such as CO2, NH3, pyridine (C5H5N), acetonitrile (CH3CN), benzonitrile (C6H5CN), CO and NO, amongst others.It is possible to quantify the adsorption capacity from the volume adsorbed at a given pressure and temperature. In the case of NH3 adsorption, the ASTM D 4824–93 method proposes the adsorbed volume to be representative of that obtained at a pressure of 150 torr and at a temperature of 175 °C [17]. However, other parameters that allow the properties of adsorbents and catalysts to be characterized can also be calculated. Thus, Henry’s constant is an important characteristic of adsorption because it provides an indication of the strength of adsorption and the isosteric heat of adsorption at low pressure. Although there are several possibilities for calculating Henry’s constant [89], when it is obtained directly from the isotherm, this method is more accurate that others if sufficient data are available in the low pressure region.The heat effects produced during adsorption processes can be described by the isosteric heat of adsorption and can be determined from the amount of gas adsorbed at several temperatures. The isosteric heat (q st ) defines the energy change resulting from the phase change of an infinitesimal number of molecules at constant pressure and temperature and a specific adsorbate loading. One method for calculating the isosteric heat of adsorption involves application of the Clausius–Clapeyron equation [89], which relates the isosteric heat to the pressure change of the bulk gas phase as a consequence of a temperature change for a constant amount adsorbed [89]: (13) q s t = − R · [ ∂ ln p ∂ ( 1 / T ) ] n where p (kPa) is the equilibrium pressure, n is the amount of gas adsorbed at temperature T (K), and R (kJ/mol·K) is the universal gas constant. The isosteric heat can be obtained from the experimental isotherms at various temperatures by plotting ln (p) versus 1/T for a constant loading n. The isosteric heat corresponds to the slope of the amount adsorbed by the materials, and the dependence of the isosteric heats of adsorption on the amount adsorbed can indicate the effect of surface loading. Indeed, in some cases, a maximum can be observed in the isosteric heats of adsorption in the presence of such a loading (see Fig. 10) [90]. This behavior can be related to the coating of the surface and subsequent formation of multilayers. Similarly, the limiting heat (q st 0 ) can also be obtained from the temperature dependence of Henry’s constant (H i ) by applying the Clausius–Clapeyron equation in the low-pressure region, where the isotherm obeys Henry’s law. (14) q s t 0 = − R · [ d ln H i d ( 1 / T ) ] n = 0 The isosteric heats obtained from this last equation, and the values found from the isosteric heats at zero coverage, should be similar [90].The techniques and procedures presented in this work allow the characterization, evaluation, and determination of the qualitative and quantitative surface properties of adsorbents and supported metal catalysts by way of selective chemisorption processes.The reaction behavior of a supported metal catalyst depends on the metal surface, the size of the metal particles, and how these particles are distributed on the surface of the catalyst support. Measurement of these properties using a chemisorption or selective adsorption technique requires careful selection of the operating conditions. Once established, however, chemisorption can be considered to be a method for routine measurement of the dispersion of supported metal catalysts. However, to measure the dispersion and particle size from the amount of an adsorbed gas, a series of assumptions are required, and it depends on the preparation and pretreatment conditions of the catalyst. A good practice, if possible, would be to use several adsorbates (H2, CO, O2), as well as to combine O2/H2 cycles and compare the results obtained. It will also be necessary to determine the possible effects of spillover, SMSI, presence of contaminants, and reversible adsorption. Among the techniques proposed, the isothermal dynamic procedure is the most popular since it allows faster measurements compared to the time needed to perform the volumetric measurements. In addition, in this case it is not necessary to volumetrically calibrate the equipment before or after the measurements. However, it has the drawback of only evaluating the centers where there is a strong interaction between the adsorbent gas molecule and the adsorption center.Adsorbents and catalysts are characterized by having acid and basic centers that are involved in a large number of processes related to petroleum refining processes, amongst others. Two types of centers can be distinguished: Lewis and Brönsted. The most common technique to qualitatively characterize this type of center is to adsorb an acidic or basic gas molecule (NH3 or CO2) and perform its desorption in a programmed temperature ramp. However, the types of acid or basic adsorption centers cannot be differentiated using this procedure, therefore characterization is only qualitative. It is possible to characterize the desorption forces from the activation energy of desorption by modifying the heating rate. To be able to differentiate between adsorption centers, and even perform quantification, it is necessary to adsorb a molecule (pyridine, acetonitrile, benzonitrile, etc.) and conduct an analysis using IR spectroscopy. To characterize this type of center, it is increasingly common to use the static volumetric procedure, which allows the amount of adsorbed gas as a function of the equilibrium pressure at a constant adsorption temperature to be obtained. In addition to being able to quantify the adsorption capacity from the volume adsorbed at a given pressure and temperature, it is possible to obtain Henry’s constant and the isosteric heat of adsorption. The dependence of the isosteric heats of adsorption on the amount adsorbed can indicate the effects of surface loading. A.Gil: is the only author of this work. Conceptualization; Formal analysis; Investigation; Methodology; Resources; Supervision; Validation; Visualization; 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 author express its gratitude to Dr Simón Yunes for valuable discussions and critical reading of the manuscript. The author is grateful for financial support from the Spanish Ministry of Science and Innovation (AEI/MINECO) and Government of Navarra through projects PID2020-112656RB-C21 and 0011-3673-2021-000004. Open access funding provided by Universidad Pública de Navarra. AG also thanks Santander Bank for funding via the Research Intensification Program.

The adsorption phenomenon has been used extensively to achieve and explain solid-state reactions, control contamination, and purify liquids and gases. This process implies the use of a porous medium or a material with specific adsorption centers where the interactions with the reagents occur. Determination of the properties of adsorbent or catalyst materials that do not contain specific adsorption sites by physical gas adsorption is a well-established procedure in most research and quality-control laboratories. However, characterizing the specific centers by selective adsorption—chemisorption—remains an open question for discussion and study. The specific centers involved are often acidic/basic and metallic; in most cases, reagents are adsorbed and desorbed in these centers, whose determination allows controlling the processes and comparing the materials. The techniques and procedures presented herein facilitate the evaluation and the qualitative and quantitative determination of the surface properties of the materials using chemisorption processes for metallic and acidic/basic sites. The aim of this work is to review these techniques and procedures, including the updates published by several researchers, who mostly strive to explain the results of bifunctional metallic and acid–base catalytic behavior.

Concerns on the increasing demand for energy and strict environmental regulations have ignited interests in producing renewable chemicals and fuels [1]. Biomass, as the sole source of renewable organic carbon, has captured widespread attention because of its ability to be converted into various valuable chemicals [2,3]. As a platform chemical derived from biomass, 5-hydroxymethylfurfural (HMF) could be converted into diverse chemicals (biofuels, functional macromolecular polymers) through different reaction pathways [4,5]. Notably, the oxidation product, 2,5-furandicarboxylic acid (FDCA) which is one of the top-12 value-added chemicals has attracted enormous interests, because it is a crucial building block for the production of bio-based polymers [3,6–10].Given the multitude of published references, metal species of heterogeneous catalysts is the main active site [11–14], and molecular oxygen (O2) serves as the oxidant, offering advantages of availability and benignity to the environment [6]. Owing to the difficulty in oxygen activation, noble metals [15–20], especially Au-based catalysts [21–24] are widely used in HMF oxidation on account of their excellent activity and product selectivity under relatively mild conditions. As reported by Corma [25], the catalytic performance of Au catalysts is strongly affected by the support: higher FDCA yields could be achieved over TiO2 and CeO2 supported Au catalysts. Yield of 99% was achieved over Au/CeO2 and Au/TiO2 (Au loading amount: 2.6 wt% in Au/CeO2 and 1.0 wt% in Au/TiO2), with 4 equiv of NaOH and 10 bar air in 5 h at 65 °C [25]. Xu and coworkers performed the reaction at 60 °C, 0.3 MPa O2 with 4 equiv of NaOH and achieved 85% FDCA yield over Au/TiO2 (1.5 wt% Au) catalyst after 6 h [26]. Besides the supporting material, the amount of base also plays a significant role in determining the performance of Au-based catalysts. As reported by Riisager [27], 71% FDCA yield could be obtained over Au/TiO2 (1.0 wt% Au) with 20 equiv of NaOH. However, when low amount of base (less than 5 equiv) was applied, the main product changed from FDCA to HMFCA. Besides, the FDCA yield was only 1% under base-free conditions.Despite the great progresses, there are still severe challenges for Au-based catalysts during HMF oxidation. First, the high loading amount of Au limits their industrial application, rendering maintaining and even promoting the performance of Au/TiO2 catalysts with low Au loading amount an urgent demand. Second, HMF oxidation over Au-based catalysts are usually carried out in the presence of excessive base, leading to both economic and environmental issues. Correspondingly, base-free catalysis attracted more attention [28,29]. In the elegant work of Zhang [30], it is well demonstrated that the Fe–Zr–O exhibited a 60.6% FDCA yield under base-free conditions, which is a excellent result for HMF to FDCA using molecular oxygen as an oxidant. Although base-free oxidation of HMF could be achieved when solid base is utilized as the supports, such as alkaline hydrotalcite (HT), severe leaching of Mg2+ ions from HT occurs inevitably, resulting from the chemical interaction between the basic support and the generated FDCA [31]. Third, the knowledge of the intrinsic active sites and the active oxygen species are still under controversy. It is reported that hydroxide ions in alkaline solution facilitate the activation of the C–H and H–O bond in the alcoholic group to form the formyl intermediate [7,31,32], while Yu and coworkers propose that radicals, instead of hydroxide ions promote the alcohol oxidation step [33].Therefore, from the viewpoint of green and sustainable chemistry, it is of significant importance to prepare catalysts with low Au loading amount and high catalytic performances. Besides, it is greatly imperative to take a deep insight into the catalytic mechanism with regard to the active oxygen species and intrinsic active sites.In this work, a series of metal oxide modified MO x -Au/TiO2 (M = Fe, Co, Ni) catalysts were synthesized and evaluated in HMF oxidation. There may be three possible advantages of the as-prepared MO x -Au/TiO2 catalysts: first, low loading amount of Au (0.5 wt%) and high catalytic performance, being promising for industrial applications; second, satisfying FDCA yield could be obtained under base-free conditions, complying with the green and sustainable chemistry principles; third, addition of transition metal oxides promotes electron transfer and generation of Au δ−–Ov–Ti3+ interface, accelerating the adsorption and activation of the reactants. In order to take a deep insight into the cause for the differences in catalytic activities, the adsorption properties, kinetic study, active sites for the rate-determining step, and the active oxygen species were investigated.Ni(NO3)2·6H2O, Co(NO3)2·6H2O, Fe(NO3)3·9H2O, titanium dioxide (TiO2), HAuCl4·3H2O (99.9 wt% analytical purity), 5-hydroxymethylfurfural (HMF, 98.0 wt% analytical purity), 5-hydroxymethyl-2-furancarboxylic acid (HMFCA, 98.0 wt% analytical purity), 5-formyl-2-furancarboxylic acid (FFCA, 98.0 wt% analytical purity) and 2,5-furandicarboxylic acid (FDCA, 98.0 wt% analytical purity) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. All items were used as received without further purification.The details about the catalyst preparation are given in the Supporting Information.XRD, AC-HAADF-STEM with energy dispersive spectroscopy (EDS), inductively coupled high-frequency plasma (ICP), X-ray photoelectron spectra (XPS), temperature programmed reduction (TPR) with H2, temperature programmed desorption (TPD) with O2 were used, and the relational details are depicted in the Supporting Information.The HMF oxidation was evaluated in a high-pressure stainless-steel autoclave equipped with a stirring function and the heating function. Detailed operating procedures and calculation methods are described in the Supporting Information.XRD patterns of Au/TiO2 and MO x -Au/TiO2 are shown in Fig. 1 . The pure TiO2 displays three peaks at 25.3°, 37.8°, and 53.9° (PDF#21-1272), corresponding to the characteristic peaks of anatase crystal phase. No obvious peaks for Au or MO x are observed, implying that Au and MO x may be highly dispersed on the surface of TiO2. The XRD lattice parameters of MO x -Au/TiO2 are intensely similar to those of Au/TiO2, indicating that the transition metal species did not enter the TiO2 crystal lattice and no solid solution was formed between them.High-angle annular dark field scanning transmission electron microscope (HAADF-STEM) was conducted to further investigate the morphological and structural characteristics of the catalysts. As shown in Fig. 2 , Au and Co species are uniformly dispersed on the surface of TiO2, in agreement with the XRD result. The content of Au was determined by ICP, and the result is listed in Table S1 in the supporting information.In order to fully understand the electronic structure and local environment of the catalysts, they were characterized by XPS, Raman and H2-TPR. As shown in Fig. 3 A, except for Au/TiO2, the binding energies of Au 4f7/2 for the other three catalysts all shift towards the lower direction. Au 4f7/2 could be deconvolved into two peaks, the one at ∼83.5 eV corresponds to the Au0 species [34], and the other at ∼82.6 eV is ascribed to the electron-rich Au species (Au δ−). After modification with different transition metal oxides, the Au δ− content increases from 0 (Au/TiO2), 31.6% (FeO x -Au/TiO2), 48.7% (NiO x -Au/TiO2) to 58.1%（CoO x -Au/TiO2）(Table 1 ). The Ti 2p3/2 spectrum in Fig. 3B was deconvolved into two peaks of ∼458.5 and ∼457.9 eV, corresponding to Ti3+ and Ti4+ [35], respectively. After modification by transition metal oxides, the Ti3+ contents rise obviously, reaching the highest value of 58.5% in CoO x -Au/TiO2. Fig. 3C, D and E show the XPS spectra of Fe 2p, Ni 2p and Co 2p, respectively. Ni and Co species exist on the catalyst in the form of both divalent and trivalent ions, while Fe species exists in the form of trivalent and tetravalent ions [36–40]. This result confirms that electrons transfer from the transition metal species (Fe, Ni and Co) to Au and Ti species. Fig. 3F shows the XPS spectra of O 1s, there are four peaks around 532.1, 531.0, 530.1 and 529.4 eV, which could be assigned to surface hydroxyl groups, surface hydroxyl groups, lattice oxygen and oxygen vacancy (Ov), respectively [41,42]. Fig. 4 shows the Raman spectra. The five peaks located at 146, 197, 390, 513, and 639 cm−1, correspond to Eg, Eg, B1g, A1g (or B1g) and Eg modes respectively [43,44], which are characteristic peaks of anatase TiO2. The peaks of FeO x , CoO x and NiO x are not detected, confirming their high dispersion on TiO2, in line with the XRD results. After modification with transition metal oxides, the intensities of all the five characteristic peaks attributed to Au/TiO2 reduce significantly d, which may result from the generation of Ov, since part of the Ti–O–Ti is replaced by Ti–Ov–Ti [45]. In addition, the Eg mode in the range of 100–200 cm−1 is presented in Fig. 4B. After addition of transition metal oxides, the peak moves to a higher frequency direction, besides, the full width half maximum (FWHM) increases slightly. CoO x -Au/TiO2 features the highest frequency and the largest FWHM, suggesting the largest amount of Ov [43]. Similar results have also been reported by Han [11], Ov concentration could be significantly improved via doping. Given that Ov could directly serves as adsorption sites for reactant molecules [46], the largest amount of Ov in CoO x -Au/TiO2 would lead to the best adsorptive performances for O2 and HMF.Besides XPS and Raman, H2-TPR was also conducted to evaluate the surface oxygen reducibility of the catalysts (Fig. 5 ). As demonstrated in previous studies [47–49], TiO2-supported VIII group metals (Fe, Co, Ni) exhibited characteristic peaks of hydrogen consumption with a maximum over 300 °C. This indicates the two peaks could be assigned to the reduction of Au2O3 (50-200 °C) and Ti-MO x (500-600 °C) [50], respectively. The incorporation of MO x leads to significant shifts of peak maximums towards lower temperature, which could be attributed to massive oxygen adsorption on catalysts. Au δ− may facilitates activation adsorption of small molecule gas such as O2. With more Au δ− formation, the hydrogen consumption of Co dopped catalysts is higher than Ni and Fe, and all these samples spend more hydrogen than Au/TiO2, which is consistent with the XPS result.Combining the XPS, Raman and H2-TPR results, it is reasonable to conclude that addition of the transition metal oxides contributes to electron transfer in the catalyst, generating the Au δ−–Ov–Ti3+ interface.The catalytic performances for HMF oxidation were investigated, as shown in Fig. 6 A, 100% HMF conversion could be achieved over all the as-prepared catalysts. However, there is blatant differences in FDCA yield: Au/TiO2 exhibits the FDCA yield of only 53.9%, which is the lowest among the four catalysts. FDCA yield of the catalyst modified by FeO x , NiO x , and CoO x is 71.6%, 82.5% and 90.2%, respectively. The CoO x -Au/TiO2 catalyst was recycled, and the result is depicted in Fig. 6B. After five recycles of HMF oxidation, the catalytic performance of CoO x -Au/TiO2 remained basically unchanged, showing excellent stability (Fig. 6B). The content of Au and Co are lower than 10 ppm in the liquid, confirming the good stability of the as-prepared catalysts. Given that HMF is a notoriously labile compound and readily converts into degradation by-products at high temperature under alkaline environment, lacking active sites for HMF oxidation would result in low FDCA selectivity and yield. This explains the suboptimal FDCA yield of Au/TiO2, which may be due to the low Au loading amount and insufficient active sites for HMF oxidation. With the modification of FeO x , NiO x , and CoO x , higher FDCA yields are obtained, and the reasons will be discussed in detail in the following part.Considering that the adsorption of reactants plays a significant role during heterogeneous catalysis, the adsorption properties for both HMF and O2 over different catalysts were investigated firstly. As shown in Fig. 7 A, catalysts modified by transition metal oxides exhibit significantly enhanced HMF adsorption capacity in the order of CoO x -Au/TiO2 > NiO x -Au/TiO2 > FeO x -Au/TiO2. Fig. 7B illustrates the O2-TPD profile, which could be deconvoluted to three peaks via a Gaussian peak fitting method according to the desorption temperature: adsorbed oxygen species (<300 °C), lattice oxygen species (300–400 °C), and bulk lattice oxygen species (>400 °C) [51–53]. As listed in Table 1, similarly, the as-prepared catalysts present the same trend for the adsorption capacities for O2. CoO x -Au/TiO2 shows both the highest adsorption capacity of 266.8 μmol/g and the ratio of adsorbed oxygen species to all oxygen species (24.9%). Among the three kinds of oxygen species, only the adsorbed oxygen species can directly participate in and play a vital role in the reaction [52]. The promoted O2 and HMF adsorption capacity after transition metal oxides may result from generation of Au δ− and Ov: O2 adsorption and activation could be enhanced over negatively charged metal species through the donation of electrons from the metal to the antibonding π∗ orbital of O2 [54–56], at the same time, as demonstrated by first-principle calculations and experimental results, Ov could directly serve as the adsorption sites for O2 and alcohols [46,57–60]. The outstanding adsorption property for HMF and the large ratio for adsorbed oxygen species over CoO x -Au/TiO2 may contribute to its excellent catalytic performance, owing to more reactants gathering around the active site and accelerating the production of FDCA.In order to take a deep insight into the reactive sites, the time course of HMF oxidation on different catalysts were investigated, and they follow the same reaction pathway (Fig. 8 ). As depicted in Scheme 1 , there are three steps for FDCA production form HMF: first, oxidation of the formyl group, generating HMFCA with a rate constant of k 1, second, oxidation of the hydroxyl group in HMFCA to produce FFCA (k 2), third, transformation of formyl group of FFCA to carboxyl group with the rate constant of k 3. The content of FFCA is low over the four catalysts, revealing that FFCA could be quickly converted into FDCA under the reaction conditions. This reaction path is consistent with the previous reports using Au catalysts [61,62]. After immobilization of transition metal oxides, the consumption rate of HMFCA is accelerated distinctly, accompanied with the remarkable increase in FDCA production rate.Rate constants for each step are fitted by quasi-first order reaction kinetics. As depicted in Table 2 , k 2 is much smaller than k 1 and k 3, providing clear evidence that oxidation of HMFCA into FFCA is the rate determining step of the whole oxidation process. After modification with FeO x , CoO x and NiO x , the rate constant of each step grows obviously, especially k 2 rises from 0.02 min−1 (Au/TiO2) to 0.30 min−1 (CoO x -Au/TiO2), demonstrating that oxidation of the hydroxyl group in HMF is intensively enhanced after modification by transition metal oxide (Table 3 ).Given that oxidation of the hydroxyl group in HMF is the rate-determining step, researches about the catalytic mechanism (active oxygen species and active sites) and the underlying cause for different performances of the four as-prepared catalysts would be carried out aiming at the step of HMFCA→FFCA in the following part.Despite the extensive researches, it is still controversial whether base is the key factor for oxidation of hydroxyl group in HMF [33]. Therefore, experiments have been designed to reveal the effect of base. As shown in Fig. 9 , in the first 30 min, the HMFCA conversion rates under base and base-free conditions are roughly the same (difference is less than 1%), but the concentrations of FFCA and FDCA are quite different. Under base conditions (Fig. 9A), FFCA converts into FDCA as soon as being generated, so only a small amount of FFCA could be detected during the continuous sampling process, which is consistent with the results of time course study. While when there is a trace amount base or no base in the solution (Fig. 9B and C), the generated FFCA transforms into FDCA at a slow rate. The difference in FFCA conversion rate may be caused by the weak solubility of FDCA in trace alkali and non-alkaline solution. The above experiment undisputedly substantiating that base is not the key factor for hydroxyl group oxidation, in good agreement with results reported by Yu [33].This scenario motivates us to go a step further by focusing our interest in identifying the active oxygen species. As shown in Fig. 10 , EPR was used to detect hydroxyl radicals (OH−) and superoxide radicals (O2 −). The four characteristic peaks with the intensity ratio of 1:2:2:1 attributes to the signal of DMPO-OH (Fig. 10A), and the characteristic peak with a signal intensity ratio of 1:1:1:1 corresponding to DMPO-O2 (Fig. 10B) [33]. The EPR signals of the two adducts are clearly observed over the four samples, indicating that both of the two oxygen-containing free radicals are generated. Among the four catalysts, CoO x -Au/TiO2 exhibits the highest concentration for both hydroxyl radicals and superoxide radicals.In order to further investigate which free radical governing the catalytic performance during oxidation of the hydroxyl group, a selective poisoning experiment was carried out. Isopropanol and p-benzoquinone are added as the scavenger for hydroxyl radicals and superoxide radicals, respectively. The FDCA yield is basically unchanged, regardless of the amount of isopropanol (Fig. 11 A), on the other hand, the FDCA yield is sensitive to the addition of p-benzoquinone (Fig. 11B), illustrating that superoxide radical is the exclusive factor determining the catalytic performance. Oxygen is first adsorbed on the Au δ− and Ov–Ti3+ sites of the catalyst in the reaction system, and transforms into superoxide radicals (O2 −) after being activated by the catalyst, and then combines with H2O on the catalytic interface generating OOH− and OH− (Eqs (1) and (2)). Similar results have also been reported in the work of Liu and Yu [17,33]. According to DFT calculations, active oxygen species promote the reaction (from HMFCA to FFCA) by reducing the energy barrier of hydroxyl dehydrogenation. It is well accepted that the incorporated O atoms in FDCA are provided by H2O [63], while much less is known with regard to the detailed contribution of O2, which has been exactly unveiled in this work. (1) O2 + e− = O2 − (2) O2 − + H2O + e− = OOH− + OH− Considering that superoxide radicals stem from oxygen adsorbed on catalysts, O2-TPO was used to evaluate the reactivity of oxygen species (Fig. S1). The peak can be divided into two parts: 50–200 °C and higher than 200 °C, which are attributed to the ultimate oxygen storage capacity (OSC) and the desorption of lattice oxygen caused by high temperature [64,65]. As shown in Table 4 , after the modification with transition metal oxide, the OSC value of the catalyst has been significantly improved, with the order of CoO x -Au/TiO2 (62.8 μmol/g) > NiO x -Au/TiO2 (48.0 μmol/g) > FeO x -Au/TiO2 (36.6 μmol/g). According to previous reports, the adsorption strength of O2, CO2 and other small molecule gases on the metal surface is related to the electron density [49,66,67]. The strong activation ability of O2 on Au δ− and Ov–Ti3+ is one of the main factors to ensure the high activity of the catalysts.Considering that TiO2 is widely applied as photocatalyst, photocatalytic oxidation of HMF was also conducted over CoO x -Au/TiO2 catalysts. As depicted in Table 5 , The FDCA yields obtained over the CoO x -Au/TiO2 (Au 8.0 wt%) catalyst under irradiation for 2 h and dark reaction conditions for 24 h are basically the same, which means that the reaction under light illumination is probably ten times faster than under dark conditions. Besides, after optimization in the amount of both catalyst and base, FDCA yields of 13% and 3% were achieved over CoO x -Au/TiO2 catalysts with the Au loading amount of 0.5 wt% and 8.0 wt%, respectively. Combined with the EPR characterization and the result of radical scavenger test, these experiments confirm that HMF oxidation follows the radical mechanism over CoO x -Au/TiO2 catalysts. The detailed mechanism will be carried out in our following work.For an environmentally friendly industrial application, inspired by Fu and Wang [18,19], we carried out HMF oxidation in a base-free environment (Fig. 12 ), and a FDCA yield of 71.2% was obtained after prolonging the reaction time. This promotes the process of green chemistry, which demonstrates a great significance to environmental protection.The elegant work of Zhou and coworkers [68] give us the inspiration that the interface between Au and the support may be the active sites for the hydroxyl group in HMF. In order to clarify the relationship between the Au δ−–Ov–Ti3+ interface and the catalytic activity clearly, the rate constant of the rate-determining step (k 2) is correlated with the surface Au δ−/(Au δ− + Au0) ratio and Ti3+/(Ti3++Ti4+) ratio, respectively. As illustrated in Fig. 13 A and B, Au δ−/(Au δ− + Au0) ratio and Ti3+/(Ti3++Ti4+) ratio display a concave and convex monotonic increase trend with k 2, respectively, suggesting that Au δ− and Ov–Ti3+ govern the catalytic activity cooperatively. It strongly confirms that the Au δ−–Ov–Ti3+ interface site acts as the intrinsic active center toward oxidation of the hydroxyl group. Au δ− site enhances O2 adsorption through the donation of electrons from the metal to the antibonding π∗ orbital of O2 [54–56]. At the same time, Ov–Ti3+ site plays multiple roles: first, it serves as the adsorption site for both O2 and the alcohol [46,57–60], the adsorbed oxygen combines with electrons transforming into O2 − and the adsorption of HMFCA enhances the oxidation of the hydroxyl group, similar result has also been reported by Wang [22]. Second, the activation and dissociation of H2O would be accelerated over the Ov–Ti3+ site [69–71], more superoxide radicals (O2 −) combine with dissociated H2O to accelerate the formation of strong oxidizing species (∗H2O2) and promotes the rate-determining step, namely oxidation of the hydroxyl group.Based on previous research [33,72], a mechanism for HMFCA oxidation on MO x -Au/TiO2 catalyst is proposed (Scheme 2 ). The oxygen vacancy on the catalyst promotes the adsorption of both H2O and HMFCA molecules. O2 combines with the electrons on Ov–Ti3+ or Au δ− to form superoxide radicals (O2 −) and reacts with the H2O dissociated on the oxygen vacancy to generate OOH− and OH−. After that, OOH− is further combined with H2O to form strong oxidizing species (∗H2O2) [17], which may aid the cleavage of C–H and H–O bond in HMFCA, generating FFCA and H2O.In this work, we have prepared Au/TiO2 catalysts modified with transition metal oxides MO x (M = Fe, Co, Ni) for HMF oxidation. Physical characterizations confirm the electron transfer from the transition metal species to Au and Ti, generating Au δ−–Ov–Ti3+ interface. The kinetic study reveals that the oxidation of hydroxyl group is the rate-determining step during FDCA production from HMF. The selective poisoning experiment demonstrates that superoxide free radicals stem from O2 instead of base is the dominant factor governing the catalytic activity. On this basis, HMF oxidation under base-free conditions has been carried out, achieving a FDCA yield of 71.2%. Studies on structure–performance unveil that the Au δ−–Ov–Ti3+ interface is the active sites for hydroxyl group oxidation: Au δ− sites enhance O2 adsorption and activation on the catalysts surface, and Ov–Ti3+ sites act as the role of “killing two birds with one stone”: enhancing adsorption of both HMF and O2, and accelerating the activation and dissociation of H2O. Therefore, this work demonstrates the synergic catalysis during HMF oxidation and achieves a better understanding of the reaction mechanism, which would be constructive for rational design of other heterogeneous catalysts.The authors declare no competing financial interest.We gratefully acknowledge the support of State Key Laboratory of Chemical Engineering (No. SKL-ChE-20A02), and the support of International Clean Energy Talent Program by China Scholarship Council.The following is/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.gee.2021.09.006.

Despite wide applications of noble metal-based catalysts in 5-hydroxymethylfurfural (HMF) oxidation, promoting the catalytic performance at low loading amounts still remains a significant challenge. Herein, a series of metal oxide modified MO x -Au/TiO2 (M = Fe, Co, Ni) catalysts with low Au loading amount of 0.5 wt% were synthesized. Addition of transition metal oxides promotes electron transfer and generation of the Au δ−–Ov–Ti3+ interface. A combination study reveals that the dual-active site (Au δ−-Ov-Ti3+) governs the catalytic performance of the rate-determining step, namely hydroxyl group oxidation. Au δ− site facilitates chemisorption and activation of O2 molecules. At the same time, Ov-Ti3+ site acts as the role of “killing two birds with one stone”: enhancing adsorption of both reactants, accelerating the activation and dissociation of H2O, and facilitating activation of the adsorbed O2. Besides, superoxide radicals instead of base is the active oxygen species during the rate-determining step. On this basis, a FDCA yield of 71.2% was achieved under base-free conditions, complying with the “green chemistry” principle. This work provides a new strategy for the transition metal oxides modification of Au-based catalysts, which would be constructive for the rational design of other heterogeneous catalysts.

No data was used for the research described in the article.With the increasing concern of climate change, the demand of renewable and carbon–neutral energy is also arising. The IEA (International Energy Agency) scenarios approaching the carbon neutral policy advocated the need of electrification but also steady requirement of conventional liquid fuel in the transportation sector. Even the usage of renewable energy is the highest in NZE (Net Zero Emissions by 2050) scenario, the market share of EVs (Electric Vehicle) is predicted to reach only 60 %. Carbon neutral energy such as hydrogen, ammonia and biofuels should redeem the shortage. Among the biofuels, biodiesel is anticipated to be a carbon neutral liquid fuel that can fulfill the surplus energy demand [1].Biodiesel can be obtained from transesterification of triglyceride (TG) with alcohol in the presence of base catalyst. Generally, the commercial biodiesel production is based on homogeneous base catalysts such as sodium or potassium hydroxide due to their high reaction rate and reasonable cost. Unfortunately, those catalysts are difficult to be extracted and recovered from the product. On the contrary, heterogenous catalysts can mitigate these problems by simplifying the separation between the product and catalyst. Various heterogeneous base catalysts have been studied for biodiesel production for instance alkali-doped materials, alkaline earth metal oxides, transition metal oxides and natural clays [2] to obtain sufficient initial activity, stability and life time. The results of previous studies on the literature are summarized in Table 1 . Particularly, the catalyst stability and life time can be simulated by the sustained conversions over the repeated batch reactions with the recovered catalysts from previous run.Alkali and alkaline earth metal oxides have been widely used for biodiesel production. Na doped aluminate was the first alkali metal applied to biodiesel production. Highly dispersed Na in Na/NaOH/γ-Al2O3 performed almost the same activity as that of homogeneous NaOH catalyst [37]. Li et al. impregnated Li over NaY zeolite [5]. The catalyst showed initial biodiesel yield of 98.6 % which declined to below 80 % after 6 repeated cycles due to the hydrolyzation of active components. Even though some of the catalytic activity could be recovered through the additional calcination, approximately 10 % loss of initial activity was inevitable. Lani et al. prepared SiO2 impregnated CaO as heterogeneous catalyst for biodiesel production [18]. The yield of biodiesel reached 90 % in the first run with a remarkable reduction from fifth to tenth use of catalysts dropping to 52.5 %. The deactivation of the catalyst might be resulted from the bond breaking leading to leaching of active metal and the formation of inactive intermediate species such as Ca(OH)2 and calcium diglyceroxide. Dai et al. calcinated Li2CO3 with TiO2 from 700 to 1000 °C producing active Li-O-Ti species present in the new crystal structure. Li present in the crystal structure could reinforce the activity and prevent leaching of the species. The conversion showed 98.2 % at the first batch run which decreased to 80 % over 10 repeated cycles [15].With the significant efforts, the deactivation caused by leaching and deformation of active components is still remained to be solved to develop active and stable heterogeneous catalysts. In this study, we introduced Na on the graphitic carbon nitride (GCN) as a novel heterogenous base catalyst suitable for transesterification of soybean oil. GCN was known to have graphene-like structure comprised of C and N atoms with H impurities [38]. The N atoms having lone-paired electrons could donate them performing as Lewis base sites. There have been few studies using GCN for biodiesel production focusing mainly on the initial activity of the catalysts prepared by multi steps using expensive precursors [31,39]. The present Na co-polymerized GCN showed stronger basicity compared to the bare GCN, which could be prepared through a single step using cost-effective precursors. The catalyst showed initial activity of 90 % conversion which was retained over 10 consecutive batch cycles at 70 °C. The catalyst was characterized using FT-IR, XRD, XPS and CO2-DRIFT. The characterization and DFT calculation results showed that strong electron dislocation and rigid bond between Na and N induced during the single step synthesis contributed to catalytic activity and the leaching resistance, respectively. The developed Na co-polymerized GCN catalysts might be widely applied to the newly constructed biodiesel production facilities.Pristine graphitic carbon nitride (GCN) was synthesized by the co-thermal polymerization of melamine (Sigma Aldrich) at 550 °C (10 °C/min) for 4 h in a crucible with a cover. The pale-yellow powder obtained was ground and sieved, and particles between 300 and 400 µm were collected.Suitable amounts of NaOH and melamine were ball-milled and the white solid solution obtained was ground and thermally polymerized using the same procedure used for Pristine GCN preparation. The catalysts were labeled as Na-GCN-n-copol, where n indicates the gram of NaOH introduced when the weight of melamine is 1 g.For comparison, Na impregnated catalyst was prepared by adding pristine GCN to NaOH aqueous solution. The mixture was sonicated for 1 h and transferred to rotary evaporator followed by calcination under N2 flow at 550 °C for 2 h. The concentration of NaOH solution was controlled to obtain Na impregnated catalyst with the identical amount of Na present in Na-GCN-0.25-copol catalyst assuming that all the Na in the solution was deposited on the pristine GCN. Thus, obtained catalyst was denoted as Na/GCN-imp.29.1 g of soybean oil, 71.3 g of methanol and 11 wt% of catalyst were transferred to a round-bottom flask and stirred magnetically. The assemblies were placed on a hotplate and heated to 70 °C while mixing continuously at a rate of 375 rpm. No weight difference was observed during the reaction.Following the reaction, the final liquid products was filtered using a glass vacuum filtration apparatus. The liquid products were diluted with methanol prior to the GC injection. For the reusability test, the used catalysts were collected and recycled after washing them with hexane (100 ml/g) and then drying them at 60 °C.The amount of fatty acid methyl ester (FAME) was measured by gas chromatography (GC, Agilent 6890, Agilent) equipped with an INNOWax capillary column (30 m × 0.32 mm × 0.5 μm, Agilent) and flame ionization detector (FID). The injector was set at 250 °C with an injection volume of 1 μl and a split ratio of 80:1. The oven temperature was held constant at 210 °C for 9 min, increased to 230 °C at a speed of 20 °C/min and then kept constant for 10 min. The biodiesel production yield was calculated using the following equation (1): (1) Yield [%] = (FAME product [g])/(soybean oilfeed [g]) × 100 FAMEproduct [g] =. ∑ ( F A M E ) conc . × M W × t o t a l p r o d u c t v o l u m e ( F A M E ) conc . : FAME molar concentration measured by GC for each methyl esters contained in soybean oil. MW : molecular weight of each methyl esters.The determination of biodiesel yield may be calculated following the internal standard method using methyl heptadecanoate (C17) as described in the European Standard EN 14103:2020 and in many references [40,41]. The internal standard method ensures proper accuracy when the analyzing sample contains ester content of FAME greater than 90 %. In the present work, depending on the catalysts, the ester content in the samples varied from 25 to 95 %. Hence, direct measurement of FAME concentration was adopted as mentioned in many recent publication [21,29,35,37,42–44]. The calibration curves of the standard solution of the five main methyl esters contained in soybean oil were obtained as shown in Fig. S2, which confirmed linearity and precise accuracy. The total weight of the produced FAME was calculated by adding the product of FAME concentration, molecular weight and the total product volume as given by Eq.(1). The molecular weight of methyl palmitate, methyl stearate, methyl oleate, methyl linoleate and methyl linolenate was 270.5, 298.5, 296.5, 294.5 and 292.5 g/mol, respectively.FT-IR spectra were obtained using a Nicolet iS50 spectrometer (Thermo Fisher Scientific). Prior to measurement, the catalyst sample was diluted with KBr powder and pressurized at 400 bars for 30 min. In situ DRIFT spectra were collected under CO2 flow (30 ml/min) in order to analytically confirm the basicity of the catalysts. Prior to the characterization, the sample cell was fully purged with Ar followed by the heat treatment at 110 °C for 1 h to remove the moisture adsorbed on the sample and cooled to 30 °C. Then the flow gas was switched to CO2 and maintained until saturation. The temperature of the sample cell was raised to the 70 °C to measure the CO2 adsorption peak. The XRD was measured with D/MAX 2500 V (Rigaku) using Ni-filtered Cu Kα radiation (λ = 0.154 nm) at 40 kV and 200 mA. The interlayer distance and crystal size were calculated using the Bragg and Scherrer equations, respectively [45]. The Brunauer–Emmett–Teller (BET) surface area was obtained by nitrogen sorption experiments conducted at −196.15 °C using a Micrometrics instrument. The sample was heated at 200 °C for 2 h at 5 °C/min before analysis in liquid nitrogen. X-ray photoelectron spectroscopy (XPS) measurements were performed using an AXIS Ultra with a delay-line detector (DLD) (Kratos Analytical) and monochromatic Al Kα (1486.6 eV) X-ray radiation. The value of the C1s core level (284.6 eV) was used as the standard peak to calibrate the chemical shift. The amount of Na present was obtained using a liquid ICP AVIO500 (PerkinElmer). The identified amount of liquid produced was vigorously mixed with DI water and then allowed to settle until the two layers were clearly distinct. The volume of the water layer was measured and used for liquid ICP analysis.In this study, all first-principle calculations were performed based on the Kohn–Sham density functional theory (KS-DFT) as implemented in the Vienna Ab initio Simulation Package (VASP) [46]. The generalized gradient approximation within the Perdew–Burke–Ernzerhof (PBE) functional form was used to exchange the correlation energy [47,48]. Plane-wave basis sets with a kinetic energy cutoff of 400 eV were used to expand the valence electron wave functions. For all structural relaxations, the convergence criterion for the energy in electronic SCF iterations, and the Hellmann–Feynman force in ionic step iterations, was 1.0 × 10-5 eV and −0.05 eV Å−1. To reduce the interaction between neighboring layers, a large vacuum space of at least 15 Å was introduced along the z-axis. A Monkhorst–Pack special k-point mesh of 1 × 1 × 1 was used to sample the first irreducible Brillouin zone. Furthermore, we analyzed the electronic structure properties, which may provide insight into the catalytic process of the reaction trajectories. In this regard, we predicted the energetic stability of the reactants and products by computing the adsorption energies of methanol, methoxyl and hydrogen on a 3 × 3 × 1 supercell of g-C3N4 with 126 atoms using the following equation: (2) Ead = ET – Eorganic –E GCN From the adsorption energies of the reactant and product, we calculated the reaction energy as follows: (3) △E = Ead(CH3O) + Ead(H) − Ead(CH3OH) − E(slab) where △E, Ead(CH3O), Ead(H), and Ead(CH3OH) were the reaction energy, adsorption energies of methoxyl, hydrogen and methanol, respectively, and E(slab) was the slab energy.FTIR spectra of the catalysts were shown in Fig. 1 . The spectra of pristine GCN was revealed to be similar with those of previous studies containing distinct CN heterocycles and tri-s- heptazine structure at 1150 to 1700 and 808 cm−1, respectively. The broad bands at 3000–3300 cm−1 were attributed to amine and hydroxyl groups derived from uncondensed melamine moiety and oxygen from air during polymerization. The intensity of CN heterocycles and amine was found to be reduced on both Na copolymerized and impregnated catalysts. Furthermore, this phenomenon was notably accelerated as the Na content was increased. The bands at 1100, 2900 and 2710 cm−1 were assigned to alkyl groups and cyano groups. During the preparation of KOH modified GCN, OH– and K+ ions were reported to interact with amine groups and break the CN heterocycle on the edge of the pristine GCN plane generating cyano groups during the copolymerization [49,50].The structural reconstruction according to the addition of Na was confirmed by XRD analysis as shown in Fig. 2 . The pristine GCN exhibited its typical diffraction pattern at 13.3° and 27.4°, corresponding to the (100) and (002) planes, respectively. The diffraction pattern of (100) plane could be understood as the GCN structure consisted of tri-s-triazine motifs, while the (002) pattern corresponded to the periodic stacking of GCN planes in c-axis [51]. The (100) peak of Na copolymerized catalysts became broader with the increasing of Na content and shifted toward lower angle, which suggested the partial cleavage of CN bonds on GCN planes. On the other hand, the for Na/GCN-imp catalyst, the diffraction pattern of (100) was not observed, which implied the full opening of tri-s-triazine structure comprising the GCN plane.Similarly, with the increase of Na content, the (002) peak of Na copolymerized catalysts became broader at the fixed position, indicating the continuous decrease of crystal size of 27.9, 9.1 and 3.2 nm for pristine GCN and Na-GCN-0.1-copol, 0.25-copol, respectively, with the constant interlayer distance of 0.32 nm. On the other hand for Na/GCN-imp catalyst, another peak at 25.2° was additionally observed, which could be resulted from the intercalated Na present between GCN layers [52] with the interlayer distance of 0.35 nm. Due to the structural modification, the BET surface area tended to decrease with the addition of Na. Pristine GCN showed BET surface area of 6.9 m2/g while that of Na-GCN-0.1-copol, 0.25-copol and Na/GCN-imp decreased to 3.8, 0.7 and 3.29 m2/g, respectively.The chemical bonding of pristine GCN and Na introduced catalysts was further investigated using XPS analysis as presented in Table 2 and Fig. 3 . The total amount of Na in the catalysts was found to be 3.5, 12.8 and 8.5 wt% for Na-GCN-0.1-copol, 0.25-copol and Na/GCN-imp, respectively. According to the previous work on literature [53], peaks at 1071 eV and 1072 eV from Na1s XPS spectra could be assigned to Na-N and Na-O bonds, respectively. As summarized in Table 2, the content of Na-N and Na-O species present in fresh catalysts was found to be 2.0, 1.5 wt% and 8.6, 4.2 wt% and 4.9, 3.6 wt% in Na-GCN-0.1-copol, Na-GCN-0.25-copol and the Na/GCN-imp, respectively. In the case of used catalysts, the residual Na was detected mainly as Na-N with the complete loss of Na-O. The Na-N content of the used catalysts was preserved to be 8.3 and 3.8 wt% for Na-GCN-0.25-copol and the Na/GCN-imp.Generally, the growth of GCN crystals is known to proceed with the bonding between strands of –NH2 groups on melamine-derivative intermediate species such as melam, melem, and melon [54] as illustrated in Scheme 1 .During the synthesis of Na copolymerized catalysts, the introduced NaOH was first dehydrogenated to Na2O, as shown in Eq.(4). The formed Na2O moiety might eliminate the NH2 groups and form Na-N bond as described in Eq. (5). (4) NaOH → Na2O + H2O (5) Na2O + –NH2 + 1/2 O2 → 2Na–N + 2 H2O Considering that the polymerization and crystal growth started from the condensation of NH2 species, the loss of those groups provoked the inhibition of further polymerization [52]. This phenomenon could explain the withdrawal of amine groups and the suppression of crystal size growth with Na content as previously discussed with the FTIR and XRD results. When the content of NaOH was further increased up to 50 wt% in the precursors (NaOH + melamine) mixture, the formation of GCN structure was totally inhibited generating only sodium cyanate (NaOCN) as shown in Fig. S1. With the proper amount of Na and melamine under mild oxidation condition, Na2O might be bonded to the NH2 strands in the intermediate species, generating NaN bonds, as described in Eq.(5). On the Na-GCN-0.1-copol and 0.25-copol, the tri-s-triazine and stacking structure were preserved as confirmed from XRD analysis results, although the elimination of NH2 species could terminate the polymerization steps generating smaller GCN crystals. This hypothesis explained the opposite inclination between the crystal size of GCN and the amount of Na in the Na co-polymerized GCN catalysts.However, during the preparation of Na/GCN-imp, NaOH solution diffused into the pre-existing stacked GCN layers with the aid of sonication. During the calcination, introduced NaOH could be converted into Na-N by forming bonding with amine group as described in Eq.(5) or coordinated with the pyridinic N atoms in the tri-s-triazine hole of the GCN plane [52]. The tri-s-triazine ring along the GCN layers could be opened through the Na introduction, which was previously confirmed by the disappearance of (100) diffraction pattern as shown in Fig. 2. The uncoordinated Na atoms located in the outer plane and between interlayers could remain as Na2O species and enlarge the interlayer distance because of the larger atomic radius of Na (∼100 pm) than N and C (70 and 60 pm) [53,55].The inherent basicity of Na-N species was further characterized using CO2-DRIFT analysis as shown on Fig. 4 . Liu et al. reported the hydrogenation of CO2 to formate over a Schiff base mediated gold nanocatalyst. CO2 was adsorbed to the Schiff base forming the carbamate (NCOO–) zwitterion, which could be measured at 1712 cm−1 on CO2-DRIFT analysis [56]. Similarly, in the present study, the catalysts showed adsorption peak between 1687 and 1706 cm−1, as shown in the Fig. 4. The basicity of the catalysts could be confirmed from the formation of carbamate species.To better understand the enhanced basicity of the Na modified GCN catalyst and the possible effect on biodiesel production, density functional theory (DFT) calculations were performed. For this purpose we examined the methanol decomposition into methoxy anion and proton (CH3OH → CH3O– + H+) on the heterogeneous catalyst surface of pristine GCN and Na modified GCN catalysts, which is believed to be the first step of the transesterification reaction [57–59]. As shown in Table 3 and Fig. S3, the reaction energy of the Na modified GCN catalyst was found to be lower than that of the pristine GCN, suggesting that the Na modified GCN catalyst had higher activity on biodiesel production. This enhancement was related to electron charge redistribution by the addition of Na to GCN. According to Bader charge analysis, the Na atom (Δσ = +0.825e) lost electrons to the neighboring N atoms (Δσ =  − 1.126e), making Na and N atoms Lewis acid and base sites, respectively. This led to the increase of binding strength of CH3O– (Ead =  − 5.87 eV) and H+ (Ead =  − 6.64 eV) on the surface of the Na modified GCN catalyst compared to the pristine GCN one (Ead =  − 4.54 eV for CH3O– and − 3.30 eV for H+) and, in turn, boosted the CH3OH dissociation reaction.The transesterification of soybean oil with methanol using GCN catalysts was carried out under identical reaction conditions which was presented in Fig. 5 . The pristine GCN had almost no transesterification activity under our reaction conditions due to low basicity. Na-GCN-0.1-copol, −0.25-copol and Na/GCN-imp showed biodiesel yield of 26.8, 90.6 and 46.1 %, respectively at the first batch run. As mentioned in Section 3.2, the base catalyst is known to activate methanol by the cleavage into the methoxide anion (CH3O–) and proton (H+). The methoxide ion, a strong base, attacks the carbonyl carbon of triglycerides, producing tetrahedral alkoxy carbonyl intermediates. CO cleavage on the tetrahedral intermediate yields methyl esters and diglycerides, respectively. Subsequently, diglyceride is further converted to monoglyceride through the nucleophilic attack of the methoxide ion, eventually producing three moles of methyl esters and one mole of glycerol. The generation of methoxide anion by base catalysts from methanol is known to be the rate-determining step, which can be accelerated with the sufficient basicity of the catalysts. The strong basic sites in the Na-GCN-0.25-copol could be expected to effectively promote the generation of methoxide anions.The catalyst reusability and stability were further investigated under 10 repeated cycle tests, as shown in Fig. 6 . The Na-GCN-0.25-copol showed constant activity over 90 % biodiesel yield under 10 cycles. From the deconvolution of Na1s XPS spectra for the used catalysts as shown in Table 2, the total amount of Na in Na-GCN-0.25-copol decreased from 12.8 % to 8.3 wt%, while Na/GCN-imp from 8.5 to 4.0 wt%. Interestingly, Na-O species present in the fresh catalysts was almost not detected in the used ones [16]. Considering the catalyst preparation condition, the introduced NaOH was believed to be converted to Na2O as previously mentioned in Eq.(4). Since the amount of Na2O measured by XPS analysis was less than 3 %, distinctive diffraction pattern of Na2O crystallite was not observed for all catalysts. However, Na2O is known to be a strong base to activate methanol generating sodium methoxide (CH3O– Na+) as Eq.(6), which will further attack the triglyceride molecule during the biodiesel production reaction. (6) Na2O + 2 CH3OH → 2 CH3O-Na+ + H2O As the introduced Na was transformed to ionic Na dissolved into reaction medium, Na2O species could be easily leached out from the catalyst surface [60]. From the ICP analysis of the liquid medium after the first batch run of Na-GCN-0.25-copol, a considerable amount of Na was detected, which corresponded to 94 % of its initial Na-O content in the fresh one. As a result, Na2O species contributed as a pseudo-homogenous catalyst only at the first batch run. This phenomenon was dominant on the Na/GCN-imp with relatively high Na2O content. The presence of a high amount of Na cation in the product fuel might cause metal corrosion as well as saponification of the biodiesel phase.However, the amount of Na-N species in the used catalysts was preserved as shown in Table 2. 96.5 and 77.6 % of Na-N content in fresh Na-GCN-0.25-copol and Na/GCN-imp were retained on the used catalysts. This result asserted the high stability of Na-N species at the current reaction conditions. As mentioned in section 3.2, the introduced Na enhanced the electron charge dislocation of the GCN structure forming the coordinate covalent bond with the neighboring N atoms. The chemical bond between Na and N atoms not only increased essentially the basicity of the N atoms but also granted the resistance against leaching of the alkali metals toward the liquid reaction medium, which was reported as the main obstacle in developing active and stable heterogeneous catalyst for commercial biodiesel production.Taking all phenomena into account, both Na2O and Na-N species were presumed to activate the biodiesel production reaction on the first batch run. As Na2O was leached out to the reaction medium and removed from the catalyst surface, Na-N species was the only available basic site for repeated batch cycles.Discriminatively for Na/GCN-imp catalyst, since the stacked structure of GCN layers were retained, Na-N species generated inside the tri-s-triazine hole in inner layers were assumed to be unaccessable as the bulky TG molecules might not be able to diffuse within the layers spacing of 0.35 nm. As a result, both fresh and used Na/GCN-imp catalysts showed low yield of biodiesel considering the relatively large amount of present Na-N species. Hence, rapid deactivation of Na/GCN-imp catalyst over repeated batch cycles was observed as Na2O species were not available due to leaching, eventhough considerable amount of Na-N species were present.Herein, a novel and economic Na-modified GCN catalyst with enhanced basicity was firstly fabricated and applied to the transesterification of soybean oil. Copolymerization of melamine and NaOH was found to be effective in generating stable and active basic heterogeneous catalysts. The Na atoms were believed to transfer electrons forming rigid bond with neighboring N atoms, which enhanced the Lewis basicity and the stability of the Na-GCN-copol catalysts. While the majority of Na species in impregnated catalysts existed in the form of Na-O, which was easily leached out to the reaction medium showing rapid catalyst deactivation over the repeated cycles. As a matter of fact, the Na-GCN-copol catalyst maintained over 90 % of biodiesel yield during 10 repeated cycles with the aid of its leaching resistance property. Sung Eun Kim: Methodology, Data curation, Writing – original draft, Formal analysis. Ji Hu Kim: Formal analysis, Methodology. Deog Keun Kim: Supervision, Project administration. Hyung Chul Ham: Supervision, Data curation, Writing – original draft. Kwan-Young Lee: Supervision. Hak Joo Kim: Writing – review & editing, Conceptualization, 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 a Grant-in-Aid from the National Research Council of Science & Technology and Korea Institute of Energy Research (Project C2-2431, Development of hetero-catalytic system for multi-feedstock biodiesel and platform chemical production).Supplementary data to this article can be found online at https://doi.org/10.1016/j.fuel.2023.127548.The following are the Supplementary data to this article: Supplementary data 1

Na-modified graphitic carbon nitrides were utilized for the transesterification of soybean oil and methanol. Graphitic carbon nitrides have not yet been widely applied in biodiesel production, despite their chemical stability and basicity. The catalysts were obtained via the co-thermal polymerization of NaOH and melamine. Catalyst prepared using conventional impregnation method was applied for comparison. The copolymerized catalyst with the optimum Na content showed over 90% biodiesel yield for 10 repeated cycles. The prepared catalysts were characterized by Fourier transform infrared spectrometry, X-ray diffraction, scanning electron microscopy/energy dispersive X-ray spectroscopy, CO2-diffuse reflectance infrared Fourier transform spectroscopy, and X-ray photoelectron spectroscopy. The elaborate rigid bonds between Na and N contributed to the leaching-resistance and catalytic activity. While the majority of Na species in impregnated catalysts existed in the form of Na-O, which was easily leached out to the reaction medium showing rapid catalyst deactivation over the repeated cycles. The basicity derived from the electron transfer from the Na to N atoms was confirmed from the density functional theory and CO2-diffuse reflectance infrared Fourier transform spectroscopy.

Fast pyrolysis of biomass with a rapid heating rate (>500 °C/s) to intermediate temperatures (400–600 °C) is a promising way to generate bio-oil from the fast decomposition of biomass in the absence of oxygen, by which the short vapor residence time can lead to high bio-oil yield with less other products like gas and solid char [1,2]. For example, flash pyrolysis or very fast pyrolysis with rapid heating rate (>1000 °C/s) has been proven to provide high yield of bio-oil as well as high conversion efficiency even over 70% [3]. However, the obtained bio-oil obtained from the fast pyrolysis always has a dark brown appearance with a distinctive smoky smell. The physical properties of bio-oils have been reported by many researchers [4,5], which are determined by the chemical compositions in the pyrolysis bio-oils. Usually, there are several hundreds of organic compounds in the raw bio-oils, which mainly include phenols, acids, aldehydes, alcohols, esters, ketones, and other macromolecules and can be classified to three major groups: (I) small carbonyl compounds including carboxylic acids, hydroxyaldehydes, hydroxyketones, acetone, acetaldehyde and acetic acids; (II) sugar-derived compounds including levoglucosan, furfural, anhydrosugars and furan/pyran ring-containing compounds; and (III) lignin-derived compounds, majorly including guaiacols and phenols [6]. Besides, oligomers with molecular weights in a range of 900–2500 also exist in the bio-oil with a large amount [7,8]. These compounds distribution is mainly determined by biomass type and pyrolysis route, which is related to the physicochemical properties of bio-oil [9,10]. The basic properties of raw bio-oil and the petroleum fuel oil are compared in Table 1 [11,12]. Obviously, the bio-oil contains much more oxygen and H2O contents, which leads to lower heating value. Higher heating values (HHV, MJ/kg) of the pyrolysis bio-oils from wood are usually ranged from 16 to 19 MJ/kg, which are just about half of the petroleum fuel oil (40 MJ/kg) even in the highest case. It can be concluded that the low quality of raw bio-oil is resulted from its high oxygen and H2O contents with some solids components, high viscosity, low pH value, thermal instability with poor combustion property [13]. Especially, the high oxygen content is the main reason for its low heating value. In addition, the unsaturated components such as phenols and aldehydes in it are unstable, which can easily transform into macromolecules through polymerization, particularly in those acid conditions, increasing the viscosity and reducing liquidity. Thus, the application of it is still limited by these characterizations. However, despite these shortcomings, the bio-oil also behaves some advantages like less toxicity, easier biodegradation and better lubricity than the petroleum fuel. Hence, it is desired to improve the quality of the bio-oil so that it can replace fossil fuels in the future [14].Generally, there are two types of operation modes for the bio-oil upgrading [2,15–17]. One is in situ catalytic pyrolysis, in which the biomass and catalysts are thoroughly mixed. In this case, the catalysts are in situ exposed to the pyrolysis vapor, where the pyrolysis vapor diffuses promptly into the catalyst pore, undergoing a series of reaction processes including cracking, deoxygenation, aromatization and condensation [2,15,16]. However, only the pyrolysis vapor passes through the catalyst layer, which is separated from the biomass on the upside by quartz wool. The name given to this process is in situ catalytic upgrading. The schematic diagram of the experimental setup is shown in Fig. 1. The other mode is ex situ upgrading, in which biomass and catalysts are located separately in one reactor with two reaction zones or the biomass pyrolysis and catalytic bio-oil upgrading are performed in two reactors separately [17]. In this case, the temperatures for biomass pyrolysis and catalytic bio-oil upgrading can be regulated individually to achieve the best operation conditions for the two processes, which allows for well control of product distribution and selectivity.Cracking of bio-oils over porous solid catalysts such as zeolite-based catalysts at ambient pressure is considered one of effective ways for the bio-oil upgrading, especially in which hydrogen gas is not necessary. The key for the catalytic cracking of bio-oil is the development of high-performance catalysts. Herein, zeolite-based catalysts for the upgrading of pyrolysis bio-oils are critically reviewed. The effects of porous structure, acidity and other parameters including biomass type, catalyst amount and reaction temperature on cracking activity, selectivity, stability and deactivation are summarized. While, the proposed mechanisms on the bio-oil upgrading over the zeolite-based catalysts for raising the contents of hydrocarbons like benzene, toluene and xylenes (BTXs) and hindering the generation of those by-products like coke and polyaromatics are discussed. Furthermore, the main strategies such as metal modification, construction of zeolites with a hierarchical structure and synthesis of special morphologies with hollow structure or core/shell structure for the improvement of deoxygenation property performance are introduced. It is expected to provide a guidance for the design and fabricate more excellent zeolite-based catalysts and their application for production of high-quality bio-oil from the fast pyrolysis of biomass.Zeolites are microporous aluminosilicate solids named as “molecular sieves”, which can accommodate various cations such as Na+, K+, Ca2+ and Mg2+ and adsorb those molecules with comparable sizes corresponding to their pore window sizes. Zeolites have been widely used as commercial ion-exchange materials, adsorbents and catalysts. Zeolites have been applied to catalyze various reactions owing to their special acidity-basicity as well as shape selectivity. Herein, the shape selectivity could be resulted from either the transition state effect or mass transfer [18]. The various micropore sizes (0.5–1.2 nm) can affect the mass transfer, thereby excluding certain reactant molecules and limiting the formation of products larger than the zeolite pore size. While, the confined space in the zeolite pore can restrict certain transition states, thereby influencing the reaction routes. In addition, zeolites also have “solvent effect” or “confinement effect”, where some reactants inside the zeolite pore have higher concentrations than those outside the pores [18]. To date, various zeolites including Beta zeolite, Y zeolite and SSZ-55 with large pores, ZSM-5, ZSM-23, ZSM-11, IM-5, TNU-9, Ferrierite with medium pores and ZK-5, SAPO-34 with small pores have been investigated for the bio-oil upgrading [18]. It is found that most of these zeolites could enhance the formation of aromatics during the upgrading process, and some of them, especially the protonated ZSM-5 (HZSM-5) as well as Beta zeolite (Hβ) always gave higher aromatics yields. Since the effects of various parameters for the bio-oil upgrading over various zeolite catalysts are similar, herein, the bio-oil upgrading over HZSM-5 is mainly reviewed.HZSM-5 is a microporous aluminosilicate zeolite with characteristics suitable for aromatics and olefins production in the petrochemical industries due to its adequate combination of acid strength and shape selectivity. Similarly, HZSM-5 based catalysts have been widely employed in the upgrading of bio-oil obtained from the biomass pyrolysis by enhancing those reactions relating to deoxygenation including decarboxylation, decarbonylation, dehydration, isomerization, and aromatization [19]. In our previous study [20], in-situ catalytic upgrading of bio-oil derived from fast pyrolysis of lignin over different zeolite catalysts was investigated, and found that HZSM-5 was more active for the improvement of bio-oil quality in terms of the highest selectivity towards monoaromatic hydrocarbons. Simultaneously, the utilization of ZSM-5 resulted in the highest yield of light oil and the lowest yield of coke among all the applied zeolite catalysts. Engtrakul et al. [21] studied the catalytic pyrolysis of pine wood biomass in a fluidized bed reactor at 450 °C utilizing various zeolite catalysts including Beta, Y, ZSM-5, and Mordenite. By the using of ZSM-5, lower acid and alcohol contents were contained in the liquid products. While, coke deposition on ZSM-5 appeared to be lower than that on other zeolites. It is considered that adequate balance between acid strength and shape selectivity of ZSM-5 should be beneficial for the conversion of biomass-derived oxygenates into aromatic hydrocarbons.The acid strength and acid site density of HZSM-5 catalysts always vary with the SiO2/Al2O3 ratio, which can determine the catalytic activity, deactivation, and product distribution [21]. There are two types of acid sites on HZSM-5, i.e., Brønsted acid site and Lewis acid site. High acidity with low Si/Al ratio is controlled especially by the Brønsted acid site, which becomes more active in the cracking process, leading to the production of more aromatics such as benzene, toluene and xylene (BTX) and light olefins with the reducing of heavy oil fraction [22,23]. However, the higher acidity will result in further secondary reactions to produce more polyaromatics and even form coke on the zeolite surface, thereby causing the deactivation of the catalyst. Polyaromatic hydrocarbons (PAH) have been reported as the precursor of coke. With the decrease in the SiO2/Al2O3 molar ratio of zeolite, more polyaromatic hydrocarbons (PAH) and water will be generated with the reduced amount of light bio-oil yield [22,24]. Thus, proper adjustment of acidity to make HZSM-5 suitable for the aromatic production has been proposed by using several techniques such as ion exchange, desilication, and dealumination [25–27.Surface area is one of main factors affecting catalytic efficiency. Typically, various HZSM-5 zeolites have specific BET surface areas in the range of 350–450 m2/g [28]. The surface area is strongly correlated with the acidity, thereby affecting catalytic activity. In general, ZSM-5 is composed of the external surface and internal surface. The external surface area corresponds to the number of the entrances to the pores, which determines the effective internal surface area, and acidic sites existing on the surface [29]. The selectively chemical reactions catalyzed by zeolites always tend to occur within the pores, and thus the pore dimension and structure of zeolite have shape selectivity to the product. For bio-oil upgrading, catalytic cracking of large molecules tends to occur on the acid sites of the external surface, where the acid sites on the internal surface cannot effectively catalyze macromolecular cracking due to the pore size limitation [30]. In this case, the surface area is related to the formation of coke on both the exterior and internal surfaces of the zeolite and especially, the production of coke on the interior surface could lead to a greater deactivation rate of zeolite by the covering of acidic surface and blocking of pores. Therefore, zeolite catalysts with a smaller outer surface area but a larger internal surface area may have a higher rate of deactivation [29]. On the other hand, those large molecules in the bio-oil can readily diffuse into the pore and access more active sites within the pores when the surface area and pore size improve simultaneously. As a result, they can be rapidly converted to hydrocarbons and consequently reducing the formation of coke. Especially, HZSM-5 catalyst with a high surface area could be more easily modified by metal loading with less significantly reduction of the surface area [31]. In addition, a large surface area could delay the deactivation of HZSM-5 catalysts caused by coke formation and deposition on the exterior surface. It should be noted that the coke deposition within the zeolite pores could inhibit the capillary and diffusion flow of reactants, finally lowering the aromatics yielding reaction rate [32]. Thus, it is better to avoid the occurring of the coke deposition within the zeolite pores. The HZSM-5 catalyst has a higher surface area and a larger pore size, with a shorter diffusion length, resulting in reactions over it, such as biomass depolymerization to produce more monocyclic aromatics like BTX with less coke. It is reported that the nanosheet ZSM-5 catalysts had better textural properties with higher BET surface area, larger mesopore volume, and higher concentration of external Brønsted acid sites than the conventional HZSM-5, thereby resulting in better performance for catalytic cracking reactions [33].The mass transfer ability of bulky reactants and products to pass through the micropore in the zeolite is determined by the pore size, thereby significantly affecting the catalytic activity. The chemical reactions catalyzed by zeolites are thought to occur mainly within the internal pore of the zeolite. As a result, a reactant with a molecule size larger than that of the zeolite pore cannot diffuse into the zeolite pore. Similarly, the product formed within the zeolite pores cannot be larger than the pore size. Therefore, the pore size and structure of the zeolite have a substantial influence on the production of products in the pores and the diffusion of products from the pores. HZSM-5 has micropores consisting of two intersecting three-dimensional channels of 10-membered rings with one straight channels (5.1 Å × 5.5 Å) and sinusoidal channels (5.3 Å × 5.6 Å) [34,35]. When the cracking reaction occurs within the pore, if the formed product can only diffuse slowly out of the pore, a small amount of product will be obtained. For those molecules in the raw bio-oil larger than the pore size of HZSM-5 such as levoglucosan and 5-hydroxymethyl furfural, they could be more likely converted to coke outside the pores since they cannot enter the pores [36]. Benzene, toluene, indene, ethylbenzene, and p-xylene have been reported to be pyrolyzed to intermediate intermediates that can easily diffuse into the pore due to the typical kinetic diameter of HZSM-5 [18]. Thus, the mass transfer will be improved by increasing the pore size in the HZSM-5 based catalysts [37,38]. However, it has also been reported that the zeolites with large pores have low stability, leading to the generation of undesirable compounds, which is also the primary cause of the deactivation [12].Zeolite is a crystalline substance with a structure characterized by a framework of linked tetrahedra (i.e., AlO4 and SiO4), each consisting of four O atoms surrounding a cation. This framework contains open cavities in the form of channels and cages which are large enough to allow the passage of those suitable guest species [39]. In general, there are several zeolite structures which are classified according to pore diameter and ring size and are given different names as shown in Table 2 . These dimensional properties are important for their role in determining aromatic yield and especially, the "n" membered ring shown in Fig. 2 can determine the zeolite structure and pore shape.The pore shape of zeolite is one factor that greatly influences aromatics production.. Micro-pore zeolites such as SAPO-34 with a CHA structure connected by 8-ring channels always cannot produce any aromatics but generate CO, CO2 and coke, which show the performances similar to pyrolysis bio-oil derived by non-catalyst pyrolysis. In contrast, those zeolites such as ZSM-5 and ZSM-11 with medium pore sizes are very suitable for biomass conversion into aromatics because they have moderate pore openings (0.5–0.6 nm) which are favor of aromatics production [40]. However, although ZSM-5 and ZSM-11 have similar framework and pore size, they also have some differences. ZSM-5 zeolite has an MFI crystal structure consisting of two perpendicularly intersecting channels of 10-membered rings (straight channels and zigzag or sinusoidal channels) (Fig. 3 ) [41,42]. These channels are connected at right angles to straight channels at zigzag angles to form 3 intersections per unit cell. Whereas, ZSM-11 has only straight channels along a- and b-axis, the absence of opening channels along the c-axis in ZSM-11 limits reactant molecules entering into the channels [43]. Thus, these two zeolites with similar physicochemical properties in pore size and acidity display different molecular shape-selective properties due to their difference in channel tortuosity. Jae et al. [18] studied the conversion of glucose to aromatics over various zeolites such as SAPO-34, ZSM-11, ZSM-5, ferrierite, Beta and Y zeolites with different pore shapes and sizes. The results confirmed that ZSM-5 provided the highest yield of aromatics with the lowest coke formation.On the other hand, those zeolites with larger pores such as BEA and FAU (beta, X, Y zeolite) have a three-dimensional (3D) pore system of 12- and 12-membered ring channels with increased accessibility of more intermediates generated from biomass pyrolysis to the pores. However, the secondary reactions in the pores could cause pore blockage and coke deposition, consequently resulting in low aromatic yield [44]. It is reported that among zeolite catalysts of H-β, HY, H-USY and HZSM-5, HZSM-5 is the most effective catalyst in promoting the yield of aromatics for the upgrading of bio-oil from lignin due to its well-balanced acidity and shape selectivity [42]. As stated above, the conversion of biomass-derived oxygenates to aromatics over the zeolite catalysts depends on the reactants entering the pore, converting in the pore, and pore shape selection of the generated products to diffuse out of the zeolite pores [45]. Thus, poor shape selectivity and diffusion limitation could hinder the reactants to access the catalytic active sites of the zeolite, which is a common issue with zeolite catalysts for the desired products formation in terms of quantity and quality. That is, only the oxygenated components in the pyrolysis bio-oil captured by the narrow pores of HZSM-5 can convert to aromatic hydrocarbons due to their shape selectivity.Generally, the upgrading of bio-oil could be influenced by not only the properties of the ZSM-5 based catalysts in terms of shape selectivity, pore size, surface area and acidity, but also the biomass type, catalyst to biomass ratio and reaction temperature.The kind of biomass has great impact on the compositions of the raw bio-oil since different kinds of biomass has different compositions of cellulose, hemicellulose, lignin, ash contents with different inorganic minerals especially alkali and alkaline earth metal (AAEM) species (Table 3 ). Biomass can be classified into hardwood (e.g., oak, beech), softwood (e.g., pine, cedar, corn stalk), and grasses (e.g., barley straw, bagasse). In general, softwood contains more lignin than hardwood while grass biomass contains less lignin than woody biomass but has a higher ash content [46]. The decompositions of cellulose and hemicellulose occur at relatively lower temperatures than that of lignin. Lignin is a more stable component, which normally begins to decompose at a temperature above 200 °C and leaves 40% of residual solid product at the end of pyrolysis. In addition, lignin is known to contain phenolic compounds, which is the most abundant source of aromatic hydrocarbons produced from the biomass [47]. Thus, it should be more suitable to select biomass with more lignin for the production of bio-oils with more aromatic hydrocarbon contents when using HZSM-5 based catalysts. Terry et al. [48] reviewed bio-oil production from pyrolysis of oil palm biomass by using the different parts of oil palm. It is found that the different bio-oil compositions obtained from the different parts of oil palm. For example, oil palm trunk (OPT) has relatively high cellulose and lignin contents, which contributes to the formations of acids and phenols. While, palm kernel shell (PKS) has a higher lignin content, which contributes to the formation of phenolic compounds. Especially, with the catalysis of HZSM-5, those compounds such as acids and phenols will be converted into aromatic compounds.For the upgrading of bio-oils from in-situ pyrolysis of biomass, it is always critical to choose a suitable catalyst-to-biomass ratio for the upgrading of bio-oil. When the proportion of the catalyst used is too small, the in-situ generated raw bio-oil cannot completely contact the catalyst. Especially, a part of catalysts could have already deactivated during the initial stage of pyrolysis by cracking of previous pyrolysis vapor. As such, the obtained bio-oil will consist of both upgraded oil and non-upgraded oil, resulting in under-desired bio-oil. Thus, it is important to optimize the catalyst-to-biomass ratio during the bio-oil upgrading process.The total products obtained from the pyrolysis process are heavily influenced by the reaction temperature. Pyrolysis is typically performed at temperatures in the range of 350–650 °C. At a lower temperature (<350 °C), it is always difficult to completely devolatilized the volatile compounds in the biomass. Moreover, a certain increase in vapor residence time as the temperature rises from 350 to 500 °C will result in a higher bio-oil yield. As the reaction temperature rises, more bio-oil will be generated. However, with an increase in temperature over 650 °C, some secondary reactions also occur, which will decrease the generation of more bio-oil since the secondary reactions predominate with the continuous increasing of gaseous products as shown in Fig. 4 [57] The stability of HZSM-5 catalysts is of great importance to their performances. The deactivation of HZSM-5 catalysts is always caused by coke-induced blockage of reactants and products on active sites and/or the sintering of active species on the catalyst surface [58]. Thus, the high-performance HZSM-5-based catalysts must be durable and capable of preventing self-accumulating coke or sintering, or even in the case that coke deposition or sintering does occur, it has little effect on the catalytic activity and still has good catalytic efficiency.During catalytic bio-oil upgrading, various reactions occur, of which the coke formation is mainly resulted from the polymerization of aromatics and olefin, leading to the blockage of pore opening and finally the deactivation of HZSM-5 catalysts [59]. Herein, the acidity affects not only the catalytic activity but also the deactivation by the promoting of coke deposition. The HZSM-5-based catalysts with high stability could be achieved by tuning the acid sites by adjusting the aluminum speciation on the external surface [60,61]. For example, dealumination from HZSM-5 structure can be realized by treatment with acid solutions (e.g., HCl, HNO3 and HF solutions) [60,62,63]. The dealumination can decrease Brønsted acidity, resulting in slower polymerization and less coke formation [63]. While, the introduction of mesopore into the structure of ZSM-5 catalysts by post-treatment with alkaline solution is also one way to reduce the coke formation. In this case, the bulk Si/Al ratio, micropore volume, and crystallinity can be decreased while the Lewis acidity is increased to improve the coking resistance [64].For those active metal modified HZSM-5 based catalysts, the agglomeration of metal species on the HZSM-5 surface, i.e., catalysis species sintering, during the upgrading process is always one of the reasons for the deactivation. It is reported that some noble metal species doped on the HZSM-5 catalysts are easily accumulated, causing the deactivation since the sintering results in the losing of active sites and blockage of the zeolite pores [63]. Especially, excessive amount of metal loading and low metal species dispersion could lead to the high accumulation of metal species during the reaction, leading to the decreases in mass transfer as well as catalytic activity.There are three common types of deoxygenation reactions over the zeolite catalysts: dehydration, decarboxylation and decarbonylation. The bio-oil always contains a considerable number of components with -OH group. The dehydration is a process to remove oxygen from those oxygenated compounds in the form of water. When the dehydration occurs on the zeolite catalysts, Brønsted acid sites play the primary role, that is, they can donate protons to the hydroxyl group of oxygenates to generate water. The decarboxylation is a process to remove oxygen from those fatty acids and fatty acid methyl esters in the form of carbon dioxide. The decarbonylation is a process to remove oxygen in the form of carbon monoxide, in which the carbonyl groups can be removed from those aldehydes and ketone compounds. Either the decarboxylation or the decarbonylation could be affected by the acidity of zeolite. Meanwhile, some carbon and hydrogen could be lost by coke formation and/or the generation of gaseous hydrocarbons by the bio-oil vapor cracking over the zeolite catalysts [65]. There are also other reactions involved in the upgrading of bio-oils, such as cracking, aldol condensation, ketonization and aromatization. Such a combined complex upgrading process will finally achieve the conversion of oxygenated compounds to hydrocarbons, thereby enhancing bio-oil quality [66].While, the aromatization on the zeolite catalysts also occurs, by which those olefins and low molecular weight oxygenates (e.g., acids, aldehydes, alcohols, esters, furans and ethers) will convert to aromatic hydrocarbons. During the aromatization process, other reactions may occur simultaneously, such as cracking, dehydrogenation, oligomerization and cyclization. Aromatization generally takes place within the pores of zeolites. Two typical examples of aromatization are: the combination of propylene and furan to produce toluene or the reaction of benzene with furan to the form naphthalene via Diels-Alder condensation reaction [32]. Fig. 5 shows the possible reactions in bio-oil upgrading over catalysts. Fig. 6 shows a possible reaction network for upgrading of bio-oil derived from the pyrolysis of biomass over the catalysts [67]. In order to achieve a good upgraded products distribution, it is important to choose a proper catalyst with excellent selectivity and high activity. During the pyrolysis of cellulose and hemicellulose, anhydrosugars are firstly generated by the depolymerized with the deoxygenation in the forms of CO2, CO and H2O by cracking and dehydration. Then, these generated sugars are dehydrated and re-arranged, resulting in the formation of furans and some small oxygenated species [68]. These intermediates can be further deoxygenated to hydrocarbons with the assistance of catalyst. While, for the pyrolysis of lignin, those phenol alkoxy compounds can be generated by cracking, dehydration and depolymerization [69]. Here, it should be noted that the char is always more easily produced from the pyrolysis of lignin than that from cellulose or hemicellulose. Similarly, those oxygenated compounds from the pyrolysis of lignin can be deoxygenated on the active sites of catalysts. Herein, stronger acidity is more beneficial for the cleavages of C-C and C-O bonds prior to deoxygenation, thereby increasing the generation of more small hydrocarbons like BTXs. On the other hand, further polymerization and aromatization of those small hydrocarbons could occur in the presence of catalysts with higher acidity, leading to the coke formation on the active sites. Therefore, it is important to adjust the acidity of catalysts in order to enhance the performance and avoid coke formation.Despite that HZSM-5 is the most popular zeolite among other types for deoxygenation of bio-oils to make aromatics since it has suitable acidity, excellent heat tolerance, strong selective cracking ability with well isomerization property, the small micropore structure of the ZSM-5 restricts the mass transfers of the reactant and product in the pore, which makes carbon deposition and catalyst deactivation easier. More crucially, the location of a large amount of acid sites inside the pore affected catalytic efficiency, always resulting in a lower yield of the desired product. To resolve these issues, the parent HZSM-5 can be restructured by introducing relatively larger mesopore or added new active sites by metal loading to increase the resistance to carbon deposition. Moreover, great efforts have been made to modify the Si/Al ratio framework by dealumination/alumination or desilication as it is directly related to the catalytic performance.Although the parent HZSM-5 is the most efficient catalyst for the bio-oil upgrading, the coking rate over it is also quite high, leading to a significant deactivation problem. As such, the aromatics generation rate is always significantly low. Thus, the parent HZSM-5 catalyst needs to be further modified to improve the upgrading performance. Currently, the performance of parent HZSM-5 catalyst is usually improved by metal loading, which is an easy way not only for the catalyst performance improving but also the coking resistance due to the simple preparation procedure and high ability to alter the acidity of HZSM-5 for achieving the optimal upgrading result [70]. To date, the types of doping metal have been widely investigated. By doping of transition metal or noble metal on HZSM-5, it is feasible to boost deoxygenation capacity and produce more carbon oxides with less water, resulting in more hydrogen available for incorporation into hydrocarbons [71]. While, alkaline earth metals such as Mg and Ca doping on HZSM-5 can behave as bases and their metal cations could function as Lewis acid sites, which allow tailoring the zeolite activity to avoid excessive cracking of the bio-oil, and in turn result in a higher yield of the deoxygenated compounds in the upgraded bio-oil with the decreasing of the formation of undesired polyaromatic hydrocarbons and coke [65].Transition metal modified HZSM-5 catalysts for bio-oil upgrading have been widely reported. For example, Ni- and Co-modified HZSM-5 catalysts especially the HZSM-5 modified by Ni can effectively increase the aromatic hydrocarbons content in the upgraded bio-oil [71]. While, the HZSM-5 modified by 2 wt% of Zn resulted in the increase in the strong acid site content, thereby increasing the BTX yield during the bio-oil upgrading process. However, further doping of Zn (e.g.,10 wt.%) reduced the acidity and physical characteristics of the catalyst, leading to poor reactant and product diffusions in the zeolite pore, thereby reducing the BTX yield [72]. Razzaq et al. [73] modified ZSM-5 by various metal species, i.e., Co, Ni, Zn and Fe, and found that the metal modification, particularly Fe-modified ZSM-5, improved the catalytic selectivity towards monoaromatic hydrocarbons (MAHs). Sun et al. [74] also confirmed that Fe-modified ZSM-5 catalyst had better deoxygenation activity than the parent ZSM-5 due to the formation of new active sites and inhibiting of repolymerization, leading to a larger amount of aromatics and less coke formation with a higher BTX selectivity. Yung et al. [75] reported that the Ga-modified ZSM-5 can result in the increase of hydrocarbons by about 30% over the parent one in the upgrading of bio-oil since the incorporation of Ga increased the dehydrogenation activity. While, Zheng et al. [70] also reported that Ga modified ZSM-5 catalysts can lead to a higher bio-oil yield with a lower amount of coke when compared with the parent one. In addition, when the HZSM-5 was modified by 1 wt.% of Mo had the potential to produce a higher yield of aromatic hydrocarbons than the unmodified one [76]. In our recent study [77], a commercial HZSM-5 zeolite with a Si/Al molar ration of 24 was modified by Cu species with a wet impregnation method, and used for the in-situ upgrading of bio-oil from the fast pyrolysis of biomass with a biomass to catalyst weight ratio of 1:1(Fig. 7 ). It is observed that Cu/HZSM-5 with low Cu modification amounts can maintain the parent HZSM-5 crystalline structure and its acid sites. The 0.5 wt.% Cu/HZSM-5 showed a highest catalytic performance with a highly relative aromatic hydrocarbons amount of 73.2% and specific aromatic hydrocarbons yield of 56.5 mg/g-biomass (d.a.f), which are much higher than those by the parent HZSM-5 (55.0% and 26.0 mg/ g-biomass (d.a.f)). Besides, the 0.5 wt.% Cu/HZSM-5 catalyst also exhibited excellent catalytic reusability and regeneration property. Herein, the suitable acidity and best textural properties for the deoxygenation of the bio-oil should be attributed to the optimum Cu loading amount. Thus, transition metal modification should be an effective way for the improvement of HZSM-5 catalyst performance. However, it should be noted that the selection of metal species and its doping amount is also important. Table 4 summarizes the reported HZSM-5 and transition metal modified HZSM-5 catalysts for the bio-oil upgrading, in which selectivity towards aromatic hydrocarbons and oil yield are two key parameters for the evaluation of the catalyst performance. It is found that most metal modified HZSM-5 catalysts had low oil yields (< 30%) and/or low selectivity to aromatic hydrocarbons (<60%).In general, HZSM-5 modified by alkaline metal can change acid strength by increasing Lewis acid sites and decreasing in Brønsted acid sites, which could effectively prevent excessive cracking of the bio-oil, resulting in a higher yield of the deoxygenated compounds in the upgraded bio-oil with the extension of catalyst lifetime. AAEMs including Na, Mg and Ca have been applied to modify HZSM-5 for the upgrading of bio-oils. For example, when Mg was loaded on HZSM-5 in the form of MgO, the obtained catalysts exhibited better selectivity towards monocyclic aromatics due to the creation of new Lewis acid sites and the reducing of Brønsted acid sites [65]. Ca loaded on HZSM-5 also altered acid strength by reducing of strong acid sites and increasing of weak acid sites. Due to the obvious acid strength change and BET surface area decrease, it promoted the production of xylenes but lowered BTX production in comparison to the parent HZSM-5 [67]. Williams and Horne [89] investigated catalytic upgrading of bio-oil derived from biomass pyrolysis over Na modified HZSM-5 (Na-ZSM-5) and compared with the parent HZSM-5, and found that the yield of single ring aromatic compounds in the upgraded bio-oils, especially BTX, increased from 15.9 wt.% for the HZSM-5 catalyst to 21.3 wt.% for the Na-ZSM-5 catalyst. Thus, AAEM modification should be also an effective way for the improvement of HZSM-5 catalyst performance.As stated above, the catalytic performance of HZSM-5 can be improved by metal modification in the upgrading of bio-oils. There are various methods for the preparation of metal modified HZSM-5 catalysts, and the preparation method could influence the physicochemical properties and catalytic upgrading performance of the obtained catalysts even using the same metal for the modification. The typical preparation methods include impregnation, ion exchange, precipitation, sol-gel, hydrothermal and the temperature programmed reaction methods. Especially, the impregnation and precipitation methods have been extensively utilized. The impregnation method can be classified as wet impregnation, in which an excessive amount of metal solution is used, and the dry impregnation, in which the volume of metal solution equals to the entire pore volume of HZSM-5 catalysts. It is reported that impregnation time and the following drying way could affect the metal modification efficiency on the HZSM-5 catalysts [77,90]. The short impregnation and drying time may cause weakly adsorbing metal species, leading to the metal deposition mainly on the zeolite surface rather than within pores. In the ion-exchange method, the excess metal precursor after ion-exchange process will be washed out by deionized water so that only the exchanged metal ions remain in the zeolite, which can result in well dispersion of metal species on the zeolite and decrease the aggregation of active species during the bio-oil upgrading process [91]. Furthermore, co-impregnation, a process for manufacturing bimetal modified HZSM-5 zeolites, is more challenging than the single metal impregnation since different metal species with different solubility and diffusivity could lead to different degrees of precipitation within the pores. However, the suitable bimetal modification could obtain more excellent zeolite catalysts for the bio-oil upgrading than the single metal modification [92]. While, due to the high dispersion ability of the precipitation method, it is more suitable for the preparation of metal-modified HZSM-5 catalysts with a high metal loading amount [77,90]. Herein, it should be noted that the nucleation and growth of metal particles will be induced by the supersaturation of precursor solution. Therefore, it is important to select suitable metal modification method for the metal-modified HZSM-5 catalyst preparation in the bio-oil upgrading.Hierarchical zeolites are characterized by the presence of a bi- or multimodal porous structure, especially containing micro-, meso- and macropores together in the HZSM-5 based catalysts. The exact definition of hierarchical structure of catalysts is a pore system of bi- or multimodal pores with different pore size where the large pores connect the small pores, i.e., small pores branch off from a continuous large pore [93]. Because of their unique properties, such materials have been attracted great attentions. Hierarchical HZSM-5 differ significantly from the conventional one in terms of improved diffusion, promoted mass transfer, enhanced resistance to deactivation [94]. The classical methods for introducing multimodal porous structure into the HZSM-5 catalysts include single templating, double templating with soft/hard-template and post-treatment.In general, the single templating method can only lead to the generation of micropores during the zeolite synthesis. Typically, tetrapropylammonium hydroxide (TPAOH) is applied as a structure-directing agent as well as a micropores template for the preparation of ZSM-5. In the case of the addition of TPAOH alone during ZSM-5 synthesis, to obtain hierarchical structured ZSM-5, it requires base etching as the post-treatment process to generate mesopores as stated in the following Section 4.2.3 [95].In the synthesis of zeolites with mesopores, two types of templates are generally selected, in which one should have a three-dimensionally structured mesopore system and it is mixed with the precursor chemicals including structure-directing agent of ZSM-5. In this case, mesopores can be produced during the crystallization stage, and the template will be decomposed by a calcination stage after the hydrothermal synthesis process [30]. It is reported that using a small amount of mesopore template during the synthesis process always results in insufficient mesopore generation whereas using a large amount of template will hinder the crystal nucleation and growth [96]. Thus, it is needed to search the optimum mesopore template amount during the synthesis. In the production of hierarchical zeolites, both hard template such as carbon black, carbon fibers, aerogels, and polymer aerogel and soft template such as cationic polymers, amphiphilic organosilane surfactants, and silylated polymers will be applied [97,98], by which the morphology of mesopores may be accurately regulated. Compared to the chemical etching (acid or base treatment) for the generation of mesoporous or microporous structure as as stated in the following Section 4.2.3, the employing of a secondary template could raise the cost and a calcination process to eliminate the template is also necessary.In comparison to direct synthesis ways, the post-treatment method is more convenient, simple, and cost-effective for introducing secondary meso- and/or macro-pores in the zeolite catalyst structure by extraction of elements on the framework. Generally, an acid solution was employed for extracting Al atom while a base solution was employed for extracting Si atom. Both of which can generate mesopores in the zeolite structure. The disadvantage of this method is the loss of micropore structure after the generation of mesopore porosity, or the collapse of structure to lose the surface area in the case of over extraction. The first method is the use of base or alkaline solution, which can be called “desilication/alkaline etching”. The preparation of hierarchical HZSM-5 catalysts by the alkaline etching method could result in improved surface properties. Since the small pores of the parent ZSM-5 always limit the mass transfer and diffusion of reactants and products, by introducing mesopores into the structure, the accessibility of acid sites can be improved, the diffusion length can be shortened and finally the catalyst lifetime can be enhanced [99]. Popular alkaline solutions used for the etching of HZSM-5 include sodium hydroxide (NaOH), tetrapropylammonium hydroxide (TPAOH), tetrabutylammonium hydroxide (TBAOH), sodium carbonate (Na2CO3) with NaOH solutions [100]. During the alkaline etching, some Si-Al bonds may be disrupted, causing the zeolite to lose its location adjacent to Al and decrease the acidity. While, the obtained catalysts usually have a wide pore size distribution. For the bio-oil upgrading, it is reported that the ideal Si/Al ratio in the original ZSM-5 for the construction of a hierarchical structure by using alkaline solution should be 25–50 [101]. Herein, since the excessive aluminum content in ZSM-5 can surround the silicon to prevent the desilication for the mesopore creation. On the other hand, when the aluminum content is too low, a large amount of Si can be also dissolved due to the instability of structure, resulting in low yield of zeolite and too many large mesopores, which always negatively affect the catalytic cracking activity. Organic hydroxides such as TPAOH and TBAOH have lower ability for the Si dissolution than those inorganic hydroxides, which can better control Si species dissolution so that only a little change occur in the acid property after the alkaline treatment [102]. Furthermore, it is reported that the hierarchical HZSM-5 catalysts by post-treatment using a mixture of inorganic and organic base solutions can result in a better control of the mesopore formation since the assistance of the organic base solution prevented the excessive extraction to maintain a better zeolite structure as shown in Fig. 8 [103].In our previous study [104], hierarchical HZSM-5 zeolites were fabricated via desilication of conventional HZSM-5 in various NaOH solutions in the presence of TPAOH for the bio-oil upgrading. It is found that the hierarchical HZSM-5 prepared by using 0.2 M NaOH with 0.25 M TPAOH etching had the most excellent catalytic performance with detected aromatics yield as high as 45.2 mg/g-bio-oil. Herein, in the presence of 0.25 M TPAOH, the mesopore formation can be well controlled, leading to the rise in surface area but maintaining enough acidity. Furthermore, to increase the aromatic hydrocarbons as well as the coking resistance, the hierarchical HZSM-5 was further modified by various metals. It is observed that 0.25 wt.% Cu/HZSM-5 resulted in the increase of the aromatic hydrocarbons yield (∼54.5 mg/g-bio-oil) with a better coking resistance performance (Fig. 9 ).The second method is the use of acid solution, which is called “dealumination/acid leaching”. Dealumination is the process of removing Al from the zeolite framework using an acid solution. It is most typically employed for the post-treatment of parent HZSM-5 zeolites with a low Si/Al ratio (2.5-5), which usually have high acid density and strong acid strength and favor the coke generation, thereby causing catalyst deactivation [105,106]. After the dealumination using acid solution, the Si/Al ratio in the zeolite framework will be increased, and a hierarchical structure with existing mesoporosity will be generated. In this case, the adjustment of zeolite acidity becomes more controllable. In addition, even if the zeolite is extremely dealuminated, partial extra-framework Al can be reinserted into the structure using the hydrothermal technique while the zeolite structure is preserved better than the case employing the alkaline etching method [107,108]. However, the use of strong acid concentration will cause a decrease of Brønsted acid sites due to a significant decrease in the framework Al content, and an increase in the Lewis acid site due to the detached Al which becomes an extra-framework Al floating in the environment as an extra-framework Al atom [109]. The most commonly used acids in the dealumination are hydrochloric acid (HCl), nitric acid (HNO3), sulfuric acid (H2SO4) and hydrofluoric acid (HF) [110]. Herein, chemical etching with fluoride ions generated from NH4-HF has been reported to create secondary porosity and increase the exterior surface area of hierarchical ZSM-5. However, the use of extremely concentrated HF solution could result in the decrease of Brønsted acidity due to the indiscriminate extraction of Al and Si [111]. Fig. 10 presents an example of an SEM image of hierarchical ZSM-5 treated with an acid solution.As stated above, owing to its acidity and shape-selective property of HZSM-5, it is considered as one of the most effective catalysts for the deoxygenation of bio-oil, especially in the production of light aromatic hydrocarbons such as BTXs. However, one of the major drawbacks of HZSM-5 is the fast deactivation due to the coke formation. In the upgrading of bio-oils, the mass transfer of large intermediate oxygenates in the bio-oil is restricted by the small pore diameter of HZSM-5 so that it is difficult to diffuse into the pore for reaction, which will result in the coking to lock the inner pore and finally cause HZSM-5 catalysts to be deactivated. Thus, HZSM-5 catalysts with special morphologies have been designed to reduce the diffusion length or increase the diffusion efficiency. To date, hollow ZSM-5 catalysts, core-shell ZSM-5/mesopore composite catalysts, nanosheet ZSM-5 catalysts have been developed.To improve the catalytic effectiveness and keep the shape-selective ability of micropores in the zeolites, interior hollow structured zeolites have been designed and fabricated. In this case, the hollows within the zeolite catalysts will assist to shorten the diffusion distance with a decrease in the unutilized quantity within the center of zeolites. Two ways have been applied to fabricate hollow zeolites [112]. The first one is to assemble zeolite crystals with polycrystalline shells using a template followed by calcination at a high temperature to remove the template as shown in Fig. 11 [112]. Herein, the zeolite crystals are aggregated at random to form an interparticle cavity. Since the obtained shell thickness is only a few microns, the reactants and products could be more easily pass through the shell. Fig. 12 shows one obtained hollow zeolite catalysts by using mesoporous silica spheres as the sacrifice template, on which zeolite nanoparticles subsequently grow during the synthesis process [113]. Herein, the inner part of the particle is a jujube-like mesoporous silica sphere as Si and pre-seeded source for the preparation of hollow zeolite.The another approach is to employ chemical etching to create hollow cavities in the zeolite catalysts. This method involves two-step: (i) using sol-gel method to grow large single zeolite crystal from a single nucleus; (ii) constructing the hollow structure in the center of the obtained zeolite crystal with a shell by etching using various chemical concentrations. Fig. 13 shows ZSM-5 zeolites with an internal hollow structure, where the parent ZSM-5 crystal was desilicated by TPAOH [114].In our previous study [115], hollow HZSM-5 catalysts with a mesoporous shell were fabricated by a hydrothermal process combined followed with TPAOH etching. When it used for in-situ catalyst upgrading of bio-oil from rapid pyrolysis of biomass, the obtained best hollow HZSM-5 catalyst led to aromatic hydrocarbon yields in the range of 78.49–78.67% for cellulose and hemicellulose, which is much higher than those using the parent HZSM-5 (61.06–68.26%). While, when the biomass(cedar)/catalyst weight ratio was 1:2, up to 80.16% of the aromatic hydrocarbon yield was achieved. Besides, this catalyst had good reusability and regeneration property since the accessibility to the acid sites in the prepared hollow HZSM-5 catalysts was greatly enhanced, which effectively increased the reaction rate as well as the coking resistance (Fig. 14 ).Preparation of HZSM-5/mesoporous material composite with a core/shell structure is another way to improve the performance of HZSM-5 based catalysts, which is considered to have following advantages: (i) the mesoporous structure on the shell is beneficial for the better diffusion and can hinder the coke formation on the surface of ZSM-5 catalysts; (ii) the acidity and shape selectivity of ZSM-5 suitable for aromatic hydrocarbons production in catalytic upgrading process can be maintained [116–118]. Fig. 15 shows an illustration for the preparation of the core/shell ZSM-5/mesoporous material composite [112], which is a ZSM-5 catalyst coated with a thin mesoporous MCM-41 layer. When it was applied for the in-situ and ex-situ catalytic fast pyrolysis of biomass, the equivalent hydrocarbon yield as that from the pure ZSM-5 was obtained, but the MCM-41 shell worked as a barrier layer for coke deposition, which effectively protected the ZSM-5 from the severe coke formation.The nanosheet HZSM-5 has a hierarchical pore structure with a thin sheet structure, which can overcome the disadvantages of the small and long porous diffusion path in the conventional HZSM-5 zeolite [119–121]. Due to its size (only 5–10 nm) as small as the size of a unit cell, the diffusion length can allow the reactants and products to diffuse in the HZSM-5 more easily, which could improve catalytic activity and lifetime of catalysts [119–121]. A mesoporous HZSM-5 nanosheet was synthesized by using a surfactant as the structure-directing agent and applied to upgrade the bio-oil in the catalytic pyrolysis of cellulose. In this case, although the obtained HZSM-5 nanosheet resulted in similar aromatic hydrocarbons and olefins yields as those by using conventional HZSM-5 catalysts, it demonstrated a longer lifetime even though the coke content was also higher than those cases using the conventional HZSM-5 catalysts since the mesopores still enabled better accessibility to active acid sites. Furthermore, because such a nanosheet zeolite has a larger surface area, it could be an outstanding player for loading metal nanoparticles to create a multifunctional zeolite catalyst. With increasing metal loading amounts, the metal nanoparticles could distribute uniformly within zeolite crystals and prevent aggregation during the reaction.As is known, biomass is the only renewable energy source and is considered a potential alternative to fossil fuels. Efficient biomass utilization is full of challenges, and among the available technologies, fast pyrolysis is effective in producing bio-oil. However, pyrolysis bio-oil needs to be upgraded before use as the transport fuels or chemical feedstock due to its low quality as fuels and its low value as the chemicals. Catalytic upgrading of pyrolysis bio-oils to increase the quality is desired in recent year and the catalyst with high catalytic activity, excellent selectivity and long life-time is the foundation of this process. The HZSM-5 based catalysts are currently the most widely used and effective catalysts for biomass conversion in aromatics production due to its outstanding acidity, heat-resistant properties, adequate pore size. However, the mass transfer capacity of large reactants and products through the small pores of HZSM-5 is limited which had a significant negative impact on the catalytic performance. Due to the most cracking reaction occurring within the pore, the generated products will diffuse out slowly from the pores, resulting in a low-yield of product. In addition, the HZSM-5 based catalysts often suffer from coke formation due to its high acidity. Thus, HZSM-5 based catalysts should be more improved and modified.In this article, HZSM-5 based catalysts for the upgrading of pyrolysis bio-oils are critically reviewed. The effects of porous structure, acidity and their interactions on biou-oil upgrading activity, selectivity, stability and deactivation are summarized. The proposed mechanisms on the bio-oil upgrading over the catalysts are discussed. In particular, the main strategies including metal modification, construction of zeolites with a hierarchical structure and synthesis of special morphologies with hollow structure or core/shell and nanosheet structures for the improvement of deoxygenation property performance are introduced. In fact, each of the strategy investigated has its combination of advantages and disadvantages. It should take the advantages of each technique and continue to improve the HZSM-5 based catalyst to make it more efficient. Based on the critical review above, the diffusion path and acidity property should be moderately modified to preserve catalytic activity and meanwhile to avoid secondary reactions that lead to coke and catalyst deactivation. However, those reported studies for the development of HZSM-5 based catalysts to optimize catalytic upgrading of bio-oil remain at a laboratory scale. Therefore, it would be of great benefit to the industrial application if it is cost-effectively scaled up. Finally, the improved HZSM-5 based catalysts were not only suitable for the catalytic upgrading of bio-oil, as the original ZSM-5, they should be also served as the excellent catalysts for a variety of other processes. It is expected that this review will assist not only those who are interested in catalytic upgrading of bio-oil processes, but also those who are interested in applying my findings to other research areas (Fig. 9).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 JST Grant Number JPMJPF2104 and Hirosaki University Fund, Japan. N. Chaihad gratefully acknowledges the scholarship from the Ministry of Education, Culture, Sport, Science and Technology (MEXT) of Japan.

Fast pyrolysis of biomass is an attractive way to produce bio-oil since it can convert most of biomass components directly into liquid fuel. However, the bio-oils obtained from such a fast pyrolysis process always have highly complex oxygenated compounds with high viscosity, serious corrosivity, and rather instability. Thus, before the raw bio-oils are used as fuel or chemical feedstock, they must be upgraded, especially deoxygenated. Cracking of bio-oils over porous solid catalysts such as zeolite-based catalysts at ambient pressure is considered one of effective ways for the bio-oil upgrading, especially in which hydrogen gas is not necessary. Herein, zeolite-based catalysts (mainly HZSM-5 based catalysts) for the upgrading of pyrolysis bio-oils are critically reviewed. The effects of porous structure, acidity and other parameters including biomass type, biomass/catalyst ratio and operation temperature on cracking activity, selectivity, stability and deactivation are summarized. While, the proposed mechanisms on the bio-oil upgrading over the zeolite-based catalysts and the possibility for the application of the developed catalysts in the industrial process are discussed. Furthermore, the main strategies including metal modification, construction of zeolites with a hierarchical structure and synthesis of special morphologies with hollow structure or core/shell structure and nanosheet structures for the improvement of deoxygenation property performance are introduced. It is expected to provide a guidance for the design and fabricate more excellent zeolite-based catalysts and their application for high-quality bio-oil production from fast biomass pyrolysis.

2-MethylindoleTetrahydro-N-ethylcarbazoleOctahydro-N-ethylcarbazoleOctahydro-2-methylindoleDodecahydro-N-ethylcarbazoleDodecahydro-N-propylcarbazolePerhydro-dibenzytolueneBenzene, toluene, and xyleneCatalytic fast pyrolysisCompressed hydrogen storageCarbon material hydrogen storageUnited States Department of EnergyLiquid ammonia hydrogen storageLiquid hydrogen storageLiquid organic hydrogen carriersMetal alloy hydrogen storageMethylcyclohexaneMethanol-to-aromaticsN-ethylcarbazoleN-propylcarbazoleThermal catalytic conversion and ammonificationTolueneUnited States of AmericaAs living standards around the world improve, there is an increasing reliance on energy, which exacerbates the global energy challenge. Renewable energy has developed into a clean and efficient alternative to existing energy sources. Furthermore, the worldwide focus on “Emission Peak and Carbon Neutrality” and the adoption of numerous legislations have made renewable energy the unavoidable path for the transformation of the current energy system [1–3]. Therefore, it is an inevitable trend to accelerate the development of renewable energy. At present, the use and storage of hydrogen is a potential route to the current development of renewable energy, as hydrogen is a clean energy source that does not produce any pollutants [4]. “Hydrogen economy” is also a hot topic of sustainable development, and it is necessary to determine the direction and strategy of future development based on the conditions of the world. Technical methods for hydrogen production and consumption are reasonably established and well-developed in the supply chain. Hydrogen energy storage, hydrogen-powered automobiles, and hydrogen-powered ships are common applications [5,6]. Due to low volumetric density (0.0899 kg/m3), volumetric energy density (0.003 kW·h/L), and gravimetric energy density (33 kW·h/kg), hydrogen energy usage faces significant storage and transportation constraints. The flammable and explosive qualities of hydrogen at normal temperature and pressure have also hampered large-scale and commercial hydrogen energy uses [7–9]. For on-board hydrogen sources, the US Department of Energy (DOE) has proposed objectives of 5.5 wt% and 62 kg/m3 for gravimetric and volumetric hydrogen capacity, respectively [10].Hence, the development of efficient hydrogen storage technology is currently a popular focus of research. Improving hydrogen storage capacity and rate while reducing energy consumption are major characteristics of hydrogen storage technology. Compressed hydrogen storage (CH2), liquid hydrogen storage (LH2), liquid organic hydrogen carriers (LOHCs), liquid ammonia hydrogen storage (LAH2), metal alloy hydrogen storage (MAH2), and carbon material hydrogen storage (CMH2) have all been the subject of much investigation to address above problems [11,12]. Among them, LOHCs technology is recognized to be excellent for long-distance and large-scale hydrogen storage and transportation due to its high hydrogen storage capacity, environmental friendliness, safety, and efficiency [13,14]. According to previous research, hydrogen storage liquids are generally high-purity single aromatic or N-doped compounds. Complex refining methods are often used to create these compounds from nonrenewable fossil energy sources. For instance, aromatic compounds can be made through naphtha reforming and petroleum cracking. Potentially, LOHCs can be obtained from biomass been turned into valuable liquid fuels and aromatic compounds through a variety of conversion methods. The lignocellulosic biomass is composed of cellulose (40%-50%), hemicellulose (25%-30%), and lignin (15%-20%) [15,16]. Take lignin as an example, it is a unique, renewable natural polymer with aromatic structures. Through the thermochemical conversion pathway, lignin can be depolymerized into aromatic compound intermediates, and can also be used to produce small molecular compounds such as biomass fuels and light aromatics [17]. Theoretically, these compounds can be used for the storage and release of hydrogen via a pair of reversible reactions in LOHCs technology. Thermal conversion conditions of biomass are expected to optimize the target product as a major component of organic hydrogen storage liquids, including targeted deconstruction, nitrogen doping, better catalysts, reaction condition, etc.In all, this article provides an overview of LOHCs technology, including basic principles, technical approaches, and applications. This review also proposes the concept of biomass-derived renewable LOHCs to demonstrate the potential of biomass as a carbon-neutral energy carrier for hydrogen storage. Combined with the current thermochemical conversion technologies of biomass, the preparation and development of aromatic hydrocarbons and N-doped compounds are briefly summarized and exhibited. In addition, the technological route, feasibility, and challenges of biomass-based LOHCs were evaluated.Based on the reaction principle, the categories of hydrogen storage are mainly composed of physical hydrogen storage, physical adsorption hydrogen storage, and chemical adsorption hydrogen storage. CH2 [18] and LH2 [19] are two types of physical methods. The physical adsorption of hydrogen [20] is usually composed of carbon materials, zeolite, and metallic organic framework materials. LOHCs, LAH2 [21], electrochemical hydrogen storage [22], and MAH2 [23] are examples of chemical adsorption storage methods. It is worth mentioning that hydrate hydrogen storage is also a physicochemical method, where H2 capture occurs via the formation of a hydrate shell with hydrogen bonds between water molecules, with H2 being kept by the topology of the cavity [24]. The characteristics, advantages, and disadvantages of several common types of existing main hydrogen storage technologies are summarized in Table 1 . Pure steel metal (17.5∼20 MPa), steel liner fiber wound (26.3∼30 MPa), aluminum liner fiber wound (30∼70 MPa), and plastic liner fiber wound (>70 MPa) are the four pressure categories for gas cylinders [25]. The application of some countries is relatively mature. For instance, the all-steel bottle container produced by JFE in Japan and the carbon fiber-wound hydrogen storage container with steel liner developed in the USA have been used for hydrogen refueling stations. Currently, bottle leakage, liner and interface sealing, and the design of transportable hydrogen cylinders for transportation are all issues that researchers are grappling with.CH2 technology is relatively easy to industrialize and offers fast charge and discharge rates, it is widely used for onboard hydrogen storage in new energy vehicles. Compared with CH2 technology, the storage capacity of LH2 technology has been greatly improved. LH2 technology has a density of 70.85 kg/m3, which is 1/800 of the volume of gaseous hydrogen, making it easier to transport large volumes over long distances, considerably improving transport efficiency [26]. This type of technology is universally used for cryogenic rocket propellants [27,28]. Although LH2 technology has sufficient advantages in terms of storage and transport capacity, it is based on a liquid-phase state formed at extremely low temperatures (< -253°C). As a result of its peculiar working conditions, LH2 technology has some drawbacks. On one hand, energy consumption is extraordinarily high as a result of the need to transform gaseous hydrogen into liquid through a series of technical means. Liquid hydrogen, on the other hand, absorbs heat continuously to form evaporative gases, which necessitate high insulation in storage facilities [29]. LAH2 technology tends to utilize ammonia as the hydrogen carrier, and a high hydrogen storage capacity is obtained (17.8 wt%), 1.7 times higher than that of LH2. Due to the high stability, liquid ammonia can meet the need for energy storage in time and in space [30]. However, LAH2 technology should be considered for the need of high energy input and toxicity, and potential hazards to equipment, the human body, and the and environment during long-term and long-distance storage and transportation [31].In contrast with traditional hydrogen storage technologies, LOHCs technology has following major advantages: (1) it has a prominent capacity for hydrogen storage, as well as excellent performance (meeting DOE index requirements); (2) most of the substances have a high boiling point and low melting point, allowing them to maintain a stable liquid phase at room temperature while remaining nonvolatile; (3) this system has stable catalytic hydrogenation and dehydrogenation processes, and the reactants and catalysts can be recycled; (4) the storage, transportation, and maintenance of hydrogen-storage materials are safer and more convenient, allowing for large-scale and long-distance distribution; (5) current gasoline and diesel delivery techniques and gas station buildings may be immediately implemented. However, there are also several disadvantages, mainly including: (1) the reaction requires professional hydrogenation and dehydrogenation equipment, which has high investment costs; (2) the dehydrogenation reaction must be carried out at high temperature, which is likely to result in catalyst coking and deactivation; (3) the reaction process consumes large amounts of energy, and the performance decreases after several cycles; (4) the hydrogen produced by the dehydrogenation reaction is not of high purity, and inappropriate conditions and reactants are more likely to induce side reactions; (5) the initial cost to purchase LOHCs materials is extremely high.What's more, some studies have found a higher energy demand of LOHCs than CH2, LH2, MAH2, and LAH2 based on the 0-dimensional simulation [32]. However, in terms of energy storage of regenerative hydrogen in the cell system, LOHCs technology showed efficiency with the increase of the energy storage cycle, confirming the suitability for long-term hydrogen storage. Assuming that large-scale storage and transport across oceans are targeted for applications in the field of hydrogen storage, then there will be a wider market for LOHCs technology. In addition to the characteristics of high hydrogen storage capacity, carbon cycle, and suitability for long-term utilization, the safety performance is also superior to that of other hydrogen storage methods. Since the concept of LOHCs was proposed, the technology has also been continuously optimized. In other words, LOHCs technology is exactly promising and marketable for hydrogen storage today.LOHCs technology is based on reversible hydrogen storage and release reactions using unsaturated liquid organics (e.g., toluene, naphthalene, and N-ethylcarbazole) as hydrogen storage agents and the corresponding saturates (e.g., methylcyclohexane, decalin, and dodecahydro-N-ethylcarbazole) as hydrogen carriers [7,37]. The fundamental principle of the reactions in LOHCs technology is shown in Fig. 1 . The hydrogenation process is an exothermic reaction in which the organic hydrogen storage liquid is mixed with raw hydrogen in the reactor. The system is then heated to a specific temperature under the influence of the catalyst to form the corresponding saturated hydride. The products of the hydrogenation reaction are called hydrogen carriers (Hx-LOHCs). Essentially, it is a catalytic mechanism that uses hydrogen to transform unsaturated bonds into saturated ones [14,26]. From the perspective of chemical equilibrium, both low temperatures and high pressures are more favorable to hydrogenation. As the inverse of hydrogenation, dehydrogenation is an endothermic reaction. In the presence of the catalyst, hydrogen is extracted from Hx-LOHCs in the dehydrogenation device. The process involves continual absorption of external heat due to the energy difference between the energy required for the dissociation of hydrogen atoms and the activation energy of the C-H bond [38]. It is necessary to focus on the large energy consumption caused by the difference in temperature during the reaction, as well as the reduction of catalytic activity. Then, for long-distance transportation of hydrogen carriers, existing liquid fuel transportation (pipelines, ships, trucks, etc.) can be employed. It was found that unsaturated aromatics and corresponding hydrides can be hydrogenated and dehydrogenated without destroying the main structure of the carbon ring [39].To increase the conversion rate and selectivity, as well as the recycling efficiency and activity of the catalysts, the composition of the compounds and conditions can be tweaked. Consequently, a high-performance LOHCs system should have the following performance indices [40]: (1) low melting point and high boiling point; (2) large hydrogen storage capacity (volume: >56 kg/m3, gravity: >6 wt%); (3) high stability of ring chain during dehydrogenation and high purity of hydrogen discharged; (4) low heat uptake and mild dehydrogenation conditions; (5) cheap cost and readily available materials; (6) long cycle life and selective dehydrogenation; (7) enhanced stability during use and transportation, low toxicity, and environmental friendliness.LOHCs technology was first proposed as a non-cryogenic approach in 1975. Such technologies are more inclined to use aromatic compounds such as benzene and toluene for vehicle fuels as hydrogen storage carriers. As research progressed, scholars discovered that the enthalpy and temperature can be effectively reduced when an appropriate number of heteroatoms, such as N atoms, intervene and replace C atoms. Furthermore, as the number of N atoms in an organic compound grows, the dehydrogenation temperature also increases [42–44]. The most investigated organic compounds at the moment are aromatic and N-doped compounds. Simultaneously, a growing number of organic compounds are being discovered. Table 2 lists the physicochemical parameters and reaction equations for several regularly used LOHCs systems. The main systems found in the literature are toluene (TOL)/methylcyclohexane (MCH) [9], N-ethylcarbazole/dodecahydro-N-ethylcarbazole [45,46], naphthalene/decalin [47], dibenzyltoluene/perhydro-dibenzyltoluene [48], biphenyl/bicyclohexyl, and diphenylmethane/dicyclohexylmethane. Many scholars have studied the reaction mechanism of these hydrogen storage systems through molecular dynamics, nuclear magnetic resonance and other methods, which are summarized in Fig. 2 . It is discovered that the steric effect will be strongly impacted by the existence of molecular size, methyl, heteroatoms, etc., thereby affecting the priority of bond hydrogenation and dehydrogenation.Furthermore, a few corporations in wealthy countries such as Germany and Japan have already commercialized many systems. For example, dibenzyltoluene, which has a maximum hydrogen storage capacity of 57 kg/m3, has been the subject of research by the German business “Hydrogenious LOHC Technologies GmbH”. Japanese enterprises, such as “Chiyoda Chemical Construction”, have already integrated and developed in a variety of disciplines, including ocean-going hydrogen transport, miniaturization of hydrogenation and dehydrogenation, hydrogen refueling stations, and distributed energy delivery. Overall, Japanese industries have concentrated their research and development efforts on toluene/methylcyclohexane. Some Chinese firms have entered the LOHCs market. The most established company is “Hynertech” in Wuhan, where the N-ethylcarbazole/dodecahydro-N-ethylcarbazole system is the primary research focus. They have been concentrating on the creation of “hydrogen oil”, a liquid organic hydrogen storage solution that combines the benefits of safety and stability with high hydrogen storage capacity. It's worth noting that “Hynertech” has demonstrated high-temperature waste gasification to “hydrogen oil” and the hydrogen energy business.Although olefins, alkynes, and aromatic hydrocarbons can all be employed as hydrogen storage liquids, studies have confirmed that aromatic compounds are the best choice for hydrogen storage. Aromatic compounds can be applied as LOHCs because of the unique resonance interaction of aromatic rings which makes them more likely to hydrogenate and dehydrogenate than other organic molecules [65]. The first system studied was benzene/cyclohexane [65,66], which has a high gravimetric hydrogen storage capacity of 7.2 wt% and a volumetric hydrogen storage capacity of 55.9 kg/m3. The major disadvantage, however, is the high dehydrogenation temperature. The dehydrogenation reaction needs to be completed at a temperature of close to 300℃, resulting in considerable energy consumption. Fig. 3 a illustrates the hydrogenation and dehydrogenation reactions of the benzene/cyclohexane system [65]. In a catalyzed hydrogenation process, benzene and hydrogen are introduced into the reactor at a pressure of 4 MPa and a temperature of 150°C to create saturated cyclohexane. During the reaction, approximately 68.8 kJ/mol of energy is produced. At 300°C and 0.1 MPa, cyclohexane absorbs roughly 68.8 kJ/mol of energy, with the gradual release of hydrogen and eventually dehydrogenates to benzene [67]. To find a better reaction system, Itoh et al. [68] used Pt/Al2O3 as the catalyst to compare the conversion and dehydrogenation temperatures of cyclohexane and methylcyclohexane, two types of hydrides: benzene and toluene. Because of the presence of methyl as an alkyl group, the dehydrogenation temperature was lower than that of cyclohexane. The mixture of methylcyclohexane and cyclohexane has also been shown to be a hybrid chemical hydride, which can be researched for hydrogen storage [68].The reversible process of the toluene/methylcyclohexane system is shown in Fig. 3b, where a reduced storage capacity is accompanied by a lower dehydrogenation temperature. Overall, the benzene/cyclohexane and toluene/methylcyclohexane systems exhibited excellent hydrogen storage capacities. However, a higher dehydrogenation temperature remains a critical factor affecting the energy consumption of the reaction. Naphthalene, a cheap and simple condensed aromatic hydrocarbon with excellent hydrogen storage capacity, was shown to be equally suitable for hydrogen storage [69–71]. The naphthalene/decalin system reaction is shown in Fig. 3c [58]. As naphthalene is prone to many side reactions during hydrogenation, there are distinctions between the resulting naphthenic compounds such as trans-decalin and cis-decalin [72]. Many factors influence the rate of reaction, as well as the separation and purification of the product, such as the number of rings opened, organic solvents, and catalysts. Terribly, the high temperature and vapor pressure during the dehydrogenation of decalin will lead to ring-opening and cracking after several cycles. This will make more tar and coke adhere to the equipment which is difficult to remove [72]. In addition, to ensure the reactivity and state of the liquid phase, it is necessary to equip the transport with heating facilities, which increases the complexity and cost of storage and transportation.Dibenzyltoluene (DBT) is the principal component of “thermal conductive oil” in daily production, in addition to the three systems mentioned above. The physical and chemical qualities of “thermal conductive oil” are stable, and it is distinguished by its high boiling point, low melting point, and low toxicity. The dibenzyltoluene/perhydro-dibenzytoluene system has become a hot topic within current aromatic hydrogen storage liquid compounds [59,73]. As illustrated in Fig. 3d, DBT interacted with hydrogen in the presence of the catalyst to produce perhydro-dibenzyltoluene under certain conditions. What's more, DBT is a prominent research target for aromatic organic hydrogen storage liquids due to its exceptional physicochemical features. DBT may be recycled numerous times when used as the hydrogen storage carrier, and its fixed cost for hydrogen storage is lower than that of liquid chemical plants. However, the use of DBT has drawbacks of high energy consumption and slow reaction rate in dehydrogenation, and H2 needs to be purified when released [42]. In addition, the technical bottlenecks in developing efficient and low-cost dehydrogenation catalysts also limit the application of DBT in liquid hydrogen storage to some extent.In addition to the systems mentioned above, there are several studied on LOHCs utilizing biphenyl/bicyclohexyl and diphenylmethane/dicyclohexylmethane as the research subjects. Biphenly and diphenylmethane are particularly popular as hydrogen carriers because of the excellent hydrogen capacity, stability and economic performance [74]. However, the solid physical state under ambient environment has limited the application for hydrogen storage. Hence, a growing number of studies have proposed the low-eutectic mixture (such as the combination of biphenyl and diphenylmethane) as potential hydrogen carrier, realizing liquid state at room temperature and atmospheric pressure [75,76]. It has been demonstrated that an optimum composition of biphenyl (C12H10, 35 wt%) and diphenylmethane (C13H12, 65 wt%) can be formed and the hydrogen storage capacity can be maximized (6.9 wt% and 69.1 gL-1) [77].It has been confirmed that the incorporation of heteroatoms such as N, P, and O into aromatics can significantly affect dehydrogenation thermodynamics [43]. To better understand the relationship between the heterocyclic structure and the enthalpy of dehydrogenation, Clot et al. [63] investigated how the substituted positions of different heteroatoms affect the dehydrogenation reaction. They eventually discovered that substituting heteroatoms at the ring's 1-position can effectively lower the dehydrogenation temperature of the hydrogen storage carriers. In addition, the addition of heteroatoms to the five- and six-membered rings can reduce the enthalpy of dehydrogenation to a significant extent. Recently, an increasing number of studies have discovered that hydrides in N-doped systems of N heterocyclic compounds (e.g., indoles [78,79], pyridines [80,81], and carbazoles) exhibit lower dehydrogenation temperatures than cyclic olefins. N-ethylcarbazole (NECZ)/dodecahydro-N-ethylcarbazole (12H-NECZ) [63] is the most frequent in this sort of system. The dehydrogenation temperature gradually lowered to around 200℃ as the number of heteroatoms in the organic matter increased. The hydrogen storage and release reactions of the NECZ/12H-NECZ system are shown in Fig. 4 a. Under catalytic action, hydrogenation of NECZ can be achieved within 150°C - 200°C, resulting in the formation of 12H-NECZ. At a pressure of 0.1 MPa and a temperature around 200°C, 12H-NECZ could be reduced to NECZ. When entirely hydrogenated, the gravimetric hydrogen storage capacity of 6.7 wt% for carbazole and 5.8 wt% for NECZ are lower than that of the aromatic-alkane system. However, there was a significant decline in dehydrogenation temperature. The difference in temperature and enthalpy of the reaction between carbazole and NECZ is rather modest. The addition of ethyl in NECZ slightly decreases the hydrogen storage capacity. However, the passivation effect of N atom on the catalyst is effectively reduced to maintain the activity [82]. Moreover, the melting point of NECZ is just 69°C, significantly decreasing the number of additional operating steps owing to the higher melting point. Because carbazole and NECZ are solids at room temperature, some organics, such as hot ethanol and ether, need to be added to dissolve the solid or lower the melting point to enable a smooth reaction. For the dehydrogenation of 12H-NECZ, an inadequate reaction produces additional by-products. Taking 4H-NECZ and 8H-NECZ as examples, these substances decrease the efficiency of transformation. In many cases, the presence of stereoisomers of semi-hydrogenated products with different reactivity results in reduced and incomplete conversion, even though the thermodynamic is favorable [83]. Accordingly, suitable catalysts for hydrogenation and dehydrogenation must be selected to address the problem of low efficiency.In addition to carbazole, several N-doped aromatic heterocyclic derivatives (e.g., indole [79,84], pyridine [44,81], and pyrrole) can be hydrogenated and dehydrogenated. Despite the related studies being less frequent than those of carbazole, more studies need to be conducted. Fig. 4b and Fig. 4c show the relative reaction of 2-methylindole (2-MID)/octahydro-2-methylindole (8H-2-MID) and N-propylcarbazole (NPCZ)/dodecahydro-N-propylcarbazole (12H-NPCZ) systems [14]. When used as the organic hydrogen storage liquid, 2-MID has a hydrogen storage capacity of 5.76 wt% and can be hydrogenated to 8H-2-MID with catalysts such as Ru/Al2O3. Thus, 8H-2-MID can be completely dehydrogenated to form 2-MID. Similarly, NPCZ can be catalytically hydrogenated to 12H-NPCZ at 120°C-150°C with a hydrogen storage capacity of 5.43 wt%. The class of N-doped compounds has been the subject of much research, including the examination of some of the compounds produced during the hydrogenation process.Similar to mixed aromatic systems such as biphenyl/diphenylmethane, owing to the limitations of the physical and chemical properties of pure substances, some studies have explored the effects of hybrid systems of N-doped compounds [68]. Compared to a single compound, the lower freezing point of the mixture made it more beneficial for use in colder climates. The melting points of combinations containing various alkyl compounds were examined by Stark et al. [85]. They realized that the melting points might be lowered by using the appropriate combination of N-alkylcarbazole. Shuang et al. [86] investigated mixed liquid hydrogen storage systems and the impact of mixing in different proportions on the performance of the system. Consequently, they discovered that the melting point of the mixture dropped to 25°C when the system was mixed with 40 wt% 2-MID, 36 wt% NPCZ, and 24 wt% NECZ. With a solvent-free catalytic process and a high hydrogen storage rate, the hydrogen storage capacity could reach 5.64 wt%.Taking various promising LOHCs as the research object, the dehydrogenation performance differs significantly in technical, economic, and environmental aspects [87]. For the production rate of H2, TOL and DBT tended to be better than NECZ. For the economic feasibility, the use of NECZ took the higher investing cost to a large extent than DBT and TOL, with almost unit H2 production cost of 264.47 $ kgH2 -1 while 54.94 $ kgH2 -1 for DBT, and 19.94 $ kgH2 -1 for MCL. Meanwhile, compared with DBT and MCH, the CO2 emissions per produced H2 were revealed to be showing environmental drawbacks of NEZ. Hence, the techno-economic performance of LOHCs should be comprehensively considered.When comparing the strengths and drawbacks of the LOHCs technology processes, it becomes obvious that a number of reasons can limit large-scale commercialization. The key obstacles include the high energy consumption, the difficulty of developing dehydrogenation catalysts, and the decrease of hydrogen storage performance as the number of cycles increases [88–90]. As a result, current research focuses on lowering energy consumption and developing high-performance catalysts. Major dehydrogenation catalysts are supported metal catalysts and tend to be prepared by impregnation [91], deposition precipitation [92,93], one-pot [94], and sol-gel methods [95], etc. Generally, these catalysts are loaded on carbon-based materials, Al2O3, TiO2, zeolite, and other carriers [96,97]. Since the hydrogenation and dehydrogenation of hydrogen storage liquids are reversible reactions, catalysts with high hydrogenation activity also perform well in dehydrogenation reactions. Catalysts are often categorized based on how metals are mixed. Monometallic catalysts (noble metal catalysts, non-noble metal catalysts [98–100]), polymetallic catalysts [101], and other catalysts (such as boron nitride and metal complexes) are examples of different types of catalysts.(1) Monometallic catalystsMonometallic catalysts are normally categorized into metallic and non-metallic catalysts based on metal activity [89]. Palladium (Pd), platinum (Pt), ruthenium (Ru), rhodium (Rh), nickel (Ni), molybdenum (Mo), and copper (Cu) have all been extensively researched. The choice of the catalyst has a critical influence on the products and activity of the reaction. The proportion of different forms of d-electrons has been demonstrated to be related to the active component. In the case of noble metals, for example, Pd, Pt, Ru, and Rh all occupy 40% of the d-electrons in their atoms, and their activity is higher [102,103]. With high activity, Pd and Pt can significantly decrease the dehydrogenation temperature. The activity of Pd was maintained after a few cycles, demonstrating a distinct advantage in all types of LOHCs systems. Wang et al. [91] performed a comparative experimental study on the dehydrogenation of 12H-NECZ with graphene (rGO)-supported noble metal catalysts. The catalytic activity was found to follow the order Pd/rGO > Pt/rGO > Ru/rGO > Au/rGO in the experiments. The authors also compared Pd/rGO with the commercial catalyst Pd/Al2O3. Pd/rGO was shown to be superior to Pd/Al2O3 in many ways, and its catalytic activity remained unchanged within a certain range of reaction temperature. Sharma et al. [104] employed ruthenium metal as the active component of the catalyst. They discovered that at 120°C and a partial pressure of H2 of 60 atm for 2 hours, 100% conversion of benzene with 100% selectivity could be achieved. Despite the benefits of noble metal-supported catalysts in terms of activity, reaction rate, and service life, there is a demand for low-cost, high-efficiency catalysts for LOHCs.For the storage and release of hydrogen from LOHCs systems, Ni-based catalysts also exhibit outstanding catalytic activity. These catalysts are generally utilized in the form of supported or skeletal Ni for reactions [105]. Raney-Ni is widely utilized for skeletal Ni and shows a high activity due to its large specific surface area. Nevertheless, the conversion rate tended to drop gradually as the number of catalytic cycles and time increase. When it comes to the supported Ni, the catalysts Ni/SiO2 [67], Ni/Al2O3-TiO2 [106], and Ni2P/Al2O3 [67] are commonly utilized. In addition to some supported metals such as Ni, Mo have also been studied as a non-noble metal, and its loading effect has a significant impact on the surface properties of the catalyst. However, Mo frequently exhibits a decrease in activity and produces more coking materials. Generally, non-noble metals have disadvantages in terms of reactivity, temperature, and service life when compared with noble metals, but they offer a remarkable economic advantage.(2) Polymetallic catalystsFor the features obtained by the different physicochemical qualities of single components of monometallic catalysts, the use of polymetallic or transition-metal catalysts can completely exploit the strengths of each component. This sort of catalyst can be improved to stimulate the breakdown of C-H bonds, improve the activity of intermediates, speed up the reaction, and even increase the selectivity of products. The addition of Ni, Cu, Sn, and other second components to noble metal catalysts can take advantage of the involved synergistic effects, such as AuxPdy/rGo [107] and Pd-M/TiO2 [92]. Doping with polymetallic materials can considerably reduce the number of noble metals and economic pressure, while maintaining high reactivity and cycle stability. Wang et al. [94] studied the dehydrogenation performance of 12H-NECZ by using rGO as a carrier to prepare Pd and Cu bimetallic catalysts via a one-pot method. The Pd1.2Cu/rGO catalyst had the maximum catalytic activity, with 100% conversion of 12H-NECZ after 1 hour of reaction and 100% product selectivity after 7 hours. Yuki et al. [108] developed the binary alloy for the dehydrogenation of TOL/MCH, and they finally found that Pt3Fe/SiO2 acts as a highly active and durable heterogeneous catalyst with excellent toluene selectivity (>99%) and long-term durability. The core is that the excellent performance was derived from the synergy of each element (C-H activation of Pt, decoking of Fe, and toluene desorption of Zn). Numerous investigations have shown that the introduction of other metals into monometallic materials can result in positive synergy, improved catalyst activity and stability, and cost benefits [109–112]. However, the characteristics of the catalysts are strongly affected by the processing procedures and types of the carriers. For the hydrogenation and dehydrogenation of various LOHCs systems, the introduction of the second or third metal should serve as the reference object. Above all, it is necessary to consider whether the addition of metals and the preparation procedures can interact positively with the hydrogen carriers.In addition to the active component, the metal-supported carrier is a crucial element that affects the performance of the catalyst. Several studies have discovered that catalysts with metal nanoparticles on the surface of carbon-based materials (e.g., activated carbon, graphene, and carbon nanotubes) can have strong hydrogenation and dehydrogenation activity [113,114]. Catalyst activity is increased when metal links are modified, allowing researchers to better regulate the reaction process and facilitate research for industrial applications. Above all, it is essential to assess whether the different metals and preparation processes used in the catalysts could lead to positive system synergy. Apart from supported catalysts, a growing number of other types of catalysts, such as boron nitride and metal complexes, have gradually become research hotspots in recent years. However, many of these catalysts have challenging production procedures and are difficult to apply in industrial settings.Overall, the reactivity of hydrogen storage qualities is affected by the types of metal, support, and carriers. In both monometallic and bimetallic systems, noble metals exhibit excellent catalytic characteristics. However, it is still necessary to balance costs and benefits. Bimetallic and polymetallic catalysts are currently recognized as the most promising strategies to improve the process. The possibility of lowering the quantity of noble metals while retaining catalytic activity holds a lot of promise. In addition to changing the catalysts in terms of the active component, carriers, and supporting mode, many researchers employ sulfur in conjunction with catalysts modification techniques in the petrochemical industry to optimize the catalytic activity [115]. In order to prepare the Pt/Al2O3 dehydrogenation catalyst in the dibenzyltoluene/perhydro-dibenzyltoluene system, Wasserscheid's team added a specific amount of sulfide. They found that sulfur occupied a low coordination sites of the supported Pt nanoparticles, which significantly boosted the dehydrogenation efficiency and lowered the production of byproducts [116]. Furthermore, by increasing the dispersion of active components, lowering the surface charge density, and maximizing the H2 spillover effect, the catalytic activity in the reaction of polycyclic aromatic hydrocarbons and heteroatom-doped compounds can be boosted [117–119].Hydrogenation and dehydrogenation reactors are key components of the whole system, with dehydrogenation reactors receiving more research and development. The reactor has more rigorous pressure requirements since the hydrogenation process necessitates greater pressure. Stainless steel autoclaves, along with fixed-bed reactors and other laboratory equipment, are more commonly utilized for easy operation [65,83,120]. When the reaction is accompanied by tandem side reactions, the fixed-bed reactor allows continuous hydrogenation and effective interaction between the reactants and catalyst, resulting in high selectivity. Individual reactor units are utilized for batch hydrogenation in stainless steel autoclaves, which are relatively straightforward to operate. To maintain the reaction at a consistent pressure, a hydrogen reservoir was attached to the reactor during the reaction. According to the classification of reactants and reaction conditions, dehydrogenation reactors can be divided into two types: steady-state and non-steady-state [121,122]. Dehydrogenation reactions are typically categorized into three groups based on the phase of organic liquid hydride: gas-phase, liquid-phase, and “wet-dry” multi-phase dehydrogenation. The advantages and disadvantages of several types of reactors are listed in Table 3 . Among what has been mentioned in the list, principal dehydrogenation reactors at the laboratory-research stage are “oil bath pot - three mouth flask” [114,123], fixed-bed reactor [47], pulse jet reactor, and membrane catalytic reactor [124]. The “oil bath pot - three mouth flask” apparatus is a batch reactor. The hydrogen-rich liquid is injected into the container after it has been heated to the temperature of the dehydrogenation reaction in the oil bath. The hydrogen removed by the reaction was cooled, separated by a serpentine condenser, and eventually collected. “Oil bath pot - three mouth flask” is a simple-structured device of the reaction which enables the effective separation of reactants and products. However, the temperature of the dehydrogenation reaction is frequently a limiting factor, and it is more commonly utilized in systems with lower dehydrogenation temperatures, such as NECZ. Fixed-bed [125–127], membrane reactors, and stainless steel autoclaves [128] are more often employed for systems with higher dehydrogenation temperatures and requirements for continuity. In a fixed-bed reactor, similar to hydrogenation, dehydrogenation is conducted by heating the catalyst to a given temperature and then transferring the organic hydrogen carrier into the unit. In the case of MCH, after being heated in the gasification chamber, it traveled through the catalyst bed in a gaseous condition and then reacted. Finally, the hydrogen was extracted and gathered.Therefore, a suitable reactor must be selected for the phase states of different reactants and mechanisms of dehydrogenation. There are numerous methods for optimizing and upgrading the reactor on a regular basis. To improve the dehydrogenation rate and purity of the produced hydrogen, multiphase dehydrogenation can be used instead of single-phase dehydrogenation. Microreactors can also be utilized to keep a consistent dehydrogenation temperature [121]. Some bottleneck difficulties in pulse injection reactors [129] and membrane-catalyzed reactors remain addressed, and commercial applications are still a long way off. Fixed-bed reactors offer a high conversion efficiency and can be utilized for continuous feeding. However, the diffusion of gas, which easily causes coke production and catalyst degradation, currently limits the reaction rate of fixed-bed reactors. During the dehydrogenation, it is necessary to take into account the activity of the catalyst, temperature, and reaction efficiency to control completion of the reaction. When the reaction is insufficient, it is also critical to avoid a reverse reaction.The development of several types of reactors for independent hydrogenation and dehydrogenation processes is progressing. Under ideal conditions, a high level of cycling performance can be reached by considering the full response system holistically. LOHCs technology is a three-step closed-cycle reaction that includes hydrogenation reactions, hydrogen carrier conveyance, and dehydrogenation reactions. Among numerous nations with relatively advanced and widespread hydrogen energy development, Japan has essentially completed the demonstration project of hydrogen storage and release, addressing hydrogenation generation and Gas-to-Liquid technologies and benefiting from hybrid fuel cells [133]. The integrated system for hydrogen storage and release is depicted in Fig. 5 , which is a flow chart at the level of laboratory or small and medium-sized hydrogenation and dehydrogenation equipment. As shown in the Fig. 5, when completely hydrogenated, the product is condensed by employing a condenser, which discharges the excess exhaust gas and produces hydrogen storage carriers. Fully hydrogenated organic liquids can be stored and transported efficiently because of their high stability and cyclic performance. When the hydrogen is required, LOHCs are transferred by liquid storage tanks or other equipment to the dehydrogenation unit. The dehydrogenation is then carried out under the action of a specific condition. Finally, hydrogen is discharged from the hydrogen outlet of the unit and stored in a gas storage tank for later use. The organic liquid can be processed, transported, and recovered after dehydrogenation for further hydrogen storage.Owing to the advantages of safety, compatibility, and high hydrogen storage capacity of current liquid storage facilities, multiple energy sources can be transported and preserved. These features distinguish LOHCs technology from conventional hydrogen-storage systems. This aligns with the present call for a firm guarantee for the development of clean, low-carbon, safe, and efficient energy systems. This is expected to alleviate the problem of uneven distribution of energy in space and time.Since the reversible processes of LOHCs technology require specific reaction conditions, this ensures a high degree of stability for liquid organic hydrogen storage carriers. Because of the flammable and explosive characteristics and low density of hydrogen, its transport across oceans or other large-scale applications is particularly challenging. Liquid hydrogen, liquid ammonia hydrogen, and LOHCs are the three main technologies for bulk hydrogen transport now in use [10,134]. For the delivery of hydrogen, traditional LH2 technology necessitates absolute temperature control. The storage vessel is insulated as a result of the conditions of use, which can will increase the cost greatly. In the case of LAH2 technology, traces of ammonia remaining in the hydrogen after dehydrogenation cause severe degradation of the performance. In summary, high-pressure and liquid hydrogen are more suitable for short-haul transportation, but both systems have large upfront expenditures and administrative restrictions. As for LOHCs technology, hydrogen carriers can be stored and transported with existing oil and gas transmission pipelines, tankers, and storage tanks. The usage of LOHCs technology aims to store hydrogen through organic hydrogen storage liquids from raw hydrogen resources. Hydrogen storage tankers, tankers, pipelines, and ships are used to carry the carriers to their final destination [10,135,136]. Finally, catalytic dehydrogenation units release hydrogen for use in fuel cells, hydrogen refueling stations, and industrial production. At the same time, after cooling, the hydrogen-leaved liquids used for hydrogen storage can be recycled and stored for future use.“Chiyoda Chemical Construction” in Japan has carried out the research and engineering test of large-scale LOHCs technology based on TOL/MCH, with the dehydrogenation conversion of MCH over 99.9%, selectivity of TOL over 99.9%, and catalyst life over 10,000 hours. As the representative enterprise, “Chiyoda Chemical Construction” imported a total of 210 metric tons of hydrogen from Brunei to realize the transfer of hydrogen across the oceans in 2020. The first project of the worldwide hydrogen-energy supply chain has completed its demonstration phase. This chain was based on the production of hydrogen through natural gas reforming at the Brunei plant, employing a stable chemical as the carrier and then using conventional transportation to convey hydrogen to Japan across long distances. The goal of this project is to feed hydrogen to turbines to create electricity.In addition to being used for the bulk and transoceanic transport of hydrogen energy, LOHCs technology can also be applied to existing hydrogen energy vehicles [73,137]. It is possible to load hydrogenated cyclohexane straight into the vehicle and dehydrate it using an onboard dehydrogenation device in the case of hydrogenated cyclohexane. “Hynertech” in Wuhan, the representative firm among the LOHCs technology-related enterprises in China has demonstrated a project for 1,000-ton NECZ plant using “NECZ/12H-NECZ”. The business has proposed the world's first fuel cell model that relied on liquid organic hydrogen as a source of energy in 2016. As mentioned in section 3.2, the hydrogen storage liquid developed in “Hynertech” of is known as “hydrogen oil”. Ultra-high-temperature gasification technology has been investigated for the production of “hydrogen oil” in accordance with Chinese policy. This technology makes use of municipal and industrial solid waste, which is gasified at extremely high temperatures to produce hydrogen gas and “hydrogen oil”. This not only reduces the environmental impact of hazardous waste, but it also improves energy efficiency and allows biomass to be combined with other emerging energy industries.In the context of carbon neutrality, the renewable energy industry in China is gaining traction and helping to lessen the reliance on imported energy. When it comes to long-distance transportation, conventional energy storage technologies (such as liquid fuels, electricity, and thermal energy) have limited by storage periods, high energy consumption, and low safety. As a result, positive energy storage technology research is required to realize the exploitation, transfer, and storage of important renewable energy supplies. In the case of hydrogen energy, the production and preparation of hydrogen usually consist of three categories: “blue hydrogen”, “grey hydrogen” and “green hydrogen” [138]. Hydrogen is a prospective energy carrier that can be employed as a conversion medium for a variety of energy sources. LOHCs technology allows for efficient hydrogen energy storage and transmission, and it can also be used to collect renewable energy, large-scale distributed generation, and hydrogen. This one-of-a-kind connection makes it suited for long-term and large-scale storage and usage, allowing collaborative connectivity between diverse components of the energy network [135]. For the production of hydrogen, there are an increasing number of studies on renewable energy methods such as water electrolysis/solar photolysis, biomass fermentation, bioethanol reforming, and biomass chemical cracking [139]. Electrolysis of hydrogen from renewable energy sources and the processing of biomass feedstock have been shown in a growing number of studies to be key sources of hydrogen for increasing the amount of hydrogen produced [41,135]. When comparing the investment expenses of CH2, LH2, and LOHCs, the latter is only 32% of the former [140]. Germany has proposed the “GET H2” in 2020. The project seeks to develop industrial-scale green hydrogen production in regions with abundant wind and solar energy resources, and connect with downstream application. Fig. 6 shows one of the methods in which energy is stored in a renewable system by LOHCs technology in the form of a hydrogen supply chain.Aromatics and N dopants are the critical systems available for LOHCs technology. Existing hydrogen storage liquids are commonly prepared from coal and petroleum. Pure chemicals such as triphenylbenzene [benzene, toluene, and xylene, (BTX)] tend to obtained from coal by thermal fractional distillation, and purification in turn [141,142]. When prepared from petroleum, the fractionated products contained fewer aromatic compounds and a large number of alkanes. As a result, numerous bond-breaking reforming and aromatization procedures must be used to transform them into aromatic hydrocarbons [143]. Traditional fossil energy sources utilized as raw materials are not infinite, and the cost is considerably large. To create aromatic and N-doped compounds for chemical or other applications, economical and sustainable sources must be investigated. Biomass resources, which are primarily made up of elements like carbon, hydrogen, and oxygen, offer a lot of promise for development as carbon-neutral renewable energy sources [144,145]. The three primary components of biomass, cellulose, hemicellulose, and lignin, have varied structural qualities and hence require different conversion methods and applications. Within cellulose and hemicellulose, equipped with a five-membered or six-membered cyclic polysaccharide structure, hemicellulose acts as a molecular binder bound between the cellulose and lignin [146]. Lignin, a three dimensional reticulated aromatic ring structure wrapping and reinforcing cellulose and hemicellulose, is a kind of nature organic compound made up of interlaced C-C and C-O bonds with a complex structure [147]. The aromatic and furan rings found in direct pyrolysis products of lignin can be exploited to make biomass fuels, light aromatics, and other small-molecule compounds [148,149]. Benzene is a basic carbon frame structure of biomass, consisting of a “benzene ring” encircled by six C atoms [150]. The basic structure of almost all of the products produced by various biomass conversion processes is benzene rings [151]. As a result, using biomass as the source of reaction carrier in the LOHCs system has a certain amount of practicality and economic benefit.Based on the technical principles of aromatics extraction from biomass, it can be divided into direct and indirect routes for preparation. Direct preparation refers to the direct conversion of biomass into aromatic products in a reactor without further processing. This is the biomass-catalyzed thermal cracking method that is currently being explored and used the most. The conversion of biomass into intermediate products via a variety of techniques, followed by the manufacture of aromatics, is referred to as indirect preparation technology [152]. The second method has the benefit of allowing for the targeted conversion of intermediates to aromatics using existing conversion technologies with high yields and efficiency. However, the production method is lengthy, resulting in significant raw material waste and energy usage.When using intermediates in the form of syngas (CO, H2), there are three major pathways for the preparation of aromatics: methanol/dimethyl ether, Fischer-Tropsch synthesis, and direct preparation using aromatization catalysts. Methanol-to-aromatics (MTA) technology [155–158] is one of the ways listed above that has been developed and applied in industry. The aromatic compounds were selective up to about 80% during the process, and the methanol is almost completely converted. The main products of syngas synthesis by Fischer-Tropsch are alkanes and olefins, with lesser yields of aromatics. Another technology for the direct preparation of aromatics from syngas is mainly obtained through improving the catalysts. The products can be directly converted to aromatics by selecting aromatized catalysts in the above two preparation procedures [157]. As indicated in Fig. 7 , the synthesis of aromatic hydrocarbons entails various processes. First of all, through methods such as hydrolysis, hydrogenation, or fermentation of biomass, oxygenated compounds, such as sugars, aldehydes, and alcohols are produced by the action of microorganisms. Subsequently, a series of operations including reform, dehydrogenation, and cyclization, were conducted to produce aromatics. This is attributable to the fact that the three primary components of biomass can be hydrodeoxygenated or enzymatically broken down into tiny molecules like alcohols, furans, and phenolic aldehydes. Under appropriate reaction circumstances, these molecules can be continually transformed into aromatic compounds [153]. Technologies of bioFormingTM and bio-based isobutanol to aromatics, which produce aromatic compounds from 100% renewable plants and sugars, are already commercially viable in this field. Overall, the method of producing indirect aromatics from biomass is time-consuming, with additional intermediate steps in the reaction resulting in poorer product yields, higher reaction energy consumption, and feedstock waste. To put it another way, commercial applications are extremely tough to implement, necessitating further refinement and optimization.Fig 8. Aromatic products like BTX and olefins can be synthesized from biomass feedstock (e.g., wood, agricultural products, organic solid waste, and fiber waste) via catalytic thermal cracking technology, which involves a series of complex reactions like depolymerization, isomerization, and polymerization. The products of catalytic pyrolysis of biomass include bio-oil, coke, and combustible gases. Depending on the reaction conditions, the proportion of products obtained varied. Catalytic slow pyrolysis mostly produces coke, while catalytic medium-speed pyrolysis generates combustible gas and catalytic fast pyrolysis (CFP) generates bio-oil [159–161]. The manufacture of aromatic compounds by direct catalytic pyrolysis is primarily performed through the CFP of biomass due to the high amount of aromatic products with the most basic benzene ring structure.Owing to the different structures and types of biomass feedstock, the contents of the three main components vary accordingly. Lignin has a higher selectivity for aromatic compounds in catalytic cracking products than the other two primary components of biomass due to its high H/C ratio. During pyrolysis, cellulose is the first to undergo bond breakdown, resulting in aromatic compounds and olefins with a high added value. As a result, biomass feedstock with high lignin and cellulose content offers a better potential for aromatic chemical production employing catalytic pyrolysis procedures [162]. It has been reported that the cellulose content of biomass feedstock affects the bio-oil content. More bio-oil can be produced with a greater cellulose ratio, which contributes to the generation of aromatics. Zhang et al. [163] chose ZSM-5 as a catalyst to evaluate the yield of CFP aromatic compounds products with varying pine-to-alcohol ratios. They discovered that the yield of aromatic compounds could be enhanced with the increase of the H/C ratio of the reactants. The conversion of organic matter to aromatic substances within the bio-oil can be facilitated by an increased amount of hydrogen. However, because the type of raw material has a considerable influence on the product, numerous studies have been conducted on model compounds or single components [164]. In addition, model compounds such as furan, furfural, and glucose are commonly employed to research pyrolysis mechanisms, products, and reaction routes. In the hydrodeoxygenation of lignin, Diao et al. [146] prepared MoCo9S8/Al2O3 as a catalytic material with a balance between economy and temperature stability. The easy deactivation of sulfide catalysts was focused on to achieve an efficient one-step conversion of lignin to aromatic compounds, with 99.8% conversion and 91.0% yield of benzene.Alkali metals, alkaline earth metals, metal oxides, and molecular sieves are the most typical catalysts employed in biomass catalytic pyrolysis reactions. Molecular sieve catalysts are widely used in the field of catalytic pyrolysis of biomass to prepare aromatic compounds. Because of its unique internal aperture structure and acidic sites on the surface, ZSM-5, for example, can reduce the carbon build-up problem and promote selectivity of the target product to some extent [159]. The aperture structure, acidic sites, silica to aluminum (Si/Al) ratio, and particle size are also critical factors that affect catalytic activity. More studies on modification approaches, such as increasing metal supported [165,166], employing mesoporous catalysts [162,167], and modifying the acid-base treatment order, are needed in the future.Thermal catalytic conversion and ammonification is the process of making N-doped chemicals from biomass using catalytic pyrolysis in the presence of a nitrogen donor (TCC-A). N-doped compounds include amines, nitro substituted, nitrile, and some N heterocyclic compounds with aromatic structures, such as indoles, pyridines, and pyrroles [149]. Nitrogen donors can be divided into three types: gaseous ammonia, high nitrogen biomass, and solid ammonia sources. Additionally, the ammonium salt is a nitrogen donor since it creates ammonia gas when heated. In LOHCs technology, N-heterocyclic aromatic compounds generated by heat catalysis and ammonification can be seen as hydrogen storage agents.The indoles have a chemical structure that is similar to that of furans. Therefore, by introducing an external ammonia source, catalytic pyrolysis can be used to prepare indoles to catalyze the formation of furans from biomass. For the production of indoles, several furan derivatives such as 2-methylfuran and 2-methylfurfural can be employed as intermediates or source materials. TCC-A technology has been claimed to be capable of directly converting biomass-derived furans and furfurals to indoles for the manufacture of N-doped chemicals from biomass [168]. Xu et al. [149] experimentally discovered that TCC-A technology could transform raw biomass with complex compositions into N-doped compounds and N-containing biochar. By altering the reaction conditions, the percentage of pyridines and indoles could be selectively controlled. Yao et al. [169] exploited furfural, which is obtained from biomass, as the feedstock for thermal catalytic transformation and zeolite ammonification to convert furfural to indole compounds. Ultimately, they concluded that the conversion pathway was “furfural-'furfural-imine' furan-pyrrole-indole”. Lactose, acetylation, and furan amination are the most common processes for making pyrroles. Pyrroles prepared from biomass can be obtained by rapid in situ pyrolysis of cellulose under ammonia atmosphere using HZSM-5 [170–172]. In industrial settings, pyrrole is usually prepared from furans derived from petroleum under the catalysis of solid acid catalysts. Yao et al. [173] found that the catalytic fast pyrolysis of cellulose in a low-temperature ammonia atmosphere is an effective way to selectively prepare pyrroles. When the reaction temperature was controlled at 400°C, they took γ-Al2O3 as the catalyst and a selectivity of up to 89.5% was achieved with a catalyst/cellulose ratio of 2. The classic method of “catalytic synthesis” was used to make pyridine compounds, and the raw material used was primarily glycerol. Pyridine tends to be synthesized by aldehydes, alcohols, and unsaturated hydrocarbons in an ammonia-rich environment. As a result, when pyridines are utilized as target products, biomass can be first converted into intermediate products such aldehydes, ketones, and alcohols.Therefore, by utilizing the current technologies of biomass conversion, suitable and prospective hydrogen carriers can be purposefully and selectively prepared for LOHCs technology in the future. Overall, the utilization of biomass in the synthesis of aromatic and N-heterocyclic compounds to be used as organic hydrogen storage liquids has a great prospect. However, further studies are required to create breakthroughs in terms of enhancing the stability and selectivity of the reaction.The discrepancies between the raw biomass pyrolysis products and target carriers of the LOHCs system should be compared to directionally regulate the parameters of each process. Furthermore, according to the type of selected biomass, different catalytic reaction routes and pretreatment modes are necessary to optimize the yield of the targeted product. During the catalytic pyrolysis processes of biomass, it is important to focus on the pathways (e.g., purification and ammonification) that break bonds to form monomeric compounds for subsequent operations. On the basis of the positive or negative feedback of the reactions of biomass-based organic hydrogen storage liquids, it is necessary to adjust the conditions to optimize the characteristics within the operating range. At present, the technology for the catalytic pyrolysis of biomass for the preparation of aromatic and N-doped compounds is relatively advanced. Major studies are more likely to explore the influences of the feedstock type, reaction conditions, catalysts, and other factors. Meanwhile, the advancement of LOHCs technology has been steadily advancing, with more research currently concentrated on lowering the dehydrogenation temperature, improving catalysts, and reducing the energy consumption. In addition, certain businesses around the world have already succeeded in commercializing and trading LOHCs technology. This pattern suggests that the technology has reached a high level of maturity and usability. However, the technologies producing LOHCs have abundant disadvantages in terms of sustainability or material expense. Therefore, it is necessary to explore economic and sustainable renewable carriers. Through the review of previous researches, there are few studies on the generation of hydrogen carriers through biomass conversion technology. Combined with the reaction principles of the various LOHCs systems, biomass can be applied to LOHCs through existing methods of conversion. Future research in this field should improve the catalysts and other factors through targeted modifications to produce organic hydrogen storage liquids.LOHCs technology can effectively avoid some of the shortcomings of conventional hydrogen storage technologies, which is aimed at meeting the requirements of long-term and large-scale hydrogen storage in an ambient environment. Fortunately, this technology possesses excellent compatibility with existing equipment for oil and gas production, has tremendous development prospects, and is latent. Because of the unique feature of carbon neutrality and the structure of its basic components, biomass is widely used in the clean energy sector. The most common use of biomass is to make bio-liquid fuels with aromatics as the most basic structure, which can be prepared by a variety of depolymerization processes. The products can also be converted into other chemicals through advanced processes to realize high-value utilization. Petroleum, coal, and other raw materials are commonly used to make hydrogen storage liquids in existing LOHCs systems. However, because of the limited amount of traditional energy, newer manufacturing methods should be investigated. Based on the above review, there are still several challenges for the development LOHCs technology. (1) The temperature of dehydrogenation and the energy consumption are difficult to decrease. (2) Stable, efficient, and economic catalysts used for hydrogenation and dehydrogenation need further development. (3) The initial cost to purchase LOHCs materials is extremely high. (5) It is necessary to comprehensively consider the technical, economic, and environmental performance of the system.Taking advantage of the fact that benzene is the fundamental unit in biomass, coupling LOHCs with technologies related to the catalytic pyrolysis of biomass is promising. Switching the orientation and research ideas is critical for biomass-based organic hydrogen storage liquids. To prepare this type of liquid according to the specific reaction equipment, appropriate technologies must be established. Therefore, there are numerous difficulties to solve in the utilization of renewable biomass-based hydrogen carriers. (1) According to the findings of this paper, there are numerous challenges to overcome when combining catalytic pyrolysis of biomass for the manufacture of aromatic and N-doped chemicals with LOHCs technology. Due to the coverage of active sites caused by coking, the blockage of channels, and the agglomeration of active metals, catalysts are easy to inactivate. Hence, it is necessary to prepare high-performance catalysts to ensure the directional, efficient and stable conversion of biomass pyrolysis. (2) Distinct components of biomass have different conversion processes. On one hand, Multifunctional catalysts should be designed to achieve multiple conversion paths. On the other hand, to separate components without significantly affecting the structure, advanced pretreatment technology is necessary. (3) A few components in the products prepared by catalytic pyrolysis may not have the ability to store hydrogen, which requires to explore the technology of further separation and purification. (4) Most of the existing catalysts are used for LOHCs of single substance, which may not be applicable to the LOHCs prepared by catalytic pyrolysis of biomass. Therefore, it is necessary to design suitable and efficient hydrogenation and dehydrogenation catalysts according to the characteristics of the products. According to the findings of this paper, there are numerous challenges to overcome when combining catalytic pyrolysis of biomass for the manufacture of aromatic and N-doped chemicals with LOHCs technology. Due to the coverage of active sites caused by coking, the blockage of channels, and the agglomeration of active metals, catalysts are easy to inactivate. Hence, it is necessary to prepare high-performance catalysts to ensure the directional, efficient and stable conversion of biomass pyrolysis.Distinct components of biomass have different conversion processes. On one hand, Multifunctional catalysts should be designed to achieve multiple conversion paths. On the other hand, to separate components without significantly affecting the structure, advanced pretreatment technology is necessary.A few components in the products prepared by catalytic pyrolysis may not have the ability to store hydrogen, which requires to explore the technology of further separation and purification.Most of the existing catalysts are used for LOHCs of single substance, which may not be applicable to the LOHCs prepared by catalytic pyrolysis of biomass. Therefore, it is necessary to design suitable and efficient hydrogenation and dehydrogenation catalysts according to the characteristics of the products.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 National Natural Science Fund for Excellent Young Scholars (China) (Grant No. 51822604).

Hydrogen has attracted widespread attention as a carbon-neutral energy source, but developing efficient and safe hydrogen storage technologies remains a huge challenge. Recently, liquid organic hydrogen carriers (LOHCs) technology has shown great potential for efficient and stable hydrogen storage and transport. This technology allows for safe and economical large-scale transoceanic transportation and long-cycle hydrogen storage. In particular, traditional organic hydrogen storage liquids are derived from nonrenewable fossil fuels through costly refining procedures, resulting in unavoidable environmental contamination. Biomass holds great promise for the preparation of LOHCs due to its unique carbon-balance properties and feasibility to manufacture aromatic and nitrogen-doped compounds. According to recent studies, almost 100% conversion and 92% yield of benzene could be obtained through advanced biomass conversion technologies, showing great potential in preparing biomass-based LOHCs. Overall, the present LOHCs systems and their unique applications are introduced in this review, and the technical paths are summarized. Furthermore, this paper provides an outlook on the future development of LOHCs technology, focusing on biomass-derived aromatic and N-doped compounds and their applications in hydrogen storage.

Rising global warming due to CO2 emission from fossil fuel combustion and increasing energy demands have led researchers all over the world to focus on developing technologies for clean energy production and storage, such as fuel cells and batteries. Unitised regenerative fuel cells (URFCs) represent systems that can in one instance work as electrolysers (splitting water molecules to produce hydrogen fuel and oxygen) and in the next as fuel cells (chemically “combusting” H2 fuel to produce electricity) [1,2]. For URFCs to produce high currents, high-performing bifunctional catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are of great importance [3–5]. However, producing catalysts that can sustain this transition from one operating mode to another has proven to be very difficult.Currently, the most efficient OER catalysts are considered to be ruthenium (Ru) and iridium (Ir) oxides, while platinum (Pt) has been regarded as the best performing catalyst for the ORR [6,7]. However, their high cost and low natural abundance rule out these metals for applications in renewable energy technologies in the long term.ORR is considered as the main limiting step in the fuel cell mechanism due to its sluggish kinetics and the high overpotential required [8]. The thermodynamic potential of O2 reduction versus the normal hydrogen electrode is 1.23 V. Electrode materials undergo oxidation at such a high potential, so that the electrode surface is no longer composed of a pure metal catalyst, but also of metal oxide. Thus, the Pt surface at high potentials is a mixture of Pt and PtO, resulting in an open circuit potential lower than 1.23 V, depending on the Pt to PtO ratio. The catalytic activity of Pt towards the ORR strongly depends on the O2 adsorption energy, the dissociation energy of the O-O bond, and the binding energy of OH to the Pt surface.Due to the high cost of Pt-based catalysts, researchers have focused their attention on nickel (Ni), a metal from the same group as Pt, but much more naturally abundant [9]. In recent years, Ni-based electrocatalysts in different forms (e.g., nanoparticles (NPs), foams, alloys, oxides, phosphides, metal–organic frameworks) have been proposed as efficient substitutes for expensive metals as catalysts for the OER, the ORR, and the hydrogen evolution reaction (HER) [10]. Thus, Ni-based catalysts have been reported to exhibit high susceptibility toward surface adsorption of O2, a crucial step in the ORR. The work that pointed out the potential of Ni and encouraged Pt alloying was carried out by Stamenković et al. [11]. A density functional theory study of the adsorption of OH and H2O on Pt3Ni(111) surface showed that in the presence of a Ni sublayer with 50 at.% Ni, adsorbed OH reacted with H+ to form H2O with a positive shift of 0.1 V in the reversible potential.In the present study, we investigate the possibility of increasing the catalyst’s activity while simultaneously reducing its cost by lowering the amount of Pt and replacing it with Ni. Introducing another metal to Pt can have a strong effect on the electronic structure of the Pt catalyst and the Pt–Pt interatomic distance, thus changing the electrochemical activity of the Pt catalyst. Metal NPs have been supported on three different metal oxides in order to overcome the problem of support degradation, another bottleneck in the commercialisation of URFCs. Pt-based catalysts are typically supported on carbon black, but advanced carbon materials, as well as non-carbon supports including transition metal oxides, carbides, and nitrides, have been recently suggested to improve the support/catalyst’s durability [12].Details of the preparation and characterisation of the catalysts are given in the Supplementary information (SI). The nominal Pt loading on the supports was set to 20 wt% and for the bimetallic catalyst Pt:Ni the weight percentage was set to 10:10. The obtained loadings were confirmed by inductively coupled plasma mass spectrometry (ICP-MS) and the composition, structure, and morphology were further examined by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Details of the electrochemical measurements are also provided in the SI.ICP analysis results reveal that the metal loading was close to the targeted nominal values (Table S1). The higher content of Ni in the PtNi/Mn2O3-NiO catalyst can be attributed to the Ni content in the support material itself.XRD patterns reveal the diffraction peaks of spinel Mn2O3 (Fig. 1 ), with additional details being given in the SI. Diffraction peaks of anatase TiO2 and NiO were observed for Mn2O3-TiO2- and Mn2O3-NiO-supported catalysts, respectively. The characteristic peak of Pt was clearly observed for the Pt-based catalysts, while no Ni peaks were observed for the PtNi-based catalysts.XPS analysis (Fig. 2 ) revealed sharp peaks at binding energies of 643.5, 655.8, and 530 eV, attributed to the Mn2p3/2, Mn2p1/2, and O1s regions, respectively [13,14]. The presence of Pt is confirmed by the peaks at 314 and 331 eV as Pt4d5/2 and Pt4d3/2 shake-up (satellite) peaks, and also at 71 and 74 eV as Pt4f7/2 and Pt4f5/2 shake-up (satellite) peaks, respectively [15,16]. The Ni2p peaks were observed between 855 and 879 eV [17] with their intensity increasing upon addition of NiO to the Mn2O3 support. PtNi alloy formation was confirmed by the positive chemical shifts of Pt4d5/2 and Pt4d3/2 when compared to the unmodified Pt catalysts (Fig. 2 b,d,f) [18].Formation of Pt and PtNi nanoparticles of size < 4 nm was confirmed by TEM analysis (Fig. 3 ). Somewhat bigger nanoparticles (4 to 13 nm) were observed in the case of Pt/Mn2O3-TiO2.Cyclic voltammograms (CVs) recorded in O2-saturated 0.1 M KOH reveal a clear peak corresponding to O2 reduction (Fig. 4 , S1, S2, S3). The expected enhancement of the activity of the catalysts towards the ORR, in terms of lower onset potential, E onset, and higher current density, could be observed upon grafting Pt NPs (20 wt%) on the oxide supports [19]. Further comparison of catalysts with Pt NPs (20 wt%) with those where 10 wt% of Pt was replaced with 10 wt% of Ni, showed similar diffusion-limited current densities. Rotating disc electrode voltammograms indicated the highest activity for the ORR in the case of Mn2O3-NiO-supported Pt and PtNi NPs in terms of the highest current density at a given rpm. The activity of metal NPs supported on metal oxides has been reported to depend on the size of the NPs, the oxide crystal phase/morphology (being in an appropriate oxygen adsorption mode) [20] and the support-induced modification in the electronic properties of the metal NPs [21].Furthermore, these two catalysts showed promising results when compared to the reference Pt/C (40 wt% Pt) sample, as diffusion-limited current densities of −6.44, −4.48, and −4.32 mA cm−2 were recorded at 1800 rpm for Pt/C, PtNi/Mn2O3-NiO, and Pt/Mn2O3-NiO, respectively (Table 1 ), with the catalysts studied herein containing two or four times less Pt than the commercial Pt/C.ORR Tafel analysis (Fig. 5, S4-S6 ) revealed comparable Tafel slope, b, values for Pt- and PtNi-based catalysts, comparable with or even lower than that of Pt/C (Table 1). The lowest b value was found in the case of PtNi/Mn2O3-NiO, indicating its somewhat higher activity compared to the other catalysts. Note that dual Tafel slope values were observed, indicating different reaction mechanisms at different potentials. Detailed analysis of the changes in Tafel slopes conducted by Shinagawa et al. [27] showed that the slope depends on the geometry of the sample as well as the rate-determining step.The number of electrons exchanged, n, during ORR at the studied catalysts was determined by Koutecky-Levich analysis to be between 3 and 4 (Fig. 5, S4-S6, and Table 1), suggesting that the direct 4e--mechanism (see SI) was the preferred one.The double-layer capacitance, C dl, determined from the CV study (Fig. 6 , S7), was found to be the highest for Pt/C (3.10 mF cm−2), reflecting its largest electrochemical active surface area, followed by PtNi/Mn2O3-NiO (2.67 mF cm−2). The C dl values of PtNi/Mn2O3 (0.58 mF cm−2) and PtNi/Mn2O3-TiO2 (0.29 mF cm−2) were found to be notably lower.Although all catalysts showed good stability, i.e., constant current density with time, it is worth noting that PtNi/Mn2O3-NiO produced the highest current density seen throughout the entire study (Fig. 7 ).Samples containing only metal oxides showed high activity for the OER (Fig. 8 ), as suggested by Song et al. [28]. Pt-based catalysts showed slightly decreased catalytic activity compared to the pure oxides. Damjanovic et al. [29] have established that in acidic media, oxide films forming on the Pt surface effectively reduce the catalytic activity of Pt for OER in an exponential manner. Oxide film formation is also present in alkaline media at potentials over the OER onset potential, thus decreasing the activity of Pt. The obtained results suggest that NiO is the active site for the OER, unlike the ORR where PtNi NPs provide the active sites. The high OER activity of Ni and NiMn oxides, surpassing that of Pt/C, has been demonstrated previously [9,30,31].OER Tafel regions are shown in Fig. 8 (d,e,f); the slight curvature at higher current densities indicates where IR becomes significant [6]. The Tafel slopes were found to be comparable for the catalysts studied, with the exception of PtNi/Mn2O3-TiO2 (Table 2 ), indicating their similar activity for OER. The Mn2O3-NiO-supported catalysts stand out for the lowest values of b and E onset. Furthermore, these catalysts have lower overpotential values at a current density of 10 mA cm−2 , η10, compared with the Mn2O3- or Mn2O3-TiO2-supported ones. Using the Mn2O3-NiO samples, the current density at an overpotential of 0.4 V, j 400, reached values a few times higher than those obtained using the other oxides. The highest j 400 was recorded using the PtNi/Mn2O3-NiO catalyst.The EIS study showed that Pt/Mn2O3-NiO had the lowest R ct (78 Ω), followed by PtNi/Mn2O3-NiO (106 Ω) (Fig. 9 and Table S2). PtNi/Mn2O3-TiO2 had a significantly higher value of R ct ≈ 770 Ω. For reference, commercial Pt/C has R ct of ca. 433 Ω. The electrolyte resistance, R s, (45–62 Ω) values reflect small variations in the electrode distances and cell geometry.The stability tests of the Pt- and PtNi-based catalysts, as well as Pt/C, showed a decrease in OER current densities with time (Fig. 10 a). Still, it is worth noting that this decrease was the least pronounced in the case of PtNi/Mn2O3-NiO (43%). Interaction between metal NPs and the metal oxide support has been reported to prevent migration or agglomeration of metal NPs, or their detachment from the support [12]. A drop in current density as high as 87% was recorded in the case of commercial Pt/C.To simulate the two operation modes of a URFC, an experiment switching between OER and ORR modes was run with PtNi/Mn2O3-NiO, where the O2 generated during the OER mode was subsequently reduced during the ORR mode (Fig. 10 b). The ORR currents were steady, but a decrease in the OER current density in the first hour was observed. Still, after the initial drop, the OER current density was relatively stable.Nine different samples of Pt and Pt Ni NPs supported on binary metal oxides were tested as bifunctional electrocatalysts for the ORR and the OER, for possible use in URFC technology. The introduction of Ni resulted in higher electrocatalytic activity at a lower cost, while the introduction of metal oxide supports led to improved stability. Although all samples showed good activity for both reactions, the PtNi/Mn2O3-NiO sample showed the highest activity for the ORR in terms of the lowest onset potential, the highest diffusion-limited current density, and the lowest Tafel slope. This catalyst also showed the highest peak current density corresponding to O2 reduction and the highest stability during the ORR. The results indicated a diversity of active sites for O2 reduction and evolution; while PtNi NPs act as highly active catalytic sites for the ORR, the NiO active sites boost catalyst activity for the OER. All samples showed a decrease in stability with time under OER conditions. Nevertheless, PtNi/Mn2O3-NiO showed the lowest current drop, so further studies should focus on improving its stability in the electrolysis mode. Dušan Mladenović: Investigation, Formal analysis, Visualisation, Writing - original draft. Diogo M.F. Santos: Conceptualisation, Visualisation, Writing - review & editing. Gamze Bozkurt: Investigation, Formal analysis, Writing - original draft. Gulin S.P. Soylu: Investigation, Formal analysis, Writing - original draft. Ayşe B. Yurtcan: Conceptualisation, Investigation, Writing - original draft. Šćepan Miljanić: Conceptualisation, Supervision. Biljana Šljukić: Conceptualisation, 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.The authors would like to thank the Ministry of Education, Science and Technological Development of Republic of Serbia (contract no. 451-03-68/2020-14/200146), as well as Fundação para a Ciência e a Tecnologia, Portugal, for a research contract in the scope of programmatic funding UIDP/04540/2020 (D.M.F. Santos) and contract no. IST-ID/156-2018 (B. Šljukić).Supplementary data to this article can be found online at https://doi.org/10.1016/j.elecom.2021.106963.The following are the Supplementary data to this article: Supplementary Data 1

Three different metal oxides based on Mn2O3 with TiO2 or NiO were synthesised. Pt or PtNi nanoparticles were anchored on each support, creating a set of nine samples that were tested for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). PtNi/Mn2O3-NiO showed the most promising results for ORR as evidenced by the lowest Tafel slope, the highest diffusion-limited current density and number of electrons exchanged, along with the highest stability. The best performance of PtNi/Mn2O3-NiO reflects its highest electrochemical surface area and the lowest charge-transfer resistance. Furthermore, this catalyst showed high activity for the OER as evidenced by the low Tafel slope and high current density at an overpotential of 400 mV. The present study indicated different active sites for the two reactions, i.e., PtNi NPs for the ORR and NiO for the OER.

Carbon-free on-site hydrogen production and hydrogen storage technologies have recently attracted much attention for fuel cell applications that require high-purity hydrogen to generate environmentally clean electricity [1–4]. In general, hydrogen produced from on-site natural gas reforming contains carbonaceous substances (CO, CO2) that degrade cell performance even at extremely low concentrations [5–8]. Therefore, a subsequent hydrogen purification system is essential after the reformer. Alternatively, the production of COx-free hydrogen via catalytic ammonia decomposition, which only produces hydrogen and nitrogen gases, is considered a more desirable option for direct use as a fuel in proton exchange membrane fuel cells (PEMFC). Furthermore, ammonia has significantly higher gravimetric and volumetric hydrogen storage capacities (17.7 wt% and 108 g⋅L−1, respectively, at 20 °C and 857 kPa), making it readily stored and transported, which is beneficial for on-site hydrogen production [9,10].In general, ammonia decomposition requires a high energy input (>400 °C) owing to thermodynamic limitations [11]. Thus, a well-designed catalyst must be utilized to increase ammonia conversion and hydrogen yields at relatively low reaction temperatures. Many noble and non-noble metals such as Ru [12–16], Ni [17–21], Rh [22,23], Co [10,24], Ir [25,26], and Fe [27,28] have been studied for ammonia decomposition. Among them, Ru-based catalysts, such as Ru/Al2O3 [29,30], Ru/CNT [13,31–33], Ru/zeolite Y [9], Ru/SiO2 [2,34], and Ru/TiO2 [28] have shown promising catalytic activity. In particular, the Ru/CNT catalyst exhibited the highest ammonia conversion; however, low thermal stability under a hydrogen atmosphere is an issue, which can be attributed to the methanation reaction of the carbon sources of the support itself [13,35].In addition to incorporating different types of metals, numerous studies have been conducted to adjust the basicity of the support surface to further improve catalytic activity. This is because the well-known recombinative nitrogen desorption step, which is considered the rate-determining step (RDS) for ammonia decomposition over Ru-based catalysts [36], is strongly dependent on the basicity of the catalyst support [37]. For example, adding promoters such as La, Ce, K, and Mg to the catalyst contributes to an increase in the rate of desorption of nitrogen, which is attributed to the increase in surface basicity [38–40]. The increase in the electron density of Ru due to the electron-donating catalyst surface weakens the Ru-N binding energy, thereby increasing catalytic activity.In our previous studies, Ru impregnated on a La-doped Al2O3 catalyst was developed in powder form, and was highly active and stable under ammonia decomposition conditions [41]. A significant increase in ammonia conversion was obtained when more than 10 mol% of La was incorporated into the Al2O3 support. This was predominantly attributed to the formation of the LaAlO3 phase, which strongly interacted with the Ru active sites (a strong metal-support interaction), thus limiting the agglomeration of Ru particles and enhancing the catalytic activity. Al2O3 has been extensively utilized as a commercial catalyst support for ammonia decomposition because of its high surface area, thermal stability, and chemical resistance under high temperatures conditions [29,42]. Ammonia decomposition is a structure-sensitive reaction; the formation of well-known active sites of Ru (B5 sites) is reported over the Al2O3 support [15]. Recent research on Al2O3-based catalysts has been conducted extensively on the non-noble metals such as Ni, Co and Fe [43–46]. Despite the advantages, moderate acidity of Al2O3 surface is an issue where higher basicity of the support is required to facilitate nitrogen desorption. With respect to Ru/La-Al2O3 catalyst, these acidic sites of the Al2O3 support were covered by bulky La particles, which increased the basicity of the Ru/La-Al2O3 catalyst. The catalyst was stable for more than 120 h under a gas hourly space velocity (GHSV) of 10,000 mL/gcat⋅h and a reaction temperature of 550 °C. The catalyst was further pelletized in a follow-up study in which it was utilized in a COx-free 1 kW-class hydrogen power pack, including a dehydrogenation reactor, an adsorbent tower, and a 1 kW-class polymer electrolyte membrane fuel cell [47]. The system was tethered to a drone with a flight time of over 2 h. For this application, the weight loading of La was optimized to 20 mol% (tested for 0, 10, 20, 30, 40 and 50 mol%) where a volcano-shaped activity trend was observed. The results indicate that a significant loss of Ru active sites occurs at high La loadings.In the case of the pelletized catalyst, the metal nanoparticles tended to penetrate the bulk support during the synthesis process, making them inaccessible to the incoming reactant molecules especially under high space velocity conditions. For the La-Al2O3 support, penetration of Ru from the catalyst surface into the bulk La-rich Al2O3 center poses a problem. A recent study conducted by Li et al. aiming to overcome this issue reported a modified synthesis strategy in which the surface of powder Al2O3 was coated by La2O2CO3 during La impregnation [48]. The authors stated that the formation of an La2O2CO3 phase over the catalyst prevented Ni metal from entering the bulk Al2O3 by instead forming NiAl2O4. This led to a higher number of exposed Ni active sites for the dry reforming reaction. The formation of La2O2CO3 has been observed in several other studies [49–52], where this species significantly enhanced catalytic activity by strong metal-support interactions and reductions in coke formation during dry and steam reforming as well as CO2 methanation.In the current study, Ru supported on an La carbonate-rich Al2O3 catalyst (Ru/La2O2CO3-Al2O3) was synthesized in the form of catalyst beads, and its reactivity for ammonia decomposition was examined. The catalytic activity of Ru/La2O3-Al2O3 and Ru/Al2O3 beads was also investigated to elucidate whether the La oxycarbonate species (La2O2CO3) acts as a structural stabilizer to prevent the loss of Ru metal to inner the bulk region of Al2O3 catalyst beads. Several analytical techniques were utilized to understand the physical and chemical properties of the catalysts, including Brunauer–Emmett–Teller (BET) physisorption, X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and CO chemisorption. The electronic properties of the catalysts were analyzed by X-ray photoelectron spectroscopy (XPS), and H2-TPR (temperature-programmed reduction). Finally, the cross-sectional area of the Ru/La2O2CO3-Al2O3 beads was examined by scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) to determine the distribution of Ru molecules between the surface and bulk phases of the catalyst beads.Aluminum oxide (Alfa Aesar, 1/8″, beads), lanthanum (III) nitrate hydrate (Sigma Aldrich, 99.9%), and ruthenium (III) chloride hydrate (Sigma Aldrich, 99.98%) were purchased commercially. All chemicals and raw materials were used as received without further purification.The La-Al2O3 beads were prepared using the wet impregnation method. First, 3.5408 g of La precursor solid (10 mol%) was dissolved in 25 mL of deionized water. The solution was then added to dried aluminum oxide beads (10 g) and heated to 40 °C in a vacuum oven for 2 h under mild vacuum conditions (0.8 bar). The suspension was then rigorously stirred and evacuated under a 0.3–0.6 bar vacuum, where the temperature was heated to 100 °C at a ramp rate of 15 °C/h. The suspension was dried at 100 °C for an additional 3 h, and the resulting solid beads were calcined at 600 °C for 3 h in air to yield the La2O3-Al2O3 catalyst. La2O2CO3-Al2O3 was prepared using the same impregnation method, except that calcination was performed in a CO2 atmosphere as reported by Li et al. [48].Ru metal was loaded on both La2O3-Al2O3 and La2O2CO3-Al2O3 bead supports using the wet impregnation method. Ruthenium(III) chloride hydrate (0.4576 g, 2 wt%) was first dissolved in 25 mL of deionized water. The Ru solution was mixed with 10 g of each bead support. The drying procedures were identical to those described above for the loading of La. The resulting catalyst beads were dried at 100 °C for 12 h in air to obtain the Ru/La2O3-Al2O3 and Ru/La2O2CO3-Al2O3 catalyst beads.The Ru/Al2O3 catalyst beads were also prepared using the wet impregnation method. Prior to the loading of Ru, the Al2O3 beads were calcined under a CO2 atmosphere at 600 °C for 3 h. Ru was then impregnated on the Al2O3 beads following the identical synthesis procedure used for the Ru/La-based Al2O3 catalyst beads. All the ex-situ reduced Ru-based catalyst beads were treated at 500 °C for 1 h under a 75% H2/N2 flow ( Scheme 1).All catalyst characterization techniques were conducted with a sample of whole, unground beads. The only exceptions were XRD, in which a bead was ground until a homogeneous mixture was achieved, and STEM/TEM, in which the outer surface of the bead was scraped off and collected for the sample.The surface area, pore volume, and pore size distribution of the samples were determined via nitrogen physisorption at − 196 °C (Micromeritics ASAP 2000 volumetric adsorption system). Prior to nitrogen adsorption, 100 mg of each sample was degassed under vacuum at 250 °C for 8 h to remove moisture and impurities from the sample surface. The specific surface areas were calculated using the BET method. The Barrett–Joyner–Halenda (BJH) method was used to obtain the pore volume and pore size distribution from the desorption branch of the nitrogen isotherm.The amounts of Ru and La in the samples were measured using inductively coupled plasma-optical emission spectrometry (ICP-OES; Agilent 5100). For the elemental analysis, a digestion process was employed using a mixture of HNO3 and HCl with a microwave oven.Pulse CO chemisorption of the samples was conducted using a chemisorption analyzer (Micromeritics AutoChem II 2920). Approximately 0.2 g of sample was placed in a sample tube. The sample was heated to 500 °C at a ramping rate of 5 °C/min and maintained for 1 h under pure He (99.999%) to remove moisture and impurities from its surface. The sample was then reduced in 10% H2/Ar at 500 °C for 1 h. When the reduction was completed, the temperature was decreased to 50 °C under degassing conditions. 10% CO/He gas was then injected in pulses into the sample tube every 5 min (cycle of injection-evacuation) until the sample surface was saturated with CO molecules. Ru dispersion was calculated based on the amount of adsorbed CO molecules. The stoichiometric molar ratio of adsorption was assumed to be Ru/CO = 1 (1) R u D i s p e r s i o n ( % ) = M oles of adsorbed CO M oles of Ru in the catalyst ( ICP ) × 100 The powder XRD patterns of the samples were recorded using an X-ray diffractometer (Shimadzu, XRD-6100) with Cu Kα radiation (λ = 1.5418 Å), with the accelerating voltage and current set to 40 kV and 30 mA, respectively. A continuous scan at a rate of 2θ = 2°/min was conducted in the range of 10–80°.TEM (Thermo Scientific™ Talos F200X) was employed to visualize the Ru particles and estimate their size and morphology. The sample was first suspended in ethanol for 30 min under ultrasonic treatment, and the solution was then deposited onto carbon film-coated copper grids. High-angle annular dark-field imaging (HAADF) in STEM mode and elemental mapping images measured by EDS were used to identify Ru, La, and Al dispersions on the sample surface. The particle size distribution of Ru was determined using ImageJ software to measure the size of 80 Ru particles from different locations across the sample surface.The XPS spectra of the samples were collected using an Thermo Scientific K-Alpha+ equipped with monochromatic Al Kα radiation (1486.6 eV, 12 kV and 1.16 mA). The binding energy calibration of the XPS spectra was performed using C 1s (284.8 eV) prior to XPS fitting (Avantage software program). Spectral regions corresponding to the C 1s, O 1s, La 3d, and Ru 3p core levels were recorded for each sample.The cross-sectional distribution of the Ru concentration in the sample bead was examined using FE-SEM/EDS (FEI Inspect F). Prior to the analysis, a sample bead was split in half, and the cross sections were covered with Pt by electrically conductive coating (15 mA, 60 s) for optimal imaging and analysis.TPR experiments were performed using a BELFAT-M chemisorption analyzer (MicrotracBEL Corp.) as described previously [9]. First, the sample (100 mg) was pretreated at 300 °C for 1 h under pure Ar (99.999%) gas to remove moisture and impurities inside the pores. The temperature was then decreased to 50 °C and maintained until the thermal conductivity detector (TCD) signal was stabilized. After switching the gas to 10% H2/Ar, the temperature was increased to 500 °C at a rate of 3 °C/min. The outlet gas stream was continuously measured using the TCD. A water trap (zeolite 13X) was placed between the sample tube and the detector to remove the water formed during the measurement.The catalytic activities of Ru/La2O2CO3-Al2O3, Ru/La2O3-Al2O3, and Ru/Al2O3 catalyst beads for ammonia decomposition were evaluated in a fixed-bed quartz reactor under a flow of pure NH3 at atmospheric pressure. A quartz frit was placed in the middle of the reactor to hold the catalyst beads. The reaction temperature was measured using a thermocouple located near the catalyst bed. The gas flow rates of NH3, H2 and N2 were controlled using a mass-flow controller. Prior to the reaction, the catalyst beads (0.180 g) were placed in the reactor and reduced in situ in a 75% H2/N2 flow at 500 °C for 1 h. The reaction was conducted at a GHSV of 10,000 mLNH3/gcat⋅h in the range of 350–500 °C where the temperature was decreased at intervals of 50 °C (maintained for 1 h at each temperature). The effluent gas was analyzed using an online gas chromatograph (GC, Agilent 7890 A) equipped with two thermal conductivity detectors (TCDs) and two columns. The front TCD (carrier gas: He) was connected to a CP-Volamine column to identify NH3 and N2. The second carrier gas (Ar) was linked to a 19091 P-MS4 J&W PoraPLOT Amines column for H2 detection. The ammonia conversion was calculated using (Eq. (2)): (2) Conversion NH 3 % = C N H 3 , i n − C N H 3 , o u t C N H 3 , i n × 100 where C NH 3 , in and C NH 3 , out refer to the concentrations of NH3 in the feed and product gas, respectively. The ammonia conversion at each temperature was obtained by averaging the conversion values over the corresponding hour of constant reaction temperature. The increase in the total gas volume as a result of the increase in the total number of moles of the reaction (2NH3 → N2 + 3H2) was reflected in the final ammonia conversion calculation. No by-products were detected other than N2, H2 and unreacted NH3.N2 adsorption isotherms at − 196 °C were obtained to investigate the structural properties of the as-prepared La2O2CO3-Al2O3, La2O3-Al2O3, and Al2O3 and ex-situ reduced Ru/La2O2CO3-Al2O3, Ru/La2O3-Al2O3, and Ru/Al2O3 catalyst beads. Physisorption was conducted using whole, unground catalyst beads. The isotherms and the corresponding BJH pore size distribution curves are shown in Fig. 1 . The calculated BET surface areas and pore volumes are summarized in Table 1. All isotherms showed type IV with H1 hysteresis, indicating mesoporous and open-pore structures [53–55]. The BJH pore size distribution curve reaffirmed the range of pore widths for the mesopores. When La was added to the Al2O3 catalyst beads, a narrower range of pore size distribution was obtained. Additionally, the BET surface area for the starting material, a commercial Al2O3 catalyst bead, was SBET = 227.1 m2/g and the surface area, total pore volume, and average pore diameter decreased slightly after impregnation with La species. This is because the surface of Al2O3 was covered by La species with a low specific surface area (La2O3 SBET = 21.8 m2/g [56], La2O2CO3 SBET = 47 m2/g [57]) [41]. However, all the isotherms of the La2O2CO3-Al2O3, La2O3-Al2O3, and Al2O3 catalyst bead supports were similar, indicating that the structural properties of the support were maintained after the loading of La species on Al2O3. After the impregnation of Ru particles, the surface area increased slightly, possibly attributable to the metallic Ru dispersed on the outer surface of the catalyst beads [54]. The estimated total pore volume was very similar for all the Ru-based catalysts implying that the Ru particles were uniformly dispersed on the alumina support and that no significant sintering occurred, thus maintaining the porosity of the catalyst beads.CO chemisorption measurements were taken to determine the degree of Ru dispersion over the catalyst beads (Table 1). A higher metal dispersion in the catalyst generally indicates a greater number of active metals that can participate in the reaction; thus, it is often employed as an important factor in evaluating the performance of the catalyst. From the CO chemisorption results, the initial dispersion values were obtained as 33.3%, 23.3% and 30.6% for Ru/Al2O3, Ru/La2O3-Al2O3 and Ru/La2O2CO3-Al2O3, respectively. These dispersion values were corrected based on the actual Ru weight loading from the ICP results later on. The highest Ru dispersion was obtained for the Ru/La2O2CO3-Al2O3 catalyst beads (32.2%). It should be noted that a similar Ru weight loading was achieved for all Ru-based catalyst beads (approximately 1 wt%), with the error between measurements sufficiently small to yield a fair comparison of catalytic activity. Furthermore, the La weight loading of both La-loaded samples was approximately 12 wt%.XRD patterns were obtained for the ex-situ reduced Ru-based catalyst beads (in powder form) to acquire structural information ( Fig. 2). All XRD patterns presented a wide diffraction peak centered at 2θ = 67.5°, which was assigned to amorphous γ-alumina (JCPDS 10–0425). For the Ru/La2O3-Al2O3 catalyst, characteristic diffractions of La2O3 (JCPDS 05–0602) were not observed. The peaks related to the formation of La2O3 had very low intensities, possibly because of the strong interaction between La2O3 and Al2O3 [48,58]. This resulted in a high dispersion of La species over the catalyst surface which was later confirmed by HAADF-STEM. In contrast, for Ru/La2O2CO3-Al2O3, the peaks centered at 13.1°, 25.3°, 30.9° and 34.0° were assigned to La2O2CO3 with a monoclinic structure (JCPDS 48–1113). Unlike the Ru/La2O3-Al2O3 catalyst, La2O2CO3 showed high crystallinity and formed through calcination under CO2 conditions. Notably, the La2O2CO3 phase was stable throughout the La and Ru impregnation processes that involved high temperature treatment at 600 °C. Metallic Ru was observed in both the Ru/La2O3-Al2O3 and Ru/La2O2CO3-Al2O3 catalysts. However, no peaks were identified for Ru/Al2O3 likely attributable to the low average Ru particle size leading to the absence of long-range order of Ru.STEM-EDS images of the ex-situ reduced Ru-based catalyst beads were collected to estimate the dispersity of the metal elements (Ru, Al, and La) over the catalyst surface. For sample preparation, the outer surface of the bead was scraped off and collected. The HAADF-STEM and elemental mapping images in Fig. 3 clearly indicate highly dispersed Ru and La particles over all the catalysts. Furthermore, TEM images were taken over all three catalysts ( Fig. 4, Ru/Al2O3, Ru/La2O3-Al2O3 and Ru/La2O2CO3-Al2O3). Similar to the STEM-EDS images, Ru particles indicated as black dots were highly dispersed on the amorphous Al2O3 or mixed La-Al2O3 support. The Ru particle size distribution obtained from the TEM images show narrow distribution curves between the 0.4 and 4.0 nm for all the catalysts. Among the catalysts, Ru/Al2O3 possessed the smallest average particle size (0.96 nm) which was in good agreement with the XRD results. In contrast, for the La-containing catalysts (Ru/La2O3-Al2O3 and Ru/La2O2CO3-Al2O3), a larger average Ru particle size was observed compared with that of Ru particles impregnated solely on the Al2O3 support. These results may be attributable to the presence of large La particles on the catalyst surface, resulting in a lower specific surface area and smaller average pore diameter, and thus, a greater extent of Ru agglomeration.XPS spectra of the as-prepared La2O3-Al2O3 and La2O2CO3-Al2O3 bead supports were obtained to verify the formation of La2O2CO2 over the Al2O3 surface. The X-ray beam was focused on the whole catalyst bead (without grinding it). The full-range spectrum confirmed the chemical purity of the La2O2CO3-Al2O3 sample, consisting of Al, La, C, and O ( Fig. 5 (a)). As shown in Fig. 5 (b), the La2O2CO3-Al2O3 support primarily contained peaks for the 3d orbits of the La oxide species. The electronic states of carbon and oxygen in both samples were clearly distinguishable because of the differences in their carbon-containing structures. In the C 1s spectra (Fig. 5 (c-d)), a C-C peak was detected for both samples because of adventitious carbon contamination during the exposure to the atmosphere. However, the peak centered at 289.3 eV only existed in the La2O2CO3-Al2O3 bead support and was assigned to the O-CO bond induced by (CO3)2- resonance [59]. In addition, the difference in the electron state of oxygen between both samples was more apparent than that of the carbon electron state. As shown in Fig. 5 (e-f), the appearance of two peaks was confirmed: one at 531.8 eV and the other centered at 528.8 eV. The peak at 528.8 eV was concluded to be related to lattice oxygen in La2O3 and Al2O3 and the other was assigned to the adsorbed oxygen-like surface (CO3)2-. It should be noted that the predominant peak at 531.8 eV was only detected for the La2O2CO3-Al2O3 support [60] indicating that the La2O2CO3 structure formed well on the Al2O3 support during CO2 calcination.Furthermore, the XPS spectra of the Ru 3p region clearly indicated that the Ru surface concentration of the Ru/La2O2CO3-Al2O3 catalyst beads was higher than that of the Ru/La2O3-Al2O3 beads, evidenced by the larger Ru peak intensity (3.1 times higher; Fig. 6). This seems to suggest that the La2O2CO3 interface can prevent Ru from entering the support, thereby making Ru metals more likely to be located on the surface of the bead support.A transverse section of each catalyst bead sample was prepared to validate the actual surface distribution of Ru metals over the ex-situ reduced Ru/La2O3-Al2O3, Ru/La2O2CO3-Al2O3 and Ru/Al2O3. The Ru concentration in the cross-sectional interior of the catalyst was observed by SEM-EDS line scanning. As shown in Fig. 7 , the highest Ru concentration was achieved on the bead surface for all samples. The concentration of Ru decreased towards the inside of the bead core, but the distribution trends differed for the three types of catalyst beads. To accurately compare the ratio of surface/bulk Ru concentration, the ‘surface’ was defined from the edge up to 100 µm in depth, and the ‘bulk’ was limited to the region with a diameter of 500 µm measured from the catalyst center. The average Ru concentration over each region was calculated to obtain the surface/bulk Ru concentration ratio. The results were 1.8, 2.4 and 3.8 for Ru/Al2O3, Ru/La2O3-Al2O3 and Ru/La2O2CO3-Al2O3, respectively. For Ru/Al2O3, the average Ru concentration in the surface region was the lowest (5.6 wt%) compared to the other catalyst beads, while the average concentration in the bulk region was the highest (3.2 wt%). In contrast, the average concentrations of Ru in the surface regions of Ru/La2O3-Al2O3 and Ru/La2O2CO3-Al2O3 were 7.2 wt% and 8.4 wt%, respectively.These results serve as compelling evidence for the pivotal role that the La carbonate-rich Al2O3 oxycarbonate species plays in preventing the penetration of Ru particles into the catalyst bead core and thus minimizing the loss of Ru metals to the bulk phase. It is speculated that there could be a steric hindrance effect in the macropore diffusion of Ru chloride species during catalyst synthesis, where the higher crystallinity and bulkier matrix of the La2O2CO3-Al2O3 structure aid the formation of Ru particles on the surface of the catalyst bead.H2-TPR experiments were performed to understand the reduction behavior of the non-reduced Ru-based catalyst beads. These types of experiments are useful to estimate the extent of the metal-support interaction by considering shifts in the reduction temperature [9]. As shown in Fig. 8 , the reduction of Ru/Al2O3 occurred as a single broad peak at 118 °C assigned to the reduction of RuOx to metallic Ru [41,61] indicating that the support was composed of a single component of alumina. The Ru/La2O3-Al2O3 catalyst exhibited two major reduction peaks at 131 and 209 °C. The reduction of Ru at temperatures higher than that of Ru/Al2O3 was ascribed to strong metal-support interactions between Ru particles and the La-Al species [41] meaning that the Ru particles located in the bulk phase of the catalyst beads required a higher temperature for complete reduction to metallic metal.The characteristic reduction peaks of Ru/La2O2CO3-Al2O3 were observed at 109 and 135 °C. The reduction peak for Ru shifted to a lower temperature than that of the Ru/La2O3-Al2O3 catalyst, indicating a higher concentration of Ru on the bead surface weakly interacting with the support [46,49,62]. Similar TPR results were observed by Li and coworkers [48] where higher extent of Ni reduction in the lower temperature region was obtained over the La2O2CO3-Al2O3 in comparison to the Al2O3 indicating higher concentration of the surface Ni active sites for dry reforming of methane. Ultimately, this catalyst characterization supported previous analysis in showing that the incorporation of the La oxycarbonate species leads to a higher number of Ru active sites on the surface of Ru-based catalyst beads.The Ru-based catalyst beads prepared using three different synthesis methods were investigated with respect to catalytic activity for low temperature (350−500 °C) ammonia decomposition at a GHSV of 10,000 mLNH3/gcat⋅h. As shown in Fig. 9, ammonia conversion, an exothermic reaction, increased with an increase in reaction temperature for all catalyst beads. The catalytic performance ranked from best to worst was: Ru/La2O2CO3-Al2O3 > Ru/La2O3-Al2O3 > Ru/Al2O3 with a margin of error of ± 3%. It should be noted that although a higher Ru dispersion was observed in Ru/Al2O3 compared with Ru/La2O3-Al2O3 beads, Ru/Al2O3 displayed lower catalytic activity. This suggests a promotion effect of La-addition to an Ru/Al2O3 catalyst for ammonia decomposition. Electron donation from La to Ru due to electronegativity differences increases the electron density of Ru, significantly increasing the kinetics of the ammonia decomposition reaction [41,63], specifically speeding up the cleavage of the N-H bond and the recombinative desorption of N2 [63].As expected, the highest ammonia conversion was obtained for the Ru/La2O2CO3-Al2O3 catalyst beads over the full range of reaction temperatures tested. The high Ru surface concentration and Ru dispersion of La-containing catalysts resulted in a higher ammonia decomposition activity and faster reaction rate. Overall, it is likely that the formation of Ru particles was more favorable on the surface of the catalyst beads when a La oxycarbonate-rich surface (La2O2CO3) was present in comparison to the La oxide surface (La2O3). In other words, the higher structural density of La2O2CO3 prevented the Ru salts from traveling from the bead surface into the bulk region. We believe that an La2O2CO3 surface-coating can be utilized for catalyst synthesis, particularly for beads, pellets, monoliths, and other structured catalysts where a high surface concentration of metal is essential.In this study, we investigated the effect of La oxycarbonate (La2O2CO3) on the catalytic activity of Ru/La-based Al2O3 beads for ammonia decomposition. Three Ru-based catalyst beads were prepared: Ru/Al2O3 with a boundary layer of an Ru-Al mixed oxide phase and Ru/La2O2CO3-Al2O3 and Ru/La2O3-Al2O3 with a Ru-La-Al ternary layer including La2O2CO3 and La2O3, respectively. The Ru/La2O2CO3-Al2O3 catalyst beads showed the highest activity for ammonia decomposition (80.1% conversion) compared with Ru/La2O3-Al2O3 (72.1%) and Ru/Al2O3 (51.4%) at a reaction temperature of 500 °C. According to the catalyst characterization results, the highest Ru concentration and dispersion was observed on the surface of La2O2CO3-Al2O3, which was attributed to the boundary layer of La2O2CO3 preventing the loss of Ru metals to the bulk phase of the Al2O3 catalyst beads. In other words, the formation of metallic Ru particles on the surface of the La2O2CO3-Al2O3 beads was more favorable than that on La2O3-Al2O3 because of the density of the macroscale structure of La2O2CO3-Al2O3 (acting as a structural stabilizer) and appropriate metal-support interactions, limiting the penetration of Ru particles into the catalyst bead core. The addition of an La layer between the Ru and Al2O3 beads promoted the catalytic activity for ammonia decomposition, believed to be the result of an increase in the Ru electron density, thereby leading to the faster recombinative desorption of N2. Ah-Reum Kim: Writing - original draft, Investigation. Junyoung Cha: Writing - original draft, Investigation. Jin Su Kim: Validation. Chang-Il Ahn: Formal analysis. Yongmin Kim: Methodology. Hyangsoo Jeong: Data curation. Sun Hee Choi: Resources. Suk Woo Nam: Conceptualization Chang Won Yoon: Project administration. Hyuntae Sohn: 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 work was supported by the National Research Foundation of Korea (NRF), funded by the Government of Republic of Korea (Ministry of Science and ICT, Hydrogen Energy Innovation Technology Development Program [No. 2019M3E6A1064611, No. 2019M3E6A1104113]), and the Korea Institute of Science and Technology (KIST) Institutional Program (No. 2E31872).

Three ruthenium-supported catalyst beads (Ru/Al2O3, Ru/La2O3-Al2O3 and Ru/La2O2CO3-Al2O3) were synthesized and tested for ammonia decomposition. The catalytic activity of the Ru/La2O2CO3-Al2O3 beads was significantly higher than that of the Ru/Al2O3 and Ru/La2O3-Al2O3 beads. This was primarily attributed to the addition of La, which encouraged electron donation from the bead surface to the Ru particles, increasing the rate of N2 desorption. In particular, a higher Ru surface concentration was achieved over the boundary layer of La2O2CO3 compared with La2O3. This is thought to be a result of steric hindrance, with the crystalline surface of La2O2CO3 acting as a structural stabilizer to significantly limit the penetration of Ru particles into the catalyst bead core. SEM-EDS line scanning of transverse sections of the catalyst beads confirmed a higher Ru concentration on the surface of the catalyst beads for Ru/La2O2CO3-Al2O3 compared with Ru/La2O3-Al2O3 and Ru/Al2O3. In fact, the ratio of surface/bulk Ru concentration in Ru/La2O2CO3-Al2O3 was more than twice that of Ru/Al2O3 at equal Ru loadings. The favorable properties of an La2O2CO3 surface-coating can benefit industrial catalyst synthesis, increasing the surface metal concentration compared with traditional La-based Al2O3 beads and pellets.

Sustainable and economically-efficient ways to produce and store energy are crucial for the transition to a society independent of fossil fuels. Hydrogen as energy carrier is expected to play an important role in this transformation, finding applications in powering vehicles, electronic devices or homes [1,2]. Moreover, the combustion of H2 is a clean process releasing only water unlike the currently used fossil fuels. A sustainable, environmentally friendly and scalable production of hydrogen could be achieved via alcohol electrolysis where, in addition to hydrogen gas, valuable chemicals could be obtained in the anodic reaction [3]. Examples of alcohols that attract attention are methanol, ethanol and particularly glycerol, once these can be obtained from different biomass sources and industrial processes, such as wood-based Kraft pulping [4–8]. This strategy enables the valorization of the pulp waste product or glycerol from biodiesel production through transformation into economically valuable products in addition to storing energy in hydrogen gas.The methanol oxidation reaction (MOR) serves as a prototype to understand the interactions of alcohols and metal electrocatalysts. In general, formaldehyde, formic acid and CO2 are the main products obtained during the methanol oxidation reaction (Scheme 1). A greater H2 production rate (in electrolysis) might be obtained for the full oxidation of methanol to CO2. This is due to the larger number of electrons involved for the full reaction (6 e−) as compared to the other products (2 e− and 4 e−, respectively). Platinum has been identified as a remarkable catalyst for methanol oxidation, but its high cost is an obstacle to large-scale usage. Furthermore, platinum shows some level of CO poisoning during the methanol oxidation reaction, interrupting the electrocatalytic activity [9]. This has motivated investigations to seek alternative catalysts with low production cost to improve the viability for large-scale applications. In this context, a wide range of possible single-metal catalysts, like Cu, Ni and Pd [10–13] or bimetallic catalysts, Pt-Ru, Pt-Ni, Pt-Pd, Pt-Cu and Pt-Sn, have been studied for methanol oxidation. [14–18]. Interestingly, monometallic Pd has shown to be less active for methanol oxidation than Pt catalysts [19]. However, bimetallic Pd catalysts seem to enhance the catalytic performance towards this reaction, and thus, they may become competitive with state-of-the-art Pt catalysts [20–27]. Bimetallic alloys may benefit from the bifunctional effect – possible oxidative removal of CO from the catalytic surface, [28] which would act positively for the methanol oxidation reaction [29–31]. Among these, Pd-Ni alloys provide acceptable catalytic performance for the oxidation of different alcohol-based compounds, such as methanol [20–22], ethanol [32,33], glycerol [22] and lactic acid [8].Rationalizing the effects of alloying Pd-Ni for the oxidation of alcohols is crucial for the development of novel catalysts. Among the published works on alcohol oxidation on Pd-Ni, Carvalho et al. [22] reported an increased catalytic activity for methanol oxidation as a function of the Ni content in the employed catalyst while Qiu et al. [34] showed that the composition, structure and surface morphology of PdNi play important roles during the oxidative reaction. The strategy of having PdNi supported on TiO2 nanotubes for methanol oxidation in a direct methanol fuel cell has been tested which resulted in higher electroactivity than the state-of-the-art PtRu/C catalyst. [35] Miao et al. [36] employed the framework of density functional theory (DFT) to understand the effects of the Ni position (if beneath Pd or exposed on the surface) for ethanol oxidation. When Ni is exposed on the catalyst surface, lower activation barriers were reported for C-C cleavage and higher barriers for C-O bond breaking, hence enhancing the performance of PdNi for the electrooxidation of ethanol. Other experimental and theoretical studies have also reported favorable effects of Pd-Ni over pure Pd for ethanol oxidation where the central hypothesis explored uses the oxophilic property of Ni that provides a greater number of OHads species on the catalyst surface together with an improved tolerance against CO poisoning [37–39].Several experimental and theoretical works have reported on Pd-Ni alloys providing important accumulated knowledge for application to catalytic electrooxidation of alcohols. Yet, a systematic investigation unveiling the effects of Pd-Ni alloying and linking that to the CO poisoning phenomenon and also the methanol oxidation mechanism itself has not been reported, to the best of our knowledge. Thus, to shed light on the effects on the methanol electro-oxidation when using Pd-Ni alloys as catalyst, experimental measurements and theoretical calculations were performed to obtain atomistic level insight into: i) the nature of the Pd-Ni surface morphology and coverage; ii) the effect of Ni concentration on the performance of the catalyst; iii) the effects of CO poisoning; iv) the effects of OH− concentration in the electrolyte (pH change) on the methanol oxidation activity and, finally, v) linking these to the greater observed activity delivered by the Pd-Ni alloy.More specifically, the present study combines experiments and DFT calculations to clarify the methanol electrooxidation activity using Pd, Pd3Ni and PdNi catalysts. Energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) were employed to characterize the morphology of the catalysts. Cyclic voltammograms (CVs) were performed to elucidate the electrochemical performance of each studied catalyst towards methanol electrooxidation and also to clarify the CO poisoning phenomenon and effect of pH on the reaction. In conjunction with the CVs, Pourbaix diagrams were calculated to predict the catalytic surface coverage. Moreover, the methanol oxidation reaction pathways were explored using DFT and the climbing image nudged elastic band method (CI-NEB) for barriers in combination with measurements of product distributions using High-Performance Liquid Chromatography (HPLC) to elucidate the most likely oxidative paths on the studied catalysts. Based on these experiments and calculations, we propose an explanation for the higher activity toward methanol electrooxidation displayed for the alloys compared to the pure Pd catalyst.Density functional theory has been employed within the projected augmented wave method as implemented in the Vienna Ab Initio Simulation Package (VASP) [40,41]. The optB86b-vdW functional was selected to describe the exchange and correlation term of the Kohn-Sham equation as well as an estimate of the van der Waals contribution [42–44]. This functional has been shown to, e.g., properly describe the adsorption of methanol on gold [45]. A cutoff energy of 450 eV was used for the plane-wave expansion with a grid for the sampling of the Brillouin zone that depended on the supercell size. For bulk calculations, a Monkhorst−Pack grid of 15 × 15 × 15 was employed, while for the slabs, a grid of 2 × 2 × 1 was employed due to the large supercell size. For the gas phase molecule, the Γ point was used. The spin-polarized approach was used with the electron partial occupancies obtained within the Methfessel-Paxton scheme of order 2 and a smearing parameter of 0.2 eV. Vibrational modes were computed through the finite difference approximation and the self-consistent field energies were corrected for zero-point energies (ZPE). Moreover, the rotational, translational and vibrational contributions to the entropy and enthalpy were considered for gas-phase species where we furthermore set PV = k B T (see eq. 6), where P and V are pressure and volume, respectively, while T and kB are temperature and the Boltzmann constant, respectively.Pd was modeled in a fcc unit cell while the Pd3Ni and PdNi alloy-like structures were constructed by replacing Pd atoms by Ni atoms in the Pd fcc lattice. Starting from pre-optimized bulk structures the surface models were built as follows: The (111) facet of the bulk structures investigated were modeled as a four-layer slab with a p(4 × 4) supercell and 20 Å vacuum to avoid interactions between the periodic images (Fig. 1 ). We have allowed atomic relaxations of the top two layers and fixed the bottom two layers of the slabs. Gas-phase molecules were modelled in a 20 Å cubic cell.The computational hydrogen electrode approach, as proposed by Nørskov [46], was applied to model the electrochemical reactions. This approach assumes a coupled electron-proton transfer simplifying the demanding calculation of solvation energies of ionic species. The formation energy of the intermediates (Ead) was calculated as: (1) E a d = E a d s o r b a t e * − E * − ∑ i n i μ i where E a d s o r b a t e * is the self-consistent-field (SCF) energy of the adsorbed intermediate corrected by the zero-point energy (ZPE) of the adsorbate, E* is the SCF energy of the pure slab and ni is the number of species i with chemical potential μi . Moreover, μH , μ H 2 O , μO and μC are the chemical potentials of hydrogen, water, oxygen and carbon, respectively, that are obtained as: (2) μ H = 1 2 E H 2 + e U S H E − k B T l n ( 10 ) × p H = 1 2 E H 2 + e U R H E (3) μ H 2 O = E H 2 O (4) μ O = μ H 2 O − 2 μ H (5) μ C = E C H 3 O H − μ O − 4 μ H (6) E H 2 , H 2 O , C H 3 O H = E s c f + Z P E + ( H v i b + H t r a n s + H r o t ) − T ( S v i b + S t r a n s + S r o t ) + P V Here, pH dependence is incorporated in the potential of the proton electron transfer where URHE is the potential measured against the reversible hydrogen electrode (RHE) and USHE is the potential versus the standard hydrogen electrode (SHE). E C H 3 O H , E H 2 O , E C H 3 O H are the gas-phase Gibbs free energies computed as shown in eq. 6. Escf is the SCF energy, H refers to the enthalpic thermal contribution and S to the entropic thermal contributions.We define coverage as the ratio between the number of adsorbates and the number of surface atoms.Activation barriers (Ea) for the deprotonation reaction were computed using the climbing image Nudged Elastic Band method (CI-NEB) where the number of intermediate images varies from four to seven depending on the studied reaction [47]. Forces on the atoms were minimized to 0.05 eV/Å. Minimum energy paths (MEP) of the deprotonation reaction were modeled by adding four water molecules (explicit solvation model). The structure of the four-water cluster was obtained by checking different configurations on the surfaces. The lowest-energy structure is then used for the oxidation reaction where intermediates are added to the surface plus water structure. This model ensures that the formation of O-H polar bonds stabilizes the electron transfer process and possibly lowers activation barriers through water-assisted H transfer. In contrast, in the case of C - H breaking the presence of water tends to produce higher activation barriers [48]. The energies of the transition states have been corrected to mimic the effect of the electrode potential on the obtained activation barriers [49]. Here, we assumed an adiabatic charge transfer along the MEP and the transition state energies were corrected accordingly to: E a ( U ) = E a − λ e U , where λ is the reaction symmetry factor - we approximate λ to 0.5 [50], U is the electrode potential, e is the (positive) elementary charge and Ea is the activation barrier. The reaction barrier of the OH coupling assume that an OH is removed from the catalytic surface and coupled to the intermediate resulting in no effect of the electrode potential on the activation barrier.Palladium (II) chloride (> 59.0% Pd; >99.9%, metal basis), nickel chloride hexahydrate, sodium citrate, sodium hydroxide, sulfuric acid (HPLC grade) were purchased from VWR. Sodium borohydride and Nafion solution (5% wt) were purchased from Sigma-Aldrich. Isopropanol, methanol, formaldehyde and formic acid were obtained from Merck. Super P conductive carbon was obtained from Timcal Graphite & Carbon. Ultrapure water obtained with a Millipore DirectQ3 purification system from Millipore was used throughout this work.Catalysts (Pd, Pd3Ni and PdNi) were prepared following a method previously reported [51]. Briefly, defined amounts of PdCl2 and NiCl2•6H2O to have a specific Pd:Ni molar ratio (1:0, 3:1, 1:1) were dissolved in 30 mL ultrapure water with a total metal content of 0.2 mmol. Then, sodium citrate (0.35 mmol) as stabilizer was dissolved into the same solution. N2 was bubbled to remove dissolved O2 from the solution (for 15 min) and was left continuously running to prevent new O2 entering the solution during the reaction. Then, 20 mL of 0.1 M NaBH4 (2 mmol) was added slowly (dropwise) to the mixture, and the solution was kept stirring for 1 h. The solid product was decanted, washed with ultrapure H2O and ethanol several times, and dried at 60°C for 30 min. The catalyst ink was prepared by dispersing 2 mg of the catalyst product in 50 µL Nafion (5%), 1500 µL isopropanol, and 450 µL ultrapure water in an ultrasonic bath (2 mg/mL as catalyst concentration). When the catalyst was well dispersed in this solution, 3 mg of carbon black was added, and the solution was kept in the ultrasonic bath for 1 h. A glassy carbon electrode (GCE) was modified by adding 10 µL of the catalyst ink twice and left to dry before rinsing it with ultrapure water.Electrochemical measurements were performed using a PAR273A potentiostat/galvanostat from Ametek in a 100 mL glass three-electrode cell (15 mL when product analysis was performed). A Pt mesh was used as counter electrode and a Hg/HgO electrode as reference (RE-A6P, Bio-Logic, 1 M NaOH). A glassy carbon electrode (GCE) of 0.6 cm diameter (~0.28 cm2 geometric area) was used as working electrode after addition of the catalyst ink. Current densities are presented normalized by the electrochemical surface area (ECSA). All measurements were performed at controlled temperature (25 ± 1°C) using a water bath and in a N2-saturated solution. Potentials were converted to the reversible hydrogen electrode (RHE) scale using the following equation: (7) Evs. RHE (mV) = Evs. Hg/HgO + 0.059 pH + 0.140 where pH was considered to be 13, 13.7 and 14 for 0.1, 0.5 and 1 M NaOH solutions, respectively. Electrodes were cycled in 1 M NaOH between 0.05 V and 0.95 V (vs RHE) at a scan rate of 10 mV/s in order to achieve a stable electrochemical response (5 cycles). Overpotentials are given with respect to the standard potential for methanol full oxidation to CO2 (Scheme 1), which is +0.02 V (vs RHE at pH 14) [52]. For the CO stripping experiments, CO pre-adsorption was carried out by bubbling CO gas into a 1 M NaOH solution with the working electrode at +0.1 V (vs RHE) for 20 min. Then, the solution was purged with argon for another 20 min to remove dissolved CO gas before the stripping experiment was carried out.The catalysts were characterized by Energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) in order to corroborate their elemental composition (Pd:Ni atomic ratio) and the crystalline structure, which agreed well with the data previously reported following the same preparation method [51]. EDS was carried out using the integrated detector of a JEOL JSM-7000F instrument at an acceleration voltage of 15 kV. Fig. S1 shows the EDS spectra obtained for the different catalysts (Pd, Pd3Ni, PdNi) and Table S1 summarizes the calculated values for the elemental compositional ratio. XRD patterns were recorded with a PANalytical PRO MPD diffractometer in Bragg-Brentano geometry with 1.5406 Å Cu Kα1 radiation, using a 2θ range of 14°–90° and a step size of 0.016°. The XRD patterns (Fig. S2) show that all the catalysts present the fcc Pd crystalline structure, similar to other PdNi catalysts reported previously [51,53,54] and the shifting of the 2θ peaks with increasing amount of Ni suggests successful alloy formation between the two metals [53].Identification and quantification of the oxidation products generated during the methanol oxidation reaction (MOR) for the different catalysts were carried out by High-Performance Liquid Chromatography (HPLC) on an Agilent 1260 Infinity II system with an Agilent Hi-Plex H column (250 × 4.6 mm) and a refractive index detector (Agilent 1290 Infinity II RID) set on positive polarity. A sample volume of 10 µL was injected into the column using the autosampler. Eluent was 5 mM HPLC-grade H2SO4 at a flow rate of 0.4 mL min−1. Column and detector temperatures were 30°C. The counter electrode was separated from the anodic compartment by a glass frit to avoid side reactions of the generated products. To calculate the product selectivity, the number of moles of generated products (formaldehyde and formic acid) and the number of moles of methanol consumed were obtained by HPLC after calibration using standard solutions. Since the HPLC method could only be able to detect formaldehyde and formic acid, but not CO3 2−, the amount of CO2 formed during the MOR was estimated by difference assuming that the number of moles of methanol consumed is the same as the number of moles produced for all the three products: (8) n (methanol consumed) = n (formic acid produced) + n (formaldehyde produced)+ n (CO2 produced) Firstly, the surface electrochemistry of the investigated catalysts is studied by employing cyclic voltammograms and surface coverage analysis (section 3.1). Subsequently, adsorption energies of the intermediates are discussed as a function of the Ni content in the catalyst (3.2). Catalytic activity towards methanol electrooxidation on Pd, Pd3Ni and PdNi and HPLC are given in Section 3.3. CO stripping has been investigated to check catalyst tolerance (3.4). The MOR mechanism is elucidated for each case in section 3.6. Section 3.7 is dedicated to showing the effects of the solution pH and, finally, in section 3.8 a general discussion follows.The surface electrochemistry of the different catalysts (Pd, Pd3Ni and PdNi) is studied by recording cyclic voltammograms in 1 M NaOH (Fig. 2 (a)). Several processes are observed in the forward and backward voltammetric sweeps as a result of the typical surface reactions of Pd-based electrodes in alkaline media. Moreover, stable states of the catalytic surface with H, OH, O and mixed states with H2O are calculated using DFT as a function of the electrode potential and displayed in the form of Pourbaix diagrams [55] for the three investigated catalysts Pd, Pd3Ni and PdNi (Fig. 2 (b)) on the (1 1 1) facet. The molecular states were averaged as a function of the electrode potential U using a Boltzmann distribution to properly illustrate the surface coverage and obtain insights on the onset potential where coverages change with the electrode potential (Fig. 2 (c)).In the Pourbaix diagrams (Fig. 2b), the thermodynamically stable phases are found for the curves showing the lower formation energy as function of the electrode potential U. As expected, and also shown in the voltammograms (Fig. 2), at low potentials we find the catalyst surfaces mostly covered by hydrogen. The Boltzmann distributions on Pd, Pd3Ni and PdNi display a shift down with increasing Ni content of the onset potential at which H is the dominating component (black line in Fig. 2b) with values of, respectively, 0.48, 0.39 and 0.28 V vs. RHE.With increasing electrode potential, the adsorption of OH and also H2O-OH mixtures gets more likely (blue and yellow lines in Fig. 2b). Ni as an oxophilic element provides more OH on the surfaces. This effect correlates with the computed adsorption energies of OH (Table 1 ), showing that a higher concentration of Ni in the catalyst produces stronger OH adsorption. Therefore, between the potentials 0.48 and 0.78 V vs. RHE, the pure Pd(111) catalyst must be mostly covered by OH, OH-H2O or OH-O-H2O mixtures, while for the Pd3Ni(111), it goes between 0.39 and 0.61 V vs. RHE and, finally, for PdNi(111) the interval is from 0.28 to 0.81 V vs. RHE. A wider interval is, hence, obtained for PdNi as compared to the Pd(111) and Pd3Ni(111) counterparts. Empirically assigning the OH adsorption process in the voltammetric curve is challenging for polycrystalline electrodes, but OH adsorption has also been reported to occur on Pd-based electrodes and this is usually attributed to an anodic process observed between the H- and O-regions [56].Oxygen-involved processes (O-region) are observed at higher potentials in the measured CV's, with the formation of Pd oxides occurring during the anodic sweep, and PdO reduction occurring during the cathodic sweep leading to a sharp peak at a potential near +0.78 V vs. RHE (Fig. 2c). The performed calculations are in good agreement where oxygen emerges as the dominant component on the catalytic surfaces after 0.78 V vs. RHE for Pd and Pd3Ni and after 0.81 V vs. RHE for PdNi.Overall, the number of empty sites for the methanol electrooxidation (red lines in Fig. 2c) is similar for all three catalysts. The interesting point here is that, on the alloys, H2O, O and OH adsorb preferentially on Ni sites (Table 1) and this would facilitate the methanol catalytic oxidation on the free Pd sites. To illustrate this fact, the coverage containing 0.5 ML OH+H2O is displayed for the three cases in Fig. 3 . On the Pd(111) surface, the molecular state tends to form a hexagonal structure that maximizes the formation of hydrogen bonds. This geometry is broken on Pd3Ni due to the presence of Ni. Furthermore, a chain of water and OH molecules following the Ni sites is obtained for the PdNi catalyst, thus leaving free Pd sites for further methanol oxidation.Evaluation of the surface electrochemistry allows determining the ECSA. In this regard, measuring the charge involved during the reduction of PdO is the most employed method to determine the ECSA for Pd-based electrodes in alkaline media [57] considering that the reduction of a PdO monolayer takes 0.405 mC cm−2 [58]. Table S1 shows the ECSA values calculated for the different catalysts in terms of electrode area and Pd mass. The ECSA was larger for the monometallic catalyst, but this is mainly due to the presence of a larger amount of Pd metal since the mass-normalized ECSA was similar (5.1–5.6 cm2 g−1 Pd) for all the catalysts. Accordingly, any differences observed in the MOR activity (vide infra) should be related to the influence of Ni on the reaction and not to a change in the electrochemical surface area.Here, the adsorption energies of methanol and the stable intermediate states along the methanol decomposition are computed as in eq. 1 for potential 0 vs. RHE and shown in Table 1 – more negative values mean stronger adsorption. The symmetric sites considered for the adsorption are: two top sites (TPd, TNi), three bridge sites (B2Ni, B2Pd, BPdNi), two fcc (F2NiPd, F2PdNi) and two hcp hollow sites (H2PdNi, H2NiPd); the superscripts indicate the atomic character of each site. Moreover, the last column of Table 1 shows the trends of Ead vs. the Ni concentration where the ⇑ symbol means that adsorption strength (Ead) increases with increasing Ni content while ⇓ indicates a decrease and the first symbol refers to the changes from Pd to Pd3Ni and the second from Pd3Ni to PdNi. This provides insights into which species are more stable and which become less stable with the Ni alloying.The computed adsorption energy of CH3OH showed the top site as the most stable position for Pd(111), agreeing with other investigations using a different DFT functional [59,60] and also with cases like Pt(111) and Au(111) [45,49]. When considering the catalysts Pd3Ni(111) and PdNi(111), methanol prefers to adsorb on-top of Ni atoms instead of Pd atoms, but with lower adsorption energy as compared to the adsorption on Pd(111). Methanol bonds to the catalyst surface through a dative bond involving the oxygen lone pairs which explains the preference for binding to Ni instead of Pd. Furthermore, comparing Ead of methanol on Pd3Ni(111) and PdNi(111) a less negative value is obtained for the case of PdNi - weaker adsorption. Bader population analysis of Ni atoms in the investigated catalysts showed that the lower Ni concentration of Pd3Ni(111) leads to higher charge transfer from Ni to Pd atoms as compared to the PdNi case, i.e. Ni atoms are more positive on Pd3Ni(111), see Table S2. This produces a stronger interaction between the CH3OH and the metal surface. The same effect is also observed for water and formic acid (Table 1).Another effect to be pointed out is the difference in the adsorption energy of a water molecule and a methanol molecule. During the electrochemical process, the first step to account for is the molecular adsorption of methanol on the catalytic surface. However, methanol and water come with similar adsorption energies leading to competitive adsorption with methanol interacting slightly more strongly with the metal surface by 0.02, 0.06 and 0.04 eV, for Pd, Pd3Ni and PdNi, respectively. The alloying process thus slightly favors the adsorption of methanol over water which is a positive sign for the electrocatalytic reaction.In general, adsorption energies show that intermediates with O bonding to the metal surface have their Ead strengthened when Ni is alloyed with Pd (including H2O, OH and O). For intermediates like CH2OH and CHOH the opposite trend is observed – CH2OH, for instance, prefers to adsorb at B2Pd positions highlighting the preference of such intermediates to interact with Pd instead of Ni. This configuration favors the formation of a σ bond between the C and the Pd [60]. The stabilization of O-containing species, and also the lowering of C interaction strength with the metals, have been reported for Sn alloyed with Pt and Ir alloyed with Pt [29,61]. Moreover, reduction of the CO adsorption energy is obtained where the CO bond strength is reduced when comparing Pd(111) and PdNi(111) catalysts.The MOR was experimentally studied on the different catalysts by recording cyclic voltammograms with 0.5 M CH3OH in 1 M NaOH (Fig. 4). The response normalized by the ECSA clearly shows the intrinsic activity of the catalysts under these experimental conditions. Significantly higher peak current densities were recorded with the bimetallic catalysts, which demonstrates the positive role of Ni to increase the MOR activity. Peak currents of 0.30, 1.31 and 1.88 mA cm−2 were obtained for the Pd, Pd3Ni and PdNi catalysts, respectively. A 6X increase was thus obtained with the bimetallic PdNi catalyst versus the monometallic Pd catalyst. Similarly, the overpotential required to produce the MOR at a specific current density decreased by addition of Ni. For instance, overpotentials of 845, 811 and 778 mV were obtained at 0.3 mA cm−2 (ECSA-normalized) for Pd, Pd3Ni and PdNi, respectively. This behavior agrees well with other studies reported in the literature, where bimetallic Pd catalysts have shown increased MOR activity [22].Moreover, product analysis was carried out and determined by HPLC as shown in Fig. 5. The electrochemical experiments were performed at a constant potential (+0.85 V) until reaching the same accumulated anodic charge (12 C) for all three catalysts, which took longer for the monometallic Pd catalyst as a result of its lower MOR activity. HPLC measurements were performed to determine the amount of formic acid and formaldehyde generated and methanol consumed during the reaction. The amount of CO2 generated was estimated by difference as described in the experimental section. Figure S3 (a) shows a typical chromatogram obtained for the reaction products, while Figure S3 (b) shows standard chromatograms of formic acid and formaldehyde employed to identify the products in the sample solution. Fig. 5 shows the MOR product distribution obtained for the different catalysts. In all cases, the main products were formic acid and CO2, while formaldehyde was found in lower amounts. Formaldehyde selectivity was increased for the monometallic Pd catalyst while CO2 selectivity became higher for the Pd-Ni alloys. This suggests that the presence of Ni results in products of higher state oxidation, which would involve the transfer of a larger number of electrons, and could be one reason why larger currents are observed for the bimetallic catalysts [20,62].CO stripping experiments were carried out for the Pd, Pd3Ni and PdNi catalysts and the results are displayed in Fig. 6  in the form of voltammograms. Slightly different CO oxidation responses were observed for Pd, Pd3Ni and PdNi. Sharp stripping peaks were obtained for CO oxidation using Pd3Ni and PdNi catalysts, with just a small shifting of the peak potential to +0.83 and +0.80 V, respectively. This difference would likely not have a significant effect on the MOR since the peak potential is lower than that observed for the MOR and, therefore, the CO oxidation in these catalysts during a MOR experiment is likely occurring efficiently. For the monometallic Pd catalyst, although the onset potential was similar, a larger value of the peak potential is observed (+0.87 V), and most interestingly, the CO stripping peak was broader. This fact suggests that CO stripping on the Pd catalyst is less efficient than on the PdNi catalysts, and the broader peak also indicates that the full CO oxidation is shifted to higher anodic potentials.The first effect explaining the higher tolerance towards CO-poisoning shown by the alloys is correlated to the adsorption energies of the intermediates and the electrode potential effect on the adsorption. Therefore, the effect of the electrode potential on the adsorption energies of selected intermediates is investigated for Pd(111), Pd3Ni(111) and PdNi(111) and shown in Fig. 7 , where the most thermodynamically stable products for each potential can be found.CO is found to be the most stable reaction product for potentials below 0.63 V vs. RHE for the pure Pd catalyst. Just above that, the formation of CO2 becomes more likely providing full oxidation of the reactant. A small downward potential shift is calculated for Pd3Ni and PdNi with values of 0.57 V vs. RHE and 0.56 V vs. RHE, respectively. This difference is mostly a result of the balance between the CO adsorption energies (Table 1) and the CO2 adsorption energies. CO is 0.08 eV more strongly bound on Pd compared to Pd3Ni, and 0.01 eV more strongly on Pd3Ni compared to PdNi. On the other hand, the CO2 interaction strength is higher for the cases with Ni which changes the balance between the more likely products towards CO2. This agrees well with the peak potential shifts shown in Fig. 6 resulting in a greater shift when going from Pd to Pd3Ni and a smaller shift when going from Pd3Ni to PdNi. Moreover, CO shows shorter bond distance for Ni-Cco than Pd-Cco in PdNi (Fig. 8 ). This indicates that CO may bind close to Ni sites and possibly leaving Pd sites more available for the further oxidation processes.CHOO is also obtained as the most likely intermediate for PdNi for potentials between 0.54 and 0.56 V vs. RHE. In fact, highly oxidized intermediates like CHOO and COOH display stronger adsorption at higher potentials on PdNi compared to Pd. For instance, at 0.85 V vs. RHE (the potential used for the HPLC experiment), CHOO is the second most energetically stable intermediate for Pd3Ni and PdNi while, on the other hand, CO is the second most likely intermediate for Pd. Moreover, COOH emerged as the third most stable intermediate for PdNi. This emphasizes the tendency to obtain highly oxidized intermediates on the alloys providing greater number of electrons to the electrochemical reaction (in agreement with the voltammograms, Fig. 4) and previous works [20].The solution pH (OH− ion concentration) could play a significant role in the reaction since the OH− ion seems to be involved in several intermediate steps and particularly important for the generation of products of high oxidation states, such as formic acid and CO2, where it also acts as a source of oxygen atoms required to generate the final product. Therefore, the role of OH− in the MOR activity for the different catalysts was also evaluated. Figure S4 shows the voltammograms recorded for 0.5 M methanol at different NaOH concentrations (0.1, 0.5 and 1 M) for the Pd and PdNi catalysts. MOR activity generally increased with the OH− concentration, demonstrating that OH− is involved in the reaction (scheme 1), and it plays a positive role to obtain larger current densities. Notably, the ratio between anodic peak currents for the MOR between the PdNi and Pd catalysts (Figure S5) increased with increasing NaOH concentration in solution. This shows that the PdNi catalyst seems to be more tolerant or further benefitting from increased OH− concentrations.To determine the mechanism behind the higher activity of Pd-Ni alloys towards MOR, all elementary steps of the methanol oxidation reaction network are evaluated by DFT calculations and shown in Fig. 9 at 0 V vs. RHE for the investigated catalysts. The lower energy pathways are shown by the red arrows. Here we consider scission of C-H and O-H bonds while neglecting C-O cleavage and formation of methane due to the high barrier associated with this process [61]. The most stable configuration (site position) of the intermediates was used to build up these reaction networks and are shown in Figure S6. Moreover, activation barriers are calculated for specific reaction steps that we believe are the bottlenecks for the MOR and shown in Table 2 . The MOR mechanisms, considering also activation energies, are shown in green and blue color in Fig. 9.The initial decomposition of methanol is characterized by the competition between losing a hydrogen from the O-H bond and forming methoxy (CH3O) or breaking a C-H bond and forming H2COH (hydroxymethyl). As discussed earlier, oxygen binds strongly to the catalysts containing Ni, hence, the formation of methoxy (CH3O) is preferable on Pd3Ni(111) and PdNi(111). This has also been reported for PtSn(111) and other compounds like Cu(111) and Ni(111) [61,63,64]. Pd(111) shows preference for C-H cleavage similar to Pt(111) [64]. Hence, based on thermodynamics, the methanol oxidation pathways on Pd(111) and on Pd-Ni alloys are different such that the subsequent intermediates on Pd(111) might be HCOH and COH while the alloys must form CH2O (formaldehyde) and HCO (formyl). In all cases, CO appears as the next deprotonation (abstraction from COH or CHO) and, hence, indicating an indirect oxidation path for the investigated catalysts towards the formation of CO2. CH3OH → CH2OH + H or CH3O + H on Pd: The results provided by the HPLC show a reasonable quantity of formaldehyde formed in case of the Pd catalyst by applying 0.85 V vs. RHE. This indicates that checking only the thermodynamics of the reaction does not ensure the correct oxidative mechanism, since CH2O is not an intermediate on the minimum energy path for Pd(111) (highlighted in red in Fig. 9a). The decomposition of methanol on Pt(111) has been experimentally investigated by Kruse et al. [65] using static secondary ion mass spectrometry (SSIMS), XPS, and pulsed-field desorption mass spectrometry (PFDMS). They proposed that the first H lost is based on the O-H bond breaking and forming methoxy (CH3O) that must further decay to form CH2O. Davis et al. [66] have used high-resolution electron energy loss spectroscopy (HREELS) to detect the presence of CH2O. These experimental investigations are in agreement with our HPLC results. Moreover, Jiang et al. [60] have used DFT to calculate the minimum energy pathway for methanol oxidation on Pd(111) and suggested that the activation barriers play a role. Yang et al. [59] studied methanol deprotonation in alkaline media and showed that the barrier of O-H bond breaking is reduced through the assistance of a hydroxyl group. Our experiments are performed in alkaline media where water is the main solvent. Therefore, H-bond formation between water molecules and adsorbed methanol is expected. Such a bond stabilizes the proton transfer (similar to a Grotthus mechanism) lowering its activation barrier. C-H bond breaking is also affected, but leading to increased activation barriers [48]. Hence, calculation of the activation barriers of the reactions CH3OH→ CH2OH and CH3OH → CH3O are performed using an explicit solvation model, as described in the computational details. The reaction CH3OH→CH3O+H yielded a barrier of 0.5 eV barrier at 0.85 V while the C-H cleavage showed a barrier of 0.9 eV at 0.85 V (Table 2), thus indicating a methanol oxidation path through methoxy and further going to CH2O (green line in Fig. 9 (a)). The OH coupling with CH2O, CHO and CO: As previously mentioned, the oxidation of intermediates plays an important role towards MOR. We have studied the activation barrier of the intermediates CH2O, CHO and also CO coupling to OH since these are the three possible channels where MOR can proceed to finally form CO2. The computed activation barriers (no solvation waters included) for the CO oxidation reaction – formation of COOH – are 1.1 eV, 1.2 eV and 1.5 eV for Pd, Pd3Ni and PdNi, respectively. Thus, CO removal emerges as a bottleneck to achieve the full deprotonation and oxidation to CO2. The oxidation of CH2O to CH2OOH showed barriers of 0.4 eV, 0.4 eV and 0.8 eV for Pd, Pd3Ni and PdNi, respectively, while the reaction CHO+OH→ CHOOH displays a barrier of 0.2 eV for all studied catalysts. Deprotonation of CH2O and CHO: The deprotonation of CH2O and CHO to form CHO and CO competes with the OH-coupling during MOR (see Fig. 9) and, hence, could affect the general reaction mechanism. The reaction CH2O→CHO+H shows barriers of 0.3 eV, 0.8 eV and 1.2 eV for Pd, Pd3Ni and PdNi, respectively. The differences among the obtained barriers of the oxidation (OH coupling) and deprotonation of CH2O are-0.1 eV, 0.4 eV and 0.4 eV, for Pd, Pd3Ni and PdNi (with positive values meaning lower barrier for OH coupling), hence, it highlights that the oxidative reaction plays a more important role for Ni-containing catalysts. The next is the CHO deprotonation to form the poisoning CO intermediate. Spontaneous reactions were obtained for Pd and Pd3Ni, while PdNi displayed a 0.4 eV barrier. For PdNi, the OH coupling with CHO has a barrier of 0.2 eV and, hence, the oxidative reaction is more feasible than the deprotonation reaction leading to less CO poisoning on PdNi while the spontaneous deprotonation reaction must lead to a higher susceptibility to CO poisoning for Pd and Pd3Ni.In summary, the obtained activation barriers for Pd have redirected the MOR mechanism towards the formation of formaldehyde. Further, the deprotonation of CH2O proceeds as the more likely reaction until the formation of CO (green line in Fig. 9 (a)). CO could, further, be oxidized to CO2, but, with the high activation barrier obtained for the reaction CO+OH →COOH (1.1 eV), it is more likely that CO sticks to the catalytic surfaces forming a poisoning product. The oxidation of CH2O on Pd3Ni emerged as an alternative channel (blue arrow in Fig. 9 (b)) that partially circumvents the formation of CO. On the other hand, any remaining CHO intermediate during MOR must directly go to CO, still forming CO with a high activation barrier for the oxidative removal to happen, even though in lesser quantity than Pd. The activation barriers computed on PdNi show that the oxidation of CH2O and CHO are more likely than the deprotonation (Table 2) – differently from Pd and Pd3Ni (Fig. 9 (c) green and blue). This yields less poisoning compared to the Pd and Pd3Ni counterparts and also a higher current density on the CVs due to the more effective reaction with OH occurring via two channels (also important to highlight that PdNi displays more OHads than Pd or Pd3Ni – see Fig. 2).The aspects of the MOR mechanism on Pd, Pd3Ni and PdNi, studied based on a combination of experiments and DFT calculations, have indicated specific characteristics able to provide insights into their MOR activity and selectivity. Here, we discuss these features and the links between the experimental results and the DFT calculations. Pd: The Pd catalyst showed lower tolerance towards CO poisoning compared to Pd3Ni and PdNi. Moreover, the reaction mechanism indicated in Fig. 9 (a), and also the effect of potential on the adsorption energy of the intermediates, have confirmed the higher tendency of finding less oxidized intermediates on pure Pd as compared to the bimetallic alloys. These trends result in the higher selectivity of Pd towards formaldehyde, as also found in the HPLC experiment (formaldehyde is less oxidized than formic acid or CO2). Another effect that might play a role for the Pd higher selectivity towards formaldehyde is the CO poisoning. During the MOR reaction, CO must partially poison the Pd catalyst to a greater extent than the Pd-Ni alloys. This causes steric interactions between the adsorbed CO with the intermediates blocking the reaction towards CO2. Thus, earlier products like CH2O might appear in greater quantities on this catalyst compared to on the Pd-Ni alloys. Pd3Ni: Pd3Ni has an alternative MOR route towards the formation of CO2 that is free of CO formation – oxidation of CH2O forming CH2OOH. That means a higher tolerance towards CO poisoning compared to Pd and, hence, a higher activity due to the higher number of electrons involved in the reaction (see Scheme 1). The HPLC experiment found a lower quantity of CH2O on Pd3Ni compared to on Pd together with a higher amount of CO2, which might be due to the lower probability for formation of CO on the catalytic surface. It has also been shown that the oxidative removal of CO on Pd3Ni is more effective than on Pd. However, the high adsorption energy of CO governs the kinetics of the CO oxidation together with the availability of OHads species [9]. Due to the strong CO adsorption, a high activation barrier for the reaction CO+OHads → COOH is found and, hence, this step might occur slowly. This indicates that the high activity displayed by Pd3Ni compared to Pd is, in fact, more related to the direct route towards the formation of CO2 and less related to the CO oxidation process. Jones et al. [67] proposed a direct non-CO MOR mechanism on a Pt-Ru alloy when the Ru concentration increases and leading to formation of formate (CHOO). The results shown here also indicate that the higher concentration of Ni changes MOR from an indirect to a direct mechanism. PdNi: Three main features must be emphasized for the PdNi catalyst: 1) PdNi has a strong tendency to form highly oxidized intermediates like CHOO (Fig. 7 (c)). 2) The deprotonation reaction CHO→CO+H exhibits higher barrier than the OHads coupling with CHO, hence providing an alternative route towards the formation of CO2. 3) Similar to for Pd3Ni, CH2O oxidation is a possible mechanism for the MOR on PdNi. The two possible direct routes (green and blue in Fig. 9) indicate a higher tolerance against CO poisoning for this catalyst as compared to Pd and Pd3Ni. Finally, the fact that the highest activity towards MOR is displayed by PdNi is due to the two direct non-CO routes towards CO2 and also the tendency towards formation of highly oxidized intermediates as also confirmed by the HPLC experiment. It is important to highlight that, based on the Pourbaix diagram, this catalyst is the one with more adsorbed OH species, which effectively fuels the oxidation reactions of CH2O and CHO.The obtained results of the DFT calculations indicate the non-CO routes displayed by PdNi as the main reason for the higher activity towards MOR presented by this bimetallic alloy vs. pure Pd. The OH− concentration experiment supports this hypothesis since PdNi must be benefited by the OH− concentration to a greater degree than Pd due to the effective fueling of the non-CO paths on PdNi and, contrarily, the CO poisoning must act to block the MOR on Pd. Thus, the higher OH− concentration should enhance the MOR activity on PdNi more effectively than on Pd. The experimental results displayed a ratio between anodic peak currents for the MOR between the PdNi and Pd catalysts that increases with the OH− concentration, as expected. This, therefore, corroborates our DFT results and, moreover, shows that the enhanced coverage by adsorbed OH species leads to even greater performance of the bimetallic alloys towards MOR activity.The used models to describe the MOR contain simplifications that need to be considered. For instance, the structure of the surfaces, as shown in Fig. 1 for Pd, Pd3Ni and PdNi may be expected to be subjected to variations due to strong interaction with intermediates like CO and thus the Ni/Pd distribution at the interface could be different from the one used in our model. Moreover, the experiments were performed with polycrystalline catalysts, which may add some differences to what we obtain with our model. Even if such differences can be of importance, the obtained results are in good agreement with the experimental data, which we assign to the local atomic environment dominating the interaction and different facets (with similar occurrence in the different samples) not significantly affecting the relative energies in the comparison of Pd, Pd3Ni and PdNi.Combining experiments and DFT calculations, we have successfully elucidated the benefits of a Pd-Ni bimetallic electrocatalyst towards the methanol electrochemical oxidation. We have firstly evaluated the catalytic surface coverages based on cyclic voltammograms and Pourbaix diagrams. The results revealed a shift of the onset potential where oxidation of the catalyst surface becomes the more likely process when Pd-Ni is considered. Moreover, the amount of OHads species on the catalytic surface is increased on PdNi as compared to Pd and Pd3Ni. This provides an effective OH fueling for the intermediate steps of the MOR involving OH. Higher activity and selectivity towards CO2 were also obtained for the bimetallic alloys in the cyclic voltammetry and HPLC experiments. These results are attributed to: i) different MOR mechanisms for Pd, Pd3Ni and PdNi, where Pd-Ni alloys emerged with non-CO routes for the methanol oxidation while Pd displayed a direct reaction. Notably, the PdNi catalysts showed that OHads can easily react to CH2O and CHO opening two routes towards CO2 that are free from CO. For Pd3Ni, differently, there is no barrier for the CHO → CO+H+ reaction making the coupling of OHads with CHO less likely. ii) The oxidative removal of CO on Pd is inefficient and, therefore, this catalyst suffers CO poisoning to a greater extent than Pd-Ni. iii) The oxophilic property of Ni that makes highly oxidized intermediates more likely during MOR. Finally, the solution's pH – higher OH− concentration - benefits PdNi more than Pd due to the more important role found of the OH coupling for this electrocatalyst. Overall, alloying Pd with Ni shows to be an effective strategy towards an alternative electrocatalyst that delivers good performance towards alcohol oxidation at a reasonable price.RBA and ECdS performed the calculations and DMY the experiments. AC and LGMP conceived and supervised the study. All authors contributed to the writing of the 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 work was supported by the Swedish Foundation for Strategic Research (SSF) through grant number EM16-0010 and by the Swedish Energy Agency (Project 44666-1). The computations were enabled by resources provided by the Swedish National Infrastructure for Computing (SNIC) at the PDC and NSC centers partially funded by the Swedish Research Council through grant agreement no. 2016-07213.Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2020.136954. Image, application 1

Amongst promising available technologies enabling the transition to renewable energy sources, electrochemical oxidation of alcohols, in a direct fuel cell or in an electrolysis reaction (H2 production), can be an economically and sustainable alternative to currently used technologies. In this work, we highlight the advantages of a Pd-Ni bimetallic electrocatalyst for methanol electrooxidation - a convenient choice due to the low cost of Ni combined with the observed acceptable catalytic performance of Pd. We report a synergistic effort between experiments and theoretical calculations based on density functional theory to provide an in-depth understanding - at the atomistic level - of the origin of the enhanced electrochemical activity of methanol electrooxidation using the bimetallic catalysts Pd3Ni and PdNi over pure Pd. Cyclic voltammograms and High-Performance Liquid Chromatography (HPLC) demonstrate higher activity towards methanol electrooxidation with increased Ni concentration and, furthermore, higher selectivity for CO2. These effects are understood by: 1) changes in the methanol oxidation reaction mechanism. 2) Mitigation or suppression of CO poisoning on the Pd-Ni alloys as compared to the pure Pd catalyst. 3) A stronger tendency towards highly oxidized intermediates for the alloys. These findings elucidate the effects of a bimetallic electrocatalyst for alcohol electrooxidation as well as unambiguously suggest PdNi as a more cost-effective alternative electrocatalyst.

No data was used for the research described in the article.Since the 1950s, the amount of municipal waste generated by the manufacturing sector and home consumption has been increasing faster due to the expanding population and increased human activities. Common municipal waste includes plastics, used tires, old clothing, kitchen waste, paper, etc. [1]. The average weight of waste plastic, the polymerization or polycondensation of monomers, creates macromolecular; it makes up 10% of the annual worldwide trash production [2]. One of the essential plastic wastes is PSW, which makes a wide range of consumer goods. The petroleum sector typically provides the monomers, as in the case of the ethylene-based synthesis of PSW. Therefore, the production of polyethylene, polypropylene, polystyrene, polyethylene terephthalate, polyvinyl alcohol, and polyvinyl chloride has increased more than other plastics. PSW plastic is frequently used in items that demand clarity, such as food packaging and laboratory equipment, as it is a complex, solid plastic. Recent scientific reports claim that 7–8 billion tons of solid waste are created annually around the globe, posing a significant problem for the scientific community due to the numerous negative impacts on the environment and human health [3]. In waste-to-energy plants, very little of the plastic that we discard every day gets recycled. A large portion of it is disposed of in landfills, where it might take up to 1000 years to degrade and release potentially harmful materials into the soil and water. Burning, another way to dispose of plastic waste, is detrimental to the environment since burning releases hazardous chemicals into the air. Thus, finding the right recycling solution is necessary since burning and burying plastic waste plastic is extremely harmful to the environment.In view of this scenario, waste plastic treatment means must be applied to improve their valorization rate. The thermochemical methods are among the most promising for application, and growing interest is being shown in the thermal treatment alternatives of pyrolysis, steam reforming, and combined pyrolysis/reforming systems as feasible alternative environmental and financial solutions for plastic waste processing. Compared to landfilling or conventional waste incineration, the in-situ pyrolysis-catalytic steam reforming reaction method provides various benefits, such as conversion plastic wastes and producing H2 and liquid fuels rather than harming the environment. Accordingly, recycling of PSW is substantial for environmental remediation and moves towards a more circular plastic economy. Pyrolysis manages plastic waste sustainably while producing solid char, gases, and liquid oil as energy sources [4]. Complex compounds or long-chain hydrocarbons are thermally broken down into simpler molecules or shorter-chain hydrocarbons. Additionally, using raw bio-oil from pyrolysis is difficult since it has a high oxygen concentration that concurrently lowers its energy content [5]. Furthermore, PSW temperatures seldom reach the levels required for thermal deterioration; hence this phenomenon is uncommon [6]. As a result, dissolving PSW in a dissolving agent may be a great answer to PSW problems and the creation of clean and renewable energy. As previously investigated [7,8], phenol is an effective dissolving agent since it is acidic to certain plastics, rubber, aluminium, its alloys, and lead. Phenolic constituents often result from the production of petrochemical by-products [9] and makeup around 38% of the unwanted pyrolysis oil ingredient [10]. In addition to the practical liquid fuel generation, the employment of phenol can also allow H2 generation from in situ pyrolysis-catalytic steam reforming reaction. The chemical H2 is essential for many industrial processes and may one day serve as a source of clean energy. Thus, using PSW plastics as feedstocks for valuable liquid goods and phenol as a source for chemicals like H2 will encourage the recycling of plastic waste, stop the difficulties created by the waste plastics, and act as an alternative supply of chemicals, enabling a circular economy.In the pyrolysis-catalytic steam reforming reaction, catalysts are frequently employed to improve product dispersion and raise product selectivity. At the in-situ catalytic pyrolysis processes, the catalyst and feedstock are mixed, and the pyrolysis and vapour catalytic reforming/cracking processes take place in the same reactor; therefore, the capital and operating costs are reduced. Additionally, catalysts have been used to upgrade pyrolysis products such that the hydrocarbon distribution is improved and has characteristics comparable to traditional fuels like diesel and gasoline [11,12]. For the sustainable production of H2 and liquid fuel, large-scale commercial use of noble metals such as Pd [13–15], Rh [16,17], and Pt [18,19] was employed and demonstrated the best catalytic activities; however, they are prohibitively costly. As an alternative, lots of studies on the design of non-precious metal-based catalysts for the H2 generation, such as bimetallic Ni-Co [10,20–24], Al [25,26], and Fe [27,28], have been investigated. In contrast, significant challenges such as unstable changes in morphology after the reaction, low stability, and selectivity remain. Specifically, as appealing substitutes to traditional noble metal-based catalysts, noble metal-free catalysts could have a promising future in initiating reforming and cracking reactions. Recently, the Ti@TiO2 core-shell nanoparticles were stated as a precious metal-free photocatalyst for the photothermal H2 production from aqueous glycerol solutions [29]. Yancheng et al. [30] used porous copper (Cu) foam as catalyst support for H2 production from methanol steam reforming reaction. Though its overall efficiency was lower, the microreactor utilizing Cu foam covered with 0.6 g of catalyst had more excellent methanol conversion and H2 generation rates per catalyst weight than when foams with higher catalyst loading were utilized. The previous investigation on the addition of Cu to Ni/Al2O3 catalyst found that low loadings of Cu served to lessen the alloying impact brought on by Cu enrichment, which helped prevent the production of carbon on the catalyst [31]. Additionally, it has been claimed that the low-valence Cu species (Cuo or Cu+) in the majority of Cu catalysts are active species [32]. Developing and preparing effective precious metal-free catalysts for H2 production from the in-situ pyrolysis-catalytic steam reforming reaction of PSW dissolved in phenol is the most difficult challenge in this sector. There is a sizable possibility for carbon to develop and be deposited on the catalyst's surface since this process requires the removal of H2 from phenol and liquid fuel hydrocarbons from PSW. Therefore, additional precious metal-free catalysts must be developed to present potential candidates as precious metal catalyst substitutes.To the best of our knowledge, studies are deficient for explaining the effect of the chemical and physical properties on the selectivity and coking resistance of the Ti-Cu nano-catalyst in in-situ pyrolysis-catalytic steam reforming conditions. Herein, we report the facile synthesis and characterization of a precious metal-free Ti-Cu nano-catalyst and their favourable catalytic properties towards H2 and liquid fuel generation from in-situ pyrolysis-catalytic steam reforming reaction of PSW dissolved in phenol. Catalysts before and after reaction were characterized by several methods correlating their structural and textural characteristics with catalytic activity. In the present work, three precious metal-free nano-catalysts, Ti, 4Ti3Cu (4 g Ti with 3 g Cu) and 3Ti3Cu (3 g Ti with 3 g Cu), were prepared by hydrothermal and impregnation methods to research the influences of Cu addition on the catalytic performance of the catalysts. The crystallinity and synergistic effect of calcined Ti–Cu on the inhibition of materials sintering and the surface area along with pore size distribution of the fresh catalysts were characterized by X-ray diffraction (XRD) and the Brunauer, Emmett and Teller theory (BET), respectively. The basicity study was conducted by the temperature programmed desorption of carbon dioxide (CO2-TPD), pyrrole-differential thermogravimetric analysis (DTG) curves, and pyrrole-FTIR spectra. Pyridine FTIR spectra and pyridine - DTG curves of the fresh catalysts were used to illustrate the Brønsted and Lewis acid sites. Transmission electron microscopy (TEM) and H2 temperature-programmed reduction (H2-TPR) were used to demonstrate the benefits of Cu doping on the dispersion and reducibility of Ti. Fourier-transform infrared spectroscopy-potassium bromide (FTIR-KBr) was used to examine functional groups present in the synthesized catalysts. Catalysts were tested in a fixed-bed reactor that is modified for higher PSW plastic waste to be reacted compared with our previous research [14,33–35], and the optimum catalyst base on the highest phenol conversion and H2 yield was tested at 500–800 oC and 45 h on stream. The produced liquid product samples were characterized by gas chromatography/mass spectrometry (GC/MS) and FTIR systems. Used catalysts after experiments were also collected and characterized by thermogravimetric analysis (TGA), FTIR-KBr, BET, TEM and CHNS.Nano catalysts Ti, 4Ti3Cu, and 3Ti3Cu were synthesized by hydrothermal technique with the Ti to Cu mass ratio of 1, 3:3, and 4:3, respectively. The starting reagents of titanium and copper were titanium (IV) oxide (TiO2, with the purity of 99.8%) and copper (II) nitrate trihydrate (Cu(NO3)2.3H2O, with the puriss. p.a. grade of 99–104%) which were acquired from Sigma-Aldrich and the synthesis stages are shown in Fig. S1. In accordance with our previous exploration [36,37], nano-sized Ti and Cu catalysts were separately gone through hydrothermal treatment, in which those materials were first stirred with 100 mL of deionized water at room temperature. 5 M of sodium hydroxide (NaOH) was gently dissolved with the solution to improve the nucleation and growth rates of the nanoparticles [38] and stirred for three hours at room temperature to form a clear mixture. Then, the mixture was transferred into a 100 mL Teflon-lined autoclave reactor and was hydrothermally treated at 160 °C for two days. The solid precipitate was repeatedly centrifuged (400 rpm) to separate the solid products from the liquid phase, filtered and washed with deionized water 15 times via filter paper on a Buchner funnel that was sealed with a rubber bung on the top of a side arm conical flask. The side arm of the flusk was connected with a vacuum pump to speed the filtration and washing process of the samples, followed by drying at 110 °C overnight and then calcination for three hours at 800 oC. In order to cure and harden catalysts for industrial use, remove impurities, and drive out chemically bonded moisture, calcination is a crucial step in the process of making catalysts. The prepared nano-sized Ti and Cu particles then went through the conventional impregnation method for synthesizing 4Ti3Cu and 3Ti3Cu. The detail of preparation is explained in our previous research [23,24]. In brief, a specific quantity of the calcined Ti was mixed in 150 mL of deionized water and stirred for an hour at 90 oC, and then the calcined Cu was introduced into the mixture. After vigorous stirring for a few hours, the liquid was evaporated, and a slurry was produced and dried overnight in an oven at 110 oC. Lastly, the acquired dried solid was calcined in an oven (Model Ney Vulcan D-130) at 800 °C for 3 h (30 °C min−1).XRD curves were obtained employing D8 ADVANCE Bruker X-ray diffractometer operated at 40 mA and 40 kV with Cu Ka radiation at 2 theta of 10–100°. The crystalline phases were classified by JCPDS (Joint Committee on Powder Diffraction Standards) using X'Pert Highscore Plus software, and crystal sizes were estimated from diffraction line widths using the Scherrer equation. Nitrogen (N2) adsorption–desorption performances for fresh and used samples were obtained in a Beckman Coulter SA3100™ apparatus using liquid N2 at − 196 °C. Each catalyst was degassed at 200 oC under a vacuum for 3 h before the adsorption experiments. The BET technique was used to evaluate the specific surface area. At the same time, the average pore size was calculated using the Barrett-Joyner-Halenda (BJH) technique utilizing the adsorption curve to get the total pore volume at relative pressure P/Po = 0.99. The fresh and used TEM images were acquired using a JEOL JEM-1011 microscope that functioned at 80 kV. TEM specimens were equipped by dispersing the catalyst powder in acetone with sonication and dropping it onto an ultrathin carbon-coated Cu grid. TGA-DTG analysis of the used catalysts was carried out using a Shimadzu TG-50 thermogravimetric analyzer via the flow of N2 to heat the samples from 30° to 800°C with a heating rate of 20 °C min−1. The H2-TPR was accomplished on a Micromeritics Chemisorb 2720 apparatus, and the analysis was carried out in a pure H2 at a flow rate of 30 mL/min, and the temperature was increased from room temperature to 900 °C with a heating rate of 20 °C min− 1. CO2-TPD was also conducted on the same device to detect the basicity of the catalyst. The samples were put into a quartz tube and pretreated in a Helium (He) flow at 250 °C for 1 h and then cooled down to room temperature naturally. The catalyst samples were exposed to the CO2 environment at 110 oC after the pre-treatment step until their surface sites reached their saturation state. After attaining saturation, the samples were flushed with inert gas He. Further, the temperature was increased to 900 oC with a ramp rate of 20 oC/min to determine the quantity of desorbed CO2 from the surface basicity sites using a thermal conductivity detector (TCD). The elemental linkage information of the fresh and used samples was studied by FTIR curves detailed via a Shimadzu IR-Prestige-21 model spectrometer using pure KBr as a reference background record with a scanning range of 400–4000 cm−1. The KBr pellet was prepared by mixing the catalyst with KBr with a mass ratio of 100:1, and the excellently prepared combination was pressed to procedure a 13 mm diameter pellet. The same apparatus was used to determine the functional cluster presented in the liquid products in addition to the GC/MS (Agilent 7890B).The reaction performance of the prepared Ti, 4Ti3Cu, and 3Ti3Cu was investigated via the combination of a fixed-bed reactor and online mass spectrometry. The case length was 300 mm, and the internal diameter was 8 mm at atmospheric pressure; the diagram of the experimental apparatus is illustrated in Fig. 1. 0.2 g of the catalysts were located inside the reactor, and the temperature of the catalyst bed was measured and controlled by a K-type thermocouple, which was linked to a temperature controller. The catalyst was reduced in place for one hour at 600 oC using 30 mL/min of pure H2 after flushing the catalyst bed with N2 at 300 oC. The water was fed into the pre-heater using a high-performance liquid chromatography pump (HPLC Bio-RadTM, Series 1350) to inject the fuel with 0.36 mL/min before mixing with carrier N2 (30 mL/min). In our previous research [14,33–35], we used a very small amount of plastic waste dissolved in phenol to avoid the blockage of the line before the reactor and experimental limitation. To increase the feasibility of the reaction and the amount of plastic waste in the reaction, we modified the experimental rig with a Parr Benchtop Reactor. Herein, the slurry of phenol and PSW plastic with the volumetric ratio of 5:1 was mixed with water vapour molecules and fed to the reactor using a pressurized Parr Benchtop Reactor that kept the PSW-phenol slurry in an aqueous phase at 70 oC, and transfer pipes were swathed using glass fibre heating tape and preheated at 200 oC. With the volumetric ratio of water to PSW-phenol solution of 10:1 with two mass flow controllers individually equipped for phenol line and water line that precisely monitored the flowrates of reactants; the water to PSW-phenol vapour was pumped into the reactor. For activity testing, all catalysts were teated at 500 oC. The optimum catalysts were tested in reaction temperatures ranging from 500 oC to 800 oC with a gap of 100 oC, and the relevant performance results were collected in steady-state situations. Likewise, the constancy examination of the optimum catalyst was conducted at 500 oC for 45 h. After the reactor, a condenser was installed and connected with a circular cooling system at 10 oC to liquefy the condensable liquid molecules, followed by a liquid gas separator. The components present in the gas products were analyzed online employing a GC-TCD (Agilent 6890 N), and the liquid product was analyzed using a GC-FID (HP 5890 Series II) equipped with a 0.53 mm × 30 m CP-Wax capillary column and GC/MS (Agilent 7890B). Each run was repeated at least six times to ensure accuracy and reproducibility. The result analyses, such as phenol conversion, produced gas composition in yield, were calculated following our previous research [14] and as shown in Eqs. (1), (2), (3), and (4). (1) Phenol conversion ( % ) = [ Phenol ] in − [ Phenol ] out [ Phenol ] in × 100 (2) H 2 yield ( % ) = moles of H 2 obtained moles of H 2 stoichiometric × 100 (3) CO yield % = moles of CO obtained moles of CO stoichiometric × 100 (4) CO 2 yield % = moles of CO 2 obtained moles of CO 2 stoichiometric × 100 The quantity of chemicals that react for the reaction to be fully catalyzed is known as the stoichiometric moles. So, for example, Eq. 5 represents the balancing steam reforming equation. (5) C 6 H 5 OH + 11 H 2 O ↔ 6 C O 2 + 14 H 2 Δ H o = 463.65 kJ / mol (6) C 6 H 5 OH + 5 H 2 O ⟶ 6 CO + 8 H 2 Δ H o = 710.91 kJ / mol (7) CO + H 2 O ↔ C O 2 + H 2 Δ H o = − 41.15 kJ / mol Table 1 depicts the surface area, pore volume and pore diameter of Ti, 4Ti3Cu, and 3Ti3Cu catalysts, defined by N2 adsorption–desorption investigation. As can be seen in Table 1, introducing Cu causes produce different surface areas, pore sizes and pore volumes. The BET surface area of Ti, 4Ti3Cu, and 3Ti3Cu is 15.912 m2/g, 5.374 m2/g, and 3.73 m2/g, respectively. Obviously, higher Cu contents nano-catalysts cause to decrease in the surface area, partial collapse of ordered mesoporous structure and the rise of the pore diameter (Dp). Typically, in comparison to bare Ti catalyst, 3Ti3Cu catalyst only displayed a surface area of 3.73 m2/g, and its pore volume was low. The sintering of nanoparticles during the calcination process might be significantly delayed by Ti components having a mesoporous structure, preserving a greater specific surface area. This decrease is caused by the inclusion of Cu, which partially covers the catalysts' surfaces and blocks some of their pores. The catalyst made of Cu may be simple to sinter at 600 °C in a reducing atmosphere, which leads to the growth and agglomeration of the active particles and a reduction in the specific surface area [39]. Fig. 2 displays the pore size distribution and the N2 isotherms for the fresh catalysts. Ti and 4Ti3Cu isotherms can be categorized as type-IV ascribed to the mesoporous structure [40]. However, the Ti and 4Ti3Cu samples' distribution pore size curve displayed a peak in the area above 50 nm, indicating the presence of macropores in their structure. The 3Ti3Cu material, on the other hand, exhibits a type IV isotherm and an H4 hysteresis curve, both of which are typical of slit-shaped pores [41]. This phenomenon is caused by developing tiny CuO micro crystallites that partially obstruct mesopore access.The XRD outlines of as-synthesized materials are displayed in Fig. 3; estimated crystallite sizes are included in Table 1, and the crystal sizes are diverse from 67 to 93 nm for all nano-catalysts. Diffraction peaks are prominently located at about 39.2° (200), 70.6° (122) and 76.5° (202) and marked with blue stars, matched with the standard XRD pattern of rutile Ti2O4 (JCPDS, No. 96–900–7433) and match with the crystallite sizes of 80.8 nm. The characteristic diffraction peaks of spinel phases at 2θ of 25.7°, 37.5°, 38.4°, and 69.04° were observed (marked with red hearts) and can be ascribed to representative peaks of (101), (103), (004), and (116) crystal phases and corresponding to the JCPDS number of 96–101–0943 for anatase Ti4O8, and equal with the crystal size of 42.3 nm. Meanwhile, the peaks appearing at 48.6°, 54.3°, 55.7°, 63°, 75.9° and 82.9° could be assigned to the characteristic peaks of 202, 023, 151, 061, 151 and 402 crystal phases and correspond to the JCPDS number of 96–900–9088 for orthorhombic phase structure of brookite Ti8O16, and correspond with the crystallite size of 98.2 nm. After introducing CuO to the TiO2, multiple new diffraction peaks for the tenorite (Cu4O4) (JCPDS, No. 96–110–0029) were detected (marked with green trefoil shapes) at 35.6°, 49.1°, 58.7°, 61.9°, 66.1°, and 72.9° which could be assigned to the diffraction peaks of 002, 202, 202, 113, 022 and 311 monoclinic phase structures, respectively, and equal to the crystallite size of 47 nm. The decrease in intensities after introducing the CuO to TiO2 reveals that the Cu is highly dispersed in the catalysts or causes a crystal size reduction. The XRD patterns of 4Ti3Cu and 3Ti3Cu catalyst resulted in two more new peaks at 2θ of 27.6 and 68.6, consistent with 110 and 126 crystal planes, which can be attributed to the monoclinic [JCPDS 96–153–9683] structure of TiO2 and marked with purple triangles (∼70 nm). The catalysts synthesized by the hydrothermal method showed in addition to obvious copper oxide and titanium dioxide phases, the bimetallic oxide catalyst Cu–Ti was also successfully synthesized in this experiment. This phenomenon can be approved by the appearance of the peak with the blue circle at 32.9° (∼60.6 nm) and can also be attributed to the diffraction peaks of 101 tetragonal phase structures of CuTi3 alloy. However, for the 3Ti3Cu catalyst, the diffraction peaks' intensity is low, meaning that the Ti might be highly dispersed on the Cu. It might be the "combustion" process that gives the catalyst with enhanced Ti dispersion and smaller crystal sizes. Considering the low intensity of XRD peaks of the 3Ti3Cu catalyst, one can say that Ti and Cu are present mainly in the amorphous phase in the synthesized catalysts. This also could suggest that the hydrothermal method (for Ti catalyst) facilitated the crystal growth during material preparation. In the impregnation method (for 4Ti3Cu and 3Ti3Cu), the metals are distributed over the lower layer of the support of the catalyst; this causes the intensity of the peak to decrease compared to that prepared by the single component (Ti) hydrothermal method.In this work, the TEM technique was employed to identify the position of the particles, size and morphologic features in the 3Ti3Cu nano-catalyst; corresponding results are shown in Fig. 4. The nano-sized catalysts constituted a rectangular-like shape of TiO2 with an average diameter of ∼200 nm, indicating that the TiO2 core is highly crystallized with better dispersion. However, Cu presented an uneven element form with less particle scattering. It also comprises segregated particles and appears in spherical shapes in mostly amorphous phases that are in good agreement with XRD results. This finding might be because of the less BET surface area and XRD crystal size of 3Ti3Cu compared with Ti catalyst, which was reported in previous sections. Less crystal size plays a substantial part in minimizing the coke production and deposition and enhancing the catalyst lifetime throughout the in-situ pyrolysis-catalytic steam reforming reaction process. Furthermore, as shown from the TEM micrograph, the contact between Ti and Cu particles was lower than those Ti particles. This suggests that Ti-Cu ensembles may be formed due to the interaction between Ti and Cu. And it is also confirmed that the highly dispersed nano-catalysts could be obtained with inexpensive materials via this simple hydrothermal-impregnation method.As shown in Fig. 5, the FTIR spectrum utilizing the KBr pellet approach was obtained in the wavenumber range of 4000–400 cm−1 to analyze the practical clusters in the manufactured catalysts. The phenyl ring vibrations, such as γ(C−C−C), are identified at 1265 cm−1 [42]. This peak was shifted to 1203 cm–1 after introducing a CuO component to the TiO2. Giuseppe et al. [43] mentioned that the 1265 cm−1 peak could be assigned to the C–O vibration in guaiacyl rings that clearly shifted to a pronounced broad and strong band around 1203 cm–1 for the 4Ti3Cu and 3Ti3Cu catalysts due to the bending vibrations of amino acids side chains [44] and attributed to the asymmetric stretching vibrations of C–O–C in all samples [45]. A weak peak corresponding to CO is observed at 617 cm–1 for the bare Ti catalyst that can be assigned to ωO1-Ti3-O2 [46]. After introducing the Cu, the intensity of this peak is slightly increased and matches the metal oxide stretching, i.e., Cu–O bond in the monoclinic phase, which specifies CuO nanoparticles formation [47]. The band at 1018 cm−1 (symmetric O–C–O stretching [48]) belongs to the C−H and N − H in-plane deformation vibrations [49], and might also attribute to alkoxy groups attached to titanium ions in the catalysts [50]. FTIR bands at 1689 cm−1 are attributed to CN vibrations modes [51]. Likewise, this band could also correspond to the ν (C−O) mode of a carbonyl compound formed after adsorption [52].H2-TPR characterization was implemented on all calcined samples to study the effects of adding Cu to Ti on the catalyst reducibility and the interaction between metal and support. It had been reported that the TPR profiles of Ti-based catalysts were affected by the interaction between Ti and Cu. The reduction profiles of all catalysts are shown in Fig. 6(a), and the H2 consumption are listed in Table 1. The less H2 consumption corresponds to the poor reducibility of the catalyst. Compared to 4Ti3Cu and 3Ti3Cu samples, only a small trace of H2 uptake in the Ti sample was detected, which might be associated with the reduction of the remaining TiOx species in deficient concentration. The fact that the mixed metal oxides had solidified into a solution and the synergetic effects had boosted the reducibility is another potential explanation for the absence of pure Ti's reduction peaks. The observation of the low-temperature shoulders (281 °C and 315 °C), detected in reduction profiles of 4Ti3Cu and 3Ti3Cu, is possibly ascribed to the reduction of Cu2+ to Cu0 in aggregated copper oxide species. These characteristics specify a highly distributed Cu2+ species in 4Ti3Cu and 3Ti3Cu catalysts, and the presence of these species can assign to the strong interaction among Cu and Ti. The two reduction peaks at 443 °C and 447 °C could be the reduction of monomeric Cu+ to Cu0 [53,54]. As a result, the species are reduced at much greater temperatures than Cu2+ species, which are associated with titanium copper alloy. As a result, we may conclude that the impregnated catalyst mainly comprises the copper oxides copper oxide and Cu-Ti. Due to the limited quantity of alloy or the high metal dispersion on the support, the Cu-Ti alloy peak is very weakly visible in the XRD examination. As presented in the figure, two main reduction peaks of the 4Ti3Cu catalyst were detected at about 443 °C, and 545 °C, which might be attributed to the reduction of surface oxygen and bulk oxygen of Cu, respectively, or may account for two overlapping reductions steps of copper oxide into Cuo. Noticeably, compared with the 3Ti3Cu catalyst, the reduction peaks of the 4Ti3Cu catalyst obviously shifted to a lower-temperature direction. These results reveal that the Ti species over 3Ti3Cu and 4Ti3Cu catalysts are more reducible, indicating that the reducibility of catalysts is promoted with higher Ti content. The lower the peak temperature, the better the reduction performance of the catalyst is. However, the peak area of the 3Ti3Cu catalyst was the largest, which can be concluded that the introduction of the Cu component promotes the reduction of the 3Ti3Cu catalyst, has the best redox capacity, largest H2 consumption, oxygen storage capacity and inexplicable interactions among Ti and Cu as proven by the reduction peak at 600 °C.The CO2-TPD technique was employed to rationalize surface basicity and investigate basic sites' strength and distribution. The basicity curve is shown in Fig. 6(b). The quantitative analysis of surface-adsorbed CO2 was conducted using the total peak area under the curves, and the findings are reported in Table 1. Nuanced surface characterization of porous materials is made possible by the adsorption of probe molecules, and molecules with certain characteristics (such as basic or acidic) can interact with the surface active sites that are inside the pores or between the layers. Therefore, we further characterized the basic sites by using pyrrole as a probe molecule in the DTG curve (Fig. 6(c)) and FTIR-KBr curve (Fig. 6(d)). The proportion of basic sites may be used to gauge the total basicness, leading to the following pattern: 3Ti3Cu > 4Ti3Cu > Ti. This trend proves that the pure Ti had the weakest desorption peaks of CO2, indicating its basicity was very weak and more basic sites are in the sample with higher Cu containing, while the basic strength of the sites is nearly the same. As shown in Fig. 6(b), the CO2-TPD curves can be separated into three sorts of peaks equivalent to weak (50–250 °C), moderate (250–600 °C) and strong (>600 °C) basic phases for the catalysts. By moving the basic site peaks from 675 oC to the stronger area at 698 oC, the addition of Cu2+ to TiO2 not only increases the value of overall basicity but also alters the distribution of basic sites. Catalysts with higher Ti concentrations (Ti and 4Ti3Cu) occupied strong basic strength within the weak, medium, and strong areas. However, the 3Ti3Cu catalyst had almost the same peaks at weak and medium regions, but one big intensity in the high-temperature range after 600 °C. This finding implies that because of the fundamental properties of the catalysts, the impregnation of Cu and Ti may promote additional surface sites for CO2 adsorption.The Pyrrole-DTG profile (Fig. 6(c)) illustrated that the peak intensities observed at the 200–500 °C region match the trend of CO2-TPD and Pyrrole-FTIR spectra (3Ti3Cu > 4Ti3Cu > Ti). Pyrrole forms an adsorption layer on the Brønsted sites via the cycle's two subsequent carbon atoms, and these layers have substantially lower adsorption energies than Lewis sites. Pyrrole is an amphoteric molecule that may function as a proton acceptor through its π electron orbital or a proton donor to interact with basic sites on the surface. Surface basic sites interacting with pyrrole are caused by the stretching and bending vibrational modes of surface formate (both −CH and −COO) and carbonate species over CuO and TiO2 constituents, according to the Pyrrole-FTIR spectra. Bands at 1165 cm−1, 1481 cm–1, and 1736 cm–1 were attributable to the C–CO–C stretch and bending, absorption of the phenyl ring, and C═O groups in amorphous Cu, respectively. There is a small shoulder at 1442 cm–1, which can be proven that the Cu cause to increase in the Lewis basicity of the catalysts. This observation could explain that the increase in the basicity determined by CO2-TPD with higher Cu charges is attached to the higher contribution of Lewis basic sites. The prevailing consensus is that Lewis basic sites correspond to oxygen anions with poor coordination formed as basic sites following the calcination stage. The coordination of the Lewis sites linked to O−2 anions determines their basicity. The oxygen atoms in the crystal corners have to be more basic than the oxygen atoms on the crystal faces or the edges [55]. The samples with smaller "crystal sizes" (as displayed in Table 1) should consequently have larger concentrations of Lewis sites with a low coordination number, and as a result, the basicity should be higher. Both Lewis acidic sites and Brønsted acidic sites may interact with pyrrole, as shown by the adsorption of this molecule on catalysts in its H+ form and on components in its alkali cation form, however, only the framework oxygen atoms can connect with this molecule at basic sites [56]. Hence, we further studied the Pyridine-FTIR and Pyridine-DTG for the acidity analysis.However, it is noteworthy that Brønsted and Lewis's acid sites detected upon Pyridine-FTIR spectra and Pyridine-DTG curves exhibited low acidity compared to Ti profile, probably due to the acidic –OH on the catalyst surface. It can be seen that the band at 1542 cm−1 for the Ti profile ( Fig. 7(a)) verified the existence of weak Brønsted acidic sites by developing pyridinium ions which also proved the existence of surface hydroxyl clusters [57]. Meanwhile, the incorporation of Cu on the Ti by impregnation technique increases the number of basic sites, weakening the acid possessions of 3Ti3Cu and 4Ti3Cu catalysts (Fig. 7(a) and (b)). The impregnation procedure resulted in the insertion of the Cu into the Ti structure and the creation of tiny Cu particles, which affects the number of acid sites due to the contact interface with other oxides, according to XRD data (Fig. 3). This weak Brønsted acidity hinges on the character of Cu phase species and influences plastic cracking reactions, including C–N bond cleavage. Another possibility is that the collapse of Cu's layered structure decreased the total acidity, which originated from the interlayer protons [58]. Thus, it is anticipated that the basic molecule's characteristics, the type of edge, and Ti's presence would affect how the proton transfers.In a fixed-bed reactor setup, all of the reforming tests were conducted. Compared to other methods for producing H2 fuel, in-situ pyrolysis-catalytic steam reforming is more complicated, primarily due to many coke precursors and carbon in plastic and phenol. All catalysts were first applied to a continuous reaction. The conversions of phenol and the production of H2, Co and CO2 (in yield and mole percent) are shown in Fig. 8. The properties of the pyrolysis products for the 3Ti3Cu catalyst are listed in Fig. S2. Based on these observations, the conversion of phenol could be attributed to the transformation of the phenol molecule into H2 formation via steam reforming reaction (Eq. 5). Low reforming performance was achieved under catalyst-free conditions (not shown). The H2 yield and mole percent of pyrolysis-catalytic steam reforming after adding the bare Ti catalyst was 56.8% and 68.4%, respectively. H2 production of the 4Ti3Cu catalyst was higher than the Ti sample, indicating that the Cu component played a key role in pyrolysis-catalytic steam reforming for H2 production. At the same time, by associating the catalytic performance of Ti, 4Ti3Cu, and 3Ti3Cu, it can be found that the phenol conversion for the bare Ti catalyst was 84.9%, which was lower than those of Cu -added catalysts. The reason behind this might be due to the detection of Ti-Cu alloy as shown in the XRD analysis. Ashish and Qiang [59] mentioned that the bimetallic alloys cause higher catalytic effectiveness than their monometallic complements, owing to strong interaction among the metals. Therefore, for the 4Ti3Cu catalyst, the phenol conversions increased significantly compared to the pure Ti. Meanwhile, 3Ti3Cu showed the highest phenol conversion (92.6%) and H2 yield (67.8%), indicating that higher Cu loading has a catalytic activity in the in-situ pyrolysis-catalytic steam reforming process. This heightened activity of reducible 3Ti3Cu catalyst could be because of the strong metal-support interaction, higher reducibility, strong basicity and higher amount of sites analyzed by H2-TPR and CO2-TPD, respectively. Additionally, the phenol conversion result indicates that the 3Ti3Cu catalyst with a smaller crystallite size quickly adsorbs and activates phenol molecules compared to the catalyst with a large crystallite size. It means that catalysts with large crystallite sizes (as shown in Table 1 from XRD analysis) are not preferred because they cause higher coke formation [60,61] and cause catalyst deactivation. The higher basic site results in increasing the reaction rate [62]. The results as mentioned above suggest that the strong basic sites should be catalytically active sites in this base-catalytic reaction. When Cu was added to the catalyst, more surface basic sites were created, which improved CO2 adsorption and carbonate synthesis. The carbonate was then hydrogenated by H2 that was adsorbed on and activated by the Ti metallic sites. Strong basic sites created by the introduction of Cu species activated the hydroxyl group in phenol, increasing phenol conversion and H2 selectivity. In the H2-TPR, the Cu2+ site was formed in the 3Ti3Cu catalyst due to the reducible characteristics of the Cu material. This resulted in a strong electron-donating property, a typical strong metal support interaction effect that was beneficial to the catalytic activity. Previous research [63,64] stated that strong metal support interaction effect in good decoration of metal on the support, which is favourable in catalytic performance. Since basicity was shown to grow throughout the 3Ti3Cu sample, it can be inferred from the data above that basicity and activity are directly related. In fact, significant basicity that correlated with the most active catalyst was detected at high Cu concentrations. Therefore, Cu addition modifies the catalyst's acid function, which is primarily responsible for the carbon production, and positively impacts activity and catalytic stability by preventing carbon deposition on the catalyst's surface. Fig. 14 displays the results of a TGA study of the utilized catalysts, further supporting this claim. We performed the influence of temperature and time on stream tests, as shown in Fig. 9 and Fig. 10, believing that the increase in stability is predicted given the combination of the chemical and physical features of the 3Ti3Cu nano-catalyst.The effects of reaction temperature on the catalytic performance of 3Ti3Cu are shown in Fig. 9. The H2, CO and CO2 yields, mole percent and phenol conversions were obtained after a 6 h evaluation for each temperature (6 runs and each run around 1 h) as a function of temperature. The slightly reverse water gas shift reaction (Eq. 7: CO2 +H2→CO + H2O) was promoted by increasing the temperatures. The situation that CO contents after reforming also increased with increasing temperatures could be related to the slight promotion of endothermic reactions. However, CO and CO2 yield generally does not change appreciably compared to H2 yield within the whole temperature range. The enhancement of phenol steam reforming reaction (Eq. 6: C6H5OH+5 H2O⟶6CO+8 H2) by temperature can be observed as phenol conversion, and H2 yield increased from 92.6% and 67.7% at 500 oC to 99.7% and 97.90% at 800 oC, respectively. This result suggests that the reaction process was dominated by phenol steam reforming reaction (PSR) (Eq. 6), which controls the final product distribution. Fig. 9 and Fig. 9 show that the reaction temperature of the 3Ti3Cu catalyst for the reforming of phenol may be reduced and that improved catalytic activity can be achieved, resulting in a decrease in the cost of catalyst manufacturing. In light of this, it is clear that phenol steam catalytic reforming for the generation of H2 is advantageous and that low-temperature catalytic reforming is feasible. Hence, this study confirms the feasibility of H2 generation from the in-situ pyrolysis-catalytic steam reforming reaction of PSW dissolved in phenol at temperatures less than 600 °C, which is practically more sustainable and requires less energy.To further investigate the stability of the precious metal-free Ti-Cu nano-catalyst, the endurance experiments were conducted over the 3Ti3C catalyst at 500 °C for 45 h. The changes in phenol conversion and H2 yields and mole percent as a function of time on stream during the in-situ pyrolysis-catalytic steam reforming reaction of PSW dissolved in phenol are shown in Fig. 10. The catalyst shows a highly stable behavior for all time ranges in terms of mole percentage. The yields of CO and CO2 hardly changed over 3Ti3C, evidencing better stability. In contrast, H2 yield and phenol conversion decreased from 67.8% and 92.7% at the first hour to 59% and 89.9% after 25 h time-on-stream. The rapid decrease of the conversion into the gas phase indicates the deactivation of the 3Ti3C catalyst; consequently, this pattern during a 25 run has prompted us to conduct a longer-term evaluation of the catalyst's resistance to the reaction. In spite of the severe reaction conditions and decrease in catalytic performance after 25 h of time-on-stream, an improvement can be observed at 30–45 h with a slightly decreased in H2 yield and phenol conversion, which may be related to an occurrence of slight deactivation of catalysts by carbon deposits [65,66]. The article will go into more detail on how the deactivation of the catalyst is related to the deposition of coke, the sintering and aggregation of active metal particles, and other factors.The main products analyzed from the prominent peaks in the GC/MS chromatogram are listed in Fig. S2. The chromatogram of GC–MS technique for the 3Ti3Cu nano-catalyst confirmed that three value-added components were produced upon the pyrolysis reaction of PSW-phenol. Nevertheless, the mass spectrometer's limitations prevent it from detecting low molecular weight gases. The catalytic pyrolysis products were classified into aromatic compounds such as ethylamine and oxygenated aromatics such as tert-butyl hydroperoxide and benzene, (1,1-dimethylethoxy) (BDE). Tert-butyl hydroperoxide (TBHP), an alkyl hydroperoxide with a tert-butyl group, was found to be the major liquid result of the pyrolysis process of PSW-phenol. It is frequently employed in several oxidation processes. It functions as an oxidizing agent and an antibacterial agent. For example, to produce chain-elongated peroxides, Chuan et al. [67] described a practical Fe-catalyzed decarbonylative alkylation-peroxidation of alkenes using aliphatic aldehydes and TBHP. To produce α-ketoamides, Xiaobin and Lei [68] described a brand-new and effective TBHP/I2-promoted oxidative coupling process of acetophenones with amines. Therefore, in addition to a small amount of BDE, value-added components such as TBHP can also be found in the pyrolysis liquid product of PSW dissolved in phenol. The pyrolytic products were further analyzed by FTIR analysis, and the results are shown in Fig. 11.The produced liquid component from the in-situ pyrolysis-catalytic steam reforming reaction of PSW dissolved in phenol using 3Ti3Cu nano-catalysts was also straight analyzed by FTIR to categorize the dominating functional clusters, and the results are shown in Fig. 11. FTIR can be used to analyze the chemical composition and the optical properties of the material and to better understand the release characteristics of liquid products after the reaction. All samples showed a broad band spectrum at 401, 424, 447, 864, 1049, 1165, 1296, 1620, and 2955 cm−1 wavenumber and FTIR results with the peak intensities are shown in Fig. 13. Below 3000 cm−1, one band is detected at 2955 cm−1, which is attributed to aliphatic vibrations −CH2− [69,70]. The absorption bands at 1620 cm–1 correspond to the CC stretching mode of carbonyls and indicate the presence of compounds containing aromatic rings [71,72] and benzene (C═C) in specific [73]. Also, aliphatic C–O stretching was observed at 1296 cm–1 [74]. This peak is also ascribed to the detection of the aldehydes, alkanes, and ethers [75]. The band at 1165 cm−1 was assigned to the stretching vibration of C–O–C [76]. In accordance with the previous research [77], C–H stretch in methyl, methylene, and methyne groups can be confirmed at the spectral range around the band at 1049 cm–1. Rainer et al. [78] stated that this peak could also be assigned to the C–O–C symmetric stretching in aliphatic groups and acid derivatives. In addition, there were four autocorrelated peaks at 864, 447, 424, and 401 cm–1 that confirmed the formation of out-of-plane C–H bending vibration peak for the aromatic ring [79], νCN stretching band (νCN) [80], τRing (ring torsion) and γCN (out-of-plane bending or wagging) [81], and deformation modes [82] (which is connected with torsions and bending of benzene ring [83]), respectively. The C–C group is hardly ever present throughout the pyrolysis process, indicating that the C–C break is exceedingly challenging. Heating rates can impact the CO and C–H groups.Several characterizations, including BET surface area, N2 adsorption-desorption isotherms and pore size distribution, TGA, DTG, CHNS, and TEM, were performed to investigate the deposited carbon and metal sintering over the used catalysts. An evaluation of the textural characteristics of fresh and spent catalysts (surface areas, pore volume, and average pore size) is presented in Table 2. N2 adsorption–desorption isotherms and pore size distribution of spent catalysts are also illustrated in Fig. 12. The monolayer-multilayer adsorption on the material's interior surfaces is responsible for the initial portion of the curve (at low P/Po). It is possible to explain the sharp rise in isotherm slope for high P/Po over 0.90 by capillary condensation inside the pores, followed by saturation when the pores get saturated with liquid [84,85]. All the spent catalysts present almost the same Type-IV isotherm and an H1 hysteresis loop to the Ti and 4Ti3Cu and double-hysteresis loops for the 3Ti3Cu. This means the mesoporous nature of the catalysts after the reaction is maintained, and the reactions did not substantially impact the main pore structures of the catalysts. The pore size distribution for spent Ti and 4Ti3Cu catalysts was positioned in the region of 5 − 15 nm, while it was 17 − 35 nm for the 3Ti3Cu catalyst. The decrease in specific surface area of Ti is large (> 70%) with the smallest diameter pores (20 nm), which indicated the surface and pores of the Ti catalyst were blocked with ashes, carbon, residues or poison reactants. The weakened surface area of the Ti catalyst in comparison with the fresh catalyst might also be because of the collapsing of microchannels in the catalyst structure by the collision of energetic particles during the reaction procedure [86,87]. The surface area and total pore volume of the 4Ti3Cu and 3Ti3Cu catalysts increased after the reaction; probably, an acidic treatment by phenol compound occurred during the reaction, which resulted in the removal of blockage metal on pores. This opens up the pores in the used catalyst that were closed by dangerous metals, increasing the specific surface area of the used catalyst in comparison to the new catalyst. This would increase the access of reactant molecules to the active site of the catalyst within the pores and increase the activity. The better performance of the 3Ti3Cu catalyst could be due to their enhanced basicity strength and the stable catalytic performance compared to the Ti, and 4Ti3Cu catalysts could be attributed to their structural stability during the reforming reaction.One of the traditional techniques in FTIR spectroscopy has been around from the beginning and includes pressing or grinding a tiny quantity of a dry solid sample with powdered IR-transparent substances like halide salts, with KBr being the most popular [88]. Fig. 13 shows the outcomes of using FTIR as a surface analysis approach to detect functional groups in the used catalyst, focusing on unsaturated hydrocarbons (CC linkages) and aromatics. All samples of spent catalyst were pelletized with KBr following the same procedure described in Section 2.2. The FTIR peak of the spent catalysts in 3757 cm−1 fits to the surface-linked hydroxyl groups; this peak is steadily enhanced with higher Cu content, proving the catalysts have been regenerated. Also, the samples present a slender but distinct vibration near 2376 cm−1, which is characteristic of the ν3 elongation mode of linearly adsorbed CO2 on Cu2+. This peak can also be assigned to stretching vibrations of the reactants' secondary –N = H– groups [89]. Due to the C═O stretching of the carboxyl groups, the spent catalysts displayed distinct bands at 1628 cm−1. As the intensities increase, more carboxyl groups are produced. The fact that this peak has a higher intensity indicates that Cu 's incorporation causes the creation of basic sites, which is compatible with its amphoteric capabilities. These bands could be related to species of H2 carbonates created when CO2 interacts with hydroxyl groups. A very weak sharp absorbance peak at 926 cm−1 peaks could be ascribed to the asymmetric stretching vibrations of Ti–O–Ti bonds that increased in intensity by additional Cu. The absorbance peaks placed at 548 cm–1 and 494 cm−1 corresponded to the TiO2 lattice and signified the presence of functional groups like metals [90] and related to symmetric stretching vibrational modes [91], respectively. The 548 cm–1 band might also be associated with O–Ti–O and Ti–O–Ti modes overlapping with O–Cu and Cu–O–Ti bonds because it has high intensities for the, spent 3Ti3Cu and 4Ti3Cu catalysts. The two absorbance peaks at 525 and 656 cm–1 for the bare Ti and 4Ti3Cu spent catalysts correspond to the bending of Ti−O vibrations ( and could also be related to the detection of V═O bonds [92]) and the bending of –OH in the –C–OH group [93], respectively. In the 3Ti3Cu spent sample, two strong absorption peaks at 849 cm−1 (probably related to the vibrations of the aromatic cycle [94]) and 710 cm−1 (can be ascribed to rocking modes of −(CH2)n– alkyl chains with n ≥ 4 [95]) were detected which have not appeared for the Ti and 4Ti3Cu catalysts. The bands at 849 cm–1 are involved in Ti═O stretching vibrations of orthorhombic Ti8O16 and could also represent the symmetric Ti–O stretching ν1. The band at 710 cm−1 might also be originated from in-plane bending of the CO3 2− group [96].Large organic compounds that contain C−H or C−C bonds might be trapped in the cages of acidic catalysts, which invariably results in catalyst deactivation under reaction circumstances. The in-situ pyrolysis-catalytic steam reforming process generates large organic compounds. Therefore, one crucial economic goal for the industrial use of these catalysts is to comprehend coke production. Herein, we analyzed the coke formation after reaction by TGA, DTG and CHNS techniques. Thermogravimetric analysis (TGA) is a thermal analytical technique extensively used to understand and measure the amount of coke deposited on the catalyst after the reaction. DTG gives information about the rate at which these coke are removed concerning time or temperature. The elemental analyzer (CHNS) is a technique used for quantitative determination in organic samples containing carbon and other basic elements of nature. The TGA and DTG curves of the spent catalysts are shown in Fig. 14, and quantitative weight loss data (taken from TGA and CHNS) is shown in Table 2. The weight loss processes could be separated into three phases and mentioned by weight loss (WL). The vaporization of moisture caused the first phase below about 200 °C, the second phase (WL2) between 200 and 600 °C was attributed to the burning of deposited coke, and the third phase (WL3) above about 600 °C belonged to the decomposition of remaining residues and heavy carbonaceous species. The in-situ pyrolysis-catalytic steam reforming reaction of PSW-phenol over Ti catalyst formed the highest amount of coke (8.2 wt%) with 56.2% of the total weight loss. The most significant mass drop for the Ti spent sample was at 77 oC, caused by the release of phenol and water molecules. The coke analysis results obtained for the Ti catalyst revealed that there is a significant interaction between Ti catalyst pore structure (pore size and shape) (as analyzed by pore size distribution in Fig. 2(b), XRD crystallinity in Fig. 3 and Table 1). The intensity of acid sites (Fig. 8) greatly affects Ti coke formation and deposition in converting PSW-phenol to H2. In the first stage, the weight loss for the 4Ti3Cu and 3Ti3Cu took place at the temperature of 91.7 oC and was 7.6% and 6.6%, respectively. The DTG curve of the 4Ti3Cu catalyst showed a broad peak at 752 °C, which was associated with light and heavy composites inside the 4Ti3Cu catalyst. The pyrolysis of phenol and the production of tiny derivatives from the fracture of PSW should yield light chemicals. Nevertheless, the bio-oil derivatives or the big aromatics made from the PSW structure were given credit for the heavier compounds. In both WL2 and WL3 phases, there was no coke development on the surface of the TGA curves of the Ti and 3Ti3Cu samples. The weight rise in this catalyst profile may have been caused by the oxidation of the metallic, active sites [10,23]. The DTG curve of the 3Ti3Cu sample shows almost straight like in all stages, indicating that the sample was stable in the entire temperature range. The lack of mass loss percentage in the experiment with 3Ti3Cu can be due to its low BET surface area (see Table 1) and slit-shaped pores structure. The slit-shaped pores structure has an advantage in contribution to the diffusion of oxygen during coke combustion [97]. This is perhaps due to the fact that the high content of Cu increases the basic site on the catalyst surface too much, which is not conducive to the phenol reforming reaction. The lowest weight loss for the 3Ti3Cu might also be due to the strongest metal-support interaction and strongest basic sites. The carbon content of the spent catalyst also follows the catalyst reducibility and basicity (Fig. 6) characterizations. The lowest carbon content has been taken place for the catalyst with the highest H2 consumption and CO2 uptake (Table 1). As a result of the intrinsic fundamental properties of 3Ti3Cu, which operate as a sponge for CO2 absorption, local gradients in gas concentration are generated, which promotes improved process efficiency. In fact, relative to rates of cracking side reactions, basic sites are thought to produce substantially higher rates of products and reagents desorbing from the catalyst surface [98]. The aforementioned characteristics of 3Ti3Cu made it an optimum catalyst for the in-situ pyrolysis-catalytic steam reforming reaction of PSW dissolved in phenol.Using the TEM method, we investigated the coke deposition over the employed catalysts in this study. The three forms of carbon that are deposited on the catalysts are amorphous (T ≤ 570 °C), filamentous (570 °C < T < 1000 °C), and graphitic (≥1000 °C). At the lowest temperature range of T570°C, amorphous carbon is produced, then filamentous carbon at T570°C to T1000°C, and lastly, graphitic carbon at T1000°C [99]. The morphology results of the spent 3Ti3Cu catalyst are depicted in Fig. 15, which illustrates that the reaction produced carbon with random shapes and morphologies, but mainly filamentous carbon or carbon nanofibers (CNFs) on the catalyst surface were observed. The diameters of CNFs are in the range of 20–80 nm, and pores have widened due to the pores filling. The 3Ti3Cu catalyst exhibits a similar tubular shape and sphere-like geometry with particle sizes of 20–50 nm. The catalyst revealed the existence of Cu particles on the surface, which were corroborated by well-dispersed, small particle TEM images. In contrast, the grey forms on the surface are associated with TiO2 components. The CNFs and filamentous coke did not block catalyst particles, although its progressive deposition may hinder the contact between reactants and catalyst active sites. Additionally, due to the strong Cu–Ti interaction, the Cu was not removed by the growing CNFs, and very low fragmented particles were observed. A catalyst may get fragmented during the in-situ pyrolysis-catalytic steam reforming reaction process as a result of internal catalyst reactions where carbons are first created and then developed. Smaller metal particle sizes likely helped to generate the thinner CNFs, and the thinner CNFs had greater surface energies, making them less stable. This makes it very simple to remove CNFs made with smaller Cu particles, which may be why the 3Ti3Cu catalyst exhibits the least carbon deposition in TGA. The figure clearly illustrates a nearly perfect core–shell morphology was obtained. The different shell thicknesses with spherical cores with around 30 nm diameters result from the reaction. This reaction produced a core shell-like structure with an approximate shell thickness of 8.2 nm. The key benefits of core-shell nanoparticles are that the core material's qualities, such as reactivity, may be reduced, and its thermal stability can be altered, increasing the core particle's overall stability and dispersibility. The shell materials, which can provide surface chemistry for further modifying and functionalizing the nanoparticles, are another advantage of core-shell structures. In conclusion, the in-situ pyrolysis-catalytic steam reforming process of PSW dissolved in phenol may be sustained by catalysts with increased basicity, and the catalyst can prevent complex carbon deposition.To summarize, nano-sized precious metal-free Ti-Cu catalysts were successfully synthesized by direct hydrothermal conditions and impregnation method and innovatively applied in the in-situ pyrolysis-catalytic steam reforming reaction of PSW liquefied in phenol. This study is expected to offer an essential research reference for designing precious metal-free Ti-Cu and producing H2 and valuable liquid fuel from the abovementioned reaction. As-prepared samples were analyzed by XRD, BET, CO2-TPD, pyrrole-DTG, pyrrole-FTIR, pyridine-FTIR, pyridine-DTG, TEM, H2-TPR, FTIR-KBr and the used samples were characterized by TGA-DTG, FTIR-KBr, BET, TEM and CHNS. The liquid product was also analyzed by GC/MS and FTIR systems. The morphology study suggested that Ti-Cu ensembles may be formed due to the interaction between Ti and Cu that lead to the structure of CuTi3 alloy, as confirmed by XRD analysis. Samples with higher Cu contents cause to decrease in the surface area, crystal sizes, partial collapse of ordered mesoporous structure and the growth of the average pore diameter. However, Cu results in higher reducibility, metal support interaction and basic sites of the catalysts with highly dispersed Cu2+ species in 4Ti3Cu and 3Ti3Cu samples. Cu2+ insertion into TiO2 changes the distribution of basic sites as well as the total basicity value. The 3Ti3Cu performed the best catalysis performance, highest phenol conversion, H2 production and lowest coke formation due to strong metal-support interaction, higher reducibility, strong basicity and higher amount of sites analyzed by H2-TPR and CO2-TPD, respectively. The H2 selectivity rose, and the coke quantity dropped from 4Ti3Cu and 3Ti3Cu due to decreased acid sites and increased basic sites, providing increasingly continuous catalytic activity. The catalytic pyrolysis products were classified into aromatic compounds, such as ethylamine, and oxygenated aromatics, such as tert-butyl hydroperoxide and benzene, (1,1-dimethylethoxy). These results may help in the development of more effective solid base catalysts for a variety of base-catalyzed processes.W. Nabgan First author, contents development, writing. H. Alqaraghuli Experimental section. B. Nabgan Writing, editing. T.A. Tuan Abdullah Supervision, English editing. M. Ikram Writing (Section 2), editing. F. Medina Comments and editing. R. Djellabi Writing, Revision, response to reviewer, proofreading and English 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 principal author, Walid Nabgan, is thankful for the support from Universitat Rovira i Virgili under the Maria Zambrano Programme (Reference number: 2021URV-MZ-10), Proyectos de Generación de Conocimiento AEI/MCIN (PID2021-123665OB-I00), and the project reference number of TED2021–129343B-I00. The authors are also grateful for the support given by Universiti Teknologi Malaysia (UTM) allocation budget in Pusat Pengurusan Makmal Universiti (PPMU) laboratory.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcatb.2022.122279. Supplementary material .

In-situ pyrolysis-catalytic steam reforming reaction of polystyrene waste (PSW) liquefied in phenol can generate hydrogen from phenol and valuable liquid fuel from the PSW and thus has been studied recently. However, due to the complexity of phenol compounds and plastic waste, this reaction suffers from high energy consumption and coking. Herein, Ti, 4Ti3Cu and 3Ti3Cu nano-catalysts were facilitatively prepared using hydrothermal and impregnation methods and the physical and chemical properties of the fresh and used samples were deeply characterized. The experimental results show that almost complete phenol conversion with 97.90% H2 yield was achieved at 800 oC using 3Ti3Cu nano-catalyst. The catalytic pyrolysis products were ethylamine, tert-butyl hydroperoxide (TBHP) and benzene (1,1-dimethylethoxy) (BDE). The correspondence of the preparation, morphology and catalytic activity in this research elucidates the synthesis of anti-coking and stable nano-catalysts for in-situ pyrolysis-catalytic steam reforming reaction.

The authors declare that the data supporting the findings of this study are available within the article and the supplemental information. The data and results supporting the present study are available from the lead contact upon request.Because of the rapid expansion of the world’s population and industrialization, which cause rapid increases in the demand for energy, new energy sources are desired that should be of great abundance and have less of an impact on the environment. 1 , 2 , 3 As a green, promising protocol, energy conversion via the electrochemical route is attracting more and more attention. 4 , 5 , 6 Electrocatalysts that are high performing and cost economic are critical for large-scale implementation of various electrochemical processes like water splitting and energy transformation in fuel cells. For the latter, unfortunately, the sluggish kinetics of the oxygen reduction reaction (ORR) on the cathode are a major limiting factor for the electrocatalytic cycles. 7 Up until now, Pt-based catalysts have proven to be the best choice in mediating the ORR process, while use of noble metal prevents these catalytic systems from industrial applications. 8 , 9 Thus, great effort has been made in development of low-Pt-based or non-precious metal electrocatalysts in efficient ORR processes.Transition metal-based M-N-C catalysts possess significant ORR activities with their definite coordination structures for active sites and highly adjustable electronic states and are regarded as the most promising alternatives to Pt-based precious-metal catalysts. 10 , 11 Among them, 3d transition metal-based catalysts, such as Fe, Co, Ni, and Cu, as active sites, hosted by carbon frameworks represent the frontier design of electrocatalysts. 12 Active metal sites with unoccupied 3d orbitals efficiently interact with oxygen intermediates for ultrahigh intrinsic activity, and conductive and porous carbon skeletons promote electronic conduction. 13 However, the formation of two-electron reaction products with lower efficiency as well as the fast performance degradation because of leaching of 3d transition metals have not been completely resolved. 14 In contrast, M-N-C catalysts based on 4d and 5d transition metal centers exhibit better long-term stability because of the stable arrangement of outer shell electrons, while their unsatisfactory sluggish ORR kinetics activity is caused by the coordination and electronic structure of the active metal site. 15 As a 5d Group 6 transition metal with a valency between −2 and +6, tungsten shows on the left side of the volcano plot, possessing strong oxygen binding energy. 5 , 16 Hence, tungsten (W) affords unusual performance in the ORR because the O and OH species are difficult to desorb from the W center. 17 In fact, W-N-centered single-atom catalysts (SACs) have been reported previously to be promising in the ORR. For example, Chen et al. 18 reported that the highest ORR catalytic activity could be obtained by a coordination number 5 when W coordinates only with N. Bisen et al. 19 prepared an efficient and durable ORR electrocatalyst with a WN2C2 center, which adsorbs oxygen and the reaction intermediates with moderate binding energies. In addition, Jiang et al. 17 also reported a W-N-C catalyst prepared via pyrolysis of W-doped ZIF-8 material. At present, the development of W-SACs regarding the ORR faces two challenges: (1) precise regulation of the coordination environments for atomic W and (2) substantial improvement of the catalytic activity relative to the Pt-based catalysts. 20 On the other hand, transition metal nitrides (TMNs) may serve as alternative functional materials in energy storage and electrocatalysis because of their excellent physicochemical properties. 21 , 22 Wang et al. 23 reported a facile and in situ nitriding method to obtain a unique W nitride cluster loaded on a 2D conductive g-C3N4 material (WN@g-C3N4-750); here, the cubic-phase WN and the small clusters over the g-C3N4 layer are responsible for the excellent ORR performance of the catalyst. Indeed, the electronic structure of W atoms can be tuned by nitrogen atoms, and the catalytic activity be promoted by increasing the number of W-N bonds. 24 , 25 In spite of this, most TMNs show invalid catalytic activity because of their poor intrinsic activity and low density of active sites. 22 The intrinsic activities of nitrides mainly rely on their crystalline phase, which is related to the crystal orientation and the particle size. 26 , 27 Here, we report an elaborately designed ORR catalyst with a concerted W-N single-atom site and WN nanoclusters. By complexation with phthalonitrile, W atoms are first anchored in a phthalocyanine-typed material (WPc). g-C3N4 synthesized from melamine, the most common raw material, was mixed and pyrolyzed at 700°C. The W ions are first chelated by phthalonitrile and then anchored onto g-C3N4 support after further pyrolysis destroys the WPc structures; consequently, the W-N single-atom sites and WN nanoclusters are afforded and anchored on the 2D N-doped carbon materials. Such a material exhibits performance comparable with commercial Pt catalysts while benefitting from low cost and high durability. The origins of the excellent performance of the W-N catalysts are discussed.A large number of MN4-macrocyclic compounds, such as metallo-porphyrin, metallo-corrole, and metallo-phthalocyanine complexes, have been used in electrocatalysis. 28 , 29 Transition-metal phthalocyanine complexes (MPcs) possess a highly conjugated macro-cyclic planar structure that exhibits excellent chemical stability in electrolysis. 30 , 31 Further, the delocalized π electrons over the entire macrocycle participate in MPc-mediated oxidation and reduction, exhibiting unique activity. 31 Thus, The MPc species may serve not only as excellent catalysts but also as raw materials to prepare structurally well-defined W-SACs. With a defective 2D structure, g-C3N4 is widely used as a photocatalyst and in energy conversion devices. 32 The six-fold cavities of g-C3N4 can not only anchor single atoms or clusters but also metal complexes. 33 , 34 , 35 Further, although pure g-C3N4 is of low conductivity, a certain degree of decomposition of g-C3N4 at suitable temperatures produces in situ amorphous, graphite-like structures, thus enhancing its conductivity. Our conception originates from the combination of the characteristics of WPc and g-C3N4.First, WCl6, phthalonitrile, DBU, and 1-pentanol were added to a hydrothermal kettle along with nitrogen at 0.5 MPa and heated to 220°C for 4 h (Figure 1 A), yielding a dark-green dyestuff substance. After precipitation overnight, green W species (WPc) were obtained via filtration and washing with methanol. The control sample without metal W (denoted NPc) was also synthesized by an identical procedure as WPc, without adding W. As shown in Figures 1B and 1C, the Fourier transform infrared (FTIR) spectrum of this substance shows a peak at 950.3 cm−1, indicative of a W-N bond, 36 whereas the X-ray diffraction (XRD) pattern of WPc is rather different from that of NPc. The W 4f spectrum (Figure 1D) in the X-ray photoelectron spectroscopy (XPS) shows that W resides in an unsaturated coordination environment, pointing to the presence of WPc. The g-C3N4 was prepared by heating melamine to 550°C and holding it for 4 h. WPc and g-C3N4 were mixed thoroughly in methanol at a certain ratio for 4 h to form a light-green homogeneous W-nitrogen precursor (WPc/gC3N4). The mixture was next placed in a tube furnace and heated under the protection of argon to obtain the final product, named WNPc/gC3N4-T (T is the pyrolysis temperature).The XRD patterns of WNPc/gC3N4-T (T = 600, 700, and 800) and other relevant reference samples are shown in Figure 2 A. The pyrolysis precursors WPc/gC3N4 and WNPc/gC3N4-600 have two distinct diffraction peaks with angles of 13.7° and 27.5°, respectively. The diffraction peak at 13.7° is caused by repetition of the 3-s-triazine structure in the graphite-phase carbon nitride structure, while the sharp peak at 27.5° is caused by stacking of the (002) crystal plane in the graphite-phase carbon nitride conjugate plane without W. It shows that the W species is uniformly distributed on the g-C3N4 carrier, and aggregation does not occur. In addition, the main structure of W2NPc/gC3N4-600 is proven stable at 600°C despite the mass loss, as indicated by the thermogravimetric (TG) analysis (Figure 2B). With the continuous increase in temperature, g-C3N4 was severely decomposed and graphitized; meanwhile, the so-released carbon- and nitrogen-containing fragments interact with the WPc raw material to form a brand-new structure. WNPc/gC3N4-700 shows distinct peaks arising from graphitic carbon compared with WPc/gC3N4 and WNPc/gC3N4-600. It is worth noting that faint peaks appeared around 36.76°, 62.42°, and75.22°, closely following the WN (PDF#75-1012). Interestingly, these peaks are lower than the standard pattern of WN (PDF#75-1012) because of the extreme distortion of the atomic clusters and lattices via strong interaction with the carriers. 37 Another small peak at 39.62° may belong to the W2C (PDF#35-0776), corresponding to a rather low content. 38 The loading content of W in WNPc/gC3N4-700 is around 4.22 wt %, as analyzed by inductively coupled plasma mass spectrometry (ICP-MS). Part of the formation of W carbide is inevitable because of the possibility of reaction between W and carbon atoms at high temperature. Data on appropriate precursor mixing ratios are presented in Table S1 and Figure S1, which is crucial to get the perfect WNPc/gC3N4-700. Different mixing ratios of WPc and g-C3N4 affect the W content of the product after high-temperature pyrolysis. Only at a specific ratio can the best WNPc/gC3N4-700 be obtained. Because of the high mass loss in the pyrolysis steps, too low or too high a mixing ratio will result in high W content in the final product, leading to aggregation of W elements, subsequently affecting catalytic performance. Ammonia treatment can further modify the coordination environment of W 18 and facilitate formation of WNs, as indicated by comparison of the spectra of WNPc/gC3N4-NH3-700 and WNPc/gC3N4-700. When the temperature continues to rise, the peaks around 31.62°, 35.78°, 48.5°, 64.38°, 65.88°, 73.74°, 76.12°, and 77.44° in WNPc/gC3N4-800 indicate that the W atoms are completely transformed into WN (PDF#25-1256). In the case of WPc-700 obtained via direct pyrolysis of WPc, there were more diffraction peaks, indicating that addition of g-C3N4 facilitates the dispersion of WNPc/gC3N4-700 samples that were prepared by direct heating of g-C3N4 and phthalocyanine without W, and only graphitized peaks at 26° and 44° were observed in XRD spectra.The double D band (1,342 cm−1) and G band (1,599 cm−1) in Raman spectra of WNPc/gC3N4-700 and NPc/gC3N4-700 (Figure 2C) confirm partial graphitization of the carbon support, and the intensity ratio (ID/IG = 1.08 versus 1.13) also indicates a higher defect level.The nitrogen adsorption-desorption isotherms of WNPc/gC3N4-700 and NPc/gC3N4-700 are depicted in Figures 2D and 2E. The specific surface areas, calculated using the BET method, are 177.173 and 455.672 m2/g, respectively. They exhibit similar type IV isotherms and a slightly different pore size. This suggests that the higher temperature and metal phthalocyanine lead to shrinkage or disappearance of the pore of g-C3N4. 32 , 37 , 39 XPS studies were carried out to analyze the surface chemical composition and elemental valence states. The peaks belonging to the different orbitals of C, O, N, and W can be distinguished from each other (Figure 3 A). WNPc/gC3N4-700 shows a high ratio of N originated from the raw materials; O comes from the residual alcohol solvent, and the presence of air or free oxygen in the instrument is also responsible for the predominance of C and O elements in the sample. 23 The C 1s spectrum of WNPc/gC3N4-700 (Figure 3C) reveals sharp peaks corresponding to 283.60 eV (W-C), 284.8 eV (C-C), 285.82 eV (C−N), 287.57 eV (C=N), and 289.60 eV(C=O), indicating successful doping of N in the sample and generation of W carbide. 19 , 40 The N 1s spectrum (Figure 3D) can be divided into four peaks located at 398.34 eV (pyridinic N), 399.49 eV (W-N), 400.73 eV (graphitic N), and 403.23 eV (oxidized N), respectively. Graphitic N can greatly increase the limiting current density, while pyridinic N might convert the ORR mechanism from 2e−- to 4e−-dominated processes. 41 The difference between WNPc/gC3N4-700 and NPc/gC3N4-700 indicates a binding energy of 399.49 eV, which is attributed to W-N coordination. 23 , 42 It is widely accepted that the pyridinic N and pyrrolic N moieties are prone to coordinate with W to form WN x complexes. 19 The W 4f spectrum of WNPc/gC3N4-700 (Figure 3B) features four pronounced peaks: the peaks at 32.4 eV and 34.47 eV are mainly indicative of W-C bonding and attributed to W carbide impurities. 43 By contrast, the peak aera of the W-N bond is greater than that of W-C (86.54% versus 13.46%), while the peaks at 35.57 eV and 37.64 eV are responsible for the W-N site. In addition, the W-N bonding in WNPc/gC3N4-700 is slightly lower than that in WPc, indicative of more unpaired electrons on coordinatively unsaturated sites. 18 Further, the structure and morphology of WNPc/gC3N4-700 were investigated by scanning electron microscopy (SEM). The morphology of the sample shown in Figure 4 C is a graphene-like substance after partial pyrolysis at high temperature, and the large number of sheet-like folds shows that pyrolysis results in deformation and shrinkage of the surface. 32 , 37 Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was used to observe the samples. A HAADF-STEM image (Figures 4D–4F) at higher magnification shows a large number of bright atomic dots and gathered light spots, indicating the presence of single atoms (yellow circle) and nanoclusters (red circle, diameter size in the range of 0.52–1.42 nm). The selected area electron diffraction (SAED) pattern proves the absence of a crystal phase. Well-dispersed W-N single atom and cluster sites are thus justified. We also characterized WNPc/gC3N4-NH3-700 that was obtained via ammonia treatment at high temperature (Figures 4G and 4H); the dispersed W started to agglomerate, showing a thin WN-related crystal, as indicated by the XRD pattern.X-ray absorption near-edge structure (XANES) and X-ray absorption fine structure (XAFS) spectroscopy were performed to study the electronic structure and local chemical configuration of the WNPc/gC3N4-700 sample. The XANES spectra of WNPc/gC3N4-700 were different from those of WPc, WO3, WC, and W powder (Figure 4A). Furthermore, the Fourier-transformed (FT) k3-weighted χ(k)-function of the EXAFS spectra in R-space suggested that the W species were W-N and W-N-W in WNPc/gC3N4-700 (Figure 4B). Among them, the peak at nearly 1.78 Å is attributed to the W-N in W single-atom sites (W-SACs) and WN nanoclusters (WN-NPs), whereas the peak at nearly 2.67 Å suggested W-N-W in WN-NPs. Least-squares curve fitting was conducted to acquire quantitative structural parameters around W atoms (Figure S2); the corresponding fitting results are shown in Table S2. XAFS spectra of WNPc/gC3N4-700 with different W content are shown in Figure S3. These characterizations prove that the W atoms exist as mononuclear and multinuclear centers.Cyclic voltammetry (CV) measurements for 20% Pt/C and WNPc/gC3N4-700 in alkaline electrolytes were performed to clarify the catalytic performance, as illustrated in Figure 5 A. Both electrodes display an ORR peak around 0.80 V in 0.1 M KOH solution, indicating high ORR activity. The ORR performance of different samples was evaluated systematically in O2-saturated 0.1 M KOH at 1,600 rpm and a scan rate of 10 mV/s using rotation disk electrode (RDE) measurement. As shown in Figure 5B, the pyrolysis temperature is an influential factor, and the activity of WNPc/gC3N4-700 is better than that of WNPc/gC3N4 (T = 600°C, 800°C). Notably, WNPc/gC3N4-700 exhibits extraordinary catalytic activity: the half-wave potential E1/2 (0.835V) is comparable with that of 20% Pt/C, while the mass diffusion-limited current (0.596 mA/cm2) is even better than that of 20% Pt/C that reaches the upper limit in alkaline solutions. At the same temperature, the activity of WPc-700 is much lower than that of WNPc/gC3N4-700, indicating the important role of the g-C3N4 carrier. Compared with previous reports on ORR processes mediated by W-SAC or WNs, the synergy of atomic W and small WN clusters is most likely responsible for the rather high activity of WNPc/gC3N4-700 toward the ORR. The reduced activity of the WNPc/gC3N4-NH3-700 sample can be attributed to the loss of single-atom sites and small WN clusters. The reason why it still has relatively good catalytic activity is that there still exist a small number of monatomic active sites in the catalyst, and the crystal-phase WN also has certain activity. 23 The double-layer capacitance (Cdl) evaluated by CV at different scan rates of 20–100 mV/s in the non-faradic region is followed by the respective electrochemical active surface area (ECSA) and roughness factor (Rf), and it turned out that the combination of WPc and g-C3N4 at high temperature favors generation of active areas (Figure S4; Table S3). The similar ECSA before and after NH3 treatment also indicated that differences in catalytic activity arise from changes in the active site rather than the active surface area.Usually, the low Tafel slope means that the rate-limiting step following the first electron transfer and easy-to-achieve high current at low overpotential are beneficial for the ORR. 18 , 44 Interestingly, as shown in Figure 5C, the Tafel slope of WNPc/gC3N4-700 is 40.47 mV dec−1, which is lower than that of other samples and is almost half that of 20% Pt/C (75.52 mV/dec), confirming the O2 adsorption/desorption step as the kinetically fastest process in the ORR at WNPc/gC3N4-700. 45 Rotating ring disk electrode (RRDE) measurement was conducted to investigate the ORR mechanism of the samples (Figures 5E and S5); this method has been used to evaluate the electron transfer and selectivity in electrochemical reactions. 18 The electron transfer number of WNPc/gC3N4-700 reaches 3.90 at 0.25–0.8 V with a yield of peroxide species (HO2 −) below 5%, only slightly lower than that of Pt/C (3.90–3.95) at the same range; this indicates a dominant 4-electron process with high energy conversion efficiency. The electron transfer number of WNPc/gC3N4-700 was calculated by the K-L equation, being 3.92 at 0.3–0.7 V (Figure S6), suggesting that the 4-electron transfer process dominates the electrochemical reaction.Durability and stability are also important metrics for a catalyst. Figure 5F shows the ORR polarization curves of the best-performing electrocatalyst WNPc/gC3N4-700 and commercial Pt/C before and after 5,000-cycle voltammograms. There is an almost similar negative shift of polarization curves. On the other hand, chronoamperometry measurement was adopted to test the durability of WNPc/gC3N4-700. As shown in Figure 5G, the WNPc/gC3N4-700 electrode exhibits better stability, with only 12.52% attenuation of activity compared with 42.25% current decay of 20% Pt/C after 60,000 s. Given the methanol tolerance problems of commercial Pt/C, WNPc/gC3N4-700 and 20% Pt/C were subjected to a methanol tolerance test. As shown in Figure 5H, in contrast to the sharp current density decay for the Pt/C electrode, the current density in the experiments with WNPc/gC3N4-700 is unaffected by methanol injection, indicating that WNPc/gC3N4-700 shows good methanol tolerance in the ORR process. Thus, WNPc/gC3N4-700 outperforms 20% Pt/C in terms of stability and durability.Furthermore, the ORR activities of these catalysts are also evaluated in acidic electrolyte (0.1 M HClO4) at a rotation speed of 1,600 rpm. As shown in Figure 5D, the performance of WNPc/gC3N4-700 is close to that of 20% Pt/C. It is noteworthy that the difference in polarization curves between WNPc/gC3N4-NH3-700 and WNPc/gC3N4-700 under acidic compared with basic conditions is much higher, further illustrating the importance of W single-atom sites and small WN clusters.Next, to justify the origins of the excellent performance of WNPc/gC3N4-700, the reaction energy profiles and the associated electronic structures were interrogated with theoretical calculations, as displayed in Figures 6 , S7, and S8. Based on the characterization results, two models were proposed as the possible reaction center: the W single-atom sites with pyrrolic-N2 coordination (W-SAC) and a WN particle size of ∼1nm (WN-NP). As shown in Figure 6, the ORR process may undergo six coordinates: (1) 4 ( H + + e − ) + O 2 + ∗ , (2) 4 ( H + + e − ) + O 2 ∗ , (3) 3 ( H + + e − ) + O O H ∗ , (4) 2 ( H + + e − ) + O + H 2 O ∗ , (5) ( H + + e − ) + O H + H 2 O ∗ , and (6) ∗ + 2 H 2 O . In general, W-SAC and WN-NP matter for different steps of the ORR process. According to the computational results, WN-NP outperforms W-SAC on the first two steps; i.e., adsorption of O2 and initial formation of the O–H bond. For example, at the WN-NP site, the adsorption energy of O2 is much higher, while the barrier for generating ∗OOH is much lower. A reversal takes place when coming to the last two steps: in formation of ∗OH from ∗O, the associated barrier at W-SAC is lower compared with the one at WN-NP, but the difference is tiny; finally, the reduction of ∗OH to release water at W-SAC is energetically more favorable. Considering that the step O H + e − → O H − ∗ is barrierless and reversible, WN-NP most likely facilitates the reduction of O2 to ∗OH, and W-SAC accelerates the final water release step.In summary, a polymorphic W system has been elaborately constructed using WPcs and g-C3N4 as the precursors under accurately controlled temperature. The so-prepared W-N catalyst performs encouragingly in catalyzing the ORR process. The WNPc/gC3N4-700 species exhibits efficiency comparable with the commercial Pt/C catalyst but is much more durable. This excellent performance has been attributed to the synergy of the W-SACs) and WN-NPs). Our work may point out an interesting possibility to design and prepare active, stable, non-noble-metal ORR catalysts. Further exploration is indicated to fine tune the SAC:NP ratio for commercial and industrial availability.Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Shaodong Zhou (szhou@zju.edu.cn).This study did not generate new unique materials.In a typical synthesis, W chloride (1.0 g), phthalonitrile (1.29 g), and 1.0 mL of 1,8-diazabicyclo(5.4.0)undec-7-ene were dissolved in 20 mL of 1-pentanol in a hydrothermal kettle (the volume was 100 mL). Before the reaction, nitrogen was introduced into the hydrothermal kettle to replace the air, and the nitrogen pressure in the kettle was controlled at 0.5 MPa. Then, the mixture was slowly heated for 50–60 min from room temperature to 220°C, and the temperature was then held for a further 180 min. The mixture of products was then cooled, transferred to a large flask with added excess methanol, and stayed overnight. The powder precursor was separated by suction filtration and washed several times with methanol until the color of the filtrate turned light green. Finally, the product was dried into a green powder at 60°C.In a typical synthesis, melamine (10 g) was heat-treated at 550°C with a heating rate of 3°C under flowing Ar gas for 240 min to obtain the g-C3N4.In a typical synthesis, 0.03 g WPc and 0.65 g g-C3N4 are mixed in methanol solution at 60°C for 4 h, evaporated, and dried at 70°C.The WPc/gC3N4 precursor was pyrolyzed at T (T = 600°C, 700°C, 800°C) with a heating rate of 6°C under flowing Ar gas for 120 min to obtain the WNPc/gC3N4-600, WNPc/gC3N4-700, and WNPc/gC3N4-800.The chemical information and preparation of other samples are shown in the supplemental experimental procedures.XRD patterns were conducted on a Rigaku Ultima IV using nickel-filtered Cu Kα radiation of wavelength 1.5406 Å with a scanning angle (2θ) of ∼10°–80°, operated at 40 kV and 40 mA.XPS measurements were carried out on a Thermo Scientific K-Alpha equipped with an Al anode (Al Kα = 1,486.6 eV), operated at 12 kV and 6 mA. Energy calibration was carried out using the C1s peak of adventitious C at 284.80 eV.The Raman spectra were recorded on a Horiba HRE Volution with an excitation laser of 532 nm.FTIR measurements were carried out by Thermo Scientific Nicolet iS20, and the scans (number of scans, 32) were collected with a resolution of 4 cm−1 from 4,000 to 400 cm−1.SEM measurements were performed with Sigma300 field-emission scanning electron microscope.The weight ratios of W elements were measured by ICP-MS measurements on an Agilent 7700(MS).TG was performed on a TA-Q500 at a temperature range from 50°C–850 °C at a heating rate of 10°C min−1.BET surface area analysis was performed using an AUTOSORB-IQ2-MP BET surface analyzer.The HAADF-STEM images were obtained in FEI Titan G2 80-200 ChemiSTEM operated at an acceleration voltage of 200 kV, which was equipped with a high-brightness field-emission gun (X-FEG), double spherical aberration corrector, and monochromator.Electrochemical characterization for linear scan voltammetry was performed on a CHI760D electrochemical station (Shanghai Chenhua, China) using a standard three-electrode system in 0.1 M KOH/HClO4 solution. Typically, the work electrode was a glassy-carbon rotating disk electrode (RDE; 0.196 cm2) and a glassy-carbon RRDE (0.247 cm2) with a Pt ring, and the counter and reference electrodes were graphite rod and Ag/AgCl electrodes (3.5 M KCl solution), respectively.A dispersion including 3 mg non-precious catalysts, 1.5 mg carbon black (Super-P), 0.490 mL H2O, 0.490 mL ethanol, and 0.02 mL 5 wt % Nafion was sonicated for 30 min to get a homogeneous ink. Then the prepared catalyst ink with a loading of 0.3 mg cm−2 was transferred to the glassy carbon electrode and dried naturally. The RDE polarization curves were recorded with a scan rate of 10 mV/s at 1,600 rpm in O2-saturated 0.1 M KOH/HClO4 electrolyte. The net current was calculated by subtracting the background capacitance measured in Ar-saturated solution. All potentials were recorded with iR compensation and then referred to the RHE ( E R H E = E A g / A g C l + 0.059 ∗ p H + 0.2046 V ). The electron transfer number (n) and hydrogen peroxide yield (H2O2%) can be calculated by the following equations, where N is the collection efficiency of the Pt ring (N = 0.37), and Id and Ir are the disk and ring current, respectively: (Equation 1) n = 4 × I d I d + I r N (Equation 2) H 2 O 2 ( % ) = 200 × I r N I d + I r N The kinetic current density (jK) was calculated based on the following Koutecky-Levich equations: (Equation 3) 1 j = 1 j L + 1 j K = 1 B ω 1 / 2 + 1 j K (Equation 4) B = 0.62 n F C 0 D 0 2 / 3 γ − 1 / 6 where j is the measured current density, jL is the diffusion-limiting current density, ω is the angular velocity of the disk electrode (rpm), F is the Faraday constant (96,485 C mol−1), n is the electron transfer number in the reaction process, C0 represents the bulk O2 concentration (1.2 × 10−3 mol L−1), D0 is the diffusion coefficient of O2 in electrolyte (1.9 × 10−5 cm2 s−1), and γ is the kinematic viscosity of 0.1 M KOH (0.01 cm2 s−1).The accelerated stability testing was measured in O2-saturated 0.1 M KOH solution at a scan rate of 50 mV/s and rotation speed of 1,600 rpm.The accelerated durability testing was performed in O2-saturated 0.1 M KOH solution using the chronoamperometric method at 0.60 V for 60,000 s.The ECSA and Rf was calculated by the CV curves with different scanning rates. The non-faradic current measured was plotted as a function of the scan rate to obtain Cdl. Then, the ECSA was calculated according to the equations (Equation 5) E C S A = C d l C s (Equation 6) R f = E C S A S where Cdl is the capacitance for the sample, the value of specific capacitance (Cs) is 0.04 mF/cm2 in alkaline solution, and S is the geometric area of GCE (0.196 cm2).The X-ray absorption fine structure spectra (W L3-edge) were collected at the 1W1B station in the Beijing Synchrotron Radiation Facility (BSRF). The storage rings of the BSRF were operated at 2.5 GeV with an average current of 250 mA. Using a Si (111) double-crystal monochromator, the data collection was carried out in transmission/fluorescence mode using an ionization chamber. All spectra were collected under ambient conditions. The acquired EXAFS data were processed according to standard procedures using the ATHENA module implemented in the IFEFFIT software package. The k3-weighted EXAFS spectra were obtained by subtracting the post-edge background from the overall absorption and then normalizing with respect to the edge-jump step. Subsequently, k3-weighted χ(k) data of W L3-edge were Fourier transformed to real (R) space using a Hanning window (dk = 1.0 Å–1) to separate the EXAFS contributions from different coordination shells. To obtain the quantitative structural parameters around central atoms, least-squares curve parameter fitting was performed using the ARTEMIS module of the IFEFFIT software package. A k3 weighting, k-range of 2.8–12.8 Ǻ−1, and R range of 1.35–3.35 Å were used for the fitting. The four parameters coordination number, bond length, Debye-Waller factor, and E0 shift (CN, R, σ2, and ΔE0, respectively) were fitted without any one being fixed, constrained, or correlated. 46 , 47 , 48 The structural optimization and frequency analysis were performed at the GFN2-xTB level of the xTB package (v.6.2). 49 , 50 More accurate single-point energies were obtained at the r2SCAN-3c 51 level of theory using the ORCA 5.0 package. 52 Stationary points were optimized without symmetry constraint, and their nature was confirmed by vibrational frequency analysis. Unscaled vibrational frequencies were used to correct the relative energies for zero-point vibrational energy (ZPVE) contributions. The PDOS analysis was performed using the Multiwfn 3.7 package. 53 We acknowledge financial support from the National Natural Science Foundation of China (21878265). We are also grateful to the Institute of High Energy Physics, Chinese Academy of Science, for XAFS measurements and help with XAFS measurements collected at the 1W1B station in the Beijing Synchrotron Radiation Facility (BSRF), China.Conceptualization, C.Z. and S.Z.; methodology, C.Z., W.A., and L.Y.; formal analysis, C.Z., S.Z., and L.Z.; investigation, C.Z., W.A., and L.Y.; resources, S.Z., C.Q., and M.L.; writing – original draft, C.Z., S.Z., and L.Z.; writing – review & editing, C.Z. and S.Z.; funding acquisition, S.Z., C.Q., M.L., and L.Z.; supervision, S.Z.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.xcrp.2023.101288. Document S1. Figures S1–S8, Table S1, and supplemental experimental procedures Document S2. Article plus supplemental information

The oxygen reduction reaction (ORR) constitutes an important transformation process in fuel cells. Here we report a polymorphic tungsten catalyst with a concerted tungsten single-atom site (W-SAC) and tungsten nitride nanoclusters (WN-NPs) for mediating the ORR processes. The catalyst is prepared via pyrolysis of phthalocyanine-typed material (WPc) with g-C3N4. Synergy of the W-SAC and WN-NP sites over the designed 2D g-C3N4 layer is responsible for the competitive ORR performance of the catalysts. Consequently, the WNPc/gC3N4-700 sample gives a half-wave potential of 0.835 V, a Tafel slope of 40.47mV dec−1, and a mass diffusion-limited current of 0.596 mA/cm2; such results are comparable with and even better than that of 20% Pt/C. Furthermore, WNPc/gC3N4-700 also outperforms 20% Pt/C in terms of stability and durability. Our work may point toward a design strategy for active, stable, non-noble-metal ORR catalysts.

Data will be made available on request.Faced with the forthcoming depletion of fossil resources and the huge environmental impact related to their consumption, there is an urgent demand for an irreversible transition where these fossil resources are replaced by more sustainable compounds. One of the fundamental axes to achieve this green revolution is based on the transformation of residual biomass, based on oxygenated feedstock, into biocompounds that partially replace oil and petrochemical resources [1–3]. Within the wide variety of biocompounds [4], bioalcohols are easily produced with high yields and offer diverse and attractive functionalities, positioning themselves as one of the most important biomass derivatives blocks [1]. In recent years, four carbon linear alcohols, specially the straight-chain alcohol with the -OH group in the terminal position, 1-butanol (hereafter referred just as butanol or BuOH), is attracting growing attention due to its advantages as a fuel compared to ethanol, the bioalcohol with the highest implanted production. For instance, butanol has a higher energy density, assimilable to that of gasoline, higher heating value, lower volatility, is less hygroscopic, less corrosive and has a higher viscosity than ethanol. Furthermore, it is compatible with the currently used ignition engines which means it is easier to implement with some safety, sustainable and economic advantages compared to bioethanol [5,6]. Therefore, in recent years global scientific purposes have merged to overcome the higher production costs compared to ethanol when lignocellulosic feedstocks are used as raw materials [7]. These efforts have been mainly focused on the optimization of the metabolic bioengineering of the ABE (Acetone-Butanol-Ethanol) fermentation process in order to upgrade the tolerance of the microorganism to a higher butanol concentration [8–12]. Other efforts have been devoted to improving the sustainable butanol yields in the catalytic chemical routes [13], standing out the ethanol Guerbert condensation reaction [14,15].Moreover, butanol exhibits interesting applications not only in the fuel industry but also for the generation of added-value products [16,17]. On one hand, some valorization reactions where the oxygen functionality is retained allow the synthesis of aldehydes, esters, ethers or fatty acids such as butyraldehyde [18], butyric acid or the corresponding linear ether, with wide applications in the solvents, polymers or lubricant markets [17]. On the other hand, the most promising route for butanol upgrading implies the intermolecular dehydration giving rise to 1-butene, which is easily isomerised to a C4 olefins mixture, either directly or going through the di-n-butyl-ether (DNBE) as an intermediate ( Scheme 1). Nowadays, the extensively used C4 olefins (n-butenes, i-butene, butadiene) are mostly obtained as by-products in FCC units and classified in function of its composition in the so called C4 raffinates. Some predictions point to a further demand rise outpacing the supplies growth from fossil feedstocks [19], making it increasingly profitable to source via a sustainable route. In addition, this butanol transformation is the first step in the alcohol-to-jet-fuel process; once butanol is dehydrated into C4 olefins these can be further oligomerized and hydrogenated to obtain jet fuel cuts [20]. Nowadays, the C4 stream is usually subjected to a separation process to maximize the olefin composition to n-butenes, i-butene or butadiene. However, when butanol is used as a feedstock, the olefins composition can be tailored by modifying the reaction conditions. For instance, the single-step production of i-butene is thermodynamically limited to high temperatures (>350 °C) and requires stronger acid sites than those needed for linear olefins [21]. While an oxidizing atmosphere and a dehydrogenation functionality favour the butadiene yield from butanol [22].As it is generally accepted, alcohol dehydration reactions are catalyzed by acidic sites. In the case of butanol dehydration to butenes (1-butene and the two isomers of 2-butene), it is usually performed under atmospheric pressure and mild temperature conditions. Although enough temperature is required to restrict the DNBE yield, which is thermodynamically favoured at lower temperatures [23–25]. Different catalysts have been tested, either based on Lewis or Brønsted acid sites, being both active but with a clear different catalytic behaviour. Grouping the catalysts by the nature of their acid sites, those mainly formed by Lewis or Brønsted acid sites, the model studied catalysts are represented by γ-alumina [26–28] or the acidic form of the zeolite ZSM-5, respectively. Although the former exhibits satisfactory reaction stability, much further contact times and higher reaction temperatures are usually needed to achieve high butenes selectivity [24,29–31]. However, this required elevated temperatures can lead to a decrease in butenes selectivity due to the formation of lower molecular weight compounds such as methane, ethylene or propylene [32]. Although catalysts based on Brønsted acid sites are much more active [30,33], they frequently exhibit high deactivation rates, accompanied by a loss of butenes selectivity due to secondary reactions [32,34]. For example, Palla et al. [34] obtained a full conversion (WHSV=15 h−1, 250 °C) but with a moderate selectivity towards butene (75 %). In this context, some authors have addressed this issue by combining both types of acid sites in the catalyst’s formula. De Reviere et al. [29] obtained slightly lower conversion values employing a hybrid catalyst based on nano H-ZSM5/Al2O3 in comparison with the same zeolite (60 % vs 80 %) at 240 °C, but with substantial improvements in terms of stability. Other researchers have modified the catalyst surface by the introduction of a higher quantity of Lewis acid sites, some of them with a greater strength [32]. Almost underrated in the literature, catalyst’s deactivation plays a critical role to be considered in the industrial implementation of 1-butanol dehydration process. Butanol is a less reactive molecule in comparison to its C4 isomers and shorter chain alcohols, therefore higher catalyst’s acidity is required to be activated. This fact, along with its higher molecular weight, multiplies the tendency to form coke precursors that lead to significant deactivation rates during the dehydration reaction [35,36].Acidic zeolites are by far the most studied materials for this dehydration reaction with a widespread use of the acidic form of the zeolite ZSM-5 [30,36–41]. Furthermore, the larger molecular size of butanol in comparison to lower alcohols, has revealed clear influences in terms of selectivity and stability when different zeolite topologies are used, obtaining the best performance with a MFI zeolite [38,42]. In addition to the examples already given, other acidic catalysts for the synthesis of butenes through butanol dehydration have been explored, such as amorphous aluminosilicate [41], CeO2-TiO2/carbon composites [43,44], tungstated zirconium [33], modified mesoporous silica [30] or phosphate modified carbon nanotubes [45]. A group of solid acid catalysts that have attracted much attention in recent years, due to their outstanding structure and strong Brønsted acidity, are the polyoxometalates (POM), also known as heteropolyacids (HPA). Among them, those with Keggin structure stand out for their extraordinary performance in several industrial acid-catalyzed reactions and oxidation catalytic processes [46,47]. The Keggin structure contains an heteropolyanion stabilized by acidic protons with the formula [XM12O40]n-, standing X for the heteroatom (usually P5+,Si4+) and M for the addenda atom (Mo6+, W6+). In detail, the heteropolyanion is composed of a central tetrahedron XO4 surrounded by 12 MO6 octahedra. These Keggin units in their hydrated form are coordinated with crystallization water molecules forming a body-centered cubic structure with the Keggin anions at the lattice points and acidic H2O5 + bridges along the faces [48]. On one hand, the main advantages offered by this complex structure are strong and uniform acid sites, as well as their ease of tuning by modifying their constituents. On the other hand, the major shortcomings of HPAs are related to their very low surface area and poor thermal stability, unable to cope with the highly demanding regeneration treatments typically required in these acid-catalyzed reactions. Several strategies have been devoted to overcome this matter related to coke formation, ranging from the development of thermally resistant HPAs to the performance of structural modifications to avoid coke formation or to enhance coke combustion [49].One of the unique features of HPAs is their so-called pseudoliquid behavior, which allows polar molecules to enter the crystal structure and react in the bulk. In a series of articles [48,50–52], Gaigneaux's group deeply explored this pseudoliquid performance, resulting not only in the exploiting of the HPAs bulk but also favouring the coke inhibition and reversing its deposition thanks to a smart pre-treatment strategy. They applied this approach in the gas phase methanol dehydration to dimethylether (DME), which involved subjecting the HPA to proper conditions to partially replace the water of crystallization with methanol. Unfortunately, the same authors stated that this treatment was not extensible to butanol dehydration, due to the comparative increase in hydrophobicity and molecule dimensions. Accordingly, they attempted to partially substitute the acidic protons with NH4 + to activate the bulk of the HPAs [53]. Although they achieve to exploit the bulk of the HPA, the strategy was just totally successful at low temperature, thus only producing the intermolecular dehydration product, DNBE. Consequently, it seems that to develop a sufficiently active catalyst for butanol dehydration, the remaining and plausible strategy to exploit most of the acid sites in HPAs is still their dispersion on a suitable support. The use of HPAs in butanol dehydration have been mainly focused on obtaining DNBE, showing activity and selectivity values similar to the model catalysts, Amberlite [54,55]. The HPAs have been also used unsupported, taking advantage of its uniform strong Brønsted acidity to modulate the reaction mechanism [56–58]. Recently, the effect of the HPA loading have been studied supported on silica, reaching almost completely conversion and selectivity to linear butenes (300 °C, WHSV of 37,4 h−1 ) [59] and on TiO2 [60], showing high improvements of the catalytic activity in a photoassisted catalytic reaction.Here we report the catalytic performance −in terms of activity, selectivity, stability, and regeneration ability− of a series of the synthesized catalysts, emphasizing the role of HPA-support interactions, during the gas phase dehydration reaction of butanol to butenes. Two HPAs namely H4SiW12O40 (STA) and H3PW12O40 (TPA) were deposited on two commercial carbonaceous supports: an activated carbon (AC) and a high surface area graphite (HSAG). The STA was also dispersed over silica, alumina and zirconium oxide, for comparative purposes. The loading of the HPAs was as low as 15 % to evaluate the HPA-support interactions. The catalyst activity was compared against a zeolite HZSM-5 with a Si/Al ratio= 23. Our results evidence that graphite-STA interactions efficiently tailored the acidity resulting in an active regenerable catalyst.Heterogeneous HPAs based catalysts have been synthesized by incipient wetness impregnation from a EtOH/H2O solution with a 1:1 volumetric ratio containing the appropriate polyoxometalate quantity. The HPAs employed in this work were those with a remarked high acid nature, the Tungstophosphoric acid (TPA) (H3PW12O40·nH2O; Sigma-Aldrich) and the Silicotungstic acid (STA) (H4SiW12O40·nH2O; Sigma-Aldrich). The minimum amount of solvent to fill the pores of each support was used to solve the amount of HPA, reaching a nominal content of 15 wt%. The solution was carefully deposited dropwise on the two carbonaceous materials. The commercial activated carbon (AC, SBET= 1190 m2/g) was produced from olive stones by Oleicola el Tejar, Córdoba (Spain) and pretreated with hydrochloric acid to remove the inorganic impurities. The high surface area graphite was provided by Timcal (HSAG, SBET=400 m2/g). These carbonaceous supports were impregnated with both HPAs obtaining the following catalysts: TPA/HSAG, TPA/AC, STA/HSAG and STA/AC. The support effect was studied by immobilizing the STA on different oxide supports, that is: alumina purchased from Degussa (Al2O3, SBET= 173 m2/g), zirconium oxide supplied by Melcat (ZrO2, SBET=100 m2/g) and silica from Fluka (SiO2, SBET= 433 m2/g), giving rise to the following catalysts: STA/Al2O3, STA/ZrO2 and STA/SiO2, respectively. Upon impregnation, these samples were dried in an air oven at 110 °C overnight to drive off the volatile components within the solution. In order to obtain the acidic form of the ZSM-5 zeolite, the ammonium form (CB 2314, Zeolyst international) was subjected to thermal treatment under static air (5 h, 550 °C).Textural properties of the supports and catalysts were evaluated from N2 adsorption-desorption isotherms. The measurements were carried out at 77 K using a Micromeritics 2020 ASAP equipment, before the measurement the samples were outgassed in vacuum at 423 K for 16 h. The Brunauer-Emmet-Teller method was used to calculate the mesoporous external specific surface area, while the microporous surface was evaluated using t-plot method. The total pore volume was estimated from the adsorbed amount at a relative pressure of 0.97 and the average pore size was determined by the BJH method using the desorption branch.X-ray diffraction analysis (XRD) were performed on a Polycristal X ´ Pert Pro Pananalytical diffractometer with Ni-filtered Cu/Kα radiation (λ = 0.1544 nm) operating at 45 kV and 40 mA. In each measure, Bragg´s angles between 4 and 90º were scanned at a rate of 0.04 deg/sec. Structural properties were also characterized by Fourier transform infrared spectroscopy in attenuated total reflectance disposition (FTIR-ATR) with a JASCO FT/IR 4800 spectrometer equipped with a DTGS detector and a germanium crystal. A total of 170 scans per spectrum were recorded between 400 and 4000 cm−1 with a resolution of 4 cm−1.Acidity characterization of the samples was done in a Micromeritics Autochem II 2920 equipment. The samples were pre-treated in helium flow at 623 K in situ prior to the analysis. Subsequently, total number of acid sites were established by NH3 pulse chemisorption analysis, at 393 K to avoid physisorption artefacts, until complete saturation of the sample. Finally, the samples were flushed in helium for 1 h, then the temperature was raised to 623 K and hold for 1 h whilst NH3 desorption were recorded (NH3-TPD). Acid site strengths were classified into weak (393–523 K), moderate (523–623 K) and strong acid sites (>623 K).The 3,3-dimethyl-1-butene (33DM1BN) isomerisation model reaction was carried out to evaluate the Brønsted acidity of the catalysts. In a typical experiment, 100 mg of the catalyst (sieved between 0.35 and 0.5 mm) was fixed with glass wool in a U-shape glass reactor and pre-treated in situ at 275 ºC during 30 min under continuous N2 flow (30 cm3/min). Thereafter, the catalyst was stabilized at the reaction temperature (150ºC) performing the reaction with the same N2 flow saturated after bubbling into pure 33DM1BN at 0ºC (P33DM1BN=20.4 KPa). Not converted reactant and the isomerized products were analyzed by online gas chromatography (Varian 3400) equipped with a flame ionization detector (FID) and a 20 % BMEA S/Chrom p.80/100 column. The catalytic activity (A) normalized per gram of active phase (per gram of HPA or zeolite) was calculated as below: A mmol 33 DM 1 BN min ∙ g a . p . = C 33 DM 1 BN ∙ F 33 DM 1 BN in 100 x g a . p . where F 33 DM 1 BN in and g a . p . stands for the 33DM1BN inlet flow expressed in mmol/h and the HPA or zeolite weight in each sample (active phase), respectively. The initial activity values were obtained at time= 0 after linearization of the catalytic activity versus time. C 33 DM 1 BN is the conversion calculated using the equation: C 33 DM 1 BN ( % ) = F 33 DM 1 BN in − F 33 DM 1 BN out F 33 DM 1 BN in ∙ 100 where F 33 DM 1 BN out is the 33DM1BN molar flow at the outlet.Thermogravimetric analysis of the fresh and used catalysts were done using a SDT Q600 (TA instruments) under synthetic air atmosphere (Fair = 100 mL/min) from 30ºC to 800ºC and a 10ºC/min heating ramp.The typical experiment was carried out into a stainless steel fixed-bed reactor. The reaction was tested at 275 ºC under atmospheric pressure, using 30 mg of catalyst and a constant flow of inert gas (He, 100 mL/min) and reactant (liquid BuOH, 0.04 mL/min injected by HPLC pump). The weight of the catalyst was adjusted in the selectivity studies performed at different temperatures, aiming to obtain similar conversion values. Before reaction all catalysts were treated at the same reaction temperature (275 ºC) during 30 min. The gas products were analysed online by means of a gas chromatograph (450-GC) equipped with thermal conductivity (TCD) and flame ionization (FID) detectors. The compounds with high boiling point (BuOH and DNBE) were condensed and collected every reaction hour. These liquid samples were weighted and analysed using the same gas chromatograph, but in this case only the FID detector was used.The butanol conversion C BuOH ( % )  and selectivity to a product i ( S i % were estimated employing the following equations, respectively: C BuOH ( % ) = F BuOH in − F BuOH out F BuOH in ∙ 100 S i % = n i F i F BuOH in − F BuOH out · 100 where F BuOH in represents the molar flow of butanol fed to the reactor, F BuOH out is the butanol molar flow at the reactor exit expressed in mmol/min, F i is the molar flow of the product i and n i the stoichiometric factor of the product i relative to BuOH. The catalytic activity (A) was defined as: A mmol BuOH min ∙ g a . p . = C BuOH ∙ F BuOH in 100 x g a . p . The carbon mass balance (CB) was determined as: CB % = Σ i n i F i + F BuOH out F BuOH in · 100 The nitrogen adsorption-desorption isotherms at 77 K for the bare supports and the synthesized catalysts are depicted in Figs. S1-S5. A summary of the textural properties, including the pore volume and the average pore size of the supports and catalysts, is listed in Table 1. Although the diverse nature of the supports induced great differences in the shape of their hysteresis loops, the isotherms of all the supports can be classified as type IVa (IUPAC), characteristic of mesoporous materials, except for the AC. The latter constitutes a typical type I isotherm although there is also a contribution of a type IVa isotherm due to the existence of mesoporosity, as denoted by the presence of the H4 type hysteresis loop. These textural features are distinctive of this category of micro-mesoporous activated carbons [61]. As for the HSAG, exhibited a type H3 loop typical of materials where the mesoporosity is caused by plate shape sheets, while the oxide supports, Al2O3, ZrO2 and SiO2, generate a H2 type loop. All supports denoted relatively high specific surface areas, ranging from 100 m2/g for the zirconia to 1025 m2/g for the activated carbon (Table 1). Even though the AC surface seems to stand out, the mesoporosity contribution was in the same range as the rest of the supports. After HPAs deposition, apart from the reduction of the microporous branch in the AC-based catalysts, the shape of the isotherms remained like those of the supports (Figs. S1-S5), although significant changes were detected in the textural parameters, summarized in Table 1. All samples manifested some reduction of the specific surface area, being especially remarkable for the microporous surface contribution of the TPA/AC catalyst. This loss of porosity suggests the partial blockage of micropores by HPA particles. As for the plate-shape material, HSAG, the incorporation of the HPA prompted a reduction in the BET surface area from 396 m2/g to 286 m2/g (TPA/HSAG) and to 272 m2/g (STA/HSAG). Although less obvious, the mesoporous oxides (SiO2, Al2O3 and ZrO2) likewise exhibited a reduction in the surface area after HPA impregnation, coherent with the partial coverage of their mesopores.The X-ray diffractograms of the carbon-supported HPA catalysts are represented in Fig. 1, along with the pristine HPA and either the AC (Fig. 1a) or the HSAG (Fig. 1b) support. In general, the XRD patterns of the HPA-catalysts did not show the diffraction maxima characteristic of the HPAs except in the case of TPA/AC (Fig. 1.a). This latter displayed some distinctive reflections of the TPA, which suggests the presence of larger crystallite particles of this heteropolyacid. In every case a broad and diffuse peak centered around 2Ɵ= 7° is appreciated, typical of the HPA with a raised level of hydration [31]. The same trend is observed in the X-ray diffraction patterns of the oxide-supported HPAs (Figs. S6-S8), where no diffraction lines characteristic neither the HPAs nor other species, such as those of the former oxides e.g. WO3 (with reflection lines at 2Ɵ= 23º, 24º, 34, 42º), are appreciated. Hence, these diffractograms point to a finely dispersion of the HPAs thorough the supports. This is in concordance with previous reported studies, where no crystallinity is reported until high HPA loadings are reached; for example, above 20 wt% on silica [62] or greater than 40 wt% if a carbon support is employed [63].FTIR-ATR spectroscopy is a sensitive technique to detect the vibrations of the molecular bonds, thus was performed aiming to confirm the retention of the Keggin structure in the synthesized catalysts. A magnification of the characteristic fingerprint region for the TPA-based catalysts (700–1200 cm−1) is represented in Fig. 2.a. The bands that appeared at 1071, 970, 900, 761 cm−1 in the spectrum of the pristine TPA are in good agreement with the literature findings which assign them to the stretching vibrations of P-O, WOd, W-Ob-W (corner sharing), W-Oc-W (edge sharing), respectively, characteristics of the Keggin structure. The TPA/AC and TPA/HSAG spectra showed the preservation of the Keggin structure in both samples, although a slight shift is detected for the TPA-graphite supported. This displacement towards higher wavenumbers may be a consequence of a strong HPA-support interaction, typically observed with carbon materials [64–67]. This can be interpreted as a charge transfer between the basal planes or the functional groups of the graphite and the HPA that ultimately stabilizes the charge that would otherwise be stabilized with the neighbouring polyanions [68]. This is reinforced by the lack of crystallinity which can ultimately lead to a high dispersion even at high loadings on carbon materials, and specifically for the HSAG-supported catalysts. As for the STA spectra (Fig. 2.b), the fingerprint bands obtained for the pristine HPA at 1016, 978, 906 and 736 cm−1 are assigned to the typical antisymmetric stretching vibrations WO, Si-Od, W-Ob-W (corner sharing) and W-Oc-W (edge sharing), respectively [69]. Fig. 2.b and Fig. 2.c represent a magnification of this region (700–1200 cm−1) for the STA-carbon catalysts and STA-oxides, respectively. In the STA and the STA-based catalysts spectra the characteristics vibration bands of the HPA bonds are appreciated, and no signals of other species are detected. As already mentioned for the TPA-HSAG sample, the interaction effects between the STA and the graphite support are also evidenced in its corresponding spectrum (Fig. 2.b). Except for the STA/Al2O3 sample, whose spectrum does not clearly evidence the characteristic Keggin vibration bands (Fig. 2.c), the FTIR analysis confirms the preservation of the Keggin structure in the synthesized catalysts.Hence, these FTIR-ATR studies along with the XRD patterns and the nitrogen physisorption isotherms, suggest the HPA are not deposited on the surface as crystalline or isolated species but rather forming clusters of a few highly dispersed units that allow the conservation of a large part of the specific surface area of the supports.The structure, physicochemical properties and therefore the catalytic performance of HPAs are strongly dependent of the temperature. For instance, the pristine TPA, the most stable Keggin HPA, loses the crystallization water from the stable hexahydrate structure within the temperature range of 100–280 ºC, while the total deprotonation to the unstable anhydride form (hence, the Keggin structure decomposition), starts around 370 ºC [46]. These processes take place at lower degrees for the pristine STA [46] and can shift to upper or lower temperatures according to HPA-support interactions [70]. This means that HPA decomposition is accompanied by water desorption resulting in chemisorbed NH3 release at lower than expected temperatures. Consequently, acidity characterization by classical NH3-TPD is challenging and may be misinterpreted, since the strength of the acid sites could be underestimated.Bearing in mind the decomposition temperatures above described, the total amount of acid sites was measured following the next procedure. After a pre-treatment stage at 350 ºC, the sample was subjected to a NH3 pulse chemisorption process at 120 ºC. The temperature was then raised recording the desorbed NH3 with time. The number of strong acid sites was estimated from the subtraction of the total number established by NH3 pulse chemisorption from those registered by the TPD. The same procedure was applied to the bare supports. NH3-TPD profiles of the HPA catalysts are represented in Fig. 3 while those of the bare supports are depicted in Figs. S9-S12. The total number of acid sites and their distribution according to the strength degree are summarized in Table 2.In terms of their acidic strength, the different nature of the used supports provides some properties into the catalysts: while carbon materials and silica contribute with a limited amount of acid sites, the Al2O3 and ZrO2 acidic oxides significantly incorporate weak-moderate strength centers (Table 2). The developed acidity by a supported HPA is the result of the combination of various parameters: (i) the number of acid sites, (ii) the acid-base reaction between the HPA and the support and (iii) the aggregation degree of the Keggin clusters which ultimately provide the acidic protons [71]. The larger the HPA crystallites formed, the higher similarity to the pristine HPA, thus the greater the strength. The extent of these acidic protons has been measured quantitatively, being highly dependent on the superficial properties of the support in a wide range of nominal monolayer coverage [72]. Therefore, the combination of these variables allows to finely tune the acidity of the catalyst based on the requirements of the reaction [72].As expected, the incorporation of the HPA gave rise to a large increase in the amount and strength of acid sites (particularly the moderate and strong ones), for all the samples except STA/ZrO2. It should be noted that although ZrO2 is the support with the lowest surface area (Table 1), the STA coverage is far from the monolayer coverage. For this catalyst, the decrease in the total number of acid sites is probably due to the reduction in the specific surface area, added to the fact that the basic sites present in this oxide tend to neutralize the more acidic protons. At the same time, the polar surface is prone to generate a homogeneous dispersion of the HPA. In any case, supporting the HPA resulted in a relative increase of the acid strength, as registered in Table 2. At the other extreme, the STA/Al2O3 catalyst has generated the highest number of acid centers after the incorporation of STA, especially those of weak-moderate strength. As for the silica, STA functionalization has generated less acidity than for carbon materials (Table 2).Establishing a comparison between the two types of heteropolyacids, TPA-based catalysts developed less amount of total acid sites but with higher strength than their STA-based counterparts. This is in concordance with a lower number of acidic protons per cluster but higher strength of the pristine TPA in comparison with the STA [46]. If the carbon supported catalysts are compared, both STA/AC and TPA/AC exhibited a slight major amount of acid sites than their graphite-supported equivalents (Table 2). Nonetheless, as revealed by the NH3-TPD profiles, the developed acidity in the HPA/HSAG catalysts is less strong or shifts to a relatively higher amount of moderate-strength acid sites.NH3 is an unspecific base that indistinctly chemisorbs in Lewis or Brønsted acid sites. Due to the importance of Brønsted acid sites in the butanol dehydration reaction, [33,40] the characterization of this type of acid centers was performed by the isomerization reaction of 3,3-dymethyl-1-butene (33DM1BN). The use of model reactions allows the study of the catalyst in thermal and atmospheric conditions equal to or similar to those of the reaction under study. 33DM1BN isomerization proceeds through a pure protonic mechanism due to the substitution degree of the molecule with a quaternary carbon atom which impossibilities the π-allylic intermediate formed in Lewis based isomerization reactions mechanism [73]. If the reaction is performed at temperatures below 300 ºC often leads to two single products, 2,3-dymethylbut-1-ene (23DM1BN) and 2,3-dymethyl-but-2ene (23DM2BN) ( Scheme 2), being successfully applied to study Brønsted acid sites of weak to medium strength [74–76].However, the isomerization of 33DM1BN is a susceptible reaction to coke deposition and the catalyst undergoes deactivation from very early reaction time. Aiming to avoid this deactivation effect on the comparison among catalytic activities, conversions were measured every 4 min during a brief period (20 min) and the activity was then linearized and extrapolated to zero time. For comparative purposes, the widely used zeolite ZSM-5 was also evaluated in this reaction, and the obtained catalytic performance is displayed in Fig. 4, along with those of the HPA-based catalysts. Firstly, the supported HPAs catalysts exhibited higher initial activity than the model zeolite ZSM-5, which suggests the presence of higher quantity of Brønsted acid sites in the formers. Secondly, some interesting deductions can be established by comparing these catalytic results with the acidity characterization revealed by NH3-TPD. On one hand, both STA/AC and TPA/AC displayed somewhat higher initial activity than those supported on the HSAG, in good agreement with the highest number of acid sites measured by NH3 chemisorption. On the other hand, and contrary to that one would deduce from the NH3-TPD profiles, STA/SiO2, and STA/ZrO2 exhibited a similar catalytic performance quite comparable to that of the STA/HSAG. In addition, despite STA/Al2O3 was at the top in the number of acid sites (Table 2), it displayed the worst initial activity among the HPA-based catalysts, suggesting the presence of a lower quantity of weak-medium Brønsted acid sites.Prior to the catalytic tests, the absence of catalytic activity in the blank and bare support tests was verified at 275ºC, obtaining conversion values close to 2% with the alumina, the most active support.As already mentioned in the introduction section, the 1-butanol dehydration reaction towards butene isomers corresponds to the intramolecular dehydration (Scheme 1). Whereas intermolecular dehydration gives rise to DNBE and is favoured at lower temperatures from a thermodynamic point of view. To evaluate the effect of temperature on product distribution, catalytic tests were performed on STA/HSAG using different WHSV values in order to compare selectivity values under quasi-isoconversion conditions. ( Fig. 5). Within the studied temperature range (175–225 ºC), the catalyst was highly selective to dehydration products (>99 %) either the intermolecular or intramolecular dehydration product, obtaining a mixture of linear butenes and DNBE. The following major compound was the dehydrogenation product, butyraldehyde, with selectivity values below 1 % in all cases. No signals of permanent gases (CO, CO2, H2, CH4), skeletal isomerization to i-butene or cracking products were registered. Only trace amounts of unknown heavier molecular weight compounds were detected although not relevant as carbon balance was kept above 98 % at every run.As depicted in Fig. 5, the selectivity towards linear butenes rises with temperature from the initial 40 % at 175 ºC to approximately 98 % at 275 ºC, increasing at the same time the isomerization to 2-butenes although maintaining a similar cis to trans ratio (close to 1). As observed, the dehydration reaction is strongly dependent on temperature not only in terms of activity (very different WHSV values were required to achieve similar conversion levels) but also of selectivity.Considering the highest selectivity to butenes, 275 ºC and a WHSV of 64.8 h−1 were the conditions selected to compare the catalytic performance among our different synthesized catalysts.In Fig. 6.a the catalytic activity with time on stream of both HPAs carbon-supported is compared. Except TPA/HSAG (26.7 mmolBuOH∙min−1∙g−1 a.p), the HPA/carbon catalysts showed quite similar initial catalytic activities in 1-butanol dehydration reaction (37.7, 38.7 y 40.0 mmolBuOH∙min−1∙g−1 a.p for TPA/AC, STA/HSAG and STA/AC, respectively), which tend to converge to more comparable values with time on stream. Taking into account the acidity characterization results (Table 2), the higher amount of Brønsted acid sites exhibited by the STA-based catalysts (Fig. 4) seems to compensate the superior acid strength characteristic of TPA [46], resulting in rather comparable activities in the butanol dehydration reaction. As frequently reported for this reaction, all catalysts suffered from a high deactivation rate. It is noticeable the higher deactivation rate exhibited by the STA/AC catalyst compared to that of the TPA/AC. This is probably due to coke deposition on the smallest micropores which were already blocked after the synthesis stage of TPA/AC, as the textural characterization suggested (Table 1).Support’s nature has a great influence on the catalytic activity as observed on Fig. 6.b, where the catalytic activity with time on stream is represented for the STA dispersed on different supports and the model ZSM-5. Except STA/Al2O3, these catalysts showed an outstanding catalytic performance clearly surpassing one of the most widely studied zeolites. This has been already reported for other heterogeneous catalytic acid-demanding reactions [46]. Bearing in mind the relative low strength and amount of acid sites determined by TPD-NH3, STA/SiO2 exhibited a surprisingly high initial activity, although Brønsted activity already denoted by the 33DM1BN model reaction was in the same range than the rest (Fig. 4 and Table 2). On the other extreme, STA/Al2O3 displayed the lowest catalytic activity despite presenting the largest number of acidic centers (characterized by TPD-NH3), although the lowest Brønsted acidity (as revealed by the results of the 33DM1BN isomerization reaction). This fact, along with the non-appearance of the characteristic HPA reflections in the XRD pattern (Fig. 1) and the absence of characteristic Keggin vibration bands on the FTIR spectrum (Fig. 2c) led us to think on a plausible partial decomposition of the Keggin structure, which could explain this dropping in Brønsted acidity. This latter effect has already been stated by Pizzio et al. [77] and attributed its worse catalytic performance on the i-propanol dehydration to this fact. They reported the TPA decomposition into a polymeric anion [P2W21O71]7- and WO3 when a TPA solution is in contact with an Al2O3, which was confirmed by different techniques.Interestingly, although the initial catalytic activities of the STA/ZrO2, STA/HSAG and STA/AC catalysts are well correlated to those obtained in the 33DM1BN model reaction, the deactivation rate displayed for each catalyst differs greatly. The large activity drop exhibited by STA/AC has been above mentioned and can be easily disclosed in terms of the support textural features. On the opposite, the low deactivation rate of the STA/ZrO2 catalyst compared to the rest is striking. Since the porosity of the ZrO2 differs from the other supports, a steric hindrance effect to inhibit coke precursors cannot be completely discard, but a close look to the products distribution ( Fig. 7) points to a differentiating fact with respect of ZrO2, which will be discussed below.Different reaction mechanisms have been proposed for the dehydration of alcohols and specifically for BuOH. They can be disclosed into an E1, E1cB or E2, standing for a unimolecular, concerted or bimolecular elimination mechanism, respectively, which take place on acidic, basic, or acid-base sites [78], respectively. Although they are competitive mechanisms, the 1-butene to 2-butene isomerization ratio is supposed to be closely related to the dominant path, being lower when the active sites are of basic nature. Moreover, a 2-butene cis/trans ratio lower than 1, as it is the case for STA/ZrO2 (rcis/trans=0.69), is distinctive of basic isomerization catalysts. Indeed, 1-butene isomerization has long been used as a model test of acid-basic catalytic sites [79]. In this catalyst, HPA may be highly dispersed near to a basic center developing a strong acid-weak base interaction (otherwise if a strong base were involved the dehydrogenation to 1-butanal would be more relevant). This acid-base interaction could not only stabilize the reaction intermediate but also aiding in a fast product desorption, avoiding or shortening the contact time with the strong acid, which is ultimately responsible of the catalytic activity (as the ZrO2 itself has no activity under these conditions). Except for STA/Al2O3 and STA/ZrO2, the obtained product distributions are rather similar and characteristic of Brønsted acid sites as is the nature of HPA. Thus, in the mentioned exceptions, the support can tailor at some extent the acid strength and interaction or desorption of the reaction intermediates but not modifying the acidity nature of the active site involved in the reaction mechanism of the alcohol dehydration reaction.Finally, the regeneration capacity of our catalysts was studied. As already indicated in the introduction section, this is a subject frequently underestimated in literature and especially in the recent objective of biobutanol valorization. However, it is a relevant topic to be discussed that can determine the implementation feasibility of a large-scale production process [80]. As shown above, all the studied catalysts exhibited a more or less pronounced activity drop which tends to stabilize after some hours on stream, which is characteristic of acid-catalyzed reactions [46]. Moreover, it is one of the major limitations when HPA catalysts are used [46,81]. We attempted to address this catalyst’s deactivation matter by employing some different strategies. Initially, despite described fruitful for similar catalytic systems [82], the introduction of a metallic functionalization to oxidize coke species or testing with different gas streams in the feed (for instance, reductive atmosphere) were not successful.An interesting regeneration treatment consisted of a mild oxidation process of the deactivated catalyst in synthetic air at 400 ºC for 90 min, and the obtained results for STA/HSAG and STA/SiO2 are depicted in Fig. 8. As observed, while the initial catalytic activity of the STA/HSAG was fully recovered after the treatment, the silica-supported catalyst not only did not recuperate its initial activity, but it decreased even more. It must be mentioned that the rest of catalysts subjected to this regeneration process (not shown for the sake of brevity) exhibited a similar behaviour than that of STA/SiO2; thus, being STA/HSAG the only regenerable catalyst. It should be noted that the recovery of the catalytic activity for STA/HSAG is not due to an effect of increase in the intrinsic activity in the oxidation process, because the same treatment to the fresh catalyst did not rise the initial activity already obtained with the untreated sample (not shown for the sake of brevity).The already STA-graphite interactions demonstrated (displacement of the HPA signal towards higher wavenumbers in the FTIR-ATR spectrum), the optimal degree of HPA crystallites agglomeration and the consequent decrease in the acid sites strength in comparison to that of silica, may be the reasons to explain the feasible regeneration of this catalyst.In order to get more insights to explain the different behaviour detected among our catalytic systems after the regeneration treatment, thermogravimetric analysis of fresh and used samples was performed and plotted in Fig. 9. As for the fresh STA/SiO2 (Fig. 9.a), apart from the weight loss characteristic of physiosorbed water until 100 ºC, the thermogravimetric analysis profile registered a weight loss starting a few degrees above 350 ºC that corresponds to the 1.5 H2O molecules release that gives rise to the decomposition of the Keggin structure [46]. For the used catalyst, the derivative curve evidences an asymmetric peak, whose maximum shifted a few degrees to higher temperatures in comparison to the fresh catalyst (from 435 to 462 °C). This contribution may be clearly deconvoluted into two peaks: one characteristic of the Keggin structure decomposition and the other one of the strongly adsorbed coke oxidation reaction. This clearly evidences the concurrence of the HPA decomposition and coke oxidation processes, which explains the unsuccessful application of the thermal regeneration treatment for this catalyst (Fig. 8). Interestingly, a weight loss at around 260 °C, very close to that of the BuOH dehydration reaction, which is absent in the fresh catalysts, is also observed. This can be attributed to the desorption of reaction intermediates, more precisely to DNBE. Thus, the fast activity drop produced in the first reaction hours can be ascribed to the time needed to reach a steady-state equilibrium of the different species implied in the reaction pathway, which blocked the active sites. These species are removed after the regeneration treatment and the initial fast deactivation takes place again.Curiously, the fresh and used STA/HSAG catalysts exhibited a totally different thermogravimetric analysis profile (Fig. 9.b). HPA decomposition commences at temperatures around 510 ºC, once the combustion of the graphitic support has already started, either for the used or the fresh catalyst. Indeed, this decomposition generates two different combustion rates of the HSAG. The combustion of the formed carbon deposits on the spent catalyst was confirmed from the TGA curve, which shows a weight loss at about 350 °C, absent in that of the fresh catalyst. This explains the total regeneration of this catalyst with the employed thermal treatment at 400ºC. An intermediate behaviour occurred for STA/ZrO2 fresh and used catalyst (Fig. 9.c), which finally means an incomplete recover of the catalytic activity, depicted in Fig S14. In the same manner, the oxidation treatment performed on STA/AC was not successful and the catalytic activity did not recover (Fig. S15).Hence, although SiO2 is a widespread employed support for HPAs, due to its relatively inertness which allows to maintain the former HPA acidity, when applied in the BuOH dehydration reaction an important high deactivation is observed. In our case the low decomposition temperature observed for STA/SiO2 seems not to be operative for the catalyst reactivation. Contrarily, taking advantages of the strong carbon-HPA interactions [83,84], and the thermal stability of graphite under oxidative conditions, it is possible to regenerate the softer coke and to restore the catalytic properties of STA/HSAG catalyst.The successful incorporation and high dispersion of the HPAs on the carbon supports was confirmed by means of N2 physisorption, X-ray diffraction and FTIR-ATR, which evidenced the slight and coherent decrease in the surface area, the absence of crystallinity and the preservation of the Kegging structure, respectively. The spectroscopic technique also revealed a strong interaction between the HPA and the graphitic support on HPA/HSAG samples.As expected, and as revealed by the NH3-TPD profiles, the incorporation of HPAs led to a huge increase in the amount and strength of acid sites, displaying the TPA-based catalysts less amount of total acid sites but higher strength than their STA-based counterparts. Comparison between the explored carbon supports discloses that HPA catalysts supported on AC exhibit a somewhat major amount of acid sites than those over HSAG. This was also evidenced by the higher initial activity displayed by the formers during the 33DM1BN isomerization, which is a sensible reaction to Brønsted acid sites of weak or moderate strength.During the gas phase dehydration reaction of butanol, all the explored catalyst systems were highly selective to the target products, linear butenes, and at the same time all were susceptible to deactivation by coke deposition. However, the support’s nature has shown to modulate at some extent the catalytic activity, isomers distributions, stability degree, and regeneration ability. On one hand, STA/SiO2 developed the highest catalytic activity but could not be regenerated by means of combustion. On the other hand, thanks to the strong HPA-carbon interaction, the thermal stability of graphite and HPA along with the deposition of a softer coke, STA/HSAG resulted to be a highly selective and fully regenerable catalyst following well selected oxidation treatments. J.M. Conesa: Conceptualization, Methodology, Investigation, Writing – original draft. M.V. Morales: Writing – original draft, Supervision. N. García-Bosch: Investigation. I. Rodríguez-Ramos: Writing – review & editing, Supervision, Project administration, Funding acquisition. A. Guerrero-Ruiz: Conceptualization, Supervision, Funding acquisition. All authors have given approval to the final version of the 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.The Financial support from the Spanish Agencia Estatal de Investigación (AEI) and the EU (FEDER) (projects PID2020-119160RB-C21 and -C22) is gratefully acknowledged. J.M. Conesa gratefully acknowledge the funding provided by UNED to carry out his Ph.D. (EIDUNED; jconesa61@alumno.uned.es).Supplementary data associated with this article can be found in the online version at doi:10.1016/j.cattod.2023.01.024. Supplementary material. .

1-butanol dehydration reaction has recently emerged as a sustainable route to produce butenes which can be further oligomerized to be applied as jet fuel. However, the high catalyst deactivation rates observed during this reaction due to coke deposition is still a pending matter. As promising catalysts for this reaction, we have supported two heteropolyacids (HPA), i.e. H4SiW12O40 (STA) and H3PW12O40 (TPA), on two commercial carbon materials: an activated carbon (AC) and a high surface area graphite (HSAG). Aiming to evaluate the role of HPA-support interactions, the STA was also dispersed over metallic oxides of different acidic nature, namely SiO2, Al2O3 and ZrO2. An exhaustive physicochemical characterization revealed that after the HPA dispersion thorough the support, the Keggin structure was maintained and an increase in the amount and strength of acid sites was provoked, but in different degree according to the HPA type and support’s nature. While the TPA-based catalysts developed less quantity of total acid sites, but higher strength than their STA-carbon counterparts, the STA/AC and TPA/AC samples exhibited a slight major amount of acid sites than STA/HSAG and TPA/HSAG. The HPA-support interactions have ultimately modulated to some extent the activity, selectivity, stability and regeneration ability of the synthesized catalysts, when applied in the gas phase butanol dehydration reaction at 275 °C. The higher STA decomposition temperature prompted by the graphitic support, among other factors, allowed the total regeneration of the highly active (39 mmolBuOH∙min−1∙ga.p) and n-butenes selective (>98 %) STA/HSAG catalyst by means of combustion of carbon deposits at 400 °C.

Ceramic matrix composites (CMCs) are a frequently used category of materials in nowadays high-tech industries [1,2]. Ceramic-metals, also called cermets, are a prominent subcategory of CMCs, which can be a solution for ceramic-property limitations such as brittle failure, low fracture toughness, and sensitivity to thermal shock [3,4]. They are widely used in space vehicles, gas turbines, automotive parts, slide bearings, etc. [5–7]. Alumina is one of the commonly used ceramics in the fabrication of CMCs [8], which is proven to encompass impressive behavior at elevated temperatures, excellent thermal conductivity, high hardness, significant corrosion, wearing and abrasion resistance, and relatively low density (nearly half of that of steel) [9]. Alumina is an ideal material for high-temperature insulators, substrates, and circuit boards [10]. Also, the high hardness and wear resistance of alumina or alumina-based composites, makes them ideal for use in cutting tools or high-resistance coatings, especially when heat resistance is required [11]. Moreover, alumina is biocompatible, which means it can be used in medical implants without causing an immune response. Porous alumina-based implants are used in hip and knee replacements, dental implants, and other medical applications [12,13]. Porous alumina-based materials are also widely used as catalyst converters or to purify air or water [14,15].On the other hand, alumina is an excellent candidate to combine with steel, considering the importance of various types of steel in industries [16]. For instance, SiC/steel composites are almost impossible to fabricate, as the oxide coatings on SiC are not protective enough to shield the ceramic against corrosion caused by the melted metal. Therefore, the dissolution of SiC in the melt occurs. Consequently, alumina as an oxide ceramic is a well-deserved option to compound with high-melting alloys (e.g., steels or Ni-based superalloys). In such composites, the interface of steel-alumina has proven to show satisfactory bonding, with no cracks [17–19]. Koopmann et al. [20] also demonstrated that the good interface behavior of alumina-steel can be observed even for additively manufactured samples. Moreover, for high-tech applications, there is an increasing necessity to develop novel materials specifically designed for exclusive purposes [21,22]. To this end, various innovative materials, particularly composites, were introduced through different fabrication techniques [23–25]. Considering the broad utilization of alumina in industries, a favorable category of composites can be achieved by developing alumina/metallic composites. It enables us to obtain superior material behavior, encompassing selected properties of alumina and metals (e.g., metals act as the reinforcement phase or a coolant agent in the structure, and alumina adds high abrasion resistance to it, etc.).Such composites can also be produced through powder metallurgy (PM) [26], hot pressing (HP) [27], hot isostatic pressing (HIP) [28], spark plasma sintering (SPS) [29], and casting methods [30]. However, fabrication of the CMCs via conventional techniques can be costly and geometrically limited. Besides, the aforementioned processes sometimes prolong the path to the desired parts. To overcome these barriers, additive manufacturing (AM) techniques can be introduced as an alternative solution to produce CMCs [31]. In these methods, the components are produced through a layer-upon-layer material deposition until the desired shape is made [32]. It is required to study the potential of AM to produce CMCs. The additively manufactured CMC samples were also compared to those fabricated through conventional techniques [33–35]. One major AM category is laser powder-bed fusion (LPBF), in which a laser beam is utilized to selectively melt or sinter and fuse the powder particles [36]. The principle of this method is that a thin layer of powder is uniformly deposited on the printing bed through a blade coater. The laser beam scans the selected area via the intended parameters and the desired scanning pattern. The adjacent powders are then fused together under the influence of the heat induced by the laser beam. Subsequently, the printing bed moves downwards on the z-axis, and another layer of powder is coated on the top. This procedure is repeated until the final geometry is obtained. The schematic illustration of the LPBF process is shown in Fig. 1 . Despite considerable advancement in this area, the LPBF technique is still in the early stages of development. A considerable number of researches were carried out to develop new materials for the LPBF, including iron-based alloys [37,38], aluminum-based alloys [39–41], metal-ceramic composites [42–44], and metal matrix composites [45–48].During the past few years, a limited number of researches are conducted on AM of metal-reinforced CMCs, most of which are associated with AM of WC-Co composites. In literature, LPBF [49–51], direct laser deposition [52], binder jetting [53,54], and laser engineering net shaping (LENS) [55,56] AM techniques have been used to fabricate CMCs. Among the mentioned techniques, LPBF includes most of the research. A major problem in LPBF of the metal-reinforced CMCs is the formation of cracks due to the thermal gradients [57]. Grigoriev et al. [50] used LPBF to additively manufacture WC-6 wt% Co samples and reported crack formation, which is attributed to the significant difference between the melting point of the ceramic and the metal. Khmyrov et al. [58]} studied the formation of various phases during the LPBF of WC with different Co contents (6, 50, 72.4, and 75 wt%). They reported the presence of WC and the formation of W2C in the structure for samples with 6 wt% Co. For any other sample, they observed a full dissolution of WC. A small amount of W4Co2C was revealed in samples containing 75 and 72.4 wt% Co, while samples with 50 wt% Co contained W3Co3C.On the other hand, some difficulties are faced in the laser processing of ceramics related to their low optical absorption coefficient and, subsequently, the problematic absorbing ratio of the laser energy during the process. Compounding the ceramic with metal particles is an effective solution to enhance light absorption. Besides, ceramics usually have high melting points and do not melt during laser processing. As a result, they do not have the proper flowability to fill the pores during printing. Adding metallic particles to ceramics can reduce the porosity ratio, as the metals have the potential to melt, flow, and fill the gaps at lower melting temperatures.With regard to the reasons mentioned above, this research aimed to additively manufacture the alumina/standard EOS steel alloy (DS20) composite through the LPBF technique. It is worth noting that parameters such as flowability, density, size, shape, and conductivity of the powder are critical characteristics in the broad implementation of the technique. After accomplishing the manufacturing phase, the samples' microstructure variation, porosity analysis, hardness testing, and compressive strength were addressed. Alumina-based materials have a broad range of industrial applications, especially alumina/metal composites. Previous research by Koopmann et al. [20] reported zirconia-alumina ceramic coating on a steel substrate. However, to our knowledge, this study presents the first development of an alumina/steel ceramic matrix composite using LPBF, demonstrating the feasibility of printing cermets. The advantages of additive manufacturing, including design flexibility, energy efficiency, and reduced post-processing and machining requirements, make this breakthrough approach a promising option for the fabrication of alumina-based materials. This research offers new avenues for the additive manufacturing of ceramic-based materials in general.To prepare the composite powder, an aluminum oxide powder (GF54503557, Aldrich, United States) and a standard EOS steel alloy (DS20, EOS GmbH, Germany) powders were carefully sieved through a mesh size of < 45 μm to eliminate any agglomerated particles. Using a dry ball milling setup, the powders were mixed at a ratio of 80 wt% alumina and 20 wt% steel for 4 hours. The composition of steel powder was investigated by the inductively coupled plasma–optical emission spectrometry (ICP-OES) analysis through the Sciex Elan 6100 (PerkinElmer, USA) system.A direct metal laser sintering (DMLS) machine (M250 XTENDED DMLS 3D printer, EOS GmbH, Germany) was used to fabricate alumina/steel cylindrical specimens with a diameter and height of 5 and 15 mm, respectively. The optimal printing parameters (Table 1 ), including laser power, laser scanning speed, layer thickness, scanning pattern, and hatch space, were adjusted, starting with the values derived from previous studies [59]. The as-printed specimens were then sintered in a graphite furnace under an inert gas (argon) atmosphere to improve the bonding within the structure. The sintering heating program is presented in Fig. 2 .After the sintering process, a polymeric resin (Dichtol WTF 1532, Metaplastic, Germany) was used to infiltrate into the cracks and pores of the specimens. According to the data provided by the resin manufacturer, Dichtol polymer has a service temperature of -40 °C to +300 °C. In addition, the low viscosity of the resin at room temperature could enhance the infiltration process. For the infiltration process, specimens were immersed in the liquid resin for 60 min and then cured for 4 hours at room temperature according to the guideline provided by the manufacturer. This process was repeated three times to ensure complete penetration of the resin.Simultaneous thermal analysis (STA 504, BÄHR-Thermoanalyse GmbH, Germany) up to 1600 °C and in an argon atmosphere was used to investigate the steel powder's thermal properties and determine a suitable sintering temperature.Microstructural studies were performed on both as-received powders and processed samples (as-printed, as-sintered, and polymer-impregnated). Field-emission scanning electron microscopy (FE-SEM) (Mira3, TESCAN, Czech Republic) was used to study the polished cross-sections and the fracture surface of the fabricated specimens. In addition, an energy-dispersive x-ray spectrometer (EDS) was utilized to characterize their chemical compositions.Mercury porosimetry distribution analysis was conducted in two different ranges. To investigate the distribution of pores below D: 10 μm and pores above D: 10 μm, a Pascal 440 and a Pascal 140 Mercury Porosimeters (Thermo Fisher Scientific, United States) were used, respectively.To evaluate the mechanical properties, cylindrical samples with a diameter and height of 5 and 7.5 mm, respectively, were prepared and subjected to the compression test using a universal testing machine (2000KPX, Instron, United States); the test was conducted in accordance with the ASTM-C1424 standard at a 1 mm/min rate for all samples. The as-fractured samples were studied to investigate the fracture behavior. To check the hardness of the specimens, a Vickers microhardness indenter was used (MMT-X, Matsuzawa, Japan) under a force and duration of 50g and 20 seconds, respectively. The microhardness test was carried out at different locations along the surface of the specimens with a spacing distance of 500 microns between adjacent indentation points.Powder characteristics, including morphology and particle size, were studied using FE-SEM (Fig. 3 ). The as-received alumina particles showed an elongated polygonal shape, and most of the particles were in the range of 30-45 μ m (Fig. 3(a)). The DS20 steel particles exhibited a spherical morphology with an average particle size below 20 μm. Some agglomerates could be noted in the initial steel powder, which could be removed by ball milling and sieving (Fig. 3(c)). The relatively small diameter of the steel particles and their large specific surface area led to the enhancement in light absorption properties during DMLS, which raised the heat of the particles and facilitated the sintering properties. Furthermore, the spherical morphology of the metal particles could lead to a smoother and more uniform layer during the powder deposition [60].The composition of DS20 steel powder obtained from ICP-OES analysis is listed in Table 2 .The homogenous distribution of steel particles in the dominant Al2O3 powder was found using SEM of the as-milled powder mixtures (Fig. 4 ). The even distribution was necessary to increase the laser absorption rate and enhance the mixture's flowing characteristic during the printing process. It could also improve liquid wetting characteristics and reordering of the particles.Different powder ratios were investigated to obtain the optimum mass ratio of each component in the fabricated specimens. Any defective ones with visible cracks or large porosities were ruled out. The printed specimen with the composition of 80 wt% of Al2O3 powder and 20 wt% of the DS20 steel particles showed no visible defects. The DMLS process parameters were also determined. Different parameters, including the laser scan pattern, laser energy density, and heat treatment program, were selected as variables, and optimum parameters were obtained based on the printed specimens' characteristics, which are reported in Table. 2. Then, the DMLS was successfully used to manufacture cylindrical specimens with the mentioned composition. The fabricated cylindrical specimens are shown in Fig. 5 .The sintering process was then conducted to improve the strength of the as-printed cylinders and relieve their associated residual stresses that formed due to fast localized cooling during printing [61]. The sintering temperature for alumina was determined in different works to be about 1300 °C to 1600 °C [62]. However, due to the existence of the metallic component in the CMC structure, the melting point of this component played an important role in determining its sintering temperature. To select a suitable maximum temperature for the sintering phase, the STA analysis was performed on the steel powder, presented in Fig. 6 . Based on the signals, an endothermic peak is evident at 1370 °C, possibly associated with the liquid formation. Accordingly, this temperature (1370 °C) was selected for the sintering process. Fig. 7 displays backscattered electron (BSE) SEM micrographs of the polished surface of the samples. The as-printed sample showed uniform distribution with no accumulation of the steel particles over the Al2O3 matrix. Besides, the diffusion of the steel alloys in the Al2O3 matrix is not evident from the images (Fig. 7(b)). The as-printed body is a porous structure and contains micro-cracks mainly formed between the porosities. From the fracture cross-section (Fig. 8 (a)), it can be observed that these cracks can propagate through the structure when the load is applied. The formation of micro-cracks in the sample is attributed to the brittleness of the alumina matrix and the differences in melting points and thermal expansion coefficients of alumina and steel components [51,63]. Due to the weak adhesion between steel and alumina matrix, steel particles were removed during polishing in some areas; these are distinguished in dark circles in the micrograph. It also can be understood from Fig. 7(c) that some of the alumina particles were melted during the printing process. A similar effect was noted on the fracture surface of the bars, as highlighted in Fig. 8(a). Besides, a long crack formed due to thermal gradients during solidification. Fig. 7(d-f) illustrates BSE images of the sintered sample.The images of the as-printed and as-sintered samples show no discernible difference between them. Due to the existence of the steel component in the fabricated composite and considering the steel area being melted at 1370 °C , liquid phase sintering must have occurred during the heat-treatment to achieve full densification. However, the mentioned phenomenon did not happen because of the poor wettability of the steel and alumina, the high interfacial energy between steel and alumina, and the presence of a thin layer of oxide on the surface of the steel, which prevents melted steel from moving between alumina particles [64–66]. From the fracture surface analysis (Fig. 8(c) & (d)), spherical steel particles can still be observed, confirming the stated phenomenon. It is evident from the SEM images of the polished cross-section that the melted metallic particles did not rearrange the alumina particles due to capillary action. On the other hand, it could be noticed that the as-sintered samples have relatively higher densification than the as-printed body. The reason could be associated with the densification of the alumina component during sintering. The onset of sintering temperature for alumina is around 1000 °C. Hence, alumina particles started to densify at this temperature through the solid-state sintering mechanism. Since the sintering temperature of the CMC was selected to be at 1370 °C, which was 370 °C higher than the onset of the sintering temperature of alumina, more densification was expected. However, the complete densification of the CMC was not achieved since the ideal sintering temperature of alumina is between 1500 to 1700 °C, which was impossible to implement due to the presence of the metal in the structure. Considering the STA analysis of the metallic phase (fig. 6), the formation of gas at temperatures above 1370 °C could introduce defects into the structure. The incomplete densification of alumina may also be linked to the sintering atmosphere chosen. This is because argon has a lower solubility in alumina than oxygen, which may lead to less efficient sintering [67]. It is worth noting that a noble gas was chosen as the atmosphere due to the presence of metallic particles, and the risk of oxidation during sintering, which could have had a detrimental effect. It should be considered that solidification occurred so fast when the alumina particles were melted during the printing process. So, densification could not be achieved entirely. Therefore, the particles only stuck together. By comparing the steel and alumina interaction for the as-printed body and the sintered samples in Fig. 7(b) & (e), it could be concluded that the pores inside the steel area are removed after sintering.Despite performing the sintering process and the resultant relative increase in the strength of the areas, the samples remained brittle due to the porosities in their structures. So, they could not be used under relatively high mechanical loads. Therefore, an attempt was made to remove the cracks and reduce the porosity in the samples by penetrating a suitable polymer into the matrix to improve their strength. The specimens were immersed in the Dichtol WFT 1532 resin. This polymeric solution was specifically designed to fill the pores (0–0.1 mm) and subsequently cause an improvement in the material’s mechanical and corrosion behavior. The micrographs of the samples immersed in the resin, “the polymer-impregnated sample”, are shown in Fig. 7(g-i). From the polished cross-section, almost difference can be noted between the images of the samples before and after the polymer infiltration process. However, as highlighted in Fig. 8(f), the polymeric solution could close some micro-cracks due to the low viscosity and easy penetration characteristics. Larger cracks and porosities cannot be filled in, but a thin layer on the defects' walls can be formed, improving the properties. It is worth noting that while the sintering phase was unable to eliminate large pores or voids, it was a crucial step that could not be omitted. Laser Powder Bed Fusion (LPBF) only provided initial adhesion between the particles to form the desired geometry. Therefore, the as-printed samples had little strength, and sintering was necessary to enhance the particle bonding before polymer impregnation.EDS was also employed to reveal elemental analysis and distribution of the elements within the microstructure after sintering the printed sample. The EDS analyses in three different regions of the sintered sample are shown in Fig. 9 . In region A (Fig. 9), the highest weight percentages belong to Fe and Ni elements, which confirms the presence of DS20 steel alloy. This region also has C, O, Al, Si, P, and Cu in low percentages. About 90 wt% consists of Al and O in region B, proving this area comprises alumina. The other local elements are C, Si, P, Fe, Ni, and Cu, which possess 10 wt% of the composition. In region C, which is created during the printing process or due to the removal of steel areas during the polishing of samples, C and O have the highest weight percentages. For polishing, samples were cold-mounted using polymeric materials. This polymer can smear off from the cold mount during the polishing and penetrate through the porosities and cracks. This can be the reason for the existence of high contents of C and O in dark areas. All in all, EDS analyses of the different regions of the surface further revealed the poor diffusion of the molten steel inside the alumina during the printing and sintering processes.EDS x-ray maps of the sintered sample are shown in Fig. 10 . These maps show the distribution of the elements in the microstructure. As it is clear from the images, the steel alloy elements are distributed randomly and uniformly in the matrix. Furthermore, the semi-dendritic regions are enriched with alumina, which is caused by the non-equilibrium solidification of the composite during the DMLS process. Fig. 11 presents the cumulative volume vs. the pore diameter for the as-printed and as-sintered samples before and after the polymer impregnation. The test was conducted in two ranges porosities of >10 μm and <10 μm. After sintering, the cumulative volume of the pores below 10 μm reduced by up to 16%. However, the cumulative volume of pores above 10 μm increased up to 17%, attributed to the higher thermal expansion of the metal components and causing new defects in the structure. Only a slight change in the overall percentage of porosities could be noted upon sintering the as-printed body, which further ensured the poor wettability of alumina by steel alloy. Fig. 11 also displays that the cumulative volume of porosities below 10 μm decreased up to 34% after immersing the samples in the polymeric resin, as the tiny pores and microcracks were filled during the infiltration. Moreover, polymer impregnation decreased the porosity related to the pores larger than 10 μm by 31%. To summarize, the overall porosity percentage in the structure decreased from 36% to 27% after polymer infiltration. It should be mentioned that the goal of the polymer infiltration was to improve the fabricated composite's mechanical properties and corrosion behavior. Polymer impregnation did not aim to remove porosities entirely, as it would significantly increase the composite’s weight. Besides that, it could noticeably reduce the heat transfer coefficient, which is considered a disadvantage as regards the application, requiring wear resistance and could lead to a considerable temperature increase in such processes. Finally, the main reason for the existence of porosities may relate to the irregular shapes of the initial alumina powder, which was also shown by Chen et al. [49]. The consequence of this problematic factor could be partially compensated if the wettability of alumina and the molten steel were sufficient to fill the matrix pores with steel.Compression and Vickers microhardness tests were conducted to investigate the mechanical properties of the printed samples. Fig. 12 presents the microhardness test results of the as-printed and as-sintered samples. The as-sintered sample recorded higher microhardness values than the as-printed one, which is attributed to defining a proper sintering scenario. Owing to the elevated temperatures during the sintering process, the bonding of the ceramic particles happened suitably. This improved bonding could consequently lead to the reduction of the small pores and an increase in the hardness of the composite.Moreover, the main reason the as-printed sample had more microcracks in the ceramic-metal interaction areas was their lower microhardness values [68]. Besides, the distribution of fine phases in the as-sintered composite was another critical contributing factor to the higher hardness values [57]. The results were consistent with other research works [69]. As it is clear from Fig. 12, the distribution of the microhardness value along the investigated line in the sintered sample is more uniform. This could be attributed to holding the sample at an elevated temperature for a long time in the sintering process [70]. Due to the much lower hardness of the infiltrating polymer and its poor effect on the microhardness of the CMC, the microhardness values were not presented for the polymer-impregnated sample.The results of the uniaxial compression test for both as-sintered and polymer-impregnated samples are presented in Fig. 13 . As predicted, the polymer-impregnated sample showed a higher compressive strength. The slope of the compression-elongation curves in the elastic region is almost the same for both samples, indicating no considerable change in the compressive elastic modulus. The matrix-reinforcement interfaces and the porosity are two determinative factors for the compressive strength in CMCs. By immersing the as-sintered sample in the polymer, the compressive strength increased from 56 MPa to 120 MPa, indicating a sharp increase (more than twice). Polymer's presence improved the mechanical behavior of the composite by reducing the porosity, voids, and micro-cracks. The process of polymer impregnation managed to enhance the bonding between the ceramic grains by filling the gaps with the polymer, resulting in a stronger structure. The polymer was able to reinforce the ceramic material by providing extra strength and toughness. The polymer acts as a stress-relieving layer that disperses and absorbs stress, preventing cracks from spreading and improving the fracture toughness of the ceramic porous samples [71,72]. The polymer’s role in enhancing ductility, toughness (the area under the stress-strain curve), and elongation is well-known.In summary, this study aimed to develop a composite material of alumina and Fe-Ni (steel) alloy using laser powder bed fusion additive manufacturing technology. The microstructural analysis showed a homogenous distribution of steel particles in the alumina matrix, demonstrating the effectiveness of the mixing strategy. Sintering the samples at 1370°C improved the Vickers microhardness from approximately 1475 to 1960 HV, indicating enhanced mechanical properties due to better particle bonding. Despite this improvement, the samples still contained porosity and microcracks after sintering. By utilizing polymer impregnation, the overall porosity was reduced from 36 to 27%, microcracks were eliminated, and the compressive strength increased sharply from 56 to 120 MPa, without any considerable weight gain or decrease in thermal isolation.This research presents a practical method for manufacturing alumina-based materials, which have broad applications in areas like the fabrication of electronic components, cutting tools, biomedical implants, and catalyst converters due to their biocompatibility, low density, high hardness, and corrosion and wear resistance. The promising properties of the developed samples suggest that ceramic matrix composites reinforced by particulate metallic materials, in general, could be a promising research direction for materials development in additive manufacturing. Future research could explore the corrosion resistance of these samples and the possibility of scaling up production for industrial use.The authors declare that they have no conflict of interest.

Additive Manufacturing (AM) plays a key role in meeting the vital demands of Industry. The AM industry needs the range of applicable materials to be expanded by conducting research on novel ones. In the present investigation, alumina/Fe-Ni (steel) ceramic matrix particulate composite was fabricated employing laser powder bed fusion (LPBF) additive manufacturing (AM) technology. The quality of the printed samples was associated with the LPBF process parameters, which were optimized for this process. In general, the fabricated samples showed a microstructure of alumina matrix with uniform distribution of steel (Fe-Ni) particles. The as-printed samples exhibited pores. Thus, they were subjected to a sintering heat treatment cycle under an inert atmosphere. Although the sintering cycle considerably increased the average Vickers hardness, pores were not eliminated entirely. Therefore, polymer impregnation of the as-sintered samples was carried out to reduce porosities and microcracks. The mercury porosimeter showed that the porosity decreased sequentially after sintering and polymer impregnation. In addition, mechanical investigations revealed that the polymer impregnation improved the compressive strength of the sintered samples (from 56 to 120 MPa). Alumina-based materials find wide applications in various fields, including the manufacturing of electronic components, cutting tools, biomedical implants, and catalyst converters, owing to their low density, high hardness, wear and corrosion resistance, and biocompatibility. This study presents a viable approach for the fabrication of these materials, with developed samples exhibiting promising properties. The study emphasizes the potential of additive manufacturing as an approach for the fabrication of ceramic matrix composites reinforced with metallic particulates in future research.

Meta data for XRD, PDF and XPS is available at DOI:10.17028/rd.lboro.20170784.The consumption of fossil fuels, such as oil and gas, has led to the release of large amounts of greenhouse gases which is resulting in severe environmental problems [1]. Therefore, the use of a clean energy carrier such as hydrogen is important, due to reduced CO2 emissions from its combustion and utilisation. However, this is dependent on the production method of said hydrogen. This has led to the description of hydrogen and its production methods by colours. The colour chosen is dependent on the CO2 emissions of the process, with steam methane reforming classed as grey hydrogen, due to the release of excess CO2 [2]. Potentially, renewable hydrogen can be produced from sustainable biomass sources [3]. Whilst debated, hydrogen production from biomass can be considered as green hydrogen due to the carbon neutrality of the overall process [2,4]. One such production method, which has generated much interest, is the aqueous phase reforming (APR) process, developed by Dumesic and co-workers [5–7].APR utilises a range of waste aqueous phase oxygenates derived from biomass sources, such as ethanol, methanol, ethylene glycol and glycerol [8–10]. APR is advantageous when compared to traditional methane reforming and waste oxygenate steam reforming due to the low operating temperature (200–250 °C), intermediate pressures (15–50 bar), and no requirement to vaporise the solvent which results in a lower energy demand and thermodynamically favours the water gas shift (WGS) reaction [11–13]. The latter reaction results in low CO concentrations in the effluent, as required for many industrial applications, when compared to the traditional methane steam reforming production method [14]. Other sustainable methods that reduce the CO concentration in the effluent include sorption enhanced glycerol steam reforming with in-situ CO2 removal [15,16].Many active metals have been applied to the APR process, with Pt being a promising choice of active metal due to its high activity for the WGS reaction, C–C bond scission, and low-methanation activity [17–19]. Further control over reaction pathways and increased hydrogen selectivity can be achieved through catalyst design and development of stable support materials [20–22]. Basic materials have been shown to facilitate the WGS reaction, which leads to higher hydrogen selectivity. However, these materials suffer from low hydrothermal stability and undergo restructuring and phase changes resulting in catalyst deactivation [23,24]. In contrast, acidic materials such as zeolites, favour dehydration pathways and increased alkane production [25,26]. Therefore, designing stable and favourable pathway promoting support materials is key for the viability of the APR process.Perovskites with the structural formula AMO3 have high thermal stability and wide structural versatility, which has led to a range of applications [27–29]. Previously, we have applied Pt/LaMO3 (where M = Al, Cr, Mn, Fe, Co, Ni) catalysts in the APR of glycerol, and most of the materials, apart from Pt/LaCrO3, were found to be undergo phase transformation under the reaction conditions [30]. The formation of hexagonal LaCO3OH phases, alongside M site oxides, decorated with Pt nanoparticles, were observed in these catalysts and these phases were found be active and stable catalysts in their own right. Mao and co-workers verified this finding through an extended APR reaction under flow conditions with methanol as the feedstock [31]. Therefore, the perovskite can be considered as a precursor to prepare stable and active catalysts [32]. Interestingly, these findings are contradictory to previous reports of perovskite phase stability in Ni/LaAlO3 catalysts in APR, with comparable glycerol concentration and reaction temperatures [33,34].Given this discrepancy, we have investigated the effect of the phase purity of the LaAlO3 support through alteration of the calcination conditions required to produce it. The catalytic performance and subsequent stability of the materials when applied to APR, hydrothermal conditions and acidic reaction intermediates provided insight into the importance of phase purity.Al(NO3)3.9H2O (99+% Acros Organic), La(NO3)3.6H2O (99.9% Alfa Aesar), PtCl4 (99.99% Alfa Aesar), glycerol (99% Fisher Chemical), lactic acid (≥88% Fisher Chemical), LaB6 (99.5% Thermo Scientific), citric acid monohydrate (99.9% VWR Chemicals), ammonia (32% v/w VWR Chemicals). All chemicals were used without further purification.LaAlO3 perovskite materials were prepared by sol-gel combustion method [35]. La(NO3)3.6H2O (6.073 g) and Al(NO3)3.9H2O (5.262 g) were used in stoichiometric amounts. Citric acid (11.790 g) was then added in a 2:1 ratio to metal nitrates and dissolved in deionised water (15 mL). The pH of the resultant solution was adjusted to 7 using aqueous ammonia solution (3 M) and aged at 130 °C until gel formation. The gel was then combusted at 400 °C for 10 min and further calcined for 2 h at 700, 900 or 1100 °C and labelled LaAlO3-700, LaAlO3-900, and LaAlO3-1100.Preparation of 1 wt% Pt/LaAlO3-c (c=700, 900, 1100) catalysts by a conventional wet impregnation method was as follows: Prepared PtCl4 solution (Pt content: 4.8 mg/mL; 2.082 mL) and deionised water was dispensed to give an overall solution of 16 ml. The mixture was stirred (800 RPM) at 60 °C. The support (0.99 g) was periodically added slowly over a period of 10 min. The resulting slurry was stirred for a further 15 min before heating to 95 °C and dried overnight. The dried powder was then ground and calcined in air at 450 °C (2 h, ramp rate: 10 °C/min).X-ray powder (XRD) patterns of the materials and catalysts were recorded with an LaB6 internal standard (33 wt%) using a Bruker d8 discover operating at 35 kV and 40 mA with a monochromated Co source (λ = 1.79 Å) and a Vantec detector (scan range: 20–100°; step size: 0.014°; step count: 1 s unless specified). Patterns were matched to ICDD PDF database patterns, the list of database patterns is given in Table S1. The refinement of the fresh Pt/LaAlO3- c was performed by the Rietveld method, using TOPAS v5 software and ICSD database crystal patterns given in Table S2. Atom positions, occupancies, and thermal parameters were not refined.N2 adsorption experiments were performed at −196 °C using a Micromeritics Gemini VII to obtain surface areas determined by BET method. Before measurements, the required amount of sample was measured and degassed under vacuum overnight at 90 °C.Thermogravimetric analysis (TGA) of the combusted gel of the LaAlO3 precursor and LaAlO3- c was carried out using a TA SDT Q600 to investigate the formation temperature of the perovskite. The samples were heated in air (ramp rate: 10 °C/min) to 1200 °C and 1000 °C respectively.CO chemisorption measurements were performed using an Altamira AMI-300Lite. Approximately 100 mg of catalyst sample was loaded in-between quartz wool and reduced under 5% H2/Ar flow (50 SCCM) at 240 °C (2 h, ramp rate: 10 °C/min). For analysis, conducted at RT, the sample was titrated with 10% CO/Ar by pulsing through a 574 μL sample loop.ICP-AES experiments were conducted to determine the extent of metal (La and Pt) leaching into reaction filtrates and Pt weight loadings in the fresh catalysts using an Agilent 4210 MP-AES fitted with a SPS4 autosampler. Attenuated total reflection infrared spectroscopy (ATR-IR) was collected using a Shimadzu IR Affinity-1 fitted with an ATR stage on fresh and used Pt/LaAlO3- c . The spectra were recorded between 340 and 4700 cm−1 with a 2 cm−1 resolution for 60 scans for background and spectra.XPS analysis was performed using a Thermo NEXSA XPS fitted with a monochromated Al kα X-ray source (1486.7 eV), a spherical sector analyser and 3 multichannel resistive plate, 128 channel delay line detectors. All data was recorded at 19.2 W and an X-ray beam size of 200 × 100 μm. Survey scans were recorded at a pass energy of 160 eV, and high-resolution scans recorded at a pass energy of 20 eV. Electronic charge neutralisation was achieved using a Dual-beam low-energy electron/ion source (Thermo Scientific FG-03). Ion gun current = 150 μA. Ion gun voltage = 45 V. All sample data was recorded at a pressure below 10−8 Torr and a room temperature of 294 K. Data was analysed using CasaXPS v2.3.19PR1.0. Peaks were fit with a Shirley background prior to component analysis. Line-shapes of LA (1.53243) were used to fit components.Total scattering X-ray data were collected on three LaAlO3 (700 °C fresh, 1100 °C LA/Glycerol and 700 °C LA/Glycerol/CO2) samples using the I15-1 beamline at the Diamond Light Source (Didcot, UK). The samples were loaded into 1.5 mm borosilicate capillaries with scattering collected on the sample, container and empty beamline using an energy of 76.69 keV (λ = 0.161669 Å). Corrections for background, fluorescence, absorption, multiple scattering and Compton scattering were performed using GudrunX [36]. The corrected scattering was Fourier transformed to obtain the pair distribution function (PDF) using a Q-range 0.4 < Q < 30 Å−1. PDF were refined using TOPAS v7 and ICSD database crystal patterns given in Table S2 [37]. Instrumental parameters obtained by Rietveld refinement of a standard Si 640f yielding a dQ = 0.0605. PDF refinements were conducted using a fixed dQ and refined lattice parameters, scale, thermal parameters, and spherical dampening.A FEI Tecnai F20 field emission gun transmission electron microscope (FEG-TEM) was used to image the Pt nanoparticles in conventional high resolution TEM mode. The images were recorded using the Gatan Orius SC200 CCD camera equipped to the FEG-TEM at full CCD size of 2048 × 2048 pixels. Enough frames from typical areas were recorded to ensure >300 particles are available to render a statistical representation of the Pt particle size. TEM specimens were prepared by drop casting of catalysts powder suspension in isopropanol onto standard holey carbon (300 mesh) TEM support grids.Support and Pt catalyst testing was carried out using a Parr 5500 series bench top micro reactor (50 mL) equipped with a Parr 4848 reactor controller system. The catalysts were tested using a standard procedure: 10 wt% glycerol solution (20 mL) and the catalyst (60 mg) were loaded into the autoclave and the system was purged multiple times with argon. The reaction was then carried out for 2 h at 240 °C, 42 bar, 1000 RPM. The gas products (H2, CO, CO2, and CH4) were collected and analysed using an Agilent 8860 GC equipped with a TCD detector and a Shincarbon ST column. The reactant and products in the liquid phase were analysed by HPLC using a Hitachi Chromaster equipped with an Agilent Metacarb 67H column and a refractive index detector. The liquid phase products observed were Lactic Acid (LA), Ethylene Glycol (EG), Hydroxyacetone (HA), 1,2-Propanediol (1,2-PD), 2-Propanol (2-P), 1-Propanol (1-P), and Ethanol (E). Simulated reactions without dispersed Pt species were undertaken, using the standard procedure, with LaAlO3- c (120 mg) and 10 wt% glycerol or 1 wt% LA/9 wt% Glycerol mixture (20 ml) and additional 1 bar of PCO2.Calculations for reactions were carried out as follows: Conversion X ( % ) = ( [ g l y c e r o l ] i n − [ g l y c e r o l ] o u t ) [ g l y c e r o l ] i n × 100 Conversion to gas X g a s ( % ) = ( Σ ( m o l g a s p r o d u c e d ) Σ ( m o l t h e o r e t i c a l g a s ) ) × 100 Turnover frequency T O F ( h − 1 ) = ( ( ( [ g l y c e r o l ] i n − [ g l y c e r o l ] o u t ) / m o l P t ) t i m e ) Hydrogen selectivity S H 2 % = m o l H 2 p r o d u c e d Σ m o l g a s p r o d u c t s × 100 Carbon product selectivity S ( C i ) ( % ) = ( ( m o l P s × C n ) ( ∑ ( m o l P i × C n ) ) × 100 where Ps = specified carbon product; Pi = carbon product; Cn = carbon number. Hydrogen formation rate . r ( H 2 ) ( μ m o l m i n − 1 g c a t − 1 ) = μ m o l ( H 2 ) / min g ( c a t a l y s t ) TGA analysis of the combusted gel powder precursor of LaAlO3, given in Fig. 1 a, was used to determine the formation temperature of the perovskite. The TG curve shows continuous weight loss (7.0 wt%) up to ∼300 °C followed by a major weight loss (37.4 wt%) between 300 °C and 600 °C, with a final further minor weight loss (7.3 wt%) beginning at ∼650 °C. Above 900 °C, no further weight loss is noted up to 1200 °C. The first weight loss can be identified as the loss of water through dehydration; the major weight loss can be ascribed to the decomposition of precursors and formation of an intermediate decomposition product. The final recorded high temperature weight loss suggests a removal of impurity amorphous carbonate phases (further evidence from FTIR infra vide) [38]. The amorphous carbonate materials arise from the synthesis procedure and the use of citric acid as a carbon fuel source. Calcination temperatures were then chosen either side of the final weight loss for investigation. Further TGA analysis of the LaAlO3- c materials, given in Fig. 1b, shows the presence of a small amount of amorphous impurity (4.8 wt%) in the LaAlO3-700 samples that is not present in the materials (LaAlO3-900 and LaAlO3-1100) calcined at a higher temperature. However, the final weight loss of amorphous content in the LaAlO3-700 was less than the original 7.2 wt% seen from the TGA of the combusted gel precursor, suggesting that extended isotherms at 700 °C could remove more of this impurity.Powder XRD patterns of the synthesised Pt/LaAlO3- c are given in Fig. 2 a. For each of the catalysts, no peaks for platinum species PtOx or Pt(0) were found, due to the low loading and the possible small and well-dispersed nature of the nanoparticles. The predominant phases in all the materials were the perovskite LaAlO3 (ICDD PDF 31-0022) phase with no other crystalline phases present, apart from the internal standard LaB6 (ICDD PDF 34-0427). The internal standard LaB6 (33 wt%) was used to determine the crystallinity of the samples as a ratio of LaAlO3(100): LaB6(110) reflections, which are given in Table 1 along with the physiochemical properties of the materials. It is evident that significant amorphous content was present in Pt/LaAlO3-700, which then notably decreases as the calcination temperature was increased, as seen from LaAlO3:LaB6 peak ratios changing from 1.1 in Pt/LaAlO3-700 to 2.3 and 2.4 for Pt/LaAlO3-900 and Pt/LaAlO3-1100, in agreement with findings from the TGA. However, it should be noted, that a decrease in crystallinity was seen in LaAlO3-700 (LaAlO3:LaB6 2.0 to 1.1) upon impregnation with PtCl4 solution, showing that the acidity of the Pt precursor solution could facilitate loss of crystallinity in the material. The decrease in crystallinity was not seen in the higher calcined materials suggesting higher stability of these materials. The analysis presented makes it clear that simple XRD database pattern matching, without an attempt to quantify amorphous content, leads to a lack of full understanding of the catalyst support. The LaAlO3- c particle size was calculated using the Rietveld method and calculation described by Balzar et al., assuming monodisperse distribution of particles and negligible strain [39]. The calculated fits are given in Figs. S1–S3 and Tables S3–5. As anticipated, the calculated particle size moderately increased with calcination temperature due to sintering of the particles at higher temperatures, further leading to a reduction in surface area (Table 1).To further elucidate structural defects in the LaAlO3-700 samples, PDF refinement was used to compare known models and experimental PDF data to identify the phases present within the sample. A good agreement between the experimental PDF and the model was found and is shown in Fig. 2b. For this refinement, it was found that a two-phase model, with phases LaAlO3 and La2(CO3)2OH2, was required to obtain a good fit (Rwp = 18.24%), with each phases scale allowed to refine, to obtain 52.8% and 14.6% respectively. This gives confirmation that the major phase formed in reaction is LaAlO3. However, this phase alone is not sufficient to fully explain the structure, which is in agreement with the lower crystallinity (LaAlO3:LaB6 ratio = 2.0) in the XRD patterns. Upon addition of La2(CO3)2OH2 to the refinement, the Rwp was improved by 1–2%, with the features in the r = 1–10 Å showing a much-improved fit to the peak shapes, therefore suggesting that a carbonate phase is also present in the 700 °C calcined material. Repeating the multi-phase refinement with other lanthanum carbonate containing species (LaC2O2 and La2O2CO3; Table S6, Fig. S4) also yields an improved model fit to the experimental data. It should also be noted that the existence of lanthanum carbonate species also suggests the existence of equivalent amounts of Al2O3 in the system, albeit at a lower % scale due to atomic mass and hence are not refined. This analysis shows that whilst PDF can confirm the presence of lanthanum carbonate phases and incomplete perovskite formation, it is difficult to deconvolute the phases and PDF alone is not a sufficient technique to definitively identify the carbonate phases present.Further evidence of the nature of the amorphous impurity was provided by IR, shown in Fig. 3 , with bands at 1405 and 1477 cm−1 associated with the ν 3 mode of carbonate only being seen in Pt/LaAlO3-700. The moderate splitting of the ν 3 mode (Δ72 cm−1) shows that the D3h symmetry of the carbonate anion, while not completely retained, has not been significantly lowered. This indicates that there is a minor interaction between the metal centre and carbonate in Pt/LaAlO3-700 suggesting carbonate species that are not free. Note as a point of comparison, the crystalline hexagonal LaCO3OH, given in Fig. S5, has multiple stretching bands for carbonate at 1510, 1430, 1081, 870, 846, 725, and 680 cm−1 with ν 3 splitting (Δ80 cm−1) not dissimilar to the Pt/LaAlO3 sample, however, there are multiple other carbonate bands in the crystalline LaCO3OH that are not seen in Pt/LaAlO3-700 [40]. The IR therefore supports the incomplete formation of perovskite, as shown in PDF and the existence of amorphous La carbonate species.Pt dispersion measurements by CO chemisorption, show poor dispersion (3%) for the Pt/LaAlO3-700 compared to the Pt/LaAlO3-900 (48%) and Pt/LaAlO3-1100 (32%). Pt particle sizes were also determined from TEM micrographs, the values of which are given in Table 1, with histograms and micrographs in Fig. S6, and show similar particles sizes for each catalyst but with an increased standard deviation for Pt/LaAlO3-700. The similarity in particle size is in disagreement with the Pt dispersion measured in the Pt/LaAlO3-700, and this suggests that whilst there are small particles of Pt dispersed on LaAlO3-700, there are larger particles, as suggested by the large standard deviation, of Pt that are not imaged in the TEM micrographs. Alternatively, Pt particles supported on LaAlO3-700 may be highly unstable and sinter during the reduction process prior to CO chemisorption. Despite discrepancy in Pt particle dispersion, wt% loadings determined by MP-AES show all the catalysts are 1 wt% loading within error. The determined values are given in Table 1.XPS analysis of the Pt 4f, Al 2p and O1s levels of fresh Pt/LaAlO3- c are given in Fig. 4 . Atom percent and component binding energy is given in Table S7. The Al 2p region and Pt 4f region overlap in energy and it should be noted that shifts in Al 2p binding energy can be caused by oxidation number, ligand type and coordination, and therefore the chemical shifts in binding energies of oxides, hydroxides, and oxyhydroxides can be difficult to deconvolute [41,42]. All the materials show doublet peaks which correspond to Al-O, similar to those reported for LaAlO3 and Al2O3 [30,43]. The Pt/LaAlO3-700 material shows doublet peaks at 72.57 eV, which can be assigned to PtO [44]. As well as PtO, Pt/LaAlO3-900 and Pt/LaAlO3-1100 also show doublet peaks at 74.27 and 74.57 eV respectively, corresponding to PtO2 species [44]. The contribution of higher oxidation state Pt species indicates a different and stronger interaction and stabilisation of Pt species in the higher calcined LaAlO3 materials when compared to the Pt/LaAlO3-700 [45,46]. Pt/LaAlO3-700 also shows a lower at.% (0.54 at.%) when compared the Pt/LaAlO3-900 (1.51 at.%) and Pt/LaAlO3-1100 (0.77 at.%) agreeing with lower Pt dispersion and potentially larger particles in the catalyst.The O 1s levels for each of the materials can be considered as a combination of Olattice and Oads. Olattice encompasses M−O, M–CO3, and M–OH that can be present in the lattice [47]. Binding energies for metal hydroxides and carbonates are similar and therefore are not distinguished in peak fitting due to the potential for both components in the materials. Oads can include adsorbed oxygen from various species including hydroxyl, carbonate, water and bound reaction species [48,49]. The O 1s region of the fresh Pt/LaAlO3-700 show three peaks at 529.21, 530.66, and 532.24 eV corresponding to M–Ox, M–O, and M–CO3/M–OH respectively and the assignment remains the same for the Pt/LaAlO3-900 and Pt/LaAlO3-1100. However, a higher contribution for the M-CO3/M–OH peak (13.72 at.%) is seen with Pt/LaAlO3-700 than with the higher calcined materials (8.83–9.11 at.%), possibly due to the presence of the amorphous carbonates confirmed with IR and PDF. The La 3d levels of the fresh materials, given in Fig. S7, have doublet peaks with a multiplet splitting of 3.9 eV which can be assigned to La(OH)3 and is reported in single oxide and perovskite materials [50,51].In summary, calcination of LaAlO3 precursor at 700 °C leads to a crystalline perovskite phase, however evidence from LaB6 doping, PDF, IR, and XPS shows incomplete perovskite formation from the precursors with amorphous lanthanum carbonate impurities. These impurities can be removed by calcination at higher temperatures albeit at the expense of surface area and LaAlO3 particle sintering. PtOx nanoparticles were successfully dispersed on the LaAlO3 support materials with potentially residual amounts of Cl− arising from the wet impregnation synthesis method, which unfortunately were difficult to quantify due to the overlap of Cl 2p and La 4d features in the XPS. The phase purity affects the dispersion and speciation of Pt nanoparticles on the surface of the support, with a reduced support interaction for Pt/LaAlO3-700.The catalytic performance of the Pt/LaAlO3- c catalysts was investigated in the APR of 10 wt% aqueous glycerol under optimised conditions in a batch reaction as determined by Subramanian et al. for Pt/γ-Al2O3 [18]. The catalyst performance over 2 h reaction times is shown in Table 2 . The catalytic active site can be regarded as the Pt nanoparticles, due to the perovskite materials LaAlO3- c not showing any activity for the APR reaction [30].The catalyst performance was found to be better for catalyst supports that had been subjected to higher calcination temperature during the perovskite synthesis, with the highest glycerol conversion being with the Pt/LaAlO3-1100 at 20.4% (TOF = 686.4 h−1). The modestly higher reactivity of Pt/LaAlO3-1100 vs Pt/LaAlO3-900, whist the latter had a higher initial Pt dispersion, showed that there is no strong correlation with this parameter and activity. The Pt/LaAlO3-700 had the lowest conversion (5.4%) showing that the amorphous impurity has affected the catalyst activity. The H2:CO2 ratios for the Pt/LaAlO3-900 and Pt/LaAlO3-1100 were lower than the ideal ratio (2.33) suggesting competing hydrogen consumption reactions of unsaturated intermediates, as observed by Wawrzetz et al. for a Pt/γ-Al2O3 catalyst [52]. The higher than ideal ratio observed for the Pt/LaAlO3-700 catalyst suggest other hydrogen production reactions, such as dehydrogenation, are promoted over the reforming reaction at low conversions, which was observed for Pt/LaMO3 catalysts with low activity [30]. The combination of hydrogen production and consumption reactions lead to hydrogen formation rates that are similar for each Pt/LaAlO3 catalyst despite differences in H2:CO2 ratios.The carbon product selectivity for the observed liquid and gas phase carbon products is shown in Fig. 5 . The selectivity profile was, within error, identical between Pt/LaAlO3-900 and Pt/LaAlO3-1100, while Pt/LaAlO3-700 gave higher lactic acid (LA) and ethylene glycol (EG) selectivity at the expense of hydroxyacetone (HA) and ethanol. HA is proposed as the first intermediate from glycerol dehydration, which is a more reactive substrate than glycerol, and can be readily converted to 1,2-PD. LA is also produced from HA and the dehydrogenated first intermediate glyceraldehyde and LA is achieved in high selectivity over the Pt/LaAlO3-700, suggesting dehydration/dehydrogenation reactions are favoured at low conversions over reforming [21,52]. This agrees with the gas analysis suggesting hydrogen production reactions are favoured over reforming and agrees with previous studies on Pt/LaMO3 catalysts [30]. Minimal amounts of CO were recorded for all catalysts suggesting high WGS shift activity of the catalysts under APR conditions [11,53].MP-AES analysis of the reaction effluents, shown in Table 3 , showed limited leaching of Pt during the reaction suggesting strong metal support interaction between the particles and the support material. Leaching of La was highest for Pt/LaAlO3-700 and decreased in-line with increasing calcination of the perovskite suggesting higher stability. However, the high percentage of La leaching is still present in the Pt/LaAlO3-1100 sample, suggesting dissolution of the perovskite structure in all samples. Al leaching is not reported due to its stability under acidic media and the formation and restructuring of Al2O3 to boehmite under reaction conditions [24].XRD patterns, with 33 wt% LaB6, of the recovered Pt/LaAlO3- c after 2 h APR reaction are given in Fig. 6 a. Perovskite LaAlO3 is the dominant crystalline phase in the 900 and 1100 samples, with trace LaCO3OH (ICDD PDF 26–0815) in the 1100 sample. The small amount of LaCO3OH can be correlated to the higher activity of the Pt/LaAlO3-1100, with higher turnover for WGS and hence increased CO2 production. The XRD pattern for the Pt/LaAlO3-700 also shows loss of perovskite phase crystallinity and the formation of hexagonal LaCO3OH phase and La2O(CO3)2. x H2O (ICDD PDF 28–0512) phase, which can be seen in Fig. 6b. The lanthanum oxide carbonate phase is a precursor carbonate phase that, under hydrothermal reaction conditions, dissolves and recrystalises in the eventual formation of LaCO3OH. The loss of crystallinity within all samples, evidenced by the reduction of the apparent LaAlO3:LaB6 ratios (Table 3), agrees with the AES results that significant leaching has occurred during the initial 2 h reaction. The minimal evidence of crystalline by-phases post reaction for the 900 and 1100 samples explains the apparent contradiction in the literature surrounding LaAlO3 stability, i. e that perovskite decomposition produces amorphous phases not detectable by XRD. However, in the IR spectrum of all the recovered catalysts, given in Fig. S8, show bands at 1562, 1426, and 1406 cm−1 which are assigned to lower symmetry v 3 stretching modes which both relate to crystalline LaCO3OH (Fig. S5) and are indicative of the formation of this species. Importantly, it should be noted that bands observed at 3309, 3088, 1650 and 1066 cm−1 corresponding to O-H, C-H, C=O, and C-O stretches respectively as well as carbonate bands and this suggests binding of organic carbon species (CxHyOz) on the catalyst surface and potential blockage of sites.The Pt particle size, size distributions of which are given in Fig. S9 and values in Table 3, show an increase in Pt particle size for all samples when compared to the fresh samples. This is not without precedent, as Pt particle sintering, blockage of sites and particle migration is common under APR conditions and support material phase transformation [21]. The reduced stability of Pt/LaAlO3-700 can be shown with larger Pt particles and wider standard deviation than the other two samples. Evidence of the formation of poorly crystalline phases and subsequent Pt particle migration and are also seen in TEM images of Pt/LaAlO3-1100, given in Fig. S10, which confirm restructuring of the catalyst is occurring within the reaction timeframe.XPS analysis of the used catalysts confirm multiple changes and reconstruction on the catalysts surface upon catalytic APR testing. The O 1s and Pt 4f/Al 2p levels are given in Fig. 7 and the surface analysis data for the used catalysts is given in Table S8. In the Pt 4f/Al 2p region of all catalysts, a shift in the binding energy of Pt doublet peaks show reduction of the Pt species by reaction products, such as evolved H2(g), to Pt(0) from PtO and PtO2 [44,54]. A shift in binding energy is also noted for each catalyst in the Al-O doublet peaks, possibly corresponding to hydroxylation to Al species to AlO(OH) due to the acidic conditions of APR [24,55]. The O 1s region also shows higher energy peaks at 532.60–533.5 eV for each sample which correspond to adsorbed oxygen species. The presence of Oads correlates with the IR spectra showing bound surface species, which can lead to site blocking and catalyst deactivation. The La 3d levels, given in Fig. S11, show changes in the La environment of Pt/LaAlO3-700 and Pt/LaAlO3-1100 with a shift in the binding energy and reduction of multiplet splitting from 3.9 to 3.5 eV, which can be assigned as changes from La(OH)3 to La2(CO3)3 [30,56]. This is consistent with the dissolution of perovskite phase and eventual formation of LaCO3OH phase. The at.% of La also decreases and Al at.% increases consistent with La leaching in the AES.To further confirm the structural evolution of Pt/LaAlO3 into Pt/LaCO3OH-AlO(OH) under reactions conditions, Pt/LaAlO3-1100 was chosen to be tested under APR conditions at an extended reaction time (4 h). The XRD pattern, given in Fig. S12, of the recovered catalyst mixed with 33 wt% LaB6 show clear reflections for the crystalline hexagonal LaCO3OH phase alongside little amounts of the LaAlO3 phase.To further elucidate the breakdown and restructuring of the perovskite LaAlO3 materials, simulated reactions at standard reaction conditions (240 °C, 2 h, 42 bar, 1000 RPM) without impregnated Pt nanoparticles were undertaken. Reactions were chosen to investigate the effect of reaction conditions and of acidic products. Initially, LaAlO3- c was tested with 10 wt% glycerol and the XRD patterns, given in Fig. S13, show little changes in the 900 and 1100 samples, whereas the 700 sample has a small amount of lanthanum oxide carbonate phase present as well as LaAlO3 and LaCO3OH. This shows that the remnant carbonates within the LaAlO3-700 crystallise under reaction conditions. XPS analysis (Table S8) of the glycerol treated materials show limited changes in the LaAlO3-900 and LaAlO3-1100 materials when compared to the fresh material indicating surface stability under hydrothermal conditions. However, for the La 3d region of LaAlO3-700, given in Fig. 8 a(ii), a reduction in La at.% and multiplet splitting indicates the loss of surface La and formation of La2(CO3)3 species, in-agreement with the XRD and formation of lanthanum carbonate species. This was confirmed by MP-AES of the filtrate, given in Table 4 , which shows increased La leaching (17.9%) compared to the higher calcined materials (6–7%).Reactions with lactic acid were then chosen, as LA is one of the main acidic products that is formed under batch reaction conditions and can therefore contribute to the breakdown of the perovskite. The XRD of the recovered samples, shown in Fig. 9 a, after standard reaction with 1 wt% LA and 9 wt% glycerol shows crystalline perovskite was retained in all the samples, with minimal evidence of crystalline LaCO3OH. However, similarly with the post APR recovered catalysts, there is a dramatic reduction in crystallinity, evidenced by reduction of the LaAlO3:LaB6 ratio (Table 4), of the samples indicating the formation of amorphous phases. Elemental analysis of the effluent after the simulated reaction shows La leaching up to 86% of the original content for all samples. It is clear that the perovskite phase is highly unstable in the presence of lactic acid and in the absence of a carbonate source does not form crystalline carbonate biproducts.Confirmation of further structural changes was given by PDF, shown in Fig. 10 a, by refining a model against the LaAlO3-1100 after LA and glycerol reaction, which yields a good fit with an Rwp = 21.06%. Similarly, to the fresh LaAlO3-700 refinement, two phases were needed to fully describe the data, in this instance, LaAlO3 and AlO(OH). This multiphase approach allowed identification of the bulk of the sample, which remains the LaAlO3 phase, with a scale 88.68%. This shows that the perovskite phase is retained despite the reduction in crystallinity post treatment. However, the presence of AlO(OH) phase shows the creation of this poorly crystalline phase under reaction conditions which agrees with the La leaching under acidic conditions and previous studies of alumina transformation into hydroxylated species [24].XPS analysis of the recovered materials show shifts in Al 2p peaks (Table S8), possibly from hydroxylation of the Al species, in agreement with the PDF, and an increase in surface Al at.%. In the La 3d region, a large at.% reduction is noted in the La 3d region of all samples, indicating high La leaching. Although, the speciation only changes for the LaAlO3-700 to La2(CO3)3, as shown in Fig. 8b(iii). Therefore, a clear picture emerges regarding support stability in the presence of organic acids; most of the La has dissolved into the filtrate in all samples, with the remainder locked into residual crystalline perovskite phase and the exsolved aluminium is present as a disordered AlO(OH). Again, it is important to highlight that simple fingerprinting of the XRD of post reaction materials would reveal only the perovskite phase which could easily be misinterpreted as a stable phase, when it is in fact, as supported by XPS/PDF and LaB6 doping studies, it is clearly not.CO2 is an important product in the APR of glycerol and can readily dissolve into water forming carbonic acid [57]. Simulated reactions were carried out with a partial pressure of 1 bar PCO2, which equates to 2.3% glycerol conversion assuming an ideal reforming reaction (i.e. compete product selectivity to CO2 and H2). However, as seen from the catalytic results (Table 2 and Fig. 5 vide supra), selectivity towards complete reforming is low. Therefore, the PCO2 partial pressure added in simulated experiments equates to similar amounts produced in the Pt/LaAlO3-1100 batch reactions at 20% conversion. The XRD patterns of reactions with 1 wt% LA and 1 bar PCO2 are shown in Fig. 9b, with crystalline phases LaCO3OH and LaAlO3 clearly present in LaAlO3-900 and LaAlO3-1100, with only LaCO3OH and no LaAlO3 remaining in the LaAlO3-700 sample. PDF refinement of LaAlO3-700 after the LA, glycerol and PCO2 reaction, shown in Fig. 10b, yields a well-fitting model, with an Rwp = 21.44%, with this sample being dominated by the LaCO3OH phase with a smaller contribution from AlO(OH). This confirms that the treatment with PCO2 facilitates the phase transformation of LaAlO3 into LaCO3OH-AlO(OH) phases.The presence of Al species in the XPS in all samples (Table S8), under acidic conditions and PCO2 atmosphere, in significant amounts (30.07–31.80 at.%), despite limited crystalline Al phases in the XRD patterns also suggests the formation of amorphous or poorly crystalline AlO(OH) species. The La 3d region for the samples, given in Fig. 8b(iv), also confirm the change of surface speciation to La2(CO3)3. From the MP-AES (Table 4), the addition of PCO2 into the reaction mixture leads to a drop of La in the filtrate, relative to the 1 wt% lactic acid solution (42–53% vs 85–86%). The increased stability of La being due to the formation of the stable LaCO3OH phase under an overpressure of CO2. However, a large percentage of La remains in solution, indicating lanthanum carbonate hydroxide phase formation is incomplete under the reaction timescale (2 h). It is important to note, that the rate of LaAlO3 decomposition is different for each of the LaAlO3- c samples, which has an impact on Pt particle migration during the APR reaction. It is clear that the amorphous content present in the LaAlO3-700 significantly affects the stability of the material more than increasing particle size arising from the different calcination temperatures. This is shown in the differences in LaAlO3:LaB6 peak ratios in Table 4.Interestingly, testing of LaAlO3-900 with 10 wt% glycerol and 1 bar PCO2, in the absence of an organic acid, also formed LaCO3OH, evidenced by the XRD pattern in Fig. S15. This suggests that carbonic acid is acidic enough to decompose the perovskite and force the formation and stabilisation of the LaCO3OH phase, whilst sequestering some of the CO2 present. The amount of CO2 in the simulated reactions, however, is no more than what is produced under real catalytic reactions, and this suggests that residence time under acidic environment, as well as amount of CO2 is also important in material stability.To summarise, upon testing under hydrothermal conditions (glycerol), LaAlO3-700 decomposed slightly with lanthanum carbonate phases present, as well as perovskite, whereas the higher temperature calcined materials remained stable. Testing with organic acidic products (lactic acid), without a carbonate source, led to a reduction in perovskite crystallinity, extensive lanthanum leaching and AlO(OH) formation in all samples (Equation (1)). This confirms the instability of perovskite in acidic media, as predicted by Pourbaix diagrams (Fig. S16) [58–60]. (1) LaAlO3(s) + 3H+ (aq) + H2O(l) → La3+ (aq) + AlO(OH)(s) + 2H2O(l) (2) CO2(g) + H2O(l) → HCO3H(aq) → CO3 2− (aq) + 2H+ (aq) While other acids facilitate phase dissolution and segregation, it is the presence of a carbonate source (CO2), which dissolves as carbonic acid and dissociates to the carbonate species (Equation (2)), that facilitates the formation of lanthanum carbonate crystalline phases La2O(CO3)2. x H2O and LaCO3OH (Equations (3) and (4)). Surface terminating La(OH)3 can also be readily converted to lanthanum carbonate phases in the presence of a carbonate source (Equations (5) and (6)) [30]. The La2O(CO3)2. x H2O phase dissolves and recrystallises under hydrothermal conditions in the eventual formation of crystalline LaCO3OH (Equation (7)). It is important to note that these reactions are happening concurrently until the formation of crystalline LaCO3OH and AlO(OH). It has also been reported that glycerol can mediate the formation of LaCO3OH in combination with a carbonate source [61]. (3) 2La3+ (aq) + 2CO3 2− (aq) + yH2O(l) → La2O(CO3)2.xH2O(s) +zH+ (aq) (4) La3+ (aq) + CO3 2− (aq) + H2O(l) → LaCO3OH(s) + H+ (aq) (5) 2 La(OH)3 (s) + 2 CO3 2− (aq) → La2O(CO3)2. xH2O(s) (6) La(OH)3 (s) + CO3 2− (aq) → LaCO3OH(s) + 2 OH− (aq) (7) La2O(CO3)2.xH2O(s) → 2 LaCO3OH(s) + yH2O(l) The decomposition rates for each calcination temperature are different and this would impact Pt nanoparticle redistribution in real catalytic systems. It is also important to consider residence time in acidic products, which can be further investigated using a flow reactor.The stability of LaAlO3 perovskite supports, during the Pt catalysed aqueous phase reforming of glycerol, has been investigated with respect to the original calcination temperature used to produce the perovskite. Analysis of the perovskite precursor decomposition shows that whilst a calcination temperature of 700 °C yields the perovskite phase as the sole crystalline phase, there was significant amounts of amorphous carbonate material, as identified by PDF analysis. The amorphous carbonate could be expelled from the structure upon increasing calcination temperature above 850 °C. 1 wt% Pt/LaAlO3, prepared using a perovskite support calcined at 700 °C, had a notably poorer catalytic activity during aqueous phase reforming than catalysts prepared using a support calcination temperature of 900 °C or 1100 °C. Such low activity could be attributed to poor Pt interaction with the support containing amorphous content. The highest conversion being with the Pt/LaAlO3-1100 at 20.4% (TOF 686 h−1).Characterisation of the materials after batch APR reactions, and hydrothermal exposure of supports to simulated product mixtures, showed that the perovskite is replaced by amorphous content alongside La leaching under acidic conditions. Organic acids (i.e. lactic acid) attack the structure and chelate La causing leaching. This was observed for catalysts prepared at each of the three calcination temperatures, although the process was notably faster when using the perovskite containing residual carbonate (700 °C). The smaller size of the perovskite particles at lower temperature may also have some influence on stability, however, the presence of amorphous carbonate had a more pronounced effect. The support phase purity is an important factor in catalyst activity through material stability and Pt particle interaction.Formation of crystalline LaCO3OH was only observed when a ready source of carbonate, from CO2 production by reforming, was present. This phase, alongside nanocrystalline Al2O3 or AlO(OH), was consistently formed regardless of LaAlO3 calcination temperature, however, the rate of transformation was slowed with increasing calcination temperature. The effect of calcination temperature on the rate of transformation through phase purity and particle size is an important factor to consider when producing stable and active catalysts through support phase restructuring. When the crystalline LaCO3OH was formed leaching of La significantly dropped, showing that carbonic acid provides a stabilising effect vs organic acid, which enhance leaching.It is essential to note, that simple fingerprint analysis of the support by XRD would have missed the complexity and early stages of LaAlO3 decomposition. The identification of amorphous content and its structural determination by X-ray PDF and XPS analysis was required to fully understand the changes to support structure during reactions. Donald R. Inns: Conceptualization, methodology, investigation, formal analysis and visualization, writing – original draft; Xuetong Pei: Investigation, formal analysis; Zhaoxia Zhou: Investigation, Resources; Daniel J. M. Irving: Formal analysis and visualization, writing – original draft; Simon A. Kondrat: Conceptualization, project administration, supervision, funding acquisition, writing – review 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.We would like to acknowledge funding from the EPSRC CDT ‘fuel cells and their fuels’ (EP/L015749/1). The X-ray photoelectron (XPS) data collection was performed at the EPSRC National Facility for XPS (‘HarwellXPS’), operated by Cardiff University and UCL, under Contract No. PR16195. We would also like to acknowledge the use of the facilities within the Loughborough Materials Characterisation Centre (LMCC). Finally, we acknowledge Diamond Light Source, U.K., for access to beamline I15-1 as part of the Catalysis Hub BAG proposal (CY29757). Meta data for XRD, PDF and XPS is available at https://doi.org/10.17028/rd.lboro.20170784.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.2022.101230.

Aqueous phase reforming (APR) of waste oxygenates offers the potential for sustainable hydrogen production. However, catalyst stability remains elusive, due to the aggressive hydrothermal conditions employed. Herein, we show that the catalytic performance and stability of Pt supported on LaAlO3 catalysts for glycerol APR is strongly influenced by the phase purity of LaAlO3. Calcination of the support at 700 °C produces the LaAlO3 perovskite phase and an amorphous lanthanum carbonate phase, which can be removed by calcination at higher temperature. Catalysts comprised of phase pure LaAlO3 were notably more active, with a support calcination temperature of 1100 °C resulting in 20.4% glycerol conversion (TOF 686 h−1) in a 2 h batch reaction. Interestingly, all the catalysts, regardless of LaAlO3 phase purity, eventually transform into Pt/LaCO3OH-AlO(OH) during reaction, but only in the presence of evolved carbon dioxide, itself produced from glycerol reforming. Studies using simulated reaction products showed that organic acid products (lactic acid), in the absence of CO2, facilitated La leaching and loss of crystallinity. A carbonate source (CO2) is essential to limit La leaching and form stable Pt/LaCO3OH. Pt supported on LaCO3OH and AlO(OH) are stable and active catalysts during APR reactions. Yet, the rate of perovskite phase decomposition strongly influences the final catalyst performance, with the initially phase impure LaAlO3 decomposing too quickly to facilitate Pt redistribution. LaAlO3 calcined at higher temperatures evolved more slowly and consequently produced more active catalysts.

The authors are unable or have chosen not to specify which data has been used.The rising CO2 emissions, global warming and the risks posed by climate change have put the carbon dioxide capture and utilization (CCU) technologies in the limelight. In this scenario, the catalytic processes, especially the catalytic hydrogenation of CO2 for the production of liquid fuels and bulk chemicals, receive a great deal of attention [1–4]. The interest of these processes leads in their contribution to the circular carbon economy, by replacing the fossil sources by CO2 as carbon source, and using green hydrogen and renewable energy for the products with an increasing demand in the petrochemical industry [5].The processes for the direct synthesis of hydrocarbons from CO2 (in a single reactor) show several thermodynamic benefits as opposed to the indirect routes (in two stages). Moreover, lower capital investment and operating cost are required. Two main routes can be distinguished for the direct conversion of CO2 into hydrocarbons, and both of them are carried out by means of tandem catalysts [6–9]. In the Modified Fischer Tropsch (MFT) synthesis, CO2 reacts according to the Anderson-Schulz-Flory (ASF) mechanism, characteristic of the FT synthesis, using Fe- or Co-based catalysts. The products are in situ reformed over a zeotype providing the adequate acidity and shape selectivity to the selective production of the desired hydrocarbon fraction [10,11]. On the other hand, in the route with oxygenates (methanol/dimethyl ether (DME)) as intermediates, OX/ZEO (metallic oxide/zeotype) catalysts are employed, in which the metallic oxide is the responsible of the formation of the oxygenates, and the zeotype is used for the selective conversion of these into hydrocarbons [12,13]. In both routes, the integration of the two reaction stages helps to: (i) diminish the required investment and the operating costs in contrast to the processes with two reactors; and (ii) to benefit the thermodynamics, because of the shifting of the equilibrium of the CO2 conversion step. Guo et al. [14] made a comprehensive thermodynamic study of the hydrogenation of CO2 to alcohols and hydrocarbons (ethylene, propylene, benzene), proving the thermodynamic feasibility of these processes and pointing out thermodynamics as the necessary preliminary step to establish the appropriate reaction conditions and catalyst to maximize CO2 conversion and hydrocarbon yield.The knowledge on the catalysts and mechanisms of the routes for the direct conversion of CO2 into hydrocarbons is based on the prior knowledge of the individual stages in the indirect routes. In that way, for methanol/DME synthesis, Cu-based metallic catalysts (mainly Cu-ZnO-Al2O3) have been used in the industry (originally with syngas and more recently for CO2 hydrogenation), due to their low cost and high performance [15–17]. The role of the ZnO is key to increase the dispersion of Cu and to minimize its sintering [15,18]. The presence of Al2O3 as promoter provides surface area and eases the separation of Cu-ZnO sites, resulting in an increase of the stability as well as mechanical resistance [19]. The replacement of Al2O3 by ZrO2 forming Cu-ZnO-ZrO2 catalysts resulted to improve the stability of the catalyst, which is especially interesting in the CO2 hydrogenation due to the high content of H2O in the medium [20,21]. The hydrogenation of CO2 to methanol over Cu-based catalysts is considered to proceed according to the formate species (HCOO*) as first intermediate species and successive hydrogenations [22–24]: CO2 → HCOO* → HCOOH* → H2COOH* → H2CO* → H3CO* → CH3OH* → CH3OH. The smallest extent of the alternative route, i.e., the conversion of CO2 into CO by means of the reverse Water Gas Shift (rWGS) reaction and the hydrogenation of CO to formyl species, is explained by the instability of this intermediate, that is decomposed to form CO and H2.Among the catalysts developed to avoid the limitations (temperature, H2O concentration) of Cu-based catalysts in the direct synthesis of hydrocarbons from CO2, those based on In2O3 have received great attention due to their high activity for the conversion of CO2 into methanol and their stability at the temperature required (>350 °C) for the conversion of methanol/DME to hydrocarbons [6,25,26]. Moreover, In2O3 is known to suppress the rWGS reaction, avoiding the initial CO2 to CO shift taking place over the Cu-based catalysts [27]. In In2O3 catalysts, CO2 is adsorbed and activated in the oxygen vacancies, and produces formate species following the sequence [28,29]: CO2 → HCOO* → HCOOH* → H2COOH* → H2COHOH* → H2CO* → H3CO* → CH3OH* → CH3OH. The incorporation of ZrO2 as promoter boosts the formation of additional oxygen vacancies and increases the stability of In2O3 [27,30,31]. The properties of In2O3-ZrO2 have been improved by incorporating Ni [32] and noble metals such as Pd [33,34], Rh [35], Pt [36] or Au [37].Zn containing oxides have also pointed out among methanol synthesis catalysts by providing high CO2 conversion and methanol selectivity, especially when combined with ZrO2 as support, which helps to increase methanol selectivity [38,39]. The properties of ZnO-ZrO2 catalysts are enhanced with the incorporation of noble metals [40]. Wang et al. [41] established that the reaction mechanism of CO2 hydrogenation to methanol over ZnO-ZrO2 based catalyst are both formate and CO reaction pathway. In addition to the great performance, this catalyst has shown to be highly stable, due to the formation of the ZrZnOx solid solution, and it does not undergo deactivation in long catalytic runs (up to 500 h).As aforementioned, in the direct synthesis of hydrocarbons from CO2, for the methanol/DME conversion into hydrocarbons (second reaction stage), zeolite-based catalysts are used. The activity, selectivity and stability of the zeolites are a direct consequence of their properties, especially of the shape selectivity and the acidity [42]. It is well established the dual cycle mechanism for the conversion of methanol/DME [43,44]. This mechanism takes place by the formation of light olefins as primary products by means of the cycles of alkylation/dealkylation of the intermediate polymethylbenzenes confined in the catalysts, and of oligomerization/cracking of the light olefins. The extent of the secondary reactions (alkylation, isomerization, condensation to aromatics) favors the formation of light paraffins (by hydrogen transfer and cracking), BTX aromatics and not aromatic C5+ hydrocarbons, especially interesting for their use as green gasoline. Therefore, the main challenge is the election of a selective catalyst for each aim. For the selective production of light olefins, SAPO-34 (CHA framework) is highly employed [42,45–47]. As an example of good results in the literature, Zhang et al. obtained with GamCrOx/HSAPO-34 catalyst a CO2 conversion of 11.9% and a selectivity of light olefins of 87.3% (excluding CO) at 350 °C and a selectivity of 34.5% to CO, at 350 °C and 30 bar [48]. On the other hand, the drawback of the rapid deactivation by coke deposition (assisted by the easy confinement of the polymethylbenzenes in the cages of the porous structure of SAPO-34) is lessened with the particular operating conditions (high H2 and H2O partial pressure) [49].HZSM-5 zeolite is the most studied catalyst for the production of higher hydrocarbons (such as aromatics or linear paraffins in the gasoline-range) from CO2 [13,50,51], owing to its MFI structure, that eases a major extent of the dual cycle mechanism in the conversion of methanol/DME and also of some of the secondary reactions. Moreover, its versatility towards different products in the conversion of methanol/DME by the generation of hierarchical porous structures, the adjustment of the acidity and the incorporation of metals is well established [52–54]. The porous structure of HZSM-5 zeolite (without cages in the intersections) facilitates the diffusion of the intermediate coke precursors, delaying their confinement and attenuating the deactivation [55]. Ticali et al. [50] related the higher interest of ZnZrO2/HZSM-5 catalyst for the production of aliphatic compounds in contrast to ZnZrO2/SAPO-34 highlighting higher conversion and stability of HZSM-5 at lower temperature and space time.After the development of the direct synthesis of hydrocarbons from syngas, the direct conversion of hybrid feeds (H2/CO2/CO) is gaining awareness [46,56], on account of the interest in terms of sustainability and joint valorization of CO2 with syngas obtained from biomass [57,58] or waste [59,60]. Moreover, syngas co-feeding partially provides the required hydrogen. Additionally, CO co-feeding also favors thermodynamically the production of methanol as compared to the hydrogenation of CO2 by attenuating the extent of the reverse Water Gas Shift reaction [14,61]. Moreover, the differences in the role of CO when it is formed by the rWGS reaction as a byproduct or when it is co-fed with CO2 has been assessed [14].In this context, the performance of three different metallic oxides (CuO-ZnO-ZrO2, In2O3-ZrO2, ZnO-ZrO2) was compared for their interest to activate the methanol synthesis step in the direct production of gasoline-range C5+ hydrocarbons from CO2 and mixtures of CO2/CO. The results with these catalysts in the synthesis of methanol are continued in this manuscript with those obtained using them in tandem together with HZSM-5 zeolite, aiming at selecting both the metallic oxide and the appropriate operating conditions for the selective production of isoparaffinic gasoline with commercial interest as a fuel. The results allow to assess the prospects of a ZnO-ZrO2/HZSM-5 catalyst for an attractive target (gasoline production), that has received less attention in the literature, and which is complementary to other goals in the catalytic CO2 valorization processes, such as the production of light olefins or aromatics. Considering the importance of the results for the decarbonization objective, attention will also be paid to the CO2 and COx conversion results, attending to the interest of also valorizing the syngas obtained from biomass or wastes.The metallic catalysts for the methanol synthesis step, i.e., CuO-ZnO-ZrO2, In2O3-ZrO2 and ZnO-ZrO2, named in a simplified way CZZ, IZ and ZZ, respectively, were synthesized following a co-precipitation method. CZZ catalyst was prepared with a Cu/Zn/Zr atomic ratio of 2/1/1 following the method described by Sánchez-Contador et al. [20]. IZ catalyst, with an atomic In/Zr ratio of 2/1, was prepared following the method described by Portillo et al. [30]. For the synthesis of ZZ, a metal nitrate solution with 6.00 g of Zn(NO3)2·6H2O (Sigma-Aldrich) and 13.69 g of ZrO(NO3)2·6H2O (Sigma-Aldrich) was co-precipitated over 59.12 mL of deionized water, and a (NH4)2CO3 solution (VWR Chemicals, 1 M) was added dropwise under continuous stirring to form a precipitate with an atomic Zn/Zr ratio of 1/2.5. This synthesis method was based on a previous work of Li et al. [62] and slight modifications were considered. The three catalysts were prepared at 70 °C and neutral pH. After the co-precipitation, the catalysts were aged, filtered and washed with deionized water. Afterwards, the catalysts were dried and calcined (at 300 °C for 10 h, at 500 °C for 1 h and at 500 °C for 5 h for CZZ, IZ and ZZ catalysts, respectively) in order to provide the corresponding metal oxides, according to the protocols established for each catalyst [20,30,62]. The resulting powders were pelletized to provide higher mechanical resistance, and sieved to a particle size in the 125–250 μm range.As acid catalyst, a commercial HZSM-5 zeolite (Zeolyst International) with a Si/Al ratio of 140 was used. The election of the zeolite is a complex decision. This Si/Al ratio was selected to minimize cracking reactions and to increase the gasoline yield. The zeolite, provided in ammonium form was calcined at 575 °C for 2 h to obtain the acid form, pelletized and sieved to a particle size between 300 and 400 μm. These calcination temperature is appropriate for equilibrating the catalyst, so that it can recover its activity when used in reaction-regeneration cycles, after the elimination of the coke by air combustion at 550 °C [63]. The different particle size of both catalysts was selected as to ensure the easy separation after the reaction, having formerly proved that no diffusional limitations occur with these sizes. The tandem catalysts (CZZ/HZSM-5, IZ/HZSM-5 and ZZ/HZSM-5) were prepared by physical mixture of both metallic and acid catalysts, with a metal/acid mass ratio of 1/1.The physical properties of the catalysts (Table 1 ) were determined by N2 adsorption-desorption isotherms (Micromeritics ASAP 2010). For this, the samples were degassed in vacuum conditions prior to the analysis, in order to remove impurities and H2O adsorbed on the surface of the catalyst. Afterwards, N2 adsorption-desorption equilibrium stages were conducted at −196 °C. It is remarkable that among the metallic catalysts, CZZ has a more favorable porous structure for the access of the reactants and diffusion of the intermediates and products, with higher values of BET surface area (SBET), pore volume and mean size of pore diameter, whereas the ZZ catalyst has the lowest values of these properties. The properties of the HZSM-5 catalyst are characteristic of this zeolite, and correspond to a mostly microporous structure, whereas the presence of mesopores is due to the pelletization step. It should be noted that the kinetic results in Section 3.2 highlight the minor importance of these properties and the fundamental role of the different activity of the active sites of the catalysts in the corresponding reaction mechanism.The chemical composition and atomic ratios were quantified by X-Ray fluorescence (XRF), by means of a PANalytical wavelength dispersive X-ray fluorescence sequential spectrometer (WDXRF), model AXIOS, equipped with a Rh tube and three detectors (gas flow, scintillation and Xe sealing). Results are shown in Table S1. The structure was determined by X-Ray diffraction (XRD) with a PANalytical Xpert PRO diffractometer, equipped with copper tube (λCuKα = 1.5418 Å), a vertical goniometer (Bragg-Brentano geometry), secondary monochromator and PixCel detector. The measurement conditions were 40 kV/40 mA and the pattern was recorded in a 5 < 2θ < 60 range for CZZ catalyst and in a 5 < 2θ < 80 range for IZ and ZZ catalysts.The normalized XRD patterns of the three metallic catalysts are gathered in Fig. 1 . According to the diffractograms, IZ comprises cubic structure for both In2O3 and ZrO2 oxides (in accordance with ICDD (International Center for Diffractional Data) #71–2195 and #49–1642, respectively), corresponding to the state with the highest catalytic activity [64]. On the other hand, hexagonal ZnO (in accordance with ICDD #36–1451) and cubic ZrO2 (in accordance with ICDD #49–1642) structures were found in ZZ catalyst. Regarding the traditional CZZ catalyst, its structure was described thoroughly elsewhere [20]. Briefly, the characteristic peaks of CuO and ZnO oxides were visible on the spectra, while ZrO2 peaks were not noticeable due to the high dispersion and small size of the crystals.H2 temperature programmed reduction (H2-TPR) analyses were carried out (Micromeritics Autochem 2920) to study the reducibility of the catalysts. For this assay, 100 mg of sample were treated previous to the reduction by sweeping with He, to remove possible impurities and H2O. The H2-TPR analysis was carried out heating the sample up to 800 °C at a 2 °C min −1 rate in a diluted H2 stream (10% H2 in Ar). Attending to the TPR profiles (Fig. S1), CZZ is completely reduced at temperatures above 200 °C, whereas IZ and ZZ require higher temperature, so they might be in their oxide form at the beginning of the reactions. The same equipment was used for measuring the acidity by means of NH3-TPD analyses. 50 μL min −1 NH3 were injected at 150 °C until the saturation of the sample. The desorption step was conducted with a 5 °C min −1 rate up to 550 °C in a He stream. Fig. S2 exhibits the NH3-TPD profile of the HZSM-5 catalyst. The total acidity of this zeolite accounts for 62 μmolNH3 gcat −1, with a peak at 190 °C and a higher one at 320 °C, stating low total acidity but a great presence of strong acid sites according to the classification in the literature [65].The catalytic runs were performed in an isothermal packed bed reactor (PID Eng & Tech Microactivity Reference). The reactor is made of 316 stainless steel and has a ceramic coating to avoid the contact of the reactants with the steel and so, any possible side reaction. The internal diameter of the reactor is of 9 mm and it has an effective length of 10 cm. This equipment can operate at a pressure up to 100 bar and temperatures up to 700 °C. The catalytic bed is composed of a mixture of the catalyst and an inert solid (SiC), in order to ensure isothermal conditions of the bed, to avoid preferential flow paths and to achieve sufficient bed height when operating at low space time values.The feed and reaction product streams were analyzed on-line in a micro chromatograph (Varian CP-4900, Agilent), equipped with three analysis modules composed of TCD detectors and the following columns: (i) molecular sieve (MS-5) (10 m × 12 μm) for the quantification of H2, O2, N2 and CO; (ii) Porapak Q (PPQ) (10 m × 20 μm) for the quantification of CO2, methane, H2O, C2-C4 hydrocarbons, methanol and DME; and (iii) 5 CB (CPSiL) (8 m × 2 μm) for the quantification of C5+ hydrocarbons.The reaction runs of methanol synthesis (with the CZZ, IZ and ZZ metallic catalysts) were carried out under the following conditions: 250–430 °C; 50 bar; space time, 6 gcat h molC −1; CO2/COx molar ratio in the feed, 0, 0.5 and 1; H2/COx molar ratio in the feed, 3. The reactions for the direct synthesis of hydrocarbons (with the CZZ/HZSM-5, IZ/HZSM-5 and ZZ/HZSM-5 tandem catalysts) were performed under the following conditions: 340, 380 and 420 °C; 30 and 50 bar; space time, 12 gcat h molC −1; CO2/COx ratio in the feed, 0.5 and 1; H2/COx ratio in the feed, 3. Prior to all the reaction runs, the catalysts were subjected to a partial reduction in a H2 and N2 stream (1 h at 350 °C, 2 bar and with a flow rate of 30 cm3 H2 min−1 and 30 cm3 N2 min−1).The conversions of CO2 (XCO2) and of COx (XCOx) were defined according to the expressions: (1) X C O 2 = F C O 2 0 − F C O 2 F C O 2 0 · 100 (2) X C O x = F C O x 0 − F C O x F C O x 0 · 100 where F0 CO2 and F0 COx are the molar flow rates of CO2 and COx at the inlet of the reactor, respectively, and FCO2 and FCOx are the corresponding values at the reactor outlet stream.The yield and selectivity of each i product (Yi and Si, respectively) excluding CO2 and CO, were calculated as: (3) Y i = n i · F i F C O x 0 · 100 (4) S i = n i · F i ∑ i n i · F i · 100 where ni is the number of carbon atoms of the i compound and Fi the molar flow rate of the i compound in the products stream in content C atoms. It should be noted that with the definition of yields in Eq. (2), XCOx is the sum of the yields.The reactions presented in this section were conducted without zeolite, with the aim of testing the metallic catalysts alone in the first stage of the gasoline production (synthesis of methanol) in the 250–430 °C range. Fig. 2 shows the effect of the temperature on the conversion (XCOx) of an equimolar mixture of CO2 and CO, and on the selectivity of methanol and other byproducts (CH4, C2-C4 paraffins and C2-C4 olefins) with the three catalysts. Comparing the results, notable differences are observed. With CZZ catalyst (Fig. 2a), high conversion was reached at low temperature. In fact, XCOx accounted for 19% at 280 °C, which corresponds with the thermodynamics prediction [14,66–68]. Moreover, the selectivity of oxygenates (mainly methanol, with an insignificant DME content) was 100%. Nonetheless, XCOx decreased steadily with increasing temperature, until declining to 3.5% at 430 °C, due to the thermodynamic limitation. In addition, selectivity of methanol also decreased at high temperature due to the favoring of CO formation by the rWGS reaction. These results are in accordance with the prediction of thermodynamic studies in the literature [14,66–68]. The presence of C2-C4 paraffins is explained by the hydrogenation of the light olefins formed from the conversion of oxygenates, and the presence of CH4 over 340 °C exposes the activity of CZZ in the methanation at this temperature.XCOx values were lower with the IZ (Fig. 2b) and ZZ (Fig. 2c) catalysts. These catalysts had similar activity, lower than that of the CZZ catalyst. The XCOx reached a maximum value of 4.7% at 340 °C with IZ catalyst, and between 340 and 370 °C with ZZ catalyst, which decreased above these temperatures due to thermodynamic limitations, that also affect to the conversion of CO [14,66,67]. It is noteworthy that methanol selectivity was higher with IZ and ZZ catalysts than with the CZZ catalyst (Fig. 2a). In this sense, the best performance corresponded to the ZZ catalyst, with a methanol selectivity of almost 100% in the 340–370 °C range. Indeed, selectivity only decreased slightly with increasing temperature up to 430 °C. Considering the aforementioned results, the higher activity of CZZ catalyst for methanol production below 300 °C (with a maximum at 280 °C) is of arguable interest from the perspective of its use in the direct conversion of CO2/CO mixtures to hydrocarbons, since this reaction must be performed at higher temperature to achieve the extent of the dual cycle mechanism to obtain C5+ hydrocarbons. In this regard, the reduced methanation activity of the ZZ catalyst in the 300–400 °C is of particular interest. It is noteworthy that this better performance of the ZZ catalyst with respect to the other catalysts cannot be attributed to the properties of its porous structure (Table 1), because these are less favorable for the diffusion of the reactants and products. Consequently, it should be attributed to the high activity and selectivity of the active sites of the ZZ catalyst in the methanol formation mechanism explained by Wang et al. [41] with formate ions and CO as intermediates in the reaction pathway.In Fig. 3 the effect of the feed composition (CO2/COx ratio) on methanol yield is shown. The results correspond to 350 °C, temperature considered as limit to avoid the sintering of the Cu on CZZ catalyst [27]. This catalyst is the most active for CO hydrogenation, with a methanol yield of 21%, higher than with IZ (4%) and ZZ (2%) catalysts. At higher CO2/COx ratio, methanol yield remarkably decreased with CZZ catalyst. This trend fits with previous findings regarding the effect of the CO2 content in the feed [66,69] in high conversion conditions (high concentration of methanol), for which the presence of CO is preferable to CO2, as it eases the H2O removal by means of the WGS reaction. As could be expected, the methanol yield in the CO2 conversion is lower than that obtained in the literature with catalysts of similar composition under optimal conditions for methanol synthesis, i.e., lower temperature and higher pressure than those used [70]. On the other hand, the results with IZ and ZZ catalysts showed a similar trend. They both exhibited the highest methanol yield when the carbon source of the feed was 50% CO and 50% CO2. This concurs well with previous works in the literature with the IZ catalyst [25,56]. This occurs because the reaction mechanism lies on the creation and eradication of oxygen vacancies, and the joint feed boosts this process and, additionally, favors the preservation of the oxygen vacancies.It is also outstanding in Fig. 3 that, for the hydrogenation of CO2 (CO2/COx of 1), the obtained methanol yield was similar with the three catalysts. This result evidences the aforementioned limitation of the equilibrium conversion, and that this conversion is low in CO2 hydrogenation. This is in accordance with thermodynamic studies in the literature [14,66–68]. It is also observed that with IZ catalyst methanol yield was similar in CO and CO2 hydrogenation.With the purpose of assessing the performance of the metallic catalysts used in tandem, in Figs. 4 and 5 corresponding to IZ/HZSM-5 and ZZ/HZSM-5, respectively, the effect of temperature (340–420 °C range) and pressure (30 and 50 bar) on the conversion of COx (sum of the products yields, Eq. (2)) and CO2 and on the different products yield is shown. The results correspond in both cases to an equimolar feed of CO2 and CO (CO2/COx of 0.5) and hydrogen. It should be noted that the results for the CZZ/HZSM-5 catalyst are not shown because the sintering of Cu above 320 °C was verified. In fact, an increase of the crystal size from ∼10 nm (fresh catalyst) to ∼35 nm was determined by XRD analysis of the spent catalyst (Table S2). On the contrary, IZ/HZSM-5 and ZZ/HZSM-5 spent catalysts maintained constant their properties in long reaction runs at these temperatures. Consequently, the attention was focused in these two catalysts because of their stability in the required temperature range.It is also remarkable (in Figs. 4 and 5) that the C5+ hydrocarbons are the main products for the two catalysts and the oxygenates are almost completely converted. At higher pressure, the results upturned, boosting the overall COx conversion. Regarding the IZ/HZSM-5 catalyst (Fig. 4) at 30 bar, the influence of the temperature was more subdued. COx conversion did not increase >2% when rising temperature from 380 to 420 °C. Regarding the CO2 conversion, it was more affected by temperature at the lower pressure of 30 bar, rising from 8% to 23% by increasing temperature from 340 to 420 °C. For its part, at 50 bar, the CO2 conversion reached 28% at 420 °C. At 420 °C and lower pressure (30 bar), the C5+ hydrocarbons yield was of approximately 7%, with a COx conversion of 22%. Nonetheless, under a pressure of 50 bar and at the same temperature, the obtained products were highly interesting for the insight into sustainable fuels production. With almost no methane yield (<0.5% at 30 bar), and nearly complete oxygenates conversion, the remaining products were composed of C2-C4 paraffins (with a yield of 3.5%), C2-C4 olefins (2%) and C5+ heavier compounds (17.3%) at the optimal conditions. The presence of olefins was not particularly outstanding, as they are chiefly hydrogenated due to the high H2 partial pressure.For ZZ/HZSM-5 catalyst (Fig. 5) the result of C5+ hydrocarbon yield was even improved compared to IZ/HZSM-5. The CO2 conversion boosted from 8.1% to 28.3% when increasing the temperature from 340 to 420 °C (at 30 bar); and the COx conversion enhanced from 12.8% to 28.3% when rising the operating pressure from 30 to 50 bar (at 420 °C). In addition, the CO2 conversion reached 40% under 420 °C and 50 bar, since, unlike IZ, ZZ catalyst hardly inhibits the rWGS reaction. Under such conditions, besides methane and methanol (whose yield did not exceed 1%), C2-C4 paraffins, C2-C4 olefins and C5+ hydrocarbons yields accounted for 5.1%, 1.5% and 20.7%, respectively. These hydrocarbons were mainly composed by 5 and 6 carbon number isoparaffins and some cyclic hydrocarbons that will be further itemized below. Fig. 6 shows the CO2 conversion and the product distribution (in yield terms) achieved with each catalyst in the optimal conditions (420 °C and 50 bar) for the hydrogenation of CO2 and of an equimolar mixture of CO2 and CO. There are some remarkable aspects to highlight in these results that evidence the better performance of the ZZ/HZSM-5 catalyst. As mentioned above, CZZ was not an applicable catalyst for H2 + CO2 valorization. At temperatures above 350 °C Cu sintered, because of both temperature and water content (especially high with CO2 content feeds). Nevertheless, the results with this catalyst are summarized in Fig. S3. The COx conversion at 420 °C and 50 bar did not reach 2.5% with the CZZ/HZSM-5 catalyst, since almost no oxygenates were produced at these conditions. On the other hand, with the hybrid feed (CO2/COx = 0.5) there was a higher content of oxygenates, which, however, were not successfully converted into C5+ hydrocarbons (<3%), as roughly all the hydrocarbons remained as C2-C4 paraffins, due to the poor synergy between sintered CZZ and the HZSM-5. This poor performance of CZZ is explained by the accumulation of unfavorable circumstances such as the sintering of Cu in the catalyst (Table S2) and the reduced activity of Cu catalysts for CO2 conversion. These circumstances further deteriorate under the used reaction conditions (unfavorable for the methanol synthesis step according to thermodynamics) [14,66–68]. With regard to IZ/HZSM-5 catalyst, as noted above, it showed better performance with a mixture of CO and CO2 in the feed, which is in agreement with the finding of Araújo et al. [56] about the better preservation of the oxygen vacancies for higher CO content in the feed than for a CO2 and hydrogen feed. Besides the reduced conversion, the production of gasoline-range hydrocarbons fell sharply for the H2 + CO2 feed, revealing that IZ might not be the best metallic catalyst for gasoline-range hydrocarbon production in these operating conditions. In fact, the conversions (XCO2 and XCOx) and C5+ hydrocarbons yield was higher with ZZ/HZSM-5 catalyst for both feeds. The values obtained for these indices with the CO2/CO mixture were of 39.7%, 28.4% and 20.7%, respectively. Additionally, ZZ/HZSM-5 was not affected by the higher content of CO2 in the feed in such manner. Actually, the COx conversion fell merely from 28% to 26% for H2 + CO2 feed. All this evidences the powerful interest of the ZZ/HZSM-5 as a feasible industry catalyst, as it could cope adequately with the current fluctuations of the feed composition in this process.In order to assess the importance of the synergy of the tandem catalysts on the reaction mechanisms, both in the synthesis of oxygenates and in the conversion of these into hydrocarbons, in Fig. 7 the effect of the temperature on the COx conversion for IZ and ZZ metallic catalysts and for IZ/HZSM-5 and ZZ/HZSM-5 tandem catalysts is compared. The results correspond to the hydrogenation of the equimolar mixture of CO2 and CO. As aforementioned in the synthesis of methanol (Fig. 2), the results were similar for the two catalysts above 350 °C as a consequence of the thermodynamic constraints. These constraints are removed with the presence of the HZSM-5 zeolite in the tandem catalysts, due to the shift of the equilibrium by the immediate conversion of the oxygenates. When comparing the results of the two tandem catalysts, the benefit of the synergy between the two reaction steps was more remarkable with the ZZ/HZSM-5 catalyst. At optimal conditions for the integrated process (420 °C and 50 bar), COx conversion was multiplied ∼12 times (from 1.8% to 23%) with IZ/HZSM-5 with respect to the synthesis of methanol with IZ catalyst, whereas it increased a factor of >15 with the ZZ/HZSM-5 catalyst (from 1.8% to 28%) with respect to ZZ catalyst. Fig. 8 exhibits the yield of the different hydrocarbons in the products stream with IZ/HZSM-5 (Fig. 8a) and ZZ/HZSM-5 (Fig. 8b) catalysts. These results allow to compare the performance of the two catalysts from the perspective of product interest. Additionally, the comparison of the results in the hydrogenation of the equimolar mixture of CO2 and CO, and of CO2 was assessed. The majority of hydrocarbons produced with both catalysts were C6, C5 and C4 (in this order from highest to lowest). The highest yields (9.8%, 8.6%, and 4.1%, respectively) were obtained with ZZ/HZSM-5 catalyst for the equimolar mixture. In addition, with this catalyst the yield of C6 fraction was virtually the same in the hydrogenation of CO2 and of the CO2/CO mixture, which evidences that ZZ/HZSM-5 catalyst withstands in a good way the fluctuations in the feed. It is also remarkable that the gasoline fraction (C5+ with a yield of 20.7% with ZZ/HZSM-5 catalyst) was mainly isoparaffinic with both catalysts. This elevated isoparaffin content is in accordance with the well-established activity of the HZSM-5 zeolite-based catalysts for the isomerization of the corresponding linear paraffins [71]. In addition, using HZSM-5 catalysts doped with Zn (by ion exchange or isomorphically substituted) in the conversion of DME at high pressure and in the presence of H2, the high hydroisomerization activity of Zn, favored by its capacity for H2 dissociation and surface H generation, has been determined [72,73]. Because of the favorable conditions, the total yield of C5 and C6 isoparaffins (2- methylbutane, 2-methylpentane and 3-methylpentane) reached nearly 20% in these operating conditions, with almost no C4+ n-paraffin production. Besides, the high temperature and the elevated hydrogen content hinder the dehydrocyclization and aromatization reactions, resulting in low yield of cycloalkanes (2.6%) and aromatics (0.1%). On the other hand, compounds of >7 carbon atoms were not very significant (with a total yield of 2.3%). These results are a consequence of the properties of the metallic oxide and the zeolite used in the tandem catalyst. In this way, the hydrogenating activity of the Zn-based metallic catalyst in high pressure conditions and with the presence of H2 hindered the formation of aromatics [73]. This behavior of the Zn is different from that without the presence of H2 in the feed, where the presence of CO2 favors the formation of aromatics from methanol [74]. On the other hand, the moderate total acidity and the Brönsted/Lewis ratio of HZSM-5 limited the extent of heavier hydrocarbon formation reactions [74]. Consequently, the high isoparaffin content and the low presence of linear paraffins resulted in a high Research Octane Number (RON) hydrocarbon mixture with ZZ/HZSM-5 catalyst (RON of 91.8 determined according to the method proposed by Anderson, Sharkey and Walsh [75]), indicating high quality gasoline fraction. Its characteristic composition, without the presence of aromatic compounds, is of great interest for its incorporation to the refinery gasoline pool.As concluded in preceding results in Fig. 8, comparing the catalysts, ZZ showed better performance than IZ when operating in tandem with HZSM-5. Nonetheless, for each feed composition, the trend for both catalysts was virtually the same. However, it is observed that IZ/HZSM-5 catalyst was more afflicted by alterations in feed compositions (Fig. 8a), resulting in considerably lower yield of isoparaffins with an increasing CO2/COx ratio, whereas ZZ/HZSM-5 catalyst withstood better the changes in feed, maintaining almost unchanged the production of isoparaffins (Fig. 8b).The results ratify the good performance (high activity, selectivity of methanol and stability) of In2O3-ZrO2 and ZnO-ZrO2 catalysts in methanol synthesis, especially from CO2, and also from CO2/CO mixtures, at appropriate conditions for the direct synthesis of hydrocarbons. Moreover, both catalysts showed great performance when used in tandem together with a HZSM-5 zeolite, exposing the effective synergy between the mechanisms of methanol formation and its conversion into hydrocarbons, obtaining a high yield of C5+ hydrocarbons.It is especially significant the performance of ZnO-ZrO2/HZSM-5 catalyst, with which at 420 °C, 50 bar, CO2/COx of 0.5 and H2/COx of 3, a yield of C5+ of 20.7% was obtained. Under such conditions, CO2 and COx conversions were very high, of 39.7% and 28.4%, respectively. An interesting advantage of this catalyst with respect to In2O3-ZrO2/HZSM-5 is the low dependence of the results to the CO2/COx ratio in the feed, which provides high versatility in the operation, combining the targets of valorizing CO2 and syngas derived from gasification of biomass or waste from the consumer society.The good results of gasoline production with ZnO-ZrO2/HZSM-5 catalyst from CO2 and mixtures of CO2/CO allow to value positively the interest of this route as a complementary route to others studied in the literature for the production of other hydrocarbons, such as light olefins, light paraffins or aromatics. The C5+ fraction obtained consisted mainly of C5 and C6 isoparaffins, with a yield of isoparaffins of 20% and 0.1% of aromatics. With a RON of 91.8, the obtained product had a very interesting composition for its incorporation into the refinery gasoline pool. Therefore, it can be combined with other streams which, like those derived from fluidized catalytic cracking (FCC), have a content of aromatics and olefins that exceeds legal limitations. In addition, the results can be considered pioneering for this purpose with this catalyst, and they provide good prospects for improvements in the catalyst and in the optimization of the reaction conditions. Onintze Parra: Conceptualization, Investigation, Writing – original draft, Writing – review & editing. Ander Portillo: Validation, Visualization, Methodology, Writing – review & editing. Javier Ereña: Project administration, Funding acquisition. Andrés T. Aguayo: Methodology, Resources, Supervision, Project administration, Funding acquisition. Javier Bilbao: Conceptualization, Writing – original draft, Writing – review & editing, Project administration, Funding acquisition. Ainara Ateka: Conceptualization, 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.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). O. Parra is grateful for the financial support of the grant of the Basque Government (PRE_2021_1_0014) and A. Portillo is grateful for the grant from the Ministry of Science, Innovation and Universities of the Spanish Government (BES2017-081135). The authors thank for technical and human support provided by SGIker (UPV/EHU). Supplementary material Image 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.fuproc.2023.107745.

The direct production of C5+ hydrocarbons from CO2/CO mixtures with methanol as intermediate is an attractive alternative for the production of gasoline from CO2 and syngas derived from biomass. With this purpose, the performance of CuO-ZnO-ZrO2 (CZZ), In2O3-ZrO2 (IZ) and ZnO-ZrO2 (ZZ) metallic oxides was compared by using them in tandem with a HZSM-5 zeolite. The catalysts were analyzed by means of N2 adsorption-desorption, XRD, XRF, H2-TPR and NH3-TPD. Two series of runs were performed in a packed bed reactor: (i) the methanol synthesis with the metallic oxides as catalysts, at 250–430 °C; 50 bar; CO2/COx, 0–1; H2/COx, 3; space time 6 gcat h molC −1; and (ii) the synthesis of hydrocarbons with the tandem catalysts with a metallic oxide/zeolite mass ratio of 1/1, at 340, 380 and 420 °C; 30 and 50 bar; CO2/COx, 0.5 and 1; H2/COx, 3; space time 12 gcat h molC −1. The results were quantified in terms of yield and selectivity of the product fractions and CO2 and COx (CO2 + CO) conversion. The higher methanol yield attained with the CZZ catalyst for the CO + H2 feed and its mixing with CO2 was faded by the problem of its sintering above 350 °C (minimum temperature required for the extent of methanol conversion to hydrocarbons). The IZ and ZZ catalysts were active, selective to methanol and stable both in the methanol synthesis and when used in IZ/HZSM-5 and ZZ/HZSM-5 tandem catalysts. Excellent results were obtained with the latter, which resulted in a 20.7% yield of C5+ hydrocarbon fraction at 420 °C and 50 bar, with CO2 and COx conversion of 39.7% and 28.4%, respectively. This fraction corresponded to isoparaffinic gasoline, with isoparaffin yield (mainly C5 and C6) surpassing 20% and low concentration of aromatics (0.1%) that led to a Research Octane Number of 91.8. This composition results attractive for its integration into the refineries gasoline pool. Furthermore, the changes of the CO2/COx ratio in the feed barely affected the yield and composition of the gasoline obtained with the ZZ/HZSM-5 catalyst, stating its great versatility.

Data will be made available on request.Endocrine-disrupting chemicals (EDCs) are frequently detected in the environment and are of concern due to their potentially harmful effects on human health and the ecosystem [1,2]. Recently, SO4 ∙ −-based advanced oxidation processes (SR-AOPs) are gaining enormous attention as an efficient technology to degrade and mineralize recalcitrant organic pollutants in water [3,4]. SO4 ∙ − (E0 = 2.5–3.1 VNHE) has higher standard redox potential and longer half-life time than ∙OH (E0 = 1.8–2.8 VNHE) [5,6]. Generally, SO4 ∙ − can be generated through activating peroxymonosulfate (PMS) or peroxodisulfate (PS) by heating, ultraviolet radiation, electricity, and ultrasound [7]. However, these activation methods require high energy. To reduce energy consumption, transition metals (Co, Fe, Mn, Cu, etc.)-based materials were used as heterogeneous catalysts for PMS activation [8,9]. Among them, Fe-based materials have shown great potential in the degradation of organic pollutants due to their advantages of being cost-effective, high-efficient, eco-friendly characters, and easily accessible as the second most abundant metallic element of the earth's crust [10]. Most Fe-based materials are magnetically separable, making them easier to recycle [11]. Notably, nanoscale zero-valent iron (Fe0) has been recognized as an efficient catalyst for activating PMS due to its nano size and high surface reactivity [12]. Nevertheless, Fe0 has the characteristics of its high surface energies and inherent magnetism, which lead to the formation of larger particles and subsequently reduce the catalytic activity [13]. To overcome these shortcomings, many kinds of carbon materials such as active carbon (AC), biochar (BC), graphene oxide (GO), and N-doped carbon (NC) with highly porous structures have been used as favorable support materials for loading Fe0 [9,10,14]. Nevertheless, these Fe/C composites still showed limited catalytic performances due to the thermodynamic instability and aggregation of Fe0 on carbon supports [15,16]. Therefore, seeking a better Fe/C-based catalyst with adequate activity and stability is necessary.Metal-organic frameworks (MOFs) are highly ordered porous materials, with metal ions or clusters in their center and organic ligands as linkers [17]. Their structures can be precisely engineered into a variety of multilevel nanoarchitectures with desired size, porosity, and functional groups by controlling the geometry of the constituent components through various synthetic methods [18]. In recent years, pyrolysis of Fe-based MOFs (Fe-MOFs) to form core-shell-like carbon nanostructures has become one of the most promising platforms for fabricating active and stable Fe/C-based catalysts [19]. For instance, Zhao et al. prepared Fe0/Fe3C/Fe-Nx decorated NC nanotubes by pyrolysis of MIL-88B(Fe) and melamine (MM) at 900 °C in N2 atmosphere [20]. Zhang et al. reported Fe3O4/ZnO incorporated carbon spheres by pyrolysis of Zn/Fe-MOFs at 650 °C in N2 atmosphere [21]. Chen et al. synthesize Fe0/Fe3C/Fe-Nx inside NC nanofibers by pyrolysis of polyacrylonitrile (PAN) modified Fe-MIL-101 at 900 °C in N2 atmosphere [22]. It is generally accepted that altering the annealing temperature can change catalyst characteristics, including elemental composition, metallic phase, pore structure, and graphitization degree [23,24]. These affect the active site, specific surface area, and electrical conductivity, influencing the catalytic activity of the Fe/C-based materials [25]. However, most reported Fe-MOFs derived SR-AOPs catalysts were doped with nitrogen (N) or other metallic elements (Co, Cu, Ni, etc.). The effect of annealing temperature on the major elements (Fe, C) in the catalytic materials becomes unclear [22,26].To the best of our knowledge, the effect of annealing temperature on characteristics and catalytic activity of Fe-MOFs derived catalysts with neither N nor other metallic elements have not been investigated in PMS-based SR-AOPs. Thus, in this work, we synthesized composites with Fe0/Fe3C/Fe3O4 wrapped in porous carbon shell (CC-Fe/C) at different annealing temperatures (700, 800, and 900 °C). To exclude the influence of other elements, an unmodified MIL-88B(Fe) was prepared as the only precursor, and the pyrolysis was performed under an inert atmosphere (Ar) instead of N2. The effects of annealing temperature on morphology, elemental composition, crystal phase, and pore structure of the CC-Fe/C catalysts were investigated. The catalytic activity of samples for PMS activation was evaluated by the removal of bisphenol A (BPA), a typical EDC. Results suggested that an optimal annealing temperature of 800 °C led to multiple Fe-based active sites (Fe0, Fe3C, and Fe3O4) on CC-Fe/C-800 surface, synergistically promoting its catalytic activity with good reusability (up to the third cycle) and practicability (in tap water/treated wastewater). Finally, the mechanisms (including synergistic activation of PMS, dominated reactive oxygen species, and acceleration of the Fe3+/Fe2+ cycles) involved in the catalytic oxidation of BPA were proposed. Based on the progress, this work may offer a good reference to tune the physicochemical properties of the CC-Fe/C catalyst for enhancing its catalytic activity and a new insight into the underlying catalytic reaction mechanism of PMS + CC-Fe/C system for BPA removal.All chemicals used in this work (listed in Table S1) were of analytical grade without any purification. All solutions were prepared with deionized (DI) water, otherwise indicated. Tap water and treated wastewater were obtained from our laboratory and a wastewater treatment plant in Hangzhou, respectively.MIL-88B(Fe) was synthesized by a modified solvothermal method [27]. Typically, 0.54 g (2 mmol) of FeCl3∙6H2O and 0.664 g (4 mmol) of terephthalic acid were added into a solution of 20 mL of N, N-Dimethylformamide (DMF) and 3.2 mL of 1 M NaOH. After stirring, the mixture was transferred into a 100-mL Teflon-lined stainless-steel reactor and heated at 100 °C for 24 h. The orange precipitates [MIL-88B(Fe)], were collected by centrifugation, consecutively washed with 50 mL pure ethanol and 50 mL DI water three times, respectively, and dried at 60 °C in a vacuum. The magnetic CC-Fe/C was synthesized by annealing MIL-88B(Fe) under an Ar atmosphere with a heating rate of 5 °C/min at different temperatures (700, 800, or 900 °C) for 5 h. Then the black products were cooled down to room temperature, collected, and labeled as CC-Fe/C-X, where CC refers to coral-like core-shell, and X refers to annealing temperature (See Scheme 1 ).The morphology of the catalyst was characterized by a field emission scanning electron microscope (FESEM, Sigma 300, Zeiss, Germany) with energy dispersive X-ray spectroscopy (EDS) and a high-resolution transmission electron microscope (HRTEM, Tecnai G2 F30, FEI, America). The thermogravimetric analysis (TGA) was performed on a HITACHI STA200 simultaneous thermal analyzer with a heating rate of 5 °C/min from room temperature to 1000 °C in Ar. The crystal phase of the catalyst was analyzed by an X-ray diffractometer (XRD, D8 advance, Bruker, Germany) equipped with a Ni-filtered Cu K radiation. The Brunauer-Emmett-Teller (BET) surface area and pore structure of the catalyst was determined by the N2 adsorption/desorption method on a Genini 2390 analyzer (Micromeritics, America). The surface composition of the materials was characterized by X-ray photoelectron microscopy (XPS, Thermo Scientific K-Alpha, Thermo Fisher, America) with Al-Kα (1486.6 eV) radiation. The microstructural properties of samples were analyzed by Raman spectroscopy (Thermo Scientific DXR 3Xi, America). The residual BPA after the catalytic reaction was analyzed in triplicates by an HPLC Agilent 1260, America) equipped with a C18 column. The mobile phase consisted of water/acetonitrile (v/v, 40:60) at a flow rate of 1.0 mL/min, and the UV detection wavelength was 276 nm. The instrument detection limit and the limit of quantification were 0.2 μg/mL and 50 μg/mL, respectively. The concentration of total iron ions leached out was measured by an inductively coupled plasma mass spectrometer (ICP-MS, NexION-300X, PerkinElmer). The reactive oxygen species (ROS) generated during PMS activation were identified by the electron paramagnetic resonance (EPR) spectra (Bruker EMXplus-6, Germany). PMS was colorimetrically determined based on the amount of iodine (λ max = 352 nm) formed via the oxidation of iodide by PMS.The catalytic performance of CC-Fe/C samples was evaluated by degrading BPA, as the model pollutant. The degradation process was performed in a 250 mL conical flask on a constant temperature shaker with a speed of 250 r/min in triplicate at room temperature. In a typical run, 100 mL of 10 mg/L BPA solution was added into a 250 mL conical flask. Then, 10 mg of as-prepared catalytic materials were added to the above solution. The mixture was sonicated for 1 min and left standing for 30 min to achieve adsorption equilibrium. Afterward, 1.0 mL 10 mg/mL PMS solution was added to initiate the reaction, and the initial concentration of KHSO5 was 0.31 mM in the reaction solution. At given intervals, 1.0 mL of the reaction liquid was withdrawn, quenched with 0.1 mL (0.05 M) Na2S2O3, and filtered with a 0.22 μm membrane before analysis. To evaluate the influence of the initial solution pH on the degradation process, PMS and BPA were added to the working solution before adjusting the pH using 0.1 M HCl and NaOH. Then, the catalyst was added to start the reaction.FESEM was used to observe the morphologies of the synthesized MIL-88B(Fe) and the CC-Fe/C catalysts prepared at different annealing temperatures. Fig. 1a and e revealed the high uniformity of the MIL-88B(Fe) crystals. The MIL-88B(Fe) crystal appeared as a hexagonal prism with a truncated hexagonal cone at each end. The lateral dimension was about 500 nm. After the pyrolysis process, the MIL-88B(Fe) transformed into CC-Fe/C catalysts and its uniform crystal structure disappeared, indicating the complete structural decomposition of the MIL-88B(Fe). The CC-Fe/C-700 depicted a coral-like porous morphology (Fig. 1b), similar to the cauliflower coral (inset of Fig. 1c). The graphite-like clusters assembled the coral-like nanoarchitecture (Fig. 1f). Notably, CC-Fe/C-800 and CC-Fe/C-900 (Fig. 1c and d) exhibited coral-like structures similar to that of CC-Fe/C-700, except they had the more compact and blockier pattern. Furthermore, Fig. 1f showed visible pores on the surface of CC-Fe/C-700. The pore structure of samples with higher annealing temperatures (i.e., CC-Fe/C-800 and CC-Fe/C-900) became invisible (Fig. 1g and h). This phenomenon suggested that annealing temperature might influence the pore structure of the CC-Fe/C catalysts, which would be confirmed via the N2 adsorption-desorption isotherm (Fig. 4a). The elemental mappings (Fig. 1i-l) of CC-Fe/C-800 showed the presence of Fe, C, and O. Fe atoms in CC-Fe/C-800 were distributed uniformly, which was important for promoting the catalytic activity.The EDS results (Table S2) showed that, after annealing at 800 °C, the weight ratios of C and O in the MIL-88B(Fe) decreased from 51.61% and 26.05% to 36.85% and 3.08% in CC-Fe/C-800, respectively. This may be due to the evaporation or decomposition of organic components in the MIL-88B(Fe). Thus, TGA was conducted in an argon atmosphere to gain insight into the thermal decomposition demeanor of the MIL-88B(Fe). As exhibited in Fig. 1m, the weight loss of the MIL-88B(Fe) can be divided into two main stages. The first stage occurred between 25 °C and 300 °C, with loss of weight of about 20 wt%, can be ascribed to the evaporation and decomposition of solvent molecules (such as DMF, ethanol, and water) absorbed in the porous channel of the MIL-88B(Fe) [28]. The second 43 wt% weight loss stage located at 300–650 °C was due to the complete structural decomposition of the MIL-88B(Fe) [19]. While the weight of the sample had no significant change with temperature increased from 650 °C to 900 °C. Therefore, 700, 800, and 900 °C were chosen as the annealing temperatures to ensure that CC-Fe/C catalysts were free of any residual solvents and the MIL-88B(Fe) precursor.According to XRD results (Fig. 2a), the synthesized MIL-88B(Fe) exhibited typical diffraction peaks at 9.2°, 10.2°, 16.8°, 18.3°, and 20.4°, which were consistent with the literature report [28]. After pyrolysis at 700–900 °C, the diffraction peaks of MIL-88B(Fe) disappeared in the spectrum of CC-Fe/C samples. It suggested that the MIL-88B(Fe) has completely decomposed during the high-temperature pyrolysis. Meanwhile, the new diffraction peak at 26.4° corresponded to the (002) plane of graphitic carbon, indicating the presence of graphitic structures in all three CC-Fe/C samples [29]. The graphitic peak intensity of CC-Fe/C-900 was slightly stronger than those of the other two samples due to its higher degree of graphitization resulting from elevated annealing temperature [30]. The peaks of sole Fe0 at 2θ = 44.7°, 65.0°, and 82.3° were assigned to the (110), (200), and (211) planes (JCPDF No. 06–0696), respectively [31]. The peaks at 2θ = 37.8°, 39.9°, 41.0°, and 30.2°, 35.5°, 43.2°, 62.9° were assigned to the crystalline phase of Fe3C (JCPDS 35–0772) [32], and spinel Fe3O4 (JCPDS No. 19–0629) [33], respectively, indicating that the Fe species in MIL-88B(Fe) were converted into different iron species during the pyrolysis. In the sample obtained at 900 °C (CC-Fe/C-900), the diffraction peaks of Fe3O4 nearly disappeared, while the peaks of Fe0 became sharper. Besides, according to the EDS results (Table S2), the atomic contents of C and O decreased with the annealing temperature rising from 700 °C to 900 °C. The ratio of the decreased C atomic content (1.07%) to the decreased O atomic content (0.88%) was 1:0.82, which was close to the stoichiometric ratio of C to O in CO (1:1). This phenomenon implied that the sharper XRD diffraction peaks of Fe0 in CC-Fe/C-900 was probably due to the reduction of Fe3O4 (Fe3O4 + 4C → 3Fe + 4CO) with the annealing temperature increased [34]. The above XRD results showed that the CC-Fe/C catalyst was a hybrid material composed of graphitic carbon, Fe0, Fe3C, and Fe3O4. The contents of the four components were affected by annealing temperature due to the redox reaction between these Fe and C species. The approximate weight contents of the four components in CC-Fe/C-800 sample were calculated (Text S1, Table S3). As shown in Fig. S1, the weight contents of graphitic carbon, Fe0, Fe3C, and Fe3O4 in CC-Fe/C-800 were 36.41%, 49.31%, 3.07%, and 11.02%, respectively. Finally, the magnetic CC-Fe/C catalyst can be easily recovered after using an external magnetic field (Fig. S2).Raman spectroscopy was used to evaluate the carbon matrix with graphite crystal structures in the CC-Fe/C samples. The D-band (∼1350 cm−1) and the G-band (∼1580 cm−1) were related to the disordered sp3 C atoms (or amorphous carbon) and the sp2 C atoms in both rings and chains, respectively [8]. The intensity ratio of the G to D band (I G/I D) indicates the degree of graphitic order in a carbon material [35]. According to Fig. 2b, the I G/I D ratios of CC-Fe/C-700, CC-Fe/C-800, and CC-Fe/C-900 were 0.87, 1.00, and 1.15, respectively. These results demonstrated that the graphitization degree was improved with increasing annealing temperature, which was in agreement with the XRD results (Fig. 2a). The 2D-band (∼2680 cm−1) is often used to estimate the number of graphene layers [36]. All three samples showed the characteristic 2D band. The CC-Fe/C-900 depicted the sharpest diffraction pattern, suggesting that increasing annealing temperature can promote the degree of graphitization and a high level of internal ordering.The TEM images (Fig. 3a and b) show variation in the contrast between the dark core and the light shell. As observed, the dark Fe-like cores (marked with yellow polygons) were well distributed in the carbon matrix. In the HRTEM image of CC-Fe/C-800 (Fig. 3c), the lattice distances of 0.241 nm and 0.294 nm were ascribed to the (210) and (220) planes of Fe3C and Fe3O4, respectively, which were consistent with the XRD analyses (Fig. 2a) [9,13]. According to the HRTEM images of the core-shell structure (Fig. S3), the lattice distances of 0.202 nm in the dark cores were ascribed to the (110) plane of Fe0, suggesting that the cores of CC-Fe/C-800 were Fe0 nanoparticles. Furthermore, the lattice spacing (0.335 nm) in the outer shell (Fig. 3d) was assigned to the (002) facet of graphitic carbon [32], indicating that the graphitic structure may endow Fe0 cores with an excellent protective ability which effectively prevented Fe leaching. In summary, the EDS, XRD, and HRTEM results together suggested that the main components of CC-Fe/C samples changed along with the thickness of the coating layer: the outermost shell consisted primarily of graphitic carbon; the sub-outer layer was made up of Fe3C and Fe3O4; while the interior cores were mainly Fe0 nanoparticles.The porosity and BET surface area (SBET) of the synthesized MIL-88B(Fe) and three CC-Fe/C samples were analyzed by N2 adsorption-desorption isotherms (Fig. 4a and Table 1 ). The pore size distributions were calculated by BJH (Barrett-Joyner-Halenda) method (Fig. 4b). As observed in Table 1, the pore volume of the four samples ranged from 0.089 to 0.266 cm3/g, indicating the existence of pores in them. Moreover, all four samples exhibited type IV isotherm with H3 hysteresis loops (Fig. 4a), reflecting the mesoporous structure of them [37]. While the iron-based nanoparticles with high crystallinity were likely non-porous, it suggested that the mesoporous characteristic mainly stemmed from the shell (porous graphitic carbon). The mesoporous shell can provide channels and facilitate the mass transfer of the reactants (such as PMS and BPA) from the catalyst surface to the interior cores [38]. Thus, depending on the porosity of the shell, the interior active sites (such as Fe0) can play the catalysis role directly.The SBET value (44.4 m2/g) and pore volume (0.102 cm3/g) of the MIL-88B(Fe) were small since the breathing nature of the MIL-88B(Fe) which presented closed pore structures in dry state [28]. However, after pyrolysis at 700 °C, the SBET value and pore volume of CC-Fe/C-700 increased to 135.14 m2/g and 0.266 cm3/g, respectively. It suggested that the decomposition of the MIL-88B(Fe) during the pyrolysis process was more likely to form pores, leading to an increase in SBET as well as pore volume. The SBET (46.2–135.1 m2/g) and average pore width (4.7–5.9 nm) of the three CC-Fe/C samples were close to those of previously reported Fe/C porous materials (Table S4). Among the three catalytic samples, CC-Fe/C-900 obtained at 900 °C had fewer 2–3 nm and more 3–4 nm sized mesoporous. Besides, the CC-Fe/C-900 with the highest graphitization degree (based on the XRD and Raman results, Fig. 2) presented a lower SBET of 46.16 m2/g and a smaller pore volume of 0.089 cm3/g than CC-Fe/C-700 (135.14 m2/g, 0.266 cm3/g) and CC-Fe/C-800 (73.29 m2/g, 0.136 cm3/g). This phenomenon might be due to the increase of graphitization degree driven by the growing annealing temperature, which caused a decrease in the defects in graphitic layers and, thus, decreased SBET and pore volume [24].BPA was selected as the model pollutant to assess the catalytic activities of the four prepared samples (i.e., MIL-88B(Fe), CC-Fe/C-700, CC-Fe/C-800, and CC-Fe/C-900). After 30 min of the adsorption without PMS, the removal efficiencies of BPA were insignificant (less than 4%), indicating that all four samples were ineffective in adsorbing molecular BPA (Fig. 5a). When PMS was added, only 17.7% of BPA was removed by the PMS + MIL-88B(Fe) system within 60 min. The degradation efficiency was far less than those achieved by the CC-Fe/C samples (85.1 to 100%), elucidating that the annealing process significantly improved the catalytic activity of the MIL-88B(Fe) precursor. Herein, the influence of the annealing temperature of CC-Fe/C on PMS activation was evaluated. As shown in Fig. 5a, the removal rate of BPA reached 100%, 78.3%, and 59.7%, in 20 min in PMS + CC-Fe/C-800, PMS + CC-Fe/C-900, and PMS + CC-Fe/C-700 systems, respectively. The degradation kinetics can be described using a pseudo-second-order model (Fig. S4a). It was found that the rate constant k of CC-Fe/C-800 (1.18 mM−1 min−1) was much higher than those of CC-Fe/C-700 (0.17 mM−1 min−1) and CC-Fe/C-900 (0.35 mM−1 min−1). Among the three samples, CC-Fe/C-700 exhibited the lowest catalytic activity, although it owned the largest SBET and pore volume. Herein, the specific activity comparison among the three catalysts was undertaken. As depicted in Table 1, the specific activity of CC-Fe/C-800 (0.0161 mM min−1 m−2) was higher than those of CC-Fe/C-700 (0.0013 mM min−1 m−2) and CC-Fe/C-900 (0.0075 mM min−1 m−2). It implied that the specific surface area of the catalysts was not the key factor affecting the catalytic activity. The high catalytic performance of CC-Fe/C-800 was due to the multiple Fe-based active sites (Fe0, Fe3C, and Fe3O4) generated by the appropriate annealing temperature (800 °C) rather than its relatively high SBET. Based on its excellent catalytic activity, CC-Fe/C-800 was selected in the following experiments.The effects of catalyst dosage, PMS concentration, initial BPA concentration, and solution pH on BPA removal were then investigated. Changing the CC-Fe/C-800 dosage had a negligible impact on the adsorption ability of BPA but significantly influenced the BPA degradation (Fig. 5b). Increasing the catalyst dosage resulted in a higher BPA degradation. After applying 0.06 and 0.08 g/L of CC-Fe/C-800, the removal rate was about 67.7 and 91.3% in 60 min, respectively. While at 0.10 and 0.12 g/L of catalyst dosage, BPA was completely degraded in 40 and 20 min with rate constants k of 1.18 and 7.04 mM−1 min−1 (Fig. S4b), respectively. The enhanced degradation efficiency can be ascribed to the availability of abundant active sites with increasing catalyst dosage, which ultimately activated PMS into more ROS [26]. Furthermore, the effect of PMS concentration on BPA removal was studied (Fig. 5c). With PMS concentration at 0.19–0.38 mM, 100% of BPA was degraded in 20 min. However, the removal efficiency decreased (78.6% in 60 min) at a PMS dosage of 0.09 mM, probably due to the total consumption of oxidant that can not continuously provide ROS to degrade BPA [35].The effect of changing the initial BPA concentrations (5 to 30 mg/L) on the degradation efficiency was shown in Fig. 5d. BPA removal rate decreased with increasing its initial concentration. For low initial concentrations (5 and 10 mg/L), complete degradation was achieved in 20 min. However, the degradation rate dropped to 79.8% and 54.1% in 60 min at initial BPA concentrations of 20 and 30 mg/L, respectively. This phenomenon might be attributed to the possible coverage of reactive surface sites by excess BPA molecules, leading to decreased PMS activation and an insufficient amount of ROS generated [35]. Due to the solution pH significantly influencing the catalytic performance of the SR-AOPs system [39], the catalytic activity at a wide pH range (3.8–11.4) was investigated. As shown in Fig. S5, the degradation efficiency was reduced obviously with increasing initial solution pH from acidic (pH = 3.8) to alkaline (pH = 11.4) conditions. Complete removal of BPA can be achieved in a 60 min reaction time at initial pH = 3.8, 6.7, and 9.3. However, when the initial solution pH was adjusted to 11.4, this SR-AOPs system exhibited a relatively lower removal (67.8% in 60 min) of BPA. For the strong alkaline condition (pH = 11.4), the possible hydroxylation of the catalyst surface by OH− might be unfavorable to the PMS adsorption due to the electrostatic repulsion and thus inhibited the degradation of BPA [40].The catalyst was recycled by a magnet and used in three cycles of BPA degradation under the same operating conditions. The degradation efficiencies of BPA after the first, second, and third cycles were 100%, 95.4%, and 81.7%, respectively (Fig. 6a). The used CC-Fe/C-800 was regenerated by vacuum drying (60 °C for 12 h) or thermal treatment (800 °C/h in Ar flow). As expected, the catalytic activity was partially recovered (86.9% in 60 min) by the vacuum-dried process. Significantly, the catalyst regenerated by thermal treatment in Ar degraded nearly 99.0% of BPA, showing its good renewability. According to the XRD patterns of the pristine and recycled CC-Fe/C-800 samples (Fig. S6), the diffraction peak intensities of Fe0 and Fe3C became weaker while those of Fe3O4 became stronger after the reaction. Even so, the diffraction peaks of all three iron-components (i.e., Fe0, Fe3C, and Fe3O4) still can be observed in the recycled catalyst sample. It suggested that the graphitic carbon shell of CC-Fe/C-800 may protect the interior Fe0/Fe3C/Fe3O4 nanoparticles, avoiding too much iron ion leaching from the catalyst into the solution. As expected, the ICP-MS results for Fe (Fig. S7) showed that the concentrations of leached iron ions in the catalytic system at a pH range of 5.0–10.8 were lower than 1 mg/L, which is within the safe limit as per the discharge standard of iron [41]. To further examine the catalyst's potential for practical application, the catalytic tests were performed in two kinds of actual water substrates (tap water and treated wastewater). As displayed in Fig. 6b, the removal rate slightly decreased from 100% to 98.8% and 95.0% using tap water and treated wastewater, respectively. The above results demonstrated that CC-Fe/C-800 depicted good reusability, stability, and a great prospect for water treatment.For identifying the possible ROS generated in the PMS + CC-Fe/C-800 system, an EPR test was conducted using DMPO (5,5-dimethyl-1-pyrrolidine N-oxide) as the spin trap agent. As shown in Fig. 7a, no peaks were detected in PMS alone solution in 1 min reaction, but characteristic peaks of DMPO-OH and DMPO-SO4 adducts appeared after adding CC-Fe/C-800 [42], indicating the generation of ∙OH and SO4 ∙ −. Prolonging the reaction to 10 min, the peak intensities of DMPO-OH and DMPO-SO4 adducts showed little change, suggesting that CC-Fe/C-800 can continuously activate PMS. Especially, the slight decrease in intensity of DMPO-OH peaks might be attributed to the transformation of ∙OH to 1O2 after a series of radical reactions [43]. Thus, TEMP (2, 2, 6, 6-tetra-methyl-4-piperidone) was added into the reaction solution as a sacrificial agent for 1O2 [44]. In Fig. 7b, a weak 1:1:1 triplet signal appeared in the first minute, indicating the presence of 1O2 in the SR-AOPs system. The intensity of these signals increased significantly in 10 min, which demonstrated that 1O2 was continuously generated.To further confirm the role of different ROS in the PMS + CC-Fe/C-800 system, a series of quenching experiments were performed by adding different concentrations of tert-butyl alcohol (TBA), methanol (MeOH), L-histidine (L-His), and p-benzoquinone (p-BQ), respectively. TBA could only effectively scavenge ∙OH, whereas MeOH can react with both SO4 ∙ − and ∙OH [45,46]. As shown in Fig. 8a, the BPA removal efficiency decreased from 100% to 95.8%, 79.2%, and 56.5% within 60 min after adding 0.1 M, 0.3 M, and 0.5 M TBA, respectively. It suggests that ∙OH was involved in the degradation but not the dominant reactive species. When excessive 0.1 M, 0.3 M, and 0.5 M MeOH were added (Fig. 8b), the removal efficiency of BPA decreased dramatically from 100% to 49.3%, 31.2%, and 18.7%, respectively. This significant inhibition phenomenon indicated that SO4 ∙ − rather than ∙OH would play an important role in the degradation of BPA. In the presence of L-His (a commonly used scavenger for 1O2), the reaction was also inhibited, as displayed in Fig. 8c. However, L-His greatly accelerated PMS loss in the catalytic system, inhibiting the degradation of BPA by complete consuming of PMS in 20 min (Fig. S8). Herein, another 1O2 scavenger, β-carotene, was used to evaluate the role of 1O2 in the catalytic reaction [47]. As displayed in Fig. 8d, the degradation efficiency of BPA decreased from 100% to 70.8% within 60 min in the presence of β-carotene, implying that 1O2 contributed to the BPA degradation. In addition, the degradation experiments were conducted using deuteroxide (D2O) as a solvent because 1O2 has a longer lifetime in D2O than in H2O due to the slower decaying rate as 1O2 in D2O [44]. The result in Fig. S9 showed that 92.3% of BPA was decontaminated in D2O in 10 min compared with 86.7% accomplished in H2O, which further proved the contribution of 1O2 to BPA degradation in the PMS + CC-Fe/C-800 system. Besides, the degradation efficiencies were slightly affected in the presence of p-BQ (scavenger of O2 − ∙) with different concentrations, indicating the low generation of O2 − ∙ during the PMS activation process (Fig. 8e) [48]. To explore the dominant ROS responsible for the oxidation of BPA, the inhibition rates of SO4 ∙ −, 1O2, ∙OH, and O2 − ∙ with BPA were compared according to the quenching test results (Fig. 8a-e) [49]. As shown in Fig. 8f, the inhibition ratio of BPA degradation by different scavengers was ranked as follows: MeOH (81.3%) > TBA (43.5%) > β-carotene (29.2%) > p-BQ (8.5%). It suggested that the contribution of different ROS to BPA degradation in the PMS + CC-Fe/C-800 system should follow the order as SO4 ∙ − > ∙OH > 1O2 > O2 − ∙.XPS was then applied to investigate the surface state changes of the CC-Fe/C-800 after activating PMS for BPA removal. According to the survey spectrum of fresh CC-Fe/C-800 (Fig. S10a and Table S5), three elements of Fe, C, and O were contained in the composite with surface atomic contents of 3.1%, 88.8%, and 8.1%, respectively. Significantly, the Fe content (3.1 atom%) was much lower than by EDS analysis (25.0 atom%, Table S2) since XPS can only examine the outer 3–5 nm of samples. In contrast, EDS can probe several micron depths beneath the material. This phenomenon suggested that CC-Fe/C-800 is a composite of an iron core covered by a thin graphitic carbon layer, consistent with the previous discussion (Section 3.1). After the catalytic reaction, the surface C content slightly decreased (from 88.8% to 76.2%), whereas the Fe and O contents increased to various degrees (Table S5), implying that the carbon (such as Fe3C) might be involved in the PMS activation.It is well known that Fe species can activate PMS into ROS [8]. Thus, the high-resolution Fe 2p XPS spectra of fresh and used CC-Fe/C-800 were carried out to estimate the possible catalytic mechanism. For the fresh sample (Fig. 9a), the peaks at binding energies of 708.9, 710.6, 713.4, 723.9, and 727.6 eV were assigned to Fe3C, Fe2+ 2p3/2, Fe3+ 2p3/2, Fe2+ 2p1/2, and Fe3+ 2p1/2, respectively [9]. Two satellite peaks of Fe 2p at 718.1 and 734.0 eV were fitted. The peak of Fe0 at around 706 eV was not detected in the XPS spectrum due to the rapid formation of iron oxide on the catalyst surface, and the XPS is only limited to surface detection [50]. The Fe 2p XPS spectrum of the used CC-Fe/C-800 (Fig. 9b) revealed an increase in the content of Fe3+. The weakening of the Fe3C and Fe2+ signals indicated that part of Fe3C and Fe2+ transformed to Fe3+. However, the percentage of Fe2+ did not significantly decrease (42.1% to 40.5%) after the reaction. This phenomenon might be due to the FeC bond and graphitic carbon in CC-Fe/C-800 that facilitated the electron transfer from Fe0 to Fe3+, leading to an efficient regeneration of Fe2+ [51,52]. In other words, there was Fe2+/Fe3+ cycling on the catalyst surface during the catalytic process [53].The C 1s spectra for the fresh and used CC-Fe/C-800 showed four characteristic peaks (Fig. 9c and d). The strong peaks at about 283.7 eV were ascribed to the CFe bonds (from the Fe3C phase in the carbon support) [54], confirming the presence of the Fe3C in the CC-Fe/C catalyst. The peaks at around 284.4 eV were attributed to CC derived from the sp2 hybrid graphite, revealing that another major part of C was present in cross-lined cellular lattices [53]. Moreover, two weak peaks of CO (at 285.2 eV) and CO (at 288.3 eV) were also observed [15]. After the reaction, the CO peak intensity enhanced, indicating the catalyst surface was hydroxylated. Meanwhile, the intensity of the CFe bond decreased from 74.0% to 56.0%, implying the involvement of Fe3C in the reaction through enhancing positive charge density on the adjacent carbon atoms that improved the nucleophilic addition of PMS for 1O2 generation [55].In the O 1s spectra (Fig. 9e and f), the peaks at 529.4, 531.9, and 532.8 eV, were assigned to the lattice oxygen from metal oxides (Olat), the adsorbed oxygen (Oads), and the adsorbed water molecule (Osurf), which accounted for 43.1%, 45.9%, and 11.0% before reaction, respectively [56]. After the reaction, the contents of Olat, Oads, and Osurf changed to 34.2%, 44.4%, and 21.3%, respectively. The consumption of Olat might be due to the reduction of Fe3+ to Fe2+ by electrons donated from the Fe0 inside CC-Fe/C-800 [52], and the increase in Osurf (11.0% to 21.3%) can be assigned to the H2O adsorbed on the catalyst surface after the heterogeneous reaction.Combined with the results of EPR, quenching, and XPS tests, the possible reaction mechanisms of PMS activation on CC-Fe/C-800 can be proposed, as exhibited in Fig. 10 . Firstly, HSO5 − (dissolved PMS) and BPA molecules were adsorbed onto the porous carbon surface of CC-Fe/C-800. At the same time, Fe3C in the sub-outer layer could modify the electron states of the adjacent carbon regions, resulting in a higher positive charge density on the adjacent carbon atoms (Eq. (1)) [57]. Then, the positively charged carbon was prone to nucleophilic addition of HSO5 − for generating 1O2 (Eq. (2)) [55]. Secondly, Fe3C nanoparticles could react with HSO5 − via the transformation of Fe0 and Fe3+ and subsequently undergo a series of complex radical chain reactions to produce O2 − ∙ (Eq. (3)). The latter will recombine with another O2 − ∙ and generate 1O2 via Eq. (4) [58]. (1) Fe 3 C - C - e − → Fe 3 C − C + (2) 2 HSO 5 − → Fe 3 C − C + O 1 2 + 2 H + + 2 HSO 4 2 − (3) Fe 3 C → HSO 5 − Fe 0 and Fe 3 + → HSO 5 − O 2 − ⋅ (4) 2 O 2 − ⋅ + 2 H + → O 1 2 + H 2 O 2 Meanwhile, the Fe0 cores inside CC-Fe/C-800 also participated in the activation of PMS. According to the previous literature [59,60], nano Fe0 was easily oxidized by dissolved oxygen, leading to a passivating oxide layer (≡Fe2+) on the catalyst surface and the release of dissolved Fe2+ via Eq. (5). Subsequently, these as-formed ≡Fe2+ and dissolved Fe2+ activated HSO5 − to produce SO4 ∙ − or ∙OH according to Eq. (6–9), respectively [39]. The EPR and quenching test (Fig. 7a and Fig. 8a) showed that some of SO4 ∙ − would react with H2O to produce ∙OH via Eq. (10) [61]. In addition to Fe3C and Fe0, the ≡Fe2+-containing Fe3O4 nanoparticles can also contribute to PMS activation through Eq. (6) and (7) [10,21]. (5) Fe 0 + O 2 + 4 H + → 2 ≡ Fe 2 + / Fe 2 + + H 2 O (6) ≡ Fe 2 + + HSO 5 − → ≡ Fe 3 + + SO 4 − ⋅ + OH − (7) ≡ Fe 2 + + HSO 5 − → ≡ Fe 3 + + SO 4 2 − + ⋅ OH (8) Fe 2 + + HSO 5 − → Fe 3 + + SO 4 − ⋅ + OH − (9) Fe 2 + + HSO 5 − → Fe 3 + + SO 4 2 − + ⋅ OH (10) SO 4 − ⋅ + H 2 O → SO 4 2 − + ⋅ OH + H + Furthermore, the Fe0 cores would not only participate in activating HSO5 − but also act as a cocatalyst to promote Fe2+ regeneration. It is generally accepted that Fe2+ can be regenerated by the reaction between Fe3+ and excess HSO5 − (Eq. (11)), but the reaction rate is very slow [62]. However, the Fe0 in CC-Fe/C-800 can efficiently facilitate Fe3+ reduction to Fe2+ via Eq. (12) and accelerate the Fe3+/Fe2+ cycle [52], significantly improving the catalytic activity. Besides, the graphitic carbon, an excellent platform, and electron reservoir would enhance the electron transfer between the above-mentioned Fe active sites (Fe3C, Fe0, and Fe3O4) and HSO5 −, leading to the generation of more ROS. Afterward, the ROS (i.e. SO4 ∙ −, ∙OH, 1O2, and O2 − ∙) took part in the degradation of BPA into intermediates (such as muconic, oxalic, and malonic acids), which were finally mineralized to CO2 and H2O (Eq. (13)) [35]. (11) Fe 3 + + HSO 5 − → Fe 2 + + SO 5 − ⋅ + H + (12) Fe 0 + 2 Fe 3 + → 3 Fe 3 + (13) BPA → ROS intermediates → ROS CO 2 + H 2 O In summary, coral-like CC-Fe/C catalysts with Fe0/Fe3C/Fe3O4 nanoparticles wrapped in porous carbon shell were synthesized for PMS activation. Characterization results showed that increasing the annealing temperature (700–900 °C) would increase Fe0 (main active site) content but reduce the specific surface area of CC-Fe/C catalysts. CC-Fe/C-800 prepared at 800 °C exhibited the best catalytic activity toward a wide pH condition (5.0–10.8) with low Fe leaching (less than 1 mg/L). 10 mg/L BPA was completely degraded in PMS + CC-Fe/C-800 system in 20 min. Moreover, the catalyst can be easily recovered using a magnetic field and showed good practicability. Mechanism study indicated that the excellent catalytic activity of CC-Fe/C-800 was mainly attributed to the synergistic effect of multiple active sites (Fe0, Fe3C, and Fe3O4) on its surface. During the catalytic oxidation of BPA process, SO4 ∙ − played a dominant role rather than ∙OH, 1O2, or O2 − ∙. The Fe0 cores in CC-Fe/C-800 were beneficial for Fe3+/Fe2+ recycling. This work may offer a good reference to tune the physicochemical properties of Fe-MOFs derived Fe/C materials in terms of annealing temperature for improving the catalytic activity and provide new insight into the underlying reaction mechanism of the heterogeneous SR-AOPs system. Jie Yu: Conceptualization, Supervision, Writing – original draft, Funding acquisition. Shahzad Afzal: Investigation, Writing – original draft. Tao Zeng: Investigation, Writing – review & editing, Formal analysis. He Wang: Methodology, Validation, Writing – review & editing. Hailu Fu: 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 financially supported by the National Natural Science Foundation of China (No. 22108265, 22276172), Zhejiang Provincial Natural Science Foundation of China (No. LTGS23E080006, LR21E080001), and the Fundamental Research Funding Project of Zhejiang Province (No. 2022YW25). Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2023.106660.

Fe0/Fe3C/Fe3O4 nanoparticles wrapped in graphite shells (CC-Fe/C) were synthesized via pyrolyzing MIL-88B(Fe) at 700, 800, and 900 °C. CC-Fe/C-800 prepared at 800 °C exhibited the best performance for activating peroxymonosulfate with complete removal of bisphenol A at 10 mg/L in 20 min, attributing to the synergistic effect of multiple active sites (Fe0, Fe3C, and Fe3O4) on its surface. Mechanism study suggested that SO4 ∙ −, ∙OH, 1O2, and O2 − ∙ were involved in the degradation. Fe0 cores could act as cocatalysts to promote the regeneration of Fe2+, enhancing the catalytic activity. Finally, CC-Fe/C-800 showed good reusability and practicability.

Biomass-to-GasBrunauer-Emmett-Teller modelDual fluidized bedGreenhouse gasKey performance indicatorspiping and instrumentation diagramPower-to-Gasraw synthetic natural gas after methanation/before upgradingsorption enhanced reformingstoichiometric number of the feed gassynthetic natural gastemperature-programmed oxidationtemperature-programmed reductionweight hourly space velocity in Nl/g hdry basismean Sauter diameter in μmin the feed gas to the methanation reactorreaction enthalpy at standard conditions in kJ/molmolar flow of species i in mol/snumber of carbon atoms in species i in the outlet of the methanation reactorselectivity of CO towards CO2 in %selectivity of C2H6 towards C2H4 in %minimum fluidization velocitycarbon monoxide conversion in %carbon dioxide conversion in %hydrogen conversion in %methane yield in %molar fraction of species i bulk density in kg/m3 Many industrial high-temperature processes and domestic residences rely on the supply of natural gas as an energy carrier [1,2]. However, the targets formulated by the European Commission will require a substantial reduction in the use of fossil fuels in the future [3]. The conversion of biogenic feedstock to renewable synthetic natural gas (SNG) offers the possibility of producing a chemically and physically almost identical gas that can be transported in the already existing gas distribution infrastructure and utilized with already established end-use technologies [4].Catalytic methanation processes have been studied and developed for more than 100 years since Sabatier and Senders first discovered that noble metals catalyze methanation reactions. In the 1970s and 1980s, the primary focus lay on the conversion of coal to SNG. Due to the rising awareness of climate change and the urgent need to reduce GHG emissions, renewable alternatives for SNG production have been developed. Biomass-to-Gas (BtG) as well as Power-to-Gas (PtG) routes gained importance [4,5]. Besides catalytic methanation concepts, biological methanation approaches attract more and more attention [6,7]. Today, various process concepts exist that aim at an optimized production of SNG. One possible production route is the dual fluidized bed (DFB) gasification of woody biomass or waste materials and consecutive fluidized bed methanation. A primary advantage of the DFB process is that it produces a nitrogen-free syngas that is well suited for downstream synthesis processes. At TU Wien, a 100 kWth advanced DFB pilot plant has been developed and extensively investigated [8]. The investigations show that the new design allows the utilization of various waste resources and significantly impacts the quality of the syngas, which in turn affects the downstream synthesis processes [9,10]. However, due to the typical composition of woody biomass, the production of a stoichiometric syngas for methanation with a H2/CO ratio of three is impracticable and thus further measures must be taken. Sorption enhanced reforming (SER) is an alternative operation mode of the advanced DFB process, where the stoichiometric ratio of H2 to CO and CO2 is influenced and can be adapted to the needs of the downstream synthesis process. Fuchs et al. showed that both the gasification temperature [11] and the bed material circulation rate [12] have a major impact on the gas composition. By utilizing this syngas in the methanation reactor, it is theoretically possible to produce grid feedable SNG without the need for a CO2 separation unit [13]. So far, the investigations on the suitability of these syngases (advanced DFB and SER) for methanation have only been of theoretical nature [13–15]. In a modelling approach, both Bartik et al. [13] and Brellochs [14] state optimal gasification temperatures in the range of 680 °C–700 °C for the production of SNG via SER. Since these studies are of theoretical nature, an objective of this work is the experimental investigation and evaluation of syngas from the advanced DFB pilot plant in a fluidized bed methanation unit. The fluidized bed methanation unit is designed to allow an isothermal operation of the methanation process through internal particle circulation while not disturbing the bubble formation and the gas/solid contact. A more detailed description of the reactor is shown in Section 2.1. Since the DFB and the SER processes are not part of the experimental investigations in this study, literature is referred to [8,9,16–21].During catalytic methanation, H2 and CO react to CH4 and H2O according to Eq. 1. The water-gas shift reaction (Eq. 2) leads to the formation of CO2 and H2 if the syngas shows a low H2/CO ratio. Vice versa, if the syngas shows an overstoichiometric composition, CO2 and H2 react via the reversed water-gas shift reaction and form CH4 and H2O in combination with Eq. 1. (1) CO + 3 H 2 ⇌ C H 4 + H 2 O Δ H R 0 = − 206 kJ mol (2) CO + H 2 O ⇌ C O 2 + H 2 Δ H R 0 = − 41 kJ mol Especially for syngas with a low H2/CO ratio, the Boudouard reaction (Eq. 3) plays an important role. Carbon may form on the catalyst surface and block or infiltrate active reaction sites [22]. (3) 2 CO ⇌ C O 2 + C s Δ H R 0 = − 172 kJ mol Other species often found in the syngas of the DFB or SER process are hydrocarbons like ethylene (C2H4). Ethylene can be hydrogenated to ethane (C2H6) (Eq. 4) and further to methane (Eq. 5) but can also lead to coke deposits on the catalyst [23]. The behavior of ethylene very much depends on the applied conditions and the type of reactor [24]. (4) C 2 H 4 + H 2 → C 2 H 6 Δ H R 0 = − 137 kJ mol (5) C 2 H 4 + 2 H 2 → 2 C H 4 Δ H R 0 = − 202 kJ mol All these reaction equations are highly exothermic, and large quantities of heat need to be removed. For this purpose, reactor concepts have been developed to cope with this issue [25]. Fluidized beds are known for their high heat and mass transfer capabilities due to the movement of the particles [26]. Hence, fluidized beds have been under investigation for catalytic methanation processes since 1950, as a review by Kopyscinski describes in detail [4]. The Paul Scherrer Institute recently picked up on the developments and investigated the fluidized methanation process more closely. They applied spatially resolved concentration and temperature measurements along the height of the catalytic bed. The results show that the particle movement leads to an in-situ regeneration of the catalyst particles and therefore reduces the risk for carbon depositions even in the presence of ethylene [24,27]. Furthermore, they concluded that the mass transfer between the bubble phase and the dense phase is a limiting factor in the upper part of the bed [28]. Seemann et al. [29] utilized a 10 kW fluidized bed reactor and demonstrated the conversion of syngas from the 8 MW DFB plant in Güssing. They reached around 40 vol.-% CH4 for a period of 200 h until a sulfur breakthrough was detected. Witte et al. [30] applied the same reactor setup to convert biogas from a digester with hydrogen to SNG and showed a stable long-term operation for >1000 h. On a larger scale, Hervy et al. [31] demonstrated CO2 methanation in a 400 kW fluidized bed methanation reactor. They proved that a high conversion efficiency could be maintained despite temperature and load variations.Despite these advantages, fluidized beds impose mechanical stress on catalyst particles. Thus, the development of an attrition-resistant catalyst with a proper fluidization behavior is necessary, which has not been considered or documented in the investigations mentioned above. In general, a significant amount of research has been put into the development of methanation catalysts, as some reviews show [32,33]. However, only a few investigations focus on the application in fluidized beds or the use of α-Al2O3 as catalyst support. Typically, γ-Al2O3 is used because of its high surface area and the highly dispersed metal particles, while α-Al2O3 is often disregarded because of its low surface area and weak metal-support interaction [33]. For fluidized bed applications, Cui et al. [34] added different binders to improve the attrition resistance of the produced catalyst and found that acidic silica sol showed the highest resistance. Other investigations proved the superiority of fluidized beds over fixed beds in terms of conversion rates and coking resistance in small lab-scale test rigs [35,36]. However, a holistic approach, considering the catalytic activity in combination with an optimal fluidization behavior in a representative fluidized bed reactor scale, seems to be missing.This work investigates the catalytic methanation process in a 10 kW bubbling fluidized bed methanation reactor utilizing an optimized catalyst for fluidized bed applications. In contrast to the commonly used γ-Al2O3, an attrition-resistant α-Al2O3 with a high specific surface area and improved fluidization properties is utilized. Besides the determination of the reactor and catalyst performance, the proposed concept is applied to systematically investigate the methanation of premixed gases imitating syngas from the advanced DFB pilot plant. The goal is to demonstrate that (raw) SNG production via SER and fluidized bed methanation with a tailored catalyst can lead to an optimized process chain with technical and economic advantages. Fig. 1 shows a 3D-CAD drawing of the fluidized bed reactor setup designed for the catalytic methanation of syngas. The reactor consists of two separate reaction zones operated in the bubbling fluidization regime. Both zones can be fluidized individually via two separate wind boxes. The gas distributor consists of nozzles, which provide the necessary pressure drop for uniform gas distribution. Both reaction zones are cooled individually to manage the heat released by the exothermic reaction. An air perfused coil cools the inner reaction zone, while a cooling jacket is used to cool the annular reaction zone. Thus, an isothermal operation of the methanation reactor is ensured. At the same time, the fluidization in the two reaction zones is not disturbed by internals. The catalyst, however, can move freely between the zones through the ‘upper gap’ and the ‘lower gap’ as denoted in Fig. 1. More information on the reactor setup is documented in [37]. Fig. 2 shows a simplified piping and instrumentation (P&I) diagram of the reactor setup. In the lower-left part of the diagram, the gases (N2/H2/CO/CO2/CH4/C2H4) are withdrawn from gas cylinders and premixed according to the volume flow set by valves and rotameters. After splitting and preheating, the gas stream enters the wind boxes. Here, water vapor can be added if the syngas composition requires so. The reaction zones are equipped with thermocouples type K to measure the axial temperature distribution along the reactor height. The gas outlet is equipped with a particle filter and downstream the raw-SNG is burnt in a flare. The gas compositions of the syngas input and the raw-SNG output are analyzed online, as described in section 2.3.The catalyst contained 20 wt.-% NiO and 2 wt.-% MgO and was produced in 6 batches, following the preparation method of Hu et al. [38]. The reagents used for this are listed in the supplementary material (chapter A). Nickel nitrate hexahydrate and magnesium nitrate hexahydrate were dissolved (approx. 300 ml) in water and afterward the support was added. The solution was heated and stirred until the excess water was evaporated. The powder was dried overnight at 120 °C and calcined for 4 h at 500 °C with a heating ramp of 5 °C/min. The used support was a Puralox SCCa-150/200 α-Al2O3 from SASOL, which is in particular designed for fluidized bed applications and thus exhibits a high level of attrition resistance. Despite the high calcination temperatures typical for α-Al2O3, the material is reported to have a high surface area [39].A MicrotracBEL Catalyst Analyzer Belcat-II was used for the temperature-programmed reduction (TPR) and the pulse chemisorption measurements of CO and H2 on the catalyst sample. N2 physisorption was performed in a Micromeritics ASAP 2020 Serial # 1455 for the measurement of the surface area using the BET model. To determine possible carbon depositions on the catalyst, temperature-programmed oxidation (TPO) experiments were carried out. More information on the experimental measurement procedure can be found in the supplementary material (chapter B).The particle size distribution and the Sauter diameter (dSV) of the catalyst are determined with a Malvern Instruments Mastersizer 2000 laser diffraction particle size analyzer. A Rosemount NGA 2000 gas analyzer is used to measure H2, CO, CO2 and CH4 concentrations in the feed gas and the raw-SNG. Another NGA 2000 module with a low measurement range (< 5000 ppmv) is used for the reliable detection of low CO concentrations in the raw-SNG. Additionally, a Perkin Elmer ARNEL – Clarus 500 gas chromatograph (GC) detects ethane (C2H6), ethylene (C2H4), acetylene (C2H2), propane (C3H8), and nitrogen (N2) quantitatively. Higher hydrocarbons (C3+) are qualitatively detected. Furthermore, the GC redundantly measures the CO, CO2, and CH4 concentrations, which are included in the data evaluation.For the fluidized bed methanation experiments, 1.6 kg of the prepared Ni/α-Al2O3 catalyst and 1.5 kg of the unimpregnated α-Al2O3 support were used in the reactor. This amounts to an unfluidized bed height of about 20 cm. The catalyst was heated up at a rate of approximately 3 °C/min and reduced at 500 °C for 6 h in a 9:1 volume-based hydrogen to nitrogen atmosphere.For the purpose of this investigation, the gas velocity and the mean temperature in the inner and the annular reaction zone were kept equal to each other for all experiments. Furthermore, the gas preheating temperature was set to 250 °C, and the pressure was equal to ambient conditions for all experiments. All temperatures given in section 3 are mean values computed from two measurements in the inner reaction zone and three measurements in the annular reaction zone, as indicated in Fig. 2.The following syngas compositions are tested: • Stoichiometric H2/CO syngas for the determination of the catalyst and reactor performance (H2/CO). • Typical DFB syngas composition from the 100 kWth advanced DFB pilot plant (DFB). • The flexible SER syngas composition from the 100 kWth advanced DFB pilot plant (SER). Stoichiometric H2/CO syngas for the determination of the catalyst and reactor performance (H2/CO).Typical DFB syngas composition from the 100 kWth advanced DFB pilot plant (DFB).The flexible SER syngas composition from the 100 kWth advanced DFB pilot plant (SER). Table 1 lists the gas compositions for these cases. The exact compositions of the SER syngases are given in the supplementary material (chapter C), following the work of Fuchs et al. [11]. Additionally, the parameter variations carried out for each gas composition are depicted. A variation of the C2H4 content was investigated because the syngas of the advanced DFB gasifier exhibits varying concentrations depending on factors such as temperature, bed material, and type of biomass [40]. Because of the different gas compositions and reaction conditions, the fluidization number varies between 1.8 and 7.8 u mf . Especially the variation of the weight hourly space velocity (WHSV) results in the most pronounced influence on the fluidization number.All gas analysis measurements are validated by the calculation of mass and energy balances around the reactor. For this purpose, a mathematical model of the reactor is created in the process simulation tool IPSEpro. In addition to the mass and energy balances, the model also calculates the fluid dynamic properties of the fluidized bed as well as the key performance indicators (KPI's) defined in Eqs. 6–12. All presented results in this paper reflect the balanced solution obtained from IPSEpro and not the direct measurement values.The methane yield (Y CH 4 ) is calculated according to Eq. 6, where n ̇ is the molar flow of species i and N is the number of carbon atoms in the respective gas component in the feed gas (feed) and the raw-SNG (out). Eqs. 7, 8, and 9 define the CO conversion (X CO ), the CO2 conversion (X CO 2 ) and the H2 conversion (X H 2 ), respectively. (6) Y C H 4 = n ̇ C H 4 , out ∑ i N i n ̇ i , feed ∗ 100 (7) X CO = n ̇ CO , feed − n ̇ CO , out n ̇ CO , feed ∗ 100 (8) X C O 2 = n ̇ C O 2 , feed − n ̇ C O 2 , out n ̇ C O 2 , fee d ∗ 100 (9) X H 2 = n ̇ H 2 , feed − n ̇ H 2 , out n ̇ H 2 , feed ∗ 100 Eqs. 10 and 11 show the calculation of the CO2 selectivity (S CO 2 ) and the C2H6 selectivity (S C 2 H 6 ), respectively. Eq. 10 is based on the assumption that CO2 is only formed from CO via the water-gas shift reaction (Eq. 2). The C2H6 selectivity only considers the formation of C2H6 via the hydrogenation of C2H4 (Eq. 4). However, the catalyst also shows a slight selectivity of CO towards C2H6 under certain reaction conditions. Therefore, the C2H6 selectivity is only depicted if the formation via CO does not occur. (10) S C O 2 = n ̇ CO 2 , out − n ̇ CO 2 , feed n ̇ CO , feed − n ̇ CO , out ∗ 100 (11) S C 2 H 6 = n ̇ C 2 H 6 , out n ̇ C 2 H 4 , feed − n ̇ C 2 H 4 , out ∗ 100 The stoichiometric number (SN) is calculated according to Eq. 12. It assesses the stoichiometry of the feed gas for methanation according to the reaction equations Eqs. 1, 2 and 5. (12) SN = y H 2 3 y CO + 4 y C O 2 + 2 y C 2 H 4 Table 2 gives an overview of the properties of the α-Al2O3 support and the prepared NiO/α-Al2O3 catalyst. From a fluid dynamic point of view, both can be classified as group B particles close to the transition area to group A, according to Geldart [41]. The Sauter diameter (dSV) and the bulk density (ρb) increase through the impregnation of the support with NiO. Nevertheless, the uniform and narrow particle size distribution of the support is maintained. 99% of the particles are sized between 80 and 280 μm (cf. Fig. 4). Since the particles are also approximately spherical, they are deemed well suited for an optimal fluidization behavior [26]. Additionally, a very high BET surface area was measured for the support, despite literature reports, which attribute Al2O3 in the alpha configuration a rather low surface area due to the high calcination temperature. Liu et al. [42] for example, achieved a surface area of 44 m2/g, while other supports exhibit values around 10 m2/g [43,44]. By impregnating the support with nickel, the surface area is reduced by about 22%, most likely through the blockage of pores with NiO particles [43]. Nevertheless, the resulting catalyst shows a very high surface area at around 140 m2/g, which is even in the range of commonly used γ-Al2O3-based catalysts [28,38]. The average Ni particle size at 37 nm is in the upper part of the spectrum but within the expected range. The somewhat larger Ni particles may result from a weaker catalyst/support interaction and the preparation conditions [43]. Fig. 3 illustrates the H2 consumption during the TPR of the catalyst. Two main reduction temperatures at 595 °C and 766 °C were identified. The hydrogen uptake was 1.5 mmol/g and 0.34 mmol/g for the first and the second peak, respectively. The lower temperature can be assigned to a less strongly bounded NiO on Al2O3 and the NiO that is reduced at 766 °C to Ni-aluminate spinels. MgO is known to be responsible for a higher amount of NiO being present in the form that is easier to reduce. MgO was not likely to be reduced under these conditions [45]. Additionally, the Al2O3 in the alpha configuration leads to less strongly bound NiO, which lowers the reduction temperature [43]. The H2 consumption per Ni atom was approximately 0.7, indicating a core-shell structure of Ni, where the core was still oxidized and only the shell atoms were in metallic form after reduction.In order to further evaluate the performance of the catalyst, the mechanical and chemical stability was evaluated. Fig. 4 (left) depicts the particle size distribution and the mean Sauter diameter (dSV) of the fresh and the used catalyst. No significant attrition of the catalyst was detected during approximately 200 h of operation under fluidized bed conditions. The narrow and uniform particle size distribution of the fresh catalyst could be maintained. Only a slightly smaller Sauter diameter was measured for the used catalyst. The deviation is, however, too small to state significant attrition of the catalyst. On the one hand the measurement accuracy is lower than the deviation and on the other hand the reduction of the catalyst is not accounted for since the fresh catalyst was in the original, oxidized state when the measurement was performed. The measurement accuracy is deemed suitable for the statement of no significant attrition, especially considering the relatively high number of operating hours. For a finite statement, even longer-term experiments could show if relevant attrition occurs. Furthermore, no chemical deactivation of the catalyst occurred during the methanation experiments over a period of approximately 100 h. The chemical stability was determined by repeatedly carrying out methanation experiments under the same process conditions and comparing the raw-SNG gas composition (see supplementary material chapter B). Nevertheless, some carbon deposition was found on the catalyst by performing TPO analysis of the used catalyst (Fig. 4 right). The TPO curve suggests that only very little amounts of carbon were deposited on the catalyst. Furthermore, the CO2 peak at around 350 °C suggests amorphous carbon, which is rather weakly bound [46]. In general, literature reports under fixed bed conditions indicate that larger Ni particles on α-Al2O3 are more stable and active over time than smaller particles on low-temperature calcined supports [43].In this section, the performance of the catalyst and the reactor is investigated by carrying out stoichiometric H2/CO methanation experiments. Fig. 5 shows the raw-SNG composition for varying temperatures and WHSV's in comparison to the maximum thermodynamic values (dotted lines). There is a clear influence of both parameters visible. At high temperatures, the experimental values approach the thermodynamic equilibrium independent of the applied WHSV. However, a small deviation remains, which is attributed to the back-mixing behavior of fluidized beds. Additionally, the results also confirm the findings of Kopyscinski et al. [28], who describe that the mass transfer between the bubble phase and the dense emulsion phase is a limiting factor. Above the surface of the fluidized bed, less reacted gas of the bubble phase mixes with gas from the dense emulsion phase, which overall results in a below-maximum conversion. At lower temperatures, kinetic limitations take over and lead to a pronounced deviation from the thermodynamic equilibrium. Thus, also the WHSV has a greater influence on the gas composition. The maximum CH4 content of 76.5 vol.-%db is reached at a temperature of 320 °C and a WHSV of 0.8 Nl/g h. At the same time, the H2 and CO contents are minimal at 18.9 vol.-%db and 400 ppmdb, respectively. Accordingly, the maxima and minima shift towards higher temperatures for higher WHSV, following the kinetic limitation. CO2 is produced via the water-gas shift reaction (Eq. 2), yielding around 5 to 8 vol.-%db. Additionally, the amount of CO2 is higher and the CO2 selectivity increases from around 6 to 8% with a higher WHSV. Both assertions indicate that the catalyst is very active towards the water-gas shift reaction (Eq. 2). Furthermore, the catalyst shows a slight selectivity of CO towards ethane below 330 °C. Up to 0.6 vol.-%db ethane were detected.During the experiments, the temperature distribution along the height of the catalytic bed is monitored to determine the isothermal operation capabilities of the reactor. Since the stoichiometric H2/CO methanation yields the highest specific heat amount compared to the other investigated gas compositions, the maximum temperature gradients also occur in this case. A maximum gradient of 10 °C was measured at 280 °C and 1 Nl/g h. In general, the gradient is much lower. Especially at temperatures above 320 °C a deviation of only around 2 °C was measured. Thus, an isothermal operation with the applied reactor and catalyst combination was shown to be feasible and the thermal stress on the catalyst is kept at a minimum. An additional particle mixing by induced solid circulation due to different fluidization velocities in the inner and annular reaction zones was not needed.In this section, typical DFB and SER syngas compositions from the advanced 100 kWth DFB pilot plant at TU Wien are investigated in the fluidized bed methanation reactor. The utilized syngas compositions are defined in Table 1 and the figure headings. The exact syngas compositions for the SER methanation experiments are listed in the supplementary material (chapter C). Fig. 6 shows the raw-SNG composition for varying temperatures and WHSV's in comparison to the maximum thermodynamic values (dotted lines). Because of the understoichiometric composition of the DFB syngas (SN = 0.26), 20 vol.-% water vapor was added to the syngas. Both the added water vapor and the water produced through the methanation reaction (Eq. 1) lead to a shift of the gas (Eq. 2) and the production of CO2. Together with the CO2 already present in the syngas, it is the component with the highest concentration in the raw-SNG. A maximum CH4 concentration of 43.4 vol.-%db and a minimum H2 and CO concentration of 8.8 vol.-%db and 0.32 vol.-%db, respectively, was measured at a temperature of 320 °C and a WHSV of 1 Nl/g h. Similar to Fig. 5, there is an influence of temperature and WHSV visible on the gas composition. At higher temperatures and lower WHSV's, the thermodynamic equilibrium is approached. However, the influence of both parameters is less pronounced compared to the stoichiometric CO methanation experiments. Between 300 and 400 °C, the CH4 concentration only varies by 2.7 vol.-%db at 1.5 Nl/g h. Interestingly, the CO2 and CO concentrations at high temperatures are closer to the thermodynamic equilibrium than the CH4 and H2 concentrations. Again, this could indicate that the water-gas shift reaction is favored over the methanation reaction, as Kopyscinski et al. already discussed in [24,28] for a different nickel catalyst. However, at low temperatures, Kopyscinski et al. [47] state that the water-gas shift reaction is negligible. This does not seem to be the case for the investigated catalyst since the CO2 concentrations are especially high at low temperatures. Ethylene was also added to the feed gas and is fully converted to ethane and other components like methane. The concentration of ethane strongly depends on the temperature and, to a lesser extent, on the WHSV. Fig. 7 visualizes the C2H6 selectivity (S C 2 H 6 ) and the CO2 selectivity (S CO 2 ). At temperatures around 400 °C, almost no ethane is formed, whereas at a temperature of 280 °C the C2H6 selectivity is as high as 65%. Additionally, a lower WHSV leads to a lower selectivity. This shows that the reaction of ethane to methane is kinetically inhibited at lower temperatures, whereas the conversion of ethylene is complete for all applied conditions. On the other hand, the concentration of ethylene in the syngas does not seem to have any influence on the C2H6 selectivity. At 360 °C, the same selectivities were achieved for all investigated concentrations. The course of the C2H6 selectivity over temperature agrees very well with the results published by Kopyscinski et al. [24], who found similar C2H6 selectivities even though they used a catalyst with 50 wt.-% NiO and a syngas composition with higher CO and C2H4 contents. However, above 350 °C, their C2H6 selectivity approaches zero, whereas Fig. 7 still shows some selectivity.Furthermore, the measurement data shows that no other hydrocarbons are formed under the applied reaction conditions. Reference experiments without ethylene in the syngas also show that CO is not selective towards C2H6 over the whole temperature range for a typical DFB syngas (see supplementary material chapter D). This is in contrast to the stoichiometric H2/CO methanation experiments in Fig. 5 where CO shows some selectivity towards C2H6. Both the added steam and the understoichiometric composition of the syngas can be responsible for the suppression of ethane formation from CO. In general, the formation of ethane to a certain amount is desirable because of the higher energy density and increased heating value of the resulting SNG. This, in turn, increases the leeway for fulfilling the specifications of the gas grid, since the minimum required heating value can be reached more easily. However, ethane and other hydrocarbons usually present in the DFB syngas can lead to coke formation. Even though this issue is less pronounced in fluidized beds, it can still lead to a coverage of the catalyst surface with carbon species and deactivation of the catalyst [48,49]. Carbon deposition is, however, strongly influenced by the reaction temperature and low temperatures (<380 °C) were found to suppress carbon formation to a large extent [24]. Within this work, carbon deposition was detected after an operation period of 100 h under different operating conditions. However, the deposited carbon did not lead to a deactivation of the catalyst (also see section 3.1.1). A more detailed investigation under specified conditions would be required to allow a final statement concerning this topic.A look at the CO2 selectivity shows that it is relatively constant over the investigated temperature range, which is again in agreement with [24]. For higher temperatures only a minor increase can be observed. A lower WHSV decreases the CO2 selectivity as is the case for the stoichiometric H2/CO methanation in section 3.1.2. This is explained by the fact that the methanation reactions gain importance at lower WHSV and lead to an increased CO conversion to CH4. The higher CO2 concentrations for lower WHSV in Fig. 6 are therefore attributed to dilution effects.The feed water content was set to 20 vol.-% to prevent carbon depositions and was chosen according to thermodynamic consideration as well as literature values. However, the amount of water also shows an influence on the raw-SNG composition since it is a reaction product of the methanation reactions but an educt of the water-gas shift reaction. Accordingly, the CH4 and CO concentrations decrease with increasing water content, while the H2 concentration and the CO2 selectivity increase as experiments (see supplementary material chapter D) and Kopyscinski et al. [24] confirm. Additionally, higher water concentrations can lead to a hydrothermal deactivation of the catalyst, which involves grain growth of the catalytic phase, especially at higher temperatures [22]. Therefore, an optimization of the feed water content depends on the reaction conditions, the syngas composition, and the catalyst and is a tradeoff between the raw-SNG composition and catalyst deactivation. In this work, no further investigations on the hydrothermal deactivation were carried out.Syngas from the SER process is very flexible in its composition, which can be taken advantage of in methanation processes. Fig. 8 depicts the raw-SNG composition over the stoichiometric number SN (Eq. 12) of the syngas at a methanation temperature of 360 °C and a WHSV of 1.5 Nl/g h. Additionally, the thermodynamic values are given (dotted lines). The syngas compositions range from widely understoichiometric to overstoichiometric compositions (SN = 0.4–1.6). Accordingly, also the raw-SNG composition changes with SN. In general, the experimentally determined values follow the course of the thermodynamic prediction well. For a high SN, the distance to the thermodynamic equilibrium decreases further, due to the diminishing amounts of CO and CO2, which need to be methanated, despite the constant WHSV. Furthermore, the CO2 concentration is very close to the thermodynamic equilibrium, which again indicates a preference of the catalyst towards the water-gas shift reaction. The highest methane concentrations are found for an almost stoichiometric composition containing 71 vol.-%db CH4. A lower SN leads to an excess of CO2 and higher CO concentrations but to a lower H2 content. In this case, not enough H2 is available to convert the CO2 in the syngas. On the other hand, a higher SN and therefore a higher H2 partial pressure in the syngas allows an almost complete conversion of CO2 and CO. No CO2 separation unit for the upgrading of the raw-SNG is necessary in this case. This is true for a SN ≥ 1.2 under the considered reaction conditions. However, excessive amounts of hydrogen are left in the raw-SNG, which need to be separated or recirculated before grid feeding. Hydrogen separation is necessary for all investigated compositions, since the limit of 10 vol.-%db according to the regulations [50] was not reached. Ethylene is again fully converted, but a certain amount remains as ethane in the raw-SNG. Fig. 9 depicts the key figures Y CH 4 , X CO , X CO 2 , X H 2 , and S C 2 H 6 over SN for the methanation of the SER syngas. Y CH 4 , X CO , and X CO 2 increase with a higher SN and reach almost 100%. The H2 conversion, on the other hand, decreases from left to right. Especially when looking at the CO2 conversion, the effect of the different syngas compositions becomes evident. While the CO2 conversion is almost zero on the left side of the diagram, it is nearly complete for the highest depicted SN. Overall, the performance improvement is most pronounced on the left side of the diagram, i.e., when increasing SN from 0.4 to 1. Higher SN lead to a lower increase of the key figures at the expense of a more pronounced decline in H2 conversion. In other words, the driving force for the reaction decreases. Interestingly, the ethane selectivity is relatively constant over the whole SN range even though the syngas composition varies considerably. Only a slight decrease is observed towards higher SN.This chapter compares the raw-SNG composition (Fig. 10 left) and the key figures (Fig. 10 right) of the DFB and SER syngas methanation experiments. The two displayed datasets were recorded under the same reaction conditions at 360 °C and a WHSV of 1 Nl/g h with a SN of 1.05 for the SER syngas. The CH4 content increases by more than 30 percentage points by utilizing the SER syngas, while the CO and CO2 concentrations decrease to low levels. However, the residual H2 content more than doubles. This is not because of a lower H2 conversion but due to the dilution of the DFB raw-SNG with CO2. After CO2 separation, the residual H2 concentrations are in a similar range. Nevertheless, both raw-SNG gases require an H2 and CO2 separation unit before grid feeding under the considered reaction conditions. On the contrary, the residual CO concentration of the SER raw-SNG is within the limit of 0.1 mol.-% according to the regulations [50], whereas a further reduction for the DFB raw-SNG is required. In general, the SER raw-SNG is much closer to the specifications of the gas grid and an injection to the gas grid is possible without CO or CO2 separation, as Fig. 8 shows.Thermodynamic investigations predict that grid feeding even without H2, CO and CO2 separation is theoretically possible under the right process conditions. This includes a further reduction of reaction temperature and a pressurized application. For the depicted, slightly overstoichiometric composition, this could be thermodynamically achieved at around 300 °C and 4 bara [13]. Alternatively, a two-stage methanation process could be applied, as some theoretical considerations show [51]. Fig. 10 (right) displays the key figures defined in Eqs. 6–9 and 11. The SER syngas methanation allows a doubling of the CH4 yield and a substantial amount of CO2 conversion. Simultaneously, the H2 and CO conversions are slightly increased as well. The DFB syngas, on the other hand, leads to the production of additional CO2 through the shift of the gas via the water-gas shift reaction. However, the ethane selectivity is lower in the case of the SER syngas. This is attributed to the high H2 partial pressure of the SER syngas, which influences the selectivity to some extent (cf. Fig. 9).Overall, it is necessary to look at the performance of the whole process chain and not only at the methanation itself. The methanation KPI's are much more favorable for the SER syngas. However, this should not create the illusion that the performance of the whole process from biomass to SNG is more favorable as well. The high methane yield for the SER syngas methanation is only possible because of the different process characteristics of the SER and the DFB operation modes of the gasifier. Therefore, a comparison of the whole process chain is necessary.This study shows experimental results of syngas methanation in a fluidized bed methanation reactor. The main focus points are the development and testing of a stable catalyst for fluidized beds, the methanation reactor design and the detailed investigation of the fluidized bed methanation process characteristics through parameter variations. Furthermore, optimized process concepts are investigated through the methanation of flexible syngas compositions from the advanced dual fluidized bed technology.The following results can be summarized: (i) The synthesized catalyst performs well in terms of avoidance of mechanical attrition and chemical deactivation and, therefore, maintaining a proper fluidization behavior. No significant mechanical attrition and chemical deactivation of the catalyst was detected during 200 h under fluidized bed conditions and 100 h under methanation conditions. Simultaneously, the catalyst showed a sufficient catalytic activity and selectivity towards the methanation reactions. The results were achieved by utilizing a highly porous and inert α-Al2O3 as support. (ii) The reactor design, in combination with the dilution of the catalyst with support material, allowed an isothermal operation of the process with temperature gradients as low as 2 °C. Between 280 °C and 420 °C, a clear transition between thermodynamic and kinetic limitations could be observed. The optimal tradeoff between these limitations was found to be in a temperature range between 320 and 360 °C. (iii) A maximum of 43 vol.-%db CH4 was reached through the methanation of a typical syngas composition from the advanced 100 kW DFB gasification pilot plant. Because of the understoichiometric composition of the syngas from the DFB process, CO2 was produced through the water-gas shift reaction and constituted the main component in the raw-SNG. (iv) Under all conditions, a full conversion of ethylene to ethane and other components was shown. At low temperatures, kinetic limitations favored the production of ethane while higher temperatures allowed a complete conversion to other components. Neither the ethylene concentration in the syngas nor the syngas composition (DFB or SER) showed a significant influence on the selectivity of ethylene towards ethane. (v) The SER syngas methanation was shown to yield a much more favorable composition for grid feeding and higher methane yields, while simultaneously improving the H2, CO and CO2 conversions, compared to DFB syngas methanation. A maximum CH4 content of 73 vol.-%db could be reached, which represents an increase of 30 vol.-%db compared to DFB syngas methanation. (vi) An almost complete conversion of CO and CO2 was achieved through the methanation of an overstoichiometric SER syngas (SN ≥ 1.2), allowing grid feeding without the need for an expensive CO2 separation unit. Hydrogen separation and recirculation is, however, necessary under the investigated reaction conditions. The synthesized catalyst performs well in terms of avoidance of mechanical attrition and chemical deactivation and, therefore, maintaining a proper fluidization behavior. No significant mechanical attrition and chemical deactivation of the catalyst was detected during 200 h under fluidized bed conditions and 100 h under methanation conditions. Simultaneously, the catalyst showed a sufficient catalytic activity and selectivity towards the methanation reactions. The results were achieved by utilizing a highly porous and inert α-Al2O3 as support.The reactor design, in combination with the dilution of the catalyst with support material, allowed an isothermal operation of the process with temperature gradients as low as 2 °C. Between 280 °C and 420 °C, a clear transition between thermodynamic and kinetic limitations could be observed. The optimal tradeoff between these limitations was found to be in a temperature range between 320 and 360 °C.A maximum of 43 vol.-%db CH4 was reached through the methanation of a typical syngas composition from the advanced 100 kW DFB gasification pilot plant. Because of the understoichiometric composition of the syngas from the DFB process, CO2 was produced through the water-gas shift reaction and constituted the main component in the raw-SNG.Under all conditions, a full conversion of ethylene to ethane and other components was shown. At low temperatures, kinetic limitations favored the production of ethane while higher temperatures allowed a complete conversion to other components. Neither the ethylene concentration in the syngas nor the syngas composition (DFB or SER) showed a significant influence on the selectivity of ethylene towards ethane.The SER syngas methanation was shown to yield a much more favorable composition for grid feeding and higher methane yields, while simultaneously improving the H2, CO and CO2 conversions, compared to DFB syngas methanation. A maximum CH4 content of 73 vol.-%db could be reached, which represents an increase of 30 vol.-%db compared to DFB syngas methanation.An almost complete conversion of CO and CO2 was achieved through the methanation of an overstoichiometric SER syngas (SN ≥ 1.2), allowing grid feeding without the need for an expensive CO2 separation unit. Hydrogen separation and recirculation is, however, necessary under the investigated reaction conditions.In this work, the catalytic methanation of syngas from the advanced DFB technology was experimentally investigated, combining a bubbling fluidized bed methanation reactor with an optimized methanation catalyst for fluidized bed applications. Fluidized bed reactors can be advantageously applied to SNG production because of the high heat and mass transfer capabilities. This leads to several advantages compared to the commercially utilized fixed bed methanation reactors, provided that the catalyst shows a proper fluidization behavior.To this end, the following conclusions and recommendations can be drawn from the conducted experiments: (i) α-Al2O3 was shown to be a viable catalyst support for methanation reactions. Especially in fluidized bed applications, it could be advantageously used as an alternative to the commonly utilized γ-Al2O3 because of the high mechanical and chemical stability and a proper fluidization behavior of the prepared catalyst. (ii) The stress on the catalyst was minimized due to nearly isothermal operating conditions. This is a result of the special reactor design and the dilution of the catalyst with inert support material. α-Al2O3 was shown to be a viable catalyst support for methanation reactions. Especially in fluidized bed applications, it could be advantageously used as an alternative to the commonly utilized γ-Al2O3 because of the high mechanical and chemical stability and a proper fluidization behavior of the prepared catalyst.The stress on the catalyst was minimized due to nearly isothermal operating conditions. This is a result of the special reactor design and the dilution of the catalyst with inert support material.Additionally, the fluidized bed methanation reactor and the synthesized catalyst were applied to the methanation of syngas from advanced DFB gasification and the SER process. The SER process allows the production of syngas with a suitable stoichiometric ratio of H2 to CO and CO2, yielding the following conclusions for fluidized bed methanation: (iii) The SER process in combination with fluidized bed methanation could lead to an improved and more cost-effective route for SNG production. No separation of excessive CO2 or CO from the raw-SNG is required for grid-feeding when selecting a suitable syngas composition. Compared to conventional DFB steam gasification, the methane yield is doubled (up to 95%) and the H2, CO and CO2 conversions are improved. Especially if no external hydrogen is available, the direct methanation of SER syngas could lead to a simpler and more efficient process route for SNG production. The SER process in combination with fluidized bed methanation could lead to an improved and more cost-effective route for SNG production. No separation of excessive CO2 or CO from the raw-SNG is required for grid-feeding when selecting a suitable syngas composition. Compared to conventional DFB steam gasification, the methane yield is doubled (up to 95%) and the H2, CO and CO2 conversions are improved. Especially if no external hydrogen is available, the direct methanation of SER syngas could lead to a simpler and more efficient process route for SNG production.For a full comparison of the DFB and SER syngas methanation, further investigations on the optimal process conditions and the performance of the whole process chain are necessary. Additionally, the long-term mechanical and chemical stability of the catalyst should be examined. Alexander Bartik: Conceptualization, Methodology, Investigation, Writing – review & editing, Visualization. Josef Fuchs: Methodology, Validation, Writing – review & editing. Gernot Pacholik: Investigation, Writing – review & editing. Karin Föttinger: Validation, Writing – review & editing, Supervision. Hermann Hofbauer: Conceptualization, Writing – review & editing, Supervision, Project administration, Funding acquisition. Stefan Müller: Validation, Writing – review & editing, Supervision, Funding acquisition. Florian Benedikt: Conceptualization, Methodology, 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 study was carried out within the doctoral college CO2Refinery at TU Wien. It is also part of the research project ReGas4Industry (871732) and receives financial support from the research program “Energieforschung” funded by the Austrian Climate and Energy Fund. Furthermore, the authors would like to thank Jonas Hauser for the technical assistance, Wolfgang Ipsmiller for the assistance and rental of the particle size analyzer, Johannes Schmid for the support during the conceptualization of the reactor and SASOL for the supply of the catalyst support. The authors acknowledge TU Wien Bibliothek for financial support through its Open Access Funding Programme. Supplementary material. Image 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.fuproc.2022.107402.

Catalytic methanation processes allow the production of natural gas substitutes on a sustainable and renewable basis. This study investigates the catalytic methanation of syngas from dual fluidized bed steam gasification of biomass in an innovative bubbling fluidized bed methanation reactor with an optimized catalyst. Syngas from conventional gasification and a novel combination with syngas from sorption enhanced reforming were investigated. The applied fluidized bed reactor allowed an almost isothermal operation with optimal reaction temperatures between 320 °C–360 °C. Simultaneously, no chemical deactivation or mechanical attrition during 200 h of operation indicates a high long-term stability of the catalyst. The methane concentration downstream the methanation reactor increased from 43 to 74 vol.-%db through the methanation of a hydrogen-rich syngas produced via sorption enhanced reforming. Simultaneously, the methane yield is doubled to 95% and the hydrogen, carbon monoxide and carbon dioxide conversions are improved. Furthermore, it could be shown that a CO2 content below 1 vol.-%db is feasible in the (raw) synthetic natural gas, allowing grid injection without CO2 separation. The results indicate that sorption enhanced reforming in combination with an optimized fluidized bed methanation can lead to technical and economic improvements in sustainable synthetic natural gas production.

In the last decades, hydrogen fuel cell technology has become an alternative to energy production systems in fixed and mobile devices [1]. The polymer-electrolyte-membranes fuel cells (PEMFCs) has turned into the most hopeful energy converters due to their high efficiency, no pollution, and energy suitability. Nowadays, H2 is produced mainly by hydrocarbons steam reforming, which inherently includes a large CO concentration (0.5%–10%) in the H2 stream [2]. However, high purity hydrogen is necessary since Pt anodes are very sensitive to impurities such as carbon monoxide and sulfides [2,3]. The CO presents a poisoning effect since it is adsorbed much easier than hydrogen on the surface of the Pt anode. Furthermore, CO is difficult to be eliminated from the active sites [4,5].Different strategies such as pressure swing adsorption (PSA), membrane separation, water-gas shift (WGS) reaction, selective methanation, and preferential oxidation have been proposed to minimize and remove the CO concentrations in H2 streams [2]. Among them, selective methanation of carbon monoxide (S-MET) where CO reacts preferentially to form CH4 without CO2 conversion, has been reported as a promissory strategy in hydrogen purification (Eqs. (1) and (2)) [6,7]: (1) C O + 3 H 2 ↔ C H 4 + H 2 O Δ H 0 = − 206 kJ mol - 1 (2) C O 2 + 4 H 2 ↔ C H 4 + 2 H 2 Δ H 0 = − 165 kJ mol - 1 In this sense, most of the catalysts studied in this reaction can be classified as Ni- or Ru-based catalysts [8,9]. The role of the support materials and the promoters in nickel (Ni) and ruthenium (Ru) catalysts have proved to play an essential part in the enhancement of the selectivity of CO due to their direct relationship to the electron density that inherently influences the activity of CO methanation [10–13]. Due to this, if the electron density is enhanced, CO adsorbed on the metal surface is more easily dissociated by enhanced d π -p π ∗ back bonding and by consequence, CO methanation is improved [14,15]. In addition, it has been reported that in Ru-based catalyst, the CO methanation activity has been improved using supports such as TiO2 compared to others such as Al2O3, CeO2, YSZ, SiO2, ZrO2, and MgO due to the synergy between the support and the electron density of the active phase [16,17]. Indeed, Tada and Kikuchi described a mechanistic study over Ru/TiO2 catalyst for selective carbon monoxide methanation where the control of the interfaces between active metals and support materials was observed to be a key step. Due to this, the choice of suitable support materials such as TiO2 and promoters are necessary to improve CO methanation [11].The addition of small amounts of metals in Ru-based catalysts has been reported as a strategy to enhance the selectivity in the hydrogenation of C–O towards C–C bonding through the combination of electropositive metals giving place to particles with improved redox properties [18]. This improvement can be explained in two pathways: (i) the most electropositive metal acts as a Lewis base that increases the density in the other one, therefore decreasing the bond energy of the adsorbed species, in particular, the C–C bond, and favouring the C–O hydrogenation towards the C–C one [19]; (ii) the metal active phase act as electrophilic or Lewis acid sites for CO adsorption and activation of C–O bond through the oxygen lone pair of electrons.Among the different metal promoters, rhodium (Rh) has proved to enhance catalytic activity in different reactions due to its ability to form solid solutions in a Ru matrix [20–22]. Additionally, Rh possess one extra electron in its electronic configuration, which can increase the electronic density to the active sites, therefore making the C–O bonding breakage becomes easier due to the back-bonding effect in the adsorbed metal. In a similar route, Platinum (Pt) is also able to form alloys with Ru, adopting an hcp structure at high Pt concentrations (>80%), that is, when forming solid solution Pt-Ru [23]. Pt possesses even more electronic density than Rh, which facilitates the breakage of adsorbed CO in comparison with Rh. Both Rh and Pt have intrinsic properties as methanation catalysts [24].According to these premises, in this work we synthesize a series of active Ru/TiO2 catalysts promoted with Rh and Pt to evaluate the effect of small amounts of these metals in the catalytic performance in the S-MET reaction using a real composition H2 stream to simulate the streams outlines from the WGS and PROX units.Ru/TiO2 parent catalyst was prepared by wet impregnation method based on a similar procedure as reported elsewhere [25]. Typically, 13.2 g of nitrosyl nitrate solution (14.391 wt% of Ru, Johnson Matthey) were mixed in the necessary amount of water to impregnate 19 g of Aeroxide® TiO2 P25 (Evonik) support to obtain a nominal loading of 9.5 wt% of Ru. The solid catalyst was dried at 130 °C for 24 h and finally calcined at 400 °C for 2 h.Analogously, the bimetallic catalysts were prepared by co-impregnation of both ruthenium and platinum or rhodium precursors in order to achieve a nominal loading value of 9.5 wt% of Ru and 0.5 wt% of Pt or Rh. Rh(NO3)3 from Alfa Aesar and Pt(NH3)4(NO3)2 from Johnson Matthey were employed as metallic precursors. The fresh Rh-based catalyst was then calcined at 400 °C for 2 h while Pt-based catalyst was calcined at 350 °C for 8 h based on previous results of calcination conditions reported in the literature [26,27]. For clarity, the catalysts were designated as Ru–TiO2, RhRu–TiO2 and PtRu–TiO2.The structural analysis of the synthesized catalysts was elucidated by powder X-ray diffraction on an X-Pert Pro PANalytical (Malvern PANalytical Ltd, Malvern, UK). X-ray diffraction patterns were collected using Cu Kα as a radiation source in the range of 2θ = 10°–80° with a step size of 0.05° and a step time of 80 s.The quantitative analysis of the metal loading was performed using X-Ray Fluorescence in an Axios low-power (1 kW) wavelength dispersive XRF (WDXRF) spectrometer equipped with a Rh cathode.The textural properties of the catalyst were evaluated using N2 adsorption isotherms at 77 K in a Micromeritics ASAP 2010 instrument. Previous to analysis, the samples were outgassed under dynamic vacuum at 250 °C for 2 h to eliminate adsorbed molecules and impurities. The specific surface area of each solid was determined according to using the BET method. Pore volumes were determined by BJH desorption method.Hydrogen temperature-programmed reduction (H2-TPR) experiments were carried out placing the catalyst (0.1 g) in a U-shape quartz tube reactor and initially pretreated at 300 °C (rate 10 °C min−1) in Ar atmosphere for 1 h to remove adsorbed water and then cooled to 75 °C in Ar followed by heating to 700 °C at a heating rate of 10 °C min−1 in a flow of 10% H2/Ar. A thermal conductivity detector (TCD) was used to quantify the H2 consumption. By using the same experimental set-up, the metallic particles average size distribution was determined through hydrogen chemisorption (H2-TPD) experiments. Prior to the analysis, the samples were heated up to 300 °C under Ar flow for 45 min and then cooled down to 75 °C. Finally, the adsorption of H2 took place and then Ar was introduced to perform the TPD measurements with a temperature ramp of 10 °C min−1 to 300 °C. The dispersion of Ru (D) was calculated based on the volume of chemisorbed H2 using the following simplified equation (Eq. (3)): (3) D ( % ) = 100 ( V S T P · S i 22414 ) ( F i P M i ) − 1 where V STP denotes the total volume of H2 consumed (mL g−1), S i is the stochiometric factor of H2 to Ru, which is considered S i  = 2 as previously reported in the literature [28,29], PM i is the molecular weight of the active phase (mmol g−1) and F i corresponds to the fraction of active phase per gram of sample. Finally, the crystallite size was estimated using the equation (Eq. (4)): (4) d i = 6 · V i D · a i assuming the geometry of Ru particles as hemispherical, where V i is the average volume of a metallic bulk particle and a i is the exposed area of a metallic atom in the surface. For the calculation, a i for ruthenium particles was considered as a Ru  = 9.09 × 10−2 nm3 instead of 6.29 × 10−2 nm3 based on the previous work reported by Shen et al. that considered that metallic Ru is able to expose indistinctly (100), (001) and (110) planes due to their similar superficial energy [28].X-ray Photoelectron Spectroscopy (XPS) analysis was carried out using a LEYBOLD-HEREUS model LHS-10/20 device equipped with Al-Kα radiation (1486.6 eV) and a twin crystal monochromator to produce a focused X-ray spot at 30 mA × 11 kV (400 μm major axis length of the elliptical shape). The alpha hemispherical analyzer was operated at the constant energy mode with survey scan pass energies of 200 eV to measure the whole energy band and 50 eV in a narrow scan to selectively measure specific elements. The reference binding energy was the C 1s core level at 284.6 eV.The catalytic activity was measured in a continuous flow fixed-bed stainless steel reactor (i.d. 9 mm) coupled to a Microactivity Reference Unit (PID Eng&Tech®). For each experiment, 150 mg of sieved catalyst (100–200 μm) was mixed with commercial SiC (125 μm – VWR Prolabo®) up to a volume bed of 0.32 cm3. Prior to the reaction, the catalyst was activated in pure H2 flow of 60 mL min−1 at 300 °C for 1 h and subsequent cooled up to reaction temperature. The reaction was conducted at atmospheric pressure increasing the temperature from 180 °C to 300 °C with heating rate of 10 °C min−1. A simulated mixture of real reformate stream containing H2 (50%), CO2 (15%), H2O (15%) and CO (1% or 300 ppm) balanced with N2 was fed at flow rate of 200 mL min−1 (WHSV = 80 L g−1 h−1). The effluents were on-line analyzed in a gas micro-chromatograph Varian 4900. The CO2 amount at the outlet was determined by a CO2 detector Vaisala CARBOCAP GMT220. The conversion (X i ) and selectivity (S i ) values were estimated according to the following equations (Eq. (5) to Eq. (7)): (5) X C O ( % ) = ( F C O i n − F C O o u t ) F C O i n ∗ 100 (6) X C O 2 ( % ) = ( F C O 2 i n − F C O 2 o u t ) F C O 2 i n ∗ 100 (7) S C O m e t h a n a t i o n ( % ) = ( X C O · F C O i n F C H 4 o u t ) · 100 being F C O , F C O 2 and F C H 4 the flow in mL min−1 of CO, CO2 and CH4, respectively, at the inlet (in) or the outlet (out) flow. C i , o u t corresponds to the concentration of product i in the outlet and ν i is the carbon number according to its chemical formula.In all cases, the methane selectivity estimated from Eq. (8) was higher than 95% and only small traces of ethane and ethylene were detected. The carbon balance resulted to be better than 97%. (8) S C H 4 ( % ) = ( C C H 4 o u t / ν C H 4 ∑ i C i o u t / ν i ) · 100 C i o u t is the product i concentration in the outlet and ν i is the carbon numbers according to its chemical formula. Table 1 includes the wt.% metal loading values obtained by XRF analysis. As can be observed, the measured metal contents were close to the nominal values confirming that the synthesis method was successful. However, it is noticeable that Ru loading was slightly lower than that of expected one in the bimetallic catalysts. Although these differences in Ru content may seem significant, this variation can be assumed with the uncertainty of the measurement, where variations between 0.5 and 1 wt% have been reported [30]. Fig. 1 a shows the XRD patterns of the as-synthesized catalysts in which are observed the typical diffraction lines of anatase (JCPDS 73-1764) and rutile (JCPDS 78-1510) phases present in the P25 titania support. Additionally, diffraction peaks ascribed to RuO2 (JCPDS 21-1172) are observed in all Ru-loaded patterns. However, after the incorporation of Rh and Pt in the Ru-loaded catalysts, no additional peaks were observed in the XRD patterns possibly due to the small amount of metal added to the catalyst but also possibly due to the high dispersion of the metal. Similarly, the XRD diffraction measurements were performed in the catalyst after a reduction process in H2 atmosphere to evaluate the structural changes taking place due to the reduction of the metals. Fig. 1b presents the XRD patterns of the reduced samples where the diffraction peaks related to the metallic Ru reflection planes (100), (002) and (101) are observed close to ca. 2θ = 41.4°, 43.7° and 44.1°, which confirms the complete reduction of RuO2 at the evaluation conditions. However, the diffraction peaks related to the dopant metals are not present in the patterns mostly attributed to the high dispersion of those species. The Ru metal average crystal size of the three reduced catalysts was calculated using the Debye-Scherrer method. As summarized in Table 1, it is observed a slight increase in the crystal size in Ru-based catalyst after the incorporation of the Rh and Pt promoters which may be attributed to the interactions Ru–Pt and Ru–Rh able to form bimetallic alloys.The textural properties of the catalyst were evaluated through nitrogen adsorption-desorption isotherms at −196 °C as displayed in Fig. 2 a. It can be observed that all the isotherms present a type III shape according to the IUPAC classification, which is mainly attributed to non-porous or macroporous materials and hysteresis loop type H3 usually found in materials with a wide distribution of pore size [31]. The specific surface area and pore volume of the catalysts were calculated using the BET model and D-R model, respectively. As depicted in Table 1, the incorporation of the second metal does not promote any change in the surface area, which may confirm the high dispersion of the active phase in all catalysts. Additionally, the pore volume all catalyst remains similar. However, in the bimetallic catalysts, it is observed an increase in the average particle size as a result of the possible formation of alloys [32]. The pore distribution calculated using the BJH method is shown in Fig. 2b, where a similar distribution is observed among the monometallic catalyst and the bimetallic Rh and Pt bimetallic catalysts showing a narrower pore distribution with a maximum displaced to bigger pore diameter as an effect of alloys formation.The hydrogen consumption profiles obtained in the H2-TPR measurements for all the as-synthesized catalysts are shown in Fig. 3 . In general terms, the ruthenium oxide reduction can be represented by Eq. (9): (9) R u O x + x H 2 → R u 0 + x H 2 O where x identifies the possible different ruthenium oxides that may be present in the sample. If we assume that the oxidation state of Ru is +4 for the determination of the reducibility degree, the Eq. (10) indicates that to reduce 1 mol of ruthenium oxide, 2 mol of H2 are required. (10) R u O 2 + 2 H 2 → R u 0 + 2 H 2 O Based on previous results reported in the literature related to Ru/TiO2, these types of catalysts show three TPR signals in their profiles attributed to three different reduction zones [33,34]. The signal at lowest temperature (approximately at 105 °C) has been established to be related to RuOx well dispersed amorphous species. The second reduction zone, at approximately 125 °C, is attributed to the RuOx species within the bulk. Finally, the peak at 150 °C was attributed to the reduction of the RuOx species strongly interacting with the support, being these ones the hardest to be reduced.All the catalysts present the typical reduction profiles described above. However, the bimetallic catalysts present an additional reduction zone at low temperature (<100 °C), mainly attributed to the so-called spillover effect promoted by those noble species. For instance, Kim et al. [35] reported the synthesis of 1 wt% Pt/TiO2 calcined at several temperatures, obtaining in the reduction profile three reduction process at 100, 180 and 300 °C This agrees with the first reduction profile that can be observed in the PtRu–TiO2 and RhRu–TiO2 catalyst, which is attributed to well dispersed Pt species. Additionally, the RhRu–TiO2 catalyst shows an extra reduction process at 500 °C. Wang and Ruckenstein reported the reduction of Rh in a 1% Rh coated MgO catalysts describing mainly two reduction processes, the first at 350 °C attributed to the reduction of MgRu2O4 species and the second one at 520 °C due to the reduction of Ru2O3 species [36]. However, the reduction process present in the RhRu–TiO2 is shifted to slightly lower values likely due to the higher dispersion of the Rh2O3 species. Besides, in the noble metal on reducible supports TPR profiles, a reduction zone at 180 °C is frequently found and attributed to M-Ov-Ti3+ species related to strong metal-support interaction (SMSI) effect, being Ov oxygen vacancies [37,38]. That is a common effect for Ru, Rh and Pt. All samples presented similar reducibility degree of about 100%. Table 2 shows the metal dispersion calculated from H2 chemisorption where is observed a slight decrease in the bimetallic catalyst, which may be attributed to the formation of bimetallic alloys, as was observed in above mentioned results. Komaya et al. reported the limitations of hydrogen chemisorption for the determination of the particle applied to a Ru/TiO2 sample, where it was concluded that the dispersion could be overestimated due to a fraction of H2 adsorbed that suffers spillover to the support. As a consequence, the number of active sites can be apparently higher. Additionally, if the sample is treated at high temperature reduction treatments, the metal could be partially encapsulated by the support, underestimating the mean particle size [39]. Table 2 also shows how the introduction of little Pt diminished the total amount of chemisorbed hydrogen to form a monolayer compared to the monometallic catalyst. Aguilar-Ríos et al. [40] obtained in their work similar results for Sn-modified Pt catalysts. After being doped with a ratio Sn/Pt < 1, the chemisorbed hydrogen increases. However, for Sn/Pt > 1 the H2-monolayer value diminished. That may be explained in back-bonding terms. In the monometallic Pt catalyst, the occupied σ orbital of H2 donates electronic density to the 6s Pt's orbital. For its part, the Pt donates electronic density from its 5d yz to the antibonding σ orbital (back bonding), destabilizing and finally dissociating the bond. The addition of Sn produces a reduction of the electronic density transferred to the H2 σ∗ orbital, diminishing the destabilization of the H–H and therefore making it less active. This explained the decrease in the H2 chemisorbed when Sn/Pt > 1 but not the increase when the ratio is smaller than 1. Aguilar-Ríos et al. [40] also explained this suggesting that small amounts of Sn favours the Pt dispersion acting as anchoring sites for Pt due to the affinity that Pt has for Sn (and by extension every metal of Pt group - Ru, Rh, Pd, Os and Ir) according to theoretical calculations. However, according to these authors, despite the higher dispersion, every new created site must be less active. Fig. 4 shows the TPD-H2 profiles for the samples after chemisorption experiments in which an H2 monolayer was firstly adsorbed. The two reduction processes observed are characteristics from transition metals and have been classified as H∗w (weakly adsorbed hydrogen) and H∗s (strongly adsorbed hydrogen). Sayari et al. [41] correlated a larger amount of H∗w for particles between 0.9 and 2.2 nm, which is in good agreement with the present work for Ru and RhRu samples. This may be related to the fact that the H2 adsorption/desorption is not dissociative, according to the work of Lin et al. where similar profiles were obtained [42]. By contrast, PtRu sample contains a large amount of H∗s species and metallic particles are bigger than 2.2 nm (3.1 nm in as shown in Table 2). This suggest that Pt could favors the dissociative adsorption of H2.To quantitatively and qualitatively analyze the surface species and their oxidation states before reaction, Ru, RhRu, and PtRu catalysts were ex situ reduced at 300 °C for 1 h in H2 and characterized by XPS. Fig. 5 shows the Survey spectra of all the samples, where the peaks of the main elements, titanium, oxygen and carbon, are detected. As for the ruthenium peaks, the most intense are Ru3d, followed by Ru3p. Ru3p are very weak in which Ru3p3/2 is practically masked by the intense Ti2p3/2 peak. The Ru3d peaks are located in the C1s zone [43]. The surface composition is included in Table 3 . Apparently, the surface chemical composition is similar in all the samples.As can be expected, Fig. 6 a shows that the O1s peak recorded at 530.3 eV is characteristic for oxides and is well suited with the peaks detected at 459.1 Ti 2p3/2 and 464.8 Ti 2p1/2 eV, with 2p doublet splitting of 5.7 eV, which is typical for TiO2 (Fig. 6b) [43–45]. As seen in Fig. 6c, there is no peak at 461.2(3) eV for Ru 3p3/2 in any of the spectra of the three samples [45]. This could be related to the low surface concentration of ruthenium as also reflected in the Table 3 of surface composition. Ruthenium was detected in all catalysts by the peak at Ru 3d5/2 at 279.7 eV. As shown in Fig. 6c, the position of the peak is not affected by the incorporation of Rh or Pt. The regions Rh 3d and Pt 4f are also shown in Fig. 6d and e, respectively. From the position of the peaks for Ru 3d5/2 (279.7 eV) as well as for Rh 3d (307.3 eV with DS 4.7 eV) and for Pt 4f (70.2 eV with DS 3.4 eV), we can conclude that these are metallic species [44–46].The catalysts were tested in selective CO methanation using two reaction gas compositions simulating a typical output stream of a water-gas shift (WGS) unit: H2 (50 vol%), CO2 (15 vol%), H2O (15 vol%) and CO (1 vol% or 300 ppm) balanced with N2. Fig. 7 shows the performance in terms of CO and CO2 conversion and CO methanation selectivity as a function of reaction temperature for all the catalysts when the reaction was performed with 1 vol% of CO inlet. As can be noticed, increasing the temperature leads to an exponential growth of CO conversion between 200 and 240 °C (Fig. 7a). Meanwhile, Fig. 7b shows that CO2 conversion was initiated at about 220 °C and increasing the temperature above 260 °C a drop in CO conversion become simultaneously evidenced (Fig. 7a inlet). This negative CO conversion as temperature was increased suggests that CO is produced in parallel via reverse water gas shift (RWGS) reaction. Consequently, CO methanation is thermodynamically unfavorable at higher temperatures (Fig. 7c) and the effluent CO concentration is increased with increasing temperature. Although the three catalysts presented similar activity, it can be noticed that PtRu catalyst shows minor CO conversion at high temperature (Fig. 7a inlet). Furthermore, Fig. 7c clearly shows that Pt addition affects negatively the selectivity of CO methanation. According to Xu et al. [47], the reason for this low CH4 production in PtRu catalyst can be related to the alloy formation that prevents the C–O bond cleavage of and subsequent hydrogenation of C to CH4. This argument is coherent with our results of characterization discussed above. On the other hand, it is also significant in Fig. 7b inlet that Rh addition increases slightly the CO2 conversion although selectivity of CO methanation is hardly affected. This indicates that Rh favors CO2 methanation against reverse water gas shift. It is well known that Rh is an excellent active metal for CO2 methanation [48].Subsequently and without reactivation, the catalysts were tested in the same conditions but with a lower concentration of CO (300 ppm) balanced with N2. As shown in Fig. 8 , the effect of the metal doping only is positive in the case of Rh doped catalyst, which increases the catalytic activity and selectivity in the whole temperature range. Similarly for the three samples, CO methanation was initiated at relatively low temperatures achieving conversion about 70% at 130 °C and increasing with temperature until completed CO conversion. On the other side, the CO2 methanation initiates once the CO conversion has reached approximately the 90% in every case. It is noteworthy that CO conversion decreases at higher temperatures due to the RWGS contribution, which tend to produce CO from CO2 and hydrogen, and which is highly visible in PtRu but not appreciated in the Rh doped one.It has been tested that polymer-electrolyte-membranes fuel cells (PEMFCs) only tolerates H2 stream with CO concentration below 20 ppm. Considering that inlet steam contains 300 ppm of CO, a minimum conversion of 93% will be required to decrease the concentration below 20 ppm in the outlet stream. In order to compare the three catalysts, we have defined T93 as the required temperature to achieve CO conversion of 93%. Remarkably, Fig. 8 inlet shows that RuRh bimetallic catalyst decreases the temperature T93 of selective CO methanation in absence of CO2 methanation. In comparison to monometallic Ru and bimetallic PtRu, this variation decrease the total consumption of hydrogen and it could become important in industrial-volume streams.In this work, a Ru supported on TiO2 catalyst was doped with Rh and Pt by wet impregnation method and tested in CO selective methanation using two simulated mixtures from reforming reactors. The obtained catalytic results showed that with a lower concentration of CO (300 ppm) the addition of Rh as dopant results to be advantageous in the methanation catalyst while that Pt has a negative effect since it promotes the reverse water gas shift reaction decreasing the selectivity of methanation. At higher concentration values of CO (1 vol%), Pt addition is also detrimental for CO selective methanation whereas that Ru and RuRh catalysts are practically identical. Although the positive effect of Rh observed at low CO concentrations is relatively mild and may not compensate for its use in this particular case due to the expensive nature of this metal, this approach takes one step further for a better understanding of the promotion effect of other noble metals in a Ru methanation 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.Financial support for this work has been obtained from the Spanish Ministerio de Ciencia, Innovación y Universidades (Grant: RTI2018-096294-B-C33) and Junta de Andalucía project with reference US-1263288, both programs being co-funded by the European Union FEDER.

Selective CO methanation from H2-rich stream has been regarded as a promising route for deep removal of low CO concentration and catalytic hydrogen purification processes. This work is focused on the development of more efficient catalysts applied in practical conditions. For this purpose, we prepared a series of catalysts based on Ru supported over titania and promoted with small amounts of Rh and Pt. Characterization details revealed that Rh and Pt modify the electronic properties of Ru. The results of catalytic activity showed that Pt has a negative effect since it promotes the reverse water gas shift reaction decreasing the selectivity of methanation but Rh increases remarkably the activity and selectivity of CO methanation. The obtained results suggest that RuRh-based catalyst could become important for the treatment of industrial-volume streams.

Hydrogen is considered the most promising fuel applied in fuel-cell vehicle because of the wide applicability, high application efficiency, safety, environmentally friendly and inexhaustible reserves characteristics of it [1–3]. An accepted view considers that the widespread utilization of clean renewable energy vehicles is an effective measure to deal with environmental pollution and excessive consumption of the limited fossil fuels because transportation consumes about a quarter of global energy while there is about 23% carbon dioxide emission originating from the combustion of fossil fuels in the world [4–6]. However, the chemical energy of unit volume hydrogen is low, which limits the commercialization of it. Therefore, a lot of investigations pay attention to dealing with this problem and finding the safe, suitable hydrogen storing materials that can meet the needs of using hydrogen in the field of motor vehicle and electronic products [7–11]. The ideal hydrogen storage methods should have the following characteristics, large capacity, low-temperature fast adsorption rate and high cycle survivability [12]. In present, hydrogen storage methods include high-pressure storage, liquid storage, physical adsorption storage and hydride storage. Generally, the gas storage needs 70 MPa pressure for reaching only 4.8 wt.% H2 capacity [13]. The condition of liquid storage is low-temperature (20 K) that brings about huge energy consumption (about 30% for filling) and short boiling storage duration [14]. Solid hydrogen storage is reckoned as the best one among the various methods [15,16].Among the metal hydrides, MgH2 with high hydrogen storage capacity (7.6 wt.%), low cost, non-toxicity, abundant reserves and superior reversibility characteristics is the most promising material for hydrogen storage [17,18]. Nonetheless, the attempt of the widespread application of commercial MgH2 is retarded by its some disadvantages, such as high thermodynamic stability (i.e. the high strength of the Mg-H bonds), the complicated activation procedure, the high dissociation temperature and the sluggish hydrogenation/dehydrogenation kinetics [19,20]. To copy with the dilemma, many studies have been conducted to reduce the decomposition temperature, accelerate the adsorption kinetics and change the reaction thermodynamics by the method of grain refinement [11,21], mixing catalysts [22–24], ball-milling [25,26], alloying [27,28], surface modification [29,30] and the other [31]. The mechanical grinding and melt-spinning [32,33] are very efficient means to reduce the particle or grain size. Especially, the mechanical milling can realize the refinement of grains and particles simultaneously [38]. In addition, mechanical milling brings on the formation of crystal defects and modifies the superficial characteristics of alloy particles [1]. Hence, mechanical grinding is reckoned as the best way to improve the hydrogen storing performance of magnesium based materials [34]. Besides, refining the particle to very small size will introduce the capillarity effect that can ameliorate the hydrogen absorption and desorption thermodynamics in theory. The calculation results indicate that a particle radius on the order of 5 nm will reduce the dehydrogenation enthalpy of pure Mg by about 10% [35]. As well known, there is considerable energy used for the hydrogen dissociation on the magnesium surface and this segment is regarded as a rate controlling factor in the procedure of hydrogen absorption [36]. Transition metals either in their pure form (e.g.: Ni, Ti, Nb, Fe, Co, Al etc.) [37,38] or as oxides (e.g.: Nb2O5, Fe2O3, TiO2 etc.) [39–42], hydrides (e.g.: TiH2, ZrFe2Hx, etc.) [43,44], fluoride (e.g.: FeF3, TiF3, NiF2, NbF5 etc.) [45,46] or intermetallics [47,48], can act as the catalysts because they can weaken this dissociation energy. As studied by Du et al. [49] and Pozzo and Alfe [50], the hydrogen dissociation energy on magnesium surface is 1.15 eV but it can be decreased to 0.03, 0.06, 0.56, 0.39 eV by the addition of Co, Ni, Cu, Pd, severally. Daryani et al. [51] researched that adding 6 mol% TiO2 could improve the hydrogen absorbing kinetics and reduce the decomposition temperature of as-milled Magnesium hydride by 100 K. According to the research of Shahi et al. [52], the composite MgH2 −5 wt.% Ni absorbs 5.0 wt.% H2 hydrogen at the temperature of 443 K in 15 min and at the temperature 613 K it starts to decompose. As investigated by Hou et al. [53], Mg2NiH4 with the catalyst composed of MWCNTs and TiF3 has the 503 K (516.6 K for pure Mg2NiH4) hydrogen releasing temperature (T D) and the activation energy (E a) of it is 53.24 kJ/mol (90.13 kJ/mol for pure Mg2NiH4). In particular, adding appropriate rare earths or their oxides can obviously make the Mg-based hydrides unstable and accelerate the rate of dehydrogenation reaction [54,56]. Lass [56] found that the Mg85Ni15- x M x (M= La, x = 0 or 5) alloys possess a lower enthalpy change in the reaction of producing MgH2 and Mg2NiH4. On the basis of the investigation of Luo et al. [55], the element Y is beneficial to improve the thermodynamics property of magnesium based materials and the composite Mg90In5Y5 has a lower ΔH (about 62.9 kJ/(mol H2)) in comparison with the Mg95In5 binary alloy (about 67.9 kJ/(mol H2)) and pure Mg (about 74.9 kJ/(mol H2)). Sadhasivam et al. [57] found that the original desorption temperature of the composite MgH2 −5 wt.% Mm-oxide was reduced by 76 K from 654 to 578 K. Kalinichenka et al. [58] researched the improved reaction kinetics of Mg90Ni8RE2 (RE = Y, Nd, Gd) and found that the activated Mg90Ni8RE2 could reversible absorb and release 5.5 wt.% H2 within 20 min.According to our investigation on REMg11Ni (RE = Sm, Y) + 5 wt.% M (M = MoS2, CeO2) composites, the additives MoS2 and CeO2 play a catalytic role in improving hydrogen storing performance [59,60] and 5 wt.% addition of catalysts is optimal as studied in this reference [33]. It must be very interesting to compare the effects of TiO2 and La2O3 additives with high hardness on the hydrogen storing performance of ball milling magnesium based materials. Thereby, the alloys La7Sm3Mg80Ni10–5 M (M = None, TiO2, La2O3) were fabricated by mechanical milling. The thermodynamics and dynamics of the experimental alloys were investigated. A comprehensive comparison of the impacts of different catalysts on the structure and hydrogen storing performances of the alloys is conducted.The La7Sm3Mg80Ni10 material was fabricated by inductive melting La, Sm, Mg, Ni (purity ≥ 99.9%) under 0.04 MPa He (purity ≥ 99.999%) to inhibit the volatilization of magnesium. To compensate the melting losses, additional magnesium (8 wt.%) and RE (RE = La, Sm) (5 wt.%) are required. The above-mentioned materials were all provided by CISRI Corporation. A Varian Liberty 100 inductively-coupled plasma (ICP) was applied to determining the chemical composition of experimental alloys. Then the ingot was mechanically crushed and ground to the 200–400 meshes powders. The obtained power with 5 wt.% TiO2 or La2O3 (purity ≥ 99.9%) catalyst was mechanically ground by a mill crusher at the speed of 350 rpm (the weight ratio of specimen and balls is 1: 40). The milling duration is set at 20 h. Thus, the chemical compositions of the as-milled powder were La7Sm3Mg80Ni10–5 M (M = None, TiO2, La2O3). In order to heat dissipation and reduce the cold welding of powder in the process of milling, the working mechanism of ball mill is to stop half an hour every 3 h and the powder adhered to the milling chamber walls and grinding balls needs to be scrapped in due course of time all of these operations were operated under the protective atmosphere of Ar.X-ray diffraction (XRD) (D/max/2400) determined the phase structures and compositions of the alloys. The experimental parameters were 40 kV, 160 mA, and 2°/min with 2θ changing from 20° to 90°. The radiation was CuKα1 filtered by graphite. The particles morphology observation was completed by a scanning electron microscope (SEM) (QUANTA 400). A high resolution transmission electron microscope (HRTEM) (JEM-2100F, operated at 200 kV) was utilized to the characterization of microstructure and crystalline state.Hydrogenation and dehydrogenation kinetics curves of the as-milled specimens were tested by automatic Sieverts apparatus. Prior to measuring, the sample need to be activated by six hydriding/dehydriding cycles (633 K and original hydrogen pressure of 3 MPa for hydrogen absorption, 633 K and 1 × 10−4 MPa original pressure for hydrogen desorption). The temperature of hydrogen absorption was set as 473, 513, 533, 553, 573, 593, 613 and 633 K, severally, while 553, 573, 593, 613 and 633 K for hydrogen desorption. The setting of initial hydrogen pressure is the same as activation. The sample mass required for every determination was 300 mg. Non-isothermal hydrogen desorption property was researched by utilizing thermogravimetry (TGA) and differential scanning calorimetry (DSC) (SDTQ600) whose heating rates were 5, 10, 15 and 20 K/min. Fig. 1 gives the X-ray diffraction of the as-milled La7Sm3Mg80Ni10–5 M (M = none, TiO2, La2O3) composites before and after hydrogen absorption and desorption under the condition of 633 K, 3 MPa and 633 K, 1 × 10−4 MPa, severally. ICDD (International Centre for Diffraction Data) identification of X-ray diffraction patterns shows that the as-milled M = none specimen consists of the major phase La2Mg17 and secondary phases Mg2Ni and La2Ni3. The addition of catalysts TiO2 and La2O3 do not introduce any new phase, indicating that these additives are not involved in the reaction with alloy. Moreover, it is visible to observe the broadened diffraction peaks representing the typical nanocrystalline and amorphous structures of as-milled specimens in comparison with that of the as-cast one (XRD patterns are not show here). After hydrogen absorption, the diffraction peaks get narrow and sharp. Meanwhile there are four hydrides become visible and emerge in the specimens, including MgH2, Mg2NiH4, LaH3 and Sm3H7. The reaction relationship between the elements is as follows: La2Mg17 + H2 → LaH3 + MgH2 Mg2Ni + H2 → Mg2NiH4 La2Ni3 + La2Mg17 + H2 → LaH3 + Mg2NiH4 + MgH2 Sm + H2 → Sm3H7 After dehydrogenated, the four phases Mg, Mg2Ni, LaH3 and Sm3H7 can be found. Evidently, the LaH3 and Sm3H7 phases are not decomposed because of the high thermal steadiness of them. Hence, the hydrogen desorption reactions are summarized as the following two equations: MgH2 → Mg + H2 Mg2NiH4 → Mg2Ni + H2 According to the above inference, we can see that in the process of hydrogenation and dehydrogenation, the reversible reactions of activated composites include Mg + H2 ↔ MgH2 Mg2Ni + H2 ↔ Mg2NiH4 Through a careful observation, we find that the width of the XRD peak narrows down after dehydrogenation compared with that after hydrogen absorption, which was owing to the cell volume reduction and stress relief rendered by hydrogen desorption. As found by Montone et al. [61], the volume of a metallic Mg atom is about 33% smaller than that of Mg atom in MgH2. It has been reported in the literature [62,63] that lattice distortion along with expansion and contraction of cell volume are inevitable in hydrogen storage materials during hydrogen absorption and desorption, which will cause many lattice defects such as vacancy and dislocation. The formation of the defects will have a beneficial effect on the hydrogen absorption and desorption property of the alloy. Mechanical milling of Mg-based alloy with TiO2 and La2O3 catalysts creates the defects on the surface and inside the magnesium matrix, which generate reactive clean surfaces and shrink the particle size of Mg. The creation of defects facilitates nucleation, the production of the reactive clean surface enhances the superficial reactivity, and the diminution of particle decreases the diffusion distances of hydrogen atoms. These effects ameliorate the hydriding and dehydriding kinetics of magnesium based alloy significantly.The morphologies of the as-milled La7Sm3Mg80Ni10–5 M (M = none, TiO2, La2O3) alloy powder are observed by SEM and presented in Fig. 2 . As can be observed, the alloy particles have the typical morphology of ball-milling powder and the size of them is in a range of 1–10 µm. After careful observation, it is failed to find the catalysts TiO2 and La2O3 particles, which indicates that the TiO2 and La2O3 particles are not appear on the superficial part of alloy, but is wrapped in them. Evidently, the agglomeration tendency of the as-milled particles of the M = TiO2 and M = La2O3 alloys was decreased (Fig. 2b and c) with smaller size than that of M = none alloy. It means that adding a certain amount of TiO2 and La2O3 can significantly improve the efficiency of ball milling. After comparing the particles with different catalysts, we found no obvious difference in particle size, suggesting that two catalysts have similar effect on the efficiency of ball milling. As considered by Floriano et al. [64], some catalysts with high hardness, e.g. La2O3, CeO2, TiO2 Nd2O5, etc. can act as lubricants, dispersants and/or cracking agents in the procedure of milling and are helpful to further reduce refine the particles of as-milled alloy. A very similar result also appears in the investigation of Daryani et al. [51] and Aguey-Zinsou et al. [65].The HRTEM micrographs and SAED (Selected Area Electron Diffraction) patterns of the as-milled La7Sm3Mg80Ni10 −5 M (M = none, TiO2, La2O3) materials are presented in Fig. 3 . We have noticed the nanocrystalline and amorphous structures of the as-milled alloys and the emergence of crystal defects. The SAED patterns also prove the existence of La2Mg17, Mg2Ni and La2Ni3 phases, and there is no any new phase caused by adding TiO2 and La2O3. After hydrogen absorption, the structures of as-milled alloy still are amorphous and nanocrystalline (Fig. 3b, e and h), but there is an observably decrease in amorphous phase, meaning that the dehydrogenation promotes the crystallization reaction. Four hydrides MgH2, Mg2NiH4 LaH3 and Sm3H7 also can be identified after hydrogen desorption by SAED patterns. According to Fig. 3(c), (f) and (i), we can see that the alloys after hydrogen desorption exhibit an entirely crystal structure, and the size of grain evidently increase, and Pukazhselvan et al. [66] also had the similar report. The SAED rings of dehydrogenated alloys reflect the existence of Mg, Mg2Ni, LaH3 and Sm3H7. Apparently, it is consistent with the result of XRD, the LaH3 and Sm3H7 phases still exist after dehydrogenation. In addition, it is found from Fig. 2 that the LaH3, Sm3H7, TiO2 and La2O3 nanoparticals distribute in Mg matrix dispersedly and uniformly, which is considered to be the preferred nucleation sites for hydride formation/decomposition. The phase interfaces of LaH3 (or Sm3H7, TiO2 and La2O3)/Mg (or MgH2) provide channels for the diffusion of hydrogen atoms. Therefore, the additives can be regarded as catalysts to improve the hydrogen storage performance of magnesium and Mg-based alloy [67].In this investigation, it was found that the alloy powder prepared by traditional mechanical milling can hardly absorb hydrogen because the alloy powder exposed to air easily forms an oxide film on the particle surface that blocks the contact between H2 molecules and alloy surface and prevents H2 from dissociating into H atoms. As well known, this dissociation process is the basic step in the phase transformation from metallic Mg to MgH2 and it is necessary for the incorporation of H atoms into the Mg lattice. Fortunately, it is found that when the alloy sample is kept under proper temperature and hydrogen pressure for a long time, the oxide film formed can be broken gradually, which results in exposing the fresh alloy surface and restoring the hydrogen absorption capability of the alloys. This process is called as activation. The activation performance of specimens is greater if it needs less cycle numbers. Fig. 4 demonstrates the isothermal hydrogen absorption and desorption curves of the activated La7Sm3Mg80Ni10–5 M (M = none, TiO2, La2O3) materials. It is found from Fig. 4(a), (b) and (c), the alloys are almost fully activated after the first cycle since the activation curves of the next five cycles are almost identical. It is noted that the time spent on the first hydrogen absorption to a saturated state is long. It takes 21716 s for the M = none alloy, 14568 s for the M = TiO2 alloy and 13340 s for the M = La2O3 alloy to achieve the saturated capacity of 5.15 wt.%, 5.052 wt.% and 4.916 wt.%, severally, suggesting that the activation property of the alloy is considerably improved by adding TiO2 and La2O3. For a given hydrogen absorption capacity of 4 wt.%, by which the time required is 4820, 4624 and 4758 s corresponding to the as-milled M = none, M = TiO2 and M = La2O3 alloys, respectively. It indicates that the hydrogenation rate is in the order M = TiO2 > M = La2O3 > M = none. The improved activation ability by adding TiO2 and La2O3 was attributed to the modified particles surface state and the increased defect density of the crystals resulted from adding catalysts TiO2 and La2O3. The catalyst nanoparticles distributing on the particles surface significantly increase the dissociation rate (limiting factor of hydrogen absorption rate) of hydrogen molecules.Generally speaking, the first activation reaction is a long procedure [68] and in this process, the H atoms penetrate the formed oxide layer to form mental hydrides. What's more, the attendant mechanical stress and lattice distortion are unfavorable to the absorption of hydrogen [69]. As well known, the nucleation of MgH2 on the superficial sites of alloy is retarded by the thin oxide layers [70]. Although the operation is conducted at inert gas atmospheres the oxide layers with 3–4 nm thickness still can easily form [69]. The sluggish dissociation of H2 on the alloy surface is another reason to explain the slow hydrogen absorption rate [71]. The dissociation on pure Mg surface needs high energy [48]. Besides, the diffusion of H atoms in the metal hydrides is difficult [56, 57]. The growth rate of MgH2 is decided by the hydrogen pressure due to the fact that higher pressure provides the greater the thermodynamics driving force for the hydrogen absorption. Nevertheless, if the original hydrogenation process is fast enough, a superficial layer of magnesium hydride will form to retard the hydrogen permeation [71]. Because hydrogen diffuses along the interfaces but not along the Mg hydride layer [72], the MgH2 hydride grows up in the form of slow Mg/Mg hydride interface movement. When the thickness reaches a certain value (30–50 µm), the hydrogen absorption reaction stops [73], indicating that powdered magnesium used for hydrogenation changes into massive magnesium. So the hydrogen absorption rate is affected by the powder size [74]. The hydrogenation kinetics is markedly enhanced after the first hydrogen absorption and desorption cycle. With the increase in the cycle number, the hydrogen absorption kinetics curves have little change, which means the great activation performance of experimental alloys. Noticeably, the hydrogen absorbing capacity of all the specimens after first cycle first is no more than 4.8 wt.%, which represents a visible decline in the capacity. The formation of stable hydrides LaH3 and Sm3H7 is most likely responsible for the 0.25 wt.% capacity loss.The dehydrogenation curves of the alloys are provided in Fig. 4(d), (e) and (f). It is visible that the dehydrogenation rate is fast and the dehydrogenation kinetics of the alloy was markedly improved by adding TiO2 and La2O3. In particular, the first hydrogen desorption took less time. For a given hydrogen desorption capacity of 3 wt.%, by which the time required is 193, 162 and 175 s corresponding to the as-milled M = none, M = TiO2 and M = La2O3 alloys, respectively. Evidently, the dehydrogenation rate is in the order M = TiO2 > M = La2O3 > M = none. The improved activation performance is deemed to be related to the decreased particle size, the surface modification and the weakening effect of Mg-H bond strength resulted from the additive TiO2 or La2O3. The improvement of thermodynamics performance is directly associated with the weakening of Mg-H bond strength. The co doping of multi elements, especially the transition element (or their compounds) and rare element (or their compounds) is beneficial to the reduction of thermal stability of MgH2 [75,76]. The reduction of particle observably enhances the decomposition rate of H2 on the particle surface and is beneficial to the H atoms diffusion thus enhance the activation performance [77]. Particularly, because TiO2 and La2O3 are high hardness particles, they are likely to cut into the alloy particles and form a new interface under the action of high impact stress in the ball-milling process, which may become the nucleation sites of hydride, acting as rapid paths for atoms diffusion [78]. So the addition of TiO2 and La2O3 not only enhance the efficiency of mechanical milling, make the particle size decrease but also modify the surface of alloy particles, make the nucleation of hydrides more easy.After the activation treatment, the hydrogen absorbing and desorbing properties of experimental composites were improved significantly. It is necessary to explore the change of structures in the process of activation. With the help of SEM, the morphological variations of the experimental composites before and after activation process are provided in Fig. 5 . Clearly, the particles show irregular morphologies with the very rough surface. After six hydriding and dehydriding cycles, the particle morphologies of the alloy have a dramatically change. It is very evident that many cracks appear on powder surface due to the lattice stress forming in the process of hydrogen absorption. When the lattice stress exceeds the fracture strength of the material, the pulverization of the alloy is inevitable and results in the improved properties. Through the above structural analysis, we believe that the activation is significant to the formation and decomposition of hydrides, the oxide film on the surface of alloy particles breaks, and along with the cracking of alloy particles, the specific surface area of the alloy is increased, thus improving the hydrogen absorbing and desorbing properties.To explore the influence of the different catalysts, the hydrogen absorption curves of the as-milled La7Sm3Mg80Ni10–5 M (M = none, TiO2, La2O3) composites were tested at different temperatures from 423 to 633 K and 3 MPa, as shown in Fig. 6 . It is noted that at the initial stage corresponding to the rapid formation of hydride layer near the surface, the rate is very fast and hydrogen absorption capacity can reach more than 85% of saturated capacity in less than 200 s, while in the following stage it takes long time to achieve the saturated state due to the hindrance of formed hydride layer acting on the hydrogen diffusion. Freidlmeier et al. [79] considered that when the thickness of hydride layer reached to a certain value (100 nm), the rate of hydrogen absorption tended to 0. Aiming at investigate the hydrogen absorption kinetics more deeply, the time required to absorb 4 wt.% hydrogen was calculated and compared. As obtained from Fig. 6, the spent time is 108, 67 and 56 s at 473, 513 and 533 K for the M = none specimen, 96, 62 and 48 s for the M = TiO2 composite, and 103, 64 and 53 s for the M = La2O3 material, respectively. Apparently, the hydrogenation rate of the composites is in order of M = TiO2 > M = La2O3 > M = none, which suggests that the added TiO2 and La2O3 notably ameliorate the hydrogen absorption kinetics, but this favorable effect decreases rapidly with hydrogen absorption temperature rising. Noticeably, the alloys almost display the same hydrogen absorption kinetics when the temperature exceeds 513 K, indicating the predominant role of temperature among many factors acting on the rate of hydrogen absorption. As we all know, there are three steps happening in the hydrogenation procedure of magnesium [80], namely a) the dissociation of superficial H2 molecules, b) the diffusing of H atoms through grain boundaries, c) the combination of H atoms and Mg atoms to form MgH2 on the Mg/catalyst interfaces. Because the hydrogen dissociation needs quite high energy, it is reckoned as the rate-controlling step [81]. As confirmed by Sakintuna et al. [35], the additives transition metals or their oxides in magnesium can act as the catalysts to decrease the dissociation energy. Liu et al. [82] considered that theoretically, the substitution atoms weakened the stability of Mg-H bond owing to the interaction between the valence electrons of H and the unsaturated d/f electron shell of the transition metals or oxides, improving the hydrogen absorption performance. Agarwal et al. [47] reported that it is difficult to refine the grains of Mg by mechanical milling due to the inevitable agglomeration of particles. The additive of brittle oxides or intermetallics provides convenience for reducing the particle size of Mg. The refined particles ameliorate the hydrogen absorbing and releasing properties due to the decreased diffusion length and the larger reactive surfaces of H2 caused by particle refinement [83]. Compared with the experimental alloy, TiO2 and La2O3 have higher hardness. Therefore, the existence of TiO2 or La2O3 nanoparticles increases the brittleness of alloy and eventually makes the equilibrium between fragmentation and agglomeration change to a reduced particle size, as stated by Rafi-ud-din et al. [84]. The shorter diffusion channels for H atoms and larger specific surface for H2 dissociation caused by particle refinement facilitate to enhance hydrogen absorption kinetics [85].To investigate the hydrogenation degree of the alloys and the phase structure changes during hydrogenation, The Rietveld refinements of the XRD patterns of as-milled La7Sm3Mg80Ni10 alloy hydrogenated at 3 MPa and 593 K are provided, as illustrated in Fig. 6(d). The milled alloys cannot be fitted with the Rietveld method because their XRD detections are amorphous. After hydrogenation, the amorphous phase is completely crystallized. Thus, the Rietveld method can be used analyzed the evolution of the phases of the as-milled alloy after hydrogenation. The result reveals that the as-milled hydrogenated alloy is composed of four hydrides, viz. MgH2, Mg2NiH4, LaH3 and Sm3H7 and the relative content of each phase is 56.1, 25.9, 11.5 and 6.5%, respectively. It suggested that the alloy is in saturated hydrogenation state.In order to research the relationships between the catalysts TiO2, La2O3 and hydride stability, the temperature programmed desorption and DSC of the as-milled La7Sm3Mg80Ni10–5 M (M = none, TiO2, La2O3) after complete hydrogen absorption were measured at the heating rate of 5 K/min, as presented in Fig. 7 . It is observed that the adding catalyst renders an obvious effect on the hydride stability. The onset dehydrogenation temperature of La7Sm3Mg80Ni10–5 M (M = none, TiO2, La2O3) composites after hydrogenation is 547.4, 537.2 and 540.1 K, severally. The temperatures of endothermic peaks in DSC curves of the alloys are 553.2, 546.4 and 548.9 K, respectively. The change of initial hydrogen desorption temperature can reflect the hydrides stability. Evidently, the stability of the hydrogenated La7Sm3Mg80Ni10–5 M (M = none, TiO2, La2O3) composites is the following order M = none > M = La2O3 > M = TiO2. The decreased stability is attributed to the decline in Mg-H bond energy. The additives transition metals [86] or rare-earth elements can reduce the Mg-H bond energy and act as the catalysts owing to the electronic exchange reaction between these catalysts and magnesium hydride [57]. Pighin et al. [87] investigated the function of various catalysts on Mg-H bond energy and found that the addition of transition metals or their oxides effectively decreased this bond energy, the magnesium hydride stability and the dehydrogenation temperature. According to the research of Abdellaoui et al. [12], the existence of new bonds weakens the bond strength between Mg and H, which can help us to understand the system instability and the decreased hydrogen desorption temperature mentioned above.Aiming at studying the hydrogen desorption kinetics of the composites with additives TiO2 and La2O3, the plots of capacity versus time were tested at 553, 573, 593, 613 and 633 K and presented in Fig. 8 . As can be observed, the reaction temperature greatly affects the hydrogen desorption kinetics of experimental alloys. Under the high-temperature conditions, all the alloys have a very fast reaction rate. In addition, it is noted that the adding catalysts TiO2 and La2O3 generates a favorable impact on the isothermal dehydrogenation kinetics. For further making sense of the influence of adding TiO2 and La2O3 on the kinetics, the time inquired by releasing 3 wt.% hydrogen is regarded as a reference standard. As shown in Fig. 8, the time required by releasing 3 wt.% hydrogen at the temperatures of 553, 573, 593, 613 and 633 K is 988, 553, 419, 227 and 152 s for the M = none alloy, and 578, 352, 286, 188, and 112 s for the M = TiO2 alloy, and 594, 366, 301, 197 and 132 s for the M = La2O3 specimen, respectively. Obviously, the dehydrogenation rate is in the order M = TiO2 > M = La2O3 > M = none. Based on the above data, the relationship between the time needed by desorbing 3 wt.% H2 and temperature can be constructed, as displayed in Fig. 8(d). It indicates that the additives TiO2 and La2O3 markedly ameliorate the hydrogen absorption kinetics, but the positive contribution decreases rapidly with the increase of hydrogen desorption temperature, which suggests that among all the factors affecting the dehydrogenation kinetics of alloys, temperature is predominate. It has come to light that the hydrogen desorption of MgH2 is completed through three stages: (a) magnesium phase nucleates and grows, (b) hydrogen diffuses from the magnesium hydride matrix to the surfaces, and (c) two adjacent hydrogen atoms combine to form hydrogen molecules [88]. The improved hydrogen desorption kinetics of magnesium based alloys is most likely attributed to the decline in hydride stability, the diminution of the particles size and the increase of the defect density on the particle surface of the alloys caused by adding catalysts TiO2 and La2O3. As mentioned above, the rare-earth elements and transition metals or their oxides can reduce the bond energy of Mg-H, thus, weaken the magnesium hydrides stability and accelerate the hydrides decomposition [47]. High hardness catalyst particles are likely to be embedded into the interior of alloy particles under the action of repeated impact stress in the process of ball grinding, so the alloy particles broken and particle size greatly reduced, as considered by Jain et al. [89]. Meanwhile, the additive high-hardness catalyst induces the surface defects and brings on the particle refinement of alloy in the procedure of mechanical milling [84]. The induced defects are favorable to the nucleation and increase the surface reactivity. Besides, the decreased particle size makes the length of hydrogen diffusion channels shorten. These factors definitely accelerate the hydrogenation and dehydration of the Mg-based materials [90]. Nevertheless, the improved kinetics because of adding TiO2 and La2O3 particles is not only due to the decline in particles size. Other factors such as the nature of the added oxides and the local electronic structure and the reduction of the oxide during heating should also be considered. Partially reduced oxides are expected to have different valence states and may act on ameliorating the hydrogen desorption property. Generally speaking, the main catalysis of transition metal-based catalysts is engendered by the transition metal ions and their ability to form hydrogen bonds. In this way, transition metal-based catalysts provide a faster route for H atoms diffusion. It has been reported that oxygen vacancies (also known as anoxic surfaces) on oxide surfaces also have catalytic activity [84]. Hence, we believe that the hydrogen desorption results from thermodynamically induced surface vacancies. In summary, the improved kinetics may be due to the uniform dispersion of these anoxic oxide particles, which shorten the diffusion path between reaction ions. The oxygen vacancies act as the sites for nucleation and growth of dehydrogenation products, and promote the dehydrogenation process.To investigate the dehydrogenation degree of the hydrides and the phase structure changes during dehydrogenation, The Rietveld refinements of the XRD patterns of as-milled saturated hydrides dehydrogenated at 1 × 10−4 MPa and 633 K are given, as illustrated in Fig. 8(d). It reveals that the phase Mg, Mg2Ni, LaH3 and Sm3H7 exist in the dehydrogenated alloy. It is very clear that rare earth hydrides LaH3 and Sm3H7 remain undecomposed at experimental temperatures and pressures. The relative content of each phase in the alloy is 55.7, 26.1, 11.6 and 6.6%, respectively.Generally, the occurrence of gas-solid reaction needs to overcome a total energy barrier that can be reflected in terms of the apparent activation energy. Hence, when the activation energy reaches a certain requirement, the reaction can take place smoothly. The apparent activation energy of the hydrogenated La7Sm3Mg80Ni10–5 M (M = none, TiO2, La2O3) alloys in hydrogen desorption is evaluated by the Arrhenius and Kissinger methods. As well known, the nucleation and growth of dehydrogenation products are the crucial factors that control the hydrogen desorption reaction of magnesium based materials [91]. In general, the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model can simulate this solid-state reaction [92]: (1) ln [ − ln ( 1 − α ) ] = η ln k + η ln t a — the reaction fraction; n — the avrami index; k — the dehydrogenation rate constant; t — the reaction time.According to Fig. 8, the fitting curves ln [-ln (1-α)] vs. lnt at 573, 593, 613 and 633 K can be ploted, as provided in Fig. 9 . As can be observed, the JMAK sketches are almost linear, suggesting the dehydrogenation of the composite is composed of two steps, including the first stage instantaneous nucleation and the second stage 3D growth controlled by interface [93]. The η and ηlnk values were obtained according to the slope and intercept in fitting curves at the corresponding temperature. Thus the value of k can be acquired. The apparent activation energy ( E a de ) of dehydrogenation reaction was estimated gained by using Arrhenius formula [57]: (2) k = A exp ( − E a de R T ) A — a temperature independent coefficient; R — the gas constant (8.3145 J/mol/K); T — the absolute temperature of reaction; k — the dehydrogenation rate constant.The Arrhenius plots of lnk vs. 1/T of the alloys are sketched, as presented in Fig. 9. The apparent activation energy E a de of the as-milled alloys was acquired from the slopes of the Arrhenius plots. 68.1, 62.1 and 63.6 kJ/mol correspond to the La7Sm3Mg80Ni10–5 M (M = none, TiO2, La2O3) alloys, severally. Obviously, the apparent activation energy of the dehydrogenation of the alloys is in sequenced M = TiO2 < M = La2O3 < M = none. As reported by Rafi-ud-din et al. [84], the catalysts TiO2 added by mechanical milling has shown further enhancement in the reaction kinetics by reducing the H2 dissociation activation energy. Mustafa and Ismail [83] considered that the improved ability of MgH2 decomposition was originated from the decline in this activation energy. Hou et al. [53] reported that the proper catalysts were proved to be an efficient strategy to decrease apparent activation energy E a of MgH2 hydrides.In order to compare with the JMAK model, the Kissinger method is employed to estimate the activation energy, as following equation [94]: (3) d [ ln ( β / T P 2 ) ] d ( 1 / T P ) = − E k de R β — the heating rate; E k de —the activation energy; T P — the absolute temperature corresponding to the maximal desorption rate, R — the gas constant (8.3145 J/mol/K).The as-milled La7Sm3Mg80Ni10–5 M (M = none, TiO2, La2O3) alloys need to absorb H2 to the saturated state before the measure of DSC. Fig. 10 demonstrates the non-isothermal dehydrogenation curves tested at the heating rates of 5, 10, 15 and 20 K/min, severally. As we can see, an endothermic peak exists in each DSC curve, suggesting the same reaction procedure of each specimen. According to Fig. 10, the graphs of ln ( β / T P 2 ) vs. 1/T P can be sketched, as presented in Fig. 10. It is noted that ln ( β / T P 2 ) vs. 1/T P plot is almost linear, so from the slopes of it, the activation energy E k de was obtained. According to the calculation, apparent activation energies of the La7Sm3Mg80Ni10–5 M (M = none, TiO2, La2O3) alloys are 65.5, 60.2 and 61.7 kJ/mol and in order of M = TiO2 < M = La2O3 < M = none. The addition of TiO2 or La2O3 distinctly reduces the apparent activation energy. Fan et al. [95] believe that as a significant index to evaluate the hydrogen desorption property, the decreased activation energy indicates the decline in the barrier of hydrogen desorption. Thus, it can be concluded, the additives TiO2 or La2O3 can decrease the hydrogen releasing activation energy, which is the essence behind the improved dehydrogenation kinetics.Reducing the thermal stability of magnesium hydride is the main goal to improve its hydrogen storage performance and realize its practical application. To inspect the effect of adding TiO2 and La2O3 on the thermodynamics, the P-C-T curves of the as-milled specimens were tested at the temperature of 593, 613 and 633 K and given in Fig. 11 . Obviously, the pressure platform is quite flat and the hysteresis coefficient (H f = ln (P a/P d)) is small. The catalyst TiO2 or La2O3 has no evidently change in the platform characteristic reflected in the P-C-T curves of alloys. As we can observe, two pressure plateaus emerge in every P-C-T curve and the higher and lower platform pressures stand for the formation/dissociation of the Mg2NiH4 and MgH2 hydrides, severally [96,97]. According to the plateau pressures (P a and P d) in Fig. 11, the thermodynamics parameters enthalpy change ΔH and entropy change ΔS are evaluated by Van't Hoff equation [98]: (4) ln ( P H 2 P 0 ) = Δ H R T − Δ S R P H2 — the equilibrium hydrogen gas pressure corresponding to MgH2; P 0 — the standard atmospheric pressure; R — the gas constant (8.3145 J/mol/K); T — the absolute temperature of reaction.The Van't Hoff graphs of ln P H 2 / P 0 vs. 1/T for the as-milled La7Sm3Mg80Ni10–5 M (M = none, TiO2, La2O3) composites can be sketched. Hence, the ΔH and ΔS can be calculated according to the slopes and intercepts in Van't Hoff diagrams and listed in Table 1 . It uncovers that the addition of TiO2 and La2O3 has not notably impact on the improvement of the experimental materials’ thermodynamics and the reduction of corresponding hydrides stability. The addition of catalysts TiO2 or La2O3 decreases  the stability magnesium hydride. A similar result also emerged in the investigations of Anik et al. [99] and Bououdina et al. [39]. Obviously, the absolute values of dehydrogenation enthalpy change ΔH de of the alloys are in following order M = none > M = La2O3 > M = TiO2. Based on the above results, we can find that both isothermal and non-isothermal analyses reveal that the catalysts TiO2 and La2O3 weaken the magnesium hydride stability and improve the hydrogen absorption and desorption kinetics. The positive contribution to the hydrogen storing thermodynamic and dynamics of the specimens caused by two catalysts is in following order M = TiO2 > M = La2O3 > M = none. (1) The addition of TiO2 and La2O3 has no change in the phase composition but reduces the agglomeration tendency of particles in the process of mechanical milling and make the particle size of the as-milled alloy markedly decreased. It is this modification of the microstructure that remarkably enhances the hydrogen absorption and desorption performances. (2) The addition of TiO2 and La2O3 have obviously positive contribution to the hydrogenation and dehydrogenation kinetics of the experimental alloys, which is in order M = TiO2 > M = La2O3 > M = none. It is ascribed to the decline in the size of grains and particles, the generation of the fresh surface and the creation of the various crystal defects derived from ball milling and adding catalysts. (3) The addition of TiO2 and La2O3 catalysts has a slightly favorable influence on the improvement of the thermodynamics of alloy and the stability of the hydride, which is in sequence M = TiO2 > M = La2O3 > M = none. The addition of TiO2 and La2O3 has no change in the phase composition but reduces the agglomeration tendency of particles in the process of mechanical milling and make the particle size of the as-milled alloy markedly decreased. It is this modification of the microstructure that remarkably enhances the hydrogen absorption and desorption performances.The addition of TiO2 and La2O3 have obviously positive contribution to the hydrogenation and dehydrogenation kinetics of the experimental alloys, which is in order M = TiO2 > M = La2O3 > M = none. It is ascribed to the decline in the size of grains and particles, the generation of the fresh surface and the creation of the various crystal defects derived from ball milling and adding catalysts.The addition of TiO2 and La2O3 catalysts has a slightly favorable influence on the improvement of the thermodynamics of alloy and the stability of the hydride, which is in sequence M = TiO2 > M = La2O3 > M = none.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 that could be construed as influencing the position presented in, or the review of the manuscript entitled.This study was financially supported by the National Natural Science Foundation of China (Nos. 51901105, 51871125, and 51761032), Natural Science Foundation of Inner Mongolia, China (2019BS05005), Inner Mongolia University of Science and Technology Innovation Fund (2019QDL-B11) and Major Science and Technology Innovation Projects in Shandong Province (2019JZZY010320).

In this investigation, mechanical grinding was applied to fabricating the Mg-based alloys La7Sm3Mg80Ni10 + 5 wt.% M (M = None, TiO2, La2O3) (named La7Sm3Mg80Ni10–5 M (M = None, TiO2, La2O3)). The result reveals that the structures of as-milled alloys consist of amorphous and nanocrystalline. The particle sizes of the added M (M = TiO2, La2O3) alloys obviously diminish in comparison with the M = None specimen, suggesting that the catalysts TiO2 and La2O3 can enhance the grinding efficiency. What's more, the additives TiO2 and La2O3 observably improve the activation performance and reaction kinetics of the composite. The time required by releasing 3 wt.% hydrogen at 553, 573 and 593 K is 988, 553 and 419 s for the M= None sample, and 578, 352 and 286 s for the M = TiO2 composite, and 594, 366, 301 s for the La2O3 containing alloy, respectively. The absolute value of hydrogenation enthalpy change |ΔH| of the M (M = None, TiO2, La2O3) alloys is 77.13, 74.28 and 75.28 kJ/mol. Furthermore, the addition of catalysts reduces the hydrogen desorption activation energy ( E a de ).

The aqueous electrocatalytic reduction of CO2 into energy-dense industrial chemical fuels and feedstocks has been proposed as a promising strategy to mitigate the challenge of CO2-induced global warming [1–3]. A wide range of carbon compounds (such as CO, CH4 and HCOOH) are possible products of this process, allowing an efficient pathway to simultaneous CO2 fixation and storage of a renewable energy source under ambient conditions [4,5]. However, due to the intrinsic thermodynamic stability of CO2 and the complex reaction pathway of the CO2 reduction reaction (CRR), it is still challenging to find a cost-effective and stable electrocatalyst that can directly reduce CO2 to carbonaceous products [6–9]. In an attempt to solve this problem, many electrocatalysts have been considered for CRR; however, even the most well-known Au and Ag-based catalysts cannot meet the criteria, due to their high overpotential and high cost, as well as their easy deactivation during the CRR process [10,11]. Hence, rational design of non-noble metal electrocatalysts with high selectivity and stability is critical for the application of CO2 electroreduction technology.Achieving atomic-level regulation of active transition metal atoms is important in designing an efficient catalyst [12–14]. In this regard, single-atom catalysts (SACs) have emerged as a highly promising category of electrocatalyst owing to their optimized atomic utilization, strong metal–substrate interactions and highly unsaturated coordination environment [15–17]. Moreover, there is the possibility of chemical potential tuning, in which the size, structure, shape and composition of materials can be controlled to alter the electronic structure of the SACs [18,19]. These characteristics thus make SACs ideally suited as electrocatalysts for a series of reactions including the hydrogen evolution reaction (HER) [20], oxygen reduction reaction (ORR) [21] and CO2 reduction reaction [22,23]. The preparation of SACs with a controllable microstructure and highly exposed active metal atoms is thus highly desirable.In this work, we demonstrate the successful synthesis of single-atom-Ni-decorated, nitrogen-doped carbon (denoted SA-Ni@NC) layers by carbonizing the precursor: layers of a two-dimensional bimetallic zeolite imidazolate framework (ZnNi-ZIF). After the selective etching of Zn atoms at high temperature, single atoms of Ni can be preserved and simultaneously immobilized on the resulting nitrogen-doped carbon layers via Ni–N bonds. Unlike general bulk ZIF precursors, which are nearly one hundred nanometers in size, our 2D ZIF layers allow single metal atoms in the prepared ultrathin SA-Ni@NC to be fully exposed to the electrolyte. As a result, the SA-Ni@NC layers exhibit excellent electrocatalytic activity for CRR: a high Faradaic efficiency (FE) of 86.2% is achieved at −0.6 V (vs. reversible hydrogen electrode (RHE)). A single-atom catalyst of this type with a highly exposed active site and high catalytic activity may open a new approach to the design of other CRR electrocatalysts.Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), zinc nitrate hexahydrate (Zn(NO3)2·6H2O), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), methanol and 2-methylimidazole were purchased from Beijing Innochem Technology Co. Ltd.Typically, 900.0 mg of 2-methylimidazole was added to 60 mL of deionized water, producing solution A. After that, 300.0 mg of Zn(NO3)2·6H2O and 29.3 mg of Ni(NO3)2·6H2O were added to 60 mL of deionized water, and the resulting solution was then added to solution A under continuous stirring (600 rpm) for 12 h. Finally, Zn1Ni0.1-ZIF layers were obtained after washing and drying treatments. The Zn1Ni0.1-ZIF layers were then annealed at 1000 °C for 3 h at a heating rate of 10 °C min−1 under a flow of Ar/H2 (v/v = 9:1) gas mixture. Finally, the black product (SA-Ni@NC) was collected after cooling to room temperature. The procedure for synthesizing NC, SA-Ni-2@NC, P-Ni-2@NC and P-Ni@NC was similar to that for SA-Ni@NC, except that the amount of Ni(NO3)2·6H2O was changed to 0, 58.6, 117.2 and 293.0 mg, respectively.Typically, 900.0 mg of 2-methylimidazole was added to 60 mL of deionized water, producing solution A. After that, 300.0 mg of Zn(NO3)2·6H2O and 29.3 mg of Co(NO3)2·6H2O were added to 60 mL of deionized water, and the resulting solution was then added to solution A under continuous stirring (600 rpm) for 12 h. Finally, Zn1Co0.1-ZIF layers were obtained after washing and drying treatments. The as-prepared Zn1Co0.1-ZIF layers were then annealed at 1000 °C for 3 h at a heating rate of 10 °C min−1 under a flow of Ar/H2 (v/v = 9:1) gas mixture. Finally, the black product (SA-Co@NC) was collected after cooling to room temperature. The synthetic procedure for P-Co@NC was similar to that of SA-Co@NC, except that the amount of Co(NO3)2·6H2O was changed to 293.0 mg.Typically, 900.0 mg of 2-methylimidazole was added to 30 mL of methanol, producing solution A. After that, 300.0 mg of Zn(NO3)2·6H2O and 29.3 mg of Ni(NO3)2·6H2O were added to 30 mL of methanol, and the resulting solution was then added to solution A under continuous stirring (600 rpm) for 10 min. The mixed solution was then transferred to 100 mL Teflon-lined stainless-steel autoclaves and heated at 120 °C for 6 h. Zn1Ni0.1-ZIF-particles were then obtained after washing and drying treatments. After that, the Zn1Ni0.1-ZIF-particles were annealed at 1000 °C for 3 h at a heating rate of 10 °C min−1 under a flow of Ar/H2 (v/v = 9:1) gas mixture. Finally, the black product (SA-Ni@3D-NC) was collected after cooling to room temperature.The morphologies and microstructures of the samples were characterized by field emission scanning electron microscopy (FE-SEM JEOL-7500) and high-resolution transmission electron microscopy (HRTEM, JEOL, NEM-2100F). Chemical states were characterized by X-ray photoelectron spectroscopy (XPS, Thermo escalab 250Xi). X-ray diffraction (XRD, Rigaku D/max2500PC) was carried out using Cu Kα radiation over the range 5–80°. X-ray absorption fine structure (XAFS) spectroscopy was conducted at the 1W1B station of the Beijing Synchrotron Radiation Facility. The storage rings of the Beijing Synchrotron Radiation Facility were used at 2.5 GeV with a maximum current of 250 mA. A Veeco/Bruker (ICON) atomic force microscope was used to measure the thickness of the sample. The Ni content of the SA-Ni@NC was measured by ICP-ASE (Optima-7000DV). Nitrogen sorption isotherms and BET surface area were measured by Qudrasorb at 77 K.The fabrication of single-atom-Ni-decorated, nitrogen-doped carbon layers is schematically illustrated in Fig. 1 a. Initially, zinc–nickel bimetallic ZIF layers were synthesized by mixing Zn2+ and Ni2+-containing solutions with 2-methylimidazole. This process ensures the mutual diffusion of Zn and Ni atoms in the resulting ZIF layers, and the successful preparation of the ZIF crystal was confirmed by X-ray diffraction (XRD) (Fig. 1b) [24,25]. Moreover, scanning electron microscopy (SEM) images show the ultrathin 2D morphology of the resulting ZIF layers (Figs. 1c and S1). After that, the ZnNi-ZIF layered precursor was annealed at 1000 °C in a mixture of Ar/H2 (v/v = 9/1). Since the saturated vapor pressure of Zn is much higher than that of Ni (Fig. S2), Zn atoms rather than Ni atoms are selectively volatilized from the ZnNi-ZIF layers during the high temperature treatment, generating ultrathin SA-Ni@NC layers (Figs. 1d and S3). Meanwhile, the Ni atoms are preserved and coordinated with the N atoms in the nitrogen-doped layers to form Ni–N bonds, generating single-atom-Ni-decorated, nitrogen-doped carbon layers. It should be noted that the atomic ratio of Ni to Zn has a great influence on the product. By adjusting the atomic ratio of Ni to Zn, nitrogen-doped carbon (denoted NC, Ni/Zn = 0) layers and Ni-particles-decorated, nitrogen-doped carbon (denoted P-Ni@NC, Ni/Zn = 1:1) layers were also prepared (Figs. S4 and S5).The morphology and microstructure of the prepared SA-Ni@NC layers were investigated via transmission electron microscopy (TEM). Abundant thin and transparent layers were obtained, as shown in Fig. 2 a and b, indicating the ultrathin nature of the SA-Ni@NC layers. The lateral sizes of these carbon layers are typically in the range from 500 nm to several micrometers. These layers have good flexibility, possibly originating from their intrinsically flexible nature and/or from the defective structures formed during the synthesis process. Moreover, as shown in Fig. 2c, the high resolution-TEM (HR-TEM) image reveals the obviously amorphous structure of the SA-Ni@NC layers, which can be further confirmed by the corresponding fast Fourier transform (FFT) patterns (inset in Fig. 2c). No distinct nanoparticles or clusters can be seen on the surface of the layers, suggesting that the Ni atoms might be present in the form of single atoms [26–28]. In contrast, numerous Ni particles are observed on the surface of the P-Ni@NC layers (Fig. S7c), and the existence of Ni particles in the P-Ni@NC sample can be further confirmed by the XRD patterns (Fig. 2d). The atomic force microscopy (AFM) image (Fig. 2e) and the corresponding thickness analyses (Fig. 2f) further reveal that the SA-Ni@NC layers have a thickness of ∼2.5 nm.To verify the isolated, dispersed nature of Ni atoms in the SA-Ni@NC layers, synchrotron-based X-ray absorption fine structure (XAFS) measurements were also conducted. As shown in the X-ray absorption near-edge structure (XANES) spectra (Fig. 3 a and b), the position of the blue line for SA-Ni@NC layers is located between those for the Ni foil (black line) and NiO (red line), clearly suggesting the typical electronic structure of Ni δ + (0 < δ < 2) [29]. Further Fourier transforms of R space for Ni K-edge EXAFS were conducted, and compared with Ni foil, NiO and phthalein cyanide nickel (Ni–Pc) as references. As shown in Fig. 3c, SA-Ni@NC layers exhibit a dominant Ni–N coordination peak at 1.41 Å, which is nearly identical to the peak of the Ni–Pc reference sample (1.45 Å), suggesting an interaction between Ni and N atoms in the nitrogen-doped carbon layers [30]. In contrast, the P-Ni@NC layers exhibit an obvious peak at 2.02 Å, showing the presence of Ni particles in the sample. Moreover, the single atom nature was further confirmed by wavelet transform (WT) of Ni K-edge EXAFS oscillation. As shown in Fig. 3d, there is only one intensity maximum at 6 Å−1 for SA-Ni@NC layers, which can be ascribed to the Ni–N bonding. We also carried out least-squares curve fitting to obtain the quantitative structural parameters of Ni in the SA-Ni@NC layers, and the fitting curves are shown in Fig. 3e. According to the fitting, the coordination number of Ni is 3.3 (Table S1), indicating that the Ni atoms mainly have three-fold coordination with N atoms.Additionally, the presence of Ni–N bonds was further confirmed by the XPS spectrum of the SA-Ni@NC layers. As shown in Fig. 3f, the high-resolution N 1 s spectrum for SA-Ni@NC can be deconvoluted into five peaks corresponding to oxidized N (402.5 eV), graphitic N (401.3 eV), pyrrolic N (400.6 eV), Ni–N (399.1 eV) and pyridinic N (398.5 eV) species [31]. The existence of Ni–N bonds is in excellent agreement with our EXAFS analysis. These results suggest that the Ni atoms are atomically dispersed in the nitrogen-doped carbon layers through Ni–N bonds. More importantly, this method can be extended to produce a series of SACs (such as single-atom-Co-decorated, nitrogen-doped carbon (SA-Co@NC) layers), verifying the generality of this simple method (Figs. S10−S12).The porous nature of the SA-Ni@NC layers was validated by nitrogen physisorption measurements (Fig. S13a). A high specific surface area of 449.0 m2 g−1 was obtained for the SA-Ni@NC layers, which is much higher than that of P-Ni@NC layers (246.7 m2 g−1). Moreover, the corresponding pore-size distribution curves demonstrate the existence of both micropores and mesopores in the SA-Ni@NC layers (Fig. S13b). These micropores and mesopores originate from the inheritance of ZIF precursors and the evaporation of Zn during the annealing treatment, respectively [32]. The Ni content of the SA-Ni@NC layers was found to be ∼1.61 wt% using the inductively coupled plasma-atomic emission spectrometry analysis (ICP-AES) measurement.The electrocatalytic activity of the SA-Ni@NC layers for CRR was first examined by linear sweep voltammetry (LSV) in Ar and CO2-saturated 0.1 M KHCO3 electrolytes. As presented in Fig. 4 a, under CO2-saturated conditions, the current density of SA-Ni@NC layers reached a maximum of −10.1 mA cm−2 at −1.0 V, much higher than that obtained in the Ar-saturated electrolyte (-6.7 mA cm−2). The excess current density is ascribed to the occurrence of CO2 reduction. In addition, the SA-Ni@NC layers exhibit the greatest reduction current density in the CO2-saturated electrolyte, which was approximately 2.1 and 1.7 times higher than those for NC layers (−4.8 mA cm−2) and P-Ni@NC layers (−5.8 mA cm−2) catalysts at −1.0 V, respectively (Fig. 4b).Apart from current density, selectivity is another important criterion for CRR electrocatalysts. Thus, for each sample, potentiostatic electrolysis was conducted at various potentials, and the gas and liquid products were identified by gas chromatography (GC) and 1H nuclear magnetic resonance (NMR), respectively. It is found that the only carbon-containing product detected was CO, and no liquid products were detected by NMR (Fig. S15). Fig. 4c shows the FE of CO production at various applied potentials, and it can be seen that the applied potential greatly affects the distribution of the reduction product. For the SA-Ni@NC catalyst, the production of CO starts at −0.3 V. Furthermore, SA-Ni@NC achieves the highest FE value for the production of CO: 86.2% at a potential of −0.6 V, which is much higher than the FE values obtained using NC (26.5%) and P-Ni@NC layers (30.2%), demonstrating the superior selectivity of the SA-Ni@NC layers. Moreover, the FE can be further improved by increasing the content of single Ni atoms in the nitrogen-doped layers. As shown in Fig. S18, SA-Ni-2@NC with a high content of single Ni atoms achieves the highest FE value for the production of CO: 98.1% at a potential of −0.6 V vs. RHE, which is higher than that of the SA-Ni@NC catalyst (86.2%). The FE of SA-Ni@NC is also higher than that of single-atom-Ni-decorated, 3D nitrogen-doped carbon (67.9%) at −0.6 V vs. RHE, Fig. S19, thus demonstrating the importance of designing ultrathin support materials to improve the activity of single-atom catalysts.SA-Ni@NC layers also possess good stability for the electrocatalytic reduction of CO2. As shown in Fig. 4d, both the current density and the corresponding FE for CO production show negligible decay for continuous catalysis over at least 10 h. Moreover, the single atoms of Ni are also stable after electrocatalytic CRR tests (Fig. 4e), showing the good structural stability of the SA-Ni@NC layers. These results indicate that the SA-Ni@NC layers possess both excellent activity and stability towards CO2 reduction. Electrochemical impedance spectroscopy (EIS) measurements were conducted to gain further insight into the kinetics of the CO2 reduction process. As shown in Fig. 4f, SA-Ni@NC shows a much faster charge-transfer rate during the CRR process than the P-Ni@NC and NC samples, which greatly contributes to its catalytic activity.To elucidate the origin of the excellent CO2 reduction properties of the SA-Ni@NC catalyst, DFT calculations were performed on three different sites in SA-Ni@NC, i.e., pristine N-doped carbon (NC), Ni-N3-C (Ni atom with three-fold coordination with N atoms) and Ni-N4-C (Ni atom with four-fold coordination with N atoms). Fig. 5 a depicts the calculated free-energy profiles for CO2 reduction at the three sites, in which a *COOH intermediate with two transferred proton-electrons was considered. It was found that the reaction ∗ C O 2 + H + + e - → ∗ C O O H is the potential limiting step for the pathways at the three sites, in which the lowest barrier of 0.78 eV is obtained at Ni-N3-C site, compared to a barrier of 2.90 eV at pristine NC and one of 1.57 eV at the Ni-N4-C site. Fig. 5b–5e show snapshots of the reaction pathway for CO2 reduction at the Ni-N3-C site. It can be seen that the Ni-N3-C site is the active site, facilitating the transformation from CO2 to CO. Local density of states (LDOS) values were calculated to further interpret the electronic origin of the high catalytic activity at the Ni-N3-C site. Fig. S20 presents the calculated local density of states (LDOS) for the atom (N in NC or Ni in Ni-N3-C/Ni-N4-C) bonded to *COOH/CO, as well as that of the group *COOH/CO. It can be seen that there is a stronger hybridization between Ni and *COOH/CO than between N and *COOH/CO, indicating a stronger interaction between Ni and *COOH/CO. Remarkably, a significant DOS distribution consisting of Ni atom at the Fermi level can be observed at the Ni-N4-C site, suggesting its low stability. In contrast, a splitting of DOS from Ni atoms at the Fermi level emerges at the Ni-N3-C site, which might be responsible for its high catalytic activity.In conclusion, we have demonstrated an effective electrocatalyst towards CRR based on single-atom-Ni-decorated, nitrogen-doped carbon layers. The synthesized SA-Ni@NC not only possesses highly active Ni-N3-C sites, but also has a unique 2D structure with a high surface area, multilevel pores and thin walls. Such features not only provide large amounts of available active sites for electrocatalytic CO2 reduction, but also favor fast mass transport during the catalysis. As a consequence, SA-Ni@NC layers exhibit high CRR activity for the production of CO, and also show good stability. Moreover, this simple method can also be used for the preparation of a series of single-atom catalysts (such as SA-Co@NC). In view of the large family of zeolite imidazolate frameworks, we anticipate that a series of single-atom-decorated, nitrogen-doped carbon layers could be fabricated with broader applications in the area of energy conversion technology. Chao Zhang: Conceptualization, Methodology, Software, Data curation, Writing - review & editing. Zhongheng Fu: Methodology, Software. Qi Zhao: Software, Data curation. Zhiguo Du: Methodology, Software. Ruifeng Zhang: Methodology, Data curation. Songmei Li: Conceptualization, Methodology, Supervision, Validation, Project administration.This work was financially supported by National Natural Science Foundation of China (No. 51622203).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.Supplementary data to this article can be found online at https://doi.org/10.1016/j.elecom.2020.106758.The following are the Supplementary data to this article: Supplementary data 1

Efficient selective electroreduction of carbon dioxide into energy-dense carbonaceous fuel products is highly desirable to mitigate environmental and energy-related problems. However, there is still a need to design an electrocatalyst with high selectivity and stability towards the CO2 reduction reaction (CRR). Here, we present the promising performance of single-atom-Ni-decorated, nitrogen-doped carbon layers (SA-Ni@NC) as an efficient electrocatalyst for CRR. In this catalyst the Ni atoms are atomically dispersed and most have three-fold coordination with the N atoms in the carbon layers. Theoretical calculations show that the Ni-N3-C site can act as a highly active site for the reduction of CO2 owing to the low energy barrier for the formation of *COOH intermediates. As a consequence, SA-Ni@NC exhibits a high Faradaic efficiency (up to 86.2%) for the production of CO at a potential of −0.6 V versus the reversible hydrogen electrode. Moreover, this simple method can also be used to produce a range of single-atom catalysts (such as SA-Co@NC). In view of the large family of zeolite imidazolate frameworks, we anticipate that our strategy will be extended to a variety of single-atom-decorated, nitrogen-doped carbon layers with a broad range of applications in energy conversion systems.

Excessive consumption of fossil fuels and rapid population growth have led to several environmental problems, including greenhouse gas emission, SOx, NOx, acid rain, global warming, and urban smog (Abas et al., 2015; Abokyi et al., 2019; Zhang et al., 2018). Furthermore, the fluctuation of fossil fuels prices and the heavy reliance of energy and chemical sectors on fossil fuels have caused a dramatic increase in demand for alternative, renewable and sustainable energy. Biomasses stand out as a suitable renewable energy source to produce liquid fuels due to their environmental benefits, such as abundant availability, renewability, low cost and carbon neutral (Long et al., 2013). About 220 billion metric tons of lignocellulosic biomass are generated annually worldwide, making biomass the world's largest renewable source of energy (Hassan et al., 2016). Biomass-derived bio-oil can be an alternative to fossil fuels to produce value-added chemical, heat, electricity, and energy (Yaman et al., 2018). In 2016, lignocellulosic biomass constitutes about 70 % of the total primary energy supply, which was equivalent to 56.5 EJ as shown in Fig. 1 (Global Bioenergy Statistics, 2018). Currently, numerous countries have imposed strong policies on the utilization of renewable biofuels. For example, European Union (EU) Commission demands more than 20 % of the entire automotive fuel usage to be consisted of biofuels by 2020. The U.S governmental departments also have set an aim to achieve 25 % of oil-based chemicals and 20 % of transport energy with biofuel-based alternatives by 2030 (Liang et al., 2021).Over the past two decades, increasing population and consumption have driven a massive increase in plastic demand due to its excellent characteristics of durability, light of weight, easy manufacturing, ease of use and resistance to corrosion. The global production of plastics is expected to expand from 300 million metric tons in 2015 to 1.8 billion metric in 2050 (Lee et al., 2021). In 2020, the global plastic production has reached 370 million tonnes, with Asian region contributing to about half of it (PlasticsEurope, 2020). Plastics including polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET), polystyrene (PS), and polyvinyl chloride (PVC) are extensively utilized in diverse areas, including packaging, construction, electronics, households, automobiles and others (Wang et al., 2021). The incessant growth of plastics demands has resulted in the increase of plastic solid waste (PSW) deposit every year. Municipal solid waste (MSW) accounts for around 30–35 % of the total plastic wastes in industrialized country (Tencati et al., 2016). At present, the traditional recycling methods, including incineration and landfills pose a serious threat to the environment via water resource pollution, air pollution and damages to marine ecosystems and terrestrial habitats (Ghayebzadeh et al., 2020). In addition, the natural degradation of plastic needs 400 to 1000 years, causing a major negative impact to the environment. Therefore, an alternative approach that can convert the abundant plastic waste into a more value-added product and protect the environment and human health needs to be explored.Co-feeding hydrogen-rich materials to the oxygen-rich biomass has recently paved the way to upgrade bio-oil quality. The co-pyrolysis process is highly similar to pyrolysis because it can deliver high quality bio-oil, but it involves the combination of two or more feed materials. This technique can compensate the flaws of biomass-derived bio-oil, and provide safe and effective waste treatment (Chen et al., 2020). Hydrogen-rich materials such as plastics, tires and lubricant oil can act as hydrogen donor, increase the hydrogen-to-carbon ratio of feedstock and induce positive synergistic interaction with biomass to enhance the oil quality. The interactions between the intermediates of lignocellulosic biomass and synthetic polymers during co-pyrolysis can produce bio-oil with high carbon yields, high calorific value, aromatic selectivity and hydrocarbon (Dorado et al., 2015; Lu et al., 2018b). Furthermore, co-pyrolysis offers economic advantages since it requires less energy than the pyrolysis of biomass and plastic alone (Chen et al., 2020). Suriapparao and Vinu, (2021) investigated the synergistic effects between biomass (rice husk, groundnut shell, bagasse, mixed wood sawdust and Prosopis juliflora) and hydrogen-rich plastics (Polyisoprene (PIP) and low-density polyethylene (LDPE)). The study deduced that co-pyrolysis significantly boosted the calorific value of bio-oil. The heating value of co-pyrolysis oil varied from 38 to 42 MJ/kg as compared to the heating value of biomass pyrolysis oil of 20 to 28 MJ/kg. In addition, the deoxygenation degree also increased due to the synergistic effects. Rahman et al. (2021) carried out pyrolysis for mixtures of pine and HDPE in a double-column staged reactor and observed that the addition of HDPE to pine could increase the pyrolysis oil yield up to threefold compared to pyrolysis oil of pine alone. In addition, the oil produced was rich in hydrocarbon with 99 % selectivity. Adding the catalyst to the co-pyrolysis process could facilitate the cracking of pyrolysis vapor and deoxygenate the oxygenated compounds via dehydration, decarbonylation and decarboxylation reactions, improving selectivity towards the desired compounds, such as hydrocarbon (Dyer et al., 2021).The integration of co-pyrolysis and microwave radiation could enhance the yield and properties of liquid fuel product with less energy input in a single step, and prevent the need of an additional upgrading reactor network. Microwave co-pyrolysis technique are advantageous compared to other pyrolysis techniques, including high heating efficiency, uniform heating, energy saving, fast response and better heat and mass transport even with large particle size (Chen et al., 2022; Suriapparao, et al., 2022). Xia et al. (2021) claimed that this technique could be one of the future trends in advanced pyrolysis technique. Microwave co-pyrolysis can resolve the limitations of conventional microwave pyrolysis of plastic, which produces highly viscous bio-oil consisting of heavy hydrocarbon (waxy paraffinic components) that results from the insufficient decomposition of long-chain hydrocarbon (Wan Mahari et al., 2022). The addition of microwave absorbent can enhance the pyrolysis efficiency since the plastic and biomass waste have weak microwave absorption capacity. The bio-oil obtained from microwave co-pyrolysis has high heating value and H/C ratio with low viscosity (Suriapparao, et al., 2020). This technique also accelerates the dehydration reactions with low energy utilization, resulting in bio-oil that has low moisture content and acidity. Nevertheless, the cracking of long-chain molecules into short-chain compounds and the conversion of oxygenated compounds which are achievable through catalytic up-gradation can enhance the product selectivity and properties.Catalytic co-pyrolysis of biomass-plastic mixture can be a more reliable method compared to the catalytic pyrolysis of single biomass due to the catalyst deactivation resulted from hydrogen deficiency properties of biomass. Catalyst offers an alternative pathway with lower energy requirement for selective product generation. During pyrolysis, catalyst can accelerate the reactions, including cracking, hydrocracking, decarboxylation, alkylation, aromatization, decarboxylation, and Diels-Alder reactions for better product selectivity and quality (Suriapparao et al., 2022). It is critical to deeply understand the characteristics of the catalyst in order to select an appropriate catalyst for an effective co-pyrolysis process. Several comprehensive reviews have summarized the catalytic co-pyrolysis of lignocellulosic biomass and waste plastics. Gin et al. (2021) summarized and discussed the impact of heating systems, experimental conditions, and synergistic effects of the co-pyrolysis of plastic and biomass wastes. In addition, the reaction pathway and the kinetics of the catalytic co-pyrolysis version of the same feedstocks were exclusively presented. In another review, Ryu et al. (2020) summarized the latest progress in catalytic co-pyrolysis of biomass and plastic in terms of feedstock pre-treatment, properties of feedstock and catalyst on the production of the biofuels and desired chemicals, such as aromatic hydrocarbon. However, to the best of the author’s knowledge, a review on the influence of various types of plastic as co-reactant in co-pyrolysis with solid biomass to produce chemicals and liquid fuels is still lacking. The quality and product distribution of co-pyrolysis process depends on the biomass, plastic types and properties, and processing conditions, such as temperature, particle size, residence time, reactor type and catalyst addition. Therefore, this review paper focused on the influence of different types of plastic as the co-reactant in co-pyrolysis with solid biomass on the product distribution, synergistic effect, and quality of bio-oil. Furthermore, this review also provided concise information on the critical role of zeolite-based catalyst (microporous, mesoporous, hierarchical, and metal modified zeolite) and low-cost mineral-based catalyst in upgrading the yield and quality of liquid oil. The characteristics, synthesis methods, advantages, disadvantages, and performance of each catalyst in upgrading the bio-oil through the co-pyrolysis of biomass and plastic were compared in detail. Lastly, the potential challenges and future directions for this technique were also suggested.High density polyethylene (HDPE) is a common waste in municipal solid waste (MSW), with a H/Ceff ratio of 2 (He et al., 2021). HDPE is widely used to produce sturdy bottles, flexible pipes, toys, geomembranes, ropes, cutting boards and others. Due to its less fixed carbon with no oxygen, the addition of HDPE in co-pyrolysis of biomass can lower the formation of coke on catalyst and oxygenated compound in pyrolysis oil. Furthermore, HDPE is favourable for pyrolysis process since it possesses greater than 99 % volatile content with nil moisture (Rahman et al., 2021).Few studies have reported that the incorporation of HDPE can considerably improve the quality of pyrolysis oil in terms of olefins and monoaromatic hydrocarbon while reducing the undesired product of polycyclic aromatic hydrocarbons (PAHs), which are toxic, carcinogenic, and mutagenic (Chen et al., 2016). He et al. (2021) investigated the co-pyrolysis of HDPE and corn stalk over HZSM-5 using Py-GC/MS in the temperature range and biomass-to-HDPE ratio of 550–800 °C and 1.0–0:1, respectively. The result revealed that the addition of HDPE to corn stalk sharply reduced the oxygenated compounds from 97.02 % to 42.03 %. In addition, significant synergistic effect on condensable volatile organic products (CVOPs) and hydrocarbon was observed as the experimental value of CVOPs and hydrocarbon was higher than the calculated value. During co-pyrolysis, the corn stalk-derived oxygenates could interact with HDPE-derived olefins to form hydrocarbon via Diels-Alder reactions, enhancing the hydrocarbon production while reducing the oxygenated compounds. In addition, the hydrogen atoms transferred from HDPE could promote the hydrocarbon production by enhancing the cracking and deoxidation (decarbonylation, decarboxylation and dehydration) reaction of corn stalk-derived oxygenates to hydrocarbon. Furthermore, the addition of HDPE to corn stalk as co-reactant could inhibit the coke formation by stabilizing the corn stalk-derived oxygenates (lignin-derived phenolic compound) and preventing it from undergoing polymerization on the surface of HZSM-5. A comparable trend was observed by Rahman et al. (2021) who attributed the improvement in gasoline range hydrocarbon to proton supplement provided by HDPE. The highest selectivity of gasoline range hydrocarbon (77 %) was found at pine-to-HDPE ratio and temperature of 50:50 and 550 °C, respectively. The authors also highlighted that the amount of oxygenated compounds reduced from 31.11 % to 0 as the pine-to-HDPE ratio surged from 100/0 to 0/100 because H/Ceff ratio increased as the HDPE ratio in the feedstock increased, promoting the cracking of oxygenated compounds such as phenolic to gasoline-equivalent hydrocarbons (C6-C12). Yuan et al. (2018) carried out the co-pyrolysis of cellulose and HDPE at different ratios and reported that the synergistic effects in the co-pyrolysis accelerated the generation of small molecule volatiles, including H2O, CO/C2H4, and CO2. The decomposition of HDPE via chain-end and random scission can transfer hydrogen for the decomposition of cellulose-derived anhydrosugars to aldehyde and ketone while cellulose-derived oxygenated compounds, which act as acceptor, promote the scission of HDPE to alkane and alkene groups. During co-pyrolysis, aldehyde and ketone can be further decomposed to hydrocarbon. Fig. 2 shows the reaction mechanism between cellulose and HDPE at different biomass-to-plastic ratio. In co-pyrolysis of biomass with HDPE, HDPE generally provides positive synergistic effect on bio-oil yield (Hassan et al., 2020; Önal et al., 2014; Rahman et al., 2021). Co-pyrolysis of discarded newspaper and HDPE produced more bio-oil and less gas than the theoretical value due to cross reaction between newspaper and HDPE which interfered with the degradation of functional groups attached to the cellulose structure of WP (Chen et al., 2016). This condition inhibited the production of gases of low molecular weight and favoured the production of oil. The highest oil yield of 68.43 wt% was achieved at newspaper-to-HDPE blend ratio of 1:2, and it was 31.59 % higher than the theoretical data based on weighted averages. Positive synergistic effects on fuel properties were also observed in terms of significant reduction of total acid number and viscosity by 216 % and 76 %, respectively. In addition, the quality of bio-oil was also enhanced with maximum hydrocarbon and alcohol yield of 85.88 %, which was obtained at WP:HDPE ratio of 50:50.LDPE represents the second biggest portion of plastic waste with the approximate consumption of 415 million in 2015. This value is expected to increase by 4 % in the following years. LDPE is widely used as plastic bags and packaging due to its excellent characteristics of flexibility, ease of processing and low cost (Duan et al., 2021). Compared to HDPE, LDPE has more branching (2 % of carbon atom) and weaker intermolecular forces. Suriapparao and Vinu, (2021) examined the co-pyrolysis of LDPE with five different biomass and found that the experimental bio-oil yield (13.2 – 32.3 wt%) was less than the theoretical value (42–47.5 wt%). Excessive cracking of heavier molecules into lighter gases contributed to low bio-oil yield. Although the yield of bio-oil was low, the heating value of bio-oil (37.6–41 MJ/kg) was better than the theoretical value (32.6–37.8 MJ/kg) due to the interaction between oxygen transfer from condensable phase to gas phase and hydrogen release from LDPE vapours. Substantial progress has been observed in the selectivity of naphthalene derivatives, including methylnaphthalene and 2-methylnaphthalene produced from catalytic co-pyrolysis of LDPE, and cellulose and pine wood. LDPE was a high molecular weight polyolefin, and its pyrolysis and chain scission were incomplete, resulting in the production of larger molecules. Furthermore, the addition of LDPE could inhibit the coke formation during pyrolysis of biomass due to the breakdown of LDPE and biomass via free radical. Compared to LDPE, biomass could decompose earlier due to its poor thermal stability to produce primary radicals. As the temperature increased, the LDPE started to decompose to hydrocarbon that was rich in hydrogen (H) radical. The LDPE-derived H radical could promote secondary decomposition of biomass to generate volatile substances. These volatile substances prevented the LDPE from covering the biomass by melting down at high temperature. Furthermore, the coke growth was also hindered due to the inhibition of free radical polymerization by the precipitation of volatile substances (Zheng et al., 2018). Al-Maari et al. (2021) studied the co-pyrolysis of empty fruit bunch (EFB) and oil palm frond (PF) with LDPE for bio-oil production. The results showed positive synergistic interaction on the production of aliphatic hydrocarbons and inhibition of oxygenated compounds. The hydrogen released from LDPE enhanced the decarbonylation of carbonyls and sugar, and decarboxylation of acid to hydrocarbon due to oxygen removal via CO and CO2, respectively. In addition, significant synergistic interaction between EFB and PF with LDPE on the production of bio-oil has also been observed. The positive synergistic effect could be attributed to the secondary radical reaction, leading to the condensation of non-condensable fragments. Furthermore, LDPE that acted as the hydrogenation medium for biomass could inhibit the cross-linking reactions and polymerization of biomass, leading to greater biomass weight loss (Aboulkas et al., 2012; Yuan et al., 2018). Co-feeding LDPE and sugarcane bagasse yielded pyrolysis oil which mainly consisted of aliphatic compounds with fewer aromatic compounds as compared to individual biomass with high calorific value of 40 MJ/kg. The addition of LDPE to sugarcane bagasse enhanced the H/C ratio from 1.25 to 1.47 and boosted the formation of saturated hydrocarbon in the range of C6 – C25 (Dewangan et al., 2016).PET is the third largest thermoplastic consumed in Europe after polypropylene and LDPE. PET is usually used in a variety of consumer goods, including synthetic polyester fibres, bottles and films due to its characteristics of clear and strong thermoplastic (Choi et al., 2021; Dhahak et al., 2020). Özsin and Pütün, (2018) investigated the co-pyrolysis of PET blended with peach stones and walnut shells using a fixed bed reactor, and observed increased ester and acid compounds and decreased phenolic compound. Maximum acid and ester yield of 65.87 % and 63.11 % were achieved in co-pyrolysis of PET with walnut shells and peach stones, respectively. The liquid was dominated by benzenecarboxylic acid with more than 40 % yield for both co-pyrolysis blend. Benzenecarboxylic acid and vinyl benzoate were formed when the ester link of carboxylic group was broken via beta scission, initiating the decomposition of PET. One of the biggest challenges regarding pyrolysis oil from PET is the high acid content such as benzoic acid. The acidic characteristic of pyrolysis oil can lead to corrosiveness, depreciating the fuel quality. In addition, benzoic acid can clog the pipelines and heat exchanger, triggering issues during operation at industrial scale (Lee et al., 2017). Despite the disadvantages, it is noteworthy that benzoic acid is a valuable precursor/feedstock for various industries (Çepelioǧullar and Pütün., 2014). Chen et al. (2017) investigated the synergistic interaction effects on char morphology and thermal behaviour during co-pyrolysis of PET with paulownia wood (PAW) using TGA. Their result showed a remarkable deviation between the experimental and calculated value on volatile release. Higher char yield was obtained from the PAW/PET blends at final decomposition temperature of 530 °C with ΔW above zero. In addition, the char yield increased as the PET blending ratio increased. With the increment of PET ratio in the feedstock, more cross-link reaction between PET-derived products and PAW-derived char occurred, leading to greater char production. The PET decomposition played a role as a limiting factor for the cross-linking reaction. Meanwhile, the addition of PET to PAW resulted in the agglomeration morphology of char. Ablative surface and granule cohesion were observed on the char topography as the PET blending ratio increased due to the reaction between PET decomposition products and PAW-derived initial char. PET typically decomposed at temperature between 370 °C and 460 °C. Non-catalytic pyrolysis of PET produced a liquid containing terephthalic acid and benzoic acid along with CO and CO2 gas whereas co-pyrolysis of PET and biomass formed mainly acid and esters. The upgraded bio-oil from catalytic co-pyrolysis of biomass and PET demonstrated high content of aromatic compounds in the range of C5-C12 of carbon number fuel range (Dyer et al., 2021).Polycarbonates (PC) are a group of thermoplastic polymers that contain carbonate groups in their chemical structures. PC is a prominent engineering plastic due to its characteristics, such as high impact strength, superb thermal resistance, and exceptional electrical insulation properties; and it is widely used in automobile industry, building and construction, and data storage devices such as compact disc and DVDs (Antonakou et al., 2014; Bai et al., 2020). In 2017, the global PC production has reached 6 million metric tons (Do et al., 2018). PC is unrecyclable due to its superior opposition to chemical attacks and difficulty to be extracted from the waste stream. Landfilling PC could pose environment threats due to the leaching of bisphenol. Bisphenol A (BPA) and diphenyl carbonate (DPC) substance contained in PC are regarded as endocrine disruptors that cause serious illnesses, including cancer, threaten adult health and interfere with infant hormones (Bai et al., 2020). Liu et al. (2021) researched the co-pyrolysis of pinewood blended with PC to determine the synergistic effect. The extent of synergistic effect was determined via comparison between the experimental result of co-pyrolysis of pinewood-PC mixture with the weighted average values from individual feedstock pyrolysis. Positive synergy between pinewood and PC was obtained due to the enhancement of H2, CO and total syngas yield of 33 %, 36 % and 19 %, respectively, compared to the theoretical value from individual pyrolysis. However, negative synergistic effect was noticed in the formation of CnHm. The variation in synergistic behaviour of different gas components could be attributed to the interactions between PW and PC intermediates during co-pyrolysis, producing more oxygenated compounds (alcohols, carboxylic acids, and aldehydes) with less hydrocarbons. In addition, co-pyrolysis of PW and PC remarkably enhanced the gas production yield (from 67.6 wt% to 77.2 wt%) but reduced the tar and char yield compared to the theoretical values from individual feedstock pyrolysis. This phenomenon suggested that the synergistic effects of co-pyrolysis of PW and PC involved both mutual interaction of volatile in gas phase and volatile-solid interaction which enhanced the total conversion of solid feedstock to gases. The pyrolysis of PC tends to generate more phenol via oxygen removal as CO and CO2 (Burra and Gupta, 2018). Decomposition of PC mainly occurs via chain scission mechanism which can be divided into two main reactions: primary step in cyclic oligomers production by an intramolecular exchange reaction and hydrolytic cleavage of the carbonate group, generating hydroxyl-terminated oligomers and CO2 at 400 to 500 °C temperature (Jin et al., 2016). Blending pinewood (PW) with PC could enhance this pathway, and stable phenolic intermediates could be formed with the lignin portion, enhancing the breakdown and conversion of PW to low molecular weight aromatics that exist as volatiles, and decreasing the char formation at about 10 % (Burra and Gupta, 2018). On the other hand, addition of lignin to PC pyrolysis can escalate the decomposition of PC to phenolic type compounds by enhancing the release of CO during co-pyrolysis while inhibiting the aromatic compound (Jin et al., 2016).Polyvinyl chloride (PVC) is widely used in the production of cable and wire insulation, fashion and footwear, packaging, window frames, and water pipes. PVC has a longer lifespan than other packaging plastics. About 44.3 million metric tons of PVC was produced globally in 2018, and by 2025 the world’s market size of PVC is expected to grow to nearly 60 million metric tons (Statistica.com, 2021). PVC is the main source of chlorine in municipal solid waste (MSW) and one of the problematic plastics in the feed. Its presence in the feedstock is limited to less than 5 % and generally around 1 to 2 %. The release of chlorinated hydrocarbons and HCl in PVC results in corrosion in the reactor and renders the oil halogenated (Qureshi et al., 2020). As there is no public recycling system for PVC, the proportion of the recovered PVC is relatively low. Moreover, PVC needs to be treated using hydrochloride scrubber for PVC cracking as chloride is not desired in the fuels (Xue et al., 2017). Özsin and Pütün, (2018) analysed the synergistic effects during co-pyrolysis of PVC with two solid biomasses (walnut shell and peach stones). Negative synergistic interaction on the liquid yields were observed as the liquid yield during the co-pyrolysis (14.70 – 17.60 wt%) was lower than the aggregate values (17.21 – 18.64 wt%). On the other hand, positive synergistic effect was observed with higher aromaticity of tars in co-pyrolysis yields than biomass alone. 1H NMR result showed that both aromatic protons comprised of guaiacyl units (ArH and HC = C-(conjugated)); and ɑ-hydrogen atoms of the branched chain of aromatic ring carbons, methoxy and aliphatic hydroxyl were increased when PVC was added into the biomasses. Polyenes condensation and aromatization during PVC decomposition contributed to the enhanced formation of tars aromatic. It has been well established that chlorine radicals generated during PVC decomposition could initiate condensation reaction, cyclization and aromatization. In addition, considerable value of PAHs was observed during co-pyrolysis of PVC with walnut shell (64.40 %) and peach stones (59.06 %). The decomposition of PVC favoured aromatization reaction and creation of heavier tar compounds via dichlorination, followed by inter-molecular chain transfer; the aromatic chain scission generated two or three aromatic-ring side chain before the coke deposition. HCl release during co-pyrolysis of PVC blends escalated the progression of light tar portions to heavy portions, resulting in the generation of higher molecular weight substances, such as PAHs (Tang et al., 2018).The addition of PVC could instigate the decomposition of pinewood (PW) at lower temperature range due to the acceleration of PW decomposition by HCl from the dehydrochlorination of PVC. In addition, the co-pyrolysis of PW and PVC yielded more char and less liquid compared to the theoretical data. HCl generation from PVC at lower temperature range (230–300 °C) promoted the dehydration of cellulose to aldehyde compound which was confirmed from the cleavage of glycosidic units. The hydrogen and oxygen atoms in cellulose were lessened due to the dehydration at low temperature, leading to higher char yield. Furthermore, the dehydration also reduced the tendency of depolymerization, consequently reducing the liquid yield. Furthermore, the PW-derived solid char could also act as a catalyst owing to the presence of some inorganics, such as Cao, K2O and NaO that promoted the secondary cracking of PVC oil to generate more char and gas (Lu et al., 2018a). The presence of PVC could influence the reactivity and activation energy of lignocellulosic biomass. The magnitude of reactivity of co-pyrolysis of cherry seed (CS) and PVC was nearly-two orders higher than the pyrolysis of CS at all heating rates. This observation was credited to the chemical structure of PVC which contained high electronegative chloride ions. The activation energy of co-pyrolysis of CS/PVC fell between CS and PVC value. The deviation between theoretical and experimental value of activation energy signifies the occurrence of synergistic effect between CS and PCV during co-pyrolysis (Özsin and Pütün, 2019).Generally, the addition of PS to biomass can enhance the liquid yield while decreasing the gas and char yield. (Stančin et al., 2021) reported that an addition of 25 % of PS to sawdust (SD) could double the yield of pyrolysis oil from 31 % to 62 %, specifically on the expense of gas formation, indicating the occurrence of synergistic effect in the process. Moreover, blending 25 % of PS with SD could enhance the quality of bio-oil in terms of reduction of oxygenates and PAHs while promoting the aromatic hydrocarbon. However, when the ratio of PS exceeded 25 %, a higher generation of undesired benzene derivatives and toxic PAHs became noticeable due to the secondary cracking of PS-derived styrene monomer accelerated by the interaction with biomass feedstock. Benzene derivatives in bio-oil limit its further utilization since such compounds are categorized as carcinogenic. Samal et al. (2021) examined the co-pyrolysis of eucalyptus biomass and polystyrene waste on the physiochemical and thermal characteristic of the solid char. Two distinct physiochemical and thermal characteristics of char have been observed basically at temperature below and above 450 °C. The char generated below 450 °C has high heating value and volatile content with low fixed carbon because of the polystyrene coating on the char surface. The melting polystyrene waste could deposit over biomass at temperature below 450 °C, go through volatilization with additional increase in temperature, and be transformed to liquid oil and syngas. Solid fuels with high volatile content and low fixed carbon generally possess low ignition and burnout temperatures and a higher mass-loss rate, making them unstable. However, the increased high heating value due to the existence of waste plastic coating could ease in enhancing the combustion efficiency of the fuel. In contrast, the produced chars at temperature 450 °C and above possessed more high heating value and fixed carbon with low volatile content. This kind of solid fuel demonstrates superior combustible behaviour with broader temperature range and longer time for complete combustion, all of which signify an excellent solid fuel.The addition of PS enhances the yield and property of pyrolysis oil. In contrast to pyrolysis oil from biomass (Mahua seeds) alone, the addition of 20 wt% of PS in co-pyrolysis enhanced the liquid yield from 39.26 wt% to 45.89 wt%(Mishra and Mohanty, 2020). At 20 % blending ratio, the plastic could have a maximum synergistic interaction between particles which subsequently maximize the generation of hot volatiles that could be further transformed to liquid form. At this state, greater heat and mass interaction happened between biomass and plastic particles. However, at 10 wt% and 30 wt% blending ratios, the interaction between biomass and plastic particles created negative synergistic effect, reducing the formation of hot volatiles and the production of liquid oil. Furthermore, the higher plastic ratio in the feedstock could cause the plastic melting, which would coat the biomass surface, eventually creating resistance for the discharge of hot volatiles and reducing the liquid yield. The NMR study showed the increment of aromatic and olefinic percentage in the co-pyrolysis oil (as confirmed in the FTIR diagnostics showing the peak of 1650 cm−1 –1580 cm−1 attributed to CC stretching vibration). Meanwhile, the GC–MS results revealed that an addition of 20 wt% of PS as co-reactant substantially enhanced the hydrocarbon compounds and reduced the oxygenate derivatives such as acid, making it attractive compared to thermal pyrolysis oil. However, further upgrading technique is needed due to higher viscosity value than diesel fraction. Van Nguyen et al. (2021) examined the co-pyrolysis of waste PS and coffee-grounds at various blending ratio of 75:25, 50:50, and 25:75. The results revealed that co-pyrolysis could accelerate the deoxygenation reaction, causing a reduction of oxygenated compounds and enhancement in carbon content. The effect was strongest at the PS ratio of 75 % with reduction of oxygen content to 5.68 wt%. This condition contributed to an improvement in the calorific value (39.66 MJ/kg) of pyrolysis oil which was comparable to the heating value of conventional fuel. Table 1 shows the yields and quality of bio-oil obtained from co-pyrolysis of various biomasses and plastics.Employing suitable catalyst in co-pyrolysis is beneficial to the thermochemical decomposition of biomass and plastic by tailoring the products composition and lowering the activation energy of the reaction. The benefits of catalyst addition in the degradation process include shortening the reaction time, lowering the degradation temperature, promoting the extend of degradation, reducing the amount of solid residue in final products and narrowing the product distribution (Antonakou et al., 2014). In addition, the catalyst helps to direct the reaction toward the desired products via interactions between its structure, and the reaction pyrolyzates and products (Rocha et al., 2020). The effectiveness of a catalyst depends on its acidic characteristics, redox properties, and porosity. Tuning the catalyst acidity based on its density, strength, and type is vital in designing the catalyst as each of these elements have particular influence on the activity, product selectivity and reaction pathway (Antonakou et al., 2014).Zeolite is recognized as the most efficient catalyst to produce high-value chemicals because of its high acidity, high specific surface area, high adsorption ability and shape selectivity (Han et al., 2020; Ryu et al., 2020). Its unique pore structure with strong acidity favours aromatic selectivity with excellent cracking and deoxygenation ability (Hassan et al., 2016). The acidity of zeolite which is expressed by the Si/Al ratio determines their reactivity and affects the end products of pyrolysis process with low ratio, indicating high acidity (Chi et al., 2018). Generally, the introduction of microporous zeolite in the pyrolysis is usually favourable to enhance the aromatic production.It is well established that the introduction of microporous zeolite in the pyrolysis is favourable to enhance the aromatic production. Park et al. (2019b) investigated the co-pyrolysis of Quercus variabilis (Q. variabilis) and waste plastic films (PFs) over two microporous zeolites (HZSM-5 and HY) of different acidity and surface area. The acidity (SiO2/Al2O3) of HZSM-5 and HY zeolite was adjusted to 30 and 23, respectively. The result showed that HZSM-5 with higher and stronger acidity could enhance the aromatics production than HY catalyst during the co-pyrolysis at 600 °C due to higher cracking efficiency of pyrolyzates. In addition, more appropriate shape selectivity of HZSM-5 which has medium pore size, appropriate pore window size and internal pore volume together with steric hindrance characteristic could favour the production of aromatics (Jae et al., 2011). On the other hand, higher formation of coke was observed for HY catalyst due to the more space provided as it had higher surface area (780 m2/g) than HZSM-5 (425 m2/g). In contrast, Kim et al. (2016) observed greater aromatic production over HZSM-5 catalyst at high temperature and catalyst-to-reactant ratio compared to HY catalyst during catalytic co-pyrolysis of cellulose-PP/LDPE mixture. HZSM-5 which had strong acidity was advantageous for aromatic production while high catalyst-to-reactant ratio of 1:10 could provide a large number of active sites for aromatization reaction. The authors emphasized that the properties of catalyst, specifically acidity and pore size, are crucial in determining the aromatic production efficacy during catalytic co-pyrolysis reaction. On the other hand, low temperature and less catalyst-to-reactant ratio were applied for HY catalyst since the reaction intermediates could diffuse easily into its pore and make intimate contact with active sites to undergo further reaction to form aromatic.Coke deposition and limitation of mass transfer and reactant flow diffusion are among the major challenges of pyrolysis over microporous zeolite (Kim et al., 2017b). The small pore size (less than2 nm) of microporous HZSM-5 zeolite inhibits the diffusion of large biomass and plastic reaction intermediates produced during the initial stage of pyrolysis into its internal acidic sites. The large molecules of biomass and plastic pyrolysis intermediates formed during the initial stage of pyrolysis cannot pass through the inner pores and contact the active sites of HZSM-5 since their kinetic diameter is greater than the pore size of ZSM-5 (Hassan et al., 2019). Furthermore, pore blockage from polymerization and polycondensation reactions due to acidic properties of zeolite causes deactivation of the catalyst and reduces the catalyst lifetime. Shao et al. (2017) reported that the parallel side reactions of anhydrosugars, furans, and other organic molecules in the hydrocarbon pool could lead to the coke formation on the interior surface of zeolite while Custodis et al. (2014) stated that the competing side reaction of phenol repolymerization and lignin polycondensation could cause the coke deposition.Mesoporous zeolite catalysis has been recognized as an efficient approach to attenuate the diffusion restriction of bulky biomass and plastic molecules and expand the production of aromatic hydrocarbon through the larger pore size. High surface area of mesoporous zeolite provides greater access to active sites and enhance the catalytic interaction between the co-pyrolyzed reactants, causing higher conversion rate of oxygenates to aromatic hydrocarbon. Hong et al. (2017) reported the influence of microporous and mesoporous HZSM-5 during co-pyrolysis of cellulose and polypropylene on the aromatic formation efficiency. The result showed that mesoporous HZSM-5 by ZSM-5 desilication could offer better catalytic activity than microporous HZSM-5 in term of aromatic yield. Larger pore opening obtained by desilication can enhance the diffusion of bulky intermediates to active sites of catalyst to undergo further reactions to aromatics. In addition, mesoporosity can be allocated into the zeolite core-structure via post-synthesis treatments, such as steaming and leaching with acidic or basic media (Zhu et al., 2013).MCM-41 is a type of mesoporous zeolite that has bigger pore size, making it suitable for adsorption, separation and macromolecular catalysis. Its larger pore size could ease the diffusion limitation in pores. MCM-41 could provide enough active sites for adsorption and catalytic reaction due to its high specific surface area greater than 1000 m2/g. Chi et al. (2018) conducted co-pyrolysis of cellulose and PP in the presence of MCM-41 and Al-MCM-41. The cracking of oxygenated compounds was heightened by the strong acidity originated from the inclusion of Al onto the mesoporous MCM-41. The results indicated that the production of olefins and aromatics were enhanced by using Al-MCM-41, inferring that Al-MCM-41 had superior cracking and deoxygenation effect. The aromatic formation during the co-pyrolysis was governed by internal acid sites, hydrocarbon pool, and Diels-Alder reaction (Fig. 3 ). Cellulose was decomposed earlier compared to polypropylene as it had lower decomposition temperature. Numerous oxygenated compounds and penta heterocyclic furans were produced via ring cleavage and catalytic cracking to break its hexa heterocyclic, followed by dehydration and cyclization. During the catalytic co-pyrolysis, olefin was produced from direct cracking of polypropylene via carbonium ion and β-scission and deoxygenation of oxygenated compounds at acid sites via dehydration, decarbonylation, and decarboxylation reactions. These intermediates (olefins and oxygenates) participated in deoxygenation and oligomerization to form carbocation hydrocarbon pool where the aromatic and olefins were formed. Along with hydrocarbon pool mechanism, the monocyclic aromatic hydrocarbon can be formed via Diels-Alder reaction between cellulose-derived furans and polypropylene-derived olefin. Kim et al. (2017c) investigated the impact of acidity and molecular diameter on the formation of aromatic hydrocarbon in co-pyrolysis of carbohydrates with linear LDPE. They assessed the catalytic activity of microporous and mesoporous ZSM-5 with high mesoporosity and poor acidity Al-SBA-15. Higher yield of monoaromatic hydrocarbons was obtained under catalysis of ZSM-5 due to the combination of micropores and mesopores structure. This framework is suitable for the shape selectivity of aromatic production and to improve diffusivity of bulky molecular pyrolysis intermediates into the catalyst pore. The finding of this study indicated that catalyst with higher acidity together with an appropriate structure and pore diameter was an ideal catalyst for aromatic formation in co-pyrolysis reaction. Similar trend was found in the catalytic co-pyrolysis of yellow poplar and HDPE over three types of mesoporous catalysts, including hierarchical mesoporous MFI, hierarchical mesoporous Y, and Al-SBA-15 (Rezaei et al., 2017). Hierarchical mesoporous MFI which had large mesopores and strong acidity delivered the highest yield of olefins and aromatic hydrocarbons attributable to the efficient hydrocarbon pool mechanism. The yield of solid residue (char/coke) decreased for all three types of mesoporous catalyst. The lifespan of catalyst could be enhanced by reducing the coke deposition.Metal addition could modify the textural characteristics and acid sites, and enhance thermal stability of the catalyst. This process aids in decreasing the rate of coke growth over the catalyst and enhancing the liquid production (Botas et al., 2014; Iliopoulou et al., 2012). Razzaq et al. (2019) observed that anchoring of Ni, Co, Zn and Fe oxides onto the HZSM-5 framework by wet impregnation technique could reduce the coke yield by 50 % compared to intrinsic HZSM-5 during the co-pyrolysis of wheat straw and polystyrene. This was due to the moderate acidic strength of metal-modified zeolite which was helpful in decreasing the coke formation over the zeolite. In addition, pyrolytic oil catalysed over metal-modified zeolite contained relatively higher organic phase yield instead of aqueous phase as compared to unmodified HZSM-5. The presence of metal-modified zeolite could enhance the decarboxylation and decarbonylation while inhibiting the dehydration reaction. French and Czernik, (2010) reported that incorporation of metal sites onto the zeolite framework could alter the deoxygenation pathway so that it favourably released more oxygen in the form of carbon monoxide instead of carbon dioxide and water, thereby offering more hydrogen available for aromatic production. The presence of metals boosted the aromatic selectivity towards high value mono-aromatic hydrocarbon (MAHs) and supressed the formation of oxygenated compounds. Kim et al. (2017b) investigated HZSM-5, mesoporous MFI, Pt/mesoporous MFI and Al-SBA-16 catalyst effect for the Laminaria Japonica and PP co-pyrolysis. Pt/mesoporous MFI showed higher aromatic yield and oxygenate removal efficiency than the other catalysts due to the strong Brönsted acid sites and large pore size as well as catalytic effect resulted from the incorporation of Pt. Pt promoted the cracking and deoxygenation of oxygenated compounds to aromatic. The authors also highlighted that the strength of acidity played more essential role than the pore size in the production of aromatic hydrocarbon. Coupling of weak acid sites and large mesopores lowered the catalytic performance of Al-SBA-16. Conversely, mesoporous Al-SBA-15 with weak acidity showed a better oxygen removal efficiency than HZSM-5, concluding that the pore size played an important role during the cracking of large oxygenate molecules.Impregnation of phosphorous onto the zeolite framework could enhance the hydrothermal stability and anti-coking properties of zeolite and ease the transformation of alkane to olefin, which was subsequently converted to aromatic. Yao et al. (2015) found that the modification of ZSM-5 with phosphorous (P) and nickel (Ni) increased the production of valuable aromatic hydrocarbons and olefins in the catalytic fast co-pyrolysis of pine wood and LDPE due to the enhanced zeolite’s Lewis acid sites which acted as electron pair acceptor and which had a high tendency to accept the hydride ions generated during the conversion of alkanes to olefins. Higher content of aromatic hydrocarbon boosts the commercial value of bio-oil as the aromatic compound is vastly used as additives in transportation fuel and feedstock materials in the petrochemical industry (Kim et al., 2017a). In addition, the rate of coke-induced catalyst deactivation, which is the main concern in catalytic fast pyrolysis, has also been reduced due to the impregnation of ZSM-5 with P and P/Ni cation. The incorporation of P and Ni onto the ZSM-5 significantly decreased the strong Bronsted acid sites of zeolite, in turn reducing the coke deposition. Gallium (Ga) altered the texture characteristic and acidity of zeolite by reducing the pore volume and surface area of zeolite (Li et al., 2015). Ga was introduced into the zeolite framework via incipient wetness impregnation. The Ga decreased the density of Brönsted acid sites due to the replacement of some Brönsted acid sites by Ga. Ga-containing zeolite substantially increased the production of olefin and/or monoaromatic hydrocarbons at the expense of less valuable alkane during the catalytic co-pyrolysis of pine wood and LDPE. Non-framework Ga provided a new route for dehydrogenation of alkane to olefin, which is subsequently converted to aromatic.In an effort to enhance the catalytic activity of the zeolite catalysts, the incorporation of hierarchical porosity or alteration through metals and oxide supplement has been frequently reported (Han et al., 2020; Jin et al., 2016; W. Wang et al., 2019). Although the mesoporous materials are synthesized to solve the problem of diffusion limitations, it has poor surface acidity and unstable structure property, bringing about unsatisfactory activity in acid-catalysed reactions. To solve this shortcoming, researchers have combined the advantages of microporous molecular sieve and mesoporous material, producing zeolites with hierarchical micro-mesoporous composite (Talebian-Kiakalaieh and Tarighi, 2020). It works in the way that the external mesopores capture molecules in several directions and concentrate them towards the zeolite micropores. The mesoporous structure could enhance mass transfer and cracking of large molecular pyrolysis vapours, which are hard to diffuse into the microporous zeolite (Kim et al., 2019). Furthermore, every mesopore behaves as a funnel and enables the effective penetration of molecules within the narrow one-dimensional micropore system. Such a combination of the properties of both porous systems would make the hierarchical aluminosilicates a versatile material for many applications. Five different approaches to synthesize hierarchical micro-mesoporous include recrystallization of ordered mesoporous silicas, zeolite-seeding, mesoporous carbon templating during crystallization, alkaline extraction of zeolites, and combining mesostructure and microstructure-directing agents (Enterría et al., 2014).Several studies reported that the hierarchical zeolites could substantially resolve the limitations of the conventional zeolite, such as low mass transfer problem, deactivation of catalytic activity and low activity to bulky substrates in different chemical reactions due to significant deoxygenation and excellent aromatic selectivity (Ahmed et al., 2020; Chi et al., 2018). Moreover, Song et al. (2018) mentioned that more advantages from hierarchical zeolites could be observed, such as shortened diffusion path length, abundant external acid sites and surface area, and excellent hydrothermal stability. Combination of mesoporosity and traditional zeolites of hierarchical zeolite could broaden its application in catalysis.The catalytic activity of hierarchical zeolite is mainly dependent on the synthesis method. Desilication (removing silica) and dealumination (removing aluminum) are an efficient approach to generate mesoporosity though it may result in a considerable shift in acidic properties (Ahmed et al., 2020). The alteration of zeolite structure during desilication and dealumination of zeolite is shown in Fig. 4 . Proper acid sites distribution and mesoporosity resulted from the alteration in acidity could benefit the reaction pathway and intermediates stabilization as reported by (Hong et al., 2017). The desilicated ZSM-5 showed superior catalytic activity in term of aromatic selectivity (33.50 wt%) compared to parent ZSM-5 during co-pyrolysis of cellulose and polypropylene. The treatment enlarged the pore for better diffusion while retaining its strong acidity. The desilication enhanced the weak acid sites, thus improving the liquid products yield. In addition, the weak acid sites also fostered the deoxygenation of furan via reaction with olefins to produce more aromatics. Hierarchical zeolite has great potential in catalytic reactions related to bulky molecules due to the presence of microporous and mesoporous structure (Lv et al., 2020). A hierarchical pore structure could be created in ZSM-5 by including larger pore structures namely mesopore linked to the core microporous framework as an endeavour to inhibit the coke deposition and attain higher transformation of bulky oxygenates (Feliczak-Guzik, 2018). The mesoporous structure could enhance mass transfer and cracking of large molecular pyrolysis vapours, which were hard to be diffused into the microporous zeolite (Kim et al., 2019). Alkaline treatment is well-known and established as a post-synthetic technique comprised of fractional desilication of the zeolite structure to create secondary mesopores in ZSM-5 with bigger pore opening and outer surface area (Li et al., 2014). Apart from alkaline treatment, re-assembly aided with organic templating agent permits restructuring and redeposition of silicate and aluminosilicate fragment into the mesoporous material while maintaining the weight and/or acidity in basic medium (Chen et al., 2018). Lin et al. (2021) developed a series of hierarchical HZSM-5 with various alkaline solutions ranging from 0.2 to 0.4 mol/L and found that low alkaline solution (≤0.3 mol/L) accelerated the formation of monoaromatics from 63.79 % catalysed by HZSM-5 to 71.75 % for 0.3-HZSM-5 while higher alkaline solution diminished the framework of HZSM-5, leading to reduction of aromatic production. The alkaline treatment enhanced the mesoporosity of the zeolite so that the larger intermediates including oxygenated compounds and aliphatic hydrocarbons could effortlessly access the acid sites of hierarchical zeolite to form aromatics. Furthermore, the alkaline treatment reduced the polyaromatic hydrocarbons (PAHs) formation due to shorter diffusion path distance of molecules in the hierarchical HZSM-5 zeolites, retarding the secondary polymerization reactions of mono-aromatics inside the catalyst channel. Conversely, the selectivity of aliphatic hydrocarbons and oxygenated compounds were reduced as the alkaline concentration reached 0.3 mol/L, probably due to the conversion to aromatics at the catalyst pores via a series of reactions. Li et al. (2020) investigated the catalytic fast co-pyrolysis of waste greenhouse plastic films and rice husk over hierarchical micro-mesoporous zeolite with HZSM-5 as core and MCM-41 as shell (HZSM-5/MCM-41). The result showed that the relative content of hydrocarbons and CO2 were higher than for the non-catalytic pyrolysis, suggesting that HZSM-5/MCM-41 promoted the conversion of pyrolyzates to aromatic and decarboxylation becoming one of the routes that governed the conversion. The addition of MCM-41 mesopore around the HZSM-5 crystal particles assisted in cracking the large-molecular weight volatile to small molecular compound (Lin et al., 2021). Zhang et al. (2018) reported that hierarchical HZSM-5/MCM-41 which contained a moderate amount of mesopore was effective for pyrolysis intermediate upgrading while reducing the coke formation simultaneously. Qian et al. (2021) synthesized a novel hierarchical zeolite with the aid of alkaline lignin in the re-organization of alkaline treatment core material. The result revealed that the yield of bio-oil and gas was enhanced at the expense of solid residue. Higher transformation of main pyrolyzates derived from co-reactant and inhibition of char was observed due to higher acidity and hierarchical pore system of the catalyst. In addition, coke yield also decreased due to the enhanced diffusion capability of the feedstock and coke precursor, and shorter diffusion path length in the ZSM-5 structure. Deactivation rate could be reduced as no secondary reaction was produced resulting from the short residence time (Serrano et al., 2013). More particularly, the abundant reactive species of pyrolyzates from biomass-plastic mixture rapidly traverse the catalyst layer by hierarchical pore structure prior to absorption, producing solid residue. Diffusion through hierarchical zeolite crystals is faster in a manner that is closely related to Knudsen regime since the diffusion through mesoporous materials proceeds by molecule-to-molecule interaction as well as molecule-to-pore interaction.Extensive efforts are being made to develop new catalyst with low-cost, good catalytic performance and environmental friendliness. The utilization of natural ore and industrial waste as low-cost and high-activity catalyst in the production of value-added bio-oil can pave ways for recycling and reusing those mineral and waste. Red mud (RM) is a waste residue generated from aluminium industries by the Bayer process of alumina production from bauxite (Wang et al., 2019). It comprises a complex mixture of metal oxides, notably iron oxides and small amounts of alkali earth metals (Das and Mohanty, 2019). Recently, there are significant interest in making use of red mud as a catalyst in pyrolysis of biomass due to its compositional properties containing metal oxides, including CaO, TiO2, Fe2O3, Al2O3, MgO and SiO2. (Chang et al., 2020) investigated the catalytic pyrolysis of palm kernel shell over red mud using a bench scale fixed bed reactor. The result indicated that the presence of RM could enhance the cleavage of oxygen-containing double bonds and functional groups in-side chains on benzene ring to phenol and aromatic. Duman et al. (2013) studied the catalytic pyrolysis of safflower oil cake over RM in a dual reactor system and found that the RM was an effective catalyst in deoxygenation reaction, enhancing the aromatic selectivity. Although the base property of RM could provide the additional cracking efficiency, high production of aromatic could not be achieved since RM did not possess strong acid sites and shape selectivity that were able to limit the diffusivity of longer chain intermediates into the active sites and to foster the secondary reactions including isomerization and aromatization to produce aromatic hydrocarbon (Kelkar et al., 2015). Therefore, the combination of low-cost alkaline catalyst and acidic catalyst is regarded as an ideal approach to achieve higher formation of aromatic hydrocarbon and enhance the zeolite lifetime. Yathavan and Agblevor, (2013) pyrolyzed pinyon − juniper (PJ) woody biomass over HZSM-5 and RM catalyst. The addition of RM as fractional catalyst could enhance the deoxygenation process in which the oxygen was rejected via decarboxylation (CO2) process instead of decarbonylation (CO) and dehydration (H2O) process. This process could enhance the overall carbon and hydrogen efficiency and thus, more hydrogens are available for aromatic production. Furthermore, the pyrolysis oil catalysed by RM has relatively lower viscosities than HZSM-5 catalyst. In another study, in-situ RM was used in the catalytic fast co-pyrolysis (CFCP) of organosolv lignin (OL) and polypropylene (PP) over ex-situ HZSM-5. The authors reported an increase in the cracking efficiency of OL/OP intermediates as well as an enhancement of aromatic selectivity due to the effective interaction between pyrolyzates. The presence of RM in the in-situ catalytic reactor could improve the formation of selected hydrocarbon that acted as precursor to produce aromatics over ex-situ HZSM-5 in second reactor (Ryu et al., 2020a).Coal fly ash (CFA), a by-product of coal-fired thermal power plants (TPP) is often disposed in the landfill, causing environmental and economic issues. One of the key features of CFA is that it consists of aluminosilicates, such as SiO2 and Al2O3, making it appealing as a precursor to produce zeolite-based catalysts (Supelano et al., 2020). Vichaphund et al. (2019) successfully synthesized ZSM-5 from CFA (HZSM5-FA) via consecutive alkaline fusion and hydrothermal treatment (Fig. 5 ). The zeolite crystallization time was varied at 24 hr (HZSM5-FA −24) and 72 hr (HZSM5-FA-72). The catalytic activity of the HZSM5-FA was determined in catalytic fast pyrolysis of Jatropha waste at the temperature of 500 °C and Jatropha-to-catalyst ratio of 1:1–1:10. The addition of HZSM5-FA considerably enhanced the aromatic selectivity up to 97.2 % and reduced the undesired oxygenated and N-containing compounds via deoxygenation and denitrogenation reactions. HZSM5-FA promoted the cracking of large oxygenates and nitrogenated species and further converted them to olefins and aromatic via a series of reactions, including decarbonylation, decarboxylation, dehydration, cyclization, aromatization, dehydronitration, deamination, and hydrogenation. On the other hand, HZSM5-FA-72 produced low amounts of aromatics compared to HZSM5-FA-24 due to both low acidity and high mesopore volume. Low acidity zeolite had low number of active sites (Bronsted acid sites) which were responsible to convert oxygenated compounds to aromatic compounds within the framework of zeolite catalyst while high mesopore volume could limit the molecular diffusion of pyrolyzates to inner pore of zeolite to further undergo the series of reactions for aromatic formation. Based on this result, it can be concluded that the pore structure and type of acidity play an important role for aromatic formation.Steel-slag is a waste by-product derived from steel-making process which accounts about 15 % of the total crude steel output. Most of the steel slags are accumulated heavily in landfill, becoming environmental hazards due to the leaching of heavy metals, particularly mercury (Hg), lead (Pb), chromium (Cr), cadmium (Cd), and arsenic (As) (Song et al., 2021). The higher activity of Faujasite zeolite derived from steel slag in hydrocarbon production was described by Hassan et al. (2019) in the co-pyrolysis of sugarcane bagasse and HDPE. The origin of Faujasite zeolite influenced the formation of mesopore with an average size of 45 nm and a modest surface area of 39.6 m2/g. Even though the surface area of the zeolite is quite low, the NH3-TPD measurement showed a relatively strong acidity. Promotion to the hydrocarbon pool and deoxygenation reaction takes place due to the fact that the microporous structure enables intimate contact to the strong acid sites. Due to the lack of weak acidity in the catalyst, thermal condition is thought to be responsible for the decomposition and cracking of biomass and HDPE molecules. In increasing the pyrolysis temperature, they have been able to compensate the inadequacy of weak acidity while the strong acidity contributes to the upgrading of bio-oil pyrolyzates through a succession of dehydration, decarbonylation, decarboxylation, and oligomerization reactions. Nonetheless, at the reaction temperature of 500 °C and above, reverse Diels-Alder reaction would begin to occur, hindering further upgrading of the product by favouring the generation of olefins in place of aromatics.With its distinctive pore structure and high acidity, microporous zeolite has exceptional cracking and deoxygenation abilities which favour the aromatic selectivity. However, a very small pore size (0.54–0.56 nm) of microporous zeolite resulted to coke formation, mass transfer limitation, and slow diffusion of large molecules into its inner active sites, preventing further reactions of macromolecules to valuable aromatic. Acidity and porosity are two paramount factors that influence the catalytic activity of zeolite catalyst. Mesoporous zeolite has been recognised as an effective method for reducing the diffusion restriction of bulky biomass and plastic molecules, and increasing aromatic hydrocarbon production due to the larger pore size. Although mesoporous materials are synthesised to address the issue of diffusion limitations, they have poor surface acidity and an unstable structure, resulting in inadequate activity in acid-catalysed reactions. To solve this challenge, introduction of new mesoporosity in the micropore of zeolite produces zeolites with hierarchical micro-mesoporous composite. Hierarchical zeolites can substantially resolve the limitations of the conventional zeolite, such as low mass transfer problem, deactivation of catalytic activity and low activity to bulky substrates in different chemical reactions due to the combination of two levels of porosity. Desilication and dealumination are the effective methods to create mesoporous structure with large pore size for better diffusion of bulky intermediates to active sites of catalyst to undergo further reactions to aromatic. In addition, the acidity could also be altered to foster the deoxygenation reaction. Incorporation of metal into the zeolite could alter the pore size and total number of acidic sites, and enhance the thermal stability of the catalyst, all of which are helpful to decrease the coke formation of zeolite catalyst. With the addition of metal, new enhanced Lewis acid sites of zeolite were generated, which boosted the aromatic production. Ubiquitous, low cost and a complex mixture of metal oxides, natural ores such as red mud could be utilized as a catalyst in pyrolysis of biomass. The base properties of red mud could provide additional cracking and deoxygenation for aromatic production. However, the result was still unsatisfactory due to the lack of strong acid sites and shape selectivity as compared to the conventional zeolite catalyst. Therefore, it is advisable to combine low-cost base catalyst with acidic catalyst to achieve higher production of aromatic. Natural mineral wastes including coal fly ash and steel slag consisting of aluminosilicates, such as SiO2 and Al2O3 could be exploited as precursors for the synthesis of zeolite. However, two paramount factors that need to be considered to ensure high production of aromatics are pore structure and type of acidity. A zeolite catalyst with high acidity with an appropriate pore structure needs to be tailored to obtain high cracking and deoxygenation efficiency for the production of aromatics. The performance of different types of catalysts in the catalytic co-pyrolysis of biomass and plastic is summarized in Table 2 .Co-feeding hydrogen-rich plastic to the oxygen-rich biomass offers a promising technique for the production of chemicals and bio-oil. Utilization of biomass and plastic waste in co-pyrolysis process could bring a positive impact to the environment and human being since a large amount of waste polymer could be reduced and value-added fuels and chemicals could be produced. However, to be able to fully exploit this technique, further research and development are required.Co-pyrolysis of biomass and various plastic mixture needs to be considered as a feedstock in the future as the waste materials generally are not collected separately according to their criterion. According to waste management situation, the separation of biomass and plastic waste from each other during the recycling stage is not feasible and uneconomical. Many studies have focused on the co-pyrolysis of binary mixtures instead of multi-component mixtures. Therefore, multi-component feedstocks with the optimal reaction conditions needs to be investigated Furthermore, the gas emission associated with the multi-component pyrolysis needs to be examined to fully optimize the pyrolysis technology to achieve high quantity and quality of bio-oil.Although co-pyrolysis of biomass with plastic remarkably supresses the coke formation as compared to pyrolysis of individual biomass, catalyst deactivation remains a great challenge. Selecting suitable catalysts that have high catalytic activity as well as stability is of a great importance. Acidity/basicity, shape selectivity, porous structure and number of active sites are paramount factors that need to be considered when designing a catalyst. Bifunctional catalyst that possesses acid and base properties should be developed. Base catalyst promotes the fragmentation of oxygenates which can easily diffuse into the pores of acidic zeolite. The oxygenates will then be converted to aromatic hydrocarbon via cracking and deoxygenation reaction induced by acid catalyst. Furthermore, the detailed catalytic pyrolysis and catalytic co-pyrolysis reaction mechanisms of pyrolyzates on the external surface and inner pores of catalyst also need to be understood.Pre-treatment of biomass such as torrefaction and hydrothermal could be a solution to enhance the physicochemical characteristics of the biomass which could lead to the enhancement of conversion efficiency, reduction of the coke formation and improvement to the aromatic production during the catalytic pyrolysis of biomass. For example, torrefaction can enhance the cellulose content and physicochemical properties of biomass including less oxygenated compounds and high heating value to produce bio-oil with low oxygenated compounds, low acidity, high energy content and high monoaromatic hydrocarbons (Boateng and Mullen, 2013; Ryu et al., 2020). Hydrothermal treatment can produce crystalline cellulose and remove the alkali and alkaline metals, especially the K and Na metals, which provide a suitable medium for aromatic formation (Wang et al., 2021).Synergistic effect mechanism in catalytic co-pyrolysis of biomass and plastic is extremely complex reaction pathway, dominated by free radical fragments at high temperature. During catalytic co-pyrolysis, different free radicals that act as reaction intermediates are participating in hundreds of parallel or continuous reaction pathways. However, the detailed knowledge on the evolution of free radicals as reaction intermediates during catalytic co-pyrolysis is limited and unclear as it is hard to be obtained by the conventional experimental methods alone. Most researchers propose the synergistic mechanism based on the weight loss and final product obtained via TGA and GC–MS instead of reaction intermediate verification. Until now, there has been no solid and unequivocal hypothesis explaining the synergistic effect mechanism involved in radical-induced catalytic co-pyrolysis of biomass and plastic. Therefore, it is important to identify the type and composition of free radicals present during catalytic co-pyrolysis of biomass and plastic.The authors thankfully acknowledge the support obtained from Lotte Chemical Titan (M) Sdn. Bhd. and Universiti Sains Malaysia (Grant No: 304/PJKIMIA/6050422/L128), in the form of research grant and facilities which brought forth this article. The first and second authors also acknowledge the research grant provided by Universiti Teknologi MARA, under Research Incentive Grant (Grant No: 600-RMC/GIP 5/3 (045/2021)) that has resulted in this article.The authors confirm that the data supporting the findings of this study are available within the article. Raw data that support the findings of this study are available from the corresponding author, upon reasonable 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.

The increasing global fuel consumption and growing environmental concerns are the impetuses to explore alternative energy that is clean and renewable for fuel production. Converting biomass and plastic waste into high-value fuel and chemicals via co-pyrolysis technique may provide a sustainable remediation to this problem. This review critically discussed the influence of various types of plastic wastes as co-reactant in co-pyrolysis with biomass on the product distribution, synergistic effect, and quality of bio-oil. The outcome of this review revealed that the addition of plastic enhanced the yield and quality of bio-oil and inhibited the production of oxygenated compound and coke formation. Next, the critical role of zeolite-based catalyst (microporous, mesoporous, hierarchical, and metal modified zeolite) and low-cost mineral-based catalyst in upgrading the yield and quality of liquid fuel were compared and discussed. The characteristic, synthesis method, strength, and limitation of each catalyst in upgrading the products were summarized. Hierarchical zeolites can resolve the problems of mass transfer, and diffusion limitation of large molecules into active sites associated with conventional zeolite due to the combination of two levels of porosity. Finally, the potential challenges and future directions for this technique were also suggested.

Data will be made available on request.Conversion of renewable resources to biofuels and chemicals is becoming an increasingly attractive option given the depletion of fossil fuels and the environmental problems caused by the excessive use of unsustainable energy resources. Lignin is typically regarded as one of the significant sustainable biomass sources for the production of fuels and chemicals, which continues to drive human development and utilization of them from huge abundance of agriculture and forest waste [1]. In general, lignin is mainly a three-dimensional network polymer composed of the following three phenylpropane monomers: guaiacyl, p-hydroxyphenyl and syringyl. Phenolic compounds such as guaiacol, phenol and 2,6-dimethoxyphenol can be directly obtained from the fast pyrolysis of lignin [2], but their high oxygen content, poor chemical stability, corrosivity and immiscibility with hydrocarbon fuels make them unsuitable for direct use as engine fuels [3]. Therefore, for better uses of phenolic compounds derived from lignin, the process of hydrodeoxygenation (HDO) is needed.The current literatures are keen to convert the lignin model compound guaiacol into various high-valued chemicals such as cyclohexanol, cyclohexane, benzene, 2-methoxycyclohexanol, phenol, catechol, etc. [4]. Among them, cyclohexanol is a versatile petrochemical and high-quality oxygen-containing fuel, which also can be widely used in pesticides, medicines, cosmetics and other fields. Unfortunately, the selective HDO of guaiacol to cyclohexanol is difficult due to involving the hydrogenation of the CC unsaturated bond of aromatic ring, the cleavage of the CAR-OCH3 bond, while retaining the CAR-OH [5]. According to the dissociation energy of the three bonds (CAR-OH > CAR-OCH3 > CARO-CH3), the CARO-CH3 bond of guaiacol is more easily dissociated (demethylation), resulting in catechol [6]. There are two pathways from the HDO of guaiacol to form cyclohexanol (Fig. S1) [7]. The 2-methoxycyclohexanol generated by Route II limits the cleavage of the CAR-OCH3 bond due to steric hindrance and electronic effects. And Route II needs to be carried out at higher temperature and H2 pressure, which is prone to generate cyclohexane from cyclohexanol. Route I, by comparison, can be conducted at lower temperature, which meets the needs of economic development and is the better way to synthesize cyclohexanol.The key factor for guaiacol HDO to cyclohexanol is to design an efficient reaction system conducive to the demethoxylation of guaiacol. The product selectivity and total activity in HDO reactions are greatly influenced by the properties of solvents used. Water is the greenest and the mostly being used solvent for guaiacol HDO, which is reported to be effective at promoting CO cleavage and reducing the undesirable thermal degradation [8].The more important point is to develop effective catalyst for guaiacol HDO to cyclohexanol. A series of metal catalysts have been developed and investigated for the HDO of guaiacol to cyclohexanol, including metal hybrids (sulfides, carbides, nitrides, etc.), non-noble metals and noble metals [9]. Metal hybrids exhibited satisfactory catalytic activity for the HDO reaction, but their reusability was not good enough. Non-noble metals such as Ni and Co usually show low activity. To obtain a high yield of cyclohexanol, a high metal loading, high temperature (> 200 °C), and high H2 pressure are needed. Noble metal catalysts Ru, Pd and Pt, especially Ru-based catalysts showed great potential in HDO reactions. The combination of Ru metal sites and a base such as MgO or MnOx could achieve high cyclohexanol yield (79% and 81%, respectively) owing to the presence of the base, which suppresses the unselective CO dissociation by Ru catalyst, and may also promote the demethoxylation step via stabilizing the produced phenol [10,11]. Although Ru-based catalyst with base sites gave a ∼ 80% yield of cyclohexanol for guaiacol HDO, a great amount of 2-methoxycyclohexanol (> 13%) was generated, which inhibits the increase in cyclohexanol selectivity. Highly selective HDO of lignin-derived phenols to cyclohexanol is still challenging. Besides base sites, suitable acid sites also facilitate cleavage of the CO bond [4]. Based on the above reported results, we postulate that acidic carrier supported Ru-based multifunctional catalyst with metal sites and acid/base sites will be more beneficial to the demethoxylation step in the selective HDO of lignin-derived phenols, thereby increasing the selectivity to cyclohexanol. As we know, γ-Al2O3 is a common solid acid carrier, however, it is often accompanied by structural changes in water with significantly decreased acidity and surface area that trigger catalyst deactivation [12]. The introduction of SiO2 can effectively inhibit the hydration of γ-Al2O3 support and prevent the active decay of γ-Al2O3 supported catalyst. Al2O3-SiO2 composite is a carrier with excellent performance owing to its developed pore structure, large specific surface area and strong thermal stability, and its surface acidity is easy to control [13]. The structure of Al2O3-SiO2 supported catalyst affects its separation from the reaction solution. Microspheres are easy to be separated from the aqueous phase. In addition, the Al2O3-SiO2 composite microspheres with high pore volume and specific surface area are conducive to the uniform dispersion of the noble metal [14].Based on the above considerations, herein, we designed an efficient and reusable Al2O3-SiO2 acidic uniform microspheres supported RuMn multifunctional microsphere catalyst for the highly selective HDO of guaiacol and 2,6-dimethoxyphenol to cyclohexanol in water. The choice of metal Mn as the second metal component is mainly based on the following considerations. Besides base sites, MnOx have many oxygen vacancies, which facilitates the adsorption of oxygen-containing functional groups, being conducive to the deoxidation step in the hydrodeoxygenation reaction process [15]. In our recent work, we prepared MnOx modified Ni/AC catalyst and found that the catalyst exhibited excellent performance for the HDO of 5-hydroxymethylfurfural to 2,5-dimethylfuran, owing to the synergistic effect of Ni and MnOx [16]. In this work, it was observed that, compared with the reported Ru-based catalysts, our RuMn/Al2O3-SiO2 catalyst showed higher catalytic activity and selectivity for the partial HDO of guaiacol and 2,6-dimethoxyphenol to cyclohexanol under low Ru amount and mild reaction conditions. And it exhibited excellent performance for the hydrogenation of phenol to cyclohexanol. The catalyst was easy to be separated from the aqueous solution and showed good reusability. The catalyst was well characterized by different techniques and the synergistic effect among Ru, Mn and Al2O3-SiO2 was investigated. Furthermore, possible reaction pathways of guaiacol over RuMn/Al2O3-SiO2 catalyst was proposed based on the product distribution.Al2O3-SiO2 microspheres were prepared as follows. 1 g of γ-Al2O3 was added to 80 mL of isopropanol solvent. The mixture was stirred for 30 min to ensure that γ-Al2O3 was uniformly dispersed. Then, 8 mL of distilled water, 5 mL of ammonia water and 1 mL of tetraethylorthosilicate (TEOS) were added into the above solution in sequence, and the mixture was stirred at room temperature for 10 min and kept at room temperature for 8 h. The white precipitate obtained was subsequently filtered. The obtained solid was washed with distilled water for 5 times and then centrifuged. The resulted sample was dried in an oven at 110 °C for 12 h, followed by calcination in a muffle furnace at 500 °C for 3 h. The prepared Al2O3-SiO2 sample (Si/Al = 0.2) was ground into flour and passed through a 120-mesh sieve for use.A series of RuMn catalysts supported on Al2O3-SiO2 microspheres obtained by the above-described method was prepared by an incipient wetness impregnation method. In a typical procedure, an appropriate amount of Al2O3-SiO2 was added into the aqueous solution containing the required amount of RuCl3·3H2O and C4H6MnO4∙4H2O in a beaker. After impregnated for 24 h, the mixture was dried at 110 °C for 10 h and finally reduced at 300 °C in a tubular furnace under hydrogen flow for 3 h to obtain the target catalyst, which was denoted as RuMn(x:y)/Al2O3-SiO2, where x:y means the molar ratio of Ru to Mn. The molar ratio of Ru to Mn was varied from (4:1) to (1:2) by changing the Mn content and using a fixed amount of Ru (3.0 wt%). Two monometallic catalysts 3.0 wt% Ru/Al2O3-SiO2 and 3.0 wt% Mn/Al2O3-SiO2, and active carbon (AC), SiO2 and γ-Al2O3 supported Ru or RuMn catalysts were prepared by using the same method for comparison. The details of the chemical reagents and the catalyst preparation including information on the content of Ru and Mn in the corresponding catalyst are described in the Supporting Information (Table S1).The detailed information of characterization methods including X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM), H2 temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption of CO2 (CO2-TPD), ammonia temperature-programmed desorption (NH3-TPD), pyridine adsorbed IR spectroscopy (Py-IR) are described in the Supporting Information.The catalytic hydrodeoxygenation of phenols was performed in a 25 mL stainless-steel autoclave equipped with magnetic stirring. In a typical experiment, 30 mg of catalyst, 500 mg of guaiacol and 15 mL of solvent water was added into the reactor. The reactor was sealed, purged with H2 for five times, and pressurized with H2 to the required pressure. The autoclave was then heated to the required temperature and kept at this temperature for the required time under the continuous stirring speed of 1000 rpm. After the reaction, the autoclave was quickly cooled to room temperature, the gases were collected in a gas bag and the reaction products were separated from the catalyst by centrifugation. The liquid reaction products were extracted by ethyl acetate, and then quantitatively analyzed with an Agilent 7890A gas chromatography equipped with an HP-5 capillary column (30.0 m × 0.32 mm × 0.25 μm) and a flame ionization detector (FID) using n-dodecane as an internal standard, and identified by an Agilent 6890 GC system coupled to a mass spectrometer equipped with an Agilent 5973 quadrupole mass analyzer. Conversion of guaiacol, selectivities and yields were calculated as follows (Eqs. (1)–(3)), based on the number of C6 rings in the substrate and products other than C1 products (methanol and methane). Methanol was also a major product, but because of its high volatility, its concentration is difficult to be accurately measured. In addition, the formation of methanol does not affect the quantification of products containing C6 rings. The gaseous product collected by a gas bag was analyzed using Agilent 7890A gas chromatography with FID and no methane was detected. Carbon balance = (moles of C6 ring in liquid products and unreacted reactant/initial moles of C6 ring in the reactant) × 100%, and all the carbon balance were higher than 95%, so the experimental results were reliable. (1) Conversion % = initial conc . of guaiacol − final conc . of guaiacol initial conc . of guaiacol × 100 (2) Selectivity % = moles of desired product formed moles of guaiacol consumed × 100 (3) Yield % = Conversion × Selectivity × 100 The guaiacol conversion and cyclohexanol selectivity over various carries supported Ru and RuMn catalysts were compared, and the results are listed in Table 1 . The Ru loading amount was fixed at 3.0 wt%, and the HDO activity was measured at reaction conditions of 30 mg catalyst, 180 °C and 2 MPa H2 for 0.5 and 4 h, respectively. Methanol was also a major product for guaiacol HDO, but because of its high volatility, its concentration was difficult to be accurately measured. Therefore, we did not show the data of methanol in Table 1. Both hydrodeoxygenation and hydrogenation reactions proceeded, and the main products were cyclohexanol and 2-methoxycyclohexanol over all the Ru-based catalysts, which indicated that Ru is the active sites for guaiacol HDO. Phenol, which is a precursor of cyclohexanol, was detected in shorter reaction time (0.5 h). It can be seen that although guaiacol could be completely converted over AC, SiO2 and γ-Al2O3 supported Ru catalysts for 4 h, only 66.4%, 65.4%, 74.4% and 68.6% of cyclohexanol yields were obtained, respectively, and a large amount of a saturated product of 2-methoxycyclohexanol was detected (entries 1–4). It was worth noting that Al2O3-SiO2 composite microspheres supported Ru catalyst gave higher selectivity to cyclohexanol (86.3%) and lower selectivity to 2-methoxycyclohexanol than Ru/AC, Ru/SiO2 and Ru/γ-Al2O3 under the same reaction conditions (entry 5). Monometallic Mn/Al2O3-SiO2 showed much lower catalytic activity for the HDO of guaiacol (entry 6), indicating that Mn species are inactive for guaiacol HDO. The addition of Mn obviously improved the activity and the selectivity of Ru/Al2O3-SiO2 catalysts for the selective HDO of guaiacol to cyclohexanol and decreased the selectivity to 2-methoxycyclohexanol, which was consistent with the previous report that the additive MnOx in the catalyst could accelerate the dissociation of CO [17]. The conversion of guaiacol and the selectivity to cyclohexanol increased with the decrease in Ru:Mn molar ratio from 4:1 to 2:1 (entries 7 and 8), and the highest cyclohexanol yield of 91.3% was achieved over RuMn(2:1)/Al2O3-SiO2 catalyst after reaction 4 h, much higher than those of AC, SiO2 and /γ-Al2O3 supported RuMn(2:1) catalysts (entries 11–13), and much higher than those of the reported Ru/C + MgO and Ru-MnOx/C catalytic systems (entries 14 and 15) [10,11]. Further increasing the Mn content, however, the activity and the selectivity of RuMn/Al2O3-SiO2 catalyst to cyclohexanol decreased, and thus cyclohexanol yield decreased (entries 9 and 10). Therefore, RuMn(2:1)/Al2O3-SiO2 catalyst was chosen for further investigation.The HDO of guaiacol was carried out at varied initial hydrogen pressure from 1 to 4 MPa at 180 °C to investigate the catalytic performance of the RuMn(2:1)/Al2O3-SiO2 catalyst. As shown in Fig. S2, the initial hydrogen pressure had obvious effect on both guaiacol conversion and product distribution. Under lower initial hydrogen pressure (< 2 MPa), cyclohexanone was detected, guaiacol conversion and cyclohexanol selectivity increased with the increase in initial hydrogen pressure and a maximum cyclohexanol selectivity of 91.3% with a guaiacol conversion of 100% was achieved when the reaction was conducted at an initial hydrogen pressure of 2 MPa. These results were in good agreement with Tomishige's study, which also reported that lower initial hydrogen pressure was beneficial to higher cyclohexanol selectivity over Ru/C + MgO catalyst system [10]. It was because that H2 was involved in the hydrogenation of the benzene ring and the cleavage of the CO bond in guaiacol. And the increase in hydrogen pressure meant the increase in the concentration of dissolved hydrogen in the solution and the hydrogen atoms adsorbed on the catalyst surface, thereby promoting the hydrogenation of the benzene ring and the cleavage of the CO bond in guaiacol [4]. While higher initial hydrogen pressure suppressed the formation of cyclohexanol and enhanced the formation of the by-product 2-methoxycyclohexanol, which was consistent with the literature report [18]. Finally, 2 MPa was selected as the optimal reaction pressure considering guaiacol conversion and cyclohexanol selectivity based on the above results.The effect of reaction temperature on the HDO of guaiacol over RuMn(2:1)/Al2O3-SiO2 catalyst was investigated at 2 MPa H2 for 4 h and the results are shown in Fig. S3. The conversion of guaiacol reached 100% at all the tested temperatures from 120 to 220 °C, while the product distribution was different. It was found that at a low temperature of 120 °C, the HDO of guaiacol gave 2-methoxycyclohexanol as a dominating product, indicating the occurrence of hydrogenation of the aromatic ring of guaiacol. Our experimental results well confirmed the report, in which low reaction temperature was beneficial to the hydrogenation of aromatic rings in the HDO of guaiacol over supported noble metal catalysts [19]. With the increase in reaction temperature, the selectivity to cyclohexanol increased gradually and arrived at its maximum (91.3%) at 180 °C, which indicated that the HDO of guaiacol to cyclohexanol might happen at relatively higher reaction temperature, being in good accordance with the work of Zhou et al. [20]. A continuous increase in the reaction temperature led to a decrease in the selectivity to cyclohexanol and an increase in the selectivity to cyclohexane. This indicated that excessive reaction temperature (> 200 °C) could promote the cleavage of the CO bond on the aliphatic ring of cyclohexanol, resulting in the formation of excessive hydrogenolysis product cyclohexane [18]. Meanwhile, intermediate cyclohexanone and unknown substances appeared and increased with the increase in reaction temperature, in good accordance with the observation by Wang et al. [21]. Based on the above results, 180 °C was finally chosen as the optimum reaction temperature considering both guaiacol conversion and cyclohexanol selectivity.To investigate the effect of catalyst dosage for the HDO of guaiacol to cyclohexanol, the time course experiments within 24 h were carried out over RuMn(2:1)/Al2O3-SiO2 with a dosage range of 10 to 70 mg (0.07 to 0.52 mol%), and the results are shown in Fig. 1 . It can be seen that RuMn(2:1)/Al2O3-SiO2 showed good performance for guaiacol HDO even at a dosage as low as 10 mg (Fig. 1a), under which the conversion of guaiacol could reach 100% and the yield of cyclohexanol was up to 82.1% after 2 h. Prolonging the reaction time, the yield of cyclohexanol didn't obviously increase due to the forming of a large amount of saturated product of 2-methoxycyclohexanol, which was difficult to be hydrogenated to cyclohexanol. Contrary to the literature [22], under our catalytic conditions, cyclohexanol was not further dehydroxylated to cyclohexane, the over‑hydrogenated product. This observation is consistent with our assumption that Al2O3-SiO2 support is helpful to promote the demethoxylation and inhibit the hydrogenolysis of cyclohexanol to cyclohexane. The yield of cyclohexanol was significantly improved with the increase of catalyst dosage. With the catalyst dosage of 70 mg (Fig. 1d), the cyclohexanol yield was up to 96.8% after 4 h, and the highest yield of cyclohexanol could reach 99.9% after 24 h. Besides, RuMn(2:1)/Al2O3-SiO2 exhibited a higher TOF value (861.1 h−1) than that of many catalysts (Table S2).RuMn(2:1)/Al2O3-SiO2 was further applied to other two lignin-related monomers, phenol and 2,6-dimethoxyphenol, and the results are shown in Fig. S4 and Fig. S5, respectively. It was found that phenol could be converted to cyclohexanol with 100% yield at a catalyst dosage of 10 mg for 4 h (Fig. S4), which is the highest yield under the lowest dosage reported. When the dosage of the catalyst increased to 50 mg, 100% cyclohexanol yield was achieved after 40 min. Zhan et al. [23] proposed that the introduction of Lewis acid sites favored the activation of the aromatic ring and promoted the hydrogenation of phenol, thereby improving the selectivity to cyclohexanol. As shown in Fig. S5, 2,6-dimethoxyphenol could also be well converted to cyclohexanol (a yield of 81.8% at the catalyst dosage of 50 mg and 94.5% at the catalyst dosage of 100 mg). These results indicated that RuMn(2:1)/Al2O3-SiO2 catalyst was efficient for CAR-OCH3 cleavage and benzene ring hydrogenation while protecting the CAR-OH.Reusability is one of the important aspects for the practical application of the heterogeneous catalyst. Subsequently, we investigated the stability of RuMn(2:1)/Al2O3-SiO2 catalyst for the HDO of guaiacol to cyclohexanol for 1 h and 4 h, respectively. During each cycle, after a complete reaction at 180 °C and 2.0 MPa H2, the catalyst was centrifuged, washed with water for five times, and reused for the next runs. As shown in Fig. S6, the RuMn(2:1)/Al2O3-SiO2 catalyst kept its good performance for the HDO of guaiacol to cyclohexanol during recycling. The conversion of guaiacol and the yield of cyclohexanol decreased slightly when the catalyst was recycled for four runs in 1 h (Fig. S6a). And guaiacol was still completely converted and the yield of cyclohexanol was 90.1% after four runs in 4 h (Fig. S6b). XRD patterns showed that there was no obvious change in the structure of the recovered catalyst (Fig. S7). These data indicated that the RuMn/Al2O3-SiO2 catalyst was essentially reusable in the aqueous phase HDO of guaiacol.Fig. S8 illustrates the XRD patterns of the support Al2O3-SiO2 and the reduced catalysts Mn/Al2O3-SiO2, Ru/Al2O3-SiO2, and RuMn(2:1)/Al2O3-SiO2. The broad peak observed at 23° for all the samples is assigned to amorphous SiO2 [24]. The XRD peaks that are observed at 2θ = 32.4°, 37.2°, 39.5°, 45.9°, 61.1° and 67.0° are assigned to γ-Al2O3 belonging to these hkl values (220), (311), (222), (400), (500) and (440), respectively [25]. In the cases of Mn/Al2O3-SiO2 and RuMn(2:1)/Al2O3-SiO2, no reflections due to Mn species were observed. And the XRD patterns of both Ru/Al2O3-SiO2 and RuMn(2:1)/Al2O3-SiO2 catalysts showed no Ru species reflections. These are likely due to the low metal loadings or the well-dispersion of Ru and Mn species. It can be observed that the XRD pattern of Al2O3-SiO2 was not changed after the addition of metals, indicating that the addition of Ru and Mn species did not compromise the structure of the support.Nitrogen adsorption-desorption isotherms and pore size distribution of Al2O3-SiO2 support and RuMn(2:1)/Al2O3-SiO2 catalyst are presented in Fig. S9, and the relative textural properties are listed in Table S3. Both the two samples exhibited type IV isotherms, a typical characteristic for mesoporous materials [26], which is attributed to the HDO of guaiacol [4]. A broad pore size distribution was observed in Al2O3-SiO2 and RuMn(2:1)/Al2O3-SiO2, consisting mainly of 20 nm pores, which exhibited mesoporous properties (2–50 nm) [27]. It can be seen from Table S3 that both Al2O3-SiO2 and RuMn(2:1)/Al2O3-SiO2 have high specific surface areas. Compared with Al2O3-SiO2, the specific surface area of RuMn(2:1)/Al2O3-SiO2 reduced from 130.8 to 117.5 m2·g−1 due to the addition of Ru and Mn species, and the average pore size and the total pore volume also slightly reduced, which may be due to the deformation of the mesoporous channels caused by the doping of Ru and Mn atoms in the support framework [28].The reducibility of the dried catalysts was investigated by hydrogen temperature-programmed reduction. Fig. 2 shows the H2-TPR profiles of Mn/Al2O3-SiO2, Ru/Al2O3-SiO2 and RuMn(2:1)/Al2O3-SiO2 catalysts. Mn/Al2O3-SiO2 catalyst exhibited two broad peaks at 281 and 436 °C, which likely belonged to the reduction of MnO2 to Mn3O4 and Mn3O4 to MnO, respectively. And it was difficult for MnOx to generate metallic Mn in a hydrogen atmosphere at a temperature lower than 900 °C [29]. The Ru/Al2O3-SiO2 profile showed a strong reduction peak at 137 °C and a weaker reduction peak at 318 °C, which were attributed to the reduction of RuCl3 and RuO2 to Ru0, respectively [30]. In the case of RuMn(2:1)/Al2O3-SiO2, two peaks were observed at 176 and 316 °C, which were ascribed to the reduction of Ru species to Ru0. In comparison with the monometallic catalyst Ru/Al2O3-SiO2, the peak at low temperature moved towards high temperature and the peak intensity at high temperature was stronger, indicating a clear interaction between Ru and Mn species, and the addition of Mn promotes the reduction of RuO2. In addition, the peak intensity of RuMn(2:1)/Al2O3-SiO2 at higher temperature was stronger than that of Ru/Al2O3-SiO2, suggesting that Mn species are simultaneously reduced to some extent. Ishikawa et al. also evidenced the reduction of Mn species to MnO in Ru-MnOx/C catalyst [11].The morphologies of the catalysts were characterized by TEM (Fig. 3 ). Fig. 3a clearly showed that Al2O3-SiO2 support had a uniform spherical morphology with an average diameter of 0.36 μm. After immersing Ru and Mn, the morphology of the carrier did not change significantly (Fig. 3b, c and d). For Ru/Al2O3-SiO2 and RuMn(2:1)/Al2O3-SiO2 catalysts, the diameters of about 100 metal particles were randomly selected to obtain the corresponding particle size distributions. According to the measurement, an average Ru metal nanoparticle size of 2.26 nm is obtained on the Ru/Al2O3-SiO2 catalyst (Fig. 3c), and the average metal nanoparticle size on RuMn(2:1)/Al2O3-SiO2 is 1.25 nm (Fig. 3d), which indicates that the addition of Mn improved the dispersion of metal Ru. Fig. S10 showed the HAADF-STEM elemental mapping of RuMn(2:1)/Al2O3-SiO2. It can be seen that the RuMn(2:1)/Al2O3-SiO2 micro-area contained Al, Si, O, Mn and Ru. The signals from both Mn and Ru metals were clearly detected along the particles, indicating that the MnOx species were present nearby Ru nanoparticles.XPS technique was performed to investigate the chemical state and surface composition of RuMn/Al2O3-SiO2 catalyst. The results are shown in Fig. 4 and XPS relative quantitative analysis are listed in Table S4. The survey spectrum (Fig. 4a) indicated the presence of elements Si, Al, O, C, Ru and Mn. It was generally known that the Ru 3d spectrum was not clear because it was often obscured by the strong C 1 s signal [31], as shown in Fig. 4b. The Ru 3p XPS spectrum was usually used to characterize the chemical states of Ru particles, and the test was performed between 450 and 500 eV (Fig. 4c). It can be seen that the Ru 3p spin split into pairs of Ru 3p3/2 and Ru 3p1/2. And the peaks at 462.2 and 465.8 eV in the Ru 3p3/2 were assigned to Ru0 and RuO2. The higher energy (484.3 and 486.5 eV) peaks in the Ru 3p1/2 were also contributed by Ru0 and RuO2, respectively [5]. The observation of RuO2 indicates that the oxidation of metallic Ru occurred during its exposure to air. The Mn 2p3/2 spectra of RuMn catalysts (Fig. 4d) showed peaks at 642.2 and 646.1 eV, which were assigned to Mn2+ and Mn4+, respectively. And the peaks at 484.6 and 487.0 eV in the Mn 2p1/2 were also contributed by Mn2+ and Mn4+, respectively, indicating that Mn mainly exists in the form of MnOx: MnO and MnO2. Compared to their monometallic counterparts, the binding energies of the Ru species in RuMn/Al2O3-SiO2 catalyst ascend by ca. 0.3 eV, while the binding energies of Mn species descend by ca. 0.3 eV.CO2-TPD of Ru/Al2O3-SiO2 and RuMn(2:1)/Al2O3-SiO2 were measured to investigate the basic character of RuMn(2:1)/Al2O3-SiO2 and the results are shown in Fig. S11. Compared with Ru/Al2O3-SiO2, the CO2 desorption peak of RuMn(2:1)/Al2O3-SiO2 at 414 °C became stronger, and a new peak at 516 °C, indicating the presence of moderate and strong base sites, which was attributed to the addition of Mn oxides on the catalyst surface [32].It has been proposed that the acid sites on catalyst surface play a crucial role in HDO reactions. Thus, NH3-TPD profiles of Ru/Al2O3-SiO2 and RuMn(2:1)/Al2O3-SiO2 were measured to investigated acidity, as shown in Fig. 5a. It could be observed that both the catalysts had acidity, and the acid site distribution of RuMn(2:1)/Al2O3-SiO2 was wider, indicating that the addition of Mn oxide can broad the acid site distribution. Two peaks at 128 °C and 245 °C were assigned to weak and moderate acid sites, respectively [33]. Typically, the peak at low temperature (< 300 °C) was attributed to ammonia coordinated to Lewis acid sites. The Lewis acidic active sites were favorable for CO bond cleavage, which was one of the reasons for the better catalytic performance of RuMn(2:1)/Al2O3-SiO2 catalysts.We further investigated the acid species and their distribution in the RuMn(2:1)/Al2O3-SiO2 catalyst by FT-IR spectra of adsorbed pyridine at 50, 150 and 250°C, respectively, and the FT-IR spectra of the samples were recorded at 1400–1700 cm−1. As shown in Fig. 5b, the bands at 1606, 1575 and 1444 cm−1 are assigned to Lewis acid, the band at 1643 cm−1 is assigned to Brønsted acid, and the band at 1492 cm−1 is assigned to the common absorption peak of Bronsted and Lewis acids [34]. The acid content of the samples was calculated, and the results are listed in Table S5. The Lewis acid content in the RuMn(2:1)/Al2O3-SiO2 sample was significantly higher than the Brønsted acid content. The Lewis acid sites detected at 150 and 250°C were attributed to moderate acid sites, which is consistent with the above NH3-TPD results. Combined with the performance of the above RuMn catalysts, we speculate that the moderately strong Lewis acid sites are the key to the selective demethoxylation of guaiacol.According to the first step of guaiacol HDO, there were four possible pathways, as shown in Fig. S12. (1) Guaiacol was first demethylated to catechol, then hydrogenated to 1,2-cyclohexanediol, and finally hydrogenolyzed to cyclohexanol (Eq. (1)). In this work, no catechol and 1,2-cyclohexanediol were observed during guaiacol HDO to cyclohexanol. In addition, the literature pointed out that the binding ability of catechol or 1,2-cyclohexanediol to the catalyst was stronger than that of guaiacol, which would hinder the reaction of guaiacol [35]. Therefore, the route (1) was unreasonable. (2) Guaiacol was first dehydroxylated to anisole, then hydrogenated to methoxycyclohexane, and finally demethylated to cyclohexanol (Eq. (2)). During the guaiacol HDO reaction over RuMn/Al2O3-SiO2, we detected no anisole and only a trace of methoxycyclohexane. Over time, methoxycyclohexane was not further converted to cyclohexanol, thus ruling out the route (2). (3) Guaiacol was first demethoxylated to phenol, and then phenol was hydrogenated to cyclohexanone and cyclohexanol in turn (Eq. (3)). Considering that a certain amount of phenol was detected in guaiacol hydrodeoxygenation at short reaction time (Fig. 1), phenol was an intermediate of cyclohexanol from guaiacol. Furthermore, under our reaction conditions, phenol can be converted to cyclohexanone and cyclohexanol (Fig. S4). Therefore, it can be inferred that guaiacol mainly converts cyclohexanol through route (3) over RuMn/Al2O3-SiO2 catalyst. And (4) guaiacol was first hydrogenated to 2-methoxycyclohexanol, and then demethoxylated to cyclohexanol (eq. 4). 2-Methoxycyclohexanol was the main by-product in the reaction of guaiacol HDO to cyclohexanol. When guaiacol was completely converted, 2-methoxycyclohexanol gradually decreased with reaction time, and the yield of cyclohexanol increased slowly. In addition, we investigated the reaction time curves of HDO of 2-methoxycyclohexanol and cyclohexanol under the optimized conditions (2 MPa H2, 180 °C), respectively, as shown in Fig. S13 and Fig. S14. The results in Fig. S13 showed that 2-methoxycyclohexanol was mainly converted to cyclohexanol. However, the conversion rate was too slow, and the yield of cyclohexanol was only 8.06% after 24 h of reaction. Cyclohexanol was hardly converted, and the conversion was only 0.27% after 24 h under the reaction conditions (Fig. S14). Therefore, a small part of cyclohexanol can be obtained through route (4), but this route was not the main route for the guaiacol HDO to cyclohexanol over RuMn/Al2O3-SiO2.According to the above analysis, we proposed the reaction pathways of guaiacol HDO to cyclohexanol over RuMn/Al2O3-SiO2, as shown in Scheme 1 . In this mechanism, the demethoxylation of guaiacol to phenol and the hydrogenation of guaiacol to 2-methoxycyclohexanol can simultaneously proceed, and the relative rate of the demethoxylation (step (i)) to hydrogenation (step (ii)) of guaiacol critically determines the selectivity to cyclohexanol. Cyclohexanol was produced rapidly with a high yield from phenol, while the reaction rate from 2-methoxycyclohexanol to cyclohexanol was slow. This reaction mechanism is consistent with that proposed by Tomishige et al. [11]. The combination of appropriate acidity of the support Al2O3-SiO2 and the addition of Mn was beneficial to promote the demethoxylation of guaiacol to phenol, thereby improving the yield of cyclohexanol.Al2O3-SiO2 composite microspheres were prepared. Using it as support, RuMn multifunctional catalysts with metal active sites and acid/base sites were successfully prepared by a wetness impregnation method and applied to the aqueous phase selective HDO of lignin-derived guaiacol and 2,6-dimethoxyphenol and the hydrogenation of phenol to form cyclohexanol. Under optimized mild reaction conditions of 0.52 mol% Ru, 180 °C, 2.0 MPa H2 for 4 h, a cyclohexanol yield of 96.8% was achieved from guaiacol, and the yield of cyclohexanol could reach 99.9% by prolonging the reaction time to 24 h over RuMn(2:1)/Al2O3-SiO2 catalyst. Meanwhile, phenol and 2,6-dimethoxyphenol could also be converted to cyclohexanol with high yields of 100% and 94.5%, respectively. The catalyst was easily separated from the aqueous solution and can be reused 4 times without obvious loss of activity. The Al2O3-SiO2 composite support provided appropriate acid sites, large surface area and modifiable pores, and the addition of Mn improved the dispersion of Ru, and provides moderate acid-base sites, which were all attributed to the high performance of RuMn/Al2O3-SiO2. Therefore, the present strategy has great potential for application in high-quality biofuel production from renewable lignocellulosic biomass due to its high efficiency, little catalyst dosage, green solvent, and mild conditions. Mengting Chen: Conceptualization, Methodology, Investigation, Writing – original draft, Writing – review & editing. Qifeng Zhong: Methodology, Resources, Writing – review & editing. Meihua Zhang: Investigation, Writing – review & editing. Hao Huang: Writing – review & editing. Yingxin Liu: Conceptualization, Methodology, Writing – review & editing, Visualization, Supervision. Zuojun Wei: Conceptualization, Methodology, Writing – review & editing, Visualization, 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 research was supported by the National Science Foundation of China (21878269, 21476211), the Zhejiang Provincial Natural Science Foundation of China (LY18B060016) and Jiangxi Qilin Chemical Industry Co., Ltd. (YX-[2012]008@). Supplementary material Image 6 Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2022.106550.

Herein, RuMn/Al2O3-SiO2 multifunctional catalyst was prepared and efficiently converted lignin-derived phenols into cyclohexanol by aqueous partial hydrodeoxygenation (HDO). The optimal RuMn/Al2O3-SiO2 (Ru/Mn = 2) catalyst achieved 96.8% of cyclohexanol yield with 100% guaiacol conversion under mild reaction conditions (180 °C and 2.0 MPa H2) for 4 h, with a turnover frequency (TOF) of 861.1 h−1, which is better than most results reported. The high performance of RuMn/Al2O3-SiO2 was attributed to the synergic effect of highly dispersed Ru nanoparticles with small size (1.25 nm), the microsphere structure, the appropriate acid sites from Al2O3-SiO2 and acid-base sites from MnOx.

Energy conservation and environmental protection issues [1–5] and lightweight and recyclable materials have become common in the industry. Magnesium (Mg) alloy has low density, high rigidity and can improve energy efficiency in the transportation industry. The electromagnetic shielding is beneficial for 3C products that are lightweight. It has good mechanical properties but it has exceptionally high activity and is prone to severe corrosion in humid environments, which limits its application. Many surface treatments and coating systems are used to increase the corrosion resistance of Mg alloys. These methods include conversion coating [6–10], organic coating [11,12], electroplating [13–16], electroless plating [17–20], anodizing, micro-arc oxidation (MAO) [14,15,17] and other surface treatments [21–26].Electroless nickel-phosphorus (Ni–P) plating is an appropriate process for forming amorphous metallic alloys. Due to its excellent corrosion and wear resistance, quickly expanding its range of applications, electroless Ni–P plating has recently received wide attention and research interest [27,28]. Electroless Ni–P plating is also popularly used in Mg alloys to increase their corrosion resistance and wear resistance properties [29–35]. However, when Mg is in electrical contact with nickel, there is dissimilar metal corrosion (galvanic corrosion) [35–37]. Studies show that a stable intermediate layer gives good protection for the substrate and increases the consistency of the nickel plating adhesion [17,38]. Common surface treatment methods that are used to improve corrosion resistance and stable intermediate layers are conversion coating and MAO. A manganese-vanadium conversion coating has been used for electroless Ni–P plating on Mg alloys [6,39,40]. To increase resistance to corrosion properties, phosphoric acid is used for conversion coating/electroless nickel plating [41,42].MAO treatment is an environmentally friendly technology widely used to create corrosion resistant ceramic coatings on metals and alloys [43,44]. A previous study [17] by the authors showed that a treated phosphate-free-MAO/Ni–P coating has significantly better corrosion resistance because there is good adhesion between the electroless Ni–P plating layer and the MAO layer. Furthermore, there is a good mechanical locking between the electroless Ni–P plating layer and the MAO layer, which increases corrosion resistance, mechanical and chemical properties [17].Varying the composition of electroless Ni–P plating solution generates samples with amorphous, mix-structure, and nanocrystalline films [45,46]. In general, the corrosion resistance of the coating is affected by various deposited parameters of the electroless plating system [47,48]. Most studies add fluoride to the electroless plating system in order to achieve a uniform coating for the electroless Ni–P plating solution because F– ions retard the corrosion of Mg alloy [49–53]. However, excessive F– ions reduce the growth rate of the coating, so it does not protect the substrate [50–52]. Few studies determine the effect of fluoride (F– ion) on the electroless Ni–P plating of Mg alloys and no studies investigate the effect of adding F– ions to the MAO layer of Mg alloys during the electroless Ni–P plating process. This study evaluates the effect of fluoride addition to the electroless Ni–P plating solution on the MAO/Ni–P composite coating properties and gives a better understanding of the growth mechanisms during electroless Ni–P plating in the MAO layer.This study concerns the electroless Ni–P plating on AZ31B Mg alloy with increased corrosion resistance after the substrate undergoes MAO treatment and compares the characteristic properties of chemical electroless Ni–P plating solutions with/without fluoride content to form a MAO/Ni–P composite coating. The microstructure is observed using SEM and TEM and the coating elements are analyzed using EDS and XRD. The corrosion resistance of the coating is tested using a potentiodynamic polarization curve, electrochemical impedance spectroscopy (EIS) and a salt spray test (SST). Finally, the effect of fluoride on the formation mechanism for the electroless Ni–P plating film on MAO-coated AZ31B Mg alloy is explained in detail.Rectangular pieces (50 × 50 × 2 mm3) of AZ31B Mg alloy were used as the substrate, with 2.89 wt.% of Al, 0.897 wt.% of Zn, 0.282wt.% of Mn, 0.055wt.% of Si, 0.03wt.% of Fe. The remainder is Mg. Before the MAO process, all samples were ground with sandpaper of 400#, 800#, and 1200#, washed with deionized water, degreased with alcohol and dried at room temperature. The MAO process used a pulsed DC power supply with a duty cycle of 30％ and a constant voltage of 400V in an alkaline solution for 7 min. The electrolytes for MAO treatment were Na2SiO36 g/L, NaOH 1.5 g/L and NaF 3 g/L. The temperature of the electrolytes was maintained at 10 ± 5 °C using a water cooling system.After MAO treatment, the sample was cleaned with deionized water, wiped with alcohol and dried at room temperature. The MAO samples were then immersed in a laboratory-developed catalyst ink for 10 s and dried in a 90 °C oven before electroless Ni–P planting. The catalyst ink consists of a metal palladium nanoparticle that easily penetrates MAO holes and initiates electroless Ni–P plating. The electrolyte composition and processing conditions for electroless Ni–P plating are listed in Table 1 .Scanning electron microscopy (SEM) using a JEOL JSM-IT100microscope equipped with Energy Dispersive Spectrometer (EDS) was used to observe the surface and cross-section morphology of the MAO and MAO/Ni–P composite coatings. FIB-SII 3050SE was used as a sample cutting. The transmission electron microscope (TEM) was the Philips Tecnai F30 of National Taiwan University, which uses an acceleration voltage from a LaB6 gun of 300 keV. The chemical composition of the coating was measured using EDS and the TEM. The XRD experiments used a Bruker D2 PHASER X-ray diffractometer (λ = 1.54184 Å, 30 kV and 10 mA) with Cu Kα radiation. The scanning range for the diffraction angle (2θ) was 10° and 90°, with a step width of 0.05° and a time step of 0.5 s.All Electrochemical tests used a Versa STAT 4 potentiostat/frequency to analyze the corrosion behavior of the MAO and MAO/Ni–P composite coatings. A three-electrode cell, with a saturated Hg/Hg2Cl2/KCl was used as the reference electrode and a Pt flake as the counter electrode, with the sample as the working electrode (a circle with a diameter of 1 cm is the measurement area). Potentiodynamic polarization specimens were measured in 3.5 wt.% NaCl solution. Before the test, all samples were tested for 600 s at the open circuit potential (OCP). The scanning range of the polarization curve is relative to the open circuit potential from −0.3V to +0.5 V. The scanning rate is 0.005 V/s and the 50 mV interval between the upper and lower corrosion potentials is used for the Tafel approximation method to calculate the corrosion current density. An EIS test was performed after immersion in OCP for 10 min at frequencies of 100 kHz to 10 mHz and using a sinusoidal AC perturbation of 10 mV amplitude. A salt spray test was conducted according to ASTM B-117 [54–58]. The specimens were placed in a 5 wt.% NaCl solution at a pH of 6.5–7.2 and atomized into a mist. The heating chamber was maintained at 35 °C.The microstructure results for the MAO coating are shown in Fig. 1 . The coating consists of two layers, as shown in Fig. 1(a). The outer-porous layer is located on the surface and has many pores. The second layer, called the compact layer, is between the outer layer and the substrate. The coating thickness is about 6–8 μm. The SEM image for the surface of the MAO coating in Fig. 1(b) displays that the micro-pores size is similar or the same. The micro-pores form when the voltage penetrates through the coating, and their size is approximately 1.0 ± 0.3 μm.The corrosion resistance of the MAO coating is measured using an electrochemical test in a 3.5 wt.% NaCl solution. Bare AZ31B was also used in the test for comparison. Polarization curves and an EIS test for bare AZ31B and the MAO-coated AZ31B specimens are respectively shown in Figs. 2 (a) and (b). The MAO-coated specimen has better corrosion resistance than the uncoated sample. In terms of the corrosion rate, the results indicate that the corrosion resistance of the coated specimen (1.01 × 10−8 A/cm2) is better than that for bare AZ31B (3.66 × 10−5 A/cm2). These coatings reduce the corrosion current by approximately 2–3 orders of magnitude in the potentiodynamic polarization tests.For this EIS measurement, the absolute impedance (|Z| f = 0.01 Hz ) of the MAO-coated AZ31B specimens increases to 4.3 × 106 Ω‧cm2 from the value of 1.7 × 103 for bare AZ31B, as shown in Fig. 2(b). This increase is consistent with the result for the polarization curves in Fig. 2(a). The MAO-coated AZ31B specimens show superior corrosion resistance. SST was used to determine the corrosion resistance. The corrosive medium attacks weak points on the sample surface. If bare AZ31B is subject to SST for 96 h, the corroded area fraction is 100%, as shown in Fig. 3 (a). Fig. 3(b) shows the overall morphology of the MAO-coated AZ31B specimens after 96 h of SST. The results show that the MAO-coated AZ31B specimens have excellent corrosion resistance and the corroded area fraction is approximately 0%.The characteristics of the initial deposition stage (30 s, 3 min, and 10 min) are studied to determine the role of fluoride in the electroless Ni–P plating solution. Fig. 4 shows the substrate after MAO treatment and compares the surface morphologies for the two chemical electroless Ni–P plating solutions, which are used to form a MAO/Ni–P composite coating for 30 s. Fig. 4(a) shows the SEM image of the MAO specimens that are treated in the fluoride-free solution for 30 s. The MAO surface has a discontinuous coating. The micro-pores are larger than the original and the inside of the discharge holes feature rosettes. Fig. 4(c) shows the backscattered electron image (BSE) of MAO samples treated in the fluoride-free solution for 30 s. The dark matrix in the SEM/BSE image is the Mg chemical compound with a low atomic number and the bright matrix is the Ni chemical compound, which has a higher atomic number. MAO specimens treated in the fluoride-containing solution for 30 s exhibit the MAO surface completely covered with a Ni–P coating, shown in Figs. 4(b) and (d). Table 2 shows the composition of the various electroless Ni–P plating treatments for MAO specimens for SEM/EDS taken from the areas marked as 1, 2, 3 and 4 in Figs. 4(c) and (d). The coating formed in the presence of fluoride contains more Ni species than that formed in the absence of fluoride. There is only a minimal amount of Ni in Area 1. Fig. 5 shows SEM surface morphology of samples that undergo electroless Ni–P plating for 3min and 10 min, with and without the addition of fluoride. The surface morphology shows an incomplete coating, as shown in Fig. 5(a). Due to the fluorine-free protection, the MAO surface is incomplete during the initial 30 s, so the electroless Ni–P plating solution invades the substrate and the Mg alloy produces a displacement reaction [59–62]. As the plating time increases, the replacement film continues to form and the MAO coating at the damaged section becomes more broken. Fig. 5(a) shows the disintegration of the middle MAO layer and the outer ring is a Ni–P layer that grows unevenly. Fig. 5(c) shows the MAO specimens treated in the electroless Ni–P plating bath with fluoride for 3 min. The Ni–P coating gradually covers the MAO layer and this coating is uniform and smooth. The surface of the MAO features only some micro-pores that are not filled with the Ni–P coating. When treatment time increases to 10 min, the surface morphology of the MAO specimens treated in the electroless Ni–P plating bath without fluoride shows an imperfect film, as shown in Fig. 5(b). The surface film peels off due to poor adhesion. MAO specimens treated in the fluoride-containing solution for 10 min are shown in Fig. 5(d). The MAO surface is completely sealed and the Ni–P nodules are larger than the nodules in the fluoride-free bath solution. The joints of the nodules are closely attached and the surface is complete and evenly coated. Fig. 6 shows the cross-sectional SEM image of a sample that undergoes electroless Ni–P plating for 3min and 10 min, with and without fluoride addition. Fig. 6(a) shows that the MAO layer is broken and the structure is destroyed. The Ni–P layer covered by the MAO upper layer is thinner and the plating layer features large undulations. The thickness is about 1.25 ± 0.2 μm. Fig. 6(b) shows that the MAO layer has a complete structure, which is conducive to the batching of nickel-phosphorus coatings. The thickness is 1.60 ± 0.1 μm. The coating becomes thicker with the addition of fluorine, which is consistent with the surface topography results.For a treatment time of 10 min, the cross-sectional SEM image of MAO specimens that are treated in an electroless Ni–P plating bath without fluoride is shown in Fig. 6(c). There is more significant internal corrosion of the MAO and there is thinning and embedding of the Ni–P layer. This is not conducive to nickel-phosphorus layer batching. The thickness of the coating is about 2.1 ± 0.1 μm. MAO specimens that are treated in the fluoride-containing solution for 10 min are shown in Fig. 6(d). The Ni–P coating is completely batched on the MAO coating and has good binding properties so the layer thickness is about 4.5 ± 0.2 μm.These cross-sectional characterizations show that the structure of the fluorine-containing composite coating is complete but the Ni–P layer and MAO coating which are obtained from fluoride-free bath solution do not achieve a good mechanical locking force. Previous studies [17,50,63] showed that there is a degradation of corrosion resistance for MAO-coated Mg alloy that is produced by electroless Ni–P plating. With the initiation of Ni–P deposition, H+ ions form from the oxidation of the reducing agent that is adsorbed on the inner layer surface to dissolve MgO. Therefore, H+ ions accelerate damage to the MAO coating and reduce its corrosion resistance.If a fluorine compound is formed on the MAO coating, it gives better protection. During the process of continuous nickel reduction, the fluorine-free solution damages the plating solution for about 10 min. This results in severe damage to the MAO layer due to the absence of nickel fluoride. Loss of the structure that retains the catalyst ink causes the nuclei to fall to the bottom of the beaker of plating solution and this reaction quickly destroys the plating solution, so the experiment is terminated.The plating layer of the test piece without nickel fluoride shows significant peeling and cracking. No fluorine is added because during electroless Ni–P plating, the MAO layer loses fluoride protection, so the MAO layer is not conducive to the Ni–P batch coating. The plating layer peels off at the bottom of the burned back area; regarding this situation, nickel ions form in the plating solution. Therefore, the plating solution was destroyed and blackened for about 10 min, so the experiment was not continued. Fig. 7 (a) shows the polarization curves for electroless Ni–P plating for 30 s, 3 min and 10 min, with and without fluoride addition. The polarization test results show the corrosion potential (E corr ) and the corrosion current density (i corr ) in Table 3 . Regardless of the treatment time, the corrosion resistance of the fluorine-containing treatment is far better than that which does not use fluorine. Fig. 7(b) represents the Bode plots for various electroless Ni–P plating samples using the same experimental parameters and process as Fig. 7(a). The Bode plots show that the absolute impedance of the fluorine-containing treatment is higher than that for coatings without fluoride. The absolute impedance (|Z| f = 0.01   Hz ) results are also listed in Table 3. The SEM surface morphology is shown in Figs. 4 and 5. The fluorine-containing treatments produce fewer defects than coatings without fluoride. The NaCl solution corrodes and penetrates through the grain boundaries and reacts with the electroless Ni–P plating layer. If the defects on the electroless Ni–P plating layer are obvious, the electroless Ni–P plating corrodes more significantly. Fig. 8 shows the complete process for an electroless Ni–P plating time of 40 min, with and without fluoride addition. Figs. 8(a) and (c) show that the MAO porous layer and the dense layer are damaged and loose and have no structure, leaving only the partially replaced nickel-phosphorus nodules interspersed in the defects. MAO specimens that are treated in a fluoride-containing solution for 40 min are shown in Figs. 8(b) and (d). The Ni–P coating is intact, the surface is flat and the boundaries of the Ni–P nodules are tightly connected, so this Ni–P coating is uniform and balanced, with a thickness of 6.60 ± 0.4 μm. Fig. 9 shows the EDS mapping results for the surface of the test piece for electroless Ni–P plating for 40 min. The broken morphology of the discontinuous Ni–P coating is verified by the presence of O and Mg to be a MAO layer. The Ni signal and the middle part are shown in Fig. 9(a). The Ni–P coating is uniformly and densely coated, no MAO layer is exposed and both Ni and P signals are shown in Fig. 9(b). The surface composition shows that the non-nickel fluoride damages the MAO coating, causing the Ni–P coating to exhibit a discontinuous film structure. On the other hand, the fluoride-containing treatment maintains the integrity of the MAO coating; thus, a continuous and dense Ni–P coating forms. Fig. 10 shows MAO specimens treated in an electroless Ni–P plating bath for 40 min after 96 h SST. The Ni–P layer is not completely deposited on the surface of the MAO coating when a fluoride-free solution is used, so there is severe corrosion. MAO specimens treated in the fluoride-containing solution for 40 min show only one corrosion spot after 96 h SST. The addition of fluorine affects the electroless Ni–P plating.A previous study by the authors [17] used a pull-off test [64–66] to verify the adhesion of Ni–P coatings on MAO-coated samples. The pull-off results are listed in Table 4 . The fluoride-containing treatment produces better adhesion than the fluoride-free treatment and the surface morphology and the integrity of the micro-pores is maintained. The uniform initial particle coating is also conducive to the subsequent coating of the Ni–P layer and a mechanical combination with the MAO coating, as shown by the microstructure results. The fluoride-containing treatment protects the MAO-coated AZ31B alloy. The addition of fluoride in an electroless Ni–P plating bath gives good mechanical strength and better corrosion resistance for the MAO coating.XRD patterns were evaluated to determine the crystal structure of Ni–P coatings on MAO-coated samples. The results are shown in Fig. 11 . The XRD results for the MAO coating for the different electroless Ni–P plating solutions for 30 s show that the main structure of the coatings is Mg, MgO, MgF2 and NaMgF3. However, other structures appear in the MAO coating for the fluoride-containing solution, with signals for peaks at 33.48° and 47.04° being attributed to NaMgF3. The coating time is only 30 s, so insufficient elemental Ni is deposited to be detected in the XRD. However, the SEM/EDS and TEM/EDS analyses for all coating times verify the existence of elemental Ni. The XRD analysis shows that there is a signal for MgF2, as shown in Fig. 11. The chemical reaction produces NaMgF3 and MgF2 with orthogonal, tetragonal and cubic crystal structures, respectively. The lattice parameters for NaMgF3 are a = 5.3603 Å, b = 5.4884 Å, and c = 7.666 Å. The XRD semi-quantitative analysis shows that, due to the small number of particles that is produced in the initial reaction, the film is not completely formed, so the peak amplitude is small. The initial reaction for the fluoride-containing solution also produces a NaMgF3 cubic lattice. The EDS composition analysis also shows that the square morphology of the initial surface is MgF2 and NaMgF3 cubic crystal.In order to confirm the MgF2 and NaMgF3 that are observed by XRD, TEM was used for microstructure analysis. Fig. 12 shows the cross-sectional TEM images for MAO specimens that are treated in an electroless Ni–P plating bath with fluoride for 30 s MgF2 particles and NaMgF3 cubic lattices are present on the surface and the inner wall of the hole over a large area, which grow with nickel and phosphorus particles. For a specific area of the inner wall of the hole, an EDS dot analysis was performed on the particles that are distributed in the hole.The elemental composition is shown in Table 5 . The initial reaction containing fluorine produces MgF2 particles and NaMgF3 cubic lattice and Ni–P particles. When fluorine is added in the Ni–P solution, the MAO layer morphology is protected and the integrity of the discharge holes is maintained. The uniform initial particle coating is also beneficial to the subsequent coating of Ni–P and allows mechanical combination with the MAO layer.The microstructural characterizations show that the fluoride ions in the electroless Ni–P plating bath are crucial. The initial reaction for electroless Ni–P plating produces H+ ions, which dissolve the MAO layer to produce Mg ions. The fluoride ions, sodium ions and magnesium ions in the electroless Ni–P plating solution form NaMgF3, which prevent the MAO layer from continuing to dissolve, which is beneficial to the subsequent batch coating of the Ni–P coating.The generated H+ ions corrode the MAO layer and MgO is converted to Mg(OH)2, so the MAO is less resistant to corrosion. Fluoride ions in the electroless Ni–P plating bath form MgF2 and NaMgF3 thin films to protect the MAO layer but the low fluorine content does not allow the thin film to form quickly; consequently, there appears an uneven coating and poor adhesion of the Ni–P coating. Corrosive mediums can easily penetrate the substrate through defects, so the coating lacks protection and pitting corrosion ensues. As the time for SST increases, corrosion increases the MAO layer expands. Furthermore, the electroless Ni–P coating peels off and loses its ability to protect. Excessive fluorine content also leads to excessive growth of NaMgF3, so the film becomes discontinuous and breaks.Hydrogen ions continue to corrode the MAO layer through the defects. The chloride ion is a strong adsorptive anion, which easily replaces oxygen and water molecules and is preferentially adsorbed onto the surface of the electroless Ni–P plating layer. Ni–P ions are in a dynamic balance: they are destroyed to form soluble NiCl2, which leads to the destruction of the coating morphology and the production of corrosion points. The formula for this process is: (1) Ni → Ni2+ + 2e– (2) Ni2+ + 2Cl– → NiCl2 The schematic diagram is shown to determine the configuration of the complex and the mechanism for MAO ceramic growth in Fig. 13 . At the beginning of the electroless Ni–P plating reaction, the hydrogen ions that are generated at the interface have a detrimental effect and the pH value at the interface reaction decreases, so the MAO layer corrodes and the surface morphology is destroyed. If there is no added fluorine protection in the electroless Ni–P plating solution, the MAO becomes loose and the catalyst does not flow in the hole (Fig. 13), so the electroless Ni–P plating solution reaches the substrate and a displacement reaction. It then completely coats the electroless Ni–P plating and the MAO layer. This coating adheres poorly and peels off; thus, corrosive media infiltrate from the cracks and the peeled areas on the coating. Finally, there is severe corrosion of the substrate, as shown in Table 4.If the fluorine content is insufficient in the electroplating bath, the resulting MgF2 particles and the cubic lattice of NaMgF3 are unevenly distributed and some MAO is not protected, resulting in defects. The subsequent electroless Ni–P coating layer grows along the grains; accordingly, so there are undulations and uneven coating in some areas. The uneven growth of the coating of the electroless Ni–P plating creates cracks, so corrosive media flow from micro-defects and corrosion occurs.This study determines the effect of fluoride on the formation of an electroless Ni–P plating film on MAO-coated AZ31B Mg alloy using Ni–P plating solutions with/without fluoride.Preliminary observation of the surface of electroless Ni–P plating for a short period of time (30 s, 3 min and 10 min) shows that the initial surface of the MAO specimens treated in the fluoride-containing solution contains MgF2 particles and NaMgF3 cubic lattice. MgF2 particles and NaMgF3 cubic lattice distribution impact the Ni–P coating during the electroless plating process. Hydrogen that is formed at the interface during the electroless coating process reduces the pH value. The fluoride-free electroless Ni–P plating solution induces the MAO layer to become loose due to a lack of fluoride protection. Therefore, the MAO sample treated in a fluoride-free bath suffers peel-off of the coating and the electroless Ni–P plating does not cover all of the surface due to a lack of fluoride protection.On the contrary, the MAO sample treated in the fluoride-containing solution has the best corrosion resistance and only one corrosion spot after 48 h SST. With the initiation of Ni–P deposition, H+ ions are formed by the oxidation of the reducing agent which is adsorbed onto the inner layer surface and dissolve MgO. H+ ions accelerate damage to the MAO coating and reduce its corrosion resistance. This study demonstrated that the formation of a fluorine compound on the MAO coating protects the coating and the electroless Ni–P plating coating completely covers the surface.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 study was financially supported by the Ministry of Science and Technology of Taiwan, Republic of China, under Grant No.MOST 109-2221-E-606 -009 -MY3.

This study adds fluoride to the electroless nickel-phosphorus (Ni–P) plating solution to prevent the deterioration of MAO-coated AZ31B Magnesium alloy after contact with an electroless plating bath. During the electroless Ni–P plating process, fluoride reacts with Ni2+ ions and the MAO coating to form interphases (NaMgF3), which exhibit good bonding and corrosion resistance. NaMgF3 buffers H+ ions formed from the initiation of Ni–P deposition, preventing the interface of materials from damaging the MAO coating with H+ ions. As immersion time increases, nickel is scattered over the coating. The fundamental data for MAO/Ni–P coated AZ31B Mg alloy determines whether there is fluoride in the electroless Ni–P plating solution. The results show that the coating for a fluoride-containing solution is more resistant to corrosion than those in fluoride-free solution. The compositions, structure and morphology of the MAO/Ni–P coatings that formed for different working parameters are determined using energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The corrosion resistance of the MAO/Ni–P coatings is calculated in a 3.5 wt.% NaCl solution using a potentiodynamic polarization test, electrochemical impedance spectroscopy (EIS) and a salt spray test (SST).

C2H2 semi-hydrogenation to C2H4 is a commonly used method to remove small quantity of C2H2 impurities in C2H4-rich stream, in which C2H2 acts as an impurity that poisons the catalysts used subsequently for the polymerization of ethylene [1–3]. In C2H2 semi-hydrogenation, the most effective catalyst was found to be Pd-based catalysts owing to high C2H2 conversion occurred at low temperature, however, C2H4 selectivity still needs to be improved because of C2H4 over-hydrogenation to ethane [4–6]. Thus, the development of Pd-based catalysts with high selectivity and activity in C2H2 semi-hydrogenation is still highly expected.Nowadays, in order to effectively utilize the active components of the catalyst, the metal catalyst is usually dispersed on the support with high specific surface area. However, only a small part of surface metal atoms participate in the catalytic reaction and the atom utilization rate is low. Thus, small size of supported metal clusters have been reported due to high atom utilization and more unique catalytic performance, however, the particle size in the subnanometer range is sensitive to its structure with particular electronic properties. For example, Liu et al. [7] theoretically found that the size of Pd n clusters affects H2 dissociation activity on Pd n (n = 4,6,13,19,55) clusters. Mercedes et al. [8] calculated the LUMO and HOMO orbitals of Au1∼Au38 clusters, and predicted that Au1, Au3 and Au38 have the best activity. Zhang et al. [9] prepared the atomically dispersed Pt3 clusters anchored over the core–shell nanodiamond@graphene, which presented excellent catalytic performance for n-butane direct dehydrogenation at a temperature as low as 450 °C. On the other hand, for C2H2 semi-hydrogenation, Gluhoi et al. [10] observed that when Au particle size was less than 3 nm, Au/Al2O3 showed high C2H2 conversion and C2H4 selectivity. Abdollahi et al. [11] found that Pd2 cluster have higher activity than Pd12 cluster, indicating that Pd2 cluster is more suitable. Density functional theory (DFT) studies by Xiao et al. [12] found that H2 adsorption capacity on the graphene supported Pd n (n = 1–5) clusters was stronger than that on Pd (111) surface. Huang et al. [13] prepared single atom Cu catalyst supported by the nanodiamond-graphene, C2H2 conversion is 95%, C2H4 selectivity is 98%, and the catalyst has good stability. Shi et al. [14] experimentally synthesized Cu single atom and nanoparticles corresponding to the sizes of about 3.4, 7.3 and 9.3 nm over Al2O3 support using atomic layer deposition, indicating that a size decrease of Cu nanoparticle obviously reduces the activity of C2H2 semi-hydrogenation but gradually improves both C2H4 selectivity and durability. The experiments by Huang et al. [15] prepared Pd1/ND@G catalyst with the atomically dispersed Pd over a defective nanodiamond-graphene (ND@G), which showed significantly high C2H4 selectivity (90%) and C2H2 conversion (100%) in C2H2 semi-hydrogenation. Thus, the studies on small size of metal cluster presenting particular active sites are of great significance in C2H2 semi-hydrogenation.Recently, graphdiyne (GDY), a new allotrope carbon material including C atoms with sp and sp2 hybrid, has attracted broad attentions [16–21]. The C atoms of GDY show a π–π conjugate system, which is highly delocalized in the whole planar framework [16,22,23]. Moreover, in comparison with grapheme, GDY has a unique pore structure to provide abundant adsorption site and more open storage space for molecular adsorption. As shown in Fig. 1 , the basic geometry of GDY is 6-membered ring (6 MR) and 18-membered ring (18 MR). The 18 MR structure provides a natural framework for anchoring metal clusters through a strong metal carbon covalent bond, thus forming a stable and isolated structure. For example, Lu et al. [24] systematically examined the adsorption of single atom Pd, Pt, Rh or Ir on the GDY, and found that the single atom was favorable for embedding into 18 MR of GDY and combining with four carbon atoms. The adsorption and diffusion behavior of Au, Cu, Fe, Ni or Pt atoms on the GDY were also researched by Lin et al. [25] using theoretical calculation, and found that the metal atoms have good thermal stability and very small overflow rate on the GDY even at 900 K. Chen et al. [26] studied CO oxidation reaction on the GDY supported Ag38 cluster, which showed the excellent performance due to the unique combination of the cluster and GDY.Further, the metal atoms and clusters supported by GDY are also ideal catalysts, which have been widely applied for the hydrogenation reaction of unsaturated hydrocarbons. For example, Xing et al. [27] investigated C2H2 semi-hydrogenation over the catalysts with the cluster MxN3-x (M, N = Ru, Os) supported by GDY, indicating that three atom metal clusters can be firmly anchored on the 18 MR of GDY, and effectively catalyze C2H2 semi-hydrogenation to produce C2H4; the synergistic effect between the metal cluster and GDY as a charge buffer contributes to the improvement of catalytic performance. However, up to now, few studies about C2H2 semi-hydrogenation focus on the catalysts with GDY supported small size of metal clusters PdxMy, meanwhile, the effects of cluster composition and size on the activity and selectivity are still unknown, which would provide an open space for designing highly-efficient GDY supported Pd-based catalyst in C2H2 semi-hydrogenation. Moreover, previous studies [28] showed that a Group IB metal (Cu, Ag or Au) doped into Pd to form Pd-based bimetallic alloys can well improve C2H4 selectivity in C2H2 semi-hydrogenation; meanwhile, Abdollahi et al. [29] theoretically demonstrated that among the Nin(n = 2–10) nanoclusters, Ni6 nanocluster could be used as a suitable catalyst in C2H2 semi-hydrogenation. Wongwaranon et al. [30] experimentally observed that C2H4 selectivity was improved on the Pd/Ni-modified α-Al2O3 catalysts in the presence of Ni atoms. Jin et al. [31] experimentally claimed that PdNi catalyst possessed high selectivity and stability for C2H2 semi-hydrogenation. Thus, the bimetallic PdM(M = Cu, Ag, Au, Ni) catalysts can be well applied in C2H2 semi-hydrogenation to improve its catalytic performance.In this study, a large number of PdxMy/GDY catalysts using GDY supported different sizes of PdxMy (M = Cu, Ag, Au, Ni; x + y = 1–3) bimetallic clusters have been for the first time designed; Then, the underlying mechanism of the hydrogenation process of C2H2 on the PdxMy/GDY catalysts were fully investigated using DFT calculations, the obtained results were expected to illustrate the effects of cluster composition and size in the PdxMy/GDY catalysts on the activity and selectivity of C2H4 formation. This study would provide a good clue for designing and screening out the potential catalysts with GDY supported small sizes of PdxMy clusters and other metal clusters in C2H2 hydrogenation process.Dmol3 code [32,33] in Materials Studio 8.0 were carried out for the performance of all DFT calculations. The exchange-correlation functional PBE with generalized gradient approximation (GGA) [34,35] was used. The double-numeric polarized (DNP) basis set was used to expand valence electron functions [36,37]. The van der Waals correction (DFT-D) [38] method was used to correct the weak adsorption free energy underestimated by the GGA functional. The all electron and effective core potential (ECP) basis set were used to treat the non-metal atoms and the inner electrons of metal atoms, respectively [39,40]. The k-point 4 × 4 × 1 was considered together with a smearing of 0.005 Ha for all calculations. The complete LST/QST techniques were used to obtain transition state of an elementary step [41,42], which was confirmed by the methods of Frequency analysis and TS confirmation implemented in Dmol3 code.A lots of studies showed that the type of active metals [43], the temperature [44] and the H2/C2H2 ratio [45–48] in C2H2 semi-hydrogenation can all obviously affect the formation of green oil over Pd-based catalyst, suggesting that the high temperature and high H2/C2H2 ratio can prevent “green oil” formation leading to the deactivation of polymerization catalysts [6,49,50]. This study only focus on the investigations about the effect of the types of active metals including the metal cluster composition and size on the activity and selectivity of C2H2 hydrogenation process, as a result, the effect of green oil formation on the activity and selectivity of C2H2 hydrogenation process are expected to be ignored, a high temperature of 425 K and a high H2: C2H2 ratio of 10 corresponding to C2H4, C2H2 and H2 partial pressures of 0.89, 0.01 and 0.1 atm were performed. Thus, all energies in the process of adsorption and reaction were the values at 425 K in this study (see details in the Supplementary Material).For the protocell of GDY, see Fig. 1, the C–C bond length on the 6 MR is 1.430 Å, the C−C and C=C bond lengths on the 18 MR are 1.390 and 1.232 Å, respectively, which agree with the reported values of 1.430, 1.390 and 1.240 Å [24,51]. For the supercell of GDY, a single-layer p (2 × 2) structure with a 30 Å vacuum thickness was constructed, and the lattice constant obtained through structure optimization is 18.880 Å. During the calculations, the edge C atoms of GDY denoted as the red balls in Fig. 1(b) were fixed; the C atoms of two 18 MR were fully relaxed.For PdxMy/GDY catalysts, as shown in Fig. 2 , five types of single metal catalysts, Pd, Cu, Ag, Au and Ni, are used to form metal clusters with the atom numbers of one, two and three, respectively; four types of bimetallic catalysts, Cu, Ag, Au, Ni alloyed with Pd to form PdxMy bimetallic clusters with the atom numbers of two and three, respectively. As a result, there are twenty-seven kinds of PdxMy/GDY catalysts, named as Pd1/GDY, Pd2/GDY, Pd3/GDY, Cu1/GDY, Cu2/GDY, Cu3/GDY, Pd1Cu1/GDY, Pd1Cu2/GDY, Pd2Cu1/GDY, Ag1/GDY, Ag2/GDY, Ag3/GDY, Pd1Ag1/GDY, Pd1Ag2/GDY, Pd2Ag1/GDY, Au1/GDY, Au2/GDY, Au3/GDY, Pd1Au1/GDY, Pd1Au2/GDY, Pd2Au1/GDY, Ni1/GDY, Ni2/GDY, Ni3/GDY, Pd1Ni1/GDY, Pd1Ni2/GDY and Pd2Ni1/GDY, respectively. Further, the interaction between PdxMy cluster and GDY is calculated [52,53] (see details in Table S1).C2H2 semi-hydrogenation follows the continuous hydrogenation of hydrocarbons [54]. Hydrogenation of C2H2 may occur by three routes [55–58], as presented in Fig. 3 , the first is that C2H2(ad) is successively hydrogenated to form C2H4(ad) via C2H3(ad) intermediate, then, C2H4(ad) desorb from the catalyst surface, which is the desired route for C2H2 semi-hydrogenation defined as C2H4 desorption route. The latter two is that C2H2(ad) hydrogenation via the common C2H3(ad) intermediate produces C2H4(ad) or CHCH3(ad), which could be further hydrogenated to form ethane via C2H5(ad) intermediate; these two routes, called as C2H4 hydrogenation route and CHCH3 hydrogenation route, are expected to be suppressed to facilitate the semi-hydrogenation of C2H2 to form C2H4.For PdxMy/GDY catalysts, to identify whether C2H4 desorption route prefers to occur in C2H2 semi-hydrogenation, firstly, it is necessary to calculate the priority between C2H4 desorption and its hydrogenation, if C2H4 desorption is more favored than C2H4 hydrogenation, namely, C2H4 desorption route is superior to C2H4 hydrogenation route; Then, judging whether C2H4 desorption route also prefers to occur compared to CHCH3 hydrogenation route. Based on above two aspects of analysis, we can confirm the catalysts with better C2H4 selectivity, on which C2H4 desorption route is the dominant among three routes.C2H4 feed produced by steam-cracking process is known to contain about 0.1–1% of C2H2 [59], only when C2H2 adsorption is stronger than C2H4 adsorption over the catalysts, the removal of trace C2H2 in C2H4-rich feed gas could be achieved on the catalysts. The intuitional comparison between C2H4 and C2H2 adsorption energies on PdxMy/GDY catalysts is shown in Fig. 4 (see details in Table S2 and Fig. S1). For H, C2H3, CHCH3 and C2H5 species, Fig. S2 and Table S3 give out the adsorption energies and stable configurations on above PdxMy/GDY catalysts.For PdxMy/GDY catalysts, Ag3/GDY is seriously deformed and unstable when the C2H2 or C2H4 species were adsorbed. On the Ag2/GDY and Ni3/GDY, C2H2 is not effectively adsorbed due to the weak physisoption (4.9 and 8.7 kJ mol−1). On the Ag1/GDY, Au1/GDY and Pd1Au2/GDY, the adsorption ability of C2H2 and C2H4 species are close (26.9 and 22.2 kJ mol−1, 125.4 and 129.9 kJ mol−1, 55.2 and 53.9 kJ mol−1), namely, trace C2H2 in C2H4-rich stream cannot be sufficiently adsorbed. However, as listed in Table S2, C2H2 has stronger adsorption ability than C2H4 on twenty-one kinds of PdxMy/GDY catalysts, including Pd1/GDY, Pd2/GDY, Pd3/GDY, Cu1/GDY, Cu2/GDY, Cu3/GDY, Pd1Cu1/GDY, Pd1Cu2/GDY, Pd2Cu1/GDY, Pd1Ag1/GDY, Pd1Ag2/GDY, Pd2Ag1/GDY, Au2/GDY, Au3/GDY, Pd1Au1/GDY, Pd2Au1/GDY, Ni1/GDY, Ni2/GDY, Pd1Ni1/GDY, Pd1Ni2/GDY and Pd2Ni1/GDY, namely, trace C2H2 to participate into the subsequent hydrogenation reaction can be sufficiently adsorbed in C2H4-rich stream.Based on above analysis, six kinds of Ag3/GDY, Ag2/GDY, Ni3/GDY, Ag1/GDY, Au1/GDY and Pd1Au2/GDY catalysts are excluded in C2H2 semi-hydrogenation on the basis of the weak C2H2 physisoption or the close adsorption ability between C2H2 and C2H4 species. Further, on other twenty-one kinds of PdxMy/GDY catalysts, C2H2 has stronger adsorption than C2H4, which favors the hydrogenation of C2H2.For PdxMy/GDY catalysts with stronger adsorption ability of C2H2 than C2H4, it is needed to firstly identify the priority of C2H4 between its desorption and hydrogenation, as listed in Table 1 (see the structures in Fig. S3).For PdxMy/GDY catalysts, C2H4 + H→C2H5 is more favored or competitive compared to C2H4 desorption on the Pd2/GDY, Pd3/GDY, Cu1/GDY, Pd1Cu1/GDY, Pd1Ag1/GDY, Pd2Ag1/GDY, Pd1Au1/GDY and Ni1/GDY catalysts, which easily leads to ethane, thus, these eight kinds of PdxMy/GDY catalysts exhibit poor C2H4 selectivity, CHCH3 hydrogenation route does not need to be considered. However, as listed in Table 1, C2H4 desorption would be superior to its hydrogenation to C2H5 on thirteen kinds of PdxMy/GDY catalysts, including Pd1/GDY, Cu2/GDY, Cu3/GDY, Pd1Cu2/GDY, Pd2Cu1/GDY, Pd1Ag2/GDY, Au2/GDY, Au3/GDY, Pd2Au1/GDY, Ni2/GDY, Pd1Ni1/GDY, Pd1Ni2/GDY and Pd2Ni1/GDY, which would be favor C2H2 semi-hydrogenation to gaseous C2H4, and suppress its over-hydrogenation to ethane.As mentioned above, C2H4 preferred to be desorption instead of its hydrogenation on thirteen kinds of PdxMy/GDY catalysts, namely, C2H4 desorption route become dominant compared to C2H4 hydrogenation route, thus, we need to further identify whether C2H4 desorption route also prefers to occur compared to CHCH3 hydrogenation route on these thirteen kinds of PdxMy/GDY catalysts (see details in Figs. S4–S15). The energy profiles of C2H2 semi-hydrogenation on Pd1/GDY catalyst is shown in Fig. 5 as an example.On Pd1/GDY catalyst, C2H2 + H→C2H3 has the activation barrier of 38.4 kJ mol−1, and it is exothermic by 73.9 kJ mol−1; starting from C2H3 intermediate, C2H4 formation is superior to CHCH3 formation in kinetics (1.0 vs. 116.4 kJ mol−1); further, C2H4 desorption would be much preferred kinetically compared to C2H4 + H→C2H5 (65.1 vs. 326.5 kJ mol−1), suggesting that Pd1/GDY catalyst is in favor of C2H2 semi-hydrogenation to produce gaseous C2H4. Similarly, the easy formation of gaseous C2H4 also occurs on the Cu2/GDY (Fig. S4), Cu3/GDY (Fig. S5), Pd1Cu2/GDY (Fig. S6), Pd2Cu1/GDY (Fig. S7), Pd1Ag2/GDY (Fig. S8), Au2/GDY (Fig. S9), Au3/GDY (Fig. S10), Pd2Au1/GDY (Fig. S11) and Pd1Ni2/GDY catalysts (Fig. S14). However, CHCH3 formation leading to ethane is much easier than C2H4 formation on the Ni2/GDY (Fig. S12, 62.0 and 105.7 kJ mol−1), Pd1Ni1/GDY (Fig. S13, 70.3 and 137.9 kJ mol−1) and Pd2Ni1/GDY catalysts (Fig. S15, 40.9 and 115.2 kJ mol−1), as a result, these three types of catalysts present poor C2H4 selectivity due to the formation of ethane.The energy difference between C2H4 hydrogenation and its adsorption was used to quantitatively describe the selectivity of C2H4 (ΔG s) using the Eq. (1), which has been widely applied in many previous studies [6,28,60,61]. (1) ΔG s = ΔG a−|G ads| Where G ads and ΔG a correspond to C2H4 adsorption free energy and the activation barrier of C2H4 hydrogenation to C2H5, respectively; the positive and large value of ΔG sel means that the catalyst exhibits better C2H4 selectivity. As mentioned above, ten kinds of PdxMy/GDY catalysts have better C2H4 selectivity, including Pd1/GDY, Cu2/GDY, Cu3/GDY, Pd1Cu2/GDY, Pd2Cu1/GDY, Pd1Ag2/GDY, Au2/GDY, Au3/GDY, Pd2Au1/GDY and Pd1Ni2/GDY.Meanwhile, according to the two-step model widely used in the previous work [56,58,62,63] (see details in the Supplementary Material), the reaction rate of C2H4 formation was calculated to evaluate the catalytic activity on these ten kinds of PdxMy/GDY catalysts.As listed in Table 2 , the selectivity of C2H4 over ten kinds of PdxMy/GDY catalysts, Pd1/GDY, Cu2/GDY, Cu3/GDY, Pd1Cu2/GDY, Pd2Cu1/GDY, Pd1Ag2/GDY, Au2/GDY, Au3/GDY, Pd2Au1/GDY and Pd1Ni2/GDY, are 261.4, 114.5, 162.6, 309.1, 67.1, 19.3, 384.2, 54.1, 104.7 and 85.5 kJ mol−1, respectively, the corresponding activity of C2H4 formation are 1.69 × 108, 2.94 × 106, 7.63 × 10−4, 3.45 × 10−1, 3.45 × 10−1, 4.25 × 109, 1.55 × 108, 8.71 × 10−28, 2.16 × 10−1, 2.26 × 107 and 3.63 × 1010 s−1 site−1, respectively.Further, H2 dissociation may affect the activity of C2H4 formation; H2 adsorption and dissociation were calculated on above ten kinds of PdxMy/GDY catalysts with better C2H4 selectivity (see Table S4 and Fig. S19). Our results show that only on the Au3/GDY, Pd2Au1/GDY and Pd1Ni2/GDY catalysts, the activation free energies of the rate-determining in C2H4 desorption route is lower than those of H2 dissociation, namely, H2 dissociation affects the catalytic activity toward C2H2 semi-hydrogenation to C2H4. Whereas it does not affect the catalytic activity of C2H4 formation on other seven types of Pd1/GDY, Cu2/GDY, Cu3/GDY, Pd1Cu2/GDY, Pd2Cu1/GDY, Pd1Ag2/GDY and Au2/GDY catalysts. Moreover, since this study only focus on the investigations about the effect of active metal types including the metal cluster composition and size on activity and selectivity of C2H2 semi-hydrogenation process, the effect of the initial H2 dissociation activity on the catalytic performance of C2H2 semi-hydrogenation process will be considered in our next work.For GDY supported single-metal catalysts, firstly, when the supported metal is the single atom, only Pd1/GDY is favorable for C2H4 formation, while on the Cu1/GDY, Ag1/GDY, Au1/GDY and Ni1/GDY, C2H2 could not be effectively adsorbed or the over-hydrogenation of C2H4 to ethane occurs. Secondly, when the supported metal is the double and three atoms cluster, only the Cu and Au clusters (Cu2/GDY, Cu3/GDY, Au2/GDY, Au3/GDY) are favorable for C2H4 formation; while on the Pd, Ag and Ni clusters, C2H2 cannot be effectively adsorbed or C2H4 is inclined to be over-hydrogenated to ethane. Thus, only five kinds of single metal catalysts including Pd1/GDY, Cu2/GDY, Cu3/GDY, Au2/GDY and Au3/GDY are favorable for C2H2 semi-hydrogenation to form gas phase C2H4.For GDY supported bimetallic catalysts, firstly, when the supported metal is double atoms cluster, all catalysts including Pd1Cu1/GDY, Pd1Ag1/GDY, Pd1Au1/GDY, Pd1Ni1/GDY are not favorable for C2H2 semi-hydrogenation to C2H4. However, when the supported metal is three atoms cluster, only five kinds of the catalysts including Pd1Cu2/GDY, Pd2Cu1/GDY, Pd1Ag2/GDY, Pd2Au1/GDY and Pd1Ni2/GDY are favorable for C2H2 semi-hydrogenation to C2H4. Fig. 6 shows C2H4 selectivity and its formation activity over ten kinds of PdxMy/GDY catalysts favored the formation of gaseous C2H4 (Pd1/GDY, Cu2/GDY, Cu3/GDY, Pd1Cu2/GDY, Pd2Cu1/GDY, Pd1Ag2/GDY, Au2/GDY, Au3/GDY, Pd2Au1/GDY and Pd1Ni2/GDY), among them, the catalysts with high C2H4 selectivity are Au2/GDY (384.2 kJ mol−1), Pd1Cu2/GDY (309.1 kJ mol−1), Pd1/GDY (261.4 kJ mol−1) and Cu3/GDY (162.6 kJ mol−1), accordingly, C2H4 formation activity are 8.71 × 10−28, 3.45 × 10−1, 1.69 × 108 and 7.63 × 10−4 s−1 site−1, respectively, indicating that Pd1/GDY has the highest C2H4 formation activity, while Au2/GDY, Pd1Cu2/GDY and Cu3/GDY presents poor C2H4 formation activity. On the other hand, the catalysts with high C2H4 formation activity are Pd1Ni2/GDY (3.63 × 1010 s−1 site−1), Pd2Cu1/GDY (4.25 × 109 s−1 site−1), Pd1/GDY (1.69 × 108 s−1 site−1) and Pd1Ag2/GDY (1.55 × 108 s−1 site−1), accordingly, C2H4 selectivity are 85.5, 67.1, 261.4 and 19.3 kJ mol−1, respectively, thus, Pd1/GDY catalyst presents the highest C2H4 selectivity. As shown in Fig. S20, generally, there is not a seesaw effect for the activity and selectivity, only both Au2/GDY and Pd1Cu2/GDY have higher C2H4 selectivity (384.2 and 309.1 kJ mol−1), while these two catalysts have lower activity (8.71 × 10−28 and 3.45 × 10−1 s−1 site−1). On the contrary, Pd1Ni2/GDY, Pd2Cu1/GDY and Pd1Ag2/GDY catalysts have higher activity of C2H4 formation (3.63 × 1010, 4.25 × 109 and 1.55 × 108 s−1 site−1), while the lower C2H4 selectivity (85.5, 67.1 and 19.3 kJ mol−1).Based on above analysis, the composition and size of supported metal cluster PdxMy in PdxMy/GDY catalysts present the sensitivity toward the selectivity and activity of C2H2 semi-hydrogenation. Among them, taking C2H4 selectivity and its formation activity into consideration, GDY supported single atom Pd catalyst (Pd1/GDY) in this study should provide the best selectivity (261.4 kJ mol−1) and excellent catalytic activity (1.69 × 108 s−1 site−1) for C2H2 semi-hydrogenation to gaseous C2H4.Further, the catalytic origin of Pd1/GDY catalyst with the highest activity is revealed. The high C2H4 selectivity should be attributed to the structural confinement of single atom Pd in Pd1/GDY leading to the much weaker C2H4-π bonding interactions (65.1 kJ mol−1) compared to the stronger C2H4 adsorption on the large size of Pd55 cluster (189.2 kJ mol−1) [63]. The weaker C2H4-π bonding interactions do not facilitate C2H4 activation and hydrogenation. The C3H6-π bonding characteristics between C3H6 and V1/g-C3N4 catalyst were also obtained [64]. As a result, C2H4 hydrogenation (326.5 kJ mol−1) is much difficult than C2H4 desorption (65.1 kJ mol−1) on Pd1/GDY catalyst, the produced C2H4 will easily desorb from Pd1/GDY catalyst to become the dominant product. Meanwhile, compared to the large size of metal Pd55 cluster, the faster desorption rate of C2H4 on Pd1/GDY catalyst enhances C2H4 selectivity.As shown in Fig. 7 , the metal-support interaction (E MSI/kJ mol−1) of PdxMy/GDY catalyst showed that when E MSI value was weak (−200∼−400 kJ mol−1) or strong (−600∼−800 kJ mol−1), for example, Ag3/GDY (−236.0), Au1/GDY (−265.5), Ag2/GDY (−321.5), Pd1Au1/GDY (−322.6), Ag1/GDY (−338.5), Pd1Au2/GDY (−381.5), Pd1Ag1/GDY (−400.7), Pd1Ni1/GDY (−604.0), Ni2/GDY (−654.4), Pd3/GDY (−654.8), Pd2Ni1/GDY (-667.8) and Ni3/GDY (−772.1), these catalysts could not adsorb C2H2 preferentially or were not conducive to C2H4 formation; however, when the values of E MSI were moderate (−400∼−600 kJ mol−1), the catalyst could adsorb C2H2 preferentially and realize C2H4 formation, for example, Au3/GDY (−418.3), Cu2/GDY (−440.6), Pd1Ag2/GDY (−463.9), Pd1/GDY (−481.1), Pd2Au1/GDY (−485.5), Pd1Cu2/GDY (−561.4) and Pd2Cu1/GDY (−595.1) catalysts are all favorable for C2H4 production.As listed in Table 2, Bader charge indicates that the Pd, Cu, Ag, Au or Ni atoms transfer electrons to the C atom of GDY. For PdxMy/GDY with better C2H4 selectivity, when the average charge of metal atoms is small, such as Au2/GDY (0.032), Pd1Cu2/GDY (0.112) and Au3/GDY (0.135), these catalysts have low C2H4 formation activity of 8.71 × 10−28, 3.45 × 10−1 and 2.16 × 10−1 s−1 site−1, respectively. When the average charge of metal atoms is large, such as Cu3/GDY (0.349) and Cu2/GDY (0.387), both catalysts also have low C2H4 formation activity of 7.63 × 10−4 and 2.94 × 106 s−1 site−1, respectively. Only when the average charge of metal atoms is moderate, such as Pd1Ag2/GDY (0.148), Pd1Ni2/GDY (0.156), Pd2Cu1/GDY (0.177), Pd2Au1/GDY (0.228) and Pd1/GDY (0.277), these catalysts have higher C2H4 formation activity of 1.55 × 108, 3.63 × 1010, 4.25 × 109 and 2.26 × 107 s−1 site−1, respectively. Thus, the average charge amount of metal atom is closely related to C2H4 formation activity, namely, the average charge amount of metal atom is less or more, C2H4 formation activity is low; whereas the average charge amount of metal atom is moderate, C2H4 formation activity is high.On the other hand, Huang et al. [65] implied that coke formation on the single-atom Pd1/C3N4 is markedly inhibited compared to Pd NP catalysts in C2H2 hydrogenation, the geometric effect improved coking-resistance. The oligomerization of C2H2 can be avoided on Pd1/ND@G catalyst, which is attributed to the pyramidal geometry between Pd and C atoms [15]. Indeed, C2H2 polymerization to form green oil or coke requires multiple adjacent adsorption sites that cannot be available for Pd single atom, thus, coke formation is suppressed on the Pd single atoms compared to that on the Pd NP catalyst. Similarly, Pd1/GDY catalyst in the present study only has a single active site, which can suppress the green oil or coke formation due to its unique geometric effect. Further, previous DFT studies [6] have revealed that Pd (111) with surface or subsurface C atom make the shift of the d-projected density of states of the surface Pd atoms to lower energy level, which weakens C2H4 adsorption compared to those on clean Pd (111) surface, meanwhile, the activity of Pd (111) surface slightly increase in the presence of subsurface carbon species. As a result, C2H4 desorption becomes easier, and the selectivity of C2H4 increase in the presence of surface and subsurface carbon. The projected density of states (pDOS) plots for the d-orbitals of Pd atom on the Pd (111)-surface C, Pd (111)-subsurface C, Pd1/GDY and Pd (111) catalysts are calculated, as shown in Fig. S21, similar to Pd (111) in the presence of surface or subsurface carbon species, compared to the pure Pd (111) surface, the shift of the d-projected density of states for surface Pd atoms to lower energy level also occur on Pd1/GDY, which also weaken C2H4 adsorption to increases its selectivity.To deeply illustrate the excellent activity and selectivity of Pd1/GDY catalyst, the comparisons for the activity and selectivity of C2H4 formation between Pd1/GDY and the reported catalysts in the literatures were carried out. On Pd1/GDY catalyst, C2H4 selectivity was 261.4 kJ mol−1, both C2H2 + H→C2H3 and C2H3 + H→C2H4 reactions have the activation barriers of 36.0 and 1.0 kJ mol−1, respectively; the overall barrier of C2H2 + 2H→C2H4 was 38.4 kJ mol−1.As shown in Fig. 8 (a), for the Pd-based intermetallic compounds (IMCs), the single atom Pd active site can be completely isolated by the second metal, Zhou et al. [66] found that PdZn IMCs had highly active and selective for C2H2 semi-hydrogenation, DFT results showed that C2H4 selectivity was 36.0 kJ mol−1, meanwhile, the activation barriers of C2H2 + H→C2H3 and C2H3 + H→C2H4 were 55.0 and 56.0 kJ mol−1, respectively. Feng et al. [67] found that the single atom Pd active site in PdIn IMCs had C2H4 selectivity of 34.0 kJ mol−1; the activation barriers of C2H2 + H→C2H3 and C2H3 + H→C2H4 were 36.0 and 34.0 kJ mol−1, respectively. Sandoval et al. [68] calculated that the activation barriers of C2H2 + H→C2H3 and C2H3 + H→C2H4 on PdGa IMCs were 70.0 and 75.0 kJ mol−1, respectively. Hence, compared to Pd1/GDY catalyst in this study, the activity and selectivity of C2H4 formation over these reported Pd-based IMCs catalysts are lower.As shown in Fig. 8(b), for the single atom Pd doped into metal surface, Zhang et al. [57,58] studied the hydrogenation of C2H2 on the single atom Pd-doped Cu(111), Cu(211) or Cu2O(111) surfaces, C2H4 selectivity and the overall activation barrier of C2H2 + 2H→C2H4 on Pd1/Cu(111) are 42.6 and 47.5 kJ mol−1, respectively; those on Pd1/Cu(211) are 36.4 and 78.8 kJ mol−1, respectively; on Pd1/Cu2O(111), C2H2 is easily over-hydrogenated to ethane via CHCH3 intermediate. On the other hand, for the single Pd atom doped-Cu13, Cu38 or Cu55 clusters [69], C2H4 is easily hydrogenated to ethane. Wang et al. [70] obtained that the single atom Pd-doped Ag surface can facilitate the over-hydrogenation of C2H4 to produce ethane. Yang et al. [6] found that when the surface coverage of Cu, Ag or Au on Pd (111) is increased to present the single atom Pd, C2H4 was prone to be over-hydrogenated to ethane. Yang et al. [71] showed that trimetallic PdAg2Au/Pd (111) surface showed C2H4 selectivity of 24.0 kJ mol−1, the activation barriers of C2H2 + H→C2H3 and C2H3 + H→C2H4 were 41.0 and 58.0 kJ mol−1, respectively. Thus, the selectivity and activity of C2H4 formation over the catalysts doping the single atom Pd into metal surface are still lower than those on Pd1/GDY catalyst.As shown in Fig. 8(c), for the supported single atom Pd catalysts, Wei et al. [72] experimentally prepared the thermodynamically stable Pd1–N4 structure with the single atom Pd anchored on the defects of nitrogen-doped carbon, then, DFT results show that C2H4 selectivity is 91.0 kJ mol−1; the activation barriers of C2H2 hydrogenation and C2H3 hydrogenation are 37.0 and 94.0 kJ mol−1, respectively. The experiments by Huang et al. [15] prepared the atomically dispersed Pd over a defective nanodiamond-graphene (Pd1/ND@G catalyst), DFT results show that C2H4 selectivity is 51.0 kJ mol−1; the activation barriers of C2H2 + H→C2H3 and C2H3 + H→C2H4 are 110.0 and 85.0 kJ mol−1, respectively. Huang et al. [13] experimentally prepared Cu1/ND@G catalyst, which also exhibits excellent catalytic performance for C2H2 + 2H→C2H4, and DFT results show that C2H4 selectivity is only 19.0 kJ mol−1; the activation barrier of C2H2 hydrogenation to C2H3 is 136.0 kJ mol−1. Zhou et al. [73] found that atomically dispersed Pd on nitrogen-doped graphene (Pd1/N-graphene) exhibits better activity and selectivity for C2H2 + 2H→C2H4, and DFT results show that C2H4 selectivity is 88.0 kJ mol−1, which is much lower than that on Pd1/GDY catalyst. Further, C2H2 semi-hydrogenation on the Pd1/SVG catalyst with the single atom Pd supported by a single vacancy graphene (SVG) is calculated in this study (see details in Figs. S16 and S17), the results show that C2H4 is easily over-hydrogenated to C2H5 in kinetically instead of its desorption (10.6 vs. 43.2 kJ mol−1). Similarly, the selectivity and activity of C2H4 formation over these supported single atom Pd catalysts reported in the literatures are still lower than those on Pd1/GDY catalyst.Based on above analysis, surprisingly, we found that GDY supported single atom Pd catalyst (Pd1/GDY) in this study should so far provide the best selectivity and activity toward C2H4 formation in C2H2 semi-hydrogenation in comparison with other types of the single atom Pd or Cu catalysts previously reported in the literatures. Moreover, the experiments by Qi et al. [74] found that the direct oxidation–reduction reaction of GDY and PdCl4 2− could realize the chemical deposition of Pd nanoparticles on GDY, which provided an important guidance for the preparation of GDY supported single atom Pd catalyst in the experiment.In summary, the activity and selectivity of a series of the designed PdxMy (M = Cu, Ag, Au, Ni; x + y = 1–3) clusters anchored on GDY (PdxMy/GDY catalysts) in C2H2 semi-hydrogenation have been fully examined using DFT calculations. Our results show that the activity and selectivity of C2H4 formation in C2H2 semi-hydrogenation on the PdxMy/GDY catalysts strongly depend on the composition and size of supported metal cluster, which has the relationship with the metal-support interaction of PdxMy/GDY catalysts and electronic properties. The supported metal is single atom, only Pd1/GDY is favorable for C2H4 formation; the supported metal is two atoms cluster, only Cu2/GDY and Au2/GDY are favorable for C2H4 formation; the supported metal is three atoms cluster, seven kinds of the catalysts including Cu3/GDY, Au3/GDY, Pd1Cu2/GDY, Pd2Cu1/GDY, Pd1Ag2/GDY, Pd2Au1/GDY and Pd1Ni2/GDY are favorable for C2H4 formation. Moreover, aiming at realizing the balance between the activity and selectivity of C2H4 formation, the metal-support interaction of PdxMy/GDY catalysts and the average charge amount of metal atoms should be maintain in a moderate range.Surprisingly, the comparisons among the catalysts considered in this study and previously reported in the literature showed for the first time that Pd1/GDY catalyst exhibits the high catalytic activity and selectivity toward C2H4 formation in C2H2 semi-hydrogenation. The high activity of Pd1/GDY is ascribed to Pd inherent properties toward C2H2 hydrogenation, however, the high C2H4 selectivity is attributed to the structural confinement of single atom Pd in Pd1/GDY catalyst leading to the much weaker C2H4-π bonding interactions, which is not favorable for C2H4 activation and hydrogenation, thus, the faster desorption rate of the produced C2H4 on Pd1/GDY catalyst than its hydrogenation rate enhanced C2H4 selectivity in C2H2 semi-hydrogenation. The obtained results could provide good clues for designing and screening out the potential catalysts with GDY supported small sizes of metal clusters for selective hydrogenation of alkanes.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 (No. 21776193 and 21736007) and U.S. NSF-sponsored NCAR-Wyoming Supercomputing Center (NWSC).The following is the Supplementary data to this article:The calculations methods of metal-support interaction and Gibbs free energy, C2H x (x = 2–5) and H adsorption, the energy profile of C2H2 semi-hydrogenation on the PdxMy/GDY catalysts with better C2H4 selectivity and Pd1/SVG catalyst, as well as the calculations of C2H4 formation activity on PdxMy/GDY catalysts are described. Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.gee.2020.10.020.

C2H2 semi-hydrogenation has been widely applied in industry to eliminate trace C2H2 from C2H4 feed. C2H2 semi-hydrogenation to C2H4 on a series of the newly designed catalysts, graphdiyne (GDY) as a new carbon allotrope supported different sizes of PdxMy clusters (PdxMy/GDY, M = Cu, Ag, Au, Ni; x+y = 1–3), were studied using DFT calculations. The results found that C2H2 semi-hydrogenation to C2H4 on PdxMy/GDY catalysts exhibits that both the activity and selectivity greatly depend on the composition and size of PdxMy/GDY catalysts. Surprisingly, our results for the first time discovered the Pd1/GDY catalyst with GDY supported the single atom Pd that presents the best selectivity and activity toward C2H4 formation compared to the previously reported catalysts so far in C2H2 semi-hydrogenation. This study would provide a theoretical clue for designing and screening out the potential catalysts with GDY supported small sizes of PdxMy and other metal clusters in C2H2 hydrogenation.

No data was used for the research described in the article.Brunauer-Emmet-TellerBarrett-Joyner-HalendaComputational fluid dynamicsCatalytic pyrolysisDistributed activation energyDifferential friedman Delonix regia Differential thermogravimetryEnvironmental protection agencyHigher heating valueInternational confederation for thermal analysis and calorimetryIndices for pyrolysis performanceKissinger-akahira-sunoseKinetic factorsSodium-Y zeoliteOzawa-flynn-wallPlatinum (10 wt %) on activated carbonSurface areaStarinkThermogravimetric analyzerTitanium oxideThermodynamic parametersZinc oxideFlammability indexBurnout indexIgnition indexDevolatilization indexActivation energyConversion-dependent reaction model (differential)Conversion-dependent reaction model (integral)Change in Gibbs free energyPlanck's constantChange in enthalpyBoltzmann constantFrequency factorReaction rate constantMass of catalytic DR mixture at the final timeMass of catalytic DR mixture at the initial timeMass of catalytic DR mixture at timeReaction orderTemperature approximation (integral)Universal gas constantMaximum decomposition rateMean decomposition rateCombustion indexChange in entropyAbsolute temperatureTimeBurnout temperatureBurnout timeThermogram decomposition temperatureIgnition temperatureIgnition timeMaximum decomposition temperatureMaximum decomposition timeTemperature interval at half of R p Time interval at half of the half of R p Temperature at αReaction pathwayExtent of conversionHeating rateGlobal diminishment of fossil fuels, as well as intensifying environmental pollution, necessitate exploring alternative energy sources. Biomass has received a lot of attention as the only abundant renewable resource that can be used to produce sustainable biofuels. As per the EPA [1] data, the global production of lignocellulosic biomass solid waste in 2020 was 18.1 MT. Out of this huge quantity, 17.1% is used for energy recovery, 15.7% is employed for combustion, and 67.2% is utilized for other purposes such as disposal and landfills. Delonix regia (DR) is a common lignocellulosic biomass found in the Fabaceae family [2], and its residue is used to produce bio-oil by several researchers [3–7]. However, biomass conversion to bio-oil via the pyrolysis process still faces several challenges due to the thermal instability of bio-oil, and its higher oxygen and viscosity contents. As a result, a catalyst is required to enhance the properties for a better quality of bio-oil [8]. In addition, other pyrolytic processes variables such as residence time, temperature, the ratio of biomass by a catalyst, and type of catalyst significantly influence the generation of higher yields and selectivity of bio-oil during the catalytic pyrolysis of the biomass [9–11].The following three approaches are used to employ a catalyst in the pyro–catalytic process: first, mixing the catalyst with the raw material; second, introducing the catalyst at the top of the reactor to facilitate vapor–catalyst contact; and third, placing the catalyst into a secondary reactor after primary (pyrolizer) reactor. The first two approaches are known as in–situ, whereas the third is known as ex–situ pyro–catalytic processes; each approach has a distinct effect [12–14]. Thus, selecting a proper catalyst to enhance the pyrolysis process is an alternate method for minimizing the total energy utilization of the process [9–11]. The conversion of biomass into pyrolytic products are determined mainly by the kinetic rates of the reactions that occur during pyrolysis. It has been demonstrated that an accurate kinetic technique is vital to design an effective pyrolysis process [15,16].Thermogravimetric analysis (TGA) is an analytic tool employed for assessing the pyrolytic degradation composition of heterogeneous biomass [12–14]. Understanding the pyrolytic decomposition of biomass is highly significant due to the kinetic factors (activation energy, E α ; frequency factor, k o ; and reaction pathway, Z α ) being inherently associated with the degradation mechanisms [9–11]. TGA can quantify the mass loss caused by devolatilization during the pyrolytic disintegration of biomass at a given heating rate in relation to temperature and time [8]. Furthermore, the first derivate of the TG profiles (dm/dt), commonly termed differential thermogravimetry (DTG), can be used to calculate the maximum rate of reaction [8]. Numerous kinetic and thermodynamic investigations [17–24] for biomass pyrolysis employ a first order reaction method. In this method, the parameters such as the extent of conversion (α), frequency factor (k o ), and reaction rate constant k(T) are reported by using the TGA data [17–24]. Hence kinetic factors from TGA can be measured accurately. TGA–derived kinetic factors serve as the basis for the modeling and optimization of the pyrolysis process. Consequently, kinetic factors knowledge is essential for comprehending and developing the catalytic pyrolysis process. Furthermore, the International Confederation for Thermal Analysis and Calorimetry (ICTAC) Kinetics Committee suggested multiple iso–conversional techniques for evaluating kinetic factors using TGA data [25,26]. The most used iso–conversional techniques throughout this work are Differential Friedman (DFM), Kissinger–Akahira–Sunose (KAS), Ozawa–Flynn–Wall (OFW), Starink (STK), and Distributed Activation Energy (DAE). Using iso–conversional techniques, it is possible to precisely predict the kinetic factors and thermodynamics of the biomass pyrolysis [17–24,27–31]. Furthermore, understanding of thermodynamic properties including changes in enthalpy and Gibbs free energy are critical for establishing the feasibility and energy requirement of biomass pyrolysis [32–41].The literature study indicates that no studies have been published that examined kinetic factors and thermodynamic parameters for catalytic pyrolysis (CP) of DR biomass employing iso-conversional techniques. However, various feedstock, including wheat bran [42]; poplar sawdust [30]; pine needle [43]; coconut copra and rice husk [44]; and torrified bamboo [45], have been explored with different catalysts, respectively, such as ZSM-5, Pt/C and Pd/C; Fe–Ni/ZSM-5; Ni/Al2O3; Ni–Ce/Al2O3; and HZSM-5. Furthermore, only a few investigations on CP of DR biomass for the production of bio-oil have been reported [3–7]; however, they have not reported the corresponding kinetics and thermodynamics of the process. According to the aforementioned rudimentary information, the CP of DR biomass for kinetics and thermodynamic analysis has yet to be extensively researched; therefore, the current research focuses on this investigation.Briefly, this work involved the systematic exploration of kinetic factors and thermodynamics during the catalytic pyrolysis of Delonix regia (DR) biomass over three different catalysts (Na–Y zeolite, 10 wt % of Pt/C, and TiO2–ZnO) using a thermogravimetric analyzer (TGA). The TGA of DR biomass is conducted with each catalyst loading (30, 20, and 10 wt %) at multiple heating rates (5, 10, 20, 35, and 55 °C min−1), and associated kinetic factors are ascertained that includes activation energy (E α ) determined employing five iso–conversional techniques (DFM, OFW, KAS, STK, and DAE) followed by frequency factor (k o ). Further, the variations of reaction pathways are evaluated using Criado's master plot technique. Finally, the thermodynamic properties of catalytic pyrolysis of DR biomass are thoroughly examined. This report shall be critical for elucidating the influence of different types of catalysts (Na–Y, Pt/C, and TiO2–ZnO) for biofuel generation, supporting scientific sources for pyrolyzer modeling, and optimizing the pyrolysis process.Material including Delonix regia (DR) biomass was sourced from solid wastes at IIT Guwahati. Catalysts were procured: zeolite Na–Y (SAR-5.1: 1) from alfa aesar in the USA, platinum on activated carbon (10 wt %) from sigma-aldrich in the USA, titanium oxide (>99.5 wt %), and zinc oxide (>99.9 wt %) from Sisco research laboratories in India.The methodologies of preparation and results of physico–chemical properties, including grinding/crushing, washing, drying, and higher heating value, and ultimate/proximate analysis of DR material were discussed elsewhere [3–7,46,47].Pore size distribution such as size and volume of catalysts were determined using an N2 sorption (Model: Tristar II, Make: Micromeritics, USA) analyzer. Before the analysis catalysts (three) were degassed for 6 h at 180 °C in a vacuum to eliminate moisture and volatiles. The surface area (SA) of each catalyst was calculated utilizing the Brunauer-Emmet-Teller (BET) technique. The Barrett-Joyner-Halenda (BJH) technique was employed to calculate pore size distribution. The pore size (nm) and pore volume (cm3 g−1) were 4.91 and 0.07; 5.64 and 0.07; and 3.67 and 0.51, respectively, for Na–Y, Pt/C, and TiO2–ZnO.The catalytic pyrolysis (CP) of DR biomass experiments was conducted in a thermogravimetric (Model: TG209F1, Make: Netsch, Germany) analyzer. In all experiments, DR material and catalysts were homogenized by mechanically blending them with the aid of mortar and pestle at various catalyst ratios (30, 20, and 10 wt %). For all experiments, approximately 6 mg of sample was used and heated from temperatures ranging between 25 and 1000 °C. For all experiments, 40 mL min−1 of nitrogen gas was maintained and the heating rates (β) were: 5, 10, 20, 35, and 55 °C min−1. TGA experimental runs were undertaken at β ranging from 5 to 55 °C min−1 to explore the CP of DR biomass between slow and fast pyrolysis range.In this study, kinetic factors (KF) including activation energy E α (kJ mol−1), frequency factor, k o (s−1), and pathway of reaction (Z α ) for catalytic pyrolysis (CP) of DR biomass were evaluated. All calculations by the following techniques were conducted using MATLAB (Version: R2021a): Differential Friedman (DFM), Ozawa-Flynn-Wall (OFW), Kissinger-Akahira-Sunose (KAS), Starink (STK), and Distributed Activation Energy (DAE).Calculations of KF of catalytic pyrolysis (CP) of Delonix regia (DR) were provided below: D e l o n i x R e g i a → N a − Y , P t / C , T i O 2 − Z n O V o l a t i l e s + B i o c h a r The following was the conversion rate of CP of DR: (1) d α d t = k ( T ) f ( α ) where k(T) denotes reaction rate constant, and f(α) denotes conversion–dependent reaction model (differential).Extent of conversion (α) of CP of DR was given by: (2) α = m i − m t m i − m f where m i , m f , and m t represent the mass of catalytic DR mixture at the initial, final, and any time respectively.Reaction rate constant for CP of DR was defined by Arrhenius Eq.: (3) k ( T ) = k 0 exp ( − E α R T ) where k o , R, T, and E α denotes frequency factor (s−1), universal gas constant (J mol−1 K−1), absolute temperature (K), and activation energy (kJ mol−1).Making use of Eqs. (1) and (3) gave following equation: (4) d α d t = k o exp ( − E α R T ) f ( α ) Now, consider introducing the rate of heating (β) as: (5) β = d T d t = d T d α × d α d t From Eqs. (4) and (5): (6) g ( α ) = ∫ 0 α d α f ( α ) = k o β ∫ T 0 T exp ( − E α R T ) d T = k o E α β R ∫ x ∞ u − 2 exp − u d u = k o E α β R p ( x ) Where, x = E α R T .The reaction model (integral) expression was Eq. (6), which does not have a numerical solution; nevertheless, it can be approximated using multiple iso-conversional techniques.Iso–conversional techniques: DFM [48], OFW [49,50], KAS [51], STK [52], and DAE [53] were utilized for evaluating KF for CP of DR biomass.The rearranged mathematical expressions for various iso–conversional techniques were as follows: (7) ln [ β ( d α d T ) ] = ln [ k o f ( α ) n ] − E α R T α D F M (8) ln ( β T α 2 ) = ln [ k o R E α g ( α ) ] − E α R T α K A S (9) ln ⁡ ( β ) = ln [ k o E α R g ( α ) ] − 5.331 − 1.0516 E α R T α O F W (10) ln ⁡ ( β T α 1.92 ) = ln [ k o R 0.92 E α 0.92 g ( α ) ] − 0.312 − 1.0008 E α R T α S T K (11) ln ( β T α 2 ) = ln [ k o R E α ] + 0.6075 − [ E α R T α ] D A E Various approximations such as Doyle, Miura and Maki, Perez–Maqueda, and Starink, p ( x ) = x − 2 exp − x , p ( x ) = exp ( − 1.0516 x − 5.331 ) , p ( x ) = exp ( − 1.0008 x − 0.312 ) x 1.92 , and p ( x ) = 0.6075 − x were utilized to get Eqs. (8 - 11) respectively [52–55]. Reaction pathway: The reaction pathway (Z α ) of CP of DR was analysed by Criado's technique [56]. Table 1 included the numerous reaction pathways expressions, which comprise models of differential (f(α)) and integral (g(α)) categories. The theoretical and experimental equations (12–15) of Z α were provided below: (12) Z ( α ) T h e o = f ( α ) T h e o × g ( α ) T h e o (13) Z ( α ) Exp = ( d α d t ) × exp ( E α R T ) × ∫ T 0 T exp ( − E α R T ) d T (14) Z ( α ) Exp = ( d α d t ) × E α R × exp ( E α R T ) × p ( x ) (15) Z ( 0.5 ) Exp = ( d α d t ) × E 0.5 R × exp ( E 0.5 R T ) × p ( x ) For above, p ( x ) = 0.00484 exp ⁡ ( − 1.0516 x ) Thermodynamic parameters (TP) of CP of DR, comprising changes in Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) were evaluated utilizing the following governing equations: (16) k o = β × E α × e ( E α R T m ) R T m 2 (17) Δ G = E α + R T m × ln ( K B T m h k o ) (18) Δ H = E α − R T m (19) Δ S = Δ H − Δ G T m where T m  = thermogram decomposition temperature (K), K B = Boltzmann (1.381 × 10−23 J K−1) constant, and h = Planck's (6.626 × 10−34 Js) constant.For the CP of DR, various indices including flammability index (C, K−2 min−1), burnout index (D b , min−4), ignition index (D i , min−3), devolatilization index (D v , K−3 min−1), and combustion index (S, K−3 min−2) were determined. The following equations Eq 20, 21, 22, 23 and 24 describe the definitions of these indices: (20) C = ( − R p T i 2 ) (21) D i = ( [ − R p ] t i × t p ) (22) D b = ( [ − R p ] Δ t 0.5 × t b × t p ) (23) S = ( [ − R p ] × [ − R v ] T i 2 × T b ) (24) D v = ( [ − R p ] × [ − R v ] Δ T 0.5 × T i × T p ) where, R p  = maximum decomposition rate (wt. % min−1), T i  = ignition temperature (K), t i  = ignition time (min), t p  = maximum degradation time (min), Δt0.5 = interval at half of the R p , t b  = burnout time (min), R v  = mean decomposition rate (wt. % min−1), T b  = burnout temperature (K), ΔT0.5 = temperature (K) interval at half of R p , and T p  = maximum decomposition temperature (K). The logical process flow diagram of the current research was illustrated in Fig. 1 . Whereas Fig. 2 presented TGA and DTG distribution of DR biomass and three catalysts at 10 °C min−1 of heating rate. Finally, it should be noted that Eqs. (16)–(19) are derived from and applicable only to unimolecular reactions but not to heteromolecular biomass samples although these equations are extensively used for the pyrolysis of biomass. Fig. 3 (A to C), Fig. 3 (D to F), and Fig. 3 (G to I) represents TG–DTG pyrograms of catalytic pyrolysis (CP) of DR biomass using three different catalysts namely zeolite (Na–Y), 10 wt % (Pt/C), and 1–1 wt. % (TiO2–ZnO)respectively at 1:30, 1:20 and 1:10 wt %. From Fig. 3, it was noticed that the CP of DR biomass occurred in three zones. The primary zone was formed within the temperature range (25–150 °C) which mainly liberates moisture and light molecular weight compounds. The second pyrolytic zone occurred at the temperature ranges of 150–587 °C for 30 wt% Na–Y; 150–592 °C for 20 wt% Na–Y; 150–598 °C for 10 wt% Na–Y; 150–572 °C for 30 wt% TiO2–ZnO; 150–590 °C for 20 wt% TiO2–ZnO; 150–598 °C for 30 wt% TiO2–ZnO; 150–640 °C for 30 wt% Pt/C; 150–632 °C for 20 wt% Pt/C; and 150–610 °C for 10 wt% Pt/C. This was generally identified as an active pyrolytic zone with the decomposition of major compounds such as hemicellulose and cellulose [30,42,57]. Additionally, it was emphasized that lignin decomposition also starts in this zone. At the same time, a significant part of lignin decomposition was noted in the third zone at temperatures of >500 °C [46,58]. In the third stage, biomass degradation was slow and the lignin compound contributed such a trend. Furthermore, from Fig. 2, it was observed that at a heating rate (10 °C min−1) the catalysts were stable with 76.4 wt %, 56.2 wt %, and 98.8 wt % mass of Na–Y, Pt/C and TiO2–ZnO remaining, respectively, even after 800 °C. Fig. 3 (A-C) represent the TG–DTG pyrograms of CP of DR biomass with 30, 20, and 10 wt % load of Na–Y. TG pyrograms between 5 and 55 °C min−1 showed mass loss of 53–51 wt % by using 30 wt % Na–Y; 58–56 wt % by use of 20 wt % Na–Y; and 62–60 wt % by use of 10 wt % Na–Y along with DR in the pyrolyzer. DTG pyrograms by 5–55 °C min−1 showed the different maximum decomposition temperatures were 323–362 °C by 30 wt % Na–Y; 326–366 °C by 20 wt % Na–Y; and 326–368 °C by 10 wt % Na–Y, respectively. It was observed that as the catalyst (Na–Y) loading increased, the mass loss decreased significantly and was comparable to other reports [30,59]. Moreover, an increasing heating rate causes variations of TG–DTG pyrograms for non-catalytic pyrolysis, indicating an improper heat transfer between the particles [9,46]. Likewise, change in TG–DTG pyrograms were observed for the catalytic pyrolysis. This change of pyrograms for a catalytic pyrolysis process also depends on the pore size characteristics, surface area and acid sites (combination of Lewis and Brønsted sites) availability in zeolite catalyst [30,32,34,41,44,60]. Fig. 3 (D-F) represent the TG–DTG pyrograms for CP of DR biomass with 30, 20, and 10 wt % of Pt/C. TG pyrograms between 5 and 55 °C min−1 indicated a mass loss of 55–53 wt % by 30 wt % Pt/C; 63–60 wt % by 20 wt % Pt/C; and 64–59 wt % by 10 wt % of Pt/C. It was observed as Pt/C ratio increased, mass loss decreased significantly. DTG pyrograms at 5–55 °C min−1 showed the different maximum decomposition temperatures: 334–372 °C by 30 wt % Pt/C; 334–372 °C by 20 wt % Pt/C; and 334–373 °C by 10 wt % Pt/C, respectively. The significant reactions contributing to the weight loss were decarbonylation, decarboxylation, and dehydrogenation [30,42,61]. The lower catalytic activity of the Pt/c in comparison to the Na–Y was due to lower surface area (55 m2 g−1) and lower pore volume [42,61]. Fig. 3 (G-I) represents the TG–DTG pyrograms for CP of DR biomass at 30, 20, and 10 wt % of TiO2–ZnO. TG pyrograms between 5 and 55 °C min−1 noticed a mass loss of 54–50 wt % by 30 wt % TiO2–ZnO; 56–55 wt % by 20 wt % TiO2–ZnO; and 63–62 wt % by 10 wt % TiO2–ZnO respectively. It was ascertained as the catalyst (TiO2–ZnO) ratio increased, the mass loss decreased significantly [30,62]. DTG pyrograms at 5–55 °C min−1 showed the different maximum decomposition temperatures: 332–371 °C by 30 wt % TiO2–ZnO; 332–371 °C by 20 wt % TiO2–ZnO; and 332–370 °C by 10 wt % TiO2–ZnO.At lower loadings (10 and 20 wt %), catalysts provided better thermal decomposition of biomass when compared to loading of 30 wt %. Such a trend can be attributed to the coke deposition on the surface of the catalyst [14,43]. Whereas for the non-catalytic pyrolysis process, the thermal degradation of DR biomass was higher as the remaining mass was 30.2–25.5 wt % against the increasing heating rates in the range of 5–55 °C min−1 [46].From surface area analysis of any catalyst, it was known that as the catalyst surface area diminishes, the number of active sites available for the reaction also reduces which states that the activity of the catalyst was less [30,63]. Additionally, the thermal decomposition of a catalyst indicates that the rigid walls of the pores offer the active sites significant strength, thus enhancing the catalyst's stability at high temperatures. This facilitates the usage of catalyst at higher temperatures (>1000 °C) without losing its properties [30,63]. Moreover, during the prolonged catalytic reactions, higher fractions (gaseous) were adsorbed over active sites by blocking pores that enhance the coke formation on the catalyst thereby reducing the number of active sites available for the reaction [30,62]. This also increases the activation energy (E α ) required for initiating the reaction. Fig. 4 A and Table 2 present the variation of E α with zeolite Na–Y at 30, 20, and 10 wt % loading. From the kinetic triplet analysis, according to DFM (Fig. 4A), it was observed that using Na–Y catalyst, (mean E α (kJ mol−1) and k o (min−1) factors) decreased in the range of (203 and 7.59 E+17) – (182 and 1.33 E+16) as the catalyst loading was varied from 30 to 10 wt %. When contrasted to the corresponding non–catalytic pyrolysis of DR biomass at a mean E α of 195 kJ mol−1 [46], Na–Y catalyst process provided lower mean E α 181 kJ mol−1. While the remaining four model–free techniques (KAS, OFW, STK and DAE) yielded (mean E α (kJ mol−1) and k o (min−1) factors) of (205 and 9.70 E+19) at 30 wt % Na–Y; (191 and 1.96 E+19) at 20 wt % Na–Y; and (181 and 1.83 E+17) at 10 wt % Na–Y. It was observed that the mean E α factor decreased (181 kJ mol−1) with decreasing Na–Y ratio (10 wt%). Hence, Na–Y with lower loading (10 wt%) was found to be suitable catalyst for catalytic pyrolysis of DR biomass.For Pt/C catalyst, the findings revealedthat mean E α (kJ mol−1) and k o (min−1) factors decreased as (218 and 1.28 E+21) at 30 wt % Pt/C to (200 and 9.62 E+17) at 10 wt % Pt/C using model–fitting (DFM) technique. While the remaining four model–free techniques (KAS, OFW, STK and DAE) yielded (mean E α (kJ mol−1) and k o (min−1) factors) as (211 and 4.73 E+18) at 30 wt % Pt/C; (204 and 1.06 E+18) at 20 wt % Pt/C; and (204 and 2.03 E+19) at 10 wt % Pt/C, respectively. It was observed that the mean E α factor decreased with decreasing Pt/C ratio [42,61]. Fig. 4B and Table 3 show the alteration of E α with Pt/C catalyst at 30, 20, and 10 wt % loading. Further, Fig. 4C and Table 4 demonstrate the variation of E α with TiO2–ZnO (1–1 wt. %) catalyst at 30, 20, and 10 wt % loading. Findings showed that the (mean E α (kJ mol−1) and k o (min−1) factors) decreased between (196 and 1.03 E+17) at 30 wt % TiO2–ZnO to (191 and 7.34 E+16) at 20 wt % TiO2–ZnO and increased to (201 and 3.41 E+17) at 10 wt % TiO2–ZnO utilizing model–fitting (DFM) technique. While remaining four model–free techniques (KAS, OFW, STK and DAE) yielded (mean E α (kJ mol−1) and k o (min−1) factors) of (192 and 3.72 E+17) at 30 wt % TiO2–ZnO; (194 and 2.30 E+18) at 20 wt % TiO2–ZnO; and (204 and 6.17 E+19) at 10 wt % TiO2–ZnO. It was observed that the mean E α factor increased with decreasing TiO2–ZnO ratio. The probable reasons for such alteration of mean E α factors can be attributed to the reasons stated earlier.The above findings clearly define the effect of the catalyst in determining the kinetic factors. Though the E α factor during pyrolysis increased at conversion (α) 0.1 to 0.4, there was a slight decrease at a conversion of 0.5–0.6 and then increased up to 0.7. This was due to the hemicellulose compounds degradation in the conversion range of 0.1–0.4, while cellulose degradation at 0.5–0.6 conversion and lignin degradation at conversion >0.7 [46,64]. At higher temperatures (>500 °C), lignin decomposition was prominent but increased the E α requirement because of the denser molecules nature of lignin. As the catalyst loading to the feedstock ratio increases, more coke deposition occurs on the catalyst's surface, leading to increased activation energy. The results of the present study follow a similar trend observed in recent works by Refs. [14,30,43,65].Considering all the catalysts used in the present study, each catalyst contributes differently to the pyrolysis reactions. The Pt/C catalyst was known for hydrogenation and higher deoxygenation reactions [42,61]. In comparison, TiO2–ZnO catalyst enhances the dehydration of the alcohol to olefins and leads to lower hydrocarbon production. Furthermore, the TiO2 catalyst helps convert xylene, an intermediate of hemicellulose decomposition [66]. Additionally, the zeolite catalyst provides a higher acid site, promoting the decarbonylation and deoxygenation reactions [43]. Hence, it was apparent that each of these catalysts plays a different role in the decomposition of biomass.Reaction pathways of catalytic pyrolysis (CP) of DR biomass were analysed with Criado's technique. All graphs were obtained using Eqs. (12 – 15) for conversion range (α = 0.1–0.7) at a heating rate of 10 °C min–1, shown in Fig. 5 (A-C) and Table 5 . Fig. 5A indicated the reaction pathways of F2, P4, 3A, F0, A4, F3 for 1:30 wt % of DR:Na–Y; F2, P4, R2, F2, F1, A4 for 1:20 wt % of DR:Na–Y; and F2, P4, R2, F0, R3, F4 for 1:10 wt% of DR:Na–Y. Fig. 5B depicted the reaction pathways of F4, F3, F3, F4, D1, P2 for 1:30 wt % of DR:Pt/C; P4, D3, P3, F0, F2, A4 for 1:20 wt % of DR:Pt/C; and P4, D2, P3, F0, F5, F3 for 1:10 wt% of DR:Pt/C. Fig. 5C exhibited the pathway of reaction as F4, F1, F3, F4, D3, R2 for 1:30 wt % of DR:TiO2–ZnO; A4, P4, P3, D3, F4, F2 for 1:20 wt % of DR:TiO2–ZnO; and F2, R2, P3, F3, D3, F1 for 1:30 wt% of DR:TiO2–ZnO. Therefore, the CP of DR biomass was revealed to follow a multistep reaction pathway rather than a single reaction pathway for all three catalysts of each loading [30,31,46]. Fig. 6 (A-C), shows variations of enthalpy change (ΔH) of catalytic pyrolysis (CP) of DR biomass with Na–Y, Pt/C, TiO2–ZnO respectively from the DFM technique. The change in enthalpy (ΔH) was defined as the energy necessary to move a molecule to a higher energy level from a lower energy level [30,63,64]. From the thermodynamic parameter analysis, it was found that the ΔH varied from 177 for 10 wt % Na–Y; 195 for 10 wt % Pt/C; and 196 kJ mol−1 for 10 wt % TiO2–ZnO. Additionally, it varied as 179kJ mol−1 for 20 wt % Na–Y; 200 kJ mol−1 for 20 wt % Pt/C; and 186 kJ mol−1 for 20 wt % TiO2–ZnO catalyst loadings. Whereas, it can be seen that the variation in enthalpy change can be noted as 198 kJ mol−1 for 30 wt % Na–Y; 212 kJ mol−1 for 30 wt % Pt/C; and 190kJ mol−1 for 30 wt % TiO2–ZnO. The change in enthalpy during the non-catalytic pyrolysis of DR biomass was identified as 190 kJ mol−1 [46]. Results indicate that lower ΔH factors were ascertained for 10 and 20 wt % of Na–Y and TiO2–ZnO catalyst loadings when compared with the pyrolysis of DR biomass without catalysts. Subsequently, it can also be seen that positive ΔH factors signify the indigenous endothermic reactions. Though initially, during the beginning of the reaction, the ΔH factors were noted to be lower, there was a slight increase for α ≥ 0.6 indicating that requirement of energy was more to the reaction to persist at higher conversion [30]. Fig. 6 (D-F), shows variations in change in Gibbs free energy (ΔG) for catalytic pyrolysis (CP) of DR biomass in the presence of zeolite Na–Y, Pt/C, and TiO2–ZnO catalysts with feedstock to catalyst ratio of 30, 20, and 10 wt % for the DFM technique. The ΔG was defined as an increase in the overall energy of the process to constitute an activated complex [30,63,64]. The calculations of ΔG of the current study show that it has varied from 178,178,176 kJ mol−1 for all three catalysts at catalyst loading from 30, 20, 10 wt % respectively. The ΔG factor obtained during non-catalytic pyrolysis of DR biomass was 180 kJ mol−1 [46]. Also, ΔH, and ΔG were found to be positive implying that additional energy was needed to perform the CP of DR biomass. Fig. 6 (G-I), shows variations in change in entropy (ΔS) for catalytic pyrolysis (CP) of DR biomass in the presence of zeolite Na–Y, Pt/C, and TiO2–ZnO, catalysts for the feedstock to catalyst ratio of 30, 20, and 10 wt % by DFM technique. ΔS was an essential parameter that measures the disordeness of the pyrolysis process. ΔS < 0 values were described as “slow pyrolysis,” and in this work, the CP of DR biomass experiences ΔS close to zero, implying close to thermodynamic equilibrium conditions [40]. This scenario was observed for Na–Y 10 wt% with decreased diorderness. Whereas ΔS > 0 values were referred to as “fast pyrolysis,” which indicates that the CP of DR biomass undergoes maximal physicochemical changes with more significant reactions suggesting the process was far from thermodynamic equilibrium. This scenario was observed at the higher loading of catalysts (30–20 wt%) for all three catalysts with increased disorderness. The obtained results of ΔS were also consistent with other studies [30,43].The indices of pyrolysis performance (IPP) of CP of DR biomass at three catalysts including zeolite Na–Y, Pt/C (10 wt %), and TiO2–ZnO (1–1 wt. %) at 30 to 10 wt% loading of each was shown in Table 6 . According to the IPP findings, all parameters (C, D i , D b , S and D v ) were increased with increasing heating rates (5–55 °C min−1) for three catalysts. Furthermore, in comparison to the other catalyst loadings, 10 wt % of zeolite Na–Y exhibited the lowest IPP parameters. This might occur because the zeolite Na–Y has a larger surface area and was stable at higher temperatures [67]. The higher flammability index (C = 8.78 × 10 − 5 ), suggested a lower moisture content and larger heating factor. The higher ignition and burnout indices (D i  =  944 × 10 − 3 and D b = 4540 × 10 − 5 ), indicated the higher combustibility. The higher combustion index (S = 26.5 × 10 − 8 ), exhibited stronger combustion characteristics. The higher devolatilization index (D v = 48.3 × 10 − 8 ), demonstrated the generation of a significant quantity of volatile content during the catalytic pyrolysis (CP) of DR biomass [46,64,68].This work has investigated the catalytic pyrolysis (CP) of Delonix regia (DR) biomass using three different kinds of catalysts: Na–Y, TiO2–ZnO, and Pt/C, with loading ranging from 30 to 10 wt%. According to the kinetic and thermodynamic results, it has been concluded that the Na–Y catalyst with a loading of 10 wt% showed better catalytic activity for CP of DR. From kinetic studies, the KAS technique at DR: Na–Y (1:10 wt %) has yielded the lowest mean factors of E α (kJ mol−1) 181.29 and k o (min−1) 2.10 E+16. Therefore, inexpensive, and higher thermal stability catalyst, zeolite Na–Y (load of 10 wt %) should be considered further for the bio-oils production from CP of DR biomass compared to other loads of catalysts of this study. Moreover, based on the findings, it has been observed that higher loadings (30 wt %) of all three catalysts are not recommended for a bio-oil generation because of the possible accumulation of coke on the surface of the catalysts. Criado's plots have revealed that the CP of DR biomass has followed a multistep reaction pathway instead of a single reaction pathway during the process. From thermodynamic findings, a 5 kJ mol−1 discrepancy between E α and ΔH has been noticed from the DFM technique, which has indicated that a large quantity of energy, i.e., at least equivalent to ΔH or more, should be provided for the pyrolysis to occur. Furthermore, ΔG and ΔS results have revealed that the CP of DR biomass undergoes non-spontaneous reactions. Further, more effective catalysts with various loadings should be investigated in the future to evaluate the best kinetic and thermodynamic characteristics. In addition, simulation studies such as computational fluid dynamics (CFD) also facilitate new dimensional work to analyse transportation and other insightful phenomena of such pyrolysis processes using the kinetics developed in the present work.D Rammohan: Investigation; Methodology; Validation, Writing Original Draft. N Kishore: Conceptualization; Resources; Writing Review & Editing; Supervision; Fund Acquisition. RVS Uppaluri: Supervision; 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.

Renewable energy from biomass waste shall become an effective alternative to faster-depleting fossil fuels and pyrolysis is a suitable approach to produce renewable energy from biomass. Further, kinetics parameters are essential in designing reactors for pyrolysis and this work provides kinetics of catalytic pyrolysis (CP) of Delonix regia (DR) biomass. In addition to this significance, the novelty of this work includes the utilization of catalysts of three different kinds such as zeolite (Na–Y), mixed metal oxides (TiO2+ZnO), and supported noble metal (Pt/C) catalysts at varying loads of 30–10 wt %. All experiments were performed in a micro pyrolyzer (thermogravimetry analyzer) under non-isothermal conditions at five heating rates (5, 10, 20, 35, and 55 °C min−1) in a temperature range of 25–1000 °C. To estimate kinetic factors (KF) and thermodynamic parameters (TP), five iso-conversional techniques such as Differential Friedman (DFM), Kissinger–Akahira–Sunose (KAS), Ozawa–Flynn–Wall (OFW), Starink (STK), and Distributed Activation Energy (DAE) were employed. KAS technique yielded the lowest mean activation energy, E α (181.29 kJ mol−1), and frequency factor, k o (2.10 E+16 s−1) factors by use of Na–Y zeolite of load 10 wt % whereas the corresponding change in enthalpy is 177 kJ mol−1, change in Gibbs free energy is 178 kJ mol−1, and change in entropy is −9.58 E−04 kJ mol−1 K−1. Criado's master plots confirmed the reaction pathway as: second order (F2), power-law (P4), contraction area (R2), zero order (F0), contraction volume (R3), and fourth order (F4) for 20 °C min−1 from DFM technique for CP of DR by using Na–Y zeolite catalyst of load 10 wt %.

The oxygen evolution reaction (OER) is an essential half-reaction used in energy conversion systems, such as in an electrochemical water splitting system [1,2]. The currently used OER electrocatalysts are based on noble metals, e.g. RuO2 and IrO2; however, their application is limited because they are expensive and rare and show sluggish kinetics [3,4]. To overcome such issues, many studies have attempted to develop non-noble metal-based OER catalysts, such as those based on transition metals (e.g., Ni, Fe, and Co) because of their natural abundance, low cost, and good chemical stability [5-7]. Among the transition metals, Ni-based electrocatalysts have shown excellent catalytic performance for OER owing to their suitable bond strength with neighboring active components [8]. However, they exhibit low conductivity and insufficient active sites, which hinder their electrocatalytic behavior [9].Recently, Ni-doped carbon nanostructures have attracted attention as OER electrocatalysts as they exhibit excellent electrical conductivity, remarkable chemical stability, and high number of active sites [10]. Until now, Ni-based carbon structures have been synthesized by harsh chemical methods, such as hydrothermal and solvothermal methods, which necessitates a further purification process [4,7,11]. In addition, these methods often change the electronic structures of conductive supports, which lead to reduced electron charge transport [12]. Therefore, it is challenging but highly desirable to develop a facile and green way for synthesizing Ni-doped carbon nanostructures.An alternative to the above-mentioned harsh chemical methods for the synthesis of functional carbon nanomaterials is pulsed laser ablation (PLA) process [13-16]. PLA process is simple and environment-friendly because it does not require a harsh chemical reactant and post-purifying steps [17]. Moreover, the remarkable rapid reaction associated with PLA process at high temperatures and high pressure can lead to the formation of a unique carbon nanostructure, such as heteroatom-doped graphene quantum dots and surface-functionalized carbon nanotubes [14,18,19]. Therefore, this robust process can effectively incorporate transition metals (i.e., Ni) in the carbon framework, resulting in further enhancement of the electrocatalytic OER performance. However, there have been few reports on the use of PLA process for fabricating OER catalysts; thus, herein we develop Ni-doped multi-walled carbon nanotubes (MWCNTs; Ni-MWCNTs) as high-performance OER catalysts by PLA process. The Ni-MWCNTs were characterized by high-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction analysis, and linear sweep voltammetry (LSV).Pristine MWCNTs were purchased from Hanwha Chemical (Republic of Korea). High-purity ethanol (99.9%) and nickel chloride (NiCl2) were purchased from Sigma Aldrich (USA). First, 50 mg of MWCNTs and 2 g of NiCl2 were dispersed in 500 mL of anhydrous ethanol. The PLA process was performed on the mixed solution for 1 h using a Q-switch ND:YAG pulsed laser system. Simultaneously, tip-type sonication was performed for achieving homogeneous dispersion of precursors. The mixed solution (of MWCNT and NiCl2) was ablated by pulsed laser beam (355 nm, third harmonic) at a repetition rate of 10 Hz. The pulse width was 10 nm, and the pulsed laser energy was 1 J. After the PLA process, subsequent centrifuging and drying process yielded Ni-MWCNTs.HR-TEM and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were performed using TALOS F200X (Thermo Fisher Scientific, 200 kV). XPS measurements were recorded using VG ESCALAB 200i (Thermo Fisher Scientific), where survey and high-resolution scans were obtained at pass energies of 100 and 20 eV, respectively. All electrochemical measurements (Autolab PGSTAT, Metrohm) were recorded in 1.0 M KOH (pH ≅ 13.7) electrolyte using a three-electrode electrochemical system cell with a rotating disk electrode (RDE). LSV was performed at a scan rate of 5 mV s−1. The potentials were calibrated against reversible hydrogen electrode, and all the polarization curves were iR-compensated. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range 0.1 Hz – 100 kHz, by applying a sinusoidal voltage with amplitude of 5 mV. Fig. 1 shows the possible formation mechanism of Ni-MWCNTs via PLA process. Briefly, when the pulsed laser is injected into the mixed solution (i.e., MWCNT, NiCl2, and ethanol), extremely harsh environments, such as high pressure and high temperature, are created owing to multi-photon absorption ionization [17]. Subsequently, an intrinsic phenomenon (e.g., plasma plume and cavitation bubble) of the PLA process occurs in the mixed solution, which leads to the generation of Ni molecules (i.e., derived from NiCl2), oxygen, and hydrogen (i.e., derived from ethanol in this study). Simultaneously, the MWCNT structure can partially collapse by the strong pulsed laser. Finally, these Ni molecules, oxygen, and hydrogen are unstable because of their high surface energy, resulting in their incorporation into the carbon framework (Ni-MWCNTs) [18].TEM and HR-TEM images of pristine MWCNTs (P-MWCNTs) and Ni-MWCNTs are shown in Fig. 2 . Both P-MWCNTs and Ni-MWCNTs have tubular structures and show negligible changes after the PLA process, as seen in low-resolution TEM images (Fig. 2 a,d). However, the HR-TEM images of Ni-MWCNTs show that the walls of the MWCNT structure were collapsed, overlapping with the walls, which were also partially distorted (Fig. 2 e,f). This phenomenon usually observed when the precisely controlled pulsed laser energy injected into the MWCNT, which leads to the increased surface area of MWCNTs [14]. Although the outer wall of the MWCNTs is partially collapsed, the inner walls of the MWCNTs still retain the tubular structure (Fig. 2 b,c). Finally, we confirmed the Ni was successfully incorporated in the entire MWCNT structure, as shown in the energy-dispersive X-ray spectroscopy mapping images (Fig. 2 g).We investigated the chemical composition of Ni-MWCNTs using XPS. For comparison, we prepared control samples of surface-modified carbon nanotubes (SMCNTs) using the same process, but without a Ni precursor. Fig. 3 a-c shows the C1s peaks of P-MWCNTs, SMCNTs, and Ni-MWCNTs, respectively. Both SMCNTs and Ni-MWCNTs showed the presence of oxygen-rich functional groups, such as hydroxyl and carboxyl, whereas P-MWCNTs showed only a low-intensity hydroxyl peak. Deconvolution of the Ni2p spectrum shows two main peaks at 856 and 876 eV [20]. In addition, Ni-O peaks are observed at 853 and 872 eV. These results indicated that the PLA process modulated the structure of MWCNTs, including the incorporation of Ni.The electrocatalytic performance of Ni-MWCNTs was analyzed in 1 M aqueous KOH electrolyte using a three-electrode system with an RDE, as shown in Fig. 4 . For comparison, the electrocatalytic performances of P-MWCNTs, SMCNTs, and the commercial catalyst RuO2 were analyzed under the same conditions (Fig. 4 a). LSV was performed on all the samples, and results showed that Ni-MWCNTs exhibited superior electrocatalytic activity for water oxidation. The overpotential (η) for delivering a current density of 10 mA cm−2 (η 10) was 320 mV for Ni-MWCNTs. Importantly, the commercial RuO2 catalyst exhibited the same current density (10 mA cm−2) at 360 mV, revealing that Ni-MWCNTs had higher electrocatalytic OER activity than that of commercial RuO2 catalyst. This superior OER activity of Ni-MWCNTs is related to the oxygen-rich functional groups, such as carboxyl and hydroxyl, on their surface. The presence of abundant oxygen groups effectively promoted interactions with H-carrying OER intermediates, such as OH* and OOH*, leading to enhanced OER activity [21]. Simultaneously, the successful doping of substitutional Ni results in more efficient OER activity due to the suitable bond strength with neighboring carbon framework and three-dimensional electronic intrinsic structure [22]. In addition, the Tafel slope was determined to calculate the kinetics of the OER rate-determining step [23]. Ni-MWCNTs gave the smallest Tafel slope (30.085 mV dec-1), followed by SMCNTs (37.035 mV dec-1), RuO2 (46.16 mV dec-1), and P-MWCNTs (110 mV dec-1). The low Tafel slope of 30.085 mV dec-1 suggests that OH* absorption is favorable on the surface of Ni-MWCNTs, which leads to excellent OER activity. EIS analysis was performed for Ni-MWCNTs and the control samples to calculate the charge transfer resistance (Rct ) between the surfaces of catalysts (Fig. 4 c). Generally, the smaller diameters of semicircle correspond to lower Rct values [24]. SMCNTs show a large Rct value of 63 Ω, while low Rct values were obtained for RuO2 (50 Ω), Ni-MWCNTs (30 Ω), and P-MWCNTs (22 Ω). Thus, the highly conductive MWCNTs are believed to be one of the reasons for the increased charge transfer between the catalysts and the reactants. Although the SMCNTs exhibited a larger Rct value than P-MWCNT due to low electron transfer by the collapsed outer wall of MWCNT (Fig. 1), the SMCNTs have oxygen-rich functional groups and high-surface area after the PLA process resulting in the high number of active site (Fig. 3). Consequently, the SMCNTs showed higher electrocatalytic performance than the P-MWCNT. To further investigate the OER activity, we performed cyclic voltammetry (CV) measurements for determining the electrochemically active surface area (ECSA) and turn over frequency (TOF), as shown in Fig. 4 d,e. Generally, the number of active sites is proportional to the ECSA value [25]. In SMCNTs, the number of active sites increased owing to enhanced surface area and formation of oxygen functional groups by PLA process, as shown in Fig. 4 d [14]. Subsequently, the incorporation of Ni into the MWCNT structure exhibited the highest active site. This result indicates that Ni-MWCNTs have the highest number of active sites, which is attributed to generation of reactive centers by incorporation of Ni. The calculated TOF values of all samples are shown in Fig. 4 e. Typically, a higher TOF value could be obtained from high density of catalytic activity, which was generally indicated the number of electrons produced per active site. The TOF for Ni-MWCNTs was found to be 0.035 s−1 at an overpotential of 320 mV, which is three times higher than that of commercial RuO2. In addition, the electrocatalytic durability of Ni-MWCNTs for OER was analyzed by chronoamperometry measurements. After 10 h of water oxidation at 1.54 V in 1 M KOH, Ni-MWCNTs retained 100% of initial current density, as shown in Fig. 4 f.In summary, a highly efficient and chemically stable Ni-MWCNT electrocatalysts were synthesized by a simple and green PLA process. The Ni molecules derived from NiCl2 are successfully incorporated in the entire MWCNT structure. Partial decomposition of MWCNTs and the formation of oxygen functional groups on the MWCNT surface occurred simultaneously. As a result, we observed a significant enhancement in the OER performance of Ni-MWCNTs due to the synergetic effect of Ni doping and MWCNT structural modification. Thus, our study offers an efficient and simple way of preparing highly efficient OER electrocatalysts, which can also assist in designing new functional carbon catalysts. Sukhyun Kang: Conceptualization, Writing - original draft. HyukSu Han: Conceptualization, Writing - original draft, Supervision, Writing - review & editing. Sungwook Mhin: . Hui Ra Chae: . Won Rae Kim: . Kang Min Kim: Conceptualization, Writing - original draft, 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.This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIT) (No. 2020R1A2C1102079).

Transition–metal-doped carbon-based electrocatalysts have attracted attention as alternatives to noble metal electrocatalysts (e.g. IrO2 and RuO2) for oxygen evolution reaction (OER) because they are inexpensive and highly efficient. However, their poor catalytic activity and time-consuming synthesis remain a challenge. Herein, we report a facile and green technique using pulsed laser ablation for preparing Ni-doped multi-walled carbon nanotubes (Ni-MWCNTs) as OER catalysts. Ni-MWCNTs exhibit high surface area, oxygen-rich functional groups (e.g., hydroxyl and carboxyl), and successful doping of Ni in the carbon framework. The as-prepared Ni-MWCNTs exhibited excellent OER catalytic performance, with an overpotential of 320 mV at the current density of 10 mA cm−2 in an alkaline medium, which is lower than that of the commercial RuO2 catalyst. Furthermore, Ni-MWCNTs displayed the initial electrocatalytic activity after 10-h stability tests, demonstrating good electrochemical durability. We believe that this work provides a simple protocol for fabricating heteroatom-doped carbon nanotubes as high-performance OER electrocatalysts.

No data was used for the research described in the article.The Biginelli reaction is one of the simple and direct methods for the synthesis of tetrahydropyrimidines, originally reported by Biginelli [1]. Regarding the importance of the Biginelli reaction products, much work on improving the yield and reaction conditions has been actively pursued. For example, using Lewis acids as a catalyst such as Cu(OTf)2 [2], Sc(OTf)3 or La(OTf)3, Yb(OTf)3 [3], Triethylammonium hydrogen sulfate [4], BiCl3 [5], and Mn(OAc)3·2H2O [6] instead of acidic reagents significantly improved the reaction output with reduced reaction times. Some other routes have been reported for the synthesis of Biginelli reaction products using various catalysts [7–13]. On the other hands, metal catalysts play very important role in organic synthetic reactions, especially in Biginelli reactions which are reducing reaction time and increases product yields, some of previously reported metal catalysts as H3PO3/Pd, NH4VO3 CeCl3·7H2O, Sm(ClO4)3, ((NH4)2Ce(NO3)6), NiCl2/KI, GaI3, TiO2, FeCl3·6H2O Cu(NO3)2·3H2O, Ce(SO4)2-SiO2, Fe(NO3)3·9H2O, ZnO, copper nitrate, ZrCl4, LnCl3·7H2O, ZrO2-nanopowder, and ZrOCl2·8H2O or ZrCl4/neat, etc [8–13]. These reactions were reported by use of ionic liquids, microwave irradiation, solid phase reagents, baker’s yeast, polymer-supported catalysts, zeolites, surfactants, and PEG, etc. [8–13].However, these previous methodologies have various drawbacks as long reaction times, work up complexity, tedious reaction conditions, highly economical with low yields. In addition, the multi-component reactions have been known to be powerful tools over conventional multi-step reactions and have emerged as a novel promotion in organic synthesis [7]. These reactions offer a direct fast route and enable the assembly of highly complicated and diversified molecules in a one-pot single-step process with enhanced atom economy [7]. The compounds of tetrahydropyrimidines and their derivatives have attracted considerable interest in medicinal chemistry due to their broad spectrum of pharmacological activities such as antibacterial [14], antifungal [15], anticancer [16], anti-inflammatory, analgesic [17], anti-HIV [18], antihypertensive [19], antimalarial activities [20]. These compounds exhibit specifically broad range of therapeutic and pharmacological properties, namely anticancer, antihypertensive, antiviral, and antifungal. tetrahydropyrimidines derivatives which are found as core units in many marine alkaloids, have been found to be potent HIV gp-120CD4 inhibitors [14–20]. Considering the emerging properties of tetrahydropyrimidines derivatives, the progress of advanced, clean, and uncomplicated methodologies for the efficient catalytic synthesis of these compounds with accessible reagents is of great importance.Although most of the reported routes have some benefits, however, some disadvantages are also combined with many of them using environmentally toxic organic solvents, expensive mediators, long reaction time, corrosive nature, tedious work-up, non-recyclable catalysts, limited substrate scope, high temperature, and low yields. Therefore, the development of effective and environmental benign methods is desirable for the synthesis of tetrahydropyrimidines compounds [21–23]. Therefore, we have opted to synthesis tetrahydropyrimidines derivatives via a green approach by using ZrO2/La2O3 catalyst as multi-component reactions. According to this scenario, several catalysts including metal oxides have been reported as nanocatalysts for the synthessis of chromenes and its derivatives as well as Biginelli products including Bi2V2O7 [24], NiO@TPP-HPA [25], CuO [26,27], MoO3–ZrO2 nanocomposite [28], MnO2–MWCNT [28], Cu2O [29], Mg–Al–CO3 and Ca–Al–CO3 hydrotalcite [30], Bi2O3/ZrO2 [31], ZrO2–Al2O3–Fe3O4 [32], histaminium tetrachlorozincate [33], Alumina supported MoO3 [34], ZrO2-pillared clay [35], Zn(l-proline)2 [36], Fe3O4-CNT [37], TiO2-MWCNT [38], Melem@Ni-HPA [39], RuO2 [40] and SiO2/H3PW12O40 [41].So far, many researchers have been reported Biginelli reaction by using various metal oxide systems at different reaction conditions. The various materials, including transition metals were necessitated to enhance the qualities and efficacy of ZrO2 or La2O3 nanoparticles. The surfaces of these particles were indeed transmogrified as a consequence of transition metal oxides doping to demonstrate significantly larger functionalities such as a higher surface area, and compactness, enabling them to actively participate catalytic applications. To the best of our knowledge, there is no report on the synthesis of the discerning weight composition of ZrO2/La2O3 heterogeneous catalyst. Besides, there is no report on the catalytic application of the obtained tetrahydropyrimidine derivatives in the Biginelli reaction and the investigation of the correlation of the reaction conditions including solvent-free conditions with the catalytic application. So, it was rationalized that the hard/soft natures of the metal ions would play important role in the catalytic activity of such catalysts. The current synthetic protocol furnishes several advantages like short reaction time, high purity of the isolated products, ease of reaction handling, simple separation and high yield of the desired products along with the synthesized nanocatalyst.In a typical synthesis, dissolve Lanthanum nitrate hexahydrate (5.0 g) in 100 ml of double distilled water and 0.1 M ammonium hydroxide was added to form white precipitation. The precipitate compound was filtered using Buchner funnel and washed with ammonium hydroxide then the obtained compound dried in an oven at 150 °C for 12 h. Then, the pulverised Lanthanum (III) hydroxide (2.5 g) was dissolved in double distilled water and added zirconium(IV) oxynitrate hydrate, then heated on water bath up to dry precipitate is formation. The compound dried in an oven 150 °C for overnight and further calcinations at 650 °C for 4 h to obtain pure pulverised 5 % of ZrO2/La2O3.To estimate the surface modification of the synthesized catalysts, FTIR (KBr) spectra were recorded on a Shimadzu FT-IR-8400s spectrophotometer. The powder X-ray diffraction pattern has been recorded on a Siemens D-5000 diffract meter by using Cu K radiation source and a Scintillation counter detector. The XRD phases present in the samples were identified with the help of JCPDS data files. The BET surface area was determined by N2 adsorption–desorption isotherms at liquid N2 temperature on a Micromeritics Gemini 2360 instrument. Prior to physical measurements, the synthesized compounds were dried in an oven at 393 K for 10 h and flushed with Argon gas for 1 hr. The melting points were determined in open capillary tubes and are uncorrected. The purity of the compounds was checked by TLC using pre-coated silica gel plates 60254(Merck). 1H NMR and 13C NMR spectra were recorded on Bruker Avance II 400 MHz spectrometer using tetramethylsilane as an internal standard. Mass spectra were recorded on a GCMS-QP 1000 EX mass spectrometer.In a typical procedure, a mixture of aldehydes (Ia-n, 1 mmol), acetoacetate (II, 1 mmol), urea (III, 1 mmol) and the synthesized 5.0 % ZrO2/La2O3 (30 mg) were placed in a round-bottom flask under solvent free conditions. The suspension was stirred at 80 °C for 30–40 min. The progress of the reaction was monitored by thin layer chromatography (TLC) [6:4 hexane:ethylacetate]. After completion of the reaction, the solid crude product was washed with deionized water to separate the unreacted raw materials. The precipitated solid was then collected and dissolved in ethanol to separate the solid catalyst. The filtrate was left undisturbed at room temperature to afford the crystals of the pure products such as tetrahydropyrimidine derivatives (IVa-n). The structures of all of the products were verified using 1H and 13C NMR spectral information.The synthesized ZrO2/La2O3 catalyst were generally characterized by the investigating their size, shape, morphology, optical band gap, and surface area by various techniques. A homogeneity in these properties results in the advancement in applications of nanoparticles.The surface and structural changes of the synthesized La2O3 and ZrO2/La2O3 catalysts were characterized by FT-IR spectra. It shows majorly-two characteristic stretching frequencies at peaks around 1460 and 856 cm−1 as shown in Fig. 1 . The peak around 1460 cm−1 is representing to presence oxide and the peak found at 856 cm−1 is characterizing the crystalline La2O3. The very weak absorption bands at 3604 cm−1 are assigned to OH symmetric stretching vibration of water molecules which is obtained due to hygroscopic nature of lanthanum oxide. Basis on the obtained FT-IR data, it concludes that the crystalline La2O3 do not changes with ZrO2 in the synthesized ZrO2/La2O3 catalyst.The X-ray diffraction patterns for the synthesized La2O3 and ZrO2/La2O3 catalysts were illustrated in Fig. 2 . In the XRD spectrum, it can be clearly seen that the highly narrow sharp lines for the formation of La2O3. The Braggs reflection pattern can be assigned for the formation of lanthanum oxide with cubic phase. The intense diffraction peaks obtained from XRD data include the 2θ values 15.66, 27.31, 27.98, 31.63, 39.49, and 48.62. A strong intensity peak (Fig. 2) is detected at a diffraction angle of 31.63, which is assigned to (101) plane of La2O3. The other peaks are assigned to (002), (102), and (110) lattice planes belonging to cubic crystalline phase of La2O3 and agreed with the previous report [42]. In the XRD pattern of the synthesized ZrO2/La2O3 catalyst, in addition, the strongest intensity peaks appeared at 2θ values 25.12, 30.58 and 44.05 these are associated with (111), (200), and (201) planes respectively, which indicates ZrO2 doped La2O3, which is suggested to the hexagonal phase. The average particle size was calculated using the Debye-Scherer equation D = 0.9 λ/β cosθ (where D is the average crystalline size, λ is X-ray wavelength, β is (FWHM) diffraction line and θ is the diffraction angle). The average crystalline size for La2O3 and ZrO2/La2O3 catalysts was found to be 35 ± 5 nm.The surface and structural morphology of prepared La2O3 and ZrO2/La2O3 catalysts were characterized by SEM. In the SEM images of La2O3 and ZrO2/La2O3 as shown in Fig. 3 , these nanoparticles are found to be very effective to the surface area contribution and similar to each other. The average crystalline size of the particles was also found to be less than 50 nm. It indicates that the particles were uniformly distributed all over the surface and spherical in shape and this result was agreement with XRD results hexagonal phase with same crystallite size. SEM micrographs indicated that these nanoparticles are comprised uneven spheres. It is obvious that there is some aggregation occurring in these nanoparticles. The SEM micrographs further disclose the materials porosity, which is necessary for catalytic applications.The energy-dispersive spectra (EDS) was analyzed for La2O3 and ZrO2/La2O3 catalysts to confirm the elemental composition of prepared La2O3 and ZrO2 nanoparticles. The EDS spectrum of La2O3 and ZrO2/La2O3 catalysts were illustrated in Fig. 4 , it clearly indicating the presence of elemental lanthanum, zirconium, and oxygen at 4.3 keV, 2.1 keV, and 0.3 keV, respectively, which is in good agreement with the reported status of ZrO2 doped La2O3. In the molecular formula of ZrO2/La2O3 catalyst, the stoichiometric atomic weight percentage for lanthanum to oxygen is 1:3, and in the present synthesis, the stoichiometric atomic weight percentage exactly matches with the ideal composition of ZrO2/La2O3 catalyst material.The UV–visible spectra of the synthesized La2O3 and ZrO2/La2O3 catalysts were presented in Fig. 5 . The absorption edges obtained from the plots of absorbance vs wavelength. (The interception of the tangent on the descending part of the absorption peak of the wavelength axis gives the value of diffuse absorption edge in nm). The UV–visible spectrum of La2O3 shows absorption peak in visible region, the wavelength observed at 380 nm with band gap 3.26 eV (The band gap was measured using Eg = 1240/λ formula, where Eg is the band gap energy and λ is the wavelength of the absorption edge). The UV–visible spectrum of the synthesized ZrO2/La2O3 catalyst shows a trivial red shift with compared to La2O3 at observed wavelength at 385 nm with band gap of 3.21 eV, the red shift in UV–visible DRS spectrum clearly indicate incorporation of ZrO2 on La2O3 in ZrO2/La2O3 catalyst.The surface area is an important aspect for the important applications such as surface adsorption and catalytic phenomenon. The synthesized La2O3 and ZrO2/La2O3 catalysts were investigated by nitrogen adsorption/desorption isotherms for the determining the quantitative aspect such as surface area. The specific surface area of the synthesized La2O3 and ZrO2/La2O3 catalysts were found to be 7.0157 m2/g and 11.0191 m2/g, respectively as shown in Fig. 6 . The specific surface area of ZrO2/La2O3 catalyst was superior than that of pure La2O3, the better surface area may due to impression of ZrO2 on the surface of La2O3 in the ZrO2/La2O3 catalyst. It clearly represent the ZrO2 is strongly influences the surface area of pure La2O3 in the ZrO2/La2O3 catalyst.In the application part, all the experiments were performed at the optimized concentration of catalyst. For catalytic study, the standard reaction between of benzaldehyde, urea, and acetoacetate as well as effect of solvent was investigated and summarized in Tables 1 and 2 . By varying the catalyst concentration from 2 to 10 mol%, the best outcome was obtained using concentration of 5.0 mol% catalyst under ultrasonic irradiation at 50 °C. The catalyst loading and selection of solvent were the important tasks during the present study. The increase in catalyst loading was found to exert substantial effect on product yield. There was not much difference in the yield and reaction time when catalyst loading was changed from 5.0 to 10 mol%. However, in terms of catalytic efficiency, 5.0 mol% was found to be the better choice, and therefore, the remaining experiments were performed at the concentration of 5.0 mol%. During the solvent effect study, when we switched our attention is without solvent to other solvents (Table 2), there was marked effect on the yield of the product. The scope and generality of this protocol were studied by performing the experiments with broad range of aromatic aldehydes. Importantly, we found that benzaldehydes with variety of substitution pattern did not show large difference in the yield of tetrahydropyrimidine derivatives in this novel route.The prepared 5.0 % ZrO2/La2O3 catalyst was efficaciously used to orchestrate tetrahydropyrimidine derivatives under reflux conditions. A standard reaction encompassing benzaldehyde, urea, and acetoacetate was inspected for catalyst optimization as shown in Scheme 1 . Having followed a literature review, it was encountered that ethanol was the most prevalently utilized solvent for the synthesis of a wide range of heterocyclic compounds prompting us to utilize ethanol solvent. It was encountered that a catalyst potency of 50 mg produces better performance. One of the most critical factors throughout this investigation became to optimize the catalyst loading and pick the right solvent. We actually started our exploration without a catalyst and afterward progressively augmented the dose of the catalyst. The proportion of the 5.0 % ZrO2/La2O3 catalyst dose was found to be superior in terms of catalytic proficiency and yield of the product under solvent free conditions, the remaining reactions with various aldehydes were likewise accomplished with a 5.0 % ZrO2/La2O3 catalyst loading and the obtained results were summarized in Table 1.The validity of present protocol was tested by employing the optimization conditions to synthesize broad range of tetrahydropyrimidine derivatives using aromatic aldehydes comprising various types of substituents (Table 1). All types of aromatic aldehydes furnished resulted in good yield within short reaction time without purification required. Among the heterocyclic aldehydes, the compounds of aldehydes (formula Ic, Ig, Ij, and Ii) was also used to check whether heterocyclic ring remains intact or not and to our credit here also product yield was quite higher with good stability of the resulting product. Besides all the aldehydes (formula Ib, Id, Ie, and Ih) were excellent in producing tetrahydropyrimidine derivatives without affecting the yield and reactions completed within one hour. Table 1 is completely depicts the physico-chemical data of the synthesized tetrahydropyrimidine derivatives by using the synthesized 5.0 % ZrO2/La2O3 catalyst.FTIR spectrum, ν, cm−1: 758, 1219, 1643, 1724 and 3242; 1H NMR spectrum, δ, ppm: 0.93–0.95 (t, 3H), 2.11 (s, 3H), 3.84–3.86 (q, 2H), 5.17 (s, 1H), 5.84 (s, 1H), 7.05–7.08 (m, 5H), 8.15 (s, 1H); M 261 [M+H]+.FTIR spectrum, ν, cm−1: 783, 1219, 1645, 1703 and 3240; 1H NMR spectrum, δ, ppm: 1.07–1.11 (t,3), 2.24 (s, 3H), 3.70 (s, 3H), 3.97–4.01 (q, 2H), 5.26 (s, 1H), 5.95 (s, 1H), 6.73–6.75 (d, 2H), 7.14–7.16 (d, 2H), 8.38 (s, 1H); M 291 [M+H]+.FTIR spectrum, ν, cm−1: 792, 1226, 1637, 1693 and 3348; 1H NMR spectrum, δ, ppm: 0.81–0.83 (t, 3H), 2.20 (s, 3H), 3.77–3.99 (q, 2H), 5.51 (s, 1H), 5.64 (s, 1H), 6.99–7.14 (m, 4H), 8.03 (s, 1H); M 295 [M+H]+.FTIR spectrum, ν, cm−1: 779, 1221, 1645, 1706 and 3240; 1H NMR spectrum, δ, ppm: 0.95–0.95 (t, 3H), 2.10 (s, 3H), 3.85–3.87 (q, 2H), 5.16 (s, 1H), 5.79 (s, 1H), 7.01–7.04 (m, 4H), 8.15 (s, 1H); M 295 [M+H]+.FTIR spectrum, ν, cm−1: 775, 1226, 1647, 1699 and 3360; 1H NMR spectrum, δ, ppm: 1.20–1.23 (t, 3H), 2.22 (s, 3H), 2.30 (s, 3H), 4.12–4.14 (q, 2H), 4.95 (s, 1H), 5.68 (s, 1H), 7.01–7.03 7.05–7.07 (d, 2H), 7.95 (s, 1H); M 275 [M + H]+.FTIR spectrum, ν, cm−1: 734, 1228, 1668, 1712 and 3342; 1H NMR spectrum, δ, ppm: 1.12–1.15 (t, 3H), 2.28 (s, 3H), 3.89–3.92 (q, 2H), 5.46 (s, 1H), 5.99 (s, 1H), 7.65–7.69 (m, 3H), 8.16 (s, 1H); M 306 [M+H]+.FTIR spectrum, ν, cm−1: 781, 1217, 1674, 1721 and 3340; 1H NMR spectrum, δ, ppm: 1.25–1.28 (t, 3H), 2.35 (s, 3H), 4.12–4.15 (q, 2H), 5.55 (s, 1H), 5.69 (s, 1H), 7.41–7.43 (d, 2H), 7.56–7.58 (d, 2H), 8.38 (s, 1H); M 306 [M+H]+.FTIR spectrum, ν, cm−1: 787, 1223, 1651, 1726 and 3329; 1H NMR spectrum, δ, ppm: 1.24–1.27 (t, 3H), 2.35 (s, 3H), 3.85 (s, 3H), 4.07–4.10 (q, 2H), 5.25 (s, 1H), 5.75 (s, 1H), 6.80–6.81 (d, 1H), 6.89 (s, 1H), 7.25 (d, 1H), 8.11 (s, 1H); M 307 [M+H]+.FTIR spectrum, ν, cm−1: 787, 1212, 1651, 1725 and 3346; 1H NMR spectrum, δ, ppm: 1.18–1.21 (t, 3H), 2.35 (s, 3H), 3.85 (s, 3H), 3.87 (s, 3H), 4.11–4.14 (q, 2H), 5.37 (s, 1H), 5.56 (s, 1H), 6.58 (s, 1H), 6.88–6.90 (m, 2H), 8.15 (s, 1H); M 321 [M+H]+.FTIR spectrum, ν, cm−1: 792, 1222, 1648, 1718 and 3322; 1H NMR spectrum, δ, ppm: 1.24–1.27 (t, 3H), 2.35 (s, 3H), 3.85 (s, 9H), 4.11–4.14 (q, 2H), 5.35 (s, 1H), 5.69 (s, 1H), 6.59 (s, 2H), 8.07 (s, 1H); M 351 [M+H]+.FTIR spectrum, ν, cm−1: 754, 1228, 1666, 1712 and 3348; 1H NMR spectrum, δ, ppm: 1.13–1.16 (t, 3H), 2.30 (s, 3H), 4.11–4.14 (q, 2H), 5.42 (s, 1H), 5.99 (s, 1H), 7.21–7.22 (d, 1H), 7.33–7.34 (d, 2H), 7.41 (s, 1H), 8.21 (s, 1H); M 329 [M+H]+.FTIR spectrum, ν, cm−1: 785, 1232, 1658, 1720 and 3337; 1H NMR spectrum, δ, ppm: 1.18–1.21 (t, 3H), 2.35 (s, 3H), 4.01–4.04 (q, 2H), 5.51 (s, 1H), 5.68 (s, 1H), 6.29 (d, 1H), 6.99–702 (m, 2H), 8.00 (s, 1H); M 251 [M+H]+.FTIR spectrum, ν, cm−1: 788, 1215, 1660, 1737 and 3328; 1H NMR spectrum, δ, ppm: 1.16–1.19 (t, 3H), 2.35 (s, 3H), 4.12–4.15 (q, 2H), 5.60 (s, 1H), 5.61 (s, 1H), 6.29–6.31 (d, 1H), 6.41–6.43 (d, 1H), 7.22–7.35 (m, 5H), 7.98 (s, 1H); M 174 [M+H]+.FTIR spectrum, ν, cm−1: 775, 1224, 1645, 1697 and 3346; 1H NMR spectrum, δ, ppm: 1.08–1.12 (t, 3H), 2.26 (s, 3H), 3.98–4.03 (q, 2H), 5.28 (s, 1H), 5.89 (s, 1H), 6.70–6.79 (m, 3H), 6.89–6.92 (d, 2H), 7.19 (s, 1H),7.34–7.40 (m, 3H), 8.18 (s, 1H); M 311 [M+H]+.In conclusion, we represent multicomponent synthesis of broad range of tetrahydropyrimidine derivatives employing robust and green 5.0 % ZrO2/La2O3 catalyst under neat reaction conditions from commonly accessible aromatic aldehydes, acetoacetate, and urea. Moreover, the 5.0 % ZrO2/La2O3 catalyst was synthesized by using a very simple co-precipitation approach that furnished highly pure product with high yield. FTIR, UV–vis, XRD, SEM, EDS, and BET, have been used to investigate the physical, morphological, and surface characteristics of La2O3 and ZrO2/La2O3 catalysts. The characterization study revealed that synthesized catalyst possessing hexagonal structure and having high porosity for catalytic activity. The synthesized tetrahydropyrimidine were characterized by 1H NMR and 13C NMR spectroscopic techniques to confirm their formation. The catalyst loading was optimized and we disclosed that 5.0 % ZrO2/La2O3 catalysts furnished more than 90 % product yield that too within short time span. The present protocol validates ample substrate scope comprising wide range of aldehydes and acetoacetatee in combination with urea. The benefits of this environmentally friendly framework encompass simple approach of synthesis of nanocatalyst, the heterogeneous nature of the nanocatalyst and its simple separation from the reaction mass, quick reaction times, simple isolation, high yields, and clear and simple work-up procedures. G. Balraj: Data curation, Investigation, Methodology, Software, Writing – original draft, Writing – review & editing. Kurva Rammohan: Data curation, Methodology, Conceptualization, Funding acquisition, Project administration, Validation. Ambala Anilkumar: Data curation, Investigation, Methodology, Software, Writing – original draft, Writing – review & editing. M. Sharath Babu: Data curation, Methodology, Conceptualization, Funding acquisition, Project administration, Validation. Dasari Ayodhya: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, 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 authors would like to thankful to the Head, Department of Chemistry, Osmania University, Telangana state, India for continuous encouragement and providing necessary facilities.Not applicable.

The objective of the present study is focused on the facile synthesis of ZrO2/La2O3 catalyst and the catalytic performance was evaluated towards one-pot synthesis of tetrahydropyrimidne derivatives through Biginelli condensation reaction. The synthesized catalyst was characterized by using FTIR, XRD, BET, SEM, EDX and UV-DRS techniques. All catalysts perform better than the without catalyst based reactions, which is the formation of tetrahydropyrimidne derivatives (IVa-n) conversion averaging 85–95 % at 50 °C temperature and 3–40 min under identical reaction conditions. Also, it maintains good stability. The relative ease of reducing the catalyst plus its porous nature are responsible for its performance. All the products were characterized by comparing their physical and spectral data including FTIR, 1H NMR, and ESI-MS with those of authentic compounds reported in the literature.

Efficient and cheap renewable hydrogen production from water electrolysis is a crucial challenge for a sustainable society [1,2]. Anion exchange membrane (AEM) water electrolysis aims to combine the low cost of alkaline with the advantages of proton exchange membrane (PEM) electrolyzers [3,4]. Nickel is the most active and cheapest non-noble metal catalyst reported for hydrogen evolution reaction (HER) in alkaline electrolysis [5,6]. Nickel initially shows a high HER activity, but the activity deteriorates rapidly over time due to the formation of metal hydride under HER conditions [7]. Alloying nickel with other elements such as Mo, P, S, Cu increases HER activity and stability [8]. Sluggish HER kinetics in alkaline electrolytes causes high overpotentials thus remains a challenge to develop a highly active and stable HER catalyst [9,10]. The mechanism of the HER in alkaline media is usually discussed in terms of the Volmer, Heyrovsky, and Tafel reactions [11], and a fundamental understanding of the factors influencing the rates of these steps may provide important clues for catalyst design.NiCu has been reported as an active HER catalyst in alkaline electrolytes. NiCu catalysts have been synthesized using different processes such as freeze casting [12] electrodeposition [13] and powder metallurgy [14]. He et al. obtained a current of −10 mA/cm2 at 117 mV with NiCu synthesized by galvanostatic deposition and with an atomic ratio of Ni to Cu equal to one [15]. Electrodeposited NiCu nanosheets exhibited enhanced HER activity with an onset potential of 48 mV vs. RHE [16]. Solmaz et al. reported that NiCu showed higher HER activity than Ni and Cu due to the roughness effect and synergistic interaction between Ni and Cu atoms [17,18]. Oshchepkov et al. found that high mass activity of Ni0.95Cu0.05/C is due to the electronic influence of Cu on Ni [19]. However, there is a deficiency in the literature of results demonstrating NiCu catalyst as a cathode in real alkaline electrolyzers.Mixed metal oxide (Ni/NiO-transition metal oxide (TMO)) composite structures exhibit superior HER activity [20–22] NiO attracts OHads while the metallic Ni attracts Hads intermediate during HER, thus lowering the free energy of the first step in the HER, viz. the formation of adsorbed hydrogen through the Volmer step of the reaction. The other TMO oxide, such as Cr2O3 or Fe3O4 appears to stabilize the composite NiO component under HER conditions [20–23]. Therefore, it is beneficial to design HER catalyst containing both a catalytic element and elements (or oxides) stabilizing mixed oxidation states for the catalytic element. Below we suggest NiCu mixed metal oxide (MMO) to catalyze the HER so that the catalyst contains both Ni (which has an affinity for Had) and NiO (which has an affinity for OHad), the simultaneous presence of these being stabilized by CuO under HER conditions.Ionomers are frequently employed in electrochemical testing is to promote ink uniformity and coating quality [24]. The presence of the ionomer may also increase ionic conductivity and minimizes mass-transport limitations related to the diffusion of the ionic species [24]. The literature has reported that catalytic layers containing Nafion ionomer result in higher HER activity compared to catalytic layers with other anion exchange ionomers [4,21]. The activity difference has been attributed to several factors, such as ionomer head groups, and ionomer backbone chemistry [4,21]. However, tuning ionomer to catalyst ratio is required for optimum catalyst utilization [25]. Not only its activity and stability but also the interaction of NiCu metal mixed oxide catalyst with ionomers and effects of an aqueous electrolyte, is therefore also important if this catalyst is to play a role in upscaling to AEM water electrolyzer devices, preferably including not only experiments in aqueous cells but also electrolyzer testing.In this work, we investigate the HER activity of various nickel-copper catalysts such as NiCu alloy, NiCu oxide, and NiCu mixed metal oxide (MMO) synthesized by chemical reduction. As we will show below, NiCu MMO shows an exceptional HER activity in alkaline media. In situ Raman spectroscopy under HER conditions was carried out to investigate the state of copper and nickel species present and how these states vary over time for various nickel copper catalysts and correlate this with their activity for the HER. The electrochemical activity of NiCu MMO was further optimized in terms of the type of ionomer binder, KOH concentration in the aqueous electrolyte, and the ionomer to carbon ratio. An AEM water electrolyzer based on membrane electrode assembly (MEA) of NiCu MMO at the cathode and Ir black at the anode was fabricated, tested, and compared to Pt/Ir MEA. The NiCu MMO/Ir MEA shows comparable performance to Pt/Ir MEA which indicates that it could replace scarce and expensive Pt catalyst.NiCu alloy and NiCu oxide were synthesized by mixing 10 mmol of nickel nitrate hexahydrate Ni(NO3)2.6H2O (97.0%, Sigma Aldrich) and 10 mmol Copper(II) sulfate pentahydrate (98.0%, Sigma Aldrich) in 500 ml water (18.2 MΩ cm, 3 ppb TOC, Milli-Q ultrapure water). The precursor mixture was stirred for 15 min at 750 rpm. 200 ml of 0.15 M NaBH4 (98%, Sigma Aldrich) was added dropwise while bubbles were observed. The solution mixture was stirred for another 1 hour to ensure the complete chemical reduction of precursors. The resulting precipitate was centrifuged 5 times at 8000 rpm for 6 min and cleaned with water and ethanol three times. The produced precipitate was dried in a vacuum oven at 80 °C overnight. The dried powder was annealed in an air atmosphere to obtain NiCu oxide or 5%H2/Ar to obtain NiCu alloy. The annealing was done at 500 °C for 6 h with a ramping rate of 10 °C/min.In order to make NiCu MMO, 100 ml of 1 M Na2CO3 (≥99.5%, Sigma-Aldrich) were added to nickel-copper precursors solution until the solution became milky and pH reached 10. The mixture was then stirred for another 15 min, followed by the addition of NaBH4 dropwise. The produced catalyst was subjected to the same procedure for cleaning and drying as above. The dried powder was annealed in 5%H2/Ar atmosphere at 500 °C for 6 h with a ramping rate of 10 °C/min.For catalyst supported on carbon, Ketjen black EC-600JD (AkzoNobel) was dispersed in the precursors' solution mixture to get (60 wt% catalyst supported on carbon) and stirred for another 1 hour before adding NaBH4 and complete the chemical reduction step.Scanning electron microscopy (SEM, Carl Zeiss supra 55) and energy dispersive X-ray (EDX) spectroscopy in the SEM device were used to study the morphology and elemental composition of catalysts. The catalyst morphology was further studied using Hitachi S-5500 via scanning transmission electron microscopy (STEM) mode. Bruker D8 A25 DaVinci X-ray device (Cu-Kα radiation with a wavelength of 1.5425 Å) was used to examine the crystalline characteristics of catalysts. X-ray diffraction (XRD) patterns were taken between 15 [2θ] and 75 [2θ] using a step size of 0.3 [2θ]. WITec alpha300 R Confocal Raman device with a 532 nm laser was used to collect the Raman vibrational characterstics of catalyst powders. X-ray photoelectron spectroscopy (XPS) was done via an Axis Ultra DLD instrument (Kratos Analytical) equipped with Al X-ray monochromatic source.Electrochemical investigation of the catalysts was carried out in a three-electrode cell using a rotating disk electrode (Pine Research,) with an (Ivium-n-Stat) potentiostat. Carbon paper (Toray 090, Fuel cell store) was used as the counter-electrode while Hg/HgO electrode (Pine Research) was served as the reference electrode. The working electrode was catalyst deposited on glassy carbon (GC) electrodes (5 mm diameter, Pine Research). The GC electrode was polished using alumina suspension (5 and 0.05 μm, Allied High-Tech Products, Inc.) on polishing pads. The GC electrode was then washed, sonicated in 1 M KOH for 5 min, and finally rinsed with water. The catalyst ink was prepared by dispersing 10 mg catalyst powder in 1.0 mL of a solution [500 μL water, 500 μL isopropanol]. The ionomer used was either Nafion (5 wt%, Alfa Aesar) or anion exchange ionomer Fumion FAA-3 (10 wt% fumatech) with an ionomer to catalyst weight ratio of 0.2. The Nafion ionomer to catalyst weight ratio in the ink was then optimized from a selection of weight ratios equal to 0.1, 0.3, 0.5, 0.7, and 0.9. The ink was then sonicated for 30 min in an ice bath. Catalyst loading on the GC surface was kept 250 µg/cm2.The catalyst ink was spin-coated on a GC electrode turned upside down and rotated to assure a homogenous catalyst distribution. A water drop was deposited on the electrode before immersed in the electrolyte to prevent air bubbles from forming at the electrode surface. All the electrochemical measurements were conducted in N2-saturated 1 M KOH electrolyte at room temperature (20 ± 2). The electrolyte was purged for 30 min with N2 gas before using and during the experiment to remove any dissolved gasses during electrochemical measurements. The electrolyte was prepared by using KOH (Sigma Aldrich, 85%), and water (18.2 MΩ cm, Milli-QⓇ Integral ultrapure water). The electrolyte was purified according to the procedure reported by Trotochaud et al. [26].The working electrode underwent electrochemical activation by cycling between −0.8 to −1.5 V vs Hg/HgO at a scan rate of 100 mV/s for 50 cycles. The linear sweep voltammetry (LSV) polarization curves were recorded in a potential range of −0.8 to −1.5 V vs. Hg/HgO at 1 mV/s sweep rate under continuous stirring at 1600 rpm to avoid the accumulation of gas bubbles over the GC electrode. The electrochemical impedance spectroscopy (EIS) measurements were collected at specific overpotentials (−100 to −250 mV) in a frequency range of 0.1 − 105 Hz with an amplitude of 10 mV alternative current (AC) perturbation. In this work, ohmic resistance (IR) drop was compensated at 85% of high-frequency resistance, which was measured by the EIS technique. The potential was compensated by the following equation: (1) E compensated = E measured − i R where E compensated and E measured are compensated and measured potentials, respectively.The Hg/HgO potentials were converted to RHE by measuring the voltage at zero current of the HER curve in a hydrogen-saturated electrolyte on Pt electrodes. The Hg/HgO reference electrode potential was converted to RHE in 1 M KOH using the following equation: (2) E vs RHE = E vs Hg / HgO + 0.9 All the reported current densities were normalized to the geometric area of the electrode.The electrochemical active surface area (ECSA) was measured by the electrochemical double-layer capacitance method. Then capacitance from 0.9 to 1 V vs RHE at scan rates of 50, 100, 150, 200, 250 mV/s. The CV used for electrochemical double-layer capacitance (Cdl) calculation was acquired in a potential window where no Faradaic process occurred. To derive the Cdl, the following equation was used: (3) C d l = I c ν where Cdl is the double-layer capacitance (mF/cm2) of the electroactive materials, Ic is the charging current (mA/cm2), and ν is the scan rate (mV/s).Chronoamperometry was measured at a fixed potential (−0.4 V vs. RHE) for 30 h. The stability of the catalyst material was also evaluated using an accelerated stress test (AST). AST was carried out by cycling the electrode between −0.8 to −1.3 V at a scan rate of 100 mA/cm2 for 5000 cycles. The Hg/HgO reference electrode was calibrated versus a reversible hydrogen electrode (RHE) in 1 and 0.1 M KOH. The electrochemical data shown are average data from 3 inks from every powder for each catalyst.In situ Raman measurements were carried out with a lab-made Teflon cell. The catalyst deposited on GC (pine research), a carbon paper (fuel cell store), and Hg/HgO (Pine Research) was used as a working, counter, and reference electrode, respectively as in Fig. 1 . In situ Raman spectra were collected using a WITec alpha300 R Confocal Raman microscope [532 nm laser with a power of 5.0 mW] coupled with Zeiss EC Epiplan 10x objective and G1: 600 g/mm BLZ=500 nm grating. The GC surface was polished with μm-sized alumina powders, then sonicated in 1 M KOH for 5 min and then rinsed with water and dried in air. The experiments was carried out using purified N2-saturated 1 M KOH electrolytes. The laser is emitted on the working electrode through a transparent quartz glass window that reduces contamination and interference. All the experiments were conducted at room temperature (20 ± 2 °C). All the data points were processed using origin software.In situ Raman-chronoamperometry study was done at −0.4 V vs. RHE for 30,000 s for NiCu catalysts. The Raman spectra were collected at the applied potential in 1 M KOH every 10 sweeps (10 s/sweep) from 100 to 2000 cm−1. The spectrum shift of silicon wafer Raman peak at 520.7 cm−1 was used for calibration.Catalyst inks were fabricated by mixing catalyst powder with water: isopropanol (1:1), and ionomer (Fumion FAA-3-SOLUT-10 (Fuel Cell Store)). The solution was sonicated for 30 min to ensure fine and well-dispersed ink. Cathode catalysts loadings were 1 mg/cm2 for Pt/C (60 wt% metal on support, Alfa Aesar) and 5 mg/cm2 for 60 wt% NiCu MMO/Ketjen black. An Ir black benchmark catalyst with a loading of 3 mg/cm2 (Alfa Aesar) was used at the anode for all MEAs. Catalyst layers were sprayed at 60 °C using a Coltech airbrush (0.35 mm nozzle) on Toray 090 carbon paper (25 cm2, Fuel Cell Store) for the cathode, and Ti felt (Bekaert Inc.) coated with Au for anode as catalyst coated substrates (CCSs). The area of carbon paper equals the area of Ti felt and represents the electrode surface area (25 cm2). The Ti felt was pretreated by etching in HCl (37 wt%, Sigma Aldrich) for 2 min to remove the non conductive surface oxide and then sonicated for 5 min in water and ethanol before being sputter-coated with Au using an Edwards sputter coater to reduce interfacial contact resistance (ICR) within the cell. The coating was carried out at a vapor deposition pressure of 0.15 atm at 20 mA for 2 min on each side. The ionomer content amounted to 25 and 7 wt% of the total solids in ink for cathode and anode, respectively. The membrane, Fumapem-3-PE-30, was sandwiched between cathode and anode gas diffusion electrodes as in Fig. 2 . The MEAs were conditioned and exchanged to the OH form in 1 M KOH overnight. The AEM water electrolyzer setup consisted of a 5 L Teflon tank with heaters and a peristaltic pump. Tests were conducted at T = 50 °C. The concentrations of KOH employed were 1 and 0.1 M KOH (ACS reagent, ≥85%, pellets, Sigma Aldrich). The flow rates of the pumps were 250 ml/min.A high-current potentiostat (HCP-803, Biologic) was used to control cell voltage and measure impedance in the single-cell measurements. The polarization curve was recorded galvanostatically, ramping the current from 0 to 2 A/cm2 at a rate of 80 mA/cm2 per minute. Electrochemical impedance spectroscopy (EIS) was employed to determine the cell resistances and performed at different current densities, such as 0.2 A/cm2, in the AC frequency range of 100 kHz–1 Hz. The NiCu MMO catalytic layers were post analyzed by SEM and EDX.SEM and STEM images of the nickel-copper catalyst synthesized by chemical reduction with the addition of Na2CO3 and annealed in 5% H2/Ar (NiCu MMO) are shown in Fig. 3 . The Figs. 3a and 3b show that NiCu MMO catalysts have dense areas of agglomerated nanosheet morphology. The STEM image in Fig. 3c displays that NiCu MMO nanosheets are loaded on the carbon support (Ketjen black EC-600JD) with a dark thick region of NiCu nanosheets. Fig. 3d confirmed the loosely stacked nanosheets morphology of NiCu MMO catalysts. Similar catalyst morphology produced by chemical reduction by sodium borohydride has given various names from nanocotton [29], nanosponges [30–32, and nanosheets [33–39] and in this work, we will refer to these catalysts as nanosheets. During the chemical reduction process, sodium borohydride reacts quickly with transition metal cations to precipitate metal boride MxBy species [39–42]. In the case of NiCu MMO, Na2CO3 was added during the synthesis process to precipitate oxide species [21]. We investigated another nickel-copper catalyst without the addition of Na2CO3 and annealed the resulted powder in the air (NiCu oxide) and 5% H2/Ar (NiCu alloy) and they exhibited also an agglomerated nanosheets morphology similar to NiCu MMO as seen in (Fig. S1). Energy dispersive x-ray spectroscopy (EDX) of NiCu MMO is shown in Fig. 4 a. The EDX spectrum displays peaks corresponding to Ni, Cu, O, and C with Ni: Cu weight percentage as 52.3:47.7, which is in good agreement with precursors percentage. The EDX spectrum displays peaks corresponding to Ni, Cu, O, and C. Impurities or remaining elements from the synthesis process appear to be absent.The XRD pattern of NiCu MMO in Fig. 4b shows peaks at 2θ values of 32.5°, 35.6°, 37.2°, 38.9°, 43.2°, 44.56°, 48.9°, 51.93°, and 62.8°. The diffraction peaks at 2θ values of 44.5° and 51.93° are associated with Ni (111) and Ni(200) crystal planes of nickel face-centered cubic (FCC) structure with (JCPDS card No. #04–0850) [43]. The peaks at 2θ values at 37.2°, 43.2°, and 62.8° correspond to (111), (200), and (220) diffraction planes of NiO (JCPDS card no. #47–1049) [44]. The diffraction peaks at 2θ values of 32.5°, 35.6°, 38.9°, 48.9° values correspond to CuO crystal structure (JCPDS card no. #80–0076) [45].NiCu alloy shows peaks at 2θ values of 44.5° and 51.93° that correspond to pure Ni (JCPDS No. 04–0850) [43] while the peaks at 44° and 51.2° correspond to pure Cu (JCPDS No. 04–0836) [46]. While NiCu oxide shows peaks at 2θ values of 37.2°, 43.2°, and 62.8° correspond to NiO (JCPDS card no. #47–1049) and peaks at 2θ values of 32.5°, 35.6°, 38.9°, 48.9° of CuO crystal structure (JCPDS card No. #80–0076) [44,45]. The NiCu MMO vibrational modes were characterized by Raman spectroscopy in Fig. 4c. The Raman spectrum in Fig. 4c shows Raman peaks at 490, 606, 810, 1020, and 1100 cm−1 respectively. The Raman peak at 490 cm−1 corresponds to Cu(OH)2 while the Raman peak 606 cm−1 corresponds to the Bg Raman mode of CuO [47–50. The Raman peaks at 810, 1020, and 1100 cm−1 correspond to two-phonon (2P) NiO vibrational modes [51–55.XPS analysis provides sensitive information about the surface chemical composition of NiCu MMO catalyst. NiCu MMO survey spectrum is shown in Fig. 4d. The survey spectrum indicates the presence of Ni, Cu, B, O, and C peaks. Ni 2p high-resolution XPS spectrum is shown in Fig. 5 a and 5b. The Ni 2p XPS spectrum is divided into two main peaks (Ni-2p1/2 and Ni-2p3/2) due to the spin-orbit effect and two oxidation states for nickel (Ni0 and Ni2+) can be deconvoluted. The XPS peaks at 853.8 eV and 871.4 eV can be assigned to Ni 2p3/2 and Ni 2p1/2 of Ni0 [20,56]. The XPS peaks located at 855.4 eV with a satellite at 860.9 eV correspond to Ni 2p3/2 of Ni2+. The peak at 872.5 eV with a satellite at 879.4 eV can be attributed to Ni 2p1/2 of Ni2+ [20,56] Cu-2p high-resolution spectrum is shown in Fig. 5c and 5d. The XPS peaks at 932.6 eV and 952.4 eV correspond to Cu 2p3/2 and Cu 2p1/2 of Cu0 [57]. The XPS peak at 933.7 eV corresponds to CuO [58]. The peaks at 934.8 and 954.4 eV are associated with Cu(OH)2 [58]. Cu(OH)2 appears to form due to CuO reaction with chemisorbed water on the catalyst surface.The high-resolution Ni 2p XPS spectrum in NiCu alloy exhibits peaks at 852.4 and 869.5 eV which correspond to Ni 2p3/2 and Ni 2p1/2 peaks of metallic Nio) Fig. S2a ([59]. The Cu 2p spectrum shows two peaks at 932.5 and 952.3 eV which are assigned to Cu 2p3/2 and Cu 2p1/2 of metallic Cu0 (Fig. S2b) [16].The high-resolution XPS spectrum of Ni 2p in NiCu oxide shows that the Ni 2p3/2 main peak and its satellite at 854 and 862 eV, and the Ni 2p1/2 main peak and its satellite at 872 and 879 eV, respectively confirming the presence of Ni+2 state (Fig. S2c) [60]. The high-resolution XPS spectrum of the Cu 2p spectrum of NiCu oxide shows peaks at 933.7, 943.1, 954.3, 962.9 eV. The peaks at 933.7 and 954.3 eV correspond to the Cu 2p3/2 and Cu 2p1/2, respectively. Also, there are two satellite peaks centered at about 943.1 and 962.9 eV, demonstrating the presence of Cu+2 state (Fig. S2d) [61].Based on the structural characterization. NiCu mixed metal oxide (MMO) nanosheets have Ni, NiO, CuO phases and hydroxide species such as Cu(OH)2 which can be beneficial for HER in alkaline electrolytes [62] as we will see from the electrochemical measurements. NiCu alloy contains pure Ni and pure Cu phases while NiCu oxide contains NiO and CuO phases. Fig. 6 a shows linear sweep voltammetry (LSV) curves of NiCu alloy, NiCu MMO, and NiCu oxide in 1 and 0.1 M KOH. All catalyst loadings were equal to 250 µg/cm2. NiCu MMO has the highest HER activity in 1 M KOH by achieving −10 mA/cm2 at −200 mV compared to the −250 and −300 mV for NiCu alloy and NiCu oxide, respectively, to obtain the same current density. As seen from Fig. 6a, the current density normalized to geometric surface area for NiCu MMO at −0.35 V vs RHE in 1 M KOH is five times higher than 0.1 M KOH. However, the activity trend for the nickel-copper catalysts is the same in 0.1 M KOH. Fig. 6b shows a comparison between the NiCu MMO HER activity and data from the literature. The NiCu MMO shows one of the best mass active HER catalytic activities reported in Table S1, Table S2, and Fig. 6b.The LSV curves in Fig. 6a show that the HER activity increases with increasing KOH electrolyte concentration, which in agreement with literature [63,64]. Lasia et al. found that the rate constants of Volmer and Heyrovsky reactions depend on the bulk OH−concentrations [65]. An appropriate rise of the KOH electrolyte concentration increases hydroxide ion activity [64–67]. Recently Wang et al. showed that the high HER activity at high KOH concentration is due to H3O+ intermediates generated on nanocatalyst surface [68].Electrocatalytic active surface area (ECSA) measurements were carried out to evaluate the intrinsic catalytic activity of nickel-copper catalysts. The ECSA was estimated by measuring the electrochemical double-layer capacitance (Cd l) from cyclic voltammograms at various scan rates over a non-faradaic (totally polarized) potential range, as in Fig. S3 in the Supplementary Information. The NiCu MMO catalysts exhibit the largest double-layer capacitance Cdl of 9.16 (mF/cm2) compared to those of the NiCu alloy (6.58 mF/cm2) and the NiCu oxide (3.80 mF/cm2), showing that a larger ECSA of NiCu MMO allows more exposed active sites to promote HER performance. The specific surface area of the NiCu catalysts was also investigated with Brunauer–Emmett–Teller (BET) measurement (Figure S3, ESI†). The specific surface area has a similar trend as ECSA. NiCu MMO possesses a surface area of 156 m2/g which is far higher than that of NiCu alloy (112 m2/g) and NiCu oxide (92 m2/g). When normalized to electrochemical surface area (Fig. S3, ESI†) the differences in catalyst activity become less, especially in the lower potential range. However, NiCu MMO still possesses the highest intrinsic activity.The linear regions of Tafel plots in Fig. 6c are fitted to the Tafel equation, yielding Tafel slopes of 120, 130, and 195 mV/dec for NiCu MMO, NiCu alloy, and NiCu oxide respectively. The kinetic parameters for the nickel-copper catalysts (jo and b) presented in Table 1 were derived from the Tafel equation: (4) η = a + b log j Where η (V) is the applied overpotential, j (mA/cm2) is the current density, b (V/dec) is the Tafel slope, and a (V) is the intercept.The exchange of current density jo can be obtained by extrapolating the Tafel plots to the x-axis or assuming η is zero. (5) a = ( 2.3 RT ) / ( α F ) log j o b = ( 2.3 RT ) / ( α F ) where R is the gas constant (8.314 kJ mol−1 K − 1), T is the temperature in K, α is the charge-transfer coefficient, and F is the Faraday constant (96,485 C mol−1). Table 1 summarizes the kinetic parameters for nickel-copper catalysts. NiCu MMO shows the lowest Tafel slope and highest charge transfer coefficient and exchange current density over NiCu alloy and NiCu oxide which confirms the superior activity of NiCu MMO.In view of the Tafel slope being close to 120 mV/dec, it is likely that the charge transfer coefficient represents the symmetry factor of the Volmer step in this case. The Tafel slopes reflect an intensive property of the HER catalysts from which some indication about the reaction mechanism of the HER and the rate-determining step (rds) can be obtained. The Volmer reaction involves the electroreduction of water molecules with hydrogen adsorption as in Eq. (6), while the Heyrovsky's reaction involves electrochemical hydrogen desorption eq (7). The Tafel reaction involves chemical desorption Eq. (8) [65]. (6) M + H 2 O + e − ↔ M H ads + O H − Volmer (7) MH ads + H 2 O + e − ↔ H 2 + M + O H − Heyrovsky (8) MH ads + MH ads ↔ H 2 + M Tafel A detailed analysis shows that rds for the HER at NiCu MMO is the Volmer reaction, then a Tafel slope in the order of 120 mV would result. Whether the next step in the reaction sequence is the Heyrovsky or Tafel step [65] cannot be determined by this analysis, however. Fig. 6d shows impedance complex plane plots of NiCu alloy, NiCu oxide, and NiCu MMO in 0.1 and 1 M KOH at an applied potential of −250 mV vs. RHE after subtracting ohmic resistance. In the complex plane plots, only one semicircle is observed, which can be attributed to a charge transfer process related to the HER [70–72]. The charge transfer resistance (Rct) is represented by the diameter of the semicircle. The radius of the semicircle decreases at higher KOH concentration, signifying a lower charge transfer resistance (Rct) and a higher rate of hydrogen evolution. NiCu MMO exhibit Rct value of 6.96 Ω at an applied potential of −250 mV compared to NiCu alloy (10.38 Ω) and oxide (13.81 Ω) which further confirm the superior activity, faster reaction kinetic and high electron transfer efficiency of NiCu MMO [72].The equivalent circuit for the NiCu alloy, NiCu MMO, and NiCu oxide in Fig. 6d is characterized by a single time constant, and we modeled the impedance by a series resistance (Rs, related to ohmic solution resistance), in series with one parallel circuit consisting of a charge transfer resistance (Rct) and a constant phase element (CPE) related to the double-layer capacitance. This equivalent circuit has previously been used in literature to describe HER on polycrystalline Ni and Ni-based materials [73]. (The constant phase element (CPE) was used instead of capacitance due to frequency dispersion and the appearance of depressed semicircles in the impedance plane plots). The charge transfer resistance (Rct) represents the kinetics of the HER at the electrode/electrolyte interface. The absence of Warburg impedance indicates that mass transport is rapid enough so that the reaction is kinetically controlled [70,71,74]. The impedance complex plane plots for different applied overpotentials are shown in Fig. S4a in the ESI†, and these show that the Rct decreases with increasing potential as it would if the current-voltage relationship is described by Eq. (4) above. (The lower Rct value at higher potential reflects the exponential dependence of the current on the overpotential and thus the accelerated electron transfer and higher rates of the HER at higher overpotential [70,71,74]. As can be shown by a simple differentiation of Eqn. 4 above, the Tafel slope may be obtained from plots of potential vs. log (Rct)−1. From our plots, we obtain 120 mV/dec, see Supplementary Information, Figure S4b, which is the same as that obtained from the LSV curves. The same Tafel slope being obtained with impedance spectroscopy thus validates the iR-corrected linear-sweep voltammograms. Fig. 7 a shows the current vs. time recorded in chronoamperometric measurements performed by applying a constant potential of −400 mV for 30 h on NiCu alloy, NiCu MMO, and NiCu oxide. For all samples but NiCu MMO in 0.1 M KOH, the current density decreases (i.e. the activity decreases) rapidly during the first few minutes. For NiCu MMO in 0.1 M KOH and NiCu oxide in 1 M KOH, the current density then levels off and remains constant at approximately −11 mA/cm2 and −50 mA/cm2, respectively. NiCu oxide shows stable performance at −50 mA/cm2 for 30 h. For NiCu alloy in 1 M KOH, the current density slowly increases (activity increases) with time from approximately −90 mA/cm2 to −100 mA/cm2 after 30 h. For the NiCu MMO sample in 1 M KOH, the current density reaches a minimum activity after approximately 30 min at which the current density is approximately −180 mA/cm2, and then slowly increases to a little below −200 mA/cm2 at 30 h. The chronoamperometric measurements confirm the higher HER activity of NiCu MMO in 1 M KOH than in 0.1 M KOH and over NiCu alloy and NiCu oxide in the same electrolyte. The current densities observed from chronoamperometry are in good agreement with those observed in the LSVs. Fig. 7b shows in-situ Raman spectra of NiCu MMO under an applied potential of −0.4 V vs RHE at different time intervals. All the spectra in Fig. 7b display peaks at 292, 530, 1060, 1350, and 1585 cm−1. The peak at 292 cm−1 can be assigned to copper hydroxide Cu(OH)2 species [47] while the peak at 530 cm−1 can be assigned to nickel hydroxide Ni(OH)2 [75]. The peak at 1060 cm−1 can be assigned to carbonates [76] while the peaks at 1346, and 1585 cm−1 correspond D band, and G band peaks of carbon respectively [77,78]. The spectra show clear peaks of Ni(OH)2 and Cu(OH)2 at the beginning of HER. However, the Cu(OH)2 peaks decreased more significantly than the Ni(OH)2 peaks. In other words, whereas both Cu(OH)2 and Ni(OH)2 both exist during the entire period of 30,000 s, both Cu and Ni hydroxides get slowly reduced, but Cu more so than Ni. The reduction of these elements is consistent with the Pourbaix diagrams of Cu and Ni which predict that both Ni(OH)2 and Cu(OH)2 would be reduced to metallic nickel and copper at this potential [79].The NiCu oxide also displayed peaks corresponding to Ni(OH)2 and Cu(OH)2, but these peaks disappeared completely after 15,000 s, as shown in Fig. 7c. However, the HER activity of the NiCu oxide is much lower than that of NiCu MMO. This confirms the importance of the presence of metallic species on the catalyst surface, as found by Danilovic et al. [80], for superior HER activity.Finally, the Raman spectrum for the NiCu alloy catalyst also shows peaks related to Ni(OH)2 and Cu(OH)2 surface species when the catalyst is immersed in KOH, which confirmed the hypothesis that Ni metal will convert to oxide species once in contact with KOH [80,81]. The hydroxide species on the NiCu alloy gets reduced rather rapidly (< 5000 s) at the surface, as shown in Fig. 7d, compared to NiCu MMO and NiCu oxide and did not lead to exceptional activity compared to NiCu MMO.NiCu MMO thus showed the best HER activity in the alkaline electrolytes with a Tafel slope of 120 mV/dec. The bifunctional system of NiCu MMO catalyst includes Ni metal, NiO, and CuO oxides, and provide a rapid Volmer step and thus rapid overall HER reaction kinetics. The improved HER kinetics of the NiCu MMO can be attributed to the presence of both Ni and NiO where NiO sites to facilitate water dissociation and bind OHad while Ni metallic binds Hads and CuO stabilizes NiO under HER conditions. Similarly, Bates et al. found that the synergistic HER enhancement of Ni/NiO is due to NiO content and Cr2O3 appears to stabilize NiO under HER conditions [82]. The in situ Raman results show that the presence of both metal and oxide phases is essential to sustain a high HER activity, the performance of NiCu MMO relative to that of NiCu alloy or NiCu oxide. We relate this to the in situ Raman data showing that copper hydroxide gets reduced and nickel hydroxide is to some extent preserved under HER conditions.We attribute the rapid decay in electrocatalytic activity in all samples to an initial and rapid adjustment of the surface state of all catalysts, whereas the long-term behavior is more complex. For the NiCu oxide, there is no further change in the surface state after 15000s (Fig. 7c), and the electrocatalytic activity remains the same as that immediately after the initial transient. The current transient is thus fully consistent with the Raman spectra for NiCu oxide in Fig. 7c. Since the Raman spectra of the NiCu alloy (Fig. 7d) indicate a surface at which hydroxides are completely absent after 5000 s, however, the slow increase in catalytic activity with time in Fig. 7a for this catalyst may be related to a slow change in the composition or surface area, i.e. to catalyst instability. For NiCu MMO in 1 M KOH, the initial transient is followed by a slower increase in catalytic activity. A correspondingly slow change in the surface state, c.f. the Raman spectra in Fig. 7b, appear to persist throughout the chronoamperometry experiment.We relate this difference to the synthesis. For the NiCu alloy, only a thin layer of the hydroxides will form as the NiCu alloy is exposed to the KOH solution. This layer is rapidly reduced as the catalyst is subjected to a negative potential. However, since there is no indication of any metal phase in the NiCu oxide in the diffractograms, we may assume that during exposure to negative potentials these catalysts will be reduced continuously until the entire catalyst is converted to metal. For the NiCu MMO, this seems to have combined behavior (mixed metal oxide (Ni-NiO-CuO) catalyst), since the oxidation due to the annealing is not complete, c.f. the diffractograms in Fig. 4b which displays a substantial peak corresponding to Ni(111). This catalyst heterogeneity of metallic and oxide phases will cause mixed behavior of a continuous but slow change in the surface state throughout the experiments, which may be related to a slow diffusion-limited process in the sample. The surface is therefore also slowly reorganized and will consist of a mix of phases and a slowly changing activity.NiCu MMO also showed good stability during an accelerated stress test consisting of 5000 potential cycles from between −0.8 to −1.3 V at a scan rate of 100 mV/s. The LSV for NiCu MMO before and after the procedure showed only a 20 mV difference in the potential required to achieve - 100 mA/cm2 as shown in Fig. S5 ESI†. Fig. 7e shows The HER activity using Nafion and anion exchange ionomer (Fumion ionomer) of NiCu alloy, NiCu MMO, and NiCu oxide. The activity for the HER of nickel-copper catalysts decreased if Fumion ionomer replaced Nafion in the catalyst ink, and resulted in a potential shift of 30 mV at −100 mA/cm2 as compared to the Nafion ionomer. Catalyst inks with Nafion resulted in higher HER activity compared to catalyst inks with Fumion ionomer. We assign the difference in activity between catalysts in inks with Nafion and those with Fumion ionomers to the nature of the ionomer backbone and its chemistry (ammonium-, imidazolium-, phosphonium-based compounds in anion exchange ionomers such as Fumion, or sulphonic acid groups (SO3 −) in Nafion) [83,84]. The SO3 − moiety in Nafion interacts only weakly with the catalyst surface, and the effect of SO3 − adsorption on electrocatalyst performance is expected to be negligible, particularly in the HER region where the negative charge on the catalyst surface would repel sulfonate species [21]. The quaternary ammonium (QA) functional group used for OH− transport in anion exchange ionomer (AEI), on the other hand, appears to poison NiCu MMO catalyst and block active catalyst sites. Fumion ionomer shows higher total polarization resistance than Nafion as shown in the impedance complex plane plot of NiCu MMO using Nafion and Fumion ionomers (Fig. S6.a). The small semicircle at the low-frequency region for Fumion ionomer (Fig. S6.a) has been suggested to correspond to quaternary ammonium adsorption [85]. The results show that the anion ionomer not only serves as a binder but also affects the electrocatalyst's HER activity [4].We consequently investigated the impact of the Nafion ionomer content to find the composition at which the HER performance peaks for NiCu MMO. The results are shown in Fig. 7f. The HER activity thus increases with increasing Nafion ionomer to the catalyst weight ratio (I/C), and a maximum appears at a weight ratio of I:C of 0.5. The NiCu MMO at I/C = 0.5 achieves −10 mA/cm2 at 170 mV, which indicates better catalyst utilization, lower total polarization resistance, and optimum HER performance as shown in Fig. S6b and S6c. The low performance with a low ionomer content is attributed to the poor dispersion of the ink. At high ionomer content, the HER activity is small due to increased aggregation of Nafion and the associated blocking of mass transport and active sites [25]. The moderate I/C ratio indicates that Nafion improves the catalyst dispersion and distribution and reduced transfer resistance. The optimized ionomer content provides an efficient pathway for OH− (in the aqueous electrolyte) and electrons and forms a stable reaction interface [86].To test the activity of NiCu MMO in an actual AEM electrolysis environment, NiCu MMO and Ir black MEAs were fabricated and mounted in an AEM water electrolysis cell as explained in the Supplementary Information and Fig. S7 ESI†. Two types of MEAs will be mentioned in Results Pt/C cell and NiCu MMO cell for NiCu MMO-Ir and Pt/C-Ir cells respectively. Fig. 8 shows the impedance complex-plane plot at 0.2 A/cm2 for NiCu MMO cells (Fig. 8a) and Pt/C cells (Fig. 8b) in 0.1 and 1 M KOH. The impedance complex-plane plots appear to consist of two partly overlapping and depressed semicircles. The ohmic resistance of the cell was determined from the high-frequency resistance (HFR), i.e., from the intercept with the real (Re) axes [87].In Fig. 8, we show the equivalent circuit that is used to fit the impedance data taken at 0.2 A/cm2 in both NiCu MMO and Pt/C cells. We assign the low-frequency arc to mass transport [87,88] and the high-frequency arc to electrode kinetics contributions to the cell voltage from the NiCu MMO and Pt/C cathodes. The fitted electrical circuit is comprised of a series combination of two parallel circuits each consisting of a resistance and a constant phase element (CPE), in series with a resistor, RΩ. The RΩ corresponds to the ohmic resistance of the cell (catalyst layer, current collectors and membrane). The Rct describes the charge transfer resistance of the cathode and anode. CPE1 is the constant phase element that represents the electrode roughness. The circuit has an additional RC combination, constant phase element, and the resistance (CPE2 and R1), which is suggested to describe the mass transport related to bubble formation at the electrode-electrolyte interface [88]. All parameters extracted from the fitting of the impedance data to are presented in Table S3. For 1 M KOH, NiCu MMO cell has an HFR of 0.195 Ω.cm2 while Pt/C based AEMWE cell has an HFR of 0.115 Ω.cm2. NiCu MMO cell displays an HFR of 0.295 Ω.cm2 while Pt/C achieves 0.225 Ω.cm2 in 0.1 M KOH. NiCu MMO (5 mg/cm2) higher loadings resulted in thicker catalyst layers and higher HFR compared to Pt/C. The HFR increases as KOH concentration decreases to 0.1 M KOH. This HFR increase with decreasing KOH concentration may indicate insufficient ionic conductivity of the membrane [26].The impedance data were converted to Tafel impedance. The Tafel slope can be estimated from the Tafel impedance, for a kinetically limited process, as the diameter of the impedance arc [89]. The Tafel impedance shown in Fig. S8 ESI† is the impedance multiplied with the steady-state current density at which it was obtained. We thus estimate the Tafel slope in 1 M KOH to be 40 mV for Pt and 65 mV for NiCu MMO at 0.2 A/cm2. The Tafel slope from the impedance data is in the range of 50 millivolts, whereas the slopes from the polarization curve are twice this value (see Fig. S9 ESI) suggested that the polarization curves are dominated by the ohmic resistance. Fig. 8c and 8d show the potentiostatic polarization curves of both HFR-corrected and uncorrected voltages for the AEMWE at different KOH concentrations for NiCu MMO and Pt/C cells. Fig. 8c and 8d show the AEM electrolyzer performance of NiCu MMO and Pt/C cathode catalysts in 1 and 0.1 M KOH at 50 °C using Ir black as an anode. In 1 M KOH, with NiCu MMO a cell performance of 1.85 A/cm2 at 2 V achieved, which may be compared to Pt /Ir cell that delivers 2 A/cm2 at 2 V in 1 M KOH while both cells achieved 1 A/cm2 at 2 V in 0.1 M KOH. The increase in KOH electrolyte concentration leads to a higher AEM electrolyzer performance. Fig. 8d showed that NiCu MMO cell exhibits higher performance than Pt/C catalyst when HFR-corrected. NiCu MMO (5 mg/cm2) shows higher resistance than Pt/C (1 mg/cm2) in 1 and 0.1 M KOH (Fig. 8a and 8b). Since the cell hardware, components, electrolyte, temperature is the same and the only difference is the cathode catalyst, the origin of high resistance is the higher loading and the presence of oxide species in NiCu MMO (Ni-NiO-CuO). This leads to a higher resistance in the NiCu MMO catalytic layer itself as compared to the Pt/C, with its lower loading and metallic conductivity.The results suggest that the differences in the activity of the samples (Fig. 8c) are not merely due to their different intrinsic activities, but also partly due to low electronic resistance in the catalytic layer. This contribution to the resistance will be particularly significant for poorly conducting oxides such as those of NiCu MMO. The high-frequency resistance (HFR) corrected polarization curves in Fig. 8d confirm that the electronic resistance of the cathode catalyst layer significantly affects cell performance. Similar results can be found in the literature. Yu et al. [90] showed that for catalysts with widely different conductivity the ranking depends on whether iR compensation is applied or not. Xu et al. [91] referred the differences in AEM electrolyzer performance partially to differences in the OER catalyst phases electrical conductivity. Finally, D. Chung et al. [92] showed that poorly conductive MoS2 HER activity is affected by the ohmic losses and recommend that electrical conductivity should be considered when designing active catalysts for water electrolysis.The NiCu MMO/Ir MEA activity shows a good reproducibility for three different MEAs in 1 M KOH at 50 °C as in Fig. S10 ESI†. The post-mortem analysis of NiCu MMO catalytic layers shows no visible cracks which prove the stability of catalytic layers during AEM water electrolysis as indicated in Fig. S11. Energy dispersive X-ray (EDX) mapping of NiCu MMO catalytic layers and cross-section shows the presence of nickel, copper, carbon, and a thin potassium layer after the electrolysis experiment Fig. S12, and S13 ESI†.The excellent performance of 1.85 A/cm2 at 2 V in 1 M KOH obtained for the NiCu MMO hydrogen catalyst outperforms most of those summarized in (Fig. S14 and Table S4 ESI†) allows for an active and cheap catalyst for AEM water electrolysis operation on a commercial scale [93,94] and comparable to the state of the art performance of PEM electrolysis as summarized by Ayers et al. [95].NiCu mixed metal oxide (MMO) nanosheets synthesized by chemical reduction showed an exceptional activity for the HER compared to NiCu alloy and NiCu oxide catalysts, with higher performance in 1 M KOH than 0.1 M KOH. The improved HER kinetics of the NiCu MMO bifunctional system can be attributed to the presence of both Ni and NiO where NiO sites to facilitate water dissociation and bind OHad while Ni metallic binds Hads and CuO stabilizes NiO under HER conditions. In situ Raman spectroscopy at the NiCu MMO catalysts showed that a substantial fraction of in situ formed nickel hydroxide remained after 30,000 s at HER conditions, which may explain why the NiCu MMO is able to maintain its very high activity as compared to that of NiCu alloy and NiCu oxide over longer periods of time. Despite that anion exchange ionomers would be expected to be suitable ionomers in an AEM environment, the application of anion exchange ionomers in catalytic layers resulted in a lower HER activity as compared to catalytic layers with Nafion as the ionomer. Using Ir black as an anode catalyst, cells with NiCu MMO nanosheets as cathode catalyst achieved AEM electrolyzer performance of 1.85 A/cm2 at 2 V in 1 M KOH at 50 °C. Alaa Y. Faid: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing. Alejandro Oyarce Barnett: Funding acquisition, Supervision, Writing - review & editing. Frode Seland: Supervision, Writing - review & editing. Svein Sunde: Funding acquisition, Supervision, Writing - review & editing.“There are no conflicts to declare.”This work was performed within HAPEEL project “Hydrogen Production by Alkaline Polymer Electrolyte Electrolysis” financially supported by the Research Council of Norway-ENERGIX program contract number 268019 and the INTPART project 261620. The Research Council of Norway is acknowledged for the support to the Norwegian Micro- and Nano-Fabrication Facility, NorFab, project number 245963/F50.Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2021.137837. Image, application 1

We report on the optimization of nickel-copper catalysts for superior performance as a cathode catalyst in anion exchange membrane (AEM) water electrolysis. The bifunctional system of NiCu mixed metal oxide (MMO) nanosheets includes Ni metallic, NiO, and CuO oxides provide rapid kinetics for the hydrogen-evolution reaction (HER) of the Volmer step. In-situ Raman spectroscopy for NiCu MMO proved that nickel hydroxide was sustained under HER conditions for at least 30,000 s, which may explain why the exceptional activity of NiCu MMO as compared to other nickel-copper catalysts is maintained over time. The activity of the NiCu MMO for the HER activity in alkaline electrolytes increased as KOH concentration raised from 0.1 M to 1 M. The NiCu MMO nanosheets showed superior stability under alkaline HER conditions for 30 h. The use of Nafion ionomer in the ink resulted in a higher HER current density as compared to inks with a Fumion anion ionomer. The maximum HER performance was achieved at a Nafion ionomer to catalyst weight ratio of 0.5. Using Ir black as the anode, the NiCu MMO cathode gave an AEM electrolyzer performance of 1.85 A/cm2 at 2 V in 1 M KOH at 50 °C. The NiCu MMO catalyst developed here delivers AEM performance comparable to PEM water electrolysis.

The environmental benefit of hydrogen (H2) as an energy vector stems from its zero carbon footprint [1–4]. However, more than 95% of H2 is currently produced from fossil fuels worldwide, leading to even more production of carbon dioxide (CO2) than that produced from the direct use of fossil fuels [5]. Hence, it is necessary to minimize the dependence on fossil fuels and to shift toward renewable and clean resources for hydrogen production [6,7]. Ethanol (C2H5OH) is the most widely used liquid fuel made from renewable biomass and has a relatively high H/C ratio, which is desirable for hydrogen production [8]. Ethanol can be reacted directly with water through steam reforming to produce a H2-rich gas over 3d transition metal catalysts (Eq. (1)) [9]. This process utilizes the raw product of bioethanol, which avoids the energy-consuming distillation separation of the ethanol–water mixture [9–11]. (1) C 2 H 5 OH + 3 H 2 O → 2 CO 2 + 6 H 2 , Δ H 298 K ⊖ = 173 kJ · mol - 1 However, ethanol steam reforming is a strongly endothermic reaction. Chemical looping steam reforming (CLSR), as a process intensification technology, can be employed to promote the efficiency of the steam reforming process [12,13]. In the CLSR of ethanol, the oxygen carrier (OC) is first reduced by ethanol in a reforming reactor. For example, when NiO is used as the OC, the redox reaction between C2H5OH and NiO is carried out as shown in Eq. (2). (2) C 2 H 5 OH + 6 NiO s → 2 CO 2 + 3 H 2 O + 6 Ni s , Δ H 298 K ⊖ = 150 kJ · mol - 1 The Ni2+ in NiO is reduced to metallic nickel (Ni) with the depletion of oxygen (O). Next, ethanol steam reforming occurs with the catalysis of metallic Ni (Eq. (1)). The thermal decomposition of ethanol is also carried out on the surface of the Ni when the steam-to-carbon ratio (S/C) is low (Eq. (3)): (3) C 2 H 5 OH → C s + CO + 3 H 2 , Δ H 298 K ⊖ = 136 kJ · mol - 1 All of Eqs. (1)–(3) are endothermic. Ni is then re-oxidized by air in a regeneration reactor (Eq. (4)). The deposited carbon (C) formed during ethanol steam reforming is also gasified (Eq. (5)). (4) Ni s + 0.5 O 2 → NiO s , Δ H 298 K ⊖ = - 187 kJ · mol - 1 (5) C s + O 2 → CO 2 , Δ H 298 K ⊖ = - 395 kJ · mol - 1 The heat required for endothermic steam reforming can be supplied from the oxidation of OCs (Eq. (4)) and deposited carbon (Eq. (5)) in the regeneration reactor. Therefore, the excess heat required from an external burner can be minimized. The overall reaction of the CLSR of ethanol can be regarded as the sum of ethanol steam reforming and the complete oxidation of ethanol (Eq. (6)). (6) C2H5OH + xH2O + (1.5 − 0.5x)O2 → 2CO2 + (3 + x)H2 The OCs, which are normally reducible metal oxides, play essential roles in the CLSR of ethanol. The use of metal oxides instead of gaseous oxygen (O2) as the OCs help to avoid safety risks during operation [8]. The extra oxygen from the OCs can remarkably reduce the S/C of the CLSR, which may lead to autothermal hydrogen production from renewable feedstock with an appropriate ratio of ethanol to OC. The OCs in chemical looping processes must meet a number of criteria for practical applications [14,15]. They must exhibit long-term redox stability and provide oxygen species with suitable activity [16]. NiO, as an outstanding candidate, has been investigated for use as the OC in various chemical looping processes [17]. Jiang et al. [18] applied NiO/montmorillonite in the CLSR of ethanol and achieved greater than 60% H2 selectivity in 20 cycles. However, the oxygen release of bulk NiO is too drastic, and the dispersion of the Ni derived from bulk NiO is relatively inadequate for the activation of reactive species and long-term operation, which limits the stability of Ni-based OCs [19–21]. The regulation of the reduction kinetics is the key to obtaining highly dispersed Ni and further promoting the performance of Ni-based OCs.Due to the similar atomic sizes of Ni2+ (69 pm) and Mg2+ (72 pm), a substitutional Ni x Mg1− x O solid solution in any proportion (0 ≤ x ≤ 1) can be formed by means of an adequate calcination temperature [22–24]. The Ni–Ni boundary is isolated by the Mg2+ in Ni x Mg1− x O; thus, the rapid movement of Ni2+ is inhibited in the solid solution [25]. The reduction of the solid solution is related to the rate of bulk Ni2+ diffusion and can be tuned by the concentration of Ni2+ in Ni x Mg1− x O [26]. Huang et al. [27] designed Mg–Ni–Al–O OCs with a solid solution structure and achieved excellent performance in chemical looping combustion. Ni x Mg1− x O shows great potential for chemical looping processes, although the applicability of Ni x Mg1− x O solid solutions as OCs in the CLSR of ethanol remains unclear.In this work, Ni x Mg1− x O solid solutions with different chemical compositions were synthesized as OCs to investigate the modulation effect of Mg2+ on the CLSR of ethanol. With the introduction of Mg2+, the oxygen release of Ni-based OCs was tunable. The relationship between the structural evolution of Ni x Mg1− x O and the mechanism of the surface reaction was investigated. Ethanol–water pulse and H2-temperature-programmed reduction (TPR) experiments were applied to explore the oxygen release of Ni x Mg1− x O. An in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) experiment was also carried out to determine the changes in intermediates during the CLSR of ethanol.A series of Ni x Mg1− x O (x = 0.2, 0.4, 0.6, and 0.8) solid solutions was synthesized using a co-precipitation method. Typically, a mixture of Mg(NO3)2·6H2O (98%, J&K Scientific Co., Ltd., China) and Ni(NO3)2·6H2O (99%, Aladdin Biological Technology Co., Ltd., China) was dissolved in 150 mL of deionized water (18.25 MΩ) with a total metal molarity of 2 mol·L−1. Then, 100 mL of as-prepared 6 mol·L−1 NaOH (99%, Aladdin Biological Technology Co., Ltd.) solution was used as the precipitant. The formed precipitate was aged for 12 h, and the products were filtered and washed thoroughly with hot water to remove sodium. The obtained samples were dried in an oven at 125 °C for 24 h, and then calcined at 700 °C in air for 4 h with a heating rate of 10 °C·min−1. NiO and MgO were also synthesized by the precipitation method for reference.The crystalline structures of the samples were measured using powder X-ray diffraction (XRD; Bruker Corp., USA) with a Bruker D8 Focus equipped with Cu Kα radiation (λ = 1.54056 Å, 1 Å = 10−10 m). The diffraction angle 2θ ranged from 20° to 80° with a scan speed of 8° per minute. The texture and morphology of the samples were acquired from transmission electron microscopy (TEM) characterization on a JEM-2100F transmission electron microscope (Japan Electronic Materials Corp., Japan) operated at 200 kV. The samples for TEM analysis were sonicated in ethanol and subsequently supported on copper grid-supported transparent carbon foil. The transmission electron microscope was also equipped with an energy-dispersive X-ray spectroscopy (EDS) detector (Ultim Max, Oxford Instruments, UK) for elemental analysis.The specific surface area and pore volume of the OCs were measured by nitrogen (N2) adsorption–desorption at −196 °C using a Micromeritics Tristar II 3020 analyzer (Micromeritics Instrument Corp., USA), based on the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively. Before the tests, all the materials were degassed at 300 °C for 3 h. Elemental compositions of the prepared OCs were determined by means of inductively coupled plasma optical emission spectroscopy (ICP-OES) (VISTA-MPX, Varian, UK). Prior to the measurements, the samples were dissolved in HNO3 solutions.The reduction behavior of the OCs was determined by H2-TPR measurement. The experiments were performed on a Micromeritics Autochem II 2920 instrument equipped with a thermal conductivity detector (TCD; Micromeritics Instrumen Corp., USA). In a typical experiment, the sample (100 mg) was pretreated at 300 °C for 1 h under flowing argon (Ar; 30 mL·min−1). After the sample had cooled to 100 °C, the analysis was carried out in a mixture of 10 vol% H2 in Ar (30 mL·min−1) from 100 to 950 °C at 10 °C·min−1.To determine the transfer of oxygen species during the CLSR of ethanol, the C2H5OH–H2O mixture and O2-pulse experiments were measured on a Micromeritics Autochem II 2920 instrument equipped with a Hiden QIC-20 mass spectrometer (Hiden Analytical, USA). Prior to the experiments, all the samples were pretreated in situ using a flow of Ar (30 mL·min−1) at 300 °C for 1 h. Subsequently, pulses of the mixture of C2H5OH and H2O in Ar or 2% O2 in helium (He) were admitted to the reactor. The loop volume was 0.5031 mL, and the time interval between different pulses was 3 min, excluding the interference of contiguous pulses. The reactor effluent was continuously monitored by the mass spectrometer, and the gas-phase composition was calculated from the mass spectrometer signal at mass-to-charge ratios (m/z) of 44, 31, 29, 28, 27, 18, 16, and 2 for CO2, C2H5OH, acetaldehyde (CH3CHO), carbon monoxide (CO), ethene (C2H4), water (H2O), methane (CH4), and H2, respectively.To detect the transformation of intermediates in the CLSR process, in situ DRIFTS experiments were performed on a Nicolet iS50 spectrometer (Nicolet iS50, Thermo Scientific, USA) equipped with a Harrick Scientific diffuse reflection accessory and a mercury–cadmium–telluride (MCT) detector cooled by liquid N2. All samples were pretreated at 600 °C under an Ar flow for 0.5 h, followed by purging with Ar for 1 h, and were then cooled to 400 °C to obtain a background spectrum. This collected spectrum was then subtracted from the sample spectrum for each measurement under CLSR conditions.The carbon formation on the OCs was characterized by thermogravimetric analysis (TGA; TGS-2A, Yuanbo, China) and temperature-programmed oxidation (TPO). The TGA experiment was carried out by filling 20 mg of OC into an alumina crucible. The temperature and weight change were then recorded when the temperature was increased from 50 to 900 °C with a heating rate of 10 °C·min−1 under air flow (50 mL·min−1). The TPO profiles of the spent OCs were obtained from the same apparatus, as described for the C2H5OH-pulse experiment. The OC (50 mg) was pretreated at 300 °C for 0.5 h under flowing Ar (30 mL·min−1). After the OC was cooled to 50 °C, a flow rate of 30 mL·min−1 of 10 vol% O2/He was used for oxidation, and the temperature was increased linearly from 50 to 900 °C. The CO2 in the effluent was monitored and recorded online using a mass spectrometer.CLSR tests were conducted in a stainless-steel tubular fixed-bed reactor with an internal diameter of 20 mm and a length of 400 mm. Two grams of OC (20–40 mesh) was used for the CLSR reaction. Prior to the test, the OCs were pretreated at 600 °C for 1 h under pure N2 (200 mL·min−1). After purging with N2, the bed was subsequently adjusted to the designed temperature. An ethanol–water mixture with a flow rate of 0.03 mL·min−1 and a specific S/C of 1 was fed through a pump (P230, Elite, China) into a heated chamber (200 °C), where the mixture was completely evaporated in a stream of N2 (100 mL·min−1) to start the CLSR reaction for 1 h. Then, the reactor was heated to the desired oxidation temperature under air flow (200 mL·min−1) to regenerate the OC for 10 min. The gaseous products were analyzed online by an Agilent 490 Micro gas chromatograph. The gas chromatograph consisted of two different channels for gaseous product analysis. Channel 1 was equipped with a 10 m Molecular Sieve 5A column, with Ar used as the carrier gas for the quantification of permanent gases except for CO2 (H2, N2, CO, and CH4). Channel 2 was equipped with a 10 m PoraPlot Q column, with He used as the carrier gas for the detection of CO2 and C1–C3 hydrocarbons. All the gaseous products were quantified using the micro-machined thermal conductivity detectors (μTCDs) included in each channel. Liquid products were collected and analyzed over an Agilent 7890A gas chromatograph equipped with a flame ionization detector (FID). Possible liquid products including C2H5OH, CH3CHO, and acetone (CH3COCH3) were quantified over the FID with a Porapak-Q column using N2 as the carrier gas. The selectivities (Si ) of the carbon-containing products were calculated by the following: (7) S i = [ i ] CO 2 + CO + CH 4 × 100 % where i represents the different species in the products, and [i] represents the molar concentration of i in the products.The H2 selectivity ( S H 2 ) was calculated as follows: (8) S H 2 = F H 2 F C 2 H 5 OH-in where F H 2 represents the molar flow rate of hydrogen in the products, and F C 2 H 2 OH-in represents the molar flow rate of ethanol in the reactants.Product distributions (Pi ) were calculated as follows: (9) P i = i H 2 + CO 2 + CO + CH 4 × 100 % The physicochemical properties of the as-prepared Ni x Mg1− x O are shown in Table 1 . The specific surface area characterized by the BET method is in the range of 15–30 m2·g−1, and the pore volume is in the range of 0.03–0.06 cm3·g−1. The XRD patterns of NiO, MgO, and Ni x Mg1− x O are shown in Fig. 1 (a). NiO, MgO, and Ni x Mg1− x O possess a rock salt structure. The crystalline sizes of Ni x Mg1− x O, as calculated by the Scherrer equation, are similar. To show the influence of the content of Ni on the lattice parameter, the XRD patterns in the range of 40°–46° are provided in Fig. 1(b). The (200) peak of Ni x Mg1− x O shifts from 42.8° to 43.2° with increasing Ni content (i.e., from MgO to NiO). The lattice parameter of Ni x Mg1− x O can be calculated from the peak position based on Bragg’s law (Table 1). When the lattice parameter of Ni x Mg1− x O is correlated with the content of Ni in Ni x Mg1− x O, a linear relationship can be verified (Fig. 1(c)), which indicates the formation of Ni x Mg1− x O solid solutions in the corresponding Ni/Mg proportions [22].Ni0.4Mg0.6O is selected as an example to observe the morphology of the solid solution. TEM images of Ni0.4Mg0.6O are given in Figs. 1(d)–(f). According to Figs. 1(d) and (e), the particle size of Ni0.4Mg0.6O is in the range of 10–20 nm. No segregated NiO crystals are observed. The (200) plane of Ni x Mg1− x O with a lattice spacing of 4.215 Å can also be measured in Fig. 1(f), which is in accordance with the results of the XRD characterizations. EDS mapping was applied to probe the elemental dispersion. According to Figs. 1(g)–(i), the distribution of Ni, Mg, and O in Ni x Mg1− x O is homogeneous, indicating the formation of a substitutional solid solution of Ni–Mg oxide.To achieve efficient hydrogen production, 400 °C was chosen as the temperature for the CLSR reaction (Fig. S1 in Appendix A). The S/C was set to 1. The selectivities of the carbon-containing products and H2 are given in Fig. 2 (a). As the content of Ni increases, more CH4 is generated, which is detrimental to H2 selectivity. This phenomenon can be attributed to the poor dispersion of Ni (Table 1). CO selectivity over Ni0.2Mg0.8O is the highest among various Ni x Mg1− x O solid solutions. The generation of CO hinders the purity of H2. In this study, Ni0.4Mg0.6O presents the maximum H2 selectivity of 4.72 mol H2 per mole ethanol.We further studied the properties of Ni0.4Mg0.6O. The results from the time-on-stream test of ethanol CLSR over Ni0.4Mg0.6O at 400 °C in a single cycle are given in Fig. 2(b). The CLSR of ethanol can be generally divided into three stages based on the changes in the distribution of products. In stage I (from the start of the reaction to 6 min), CO2 is the main product. Ethanol is completely oxidized by the surface oxygen of Ni0.4Mg0.6O. In stage II (from 6 to 33 min), as more Ni2+ ions are gradually reduced to metallic Ni, the decomposition of ethanol occurs over the surface of Ni to produce H2 and CH4. The selectivity of the gaseous products is then steadily maintained. The selectivity of H2 reaches its maximum and the CO concentration is suppressed to 1% in stage II. In stage III (after 33 min) the conversion of ethanol and the selectivities of H2 and CO2 decrease with the generation of more CO and CH4. The deactivation of Ni0.4Mg0.6O occurs in this stage.A cyclic stability test was carried out on Ni0.4Mg0.6O. After the CLSR reaction at 400 °C, the reduced Ni0.4Mg0.6O was re-oxidized and the carbon was combusted in the air at 600 °C for 10 min. This process is referred to as the “regeneration step” in our study. One cyclic test constituted 60 min of the CLSR of ethanol and 10 min of regeneration. The performance of Ni0.4Mg0.6O in the cyclic test is shown in Fig. 2(c). The selectivity of H2 over Ni0.4Mg0.6O only drops by about 3% in 30 cycles, indicating that the regeneration can recover the Ni0.4Mg0.6O. The structure of Ni x Mg1− x O after 30 cycles was characterized by TEM and XRD (Table 1, Fig. 2(d), and Appendix A Fig. S2). The morphology and crystal structure of Ni0.4Mg0.6O remained the same after the long-term test. The solid-solution OC exists in the form of particles, without the occurrence of sintering. The crystalline size of Ni0.4Mg0.6O after the stability test was 14.2 nm, which is similar to that of fresh Ni0.4Mg0.6O. These results verify the recovery of Ni0.4Mg0.6O in the regeneration step and demonstrate the superior stability of this solid solution in the CLSR of ethanol.A pulse experiment with an ethanol–water mixture (S/C = 1) over Ni0.4Mg0.6O at 400 °C was conducted in order to explore the oxygen-release process of Ni x Mg1− x O (Fig. 3 (a)). During the first five pulses, the peaks of H2 were not obvious and CO2 was the main product. This phenomenon indicates that the redox reaction between Ni0.4Mg0.6O and ethanol is dominant in this period, which corresponds to the stage I observed in the time-on-stream test of Ni0.4Mg0.6O (Fig. 2(b)). Afterward, the H2 peaks were enlarged and remained stable. CO2 became the dominant carbonaceous product, which represents the characteristics of stage II.XRD was applied to detect the change in the composition of Ni0.4Mg0.6O during the pulse experiment. Since there may be a diffraction peak of metallic Ni at 44° near the peak, corresponding to the (200) plane of Ni x Mg1− x O, the second strongest peak for the (220) plane of Ni x Mg1− x O was analyzed. The XRD patterns in the range of 60°–64° for Ni0.4Mg0.6O after different pulses of the ethanol–water mixture are given in Fig. 3(b). The lattice parameter of the reduced Ni0.4Mg0.6O was calculated according to the peak position. If the distribution of Ni2+ and Mg2+ in Ni x Mg1− x O is homogeneous, then the Ni content, x, of such a solid solution can be calculated according to Vegard’s law [28]: (10) a Ni x Mg 1 - x O = x a NiO + ( 1 - x ) a MgO where a Ni x Mg 1 - x O is the lattice constant of Ni x Mg1− x O, and the lattice constants of NiO (a NiO) and MgO (a MgO) were obtained from pure oxides (powder diffraction file (PDF) No. 47–1049 for NiO and PDF No. 45–0946 for MgO). Based on the calculated lattice constants of the reduced Ni0.4Mg0.6O, we obtained the Ni contents and degree of reduction of Ni0.4Mg0.6O (Figs. 3(c) and (d)).The change in the degree of reduction of Ni0.4Mg0.6O is in accordance with the findings from the pulse experiment. In stage I, the degree of reduction of Ni0.4Mg0.6O increases rapidly. The complete oxidation of ethanol is dominant, with the generation of CO2. In stage II, H2 is formed consistently in the last three pulses. Simultaneously, oxygen release continues according to the change in the degree of reduction of Ni0.4Mg0.6O. In comparison with stage I, the rate of oxygen release in stage II drops, indicating that the oxygen from Ni0.4Mg0.6O participates in the reaction between ethanol and water to produce H2. H2 selectivity is increased due to the occurrence of water gas shift. The stoichiometric S/C in ethanol steam reforming (Eq. (1)) is 1.5, which is larger than the S/C in the CLSR and pulse experiment. Therefore, additional oxygen is necessary for stable hydrogen production in stage II. Stage II in the CLSR is carried out as follows: (11) C2H5OH + 2H2O + [O] → 2CO2 + 5H2 where [O] represents the oxygen from Ni x Mg1− x O.When the active oxygen from Ni0.4Mg0.6O is depleted, the low S/C provides insufficient oxidation capacity for the steam reforming, resulting in decreased selectivity toward H2 and CO2 (stage III in the CLSR test). Meanwhile, ethanol is decomposed to carbon, which covers the surface of the OC and leads to deactivation. TGA and O2-TPO experiments were conducted to verify this process (Fig. S3 in Appendix A). The mass increase at the beginning of the TGA analysis of the reacted Ni0.4Mg0.6O after one cycle can be attributed to the oxidation of Ni (Fig. S3(a)). The subsequent mass loss is in accordance with the peak position of CO2 in the O2-TPO, which corresponds to the gasification of the deposited carbon (Fig. S3(b)). The carbon deposition is considered to be the cause of deactivation in stage III. The results also show that the coke generated in the CLSR of ethanol can be eliminated at 600 °C in the regeneration step.To further investigate the modulation effects of Mg2+ on Ni x Mg1− x O, H2-TPR experiments were performed to detect the reactivity of different oxygen species in the solid solution (Fig. 4 (a)). No reduction peak was observed over pure bulk MgO up to 900 °C. The reduction peak of NiO is very broad at 200–400 °C. The H2-TPR profiles of Ni x Mg1− x O mainly consist of a low-temperature reduction peak at around 300 °C and a large reduction peak at 400–800 °C, indicating the existence of two types of oxygen species with different reactivities. Based on the reduction profile of NiO and the structure of Ni x Mg1− x O, the low-temperature reduction peak of Ni x Mg1− x O can be attributed to the release of surface oxygen. The large peak in the high-temperature range corresponds to the reduction of Ni2+ in the bulk of Ni x Mg1− x O [29]. The temperatures of the different reduction peaks of Ni x Mg1− x O are summarized in Fig. 4(b). The reactivity of the surface oxygen is enhanced with the increase of Ni concentration in Ni x Mg1− x O. The reducibility of the metal oxide is related to the band gap between the valence and conduction bands [30]. Closer valence and conduction bands make metal oxides more easily reduced [30]. Previous research indicates that, when the Ni content x is greater than 0.074, the band gap of Ni x Mg1− x O decreases linearly with x [31]. Therefore, the oxygen-release process of Ni x Mg1− x O is inhibited with increased Mg2+ content, which aligns with the results from H2-TPR. Moreover, the coefficient of the Ni2+–Mg2+ interdiffusion increases exponentially with the concentration of Ni2+ in the air [32]. In conclusion, Ni2+ diffusion in Ni x Mg1− x O is suppressed by the lattice confinement of Mg2+. Therefore, the reactivity of bulk oxygen decreases with the enrichment of Mg2+ in Ni x Mg1− x O, which can be reflected by the increased reduction temperature of bulk oxygen. For the Ni0.2Mg0.8O sample, the reduction temperature for oxygen in the bulk is slightly lower than that of Ni0.4Mg0.6O. MgO formed in the surface layer prevents the deeper reduction of bulk Ni0.2Mg0.8O, lowering the apparent reduction temperature of bulk oxygen and resulting in a lower degree of reduction (Table 1).To investigate the reaction pathway over Ni x Mg1− x O, in situ DRIFTS experiments were carried out (Fig. S4 in Appendix A). The spectra collected at different times during the reaction were divided into three distinct stages. To observe the changes of the C-containing surface species over Ni0.4Mg0.6O, the in situ DRIFTS spectra in the range from 2400 to 800 cm−1 were obtained, and are presented in Fig. 5 .At the beginning of the reaction, the infrared (IR) peaks of gaseous CO2 at 2350 cm−1 and CO3 2− at 1510 and 1240 cm−1 were observed [33]. The generation of CO2 and CO3 2− can be attributed to the complete oxidation of ethanol by surface oxygen, corresponding to stage I observed in the time-on-stream test. As the reaction proceeds, CO is generated, according to the appearance of the peak at 2170 cm−1. The C–O bond in CH3CH2O* at 1030 cm−1 can be seen [34]. The peaks at 1740 and 1580 cm−1 are assigned to the C=O bond in CH3COO*, which is a characteristic intermediate over Ni-based catalysts in ethanol steam reforming, corresponding to stage II [35]. The IR peaks indicate that the decomposition of ethanol into CH3CH2O* occurs over metallic Ni, and the CH3CH2O* is further oxidized to CH3COO*. According to the evidence of the changes in the degree of reduction, water may work collaboratively with the bulk oxygen of Ni x Mg1− x O to oxidize ethanol in stage II. In stage III, the CO3 2− peak disappears gradually, and the intensity of the acetate peak increases. Moreover, the peak at 880 cm−1 for the C–H bond in gaseous CH4 also appears in this stage. The multiple peaks in the range from 1600 to 1400 cm−1 correspond to the C–H vibration of the deposited carbon [36]. The changes in the intermediates indicate the occurrence of the decomposition of ethanol to generate CH4 and carbon in this stage. Due to the low S/C, the oxidation capacity of water is insufficient to convert the surface C-containing species to CO2. The proposed surface reaction pathway of ethanol for the CLSR of ethanol over Ni x Mg1− x O is in line with the structural evolution of solid solution (Fig. 6 ).Ni x Mg1− x O solid solution was applied as a novel OC in the CLSR of ethanol for hydrogen production. The oxygen release of Ni x Mg1− x O is regulated with the lattice confinement by Mg2+. As a result, the optimum OC, Ni0.4Mg0.6O, was found to exhibit a robust performance toward hydrogen production (4.72 mol of H2 per mole of ethanol), with an S/C of 1. A three-stage reaction mechanism of the CLSR process was proposed. In stage I, ethanol is completely oxidized by the surface oxygen of Ni x Mg1− x O. After the depletion of the surface oxygen and the formation of surface Ni sites, ethanol is oxidized by H2O and the bulk oxygen from Ni x Mg1− x O collaboratively, achieving the maximum efficiency for hydrogen production in stage II. Without the participation of oxygen species, ethanol steam reforming becomes the dominant process in stage III. The CLSR of ethanol using Ni x Mg1− x O as the OC could potentially reduce the S/C in comparison with conventional steam reforming and achieve renewable hydrogen production from biomass with a minimum external heat supply. This research provides a feasible strategy for the design of a novel OC in diverse chemical looping processes with improved performance and structural stability.This work was supported by National Natural Science Foundation of China (U20B6002, 51761145012, and 21525626) and the Program of Introducing Talents of Discipline to Universities (BP0618007) for financial support.Hao Tian, Chunlei Pei, Sai Chen, Yang Wu, Zhjian Zhao, and Jinlong Gong declare that they have no conflict of interest or financial conflicts to disclose.Supplementary data to this article can be found online at https://doi.org/10.1016/j.eng.2020.08.029.The following are the Supplementary data to this article: Supplementary data 1

The chemical looping steam reforming (CLSR) of bioethanol is an energy-efficient and carbon-neutral approach of hydrogen production. This paper describes the use of a Ni x Mg1− x O solid solution as the oxygen carrier (OC) in the CLSR of bioethanol. Due to the regulation effect of Mg2+ in Ni x Mg1− x O, a three-stage reaction mechanism of the CLSR process is proposed. The surface oxygen of Ni x Mg1− x O initially causes complete oxidation of the ethanol. Subsequently, H2O and bulk oxygen confined by Mg2+ react with ethanol to form CH3COO* followed by H2 over partially reduced Ni x Mg1− x O. Once the bulk oxygen is consumed, the ethanol steam reforming process is promoted by the metallic nickel in the stage III. As a result, Ni0.4Mg0.6O exhibits a high H2 selectivity (4.72 mol H2 per mole ethanol) with a low steam-to-carbon molar ratio of 1, and remains stable over 30 CLSR cycles. The design of this solid-solution OC provides a versatile strategy for manipulating the chemical looping process.

Experimental data relating to the figures and tables presented in this manuscript have been deposited at Zenodo under the https://doi.org/10.5281/zenodo.7199360 and are publicly available as of the date of publication. Supporting DFT datasets have been deposited at ioChem-BD 45 under the https://doi.org/10.19061/iochem-bd-1-258 and are publicly available as of the date of publication. This paper does not report original code. Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon reasonable request.The carbon dioxide (CO2) electroreduction (eCO2R) toward simple products like carbon monoxide 1 and formate 2 has attracted wider research efforts and more recently the interest of chemical industries. However, more reduced products cannot be generated yet at promising yields under relevant operating conditions, with the single exception of ethylene. 3 , 4 Besides the initial proof of concept for recently discovered inorganic Ni oxygenates, 5 the only catalysts sustaining more than two proton-electron transfers beyond trace rates 6 are copper-based (Cu-based) materials. However, the tendency of Cu-based electrocatalysts to promote numerous products has driven an intense quest for strategies enabling selectivity control. For this purpose, nanostructure and compositional modifications have been suggested. Early observations 7 , 8 revealed that halogen-modified catalytic systems display leveraged selectivity toward ethylene, which triggered follow-up efforts to explore their potential. 9 Two strategies based on materials (copper halide catalysts) and electrolytes (halogen-containing) have been practiced so far.Regarding halogen-containing electrolytes, the positive effect of adding chlorinated, brominated, or iodinated salts like KX (X = Cl, Br, I) to the aqueous medium has been often reported. Most works claim selectivity increases toward C2+ products, 10 , 11 , 12 though a few claimed exclusively larger activity toward carbon monoxide. 13 , 14 In spite of some rationalization efforts, the influence of the halide anion coming from the electrolyte is not well understood, as it is hard to deconvolute from that of its compensating cation, which affects the medium conductivity and double layer structure as well as the CO2 activation itself. 9 , 15 Alternatively, copper halide catalysts (CuX, X = Cl, Br, I, with special focus on CuCl) represent the most pursued route through a variety of synthetic strategies, with wet bench methods 16 , 17 and electrodeposition 8 , 18 , 19 , 20 being the two most commonly used. Fluorine-containing halides have been less extensively studied. 21 , 22 Overall, reported C2+ Faradaic efficiencies outcompete halogen-free copper systems, reaching up to 80% when operated under optimized conditions, 20 , 21 though noticeably different performances have been reported over seemingly similar systems. 10 , 14 It is commonly accepted that copper halides act as catalyst precursors. A common preparation route is exposure of copper halides to reductive potentials to obtain metallic copper matrices with enhanced performance. 8 , 20 , 23 , 24 , 25 Alternatively, an intermediate step transforming copper halides into copper oxides has been introduced to increase the population of defects present upon reduction. 17 , 19 Similarly to the case of modified electrolytes, the role of halogens in the lattice is still unknown, despite pioneering efforts correlating copper halide structures with catalytic performance. 26 Halogen atoms remaining in the copper structure after reconstruction have been proposed to enhance the stability of oxidic Cu centers, 21 , 22 while other simulations predict that the adsorption of leached halide ions may alter surface electronic properties 20 or hinder the parasitic hydrogen evolution reaction. 9 On the other hand, most reports consider halogens as structure-directing agents toward specific copper facets 17 , 23 or nanostructures. 18 In this context, a key pending task is determining synthesis-structure-performance relations linked to the halogenation degree especially for the most widely explored chlorine-based systems. Chlorine exhibits advantageous features compared with bromine, the second most commonly studied halogen to this end. Its more sustainable character and frequent use in industry makes it a preferential target for practical applications. Closely related, bromination of copper phases is much faster than chlorination, making the controlled incorporation of the latter extremely challenging. 27 However, finding a synthetic method enabling controlled chlorination of copper has still remained challenging due to the strong affinity between the two elements, a well-known cause of catalyst deactivation in the Deacon process. 28 As a result, the inability to modulate chlorine content has limited the study of this family of materials to copper chloride, precluding effective catalyst design. This work develops a gas-phase treatment based on exposure to HCl at different temperatures enabling fine control of chlorine incorporation. Partially chlorinated copper systems (Cu, Cu2O, CuO) were synthesized to reveal a maximal promotion toward highly reduced products (HRPs, i.e., those requiring more than 2e− transfer and comprising CH4, C2, and C3 compounds) for a Cu2O-based catalyst containing roughly equal Cu2O and CuCl, underscoring the relevance of selecting the copper phase and chlorination degree. A direct correlation between Faradaic efficiency for HRPs and post-reaction surface chlorine content emerged. Simulations show that the origin of the stability and enhanced performance relies on a large material reconstruction, generating partially chlorinated metallic surfaces and copper oxychloride ensembles showing reduced barriers toward HRPs. These insights thus provide tools and guidelines for the design of enhanced chlorine-promoted copper catalysts.The first step toward copper catalysts with controlled degree of chlorination was to develop two oxidation treatment protocols (Figure 1 A, conditions in Table S1) to obtain bulk Cu2O or CuO starting from Cu foils. The reason for targeting the two oxidic phases is their different reactivities toward HCl due to the mechanism of lattice O-HCl interaction in the gas phase. 29 Exposure of pretreated Cu foils to the harsher oxidation conditions of 50 vol % O2/He at 673 K for 30 min was sufficient to predominantly form surface CuO domains as indicated by the presence of (002) and (111) reflections at 35.6° and 38.8° 2θ respectively in the X-ray diffraction (XRD) profiles (Figure S1), in addition to small traces of partially oxidized Cu2O(111) at 36.7° 2θ. Milder exposure to 20 vol % O2/He at 553 K for 5 min formed primarily surface Cu2O as well as traces of over-oxidized CuO domains, as indicated by the Cu2O(111) reflection at 35.6° 2θ shifting toward lower angles compared with the reference due to lattice strain (Figure S1).Preliminary tests suggested temperature as the variable enabling better control of the chlorination degree. Chlorination treatment of clean Cu and CuO x in 2 vol % HCl/He for 30 min at various temperatures resulted in systems with variable copper chlorides content, most notably CuCl as indicated by the (111) reflection at 28.6° 2θ in the XRD profiles of chlorinated samples (Figure 1B). So-prepared samples are coded according to the initial copper phase and chlorination temperature CuO x -HCl(T). While the relative proportion of bulk chloride to oxide phases steadily increases with T for Cu2O-HCl(T) (Figure 1C), two distinct regimes are observed for CuO-HCl(T), separated by the onset temperature of approximately 450 K, above which copper oxychloride phases form (Figures S2 and S3). Full chlorination of both oxides occurs above 523 K. The overall low bulk CuCl content in the Cu-HCl(T) family does not display a strong correlation with chlorination temperature, corroborating the less marked reactivity of the metallic phase. The relative proportion of surface Cl quantified using X-ray photoelectron spectroscopy (XPS) Cl 2p signals (Table S2 and Figure S4) followed the general trend in the bulk chlorination degree of the freshly chlorinated CuO-HCl(T) and Cu2O-HCl(T) systems. Similarly, assignment of the most intense Cu LMM Auger signals for Cu2O-HCl(T) identified a relative decrease and increase of Cu2O and CuCl, respectively, with increasing chlorination temperature (vide infra).Once the set of copper catalysts with controlled chlorination degrees was available, their catalytic performance was evaluated in a two-compartment cell containing CO2-saturated 0.1 M KHCO3 at −0.8 V vs. RHE, a mild potential at which complex products are not favored on the reference metallic Cu surface. 30 Extended details can be found in the supplemental experimental procedures. Figure 2 displays product distributions ordered by chlorination temperature for the three catalyst families (see Tables S3–S5 for numerical values and Figure S5 for a typical chronoamperometry profile). Figure 2A reveals that the reference family Cu-HCl(T), showing a low and uniform chlorination degree (Figure 1C), displayed a very mild promotional effect. Formate production was slightly enhanced at most temperatures with no discernible pattern. Modest ethylene formation was observed around Cu-HCl(323), aligned with reports claiming enhanced ethylene production on copper halide surfaces. 9 The set of CuO-HCl(T) catalysts yielded Faradaic efficiencies toward HRPs seemly independent from the chlorination temperature and similar to that of unmodified CuO, as can be seen in Figure 2B. Faradaic efficiency toward simple compounds, and especially to formate, was favored at intermediate chlorination temperatures. Contrary to the case of metallic copper, the CuCl content varied from 0 to 100% in this set of samples (Figure 1C), suggesting that the initial chlorination degree of CuO-based systems does not influence the ability of the catalyst to produce HRPs.A different picture emerged for the case of Cu2O-HCl(T) catalysts. Pristine Cu2O exhibited predominant formation of carbon monoxide and formate with only trace HRPs. Formate production was sharply favored for the Cu2O-HCl(323) sample. As chlorination temperature rises, HRPs become increasingly predominant up to Cu2O-HCl(373), for which HRP Faradaic efficiency peaks (ca. 14%, Figure 2A). Of note, production of all compounds gathered under the HRP acronym (methane, C2 and C3) increase with chlorination temperature, though the surge of methane must be highlighted. At higher chlorination temperatures the promotional effect is still observable, though to a lesser extent, and it becomes approximately temperature independent. According to the quasi-linear CuCl content-temperature dependence registered for Cu2O (Figure 1C), this result hinted to an optimized CuCl content of ca. 40% in the system prior to testing. The survey of other potentials suggested that the promotional effect was optimal at potentials around −0.8 V vs. RHE (Table S6), arguably due to the insufficient overpotential available at less cathodic potentials, as well as the likely instability of adsorbed Cl at highly cathodic potentials. Additional experiments upon bromination at room temperature of Cu2O and CuO shown in Figure S6 disclosed a milder promotional effect toward HRPs than their chlorinated counterparts showing similar degree of halogenation, reinforcing the interest of chlorine promotion.These distinctive behaviors already disclosed the relevance of the copper source, and that complete chlorination may not be associated with optimal promotional effect in copper materials. The next sections are devoted to elucidating the effect of chlorine on the materials’ physico-chemical properties and developing mechanistic insights supporting observed patterns.The first step was to investigate the influence on the promotional effect of surface morphologies introduced by the different synthesis conditions of the catalysts. Differences in surface roughness, which may impact the local chemical environment and thus selectivity, 3 , 31 were visualized by scanning electron microscopy (SEM, Figure 3 A). Micrographs of CuO-HCl(T) and Cu2O-HCl(T) catalyst surfaces chlorinated at low, moderate, and high temperatures did not show notably different microscale features. In parallel, changes in electrochemically active surface area quantified from double-layer capacitance measurements did not show any discernible correlation with FE HRP (Figure 3B). Since surface morphology may not be a decisive factor explaining the promotional effect, the distinctive trends observed between the three catalyst families may exhibit a chemical ground. Predictably, the density of defects, such as undercoordinated atoms or grain boundaries, or the relative population of facets could also have a relevant role to determine differences in catalytic performance among these systems. Due to the highly dynamic nature of copper surface under operation conditions, 25 its operando monitoring would be required, which still represents an experimental challenge. For the case of porous materials like catalytic layers in gas diffusion electrodes, a parallel study considering porosity should be considered.The concentration and nature of chlorinated species at the reaction interface, which evolved from the fresh structure upon exposure to eCO2R reaction conditions, were primarily analyzed and quantified using XPS and Auger signals of relevant Cl and Cu regions of used catalyst samples. For all CuO-HCl(T) and Cu2O-HCl(T) catalysts, small but quantifiable amounts of Cl reaching up to ca. 0.7 atom % and 1.8 atom %, respectively, remained after reaction (Figures 4A and S7). For Cu2O-HCl(T), a narrow range of temperature with increasing values around the maximum at 373 K for both FE HRP (Figure 2) and surface Cl content (Figure 4B) is evident, suggesting that the choice of treatment temperatures crucially affects both inter-linked phenomena. These observations are combined in Figure 4C, where an apparent linear correlation between FE HRP and surface Cl content is shown for the Cu2O-HCl(T) family.Note that the dependences of the Cl content with chlorination temperature in fresh (Figure 1C and Table S2) and used materials (Figure 4B) are notably different. Indeed, mild initial chlorination degrees are associated with the largest capability of retaining Cl during reaction. Cu LMM Auger spectra of fresh materials (Figure 5 A) show the expected increase of the CuCl signal with temperature. Fresh Cu2O-HCl(373) exhibits comparable intensities for both Cu2O and CuCl, in line with XRD analysis (Figure 1C). After eCO2R, metallic Cu is the predominant species with small shoulders assigned to CuCl appearing upon increased sputtering time of the samples. Only Cu2O-HCl(373) features stronger signals pertaining to CuCl and Cu2O, suggesting that the presence of both phases leads to a distinct (sub)surface state upon exposure to reaction conditions. However, the detection of Cu2O formed during the exposure of the sample to air cannot be discarded at this point. The surface analysis was complemented by XRD observations (Figure 5B). Since no reflections could be assigned to CuCl in Cu2O-HCl(T) after reaction, it is reasonable to expect that Cl is present as part of smaller surface and subsurface domains without crystalline order or strongly adsorbed on the surface. We thus postulated that in situ reduction of Cu oxide and chloride results in the stabilization of Cl around the reduced, defective oxidic surface ensembles formed during eCO2R, 3 which we denote in general as “copper oxychloride-like ensembles.”The parallel analysis for CuO-HCl(T) and Cu-HCl(T) catalysts showed different results. They exhibited a very mild dependence of surface Cl content after reaction with chlorination temperature (Figure 4B), with FE HRP largely unchanged compared with their untreated counterparts (Figure 2), leading to no performance-Cl content correlation (Figure 4C). Despite the clear formation of copper chloride and oxychloride phases in the bulk of the used samples that left detectable crystallites as measured by XRD for CuO-HCl(423) and CuO-HCl(473) (Figure 5B), the surface Cl content kept fairly constant around 1 atom % (Figure 4B). This suggests that most of the Cl could be locked in bulk chloride-containing phases outside the XPS measurement depth. The nature of such species and that of surface “copper oxychloride-like ensembles” would conceivably be different for the CuO-HCl(T) family, as reflected by their selectivity patterns.Overall, these results suggest structural differences among families upon exposure to reaction conditions (Figure 5C). For Cu-HCl(T), the low Cl content initially incorporated may lead to very sparse Cl coverage after reaction with no significant effect on HRP formation. For CuO-HCl(T), stabilization of inactive subsurface (oxy)chlorides may be responsible for the lack of promotion effect toward HRPs. Finally, for Cu2O-HCl(T), moderate initial chlorination degrees and subsequent exposure to eCO2R conditions favor higher Cl contents that may be stabilized at the defective, oxygen-containing copper matrix with distinctive electronic features for HRP formation. The combination of the optimum Cu phase and the accuracy of the chlorination procedure are thus crucial.Density functional theory (DFT) simulations contributed to unravel the structural and compositional features affecting HRP formation. The consensus mechanisms in eCO2R starts with CO2 adsorption promoted via electron transfer, followed by protonation to ∗COOH and a proton-coupled electron transfer to produce water and ∗CO. 3 From there, CO can either desorb or diverge toward C1 and C2 HRP formation. Thus, product distribution toward CO, methane, and C2+ products is controlled by the energy of ∗CO desorption, ∗CO protonation to ∗COH, and ∗CO-CO coupling, 32 , 33 , 34 which are taken as descriptors for selectivity.The nature of the active sites in native copper catalysts under eCO2R is controversial due to the deep structural changes depending on the reaction conditions and the initial oxidation state of the material. 35 , 36 , 37 Recent studies 25 , 38 suggest that the existence of metallic Cu0, oxidic Cu+, and polarized Cu δ+ might be responsible for the C-C coupling ability. Similarly to oxygen, chlorine can also act as modifier in similar terms due to their similar electronegativity. Therefore, the ability to present variable oxidation states in the chlorinated materials may be a favorable feature for enhancing reactivity.Thermodynamic properties of relevant intermediates were analyzed on different Cl-modified copper surfaces. First, the stability and electronic properties of metallic copper surfaces showing increasing chlorine coverages was explored considering the desorption of chlorine atoms on the lowest surface energy facets Cu(111), Cu(100), and Cu(110), 39 as well as on Cu(211) step models. Although electrolyte-metal interface studies 40 have shown the key role of halide adlayers on Cu surfaces, they might be modestly stable under eCO2R conditions. 41 However, these structures constitute the simplest model to understand the electronic effect exerted by Cl on its nearest Cu atoms. Given the maximum XPS-derived Cl coverage of ca. 0.20 ML after reaction (Figure 4), 42 simulations covered the 0.00–0.33 ML range at −0.80 V vs. RHE with implicit solvation. The results on Cu(100) show that chlorine desorption energies are independent of coverage within this range (Table S7). Chlorine adatoms on Cu(100) are marginally stable (Tables S8 and S9) since desorption as aqueous Cl− is close to thermoneutrality (ΔG des = −0.23 eV, U = −0.80 V vs. RHE), similar to the case of Cu(110) and Cu(211) (ΔG des = −0.21 eV and −0.13 eV, respectively), while Cl− on Cu(111) is even less stable (ΔG des = −0.54 eV). Desorption energies of Cl from Cu(100) and ab initio thermodynamics-derived adsorption energies per unit area (Figure S8) both indicate reduced stability at potentials more negative than U = −0.57 V vs. RHE. As ∗Cl-∗Cl lateral interactions are negligible, effects of chlorine incorporation are localized, modifying the electronic density of the nearest neighboring Cu atoms as shown by the Bader charge of 0.10–0.15 |e−| (Figure 6 A and Table S10) and supported by d-band analyses (Figure S9 and Table S11). Therefore, surface chlorination may generate polarized Cuδ+ species, known to have an important role in eCO2R. 43 , 44 As charging creates asymmetric Cu-Cuδ+ pairs and ∗CO adsorption energy on Cu sites close to Cl atoms is smaller than on regular Cu sites (Figure 6B), the ∗CO-CO coupling energy is lowered compared with clean Cu without significantly altering other steps, enhancing C2 HRP formation (Figure 6C). For Cl coverages above 0.17 ML, the decreasing frequency of these pairs diminishes this effect, promoting CO desorption instead (Figure 6B). ∗H adsorption strength was however found to be unaffected with respect to Cl coverage (Figure S10, E ∗H,ads = −0.90 eV), suggesting that the competing hydrogen evolution reaction remains unaltered. Energy profiles leading to different HRPs (Figures S11 and S12) demonstrate that the applied potential is crucial in achieving C-C bond formation. However, this modification does not lead to enhanced formation of C1 HRPs (namely methane) (Figures S13–S16).To discover stable structures able to generate asymmetric Cu-Cu δ+ pairs and promote C1 HRP formation, a heuristic approach involving model systems mirroring the structural complexity resulting from the chlorination treatment protocol was devised. Noting the optimal surface Cl content for Cu2O-HCl(373) where both oxidic and chloride phases coexisted prior to eCO2R (Figure 5), a total of 68 structural models representing a wide compositional range were built upon Cu2O(111) as a reference, with certain oxygen atoms removed and (H)Cl incorporated, to assess stable copper (hydr)oxychloride phases (see supplemental experimental procedures and Figure S17 for full details). 25 All structures were optimized via DFT, and their computed energies were compared with a predicted energy dependent on only the stoichiometry, using a general equation with the form of Equation S21. Multivariate linear regression and subsequent refinement of the variable selection resulted in a final regression model iteration that uses the numbers of Cl, H, and O atoms, as well as ∗Cl adatoms, as independent variables (see supplemental experimental procedures and Table S12). The independence of variables precluding interaction-dependent terms implies that structures with DFT energies (E DFT) lower than these predicted by the regression model (E pred) show synergetic effects that render them more stable (Figure S18A). The cooperative effects (up to −1.76 eV) appear to be associated with structural motifs that locally resemble Cu2OCl2-like (Figure S18B) or CuOHCl-like (Figure S18C) bulk structures. 25 The stability of these copper (hydroxy)chloride ensembles was ascertained by calculating energies of chlorine desorption to Cl− for the three most stable structures identified. Over the Cu2OCl2-like ensemble, the energy of both subsurface Cl atoms is ΔG des = −0.26 eV at U = −0.80 V vs. RHE, and kinetic trapping due to poor Cl− diffusion to the surface is unlikely (process is endergonic by 0.2 eV). This suggests that Cl atoms are strongly stabilized if ensembles are generated at the surface, as their desorption Gibbs free energy is much higher (1.0–2.5 eV more endergonic) than those of other subsurface Cl atoms in other structures and also higher (0.2–1.5 eV more endergonic) than surface-stabilized Cl atoms (Table S13). Despite the suggested stability of CuOHCl-like ensembles by the heuristic model, the desorption energy of Cl subsurface atoms on the structures (ΔG des = −1.27 eV) suggests their low stability under applied potential. The unique stability of the Cu2OCl2-like ensembles mirrors that of their bulk counterparts as detected by XRD (Figure S3) and XPS (Figure 5A) measurements of used CuO-HCl(T) catalysts, while CuOHCl initially present disappears under reaction conditions (Figure S3). Cu2OCl2-like ensembles induce changes in the neighboring Cu d-band centers with respect to Cu atoms located further away, by Δε d-band = 0.32 eV (Figure S19), in opposition to Cu(100). Moreover, the most stable Cu2OCl2-like ensemble showed a significant promotional effect toward CH4 formation (Figure 6D), as CO2 adsorption is more favorable by −0.57 eV compared with over Cu(100). Subsequent protonation and dehydroxylation steps leading to ∗C are also slightly more favored by −0.10 eV and −0.35 eV, respectively, with a stronger CO adsorption by 0.73 eV. Thus, Cu2OCl2-like ensembles could be assigned as sites responsible for C1 HRPs, while for the metastable CuOHCl-like ensembles, no promotion is found (Figure S20). Also, Bader charge analysis demonstrates the generation of surface Cu-Cu δ+ pairs in Cu2OCl2-like ensembles able to promote C2+ products (Figure S21 and Table S14). From a wider perspective, the nature of herein suggested active sites for the promotion of methane explains why chlorine-modified copper catalysts reported in the literature have exclusively been claimed to favor multi-carbon products. Since all previous reports were based on copper chloride due to lack of a method to control chlorination degree, the virtual absence of oxygen in the initial material precludes the formation of Cu2OCl2-like ensembles.In summary, this work developed a method to control the chlorination degree of copper electrocatalysts and applied it to Cu, CuO, and Cu2O to investigate structure-performance correlations. Surface chlorine content upon reaction correlates with FE HRP and could be maximized for mildly chlorinated Cu2O materials, revealing the importance of both the copper phase and the synthesis procedure. This work also reveals the potential of chlorine-promoted copper catalysts for the production of methane. Computational studies predict two types of sites to explain observed performance patterns. Chlorine incorporation both on Cu surfaces and in stable copper oxychloride phases showed that ∗CO-CO coupling could be enhanced by mild Cl contents giving rise to asymmetric Cu-Cu δ+ pairs with higher reactivity, while methane formation is thermodynamically favored over copper oxychloride-containing structures. These tools and fundamental insights gathered are expected to contribute to the design of the next generation of technical chlorine-promoted copper electrocatalysts.Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Javier Pérez-Ramírez (jpr@chem.ethz.ch).This study did not generate new unique materials.Polished Cu foils were oxidized in a flow of dilute oxygen at tailored conditions, followed by chlorination in a gas flow of 2 vol % hydrogen chloride diluted in helium at temperatures between 298 K and 523 K, to obtain three families of catalysts: Cu-HCl(T), Cu2O-HCl(T), and CuO-HCl(T). Further details on catalyst preparation are provided in the supplemental experimental procedures with the conditions applied in the oxidation and chlorination treatments fully detailed in Table S1.XRD analysis was used to identify and quantify bulk phases and investigate their crystallinity. XPS measurements were carried out to identify copper phases and quantify chlorine content. SEM disclosed structural features at the microscale. Further details for each of the characterization techniques are provided in the supplemental experimental procedures.Catalyst evaluation was performed in a gas-tight two-compartment cell, mounted with a microporous carbon layer as counter electrode and Ag/AgCl 3 M as reference electrode at −0.8 V vs. RHE in CO2-saturated 0.1 M KHCO3. On-line gaseous products quantification was performed by gas chromatography every 15 min, whereas liquid products were analyzed by proton nuclear magnetic resonance (1H-NMR) spectrometry after reaction. An extended description can be found in the supplemental experimental procedures.DFT calculations were performed with the Vienna Ab initio Simulation Package (VASP) 46 , 47 using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional 48  with dispersion included through the DFT-D2 method. 49 Inner electrons were represented through projector augmented wave (PAW) pseudopotentials 50 with a plane-wave energy cutoff of 450 eV. Mechanistic studies were performed on the chlorine adatom and heuristically computed copper (hydr)oxychloride models using the computational hydrogen electrode (CHE) model 51  and solvation. 52 Further details on DFT parameters, model generation, and energy calculations are provided in the supplemental experimental procedures.The authors acknowledge financial support from the Swiss National Science Foundation through the National Center of Competence in Research NCCR Catalysis (grant 180544), ETH grant ETH-47 19-1, and from the Spanish Ministry of Science and Innovation (PRE2021-097615, PID2021-122516OB-I00, Severo Ochoa Center of Excellence CEX2019-000925-S 10.13039/501100011033). The Barcelona Supercomputing Centre-MareNostrum (BSC-RES) is acknowledged for providing generous computational resources. T.Z. thanks the Agency for Science, Technology and Research (A∗STAR) Singapore for support through a graduate fellowship. The authors thank Thaylan P. Araújo and Dr. Simon Büchele for assistance with XPS measurements and Shibashish Jaydev for assistance with visualizations.T.Z. and F.L.P.V., methodology, data curation, investigation, visualization, writing – original draft. E.I.-A, data curation, investigation, visualization, writing – original draft. R.G.-M. and G.Z., methodology. A.J.M., methodology, investigation, visualization, validation, supervision, writing – original draft. N.L., conceptualization, funding acquisition, supervision, project administration, writing – review & editing. J.P.-R., conceptualization, funding acquisition, supervision, project administration, writing – review & editing.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.xcrp.2023.101294. Document S1. Supplemental experimental procedures, Figures S1–S21, and Tables S1–S14 Document S2. Article plus supplemental information

Chlorinated copper catalysts have shown promise for electroreduction of carbon dioxide to complex products, but the challenging control of chlorination keeps shaded the potential of chlorine as a selectivity promoter. This work develops a gas-phase chlorination strategy based on exposure to diluted hydrochloric acid at different temperatures to study the effect of chlorine content in copper (II) oxide (CuO), copper (I) oxide (Cu2O), and metallic copper (Cu) foils. Contrary to CuO and Cu, chlorination of Cu2O enhances the formation of highly reduced products (those requiring more than two electron transfers). Faradaic efficiency toward these products (0%–14% at -0.8 V vs. the reversible hydrogen electrode) correlates with the surface chlorine content after reaction (0 to 1.8 atom % chlorine), which is maximized for mild initial chlorination degrees (Cu2O:CuCl∼1). Experimental and computational studies suggest metallic copper surfaces with moderate chlorine coverage and oxychloride-like clusters are active sites responsible for the promotional effect. These findings may facilitate structure-performance relationships, forwarding the next generation of this family of catalysts.

Data will be made available on request.The nanomaterial displays an exceptional property such as surface to volume ratio, desirable electrical and optical possessions versus their bulk counterparts. Additionally, due to these unique possessions, the NPs have potential applications in diverse fields with highly promising efficiency, i.e., biomedical, photocatalysis, food industry, energy and environment etc [1–4]. Among the different kinds of materials explored, ferrites are a progressive material based on their incredible response and application in diversified fields. Ferrites based on the bandgap energy have phenomenal light absorption efficacy and have high capability of charge shifting. Thus, they are auspicious for application in photocatalysts, sunlight-based gadgets, and electronic devices [5–7]. Hence, these materials have been emerged a one of important class of materials for application in photocatalysis, magnetic, optical and electronic materials. Depending on the magnetization, these have two types, i.e., hard and soft ferrites. Based on geometry, these materials have spinel, orthogonal, garnet, and hexaferrites types and are classified into X, Y, Z, W, U, M, and R types depending on the crystal structures [8,9]. Among the different categories of ferrites, the Sr-hexaferrites, SrFe12O19, are the most interesting and auspicious materials in response of the unique properties. These properties make Sr-hexaferrites appropriate for application as microwave absorbers, recording media, supercapacitors, in spintronics devices, solar based cells, and radiofrequency gadgets [10,11]. Sr-hexaferrites are presented as MFe12O19, having M a divalent cation. Structurally, Sr-hexaferrites including a spinel and R block comprise of oxygen, iron, and strontium ions on tetrahedral, octahedral, and interstitial sites and have RSR*S* sequence in the c-axis [5,12,13]. Doping with di and tri-valent metal cations is regarded as a useful way to deal with the alteration of electrical, optical, magnetic, photocatalytic properties of Sr-ferrites. The optoelectronic properties of Sr-ferrites are mainly dependent on their electronic structures, crystallite size, composition, annealing temperature, dopant type and fabrication routes [14–17]. Different synthetic routes have been employed for the fabrication of Sr-hexaferrites, like the facile micro-emulsion route, co-precipitation route, hydrothermal method, sol–gel method, and green synthesis approaches. These synthetic approaches have their own advantages and disadvantages [5,6], which have been applied in various fields and responses reported were highly promising. In this scenario, the pollution a serious issue due to dyes and other toxic pollutants and need to be tackled efficiently. The photocatalytic treatment is one of advanced techniques, which uses photocatalytic process and degrade the organic toxic pollutant non-selectively and convert into non-toxic end products like H2O, CO2 and inorganic ions, which offer various advantages versus conventional wastewater treatment approaches [18–21].Based on aforesaid facts and due to the simplicity, easy approach, versatility and cost-effectiveness, the micro-emulsion strategy was used for the fabrication of Zn and Ni doped Sr-hexaferrites in the current study. The doping effect was investigated on the structural, optical, dielectric and photocatalytic properties basis. The photocatalytic activity of Sr1-xZnxFe1-yNiyO19 and SrFe12O19 was appraised for MG dye under solar light exposure.Analytical grade Fe(NO3)3·9H2O (≥98 %), Sr(NO3)2 (99.99 %) and Zn(NO3)2·6H2O (98 %) were obtained from Sigma Aldrich. The Ni(NO3)2·6H2O (≥98 %) and MG dye were obtained from Merck, while C19H42BrN and ammonia solution (35 % by weight) were acquired from AnalaR.Sr-hexaferrites Sr1-xZnxFe12-yNiyO19 NPs were prepared via micro-emulsion route. Sr-hexaferrites Sr1-x Znx Fe12-y NiyO19 NPs with composition of ×  = 0.0, 0.15, 0.30, 0.45, 0.60 and y = 0.0, 0.2, 0.3, 0.4, 0.5 were synthesized by micro emulsion procedure. The salts amounts (stoichiometric) were mixed in de-ionized water and stirred on a hot plate at 45–55 0C for 2 h. The CTAB solution was added into each composition. The pH of the mixtures was adjusted at about 11–12 with the help of ammonia solution. The mixtures were stirred for 6–7 h. The precipitates of Sr1-x ZnxFe12-yNiyO19 NPs formed are rinsed with de-ionized water repeatedly till neutral pH and drying was done at 150 ℃ in an oven for about 5 h. The crystals of Sr1-xZnxFe12 yNiyO19 NPs were ground to fine powder and annealed at 900 ℃ for 7–8 h in Vulcan A-550 furnace A schematic illustration of the synthesis protocol of Sr1-xZnx Fe12-yNiyO19 NPs is given in Fig. 1 .The PCE of Sr1-xZnxFe12-yNiyO19 NPs was estimated for MG dye under the exposure of visible light. A 3 mg of the as-synthesized Sr1-xZnxFe12-yNiyO19 photocatalyst material was added in 50 mL of MG dye solution having 10 mg/L concentration. The mixture with MG dye was stirred for half an hour in the dark, which then, was exposed to visible light (200Watt Argon lamp with cutoff filter 420 nm). For A given specified time, a 5 mL sample was taken from the mixtures, filtered and then the absorption was recorded at 632 nm and MG dye removal (%) was estimated as depicted in Eq. (1), where At and A0 are the absorbance values of MG dye solution after and before the irradiation, respectively. (1) D e g r a d a t i o n % = 1 - A t A 0 The powder XRD of Sr1-xZnxFe12-y NiyO19 NPs was carried out using Philips x-pert (PRO-3040/60) X-ray diffractometer at room temperature within a wavelength range of 2θ = 20-80° by CuKα λ = 0.15406 nm. FTIR spectra of the sample was recorded using Alpha-Bruker-ATR using OPUS-Mentor software in the range of 390–4200 cm−1. Dielectric properties of Sr1-x Znx Fe12-y NiyO19 NPs were measured using 4287-A RF LCR-meter. UV visible was accomplished using SHIMADZU-3101 spectrophotometer.The composition, crystallinity and phase of SrFe12O19 (SF-1), Sr0.85Zn0.15Fe11.8Ni0.2O19 (SF-2), Sr0.7Zn0.3Fe11.7Ni0.3O19 (SF-3), Sr0.55Zn0.45Fe11.6Ni0.4O19 (SF-4) and Sr0.4Zn0.6Fe11.5Ni0.5O19 (SF-5) were investigated by XRD analysis (Fig. 2 a). The diffraction peaks show the hexagonal structure of SrFe12O19 NPs, according to standard pattern (JCPDS# 00–033-1340) [22]. As depicted from the XRD pattern, after doping zinc (Zn2+) and nickel (Ni3+) ions in SrFe12O19 NPs with crystal structure, the peak position at 32.9° for (107) plane shifted towards the lower angle of diffraction [23]. The XRD pattern of SF-2 that might be due to growing larger ionic radii of Zn2+ ions (Zn2+ = 88 pm) in strontium ferrite lattice Fig. 2(b). In the XRD pattern of SF-3, the peak position at 32.9° for (107) plane shifted towards the larger angle of diffraction, which again might be due to growing smaller ionic radii of Ni3+ ions (Ni3+ = 74 pm) in strontium ferrite lattice [24] Fig. 2(b). The XRD pattern of SF-4 was similar to that of SF-2 in which the peak position was shifted to lower diffraction angle. However, the XRD pattern of SF-5 was quite similar to that of SF-1. The unit cell parameters such as cell volume and side lengths ‘a’ & ‘c’ and crystallite size were found to be in the range from SF-1 to SF-5 V = 673.315 to 663.512 (Å)3, a = 5.8702 to 5.8601Ǻ and c = 22.5629 to 22.3111Ǻ and 21.023 to 14.318 nm, respectively Table 1 . It was observed from the Table 1 that the lattice constants ‘a’ & ‘c’ and crystallite size decreases which was attributed because of the substitution of larger host (Sr and Fe) ions with smaller dopant (Zn and Ni) ions Fig. 3 (a-b). The crystallite size is appraised as per Eq. (2). (2) D = k λ / β c o s θ Where ‘k’ is the Scherrer factor (∼0.99). ‘λ’ is the beam wavelength of X-ray, ‘β’ is the FWHM and ‘θ’ represents angle (Bragg’s) [25]. All the peaks were well indexed at 2θ value of 21.87°, 23.82°, 25.62°, 28.53°, 32.98°, 35.33°, 38.66°, 40.61°, 42.41°, 49.08°, 53.79° and 57.40°, indicating (103), (104), (105), (106), (107), (103), (201), (116), (205), (206), (213), (300) and (1112) planes of crystal respectively. With increase in the amount of doped metals, the sharpness of the peak at about 32.9° angle was more strengthened.The FTIR spectra of SrFe12O19 (SF-1), Sr0.85Zn0.15Fe11.8Ni0.2O19 (SF-2), Sr0.7Zn0.3Fe11.7Ni0.3O19 (SF-3), Sr0.55Zn0.45Fe11.6Ni0.4O19 (SF-4) and Sr0.4Zn0.6Fe11.5Ni0.5O19 (SF-5) were observed in 400–4000 cm−1 wavenumber range (Fig. 4 ). For strontium ferrites, four different types of absorption peaks in 450–3800 cm−1 range were observed; two bands were of high frequency abbreviated as f1 and f2, while the other two vibrational modes were of low frequency represented as f́1 and f́2. The vibrational modes of high-frequency, f1 and f2 are attributed to tetrahedral and octahedral stretching vibration of M ↔ O, while the remaining two have low vibrational modes, f́1 & f́2 are attributed to lattice vibrations [26]. Fig. 3 depicts that the vibrational mode of 469 cm−1 is represented as f2 (Mocta ↔ O), while the band at 568 cm−1 represented as f1 (Mtetra ↔ O). The tetrahedral sites show variation in band position due to the doping of zinc and nickel ions in SrFe12O19 lattice. By comparing the SrFe12O19 FTIR spectra with some [24,27], there is peak shifting from region of low to high frequency when SrFe12O19 is doped by zinc and nickel ions. The reason is that grain size is reduced by doping zinc and nickel ions. For NPs, it is a common phenomenon that there is a change in the characteristic vibrational frequencies in functional groups due to small variation in the environment. Due to decrease in the size of grain, there is an increase in vibrational frequency [13].The electrical properties of SrFe12O19 (SF-1), Sr0.85Zn0.15Fe11.8Ni0.2O19 (SF-2), Sr0.7Zn0.3Fe11.7Ni0.3O19 (SF-3), Sr0.55Zn0.45Fe11.6Ni0.4O19 (SF-4) and Sr0.4Zn0.6Fe11.5Ni0.5O19 (SF-5) were measured with current voltage (I–V) analysis at normal temperature, as shown in Fig. 5 . The curves show the semiconducting behavior of SrFe12O19 semiconductors has been tailored in their Sr1-xZnxFe12-yNiyO19 ferrites. The curve of current–voltage related to Sr1-xZnxFe12-yNiyO19 ferrites demonstrates a rectifying behavior, and shows the probable progress of diode heterojunction for Sr1-xZnxFe12-yNiyO19 ferrites [28]. The value of DC resistivity for SrFe12O19 NPs and Sr1-xZnxFe12-yNiyO19 ferrites was appraised as per Eq. (3). (3) P = R × A / l Where “ρ” represents the value of DC resistivity, “R” is the resistance, pellets thickness and the area of SrFe12O19 NPs and Sr1-xZnxFe12-yNiyO19 ferrites is represented by “A” and “l” respectively. The values of DC resistivity for SrFe12O19 NPs and Sr1-xZnxFe12-yNiyO19 ferrites were 9.59⨯107, 5.11⨯104, 8.11⨯106, 1.11⨯105, 1.06⨯106 Ωcm respectively. These results clearly show that the value of resistivity decreased to 5.11⨯104, 8.11⨯106, 1.11⨯105, 1.06⨯106 Ω cm in Sr1-xZnxFe12-yNiyO19 ferrites. The decrease in the resistivity value observed for Sr1-xZnxFe12-yNiyO19 ferrites may be due to the doping of Zn2+ and Ni3+ ions in the interstitial regions of the crystal lattice formed by SrFe12O19 that generated structural deformation. The DC resistivity values for SrFe12O19 NPs and Sr1-xZnxFe12-yNiyO19 ferrites were changed into conductivity values. Finally, the values of electrical conductivity for SrFe12O19 NPs and Sr1-xZnxFe12-yNiyO19 ferrites were found to be 1.4⨯10-8, 1.95⨯10-5, 1.23⨯10-7, 9.0⨯10-6, 9.4⨯10-7 Scm−1, respectively. The increase in the conductivity value of Sr1-xZnxFe12-yNiyO19 ferrites may be due to the following reasons, (i) increase in scattering of crystallite boundary and (ii) an increase in the electron motion due to development in crystallite size [29]. The values of slope, thickness, area, resistivity and conductivity of the samples are provided in Table 2 . Fig. 6 (a) shows the dielectric constant versus frequency of Zn and Ni-doped SrFe12O19 NPs. The change in dielectric constant value versus frequency shows that it decreases by increasing the frequency range up to a certain frequency region, after which it becomes constant. This behavour of the dielectric constant was explained by Maxwell Wagner and Koop’s phenomenal theory [30]. According to this theory, the dielectric nature of the ferrite materials was based on two layer models, the grain boundaries consisting of high and low conducting properties. Electrons enter these grain boundaries from the varoius interstitial sites causing a barrier by hopping conduction mechanism. At these interstitial sites (boundaries) the resistance offered results in polarization which increases the dielectric constant. While at the high-frequency region the induced polarization decreases due to the decrease in the movement of electrons towards these boundaries, which decreases the dielectric constant at high-frequency region [31,32]. By adding the dopants (Zn and Ni) in the doped material, it was observed that the dielectric constant increased as the concentrations of the dopants increased in SrFe12O19 NPs, as depicted in Fig. 6(b). This might be attributed to the composition of the lattice structure and its changes from the octahedral to tetrahedral sites. This reduces the host (Sr and Fe) content ultimately decreasing the hoping conduction from Fe3+ to Fe2+ ions in the doped materials [33]. Fig. 7 (a-b) depicts tangent and dielectric loss versus shows the frequency of the SrFe12O19 NPs and Zn and Ni doped ferrites materials. The tangent and dielectric loss factor show a similar trend as depicted by the dielectric constant at different frequency regions. It was observed that the tangent and dielectric loss was high at low frequency region and shows decreasing trend at high frequency region. The decrease of the tangent and dielctric loss with frequency was ascribed to the reduction in the domain wall motion, magnetization and space charge polarization. These, resulted in the decrease of tangent and dielectric losses at high frequency range [5]. The effect of dopant content on the variation of tangent and dielectric loss reveals that the doping of Zn and Ni in Sr-hexaferrites decreases the dielectric and tangent loss. It was observed that the tangent and dielectric loss was low for the doped material compared to Sr-hexaferrites. The very low loss value for the doped material was attributed to smaller oxygen vacancies, low resistance offered, hopping conduction mechanism, grain interfaces and the grain boundaries. The Zn and Ni doped Sr-hexaferrites material exhibits low resistivity, which needs small amount of energy to exchange the electrons from Fe3+ to Fe2+ ions resulting in the reduction of the dielectric and tangent loss values. The doped Sr-hexaferrites material with smaller values of tangent and dielectric loss exhibit a very small leakage current. This makes this material to be potentially suitable for energy storage, electronic, microwave and energy storage devices [34].The material possesses AC conductivity due to electrons transfer in various valance states. Fig. 8 (a) depicts AC conductivity of Zn and Ni doped and undoped Sr-hexaferrites. AC conductivity value was enhaced as the concentration of the dopants (Zn, and Ni) was increased as well as frequency. This increase in the AC conductivity values was attributed to the high conduction power of dopant metal ions which can donate electrons easily compared to the host metals. This results in increase in polarization, which might be due to relaxation and reorientational phenomenon, and dispersion effects as illustrated by the jumping relaxation model [13]. The electrical resistivity measurements of the Zn and Ni doped and undoped Sr-hexaferrites materials are illustrated in Fig. 8(b). The electrical resistivty values declined with frequency and concentrations of the dopants. The resistivity value was higher in low-frequency region and decreased onwards with increased frequency. The decreased values of electrical resistivity upon doping might be attributed to reduction in the resistance offered by the dopant ions in the movement charge carriers species, that is electron which results in increase in the hoping conduction mechanism. The material with small values of electrical resistance at ordinary conductions are the potentail candidates for electrical and electronic devices applications [5].UV–Visible absorption spectrum of SrFe12O19 (SF-1), Sr0.85Zn0.15Fe11.8Ni0.2O19 (SF-2), Sr0.7Zn0.3Fe11.7Ni0.3O19 (SF-3), Sr0.55Zn0.45Fe11.6Ni0.4O19 (SF-4) and Sr0.4Zn0.6Fe11.5Ni0.5O19 (SF-5) NPs was analyzed in 200–800 nm range. Obviously, all the samples absorb both in UV and visible range and four distinct absorption bands (Fig. 9 ). One absorption band was in the visible light range and the remaining three were observed in UV light region; however, the position of these absorption bands was changed by changing the concentration of the dopants Zn and Ni metal ions in SrFe12O19 lattice Table 3 . Hence, the variation observed in the absorption bands of pure & zinc- nickel doped strontium ferrites NPs might be due to changing some aspects such as oxygen deficiency, crystallite size and structural defects [35].The value of band gap energy of all samples were calculated using Eq. (4). (4) α h υ 2 = K h υ - E g n Where ‘hʋ’, ‘Eg’, ‘α’, ‘K’ and ‘n’ are indicating photon energy, band gap energy, coefficient of absorption, constant and transition type (indirect, direct, forbidden, and allowed), respectively. The value direct band gap energy (n = 2) for pure and doped strontium ferrite NPs was calculated by extra-plotting (αhʋ)2 verses hʋ (Fig. 9) and band gap energy values are provided in Table 3. The band gap energy values of SF-1, SF-2, SF-3, SF-4 and SF-5 were calculated to be 1.24(eV), 1.21(eV), 1.18(eV), 1.18(eV) and 1.09(eV), respectively. It was observed from the Table 3 that the bandgap values decreases from un-doped SF-1 to the highly doped SF-5. This variation in bandgap energy might be ascribed to the formation of sub-energy levels and the defects caused by doping Zn and Ni ions in Sr-ferrite [5].The PCE of the synthesized photo-catalyst Sr1-xZnxFe12-yNiyO19 NPs material with highly dopant content Zn, and Ni (x = 0.6, y = 0.5) was evaluated using MG dye, as depicted in Fig. 10 . From the absorption spectrum it was observed that the as-fabricated Sr1-x Znx Fe12-y NiyO19 NPs doped material degraded almost 72.23 % of the MG dye in one hour under visible light-irradiation. The rate constant for degrading the MG dye by the as-fabricated Sr1-xZnxFe12-yNiyO19 NPs was observed to be 0.03163 min−1 as illustrated in Fig. 11 (a-b).The photodegradation mechanism of dye by the as-fabricated Sr1-xZnxFe12-yNiyO19 NPs is described in Fig. 12 . Photo-catalytic mechanism involves the activation of active sites at the surface of the photo-catalyst materials exposed to visible light irradiation. As a result of light exposure, e- are excited from VB to CB generating an electron-hole pair. It converts the water molecules for generating OḢ radical, which has oxidizing nature [13]. The (OḢ) radical thus formed oxidizes MG dye to non-toxic small inorganic and organic end-products. On the other hand, oxygen takes up electrons forming super-oxide (O2 –) anionic radical, which after being protonated forms H2O2. The H2O2 produced further associates and produce OḢ radical, which interacts with dye structure and causes the degradation of MG dye molecule to simple non-toxic degraded end-products [5], as depicted in Eqs. 5–8. Table 4 shows a comparison of photo-catalytic activities of related photocatalytic material versus present study and analysis revealed that Sr1-xZnxFe1-yNiyO19 NPs has promising photocatalytic activity and can be employed to the remedy of coloring agents in the textile effluents under visible light exposure and and under the current situation of water contamination with diverse type of pollutants [36–40], there is need to develop and adopt eco-benign methods for wastewater treatment and photocatalytic processes under solar light irradiation is one of promising in this regard. (5) P h o t o c a t a l y s t + I r r a d i a t i o n → h + C B + e - C B (6) e - + O 2 → O 2 - ∙ (7) h + C B + O H - → O H ∙ (8) O H ∙ + O 2 - ∙ + M G d y e → d e g r a d e d p r o u c t s A photocatalyst's stability is crucial for commercial application. As a result, the reusability and stability of Sr1-xZnxFe12-yNiyO19 photocatalyst was tested four times using repeated cycle runs. Following each experiment, the photocatalyst was removed from the MG aqueous solution by ultracentrifugation and dried at 50 °C. The recycling studies, givenin Fig. 13 , show that there was slight decline in the photocatalytic activity of Sr1-xZnxFe12-yNiyO19 after each run which can be attributed to catalyst loss due to deactivation of active sites and during separation process for next cycle and aggregation or leaching of the photocatalyst's surface as a result of subsequent heat treatment [45].Sr1-xZnxFe12-yNiyO19 NPs were successively synthesized by micro-emulsion method. Analysis of the synthesized Sr1-xZnxFe12-yNiyO19 NPs was performed by different techniques along with photocatalytic application studies. The XRD analysis confirmed the growth of SrFe12O19 with hexagonal crystal lattice. The doping of Zn and Ni metals in SrFe12O19 crystal lattice was confirmed by shifting of peaks in the XRD pattern of Sr1-xZnxFe12-yNiyO19 NPs. The dielectric properties of the doped and undoped material reveal the potential uses in electronic and electrical devices. The study of the optical properties of pristine SrFe12O19 and doped Sr1-xZnxFe12-xNiyO19 NPs proved the change in the absorption due to, the engineering of bandgap energies. The minimum bandgap energy of 1.09 eV was achieved for SF-2 by controlling Zn and Ni concentrations. The PCA of Zn and Ni-doped material was evaluated for MG dye and 72.23 % dye was degraded in 60 min under visible light irradiation. The doped material might have potential applications as photocatalyst under visible light irradiation, which will make the process economical for application at pilot scale.This research was funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project (PNURSP2022R26), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia. The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University, Saudi Arabia for funding this work through Research Groups Program under Grant No R.G.P.2: 33/43. Ismat Bibi: Conceptualization, Supervision. Shahid Iqbal: Investigation, Writing – original draft. Shagufta Kamal: Validation. Qasim Raza: Methodology. Mongi Amami: Investigation, Data curation. Khadijah M. Katubi: Funding acquisition, Data curation, Resources. Norah Alwadai: Software, Resources. Munawar Iqbal: Writing – review & editing, Writing – original draft, Visualization.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 (PNURSP2022R26), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia. The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University, Saudi Arabia for funding this work through Research Groups Program under grant number R.G.P.2: 33/43.

A series of Sr1-xZnxFe1-yNiyO19 with composition of x = 0.0, 0.15, 0.30, 0.45, 0.60 and y = 0.0, 0.2, 0.3, 0.4, 0.5 were prepared using micro-emulsion approach and characteristics were studied by XRD, FTIR and UV–vis techniques. The effect of doping was investigated based on the optical, dielectric, structural and photocatalytic properties of Sr1-xZnxFe1-yNiyO19. XRD analyses confirmed the hexagonal crystal structure of pure and doped SrFe12O19 NPs. The lattice parameters, X-ray density, porosity, bulk density and crystallite size of Sr1-xZnxFe1-yNiyO19 and SrFe12O19 were compared. The FTIR analysis confirmed that there was shifting of peaks from region of lower to higher frequency in Sr1-xZnxFe1-yNiyO19, due to the reduction of the size of grains after doping. The current–voltage I-V data shows that there is gradual increase in the conductivity of as prepared doped-material, but the reverse was the case for resistivity, which exhibit good conducting behavior of the fabricated material. Evaluation of the dielectric properties of the samples shows that the AC conductivity and dielectric constant enhanced, while tangent, dielectric loss and resistivity decreased with concentration of the dopants (Zn and Ni). These excellent properties of doped material can be used for the fabrication of various electronic, electrical, microwave and high frequency devices applications. The photocatalytic activity of Sr1-xZnxFe1-yNiyO19 and SrFe12O19 was tested against methyl green (MG) dye. Results revealed that Sr1-xZnxFe1-yNiyO19 degraded 72.23 % of MG dye in 60 min under the exposure of solar light. The recyclability and usability tests revealed that there was a minute loss after successive four runs. Sr1-xZnxFe1-yNiyO19 might have potential applications as a photo-catalyst under solar light irradiation.

Functional amines, among different kinds of chemicals, are considered as the highly valuable precursors widely applied in many fields of biology, medicine, and material [1–3]. For instance, the top-selling drugs reported in 2020 usually contain the nitrogen and/or amino groups as the integral parts that play an important role in their activities [3,4]. Various strategies for the synthesis of functional amines are thus developed, including but not limited to the reduction of functional groups containing nitrogen, the alkylation of ammonia and amines, the Gabriel synthesis, and the reductive amination of carbonyl compounds [1,5]. Among them, the last one represents the most resourceful method adopted in both academic laboratory and industry as it uses molecular hydrogen to be the reducing agent, which is beneficial for the atomic balance and efficiency [4,6,7]. To our knowledge, reductive amination is a cascade reaction [1], where the initial step forms the carbinol amine that loses water to offer imine or iminium ion; subsequently, these intermediates will be further reduced to produce the specific amines. Screening appropriate catalysts in this kind of reaction is thus important because they have to selectively convert the imine without considerably affecting the primary aldehyde or ketone or other available reducible groups.In this regard, Fernandes et al. [8] reported a strategy for the direct reductive amination of aldehydes by the catalysis of various high valence oxorhenium (V and VII) complexes. Shortly afterward, Fischmeister et al. [9] also described a zwitter-ionic iridium complex catalyst with a 2, 2′-dipyridylamine ligand as effective for the reductive amination of lactic acid (LA) with 4-methoxyaniline. However, Liu et al. [10,11], Beller et al. [12], and Huang et al. [13] clearly stated that the synthesis and application of homogeneous catalysts are usually limited by the disadvantages in terms of environmental safety, stability, and recyclability. For the advancement of sustainable and cost-effective processes, it is preferable to develop non-noble metal-based heterogeneous catalysts, whose catalytic activity is mostly characterized by the support features and metallic phases. Herein, the carbon-based supports, especially for those derived from biomass [14–17], have recently received growing attention as they are easily prepared and abundant in nature. Some strategies are also adopted to enhance the interaction of metallic nanoparticles with carbonaceous structures for better catalytic activity, and one of the common ways is the encapsulation of metal nanoparticles into the carbonaceous structure [11,12,18]. Yan et al. [19] suggested the synthesis of carbon-encapsulated iron nanoparticles using pyrochar from fast pyrolysis of pine wood as the supporting materials, which possesses a high activity for the conversion of biomass-derived chemicals to liquid hydrocarbons. Similarly, several groups have demonstrated that the carbonized cellulose or other nanostructured carbonaceous materials fabricated with metallic nanoparticles exhibit the desirable performance in the reaction of reduction hydrogenation [12,16]. The coating of metallic nanoparticles on such carbonaceous materials is prepared by the surface precipitation of metal precursor to the hard templates, and the loading process heavily relies on the porous carbonaceous materials. This process technically belongs to the physical adsorption, and the whole procedure is limited by the tedious and complex preparation processes because a series of treatments such as pyrolysis, loading, and reduction is necessary.Very recently, the hydrochar prepared by the facile and mild hydrothermal process of lignocellulose has been used as another sustainable carbonaceous support for the applications of material science [14,15,17,19–21]. In contrast to the pyrochar, the hydrochar is easily prepared and characterized by the oxygenated functional groups, even though its specific surface area is relatively low. Biradar et al. [14] utilized a straightforward route of synthesis for the flower-like nanoparticles originated from waste bagasse, and this catalyst facilitates the reductive amination of aldehydes with nitroarene. Similar hydrochar-supported catalyst was also reported by Gai et al. [17], who uses pinewood sawdust as the carbon resource. Furthermore, Titirici et al. [20], Ravi et al. [14] and Hu et al. [21] stated that such carbonaceous support (i.e., hydrochar) can cooperate with a metal precursor to enhance the dispersion and promote simultaneous/mutually reactions because of the presence of various functional groups, especially for the oxygenated complexes (e.g., hydroxyl, carboxyl, carbonyl). These functionalities can improve the access of metal solutions into carbonaceous matrix as its decrease in hydrophobicity [22]; in addition, its surface functionalities can also provide the anchorage sites for metal precursor and act as the active centers in multifunctional catalysts caused by their acid-base or red-ox properties [23,24]. The loading process of metal precursor in this carbonaceous support intrinsically belongs to the chemical adsorption, but unfortunately, there is still a large uncertainty in this field. The relationship between the craft conditions and the catalytic properties of prepared materials, such as the hydrothermal severity, the loading process, and the post-treatment, have been rarely reported so far. And also, the selection of real biomass in previous studies is another obstacle, since the natural and complex components make it difficult to control the structural features of hydrochar-supported catalysts.In this study, we prefer to use biomass-derived glucose as the carbon source for the preparation of support via the hydrothermal process; meanwhile, the exploration on the influence of synthesis routes or conditions towards the performance of hydrochar-support catalysts are identified. Simple handling of the catalyst and separation from the medium is possible with the applied support, and the preparation process is sustainable since the applied support is very common and available in large quantities. The structural features of catalysts associated with its catalytic activity are characterized by several techniques, and the selected catalysts are next applied to the reductive amination of benzaldehyde as a simple model reaction under varied conditions. Subsequently, with the optimized conditions in hand, we conduct the reductive amination protocol to gram-scale synthesis as well as the lifecycle performance in a batch reactor to prove the potential in industrial application, which may provide an available preparation of simple but highly efficient catalyst for the reductive amination in near future.The chemicals and solvents used in this study were purchased from the certified companies registered in the China Academy Science On-line market systems. For instance, nickel nitrate, cobalt acetate, and ferric nitrate were purchased from Sigma-Aldrich Co., Ltd., whose purification was higher than 99.5%; Methanol (AR, 99.7%), and ethanol (GC, 99.9%) were obtained from Aladdin (Shanghai) Chemical Technology Co., Ltd.; Glucose (AR, 99%) and citric acid (AR, 99%) were purchased from Shanghai Macklin Biochemical Co., Ltd. More specifically, the metal salts and glucose were used as the precursor and carbon source, respectively, while the citric acid was adopted as the inducer which can improve the carbonization degree of carbonaceous support; all of them were applied without any further purification after receiving.Biomass-derived glucose was applied to produce hydrochar as the support for metallic nanoparticles, which was heterogeneously loaded on the carbonaceous matrix via two protocols, as explained below. The catalysts prepared by the impregnation process (method A) and one-pot synthesis (method B) were labeled as x@HCIm-y or x@HCOp-y, respectively. In here, x was the type of metal while y was the hydrothermal temperatures, and the procedure was illustrated in Fig. 1 for reference.Hydrothermal process was conducted on a stainless steel autoclave reactor (as shown in Fig. S1), and the detailed experimental procedure can be referred to our previous literatures [10,11,25]. Briefly, 3 g of glucose, 0.15 g of citric acids, and 30 mL of deionized water were loaded into the reactor to keep the solid-liquid ratio at approximately 1:10; afterward, the reactor was sealed and a high-purity N2 flow (99.999%) was injected into the reactor through the inlet pipeline to create an inert atmosphere for subsequent carbonization. The hydrothermal temperatures were in a range of 180–240 °C (with an interval of 30 °C), while the holding period and heating rate were fixed at 16 h and 5 °C min−1, respectively, to avoid the effects caused by secondary factors. Additionally, the magnetic stirrer was operated at a constant rate of 300 rpm throughout the whole hydrothermal process to attain the homogeneous heating. Once the carbonization stage was finished, the reactor was rapidly cooled down, and the hydrochar was separated from the resultant mixture by vacuum filtration and dried in an oven at 105 °C for at least 12 h.In the impregnation method, around 5 g of the hydrochar was impregnated into a 50 mL solution with a metal concentration of 0.50 mol L−1. Subsequently, the aqueous system was kept at 55 °C for slow evaporation, meanwhile, the samples were magnetically stirred to promote the uniform deposition of metal ions into carbonaceous matrix. Importantly, EtOH was used as the solvent in this aqueous system, which not only leads to a well-distributed of metallic atoms but also makes the present protocol safer than other in which highly dangerous solvent was used. Following the impregnation step, the samples were ground and calcined at 600 °C for 3 h, with an increasing rate of 5 °C min−1 and a constant flow of N2 (99.999%) at 60 mL min−1. The residual solid was washed with deionized water and then vacuum freeze-dried at −41 °C overnight to obtain the hydrochar-supported catalyst, which was stored in a vacuum vessel waiting for analysis.In the one-pot synthesis, the general procedure was similar to that of the hydrochar preparation described in Section 2.2.1, but the metal precursor was added into the aqueous system (0.50 mol L−1) prior to the hydrothermal process, which might induce the chemical linking between the metal ions and the functional groups during the synthesis of hydrochar. Herein, the metal precursor can be in-situ reduced and anchored by the functional groups in hydrochar, thereby leading to the formation of inner-sphere surface complexes with metallic nanoparticles. Afterward, the obtained solids were filtered from the resultant mixture and washed with deionized water to remove the unloaded metals. Finally, the prepared catalysts were vacuum freeze-dried and stored under identical conditions as introduced in method A.Generally speaking, the performance of heterogeneous catalysts is characterized by two aspects: one is relevant to the structural features of carbonaceous support, and another is closely related to the metallic nanoparticles. In an attempt to explore the comprehensive information, both aspects were carefully analyzed with the help of several techniques as described below.For the structural features of carbonaceous support: 1) the morphological information was measured using a high resolution transmission electron microscopy (HRTEM), and another high-angle annular darkfield scanning transmission electron microscope (HAADF-STEM; JEM-2100F, Japan) was coupled with an energy dispersive X-ray spectrometry (EDS; Thermo Scientific, Waltham, MA) to measure the distribution of metallic atoms. At the same time, the particle surface was investigated by scanning electron microscopy (SEM, S-4800, HITACHI, Japan). The surface area and pore structure were also analyzed by a Micromeritics Gemini VII 2390 gas-adsorption analyzer according to the N2 isothermal adsorption/desorption at −186 °C in the relative pressure (P/P o ) between 0.01 and 0.99; 2) the crystal structure was explored by XRD (PANalytical, X'Pert PRO, Netherlands) with a Cu Kα radiation (λ = 0.15406 nm), and the scanning conditions were adjusted from 20° to 80° in 2θ range with 0.0167° step interval. In addition, given the correlation of Raman spectra with the parameters calculated from XRD, a Raman spectrometer (LabRAM HR800-LS55, France) was applied for the specific microstructure information of metal-based catalysts. The light source was provided by Nd-YAG at 532 nm, and the scanning region was selected between 1800 cm−1 and 1000 cm−1. XPS analysis was adopted to characterize the surface chemistry, especially for the metallic states and carbon functionalities, of solid samples to a depth of around 0.1–1 nm. XPS spectra were obtained on a Thermo Scientific ESCALAB 250Xi spectrometer, which was equipped with Al (Kα) X-ray radiation source (hv = 1486.6 eV) at a 20 eV pass energy, a 0.1 eV energy step, and 0.1 s dwelling time.For the catalytic activity of metallic nanoparticles: 1) the total acidity and acid strength distribution were determined by temperature-programmed desorption of ammonia (NH3-TPD) with an ASIQACIV200-2 automatic physical/chemical adsorption analyzer (Quantachrome, U.S.). The sample (150 mg) was loaded into the U-tube quartz reactor and heated to 200 °C for 30 min (heating ramp of 10 °C min−1) under 30 mL min−1 of He flows, with the purpose of moisture removal. The system was cooled back to 80 °C, and the 8% NH3/He mixed gas was switched for the chemical adsorption of NH3 on the sample, following with another He flow of 30 mL min−1 to remove the physically adsorbed NH3. Next, the system was heated to 800 °C with a heating ramp of 10 °C min−1 under the same He flows; 2) the distribution of Bronsted (B) and Lewis (L) acidity was examined by the pyridine-absorbed Fourier transform infrared spectrometer (Py-FTIR; Nicolet 6700, Orlando, FL). A 100 mg sample was vacuum-activated (1 × 10−4 mmHg) at 200 °C for 60 min to remove the moisture, and the background spectrum was recorded after the sample was cooled back to 50 °C. Subsequently, the sample was exposed to the pyridine (Aldrich, GC, purity ≥99.5%) vapor for adsorption within 15 min, and the experimental spectrum was recorded several times after the extra pyridine was completely removed by vacuum-pumping.The reductive amination of benzaldehyde was conducted in a stainless autoclave reactor (NS6-20D-SS1, Anhui Kemi Instrument Co., Ltd.), coupled with six independent channels for different substrates. In a typical run, 0.5 mmol benzaldehyde, 10 mg catalyst, and 5 mL 7 mol L−1 NH3 (in MeOH) were accurately weighed into one of the channels; next, the reactor was flushed with H2 several times to remove air and charged 2 MPa H2 at the final time. The reactor was then heated up to the target temperature for a given period, and the magnetic stirrer was rotated at a constant speed of 300 rpm throughout the whole period to ensure a homogeneous reaction. Afterward, the products in the mixture were filtered and detected by gas chromatography to primarily judge the feasibility of the reactions. The target products were further identified by GC-MS (Thermo Trace 1300-ISQ QD, USA) and 1H NMR (Avance III 400 MHz NMR, Bruker, Germany) to calculate the corresponding conversion and selectivity, where the 1, 3, 5-trimethoxybenzene was used as the internal standard.It is generally agreed that the catalytic activity of heterogeneous catalysts is the result of a complex interplay among multiple factors, including not only the structural features of carbonaceous matrix but also the specific activities of metallic phases in samples [26]. First of all, the SEM images exhibit the morphologies of the prepared catalyst, and it is obvious to find that upon hydrothermal carbonization of glucose, micrometer-sized carbon spheres are observed in all samples. In the present case, the addition of citric acid induces a change in the particle morphology, that is, the microspheres are formed out of small aggregated particles. This observation is confirmed by Titirici et al. [27] who stated that citric acid seems to stabilize the first formed small droplet, thereby preventing them from further growth, as might occur in the pure glucose case. Later on in the process, the primary polymerized particles assemble into a micrometer-sized “raspberry”-like structure, as depicted in Fig. 2 . And also, the type of the loaded metals affected the surface structures to a different extent by chemically changing the carbonization degree, as the Ni@HCIm-180 possess relatively smoother spheres than that of Co@HCIm-180 and Fe@HCIm-180. This situation might be relevant to the specific amount of metal coupling with the functional groups, as their concentration in the catalyst systems is Ni > Co > Fe, reaching 74.9 wt%, 17.0 wt%, and 5.4 wt%, respectively (as shown in Fig. 2(a)–(c)). In addition, concerning the cobalt-loaded catalysts under different temperatures, it can also be found that the “raspberry”-like structure was slowly destroyed as a result of the serious decomposition [18,28], leading to more porous microstructures in Co@HCIm-210 (Fig. 2(d)) and Co@HCIm-240 (Fig. 2(e)). In contrast to Co@HCIm-180 prepared by the impregnation method (method A), a smoother structure in the nanoparticles prepared by one-pot synthesis (method B) was clearly observed in Fig. 2(f), which may inhibit the interactions between the active sites and the reactants during catalytic reactions.In addition, the TEM images were in good agreement with the above finding and further confirms the solid characteristic. As shown in Fig. 3 (a), Co-based nanoparticles are well-dispersed and surrounded by a combination of some graphitic layers and short-range ordered graphitic spherical shells with an average size of 6.03 nm, while Ni-based (Fig. S2) and Fe-based (Fig. S3) nanoparticles are easily sintered when operating at the same temperature (i.e. 600 °C). An aggregation of Co@HCIm into larger particles can be observed when hydrothermally treated at higher temperatures, increasing to 8.47 nm and 12.11 nm in Co@HCIm-210 and Co@HCIm-240, respectively. However, some of the large particles with size over approximately 100 nm can also be observed in Fig. 3(a), which might be attributed to the slight agglomeration of partial nanoparticles on the surface. Their catalytic activity is limited by the particle size [22,26]; as a rule, the formation of large particles of the active phase in catalysts is undesirable because of the low reactivity, which redounds in detrimental economic consequences. Not only that, a high degree of dispersion of the active phase is essential since it allows the efficient diffusion, transportation, and transfer of reactants to the catalytic active sites [29]. In this consideration, hydrochar is used as support because its oxygenated functional groups allow the preparation of nanomaterial with good dispersion of the active phase. These groups can serve as bridges or coordinating sites to capture and anchor metal ions, followed by in-situ metallic nanoparticles formation after the calcination in N2, which leads to the robust fixation and high dispersion of metallic nanoparticles [17,20], as depicted in Fig. 3(c)–(f). Meanwhile, Fig. 3(b) demonstrated that the Co2+ and Co0 metal existed in one plane, which reveals the presence of the (1 1 1) plane of metallic Co with an inter-planar spacing of 0.20 nm and (2 2 0) plane of CoO with an inter-planar spacing of 0.21 nm. The corresponding elemental mapping images of the Co@HCIm also illustrate the coexistence of carbon, oxygen, and cobalt with a mass fraction of 72.8 wt%, 9.1 wt%, and 18.1 wt%, respectively, which could be caused by the superficial oxidation of cobalt in air, consistent with the observations on metallic Co-based materials previously reported [29,30].On the other hand, the morphology of the Co@HCOp catalysts was also characterized by the TEM technique. As shown in Fig. 4 , the amount of micro-sized particles with nearly spherical morphology are observed in the sample; however, there is no cobalt nanoparticles can be found on the carbon surface although the elemental mapping did reflect the existence of cobalt. This phenomenon can be explained by the reason that the relatively mild conditions provided by the hydrothermal process are insufficient for the crystallization of metallic atoms, where the metal precursor prefer to form the amorphous states during one-pot synthesis [31]. As reported by Gai et al. [17], the catalysts prepared by the one-pot synthesis have a more uniform size and spherical shape than those prepared by the impregnation method. Hu et al. [21] also stated that metal ions can effectively accelerate the hydrothermal process of carbohydrates to form carbonaceous spheres, while the agglomeration and sintering of metallic nanoparticles may destroy the catalytic activity. Thus, we presume that during the one-pot synthesis, hydrochar can serve as the stabilizing ligands for coating of the out layer of the metallic nanoparticles to prevent the agglomeration and facilitate the formation of highly dispersed metallic nanoparticles on the surface, but further studies are necessary to measure the catalytic activity of metallic cobalt in amorphous states.Subsequently, the Brunauer -Emmett -Teller (BET) and Barrett -Joyner -Halenda (BJH) methods were applied to analyze the different pretreated carbon catalysts, which allows reliable pore size, porosity, and surface area characteristics to be calculated in the pore width range from micro-to mesoporous structures [18,29], as shown in Fig. 5 and Table 1 . Following with the N2 adsorption-desorption isotherms at −196 °C, it can be calculated that the specific surface areas of the prepared catalysts are in a sequence of Fe@HCIm-180 (338.17 m2 g−1) > Ni@HCIm-180 (331.09 m2 g−1) > Co@HCIm-180 (290.46 m2 g−1). In addition, all of them present a type IV isotherm as a capillary condensation step in the adsorption branch is clearly observed, which is attributed to the presence of slit-shaped pores [18]. This situation corresponds to an irregular but well-developed porous structure, as a large amount of N2 is adsorbed in the entire relative pressure range, which is also confirmed by their higher surface areas and larger pore volume. Interestingly, some early studies demonstrated that the catalyst containing porous structures facilitates the rapid transportation of products and reactants, thus playing a key role in regulating product distribution [29,32]. For the series catalysts of Co@HCIm, the value of specific surface area and pore volume gradually enlarge with the increased hydrothermal severity, but it will also cause the agglomeration of cobalt atoms at the same time. Concerning the Co@HCOp, a great decrease of the porous structure is found with only 12.02–14.41 m2 g−1 of specific surface area and 0.014–0.024 cm3 g−1 of total pore volume, which can be explained by the formation route of hydrochar during one-pot synthesis as discussed above [27,28]. Furthermore, Fig. 5 also exhibits the pore size distribution of the prepared catalysts. It is noted that these catalysts display a peak centered at around 2–10 nm, indicating that the hydrochar is a mesoporous material that can enhance catalytic activity and provide efficient diffusion [17]. The average pore diameters of Ni@HCIm-180, Fe@HCIm-180, and Co@HCIm-180 are calculated to be 3.432 nm, 3.814 nm, and 3.075 nm, while their corresponding total pore volumes are 0.169 cm3 g−1, 0.114 cm3 g−1, and 0.065 cm3 g−1, respectively. And also, the pore diameter and pore volume of the Co@HCIm-180 is larger than those of the Co@HCOp-180, which is supported by their trends in specific surface area.To further investigate the structure-activity relationship, XRD and Raman techniques were adapted together as they can provide comprehensive insight into the structural features [28], such as the crystallization and aromatization of samples. As can be seen from Fig. 6 (a), the types of metal precursors will cause differences in XRD patterns. 1) For Ni@HCIm-180, nickel nitrate will decompose into metallic Ni when treating with high temperature without any additional reductant, which is possibly caused by the in-situ reduction of nickel oxide during the carbonization of hydrochar in N2 [10,11]. As a result, the strong peaks of Ni (111), Ni (200), and Ni (220) are presented at the same time. 2) For Co@HCIm-180, the diffractions at 2θ = 44.3°, 51.4° and 75.8° belong to the diffraction of (111), (200) and (220) facets of metallic Co, respectively, while another peak associated with 2θ = 36.6° assign to the (111) planes of CoO, meaning that Co2+ and Co0 metal are co-existed in the samples [4,12,29,30]. Furthermore, with the increase in hydrothermal temperatures, the intensities of the metallic phase increased slightly, indicating the increase of the crystallinity, as depicted in Fig. 6(b); 3) For Fe@HCIm-180, there are several prominent diffraction peaks located at 2θ = 35.4°, 62.6°, and 44.7°, which can be well indexed to characteristic (311) and (440) reflections of Fe2O3 and (110) reflections of metallic Fe, respectively [17,19]. An interstitial compound of iron carbide is also observed, as the diffraction peaks around 42.9° correspond to (211) planes of Fe3C. The XRD spectra of all samples exhibit the strong and broad C (002) peak [10,32], which indicates that the metallic nanoparticles have been adsorbed and co-exist with the carbonaceous matrix on the support. In contrast, no obvious peaks or differences are observed among the Co@HCOp catalysts due to the amorphous states of metallic phase and carbonaceous matrix, as depicted in Fig. 6(c), corresponding with the statistical analysis of the elemental mapping in TEM.The carbonaceous structure of the prepared catalysts was further characterized with the help of Raman spectra, whose results are illustrated in Fig. 6(d). All of the hydrochar-supported catalysts exhibit a D band around 1360 cm−1 due to the disordered arrangement and low symmetry of graphite lattice structure, while another G band at 1580 cm−1 is a scattering peak attributed to the stretching of all sp 2 atomic pairs on carbon ring or long carbon-chain in graphite [27,28]. The intensity of the D band to G band (ID/IG) of the prepared catalysts is calculated to be 1.98–2.26 (in method A) and 2.43–2.88 (in method B), suggesting much more defects are found in the graphitic network. This situation confirms the result of SEM image that the materials obtained via the hydrothermal carbonization of glucose in the presence of citric acid are highly carbonized. The value of La (R = ID/IG, La = 44/R) was also calculated to characterize the degree of aromatization level [33], reaching 22.2, 19.5, and 21.6 for Ni@HCIm-180, Co@HCIm-180, and Fe@HCIm-180, respectively. This difference indicates the type of metal precursor might affect the aromatization of carbonaceous matrix. In addition, it is noted that both intensities in the Co@HCIm and Co@HCOp increase gradually at higher hydrothermal temperatures, indicating that carbon accumulation occurs and leads to the much higher number of covering graphitic layers on the surface.XPS was employed to investigate the elemental compositions and chemical states of the specific species on the samples. As can be seen from Fig. 7 (a), (d), and (g), the types of metal precursors will cause differences in the patterns, but a similar trend can be observed: both of the metallic and oxidized states co-existed on the surface of the catalyst, but the former occupies the dominant parts. For instance, the chemical state of Co 2p 3/2 is mainly in metallic form with the binding energy at around 777.8 eV, while that of Co 2p 3/2  at 780.5 eV belongs to the oxidized form of Co2+; a strong Co 2p 3/2 satellite peak at 783.7 eV and Co 2p 1/2 satellite peak at 799.8 eV are found, which has been used for the identification of cobalt species [34,35]. By calculating and comparing the corresponding peak areas of different Co species in the series samples of Co@HCIm and Co@HCOp, the Co species are mainly in the form of metallic cobalt in Co@HCIm rather than Co@HCOp, but the increase of hydrothermal temperatures might convert part of Co0 into Co2+, as depicted in Table S1. In the C 1s spectra (Fig. 7(b), (e), and (h)), the wide peak ranging from 282 to 292 eV can be resolved into five individual peaks corresponding to the CH bonds CC sp 3 graphene bonds, CC sp 3 graphene oxide bonds, CO bonds, and OCO bonds, respectively [28,36]. Among them, the graphitic structure of CC occupies the dominant role while the graphene oxide bond ranks second, suggesting that the prepared catalysts are highly carbonized but might undergo surface oxidation during the preparation [29]. Further evidence of the surface oxidation can be found through the peaks of O2, O3, and O4 in the O 1s spectra (Fig. 7(c), (f), and (i)); not only that, the O1 peak at 529.3 eV is representative of a metallic oxide network [30], which supports the results of elemental mapping and XRD.In an attempt to identify the effect of acidity on catalytic activity, the relevant types and total amount of acid sites were collaboratively examined by Pyridine adsorption and NH3-TPD, whose profiles are depicted in Fig. S4. In general, the Py-FTIR spectra of prepared catalysts are detected between 1400 and 1650 cm−1. The reflection bands at approximately 1450 cm−1 and 1590 cm−1, which is ascribed to the adsorption of pyridine on the Lewis acid sites, are all observed for Ni@HCIm-180, Fe@HCIm-180, and Co@HCIm-180; at the same time, only a tiny reflection bond at 1540 cm−1 is found, which indicates that the Brønsted acid sites can almost be negligible [37,38]. The Brønsted acid sites can be attributed to the hydroxy groups on the support and easy to be dissociated at high temperatures, while the reduced metal species can act as Lewis acid sites or electrophilic sites to polarize and facilitate the cleavage of C–O bond [33]. It is reasonable that most of the acid sites on the surface of prepared catalysts belong to Lewis acid sites, which is the dominant species of these catalysts regardless of the type of metal precursors. Afterward, Co@HCIm-180 is taken as an example to further determine the specific acid sites by NH3 adsorption, which can be classified into weak (<250 °C), medium (250–450 °C), and strong (>400 °C) acidic sites based on the procedure temperatures [36,38]. As a result, it is found from the NH3-TPD profile that the catalyst has a relatively lower amount of medium acid site and a much high amount of strong acid sites. Meanwhile, Lv et al. [36] also reported that the number of acid sites identified by NH3 adsorption will be decreased with the increase of hydrothermal temperatures, indicating that the higher hydrothermal severity reduces the surface acidity.All of the prepared catalysts as well as some commercial catalysts were tested by the reductive amination of benzaldehyde in the presence of ammonia solution and H2 to produce the benzylamine, which is presented as a structural motif in several bioactive molecules [4,39]. According to the primary experiments, as shown in Table 2 , Co@HCIm-180 (Entry 3) exhibits the best catalytic activity and selectivity towards 1a when compared to most commercial catalysts (Entry 8–13) and other catalysts prepared by different metal precursors (Entry 1: Ni@HCIm-180; Entry 2: Fe@HCIm-180), hydrothermal temperatures (Entry 4: Co@HCIm-210; Entry 5: Co@HCIm-240) or procedures (Entry 6: Co@HCOp-180), reaching a satisfying conversion and yield of approximately 99% and 82%, respectively.First of all, Gould et al. [40] identified that the catalytic ability of transition metals in terms of reductive amination follow an order of Fe < Ni < Co, which confirms the present results described above (Entry 1–3). Besides, the hydrothermal carbon in this catalyst systems provides abundant adsorption sites for metallic nanoparticles but interacts weakly when compared to other oxide supports, thereby overcoming the drawbacks associated with the formation of inactive mixed oxides [41]. The cobalt oxide can be auto-reduced upon heating under an inert environment, thus leading to a smaller cobalt particle size than that reduced under H2. This auto-reduction is related to two aspects [41,42]: one is the volatiles released by hydrochar during thermal decomposition, which contains CO, CH4, or H2 that can reduce Co2+ to Co0, and another is the formation of smaller cobalt nanoparticles under an inert environment, which is attributed to the rapid diffusion of oxygenated functionalities that induces the migration of cobalt atoms on the carbon surface at higher temperatures. As a result, it is easy to understand that the catalytic selectivity towards 1a decreases with the increase of hydrothermal temperatures as most of the volatile matters are previously decomposed at the hydrothermal stage [43]. Additionally, it is found that the O/C ratio is weakened by increasing the hydrothermal temperatures, leading to a downtrend in the density of carboxylic and carbonyl groups. Less metal precursor is thus impregnated in Co@HCIm-240 when compared to Co@HCIm-180.Second, the one-pot synthesis also provide suitable conditions for the in-situ reduction caused by the oxygenated functional groups in the hydrochar [44], which is already proven by the XRD and XPS results. This reduction may proceed prior to the intermolecular dehydration and aldol condensation during the hydrothermal process, so that the metallic nanoparticles tend to be preferentially in-situ dispersed in the hydrophobic core of the hydrochar. However, only amorphous metals are observed on the prepared catalysts of Co@HCOp, which is probably caused by the fact that the hydrothermal temperatures below 240 °C is insufficient for the crystallization of metal. This difference in the chemical states of cobalt provides a plausible explanation for the better catalytic performance on the hydrochar-supported catalysts prepared by the impregnation method (Entry 3) rather than the one-pot synthesis (Entry 6).With the best catalyst (i.e., Co@HCIm-180) in hand, we next optimized the reaction conditions to get the highest yield of target products; meanwhile, the reaction route of the reductive amination of benzaldehyde was also explored to investigate the influence of specific parameters. As is known to all, side reactions are one of the main reasons for carbon loss [25,45], as illustrated in Fig. 8 (a). In addition to benzylamine, the condensation product of N-benzylidenebenzylamine (i.e., 1c) is identified to be one of the intermediates for the reductive amination of benzaldehyde. The hydrogenation of N-benzylidenebenzylamine can easily produce the by-product of dibenzylamine (i.e., 1d), while the subsequent thermal cyclization to 2, 4, 5-triphenyl-4, 5-dihydro-1H-imidazole (i.e., 1e) has also been achieved. Interestingly, the hydrogenation of benzaldehyde into phenylcarbinol (i.e., 1b) is not detected in our catalyst system, which might be owing to the fact that the cobalt-based catalysts are inactive towards the hydrogenation of benzaldehyde [40].Following this route, the effect of reaction temperature is firstly studied under 2 MPa H2 and 5 mL of 7 mol L−1 NH3 solution (in MeOH), and the results of yield are depicted in Fig. 8(b). As can be seen, benzaldehyde can be completely converted even at room temperature, but no target product is observed. By increasing the temperature to the boiling point of the solvent or over it, the yield of 1a steadily grow up to 89.5% as the molecular reactions are accelerated at gas-liquid mixed state; moreover, higher temperatures also provide more heat to overcome the activation energy required by reductive amination. On another hand, the influence of reaction period exhibits the similar trend, where the zero hour can only catalyze a few of the reactant but the 4 h are sufficient enough to convert most of the benzaldehyde into benzylamine. It seems that the reaction temperature and period are the primary factors, as the influence of the NH3 concentration, the H2 pressure, and the amount of catalyst are relatively less important towards the yield of 1a, ranging from approximately 84.4%–93.7%.After developing the successful synthesis of primary amines by Co@HCIm-180, we hope to apply this method in the future industry. And here, we are delighted to show that the conversion of benzaldehyde to benzylamine under three gram-scale tests, including 0.25-g scale, 0.50-g scale, and 1-g scale, are well performed in this catalyst systems based on the optimal reaction conditions (i.e., 90 °C, 4 h, 7M NH3 solution, 2 MPa H2, and 10 mg catalyst). They are similar to those microgram-scale tests, and the excellent yields of 1a are obtained for all the tested scales, as depicted in Fig. S5(a). Furthermore, we also evaluated the catalyst stability and recycle ability using the benchmark reaction under similar experimental conditions. As expected, in comparison to the previously reported catalysts, the hydrochar-supported catalyst can be recycled at least 5 times without the significant loss of activity, although the downtrend become more serious after the 5th cycle (Fig. S5(b)). More importantly, owing to the magnetic properties offered by the metallic cobalt, these catalysts can be easily separated from the aqueous systems by a magnetic bar [10,29], which is considered as an advantage in industrial application.In summary, we have successfully prepared the hydrochar-supported catalysts through two sustainable routes, i.e., the impregnation method and the one-pot synthesis, by using glucose as carbon source, citric acids as inducer, and different metal salts as the catalytic actives. According to the structural analysis, differences between two types of catalysts were carefully revealed, which affects its catalytic performance to a large extent. The impregnation method at the atmospheric pressure might favor the electrostatic attraction with the outer hydration shell of the metal cations, leading to the formation of outer-sphere surface complexes with well-distributed metallic nanoparticles. By contrast, the one-pot synthesis might provide suitable conditions for the formation of inner-sphere surface complexes as the chemical adsorption of metal cations on hydrochar proceed prior to the intermolecular dehydration and aldol condensation. Unfortunately, as a result of the relatively lower carbonization temperature, its catalytic activity on the reductive amination was limited when compared to the former type of catalyst, which is reflected in two aspects: the inferior porous structures and the amorphous state of metal atoms. Subsequently, the catalyst prepared by the impregnation method was adopted to analyze the influence of reaction conditions, which indicates that the reaction temperature and period are the primary factors. And also, this protocol could exhibit a similar reactivity during the gram-scale and had a long lifecycle as it can be easily recycled and reused up to five times without the significant loss of catalytic activity and selectivity. By and large, the above findings confirm an available and sustainable preparation of simple but highly efficient catalysts on the production of functional amines, and future work will be directed towards the identification of catalytic mechanisms on various substances with different functional groups.J. G. L. and L. L. M supervised and designed the research. X. Z. Z. performed most of the experiments and wrote the original paper. J. G. L. reviewed and corrected the original manuscript. All authors discussed the results and assisted during manuscript preparation.Correspondence and requests for materials should be addressed to L. L. M or J. G.L.The authors declare no competing financial interests.This work was supported financially by the National Key R&D Program of China (2018YFB1501500), and National Natural Science Foundation of China (51976225).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.2022.01.012.

Since the utilization of abundant biomass to develop advanced materials has become an utmost priority in recent years, we developed two sustainable routes (i.e., the impregnation method and the one-pot synthesis) to prepare the hydrochar-supported catalysts and tested its catalytic performance on the reductive amination. Several techniques, such as TEM, XRD and XPS, were adopted to characterize the structural and catalytic features of samples. Results indicated that the impregnation method favors the formation of outer-sphere surface complexes with porous structure as well as well-distributed metallic nanoparticles, while the one-pot synthesis tends to form the inner-sphere surface complexes with relatively smooth appearance and amorphous metals. This difference explains the better activity of catalysts prepared by the impregnation method which can selectively convert benzaldehyde to benzylamine with an excellent yield of 93.7% under the optimal reaction conditions; in contrast, the catalyst prepared by the one-pot synthesis only exhibits a low selectivity near to zero. Furthermore, the gram-scale test catalyzed by the same catalysts exhibits a similar yield of benzylamine in comparison to its smaller scale, which is comparable to the previously reported heterogeneous noble-based catalysts. More surprisingly, the prepared catalysts can be expediently recycled by a magnetic bar and remain the satisfying catalytic activity after reusing up to five times. In conclusion, these developed catalysts enable the synthesis of functional amines with excellent selectivity and carbon balance, proving cost-effective and sustainable access to the wide application of reductive amination.

Energetic crisis is a critical global issue, particularly for the development of hydrogen generation technologies, since molecular hydrogen has a relevant energy density but is also considered as an environmentally friendly fuel, with theoretically zero-emissions of greenhouse gases, allowing to also address the nowadays environmental sustainability concern [1]. In this context, ammonia borane (AB) is an excellent hydrogen storage molecule, being considered as solid-state hydrogen, mainly because of its high hydrogen gravimetric content (near 19.6 %) and good stability under normal conditions [2]. Hydrogen can be generated from this molecule by a hydrolytic reaction using a catalytic material to accelerate its cleavage. Nevertheless, most catalysts employed for efficient AB hydrolysis are based on noble metal nanoparticles such as Pt [3,4], Pd [5], Rh [6], and Ru [7]. Within this context, it is well-known that noble metals are non-abundant and expensive materials, which could limit their practical applications. In this sense, a facile approach to reduce the cost of catalytic materials consists in combining highly active noble metals with earth-abundant metals into nanostructured systems, such as alloyed bimetallic nanoparticles. Additionally, the incorporation of metals into nanostructured systems has promoted several improvements, especially in catalysis, due to synergistic enhancements. This has been observed, for instance, in CuAu nanoparticles for the hydroxylation of aromatic compounds [8] or CoPd nanoparticles for CO oxidation, which has also showed a composition-dependence selectivity [9].Therefore, it results important to select catalytically active earth-abundant metals for this purpose and avoid a dramatic decrease in the catalytic efficiency of the bimetallic material. An excellent candidate for this task is nickel since it has been demonstrated to have an active performance in hydrogen generation reactions [10,11]. Besides, nickel is the fourth most abundant transition metal in nature, after Fe, Ti, and Zr, and is much cheaper than noble metals such as platinum. The latter is the most active metal of its periodic group for hydrogen generation reactions among several catalytic processes, but it is also almost 1600 times more expensive than nickel. In this sense, the combination of both metals, nickel and platinum, into nanostructured materials is of interest since both elements have been demonstrated to be active towards hydrogen generation reactions and share similar properties for being in the same periodic group, enabling them to be easily alloyed [12,13]. Therefore, the development of bimetallic nanomaterials has undergone significant growth, with the appearance of different metallic combinations and compositions, in order to tune and improve the catalytic properties of the final material [14,15]. In this context, combining a catalytically active metal, like platinum, with earth-abundant metals, such as nickel, would primarily allow an overall material cost reduction. Furthermore, NiPt alloyed bimetallic systems tend to present synergic behaviors, especially for catalytic applications due to modifications to the electronic, structural or textural properties of the material [16,17], making bimetallic systems an attractive and versatile option to carry out catalytic processes.Additionally, the use of different supports is a simple and efficient strategy to improve the stability and reusability of catalytic materials. In this regard, biopolymers are highly abundant in nature, biodegradable, non-toxicity, and have a wide diversity of functional groups that are crucial for stabilizing nanostructured systems [18]. However, despite these promising properties, there is a lack of information on the implementation of biopolymers for the synthesis of alloyed bimetallic systems for hydrogen generation purposes. In this context, alginate constitute an interesting biopolymer, commonly extracted from brown algae, that displays outstanding interactions with divalent metal ions. An excellent example is the combination of alginate with calcium ions, which drives the facile formation of alginate hydrogel in different formats such as spherical beads [19–21]. Additionally, the use of hydrogel-based systems derived from biopolymers is appealing due to the low amounts of the biopolymer necessary to generate a 3D network upon swelling in large amounts of water (e.g., up to 90 % w/w of the final material) [22]. Due to the above, alginate-based materials are interesting candidates to assist the formation of bimetallic nanoparticles, acting as a support system for their formation, as well as for the conservation of the properties of the nanostructures for recyclability.With this in mind, the aim of this work was to establish: i) a simple method for the synthesis of catalytically active NiPt alloyed bimetallic nanoparticles on alginate hydrogel beads, and ii) the use of this hybrid system as a heterogeneous catalyst in the hydrolysis of AB for the efficient production of hydrogen. Herein, we wish to highlight bimetallic nanoparticles supported by non-toxic hydrogels, derived from renewable feedstocks, as an attractive option for sustainable technological applications such as catalytic hydrogen evolution.Sodium alginate (MW = 380.000 g/mol, G:M = 25:75), nickel chloride (II) (NiCl2, 98 %, Sigma-Aldrich), potassium tetrachloroplatinate (K2PtCl4, 99.9 %, Sigma-Aldrich), sodium borohydride (NaBH4, 98 %, Merck), ammonia borane complex (NH3BH3, 97 %, Sigma-Aldrich), and calcium chloride (CaCl2, 99 %, Merck) were used as purchased with no further purification. Milli-Q water 18.2 MΩ cm−1 was used for all experiments.Alginate beads were prepared following a previously reported method [23]. Briefly, 10 mL of a 3.0 % w/v sodium alginate solution was slowly added dropwise employing a syringe pump at 0.3 mL/min provided with a G25 syringe needle on 90 mL of a CaCl2 5 % w/v solution as a gelling medium. As the sodium alginate solution was added, the calcium solution was gently stirred in order to conserve the formed alginate beads. Once the alginate solution was completely added, the hydrogel beads were kept in the gelling medium without stirring for 1 h to allow the formation and proper maturation of the beads, which were subsequently washed water (3 × 100 mL) to remove the excess of calcium ions. Finally, so-obtained alginate hydrogel beads were stored in water until their use on-demand.Alginate beads loaded with mono- and bimetallic nanoparticles were synthesized by a two-stage procedure that consisted of the adsorption of metallic precursors on hydrogel beads, followed by the reduction into the corresponding nanoparticles. The first stage was carried out by immersing 2 g of wet alginate hydrogel beads into 50 mL of water, in which aliquots of the nickel and platinum solutions, 74.8 mM and 24.9 mM respectively, were added (aliquot amounts are specified in Table S1) and kept under continuous stirring for 48 h. Once the metal adsorption stage was completed, the beads were washed with water (3 × 100 mL) to remove the excess of ions.The reduction step was subsequently performed by immersing 500 mg of hydrogel beads loaded with metallic ions into 10 mL of water in a sealed Schlenk tube, which was purged with argon for 10 min. After this time, 500 μL of a NaBH4 60 mg/mL solution was slowly added to the reaction vessel, which was placed in an oil bath at 60 °C and stirred at 700 rpm for 2 h. After this time, the beads were removed, washed with abundant water and stored prior to characterization and catalysis testing.Hydrogen generation reactions were performed using a sealed Schlenk flask connected to a gas burette system. In a typical experiment, 80 mg of catalyst were added to a sealed Schlenk flask containing 2 mL of water. Then, 400 μL of a 10 mg/mL AB aqueous solution was injected to start the reaction. The hydrogen generation kinetics were monitored by the volume displacement observed in the gas burette. All experiments were performed at room temperature unless otherwise stated.Turn over frequency (TOF) values were calculated using Eq. (1), considering the amount of hydrogen produced, the metal content incorporated to the reaction and the time t at which each catalyzed reaction reached a plateau on its hydrogen evolution profile. (1) TOF = mmol H 2 mmol cat x t Infrared spectroscopy was performed using a PerkinElmer UATR spectrometer by directly inserting the sample in the ATR probe. Spectra were recorded between 4000 and 400 cm−1, with a resolution of 1 cm−1. Thermogravimetric analyses (TGA) were performed in a Mettler thermogravimetric analyzer with a nitrogen flux of 20 mL/min. Rheologic measurements were performed in a discovery hybrid rheometer HR 20 with a 40 mm stain plate, by homogeneously dispersing 500 mg of alginate beads on the testing plate. The rheological experiments were conducted at room temperature and a constant shear frequency of 10 s−1, using an oscillatory strain window between 0.001 and 100 %. The metallic content in the beads was determined with an Agilent atomic absorption spectrophotometer (AAS) GTA 120 and the data were processed using a SpectrAA 240Z. TEM images were obtained with a JEOL JEM 1010 microscope, with a resolution of 0.4 nm, placing the samples on copper grids. TEM images were processed using ImageJ software. Finally, XPS analyses were measured in a “Hippolyta” Devi-sim (SPECS) near ambient pressure X-ray photoelectron spectrometer under ultra-high vacuum equipped with a PHOIBOS NAP-150 analyzer and a 2D-DLD detector, in which the samples were irradiated with a monochromatized Al source (Kα hν = 1486.7 eV) and a flood gun for charge compensation. Data treatment was carried out using XPSPEAK software version 4.1, adjusting a Shirley baseline for each region and referencing the signals with respect to the C 1s peak at 284.8 eV.Synthesis of the hydrogel beads loaded with mono- and bimetallic NiPt nanoparticles was carried out employing an easy procedure that allowed the obtention of the nanocomposites in a controllable and reproducible way (Fig. 1 ). Firstly, the adsorption step was performed taking advantage of the good interaction of alginate chains with divalent ions [18], which favors the adsorption of Ni (II) and Pt (II) ions. Subsequently, the reducing step was performed employing a strong reducing environment, with the addition of a concentrated solution of NaBH4, causing a notable color change in the material going from pale brown to black beads, indicating the formation of the desired nanoparticles. The conditions were selected due to the difficulty of reducing nickel ions into a zero-valence state, because of its negative reducing potential (E0 = −0.257 V). [24] In this case, based on the Marcus theory and taking into consideration the mismatch between the reduction potential of Pt (II) and Ni (II), a strongly reducing environment should be required to kinetically favor the formation of alloyed bimetallic nanoparticles [25,26]. With this synthetic procedure we expect that the nanoparticles are formed with an adequate availability on the outer surface of the bead to work as a supported catalyst. Nevertheless, some nanoparticles may also diffuse to the inner part of the beads and contribute to the potential catalysis, at least to some extent.The presence of the nanoparticles was confirmed by TEM analysis (Fig. 2 ), in which the size distribution of the synthesized nanoparticles was between 2.72 ± 0.69 nm and 5.24 ± 1.14 nm. These results confirmed that the proposed reaction protocol allowed the obtention of nanoparticles with narrow size distributions. The metallic content in the material was calculated by atomic absorption spectroscopy (AAS), revealing values around 1.87 ± 0.03 μmol of metal per gram of hydrogel, values that are also in good agreement with the initial targeted compositions for each system (Fig. 3 ).Subsequently, hydrogels loaded with Ni (II), Pt (II) and the corresponding mono- and bimetallic nanoparticles were characterized by FT-IR, in order to gain an insight into the role that alginate functional groups plays over the adsorption and stabilization of the metallic ions and nanoparticles (Fig. 4 ). The spectra showed the main signals attributed to the alginate backbone, that is the most abundant part of the nanocomposite in comparison with the metallic load of the material. Bands centered at 3320, 2920, 1598, 1412 and 1019 cm−1 correspond to the stretching vibrations of OH, CH, asymmetric and symmetric CO stretching, and to the CO vibrations of the pyranose rings. After the reduction steps, the main signals corresponding to the alginate chains remained in the spectra of all materials, indicating that alginate structure remains unaltered after the reductive process. Nevertheless, there was an increment in the intensity of the signals of OH and CO bonds, which was more noticeable as they were proportionally compared with the intensity of the CH band at 2900 cm−1, indicating the particular importance of these groups during the adsorption of metallic ions and stabilization of metallic nanoparticles [27,28]. The spectral changes observed in the FT-IR suggest that functional groups of alginate were adequately preserved during the protocol, maintaining a number of hydroxyl and carboxylic groups still available to interact with metallic ions and adsorb them onto the hydrogels, which represents a key step in this protocol.Considering that the functional groups of alginates interact in a different way with metallic ions and nanoparticles, they may also influence other properties of the materials such as the mechanical properties of the beads. For this reason, samples of the material at the different stages of the synthetic process were analyzed by oscillatory rheometry. Recently, Posbeyikian et al [29] applied a detailed rheometric method to deeply understand the crosslinking process during the formation of alginate beads. Following that method, we carried out rheological analysis on pristine synthesized alginate beads, beads loaded with adsorbed metallic ions, and beads loaded with reduced metallic nanoparticles (Fig. 5 ). Rheological profiles showed a dominant solid-like behavior during a large part of the analysis, with a storage modulus (G') almost 10 times higher than the loss modulus (G") in all cases. Tan (δ) plots of the materials showed that all materials behave similarly in terms of damping properties, maintaining a phase angle close to zero, typical of solid-liked materials, which demonstrates a notable increase at higher oscillating stress values, and suggesting the presence of a yield stress, accusing the start of the viscous region. However, there is a notable decrease in the plateau of G' (material strength) once the reduction reaction was performed in comparison to the G' values obtained for the pristine alginate beads or beads with adsorbed ions. In this regard, a reduction of the strength of the material could be related to a decrease in the interaction strength between the polymer chains that conform the 3D polymer network. This phenomenon can be related to the stabilization of the formed nanoparticles by the alginate functional groups, since the stabilization process requires their interactions with the surface of the metallic nanoparticles in order the avoid agglomeration or leaching.Further characterizations indicate that the thermal stability of the hydrogel beads loaded with mono- and bimetallic nanoparticles was limited by the evaporation of the water that forms part of the 3D hydrogel network (Fig. 6 ). This was distinguished by a significant weight loss stage centered approximately at 100 °C, confirming that the material is composed of a high amount of water, followed by a moderate second weight loss stage approximately at 200 °C corresponding to the degradation of the remanent polymer content. A similar thermal–behavior has been observed in previous reports, where a first degradation step taking place at temperatures below 150 °C has been previously assigned to the evaporation of the water content present in alginate beads [30,31]. Despite the rapid weight loss experienced by the materials, these can still be useful to perform reactions employing water as a reaction medium and near room temperature, which is highly desirable for sustainable applications.After the complete characterization of the materials, they were tested as catalysts for the hydrolysis of AB as a model reaction for the generation of hydrogen (Fig. 7a). It is important to highlight that hydrogen generation from the hydrolytic reaction was not detected at room temperature in the absence of alginate beads loaded with metallic nanoparticles (Fig. S2). The catalytic properties were determined using a metal loading near 0.1 % with respect to the initial AB amount in each run. In order to compare the released equivalents of hydrogen against the initial equivalents of AB, the obtained data were normalized by the initial amount of AB placed into the reactor. The hydrogen generation profiles showed that the reaction started immediately after the AB addition, without any induction period, suggesting a fast activity of the evaluated catalysts (Fig. 7b). Furthermore, almost all prepared nanocatalysts were highly active, reaching conversion values around 3.0 equivalents at room temperature. This value corresponds to a theoretical complete hydrolysis of 1.0 equivalent of AB, except for the case of alginate beads loaded with NiPt 3:1 and pure monometallic Ni nanoparticles. The latter being the least active in the series of evaluated catalysts.The obtained data were fitted using the well-known pseudo-order models (zero, first and second order, Fig. S3) aiming to have better insight into the kinetic mechanism that governs the reaction. The best fit was achieved using the pseudo-zero order model with a R2 value near 0.996, indicating a zero order of AB in the rate law (Eq. (2)). (2) − d AB dt = k app → AB t = AB 0 − k app t in which, [AB] t, [AB] 0 , k app and t are the time-dependent and initial concentrations of ammonia borane, apparent kinetic rate constant, and reaction time, respectively. To confirm the zero order of AB in the hydrogen generation reaction, different initial concentrations of AB were evaluated. However, the kinetic profiles (Fig. 7c) showed different reaction rates as the initial concentration varied from 39 to 56 mM, being opposite to the expected tendency from the previous model, since a change in the initial concentration should not modify the reaction rate. In this regard, Figen et al [32] previously reviewed the most common models used to fit the kinetic data of hydrogen generation from AB hydrolysis, in which zero order is the most used. Nevertheless, some articles highlight that the catalyzed reaction has little dependence on the AB concentration, disagreeing from the zero order kinetics [33].With these considerations in mind, the Langmuir-Hinshelwood model was applied, taking into account the heterogeneous nature of the catalyst [34]. This model is commonly used for bimolecular reactions that take place at the surface of catalytic materials, in which both reactants are adsorbed on neighboring sites of the catalyst to subsequently accomplish a surface chemical reaction between them, which is usually the rate limiting step of the reaction. The model ends with the desorption of the generated products [35]. The general Langmuir-Hinshelwood expression that explains this phenomenon is given by Eq. (3): (3) − d AB dt = k r K AB 1 + K AB in which k r and K are the superficial chemical reaction and the equilibrium adsorption constant of AB onto the catalyst, respectively [36]. This equation can be integrated as following (Eq. (4)): (4) Ln AB 0 AB + K AB 0 − AB = k r Kt However, the integrated Langmuir-Hinshelwood equation was difficult to use, due its non-linear nature. For that reason, instead of applying the integrated method, Eq. (3) was linearized into the following expression: (5) 1 r = 1 k r + 1 k r K AB in which r represents the rate of the reaction. The data adequately fit into Eq. (5), in which k r and K could be extracted from the reciprocal values of the slope and the intercept of the fitted Langmuir-Hinshelwood curves, respectively. Results are summarized in Table 1 .The obtained values revealed notable changes for both k r and K parameters as the metal composition of nanoentities was modified. Firstly, K[AB] values were calculated considering the initial concentration of AB loaded in the reaction vessel. This was carried out considering the importance of K[AB] as a determining factor, because of its influence in the establishment of limiting conditions for the model. Mathematically, when K[AB] > 1 the general Langmuir-Hinshelwood equation can be approximated to a rate law equation only affected by the k r of the reaction. On the other hand, when K[AB] < 1, the rate law can be expressed as a first order expression on AB concentration. In this regard, all K[AB] values were in better agreement with the first limit case, which, from a mechanistic point of view, suggests an initial fast adsorption of AB molecules on the surface of the material, closely related to the Langmuir-Hinshelwood mechanism. These results indicate that the rate limiting step of the hydrogen generation is the surface reaction that takes place on the catalyst, characterized by k r , which shows a notable increase by the formation of the alloyed systems, a phenomenon related to a synergistic effect promoted by the presence of both metals forming part of the catalysts.Further testing of the materials on recyclability and leaching studies were performed, which are especially important to evaluate the efficiency of the bio-based support to maintain the catalytic properties of the nanoparticles and to avoid their desorption into the reaction medium. To accomplish this, the reaction was performed in multiple catalytic cycles using alginate beads containing NiPt 1:3 (Fig. 8a). The reaction profiles showed minor differences throughout the evaluated cycles, with a slight decrease on the kr which was below 10 % during the evaluated cycles. Additionally, leaching of catalytically active nanoparticles form the hydrogel was also evaluated (Fig. 8b). Initially, a cycle of the reaction was performed using hydrogels loaded with NiPt 1:3 nanoparticles as an example. Then, once the reaction was completed, the beads were removed from the medium and the reaction was performed only with the supernatant of the previous reaction. This demonstrated that the supernatant of the first reaction did not show a water displacement in the gas burette, suggesting that there is no presence of an active material able to perform the hydrogen generation reaction, and that the nanoparticles are well retained by the alginate beads sustaining its use as an adequate support for these systems, facilitating the easy recovery of the material from the reaction medium, and its subsequent utilization without further purification processes, in contrast with other colloidal dispersed systems [37].Additionally, temperature effect on the reaction was evaluated for beads loaded with NiPt 1:3 nanoparticles (Fig. 8c). The results confirmed a gradual increase in the reaction rate with the temperature. This was in good agreement with Arrhenius model (Fig. 8d), in which activation energy of the reaction using this catalyst was extracted with a value of 50.24 kJ/mol, which is comparable to the value reported for other monometallic systems such as Pd [35] and Pt [38] catalysts, with Ea values of 41.50 and 50.35 kJ/mol, respectively.Then, TOF values were calculated to provide a more accurate comparison between the obtained catalysts and other systems already reported in the literature (Fig. 9 ). The hydrogels containing alloyed systems presented higher TOFs than their monometallic counterparts, showing a volcano-shaped trend over the complete composition range studied. These results are in good agreement with the synergic behavior for the catalytic hydrogen generation [10,39,40].In order to gain a better insight into the catalytic synergy obtained by the combination of both metals, XPS analysis was carried out on the NiPt 1:3 system, since it had the best performance among the evaluated materials. Fig. 10 a-c show the XPS spectra of the regions corresponding to carbon 1s, platinum 4f and nickel 2p regions. Fig. 10a shows carbon 1s region of several peaks at 284.8, 286.6, 287.9 and 289.0 eV regarding to several carbon atoms in different environments with increasing electronegative surroundings, related to carbon atoms in alginate chains bonded to hydroxyl groups (COH) or forming part of carboxylate moieties (OCO). Fig. 10b reveals the presence of two pairs of signals, with its 4f7/2 peaks at 70.7 and 72.8 eV, each with its characteristic spin-orbit split of 3.3 eV. This result suggests the presence of platinum atoms in two oxidation states at the surface of the material, mostly in the form of Pt(0) and Pt(II), being the latter related to PtO moieties. Finally, Fig. 10c illustrates the signals related to the nickel 2p region, with a Ni 2p3/2 at 854.1 eV, with a spin-orbit split of 17.6 eV, and a shake-up satellite peak at 871.7 eV. The observed binding energy probably indicate the presence of nickel atoms in a high oxidation state, which could be attributed to the presence of Ni(OH)2 species at the surface of the nanoparticles due to the observed chemical shift. Previously, Fu et al [10] evaluated the activity of NiPt bimetallic nanoparticles increasing the alkalinity of the reaction medium, in which the hydrolytic reaction was enhanced varying the pH of the media. Furthermore, Zhao et al [41] previously modeled water adsorption on nickel oxide species at the surfaces, evidencing a dissociative pathway for its adsorption, accompanied by an notable exothermic ΔH. In this regard, a plausible mechanism to explain the surface enhancement of AB hydrolysis could be related to the presence of preformed hydroxylated species at the surface of the catalyst, preferentially on nickel atoms as suggested by XPS analysis. This would facilitate the surface cleavage of AB molecules that are preferentially adsorbed on platinum atoms, a phenomenon that might explain the greater adsorption constant calculated by the Langmuir-Hinshelwood model in platinum-rich materials.At this point, the TOF value calculated for alginate beads containing NiPt 1:3 catalyst was compared other supported and also unsupported catalysts from the literature (Table 2 ). Our system showed a competitive behavior against other similar bimetallic systems based on the combination of noble Pt atoms with earth abundant metals such as Ni or Co. The beads containing NiPt 1:3 bimetallic nanoparticles showed a TOF above some non-supported systems such as Pt0.65Ni0.35 or Pt0.01Ni0.99. These systems tend to present higher activity in comparison with supported materials since they behave more similar to homogeneous catalysts, demonstrating a better performance in comparison to unsupported bimetallic nanoparticles formed by only noble metals such as Ag@Pd nanoparticles [42]. Additionally, the biobased support afforded a good dispersity of the catalytic nanoparticles, exhibiting a TOF value similar to that obtained for CoPt bimetallic nanoparticles on nanoporous graphene sheets [43], a system characterized by its wide surface area.Biohydrogel beads loaded with NiPt bimetallic nanoparticles were successfully synthesized using an easy and cost-effective method, achieving homogeneously distributed mono- and bimetallic nanoparticles. These materials constitute a promising bio-based candidate for hydrogen generation reactions, being highly active for the hydrolysis of AB, reaching a quantitative hydrogen generation employing a catalyst concentration near to 0.1 % with respect to the AB loading. The reaction mechanism consists of an initial fast adsorption of reactants at the surface of the nanoparticles, followed by a surface reaction which is enhanced by the combination of nickel and platinum atoms. This is attributed to the synergic behavior of NiPt nanoalloys, which affords a TOF value of 84.1 min−1 for alginate beads containing NiPt 1:3 bimetallic nanoparticles. Finally, the synthesized bio-based materials are also easily recovered thanks to the properties of the biohydrogel support. Oscar Ramírez: Investigation, Conceptualization, Methodology, Formal analysis, Writing – original draft. Sebastian Bonardd: Supervision, Methodology, Writing – review & editing. César Saldías: Writing – review & editing. Macarena Kroff: Resources, Formal analysis. James N. O'Shea: Resources, Formal analysis. David Díaz Díaz: Supervision, Project administration, Writing – review & editing, Funding acquisition. Angel Leiva: Supervision, Project administration, Writing – review & 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.O. Ramírez thanks Beca doctorado nacional ANID 21191002. S. Bonardd thanks MINECO for a Juan de la Cierva – Formación contract FJC2019-039515-I. C. Saldías thanks Fondecyt Project 1211022. M. Kroff thanks to Beca doctorado nacional ANID 21180627. J. N. O'Shea acknowledges and thanks Innovate UK through the Energy Research Accelerator, the Engineering and Physical Sciences Research Council (EPSRC), and the University of Nottingham Propulsion Futures Beacon for funding. A. Leiva thanks to FONDECYT 1211124 and FONDAP 15110019 projects for the financial support of the research. D. D. Díaz thanks financial support from the Spanish Government for the Senior Beatriz Galindo Award (BEAGAL18/00166) and the project PID2019-105391GB-C21/AEI/10.13039/501100011033. The authors thank NANOtec, INTech, Cabildo de Tenerife and ULL for laboratory facilities. Supplementary material Image 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijbiomac.2022.11.106.

Alginate hydrogel beads were loaded with bimetallic NiPt nanoparticles by in situ reduction of the respective polymer matrix containing precursor metallic ions using a NaBH4 aqueous solution. The alginate hydrogel beads loaded with NiPt nanoparticles were characterized by TEM, AAS, FT-IR, TGA, XPS, and oscillatory rheometry. The prepared hybrid hydrogels were proven to be effective as catalytic materials for the hydrolysis of ammonia borane (AB) for quantitative hydrogen generation using catalytic loadings of 0.1 mol%. In addition, the reaction mechanism of the hydrolytic reaction using NiPt loaded alginate hydrogel beads was determined by Langmuir-Hinshelwood model. The experimental results showed that the reaction mechanism consisted of an initial fast adsorption of reactants at the surface of the nanoparticles, followed by a rate-limiting surface reaction. The NiPt nanoalloys exhibited an enhanced behavior for hydrogen generation with a maximum TOF of 84.1 min−1, almost 71 % higher compared to monometallic platinum atoms, and likely related to a synergistic interaction between both metals. Finally, the hydrogel matrix enabled the material to be easily recovered from the reaction medium and reused in further catalytic cycles without desorption of active nanoparticles from the material.

The advent of industrialization over the last 200 years gives rise to many anthropogenic activities that lead to the destruction of the natural environment. Increased levels of carbon dioxide (CO2) in the atmosphere caused by the consumption of fossil fuels play a leading role in the greenhouse effect and subsequent global warming. Over the last few decades, extensive research has been carried out to explore alternative energy carriers such as hydrogen that can alleviate the role of CO2 in the atmosphere [1–6]. Different strategies have been explored to produce H2 in a sustainable and renewable way such as solar water splitting, but the storage and transportation of hydrogen results in additional costs for the commercialization of this technology. As a promising alternative, the future hydrogen economy can be enhanced by the utilization of ammonia (NH3) as a carrier due to its high hydrogen content (1.4 % greater mass fraction than methanol) and volumetric energy density as compared to liquid hydrogen [7]. Also, ammonia is an important commodity in the food and energy supply chains with high annual consumption of 200 million tons. In addition to the production of fertilizers, ammonia is also a feedstock in the production of explosives, plastics, resins, synthetic fibers, and refrigerants (Fig. 1 ).In nature, nitrogen fixation at ambient conditions is carried out by the nitrogenase enzyme present in microorganisms, comprised of a molybdenum-iron (MoFe) protein (Equation 1) [8–10]. This process is carried out by a proton-coupled electron transfer (PCET) reaction with a significant input of energy by the hydrolysis of adenosine triphosphate (ATP) into adenosine diphosphate (ADP) and inorganic phosphate (Pi) with intrinsic hydrogen production. In industry, the Haber–Bosch (HB) process utilizes iron (Fe) and ruthenium (Ru) based catalysts to reduce nitrogen with hydrogen (from steam reforming) at high temperature (400–500 °C) and pressure (130–150 bar) to produce ammonia [11]. The high temperature and pressure are required to overcome thermodynamic constraints related to the bond dissociation energy of the nitrogen molecule (911 kJ mol−1) and for an increased rate of production. Moreover, the production, purification, compression, and transportation of reactant gases add to the high cost and energy consumption of this process [4,12]. Therefore, there is a strong demand to explore less energy-intensive, eco-friendly, and economically viable alternative strategies for facilitated ammonia production under ambient conditions. (1) N2 + 8 H+ + 8 e− + 16 ATP → 2 NH3 + H2 + 16 ADP + 16 Pi For green and sustainable ammonia production, different strategies have been employed in the recent past including, molecular catalysis, electrochemical and photo-electrochemical reduction of nitrogen to produce ammonia [9,14–17]. Inspired by the nitrogenase enzymes, different transition-metal-based molecular catalysts have been developed and assessed for ammonia production. The mechanism of nitrogen reduction catalyzed by molybdenum and iron-based phosphine and trisamidoamine complexes has been studied [18–22]. The mechanism's elucidation, as well as the catalyst's stability and regeneration, are issues that need to be addressed. Photo-electrochemical nitrogen reduction and nitrogen oxidation into ammonia and nitrates, respectively, have also been explored using the visible light spectrum. Photosensitive materials of various sorts have been used, primarily titania (TiO2), but also transition metal-based semiconductors, noble metals, chalcogenides-supported biomaterials, and polymeric materials [20,23–26]. The use of this method on a wide scale has been limited due to the limitations of different photocatalytic materials such as low solar spectrum absorption and significant charge recombination.Electrochemical reduction of nitrogen to ammonia (eNRR) at ambient conditions is recognized as one of the most promising approaches toward a CO2-free product due to the abundance of carbon-free reactants (nitrogen and water) and the benefits of heterogeneous catalysis along with the use of clean and sustainable energy sources (Equations 2-4) [27]. (2) Cathode: N2 + 6H2O + 6e − → 2NH3 + 6OH− −0.77 V vs. SHE (3) Anode: 2OH− + H2 → 2H2O + 2e − +0.82 V vs. SHE (4) Overall: N2 + 3H2 → 2NH3 +0.06 V Modern research on eNRR opens new possibilities for reducing copious atmospheric nitrogen to ammonia using a range of efficient electrocatalysts (Table 1 ) [28–31]. Electrochemical ammonia production may be implemented as a small-scale decentralized system, which promises to be a breakthrough for the industry of underdeveloped countries. The facile transportation and storage of liquid ammonia further make it a promising low-cost technology [32,33].Several factors mitigate against an efficient eNRR. The availability of dinitrogen at the electrode is limited by its low solubility in aqueous solutions. Without a suitable catalyst, the strong non-polar bonding (E d = 911 kJ mol−1) and high activation barrier for dissociation (941 kJ mol−1) of N≡N result in a low rate of ammonia production, and competition from the electrochemical hydrogen evolution reaction (HER, Equation 5) lead to a low faradaic efficiency (FE) [34]. Therefore, the development of an efficient and selective catalyst that can promote the reduction of nitrogen to ammonia while suppressing the HER is of great importance to the modern world. (5) 2H2O + 2e → H2 + 2 OH− To attain this purpose, rational catalyst design is essential and must address both the intrinsic and extrinsic catalytic activity of the electrocatalyst. The intrinsic catalytic activity of an electrocatalyst directly affects the overall performance of the reaction. Electronic structures dictate the intrinsic catalytic properties of different metal and non-metal surfaces that can be further altered by doping of heteroatoms, alloying with other metals, core-shell structures formation, facet engineering, creation of defects, and intercalation of different anions or cations [35–39]. Extrinsic catalytic properties are determined by structural and morphological parameters that can affect the kinetics of the reaction. The shape, size, and morphology of the catalytic material dictate its active site density, which should be finely engineered to enhance overall catalytic performance (Fig. 2 ). Moreover, rational electrode design could also improve the mass transport and charge transfer of the reaction by an enhancement in the surface area, the development of porous structures, and the use of different support materials [16,35].Theoretical insights into the mechanistic approach provide a framework for the rational design of a catalyst with effective catalytic activity towards eNRR. A systematic account of the structure-to-performance relationship of a catalyst for eNRR is presented in this review. This review summarizes the latest work in rational catalyst design strategies and mechanistic analyses for eNRR in ambient settings by taking theoretical to experimental approaches. Systematic illustrations of recent theoretical and experimental data on catalyst design will provide a full account of important challenges and constraints of this technique, as well as the ability to envision many potential future pathways. A framework that is supported by theoretical studies that have led to the optimal selection and development of eNRR catalysts in the recent past will be described. Following a discussion of recently-developed electrocatalysts for eNRR in ambient conditions, we outline various recently-published experimental methodologies to improve the intrinsic and extrinsic catalytic activity of advanced electrocatalysts for eNRR. Finally, we discuss some key findings and projections for this developing field.Electrochemical processes are generally driven by reactant adsorption, followed by activation, polarization, and transformation of reactants into products via various PCET reactions. Theoretical and experimental elucidation of several mechanistic paths for eNRR is first discussed in this context, as well as the analysis of thermodynamic and kinetic variables during the process.Molecular nitrogen is one of the most thermodynamically stable and inert molecules attributed to the short bond length (109.67 pm) and high dissociation energy (911 kJ mol−1) of the triple bond. The large energy gap (10.82 eV) between the highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO) of nitrogen with its low polarizability, impedes electron transfer and reaction kinetics for ammonia production. Also, thermodynamic constraints related to various intermediates (NH, NH2, NH3) of this process add to the challenge. This process under ambient conditions is hampered by the competitive hydrogen evolution reaction. The affinity of available catalysts toward the hydrogen atom is greater than the non-polarizable nitrogen molecule. Due to the high redox potentials of intermediates and high activation energy requirement, this process is energy-intensive. However, the total Gibbs energy for ammonia formation favors the eNRR over hydrogen evolution. Also, an intrinsic challenge related to the low solubility of nitrogen (0.66 mmol L−1) and high concentration of water (55 mol L−1) in aqueous media limits the interaction of reactant nitrogen molecules with the active surface of the catalyst resulting in the low selectivity for ammonia formation. In this regard, active and selective catalyst design is a priority to overcome the above-mentioned challenges.Mechanistic investigations of the chemical process are highly desirable to develop an active selective and stable catalyst. Electrochemical NRR is governed by several PCET reactions and can be explained by different pathways. Firstly, the adsorption of nitrogen molecules or adatoms on the surface of the catalyst is the first step in the process that is followed by the PCET reaction, and after several PCET reactions, the desorption of the product takes place.Electrochemical NRR is generally believed to proceed via two types of reaction mechanisms, i.e., associative, and dissociative mechanisms (Fig. 3 ) [40–42]. In the associative mechanism, the hydrogenation of adsorbed nitrogen molecules on the surface of the electrocatalyst takes place with two nitrogen atoms bound together. After two successive hydrogenations on the distal nitrogen atom, the final cleavage of the N≡N bond takes place resulting in the release of the first ammonia molecule (distal pathway). The second ammonia molecule is released after successive hydrogenations of the second adsorbed nitrogen atom. Moreover, successive alternating hydrogenations could occur on each nitrogen atom resulting in the hydrogenation of both N-atoms before N≡N bond dissociation (alternating pathway). Different types of adsorption symmetry of nitrogen molecules on the surface of the active site decide the pathway for the associative mechanism. If the adsorption of a nitrogen molecule is end-on, then the associative distal pathway is possible, and if the adsorption is side-on, then the alternating pathway is possible. For the dissociative mechanism, N≡N bond dissociation, a step that requires a large amount of energy, takes place before the hydrogenation of the nitrogen atom.For catalysis by transition metal nitrides, ammonia is proposed to form by hydrogenation of a nitrogen atom in the transition metal nitride structure; the Mars–van Krevelen mechanism (MVK) [43,44]. This results in the creation of a nitrogen vacancy in the structure that is then occupied by N from the applied nitrogen gas. Successive hydrogenations result in the formation of ammonia. Further exploration with experimental evidence is important for an improved understanding of this process particularly the rate of vacancy creation and consumption.To achieve the above-mentioned mechanistic pathways, an enhanced interaction of nitrogen with an active site is required. However, despite the weak interaction between nitrogen and noble metal catalysts (Pd, Au) at low potentials, reasonable performance is seen with these catalysts. For these catalysts, surface hydrogenation is considered an important factor in the formation of ammonia [45]. Density functional theory (DFT) calculations revealed that the adsorption of nitrogen is outcompeted by the reduction of proton (H+) in the first step providing high coverage of adsorbed hydrogen (∗H) (Fig. 4 ). Nitrogen can be activated by this hydrogenated surface to form ammonia at low overpotentials. A synergistic role of surface hydrogenation and the activation energy on the surface of the catalyst has been elucidated and the reduction of H+ is confirmed as the potential-determining step.Mechanistic attributes of the eNRR process can be better understood by using in-situ and operando characterization tools. Yao et al. recently conducted a spectroscopic study of eNRR on gold (Au) and platinum (Pt) nano surfaces and proposed an associative mechanism for eNRR on Au thin films using results from surface-enhanced infrared absorption spectroscopy (SEIRAS) [46]. N2H species were detected by SEIRAS at potentials below 0 V vs. RHE which is a clear indication of the associative pathway for eNRR. Moreover, no intermediate was detected for Pt under the same eNRR conditions that demonstrated the highest activity of Pt as a HER catalyst. Similarly, rhodium (Rh) surfaces were also explored as an eNRR catalyst by SEIRAS, and a new reaction mechanism was proposed [47]. Firstly, the electrochemical formation of N2H2 by a two-electron pathway was observed and then the formation of ammonia after successive hydrogenation was completed. The IR bands for the hydroxyl group are observed at 3250 cm−1 and 1612 cm−1 which can be attributed to the 1st layer of water (Fig. 5 a). At potentials from 0.2 to −0.4 V vs. RHE, the N=N stretching band at 1997–2036 cm−1 is allotted to adsorbed N2H x (0≤ x ≤ 2). Furthermore, surface hydrogenation, which varies with applied potential, was also confirmed by a weak band at 1865 cm−1 due to adsorbed hydrogen atoms. Lai et al. reported superior eNRR performance for rhenium sulfide (ReS2) when doped with Fe on the surface of N-doped carbon which was verified by in-situ attenuated total reflectance infrared (ATR-IR) analysis (Fig. 5b) [48]. A positive shift in the IR bands for O–H stretching was observed from 0 to −0.3 V vs. RHE which was ascribed to the change in the adsorption configuration of water. Furthermore, the stability of the catalyst was also corroborated by SEIRAS spectra at −0.2 V vs. RHE with similar absorption results.Fu et al. demonstrated the enhancement in the selectivity of this process by a dual atom catalytic system [49]. Re2MnS6 nanosheets were developed and the mechanism was evaluated for eNRR by in-situ Raman analysis. The presence of a Raman mode at 658 cm−1 under applied voltage in the presence of N2 confirmed the formation of NH3 (Fig. 5c). Maximum production of ammonia was obtained at −0.30 V vs. SHE and no ammonia was detected in argon (Ar) on a Fe doped ReS2 surface. A weak band for ReS2 was observed with a blue shift to 709 cm−1 that was attributed to the synergistic effect of dual active sites (Fig. 5d). Dual sites provided enhanced interaction with nitrogen due to the delocalized electronic structure and different adsorption sites. Several other authors have employed similar methodologies to explore the eNRR mechanism by different in-situ techniques [50–56]. By providing evidence of the mechanism these techniques are helpful in the proper selection of material in rational catalyst design for eNRR.To screen promising candidates, understand reaction processes, and optimize catalysts, ab initio simulations such as Hartree–Fock, and multi-reference approaches can be utilized. DFT compared to other ab initio calculations, offers information about the energy, structure, and electronic configurations of a particular set of compositions with a lesser computational cost and higher accuracy [57]. Herein, the most relevant DFT results for electrochemical nitrogen reduction under ambient conditions, such as Volcan plots, Gibbs free energy diagrams, the density of states (DOS), and charge analysis are discussed. The utilization of these results to understand experimental results or predict novel catalyst compositions is highlighted. The framework provided by theoretical studies is envisaged to facilitate the proper selection of material for advanced electrocatalyst design.Extensive theoretical studies have been carried out for the exploration of the eNRR reaction mechanism on a variety of different surfaces [58–62]. Based on free energy changes and adsorption strength of possible intermediates for the eNRR process, different endergonic steps have been identified that limit the rate of reaction. A careful analysis of adsorption energies of different intermediate leads to the development of a volcano plot that gives a basic selection criterion for different types of suitable surfaces. It is a quantitative illustration of the Sabatier principle [3] in terms of the optimum interaction of different reactants, intermediates, and products with the catalyst surface. A good catalyst has optimized binding energies for both reactants and products that direct the overall process towards spontaneity. Moreover, free energy profiles provide basic insight into the mechanism and kinetics of the reaction by elucidation of rate-limiting steps. These described approaches are robust and powerful in the screening of a broad database and using thermodynamic properties as descriptors.DFT calculations on first and second-row transition metals of two different surfaces (flat and stepped) have been conducted for eNRR, and a volcano plot was established [63]. The volcano plot in Fig. 6 a illustrates the free energy relationship of adsorbed N atoms on both flat and stepped surfaces of different transition metals with the applied potential difference. Metals on the right side are limited by the adsorption of nitrogen molecules on the surfaces with the first PCET reaction (an associative mechanism) and nitrogen dissociation (a dissociative mechanism) as the rate-limiting steps. However, the left side of the plot depicted the same rate-determining step for both associative and dissociative mechanisms but different reaction pathways depending on the metal surfaces. For flat surfaces the second PCET reaction and for stepped surfaces, the final PCET reaction or desorption of ammonia are the rate-determining steps. Moreover, early transition metals were predicted to have an optimum binding for N-adatoms as compared to H-adatoms, they are preferred over late transition metals for eNRR at operating potentials of around −1.0 to −1.5 V vs. SHE. Therefore, the low rate of ammonia formation could be attributed to the competing HER reaction on the late transition metals. By this approach, one can predict the most suitable transition metal for eNRR at ambient conditions based on optimal nitrogen binding and rate-determining steps.Montoya et al. explained the linear scaling relationship of adsorption energies of eNRR intermediates on pure noble metals (Ag, Ru, Re) [64]. They employed a two-variables description of theoretical overpotentials that resulted in the exploration of the fundamental limitations of this process. It helped in the optimization of required overpotentials for different steps in the eNRR process that led to the selective design of electrocatalysts for eNRR. For eNRR, NH2 and N2H have relatively large adsorption energies that lead to the scaling relation with an overpotential of 0.5 V [61]. In this regard, strategies have been proposed to enhance eNRR activity including selective stabilization and destabilization of N2H and NH2 at different catalyst surfaces, functionalization of the surfaces by co-adsorbed molecules, and the solvation effect [65]. This can be achieved by rational catalyst design strategies that tune the catalyst surface according to the reaction requirements.DFT calculations have been also conducted to envisage the role of metal oxides in the activation of nitrogen [66]. As illustrated in Fig. 6b, most of the metal oxides have endergonic nitrogen binding energies except tantalum oxide (TaO2) and rhenium oxide (ReO2), so the first step in the eNRR process is rate-limiting. Moreover, the effect of HER was also observed by examining the adsorption energies for hydrogen and nitrogen on various metal oxides. Except for ReO2 and TaO2, all metal oxides are more susceptible to adsorb hydrogen than nitrogen, making them less suitable for the eNRR process. As adsorption energies dictate various steps to be the rate-limiting step for any reaction, so the respective energy profiles will give a better understanding of reaction mechanisms. For instance, ammonia formation on Ru@C2N was theoretically analyzed by free energy profiles for different mechanisms [67]. In the associative mechanism by distal pathway, nitrogen adsorption is the first step on the electrocatalytic surfaces that resulted in the decrease in free energy for conversion from gas to the chemisorbed molecule. At applied bias U = 0 V, the first hydrogenation of the nitrogen molecule requires energy making it an endergonic step in this case. This endergonic step can become spontaneous when the bias of U = −0.96 V is applied (Fig. 6c). All other steps become spontaneous with no change or decrease in free energy. Moreover, in the dissociative mechanism, the most endergonic step is the dissociation of nitrogen molecules on the surface of the catalyst as it is restricted by the high free energy barrier (2.6 eV). Therefore, the associative mechanism is more plausible for eNRR on this surface. In addition, four pathways are proposed for the dissociative mechanism including (A) the direct dissociation of the N≡N bond caused by the first PCET reaction on the ∗NNH species, (B) the dissociation of the ∗NNH2 species into ∗N and ∗NH2, (C) the decomposition of the ∗NHNH2 into ∗NH and ∗NH2; and (D) the ∗NH2NH2 separating into two ∗NH2. For Ru@C2N-DM, after the adsorption of the nitrogen molecule and first proton transfer, the free energy difference is small in pathways C and D which drives the reaction downhill after every proton transfer under a bias of −0.96 V (Fig. 6d). After the formation of NHNH2 species, the dissociation of nitrogen molecules takes place which further lowers the energy. Therefore, a careful design of the catalyst surface is necessary to make it more favorable for nitrogen adsorption as compared to the H adsorption.DOS is a non-self-consistent computation that is used to determine the density of occupied electronic energy levels following structural optimization and static calculation. The DOS is a semi-quantitative indicator of a catalyst’s electronic conductivity, particularly for catalysts with similar compositions and structures. The lower bandgap between the valence and conduction bands, as well as the greater DOS near the Fermi level, indicate a higher charge carrier concentration, which favors higher electronic conductivity.Wu et al. investigated the nitrogen reduction reaction by heteronuclear metal-free double-atom catalysts [68]. A set of 36 catalysts was evaluated for the activity and selectivity of the nitrogen reduction process by using DFT. Firstly, four different non-metallic substrates were used to evaluate the performance of this process. Among carbon nitrides (g-C3N4, g-CN, g-C2N), and boron phosphides (BP1, BP2, and BP3), Boron-based substrates were found to be more active in nitrogen reduction. A synergistic role of boron and silicon was predicted to be a suitable composition for this process. B–Si@BP1 and B–Si@BP3 were the most active catalysts based on the adsorption energies for different reaction intermediates. The conductive behavior of these catalysts was elucidated by the density of states. As illustrated in Fig. 7 a the Fermi level was traversed by the 2π∗ orbital of adsorbed dinitrogen on B–Si@BP1, showing that this component of the antibonding orbital of nitrogen was easily filled by electrons from the catalyst, resulting in the activation of dinitrogen. However, for B–Si@ BP3, the electronic density location of 2π∗ was at a slightly higher energy than that of adsorbed nitrogen on BSi@BP1, which confirmed the ease in side-on adsorption than end-on adsorption to feedback electrons to the antibonding orbital of nitrogen (Fig. 7b).Li et al. systematically investigated several transition metal single-atom catalysts on tungsten sulfides (WS2) monolayers for nitrogen reduction by DFT studies and kinetic modeling [69]. N2H adsorption was chosen as the activity descriptor and found three catalysts to have the highest activity, Re@WS2, Os@WS2, and Ir@WS2. The presence of vacant and occupied 5d orbitals in these catalysts facilitated donations and back donation of electronic density, resulting in enhanced nitrogen activation. The introduction of transition metals on the monolayer WS2 induced spin polarization with the mid-gap states formed by the 5d states of the transition metal. These orbitals hybridized with the W5d and S3p to form a strong metal-support interaction. This visualization of electronic states promises a facile approach for the effective screening of catalysts.Li et al. also demonstrated the doping effect of carbon in hexagonal boron nitride ribbons by DOS investigations [70]. A decrease in the overall nitrogen overpotential from 1.14 to 0.39 V was observed with a modulation in the adsorption energy of different reaction intermediates after the introduction of carbon atoms, which was attributed to the changes in the electronic structure of boron nitride. To further the investigation, ‘zigzag’ and ‘armchair’ nanoribbons were evaluated for the electronic state modulation with nitrogen adsorption. Fig. 7c shows confirmation of the presence of a partial wave at the Fermi level for the zigzag configuration with improved conductivity that reflected the affected electron transfer in this case. A similar trend was observed for the armchair configuration that demonstrated both configurations as the active materials for nitrogen reduction (Fig. 7d). However, the zigzag configuration showed magnetism due to the unsymmetrical nature of the spin-up and spin-down density of states. This spin-polarized magnetism in the zigzag configuration provided localized charge density sites on the surface of the catalysts that improved the interaction of reactants with the active site. This result demonstrated the theory-assisted rational catalyst design is a desirable approach for the development of an active, selective, and efficient catalyst for nitrogen reduction.The spatial distribution of electron density in a catalytic structure is determined as a charge density distribution. On the catalyst's active sites, which are nothing more than the localized or delocalized charge density sites, numerous surface reactions occur during electrocatalytic processes. The formation of the active site and the successive adsorption of the reactant moiety are directly related to the charge density sites in the overall structure. So, the investigation of these charge distributions is desirable for understanding the role of the active site with the plausible mechanism of a process. The charge density difference can be used to study electronic interactions and redistributions when new bonds, interfaces, or heterojunctions occur.Bader charge analysis provides evidence of the charge distribution on the surface of the catalyst [71]. This analysis demonstrates the charge distribution due to any modulation of the catalyst structure or a chemical change during a process. For instance, Zhang et al. evaluated Fe2 clusters on molybdenum sulfides (Fe2/MoS2) for nitrogen reduction by Bader charge analysis in an enzymatic pathway [72]. The variation of the Bader charge was calculated for every elementary step of this process on three different moieties including the adsorbed N x H y (x = 1, 2; y = 1–3) species, the Fe2 cluster, and the MoS2 substrate (Fig. 8 a). The charge fluctuation on the Fe2 cluster is less than on N x H y and the MoS2 substrates. During the eNRR process, the MoS2 substrate serves as an electron reservoir, while the Fe2 cluster serves as a charge transmitter between the adsorbed N x H y species and the MoS2 substrate.Arachchige et al. employed Bader charge analysis to investigate the role of dual metal atoms on the surface of graphdyine [73]. The Bader charge analysis for cobalt-nickel on graphdyine (CoNi@GDY) during the distal mechanism revealed a charge distribution throughout this mechanism (Fig. 8b). The positive charge of CoNi@GDY was increased due to electron donation to nitrogen during the adsorption step. The charge plot for the first and second PCET revealed electron accumulation on CoC4 and electron exchange from NiC4 to adsorbed N x H y . Then, for the remaining steps, both CoC4 and NiC4 demonstrated identical charge distribution, apart from the fifth PCET, when charge fluctuation was in the other direction. The synergistic effects of two metal atoms, Co, and Ni, were established by such charge change, underscoring the necessity of a double atom catalytic site.Theoretical studies provide a solid framework for rational catalyst design and the proper selection of efficient and stable catalysts. Different surfaces have different capabilities to activate nitrogen molecules for eNRR. Identification of rate-limiting steps and scaling relations between different intermediates on catalytic surfaces help in the better design of a catalyst [60,68,74,75]. Moreover, kinetic barriers that are directly affecting the rate of reaction could be removed by an enhancement in the extrinsic properties of the catalyst. However, linear scaling relations depend mainly on the type of catalytic surface under study. Therefore, in addition to these approaches, the gradient ascent method and microkinetic modeling should be employed to assess the overall turnover frequency of a catalyst [39,71].In recent years, a variety of materials have been investigated for nitrogen reduction at ambient conditions. To have a comprehensive description of different categories, herein, we have divided electrocatalysts into four types based on composition. These include (i) noble metal-based, (ii) non-precious metal-based, and (iii) metal-free electrocatalysts.Noble metal catalysts have been employed as active catalysts for a variety of applications due to their high conductivity, active crystalline surfaces, enhanced interaction, activation, and polarization of different reactants [76]. These versatile characteristics make them a superior class of materials in catalysis. Various noble metal-based catalysts have also been explored in the recent past for eNRR [9,77–79]. Herein, we discuss some representative reports based on ruthenium, gold and palladium (Ru, Au, Pd) metals for eNRR at ambient conditions.Corroborated by theoretical studies, Ru is one of the most active materials and was first evaluated for eNRR at ambient conditions by Kordali et al. [80]. They electrochemically deposited nanosized Ru on carbon felt and evaluated the electrode for eNRR. At −1.10 V vs. Ag|AgCl in a nitrogen atmosphere, 20 °C, and atmospheric pressure, a rate of 0.21 μg h−1 cm−2 and a current efficiency of 0.28 % for ammonia production were achieved. Similarly, a nanosized Ru-based electrocatalyst was synthesized by oleate-mediated synthesis and evaluated for eNRR in an acidic environment [81]. A greater yield of ammonia (5.5 μg h−1 m−2) with a FE of around 5.4 % at 0.01 V vs. RHE was obtained. This selective ammonia conversion at low overpotential could be attributed to well-dispersed nanosized Ru particles on the surface of carbon fiber paper (CFP). DFT calculations highlighted the enhanced interaction of nitrogen with Ru (001) surfaces that resulted in the exergonic adsorption of nitrogen (Fig. 9 a). In a recent report by the same group, Ru/MoS2 was demonstrated as a selective catalyst for eNRR [82]. MoS2 polymorphs were employed to control the HER and a FE of 17.6 % with a high yield rate of ammonia (1.14 × 10−10 mol s−1 cm−2) is achieved. Theoretical calculations confirmed a hypothesis that nitrogen activation took place at Ru nanoclusters and a synergistic coupling effect with S-vacancies on the 2H–MoS2 resulted in the hydrogenation of N adatoms.For effective electrocatalytic ammonia production, ruthenium-doped defect-rich tin oxide nanoparticles on carbon cloth (Ru–SnO2/CC) were developed by Sun et al. [83]. In 0.1 mol L−1 sodium sulfate (Na2SO4), it produced 4.83 μg h−1 cm−2 of ammonia with a FE of 17 %. The developed synergy by the combination of Ru species, SnO2, and oxygen vacancies resulted in a system with enhanced Ru stabilization and suppression of HER, while oxygen vacancies in the SnO2 lattice improved nitrogen adsorption and enhanced the activity of the Ru active center (Fig. 9b).Shi et al. developed Au sub-nanoclusters supported by TiO2 for nitrogen reduction by the tannic acid reduction method [84]. Au sub-nanoclusters (0.5 nm) decorated TiO2 demonstrated a high production yield (21.4 μg h−1 mg−1) of ammonia with a FE of 8 % at −0.2 V vs. RHE. This efficient activity of isolated Au nanoparticles could be attributed to the sub-nanometre dimensions that provide the enhanced exposure of active sites. Besides intrinsic catalytic activity improvement at sub nanometric dimensions, the dispersion of these isolated metal atoms on oxide supports also improved the active site density and stability of nanoparticles. In a recent report, the micelle-assisted electrodeposition approach was used for the direct fabrication of porous Au on Ni foam [85]. This catalyst exhibited a FE of 13 % with an improved rate of ammonia production (9.42 μg h−1 cm−2). Enhanced performance could be attributed to the porous nature of the catalyst that increased the interaction of reactants by nanoconfinement. Moreover, exposure to more active sites resulted in improved Au intrinsic activity.Au nanoparticles dispersed on bismuth telluride (Bi2Te3) nanosheets to form two-dimensional (2D) heterojunction. Au–Bi2Te3 nanosheets have shown activity for the eNRR [79]. This could be attributed to the enhanced active site density due to the good dispersion of Au nanoparticles and the synergistic effect of the heterojunction composite. An ammonia yield rate of 32 μg h−1 mg−1 with a FE of 20 % in a 0.1 mol L−1 Na2SO4 electrolyte was obtained. The proposed method for building high-performance heterojunction electrocatalysts for electrochemical ammonia production is a viable option for rational catalyst design. Similarly, Wang et al. studied the effect of hydrogenation on Au/TiO2 composite for the eNRR [86]. A hydrogen plasma was used to create defects and vacancies in the composite. The treated sample (H–Au/TiO2) demonstrated a FE of 2.7 % at –0.1 V vs. RHE that could be attributed to disordered patches on the surface of TiO2 nanoparticles that emerged after hydrogenation treatment, and a substantial number of oxygen vacancies were developed into TiO2 crystalline structures.Pd-based catalysts have been explored for nitrogen reduction due to hydride formation at certain voltages, which promotes different hydrogenation reactions at the Pd surface. Nano-sized Pd decorated on carbon was synthesized by the polyol reduction method [87]. In nitrogen-saturated phosphate buffer solution (PBS), a yield of 4.5 μg h−1 mg−1 with a high FE of 8 % was obtained at –0.1 V vs. RHE with lower overpotential. This is due to the dispersion of active sites with localized charge density, and the Grotthuss-like hydride transfer mechanism that controls the rate-limiting step of this process. DFT calculations corroborated the proposed hydride mechanism on the Pd surface. PCET or direct hydrogenations are thermodynamically less favorable as compared to the in-situ formed palladium hydride (α-PdH) that allows the activation of a nitrogen molecule on the Pd surface. Similarly, strong interfacial interactions were developed between PdO–Pd that enhanced the nitrogen interaction and eNRR performance [88]. Laser-irradiated PdO/Pd heterojunctions on CNTs exhibited a high FE (11 %) with a high rate of ammonia production (18.2 μg h−1 mg−1) at 0.1 V vs. RHE. A synergistic effect of PdO and Pd decreased the proton transfer rate and reduced the overpotential.In a recent report by Chen et al. electrodeposited Pd/PdO electrocatalysts were developed to regulate oxygen levels in various gas atmospheres [89]. The inclusion of an oxygen atom into a pure Pd catalyst modulated the electron density of the Pd/PdO heterojunction that increased the adsorption energy for nitrogen and hydrogen, as corroborated by theoretical calculations. Experimental data revealed that a moderate oxygen content led to improved performance, with an ammonia yield of 11 μg h−1 mg−1 and a FE of 22 %. This was ascribed to the moderate adsorption of nitrogen on the Pd surface along with hydrogen suppression due to the formation of defects.Due to the high cost and scarcity of noble metals, non-precious metal-based catalysts must be investigated. Similar electronic properties of transition metals and their surface chemical bonds make non-precious metals a good choice for gas-phase reactions and as an alternative for noble metals. In the recent past, enormous research has been done on non-precious metal-based electrocatalysts for various applications [90–92].Based on theoretical studies, Fe is predicted to be a good catalyst candidate for eNRR at ambient conditions. Moreover, its interactions with nitrogen and other intermediates promise feasible pathways for eNRR. Different Fe-based nanocatalysts have been investigated for eNRR at ambient conditions. Zhou et al. achieved a FE of 60 % for ammonia on a nanostructured Fe-based electrocatalyst [93]. They employed phosphonium-based ionic liquids with high nitrogen solubility to improve its availability in the system. The selectivity can be improved by controlling the water content in the reaction media that provides hydrogen for the competing HER. At around −0.8 V vs. NHE, FE of more than 60 % and a rate of 4.7 × 10−12 mol s−1 cm−2 was achieved for Fe-FTO catalyst in [P6,6,6,14][eFAP] ionic liquid with low current densities. Fe–N3 sites were explored as active sites for eNRR at ambient conditions [37]. Zeolitic imidazole framework (ZIF) and carbon nanotubes (CNT) derived Fe–N/C-CNT hybrid exhibited a FE of 9.2 % with a 34 μg h−1 mg−1 rate of ammonia production at −0.2 V vs. NHE. As corroborated with DFT calculations, Fe–N3 sites were active centers for enhanced nitrogen activation and polarization.Due to its high electron-donating capability and empty 6d orbital, bismuth (Bi) has been explored as an active material for different gas-phase reactions where the activation and polarization of reactants are conducted by transfer of electron density back and forth [94]. Its semiconducting nature provides localized density states near the Fermi level that act as localized charge sites that help in the donation of p-electrons to nitrogen for activation. In this regard, different Bi-based catalysts have been explored for eNRR. 2D mosaic Bi nanosheets (BiNS) were developed for eNRR in neutral media [95]. The in-situ electrochemical reduction process resulted in the formation of mosaic BiNS that exhibited enhanced eNRR performance. A FE of 10 % with an ammonia rate of 13 μg h−1 mg−1 was achieved at −0.8 V vs. RHE. The 2D structure provided the enhanced exposure of active sites that synergistically improved the intrinsic catalytic property of Bi by effective p-electron delocalization in BiNS. In another report, a Bi nanosheet array was developed electrochemically and evaluated for eNRR at ambient conditions [94]. A FE of 10 % with an ammonia production rate of 6.89 × 10−11 mol s−1 cm−2 at −0.5 V vs. RHE in acidic media. DFT calculations revealed the enhanced activation of nitrogen on Bi and an associative alternating pathway was proposed for nitrogen reduction.Xue et al. employed a wet-chemical approach using sodium citrate as a stabilizing agent to grow ultrafine tin (Sn) nanoparticles on carbon black [96]. This catalyst exhibited a FE of 22 % with an ammonia yield rate of 17 μg h−1 mg−1 in the 0.1 mol L−1 Na2SO4 electrolyte which was attributed to the small size of the particle improved the active site density and intrinsic catalytic activity of the material. The role of Sn as a sacrificial species for the hydrogen evolution reaction on the surface of phosphorene was explored by Liu et al. [97]. This improved the overall conservation and separation of nitrogen adsorption sites on the surface of phosphorene which resulted in the improved selectivity toward ammonia formation. At a low overpotential, Sn-phosphorene obtained a FE of 36 % and a notable ammonia production rate of 26.9 μg h−1 mg−1. After doping with Sn, DFT calculations revealed that different adsorption sites for water and nitrogen were available, with water adsorbing preferentially onto the Sn sites. Similarly, antimony (Sb) based catalysts were also evaluated for nitrogen reduction and found to be active at ambient conditions. The coupling of Sb and metallocene (Nb2CT x ) developed a localized electron-rich interface as confirmed by the DFT calculations [98]. This resulted in the modulation of the Sb band structure which improved nitrogen activation and hydrogenation. An ammonia production rate of 49.8 μg h−1 mg−1 was achieved with a FE of 27 % at ambient conditions.Owing to the better intrinsic catalytic activity of different metals as suggested by theoretical approaches, the derivatization of these metals opens a new window for enhanced properties. For instance, Mo is an active entity in the HER process but to improve its intrinsic catalytic properties variety of derivatives have been developed including phosphides, carbides, and sulfides [99]. Modification of the electronic properties of the parent metal resulted in better interaction with reactant molecules (H2O, N2, O2, CO2) due to the effective polarization of electronic density by the incoming group. In this regard, different transition metal derivatives have been explored in the recent past and can be categorized into i) transition metal oxides, ii) transition metal nitrides, iii) transition metal carbides, and iv) transition metal sulfides. Representative candidates of these types are illustrated in the sections below.Due to lesser conductivity than the parent metals, transition metal oxides tend to have lower performance in some electrochemical processes. Theoretical calculations revealed that early transition metals have a high tendency to interact with nitrogen as compared to late transition metals, resulting in the ease of nitrogen activation and polarization. To investigate the catalytic ability of metal oxides for eNRR at ambient conditions, Skulason and co-workers conducted DFT calculations of rutile-type transition metal oxides [66]. A stability diagram for each transition of metal oxide was developed by adsorption of different species on its (110) facet, and then the adsorption energies were evaluated as a function of applied potential. Thermodynamic aspects were also elucidated by free energy diagrams of the eNRR process. Based on the binding energies of N2H species on different transition metal oxides, a volcano plot was developed reflecting the catalytic activity and selectivity of different surfaces for eNRR. The most promising transition metal oxides for nitrogen reduction are NbO2, ReO2, and TaO2 with low onset potentials. Iridium oxide (IrO2) was found to be the most active oxide for this process with an onset potential of −0.36 V vs. SHE, but it is prone to be poisoned by hydrogen atoms, giving a lower selectivity towards eNRR.Rational catalyst design leads to improved intrinsic catalytic properties of a catalyst [100]. Fe/Fe-oxide catalysts were prepared by the oxidation of Fe foil at high temperatures and then electrochemically reduced to different interfaces containing Fe–Fe3O4 and Fe–Fe2O3 hybrids. Due to the less conductive nature of Fe-oxides, suppression of HER takes place with enhancement in the intrinsic catalytic property of Fe towards eNRR at ambient conditions. At −0.3 V vs. RHE, a FE of 8 % was obtained with an ammonia yield rate of 0.20 μg h−1 cm−2. Multishelled hollow chromium oxide (Cr2O3) microspheres were synthesized by the hydrothermal approach and evaluated as an efficient eNRR electrocatalyst [101]. A FE of 6 % in 0.1 mol L−1 Na2SO4 was obtained with a total ammonia production yield of 23.5 μg h−1 mg−1 cat at −0.9 V vs. RHE. This performance was attributed to the rational design of this non-noble metal catalyst with a hollow texture. This hollow structure facilitated the diffusion of nitrogen and desorption of products that enhance the mass transport with the improved kinetics of the reaction.Niobium oxide (Nb2O5) nanofibers showed excellent performance towards eNRR [102]. In 0.1 mol L−1 hydrochloric acid (HCl), an average yield of 43.6 μg h−1 mg−1 with a FE of 9 % at −0.55 V vs. RHE was achieved. Intrinsic catalytic properties of Nb due to the enhanced interaction with nitrogen in an exergonic step are the source of the improved performance. Also, DFT calculations predicted that Nb edge atoms were involved in the activation and polarization of the dinitrogen molecule, and charge transfer takes place between the surface Nb atom and dinitrogen molecule that was compensated by the neighboring Nb atom by back donation, hence, weakening the N≡N bond. Interestingly, NbO2 exhibited better nitrogen reduction performance than Nb2O5 [103]. At −0.6 V vs. RHE in acid media, a FE of 32 % was achieved, and at −0.65 V vs. RHE, a high rate of ammonia production (11.6 μg h−1 mg−1) was obtained that could be assigned to the different linkage of Nb atoms in the crystal structure. In NbO2, there is an availability of empty d-orbitals that caused the enhanced interaction of nitrogen molecules on the surface by receiving the electronic density from nitrogen. Moreover, a single d-electron participated in the activation of N≡N by back donation resulting in the high performance of the material towards nitrogen reduction.Similarly, layered 2D perovskites have anomalous electronic properties that make them an interesting choice for different electrochemical applications. Atomically disassembled nanosheets of La2Ti2O7 were developed by the hydrothermal method and their electrochemical nitrogen reduction performances were investigated [104]. A high ammonia yield of 25 μg h−1 mg−1 with a FE of 4.5 % was achieved at −0.55 V vs. RHE in the acid electrolyte with good stability. It could be attributed to the 2D layered structure of La2Ti2O7 that enhanced the interaction of reactants with catalysts by exposing more active sites. In a recent report, doped-LaFeO3 was developed by the sol-gel method and evaluated for nitrogen reduction [105]. The ammonia formation rate of 13 μg h−1 mg−1 was achieved at a cell voltage of 2.4 V, while, a FE of 1.9 % was obtained at 1.8 V. Enhanced eNRR performance could be attributed to the Cs and Ni doping in the LaFeO3 structure along with oxygen vacancies.Modified electronic structures of metal nitrides along with N-vacancy tailored structures provide enhanced interaction of nitrogen with metal nitride surfaces. Extensive theoretical studies have been carried out on transition metal nitrides to evaluate the eNRR. Abghoui et al. extended this approach by investigating the reaction mechanism and the dissociation barrier of nitrogen on transition metal nitrides [44,106]. Four promising candidates were identified including vanadium nitride, zirconium nitride, chromium nitride, and niobium nitride (VN, ZrN, CrN, NbN). These metal nitrides were proposed to follow the Mars-van Krevelen mechanism in which an ammonia molecule is formed by the reduction of surface nitrogen of a nitride creating a N-vacancy that was filled by dissolved nitrogen from the electrolyte. They proposed that low-index facets of these transition metal nitrides were stable towards poisoning, decomposition, and suppression of activity by adsorbed oxygen and hydrogen molecules and were expected to produce ammonia with high current densities. Computation by the hydrogen electron method determined that the Mars-van Krevelen mechanism (MvK) was most plausible for eNRR on transition metal nitrides (Fig. 10 a and b) [107]. Cobalt molybdenum nitride (Co3Mo3N) is reported to be the most active catalyst for nitrogen reduction at 400 °C [108]. Various theoretical studies were conducted on the understanding of its mechanism at the atomic level by Yazdi et al. [109]. Dispersion-corrected DFT calculations were performed over Co3Mo3N surfaces with defects to elucidate the mechanism of ammonia formation at different temperatures and the MvK mechanism was confirmed.The activity of VN nanoparticles towards nitrogen reduction was studied in a membrane electrode assembly in a fuel cell arrangement with hydrogen fed at the anode as the source of protons [110]. At −0.1 V vs. RHE, a FE of 6 % with an ammonia production rate of 3.3 × 10−10 mol s−1 cm−2 was achieved and 15N isotope labeling experiments confirmed the MvK mechanism. X-ray photoelectron spectroscopy (XPS) analysis of fresh and spent catalysts observed the phase transformation from VN to VN1-x O x and confirmed the active site for this process. Structural N-atoms with adjacent O-atom were susceptible to hydrogenation which leads to ammonia formation. In this regard, in-situ and operando characterizations are crucial for the explanation of structure performance relationships. In a recent report, titanium oxynitride (TiON) was evaluated as an efficient catalyst for eNRR [111]. Commercial titanium nitride (TiN) was etched by a plasma-enhanced approach and a TiON phase formed that enhanced the overall performance. This catalyst exhibited a FE of 9 % with an ammonia production rate of 3.32 × 10−10 mol s−1 cm−2 at −0.6 V vs. RHE in neutral media.Despite various theoretical studies that predict transition metal nitrides as active nitrogen reduction catalysts, experimental evidence is lacking in this regard. Recently, MacFarlane and coworkers evaluated Nb4N5 and VN for nitrogen reduction at ambient conditions and found interesting results [43]. According to their deductions after various experiments, VN and Nb4N5 were inactive for eNRR but they were producing ammonia by reductive decomposition of lattice nitrogen. Nb and V hydroxide precursors were developed by hydrothermal methods and their further nitridation was done at high temperature in the presence of an ammonia environment. After evaluating nitrogen reduction in different control experiments (dinitrogen, Ar, open circuit), no great difference was observed in ammonia production. However, soaking the electrode in acid resulted in the production of a great amount of ammonia which was attributed to the non-catalytic reductive decomposition of lattice nitrogen. Different control experiments were also conducted to elucidate the exact origin of ammonia on these transition metal nitrides. It was confirmed that the produced ammonia was from the decomposition of lattice nitrogen by the formation of irrecoverable nitrogen vacancies. In another recent report, similar results were obtained for Mo2N at ambient conditions [113]. Different control experiments highlighted the decomposition of Mo2N under these reductive conditions and proved to be inactive for eNRR at ambient conditions. These results illustrated that careful insight is required towards ammonia production by N-containing materials.Like 2D nitrides, 2D transition metal carbides such as M3C2 also have the potential to capture and reduce nitrogen to ammonia. DFT calculations were conducted by Azofra et al. to explore metal carbides for nitrogen reduction [112]. The efficient stabilization of nitrogen molecules on metal carbides leads to its activation by elongation and weakening of its bonds, resulting in the high performance of ammonia formation. Moreover, the first PCET reaction was the rate-determining step for nitrogen reduction that required low activation barrier, for instance, vanadium carbide (V3C2) required 0.64 eV. Metal carbides of d2, d3, and d4 are proposed to be more active for the stabilization of nitrogen as indicated by the spontaneous chemisorption energies (Fig. 10c). In a recent report, molybdenum carbide (Mo2C) nanodots were developed by molten salt synthesis and evaluated for eNRR [114]. At −0.3 V vs. RHE, and ammonia yield of 11 μg h−1 mg−1 was achieved with a FE of 7.8 % on a hydrophilic substrate. Similarly, Mo2C nanorods also exhibited eNRR activity at ambient conditions [115]. A Mo-based precursor (Mo3O10(C6H8N)2·2H2O) was pyrolyzed at high temperatures to form Mo2C nanorods. A FE of 8 % with an ammonia yield rate of 95.1 μg h−1 mg−1 was obtained at −0.3 V vs. RHE in acidic media.2D transition-metal carbide provide the metallic properties of carbides along with the hydrophilic nature of the termination (OH, O) that make them a suitable class of compounds for versatile applications [116,117]. 2D Ti3C2T x (T = functional group termination like F, OH, x = 1–3) nanosheets were developed by the delamination technique and evaluated for nitrogen reduction [118]. At −0.4 V vs. RHE in acidic media, a FE of 9 % with an ammonia yield of 20 μg h−1 mg−1 was obtained which is attributed to the improved selectivity for nitrogen reduction. Enhanced chemisorption capabilities of nitrogen on Ti3C2Tx were confirmed by DFT calculations that resulted in the activation and polarization of the N≡N bond. High specific surface area and abundant active sites were the key improvements by nanostructuring. Similarly, a few-layered MoSe2 on Ti3C2Tx MXene (MoSe2/Ti3C2Tx) was fabricated via a hydrothermal synthesis and thermal annealing as an active electrochemical nitrogen reduction catalyst [119]. DFT studies elucidated Mo atoms as active centers for nitrogen reduction with the distal pathway for ammonia production. MXene provided the platform for the exposure of active sites by improved dispersion and conductivity. The developed heterostructure exhibited an ammonia yield rate of 56 μg h−1 mg−1 at −0.50 V vs. RHE and a FE of 14 % −0.35 V vs. RHE.Like other derivatives, TMS also have been investigated for nitrogen reduction at ambient conditions. Fe-atom decorated 2D MoS2 sheets for ammonia formation at ambient conditions were investigated by theoretical studies [10]. Like the FeMo cofactor in nitrogenase, the Fe–Mo linkage is supposed to have high selectivity for nitrogen, which facilitates the nitrogen reduction endergonic steps. DFT calculations demonstrated that the stabilization of the nitrogen molecule on a Fe site could be ascribed to the exchange of mutual electronic density between Fe and N atoms activating the N≡N bond. A low activation barrier of 1.02 eV was calculated for the first PCET reaction which was the most endergonic step on the Fe–MoS2 surface. Also, the lower binding energy between ammonia and this surface resulted in the ease of product desorption that regenerated the catalytic active sites.Recently, MoS2 was synthesized by the hydrothermal method on carbon cloth as an electrode for nitrogen reduction [1]. DFT calculations confirmed that positively charged Mo edges were the main catalytic sites for eNRR as they stabilized nitrogen by receiving electronic density. Also, the first hydrogenation step was the rate-determining step that requires 0.68 eV free energy in the absence of applied potential. A rate of 8.8 × 10−11 mol s−1 cm−2 for ammonia production was achieved with a FE of 1.1 % in 0.5 mol L−1 Na2SO4 at −0.5 V vs. RHE. Wu et al. developed a unique sub-monolayer MoS2−x structure that selectively adsorbed, activated, and dynamically hydrogenated nitrogen [120]. This selective interaction improved the selectivity of the catalyst by controlling the scaling relation. Experimental and theoretical results demonstrated that the developed surfaces modulated the surface binding of nitrogen intermediates which resulted in improved ammonia formation. By the rational catalyst design, a FE of 24.7 % with an ammonia formation rate of 17 μg h−1 mg−1 was achieved with enzymatic side-on nitrogen adsorption.Nitrogen-doped carbons are viewed as an efficient class of material due to their effective role in various catalytic reactions. Due to the redistribution of electronic density in the structure after N-doping, improvement in the number and type of active sites for enhanced interaction of reactant nitrogen molecules is observed [121]. Specifically, a higher content of pyridinic and pyrrolic N-sites contributes to the performance. A wide interest has been developed in the derivatization of carbon from different porous materials like metal-organic frameworks (MOFs) and Zeolitic imidazole frameworks (ZIFs) as they inherit the parent porosity of material to the formed carbon. These porous structures improve mass transport and increase the charge density of the material. Also, high conductivity and large potential window favor their applicability in nitrogen reduction.Porous nitrogen-doped carbon was synthesized by the carbonization of ZIF-8 and evaluated for nitrogen reduction at ambient conditions [122]. Ammonia production rate of 1.4 mmol g−1 h−1 was achieved at −0.9 V vs. RHE in an aqueous electrolyte. DFT calculations proposed an associative alternative pathway for ammonia formation by porous N-doped carbon where successive hydrogenations take place on alternate N atoms. Similarly, nanoporous nitrogen-doped carbon derived from ZIF-8 was evaluated for nitrogen reduction [123]. An improved ammonia production rate of 3.4 × 10−6 mol s−1 cm−2 was obtained with a FE of 10 % at −0.3 V vs. RHE. Pyridinic-N sites adjacent to C-vacancy were more prone to interact with nitrogen molecules which lead to the dissociation of N≡N bond with the subsequent hydrogenations, as revealed by DFT.Biomass-derived porous carbons are gaining much interest due to the large abundance of different types of natural sources, their non-toxic nature, and their low cost. Also, the presence of other heteroatoms (N, B) improved their intrinsic catalytic properties. Cicada sloughs were used to develop porous NDC by a high-temperature annealing process [124]. These developed carbons were assigned as an effective and stable nitrogen reduction catalyst with an ammonia yield of 15.7 μg h−1 mg−1 and a FE of 1.4 % at −0.2 V vs. RHE. This could be attributed to the synergistic effect of doping and the high surface area of developed carbons (1547 m2 g−1) that increased the number of active sites with improved mass transport. Similarly, using tannin as a precursor, biomass-derived O-doped carbon nanosheets were developed and their nitrogen reduction performance was investigated [125]. At −0.6 V vs. RHE, a FE of 4.9 % was obtained with an ammonia yield rate of 20 μg h−1 mg−1 with high electrochemical stability.Different types of active sites for enhanced interaction with nitrogen were achieved by boron doping. B-doped 2D graphene was synthesized by high-temperature treatment in the presence of a B-precursor and evaluated for eNRR at ambient conditions [126]. The redistribution of electronic density in the graphene structure resulted in the formation of Lewis acid sites that could interact well with the nitrogen (Lewis base). Moreover, an empty orbital in B readily accepted the electronic density of nitrogen resulting in the activation and polarization of the nitrogen molecule. At −0.5 V vs. RHE, a FE of 10.8 % with an ammonia yield of 9.8 μg h−1 cm−2 was observed with 6.2 % B-doping in the graphene structure.Song et al. developed N-doped carbon nanospikes for eNRR in aqueous media as a physical catalyst [127]. At −1.19 V vs. RHE, a FE of more than 11 % with a 97 μg h−1 cm−2 rate of ammonia production was obtained. Due to the absence of any metal, performance could be attributed to the confined electric charge developed at the surface of nanospikes as proven by the comparison of performance with O-etched blunt carbon tips that produced very low ammonia. Also, HER was suppressed by the formation of a dehydrated cationic layer near the surface of tips that allowed only nitrogen to pass on and excluded water.The catalytic activity of any material depends on its chemical nature and the composition of the catalyst which dictate the electronic properties that in return validate it as a good or a bad catalyst. The systematic investigation of intrinsic catalytic activity can provide new insight into a high-performance catalyst design. The rate of a chemical reaction depends on the exposed active sites as the collisions and interactions of reactant molecules with these active sites result in the formation of products. Towards the improvement in reaction kinetics, an increase in active site density is a prerequisite for enhanced catalytic activity. It has been confirmed that multifold enhancement in the performance of material with fast kinetics has been achieved by tuning the number of active sites. Herein, we describe recent strategies for rational catalyst design for eNRR by theoretical and experimental approaches. We shed light on (i) nanostructuring of materials, (ii) single atom formation, (iii) electronic structure modifications, and (iv) surface modifications.The reduction of particle size from bulk to the nano level modifies the fundamental properties of the material including physical, electronic, and chemical properties. Confinement of particles in nano dimensions introduces different energy levels that show the inclusive alteration in the electronic structure of the material. Due to greater exposure to a total number of atoms in nano dimensions, materials behave differently based on the size and number of particles due to the increased availability of active centers. Further, certain areas with localized charge density can act as sites for enhanced interaction with the reactant moiety [128]. Due to the highly conductive nature and the availability of free (d orbital) electrons, nanosized noble metals have excellent catalytic activity towards a variety of reactions related to energy applications. Small molecules (N2, H2, O2, CO2, etc.) can be easily stabilized by the transfer of electronic density from d-orbitals of metals to the vacant orbitals (antibonding) of these small molecules.Nanosized non-noble transition metals have also been employed as catalysts for different applications as they can have similar electronic properties as noble metals [61,62,129]. Improvement in the active site density and mass transport properties by nanostructuring during nitrogen reduction has led to enhancement in FE towards ammonia production with high yield. Within the nanoscale, further optimization of the shape and morphology of a catalyst along with the exposure of specific facets inclined toward enhanced catalytic activity. Herein, we highlight these strategies from recent literature for enhanced eNRR performance.The peculiar properties of metal and non-metal catalysts are observed in different shapes and sizes, and it is a fundamental goal of material science to tailor the shape of particles [130–133]. Anisotropic metal nanoparticles (NPs) are important in the fabrication of different smart devices including a wide range from electronics to biological applications. Different shapes have various surfaces that interact differently with the reactant molecules as the specific electronic environment is available for a specific shape.Nazemi et al. developed and evaluated Au hollow nanocages (AuHNCs) for the eNRR (Fig. 11 a and b) [134]. In 0.5 mol L−1 lithium chlorate (LiClO4) solution, at −0.4 V vs. RHE, a FE of 30 % was obtained and the highest yield of ammonia production (3.9 μg h−1 cm−2) was achieved at −0.5 V vs. RHE. This high performance was attributed to the hollow structure that provided high surface area and the confinement effect resulted in improvement in mass transport and active site density. Further to this approach, the same group elucidated the effect of pore size and density on the walls of AuHNCs on eNRR [135]. In a recent report by the same group, the role of oxidation of silver (Ag) in Au–Ag HNCs was elucidated [136]. Resultant Ag2O–Au nanocages exhibited a FE of 23.4 % at −0.4 V with an ammonia production rate of 2.14 μg h−1 cm−2. This study emphasized the need for an O2-free environment in electrochemical nitrogen reduction for stable ammonia formation. Moreover, the role of Ag in bimetallic Au–Ag nanocages for the improved selectivity and activity towards nitrogen reduction was confirmed to be vital.Cobalt phosphide (CoP) hollow nanocages (HNCs) were developed by the self-assembly of MOF-derived ultrathin CoP nanosheets (Fig. 11c and d) [137]. The shape-controlled growth of these nanoarchitectures shows a synergistic effect for nitrogen reduction with coordinatively unsaturated catalytic sites of phosphide. A FE of 7.3 % was achieved at 0 V vs. RHE with an ammonia yield of 2 μg h−1 mg−1 in 1 mol L−1 potassium hydroxide (KOH). However, an exponential increase in ammonia rate was observed until −0.4 V vs. RHE, and a rate of ammonia formation of 10.78 μg h−1 mg−1 was obtained. It was ascribed to the inhomogeneous surfaces with charge-separated sites on the CoP catalyst due to hollow nanocages.Tuning the morphology of material with hierarchical structures at the nanoscale also improved the active site density and confinement effect. Also, enhancement in the mass transport of material was achieved with an improved rate of reaction. Recently, hierarchical Au flower-like microstructures were synthesized by a soft templating method using gum Arabic as a capping agent and evaluated for nitrogen reduction [138]. As compared to spherical Au particles, a high ammonia yield (25 μg h−1 mg−1) with a FE of around 6 % was achieved at −0.2 V vs. RHE which was ascribed to the enhanced exposure of active sites in a hierarchical structure. Similarly, atomically distributed nanosheets nano-assemblies of Rh metal were synthesized by cyanogel-assisted method and evaluated for nitrogen reduction [139]. High specific surface area and modified electronic structure by 2D-nanosheet structure were the attributes of improved performance. In alkaline media, an ammonia production rate of 23 μg h−1 mg−1 and improved selectivity were obtained at low potential (−0.2 V vs. RHE). Different shapes and morphologies improved the exposure of active sites in a catalytic material. In aqueous nitrogen reduction, challenges related to the low solubility of the nitrogen could be overcome by the confinement of reactants in hollow and porous structures. This results in the shortened mass and electron pathway with the enhancement in the rate of a catalytic process.The development of nanocrystals with specific facets provides coordinatively saturated and unsaturated active sites and surfaces that can be tailored according to the framework provided by the computational approach. The lattice mismatch at different scales by facet engineering develops strain in the structure of the material. This strain can propagate in the structure and gradually fades away from the interface. This resulted in the improved energetics of the catalytic reaction at the interface. High index and low index facets of material interact differently with a reactant. Low index facets are energetically more stable as compared to high index facets but show less intrinsic catalytic activity [140,141]. So, a necessary insight by theoretically assisted experimental studies towards facet control synthesis of a variety of materials for nitrogen reduction is outlined here.Theoretically, nitrogen reduction on the molybdenum nitride (γ-Mo2N) surface with different facets was evaluated by Metanovic et al. [142]. Free energy profiles confirmed the eNRR process proceeds by both (associative and dissociative) pathways, with a series of reactivity decreasing in the order of (111) > (101) > (100) ∼ (001) with a range of overpotential between −0.7 V to −1.4 V. Due to the high interaction of (111) surface with the nitrogen as compared to H-adatoms and side-on adsorption of nitrogen molecule resulted in the higher activity of this facet.Facet control synthesis of Mo-nanofilm was achieved by combined electrochemical anodization and reduction process and was further evaluated for ammonia formation [143]. At an overpotential of −0.14 V vs. RHE, a FE of 0.72 % was obtained with the 3.09 × 10−11 mol s−1 cm−2 rate of ammonia production. Similarly, tetrahexahedral Au (THH) nanorods with a high index facet (730) were prepared and had a 1.6 μg h−1 cm−2 rate of ammonia production at −0.2 V vs. RHE [144]. Moreover, a FE of around 4 % was obtained for ammonia along with the production of hydrazine. Briefly, the interaction of nitrogen with the highly coordinatively unsaturated surfaces (730) on terraces was the first step of this mechanism. Later, the nitrogen bond dissociated and the N atom was chemisorbed. Successive PCET reactions lead to the complete reduction process. The improved eNRR performance on the high index facet is attributed to the facet engineering that provided the coordinatively unsaturated active sites.Single atoms due to a small size (below 1 nm) and localized electronic density behave in a different pattern (like the molecular catalyst) as compared to agglomerated nanostructures for a catalytic process. Enhanced atomic-level interactions of the catalyst with reaction intermediates and localized electronic densities of atomically dispersed transition metals result in effective activation and polarization of small molecules with an improvement in the selectivity of the reaction.DFT calculations suggested the atomically distributed Mo on boron nitride (BN) monolayer to be an efficient catalyst for eNRR [145]. Exploration of different other transition metals (Sc to Zn, Ru, Rh, Pd, and Ag) as a single atom on a BN monolayer as a nitrogen reduction catalyst had also been done. Single Mo atoms supported by a BN monolayer are found to be highly active for nitrogen reduction with an overpotential of 0.19 V vs. RHE, which is much lower than other investigated Mo-based catalysts. This high activity could be attributed to the high spin-polarization, selective stabilization of N2H, or destabilization of NH2 intermediate on this surface.Au single atomic sites were developed on N-doped porous carbon (NDPC) by a template-assisted, impregnation method followed by reductive annealing at high temperatures [146]. The NDPC exhibited improvement in overall mass transport and metal catalyst stabilization. Electronic polarization by Lewis acidic and basic sites resulted in the activation of nitrogen and improved efficiency. At −0.2 V vs. RHE, a FE of 12.3 % was achieved with a yield rate of 2.32 μg h−1 cm−2 for ammonia production. Single-atom Ru sites on NDPC were also developed by thermal treatment of MOF-based precursors [147]. eNRR performance in aqueous media at −0.1 V vs. RHE revealed a FE of 21 % with an ammonia yield rate per mg of Ru of 3.66 mg h−1. Importantly, the addition of zirconium oxide (ZrO2) suppressed the hydrogen evolution reaction. Being an early transition metal, it exhibited high interaction with nitrogen which improved the overall ammonia production.Similarly, isolated single-atom Fe anchored on N-doped carbon exhibited nitrogen reduction activity [148]. DFT calculations confirmed the stabilization of Fe by N in the Fe–N4 configuration which is considered an active site for nitrogen activation. At −0.4 V vs. RHE, this catalyst achieved a FE of 18 % with an ammonia yield rate of 62.9 μg h−1 mg−1. In another report, a positive shift in the onset potential of ammonia was achieved on single atom Fe anchored on N–C derived from pyrrole [149]. At 0.193 V vs. RHE over 56.55 % FE was achieved which could be attributed to the enhancement in active sites and the synergistic effect of N–C. Molecular dynamic simulations revealed the enhanced access to nitrogen on the single-atom surfaces that resulted in the suppression of HER. Similarly, a cost-effective Mo-based single-atom catalyst was developed and evaluated for eNRR [150]. Single-atom Mo anchored on nanoporous carbon (NPC) achieved a FE of 14.6 % with a high rate of ammonia yield of 34 μg h−1 mg−1. Moreover, this catalyst exhibited long-term durability and electrochemical stability.The fundamental shift in energy level and electron structure is the effect of reducing the size of nanomaterials to cluster- or atom-level. Theoretical and experimental results show that single atoms or smaller clusters have superior catalytic activity or selectivity than larger clusters. This catalytic design is facing a few challenges that need more attention for rational catalyst design. For instance, the role of support in the confinement of single atoms and its interaction with the metal moiety to control the reaction progress need to be elucidated. It is highly desirable to develop a multi-functional single-atom catalyst as most electrocatalytic reactions are multi-step processes and are affected by the scaling relation. Different adsorption sites in a catalytic structure could provide a tandem-like structure that results in a domino effect. Importantly, single-atom catalysts are less stable and hard to produce at a large scale which dictates the need for the development of high-throughput methods for their development [151].For rational catalyst design, improvement in the intrinsic catalytic activity of the material is a fundamental requirement. The electronic structure of a catalyst dictates its intrinsic catalytic activity. Therefore, the electronic properties of the material could be modified after considering of reaction mechanisms suggested by the theoretical framework. Thermodynamic and kinetic reaction parameters can be well optimized on different types of catalytic surfaces. Enhanced interaction of reactants and products with the surface of the catalyst is vital for the spontaneity of a reaction. In this regard, different types of active sites on the surface of the catalyst will improve the overall interaction of reactants and products with the catalyst surface. Electronic properties could be improved by i) doping, ii) alloying and ii) core-shell structures. Herein, the exploitation of these strategies for nitrogen reduction is highlighted.The deliberate introduction of heteroatoms into a parent material to vary its electrical and structural properties is known as doping. It modifies the charge density of the parent material by the inclusive effect of the electronic density of the dopant and the creation of defects. Metal and non-metal doping of material produce localized charge sites that help in the enhanced interaction of catalysts with the reactant molecule. For instance, doping of carbon or carbon-based material with the more electronegative element (N) distributed the electronic density of carbon away from it resulting in the creation of positively charged carbon sites [152]. These positively charged sites are available to receive an electronic density of reactant molecules (N2, O2) that causes their stabilization.Aluminium-doped graphene was explored as a catalyst for eNRR by DFT calculations [153]. Aluminium provided adsorption and binding sites for the reactants and graphene facilitated efficient electron transport. The proposed mechanism described the hydrogenation of adsorbed nitrogen moiety on the aluminium site like the internal hydrogen transfer in homogeneous catalysis. Interestingly, B-doped TiO2 exhibited eNRR performance in neutral media [154]. In neutral electrolyte, an ammonia formation rate of 14.4 μg h−1 mg−1 was obtained with a FE of 3.4 % at −0.8 V vs. RHE. This performance could be attributed to the positive charge developed on the TiO2 due to the electron-deficient nature of B, which resulted in the enhanced interaction with nitrogen. Similarly, fluorine-doped iron double hydroxides (β-FeOOH) nanorods displayed an improved nitrogen reduction performance [155]. At −0.6 V vs. RHE, a FE of 9 % with an ammonia yield rate of 42 μg h−1 mg−1 was achieved. DFT calculations revealed that substitution of the hydroxide (OH) group with fluorine (F) helped in the lowering of adsorption energy that improved the kinetics of the reaction.Various efforts have been employed for the alteration of carbon structure with heteroatom doping. Sulfur-doped carbon nanospheres were developed by hydrothermal method followed by annealing in Ar using glucose as a carbon source [156]. At −0.7 V vs. RHE, a FE of 7 % with an ammonia rate of 19 μg h−1 mg−1 was achieved in 0.1 mol L−1 Na2SO4. A similar effect has been explained by O-doped and biomass-derived N-doped porous carbons towards nitrogen reduction [157,158].To exploit the improved electronic environment produced by the co-doping strategy, B–N-rich defective carbon nanosheets were synthesized by thermal treatment of graphene oxide and boric acid in an ammonia environment (Fig. 12 a) [159]. DFT calculations elucidated the B–N pair as a trigger and the adjacent C atom as an active site for the eNRR process. Due to the presence of extended active sites, enhanced nitrogen reduction performance was achieved with a FE of 13 % and an ammonia production rate of 7.7 μg h−1 mg−1 at −0.3 V vs. RHE. Accurate quantification of ammonia was obtained by rigorous control experiments (Fig. 12b). In another report, N–P co-doped carbon was developed by thermal treatment of polyaniline (PANi) in phytic acid [160].Hierarchical nanocarbon foams were obtained and evaluated for nitrogen reduction in acidic electrolyte. A FE of 4.2 % with an ammonia yield rate of 0.97 μg h−1 mg−1 was achieved at −0.2 V vs. RHE. In-situ FTIR confirmed the associative alternating pathway for ammonia formation on this surface, as hydrazine formed during this process. This performance was attributed to the synergy developed by the co-doping.Modified electronic properties of different metals could be achieved by alloy formation. Theoretical studies persuaded the utilization of more than one metal as the active site due to the difference in the interaction capability of reactant molecules with different metals. Importantly, ligand and strain effect due to the formation of hetero-atom bond and altered bond length improved the electrocatalytic properties of catalysts. Moreover, alloying an expensive metal with a non-precious metal could be cost-effective.The effect of alloy formation on eNRR was studied by Manjunatha et al. by employing RuPt alloys as nitrogen reduction catalysts at ambient conditions [161]. The synergistic effect of two noble metal catalysts resulted in the improved interaction with the nitrogen molecule by subsequent electron addition to antibonding orbitals of nitrogen. An improved activity with an ammonia production rate of 5.1 × 10−9 g s−1 cm−2 and a FE of 13 % at −0.123 V vs. RHE was achieved. Similarly, a PdCu/RGO system was developed by the facile reduction method for eNRR [162]. The focus was on the improvement of intrinsic catalytic properties of non-noble transition metal by alloying with a small quantity of noble metal. The improved efficiency towards ammonia formation was achieved by Pd0·2Cu0.8/RGO system at −0.2 V vs. RHE with a FE of less than 1 % and an ammonia yield rate of 2.80 μg h−1 mg−1 and attributed to the enhanced intrinsic catalytic property by alloying. Bimodal palladium-copper (PdCu) alloys were also developed recently for eNRR evaluation (Fig. 12c and d) [163]. Due to the interconnected porous structure and appropriate Pd/Cu ratio, an ammonia yield rate of 40 μg h−1 mg−1 was achieved. Palladium-ruthenium (PdRu) with tripod structure was investigated for eNRR and a high yield of ammonia was achieved in 0.1 mol L−1 KOH electrolyte [164]. The tripod structure along with bimetallic composition improved the overall ammonia production. Exposure of active sites in specific crystal orientations enhanced the intrinsic property of a catalyst.High-entropy alloys (HEAs) are a new class of multicomponent alloys that have found widespread application as electrocatalysts [59,129,165,166]. HEA design has already overcome the limitations of primitive alloy materials. Because of their variable element compositions, HEAs open a plethora of possibilities for the design of electrocatalysts (particularly multifunctional electrocatalysts). Zhang et al. developed RuFeCoNiCu HEA nanoparticles as an electrocatalyst for eNRR [167]. The developed HEA nanoparticles demonstrated an ammonia yield of 57.1 μg h−1 mg−1 with a FE of 38.5 %. Furthermore, these electrocatalysts demonstrated excellent electrochemical activity in other electrolytes as well as excellent stability for 100 h. This improved performance is attributed to a multi-site cooperative catalytic mechanism on the surface of HEA alloys.Creating defects and different vacancies results in the formation of coordinatively unsaturated sites in the structure of the catalyst that is more active due to localized charge density and free electrons [168]. Free electrons available on these sites could be back donated to the reactant molecules to activate and polarize them. For instance, oxygen vacancies (Vo) created in metal oxides provide a localized charge site that stabilizes and activates small molecules by accepting electronic density. Herein, we shed light on the role of vacancies and defects on the surface of the catalyst for nitrogen reduction.Improvement in surface oxygen vacancies of ferric oxide (Fe2O3) was achieved by calcination in an inert environment [169]. These modified structures were further evaluated for nitrogen reduction and an improvement in the eNRR performance was observed as compared to the non-modified sample. A FE of 6 % was achieved at −0.9 V vs. Ag/AgCl with an average ammonia production rate of 0.46 μg h−1 cm−2. Oxygen vacancy-dependent nitrogen reduction performance was also evaluated by Lv et al. They synthesized ceria-assisted amorphous bismuth vanadate (Bi4V2O11) hybrid by electrospinning method (Fig. 13 a and b) [170]. Due to the proper alignment of electronic bands, interfacial electron transfer was more plausible in this structure which enhanced the electrochemical performance of the material. Intrinsic oxygen vacancy (Vo), due to reduced vanadium provided the localized electron density that was proposed to be donated back to the nitrogen antibonding molecular orbitals for enhanced activation and polarization. A high average yield of ammonia (23 μg h−1 mg−1) was achieved with FE >10 % at −0.2 V vs. RHE. This could be attributed to the enhanced interaction of nitrogen with the catalyst surface and special active sites on the V atom due to its low coordination and high spin polarization after the creation of oxygen vacancies. In a recent report, Vo-derived tantalum oxide (Ta2O5) nanorods were proven to be an efficient catalyst for nitrogen reduction with an ammonia yield rate of 15.9 μg h−1 mg−1 and a FE of 8.9 % [171]. As corroborated by the DFT studies, Vo supported the activation and polarization of reactant molecules by the transfer of electron density, hence improvement in the kinetics of the reaction obtained.Interestingly, atomic layers of Vo-derived MoO2 were developed by the CVD approach [172]. This Vo-modified catalyst exhibited a high selectivity and efficiency towards ammonia formation with a yield rate of 12.20 μg h−1 mg−1and a FE of 8.2 % at −0.15 V vs. RHE. Based on adsorption experiments and theoretical studies, enhanced interaction of nitrogen was observed. Tuning of Vo at the atomic level improves the performance by a complete utilization of active sites on the surface. Similarly, Vo-derived Cr-doped CeO2 was developed by a hydrothermal approach and evaluated for nitrogen reduction in 0.1 mol L−1 Na2SO4 [173]. At −0.7 V vs. RHE, an ammonia yield of 16 μg h−1 mg−1 with a FE of 4 % was obtained.In a recent report, Vo-derived TiO2 in situ grown on Ti3C2Tx was evaluated for the nitrogen reduction reaction [176]. Abundant surface defects and high conductivity with a large surface area were the main attributes of ammonia production. An ammonia yield of 32.17 μg h−1 mg−1 was achieved at −0.55 V vs. RHE with a FE of 16 % at −0.45 V vs. RHE. DFT calculations confirmed the role of Ti edge atoms and Vo on eNRR performance in an associative distal mechanism. Similarly, Vo-derived CeO2 nanorods also exhibited improved eNRR performance as compared to a bulk counterpart in neutral media [177]. An ammonia yield rate of 16.4 μg h−1 mg−1 was obtained at −0.5 V vs. RHE, while a FE of 3.7 % was achieved at −0.4 V vs. RHE.Oxygen vacancy enhances the localized charge density on the surface of the catalyst and contributes to the overall performance of the catalyst. However, the role of oxygen vacancy concentration and stability in the improved activity of the catalyst is lacking in the literature. It is recommended to evaluate the structural evolution of a material with oxygen vacancy during the process to understand more about its role.The defect engineering strategy produced localized charge density sites due to electronic density redistribution that facilitated the enhanced nitrogen confinement at N-vacancy sites. Defective polymeric carbon nitride (PCN) was developed and evaluated for nitrogen reduction (Fig. 13c and d) [174]. A FE of 11 % with an ammonia yield of 8 μg h−1 cm−2 at −0.2 V was achieved. High performance could be attributed to the strong activation of nitrogen on the N-vacancy site as corroborated well with DFT calculations. Like transition metal catalysts, back donation of charges found in this system caused the activation of N≡N and rendered nitrogen reduction. Similarly, defective fluorographene was developed by a hydrothermal approach and evaluated for eNRR [178]. At −0.7 V vs. RHE, a FE of 4.2 % with an ammonia yield rate of 9.3 μg h−1 mg−1 was achieved.Defect-rich MoS2 nanoflowers were also developed and evaluated for nitrogen reduction [179]. At −0.4 V vs. RHE, a FE of 8.3 % was achieved with a yield rate of 29.28 μg h−1 mg−1. As corroborated well with the DFT calculations, defects were the active sites for eNRR. Based on the Hydrogen model, due to the presence of defects, a bare Mo atom could activate a nitrogen molecule with an associative distal hydrogenation step that led to the production of ammonia. However, defects also showed strong interactions with ammonia making the desorption step more endothermic. In another theoretical-assisted experimental approach, a defective TiO2 catalyst was developed for eNRR [180]. It exhibited improved eNRR performance as compared to non-defective counterparts with an ammonia formation rate of 1.24 × 10−10 mol cm−2 s−1 and a FE of 9 % at −0.15 V vs. RHE.Overwhelming HER competition could be minimized by a variety of approaches. Importantly, surface modification was done by a hydrophobic coating of ZIFs on the noble metal catalyst (Ag–Au) that suppressed the HER process and improved the performance of noble metals (Fig. 13e and f) [175]. The hydrophobic nature and high gas sorption ability of ZIFs were the main attributes of high ammonia selectivity (90 %) over hydrogen production. Deposition of Ag-nanocubes on Au electrodes and subsequent coating of hydrophobic ZIFs by wet chemical deposition was performed. Confinement of electroactive species and the inhibition of water access by ZIFs film resulted in the 10 pmol cm−2 s−1 rates of ammonia production at −2.9 V vs. Ag/AgCl with a FE of 18 % in an aprotic solvent (THF) containing lithium triflouromethanesulfonate as the electrolyte. Despite this approach, eNRR performance is still impeded by the HER process. Moreover, the role of an aprotic solvent, Li-ion incorporation, and flouro-based additives in the electrolyte need to explore more in this regard.Core-shell type nanostructures provide two different interaction sites depending on the core and the shell. The synergistic effect of core-shell morphology and composition resulted in the unique properties of the material [181–184]. Mostly, the stability of reactive cores could be tuned up by a coating of the shell. Moreover, these core-shell type of structures also provides localized charge density sites for enhanced interaction with the reactant molecules.Core-shell type of α-Fe nanorods@Fe3O4 was grown on carbon fiber paper by the hydrothermal approach and evaluated for ammonia formation in the aprotic solvent-ILs mixture [185]. An enhancement in the eNRR activity was achieved with an ammonia production rate of 2.35 × 10−11 mol s−1 cm−2 and a FE of 32 %. A smoother energy profile was obtained by DFT calculations, highlighting the associative distal pathway as the most plausible mechanism for these structures. Au@CeO2 core-shell structures were developed by room temperature spontaneous redox approach with 3 % loading of Au [183]. Au cores below 10 nm were the main active sites for nitrogen reduction, but oxygen vacancies created on CeO2 also enhanced the localized charge density. In acidic media, this catalyst exhibited a FE of 9.5 % with an ammonia yield rate of 10.6 μg h−1 mg−1 at −0.4 V vs. RHE.Electrochemical nitrogen reduction is regarded as a possible avenue for storing renewable energy in chemical bonds, as well as an environmentally acceptable alternative to the century-old Haber–Bosch process. For the last few years, scientists have been working hard to improve the rate and efficiency of this reaction, and they have achieved significant progress. In this review, recent theoretical and experimental studies for eNRR are summarized to provide a fundamental basis for the rational design of an electrocatalyst. A detailed theoretical framework is explained that helps in the rational catalyst design based on different plausible mechanisms for eNRR. This framework involves the selection of an active material based on the reactant adsorption and desorption kinetics along with the elucidation of scaling relation by the adsorption and desorption of reaction intermediates. Furthermore, explorations related to the development of new catalytic compositions for nitrogen reduction could be achieved by the evaluation of the density of states and charge distribution. It has been deduced from theoretical studies and verified experimentally, that electrocatalytic nitrogen reduction on the surface of transition metal catalysts requires higher overpotentials than the hydrogen evolution reaction, which results in low FE and ammonia production. However, transition metal derivatives and single-atom catalysts are predicted by theoretical studies to be more active than their parent bulk metals for nitrogen reduction. Based on the intrinsic and extrinsic catalytic properties of a catalyst, several strategies are suggested by consideration of a theoretical-directed experimental approach. Different compositions designed by rational catalyst design showed enhanced FE for ammonia including transition metal oxides, carbides, sulfides, and metal-free carbon-based materials. Synergism between theoretical and experimental studies will lead to a better understanding of the mechanism that eventually leads to rational catalyst design.Based on our understanding of this reaction, herein, we present several outlooks for the development of this field. (1) Catalyst development methods: In the literature, there is a lack of reports of highly active and efficient catalysts for this process. This may be attributed to the limitations related to the catalyst development method. For instance, achieving homogeneity in the catalyst structure is the biggest challenge in several catalytic methods. Importantly, electrochemical reactions are surface-based interactions of reactant moieties with the catalyst surface on flat electrodes that limit access to deeper catalytic content. Several methods are hard to scale up for industrial-scale production of the catalyst. Moreover, challenges related to the optimization of reaction parameters hamper the production of active and efficient catalysts. In this regard, the vital paradigm shift in the chemistry of this process is possible by theory-guided rational catalyst design. Thus, elucidation of the mechanism by theoretical studies is required, especially those related to heterogeneous catalyst surfaces and atomic-level electrocatalysis. More precise control of the content, structure, and active sites is required. Although external dopants, intrinsic defects, and alloy formation have shown to be useful in altering the electronic structure, accurately controlling the appropriate doping site/type, defect quantity, and alloy content, which is important to determine the catalytic activity, remains a challenge. It is desirable to have new catalyst development methods that are facile and sustainable to control the desired target catalytic properties. Moreover, advanced strategies for catalyst development that have been employed at the lab scale need optimization to work at a larger scale. Finally, artificial intelligence and machine learning might improve the search for new efficient electrocatalysts. (2) Selectivity and rate of ammonia production: Selectivity is one of the challenges reported for eNRR in aqueous media that is attributed to the enhanced tendency of transition metals for the adsorption of hydrogen as compared to the nitrogen moiety. Moreover, the competition for adsorption on the surface of the catalyst between reactants and different reaction intermediates resulted in low rates of ammonia production. Though an improved FE had been reported by rational catalyst design strategy, the overall rate of ammonia production was low (>10−9 mol s−1 cm−2). It is highly desirable to put effort into the improvement of the rate of this process after the selection of an efficient catalyst. Furthermore, catalyst stability and durability for long-time operation are also important to make this process economically feasible. For instance, surface reconstruction of the catalyst has gained attention in the recent past during the electrolysis at high reductive/oxidative potentials [186–190]. As a result, during electrocatalytic activities, the surface structures and compositions of catalysts are dynamically reconstructed. With advances in in-situ and operando techniques, it has been discovered that during electrolysis, electrocatalysts undergo surface reconstruction to form the actual active species, accompanied by a change in their oxidation state. As a result, establishing unambiguous structure-composition-property relationships in the pursuit of high-efficiency electrocatalysts requires a thorough understanding of the surface reconstruction process. (3) In-situ characterization and product quantification: It is highly desirable to employ a rigorous protocol for the measurement of the amount of ammonia produced from dinitrogen by eNRR to confirm the exact origin of the N-source. Andersen et al. reported a benchmarking protocol for the eNRR evaluation at ambient conditions [5]. As illustrated in Fig. 14 , the amount of ammonia and other nitrogen-containing compounds in the setup due to background contamination needs screening. If the contamination levels measured are within an order of magnitude of the ammonia produced different measures are required to eliminate the interference including cleaning of membrane, electrochemical setup, and the developed material. Moreover, quantitative isotope-sensitive measurements of produced ammonia are recommended to validate the exact origin of the N-source. In-situ and operando characterization tools are crucial to closing the gap between mechanistic understanding and the performance of the catalyst for eNRR. Real-time mechanistic evaluations are recommended for this process to understand challenges and explore solutions. This will improve the deeper understanding of this process in real-time and a correlation between the structure to performance of the catalyst will be developed. Elucidation of electrochemical reactions during nitrogen reduction regarding kinetics and solid-liquid interface is the need of time. Moreover, ammonia detection techniques are required for the rigorous validation of this process. In this regard, validated electrochemical, spectroscopic, and chromatographic techniques with high sensitivity are required. (4) Device fabrication: The sluggish rate of NH3 production needs improvement with high current densities to be used as a commercial process for eNRR at ambient conditions. Such kinetic studies related to mass and electron transfer mechanisms are particularly significant. For the large-scale application of this process, a device is required that can work in harsh conditions to produce high rates of ammonia. In this regard, a gas diffusion electrode-based flow cell type eNRR electrolyzer is expected that can operate at high current density. Catalyst development methods: In the literature, there is a lack of reports of highly active and efficient catalysts for this process. This may be attributed to the limitations related to the catalyst development method. For instance, achieving homogeneity in the catalyst structure is the biggest challenge in several catalytic methods. Importantly, electrochemical reactions are surface-based interactions of reactant moieties with the catalyst surface on flat electrodes that limit access to deeper catalytic content. Several methods are hard to scale up for industrial-scale production of the catalyst. Moreover, challenges related to the optimization of reaction parameters hamper the production of active and efficient catalysts. In this regard, the vital paradigm shift in the chemistry of this process is possible by theory-guided rational catalyst design. Thus, elucidation of the mechanism by theoretical studies is required, especially those related to heterogeneous catalyst surfaces and atomic-level electrocatalysis. More precise control of the content, structure, and active sites is required. Although external dopants, intrinsic defects, and alloy formation have shown to be useful in altering the electronic structure, accurately controlling the appropriate doping site/type, defect quantity, and alloy content, which is important to determine the catalytic activity, remains a challenge. It is desirable to have new catalyst development methods that are facile and sustainable to control the desired target catalytic properties. Moreover, advanced strategies for catalyst development that have been employed at the lab scale need optimization to work at a larger scale. Finally, artificial intelligence and machine learning might improve the search for new efficient electrocatalysts. Selectivity and rate of ammonia production: Selectivity is one of the challenges reported for eNRR in aqueous media that is attributed to the enhanced tendency of transition metals for the adsorption of hydrogen as compared to the nitrogen moiety. Moreover, the competition for adsorption on the surface of the catalyst between reactants and different reaction intermediates resulted in low rates of ammonia production. Though an improved FE had been reported by rational catalyst design strategy, the overall rate of ammonia production was low (>10−9 mol s−1 cm−2). It is highly desirable to put effort into the improvement of the rate of this process after the selection of an efficient catalyst. Furthermore, catalyst stability and durability for long-time operation are also important to make this process economically feasible. For instance, surface reconstruction of the catalyst has gained attention in the recent past during the electrolysis at high reductive/oxidative potentials [186–190]. As a result, during electrocatalytic activities, the surface structures and compositions of catalysts are dynamically reconstructed. With advances in in-situ and operando techniques, it has been discovered that during electrolysis, electrocatalysts undergo surface reconstruction to form the actual active species, accompanied by a change in their oxidation state. As a result, establishing unambiguous structure-composition-property relationships in the pursuit of high-efficiency electrocatalysts requires a thorough understanding of the surface reconstruction process. (3) In-situ characterization and product quantification: It is highly desirable to employ a rigorous protocol for the measurement of the amount of ammonia produced from dinitrogen by eNRR to confirm the exact origin of the N-source. Andersen et al. reported a benchmarking protocol for the eNRR evaluation at ambient conditions [5]. As illustrated in Fig. 14 , the amount of ammonia and other nitrogen-containing compounds in the setup due to background contamination needs screening. If the contamination levels measured are within an order of magnitude of the ammonia produced different measures are required to eliminate the interference including cleaning of membrane, electrochemical setup, and the developed material. Moreover, quantitative isotope-sensitive measurements of produced ammonia are recommended to validate the exact origin of the N-source. In-situ and operando characterization tools are crucial to closing the gap between mechanistic understanding and the performance of the catalyst for eNRR. Real-time mechanistic evaluations are recommended for this process to understand challenges and explore solutions. This will improve the deeper understanding of this process in real-time and a correlation between the structure to performance of the catalyst will be developed. Elucidation of electrochemical reactions during nitrogen reduction regarding kinetics and solid-liquid interface is the need of time. Moreover, ammonia detection techniques are required for the rigorous validation of this process. In this regard, validated electrochemical, spectroscopic, and chromatographic techniques with high sensitivity are required. Device fabrication: The sluggish rate of NH3 production needs improvement with high current densities to be used as a commercial process for eNRR at ambient conditions. Such kinetic studies related to mass and electron transfer mechanisms are particularly significant. For the large-scale application of this process, a device is required that can work in harsh conditions to produce high rates of ammonia. In this regard, a gas diffusion electrode-based flow cell type eNRR electrolyzer is expected that can operate at high current density.The authors declare no conflict of interest.The study is supported by Australian Research Council (DP210103892). C.Z. also thanks Australian Research Council for the award of Future Fellowship (FT170100224).

Ammonia (NH3), a carbon-free hydrogen carrier, is an important commodity for the food supply chain owing to its high energy capacity and ease of storage and transport. The Haber–Bosch process is currently the favored industrial method for large-scale ammonia production but requires energy-intensive and sophisticated infrastructure which hampers its utilization in a sustainable and decentralized system of manufacture. The electrochemical nitrogen reduction reaction (eNRR) at ambient conditions holds great potential for sustainable production of ammonia using electricity generated from renewable energy sources such as solar and wind. However, this approach is limited by a low rate of ammonia production with high overpotential and the competing hydrogen evolution reaction (HER). For a better understanding and utilization of eNRR as a sustainable process, insight into rational catalyst design and mechanistic evaluations by a theoretically-directed experimental approach is imperative. Herein, recent insights into rational catalyst design and mechanisms, based on intrinsic and extrinsic catalytic activity are articulated. Following the elucidation of basic principles and mechanisms, a framework supplied by theoretical studies that lead to the optimal selection and development of eNRR catalysts is presented. Following a discussion of recently developed electrocatalysts for eNRR, we outline various recently-used theoretical and experimental methodologies to improve the intrinsic and extrinsic catalytic activity of advanced electrocatalysts. This review is anticipated to contribute to the development of active, selective, and efficient catalysts for nitrogen reduction.

The photocatalytic process is one of the cleanest technologies from an environmental perspective. It has been investigated intensively over the last two decades because of its potential application in waste treatment and the production of new sources of energy [1,2]. TiO2 is one of the most attractive UV-photocatalysts with application in many fields, especially wastewater treatment [3,4]. However, the photocatalytic efficiency of TiO2 is prone to decline over time, mainly due to its limited light-gathering capacity and the easy recombination of electron–hole pairs [3,5].Using sunlight as the energy source for the photocatalytic reaction is a key target in studies of photocatalysis. The development of small band gap photocatalysts is a promising way to approach this target and has been the subject of considerable interest in recent years. A suitable high-efficiency semiconductor for visible light photocatalysts needs a band gap (E G < 3.0 eV) that is sufficiently narrow to harvest visible light but is large enough (E G > 1.23 eV) to provide energetic electrons [6]. Further, a significant reduction in the band gap energy enhances the recombination of electrons and holes, dictating an optimum value for band gap energy [7]. Other important parameters determining photocatalytic efficiency are the morphology, crystal structure, and particle size of the photocatalyst [8]. It has been reported that the photocatalytic activity of semiconductors can be improved by increasing crystallinity and enhancing the specific surface area [9].The main ways of improving the performance of existing materials are: ion-doping or defect modification [10], fabrication of a composite material consisting of a photocatalyst and a highly conductive material to suppress the recombination of electron–hole pairs by enabling fast photogenerated electron transfer [11], and creation of heterojunctions between two semiconductors with different band structures to inhibit electron–hole recombination and improve photocatalytic performance [12]. Recent studies have used different approaches to improving the photocatalytic performance of UV–TiO2 catalysts, including doping with metal ions (Ag, Fe, V, Au, Pt, Ni, Co, Cu, Nb) [13,14], non-metal (N, S, C, B, P, I, F) [13,15], mixed oxides with p–n junction characteristics [16,17] and combining TiO2 with smaller band gap energy semiconductors, such as WO3, SiC, Cu(OH)2, CuOx, Ni(OH)2, NiO, Si, CdS, and SrTiO3 [18–20].Because of their high level of photocatalytic activity under UV irradiation and visible light, perovskite-type oxides, such as tantalates and titanates, have recently attracted much attention [21,22]. Ti-based materials, and especially perovskite titanate ATiO3 (A = Ba, Sr, Fe, and Ni), are promising new photocatalysts with notable advantages [23] that have been the subject of intensive research [24,25]. Among perovskite titanates, NiTiO3 has an ilmenite-type crystal structure in which both Ni and Ti are in octahedral coordination, with alternating cation layers occupied by Ni and Ti [26]. NiTiO3 has attracted considerable attention due to its superior photocatalytic and electro-optical properties and low dielectric constant [25,27]. NiTiO3 is an n-type semiconductor material with antiferromagnetic properties [28] is highly stable in an oxidizing environment and under light irradiation. In addition, NiTiO3 has an optical absorption spectrum with band gap energy of around 2.2 eV, which means that it has excellent potential for use in photocatalytic applications in visible light [28]. There have been numerous reports on the investigation of the photocatalytic activity of NiTiO3, including degradation of Tergitol, Safranine T [29], and Rhodamine B [30]. The photobleaching of methyl orange by NiTiO3 confirms its good photocatalytic properties in visible light [31]. However, its low band gap energy reduces its quantum efficiency when used as an individual photocatalyst [32].An effective strategy for improving the photocatalytic performance of TiO2 is to combine it with other semiconductors to form a heterostructured photocatalyst [18,33]. TiO2-based heterostructure systems, i.e. SrTiO3–TiO2 and BaTiO3–TiO2, have been shown to promote cation charge and hole transport, with a narrowing of the band gap energy of TiO2 and enhanced electron–hole pair separation [20,34], which improve photochemical efficiency. However, both BaTiO3 and SrTiO3 have relatively large band gap energy of about 3.4 eV, limiting the utilization of sunlight sources. TiO2-coupled nickel titanate has recently been reported as an efficient photocatalyst in visible light for the decomposition of methylene blue [35]. NiO/NiTiO3 composites present a higher degradation rate than pure NiTiO3 regardless of the amount of NiO present [36].The doping of TiO2 with NiTiO3 is a promising solution for improving the photocatalytic performance of TiO2 and expanding absorption to the visible light region [27]. However, few studies have used NiTiO3-doped TiO2 catalysts for wastewater treatment, especially the treatment of persistent organic compounds. Therefore, it is desirable to develop NiTiO3-doped TiO2 photocatalysts for this purpose. In this study, TiO2 catalysts doped with various NiTiO3 concentrations were prepared by combining the sol–gel and the hydrothermal method using water as an eco-friendly solvent in the synthesis process. The physicochemical and photochemical properties of doped catalysts were determined using modern physico-chemical analysis techniques. In addition, using cinnamic acid (CA) as the representative persistent compound, we further demonstrated that the modified NiTiO3–TiO2 catalysts are excellent UV-A-photocatalysts for the photodegradation of persistent organic pollutants.NiTiO3 perovskite was synthesized by the sol–gel method. First, 2.91 g of nickel nitrate hexahydrate (Ni(NO3)2·6H2O, Merck, >99%) and 2.10 g of citric acid monohydrate (C6H8O7·H2O, Merck, >99%) were dissolved with 5 mL of ethanol absolute (C2H5OH, Merck, >95%) and stirred at 300 rpm for 1 h to form a homogeneous mixture. Next, 3 mL of titanium (IV) isopropoxide (Ti(OC3H7)4, Sigma-Aldrich, >97%) was added drop by drop while stirring and then held for 1 h to form a transparent sol mixture. The synthetic sol was heated slowly to 60 °C and dried for 24 h to produce a bright green puffy porous gel. Finally, the porous gel was heated at 700 °C for 2 h to form the required NiTiO3 nanoparticles (NTO).NiTiO3–TiO2 mixed catalysts with different NiTiO3 concentrations were synthesized in an eco-friendly neutral medium. First, 3 mL of Ti(OC3H7)4 was added drop by drop to the distilled water and stirred for 30 min at 300 rpm. Next, m grams of NiTiO3 synthesized in the above step were added to the solution. The mixture was then hydrated in a steel-lined Teflon container at 160 °C for 12 h. The solids were then filtered and washed three times with distilled water and ethanol. Subsequently, the solids were dried at 60 °C for 12 h to obtain NiTiO3–TiO2 mixed catalysts. The catalysts were denoted as xNTO–Ti, representing the NTO content of the mixed catalysts, x = 0.5, 1.0, 1.5, and 2.0 wt.%.The physicochemical characteristics of the samples were studied using Χ-ray diffraction (ΧRD), Raman spectroscopy, scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FT-IR), Brunauer–Emmett–Teller adsorption (BET), energy dispersive X-ray (EDX), UV–vis absorption spectra techniques, and point of zero charges (PZC). The methods are detailed in our previous study [37].In this study, CA, a recalcitrant phenolic acid, was selected as the representative agent for the persistent organic matter to be researched. The photocatalytic activity of the samples was studied using the batch method for the photodecomposition of CA, as described in our previous study [37]. The effects of the NiTiO3 loading (0.5, 1.0, 1.5, and 2.0 wt.%) in the modified samples on CA photodegradation were investigated, along with the operating parameters including, the airflow rate (0.2, 0.3, 0.4, and 0.5 L/min), the catalyst dosage (0.75, 1.00, 1.25, 1.50 and 1.75 g/L), and the initial pH of the solution (3.8, 5.0, and 7.0). The reaction temperature was fixed at 25 °C by heat exchange with cyclic cold water. The initial pH of the solution was adjusted using a buffer solution pH = 10 ± 0.01.The XRD patterns of the catalysts are presented in Fig. 1 . The XRD pattern of the TiO2 synthesized in the neutral medium of water contained the characteristic peaks of the anatase phase at 2θ = 25.6, 38.2, 48.3 and 54.7° (JCPDS 21-1272), with the strongest intensity at 2θ = 25.6° [37]. The XRD patterns of the NTO–Ti samples contained the characteristic peaks of the high crystallinity anatase phase at 2θ = 25.4°; 38.1°; 48.3°; 54.3°; and 62.6°. In addition, a small amount of rutile (110) at 2θ = 27.4 (JCPDS card 21-1276) [38] and brookite crystalline phases of TiO2 appear at 31.3° [39]. On samples 0.5NTO–Ti and 1.0NTO–Ti, almost no characteristic peaks of NiTiO3 appear because of the relatively low concentration; this proves that NiTiO3 is well dispersed on the TiO2 surface of these samples (as can be seen from SEM images in Fig. S1). However, the diffraction planes (102), (104), (110), (116), (214) and (300) situated at 2θ = 23.9°; 32.8°; 35.4°; 53.4°; 61.9°; and 63.5°, respectively, reveal the single ilmenite phase NiTiO3 [38] corresponding to JCPDS 75-3757 [40] and were observed with the 1.5NTO–Ti and 2.0NTO–Ti samples that had higher concentrations of NiTiO3. No peaks corresponding to the NiO (111), (200), (220) crystal plane, respectively at 2θ = 37.2°, 43.3°, and 62.8° (JCPDS card no. 47-1049) [36,38] were observed, indicating that Ni exists solely in the structure of the NiO6 octahedra of NiTiO3. In the NTO–Ti modified sample, no shift in the peak characteristics of the phases of TiO2 relative to the single TiO2 was observed, whereas a right-shift in the XRD peaks of NiTiO3 compared to pure NiTiO3 can be clearly seen; this is evidence of the fusion of NTO and TiO2.Based on the XRD results at 2θ = 25.4° of the (101) anatase TiO2 plane, the average crystal size of the catalysts is calculated by the Debye–Scherrer equation with K = 0.94 to be 34.8 nm, 8.6 nm, 9.0 nm, 9.0 nm and 8.7 nm for TiO2 and the 0.5NTO–Ti, 1.0NTO–Ti, 1.5NTO–Ti, and 2.0NTO–Ti samples, respectively. It appears that the addition of NTO significantly reduces the crystal size of TiO2 and simultaneously enables the transfer of a small amount of the anatase phase to rutile and brookite phases under hydrothermal conditions at 160 °C.Additional information regarding the crystalline structure was obtained through Raman spectroscopy (Fig. 2 a). On the bare TiO2 sample synthesized in the neutral medium of water (denoted as Ti(w)), the peaks of the anatase phase are observed at 147, 402, 517 and 639 cm−1. For the NTO–Ti samples, the characteristic peaks of TiO2 anatase still appear, with a clear shift relative to pure TiO2, specifically 2θ = 146, 198, 320, 398, 515, 640 cm−1. The peaks at 146,198, 640 cm−1 with the strongest signal intensity at 146 cm−1 are characteristic of the E g spectrum; 398 cm−1 is attributed to the B 1g spectrum, and 515 cm−1 is assigned to A 1g. E g is the asymmetric stretching vibration, while B 1g and A g correspond to the asymmetric and symmetric bending vibration of TiO2 anatase. In addition, Raman spectra also detected the E g asymmetric stretching vibration of the rutile crystalline phase at wavenumber 238 cm−1 [41]. Meanwhile, peaks at wavenumbers 246, 284, 351, 552, and 706 cm−1 are typical of the appearance of NiTiO3 [42]. A weak Raman peak is observed at around 706 cm−1 in the NTO–Ti modified catalysts, which can be ascribed to the NTO phase with a hexagonal structure [43]. The absence of wavenumber 547 cm−1 originating from Ni–O bonds in the Raman spectrum suggests that almost all the Ni had been utilized to form NiTiO3 nanocrystallites [44], confirming the results of the XRD analysis. The Raman spectra indicated that adding of NTO to TiO2 leads to a shift in the oscillation peaks of TiO2 and NTO relative to bare TiO2 and NTO. These findings indicate a strong interaction between NTO and TiO2, resulting in the fusion of NTO into the structure of TiO2.The FTIR spectra of the NTO–Ti samples recorded in the wavenumber region 400–4000 cm−1 are shown in Fig. 2b. It can be observed that there are three main absorption regions for all the samples, in the ranges 2500–3600 cm−1, 1500–1650 cm−1, and 400–800 cm−1, respectively. The peaks at 2500–3600 cm−1 with the highest intensity at 3405 cm−1 are typical of stretching vibration of –OH in the hydroxyl group [45]. It follows from Fig. 2b that the addition of NTO into TiO2 leads to a reduction in the intensity of the OH groups; further, the greater the amount of perovskite added, the greater the production in the hydroxyl groups. The characteristic bands discovered at 1500–1650 cm−1 are H–OH groups adsorbed on the catalyst surface. Previous studies have shown that the presence of OH and H–OH groups has an essential role in photocatalysis since the hydroxyl group present on the catalyst surface can react with holes to form hydroxyl radicals [46]. The peaks at about 1560 and 1395 cm−1 are due to the N–O bond vibration of NO3 − and carboxyl vibration, respectively [47]. The typical bands in the 400–800 cm−1 range correspond to the vibrations of the metal–oxygen bond [40]; the absorption peak near 440 cm−1 corresponds to the Ti–O–Ni bond [48] and the strong absorption bands at 450 and 565 cm−1 correspond to the stretching vibrations of Ti–O and bending vibrations of O–Ti–O, respectively. The absorption bands of the Ti–O octahedral appear at 675 and 500 cm−1, corresponding to the formation of the NiTiO3 phase [47]. The characteristic bands of the carbonate (867 and 1067 cm−1) do not appear in the spectrum, which means that the synthesized crystals are carbonate-free [49]. Therefore, this result demonstrates the successful synthesis of the NTO-mixed TiO2 catalyst.On the SEM image (Fig. S1(a)), the clumps of spherical-like TiO2 particles size of 10–40 nm can be observed in pure TiO2 sample, while NiTiO3 (Fig. S1(b)) appears in the form of bigger bright spherical granules, 20–50 nm in size. On mixed samples the interspersing dark and light particles of few nm in size can be seen. As the concentration of NTO in the mixture catalyst increased, the density of light-colored particles, attributed to NTO, increased. However, the particle sizes of the three samples containing 0.5–1.5% NTO were approximately the same, at about 5–10 nm. From the SEM analysis results, it can be suggested that in the mixed samples TiO2 and NiTiO3 exist in form of smaller separate particles, which is consistent with the results of the XRD analysis above.The N2 adsorption/desorption isotherms of Ti(w) and the 1.0NTO–Ti samples (Fig. 3 ) exhibit a type IV(a) isotherm curve, based on the classification by the International Union of Pure and Applied Chemistry [50]. In this type, the initial monolayer–multilayer adsorption on the mesopore walls is followed by pore condensation. In the adsorption isotherm of the TiO2 sample, the adsorption branch contains a low slope region, which is associated with multilayer adsorption on pore walls and a narrow hysteresis loop, which are indicative of a narrow distribution of uniform mesopores and limited networking effects [50]. Further, the adsorption branch ends with a plateau region, indicating that mesopores are completely filled and macroporosity is non-existent. Based on these features, the isotherm for the bare TiO2 sample is type IV(a) with an H1 hysteresis loop [51]. The H1 hysteresis has been found in networks of ink-bottle pores where the width of the neck is similar to the width of the pore/cavity [50] or corresponds to cylindrical pores with openings at both ends [52].The shape of the adsorption/desorption isotherm of the 1.0NTO–Ti sample is consistent with types H2(b) or H3. However, there is no appearance of the sharp step-down of the desorption branch, typical of the H3 type, so it can be concluded that the adsorption/desorption isotherm in this case is type H2(b) [50]. The type H2(b) loop is associated with pore blocking and indicates that the sample contains ink bottle-like pores; however, adsorption of N2 in this sample takes place to some extent in the monolayer region and is much stronger at higher p/p o values (p/p o > 0.6), indicating that the neck size is large. Thus, in both samples, there are networks of ink-bottle pores where the widths of the neck and the pore/cavity are similar. The pore diameters of the bare TiO2 and NTO–Ti mixed catalysts were determined to be around 20 Å (Fig. 3 and Table 1 ). The existence of ink bottle-like pores in TiO2 and the NTO–Ti samples is beneficial to the adsorption of oil and gas [52].The hysteresis loop type H2(b) of the NTO–Ti mixed sample [51] exhibits more complex pore structures in which network effects are important. Type H2 is characterized by a more random distribution of pores and an interconnected pore system in the heterostructure sample [53] that affects the textural properties of the NTO–Ti catalysts, such as surface area, pore volume, and pore size. Indeed, the specific surface area of the NTO–Ti samples ranges from 142.8 to 163.2 m2/g, while the pore volume is in the range 0.086–0.182 cm3/g and pore size is in the range 17.6–23.4 Å. It appears that the size and dispersion of the nanostructures obtained by the hydrothermal process depend on the NiTiO3 loading. Of the catalysts investigated, the largest pore size (23.4 Å) and pore volume (0.182 cm3/g) are found in the 1.0NTO–Ti sample, which provides superior adsorption capacity compared to the other catalysts. The values of specific surface area (159.7 m2/g), pore volume (0.178 cm3/g), and pore size (22.8 Å) of pure TiO2 are within the variation range of the corresponding quantities of the NTO–Ti samples. The textural properties of the 1.0NTO–Ti and bare TiO2 catalysts are approximately the same, showing that the structure distribution in the 1% NTO–Ti sample is the most favorable. The pore volume and pore size of the 1.0NTO–Ti sample are also comparable to those of TiO2 and are higher than those of the other two mixed samples, as can be seen in Table 1. The relatively high specific surface area of the samples may be related to the loose assembly of the primary nano-sized spherical-like particles to form porous secondary particles 30–100 nm in size, as observed in the FE-SEM images (Fig. S1).The elemental compositions of the 1.0NTO–Ti sample were determined by EDS analysis. The EDX results (Fig. S2) reveal the presence of Ti, O, and Ni in the NTO–Ti sample. This result indicates the formation of a pure 1.0NTO–Ti powder without impurities. Fig. S2a shows that the distributions of the Ti, O, and Ni elements in this sample are synchronous. Meanwhile, Fig. S2b shows the appearance of Ti at energy levels 0.28, 0.42, 4.51, and 4.92 keV; Ni at 0.95 and 5.62 keV; and O at the highest energy level of 0.53 keV. The analytical mass percentages of O, Ti, and Ni elements are 39.01%, 60.75%, and 0.24%, approximating to their theoretical mass ratios (O: 39.83%; Ti: 59.45%; Ni: 0.72%). The results confirm the formation of the NTO–Ti material. The mass percent of Ni is lower than the theoretical value because more Ni2+ ions are lost due to the higher solubility of Ni-containing precipitate and the filtering and washing processes [44].The optical absorption characteristics of the NTO–Ti catalysts were estimated by UV–vis diffuse reflectance spectroscopy at ambient temperature; the values for band gap energy (E G) were determined using the Tauc formula [54]. The band gap energy of NiTiO3 was found at 2.18 eV (corresponding to visible light at 560 nm) due to Ni2+ → Ti4+ charge-transfer bands [55]. The addition of NiTiO3 to TiO2 reduced the band gap energy from 3.2 eV (for anatase TiO2) to 3.02–3.08 depending on the NiTiO3 loading, corresponding to the extension of light absorption to the visible region (404–412 nm versus 385 nm) (Fig. 4 ). This effect can be explained as follows. Adding NTO to TiO2 makes the phase composition of the TiO2 to be changed changes, comprising anatase (E G = 3.2 eV), rutile (E G = 3.1 eV) [1] and brookite (E G = 1.86 eV) [56] crystalline phases simultaneously, as shown in the XRD analysis, while bare TiO2 exists solely in the anatase phase. In addition, the absorption band at 404–412 nm is also due to the O2− → Ti4+ charge transfer band of NiTiO3 [55]. The energy bands of the NiTiO3 align with the TiO2 to construct a heterojunction, as noted by Yue-Ying Li [41]. When NiTiO3 and TiO2 are excited by light illumination, the heterojunction promotes the photogenerated electrons of the conduction band to migrate from NiTiO3 to TiO2, with a simultaneous flow of holes from TiO2 to NiTiO3. As a result of these factors, the band gap energy of the mixed TiO2 sample reduces from 3.2 eV to a value of 3.02–3.08 eV. As shown in Fig. 4, increasing the NiTiO3 content from 0.5 to 1.0 wt.% leads to a redshift in the absorption band from 404 to 412 nm and reduces the band gap from 3.08 eV to 3.02 eV, approaching the upper threshold of the desired band gap range (3.0 eV) [6]. Meanwhile, the absorption wavelength reduces to 407 nm and the band gap energy increased slightly to 3.05 eV when the NiTiO3 concentration is increased further, to 1.5 wt.%. This result is consistent with a prior study [14]. Clearly, the enhanced absorption of the visible region may lead to higher utilization performance during the photodegradation process for organic compounds.The conversion of CA during a 120 min reaction using NTO–Ti catalysts with different NTO concentrations is shown in Fig. 5 . It was found that pure NiTiO3 exhibits very low activity in the photodegradation of CA, with 120-min removal efficiency (X120) of 3.8%, while the value of bare TiO2 is 68.7%. Similar results were obtained in the study of Li [44], which reported that NiTiO3 exhibits much lower catalytic activity for methylene blue photo-degradation than Degussa P-25 (10% methylene blue degraded in 80 min on NTO versus 100% in 20 min on P-25). TiO2 consists of TiO6 octahedra, and both valence band and conduction band consist of hybridized O 2p and Ti 3d orbitals. Thus, under UV light, most charge-transfer transitions in TiO2 are O2− → Ti4+. TiO2 absorbs little visible light but exhibits high photocatalytic activity under UV light [57]. NiTiO3 has a narrower band gap than TiO2, but the crystal structure of NiTiO3 consists of alternating NiO6 and TiO6 layers and induces a wide energy gap from the hybridized Ni 3d and O 2p orbitals to the predominant Ti 3d orbitals, blocking both Ni2+ → Ti4+ and O 2p → Ti 3d charge-transfer transitions. This effect leads to the low photocatalytic performance of NiTiO3 [44].The results in Fig. 5 show that by increasing NTO concentrations from 0.5 to 1.0 wt.%, CA removal efficiency increases significantly; the decomposition at 30 min increased from 32.5% to 49.2% and rises from 59.9% to 82.8% after 120 min. However, when the NTO concentration is increased to 1.5% and 2.0 wt.%, CA treatment efficiency decreases sharply; the 30 min conversion of CA is only 38.5 and 41.9%, respectively, while CA conversion after 120 min is 65.2 and 69.7% in the same conditions. Among the NTO–Ti catalysts, 0.5NTO–Ti has the least activity and is the sample with the smallest specific surface area, pore volume, and pore dimension. In addition, this sample also exhibits the lowest intensity of OH groups and adsorbed water and the greatest band gap energy, which is unfavorable in the photocatalytic reaction [58]. The superior properties, namely the largest pore diameter and pore volume, the smallest particle size, the highest density of OH groups and adsorbed water and the smallest band gap energy, were responsible for the outstanding performance of the catalyst contained 1.0% NTO. The CA conversion efficiency for the 1.0NTO–Ti sample was also much higher than that of pure TiO2 (X120 = 82.8% versus 68.7%). Therefore, adding NTO to TiO2 significantly improves the physicochemical and photochemical properties of the photocatalyst by reducing the crystal size, narrowing the band gap energy, enhancing UV-A light absorption and converting part of the TiO2 anatase phase to rutile that enhances the photocatalytic performance of the heterostructure catalyst. However, due to the very low photocatalytic activity of NiTiO3, the high loading of NTO content (1.5% and more) leads to a decrease in the activity of the resulting catalyst.The effects of the reaction conditions on CA photocatalytic degradation for the 1.0NTO–Ti catalyst are shown in Fig. 6 . It can be seen from Fig. 6a that airflow rate greatly influences catalytic activity in CA photodegradation. CA conversion increases sharply as airflow increases from 0.2 to 0.3 L/h. The 120-min removal efficiency of CA increased from 68.4 to 82.8%. However, CA conversion decreased with increasing airflow up to 0.4 and 0.5 L/h, and the value of X120 fell to 79.6 and 68.8%, respectively. Dissolved oxygen plays an essential role in supporting the generation of free radicals such as OH and HO2 [59]. At the same time, O2 has a critical role in capturing electrons and limiting electron–hole recombination [60]. Therefore, the airflow entering the reactor increases, leading to an increase in dissolved oxygen content, which improves the efficiency of CA decomposition. However, an increase in oxygen concentration also increases the level of HO2 which reacts with OH radicals in the solution reducing the efficiency of CA treatment [59]. In addition, foaming occurs when there is a high level of aeration. This phenomenon interferes with UV light absorption by the catalyst particle and causes a substantial disturbance to the catalyst particles moving out from the reaction volume to the solution surface, resulting in a reduction in the catalyst concentration directly involved in the reaction; consequently, the efficiency of CA oxidation reduces [61]. Thus, the most suitable airflow rate is 0.3 L/h.When increasing the catalyst dosage from 0.75 to 1.5 g/L, CA treatment efficiency increases significantly. CA decomposition at 120 min increased from 70.9% to 82.8% (Fig. 6b). Catalyst concentrations continued to rise to 1.75 g/L, and CA treatment efficiency decreased slightly, being 80.6% after 120 min. The greater the concentration of catalyst, the larger the number of active sites obtained, resulting in higher conversion efficiency. However, the agglomeration of particles at high catalyst concentrations leads to a decrease in the contact surface area. In addition, collisions between particles can inactivate the catalyst, reducing the number of active sites and the reaction efficiency. On the other hand, a greater density of particles in the solution leads to an increase in turbidity and a decrease in light transmittance into the solution [62]. Therefore, the best catalyst dosage was chosen as 1.5 g/L.The pH of the solution is an essential parameter in photocatalytic reactions because it determines the surface charge characteristics of the catalyst. The highest CA conversion rate was achieved at pH 3.8, at 82.8% after 120 min (Fig. 6c). The point of zero charges (pHPZC) of the 1.0NTO–Ti sample is 6.3 (Fig. S3). Thus, the 1.0NTO–Ti surface is positively charged in acidic media (pH < 6.3). In addition, the pKa value of CA is 4.4 [63]. At a pH lower than its pKa value, CA is deprotonated to form the anion C6H5C2H2COO−. Then, at pH < 6.3, strong CA adsorption on the 1.0NTO–Ti particles is attained since electrostatic attraction results in positively charged NTO–Ti with the anion C6H5C2H2COO−. In addition, at a lower solution pH, more H+ ions are generated and more HO2 free radicals are produced from the combination of H+ ions with O2 − free radicals [64], enhancing CA decomposition performance. At a pH higher than pHPZC (>6.3) the catalyst surface is negatively charged. A base solution with Coulomb repulsion was created between the negatively charged surface of the catalyst and OH− ions that reduced OH radical formation by reaction between the ions (OH−) and holes (h+), reducing CA conversion. Further, an alkaline environment significantly decelerates the transmission of ions in the reactive solution and reduces the possibility of beneficial free radicals forming [65], also resulting in a reduction in catalytic activity. Therefore, the positive charges of the surface of the 1.0NTO–Ti sample at pH < 6.3, especially at pH = 3.8, were favorable in attracting anions in the reaction solution, performing the CA decomposition reaction on the surface. Furthermore, 3.8 is the inherent pH value of the CA solution, facilitating its decompose without pH adjustment.Ilmenite-structural NiTiO3 consisting of alternating layers of NiO6 and TiO6 octahedra not only strongly absorbs ultraviolet light (wavelength < 360 nm) but also selectively absorbs visible light, mainly in a wavelength range 420–540 nm and above 700 nm [66] and so could be a potential photocatalyst. There are numerous reports of investigations into the photocatalytic activity of NiTiO3, including degradation of methyl orange [31], methylene blue [66], humic acid [67], Tergitol, Safranine T, and Rhodamine B [29,30]. In some cases, NiTiO3 does not exhibit obvious photocatalytic activity in the degradation of contaminants, for example, methylene blue in water [66], due to the relatively low mobility of charges [68]. Many attempts have been made to improve the photocatalytic activity of NiTiO3. In a recent publication [66], Khan et al. noted that a heterostructure catalyst system based on the combination of NiTiO3 nanofibers and porous gC3N4 sheets removed 97% of methylene blue molecules after 60 min exposure to visible light irradiation. Further, Pham et al. in [69] showed that removal of methylene blue by molybdenum (Mo)-doped NiTiO3/g-C3N4 composite photocatalysts under visible light increases by a factor of 6.5 compared with that of pristine NiTiO3. Bi4NbO8Cl modified with NiTiO3 (10% in weight) showed improved activity in the photocatalytic degradation of the organic dye Rhodamine B under ultraviolet-visible irradiation. The enhanced photocatalytic performance can be ascribed to the formation of intimate interfacial contact and type-II band alignment between NiTiO3 and Bi4NbO8Cl [70]. The heterostructure catalyst NiO/NiTiO3 showed a higher degradation rate than pure NiTiO3, on which 92.9% of the Rhodamine B was degraded after 60 min illumination under a UV– full visible spectrum [36].Currently, the use of NiTiO3–TiO2 heterostructure catalysts and their photocatalytic properties is rarely reported. TiO2-coupled NiTiO3 nanoparticles exhibit suitable photocatalytic activities under visible light irradiation and perform markedly better than that commercial P25, as reported by Xin Shu et al. [35]; with TiO2-coupled NiTiO3 nanoparticles, 73% decomposition of methylene blue in aqueous solution was achieved after 6 h against around 20% by P25 under visible light irradiation. Further, Bellam et al. [38] reported that almost equal levels of efficiency were achieved after 2.5 h. Binary phase TiO2 coupled NiTiO3 prepared by a co-precipitation method showed 75% tetracycline degradation and 35% total organic carbon (TOC) removal after 2 h [25]. Up until now, there have been no published reports that allow comparison of the activity of NTO-modified TiO2 catalysts for the treatment of the persistent organic matter, such as CA.This study provides a design strategy for incorporating small band gap nickel titanate into TiO2 semiconductors to form a highly active photocatalyst. Through combining the sol–gel and the environmentally friendly hydrothermal methods the small band gap TiO2-based photocatalyst was successfully fabricated by coupling TiO2 with NiTiO3 perovskite. The addition of a small amount of NiTiO3 to TiO2 causes the reduction of crystal size and band gap energy that promotes a red shift of absorbed light to the visible region. The textural properties and band gap of NTO–TiO2 heterostructure catalysts can be controlled by the amount of NiTiO3 loading. The TiO2 contained 1.0 wt.% of NiTiO3 sample was shown to be the most effective catalyst thank to its progressive textural properties and smallest band gap energy (3.02 eV), which is close to the upper threshold of visible-light catalysts. The reaction conditions have a significant influence on the performance of the photocatalyst. At the most favorable situation, the heterostructure catalyst 1.0NTO–Ti exhibits superior activity compared to the parent TiO2 and NiTiO3 in photodegradation of a persistent phenolic acid.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 Vietnam Academy of Science and Technology under the grant No. ĐLTE00.09/20-21.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.jsamd.2021.100407.

Mesoporous TiO2 mixed with NiTiO3 at various concentrations was synthesized by combining sol–gel and eco-friendly hydrothermal methods. The properties of the NiTiO3–TiO2 (NTO–Ti) photocatalyst were characterized using Χ-ray diffraction, Raman spectroscopy, scanning electron microscopy, Fourier-transform infrared spectroscopy, Brunauer–Emmett–Teller adsorption, energy dispersive X-ray, and UV–vis absorption spectra techniques. The photocatalytic activity of NTO–Ti catalysts was assessed by way of the photodegradation of cinnamic acid (CA) under UV-A irradiation. The effects of the operating parameters, including catalyst dosage, airflow, and initial solution pH on the photodecomposition efficiency of CA were also investigated. Research results confirm that NTO–Ti heterostructure catalysts are synthesized in the crystalline phase with high crystallinity. Compared with pure TiO2, the NTO–Ti catalysts have a smaller particle size and average crystallite size (8.6–9.0 nm versus 34.8 nm) and lower band gap energy (3.02–3.08 eV versus 3.20 eV). The catalysts also enable a redshift in the absorption band from UV (λ = 385 nm) to UV-A light (λ = 404–412 nm). The study showed that the physicochemical and photochemical properties and the photocatalytic performance of the NTO–Ti catalysts are controlled by the NiTiO3 loading. NTO–Ti with NiTiO3 1.0 wt.% was found to maximize CA photodegradation. Under the most favorable conditions, CA removal of 82.8% was obtained after 120 min, which is higher than for pure TiO2 (68.7%) and NiTiO3 (3.8%) catalysts under the same conditions.

Data will be made available on request.Over the past years, 3D printing or so-called additive manufacturing (AM) gained more and more public interest and inspired researchers to elaborate novel solutions in various application fields such as automotive/aerospace industry, bioprinting, medical/dental application or in arts. [1–5] This wide range of applications is based on the plethora of printable materials ranging from metals and plastics even to ceramics. [1,6] In addition, several AM techniques are conceivable for most processible materials, each of which featuring different advantages and disadvantages. [1] For example, in the field of ceramic 3D printing, the processes binder jetting, fused deposition of ceramics, stereolithography, selective laser sintering, or direct ink writing (DIW, also called robocasting) have proven to be suitable options. [7–11] Amongst others they have also been successfully used for ceramic 3D printing in heterogeneous catalysis, whereat direct ink writing is one of the more frequently used techniques. [12–16] The printed catalysts were shown to be active in different catalytic processes e.g. Tubío et al. printed monolithic Cu/Al2O3 structures for successful use in Ulman reactions [17], Stuecker et al. compared wash-coated printed alumina monoliths and directly printed material to commercial (wash-coated) monolithic structures for the combustion of methane [18], Middelkoop et al. investigated the activity of directly printed Ni/Al2O3 monolithic structures for the CO2 methanation reaction [19], and Xu et al. reported printed Al2O3 tubes with Pd immobilized in the porous inner Al2O3 ceramic to be used as continuous flow reactor for the reduction of 4-nitrophenol [20]. Compared to conventional shaping techniques such as tableting, granulation or extrusion, 3D printing offers access to tailor-made geometries as well as more complex shapes including e.g. lateral channels. The geometrical shape of a catalyst it influences important process parameters such as pressure drop as well as heat and mass transport within the catalyst bed and is therefore crucial for technical realization. [10,21–25]When combining additive manufacturing and heterogeneous catalysis, a distinction must be made between whether a mold is printed and later burned off, the catalytically active material or one of its precursors is printed itself, or a carrier is printed first which is afterwards loaded with the active component. [7,12,26–29] The latter two main approaches are comparable with the existing technical production methods for heterogeneous catalysts, where extrusion, tablet pressing, and granulation are the common shaping processes and either the active component itself or a carrier is used. [25,30,31] One example of unsupported catalysts is the mixture of iron oxide, potassium oxide and alumina for ammonia synthesis [32], whereas platinum (doped with rhenium) on alumina is an industrially applied supported catalyst for reforming. [30,32] Two main techniques are used to impregnate the carrier with the active component: Dry impregnation (incipient wetness impregnation) describes the method in which a quantity of impregnating solution corresponding to the exact pore volume of the support material is applied. However, if the volume of the impregnating solution exceeds the pore volume by a multiple, it is called wet impregnation. [32–35] The oxidic supports contain surface hydroxyl groups which are positively or negatively charged, depending on the pH value of the impregnation solution relative to the point of zero charge, and interact with the dissociated metal salts. [33,36] As-prepared materials are so-called egg-shell catalysts, in which the active component is in the outer layers of the shape. Competitive adsorption with other anions can be used to influence the penetration depth of the metal precursor into the catalyst leading to egg-yolk or egg-white distributions. Hereby, the respective active component is not directly on the surface but further inside the catalyst particle, which prevents catalyst poisoning. [32,33]One reaction in which platinum catalysts impregnated on alumina are used is the dehydrogenation reaction of perhydro dibenzyltoluene (18H-DBT). [37–39] This reaction has attracted more and more interest in the last years, as it is an integral part for the use of liquid organic hydrogen carriers (LOHC) since its first introduction by Brückner et al. in 2014. [37,40–42] With the help of aromatic or heteroaromatic compounds such as methylcyclohexane, dodecahydro-N-ethyl carbazole or perhydro dibenzyltoluene, it is possible to store hydrogen chemically. [42,43] For this purpose, the corresponding materials are hydrogenated at a hydrogen surplus and dehydrogenated at hydrogen demand. For the system 18H-DBT and its corresponding dehydrogenated form 0H-DBT (dibenzyltoluene) a non-toxic compound, a hydrogen storage capacity of 6.2 wt% is reported and thus proving to be well suited. [42] In literature, perhydro dibenzyltoluene dehydrogenation reactions are carried out using e.g. Pt/AlOx catalysts such as the egg shell catalysts EleMax-102D or EleMaxD 101 from Clariant which usually are mortared or ball-milled prior to reaction. [44–46] However, also different metals (Pt, Pd, Ru) and carriers (SiO2, CeO2, C, TiO2) have been investigated as catalysts. [41,47,48] As this reaction is highly endothermic, [49] it is therefore, prone to heat and mass transport limitations when performed with commercial shaped catalysts. These could be overcome by using AM as a novel shaping technique that allows tailor-made catalyst shapes for improved flow behavior within the reactor. To the best of our knowledge, so far, no 3D printed catalyst has been investigated in the dehydrogenation reaction of perhydro dibenzyltoluene.The aim of this work was to investigate the influence of shapes manufactured by the 3D printing technique direct ink writing on catalyst carriers and their subsequent wet impregnation with a platinum precursor. The effect of tailored geometries and thus surface-to-volume ratios, as well as calcination temperatures and target loadings on the impregnation with platinum was examined in detail using BET analysis, light microscopy, TEM and μCT, providing useful insight into the impregnation process. Using the prepared catalysts, the catalytic activity for the dehydrogenation reaction in a semi-batch set-up was investigated.To produce a printable paste based on boehmite (AlOOH), 45.5 wt% Disperal 60 (Sasol Germany GmbH), 19.5 wt%. Pural SB (Sasol Germany GmbH) and 35 wt% acetic acid (pH = 3, Sigma Aldrich) were mixed twice by means of a SpeedMixer® (Hauschild GmbH & Co. KG) for 2 min at a maximum speed of 3500 rpm with 3 min cooling time after each mixing cycle. The printing itself was carried out with a self-constructed lab-scale DIW printer. For printing of 4 × 4 mm cylinders the nozzle size was 0.41 mm, whereas for printing the monolithic structures with an overall size of 23 × 4 mm and a 4 mm central hole, 0.51 mm nozzles were used. To simplify the removal of the print bed for the bigger shapes, it was covered with a thin layer of Formentrennöl C (Clariant Produkte Deutschland GmbH). The structures were dried on the print bed for 24 h at room temperature prior to thermal post-treatment.Calcination was carried out in a WiseTherm®FHP-12 (witeg Labortechnik GmbH) muffle furnace with a heating rate of 1 K/min and an 1 h isothermal step at the target temperatures 1000 °C or 1100 °C respectively.Wet impregnation of the calcined carriers was performed by adding the desired amount of platinum sulfite acid (Clariant Produkte Deutschland GmbH) to the pre-wetted shapes in bidistilled water. The cylinders were placed directly in the water whereas the monolithic structures were immersed hanging to minimize contact areas. Impregnation was carried out for 3 h before removing excessive solution. A second calcination step was carried out in a WiseTherm®FHP-12 (witeg Labortechnik GmbH) muffle furnace by heating to 60 °C at 1 K/min for 1 h, followed by further temperature increase to 120 °C at a rate of 2 K/min for 3 h. The final temperature of 400 °C was reached with a rate of 2 K/min and held for another 3 h. Consecutive reduction was carried out at 400 °C for 3 h in a tube furnace R50/250/12 (Nabertherm GmbH) under constant flow of forming gas (H2/N2 = 5 /95, Westfalen GmbH) at a heating rate of 1 K/min.Size and mechanical stability of printed and calcined shapes were measured using a MultiTest 50 (Dr. Schleuniger Pharmatron). Uniaxial compression tests vertically to the cylinder axis allowed calculation of the side crushing strength σ crush from the fracture load F, the cylinder diameter d and its height h by using the equation given by Timoshenko and Goodier [50]: (1) σ crush = 2 F π dh N2 physisorption measurements were performed on a NOVAtouch analyzer (Quantachrome Instruments) at 77 K. Prior to measurement the samples were degassed under vacuum at 120 °C for 3 h. The specific surface area SBET was calculated according to the method of Brunauer, Emmett, and Teller (BET) between p/p 0 = 0.05 and 0.3. According to the method of Barrett, Joyner, Halenda (BJH), the desorption branch of the isotherm was used to determine the pore size distribution.Powder X-Ray diffraction (XRD) measurements were performed on a PANalytical Empyrean diffractometer (Malvern) using Cu Kα radiation with a voltage of 45 kV and a monochromator. The powders were scanned in the range of 5° to 90° (2Θ) with a step size of 0.0065°. Obtained data was processed using HighScore Plus.For infrared spectroscopy (IR) of adsorbed pyridine, the catalyst carrier was pelleted into a thin wafer and heated to 450 °C with a rate of 10 K/min for 1 h activation under vacuum. After cooling down to 150 °C, the apparatus was filled with pyridine until the sample was fully saturated and equilibrated for 1 h prior to another evacuation for 1 h. Scans were taken using a Nicolet 5700 FT-IR spectrometer after activation and after outgassing.Detailed images of the catalysts were taken with a MZ8 microscope (Leica) equipped with a MicroCam II (Bresser). To determine the penetration depth of platinum into the cylinders, they were embedded in epoxy resin (EpoFix and TekMek, Struers) and later polished using a Beta Grinder Polisher (Buehler).Micro-computed tomography (μCT) measurements (v|tome|x s 240, phoenix/GE) were performed to enable non-destructive analysis of the internal structure of printed cylinders as well as their impregnation behavior. The direct tube xs 240 D was operated at 70 kVp and 60 μA. To minimize beam hardening artifacts, a 0.5 mm aluminum filter was used for all measurements. The X-ray detector was a DXR-250RT with 200 μm × 200 μm pixel size on a 1000 × 1000 pixel matrix of amorphous silicon directly coupled to a CsI scintillator. The distance of the X-ray focus to the detector was 812.0 mm and the focus to object distance was 18.8 mm for all measurements. This results in a magnification of 43.25 and an effective voxel size of the reconstructed volume of 4.62 μm. 1600 projections were taken for 360° rotation of the sample. The reconstruction was done with the software xaid (MITOS, Germany). μCT images were processed using Fiji ImageJ.Even low local concentrations of platinum in a matrix of Al2O3 can very well be visualized by X-ray computed tomography because of the much stronger photo electric absorption of platinum. Fig. 5 shows XY, XZ and YZ slices through μCT scans of impregnated cylinders at different calcination temperatures and different platinum loadings. The diffusion of platinum into a homogeneous Al2O3 matrix follows Fick's second law. The diffusion constant can be determined by fitting the attenuation constant due to the position dependent platinum concentration depending to its nearest distance to the surface of the sample. Due to the many sintered particles within the Al2O3 matrix, this distance cannot be determined by the length of a straight line between the considered voxel and the surface of the sample. Instead, we used the open source software imageJ/Fiji and its macro capabilities for this task. [51] Our macro was inspired by a similar one written by O. Burri [52] and changed for our needs. The wand tracing tool of imageJ and a suitable threshold value served for tracing the sample boarder of 2D slices of the reconstructed 3D volume of each sample. With the freehand lines tool of imageJ a line was drawn between the middle of a sample and its boarder, carefully avoiding to draw through sintered particles. With the plugin “Exact Signed Euclidean Distance Transform (3D)” the shortest distance of each pixel on the freehand line to the tracing line at the boarder was determined and saved along with the grey value of the reconstructed slice at this pixel position.These data then were fitted with Fick's second law density distribution for platinum plus a constant mean grey value for the Al2O3 matrix. Fitting was done with the module lmfit 0.9.2 in python 3.6.9. Hereby, c(x,t) is the concentration of platinum in dependence of the distance x from the outer surface and the time t. N 0 is the number of particles in an infinitesimal small area A at x,t = 0, and D the diffusion coefficient. Eq. (2), Fick's second law for a diffusion from a boarder into a semi-infinite space, can be transformed to Eq. (3) leading to the fitting parameters a and b for the diffusion and the constant background c 0. [53,54] (2) c x t = N 0 A ∙ πDt ∙ e − x 2 4 Dt + c 0 (3) c x t = a πb ∙ e − x 2 4 b + c 0 Archimedes buoyancy method by means of a Jolly balance was used to determine bulk densities ρ bulk, apparent solid densities ρ app and porosities ϕ for cylinders and monoliths according to the DIN EN623–2 standard. [55] Three different masses of each sample were determined: the mass of the dry sample m dry, the mass of the completely impregnated sample m damp, and the mass when suspended in water m suspended. With the density of water ρ water the following calculations were performed [55,56]: (4) ρ bulk = m dry m damp – m suspended ∙ ρ water (5) ρ app = m dry m dry – m suspended ∙ ρ water (6) ϕ = m damp – m dry m damp – m suspended Inductively coupled plasma optical emission spectrometry (ICP-OES) was carried out on an Aglient 700 Series ICP Optical Emission Spectrometer to determine the amount of platinum on the impregnated shapes. Therefore, the respective catalysts were grounded and dissolved in aqua regia (hydrochloric acid: nitric acid (both Sigma Aldrich) = 3:1 vol%/vol%). The samples were diluted with bidistilled water and filtered via 0.45 μm PTFE syringe filters (VWR). For preparation of the metal standards, several concentrations ranging from 1 ppm to 50 ppm were prepared using a platinum AAS standard (Sigma Aldrich). For concentration determinations the wavelength 214.42 nm was used.Metal particle size as well as dispersion were examined by means of transmission electron microscopy (TEM). Grounded samples were suspended in absolute ethanol (Sigma Aldrich), dropped on Holey Multi A grids (Quantifoil Micro Tools GmbH) and dried. The measurements were performed on a JEOL JEM 1400 plus instrument at an acceleration voltage of 120 kV. Using the software Fiji ImageJ, the metal particle diameter d Pt was determined manually by measuring at least 300 particles each. Based on d Pt the metal dispersion D Pt can be calculated as following: [57]. (7) D Pt = K ∙ V Pt S Pt ∙ d Pt with the constant K reflecting the particle shape (K = 6 for spherical particles), V Pt the volume per metal atom and S Pt the average surface area of metal particles per metal atom.Dehydrogenation test reactions were carried out semi-batch-wise in a 100 mL flask equipped with two flow breakers and a thermocouple. The required amount of the reactant perhydro dibenzyltoluene (18H-DBT; Hydrogenious LOHC Technologies GmbH) was stirred and heated up to 325 °C under argon atmosphere using a heating mantle (Winkler AG). After reaching the set temperature the catalyst (nPt/n18H-DBT = 0.175 mmolPt/mol18H-DBT) was added to the reactant and the temperature was reduced to the reaction temperature of 310 °C. The higher starting temperature was chosen to counterbalance the strong temperature drop at the start of the reaction caused by the strong endothermicity of the dehydrogenation. The mass of the catalyst with 0.9 wt% loading was only 1/3 of the catalyst's mass with a loading of 0.3 wt%, while the fine tuning of the Pt to reactant ratio was carried out adjusting the reactant amount. For full particle test reactions, the cylinders and monoliths were placed in a wire cage or on a wire (V4A stainless steel) as catalyst holder, respectively. Intrinsic test reactions with catalyst powder were carried out by adding the ground catalyst powder to the reactant. Sieving of the samples was not performed to ensure that the entire sample was used and to avoid sieving out alumina or Pt, which would then affect the concentration of active species in the reaction. The reaction was monitored for 6 h using proton nuclear magnetic resonance spectroscopy (1H NMR) in acetone‑d 6 carried out using a Bruker Ascend spectrometer at 400 MHz (300K). All spectra are referred to the solvent residual signal and chemical shifts are given in δ-values (ppm). Based on the 1H NMR data, the degree of hydrogenation (DH) was calculated according to Do et al. [35] and Preuster [58] with the ratio x of the integral of the aromatic protons (7.5–6.6 ppm) to the integral of all protons (7.5–6.6 ppm, 4.8–3.6 ppm, 2.6–2.1 ppm, and 2.0–0.4 ppm): (8) DH = 1.3945 x 6 − 4.9037 x 5 + 5.6287 x 4 − 5.207 x 3 + 4.0098 x 2 − 2.9217 x + 1 For better comparison of the dehydrogenation activity, the productivity P is defined as the ratio of the mass of hydrogen m H2 evolved per mass Pt m Pt and time [59,60]: (9) P = m H 2 m Pt ∙ t = ∆ DH ∙ m 18 H − DBT ∙ M H 2 M 18 H − DBT ∙ ν H 2 ν 18 H − DBT ∆ t ∙ m cat ∙ ω Pt Herein, Δ DH is the difference in degree of hydrogenation during the time Δ t. m cat is the catalyst mass, M i and ν i are the molar mass and the stoichiometric coefficient of the respective component i and ω Pt is the platinum loading as determined via ICP-OES. For better comparison, P should be compared at the same Δ DH, therefore it was always examined between a degree of hydrogenation of 90% and 40%. Moreover, the productivity per bulk volume V bulk of the catalyst is defined as P Vbulk: (10) P V bulk = m H 2 V bulk ∙ t = ∆ DH ∙ m 18 H − DBT ∙ M H 2 M 18 H − DBT ∙ ν H 2 ν 18 H − DBT ∆ t ∙ V bulk The bulk volume V bulk of the monoliths is calculated as the volume of a cylinder with the respective diameter and height of the monoliths used. V bulk of the cylinders is determined by multiplying the wire cage base area with the measured filling height of the cylinders.In a first step, two different catalyst shapes were printed using direct ink writing of a paste consisting of boehmites and acetic acid as cheap organic binder. [61] The first ones were cylinders with a height and diameter of 4 mm whereas the second shape was a monolithic shape sized 23 × 4 mm including a 4 mm hole in the center (Fig. 2), herein referred to as monolith. While cylinders can be fabricated by extrusion or tableting, such monoliths including lateral holes are not accessible by commercial shaping techniques. Addition of these lateral holes compared to extrusion-based monoliths improves the flow tortuosity and thus catalytic activity by improving mass and heat flow. [18] Afterwards, thermal post treatment was performed to transform the printed aluminum oxide hydroxide to alumina, which acted as final carrier material. 1000 °C and 1100 °C were chosen as calcination temperatures (T calc), as within this temperature range the phase transition from γ- to α- via θ-alumina takes place [25,30,32] causing strong changes in surface area and stability [9]. While for other test reactions like the oxidation of ethanol, formation of the more stable α-Al2O3 phases is suited, [62] pure α-Al2O3 is not considered reasonable herein as it has an extremely low surface area. It emerges around 1100 °C, thus higher calcination temperatures are not used. [25,30,32,63] The formation of the desired phases has been confirmed via powder XRD measurements of the printed shapes before and after calcination (Fig. S1).Mostly due to the drying on the print bed but also to some extent caused by calcination, the final shapes turned out slightly smaller than initially aimed (Table S1). For the cylinders using common formulas, the surface-to-volume ratio (S/V) was calculated to 1.64 mm−1 and 1.72 mm−1 for T calc = 1000 °C and T calc = 1100 °C, respectively. When calculating the surface-to-volume ratio of the monoliths, the strand diameter is the most important characteristic and leads to S/V ratios of 9.70 mm−1 for T calc = 1000 °C or 10.11 mm−1 for T calc = 1100 °C (Table 1 ). For the mathematical calculations, perfect geometrical shapes were assumed leading to small inaccuracies of the obtained values. However, these inaccuracies are considered negligible small compared to the differences between cylindrical and monolithical shapes. The distance between two strands in the monoliths has been calculated to be 0.33 mm for the monoliths calcined at 1000 °C and 0.29 mm for the monoliths calcined at 1100 °C. As the overall shrinkage was increased at higher temperatures for both, monoliths and cylinders, the shapes at lower calcination temperatures had higher surface-to-volume ratios. Additionally, the surface-to-volume ratio for monoliths was about 5.9 times higher compared to the cylinders independent of the corresponding calcination temperature.Using Archimedes buoyancy method by means of a Jolly balance the overall porosity ϕ of the shapes was examined revealing values ranging from 58.4% (cylinders, 1100 °C) to 77.9% (monoliths, 1000 °C). The higher porosity at lower calcination temperature could be explained by temperature-dependent shrinkage behavior as well as the decrease of small pores. As the weight during phase transition remained constant, higher shrinkage led to a smaller porosity. Differences between cylinders and monoliths could be explained as the porosity ϕ determined via Jolly balance is defined as the fracture of the volume of open pores to the sum of the volume of open and closed pores as well as the solid density itself. [56] The higher surface-to-volume ratio of monolithic structures increased the amount of open pores and subsequently increases the porosity.Regarding the mechanical stability, the crushing strength increased from 0.7 MPa by a factor of six when changing from 1000 °C to 1100 °C calcination temperature (Table 2 ). This is in accordance with the results from Ludwig et al. [9] Generally, at both calcination temperatures shapes with sufficient crushing strength for catalytic applications can be obtained. The crushing strength can be calculated from the pressure required for compression using literature formulas based on the geometrical shape. [50] As no such formulas are present for the respective monolithic shapes, no values for them could be determined. By means of N2 physisorption, the specific surface area S BET was determined quantifying a decrease from 55 m2g−1 to 22 m2g−1 and hereby showing a similar trend as the total pore volume with increasing the calcination temperature. Surface area and pore volume are herein considered as material characteristics and thus primarily based on the calcination temperature rather than the shape printed. Overall pore size distributions determined via BJH method (Fig. 3) showed only mesopores and a bimodal curve for both calcination temperatures with pore radii of approximately 5 nm and 20 nm. However, sintering at higher temperatures led to a decreased amount of both, smaller and bigger pores with a much more prominent decline of the smaller pores. This finding is in accordance with common literature, showing sintering of smaller pores first. [31,64,65]μCT measurements provided information about the inside of the printed and calcined cylindrical carriers (Fig. S2). XZ and the YZ slices of the shapes (as depicted in Fig. 1 ) showed a rough outer surface area caused by the layer-wise manufacturing technique independently if the uncalcined so-called green part or the calcined material were scanned. Further, all samples exhibited small cracks on the flat bottom side of the cylinder that are most likely caused by anisotropic shrinkage due to drying on the print bed at room temperature directly after printing. Some small dark spots from entrapped air could be caused either by printing inaccuracies, especially at the outer layer, or form during drying and sintering. As the white spots, that could be seen for both calcined samples, are not present in the green body, these are caused by the thermal treatment and therefore sintering of the alumina particles. In general, the amount of white sintered particles is slightly higher for the samples calcined at 1100 °C. In a next step, wet impregnation with platinum sulfite acid solution of cylinders and monoliths, both calcined 1000 °C as well as at 1100 °C, was carried out. With 0.3 wt% and 0.9 wt% there are two target platinum loadings for each of the four different carriers resulting in a set of eight different impregnated catalysts (Table 3 ).Again, surface area measurements were performed of the impregnated samples. For cylinders calcined at 1000 °C as well as 1100 °C and both target Pt loadings (0.3 wt% and 0.9 wt%), physisorption measurements were carried out after impregnation and calcination of the platinum sulfites to platinum oxides as well as after the subsequent reduction. Comparison of the specific surface area S BET as well as the pore volume V p showed that those characteristics are not changed by impregnating and reduction of the carrier (Fig. S3). Only the total pore volume decreased slightly most likely caused by the fact that platinum clusters are now partly filling the pores. When comparing the pore size distributions for carriers calcined at 1000 °C, one can see the bimodality was maintained as pores with a radius of approximately 5 nm as well as the ones with 20 nm radius decrease both, which indicates that the platinum was deposited in all pores.To determine the exact amount of platinum deposited on the catalyst carrier, ICP-OES measurements were performed. The results showed that the target loading was not achieved for any of the samples as the actual loading is lower (Table 3). The achieved loading for 0.9 wt% target loadings is lower relative to the 0.3 wt% target loadings, as a higher amount of platinum was supposed to impregnate on the same overall surface. Further, the loading for monoliths was decisively higher as about 78% to 97% of the targeted loading could be obtained whereas the loading of the cylinders ranged between 37% (1100 °C, 0.9 wt%) and 82% (1000 °C, 0.3 wt%). This can be explained by the fact, that the surface-to-volume ratio of monolithic shapes is about six times greater than for cylinders. When comparing the loading of the shapes at the calcination temperatures, it is remarkably that the relative loading was higher when calcining at 1000 °C, even though the S/V ratio was higher for shapes calcined at 1100 °C. However, not only the external surface-to-volume ratio must be taken into account but also the specific surface area S BET of the carrier itself as determined via N2 physisorption. Here, the specific surface area of shapes calcined at higher temperatures was lower and therefore explaining why only lower loadings could be achieved. These findings in general correlate with literature. [66]Transmission electron microscopy (TEM) measurements were carried out to determine the diameter d Pt of the platinum clusters on the catalyst (Fig. S4, Fig. S5). In accordance with the previous results from ICP-OES measurements showing that a higher calcination temperature and therefore a lower specific surface area resulted in lower loadings, TEM measurements revealed that the platinum particle diameter was generally higher for higher calcination temperatures. The lower specific surface area as well as a lower amount of surface hydroxyl groups as derived from IR spectra of adsorbed pyridine (Fig. S6) and in accordance to literature [64,67–70] allowed only a limited number of particles to form which consequently get bigger. Despite the fact of the specific surface area, also the S/V ratio is important for the impregnation as the monoliths show smaller mean values of the platinum cluster size than the cylinders. Interestingly, the platinum cluster diameter decreased with increased loading. One reason for that might be, that if impregnated with higher amounts of platinum, the probability of ion exchange during impregnation is higher leading to more nucleation and thus overall, slightly smaller particles. Longer impregnation times might lead to loadings close to the target loading and similarly to bigger metal particles. This effect was more prominent for the cylinder samples; however, the achieved loading for these is maximum doubled with three-fold targeted loading. However, it has to be noted that the standard deviation is relatively high, so the values have to be treated with caution. As the dispersion D Pt is inversely proportional to the metal particle diameter d Pt, it showed opposing trends ranging from 74% (cyl. 1100 °C, 0.3 wt%) to 94% (monol. 1000 °C, 0.9 wt%).When examining enlarged images of the monolithic cross section derived from light microscopy (Fig. 4), it is notable that at a calcination temperature of 1000 °C the overall surface did not seem smooth but shows dark spots that were more prominent for 0.9 wt% but could also be observed at 0.3 wt%. As the metal cluster size determined via TEM was in the same range for all the samples, it can be assumed that these dark spots were caused by the higher porosity of samples calcined at 1000 °C. One possible explanation is the higher number of small pores that were observed which might cause an optical illusion and hereby just appear to be darker. However, the presence of larger Pt particles might also be a explanation thereof. Microscopic analysis of the cylinders (Fig. S7) revealed a similar coloring like the monolithic structures.Light microscopy was used additionally to examine the impregnation and its depth into the cylinders and monolithic structures. In general, a higher target loading resulted in a deeper platinum penetration for all carriers. Due to the lower specific surface area, platinum penetrated deeper into the shape at calcination temperatures of 1100 °C. Further, the increased surface-to-volume ratio of the monolithic structures led to a decreased penetration depth of the platinum. When comparing cylinder shapes with each other, the penetration depth of cylinders calcined at 1000 °C and a Pt loading of 0.9 wt% was lower than that of those calcined at 1100 °C with 0.3 wt% loading. For monoliths however, shapes calcined at 1100 °C with 0.3 wt% loading showed a smaller penetration depth compared to those calcined at 1000 °C with 0.9 wt%. As previously discussed, two characteristics, namely the surface-to-volume ratio and the specific surface area influence the impregnation with platinum and lead to opposing trends for these sets. Apparently, the high surface-to-volume ratio (compared to the platinum loading) of the monolithic structures seems to dominate penetration over the specific surface area. For the cylinder on the other hand, the surface-to-volume ratio for both calcination temperatures are so small that the decreased specific surface seems to be predominant at elevated calcination temperatures. According to literature, the layer thickness of the active material in egg-shell catalysts for the dehydrogenation reaction of perhydro dibenzyltoluene should not exceed 90 μm in order to prevent mass transport limitations. [59] This requirement was only fulfilled for monolithic structures calcined at 1000 °C with a target loading of 0.3 wt% (50 μm penetration depth) and for monolithic structures calcined at 1100 °C with a target loading of 0.3 wt% (90 μm penetration depth) (Table 3, Fig. 4).To gain a deeper understanding of the platinum penetration depth into the catalyst carrier on a macroscopical scale, μCT imaging analysis and fitting of the platinum solution diffusion and the concentration decrease into the carrier has been performed (Fig. 5). Due to the considerably higher atomic number of platinum compared to aluminum, the X-ray absorption of Pt is much higher and thus impregnated areas are depicted brighter than unimpregnated centers of the cylinders. μCT images showed that a higher calcination temperature and a higher loading increased the penetration depth, which is in accordance with the overall trends that were observed via light microscopy. Proving a homogeneous impregnation, one can see that the impregnation occurred on all outer surface areas of the cylinders. Still, the penetration did not only occur lateral but also diagonal, explaining why the rough outer surface area could not be seen as of the boundary layer within the cylinder caused by the platinum impregnation. However, the optical determination of the impregnation depth was in general more difficult for carriers calcined at 1100 °C, due to a reduced overall contrast as the concentration of platinum is more widely spread. Further, no platinum could be observed along the cracks at the bottom side of each cylinder. One explanation for this is that as the catalyst was pre-wetted prior to impregnation, capillary forces do not play an important role but only diffusion processes influence the impregnation. [33] However, it is also possible, that the capillary forces in general were not strong enough to completely fill the cracks with impregnation solution at all. [33]The penetration behavior itself was further investigated by means of a grey scale analysis of the μCT scans. Carefully avoiding the white sintered particles, a line was drawn from the center to the edge of the cylinder. The resulting grey value of each pixel as well as its shortest distance to the edge of the cylinder was recorded. These values were then fitted according to the Fick's second law for a semi-infinite cylinder (2) (Fig. S8) and for better comparison only the stacked and fitted grey values are visualized (Fig. 6). Regarding the courses of the grey value, it is obvious that cylinders with equal calcination temperatures show similar trends to each other. In accordance with the microscopic images, one can clearly see that the amount of platinum at the outer surface of the cylinders calcined at 1000 °C is higher but decreases rapidly. On the other hand, for shapes calcined at 1100 °C the overall course is flatter but the penetration deeper. This higher course at lower calcination temperatures can be explained as similar or even higher amounts of platinum are impregnated on a smaller area resulting in higher loadings. As described previously, these differences in penetration depth and loading are caused by the varying amount of surface hydroxyl groups allowing a higher platinum loading. Additionally, μCT scans show that the penetration depth increases with higher target loadings. In general, the penetration depth obtained via light microscopy approximately corresponds to a platinum density of one tenth of the value at the surface of the cylinder as measured via μCT (Fig. S9). This shows that μCT analysis as non-destructive technique is a useful tool to assess impregnation behavior and penetration depth of the catalytically active species, allowing detailed understanding of the impregnation for the preparation of highly active catalysts.Catalytic test reactions completed the evaluation of the influence of catalyst shape, calcination temperature and Pt impregnation. Therefore, the catalysts were tested for the dehydrogenation of 18H-DBT (Fig. 7). As platinum is the active component, the mass of catalyst was adjusted to the reactant mass, so that a constant Pt to 18H-DBT ratio was maintained throughout all reactions.To exclude diffusion limitations, intrinsic test reactions were executed with powdered catalysts first (Fig. 8a). These revealed that almost all catalyst samples showed the same dehydrogenation activity independently on catalyst shape, calcination temperature and Pt loading. The productivities of all catalysts are very similar and range from 6.6 gH2·gPt −1·min−1 to 8.9 gH2·gPt −1·min−1 (Table 4 ). Small differences were most likely caused by inaccuracies of the 1H NMR examination. After a reaction time of 1 h the degree of hydrogenation had already decreased from 98% to approximately 25%. A minimum degree of hydrogenation at around 7% seemed to be reached with all samples after 3.5 h. No further decline in the degree of hydrogenation was observed when extending the reaction time. There were only two curves that are not perfectly in line to other reactions, namely the monoliths calcined at 1000 °C and 1100 °C with a target loading of 0.3 wt%. Especially between 0.5 h and 3 h they showed a slightly better dehydrogenation activity. This results in higher productivities with values of 8.9 gH2·gPt −1·min−1 and 8.3 gH2·gPt −1·min−1 for 1000 °C and 1100 °C, respectively. One possible explanation is that the overall distribution of platinum on those shapes is the best, as the monolithic structures have the highest S/V ratios and a target loading of 0.3 wt% is rather low.When executing full particle test reactions, the cylinder catalysts were put in a stainless-steel wire basket whereas the monoliths were placed on a stainless-steel wire. After reaching the set temperature, the catalyst was lowered into the reaction solution and fixated, starting the reaction. The overall courses in Fig. 8 clearly show that the dehydrogenation activity of the powdered catalyst is higher than the respective activity in the full particle reactions. This can be confirmed as the productivities for the full particle test range from 1.3 gH2·gPt −1·min−1 to 4.0 gH2·gPt −1·min−1. The reactions with full particles consequently exhibit only between 16% (cyl. 1100 °C, 0.9 wt%) to 48% (monol. 1100 °C, 0.3 wt%) of the respective powdery productivities (Table 4). A degree of dehydrogenation of 25% required reaction times of approximately 3 to 4 h or even up to 6 h (cyl. 1000 °C and 1100 °C, 0.9 wt%). The enhanced performance of catalyst powder can be attributed to diffusion limitations of full particle catalysts. Throughout the whole reaction all printed shapes remain intact due to their sufficiently high crushing strength enabling easy separation of the catalyst and the reaction mixture afterwards.Comparing the activities of either the cylinders or the monolithic structures in full particle reactions to each other (Fig. 8c and Fig. 8d) it is remarkable that the performance and productivity of catalysts calcined at 1100 °C is higher than of those calcined at 1000 °C (Table 4). This effect could be observed even though the penetration depth of the platinum is higher at 1100 °C and one would expect that this hinders the diffusion leading to slower reaction speeds. However, another aspect regarding the activity is coming into play here as 18H-DBT is a relatively large molecule and hence a minimum pore diameter of around 26 nm is beneficial for the reaction, making the bigger pores more important. [59,60] As discussed previously, the pore size distributions of the two carriers (Fig. 3) differed as the number of pores with radii of 5 nm was significantly higher when calcining at 1000 °C. As a homogeneous distribution of platinum over all pores is expected, it is likely that there was some platinum present in the smaller pores and thus inaccessible for the reactant. This led to more inaccessible platinum which lowers the overall activity. Yet, when grounding the catalysts for intrinsic reactions this diffusion limitation to smaller pores seemed to be no longer prominent. Nevertheless, the trend that the calcination temperature of 1100 °C with otherwise identical parameters led to a higher activity is not applicable for the cylinders with a loading of 0.9 wt%. These showed a similar but also the lowest activity of all samples examined as well as a productivity of only 1.3 gH2·gPt −1·min−1 regardless of the calcination temperature. Even after a reaction time of 6 h the dehydrogenation was lower than for all other systems and the degree of hydrogenation achieved was at around 35%. Compared to the other catalysts, the general uptake of platinum during impregnation was the lowest with values of 56 wt% (cyl. 1000 °C, 0.9 wt%) and even lower 37 wt% (cyl. 1100 °C, 0.9 wt%). This leads to the assumption that even though the platinum particle size is comparable to the other samples, the general accessibility of platinum is hindered due to lower S/V ratios and high target loadings.For all further examinations, it is important to keep in mind that the bulk volume during the reaction was as identical as possible for cylinders and monoliths when comparing same target loadings. For 0.3 wt% V bulk was in average 3.3 ± 0.2 cm3 whereas for 0.9 wt% it was 1.4 ± 0.4 cm3. Due to higher measurement inaccuracies for cylinders with a 0.9 wt% target Pt loading as well as the relatively lower loadings compared to the target loadings, the standard deviation for the latter one is high, and values should be regarded with caution. Soley for monoliths with a target loading of 0.9 wt%, V bulk can be calculated to 1.0 ± 0.1 cm3. On average, the bulk volume for the monoliths with a loading of 0.9 wt% was only 32% compared to the respective lower loaded cylinders. In general, at the same calcination temperature and target loadings the monolithic structures showed higher activities and productivities than the cylinders. For this trend several factors came into play. As the S/V ratio of monoliths was about six times as high as for cylinders, the same amount of platinum was spread over a larger surface. In consequence, the overall penetration depth was lower and platinum particle diameters were smaller. These factors are important for the reaction, as they influence the accessibility of the platinum for the reactant molecules. Still, it cannot be distinguished how important which of the effects is because of the overlap of these effects. Beside the differences in platinum distribution, it is also likely that the overall fluid dynamics within the semi-batch reactor must be considered. The monoliths with several small channels most likely enabled better mass and heat flow through the catalyst bed than the cylinder bed and hereby also influenced the catalytic reaction beneficially.Lastly, the influence of the different loadings at similar shapes and calcination temperatures was examined. For all tested full particle reactions, the catalysts with a target loading of 0.9 wt% showed a reduced activity and lower productivities compared to those with 0.3 wt%. On first sight, this seems counterintuitive, but all reactions were carried out with the same platinum to reactant ratio. Therefore, an increased loading resulted in a decreased mass of catalyst in the reaction and therefore a reduced bulk volume. Another factor to be considered is that higher loadings also resulted in deeper penetration of the platinum and therefore were likely to increase reactant diffusion limitation to the active platinum center leading to lower activities. This effect could not be balanced by the fact that the platinum particle sizes are smaller for higher loadings as discussed previously. In general, the best activity and productivity within the full particle tests were observed for the monolith 1100 °C, 0.3 wt% with a value of 4.0 gH2·gPt −1·min−1 followed by the monolith with the same loading at the lower calcination temperature (3.5 gH2·gPt −1·min−1).Interestingly, the comparison of monolithic structures with a loading of 0.9 wt% to cylinders with a loading of 0.3 wt% (Fig. 8b) at same calcination temperatures revealed similar Pt-based productivities. However, a significant difference within this set of catalysts was the bulk volume since for similar platinum to reactant ratios the catalyst mass was reduced for the approximately three-fold increased loading. Hereby, the monoliths had only about 32% of the bulk volume compared to the respective cylinders. Only slightly better Pt-based productivities of cylinders than the respective monolithic structures were achieved (Table 4). The differences in bulk volume changed heat and mass transport within the semi-batch reactor and led to the minimally decreased activities compared to the respective cylinders, even though the platinum particle size or the penetration depth present at the monolithic samples would lead to the assumption of higher activities.The productivity in respect to the bulk volume P Vbulk it is in general higher for monoliths than for cylinders. The values for the cylinders range from 0.0029 gH2·gPt −1·min−1·cm−3 (cyl. 1100 °C, 0.9 wt%) to 0.0050 gH2·gPt −1·min−1·cm−3 (cyl. 1000 °C, 0.3 wt%). Due to the overall low activity of the cylinders with higher loading, P Vbulk for them is slightly smaller than for the ones with 0.3 wt% loading. However, when comparing the monolithic samples regarding their P Vbulk, a different trend can be observed. The higher loaded monoliths exhibit an approximately two-fold higher volume-based productivity. These contrary trends can be attributed to the differences in surface-to-volume ratio of the monoliths compared to the cylinders and the resulting differences in wet impregnation. When comparing P Vbulk for the cylinders 1000 °C and 1100 °C at 0.3 wt% (0.0050 gH2·gPt −1·min−1·cm−3 and 0.0044 gH2·gPt −1·min−1·cm−3) and the respective monolith values of 0.0139 gH2·gPt −1·min−1·cm−3 and 0.0147 gH2·gPt −1·min−1·cm−3 which exhibit similar Pt based productivities, the three fold average higher P Vbulk for the monoliths becomes apparent. This shows that by means of increased surface-to-volume ratios the activity and productivities P and P Vbulk of a catalyst can be easily increased just by improving the impregnation behavior. Hereby, AM and especially DIW are suited techniques to obtain these varied and more complex shapes with increased surface-to-volume ratios. Furthermore, advanced shape optimization is likely to increase the flow behavior, leading to even improved catalytic activities.Herein, alumina catalyst carriers were fabricated by direct ink writing, namely cylinders and monolithic structures. These shape variations and respective changes in external surface area as well as two calcination temperatures (1000 °C and 1100 °C) resulting in different specific surface areas influenced the wet impregnation behavior and consequently the activity of full particle catalysts in the dehydrogenation of 18H-DBT. The prepared shapes have in been analyzed in depth using a combination of various techniques, including BET, light microscopy, TEM and μCT. By evaluating the catalysts, the use of μCT as an advanced analysis technique offers unique advantages for the preparation of 3D printed, heterogeneous catalysts. Using these techniques, some conclusions can be drawn, which provide useful guidance for the impregnation of catalyst carriers. In general, a higher surface-to-volume ratio of the carrier resulted in a higher loading relative to the target loading as well as in smaller platinum particles and lower penetration depth. Similar trends could be observed at lower calcination temperatures and therefore higher specific surface areas as well. Higher targeted loadings on the other side cause a decrease of the platinum loading as well as the average particle size but increase the penetration depth. These deductions can be used for the preparation of well-defined catalysts by means of impregnation with an active species. Especially when focusing on very complex structures accessible via 3D printing, such findings help understanding the preparation processes, enabling tailor-made impregnation of such advanced structures.Resulting from the intrinsic and full particle catalytic dehydrogenation test reactions one can conclude that fabrication of catalysts by direct ink writing is beneficial for the catalytic reaction in general. As a higher exposed surface of the catalyst is beneficial for the catalytic performance, variation of the geometries by DIW or even printing e.g. a continuous flow reactor itself might influence our catalytic activity even further. [20,62] Overall, higher calcination temperatures were beneficial for the reaction as the pore size distribution of the carrier is enhanced hereby. This seemed to be more important than a higher penetration depth or larger platinum particle sizes. When impregnating monoliths and cylinders with the same target loading and working with similar reactor volumes, the monolithic structures showed significantly higher activities for dehydrogenation of perhydro dibenzyltoluene. Further, when aiming for similar activities with monolithic structures one can either reduce the platinum loading keeping the reactor volume constant or keep the same Pt amount but reduce the reactor volume. Both is beneficial, as it either requires a reduced amount of the very expensive noble metal platinum or decreased reactor sizes and therefore decreased operating costs. This reveals the potential that additive manufacturing and especially direct ink writing show when fabricating catalyst carriers.Based on the requirements of your journal, we want to hereby list a detailed breakdown of all authors contributions:Paula F. Großmann: conceptualization, writing - original draft, data curation, formal analysis, investigation, visualization, writing - review & editing.Markus Tonigold: conceptualization, formal analysis, writing - review & editing, resources.Norman Szesni: formal analysis, visualization, writing - review & editing.Richard W. Fischer: conceptualization, formal analysis, writing - review & editing, project administration.Alexander Seidel: formal analysis, visualization, writing - review & editing, resources.Klaus Achterhold: formal analysis, visualization, data curation, writing - review & editing.Franz Pfeiffer: supervision, writing - review & editing, project administration.Bernhard Rieger: supervision, project administration, funding acquisition, resourcing, writing - review & editing.The financial support of the Bayerische Forschungsstiftung (BFS) is gratefully acknowledged. K. Achterhold and F. Pfeiffer acknowledge financial support through the DFG Cluster of Excellence Munich-Centre for Advanced Photonics (MAP, DFG EXC-158), the DFG Gottfried Wilhelm Leibniz Program and the Center for Advanced Laser Applications (CALA).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.P. F. Großmann would like to thank Max Koch for carrying out the ICP-OES measurements and Dr. Carsten Peters and Roland Weindl for their help when carrying out TEM measurements. Furthermore, P. F. Großmann would like to thank Larissa Sommer, Marlene Viertler, Jan Meyer, Stefanie Pongratz and Mira Eggl for their help during printing and in carrying out various measurements and Moritz Kränzlein for his scientific input. Special thanks to the MuniCat team and especially Hanh My Bui for the fruitful discussions. Supplementary material Image 1 Detailed dimensions and densities for printed shapes, physisorption results of the impregnated cylinders, metal particle distributions as examined via TEM analysis, microscopic images of impregnated cylinders. Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2023.106610.

Direct ink writing as additive manufacturing technique was used to print two different boehmite based shapes, cylinders and monoliths, serving as catalyst carriers. These were wet impregnated targeting 0.3–0.9 wt% platinum loadings. ICP-OES, μCT and microscopy revealed dependencies from calcination temperature, geometry and platinum loading. Dehydrogenation reactions of perhydro dibenzyltoluene as liquid organic hydrogen carrier were performed examining the catalytic performance. Differences when executing full particle measurements led to the conclusion that direct ink writing as shaping technique for catalyst carriers and the respective impregnation is highly beneficial as more complex shapes can be obtained, resulting in higher activities.

Data will be made available on request.Zeolite is a microporous material comprising of aluminosilicates that can be discovered naturally in nature [1–3] or synthesized [4–7], with superior properties such as large surface area, molecular shape selectivity, and high thermal and chemical stability [8–11]. These properties immensely satisfy catalyst criteria. Thereby, zeolite becomes one of the most widely used heterogeneous catalysts in various applications, especially in the petrochemical industry [12–15]. However, the restricted access of the small micropore could limit the reactant and product diffusion, leading to slower catalytic reaction and accelerating the pore blocking formation [16–21]. Furthermore, it also hinders the use of bulkier reactants. Thus, the catalytic process is not appropriately utilized [22–24].A large number of studies have been focused on overcoming the shortcoming of micropore zeolite catalysts [25–27]. The strategy is to modify the zeolite structure by introducing a hierarchical structure that can be synthesized through a bottom-up or top-down process [17,28–30]. Hierarchical zeolite is defined as a zeolite with at least has two pore types. Generally, the second pore is mesopore type (2–50 nm) [31–33]. Another strategy is to extremely reduce the size of the zeolite crystals into nanosized. However, the synthesis of nanosized zeolite faces serious issues, i.e., low yield and difficulty in separation [34].Zeolites with two-dimensional structures (nanosheet) were intensively developed and demonstrated remarkable catalytic activity relative to conventional zeolite. The term of two-dimensional or nanosheet zeolite could be used if one of the dimensions of zeolite crystal is significantly less than several nanometers (about one- or two-unit cells). Typically, it contains a remarkable accessible external surface per mass/volume unit compared to the 3D one. Their open structure, as well as surface-exposed active sites, could address the mass transport limitation issues that occurred in zeolite catalysis [35,36]. Furthermore, it also has a high pore volume enabling bulk molecules to undergo catalytic reaction and shortening the diffusion pathway. Hence, the catalytic lifetime could be more prolonged [8–10]. Besides, this 2D zeolite also showed high thermal stability and strong acidity properties, which are necessary for the catalytic reaction [32].The enhanced catalytic performance in terms of activity, selectivity, and stability of nanosheet zeolite has been reported for several reactions, e.g., the benzylation of toluene [37], methanol to propylene [38], isomerization [39], and pyrolysis of biomass reaction [40]. Furthermore, for some particular reactions, the metal sometimes should be introduced in the zeolite system to supply suitable acidic strength. Thus, the reaction could be directed to generate the desired product. In this case, the nanosheet structure provides the exceptional surface area in which the metal could be well-distributed on the zeolite surface, enhancing the catalytic performance. For instance, W-substituted nanosheet zeolite shows a lower coke deposition and higher selectivity to propylene product than that of the conventional W-substituted zeolite [38]. In another experiment reported by Liu et al. [41] nanosheet zeolite could generate strong interaction with Pd nanoparticles, then restraining the Pd leaching. Moreover, it also promotes a better Pd dispersion and higher Brønsted acid sites than the conventional zeolite. In result, the higher formation rate of H2O2 product (1.9 times) and longer catalytic stability (almost 3 times) than that of the Pd-conventional zeolite are observed.From the catalyst design point of view, several synthesis strategies have been documented. For instance, Ji et al. [42] reported the successful synthesis of MFI nanosheet zeolite by employing bifunctional organic surfactant resulted in higher resistance to coke formation. Tian et al. [43] reported that pillaring by using seed-assisted secondary growth could alleviate the negative effect of calcination resulting in nanosheet zeolite with high connected mesopore which is highly selective to light olefins and minimizes the aromatic byproduct [43]. Other than those strategies, ADOR (Assembly-Disassembly-Organized-Reorganized) method was also mentioned as a potential strategy to achieve the nanosheet structure from the existing zeolite structure [44,45].From those significant progresses that have been reached regarding the nanosheet zeolite, several reviews have been published, in which it covers the discussion of 2D zeolite using different frames of reference [46,47]. Most of the publications are well-written by Roth and his group. Particularly, they have summarized and classified both the synthesis procedure and their catalytic opportunities, i.e., petrochemistry, oxidation reaction, fine chemical synthesis, and organometallic [48–50]. Very recently, wang et al. [51] also published the nanosheet zeolite for catalysis, which especially focused on the zeolite that has been synthesized from a sustainable approach. In this review, we present a comprehensive overview of the recent development of nanosheet zeolites comprising i) synthesis strategies, ii) physicochemical properties, and iii) their catalytic applications. Particularly, the relationship between the characteristic of the catalyst and the catalytic performance will be highlighted. This review also contains a broader catalytic application, e.g., the conversion of biomass and photocatalysis, in which it is essential to direct the development of the energy future.Synthesis of nanosheet zeolite has been widely reported, especially zeolite with MFI and MWW framework types. Nanosheet zeolites with other types of frameworks have also been successfully synthesized, such as FAU [37,52] and MOR [53]. Mainly, the synthesis of 2D or nanosheet zeolite could be obtained by the three different synthesis procedure, i.e., (i) the hydrothermal synthesis, in which the zeolite form is as a layered precursor that produce 3D zeolite framework upon the calcination treatment. So, the layered precursor zeolite must undergo the subsequent post-treatment for obtaining the 2D structure, e.g., pillaring, delamination. (ii) Surfactant-assisted synthesis, for example, the MFI nanosheet structure. (iii) the transformation from the 3D structure of the zeolite. It is also so-called the ADOR process, in which nanosheet zeolite was generated from existing zeolite [44,45]. The two former methods could be classified as bottom-up method. Meanwhile, the latter is the bottom-up method. Other methods, including seed-induced method and chemical etching, are also elaborated.Basically, the quaternary ammonium surfactant comprised a long chain alkyl group as a tail, and the quaternary ammonium groups as a head. The latter act as a structure-directing agent for zeolite framework formation. Meanwhile, the former control the growth of the zeolite crystal, leading to the formation of nanosheet structure [45,54]. This concept was introduced by Ryoo and Co-workers [54]. They succeeded in synthesizing multilamellar MFI nanosheet zeolite with a thickness of 2 nm using bifunctional diquaternary ammonium-type surfactant C22H45–N1(CH3)2–C6H12–N1(CH3)2–C6H13 in the bromide form (C22-6-6Br2) under crystallization temperature of 150 °C for five days. The diammonium head group directed the formation of MFI zeolite, while the hydrophobic tail induces the formation of the micelle, then created ultrathin nanosheet by inhibiting the excessive crystal growth along the b-axis as already mentioned previously in subsection 2.1. Unilamellar MFI nanosheet was also obtained by employing surfactant C22-6-6(OH)2 and reducing the Na + concentration in the synthesis mixture with a more prolonged hydrothermal time (11 days). Na+ concentration was assumed to be a crucial factor during the crystal growth in a–c axis and layer stacking along the b-axis, producing different forms of MFI nanosheet.Furthermore, Na et al. [55] reported that silica sources and ethanol strongly affect the formation of nanosheet zeolites. The presence of a small amount of ethanol could increase the surfactant movement accelerating crystal growth. Zou et al. [33] prepared a ZSM-5 nanosheet using [C18H37–N+(CH3)2–C6H12–N+(CH3)2–C6H13]Br2 (denoted as C18-6-6Br2) as a template with a similar procedure reported by Choi et al. [54]. In this case, the Na-ZSM-5 nanosheets were then ion-exchanged with Cu cations resulting in a Cu-ZSM-5 nanosheet with a large surface area (410 m2/g) and Vmeso (0.45 cm3/g) compared to conventional zeolite (381 m2/g and 0.12 cm3/g). Zhang et al. [32] also synthesized MFI nanosheet with a slightly modified procedure [54] by optimizing the amount of ethanol. The obtained MFI nanosheets showed a highly enhanced specific surface area (366–665 m2g-1) compared to the conventional MFI zeolite (293 m2g-1). Accordingly, these MFI nanosheets exhibited a greater number of micropores, mesopores, and macropores than the conventional MFI zeolite [32]. Ethanol can serve as a cosolvent, promoting the crystal growth process of nanosheet zeolites by adjusting the surfactant micelle structure. Moreover, a larger volume of ethanol can produce a disordered structure of nanosheets [55].Wei et al. [15] proposed the mechanism of self-interlocked ordered nanosheet stack (SI-ONS) in C22-6-6Br2 system with variation of anions and cations of precursors as described in Fig. 1 ; the mechanism consists of 4 steps; specifically, i) formation of micelles from the surfactant, ii) formation of silicate micelles where Br− ions are replaced by silicate ions, iii) aggregation of silicate micelles through condensation due to lower silicate surface charge, and iv) the silicate micelle aggregates turn into a solid mesophase then the transformation and crystallization occurs simultaneously. In addition to that, Wei et al. [15] also explored the effect of precursor cations and anions on the textural and acidity properties of self-interlocked ordered nanosheet stack. The result showed that variation of cations and anions influence the formation of inter-crystalline mesopore by cation order of Na+> K+> Rb+> Cs + and anion order SO4 2−> NO3 −> Cl−. Besides, XRD analysis showed improvement of crystallinity with the increase of cation size. Based on the NH3-TPD result, anions and cations of precursors showed negligible effect on the number and strength of strong acid sites in the resulting 2D MFI products.Despite the use of diquaternary ammonium surfactant, the monoquaternary has also been reported to synthesize the zeolite nanosheet by Xu et al. [56]. A single-crystalline mesostructured MFI zeolite nanosheets (SCZN) was produced by using surfactant-containing aromatic group, i.e., C6H5–C6H4–O-(CH2)m-N+(CH3)2C6H13 (Br-) coded as CPh-Ph-m-6. The variation of carbon chain length (m) from 6 to 10 results in the various interlamellar d-spacing of 1.7–2.1 nm. Biphenyl groups can interact with each other through π-π interaction, stabilizing lamellar micelles and aligning the MFI nanosheet structure. Therefore, π-π interaction and is considered an important function in preserving the shape of the nanosheet [57].However, the limitation of using bifunctional structure-directing agents (SDAs) is the reduction of ordered mesopores during the calcination process. An alternative strategy to hinder the destruction of MFI zeolite layers is by introducing pillars in the interlayer space to support the ordered mesoporous structure in the zeolite. In 2010, Ryoo and co-workers [55] reported a method to preserve the ordered multilamellar MFI nanosheet zeolites by intercalating silica pillars in the interlayer spacing of the MFI nanosheet.Despite of the MFI, the use of the quaternary ammonium surfactant was also reported for SAPO-34 nanosheet synthesis. In the study reported by Chen et al. The nanosheet morphology was assembled as the results from the existence of [3-(trimethoxysilyl)propyl] octadecyl dimethyl-ammonium chloride (TPOAC) as mesoscopic aggregation. Meanwhile, the formation of CHA structure was assisted by tetraethylammonium hydroxide. Notably, the surfactant functionedas a stabilizer of the energy surface of the morphology. The amount of TPOAC modulates the formation of the unique aggregation morphology. Furthermore, the hydrothermal synthesis condition are also strongly related to the crystal formation [58].The structure of mesopores is possibly destroyed at high-temperature forming new Si–O–Si bonds between adjacent nanosheets. The formed 2D crystalline framework leads to a partial loss of uniform mesopores and surface area by interlayer condensation. Alternatively, the pillaring method evidently reduces the calcination effect issue resulting in more preserved lamellar nanosheets after high temperatures treatment [26,55,59].Na et al. [55] introduced a nanosheet zeolite pillar through hydrolysis by adding TEOS (tetraethyl orthosilicate) into the non-calcined as-synthesized nanosheet zeolite and transforming it into silica pillars. As shown in Fig. 2 a, the surfactants form spherical micelles with a hexagonal phase which then transform into lamellar mesophase; with the process involved, the head of the two SDA facing each other will enter in a straight channel on the crystal axis-b. The resulting pillared nanosheet zeolites showed a high surface area (615 m2/g) with a large pore volume (0.44 cm3/g) as a result of the maintenance of a regular multilamellar structure during SDA removal. Apparently, the introduction of silica during the pillaring process covered the Al sites; thus, pillared nanosheet zeolite showed reduced external acid sites compared to non-pillared one. However, the reduction in external acid sites by pillaring is highly tolerable as it still showed greater external acid sites than the conventional MFI.On the other hand, a large molecule can reach zeolite micropores from any direction. Moreover, the MFI framework remains 3D pore connectivity along the crystal axis and affords a high molecular diffusion efficiency. Besides, the interlayer spacing can be easily adjusted by surfactant tail length. Another necessary aspect of this pillared zeolite is the mesopore surface, described as the nanosheet zeolite external surface, terminated with a great number of silanol (-Si-OH) groups. Thereby, the surfaces can be functionalized through a silylation process to change surface features such as hydrophilic or hydrophobic properties [55].Using the same pillaring precursor TEOS, Ali et al. [59] also prepared a self-pillared MFI nanosheet. The procedure of the synthesis process is depicted in Fig. 2b. Based on XRD measurement, the self-pillared nanosheets exhibited a very sharp peak at (501). Thus, the characteristic peak of the pillared nanosheet shows the same peak as the nanosheet that appears in the h0l plane. This occurrence is because the crystals grow in the a-c-plane direction. In addition, the characterization using high-angle annular dark-field (HAADF) STEM result exhibited that the nanosheet zeolite has a wide a-c plane with a mesospace-interlayer and thickness of ∼2 nm. Compared with conventional zeolite MFI, the unique structure of the nanosheet self-MFI nanosheet pillar showed enhanced catalytic activity, selectivity, and hydrothermal stabilityTian et al. [26] reported comparison results of pillared HZSM-5 nanosheets prepared by two different pillaring methods, dual-template and Si intercalation, by using TPAOH and TEOS as pillaring precursors, respectively. Both pillaring methods efficiently protect the mesopores of HZSM-5 nanosheets from collapse during the calcination. However, the dual-template method showed superior channel connection and more preserved acidity.Chang et al. introduced a new type of triblock SDA (N3-POn-N3) with linker tripropylene oxide (PO3) for direct synthesis of highly-branched and pillared MFI nanosheet. The branching of the MFI nanosheet was a result of the short PO6 linker of N3–PO6–N3 that limited the movement of the SDA. Accordingly, the degree of branching (density of orthogonal stacking of bundles of nanosheets) can be adjusted by the length of the POn blocks of the SDAs [60]. The use of longer linker N3–PO68–N3 led to a reduction of branching and partially generated random assembling of the stackings of nanosheet. Furthermore, adjusting the linker length of triblock SDA can control the textural properties of nanosheets, including the degrees of branching and the distance between adjacent nanosheets [61].In addition, pillaring of zeolite nanosheet to produce more open structure could be carried out through vapor phase pillarization (VPP) method as reported by Wei et al. [62]. In their work, they integrate the three typical processes in zeolite pillarization, i.e., intercalation, hydrolysis, and calcination, into single operation using only one apparatus in which the quartz “U”-shaped tube was placed in a furnace. The liquid alkoxide as a pillaring precursor was added dropwise into Teflon cup and evaporated for intercalation. The hydrolysis of the intercalated alkoxides was also performed in the same set-up. Meanwhile, the 2D zeolite was placed in a separated glass tube container. The result showed that this method required less alkoxide about ten times than the typical method. Furthermore, almost 100% efficiency and TEOS utilization for the formation of pillared zeolite were obtained with the reduction of liquid waste. As inert spacers, the use of SiO2 as pillars is associated to the partial blockage of the active site which present between the external lamellar surface. In this case, Schwanke et al. [63] has reported that the addition of active species, i.e., Niobium Oxide as a mixed pillars could improve the catalytic activity of a pillared lamellar MWW zeolites.Other than those strategies of pillaring zeolite, the epitaxially growing the layered zeolite on the bulk one has also been reported. In this case, the bulk zeolite act as support to avoid the condensation and disordering the interlayers during the calcination process. Furthermore, it also provides the shape selectivity properties and the acid site, which are very important for the reactions. Therefore, this hybrid material possesses a good catalytic performance on the methanol to propylene reaction [64,65]. However, despite pillaring effectively preserve the mesopore structure of nanosheet zeolite during the SDA removal, the silica pillars quickly disintegrated under ambient conditions. Besides, the synthesis process is complicated and time-consuming. Thus, the synthesis method delimits its practical application.Zeolite MFI nanosheet has been successfully synthesized by Jeon et al. [66] through a bottom-up process using MFI seeds with a diameter of 30 nm. In this process, nanosheet zeolite is formed through single rotational growth after the seeds reached a specific size and shape [66]. The schematic of the hierarchical ZSM-5 (Hi-ZSM-5) nanosheets can be seen in Fig. 3As shown in Fig. 3, Liu et al. [67] reported synthesizing the nanosheet zeolite using the seed method assisted hexadecyl trimethyl ammonium bromide (CTAB) as the second template. Concerning the first step, the ZSM-5 seed was destroyed to form sub-nanocrystals following the aging process. The distinct approaches for nanosheet zeolite synthesis processes included adding CTAB into the mixture to produce hierarchical ZSM-5 (Hi-ZSM-5) at high temperatures. Zeolite Hi-ZSM-5 exhibits a particle size of 2–4 mm with a honeycomb-like morphology.Shang et al. [68] prepared nanosheet zeolite by applying ZSM-5 seed in the presence of di-quaternary ammonium surfactant (C18-6-6Br2). The seed-assisted strategy provides a positive result in decreasing the crystallization time and the particle size. For instance, the crystallization times were reduced linearly from 120 h to 24 has the seed content increased from 0 to 30 wt%. Meanwhile, increasing the ZSM-5 seed content also led to a reduction in particle size. Besides, the acidity and textural properties of nanosheet zeolites are easily tuned through this method (by variation of number seed)Xing et al. [69] synthesized nanosheet zeolite through seed-induced with the addition of urea as a crystal growth inhibitor. The seed crystals promote the formation of nuclei to form small particle sizes. Different types of seeds, spherical and sheets-like ZSM-5, contributed to alterations in acid properties, pore structure, and defect sites of the nanosheet zeolites. The use of ZSM-5 sheet-like seed produced nanosheet zeolite with slightly higher surface area (437 m2/g) and total acidity 123 μmol/g compared to that synthesized from spherical ZSM-5 seed (416 m2/g, 114 μmol/g).Chemical etching is one of the top-down methods for the synthesis of nanosheets. Prech et al. [70] synthesized layered zeolite by using fluoride etching (NH4F). The concentration and strength of the acid sites and textural properties of the layered zeolite can be altered by fluoride etching. The external surface area of the as-synthesized layered zeolite increased by hardness treatment (from 86 m2/g became 92 m2/g after 30 min). In contrast, the external surface area of the calcined samples did not change with both mild and hard treatment. The fluoride treatment can remove all species that are not an integral part of the zeolite structure. Consequently, the materials treated after calcination restrain the Al framework. The addition of CTAB surfactant can increase the Lewis acid site concentration in proportion to the amount of extra Al frameworkBesides, Zhou et al. [71] introduced chemical etching using alkaline media, i.e., TPAOH, to transform the bulk microcrystal 3D ZSM-5 zeolites into hierarchical nanosheet zeolites (Fig. 4 ). Typically, a bulk zeolite crystal was treated with TPAOH solution, sonicated, and finally conducted in a hydrothermal treatment at 170 C for 18 h under a rotational convection oven. During this hydrothermal treatment, the dissolution-recrystallization mechanism may occur as indicated by declining crystallinity. As a result, a hierarchical zeolite crystal with a hollow structure was observed. Furthermore, the dissolved silicon and aluminum species may diffuse toward the external surface of hollow zeolites, and the subsequent secondary nucleation and crystal growth may exist on their a/c faces. Then, a longer etching period allows the thinning of the crystal walls progressively in all directions. Notably, the largest crystal faces in ZSM-5 zeolites were the faces perpendicular to the direction of the b-axis; therefore, only [0 k 0] faces of bulk zeolites survived, resulting in the formation nanosheet structure in 18 h of the etching period. However, the thickening of the nanosheet structure was observed in the prolonged etching period at 24 h, owing to the unbalanced rate between the dissolution and recrystallization rate.The modification of layered zeolites, such as delamination, was usually preceded by the breakage of interlayer bonds and the expansion of interlamellar space called swelling. A high concentration of surfactant and hydroxide (OH−) combined with elevated temperature are often needed for performing this process. Corma et al. [72] have applied this technique to generate the delaminated zeolite ITQ-2 from the MWW-type structure zeolite precursor, i.e., MCM-22. The precursor was firstly swollen by refluxing the slurry of the solid sample (mixture of MCM-22 + water) with an aqueous solution of 29 wt% hexadecyltrimethylammonium bromide and 40 wt% of tetrapropylammonium hydroxide for 16 h at 353 K. After the subsequent ultrasound, acid treatment, and calcination, a new structure of nanosheet zeolite ITQ-2 was obtained. The much higher well-defined external surface area in the ITQ-2 provides a more accessible acid site. Thus, it exhibits a better catalytic activity than MCM-22 or MCM-36, in which they are its parent and its pillared form of MCM-22, respectively [73]. The schematic illustration of the different synthesis technique was islustrated in Fig. 5 .Regarding the severe condition needed for the swelling process in conventional delamination zeolite, recently, the fluoride or chloride anion-promoted exfoliation is expected to realize the delaminated zeolite under milder conditions. Eilertsen et al. [74] reported the synthesis of UCB-2 zeolite from the delamination of zeolite layered precursor MCM-22 using a mixture of cetyltrimethylammonium bromide, tetrabutylammonium fluoride, and tetrabutylammonium chloride in N,N-dimethylformamide (DMF) as solvent. Further treatment using concentrated acid at room temperature could result in delaminated zeolite, i.e., UCB-2. They suggest that the delamination may be facilitated by the organic solvent in a certain manner since it was reported to successfully exfoliate the layered materials such as hydrotalcites.The use of exfoliation method has also been reported for SAPO-34 nanosheet, which is generated from lamellar SAPO-34. Practically, the lamellar zeolite was exfoliated in mild condition through the strategy of the solvent-mediated freeze-thaw process. Firstly, the lamellar SAPO-34 was dispersed in hexane and frozen in a liquid nitrogen bath. Then, subjected into a sonication process. After repeated this process for 20 times, the sonicated samples was mixed with the excessive ethanol and aged for 12 h. Notably, the unexfoliated crystal will be settled down in bottom, in which it is separated by centrifugation process. Meanwhile, the upper suspension was separated and dried for 30 min. SAPO-34 nanosheet suspension was obatined by dispersing the dried sample in ethanol and subjected to a sonication process for 2 h. Finally, the nanosheet with 4 nm of thickness is generated [75].Basically, the ADOR process contains the disassembly of the previously assembled zeolite selectively and controllably into layered building units (Fig. 6 ). Then, organize them into a suitable orientation and finally reassemble again into a new zeolite structure through a condensation. This method could be used to obtain a certain novel zeolite framework that could not be prepared through direct hydrothermal synthesis. For instance, the IPC-2 (OKO) and IPC-4 (PCR) could only be obtained through the ADOR process of germanosilicate UTL zeolite [76,77].Cejka and Coworkers [44] demonstrated the seriously altered XRD characteristic of zeolite UTL after the hydrolysis process (room temperature to 100 °C of hydrothermal treatment at pH neutral to acidic (0.1 M HCl)), which indicate the significant structural change assigned to the transformation of the 3D to 2D zeolite (also confirmed by TEM). During the hydrolysis process, the interlayer space was contracted depending on the boron content, liquid media (water or acid solution), and temperature. In addition, the D4R units and their connectivities to the original layer were broken. Finally, the nitrogen isotherm characterization of the calcined hydrolyzed UTL showed that this new material has a typical microporous profile volume of 0.095 cm3/g and BET 270 m2/g with no mesoporous character.Nanosheet zeolite has ultrathin sheet morphology with thickness ranging from 2 to 100 nm [79–81]. A model of the nanosheet zeolite structure has been proposed by Ryoo et al. [54] The model depicts that the surfactant molecule aligns along the channels of the MFI framework. An assembly is built along the b-axis to form a multilamellar structure or a random assembly to form a unilamellar structure [54]. This unique structure impacts the distribution of the active sites. Different from 3D zeolite which the active sites mainly existed in the micropore, most active sites in 2D zeolites exist on the external surface [82]. Thus, a solid understanding of the structural and chemical properties of nanosheet is indispensable to enhance catalytic performance.Numerous investigations on the morphology of nanosheet zeolites have been reported and are depicted in Fig. 7 . It should be noted, the term morphology which is used in this section was related to both of nanosheet itself and the morphology of the assembled zeolite nanosheets. A distinct difference between the conventional and nanosheet form of MFI zeolites (Fig. 7a–g) is clearly observed. Conventional ZSM-5 (Fig. 7a) shows a typical hexagonal-prismatic or coffin shape morphology [83,84]. Kadja et al. [85] reported a nearly perfect spherical morphology for conventional ZSM-5 zeolite synthesized through the low-temperature synthesis method (LTS). Similar thickness in three different directions was observed, indicating that this spherical morphology is formed from three-dimensional crystal growth. Therefore, the morphology of conventional ZSM-5 zeolite is strongly influenced by crystal growth [86]. Meanwhile, the MFI nanosheet zeolite shows a thin sheet morphology with various assemblies (Fig. 7b–g). Hu et al. [87] obtained an MFI nanosheet zeolite with particle size around 4 μm and confirmed that the obtained nanosheet zeolite has a lamellar stacking morphology with the nanosheets intergrowth in a three-dimensional direction (Fig. 7b). This morphology was achieved by using the bifunctional organic surfactant, i.e., [C18H37–N+(CH3)2–(CH2)6–N+(CH3)2–C6H13] Br2 In the seed-assisted synthesis of nanosheets, the particle size of the obtained zeolite is greatly influenced by the seed content. The use of 5–30 wt% seed in the gel resulted in particle sizes from 0.8 to 1.2 μm to∼500 nm (Fig. 7c) [68]. Using the similar approach, Fang and coworkers [88] demonstrated that the nanosheet assemblies could be tuned by adjusting the Si/Al ratio. In this case, the spherical morphology of the nanosheet zeolite with a house-of-cards structure was obtained when Si/Al ratio is 31 (Fig. 7d). The lower Si/Al ratio resulted in the formation of an irregular stack, whereas the higher Si/Al ratio generated a nanosponge-like morphology. Besides, Hao et al. [89] customize the nanosheet thickness by arranging the proportion of surfactant and tetraethyl orthosilicate (TEOS) (Fig. 7g). An enormous amount of SDA leads to a denser morphology. Further, the thickness of the nanosheet structure was related to the ratio between mesopore and micropore, in which it strongly affected their catalytic activity. A similar result was also reported by Xu et al. [81], in which the thickness of aluminosilicate ferrierite (FER) nanosheet zeolites (6–200 nm) is strongly correlated with the amount of N,N-diethyl‐cis‐2,6‐dimethylpiperidinium (DMP) as a structure directing agent in the starting gel.Very recently, Li et al. [93] demonstrated the synthesis of ultrathin FER zeolite nanosheets named SCM-37 zeolite using octyltrimethylammonium chloride (OTMAC) and 4-dimethylaminopyridine (4-DMAP) as dual templates. A study using 13C MAS NMR revealed the involving oh those two organics in the zeolite formation. Particularly, the ammonium head group of OTMA+ is located in FER cages, 8-MR, and 10-MR channels. Meanwhile, no 4-DMAP is found at those location. Thus, FER layers might be the location of the 4-DMAP, which then inhibiting the crystal growth in the a-direction.Multilamellar structure with a petal-like morphology (Fig. 7e) was observed for the MFI nanosheet zeolite synthesized from CPh-Ph-m-6 surfactant [56]. Park et al. [18] investigated the effects of the multi-quaternary ammonium surfactant structure (tail-N+(CH3)2–{spacer–N + R 2} n−1–R*) on the synthesis of MFI nanosheet zeolites including the spacer length between the ammonium, alkyl groups in the terminal ammonium (R), length of surfactant tail, and the number of ammonium groups. The variation of –C3H6-, –C6H12-, and –C8H16- spacers (CiH2i) in the C22–i N2 [i.e., C22H45–N+(CH3)2–C i H2i –N+(CH3)2–C6H13] leads to the formation of micrometer-sized bulk crystals, multilamellar mesostructure, and disordered nanosheets, respectively. In addition, alkyl group moieties in the terminal ammonium also greatly influence zeolite crystallization. The decrease in the hydrophilicity of the alkyl group tends to hinder the SDA function. Thus, in the formula of C18–6N2(R) surfactants, the MFI nanosheet zeolites are only formed when Me2, Et2, and Pr3 are used as the alkyl moieties (R). In reverse, the use of Pr2 and But2 lead to the formation of an unknown silicate phase. In the case of the length of the surfactant tail, C8 and C6 are considered not hydrophobic enough to form an MFI nanosheet. Meanwhile, the thickness of the MFI nanosheet zeolite is reportedly increased with the number of ammoniums.Several researchers have also revealed nanosheet zeolites for other framework types. FAU nanosheet shows a ball-like morphology as depicted in Fig. 7h []. Salakhum et al. [90] reported that the morphology of the FAU nanosheet could be affected by the amount of surfactant, 3-(trimethoxysilyl)propyl octadecyl dimethyl ammonium chloride (TPOAC). A high amount of TPOAC could inhibit the crystal growth obstructing the morphological assembly of nanosheet. Meanwhile, the textural properties can be controlled by the synthesis conditions such as crystallization temperature and the amount of surfactant. Typical morphology of FAU nanosheet was observed uniformly at a crystallization temperature of 85 °C and a TPOAC molar fraction of 0.030 []. Ferdov et al. [91] also reported an FAU nanosheet assembly with ball-like morphology and a diameter of 2 μm at a crystallization temperature of 65 °C for 96 h (Fig. 7i). By employing silane surfactant, Fu et al. [52] obtained a Y nanosheet with a flower-shaped cards-like morphology, the thickness of 50 nm, and particle size of 2–5 μm (Fig. 7j).Zhou et al. [92] reported a disordered 2D MWW zeolite with noticeable uniform thickness (Fig. 7k) through the one-pot synthesis in the presence of long-chain surfactant cetyltrimethylammonium (CTA). This surfactant CTA could adjust the Al position without impacting the final product of the Si/Al ratio. This technique can be applied to layered zeolites to produce high surface area nanosheets []. In the case of template-free synthesis, the nanosheet mordenite was successfully synthesized with a nanosheet stack morphology (Fig. 7l) with a thickness of about 50–100 nm assembled. A higher H2O/SiO2 molar ratio contributed to the transformation of a single plate of the building unit with a thickness of around 3 μm into a bundle of nanosheets about 50–100 nm, suggesting that the decrease in basicity (higher H2O/SiO2 ratio) favors the formation of nanosheet structure [53].For the M-substituted zeolite, the MFI nanosheet with various subtituate metals, i.e., Al, Ga, and Fe was successfully obtained by Ji et al. [42] using diquaternary ammonium-type surfactant. The as-synthesized nanosheet zeolite exhibited a petal-like morphology composed of stacked intergrown crystals (Fig. 7m-o). Those inter-grown crystals can act as a pillar that prevents the structure collapse when the SDA is removal by calcination. Besides, M-substituted MFI nanosheets with other metals, i.e., Mn, Ce, W were also reported by Hadi et al. [38] showing disordered multilamellar structures composed of interlinked ultrathin MFI type zeolite nanolayers (Fig. 7p). Shown in the image that the thicknesses of the nanolayers are relatively uniform along the crystal axis perpendicular to the nanosheet layers.TEM and HRTEM images of nanosheet zeolites are shown in Fig. 8 , showing the ultrathin morphology of the nanosheet zeolite. Multilayer piles observed in the TEM image indicate the presence of mesopores between the gaps of the nanosheet [37]. In several cases, the lamellar stacking of nanosheet zeolite was clearly observed enabling us to further identify its component. For instance, Ryoo and co-workers [54] found that the stacking was comprised of the layer of MFI framework with a thickness of 2 nm and surfactant micelle with a thickness of 2.8 nm. The former is assigned to a single unit cell (three pentasil sheet) dimension along the b axis (b = 1.9738 nm) [54]. Figure 8a shows MFI zeolite nanosheets with a thickness of 2.0 nm on the b axis and a width of 60 nm on the a-c axis [130]. A similar MFI-surfactant layer stacking was also observed by several researchers (Fig. 8b–d). According to Liu et al. [23], the thickness of the surfactant layer was affected by the surfactant used. In this case, the C6-12-diphe ( ∼ 2.5 nm) results in the thicker layer than that of C6-6-diphe ( ∼ 2.0 nm). Furthermore, the molecular structure of surfactant strongly affected the morphology of MFI nanosheet zeolite. Variation of packing parameters between surfactants contributed to the formation of different mesophases. The packing parameter (g) is expressed as the volume of the surfactant chain (V) to the effective area of headgroup (a 0 ) and length of the surfactant (l c ) [55,94]. Mainly, the surfactant packing parameters can manage the growth of the 2D structure of the zeolite. Furthermore, the properties of MFI nanosheet zeolites, including textural and morphologies, can be tunned with different alkyl spacers of surfactants [23].In the study conducted by Park et al. [18], spacers between ammonium in the diaquaternary ammonium surfactant affected the morphology of the MFI nanosheet. The multilamellar MFI nanosheets with a thickness of 2 nm in Fig. 8c were generated by employing surfactant C22–6N2. Meanwhile, when surfactant with longer alkyl linkage such as C22–8N2 was used, a disordered MFI nanosheet was generated [18]. This result is probably can be described by packing parameters. Moreover, the multilamellar morphology of the MFI nanosheet is also confirmed by another report, as depicted in Fig. 8d [31]. Furthermore, by using the procedure reported by Ref. [54], Zou et al. [33] reported a lamellar stacking with lettuce-leaf-like morphology (the thickness around 20–30 nm) with overall particle size of 500 nm for the MFI-Cu nanosheet (Fig. 8e). In contrast, conventional zeolite exhibits a sphere particle consisting of small particles with a diameter around 250–500 nm and a bigger particle (1 mm) [33].Other than the stacking of the nanosheet structure, TEM images could also reveal that nanosheet zeolite provides the compatible surface for the metal to be well-dispersed. For instance, Pd was successfully dispersed on the outer of the surface Y nanosheet (Fig. 8f, red arrow indicates the Pd particles (1–3 nm) spread on the surface nanosheet) [52]. Besides, Yutthalekha et al. [37] revealed that the FAU nanosheet zeolite which obtained by using TPOAC (3-(trimethoxysilyl)propyl octadecyl dimethyl ammonium chloride) as surfactant is not only contain the multilayer stacking of nanosheet of nanosheets with the interstitial mesopores but also the mesopores cavity as clearly shown in Fig. 8g.In the case of titanium silicalite-1 (TS-1) nanosheets prepared by using a di-quaternary ammonium template (C22-6-6Br2), a cylinder-like bulk was observed as shown in Fig. 8h with a thickness of ∼200 nm, an average width of ∼250 nm, and an interlayer distance of 3.0 nm [95]. Lu et al. [96] reported the MOR nanosheet with a highly ordered layer and 11 nm of thickness (Fig. 8i) was achieved by using a bifunctional SDA, Gemini-type amphiphilic surfactant. The benzyl quaternary ammonium cations directed the formation of MOR topology, while the hexadecyl tailing group possibly formed a hydrophobic barrier in the micelles preventing the continued crystal growth along the b-axis. In contrast, the TEM image of the MWW nanosheet (Fig. 8j) synthesized by Zhou et al. [92] shows randomly oriented nanosheets. In addition, ITQ-2 prepared by delamination of MCM-22 zeolite precursor resulted in a random aggregation of nanosheets with a thickness of 5–10 nm (Fig. 8l), while MCM-22 exhibits disc-shaped particles with thickness of 20–30 nm (Fig. 8k) [97].X-ray Diffraction is the primary tools to characterize the structural properties especially the crystallinity of both 3D and 2D zeolites. Different from 3D zeolite, 2D Nanosheet zeolite has a diffraction pattern that are often broad. A sharp reflection is appeared in the h0l lattice plane, reflecting the 2D order of the layers, in which crystal growth occurs only in the a-c plane [45]. Also, quite similar to zeolite nanoparticles, the wider peak which is observed is correlated to the smaller particle size compared to the conventional 3D zeolite [98]. Fig. 9 a demonstrates the diffraction pattern of the 2D and 3D structures of Cu-ZSM-5. A sufficient sharp of the peak was only detected for h0l reflections confirming that the thickness along the b-axis is extremely small or broadened for Cu-ZSM-5 nanosheet Meanwhile, for the conventional one, all the hkl reflections are clearly observed [33].Furthermore, the low angle x-ray diffraction could also be used to identify the presence of the periodic interlamellar structural order as reported by Xu et al. [57]. The layered MFI nanosheet structure was indexed by the peak at 2θ 1.84ο and 3.65ο (Fig. 9bi), respectively assigned to the first and second-order reflection of layered MFI. In this case, the new unit cell constant of B = 4.8 nm, whereas the A and C parameters are still the same as MFI unit. This technique was also applied by the same group to evaluate the various single head ammonium surfactant in generating the MFI nanosheet zeolite (Fig. 9bi,ii). Actually, from the high angle XRD pattern, it was already observed that the 1 and 2 samples did not produce the lamellar structure indicated by the sharp peak of Bragg reflections in all directions. For sample 3, identification of the pattern is still elusive. In this case, applying the low angle XRD could help to further confirm the presence of the nanosheet structure, although it also could be obtained by performing the SEM or TEM characterization [56].Corma and Coworkers [73] also performed XRD analysis to investigate the structural evolution of the layered precursor MWW structure during the post-synthesis process (Fig. 9c). As the precursor was swollen using CTMA+, the XRD pattern was significantly changed. Only the peak at 18ο − 28 ο was sufficiently observed. Also, the shift of the peak at 3–7 to the lower 2 theta followed by the increase of intensity is indicated the increase in the distance between the layers from 2.7 nm to 4.5 nm. Furthermore, pillaring those swollen materials results in the change again in XRD pattern due to the formation of the new material which has been known as MCM-36. On the contrary, the delamination of that swollen material removes the peak at 2 theta 3–7. Moreover, the high angle peak was much broader, and the peak intensity was much smaller compared with its MWW precursor, suggesting that both crystal size and its previous long order are the dramatically reduced. Then, a new material called ITQ-2 was generated from this process. A significant structural change also occurred on the ADOR processes. The alteration of the XRD pattern of UTL zeolite and its transformation during the hydrothermal treatment at room temperature to 100 οC was depicted in Fig. 9d. The abundance peak of 3D UTL zeolite was dramatically reduced to several low-intensity peaks, in which the dominant 2 theta position was assigned to the 1.2 ± 0.1 nm d−spacing. Furthermore, the interlayer reflections of UTL framework (hkl) are disappeared and the low intensity peaks was resulted form the reflection of (0 kl) indices. Lately, the lamellar materials called IPC-1P was identified as the product form the structurally modification of 3D zeolite materials [44].In the case of porosity, the N2 adsorption-desorption was usually performed to evaluate the pore volume and surface area. Typically, nanosheet zeolite contains a higher mesopore volume and external surface area. For instance, numerous researchers reported that MFI nanosheets exhibited type IV isotherms [23,32,98–100]. In the study reported by Zhang et al. [32], the obtained MFI nanosheets showed a hysteresis loop with the capillary condensation at the relatively high pressure from 0.4 to 0.9, indicating the presence of larger pores (mesopore and macropore). The specific surface areas of the obtained MFI nanosheet zeolite were varied in the range 366–665 m2 g−1, which are greater than that of conventional MFI zeolite (293 m2 g−1). In line with that, enhanced total volume, mesopore volume, and micropore volume of MFI nanosheet were observed in the pore-size distributions calculated from Barrett–Joyner–Halenda (BJH) method. The calcined MFI nanosheets also exhibited broad distribution of mesopore diameter, suggesting partial condensation of MFI nanosheets during the calcination [23,32]. Fig. 10 a displays nitrogen adsorption for ZSM-5 nanosheet prepared by Xiao et al. [100]. Compared to the conventional 3D one, the ZSM-5 nanosheet demonstrate a highly mesoporous structure. The surface area for the latter(515 m2/g) is much greater than conventional (307 m2/g). Besides, they also reported an external surface area of ZSM-5 nanosheets (260 m2/g). Similar character was also observed for pillared MFI nanosheet (Fig. 10c) [62]. A study by Yutthalekha et al. [37] shows that the textural properties of nanosheet zeolite (in this case, for FAU zeolites) is strongly affected by the amount of TPOAC (3-(trimethoxysilyl) propyl octadecyl-dimethylammonium chloride) as a template. The total surface area and micropore volume decrease with the increase of the TPOAC/Al2O3 ratio from 0.01 to 0.04, i.e., from 734 to 566 m2/g, and from 560 to 407 m2/g, respectively. Meanwhile, the micropore volumes of nanosheets also decrease from 0.22 to 0.16 cm3/g. Other than that, the nanosheet zeolite obtained by the pillaring method was also reported to provide a much higher mesopore volume and mesoporous surface area that of the conventional one. Moreover, it is observed that V meso of MFI nanosheet prepared by intercalation method (0.398 cm3g−1) is higher than the dual template method (0.369 cm3g-1). Meanwhile, the microporous surface area (S micro ) of the nanosheet prepared by the dual template method (221 m2g-1) is 1.38 times higher than that of the intercalation method (160 m2g-1). These results show that the intercalation method is more efficient in generating mesoporous structures, while the dual template method produces more microporous structures [26].Corma et al. [73] demonstrated the transformation of the textural properties during the post-synthesis of layered precursor MCM-22 zeolite (Fig. 10b). The progressive increase of the total pore volume and surface area was observed, in which the nanosheet zeolite obtained from the delamination process (ITQ-2) exhibited the highest value compared to the pillared one (MCM-36) and its layered precursor (MCM-22). For the ADOR synthesis process, the effect of molarity of the hydrolysis media was investigated by Čejka and Co workers [76]. By choosing the suitable molarities, the surface area and pore volume in the range of 150–590 m2g-1 and 0.06–0.22 cm3g-1, respectively, could be adjusted (Fig. 10d).Acidity property in the zeolite active sites is the most crucial part of catalytic reaction. The acidity of the zeolite is determined by the Si and Al content in the zeolite structure. Several characterization techniques have been reported to investigate the acidity properties of nanosheet zeolites, including NH3-TPD, 27Al MAS NMR, and FTIR. The acidic strength and densities of nanosheet zeolites can be examined by NH3-TPD, in which the weak and strong acidity could be observed in the peaks of 190–230 °C and 405–475 °C, respectively. Al coordination in the zeolite structure can be determined using 27Al Mas NMR. The peak at 54 ppm is assigned to the tetrahedrally coordinated Al in the zeolite framework. Meanwhile, the type of acidity/basicity could be analyzed by FTIR. The hydrogen atom is observed at peaks of 1445 and 1600 cm−1. The Lewis acid sites (LAS) can be detected at 1454, 1487, and 1625 cm−1. Meanwhile, the Brønsted acid sites (BAS) could be observed at 1487, 1540, and 1634 cm−1) [61].Acidity properties could be improved by increasing the crystallinity of the zeolite (under the same SiO2/Al2O3 ratio). It is because higher crystallinity means higher internal-framework Al sites and lower extra-framework Al sites. Meanwhile, the internal-framework Al sites contribute more significantly to the improvement of acid strength since internal-framework Al sites have better tetrahedral geometry [32]. Based on research conducted by Verheyen et al. [31], the amount of acid for nanosheet zeolite and conventional zeolite is almost the same when both 2D and 3D zeolite have the same amount of Al []. Fig. 11 a displays two desorption peaks for nanosheet zeolite (denoted NS), The seed-fused ZSM-5 nanosheets (CNS-x), x refers to the number of seeds, and B-incorporation sample (B–CNS-5) showing weak and strong acid at 180–250 °C and 405–475 °C, respectively. The addition of seed seemingly leads to weaker acidity since the strong acid peaks of CNS-x samples shift to a lower temperature, and boron incorporation further lowers the acid strength. Pyridine-adsorbed FT-IR (Py-IR) in Fig. 11b reveals the presence of BAS and LAS at around 1450 cm−1 and 1545 cm−1, respectively, for all resulting nanosheet zeolites. Besides, the pyridine adsorptions on both BAS and LAS are observed at the peak of 1490 cm−1. The non-seed-fused nanosheet zeolite showed a remarkably enhanced BAS/LAS ratio (5.57) compared to the nanosheet zeolite with seed addition. Furthermore, boron incorporation generated more total acidity but less BAS number, suggesting that boron incorporation affects the Al distribution and increases the Si/Al ratio[68]. It can be concluded that the synthesis approach of the nanosheet zeolite strongly affects Al species coordination, whereas the more content of Al in the framework can generate more Brønsted acidity.Saenluang et al. [103] measured 27Al NMR for hierarchical nano spherical ZSM-5 nanosheets and resulted in two signals at 55 ppm attributed to the presence of Al species in the tetrahedral zeolite framework and 0 ppm ascribed from an extra framework of Al species [103]. The variation of synthesis precursors contributes to different intensities at 0 ppm. The use of pure silica Nanobeads precursor generated much higher intensity at 0 ppm compared to aluminosilicate nanobeads precursor. These results indicate that pure silica nanobeads precursor, which does not provide the aluminum source, favors the formation of extra-framework aluminum. In contrast, the presence of silicon and aluminum in the aluminosilicate nanobeads precursor assists the formation of internal framework aluminum.Jung et al. [104] investigated the acidity of hierarchical zeolites synthesized through desilication and structure-directed synthesis using 31P NMR spectra (Figure 11c). The former one generated hierarchical 3D MFI zeolite, and the latter produced an MFI nanosheet (denoted SD-MFI). Although both zeolites exhibited comparable textural properties and a total number of external acid sites, nanosheet zeolites surprisingly showed a higher concentration of strong acid sites than conventional MFI (denoted MFI-0.05). Thus, weaker BAS in the desilicated-MFI zeolite may be caused by the altered rearrangement of Al–O–Si bonds during the desilication process.The acid site properties of zeolite are also determined by substituting heteroatom in zeolite. In this case, Ji et al. [42] investigated the acidity properties of isomorphous MFI nanosheet zeolites (MFI (M), M = Al, Ga, and Fe). These isomorphous MFI nanosheets showed weaker acidity compared to conventional ZSM-5. The strength of acidity for isomorphous MFI nanosheet zeolites increased in the following order: MFI-Fe < MFI-Ga < MFI-Al. Furthermore, the result of pyridine-absorbed FTIR of isomorphous MFI nanosheet zeolites shows that the amount of Lewis acid sites was lower than that of Brønsted acid sites. This indicates that most aluminum, gallium, and iron atoms existed in the framework of zeolite forming Brønsted acid sites. The presence of these metals in the MFI nanosheet framework increases the ratio of LAS/BAS ratio, which is favorable for catalytic cracking of hydrocarbon .Compared to the 3D structure, the enhanced acid sites accessibility is more expected to the 2D one. For instance, in the pillared 2D zeolite, the expanding of the two adjacent of zeolites layer could enhance results in more exposed surfaces, leading to the accessibility improvement. Typically, it is closely related to the catalytic performance. Therefore, quantification of the type of acid sites is also crucial. In this case, there are range of organic molecules that has been utilized as a probe molecule, e.g., CO, pyridine, DTBP, TPP, and TMPO. The density of total acid sites was determined from the uptake of small organic base molecules. Meanwhile, the uptake of the larger one was used to calculate the external acid concentration. Finally, the accessibility was determined by calculating the ratio between adsorbed large molecules to the small ones [82].Wu et al. [101] used the pyridine and 2,4,6-collidine as probe molecule to evaluate the total and external acidity of MFI zeolite through IR spectroscopy. Result showed that nanolayered zeolite contains the higher amount of external Brønsted acid site and Lewis acid sites than that of the bulk MFI zeolite. Meanwhile, the more defective nature on the unilamellar zeolites results in the higher of external Brønsted sites, Lewis sites and silanol sites than multilamellar zeolites. Thus, unilamellar MFI showed a lower deactivation rate although there is no correlation between external BAS with the total turnover of methanol . Choi and Coworkers [102] also evaluated the accessibility of nanosheet zeolite compared with the 3D zeolite structure. ITQ-2 and the ultrathin zeolite samples (AS-8) show a higher accessibility factor than other samples, indicating the benefits of higher mesopore surface area. It corresponds to the two-dimensional structure which provides enough surface to the higher concentration of external acid sites.Nanosheet zeolite is widely used in various catalytic applications, including methanol conversion, aromatization, isomerization, oxidation/reduction. Other reactions are also described in this section. The summary of applications of nanosheet zeolites as heterogeneous catalysts are tabulated in Table 1 .Methanol can be obtained by converting syngas (H2, CO) from natural gas, coal, or pyrolysis and gasification of biomass [105]. Methanol can be converted into a hydrocarbon to produce fuel, and the process is known as methanol to hydrocarbons (MTH), such as methanol-to-propylene (MTP) or methanol-to-olefin (MTO) [106]. Those processes usually involve catalysts to accelerate the conversion rate. Generally, the catalysts used in methanol conversion are ZSM-5 and SAPO zeolite. Many researchers focus on modifying zeolite to improve the catalytic process, such as acidity, stability, and selectivity. There are three main steps in the methanol conversion, (i) dehydration of methanol to form dimethyl ether (DME), (ii) equilibrium state consisting of a mixture of methanol, DME, and water, and (iii) conversion into olefins through dehydration [107].Hu et al. [87] reported that the nanosheet zeolite exhibited excellent catalytic activity compared with conventional zeolite. The use of zeolite MFI nanosheet catalyst in the MTP process resulted in nearly 100% conversion after a 240 h reaction and 51% selectivity toward propylene (Fig. 11a). The remarkably improved catalytic performance of MTP over nanosheet catalysts is ascribed to its unique morphology. Due to its large dimensions in the a-c plane, as previously mentioned, the nanosheet zeolite crystals end up with a higher surface fraction (010) than their conventional counterparts. Accordingly, the substrate is more feasible to reach the pore of the MFI nanosheet through the (010) surface corresponding to a straight channel. Besides, the ultrathin nanosheet provides shortened diffusion paths (especially the straight channels), hindering the formation of a larger molecule such as naphthene and aromatics that could block the zeolite channels. Consequently, the favored light olefin could be diffused out before the secondary reaction occurs, and the deactivation rate of the catalyst could be depressed. In addition, Hadi et al. [38] also reported the better performance of W-substituted MFI nanosheet zeolites. It exhibits propylene selectivity of 55.7%, with the total selectivity to light olefins reached 88.04%, and the catalyst is stable until 81 h (Figure 12b). It is higher than that of the W-substituted conventional zeolite. It corresponds to the unique morphology of the MFI nanosheet which provide a shorter diffusion path. In this case, once the product was formed, it could conveniently reach the outer, restraining the occurrence of the secondary reaction such as aromatization and hydrogen transfer.Xing et al. [69] reported the different catalytic performances of hydrothermally treated ZSM-5 nanosheet (HT-ZSM-5-SM/FM) prepared by different types of seeds, spherical (SM) and sheet-like (FM) ZSM-5. The hydrothermally treated ZSM-5 nanosheet prepared from spherical seed showed enhanced catalytic activity compared to that from sheet-like seed. According to TG analysis, the amount of coke loss in the range 200–700 °C for hydrothermally treated ZSM-5 nanosheet prepared from spherical and sheet-like seed are 32.8% and 22.6%, respectively. These demonstrated that the use of spherical seed for the synthesis of ZSM-5 nanosheet catalyst favors higher carbon capacity.The selectivity and catalyst lifetime is remarkably enhanced after hydrothermal treatment [69]. According to Lu et al. [131] MOR nanosheets exhibit greater selectivity for light olefins than traditional MOR zeolite, as demonstrated in Figure 12 c. The ethylene selectivity decreased as the MOR thickness increased. These findings suggest that ethylene is formed via an aromatic-based mechanism on particular acid sites in the MOR framework.Furthermore, Kim et al. [108] studied the catalyst performance of MFI nanosheet zeolite with varied thicknesses (2.5 nm and 7.5 nm) and different Si/Al ratios (100–700) in MTP reaction. Based on Fig. 12d, all samples possess a thickness of 2.5 nm except the H-ZSM-5 sample and NS-MFI-500 (7.5 nm). The nanosheet zeolite's thickness and Si/Al ratio can influence catalytic activity, while a higher propylene selectivity and longer lifetime were reached by nanosheet zeolite with thicknesses 2.5 nm (Fig. 12d). The nanosheet with thicknesses >7.5 nm increases the catalyst's diffusion resistance, making the reactant and product transfer more difficult. Therefore, the nanosheets with the thickness lower than 2.5 nm may lead to higher selectivity to propylene and a more robust tolerance to coke formation. In addition, the Si/Al ratio of the nanosheet zeolite is related to the amount of acidity for the MTP reaction. Besides, the number of cokes can also increase linearly with the number of acid sites. However, when the number of acid sites is too low, it is not adequate to convert higher molecules into light olefins [].Light olefin production by steam cracking is remained challenging due to several restrictions, including high reaction temperature (more than 800 °C), expensive construction materials, high energy consumption, the inflexibility of the product, and, in particular, low propene/ethene (P/E) ratio. Conversely, catalytic cracking can operate at much lower temperatures (often at 500–600 °C) with consequent energy savings. Besides, the P/E ratio can easily be tuned by appropriate catalyst design and operating variables. Therefore, catalytic cracking makes a necessary contribution to satisfy the growing demand for light olefins today. Catalyst (generally zeolite) is a key factor to achieve high activity and selectivity to light olefins [109].The crucial aspects of product selectivity are the textural properties and acidity of the catalyst. The acidity of the catalyst is the primary determining factor of conversion. Fig. 13 a shows the conversion of n-decane by using nanosheets zeolite (ZN-2), HZSM-5 nanosheet zeolite with silica-pillared (PZN-2), HZSM-5 nanosheet zeolite with dual-template synthesis method (DZN-2), and conventional HZSM-5 zeolite (CZ-500) (Fig. 13a). The olefin selectivity of the dual-template HZSM-5 nanosheet zeolite is almost two times higher than that of the nanosheets zeolite and conventional HZSM-5 zeolite. As for silica-pillared HZSM-5 nanosheet zeolite, although the B acid is reduced and decreases the n-decane conversion, the selectivity towards light olefins is significantly higher than that of the non-pillared nanosheet zeolites and conventional HZSM-5 zeolite. This indicates that the pore structure of zeolite catalyst plays a crucial role in light olefin selectivity. The deactivation rate of conventional HZSM-5 zeolite reached 68.24%, which is > 11 times higher than HZSM-5 nanosheets zeolite (5.81%) (Fig. 13a). Besides, HZSM-5 nanosheets zeolite achieved the highest yields of light olefin (∼35%) with a slight reduction after 16 h of reaction (Fig. 13b) [26]. Fig. 13c displays the conversion of n-dodecane using nanosheet zeolites synthesized with a different number (n) of TPAOH, (10(C22-6-6)/n (TPAOH). Among the catalyst, nanosheet zeolite with n = 4 shows superior catalytic activity with a conversion rate of 76.8%, TOF value of 130.92 s−1, and deactivation rate of 9.11%. The inter-connected mesopores structure may inhibit the secondary reactions such as aromatization, leading to high olefin selectivity and low selectivity of aromatic compounds. The catalytic cracking of n-dodecane was tested in a flowing reactor at 4 MPa and 550 °C, as depicted in Fig. 13d [43].In the case of the aromatization process, the pore structure and strength of acid sites are the main factors responsible for high conversion and selectivity [97]. Kim et al. [39] studied the effect of Pt/MFI thickness from bulk to nanosheet scale (300–2 nm). The catalytic result is shown in Fig. 14 a. The catalytic conversion of n-C7 was first plotted as a reaction temperature function. It can be seen that the thickness of zeolite does not give a significant effect on the conversion of n-C7 since all Pt/MFI zeolites show a relatively similar S-curves. Meanwhile, the MFI zeolite crystal thickness becomes smaller, the conversion of i-C7 was higher than others. Moreover, B-300 was used as support for Pt NPs, and the catalyst achieved the maximum i-C7 mole percent 22 mol%, C-40 resulted in 29 mol%, NC-10 reached 42%, and NS-2 given 48%. However, reduction of thickness generated a higher i-C7 mole percent. Fig. 11b shows a correlation between the total conversion and the mol percent of i-C7 for a given catalyst, as measured over the range of reaction temperatures (200–300 °C). As shown in Fig. 14b, the i-C7 mole percent increases by the order B-300 < C-40 < NC-10 < NS-2 with the maximum value of 48%. It can be inferred that nanosheet zeolite is more favorable for i-C7 isomer production. The extremely thin morphology of the nanosheet facilitates the branched isomer product to migrate out before further reaction occurs, yielding a more highly branched isomer []. Fig. 14c shows the n-pentane conversion of both conventional and nanosheet forms of Ga-ZSM-5. Ga-ZSM-5 nanosheets (GaExcZS-5-NS-X, with X, refers to the ratio of Si/Al) exhibit remarkably improved conversion of n-pentane compared to the conventional one (GaExcZS-5-CON-X). The BTEX selectivity for both Ga-ZSM-5 nanosheets with Si/Al ratios of 69 and 38 is over 40 and 43%, respectively. The catalytic performance on Ga embedded in ZSM-5 nanosheets is associated with the highly dispersed Ga species. With the increasing acidity in Ga-MFI (Fig. 14d), the catalytic activity was also increased because the high acid density can facilitate the catalytic breakdown of n-pentane in the first step of the reaction. It also activated oligomerization and further cyclization. Conversely, even at low Si/Al ratios, conventional structures still have a low conversion, low aromatic selectivity, and fast deactivation of the catalyst. This investigation confirms that an increase in the aromatic yield depends not only on the Si/Al ratio but also on the zeolite pore structure [110].Alkylation is one of the most important reactions in organic synthesis, especially in the interconversion of alkylbenzenes. Therefore, a great number of studies have been focused on the alkylation reaction [111]. Liu et al. [53] synthesized MOR nanosheets by varying the amount of water in the hydrothermal process. The sample was denoted H-MOR-x, where x refers to either the water amount (H2O/SiO2 ratio), symbol A (leaching), or C (commercial), as shown in Fig. 15a and b. Among the obtained zeolites, H-MOR nanosheet with a water amount of 26 (H-MOR-26) exhibits the highest conversion. Meanwhile, the order of the conversion for all catalyst are H-MOR-26 > H-MOR-20 > H-MOR-11 > H-MOR-11-A > H-MOR-40 > HMOR- C. This order is related to the number of strong Brønsted acid sites. Each of the various catalysts has the following Brønsted acidities 1.004, 0.893, 0.833, 0.623, 0.527, 0.140, respectively. Meanwhile, all MOR nanosheets show similar selectivity to methyl acetate (∼98%) and produce a small amount of ethanol and methanol. The DME conversion of MOR zeolite remains stable after 12 h except for H-MOR-11, which shows an extreme reduction from 39% to 9%. It is reasonable since H-MOR-11 possesses a small external surface area promoting coke deposition inside the micropore and decrease the catalyst stability.Feng et al. [112] studied the carbonylation of DME reaction to produce methyl acetate using commercially conventional H-ZSM-35 (CZ35), H-ZSM nanosheets (NZ35), and ZSM-35 hierarchical nanosheets prepared with various concentrations of NaOH 0.2 M (Hi-NZ350.2), NaOH 0.4 M (Hi-NZ350.4), and NaOH 0.6 M (Hi-NZ350.6). As shown in Fig. 15c, the catalyst ability in the carbonylation reaction increases with the order Hi-NZ350.6 < CZ35 < NZ35 < Hi-NZ350.2 < Hi-NZ350.4 with the conversion of 14.9, 22.4, 26.2, 31, and 42%, respectively. The low catalytic activity of the Hi-NZ350.6 catalyst may occur because of the damaged structure, lowest surface area, and pore volume, among other catalysts. The selectivity of all catalysts is relatively comparable, with a selectivity value over 90% to MA. The by products such as CO2 and CH4 are produced in small amounts caused by the WGSR (water-gas shift reaction) reaction. Furthermore, nanosheet zeolite shows high stability due to excellent diffusion resistance suppressing the deactivation of the catalyst [].Saenluang et al. [103] evaluated the effect of Si/Al ratio and Al distribution of hierarchical nanospherical ZSM-5 nanosheet on the catalytic performance in the alkylation of benzene. The catalytic activity of hierarchical ZSM-5 nanosheet with uniform Al distribution denoted as Hie-SZSM-5-AS (Low) for Si/Al = 59 and Hie-SZSM-5-AS (High) for Si/Al ratio = 112, hierarchical ZSM-5 nanosheet with less uniform Al distribution denoted as Hie-SZSM- 5-PS, and conventional ZSM-5 denoted as Con-ZSM-5. The catalytic activity increases by the order Hie-SZSM-5-AS (Low) > Hie-SZSM-5-PS > Con-ZSM-5> Hie-SZSM- 5-AS (High). High Si/Al ratio (112) in hierarchical nanopsherical ZSM-5 nanosheet led to lower acid density, thereby decreasing the catalytic performance that was even lower than conventional ZSM-5. Noticeably, Al distribution also contributed to the altered catalytic performance. Hierarchical ZSM-5 nanosheet with less uniform Al distribution (Hie-SZSM- 5-PS) shows lower benzene conversion than the more uniform one (Hie-SZSM-5-AS (Low)) despite both catalysts possess similar acidity and textural properties. The different Al distribution means different acid distribution. As previously discussed, the different Al distribution could be attributed to different starting materials, where the use of aluminosilicate precursor for the synthesis of Hie-SZSM-5-AS (Low) sample favors the formation of tetrahedrally coordinated Al species generating Brønsted acid sites. Meanwhile, the use of pure silica precursor promotes the formation of octahedrally coordinated Al species or extra-framework Al species. The low catalytic performance of conventional ZSM-5 compared to both SZSM-5-AS (Low) and Hie-SZSM- 5-PS demonstrates the contribution of mesopore in the hierarchical ZSM-5 nanosheet, which promotes more facile diffusion of the reactant to the active sites. In case of selectivity, all catalysts are more selective to ethylbenzene. Although the Hie-SZSM- 5-AS (High) shows the lowest benzene conversion, the selectivity to higher aromatics is the lowest among the catalysts , possibly due to the insufficient acidity to undergo further reaction.Friedel − Crafts alkylation reaction of benzyl alcohol (BA) with benzene was reported by Zhou et al. [92] using MWW nanosheet catalysts. The MWW nanosheets prepared by using cetyltrimethylammonium (CTA) (Hd-MWWx, x refers to %wt CTA), and MWW nanosheet prepared by post-synthetic exfoliation method (ITQ-2). The catalytic ability of those zeolite decreases by the order H-d-MWW8.0 > H-MCM-22 > H-d-MWW5.5 > H-ITQ-2. The excellent catalytic activity of H-d-MWW8.0 is related to the high external surface area (359 m2/g) and high site Al density. Meanwhile, the low catalytic activity of H-ITQ-2 can be caused by the damaged structure during the exfoliation process. This demonstrates that the post-synthetic exfoliation method adversely affects the catalytic performance of 2D layered MWW zeolite in Friedel − Crafts alkylation reaction [92]. Liu et al. [132] reported a hierarchically structured MFI zeolite nanosheet for benzylation reactions. Zeolite nanosheets are labeled as MZA-n with the Si/Al molar ratio (n = 15, 31, and 50), while commercial zeolite ZSM-5 is labeled as CZSM-5. The relationship between the Bronsted acid/Lewis acid ratio (B/L) and benzyl alcohol conversion and alkylation/etherification selectivity is shown in Figure 15d. The Bronsted acid/Lewis acid ratio is a critical factor for evaluating benzylation activity and selectivity in the catalyst studied, according to the linier trend between Cc/2CE and B/L. Bronsted acid and Lewis acid sites can promote benzyl alcohol conversion and alkylation product selectivity.The application of zeolites in oxidation or reduction reaction was usually coupled with metals. In this case, nanosheet zeolites provide a high surface area for metals to uniformly disperse onto their surfaces. Zou et al. [33] examined the performance of Cu-ZSM-5 nanosheets for N2O decomposition by comparing it with conventional Cu-ZSM-5. Cu-ZSM-5 nanosheet shows excellent catalytic activity and stability, as shown in Fig. 16 a. The N2O conversion of Cu-ZSM-5 nanosheets is quite stable around 80% after a 50 h reaction at 475 °C. In contrast, a sharp drop in N2O conversion from 74% to 54% is observed for conventional Cu-ZSM-5 after 50 h. The difference in the performance of the two types of catalysts is attributed to the differences in structure and properties where Cu-ZSM-5 nanosheets possess a higher surface area of 410 m2/g compared to conventional Cu-ZSM-5 (381 m2/g). Besides, from the O2-TPD data, the desorption of O2 from the Cu sites in the Cu-ZSM-5 nanosheet was more easily desorbed than the conventional one. High desorption of O2 is favorable for N2O decomposition. Thus, the Cu-ZSM-5 nanosheet showed improved catalytic activity.Structured Pt @ ZSM-5 nanosheets were prepared by Liu et al. [113] and applied in catalytic combustion of VOC. The catalytic process is carried out by loading Pt metal on three varied supports. The Pt ion was loaded on the nanosheet zeolite ca. 2 nm without calcination (Pt/PZN-2), with calcination (Pt/ZN-2), and conventional ZSM-5 ca. 500 nm (Pt/CZ-500) by impregnation method. Based on Fig. 16b, all types of catalysts reached 100% toluene conversion. However, the toluene conversion for Pt/ZN-2 catalyst and Pt/CZ-500 decreases after 180 h and 72 h, respectively. Pt particles are possibly aggregated under such conditions, particularly for Pt/ZN-2 and Pt/CZ-500 catalysts. Pt/ZN-2 was prone to collapse after 360 h of toluene combustion. Interestingly, Pt/PZN-2 remains stable after 360 h, showing the excellent hydrothermal stability of the sandwich-structures.Li et al. [114] compared the performance of 3 types of catalysts, including conventional TS-1 (CTS-1), mesoporous TS-1 (MTS-1), and hierarchical TS-1 nanosheets (HTS-1 50) with Si/Ti ratio of 50. They were applied in the catalytic epoxidation of cyclic olefins (cyclohexene and cyclooctene). As shown in Fig. 16c, the order of cyclohexene and cyclooctene conversions are HTS-1-50 > MTS-1 > CTS-1. This could be explained by the lower external surface area, which only generates a small portion of the active site. In contrast, HTS-1, which has a high external surface area, shows superior catalytic performance. All catalyst shows high selectivity toward epoxides. HTS-1-50 exhibited epoxide selectivity of 75.4% for cyclohexene. Meanwhile, the epoxide selectivity from cyclooctene is 98.3%, 96.9%, and 92.6% for CTS-1, HTS-1-50, and MTS-1, respectively. The high epoxide yield from cyclooctene could be caused by the higher electrophilicity of the carbon double bond in cyclooctene, which leads to a more stable cyclooctene epoxide. Furthermore, the HTS-1-50 catalyst showed higher epoxide yields of 17.6% and 17.4% for cyclohexene oxide and cyclooctene oxide, respectively. Meanwhile, the epoxide yield of the CTS-1 catalyst was only 3.5% and 2.5% for cyclohexene oxide and cyclooctene oxide, respectively. The more enhanced catalytic activity of hierarchical TS-1 nanosheet over the conventional TS-1 is attributed to the presence of larger porosity and a 2-dimensional form of hierarchical TS-1 nanosheet.Meng et al. [115] reported Fe/ZSM-5 nanosheet zeolite catalysts for benzene oxidation to produce phenols. The Fe/ZSM-5 nanosheets were synthesized using different SDAs, C22-6-3Br2 and C22-6-6-6-3 Br4. As shown in Fig. 16d, the samples are denoted as Fe/ZSM-5 (xN, y), where x is the number of quaternary ammonium ions (2 and 4) and y is the Si/Fe ratio (180, 360, 720). The suffix -st refers to steamed treatment. Steaming is an effective method to increase the amount of Fe2+that also acts as the active site. Thus, the steamed zeolite shows higher conversion and selectivity due to the high density of Fe2+ and FexOy aggregates in the steamed zeolite. However, steamed zeolite is more prone to deactivation than bulk zeolite. Steamed Fe/ZSM-5 nanosheet synthesized using C22-6-3Br2 (with a thickness of 3 nm) showed the highest catalytic activity by producing 185 mmol g−1 phenol after 24 h on stream. Meanwhile, the bulk zeolites exhibited lower phenol selectivity and underwent faster deactivation with the increase in the amount of Fe. Likewise, the higher the Fe content for the nanosheet zeolite, the more coke production, and the faster it is deactivated. The initial catalytic activity of the nanosheet zeolite was about 50% higher than that of bulk zeolite. This occurrence is due to the diffusion resistance of the micropore structures in the bulk zeolite. Characterization of the texture of the spent catalyst demonstrated that the carbon coke in the deactivated bulk zeolite is assembled in the micropore. In contrast, the coke of the deactivated nanosheet zeolites was mainly collected in the mesopores.Besides the thermal catalytic reduction process, zeolites have also been applied in photocatalysis reactions, e.g., the reduction of CO2 into various products such as CO [118] and CH4 [119], the H2O splitting [117], the degradation of methylene blue [120] and methyl orange [121]. It has been known that combining metal oxide into porous materials such as zeolites could generate the isolated metal oxides acting as single site photocatalysts (Fig. 17 a) [116], in which the photocatalytic activity is not related to the transfer of electron and hole in the valence band and conduction band, respectively to the photocatalyst surface as observed in the semiconductor materials [119], but it correlates to the formation of the (M(n−1)+) species as a consequence of the charge transfer from ligand to metals (LMCT)). In that sense, this confined structure provides a localized environment with a quite short distance, thus, the charge transfer process in the photocatalyst materials could be enhanced, leading to extraordinary photocatalytic activity [116].Compared to conventional zeolites, nanosheet zeolites have a higher photocatalytic activity. For instance, Liu et al. [117] reported a higher H2 evolution rate (2152,7 μmol/h) of CdS/Pt nanoparticles supported on a porous lantern-like MFI zeolite composed of 2D nanosheets (NL-MFI) than those of supported on conventional MFI zeolites (1079.3 μmol/h) and unsupported Pt/CdS (515.0 μmol/h) (Fig. 17b and c). Moreover, the Pt/CdS-NL-MFI sample also exhibits higher stability (Fig. 17d). This was assigned to the promoting effect of nanosheet structure in absorbing visible light, separating the photogenerated electron-hole pairs, and enhancing the interaction between the water molecules and the photocatalyst.Moreover, other reports suggested that the remarkable photocatalytic activity of nanosheet zeolites was accounted for their ability to facilitate the high dispersion of metal nanoparticles [120] and the abundance of exposed Al atoms as alkaline sites, which allowed the high adsorption capacity of acidic molecules like CO2. Moreover, it also offers more abundance of surface active sites, allowing the excited state electrons to be quickly captured and transferred, thus resulting in a high photocatalytic activity [119]. Furthermore, for the reaction involving bulk molecules such as methylene blue, the existence of a mesopore in the nanosheet structure accommodates the molecule to conveniently adsorbed and react on the catalyst surface [121]. Moreover, it shortens the diffusion path length of the molecules, thus improving mass transfer.Although several positive results of the nanosheet zeolite were associated with their high external surface properties, a different result was reported by Jiři Čejka and his group [118]. They demonstrated that 3D TS-1 zeolites show a higher product formation rate (3.29 × 10−4) than 2D TS-1 zeolites (3.04 × 10−4), with a selectivity of 30.8% and 30.7%, respectively. Notably, the external surface area for these two samples was 6 m2/g and 94 m2/g. It suggests that the photocatalytic reduction of CO2 was not merely driven by the specific or the external photocatalyst surface, which produces a stronger and higher interaction between the molecules and the catalyst. Yet, hydrophilicity is also strongly affected in the efficiency and the selectivity of the catalyst. This result was confirmed with the H2O TPD results, which demonstrated the best correlation between the amount of the adsorbed H2O with the hydrogen production.Lee et al. [40] reported the catalytic performance of unilamellar mesoporous MFI nanosheets (UMNs) and Al-SBA-15 for pyrolysis of lignocellulosic biomass (cellulose, xylan, and lignin). For catalytic pyrolysis of cellulose, both UMNs and Al-SBA-15 converted the cellulose into various products, with the highest produced products are oxygenates (ketone, aldehyde, alcohol, cyclo-compounds, furans, and levoglucosan). The strong Brønsted acid in UMNs catalyst promoted the conversion of levoglucosan into furan and aromatic compounds. The different catalysts resulted in different cyclo compounds. The UMNs strong acid sites are considered to facilitate the production of cyclo compounds with lower oxygen content, 2-cyclopentene-1-one. Meanwhile, Al-SBA-15 mainly produced 2-cyclopentene-1,4-diones.Furthermore, the xylan catalytic process also produced high oxygenated products (esters, ketones, aldehydes, alcohols, cyclo compounds, and furans). The increase in mono-aromatic yield occurs with the addition of UMNs. The degree of crystallinity of hemicellulose (xylan) is lower than cellulose. Thereby, xylan is easier to break down and converted into aromatics. Furthermore, the catalytic pyrolysis of lignin yields a highly phenolic product such as 2-methoxy-phenol. The yield of mono-aromatic was much higher when using UMNs than Al-SBA-15. This occurrence is feasible because Al-SBA-15 has a weaker acidity that can convert heavy compounds into light phenolics but cannot further convert light phenolics into aromatics.UMNs, which have strong acidity, showed better catalytic performance than Al-SBA-15. A tremendous amount of oxygenating has been removed, and the aromatic yield is three times higher, leading to more outstanding quality bio-oil production [40].Liu et al. [41] prepared Pd loaded on HZSM-5 nanosheets (Pd/ZN) for direct synthesis of hydrogen peroxide. The study was conducted by comparing the Pd loaded on nanosheet zeolite (Pd/ZN-50) and conventional zeolite (Pd/CZ-50) with a Si/Al ratio of 50. The catalytic performance of Pd/ZN-50 exhibited 82.3% conversion, whereas Pd/CZ-50 reached 25.7% conversion at 0.5 h. Meanwhile, the H2O2 selectivity of the catalyst Pd/ZN-50 (35.1%) is much lower than Pd/CZ-50 (62.6%). Based on the conversion and selectivity values, the productivity of H2O2 from the Pd/ZN-50 (28.9 mmol H2O2 (g cat)−1. h−1) was greater than that Pd/CZ-50 (20 mmol H2O2 (g cat)−1. h−1) after 0.5 h. The catalyst performance depends on the concentration of H+, crystal phase, Pd dispersion, and Pd0 content [122]. Pd/ZN-50 shows lower selectivity due to a higher decomposition rate compared to Pd/CZ-50. This result could also be due to the Pd+-Z- structure formed by the stronger metal-support interaction between the Pd particles and the acid sites. The higher selectivity of Pd/CZ-50 toward H2O2 is due to the lower surface charge density of the Pd atom inhibiting the dissociation of the O–O bonds in H2O2. Moreover, catalytic performance (including H2 conversion, H2O2 selectivity, and H2O2 formation rate) is enhanced with a lower Si/Al ratio. In addition, the smaller Pd nanoparticle size in the Pd/ZN-50 catalyst offers a strong ability to dissociate the O–O bonds in H2O2.Feng et al. [123] evaluated Ni/ZSM-5 nanosheet catalysts for hydroconversion of oleic acid to aviation-fuel-range-alkanes (AFRAs) by alternate the Si/Al ratio (100, 200, or 300). In addition, the external surface of nanosheet zeolite ZSM-5 was modified through CLD (Chemical liquid deposition) method. The catalytic test showed that all catalysts reached 100% oleic acid conversion. The Ni/ZSM-5 nanosheet catalyst showed high deoxygenation activity due to improved access to the active site. In addition, the decrease in the concentration of strong Brønsted acid for CLD-modified catalysts led to higher OLP (organic liquid products) production than the pristine ones. Low external Brønsted acid concentration promotes cracking reaction of the primary long-chain deoxygenated product into linear AFRA. Conversely, the high external Brønsted acid concentration supports secondary cracking of the deoxygenated products generating short-chain alkanes. Besides, low or moderate internal Brønsted acid concentration increases the isomerization of AFRA and decreases the secondary cracking.Kim et al. [80] compared the performance of three types of catalysts, specifically MFI nanosheet, amorphous silica, and bulk silica zeolite, in the gas-phase Beckmann rearrangement of cyclohexanone oxime. MFI nanosheets showed high catalytic activity with cyclohexanone oxime (CHO) conversion of 77%, a selectivity to lactam of 92%, and a long catalytic period up to 100 h with no observable change in selectivity. On the other hand, the bulk silica zeolite catalyst showed an initial conversion of 48% and underwent more rapid deactivation. The conversion considerably dropped to 15% after 20 h reaction. The longer catalytic lifetime of the MFI nanosheet can be explained by the significant difference in the coke content between deactivated nanosheet and bulk MFI (4 and 7 wt%, respectively). It suggested that the MFI nanosheet defeated the polymer species that produce coke and lead to the deactivation of the catalyst. The high selectivity of the nanosheet zeolite toward lactam is attributed to the silanol group's surface located in the crystal plane (010).Chang et al. [61] also synthesized silicalite-1 nanosheet for vapor-phase Beckmann rearrangement of cyclohexanone oxime (CHO) to produce lactams. The SDA was prepared by using poly (propylene glycol) bis(2-aminopropyl ether) (NH2(PO)nNH2) with an average molecular weight of ∼400 (ñ6) and ∼2000 (ñ33). Silicalite-1 samples synthesized with N3-POn-N3 are denoted S1-n-A. The catalytic result showed that the S1-6-A sample could convert 76% of CHO at the beginning of the reaction. It decreased to 34% after 50 h reaction time with e-caprolactam (CL) selectivity reaching 96%. Moreover, the S1-33-A catalyst arose conversion of CHO to 92% at the beginning of the reaction and decreased to 51% after 50 h, with CL selectivity reaching 97%. High CHO conversion of both catalysts can be assigned to large inter-lamellar and inter-bundle porosity that facilitate mass transport of substrate. Noticeably, a longer POn linker contributes to the improved pore volume. In addition, the S1-33-A sample has a large pore volume (0.76 cm3g− 1) compared to S1-6-A (0.34 cm3 g−1), so that S1-33-A shows higher CHO conversion. The high CL selectivity for S1-6-A and S1-33-A can be attributed to the binding sites in the hierarchical silicalite-1 nanosheet, such as the silanol groups on the external surface.Ali et al. [59] prepared a self-pillared MFI nanosheet catalyst for acrolein production through the glycerol dehydration process. Self-pillared MFI nanosheet achieved 92% selectivity toward acrolein at WHSV 4 h −1. The stability analysis shows fresh and regenerated pillared MFI nanosheets resulting in comparable catalytic activity, which means the catalyst exhibited good thermal stability. The zeolite possesses a Brønsted acid, which is the active site utilized in glycerol dehydration. Since the Brønsted acid conducts the catalytic reaction in glycerol, the acidity of the catalyst is crucial that can impact the activity, selectivity, and stability of the catalyst. Besides, the catalyst structure is also an important factor that could affect diffusion and coke capacity. Pillared MFI nanosheet showed higher TON than nanocrystalline ZSM-5 due to high catalyst stability, although they have the same crystal size. The poor selectivity of nano-ZM-5 is correlated with restricted reactant diffusion. Therefore, a self-pillared MFI nanosheet catalyst can inhibit coke formation and increase the selectivity of acrolein by reducing diffusion restrictions.To date, research on the nanosheet zeolite is under the spotlight due to the remarkable advantages offered by its two-dimensional structure. From the catalyst design point of view, several synthesis strategies have been extensively studied. Generally, the preparation of nanosheet zeolite could be conducted through bottom-up and top-down methods. The former is usually performed by hydrothermal synthesis process, in which the structure-directing agent is usually involved. Typically, it produces the so-called layered precursor, which could condense upon the calcination, e.g., MCM-22. In this case, several post-synthesis modifications could be performed to prevent the structure collapse, including pillarization, delamination, swelling, and exfoliation. Furthermore, another strategy like the use of surfactant as multifunctional template has also been reported to directly synthesize the nanosheet structure. In this case, the variation in the synthesis parameter, such as type of the surfactant as SDA, SDA amount, Si/Al ratio, crystallization temperature, and time could affect the morphology, thickness, acidity, and textural properties of nanosheet zeolite. In the case of SDA, almost every part of the structure, such as to N–N spacer length, alkyl groups of SDA, hydrophobic tail length, and the number of ammonium groups, also could influence the characteristic of nanosheet zeolite. On the other hand, the ADOR method is considered as the bottom-up strategy, in which the 2D structure was generated from the existing 3D structure. In addition, chemical etching, the top-down method, has also been reported using NH4F. The acidity and textural properties of the obtained nanosheet zeolite could be tailored by the fluoride etching.In the structural and textural analysis of nanosheet zeolite, a similar characterization technique with conventional zeolite is usually performed, i.e., XRD, FTIR, NMR, SEM, TEM, and N2 adsorption-desorption. Typically, the XRD pattern of nanosheet zeolite is broader, in which the sufficient sharp enough peak dominantly appeared for h0l indices. Moreover, the peak observation at a low angle position could also be utilized to evaluate the interlayer space of nanosheet zeolite. For the direct observation of the nanosheet morphology formation, the SEM and TEM analyses are the mandatory techniques that should be performed. In general, nanosheet zeolite exhibits a more open structure with the ultra-thin of crystal thickness, in which the composition of the stacking layers could be determined by observing the d-spacing between the layers observed in the TEM images. Since the space of the interlayers is in the mesopore scale, the nanosheet zeolite usually shows a type IV isotherm, indicating the presence of the mesopore with a large external surface. It benefits the catalysis process, which needs an accessible acidic site to facilitate the reaction. Furthermore, when the product is formed, it will easily diffuse to the outer, restraining the secondary reaction. For the metal-substituted catalyst, nanosheet zeolite provides a remarkable surface for metal to be well-distributed. Thus, the appropriate amount, strength, and distribution of acid sites could be obtained, leading to improved catalytic performance. In the case of the acid properties evaluation, the FTIR, NH3-TPD, as well as the MAS-NMR characterization was usually employed. In addition, the organic base molecule has also been reported as a probe molecule to determine the accessibility of the acid site.The catalytic activity of nanosheet zeolites in several reactions such as methanol conversion, alkylation, isomerization, cracking, oxidation/reduction exhibited excellent performance compared to conventional zeolite. Nanosheet zeolites show high conversion and selectivity as well as high stability. Due to its large dimensions in the a-c plane, as previously mentioned, the nanosheet zeolite crystals end up with a higher surface fraction (010) than their conventional counterparts. Accordingly, the substrate is more feasible to reach the pore of the nanosheet zeolite. The ultrathin nanosheet also provides shortened diffusion paths (especially the straight channels) in nanosheet zeolite catalysts. The crucial aspects for product selectivity are texture properties and acidity of the catalyst. Furthermore, nanosheet zeolite is more resistant to deactivation because the formed coke is stored in the external surface so that the active site is not easily covered by coke. Table 1 demonstrates several zeolites that have been reported to be applied in several reactions. As can be seen, MFI is the most common topology that has been applied, in which the surfactant assisted method is the most used synthesis strategy. Notably, some modification with metals is also applied to adjust the catalyst acidity and/or facilitate the redox reaction. Although the presence of the nanosheet morphology has improved the catalytic activity, other properties shall be carefully adjusted. For instance, the Si/Al ratio for the common methanol to propylene reaction (MTP) is usually higher than 200, meanwhile, it is lower for the aromatization and isomerization reaction (<100). Besides, other zeolite types such as FER, MOR, and FAU nanosheet is still limited, suggesting the availability of more opened areas to design and applied those type of zeolites. In the catalysis point of view, the application of nanosheet zeolite to support the green and sustainable process begin to be emphasized, i.e., the water splitting reaction, biomass conversion, and the CO2 reduction. The research focused on the broader area should be engineered owing to the potential capability of nanosheet zeolites.Despite nanosheet zeolites have advantages over conventional zeolite, the nanosheet zeolites still face several synthesis challenges, which are time-consuming, costly materials, and requires several steps; thus, limiting their practical applications. The diquaternary ammonium-based SDAs are not commercially available, and should be synthesized through complex, multistep organic reactions. In this sense, the seed-induced method might be the more preferred route due to its less complexity and relatively lower cost. Extensive studies should be pursued to engineer the crystal growth parameters to obtain the nanosheet zeolites with desired properties (thickness, lateral dimension, chemical composition, etc.). The use of small molecules (e.g., urea) could induce the crystal growth-inhibiting effect in particular directions; thus, promoting the nanosheet morphology. Moreover, the synthesis of nanosheet zeolites using low-cost, renewable precursors, e.g., silica extracted from agricultural waste, or natural sources, could be promising, as have been implemented in the synthesis of hierarchical zeolites [28].Finally, from the significant progress of nanosheet zeolite, several insights could be constructed. The design of a more affordable bifunctional template and the utilization of the renewable source for nanosheet zeolite synthesis should be considered. Moreover, the in-situ characterization coupled with the computational studies could be performed to precisely determine a significant factor dictating both the nanosheet formation and the catalysis reaction. Further development using machine learning techniques also allows the rationalization of physicochemical, and structural insights into the chemistry of zeolite synthesis and catalysis, leading to the understanding of empirical knowledge, classification of synthesis records, discovery of novel materials and efficient reaction route. Moreover, It could also speed up understanding the synthesis−structure relationship. From a technological point of view, applying nanosheet zeolite for catalytic membrane reactors would bring a remarkable breakthrough to the catalysis industry since its unique structure demonstrates an outstanding performance. To this end, the industrial upscale of the material synthesis, as well as its application, could drive a breakthrough in energy and fine chemical industries.Grandprix T. M. Kadja: Conceptualization, Writing – original draft, Writing – review and editing, Supervision, Funding acquisition. Azhari, Noerma J. Azhari: Writing – original draft, Writing – review and editing, Validation, Visualization. St. Mardiana: Writing – original draft, Writing – review and editing, Validation, Visualization. Neng T. U. Culsum: Writing – review and editing, Validation, Formal analysis. Ainul Maghfirah: Writing – review and editing, Validation, Formal analysis.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 Hibah PDUPT 2022 (first year) (Contract no. 083/E5/PG.02.00.PT/2022) from the Ministry of Education, Culture, Research and Technology of the Republic of Indonesia.

Recently, zeolites with two-dimensional structures, so-called nanosheet zeolites, have been intensively advanced owing to their excellent catalytic performance compared to conventional zeolite. This extremely thin nanosheet structure of zeolite provides more accessible active sites contributing to shortened diffusion pathways and primarily enables bulky molecules to undergo catalytic reactions. Several nanosheet zeolites, such as MFI, FAU, MOR, MWW, TS-1, have been successfully synthesized. Generally, they are obtained by direct synthesis method involving surfactant as structure-directing agent (SDA) and or seed. Interestingly, almost every part of the synthesis parameter conditions can alter the characteristics of this nanosheet zeolite. Thus, 2D zeolites also offer easily tunable characteristics. Up to now, nanosheet zeolites have been extensively studied as a catalyst in methanol conversion, catalytic cracking, isomerization, alkylation, oxidation/reduction, lignocellulose conversion, and other reactions. Furthermore, the modification of nanosheet zeolite could be pursued to enhance its catalytic performance. Pillaring is evidently effective in improving the textural properties of nanosheet zeolite by protecting the mesopore structure during the SDA removal. Herein, we comprehensively present the recent progress on the development of nanosheet zeolites. This review concludes with a summary of a discussion of remaining challenges and outlook for nanosheet catalyst technologies.

Recently, due to declining oil resources, research on alternative transport fossil fuels has grown rapidly in both industry and academia [1,2], and fossil fuel customers have turned to biofuels [3]. For such reason, researchers have paid special attention to hydrogen as a promising alternative to fossil fuels to overcome the energy crisis and environmental pollution, because the product of hydrogen and oxygen combustion is steam [4,5]. One of the strengths of this valuable energy carrier is its abundance [6], renewability [7], and its non-pollution and high energy density [8]. Electrochemical reduction of molecular hydrogen from water is a general process in producing pure and very clean hydrogen, but it requires considerable electrical energy to break the hydrogen and oxygen bond, which is due to the high voltage of hydrogen. Therefore, minimizing the cathode voltage and its economic aspect is very important [9]. In the last few decades, many efforts have been made to develop transition metals such as cobalt and nickel as inexpensive high-efficiency catalysts [10]. Nickel selenide possess unique electron states and exhibit excellent performance for HER [11]. Cobalt selenide is one of the catalysts that has been considered recently and due to the position of its electron layers and the inherent nature of the charge transfer material from the electrode surface to the bulk is very fast, which can create superior electrocatalytic properties [12]. For electrocatalysts, high specific surface area in nanostructured materials can increase hydrogen release activity [13,14]. Nickel foam is an attractive raw material for electrocatalytic application due to its low cost, high electrical conductivity, high porosity, and structural integrity [15], which used as substrate for catalytic applications. In previous researches to synthesis nickel–cobalt selenide CoCl2·6H2O, Co(NO3)2, Co(OH)2, Co(CH3COO)2 Co3 was used as precursor for cobalt, NiCl2·6H2O, Ni(NO3)2, Ni(OH)2 and Ni(CH3COO)2 as precursor for nickel and Se powder, Na2SeO3, Se(CH3)2 and SeO2 as precursor for selenium, which synthesized on various substrate; for example Bhat et al. [16] synthesized nickel selenide nanosheets on nickel foam using nickel hydroxide and Se powder by hydrothermal method, Kwak et al. [17] synthesized nickel–cobalt selenide nanocrystals using Co (NO3)2, Se (CH3)2, Ni (NO3)2, Ge (CH3)4 and Nafion precursors using a laser process and layered on the silicon wire substrate, Liu et al. [18] using Ni(CH3COO)2, Co(CH3COO)2 and SeO2 precursors, nickel–cobalt selenide nanoparticles were synthesized by electrochemical deposition on titanium sheet, Ming et al. [19] synthesized (Ni, Co)Se2/C-HRD nanoparticles using CO (NO3)2, Ni (NO3)2 and Se powder precursors on ZIF-67 by heating in a quartz tube space, Rezaei et al. [20] reported a two-step synthesis of nickel–cobalt selenide using CO(NO3)2, Ni (NO3)2, Na2SeO3 and N2H4 precursors on reduced graphene by hydrothermal process; but, in previous research, there has been no report on the synthesis of nickel and cobalt selenide using CoCl2·6H2O, NiCl2·6H2O, Na2SeO3 and N2H4 precursors on nickel foam using one-step hydrothermal synthesis; in addition to contradictory compounds have been reported for the most suitable nickel–cobalt–selenide composition. Bin et al. [21] and Liu et al. [18] reported the combination of Co0.11Ni0.89Se2 and Co0.13Ni0.87Se2, respectively, while Zheng Xin et al. [22] reported the combination of Co0.8Ni0.2Se as the one with the most activity in the hydrogen evaluation; Therefore, this research is based on the synthesis of nickel–cobalt selenide using CoCl2·6H2O, NiCl2·6H2O, Na2SeO3, and N2H4 precursors on nickel foam using one-step hydrothermal method. The purpose of this study was to investigate the structures of nickel selenide, cobalt selenide, and nickel–cobalt selenide as a cathode material in the process of water molecule breakdown, or in other words, hydrogen reduction in the water electrolysis process, and investigated the effect of different ratios of nickel and cobalt and the effect of hydrothermal temperature on electrocatalytic efficiency of these structures.Distilled water was used in all experiments in this study. The chemicals used included nickel chloride hexahydrate (NiCl2·6H2O), cobalt chloride hexahydrate (CoCl2·6H2O), sodium selenide (Na2SeO3), hydrazine (N2H4), hydrochloric acid (HCl), and nickel foam, which were purchased from Merck.Commercial nickel foam with 3 mm thickness cut into 10 mm × 30 mm pieces, then to remove the surface oxides and impurities and activate the nickel foam surface, in a solution containing 3 M hydrochloric acid became ultrasonic in an ultrasonic bath for 15 min and washed several times with acetone and distilled water, then dried at room temperature.Two series of hydrothermal experiments were performed at 150 and 180 °C. Sodium selenide and hydrazine were constant in all experiments and 10 ml (100 mM) and 2 ml were used, respectively. Another variable parameter is the ratio of nickel chloride to cobalt chloride. The solution was prepared by first pouring 10 ml of nickel chloride solution (100 mM) into the beaker for the first sample (NiSe-150) and then 10 ml of sodium selenite (100 mM) was added while stirring to prevent the solution from clotting during the addition of the solutions, the beaker was stirred, then 2 ml of hydrazine (as reducing agent) was added to the solution during stirring, and by adding 78 ml of distilled water, the total volume of the solution was increased to 100 ml. The solution was stirred by a magnetic stirrer for 15 min to obtain a uniform solution without light pink clots. The solution with a piece of pre-activated nickel foam was poured into a Teflon-lined autoclave and thoroughly sealed. The container containing the solution was placed in the oven and it was set at 5 h and a temperature of 150 °C. Subsequent experiments were performed by different Ni+2:Co+2 mol ratios of 8:2 (Ni8Co2Se-150), 4:6 (Ni4Co6Se-150), 6:4 (Ni6Co4Se-150), 2:8 (Ni2Co8Se-150), and finally cobalt chloride 0:10 (CoSe-150) instead of nickel chloride with the conditions of the first experiment, in which a total of 6 samples were made. Then 6 samples were synthesized again with the previous ratios at 180 °C (NiSe-180, Ni8Co2Se-180, Ni6Co4Se-180, Ni4Co6Se-180, Ni2Co8Se-180, and CoSe-180). After reaction in the autoclave and cooling to ambient temperature, the chamber was ejected from the oven and deposited precipitation with precipitation growing on the nickel foam substrate was washed several times with distilled water and dried in a vacuum at 60 °C for 5 h. The precipitated powder was separated from the solution in an autoclave process by centrifugation and perform XRD analysis with a device model Rigaku ivultim to determine the structural phases, FESEM with a device model Mira3 TESCAN-xmu and TEM with a device model Philips CM30 to evaluate the morphology and FT-IR with a device thermos-avatar model to determine the functional groups. To evaluate the amount of hydrogen release activity, the produced samples (powder precipitated on nickel foam) were electrochemically examined. For this purpose, a Vertex model Ivium potentiostat device was used in a three-electrode system with a platinum electrode as a counter, a silver electrode (Ag|AgCl/saturated with KCl) as a reference electrode, and deposited thin films for a working electrode to which the test specimen is attached and 1 M KOH solution at 25 °C was used for the electrolyte of all electrochemical tests.The results of the electrochemical measurements were recorded in the form of Nyquist, Phase, and Bode plots and the Zview software was used to determine equivalent circuits for this test. All of the LSV diagrams shown in this activity were iR modified. All the potentials measured experimentally against Ag|AgCl were shown using the Nernst equation, the same as Equation (1), converted to a reversible hydrogen electrode (RHE). (1) ERHE = EAg | AgCl + 0.059 pH + E°Ag | AgCl Here, the standard potential of Ag|AgCl at 25.1 °C is 0.197 V versus RHE.The electrochemical kinetics of the (NiCo) Se HER catalysts related to the overpotential ( η ) with current density (j) have been performed to calculate the Tafel slop using Equation (2). (2) η   =  a  + 2.3 R T α n F log   ( j ) where η is the overpotential, j is the current density and other symbols have their usual meaning. The Tafel slope is  2.3 R T / α n F . Tafel slopes were calculated from the polarization test with a scanning speed of (10 mV s−1) in 1 M KOH solution.X-ray diffraction analysis of synthesized samples at 150 °C from hydrothermal process, can be seen in Fig. 1 (a). According to the diffraction pattern of NiSe and CoSe samples, the peaks show high intensity, which indicates the crystalline structure of the electrodes, and the formation of the crystallinity structure leads to stability in long-term performances and it is a determinant factor in hydrogen evolution activity. In diffraction pattern of NiSe-150 and CoSe-150, the peaks generated in the 2θ specific correspond to the pages specified in Fig. 1(a). Due to the similar atomic diameters of nickel and cobalt, there will be no change in the plans where cobalt replaces nickel [18], finally, there will be no significant change in the angle of the peaks; e.g. peak in NiSe-150 which 2θ = 33.50, in CoSe-150, the same peak is in 2θ = 33.4 which clearly shows the overlap of the peaks. The crystallinity size calculated by Scherrer method; crystalline size was 27, 33, 31, 45, 24 and 39 for NiSe-150, Ni8Co2Se-150, Ni6Co4Se-150, Ni4Co6Se-150, Ni2Co8Se-150 and CoSe-150 respectively. Fig. 1(b) also shows the FTIR spectroscopy pattern of synthesized samples at 150 °C and confirms X-ray diffraction, and shows that at low wavelengths metal bonds are formed [23]. EDS analysis was used for Ni8Co2Se-150 to shows the amount of each existing element in the sample, the results of which can be seen in Fig. 1(c) and shows that all three elements nickel, cobalt, and selenium are presented in the structure. To investigate the surface morphology of the Ni8Co2Se sample, which was introduced as the optimum sample, TEM analysis was used and the results presented in Fig. 1(d) which shows that the powders are in the form of spheres on the surface of that structures. From the TEM pattern, it can be seen that the material is polycrystalline. This pattern indicates that the material is amorphous if the light points be a complete circle, but if it is a series of dotted points around a circle perimeter, it indicates that the material is polycrystalline.FESEM results of the synthesized samples at 150 °C are shown in Fig. 1(e–j). The resulting NiSe-150(free cobalt) image Fig. 1(e) shows that it has grown as a nanoparticle, the synthesis of nickel selenide by the hydrothermal process is usually arranged as a nano-grid [24]. These images show that the nanoparticles are in the form of plates and placed next to each other and have formed a nano-plates structure. Fig. 1(f) shows a sample of Ni8Co2Se-150 growing in the form of regular and continuous nano-plates due to the addition of a small amount of cobalt in the nickel selenide structure, which is called hydrangea structure. Porous spaces have grown uniformly in all directions [25]. Such nanostructured particles are very useful for electrocatalytic activity [26,27]. One of the factors that improves the electrocatalytic activity of electrodes is to increase the specific surface area of the electrode, because the empty space between the nano-plates is a good location to place ions in the electrolyte and increase the reaction surface in the hydrogen release. In sources, this principle is known as the roughness factor, so that the higher specific surface area of an electrode (roughness factor), active points on the surface also increases and exhibits better kinetics.Then, by increasing the ratio of cobalt to nickel, the morphology is taken out of the regular lattice state and also reduces the specific surface area of the catalyst. In Fig. 1(g), which is related to the Ni6Co4Se-150 sample, the structure is shown as irregular nano-plates with double growth of some plates, which is discontinuous and nanoparticles have grown along with it. This reduces the catalytic properties of the electrode, which can be proved by electrochemical tests, which will be described below. In the Ni4Co6Se samples and Ni2Co8Se, respectively, shown in Fig. 1(h and i), it will grow in the same way and will decrease the specific surface area with the roughness factor. In the CoSe-150 sample Fig. 1(j), regular structure growth appears again, which is visible as a sphere with a latticed surface. The nano-plate thicknesses were measured between 20 and 30 nm. In the following, in Fig. 1o, the sample of map analysis is shown, which in Fig. 1(k and n) shows the distribution of elements in the structure. This proper dispersion results in the uniform electrocatalytic activity of the sample at different stages of the electrochemical tests.The most important quantity studied in the Tafel test is the slope of the Tafel cathodic plot (bc), because the Tafel slope is directly related to the kinetics of the catalysts reaction [28], which is the angle coefficient of the logarithmic variation of the observed current density according to the applied overpotential. If other conditions are constant, the smaller Tafel slope of the test electrode exhibits higher electrocatalytic activity of the electrode in the hydrogen evolution reaction. Steady-state polarization diagrams (Tafel diagrams) obtained from the surface of the electrodes in 1 M KOH solution at 25 °C are shown in Fig. 2 (a and b). In these two diagrams number of Tafel slope is be able seen on each plot. That the Ni8Co2Se-150 sample with 61.3 mV shows the lowest slope among all samples. Increasing the cobalt/nickel ratio in the structure, increases the Tafel slope, for this reason, samples Ni6Co4Se-150, Ni4Co6Se-150, Ni2Co8Se-150 has higher slopes at both synthesis temperatures of 150 and 180 °C respectively. The higher slope indicates lower electrocatalytic activity and lower reaction kinetics. As mentioned before, one of the factors that affect the density of the exchange current is the roughness factor, and this parameter does not affect the reaction mechanism, but its effect is observed on the current density and the Tafel curve is performed without changing the slope or reaction mechanism leads to a higher current density, and as the electrode surface area increases, this principle becomes more pronounced: that attention to Tafel slope the samples is quite obvious. The Tafel slope also shows that the samples containing cobalt have the lowest potential, which confirms the increase in nickel activity by adding cobalt to a certain amount. According to the Tafel test shown in Fig. (2a and b), the activity of hydrogen evolution increases by 20% with the addition of cobalt, and with increasing the cobalt to nickel ratio over than 0.2, we see an increase in the cathodic slope. This can be attributed to the reduction in hydrogen evolution activity, which can be attributed to the reduction of surface active sites due to the reduction of the continuous structure arrangement of the nano-plates [21]. Fig. 2(c and d) show the graph of the LSV and the activation potential for the synthesized samples which syntheses at 150 and 180 °C respectively, at the applied potential of 0 to −0.5 V with a sweep rate of 10 mV s−1, and Table 1 also presents the activation potential numerically. Since the evolution activity may vary in different current densities, this parameter was investigated in three current densities, start-up activity, −10 and −20 mA cm−2. In the initial potential or in some way the hydrogen reduction potential of the Ni8Co2Se-150 sample with −62 mV has the lowest value, which indicates better activity of this sample than other samples. In continuation, this sample maintains its activity trend and has the best activity in −10 mV current density with −149 mV over potential. In summary, the Ni8Co2Se-150 sample has a minimum of ɳ10 for hydrogen evolution in alkaline environments among selenide-based studies. For example: for nickel selenide nano-plates with carbon plates at ɳ10 overpotential 184 mV [29], for nano-forest nickel selenide with nickel foam substrate at ɳ10 over potential 203 mV [30], for cobalt selenide at ɳ10 overpotential 472 mV [31], for molybdenum selenide–nickel selenide at ɳ10 overpotential 210 mV [32], for cobalt selenide with nickel-double layer hydroxide foam substrate and graphene at ɳ10 overpotential 260 mV [33] have reported.Along with the confirmation of the Tafel test, the hydrogen evolution activity decreases with increasing the amount of cobalt over than 20%, as in the Ni2Co8Se sample with −149 mV overpotential in the current density of −10 mA, we see an activity of almost half of the sample activity. By adding the amount of cobalt to the structure of electrode, we see higher activity of this catalyst, which can be due to the structure with high electrical conductivity of the catalyst and the addition of alternate cobalt at the electrode surface causes better absorption of protons and water molecules [21].In Fig. 2(e, f) the vertical axis is the imaginary impedance (z″) and the horizontal axis is the real impedance (z′). In this figure the distance of the circle center from the origin is Rs + Rp/2. In the Nyquist diagram, the lower final resistance (Rct) leads to greater hydrogen evolution activity, which the Ni8Co2Se sample has the lowest final resistance and higher electrocatalytic activity, which confirms the LSV diagram and Tafel slope polarization. Also, other samples such as CoSe, Ni2Co8Se, NiSe, Ni4Co6Se, and Ni6Co4Se have higher resistance, respectively, which indicates a lower catalytic property than the Ni8Co2Se sample. This final resistance trend also applies to the synthesized samples at 180 °C, which can be attributed to the change in crystal structure from hexagonal to rhombohedral. Also, Zhang et al. reported that the catalyst crystal structure changes when temperature or time varies in hydrothermal process [34]. Also, in the equivalent circuit for these curves using Z-view software, we see two condenser capacity and three resistors, which indicates the fracture of the layer and the penetration of active ions into the substrate by the electrolyte. Other parameters in this equivalent circuit are: Rs is the soluble resistance (uncompensated resistance), Rf is the layer resistance, Cdl is the capacitance, Cdp is the imaginary capacitance, and Rct is the final resistance [35]. Numerical quantities obtained from the electrodes using ZView software are shown in Table 2 . Another diagram that can be obtained from the EIS test is the bode diagram. This diagram consists of two separate curves, one is phase angle variations in terms of frequency logarithm, Fig. 2(g and h) and the other related to impedance logarithm variations in terms of frequency logarithm, Fig. 2(i and j). In the diagram of phase angle changes shown in Fig. 2((g) at 150 °C and (h) at 180 °C), we see two time-constant indicating existence of two capacitors in the equivalent circuit. The phase angle transfer at the maximum frequency at higher values close to 90° indicates less electrocatalytic activity in hydrogen evolution [36]. In this diagram, the Ni8Co2Se-150 sample has the lowest phase angle transfer value, which indicates the high activity of this sample in hydrogen evolution reaction. In the bode curve shown in Fig. 2(i and j), the impedance at the highest frequency on the right side of the curve is equivalent to the soluble resistance and the impedance at the lowest frequency on the left side of the curve is equivalent to the resistance of the whole system. As expected, the Ni8Co2Se sample at both synthesis temperatures shown in Fig. 2((i) at 150 °C and (j) at 180 °C) has the lowest final resistance, which is confirmed by other diagrams obtained from electrochemical tests. Another noteworthy point in this diagram is the slope between the soluble resistance and the final resistance, which is equivalent to the capacitive region. Increased capacitive region indicates the inhibitory properties against the entry of electrolytes and corrosive agents into the coating and substrate.Since the Ni8Co2Se-150 sample synthesized at 150 °C, which was known as the optimum sample in the hydrogen evolution process according to other electrochemical tests, was subjected to chronoamperometry stability test. As can be seen in the diagram in Fig. 2(k), after 12 h at a potential of 150 mV, the sample does not show a significant drop in current, indicating the catalyst stability in an alkaline environment for long periods. To compare the catalytic properties of the Ni8Co2Se-150 electrode, after stability test for 12 h, a linear sweep voltammetry test (LSV) was performed on the sample and the results of the LSV test shown in Fig. 2(l). Failure to move the LSV diagram on the horizontal axis (potential axis) indicates no dispersion and ultimately a decrease in the catalytic properties of the electrode. The fact that no significant decrease in hydrogen evolution potential occurred after 12 h indicates that this sample will remain stable for longer periods.One of the applications of cyclic voltammetry tests is to measure the reversibility of the electrode. The greater difference between the anode and cathode peaks, the electrode is less reversible. All samples were scanned at 10, 20, 30, 40, 50, 70, and 100 mV s−1 scan rates. The cyclic graph for the synthesized electrodes at 150 °C is shown in Fig. 3 (a–f) and 150 °C is shown in Fig. 3(i–n). The reactions performed during the test include the oxidation reaction (related to the anodic peak) and the reduction reaction (related to the cathodic peak). An anode peak and a cathode peak appeared in all samples. As shown in Fig. 3, the anode peak current increases with increasing scan rate, indicating the high electrocatalytic activity of the electrodes in the redox reactions. As it is known, with increasing the scan rate without any apparent change in the electrode, the current density has increased, which indicates the stability of the electrode material. The peak symmetry indicates the excellent reversibility of the electrodes. The displacement of the peaks towards positive or negative currents indicates the property of electron transfer and electrical conductivity, that is, the farther apart the anode and cathode peaks, the higher catalytic property of the sample, which separates with increasing scanning rate of the anode and cathode peaks, and the difference between the anodic and cathodic peaks is linearly proportional to the square root of the scanning rate. These behaviors suggest that faradaic reactions to store energy are a process controlled by the penetration of electrolyte ions, which may be as following: (3) ( Ni,Co ) Se 2 + 2 OH – ⇌ NiSeOH + CoSeOH + 2 e – (4) CoSeOH + OH – ⇌ CoSeO + H 2 O + e – (5) NiSeOH + OH – ⇌ NiSeO + H 2 O + e – The cyclic diagram of the synthesized electrodes at 150 °C is shown in Fig. 3(g, h) and for the synthesized electrodes at 180 °C in Fig. 3(o, p). Scan rates of 10 mV s−1 (g and o) and 100 mV s−1 (h and p) were compared in all samples and showed that by adding some cobalt to the nickel selenide structure, the area under the CV graph increases, and along with it the difference between the anodic and cathodic peaks also increases, which in both 150 and 180 °C indicates an increase in current density and thus an improvement in catalytic properties, which the area increase in the Ni8Co2Se sample is quite clear. Then, by increasing the cobalt to nickel ratio over than 20%, we encounter a decrease in the anodic and cathodic peak differences, which indicates that increasing the cobalt to nickel ratio over than 20% causes a decrease in the catalytic properties of the electrodes, which is confirmed by other electrochemical tests. Table 3 shows the comparison of the difference between anodic and cathodic peak currents (ΔJ) on the CV diagram of the synthesized samples at 150 and 180 °C at a scan rate of 10 mV s−1. This table shows that all samples synthesized at 150 °C have a higher ΔJ number than samples synthesized at 180 °C and the highest value is related to the Ni8Co2Se-150 sample.We further used the Comparative chart in Fig. 4 to demonstrate the preference of electrocatalytic activity of the Ni8Co2Se sample in compared previous research which show Ni8Co2Se sample with Release activity 187 mv Contains more optimal activity than some other catalysts.In summary, to synthesize the optimal NiCoSe composition, we synthesized different compositions at two different temperatures and examined the hydrogen evolution. We showed that by slightly increasing the amount of cobalt in this composition, the hydrogen evolution activity increases so that in Ni8Co2Se sample with an overpotential of −185 mV at current density of −10 mA cm−2 and −201 mV at a current density of −20 mA cm−2 has the highest activity in hydrogen evolution. Also, the mentioned sample has maintained its activity after 12 h in the chronoamperometry test, which is an indication of the proper conditions of this electrocatalyst.No funding was received for this work.All listed authors meet the ICMJE criteria. We attest that all authors contributed significantly to the creation of this manuscript, each having fulfilled criteria as established by the ICMJE.No conflict of interest exists.

Designing of complex nanostructure with a high specific surface area using transition metal selenide seems to be a necessity in response to the challenges of the hydrogen release process for renewable energy. Herein, we synthesized the electrocatalyst nickel foam-supported with nanostructured nickel–cobalt selenide using the hydrothermal process at 150 and 180 °C and has been investigated in hydrogen evolution reaction. Synthesis of nickel–cobalt selenide nanostructure on nickel foam carried out with different Ni+2:Co+2 mol ratios of 0:10, 2:8, 6:4, 4:6, 8:2, and 10:0 in the structure of electrodes. The XRD results indicate the formation of the nickel–cobalt selenide phase in different ratios. The FESEM and TEM results show the formation of mesoporous three-dimensional nano-lactic with petal thickness in the range of 20–30 nm. Also, to evaluate the properties and electrocatalytic efficiency, electrochemical tests of Tafel slope, cyclic voltammetry, linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) were used. The Tafel result test shows a cathodic slope of −61.3 mV and an exchange current density of 0.86 mA cm−2 in the Ni8Co2Se-150 (Ni+2:Co+2 = 8:2) sample, which LSV shows −185 mV at a current density of −10 mA cm−2. EIS test shows a polarization resistance value of 2744 Ω for mentioned sample and was selected as the best sample with the highest catalytic activity.

Hydrogen peroxide (H2O2) is one of the cornerstones of the chemical industry, with a wide range of applications in areas such as pulp bleaching, chemical synthesis, and disinfection. The demand for H2O2, considered a green oxidant that generates only O2 and H2O as by-products, is constantly increasing. 1 , 2 The global H2O2 market has a value of $3.1 billion and is predicted to reach over $4 billion by 2027 at an annual growth rate of 4.2%. 3 The history of H2O2 production dates back to the early 19th century, when it relied on the chemical reaction of BaO2 with HCl and electrolysis of H2SO4 solution. 4–6 Later, the autooxidation process became a major route for H2O2 synthesis, 7 followed by the emergence of the anthraquinone process in 1939, which remains a mainstay in the industry. 8 The anthraquinone process is currently implemented at a large scale and supplies approximately 95% of global demand. 1 , 2 , 9 However, this process requires pressurized H2 gas and Pd-based hydrogenation catalysts and involves energy-intensive distillation steps. Along with intensive energy requirements, the process further suffers from the generation of organic wastes by side reactions. Furthermore, the manufacture of highly concentrated products is more economically viable because of its centralized plant system, which, however, imposes additional costs for transportation. In addition, H2O2 decomposition occurs faster at high concentrations, which compels the use of stabilizing agents such as sodium pyrophosphate and chelating agents. Therefore, alternative processes for H2O2 synthesis are currently being widely investigated. Direct synthesis is a straightforward method in which H2 and O2 gases react to produce H2O2. 10 However, the high thermodynamic activity of this reaction obliges H2 gas to be diluted with CO2 or N2, and Pt-group-metal (PGM)-based catalysts are required to control the reactivity and reaction pathways. This method is also plagued by relatively low H2O2 selectivity and the potential risk of explosion.In this regard, the electrochemical production of H2O2 has attracted significant attention. 11–18 This method enables small-scale, on-site, and continuous production of H2O2. It is also safe and environmentally benign because no carbon-involving side products are generated, and aqueous electrolytes (H2O and H+) serve as hydrogen sources. There are two major pathways for H2O2 electrosynthesis: oxygen reduction reaction (ORR) and water oxidation reaction (WOR). The ORR can take place in several electrochemical and chemical reaction pathways, among which the desired H2O2 generation occurs by the two-electron pathway ORR (2e− ORR; Equation 1). 2 e − ORR : (Equation 1-1) O 2 + 2 H + + 2 e − → H 2 O 2 acidic (Equation 1-2) O 2 + H 2 O + 2 e − → HO 2 − + OH − ( alkaline, pH > 11.7 ) However, H2O2 production via 2e− ORR is thermodynamically unfavorable compared with 4e− ORR (Equation 2). The generated H2O2 is unstable, leading to further reduction to H2O (Equation 3) or chemical decomposition (Equation 4). 4 e − ORR : (Equation 2-1) O 2 + 4 H + + 4 e − → 2 H 2 O ( acidic ) (Equation 2-2) O 2 + 2 H 2 O + 4 e − → 4 OH − ( alkaline ) Electrochemical H2O2 reduction: (Equation 3-1) H 2 O 2 + 2 H + + 2 e − → 2 H 2 O ( acidic ) (Equation 3-2) HO 2 − + H 2 O + 2 e − → 3 OH − ( alkaline ) H2O2 disproportionation: (Equation 4) 2 H 2 O 2 → 2 H 2 O + O 2 Therefore, the development of active and selective electrocatalysts for the 2e− ORR is crucial for the successful implementation of electrochemical H2O2 production technology. The same is applied to H2O2 electrosynthesis via WOR, as it presents similar issues regarding multiple reaction pathways and controlling reaction selectivity.This paper provides a comprehensive review of selective electrocatalysts for the 2e− ORR and a current understanding of the electrocatalytic process and reactivity-determining factors. First, we present the recent progress in the design of H2O2 electrosynthesis catalysts via 2e− ORR and in the understanding of the nature of the active sites and their impact on the activity and selectivity. The electrocatalyst section is categorized according to the chemical composition of the catalysts: PGM-based atomically dispersed catalysts (ADCs), non-PGM-based ADCs, and metal-free heteroatom-doped carbon catalysts. The H2O2 electrosynthesis activity of each class of catalysts is benchmarked to understand the current status of advancement and to provide guidelines for future studies. Interfacial factors and phenomena that regulate the H2O2 production activity and selectivity are also introduced. We suggest guidelines for the accurate measurement of the catalysts’ performance on H2O2 electrosynthesis, which have been largely overlooked in the current literature. Finally, reactors designed for high-current-density operation and systems that utilize electrosynthesized H2O2 are presented.A mechanistic understanding of a specific reaction is essential for the rational design of efficient electrocatalysts. For the ORR, two major mechanisms, the associative mechanism and the dissociative mechanism, are suggested (Figure 1 ). In the associative mechanism, O2 adsorption and subsequent ∗OOH formation commonly occur regardless of the final product, according to the following equation: (Equation 5) O 2 + ∗ + ( H + + e − ) → OOH ∗ If the binding strength of the ∗OOH intermediate on a surface site is medium or weak, further proton and electron transfer occur, leading to the formation of H2O2 via the 2e− pathway (Equation 6). (Equation 6) ∗ OOH + H + + e − → H 2 O 2 + ∗ However, when the ∗OOH intermediate is strongly adsorbed, the O–O bond dissociates to form the ∗O intermediate. As a result, sequential 2e− processes complete the 4e− ORR (2e− × 2e− pathway) to generate H2O (Equation 7). (Equation 7-1) ∗ OOH + ( H + + e − ) → O ∗ + H 2 O (Equation 7-2) O ∗ + ( 2 H + + 2 e − ) → OH ∗ + ( H + + e − ) → H 2 O + ∗ In the dissociative mechanism, a pair of adjacent surface sites strongly adsorb O2 molecules, which are broken down into ∗O intermediates. Only ∗O and ∗OH intermediates are involved in this mechanism without the formation of the ∗OOH intermediate, leading to H2O product via the 4e− pathway (Equations 8 and 7-2). (Equation 8-1) O 2 + 2 ∗ → 2 ∗ O (Equation 8-2) 2 ∗ O + ( 2 H + + 2 e − ) → 2 ∗ OH Therefore, high H2O2 product selectivity from the ORR can be achieved using electrocatalysts that favor the associative mechanism. Another important factor is the ∗OOH binding energy of the catalytic active site. Active sites with low ∗OOH binding energies are unfavorable for the first O2 adsorption, according to the linear scaling relation of the reaction intermediates, reactants, and products. The weak binding thus leads to high H2O2 selectivity but a low conversion rate (high overpotential). Likewise, the scaling relation also suggests that the active sites with high affinity for ∗OOH intermediates also show high binding strength with ∗OH and ∗O intermediates, resulting in low H2O2 selectivity. According to the binding energy criteria, inert PGMs such as Au and Ag with low ∗OOH binding energies would have high H2O2 selectivity and low activity, whereas oxophilic transition metals such as Fe, Co, Ni, and Mn would show low 2e− ORR selectivity and activity. Pt and Pd, with optimal ∗O binding energies, favor the 4e− ORR. 11 The arrangement of catalyst surface atoms affects the adsorption geometry and binding strength and thus the reaction pathways. For instance, ab initio calculations revealed that the different adsorption strengths of oxygenated reaction intermediates depend on the adsorption sites on crystalline Pt surfaces. 19 Therefore, controlling the atomic arrangement can serve as an effective way of catalyst design, leading to different O2 adsorption modes and reaction mechanisms. For a geometrically isolated site, O2 is adsorbed in the end-on mode, rendering preference for the associative mechanism, whereas an ensemble site can adsorb O2 in a side-on geometry favoring the dissociative mechanism (Figure 1). In summary, ORR selectivity can be adjusted by the chemical composition and geometric structure of the active sites. Various classes of electrocatalysts, including monometallic PGMs (Au and Ag), 20 , 21 PGM-based alloys (Au–Pd, Pt–Hg, Pd–Hg, and Pd–Au), 22–24 PGM-based ADCs (Os, Ir, Rh, Ru, Pt, and Pd), 25–31 non-PGM-based ADCs (Fe, Co, Ni, and Mn), 32–46 metal compounds (oxides, sulfides, borides, etc), 47–50 and metal-free doped-carbon catalysts, 51–88 have been explored as selective electrocatalysts for the 2e− ORR. In this review, we focus on low- and non-PGM catalysts: PGM-based ADCs, non-PGM-based ADCs, and metal-free doped-carbon catalysts, which will be reviewed in the next section.In the early stage of PGM-based selective electrocatalysts for the 2e− ORR, Pt- and Pd-based alloys diluted with less catalytically active elements (i.e., Hg and Au) were demonstrated to exhibit excellent H2O2 selectivity. This reactivity can be explained by the peculiar atomic arrangements of active sites that inhibit O–O bond breakage and stabilize the ∗OOH intermediate. 22–24 However, only a small fraction of the precious metals in the alloy nanoparticles are exposed on the surface, and thus, their utilization efficiency is low. In addition, the use of toxic or other precious metals as secondary metals raises concerns from environmental and economic points of view. PGM-based ADCs allow full utilization of PGM with atomically dispersed active sites, which significantly reduces the amount of expensive PGM. Hence, considerable effort has been devoted to the development of PGM-based ADCs on appropriate supports with optimized anchoring sites. 25–31 Carbon is the most commonly used support material. However, carbon itself possesses a very low capacity for generating single atomic sites owing to its deficient anchoring sites. In this regard, electron-rich heteroelements such as N, S, and O have been deliberately introduced into the carbon lattice, as these dopants have the propensity to form coordination bonds with the metal atoms. In a notable example, Choi and co-workers developed S-doped microporous carbons as supports for atomically dispersed Pt. 25 Because S has a strong affinity for most PGMs, the S-doped carbon sustains a high loading of Pt atoms of up to 5 wt %. Unlike Pt nanoparticles, which are favorable for the 4e− ORR, the atomically dispersed Pt coordinated to S exhibited high 2e− ORR selectivity.The electrocatalytic activity and selectivity of ADCs strongly depend on the composition and structure of their active sites. Therefore, a synthesis platform that can produce ADCs across a broad compositional range without any clusters or particles is in high demand, which allows the unraveling of the trends of 2e− ORR activity and selectivity. Joo and co-workers established a general strategy for PGM-based ADCs with various types of metal centers (Os, Ru, Ir, Rh, and Pt) to study their 2e− ORR performance. 26 The “trapping-and-immobilizing” strategy developed in this work could effectively prevent the agglomeration of PGM atoms during impregnation and reductive activation steps. In this process, a carbon support was coated with an ionic-liquid-derived carbonaceous layer containing electron-rich heteroatoms. During impregnation and drying, such heteroatoms can trap metal precursors via electrostatic interactions. A sacrificial silica layer was then coated on the metal-precursor-impregnated carbon to immobilize the metal species and thereby to mitigate their agglomeration during reductive thermal activation. Thus, the synthesized ADCs (denoted as M1) generally showed higher 2e− ORR selectivity than the corresponding PGM nanoparticle (MNP) catalysts, which favored 4e− ORR. Among the M1 catalysts, Rh1 showed the highest H2O2 production activity (lowest overpotential), and Pt1 showed the highest H2O2 selectivity. The difference in H2O2 selectivity between the ADCs and NP catalysts can be explained by different reaction mechanisms; the M1 catalysts follow an associative mechanism (end-on adsorption) in which the OOH∗ intermediates are preserved, whereas the ORR over the MNP catalysts proceeds via a dissociative mechanism due to the presence of metal ensemble sites. The trend for H2O2 production among the M1 catalysts can be explained by density functional theory (DFT) calculations. The difference between the binding energies of the ∗O and ∗OOH species (ΔG O – ΔG OOH), which describes the thermodynamic stability of ∗OOH intermediates, showed a linear relationship with the H2O2 yield (Figure 2A). The oxygen-binding-energy trend was experimentally verified by an O2 temperature-programmed desorption (TPD) experiment, where the O2 desorption temperature had an inverse linear relationship with the H2O2 selectivity. The M1 catalysts exhibited higher onset potentials for the 2e− ORR than the MNP catalysts, and a volcano-shaped plot was observed between the onset potential and oxygen binding energy of the M1 catalysts. Among them, Rh1, which has an appropriate oxygen binding energy, had the highest onset potential for H2O2 production. In contrast, the MNP catalysts generally exhibited low onset potentials for 2e− ORR, which had a relatively poor correlation with the ∗O binding strength (Figure 2B).The chemical identity of the coordinating atoms is another critical factor that regulates the catalytic properties. Because the preparation of ADCs typically involves high-temperature reactions of precursor mixtures, the control of the coordinating atoms can be mostly achieved by using a precursor comprising targeted elements. Joo’s group demonstrated reversible modification of the coordination environment of PGM-based ADCs by gas-phase ligand exchange reactions. 27 When the as-synthesized PGM-based ADCs (described above) were subjected to heat treatment under CO or NH3 gas, the ligands of the as-prepared ADCs were substituted with the corresponding gas molecules. Depending on the type of ligand, the H2O2 production activity and selectivity could be noticeably tuned; CO-coordinated Rh ADCs exhibited a higher onset potential but lower selectivity toward the 2e− ORR, and the reverse trend was observed for the NH3-coordinated catalyst. Importantly, ligand exchange was reversible, which consequently modulated the oxidation state of the metal center and its electrocatalytic activity in a reversible manner (Figure 2C). DFT calculations revealed that the switching behavior of the ORR selectivity originates from the change in the electronic structure and ∗OOH binding energy with respect to the ligand type.These examples highlight the importance of the chemical identities of the metal centers and coordinating atoms. Although carbon is usually utilized as a support for ADCs, other conductive materials have great potential, although many remain unexplored. 28 , 29 Lee and co-workers investigated TiN and TiC as supports for atomically dispersed Pt (Pt1/TiN and Pt1/TiC, respectively). 29 Pt1/TiC showed higher activity and selectivity for H2O2 production than Pt1/TiN (Figure 2D). DFT calculations suggested that the high oxygen affinity of TiN could facilitate the side-on adsorption of O2 molecules on Pt and Ti atoms, decreasing the 2e− ORR selectivity. In contrast, on Pt1/TiC, O–O bonds were preserved because of favorable end-on adsorption on the Pt atom, leading to higher H2O2 selectivity (Figure 2E). Li and co-workers developed a redox-based ion-exchange method to anchor Pt atoms on a CuS x support, where the strong Pt–S coordination enabled high loading of atomically dispersed Pt up to 24.8 wt %. 30 The optimized catalyst exhibited >90% H2O2 selectivity over a wide potential range of acidic electrolytes (Figure 2F). In contrast, the Pt NP on the CuSx catalysts and CuSx support itself showed low H2O2 selectivity, suggesting that the Pt–S x sites were active motifs.Non-PGM-based ADCs (also known as M–N/C catalysts because N-doped carbons are usually used as supports) have been studied mainly as 4e− ORR catalysts for application as hydrogen fuel-cell cathodes. In this regard, M–N/C catalysts exhibiting high 2e− ORR selectivity are undesirable for the fuel-cell application because 2e− ORR decreases the conversion efficiency and the generated H2O2 accelerates the deterioration of the fuel cell. However, the rapidly increasing interest in H2O2 electrosynthesis has shed light on non-PGM ADCs with a high 2e− ORR selectivity. In M–N/C catalysts, three major structural factors critically influence ORR activity and selectivity. These include the type of metal center, type of coordination atom and geometry (first coordination shell), and presence of heteroatom species adjacent to the active sites (second coordination shell) (Figure 3A). The impact of the metal center has been intensively studied because its control is relatively simple. Liu and co-workers investigated the H2O2 activity and selectivity trends of M–N/C catalysts with controlled metal centers (Mn, Fe, Co, Ni, and Cu). 32 DFT calculations predicted that Co–N/C would exhibit the highest performance for 2e− ORR because of the optimal ∗OOH oxygen binding energy of the Co–N active sites. The experimental activity trends in terms of overpotential were found to be Co > Fe > Ni > Mn > Cu (Figure 3B), whereas the selectivity was in the order of Co > Mn > Ni > Cu > Fe (Figure 3C), which is consistent with the DFT calculations. Similar trends have been verified in other studies; 33 , 34 however, considerable discrepancies in the obtained results have been found. This difference could originate from a change in the coordination environment depending on the catalyst preparation conditions.Therefore, the catalytic effect of the first coordination shell was examined. 35–41 Wang and co-workers prepared a set of Fe-based ADCs with different coordinating atoms: one with typical N-coordination (Fe-N-CNT) and the other with C- and O-coordination (Fe-CNT). 35 The two catalysts showed dramatically different ORR selectivity trends, where Fe-N-CNT followed the 4e− ORR, whereas Fe-CNT favored the 2e− ORR (Figure 3D). This was further substantiated by demonstrating the selectivity shift from 4e− to 2e− ORR by reductive heat treatment of Fe-N-CNT, which transformed the 4e−-ORR active Fe–N center to the 2e−-ORR active Fe–C–O center (Figure 3E). The functional groups adjacent to the atomically dispersed active site, even if not directly bonded to it, can also significantly impact electrocatalytic processes.The control of the so-called second coordination shell has recently emerged as a simple way to tune activity. 41–43 Hyeon, Sung, and co-workers investigated the activity of electrochemical H2O2 production of Co–N/C catalysts with neighboring oxygen functionalities in the second coordination shell as activity and selectivity modifiers. 42 DFT calculations revealed that the epoxy groups in the second coordination shell would optimize the ∗OOH binding energy of the Co-based active center and improve the 2e− ORR activity of the Co–N/C catalyst. In contrast, the raw Co–N sites are expected to exhibit a 4e− pathway preference. Experimentally, a Co–N/C catalyst was prepared on graphene oxides rich in epoxy groups (denoted as Co1–NG(O)). As a comparative sample, the Co1–NG(O) catalyst was annealed at high temperatures under inert conditions, resulting in a Co1–NG(R) catalyst. Spectroscopic analyses identified a higher content of epoxy groups in Co1–NG(O) than in Co1–NG(R). Co1–NG(R) showed a positively shifted disk onset potential and higher diffusion-limited disk current density than Co1–NG(O), whereas Co1–NG(O) showed a higher ring onset potential and diffusion-limited ring current density than Co1–NG(R) (Figure 3F). These results suggest that Co1–NG(R) and Co1–NG(O) catalyze 4e− ORR and 2e− ORR, respectively. This was verified by the H2O2 selectivity results, where Co1–NG(O) exhibited a high H2O2 selectivity (>80%) over a wide potential range (Figure 3G). Although the promotion effect of oxygen groups in M–N/C catalysts has been shown by other groups, various combinations of the metal centers and functionalities should be further explored to obtain the best synergistic effect.Metal-free carbon-based catalysts are perhaps the most attractive class of non-PGM catalysts because of their low price, high electrical conductivity, and abundance. As such, carbon-based catalysts have been actively studied for many electrochemical reactions in the past few decades. For the 2e− ORR, a surge of research interest in carbon-based electrocatalysts has emerged after promising results were demonstrated by Cui’s and McCloskey’s groups. 51 , 52 Both revealed that oxygen functional groups on carbon nanomaterials play critical catalytic roles in H2O2 electrosynthesis from O2. Cui and co-workers showed that the 2e− ORR selectivity and activity increased linearly with the oxygen content, and these correlations have since been further confirmed with other carbon nanomaterials. 51 DFT calculations suggested that C–O–C (ether-type) and O–C=O (carboxylate-type) moieties are possible active sites. McCloskey and co-workers prepared few-layered mildly reduced graphene oxide (GO) catalysts by thermal reduction of GO at controlled temperatures (F-mrGO(X), X = temperature). 52 F-mrGO(600) exhibited a higher H2O2 electroproduction activity than F-mrGO(300) and F-mrGO (Figure 4A). The high activity of F-mrGO(600) containing a lower O content than the others indicates that the annealing step transformed the less-active O groups in F-mrGO into active species. Spectroscopic analyses suggested that the basal ethers were transformed into active edge ether groups at high temperatures (Figure 4B). Subsequent studies have suggested several oxygen functional groups, including ether, epoxy, carbonyl/quinone, and carboxyl, as active sites for the 2e− ORR. In an effort to identify reactive oxygen species, Joo and co-workers investigated the intrinsic H2O2 electrosynthesis activity of respective carboxyl, carbonyl, and phenol groups, which are typically generated during the oxidative doping process. 53 Oxidized graphitic ordered mesoporous carbon (O-GOMC) catalysts containing the three oxygen functional groups were treated with benzoic anhydride (BA), phenyl hydrazine (PH), and 2-bromo-1-phenylethanone (BrPE), which can selectively block phenol, carbonyl, and carboxyl groups, respectively, allowing assessment of the activity of the respective functional groups. The site-blocked catalysts exhibited a considerable decrease in activity compared with the pristine O-GOMC catalysts but to a different extent depending on the blocked oxygen functionality (Figure 4C). To calculate the turnover frequency (TOF), which represents the intrinsic catalytic activity per active site, the number of each functional group was determined by quantifying the number of blocking molecules. The resulting TOF values revealed that the intrinsic activity was higher in order of the carboxyl, carbonyl, and phenol groups (Figures 4D and 4E). Liu and co-workers also performed a chemical blocking experiment using O-doped carbon-nanosheet catalysts, where C=O groups were identified as the active species. 54 There are still inconsistencies in the proposed active sites, which mainly originate from the fact that many different types of functional groups are simultaneously generated during uncontrolled doping procedures.Nitrogen doping is one of the most common methods to promote the activity of carbon catalysts; however, H2O2 electrosynthesis with N-doped carbons has been relatively less explored than with O-doped carbons. Strasser and co-workers prepared a set of N-doped mesoporous carbons using a hard-templating method by changing the synthesis conditions, such as the number of precursors and the carbonization temperatures. They found that the activity trends of the prepared carbons were not well explained by the zeta potential or defect site density. Instead, a volcano-type activity behavior with the N content was found, suggesting that excessive N active species can decrease H2O2 selectivity, although its origin remains elusive. 56 Qiao and co-workers investigated the ORR activity and selectivity trends of N-doped carbon model catalysts called N-mFLG-X (X = mass ratio of melamine to glycine). 57 The precursor composition was found to control the pyrrolic-N content with a fixed relative amount of pyridinic N and graphitic N (Figure 4F). The H2O2 electrosynthesis efficiency increased as the quantity of pyrrolic N in the catalysts increased (Figures 4G and 4H). In situ X-ray absorption near-edge structure (XANES) analysis indicated that N-mFLG-8, the best catalyst, contained a larger amount of OOH∗ intermediates and fewer O∗ intermediates than N-mFLG-16, which demonstrated that N-mFLG-8 favored the 2e− ORR, whereas N-mFLG-16 favored the 4e− ORR. The electrocatalytic properties for the 2e− ORR of heteroatom-doped carbons other than O- and N-doped carbons have rarely been reported. Wang and co-workers screened various heteroatom-doped carbons (B, N, P, and S) for H2O2 electrosynthesis. 58 Doping was performed by the thermal reaction of O-doped carbon black with precursors containing the desired heteroelements. All the catalysts possessed similar structural properties, except for the type of heteroatom dopant, enabling the study of model catalysts. The doped carbons generally exhibited high 2e− ORR activity with H2O2 selectivity of over 60%, and the trends were in the order of B, N, S, and P (Figures 4J and 4K). N-doped carbon showed a relatively high onset potential but a relatively low H2O2 selectivity, whereas P-doped carbon exhibited the opposite trend. DFT calculations suggested that the thermodynamic stability of the ∗OOH intermediate on B-doped carbon may explain the high H2O2 selectivity.In addition to heteroatom doping, the H2O2 production activity of carbon electrocatalysts can be boosted by structural tuning. Structural modification includes pore structure control, which can increase the surface area and enhance the mass transport of carbon catalysts, and the associated formation of defective carbon sites and surface curvatures, which can bring changes in the electronic structure of carbons. Nabae and co-workers prepared various N-doped carbons by hard templating using mesoporous silica KIT-6 with controlled surface area and porosity. 59 They found that mesoporous carbons exhibited approximately double 2e− ORR activity than microporous carbons, highlighting the importance of facilitated mass transport in the mesopores (Figures 5A and 5B). Bao and co-workers demonstrated similar results by comparing the electrocatalytic activities of mesoporous and microporous carbons (MesoCs and MicroCs, respectively) and highly ordered pyrolytic graphite (HOPG). 60 MesoCs exhibited a higher onset potential than MicroCs; however, both catalysts possessed a similar H2O2 selectivity of approximately 70% (Figure 5C). In addition to the effect of porosity-dependent mass transport, DFT calculations suggested the possibility of enhancing the activity of porous carbons through the formation of defect sites (Figure 5D). Optimal defect structures were identified based on the binding strength of the ∗OH intermediates. Joo and co-workers exploited a highly curved surface structure in MesoCs to enhance H2O2 electrosynthesis activity. 61 They prepared GOMCs using a hard-templating method with highly aromatic carbon precursors. Periodically arrayed graphitic carbon nanorods in GOMCs, which were composed of vertically stacked graphene nanosheets, allowed maximum exposure of active graphitic carbon edges (Figure 5E). The H2O2 electrosynthesis activity of the edge-site-rich GOMC was 28 times higher than that of the basal plane-rich CNT, whereas both catalysts showed similarly high H2O2 selectivity of approximately 90% regardless of the number of edge sites (Figures 5F–5H). Oxidative treatment of GOMC selectively installed oxygen functional groups at the edge and achieved an additional 3.5-fold increase in activity.Electrocatalysis is essentially an interfacial process that occurs at the boundary between an electrocatalyst and electrolyte. Therefore, in addition to the active-site structure of a catalyst, understanding the role of interfacial species and phenomena is equally important for tuning reaction selectivity. Joo and co-workers synthesized a set of OMCs doped with various combinations of heteroatoms (N, S, and O). 62 Kelvin probe force microscopy (KPFM) was used to correlate the ORR activity and selectivity trends of the doped OMCs with the nanoscale surface charge density (Figure 6A). The electrocatalytic ORR activity increased in the order N,S,O-OMC > N,O-OMC > S,O-OMC > O-OMC > OMC, whereas the H2O2 selectivity showed the reverse order (Figure 6B). Interestingly, the work functions of the doped OMC catalysts, as assessed by KPFM measurements, were linearly correlated with their H2O2 selectivities and reaction rates (Figure 6C). The results indicate that a doped OMC with a lower work function has a lower energy barrier for donating electrons from its surface to the adsorbed oxygen, which facilitates the formation of OOH∗ species and consequently promotes the 4e− ORR, and vice versa. Hence, the work function of a doped carbon and its capability to promote the first electron transfer to generate OOH∗ species can be used as an important descriptor for designing advanced carbon-based ORR electrocatalysts. In this context, Joo’s group prepared edge-rich O-doped GOMCs with tuned O contents and measured their heterogeneous electron transfer (ET) rate constants (k obs 0) using the Nicholson method and an outer-sphere redox species, [Fe(CN)6]3−/4− (Figure 6D). 61 The H2O2 electrosynthesis activity, ET rate, and reciprocal of the charge-transfer resistance exhibited similar volcano-like trends depending on the amount of O in the catalysts (Figure 6E). The k obs 0 values and mass activity (j m) of a series of O-doped GOMCs showed a linear relationship with the 2e− ORR (Figure 6F).In typical aqueous electrolytes, the catalyst surface is charged under an applied potential, and ions are distributed to generate an electrical double layer at the interface. The type and concentration of ions modify the double-layer structure and electrocatalytic properties. Sa and co-workers investigated the effect of ion type and concentration on the 2e− ORR activity of O-doped carbons. 63 Cation was found to be a major ionic species at the interface based on potential of zero charge (PZC) measurements. ORR measurements in alkaline electrolytes with various cations (Cs+, K+, and Li+) revealed a cation-dependent activity trend, Cs+ > K+ > Li+, whereas no change in the activity was observed by the type of the anion. In addition, a higher cation concentration was effective for increasing the 2e− ORR activity. These activity trends correlated with the ET kinetics at the interface, which is known to be influenced by the structure of the cation hydration shell.The interfacial structure can also be influenced by the presence of bulky surfactant molecules, as investigated by Guo and co-workers (Figure 6G). 64 The linear sweep voltammetry (LSV) curves of carbon black (CB) for the ORR in 0.1 M KOH indicated that the addition of a cationic surfactant (cetyltrimethylammonium bromide [CTAB]) increased the H2O2 selectivity of CB, whereas the presence of an anionic surfactant (sodium dodecyl sulfate [SDS]) exerted an adverse effect on selectivity, with both surfactants having a marginal impact on the catalytic activity (Figure 6H). These phenomena could be attributed to the attractive or repulsive Coulombic forces between the product HO2 − (alkaline form of H2O2) and the cationic or anionic surfactant, respectively; the interaction with CTAB was suggested to induce faster peroxide desorption kinetics, whereas that with SDS promoted further reduction of the peroxide. Furthermore, the authors conducted a kinetic model analysis of the carboxyl (–COO) and carbonyl (C=O) functionalities using O-doped CB. The ratios of the HO2 − desorption to HO2 − electroreduction rate constants of the O-CB with or without CTAB were calculated, which indicated that an increased –COO/C=O ratio led to better H2O2 production selectivity. The density of the surface carboxyl groups affected not only the rate of H2O2 production but also the rate of peroxide desorption, which collectively facilitated the formation of H2O2. In addition, electrokinetic calculations revealed a 4-fold higher HO2 − desorption rate of the CB catalyst with CTAB than that of pristine CB (Figure 6I).Tuning the hydrophobicity/hydrophilicity of electrocatalysts may control their local mass-transport behavior and generate a special interface environment. Sun and co-workers developed honeycomb carbon nanofibers (HCNFs) with high porosity and superhydrophilicity (Figure 6J). 65 The HCNFs exhibited higher activity and selectivity for H2O2 production than non-porous solid CNFs (SCNFs). The enhanced performance of the HCNFs was attributed to the superhydrophilicity resulting from rich oxygen functionalities and the surface topography of the carbon matrix. This structure allowed the effective wetting of the catalyst by an aqueous electrolyte and sufficient interaction between the surface and electrolyte. Furthermore, the honeycomb-like pore structure entrapped O2 inside the pores, increasing the local O2 concentration (Figures 6K and 6L).As described above, significant advances have been made in H2O2 electrosynthesis over the past few years. In this section, we summarize the 2e− ORR activity of selected, high-performance catalysts, which would serve as important guidelines for assessing newly developed catalysts (Figure 7 ; Table 1 ). As key indicators, we selected O2-to-H2O2 mass activity (MA) and site-normalized activity, which are current per catalyst mass and turnover number per second per active site, respectively. The MA and site-normalized activity values were calculated by normalizing apparent activity with the catalyst loadings and concentrations of active species in the developed materials. MA and site-normalized activity represent the device-oriented and intrinsic activities, respectively.H2O2 electrosynthesis activity and selectivity are typically evaluated using a rotating ring disk electrode (RRDE) (discussed in detail in the next section). The two voltametric curves obtained from the RRDE measurements show the reaction rates for different processes: the disk current is related to the total O2 conversion rate, regardless of the product, and the ring current represents the O2-to-H2O2 conversion rate. To calculate the MA of the catalyst, the ring current was extracted and corrected using the collection efficiency. Next, the kinetic ring current (actual reaction rate) was obtained by removing the effect of diffusion-limited current. Finally, the MA value was calculated by dividing the kinetic current by the catalyst loading. A summary of the MA values of PGM-based ADCs, non-PGM-based ADCs, and metal-free heteroatom-doped carbon catalysts as a function of applied potentials is shown in Figures 7A–7D, which are categorized by the electrolyte pH. The MA values would be higher for catalysts with (1) active sites with higher intrinsic activity, (2) a larger number of active sites, (3) greater specific surface area, and (4) better mass transport. It should be noted that the MA for H2O2 production generally decreases at lower pH, i.e., acidic and neutral electrolytes (Figures 7A and 7B), which presumably arises from the pH-dependent reaction kinetics. However, in alkaline H2O2 production, H2O2 is relatively unstable and prone to chemical decomposition; hence, the operation of the H2O2 electrosynthesis system under neutral and acidic conditions is preferable for practical applications. Therefore, the development of efficient electrocatalysts for H2O2 synthesis requires an understanding of the pH-dependent reaction mechanism, which has rarely been investigated.TOF provides the most accurate information on the intrinsic activity of an individual active site. However, accurate quantification of the number of accessible active sites and thus obtaining true TOF values are challenging tasks, which arise from two major difficulties: (1) identification of genuine active sites and (2) estimation of accessibility of active sites under reaction conditions. Recently, some groups have assessed the number of active sites based on poisoning methods for calculating the exact TOF value. These methods consist of blocking the active sites via the adsorption of poisoning molecules on a catalyst and analyzing the amount of the molecules that are either attached to the active sites or remained unreacted. The blocking molecules should (1) deactivate the active sites when combined, (2) react with only a single type of active site in the same molar ratio, and (3) be stably attached under the reaction conditions. The poisoning strategy has been particularly successful for ADCs because there are several probe molecules available that have high selective affinity to metal centers. Strasser and co-workers utilized CO pulse chemisorption and subsequent TPD to estimate the accessible active-site density of non-PGM ADCs. 89 Because CO binding strength with the metal center is insufficient, the chemisorption experiment should be conducted at a low temperature (193 K). Kucernak and co-workers found that NO2 − can selectively bind the active sites of Fe–N/C catalyst and form an adduct that is stable under open circuit potential in electrolytes. 90 Poisoning with the NO2 − and the subsequent reductive stripping provided the quantitative insight. The stripping charge and the degree of deactivation were correlated to evaluate the intrinsic activity of Fe–N/C catalysts. Although the NO2 −-based method is versatile, the stability of the ion is pH sensitive, which necessitates a buffered electrolyte for the reproducible measurement, and the optimum pH (5.2) is less relevant to the practical operation conditions. Choi and co-workers developed a CN− blocking method where poisoning catalysts with CN− and determining the residual CN− concentration resulted in quantitative information about the metal center. 91 The CN−-based method not only gave a consistent result with the above-mentioned two methods but also could be used to determine the active-site density of a wide variety of catalysts including non-PGM ADCs, a Pt-based ADC, and Pt NPs.For metal-free heteroatom-doped carbon catalysts, the active-site quantification method has rarely been reported, except for the organic reaction-based site-blocking method discussed earlier in the metal-free carbon-based catalysts section. We also note that it is still challenging to estimate TOF values precisely when a catalyst contains two adjacent active sites that show a synergistic activity boost. Because the amount of accessible active sites is undetermined in most literature, we summarized the intrinsic activity of reported catalysts by normalizing with the total metal contents (metal-normalized activity [MeNA]) for ADCs and total heteroatom contents (heteroatom-normalized activity [HNA]) for metal-free doped carbon catalysts instead of TOF value (Figure 7). Because this calculation assumes that every metal atom or heteroatom is accessible and equally active, the summarized MeNA and HNA values represent the lower bound limit of a TOF. Figures 7E and 7F show the MeNA values of ADCs in acidic and neutral electrolytes. In acidic media, Pt- and Rh-based PGM ADCs exhibited the highest intrinsic 2e− ORR activity (Figure 7E). Among the non-PGM ADCs, only the Co-based ADC exhibited significant activity for acidic 2e− ORR. For the 2e− ORR in neutral media, a limited number of catalytic activities have been reported only for non-PGM ADCs (Figure 7F). We note that ADCs normally contain metal active centers as well as metal-free heteroatom dopant sites; the latter species can probably contribute significantly to the apparent activity of ADCs measured in alkaline media. The MAs of metal-free heteroatom-doped carbons surpass those of many non-PGM ADCs (Figures 7C and 7D). In this regard, the MeNA values of the ADCs in alkaline media are not shown to prevent any misleading conclusion. For metal-free carbon-based catalysts, although no robust methods for the quantification of active sites are established, it is plausible that the activity is generally proportional to the amount of heteroatom dopants (Figures 7G−7I). The HNA values provide several insights. First, like the ADC catalysts, the carbon-based catalysts also exhibit higher activity at alkaline conditions than at neutral and acidic conditions. Second, O-doped carbons show generally better H2O2 electrosynthesis activity than N-doped carbons, as the latter have a propensity for promoting 4e− ORR. The comparative study on the intrinsic dopant-dependent activity of doped carbon containing each heteroatom (O, N, S, B, etc.) will be of great scientific importance.Establishing robust evaluation protocols for H2O2 electrosynthesis is important for reliably assessing a newly developed catalyst. In this section, the best practices and pitfalls are suggested for the accurate laboratory measurements of H2O2 electrosynthesis. Currently, two major methods are widely used: RRDE and bulk electrolysis (Figure 8A and 8B).RRDE is the most convenient method for the rapid measurement of the kinetics and selectivity of a catalyst. Dissolved O2 is transported to the disk by forced convection of the rotational movement of the electrode and reduced to H2O2 and H2O at the disk. The products then diffuse out to the outer Pt ring, where an appropriate potential is applied to selectively oxidize H2O2 (1.2–1.4 V versus reversible hydrogen electrode [RHE]). The H2O2 selectivity calculated according to the following equation: (Equation 9) H 2 O 2 Selectivity ( % ) = 200 1 + N × i d i r where i d, i r, and N are the disk current, ring current, and collection efficiency, respectively. Although both the H2O2 selectivity and H2O2 Faradaic efficiency (|i r/N×i d|) represent how the catalysts selectively convert O2 to H2O2, the former is always higher. 92 The collection efficiency relates the ratio of the produced amount on the disk to the detected amount on the ring. This value depends primarily on the dimensions of the disk and ring electrodes. Although the N value is given by the manufacturer, it must be determined experimentally, as it can be influenced by the measurement conditions. 93 The collection efficiency was measured by the fast redox reaction of [Fe(CN)6]3−/4− while rotating the electrode.Measurements are usually conducted in an inert gas-saturated electrolyte with 2 mM K3[Fe(CN)6] using chronoamperometry. During the measurement, the disk and ring overpotentials should be sufficiently large to guarantee diffusion-limited conditions. For every measurement, it is recommended to use the same experimental conditions, including electrolyte, electrode rotation speed, and catalyst loading. In particular, the catalyst loading on the disk electrode critically affects both the collection efficiency and H2O2 selectivity and thus can be a source of error. Ideally, a larger amount of catalyst contains a larger number of active sites, which should increase the apparent activity, while the selectivity remains constant. However, the higher catalyst loading results in a thicker catalyst layer in which the product is trapped, inhibiting diffusion to the ring electrode. This trapping effect often underestimates the collection efficiency and H2O2 selectivity. In addition, the trapped H2O2 has a higher chance of undergoing further reduction to H2O in the presence of H2O2 reduction sites (2e− × 2e− ORR pathway). 94 On the contrary, insufficient catalyst loading may lead to a substrate effect where the apparent activity mainly originates from the substrate itself (detailed discussion presented below). In this regard, exploration of the loading effect of the catalysts and optimization the catalyst loading are important.To demonstrate the effect of the catalyst loading, we measured the collection efficiency of a commercial RRDE with our previously developed catalyst (O-GOMC) at loadings of 0, 50, 100, 300, and 600 μg cm−2. The collection efficiency (value provided by the manufacturer of 37%) decreased from 40% without the catalyst to 30% with the catalyst loading of 600 μg cm−2 (Figure 8C). We then measured the loading-dependent ORR activity and selectivity in 0.1 M KOH. As the loading increased, the onset potential shifted positively because of the enhanced reaction kinetics with a larger number of active sites (Figure 8D). However, owing to the trapping effect, the ring current decreases with increasing catalyst loading. This resulted in a lower H2O2 selectivity at higher catalyst loadings when a fixed N value (0.37) was used. This underestimation (or overestimation) can be corrected by applying a loading-dependent collection efficiency in the calculation of H2O2 selectivity. After the correction, the loading-dependent selectivity difference decreased substantially, yet discrepancy remained at low potentials (Figure 8E). This phenomenon can be attributed to the mixed reaction pathways, including H2O2 chemical decomposition in the thick catalyst layer, electrochemical reduction of produced H2O2 by multitudinous catalytic sites (2e− × 2e− pathway) in high catalyst loading, and complex mass-transport behavior. Hence, for H2O2 electrosynthesis selectivity assessments using the RRDE, (1) the collection efficiency should be determined under the same experimental conditions where the ORR measurements are conducted, and (2) a catalyst loading below 100 μg cm−2 is preferable.In addition, H2O2 oxidation at the ring should be measured in a diffusion-controlled manner (i.e., fast kinetics), for which the Pt ring should be physically and electrochemically cleaned before every measurement. Physical cleaning can be performed by traditional polishing with an alumina suspension, and the residual alumina particles on the electrode are removed by ultrasonication. The Pt surface was electrochemically cleaned via repeated potential cycling. Although this is typically carried out for the measurement of Pt-based electrocatalysts, the effect of Pt-ring cleaning on H2O2 selectivity has not been investigated. Herein, we propose a Pt-ring-cleaning protocol, which consists of applying potential cycles on the Pt ring in an N2-saturated electrolyte between 0.05–1.20 V (versus RHE) at a scan rate of 500 mV s−1. Generally, the hydrogen underpotential deposition and desorption peaks are restored during cycling (Figures 8F−8H). We also observed that the Pt ring was severely contaminated when a typical alcoholic solvent, such as isopropanol or ethanol, was used to wipe the RRDE before the electrochemical cleaning step. This effect can be attributed to the adsorption of the solvent molecule and/or its decomposition products from catalysis by Pt. To optimize the Pt-ring-cleaning protocol, the number of cycles was first controlled, and the H2O2 selectivity after 10 potential cycles was compared to that without cycling (Figure 8I). However, excessive cycling led to lower H2O2 selectivity because the prolonged potential cycles caused Pt dissolution into the electrolyte, and trace Pt ions were redeposited on the catalyst during the ORR measurement. The redeposited Pt acted as an efficient 4e−-ORR catalyst and decreased the H2O2 selectivity. To remove the dissolved Pt ions, the electrolyte was replaced with a fresh electrolyte after 50 potential cycles. An electrochemically cleaned Pt ring and a Pt-free electrolyte led to the highest H2O2 selectivity. In the same context, after cyclic voltammetry of the Pt ring, the ORR measurement should begin preferably within 10 min. Pt is spontaneously oxidized in an aqueous electrolyte, and a passivation layer is formed, which deteriorates the H2O2 oxidation kinetics of the Pt ring. As the Pt-cleaning step is followed by O2 purging of the system, at least 3 min of sparging time is necessary for O2 saturation. We tested the effect of Pt-ring passivation times of 3, 5, and 10 min after cleaning. A considerable decrease in H2O2 selectivity was observed after 10 min, whereas similar selectivities were observed after 3 and 5 min (Figure 8I). Overall, the best way to evaluate the H2O2 electrosynthesis activity is to (1) determine the collection efficiency using a low amount of catalyst-loaded RRDE in the desired electrolyte and (2) electrochemically clean the Pt ring before the ORR measurements, and O2 bubbling time should be limited up to 5 min after the Pt-ring cleaning. Finally, obtaining and reporting the H2O2 selectivity data can be supplemented by Koutecky–Levich analysis, which requires rotation-speed-dependent LSV measurements.Another issue concerns the evaluation of the reaction kinetics. The onset potential of an active 2e− ORR catalyst is usually above the standard equilibrium potential of 0.70 V. Such a phenomenon originates from the absence of H2O2 in the electrolyte until right before the 2e− ORR initiates, increasing the equilibrium potential at around 0.8 V, according to the Nernst equation. This situation may cause a fluctuation or drift of the equilibrium potential owing to the change in the local concentration of H2O2 produced in situ during electrocatalysis. We assumed that such changes could inhibit the appropriate selection of a linear region in Tafel analysis, which is typically used to extract kinetic information. Therefore, to unravel the impact of the in-situ-generated H2O2, the ORR measurements were conducted in the electrolyte with a controlled concentration of H2O2 (0–0.1 M) using the O-GOMC-5.5 catalyst. Following the Nernst equation, the LSV curves were negatively shifted as the H2O2 concentration increased (Figure 8J). The corresponding Tafel plots (Figure 8K) reveal that the selection of the linear region below the kinetic current of 0.05 mA is not appropriate, as the Tafel slopes change significantly and randomly with respect to the H2O2 concentration. Instead, selecting the region above the kinetic current of 0.1 mA is more reasonable in a kinetic sense because the Tafel slopes change monotonically with H2O2 concentration in this range. The Tafel slope of this catalyst was 73 mV dec−1 without H2O2 and increased to 110 mV dec−1 with 0.1 M H2O2. If the LSV curve measured without H2O2 is available, the apparently linear region between 0.78–0.84 V (versus RHE) may be selected, which results in the Tafel slope of 40 mV dec−1 and possibly leads to wrong conclusions.Finally, when reporting the 2e− ORR activity, the substrate activity should also be considered. This is particularly important when testing electrocatalysts using carbon-based substrates, mostly glassy carbon (GC) electrodes, in alkaline media. The GC electrode exhibited a significant 2e− ORR activity and selectivity. In addition, it becomes electrochemically oxidized during multiple uses, resulting in the introduction of O species in the GC that can promote catalytic activity. Figures 8D and 8E present the H2O2 electrosynthesis activity of a polished GC electrode (denoted as catalyst loading “0”) measured in 0.1 M KOH. Interestingly, the GC exhibited a considerably high onset potential (0.62 V versus RHE) and a maximum H2O2 selectivity of 92%.Bulk electrolysis is a more accurate method for assessing the H2O2 electrosynthesis activity of a catalyst that can quantify accumulated H2O2 in the electrolyte and respective Faradaic efficiency (FE). For H2O2 quantification, the generated H2O2 must not be consumed or decomposed during the bulk electrolysis. Therefore, the counter electrode, where an arbitrary oxidation potential is applied, must be separated from the working electrode; otherwise, the produced H2O2 is readily oxidized. An H cell is suitable for this purpose as its cell design separates the working and counter electrode chambers using an ion-exchange membrane (usually Nafion). In addition, the RRDE with a Pt ring is not suitable for H-cell experiments because Pt can act as a very efficient catalyst for chemical H2O2 decomposition. Because FE is calculated based on the ratio of the produced amount of H2O2 to the theoretical production, it is crucial to accurately measure the produced H2O2 accumulated in the electrolyte.Three major methods were used to quantify the accumulated H2O2: ultraviolet-visible (UV-vis) spectrophotometry, titration, and colorimetric test strips. 95 , 96 In the UV-vis spectrophotometry, the Beer-Lambert law describing the linear dependence of the absorbance of a target material at a specific wavelength with the concentration is used. Because H2O2 barely exhibits its color, chemicals such as cobalt(II) carbonate, I−, and Ti4+ are added to react with H2O2 to exhibit strong UV-vis absorbance. In the cobalt(II) carbonate method, Co2+ ions react with H2O2 in solution, generating a Co3+ carbonate complex that absorbs a wavelength of 260 nm. Iodometric analysis relies on the oxidation of I− by the generated H2O2, and the resulting I3 − shows absorbance at 351 nm. In the titanium method, a peroxotitanium complex is formed, which absorbs a wavelength of 400 nm under acidic conditions. Spectroscopic H2O2 quantification has the advantages of a low detection limit of 50–100 μg/L and high reliability. Hence, detection can be achieved in a short time of measurement: it takes approximately 1 min at a fixed current of 1 mA with an electrolyte volume of 100 mL (assuming 100% H2O2 FE).Second, redox titration is a simple method, as it requires neither spectroscopic devices nor calibration curves. However, its measurements take more time because of a higher detection limit (0.1–1 wt%). Similar to the spectroscopic method, titration requires compounds that change their colors by the redox reaction with H2O2, such as I−, MnO4 −, and Ce4+. Iodometric titration has similar chemical principles to the above-mentioned iodometric analysis, but starch must be added as an indicator that dramatically changes color at the endpoint. In permanganate titration, MnO4 − is reduced by H2O2, losing its characteristic purple color and thus making the use of an indicator unnecessary. However, because KMnO4 is not a primary standard, additional standardization of the KMnO4 titrant is required for this method. Cerium titration was performed using the reduction reaction of Ce4+ by the generated H2O2, forming colored Ce3+. The reliability of this method can only be guaranteed under acidic conditions and at temperatures below 10°C.Finally, colorimetric strip tests were performed by dipping the test strip into an H2O2 solution of arbitrary concentration, drying, and checking the color. Although this method is fast, simple, and intuitive, the detection limit is very high, the error is very large, and the reliability of data decreases as time passes after the measurement.There are several points to be considered in H2O2 quantification experiments. First, O2 dissolved in the electrolyte should be removed before starting the quantification. Residual O2 may react with the reagents used in the titration reaction, which leads to an inaccurate determination of the H2O2 concentration. The second is related to the selection and amount of H2O2 stabilization agents added to the electrolyte. H2O2 is an unstable compound that must be often stabilized during bulk electrolysis where the measurement is conducted for a long time, particularly in high-pH electrolytes. Notably, the additive itself undergoes or initiates redox reactions within a certain potential range. Hence, the stability and potential effects of the additive should be checked before its use in H2O2 quantification.In the development of new catalysts, the above-mentioned measurements using the RRDE are extremely useful because they allow the rapid screening of candidate materials. However, the RRDE can output a maximum current density of only a few mA cm−2, which corresponds to an H2O2 generation rate of a few mg h−1 per 1 cm2. This low production rate is not industrially viable. Furthermore, the rotation system or any agitation tool for convection increases the volume of the device and production cost. A high current density of up to a few tens of mA cm−2 can be obtained using a porous carbon-paper-based electrode in an H cell. The O2 diffusion and transport behavior at these electrodes can be improved by controlling the internal pore structure and hydrophobicity. However, the production rate remains far from acceptable ranges for industrial applications. This limitation originates from the low concentration (1 mM) and low diffusion coefficient of aqueous O2.Therefore, the design of an electrochemical reactor that performs high-rate conversion with gaseous reactants is necessary to achieve current densities over hundreds of mA cm−2. Yamanaka and co-workers demonstrated an electrochemical H2O2 synthesis reactor in which a membrane-electrode assembly (MEA), consisting of a 2e− ORR cathode, an oxygen evolution anode, and a solid polymer electrolyte (SPE; Nafion in this case), was used to separate the cathode and anode chambers filled with neutral electrolytes. 97 Electrolysis performed using aqueous O2 as the reactant was inefficient for H2O2 production with low rates and current efficiencies below 10 μmol h−1 and 2%, respectively (Figure 9A). In contrast, when the cathode chamber was half filled with the electrolyte, gaseous O2 was supplied from the exposed part of the electrode, allowing much faster H2O2 generation of 132 μmol h−1 (Figure 9B).Because similar issues are found in the area of electrochemical CO2 reduction, intensive efforts have been made to develop high-current-density reactors. For CO2 reduction, pioneering work by Kenis and co-workers demonstrated a membrane-free flow reactor (microfluidic reactor), where the gas chamber and catholyte chamber were separated by a catalyst-coated carbon-paper electrode (or gas-diffusion electrode [GDE]). 100 The Jaramillo group utilized this reactor for H2O2 electrolysis, where O2 gas was supplied from the backside of the electrode and reacted at the gas-catalyst-electrolyte boundary (Figure 9C). 98 As this system allows the reaction of gaseous O2, the limitations regarding the low concentration and diffusion coefficient of O2 can be overcome. In addition, as the catholyte flow system inhibits the transportation of the produced H2O2 to the anode, membraneless operation is possible, which can reduce the Ohmic loss. Using this reactor, a 50-mA operation was demonstrated at a total cell voltage of 1.6 V.Although the flow reactor enables high-current-density operation, the generated H2O2 is dissolved in the electrolyte, requiring a separation process to obtain pure H2O2. To address this problem, Wang and co-workers devised an electrolyte-free reactor using a porous solid electrolyte that enabled H2O2 electrosynthesis in deionized water. 99 A typical MEA is composed of a cathode, membrane electrolyte, and anode, and these components are hot pressed for close contact. In porous solid-electrolyte design, the MEA consists of a cathode, anion-exchange membrane (AEM), porous solid electrolyte, cation-exchange membrane (CEM), and anode. The 2e− ORR at the cathode and hydrogen oxidation reaction at the anode produced HO2 − and H+, respectively. The ions were then transported through the AEM and CEM to the porous solid electrolytes, in which deionized water flowed. Finally, the ions were dissolved in deionized water flowing through the solid electrolyte to produce a ready-to-use pure H2O2 solution with a maximum concentration of 20 wt % at a current density of 200 mA cm−2 (Figure 9D).The efficiency of the flow reactor and MEA-type reactor critically depends on the number of gas-catalyst-electrolyte boundaries (or triple-phase boundaries) and the transport of reactants and products. Therefore, the architecture and surface properties of GDEs should be meticulously designed, and some operation variables, such as O2 and electrolyte flow rates, should be carefully controlled.The integration of high-current reactors and renewable energy sources will enable low-cost, on-site H2O2 production systems. Electrochemically produced H2O2 can be directly utilized in many areas, such as drinking-water cleaning in developing countries (Figure 9E), 98 disinfection, and bleaching applications. 35 , 50 Wang and co-workers developed a highly selective Fe-CNT catalyst for H2O2 production, where an H2O2-containing solution was produced for 210 min of operation. 35 Despite its low concentration (1,613 ppm), the reaction solution could be directly used for bacterial disinfection without further purification (Figure 9F). Chen and co-workers prepared a tested electrolyte containing 309 mM H2O2 for 3 h by applying a current density of 35.4 mA cm−2 with a high maximum FE of 91.9% in an H cell. 50 With traces of Fe2+ ions, H2O2 is decomposed to hydroxyl radicals by the Fenton reaction, which can successfully bleach various types of dye solutions with approximately 99% removal efficiency. These examples of the utilization of electrosynthesized H2O2 should be preceded by adequate accumulation of H2O2 in the electrolyte after a few hours of operation.Recently, Jang, Joo, and co-workers developed all-in-one devices that carry out bias-free photoelectrochemical production of H2O2 and its direct in situ utilization for chemical valorization. They first demonstrated a three-compartment photo-electro-biochemical reactor for lignin conversion (Figure 9G). 44 The system was composed of three separate chambers, each containing a reduced TiO2 photoanode (H:TiO2), a Co–N/C electrocatalyst-based cathode for H2O2 production, and a lignin peroxidase enzyme for lignin upgrading. When exposed to light, the H:TiO2 photocatalyst invoked photocatalytic oxygen evolution, and the generated electrons were transported to the Co–N/C electrocatalyst, which received electrons to promote O2-to-H2O2 conversion. The produced H2O2 diffused into the adjacent enzyme chamber, enabling lignin peroxidase to catalyze lignin depolymerization with a conversion and selectivity of 93.7% and 98.7%, respectively. The authors expanded the unbiased H2O2 generation and utilization system to H2O2-involving heterogeneous catalysis (Figure 9H). 45 They applied this reactor for propylene epoxidation to propylene oxide (PO), an important chemical in the plastic industry. A Co–Pi/BiVO4 photoanode and a Co–N/C cathode electrocatalyst were used for photoelectrochemical H2O2 production. Titanium silicate (TS-1) then catalyzed the propylene epoxidation reaction using in-situ-generated H2O2 as an oxidant with a high production rate (10.6 mmol h−1 for 5 h) and a PO selectivity of 97.6%, which was maintained for 24 h. It should be noted that the integrated photo-electro-heterogeneous catalytic system enabled propylene epoxidation under ambient conditions using only sunlight and O2. As photo-electrochemical H2O2 generation was successfully demonstrated over a wide pH range (2–8), the integrated system is envisioned to be applicable for other H2O2 utilization reactions.Electrochemical H2O2 production is a highly promising carbon-free technology that enables continuous on-site H2O2 production and has recently emerged as a promising alternative method to the current anthraquinone process. This review summarized recent design strategies for efficient electrocatalysts for the 2e− ORR, including PGM-based ADCs, non-PGM-based ADCs (M–N/C), and metal-free doped carbons. Remarkable progress has been made with these catalysts over the last few years in terms of their activity and selectivity. However, several critical issues remain to be addressed. The origin of the activity of these catalysts remains largely elusive, as the chemical processes for doping of active moieties in such catalysts induce high heterogeneity of the active-site distribution in the resulting catalysts. Therefore, identification of the active-site structure and selective generation of desired active sites are essential for future progress. The role of the interfacial species should be clarified, which is hampered by the difficulty in characterizing the electrical double layer. With an improved understanding of the double-layer structure, the rational design of interfacial additives or co-catalysts can be facilitated.The summarized MA and site-normalized activity of reported catalysts suggest that future electrocatalyst research should focus on achieving low overpotentials in neutral and acidic electrolytes for practical applications of H2O2 electrosynthesis technology. This objective can be achieved by the discovery of new electrocatalysts and new insights into the pH-dependent reaction mechanism and double-layer structure. In the course of catalyst development, the ORR activity and selectivity of the developed catalysts from different laboratories should be compared on a standardized protocol for rapid advancement. In this regard, a measurement protocol for the accurate assessment of electrocatalytic properties should be established, particularly for H2O2 selectivity. Although RRDE is the most prevalent method for evaluating H2O2 selectivity, the measured value is affected not only by the intrinsic properties of the catalysts but also, to a significant degree, by the experimental conditions, including catalyst loading and cleanliness of the electrode. The critical impacts of each source of error are demonstrated in this review, which is envisioned as a measurement guideline for future research. We also emphasize that the efficiency of the developed catalysts should be tested by bulk electrolysis measurements using an H cell, which has more practical relevance.Industrial-scale reactors using GDE and MEA, which have been widely used in the CO2 electroreduction field, have recently been employed for high-rate H2O2 electrosynthesis. The acquired knowledge from previous studies should be considered to obtain a high conversion rate and long-term stability. At this stage, most of the high-current-density reactor research focuses on the optimization of the electrode architecture to maximize the triple-phase boundary and prevent electrode flooding, which deteriorates the activity. However, the effects of local pH must be studied, as it is one of the most important issues in CO2 conversion. Because the 2e− ORR is a proton-consuming process, the accumulation of OH− at the interface under the operating conditions may be utilized to enable efficient H2O2 electrosynthesis even using acidic and neutral electrolytes. Characterization of the interfacial pH and molecular dynamics simulations will aid in understanding such phenomena. Finally, the advanced design of the reactor can contribute to the economic and efficient utilization of electrosynthesized H2O2, which implies direct usage without further purification or concentration. This includes an electrolyte-free reactor producing a pure H2O2 solution and the integration of the H2O2 electrosynthesis reactor and utilization modules.This work was supported by the National Research Foundation (NRF) of Korea (NRF-2019M3E6A1064521, NRF-2019M3D1A1079306, NRF-2019M1A2A2065614, and NRF-2021R1A2C2007495 to S.H.J.; NRF-2020R1C1C1006766 to Y.J.S.).S.H.J. and Y.J.S. conceptualized this review. J.S.L. drafted the manuscript. Y.J.S. and S.H.J. revised the manuscript.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.xcrp.2022.100987. Document S1. Note S1 Document S2. Article plus supplemental information

The electrosynthesis of H2O2 via a two-electron pathway oxygen reduction reaction (2e− ORR) has emerged as a promising way of carbon-free and on-site production of H2O2. Active and selective electrocatalysts for the 2e− ORR are essential for achieving high O2-to-H2O2 conversion efficiency. In this review, we present the recent progress in the development of 2e− ORR electrocatalysts including Pt-group-metal (PGM) and non-PGM atomically dispersed catalysts and metal-free heteroatom-doped carbons. The impact of the active sites and interface structures on the electrocatalytic process is summarized. Benchmarking of the electrocatalytic activities in terms of O2-to-H2O2 mass activity and site-normalized activity is presented to understand the current status of advancement and to provide an insight into possible future research directions. In addition, some guidelines and pitfalls in typical laboratory measurements for assessing 2e− ORR performance are proposed. Finally, recent advances in high-current-density H2O2 electrosynthesis reactors and devices that exploit electrosynthesized H2O2 are introduced.

Transition metal sulfide nanoparticles have been identified as a family of very promising earthabundant and low-cost electrocatalysts, which can rival those of metal selenides, nitrides, phosphides, and carbides nanoparticles [1–3]. Due to their electrochemical properties, metal sulfide that demonstrate a potential catalyst to substitute noble metals, are expected to reduce polluted 4-nitrophenol (4-NP) to eco-friendly 4-aminophenol (4-AP) [4–7]. In addition, incorporating heteroatoms into catalytic materials, the catalytic performance will be considerably enhanced by improving the electron transfer [8–10]. In contrast, synergistic effect brought by multiple heteroatoms (such as N-, P-, S-) co-doping can verify directly with the electronic structures and polarities of the catalysts [11,12].Metal organic frameworks (MOF) possess intramolecular pores which constituted by the self-assembly of metal ions and organic linkers [13–15]. Hu et al. prepared Cu-BMOF using two organic ligands by a hydrothermal method [16]. Based on the high affinity of N, S heteroatoms with noble metal ions, the two-ligand Cu-BMOF displayed auspicious adsorption capacity of 933 mg/g for removing Au (III) from aqueous solutions. Unfortunately, most of the MOF framework easily destroyed in acidic or basic media, and will be strictly limited to practical applications. Therefore, a large number of derivatives transformed from MOF are proven to be ideal materials, which not only retain some important advantages of MOF, but also significantly generate prominent feature [17–19]. Pan et al. synthesized porous carbon derivatives through pyrolyzing the two-ligand bimetallic MOF (CoxZny-JUC-160) [20]. Their derivatives possesses uniformly dispersed active sites, hierarchical porous structure, open pore network. Actually, the multiple heteroatom-doped (such as N-, P-, S-) materials could be synthesized by one-step controlling the well-designed two-ligand MOF [21].Due to self-assembly of metal ions and organic linkers, the mixed ligand MOFs provide a practical platform to in-situ synthesize multiple heteroatoms co-doping materials with enhanced catalytic performance. In this work, a known structural [M6(TDC)6(hmt)2(DMF)6(H2O)3] (JUC-85Ni) (M = Ni; TDC = 2,5-thiophenedicarboxylic acid; hmt = hexamethylenetetramine) is synthesized through a simple solvothermal method. The JUC-85Ni precursors are further investigated to synthesize metal sulfide nanoparticles dispersed N, S-codoped carbon materials (NiS 2 @NSC). The NiS2@NSC nanocomposites provide well-balanced N, S-codoped structure, good physicochemical stability and high loading of catalytic sites (Scheme 1 ). Herein, the route adopted two-ligand MOF to synthesize catalysts who rich in metal sulfide, N and S functional groups is convenient and effective. Thus, with accurate designing, the NiS2@NSC nanocomposites can be exploited as excellent catalytic center for hydrogenation reduction of 4-NP. Eventually, the proposal catalytic mechanism for NiS2@NSC is proposed, which is of great significance toward the future design of catalysts for 4-NP reduction.The final structure and crystallinity of the as-synthesized JUC-85Ni precursors and their derivatives are investigated by PXRD. We compare the typical peaks of JUC-85Ni with the simulated one, whose metal center is cadmium [22]. Fig. S1 indicates Ni- and Cr-based two-ligand MOF are isomorphic three-dimensional structure with loh1 topology. After solvothermal sufidation reaction with TAA at 120 °C for 4 h, JUC-85Ni precursor is converted to NiS2, N, S enriched porous carbon nanocomposites. In Fig. S2, two board peaks observed at 2θ = 24o and 44o are indexed to porous carbon [23]. The obvious NiS2 peaks appear at 2θ = 27.2°, 31.6°, 35.6°, 38.8°, 45.30°, 48.0, 53.6°, 56.2°, 61.2° and 68.1°, corresponding to (111), (200), (210), (211), (220), (211), (311), (222), (321) and (410) plane of cubic vaesite NiS2 (PDF # 11-0099) [7]. Noticeably, the composition of product can be affected by the sulfidation reaction time. As shown in Fig. S3, the crystal structure of JUC-85Ni-2 h (The product of the reaction time for 2 h) is still maintained, while the decreased intensity indicate that the crystallinity change. In order to confirm the formation of NiS2 nanoparticles, sulfidation reaction time should be extended to 4 h.N2 adsorption/desorption isotherms show a type-III curve and a characteristic steep increase at high relative pressures, which demonstrate the sample has characteristic microporous structure (Fig. S4). The BET surface area of NiS2@NSC catalyst is 13.4 m2·g−1. The average pore size is 8.5 nm for NiS2@NSC, measured from the Density-Functional-Theory (DFT) method (Fig. S5).The morphology characterization of materials are explored by SEM and TEM at different magnification (Fig. 1 , Fig. S6). The JUC-85Ni precursors appear as regular nanocube shape (Fig. S6a). However, the NiS2@NSC couldn't keep the original appearance, showing irregular shape (Fig. 1). SAED images (the upper right corner) indicate lattice fringes distance is 0.254 nm, which corresponds to the (210) plane of NiS2. It is noticed that the NiS2 nanoparticles are all wrapped by a thin layer of carbon, which are likely to avoid becoming large particles (Fig. S6b). The TEM elemental mapping images reveal carbon, nitrogen, oxygen, sulfur and nickel elements are uniformly distributed in NiS2@NSC (Fig. S7).The chemical composition and coordination of NiS2@NSC are analyzed by XPS instrument. The binding energies around 289, 402, 532, 164 and 860 eV in the survey spectra are corresponding to C, N, O, S and Ni, respectively (Fig. S7). The N 1 s XPS spectra show thtee peaks at 399.6, 400.5, 401.7 eV, suggesting the existence of pyridinic-N, pyrrolic-N, graphitic-N (Fig. 2a). For S 2p XPS spectra, the two peaks (163.1 and 164.3 eV) are assign to Metal-S 2p 3/2 and Metal-S 2p 1/2, and match well with XRD data (Fig. 2b). The peaks at 161.0 and 162.0 eV could be attributed to the bond of C-S-C, indicating the presence of S in the carbon matrix [11,24]. The observed peaks at 168.6 and 169.8 eV corresponds to sulfate species from unavoidable oxidation [5]. Four sub-peaks are observed in Ni 2p XPS spectra, wherein the peaks at 853.2 eV (Ni 2p 3/2) and 870.4 eV (Ni 2p 1/2) assign to Ni2+, and the fitting peaks at 857.1 eV (Ni 2p 3/2) and 875.2 eV (Ni 2p 1/2) are Ni3+ (Fig. 2c) [25]. Added to this, there are other two satellite peaks (identified as “Sat.”) located at 861.5 and 880.4 eV [26]. The C 1 s, O1s XPS spectra are also analyzed in Fig. S8.Considering its simplify operation and mild experimental condition, the catalytic activity of NiS2@NSC are evaluated by the reduction of 4-NP to 4-AP in NaBH4 aqueous solution [5,6]. The UV–vis absorbance spectroscopy is used to monitor the change of 4-NP during the reaction. With the adding of NaBH4, the 4-NP transform to 4-nitrophenolate ions, demonstrating by absorption peak shift from 317 to 400 nm. When a small amounts of NiS2@NSC catalyst is added into the reaction solution, a peak of 4-AP appear at around 300 nm, which indicate that 4-nitrophenolate ions are reduced. When NiS2@NSC catalysts are used to reduce 4-NP, the UV–vis absorption spectra varied with time between 10 °C and 40 °C are showed in Fig. S9a-d. The catalyst behave excellent catalytic property at 40 °C with a reduction time of 45 s. However, the reduction time will be higher than 60 min or more without a catalyst (Fig. S10).To investigate the reduction of 4-NP to 4-AP is mainly through degradation, the 4-NP adsorption test is measured (Fig. S11) [27]. The 4-NP removal rates is only 4.6% at 5 min, which has low adsorption potential because of a small surface area. Compared to the catalytic reaction time of 225 s, the adsorption process plays an unimportant role in removal of 4-NP.Fig. S9e-f present the plot of C t/C 0 against reduction time (reduction time is “t”, C t is the corresponding concentration at “t”, C 0 is initial concentration of 4-NP) using NiS2@NSC catalyst under varying temperatures from 10 °C to 30 °C. The value of C t/C 0 decline quickly with reduction time, revealing the 4-NP conversion increases rapidly. As the excess concentrated NaBH4, pseudo-1st order kinetic is assumed to determine the apparent rate constant (k app = −ln(C t/C 0)/t). The k app acquired from linear correlation are calculated to be 0.0201, 0.0355 and 0.0550 s−1 at 10 °C, 20 °C and 30 °C, and summarized in the Table S1. Additionally, as temperature increases, it could be observed that the k app increases. According to our statistics (Table 1 ), the NiS2@NSC catalyst possess the higher kapp than other published, indicating that the catalytic activity is superior to noble metal, transition metal and metal sulfide [5,6,8,19,28–30]. These k app values are further adopted to calculate thermodynamic parameters [31].The turn over frequency (TOF) of NiS2@NSC catalyst catalyst can better indicates the catalytic performance compared with k app, which can be calculated by the mass of 4-NP reduced per mass of NiS2 per reaction time. The catalyst possess uniformly heteroatom-doped structure, small size and high catalytic sites, the interfacial electron will transfers faster between the catalyst and 4-NP, causing TOF value of 188.6 h−1 for NiS2@NSC catalyst at 40 °C.The thermodynamic data are conducive to deeply understand the catalytic reaction pathway. Activation energy (Ea), enthalpy (ΔH), entropy (ΔS) and gibbs free energy (ΔG) can be obtained from following equations.Arrhenius Eq. (1) reflects a relationship between T and k app, applying to calculate activation energy (Ea): (1) ln k app = − Ea R 1 T + ln A Where R is molar gas constant (8.314 J·K−1·mol−1). A straight line of lnk app versus 1/T is given in Fig. S12 (red lines), from which the Ea value is 35.74 kJ mol−1 for the NiS2@NSC catalyst.Eyring Eq. (2) is used to calculate activation enthalpy (ΔH) and entropy (ΔS): (2) ln k app T = − Δ H R 1 T + ln K B h + Δ S R Gibbs free energy (ΔG) is calculated by Eq. (3): (3) Δ G = Δ H − T Δ S The constant applied in calculation: K B is boltzmann constant (1.381 × 10−23 J·K−1), h is Planck constant (6.626 × 10−34 J·k−1·mol−1), T is absolute temperature.Fig. S12 (blue lines) reveal fitted straight lines of ln(k app/T) versus 1/T, and ΔH and ΔS value for the 4-NP reduction can be obtained from slope and intercept of the lines. The positive ΔH value catalyzed by NiS2@NSC catalyst are determined to be 33.40 kJ·mol−1, showing the endothermic nature of the catalytic reduction process. A negative ΔS value corresponding to NiS2@NSC catalyst is calculated to be −158.8 kJ mol−1 K−1, suggesting a disordered system. ΔG values for 4-NP reduction exhibits equivalent values of 83.10 kJ·mol−1 at 40 °C in case of NiS2@NSC. The positive ΔG reflects the reduction of 4-NP is not feasible and spontaneous without a catalyst.In terms of above analysis results and reference, the proposal mechanism for the 4-NP reduction reaction catalyzed by our catalysts is proposed in Scheme 2 . According to Langmuir–Hinshelwood model (L-H model), BH4 − and 4-nitrophenolate ions are absorbed on the surface of porous N,S-codoped carbon layer where is enriched with electrons [32,33]. NiS2 nanoparticles should be active of BH bond to produce surface hydrogen species (NiH species). On the other hand, the e− are transferred from BH4 − to the Ni atom. Then NO bond of 4-nitrophenolate ions will be broken by active hydrogen species to form NH bond [34], finally generating the reduced 4-AP. Simultaneously, the 4-NP accept the e− to reach the charge conservation. The L-H model regard the step of 4-nitrophenolate reacted with active hydrogen species as rate determining step due to the relatively slow speeds [31]. Eventually, the product 4-AP desorb from carbon matrix into the solution, and catalyst is ready for next cycle. The NiS2@NSC catalyst exhibit excellent catalytic activity, which cloud be summarized as follows: (I) synergetic effect between NiS2 and N, S-doped carbon facilitate the electron relaying to establish electron-enhanced areas, and more 4-NP preferentially adsorb on. (II) the dispersed and high amounts of NiS2 catalytic sites are benificial to generate active hydrogen species. (III) relatively higher porosity thin N,S-doped carbon layer support express entry to 4-NP interaction and the 4-AP release.The recycling performance is a factor to evaluate its applicability as important as catalytic activity. The reusability of the NiS2@NSC catalyst can be performed in five repeated experiments. Fig. S13 shows a catalytic efficiency of the NiS2@NSC catalyst as high as 93.0% after fifth cycles. Additionally, the TEM image show no aggregation of the used NiS2@NSC catalyst in comparison with those of the newly prepared one (Fig. S14). The BET surface area of uesd NiS2@NSC catalyst is 14.7 m2·g−1 (Fig. S15). Moreover, no obvious difference can be viewed from the XPS spectra of the used and newly prepared catalysts (Fig. S16), demonstrating a good recyclability and stability. By virtue of spatially separation of NiS2 nanoparticles in the thin NSC layer, the NiS2@NSC catalyst exhibit not only the excellent catalytic activity but also high stability, making it a promising candidate for the reduction of nitroaromatic compounds.In our previous work, a synthesized NiS nanocatalyst exhibit catalytic activity toward 4-NP hydrogenation as well as methyl orange (MO) hydrogenation [5]. Wang et al. prepared Co@NC catalyst for 4-NP and dye (RhB, MB) catalytic reduction in aqueous solution [19]. To verify the universality of Co@NC catalytic performance, simulated experiment for purification of the 4-NP and MO containing water pollutants is done according to the relevant literature [35], 50mL of 4-NP (0.12mM), MO (0.02 mM) and NaBH4 (12 mM) mixed solution was stirred, and then 0.5mL of NiS2@NSC catalyst suspension (4 mg/mL) was added into the above solution. Afterwards, the mixture was quickly filtered through an injector with nylon syringe filter. Fig. S17 demonstrate the absorbance peak of 4-NP and MO was absent, indicating NiS2@NSC catalyst can be hopful regarded as an efficient catalyst for 4-NP reduction and dye.As has been stated, we employee S-containing ligands and N-containing ligands to synthesize N, S co-doping mixed-ligand metal organic framework (JUC-85Ni). Next, the NiS2@NSC catalyst was synthesized by sulfidation of JUC-85Ni precursors with thioacetamide (TAA). When evaluating as a catalyst for 4-NP reduction, the excellent characteristics endows NiS2@NSC catalyst outstanding catalytic activities with a apparent rate constant of 0.0550 s−1 at 30 °C and a catalytic efficiency of 93.0%. According to Langmuir–Hinshelwood model, the proposal catalytic mechanism of NiS2@NSC catalyst has been proposed in our paper. We hoped these high-effective catalysts will demonstrate universal application prospects in removing pollutants from wastewater. Guozhu Zhang: Investigation, Data curation, Visualization, Formal analysis, Writing – original draft. Yuhe Wang: Investigation, Validation, Formal analysis. Fei He: Investigation, Validation. Lixin He: Investigation. Haixia Li: Writing – review & editing, Funding acquisition. Dan Xu: Conceptualization, Methodology, Visualization, Resources, Writing – original draft, Writing – review & editing, 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 financially supported by the Hainan Provincial Natural Science Foundation of China (219QN219), the Finance Science and Technology Project of Hainan Province (220RC618). Additionally, we thank the assistance from Sub-center of The Environment and Plant Protection Institute, CATAS Precision instruments Sharing Center. Supplementary material: Scheme 1. Schematic illustration for the synthesis of JUC-85Ni precursors and NiS2@NSC. Image 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2022.106454.

Herein, two-ligand nickel-based MOF (JUC-85Ni) have been prepared by choosing high N-containing and S-containing ligand. On this basis of the research, the NiS2 nanoparticles dispersed N and S co-doped carbon materials (NiS2@NSC) are fabricated through the sulfidation of JUC-85Ni precursors with thioacetamide (TAA). The rationally designed NiS2@NSC catalysts are conducted a test for catalytically reducing 4-nitrophenol (4-NP) to environment friendly 4-aminophenol (4-AP) under NaBH4 alkaline condition. The superior features endows the catalyst with outstanding catalytic activity and recycling performance, and support it great potential for wastewater treatment.

Ammonia (NH3) is a crucial chemical feedstock for the production of fertilizers, pesticides and many other chemicals. Moreover, due to its unique characteristics of a high energy density, clean combustion, and convenience for storage and transportation, NH3 is also regarded as a promising alternative clean and sustainable energy storage carrier in the future [1]. Currently, the conventional method for industrial-scale NH3 synthesis, namely the Haber-Bosch (HB) process, requires high pressures (20–40 MPa) and high temperatures (400–600 °C) in the presence of an Fe-based catalyst. As a result, the HB process consumes ∼2% of the global primary energy supply and produces ∼300 million tons of CO2 per year [2,3]. Great efforts have been devoted to developing greener and more sustainable alternatives for ammonia production at lower pressures and temperatures, including biochemical and electrochemical processes. More recently, the potential applications of decentralized ammonia (NH3) production using green hydrogen on small scales have attracted increasing interest since the process could be driven by renewable energy sources such as wind and solar power [4,5].Non-thermal plasma (NTP) is regarded as a promising and emerging technology for NH3 production from N2 and H2 at low temperatures and ambient pressure. Plasma processes can be switched on and off instantly due to fast plasma-chemical reactions, thus offering great flexibility that can be coupled with renewable energy sources especially intermittent renewable energy for decentralized NH3 production. Different types of NTP have been investigated for plasma synthesis of NH3, including dielectric barrier discharge (DBD), microwave plasma, radio-frequency plasma, etc. DBD plasma has attracted great attention in NH3 synthesis due to its system compactness and scalability, mild operation conditions and simplicity of plasma-catalyst integration, which could generate a plasma-catalytic synergy to greatly improve the performance of chemical reactions [6–8]. The enhancement mechanisms of plasma-catalysis have been attributed to the synergistic interactions between plasma and catalysts, as the packed catalysts could affect the discharge characteristics, which in turn change the chemical reactions and alter the reaction kinetics.It is widely recognized that catalysts and surface reactions play an important role in determining the reaction performance of plasma-catalytic chemical processes including CO2 conversion, oxidation of volatile organic compounds (VOCs) and NOx abatement [9–11]. More recently, various catalysts have been investigated in plasma-catalytic NH3 synthesis. Shah et al. investigated the effect of 11 transition metals and low-melting-point metals on the plasma NH3 production. Ni, Sn and Au showed superior energy efficiency in the plasma-catalytic NH3 synthesis [12]. Wang et al. studied the effect of transition metal catalysts (M/Al2O3, M = Cu, Ni and Fe) on the plasma-catalytic NH3 synthesis. Ni/Al2O3 showed outstanding catalytic activity on the NH3 production, with a 15.2% higher NH3 synthesis rate than that using Fe/Al2O3 [2]. In addition to transition metal-based catalysts, noble metal (e.g., Ru)-based catalysts have also attracted much attention in plasma-catalytic NH3 synthesis due to their high activity. Patil et al. evaluated the effect of a wide range of supported metal catalysts on the plasma-catalytic NH3 synthesis and found that the activity of these catalysts followed the order of Ru > Rh > Ni > Co > Fe > Pd ≫ Mo [21]. Kim et al. studied the plasma-catalytic NH3 synthesis over promoted Ru/AC catalysts and reported the effect of metal promoters followed the order of Mg > K > Cs > no promoter [13]. Mizushima et al. reported the use of Ru, Pt, Ni and Fe as the catalytic active phase improved the NH3 yield by 40–100% in a plasma reactor, while the presence of Ru showed the highest NH3 yield [14]. However, most previous studies focused on improving NH3 production by changing the active metals of the catalysts, while far less research has been conducted to understand the effect of different supports on the plasma-catalytic NH3 synthesis. Xie et al. reported that using L-MgO supported Ru catalysts one can reach an NH3 synthesis rate of 1.04 g s−1, ∼8% higher than that using Ru/Al2O3 at a relatively low temperature (300 °C) [15]. Gorky et al. found the presence of zeolitic imidazolate frameworks of ZIF-8 and ZIF-67 supports significantly enhanced the NH3 synthesis rate by 30–60% compared to Beta, 5A and SAPO zeolites [16]. However, the fundamental understanding of different catalyst supports and their physicochemical properties on the plasma-catalytic NH3 synthesis is still limited.In this work, we have investigated NH3 synthesis from N2 and H2 over supported Ru catalysts in a co-axial DBD plasma reactor. Activated carbon (AC), α-Al2O3, ZSM-5 and SiO2 are chosen as the supports for Ru catalysts. The effect of these supports on the discharge characteristics, NH3 concentration and energy yield of the process was investigated at different operating conditions. Catalyst characterization including N2 adsorption-desorption, X-ray diffraction (XRD) and temperature-programmed desorption of CO2 (CO2-TPD) was performed to understand the structure-performance relationship between the catalysts and NH3 synthesis. The key reaction performance of the plasma-catalytic synthesis of NH3 in this work is compared with the results reported in the literature.In this work, an ultrasonic-enhanced wet impregnation method was used for the preparation of the Ru-based catalysts. All chemicals were of analytical grade. To prepare the Ru-based catalysts, a weighed amount of RuCl3∙3H2O was firstly dissolved in deionized water and magnetically stirred for 1 h to form a transparent solution. Then, a desired amount of the support was added into the solution and treated by ultrasonication for 3 h at room temperature. After that, the mixture was heated and vigorously stirred in a water bath (80 °C) for 3 h, followed by drying in an oven at 110 °C for 12 h. The samples were then calcined in a nitrogen gas stream at 500 °C for 5 h, then crushed and sieved to 40–60 meshes. The obtained samples were reduced in a 5 vol.% H2/Ar gas stream at a total gas flow rate of 100 mL min−1 at 500 °C for 5 h. The catalysts are denoted as Ru/M where M is the catalyst support (M = AC, α-Al2O3, ZSM-5 and SiO2). The loading amount of Ru was 1 wt.% in this work.N2 adsorption-desorption experiments were conducted at 77 K to obtain the textural properties of the Ru/M catalysts (M = AC, α-Al2O3, ZSM-5 and SiO2) using a Micromeritics ASAP 2010 instrument. Each sample was degassed at 200 °C for 5 h before the measurement. The specific surface area (SBET) and pore size of the Ru/M catalysts were obtained using the Brunauer-Emmett-Teller (BET) equation, while the average pore diameter and pore volume of the samples were calculated based on the Barret-Joyner-Hallender (BJH) method. The XRD patterns of the Ru/M catalysts were obtained using a Rikagu D/max-2000 X-ray diffractometer with a Cu-Kα radiation source. All samples were scanned in the 2θ range of 10°–80° with a step size of 0.02°. The basicity of the Ru/M samples was measured by CO2-TPD. During the test, each catalyst (100 mg) was pre-treated and degassed at 250 °C in an Ar flow for 1 h before being cooled down to 50 °C. The sample was then saturated with 5 vol.% CO2/Ar at a flow rate of 40 mL min−1 for 1 h, followed by purging with pure Ar at a flow rate of 40 mL min−1 to remove any weakly adsorbed CO2. Finally, the TPD measurement was performed by heating the sample from 50 °C to 800 °C at a heating rate of 10 °C min−1 in a pure Ar flow at a flow rate of 40 mL min−1. The CO2 desorption amount was determined by integrating the CO2-TPD profile. Fig. 1 shows a schematic diagram of the experimental setup. The plasma reactor consisted of a quartz tube, two polytetrafluoroethylene (PTFE) seals and two electrodes. The quartz tube had an outer diameter of 10 mm with a wall thickness of 1 mm. The two PTFE seals were placed at both ends of the quartz tube. Aluminum foil with a length of 30 mm was wrapped tightly around the quartz tube and served as a ground electrode. A stainless steel (SS) rod (high voltage electrode) with a diameter of 4 mm was placed in the center of the quartz tube and held by the PTFE seals. The DBD plasma reactor was connected to an AC high voltage power supply (CTP-2000K, Suman, China). The Ru/M catalyst (200 mg, 40–60 meshes) was packed into the discharge region and held by glass wool in each test. The DBD reactor was fan-cooled during the experiments, and the temperature on the outer wall of the reactor was around 100–110 °C as measured by an infrared (IR) temperature camera (UTi165A, UNI-T, China).In this work, N2 (99.99%) and H2 (99.99%) were used as the reactants and their flow rates were controlled by mass flow controllers (D07-B, Sevenstars, China). N2 (99.99%) and H2 (99.99%) were pre-mixed in a mixing chamber before flowing into the DBD reactor. The gas flow rate after the plasma reaction was measured using a soap-film flowmeter. A high voltage probe (Tektronix P6015A, 1000:1, USA) was used to measure the applied voltage, while a non-source voltage probe (Tektronix TPP0500, USA) was applied for measuring the voltage drop across a capacitor (0.47 μF). All signals were recorded by a Tektronix DPO2014 digital oscilloscope. The discharge power was determined using the Q-U Lissajous figure method (Eq. (1)). (1) P W = f × C m × A where Cm is the measuring capacitance (0.47 μF), f is the frequency (10.1 kHz in this work) and A is the area of the Lissajous diagram.The specific input energy (SIE) of the plasma process is expressed as follow: (2) SIE kJ L - 1 = P W Q mL min - 1 × 60 where Q is the total flow rate.The NH3 concentration was measured online using gas chromatography (Fuli 9720, China) equipped with a thermal conductivity detector (TCD). All data was measured three times and the average value was presented in this work.The energy yieldEY) of the plasma NH3 synthesis process is calculated according to Eq. (3): (3) Energy Yield   g kWh - 1 = M × C out × Q after P where M denotes the molar mass of NH3, Cout is the NH3 concentration measured at the reactor outlet and Qafter is the gas flow rate after the reaction measured by a soap-film flowmeter. Table 1 shows the textural properties of the Ru/M catalysts based on the N2 adsorption-desorption experiment [17]. The physical properties of the Ru/M catalysts show a significant difference due to the presence of different supports. Ru/AC has the highest SBET of 1333.8 m2 g−1, followed by the ZSM-5 (358.5 m2 g−1), α-Al2O3 (6.5 m2 g−1) and SiO2 (8.3 m2 g−1) supported Ru catalysts. The Ru/AC catalyst shows the highest porous volume of 0.79 cm3 g−1, which is about 3 times of the pore volume of Ru/ZSM-5 and Ru/α-Al2O3. Ru/SiO2 has an average pore size of 14.2 nm, much larger than that of Ru/ZSM-5 and Ru/AC (2.3–2.4 nm). Similar findings regarding the natures of the catalyst supports were reported in the synthesis of SiO2, ZrO2 and YSZ supported Ag catalysts [18]. Fig. 2 shows the XRD patterns of the Ru/M catalysts. The XRD patterns of all the Ru/M catalysts show the typical diffraction peaks of the supports, namely orthorhombic ZSM-5 (JCPDS No. 44-0003), hexagonal α-Al2O3 (JCPDS No.75-1864), hexagonal SiO2 (JCPDS No. 05–0490), and hexagonal activated carbon (JCPDS No. 50–1086). No identical diffraction peaks of Ru species were observed for any of the Ru/M catalysts, which could be ascribed to the low Ru loading amount or the high dispersion of Ru particles on the catalyst surface [19]. Fig. 3 presents the CO2-TPD profiles of the Ru/M catalysts. The CO2-desorption peaks below 250 °C are attributed to the weak basic sites, while the peaks located between 250 °C and 500 °C are associated with the medium basic sites [20]. Moreover, the peaks that appeared above 500 °C can be ascribed to the presence of strong basic sites [21]. For Ru/AC, a strong CO2 desorption peak is observed at 726 °C, while a weak desorption peak is located at 389 °C. Ru/ZSM-5 shows two small peaks at 166 °C and 756 °C, indicating the co-existing of strong and weak basic sites on the surface of the Ru/ZSM-5 catalyst. For Ru/α-Al2O3 and Ru/SiO2, only faint CO2-TPD desorption peaks are observed in the tested temperature range. The CO2 desorption amount of the Ru/M catalysts is associated with the basicity of the catalysts and is determined by the CO2-TPD profiles (Table 1). The Ru/AC catalyst shows the highest CO2 desorption amount of 2.71 mmol g−1. The desorption amount of the catalysts follows the order of Ru/AC (2.71 mmol g−1) > Ru/ZSM-5 (0.33 mmol g−1) > Ru/SiO2 (0.17 mmol g−1) > Ru/α-Al2O3 (0.16 mmol g−1), indicating that Ru/AC has the highest basicity in this work.The effect of the N2/H2 molar ratio on the plasma-catalytic NH3 synthesis over the Ru/M catalysts is presented in Fig. 4 . The NH3 concentration is between 102 ppm and 251 ppm in the plasma reaction without a catalyst, while the highest NH3 concentration is obtained at the optimal N2/H2 molar ratio of 1:1. The presence of the Ru/M catalysts in the DBD reactor significantly enhances the NH3 concentration regardless of the N2/H2 molar ratio. It is reported that the generation of N radicals is crucial for NH3 synthesis since the dissociation energy (9.75 eV) of the N≡N triple bond is more than twice the dissociation energy of H2 molecules (4.52 eV) [13]. In an N2-rich condition, the probability of effective collisions between energetic electrons and N2 molecules could be increased, which may further accelerate the generation of N radicals and thus NH3 synthesis in the plasma environment. An early study by Bai et al. reported a favorable N2/H2 molar ratio of 9:10 in an MgO coated DBD reactor for NH3 synthesis [22]. Shah et al. also reported that the highest NH3 synthesis rate of 1.4 μmol min−1 was achieved at an N2/H2 ratio of 1:1 despite the presence of a 5A zeolite in the plasma reactor [23]. Fig. 5 shows the effect of gas flow rate on the plasma-catalytic NH3 synthesis at a discharge power of 9 W and an N2/H2 molar ratio of 1:1. For all cases, the NH3 concentration decreases with the increase of the total gas flow rate. Using the Ru/AC catalyst shows the highest NH3 concentration of 1544 ppm at 50 mL min−1, while further increasing the gas flow rate to 150 mL min−1 decreases the NH3 concentration to 439 ppm. Similarly, for the case of plasma only, the NH3 concentration decreases from 331 ppm to 175 ppm when raising the flow rate from 50 mL min−1 to 150 mL min−1. A higher gas flow rate reduces the residence time of reactants in the plasma-catalytic system. As a result, the possibility of effective collisions for 1) the generation of N radicals and H radicals between electrons and carrier gas molecules, and 2) the recombination of N radicals and H radicals for NH3 synthesis would be decreased in a given plasma reactor under the same reaction conditions regardless of the catalyst type, leading to a lower NH3 concentration. Similar phenomena were widely reported in plasma processes for NH3 synthesis, CO2 decomposition and oxidation of VOCs, etc. [24]. Fig. 6 shows the Lissajous figures of the discharge with and without the Ru/M catalysts at a constant discharge power of 9 W. The shape of the Lissajous figure changes from a parallelogram shape to an oval shape when the Ru/M catalyst is packed in the plasma reactor, indicating the variation of discharge mode in the presence of the Ru/M catalysts. Kim et al. reported that the presence of a supported noble metal catalyst in the plasma reactor could expand the discharge region and the discharge mode could be shifted from typical filamentary micro-discharge to a combination of surface discharge and weak micro-discharges [25,26]. At a fixed discharge power, the peak-to-peak (pk-pk) applied voltage of the DBD reactor without packing is 12.6 kVpk-pk, while it increases to 13.4 kVpk-pk for the DBD reactor packed with Ru/ZSM-5, Ru/α-Al2O3 or Ru/SiO2, indicating a decreased current in the presence of these catalysts at a fixed discharge power. This phenomenon could be ascribed to the increased dielectric constant of the plasma reactor packed with the Ru/ZSM-5, Ru/α-Al2O3 and Ru/SiO2 catalysts compared to the non-packed DBD reactor [27]. The differences of relative dielectric constants between α-Al2O3, SiO2 and AC are quite small as listed in Table 2 and the dielectric constant of ZSM-5 is around 100. In the present work, the Lissajous figures for the Ru/ZSM-5, Ru/Al2O3 and Ru/SiO2 packed-DBD reactors are almost similar, suggesting the presence of these materials provides no significant effect to the electrical characteristics of plasma. It is interesting to note that the applied voltage of the DBD coupled with Ru/AC is only 11.6 kVpk-pk, significantly lower than the other cases in this work. The lower applied voltage for the Ru/AC packed DBD reactor could be ascribed to the electrical conductivity of the activated carbon support, which could contribute to the charge transfer in the plasma environment, and consequently decrease the applied voltage. Hong et al. also reported that the charge transfer was enhanced by around 80% in a diamond-like carbon-coated Al2O3 packed plasma reactor compared to a bare-Al2O3 packed reactor at a fixed applied voltage [28].The effect of discharge power on NH3 synthesis over the Ru/M catalysts is shown in Fig. 7 . The NH3 concentration increases monotonically with the increasing discharge power for all cases. In the plasma ammonia synthesis without a catalyst, the NH3 concentration ranges from 31 ppm to 437 ppm when increasing the discharge power from 5 W to 18 W. The presence of the Ru/M catalysts in the plasma reactor considerably improves the reaction performance compared with the reaction using plasma alone. When packing Ru/AC into the DBD, the NH3 concentration is varied between 151 ppm and 1788 ppm in the same discharge power range, and the highest NH3 concentration is achieved at 18 W. The Ru/ZSM-5, Ru/α-Al2O3 and Ru/SiO2 catalysts show lower ammonia concentrations compared to Ru/AC. The energy dissipated into the plasma reactor was recognized as the driving force of plasma-induced NH3 synthesis since it could contribute to the generation of energetic electrons and consequently chemically reactive species including N and H radicals, excited N2 species and N2 + ions [2]. The increase of discharge power could increase the number density of filamentary micro-discharges and expand the discharge region, resulting in higher possibilities of effective collisions between the plasma species, enhancing the production of NH3 [31]. The energy yield of the NH3 synthesis process in the plasma-catalytic system increases within the discharge power range of 5 W–9 W, then the energy yield decreases when further increasing the discharge power. The highest energy yield of 0.64 g kWh−1 is achieved at 9 W over Ru/AC, followed by Ru/ZSM-5, Ru/SiO2 and Ru/α-Al2O3, as shown in Fig. 7b. This phenomenon could be ascribed to the dynamic equilibrium between NH3 decomposition and recombination of N and H radicals at a high discharge power. Similar trends have been reported by Peng et al. using an MCM-41 support for plasma-induced NH3 synthesis [24], and our previous work on catalyst screening for NH3 synthesis in a plasma reactor [3].The performance of the plasma-catalytic NH3 synthesis shows a distinct enhancement over the Ru/M catalysts. In the plasma-catalytic systems where the catalysts are directly in contact with the discharge, the local and average electric fields would be enhanced due to the higher dielectric constant of the catalysts, especially in the regions near the contact points between the catalyst pellets and reactor walls [32]. The intensified electric field could contribute to the generation of N and H radicals in the gas phase of the plasma region, contributing to the formation of NH x (x = 1 or 2) intermediates and NH3 molecules. Moreover, the reactions on the surfaces of the Ru/M catalysts also play a crucial role in the plasma-induced process as the radicals and intermediates could be transported and adsorbed on the catalyst surfaces and undergo a series of complex surface reactions for NH3 generation [33]. The physicochemical properties of the Ru/M catalysts may significantly affect the surface reactions in the plasma region. As shown in Table 1, the Ru/AC catalyst possesses the highest SBET, followed by Ru/ZSM-5, Ru/α-Al2O3 and Ru/SiO2. A higher SBET value could offer more adsorption sites for the reactants and intermediates including N and H radicals, excited N2 species, etc. Thus, the residence time of these species would be prolonged on the surface of Ru/AC compared to the other Ru/M catalysts, resulting in higher possibilities of effective collisions for NH3 formation. The CO2-TPD profiles of the Ru/M catalysts show two major desorption peaks except for Ru/SiO2. Ru/AC shows the highest CO2 desorption amount, indicating it has the strongest basicity among the tested Ru/M catalysts. Previous work reported that the weak basic sites are associated with the Brønsted basicity of the lattice-bond OH groups, while the medium and strong basic sites could be related to the Lewis basicity from three- or four-fold-coordinated O2− anions, showing stronger electron-donating capacity compared with the Brønsted basic sites [34]. As a result, the presence of more basic sites, particularly medium and strong basic sites, could provide electrons to Ru species during the reaction and contribute to the dissociation of N2 molecules [35]. Previous work also confirmed that N2 dissociation could be enhanced over catalysts with lower electronegativity [36]. It is worth noting that materials with a higher electronegativity tend to accept electrons during the catalytic reactions, which may inhibit N2 dissociation and reduce the formation of N species for NH3 synthesis. The adsorbed N species could react with the H radicals in the gas phase and on the catalyst surfaces to form NH3 molecules. The order of basicity of the Ru/M catalysts is in accordance with the activity of the plasma-catalytic NH3 synthesis, indicating that the basicity of the catalysts is a very important factor to tune the reaction performance of the plasma-catalytic NH3 synthesis. Fig. 8 summarizes a comparison of energy yield in the process of plasma NH3 synthesis over various catalysts. The energy yield of NH3 synthesis in the cases of plasma only ranges from 0.11 g kWh−1 to 0.28 g kWh−1 in previous works. Clearly, introducing a catalyst in the plasma reactor could considerably improve the energy yield of NH3 synthesis, and the composition of catalysts is critical to determine the reaction performance. The energy yield of NH3 synthesis over bare supports (without a metal) is much lower compared to the supported catalysts. For example, Patil et al. reported an energy yield of 0.34 g kWh−1 in the plasma-catalytic NH3 synthesis over BaTiO3 at an SIE of 1.3 kJ L−1 [37]. The presence of a supported active metal phase can significantly improve the energy yield of ammonia. Mehta et al. reported that the energy yield of ammonia production using Ni/Al2O3 was 0.89 g kWh−1, almost twice that of the bare Al2O3 support at the same SIE of 6.0 kJ L−1 [5]. The results show that the doping of active metals can significantly enhance the energy yield of the NH3 synthesis. A similar finding was also reported by Xie et al. using Ru/L-MgO which gave a higher energy yield of 1.14 g kWh−1 compared to that (∼0.3 g kWh−1) using plasma only [15]. In this work, the energy yield of ammonia production is about 200% higher than that of the reaction using plasma only. This significant enhancement can be attributed to the promotion of the dissociation of N2 and H2 molecules on the catalyst surface through the Eley–Rideal mechanism and Langmuir-Hinshelwood mechanism with the presence of Ru, which accelerates the reaction of ammonia synthesis.In this work, the highest energy yield of 0.63 g kWh−1 was achieved at an SIE of 5.4 kJ L−1 using Ru/AC, which is 21.2% higher than that of using the Ru/ZSM-5 catalyst. Our results show that the type of catalyst support could directly affect the performance of the plasma-catalytic ammonia production. It is worth noting that the value of energy yield is a bit lower when compared with our previous work. The difference could be a result of the different loading of active metal Ru since a significantly higher amount of Ru (5 wt.%) was used in our previous work [20]. However, the energy yield achieved in this work could still be optimized further. The performance of the ammonia synthesis rate in the plasma environment not only depends on the components of the catalyst but also various parameters, such as reactor configuration [38,39] and operation conditions [40,41], etc. For example, increasing the ammonia synthesis performance by increasing the amount of catalyst used does not seem economically viable as it leads to added costs for the catalyst and reactor size. To sum up, the balance between the energy yield and NH3 concentration should be taken into account in the stated studies, while the optimization of the amount of catalyst used and catalyst compositions should be considered for further development and optimization of the plasma-catalytic NH3 synthesis process.In this work, the effect of various catalyst supports on the plasma-catalytic NH3 synthesis over the Ru/M catalysts was studied in a DBD plasma reactor. The NH3 concentration and energy yield of the plasma-catalytic process were significantly affected by the different supports. Compared with the reaction using plasma alone, the presence of the Ru/M catalysts improved the NH3 concentration by 163.4%–387.6% at an SIE of 5.4 kJ L−1, and the energy yield of ammonia production was increased by 163.1%–387.0%. The reaction performances followed the order of Ru/AC > Ru/ZSM-5 > Ru/α-Al2O3 > Ru/SiO2. The results also showed that the optimum N2/H2 molar ratio for NH3 synthesis was 1:1 in this work, and lower gas flow rates benefitted NH3 production. The catalyst characterization showed that the enhancement in NH3 synthesis in the plasma reactor over the Ru/AC catalyst could be attributed to the larger specific surface area, pore volume and stronger basicity of the Ru/AC 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.This work was supported by the National Natural Science Foundation of China (No. 51976093) and K. C. Wong Magna Fund at Ningbo University. X. Tu acknowledges the support from the Engineering and Physical Sciences Research Council (No. EP/V036696/1) and the British Council Newton Fund Institutional Links Programme (No. 623389161).

In this work, we have investigated the effect of different supports (activated carbon (AC), α-Al2O3, ZSM-5 and SiO2) on the plasma-catalytic synthesis of ammonia (NH3) from N2 and H2 over Ru-based catalysts in a dielectric barrier discharge (DBD) plasma reactor. Compared with the NH3 synthesis using plasma alone, the presence of the Ru-based catalysts in the DBD reactor significantly enhanced the NH3 production and energy yield by 163%–387.6% with a sequence of Ru/AC > Ru/ZSM-5 > Ru/α-Al2O3 > Ru/SiO2. The effect of different operating parameters on the plasma-catalytic NH3 synthesis over Ru/AC was also examined. N2 adsorption-desorption experiment, X-ray diffraction analysis and temperature-programmed desorption of CO2 were performed to get insights into the structure-performance relationships between the plasma-catalytic NH3 synthesis and Ru-based catalysts with different supports. Both textural properties and the basicity of the Ru/AC catalyst contributed to the enhanced NH3 production in the hybrid plasma-catalytic system.

Water electrolysis, powered by renewable energy (e.g., solar, wind), has been identified as presently the most favorable way of producing high-purity green hydrogen (H2), which can substitute conventional fossil fuels to decarbonize different sectors of our economy [1]. It is predicted that the surge in the demand for green H2 will significantly boost the installation of many gigawatt (GW) electrolyzers worldwide by 2030 [2]. To turn this blueprint into reality, it is important to keep developing new materials and components composed of inexpensive and earth-abundant materials that can be integrated into electrolyzers to improve performance and lower H2 production cost. Particularly, there is a pressing need to replace the costly and scarce platinum group metal (PGM) catalysts with earth-abundant transition metal (TM)-based alternatives to promote the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). To this end, substantial effort has been made in the past decade, and a variety of TM-based electrocatalysts, such as phosphides [3], chalcogenides [4], nitrides and carbides [5], were reported, having shown HER and/or OER performance comparable to their PGM counterparts. Among these emerging catalysts, transition metal tellurides (TMTs) have recently attracted considerable attention, given that tellurium (Te) shows lower electronegativity and greater metallic character compared to other chalcogens like selenium (Se), sulfur (S), and oxygen (O) [6], which can give rise to enhanced electrical conductivity and a higher degree of covalency in the metal–chalcogen bonds in TMTs. Such a covalent character can lead to a favorable electronic band structure, facilitate the alignment of the valence and conduction band edges with the water oxidation/reduction potentials, and also promote the redox reactions of the transition metal center, contributing to electrocatalytic performance improvement [7–9].Silva et al. lately proposed that TMTs would show better electrocatalytic activity for OER compared to other chalcogenides, because the coordination environment around the TM atoms largely influences the TM's pre-oxidation; with a decreased electronegativity of the anion (i.e., S > Se > Te), the OER onset potential can be effectively lowered. They further experimentally verified their hypothesis using Ni chalcogenides (i.e., Ni3Te2, Ni3Se2 and Ni3S2) synthesized under similar conditions [10]. Although an opposite activity trend was observed for Chevrel-phase Mo6X8 (X = S, Se, Te) electrocatalysts toward the HER [11], using the hydrogen adsorption free energy (ΔGH) as the activity descriptor Lee et al. recently demonstrated that many TMTs (e.g., ZrTe2, TiTe2, MoTe2, and VTe2) with a proper density of anion vacancies are situated on top of the volcano plot exhibiting much better HER activity compared to the corresponding transition metal sulfides and selenides [12].This opinion article provides a short account of the advances made in the last 3–4 years in developing TMT-based electrocatalysts for use in HER, OER, and overall water splitting, with a focus on major strategies developed so far to improve the electrocatalytic performance, including nanostructure engineering, composition engineering, and heterostructuring/hybridization. An outlook about future research on the design and development of TMT catalysts is also outlined.According to previous studies [13–15], bulk TMTs only exhibit mediocre electrocatalytic activity. To improve the catalytic performance, nanostructuring has turned out to be an effective strategy because it not only allows more active sites to be exposed but also facilitates mass transfer of electrolyte and gaseous products [10,15,16]. For instance, MoTe2 nanosheets (NSs) prepared by liquid exfoliation demonstrated a notably enhanced HER activity in 0.5 M H2SO4, compared to the bulk MoTe2 [14]. Hence, much effort has been devoted in the last few years to developing various TMT nanostructures, such as hollow NiTe2 nanotubes (NTs) [17], core–shell CoTe2@NC nanoparticles (NPs) [18], and hierarchical CoTe2 nanowires (NWs) [19].Ananthara et al. synthesized NiTe2 nanostructures with two distinct morphologies, for example, NWs and nanoflakes (NFs) by a short-time hydrothermal treatment of Ni foam in the presence of Te powders and NaHTe, respectively. They found that NiTe2 NWs showed better HER activity than NiTe2 NFs in both acidic and alkaline conditions (Figure 1 a), and meanwhile exhibited a Tafel slope similar to (in 0.5 M H2SO4) or smaller than (in 1 M KOH) that of commercial Pt/C benchmarks, revealing favorable reaction kinetics [20]. They ascribed the improved performance to the high charge-transfer ability of NiTe2 NWs and their large electrochemically accessible surface area (ECSA). Nanostructuring can help effectively expose catalytically active sites. To this end, Zhang et al. recently found that although the pristine chemical vapor deposited (CVD) 1T′-MoTe2 ultrathin films showed an inconspicuous HER activity, after ion-beam etching the HER performance of the 1T′-MoTe2 films was significantly enhanced, able to achieve a current density of 100 mA cm−2 at an overpotential (η) of 296 mV and a small Tafel slope of 44 mV dec−1 in 0.5 M H2SO4 [15]. Moreover, the ion-beam etched sample revealed an improvement in catalytic stability, retaining 87% of the initial current density when compared with the pristine sample which only had 40% of the initial current density after continuous electrolysis for 3600 s [15]. The enhancement was attributed to the largely exposed active edge sites, which was confirmed by conductivity measurement, visualized copper electrodeposition, and density functional theory (DFT) calculation. While the authors claimed that the ion-beam etching method can be extended to increase the active sites of other materials, it is arguably time-consuming and perhaps economically unviable for massive production of catalysts.Metal–organic framework (MOF) has been extensively used to prepare nanostructured electrocatalysts given its spatially-ordered microstructure, large specific surface area and high nanoporosity. Using zeolitic imidazolate framework (ZIF)-67 as a template, Wang et al. developed a composite catalyst comprising CoTe2 NPs encapsulated in nitrogen-doped carbon nanotube frameworks (CoTe2@NCNTFs, Figure 1b) [21]. This MOF-derived catalyst possesses a large surface area, high conductivity, and open channels for effective gas release; moreover, it enables fast electron transport presenting a Tafel slope much smaller than that of bulk CoTe2. Consequently, CoTe2@NCNTFs was reported to show good catalytic performance, requiring overpotentials of 330 and 208 mV to achieve 10 mA cm−2 for the OER and HER in 1.0 M KOH. When used as bifunctional catalysts for overall water splitting, CoTe2@NCNTF could afford 10 mA cm−2 at a cell voltage of 1.67 V (Figure 1c). Using a similar approach, Wang et al. further demonstrated that the composition of cobalt telluride in MOF-derived nanostructures can be easily adjusted [22]. The obtained optimal Co1 · 11Te2/C catalyst had more reducible Co species and higher surface dispersion of Co ions, compared to the CoTe/C and CoTe2/C references, leading to notably enhanced HER performance (178 mV@10 mA cm−2 in 1 M KOH). The observed high activity of Co1 · 11Te2/C was explained by DFT calculations, where Co1 · 11Te2/C shows an optimal Gibbs free energy (Figure 1d).Besides, electrocatalysts in-situ grown on current collectors, which form self-supported ready-to-use electrodes, also drew considerable attention lately. Previous studies have demonstrated that such an architecture can prevent the collapse and agglomeration of nanostructured catalysts, rendering long-term stability of the electrodes [23–26]. Additionally, efficient charge transfer and mass diffusion can be achieved given the intimate, binder-free adhesion of catalysts to the current collector. In this respect, a number of self-supported TMT-based HER/OER electrodes were reported recently, for example, CoTe2 NWs array on Ti mesh [19], CoTe2 NPs on Co foam [27], Cu7Te4 arrays on Cu foil [28], NiTe2 NWs on Ni foam [20], NiTe2 NS array anchored on Ti mesh [16], and FeTex NSs on Fe foam [29], which cannot be exhaustively elaborated in this short review article. However, it is worth noting that under OER conditions, TMTs usually show unfavorable electrochemical stability. The surface Te species tend to become soluble and the TMT will be eventually converted into the corresponding metal oxyhydroxide, which serves as the real catalytically active species, as reported before [30] and verified by the CoTe nanoarrays grown on Ni foam reported recently by Yang et al. [31].While nanostructure engineering can enhance the catalytic activity by tuning catalyst's physical morphology and structure to expose more catalytically active sites, in order to further boost the performance the intrinsic catalytic activity of materials must also be improved, which can be enabled by composition engineering of catalysts. Tuning the composition of TMT catalysts may lead to changes of local coordination environment and chemical properties, enhancing the catalytic activity through the ligand and/or ensemble effects. To this end, heteroatom doping has been widely adopted to regulate the electronic structure, and several transition metal (e.g., Ni, Co, Fe) [32–34] and nonmetal (e.g., P, S) [35–37] elements were already successfully doped into TMTs to improve catalytic performance. For example, by doping Fe into Mo/Te nanorods (NRs), He et al. demonstrated that the catalytic stability of Mo/Te NRs could be largely improved, because Fe-doping promoted the formation of high valence state Mo species and induced strong electronic state modification [38]. Additionally, after Fe doping, the Tafel slope and charge transfer resistance of Fe–Mo/Te became smaller, and the Tafel analysis revealed that the main kinetics pathway involves a mixed step of the M−O or M−OOH formation [38]. Moreover, Fe-doping was also reported to be able to boost the catalytic performance of Co1 · 11Te2 NPs encapsulated in nitrogen-doped carbon nanotube frameworks (NCNTF) [39]. He et al. found that the Fe–Co1 · 11Te2@NCNTF obtained by tellurization of Fe3+-etched ZIF-67 in H2/Ar gas showed a blue-shift in binding energy in the Co2p XPS spectrum, relative to the pristine Co1 · 11Te2@NCNTF (Figure 2 a), resulting in Co species with decreased electron density and a higher intensity of Co3+ components. This rationally explained the better HER and OER performance of Fe–Co1 · 11Te2@NCNTF, which presented TOF values ten times higher than those of undoped Co1 · 11Te2 for both reactions [39]. Besides, Pan et al. demonstrated that doping Mn into 1T-VTe2 helped stabilize the 1T-phase and develop nanosheet-like structure with a high surface area and porosity, which substantially enhanced the HER and OER performance, compared to the undoped 1T-VTe2 (Figure 2b and c) [40]. Moreover, the performance could be further boosted when hybridizing Ni nanoclusters (NiNCs) with Mn-doped 1T-VTe2 NSs (NiNCs-1T-Mn-VTe2 NS) which showed markedly improved reaction kinetics among all catalysts investigated [40].Previous study disclosed that the number of dopants can influence the electrocatalytic performance [41]. This was also demonstrated in TMT-based catalysts. Based on DFT calculations, Gao et al. found that co-doping of Co and Ni into MoTe2 can readily trigger the 2H-to-1T' phase transition, compared to the monoatom doping (Figure 2d) [33]. They further experimentally proved the markedly enhanced HER performance for the Co/Ni co-doped MoTe2.Aside from cation doping, anion incorporation can also effectively improve the electrocatalytic activity. Wang et al. reported that S-doping can turn the electrocatalytically inactive 2H–MoTe2 into an active catalyst, due likely to the ligand effect induced electronic structure changes, upon which electrons accumulate on the surface S atoms such that the S sites can adsorb H∗ intermediate more readily, promoting the HER [42]. Similarly, Chen et al. demonstrated the activation of CoTe2 for OER by doping secondary P anions into Te vacancies to trigger a structural transformation from the hexagonal to the orthorhombic phase (Figure 2e) [43]. This allowed a current density of 10 mA cm−2 to be achieved at η = 241 mV, lower than the hexagonal CoTe2 (η = 308 mV@10 mA cm−2). Besides, other mixed tellurides and anion-doped TMTs, such as Ni1-xFexTe2 hierarchical nanoflake arrays [44], free-standing CoNiTe2 NSs [45], MoSxTey/Gr [46] and MoSe0 · 12Te1.79 solid solutions [47], were also explored recently as HER and OER catalysts. Most of these reports suggest that the stoichiometry between two metals or two chalcogen elements is a key factor of regulating the intrinsic catalytic activity through electronic structure modulation.Besides nanostructure and composition engineering, heterostructuring or hybridization of the active catalyst with other active components has also turned out to be an effective approach to boosting the catalytic performance. Typically, such heterostructuring/hybridization can introduce abundant interfaces, which allows for engineering the electronic structure and thus the selectivity and reactivity of the catalysts. Moreover, the ensemble effect may come into play in the exposed hetero-interfaces through the migration of adsorbed reaction species from one component to the other, unlocking unprecedented catalytic reaction pathways and thereby promoting the overall reaction rate [48,49]. In this regard, a number of TMT-based heterostructured catalysts have been recently reported to show improved HER/OER performance, such as TMT nanostructures composited with a secondary TMT [50–52], an oxide/hydroxide [53–55], a chalcogenide [56,57] or a phosphide [58,59]. For instance, Xu et al. demonstrated that Ni3Te2–CoTe hybrids grown on carbon cloth in a single-step hydrothermal process (Figure 3 a) showed better OER activity than each individual component (i.e., Ni3Te2, CoTe), capable of affording a current density of 100 mA cm−2 at η = 392 mV [60], with a low Tafel slope of 68 mV dec−1 indicating that the chemisorption of hydroxyl groups on the catalyst surface is the rate-determining step. The authors proposed that the improved performance results from the incorporation of CoTe that helps expose more Ni3Te2 active sites, reflected by the double-layer capacitance measurements (Figure 3b and c) and the high density of states near the Fermi level in the Ni3Te2 component, as suggested by DFT analysis. Additionally, Xue et al. reported a NiTe/NiS heterojunction fabricated by coupling NiS nanodots (NDs) on hydrothermally-synthesized NiTe nanoarrays in an ion-exchange process (Figure 3d) [57]. The NiS NDs were found to decorate on the surface of NiTe with a high density (Figure 3e), and the interface between NiS and NiTe could be clearly seen under high-resolution transmission electron microscopy (HRTEM) examination (Figure 3f). The introduced NiTe/NiS nanointerfaces led to notable electronic structure modulation, thus optimizing the binding energy of the ∗OOH intermediates. This could be explained by the ligand effect, given that the d-band center of Ni in NiTe/NiS shifts to low-energy level with respect to NiTe due to the triggered electron transfer from Ni to S, which decreases the binding strength of intermediates on catalyst surface, resulting in a low reaction barrier. Consequently, the hybrid catalyst only needed an overpotential of 257 mV to deliver 100 mA cm−2 and showed a Tafel slope of 49 mV dec−1 for OER in 1.0 M KOH, much lower than pure NiTe and NiS. Moreover, the catalyst also exhibited good stability of over 50 h at 50 mA cm−2 (Figure 3g), with a potential increase of around 6%. Besides, Sun et al. managed to couple RuO2 and NiFe layered double hydroxide (NiFe-LDH) on NiTe NR surfaces, forming NiTe@RuO2 and NiTe@NiFe-LDH heterostructures, which were used as cathode and anode, respectively, for overall water splitting [61]. The assembled device delivered a current density of 200 mA cm−2 at a voltage of 1.63 V and could be powered by a 1.5-V solar cell for continuous water electrolysis. This result favorably compares to the Pt/C||NiTe@FeOOH electrode pair reported by the same group [62], which showed a voltage greater than 1.6 V for the same current density.Our group recently developed heterostructured dual-phase CoP–CoTe2 NWs with abundant interfaces, which exhibited good HER and OER performance in acidic/alkaline solutions [59]. The dual-phase CoP–CoTe2 NWs were used as bifunctional catalysts for bipolar membrane water electrolysis (BPMWE). The use of a BPM allows HER and OER to take place simultaneously in their respective kinetically favorable acidic and alkaline electrolytes. Particularly, when used in the forward-bias configuration, that is, the cation-exchange layer (CEL) faces the alkaline anolyte and the anion-exchange layer (AEL) faces the acidic catholyte, electrochemical neutralization between OH− and H+ ions happens, which will assist water electrolysis by lowering the external electrical energy needed. We demonstrated that using CoP–CoTe2 as bifunctional electrocatalysts, the BPM electrolyzer in the forward-bias configuration could deliver 10 mA cm−2 at a low cell voltage of merely 1.01 V (Figure 3h), and it could operate stably for 100 h without notable degradation, presenting performance better than that of anion-exchange membrane water electrolysis (AEMWE) using the same CoP–CoTe2 electrode pairs (Figure 3i). The forward-bias BPMWE represents a promising alternative to the conventional proton-exchange membrane water electrolysis (PEMWE) and AEMWE technologies, enabling hydrogen production with minimized electrical energy consumption.Transition metal tellurides have recently emerged as a class of promising electrocatalysts for both hydrogen and oxygen evolution reactions. Although bulk TMTs usually only show inferior catalytic activity, particularly for the HER, various strategies including nanostructure engineering, composition engineering and interface engineering, have been developed to improve TMT's electrocatalytic performance, as outlined in this article. Table 1 summarizes the HER, OER and overall water splitting performance of most TMT-based nanocatalysts reported in the last couple of years. Notwithstanding remarkable achievements in catalyst design and synthesis, further improvement in electrocatalytic performance is still critically needed. In particular, most TMT catalysts reported so far were only tested at low current densities relevant to solar water splitting (e.g., 10–20 mA cm−2), and the viability of using TMTs for water electrolysis under industry-relevant conditions (e.g. current density higher than 500 mA cm−2) has not been assessed yet. Additionally, the catalytic stability of TMT-based catalysts should be substantially improved. Furthermore, TMT-based catalysts should also be validated in membrane electrode assemblies (MEAs) or single cells, instead of only in an aqueous model system. This will help evaluate the commercial viability of TMT catalysts. Many TMTs show metallic character and possess high electrical conductivity, which is advocated to facilitate charge transfer. However, high electrical conductivity is not the only determining factor of electrocatalysis. It is crucially important to tune the electronic structure of TMTs through composition and/or interface engineering to lower reaction barriers and regulate the binding energy of reaction intermediates. An in-depth understanding is yet to be achieved in this regard, and the combination of theoretical predictions with delicate experimental studies, especially using advanced in-situ and operando spectroscopic and microscopic characterization techniques, will provide fundamental new insights into the catalytic/degradation mechanisms of TMT catalysts, contributing to the rational design of TMTs with significantly improved electrocatalytic performance.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 article.L. Liu acknowledges the financial support of National Innovation Agency of Portugal through the Mobilizador programme (Baterias 2030, Grant No. POCI-01-0247-FEDER-046109). I. Amorim is thankful to Fundação para a Ciência e Tecnologia (FCT) for the support of PhD grant No. SFRH/BD/137546/2018, co-financed by the Fundo Social Europeu (FSE) through the Programa Operacional Regional Norte (Norte 2020) under Portugal 2020.

Renewable energy powered electrochemical water splitting has been recognized as a sustainable and environmentally-friendly way to produce green hydrogen, which is an important vector to decarbonize the transport sector and hard-to-abate industry, able to contribute to achieving global carbon neutrality. For large-scale deployment of water electrolyzers, it is essential to develop efficient and durable electrocatalysts—one of key components determining the electrochemical performance, based on cheap and earth-abundant materials. To this end, transition metal tellurides (TMTs) have recently emerged as a promising alternative to the conventional platinum group metals for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). This review article provides a brief account of the latest development in TMT-based HER and OER catalysts, with a focus on various strategies developed to improve the catalytic performance, such as nanostructure engineering, composition engineering, and heterostructuring/hybridization. Perspectives of future research on TMT-based catalysts are also shortly outlined.

Catalysts are applied to chemical processes within the commodity, specialty, fine, and pharmaceutical industries to decrease production costs, reduce waste generated, and improve reaction yield. On an industrial scale, heterogeneous catalysts are typically used due to their implementation versatility, ease in separation, and extensive customizability. Transition metal heterogeneous catalysis innovation focuses on active site availability, broad reaction applicability, and process activity through optimizing material combinations of metal and support systems. Furthermore, new or existing catalysts could be applied to a broad range of chemical systems to achieve previously inaccessible outcomes. Research areas within this field include synthesizing precision single-site catalysts, designing support structures, and defining single or multiple metal and metal oxide active catalytic components. A common support material found in a variety of catalysts is activated carbon, which can be produced from a variety of organic feed materials. However, various new allotropes of carbon have been discovered with great potential as catalyst supports. One support material, graphene, has received increasing attention due to its many properties: 1) native catalytic properties enabling co-catalytic functions [1–4], 2) bolstered catalytic activity of supported metals [5–13], 3) chemical functionalization to customize catalytic properties [2,14-16], 4) tough lattice structure and electric conductivity, and 5) specific enhancement of electron donor/acceptor properties of the metal [17,18].Graphene, a 2D allotrope of carbon, consists of sp2-hybridized carbon atoms packed in a hexagonal honeycomb lattice. These hybridized sp2 orbitals allow the carbon to form three covalent σ-bonds, separated by a distance of 0.142 nm, attributing to the toughness of graphene's lattice structure. The fourth unused electron of carbon atoms form a π-bond, perpendicular to the σ-bonds, enabling efficient electron transfer between atoms [19]. The π-bond is attributed to the electronic properties of graphene [15,19-24]. Fig. 1 below represents the 2D and 3D structure of graphene due to the hybridized carbon bonds.Similar to graphene, carbon nanotubes (CNTs) are essentially graphene sheets rolled into a 1D tube called single-wall CNTs (SWCNTs) or into a 3D structure with multiple tubes encased within each other called multi-wall CNTs (MWCNTs). CNTs possess exceptional mechanical, chemical, electronic, and optical properties which make these materials excellent candidates for catalytic applications [25–30]. A distinguishable factor between graphene and CNTs is the aspect ratio, or ratio between the length and width or diameter. CNTs typically have an aspect ratio in excess of 1000 with minimal defects or distortions along the length of tube [31]. Fig. 2 below represents the differences between graphene, SWCNTs, and MWCNTs.While the invention of CNTs was a great achievement for the materials research community, small volume production and excessive cost due to processing complications restrict CNTs from further development and adoption in commercial applications [32]. The presence of impurities, nonuniformity in morphology and chemical structure, limited control over length and chirality, and the discrepancy between quality and yield limit CNT implementation as a support material for commercial catalytic applications [33–36]. However, active research on CNTs as a catalyst support is still ongoing and hopefully in the future these complications will be overcome.Heterogeneous catalyst supports are an essential component of supported metal catalysts. The support-metal interaction is typically the defining feature of specific support-metal combinations and are often application dependent. The support can improve the native catalytic activity of metal nanoparticles through these interactions by increasing electron transport for the active catalyst center or by promoting better metal dispersion and narrow particle size distribution resulting in high catalytic performance [37,38]. Furthermore, support materials can behave as metal particle stabilizers which prevent agglomeration and aggregation of metal nanoparticles during catalysis, promoting catalytic activity for the desired reaction pathway and preventing deactivation. Mechanical strength, external surface area, narrow pore size distribution and internal surface area, strong thermal and chemical stability, high resistance towards metal sintering, and low-cost are among the important criteria for support selection. Carbon materials can be designed and synthesized to meet a variety of performance criteria based the desired application, which make carbon one of the most promising support materials in heterogeneous catalysis [39,40]. Table 1 below summarizes important physical properties characteristic to carbon supports. Table 1 demonstrates how different carbon material types possess different physical properties which make certain forms better suited for different applications. Carbon can be fabricated into a variety of geometric forms such as granules, extrudates, pellets, fibers, powder, and cloth. The flexibility in manufacturing of carbon materials means carbon can be synthesized to meet a variety of specifications such as pore size distribution for selective reactions, chemical modification by introduction of certain elements or reactive groups, and external surface area. Furthermore, the relative inertness, ability to adsorb metals, high thermal stability (>1000 K in inert, ∼500 K in oxygen, and ∼700 K in hydrogen), relative abundance and low cost often result in the choice of carbon over other conventional supports such as silica and alumina [39]. Graphene, and CNTs, can be synthesized and customized to meet any specific application for heterogeneous catalysts. A material property summary for both graphene and CNTs are in Table 2 below.Pristine graphene is considered an efficient electron conductor due to its hybridized bonds resulting in high electron mobility (∼200,000 cm2/Vs). This is the highest value compared to conventional semiconductor materials such as silicon (∼1400 cm2/Vs), indium antimonide (77,000 cm2/Vs), and carbon nanotubes (>100A,000 cm2/Vs) [45–47]. Graphene exhibits superior thermal conductivity (∼5000 Wm−1K−1), [48] a high surface area (2630 m2/g) [61], an excellent Young's modulus (1.0 TPa) [53], and high optical transmittance (97.7%) [52]. This electronic network, along with graphene's mechanical properties, makes graphene a robust material able to withstand repeated applications of chemical or mechanical stress [22-24,62-67].Through study of both the fundamental nature and manipulation of graphene's properties, the potential of graphene as a support system is expanding the field of transition metal heterogeneous catalysis. To catalyze future graphene research, this review focuses on the following features 1) modification of graphene to create unique structures, 2) improvement of graphene's support function due to the aforementioned modifications, 3) synthesis methods for leveraging these effects, and 4) statistical analysis focusing on future opportunities to optimize applications for graphene based catalysts.Metastable states for any material exhibit unusual behavior compared to classically stable states. This behavior is often associated with “defects” in the material that forms a kinetic, metastable state and account for the observed properties to differ from a thermodynamically stable phase. These defects, which exist as atomic vacancies and intrusions, molecular rearrangements, and grain boundary changes, are caused by applied stresses which enable graphene's properties to be tuned for specific applications. These properties are dependent on the nature of applied stress and how graphene re-hybridizes to perform repairs [23,65,66]. Graphene is typically synthesized by chemical vapor deposition of methane or other carbon sources or by exfoliating graphite [22-24,64,66,68-76]. These techniques yield either pristine or defective graphene depending on feed source, synthesis conditions, and forces applied during synthesis. Graphene defects can be characterized as point, line, and interlayer defects (Fig. 3 ) [63,65,77-79].0D point defects are single atom substitutions or additions, vacancies, and reconstructions resulting from an inserted or displaced atom. Common 0D point defects include vacancies, adatoms and substitutions, and Stone-Wales (SW) defects. Vacancies result when one or more carbon atoms are displaced from their normal position within the hexagon lattice, leading to form a variety of polygons (Fig. 4 a) [80–82]. Adatoms (same-type atom) and substitutions (different-type atom) occur when an atom is inserted into the carbon network, displacing the atoms and effectively doping graphene (Fig. 4b). SW defects are caused by the rotation of the CC bond without any loss or gain of carbon atoms. A single CC rotation transforms four adjacent hexagons into two separate pentagons and two heptagons, which share the rotated bond (Fig. 4c) [80–83]. In addition to 0D defects, 1D line defects and 2D bilayer and multilayer defects occur when applied stresses cause decreased dimensionality in graphene. 1D line defects appear as atomic dislocations or grain boundary changes where atoms are anomalously organized (Fig. 4d). 2D bilayer and multilayer defects are the manifestations of 0D and 1D defects when defect-containing graphene sheets are stacked on one another (Fig. 4e).Experiments investigating graphene bond re-hybridization have been conducted to determine how graphene re-hybridizes after applied external chemical and physical stresses. These experiments shed light on how the resulting metastable state could be tuned to enhance specific properties, such as electron transport, metal-binding strength, and durability [22-24,62-66]. Computational experiments were able to predict and explain the mechanism behind the formation of these thermodynamically metastable structures. The geometrical change of the graphene layer due to vacancy movement at high temperature is shown in Fig. 5 where a single vacancy proceeds toward another single vacancy by successive jumps that eventually form a stable 555–777 SW like defect (more details in [67]).The most common method of graphene functionalization is to introduce non-metallic elements, usually oxygen, nitrogen, phosphorous, and sulfur, to graphene through chemical modification. Functionality refers to when multiple atoms of a non-carbon element is added to graphene in a non-specific manner. Graphene oxide (GO) is a prime example of oxygen-functionalized graphene (Fig. 6 ).GO was first developed by Hummers and Offeman in 1958 in an effort to introduce oxygen functionality and exfoliate graphite [7,84]. The Hummers method involves subjecting graphite to a concentrated solution of sulfuric acid, sodium nitrate, and potassium permanganate at room temperature [24,64,84-89]. The graphite simultaneously becomes oxidized and exfoliated to form GO from this process. Due to the oxygen functionality, the GO sheets can no longer restack and thus remain as separate sheets. Characterization of the specific functional groups on GO is challenging but imperative to understanding the nature and degree of functionalization when GO is synthesized [90]. For this reason, GO is often used as a precursor for producing exfoliated graphene from graphite rather than used directly as a support. Graphene is often produced by reducing GO to form reduced GO (rGO) from graphite following an oxidation procedure much like the Hummers method. RGO is typically referred to as defective graphene since the oxidation and reduction processes tend to leave the sheet crumpled and with other defects. Graphene has also been functionalized with nitrogen [91], phosphorous [92], and sulfur [93].Graphene doping is the process of adding a single atom, typically heteroatoms such as N, B, O, S, and P, of a non-carbon element to graphene's carbon lattice. If functionalization is considered a non-selective technique, then doping is considered a precision technique. When performed correctly, dopants are typically added into graphene's defect sites, often substituting a carbon atom or filling a vacancy site. Doping into defect sites can improve electron transport across the structure, which provides the strongest binding between the doped site and defected graphene lattice [94]. While functionalization can also generally improve graphene's electron transport properties, doping specifically targets properties to enhance for a variety of applications.Graphene modification by functionalization or doping often is performed to achieve improved results for specific applications. Recently, a focus in catalysis has been to use modified graphene as a support for a variety of reactions. Metallic dopants are of particular interest due to the ability to act as catalysts or co-catalysts during reactions. Metals are excellent catalysts and single-atom catalysis is a promising field compared to traditional particle catalysis due to material savings potential. For example, Xi et Al. has recently demonstrated single-atom Pd doped graphene catalysts exhibiting high catalytic performance for two applications [95,96]. Thus, metal doped graphene can be promising single-atom catalysts or co-catalysts for a variety of applications (some examples present in Section 4).By inducing defects on graphene, its catalytic properties and the stability of metal-support binding are enhanced; controlling the defect formation mechanism is imperative to improve the catalytic potential [97–107]. Defects allow the active metal catalyst more direct access to graphene's electronic “highway” which can be tuned to modify the electron donor/acceptor property [17,18,108]. This is attributed to the bond formed between the metal particle at the defect site. Engineering graphene defect sites should be an important consideration when considering graphene as a support. As described earlier, there are a variety of modifications which can achieve desired results in a modified catalyst support. But how do those modifications change the physical nature of the material?The properties of graphene's electronic highway are derived from its chiral band structure. The 2D band structure of pristine graphene is confined by the carbon atoms and exhibits an effective bandgap of zero [15,22,23,56]. For catalysis, having a small bandgap is ideal since theoretically this means the resistance to electron excitation, or flow, is at a minimum. However, the act of depositing metal particles onto the surface changes graphene's band structure due to material interactions. Additionally, maintaining pristine graphene throughout the catalyst synthesis process is difficult and impractical. Modified graphene, containing defects, functionality, or dopants, is a far more practical starting support material for prepared metal nanoparticle catalysts.Modified graphene inherently has a different band structure compared to pristine graphene. For defective graphene, the modifications are the inherent defects which change how electrons move through the π-network. The type of defect, functional group, and dopant control or direct the band structure and ultimately change the band gap [7,15,22,63]. For example, GO has increased affinity to metal ions compared to pristine graphene, leading to increased metal uptake and potential particle nucleation sites based on the relative surface density of oxygen-containing functional groups [24,64,109-114]. This is true for all forms of modified graphene. Dopants are advantageous due to the potential for precision site engineering of band gap structures and particle nucleation site dispersion. The stresses applied during the production of modified graphene create structures and introduce chemical species that create sites to produce heterogeneous catalysts with desirable particle sizes, particle dispersion, and chemical activity [23,24,40,62,64,67,70,95,96,109,115-117].Considering the relatively low native catalytic activity of GO and rGO, a more practical endeavor is using these materials as catalytic supports. The most sought-after supports typically have high surface-areas and contain micropores, providing an abundance of deposition sites for metals and helping to control metallic particle sizes, respectively [118–120]. Defect sites and oxygen functionality typically increase surface area leading to higher metal uptake, as well as mimic micropores, limiting particle size. The synthesis of these materials and catalysts can be thought of as the means to making the next generation of materials. Ultimately, the desired material changes must be made by a controlled synthesis method. How can these materials be made, and which method is optimal for the application? These questions will be addressed in the following section where specific focus will be made to address how the synthesis method itself plays a role in the creation process of modified graphene and the final catalyst product.Carbon is widely used as a support for metal nanoparticle catalysts due to its unique properties discussed above. Graphene forms of carbon have been used as a specialty support to take advantage of its structural properties. Specific applications are discussed in a later section. The synthesis methods used for preparing supported metal nanoparticles on graphene do not differ extensively from synthesis on regular carbon supports. Some of these common methods are discussed in this section.The most common way of preparing supported metal catalysts is by simple impregnation methods. The support is immersed in an aqueous solution containing a precursor of the metal, usually a dissolved salt (Fig. 7 ). Immersion can be done with excess solution, called wet impregnation, or in incipient wetness mode, also known as pore-filling or dry impregnation [121,122]. Impregnated supports are thereafter recovered, dried, and thermally treated to generate the metal nanoparticles usually by heating under flowing gas that can be oxidizing or reducing. While simple impregnation methods can be cost effective in making graphene catalysts these typically produce large particles or agglomerates due to the poor interaction between metal precursor and support surface [123,124], thus having poor dispersion – the availability of metal surface sites relative to the amount of metal used in the catalyst. Wide size distribution of nanoparticles is also a usual feature of simple impregnation due to the mobility of the metal during nucleation. In some cases, the solution conditions during simple impregnation can result in high dispersion due to enhanced electrostatic adsorption of the metal precursor [125].The method of strong electrostatic adsorption (SEA) improves the metal dispersion in the product catalyst by enhancing precursor-support interaction [125,126]. SEA, essentially a special impregnation method, is an industrially applicable process easily done by soaking the support in a solution containing the metal precursor at the appropriate pH. A surface potential, or charge, is imparted on the support by protonation or deprotonation of surface functional groups with the pH of the impregnating solution at an optimum value, far from the support point of zero charge (PZC) (Fig. 8 ). Oppositely charged precursor ions are then electrostatically attracted to the surface while retaining hydration sheaths [127,128]. Carbon supports are abundant in surface groups that can be protonated or deprotonated [129,130]. Graphene rings on carbon can accept protons when in contact with an acidified precursor solution, creating a positively charged surface which can adsorb anionic metal precursor complexes. These rings however cannot be deprotonated from neutral charge; thus, the surface cannot be negatively charged when in contact with a basified solution [125]. Terminating groups that can be protonated (or deprotonated) can be created by functionalization of carbon surface [130,131]. In cases where impregnation without intentional SEA synthesis on carbon support resulted in highly dispersed metal particles, the impregnating solution was highly acidic and far from the PZC of carbon, promoting electrostatic adsorption of the precursor [125].The use of SEA for synthesizing carbon or graphene supported catalysts has been demonstrated for efficient utilization of expensive noble metals, ensuring most of the metal atoms are available on the surface [132–135]. In electrochemical applications, highly dispersed noble metal catalysts are sought as it corresponds to high electrochemical surface area which correlates with improved activity as shown in recent work using SEA for polymer electrolyte membrane fuel cell (PEMFC) [136,137]. In that work, catalysts prepared by SEA performed better than a comparable commercial catalyst due to higher metal dispersion shown by TEM imaging (Fig. 9 ). In Suzuki cross coupling using SEA prepared Pd/graphene oxide catalysts, the strong interaction between metal and support was cited as beneficial to catalyst performance [18].Another method of supported nanoparticle catalyst preparation is by deposition-precipitation (DP) synthesis. Similar to wet impregnation, the support is immersed in a solution containing the metal precursor but with the addition of a precipitating agent (e.g. urea, sodium hydroxide, etc.) that causes particle nucleation [138]. The precipitation is induced by an increase in pH causing highly insoluble metal hydroxide to form and adhere to the support surface (Fig. 10 ). Monitoring and controlling the pH is necessary to regulate the growth of particles. Depending on the pH and functional groups on the surface of the support, metal ion adsorption can happen and these can serve as nucleation sites for DP [139]. Much like SEA, the metal and support surface interaction during DP synthesis is credited for catalyst stability during evaluation [140].Continuous manufacturing technology presents a new frontier for heterogeneous catalysis synthesis wherein issues identified in batch processes can be minimized or eliminated. Batch-to-batch variability of continuous processes are significantly reduced compared to batch equivalent processes [141]. Product quality and consistency is paramount when synthesizing a product with a specified performance. In fact, the given tolerance associated with product performance can usually be narrowed when a continuous process is implemented. Heterogeneous catalysts have been difficult to completely produce in a single continuous manufacturing process due to limitations in handling slurries, controlling metal deposition and nucleation, and efficient product separation. Significant engineering considerations must be made in order to control parameters critical for manufacturing a heterogeneous catalyst.Examples of adapted thermochemical deposition methods for continuous synthesis have been populating journals over the past five years. Thermochemical methods are techniques suited for continuous processes due to their dependence on concentration, rapid mass- and heat-transfer, and time-dependent reaction progress. Continuous systems which contain plug-flow reactors and a laminar flow fluid are best suited for heterogeneous nanoparticle synthesis. While heterogeneous nucleation, nucleation of metal particles onto a material of different elemental composition, remain a challenge, controlled particle nucleation and growth and subsequent deposition onto graphene can be achieved. Smith, et Al. successfully synthesized core-shell particles for use in Fischer-Tropsch reactions by adapting a solvothermal procedure to a continuous plug-flow reactor [142]. A traditional plug-flow type reactor was used in the experiment to synthesize the copper based nanocomposite particles. Fig. 11 below is the schematic of the synthesis process.New reactor technology could further improve upon existing or enable new synthesis methods used for manufacturing heterogeneous catalysts at scale. One such system is the spinning disk reactor (SDR). SDRs are unique reactor systems which are rotor-stator setups with a spinning disk housed inside a stationary reactor. These reactors are typically characterized by their fast residence time, high sheer forces, and plug-flow configuration. They are an instrument for process intensification applied for pharmaceutical, particle, and polymer reactions to improve reaction kinetics at small volumes [143–145]. Computational fluid dynamic and classical models demonstrate the micromixing efficiency and fluid hydrodynamic effects found within the reactor to demonstrate the enhanced mass- and heat-transfer associated with accelerated reaction kinetics. For nanoparticle synthesis reactions, SDRs can be applied to accelerate monomer formation to favor rapid nucleation and small particle formation. An example of one such setup can be found in Fig. 12 below.Rapid nucleation of metal monomer is desirable in these systems since the resulting particles or clusters tend to be very small [146,147]. The rate of nucleation can be controlled by the disk spin speed, disk gap spacing, and overall throughput (flow rate and concentration). Recently, examples of catalyst particle synthesis have been successfully demonstrated, such as copper particle catalysts for methanol synthesis [148]. The authors were able to control the particle size by varying the spinning speed of the reactor and each condition exhibited tight size distribution. Work being performed by the Gupton group with Pd has also demonstrated that the SDR can produce small Pd particles supported on graphene.Reduction treatments are often necessary after metal deposition occurs since a variety of methods do not simultaneously or sequentially deposit and nucleate the metal particles in the same reaction solution. Typically, reduction is handled by a tube furnace flowing hydrogen or using a reducing agent, such as hydrazine, in solution to reduce the metal ions on the carbon surface or in solution to form metal nanoparticles. Microwaves offer an alternative method for reduction since the heating mechanism is fundamentally different from conventional heaters. The heating rate of a material is directly proportional to the material's (molecule, particle, or atom) dipole moment [149–152]. Metals in particular have high dipole moments, resulting in a bulk metal's surface temperature reaching close to 1000 K under ambient microwaving conditions [151,153-156]. Metal nanoparticles and dissolved metal ions or atoms will absorb microwave energy, often providing sufficient energy required for particle nucleation and growth.Microwaves also enable exploiting unique properties of materials undergoing transition states. Microwaves have been used to exfoliate pristine graphene from GO in solution [157]. In the case of graphene supported nanoparticles, microwaving graphene samples containing metal nanoparticles can form defect sites while also exfoliating the graphene. This effect has been demonstrated for palladium nanoparticular systems where the palladium catalyzes the formation of defect sites on the graphene [150]. The in situ defect formation allows the palladium to bind to a defect site allowing the metal better access to graphene's electronic network [17]. DFT calculations performed in that study corroborated this effect and explained this enhancement in catalytic activity [17,18,108]. Due to the stable bind between palladium and graphene, the leached metal content reduced compared to catalysts prepared by non-microwave methods [18,150]. Microwaves are a proven method to enhance catalytic activity as well as form solid-state ligand structures between metals and graphene. Microwaves have also been successfully applied in a flow application to synthesize heterogeneous materials [158].The unique properties of graphene and other specialty sp2 carbon that have been discussed in the previous chapters can be utilized when implemented as a catalyst or catalyst support in a reactive application. Enhanced catalytic performance can be attained by using an appropriate preparation method, to induce interaction between the specialty carbon support and the catalytic metal, or other heteroatom-doped element. The various applications cited in the succeeding sections provide examples of benefits brought about by these properties and interactions. These concepts may be expanded to other similar reactive applications.The oxygen reduction reaction (ORR) is the most crucial reaction for energy conversion devices such as fuel cells and metal-air batteries. However, the sluggish kinetics of ORR limits effective energy conversion, highlighting the necessity of active and stable electrocatalysts to enhance ORR performance [159]. Although Pt or Pt-alloy nanoparticles supported on graphitic carbons (Pt/C) more effectively catalyze ORR, the excessive cost, limited reserves, and low stability of Pt are still impeding the progress of fuel cells and metal-air batteries toward commercialization [160]. To reduce the catalyst cost, recent researches have focused on the reduction or replacement of Pt electrodes for ORR. Several experimental investigations revealed metal-free doped-graphene as promising electrocatalysts for ORR [16,161-164]. The rotating-disk (RDE) voltammograms studies by Qu et al. showed that the specific current density at metal-free N-doped graphene electrodes is about three times higher relative to Pt/C electrodes at the potential between 0.4 V and 0.8 V (see Fig. 13 a) [165]. In another ORR study, S-doped graphene exhibited 76% resistance to catalyst performance decay at the end of 6500 s stability test, which performed in O2-saturated KOH solution using current- time chronoamperometric response at −0.2 V, while graphene and Pt/C catalyst showed about 74% and 46% resistance respectively (see Fig. 13b) [162]. Both these investigations indicate that heteroatom doped graphene materials have high activity as well as stability towards ORR.To eliminate the uses of Pt electrodes for ORR, many researchers are also developing non-precious metals (M: Co, Fe, Ni, Mo, Al, Cu, Sc, etc.) doped or M-X co-doped graphene materials to effectively catalyze ORR. One of the examples of such material is Fe and N co-doped 3D graphene (Fe-N/R3DG), which exhibits higher onset potential (Eonset = 0.98 V) and half-wave potential (E1/ 2 = 0.82 V) compared to Pt/C catalyst (Eonset = 0.97 V and E1/ 2 = 0.82 V) (see Fig. 13c). Additionally, the durability of the Fe-N/R3DG catalyst was found higher since the current retention of Fe-N/R3DG catalyst was 89% after 20,000 s of continuous stability test relative to that of Pt/C catalyst (62%) (see Fig. 13d) [166]. Similar results were obtained for Co and N co-doped [167–169], Cu and N co-doped [170], Al and N co-doped [171], and Sc and N co-doped [172] graphene materials at ORR conditions.Fischer-Tropsch Synthesis (FTS) and other selective hydrogenation reactions represent important large, industrial scale hydrocarbon transformations. Much like common carbon supports (i.e. activated carbon, carbon black), graphene has been utilized as an effective support for metal catalyzed hydrocarbon synthesis. Graphene offers advantages over traditional carbon supports with its high specific surface area, inertness, and ease of functionalization. Cobalt supported on carbon has been widely studied for low temperature FTS, with deactivation of the catalyst being a major problem. When Co is supported on high surface area graphene, Co can be better dispersed thus producing smaller particles and giving higher availability of surface active sites [123]. Using graphene also decreased internal mass transfer limitations compared to carbon nanotube supported catalysts, with higher contact surface of the catalyst nanosheets, resulting in better activity and selectivity for FTS (Fig. 14 ). Nitrogen functionalization of graphene surface with ammonia, prior to addition of Co, created nucleation sites for nanoparticle growth [124]. The increased anchoring strength between Co and graphene reduced the mobility of Co particles on the graphene which prevented sintering and deactivation. High activity, C8+ selectivity, and stability for FTS have also been reported on Fe/rGO catalyst prepared by microwave synthesis [173]. Similar to the aforementioned Co studies, the formation of defect sites during chemical reduction of GO as well as during microwave treatment provided nucleation sites for nanoparticle anchoring that enhanced the Fe/rGO catalyst stability. The electronic interaction of Fe with GO was cited as a factor contributing to enhanced activity affecting adsorbate binding and catalyst fouling.High selectivities under mild conditions have been reported for graphene supported catalysts applied to hydrogenation reactions. In one study, atomically dispersed Pd on graphene made by atomic layer deposition was applied to hydrogenation of 1,3-butadiene at 50 °C giving 100% selectivity to butenes with 95% conversion (Fig. 15 ) [174]. High durability of the catalyst was recorded up to 100 h. The high selectivity was attributed to lack of Pd ensembles that fully hydrogenate butenes through a secondary reaction. In another study, atomically dispersed Pd/N-graphene was prepared via freeze-drying mediated synthesis [175]. Photothermal hydrogenation of acetylene to ethylene was achieved with a remarkable 99% conversion and 93.5% ethylene selectivity at 125 °C. Anchoring of the Pd atoms on the N-doped graphene was also credited for the high durability. Another study looked at the liquid phase hydrogenation of cinnamaldehyde using Pt/graphene catalyst [176]. Ethanol solvent was deemed best for the chemoselective reduction of the C=O bond, forming cinnamyl alcohol, achieving 73.9% conversion with 83.2% selectivity to the alcohol. This high selectivity was largely attributed to the high dispersion of the Pt catalyst on the graphene.Carbon-carbon cross-coupling transformations represent an important class of reactions for the preparation of complex organic molecules. Particularly, Pd catalyzed Suzuki couplings are of interest for pharmaceutical applications due to the mild reaction conditions and broad application across a wide substrate scope [17,18,108,150,177]. Heterogeneous Pd/graphene (Pd/G) systems have demonstrated a marketable competitive edge against commercial heterogeneous Pd/C catalysts and even some homogeneous Pd catalysts with added ligands [18]. For pharmaceutical applications, low metal contamination in the final product and high catalyst recyclability or lifetime is critical. Pd/G catalysts ensure little to no metal leaching off the catalyst and maintain high catalyst recyclability or lifetime compared to homogeneous catalysts [17]. This system achieves this through a unique set of synergistic properties, often dependent on the graphene and synthesis method.A Pd/G catalyst was prepared from co-reducing PdNO3 and GO with hydrazine in an aqueous solution using a microwave reactor. The catalyst yielded turn-over-frequencies (TOF) over 100000 hr−1 for a model Suzuki coupling reaction [9,17,18]. This high activity was attributed to the defect sites formed on graphene's surface during synthesis. This process was further optimized by changing to PdCl2 and performing SEA followed by microwave treatment. SEA was performed first to uptake the metal salt onto the support, then the dried material was microwaved to form defect sites and reduce the metal onto the support. The resulting material was highly active for cross-coupling reactions, demonstrating TOF over 200000 hr−1 [150]. These results were remarkable on their own, however computation provided greater insight into why these materials exhibited such high activity.Density Functional Theory (DFT) calculations were performed to better understand the stability of these catalysts how these defect sites played a role in the catalytic mechanism. It was found that metals were strongly immobilized and stabilized on the graphene support when the metal was anchored to a defect site (see Fig. 16 A). The strongly anchored metal particles on defect sites were found to exhibit relatively high electron charge transfer properties compared to non-bound particles (see Fig. 16B). Further, these strongly immobilized and stabilized metal particles were found to lower the activation energies of each step in the catalytic cycle compared to the non-supported metal particles (see Fig. 16C). The significance of these findings demonstrates how metal catalysts behave as charge donors and acceptors to facilitate the catalytic mechanism [18,108]. When these particles are supported on defective graphene, the charge-transfer capabilities of the metal particles are enhanced which decrease their overall reaction activation energy barrier. This concept can be applied to other reactive applications other than Suzuki reactions.Statistical design of experiments (DoE) has long been a tool available to optimize industrial processes, minimize the number of conducted studies and trials, and avoid testing or analysis bias. DoE is typically used to determine causation where the relationships between the tested variables and measured responses may be difficult to ascertain. Conducting DoE effectively requires constructing simple, informative experiments about a known system such that the results are absent of systematic error, valid across a broad range of conditions, and estimates uncertainty to assert statistical significance. Additionally, DoE can enable advanced understanding of how these conditions can synergize together in a final effect that is more than the sum of the parts. The basic principles of any DoE are: 1) true replication of experimental conditions such that repeatability is verified and the random error variance is estimated; 2) randomized sampling and test order such that systematic errors are minimized from the study; and 3) blocking or partitioning into test subsets such that the precision and range of validity are maximized. This tool however has not been adopted widely by practitioners of heterogeneous catalysis and nonexistent in the development of new graphene catalysts. Fig. 17 below displays the rate of increase of mentions of “heterogeneous catalysis” in publications ranging from 1905 to 2021, and compares that with the subset of those papers that also mention “DoE.”It is clearly seen that DoE comprises a very small portion, only 0.2% in 2020, of the methodologies utilized in the study of heterogeneous catalysis. Prior to 1980, there were no mentions of DoE in publications focused on heterogeneous catalysis. Looking towards the future, DoE principles should be applied to the field of heterogeneous graphene supported metal catalysis. This represents a fertile area of investigation considering DoE can be used to reduce development time for new catalysts, optimize catalyst synthesis conditions for desired property, improve material performance and robustness, and evaluate catalytic performance in industrial process. It is important to consider that any statistical based analysis must be compared with non-statistical based knowledge to ensure the results are meaningful to this field.Ali et al. utilized central composite design (CCD) to optimize the one-step preparation of a reduced graphene oxide-titanium carbon nanotube (rGO-TNT) visible light catalyst [178]. CCD is a specific type of design that uses a polynomial model. The percent degradation of methylene blue dye (MB) was optimized by investigating the anodization time (hr) and voltage (V) with the ranges of 1–3 hr and 30–60 V, respectively. The design space included 13 experiments, and the model was fit to the second-order polynomial Eq. (1) with coded factors (1) y = c 0 + c 1 x t + c 2 x V + c 3 x t x V + c 4 x t 2 + c 5 x V 2 where xt and xV represent the anodization time and voltage, respectively, and the remaining factors represent interacting variables. The study and statistical analysis determined all factors, including interacting factors, were significant and retained in the final model for the CCD experimental analysis [178]. Eq. (1) was used to determine predicted MB degradation percentages and the values well matched the experimental values obtained. The data was then used to perform a response surface methodology (RSM) analysis in order to find a predicted set of optimal conditions. RSM analysis projects an experimentally derived dataset to find a predicted optimal outcome which may not be contained in the space bounded by the original designed experiments. These analyses generate contour and response surface plots which demonstrate how the factors affect the desired response (MB% degradation) across a continuum (see Fig. 18 ).The 3D plot (Fig. 18) demonstrated a maxima of MB% degradation response generated by the experimental design model from varying the voltage and time. Different colors in the plot indicated different ranges for the response where the orange levels show upwards of 90%. The contour plot (Fig. 18) represents a 2D projection of the 3D response surface plot (Fig. 18). The red and tan points on the plots represent the conditions performed in each individual experiment. The RSM analysis indicated that the optimal set of conditions a reaction with a duration of 2.06 hr and a voltage of 47.74 V would achieve a predicted MB removal of 90.1%. This value had good agreement with the experimentally determined value of 91.1% at the same conditions [178]. The good agreement between predicted and experimental values demonstrate the potential of DoE to accurately and precisely predict response values when a good experimental design was conducted.While the authors give no physical basis for the significance of an interaction between anodization potential and time, it seems reasonable that any interacting effect between voltage and time may be a result of the imparted strain on the RGO-TNT catalyst substrate. The generation of electron-hole pairs was identified as the limiting step in the reaction, and therefore the use of RGO in the catalyst should increase the rate of this step. This electron-hole pair was responsible for generating the radical species in the photocatalytic reaction to degrade the organics into oxidized products [178]. As discussed previously, imparting stress into graphene to cause strain will cause the electrons within the pi-structure to shuffle which can generate a charge. Upon removal of this stress, the graphene can rehybridize to return to its initial base state, much like electron-hole pair combinations. Under photocatalytic conditions in this experiment, it is reasonable to attribute the electron-hole generation necessary for MB degradation to this inherent property of RGO-TNT and why it is superior to native TNTs in this application.DoE can capture a system remarkably well and enable researchers to predict desirable conditions accurately and precisely for an experiment rather than laboriously performing an excessive number of experiments. However, DoE is currently underutilized in the field of heterogeneous catalysis as a whole and nonexistent for designing new graphene catalysts. The consequence of statistical analysis for independent factors and their potential interactions are important to discuss within the context of the mechanisms pertinent to the chemical system and will add value to a study. In the case of omission of a factor or interaction from a model due to statistical insignificance, the researchers need to explicitly explain their reasoning for the omission to improve the quality of results. Conversely, the researchers must scientifically rationalize why specific variables or interactions are statistically significant with reference to an underlying physical mechanism.Physical interactions, particularly those that are strong enough to give statistical signals, can make a traditional scientific analysis much more difficult, sans statistical quantifications of these interactions. One example of this complexity in heterogeneous supported-metal catalysis is the physical interaction between the metal and support, and analysis of statistical interactions can help elucidate the nature of this metal-support interaction in each system. There will also be situations where either an unexpected statistical interaction may be present, or where there is no statistical indication that there is an interaction where one expects there to physically be, such as the interaction or lack thereof between the support and the metal particle. Therefore, statistical analysis must be complimented by physical and chemical knowledge to obtain significant and meaningful results. This will elevate the discussion of both the statistics and the studied material or process and lead to new physical insights.The investigations of metal catalysts supported on graphene, especially functionalized or defective graphene, demonstrate the enormous potential for leveraging enhanced catalytic properties. In this review, we have discussed the recent advances in this field, elaborated on the nature of functionalized and defective graphene, how to synthesize these materials and create metal supported catalysts, and demonstrated real world examples leveraging these properties. Graphene functionalization and defect creation represent a critical area of opportunity to produce the next generation of highly active and selective catalysts. Through advanced synthesis techniques, such as SEA and the utilization of microwaves, the rational synthesis of metal-based catalysts on functionalized and defective graphene is achievable. To date, considerable advances in this area have been made.However, limitations of these materials still exist particularly in the area of graphene doping, functionalization, and defect creation at specific sites in a controlled manner. A considerable amount of work remains to better synthesize site specific functionalization and defect sites on graphitic carbon for their use as catalyst supports. Specific attention to advancing these methods used to create these special graphene materials in a manner which creates site specific modifications should be investigated. Further, advanced characterization methods in collaboration with computational methods are necessary to elucidating the effect of graphene functionality and defects on catalytic performance of metal particles. All the above work must be done to better understand how the metal-support interaction drives chemical catalysis. The nature of the support including dopants and defects, the metal type and quantity, preparation technique, and the application as a catalyst are all dependent on the metal-support interaction. This interaction will be inherently unique to any catalyst prepared due to the complexity involved in each step of the process when designing a catalyst. The future of designed graphene containing specific functional groups and defect sites is promising. Especially in the field of chemical catalysis, these materials used as supports could provide a significant advantage to activity and selectivity in industrial applications and replace current industrial catalyst in the coming years.M.B.B. and F.B.A.R. contributed equally to the preparation of this review article. M.B.B., F.B.A.R., J.M.M.T., and E.H.C. wrote this Review. J.M.M.T, E.H.C., J.R.R., and B.F.G. were involved in editing the manuscript.The authors declare no competing financial interest.The authors are grateful for financial support by the Center for Rational Catalyst Synthesis an Industry/University Cooperative Research Center funded in part by the National Science Foundation [Industry/University Collaborative Research Center grant IIP1464595]; Nanomaterials Core Characterization Facility at Virginia Commonwealth University.

Transition metal-based heterogeneous catalysts are widely used across many industries. The prevalence of these materials across so many domains has inspired research into many different types of solid supports, the nature of which can affect catalytic performance. One support receiving increased attention because of its many desirable features is graphene. These features include 1) native catalytic properties enabling co-catalysis, 2) enhanced catalytic activity when both metal atoms and nanoparticles are supported, 3) chemical functionalization to tune catalytic properties, 4) tough lattice structure and high electric conductivity, and 5) specific solid-state ligand bond formation augmenting electron transport between graphene and the metal to name a few. Although graphene shows tremendous applicability in heterogeneous catalysis, researchers are still tuning the structure to improve its catalytic performance, such as by incorporating defects or dopants into its morphology. Another important consideration is the interaction between the graphitic support and metal catalyst particle, which in turn is highly dependent upon the nature and quality of the catalyst preparation technique. This work reviews the modification of graphene structure along with the applications of different modified graphene-supported catalysts. It also discusses some of the most used and efficient catalyst preparation techniques for both batch and continuous modes. Various examples of applications that highlight graphene properties and catalytic interactions are discussed. To strengthen our reviews, a set of statistical analysis is included.

The surge in global energy consumption, the depletion of reserves of fossil fuels and the growing concern about their harmful effects on the environment necessitates the search for alternative energy sources [1]. One of the best candidates among the possible alternative renewable and sustainable fuels is bio-oil, which can be obtained through the pyrolysis of biomass. Bio-oil is considered carbon-neutral: the carbon dioxide produced by burning bio-fuels is absorbed by plants, from which biomass and bio-oil can be obtained again; thus, the greenhouse gas emissions are significantly reduced compared to fossil fuels. However, bio-oil contains high amount of oxygen and unsaturated compounds, such as aldehydes, ketones, organic acids, phenols and their derivatives, which are thermally and chemically unstable [2]. In addition, these compounds have a low calorific value, high corrosiveness and can polymerize during storage, which limits the direct use of bio-oil as a fuel [3]. Therefore, bio-oil must be upgraded to be used as a substitute fuel or valuable chemical feedstock.One of the common ways to improve the quality of bio-oil is hydrodeoxygenation (HDO). Usually HDO is carried out in the presence of conventional hydrotreating catalysts – sulfides of CoMo and NiMo [4,5]. However, higher yields of deoxygenation can be obtained with noble-metal catalysts [6], including palladium (Pd), platinum (Pt), ruthenium (Ru), and rhodium (Rh), which retain their high activity at mild temperatures and pressures. Also, due to complexity of bio-oil composition, most studies are focused on the HDO of bio-oil model compounds – guaiacol, eugenol, phenol, anisole and vanillin [7]. Among them, vanillin occupies a special place, since it contains three oxygen-containing groups, namely: a hydroxy-, a methoxy- and a carbonyl-group. This makes vanillin an interesting substrate for studying the activity of catalysts in hydrodeoxygenation. Thus, it is possible to carry out the selective hydrogenation of vanillin to vanillin alcohol [8], or convert it to p-creosol via hydrogenation and hydrodeoxygenation [9], and obtain also cyclohexanol or cyclohexane.In addition to the nature of the metal used, the activity and selectivity of catalysts is significantly affected by the support used [10]. In particular, the chemical composition and morphology of the surface of support affect the properties of the metal nanoparticles, and hence the performance of the catalyst. As an example, one of the most active hydrodeoxygenation catalysts is metal nanoparticles supported on a carrier with Brønsted acid sites [11], which facilitates hydrogenolysis of CO bond [12]. Such carriers can include: zeolites, metal-organic frameworks (MOFs), phosphonic acid modified porous materials, and N-doped carbons. The introduction of this bifunctionality is usually carried out by the modification of catalysts supports with acidic group [13], or using the materials which already had acidic properties [12]. At the same time, another urgent task is the selective hydrogenation of vanillin to vanillin alcohol – а valuable intermediate in the synthesis of novel flavorings and fragrances, and a potential building block in the synthesis of epoxy resins [14]. In this case, on the contrary, the hydrogenolysis of the CO bond must be avoided, which can be achieved either by changing the hydrogenation reaction conditions [15], or by changing the composition of the catalyst.A convenient tool for controlling the properties of the catalyst is the modification of the support with functional groups. Thus, phosphonic acids were used [16] to modify Pd/Al2O3 towards the application of a low-temperature liquid-phase vanillin hydrodeoxygenation. Modification allowed to enhance the yield of creosol from 2.5 to 87% at 50 °C due to the creation of metal/acid bifunctional sites. In another work, the authors carried out a modification of metal-organic framework UiO-66 with amino-groups. The obtained Pd@NH2-UiO-66 had superior performance in vanillin HDO due to the cooperation between the metallic Pd sites and the amine-functionalized MOF [17].Among other supports, porous carbon polymers have attracted an increasing level of researcher's interest. These materials can integrate the advantages of both porous materials and polymers. Their main advantages over porous carbons, which were used by other authors for vanillin hydrodeoxygenation [8,9], are the greater variety of synthesis and modification methods. Porous polymers have high chemical and physical customization provided by the versatility of organic chemistry. Moreover, various functional groups can be incorporated into porous polymers by pre-synthetic and post-synthetic modifications. Compared with zeolites and MOFs that are relatively highly sensitive to acidic or basic conditions [16,17], porous polymers generally have high chemical stability. An example is the porous aromatic frameworks (PAFs), which are widely used in catalysis [18]. PAFs have rigid structure consisting of aromatic rings connected to each other [19]. They are attracting more and more the attention of researchers due to their high surface area, the possibility of varying their pore size, high thermal and mechanical stability. The aromatic structure of PAFs contributes to the stabilization of metal nanoparticles, and also makes it relatively easy to modify catalysts with functional groups [20]. Catalysts based on metal nanoparticles in the pores of PAFs are promising for the conversion of lignin and related molecules. In the current work, we investigated vanillin hydrogenation on palladium nanoparticles, supported on three porous aromatic frameworks: PAF-30 without functional groups; PAF-30-SO3H, modified with sulfo-groups; and PAF-30-TEA modified with thiethylamino-groups.The following reagents were used in the present work: Methanol CH3OH (Component-Reactiv, high-purity grade); Ethanol C2H5OH (Component-Reactiv, high-purity grade); Isopropyl alcohol (CH3)2CHOH (Component-Reactiv, high-purity grade); Tetrahydrofuran C4H8O (Component-Reactiv, high-purity grade); Diethyl ether (Component-Reactiv, high-purity grade); Dichloromethane CH2Cl2 (Component-Reactiv, high-purity grade); Acetic acid CH3COOH (Ruskhim, high-purity grade); Chlorosulfonic acid HSO3Cl (99%, Sigma-Aldrich); Triethylamine (Sigma–Aldrich, St. Louis, MO, USA, 98%); Sodium hydroxide NaOH (Reakhim, 99%); Palladium (II) acetate Pd(OAc)2 (Sigma–Aldrich, 98%); Potassium hexachloropalladate (IV) K2PdCl6 (Sigma–Aldrich, 98%); Sodium borohydride NaBH4 (Aldrich, 98%); Vanillin C8H8O3 (Rushim, 99%); Salicylaldehyde C7H6O3 (ACROS Organics, 99%); p-Anisaldehyde C8H8O2 (ACROS Organics, 99%), p-toluenesulfonic acid (Sigma-Aldrich, 99%); tetramethylammonium hydroxide pentahydrate (Sigma-Aldrich, ≥97%).Porous aromatic framework PAF-30 was prepared by Suzuki cross-coupling reaction between tetrakis-(p-bromophenyl)methane and 4,4′-biphenyldiboronic acid according to previous published procedure [21]. Sulfonation of PAF-30 was carried out using a solution of chlorosulfonic acid in dichloromethane [22]. Chlorosulfonic acid (167 μl) was added dropwise to the suspension of PAF-30 (500 mg) in dichloromethane (25 ml) at 0 °C, and the resulting mixture was stirred at room temperature for 24 h. After completing the reaction, the suspension was poured into ice, the solid product of PAF-30-SO3H was filtered, washed with water, THF, diethyl ether and finally dried in vacuum.Synthesis of PAF-30-TEA was performed in two steps. First, chloromethylation of PAF-30 was carried out according to the method reported earlier [23]. The resulted material PAF-30-CH2Cl (300 mg) was refluxed with triethylamine (30 ml) for 3 days. The solid product was then collected by filtration, washed with 1 M NaOH (50 mL), water, ethanol, and finally dried in vacuum. The obtained material PAF-30-[CH2NEt3]+OH−was named PAF-30-TEA.Immobilization of palladium on supports included the impregnation of materials with metal salts and their reduction with NaBH4. The choice of palladium salt and solvent depended on the type of carrier used. Thus, impregnation of PAF-30 was carried out from a solution of Pd(OAc)2 in chloroform, PAF-30-SO3H – from a solution of Pd(OAc)2 in methanol, and PAF-30-TEA – from a solution of K2[PdCl6] in methanol. By the general procedure, 100 mg of PAF was stirred with 10 mL of 1.9 mmol/L solution of palladium salt at room temperature for 24 h. Then, a solution of 80 mg of sodium borohydride in 5 mL of water-methanol mixture (1:1 mL) was added dropwise to the resulting suspension. The reaction mixture darkened, and then stirred for 12 h. After reaction, the resulting solid product was collected by centrifugation and washed with ethanol (2 × 50 mL), water (2 × 50 mL) and THF (2 × 50 mL). Also, the catalyst based on PAF-30-SO3H was pre-washed with acetic acid (50 mL) to remove residual Na+ cations.Nitrogen adsorption isotherms were measured on a Micromeritics Gemini VII 2390 instrument (Micromeritics, Norcross, GA, United States). All samples were degassed at 120 °C for 8 h before analysis. The specific surface area (SBET) was calculated using the Brunauer–Emmett–Teller (BET) method based on adsorption data in the relative pressure range of P/P0 = 0.05–0.25. The total pore volume (Vtot) was determined by the amount of nitrogen adsorbed at the relative pressure of P/P0 = 0.965.IR spectra were recorded with a Nicolet IR200 (Thermo Scientific) instrument using multiple distortion of the total internal reflection method with multi-reflection HATR accessories, containing a 45° ZnSe crystal for different wavelengths with a resolution of 4 cm−1 in the range of 4000–400 cm−1. All spectra were recorded by averaging 100 scans.Chemical composition (compositional weight percentage of carbon, hydrogen, palladium, sulfur and nitrogen) was determined using a CHNS elemental analyzer (Thermo Flash 2000) located in the Center for Collective Usage “Analytical Center for the Problems of Deep Refining of Oil and Petrochemistry” of A.V. Topchiev Institute of Petrochemical Synthesis, RAS.Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-2100/Cs/GIF microscope (JEOL, Tokyo, Japan) with a 0.19 nm lattice fringe resolution and an accelerating voltage of 200 kV. The processing of the micrographs and the calculation of the average particle size were conducted using the ImageJ software program. Palladium and nitrogen localization in the catalyst Pd-PAF-30-TEA was investigated using energy dispersive X-ray (EDX) analyzer (EX-24065JGT).The acidity measurement was carried out by acid-base titration. The acid catalyst was dispersed in a standard solution of NaCl (0.01 mol/L), and then in a standard solution of NaOH (0.01 mol/L) as titrant. Acid-base potentiometric titration was carried out using a PH-009(II) pH meter with the following characteristics: pH measurement ranges from 0.00 to 14.00; resolution 0.01 pH; accuracy ±0.01 pH.Hydrodeoxygenation of vanillin, salicylaldehyde and anisaldehyde was carried out in a stainless-steel batch reactor, equipped with glass tube and magnetic stirrer. An amount of 2 mg of the catalyst and 1.5 ml of 0.173 M substrate solution in isopropyl alcohol or water were loaded in the tube, which was then placed in the reactor. Reactions were carried out for 0.25–18 h at a hydrogen pressure of 10 bar and in the temperature range of 60–80 °C. After completion of the reaction, the autoclave was cooled to room temperature and depressurized. Reaction products were analysed by gas chromatography. All experiments were performed at least twice, the experimental error doesn't exceed 5%.The surface of the PAF-30 was modified with -SO3H and –[CH2NEt3]OH groups, resulting in PAF-30-SO3H and PAF-30-TEA materials as shown in Fig. 1 . Introduction of functional groups reduced the free volume of mesopores, which could be seen by the decrease of the distance between adsorption and desorption curves in modified materials (Fig. 2 ). Thus, the surface area and pore volume of PAF-30 decreased from 489 m2/g and 0.241 cm3/g to 428 m2/g and 0.192 cm3/g, respectively, in PAF-30-SO3H, and to 427 m2/g and 0.135 cm3/g in PAF-30-TEA (Table 1 ), respectively. Adsorption isotherms for all materials are typical for porous aromatic frameworks, contain steep N2 uptakes at low pressures (P/P0 = 0–0.05), and a hysteresis loop between adsorption and desorption curves. This indicates that all PAFs have micro-mesoporous nature structure.According to the elemental analysis (Table 1), PAF-30-SO3H contains 7.39 wt% of sulfur, which corresponds to an acidity of 2.41 mmol/g. PAF-30-TEA contains 0.55 wt% of nitrogen, and the concentration of –[CH2NEt3]+ was 0.39 mmol/g. FTIR spectrum of the PAF-30-SO3H material (Fig. S1, ESI) contains characteristic absorption bands at 1370, 1135–1221, 1034, 901, 221,610 cm−1 for sulfo-groups [22,24], which confirms the successful modification of PAF-30. In contrast, the FTIR spectrum of PAF-30-TEA is almost identical to the spectrum of parent PAF-30 material. On the one hand, this might be due to a lower concentration of functional groups in PAF-30-TEA compared to PAF-30-SO3H. On the other, the absorption band at 1637 cm−1, characteristic for -NEt3 + groups [25,26], has low intensity.The immobilization of palladium nanoparticles was performed by the wetness impregnation method using a solution of Pd(OAc)2 or K2[PdCl6] with subsequent metal reduction with sodium borohydride. According to the ICP-AES, the palladium content in Pd–PAF-30-TEA was 1.41 wt%, while in Pd–PAF-30 and Pd-PAF-30-SO3H it was almost two times less – 0.84 and 0.74 wt%, respectively. This could be due to the presence of triethylamine groups in PAF-30-TEA which can readily bind tetrachloropalladate species by an ion exchange mechanism [27,28].The size of palladium nanoparticles (Pd NPs) and their distribution were studied by TEM (Fig. 3 ). For the Pd-PAF-30 catalyst, Pd NPs have a broad size distribution with main maxima at 1–1.5 nm and 3–4 nm. Smaller particles were encapsulated in the pores of the support, while larger particles with an average diameter of 4–4.5 nm were distributed mainly on its external surface. Nonetheless, the number of small particles is greater than that of large ones. In contrast, particle size distribution curves for Pd–PAF-30-SO3H and Pd-PAF-30-TEA are close to normal and have maxima at 3.5 nm and 4 nm, respectively. Energy dispersive X-ray spectroscopy (EDX) of Pd-PAF-30-TEA also confirms the uniform distribution of both the metal and functional groups in the catalyst (Fig. 4 ).All catalysts were tested in the reaction of vanillin hydrodeoxygenation (Fig. 5 ), which includes sequential hydrogenation of the carbonyl group and hydrogenolysis of the CO bond in the resulting -CH2OH group. In the case of Pd-PAF-30, the complete conversion of vanillin into products, vanillyl alcohol (75%) and p-creosol (25%), was observed already after 30 min of the reaction. With an increase in the reaction time, the yield of creosol increased significantly. For 1 h of reaction, it was found to be 93%, and complete conversion of vanillin into creosol occurred within 2 h. Such high activity of Pd-PAF-30 can be associated with the small size of palladium nanoparticles (1–1.5 nm).Pd-PAF-30-SO3H showed the greatest vanillin hydrodeoxygenation activity, where the creosol yield for 30 min on Pd-PAF-30 was 25%, and on Pd-PAF-30-SO3H was 85% (Fig. 6 ). The presence of acidic -SO3H sites accelerated deoxygenation to produce the desired p-creosol, which had also been observed in many studies [16,29,30]. The exact mechanism by which acidic groups are involved is still debated. It should be mentioned that the rate of hydrogenolysis of CO bond reaction depends not only on the presence and amount of Brønsted acids [17,31], but also on reaction temperature, solvent polarity [15,32], metal dispersion [33], concentration of oxygen functionalities of support (for carbon supports) [8], and electronic properties of palladium surface [34]. However, we are close to the point of view of the authors of work [31], according to which acidic groups can affect the adsorption of [H] on the support near the metal-support interface, transfer charge to Pd particles, or acid groups are directly involved in the hydrogenolysis of the CO bond.In contrast, the reaction over Pd-PAF-30-TEA gave vanillyl alcohol as the main product, and the hydrogenation rate was lower than over Pd-PAF-30 and Pd-PAF-30-SO3H even though the palladium content was higher. The reason for the such high selectivity of Pd-PAF-30-TEA for vanillyl alcohol might be associated with the blocking of active sites on palladium nanoparticles with alkylammonium groups [35], or with the interaction of counterions of –CH2[NEt3]+ groups with the surface of palladium nanoparticles with changing the adsorption of substrate and hydrogen. Also, this could be related with the reduced acidity of this catalyst.In order to confirm the influence of functional groups, we conducted experiments with Pd-PAF-30 and monomeric analogues of PAF-30-SO3H and PAF-30-TEA – p-toluenesulfonic acid and tetraethylammonium hydroxide (Table 2 ). Reaction time was short (15 min) for Pd-PAF-30-SO3H catalyst in order to achieve a high content of vanillyl alcohol in the reaction products, and to clearly show the effect of sulfo-groups. On the contrary, for Pd-PAF-30-TEA catalyst, reaction time was long (2 h) to show a decrease in the rate of deoxygenation of vanillin alcohol in the presence of alkylammonium ions.Vanillin hydrogenation on Pd-PAF-30 for 15 min gives 75% of vanillyl alcohol and 24% of creosol. However, even a small amount of p-TsOH accelerates deoxygenation of vanillyl alcohol to creosol, leading to a formation of 80% of creosol and 20% of vanillyl alcohol. The same reaction products – 15% of vanillyl alcohol and 85% of creosol – were obtained on Pd-PAF-30-SO3H catalyst, which proves the effect of sulfo-groups in PAF-30 on the hydrodeoxygenation activity of the catalyst. Nevertheless, there seems to be no direct interaction between Pd nanoparticles and nearby acidic sites in the Pd-PAF-30-SO3H catalyst, since the addition of p-TsOH also increases the creosol yield observed.When the reaction was carried out for 2 h on the Pd-PAF-30 catalyst, complete conversion of vanillin into creosol was observed. The introduction of tetraethylammonium hydroxide completely changed the composition of the reaction products: vanillyl alcohol became the predominant product (85%), while creosol was present only in trace amounts. Similar results were observed for the catalyst Pd-PAF-30-TEA. The lower selectivity of the formation of vanillyl alcohol might be associated with an insufficient number of –[CH2NEt3]+ groups in the material, and the presence of palladium nanoparticles on the surface of which these groups are absent.Pd-PAF-30-TEA was also tested in the hydrogenation of vanillin at different temperatures (Fig. 7 ). As expected, an increase in the process temperature led to a growth in the rates of both hydrogenation and hydrogenolysis reactions. Thus, an increase in temperature by 10 °C reduced the time of complete hydrogenation of vanillin by about 1.5–2 times. The appearance of the kinetic curves of vanillin consumption is characteristic for first-order reactions, and is typical for this reaction. Creosol accumulation curves obey more complex kinetic relationships: at 60 °C the reaction rate is near to be constant, and at 70 and 80 °C it is initially high and then decreases and also becomes constant. However, despite the slowdown of vanillyl alcohol hydrodeoxygenation reaction, this does not cease: the yields of creosol after 68 h of reaction at 60, 70 and 80 °C were 45, 51 and 61%, respectively.The stability of all catalysts was studied in reuse experiments (Figs. 8 and 9 ). Pd-PAF-30 and Pd-PAF-30-SO3H catalysts gradually lost their activity, probably due to metal leaching from the external surface of PAF particles or from their pores, which is confirmed by a decrease in the content of palladium to 0.41 and 0.38 wt%, respectively. Also, in the case of Pd-PAF-30-SO3H, a slight decrease in the concentration of sulfo-groups (<0.5%) was observed after 6 recycling experiments, and isopropyl-vanillyl ether was present in the reaction products starting from the second cycle. In contrast, Pd-PAF-30-TEA shows great stability at least six times without significant loss of activity and selectivity to vanillyl alcohol. The decrease in palladium content was insignificant, and final metal content was 1.37 wt%.We have also investigated the activity of catalysts in the hydrogenation of some other aromatic carbonyl compounds – anisaldehyde and salicylaldehyde (Fig. 10 ). As before, during the reaction the substrates undergo hydrogenation and further deoxygenation. Also, in the case of vanillin, the formation of a small amount of vanillyl-isopropyl ether was observed.Complete conversion of all substrates was observed on Pd-PAF-30-SO3H and Pd-PAF-30 catalysts (Table 3 ). As expected, mainly deoxygenation products were formed on the Pd-PAF-30-SO3H catalyst due to the presence of Brønsted acid sites. In the case of Pd-PAF-30, hydrogenation of both vanillin and salicylic aldehyde led to the formation of approximately equal amounts of hydrogenation and hydrodeoxygenation products, whereas the reaction with anisaldehyde gives p-methoxytoluene as the only product. When Pd-PAF-30-TEA was used as a catalyst, the main product of anisaldehyde hydrogenation was also p-methoxytoluene, while hydrogenation of vanillin and salicylaldehyde gave the corresponding alcohols with high selectivity. However, the conversion of these substrates did not reach 100%. Due to the fact that all these aromatic aldehydes contain electron-donating groups (EHOMO values are −0.2453, −0.2482 and − 0.2551 for vanillin, anisaldehyde and salicylic aldehyde respectively [34]), we suppose that this difference in catalytic activity is largely related to the different strength of adsorption of molecules on the surface of palladium nanoparticles, and the rate of their interaction with adsorbed hydrogen. Thus, the rates of hydrogenation and deoxygenation reactions over Pd-PAF-30-SO3H are so high that after the adsorption of substrates on the palladium surface, their conversion into deoxygenated products occurs very quickly. In the case of Pd-PAF-30, the substrates are rapidly hydrogenated to the corresponding alcohols, and then desorbed from the palladium surface. The rate of further deoxygenation depends on how easily alcohol molecules will be re-adsorbed on the surface of nanoparticles. Anisaldehyde, due to the absence of steric hindrance, diffuses to palladium nanoparticles and is easily adsorbed on their surface, which leads to a high yield of the deoxygenation product. Salicylaldehyde molecule contains -OH group in the ortho position, and hydrogen atom in it forms a hydrogen bond with the oxygen of the carbonyl group. This makes the adsorption of salicylaldehyde on the surface of palladium nanoparticles more difficult, due to which the hydrogenation and hydrodeoxygenation rates of this substrate become lower. Vanillin is the most “bulk” molecule, and its diffusion to the active sites of the catalyst is expected to be slower than for the other substrates. This also applies for the Pd-PAF-30-TEA catalyst: hydrogenation of anisaldehyde proceeds much faster than vanillin and salicylic aldehyde due to the absence of diffusion or adsorption restrictions and the resulting p-methoxybenzyl alcohol, than being converted into p-methoxytoluene. In contrast, hydrogenation of vanillin and salicylic aldehyde gives 50–80% conversion with >95% selectivity to the corresponding alcohol.Considering that water is a desirable green solvent for chemical transformations, we also investigated the activity of the catalysts and results are given in Table 4 . Pd-PAF-30-SO3H exhibits the same activity in water as in isopropyl alcohol, while Pd-PAF-30-TEA demonstrates even higher deoxygenation activity. However, the activity of Pd-PAF-30 in water decreased – probably due to lower hydrophilicity of aromatic PAF-30 and poorer dispersion of the catalyst in reaction media. Nevertheless, we can conclude that the synthesized catalysts could be used in environmentally friendly process of bio-components processing.Three supported palladium catalysts based on porous aromatic frameworks with different composition of the surface were tested towards vanillin hydrogenation. All catalysts showed high hydrogenation activity, but the other properties – stability and activity in deoxygenation – depend on the structure of the PAF. Thus, the Pd-PAF-30 exhibited both moderate deoxygenation activity and stability, producing vanillin alcohol and creosol as reaction products. Modification of PAF-30 with sulfo-groups greatly enhanced the activity of the catalyst in deoxygenation, but reduces its stability. In contrast, modification of PAF with alkylammonium groups inhibited the catalytic activity in deoxygenation, making the Pd-PAF-30-TEA catalyst selective in the hydrogenation of aromatic aldehydes to alcohols. Apparently, this selectivity is associated more with the creation of steric restrictions for the adsorption of substrate molecules or with the reduced acidity of this catalyst. The observed effects of the influence of sulfo- and alkylammonium groups were also confirmed by reactions with PAF-30 and toluenesulfonic acid and tetraethylammonium hydroxide. The simplicity of PAFs modification methods and the very high chemical stability, makes it easy to tune the properties of the obtaining catalysts without loss of stability and activity. The results of catalytic tests showed that using PAFs as a support, one can easily tune the activity and selectivity of supported palladium catalysts by changing the surface composition of the porous aromatic framework. M.A. Bazhenova: Investigation, Writing – original draft. L.A. Kulikov: Project administration, Writing – review & editing, Visualization. Yu.S. Bolnykh: Investigation, Visualization. A.L. Maksimov: Formal analysis, Methodology. E.A. Karakhanov: Supervision.The authors declare no competing financial interest.This work was financially supported by the Russian Science Foundation (RSF) grant (project № 20-19-00380). Supplementary material Image 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2022.106486.

The aim of the current work was to study the activity of supported palladium catalysts based on porous aromatic frameworks (PAF) in vanillin hydrogenation, and to develop the methods for controlling their activity and selectivity by modifying the PAF structure with various functional groups. Using unmodified PAF-30, its sulfonated derivative PAF-30-SO3H, and that of PAF-30-TEA modified with CH2N+(C2H5)3 groups, palladium catalysts were synthesized and tested for HDO of lignin components. This investigation provides one potential route for the development of efficient catalysts for hydrodeoxygenation by the adjustment of the composition of the catalyst and its appropriate derived properties.

Data will be made available on request.As is well-known, carbon dioxide (CO2) is the major cause of global climate change, mainly originating from the burning of fossil fuels such as coal and petroleum [1–3]. Meanwhile, CO2 is an abundant resource, economically attractive, nontoxic, and a renewable C1 carbon source [4–6]. As shown in Fig. 1, chemical transformation of CO2 is an exciting way to reduce CO2 concentration.The structure of DMC contains some functional groups such as methyl (CH3-), methoxy (CH3O-), carbonyl (-C(O)-) and carbonyl methoxy (CH3O-C(O)-) [8,9], which can substitute toxic dimethyl sulfate and phosgene in methylation and carbonylation reactions. It is also widely used as a fuel additive [10], organic solvent [11] and electrolyte in lithium-ion battery [12], etc. Various DMC synthesis routes include methanol phosgenation, oxidative carbonylation of methanol, transesterification [13], alcoholysis of urea, direct synthesis from CO2 and methanol, etc. However, some of these processes use toxic, corrosive, flammable and explosive gases such as phosgene, hydrogen chloride, carbon monoxide. Due to the cheap raw material, the avoidance of high toxic reagents, as well as the direct utilization of greenhouse gas CO2, the direct synthesis of DMC from CO2 and methanol is promising.As previously reported, a number of homogeneous and heterogeneous catalysts including ionic liquids [14], alkali carbonates [15], transition metal oxides [16,17], heteropoly acid catalysts [18], and supported catalysts [19] have been investigated. Studies over the past two decades have provided important information on metal oxides; particularly about CeO2 and ZrO2 are the dominating catalysts for the direct synthesis of DMC. Bell et al. [20,21] investigated the mechanism of DMC for formation over zirconia using in situ infrared spectroscopy, and then established that the acid-base sites of the catalyst surface played a decisive role in the direct synthesis of DMC from CO2 and methanol. However, its catalytic performance is still hampered by low specific surface area and insufficient exposure of active sites.Metal-organic frameworks (MOFs) owing inherent large specific surface areas, high porosity and the flexibility of structure and properties design, have been widely applied to catalytic processes [22–26]. In previous studies, a Zr-based metal-organic frameworks catalyst UiO-66-X (X refers to the molar equivalent of trifluoroacetic acid modulator relative to terephthalic acid) was effective for the synthesis of DMC from CH3OH and CO2 [2]. The addition of trifluoroacetic acid (TFA) could increase the amount of active sites (Lewis acidic site, Lewis basic site and terminal hydroxyl), and provide higher specific surface area (>1479 m2 g–1) and highly developed pore structure. The UiO-66–24 catalyst showed excellent catalytic activity (0.17 mmol g-cat–1 h–1) compared with the ZrO2 (0.03 mmol g-cat–1 h–1) for direct synthesis of DMC. On this basis, Zr-based metal organic frameworks catalyst MOF-808-X (X refers to the molar ratio of ZrOCl2·8H2O/1,3,5-benzenetricarboxylic acid) was synthesized and used as the catalyst for this reaction [27]. MOF-808–4 showed the best activity with almost no redundant BTC or zirconium clusters, which outperformed previously reported Zr-based metal-organic frameworks catalyst UiO-66–24 [28]. Based on the results, HPW encapsulated inside the micropore of MOF-808 matrix (HPW@MOF-808) and HPW mainly aggregated on the outside surface of MOF-808 matrix (HPW/MOF-808) were prepared and used for the direct synthesis of DMC from CH3OH and CO2 [2]. HPW@MOF-808 exhibited higher activity than UiO-66–24. Hence, MOFs exhibited excellent catalytic for the reaction system. However, the hydrothermal stability of MOF-808 was not reliable.In order to quantitatively regulate the composition of Lewis acid-base sites on the catalyst surface UiO-66, a series of cerium modified Ce-UiO-66-X (X refers to the millimole of Ce doping) catalysts for the synthesis of DMC from CO2 and CH3OH were synthesized through cationic modification, which has not been reported in the literature. Then, the relationship between the Ce doping amount and the amount of acid and base sites of the catalyst was explored. Furthermore, the effect of 2-cyanopyridine dehydrating agent was investigated at the optimal reaction conditions. Finally, a possible reaction mechanism was deduced based on the characterization results.Zirconium chloride (ZrCl4, 98%), 1,4-benzenedicarboxylic acid (BDC, 99%), cerium nitrate hexahydrate (Ce(NO3)3·6 H2O, 99.5%) were purchased from Aladdin Industrial Inc. (Shanghai, China). N,N-dimethylformamide (DMF, 99.5%), methanol (99.5%), 1-Pentanol (99%) and dimethyl carbonate (DMC, 99%) were obtained from Deen Chemical Co. (Tianjin, China). Carbon dioxide (CO2, 99.9%), ammonia (NH3, 99.9%) and helium (He, 99.99%) were purchased Yuanzheng Gas Co. (Henan, China). All chemicals were used without further purification.UiO-66 was synthesized by a hydrothermal method described in the literature [26,29], with slight modification. Briefly, 1,4-benzenedicarboxylic acid (0.830 g, 5 mmol) and zirconium (IV) chloride (1.165 g, 5 mmol) were added in N,N‐dimethylformamide (30 mL). The mixture was stirred at room temperature for 1 h. Then the obtained mixture was transferred to a Teflon-lined autoclave and heated at 120 °C for 24 h. After cooling in air to room temperature, the white solid was filtered off, washed sequentially three times with DMF and methanol, and finally dried at 150 °C.The Ce-UiO-66 catalysts synthesized in the same way as described above, except that a certain amount of Ce(NO3)3·6 H2O (1.5 mmol, 2 mmol, 3.5 mmol or 5 mmol) was added to the above precursor ZrCl4/BDC solution and stirred at room temperature for 1 h. The Ce/Zr content is 1.5 mmol/3.5 mmol (Ce-UiO-66–1.5), 2 mmol/3 mmol (Ce-UiO-66–2), 3.5 mmol/1.5 mmol (Ce-UiO-66–3.5), 5 mmol/0 mmol (Ce-UiO-66–5) respectively. The remaining solvothermal and cleaning processes were identical to that of UiO-66.Meanwhile, in order to compare the effect of different synthesis methods on the catalytic performance, the Ce-UiO-66–2-IM and Ce-UiO-66–2-IE catalysts with the same Ce/Zr molar ratio as Ce-UiO-66–2 (ICP-AES analysis) were synthesized by impregnation and ion-exchange methods respectively (Supporting Information).The X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer operated at 40 kV and 40 mA with Cu-Kα radiation. The scan of diffraction angle was in a range of 5–80° (2θ) at a scanning speed 0.05°/s.The specific surface area was measured by nitrogen adsorption at 77 K using the Brunauer-Emmett-Teller (BET) method on a Autosorb-iQ-MP-C. Prior to measurements, the samples were desorbed at 150 °C for 12 h using a Belprep vacuum instrument. The pore size distribution of the samples was evaluated by the Density Functional Theory (DFT) method. The total pore volume (Vp) was estimated from the volume of nitrogen adsorbed at a relative pressure of 0.99.The infrared analysis of as-prepared materials has been performed using Fourier transform infrared spectroscopy (FT-IR), Vertex 70, in the range of 400–4000 cm−1.Scanning electron microscopy (SEM) analysis was performed using a JSM-7900 F equipment. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) measurements were performed on a JEM-2100 F equipment operated at 200 kV.Thermogravimetric analysis (TGA) was performed using a NETZSCH thermogravimetric analyzer with a heating rate of 10 °C min−1 from 25 to 700 °C under air environment.The acid-base properties of the samples were studied by temperature-programed desorption of ammonia (NH3-TPD) and temperature programed desorption of carbon dioxide (CO2-TPD). Typically, 100 mg of samples were used for each measurement. The sample was pretreated at 300 °C in the helium gas flow for 30 min to eliminate impurities. After cooling to 40 °C, the sample was saturated with NH3 or CO2, then weakly adsorbed NH3 or CO2 was subsequently removed by purging with He for 60 min. The desorption process was conducted in He from room temperature to 400 °C. The desorbed NH3 or CO2 was detected by TCD detector.X-ray photoelectron spectroscopy (XPS) spectra were obtained using a Thermo ESCALAB 250 spectrometer equipped with Al Ka radiation (1486.6 eV) under a binding energy was referenced to the C1s line (284.8 eV).The element content of Zr and Ce of the catalysts was determined by an inductively coupled plasma atomic emission spectrometer (ICP-AES, Thermo iCAP6300, Thermo Fisher, USA).A 60 mL autoclave batch reactor was used for the direct synthesis of dimethyl carbonate (DMC) from CO2 and CH3OH. Typically, 0.1 g catalyst and 6.4 g CH3OH were added into the autoclave, then 5.5 MPa CO2 was introduced into the autoclave at 25 °C. After reacting for a desired time, the reactor was immediately cooled by water bath and depressurized. Then, the catalyst and liquid product were separated by centrifugation. The compositions of the reaction product were identified with a Haixin GC-950 gas chromatograph equipped with a flame ionization detector. The amount of DMC formed was determined by internal method with n-amyl alcohol as the internal standard substance. For all catalytic reactions, no other substances were detected except DMC in both gas and liquid phase samples. Therefore, the selectivity of DMC was considered as 100%.The DMC yield, DMC formation rate and the turn over frequency (TOF) of DMC were calculated using the following formulas [28]: (1) DMC yield = Amount of DMC formed ( mmol ) × 2 Amount of methanol added ( mmol ) × 100 % (2) DMC formation rate = Amount of DMC formed ( mmol ) Amount of catalyst ( g ) × Reaction time ( h ) (3) TOF = Amount of DMC formed ( mmol ) Amount of metal in added catalyst mmol × Reaction time ( h ) The crystal structures of synthesized catalysts were investigated by XRD patterns. As shown in Fig. 2, the typical characteristic diffraction peaks of the as-synthesized UiO-66 were observed at 7.3°, 8.5°, 12.0°, 14.8°, 17.0°, 22.2°, 25.7° and 30.7°, which corresponded to the (111), (002), (022), (113), (004), (115), (224) and (046) planes respectively. All the diffraction peaks were found to be well-matched with the reported literatures [30,31], confirming that Zr-based metal-organic framework UiO-66 was successfully synthesized. After the addition of Ce, with the cerium content increasing from 0 to 2 mmol, the peak intensity of the XRD patterns increased, indicating the increased crystallinity of Ce-UiO-66-X. However, no new characteristic diffraction peaks of Ce-UiO-66–1.5 and 2 catalysts were observed, implying that Ce doping did not change the crystal structures of UiO-66. Based on this result, it might be deduced that the Zr element was partially substituted by Ce, and Ce atoms were successfully inserted into UiO-66 structure successfully. A further increase in Ce content to 5 mmol, the intensity of the characteristic diffraction peak gradually weakened and finally disappeared [32], and a new diffraction peak at 2θ = 9.4°, corresponding to cerium tris (3,5-diaminobenzoatel) hydrate (PDF 41–1744) appeared. Moreover, the intensity of the new diffraction peak intensity increased with increasing cerium content, which implied that the MOFs structure formation was hampered. That is to say, it was necessary to control the dosage of Ce to ensure formation of the crystalline lattices of UiO-66 MOFs [33]. Obviously, the diffraction peak intensity of the Ce-UiO-66–2 catalyst was the largest highest in the synthetic catalysts, indicating the highest crystallinity.The nitrogen adsorption-desorption isotherms and pore size distribution curves of the different catalysts are displayed in Fig. S1. UiO-66 (Fig. S1a) exhibited a typical Type Ⅰ adsorption isotherm curve, indicating that the sample presented microporous structure [34]. After adding cerium, the Ce-UiO-66–1.5 and Ce-UiO-66–2 still maintained a microporous structure, implying Ce element replaced part of the Zr atoms by one-step synthesis method successfully. A further increasing in the content of cerium, the isotherm curve of Ce-UiO-66–3.5 displayed a type IV curve with a hysteresis loop at P/P0 = 0.45–0.95, demonstrating that the material possessed a typical mesoporous structure [35]. The appearance of this mesoporous structure was probably due to the fact that excessive cerium dosage hampered the formation of UiO-66 MOFs structure. Table 1 lists the specific surface area and total pore volume of the UiO-66 and Ce-UiO-66-X catalysts. Ce-UiO-66–2 had a maximum specific surface area (1281.3 m2 g−1) and pore volume (0.724 cm3 g−1). Among these catalysts, the specific surface area and total pore volume of Ce-UiO-66–1.5 and Ce-UiO-66–2 were larger than those of UiO-66, suggesting that the introduction of Ce was beneficial to enlarge the specific surface area to some extent, which were probably because of their high crystallinity. Whereas the specific surface area and total pore volume of Ce-UiO-66–3.5 and Ce-UiO-66–5 were both smaller than those of UiO-66. Their specific surface area and total pore volume decreased sharply may be due to the formation of a new crystalline phase. Combined with XRD results, it can be concluded that a decrease in crystallinity could result in a decrease in specific surface area.FT-IR spectra of UiO-66, Ce-UiO-66–1.5, Ce-UiO-66–2, Ce-UiO-66–3.5 and Ce-UiO-66–5 catalysts are showed in Fig. 3. The broad band around 3227 cm−1 was ascribed to -OH stretching vibration from the surface adsorbed water [36]. The band at 1710 cm−1 corresponded to the uncoordinated -COOH of free BDC [27,37], corroborating the existence of unreacted BDC trapped in the micropores of Ce-UiO-66–1.5 and Ce-UiO-66–2. Obviously, the band for 1710 cm−1 didn’t emerge in FT-IR spectra of Ce-UiO-66–3.5 and Ce-UiO-66–5, indicating that unreacted BDC was washed out completely. The band at 1657 cm−1 was assigned to CO stretching in the carboxylic acid present in BDC [25,38]. The band at 1579 cm−1 was assigned to CC vibration in the aromatic ring of the organic linker [39]. The strong band at 1400 cm−1 was linked to C-O stretching vibrations in carboxylic acid [39].It was clear that the peaks of CC and C-O bonds shift to a lower wavenumber in Ce-UiO-66–3.5 and Ce-UiO-66–5 catalysts, which was attributed to the formation of a new crystalline phase, these were consistent with XRD results. The peaks at 668, 744, 810 cm−1 were the mix of O-H band and C-H vibration in the BDC ligand [40,41]. The characteristic adsorption peaks at 549 cm−1 and 485 cm−1 corresponded to Zr-(OC) asymmetric stretch and Zr-O vibration in the UiO-66, Ce-UiO-66–1.5 and Ce-UiO-66–2 respectively [30,42]. It is worth noting that the diffraction peaks at 549 cm−1 and 485 cm−1 showed the maximum diffraction peak intensity in Ce-UiO-66–2 catalyst. With the further increase of Ce content, the peaks at 549 cm−1 and 485 cm−1 disappeared and a new peak at 509 cm−1 appeared corresponding to the bond asymmetric stretching of Ce-(OC) in the spectrum of Ce-UiO-66–3.5 and Ce-UiO-66–5 [39]. Obviously, the Ce-UiO-66–2 catalyst showed the maximum diffraction peak intensity in the FT-IR spectra, revealing that the Ce-UiO-66–2 catalyst had the most regular UiO-66 MOFs structure. These results were consistent with the XRD results.The surface morphology of the samples was investigated by SEM ( Fig. 4). As depicted in Fig. 4a, it was clear that the UiO-66 catalyst showed the irregularly spherical crystal morphology. This morphology was in line with that reported in the literature [31,43]. Compared to the UiO-66 catalyst, the particles of Ce-UiO-66–1.5 and Ce-UiO-66–2 catalysts became smaller and the crystal structure became more complete gradually. Especially, Ce-UiO-66–2 catalyst (Fig. 4c) had the most regular UiO-66 MOFs structure. However, a further increase in amount of Ce, large lumps appeared on SEM images of Ce-UiO-66–3.5 and Ce-UiO-66–5 catalysts, as shown in Fig. 4d and e. This may be because that a large amount of Ce caused the protonation of terephthalic acid ligands and then restrained the formation of UiO-66 MOFs structure, finally appeared big block morphology. The results indicated that excessive cerium content formed a new crystalline phase, which was consistent with the XRD observation.Furthermore, the TEM characterization of UiO-66, Ce-UiO-66–2, Ce-UiO-66–5 catalysts are also presented in Fig. 5. As provided in Fig. 5a and b, it showed that the particle sizes of the crystalline UiO-66 and Ce-UiO-66–2 were uniform, 150–200 nm approximately. Moreover, a big block morphology was observed from the TEM image in Fig. 5c, which was consistent with the SEM observation. The elemental mapping images of the UiO-66, Ce-UiO-66–2 and Ce-UiO-66–5 materials were observed, including C, Ce, O and Zr. No conglomeration of Ce element was observed, suggesting the good dispersion of Ce on the Ce-UiO-66–2 catalyst surface.To investigate the thermal stability of Ce-UiO-66-X and UiO-66 catalysts, the materials were measured by thermal gravimetric analysis (TGA). As shown in Fig. S2, three distinct mass-loss regions were observed. A minor mass-loss below 150 °C was observed due to the removal of H2O and methanol, and the weight loss within 150–400 °C was related to the evaporation of DMF solvent and dehydroxylation of zirconium/cerium oxo-cluster [44,45]. Also, a sharp weight decline in the range of 400–540 °C (UiO-66, Ce-UiO-66–1.5 and Ce-UiO-66–2 samples) and 340–440 °C (Ce-UiO-66–3.5 and Ce-UiO-66–5 samples) were corresponded to the decomposition of organic bridging ligands (BDC) [28]. In addition, no additional weight loss was observed above 550 °C. The TGA results indicated that the UiO-66 possesses highly stable structure and the Ce-UiO-66–1.5 as well as Ce-UiO-66–2 display no obvious change on the thermal stability, which was maybe due to the Ce incorporated into the framework of UiO-66. However, the Ce-UiO-66–3.5 and Ce-UiO-66–5 samples decreased obviously, which could be related to incomplete UiO-66 MOFs structure due to the introduction of excessive cerium in the synthesis process.The number of BDC linkers in Zr6 formula unit was calculated following the method reported by Shearer [46]. In the work, the number of linkers in the ideal Zr6 formula unit is 6, and the actual number of linkers in the defective Zr6 formula unit is 5.13 for UiO-66. As displayed in Fig. 6a, the experimental weight loss (%) of BDC linkers for UiO-66 and Ce-UiO-66–2 were 42.17% and 44.51% respectively. Therefore, Ce-UiO-66–2 possessed more ideal structural units, which was consistent with the results of XRD.As shown in Fig. 7, the NH3-TPD and CO2-TPD were performed to probe the acidic and basic properties of all catalysts. Based on the area of NH3 and CO2 desorption peaks, the number of different acidic and basic sites were summarized in Table 2. Obviously, all samples showed three types of acidic sites with different intensities in Fig. 7a, which could be attributed to weak (α1 and α2 peak 50–200 °C) and medium (β peak 200–330 °C) acidic sites, respectively, similar results were also reported by Zhang et al. [47]. The acidity of UiO-66 was originating from the missing of BDC-linker in the Zr6 clusters and the exposed Zr4+ of Zr6 node [34,48–51]. As can be seen, the number of total acidic sites in UiO-66 was 3.10 mmol g-cat−1. After Ce modification, the amount of the total acidic sites increased from 3.10 mmol g-cat−1 to 5.02 mmol g-cat−1 for Ce-UiO-66–2, which probably originated from the insertion of Ce3+ ions into the zirconium clusters [44]. However, as the cerium content increased to 5 mmol, the amount of the total acidic sites decreased from 5.02 mmol g-cat−1 to 0.48 mmol g-cat−1 for Ce-UiO-66–5. This was probably because the excessive cerium dosage hampered the formation of UiO-66 MOFs structure as indicated by the XRD results. Fig. 7b shows the CO2 desorption profiles of Ce-UiO-66-X and UiO-66 catalysts, which could be assigned to weakly (peak α 200–310 °C) and moderately (peak β 310–400 °C) basic sites, respectively. Peak β could be attributed to medium (β peak) basic sites arising from the unsaturated lattice oxygen anion (Zr-O and Ce-O) and unsaturated terminal oxygen anion in zirconium/cerium cluster (Zr-O− and Ce-O−) [28]. As can be seen in the Table 2, the Ce-UiO-66–2 showed the largest amount of medium basic sites of 1.04 mmol g-cat −1. And Ce-UiO-66–5 showed no basicity virtually, which was probably attributed to the irregular structure, as indicated by the TEM and SEM results.As indicated by NH3-TPD and CO2-TPD results, compared with UiO-66, Ce modulation increased the amount of acid and basic sites in Ce-UiO-66–2 significantly. The total acidity and basicity decreased in the order of Ce-UiO-66–2 > Ce-UiO-66–1.5 > UiO-66 > Ce-UiO-66–3.5 > Ce-UiO-66–5.To investigate the surface compositions of Ce-UiO-66-X and UiO-66 catalysts, X-ray photoelectron spectroscopy (XPS) was employed and the results are shown in Fig. 8. As presented in Fig. 8a, three main peaks corresponding to Zr 3d, C 1 s, and O 1 s were observed in the XPS spectra of UiO-66. After the Ce3+ doping (Fig. 8b), new peaks appeared at 882.5, 885.5 eV and 900.8, 903.8 eV of Ce-UiO-66–3.5 corresponding to Ce 3d5/2 and Ce 3d3/2 [52,53], respectively, which indicated the incorporation of Ce3+. However, no characteristic diffraction peaks for Ce 3d appeared on the Ce-UiO-66–1.5 and Ce-UiO-66–2 catalysts, which was mainly because the highly dispersed Ce or low cerium content (below the limit of detection by the XPS) on the catalyst surface.In the Zr 3d XPS spectra of UiO-66 catalysts (Fig. 8c), the doublets in the binding energies at 182.9 eV and 185.3 eV belonged to the Zr 3d5/2 and Zr 3d3/2, respectively. The Zr 3d binding energy increased first and then decreased with the increase of cerium content, demonstrating that the bonding environment of Zr nodes was changed after the introduction of Ce3+. In addition, the result indicated that Zr4+ in the structure of UiO-66 might be partially replaced by Ce3+ [44,54].It is worth noting that there was a negative shift in the binding energy of O 1s spectra from 532.0 to 531.7 eV when the Ce3+ doping amount reached 5 mmol, indicating that a new chemical state of O1s emerged. This result was consistent with the XRD analysis. Furthermore, the O 1s (Fig. 8d) spectra can be fitted by three peaks. The peak at ∼530.3 eV could be attributed to unsaturated lattice species of Zr-O and Ce-O bonds [55]. The peak at ∼531.9 eV was ascribed to surface hydroxyl oxygen species O-H [28]. The peak at ∼533.3 eV was assigned to oxygen component in the O-CO [56]. The curve fitting results of all five samples were listed in Table 3, unsaturated lattice species (Zr-O and Ce-O) have the maximum content of 15% in Ce-UiO-66–2 catalyst, indicating it had more intermediate alkaline sites, which was consistent with the CO2-TPD results.To investigate the effect of different preparation methods on the catalytic performance, the Ce-UiO-66–2-IM and Ce-UiO-66–2-IE catalysts with the same Ce/Zr molar ratio as Ce-UiO-66–2 (ICP-AES analysis) were synthesized and evaluated in Fig. 9. The Ce-UiO-66–2 catalyst has the highest DMC yield. This was mainly due to the high dispersion of Ce into skeleton lattice of Ce-UiO-66–2 catalyst. Therefore, one-step synthesis is the best catalyst preparation method. Table 4 summarizes the catalytic performances of UiO-66 MOFs catalysts with different contents of cerium. The DMC yield first increased and then decreased with the content of Ce increasing, and it reached the maximum value when X = 2 mmol. In addition, it should be noted that DMC was not detected in the absence of catalyst, which showed the importance of catalyst in this reaction. Among the catalysts tested, the UiO-66 (none Ce) exhibited low catalytic activity. After Ce modification, the average DMC formation rate increased from 0.171 mmol g-cat−1 h−1 for pristine UiO-66 to 0.335 mmol g-cat−1 h−1 for Ce-UiO-66–2, indicating that the Ce modification significantly enhanced the catalytic activity of UiO-66. Moreover, the Ce-UiO-66–5 catalyst (pure Ce) also showed no activity for this reaction, which could be related to irregular UiO-66 structure due to the introduction of excessive cerium in the synthesis process as indicated by the XRD and SEM results.In order to eliminate the influence of the crystal structure of Ce-based UiO-66 catalyst on the catalytic performance, the UiO-66(Ce) catalyst (pure Ce) with complete UiO-66 structure was synthesized according to the method used in literature [57,58], and evaluated for the synthesis of DMC from CO2 and CH3OH, and the results are shown in Table 4. As can be seen, the DMC yield over UiO-66(Ce) was 0.047%, which has a higher catalytic activity than Ce-UiO-66–5. The results indicated that the crystal structure of Ce-based UiO-66 catalyst really do has effect on the activity of the catalyst in the aforementioned reaction. However, the catalytic activity of UiO-66 (Ce) was still lower than that of other catalysts. In addition, the Ce-UiO-66–2 catalyst exhibited the best catalytic performance (0.335 mmol g-cat−1 h−1) in this work, which could be due to the maximum amount of acidic and basic sites and the most regular UiO-66 MOFs structure.As shown in Fig. 10, the effects of different reaction parameters (reaction temperature, reaction time, and catalyst dose) on the DMC yield over Ce-UiO-66–2 catalyst were investigated.As we know, the reaction is not occurred spontaneously in nature [59]. So, the influence of the reaction temperature on the DMC yield is shown in Fig. 10a, the temperature was varied between 100 and 150 °C. There was a significant increase in the yield of DMC as the reaction temperature increased from 100 °C to 140 °C. As the temperature increased to 150 °C, the DMC yield decreased. So, it can be seen that the optimum reaction temperature is 140 °C. Fig. 10b shows the effect of the catalyst dose on the catalytic performance. The yield of DMC increased with catalyst dose, and the highest yield of DMC was 0.209% at 0.25 g of the catalyst. With a further increase in the catalyst dose > 0.25 g, the DMC yield decreased instead. Fig. 10c indicates the effect of the reaction time on DMC yield, the time was varied between 1 and 48 h. It can be seen that the DMC yield increased from 0.126% to 0.295% with the reaction time increased from 1 to 12 h. After 12 h, DMC yield was constant. This was probably due to the fact that the reaction has reached equilibrium when the reaction time is 12 h.So, the optimum reaction conditions on the Ce-UiO-66–2 catalyst are as follows: reaction temperature was 140 °C, catalyst weight was 0.25 g; reaction time was 12 h. Fig. 11 shows the catalytic results of Ce-UiO-66–2 for the synthesis of DMC from methanol and CO2 with six times reused. After each run, the catalyst was recovered by centrifugation, and further washed with methanol and dried at 150 °C for 12 h. The regenerated sample was then used for the next run under the same reaction conditions. The observed DMC yield slightly decreased with an increase in reuse cycles. While, the XRD patterns (Fig. S3) suggested that the structure of Ce-UiO-66–2 was retained after used for six cycles. Furthermore, ICP-AES analysis (Table S4) showed that only 0.2% of the total amount of Zr and Ce was leached from Ce-UiO-66–2 after six cycles. The results showed that the Ce-UiO-66–2 catalyst was stable and effective for DMC synthesis in our reaction system.Bell et al. [20,60] had pointed out that amphoteric Zr-OH hydroxyl groups and coordinately unsaturated Zr4+O2− sites as Lewis acid-base pairs played a key role in the synthesis of DMC from CO2 and CH3OH over ZrO2. Wang et al. [61] reported that spindle-like ceria exhibited the best catalytic performance due to exposing active planes and a large amount of acid-base sites. Kumara et al. [62] believed that manganese modification could enrich the amount of acid-base sites on CeO2, then, methanol could be activated to form H+ and CH3O− due to basic sites, and to form CH3 + and OH− in the presence of weak/moderate acidic sites. The catalyst based on Ce1-Mn0.125 showed the best catalytic activity with the maximum number of weak and moderate acidity and basicity. Thus, tuning the amount of acid-base sites of catalysts was an effective strategy to improve the catalytic activity of the catalyst. Fig. 12 shows the relationship between the catalytic activity and the amount of acid and medium basic sites of Ce-UiO-66-X and UiO-66 samples. It could be found that the yield of DMC increased with the number of acid and medium basic sites (Ce-UiO-66–2 > Ce-UiO-66–1.5 > UiO-66 > Ce-UiO-66–3.5 > Ce-UiO-66–5). The Ce-UiO-66–2 catalyst presented the best catalytic activity compared to other catalysts, which was due to the fact that the higher amount of acidic and medium basic sites are responsible for the formation of methyl carbonate.Xuan et al. [28] reported that the enlargement of the pore size and pore volume were also contributed to the enhancement of catalytic activity, since the mass transfer resistance in the channel with larger pore size was lower. So, in addition to the acid-base properties, the effect of the textural properties on the catalytic activity should also be considered [2]. As shown in Table 2, after Ce modification, Ce-UiO-66–2 and Ce-UiO-66–1.5 provided more active sites (acid sites: 5.02 mmol g-cat−1 and 4.97 mmol g-cat−1, medium basic sites 0.61 mmol g-cat−1 and 0.53 mmol g-cat−1) than pure UiO-66 (acid sites: 3.1 mmol g-cat−1, medium basic sites: 0.36 mmol g-cat−1) for this reaction. In addition, the average DMC formation rate increased from 0.171 mmol g-cat−1h−1 for pristine UiO-66 (Table 4) to 0.315 mmol g-cat−1h−1 and 0.335 mmol g-cat−1h−1 for Ce-UiO-66–1.5 and Ce-UiO-66–2, respectively. It is worth noting that the difference between average DMC formation rate of Ce-UiO-66–2 and Ce-UiO-66–1.5 is 0.02 (0.335 − 0.315 = 0.02), the difference between acid sites of Ce-UiO-66–2 and Ce-UiO-66–1.5 is 0.05 (5.02 − 4.97 = 0.05), and the ratio of 0.02–0.05 is 0.4. However, the ratio of the difference between average DMC formation rate of Ce-UiO-66–1.5 and UiO-66 to the difference between acid sites of Ce-UiO-66–1.5 and UiO-66 is 0.077 [(0.315 − 0.171)/ (4.97 − 3.1) = 0.077] (Fig. 12). That is to say, it is not linear correlation completely between the acid sites and the average DMC formation rate, the same is true for medium basic sites. In conclusion, the acid-base properties are not the only factor affecting catalytic activity. According to the N2 adsorption results, the specific surface area and pore volume of Ce-UiO-66–1.5 and Ce-UiO-66–2 were larger than that of pristine UiO-66, and the catalytic activity of Ce-UiO-66–1.5 and Ce-UiO-66–2 were higher. This showed that the enlargement of the specific surface area and pore volume in Ce-UiO-66-X may also contributed to the catalytic enhancement, since the large specific surface area may also contribute to provide higher accessibility for the reactant to the active sites located in the micropores.In summary, the increased amount of active sites as well as the enlargement of specific surface area originated from the addition of a suitable amount of cerium contributed to the enhanced activity of Ce-UiO-66-X.The catalytic activity of Ce-UiO-66–2 catalyst for the direct synthesis of DMC was compared with the reported catalysts under different reaction conditions ( Table 5). As can be seen in Table 5, the Ce-UiO-66–2 (Entry 1) catalyst showed the DMC yield of 0.134%. Additionally, using 2-cyanopyridine as dehydrating agent (Entry 2), DMC yield can be increased by about ten times. It can be seen that the addition of dehydrating agent has increased the yield of DMC to a certain extent, which is consistent with previous literature [27,28,63].However, the TOF over Ce-UiO-66–2 (Entry 2) was lower than that over CeO2 catalyst using 2-cyanopyridine as a recyclable dehydrating agent (Entry 12). This is because CeO2 can improve the hydration of 2-cyanopyridine, thus rapidly remove the by-product water and increase the formation of DMC. Besides, the Ce-UiO-66–2 (Entry 1) catalyst showed higher TOF than those reported without the addition of dehydrating agent. It should be noted that the TOF over 5%Cu-Ni/ZIF-8 (Entry 16) was higher than that over Ce-UiO-66–2 catalyst (Entry 1). But in the reusability test experiments, the yield of DMC over 5%Cu-Ni/ZIF-8 [70] catalyst dramatically decreased from 1.71 mmol g-cat−1 to 0.57 mmol g-cat−1 after four cycles. In contrast, Ce-UiO-66–2 was used for six times with DMC yield slightly decreased from 1.18 mmol g-cat−1 to 1.16 mmol g-cat−1, indicating the better stability of Ce-UiO-66–2 catalyst.A possible reaction mechanism for the direct synthesis of DMC from CO2 and methanol over Ce-UiO-66–2 catalyst is shown in Scheme 1. Ce-UiO-66–2 contains Lewis acid site (exposed Zr4+/Ce3+) and Lewis basic sites (unsaturated O2− anion in Zr-O, Ce-O, Zr-O− and Ce-O−).One molecule of methanol and CO2 is activated on Lewis acid-base pair of sites. Methanol binds to Lewis acidic site of metal node in Ce-UiO-66–2 to form Zr/Ce-OCH3 and releases an H atom, which then reacts rapidly with a surface OH group to form H2O. The activated CO2 is then inserted into the Zr/Ce-O bond of the Zr/Ce-OCH3 species to produce m-CH3OCOO-Ce/Zr. In addition, methanol can also be activated into methyl group and hydroxyl group by the acidic sites. The methyl group is transferred to the terminal O atom of methyl carbonate species to produce DMC, while the hydroxyl group can react with exposed Zr4+/Ce3+ to form terminal hydroxyl. The reaction mechanism is similar to some reported literature [2,19,27,28].In this study, a series of Ce-doped Zr-based metal-organic frameworks UiO-66 were synthesized via a modified hydrothermal method and investigated for the direct synthesis of DMC from CO2 and methanol. Among all the Ce-UiO-66-X catalysts, Ce-UiO-66–2 catalyst showed the highest DMC yield of 0.295% at 12 h, 140 °C, 11 MPa. The results demonstrated that the addition of cerium to UiO-66 could influence the growth of UiO-66 MOFs, and thus increased the amount of the surface acidic and moderately basic sites, which then facilitated the activation of methanol and CO2. Besides, the enlargement of the specific surface area was also contributed to the enhancement of catalytic activity. This study offers strategies to design metal doping modification of MOFs materials and provides a novel catalyst for the synthesis of dimethyl carbonate. Linmeng Huo: Conceptualization, Writing – review & editing, Data curation. Lin Wang: Software, Investigation, Data curation. Jingjie Li: Formal analysis, Visualization, Software. Yanfeng Pu: Methodology, Resources, Review, Supervision. Keng Xuan: Software, Investigation. Congzhen Qiao: Supervision, Resources. Hao Yang: Methodology, 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.This research was funded by the Technology Research Project of Henan Province (Grant No. 212102210210), the First-class Discipline Construction Project of Henan University (No. 2019YLZDCG01), the Technology Research Project of Kaifeng City (Grant No. 2001003).Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jcou.2022.102352. Supplementary material .

The disadvantage of Ce/Zr metal oxide as catalyst for direct synthesis dimethyl carbonate (DMC) from CO2 and methanol is low DMC formation rate because of its low specific surface area and active sites. In this work, it can be improved by highly uniform dispersion cerium doped Zr-based metal-organic frameworks (UiO-66 MOFs), which were synthesized via a modified hydrothermal method. The as-prepared catalysts have been extensively characterized using XRD, BET, FT-IR, SEM, TEM, TGA, NH3-TPD, CO2-TPD, XPS techniques. Experimental evaluation results indicated that the highly uniform dispersed Ce-doped UiO-66 MOFs exhibited markedly improved catalytic performance than traditional Ce/Zr metal oxide catalyst. The highest yield of DMC catalyzed by Ce-UiO-66–2 was 0.295% (reaction time was 12 h; reaction temperature was 140 °C; reaction pressure was 11 MPa). Then, it was found that doping of Ce atoms into the zirconium lattice to produce UiO-66 MOFs could effectively increase the number of acidic and medium basic sites than UiO-66 (none Ce), thus could greatly enhance the catalytic performance. Moreover, using 2-cyanopyridine as dehydrating agent, the DMC yield could be further raised greatly. Last, based on reported literature and our results, a possible reaction mechanism over Ce-UiO-66-X was proposed.

We are nearing the point where hydrogen energy is rapidly developing and aimed to replace the traditional fossil fuel with green and renewable energy [1,2]. Electrocatalytic water splitting has been proven to be a promising clean energy technology for hydrogen production. Still, its large-scale implementation is hindered by the high-cost and scarce resource of noble-metal catalysts (e.g., RuO2 and IrO2) commonly used on the anode to overcome the sluggish kinetics of oxygen evolution reaction (OER) [3]. Therefore, searching for alternative electrocatalysts with analogous OER activities but low cost and high abundance is an attractive research topic [4,5]. During recent decades, researchers have developed numerous non-noble metal-based electrocatalysts with low overpotential, small Tafel slope, and good stability, such as metal oxides [6], hydroxides [7,8], chalcogenides [8,9], nitrides [10], phosphides [11,12], and carbon-based materials [13,14].Metal-organic frameworks (MOFs) possess large specific surface area, tunable porosity, ordered structure, and flexible metal-linker compositions, accordingly, demonstrate high potential in constructing the advanced OER electrocatalysts [15–17]. Firstly, MOFs can be directly applied as anode catalysts to boost OER activities, such as MIL-88 [18], MIL-53 [19], MIL-100 [20], NiFe-MOF [21] and ZIF-67 [22,23]. Moreover, the MOF derived materials can afford unique nanostructures with enhanced catalytic performance while avoiding the intrinsic chemical instability of MOFs, such as hollow structured metal oxides [24], metal phosphides [25], and carbon-coated metal alloys [26,27]. To further optimize the activity, the second or third metal ions were deliberately incorporated into the MOF matrix to modify the coordination environments and electronic properties of active sites [28,29]. Fe-Co-Ni based ternary materials are commendable cases in the preparation of MOF-derived electrocatalysts compared with monometallic derivatives owing to the following multiple advantages: facile electronic structure manipulation to reach the optimal oxygen binding affinity; the synergetic effects between different metal sites for multistep OER process; and improvement in the electrical conductivity. For example, Lang et al. synthesized Fe-Co-Ni-MIL-53 by the solvothermal method. They acquired a current density of 10 mA cm−2 with a low overpotential of 219 mV, the electronic environment of active metal sites is efficiently modulated under the synergy effect of the ternary metals to favor enhancing the catalytic OER process [30]. Chen et al. designed a hollow multivoid nanocuboidal catalyst based on ternary Ni-Co-Fe MOF precursor, different ionic reaction rates of [Co(CN)6]3- and [Fe(CN)6]3- in the MOF precursor with S2- are exploited to produce internal interconnected voids, heteroatom doping, and a favorable electronic structure, thus generating dual-functionality toward OER and hydrogen evolution reaction (HER) [31].Generally, the one-pot synthesis method, in which the multimetal-organic frameworks are prepared by introducing all metallic sources into the organic precursors, is a conventional strategy to build electrocatalysts [32]. However, the in-situ generated multimetal-organic frameworks always possess the uniform structure as the monometal-organic frameworks, which may suffer from the chemical and structural instability during electrochemical test or post-treatment, causing severe damage to the original well-designed nanostructures, such as the aggregation of metal site, decline of surface area and destroy of pore structure [33]. For example, Zhang et al. observed that the stability of in-situ generated FeCo-MIL-88B declined seriously (retain ∼75 % of its initial activity) after 40 h electrocatalysis, while the anion-exchange (such as W and Se) treated products can retain ∼90 % of its initial activity at the same reaction conditions [18]. Han et al. found there are some large metal oxide nanoparticles forming when calcinating the in-situ generated Co-Fe MOF at 550 °C in N2 gas, and the particle size becomes larger obviously when increasing the calcination temperature [34].In this study, we report an effective strategy to build a hierarchical MOF structure through ion exchange between Fe-MIL-101-NH2 and Co, Ni ions, where 2-D ternary metal MOF layers encapsulate 3-D MOF octahedral crystals. The original octahedral skeleton structure of MOF precursor can be maintained after air calcination treatment, resulting in hierarchically structured CoNiFe spinel oxide-carbonitrides hybrids, simplified as CoNiFeOx-NC. The obtained catalyst achieved an excellent OER activity with a low overpotential of 265 mV at 50 mA cm−2 and held excellent stability of more than 40 h OER catalysis at around 12 mA cm−2. A combination of multiple characterization techniques and density functional theory (DFT) calculations was employed to investigate the structure-performance relationships. The results revealed that the enhanced performance of CoNiFeOx-NC can be attributed to the highly dispersed metal oxides nanoparticles, large surface area, and unique electronic structure modulation of Co-Ni-Fe oxide active sites. Finally, the strategy developed here may open up more novel and versatile approaches to developing non-noble electrocatalysts with high performance.2-aminaterephthalic acid (BDC-NH2, 98 %, Aladdin); Iron (III) chloride hexahydrate (FeCl3·6H2O, 99 %, Aladdin); Nickel (II) chloride hexahydrate (NiCl2·6H2O, 99 %, Aladdin); Cobaltous (II) chloride hexahydrate (CoCl2·6H2O, 99 %, Aladdin); N, N-Dimethylformamide (DMF, ≥99 %, Aladdin); Ethanol (C2H5OH, ≥99.7 %, Sinopharm Chemical Reagent Co., Ltd); Methanol (CH3OH, ≥99.5 %, Aladdin); Nafion (5 wt.%, Sigma-Aldrich). All chemicals were purchased from commercial sources and used without further treatments.The 3-D Fe-MIL-101-NH2 was prepared following previously reported procedures [35]. Typically, BDC-NH2 (0.45 g, 2.5 mmol) was dissolved in DMF (15 mL), then a mixture of FeCl3·6H2O (1.35 g, 5.0 mmol) and DMF (15 mL) was added into the above solution. After stirring for one hour at room temperature, the resultant mixture was transferred into a 50 mL Teflon-lined autoclave and placed in a 115 °C oven for 20 h. After cooling to room temperature, the powder products were centrifuged, washed with DMF and methanol several times, and finally dried under vacuum for 24 h.Co and Ni co-doped Fe-MIL-101-NH2 (labeled as CoNiFe-MOF) was synthesized by an ion-exchange method. The Fe-MIL-101-NH2 precursor (0.27 g) was dispersed in DMF solvent (15 mL). The mixture of NiCl2·6H2O (0.67 g, 2.8 mmol) and CoCl2·6H2O (1.34 g, 5.6 mmol) dissolved in DMF solvent (15 mL) was added into the above solution under continuous magnetic stirring. After stirring for one hour at room temperature, the resultant mixture was transferred into a 50 mL Teflon-lined autoclave and placed in a 90 °C oven for 48 h. After cooling to room temperature, the powder products were centrifuged, washed with DMF and methanol several times, and finally dried under vacuum for 24 h.Co-doped Fe-MIL-101-NH2 (labeled as CoFe-MOF) and Ni-doped Fe-MIL-101-NH2 (labeled as NiFe-MOF) were synthesized in the similar process as CoNiFe-MOF but changing the cation precursor to CoCl2·6H2O (2.00 g, 8.4 mmol) for CoFe-MOF and NiCl2·6H2O (1.99 g, 8.4 mmol) for NiFe-MOF. In-situ generated ternary-metal MOF (denoted as IS-CoNiFe-MOF) was synthesized in the similar process as Fe-MIL-101-NH2 but changing the cation precursor to the mixture of FeCl3·6H2O (2.5 mmol), NiCl2·6H2O (2.0 mmol) and CoCl2·6H2O (0.5 mmol).CoNiFeOx-NC was obtained through heating CoNiFe-MOF in the air. Firstly, CoNiFe-MOF frameworks were taken in a tube furnace. Then the temperature was raised to the target temperature (i.e., 200, 300, 400, and 500 °C) at a rate of 5 °C min−1 and maintained for 60 min. Finally, the pyrolysis products were naturally cooled down and collected.CoFeOx-NC, NiFeOx-NC, IS-CoNiFeOx-NC were synthesized in the same process as CoNiFeOx-NC catalyst by heating CoFe-MOF, NiFe-MOF and IS-CoNiFe-MOF, respectively.X-ray powder diffraction (XRD) was carried out on a Philips X’pert pro MPD Super diffractometer equipped with Cu Kα radiation (λ = 1.5418 Å). Transmission Electron Microscope (TEM), High-Resolution TEM (HRTEM), and Selected Area Electron Diffraction (SAED) images were acquired on JEM-2100 UHR at an acceleration voltage of 200 kV. X-ray photoelectron spectra (XPS) were recorded by Thermo Fisher ESCALAB 250 analyzer with Al Kα radiation. The element component analysis was characterized by inductive coupled plasma-atomic emission spectrometry (ICP-AES) using Agilent ICPOES 730 spectrometer. Thermogravimetric (TG) analysis was performed using a PerkinElmer Diamond TG-DTA set-sys-evolution instrumentation. IR spectra were recorded with a Bruker Alpha Platinum ATR in the 400–4000 cm−1 region. The pore properties and Brunauer-Emmett-Teller (BET) surface area of samples were characterized by a Micromeritics ASAP 2020 nitrogen adsorption apparatus at 77 K.All the electrochemical performance tests were undertaken in a typical three-electrode system using CHI660E electrochemical workstation. The carbon paper electrode coated with catalysts, Ag/AgCl, and graphite rod acted as the working electrode, reference electrode, and counter electrode, respectively. The working electrode was prepared as follows: 5 mg as-prepared catalyst and 20 μL Nafion solution were dispersed in 500 μL ethanol to form a homogeneous slurry by sonication, then 100 μL of the acquired slurry was uniformly dispersed onto the 1.0 cm2 of carbon paper and dried at room temperature. Before the measurements, Ar gas was purged into the electrolyte solution (1.0 M KOH) for at least 30 min.All the measured potentials (E Ag/AgCl) were calibrated to the reversible hydrogen electrode (RHE) potentials based on the equation: E RHE = E Ag/AgCl + 0.0591×pH + 0.198. 90 % iR-correction was done using the instrument’s available function for the linear sweep voltammetry (LSV). The electrochemical double-layer capacitance (C dl) of as-prepared samples were calculated by the CV method at different scan rates (5−50 mV s−1) within a non-faradic potential range (1.107–1.207 V).DFT + U calculations were performed using Vienna Ab-initio Simulation Package (VASP). The interactions between the valence electrons and ionic cores were described using the projector augmented wave (PAW) method. The electron exchange-correlation energy terms were evaluated using the Perdew-Burke-Ernzerhof (PBE) functional with on-site Coulomb repulsion U term on Co, Ni, and Fe 3d electrons [36]. According to the previous study, the values U(Co) = 3.0 eV, U(Ni) = 5.5 eV, and U(Fe) = 3.5 eV are found to provide an appropriate description for the properties of transition metal oxides [37]. The plane-wave cut off energy for the system was optimized as 400 eV. The Brillouin zone was sampled with a 3 × 3×1 k integrations mesh. The final forces and energy convergence criteria were set as 0.03 eV Å−1 and 1 × 10-4 eV, respectively.The γ-Fe2O3(110), CoFe2O4(110), and NiFe2O4(110) surface models were built with 15 Å of vacuum along the z-axis. For the Co-doped NiFe2O4(110) surface model, one Ni Oh atom on the NiFe2O4(110) surface was substituted by a Co atom. During the geometry optimization, the uppermost two layers of atoms were relaxed, and the other bottom atoms were fixed to represent the bulk position. All the pristine surface models were optimized to minimal energy before the adsorption calculation. For the OER process calculation, a standard four-electron reaction mechanism in alkaline condition was considered for the calculation of Gibbs free energy change according to the previous study [38]: (1) O H – + * → * OH+ e – Δ G 1 =   Δ G O H −   e U   +   Δ G H + p H (2) * OH+O H – → * O+ H 2 O+ e – Δ G 2 =   Δ G O −   Δ G O H −   e U   +   Δ G H + p H (3) * O+O H – → * OOH + e – Δ G 3 =   Δ G O O H −   Δ G O −   e U   +   Δ G H + p H (4) * OOH+O H – → * + O 2 + H 2 O + e – Δ G 4 =   4.92   −   Δ G O O H −   e U   +   Δ G H + p H where U is the potential measured against normal hydrogen electrode (NHE) at standard conditions. The theoretical overpotential is then readily defined as: (5) η = max ( Δ G 1 ,   Δ G 2 ,   Δ G 3 ,   Δ G 4 ) / e - 1.23 The Gibbs free energy of adsorbed species is defined as: (6) Δ G = Δ E a d s + Δ E Z P E - T Δ S a d s where Δ E a d s is the adsorption energy, Δ E Z P E is the zero point energy difference between adsorbed and gaseous species, and   T Δ S a d s is the corresponding entropy difference between these two states (T was set to be 298 K). The calculation results of zero point energy and entropy of the OER intermediates were listed in Table S1. Δ E a d s was calculated relative to H2O and H2 (at U = 0 and pH = 0) as: (7) Δ E O H =   E O H * -   E * - ( E H 2 O -   1 2 E H 2 ) (8) Δ E O =   E O * -   E * - ( E H 2 O - E H 2 ) (9) Δ E O O H =   E O O H * -   E * - ( 2 E H 2 O -   3 2 E H 2 ) Fig. 1 depicts the synthesis scheme of multi-metal MOFs derived hybrids via the ion-exchange method. Firstly, Fe-MIL-101-NH2 was synthesized using BDC-NH2 and FeCl3·6H2O according to the previously reported procedures [35]. The as-prepared Fe-MIL-101-NH2 displays uniform octahedron morphology with a smooth surface and an average diameter of ∼250 nm (Fig. 2 a). Secondly, Co2+ and Ni2+ were introduced into Fe-MIL-101-NH2 at 90 °C in the DMF solvent through a solvothermal method. ICP-AES results (Table S2) show that the content of Ni and Co is about 7.69 wt.% and 1.76 wt.% respectively in the as-prepared CoNiFe-MOF, and Fe content decreases from 14.96 wt.% of the original Fe-MIL-101-NH2 to 7.39 wt.%. XRD results (Fig. 2c) demonstrate that the diffraction pattern of CoNiFe-MOF is similar to that of Fe-MIL-101-NH2, suggesting the pristine MOF phase remained after ion exchange treatment. Their FT-IR spectra (Fig. S1) clearly show the adsorption band of the νas(-COO-) linking to the metal atom at 1657 cm−1 [20], indicating no free BDC-NH2 ligand in the CoNiFe-MOF structure [39]. However, the microstructure of CoNiFe-MOF has changed concerning Fe-MIL-101-NH2, as shown in Fig. 2b, the original smooth surface has turned into rough and fluffy nanosheets.Finally, the CoNiFe-MOF was pyrolyzed in the air at 300 °C in a tube furnace. As shown in Fig. 2e, the obtained sample (CoNiFeOx-NC) almost preserves the original morphology of CoNiFe-MOF. XRD results (Fig. 2f) reveal the formation of CoNiFe oxides phase with spinel structure (NiFe2O4, JCPDS no. 54-0964; CoFe2O4, JCPDS no. 22-1086; γ-Fe2O3, JCPDS no. 39-1346). The diffraction peaks of Fe-MIL-101-NH2 disappeared completely in CoNiFeOx-NC after air heating treatment. The HRTEM image of CoNiFeOx-NC (Fig. 2h) further confirms the microstructure of CoNiFe spinel oxides, where the lattice fringe with the distance of 0.213, 0.241, and 0.222 nm can be assigned to (400), (222) facet of NiFe2O4 and (321) facet of γ-Fe2O3 respectively. Furthermore, the compositional distribution of a typical CoNiFeOx-NC was investigated through STEM coupled with EDX-mapping (Fig. 2j). It can be observed that Co and Ni elements display a uniform distribution. Interestingly, the Fe element exists in the core of octahedron and appears in the nanosheets shell. This indicates that the Fe-MIL-101-NH2 precursor was etched by CoCl2 and NiCl2 during the ion exchange process [24,40], and the released Fe ions regrowth on the surface of octahedron together with Ni and Co ions, forming solid ternary solutions (CoNiFe-MOF). The broad peak at about 22° in XRD pattern of CoNiFeOx-NC (Fig. 2f) should be the characteristics peak of amorphous C-N compounds (carbonitrides) [41], which is generated from the part decomposition of organic ligands. The considerable amount of C and N elements detected by the EDX (Fig. 2i) and STEM (Fig. S2) verifies the presence of carbonitrides in the sample, which agrees with XPS results (Table S3). Besides, the substrate surrounding the metal oxide nanoparticles presents an amorphous state in the HRTEM images (Fig. S3), generally indicating the presence of amorphous carbon material. However, The FeOx-NC sample derived from pristine Fe-MIL-101-NH2 possesses a featureless morphology, including numerous large nanoparticles (Fig. 2d). The component of these nanoparticles was determined to be γ-Fe2O3, according to the XRD patterns (Fig. 2f) and HRTEM images (Fig. 2g).To further investigate the impact of Co and Ni ions on the microstructure of CoNiFe-MOF, control samples were synthesized by solely introducing Co or Ni element into Fe-MIL-101-NH2 and compared with the as-prepared ternary metal-based MOF. ICP-AES results (Table S2) verify the successful incorporation of Co and Ni in the CoFe-MOF and NiFe-MOF, respectively. As TEM images displayed in Fig. 3 a, the introduction of Ni has little influence on the morphology of Fe-MIL-101-NH2. XRD patterns further confirm the unvaried topological structure for NiFe-MOF (Fig. 3b). As a previous study revealed, the FeO6 geometries in the MIL-53 frameworks can be partly substituted by the NiO6 ones, indicative of the similar coordination ability of Ni and Fe ions with carboxylate groups [30]. However, the introduction of Co changed the original octahedral morphology to concaved octahedrons (Fig. 3a). This is also reflected in the XRD patterns of CoFe-MOF (Fig. 3b), where the diffraction peaks are distinguished from those of Fe-MIL-101-NH2. This could arise from the strain-stress induced by the Co2+ with a larger ionic radius (0.0745 nm) than Fe3+ (0.0645 nm) and Ni2+ (0.0690 nm) [42,43] or the different coordination preference with carboxylate groups between Fe and Co [44,45], in the enlightenment of the previous declaration that the substitution process in MOF is significantly metal ion-dependent [46]. Interestingly, the monometallic incorporation of Ni or Co could not afford the analogous hierarchical structure of CoNiFe-MOF, suggesting the indispensable role of mixed Co and Ni precursor during the metal ion exchange process.Fig. S4a-d show the TEM images of CoNiFe-MOF with different ion exchange times. It was observed that as the exchange time increased, more ultrathin nanosheets formed on the surface of pristine Fe-MIL-101-NH2, and the octahedral core part gradually diminished in size. Fig. S5 gives the TEM images of CoNiFe-MOF with various Co precursor amounts (0.6, 1.4, 2.8, and 5.6 mmol), which shows that decreasing Co2+ amount inhibits the formation of nanosheets. More evidences were given in a controlled ion-exchange process without Co ions addition but different amounts of HCl addition (0.9, 1.7, 3.5, and 6.0 mmol). As shown in Fig. S6, the inhibited nanosheets formation further certify the important role of Co2+ on the formation of nanosheets shell. Based on the above results, we deduce the possible growth mechanism of the hierarchical structured CoNiFe-MOF as following (Fig. 3c): During the ion exchange process, Ni ion substituted part of Fe atoms in the frameworks of Fe-MIL-101-NH2, while Co ion etched the surface of Fe-MIL-101-NH2, providing the favorable conditions for the second building of metal-organic frameworks, then the dissolved Fe ions re-coordinated with Ni, Co ions and carboxylate groups to form tri-metallic MOF nanosheets [40,47]. During the growth of tri-metallic MOF nanosheets, the similar coordination ability with carboxylate groups between Ni and Fe ions facilitates the preferred coordination of Fe ions with Ni ions and carboxylate groups rather than Co ions. As a result, the as-synthesized tri-metallic MOF nanosheets are mainly composed of Fe and Ni solid solutions with limited amount of Co dopant. Similar dissolution-coordination phenomenon has also been reported by other researchers. Xun Wang et al. revealed the in-situ transformation from Zn/Ni-MOF-5 nanocubes to Zn/Ni-MOF-2 nanosheets with pre-formed nanocubes acting as supporting template without any surfactants [48]. David Lou et al. observed a cooperative etching-coordination-reorganization process when introducing the guest metal salt as a Lewis acid into ZIF-67 [40]. It should be emphasized that there is no prominent diffraction peak corresponding to layered double hydroxide (LDH) found in the XRD patterns of CoNiFe-MOF even after 48 h of ion exchange treating (Fig. S4e). Besides, the DMF solvent that we used in the ion exchange process is not favorable for the formation of LDH.The OER electrocatalytic performances of FeOx-NC, CoFeOx-NC, NiFeOx-NC, and CoNiFeOx-NC were evaluated in Ar-saturated 1.0 M KOH solution. Firstly, the linear sweep voltammetry (LSV) was recorded in the range of 1.05–1.85 V vs. RHE at 5 mV s−1 scan rate (Fig. 4 a). According to the iR-corrected LSV results, the overpotential of FeOx-NC, CoFeOx-NC, NiFeOx-NC, and CoNiFeOx-NC catalyst at the current density of 50 mA cm-2 is 526, 361, 289, and 265 mV respectively. CoNiFeOx-NC catalyst behaves the best catalytic activity than the other samples with nearly 100 % Faradaic efficiency (Fig. S7) towards OER, which is also superior to the commercial IrO2 catalyst and among the best in recently reported advanced OER catalysts (Fig. 4f). There is a prominent oxidation peak at 1.40 V vs. RHE in the LSV curve of CoNiFeOx-NC, which should be attributed to the oxidation peak of the remaining MOF in CoNiFeOx-NC because of the protection of ternary-metal based shell during the air annealing process. Then the Tafel plots were recorded to study their OER reaction kinetics (Fig. 4b). The Tafel slope was calculated to be 99.9, 64.0, 47.6, and 64.1 mV dec−1 for FeOx-NC, CoFeOx-NC, NiFeOx-NC, and CoNiFeOx-NC, respectively, demonstrating the enhanced reaction kinetics of CoNiFeOx-NC after Co and Ni incorporation. Meanwhile, electrochemical impedance spectroscopy (EIS) experiments display that CoNiFeOx-NC electrode possesses the smallest charge transfer resistance (Rct = 1.65 Ω) compared with FeOx-NC (35.17 Ω), CoFeOx-NC (19.42 Ω), NiFeOx-NC (4.77 Ω), and IrO2 (2.24 Ω) electrodes (Fig. 4c). To estimate their electrochemical active surface area (ECSA), the double-layer capacitance (C dl, Fig. 4d) was measured by recording the cyclic voltammograms (CVs) at scan rate from 5 to 50 mV s−1 within the non-faradic voltage range (1.107–1.207 V vs. RHE, Fig. S8). According to the equation ECSA = C dl / C s, the ECSA is directly proportional to the C dl because C s is constant as the specific capacitance of per unit area of material with an atomically smooth surface. Therefore, IrO2 catalyst displays the highest ECSA, and CoNiFeOx-NC displays much higher ECSA than FeOx-NC, CoFeOx-NC, and NiFeOx-NC.Additionally, the effect of synthesis conditions on the OER performance of CoNiFeOx-NC was investigated by varying some crucial factors, such as the cation precursor ratio (NiCl2/CoCl2, Table S4) in the ion-exchange process and the calcination temperature (200, 300, 400, and 500 °C) in the pyrolysis process. As shown in Fig. S9, the NiCl2/CoCl2 ratio significantly affects the formation of nanosheets shell outside the MOF core, thus endowing different OER performances (Fig. S10). As shown in Fig. S12, CoNiFeOx-NC samples obtained at other calcination temperatures (200, 400, and 500 °C) behave the inferior OER activities than the as-prepared CoNiFeOx-NC sample at 300 °C. XRD and TEM results (Figs. S14 and S15) indicate that the high calcination temperature could result in nanoparticle agglomeration due to the collapsing of 3-D structure. This also suggests the significant role of carbonitrides part in CoNiFeOx-NC, which is providing a 3-D framework that could afford large specific surface area for CoNiFeOx active sites distribution. Nevertheless, in this catalyst, the carbonitrides part of CoNiFeOx-NC is not the main active site for OER reaction as revealed by the performance outcomes of carbonitrides itself (Fig. S18).The durability properties of electrocatalyst is another crucial factor for renewable energy application systems. Firstly, the polarization curve of the CoNiFeOx-NC electrode after 1500 consecutive CV scans was compared with the initial one (Fig. 4e), which shows a slight decrease of only 7 mV at the current density of 50 mA cm−2. Then the long-term chronoamperometry method was applied to test the stability of CoNiFeOx-NC anode at the constant voltage of 1.49 V. As shown in the inset of Fig. 4e, the CoNiFeOx-NC catalyst can retain 96.6 % of its initial activity after 40 h electrocatalysis, while the commercial IrO2 catalyst losses about 71.3 % of its pristine activity. Besides, the stability of counterpart obtained from in-situ generated CoNiFe-MOF (donated as IS-CoNiFeOx-NC) was evaluated at the constant voltage of 1.61 V (Fig. S22), and the current density of IS-CoNiFeOx-NC also deteriorates quickly during the long-term test (maintained 70.4 % activity after 40 h electrocatalysis). The crystal phase and microstructure of CoNiFeOx-NC after 40 h stability tests were analyzed by XRD and HRTEM characterizations (Fig. S23), which indicates that the spinel metal oxides still exist after the stability test. Meanwhile, the XPS spectra results of post-OER CoNiFeOx-NC also display similar information with the original CoNiFeOx-NC sample (Fig. S24). All of the above proofs confirm that the as-prepared CoNiFeOx-NC catalyst is considerably stable for the OER reaction.The excellent OER actiity of CoNiFeOx-NC could be attributed to its unique structure concerning FeOx-NC, CoFeOx-NC, and NiFeOx-NC. Firstly, the formed 2-D MOF layers encapsulating 3-D MOF octahedral cores prevents the collapse of the frameworks during air calcination. The stable 3-D framework composed of plenty of carbonitrides compound contributes to the generation of highly dispersed CoNiFeOx nanoparticles in CoNiFeOx-NC (Fig. 2h). As shown in Fig. S25, for NiFe-MOF and CoFe-MOF, the original octahedron morphology has almost disappeared after air heating treatment. Meanwhile, TG curve results (Fig. S26) also show that CoNiFe-MOF is more heat resistant than Fe-MIL-101-NH2 under the air atmosphere. Secondly, the ion exchange induced dissolution-recoordination process between Fe, Co, and Ni triggered a fluffy nanosheets structure with rich channels and large surface area. The porosity properties of FeOx-NC, CoFeOx-NC, NiFeOx-NC, and CoNiFeOx-NC were characterized by N2 adsorption-desorption isotherms. As shown in Fig. 5 a-b, all of them displayed typical IV isotherm characteristic of mesoporous materials, but CoNiFeOx-NC has the largest pore volume and BET surface area (104 m2 g−1). This result echoes the much higher ECSA surface area of CoNiFeOx-NC than that of FeOx-NC, CoFeOx-NC, and NiFeOx-NC (Fig. 4d). The expansion of pore size distribution and specific surface area in CoNiFeOx-NC may contribute to the exposure of active sites to electrolyte and effective mass transport.Thirdly, the unique modulation of electronic structure through electron transfer between Co, Ni, and Fe could optimize the adsorption energy of oxygen species on the catalyst surface. Fig. 5c shows the deconvoluted XPS spectra of the Fe 2p signals for FeOx-NC, CoFeOx-NC, NiFeOx-NC, and CoNiFeOx-NC samples. The 2p3/2 peaks at around 710.7 and 713.1 eV, along with the 2p1/2 peaks at around 724.0 and 726.0 eV should be indexed to the Fe3+ from the different binding environment (octahedral and tetrahedral sites). This is because CoFe2O4 and NiFe2O4 possess an inverse spinel structure with Co2+ or Ni2+ in octahedral sites and Fe3+ equally distributed between octahedral (Oh) and tetrahedral (Td) sites of the O2− fcc cell [26,49]. The peaks at around 718.6 and 732.6 eV are satellite peaks induced by the charge transfer from Fe to O [50]. The Fe 2p peaks shift to higher binding energy after Ni or Co incorporation, revealing the charge transfer from Fe to Co or Ni.Moreover, the binding energy of Ni 2p peaks in CoNiFeOx-NC is lower than that of NiFeOx-NC (Fig. 5d), suggesting the charge transfer from Co to Ni, which is further confirmed by the higher shift of Co 2p peaks in CoNiFeOx-NC in contrast to that of CoFeOx-NC (Fig. 5e). Therefore, a general charge transfer trend of Fe→Co→Ni was observed in the as-prepared materials. This is consistent with the electronegativity order of Fe < Co < Ni as the more electronegative Ni element attracts more electrons [51]. It should be stated that Ni and Co elements are present as the divalent form in CoNiFeOx-NC and NiFeOx-NC, corresponding to the tetrahedral binding sites in the spinel structure. However, trivalent Co was found in the Co 2p spectra of CoFeOx-NC. This is due to the formation of Co3O4 species in CoFeOx-NC sample, as seen in XRD patterns (Fig. S27). The high-resolution XPS spectra for C, O, and N are also given in Figs. S28–S31, confirming the existence of carbonitrides and metal oxides in all the samples. Additionally, as reflected in the O1s spectra of CoNiFeOx-NC (Fig S28d), the area ratio of CO and CO species is much larger than the metal oxides species in comparison to the other three samples. This also proves that the MOF core was finely protected by the outside 2-D layers in CoNiFeOx-NC during the air heating treatment.To further get insight into the electronic structure modifications and the corresponding effects on the OER activity, DFT + U calculations were deliberately performed to investigate the relationships between the electronic structure and OER mechanism in different spinel metal oxides. The spinel oxide crystal was chosen as the catalyst model since all the catalysts displayed spinel crystal structures according to Figs. 2 and S27, which is γ-Fe2O3 (110), CoFe2O4 (110) and NiFe2O4 (110) surface model for FeOx-NC, CoFeOx-NC and NiFeOx-NC catalyst. The content of Co in CoNiFeOx-NC is slight compared with the contents of Ni or Fe, and the lattice of spinel oxide in CoNiFeOx-NC expanded (Fig. 2), suggesting the incorporation of larger heteroatom such as Co. Therefore, the Co doped NiFe2O4 (110) surface model was adopted to represent CoNiFeOx-NC catalyst. Firstly, the electronic structure of Co-NiFe2O4 was analyzed by comparing it with NiFe2O4. Fig. 6 a shows the contour charge maps of the charge density distributions for NiFe2O4 and Co-NiFe2O4 along the 110 facet, in which the yellow and blue regions correspond to the electrons accumulation and depletion respectively. It’s found that the electron-deficient state around the Fe site is apparently intensified in contrast to the corresponding Ni or Co site. In Co-NiFe2O4, the evident electron depletion was also observed around Co site, whereas the electron accumulation is more pronounced around Ni than that in NiFe2O4, indicating the partial electron transfer from Co site to Ni site via O bridge. These simulation results agreed well with the XPS results where the electron transfer follows the trend of Fe→Co→Ni, and the charge density distribution of CoFe2O4 is also consistent with the above trend (Fig. S32).The diversity of electronic structure in metal oxides will lead to the different adsorption conditions for the reaction intermediates. Herein, the adsorption geometries and adsorption energies of three primary OER intermediates (OH*, O*, and OOH*) on the γ-Fe2O3 (110), CoFe2O4 (110), NiFe2O4 (110) and Co-NiFe2O4 (110) catalysts surface were calculated and compared. On each catalyst surface, both Fe Td site and Fe/Co/Ni Oh site (referring to Fe Oh site for γ-Fe2O3, Co Oh site for CoFe2O4, Ni Oh site for NiFe2O4, and Ni-Co coordinated Oh site for Co-NiFe2O4, respectively) were chosen as the adsorption sites. The optimized adsorption geometries were shown in Fig. S33, and the calculated adsorption energies were displayed in Fig. S34 (Td site) and Fig. 6b (Oh site). The adsorption energy results indicate that the oxygen species prefer to binding with Oh site with larger binding energies rather than Td site for all the catalysts, and the Gibbs free energy results (Table S5) in the following discussion also demonstrate a lower reaction energy barrier on Oh site than Td site. Therefore, the Oh site (Fe Oh site for γ-Fe2O3, Co Oh site for CoFe2O4, Ni Oh site for NiFe2O4, and Ni-Co coordinated Oh site for Co-NiFe2O4, respectively) was received as the preferred adsorption site for oxygen species in this system. As shown in Fig. 6b, the adsorption energies for three types of oxygen species on the Oh site follow the trend of γ-Fe2O3 (110)>CoFe2O4 (110)>Co-NiFe2O4 (110)>NiFe2O4 (110). According to the above electronic structure results, the electron-deficient state in spinel structure is in the order of Fe > Co > Ni. Consequently, the binding strength of Fe Oh site on γ-Fe2O3 (110) with oxygen species is the strongest, followed by the Co Oh site on CoFe2O4 (110), and the Ni Oh site on NiFe2O4 (110) is the weakest. Impressively, after Co incorporation, the Ni-Co coordinated Oh site on Co-NiFe2O4 (110) can provide moderated oxygen-binding strength (e.g. EOH* = −0.83 eV) compared with the original Ni Oh site (e.g. EOH* = −0.39 eV) and Co Oh site (e.g. EOH* = −1.01 eV). This binding strength modulation optimized the adsorption of oxygen species on Co-NiFe2O4 (110), hence causing OER intermediates transformation multi-steps more readily accessible on CoNiFeOx-NC. This assumption was further verified by the theoretical energy barrier results of four catalysts towards OER. Fig. 6c displays the Gibbs free energy diagrams along the proposed OER pathway on different catalysts. For γ-Fe2O3 (110), CoFe2O4 (110), and NiFe2O4 (110), the third elementary reaction step has the largest energy barrier of 2.44, 2.31, and 2.02 eV respectively, which can be considered as the rate-determining step. However, for Co-NiFe2O4 (110), the energy barrier of the third step decreases significantly to 1.82 eV, and the rate-determining step turns to the last step (1.92 eV). This means Co-NiFe2O4 (110) requires a lower overpotential to drive water oxidation. Meanwhile, the backward rate-determining step is commonly corresponding to the acceleration of reaction kinetics [52]. The theoretical energy barrier results agree well with the experimental results, where the overpotential at the 50 mA cm−2 is in the order of FeOx-NC > CoFeOx-NC > NiFeOx-NC > CoNiFeOx-NC. Finally, the typical OER reaction mechanism on Co-NiFe2O4 (110) was depicted in Fig. 6d.In summary, we have demonstrated a hierarchically structured CoNiFeOx-NC catalyst with the superior performance of the OER rection, derived from core-shell structured MOF. Co and Ni ions display the indispensable and cooperative role for the formation of 2-D ternary metal MOF layers encapsulating 3-D MOF octahedral crystals in the ion exchange process. Co ions provide favorable environments for the second building of metal-organic frameworks by etching the surface of Fe-MIL-101-NH2, while the dissolved Fe ions prefer to re-coordinating with Ni, Co ions, and carboxylate groups to form tri-metallic MOF nanosheets. The formed nanosheets shell not only protects the frameworks from being destroyed during the air calcination treatment but also helps maintain a large surface area and porous structure of the materials. More importantly, an electron transfer trend of Fe→Co→Ni was found in the as-prepared CoNiFeOx-NC material based on the experimental and DFT studies. This optimized electronic structure results in moderated oxygen-binding strength on Ni-Co coordinated Oh site in contrast to the original Ni Oh site and Co Oh site, thus lowering the theoretical energy barriers for OER. Benefitting from the above features, the well-designed CoNiFeOx-NC catalyst delivers high efficiency in the alkaline OER reaction. Chen Chen: Conceptualization, Methodology, Investigation. Yongxiao Tuo: Software, Formal analysis, Writing - original draft. Qing Lu: Validation, Investigation. Han Lu: Validation. Shengyang Zhang: Validation. Yan Zhou: Funding acquisition. Jun Zhang: Supervision, Funding acquisition, Writing - review & editing. Zhanning Liu: Data curation. Zixi Kang: Writing - review & editing. Xiang Feng: Writing - review & editing. De Chen: 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.This work was partially supported by the Taishan Scholar Project of Shandong Province, the National Natural Science Foundation of China (No. 21805308), the Key Research and Development Project of Shandong Province (No. 2019GSF109075), the Fundamental Research Funds for the Central Universities (No. 18CX06065A, No. 20CX06022A).Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2021.119953.The following is Supplementary data to this article:

Metal-organic frameworks (MOFs) have recently emerged as promising precursors to construct efficient non-noble metal electrocatalyst for oxygen evolution reaction (OER). Herein, a Co-Ni-Fe spinel oxide-carbonitrides hybrids (CoNiFeOx-NC) electrocatalyst with hierarchical structure was synthesized from Fe-MIL-101-NH2 through a unique ion-exchange based strategy. The ion exchange of Fe-MIL-101-NH2 with both Ni and Co ions induced a hierarchically structured 2-D ternary metal MOF shell layer encapsulated 3-D octahedral MOF crystals as a core. This prevents the collapse of MOF frameworks during the air calcination process and affords highly porous structure and large surface area. Additionally, the unique combination of Co-Ni-Fe in spinel oxides derived from calcination of the hierarchically structured core-shell MOF provides a favorable electronic environment for the adsorption of OER intermediates, which was further verified by the XPS characterizations and DFT calculations. DFT study revealed the Ni-Co coordinated Oh sites in the MFe2O4 reverse spinel structures as the main active sites, which tuned the binding strength of oxygen species with a catalyst through electron transfer of Fe→Co→Ni, thereby lowered the energy barriers for OER. As a result, the rationally designed CoNiFeOx-NC catalyst manifests superior OER performance with a low overpotential of 265 mV at 50 mA cm−2 and a decent Tafel slope of 64.1 mV dec-1. The ion-exchange based strategy may serve as a versatile platform for rational design and synthesis of multi-metallic MOF derived electrocatalysts.

A process for the selective extraction and separation of vanadium and nickel from spent-residue oil hydrotreating catalysts by a direct acid leaching−solvent extraction method was studied. The extraction and separation of vanadium(IV) and nickel(II) are divided into two stages: acid coleaching of vanadium and nickel and solvent extraction. In the acid coleaching stage, the leaching ratios of vanadium and nickel reach 88.07% and 75.58%, respectively, which can realize highly effective coleaching. In the solvent extraction stage, countercurrent experiments show that the extraction ratio of vanadium can reach 99.21% after a three-stage extraction with P204 as the high-efficiency extractant of vanadium in the acidic environment, while nickel and iron are not extracted. After the anti-extraction solution is pretreated by aluminum precipitation, the extraction ratio of nickel reaches 99.79% after a three-stage extraction with LIX84-I as a high-efficiency extractant of nickel in ammonia medium. A process flow for the recovery of vanadium and nickel is proposed, which not only can realize the separation and recovery of vanadium and nickel but also can realize the recycling of reagents.

Generally, catalysts include homogeneous catalysts and heterogeneous catalysts. Homogeneous catalysts have relatively uniform active centers, higher activity and selectivity, and fewer side reactions, but they are hard to separate, recover, and regenerate from the reaction system. Heterogeneous catalysts are easily separated from the system and can be used repeatedly, but the activity and selectivity of heterogeneous catalysts are often worse than that of homogeneous catalysts. Importantly, single-atom catalyst (SAC), a new type of catalyst, was developed, which is considered to be a bridge between homogeneous catalysts and heterogeneous catalysts due to the distinguished selectivity, catalytic activity, and easy to separation (Chen et al., 2018c; Mao et al., 2019; Sun et al., 2019b; Yang et al., 2013). In recent years, many research have contributed to the developments of single atoms catalysis, but the understanding of single atoms from atomic and electronic insights is still inadequate due to the deficiency of characterization techniques. Therefore, it is very important to comprehend the development process, synthesis methods, and coordination regulation approaches thoroughly for SACs.In 1995, Thomas and colleagues studied isolated single atom of Titanium as the active site of heterogeneous catalyst (Maschmeyer et al., 1995). In 2000, the presence of single atoms was discovered when size-selected Pdn (1 ≤ n ≤ 30) cluster supported on MgO were prepared by using a mass separation soft landing technique (Abbet et al., 2000). In 2003, single-site Au species on ceria-based catalyst for water-gas shift were reported by Fu and colleagues (Fu et al., 2003). In 2007, mesoporous Pd/Al2O3 with single sites was prepared by impregnation method for selective aerobic oxidation of allyl alcohol (Hackett et al., 2007). With the development of characterization techniques, the concept of “single atom” was first proposed by Zhang and colleagues in 2011 (Qiao et al., 2011). The isolated single Pt atoms fabricated on the surfaces of iron oxide (Pt1/FeOx) displayed high activity and selectivity in CO oxidation. In recent years, the design and preparation of atomically dispersed catalysts have attracted extensive research interests in plenty of applications, such as photocatalysis, organic catalysis, electrocatalysis, and environmental and energy aspect. Meanwhile, because of the high surface free energy of single atoms, it is still a major challenge to increase the loading capacity of single atoms. What’s more, the coordination environments, including the coordination number, the coordination atom, and the distance between the center atoms and the neighboring atoms, greatly influence the catalytic activity of single-atom catalysts (Cook and Borovik, 2015; Mao et al., 2019; Sun et al., 2019b; Tao et al., 2020). Therefore, how to systematically regulate the coordination environments is of great significance to the screening of efficient single-atom catalysts (Lang et al., 2019; Liang et al., 2019; Liu et al., 2019a; Qiao et al., 2015).The coordination environment and the loading of metal atoms are closely related to the catalytic performance of SACs. In order to further study the regulation of coordination environment and loading, a variety of materials are used as supports for single-atom catalysts, including metal and metal oxide (Cao and Lu, 2020; Ma et al., 2021), sulfide (Feng et al., 2018; Li et al., 2022), phosphide (Jiang et al., 2020), zeolites (Sun et al., 2019a), metal-organic frameworks (MOFs) (Zhou et al., 2021a), covalent organic frameworks (COFs) (Liu et al., 2020), and carbon-based materials (Guo et al., 2021), such as graphene, graphdiyne, and hexagonal boron nitride. Among these carriers, carbon-based materials are regarded as the promising candidate materials for large-scale production of SACs due to low cost, superb conductivity, tunable physicochemical property, and high specific surface area (Georgakilas et al., 2015; Shaik et al., 2019; Su et al., 2013). Therefore, the overview and summary of carbon-based-material-supported SACs are necessary for the improvement of future work.In this review, we summarized the synthesis methods of SACs supported on carbon-based materials and then highlighted the great significance to guide the coordination regulation of single atoms and improve the loading of SACs. Then, we introduced the advanced characterization techniques, including ex situ and in situ technologies, which is vital to learn about the SACs from atomic and electronic levels. Most important of all, the applications of carbon-based-material-supported SACs in electrocatalysis are discussed by combining calculations and experiments, and the coordination environment and metal loading of the SACs are emphasized, involving hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), and nitrogen reduction reaction (NRR). In the end, the challenges and development opportunities of SACs are fully discussed. We believed that this review could provide essential information to rationally construct SACs in the future.SACs usually display superb catalytic activities for electrochemical reactions, hydrogenation reactions, and so on. However, the highly dispersed single-atoms tend to migration and agglomerate during the synthesis process due to the high surface energy. How to synthesize stable and high-loaded single-atom catalysts requires continuous exploration. In this section, the methods of atomic layer deposition (ALD), impregnation strategy, electrochemical deposition, chemical vapor deposition (CVD), balling milling, and pyrolysis strategy are summarized. Notably, impregnation strategy and pyrolysis strategies are beneficial to the large-scale preparation of SACs in industry.Atomic layer deposition (ALD) has a promising future in the synthesis of small size catalysts. In the ALD process, the substrate is alternately exposed to different reactions of precursor vapors. The material could be deposited in atomic layers by sequential and self-limiting surface reactions. Previous studies have shown that ALD can control the morphologies of deposited metals, from tiny discrete nanoparticles to continuous films, by surface chemistry strategy (King et al., 2008; Liu et al., 2009). Meanwhile, ALD can precisely control the size and distribution of particles on the substrate (George, 2010; Marichy et al., 2012). In 2013, single Pt atoms on graphene nanosheets with low coordination were prepared by using ALD technique for the first time (Sun et al., 2013). The container for graphene was placed directly on the heated stage of ALD. MeCpPtMe3 acted as the precursor keeping at 65°C to provide a steady state flux to the reactor (∼800 mTorr). Gas lines were heated to 100–150°C to avoid precursor condensation. High purity O2 and N2 were used as the counter reactant and purging gas, respectively. The reaction took place at 250°C with the pulse of 1 s of MeCpPtMe3, 20 s of N2 purge, and 5 s of O2 in an ALD cycles. By increasing the number of ALD cycles, the size of prepared catalysts can be precisely regulated, including single atoms, sub-nanometer clusters, and nanoparticles, while affecting the coordination number of the metal center atom. Similarly, isolated Pt single-atoms and clusters were fabricated on the nitrogen-doped graphene nanosheets through the same ALD synthesis approach expect for the carrier (Cheng et al., 2016; Figure 1A). There are numerous individual Pt atoms and very small clusters in the ALD50Pt/NGNs catalysts, whereas some Pt clusters grow into nanoparticles after 100 cycles. Meanwhile, the HER catalytic performance decreased with the increased number of ALD cycles, and ALD Pt/NGN catalysts showed higher catalytic activity and long-term stability in comparison with the commercial Pt/C catalysts in 0.5 M H2SO4. What’s more, the Pd1/graphene with Pd-C2 coordination were prepared by anchoring the palladium hexafluoroacetylacetate (Pd(hfac)2) and formalin at 150°C on a viscous flow reactor (Yan et al., 2015a). The (Pd(hfac)2) was held at 65°C to get a sufficient vapor pressure. Likewise, the 99.999% of purity N2 act as carrier gas at a flow rate of 200 mL min−1. The manifold was kept at 110°C to avoid precursor condensation. It showed outstanding activity, selectivity, and durability than the Pd-NPs/graphene in selective hydrogenation of 1, 3-butadiene with about 100% butenes selectivity at 95% conversion at about 50°C. ALD can also be used to prepare carbon-based-material-supported nonnoble metal single-atom catalysts. The isolated Co atoms anchored on graphene (Co1/G) with tunable high metal loading and a six-coordination of Co1-O2C4 were fabricated by ALD (Yan et al., 2018b). The amount of metal loading can be modulated by simply controlling the number of turns of the ALD. As shown in Figure 1B, the first cycle of Co ALD was carried out on thermally reduced graphene oxide by exposing the carrier to CoCp2 vapor. Subsequently, the molecule O3 is injected into the cavity to remove the ligand. Although the new active site is reconstructed, another batch of Co atoms is loaded in the subsequent ALD cycle. The load density of Co1/G catalyst was precisely adjusted by controlling the cycles of Co ALD. A series of Co1/G catalysts with loading capacities of 0.4, 0.8, 1.3, 2.0, and 2.5 wt% were obtained by 1, 2, 3, 4, and 5 ALD cycles, respectively. The Co1/G SACs exhibited high activity and selectivity for the hydrogenation of nitroarenes to produce azoxy aromatic compounds.By adjusting the number of cycles, order, and type, ALD can achieve atomically fine control over the structure of catalyst active sites, providing a bottom-up strategy for precise and controllable catalyst synthesis. However, the high cost and low yield are the primary reasons for limiting the industrial application of ALD method.Impregnation method, as one of wet-chemical methods, is considered to be the most promising route for mass production due to the low price and easy operation. Impregnation method is widely used in the preparation of supported catalysts, especially low-content noble-metal-supported catalysts. Yin et al. realized coordination regulation through impregnation method and confirmed better HER performance in low coordination environment (Yin et al., 2018). Pt single-atom supported on graphdiyne (GDY) was prepared by wet-chemical route. The coordination environment is controlled by controlling facile annealing step. React GDY with K2PtCl4 aqueous solution at 0°C for 8 h (named as Pt-GDY1) and wash with plenty of water. Then annealing in Ar atmosphere at 200°C for 1 h (named as Pt-GDY2) (Figure 1C). The Pt-GDY2 with four-coordinated C2-Pt-Cl2 exhibits higher mass activity up to 3.3 times than Pt-GDY1 with five-coordinated C1-Pt-Cl4. Higher total unoccupied density of states of Pt 5d orbital and close to zero |ΔGPt H∗| value makes Pt have higher HER catalytic activity. Hetero-atom doping modification of the carrier affects the coordination environment. For example, atomically dispersed electrocatalysts (ADCs) with Ru-C5 single atoms and Ru-O4 nanoclusters were fabricated in S-doped carbon black by using impregnation strategy in room temperature (Cao et al., 2021). Activated carbon and 2, 2-bithiophene were grinded fully and then calcined in a tube furnace at 800°C for 2 h under N2 atmosphere to obtain the S-doped carbon material. Thirty milligrams of S-doped carbon material and 20 mL water were mixed intensively in a beaker; 0.05 mmol RuCl3·xH2O dissolved in 5 mL deionized water was dropped into the above solution and stirred for another 6 h. Then, the mixture was centrifuged and dried in a vacuum drying oven at room temperature to obtain the atomically dispersed Ru catalyst. Meanwhile, dual-site Ir, Rh, Pt, Au, and Mo ADCs can also be prepared by this method. The Ru ADCs show enhanced HER performance in alkaline solution due to the synergic effect between single-atoms and sub-nanoclusters. What’s more, Cu-SA/SNC with low-valence Cu(+1)-N4-C8S2 was constructed by impregnation method with single copper atoms embedded in a sulfur and nitrogen-modified carbon support (Jiang et al., 2019). Na2S·9H2O and S powder were dissolved in deionized water by ultrasonic dissolving for 5 h at ambient conditions. Then, the solution was heated at 80°C for 12 h in a Teflon autoclave to obtain S precursor. CuPc, DCDA, and trimesic acid were dissolved in deionized water. Then S precursor was dropped into the above solution for continuously stirring and drying at 80°C. Next, the mixture was annealed at 900°C for 2 h under N2 atmosphere, and the samples were leached in 0.5 M H2SO4 solution at 80°C for 24 h to remove the free-standing metallic residues. Synthesizing single atom catalysts by the impregnation method is simple, without complicated and expensive equipment (Sun et al., 2020). Therefore, this method is very suitable for large-scale synthesis of single-atom catalysts. Yang et al. successfully synthesized a series of M-SACS catalysts (M = Ni, Mn, Fe, Co, Cr, Cu, Zn, Ru, Pt, and their combinations) by complexing a series of metal cations with 1, 10-phenantholine and loading them on commercial carbon black (Yang et al., 2019a). The synthetic approach enables large-scale (>1 kg) production of single-atom catalysts with high metal loadings. The synthesized Ni single-atom catalyst exhibits excellent activity in the electrochemical reduction of carbon dioxide to carbon monoxide. It provides an important approach for large-scale preparation of SACs by impregnation method.Electrochemical methods were regarded as the effective strategies to synthesize high-purity SACs because of low cost and simple applicability. The standard three-electrode system can quickly prepare the target sample and accurately control the catalyst preparation process by adjusting the workstation parameters, which has obvious advantages over the traditional wet chemical method.Atomic dispersed Ru-doped ultrathin Co(OH)2 nanosheet arrays (CoRu@NF) was fabricated by electrochemical deposition method (Zhu et al., 2021; Figure 2A), which shows excellent catalytic performance of OER in 1.0 M KOH and 0.1 M KOH solution. A standard three-electrode system was carried out for electrochemical deposition. The nickel foam (NF) was immersed 2 cm below the liquid surface; saturated calomel electrode (SCE) and the carbon rod were used as the working electrode, reference electrode, and counter electrode. Co(NO3)2∙6H2O aqueous solution was poured into a 100 mL electrolytic cell. Cyclic voltammetry with the scanning potential of 0 ∼ −1.2 V (versus SCE), the scanning rate of 100 mV s−1, and the scanning cycles of 40 times was used. The introduction of Ru reduces the thickness of the nanosheets, exposing more active sites. Besides, single-atoms Ru were anchored on the surface of MoS2 nanosheets array supported by a carbon cloth with 3D porous structure based on theoretical predictions (Wang et al., 2019; Figure 2B). The MoS2/CC acts as the working electrode. Atomically Ru was electrodeposited by cycling MoS2/CC substrate from −0.5 to 0.4 V versus SCE at the sweep rate of 20 mV s−1 in the electrolyte containing RuCl3 and H2SO4 for 20 cycles. Finally, the Ru-MoS2/CC was taken out, washed with deionized water and dried by nitrogen flow. The catalyst displays HER catalytic performance comparable to commercial Pt/C under pH-universal conditions. What’s more, Zeng’s group reported the fabrication of SACs by electrochemical deposition method in a wide range of metals and supports (Zhang et al., 2020). The cathodic voltage was from 0.10 to −0.40 V, the anodic voltage was from 1.10 to 1.80 V, and the scanning rate was 5 mV s−1. The processes were repeated for 10 times and 3 times in cathodic and anodic deposition, respectively. The experimental results showed that SACs displayed different electronic states due to different redox reactions between the cathodes and the anodes. More than 30 different SACs can be successfully fabricated from cathodic or anodic deposition only by varying different metal precursors and supports. Interestingly, the SACs deposited by cathode have higher activity for hydrogen evolution reaction, whereas the SACs deposited by anode have higher activity for oxygen evolution reaction.In general, the species and coordination environment of SACs can be changed by varying the supports and metal precursors. However, there are still few studies on the preparation of single atoms on carbon-based materials by electrodeposition due to the influence of electrodeposition equipment.As a kind of “top-down” method, CVD is often used to synthesize single-atom catalysts. The research of CVD began in the late nineteenth century. Its principle is to introduce the reaction agent vapor and other gases required into the reaction chamber, by increasing the temperature, or other forms of energy, so that they have chemical reactions on the substrate surface to generate new solid substances deposited on the surface (Drosos and Vernardou, 2018; Zhang et al., 2019).The CVD method consists of the following four steps: (1) The reaction gas diffuses to the surface of the material; (2) the reaction gas is adsorbed on the surface of the material; (3) the chemical reaction occurs on the surface of the material; (4) the gaseous by-products are separated from the surface of the material. Due to the nucleation or growth at the molecular level, CVD is more suitable for the formation of dense and uniform thin films on the surface of irregularly shaped substrates, and the deposition speed is fast and the quality of the film is very stable (Gardecka et al., 2018). Some special films also have excellent optical, thermal, and electrical properties and thus easy to achieve mass production (Liu et al., 2019b). For example, Miroslav and colleagues (Kettner et al., 2019) synthesized Pd-Ga alloy supported on highly ordered pyrolytic graphite (HOPG) by vapor deposition under ultra-high vacuum, which is shown in Figure 2D. Pd was deposited using a commercial electron beam evaporator from a Pd wire onto the HOPG substrate that was kept at room temperature, and Gallium was evaporated from a pyrolytic boron nitride crucible in a second electron beam evaporator at an angle of approximately 45° with respect to the sample normal. The evaporation rates were 1.5 Å/min for Pd and 0.5 Å/min for Ga, respectively. Through STM/AFM characterization results, it can be seen that the HOPG-rich Pd-Ga alloy was prepared on HOPG. The Pd-Ga alloys of Ga exhibit superior Pd single-atom site properties and excellent stability. In addition, Mohammad and his team (Tavakkoli et al., 2020) successfully synthesized N, Co, and Mo single-atom-decorated highly graphitized graphene nanoflake-carbon tube (CNT) composites by a one-step reactive vapor deposition method. As shown in Figure 2E, the method first obtains the CoMo mixed catalyst by heating and calcination. Then, acetonitrile was added in the mixed atmosphere of H2/CH4 for N doping, and the carbon material was grown on the catalyst at 1000°C. A high specific surface area mesoporous material obtained by this method is favorable for the oxygen mass transfer process and exhibits high catalytic activity and stability (basic conditions) for OER and ORR. Through STEM images, single metal atoms can be clearly identified in the multilayer graphite films, as shown in Figure 2F. However, the deposition temperature of CVD is usually very high, generally between 900°C and 2000°C, so it is usually used on carbon materials. However, high temperature can easily cause great damage to common materials, such as nickel foam, which limits the choice of substrates and deposition layers. At present, two aspects of medium, low temperature and high vacuum, are the main development directions of CVD (Malarde et al., 2017).Ball milling can cut and reconstruct the chemical bonds of materials/molecules and is widely used in the preparation of carbon-based-material-supported single atoms. Moving balls with kinetic energy apply their energy to the materials, causing a single metal atom to be embedded on the surface of the carbon substrate (Yang et al., 2020b). A series of graphene-embedded FeN4 (FeN4/GN) catalysts with different Fe content were prepared via high-energy ball milling (Deng et al., 2015). Firstly, 2.0 g graphite flake and 60 g steel balls were placed in a hardened steel cylinder in a glove box, cleaned with high-purity argon for 20 min, and sealed. Various ratios of 2.0 g FePc and GN composites and 60 g steel balls (1–1.3 cm in diameter) were operated like before. Ball milling was agitated with 450 rpm for 20 h. A series of FeN4/GN samples with different Fe content were obtained. Coincidentally, single Fe atoms anchored on graphene nanosheets (FeN4/GNs) were fabricated by ball milling method for the direct conversion of methane to C1 oxygenated products at room temperature (Cui et al., 2018). Other transition metals were also prepared by this method, including Mn, Fe, Co, Ni, and Cu. A series of M-N4-coordinated SACs were obtained by simply regulating the type of metal precursor salt. Fe and/or Co atomically dispersed within the 2D conjugated aromatic networks (CAN) were synthesized with the assistance of ball milling (Yang et al., 2019b). Firstly, PMDA, urea, NH4Cl, (NH4)6Mo7O24·4H2O, and a certain amount of metal chloride (FeCl3 and CoCl2·6H2O) were mixed in a crucible. The metal polyphthalocyanine weas prepared by heating the mixture in a muffle furnace at 220°C for 3 h; 0.2 g above polyphthalocyanine and 15 mL deionized water were transferred into a zirconium dioxide capsule containing zirconium dioxide balls (0.5 mm in diameter). Ball milling was carried out at 1000 rpm for 1 h. The single-metal-atom-site density up to 10.7 wt% without agglomeration. CAN-Pc (Fe/Co) with Fe-N4 and Co-N4 coordination displays superior performance to benchmark Pt/C for ORR and Zn-air batteries. In one study, a rapid one-step mechanochemically induced self-sustaining reaction was proposed (Jin et al., 2021b). Nitrogen-doped-carbon-supported single Co atoms were prepared by direct ball milling of cobalt (II) 5,10,15,20-tetrakis-(4′-bromophenyl) porphyrin (Co-TPP-Br) and calcium carbide without the pretreatment of carbon support and further pyrolysis procedure (Figure 2C). The mechanochemical energy can ignite and propagate a self-sustaining exothermic process, leading to the direct formation of carbon matrix to stabilize metal sites. The sample Co-BM-C with CoN4 configuration prepared by ball milling (BM) showed excellent HER (η10 = 126 mV) and OER (η10 = 240 mV) performance in 1.0 M KOH, showing great potential in overall water splitting (1.60 V @ 10 mA cm−2).The pyrolysis strategy shows the merits of low price, environmental friendliness, and simplification in the synthesis procedures. The different types of metal atoms can be controlled by modulating the parameters in the synthesis process. Metal nodes in metal-organic framework (MOFs) are known to be atomically dispersed and have a well-coordinated environment, making them ideal precursor types for building SACs (Wang et al., 2018a). The single tungsten atoms supported on MOF-derived N-doped carbon matrix was achieved successfully for HER applications (Chen et al., 2018a). The W-SAC and MOF were prepared by pyrolysis strategy. Tungsten precursor (WCl5) was encapsulated in the skeleton of MOF (UiO-66-NH2) and then pyrolyzed at 950°C (Figure 3A). The excess zirconia is removed by hydrofluoric acid solution to obtain W-SAC. It is important to note that the uncoordinated amines in UIO-66-NH2 play an important role in preventing the aggregation of W species. The catalyst displays 85 mV at a current density of 10 mA cm−2 in 0.1 M KOH, where HER catalytic performance is close to that of commercial Pt/C. In addition, zeolitic-imidazolate frameworks (ZIFs) also are often used to design templates and precursor for single-atom catalysts due to its flexibility and ultrahigh surface area (Xia et al., 2015, 2016). ZIFs can be converted into amorphous or graphite-carbon frames by pyrolysis synthesis, thus providing a rich platform for the design of functional custom materials for electrocatalytic applications (Tang et al., 2015; You et al., 2015; Zheng et al., 2014). Co–Nx/C nanorod array derived from 3D ZIF nanocrystals was prepared through Zn2+ clusters that react with methylimidazole/PVP ligand to form ZIF nanocrystals, which catalyze the structural evolution of nanorods (Amiinu et al., 2018) (Figure 3B). Due to the synergistic effect of the chemical composition and abundant active sites of the nanorods, the catalysts show excellent ORR and OER performance compared with commercial Pt/C and IrO2.In order to achieve coordination regulation, Wang and colleagues prepared a series of single-Co-atoms catalysts with different nitrogen coordination numbers and studied their catalytic performance for CO2 reduction (Wang et al., 2018b). Co/Zn ZIFs were synthesized at room temperature firstly, and then Zn would be evaporated away during the pyrolysis process. The Co ions would be reduced to single Co atoms anchored on nitrogen-doped porous carbon. SACs with coordination number from 2 to 4 were prepared by controlling volatile C-N fragments to adjust the number of N around central Co site through bimetallic Co/Zn ZIFs at 1000°C, 900°C, and 800°C of pyrolysis temperatures, respectively (Figure 4A). Co nanoparticles were also prepared by pyrolysis of ZIFs containing pure Co. As can be seen from Figure 4B and C, the Co-N2 catalyst maintained the initial ZIF morphology. EDX spectrum indicates that Co atoms are uniformly distributed throughout the structure (Figure 4D). Meanwhile, atomically dispersed Co atoms can be directly observed from AC-HAADF-STEM (Figures 4E and 4F). What’s more, SAED with ring pattern indicates that the crystallinity of Co-N2 catalyst is poor (Figure 4G). The optimum selectivity and activity are shown when Co is coordinated with two N atoms with 94% CO formation Faradaic efficiency and a current density of 18.1 mA cm−2 at an overpotential of 520 mV. Meanwhile, the turnover frequency (TOF) value of the CO formation is up to 18200 h−1. The results of experiments and theoretical calculations show that Co-N2 sites can promote the formation of CO2 into CO2 − intermediates, thus enhancing the CO2RR performance. Moreover, improving the loading of SACs is another factor to promote the industrial application of single atom catalysis. Atomically dispersed transition metals anchored on nitrogen-doped carbon nanotubes (MSA-N-CNTs, where M = Ni, Co, NiCo, CoFe, and NiPt) with high loading were fabricated through a multi-step pyrolysis strategy (Cheng et al., 2018). Take the NiSA-N-CNTs for example, Ni(acac)2 was dispersed with dicyandiamide C2H8N2 in 100 mL solution and stirred for 10 h, followed by drying and grinding. Subsequently, the mixture was heated at 350°C and 650°C for 3 h under Ar atmosphere, respectively. Finally, the as-prepared yellow powder was heated in a selected temperature range of 700–900°C to obtain the target production. From the SEM and TEM image, it can be observed that the average CNT diameter is around 31 nm without metallic nanoparticles (Figure 4H and 4I). Meanwhile, the uniform distribution of N and Ni can be seen from STEM-EDS mapping (Figure 4J). In AC-STEM, bright spots corresponding to Ni atoms were uniformly distributed in CNTs, and individual Ni atoms were located on the walls of a CNT (Figure 4K and 4L). In addition to normal C6 carbon rings, C5, C7, and other nonC6 carbon rings were also formed in CNT, combining with the results of Raman spectroscopy (Figure 4M). NiSA-N-CNTs with a load of up to 20 wt% showed the best selectivity and activity for the electrochemical reduction of CO2 to CO, with TOF values two orders of magnitude higher than those of Ni nanoparticles loaded on CNTs. Han and colleagues reported the single Cu atoms dispersed on graphene through a unique confined self-initiated dispersing protocol (Han et al., 2019a). The GO/DICY was prepared by stirring dicyandiamide (DICY) graphene oxide dispersion and then freeze-drying. Then the mixture was added into a quartz boat that was tightly wrapped by a piece of Cu foil. The quartz boat was pyrolyzed at 600°C for 2 h and 800°C for 1 h under Ar atmosphere in a tube furnace. The production was treated with 0.5 M H2SO4 and then pyrolyzed at 300°C again. This novel in situ dispersion protocol produces highly reactive gaseous copper-containing intermediates that essentially circumvent the large-scale agglomeration of metal atoms in traditional processes and facilitate Cu dispersion on graphene (Figure 4N). The catalyst with the loading of 5.4 wt% showed an outstanding performance for ORR due to the abundant and highly dispersive Cu single atoms. Zhao and colleagues prepared a series of M−NC (M = Mn, Fe, Co, Ni, Cu, Mo, Pt, etc.) SACs with metal loadings up to 12.1 wt% through a cascade anchoring strategy (Zhao et al., 2019). Firstly, the metal ions are chelated by chelating agent and anchored onto oxygen-species rich porous carbon support. Then the complex bound carbon and melamine were put into a tube furnace and heated to 800°C for 2 h under Ar flow to obtain the M-NC. Furthermore, the scale-up synthesis can be achieved in parallel by the same synthesis route except for increasing the amounts of materials.In general, the precise and controllable preparation of SACs can be achieved by ALD, however, its expensive equipment and low yield limited the development of this technology. From a practical point of view, wet-chemistry synthetic methods for SACs are more desirable approaches because of its ease of operation and feasibility of large-scale manufacturing. From the methods introduced earlier, it can be seen that both impregnation method and pyrolysis method can be used to achieve the scale-up preparation of carbon-based-material-supported SACs. In particular, the impregnation method does not require complex and expensive equipment and displays the characteristics of low cost, simple operation, and easy synthesis, so it shows great potential in mass preparation.The research progress of SACs is closely related to the development of characterization technology. Advanced characterization techniques help to understand the coordination environments and electronic structures of SACs, which directly affect the catalytic performance. Hitherto, the main applied characterization methods included high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), atomic force microscope (AFM), scanning tunneling microscopy (STM), X-ray absorption spectroscopy (XAS), and Raman measurements. Furthermore, in situ characterization has also been introduced as a powerful tool for studying the real active sites, structural changes during the catalysis process, and reaction mechanisms of SACs. Thus, in order to fully understand the structure, composition, and coordination environment of SACs, it is necessary to combine a variety of advanced characterization methods. In addition, density functional theory (DFT) simulation provides an unprecedented opportunity to discover catalytic reaction mechanisms, enabling the rational design of materials with personalized activity.Nowadays, transmission electron microscopy (TEM) is a powerful tool to obtain the fine structure of nanomaterials. From the Figure 5A (Ren et al., 2020), we can see that the received signals are mainly transmitted electron beam and scattered electron in the range of θ3. In the range of θ2, Bragg scattered electrons are signals. When the received signal is θ1, the HAADF image can be obtained. Especially, supported single atoms can be directly observed through aberration-corrected HAADF-STEM, and even the atomic structure information could be obtained. The phase contrast of atoms is directly proportional to their relative atomic number Z. Because the Z value of an isolated atom is different from that of the host atom, the relative bright dots of single metal atoms can be observed clearly. Therefore, theoretically speaking, the higher the Z value of the atom on the hosts, the better the image will be. For example, the as-prepared Co SAs/N-C structure shows rhombododecahedral shape (Yin et al., 2016), as revealed by TEM and HAADF-STEM (Figures 5B and 5C). The aberration-corrected HAADF-STEM (Figures 5D and 5E) were carried out to elucidate the form of the Co atoms with sub-angstrom resolution. Due to the different Z contrast between Co and C, the isolated heavier Co SACs can be identified in the carbon support.The STM is a scanning probe microscopy tool that enables the observation and localization of individual atoms with a higher resolution than its atomic force microscope counterparts. The atomic-level sharp metal tip in STM scans the surface atomic structure based on the electron quantum tunneling effect of the tip-sample nano-gap. The effect causes the tunnel current to show an exponential relationship with the size of the gap, and an atomic-level sample surface topography characteristic image is obtained. It is commonly used in surface scientific research to examine model catalysts, such as single crystals with well-defined surface structures. What’s more, the STM can directly observe whether the surface atoms of the material have periodic surface structure features, surface reconstruction, and structural defects. However, STM cannot detect deep information and observe insulators. Atomically dispersed platinum (Pt) was synthesized by photochemical reduction method (Wei et al., 2017). The atomically dispersed Pt on ultrathin carbon films can be directly observed through STM (Figure 6A). At atomic resolution, single Pt atoms appear as single-peak protrusions with a diameter of about 0.2 nm and a height of about 0.3 nm. Besides, Wu and colleagues synthesized high-density Cu(I)-N active sites in an N-doped graphene matrix via pyrolysis of copper phthalocyanine and dicyandiamide (Wu et al., 2016). HAADF-STEM and the corresponding element mappings show the uniform distribution of Cu, N, and C (Figure 6B). From atomic resolution STM image (Figure 6C), the obvious bright spots can be observed, which indicates that the Cu atoms are atomically dispersed. STM simulations further revealed the atomic structure of this catalyst, in which atomically dispersed Cu-N2 centers are embedded in the graphene lattice (Figure 6D).AFM can not only measure the surface morphology of the sample (close to the atomic resolution) but also detect the force between the atoms on the surface, the elasticity, plasticity, hardness, adhesion, friction, and so on. From topographic image, the height of the nanoparticle or the surface roughness of the sample can be seen clearly. Reduced graphene oxide (rGO) has stable anchor sites for metal single atoms, but the anchor sites are sparse, making it difficult to prepare high-load metal single-atom catalysts. Therefore, combining rGO with two-dimensional materials with abundant connecting atoms, such as carbon nitride, is an effective strategy to deal with this challenge. Therefore, metal single atoms (Pd, Pt, Ru, Au) were fabricated on porous carbon nitride/reduced graphene oxide (C3N4/rGO) foam (Fu et al., 2020). Among these catalysts, Pd1/C3N4/rGO showed enhanced catalytic activity over its NPs counterpart for Suzuki-Miyaura reaction. From the Figures 6E–6G, the C3N4/rGO layer with an onion-like microstructure with orderly organization can be observed. The thickness of GO sheet building blocks was measured by AFM (Figure 6H). The sheet height of about 0.7 nm corresponds to the height of a single GO layer. Meanwhile, the AC-HAADF-STEM was applied to observe the samples (Figure 6J). It can be seen from the element mapping diagram (Figure 6I) that Pd element is evenly distributed in the matrix containing N and C. As shown in isolated single Pd atoms, sites can be seen clearly on the 2D C3N4/rGO sheet without aggregated Pd nanoparticles or clusters.XAS is an element-specific technique used to obtain the properties of absorbing atoms and their surroundings, resulting in a comprehensive understanding of the chemical state and structure of catalysts. It is the main technique used to characterize different coordination structures, which can be used to gain insight into the local atomic and electronic structure of single atoms. XAS includes X-ray absorption near edge structure (XANES) spectrum and extended X-ray absorption fine structure (EXAFS) spectrum. The energy of XANES spectrum ranges from the absorption edge to 30–50 eV above the absorption edge, and it is sensitive to the charge state and orbital occupancy of single metal atoms. The EXAFS spectrum represents the spectral region where the energy above absorption edge ranges from 30–50 eV to 1000 eV or more. Through Fourier transform (FT) analysis of EXAFS, the coordination number and distance between the central atom and adjacent atoms can be extracted. Wavelet transform can distinguish backscattered atoms and provide strong resolution in k and R space, which is the perfect complement to FT. Single Cu atoms coordinated with both S and N atoms in MOF-derived hierarchically porous carbon (S-Cu-ISA/SNC) was reported by atomic interface regulation (Shang et al., 2020). To better analyze the chemical state and atomic structure of the sample, synchrotron-radiation-based soft XANES and XAFS was carried out. From the analysis of Cu L-edge spectrum (Figure 7A), carbon K-edge spectrum (Figure 7B), N K-edge spectrum (Figure 7C), S L-edge, and K-edge spectra of S-Cu-ISA/SNC combining the XPS results, bonds between atoms can be obtained. Meanwhile, the interface structure at atomic scale, like the average oxidation state of Cu, can be obtained from Cu K-edge XANES spectra of S-Cu-ISA/SNC and the references (Cu foil, CuS, and CuPc) (Figure 7D). The scattering of Cu-N and Cu-S was detected by FT peaks in FT-EXAFS spectra for S-Cu-ISA/SNC, and no Cu-Cu bond was found (Figures 7E and 7G). Cu K-edge wavelet transform (WT)-EXAFS has also been applied to study the atomic configuration and the Cu-N and Cu-S contributions of S-Cu-ISA/SNC due to the strong resolution of k and R spaces (Figure 7F). These results strongly prove the existence of Cu single atoms. Based on the above analysis, the first coordination number of the central copper atom is 4, including one metal-sulfur and three metal-nitrogen bonds, in which bond lengths corresponds to 2.32 and 1.98 Å, respectively (Figure 7H).Raman spectroscopy is a nondestructive analysis technique based on the interaction of light and chemical bonds in materials. It can provide detailed information about the chemical structure, phase and morphology, crystallinity, and molecular interaction of the samples. A Raman spectrum is usually composed of a certain number of Raman peaks. Each Raman peak represents the wavelength position and intensity of the corresponding Raman scattered light. Every peak corresponds to a specific molecular bond vibration, which includes not only a single chemical bond, such as C-C, C=C, N-O, and C-H, but also the vibration of a group composed of several chemical bonds, such as the benzene ring breath vibration, long polymer chain vibration, and lattice vibration. Raman measurements are used to further analyze the structural information of SACs. Raman spectra also show the D band and G band, which can be distinguished allotropes of carbon in carbon materials. The D band represents the disordered carbon atoms and sp2-hybridized carbon atoms (Li et al., 2012, 2018; Pan et al., 2013; Zhang et al., 2017b), whereas G band is related to the tangential stretching mode of sp2 carbon atoms, indicating the existence of crystalline carbon in the carbon material (Deng et al., 2017). Wei and colleagues prepared N-decorated carbon-encapsulated W2C/WP heterostructure as an efficient HER electrocatalyst in acid and alkaline solutions (Wei et al., 2021b). The samples prepared with different precursors of (NH4)10H2(W2O7)6/NH4H2PO4, 1:0, 1:1, 1:2, 1:4, and 1:12, were labeled as W2C/W@NC, W2C/WP@NC-1, W2C/WP@NC-2, W2C/WP@NC-4, and WP@NC. The two main peaks located at 697 and 803 cm−1 in the Raman spectrum correspond to the stretching vibration of W-C (Figure 8A). The ID/IG ratios value of W2C/WP@NC-2 is 0.96 (Figure 8C), which is smaller than that of W2C/WP@NC-1 (Figure 8B) and W2C/WP@NC-4 (Figure 8D). The results showed that W2C/WP@NC-2 illustrated high conductivity and quick charge-transfer rate. Ex situ techniques are used to establish the relationship between electrochemical performance and the properties of materials. However, in situ characterization can not only provide plenty of valuable information during the dynamic change process but also assess the coordination environment of the active site accurately. Nowadays, various in situ characterizations have gradually emerged with the continuous in-depth study of single atoms.Infrared spectroscopy can directly detect the interaction between adsorbed molecules and the supporter surface, and time- and temperature-resolved Fourier transform infrared spectroscopy (FTIR) can be used to detect catalytic intermediates. By detecting the vibration frequency and intensity of the model, the characteristics of the active center can be inferred after appropriate correction. Selection of appropriate probe molecules, such as CO, NH3, pyridine, and so on, can be used to analyze the overall catalyst, which is an important strategy to analyze the SACs. This paper mainly introduces the application of infrared spectroscopy in the characterization of SACs using CO as probe molecule. FTIR measurement was performed using CO as probe molecule to analyze the dispersion and oxidation state of Pt in the sample (Qiao et al., 2011) (Figure 9A). In sample B, there are three vibration bands in 2030 cm−1, 1950 cm−1, and 1860 cm−1. The main peak at 2030 cm−1 is the linear adsorption of CO at Pt0 site, whereas the weak vibration band at 1950 cm−1 and 1860 cm−1 is caused by the adsorption of CO on the bridge of two Pt atoms and the interface between Pt clusters and the support. That is, bridge-bonded CO indicates the presence of dimer or Pt clusters. These results indicate that Pt clusters and single atoms coexist in the samples. What’s more, Hu and colleagues analyzed the existence state of Pt in Pt-SA/CsPbBr3 NCs (Hu et al., 2021). The strong vibration peak at 2058 cm−1 indicates the linear adsorption of CO at the Ptδ+ sites, which proves the existence of single Pt atoms (Figure 9B). The absence of CO bridge adsorption peak indicates that Pt atoms may not have formed Pt nanoparticles or massive Pt atoms agglomeration. In situ XAS can be used to analyze the evolution of the coordination environments during the catalytic process. Xiong et al. reported isolated single-atom Rh anchored on N-doped carbon (SA-Rh/CN) for formic acid oxidation (Xiong et al., 2020). The in situ XANES spectra of SA-Rh/CN were collected at Rh K-edge during chronoamperometry (CA) to investigate the change of oxidation state for Rh atom (Figure 9C). The results showed that the intensity of the main absorption peak at ∼ 23250 eV gradually increased with the extending of reaction time, indicating that the oxidation state of Rh atoms became higher and higher, which may be caused by the formation of oxides in the process of high potential reaction. Similarly, the structural evolution and atomic interface structure of isolated Cu sites were collected by Cu K-edge XANES (Figures 9E and 9F) and EXAFS during ORR (Shang et al., 2020). In situ XAS was carried out in electrochemical cell set-up (Figure 9D). The energy at the edge decreases gradually, along with the intensity of the white line from 1.05 V to 0.75 V. The in situ spectroscopic analysis shed light on the evolution of the electronic and atomic structures of the Cu-S1N3 moiety of S-Cu-ISA/SNC, revealing that the low-valence (+1) Cu-N-bond-shrinking HOO-Cu-S1N3, O-Cu-S1N3 and HO-Cu-S1N3 may contribute to ORR activity (Figure 9G). However, in situ XANES is not yet prevalent because of the extremely limited resources of synchrotron radiation. With the construction of more advanced synchronous light sources, in situ XANES will play an increasingly important role in scientific research. In situ Raman is a powerful analytical tool for revealing the reaction route and analyzing the reaction mechanism due to the high temporal and spatial resolution. Surface-enhanced Raman scattering (SERS) is caused by electromagnetic and the charge transfer mechanism, which means that when the analyte is adsorbed on rough metal surface, its Raman signal will be enhanced (Cialla et al., 2012). Sun and colleagues employed the in situ SERS to monitor the adsorbate-substrate interaction in the process of ORR on the Au@Pd@Pt core/shell nanoparticles, which provided the direct evidence of ∗OOH intermediate (Sun et al., 2022). Furthermore, it is proved that the introduction of Pd shells affects the strain and electronic effect, leading to enhanced ORR activity. The relationship between ORR performance and strain/electron effect was illustrated by detecting intermediates from in situ SERS technique. What’s more, time-resolved SERS (TR-SERS) was applied to reveal the dynamics of carbon dioxide (CO2) reduction reaction intermediates on Cu electrodes (An et al., 2021). The results showed the surface reconstruction of Cu and the dynamic CO surface intermediates. This technique is of great significance for understanding the dynamic information of the surface reaction during CO2 electrolysis.However, SERS is limited to metal substrates with nanostructures. Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is applicable to the surfaces of various materials and nanostructures (Li et al., 2013). SHINERS are composed of a plasma gold or silver core and an inert shell such as SiO2 and Al2O3. These shell-isolated nanoparticles are easy to manufacture and can be dispersed on the surface of analytes with different composition and morphology, which are employed to enhance the Raman vibration signal of nearby molecules (Ding et al., 2016). Wei and colleagues adopted in situ SHINERS to monitor the catalytic reaction process and kinetics of hydrogenation of nitro compounds and to characterize the structure of Pd single atoms for the first time (Wei et al., 2021a). Pd SAs were anchored on the surfaces of TiO2 or Al2O3 shells of Au-shell-isolated nanoparticles. It also revealed the nucleation process of Pd species from single atoms to nanoparticles. This work provided a new spectroscopic tool for the in situ study of SACs, especially the solid-liquid interface.With the development of science and technology, researchers have developed numerous high-performance electrocatalysts, and the understanding about these reactions is gradually deepening. Density functional theory (DFT) calculations are widely used to study the free energies of intermediates and further reveal the mechanism of enhanced reactivity. The development of DFT theoretical models and advanced characterization techniques has greatly enhanced the understanding of electrocatalyst reaction mechanisms, such as the identification of active sites and the theory design of catalysts. Pt/C is considered to be the best catalyst for HER and ORR. RuO2 and IrO2 show the best catalytic performance toward OER. However, due to the scarcity and high price of precious metals, it is urgent to develop new type catalysts to reduce the production cost. SACs display great potential in the realization of efficient and selective electrocatalytic processes because of unique electronic structure and coordination environment. The surrounding coordination atoms of the central metal atom show important effects on their catalytic activity, selectivity, and stability, which are significant indicators of catalysts. In this section, the characterization of the existence of single atoms and coordination environments and their related catalytic performance will be discussed in detail.With the consumption of fossil fuels, a series of environmental problems have aroused people’s attention. Hydrogen is considered to be the most likely alternative to fossil fuels due to its high energy density, no carbon emission, and without pollution. The method of electrolyzing water to produce hydrogen has attracted wide attention because of high efficiency, no need to consume fossil energy, and high product purity. Since Nicholson and Carlisle proposed the concept of water electrolysis in the 18th century, electrochemical water splitting has been developed for more than 200 years (Kreuter and Hofmann, 1998). Hydrogen evolution reaction (HER) is represented by the chemical formula as: 2H+ + 2e– → H2, which is a multi-step electrochemical process that occurs on the electrode surface to generate gaseous hydrogen at the cathode (Zheng et al., 2015). The reaction mechanism of HER is different in acidic and alkaline solutions, but both can be divided into two elementary reactions (Bockris and Potter, 1952; Sheng et al., 2010). The first step is electrochemical hydrogen adsorption, which is called Volmer reaction. The second step is electrochemical desorption (Heyrovsky reaction) or chemical desorption (Tafel reaction). The mechanism under acidic conditions can be expressed as (Lasia, 2010) (Equation 1) H + + M + e - ⇌M - H ∗ Volmer reaction (Equation 2) M − H ∗ + H + + e − ⇌ M + H 2 Heyrovsky reaction or (Equation 3) 2 M − H ∗ ⇌ 2 M + H 2 Tafel reaction The mechanism under alkaline conditions can be expressed as (Equation 4) H 2 O + M + e − ⇌ M − H ∗ + OH − ( Volmer reaction ) (Equation 5) M − H ∗ + H 2 O + e − ⇌ M + OH − + H 2 ( Heyrovsky reaction ) or (Equation 6) 2 M − H ∗ ⇌ 2 M + H 2 ( Tafel reaction ) where H∗ designates a hydrogen atom chemically adsorbed on an active site of the electrode surface (M).As explained in Conway and Tilak in detail, the Tafel slope value obtained by the HER polarization curve can be used to judge the possible rate-determining step during the reaction process (Conway and Tilak, 2002). From these elementary reactions, we can see that the chemical adsorption and desorption of H atoms is a complicated process. According to the different pathways of H atom desorption, the HER reaction mechanism can be divided into Volmer-Heyrovsky mechanism and Volmer-Tafel mechanism (Li et al., 2016).The bonding ability of the active sites in the catalyst and H∗ should neither be too strong nor too weak according to the Sabatier principle (Sabatier, 1920). When the bonding ability of M-H∗ is too strong, it is not conducive to the breaking of the bond and the release of hydrogen. On the contrary, it is not beneficial to the process of proton-electron transfer. However, the bond energy between the active site of the catalyst and H∗ cannot be measured directly, so it is difficult to establish a direct correlation between the intermediate H∗ and the electrochemical reaction rate (Marković and Ross, 2002). From the perspective of physics and chemistry, the adsorption free energy of H∗ (ΔGH∗) can be used to evaluate the capacity of H adsorption and the release of H2 (Nørskov et al., 2005). Therefore, ΔGH∗ is a key parameter used to evaluate the reaction rate. ΔGH∗ close to zero could be used to evaluate the efficiency of the catalyst, but not a requirement. More importantly, it is concluded that the experimental exchange current density j0 value and ΔGH∗ have a “volcano curve” correlation through DFT calculation (Skúlason et al., 2010).As can be seen from the volcano diagram (Zeradjanin et al., 2016), Pt is located at the top of the volcano diagram, indicating appropriate adsorption energy and the highest current density, which explains the optimal performance of Pt in HER (Figure 10A). However, due to the scarcity of precious metals, the application of SACs in HER has attracted wide attention from researchers in order to improve atomic utilization rate and reduce catalyst cost. Meanwhile, the development of high-active non-noble-metal catalysts is also an effective solution.The introduction of heteroatoms into carbon substrate can not only form new coordination environments but also improve metal loading, which has attracted extensive attention of researchers. The general coordination number between metal atom and N atom is 4, which may be related to the valence state and electronic structure of the central atom. For example, Ye and colleagues reported single Pt atoms anchored on aniline-stacked graphene with a Pt loading of 0.44 wt% through microwave reduction method (Ye et al., 2019). The Pt SASs/AG with unique Pt-N4 coordination not only displays high HER activity with the overpotential of 12 mV at 10 mA cm−2 in 0.5 M H2SO4, better than 20 wt% Pt/C, but also the mass current density of 22,400 AgPt −1 at the overpotential of 50 mV is 46 times higher than commercial 20 wt% Pt/C. Moreover, stability is also a significant issue in the development SACs. Cyclic stability and long-term stability can be assessed by cyclic voltammetry sweeps and chronopotentiometric measurements. The Pt SASs/AG shows negligible decay by comparing the LSV curves before and after 2000 cycles and displays outstanding long-term stability over 20 h. From the XPS spectrum of Pt 4f (Figure 10B), it shows the strong interaction of Pt and aniline-formed Ptδ+ XAFS, and DFT calculations show that the isolated Pt is coordinated with the N of four aniline molecules (Figures 10C–10E), which optimizes the electronic structure of Pt. The modulation of the d-band center and density of states (DOS) near the Fermi level of Pt atoms by aniline caused the single Pt sites to have appropriate hydrogen adsorption energy and finally enhances HER activity. In addition, other noble-metal-based SACs have also been studied for HER. The unsaturated coordination between the central Ru atom and the surrounding N atom can also significantly enhance HER performance. For example, C3N4-Ru were fabricated by thermal treatment of graphitic C3N4 nanosheets and RuCl3 in water, which shows apparent HER activity in acidic media, and the HER activity is positively correlated with loading of Ru (Peng et al., 2017). The test results showed charge transfer from C3N4, a unique functional scaffold, to the Ru center (Figure 10G). The formation of unsaturated coordination Ru-N2 moieties as effective active sites facilitated the adsorption of hydrogen from the DFT calculation (Figure 10F). Besides, single Ruthenium atoms were anchored amorphous phosphorus nitride nanotubes (Ru SAs@PN) through strong coordination interactions between the d orbitals of Ru and the lone pair electrons of N located in the HPN matrix (Yang et al., 2018b). The SACs in Ru-N3.8 coordination environment were prepared by impregnation method. The Ru SAs@PN showed excellent HER activity with overpotential of 24 mV at 10 mA cm−2 and robust long-term stability over 24 h in 0.5 M H2SO4. DFT calculations showed that the Gibbs free energy of adsorbed H∗ over the Ru SAs on PN is much closer to zero compared with the Ru/C and Ru SAs supported on carbon and C3N4 (Figure 10H). Therefore, adjusting the number of coordination atoms of metal centers is considered to be a very effective way to optimize HER performance.In recent years, non-noble-metal-based SACs have also attracted extensive attention. However, compared with Pt based catalysts, their performance still needs to be further improved. Non-noble-metal-based SACs with low coordination environment and unique electronic structure have the potential to replace Pt-based catalysts (Chen et al., 2017a; Zhang et al., 2018). Graphdiyne (GD) is a two-dimensional carbon material with monatomic thickness, which has natural uniform pores, rich triple bonds, and strong reduction ability (Li et al., 2010, 2014). It has been used in various research fields, so GD may also be an excellent candidate as support. Isolated nickel/iron atoms anchored on graphdiyne were fabricated by electrochemical synthesis method with Ni-C and Fe-C coordination, respectively (Xue et al., 2018) (Figure 11A). From the results of ICP-MS, the loading of Ni in Ni/GD is 0.278 wt% and the loading of Fe in Fe/GD is 0.680 wt%. Fe/GD with higher metal laoding exhibits the overpotential of 66 mV at 10 mA cm−2, which is smaller than Ni/GD (88 mV). And their performances are superior to the most state-of-the-art bulk nonprecious catalysts. Meanwhile, the Fe/GD displays more superior durability through 5000 cycling tests. The strong chemical interaction and electronic coupling between single atoms Ni/Fe and GD allow a high charge transfer between the catalytic active center and the support, so the performance of HER is improved.In addition to coordinating with the same type of surrounding atom, the central metal atom can also coordinate with different atoms simultaneously. Mo-based catalysts such as carbide, nitride, and sulfide have attracted a lot of attention because of good performance in HER. Chen and colleagues designed single Mo atoms supported on N-doped carbon for the first time (Chen et al., 2017a), which shows high HER performance with the overpotential of 132 mV at 10 mA cm−2 and onset overpotential of 13 mV in 0.1 M KOH. From AC-STEM and XAFS, it can be seen that Mo1N1C2 was formed by single Mo atom immobilized with one nitrogen atom and two carbon atoms (Figures 11B–11F). More importantly, the active sites Mo1N1C2 showed higher catalytic activity than Mo2C and MoN due to the lowest absolute value of ΔGH∗ in Mo1N1C2 compared with the Mo2C and MoN from the DFT calculation results. Further DOS calculations revealed that the DOS of Mo1N1C2 near the Fermi level was much higher than that of Mo2C and MoN, which was favorable for the charge transfer during the HER process because of higher carrier density. Moreover, tungsten-based catalysts, including WCx (Gong et al., 2016; Wang et al., 2020), WNx (Yan et al., 2015b), WPx (Wang et al., 2016; Xing et al., 2015), WSx (Lin et al., 2014; Lukowski et al., 2014; Merki and Hu, 2011; Voiry et al., 2013), and so on, also have outstanding properties in HER. However, in order to achieve industrial application, it is necessary to further improve HER activity and stability of the tungsten-based materials. W-SAC with W-N1C3 sites supported on MOF-derived N-doped carbon was prepared for HER in both alkaline and acidic media (Chen et al., 2018a). The W-SAC showed excellent stability without attenuation after 10,000 CV cycles. It is determined by HAADF-STEM and XAFS that the atomically dispersed W1N1C3 act as the active site, which plays a significant role in enhancing the HER performance as proved by DFT calculation results. The DOS of W-SAC near the Fermi level is much higher than of WC and WN, leading to a larger carried density for promoting charge transfer in HER (Figures 11G and 11H). Furthermore, the DOS near the Fermi level in W-SAC was mainly contributed by the W d-orbital, whereas the contributions of C and N p-orbital were negligible; this suggested that the single W dispersion as well as unique electronic structure could efficiently enhance the d-electron domination near the Fermi level and enhance the HER catalytic performance. The coordinating atom N is partially replaced by other atoms, which shows a great influence on the local chemical environment of the central atom. Thus, it can be used to optimize the electronic structure of the central atom and is an important method to enhance the activity of SACs by modulating the N coordination to form dual-atom coordination.Oxygen evolution reaction (OER) is another important half-reaction that occurs at the anode during the water electrolysis process, involving four coordinated proton-electron transfer steps. However, there is still a long way to go in the mechanism understanding and material design of OER catalysts. As shown in Figure 12A (Chen et al., 2021), the generally accepted OER reaction mechanisms are the traditional adsorbate evolution mechanism (AEM) and lattice oxygen mechanism (LOM).In AEM, the reaction mechanism of OER is similar to HER, involving the steps of adsorption and desorption. The mechanism can be expressed as follows (Man et al., 2011; Rossmeisl et al., 2007): (Equation 7) ∗ + H 2 O 1 → OH ∗ + H + + e − (Equation 8) OH ∗ → O ∗ + H + + e − (Equation 9) O ∗ + H 2 O ( 1 ) → OOH ∗ + H + + e − (Equation 10) OOH ∗ → O 2 ( g ) + H + + e − Similarly, OER also has a volcanic curve based on oxygen adsorption energy. Noble metal oxides, such as IrO2 and RuO2, showed the best OER performance due to low overpotential. Based on this mechanism, the OER performance can be effectively improved by modifying the catalyst supports, such as introducing heteroatoms through synergistic effects to optimize the electronic structure, creating defects to redistribute surface charges and so on.LOM has attracted much attention in recent years. The lattice oxygen participates in the proton-electron transfer in the reaction process (Huang et al., 2019). The mechanism can be expressed as follows (Rong et al., 2016): (Equation 11) OH ∗ → ( V O + OO ∗ ) † + H + + e − (Equation 12) V O + OO ∗ † + H 2 O 1 →O 2 g + V O +OH ∗ † + H + + e − (Equation 13) V O + OH ∗ † + H 2 O 1 → H O - site ∗ + OH ∗ † + H + + e - (Equation 14) ( H o - site ∗ + OH ∗ ) † →OH ∗ + H + + e − † Parentheses indicate that adsorbates are calculated in the same supercell.In early studies, the involvement of lattice oxygen in OER process under acidic conditions was confirmed by isotope labeling and differential electrochemical mass spectrometry (DEMS) techniques (Kötz et al., 1984; Willsau et al., 1985; Wohlfahrt-Mehrens and Heitbaum, 1987). The O in RuO2 participates in the reaction to form soluble RuO4, which is detected in the solution. Pan and colleagues reported a model system of Si-incorporated strontium cobaltite perovskite electrocatalysts in alkaline solution with similar surface transition metal properties but different oxygen diffusion rates (Pan et al., 2020). The correlation of intrinsic OER activity with oxygen ion diffusion rate and oxygen vacancy diffusion rate are shown in Figures 12B and 12C. The evolution of oxygen correlates with the contribution of the LOM mechanism at different degrees that closely relates to the oxygen ion diffusivity (Figure 12D). This work provides a reference for designing more stable perovskite surfaces to further optimize electrocatalysts. A series of perovskite OER catalysts were also tested through in situ 18O isotope labeling mass spectrometry (Grimaud et al., 2017). The results showed that the O2 generated from the lattice oxygen for some highly active oxides. In combination with experiments and DFT calculations, catalysts with lattice oxygen exchange exhibited pH-dependent OER activity, whereas those without lattice oxygen exchange displayed pH-independent OER activity. LOM shows higher OER activity than the conventional AEM as proved for the ABO3 (A = lanthanum or strontium, B = transition metal) perovskites (Yoo et al., 2018). Activity volcano plots for AEM and LOM of perovskite systems have been established by a simulation work. Furthermore, the LOM is preferred for achieving bifunctional catalysts for OER and ORR.In the HER and OER of water splitting, OER is the core of electrochemical energy conversion. However, OER displays high overpotential during the reaction because of sluggish kinetics, which is the main step of energy consumption. Therefore, high-efficiency electrocatalysts are particularly important to OER (Chen et al., 2019; Liu et al., 2021; Zhou et al., 2019). RuO2 and IrO2 exhibit high catalytic activity for OER in a wide pH value and are often used as benchmarks for OER catalyst evaluation. It was found that the coordination environments of single atom may undergo changes during the process of reaction. Ru single atoms anchored on nitrogen-carbon support (Ru-N-C) were synthesized with Ru1-N4 sites (Cao et al., 2019). The catalyst showed an efficient and durable electrocatalyst for acidic OER with overpotential of 267 mV at the current density of 10 mA cm−2, mass activity of 3571 A gmetal −1, and TOF of 3348 O2 h−1. The dissolution rate of Ru is less than 5% in acid solution due to the outstanding structural stability. The Ru-N-C was employed to measure the overall water splitting in a two-electrode system to mimic the PEMWE, showing superior activity and stability. The dynamic pre-adsorption of single oxygen atom into the formation of O-Ru1-N4 structure with more charge donations of Ru through in situ XAFS and FTIR was also identified (Figures 13A–13F). Theoretical calculations showed O-Ru1-N4 with higher Ru oxidation state as the real active site for the high OER activity in acidic solution. The formed O-Ru1-N4 moieties under operando state exhibited a low barrier of O-O coupling to form the OOH∗ intermediate. It is also an effective way to prepare SACs by creating defects on the support to regulate the coordination environment. Atomically dispersed Ni catalyst on defective graphene (a-Ni@DG) with four-coordination Ni-C4 structure was fabricated through an incipient wetness impregnation method and subsequent acid leaching (Zhang et al., 2018). The Ni loading of a-Ni@DG was around 1.24 wt% by this facile and inexpensive strategy. XAS and DFT calculation revealed that the diverse defects in graphene can induce different local electronic DOS of a-Ni, which suggested that aNi@defect serves as an active site for unique electrocatalytic reactions (Figures 13J–13L). For example, aNi@G5775 and aNi@G585 are responsible for HER and OER with low overpotential and high TOF values and stability, respectively. HAADF-STEM not only confirmed the uniform distribution of single Ni atoms but also observed that aNi trapped in the Di-vacancy provided direct evidence for the Ni-C4 configuration (Figures 13G–13I). Diverse defects can induce different local electronic density of states. Creating specific defects on the support forming various active sites can achieve good catalytic effects for different reactions at the same time.What’s more, bimetallic center SACs have gradually attracted extensive attention from researchers. The synergistic effect of bimetallic centers can optimize the adsorption and desorption of intermediates and reduce the reaction energy barrier. For example, atomically dispersed binary Co-Ni sites embedded in N-doped hollow carbon nanocubes (CoNi-SAs/NC) are synthesized for bifunctional OER and ORR (Han et al., 2019b) (Figure 13M). The rechargeable process of Zn-air batteries is realized efficiently and with low potential and robust reversibility. DFT calculation showed that the uniformly dispersed single sites and synergistic effect of adjacent Co-Ni bimetallic centers optimized the adsorption and desorption process, reduced the overall reaction energy barriers, and finally promoted the reversible oxygen electrocatalysis.In the reported literature, the activity and stability of HER and OER were measured in standard three-electrode system of lab scale. Nonetheless, the catalyst’s performance in the lab scale is somewhat different from that in practical electrolyzers. The practical electrolyzers usually require higher current density and voltage, and the operating environment is more intense. Therefore, the practical electrolyzers put forward higher requirements for the activity and stability of the catalysts.Currently, the electrolyzers used for water electrolysis include alkaline water electrolyzers (AWE), proton exchange membrane water electrolyzers (PEMWE), and anion exchange membrane water electrolyzers (AEMWE). Although traditional AWE has been fully industrialized, it is limited by its environmental friendliness and purity of hydrogen production. PEMWE and AEMWE have received extensive attention due to the high efficiency, high purity of hydrogen produced, and low energy consumption. Influenced by the properties of the membrane and the local pH, PEMWE and AEMWE are suitable for catalysts in acidic and alkaline environments, respectively. The current development of PEMWE is relatively mature in commercial-scale water electrolysis for long-term operation, but the progress of durable catalysts other than Ir-based noble metal OER catalysts in acidic environments remains a great challenge. As more OER catalysts display better performance in alkaline environment, AEMWE shows certain advantages, but the stability of membrane and the design of electrolyzer still need to be further improved to meet the requirements of long-term electrolysis.The membrane electrode assemblies (MEA), composed of the catalytic layer and the proton exchange membrane, is the main site for material transport and electrochemical reaction in the entire electrolytic cell. The characteristics and structure of the MEA directly affect the performance and life of the PEMWE. Hao and colleagues applied the prepared grain boundaries (GB)-TaxTmyIr1-x-yO2-δ nanocatalysts to PEMWE as anode in an acidic condition (Hao et al., 2021). The pretreated carbon paper- (CP) and platinum-plated titanium foam were applied as cathode and anode gas diffusion layers (GDLs), respectively. The MEA were constructed by placing the catalyst-supported Nafion 117 membrane between CP and Pt-plated Ti foam GDLs (Figure 13N). The polarization curves were measured at 50°C, showing the cell voltage of GB-Ta0.1Tm0.1Ir0.8O2-δ is 1.766V to reach the current density of 1 A cm−2 (Figure 13O). Moreover, the PEMWE using GB-Ta0.1Tm0.1Ir0.8O2-δ displayed the outstanding stability at 1.5 A cm−2 for at least 500 h without obvious attenuation (Figure 13P). However, the research of SACs on PEMWE still needs to be further explored.Oxygen reduction reaction (ORR) occurs at the cathode of electrochemical energy equipment through either a 4-electron pathway (O2+4H++4e-→2H2O) or a 2-electron pathway (O2+2H++2e-→H2O2). The four-electron reaction mechanisms are shown as follows (Nørskov et al., 2004): (15) 1 2 O 2 + ∗ → O ∗ → e - + H + HO ∗ → e − + H + H 2 O + ∗ or (16) O 2 + ∗ → e - + H + HOO ∗ → e - + H + H 2 O + O ∗ → e - + H + HO ∗ → e - + H + H 2 O + ∗ The two-electron reaction mechanisms are shown as follows: (17) O 2 + ∗ → e - + H + HOO ∗ → e - + H + H 2 O 2 + O ∗ where ∗ represents the catalytically active sites.The Equations (7) and (8) represent dissociative and associative mechanisms, respectively. Whether the O-O bond breaks during ORR process determines the selectivity of H2O2 or H2O. The 2-electronic process is clean and pollution free, and the hydrogen peroxide produced is an important fine chemical. The 4-electronic process produces water directly, which is mainly used in fuel cells and metal-air batteries (Liu et al., 2018; Tong et al., 2021; Zhou et al., 2021b). However, these two reaction pathways often occur at the same time, resulting in reduced selectivity of the desired product. Thus, the application of SACs in ORR has been extensively studied to improve selectivity, stability, and activity.The precious-metal-group (PMG) catalysts exhibit high-efficiency electrocatalytic performance for ORR. Doping heteroatoms into carbon materials can affect the coordination environment of the metal center atoms, which in turn shows an impact on the catalytic performance. M-N-C catalysts with M-N4 coordination structure are considered as potential catalysts for replacing Pt-based materials in ORR. Single Pt atoms supported on carbon black were fabricated with carbon monoxide/methanol tolerance for ORR (Liu et al., 2017). DFT calculations were used to study the synergetic effect between single Pt atoms and doped-N and the intrinsic activity of the active sites on Pt1-N/BP (carbon black) (Figures 14A and 14B). The results showed that the main effective sites are single-pyridinic-nitrogen-atom-anchored single-Pt-atom centers, displaying highly active and making it one of the most promising sustainable, large-scale alternatives to conventional Pt-NP-based electrocatalysts. The catalyst was employed as a cathode in acid single cell, and the power density reached 680 mW cm−2 at 80°C. The current of Pt1-N/BP still maintains more than 70% when used as cathode in fuel cell after working for 200 h continuously. It shows good stability compared with other non-noble-metal catalysts. Other precious metal-based SACs have also been used in ORR studies. Ir-N-C SAC with Ir-N4 configuration fabricated by host strategy exhibited orders of magnitude higher ORR activity than Ir NPs (Xiao et al., 2019) (Figure 14C). The Ir-SAC was applied in a H2/O2 fuel cell as cathode, showing a higher open circuit voltage of 0.0955V and a power density of 932 mW cm−2. The SEM and HAADF-STEM displayed the morphology and nanostructure of Ir-SAC, and the EDS mapping showed the existence of Ir embedment within the carbon matrix (Figures 14D–14H). The bright spots in Figure 14I corresponded to the atomically dispersed Ir atoms. Atomic structure characterization results and DFT calculations showed that the high activity of Ir-SAC was attributed to the moderate adsorption energy of the Ir-N4 moiety. Single Ru atoms supported on N-doped graphene oxide for ORR in acidic solution were prepared through NH3 atmosphere annealing (Zhang et al., 2017a). The Ru/N-doped graphene showed excellent four-electron ORR activity, stability, and tolerance. Combing the DFT calculations, the Ru-oxo-N4 moieties during the oxidative electrocatalytic condition are responsible for the ORR catalytic activity (Figures 14J and 14K).Recently, transition-metal-based catalysts have emerged as promising alternatives to PMG materials due to the adjustable electronic structure. Fe-N4 is considered to be the best performing non-noble-metal catalyst in ORR. Chen and colleagues performed adjustment of O coordination on Fe SACs to enhance ORR performance. Single iron atoms anchored on N-porous carbon with Fe loading of 2.16 wt% were fabricated through a cage-encapsulated-precursor pyrolysis strategy (Chen et al., 2017b). It can be seen from XANES that Fe is positively charged (Figure 15A). According to the analysis results of FT-and WT-EXAFS, Fe exists in the form of a single atom in the catalyst (Figures 15B–15E). Meanwhile, the corresponding EXAFS R space fitting curves of Fe-ISAs/CN is shown as Figure 15F. The coordination number of Fe is 5 with four N atoms and one O atom. So, the atomic structure model is shown in Figure 15G through further analysis. The Fe as isolated atoms with N coordination showed higher ORR activity than commercial Pt/C and most non-precious-metal materials, which was attributed to the high capability of the single Fe atoms in transferring electrons to the adsorbed OH species demonstrated by first principle calculations. Meanwhile, Fe-ISAs/CN showed superb durability with little change in E 1/2 for 5000 CV sweeps.What’s more, changing the metal central atom is a direct means to regulate the coordination environment, which also affects the catalytic performance. Single Co atom and N co-doped carbon nanofibers with CoN4-G coordination were reported for ORR in both acidic and basis medium (Cheng et al., 2017). TEM and HRTEM showed that the diameter of the prepared catalyst was about 150 nm, and the substrate was amorphous carbon (Figures 15H and 15I). No obvious bright regions were observed in HAADF-STEM (Figure 15J), further indicating the absence of cobalt-containing particles. EDX mapping results showed that Co, N, and C were evenly distributed in CNFs (Figure 15K), and the Co corresponding to bright spots existed as a single atom in AC-HAADF-STEM (Figures 15L and 15M). The Co-N/CNFs displays desirable ORR performance and high stability with negligible decrease of E 1/2 after 10,000 CV cycles. Moreover, the catalyst as cathode reached a power density of 16 mW cm−2 and an outstanding stability with more than 200 h, showing the potential of application. What's more, a series of M-N-C materials (M = Mn, Fe, Co, Ni, and Cu) with atomically dispersed M-Nx sites were investigated the trends in electrochemical H2O2 production from molecular first principles to bench-scale electrolyzers operating at industrial current density (Sun et al., 2019c). Co-N-C catalyst showed outstanding ORR activity and selectivity to H2O2 and more than 4 mol peroxide gcatalyst −1 h−1 at a current density of 50 mA cm−2. The relationship of activity-selectivity and the trend of M-N-C materials was further analyzed by DFT calculations, providing a molecular scale understanding of the experimental volcanic trend of four-electron and two-electron ORR (Figure 15N). Meanwhile, the binding free energy of HO∗ intermediate placed Co-N-C close to the top of the two-electron volcano, retaining catalytic activity while promoting two-electron pathway selectivity.The increasing global environmental crisis has aroused people’s attention to greenhouse gas emissions, conversion, and storage (Anagnostou et al., 2016; Mun et al., 2018; Obama, 2017). CO2 is considered to be the main cause of the greenhouse effect. Electroreduction of CO2 into high value-added products, such as CO, HCOOH, CH4, CH3OH, C2H4, and so on, is a promising route, which can mitigate environmental problems. Because water acts as the medium of CO2RR, HER inevitably becomes the side reaction of this reaction (Li et al., 2017; Yang et al., 2018a). Therefore, efficient catalysts in CO2RR should reduce HER activity and enhance CO2RR activity at the same time (Lin et al., 2019; Yan et al., 2018a).Currently, the key obstacle to the development of efficient CO2RR catalysts is the lack of a basic understanding of surface-mediated electrochemical reactions. There are many possible products in CO2RR, involving electron transfer numbers ranging from CO (2e−) and HCOOH (2e−) to CH3CH2CH2OH (18e−), so the interpretation of the reaction mechanism is more demanding (Lu and Jiao, 2016). Some typical multi-electron reactions in neutral medium are shown as follows: (Equation 18) CO 2 ( g ) + 2 H + + 2 e - →CO ( g ) + H 2 O (Equation 19) CO 2 ( g ) + 2 H + + 2 e - →HCOOH ( 1 ) (Equation 20) CO 2 ( g ) + 4 H + + 2 e - →HCOOH ( 1 ) + H 2 O (Equation 21) CO 2 ( g ) + 6 H + + 6 e - →CH 3 OH ( 1 ) + H 2 O (Equation 22) CO 2 ( g ) + 8 H + + 8 e - →CH 4 ( g ) + 2 H 2 O SACs show high activity in many reactions with the highest atomic utilization rate, especially the unsaturated coordination between the central metal atom and the surrounding atoms, which significantly enhances the catalytic performance. Meanwhile, the uniform active site and geometry structure enhance the interaction between the mental centers and the support, which helps to improve the selectivity of the catalyst (Chen et al., 2018c). Therefore, SACs display great application potential in CO2RR.Reducing the coordination number between metal center and N leads to form unsaturated coordination, which is helpful to optimize the catalytic performance. Zhao and colleagues prepared Ni atoms anchoring on N-doped porous carbon with Ni-N3 coordination by ZIF-assisted strategy for the first time in CO2RR (Zhao et al., 2017) (Figure 16A). The SAC displayed outperforming current density of 10.48 mA cm−2 at an overpotential of 0.89 V with a high turnover frequency (TOF) of 5273 h−1 and Faradaic efficiency (FE) for CO production of over 71.9%. Besides, Zheng and colleagues fabricated an unsaturated coordination copper with nitrogen sites anchored into graphene matrix (Cu-N2/GN) for CO2RR to CO production (Zheng et al., 2019). The catalyst showed higher activity and selectivity with the maximum FE of 81% at a low potential of −0.50 V and an onset potential of −0.33 V than the atomically dispersed Cu-N anchored on carbon materials reported previously. From a practical point of view, the Cu-N2/GN was applied in rechargeable Zn-CO2 battery with a peak power density of 0.6 mW cm−2, and the battery charging process can be powered by natural solar energy. Theoretical calculations showed that the moderate free energy of Cu-N2 sites promote the adsorption of CO2 molecules at the Cu-N2 site (Figures 16B–16E). The adsorption state of H2O, CO2, COOH, and CO with Cu-N2-based DFT electron density was hybridized with surface states of Cu-N2 (Figures 16F–16I). The short bond length of Cu-N2 sites caused the accelerated charge transfer from Cu-N2 site to ∗CO2, which enhanced effectively the formation of ∗COOH and CO2RR performance. Adjustment of N coordination was also applicable for controlling the coordination environment of CO2RR SACs. Bifunctional catalysis of Co and N co-doped hollow carbon for CO2RR and HER has been reported (Song et al., 2018). The loading of Co single-atoms was around 3.4 wt%, and Co-C2N2 moieties acted as the major active sites during the process of CO2 reduction. The catalyst was prepared through high temperature pyrolysis (900°C) to remain the high content of Co single atoms and prevent the loss of nitrogen. The Co-HNC possessed better catalysis performance than Co NP-SNC in 0.1 M KHCO3 (Figure 16J). The Co-HNC showed a nearly 100% FE and high formation rate of around 425 mmol g−1 h−1 at 1.0 V, with the product ration of CO/H2 approximating ideal 1/2 in the potential range from −0.7 to −1.0 V (Figure 16K). Meanwhile, the catalyst displayed the long-term stability for 24 h with negligible degradation of current density (Figure 16L) and the almost identical resistance to the Co NP-SNC (Figure 16M). Potassium thiocyanate (KSCN) poisoning experiment was carried out to confirm the selectively functioning of Co SAs and N-C groups for CO2RR and HER. The CV curves and formation rate change were recorded in Figures 16N and 16O, which showed the gaseous products increased significantly but the CO selectivity of Co-HNC decreased sharply to 9.8%. What’s more, the flow-cell electrolyzer effectively solves the limitation of CO2 dissolution and diffusion in the traditional test device, realizing the highly selective conversion of CO2 under the high current density and accelerating the industrial application of CO2RR technology (Jin et al., 2021a). Yuan et al. designed single Cu atoms anchoring on the graphediyne (Yuan et al., 2022). In situ Raman and DFT calculations revealed that the presence of Cu-C bonds leads to the formation of CH4, more facile during the process of CO2RR. The catalyst also showed high activity and CH4 FE and partial CH4 current density in flow-cell electrolyzer.Ammonia (NH3) is not only a key raw material for main agricultural fertilizers but also shows important applications in chemical engineering and pharmaceutical and synthetic fiber fields (Galloway et al., 2004, 2008; Yang et al., 2020a; Zamfirescu and Dincer, 2008). Currently, the industrial synthesis of NH3 commonly depends on the Haber-Bosch method under high temperature and pressure conditions (300–500°C, 15–30 MPa), consuming more than 1% of the global energy supply annually (Guo et al., 2018; Song et al., 2019; van der Ham et al., 2014a). Moreover, the thermodynamically limited conversion is only ∼15%. Using N2 as raw material, electrocatalytic nitrogen reduction reaction (NRR) realizes the synthesis of ammonia at room temperature and pressure, which exhibits the merits of low energy consumption and without pollution. It provides a green and low-carbon technical route for ammonia synthesis industry. NRR involves a 6e− transfer process: (Equation 23) N 2 ( g ) + 6 H + + 6 e - → 2 NH 3 ( g ) However, the high bond energy of N≡N (940.95 kJ mol−1) is a major obstacle to the NRR process, so it is necessary to develop efficient electrocatalysts, especially SACs, to reduce the reaction energy barrier and accelerate the generation of NH3 (Chen et al., 2018b; van der Ham et al., 2014b; Wang et al., 2017). What’s more, the adsorption of N2 on the catalyst surface is usually not satisfactory, which is not conducive to the formation of intermediates and limits the selectivity and yield of NH3 (Tao et al., 2019). Although many metal-based catalysts have been researched, most metals are too weakly bonded to achieve efficient N2 adsorption and activation, which is generally considered a rate-limiting step for NRR. Meanwhile, NH3 yield and faradaic efficiency (FE) are still far below the requirements of practical application.Carbon-based-material-supported SACs show great application potential toward NRR due to the abundant exposed active sites and high catalytic activity. Single Ru atoms anchored on nitrogen-doped carbon (Ru SAs/N-C) were fabricated by facile pyrolysis method. The Ru SAs/N-C achieved a recorded-high activity in NRR, which possessed an FE of 29.6% for NH3 production with partial current density of −0.13 mA cm−2 (Geng et al., 2018). More importantly, the yield of the SACs reaches 120.9 μgNH3 mgcat. −1 h−1, well above the highest number ever reported (Figures 17A–17D). The stability of Ru SAs/N-C displayed less than 7% attenuation of NH3 yield rate after 12 h potentiostatic measurement. DFT calculation showed that Ru SAs/N-C promoted N2 dissociation, resulting in increased activity relative to Ru NPs/N-C. In addition to noble metals, non-noble-metal SACs have also been studied in NRR. Copper single atoms attached in porous N-doped carbon network (Cu SAC) with Cu-N2 active sites as pH-universal catalyst showed outstanding NH3 yield rate and FE under 0.1 M KOH and 0.1 M HCl conditions (Figures 17E–17H) (Zang et al., 2019). Meanwhile, the Cu SAC also displayed excellent stability over 12 h with little current attenuation. The combination of experiment and first-principles calculations revealed that Cu-N2 coordination acts the effective active sites in NRR catalysis.Up to now, SACs have attracted extensive research interests in a wide range of catalytic fields, including photocatalysis, organic catalysis, electrocatalysis, and environmental aspect. The primary target of the rapid-developing SACs field is reducing the using of precious metals while keeping the catalytic activity. Carbon-based-material-supported SACs display great application prospect because of its low cost, high efficiency, and robustness. In this review, we introduced the synthesis methods and the advanced characterization techniques used in the identification of SACs, mainly concerning X-ray-derived spectroscopy and in situ techniques, which showed important guiding significance for coordination regulation and coordination environment recognition of SACs. In addition, the applications of carbon-based-material-supported SACs were discussed in electrocatalysis fields, including HER, OER, ORR, CO2RR, and NRR. To date, some progress has been made in enhancing catalytic performance of SACs. However, there are still many opportunities and challenges for the prospect of single atoms in the future.Firstly, the low loading of single atom in the SACs prepared by the existing synthesis strategies restricts the development of SACs. The sluggish reaction kinetics need to be overcome through exposing more active sites in catalysis. Low loading SACs may lead to the accumulation of intermediates during the reaction process, resulting in side reactions and reduced selectivity, which is not suitable for industrial scale applications. However, when the metal atom loading increases, the migration and agglomeration of single atoms tends to form nanoclusters or nanoparticles due to its high surface free energy. Therefore, it is imperative to develop SACs with high loading active sites for industrial production. In addition, it is very essential to study the interaction between metal single atoms and support, because the support shows an effect on the loading and electronic structure of single atom. For example, Xia and colleagues used graphene quantum dots as carbon substrates, which were modified with -NH2 groups to improve the coordination activity for metal ions (Xia et al., 2021). The as-prepared transition metal single-atom material achieved a loading of up to 40 wt% and excellent thermal stability. Besides carbon-based materials, two-dimensional material transition metals, such as the sulfides, selenides, phosphides, and so on, have also been studied as carriers for SACs. The electron transfer between metal and carrier can be directly regulated by electronic metal-support interaction (EMSI), thereby regulating the electronic state of the supported metal. Therefore, it is of significance to develop the novel supports of SACs with superior catalytic performance for energy conversions.Secondly, the coordination environments show a great influence on the electronic and geometric structure of the central metal atoms, which plays an important role in the catalytic properties of SACs. Nonmetal heteroatomic doping (N, O, S, P, etc.) is one of the main strategies to regulate coordination environments. However, other elements, such as SE, Te, and halogen elements, are rarely studied and may display unexpected catalytic properties. In addition, the asymmetric distribution of charges may lead to superior performance. Thus, it is imperative to study the dual or more metal center sites. In a word, rationally constructing coordination environments of SACs is significant to boost the catalytic activity, which provides a direct way to understand the intrinsic activity of SACs.Thirdly, the characterization techniques are the significant fundament for the recognition of SACs. At present, the identification of coordination environments relies heavily on XAS, whereas the technique is bulk sensitive and only provides bulk average information. Therefore, it is very important to improve the spatial resolution of characterization technology. Furthermore, in order to determine the active sites of SACs and dynamic changes during the reaction, it is necessary to combine in situ characterization techniques. The dynamic changes of coordination structures and oxidation states of SACs during the catalytic process is worthy of further exploration because it is closely related to intrinsic activity.Finally, the step process and reaction mechanism of single-atom catalytic reaction are still in the preliminary exploration stage. Constructing a reliable structure-activity relationship of catalytic reactions is crucial for designing high-performance SACs. Theoretical simulation is conducive to understanding the structure-activity relationship of catalysts at atomic level. DFT calculation is a powerful tool to explore the atomic structure and intrinsic active sites. In addition, the reaction free energy of each elementary step and the adsorption energies of the intermediates can be obtained from DFT calculation, which is of great significance to the understanding of reaction mechanism. However, some of the proposed mechanisms do not match well with experimental results. More accurate models should be developed to reflect rational catalytic processes. What’s more, DFT calculation combined with machine learning can predict efficient SACs, which shows a positive effect on the prospect of electrocatalysis. We believe that this work can promote the development of single-atom catalysis and deepen readers' understanding of single atoms.This work is supported by the National Key Research and Development Program of China (2021YFA1500500), National Natural Science Foundation of China (Grant Nos. 21822801 and 22005025), and China Postdoctoral Science Foundation (2021M700352).D. Cao and D.J. Cheng supervised the preparation of this review article. D. Cao, D.J. Cheng and H.M. Zhang conceived the topic. H.M. Zhang contributed to the most of the writing, and W.H. Liu contributed to some content and figures. H.M. Zhang and W.H. Liu revised the manuscript. D. Cao and D.J. Cheng revised and finalized the manuscript. All author approved the final version of the manuscript.The authors declare no competing interests.

In recent years, single-atom catalysts (SACs) with unique electronic structure and coordination environment have attracted much attention due to its maximum atomic efficiency in the catalysis fields. However, it is still a great challenge to rationally regulate the coordination environments of SACs and improve the loading of metal atoms for SACs during catalysis progress. Generally, carbon-based materials with excellent electrical conductivity and large specific surface area are widely used as catalyst supports to stabilize metal atoms. Meanwhile, carbon-based material-supported SACs have also been extensively studied and applied in various energy conversion reactions, such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), and nitrogen reduction reaction (NRR). Herein, rational synthesis methods and advanced characterization techniques were introduced and summarized in this review. Then, the theoretical design strategies and construction methods for carbon-based material-supported SACs in electrocatalysis applications were fully discussed, which are of great significance for guiding the coordination regulation and improving the loading of SACs. In the end, the challenges and future perspectives of SACs were proposed, which could largely contribute to the development of single atom catalysts at the turning point.

Data will be made available on request.Imines have wide health and industrial applications, for example, imines show anti-inflammatory and anticancer potential in health treatment [1] and serve as nitrogen-containing building blocks for various industrial chemicals [2]. Traditionally, imines are manufactured via condensation of primary amines and carbonyl compounds (e.g., aldehydes or ketones) [3]. Because of the disadvantages including heavy odor, easy deterioration, difficulty in conservation, handling and transportation, etc., the conventional process is not widely popularized.To overcome those issues, several new approaches have been proposed such as primary amines tethering [4], oxidative alcohols-amines coupling [5,6], secondary amines dehydrogenation [7], and amines' N-alkylation [8]. Among them, the alcohols-amines oxidative coupling method is considered as most promising [5,9]. Compared to aldehydes and ketones, alcohols are cheaper, widely available, more stable and less toxic. The reaction operates at mild temperature and ambient pressure and uses inexpensive molecular oxygen as the oxidant. Water is its sole by-product. Thus, the process is greener and more environmentally benign. So far, various supported heterogeneous catalysts such as Au [10], Pd [11], Pt [12], and Ru [13] have been examined for this reaction. Although, these noble-metal-based catalysts exhibited good performance in alcohols conversion and imine production, the high price and low availability of noble metals is a concern for large-scale application in imine synthesis. Developing transition-metal-based catalysts with comparative performance is much desired.Recently, manganese oxide (MnO x ) has received increasing attention for imine synthesis largely because of its suitable physicochemical properties and relatively low cost [14–20]. Blackburn and Taylor [14] applied manganese oxides for imine synthesis from alcohols along with 4A zeolite for dehydration. Sithambaram and coworkers [15] developed a more efficient catalytic process by using OMS-2 as the bifunctional catalyst [15]. Other manganese-oxide-based catalysts such as MnO x /HAP [5], MnCo2O4–500 [16], Mn1Zr0.5O y [17], and α-MnO2/GO [18] have also been reported. However, relatively high reaction temperature and long reaction time are still required, due to their relatively low activities.One promising approach to enhance the catalytic activity of manganese oxide catalysts is to alter their crystal structures and surface properties by doping transition metal ions into the framework or pore channels of manganese oxides [21–23]. OMS-2 is an allotrope of MnO2, consisting of a one-dimensional tunnel structure. Its 2 × 2 edge is shared with MnO6 octahedra. The size of its tunnel opening is about 0.46 nm (see Fig. 1 ). Within OMS-2, Mn exists mainly as Mn4+, along with a small number of Mn3+ and Mn2+ ions, exhibiting an average oxidation state of ∼3.8 [15]. OMS-2 shows good catalytic performance in some oxidation reactions due to its mixed valence manganese ions, large surface area and opening tunnel structures [15,24]. Doping low-valent metal ions (e.g., M2+, M3+) into the framework of OMS-2 has been widely studied, whereas only a few reported the incorporation of high-valent ions (e.g., M5+, M6+) [25,26]. High-valent metal ions have larger ionic radius and stronger Lewis acidity, which may be in favor of forming active oxygen species on the surface of manganese dioxides. For example, incorporating V5+ ions into OMS-2 enhanced the activity for aldehyde and methane combustion at low temperatures, which was attributed to the increased oxygen vacancies, Lewis acid sites and redox properties [27]. Consequently, vanadium doped OMS-2 may also work well for imine synthesis from oxidative coupling of amine and alcohols.Herein, we present our study for the first time on using vanadium doped cryptomelane-type manganese oxides (V-OMS-2) for imine synthesis via oxidative coupling of benzyl alcohol and aniline. Various characterization techniques such as X-ray diffraction powder (XRD), nitrogen physisorption (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature- programmed reduction (H2-TPR) and temperature-programmed desorption (NH3-TPD) were employed to characterize the catalyst structure. By combining with the reaction results, we were able to establish the correlation of catalyst structure and catalytic activity. The effects of vanadium doping amount and vanadium precursors on the physicochemical properties and catalytic performance of V-OMS-2 catalysts were systemically studied, and their relationship was discussed. The stability and recyclability of the catalyst was also investigated.The undoped OMS-2 was synthesized with a modified reflux method [28]. Typically, 9.9 g of MnSO4·H2O (>99%, Sinopharm) and 3.4 mL of HNO3 (68 wt%, Sinopharm) were dissolved in 35 mL of deionized water (solution-A). Then, 9.9 g of KMnO4 (>99%, Sinopharm) was dissolved in 120 mL of deionized water (solution-B). Next, solution-B was tardily added to solution-A under magnetic agitation. A brown slurry was formed immediately, which was further refluxed at 100 °C for 24 h. After naturally cooling down, the obtained brown-black precipitate was filtered, washed and recovered, and then dried at 110 °C for 12 h. The obtained solid was then treated at 250 °C in air for 2 h to completely remove adsorbed water from the pores.As for the preparation of vanadium doped OMS-2 catalysts, the same procedure was applied. A certain amount of vanadium pentoxide (V2O5, >97%, Sinopharm) or sodium metavanadate (NaVO3, >99%, Sinopharm) was added right after the completion of the solution-B addition. It should be pointed out that the presence of excess nitric acid (∼50 mmol) in the solution-A can ensure the complete dissolution of vanadium pentoxide (0.5–6 mmol) or metavanadate (∼3 mmol). The amount of vanadium pentoxide or sodium metavanadate was adjusted to get 1–12 mol% of V/Mn in the synthetic solution (i.e., vanadium concentration was 0.007–0.08 mol/L). The obtained vanadium doped catalysts are labeled as x%V-OMS-2(y), where x stands for the molar percentage of V over the input Mn in the synthetic solution and y represents the type of vanadium precursor, i.e., vanadium pentoxide (y = 1) or sodium metavanadate (y = 2). For comparison with the literature, the as-synthesized 3%V-OMS-2(1) was also calcined at 400 °C in air for 3 h, and the obtained catalyst was named as 3%V-OMS-2(1)-400.For comparison, low-valent transition metal (M) ions including Cu2+, Ni2+, Co2+, Fe3+, Cr3+ were also doped into the OMS-2 structure using the same procedure, and the molar percentage of M/Mn in the synthetic solution was fixed at ∼3 mol%.The conventional wet impregnation method was also employed to prepare vanadium-doped OMS-2 catalyst at 3 mol%. Typically, 2.0 g of OMS-2 was immersed in a 10 mL aqueous ammonium metavanadate solution (∼ 0.069 mol/L), which was stirred continuously for 2 h, then stood for 24 h, followed by 10 h of drying at 120 °C and another 3 h of roasting at 250 °C. The obtained catalyst is then termed as 3%V/OMS-2.The nitrogen adsorption-desorption isotherm was measured at −196 °C with a Micromeritics ASAP 2020 system. Before analysis, each sample was degassed at 250 °C for 3 h. The standard Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface area, and the Barrett-Joyner-Halenda (BJH) method was employed to evaluate the pore size distribution. The crystal structure of each catalyst sample was analyzed on a Bruker D8 Advance X-ray powder diffractometer using Cu Kα radiation (λ = 0.15064 nm) at 40 kV and 30 mA. The SEM images and EDX spectra were taken on a HITACHI SU8010 electron microscope to analyze the morphology and chemical component of each catalyst sample. Transmission electron spectroscopy (TEM) images were obtained on a JEOL JEM-2100 electron microscope operated at 200 kV. A Perkin Elmer ELAN DRC-e inductively coupled plasma mass spectrometer (ICP-MS) was used to obtain the elemental content of catalyst sample. X-ray photoelectron spectra (XPS) were recorded on a ESCALAB 250Xi (Thermo-Fisher) spectroscope equipped with Al Kα as the exciting X-ray source. The C1s photoelectron peak at 284.8 eV was used as the reference to correct the binding energies of other elements. Hydrogen temperature-programmed reduction (H2-TPR) was performed on a home-assembled flow system equipped with a TCD detector. Typically, 20 mg of sample was loaded in a quartz reactor and treated with air at 250 °C for 1 h, followed by purging with pure N2 (30 mL·min−1) for 0.5 h. After cooling to 30 °C, the 5% H295% Ar mixture gas (30 mL·min−1) was introduced into the reactor, and the H2-TPR profile was recorded at a heating rate of 10 °C·min−1 up to 800 °C. NH3-TPD profiles were obtained on a Micromeritics Autochem 2920 II chemisorption device equipped with a TCD detector.The oxidative coupling of benzyl alcohol with aniline was carried out in a 50 mL three-necked round-bottom flask assembled with a condenser and an air balloon. In a typical run, benzyl alcohol (BA, 0.5 mmol), aniline (0.5 mmol), catalyst (50 mg), and solvent toluene (2 mL) were added into the flask and heated to 80 °C under continuous stirring for a certain time. As shown in Fig. S1, the mass transfer limitation can be excluded when the agitation speed is 800 rpm and higher. Therefore, all experiments were carried out at the stirring speed of 800 rpm. After reaction, the reactor was cooled down immediately and the solid catalyst was recovered by centrifugation. The obtained liquid products were analyzed by a gas chromatograph (GC-7890B) assembled with a FID detector and a Rtx®-5 type capillary column (30 m × 0.32 mm × 0.25 um) in the presence of dodecane as the internal standard. The calculated carbon balance (C-balance) was above 97% in all the reactions. The benzyl alcohol conversion, imine selectivity [17], yield, and catalyst activity (the formation rate of imine) were computed via the following equations: (1) Conversion % = n BA 0 − n BA t n BA 0 × 100 % (2) Imine Selectivity % = n imine t n BA 0 − n BA t × 100 % (3) Yield % = conversion × imine selectivity × 100 % (4) Act . − W = n imine t W cat × t mmol · g − 1 · h − 1 weight − based Activity (5) Act . − A = n imine t S cat × t mmol · m − 2 · h − 1 surface area − based Activity where n BA 0 is the moles of benzyl alcohol in the feed (mmol), n BA t is the moles of benzyl alcohol (mmol) and n imine t is the amount of imine (mmol) in the liquid at the reaction time of t (h), W cat is the mass of catalyst used for the reaction (g), S cat is the total surface area of catalyst used for the reaction (m2).The N2 adsorption-desorption isotherms of V-OMS-2(1) samples with different V/Mn ratios are shown in Fig. 2 . Both the OMS-2 and 1%V-OMS-2(1) samples exhibited the IUPAC defined type-II isotherm, while it became less distinct for the 3%, 6% and 12%V-OMS-2(1) samples. Generally, the hysteresis loop in the type-II isotherm appears at the relative pressures of 0.8 < P/P0 < 1.0. Its shape represents the difference in the relative proportions of voids or pores between particles. Such a hysteresis loop for the 3%, 6% and 12%V-OMS-2(1) became much smaller. Meanwhile a new hysteresis loop formed at the relative pressure range of 0.4 < P/P0 < 0.8, indicating the emergence of mesoporous structure in these samples. Their corresponding BJH pore size distribution curves are shown in Fig. S2. Table 1 displays the physical properties of each sample including surface area (SA), pore volume and pore size. The specific surface area of undoped OMS-2 is about 86 m2·g−1, of which ∼16% is related to micropores (14 m2·g−1). Its micropore volume is only 0.0067 cm3·g−1, which is <2% of the total pore volume (0.43 cm3·g−1), suggesting that the contribution of micropores is negligible. Doping vanadium into OMS-2 increased the surface area, especially the non-microporous surface area. The surface area of V-OMS-2(1) samples reached to the highest at 6 mol% V doping (SA = 241 m2·g−1), then decreased slightly at the vanadium content of 12 mol% (SA = 206 m2·g−1). On the other hand, the pore volume and average pore size of V-OMS-2(1) samples decreased gradually with the increase of V/Mn ratio. A remarkable decrease was observed when the V/Mn ratio was higher than 6 mol%. This may be correlated with the decrease in the particle size and the crystallinity of these samples [28]. Fig. 3 compares the N2 adsorption-desorption isotherm and pore size distribution of the undoped OMS-2, 3%V-OMS-2(1), 3%V-OMS-2(2) and 3%V/OMS-2 samples. All samples display the type-II isotherm, except the 3%V-OMS-2(1) sample which exhibits the feature of type-IV isotherm. Compared to the undoped OMS-2, the pore size of 3%V-OMS-2(2) and 3%V/OMS-2 became slightly larger (Fig. 3B), whereas their surface areas and pore volumes including micropore surface area and micropore volume were smaller (Table 1 , entry 7 and 8). Over the 3%V-OMS-2(1) sample, the pore size distribution became much narrower, showing a significant decrease in the pore size (20 → 9.2 nm). On contrast, its surface area remarkably increased (86 → 179 m2·g−1), whereas the change in the pore volume was little (0.43 to 0.41 cm3·g−1).The XRD patterns of the OMS-2 and V-OMS-2(1) samples with different V/Mn ratios are presented in Fig. 4 . They all were alike to the tetragonal structure of natural cryptomelane-type manganese oxide (OMS-2, JCPDS 20–0908), confirming the success in obtaining and preserving the 2 × 2 tunnel structured manganese oxide phase. The diffraction peaks at 2θ = 12.8, 18.1, 28.8, 37.6, 42.0, and 50.1o are assigned to (110), (200), (310), (211), (301), and (411) of the OMS-2 crystal phase, respectively. With V/Mn ratio increasing, these diffraction peaks became broader while the intensities dropped significantly. It indicates that V5+ ions are embedded into the OMS-2 skeleton, resulting in a decrease in the OMS-2 crystallinity. When the V/Mn ratio was higher than 6 mol%, only one broad diffraction peak at 2θ = 38.7o was detected, showing a significant loss of the OMS-2 crystal structure. It suggested that doping large amount of vanadium could impede the formation of OMS-2 structure.It should be highlighted that no additional diffraction peaks related to any vanadium species were detected in all the samples studied, suggesting the doped vanadium species were well-dispersed either on the surface or within the framework of OMS-2 [26]. Fig. 5 compares the XRD patterns of the OMS-2 and 3%V-OMS-2 samples prepared with different vanadium precursors. All samples exhibited similar XRD patterns, matching well with the standard tetragonal structure of cryptomelane-type MnO2 (JCPDS 20–0908). The intensity of diffraction peaks decreases as follows: OMS-2, 3%V/OMS-2 > 3%V-OMS-2(2) > 3%V-OMS-2(1). The weaker diffraction peaks suggest that more vanadium ions were incorporated into the OMS-2 structure, resulting in the reduction of the OMS-2 crystallinity. Such a phenomenon was also reported over Ce-doped OMS-2 catalysts [29].TEM and high-resolution TEM (HRTEM) images of the OMS-2, 3%V-OMS-2(1), 3%V-OMS-2(2) and 6%V-OMS-2(1) samples are presented in Fig. 6 . The OMS-2 showed a nanorod-like morphology with the length of 200–400 nm. The 3%V-OMS-2(1) and 3%V-OMS-2(2) samples both exhibited similar nanorod-like morphology as the OMS-2, but with a much shorter length of 50–100 nm and a slight larger width. Similar morphological changes can also be observed in the SEM images of these samples (Fig. S3). Through the HRTEM images, we can observe the well-defined lattice planes of these samples. Over OMS-2, the lattice fringe is 0.69 nm, which can be attributed to the (110) planes. In the 3%V-OMS-2(1) and 3%V-OMS-2(2) samples, the fringe spacing was 0.31 nm, which is assigned to the (310) plane.As for the 6%V-OMS-2(1) sample, nanoparticles sized from 5 to 20 nm were formed and served as building blocks to construct the flake or spherical agglomerates (Fig. 6D). The nano crystallites of 6%V-OMS-2(1) sample were randomly oriented, indicating extremely short-range order of lattice structure (Fig. 6D1). The lack of diffraction rings or spots in the selected area of electron diffraction pattern (Fig. 6D, inset) confirmed the poor crystal phase of this sample, which is consistent with the XRD result. No other separated phases were identified in both TEM and SEM images of all the samples, further confirming the high dispersion of vanadium species.Mn 2p, V 2p and O 1 s XPS spectra of the OMS-2 and V doped OMS-2 samples are shown in Fig. 7 . The Mn 2p3/2 spectra of all samples (Fig. 7A) are very broad and asymmetrical, suggesting the coexistence of various manganese species. Thus, we conducted the peak deconvolution based on the literature [30] and obtained three characteristic peaks at 640.5, 641.7 and 643.0 eV, corresponding to surface Mn2+, Mn3+ and Mn4+ species, respectively. Then their composition was computed accordingly and is presented in Table 2 . The undoped OMS-2 contains mainly Mn4+ on the surface (79.5%) along with 17.2% Mn3+. Doping vanadium results in the increase of Mn3+ composition. It is 21.3% Mn3+ in the 3%V-OMS-2(1) and further to 27.3% in the 6%V-OMS-2(1) sample, along with the decrease of both Mn4+ and Mn2+. It means the replacement of Mn with V favors the formation of Mn3+ ions, which may be beneficial to the catalytic activity, as the Mn3+/Mn4+ pair is important for catalyzing oxidation reaction [28].The binding energy of V 2p3/2 was 517.0 eV for all V-OMS-2 samples (Fig. 7B), close to the binding energy of vanadium pentoxide reported in the literature [31], indicating that the doped vanadium species are mainly in V5+ state. The O 1 s spectra can be deconvoluted into three peaks at 529.5 eV (α peak), 531.0 eV (β peak) and 533.0 eV (γ peak), corresponding to the lattice oxygen (Osat), the surface-adsorbed oxygen (Ounsat) and the oxygen in the surface-adsorbed water, respectively [32]. As shown in Table 2, the ratio of Ounsat/(Osat + Ounsat) for the OMS-2 was 0.293. The value increased with the increase of vanadium doping, implying that the more vanadium atoms were incorporated into the framework of the OMS-2, the more adsorbed oxygen species on the surface. The Ounsat/(Osat + Ounsat) ratio of 6%V-OMS-2(1) (0.391) was higher than that of 3%V-OMS-2(1) (0.339), indicating that the 6%V-OMS-2(1) catalyst possessed more surface-adsorbed oxygen species.EDX spectra of the undoped OMS-2, 3%V-OMS-2(1), 3%V-OMS-2(2) and 3%V/OMS-2 samples are shown in Fig. 8 . The K, Mn and O peaks are clearly observed over the undoped OMS-2 (Fig. 8A). A new peak emerged at 4.95 keV over the V doped OMS-2 samples, confirming that V was successfully introduced into OMS-2. Compared to 3%V-OMS-2(2), 3%V-OMS-2(1) had less K, O and V, but higher Mn content (Table S1), indicating a chemical composition difference when using a different vanadium precursor.The bulk V/Mn ratios for all 3%V-OMS-2 samples measured by ICP and the surface V/Mn ratios for the 3%V-OMS-2(1) and 3%V-OMS-2(2) samples estimated by XPS are summarized in Table 1. The bulk V/Mn ratios of all samples were very close to the input for the synthesis. It suggests that all vanadium from the feedstock were incorporated into OMS-2. On the other hand, the 3%V-OMS-2(2) catalyst showed identical V/Mn ratio on the surface to that in the bulk value (3.0 vs. 3.2), implying that the doped vanadium in this sample was uniformly dispersed. For the 3%V-OMS-2(1) sample, however, its V/Mn ratio on the surface was only 1.9, much smaller than that in the bulk. It indicates that more vanadium atoms located inside the channels/framework of OMS-2. Replacing framework Mn atoms with V could inhibit the growth of OMS-2 crystals, resulting in poor crystallinity and more structural defects, which is in a good agreement with XRD and TEM results. Additionally, the 3%V-OMS-2(1) catalyst showed higher Ounsat/(Osat + Ounsat) ratio (0.339) than that of 3%V-OMS-2(2) sample (0.303). It implies that vanadium pentoxide is a better vanadium precursor in terms of generating more active surface oxygen species by adsorbing oxygen molecules from air on the defective sites [33,34]. More active surface oxygen species could promote the oxidative coupling of benzyl alcohol and aniline to imine.The redox properties of the OMS-2 and V-OMS-2(1) samples with different V/Mn ratios were examined by H2-TPR, which are shown in Fig. 9 . The OMS-2 exhibited three reduction peaks at the temperature ranged from 250 to 400 °C, which is labeled as α (at 335 °C), β (at 359 °C) and γ (at 376 °C), respectively. The α peak is attributed to the reduction of the surface oxygen species [35]. Whereas the β and γ peaks are attributed to the sequential reduction of MnO2 → Mn3O4 and then Mn3O4 → MnO, respectively [36]. Doping vanadium changes the reduction behavior of the OMS-2 material (see Fig. 9 and Table 3 ). The 1%V-OMS-2(1) samples exhibited a similar H2-TPR profile to OMS-2, with a slight shift of both β and γ reduction peaks to a lower temperature. While over the 3, 6 and 12%V-OMS-2(1) samples, the H2-TPR profiles changed significantly. First, the α reduction peak gradually shifted to lower temperature and became larger with the increase of vanadium content, which means the increase in the surface oxygen species with doping vanadium into OMS-2 [28], in good agreement with the XPS results. Lower reduction temperature also suggests the increase in the mobility of oxygen species [28,37]. Second, the β and γ peaks became distinctly separated, due to a considerable shift of the γ reduction peak to a higher temperature. It indicates that with the presence of vanadium, the reduction of Mn3O4 species becomes harder. On the other hand, the β reduction peak became relatively smaller, which may suggest the decreased amount of MnO2 in OMS-2 with vanadium doping. In fact, such a change is consistent with XPS results shown in Table 2, where the Mn4+ content decreased and the Mn3+ content increased with the increase of vanadium doping. Third, a new reduction peak was observed at 550 °C over the 6%V-OMS-2(1) sample and it became larger over the 12%V-OMS-2(1) sample. Since V−O bond is stronger than MnO bond, a higher temperature is required for the reduction of vanadium oxide [37,38]. Thus, this new peak can be ascribed to the reduction of vanadium species, although they were not detected by XRD. Furthermore, the presence of vanadium oxide species on the external surface may hinder the reduction of surface-adsorbed oxygen species. As a result, only a broad reduction peak at 321 °C (i.e., merged α and β peaks) was obtained over the 12%V-OMS-2(1) sample.Based on H2-TPR profiles, the H2 consumption was estimated and is listed in Table 3. The H2 consumption was 10.9 mmol·g−1 for the undoped OMS-2, which is slightly smaller than the theoretical value for the complete reduction of MnO2 to MnO (11.5 mmol·g−1). Over the V-OMS-2(1) samples, the H2 consumption decreased with the increase of vanadium doping, being 10.7, 9.8, 9.3 and 8.5 mmol·g−1 for 1, 3, 6 and 12%V-OMS-2(1) samples, respectively. The decreased H2 consumption could be attributed to the formed Mn3+ species induced by the introduction of vanadium, as suggested by XPS.The influence of vanadium precursor on the redox property of prepared V-OMS-2 catalyst was also studied by H2-TPR. The results are shown in Fig. 10 . Although all four samples consumed similar amount of hydrogen during the reduction (see Table 3), their H2-TPR profiles are distinguishably different. The 3%V/OMS-2 sample prepared with wet impregnation method exhibited the most different reduction behavior. All the three reduction peaks (i.e., α, β, and γ peaks) are delayed to higher temperatures and the β and γ peaks are even merged into one peak at 450 °C. Considering that the reduction of vanadium oxides generally occurs above 500 °C [38], it indicates that the vanadium species introduced by impregnation method are mostly located on the external surface of OMS-2, which retards the reduction of manganese oxides in OMS-2.Compared to 3%V-OMS-2(1), 3%V-OMS-2(2) exhibited the α peak at a relatively higher temperature, but slightly lower than that of OMS-2. It suggests that the mobility of surface oxygen species over 3%V-OMS-2(2) is better than OMS-2 but worse than 3%V-OMS-2(1). The reduction temperature of both β and γ peaks shifted to a higher temperature, whereas the γ peak changed more, resulting in a relative more separation of the β and γ peaks in comparison to the OMS-2 sample, but still less than that in the 3%V-OMS-2(1) sample. It implies that the formation of Mn3+/Mn4+ pair in the 3%V-OMS-2(2) sample is more than that of OMS-2, but less than that of 3%V-OMS-2(1) sample, which is consistent with XPS results (Fig. 7 and Table 2).It is reported that weak acid sites of a catalyst play an important role in the imine synthesis in oxidative coupling of benzyl alcohol and aniline [15,39]. This is because the weak acid sites can interact strongly with aniline, which is a weak base. Consequently, the NH3-TPD experiments were carried out to determine the amount and strength of the weak acid sites on the vanadium doped OMS-2 catalysts. The obtained NH3-TPD profiles are presented in Fig. 11 and the estimated weak acid sites are listed in Table 3 (the value for individual peak is shown in Table S2). All the profiles can be deconvoluted into four peaks named as A, B, C and D. According to literature [40,41], the A and B peaks are attributed to weak Brønsted acid sites and the C and D peaks are related to Lewis acid sites.As shown in Table 3, the OMS-2 sample has more Lewis acid sites (0.50 mmol·g−1) than the weak Brønsted acid sites (0.15 mmol·g−1), which is consistent with that reported in literature [42]. After doping vanadium, both weak Brønsted acid sites and Lewis acid sites significantly increased as compared with the original OMS-2. The total weak acid sites were 1.21, 1.45, 1.40 and 1.16 mmol·g−1 for 1, 3, 6 and 12% V-OMS-2(1) samples, respectively. The maximum amount of acid sites was obtained over the 3%V-OMS-2(1) sample, which is more than double of the acid sites on the undoped OMS-2. As indicated by XRD and TEM, doping vanadium higher than 3 mol% could result in worse crystallinity and smaller particles. It may be the reason for the decrease in the acid sites over the 6 and 12% V-OMS-2 samples.We also compared the effect of vanadium precursor on acid properties of V-OMS-2 catalysts by NH3-TPD. Fig. 12 shows the NH3-TPD profiles of the undoped OMS-2, 3%V-OMS-2(1), 3%V-OMS-2(2) and 3%V/OMS-2 samples. Their corresponding acid sites are computed and listed in Table 3. The total acid sites decrease as 3%V-OMS-2(1) (1.45 mmol·g−1) > 3%V-OMS-2(2) (0.79 mmol·g−1) > OMS-2(1) (0.65 mmol·g−1) > 3%V/OMS-2 (0.57 mmol·g−1). 3%V-OMS-2(1) sample has the acid sites 80% more than the 3%V-OMS-2(2). Whereas the 3%V/OMS-2 sample shows even less acid sites than the undoped OMS-2, suggesting that loading vanadium by wet impregnation has negative impact on the acidity of OMS-2. The NH3 desorption on the 3%V-OMS-2(1) sample appears at lower temperatures among the four samples, implying that the acidity on 3%V-OMS-2(1) is the weakest. The results indicate that more and weaker acid sites can be obtained when vanadium pentoxide is used as the precursor.The catalytic performances of various transition metal doped OMS-2 catalysts have been examined for the imine synthesis from oxidative coupling of benzyl alcohol and aniline with air. The results are shown in Table S3. Compared to OMS-2 catalyst, only the V doped OMS-2 showed a significant promotion in the benzyl alcohol conversion and imine yield, while doping other transition metals including Cu, Ni, Co, Fe and Cr all resulted in a decrease in the catalytic performance. Thus, the vanadium doped OMS-2 catalysts are investigated more in detail for this reaction.As shown in Table 4 (Entry 1–8), doping vanadium enhanced catalytic activity of the V-OMS-2(1) catalysts for oxidative coupling of benzyl alcohol and aniline to imine. The benzyl alcohol conversion increased with V/Mn ratio, reached the maximum at the V/Mn ratio of 3 mol%, then dropped gradually with the further increase of vanadium content. The weight-based activity (Act.-W) also followed the same trend. The high activity of the V-OMS-2(1) catalyst could be attributed to the improvement in the surface area, surface oxygen species (or Mn3+/Mn4+ pair), weak acid sites, and relatively smaller particle size as the characterization results indicated (see Section 3.1).In order to assess the contribution of catalyst surface area, the surface-area-based activity (Act.-A) was calculated and is also presented in Table 4. The Act.-A of the undoped OMS-2 was 0.020 mmol·m−2·h−1. When 1 and 3 mol% V was doped to OMS-2, the Act.-A became 0.024 and 0.016 mmol·m−2·h−1. The results exhibited a different trend from that observed with the weight-based activity. It suggests that the surface area may play an important role in determining the catalyst activity for the imine synthesis reaction. Although the 1%V-OMS-2(1) catalyst showed a slightly better Act.-A than the 3%V-OMS-2(1) catalyst, the 3%V-OMS-2(1) catalyst exhibited a much higher benzyl alcohol conversion, imine yield and Act.-W, which can be attributed to its significantly larger surface area (179 vs. 95 m2·g−1 in Table 1).In fact, we did obtain same surface-area-based activity over the V-OMS-2(1) catalysts when the reaction was conducted over the catalysts with the same surface area (please see the discussion in Section 3.2.3).When more vanadium was doped, a decrease in the Act.-A was observed. It was 0.011 and 0.009 mmol·m−2·h−1 for the 6 and 12%V-OMS-2(1) catalyst, respectively, which is smaller than that of OMS-2 catalyst, and much smaller than that of the 1%V-OMS-2(1) catalyst. However, both the 6 and 12%V-OMS-2(1) catalysts showed better performance than OMS-2 catalyst, especially the 6%V-OMS-2(1) catalyst, which was even better than the 1%V-OMS-2(1) catalyst in terms of benzyl alcohol conversion, imine yield and Act.-W. Considering the significantly higher surface area of 6%V-OMS-2(1), it can be further concluded that the surface area plays an important role in the imine synthesis over the V-doped OMS-2 catalysts.It should be pointed that the 6%V-OMS-2(1) catalyst possessed higher surface area and more surface-adsorbed oxygen species and similar acid sites than the 3%V-OMS-2(1) catalyst as suggested by BET, XPS, H2-TPR and NH3-TPD characterizations, however, its catalytic activity was less than the 3%V-OMS-2(1) catalyst. Same phenomenon was also observed on the 12%V-OMS-2(1) catalyst versus the 1%V-OMS-2(1) catalyst. The XRD results clearly showed the loss in the OMS-2 crystal structure of the 6%V-OMS-2(1) and 12%V-OMS-2(1) catalysts (Fig. 4). It indicates that the preservation of the OMS-2 structure is also crucial for this reaction.To understand the effect of vanadium precursor, the synthesized 3%V-OMS-2(2) and 3%V/OMS-2 catalysts were also examined for the oxidative coupling of benzyl alcohol with aniline at the same reaction conditions, which are also presented in Table 4 (Entry 9–10). Over the undoped OMS-2 catalyst, the benzyl alcohol conversion was 60% and the imine yield was about 52%. Over the impregnated 3%V/OMS-2 catalyst, the catalytic activity dropped significantly to 23%. The characterization results showed that on this catalyst, the vanadium species are mainly located on the external surface of OMS-2, resulting in a decrease in the surface area. Moreover, the impregnated vanadium decreased the acid sites of the catalyst as indicated by the NH3-TPD characterization. As a result, the benzyl alcohol conversion decreased much. It indicates that the impregnated vanadium species have no promotion for the imine synthesis from oxidative coupling of benzyl alcohol and aniline. On the contrast, both 3%V-OMS-2(1) and 3%V-OMS-2(2) exhibited higher catalytic activity for the reaction, the benzyl alcohol conversion of which was 94% and 70%, respectively. Compared to the 3%V-OMS-2(2) catalyst, the 3% V-OMS-2(1) catalyst prepared with vanadium pentoxide possessed much higher catalytic activity, which could be associated with its larger surface area, better mesopore structure, shorter nanorod morphology, the substantially increased acid sites, more substitution of framework Mn species with vanadium and greatly enhanced surface reactive oxygen species as suggested by BET, XPS, SEM, TEM, H2-TPR and NH3-TPD.Since our catalysts were only treated at 250 °C, a conventionally calcined catalyst, the 3%V-OMS-2(1)-400 catalyst was also prepared by calcination at 400 °C and evaluated for the oxidative coupling of benzyl alcohol and aniline. Compared to the 3%V-OMS-2(1) catalyst, both benzyl alcohol conversion and imine yield of the 3%V-OMS-2(1)-400 catalyst were significantly lower. Although 3%V-OMS-2(1)-400 showed an improved crystal structure than 3%V-OMS-2(1) (Fig. S4), its acid sites were much less (Fig. S5). It suggests that high temperature calcination could reduce the catalyst acid sites, which may be responsible for the decrease in the catalytic activity of the 3%V-OMS-2(1)-400 catalyst.As discussed above, the 3%V-OMS-2(1) catalyst showed the best performance among the catalysts studied. Consequently, we conducted the reaction condition optimization on this catalyst. We first studied the influence of solvent. As shown in Table 5 , the solvent plays a crucial role in this reaction. Non-polar and aprotic toluene is particularly efficient in providing both high benzyl alcohol conversion and imine selectivity. Although a high benzyl alcohol conversion was obtained when acetonitrile was used, the selectivity of imine was low (Entry 7). Dichloroethane led to a high imine selectivity, but the benzyl alcohol conversion was poor (Entry 8). Other solvent, such as THF, isopropanol and dioxane, were also examined. Both the benzyl alcohol conversion and the imine selectivity were low (Entry 9–11).Second, the effect of reaction temperature was investigated. After 24 h of reaction at 30 °C, the benzyl alcohol conversion was only 29% and the selectivity of imine was 86% (Table 5 , Entry 3). With increasing temperature, both the benzyl alcohol conversion and the selectivity of imine improved greatly. For example, after 12 h of reaction at 60 °C, the benzyl conversion and the selectivity of imine were 97% and 93%, respectively (Table 5 , Entry 2). When the temperature was raised to 80 °C, 99% of benzyl alcohol conversion and 93% of imine selectivity were achieved with only 4 h of reaction (Table 5 , Entry 1).We have also examined the effect of aniline-to-BA ratio. When the aniline:BA ratio was changed from 1:1 to 1.5:1, the benzyl alcohol conversion increased from 94% to 97%, and the selectivity of imine increased from 90% to 95% (Table 5 , Entry 4 and 5). Further increasing the aniline:BA ratio to 2:1, the benzyl alcohol conversion and the imine selectivity reached 99% and 100%, respectively (Table 5 , Entry 6). Although adding more aniline is beneficial, the improvement in the conversion and imine selectivity is not substantial. Table 6 compares the catalytic performance of our 3%V-OMS-2(1) catalyst with those reported in literature [5,10,13,16,39,43]. The 3%V-OMS-2(1) catalyst considerably outperforms the other catalysts including some noble-metal-based catalysts. The reaction time to achieve the similar benzyl alcohol conversion and imine yield is remarkably shorter. Its Act.-W value is significantly better than the other catalysts (Table 6 , Entry 3–7). Over the 3%V-OMS-2(1) catalyst, the highest benzyl alcohol conversion (∼99%) was attained after 4 h of reaction, where the imine yield was as high as 92% (Table 6 , Entry 2). The highest Act.-W of 2.83 mmol·g−1·h−1 (Table 6 Entry 1) was achieved at the reaction time of 3 h in this work, which is more than double of the best Act.-W (1.24 mmol·g−1·h−1 [16]) for the Mn-based catalyst reported in literature. Although the Ru/Zn1@Ui-66 catalyst showed a higher Act.-W (4.12 mmol·g−1·h−1 [43]), it consumes more aniline and uses pure oxygen for the reaction.As discussed above, the incorporation of vanadium can induce the change in the surface area, acid sites, Mn3+ component and active surface oxygen species of the OMS-2, thus resulting in a difference catalytic performance in oxidative coupling of benzyl alcohol and aniline to imine. To clarify which factor plays a more important role, we further studied the effect of the catalyst amounts on the reaction performance over the OMS-2 and 3%V-OMS-2(1) catalysts. As shown in Table 4 (entry 1 and 2), when the amount of OMS-2 increased from 50 mg to 104 mg (so that the total surface area of OMS-2 catalyst is the same as that of 50 mg 3%V-OMS-2(1) catalyst), the conversion of benzyl alcohol increased from 60% to 83%, and the yield of imine increased from 52% to 73%. Its surface area-based activity (Act.-A) was 0.014 mmol·m−2·h−1, which is very close to that of the 3%V-OMS-2(1) catalyst at the same surface area level for the reaction. On the other hand, when we used 24 mg of 3% V-OMS-2 (the same surface area as that of 50 mg OMS-2) for the reaction, the benzyl alcohol conversion dropped to 69% and the corresponding imine yield was 60%. The Act.-A of the 3% V-OMS-2(1) catalyst became 0.023 mmol·m−2·h−1, which is also close to that of the OMS-2 catalyst (0.020 mmol·m−2·h−1,) at the same surface area level for the reaction. The results disclose that the improved catalytic performance of the 3%V-OMS-2(1) catalyst mainly related to its increased surface area. In other words, the increased surface area induced by vanadium doping plays a major contribution to the enhanced activity of the V-OMS-2 catalyst.We have further investigated the relationship between the catalytic activity of V-doped OMS-2 catalysts and their redox property (Mn3+ component and active surface oxygen species) and acidity. Fig. 13 shows the surface area-based activity (Act.-A), the total acidity, the fraction of surface Mn3+ ions and Ounsat species of the catalysts as a function of V/Mn ratio in the V-doped OMS-2 catalysts. Both the activity and the total acidity showed the same trend with the increase of V/Mn ratio (Fig. 13A), showing higher Act.-A associated with higher acidity, i.e., a proportional relationship. It suggests a positive correlation between the catalyst activity and the total acidity. On the contrast, we observed a tradeoff between the catalyst activity and the fraction of surface Mn3+ ions and Ounsat species (Fig. 13B). Both the activity and the fraction of surface Mn3+ ions and Ounsat species increased with the addition of vanadium. When the V/Mn ratio was higher than 3 mol%, although the fraction of surface Mn3+ ions and Ounsat species kept increasing, the catalyst activity decreased sharply instead. The XRD and TEM results showed a degradation of OMS-2 structure at higher V/Mn ratio. Thus, it indicates that preserving the OMS-2 structure is critical for achieving positive impact of the fraction of surface Mn3+ ions and Ounsat species on the catalyst activity.To understand the reaction pathway over the 3%V-OMS-2(1) catalyst，the dependence of catalytic activity on the reaction time was investigated. As shown in Fig. 14A, at the beginning of the reaction, the selectivity of imine was relatively low, whereas the selectivity of benzaldehyde was much higher. The initial reaction rate was about 7.1 mmol-BA·g−1·h−1. With prolonging the reaction time, the selectivity of imine increased, while the selectivity of benzaldehyde decreased. After 2 h of the reaction, the selectivity of imine and benzaldehyde changed little. The conversion of benzyl alcohol reached 99% in 4 h with the corresponding imine yield of 92%. We have also tracked the conversion of aniline and the production of related products from aniline conversion (Fig. 14B). The yield of the imine (i.e., N-benzylideneaniline, PhC=NPh) was almost overlapped with the aniline conversion, along with the detection of small amount of azobenzene (PhN=NPh) byproduct (< 3%), which is generated from the self-coupling of aniline reactant. The results indicate that imine (i.e., N-benzylideneaniline) is the main product, and the benzaldehyde is a primary product generated from the selective oxidation of benzyl alcohol.According to our experimental results and previous reports [15,18,39], the air-oxidative coupling reaction between benzyl alcohol and amine over 3%V-OMS-2(1) possibly proceeds with two consecutive steps, that is, oxidation of benzyl alcohol to benzaldehyde, followed by condensation with aniline to form imine, as depicted in Scheme 1 . In this process, the 3%V-OMS-2(1) serves as a bifunctional catalyst to catalyze these two distinct steps (i.e., selective oxidation and condensation), which is closely related to the catalyst's surface area and acid sites as discussed in the structure-activity relationship section.In order to provide further mechanistic insights, three key control experiments including (i) aerobic oxidative of benzyl alcohol without aniline, (ii) aniline without benzyl alcohol, and (iii) condensation of benzaldehyde with aniline were carried out under the same reaction conditions and the results are presented in Fig. 15 . In the control experiment (i), the benzyl alcohol was readily converted without the presence of aniline and the conversion increased with the increase of reaction time. The initial reaction rate was 5.7 mmol·g−1·h−1. Benzaldehyde was the sole product at the selectivity of 100%. The BA conversion in this experiment was lower than that with the presence of aniline (conversion in Fig. 14A). The result suggests that the presence of aniline is indispensable to the formation of N-benzylideneaniline and can promote the conversion of BA to benzaldehyde by consuming benzaldehyde to produce imine.Without the presence of benzyl alcohol, aniline can also be readily converted, but at a much slower reaction rate (control experiment (ii)). The initial reaction rate was about 2.0 mmol·g−1·h−1. The aniline conversion gradually increased within 2 h and became stable at about 25% with the reaction time suggesting a reaction equilibrium under the conditions studied. More important, the reaction product was azobenzene (100% selectivity), indicating a self-coupling reaction of aniline.Compared to single reactant reactions in control experiment (i) and (ii), we observed that the condensation of benzaldehyde and aniline to imine proceeded much faster (the control experiment (iii)). The conversion of benzaldehyde reached >90% in 1 h. The initial conversion rate was about 28.9 mmol·g−1·h−1. With prolonging the reaction time, the conversion of was stabilized at about 92%, suggesting a reaction equilibrium was reached. The imine selectivity was 100% throughout the reaction time in this experiment.Based on the above results, a reaction scheme with competing pathways has been proposed, which is illustrated in Scheme 2 . In the oxidative coupling of benzyl alcohol and aniline over the 3% V-OMS-2(1) catalyst, imine is the main product (92% selectivity), whereas benzaldehyde and azobenzene are the by-products, the selectivity of which was 6% and 2%, respectively. The initial conversion rate for the condensation of benzaldehyde and aniline reaction (∼ 28.9 mmol·g−1·h−1, Step 2 in the Scheme 1) is >5 times faster than that of the benzyl alcohol oxidation reaction (∼ 5.7 mmol·g−1·h−1, Step 1 in the Scheme 1), and is >14 times faster than that of aniline self-coupling reaction. The results clearly display that the oxidation of benzyl alcohol to benzaldehyde, i.e., the Step 1 in the Scheme 1 is the rate-determining step to the formation of imine.Since both the initial conversion rates in the oxidation of benzyl alcohol and the condensation of aldehyde with aniline reactions are much greater than that of aniline self-coupling reaction, it is reasonable that only trace amount of azobenzene was detected in the oxidative coupling of benzyl alcohol with aniline. It should also be noted that we did not observe any evidence related to disproportionation.As shown in Scheme 1, benzaldehyde is the primary product (step 1), which then quickly reacts with aniline to form imine via a fast condensation reaction (step 2). Therefore, appropriately increasing the ratio of aniline: BA at the beginning of the reaction can change the reaction equilibrium and promote the synthesis of imine. As confirmed by the results in Table 5 (entry 4–6), the imine yield increased from 85% to 99% when the aniline:BA ratio increased from 1:1 to 2:1. The azobenzene yield changed little as the reaction rate of aniline self-coupling is much slower, requiring longer time to reach the equilibrium. Reaction temperature can largely affect the reaction rate. As a result, a low reaction temperature normally requires a longer reaction time to achieve the same level of imine yield (Table 5 , entry 2–4).To further understand the role of the surface oxygen species and air atmosphere, we have performed additional experiment on the oxidative coupling of benzyl alcohol with aniline over 3%V-OMS-2(1) catalyst under an N2 atmosphere. The results are shown in Fig. 16 . The quick conversion of benzyl alcohol and the production of imine was observed for first hour of reaction. Then, the conversion of benzyl alcohol significantly slowed down and stabilized at about 45% after 3 h of reaction. The oxidation process can be speeded up again by replacing N2 with air. The benzyl alcohol conversion rapidly increased to 81% within 1 h by introducing air. The results suggest that surface oxygen species play an important role in the rate-determining step of the imine synthesis as the oxidation occurs in the Step 1. The surface-activated oxygen species on the 3%V-OMS-2(1) can oxidize benzyl alcohol to benzaldehyde under an inert atmosphere until complete consumption. The presence of molecular oxygen in air can interact with the defective sites of 3%V-OMS-2(1) to re-generate surface-activated oxygen species which keeps the oxidation reaction ongoing continuously. It proves that the regenerable surface-activated oxygen species on 3%V-OMS-2(1) are the active sites for the oxidation coupling of benzyl alcohol with aniline and the acid sites are responsible for the imine formation via benzaldehyde-aniline condensation, which is basically consistent with other manganese-based catalyst reported in literature [17].After reaction, the used 3%V-OMS-2(1) catalyst was recovered by filtration and washed sequentially with a small amount of dichloroethane and ethyl acetate for two times in turn, followed by drying overnight in a vacuum oven at 50 °C. After being treated at 250 °C in air for 2 h again, the regenerated catalyst was then used in the next run. As shown in Fig. 17 A, no obvious change in the benzyl alcohol conversion and imine yield was observed in the repeated four runs. Furthermore, the following experiment was conducted to verify whether there was a V and Mn leaching during the reaction process: the solid catalyst was removed by filtration after 1 h of reaction and then kept the filtrate to continue for another 3 h under the same reaction conditions. As shown in Fig. 17B, the reaction halted right away after the catalyst was removed. No appreciable loss of V and Mn was detected in the solution by ICP-MS.The spent catalyst was analyzed by N2 physisorption. As shown in Fig. S6, the fresh and spent catalysts possessed almost identical N2 adsorption-desorption isotherm and hysteresis loop, indicating that the pore structure of the catalyst was not affected during the reaction. As compared with the fresh catalyst, only a slight decrease in the specific surface area of the spent catalyst was observed, whereas the pore volume and the pore size were almost the same (Table 1). The decrease in the surface area may be caused by the adsorption of some products or/and reactants on the catalyst surface. The XRD patterns shown in Fig. S7 suggest that the characteristic crystal structure of the fresh catalyst was well preserved after reaction. Mn 2p, V 2p and O 1 s XPS spectra of the fresh and spent catalysts are shown in Fig. S8. The similar spectral shapes and binding energies suggest that the valence and bond states of Mn, V and O did not change after the reaction. Moreover, the molar ratios of various elements on the surface of the spent catalyst remained the same as compared with the fresh catalyst (see Tables 1 and 2 ). The above results confirm that the 3%V-OMS-2(1) catalyst is highly stable and recyclable for the reaction, which makes it promising for practical application in imine synthesis from oxidative coupling of alcohols-amines with air.The effect of vanadium doping amount on the structure and catalytic activity of the V-OMS-2 catalyst for imine synthesis from oxidative coupling of benzyl alcohol and aniline was studied in this work. Doping proper amount of vanadium not only improves the surface area and acid sites, but also increases the amount of Mn3+ component, the mobility and amount of active oxygen species on the OMS-2 surface. As a result, the catalytic activity for imine synthesis from benzyl alcohol and aniline was greatly enhanced. The high specific surface area is found to be the key contributor for the high catalytic activity of 3%V-OMS-2(1). Additionally, the crystallinity of OMS-2 may also play an important role in the imine synthesis. Doping large amount of vanadium (> 6 mol% of V/Mn) hindered the formation of OMS-2 crystal phase, leading to a drop in the catalytic activity for the imine synthesis.Furthermore, the influence of vanadium precursor on the structure and catalytic performance of 3%V-OMS-2 catalyst was also investigated. Compared to sodium metavanadate, vanadium pentoxide was a better precursor for the 3%V-OMS-2 catalyst, which exhibited much higher catalytic activity for the reaction. The enhancement could be attributed to its larger surface area, the substantially increased acid sites, unique mesoporous structure and increased active surface oxygen species.The developed 3%V-OMS-2(1) catalyst exhibited exceptional catalytic performance for imine synthesis. The high imine yield of 92% was achieved at the 99% of benzyl alcohol conversion with 4 h of reaction. The highest activity was 2.83 mmol·g−1·h−1, much better than those reported in literature. In addition, the catalyst showed high stability and good recyclability for the reaction. Thus, it is promising for practical application in the imine synthesis via air oxidative coupling of benzyl alcohol and aniline.Xiaohui Guo contributed to data curation and writing - original draft;Mengke Li contributed significantly to investigation;Peizheng Zhao contributed to methodology and resources;Xiaoxing Wang contributed to formal analysis and writing - review & editing;Qinghu Tang contributed to the conceptualization, supervision, project administration, funding acquisition and visualization.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 key research project funded by the Department of Education of Henan Province (19A150030) is greatly acknowledged. Supplementary material Image 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2022.106540.

A series of vanadium doped cryptomelane-type manganese oxide (V-OMS-2) catalysts were prepared by a simple, low-cost reflux method, and investigated for one-pot imine synthesis from oxidative coupling of benzyl alcohol and aniline with air. The physicochemical properties of the V-OMS-2 catalysts were characterized by various techniques including XRD, BET, SEM, TEM, XPS, H2-TPR and NH3-TPD. It was found that the surface area, Lewis acid sites, the amount of Mn3+ component and active surface oxygen species were much improved with vanadium doping. Consequently, the activity of V-OMS-2 catalyst for oxidative coupling of benzyl alcohol and aniline to imine was enhanced. The highest conversion and the imine yield were obtained over the 3 mol% V-OMS-2 catalyst, being ∼99% and 92%, respectively. Higher vanadium doping (≥ 6 mol%), however, hindered the preservation of OMS-2 crystal structure, leading to a drop in the catalytic performance. The high specific surface area was suggested to be the key contributor to the high catalytic activity of 3% V-OMS-2(1) catalyst. Among the vanadium precursors studied, the catalyst prepared with vanadium pentoxide exhibited a much higher catalytic activity, which can be attributed to its larger surface area, unique mesoporous structure, increased Lewis acid sites and more readily available surface oxygen species. In addition, the stability and recyclability of the catalyst were also studied, and the reaction mechanism was discussed.

In the last two decades, Metal-Organic Frameworks (MOFs) have revolutionized the application fields of nanoporous materials in both academic and even industrial contexts [1,2]. In the case of heterogeneous catalysis, the discovery of MOFs ended (at least, potentially) the limitations of having metal centers of any nature, in any proportion and in practically any chemical environment within porous networks, closing the gap between homogeneous and heterogeneous catalysis [3–5], and found correlation between their structural features and their catalytic performance [6,7]. The other great limitation of the porous solid catalysts versus homogeneous catalysts is the reactant and products diffusion [8,9]. One of the most successful strategies to combat the diffusion problems is a drastic reduction of crystal size [9,10].For so many reasons, MOF-74 is one of the most widely studied MOF materials. From a catalytic point of view, the most interesting properties of this material are the existence of open metal sites, its versatility in metal composition [11–14] and the possibility of being prepared with the smallest crystal size ever described for a porous material [10]. Fig. S1 shows the perpendicular view of the hexagonal shaped pores of Zn- and Cu-MOF-74 materials having a diameter of approximately 1 ​nm. The nanocrystalline form of the MOF-74 materials have shown to have much higher catalytic performance than their micron-sized counterparts prepared by conventional solvothermal methods [15,16]. The synthesis methodology of such nanocrystalline M-MOF-74 materials is an important advance with respect to the conventional ones in terms of economic and energetic sustainability, as it is prepared practically instantaneously, at room temperature and with high atomic economy (high yield and the metal/linker ratio coinciding with the stoichiometry found in MOF-74) [10,17]. However, the ‘Achilles heel’ of such methodology is the nature of the solvent, which is the non-volatile N,N-dimethylformamide (DMF), and which should be removed/exchanged by tedious washing procedures. Other attempts to prepare M-MOF-74 at room temperature in a solvent as sustainable as water (although in the presence of an stoichiometric amount of a base for deprotonating the organic ligand) led either to non-nanocrystalline MOF-74 [18] or to the impossibility of preparing some M-MOF-74 material such as Cu probably because of trend of Cu(II) to form Cu(OH)2 phases, even at moderate pHs [19]. In this context, more sustainable preparation methodologies of nanocrystalline M-MOF-74 materials are demanded. Fortunately, MOF-74 is also very versatile in its preparation media. For instance, it is not so common that a give MOF material could be synthesized in four different solvents: DMF, water, THF [20,21] and methanol [21,22].The use of methanol as the unique solvent for the preparation of MOF-74 at room temperature has been described only for the Cu-based material [22], whereas it has been used as a co-solvent (only for metal source) in the preparation of different M-MOF-74 at very low temperature (−78 ​°C) [21]. Compared with DMF, the most conventional solvent in the synthesis of MOF-74 [11,23,24], including at room temperature [10,15,17], methanol possesses some very attractive sustainable properties such as much lower boiling point, much lower price, much higher availability, etc. Compared with water, which is the reference solvent in terms of sustainability, methanol is more volatile (easier to be activated) and, more importantly, MOF-74 can be prepared in methanol without adding any chemical specie beyond the essential metal and linker sources, whereas the assistance of a base acting as a deprotonating agent is compulsory in the synthesis in aqueous solution. Not less, methanol is the universal solvent used in the washing protocols, which normally lasts six days, making washing procedure very much unsustainable than the synthesis procedures themselves. The preparation of MOF-74 in methanol would minimize the effort in the washing/activation protocols. In this work, attempts of preparing M-MOF-74 (M ​= ​Mg, Mn, Co, Ni, Cu, Zn and Cd) materials at room temperature in methanol are described. To make the procedure ever more sustainable, a metal/linker ratio of 2:1, which is the MOF-74 stoichiometry, was used instead of the conventional 2.6:1 ratio. The successfully-prepared Cu-, Co- and Zn-based MOF-74 materials (at room temperature, after really short times, with high yields and with small crystal size) were fully characterized and catalytically tested in the styrene oxidation to benzaldehyde.Benzaldehyde is one of the most industrially demanded aromatic aldehydes, as it is used in so many applications such as flavoring agent in the food industry, reagent for the pharmaceutical industry or an intermediate for the production of perfumes and dyes or industrial solvent [25]. It is commonly obtained as a byproduct of the oxidation of toluene in the synthesis of benzoic acid or by hydrolysis of benzylidene chloride [26]. Some recent works have focused on the production of benzaldehyde by the oxidation of the olefin styrene with different peroxide-based oxidants like H2O2 [27] o tert-butylhydroperoxide (TBHP) [28]. This reaction gives superior yields to benzaldehyde and has the extra key advantage of being heterogeneously catalyzed, which implies ease for recovery, reactivating and reusing the catalysts. Some MOF-74 materials, such as the Cu- and the Co-based ones, have been used to catalyze this reaction with O2 as the oxidant, with selectivities of 100 and 35% for benzaldehyde and conversions of 0.6 and 47% respectively [29]. On the other hand, Mn-MOF-74 gave a conversion of 95% and selectivity to benzaldehyde of 55% using TBHP as an oxidant [28]. Therefore, the influence of the nature of the metal on the catalytic activity seems to be evident, whereas it is expected that other variables such as reaction time and temperature, type and amount of oxidant, type of solvent and amount of catalyst have also marked influence. In this work, we have tested the catalysts Co-, Cu- and Zn-MOF-74 prepared at room temperature in methanol in this reaction. After optimizing different reaction conditions, the activity of Cu-MOF-74 surpassed that of their counterparts, with notable styrene conversion (57%) and selectivity to benzaldehyde (65.4%). Moreover, unlike the structural transformation of the catalyst Cu-MOF-74 described in some other Fine Chemistry reactions [15,22,30], diffraction techniques indicate that the structure is preserved after reaction.The attempts of preparing M-MOF-74 (M ​= ​Cd, Cu, Co, Mn, Mg, Ni and Zn) materials were carried out following a methodology already described for Cu-MOF-74 [22] at room temperature, in methanol (MeOH) as the unique solvent, and a metal/2,5-dihydroxyterphthalic acid (H4dhtp) ratio of 2, which is the stoichiometry found in the final structure of MOF-74. A solution of H4dhtp (0.20 ​g, 1.0 ​mmol)) in MeOH (6.67 ​g, 208 ​mmol) was added dropwise over a solution of M(CH3COOH)2·xH2O (2.0 ​mmol) in MeOH (3.33 ​g, 104 ​mmol) during a period of 10 ​min with constant agitation at room temperature (23 ​°C). Such mixture of solutions provokes the immediate appearance of a precipitate. The agitation was kept for 20 ​h, at room temperature, after which the crystalline solid was recovered by centrifugation and washed with 10 ​mL of MeOH five times. The solid was kept immersed in 10 ​mL of MeOH for 6 days and exchanged for the same amount of fresh MeOH three times.Powder X-ray diffraction (PXRD) data were collected under ambient conditions on Bruker D8 Discover diffractometer using Cu Kα1 (λ ​= ​1.5406 ​Å) and with Bragg-Brentano configuration that operates at a voltage of 40 ​kV and a current of 40 ​mA. It has a fast multichannel LYNXEYE XE-T detector that discriminates fluorescence. Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectra were recorded using a Thermo Nicolet Nexus 670 FTIR spectrometer equipped with a SensIR Technologies Duras-amplIR horizontal ATR accessory and a liquid nitrogen-cooled MCT detector. Thermogravimetric analysis (TGA) was registered from 25 to 900 ​°C with a heating rate of 20 ​°C/min under air flow using a Perkin-ElmerTGA7 instrument. Morphology studies were carried out in an ultrahigh resolution Philips XL 30 SEM instrument with a tungsten filament. N2 adsorption/desorption isotherms were performed on a Micromeritics ASAP 2420 device at −196 ​°C; previously, the samples were activated at 150 ​°C for 18 ​h under high vacuum. The surface area was estimated by applying the BET method to the experimental adsorption points registered at low p/p0. Micropore size distributions were estimated applying Hovarth-Kawazoe (cylinder geometry) method to a different N2 isotherm, being registered only the adsorption branch in a Micromeritics ASAP 2020 device a −196 ​°C from very low p/p0 (10−7) up to p/p0 ​= ​0.1.Before initiating the catalytic experiment, the M-MOF-74 material was activated at 150 ​°C and 10−3 ​bar for 1 ​h. The styrene oxidation reactions were carried out in a batch glass reactor of 50 ​mL, under atmospheric pressure and continuous stirring. The reagents were added in the following order: activated MOF-74 (100 ​mg), acetonitrile (10 ​mL), styrene (5 ​mmol, 0.52 ​g) and finally oxidant (10 ​mmol of tert-butylhydroperoxide, TBHP, 5.5 ​M in n-decane). Different aliquots were taken from the reaction media at different reaction times (between 0 and 4 ​h). Finally, the reaction mixture was centrifuged to recover the catalyst, which was dried and analyzed by PXRD. For specific catalytic tests, all of them carried out with Cu-MOF-74 as catalyst, H2O2 (50 ​wt% in aqueous solution) was used as the oxidant, TBHP/styrene molar ratio was varied to 1:1 and 3:1, the amount of catalyst was 0 ​mg, 50 or 150 ​mg, and the reaction temperature was modified to 45 or 82 ​°C.The catalyst-free aliquots were analyzed by gas chromatography (GC) using an Agilent 6890 HP instrument with a Carbowax column (25 ​m ​× ​0.25 ​mm and 0.25 ​μm), He flow of 2.5 ​mL/min, FID detector at 250 ​°C and heating ramps of 8 ​°C/min from 40 to 180 ​°C and of 10 ​°C/min from 180 to 210 ​°C.The mixture of the metal acetate solution and the H4dhtp solution, both in methanol, led to the immediate formation of a solid. The XRD patterns of the resultant solids after maintained the mixture under magnetic stirring overnight (20 ​h) are plotted in Fig. 1 . The diffractograms of the samples having MOF-74-like structure are plotted on Fig. 1-left, whereas Fig. 1-right shows the XRD patterns of the samples with either impure MOF-74 or those samples that do not contain MOF-74 phase. The XRD pattern of the Zn-based sample matches well with that generated from a Zn-MOF-74 cif file. The XRD pattern of the sample Cu-MOF-74 is substantial different to these of the other M-MOF-74 due to the structural differences arisen from the notable Jahn-Teller effect in the octahedral coordination of the Cu(II) within the framework [31,32]. Such Jahn-Teller effect, which is evident in Fig. S1, is a geometric distortion of a non-linear molecular system that reduces its symmetry and energy, being typical of octahedral coordination, particularly in Cu environments. The matching of the XRD pattern of our Cu-MOF-74 with that of the theoretical Cu-MOF-74 (Fig. S2) is also very good. The diffractogram of Co-MOF-74 scarcely contains a few reflections, which are very broad and very little intense, but they also match well with the pattern of the simulated Co-MOF-74 (Fig. S2). The reason behind such so ‘poor’ diffraction is simply the extremely nanocrystalline nature of this sample and it is not necessarily related to low quality of the sample, in good agreement with what has been described for this material prepared in DMF at room temperature [10]. The nanocrystallinity degree of these MOF-74 samples prepared at room temperature seems to depend on the nature of the metal, as the same order in crystal/domain size (Co ​< ​Cu ​< ​Zn) was found either in DMF [10,15] or in methanol (this work).The Mg-, Mn-, Ni- and Cd-dhtp samples, whose XRD patterns are shown in Fig. 1-right, were formed by other phases. Mg-dhtp phase is amorphous, Mn-dhtp is a non-identified poor crystalline phase and Cd-dhtp is an unknown crystalline phase. That is why these samples were discarded for further characterization and for testing them as catalysts. Only Ni-dhtp sample contains an important proportion of MOF-74 phase, perhaps it is even pure. This Ni-based sample was characterized by FTIR spectroscopy, TGA and N2 adsorption/desorption isotherm (non-shown) under the same conditions than these used for Co-, Cu- and Zn-MOF-74 (see below), and all the achieved results agrees the possibility of such sample could be a very nanocrystalline (pure) Ni-MOF-74. Nevertheless, because of the very low resolution of its XRD pattern together with the presence of some reflections of doubtful origin (but they could be perfectly due to a very ordered coordination of methanol to the open metal sites), we have decided not to go further with this sample in order to avoid, for instance, any misinterpretation of its catalytic behavior.Some attempts to obtain the formation of Cu-MOF-74 were also carried out in other common alcohols, particularly ethanol and isopropanol, but the phase MOF-74 was not detected. All remaining characterization is only shown for Co-, Cu- and Zn-MOF-74 samples, which were also the only ones tested in the catalytic oxidation of styrene to benzaldehyde. Fig. 2 shows the 700-1000 ​cm−1 region of the FTIR spectra of these three samples compared with that of the organic linker in its acidic form H4dhtp. Such region is quite sensitive to the conformational and/or local environment of organic molecules, in such a way that it could be considered as a fingerprint region. Because of the different environment of the organic linker dhtp when it is tetraprotonated (H4dhtp, 2,5-dihydroxyterephthalic acid) and when it is forming the MOF material (anion dhtp4−), the corresponding FTIR spectra are radically different. Moreover, the FTIR spectra of the samples Zn-MOF-74, which was unequivocally identified as MOF-74 by XRD pattern (Fig. 1), and Co-MOF-74, whose structural identification by diffraction techniques generates reasonable doubts (Fig. 1), have practically the same pattern, confirming that both samples have the same local linker environment; in other words, both have the same short-range structure. The only real difference between these two spectra is the broadening. Co-MOF-74 is formed by so small crystal size that affects the IR peak width, in spite of infrared spectroscopy provides information at relatively short range [33]. On the other hand, the FTIR spectrum of the sample Cu-MOF-74 somehow reminds these of the Zn- and Co-MOF-74 samples but, at the same time, it possesses marked differences. Such features must be due to the structural differences between them, as a consequence of the marked Jahn-Teller effect found in Cu-MOF-74 [31,32], which also makes completely different the XRD patterns beyond the two most intense and lowest angle reflections (Fig. 1 and S2).Thermogravimetric analysis (TGA) profiles of the M-MOF-74 prepared in methanol are shown in Fig. 3 . The first weight loss is due to methanol (which was both synthesis and washing solvent) within the pores. The main weight loss, which is attributed to removal/combustion of the linker dhtp, is directly related to the MOF decomposition. According to such assignment, the order of the thermal stability of the three MOFs is Cu ​< ​Co ​< ​Zn, which is in good agreement with the literature [10,34]. There is also good agreement on the temperature values at which the different M-MOF-74 decomposition takes places as well as on the shape of the TGA curves and for the linker/residual weight ratio. The thermal stability of any of these samples (decomposition of the less stable Cu-MOF-74 starts above 225 ​°C under air flow) should in principle be enough for being used as catalysts in the conversion of styrene to benzaldehyde under the reaction conditions of this work (below 82 ​°C).One of the most valued physicochemical properties of nanoporous catalysts is their textural properties. Fig. 4 compares the N2 adsorption/desorption isotherms of these sustainable M-MOF-74 materials. All of them possess outstanding (micro)porosity, reaching BET surface areas of the same order to these published in the literature for high-quality MOF-74. In addition, it is well known that the nanocrystallinity entails a decrease of the microporosity in porous materials [10,35]. In particular, the estimated BET surface area was 702, 925 and 1013 m2g-1 for Zn-, Co- and Cu-MOF-74, respectively. For some unknown reasons, it is quite common that Zn-MOF-74 material has lower surface area compared to its counterparts based on other divalent metal ions of similar atomic weight and prepared under the same experimental conditions [10,11]. That is also the case of this series of samples. Furthermore, Cu- and especially Co-MOF-74 samples, but not Zn-based one, have certain mesoporosity, as evidenced by the presence of notable hysteresis loops in their isotherms. Such mesoporosity must be of intercrystalline nature, which is in good agreement with other nanocrystalline M-MOF-74 materials prepared in DMF also at room temperature [10,15]. The so small crystals and/or crystalline domains forming these two samples are unstable in an isolated form, and then they are aggregated (rather than agglomerated) in very consistent samples (see SEM images in Fig. 5 ), leaving meso-holes of relatively homogeneous size, which provokes the appearance of the hysteresis loops in the mesoporous region of their isotherms (Fig. 4). Supporting this interpretation, the order of the amount of mesoporosity (Co ​> ​Cu ​> ​Zn) follows the inverse order to that found for crystal size (Zn ​> ​Cu ​> ​Co). Another N2 isotherm of the sample Co-MOF-74 (and also of Zn-MOF-74 for comparison purposes) was registered at low range p/p0 (10−7 – 10−1) in order to further support/deny the microporous nature of this sample that is so hard to structurally characterize by diffraction techniques (Fig. 1 and S1). Fig. S3 shows such isotherms and makes clear that the Co-dhtp is undoubtedly of microporous nature and that its pore diameter (centered at ca. 9.3 ​Å) is quite close to that expected. The relatively large width of this peak must be again attributed to their very small crystalline domains.Representative SEM images of the three M-MOF-74 are shown in Fig. 5. It is obvious that the observed particles in the samples Co- and Cu-MOF-74, whose size is well below micrometer scale, are composed by agglomerates/aggregates formed by a large number of nanocrystals. This is in good agreement with samples similarly prepared in DMF as solvent [10,15], as suggested by the large broadening of the XRD reflections (Fig. 1 and S1) and even of the FTIR bands (Fig. 2), and by the hysteresis loops in the mesopore region of the N2 isotherms (Fig. 4). In contrast, the Zn-MOF-74 material is formed by isolated needle-like crystals, with length of a few micrometers. The morphology of the sample Zn-MOF-74 is associated to two characterization features seen previously: (i) the relatively sharp reflections found in the XRD pattern of this sample (Fig. 1); and (ii) the absence of any adsorption in the mesoporosity region, leading to a type-I N2 isotherm at −196 ​°C (Fig. 4), since these isolated crystals cannot generate any intercrystalline porosity. This singularity of Zn-MOF-74 somehow is also related to a couple of precedents. On the one hand, large crystals of Zn-MOF-74 are formed in the synthesis at room temperature in water using NaOH as deprotonating agent [18]. On the other hand, in spite of the samples Zn-MOF-74 prepared in DMF at room temperature forms agglomerates like the rest of M-MOF-74 samples, it is formed by the largest crystals of series M-MOF-74 and its crystals are scarcely fused, unlike for instance the sample Co-MOF-74, in which the nanocrystalline domains are completely fused in very large particles [10].Only the three samples undoubtedly formed by MOF-74 phase and whose characterization have been described in detailed above, that is, Co-, Cu- and Zn-MOF-74 materials, were catalytically tested. The formation of benzaldehyde from styrene requires an oxidant and ideally catalytic redox centers, which are provided by the catalyst. (A scheme in the Supplementary Information -Scheme S1- shows the most accepted mechanism on the synthesis of benzaldehyde through the styrene oxidation using TBHP as an oxidant and MOF-74 or relative materials as catalysts [27,28,36]). Therefore, it is expected that the Zn-based MOF-74 sample is not active in this process, as Zn does not have redox nature. In any case, it could be used as a kind of blank experiment.Given the interest of this work in the catalytic synthesis of benzaldehyde, results and discussion of the catalytic performance will focus on the yield to benzaldehyde rather than in styrene conversion or selectivity. Before studying and comparing the catalytic activity of the three M-MOF-74 in the synthesis of benzaldehyde, the reaction conditions were optimized using Cu-MOF-74, which was selected based on its good catalytic performance in different oxidation reactions of organic compounds [15]. Fig. 6 shows the kinetics of yield to benzaldehyde under different reaction conditions. (The detailed data is given in Table S1). Thus, the nature of the oxidant (Fig. 6A) has marked influence on the yield even though the comparison was carried out between two very similar oxidant species, hydrogen peroxide H2O2 and tert-butylhydroperoxide (TBHP). Using the latter as the oxidant, a notable yield of 37.3% was achieved, which is five times higher than the yield reached when H2O2 was the oxidant. Since both species possess the same oxidant group (peroxide), we believe that the reason behind so dramatic difference in catalytic performance must be related to the ‘solvent’ of the peroxide sources, in particular, to the presence/absence of water. These peroxides must be stabilized with a solvent; otherwise they spontaneously react. The used TBHP in this work is basically water-free as it is diluted in n-decane whereas H2O2 is diluted in water (50 ​wt % H2O2). Similar behavior has been found in the oxidation of other olefin, cyclohexene, with the same family of catalysts (but prepared in DMF at room temperature) [15]. Polar H2O molecules (and presumably not these of n-decane) could be serious competitor of reactants (styrene and H2O2) to be coordinated to the Lewis acid open metal sites of the MOF-74. The negative influence of water in the reaction media of oxidation reactions has been also made clear in the oxidation of olefins catalyzed by Ti-containing nanoporous materials [37,38].Once optimized the nature of the oxidant, its content in the reaction media was also studied (Fig. 6B). A higher concentration of TBHP does not necessarily produce higher yield to benzaldehyde. A TBHP/styrene molar ratio of 2 improves the yield to benzaldehyde versus a ratio of 1, but the yield decreases when the ratio is increased to 3. In spite of the oxidation mechanism of this reaction could be not completely clear, there is a general agreement about the need of both reactants to reach a given open metal site in order to the reaction occurs; therefore, it could make sense that a disproportionate excess of one of the reactants could be counterproductive for catalytic performance purposes. Fig. 6C evidences notable influence of the amount of catalysts on the yield to benzaldehyde. Increasing the amount of the catalyst is beneficial to some extent (for instance, in the catalyst content range from 0 to 100 ​mg). However, further increase induces the contrary effect (for instance, from 100 ​mg to 150 ​mg). It must be highlighted the different shape of the kinetics curves in the presence and in the absence of any catalyst. When Cu-MOF-74 is present in the reaction media, practically all benzaldehyde is formed at the very beginning of the reaction (only during the first 30 ​min), whereas benzaldehyde production continues growing for at least 2 ​h in the blank experiment. In other words, Cu-MOF-74 indeed accelerates the formation of benzaldehyde until it somehow becomes inactive.The last optimized parameter was the reaction temperature (Fig. 6D). Once again, an increase of this parameter is initially favorable, in such a way that the yield to benzaldehyde is significantly enhanced at 75 ​°C in comparison with that achieved at 45 ​°C. However, when the reaction temperature was increased further, up to 82 ​°C, the maximum temperature as it is the boiling point of the solvent acetonitrile, the yield decreases, due to the selectivity to other undesired products in this work (such as styrene oxide or phenylacetaldehyde) is favored.Once some of the reaction parameters were optimized (Fig. 6), the three M-MOF-74 and a blank experiment (with no catalyst) were compared under such optimized conditions (TBHP as oxidant, in a molar ratio of 2 with respect to styrene, with 100 ​mg of catalysts and at 75 ​°C as reaction temperature) (Fig. 7 ). The most active catalyst is in the synthesis of benzaldehyde is Cu-MOF-74. Indeed, after 4 ​h, which is the longest tested reaction time, it is the only catalyst that surpasses the blank experiment in the reached yield to benzaldehyde. As expected, the Zn-MOF-74 catalyst, which is free of any redox center, acts as an inhibitor rather than as a catalyst [15]. On the other hand, although Co-MOF-74 could be also considered as an inhibitor because its yield to benzaldehyde is lower than that given by the blank experiment, this material, unlike Zn-MOF-74, shows indications of its real catalytic role. At reaction time shorter than 30 ​min, its yield to benzaldehyde is significantly higher than the yield detected in the experiment with no catalyst. It is important to note that the reaction conditions have been optimized for Cu-MOF-74, so it should not be ruled out that Co-MOF-74 give higher yield to benzaldehyde than the blank under its own optimized conditions. In any case, it seems clear that Cu-MOF-74 is the best MOF-74-based catalyst for this reaction, just like it was found in the related reaction oxidation of cyclohexene with TBHP.In order to value the quality of these results, Fig. 8 compares the yield to benzaldehyde obtained in this work with these achieved by other M-MOF-74 published elsewhere [28,29]. After analyzing the outstanding influence of certain reaction parameters in the final yield to benzaldehyde, the comparison shown in Fig. 8 must be taken cautiously. The conditions of the reaction published elsewhere were: (i) 80 ​°C, 20 ​h, no solvent, O2 as the oxidant for Co- and Cu-MOF-74 [29], and (ii) 75 ​°C, 6 ​h, no solvent, TBHP as the oxidant for Mn-MOF-74, whereas our conditions were: 75 ​°C, 4 ​h, acetonitrile as solvent, and TBHP as the oxidant for Co- and Cu-MOF-74. Anyway, both MOF-74 materials that can be compared with the literature data, Co- and Cu-based ones, resulted to be more active in our case. It is particularly significant the difference in yield to benzaldehyde in the case of Cu-MOF-74. Even though, the yield given by our Cu-MOF-74 is below that given by Mn-MOF-74. Unfortunately, we could not get Mn-MOF-74 by our sustainable methodology.Probably the most controversial property of MOFs when applied as heterogeneous catalysts is their low both thermal and chemical stability. This is especially true for Cu-MOF-74, the most active catalyst in this work, as it has been reported their structural transformation/decomposition during the redox [15,22] and acidic-catalyzed [30] reactions. Fig. 9 shows the XRD patterns of the three samples before and after being tested in the synthesis of benzaldehyde from the oxidation of styrene with TBHP. In all cases, the structure of MOF-74 is preserved, which is particularly important in the case of Cu-MOF-74.M-MOF-74 materials (M ​= ​Cu, Co, Zn and possibly Ni) were successfully prepared at room temperature using methanol as the unique solvent. The method is simple, quick, sustainable, gives high yields, and carried out in methanol as solvent, which is the solvent almost universally used for washing/activating this material. The resultant Cu- and especially Co-MOF-74 materials were so nanocrystalline that the phase MOF-74 by diffraction techniques could not be undoubtedly identified. However, the combination of all characterization techniques used in this work (XRD, FTIR spectroscopy, TGA, N2 isotherms and SEM) certified that these materials are (nanocrystalline) M-MOF-74. Apart from microporosity, Co- and Cu-MOF-74 materials possess intercrystalline mesoporosity given by the aggregation of nanocrystals in relatively large particles, whereas Zn-MOF-74 is formed by needle-like isolated crystals and lacks any significant mesoporosity. These three M-MOF-74 were catalytically tested in the oxidation of styrene to benzaldehyde using peroxides as oxidants. When the oxidant was H2O2 in aqueous solution, there was practically no reaction probably because of water interference, but the reaction took place when TBHP was used as the oxidant. Under the optimized conditions (amount of catalyst, TBHP/styrene ratio or reaction temperature), the highest catalytic activity was given by Cu-MOF-74 whereas Zn-MOF-74 behaved as an inhibitor rather than as a real catalyst. The yield to benzaldehyde catalyzed by the sustainable Cu-MOF-74 is competitive with the best M-MOF-74 catalysts reported so far for this reaction, and, not less, this catalyst preserved intact their structure during the reaction process.This work has been partially financed by the Spanish State Research Agency, the European Regional Development Fund (FEDER) through the Project MAT2016-77496-R (AEI/FEDER, UE), 2019AEP076 (CVCSIC-AEPP-Ayudas Extraordinarias para preparación de proyectos 2019), PAPIIT UNAM Mexico (IN101517) and CONACyT (1789) projects. This work has been also financed by the CONACyT project A1-5-30646. J. Gabriel Flores: Formal analysis, carried out most of the experimental work. In addition, he contributed to the analysis, interpretation of the results, and to preparation of the manuscript. Manuel Díaz-García: contributed to part of the experimental work, in particular, in the synthesis of materials. Ilich A. Ibarra: took part in the design of the research. This work has been partially financing by his projects. Julia Aguilar-Pliego: took part in the design of the research and coordinated the catalytic part. This work has been partially financing by her projects. Manuel Sánchez-Sánchez: Formal analysis, Writing – original draft, designed and coordinated the project, contributed to the analysis of the results, and wrote and submitted the manuscript. This work has been partially financing by his projects.The authors declare no conflict of interest.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.jssc.2021.122151.

Metal-Organic Framework (MOF) materials are promising heterogeneous catalysts in different areas including Fine Chemistry, mainly if they possess open metal sites and if they are nanocrystalline. In this work, a new sustainable methodology to obtain nanocrystalline M-MOF-74 materials at room temperature, without any energy input and in methanol as the unique solvent, is described. Amongst the seven divalent metal tested, pure MOF-74 phase was achieved in the case of Co, Cu and Zn (and maybe Ni), but not in the case of Mg, Mn and Cd. The formation of these MOFs is taken place as soon as the metal source (metal acetate) and the organic linker 2,5-dihydroxyterehpthalic acid are put together. The so-prepared Co- and Cu-MOF-74 (but not Zn-MOF-74) resulted to be nanocrystalline and having high external surface area and intercrystalline mesoporosity. They were tested as heterogeneous catalysts in the synthesis of benzaldehyde from styrene using tert-butylhydroperoxide as an oxidant. Different experimental parameters like reactants ratio, amount of catalysts or reaction temperature were optimized. Cu-MOF-74 gave the highest benzaldehyde yield and interestingly it maintained intact its structure after reaction, unlike the same catalyst used in some other catalytic processes under similar relatively mild conditions.

Heavy metals are natural elements presenting high atomic mass and density (> 5 g cm−3). Some of these metals are essential for animals, with indispensable functions for human metabolism [62]. However, several studies indicate that some heavy metals are likely to be carcinogenic (hexavalent chromium, arsenic, cobalt, nickel, antimony, vanadium and mercury), mutagenic (arsenic and vanadium), teratogenic (arsenic), allergenic (nickel) or endocrine-disrupting (silver, copper, zinc and selenium). Low levels of nickel result in reduced growth in intrauterine development, and its deficiency can reduce iron absorption, leading to anemia [51]. The main adverse effects caused by exposure to compounds containing this metal are skin allergies, lung fibrosis and lung cancer, depending on their ability to enter cells [10,98]. Despite its effects, nickel(II) is largely used in the manufacturing process of stainless steel, metallic alloys and batteries [80]. The release of this metal into the environment may occur from various industries, viz., nickel plating, zinc-based casting industry and storage batteries, silver refinery, mining and metallurgy of nickel [5,39].The presence of nickel in drinking water can also occur due to corrosion of pipes containing nickel in their composition or even to the poor removal of this metal by water treatment systems [60]. Regulatory environmental agencies establish concentration limits for nickel, owing to the risks presented by its existence in drinking water and wastewater. For instance, the World Health Organization establishes as a guideline a value of 0.07 mg L−1 for the concentration of nickel in drinking waters (WHO/SDE/WSH/07.08/55). The concentration of nickel may range from 0.5 mg L−1 to 192 mg L−1 in wastewater effluents [53]. Adsorption on several carbon-based adsorbents [80,95,96] has resulted in efficient processes for the removal of Ni. However, many studies report high uptake capacities, since the removal of Ni from waste waters is normally studied considering high loads of the heavy metal (> 50 mg L−1) [24,53,59]. The feasibility of the adsorption of nickel on carbonaceous adsorbents should also be explored at low nickel concentrations.An efficient scenario allowing to decrease the costs of the adsorption process and to reach a circular economy approach consists in the development of technologies to valorize wastes by their transformation into suitable adsorbents [25,84,89]. In this sense, the scientific community has been putting a great effort into the development of carbon-based adsorbents from biomass wastes coming from agro-industrial activities, such as fruit peels [25], shell of nuts [42], bagasses [16], among others. By using biomass waste as a carbon precursor, different carbon-based adsorbents can be obtained, viz. pyrochars, hydrochars, or activated carbons, depending on the carbonization processes applied [16]. A pyrochar (PC) is obtained through the thermal treatment of the precursor at 400–1000 ºC in an inert or oxygen-limited environment [97]. Hydrochars (HCs) can be prepared by hydrothermal carbonization (HTC), which consists in a thermochemical conversion in the presence of water at temperatures ranging from 150 to 350 ºC and autogenous pressure [71]. HTC is interesting because of its technical simplicity, low cost and energy efficiency. Activated carbons (ACs) are typically obtained through two steps: activation and carbonization. Activation can be conducted using chemical (treatment of the precursor with oxidants) or physical (steam, CO2 and air) methods [94]. As an activation step, HTC also works as an efficient process to obtain a suitable precursor (HCs) for the production of ACs [16].The chemical activation to prepare ACs from biomass waste has been studied with different activating agents, such as inorganic acids, bases or salts [2,94]. However, there are scarce studies on HTC of biomass wastes using additives to improve the physicochemical properties of the resultant HCs [16,66,71,85]. The use of chemical agents in HTC can be exploited to introduce improved surface chemistry for adsorption applications of the resultant HCs or ACs. Furthermore, chemical agents in HTC can also act as structure-directing agents to prepare carbonaceous spheres [9]. Among them, iron (III) chloride has proved to be an excellent activating agent for the preparation of carbonaceous materials [3,55,81] and as a metal doping for the adsorption of heavy metals from aqueous solution [11,27,53]. In fact, for carbonaceous adsorbents, the metals and functional groups on their surfaces, with acid or base character, play an important role in the adsorption process [80]. In this sense, HCs are rich in functional groups that can greatly improve chemical reactivity [36]. Because of this, many scientists have been testing HCs as adsorbents for the removal of heavy metals, pesticides, and drug residues [35]. However, the influence of the adsorbent’s characteristics (e.g. functionalities, morphology, or textural properties) on the adsorption of Ni has not been deeply studied so far.The properties of the carbon-based materials not only depend on the type and operating conditions of the carbonization process but also on the carbon precursor selected for their preparation. The materials obtained under the same conditions can present significant differences in their characteristics when other carbon precursors are used [16]. Therefore, the biomass waste used for the preparation of adsorbents should be carefully selected. In this sense, citrus fruit peels have shown to be efficient precursors for preparing carbon-based materials [23,25]. As the precursor contains citric acid, interesting carbon-based materials may be obtained, since citric acid is used as catalyst to develop this type of material [85,87].Citrus fruits are one of the largest fruit crops in the world. Similarly, the citrus industry is also the second largest fruit processing industry, surpassed only by the grape industry, which mainly produces wine [38]. Approximately one-third of the citrus fruits are processed for juice production, resulting in 50–60 % of organic waste, typically constituted by the peel, seeds and leaf residues [75]. It is noteworthy that due to the amount of organic matter present in citrus fruit peels, the disposal of this type of residue directly in the soil can cause damage, given its ability to change the physicochemical characteristics of the soil [79]. Currently, land space occupation and pollution with phenolic compounds due to dumping of waste are becoming problematic [26]. For this reason, the development of techniques to valorize the large amount of waste generated in the citrus juice processing industry is required. In this sense, the production of biochars from diverse citrus peels has become interesting as a low-cost alternative to obtain high-value products, avoiding the pollution of waste dumping [65,78,81,83].This work deals with the preparation of activated carbon, pyrochar and hydrochar materials using tangerine peels as carbon precursor and their assessment in the removal of Ni(II) by adsorption. Hydrochars (HCs) are prepared by HTC assisted with FeCl3, known as a catalyst of carbonization processes [55] and later used as a precursor for the preparation of activated carbons (ACs) by pyrolysis at the same conditions of pyrochar (PC) directly prepared from the tangerine peels. The different properties of the ACs, PC and HCs and how they affect the adsorption of Ni(II) are analyzed, and the kinetic and equilibrium adsorption of Ni(II) on them is modeled. To the best of our knowledge, there is a scarcity of studies dealing with the valorization of tangerine peels, as is the case of other peels, especially considering FeCl3-assisted HTC. Similarly, few studies assess ACs, PCs and HCs prepared from the same source to be applied to the adsorption of a heavy metal at similar operating conditions.The modelling of the adsorption process is invaluable, not only for the prediction of the solute adsorption onto the adsorbent at different operating conditions, but also for a better understanding of the adsorption mechanism occurring on a system [20,28]. Adsorption isotherms data (quantification of adsorbed solute per unit mass of adsorbent at a constant temperature for different solute concentrations in solution at the equilibrium) can be processed for a deep understanding of the interaction between the solute (Ni(II) in this work) and the adsorbent. The constants obtained from the different models provide important information on the affinities of the adsorbent for the removal of the pollutant and on the mechanisms of adsorption. The application of kinetic adsorption is also useful in studying the dynamics of the adsorption mechanism in terms of the order of the adsorption rate constant. Additionally, the parameters obtained as results of the fitting kinetic models allow to assess the time required to remove Ni(II) on the selected adsorbent [49,76].The Langmuir equation is a well-known isotherm model that assumes that adsorption occurs on a homogeneous surface of an adsorbent containing sites that are equally available for adsorption [52]. The separation factor (R L ) is an important parameter of the Langmuir isotherm typically used to verify whether the adsorption under study is unfavourable (R L > 1), linear (R L = 1), favourable (0 < R L < 1) or irreversible (R L = 0). Langmuir equation and R L are expressed by Eqs. (1–2), (1) q e = q m · K · C e 1 + K · C e , (2) R L = 1 1 + K · C 0 , where q e and C e refer to the solute adsorbed per mass of adsorbent (mg g−1) and adsorbate concentration in aqueous media (mg L−1) at equilibria stage, q m and K are constants (two-parameter model) measured in mg g−1 and L mg−1, respectively, R L is the separation factor (dimensionless quantity) and C 0 is the initial concentration of the adsorbate.Freundlich isotherm is an empirical equation (Eq.(3)) widely applied for heterogeneous systems with interaction between the adsorbate, representing suitably non-asymptotic adsorption curves between uptake capacity (q e ) and equilibria concentration (C e ) in the aqueous media [69]. The heterogeneity factor (n) can be employed to indicate if the adsorption is linear, chemical or a physical adsorption process (n = 1, n  <  1 or n  >  1, respectively). This two-parameter model is represented by Eq. (3), (3) q e = K · C e 1 / n , where K is the constant of Freundlich measured in L1/n mg−1/n and n is the exponent.The Sips isotherm model (Eq. (4)) is a combination of the Langmuir and Freundlich isotherms [82]. At high adsorbate concentrations, the equation provides the adsorption capacity in the monolayer, typical of the Langmuir isotherm. At low adsorbate concentrations, the Sips equation is reduced to the Freundlich equation. In the literature, it is possible to find the Sips model named as Koble-Corrigan model [28,77,93], but Koble and Corrigan used the Sips model indeed, as they described [46]. For this reason, the Koble-Corrigan model was not object of study in this work. This three-parameter model is represented by Eq. (4), (4) q e = q m · K · C e n 1 + K · C e n , where q m and K are constants measured in mg g−1 and Ln mg-n, respectively, and n is an exponent (three-parameter model).To improve the fitting of Langmuir and Freundlich equations, Redlich and Peterson developed their model [68], which is mathematically equal to the Radke and Prausnitz isotherm model developed in the adsorption of solutes from dilute aqueous solutions on activated carbon [20,67]. Redlich and Peterson isotherm model is typically expressed as Eq. (5), (5) q e = A · C e 1 + K · C e n , where A and K are constants measured in L mg−1 and Ln mg-n, respectively, and n is an exponent.The General Isotherm Equation (GIE) proposed by Tóth for all types of isotherms was developed to consider the heterogeneity, and the lateral and vertical interaction energies of the adsorbed molecules. The Tóth isotherm model usually applied in the modelling of adsorption systems for the wastewater treatment is the solution of the GIE when the dynamic equilibrium adsorption is higher for the monolayer than the subsequently formed layers [88] and it is expressed by Eq. (6), (6) q e = q m · C e 1 / K + C e n 1 / n , where q m is a constant measured in mg g−1, K is a constant (Ln mg-n), and n is an exponent, which can take values in a wide range (>0), allowing to suitably predict the adsorption isotherms.The isotherm model of Khan was developed for studying the adsorption of aromatic compounds on activated carbons from multi-component aqueous phase solutions [43–45]. The generalized model for a single solute could be formulated according to Eq. (7), (7) q e = q m · K · C e 1 + K · C e n , where q m and K are constants measured in mg g−1 and L mg−1, respectively, and n is the exponent.The model Vieth-Sladek was first proposed to model adsorption of gases in glassy polymers. Owing to the remarkable resemblance of the studied application with the adsorption on porous solids [92] it has been also used to model the adsorption of model pollutants [47,91]. The model may be expressed by Eq. (8), (8) q e = q m · K · C e 1 + K · C e + n · C e , where q m , K and n are constants measured in mg g−1 for q m and L mg−1 for both K and n.Brouers and Sotolongo proposed a Weibull distribution as a possible empirical isotherm model [8] that has been used to predict pollutants adsorption on carbon-based materials [91]. The Brouers and Sotolongo equation is formulated as Eq.(9), (9) q e = q m · 1 − exp − K · C e n , where q m and K are constants measured in mg g−1 and Ln mg-n, respectively, and n is the exponent.The Jovanović model consists of two equations developed for the physical adsorption on monolayer and multilayer adsorption. Initially, this model was developed for adsorption in the gas phase [41], but it is largely used in the adsorption of solutes from aqueous media solutions [91,93].The pseudo-first-order equation describes the adsorption rate based on the monolayer adsorption capacity [33] and it is typically represented by Eq. (10): (10) q t = q e · 1 − exp − k · t , where q t and q e refer the solute adsorbed (Ni(II)) per mass of adsorbent (mg g−1) at a time of contact t (min) and at the equilibria stage, respectively, and k represents the rate constant of the adsorption process (min−1).The pseudo-second-order model [58], also found as an hyperbolic model [20], is typically used to describe adsorption processes controlled by chemisorption, involving valence forces through sharing or exchange of electrons between the adsorbent and the adsorbate. Eq. (11) represents this model, (11) q t = 1 1 k · q e 2 · 1 t + 1 q e , where the rate constant k is measured in g mg−1 min−1.Bangham is a pore diffusion model expressed by Eq. (12): (12) q t = k · t 1 / m , where the rate constant k is measured in mg g−1 min−1/m and m is an exponent.The Elovich equation is a model based on chemical adsorption [70], typically used in the simplified form obtained by Chien and Clayton [20]. In this work, the integrated form of Elovich equation was used, as shown in Eq. (13), (13) q t = 1 β · ln α · β · t + 1 , where α and β (two-parameter model) are the Elovich constants measured in mg g−1 min−1 and g mg −1, respectively.The Dünwald-Wagner intraparticle diffusion model is typically expressed as shown in Eq. (14) [69], (14) q t = q e · 1 − exp − k · t , where the rate constant k is measured in min−1 and m is an exponent.Weber-Morris equation is another mechanistic model typically found as shown in Eq. (15), (15) q t = k · t + m , where the rate constant k is measured in mg g−1 min−1/2 and m is a parameter measured in mg g−1.The Avrami kinetic model was developed considering possible changes of the adsorption rates as a function of the initial concentration and the adsorption time, as well as the determination of fractionary kinetic orders [56] and it is expressed as in Eq.(16), (16) q t = q e · ( 1 − exp − k · t m ) , where the rate constant k is measured in min−1 and m is an exponent (only three parameter-kinetic adsorption model used in this work).Tangerine peels (TP) were obtained after domestic use. 99.995 % nitrogen was supplied from Praxair. 97 % iron (III) chloride hexahydrate (FeCl3.6 H2O) was supplied from Panreac, 95 % nickel(II) chloride hexahydrate (NiCl2.6 H2O), 98 % sodium hydroxide (NaOH) and 37 % hydrochloric acid (HCl) were obtained from Fisher chemicals. All reagents were used as received without further purification, and distilled water was used throughout the research.TP was first dried in oven at 100 ºC for 24 h, and then grinded and sieved to obtain particle sizes between 106 and 250 µm using two sieves with metallic mesh (CISA) according to ISO 3310.1 and ASTM E-11–95 (Nº 140 and 60, respectively). Hydrochar microspheres were then produced adapting the methodology described elsewhere [15,16]. Briefly, a suspension of 2.5 g of the dried and sieved TP was prepared with 20 mL of FeCl3 solution (2.5, 1.0 and 0.5 M) in a 125 mL high-pressure autoclave (Model 249 M 4744–49, Parr Instrument co., USA), heated to 200 ºC for 3 h under autogenous pressure. The recovered hydrochar microspheres were labelled as HCMS-2.5, HCMS-1.0, and HCMS-0.5, according to the concentration of FeCl3 solution used in the HTC.A pyrochar (PC) and activated carbons microspheres (ACMS-2.5, ACMS-1.0, ACMS-0.5 from HCMS-2.5, HCMS-1.0, and HCMS-0.5, respectively) were produced by pyrolysis of the TP and hydrochars, respectively, under N2 continuous flow (100 Ncm3 min−1) at 800 ºC, for 4 h, using a tubular furnace (Therm Concept).The compositions of the solid materials (PC, HCMSs and ACMSs) were determined by elemental analysis (Carlo Erba Instrument EA 1108) to know the weight percentages of carbon, nitrogen, hydrogen and sulfur. To determine ashes, the carbonaceous materials were weighted before and after calcination, conducted in static air (muffle) at 800 ºC for 4 h.The textural properties of the carbonaceous materials were determined from the analysis of N2 adsorption-desorption isotherms at 77 K, obtained in a Quantachrome NOVA TOUCH LX4 adsorption analyzer. Degasification was conducted for 16 h at 120 ºC. BET, Langmuir, external and microporous surface areas (S BET , S Langmuir , S ext and S mic , respectively), micropore volume (V mic ) and total pore volume (V Total ), were determined as described elsewhere [63].Scanning electron microscopy (SEM) images of the TP-based carbonaceous materials were obtained using a FEI Quanta 400FEG ESEM/EDAX Genesis X4Minstrument equipped with an Energy Dispersive Spectrometer (EDS).Functionalities were studied through X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FT-IR). XPS analysis was conducted in a PHI-5701 of Physical Electronics, whereas FT-IR spectra were obtained with a Perkin Elmer FT-IR spectrophotometer UATR Two with a resolution of 1 cm−1 and scan range of 3000–450 cm−1.Acidity and basicity of the carbon-based materials were determined by acid-base titration of an acid or base solution after keeping in contact with the adsorbents for 48 h, as detailed in the literature [18,73]. Surface acidity (SA) and basicity (SB) were determined considering the BET surface area of each adsorbent.Equilibrium adsorption isotherms of Ni(II) on the ACs, PC and HCs were determined by means of the equilibrium method [19]. First, 0.125 g of adsorbent were added into 50 mL of nickel(II) chloride solutions at different concentrations (5, 10, 20, 50, 80 and 100 mg L−1 of Ni(II)). The mixtures were stirred at 240 rpm for 72 h.Kinetic adsorption of Ni(II) on the TP-based materials was conducted using 0.125 g of adsorbent and 50 mL of a 5 mg L−1 nickel(II) chloride solution. The adsorption was conducted at 240 rpm. Then, different samples were withdrawn from the Erlenmeyer at the following selected times: 15, 30, 60, 120, 240 and 1440 minThe effect of pH in the adsorption of Ni(II) onto the TP-based adsorbents was assessed at pH ranging from 3 to 9. For each run, 50 mL of the 100 mg L−1 Ni(II) chloride solution was used and 0.125 g of the adsorbent was added. The pH of the solution was adjusted using 1 mol L−1 HCl and 1 mol L−1 NaOH during all runs. After 72 h, the samples were filtered in order to separate the adsorbent from the liquid fraction and the concentration of Ni (II) in the filtrate was determined.Samples withdrawn during the adsorption experiments were filtered to separate the adsorbent, and the liquid samples were analyzed by atomic absorption spectrophotometry (Varian SpectrAA 220, Steinhausen, Switzerland) to determine Ni(II) in the aliquots.The amount of Ni(II) adsorbed on the TP-based materials was determined by application of Eq. (17), (17) q t = C Ni II , 0 − C N i II , t V W adsorbent , where q t refers to the amount of Ni(II) adsorbed per unit mass of TP-based material at time t (mg g−1), C Ni(II),0 is the initial Ni(II) concentration in the aqueous solution (mg L−1), C Ni(II),t is the concentration of Ni(II) in the solution at the adsorption time t (mg L−1), W adsorbent refers to the mass (g) of TP-based material and V is the volume of the aqueous solution (L).Kinetic and isotherm adsorption models were obtained using non-linear regression since better-fitted equations are obtained than using linearized equations [49], consisting of successive numerical iterations to minimize the least sum of squared errors (SSE) of q t (cf. Eq. (18)), as detailed in previous works dealing with modeling methods [17,20], (18) SSE = ∑ i = 1 n q t , exp , i − q t , model , i 2 , where q t,exp,i (mg g−1) is the amount of Ni(II) adsorbed per unit mass of TP-based adsorbent at time t in the measured adsorption experiments (q t being expressed as q e for equilibrium runs), q t,model,i (mg g−1) the respective calculated values given by the model, i representing each value up to n values obtained in each experiment.Alternatively, different error functions were used, viz. the sum of the square of the errors (SSE), the sum of absolute errors (SAE), the hybrid error function (HYBRYD), the Marquard’s percent standard deviation (MPSD), and the average relative error (ARE) [42] to assure the good fitness of the models. SAE, HYBRYD, MPSD, and ARE error functions are respectively described by Eqs. (19–22), (19) SAE = ∑ i = 1 n q t , exp , i − q t , model , i , (20) HYBRYD = 100 n − p ∑ i = 1 n q t , exp , i − q t , model , i 2 q t , exp , i , (21) MPSD = 100 1 n − p ∑ i = 1 n q t , exp , i − q t , model , i q t , exp , i 2 , (22) ARE = 100 p ∑ i = 1 n q t , exp , i − q t , model , i q t , exp , i , where p refers to the number of parameters for each model (remaining parameters as above described for SSE).The models were also evaluated by the determination factor (r 2 ) and the adjusted determination factor (r 2 adjust. ), to take into account the degrees of freedom or the number of parameters from each model equation [28]. Table 1 summarizes the carbon, hydrogen, nitrogen, sulfur and ash contents for the precursor (TP) and for the prepared carbonaceous materials. Compared to the raw waste (42.1 wt % and 6.17 wt% of C and H, respectively), it is possible to observe that the carbon content increases (64.7–93.1 wt%) and the hydrogen composition decreases (1.06–5.21 wt%) for all TP-based materials prepared, resulting in an increment of the C/H ratio (from 6.8 in the TP precursor to 12.4–82.8 in the prepared materials). The effect is more evident in materials subjected to pyrolysis (ACMSs and PC) since the thermal process causes the release of volatile compounds, such as water and low molecular weight hydrocarbons, and the carbonization of the sample [12,16].The increase in the C/H ratio observed for hydrochars (12.4–13.9) with respect to TP (6.82) is due to aromatization, condensation or polycyclization reactions during the carbonization of TP [12,16]. ACs (ACMS-2.5, ACMS-1.5 and ACMS-0.5) prepared by sequential HTC and pyrolysis show the highest C/H ratios (77.0–82.8), due to the effect of both processes. Among the ACMSs and HCMSs, ACMS-2.5 and HCMS-2.5 show the highest C/H ratios (82.8 and 13.9), evidencing the role played by the iron catalyst during the carbonization processes.The ash content in the TP and in the carbon-based materials is a measure of the inert, inorganic and probably unusable part of the material whose presence may modify the interaction between the surface of the carbon material and the adsorbate [6]. The TP-based materials show values of ashes ranging from 1.4 to 8.0 wt%. The highest value was found for PC since pyrolysis leads to the volatilization of the organic compounds of the precursor TP, as also observed in works dealing with the production of carbon materials from other sources [16]. The same effect was observed for ACMSs (2.2–5.1 wt% of ashes) prepared by pyrolysis from HCMS (1.4–4.8 wt% of ashes). On the other hand, HTC leads to a decrease in the ash content, likely due to the leaching of alkali and alkaline earth metals present in the TP, promoted by the contact of the solid with the high temperature liquid solution, as was observed in previous works dealing with HTC or using acid solutions during the activation of ACs [16,17,71]. Only one hydrochar (HCMS-2.5) shows a slight increase of ash content (from 4.4 to 4.8 wt%), which was ascribed to the impregnation of the material with iron during HTC, as evidenced by the tendency of increasing ash content with increasing iron concentration: 4.8, 3.2 and 1.4 wt% for hydrochars prepared with 2.5, 1.0 and 0.5 M of FeCl3, respectively.The remaining content (different from C, H, N, S and ashes) is typically associated with other heteroatoms, such as oxygen. As observed, its content decreases after either pyrolysis or HTC due to the carbonization processes (those elements are released).The N2 adsorption isotherms of the studied TP-based adsorbents are depicted in Fig. S1 and the textural properties obtained through the calculation methods described in the methodology are summarized in Table 2. As observed, nitrogen adsorption isotherms show higher quantities of volume adsorbed for ACMSs, followed by PC and HCMSs, whose uptake capacity is considerably lower. Similar trend has been reported in studies dealing with the preparation of ACs, PCs and HCs and from different precursors [16].As expected, ACMSs have the highest BET (238–287 m2 g–1) and Langmuir (330–391 m2 g–1) surface areas and total pore volumes (162–282 mm3 g–1), among all adsorbents prepared. ACMSs show similar results to those reported in the literature regarding the synthesis of carbon materials prepared by diverse activation and carbonization methods of rice husk (171–280 m2 g–1) [13,64], palm shell (260–266 m2 g–1) [14,31], or coconut shell (183 m2 g–1) [14].The PC sample, obtained by pyrolysis of TP without any other treatment, reaches values considerably lower than ACMSs (104 and 146 m2 g−1 of BET and Langmuir specific surface areas, respectively and 66 mm3 g−1 of total pore volume), evidencing that HTC works as activation process for the development of ACMSs. The PC sample presents a significant microporosity (S mic = 94 m2 g–1 and V mic = 50 mm3 g–1), showing that citric peels are interesting precursors for the development of carbon-based adsorbents. Obviously, microporosity increases when HTC is used prior to pyrolysis, obtaining ACs instead of PC, resulting in materials with a microposity two times higher than that in PC (S mic = 198–217 m2 g–1 and V mic = 101–116 mm3 g–1).Moreover, the differences found among the ACMSs evidence the effect of the quantity of iron impregnating the HCs as an active catalyst for the carbonization and development of porosity, since specific surface areas and total pore volume increase in the following order ACMS-0.5 <ACMS-1.0 <ACMS-2.5. Similar values are found in the literature [16,71] for the specific surface areas and pore volume of HCs that were determined according to the nitrogen adsorption isotherms (Fig. S1).Preliminary tests related to the adsorption of Ni(II) (in 5 mg L–1 aqueous solution) over the TP-based carbonaceous materials (2.5 g L–1 of adsorbent) at room temperature reveal that is possible to remove an amount of the heavy metal from the aqueous solution per unit mass of the adsorbent ranging from 0.19 to 1.99 mg per g of adsorbent (cf. Table 2), being the highest uptake capacity of Ni(II) obtained with the PC adsorbent, i.e., the pyrochar. Although ACs show more interesting textural properties for this application (highest specific surface areas and total pore volumes); however, lower adsorption capacities are obtained with ACMSs (1.43–0.40 mg g−1) when compared to the pyrochar (1.99 mg g–1). On the other hand, HCMSs show some unexpected uptake capacity (0.60–0.19 mg g–1), when considering their textural properties. In fact, the capacity of adsorption per surface area (Q e ) of these materials is significantly higher (27.8–54.8 μg m–2) than that of PC (19.1 μg m–2) and ACMSs (1.7–5.0 μg m–2).It is noteworthy that the largest uptake capacities among ACMSs (1.43 mg g−1) and HCMSs (0.60 mg g−1) were obtained for the TP-based adsorbents prepared with the highest concentration of FeCl3 during the HTC (ACMS-2.5 and HCMS-2.5). For this reason, only these materials were selected for further investigation.Texture and morphology of three selected TP-based adsorbents (HCMS-2.5, PC and ACMS-2.5) were determined through SEM (cf. Fig. 1). The microphotographs confirm the production of microspheres by HTC using FeCl3. For HCMS-2.5, it is possible to observe particle sizes ranging from 1858 to 3615 nm at low image magnification (Fig. 1a), but many carbon microspheres were found with sizes between 50 and 350 nm (Fig. 1b).The material prepared by pyrolysis of TP without further physical-chemical treatment (PC) presents higher particle sizes (105.7–238.1 µm) than HCMS-2.5 and ACMS-2.5 (Fig. 1c). The particle size of PC is in accordance with the sieving performed to prepare the adsorbents (sieves with a metallic mesh to separate particle sizes between 106 and 250 µm). The implication is that the particle size of the precursor is not strongly affected by pyrolysis, as it is by HTC with FeCl3, which led to decrease more than 100 times the particle size. Regarding the morphology, PC shows irregular particle shape and a rough surface, likely due to the porosity developed during the pyrolysis to prepare PC.The pyrolysis of HCMS-2.5 to obtain ACMS-2.5 allowed to keep the particle size of microspheres, although it is possible to observe the sintering of some microspheres of HCMS-2.5, leading to agglomeration of activated carbon microspheres (Fig. 1e). The surface of this material is also slightly rough, probably due to the porosity development during the thermal treatment, as observed for PC.Selected regions of micrographs were analysed by EDS to determine the elemental content of HCMS-2.5, PC and ACMS-2.5 (Fig. S2). As observed, the materials mainly consist of carbon. HCMS-2.5 (Fig. S2a) and ACMS-2.5 (Fig. S2c) show a slight iron content. On the other hand, the EDS of PC adsorbent confirms the presence of alkali and alkaline earth metals (Na, Mg, K and Ca) from the precursor. The presence of this metal and the highest signal in oxygen for PC compared to ACMS-2.5 is in accordance with the CHNS-elemental analysis presented above. Alkali and alkaline earth metals were not detected for HCMS-2.5, nor ACMS-2.5, meaning that HTC allows to obtain more purified carbonized adsorbents.The adsorptive properties of carbonaceous adsorbents are influenced not only by their textural properties but also by their surface chemistry. TP-based adsorbents may contain heteroatoms such as oxygen, nitrogen and phosphorus on their surface that form organic functional groups, such as carbonyls, carboxylic acids, lactones, ethers, phenols, aldehydes, amines and phosphates, which can be neutral, acidic or basic [40]. The surface chemistry depends on the composition of the precursor and on the method of carbonization [7,30]. Fig. 2 shows FT-IR spectra obtained for the TP precursor and the selected adsorbents (ACMS-2.5, PC and HCMS-2.5). The bands were assigned to probable bounds according to FT-IR spectra of diverse carbon-based materials [90].As observed, the TP precursor presents more bands than the respective TP-based carbon materials due to the complex composition of the biomass. As a C-rich material, some bands can be ascribed to the presence of C-H (2930–2850 cm−1) and CC (1635 cm−1) bonds. However, this material stands out for the high content of surface oxygen groups (SOGs). Hydroxyl groups (-O-H) are found at 1635–1625 cm−1. Carbonyls (-CO) can be identified at 1385 cm−1 and the presence of epoxy groups are demonstrated by bands at 1050, 1075 and 1150 cm−1. The carbonized samples (HCMS-2.5, PC and ACMS-2.5) show a strong decrease in the bands representative of SOGs. In fact, PC and ACMS-2.5 only present a significant band at 1630 cm−1 that can be assigned to double bounds between carbon atoms. The absence of significant bands for CH bonds and SOGs demonstrated the success of the carbonization and a poor functionalized surface. The lower content of O and H agrees with the elemental composition found by elemental analysis. On the other hand, HCMS-2.5 show significant bands at 2930–2850 cm−1 (CH bonds) and the regions of diverse SOGs, as expected for the hydrothermal treatment at low temperatures, also agreeing with the results for elemental analysis. Fig. 3 shows the XPS spectra and the relative atomic percentage content of elements on the surface of each TP-based adsorbent. As expected, HCMS-2.5 corresponds to hydrochar microspheres rich in surface oxygen groups (SOGs), since 17.73 % of O1s was obtained. Surprisingly, PC also presents a high content of oxygen on the surface and this technique also confirms the presence of metals (K and Ca). The values obtained are close to those obtained by SEM/EDS and also similar to the elemental composition obtained by CHNS-AE. K and Ca were not detected in ACMS-2.5 and HCMS-2.5, as HTC can remove those elements, as corroborated by SEM/EDS. ACMS-2.5 shows the lowest content of oxygen and the highest content of C among the selected materials, agreeing with the results obtained by elemental analysis. As observed, most of iron was removed from the microsphere materials (HCMS-2.5 and ACMS-2.5) after washing. On the other hand, the low iron content observed for PC most likely comes from the raw TP used as a precursor.To deepen knowledge of the surface chemistry, O1s peaks of XPS spectra analysis were assessed for the materials without significant quantities of other metals, i.e., HCMS-2.5 and ACMS-2.5. O1s deconvolution curves are represented in Fig. S3 and the assignments to the relative content of SOGs are summarized in Table 3. As observed, HCMS-2.5 presents mainly hydroxyl groups (14.7 % out of 17.7 %), whereas ACMS-2.5 shows lower content of hydroxyl groups with a more predominant presence of carbonyl groups (1.5 % out of 3.9 %). The reduction of hydroxylic groups in ACMS-2.5 can be ascribed to the release of weak hydroxyl groups during the pyrolysis at 800 ºC of HCMS-2.5 to obtain ACMS-2.5.The results obtained in terms of acidity and basicity of the selected adsorbents (ACMS-2.5, PC and HCMS-2.5) are shown in Table 4. As can be seen, acid-base properties show significant differences among the adsorbents, hydrochar HCMS-2.5 being more acid (0.32 mmol g–1) than ACMS-2.5 (0.23 mmol g–1) and, especially, than PC (0.01 mmol g–1). Taking into account the specific surface area of the materials, the same order is observed for the surface acidity: HCMS-2.5 (45.7 μmol m–2) > ACMS-2.5 (0.78 μmol m–2) > PC (0.10 μmol m–2). On the opposite, the basic character of the adsorbents follows a different order when measured per mass or per surface area, highlighting the basicity of PC (1.83 mmol g–1) and HCMS-2.5 (30.4 μmol m–2). Interestingly, the same order was found for the basicity (Table 4) and the uptake capacity of the materials (Table 2) per gram (PC > ACMS-2.5 > HCMS-2.5) and per surface area (HCMS-2.5 > PC > ACMS-2.5), evidencing the strong role played by the basicity in the adsorption of Ni(II).The highest acidity of HCMS-2.5 can be ascribed to the chloride precursor (FeCl3) used as activating agent during HTC. The strong acid and basic character of this material compared to the others are due to the carbonization in the presence of water that leads to a material highly functionalized with oxygen-containing surface groups [4]. This can be expected by the oxygen content of hydrochars according to the remaining content presented in Table 1 (24.0–28.0 wt% for HCMSs, whereas 2.5–9.6 wt% was found for PC and ACMSs). The basicity of PC may be ascribed to the fact of having the highest content of ashes (8.0 wt%, whereas less than 5.1 wt% is found for other TP-based adsorbents), consisting mainly of alkali and alkaline earth metals, as observed both in SEM/EDS and XPS analysis. Fig. 4 shows the isotherm adsorption of Ni(II) on the selected adsorbents: (a) HCMS-2.5, (b) PC and (c) ACMS-2.5. As observed, PC shows the highest uptake capacity of Ni(II), reaching values of 13.9 mg g–1 at the highest tested initial concentration of Ni(II) (C Ni(II),0 = 100 mg L–1). In contrast, the highest adsorption capacity of ACMS-2.5 and HCMS-2.5 was 5.18 and 4.88 mg g–1, respectively, at the same operating conditions.The shape of the isotherms is also considerably different among the TP-based adsorbents. Giles classification is usually used to distinguish the isotherm adsorption curves according to their characteristic shapes between four isotherm classes: high affinity (H), Langmuir (L), constant partition (C) and sigmoidal-shaped (S) [29]. H and L isotherms have a convex shape. However, the slopes of H isotherms reach higher values than L isotherms because the sorption affinity of H isotherms strongly increases with decreasing concentration. S isotherms have a concave shape at low concentrations, while C isotherms are defined by a constant sorption affinity [34].Accordingly, the isotherm curve obtained with ACMS-2.5 may be classified as L1 since it describes a medium affinity at low concentrations of Ni(II) and equilibrium uptake capacities (q e ) gradually describe an asymptotic curve for C e higher than approximately 20 mg L–1. The isotherm curve obtained with PC may be classified as H1, since it describes a similar curve, but it shows a strong affinity at a low concentration of Ni(II). The isotherm curve obtained with HCMS-2.5 may be classified as C1, because of the line trend described by the equilibrium uptake capacities upon the initial concentration of Ni(II). These differences may be ascribed to the different textural and acid-base properties of the adsorbents. PC should present the highest affinity and uptake capacity for the combination of a significant porosity and basicity, whereas HCMS-2.5 has not enough specific surface area and ACMS-2.5 shows limited functionality.The isotherms for adsorption of Ni(II) onto the TP-based adsorbents were evaluated by 10 models: 7 with three parameters (Sips, Redlich-Peterson, Tóth, Khan, Vieth-Sladek, Brouers-Sotolongo and Jovanović for multilayer adsorption), and 3 with two parameters (Jovanović for monolayer adsorption, Langmuir and Freundlich) in their functions [20]. Those models were fitted by a non-linear regression method, since model parameters may be distorted by linear regressions [49,54] and adjusted determination factor (r 2 adjust. ) used to take into account the number of parameters from each model function [20]. The equations, the parameter values and the most significant statistical data obtained from the fitting of the isotherm adsorption models to the experimental data are compiled in Table 5 (isotherm curves obtained are depicted in Fig. 4). As observed, most of the isotherm models accurately fit (r 2 = 0866 – 0.996 and r 2 adjust. = 0.809 – 0.993) to the experimental data obtained with the TP-based adsorbents. The fitting with all models considered the minimization of different error functions (SSE, SAE, HYBRYD, MPSD and ARE), since other studies reported about the importance of the objective function in the fitting with the models [42,48]). However, non-significant differences were observed among the models for the parameter and statistical data (r 2 ), as exemplified by the q m values predicted using SSE, SAE, HYBRYD, MPSD, and ARE for each isotherm model in Fig. S4. For this case, the maximum difference for the predicted value of q m was 2.8 mg g−1 found for the Tóth model (simplified function). It is noteworthy that the isotherm models keep the same order of values for the predicted q m regardless of the error functions used for the fitting, as follows: Tóth > Sips > Brouers-Sotolongo > Langmuir > Jovanović (monolayer) > Vieth-Sladek > Redlich-Peterson > Jovanović (multilayer) > Khan > Freundlich. A similar trend was observed in a previous work [20], so it may be expected to predict higher values of monocape uptake capacity (q m ) with the models of Tóth (simplified model), Sips, Brouers-Sotolongo, Langmuir and Jovanović (model for monolayer adsorption). Among the TP-based materials tested, PC leads to the highest values of both K and q m , evidencing it as the material with the highest affinity and uptake capacity.The isotherm adsorption of Ni(II) on ACMS-2.5 is best represented (r 2 = 0986 – 0.989, and r 2 adjust. = 0.977 – 0.983) by hyperbolic isotherm adsorption models (Sips, Redlich-Peterson, Tóth, Khan, Vieth-Sladek and Langmuir), which is in agreement with the sorted L1 type isotherm curve according to Giles classification. Taking into account the degrees of freedom in the fitting (r 2 adjust. ), Langmuir is the best model representing the curve of ACMS-2.5 with q m = 5.44 mg g−1, K = 0.334 L mg−1, r 2 adjust. = 0.983, as also evidenced by the value taken by parameter n (close to 1 as the exponent, and close to 0 for Vieth-Sladek) in the other models. The values of the q m and K for the Langmuir model obtained through the minimization of the different error functions (SSE, SAE, HYBRYD, MPSD, and ARE) were similar (5.44–5.50 mg g−1, and 0.289–0.334 L mg−1 for q m and K, respectively).The adimensional Langmuir separation factors (R L ) predicted from the Langmuir model for each TP-based adsorbent are depicted in Fig. S5. As can be seen, the values of R L obtained for ACMS-2.5, PC, and HCMS-2.5 range from 0 to 1. Hence adsorption is favorable in all cases, but R L values show great differences among the TP-based adsorbents. HCMS-2.5 leads to values close to 1, characteristic of linear type isotherms, as is described by this adsorbent (Fig. S1). On the opposite, the adsorbent presenting the highest adsorption capacity (PC) shows values of R L close to 0, expected for irreversible adsorption.The isotherm adsorption of Ni(II) obtained with PC is also well described using hyperbolic models (r 2 = 0.930 – 0.986, and r 2 adjust. = 0.912 – 0.977) being best represented by Khan (q m = 5.62 mg g−1, K = 27.7 L mg−1, n = 0.880, r 2 adjust. = 0.977) and Vieth-Sladek models (q m = 9.26 mg g−1, K = 12.8 L mg−1, n = 0.082 L g−1, r 2 adjust. = 0.977). The differences obtained by fitting with different error functions were also negligible.For the isotherm adsorption curve of Ni(II) on HCMS-2.5, the model developed for Jovanović for multilayer adsorption (q m = 1.53 mg g−1, K = 0.137 L mg−1, n = 0.014 L mg−1) considerably fitted better (r 2 = 0.996 and r 2 adjust. = 0.993) than all other models evaluated. In a previous work, Jovanović multilayer isotherm was found to satisfactorily predict the adsorption of a pollutant onto activated carbonaceous materials [20] with the same magnitude for both K and m constant values.Considering the BET surface area of each TP-based adsorbent, the uptake capacity on the monolayer of HCMS-2.5 could be considered the highest (Q m = q m /S BET = 139 μg m–2). In this case, the values of the parameters obtained using different error functions were also similar.The adsorption of Ni(II) from aqueous solution (C Ni(II),0 = 5 mg L−1) upon time of contact with 2.5 g L−1 of ACMS-2.5, PC, and HCMS-2.5 is represented in Fig. 5. Although HCMS-2.5 shows the lowest adsorption capacity, the adsorption of Ni(II) on HCMS-2.5 is faster compared to ACMS-2.5 and PC, likely due to the absence of microporosity. In contrast, internal diffusion may be hindered in PC and ACMS-2.5. As a consequence, kinetic adsorption on HCMS-2.5 shows an asymptotic trend that is quickly reached, and the equilibrium uptake capacity should be achieved fast.The assessment of the kinetic adsorption of Ni(II) on the TP-based adsorbents was evaluated using pseudo-first-order, pseudo-second-order, Bangham, Elovich, Dünwald-Wagner, Weber-Morris, and Avrami kinetic models. The equation of these models, the kinetic constants and the statistical data resulting from their fitting to the experimental data obtained in the adsorption of Ni(II) on ACMS-2.5, PC, and HCMS-2.5 are summarized in Table 6. Those models were also fitted by a non-linear regression method [49,54] (r 2 adjust. was not presented, since only one model has three parameters – Avrami – and it is not the model best able to predict the data obtained).The kinetic adsorption curves on the TP-based adsorbents predicted by the models are represented in Fig. 5. As observed, most of the kinetic models are capable of accurately predicting the experimental data obtained and, except for the Weber-Morris for ACMS-2.5 and HCMS-2.5 (r 2 = 0.532–0.647), well fitness is available for the kinetic models to predict the kinetic adsorption curves of the TP-based adsorbents (r 2 = 0.864–0.999). As expected, all models predict higher values of equilibrium adsorption capacity for PC, followed by ACMS-2.5 and HCMS-2.5.Kinetic constants (k) for pseudo-first-order and pseudo-second-order are higher for the hydrochar (HCMS-2.5), followed by ACMS-2.5 and PC, evidencing that surface chemistry is not so determinant for the adsorption rate, as was found for the uptake capacity. The specific surface area and pore volume of the TP-based adsorbents do not show apparent relation with the kinetic constants, so the adsorption rate of Ni(II) on the ACMS-2.5, PC and HCMS-2.5 is ruled by the combination of different factors, such as textural properties and surface chemistry.In the case of HCMS-2.5, the highest adsorption rate of Ni(II) may be ascribed to the surface chemistry and to the absence of porosity (all Ni(II) is adsorbed on its external surface, and there is no internal diffusion). The model able to better predict the kinetic adsorption of Ni(II) on HCMS-2.5 was the pseudo-second-order (q e = 0.553 mg g–1, k = 0.270 g mg–1 min–1, r 2 = 0.999), due to the fast adsorption because of the affinity of HCMS-2.5 with Ni(II). ACMS-2.5 shows values of kinetic constant higher than PC, likely due to the highest specific surface area available (S ext = 70 m2 g–1 and S mic = 217 m2 g–1) and moderate affinity of ACMS-2.5 according to the surface chemistry (SA = 0.78 μmol m–2 and SB = 2.44 μmol m–2). The kinetic adsorption of Ni(II) on ACMS-2.5 was also better described by pseudo-second-order (q e = 1.07 mg g–1, k = 0.0469 g mg–1 min–1, r 2 = 0.978).It is typically assumed that the pseudo-second-order model fits well adsorption processes controlled by chemisorption, involving valence forces by sharing or exchange of electrons that may happen between Ni(II) and functional groups on the surface of HCMS-2.5 [58]. It is also reported that the adsorption of Ni on biomass-based adsorbents is well predicted by pseudo-second-order [50,84]. For the sample with the highest uptake capacity (PC), the pore diffusion model of Bangham was the kinetic model able to better predict the kinetic adsorption of Ni(II) on PC (k = 0.471 mg g–1 min–1/m, m = 6.54, r 2 = 0.996), revealing that the process may be strongly controlled by the internal diffusion of Ni(II) inside pores of PC.The parameter m of the kinetic model of Bangham may be used as an indicator of the intensity in the adsorption of Ni(II) [70]. In this sense, the value of 49.9 for HCMS-2.5 reveals a strong affinity for the Ni(II) on this adsorbent, as expected for the fast adsorption observed. The value is considerably higher when compared with previous works [20].It is noteworthy that the best kinetic models predicting the adsorption of Ni(II) on each TP-based adsorbent was the same when other error functions were considered (SAE, HYBRYD, MPSD, and ARE) to fit the kinetic models, obtaining values for the parameters close to those presented minimizing SSE.In the same sense that acid-base functionalities of carbon-based adsorbents affect the adsorption of Ni(II), the pH of the aqueous solution also plays a key role in adsorption. The alteration of the pH media may cause dissociation of acid-base groups on the surface of the adsorbent and cause a significant improvement or worsening of the efficiency of removal of Ni(II) ions. In Fig. 6 is shown the effect of pH on nickel adsorption with the TP-based adsorbents evaluated.It is notable that there is an increase in Ni adsorption at alkaline pH, as reported in literature [32,95,96]. At pH 3 there was no adsorption on any TP-based adsorbent. The adsorption of Ni is improved at a higher pH, due to the lower number of H+ ions, while at low pH there is a competition between hydronium cations and metals, reducing the adsorption of metal ions [1,57]. When the pH is higher, the concentration of H3O+ ions decreases and the sites on the surface of the carbon turn mainly into dissociated forms and can exchange H3O+ ions with metal ions in solution [86].According to the above results related to the characterization of the TP-based adsorbents and their performance, it can be concluded that textural properties and mainly surface chemistry plays a significant role in Ni(II) adsorption. HCMS-2.5 adsorbent shows a significant uptake capacity of Ni taking into account its BET surface (11 m2 g−1), reaching the highest Ni adsorption capacity per surface area (54.6 μg m−2). FT-IR and XPS spectra revealed that HCMS-2.5 is rich in SOGs mainly consisting of hydroxyl groups. PC sample, which was prepared by pyrolysis of the raw precursor at higher temperatures than HCMS-2.5, also presents a significant content of oxygen on its surface. Its functionalization, coupled with a higher porosity, make it a standout adsorbent for Ni removal. The pyrolysis of HCMS-2.5 to obtain ACMS-2.5 results in activated carbon microspheres with the highest surface area, but the adsorbent does not display a surface chemistry suitable for adsorption of Ni. A proper combination of SOGs allows adsorbing Ni by cation exchange (hydroxyl and carboxylic groups), electrostatic attraction (ester, carbonyl groups) or complexation (epoxy, ether groups) [37,96]. Taking into account that pseudo-second order model is the best kinetic model able to predict Ni adsorption on these adsorbents, it is expected that the process is controlled by chemisorption, involving valence forces by sharing or exchange of electrons because of the functional groups on their surface. Table 7 gives a comparison of the maximum Ni(II) adsorption capacity on different carbonaceous materials reported in literature. As observed, TP-based adsorbents show values of uptake capacity similar or higher than others reported taking into account the diverse operating conditions in the adsorption of Ni(II) found in literature (C Ni(II),0 = 30–150 mg L−1, C ads = 0.6–20 g L−1, 20–40 ºC, pH 3–8, 3–24 h of contact time).The successful preparation of hydrochar and activated carbon microspheres from tangerine peels has been proved by hydrothermal carbonization with FeCl3 followed by pyrolysis at mild and moderate conditions, respectively, evidencing that biomass waste, such as citrus fruit peels, may be turned into high-added value materials in the context of a circular economy approach. The presence of chemical agents, such as FeCl3 in hydrothermal carbonization, has resulted in an effective tool for tuning the morphology, surface chemistry and increased carbonization yields in hydrochars and further preparation of activated carbons from those hydrochars. Furthermore, it was demonstrated that the shape and size of the microspheres obtained by hydrothermal carbonization are maintained even after heating to obtain activated carbon microspheres with higher porosity.The role played by textural properties and surface chemistry of carbonaceous adsorbents has been elucidated, concluding that the functionality of carbon-based materials is determinant in their performance for the adsorption of Ni (the highest uptake capacity of Ni was found with pyrochar, which shows a basicity of 1.83 mmol g−1 and a S BET of 104 m2 g–1, whereas a poor adsorption capacity was observed with the activated carbon, which has a S BET of 287 m2 g–1).The best isotherm and kinetic models predicting the adsorption of Ni on the carbonaceous materials were different among them due to their unique characteristics. Langmuir, Khan, and Jovanović are models that best represent the adsorption of Ni on the activated carbon, pyrochar and hydrochar, respectively. On the other hand, kinetic adsorption of Ni was well predicted by the pseudo-second order model for activated carbon and hydrochar. In contrast, the adsorption of Ni on pyrochar was represented by the Bangham model. Jose L. Diaz de Tuesta: Investigation, Conceptualization, Methodology, Formal analysis, Writing – original draft preparation, Visualization. Fernanda F. Roman: Investigation, Validation. Vitor C. Marques: Investigation, Writing – original draft preparation. Adriano S. Silva: Investigation. Ana P.F. Silva: Investigation. Assem A. Shinibekova: Investigation. Sadenova Aknur: Investigation. Marzhan S. Kalmakhanova: Supervision. Bakytgul K. Massalimova: Supervision. Margarida Arrobas: Investigation. Tatiane C. Bosco: Supervision. Adrián M.T. Silva: Supervision, Writing – review & editing, Funding acquisition, Project administration. Helder T. Gomes: Supervision, Conceptualization, Writing – reviewing & editing, Funding acquisition, Project administration.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Helder Teixeira Gomes reports financial support was provided by European Regional Development.The authors are grateful to the FCT (Foundation for Science and Technology, Portugal) and FEDER (European Regional Development Fund) under Programme PT2020 for financial support to CIMO (UIDB/00690/2020). We would also like to thank the scientific collaboration under Base-UIDB/50020/2020 and Programmatic-UIDP/50020/2020 funding of LSRE-LCM, and LA/P/0045/2020 funding of ALiCE, funded by national funds through FCT and MCTES (Ministério da Ciência, Tecnologia e Ensino Superior, Portugal) by PIDDAC (Programa de Investimentos e Despesas de Desenvolvimento da Administração Central, Portugal). Fernanda F. Roman and Adriano dos Santos Silva acknowledge the national funding by FCT and MIT (Massachusetts Institute of Technology, USA), and the ESF (European Social Fund) for individual research grants with reference numbers of SFRH/BD/143224/2019 and SFRH/BD/151346/2021, respectively.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jece.2022.108143. Supplementary material .

The presence of heavy metals in the environment as a consequence of human activity is an issue that has caught the attention of researchers to find wastewater treatment solutions, such as adsorption. In this work, hydrochars and activated carbon microspheres are prepared from tangerine peels as carbon precursor and FeCl3 as activating and structure-directing agent in the hydrothermal carbonization, allowing to obtain hydrochar microspheres ranging from 50 to 3615 nm. In addition, a pyrochar was prepared by pyrolysis of the same precursor. The activated carbon shows the highest surface area (S BET up to 287 m2 g–1), but the basicity of the pyrochar (1.83 mmol g−1, S BET = 104 m2 g–1) was determinant in the adsorption of Ni, being considered the carbon-based material with the highest uptake capacity of Ni. Isotherm and kinetic adsorption of Ni on the most representative activated carbon microsphere, pyrochar and hydrochar microsphere are assessed by 10 and 7 models, respectively.

There is rising global concern on climate change phenomenon and the eventual depletion of non-renewable fossil fuels [1]. Industrial processes at present rely on steam reforming of methane for generation of synthesis gas for power generation or hydrogen production. Methane, i.e. natural gas is the simplest hydrocarbon molecule, having an energy density of 55.5 MJ/kg [2]. However, concerns over rising anthropogenic carbon dioxide emission into the atmosphere by consumption of fossil fuel reserves including natural gas, oil or coal has made it important to look for alternate fuel sources that are renewable and carbon neutral. Recent years have seen much work devoted to reforming of renewable resources such as biomass and biomass derived liquid fuels which are emerging as important research fields. There are several advantages of using biomass as a renewable source as it is abundantly available in many forms since it is less dependent on location and climate. As biomass absorb CO2 through photosynthesis in the daylight, the use of biomass for energy conversion via biomass gasification results in net neutral CO2 emission [3]. In light of these advantages, there is increased interest in developing biomass gasification as a solution to using biomass as a renewable source for electricity generation.Gasification of solid biomass at high temperature converts it into syngas which is a mixture of CO and H2. The gaseous syngas mixture can be catalytically transformed into industrially useful chemical intermediates and products such as methanol, dimethyl ether, olefin and paraffins or be used for power generation [4–7]. There are several configurations to perform biomass gasification such as updraft gasifier, downdraft gasifier and fluidized bed gasifier [8]. These different configurations face a common problem which is the production of tar, a mixture of polycyclic aromatic hydrocarbons compounds due to the incomplete gasification of biomass into syngas. Tar production can lead to severe maintenance problems such as clogging of reactor outlet and other downstream equipment. Moreover, formation of tar as a byproduct causes a loss in energy conversion efficiency of the gasification process since tar consists of significant amounts of energy in the form of mixture of aromatics. Tar also contains polycyclic aromatic hydrocarbons compounds which can be hazardous for human health if disposed into water sources such as rivers and underground water. These constraints have been identified as a hindrance to widespread commercialization of biomass gasification as a commercially viable alternative for sustainable energy production [6,9]. The technical and commercial feasibility of biomass-mediated syngas route for chemical intermediates and product manufacturing hinges on the effectiveness of tar elimination/conversion.For tar removal/conversion, various approaches are presently being reported in the literature, including physical removal, thermal cracking and catalytic conversion [10,11]. The physical removal technique uses either cyclone or ceramic filters to separate the particulate matter. However, after a specific duration of time the problem of pressure drop arises due to blockage of filters by the particles and build-up of pressure drop on the filter cake. Although ceramic filters are useful techniques for the removal of particles, it shows the poor performance for the removal of tar. Under hot gas filtration condition technique, the tar remains in gaseous form and escapes the filter. Therefore, ceramic filtration alone is not a promising technique for hot gas cleaning. However, it can be combined with other separation techniques such as thermal and catalytic cracking/conversion. In few instances wet scrubbers are also used for physical tar removal by impingement of tar on the water droplets. Here, the tar and liquid flows through the decanter and the tar is separated from the aqueous phase. The temperature of the liquid varies from 35 to 60 °C. Moreover, the energy content of the tars is usually wasted in this process, reducing the overall efficiency of this process, while the post disposal of collected tars is one of the disadvantages of this process. Although physical removal process offers a reliable and effective solution for tar elimination, it is more energy consuming and more expensive to operate compared to other processes. In the thermal cracking process, the tar is converted to smaller non-condensable gases at a high temperature (1000–1300 °C), and the complete conversion to small molecule depends upon the residence time and cracking temperature. Here, a higher air-to-fuel ratio is used to maintain the high temperature at the oxidation zone. The higher concentration of oxygen reduces the tar content in the product gas. Additionally, the heating value of the burnable gas is also reduced. Generally, downdraft gasifier is used for this process due to high temperature requirement. The gas composition after thermal cracking is although very high yet the tar conversion is lower even below the acceptable range. Therefore, there is a need to develop an efficient tar conversion technology to obtain clean syngas. In contrast with thermal cracking reaction, catalytic cracking/conversion of tar at high temperatures (750–900 °C) is considered as the efficient technology to achieve complete tar conversion and higher product gas composition.To eliminate tar compounds before transporting the syngas to downstream processes, the syngas stream can be either treated within the gasifier itself which is known as primary process, or it can be treated at a separate unit which is known as secondary process. For the primary process, tar is treated with catalyst within the gasifier with steam additive; thus, syngas of higher calorific value can be attained at gasifier outlet. As for secondary process, tar contaminated syngas stream is sent to an absorption tower to absorb tar compound and reduce tar content. Tar contaminated syngas stream can be alternatively sent to a catalytic steam reformer unit to convert tar into syngas which also enhances syngas calorific value. Considering the complexity of real tar, low-cost and disposable catalysts are more likely to be applied in practical gasification plants. Dolomite and olivine mineral are typically used as catalyst in primary and secondary tar elimination process due to high availability and affordability [8]. However, the catalytic stability and activity of dolomite and olivine catalyst have much to improve on. Thus, there has been substantial research on developing synthetic catalyst to attain higher tar conversion and higher catalytic stability. There are works in the literature dealing with the steam reforming of real biomass tars [10,12]. However, the complexity of tar composition makes it difficult to ascertain both the reaction mechanism and the main species responsible for catalyst deactivation. In a typical biomass tar benzene (37.9%), toluene (14.3%), naphthalene (9.6%) and other monocyclic (13.9%) and bicyclic (7.9%) aromatic hydrocarbons are major components, however the composition of individual molecules can be varied with the nature of biomass and gasification conditions [10,12]. Therefore, it is important to focus on the behavior of individual model molecules, usually toluene, benzene, phenol or naphthalene during steam reforming on metal catalysts. Furthermore, since tar is a mixture of organic compounds with different structure and molecular weight affecting product distribution and coke nature, a deeper understanding on the behavior of tar main components and their mixture will be helpful to understand the catalytic tar reforming process.There are numerous review articles has been reported on biomass gasification and catalytic tar reforming process in recent years. For instance, Tomishige et al. had presented a comprehensive review on the development of metal catalysts for steam reforming of tar [13]. Lasa et al. reviewed biomass gasification processes as well as Ni-based catalysts for steam gasification of biomass [14]. Furthermore, the background on tar evolution, main tar precursors and models that simulate tar formation and evolution was also reviewed and according to temperature increments, tar is divided into primary (acetols, and acetic acid), secondary (phenols, and toluene) and tertiary tars (naphthalene) [15]. In another review, the various types of tar reduction catalysts are reviewed by classifying them into minerals and synthetic catalysts according to their production methods [16]. Yung et al. [17] organized and discussed the investigations of catalytic conditioning of biomass-derived syngas with various catalyst formulations and also discussed the roles of catalyst additives. Guan et al. presented a myriad of catalyst support and active metals used for steam reforming of tar as well as some elucidation on the mechanistic aspect of tar reforming [18]. Xiong et al. summarized enhanced performance for the selective CC and CO/CO bond-scission reactions of bimetallic and metal carbide catalysts for biomass derived oxygenate conversion reactions [19]. Liu et al. had presented a comprehensive review on the application of catalysis-plasma system for tar reforming in biomass gasification system [20]. Recently, Hu et al. reviewed various biomass pyrolysis process and the effect of the process design, reactors and catalysts on pyrolysis process was highlighted [21]. In this review, we will be covering recent developments in the catalyst technology for steam reforming of tar model compounds with respect to the influence of their catalytic properties to the catalysis. Also highlighted are the kinetics and reaction mechanism insights for steam reforming of toluene reaction.The seminal concept of adsorption kinetic in heterogeneous catalysis, first introduced by Irwin Langmuir and refined by Sir Cyril Hinshelwood and Sir Eric Rideal and Daniel Eley, has provided the conceptual bedrock for clearer understanding and description of reaction mechanisms on catalyst surface for rational catalyst design [22–24]. With clear mechanistic understanding of the reaction, an intrinsic kinetic equation can be first derived, and effectiveness factor can be factored to account for diffusion limitations for more accurate reactor design which is crucial for safe and economical chemical process design. A good example of such application could be found in a highly cited paper by Xu and Froment where 220 experimental runs were performed for catalytic steam reforming of methane reaction. Twenty kinetic models were derived and subjected to non-linear model fitting, based on the results from 220 runs. Out of the twenty kinetic models, only one was found to be statistically significant and thermodynamically consistent [25]. The second part of his work involved determining the catalyst effectiveness factor which was factored in for simulation of steam reforming of methane reactor [26]. However, the reactive intermediate may not be included in the intrinsic reaction mechanism. A microkinetic model can be developed to include the reactive intermediate in a set of elementary reactions that are thought to be relevant in the catalytic chemical transformation. A rate equation in every elemental step contain rate constant for the forward and backward reactions. This rate constant can be determined using DFT calculations under transition state theory. Once the rate constants are determined, the instantaneous concentration of the species can be solved using numerical methods as these elementary steps are often expressed as a system of ordinary non-linear differential equations [27,28].In addition to having a fundamental understanding of the reaction mechanism, an awareness of the state of catalyst surface under actual reaction condition has to be developed. Experimental spectroscopic techniques such as X-ray absorption spectroscopy (XAS), x-ray photoelectron spectroscopy (XPS) and DRIFTS can be utilized ex-situ, in-situ or operando to evaluate the state of catalyst surface. In conjunction with the use of in-situ or operando spectroscopic technique, computational simulation techniques such as density functional theory (DFT) and molecular dynamic (MD) have been extensively utilized to provide theoretical framework to analyze the experimental results obtained from spectroscopic techniques and gain scientific insights on reaction mechanisms and catalytic trends [29,30]. However, establishing structure-function relationship of tar reforming catalyst based on microkinetic modeling, spectroscopic techniques and computational simulation using actual tar is a stimulating challenge as tar is a mixture of polyaromatic compounds. Hence, aromatic compounds such as benzene, toluene and phenol are frequently used as tar model compounds. Due to the complexity of the tar model compounds, several reactions are possible to occur simultaneously. Some of them are as follows: (1) Steam reforming S / C = 1 : C x H y + xH 2 O → xCO + x + y / 2 H 2 (2) Steam reforming S / C = 2 : C x H y + 2xH 2 O → xCO 2 + 2x + y / 2 H 2 (3) Water − gas shift : CO + H 2 O → CO 2 + H 2 (4) Dry reforming : C x H y + xCO 2 → 2xCO + y / 2H 2 (5) Steam reforming of methane : CH 4 + H 2 O → CO + 3H 2 (6) Carbon formation : C n H x → nC + x / 2 H 2 In addition to above reaction, below reactions are also possible for steam reforming of toluene reaction. (7) Hydrodealkylation : C 7 H 8 + H 2 → C 6 H 6 + CH 4 (8) Steam dealkylation : C 7 H 8 + H 2 O → C 6 H 6 + CO + 2H 2 (9) Steam dealkylation : C 7 H 8 + 2H 2 O → C 6 H 6 + CO 2 + 3H 2 Swierzcynski et al. initially hypothesized that toluene was dealkylated from the methyl group to form benzene as the main product. The adsorbed methyl group was then reformed to CO + H2. The adsorbed benzene underwent CH bond scission and CC bond scission to form linear hydrocarbon fragments which subsequently transform into CO + H2 [31].Benzene, a non-polar compound, can experience intermolecular forces when adsorbed on the metal surface. Recent DFT and surface study have shown that the discrepancy between simulated and experimental values can be reduced when intermolecular forces are accounted for in DFT simulations [32]. Mei et al. performed combined experimental and DFT study for steam reforming of benzene on Rh/MgAlO4 and Ir/MgAlO4 catalysts [33]. The TOF of Rh/MgAlO4 was reported to be higher than Ir/MgAlO4 catalyst due to reduced CC and CH bond scission while a smaller Ir nanoparticle size only affects CC bond scission. The DFT simulation performed in that study suggested that aliphatic CH and CC bonds are both competitive on Rh (111) surface due to similar reaction energy which leads to phenyl and linear C6H6 species. The CH bond scission of phenyl at the meta position is more favorable than para position. The CH bond breaking preference order for ring-opening of linear C6H6 species goes in the order of decreasing endothermicity as follows: ortho > meta > para. CC bond scission of the linear hydrocarbon chain leads to formation of C4H4 and C2H2 acetylene species [33].The findings from benzene as a tar model are also equally applicable to oxygenated aromatic compound such as phenol which is also a major constituent in tar mixture. Spencer et al. experimentally performed phenol adsorption onto Ni (111) surface and compared to DFT results. Heat of adsorption of phenol reduced as surface coverage which indicates that multi-layer coverage is formed on the nickel surface. The measured bond energy is compared with the ones obtained from DFT. The bond energy estimated by DFT is significantly smaller than the experimentally determined values by 87–94 kJ/mol. Reasons for significant underestimation are attributed to the presence of van der Waals intermolecular force and possibly the functionals used in DFT calculation. The extent of underestimation is reduced when van der Waals force is included in the DFT calculation to account for long-range intermolecular effects but the magnitude of underestimation remains significant [34]. Nonetheless, this study is an important example on including intermolecular forces in surface DFT calculation and the need to keep improving the DFT method for more accurate estimation of surface bond properties.Comparatively, toluene as a model for tar is much investigated than other model compounds because it represents the stable aromatic structure and apparent in tar formed with high-temperature processes. Quang et al. comparatively investigated toluene decomposition behavior on Ni (111) and boron-doped Ni (111) surfaces and proposed a toluene decomposition mechanism as shown in Fig. 1 [35]. The π electrons in toluene aromatic ring are transferred to metal cations which stabilize the aromatic ring and result in toluene adsorption on the catalyst surface [35,36]. The first step in toluene decomposition starts with CH bond activation of the methyl group (CH3) as the activation energy and reaction energy for this step is lower than aromatic CH dissociation [35]. This is shown in step 1 as CH3 is dehydrogenated into CH2*. Such facile CH bond dissociation of the methyl group is also observed on stepped and stepped-kinked Ni surface under ultra-high vacuum (UHV) condition [37]. This is followed by further dehydrogenation of the CH2* to C*, from step 2 to step 3. CH bond dissociations at the ortho or/and meta position occurs at step 4. From step 5 onwards, ring opening via aryl CC bond cleavage and more CC cleavages occurs to generate shorter hydrocarbon chains which become energetically possible after the ring opening steps.From these computational DFT investigations by other co-authors, toluene decomposition on catalyst surface is clearly a complex phenomenon. However, the effect of water addition on the metal surface, in the presence of toluene, cannot be ignored. Mukai et al. performed in-situ DRIFTS for steam reforming of toluene over Ni/La0.7Sr0.3AlO3 − δ catalyst in an attempt to elucidate the reaction mechanism for steam reforming of toluene [36]. The rate-determining step is suggested to change at 400 °C as the absorbance profile at that temperature between the wavenumber 1250 and 1750 cm−1 remains unchanged after steam introduction which may suggest that the peaks belong to a reactive intermediate. To determine the identity of reaction intermediate with wavenumber between 1250 and 1750 cm−1, different probe molecules such as benzene, n-heptane, ethylene and benzaldehyde are introduced to the catalyst surface. Based on the DRIFTS results after dosing with different listed molecules, the authors concluded that toluene was decomposed into C2 species to form reactive intermediate on Ni/La0.7Sr0.3AlO3 − δ surface in the presence of steam.Usman et al. performed in-situ DRIFTS and temperature-programmed surface reaction using steam and toluene to elucidate steam reforming of toluene reaction mechanism and identify the reactive intermediate [38]. The presence of aldehyde species is detected from respective CO and CH stretching signals at 1760 cm−1 and 2820–3170 cm−1 and is assumed to be the reactive intermediate. Based on the presence of aldehyde (CHO) intermediate, the reaction mechanism and Langmuir-Hinshelwood type kinetic model for steam reforming of toluene over La0.8Sr0.2Ni0.8Fe0.2 catalyst was proposed and experimentally validated using non-linear regression fitting as shown: (R1) C 7 H 8 + s 1 ↔ K 1 C 7 H 8 · s 1 (R2) C 7 H 8 · s 1 + s 1 ↔ K 2 C 6 H 6 · s 1 + C H 2 · s 1 (R3) C 6 H 6 · s 1 + 2 s 1 ↔ K 3 3 C 2 H 2 · s 1 (R4) H 2 O + s 2 ↔ K 4 O · s 2 + H 2 (R5) C 2 H 2 · s 1 + O · s 2 ↔ K 5 C 2 H 2 O · s 1 + s 2 (R6) C 2 H 2 O · s 1 + s 1 ↔ K 6 C H 2 O · s 1 + C · s 1 (R7) C H 2 · s 1 + O · s 2 ↔ K 7 C H 2 O · s 1 + s 2 (R8) C H 2 O · s 1 + s 1 ↔ K 8 CHO · s 1 + H · s 1 (R9) CHO · s 1 + s 1 ↔ K 9 CO · s 1 + H · s 1 (R10) CO · s 1 + O · s 2 ↔ K 10 C O 2 · s 2 + s 1 (R11) CHO · s 1 + O · s 2 ↔ K 11 C O 2 · s 2 + H · s 1 (R12) C · s 1 + C O 2 · s 2 ↔ K 12 2 CO · s 1 + s 2 (R13) CO · s 1 ↔ K 13 CO + s 1 (R14) C O 2 · s 2 ↔ K 14 C O 2 + s 2 (R15) 2 H · s 1 ↔ K 15 H 2 · s 1 + s 1 (R16) H 2 · s 1 ↔ K 16 H 2 + s 1 r = k f C 7 H 8 1 3 CO 2 H 2 O 4 3 CO 4 3 H 2 8 3 − k b CO 2 1 + K CO · p CO + K H 2 p H 2 + K toluene p toluene 1 + K CO 2 p CO 2 + K H 2 O p H 2 O p H 2 The reaction between the aldehyde species and oxygen species to produce CO2 (Eq. (R11)) was found to be the rate-determining step for steam reforming of toluene based on close fitting of the parity plot. The apparent activation energy of 109.64 kJ/mol was reported which was attributed by the author to the presence of lattice oxygen [38].Recently, Du et al. has performed steam reforming of toluene over Ni nanoparticles supported on pyrolyzed carbon at 600 °C and has observed that the TOF for smaller Ni nanoparticles is more than the TOF for larger nickel nanoparticles [39]. The authors assume that there are more stepped surfaces on smaller nanoparticle than larger nanoparticle. DFT simulations are performed by the authors which demonstrate that the toluene adsorption on stepped Ni (111) surface are stronger than flat Ni (111) surface as there are more un-saturated coordinated sites on stepped Ni (111) surface. The authors also proposed that toluene adsorbs parallel to the surface, followed by dehydrogenation of the methyl group in a step-wise manner. Aromatic CH bond cleavage and aromatic ring opening occurs. The ring-opened aliphatic hydrocarbon undergoes further CC bond cleavage to form C3 and C4 fragments.Novel methods for the preparation of heterogeneous catalyst systems endowed with preferred properties are important fundamental research areas. In general, the metal surface constitutes the main active site for the catalytic reforming reaction. Moreover, the surface properties of the catalyst support also play an integral role in defining the reaction mechanism [40–43]. For steam reforming of tar, the catalyst should be able to activate both the reactants that are tar and steam. The balance between these two reactions controls the activity and stability of a catalyst. Since tar is heavy aromatics the major catalyst deactivation is identified to be active metal sintering in high steam environment and encapsulation of carbon over active metal centers. In order to address these issues several approaches in developing efficient catalyst has been reported in the literature. By considering the recent reports on steam reforming of tar model reaction, in this section we have categorized the catalysts as role of support, oxygen mobility, basicity and alloying with second metal with respect to catalytic conversion and carbon suppression.Support is the backbone of any catalytic material offering several advantages including strongly binding the metal with the support which prevents leaching out of active metal, provides acidic, basic centers, oxygen vacancy and also improves the performance. Therefore, the choice of support material is crucial for steam reforming of biomass tar reactions. As discussed before, the major challenge in tar model steam reforming reactions includes metal sintering and coke deposition. Under this section, we will be discussing about the role played by different properties of support to overcome the challenge along with discussion on the current development in the new types of catalyst support. Table 1 summarizes catalytic systems reported for steam reforming of biomass tar model reactions.Alumina is one of the most commonly used catalyst support for nickel catalysts for tar reforming reaction in view of its good mechanical strength and chemical/physical stability, and availability in dispersion of the active metal phase. α- and γ-Al2O3 are the commonly used alumina phase support for toluene steam reforming [44]. He et al. studied the effect of two phases of alumina (α and γ) as a catalyst support for Ni on hydrogen production and formation of carbon nanotubes during the steam reforming of toluene. This study found that the performance of Ni/γ-Al2O3 was superior than Ni/α-Al2O3. Due to enhanced metal support interaction between Ni particle and γ-Al2O3 led to the base-growth mechanism of CNTs on Ni/γ-Al2O3 in which Ni particles located at the bottom of CNTs, thus the growth of CNTs covered Ni active sites and decreased catalytic activity of Ni/γ-Al2O3 [45]. In order to improve hydrothermal stability under aqueous phase reforming condition, silylation of metal oxide surfaces has also emerged as a promising route to tune the surface properties of inorganic materials, such as silica, zeolites and alumina [46,47]. In other study Artetxe et al. studied steam reforming activities of various biomass tar model compounds using Ni/Al2O3 catalysts and found that the highest carbon conversions and H2 potential for anisole and furfural, while methyl naphthalene gave the lowest reactivity [48]. Among all the model compounds, the amount of coke was higher with oxygenate reactants due to their higher reactivity favoring unwanted reactions that promoted its formation. F. Liu et al. modified Pt/γ-Al2O3 catalyst with silica to prevent the hydrolytic attack, as shown in Fig. 1. γ-Al2O3, silylation followed by high temperature calcination could block the surface Lewis acid Al sites that serve as initial hydration sites for boehmite formation via coordinative saturation and the formation of Al-O-Si bonds, inhibiting water adsorption [49]. The improvement in the thermal stability of Al2O3 with SiO2 doping is also observed by others [49–53]. Similarly, M.A. Adnan et al. synthesized Fe2O3/SiO2 doped Al2O3 by one-pot synthesis method and the toluene conversion of about 76% was reported for 10% Fe2O3/SiO2 doped Al2O3 catalyst at 600 °C. The Fe2O3 accelerates the γ-Al2O3 collapse and transform into other phases, leading to the loss in surface area. And presence of Fe2O3 generates new strong acid sites in Fe2O3/SiO2 doped Al2O3 catalysts. And they have observed that the strong acidic sites promote tar cracking and toluene conversion reactivity. Therefore, the toluene conversion of a catalyst is controlled by the balance between the specific strong acidic sites and surface area of the catalyst [54].Yet in another study, Castro et al. studied the effect of bare Al2O3 and CeO2 doped Al2O3 as a support for Pt metal. The addition of Ce decreases the density of acidic sites as observed from ammonia TPD data due to coverage of acidic sites of Al2O3. Due to this Ce doped Al2O3 showed highest formation of CO2 in comparison to CO due to promotion of higher WGS reaction (Fig. 2 ) [55]. In another study, the Ce/Zr ratio supported on Al2O3 was varied. The study found that for all the catalyst the toluene conversion decreased tremendously during the 22 h operation with no significant coke formation. The decrease in Ce/Zr ratio increased the acid site density and the carbon formation rate. Ce present on the catalyst surface covered the acidic centers of Al2O3 and covered the Lewis acidic centers which inhibit higher coke formation [56]. Many studies were also carried out on hexa-aluminates as catalyst support for various number of reactions [57]. A different approach to modified alumina based catalyst is the development of hexaluminates based support. La, La/Ce and Ca hexa-aluminates were synthesized using co-precipitation method. Among the three doped hexa-aluminates, La/Ce showed the superior performance. For La/Ce the nickel dispersion was independent of Ni loading and showed stable performance for 18 h operation. The Ce promoted catalyst exhibits reduction in the interaction between metal and support thus promotes more metal dispersion [58].Hydrotalcites are another class of support materials which has gained huge interest. Hydrotalcites, when calcined above 450 °C forms mixed metal oxides which exhibits various advantageous properties for catalytic applications such as large surface, higher metal dispersion and synergistic effect between metal and metal oxide support [59,60]. One of the major issue when Al2O3 alone was used as a support is higher coke deposition and metal sintering. And addition of alkaline earth metals such as MgO and CaO in the form of hydrotalcite precursor have shown improved resistant towards coke formation and metal sintering. K. Tomishige group has studied various mono- and bi-metallic catalysts derived from parent Mg-Al-Ox hydrotalcite precursor for biomass derived tar reforming [61,62]. D. Li et al. studied the effect of composition and reduction pretreatment condition of Ni/Mg/Al catalyst for steam reforming of biomass derived tar. The characterization results showed a nanocomposite was formed between Ni metal surface and Mg(Ni,Al)O particles comparatively smaller Ni particles are formed and lower coke deposition [63]. In conjugation, Ashok et.al studied the hydrotalcites derived from NiO-CaO-Al2O3 oxides and observed the established synergy between NiO and CaO species. The better performance at catalyst composition of NiO-CaO-Al2O3 (8:62:30) was attributed to high basic strength of support, metal dispersion and higher resistance towards metal sintering [64]. Likewise, Lertwittayanon et al. [65] studied the effect of promoting CaZrO3 nanoparticles on Ni/α-Al2O3. The study varied the loading amount from 0 to 15% and found that the loading of 15% showed the highest activity. Mayenite (Ca12Al14O33) type of support which also consists of CaO has also been an active and stable support for steam tar reforming and CO2 capture.Overall, for steam reforming of tar reactions, Al2O3 was considered as preferred support materials by many researchers due to its availability and ease of catalysts synthesis with various catalyst formulations. The doping of Al2O3 with various base metal oxides such as MgO, CaO, SrO, CeO2, ZrO2 and so on were investigated to suppress or neutralize the Lewis acidic centers which helps for the activation of CH bond in the hydrocarbons and polymerize the carbonaceous species to deposited as coke on the catalyst. Furthermore, hydrotalcites derived catalyst possess great potential in terms of stability and better metal dispersion due to enhancement in the basic properties of the catalysts. Similarly, modifying Al2O3 with redox metal oxides (CeO2, ZrO2) helps to improve the oxygen vacancy in the support thereby suppress the coke formation. Finally, Al2O3 is one of the widely accepted supported material for most of the steam reforming applications.Silica being the inert support offers several advantageous properties such as high surface area, thermal stability and metal sintering resistance due to the ease in the formation of metal silicate [66–69] [70–72]. There are several modifications adapted in the preparation method to improve the metal support interaction [73–76]. A very promising class of silicate, phyllosilicate, is one of the widely accepted class of mineral which unique property playing a dynamic role in catalysis. Phyllosilicate structure derived Ni/SiO2 catalyst has been reported for steam reforming of ethanol (SRE) reactions [77]. The stronger interaction between Ni and SiO2 facilitates partial reduction of Ni species, which in turn provided necessary hydroxyls species to enhance SRE activity and suppressed carbon formation. Recently, there has been extensive research based on utilization of natural minerals materials as a catalyst support being the low cost materials. Ni-phyllosilicate catalyst, this type of support has been deeply investigated by our group [78–81]. This study includes investigating the shape effect of phyllosilicate in terms of metal support interaction, coke resistant and metal sintering. Core-shell catalyst was investigated elucidating the effect of silica shell in preventing metal sintering and thus enhancing the stability and performance [82–86]. Recently, Li et al. reported the NiCo@NiCo phyllosilicate@CeO2 hollow core shell for steam reforming of toluene as a biomass tar model, as shown in Fig. 3 . The catalyst showed good catalytic activity and stability for 45 h on stream. The activity was due to strong NiCo interaction and also with the support CeO2. The synergistic effect between Ni and Co also prevented the formation of carbon on the catalyst surface [87].Mesoporous silica in the form for SBA-15 has been widely used for various applications due to high surface area and ordered, uniform hexagonal pores. Modification of SBA-15 with other lanthanide series elements such as La, Ce has been reported to increase the oxygen mobility, inhibits cracking of larger molecules into smaller and improves stability of the support. Usman et al. studied the series of La2O3 modified Ni/SBA-15 catalysts prepared by organic acid assisted synthesis method. As compared to Ni/SBA-15, La doped Ni/SBA-15 showed lower coke formation by forming oxy-carbonates thus making the catalyst stable for >30 h of operation (Fig. 4 ) [88]. In other study Ni support SBA-15 was compared with Ni/Al2O3 catalyst for steam reforming of guaiacol as a tar model. It was reported that the deactivation in Ni/Al2O3 catalyst is due to the encapsulation of amorphous carbon, while the carbon nanotubes grown on the tip of Ni particle and had less impact on catalyst deactivation for Ni/SBA-15 catalyst [89]. Another type of mesoporous support was used in the form of MCM-41. Nickel was impregnated on MCM-41 using ethylene glycol (EG) assisted co-impregnation method. An enhanced metal dispersion with higher metal support interaction was observed when compared with Ni/MCM-41 prepared by conventional wetness impregnation method. The catalyst also showed a good reusability up-to 5 cycles. In terms of H2 yield, the value increased by 8% when using 20 wt% Ni/MCM-41-EG as compared with 20 wt% Ni/MCM-41 for the steam reforming of tar.Besides Al2O3, SiO2 is one of the most commonly used catalyst support for various high temperature reforming applications due to several advantages such as availability, high surface area and thermal stability by forming stable phase such as metal silicates. One of the major issue for silica-based catalysts in high temperature steam reforming applications is the leaching of silica material in high steam reaction environment. Therefore, for commercial steam reforming applications the utilization of silica based catalysts is limited to lower SiO2 content. And there are limited studies were reported for steam reforming of biomass tar model reforming reaction using silica-based materials as catalysts. Recently, Ni/Co-phyllosilicate structured silica based materials seem to be more interesting for steam reforming applications due to the presence of hydroxyl species as structural moieties of the phyllosilicates. And the involvement of hydroxyls species in activating steam during high temperature reforming reaction makes these materials gave stable catalytic performance with reduced carbon deposition. However, the structural stability of silica groups in steam reforming environment is required detailed investigation. Until now, silica based materials are fall behind Al2O3 support for biomass tar reforming applications.Similar to hydrotalcite materials, the oxides derived from perovskite materials are also used as a catalyst and/or catalyst support for steam/CO2 reforming reactions. These kinds of materials known to offer high oxygen vacancies, presence of lattice oxygen which enhances oxidation of hydrocarbons adsorbed on metal, redox property and thermal stability [90]. Mukai et al. carried out series of studies to investigate the Ni support on perovskite catalyst for steam reforming of toluene [91]. However, highest conversion and H2 was achieved by LaAlO3 as compared to other combinations of A and B site elements. Although the performance was highest, the coke deposition on the catalyst was significant. Thus, in order to suppress the coke formation, when A-site was partially substituted with Sr the activity enhanced and the coke deposition decreased approximately 3 times that of un-doped LaAlO3. The Sr doping increased the metal support interaction which was obtained by calcining the catalyst at different temperatures. After calcination at higher temperature the percentage dispersion was decreased which suppressed the catalytic activity.Perovskite structure also consists of active metal ions such as Ni/Co/Fe in the B-site of the crystal. Therefore, it is important to study the effect of perovskite as only support with no active metal present on its structure. Laosiripojana et al. studied the effect of palygorskite, MgO–Al2O3, La0.8 Ca0.2CrO3, and La0.8Ca0.2CrO3/MgO–Al2O3 for Ni and NiFe as active metal sites. Among all the catalyst tested NiFe supported on La0.8Ca0.2CrO3/MgO–Al2O3 shows the highest value of H2 yield and resistance towards coke formation [92]. Alumina is one of the most prominently used B-site elements for reforming reaction due to its acidic nature which helps to break the CC bond. Takise et al. extensively investigated numerous perovskite as a support for cobalt metal including La0.7Sr0.3AlO3 − x (LSAO), La0.7Ca0.3AlO3 − x (LCAO), La0.7Ba0.3AlO3 − x (LBAO), LaAlO3, Sr/LaAlO3, LaAl0.7Zn0.3O3 − x, SrTiO3, SrTi0.7Fe0.3O3 − x, SrZrO3, and SrCe0.5Zr0.5O3 − x [93–95]. In this study, the catalysts are categorized at La ion and Sr ion incorporated perovskite oxide. Co/LSAO and Co/LCAO showed highest activity and stability for 300 min of operation. In Sr-ion category, Co/SrZrO3 performed highest among all other catalyst. In TPR study, the reduction peak for Co/LSAO was larger than Co/LaAlO3 ensuring the higher metal support interaction (CoO); higher dispersion of metal ion was also observed. Another study reported the Ni/La0.7Sr0.3AlO3 − x which showed highest toluene conversion of 67.5% at 650 °C. The remarkable enhancement in catalytic activity was due to the insertion of Sr which caused the lattice distortion and also helped in suppressing the coke deposition when compared with Ni/La2O3 and Ni/LaAlO3 [96].Perovskite based support has great potential in terms of providing the oxygen vacancy for the steam activation as well as gasifies the carbon formed during the reaction. However, due to low surface area and cost of the synthesis makes it difficult for commercialization. The further modification in the perovskite type of support may be focus on the improvement of surface area and finding the facile and inexpensive route for the synthesis will make this system more viable for the gasification process.Biochar, one of the by-product of biomass gasification process was reported as a potential candidate as support material for tar reforming reaction. Due to high surface area, higher pore volume, thermal stability and availability of surface functional group has greatly attracted attention towards investigating biochar as a catalyst for tar reforming reaction [97]. The char can be activated to carbon and used as a catalyst support, as the activated carbon is considered to be stable under both acidic and basic condition; it is indeed flexible enough in terms of change in textural and chemical properties. The metal dispersion and interaction between metal and carbon also affect the catalytic performance of the carbon based catalysts [98–100]. Liu et al. used biochar as catalyst for tar reforming reaction and it was reported that the tar destruction ability of biochar is influenced by O-containing functional groups in biochar [101]. Qian et al. utilized gasification derived Red cedar char as a support. Before impregnating it with metal the char was activated, then followed by impregnation with nickel precursor. The study includes the effect of nitrate precursor and pre-treatment method. Catalyst prepared by impregnating nitrate precursor showed the highest performance of around 80% conversion at 700 °C. However, the long term stability test any of the catalyst was not shown. The surface area of spent catalyst decreased and large portion of nickel appeared on the surface due to structural destruction of activated char during reforming reaction [102]. Furthermore, Zhen-Yi et al. prepared Ni/Biochar via simple one-step pyrolytic approach and investigated for steam reforming of toluene at relatively lower temperature of 600 °C (Fig. 5 ). A highly dispersed nickel particles were obtained on the supported catalyst. This was shown due to the presence of high porosity and also the presence of abundant amount of surface oxygen on the raw biomass. The nickel particle size played crucial role in enhancing the catalytic activity and stability which showed the smaller nickel particle (4.2 nm) gave a turn over frequency of 1.64 s−1 [103]. Additionally, the pyrolysis temperature played more significant role in forming smaller nickel particle sizes [39].Another strategy to improve the mechanical stability and chemical resistibility of activated char was studied by J.-P. Cao et al. [104]. In this paper, a novel porous carbon catalyst was prepared by inserting metallic nickel in D151 resin through ion exchange. The result showed that Ni dispersion depends on two parameters, pH and carbonization temperature. Ni/resin char showed higher performance as compared to commercially available catalyst Ni/Al2O3. As compared with other types of char such as bio-char and lignite char, in Ni/RC (resin char) the metal is embedded in a definite chemical structure and thus showed higher potential to be used in catalytic biomass gasification [104]. Bimetallic (NiRe) supported on sewage sludge char was prepared and investigated its performance cracking and reforming of biomass tar. At optimum temperature of 800 °C, the 3%Re-7%Ni/Char showed the highest conversion and H2 yield, and the performance was stable for 18 h of operation with only 5% coke deposition [105].A different type of char studied is the hydro-char. It is considered as an ideal support for the synthesis of nickel based nanomaterials derived from hydrothermal carbonization of carbohydrate rich biomass [106]. C. Gai et al. proposed and synthesized one pot hydrothermal carbonization of metal on hydrochar to obtain smaller size Ni and NiFe metal, as shown in Fig. 6 . With the help of characterizations such TEM, the author elucidates factors that affect the structure, composition, and size of the nanoparticles [107].Biochar based materials as catalysts for tar reforming reaction is an emerging area which is the least expensive material and versatile route to modify the catalyst. Owing to the high surface area, this type of support increases the availability of active sites for the reaction. Yet, the thermal and chemical stability of char is questionable which decreases the activity during the reaction. Recently, many researchers are focussing on this class of catalyst materials especially for commercial biomass gasification processes.Apart from mostly studied Al2O3, SiO2, perovskite and char based support used for this reaction, there are several other types of support (mixed oxide, core-shell, natural minerals) which have shown promising behavior will be discussed in this section. It has been reported in the literature that Ni/TiO2 catalyst formed a mixed oxide (NiTiO3) when calcined at high temperatures (i.e. 800 °C) [108]. After activation, NiTiO3 system enabled the coexistence of smaller particles which were more active and resistant to deactivation and sintering during SRE reaction. Similarly, the strong metal-support interactions could also be obtained between Ni species and sol-gel synthesized Al for Ni/Al2O3 catalysts which were calcined at high temperatures [109].Core shell like catalyst has been extensively investigated for various thermal reactions including reforming reactions [110]. In one study [111], confinement effect observed in Ni@ZrO2 core-shell catalyst prevented metal sintering, enriched surface active oxygen content and widened metal–support interfacial perimeter, thereby aiding the removal of carbon deposits during SRE reaction. Core shell like structure, Ni@Al2O3 was studied by Qian et al. (Fig. 7 ). The stable shells provide the unique environment around active sites and the strong interaction between Ni and the core–shell supports seems to be responsible for the catalyst activity and stability in toluene steam reforming [112].Similarly, X. Zou et al. studied the inexpensive and earth abundant materials known as Palygorskite (Pal). Fe3Ni8 was impregnated on this support Fe3Ni8/Palygorskite catalyst with high dispersion was successfully prepared and exhibited superior catalytic performance compared with those of the monometallic catalysts (Fe3/Palygorskite and Ni8/Palygorskite) and the bare Palygorskite. Olivine is a natural occurring mineral in the form of (MgxFe1 − x)SiO4. This support has been widely used for biomass steam reforming [113,114]. From all these studies, it was found that the presence of olivine in the reactor offers following advantages, decreases the tars content, increases syngas yield, and a decrease in the amount of CH4. Another preferred aspect of Olivine is due to the high sintering resistance compared with other nature ore catalysts. J. Meng et al. reported the performance of iron supported on olivine prepared via thermal fusion reaction. The effect of thermal fusion was studied and it was found that a higher thermal fusion temperature (1400 °C) enhanced the interactions between Fe and olivine supports. This lead to the formation of a new phase, (Mg, Fe) Fe2O4 which promoted the increase in a high H2 yield and also showed higher resistance towards coke formation [115]. This author reported a study where Ni was used as a major element supported on Olivine prepared via thermal fusion technique where the toluene conversion of 99.6% was achieved [116]. This study was assisted by various range of characterization to understand the physicochemical properties of both support and catalyst. Only Fe/Olivine showed higher toluene conversion whereas Fe-NI/Olivine prevented coke formation with the highest toluene conversion of 98.44% and carbon formation of 7% after 45 h of reaction. Yet in another study, Fe/Olivine [117,118] strong interaction between iron and olivine, led to the attainment of an equilibrium between Fe0/Fe2+/Fe3+ in partially reduced catalyst that was proposed to be responsible for the catalyst activity and stability.Mixed oxides were also reported for biomass gasification process. Herein, various types of supports were investigated. The enhancement in catalytic performance of Ni species in Ni supported over MgO-Al2O3 [119,120], MgO-CaO [121,122], and CeO2-ZrO2 [123,124] was mainly attributed to intimate interaction between Ni and supports. In another study, cheaper and ecofriendly source of support materials can be derived from Municipal solid waste (MSW) incineration process, which consists of wide range of metal and metal oxides to be used as catalyst [125]. According to them upon hydrothermal treating with 3 m NaOH, the incinerator bottom ash (IBA) act as better catalytic support than un-treated IBA. And the nature of the support is greatly influenced by the time of hydrothermal treatment.Naturally occurring and waster derived materials are available in abundance promising properties as a catalyst or support. This type of supports can be modified by means of synthesis method or treatment under higher temperature. Few drawbacks of these materials include low surface area, and complex metal/metal oxide composition. The determination of exact properties of these materials which leads to catalytic activity is a challenge. Therefore, the availability of in-depth characterization techniques is in scarce.The oxygen storage capacity and oxygen mobility of the catalyst support are important parameters that affect the activity and stability in reforming reactions. The presence of oxygen vacancies and mobile oxygen species in the support structure can enhance the activation of steam in steam reforming reaction and increase the transport and supply of oxygen to the active sites to oxidize active carbon species to CO. Greater amount of oxygen vacancy and higher oxygen mobility are crucial for surface carbon gasification since they promote the prevention of carbon deposition. In fact, oxides with high surface oxygen mobility aid in H2O activation, promotion of water gas shift reaction as well as the oxidative elimination of surface bound carbon present on Ni nanoclusters [40]. This ultimately improves the selectivity, activity and stability of the catalyst. Moreover, for higher hydrocarbons such as tar, the undesirable coke deposition is due to adsorption and desorption of unsaturated hydrocarbons and tars on the catalyst surface [38]. Reducible supports like perovskite oxides and ceria are known for their lattice oxygen mobility. In perovskite (ABO3) oxides, the partial substitution of A or B site metals by other elements with different valence or cationic radius gives rise to structural defects in the lattice, that generates oxygen vacancies and mobile oxygen species [38,96,126]. On the other hand, CeO2 can undergo redox cycles between CeO2 and CeO2 − x under the steam reforming atmosphere due to its unique property of being able to stabilize both Ce4+ and Ce3+ ions in its fluorite structure. The formation of partially reduced CeO2 − x gives rise to oxygen vacancies that can activate steam and absorb the active oxygen species, which can thereafter diffuse to the active metal-ceria interface and participate in the reaction. In the following section, the effect of oxygen mobility of supports in steam reforming of tar will be discussed with respect to type of materials.CeO2 is well-known for its redox nature and oxygen storage capacity (OSC). The CeO2 support can undergo rapid redox cycles and accept and release lattice oxygen in a reaction [127,128]. Addition of CeO2 to other supports has hence shown improved activity and coke resistance in steam reforming of tar. For example, Ashok and Kawi [129] reported that addition of appropriate amount of CeO2 to Ni/Ca-Al catalysts enhances activity in SRT reaction and suppresses carbon deposition by promoting the oxidation of carbon precursors deposited on nickel surface by supplying necessary oxygen species. It is also worth mentioning that the redox nature and OSC of CeO2 makes it highly active for water gas shift reaction, which in turn produced higher values of H2/CO for Ni/Ca–Al–Ce catalysts than Ni/Ca–Al catalyst.The incorporation of ZrO2 in CeO2-ZrO2 mixed oxides has been shown to increase the concentration of oxygen vacancies and mobile oxygen species in the support, and promote the support reducibility, leading to higher stability in reforming reactions [127]. Maia et al. studied the effect of doping 25%, 50% and 75% Zr in Ni/CexZr1 − xO2 and observed that the highest stability was observed on Ni/Ce0.5Zr0.5O2 in steam reforming of glycerol. OSC measurements revealed that a doping of 25–50% Zr in the CeO2 structure resulted in highest oxygen storage capacity, which could be correlated with the higher stability and lower coke formation on these catalysts [130]. Doping ceria with rare earth metal oxides like samaria, gadolinia have also been shown to increase activity and coke resistance in tar reforming by increasing the oxygen storage capacity. For instance, Laobuthee et al. observed that doping a small amount of Sm in CexSm1 − xO2 (without any metal doping) increases the reducibility and oxygen storage capacity of the oxide and increases the hydrogen yield from steam reforming of toluene from 20.2% on CeO2 to 32.8% on Ce0.85Sm0.15O2 [131]. Higher Sm doping leads to a fall in the oxygen storage capacity, possibly due to the inability to form a pure solid solution structure, and a concurrent drop in activity is observed. In another study [132], it was observed that incorporation of Mn into Ce0.75Zr0.25O2 mixed oxide has the potential to modify its redox properties by introducing more structural defects and oxygen vacancies and increasing the oxygen mobility. Upon impregnation with Ni, the presence of Mn was found to vividly decrease carbon deposition in the steam reforming of tar (using naphthalene as the model compound) by promoting surface carbon gasification and/or water gas shift reaction. Castro et al., however, reported a different trend for Pt/CeO2-ZrO2/Al2O3 catalysts where an increase on oxygen vacancies and oxygen mobility due to the addition of ZrO2 in CeO2 did not benefit the coke resistance of the catalyst. Instead, the doping of ZrO2 increased the core formation tendency in steam reforming of toluene, which was attributed to an increase in acid sites that catalyzed the oligomerization of toluene into heavier aromatics and carbon deposits. The authors inferred that the increase in oxygen storage capacity by zirconia addition did not have a beneficial effect in suppressing coke formation [133].Addition of Fe in Ni catalysts has also been shown to improve activity and coke resistance in steam reforming of tar by increasing the amount of lattice oxygen and adsorbed oxygen on the catalyst surface. Fe in the NiFe alloy can easily be converted to FeOx by steam, which can then supply lattice oxygen to oxidize carbon and other intermediates on Ni to form CO [134,135]. Ashok et al. showed that surface enrichment of iron species on nickel in Ni/Fe2O3-Al2O3 catalyst enhanced the conversion of toluene and suppressed coke formation by increasing the coverage of oxygen species on the catalyst active sites [136]. Similarly, Sun et al. studied the reaction process of different oxygen species in a series of Ni-Fe/Al2O3 catalysts during steam reforming of toluene. Based on a H2O/toluene pulse study, they proposed that adsorbed oxygen from iron species participates in oxidation of intermediates from toluene decomposition to produce CO/CO2 through a Langmuir-Hinshelwood mechanism and the lattice oxygen in FeOx oxidizes the carbon deposition or intermediates following a Mars-van Krevelen mechanism [137].Perovskite oxides are well-known for their oxygen transport capabilities through the mobility of lattice oxygen, spurring their application in oxygen permeable membranes. It has been shown that introduction of heteroatoms in the A or B sites of ABO3 perovskite oxides cause a lattice distortion and increases oxygen vacancies, leading to an improvement in redox ability and oxygen mobility. Oemar et al. studied the effect of Sr doping in the A site of LaAlO3 supported Ni catalysts and observed that Ni/La0.8Sr0.2O3 catalyst showed superior catalytic performance both in terms of activity and coke resistance in steam reforming of toluene [96]. Using XRD, TPR, XPS and O2-TPD, they showed that the superior catalytic performance of the Ni/La0.8Sr0.2AlO3 catalyst was a result of lattice distortion caused by strontium doping, which produced a higher concentration of oxygen vacancies on the catalyst surface. This lowered the activation energy of the migration of lattice oxygen, enhancing the mobility of lattice oxygen species which favored the direct partial oxidation of toluene, and also improved the adsorption abilities of gas phase oxygen species. Sekine et al. has reported that the suppression of coke and enhanced activity is possible when A-site in LaAlO3 perovskite is partially substituted with Sr oxide. The decrease in coke formation is approximately 3 times that of un-doped LaAlO3. Sr doping enhanced the mobility of lattice oxygen on the perovskite as shown by transient response test using H2 18O [138]. The Sr role and mechanism was further investigated by transient isotopic technique [95]. The role of lattice oxygen and metal support interaction was obtained by calcining the catalyst at different temperature and it was found that higher the temperature for calcination the percentage dispersion was smaller and also the catalyst activity decreased from 72.9% (750 °C) to 15.5% (1100 °C). Similarly, Mukai et al. demonstrated with transient response techniques using H2 18O that the surface lattice oxygen of La0.7Sr0.3AlO3−δ worked as active oxygen by redox mechanism at 600 °C and oxidized the adsorbed CHx species on the Ni surface directly, forming CO in steam reforming of toluene [139]. Arrhenius plots showed that the rate-determining step of the SRT reaction changed at a certain temperature at which the lattice oxygen was able to contribute to reaction over Ni/La0.7Sr0.3AlO3−δ and Ni/LaAlO3. The same authors also showed that the lattice oxygen mobility of the La0.7Sr0.3AlO3−δ support could be enhanced by increasing the Ni particle perimeter or the Ni/support interface area. By changing the Ni particle size through different calcination temperatures, it was observed that the catalytic activity of the catalyst could be correlated to the Ni particle perimeter on the support instead of the metallic surface area, and carbon decomposition increased concomitantly with a decrease in this Ni perimeter [91]. The presence of oxygen vacancies in the La0.7M0.3AlO3−δ(M = Sr, Ca, Ba) support was also postulated to create an anchoring effect on Co nanoparticles that stabilized them and prevented sintering under toluene reforming conditions [140]. The substitution of alkali metals was shown to create a more reductive La ion in the lattice, and the fixed oxygen species near reductive La is expected to be effective in Co metal anchoring. High oxygen release rate was measured on the alkali metal substituted perovskites (Fig. 8 ).Substitution of lanthanide based perovskites with redox elements such as ceria in the perovskite lattice structure can also enhance oxygen storage capacity which ultimately leads to suppression of sintering and formation of char on the surface [141,142]. For instance, toluene steam reforming activity was enhanced due to Ce substitution in La0.6Ce0.4NiO3 catalyst [143]. Recently, the high oxygen vacancy and mobility of Ce substituted perovskites have also been shown to increase the catalyst stability and resistance to sulfur poisoning, which is an important challenge in biomass tar processing [144]. In a series of LaxCe1 − xCo0.5Ti0.5O3 catalysts, it was observed from XPS that La0.8Ce0.2Co0.5Ti0.5O3 possessed the highest percentage of mobile oxygen species on the surface. The La0.8Ce0.2Co0.5Ti0.5O3 catalyst showed the highest toluene reforming activity and the best regeneration capacity after exposure to H2S. Sulfur poisoning in steam reforming of toluene mainly occurs by the conversion of the active metal into inactive sulfides and the catalytic activity can be partially restored after stopping sulfur in feed depending on the stability of the sulfide formed. It was observed that compared to other compositions, the La0.8Ce0.2Co0.5Ti0.5O3 catalyst could achieve almost 90% of its original activity after exposure to H2S, which is possibly due to its better O2 mobility nature, which makes it possible to oxidize the sulfides to respective sulfur oxide species during SRT reaction post exposure to H2S.Besides oxygen mobility, catalyst basicity is also an important property which is desirable for the breaking of CC bonds in the hydrocarbon as well as suppressing carbon formation. In steam reforming reactions, basic sites also activate steam to generate hydroxyls and these hydroxyls have the ability to reduce the carbon deposition over metal centers. Despite the fact that acidic supports such as alumina have been widely used for reforming reactions owing to their low cost and high surface area, catalyst deactivation remains a prevalent issue on alumina supported catalysts. This is inevitable due to the acidic nature of alumina which favors undesirable parallel side reactions such as the dehydration of oxygenates to olefin intermediates, eventually causing carbon formation on the surface of the catalyst [1,145]. Improving the catalyst basicity are both essential and beneficial in minimizing these side reactions. Besides, presence of basic sites can promote non-oxidative cleavage of CH bonds [146]. In order to improve the basicity of a catalysts several approaches such as change in the synthesis method to establish interactions between metal and basic oxide supports, doping with base metal oxides and so on are reported. Among the several approaches, doping the metal and/or support with base metal oxides is much explored for various reforming reactions especially steam reforming of tar model compounds. The most commonly added metal oxide can be either from alkali or alkali earth metal oxides series. Besides, optimum amount of doping with other metal oxides such as CeO2, ZrO2, MnO2 and Y2O3 also showed enhanced basicity property of a material [147]. With this scenario in this section, we will discuss categorically the influence of change in the basicity of a catalyst for steam reforming of tar model reactions with respect to nature of dopants. Most commonly reported base oxide promoted catalytic systems for biomass tar model reforming reaction are detailed in Table 2 .Alkali metals are metal belonging to group 1A of the periodic table such as lithium (Li), sodium (Na), potassium (K). Many studies proved that alkali metal catalysts are very effective in steam reforming of tar and can improve the quality of gaseous product [148,149]. However, the major disadvantage of these catalysts is their evaporation during the reaction and difficulty in recovery. In one work K-doping to Ni/dolomite catalyst was compared with Ca and MnO2 doping for steam reforming of toluene activity. Comparatively, doped catalysts exhibited higher toluene conversion with improved carbon suppression than reference Ni/dolomite catalysts [150]. In another work, Moud et al. reported the effect of K coverage on Ni/MgAl2O4 catalyst for sulfur-laden tar reforming reaction [151]. According to them the reason for improved reforming activity for both methane and tar reforming was related to K adsorption, lowering the surface coverage of S at active sites due to weakening of NiS bonds. Furthermore, in another study, K. Yip et al. prepared biochar from pyrolysis of leaf, wood and bark in a fixed bed reactor at 750 °C. The result indicated that the presence of Na, K, and Ca in bio-chars played key role in enhancing the catalytic activity with the order of K > Na > Ca present during the steam gasification of biochar [152]. Similarly, Feng et al. reported K and Ca-loaded biochar catalysts for reforming of various model compounds (such as toluene, naphthalene and phenol). At 800 °C, the release of K from biochar samples is nearly twice of Ca during the tar model reforming with 15% H2O or pure CO2 [153]. More O-containing functional groups are formed on K-loaded biochar than on Ca-loaded and H-form biochars. Recently, Ashok et al. reported the acidity of incineration bottom ash (IBA) derived from municipal solid waste can be neutralized and Lewis basic centers can be generated by hydrothermally treating with 3 M NaOH solution for 8–24 h. The treated IBA act as better support material for Ni catalysts for steam reforming of toluene reaction. Both toluene conversion and carbon suppression was enhanced significantly than un-treated Ni/IBA catalyst [125]. Furthermore, as mentioned in previous sections, production of higher H2/CO values than stoichiometric values for steam reforming of tar is inevitable due to the involvement of water-gas-shift (WGS) reaction. And it is well reported in the literature that the doping of small amounts of alkali metal oxides to supported catalysts significantly enhances WGS activity and suppressed methane formation [154–156]. Thus the role of alkali metal oxide in promoting steam reforming of tar activity is greatly associated with their activity towards WGS reaction [157]Among all the base metal oxides, the alkali earth metal oxides such as MgO and CaO are extensively investigated for steam reforming of biomass tar reaction [158–160]. It is due to the availability of these oxides, which are also major components of many naturally available minerals such as olivine, dolomite, calcite, phyllosilicates and so on. Furthermore, these oxides are used as dopants to improve the basicity of the material and stabilize the metal and support materials during high temperature reforming environment. Besides, SrO and BaO are also used as other alkaline earth metal oxide dopants for reforming applications [161]. By considering the uniqueness of these meatal oxides, the researcher selectively investigated them for various types of catalytic materials. For instance, the role of MgO in tar reforming activity is majorly studied using Mg-Al-Ox hydrotalcite derived catalyst precursors, where Mg2+ species was partially or completely replaced with other catalytically active M2+ species such as Ni2+, Cu2+ and Co2+, and Al3+ with Fe3+ species [135,162]. A uniformly distributed mono and bimetallic catalytic systems can be obtained via decomposition of hydrotalcite precursors.Similarly for steam reforming of toluene reaction, Ashok and Kawi [163] found that doping CaO to a NiFe alloy (at the optimal molar ratio of 1.5:1:2) supported on iron-alumina catalyst, resulted in activation of the water molecules at lower temperatures, resulting in enhancement of steam reforming of toluene at lower temperatures as well as suppression of carbon deposition. The function of CaO in the hydrotalcite structured NiO-CaO-Al2O3 was also explored by Ashok et al. [64] for the steam reforming of toluene reaction. CO2 sorption studies were conducted in order to elucidate the basic strength of the catalysts, and Ni-Ca-Al catalysts with the composition of Ni:Ca:Al = 8:62:30, was found to display substantially stable CO2 sorption behavior for up to 10 carbonation and de-carbonation cycles. The authors correlated the synergism between the active Ni phase and CaO phase in Ni-Ca-Al catalysts as the main factor which resulted in promising performance for the steam reforming of tar reaction. Similarly, catalytic bi-functional material Fe/CaO-Ca12Al14O33 was also reported for steam reforming and WGS reaction, where the iron phases favor the H2 production and the CaO-Ca12Al14O33 phase simultaneously captures CO2 during several cycles of carbonation-decarbonation [164,165]. In another study promotional effect of CaO doping to NiFe bimetallic catalyst was reported for steam reforming of toluene reaction. According to authors, the defect sites in CaO could be helping in H2O adsorption, which can react with adjacent carbon on Ni surface at their interface [166]. Likewise, enhancement in the Ni activity and stability from the individual roles of CaO and CeO2 in 2.5 wt% of Ni20Ca60Ce20 catalyst was also reported for reforming reaction, where CeO2 can act as redox component that provides for a higher H2O dissociation leading to carbon oxidation and the CaO can enhance the basicity of the catalyst as well as provide for a higher dispersion of Ni metal [167].In another study on Ni/CaAlOx for steam reforming of biomass, the authors varied Ca/Al ratio and observed increase in Ca loading increased the CO formation and also reduced CO2 formation, however the Ni particle size was sacrificed [168]. Effect of calcium addition was recently studied by Jin et al.; the catalyst was modified by incorporating Ca into the Ni-MgO-Al2O3 catalyst which was effective for enhancing hydrogen production. This was explained to be due to enhanced adsorption of CO2 on the CaO promoting the WGS reaction [169].Likewise, Oemar et al. [126] found that substitution of a small amount of Sr with La in the LaxSr1 − xNi0.8Fe0.2O3 (LSNFO) perovskite structure for steam reforming of toluene, especially at low steam/carbon ratio, led to good catalytic activity and stability (Fig. 9 ). This was attributed to the inherent property of Sr in the perovskite structure which can strongly adsorb water. Similar behavior of enhanced catalytic performances at low steam/carbon ratio using basic additives like CaO [170], SrO [95,171–173], MgO [174], SmO [131], BaO [58,175] was also reported for steam reforming of toluene reaction.Furthermore, Takise et al. extensively investigated numerous perovskite as a support for cobalt metal including La0.7Sr0.3AlO3 − x (LSAO), La0.7Ca0.3AlO3 − x (LCAO), La0.7Ba0.3AlO3 − x (LBAO), LaAlO3, Sr/LaAlO3, LaAl0.7Zn0.3O3 − x, SrTiO3, SrTi0.7Fe0.3O3 − x, SrZrO3, and SrCe0.5Zr0.5O3 − x [ 93–95 ] . In this paper, the catalysts are categorized at La ion and Sr ion incorporated perovskite oxide. In La ion category, Co/LSAO and Co/LCAO showed highest activity and stability for 300 min of operation. Whereas, in Sr-ion category, Co/SrZrO3 performed highest among all other catalysts. The anchoring effect between Co and support was investigated using STEM. From H2 18O SSITKA, the Co/LSAO showed higher lattice oxygen release rate than that of Co/LCAO or Co/La0.7Ba0.3AlO3 − x. Also, BaO substituted Ba–Ni-hexaaluminate (BaNixAl12 − xO19) was tested for steam-reforming of 1-methylnaphthalene. The BaNi substituted hexaaluminates show ~90% conversion at 900 °C for tar cracking, and also showed activity for water–gas-shift reaction [176,177].In addition to alkaline and alkaline earth metals, the doping with other metal oxides also gave improved basicity. For instance Abou Rached et al. [178] observed that the addition of cerium to Ni2Mg2Al4 enhanced the formation of surface carbonates which is due to the enhanced the redox nature, facilitating the CO2 transport to Mg oxides/hydroxides basic sites. In another study, the Ce/Zr ratio supported on Al2O3 was varied. The study found that for all the catalyst the toluene conversion decreased tremendously during the 22 h operation with no significant coke formation. The decrease in Ce/Zr ratio increased the acid site density and the carbon formation rate. Ce present on the catalyst surface covered the acidic centers of Al2O3 and covered the Lewis acidic centers which inhibit higher coke formation [56]. In another study Mazumde and De Lasa [179] observed that the addition of La2O3 up to 5 wt% to Ni/Al2O3 catalyst improved surface area, CO2 adsorption capacity, Ni reducibility and metal dispersion, as well as reduction in support acidity and improves basicity. Mayenite (Ca12Al14O33) type of support which also consists of CaO has also been as an active and stable support of CaO catalysts for steam tar reforming and CO2 capture [180]. E. Savuto et al. investigated the effect of Ce addition to the mayenite. The Ce doping on the catalyst was very useful as the activity for undoped Ni/Mayenite was better even in the presence of sulfur. However, the Ce doped catalyst showed lower coke deposition [181]. Likewise, Lertwittayanon et.al [65] studied the effect of promoting CaZrO3 nanoparticles on Ni/α-Al2O3. The study varied the loading amount from 0 to 15% and found that the loading of 15% performed the highest. This was attributed to oxygen vacancies present on the catalyst surface which enhanced the steam adsorption and desorption; this step led to the formation of additional amount of H+. This additional H+ increased the H2 yield and prevented coke formation by gasification process.Improvement in metallic dispersion is an important factor to constrain metal sintering which generally occur at high temperature reforming activities. Nickel based catalysts and especially bimetallic catalysts are found to exhibit higher activities than monometallic catalysts and this phenomenon is attributed to the co-existence of the well dispersed metals [182]. In view of this, alloying or bimetallic catalyst synthesis can effectually enhance resistance to metal sintering [20,183–186]. The physicochemical properties of the alloyed metals generally correlate to their composition, atomic ordering and particle size [40]. In general, volcano-type relationships between the catalyst compositions and the correlating catalytic performances have been observed. However, it is rather challenging to control composition and size of each particle in alloy nanoparticle system [187]. In this section, we mainly discuss the effect of bimetallic alloy on the catalytic performance in steam reforming of biomass tar and its model compounds in detail (Table 3 ). Throughout the literatures, Ni, Fe, and Co are the most common and promising alloying elements utilized in steam reforming of biomass tar due to their high catalytic activity and coke resistance resulting from the synergistic effect.Among bimetallic catalysts, NiFe alloy catalysts have been extensively studied for steam reforming of biomass tar and its model compounds. Oemar et al. [188] reported the synergistic interaction between Ni and Fe in forming bi-metallic NiFe particles in the perovskite oxide catalyst which conferred high activity and stability for the steam reforming of toluene. The presence of LaNi0.8Fe0.2O3 (LNFO) catalyst resulted in about 30–50% higher hydrogen production compared to the monometallic-based LaNiO3 (which showed a continual decreasing trend in hydrogen production). Moreover, the carbon deposition rate of the bimetallic LNFO catalyst was almost half of the monometallic LNO catalyst. The authors attributed the enhanced catalytic performance of the Fe substituted LNFO catalyst to the synergy between the Ni and Fe atoms on the smaller NiFe bimetallic particles. Moreover, the strong interaction formed between the metal and support further prevented metal sintering. Furthermore, Ashok and Kawi [136] reported a toluene conversion of >90% for a period of 26 h over Ni/Fe2O3–Al2O3 catalyst at 650 °C. According to XRD analysis, NiFe alloys were formed and stable throughout the reforming reaction. The surface Fe species played the role of co-catalysts by increasing the coverage of oxygen species during the reforming reaction to enhance the reaction of toluene and suppress coke formation (Fig. 10 ).Tomishige et al. [187] reported that NiFe alloy particles with uniform compositions could be produced via hydrotalcite-like precursors in Ni-Fe/Mg/Al catalysts. An optimum alloy composition of supported catalysts (Fe/Ni = 0.25) generally showed better catalytic performance over individual metal supported catalysts for biomass tar steam reforming reaction. Furthermore, they investigated the regenerability of hydrotalcite-derived Ni-Fe/Mg/Al bimetallic catalysts [61]. The behavior of NiFe alloy nanoparticles on Ni-Fe/Mg/Al catalyst during the oxidation-reduction treatment is illustrated in Fig. 11 . Upon the oxidation, NiFe alloy nanoparticles are oxidized and incorporated into the near surface of Mg(Ni, Fe, Al)O periclase. Upon the subsequent reduction, the uniform NiFe alloy nanoparticles are regenerated. In a follow-up study [189], the same group also examined the catalytic performance of Ni-Fe/Mg/Al catalysts derived from hydrotalcite towards the steam reforming of tar model compounds including benzene, toluene and phenol. Similarly, Ni-Fe/Mg/Al (Fe/Ni = 0.25) catalyst showed greater activity and effectively suppressed the carbon deposition than the one without Fe addition, specifically in the case of steam reforming of toluene and benzene. Nonetheless, carbon deposition over Ni-Fe/Mg/Al catalyst in steam reforming of phenol was relatively high due to the strong adsorption of phenol on both Fe and Ni sites and its successive decomposition to carbonaceous species.In Fig. 12 , environmentally friendly NiFe alloy catalysts supported on olivine were also investigated in steam reforming of phenol and naphthalene as tar model compounds [190,191]. By using thermal fusion (TF) method, Fe and Ni were partly fused into the structure of olivine existing in the form of (Mg, Fe) Fe2O4 and Ni2SiO4 (NiFe2O4), respectively. After reduction, active metal Fe, Ni and NiFe alloy particles uniformly dispersed on Si, Mg, O phases. NiFe alloy particles sizes on TF-Ni/Fe/olivine were thus smaller compared to those on Ni/Fe/olivine prepared by wetness impregnation method. The NiFe bimetallic catalysts supported on olivine performed well in phenol steam reforming and exhibited good stability in the initial stages of stability test. The stable active sites were attributed to strong interaction between NiFe alloys and olivine support. It was also found that the deposited carbon from phenol steam reforming consisted of Cα, which was easily eliminated by steam. In contrast, Cβ and Cγ with higher graphitization degrees were detected in the carbon deposits from naphthalene steam reforming. They were more difficult to react with steam, and hence naphthalene mostly underwent the catalytic cracking process.A majority of NiFe bimetallic catalysts are acquired from specific precursor structures such as perovskite, hydrotalcite, spinel and olivine. The synergistic effect between Ni and Fe plays an important role in catalyst performance during tar reforming. Ni sites have high ability to activate CH and CH bonds in hydrocarbon molecules, while Fe sites can promote the activation of water molecules, providing adsorbed oxygen. The incorporation of Fe to Ni catalysts can also increase the coverage of oxygen species due to the higher oxygen affinity of Fe than that of Ni. These available oxygen atoms during tar reforming can quickly react with carbonaceous species on the adjacent Ni sites, thus suppressing the coke formation. The key requirement for these bi-metallic catalysts is the formation of uniform NiFe alloy structures and the composition of the elements. The optimization of the catalysts composition is significantly depending on the catalyst synthesis method and the uniformity of the alloys helps in improving the selectivity towards desired product such as reduced carbon formation.Co-based catalysts have also been reported to be effective for tar removal. One study from Tomishige's group investigated the synergistic effect of Ni-Co/Al2O3 catalysts on steam reforming of tar from cedar wood pyrolysis [192]. Compared to monometallic catalysts, Co/Al2O3 and Ni/Al2O3, Ni-Co/Al2O3 (Co/Ni = 0.25 as an optimum alloy composition) exhibited superior activity and possessed greater coke resistance. Additionally, toluene was also used as the model compound in this work. In contrast to the result observed in steam reforming of oxygenated tar, Co/Al2O3 was highly active and stable with smallest amount of carbon deposition for steam reforming of this aromatic hydrocarbon. It was reported that monometallic Co catalyst could effectively suppress toluene decomposition and CO disproportionation, which are the main route for coke formation. In another study by Nabgan et al. [193], bimetallic NiCo catalysts supported on ZrO2 were investigated for catalytic steam reforming of phenol, which is the main component of tar formed following steam gasification of wood biomass. Monometallic Ni and Co catalysts possessed higher acidity sites compared to bimetallic catalysts, leading to lower activity towards phenol steam reforming and higher coke deposition. The existence of Co in NixCoy/ZrO2 (x = 0, 1, 2, 3, 4 where x + y = 4) not only neutralized the acidity but also caused a decrease in the crystal size and reducibility of the catalysts. Among bimetallic catalysts, Ni3Co1/ZrO2 catalyst showed highest basic site and the presence of tetragonal (t-ZrO2) phase structure was still observed. While the amount of t-ZrO2 phases in Ni2Co2/ZrO2 and Ni1Co3/ZrO2 catalysts was found to decrease. It has been reported that t-ZrO2 phase is more stable and active for chemical reactions than monoclinic (m-ZrO2) and cubic (c-ZrO2) phases [194,195]. Thus, Ni3Co1/ZrO2 catalyst presented superior catalytic activity in terms of phenol conversion and hydrogen yield with high coke resistance.The similarity in atom radius of Ni and Co would be favored for the formation of NiCo alloy nanoparticles, leading to the synergistic effect between these two metals. Specifically, the Co addition to Ni catalysts can enhance the resistance to coke deposition since the Co sites can effectively suppress coke formation reaction such as tar decomposition and CO disproportionation. In most of the studies, the deactivation in NiCo alloy catalysts is preferably oxidation of metallic Co species into Co oxides than coke deposition. Therefore, the amount of Co doping to Ni is critical to obtain stable catalytic performance with reduced coke deposition. As highlighted before for NiFe alloy catalysts, the catalyst synthesis method is important to obtain uniform NiCo alloy supported catalysts and reduction gas enriched reaction conditions helps in minimizing deactivation due to Co oxidation.Steam reforming of biomass tar and/or model compound was also conducted on CoFe bimetallic catalysts. Wang et al. [196] modified the Co/Al2O3 with Fe by co-impregnation method and reported the increase in toluene steam reforming activity due to the synergy between Co and Fe at the appropriate composition. However, the activity of Co-Fe/Al2O3 catalyst considerably decreased as a function of time while that of Co/Al2O3 catalyst remained stable. It is noteworthy that with the presence of H2 in the reactant gas, the stability of Co-Fe/Al2O3 catalyst was improved remarkably. This improvement may be attributed to the strong interaction of H2 on the bcc CoFe alloy surface, which in turn restrained the oxidation of Fe in the bcc CoFe alloy. The preservation of the bcc CoFe alloy thus contributed to high activity in steam reforming of toluene. Koike et al. [197] further unraveled the effect of the H2 addition to steam reforming of toluene. The reaction mechanism over Co–Fe/α-Al2O3 catalyst was proposed as shown in Fig. 13 . The hydrogen species was hypothesized to facilitate the activation of toluene via its methyl group.Thereafter, Wang et al. [198] attempted the synthesis of Co-Fe/Mg/Al catalysts prepared from hydrotalcite-like precursors. In this way, nanocomposite structure of the bcc CoFe alloy on MgAl2O4-based solid solution was obtained (Co/Fe/Mg/Al = 10/10/40/40) with more uniform composition than that on Co–Fe/α-Al2O3 (Fe/Co = 0.25) catalyst prepared through conventional co-impregnation method. Compared to Co–Fe/α-Al2O3 (Fe/Co = 0.25), Co/Mg/Al and Ni-Fe/Mg/Al (Fe/Ni = 0.25) catalysts, (Co/Fe/Mg/Al = 10/10/40/40) catalyst was more active and resistant to coke deposition. Moreover, the regeneration ability of (Co/Fe/Mg/Al = 10/10/40/40) catalyst was higher than that of Co–Fe/α-Al2O3 (Fe/Co = 0.25) catalyst, although its catalytic performance decreased after the repeated use.Finally, in CoFe alloy catalysts, Fe itself have low reforming activity; however, the synergistic effect between Co and Fe, especially the bcc CoFe alloy phase, can enhance tar reforming activity and suppress coke formation. Co sites can activate tar molecules, while neighboring Fe sites can supply oxygen atom to the carbonaceous intermediate.Apart from NiCo and NiFe bimetallic catalysts, alloying Ni with Cu has also been applied to enhance the catalytic performance in terms of activity and resistance to coking [199]. In one study [200], Ni-Cu/Mg/Al bimetallic catalyst was derived from hydrotalcite-like compounds through the calcination and reduction. At the optimum composition of Ni-Cu/Mg/Al (Cu/Ni = 0.25) gave almost total conversion of tar at 550 °C. It also exhibited higher activity and lower yield of coke as compared to monometallic Ni/Mg/Al and Cu/Mg/Al catalysts as well as Ni-Fe/Mg/Al (Fe/Ni = 0.25) catalyst under the same conditions in steam reforming of biomass tar reported in previous work [61]. Furthermore, high stability of Ni-Cu/Mg/Al (Cu/Ni = 0.25) could be achieved over 2 h of testing without aggregation and change in structure of NiCu alloy particles observed. Following this work, Li et al. [201] evaluated the same Ni-Cu/Mg/Al hydrotalcite-like catalyst in steam reforming of 1-methylnaphthalene so as to elucidate the effect of Cu/Ni. The volcano-type relationship between the catalyst compositions and the correlating catalytic performances was also observed in this system, in which Cu/Ni = 0.25 composition provided the highest reforming activity and satisfying selectivity towards CO + CO2. Based on the kinetic study, it was also found that 1-methylnaphthalene had strong interaction with both Ni and NiCu catalysts, whereas steam adsorbed more in the presence of NiCu alloy and possibly dissociated to create more adsorbed oxygen species. Ashok and co-workers [199] have explored the catalytic performance of Ni-Cu/SiO2p catalysts derived from phyllosilicate structures for biomass tar steam reforming using cellulose as biomass model compound. At optimum molar composition of Cu/Ni = 0.15 in 30Ni-5Cu/SiO2p catalyst, NiCu alloys could be formed along with the existence of the phyllosilicate structure. It was shown that 30Ni-5Cu/SiO2p catalyst gave the highest biomass conversion to gaseous products and possessed better stability for longer reaction times at 600 °C.Dagle et al. [202] presented the study of steam reforming of hydrocarbons from biomass gasifier (i.e. benzene and naphthalene) over MgAl2O4-supported transition metals. Particularly, novel bimetallic IrNi catalyst was examined, focusing on the theoretical modeling study to understand the nature of IrNi alloy structure, its resistance to coking and activity towards reforming reaction. As shown in Fig. 14 , Ni50 wetted the surface markedly with direct NiO contacts, whereas Ir50 exhibited fewer direct IrO contacts. A well-mixed alloy of the gas-phase Ir5Ni45 cluster was obtained, where three It atoms were lifted and occupied the surface of Ir5Ni45 particle. The relative energetics of the last reaction during methane dissociation were also measured to investigate the propensity of the cluster to form coke. It was found that small Ir clusters in IrNi alloy particles showed coke resistance ability. Additionally, small Ir (~2–3 atoms) on the surface of Ni-rich particles provided electron-rich Ir sites, enhancing activity and durability for steam reforming as compared to only small Ir clusters or Ni particles.Noble transition metal Pt has also been added into Ni-based catalysts to build up the bimetallic systems for steam reforming reactions [203]. Mukai et al. [204] reported the effect of Pt addition to Ni/La0.7Sr0.3AlO3−δ perovskite (Pt/Ni/LSAO) catalyst on toluene steam reforming. Even with no pre-reduction, high catalytic activity and low coke formation were achieved using Pt/Ni/LSAO catalyst. These results might be related to the enhancement of Ni reducibility due to the additive Pt as well as the formation of adjacent or alloy structure between Pt and Ni on Pt/Ni/LSAO catalyst. It was also discovered that different sequence of Pt and Ni impregnation resulted in different structure of supported metals, and hence affected the catalytic activity and the amount of coke formation. Since the interface between Ni and LSAO perovskite was crucial for exchanging lattice oxygen, the impregnation of Pt to Ni/LSAO catalyst was preferable to the inverse impregnation. Trace amount of noble metal Pd was also discovered to promote activity and stability of hydrotalcite-derived Ni/Mg/Al catalyst in oxidative steam reforming of biomass tar from pyrolysis [205]. With the same noble metal/Ni molar ratio, Pd showed higher catalytic performance than Pt, Au, Ru, Rh and Ir, which could be due to the enhancement of Ni reducibility and the formation of highly dispersed Pd atoms on the surface of Ni particles (Fig. 15 ). The presence of trace Pd could also prevent the oxidation of active Ni metal, resulting in more stable catalytic performance during two-hour stability test as compared to monometallic Ni catalyst.Rare transition metal rhenium (Re) was also utilized as the promoter in Ni/Char catalysts for cracking/reforming of naphthalene and biomass tar [105]. The addition of Re could improve dispersion and prevent agglomeration of Ni particles supported on char, resulting in the catalytic activity improvement. Hydrogen yield was also increased with the presence of Re probably because it facilitated water dissociation, which subsequently enhanced water gas shift activity.Higher oxygen affinity can also be obtained through the formation of small NiCu alloy nanoparticles. The higher oxygen affinity can promote the activation of steam and subsequently facilitate the reaction of oxygen atoms with activated tar molecules. Alloying Ni with other second metals can also provide synergistic effect, which increases reducibility, yields small particle size of alloys and/or enhances metal dispersion. The ample surface metal particles thus offer the large number of active sites for tar dissociation and steam activation.This review summarizes the recent efforts that have been made for the development of catalysts for steam reforming of biomass tar reactions using tar model compounds. Research efforts have been more focused on the design and influence of material properties on the catalysis with respect to reactant conversion, stability and carbon suppression. The catalytic performance of a material is also highlighted for change in the mechanism of steam reforming reactions. On a common note, the role of an efficient catalyst is pivotal for economically feasible of catalytic biomass gasification technology. The nature and properties of the catalysts such as redox or acid-base properties play an important role in determining the catalytic activity as well as selectivity towards the desired product. In general, an excellent catalyst should have high tar conversion efficiency, low cost, easy regenerability and be environment friendly. For an efficient catalyst both nature of active metal and support material plays a crucial role. With respect to availability and activity, Ni as an active metal component is widely accepted for this reforming reaction. Furthermore, the use of synergistic bimetallic catalysts has been one of the promising approaches to increase the catalytic performance and enhance coke resistance during steam reforming of biomass tar model reaction. This strategy can provide (i) the significant increase in the coverage of oxygen species by alloying either Ni or Co with Fe (ii) higher oxygen affinity through the formation of small NiCu alloy particles (iii) the enhancement of reducibility along with higher surface metal concentration and/or greater dispersion due to the synergistic effect between Ni and second metals, such as Co, Ir, Pt, Pd and Re.Based on the current state of art in the development of catalyst for biomass gasification process, the key criteria for the future modification are as follows: (i) increasing the basic strength of catalyst to achieve higher syngas yield via water gas shift reaction, (ii) incorporation of oxygen vacancies by using redox metal oxides such as CeO2, FeOx or perovskite-based support as this will help to suppress the coke formation, (iii) development of core-shell structure with porous shell to prevent the metal sintering, coke formation and improve the mass transfer of reactants and products; specifically formation of oxide shell with high oxygen mobility such as CeO2, mixture of CeZr and perovskite shell will enhance the coke resistance ability; additionally using hydrotalcite as a shell with porosity will help to enhance the basic properties of the catalyst, and (iv) enhancing the surface area of natural minerals and waste-derived catalysts by modification with structure-directing agents or optimizing the hydrothermal conditions such as the choice of base, time and temperature for the hydrothermal treatment.For the choice of nature of support, so far, most of the works considered alumina-based support is an effective support which promotes the breaking of CC bond in the hydrocarbons and enhances conversion. However, the challenge is to improve the hydrothermal stability of alumina and also to prevent coke deposition. Modification of alumina support with alkaline earth metal oxides such as MgO, CaO and inert metal oxides SiO2 helps in improving the coke deposition and improved the stability of the catalyst. For SiO2 based catalyst, phyllosilicates type of support has emerged as a promising catalyst. SiO2-based catalyst has been designed in the form of the core-shell catalyst which improves the confinement effect and prevents metal sintering problem under the high-temperature reaction condition. Synthesis of catalysts via hydrotalcites precursors also have great potential to fine tune and achieve higher activity and longer stability. In future, the core-shell structure of catalyst can be explored with CeO2, ZrO2, and mixed oxides. Another possible route to improve the coke resistance of the catalyst is by utilizing the oxygen vacancy of the support such as provided by perovskite-based support etc.So far, the commercially available catalysts for the gasification process are natural minerals (olivine, dolomite), alkali metal catalysts, and transition metal-based catalysts. With the use of natural mineral as a catalyst, the gaseous product formed needs to be improved. Moreover, this leads to the also additional gas-cleaning step as the quality of the end product is inadequate. In contrast, a wide range of studies was conducted and reported in the literature using commercial nickel-based catalysts in biomass gasification to promote steam-reforming, water–gas shift reactions and to eliminate tar. When nickel is used as a catalyst, the quality of the gaseous product can be enhanced. Ni-based catalyst is more economically attractive as both gasification and gas clean-up occur simultaneously. There are several Ni-based catalyst systems commercialized for the biomass gasification, for example, BASF developed Ni supported on CaO–Al2O3–SiO2–K2O and MgO–CaO–Al2O3–SiO2–K2O referred as G1-25/1 and G1-50, respectively [206,207]. These catalysts showed the toluene conversion of 89–99% can be achieved in the temperature range of 660–850 °C. Baker and Li et al. studied the commercial nickel catalyst G-90C as a primary catalyst for biomass gasification [208,209]. Both of these studies elucidated the advantages and disadvantages of using the commercial Ni-based catalyst. The catalyst active the quality of gaseous product and also decreased the overall tar yield. However, the catalyst suffers from deactivation after several cycles. Also, the leading cause was the coke deposition on the catalyst. In a review paper by Chan et al. described a summary and detailed description of the commercially available primary and secondary catalyst for biomass gasification process [210]. Therefore, a tremendous effort has been made in the past few years to improve the transition metal based catalyst to achieve with high tar conversion and improved resistant towards coking.A cheaper and effective catalyst will make the biomass reforming process more economical; therefore, eco-friendly materials, char based supports were also investigated. A tremendous effort has been implemented to improve the thermal and chemical resistance of the char based support for toluene steam reforming. However, the performance still needs to be improved to be comparable with other types of support. Therefore, modification of the synthesis method to improve the acidic and basic centers, improving the surface area of the char will be helpful to achieve excellent performance for biomass gasification processes. Several natural minerals and waste materials such as incinerator bottom ash are some of the emerging types of support for this steam reforming reaction. Although the catalyst showed appreciable thermal stability, the catalytic activity was inferior to that of other catalytic system. Therefore, further improvement in the catalyst surface area, fine-tuning the acidity and basicity and improving the metal dispersion via modification of the synthesis method is crucial to achieving higher catalytic activity.In the context of actual biomass tar conversion, there is a pressing demand to develop high performance, low cost and robust catalyst, with the use of harsher temperatures and feedstock that contains potential catalyst poisons such as sulfur, nitrogen and chlorine, the issues of catalytic deactivation through thermal sintering and poisoning (e.g. sulfur, nitrogen, chlorine and etc.) needs to be solved through development of thermally-stable and sulfur-tolerant catalysts for actual tar reforming applications [211]. Beside the development of efficient catalyst for tar reforming, there are some alternative approaches which are reported for efficient tar conversion. For instance, Wang et al. [212] showed integration of perovskite based membrane with catalyst and performed steam reforming of toluene reaction. The perovskite based membrane selectively separates oxygen from air and promotes partial oxidation of tar reaction. By this way the catalyst showed enhanced catalytic performance with respect to toluene conversion and carbon suppression ability. However, the stability of perovskite based membrane in the presence of poisoning compound needs to be investigated further [213]. Another approach for efficient tar conversion is performing steam reforming tar using electro chemical reforming technique [20,214,215]. Herein the tar reforming is performed in the presence of mild electric current [216]. It can be performed between 100 °C and 800 °C temperature conditions. The reports showed the catalytic performance was remarkably increased in the presence of electric current. These two processes required much more research in order to confirm their technical feasibility in biomass gasification processes. From this comprehensive review on the progress of the performance of the catalyst for steam reforming of biomass tar model, it can be established that different properties of support such as acidic sites, oxygen vacancy, and basicity, and alloying active metal improve the interaction of metal with support which eventually helps in making a robust catalyst.The authors gratefully thank the financial support from National University of Singapore, National Research Foundation-Prime Minister's office, Republic of Singapore, the National Environment Agency - Singapore under the Waste-to-Energy Competitive Research Program (WTE CRP 1501 103, WBS No. R-279-000-491-279), Agency for Science, Technology and Research (AME-IRG A1783c0016, WBS No. R-279-000-509-305) and Ministry of Education - Singapore (MOE2017-T2-2-130, WBS No. R-279-000-544-112).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 review describes recent advances in development of catalysts for steam reforming of biomass tar model reactions, using toluene, benzene and naphthalene as tar model compounds. Catalytic systems have been categorized based on their catalytic properties. The material properties such as oxygen mobility and basicity of catalysts showed great influence in their effectiveness in tar reforming. Changes in the properties such as oxygen mobility and basicity with various metal and/or support modifications and their influence on catalytic behavior with respect to reactant conversion and coke inhibition is comprehensively discussed. The activity of the catalysts derived from various synthesis methods is also introduced. The changes induced in the pathways of steam reforming reactions by catalyst modification is also highlighted together with changes in catalytic properties. Reaction pathways for steam reforming of toluene by experimental studies together with insights gained from computational DFT studies is also presented.

Waste incineration is developing rapidly in China, since this process can reduce waste volume as well as generate heat energy (Zhu et al., 2018). However, it cannot be avoidable to have pollutants, such as NO x and chlorine-containing volatile organic compounds (CVOCs) from flue gas of municipal solid waste incinerators, and the emissions are increasing in recent years in China (Kulkarni et al., 2008; Wu et al., 2016; Li et al., 2017; Shao et al., 2021). Meanwhile, NO x and CVOCs are the critical precursors of PM2.5 and O3 (Ni et al., 2009; Wu et al., 2016; Ye et al., 2022), which can cause severe environmental problems (Alzaky and Li, 2021). Therefore, how to effectively control emissions of NO x and CVOCs has been a focus of air pollution control.Selective catalytic reduction of NO x by hydrocarbons (HC‐SCR) has attracted much attention over the past two decades (More et al., 2018; Xu et al., 2020). The transition metal oxides display the good reactivity with inexpensive costs, which are becoming potential catalysts in HC-SCR, especially Mn-based catalysts. For example, Liu et.al. (Liu et al., 2019a) obtained up to 100 % NO conversion at 130℃ with MnOx/AC in C2H4-SCR.In the last few decades, it has been emerging various strategies for VOCs elimination, such as adsorption (Zhao et al., 2022), plasma (Yu et al., 2020), biodegradation (Khan et al., 2018), catalysis (Chen et al., 2022), photocatalysis (Chen et al., 2022), Photothermal Catalysis (Yang et al., 2022b). Catalytic oxidation is recognized as one of the most efficient technologies for low concentration CVOCs removal due to high removal efficiency and mineralization rate (Fang et al., 2019; He et al., 2019; Liu et al., 2019c). Interestingly, transition metal oxides-based (MnOx) catalysts with variable valance states also exhibit the potential of high reactivity in CVOCs decomposition (Kim and Shim, 2010; Santos et al., 2010; Piumetti et al., 2015; Ye et al., 2021). Sphere-Shaped Mn3O4 exhibited a complete conversion of methyl − ethyl − ketone to CO2 at 200 °C (Pan et al., 2017). Cheng et. al. (Cheng et al., 2017) also obtained 90 % dimethyl ether conversion at 238 °C on α-MnO2.Hence, MnOx catalysts shows the bifunctional ability to reduce NO x and oxidize CVOCs. However, to the best of our knowledge, the promising catalyst of α-MnO2 in these two different reaction mechanisms was seldom reported when applying for NO x reduction in HC‐SCR and CVOCs oxidation separately. It is worthy investigating the relationship between structure properties and reactivity of α-MnO2, and to discuss the physicochemical characterizations. In this present work, a series of α-MnO2 catalysts with the different physical–chemical characteristics were prepared and applied for the catalytic removal of both NO x and DCE (as the typical pollutant of CVOCs). Catalyst characterization and reactivity were investigated to illustrate the relation between properties and reactivity of α-MnO2 catalysts.All α-MnO2 samples were synthesized by hydrothermal method using Potassium permanganate (KMnO4) and Manganous acetate ((CH3COO)2Mn) as precursors. For α-MnO2-1, 1.5 g (CH3COO)2Mn and 2.5 g KMnO4 were added into 160 mL distilled water with magnetic stirring for 30 min at room temperature. Next, the mixed solution was poured into two Teflon-lined stainless-steel autoclave (100 mL), and then heated at 160 °C for 12 h. The obtained colloids were washed with deionized water and ethanol, then dried in vacuum at 80 °C for 12 h. The dried samples were calcined at 450 °C for 4 h. Finally, the catalyst was pelleted, crushed, and sieved to 40 ∼ 60 mesh granules before use.To modify the physical–chemical characteristics of α-MnO2 catalysts, the mass ratio of (CH3COO)2Mn and KMnO4 was 4.9:3.2, and the temperature of hydrothermal treatment was 140 °C. The obtained product was donated as α-MnO2-2. Also, according to the synthesis plan of α-MnO2-1, the mass ratio of (CH3COO)2Mn and KMnO4 was 9.7:6.3, and the temperature of hydrothermal treatment was 90 °C. The obtained product was donated as α-MnO2-3. To obtain α-MnO2-4, the mass ratio of (CH3COO)2Mn and KMnO4 was 0.8:3.0, and the hydrothermal treatment was carried out at 240 °C.The catalytic activity test of HC-SCR was carried out in a stainless-steel tubular (i.d.10 mm). 600 mg catalyst was evaluated under a typical feed gas included 800 ppm NO, 600 ppm C3H8, 6.5 vol% O2, and N2 as balance. The reaction temperature was conducted from 150 to 550 °C at a total flow rate of 450 mL/min, corresponding to a weigh hourly space velocity of 19 000 mL·g−1·h−1. The NO x concentration of the inlet and outlet were continuously measured by an infrared gas analyzer (Xi’an Juneng Corporation, China). NO x conversion was evaluated by Eq. (1): (1) N O x conversion \% = N O x in - N O x out N O x in × 100\% where [NO x ]in and [NO x ]out are the inlet and outlet concentration of NO x , respectively.The catalytic activity of DCE combustion was measured using a stainless-steel tubular (i.d.10 nm). 500 mg catalyst was selected for activity test. The reaction was conducted from 100 to 450 °C at a flow rate of 400 mL/min of feeding gases with 500 ppm DCE and 21 vol% O2, corresponding to a gas hourly space velocity (GHSV) of 48 000 mL·g−1·h−1. DCE and products (CO and CO2) were measured by an on-line gas chromatograph (GC9890) with ECD and FID. An on-line Cl2 and HCl detectors (PN–2000, China) was used to analyse the concentrations of Cl2 and HCl. DCE conversion, CO x (CO2 and CO) yield, HCl yield and Cl2 yield were evaluated by the following Eqs. (2) to (6), respectively. (2) N O x conversion (\%)= N O x in - N O x out N O x in × 100\% (3) C O 2 yield \% = C O 2 out 2[DC E in ] × 100\% (4) CO yield \% = C O out 2[DC E in ] × 100\% (5) HCl yield \% = HC l out 2[DC E in ] × 100\% (6) C l 2 yield \% = C l 2 out [DC E in ] × 100\% where [DCE]in is the DCE inlet concentration of DCE. [DCE] out , [CO] out , [CO2] out , [HCl] out and [Cl2] out are the outlet concentration of DCE, CO, CO2, HCl and Cl2, respectively.Fourier infrared spectrum (FT-IR) spectra was recorded in the range 400 ∼ 2000 cm−1 with a resolution of 4 cm−1 on a thermo scientific nicolet iS20 using the KBr pellet technique.X-ray powder diffraction patterns (XRD) were obtained by Panalytical X'Pert'3 Powder diffractometer, equipped with Cu Kα X-ray radiation (λ = 0.15406 nm). All the catalysts were scanned at 2θ range between 10° to 80° (rate of 2°/min).The surface areas of the synthesized materials were determined by the Brunauer − Emmett − Teller (BET) method using ASAP 2460 3.01 instrument. Nitrogen physisorption experiments were carried out at 77 K after initial pretreatment of the samples by degassing at 300 °C for 2 h.Scanning electron microscopy (SEM) images have been studied using Schottky (ZEISS Gemini 300) equipment with a resolution of 10 kV and 50 kV.Transmission electron microscopy (TEM) images were obtained from JEM-F200 electron field emission transmission electron microscope (JEOL, Japan) under 200 kV acceleration voltage.X-ray photoelectron spectra (XPS) were carried out on thermo scientific system with Al Kα radiation. The binding energy scale was corrected for surface charging by use of the C 1 s peak of contaminant carbon as reference at 284.8 eV.Hydrogen temperature programmed reduction (H2-TPR) experiment was conducted on an AutoChem1 II 2920 instrument equipped with a thermal conductivity detector (TCD) to measure the consumption of H2. Before detection by the TCD, a 50 mg sample was pretreated under N2 stream (40 mL·min−1) at 300℃ for 1 h, and then cooled to 50 °C. A mixed stream with a 10 vol% H2/Ar mixture (50 mL·min−1) was introduced into the sample, and the sample was heated from room temperature to 800 °C.The NH3 temperature-programmed desorption (NH3-TPD) was performed on an AutoChem1 II 2920 instrument. The catalyst was (100 mg) was pretreated in a N2 (50 mL∙min−1) of at 300℃ for 1 h, and then cooled to 50℃. Sample was treated with 10 % NH3 diluted in N2 (30–50 mL∙min−1) for 1 h to achieve adsorption saturation. The gas was switched back to He (30 mL∙min−1) for 1 h to purge the physically adsorbed species. Finally, the catalyst was heated from 50℃ to 800℃ at a rate of 10 °C∙min−1 in high purified N2 (30 mL∙min−1). Fig. 1 showed the different catalytic performance of α-MnO2 samples for NO x reduction and DCE oxidation at the range of 100 ∼ 450 °C. For NO x reduction, activity performance decreased as follows: α-MnO2-3 (63.5 %) > α-MnO2-4 (53.8 %) > α-MnO2-1 (45.2 %) ≈ α-MnO2-2 (42.1 %) at 250 °C. For DCE oxidation, T50 was chosen to compare the activity of these samples. T50 of DCE oxidation follows the order of α-MnO2-3 (276.4 °C) < α-MnO2-4 (310.9 °C) < α-MnO2-2 (337.3 °C) ≈ α-MnO2-1 (348.6 °C). Thus, α-MnO2-3 exhibited the highest catalytic activity for both NO x reduction and DCE oxidation.As exhibited in Fig. 2 , in yield of CO2 and CO, for α-MnO2-3, it is remarkable to reveal 84.4 % of C3H8 was decomposed into CO x . While for α-MnO2-1, α-MnO2-2 and α-MnO2-4, C element in C3H8 was about 36.8 %, 41.6 % and 51.6 % converted to CO2, and about 12.8 %, 14.6 % and 15.9 % converted to CO, respectively. In yield of HCl and Cl2, for α-MnO2-3, 53.8 % of HCl and 22.7 % of Cl2 were observed. That is, 76.5 % of DCE was total oxidized into inorganic chlorine products using α-MnO2-3. However, for α-MnO2-1, α-MnO2-2 and α-MnO2-4, Cl element in DCE were about 10.0 %, 8.6 % and 17.7 % converted to Cl2, and about 29.9 %, 34.2 % and 34.9 % converted to HCl, respectively. There are about 60.0 %, 57.1 % and 47.4 % of chlorine remained as organic chlorine. Above all, α-MnO2-3 displayed the best products selectivity.FT-IR spectra of the synthesized materials are shown in Fig. 3 a. The peaks in low wavenumbers between 800 cm−1 and 400 cm−1 are assigned to Mn-O lattice vibration (Yuan et al., 2009; Wang et al., 2019). Our samples showed the well-defined absorption peaks of MnO2 at 469 cm−1, 526 cm−1, and 720 cm−1 as well as the weak defined shoulder at 597 cm−1 (King'ondu et al., 2011; Chen et al., 2015; Liu et al., 2019b). Fig. 3b shows the XRD patterns of the as-prepared manganese oxide samples. Comparing to the XRD patterns of the standard α-MnO2 (JCPDS 44–0141) (Cheng et al., 2017; Gao et al., 2017), it can deduce that all of the four samples could be well corresponding to the tetragonal α-MnO2 phase. The diffraction peaks at 2θ = 12.9°, 18.2°, 25.8°, 28.9°, 37.6·°, 42.0°, 49.9°, 56.4°, 60.3°, 65.1°, and 69.7 °could be attributed to the (110), (200), (220), (310), (211), (301), (411), (600), (521), (002) and (541) plane, respectively. α-MnO2 presented three main peaks at 12.9°, 28.9° and 37.6°, which were assigned to the (110), (310) and (211) planes of α-MnO2 (JCPDS 44–0141). The peak intensity of (110) plane order decreased as follows: α-MnO2-3 > α-MnO2-2 > α-MnO2-4 > α-MnO2-1. The intensity order of (310) plane is as follows: α-MnO2-2 > α-MnO2-3 > α-MnO2-1 > α-MnO2-4. Interestingly, the peak intensity of (211) plane follows the order as α-MnO2-3 > α-MnO2-4 > α-MnO2-2 > α-MnO2-1, which is similar with the activity order of four samples. It indicates that (110) plane, (310) plane and (211) plane may be exposed active planes, in agreement with the findings of TEM. Besides, no diffraction peaks of other phases, such as β-MnO2 and γ-MnO2, are detected, implying that each catalyst is composed of α-MnO2 phase. This result is consistent with the result of FT-IR.The average pore size, pore volume and specific surface area of catalysts were measured by N2 adsorption–desorption, and the results are presented in Fig. 4 and Table 1 . N2 adsorption–desorption isotherms of the samples type II characteristics with well-developed H3 type hysteresis loops, confirming that the samples have mesoporous characteristics (Fig. 4a) (Sing, 1982). The samples possessed a mesopore distribution in the range of 2 ∼ 35 nm in Fig. 4b. α-MnO2-1 and α-MnO2-4 presented a wide peak centered at from 5 nm to 35 nm, while α-MnO2-2 and α-MnO2-3 presented the peak centered at ca. 2.5 nm. In Table 1, the surface area order decreased as follows: α-MnO2-4 (104.6 m2·g−1) > α-MnO2-1 (72.6 m2·g−1) > α-MnO2-3 (44.1 m2·g−1) > α-MnO2-2 (34.6 m2·g−1). The pore volume of order was as follows: α-MnO2-4 (0.6 cm3·g−1) > α-MnO2-1 (0.2 cm3·g−1) > α-MnO2-3 (0.1 cm3·g−1) = α-MnO2-2 (0.1 cm3·g−1). The difference in preparation condition leads to a big difference in surface area of the α-MnO2 samples.To further analyze the morphologies and surface structures of the catalysts, the SEM and TEM images of four samples are shown in Fig. 5 . It should be noted that the wire-like morphology can be differentiated from the rod-like morphology in terms of the bending or straight shape (Wang et al., 2012). Fig. 5 (a) and (d) showed that both α-MnO2-1 and α-MnO2-4 are presented as stacking-nanowires, while Fig. 5 (b) and (c) exhibited nanorod-like appearance of α-MnO2-2 and α-MnO2-3 with uniform distribution. It should be noted that the surface area of wire-like morphology can be much larger than the rod-like morphology. The well-identified periodic lattice fringes of 2.40 Å, 3.10 Å and 6.94 Å are corresponding to the interplanar distance of (211), (310) and (110) facets of α-MnO2, respectively. Whereas, severe blurring of the lattice fringes were also detected (highlighted by red rectangles) in α-MnO2-3. It is worth noting that large amount of point defects on α-MnO2 could obscure the distorted lattice fringes, which may result from the existence of oxygen vacancies on catalyst surfaces(Huang et al., 2018).The morphologies of samples are consistent with the findings of XRD.XPS measurements were carried out to identify the surface species of α-MnO2 samples. Fig. 6 a illustrated Mn 2p spectra of four samples. Peaks at 642.7, 641.7 and 640.4 eV can be attributed to Mn4+, Mn3+ and Mn2+, respectively (Si et al., 2015; Ma et al., 2017; Zhang et al., 2022). In Table 1, the proportion of low valence Mn (Mn3+ and Mn2+) followed the order (Table 1): α-MnO2-3 (0.7) > α-MnO2-4 (0.6) > α-MnO2-2 (0.5) = α-MnO2-1 (0.5). Low valence Mn content is an indicator of surface oxygen vacancies (Yang et al., 2020). Additionally, Mn2+-O and Mn3+-O bonds are weaker than Mn4+-O (Zhang, 1982). Large proportion of low valence Mn results in longer and weaker Mn-O bonds on the surface of α-MnO2-3 (Yang et al., 2020). It suggests that oxygen atoms on its surface are more likely to be released to participate in oxidation. In addition, the existence of surface low valence Mn would promote dissociation and activation of circumambient oxygen atoms (Yang et al., 2020).The XPS spectra of O 1 s of the samples are shown in Fig. 6b. As reported previously, the peak around 529.0 ∼ 530.0 eV is typical for Olatt in a coordinatively saturated environment, while the peak around 531.0 ∼ 532.0 eV can be attributed to the Oads in a low-coordinated environment (Tang et al., 2010; Wang et al., 2011; Yang et al., 2022a). As shown in Table 1, the Oads/Olatt molar ratio for all α-MnO2 catalysts follows the order of α-MnO2-3 (0.6) > α-MnO2-4 (0.5) > α-MnO2-2 (0.4) = α-MnO2-1 (0.4), which is consistent with the catalytic activity results. As we known that Oads performs high activities and makes an important impact in SCR reaction because of its higher mobility than Olatt (Zhang et al., 2020). On the basis of the Mars-van Krevelen mechanism, the emergence and annihilation of oxygen vacancies is the key step of VOC oxidation. Adsorbed oxygen species participate in the redox cycle of the vacancies from gaseous-adsorbed oxygen transformation (Huang et al., 2015). Surface adsorbed oxygen are relevant to the formation of Mn3+ and Mn2+ and are more active than lattice oxygen at low temperatures (Wang et al., 2012). Thus, α-MnO2-3 might be highly active in DCE oxidation owing to large numbers of low valence Mn cations and adsorbed oxygen species.H2-TPR measurement is carried out to analyze the reducibility of different α-MnO2 samples and the results are presented in Fig. 7 . As shown in Fig. 7a, Two reduction peaks (Ⅰ, Ⅱ) could be due to the reduction of Mn4+ to Mn3+ and Mn3+ to Mn2+, respectively (Yang et al., 2020). The initial H2 consumption rate was calculated to better evaluate the reducibility of these samples, as depicted in Fig. 7b. It was clearly seen that the initial H2 consumption rates of the samples decreased in the order of α-MnO2-3 > α-MnO2-4 > α-MnO2-1 > α-MnO2-2. The lower reduction temperature and the larger initial H2 consumption rate indicate a better low-temperature redox ability (Chen et al., 2017; Gong et al., 2017).NH3-TPD experiments are taken to analyze the acidities of the four types of α-MnO2 and the results are shown in Fig. 8 and Table 1. As shown in Fig. 8, NH3-TPD curves of α-MnO2 samples exhibited two desorption peaks (labeled as I and II). The desorption peak I at low temperature is attributed to the desorption of NH3 from weak acid sites and the desorption of physisorbed NH3, the desorption peak II at middle temperature is assigned to the desorption of NH3 from middle strong acid sites (Fang et al., 2013; Yao et al., 2017). It is worth noting that two desorption peaks of α-MnO2-3 and α-MnO2-4 belong to weak acid site and middle strong acid site at below 450℃. The quantitative analysis data of NH3-TPD was summarized in Table 1. It was reported that the quantity of the desorption peak was proportional to the strength of acid site (Zhang et al., 2020). Hence, as seen in Fig. 8, the quantity of peaks can be ranked by α-MnO2-3 > α-MnO2-4 > α-MnO2-1 > α-MnO2-2, implying the order of acid site numbers. α-MnO2-3 catalyst not only shows two acid sites at 145℃ and 350℃, but also presents the largest amount of acid sites among these catalysts, which is basically consistent with the activity test of NO x reduction.The possible properties-reactivity relationship of four α-MnO2 samples was illustated in Fig. 9 . In this work, Oads/Olatt, low valence Mn content and total acidity were positively related to activity of the samples. However, surface area, pore structure and redox properties are not the key factors in our study.α-MnO2-3 catalyst presented the best performance among a series of α-MnO2 for both the catalytic reduction of NO x reduction and DCE oxidation. The Oads/Olatt, the proportion of low valence Mn content and total acidity are the crucial factors for the activity of α-MnO2. The maximum conversion of NO x achieved 63.5 % at 250℃ and DCE achieved 80 % at 338℃ on α-MnO2-3 catalyst, respectively. As for the yield of carbon and chlorine, α-MnO2-3 also exhibits highest yield, which implies that α-MnO2-3 may be a potential catalyst for removal of NO x and VOCs.The authors declare that they have no know competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was financially supported by Zhejiang Provincial Natural Science Foundation of China (LGF22B070005) and Innovation and Entrepreneurship Talent Project of Jiangsu Province (JSSCRC2021236).

The reduction ability of NO to N2 and the oxidation performance of 1,2-dichloroethane (DCE) over α-MnO2 catalysts were investigated. The results show that α-MnO2-3 exhibited the highest catalytic activity in 63.5 % conversion of NO x reduction by C3H8 at 250 °C, and 80 % conversion of DCE combustion by O2 at 338 °C. It is revealed the active phase of α-MnO2-3 is tetragonal α-MnO2 with the selectively exposed plane of (211). It was proposed the high DCE decomposition of α-MnO2-3 was ascribed to the redox properties. The overall characterization results revealed that α-MnO2-3 catalyst preserves more active sites of low valence Mn and higher surface adsorbed oxygen (Oads) /lattice oxygen (Olatt) at the outermost layers, and lower reduction temperature in H2-TPR profiles than that of other catalysts. Meanwhile, NH3-TPD profile of α-MnO2-3 also shows a large number of acid sites promote NO x reduction.

As a class of the rarest elements on the planet, platinum group metals (PGMs) are indispensable materials in many critical technologies attributed to their unique physical and chemical properties. In particular, platinum, palladium and rhodium are utilized as the essential ingredients for vehicle-use catalysts (Nassar, 2015). The traditional application is in emission after-treatment devices for internal combustion engine vehicles (Shelef and McCabe, 2000; Gandhi et al., 2003; Li et al., 2010). The emerging application is in fuel cell vehicles (FCVs), which have attracted global attentions in the context of transport sector decarbonization (Kampker et al., 2020). Platinum is a core ingredient for the catalyst layer, which is indispensable in proton exchange membrane FCVs (Hao et al., 2019). Global consumption of PGMs for producing vehicle-use catalysts reached 400 t in 2020, accounting for over 60% of total consumption (Market Report, 2021).There are numerous driving factors that are expected to significantly change future PGM supply and demand (Rasmussen et al., 2019). The enforcement of stricter emission regulations will result in an increase in the demand for ICEV-related PGM catalysts. These regulations are gradually implemented around the world, such as the Low Emission Vehicle (LEV) LEV-III automotive emission regulations in North America and the national stage Ⅵ emission regulations in China (Xun et al., 2020). Meanwhile, breakthroughs have been made in the commercialization of FCVs. More than twenty thousand FCVs have been produced around the world in 2020 (Hart and Jones, 2020). Approximately 0.175 g/kW of platinum is used in the engine of the Toyota Mirai FCV (Toyota, 2020), which is higher than that contained in the catalytic converters of conventional vehicles (about 0.05 g/kW for a Euro VI gasoline engine) (Hao et al., 2019). Under such circumstances, the proportion of PGMs needed for vehicle related catalyst production to the total PGM production will remain at a high level. Recycling is another important factor for PGM supply. According to Graedel et al. (Graedel et al., 2011), the end-of-life recycling rates of PGMs were above 50% in 2011. In some developed countries, they can reach a level above 60% (Xu et al., 2018; Saidani et al., 2019). However, they are relatively lower in many developing countries (Xun et al., 2020). There is still great improvement potential for the global recycling situation, which can replace the primary supply of PGMs to a certain extent.Furthermore, a big geo-political, labor or health related incident could have a significant impact on the PGM catalyst supply chain. For instance, the COVID-19 pandemic has magnified the concerns over the vulnerability of the supply chains of many goods and services to disruptions (Roskill. , 2020). Mining companies in South Africa can only operate at 50% capacity for the moment, causing a sharp drop in mine production (WPIC, 2020; USGS, 2021). Numerous suppliers of refined PGMs and PGM catalysts have halted production (WPIC, 2020). Jowitt (Jowitt, 2020) provided an overview of the effects of COVID-19 mitigation on the mining sector, indicating that it is more likely that a decreased demand leads to further lower metal prices with negative economic impacts on mining operations. As for downstream commodities of the supply chain, the pandemic caused declined production of both ICEV-related PGM catalysts and FCV-related PGM catalysts (Market Report, 2021; Hart and Jones, 2020). The global supply chain structures of PGM catalyst may face certain risks. Thus, understanding of the historical trajectory of the entire supply chain provides the basis to reduce such supply risks.Under such a circumstance, securing the raw material supply of PGMs and assessing associated supply risks received wide attentions (Schrijvers et al., 2020). Numerous studies have evaluated supply risks and vulnerabilities of PGMs (Rasmussen et al., 2019; Graedel et al., 2015; Hayes and McCullough, 2018; Mudd and Jowitt, 2017; Jowitt et al., 2018). Mudd et al. (Mudd et al., 2018) presented a global assessment of PGM resources and analyzed the key mining trends, indicating that key problems are not geological or resource depletion, but social, economic and environmental in nature like recent social issues in South Africa and volatile global economic conditions. Jowitt et al. (Jowitt et al., 2020) analyzed global metal reserves and found that primary PGM supply will not be exhausted within several decades. They suggested similar points of view that environmental, social, and governance factors are likely to be the main source of risk in metal supply. Yuan et al. (Yuan et al., 2020) introduced an assessment framework to analyze the criticality of platinum from 1975 to 2015, showing that the supply risk of platinum is strongly influenced by South Africa’s socio-political status and dominance over global supply and reserves. All these studies show that PGM supply risks cannot be neglected which are mainly caused by the governance and environmental factors.Existing studies have laid a solid foundation for analyzing supply risks of PGM supply chain. However, they are mainly focused on the upstream stages including mining and refining. Considering manufacturing technical barriers, global production of PGM catalysts has concentrated in specific countries and regions, with significant barriers for capacity shifting (Islam et al., 2018). Identifying the supply risks of each stage along the supply chain becomes more crucial. The difference between the supply of the ICEV-related PGM catalysts and FCV-related PGM catalysts at the manufacturing stage needs more attention. Hence, this study provides a perspective of the entire supply chain, systematically assesses the global supply structures and risks of major stages throughout the production processes of PGM catalysts used in vehicles for the period 2010–2020. Traditional and new applications of PGM catalysts in the automotive industry are both considered, including ICEV-related PGM catalysts and FCV-related PGM catalysts. The government management and environmental factors are also considered in each stage. At the refining stage, this study takes the recycling into full consideration. The results indicate that significant supply risks exist in the PGM catalyst supply chain, especially in the mining stage of PGMs and the manufacturing stage of FCV-related PGM catalysts. Relevant policy suggestions are put forward to mitigate the supply risks of the supply chain.This study chooses the whole world as the spatial boundary. The main PGM mining, refining and catalyst production countries or regions with related explanations are shown in the supplementary document. The temporal boundary is set to 2010–2020. This period is chosen to reflect the status quo of the supply risks sufficiently.The entire PGM catalyst supply chain considered in this study covers three stages: mining, refining, and manufacturing. From the raw material perspective, PGMs are a major concern. Mining stage refers to the process from natural PGM resource to minerals. The mining activities always exert significant pressure on environmental and ecological systems (Kosai et al., 2021). Refining stage refers to the process from PGM minerals to intermediate products. The specific processes mainly include milling, concentrating, smelting, converting, separating and refining (Rasmussen et al., 2019). Manufacturing stage refers to the process from intermediate products to final products for end-use purpose, specifically referring to the process from refined PGMs to ICEV-related PGM catalysts and FCV-related PGM catalysts in this study (Sun et al., 2019).The supply risk normally refers to the probability of material supply disruption. The most widely used indicator is the diversity of supplying countries or regions, measured by the Herfindahl-Hirschman-Index (HHI) (Schrijvers et al., 2020). HHI is a comprehensive index to measure the degree of industrial concentration, which is widely used by economists and government regulatory departments (Silberglitt et al., 2013; Brown, 2018). In this study, we combine the HHI with the Worldwide Governance Indicator (WGI) and the Environmental Performance Index (EPI) respectively to quantitatively evaluate supply risks, focusing on the governance and environmental influence.The HHI is calculated by summing the squares of the market shares of fifty largest supply entities in a given market, as shown in Eq (1). It should be noted that, the supply entity refers to the country in this study. (1) HHI = ∑ i S i 2 where, S i is the market share of country i (in percentage unit).The WGI is an aggregate indicator for measuring six dimensions of governance for over 200 countries and regions over the period 1996–2019, including Voice and Accountability (VA), Political Stability and Absence of Violence (PV), Government Effectiveness (GE), Regulatory Quality (RQ), Rule of Law and Control of Corruption (RC) (World Bank, 0000). All the sub-indicators range from –2.5 (bad governance performance) to 2.5 (good governance performance). Among these dimensions, PV has a significant correlation with the stability of supply structures (van den Brink et al., 2020; Nassar et al., 2012; Nuss et al., 2014). The stability of supply structures significantly affects the probability of supply disruptions. The value of WGI_PV is scaled to 0–1 by using Eq (2). (2) W G I _ P V scaled = - 0.2 × W G I _ P V + 0.5 The aggregate indicator of HHI and WGI_PV is defined as HHI-WGI. It takes both the diversity and stability of supplying countries into account. The HHI-WGI is calculated by Eq (3) (Mudd et al., 2018). The higher HHI-WGI is, the higher supply risk can be identified. (3) HHI - W G I = ∑ i S i 2 × W G I _ P V i , s c a l e d where, S i is the market share of country i (in percentage unit); W G I _ P V i , s c a l e d is the scaled WGI_PV value of country i.The EPI aggregates 24 indicators covering environmental health and ecosystem vitality, and measures environmental performance of 180 countries and regions (Wendling et al., 2018). It ranges from 0 (bad environmental performance) to 100 (good environmental performance). In the production processes, unsatisfactory environmental performance can cause supply risks (van den Brink et al., 2020). The reason is that such processes may lead to a level of environmental damage that society does not considers acceptable (Graedel et al., 2012). In such a situation, the production processes can be disrupted. The value is scaled to 0–1 by using Eq (4). (4) EPI scaled = 100 - E P I 100 The aggregate indicator of HHI and EPI is defined as HHI-EPI. It considers the diversity of supplying countries and the environmental risk in the supply processes. The HHI-EPI is calculated by Eq (5). The higher HHI-EPI is, the higher environmental risk can be identified. (5) HHI - E P I = ∑ i S i 2 × EPI i , s c a l e d where, S i is the market share of country i (in percentage unit); EPI i , s c a l e d is the scaled EPI value of country i.To estimate the HHI, data are needed on PGM mining, refining and catalyst manufacturing at the national or regional level. Data on production of platinum and palladium in the mining stage are obtained from annual mineral yearbooks for the period 2010–2017, and mineral commodity summaries 2020 and 2021 by USGS (USGS, 2021; USGS, 2018; USGS, 2020). Data on production of rhodium in the mining stage are obtained from market reviews for the period 2010–2013, and PGM market report February 2021 of Johnson Matthey (Market Report, 2021; Market Report, 2013). In the refining stage, PGM primary and secondary supply data are supported by Steve Forrest Associates (SFA, 2020). The production of PGMs in this stage refers to the primary supply. When taking recycling into account, the PGM production refers to the total supply that can be calculated by Eq (6). (6) TS i = PS i + SS i where, TS i is the total supply of refined PGMs of country i; PS i is the primary supply of refined PGMs of country i; SS i is the secondary supply of refined PGMs of country i.This study uses the PGM mass contained in catalytic converters to represent the production of ICEV-related PGM catalysts in the manufacturing stage. Data on the global production distribution of ICEV-related PGM catalysts are calculated by Eq (7)-(10). (7) SA i = 1 n ∗ ∑ n AP i k ∑ i AP i k (8) AP i k = VP i k + ∑ j ( NE i , j k - NI i , j k ) (9) NE i , j k = EX i , j k ∗ β j k (10) NI i , j k = IM i , j k ∗ β j k where, SA i is the share of global ICEV-related PGM catalyst production for country i; n is the number of elements considered in this study; AP i k is the mass of element k contained in ICEV-related PGM catalysts produced in country i; VP i k is the mass of element k contained in vehicles produced in country i; NE i , j k is the mass of element k contained in commodity j exported from country i; NI i , j k is the mass of element k contained in commodity j imported to country i; EX i , j k is the mass of commodity j which contain element k exported from country i; IM i , j k is the mass of commodity j which contain element k imported to country i; β j k is the proportion of the mass of element k in commodity j of the international trade.In this study, data on PGM mass contained in vehicles are obtained from SFA’s reports (SFA, 2020). The international trade data of relevant commodities which contain PGMs are mainly obtained from the United Nations Commodity Trade Database from 2010 to 2020 (UN, 2021). It should be noted that, when two countries report different trade data, the data from the country with high WGI_PV are used in this study. Information about these commodities and corresponding proportions of PGM mass are shown in Table 1 .This study uses the platinum equivalent to represent the production of FCV-related PGM catalysts in the manufacturing stage. Country-level supplies are calculated by using a bottom-up approach as shown in Eq (11). Due to low production of FCVs, this study does not consider the stocks of these commodities. (11) TS j = ∑ i PV i , j ∗ SP i , j ∗ CD i , j - IM j where, TS j is the total supply of FCV-related PGM catalysts in all FCV types in country j; PV i , j is the production volume of FCV type i in country j; SP i , j is the fuel cell system power of FCV type i in country j; CD i , j is the demand of FCV-related PGM catalysts per unit power of FCV type i in country j; IM j is the total import of FCV-related PGM catalysts in country j.The major FCV suppliers and system powers are obtained from annual reports of numerous companies and literatures, which are shown in our previous study and the latest report (Xun et al., 2021; Yu, 2021). As for the demand of platinum per unit power of FCVs, this study estimates that it remained unchanged from 2015 to 2017, and from 2019 to 2020. Data on the demand from 2017 to 2019 can be found in technical reports presented by Xun (Xun et al., 2021). Relevant information about international trade are obtained from the same source.To estimate the HHI-WGI and HHI-EPI, data on scores of countries or regions over the period 2010–2020 are needed. They are obtained from open sources published by authorities (World Bank, 0000; Wendling et al., 2018). Detailed regional material production data, original and scaled WGI_PV and EPI index values with corresponding explanations are shown in the supplementary document. Fig. 1 shows the supply structures of each stage along the PGM catalyst supply chain in 2020, from which the reason for the status of supply risks can be found. The main producing countries for platinum are South Africa and Russia. For palladium, Russia and South Africa together occupy the main market supply. For rhodium, South Africa dominates the market, which means that the supply sources are more concentrated. According to the WGI_PV assessment results, South Africa and Russia are mid-ranking countries (ranked 138th and 153rd out of 214 countries and regions) (World Bank, 0000). As for EPI, Russia is at the relatively high level (ranked 52nd out of 180 countries and regions), while South Africa ranks much lower (ranked 142nd out of 180 countries and regions) (Wendling et al., 2018). Most ores are uneconomical for transportation because of low concentration (Reith et al., 2014). Smelters are mainly constructed in areas proximal to major mines like Noril'sk in Russia and the Bushveld in South Africa (USGS, 2018). The smelters and refiners could use both primary concentrates and recycled materials to make PGM powder or bars. The global supply structures for PGM refining are similar to those for mining.While in the manufacturing stage, the global supply structure for ICEV-related PGM catalysts is more diversified than supply structures of upstream materials. The main producing countries of ICEV-related PGM catalysts are the United States, China, Japan and Germany, which are also major producers of vehicles. The global supply structure of FCV-related PGM catalysts is very different to that of ICEV-related PGM catalysts. The main producing countries are also major producers of FCVs. The production in Korea accounts for more than 70% of global production, while the United States accounts for 13%. Other producing countries include China, Japan and Germany. These countries have better performance than South Africa and Russia in terms of the WGI_PV and EPI performances, thus the supply risks of the manufacturing stage are lower obviously. Fig. 2 shows global production situation of FCV-related PGM catalysts. Since the FCV industry is still at the initial stage of development, the gross production of FCV-related PGM catalysts is much smaller than that of ICEV-related PGM catalysts. The market-dominant country changed from Japan to Korea, which triggered such a tremendous change in supply risk of FCV-related PGM catalysts. The production in Korea had increased more than 150 times, from less than 2 kg platinum equivalent in 2015 to 326 kg in 2020. The production in Japan had fallen by 70%, from 47 kg to 14 kg during the period 2015–2020. The production in the United States, China and Germany surged from 2015 to 2019, while they suffered a sharp decline in 2020 affected by the COVID-19 pandemic. Fig. 3 (A) shows the HHI of the PGM catalyst supply chain from mining to manufacturing stage during the period 2010–2020. A larger HHI represents higher supply concentration, which implies a higher probability of supply disruption. It should be noted that, an HHI above 2500 indicates a highly concentrated market, while an HHI between 1500 and 2500 indicates a moderately concentrated market (Silberglitt et al., 2013). In the mining stage, the HHI for platinum, palladium and rhodium had been declining slowly in the recent decade, from 6103, 3490 and 7462 to 5198, 2689 and 6105, respectively. The reason could be that the Lac des Iles mine located in Canada have come onstream. Commercial production of PGM began in 1993 from the deposit known as the Roby zone, which until recently was exploited exclusively via open pit mining (Market Report, 2021). The rising production of PGM mine in countries other than South Africa has reduced supply concentration. The HHI for platinum, palladium and rhodium in the refining stage showed a similar trend, dropping from 6173, 3544 and 7144 in 2010 to 5291, 2999 and 5839 in 2020, respectively. While considering the secondary supply, there was a significant decline of HHI. The average HHI values for platinum, palladium and rhodium during the period dropped down by 39%, 32% and 43% respectively. It should be noted that, there was a sharp fall of HHI in 2020. The main reason could be that the COVID-19 led to a sharp drop in mine production of South Africa, with 10%, 13% and 28% decline for platinum, palladium and rhodium, respectively.In the manufacturing stage, the HHI were significantly different from those of the supply chain upstream. As for ICEV-related PGM catalysts, they were much lower and showed a downward trend, changing from 1433 in 2010 to 1079 in 2020. The HHI of FCV-related PGM catalysts were not present from 2010 to 2014 due to little global production. They decreased from 6760 in 2015 to 3185 in 2018, and then increased to 5937 in 2020. The main reason for such a trend was that the production in Korea had increased sharply from 2015 to 2020, while the production in Japan had experienced a significant decline. The production in other countries rose steadily. Fig. 3(B) shows the HHI-WGI of the PGM catalyst supply chain in recent years. A larger HHI-WGI represents higher supply concentration with lower governance capability of supply countries, which implies a higher chance that supplies from these countries are cut-off (Nassar et al., 2020). Similarly to HHI, the HHI-WGI for platinum had decreased from 3122 to 2839 and from 3156 to 2888 in the mining and refining stages during the last decade respectively. The HHI-WGI for palladium in the mining and refining stages were approximately the same. It should be noted that, the values in the refining stage fell about 40% when taking the secondary supply into account. So were the HHI-WGI for rhodium, the values in the refining stage nearly equaled to those of the mining stage. In the manufacturing stage, the HHI-WGI values of ICEV-related PGM catalysts decreased by 15%. The HHI-WGI of FCV-related PGM catalysts dropped from 1972 to 1312 from 2015 to 2018, and rose up to 2407 in 2020. Fig. 3(C) shows the HHI-EPI of the PGM catalyst supply chain in the last decade. A larger HHI-EPI represents higher supply concentration with higher environmental risk of supply countries, which implies a higher probability of causing negative environmental impacts in the supply processes. In the mining stage, the values for platinum, palladium and rhodium varied from 3339 to 2837, from 1579 to 1151, and from 4107 to 3348 in the last ten years respectively. The values in the refining stage were almost the same. In the manufacturing stage, the values of ICEV-related PGM catalysts were much lower than those in the upstream stages, changing from 591 to 351 during the period 2010–2020. The values of FCV-related PGM catalysts changed from 1725 to 944 during the period 2015–2018, and went up to 2220 in 2020. The HHI-EPI in the refining stage were much lower when recycling is considered, exactly as the HHI and the HHI-WGI.In the upstream stages of the PGM catalyst supply chain, the risk of rhodium was the highest among PGMs as Fig. 4 shows. Considering the mean value of HHI from 2010 to 2020, the value for rhodium was 24% and 115% higher than that for platinum and palladium respectively in the mining stage, while 21% and 112% higher in the refining stage. As for the HHI-WGI, the value for rhodium was over 20% and 90% higher than that of platinum and palladium respectively in the mining and refining stages. And the average value of HHI-EPI for rhodium was higher than that for platinum and palladium in these stages. In the downstream stages of the PGM catalyst supply chain, the supply risk of ICEV-related PGM catalysts remained stable in the last decade. However, the supply risk of FCV-related PGM catalysts changed drastically, due to the change of global production situation as shown in Fig. 2. In the last decade, the production of FCV-related PGM catalysts was initially concentrated in Japan and gradually moved to Korea. Although the global productions of FCV-related PGM catalysts were significantly less than those of ICEV-related PGM catalysts, the supply risk could not be ignored. On the one hand, the higher technical threshold of FCV-related PGM catalysts led to the more concentrated supply structure. Many countries devote great effort to developing FCV-related PGM catalysts that have not been commercialized yet. On the other hand, building production capacity needs the initial investment of time and money. The rapid development of FCVs will increase the demand of fuel-cell-related PGM catalysts. In the event of a supply shortage, only a few countries are able to respond promptly and increase the production of them.Based on the above analysis, great supply risks are found in the entire PGM catalyst supply chain. There are countries that play an important role in this supply chain, producing substantial commodities at each stage. The inaccuracy of production statistics can have a significant impact on the results of supply risk assessments. Taking the United States and South Africa as an example, this study presents the sensitivity analysis of the production on the HHI, HHI-WGI and HHI-EPI of the major stages in the PGM catalyst supply chain as shown in Fig. 5 . The United States is a typical country with strong production technology, while South Africa has abundant natural resources.This study takes the commodity outputs mentioned above along the PGM catalyst supply chain in 2020 as the baseline. The results show that the production of the United States mainly influence the supply risks of commodities in the downstream of the supply chain, while the production of South Africa mainly have impacts on upstream commodities. For instance, when assuming 10% higher production of ICEV-related PGM catalysts in the United States, the estimated HHI, HHI-WGI and HHI-EPI of ICEV-related PGM catalyst manufacturing stage change by 4%, 4% and 3% respectively. When assuming 10% lower production, the estimations change by −4%, −4% and −3% respectively. As for the upstream stages like refining, the results indicate that the assumption of 10% higher production of primary refined platinum in South Africa causes 5%, 5% and 5% changes of the estimated HHI, HHI-WGI and HHI-EPI respectively. While the assumption of 10% lower production causes −5%, −5% and −5% changes of those supply risk indicators respectively.As mentioned above, extensive studies have discussed whether the PGM resources are facing the problem of short supply. The balance between supply and demand has huge impact on supply risks. If the global demand of PGM catalysts used in vehicles cannot be met by the total PGM supply, supply risks are inevitable. So far, the consensus is that the primary PGM supply will most likely not to be the restriction for future development of the catalyst used in vehicles. However, as we found in our previous study, there could be significant supply risks due to resource location. The distributions of PGM reserves and demands are highly mismatched (Hao et al., 2019). The countries in need of PGMs are facing with the serious challenge of securing PGM supply. A high concentration of processing capacity leads to considerable potential supply risks, for the reason that the productions of commodities in the supply chain of PGM catalysts have higher technical threshold and are difficult to be replaced by alternatives once supply disruption occurs (CsA and Hausmann, 2009). Besides, the production processes are under restrictions of Environmental, Social and Governance (ESG) (Jowitt et al., 2020). The possibility of supply disruption can be reflected through the ESG performance of suppliers, which can be quantified by WGI_PV and EPI. Furthermore, the transport processes are seriously influenced by the ESG performance of suppliers. A sophisticated transportation logistics system had been built and maintained around the world. Although the system remains stable most of the time, it is also confronted with potential risks because the country with unsatisfactory ESG performance is more likely to interrupt transport.According to the results shown in Fig. 1, the global supply structures for PGM mining are highly concentrated. In the refining stage, the situation remains unchanged only considering the primary supply. While taking the secondary supply into consideration, the supply structure for palladium becomes moderately concentrated. As for the manufacturing stage, the supply structure for ICEV-related PGM catalysts becomes much less concentrated. But the supply structure of FCV-related PGM catalysts is highly concentrated. The supply risks along the entire supply chain of PGM catalysts used in vehicles cannot be ignored. Based on the analysis, several suggestions could be propounded with the aim of mitigating the risks of the PGM catalyst supply chain for countries seeking to secure the future supply of it.First, optimizing the supply structure of the upstream stages along the PGM catalyst supply chain could effectively reduce supply risks. The global supply structure for PGM mining is hard to change, for the reason that mining production distribution is the map of reserve and resource endowment to some extent (Ali et al., 2017). The richest mineral resources of PGMs are concentrated in South Africa and Russia. It is difficult to build up PGM ore stocks for the major consumers of the PGM catalyst including the United States, China, Japan, Korea and Europe. The total amount of PGMs in these countries or regions is relatively small and the ore grade is very low. While they can pay attention to overseas investment which can effectively reduce regional supply risks (Sun et al., 2019). For example, as one of the world’s largest PGM mining companies, Anglo American Platinum Limited provided 34%, 21% and 36% of the global mining production of platinum, palladium and rhodium respectively. In fact, it is incorporated in the United Kingdom (Anglo, 2019). In addition to overseas investment, establishing national stocks of refined PGM products can also help these economically advanced countries or regions to reduce import dependences.Second, striving to develop secondary supply of PGMs could be another way to face the challenge. As the results show, the values of supply risk indicators for PGM refining decrease over 30% with the secondary supply considered. In fact, the global in-use stocks of PGMs have very promising recycling potential, accounting for more than 10% of natural resources in 2015 (Nassar, 2015). Meanwhile their geographical distribution is more consistent with the distribution of demand. However, the current recycling situation of PGM materials is not optimistic. The world is facing the challenges of low end-of-life recycling rates, especially in developing countries (Xun et al., 2020). The state-of-the-art recovery rates of PGMs are relatively high, while the collection rates are much lower due to complex factors, including management at the national level and recycling profits (Jha et al., 2013; Maes et al., 2016; Hagelüken et al., 2009). In order to improve recycling actuality, further measures need to be adopted, including strengthening the supervision of the recycling processes, constructing the recycling infrastructures and so on.Third, developing low-PGM catalysts used in vehicles could address the challenge of supply risks along the supply chain. Considerable efforts have been dedicated to the exploration in catalytic converters since the last century (Li et al., 2010; Kapteijn et al., 1993). As for the future demand, many countries have already made national plans to reduce the PGM loadings of fuel cells. For instance, the United States, China and Japan published the technology roadmap, setting the targets of reaching 0.1 g/kW by 2025 (Doe, 2017), 0.125 g/kW by 2030 (SAE-China, 2016), and 0.05 g/kW by 2030 (Nedo, 2017) respectively. At the same time, countries that are striving to develop FCVs need to improve the localization rate of PGM catalyst. They can consider to take specific measures, such as increasing policy and financial support to R&D, enhancing demonstrations and applications of new low-PGM catalyst technologies, stepping up infrastructure construction, and so on.Last but not the least, appropriate optimization of the PGM catalyst supply chain plays a positive role in reducing supply risks. However, risk reduction cannot be the sole objective. Measures like the re-shoring of the manufacturing sector help reduce supply risks, but they should be assessed cautiously. In the context of economic globalization, many companies have been moving their production capacity abroad through acquisitions and building plants overseas (Sun et al., 2019). These actions help to overcome many risks, including the lack of domestic natural resource and the restriction of human resource (Dachs et al., 2019). Under the circumstance of the COVID-19 pandemic, the importance of manufacturing capacity is fully exhibited before the world, which could accelerate the re-shoring of manufacturing sector. Although the re-shoring help to increase domestic employment and reduce national supply risks, it may affect the efficiency of production due to diseconomies of scale in the industry (Delis et al., 2019; Bailey and De Propris, 2014). Protecting the resilience of supply chains is not simply to bring all production home, but to play a large role in the global supply chains. Whether to adopt measures for the re-shoring of manufacturing sector needs to be based on the endowment of domestic relevant resources and product performances (Li and Zobel, 2020).This study provides insights into supply risks along the entire supply chain of the PGM catalyst used in vehicles during the period 2010–2020. The results show that there are significant supply risks embodied in current supply chain, especially in the mining stage. The HHI of platinum, palladium and rhodium in the mining stage changed from 6103, 3490 and 7462 in 2010 to 5198, 2689 and 6105 in 2020 respectively. Enhancing recycling of PGMs can effectively reduce supply risks in the refining stage, bringing about over 30% falls in the supply risks. As for the manufacturing stage, the HHI of FCV-related PGM catalysts decreased from 6760 in 2015 to 3185 in 2018, and then increased to 5937 in 2020. Comparing to that of ICEV-related PGM catalysts, the supply risk of FCV-related PGM catalysts was apparently higher. The results are of high relevance and importance to policy makers seeking to secure the future supply of conventional vehicles or FCVs.One limitation of this study is that a high concentration of processing capacity does not necessarily mean great supply risk. A clear mechanism between the selected indicators and a resulting supply risk cannot be provided by our model. Another is that since the FCV industry is still at the initial stage of development, the analysis of supply risks along the supply chain of FCV-related PGM catalysts does not necessarily reflect future development trends. Further efforts are needed to fill this research gap and provide more accurate estimations that can lead to more relevant policy implications.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 study is sponsored by the National Key R&D Program of China (2019YFC1908501), National Natural Science Foundation of China (72122010, 71774100).

Platinum group metals (PGMs) are important catalytic materials for producing emission after-treatment devices in internal combustion engine vehicles (ICEVs) and fuel cells in fuel cell vehicles (FCVs). There are high resource and technical barriers along the supply chain of PGM catalysts, for which assessing the associated supply risks is essential to understand the potential challenges. This study maps the supply chains of PGM catalysts used in ICEVs and FCVs for the period of 2010–2020, and compares the associated supply risks. The results show that significant supply risks can be identified in both the upstream and downstream stages of PGM catalyst supply chains for ICEV and FCV uses. Specifically, the ICEV-related PGM catalyst supply chain maintained high levels of supply risks with slight decline over the assessed period. With recycling considered, supply risks in the refining stage can be reduced by over 30%. Supply risks embodied in the FCV-related PGM catalyst supply chain were relatively higher, with significant fluctuations during the period 2015–2020. Relevant suggestions are propounded aiming to mitigate supply risks of PGM catalysts used in ICEVs and FCVs.

Carbon Balance (%)Molar flow at the reactor inlet of the component i (μmol min-1)Molar flow at the reactor outlet of the component i (μmol min-1)Selectivity towards CH4 (%)CO2 stored (μmol g-1)Time (min)Temperature (°C)Sample mass (g)CH4 production (μmol g-1)CO production (μmol g-1)H2O production (μmol g-1)CO2 Capture and SequestrationCO2 Capture and UtilizationDual Function MaterialEnergy Dispersive SpectroscopyIntegration of CO2 Capture and UtilizationInductively Coupled Plasma Atomic Emission SpectroscopyLaNiO3 perovskiteNanoparticlesSynthetic Natural GasScanning Transmission Electron Microscopy-High Angle Annular Dark FieldThermal Conductivity DetectorTemperature Programmed DesorptionTemperature Programmed ReductionX-Ray DiffractionThe increase in global energy demand has led to a rapid growth in the fossil fuels consumption. As a result, the emission of greenhouse gases has been constantly increasing during the last decades, contributing to global warming and ultimately to climate change [1]. CO2, mainly emitted from the power generation sector and the industrial and transportation vehicles, is a major contributor to global warming due to its huge emission amounts [2,3]. Thus, the reduction of the CO2 emissions to the atmosphere is essential to limit global warming. In this context, carbon capture and sequestration (CCS) from industry and energy related sources as well as the increase in the efficiency of industrial processes and the widespread implementation of renewable energies, are expected to play an important role in overcoming this increasing problem [4]. However, CCS technology requires captured CO2 purification and transport to storage places and its isolation, which increases drastically the cost and the energy consumption of the process [5,6].During the last years, there is keen interest in the integration of CO2 capture and its utilization (ICCU), since this technological alternative allows reducing the cost of the overall process by eliminating transportation and storage of CO2 by its conversion to fuels or value-added chemicals [7]. Farrauto et al. [8,9] have patented in 2015 the use of dual function materials (DFMs) to convert the captured CO2 from diluted exhaust gases into methane in a single reactor. Such ICCU process can be even greener when is carried out with hydrogen obtained by the electrolysis of water using surplus renewable energies, contributing at the same time to store excess electrical energy in the form of methane. Therefore, ICCU-methanation technology reduces CO2 emissions to the atmosphere and contributes to solve the problem of intrinsic intermittency of renewable sources [10]. Moreover, the cycling operation, in contrast to the observed for the continuous hydrogenation of CO2, can be directly applied to an effluent gas without the necessity of additional heat input to perform the CO2 capture and does not need purification steps, which reduces the global costs of the process [11].The selected DFM should selectively capture CO2 from steam- and O2-containing flue gas at different temperatures (200–550 °C), depending on the application and effluent gas properties; and then hydrogenate the adsorbed species to methane with H2 in a carbon neutral cycle. The overall CO2 adsorption-hydrogenation process follows the stoichiometry of the Sabatier reaction [12], which is thermodynamically favoured at low temperature due to its strong exothermicity. (1) CO 2 + 4H 2 ⇄ CH 4 + 2H 2 O Δ H 0 = - 164 kJ mol - 1 However, the stable electronic structure of the CO2 molecule makes its activation difficult under mild conditions, such as low temperature and low pressure. The use of high reaction temperatures favours the kinetics of CO2 to CH4 conversion; however, it contributes to increase the equipment investment as well as the operational cost, which is undesirable for large-scale industrial utilization. Therefore, a high-performance catalyst that can activate CO2 and promote the reaction rate under relative low temperatures and pressures is vital for its widespread implementation. Based on the characteristics of dual operation, these catalysts require the presence of a storage material for CO2 capture and an active site for H2 activation and CO2 hydrogenation to methane. Regarding to the CO2 adsorption functionality in DFMs, a wide variety of alkali/alkaline-earth phases have been proposed as CO2 storage material, mainly Na [13,14], Ca [8,15], Mg [4,16] or K [4,17]. These storage components should be capable to reversibly operate at intermediates-high temperatures (200–450 °C) [18]. On the other hand, the catalytic and hydrogenation sites are usually based on Ni [15,19–21], Ru [13,14,22,23] or Rh [24] metals. Among them, Ni presents the best cost to activity ratio, which makes this alternative most suitable for industrial applications. Finally, both phases are usually dispersed on a high surface area carrier in order to increase the methane production. In this sense, previous studies reported that γ-Al2O3 is the most appropriate support among other materials [19,25].Ni-based catalysts prepared by conventional preparation methods often lead to large and heterogeneous particle size distribution, which limits the control over the interaction between the metal nanoparticles (NPs) and the support [26]. Hence, Ni-based catalysts present limited catalytic activity at low temperatures and can be easily deactivated due to the metal sintering occurring at high temperatures. Furthermore, Ni-based DFMs have been considered only for process at intermediate-high temperatures, since Ni can be readily oxidized during the CO2 adsorption period but not easily reduced back during the hydrogenation step at low temperatures [11,19]. Intense efforts have been made in order to design and improve Ni-based catalysts for their application as dual function materials (DFMs). The catalytic behaviour of the Ni-based materials depends on several factors such as the type of support, Ni loading, addition of a second metal and preparation method [27,28,29].Largely based on the pioneering research of Daihatsu and Toyota, the ex-solution of active metal NPs from an oxide host, such as perovskite-type lattice, has been identified as a simple way to achieve a homogeneous active sites distribution, with good reversibility and controlled interactions between metal and the support [30]. This concept has been already explored for controlling Ni particle sizes and distribution of the catalysts used in the stationary CO2 methanation process [31–33]. Specifically, LaNiO3-type perovskites, partially doped with different components (i.e., Ce, K or Ca), have been proposed as promising host materials to carry out the inside-outside ex-solution of Ni NPs. Nevertheless, non-supported perovskites exhibited rather low surface areas, which could limit the active sites dispersion and the reaction intermediates diffusion. To address the aforementioned limitations, Li et al. [34] and Wang et al. [35] distributed LaNiO3-type perovskites on silica supports. The obtained catalysts have shown improved CO2 methanation efficiency. Based on the well-known promoting effect observed for ceria in its application to the conventional Ni/CeO2 catalyst for the stationary CO2 methanation [36–38], we recently explored the viability of ceria-supported LaNiO3 perovskites as precursor of highly active and stable materials for the continuous CO2 methanation [39]. These catalysts present notably higher methane production than the conventional Ni/CeO2 catalyst and that obtained from the bulk LaNiO3 in the stationary CO2 to CH4 hydrogenation process. Nevertheless, to the best of the authors' knowledge, the use of LaNiO3 perovskite as precursor of highly efficient DFM material for CO2 capture and hydrogenation to methane has not been published to date.Considering this background, the aim of this work is to evaluate for the first time in the scientific literature the applicability of supported LaNiO3 perovskites, as precursors of efficient dual function materials for CO2 adsorption and in-situ hydrogenation to methane. For that, the previously developed 30% LaNiO3/CeO2 formulation as well as others here synthesized over conventional DFM supports, such as Al2O3 and La-Al2O3, are evaluated in cycles of CO2 adsorption and hydrogenation to CH4. Taking into account the characterization results, the interrelationships between physico-chemical properties, activity, and stability are discovered.Prior to the preparation of perovskite-based formulations, different supports were obtained. On the one hand, the ceria support was obtained by direct calcination of the Ce(NO3)3·6H2O (Sigma Aldrich, 99.9%) precursor at 500 °C for 4 h in static air. On the other hand, the 5 wt% La-Al2O3 support was obtained by wetness impregnation method over previously calcined γ-Al2O3 (650 °C, 2 h). For that, the amount of La(NO3)2·6H2O (Merck, 99.0%), neccesary to obtain a 5 wt% of La2O3 over the support, was incorporated onto γ-Al2O3 (Saint Gobain, SA6173) inside a rotary evaporator (vacuum and 35 °C).Once different supports (Al2O3, La-Al2O3 and CeO2) were obtained, supported perovskites were prepared by combining citric acid and impregnation methods, as reported elsewhere [39]. For that, nominal perovskite loading of 30 wt% was impregnated over these supports. The adopted nomenclature for the fully formulated samples was the following: LNO, 30% LNO/CeO2, 30% LNO/Al2O3 and 30% LNO/La-Al2O3. Note that this nomenclature corresponds to the precursors of the catalysts for CO2 methanation reaction. In order to obtain different DFMs, these precursors were in-situ reduced in the reaction bench.X-ray diffraction (XRD) analyses of the fresh and used samples were carried out using a Philips PW1710 diffractometer. For that, all samples were subjected to Cu Kα radiation in a continuous scan mode in the 2θ range 5–70° with 0.02° per second sampling interval. PANalytical X‘pert HighScore and Winplotr profile fitting software were used for data treatment. ICDD (International Centre for Diffraction Data) database cards were used for comparative purposes to identify the phases present in the samples.Scanning Transmission Electron Microscopy - High Angle Annular Dark Field (STEM-HAADF) images were taken for the samples after reduction and CO2 methanation reaction with a Cs-image-corrected Titan (Thermofisher Scientific). This equipment operated at a working voltage of 300 kV, and was equipped with a CCD camera (Gatan) and a HAADF detector (Fischione). The instrument has a normal field emission gun (Shottky emitter) equipped with a SuperTwin lens. Alternatively, the TEM apparatus was also equipped for X-ray Energy Dispersive Spectroscopy (EDS) experiments with an Ultim Max detector (Oxford Instruments). A 2 k × 2 k Ultrascan CCD camera (Gatan) was positioned before the filter for TEM imaging, using an energy resolution of 0.7 eV. The acquisition time for the analysis was 50 ms per spectrum and the used energy dispersion was 0.2 eV pixel−1. Prior to these experiments, the samples were sonicated in ethanol and dropped onto a holey, amorphous carbon film supported on a copper grid.Textural properties of the fresh and used samples, that is, after controlled reduction and CO2 methanation, were determined by N2 adsorption–desorption isotherms at −196 °C, using a Micromeritics TRISTAR II equipment. All samples were pretreated with flowing N2 on a Micromeritics SmartPrep instrument at 300 °C for 10 h.La, Ni, Al and Ce contents were quantitatively determined by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP–AES). The analyses were carried out using a mass spectrometer with a plasma source (Q-ICP-MS XSeries II model of Thermospectrometer). Prior to the analysis, the samples were digested at 120 °C with an acid mixture (HNO3:HCl = 1:3) inside a microwave furnace.The redox behaviour of the fresh samples was examined by means of Temperature Programmed Reduction with a 5% H2/Ar mixture (H2-TPR) in a Micromeritics AutoChem II equipment. For that, the quartz tube reactor was loaded with 0.1 g of sample, which was pretreated in 30 mL min−1 of 5% O2/He mixture at 600 °C for 30 min. Then, the sample was cooled down to 35 °C under inert conditions, and finally the temperature was increased from 35 to 950 °C in a 5% H2/Ar mixture (30 mL min−1) using a heating rate of 10 °C min−1. Water generated during samples reduction was removed by condensation in a cold trap placed before TCD detector. The outlet gas stream was continuously monitored with a Hiden Analitical HPR-20 EGA mass spectrometer.The basicity of the samples was evaluated by Temperature Programmed Desorption of CO2 (CO2-TPD) experiments, which were carried out in a Micromeritics AutoChem II equipment. The quartz tube reactor was loaded with 0.15 g of the fresh samples. Aiming to obtain a catalytic material similar to that analyzed in the activity test, bulk perovskite and ceria- and alumina-supported samples were completely reduced in a 5% H2/Ar mixture (50 mL min−1) at 650, 550 or 800 °C (2 h), respectively. Then, the reduced samples were cooled down to 40 °C, under inert conditions (He flow stream). Once this temperature was reached, the adsorption of CO2 was performed by exposing the samples to a 5% CO2/He flow stream (50 mL min−1) for 60 min. Finally, the samples were heated from 40 to 900 °C at 10C min−1 in He (50 mL min1) and the desorbed gases were continuously monitored with a Hiden Analitical HPR-20 EGA mass spectrometer.CO2 adsorption and hydrogenation cycles were carried out in a vertical stainless steel tubular reactor inside a 3-zone tube furnace. The reactor was filled with 1.0 g of pelletized (0.3–0.5 mm) fresh formulation, where the operating temperature was continuously measured through a thermocouple placed in the centre of the catalytic bed. Prior to the catalytic test, fresh samples were in-situ reduced with a stream composed of 10% H2/Ar leading to the conformation of the final DFM due to the controlled reduction of perovskite-based formulation. With that aim, the temperature was progressively increased from room temperature to 650, 550 or 800 °C (2 h) for bulk and ceria- and alumina-supported samples, respectively. Note that the reduction temperature for each support was already optimized in a previous study [39].Once the DFM was obtained, CO2 adsorption and hydrogenation experiments were carried out, increasing the reaction temperature progressively from 280 to 520 °C, in steps of 40 °C. During the adsorption period (60 s), the feed composition was 10% CO2/Ar. Then, this step was followed by a purge with Ar (120 s) to remove weakly adsorbed CO2 and prevent mixing of streams. Finally, CO2 was replaced by a 10% of H2 during the hydrogenation (methanation) period (120 s). Before starting the following CO2 adsorption period, the catalyst and the system were again purged with Ar for 60 s. The catalytic tests were carried out with a total flow rate of 1200 mL min−1. This flow corresponds to space velocities of around 45,000 and 140,000 h−1 for ceria- and alumina-supported samples, respectively. CO2, CH4, CO and H2O were continuously quantified by a MKS MultiGas 2030 FT-IR analyser.The amount of CO2 stored was calculated from Eq. (2). With that aim, the amount that leaves the reactor must be subtracted from the amount fed. To determine the amount of CO2 fed, the stream from the feed system was led directly to the analyser. The obtained profile corresponds to the actual CO2 input that was fed to the reactor. (2) S T O CO 2 μmol g - 1 = 1 W ∫ 0 t F CO 2 in t - F CO 2 out t d t On the other hand, the CH4, CO and H2O productions were calculated from the following expressions: (3) Y CH 4 μmol g - 1 = 1 W ∫ 0 t F CH 4 out t d t (4) Y CO μmol g - 1 = 1 W ∫ 0 t F CO out t d t (5) Y H 2 O μmol g - 1 = 1 W ∫ 0 t F H 2 O out t d t CH4 selectivity is determined by relating the CH4 and CO productions since they were the only two products that were detected: (6) S CH 4 % = Y CH 4 Y CH 4 + Y CO × 100 Finally, the carbon balance check was carried out from the following expression: (7) C B % = Y CH 4 + Y CO S T O _ C O 2 × 100 Fig. 1 includes XRD patterns of 30% LaNiO3/CeO2, 30% LaNiO3/Al2O3 and 30% LaNiO3/La-Al2O3 samples before (a) and after (b) CO2 methanation reaction. The corresponding XRD diffractograms of ceria (CeO2) and alumina (Al2O3) supports as well as of bulk perovskite (LNO) are also included as reference.Regarding to fresh samples (Fig. 1a), intense diffraction peaks (○) at 28.6, 33.1, 47.5, 56.3 and 59.1 °2θ are observed for ceria support, whereas wide peaks (+) at 7.6, 45.9 and 67.0 °2θ are identified for alumina support. These reflections are characteristic of a cubic highly crystalline ceria and an amorphous cubic alumina phases, respectively. On the other hand, the bulk perovskite (LNO) shows three mains diffraction peaks (Δ) at 32.9, 47.4 and 58.7 °2θ, which are characteristic of a rhombohedral LaNiO3 phase. Furthermore, this sample also shows additional peaks in form of impurities, characteristic of hexagonal La2O2CO3 (●), cubic NiO (□) and tetragonal La2NiO4 (▲) phases, respectively. Their presence suggests that a fraction of Ni2+ and La3+ is not inserted inside the perovskite structure during perovskite structure conformation, due to a limited stability of LaNiO3 oxide.Supported samples (30% LaNiO3/CeO2, 30% LaNiO3/Al2O3 and 30% LaNiO3/La-Al2O3) show an intermediate diffractogram to that described for the bulk perovskite and the corresponding support. However, it is worth mentioning that the peaks of the ceria support overlap those of the perovskite in the case of the 30% LaNiO3/CeO2 sample. In any case, the small displacement of the peak situated at 33.1 °2θ to lower 2θ positions with respect to bare ceria support suggests the coexistence of both phases in the supported samples (Figure S1). Finally, it can be noticed that the relative intensity of the characteristic diffraction peaks of La2O2CO3 (●), NiO (□), and La2NiO4 (▲) impurities, increases for Al2O3- and La-Al2O3-supported samples with respect to bulk perovskite and CeO2-supported samples. These results suggest that the LaNiO3 conformation is partially limited over alumina-supported samples, especially for bare Al2O3 support.Once LaNiO3 perovskite is conformed, Ni should be ex-solved from the perovskite host with controlled size to conform the desired DFM. In order to confirm the Ni nanoparticles (NPs) ex-solution, XRD measurements were carried out for the samples used in cyclic CO2 adsorption and in-situ hydrogenation (Fig. 1b). Note that these samples were in-situ reduced prior to the catalytic test at the temperatures specified in Section 2.3. As can be observed, all samples show intense diffraction peaks of corresponding supports (CeO2 or Al2O3), which confirm their high stability. In contrast, no diffraction peaks are discernible for LaNiO3, NiO and La2NiO4 phases. These results confirm the complete reduction of NiO, LaNiO3 and La2NiO4 phases, leading to cubic Ni0 ( ) and La2O3 formation. However, an increase in the intensity of La2O2CO3 diffraction peaks is observed, instead of La2O3 phase identification. In agreement with that reported in previous works [40,41], this fact is due to CO2 adsorption on La2O3 sites during CO2 methanation. Hence, XRD results demonstrate the controlled ex-solution of Ni0 nanoparticles from the LaNiO3 during the controlled reduction process. Ultimately, DFMs are obtained with the following general formulation: Ni-La2O3/support (with support = Al2O3, La-Al2O3 or CeO2).The Ni0 crystallite sizes are determined by applying the Scherrer equation to the peak located at 51.8 °2θ in the used samples (Table 1 ). As can be observed, the crystallite size of the 30% LNO/CeO2 sample is 7.0 nm, whereas it increases to 12.4 and 11.5 nm for 30% LNO/Al2O3 and 30% LNO/La-Al2O3 samples, respectively. In agreement with XRD results (Fig. 1a), the ceria support favours the formation of a higher proportion of LaNiO3 perovskite, instead of impurities; as a consequence, this fact increases the Ni3+ available to be ex-solved, in form of smaller Ni0 NPs, during the reducing step. In any case, these values are significantly lower than that observed for the bulk perovskite (31.7 nm). Thus, supporting the LaNiO3 perovskite over different nature supports seems to be an efficient way to promote the ex-solution of Ni NPs with smaller crystallite size.Different phase’s distribution of 30% LaNiO3/CeO2, 30% LaNiO3/Al2O3 and 30% LaNiO3/La-Al2O3 used samples were analyzed by STEM-HAADF images and representative EDS elemental maps (Fig. 2 ). Furthermore, their Ni particle sizes distribution, estimated by measuring the size of at least 100 particles identified in these images, is also included in the form of a histogram. As a general trend, Ce (dark blue colour) or Al (light blue colour) and La (green colour) elements coexist with homogeneous distribution in all analyzed areas. Furthermore, small-sized Ni NPs (red colour) uniformly distributed on La and Ce or Al surface can be identified irrespective the analyzed support. Nevertheless, the Ni particle’s agglomeration is higher for the alumina-supported samples (Fig. 2b and c), which leads to a more heterogeneous Ni particle size distribution in the right side histogram. In agreement with the Ni size estimated by XRD experiment (Table 1), the lowest average size (5.0 nm) corresponds to 30% LaNiO3/CeO2 sample (Fig. 2a), whereas both alumina-supported samples show a Ni average size above 10 nm. These observations evidenced that the utilization of ceria as support favours the formation of a DFM with smaller Ni NPs. As previously suggested by XRD diffractograms (Fig. 1), the LaNiO3 perovskite formation is favoured with respect to the formation of impurities (i.e. La2O2CO3, NiO and La2NiO4), favouring the ex-solution of Ni0 NPs with smaller size from the perovskite host. Furthermore, the ceria-supported sample requires lower reduction temperature (550 °C vs. 800 °C) to completely ex-solve the Ni0 NPs from the different Ni-based phases, which limits their sintering during reduction step. Finally, these aspects seem to favour a more homogenous distribution of the La-based phases for ceria-supported samples.The analysis of the main textural properties of the used samples was carried out by isothermal (–196 °C) N2-adsorption–desorption. As expected, all perovskite-based formulations show type IV isotherms (Figure S2) according to the IUPAC classification, which are characteristic of mesoporous materials. Table 1 summarizes the corresponding specific surface areas (S BET) and pore volumes (V p) for the used and fresh samples (in brackets). 30% LaNiO3/CeO2, 30% LaNiO3/Al2O3 and 30% LaNiO3/La-Al2O3 samples present S BET of 37, 103 and 105 m2 g−1, respectively. This trend is ascribed to the progressive overlap of the pores and the support́s surface by the deposition of the LaNiO3 perovskite, together with pores narrowing due to calcination as well as reduction at high temperatures, in line with the proportional decrease in pore volume observed. In any case, these values are much higher than that of the bulk perovskite (12 m2 g−1), which contributes to the ex-solution of smaller Ni NPs.In order to investigate the redox properties of the samples, Fig. 3 shows the hydrogen consumption profiles, normalized per sample mass unit, for fresh 30% LaNiO3/CeO2, 30% LaNiO3/Al2O3 and 30% LaNiO3/La-Al2O3 samples. For comparative purposes, the H2–TPR profiles of bulk LaNiO3 perovskite (LNO) and CeO2 support are also included.In agreement with the results in our previous work [39], the ceria-supported perovskite shows an intermediate reduction profile between the CeO2 support and bulk LaNiO3 perovskite. Specifically, two main H2 consumption regions can be identified, i.e. below and above 650 °C. As observed for the bulk LaNiO3, the low temperature region presents three main peaks centred at 225, 375 and 475 °C, which are ascribed to the progressive reduction of NiO, LaNiO3 and La2NiO4 phases following the stoichiometry of Eqs. (8–10). Note that the Ce4+ at the surface is also reduced in this temperature region. Meanwhile, the peak above 650 °C is ascribed to the final reduction of the bulk CeO2 (Eq. (11)) [42]. (8) 4LaNiO 3 + 2H 2 → La 4 Ni 3 O 10 + Ni 0 + 2H 2 O (9) La 4 Ni 3 O 10 + 3H 2 → La 2 NiO 4 + 2Ni 0 + La 2 O 3 + 2H 2 O (10) La 2 NiO 4 + H 2 → Ni 0 + La 2 O 3 + H 2 O (11) 2CeO 2 + H 2 → Ce 2 O 3 + H 2 O The reduction profiles of alumina-supported samples (30% LaNiO3/Al2O3 and 30% LaNiO3/La-Al2O3) differ from that observed for ceria-supported sample. On the one hand, the reduction of NiO, LaNiO3 and La2NiO4 phases (Eqs. 8–10) takes place more progressively. On the other hand, a new contribution, centred around 750–800 °C, is observed. Taking into account that Al2O3 and La-Al2O3 support are no reducible, this peak is associated with the reduction of highly stable NiAl2O4 phase formed during calcination step [43]. These results denote a high interaction between the Ni and alumina support, which limits the conformation of the perovskite oxide and favours the presence of impurities, as previously suggested by XRD analysis (Fig. 1).As can be observed, the reducibility is clearly enhanced for ceria-supported samples with respect to those observed for the bulk perovskite and the ceria support. In contrast, this shift is limited for the alumina-supported samples, especially for the perovskite deposited onto bare alumina. However, it is worth to mention that species below 250 °C are more easily reduced for alumina-supported samples than for ceria-supported and bulk perovskites. As previously suggested, the reduction of NiO, not inserted in the perovskite lattice, also occurs at this temperature region and is favoured by the higher specific surface area of alumina-supported samples with respect to the ceria-supported one. In any case, the concentration of the species reduced in the low temperature region (below 650 °C) is significantly higher for the ceria-supported samples. These results evidence that the redox properties of the samples are favoured with ceria as support, which leads to an easier reduction of Ni-based species as well as of ceria support [44]. Thus, synergetic effects between LaNiO3 and ceria phases are evidenced, which promote the accessibility of the former and the reducibility of the latter due to spill-over effect of activated H2 [32].To gain insight on the hydrogen consumption occurred during H2-TPR experiments, the effluent gas was analyzed by mass spectroscopy for ceria- and alumina-supported samples (Figure S3). As can be observed, a noticeable methane formation can be identified in both cases due to the hydrogenation (CO2 + 4H2 ⇄ CH4 + H2O) of the CO2 released due to La2O2CO3 decomposition on Ni0 sites, specie formed during the perovskite reduction at lower temperatures. As a result, this process implies the consumption of additional H2. Thus, the H2 consumption observed between 250 and 600 °C is not only due to the reduction of reducible species but also due to the methane formation of the adsorbed CO2 at the surface. Among different supports, the 30% LNO/CeO2 sample shows the highest CH4 production below 400 °C. This trend suggests that the activation of CO2 methanation takes place at lower temperatures for the DFMs obtained from the ceria-supported sample, in line with the higher reducibility observed during H2-TPR experiments (Fig. 3). Table 2 shows the integrated area related to the reduction of the different species per gram of sample. Based on the reduction steps described in Eqs. (8–10), 1.5 mol of hydrogen are consumed per 1 mol of LaNiO3 perovskite, whereas 0.5 mol of hydrogen are consumed in the reduction of CeO2 support (Eq. (11)). In contrast, no hydrogen consumption is expected due to the La-Al2O3 or Al2O3 supports reduction. Moreover, noticeable hydrogen consumption is related to carbonates reduction, in line with the high CH4 production identified in Figure S3. In fact, this contribution should be more relevant for ceria-supported samples. As a result, the overall H2 uptake decreases from 7507 µmol H2 g−1 for ceria-supported samples to values below 4745 µmol H2 g−1 for alumina-supported samples.It is worth to mention that the increase in H2 consumption for the 30% LNO/CeO2 sample is especially remarkable below 400 °C (Table 2). In order to explore in more detail this aspect, Fig. 4 plots the evolution of the H2 uptakes below 250 °C related to Ni content for different supported samples and bulk perovskite. Note that this hydrogen consumption was previously assigned to the partial reduction of Ni3+ in the perovskite lattice together with the reduction of highly dispersed NiO nanoparticles, since no CH4 formation is observed in this temperature region (Figure S3). As can be observed, the H2/Ni ratio progressively decreases from 0.82 for 30% LNO/CeO2 sample to 0.23 for 30% LNO/La-Al2O3 sample. This result confirms that the H2 activation during CO2 methanation is promoted at lower temperatures by the use of ceria support due to the increase in the concentration of highly reducible Ni-based species in LaNiO3 perovskite.To investigate the interaction between the CO2 molecule and the catalyst surface, the CO2–TPD profiles of the LaNiO3, 30% LaNiO3/CeO2 30% LaNiO3/Al2O3 and 30% LaNiO3/La-Al2O3 samples, as well as those of the corresponding supports (CeO2, Al2O3 and La-Al2O3) were compared (Fig. 5 ). Each sample was prereduced following a similar procedure to that achieved prior to catalyst test (Section 2.3). On the one hand, ceria support shows a single desorption peak centred at 100 °C, which is assigned to the CO2 decomposition arising from bridged and bidentate carbonates adsorbed onto ceria surface. Meanwhile, the weak signal, observed above 400 °C, is assigned to the decomposition of carbonates not eliminated during calcination and reduction steps [45]. On the other hand, bare Al2O3 and La-Al2O3 supports present an asymmetric desorption peak at 100 °C, which is assigned to CO2 desorption from weak Brönsted OH– groups [43]. It is worth to note that the shoulder at higher temperatures is slightly higher for La-Al2O3 support, due to the CO2 desorption from monodentate carbonates adsorbed on highly dispersed La2O3 formed on catalytic surface.Regarding bulk perovskite, three main desorption peaks can be observed: below 200 °C, between 200 and 550 °C and above 550 °C. In increasing order of temperature, these peaks are assigned to the decomposition of weakly adsorbed CO2 on Ni0 sites [46], and decomposition of monodentate carbonates linked to highly dispersed and bulk-like La2O3 species in the form of La2O2CO3 [27,44], respectively. As expected, supported perovskites show an intermediate CO2 desorption profile to that observed for the bulk LaNiO3 perovskite and the corresponding support. Nevertheless, two main differences can be identified. On the one hand, the desorption of the different adsorbed species takes place at lower temperatures with respect to bulk perovskite. On the other hand, a more progressive CO2 decomposition during the whole temperature range is favoured for ceria-supported sample. As previously discussed, the impregnation of the LaNiO3 perovskite on a high surface area supports limits its agglomeration during calcination. This fact promotes a more homogeneous distribution of the La2O3 phases at the surface, which favours the formation of monodentate carbonates of different stability on the La2O3 sites. However, this process is partially limited for alumina-supported samples due to the formation of NiAl2O4 in detriment of LaNiO3 perovskite conformation, in line with XRD results (Fig. 1). This fact limits the ex-solution of La2O3 from the perovskite, favouring its sintering during the reduction at high temperatures, which ultimately leads to more heterogeneous distribution of the La2O3 phase than in ceria-supported samples (Fig. 2).According to desorption temperature or chemical bond strength, basic sites can be classified into weak (T < 150 °C), medium (T = 200–550 °C) and strong (T > 550 °C). Based on this distribution, the concentration of the different species was determined after the deconvolution and integration of the different peaks identified in the desorption profile (Table 3 ). Comparing supported samples, the lower concentration of the weak basic sites corresponds to the reduced 30% LaNiO3/CeO2 sample. As observed in Table 1, this fact is ascribed to its lower specific surface area, which leads to almost total coverage of the ceria surface. In contrast, this sample shows significantly higher medium and strong basic sites concentration than alumina-supported samples (30% LaNiO3/Al2O3 and 30% LaNiO3/La-Al2O3). Specifically, the concentration of medium and strong basic sites ranges from 120.6 and 87.1 µmol CO2 g−1 for the 30% LNO/CeO2 sample to 29.0 and 58.5 µmol CO2 g−1 for the 30% LNO/Al2O3 sample, respectively. As suggested in our previous work [39], the controlled reduction of 30% LaNiO3/CeO2 sample leads to the formation of additional medium strength basic sites (NiO-CeO2 interface) for CO2 adsorption with respect to alumina-supported samples and bulk perovskite. Furthermore, the high interaction of Ni with the support leads to the presence of NiAl2O4 phase (Fig. 3), which limits the LaNiO3 perovskite conformation, increases the proportion of impurities (La2O3 and NiO) for fresh samples (Fig. 1a), and needs a higher reduction temperature (800 °C vs. 550 °C). Ultimately, these aspects limit the ex-solution of highly dispersed La2O3 basic sites from LaNiO3 host, leading to a significant decrease of the concentration of medium strength basic sites.In order to gain insight on this issue, Table 3 shows the ratio of desorbed moles of CO2 per mol of La2O3. Note that for the accurate determination of the amount desorbed from the La2O3 adsorbent, the moles of CO2 desorbed from the corresponding support were subtracted. According to decomposition reaction of lanthanum carbonate (La2O2CO3 ⇄ La2O3 + CO2) 1 mol of CO2 should be desorbed per mol of La2O3, if this compound was completely carbonated during saturation step. However, this ratio is below 1 for all samples. Among them, the lowest value (0.08) corresponds to LaNiO3 perovskite, whereas supported perovskites show values more than twice of that of bulk counterpart. Although ceria supported sample show significantly lower specific surface area (Table 1), it presents the highest value (0.28). This trend confirms the higher accessibility of La2O3 sites and the presence of additional CO2 adsorption sites in the NiO-CeO2 interface (180–360 °C). Finally, this fact leads to a significant increase of the surface density of medium basicity species, from 0.39 μmol CO2 m−2 for alumina-supported samples to 3.26 μmol CO2 m−2 for ceria-supported one. Thus, these results confirm that supporting LaNiO3 perovskite over ceria support increases the accessibility and the concentration of CO2 adsorption sites with respect to alumina-supported samples.Aiming to introduce the basic principles of the operation, Fig. 6 displays the evolution with time of the outlet concentration of CO2, CH4, CO and H2O during an entire CO2 adsorption and hydrogenation cycle at 400 °C. Although these results correspond to the DFM derived from 30% LaNiO3/CeO2 formulation, the global reaction evolution is similar for alumina-supported samples and bulk perovskite.During the adsorption cycle (1 min) a gas stream composed of 1.4% CO2/Ar is fed. To estimate the amount of CO2 adsorbed on the catalyst, the CO2 concentration profile when the reactor is bypassed is also included. As can be observed, CO2 concentration is almost negligible at the beginning of the adsorption period; in fact, no CO2 signal at the reactor outlet is detected during the first 35 s. Following, it increases rapidly achieving almost the inlet concentration at the end of the storage period. This trend reveals the progressive CO2 adsorption on storage sites, mainly La2O3 phase [47] and, in minor extent, on NiO–CeO2 interface, up to their total saturation through the following reaction: (12) La 2 O 3 + CO 2 ⇄ La 2 O 2 CO 3 Few seconds delayed, an increasing H2O signal is detected at the reactor outlet. The identification of this compound during the adsorption period reveals that CO2 is progressively displacing pre-adsorbed H2O due to its competitive adsorption on La2O3 storage sites through Eq. (13). However, it can be concluded that the CO2 adsorption preferentially occurs onto free La2O3 sites, since H2O is detected quite delayed with respect to CO2 identification. Once La2O3 adsorption sites are completely carbonated (Eq. (12)), the storage of CO2 is transferred to La(OH)3 sites (Eq. (13)). (13) 2La OH 3 + CO 2 ⇄ La 2 O 2 CO 3 + 3H 2 O Note that the desorption of a small fraction of H2O stored on ceria or alumina supports in form of hydroxyls cannot be ruled out, which can conform bicarbonates during CO2 adsorption period. However, it is well-known that their stability is limited at working temperatures, which makes this adsorption route minority with respect to that expressed by Eqs. (12–13), especially for the ceria-supported sample [38].From these data, the amount of CO2 adsorbed onto the catalyst is calculated by Eq. (2) (Table 4 ). In order to assess the stable behavior of the DFM, the corresponding values to 3 consecutive cycles is included, which results in values between 86.4 and 90.7 µmol CO2 g−1. Furthermore, an almost negligible CO peak is observed during the adsorption period, which value is determined by Eq. (4) and summarized in Table 4 (around 9 µmol g−1), which is related to the incomplete hydrogenation of adsorbed CO2 with H2 chemisorbed on the Ni0 sites during the previous hydrogenation period following the reverse water gas shift reaction (RWGS, Eq. (14)). Alternatively, other authors related CO formation to the progressive decomposition of adsorbed formate species [48]. (14) H 2 + CO 2 ⇄ CO + H 2 O Once the adsorption period is completed, the CO2 is removed from the feed stream and a constant Ar flow rate is fed during 2 min, in order to purge the catalyst as well as the reaction system. As a result, CO2 and H2O signals progressively decrease practically to zero during this period.Then, the hydrogenation period (2 min) begins with the admission of a gas stream composed of 10% H2/Ar. Immediately after the injection of 10% H2/Ar mixture, a sudden CH4 production is observed with a long tail extended during the rest of the period. Besides, H2O formation is detected around 10 s delayed from CH4 detection. This process can be described by the following reaction scheme: (15) Step 1 : La 2 O 2 CO 3 ⇄ La 2 O 3 + CO 2 (1) Step 2 : 4H 2 + CO 2 ⇄ CH 4 + 2H 2 O (16) Step 3 a : La 2 O 3 + 3H 2 O ⇄ 2La OH 3 (17) Step 3 b : Ce 2 O 3 + 3H 2 O ⇄ 2Ce OH 3 Firstly, lanthanum oxide carbonate is decomposed to form gaseous CO2 (Eq. (15)). Then, the CO2 released reacts with hydrogen to form methane and water following Sabatier reaction (Eq. (1)). Taking into account the stoichiometry of Eq. (1), 2 mol of H2O should be detected per mol of CH4; nevertheless, the experimental ratio during hydrogenation period ranges between 1.38 and 1.40, which reveals that part of H2O is stored on the surface La2O3 sites (Eq. (16)) or ceria support (Eq. (17)). As we already reported in previous work for conventional Ni/CeO2 catalysts [49], the water adsorption on ceria sites is limited due to its high oxygen mobility, which favours water desorption during the hydrogenation period. Indeed, the H2O/CH4 ratio is significantly higher than that observed for conventional Ru-CaO/Al2O3 and Ru-Na2CO3/Al2O3 DFMs (H2O/CH4 < 1.14) [13]. Finally, a small fraction of CO (around 1 µmol g−1) is also detected during the hydrogenation period due to RWGS reaction (Eq. (14)).If the entire CO2 adsorption and hydrogenation cycle is considered, a H2O/CH4 ratio ranging between 2.00 and 2.03 is obtained, that is close to the stoichiometry value (H2O/CH4 = 2) defined by Sabatier reaction (Eq. (1)). With the aim of giving more reliability to the results obtained, the carbon balance was also determined (Eq. (7)). As can be observed in Table 1, the amount of CO2 stored during the adsorption period is around 88 µmol g−1, whereas around 80 µmol g−1 of CH4 and 8 µmol g−1 of CO are produced during CO2 hydrogenation period. Thus, carbon balance closed within ± 5% since CH4 and CO are the only products detected by FTIR during the reaction.Catalytic activities of LaNiO3-derived DFMs are evaluated by analyzing the evolution of CH4 and CO production per cycle with reaction temperature (Fig. 7 ). These parameters were estimated applying Eqs. (2–3) for the data obtained from similar CO2 adsorption and hydrogenation experiments to that reported in Fig. 6. Aiming to mimic an effluent gas from a combustion process, the CO2 concentration during storage period was increased from 1.4 to 10% in these experiments, in which the carbon balance closed with an error below 5%.As can be observed in Fig. 7a, the evolution of CH4 production with reaction temperature is influenced by the type of perovskite-based formulation used as precursor of the corresponding DFM in each experiment. As expected, methane production increases up to 440 °C for DMFs obtained after the reduction of bulk LaNiO3 and 30% LaNiO3/CeO2 formulations. Above this temperature, CO2 conversion slightly decreases due to a destabilization of adsorbed carbonates. In contrast, DFMs obtained from alumina-supported perovskites (30% LaNiO3/Al2O3 and 30% LaNiO3/La-Al2O3) achieve their maximum CH4 production at 280 °C. Then, a progressive decrease in the amount of CH4 produced is observed at increasing temperatures. As previously observed, weak strength basic sites are predominant for alumina-supported DFMs (Table 3). Thus, the maximum CH4 production observed at 280 °C is related to a more efficient CO2 adsorption on weak strength basic sites, main adsorption sites at this temperature range. On the contrary, as the reaction temperature increases, the adsorbed CO2 on weak basic sites become less stable, limiting their hydrogenation for alumina-supported samples. Meanwhile, the presence of higher strength basic sites for non– and ceria-supported samples favours decomposition of a major quantity of the adsorbed CO2 species to be hydrogenated to CH4 at higher temperatures. Thus, Ni-La2O3 interface higher accessibility can be considered as a key parameter to maximize CO2 adsorption and in-situ hydrogenation at this temperature range.Regarding to CO formation (Fig. 7b), all samples show an increasing CO production with reaction temperature. This trend is ascribed to the promotion of the RWGS reaction (Eq. (17)) during the CO2 hydrogenation step. In any case, the CO production is below 31 µmol g−1 for all samples, which remarks the high selectivity towards methane of here developed materials.Among different samples, the DFM obtained after the reduction of the LaNiO3 formulation exhibits the highest CH4 production (117 µmol g−1), in line with the higher density of medium basic sites identified in Table 3. However, the DFM derived from 30% LaNiO3/CeO2 precursor maintains the highest CH4 production, if the whole temperature range is considered. Furthermore, this sample shows a CO production 3 times lower (8 vs. 31 µmol g−1) than the DFM obtained from bulk perovskite, which is ascribed to the higher strength of CO2 adsorbed species [50,51].To better understand the differences in the catalytic behaviour, Fig. 8 plots CH4 concentration profiles during complete CO2 adsorption and hydrogenation cycles at 280, 400 and 520 °C for DFMs obtained from 30% LaNiO3/CeO2, 30% LaNiO3/La-Al2O3 and LaNiO3 precursors. Profiles corresponding to the 30% LaNiO3/Al2O3 precursor have not been included since they are similar to those of La-Al2O3-supported sample. In general, the evolution of CH4 is significantly affected by DFM composition, especially at intermediates-high temperatures. The maximum CH4 production is observed at initial times for the alumina-supported sample, whereas this process is delayed and takes place more progressively for the DFM obtained from bulk perovskite. On the other hand, the ceria-supported sample shows an intermediate CH4 production profile. As previously observed in Table 1, the specific surface area was significantly higher for supported samples with respect to that observed for the DFM derived from bulk perovskite, which leads to the exsolution of Ni NPs with significantly lower average particle size than bulk counterpart (31.7 nm), especially for ceria-supported sample. Furthermore, this sample shows the higher proportion of medium basic sites with respect to strong basic sites. Taking into account that the close contact between storage component and the Ni0 NPs is regarded as the key factor to efficiently transfer of dissociated H to desorb, and subsequently to hydrogenate, adsorbed CO2, these facts explain the wider temperature window of the DFM derived from the 30% LaNiO3/CeO2 formulation. In contrast, the stability of adsorbed species is limited for alumina-supported sample, favouring only the CH4 production at the beginning of the hydrogenation period and low temperatures.To sum up, the 30% LaNiO3/CeO2 emerges as the optimal catalytic precursor, resulting in a dual function material with high efficiency to adsorb CO2 and in-situ hydrogenate it to methane. This fact is ascribed to an proper balance of different basic sites concentration, where CO2 adsorption takes places (Ni-CeO2 interface as well as highly dispersed and bulk-like La2O3), and higher accessibility of active sites for H2 activation and CO2 methanation (Ni0 NPs). Ultimately, this fact also favours a higher selectivity towards methane. Furthermore, this sample is able to produce a high fraction of methane at initial period of the hydrogenation cycle. As suggested in our previous works [52,53], the duration of the hydrogenation period should be enough to ensure high CH4 production but not too long to limit hydrogen conversion. Hence, optimal hydrogenation time will provide a best balance between more efficient use of reductant agent and CH4 production. As a result, the faster kinetics discovered with the DFM derived from 30% LaNiO3/CeO2 catalytic precursor, makes it a first-class alternative as promotes the joint optimization of H2 conversion and CH4 production. The last aspect to consider is that the Ni content is around 70% lower with respect to bulk LaNiO3, which reveals a superior intrinsic activity of this sample.In order to have a more realistic view of the relevance of the reported results, the CH4 and CO productions obtained with this DFM where compared to those obtained with 15% Ni-15% CaO/Al2O3 model DFM (Figure S4) [13], showing comparable CO2 adsorption and in-situ hydrogenation to CH4. These results remark that here developed DFMs can be considered as promising novel materials for CO2 methanation technology. The still limitation of higher CH4 production at higher temperatures is actually under study in our labs with the use of other alkaline or earth-alkaline adsorbents, such as Ca, Ba, Na and K.The real-world applicability of the DFM obtained after the controlled reduction of 30% LaNiO3/CeO2 precursor was more deeply analyzed by subjecting this DFM to long-term CO2 adsorption/hydrogenation experiments under hard operational conditions. This study was completed by evaluating the influence of the presence of O2 during adsorption period on its CO2 adsorption and hydrogenation efficiency. Fig. 9 shows the evolution of CH4 and CO productions with the number of CO2 adsorption/hydrogenation cycles for 30% LaNiO3/CeO2-derived sample at 520 °C. As can be observed, CH4 and CO productions as well as selectivity towards methane remain almost stable irrespective of time elapsed. Specifically, the CH4 production slightly decreases from 80 µmol g−1 to 78.4 µmol g−1, whereas CO production keeps at 9.3–9.4 µmol g−1. This catalytic behaviour reveals the high stability of the developed DFM towards CO2 adsorption and hydrogenation in consecutive cycles. The close contact between Ni0 NPs, La2O3 and CeO2 phases (Fig. 2), formed after the controlled reduction of 30% LaNiO3/CeO2, prevents the thermal agglomeration of Ni NPs during calcination and CO2 methanation reaction processes, in line with the observed by Wang et al. [35] for Ni-La2O3/SBA-15 catalyst.The developed DFM should also selectively capture CO2 from O2-containing flue gas at relatively high temperatures and then, hydrogenate the adsorbed species to methane with H2. With the aim of evaluating the influence of the presence of O2 during storage period a 10% of O2 is jointly fed with a 10% of CO2 during adsorption cycles. Fig. 10 plots the evolution of CH4 and CO productions with the number of CO2 adsorption/hydrogenation cycle at 400 °C for 30% LaNiO3/CeO2-derived sample. Note that the cycles 1–2 and 8–9 were carried out in the absence of O2 in the feed stream, whereas this compound was fed in cycles 3–7.Comparing cycles 1–2 with cycles 3–7, a negative effect on CH4 production can be detected for the experiments in the presence of O2 during the adsorption period. Indeed, the CH4 yield immediately decreases from 99 to 47 μmol g−1 from the 2nd to 3rd cycle, whereas no significant changes are observed for cycles 4–7. Zheng et al. [54] justified the loss of activity for O2-containing experiments by the oxidation of the active metallic phase during the adsorption step. In agreement with their results, a small CO2 signal and a significant decrease in methane production is observed at the beginning of the hydrogenation period for the oxygen-containing experiment with respect to oxygen-free experiment (Figure S5). This fact reveals that some carbonates, adsorbed during the storage period, are released without being hydrogenated due to the absence of enough Ni0 active sites to reduce them towards CH4. In any case, the decrease in methane production for O2-containing experiments is significantly lower to that observed for 10% Ni-6.1% NaO/Al2O3, where no methane formation was observed when the sample was exposed to O2 and H2O during the CO2 capture step [19]. On the other hand, it is worth to mention that, in contrast to that observed in our previous work for conventional 10% Ni–10% Na2CO3/Al2O3 [11], CO production remains invariable after inclusion of O2 in the feed stream. This trend discards the promotion of RWGS reaction (Eq. (14)) due to a partial oxidation of Ni0 to NiO.During the next cycles (i.e. from the 8th to the 9th), CO2 adsorption/hydrogenation cycles were again carried in an oxygen-free environment. Remarkably, the CH4 production is recovered immediately after the oxygen is removed from the feed stream. Note that the 8th and 9th cycles show similar CH4 and CO productions than 1st and 2nd cycles. Thus, these results reveal that the here discovered DFM has a high ability to restore activity once O2 is not fed during CO2 adsorption, which it is one of the main limitations of the conventional Ni-based formulations [19,55,56]. In agreement with the H2-TPR results, the high reducibility of different Ni species implies that Ni can be easily reduced back during the hydrogenation step at low temperature. Therefore, 30% LaNiO3/CeO2-derived DFM can be considered a superior candidate for real conditions process at intermediate-high temperatures.In summary, the confinement of Ni NPs on La2O3 or La-Ce-O interfaces prevents them from thermal agglomeration and favours their redox properties. Hence, ceria-supported LaNiO3 perovskites can be considered as an efficient precursor of highly stable and versatile dual function materials for the cyclic CO2 adsorption and hydrogenation to methane technology.This work is focused on the analysis of the viability of LaNiO3-based formulations as precursor of active, stable and versatile dual function materials for CO2 adsorption and hydrogenation into CH4. In particular, the following perovskite-based formulations are prepared by combining citric acid and wetness impregnation: LaNiO3, 30% LaNiO3/CeO2, 30% LaNiO3/Al2O3 and 30% LaNiO3/La-Al2O3. The prepared formulations are widely characterized before and after controlled reduction process. XRD experiments reveal the presence of LaNiO3 as well as of impurities in form of La2O2CO3, NiO and La2NiO4 for all calcined samples. The concentration of these impurities increases for 30% LaNiO3/Al2O3 and 30% LaNiO3/La-Al2O3 samples with respect to LaNiO3, 30% LaNiO3/CeO2 ones, whereas the specific surface area follows the opposite trend. Among different samples that supported on ceria oxide (30% LaNiO3/CeO2) shows the higher reducibility in H2-TPR experiments, whereas the redox properties are limited for 30% LaNiO3/Al2O3 and 30% LaNiO3/La-Al2O3 due to the higher perovskite-support interaction, which favours the NiAl2O4 formation instead of LaNiO3 conformation.After the controlled reducing process, Ni is exsolved from perovskite host in developed formulations, leading to the formation of a mix between Ni0 nanoparticles and La2O3 phase. However, the interaction of Ni nanoparticles with the La2O3 phase and support is controlled by the characteristic of the precursor used. The best compromise between specific surface area and LaNiO3 stability observed for ceria-supported sample (30% LaNiO3/CeO2) promotes the formation of smaller Ni0 nanoparticles and a more homogeneous La2O3 distribution. As a result, the obtained DFM presents a stronger interaction between Ni0 NPs and the La2O3 and CeO2 phases and a more modulated basicity. Finally, this fact favours the transfer of dissociated H to hydrogenate near-adsorbed CO2 at the studied operating temperature. As a result, the material obtained after the reduction of the 30% LaNiO3/CeO2 formulation exhibits the highest CH4 production, if the whole temperature range is considered. Specifically, its maximum CH4 production per cycle is 104 µmol g−1 (440 °C) and its selectivity towards CH4 formation is above 90% in the whole temperature range. Furthermore, this DFM also emerges as promising approach to promote the joint optimization of H2 conversion and CH4 production, since its present quite fast kinetics during CO2 hydrogenation to methane.The 30% LaNiO3/CeO2-derived DFM also shows promising properties for the real-world applicability. On the one hand, it demonstrates high stability during long-terms experiments under hard reactions conditions, even above than other conventional Ni-based DFMs. On the other hand, although, the presence of O2 during the CO2 capture step has a detrimental effect on CH4 production, the decrease is lower than that reported for other conventional Ni-based catalysts. Indeed, the activity recovery capacity is higher when the system comes back to an oxygen-free environment due to the enhanced redox properties of this novel DFM. Thus, ceria-supported LaNiO3 perovskites emerge as promising precursors of highly active, versatile and stable novel dual function materials for CO2 adsorption and hydrogenation to methane under wide variety of operational conditions. Jon A. Onrubia-Calvo: Conceptualization, Methodology, Validation, Writing – original draft. Alejandro Bermejo-López: Methodology, Investigation. Sonia Pérez-Vázquez: Investigation. Beñat Pereda-Ayo: Conceptualization, Methodology, Visualization, Writing – review & editing. José A. González-Marcos: Methodology, Data curation, Supervision, Funding acquisition. Juan R. González-Velasco: Conceptualization, Supervision, Funding acquisition, 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.Support for this study was provided by the Spanish Ministry of Science and Innovation (Project PID2019-105960RB-C21) and the Basque Government (Project IT1297-19). One of the authors (JAOC) acknowledges the Post-doctoral research grant (DOCREC20/49) provided by the University of the Basque Country (UPV/EHU).Supporting information includes detailed information on characterization of fresh and used samples, i.e. Enlargement of XRD results, N2 adsorption-desorption isotherms and CH4 and CO2 mass spectroscopy signals during H2-TPR experiments. A comparison of CH4 and CO productions with respect to conventional 10% Ni-15% CaO/Al2O3 during CO2 adsorption and hydrogenation cycles is also included as reference. Supplementary data to this article can be found online at https://doi.org/10.1016/j.fuel.2022.123842.The following are the Supplementary data to this article: Supplementary data 1

The valorisation of CO2 through its capture and in-situ hydrogenation to methane, using dual function materials (DFMs), emerges as promising alternative to reduce CO2 emissions to atmosphere and the global cost of current CO2 Capture and Utilization (CCU) technology. This work investigates the viability of LaNiO3-derived formulations as precursors of DFMs for CO2 capture and in-situ conversion to CH4. For this purpose, a set of DFMs obtained from 30% LaNiO3/CeO2, 30% LaNiO3/Al2O3, 30% LaNiO3/La-Al2O3 and LaNiO3 precursors were synthesized and systematically characterized before and after a controlled reduction process. Results of XRD analysis, STEM-EDX images, H2-TPR and CO2-TPD experiments reveal that the DFM obtained after reduction of 30% LaNiO3/CeO2 formulation shows the smallest Ni0 particle size (7 nm) and the highest medium-strong basic sites concentration. In fact, this DFM widens operation window with methane production ranging between 80 and 103 µmol g−1 and maintains a selectivity towards methane above 90% in the range of 280–520 °C. The best catalytic behaviour is related to a better contact between the different nature basic sites and the homogenously distributed Ni0 sites, which favours a fast spill-over of dissociated H to near CO2 adsorption sites. The applicability of this formulation is further evidenced by a highly stable CH4 production during long-term experiments and a promoted Ni0/NiO reversibility in the absence/presence of O2 during the CO2 adsorption period, which allows a fast and complete recovery of CH4 production in absence of O2. These aspects favour a versatile application of the 30% LaNiO3/CeO2-based DFM formulation to convert CO2 outlet streams from combustion flue gases of different nature.

Data will be made available on request.Lower Olefins (C2-C4) are indispensable intermediates in the chemical industry. [1] Approximately 20% of the yearly production revenue of the chemical industry is related to the olefin-based polymer industry. [2] Traditionally, olefins are produced via steam cracking of fossil feedstocks, like crude oil. [1] Due to the growing social and political awareness regarding sustainable production, non-fossil based production routes and the shift towards a circular economy gain great interest. [3,4] While part of the olefine based polymers can be recycled mechanically [5] a large portion still needs to be recycled by other means. [6] Direct chemical recycling to the olefin monomers is more or less impossible as selective breaking of the CC bond and dehydrogenation is difficult. [7] A possible way to close the carbon cycle is i) gasification or incineration of the polymer waste and ii) synthesis of ethylene and propylene through hydrogenation of the resulting CO/CO2 or solely CO2 streams. [8] While the incineration of plastic waste is commercialized [9] and recently the gasification of these waste is studied intensively [10,11], the synthesis of Olefines from the CO/CO2 streams remains the more critical challenge in this carbon cycle. Direct electrochemical reduction of CO/CO2 to ethylene is an attractive process but still from a technical point of view not mature enough. Methanol synthesis and subsequent Methanol-to-Olefin (MTO) conversion is a possible route, while for the MTO process no large-scale production plant with a capacity of >1 MTPA is existing. The Fischer-Tropsch-Synthsis (FTS) in contrary is commercialized on big scales and it is known that the product selectivity for CO as feedstock can be tuned towards high olefine yields and in a single process. [1,12] Fischer-Tropsch-to-Olefins (FTO) can thus be an important, scalable and readily available process to close the carbon cycle for polypropylene and polyethylene recycling. CO2 as feedstock, in combination with green hydrogen, would be especially interesting, as it cannot only stem from plastic waste incineration, but also other sustainable resources as well as from direct air capture. [3]As CO2 is not converted into Fischer-Tropsch-products directly, the reverse water-gas shift reaction (RWGS) to CO has to be carried out prior. The combination of this endothermic equilibrium reaction and the highly exothermic Fischer-Tropsch synthesis (FTS) can be realized either in two separate or in one combined reactor unit. Due to their opposite reaction enthalpies and process intensification aspects, the direct heat integration is one main advantage of coupling both reactions in one reactor. Additionally, as CO is converted to FTS-products in a series of irreversible consecutive reactions, a high CO2 conversion by RWGS can be obtained even at low temperatures. However, the coupling of RWGS and FTS demands a catalyst system exhibiting activity for both the shift-reaction and the FTS, which is met by using Fe as an active metal. [4]In this context, especially carbon supported iron catalysts are of increasingly growing interest. [13–15] Compared to the traditionally used oxidic support materials, such as SiO2 and Al2O3, iron species on carbon materials prove to be reducible more easily, since there is no strong metal support interaction (SMSI). This facilitates the conversion of Fe species into FT-active carbides, as the carbidisation of elemental iron is facilitated on carbon supports. [16,17] Despite the less pronounced interactions between support and active metal, carbon-supported iron systems, as demonstrated by the use of carbon nanotubes (CNT) as support, showed excellent resistance to sintering. [18,19]Regarding the utilization of carbon supported Fe in a CO2 based FTO process, Oschatz et al. reported sulfur and sodium as promising promotors for the production of olefins using ordered mesoporous carbon (CMK-3) as support route. [20–23] They found that the undesired methane formation is inhibited by sulfur doping, while the desired β-hydride elimination to short chained olefins is preferred at the same time. Addition of sodium supports chain growth and reduces selectivity to methane while inhibiting olefin hydration to paraffins. [24]Most of the existing studies dealing with Fe/C-catalysts in the FTS were carried out using research carbon materials like CNT's or CMK-3. Facing the challenges of an industrial FTS process, these materials reveal disadvantages for commercialization of derived catalysts like expensive and complicated synthesis, as well as handling on the industrial scale of these small sized powdered materials and the resulting severe pressure drop for technical fixed bed reactors. Also, so far, results are mainly presented for some hours time on stream and rarely long term stability studies for several hundred hours. Overall, both hinders an application of Fe/−catalysts in FTO strongly.The following study presents the use of beaded carbon blacks (CBs) as support for iron based CO2-FT catalyst. CB represents a material which is widely available in industrial scale at low price with additional formulation procedures to reduce pressure drop for fixed bed reactors e.g. through beading also being carried out on an industrial scale. Commercial beaded CBs varying in properties (e.g. specific surface areas from 36 m2·g−1 to 380 m2·g−1) were employed for catalyst synthesis. The catalyst performance in direct olefin synthesis for CO2 hydrogenation was assessed and for the most promising catalyst a long-term study proved stability for 170 h time on stream.Five commercially available beaded Carbon Blacks (Orion Engineered Carbons: Printex G, Printex 35, Printex 60, Printex 85, Printex 90) were used as supports in this study. The preparation strategy was modified according to Oschatz et al. [15] Carbon supported catalysts were prepared by incipient wetness impregnation, using a solution of ammonium‑iron-citrate (1.787 g; 16.5–18.5% Fe; Acros Organics), tri‑sodium-citrate dihydrate (0.038 g; 99+ %, Fisher Scientific), and iron sulfate heptahydrate (0.026 g; 99+ %; Acros Organics) in 6 mL bidistilled water. Concentrations were chosen so that a loading of 10 wt% Fe, 0.3 wt% Na and 0.1 wt% S with respect of the carbon support results. 3 g of dried carbon support (100 °C over night) was impregnated with this solution in six to eight steps depending on the capacity of the support material. Every impregnation step was followed by a two hour drying step at 100 °C combined with the homogenization of the sample in an achate mortar. After the last drying step, the raw catalyst was calcinated in a tube furnace for 5 h at 500 °C under constant nitrogen flow at atmospheric pressure. This catalyst is referred to as pristine catalyst.Optical emission spectrometry with inductively coupled plasma (ICP-OES, Optima 2000DV (Perkin Elmer)) was used to determine the loading of Fe, Na and S on the carbon supports. Sample preparation included the oxidation of the carbon support at 500 °C in a muffle furnace, followed by aqua regia digestion of the residues. N2-physisorption measurements (Quadrasorp-MP-30 (Quantachrome Instruments)) are carried at −196 °C, with samples degassed at 350 °C and 0.1 mbar for 18 h. The multi-point Brunauer-Emmett-Teller method (MBET) is used to determine the specific surface area. [25] The pore volume is determined by means of density functional theory (DFT) using the adsorption isotherm. Oil adsorption is used to determine the degree of branching of the carbon blacks studied. The determination is made by Orion Engineered Carbons according to ISO 4656 respectively ASTM D 2414 utilizing dibutyl phthalate as adsorbate. [26] TPD-MS was carried out in a STA 409 PC Luxx thermo-balance (NETZSCH) coupled with an online mass spectrometer (Omnistar, Pfeiffer Vacuum GmbH). For this purpose, 150 mg of the carbon black were initially heated to 80 °C at a flow rate of 30 NmL min−1 He with a heating rate of 5 °C min−1 and dried for 30 min. The sample was then heated from 80 °C to 1000 °C at a heating rate of 5 °C min−1. X-Ray diffraction (XRD) measurements have been performed in a Stadi-P (Stoe GmbH) using a Cu-Kα,1-source. Elemental analysis was carried out in a Vario EL III analyzer (Elementar Analysensysteme GmbH). All samples were measured five times. The determination of the primary particle size was carried out by means of dynamic light scattering by Orion Engineered Carbons. [27] The ISO 1125 respectively ASTM D 1506 standard was used to determine the ash content of the carbon blacks used by Orion Engineered Carbons. [28] Scanning transmission electron microscopy (STEM) was carried out in a JEM 2100F (JEOL) with an accelerating voltage of 200 kV and a spot diameter of 0.7 nm. STEM images were used for the determination of the iron particle size distribution, by analyzing a minimum of 50 particles per sample. Scanning electron microscopy (SEM) was carried out using a Philips XL30 FEG electron microscope with an acceleration voltage of 10–15 kV. X-ray diffraction (XRD) was performed using a Rigaku Miniflex exquipped with a D/tex Ultra detector (CuKα, 40 kV, 0.03 mm Ni-filter). Temperature programmed reduction (TPR), CO-Chemisorption as well as CO- and CO2-temperature programmed desorption (TPD) were carried out in a 3Flex (Micromeritics) chemisorption unit. TPR is performed up to 400 °C with a heating rate of 5 °C min−1 using 0.5% H2 in Argon. Following an evacuation, static CO-chemisorption is performed at 30 °C between 100 mbar and 800 mbar. Afterwards CO-TPD is realized between 30 °C and 400 °C obtaining a heating rate of 5 °C min−1. Next a second TPR is performed and the sample is again evacuated. After the CO2-saturation of the sample using 4% CO2 in He at 30 °C, CO2-TPD is performed at a temperature up to 600 °C using a heating rate of 5 °C min−1.Catalyst reduction and CO2-FTS activity measurements were carried out in the same u-shaped fixed bed reactor. 0.5 g of the pristine catalyst were placed into the reactor and fixed with the aid of two glass wool plugs. Catalyst reduction was carried out at 300 °C for 5 h at 30 bar using pure hydrogen. For FTS, a premixed H2:CO2-mixture (3:1 mol mol−1) was used as feed at a mass flow of 0.03 g min−1 (GHSV = ∼8000 h−1). 325 °C were set as standard temperature. Carbon monoxide and carbon dioxide were detected via FT-IR (Bruker alpha), hydrocarbons were analysed in a Shimadzu GC-2014 gas chromatograph equipped with two FID's. Separation of C1-C6-hydrocarbons took place at a Rt-QS-BOND Plot column (Restek) while higher hydrocarbons were separated at a Rtx-column (Restek). Resulting σi values represent the fraction of the respective substance class i of the total amount of hydrocarbons and are standardized to one. All conversions, selectivities and product distributions shown result from the mean value of three measurements after 30 h TOS.For the long-term measurement,1.5 g of the pristine catalyst (Printex 60) and 3.0 g Printex 60 carbon black as inert dilution were placed into the reactor and fixed with the aid of two glass wool plugs. Catalyst reduction was carried out at 300 °C for 5 h at 30 bar using hydrogen in helium (1:2 mol mol−1). A stoichiometric H2:CO2:CO-mixture (2.5:0.5:0.5 mol mol mol) was used for preconditioning of the catalyst for the first 48 h TOS at 325 °C. After preconditioning, the feed was set to a stoichiometric H2:CO2-mixture (3:1 mol mol−1) using 80.7 NmL min−1 H2 and 3.2 g min−1 CO2 (GHSV = ∼3200 h−1). Product analytics were performed using an Agilent GC6890N gas chromatograph equipped with one FID and one TCD. Separation of C1-C3-hydrocarbons as well as CO and CO2 took place at a Rt-QS-BOND Plot (Restek) column and a Rt-Alumina (Restek) column while higher hydrocarbons were separated at a Rtx-column (Restek). Resulting σi values represent the fraction of the respective substance class i of the total amount of hydrocarbons and are standardized to one.Carbon dioxide (X CO2) conversion is calculated dividing the difference between the initial quantity ( n ̇ 0 , CO 2 ) and the resulting quantity of carbon dioxide ( n ̇ CO 2 ) through the initial quantity (eq. 1). (1) X CO2 = n ̇ 0 , CO 2 − n ̇ CO 2 n ̇ 0 , CO 2 Selectivity to carbon monoxide (S CO,CO2) build out of carbon dioxide is calculated dividing the resulting quantity of CO ( n ̇ CO ) through the difference between the initial quantity ( n ̇ 0 , CO 2 ) and the resulting quantity of carbon dioxide ( n ̇ CO 2 ) (eq. 2). (2) S CO , CO2 = n ̇ CO n ̇ 0 , CO 2 − n ̇ CO 2 Selectivity to hydrocarbons (S HC,CO2) build out of carbon monoxide is calculated dividing the product of the resulting quantity of HC ( n ̇ HC ) and its number of carbon atoms (N C, HC) through the difference between the initial quantity ( n ̇ 0 , CO 2 ) and the resulting quantity of carbon dioxide ( n ̇ CO 2 ) (eq. 3). (3) S HC , CO2 = n ̇ HC ∙ N C , HC n ̇ 0 , CO 2 − n ̇ CO 2 Yield of product P (Y P,CO2) is calculated multiplying the selectivity of product P with the carbon dioxide conversion (eq. 4). (4) Y P , CO2 = S P , CO 2 ∙ X CO 2 Using the selectivities for hydrocarbons (S HC,r), the product fractions σHC,r are determined. This calculation (eq. 5) is based on the normalization of the product spectrum and serves the separate consideration of the resulting FT product distribution neglecting the WGS equilibrium. (5) σ HC , r = S HC , r ∑ HC S HC , r Since the focus of this work is on the production of short-chain olefins, the olefin fraction in the C2-C6 fraction of the product spectrum is introduced as a further measure (eq. 6). (6) C 2 − C 6 − olefin fraction = ̂ x C 2 − C 6 , olefins x C 2 − C 6 , total Using the logarithmic expression shown in eq. 7 and via linear regression, the chain growth probability α is determined graphically. For this purpose, log x HC is plotted against the carbon number N C,HC. (7) log x HC = N C , HC · log α + log 1 − α α Carbon blacks are known to be hierarchically build from primary particles, forming stable aggregates and loose agglomerates. The employed carbon blacks exhibit a wide variety of properties, owing to variations in their microstructure. As shown in Fig. 1A, all carbon blacks exhibit different primary particle sizes. [27] Starting with Printex G, the primary particle size decreases from 51 nm to 14 nm for Printex 90. The aggregates built from these primary particles were characterized by oil adsorption carried out with dibutyl phthalate (DBP), with the resulting DBP number representing a measure for the degree of branching (Fig. 1A). [27] With 48 mLDBP 100 g−1 and 42 mLDBP 100 g−1, Printex 35 and 85 have by far the lowest DBP number, and exhibit therefore the most linear structure. Printex 60 with 115 mLDBP 100 g−1 is the carbon black with the highest degree of branching. Due to the different primary particle sizes and structuring of the aggregates, varying textural properties result. The N2-physisorption isotherms (Fig. 1B) of Printex G, 35 and 60 represent isotherms of type III according to IUPAC nomenclature. They include almost no uptake in the relative pressure range of 0 to 0.1, and furthermore lack desorption hysteresis indicating the absence of meso- and microporosity. Since the majority of nitrogen uptake occurs at relative pressures above 0.8, and no plateau could be observed, macroporosity is dominant in these materials. In contrast, the isotherms of Printex 85 and Printex 90 show a desorption hysteresis (type H3) and a low nitrogen uptake in the relative pressure range of 0–0.1 can be observed, resulting in a classification as a mixture of Type II and Type IV. [25] The specific surface area (SSA, determined by BET) increases with decreasing primary particle size from 36 m2 g−1 (Printex G) to 380 m2 g−1 (Printex 90). With high probability, the SSA results from the geometric surface of the particles, with particle interstices as the origin of porosity. In consequence, the degree of branching plays an important role, as is illustrated by Printex 85 and Printex 90, which both have a similar primary particle size, but a clearly different structuring and thus a very different SSA.These findings are clearly supported by scanning electron microscopy (SEM) and transmission electron microscopy (TEM, Fig. 2 ). SEM imaging of Printex G and Printex 90 illustrates the different morphologies that result from the difference in primary particle size. Printex G, which exhibits a large primary particle size of 54 nm, shows clear macroporosity at the agglomerate level, while Printex 90, which has an average primary particle diameter of 14 nm, appears to largely lack porosity in the μm range (Fig. 2A, B). TEM imaging shows that the aggregates of Printex G and Printex 90 exhibit clear differences in terms of particle interstices that reach up to several tens of nanometers for Printex G, while they are significantly smaller in case of Printex 90 (Fig. 2C, D).In addition to their structure, the composition of the carbon black supports is of interest, which can be described by the carbon content, the amount of ash and amount of volatile components (Fig. S1). Carbon content of the different carbon blacks ranges from 97 wt% for Printex 85 to up to 99.7 wt% for Printex 35. In addition to the lowest carbon content, Printex 85 reaches the highest values in case of volatiles (1.2 wt%) and ash (0.8 wt%). Volatile components are often present in the form of oxygen surface groups on the carbon surface. In order to be able to identify carbon surface oxides, temperature-programmed desorption measurements with coupled mass spectrometry (TPD-MS) were carried out (Fig. S2). During TPD to 1000 °C, Printex 35 shows no detectable mass loss, while the largest loss (1.7 wt%) is observed in case of Printex 85. Regarding the detected CO, CO2 and H2O emission profiles it must be emphasised that the interpretation must be done with caution due to the very low amount of surface groups on the soot surface. In addition, due to the wide temperature ranges of the desorption of different surface species, it is generally difficult to make assignments. For Printex 85, three H2O desorption maxima can be observed at 180 °C, 300 °C and 710 °C. The detection of water might be assigned to condensation reactions, for example of two hydroxyl groups to an ether. The CO2 emission profile shows desorption maxima in comparable temperature ranges. CO2 emission in the range between 200 °C and 400 °C can be assigned to the decarboxylation of carboxylic acids and anhydrides, while high temperature CO2 evolution (>500 °C) indicates the decomposition of lactones. CO emission is detected above 600 °C. This desorption range is characteristic for the presence of hydroxyl groups, ethers, ketones or aldehyde groups. [29–31]The carbon-supported iron catalysts were prepared by incipient wetness impregnation (IWI). As described in detail in the experimental section, the target loading was 10 wt% iron (Fe), 0.3 wt% sodium (Na) and 0.1 wt% sulfur (S). Fe, Na and S loading was determined by means of optical emission spectroscopy with inductively coupled plasma (ICP-OES) (Fig. 3A). The mean loading of the samples was found to be 9.75 wt% (Fe), 0.66 wt% (Na) and 0.16 wt% (S). It should be emphasised that there are clear outliers in the form of the P_85-Fe/Na/S (sodium 0.93 wt%) and P_90-Fe/Na/S (sulfur 0.27 wt%) catalysts. On average, the iron loading is slightly below the target value of 10 wt%. At the same time, the sodium and sulfur loadings clearly exceed the respective target value. With regard to the sulfur loading, weighing error might play a role, but also the influence of sulfur impurities in the pristine carbon black is conceivable, since the industrial carbon blacks used are produced on the basis of crude oil. The sulfur content in the pristine carbon black determined by ICP-OES is up to 0.02 wt%. This form of contamination is also conceivable in the case of sodium loading. The sodium content in the pristine carbon black is <0.025% by weight. An important source of error arises regarding the sodium content in the sodium citrate dihydrate used as a precursor for Na loading. According to the manufacturer, this amounts to 16.5–18.5 wt%. In control samples examined by ICP-OES, the content was clearly higher at around 24 wt%.The pristine carbon black and final catalyst were additionally characterized by X-ray powder diffraction (XRD) (Fig. S3). Due to the high content of amorphous domains in the samples, an intensive background noise can be observed. The diffractogram of the Printex 60 carbon black shows two significant reflections at diffraction angles of 25° and 43° that can be assigned to the 002 and 100 or 101 lattice planes of graphitic domains. [32] As expected, for the support loaded with iron, sodium and sulfur, additional reflexes appear, that are characteristic for iron oxides consisting of different species. Due to the very broad reflections, which may be caused by the small crystallite sizes of the supported iron oxide particles, an exact assignment of the reflections is not possible. The most dominant reflections are at 35°, 43° and 62°. These can be assigned to the 311, 400 and 440 reflections of magnetite (Fe3O4) [33] or the 111, 200 and 220 reflections of wüstite (FeO) [34]. The low symmetry of the reflexes at 35° and 43° is also an indication of the superposition of at least two reflections of different species. Furthermore, the reflex intensities of the powder diffractograms shown do not correspond to the reflex ratios of phase-pure Fe3O4 or FeO species. [35]As a result of Fe, Na and S loading, changes in texture can occur resulting from the deposition of the active phase on the carbon blacks and from the restructuring of the agglomerates due to the water impregnation process. A comparison of the specific surface area and the pore volumes before and after Fe, Na and S loading is shown in Fig. S4. Both SSABET and VPore show comparable trends. Printex G and 35 supported catalysts show slight increases in SSABET and VPore (SSABET: P_G to P_G-Fe/Na/S (by 50% from 36 to 54 m2 g−1), P_35 to P_35-Fe/Na/S (by 22% from 59 to 72 m2 g−1); VPore: P_G to P_G-Fe/Na/S (by 66% from 0.06 to 0.10 cm3 g−1), P_35 to P_35-Fe/Na/S (by 69% from 0.16 to 0.27 cm3 g−1)). P_60 and P_60-Fe/Na/S include almost constant SSABET (∼116 m2 g−1) as well as slightly increasing VPore (by 25% from 0.20 to 0.25 cm3 g−1). The values of P_85 and P_90 behave differently, as the initial specific surface areas decrease significantly (P_85 to P_85-Fe/Na/S (by 23% from 195 to 150 m2 g−1), P_90 to P_90-Fe/Na/S (by 24% from 389 to 297 m2 g−1)). In addition to that, the loss regarding pore volume is also significant, especially for P_85-Fe/Na/S (by 47% from 0.47 to 0.25 cm3 g−1).Transmission electron microscopy was employed to determine the iron particle sizes distribution (Fig. 3B and Fig. 4 ). All materials show a narrow distribution of nanoscale iron oxide particles in the range of 2 to 15 nm. This is particularly evident from the low standard deviation of 2.4 nm in the case of P_35-Fe/Na/S. Fe particles larger than 20 nm were not found in any of the samples examined. The maximum of the Fe particle size distribution for P_90-Fe/Na/S is 3.7 nm. All other supports show average particle diameters in the range of 6.5 nm to 7.5 nm. These observations correspond well with results of Oschatz et al., who reported narrow size distributions of Fe particles deposited on CMK-3 and carbon black supported systems with comparable metal loadings and dopants. Average particle sizes were reported to be 5.58 nm and 4.2–4.7 nm for CMK-3 and carbon black supported catalysts, respectively, with standard deviations of 1.22 nm and 1.0 nm. [15,22]H2-TPR measurements (Fig. 3C) lead to comparable trends. All four examined samples include a broad reduction peak with a connected shoulder (Onset-Temperature ∼ 200 °C). The peak temperatures lie in a range between 353 °C (P_G-Fe/Na/S and P_90-Fe/Na/S) and 388 °C (P_85-Fe/Na/S). Chew et al. and Ma et al. report two reduction peaks for Fe/CNT- respectively Fe/AC-systems in the same temperature range. The first peak between 200 °C and 300 °C is related to the reduction of Fe2O3 to Fe3O4. This would indicate that P_90-Fe/Na/S includes the highest amount of Fe2O3. The second peak between 300 °C and 400 °C can be assigned to the reduction of Fe3O4 to FeO. [36,37] A third reduction peak representing the reduction of FeO to Fe in the temperature range up to 700 °C is to be expected but is not investigated here. [37] The presence of a low-temperature shoulder due to the absence of sharply separated reduction peaks in the case of this study can be related to impoverishment effects due to the low H2 concentration of 0.5%. The observed differences in the temperature of the H2 consumption maxima and the corresponding peak intensities are most likely a consequence of differences in the metal support interactions between Fe/Na/S and the different carbon black supports. In this context it is well-known that support properties influence the reducibility as well as the ratio of FeOx species that are obtained after calcination. [36]To probe the influence of the carbon support on CO2 and CO adsorption properties, CO- as well as CO2-TPD was carried out. Reduced catalysts were used in this context, it should be noted, however, that due to instrumental limitations the reduction conditions differ from those used for the catalytic activity tests. Regarding CO2-TPD of CO2-saturated iron catalysts, studies of Cheng et al., Xu, Wang et al. and Xu, Zhai et al. show strong dependencies of the support material used. [38–40] All three studies report weakly and strongly chemisorbed CO2. This is also observable regarding the Fe/C-systems used in this study (Fig. 5A). Strongly chemisorbed CO2, desorbed in the temperature range between 200 °C and 300 °C, is related to the influence of highly basic Na2O-species. [40] Desorption in lower temperature regions can also be assigned to CO2 adsorbed on plain iron. [40] The resulting CO2 binding strength of the carbon black supported catalysts lies between graphene oxide and silica supported iron systems. [38,39] In addition to CO2-TPD, also CO-TPD was carried out, and shows the influence of alkali metals (Fig. 5B). [41] Strong CO chemisorption, caused by the already mentioned effect of Na2O leads to a broad shoulder (100–150 °C) in addition to a desorption maximum at ∼82 °C. This effect can be seen most significantly in case of P_90-Fe/Na/S. A reason for that is delivered by the static CO-chemisorption (Fig. S5) conducted prior the CO-TPD. P_90-Fe/Na/S includes, by far, the highest amount of reversible chemisorbed CO (4.6 mmolCO gFe −1, Fig. 3D). Due to the high amount of irreversible chemisorbed CO in case of P_60-Fe/Na/S (7.3 mmolCO gFe −1), the highest specific iron surface area (SSA Fe) results (27.1 m2 gFe −1). The large difference between reversible (0.6 mmolCO gFe −1) and irreversible chemisorbed CO might be caused by carbonylation and therefore iron leaching at low temperature (30 °C). [34] In contrast to that P_90-Fe/Na/S exhibits a SSA Fe of 12.5 m2 gFe −1 while P_G-Fe/Na/S (1 m2 gFe −1) and P_35-Fe/Na/S (1.9 m2 gFe −1) show much lower accessible iron surfaces. Table 1 gives a comprehensive summary of the characterization of the pristine catalysts.Following the comprehensive characterization of the catalysts (Table 1) they were tested regarding their CO2-FTS activity. Considering both CO2 conversion (Fig. 6A) and CO yield (Fig. 6B), the five catalyst systems show considerable differences. While P_G-Fe/Na/S and P_35-Fe/Na/S show a FTS-typical start-up behaviour, with an increase of the conversion in the first 30 h from 22% and 15% to 25% and 21%, respectively, the catalyst supported on Printex 85 deactivates instantly after the start of the reaction. P_90-Fe/Na/S also shows a slight decrease in CO2 conversion (X CO2) over the course of the reaction time. Only P_60-Fe/Na/S shows a true steady state with an almost unchanged degree of CO2 conversion of around 35% and a constant CO yield of 12.5%. The Printex G supported catalyst reaches a steady state in terms of CO yield (Y CO,CO2) While P_35-Fe/Na/S and P_90-Fe/Na/S show largely identical Y CO,CO2 curves with an increase from 12.5% to around 17%, P_85-Fe/Na/S reaches a maximum at 10 h TOS. Beyond activity, the product spectrum also differs between the individual catalyst supports (Fig. 6C and D). The catalyst supported on Printex 85 shows a hydrocarbon yield of only 1.4%, followed closely by P_35-Fe/Na/S (4.3%) and P_G-Fe/Na/S (6.5%). The catalytic performance of P_G-Fe/Na/S and P_35-Fe/Na/S differs only minimally, both in terms of the conversion and yield values as well as in relation to the product classes formed, with selectivities to methane of 62.7% and 63.0%, to alkanes of 24.0% and 23.1% to olefins of 6.2% and 8.5% and to alcohols of 7.0% and 9.2% with Y HC,CO2 values of 15% and 20%, respectively, P_90-Fe/Na/S and P_60-Fe/Na/S show the highest FT yields. Strikingly, the two systems differ significantly in their product spectrum, which is reflected in both the α-values achieved and regarding the olefin fraction. The catalyst supported on Printex 90 with α-value of 0.32 tends to form significantly shorter carbon chains compared to the system supported on Printex 60 with 0.43. At the same time, almost exclusively saturated hydrocarbons are formed (σMet,CO2 = 48.8%, σPar,CO2 = 41.2%). Only P_60-Fe/Na/S, with an olefin content in the C2-C6 fraction of 40%, combined with the lowest selectivity to methane (24.6%) and the highest conversion of CO2 (33.5%), shows true potential for the use as catalyst in the Fischer-Tropsch-to-olefins-reaction (FTO). This is also confirmed by the chain length distribution of the different product classes (Fig. S6) as all three substance classes, C2+ alkanes, alkenes and alcohols, exhibit maxima at low chain length between C2 and C4. Above a chain length of C8, only very negligible amounts of product are detected. Table 2 gives a comprehensive overview of the catalytic performance of all carbon black supported Fe/Na/S catalysts.In order to explore changes in catalyst properties, post mortem characterization was carried out by N2 physisorption, TEM and XRD. Regarding P_G-Fe/Na/S, P_35-Fe/Na/S and P_85-Fe/Na/S no changes in specific surface area can be observed. In contrast, SSABET of P_60-Fe/Na/S and P_90-Fe/Na/S decrease slightly (from 113 m2 g−1 to 89 m2 g−1 for P_60-Fe/Na/S and 343 m2 g−1 to 294 m2 g−1 for P_90-Fe/Na/S). A possible cause of this decrease is the sintering of the supported iron particles, as a lower number of particles with increased diameter leads to a lower specific surface area. Regarding the pore volumes according to the DFT method only in case of P_35-Fe/Na/S and P_60-Fe/Na/S a slight decrease can be observed (from 0.24 cm3 g−1 to 0.19 cm3 g−1 for P_35-Fe/Na/S and 0.22 cm3 g−1 to 0.19 cm3 g−1 for P_60-Fe/Na/S). The decrease in pore volume as a result of pore filling by long-chain FTS products is known from the literature, especially for cobalt-catalysed low-temperature FTS. [42] Due to the low average chain length of the products formed in the case of iron-catalysed HT FTS, this explanation is not convincing (Fig. S6). In addition, the decrease in pore volume would be more pronounced with an increasing rate of hydrocarbon formation, which is not the case here. It appears likely that the textural properties of the examined supports play a leading role in the activity of the corresponding FTS catalysts, as they significantly influence the resulting specific iron surface. The Printex G and Printex 35 supported catalysts have the lowest specific surface area, 54 m2 g−1 and 72 m2 g−1 respectively, and at the same time the lowest SSA Fe (P_G-Fe/Na/S: 1 m2 gFe −1) leading to a low catalytic activity. In contrast, a higher catalytic activity is observed in the case of P_60-Fe/Na/S and P_90-Fe/Na/S. These materials also include a higher SSA BET of 113 m2 g−1 and 343 m2 g−1 resulting in a higher SSA Fe of 27.1 m2 gFe −1 and 12.5 m2 gFe −1. Only P_85-Fe/Na/S represents an exception, as the specific surface area of 150 m2 g−1 lies between that of P_60-Fe/Na/S and P_90-Fe/Na/S whereas this system shows low SSA Fe (1.9 m2 gFe −1) as well as the lowest FT activity.In this context, the specific surface area of the carbon black and its degree of branching appear to be the key for the observed differences. A high specific surface area in combination with a high degree of branching appears to facilitate a good initial dispersion and high specific surface areas of FeOx nanoparticles on Printex 60 as well as on Printex 90. High specific surface areas of the support, a high degree of branching as well as a high specific surface area of Fe nanoparticles all contribute to a large area of contact between the carbon black support and the FeOx nanoparticles. This large contact area might facilitate the carburization of FeOx in the first hours TOS to the FT active iron carbide phase, and thus the high hydrocarbon productivities of Fe/Na/S supported on Printex 60 and Printex 90. The other carbon black supports exhibit either a low degree of branching or comparatively low specific surface areas and do not facilitate the formation of well dispersed, accessible FeOx nanoparticles. In consequence, the area of contact between FeOx and carbon black support might be much smaller, leading to a low degree of carburization with the large remaining fraction of iron oxide species accounting for a high selectivity to CO. Oschatz et al. list sintering as the main reason for the deactivation of carbon-supported iron catalysts in the FTS. [15] TEM images of the used catalysts confirm the sintering of Fe nanoparticles over the course of 30 h of FTS (Fig. 7A, B). A broadening of the particle size distribution as well as a shift of the maximum of the distribution towards higher particle sizes is observed (Fig. 7C). This is reflected both in the mean value but also in the standard deviation. The largest increase of particle size is recorded in case of P_90-Fe/Na/S with an increase of 350% (Fig. 7D). Likewise, the averaged Fe particle size of P_60-Fe/Na/S increases by 299%. This behaviour correlates with the observations of Torres Galvis et al., who report increases in Fe particle size of 110–270% for an Al2O3-supported Fe/Na/S system after 120 h TOS. [23] It should be noted that in this study the time on stream was only 30 h, which led to a similar increase in Fe particle size. This observation is probably a consequence of different metal-support interactions, with the utilization of carbon black as a weakly interacting support leading to an increased sintering tendency. Oschatz et al., using carbon supports, also report significant increases in Fe particle sizes after 120 h or 140 h TOS. They increase on average to 25–28 nm and 16–18 nm, respectively, depending on the support material, [15,22] which translates to increases of 400% on CMK-3 and 280% on carbon black. It is also noticeable that Fe particle growth seems to correlate with catalytic activity: particle growth is significantly less pronounced regarding P_G-Fe/Na/S, P_35-Fe/Na/S and P_85-/Fe/Na/S, correlating directly with the lower FT activity of these systems.According to XRD measurements of the used catalysts two significant differences are observed (Fig. 7E, F). Additional weakly pronounced reflexes appear in the range of 40° / 2θ to 60° / 2θ. Although challenging to assign to individual Fe phases, most likely iron carbides such as the Eckstrom-Adcock carbide (Fe3C7), cementite (Θ-Fe3C) or a poorly defined iron carbide structure (FexCy) are responsible for these new reflexes. [43] Furthermore, it is striking that the iron oxide reflexes at 30° / 2θ, 35° / 2θ, 43° / 2θ, 57° / 2θ and 63° / 2θ are clearly narrower and more intense in the case of the used catalyst. Due to the intensity ratios, a phase pure FeOx species is still not present, however, the ratios known from Fe3O4 are clearly approached. Possible reasons for this observation are the transformation of FeO to Fe3O4, Fe0 as well as FeCx, while the narrowing of reflexes can be attributed to the growth of the crystallites, [36] thereby indicating sintering.In summary, P_60-Fe/Na/S emerges from the support variation as the most suitable industrial carbon black. Accordingly, this material is used as the catalyst support for the long-term stability test.The Printex 60 supported system was additionally tested for its long-term-stability over 170 h TOS (Fig. 8A, B). The conversion of CO2 reaches a steady state of about 30% directly with the setting of the operating conditions after the preconditioning of the catalyst at 48 h TOS. Over the entire test period of 170 h TOS, X CO2 fluctuates only within the range of 28.2% to 31.5%, while no deactivation is observed. A similar behaviour for K/Mn/Fe catalysts on nitrogen-doped CNTs is reported by Kangvansura et al.. Following the first 40 h of TOS, which roughly corresponds to the duration of the preconditioning performed in the present work, they report steady-state CO2 conversions of 25.4% to 31.8% for the following 20 h of TOS at 25 bar and 360 °C. The group attributes the deactivation during the induction period to the growth of the iron particles. [43] It should be noted that catalyst deactivation might also be caused by re-oxidation of FT-active iron carbide species to FeOx species. However, in case of CO2 hydrogenation to olefins this connection is non-trivial, as the presence of iron oxide species are integral to the desired reaction pathway as they catalyse the conversion of CO2 to CO via the RWGS equilibrium. As CO2 conversion as well as the yield of CO and hydrocarbons remains stable over 170 h TOS, we assume that after preconditioning a steady state between iron carburization and oxidation is established which results in a constant ratio of FeOx and FeCx species. The steady-state yields of hydrocarbons and CO are ∼16% in the case of Y HC,CO2 and ∼ 14% for Y CO,CO2, corresponding to selectivities of 53% and 46%, respectively. Thus, the CO selectivity of 48.5% published by Kangvansura et al. on a Mn/Fe/NCNT system is achieved. [43] Significant changes over the reaction time occur exclusively with regard to the products formed. In this context, from 80 h TOS, an increase in the proportion of methane from 22% to 38% can be observed. At the same time, the proportion of C2+-alkanes decreases from 23% at the beginning to 16%. Alkenes and alcohols show similar trends. Starting from initial values of 33% and 15%, respectively, both proportions increase to 42% and 18%, respectively, up to a TOS of 62 h, and then decrease.Between 100 h and 175 h TOS, both values fluctuate in very narrow ranges between 31% and 33% in the case of the alkenes and between 13% and 14% in the case of the alcohols. Consequently, regarding the whole reaction time, no significant changes for these to product classes exist. The observations described are also reflected in the chain growth probabilities and olefin fractions in the C2-C6 fraction shown in Fig. 5B. As a result of the increasing fraction of methane formed, the chain growth probability drops slightly from 0.48 to 0.45. However, it should be noted that the plotted α-values are subject to a considerable scatter in the range of 0.5 to 0.43. At the same time, due to decreasing alkane-content, the share of olefins on the C2-C6 fraction rises steadily from 42% to almost 50% from a TOS of 90 h onwards. At that point additionally studies are necessary to explain the changes regarding the product selectivities.Within the present study, iron-based Fischer-Tropsch catalysts were prepared with sodium and sulfur promoters supported on five industrially available, beaded carbon blacks. The carbon blacks exhibited different structural parameters, with specific surface areas ranging from 36 m2 g−1 to 380 m2 g−1. As a result of testing these systems regarding their suitability as CO2-FTS-catalysts with the target product short-chain olefins, clear correlations emerge between the structure of the materials and their catalytic activity. Thus, the combination of aggregates with a high degree of branching and a specific surface area in the range of 150 m2 g−1 proves to be desirable. Using the most promising system (X CO2 = 35%, σC2-C6-Ole,CO2 = 40%), the long-term stability of the Fe/C catalysts over 170 h TOS was demonstrated, whereas no decrease in CO2 conversion was observed. In this study we could demonstrate that inexpensive, industrial available beaded carbon blacks can be utilized as supports to prepare easy-to-handle Fe based FTS catalysts exhibiting high stability along with similar catalytic activity and selectivity compared to considerably more expensive carbon nanomaterials such as CNT's. Aci acids Alc alcohols CB carbon black CMK-3 ordered mesoporous carbon CNT carbon nano tubes DBP dibutyl phtalate DFT density functional theory FID flame ionisation detector FT-IR fourier-transform infrared FTO fischer-tropsch-to-olefins FTS fischer-tropsch-synthesis GC gas chromatograph GHSV gas hourly space velocity STEM scanning transmission elektron microskopy ICP-OES optical emission spectroscopy with inductiv coupled plasma IWI incipient wetness impregnation HC hydrocarbons MBET multi-point brunauereEmmett-teller-methode Met methane MTO methanol to olefins O/P olefin−/paraffin ratio OEC orion engineered carbons Ole olefins Par paraffins SMSI strong metal support interaction TCD thermal conductivity detector TPD/TPR temperature programmed desorption / reduction (R)WGS (reverse) watergas-shift-reaction XRD x-ray diffraction acidsalcoholscarbon blackordered mesoporous carboncarbon nano tubesdibutyl phtalatedensity functional theoryflame ionisation detectorfourier-transform infraredfischer-tropsch-to-olefinsfischer-tropsch-synthesisgas chromatographgas hourly space velocityscanning transmission elektron microskopyoptical emission spectroscopy with inductiv coupled plasmaincipient wetness impregnationhydrocarbonsmulti-point brunauereEmmett-teller-methodemethanemethanol to olefinsolefin−/paraffin ratioorion engineered carbonsolefinsparaffinsstrong metal support interactionthermal conductivity detectortemperature programmed desorption / reduction(reverse) watergas-shift-reactionx-ray diffraction α chain growth propability, – d P,cCB/iron carbon black / iron diameter, nm Q CO,Ads,rev/irrev quantity of reversible / irreversible adsorbed CO, mmol gFe −1 S p,r selecivity to product p from reactant r, mol mol−1 σ HC,r share of hydrocarbon HC related to all HC's, mol mol−1 S HC,r selecivity to hydrocarbon HC from reactant r, mol mol−1 SSA BET specific surface area using BET-method, m2 g−1 TOS time on stream, h V Pore pore volume, cm3 g−1 w Fe,Kat weight fraction of iron related to total catalyst mass, g g−1 X r conversion of reactant r, mol mol−1 Y p,r yield of product p out of reactant r, mol mol− chain growth propability, –carbon black / iron diameter, nmquantity of reversible / irreversible adsorbed CO, mmol gFe −1 selecivity to product p from reactant r, mol mol−1 share of hydrocarbon HC related to all HC's, mol mol−1 selecivity to hydrocarbon HC from reactant r, mol mol−1 specific surface area using BET-method, m2 g−1 time on stream, hpore volume, cm3 g−1 weight fraction of iron related to total catalyst mass, g g−1 conversion of reactant r, mol mol−1 yield of product p out of reactant r, mol mol− 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.Stephan Schultheis acknowledges a fellowship from the Darmstadt Graduate School of Excellence Energy Science and Engineering. Felix Herold acknowledges a fellowship within the Walter-Benjamin-program of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation, project number 471263729). The authors acknowledge funding from DFG within CRC 1487 (Iron, upgraded!; project number 443703006). Orion Engineered Carbons is acknowledged for providing carbon black samples. Supplementary material: Carbon black characterization by TPD-MS, XRD and N2 physisorption. Fe/Na/S catalyst characterization by, CO chemisorption, ICP-OES, XRD and N2 physisorption. Image 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2023.106622.

The Fischer-Tropsch-to-Olefins process allows to convert waste stemming CO2 with green hydrogen to olefins. Iron can catalyse both core reactions: 1) reverse-water-gas-shift as well as 2) Fischer-Tropsch. Carbon supported catalysts were reported to be highly attractive in this context, but until now mainly non technically applicable research carbons like nanotubes or ordered mesoporous carbons were studied and long term stability studies are missing. Here, beaded carbon blacks, were studied as available and inexpensive support materials for Fe catalysts in CO2-based FTO. The most promising support yielded selectivities towards olefins of almost 40% and showed for 170 h high stability.

Energy consumption in the world is rapidly increasing due to economic developments. Approximately 80% of the world's primary energy is currently provided from fossil fuels [1,2]. This excessive dependence on fossil fuels contributes to severe environmental degradation such as water, air, and soil pollution. Therefore, biomass is considered a promising feedstock for producing renewable fuels and chemicals [3,4].A potentially efficient and cost-effective method for converting biomass to liquid fuel (bio-oil) is fast pyrolysis [1,4–7]. Fast pyrolysis is a thermochemical process that decomposes biomass into bio-oil, bio-char, and gaseous products [8]. Compared to conventional petroleum-derived fuels, pyrolysis oil has acidic and corrosive properties, high water content, and a relatively low energy density, making it challenging to utilize as transportation fuels [9,10]. In addition, the high oxygen content in crude bio-oils, usually 20 to 50 wt%, results in a low heating value, poor stability and volatility, high viscosity, and corrosiveness [10–12]. Thus, bio-oil upgrading is required to lower its water and oxygen contents for further applications [11–14].Catalytic hydrodeoxygenation (HDO), one of the most effective methods for bio-oil upgrading, involves the stabilization and selective removal of oxygen from untreated bio-oil [11,13,15–18]. Identifying highly active and stable catalysts for upgrading bio-oil in pilot scale has been the objective of several studies [13,17,18]. Jahromi et al. [11] investigated hydrotreating of guaiacol (GUA) using red mud-supported nickel and commercial Ni/SiO2-Al2O3 catalysts at different reaction temperatures (300, 350, and 400 °C) and initial hydrogen pressures (4.83, 5.52, and 6.21 MPa). They found that the major products of the hydrotreating process were catechol, anisole, phenol, cyclohexane, hexane, benzene, toluene, and xylene. Ly et al. [14] examined various catalysts such as CoMoP/γ-Al2O3, Co/γ-Al2O3, Fe/γ-Al2O3, and HZSM-5 to upgrade bio-oil from Saccharina japonica alga. Among the catalysts, Co/γ-Al2O3 showed higher HDO performance than others (HHV of bio-oil was 34.41 MJ/kg). The kerosene-diesel fraction (C12–C14) increased from 36.17 to 38.62–48.92 wt% by catalytic HDO.Activated carbon (AC) has been used as a promising catalyst support due to its high surface area, which allows for the high dispersion of metal species [19–22]. Unlike common activated carbon made from other biomasses such as coconut shell, palm, or coal, bamboo-based activated carbon has relatively large pores suitable for the adsorption of large molecules due to the coarse texture of the raw bamboo. Kim et al. [23] showed that the pore size structure of bamboo-based activated carbon has a larger BET surface area (1329 m2/g), as compared to coconut-based activated carbon (1199 m2/g) and carbon cyrogel (639 m2/g). This characteristic made bamboo-based activated carbon more attractive catalyst support than those from other biomass materials. It was also reported that AC reduces the reactivity while improving the selectivity in HDO products [19,20]. Jin et al. [19] investigated “hydrogen-free” HDO of GUA in a high-pressure batch reactor. They found that the activity of Ru/C catalyst is superior to other studied catalysts (i.e., Au/C, Pd/C, and Rh/C). Using 10 wt% Fe/AC at 300 °C and atmospheric pressure, Tran et al. [20] successfully hydrodeoxygenated GUA into cresol and 1,2-dimethoxybenzene. Higher selectivity but lower HDO product diversity was obtained by Fe/AC, compared to Ni/Al2O3. Jin et al. [21] studied in-situ HDO of GUA over Ni-based nitrogen-doped activated carbon-supported catalysts (Ni/PANI-AC), improving the conversion of GUA by 8 % compared to Ni/AC catalyst. This improvement can be attributed to the acid-base properties and modified electronic properties, which promote the C-O cleavage and enhance the dispersion of Ni particles on the surface of the catalyst.Transition metals, such as Co and Fe, are often used as catalysts in HDO because they are less expensive than precious metals (e.g., Pt, Pd, Ru, Rh, Ir, etc.) [20,24–26]. Among these catalysts, Co was proposed to promote catalytic activity in HDO of 2-furyl methyl ketone (FMK) as a model compound in bio-oil from pyrolysis of Saccharina Japonica alga [27]. The conversion of FMK was found to be in the following order: Co > Mo > Ni. Using Ni-Co/γ-Al2O3 catalysts, Raikwar et al. [25] obtained 98.9 % conversion of GUA along with 35.2 % selectivity of benzene and 59.1 % selectivity of cyclohexane at 302 °C. Since Fe has been reported to have lower activity in benzene hydrogenation than other transition or precious metals (Ni, Co) [28,29], it is expected to provide a good balance between activity and selectivity [30,31]. Ly et al. [24] investigated HDO of FMK over 5 wt% Fe2P/γ-Al2O3 catalyst and obtained the highest conversion of 92.6 % into 2-allyl furan (79.34%) and methylcyclohexane (13.26%) at 400 °C.The present study aims to determine inexpensive and effective catalysts for upgrading bio-oils. Another objective of this study is to control the reaction pathways during bio-oil upgrading to increase the selectivity of valuable chemicals, especially methyl phenol derivatives, in the liquid products. Methyl phenol derivative, one of the most common components of lignocellulosic bio-oils, is an important chemical intermediate and is essential to produce various chemicals and materials such as phenolic resins, alkylphenols, and more. It is also used as a precursor to other compounds and materials, including plastics, pesticides, pharmaceuticals, and dyes [32]. However, the current technologies for producing methyl phenol derivatives are challenging because of their high cost and increased pollution to the environment [33]. For this reason, in this study, AC-supported Co, Fe, and bimetallic (Co-Fe) catalysts were investigated for HDO of pyrolysis oil from wood pallet sawdust (WPS) in an autoclave using various techniques. In addition, the performance and deactivation of various AC-supported catalysts were systematically investigated under different operating conditions.The wood pallet sawdust (WPS) was provided by Hanssem Co. LTD. (Korea). The samples were subjected to drying at 105 °C overnight to remove the moisture before the experiments. The bio-oils (organic phase) were obtained by fast pyrolysis in a fluidized bed reactor (pilot-scale 20 T/day) at 450 °C with a fluidization velocity of 2.5 × Umf (Umf = 4.5 L/min), using silica sand as bed material. The thermogravimetric analysis (TGA N-1000, SINCO) was carried out under a nitrogen flow rate of 20 mL/min to understand the thermal decomposition of the biomass and bio-oil. During this analysis, the temperature was increased from 20 to 700 °C at a heating rate 10 °C/min (Figure S1).In this study, the AC support was produced from bamboo through two steps, i.e., carbonization and activation. These process details are described elsewhere [20,22]. After cooling to room temperature, the AC was sieved to homogenize the particle size between 75 and 180 µm. The catalysts were prepared by an incipient wetness impregnation method with a 10–30 wt% metal loading (Co and Fe) on AC. The calculated amounts of cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O, 98%, Sigma-Aldrich) and iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, 98%, Sigma-Aldrich) were hydrolyzed with deionized water. Following impregnation, the catalysts were dried at 105 °C for 24 h. The catalysts on AC were pretreated with 250 mL/min of N2 at 500 °C (Co catalyst) and 550 °C (Fe catalyst) at atmospheric pressure due to high oxygen content on the surface of AC [22]. For the preparation of the bimetallic Co-Fe catalyst, 20 wt% Co-Fe with various ratios was chosen to investigate the HDO process. These pretreated catalysts are denoted as xCo-yFe, in which x and y are the weight ratio of Co and Fe (i.e., 1:1, 2:1, 3:1, and 4:1) loaded on AC, respectively.The crystallographic structure of the catalysts was examined using an X-ray diffractometer (MAC-18XHF, Rigaku, Japan) with a CuKα radiation source (λ = 1.54 A), which was operated at a scanning rate of 5°/min in 2θ range of 10° to 80°. The textural properties of the catalysts were analyzed using a N2 porosimeter (Tristar, Micromeritics, USA). The specific surface area was calculated via the multipoint Brunauer–Emmett–Teller (BET) method within a relative pressure (P/Po) range of 0.05–0.25. The morphology of the prepared catalysts was analyzed using field-emission scanning electron microscopy (FE-SEM; Leo-Supra 55, Carl Zeiss STM, Germany), while Transmission Electron Microscopy (JEM 2100, JEOL, Japan, accelerating voltage: 20 kV) was used for confirmation of carbon deposit on spent catalyst after the reaction. Apart from TEM, a thermogravimetric analyzer (TGA-Scinco 1000, Korea) was used to determine the amount of carbon on the spent catalyst, which was washed with acetone, and dried at 105 °C to remove the contaminants after the reaction. For temperature program reduction (TPR) analysis, the catalysts were treated under H2 and Ar (2:8 mL/min) at atmospheric pressure in a fixed bed reactor. The reduction process continued up to 800 °C, then cooled and re-passivated for a catalytic deoxygenation reaction [20,24,32]. Gas chromatography (GC) with a thermal conductivity detector (TCD) was employed to measure the hydrogen consumption of the catalysts. The C, H, and N contents of the AC from bamboo were determined using an elemental analyzer (Flash EA 1112, CA Instrument, USA), and the O content was determined by the difference.Temperature-programmed desorption (TPD) of ammonia was investigated using a mass spectrometer (HPR20. Hyden Analytical) to evaluate the total acidity of the catalysts [20,27]. The catalysts were treated with He gas, followed by ammonia adsorption at 100 °C with 4 vol% of NH3 in He gas. The catalyst samples were flushed with He gas to remove the adsorbed NH3. The amount of desorbed NH3 under He flow was measured using gas chromatography with a TCD. X-ray photoelectron spectroscopy (XPS, K-alpha X-ray Photoelectron Spectrometer, Thermo Scientific) measurement was also carried out to determine the oxide states of metal dopants (Co, Fe). The carbon C (1s) line was used as a reference with a binding energy value at 284.6 eV. The reduced catalyst was passivated in a 3% air in Ar at room temperature for 2 h before exposing it to the air [33].Upgrading of WPS bio-oil was carried out in an autoclave reactor. As shown in Figure S2, the system has been reported in our previous research [14]. The reactor was heated from 300 to 350 °C under different initial hydrogen pressures, ranging from 25 to 60 bar. With the autoclave submerged in the molten salt bath, a catalyst/bio-oil ratio of 1.5/10 (i.e., 3 g of catalyst with 20 g bio-oil sample) was employed in the experiments. In each experiment, the residence time was fixed at 60 min, and the reactor was removed from the bath and cooled to room temperature after each run. After HDO, the product yield was calculated from the amounts of the product and the bio-oil fed to the system. The gaseous product was collected with a gas sampling bag, and the gas yield was determined by measuring the weight difference of the bag before and after the collection. The liquid and solid products were separated using a solvent extraction technique with a microfilter paper (pore size: 0.45 μm). Then, the solid yield was calculated by weighing the solid after drying, while the liquid yield was given by difference. For all the presented calculations, each data point was an average of at least three experimental results. The elemental compositions of the upgraded bio-oils were characterized by Flash EA1112, CE Instrument [14,34], while the moisture content was measured by a Karl-Fischer (CA-200, Mitsubishi, Japan). The gas compositions were identified by gas chromatography (YL 6500GC, YL Instrument, Korea) equipped with dual detectors, a flame ionization detector (FID) with a Porapak N column to analyze hydrocarbon gases (C1–C4) and a TCD with a Molecular sieve 13X column for H2, CO, CO2, and CH4. The FID was operated at 250 °C and a flow rate of 20 mL/min N2 (ultra-high purity, 99.999%) as a carrier gas, while the TCD detector at 150 °C under the same flow rate argon (ultra-high purity, 99.999%). The gas compositions were also analyzed by a gas chromatograph/mass spectrometry (GC/MS, 7890A, Agilent, USA), with a capillary column of HP-5MS (30 m × 0.25 mm × 0.25 μm) under a constant flow rate of helium (1.0 mL/min). The oven temperature was programmed at 40 to 90 ℃ at a heating rate of 10 ℃/min, followed by 40 ℃ /min to 250 ℃ while the detector temperature was set to 280 ℃. A 13C NMR spectrometer at 300 MHz was employed to characterize functional groups of the bio-oils dissolved in dimethyl sulfoxide-d6 (DMSO‑d6). The high heating value (HHV) of the bio-oil was measured using a bomb calorimeter (SDC715 Calorimeter, Sundy). The degree of deoxygenation (DOD) describes the effectiveness of oxygen removal, indicating the quality of the produced bio-oil. The degree of deoxygenation is calculated by the following equation [35]: (1) D e g r e e o f d e o x y g e n a t i o n D O D = 1 - wt % O i n u p g r a d e d b i o - o i l wt % O i n r a w b i o - o i l × 100 % Table 1 shows the characteristic of WPS and WPS pyrolysis bio-oil. The moisture and ash contents of the WPS were measured to be 22.99±0.17 and 2.43±0.47 wt%, while those of the WPS pyrolysis bio-oil were 3.02±0.11, and 0.34±0.04 wt%, respectively. Elemental analysis shows that the C, H, N, and O contents of the WPS were 49.95±0.11, 6.07±0.03, 0.61±0.02, and 43.37±0.24 wt%, respectively. For the WPS bio-oil, the contents were 61.27±0.14, 6.45±0.22, 0.29±0.02, and 31.99±0.09 wt%. The HHV of WPS and WPS bio-oil were 18.43±0.05 and 25.69±0.14 MJ/Kg.The textual properties of the AC-supported Co and Fe catalysts are presented in Table 2 . The specific surface area (SBET), pore volume, and average pore size of the AC support pretreated at 550 °C were determined to be 604.4300 m2/g, 0.0431 cm3/g, and 2.4500 nm, respectively [20]. As the Fe content increased from 10 to 30%, the SBET decreased from 111.5020 to 44.0120 m2/g, while pore size increased from 21.2101 to 9.5506 nm, and pore volume increased from 0.0297 to 0.993 and then decreased to 0.0424 cm3/g. While increasing the Co content from 10 to 30%, the SBET, pore size, and pore volume increased from 9.1974 to 19.0026 m2/g, 4.3157 to 21.2101 nm, and 0.0087 to 0.0387 cm2/g, respectively. With increasing the ratio of Co:Fe from 1:1 to 4:1, the SBET of bimetallic Co-Fe/AC decreased from 350.8678 to 117.7973 m2/g, whereas pore size and volume increased from 2.6370 to 3.2530 nm and from 0.0303 to 0.0885 cm2/g, respectively.The X-ray diffraction (XRD) patterns of the AC-supported catalysts are shown in Fig. 1 . The diffraction peaks at 2θ = 24.0, 28.8, 30.0, 31.2, 33.9, 40.5, 50.0, and 73.7° represent the formation of KHCO3 in the AC. It is likely due to the high content of K in bamboo [20]. The diffraction peaks at 16.16 and 20.71° are attributed to the formation of CoCl2·H2O. The peaks assigned to CoCO3, CoO, and Co are observed at 32.59, 36.81, and 43,97°, respectively [38–40]. The XRD pattern of Fe/AC and Co-Fe/AC catalysts exhibit peaks assigned to Fe3O4 (35.63, 57.29, and 62.93°) and Fe (44.81°). The formation of spinel CoFe2O4 (35.40 and 62.54°) was observed from the spent catalyst (S-20 wt% 4Co-1Fe/AC).The TPR analysis was conducted to investigate the reduction of the metal oxides to metals by contact between metal species and the supports [36]. As shown in Fig. 2 , an increase in Co loading from 10 to 30% increased the reduction temperature of Co catalyst from 492 to 533 °C. A similar trend was observed with an increase of Fe loading, leading to an increase of reduction temperature from 640 to 688 °C. The increase of Co loading in Co-Fe/AC catalyst led to the increase of reduction temperature slightly. Reduction behaviors of bimetallic catalysts were influenced by their compositions (i.e., Co:Fe ratio). For example, the reduction peaks of the Co-Fe/AC catalyst were observed to shift to higher temperatures with increasing the Co:Fe ratio (from 1:1 to 4:1).The profiles for temperature-programmed desorption of ammonia (NH3-TPD) are presented in Fig. 3 . The desorption temperature illustrates the potency of the catalyst's active sites. Based on the desorption temperature, the acid sites can be classified into weak (<250 °C), moderate (250–500 °C), and strong acid sites (>500 °C) [27,37]. The loading of metal on AC resulted in changes in the distribution of active sites. The Co catalyst only has strong acid sites, while the Fe catalyst has both weak and strong acid sites. The increased loading of Co species in Co/AC catalyst resulted in a slight increase of strong acid sites. However, with changing the Fe loading amount in Fe/AC catalyst, the number of active sites was in the following order: 20 wt% Fe/AC > 30 wt% Fe/AC > 10 wt% Fe/AC. The amount of both weak and strong acid sites increased with the Co ratio in Co-Fe/AC catalyst. The acidity density of bimetallic catalysts was higher than that of the monometallic catalysts, suggesting that the bimetallic catalysts would have higher catalytic activities. It was found that 20 wt% 4Co-1Fe/AC showed the most elevated acidity among the tested catalysts.The SEM-EDS of activated carbon in Fig. 4 shows a mixture of smaller irregular geometrical (20 wt% Co/AC, 20 wt% Fe/AC), cubic (20 wt% 4Co-1Fe/AC) with Co, Co3O4, Fe, and Fe3O4 particles agglomerated together on the surface of activated carbon support (>10 μm). It is possibly due to the different amorphous phases in the sample, as confirmed by the earlier XRD analysis. The growth of the particles in the solution during the synthesis was facilitated by the Ostwald ripening, during which the smaller particles grew in size while decreasing in number as highly soluble small particles dissolved and re-precipitated. In addition, a particle with negative curvature exhibited lower solubility than another particle with positive curvature. Therefore, the latter inclined to precipitate on the surface of the former, leading to growing of the particle’s neck, strengthening the particle–particle cluster during agglomeration [38]. The TEM images of selected catalysts are shown in Figure S3 (a-e). The dark spots in Figure S3 (a-c) are attributed to the presence of metal oxides (Co3O4 and Fe3O4), which is in good agreement with the XRD analysis [39].As shown in Fig. 5 , X-ray photoelectron spectroscopy (XPS) analysis was carried out to investigate the valence state of Fe and Co in the catalysts [33]. Fig. 4 (a) shows that the XPS spectrum for Co2p can be deconvoluted into four and five peaks for 20 wt% Co/AC and 20 wt% 4Co-1Fe/AC, respectively. Two peaks centered at 780.2 and 784.6 eV correspond to Co2p3/2 and Co2p3/2sat, while the other peaks at 796.3 and 802.3 eV are attributed to Co2p1/2 and 2p1/2sat. Other observed peaks show Co2p3/2 and 2p1/2 spin–orbit doublet due to the formation of Co-oxide phases (i.e., CoO or Co3O4) [39–41]. For fresh and spent 20 wt% 4Co-1Fe/AC catalysts, the shake off satellites were detected, which occur when the valence electron is ejected from the ion completely (to the continuum). It appears as a broadening of the core level peak or contribution to the inelastic background. In addition, unfilled shell containing unpaired electrons might cause multiplet splitting, which result in two broad peaks around 785 eV. Similarly, the XPS spectrum of Fe2p is deconvoluted into three peaks, as shown in Fig. 4 (b). Peaks corresponding to Fe2p3/2 and Fe2p3/2sat were observed at binding energies of 709.8 and 714.3 eV, whereas a peak at 723.4 eV is assigned to Fe2p1/2. The peak at 709.8 eV seems to correspond to the metallic phase of Fe, while other peaks centered at 714.3 and 723.4 eV seem to be due to the formation of the iron oxide phase (i.e., Fe3O4) [39–41]. These results are in good agreement with the those from the XRD analysis. Fig. 4 clearly shows that the XPS spectrum of both Co2p and Fe2p changed after reactions, indicating that the ratio of metal and metal oxide species was altered by the reaction. These changes in the chemical state and in the oxygen species of the catalyts lead to the deactivation of the catalysts.The WPS bio-oil was upgraded under different temperatures and hydrogen pressures in the presence of AC. The product yields and elemental analysis of the upgraded oil are summarized in Table 3 . As the reaction temperature increased, the liquid yield decreased, while the solid and gas yields increased. With an increase in temperature from 300 to 350 °C at 25 bar, the liquid yield of the bio-oil decreased from 80.06 to 55.36 wt%, while the solid and gas yields increased from 13.83 to 37.18 wt% and from 6.11 to 7.46 wt%, respectively. It was also found that with increasing hydrogen pressure, the gas and liquid yields increased, but the solid yield decreased. For example, when the hydrogen pressure increased from 25 to 60 bar at 350 °C, the solid yield decreased from 37.18 to 26.79 wt%, while the gas and liquid yields increased from 7.46 to 12.53 wt% and from 55.36 to 60.68 wt%.The HHV of bio-oil is also influenced by the operating conditions. As seen in Table 3, hydrogen pressure and reactor temperature enhance the HHV of the upgraded bio-oils. Consequently, among the conditions examined in this study, HDO at 350 °C and 60 bar provided the highest HHV of 33.72 MJ/Kg, which is higher than 25.69 MJ/Kg from the raw bio-oil.The gaseous products mainly consist of CO2, CO, CH4, and small amounts of hydrocarbons (C2-C4). H2 in the gas products was not measured in this study because it is generated, while consumed simultaneously through the bio-oil upgrading process. The CO2, CO, and CH4 were produced by decarboxylation, decarbonylation, and demethylation, while other hydrocarbon gases were formed by a cracking reaction. By increasing the reaction temperature and hydrogen pressure, the selectivity of CO2 was slightly decreased from 84.14 to 80.93%. On the other hand, the selectivity of CO and CH4 increased from 4.52 and 3.99% to 9.23 and 8.15%. It is most likely that as temperature and pressure increase, CO production is further promoted through a secondary cracking of volatiles, followed by a reduction of CO2, while other hydrocarbons are stable [42]. Table 4 presents the liquid compositions of upgraded bio-oil under different operating conditions. In the raw WPS bio-oil, the main components were found to be phenolic compounds and benzenediol derivatives. In particular, the area% of phenol, methyl phenol, and benzenediol derivatives in the raw WPS bio-oil were measured to be 5.92, 15.67, and 35.09%, respectively. After the HDO at 350 °C and 60 bar, the area% of phenol and methyl phenol increased to 6.99 and 19.51%. In contrast, the area% of benzenediol derivatives decreased to 8.95%. These results indicate that a hydroxyl group (–OH) in the benzenediol derivatives is cleaved to form the phenolic compound, especially methyl phenol derivatives. The mechanism of hydroxyl group cleavage has also been elucidated using a bio-oil model compound such as guaiacol in our previous studies [10,11,20,21] and phenol [16,26]. During the upgrading process, the cracking reaction produces lighter components such as acetic acid (9.10%) and butanoic acid (3.44%). Besides, some transfers of alkyl groups indicate that alkylation proceeded during the bio-oil upgrading. The Me-O bond in the bio-oil components could be transformed into a methyl radical remaining attached to the catalyst surface [43], which allows a transalkylation in which the methyl radical is attached to the phenol in an ortho position via electrophilic substitution. This result agrees with the findings reported by Zhao et al. [44] and Bui et al. [43,45], who proposed a pathway for the formation of alkyl phenols. The moisture content of the upgraded oil was 17.30%, which is much higher than that of raw bio-oil (3.25%). Accordingly, the DOD of bio-oil increased from 40.21 to 63.66% after the HDO, suggesting that effective dehydration occurred during the bio-oil upgrading process.The yields and the distribution of HDO products for different catalysts are shown in Table 5 . For single-metal atom catalysts, 10, 20, and 30 wt% of Co or Fe were loaded on AC while 20 wt% of Co-Fe dual atoms with different ratios of Co/Fe were loaded for Co-Fe bimetallic catalysts.Among the mono-metal catalysts, a maximum liquid yield of 70.46 wt% was obtained at 20 wt% Co, while an increase of Fe loading decreased the liquid yield. For the mono Fe catalysts, a maximum liquid yield (59.33 wt%) was observed at 10 wt% Fe. As the Fe loading increased from 10 to 30 wt%, the bio-oil (liquid) yield decreased sharply from 59.33 to 36.78 wt%, while the solid and gas yields increased from 36.66 to 56.83 wt% and from 4.01 to 6.93 wt%, respectively. Excessive loading of Fe could lead to the agglomeration of the metal species and the reduction of the active sites on the catalysts. In addition, as shown in Table 2, the pore size decreased with the amount of Fe loading onto AC, which is probably due to the pore blockages caused by the growth of metal ions or the formation of bulk metal oxides during the impregnation and/or calcination step. Tran et al. [20] reported similar findings from HDO of GUA over Fe/AC and Ni/γ-Al2O3 catalysts synthesized by the impregnation method. Abu and Smith [46] also obtained comparable results using Co-added MoP and Ni2P catalysts in hydrodesulfurization of 4,6-dimethyldibenzothiophene. In this study, 20 wt% Co and 10 wt% Fe were found to be optimal by facilitating the uniform dispersion of metal atoms on the AC surface. Among the tested mono-metal catalysts, the bio-oil upgraded using 20 wt% Co/AC offered the highest HHV (34.22 MJ/Kg). Differences in the textural properties of catalysts could lead to these results.Based on the results from the mono-metal catalysts, 20 wt% of Co was chosen for bimetallic Co-Fe catalysts, and the effect of Co/Fe ratio (1 to 4) was further studied. By increasing the ratio of Co to Fe, both bio-oil and gas yields increased from 65.76 to 68.85 wt% and from 3.30 to 7.15 wt%. However, the solid yield decreased from 30.94 to 24.00 wt%. A ratio of 4Co-1Fe seems to be optimal in this study, based on the HHV (34.16 MJ/Kg) and yield (68.85%) of bio-oil. In the presence of mono- or bi-metallic catalyst, the DOD of the upgraded bio-oils ranged from 44.61 to 79.81%.According to the analysis in the composition of gas products, the gas selectivities from Co/AC and Fe/AC were quite different. In particular, when using Co/AC catalysts, CO, CO2, and CH4 were mainly produced, whereas CO production was significantly reduced with Fe/AC catalysts. With increasing Co/AC ratio in bimetallic catalysts from 1 to 4, the CO and CH4 contents in the gas products increased, while the CO2 decreased. When Fe-based catalysts were used, the moisture content in the upgraded bio-oil was 32.96–34.21%, which is higher than that of Co-based catalysts (25.86 to 26.90%). The higher moisture contents in the use of Fe-based catalysts can be explained by dehydration as well as the gas-phase reactions such as CO/CO2 methanation and reverse water–gas shift reactions presented below [15,47]. (2) C O 2 + H 2 ↔ C O + H 2 O (3) C O + 3 H 2 ↔ C H 4 + H 2 O (4) C O 2 + 4 H 2 ↔ C H 4 + 2 H 2 O Table 6 illustrates the liquid compositions of upgraded bio-oil obtained using different AC-supported catalysts at 350 °C and 60 bar. It is well known that phenol and benzenediol derivatives are favorable as liquid components in bio-oils due to their high stability and HHVs. From this context, the 20 wt% 4Co-1Fe/AC seems to be the most desirable catalyst among the tested catalysts. Table 6 clearly shows that the production of the components mentioned above improved significantly with 20 wt% 4Co-1Fe/AC. The area% of phenol, methyl phenol, and benzenediol derivatives were calculated to be 9.58, 26.40, and 8.95%, respectively. Besides, the area% of naphthalene derivatives were enhanced with 20 wt% 4Co-1Fe/AC. The Retro-Diels–Alder reaction might proceed during the HDO process. These trends have been reported in the ex-situ catalytic upgrading of bio-oil vapors over HZSM-5 and microwave-assisted pyrolysis of waste olefins [48–50]. Fig. 6 shows the carbon number distribution of raw bio-oil and upgraded bio-oil using different AC-supported catalysts at 350 °C and 60 bar. For the raw bio-oil, for example, the light (C5-C11), diesel (C12-C18), and heavy (C19-C38) fractions were 49.99, 32.30, and 17.66 wt%. The carbon numbers of upgraded bio-oil were mainly distributed in C5–C11 fractions, which were calculated to be 46.79, 46.17, and 50.66 wt% for 20 wt% Co/AC, 20 wt% Fe/AC, and 20 wt% 4Co-1Fe/AC, respectively. With 20 wt% 4Co-1Fe/AC, the highest proportion of light fraction (C5-C11) was achieved, especially with the C8 component (20.40 wt%). The C8 component of the light fraction might belong to the methyl phenol derivatives, one of the most favorable components of bio-oils. Methyl phenol derivative is an important chemical intermediate and is essential to produce various chemicals and materials, such as phenolic resins, alkylphenols, etc. However, most of the methyl phenol derivatives are currently produced from benzene through cumene process, which consumes a large amount of fossil fuels and causes environmental pollution [33]. In this study, the highest proportion of light fraction (C5-C11) was obtained with 20 wt% 4Co-1Fe/AC, suggesting that this catalyst is most promising for upgrading WPS bio-oils.Using the NMR spectroscopy, functional groups in bio-oils were identified [8,12] based on the chemical shift regions. As shown in Fig. 7 , the 13C NMR spectra of bio-oils were divided into several chemical shifts regions, 0–55, 55–95, 95–165, and around 200 ppm. The raw bio-oil mainly consisted of aromatic compounds, aliphatic hydrocarbons, and a small amount of carbohydrates, alcohols, esters, and phenolic methoxy groups. The upgraded bio-oil with Co/AC and 4Co-1Fe/AC produced high intensity of aromatic compounds, CO groups, and aliphatic hydrocarbons, whereas Fe/AC led to generate carbohydrates, alcohols, esters, and phenolic methoxy groups. Generally, aldehydes and ketones are not found in 13C NMR spectra. Relatively high intensity of aromatic compound groups observed at 90–165 ppm might be due to cleavage of the hydroxy and methoxy groups via dehydration and demethoxylation, leading to lower oxygen contents. It was found that aromatic compounds and carboxylic acids of the bio-oil with Fe/AC were less than those for Co/AC and Co-Fe/AC catalysts. This difference can be attributed to the formation of CO2 via decarboxylation, which occurs in the presence of iron oxides. It is in good agreement with the GC/MS analysis and the results from our prior work, where iron oxide catalysts were employed in the pyrolysis of spent coffee waste for upgrading sustainable bio-oil in a bubbling fluidized-bed reactor [12]. In this study, the 2D NMR HSQC spectra also was collected (Figure S4) to determine the proton-carbon single bond correlation and predict small molecular components in the bio-oil. A strong signal attributed to the aromatic carbon was observed at around 7.5 ppm, and signals at the region of 7.08-6.59 ppm appear to be contributed by the benzene ring. The chemical shift by hydroxy and ketone groups can be seen at ~2 ppm, while the peak at ~3.77 ppm shifted downfield represents the methoxy group. These observations are also consistent with the 13C NMR results in Fig. 7.Catalyst deactivation is primarily caused by the carbonaceous formation on internal and/or external surfaces [51–56]. Furthermore, the reduction in catalytic activity can be caused by blocking the active sites of catalysts when intermediate products are adsorbed onto the catalyst surfaces during the HDO process [52,54,56].In this study, the deactivation of catalysts was investigated by characterizing the fresh and spent catalysts. After HDO, as shown in Table 2, the specific surface area of Co/AC, Fe/AC, and 4Co-1Fe/AC catalysts significantly decreased to 7.1634, 12.3916, and 8.1094 m2/g from 11.1108, 93.2195, and 117.1773 m2/g. In addition, the pore volume was reduced to 0.0066, 0.0130, and 0.007 cm3/g. It is likely that the formation of coke in catalysts reduced both the specific surface area and pore volume.Thermogravimetric analysis was utilized to quantify the coke formation by measuring the difference in weight loss between fresh and spent catalysts (Fig. 8 ). The oven temperature was increased from 20 to 700 °C at a heating rate 10 °C/min under the air atmosphere. The weight loss differences for Co/AC, Fe/AC, and 4Co-1Fe/AC catalysts were 35,1, 17.5, and 46.0 wt% at 700 °C, respectively. The weight loss curve by differential thermalgravimetric (DTG) analysis was also prepared to determine the combustion temperature of the AC support in the catalyst, as shown in Fig. 8 (b). The DTG peaks of fresh catalysts confirm that the AC is burned out in the range of 380–450 °C. Hou et al. [51] reported that the weight loss of the spent HZSM-5 catalyst in the range of 400–700 °C is due to the carbon combustion. Except for the peaks in the combustion temperature range of AC, the spent catalysts showed additional DTG peaks at different temperature ranges. For the spent Fe/AC and Co-Fe/AC, the peaks were observed in the range of 225–266 °C and 330–500 °C, while Co/AC was at a relatively higher temperature (524 °C). The mass loss before 350 ℃ is mainly ascribed to the volatilization of the non-desorbed products, while other losses are due to the combustion of coke [52].Depending on the location and type of coke, its combustion temperature varies. For example, the internal coke is burned at a higher temperature than the external coke [52]. In addition, weakly bonded cokes such as linked-ring compounds are more likely to burn at lower temperatures than strongly adsorbed cokes such as condensed-ring compounds [54]. Besides, Figure S3 shows the shape and type of carbonaceous formation on the catalyst surface. Based on our thermogravimetric analysis, the carbonaceous compounds that caused the deactivation were formed on the external catalyst surface. Aside from the carbonaceous materials, the formation of spinel CoFe2O4 during HDO, which was confirmed by XRD analysis, might lead to the deactivation of catalysts [27].Pyrolysis of biomass is a promising process for producing bio-oils and valuable chemicals. Unfortunately, bio-oils produced by pyrolysis are corrosive and chemically unstable due to high water and oxygen contents. Catalytic hydrodeoxygenation (HDO) is one of the most effective technologies to improve the quality of bio-oils. In this study, HDO of bio-oils produced by pyrolysis of wood pallet sawdust (WPS) was systematically investigated using mono- (Co or Fe) and bi-metallic (Co-Fe) catalysts supported on activated carbon (AC). Based on the overall results, 20 wt% 4Co-1Fe/AC catalyst was found to be optimal among the tested catalysts. Using this catalyst, high HHV (34.16 MJ/kg) and liquid yield (68.85%) were obtained at 350 °C and 60 bar. Furthermore, the production of desirable components such as phenol derivatives was improved. Finally, with 20 wt% 4Co-1Fe/AC, the highest proportion of light fraction (C5-C11) was achieved, especially with the C8 component (20.40 wt%). The catalysts were deactivated by the formation of carbonaceous compounds on the external surface, oxidation of metal species, and blocking of active sites on catalysts. Our results suggest that the upgraded bio-oils by HDO can be used as transportation fuels or great sources for alternative fuels and valuable chemicals. Along with the results presented here, the feasibility studies, including techno-economic analysis and energy balance for the entire pyrolysis-HDO process, need to be carried out 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.This work was supported by the (NRF) grant funded by the Korea government(MSIT) (No. 2020R1A2B5B01097547). This work was supported by the Engineering Research Center of Excellence Program of the Korea Ministry of Science, ICT & Future Planning (MSIP)/National Research Foundation of Korea (NRF) (Grant NRF-2021R1A5A6002853).

The catalytic hydrodeoxygenation (HDO) processes for upgrading pyrolysis bio-oils from wood pallet sawdust (WPS) were studied using activated carbon (AC) as a support of mono- (Co/AC and Fe/AC) and bi-metallic (Co-Fe/AC) catalysts. The effects of the reaction temperature and hydrogen pressure on products and high heating value (HHV) were systematically investigated. At 350 °C and 60 bar, 20 wt% Co/AC showed the highest liquid yield (70.46 wt%) along with HHV of 34.22 MJ/Kg. Among the tested bimetallic catalysts, comparable liquid yield (68.85 wt%) and HHV (34.16 MJ/kg) were achieved with 20 wt% 4Co-1Fe/AC catalyst. Methyl phenol derivatives were found to be the main component in upgraded bio-oil. The carbon number of the upgraded bio-oil was mainly distributed in C5–C11 fraction, especially with the C8 component (20.40 wt%). The catalysts were deactivated by the formation of carbonaceous compounds on the external surface, oxidation of metal species, and blocking of active sites on catalysts.

The oxygen evolution reaction (OER) is pivotal because of its multiple prospective energy storage devices, including water electrolyzers and rechargeable metal-air batteries; 1–4 however, the OER pathways are complex and generally impose significant kinetic bottlenecks with large overpotentials (usually more than 350 mV). 5 The benchmark Ru/Ir catalysts can hardly satisfy scaled up practical utilization because of their scarcity, prohibitive cost, and detrimental environmental effects. 6 Nowadays, earth-abundant transition-metal-based compounds have comparable performance and, thus, have attracted extensive attention as alternatives. 7–9 However, most of these compounds undergo surface reconstruction under operating conditions, which makes it difficult to capture the dynamic structure and recognize real catalyst information. 10 For example, Ni-based OER electrocatalysts, 11 , 12 such as chalcogenides, nitrides, and phosphides, are thermodynamically less stable than oxides in strongly oxidative environments. The electro-derived oxidation on their surface induces reconstruction to form target OER-active oxides/(oxy)hydroxides (Table S1), which have been identified as real active species. 13 These post-OER catalysts generally show a near-surface reconstruction structure at the nanoscale, such as a core-shell structure, containing a large percentage of inactive atoms in the core. 14 Because of their limited near-surface reaction region, 15 , 16 the evolving catalysts usually exhibit incompletely developed catalytic activity. The compositional complexity of partially reconstructed catalysts greatly hinders an insightful understanding of catalytic origins. 17 Therefore, it raises curiosity and challenges in fundamental mechanistic research of the reconstruction chemistry of pre-catalysts, including origins of limited reconstruction degrees and dynamic reconstruction mechanisms.The reconstruction results of reconstruction layer thickness (RLt) and the smallest size in one dimension based on reported OER pre-catalysts are summarized in Figure 1 (see also Table S2). The RLt values for most reported pre-catalysts are less than 10 nm. Deep/complete reconstruction will maximize the number of active sites in reconstructed catalysts and thus endow high-mass-activity catalysis. To date, very few studies have focused on completely reconstructed (CR) catalysts. 18–20 Because of the fast reconstruction processes, deep comprehension of reconstruction is crucial but difficult. Despite its importance, the general synthesis method and underlying mechanism of CR catalysts have not been reported. More importantly, most current reports investigate catalyst performance in low-concentration alkali (0.1–1 M KOH) rather than in industrial-concentration 20–30 wt % KOH. 18–21 Harsh conditions may result in different reconstruction results for pre-catalysts. Evaluating catalysts in 20–30 wt % KOH for alkaline water electrolysis (AWE) can promote their commercial application. 22 Therefore, it is meaningful for performance evaluation under realistic operating conditions.As illustrated in Schemes 1A–1C, we present etching-leaching-reconstruction engineering to achieve complete reconstruction of bulk hydrate pre-catalysts. As a proof of concept, various in/ex situ technologies were employed to capture the time-resolved structural/phase evolution of NiMoO4·xH2O under electro-oxidation conditions. The reconstruction steps generally consist of complete collapse of hydrates with co-leaching of crystal water and MoO4 2− and then its reconstruction to NiOOH via electro-oxidation. Intrinsically, the loose reconstruction layer caused by co-leaching is the key to promote deep penetration of solution and, thus, complete reconstruction. However, for NiMoO4, the entire crystal structure could not be disintegrated by alkali etching, and the formed dense layer prevents electrolyte penetration for further etching and/or reconstruction. Therefore, the formed, partially reconstructed NiMoO4@NiOOH features an inert core part because of near-surface catalysis properties (Schemes 1D–1F). More importantly, the unique interconnected structure of CR catalysts endows them with ultrastable catalysis and potential AWE applications under realistic conditions.NiMoO4·xH2O nanowires (NWs) grown on conductive nickel foam (NF) were fabricated using a low-temperature hydrothermal method that has demonstrated manufacturing amplification capability (>250 cm2 in one-pot synthesis) toward commercial applications (Figure S1). These nanowires are monocrystalline with a smooth surface, confirmed by transmission electron microscopy (TEM) imaging and the corresponding selected area electron diffraction (SAED) pattern (Figure S2). Its crystal structure was determined via atomic substitution and structure optimization because the powder X-ray diffraction (XRD) pattern is similar to that of the reported analog CoMoO4·0.75H2O. 23 The loss of lattice water (LW) does not induce collapse of the NiMoO4·xH2O framework, and the obvious phase evolution occurs when the calcination temperature is higher than 400°C, which is attributed to loss of coordination water (CW) (Figure S3).Nickel foam cannot maintain its flexibility and toughness for direct non-binder catalysis when heated to more than 600°C. Therefore, a proper calcination temperature of 550°C was chosen to fabricate the anhydrous NiMoO4 nanowire arrays.Cyclic voltammetry (CV) activation was conducted at 0.924–1.724 V versus reversible hydrogen electrode (VRHE), resulting in two different geometric/phase structures of NiMoO4·xH2O and NiMoO4. For NiMoO4·xH2O, CR-NiOOH forms in a stable state during OER (Figure S4). High-angle annular dark-field scanning TEM (HAADF-STEM) imaging clearly demonstrates its morphological characterization, which is represented schematically in an inset in Figure 2 A. Such a nanowire is interconnected by sub-5-nm ultrasmall nanoparticles (NPs), resulting in visible interspaces with ∼5-nm nanopores accessible to electrolytes. All interplanar spacings of NPs within the nanowire are well indexed to the planes of NiOOH (Figure 2B). The exposed interplanar spacings of 0.158, 0.208, 0.213, 0.240, and 0.248 nm can be well assigned to the (220), (210), (111), (011), and (101) planes of orthorhombic NiOOH (Joint Committee on Powder Diffraction Standards [JCPDS] 27-956), respectively. It displays low-crystalline and polycrystalline characteristics confirmed by the SAED pattern (Figure S4D). Only two Raman peaks at 474 and 554 cm−1 belonging to the eg bending and the A1g stretching vibration of Ni-O in NiOOH 24 are observed (Figure S4E), and the O/Ni atomic ratio is 2.05 from the corresponding energy-dispersive X-ray (EDX) spectroscopy spectrum (Figure 2C), which further demonstrates the pure phase of CR-NiOOH. In addition, negligible content (0.1 atomic percentage [at.%]) of the Mo element suggests its absence within the nanowire. The color of CR-NiOOH is black (inset in Figure 2C), which is the typical color of nickel (oxy)hydroxide. Tomographic data were further analyzed to show its three-dimensional structure from multiple perspectives (Figures 2D–2G; Video S1). Electron tomography was conducted at consecutive rotational angles from −60° to 48°, and HAADF-STEM images were collected simultaneously. High homogeneity of ultrasmall NPs and no agglomerated large particles are observed from various rotational angles. Particularly, CR catalysts that featured an ultrasmall NP-interconnected structure with evenly distributed gas-permeable pores were reported first. Video S1. Electron Tomography Video of a Single CR-NiOOH Nanowire Different from NiMoO4·xH2O, NiMoO4 undergoes surface reconstruction occurs after 1-day continuous electro-oxidation, which results in core-shell NiMoO4@NiOOH nanowires. Only an ∼7-nm-thick NiOOH layer forms, and the inner continuous lattice fringes covering dozens of nanometers are indexed to the planes of NiMoO4 (JCPDS 86-0361) (Figure 2H; Figures S5A and S5B). STEM element mapping further confirms the core-shell structure as undetected Mo signals in the shell region (Figure 2I). No diffraction signals are assigned to NiOOH from the SAED pattern (Figure S5C), and Raman spectra of NiMoO4 before and after activation remain almost unchanged (Figure S5D). The undetected Raman/SAED signals of NiOOH are attributed to its ultrathin layer structure. The reported Ni-Mo nitride after activation also occurred the partial reconstruction just like NiMoO4. 25 Ex situ high-resolution TEM (HRTEM) characterizations during potential-controlled CV measurements were carried out to uncover the morphological evolution of NiMoO4·xH2O. An ∼2.66-fold current density at 1.724 VRHE is achieved when comparing the 20th cycle with the initial one, with the area of the closed curves increasing gradually (Figure 3 A). Only 20 cycles with 640-s duration are required to achieve complete reconstruction, indicating a fast reconstruction rate. The redox peak currents are gradually stabilized in the 15th–20th cycles (Figure S6), which indicates that the pre-catalyst reached the steady state. Ex situ electrochemical impedance spectroscopy (EIS) results from the same electrode show that the charge transport resistance (Rct) decreased significantly from 34 to 6.5 ohm (Ω), indicating faster charge transfer of CR-NiOOH (Figure 3B). To visualize the dynamic reconstruction process, the microstructures of intermediates at different stages were analyzed (Figures 3C–3F). After the first CV to 1.23 VRHE, which is the equilibrium potential for OER, the surface of monocrystalline NiMoO4·xH2O becomes amorphous (see Figure S7 for detailed characterization). After an anodic scan to 1.60 VRHE, where the evolution of O2 happens, the distinct three-layer region appears (Figure S8A). The outermost layer consists of ultrasmall low-crystalline NPs with an amorphous transitional interlayer and innermost NiMoO4·xH2O layer. Because of the oxygen-evolving process, the surface Ni species are oxidized to OER-stable high-valence species. The generated (oxy)hydroxide is transformed in situ from surface species rather than by a dissolution-deposition process (Figure S9). Elemental distributions and content analyses depict the uniform distribution of Ni and O elements, whereas Mo shows a gradient distribution and decreases gradually from inner to outer (Figures S8D–S8F). When back at 0.924 VRHE to achieve a complete CV cycle, the reconstruction degree deepens (Figure S10). A rough region with ∼50-nm thickness is clearly visible in TEM images. Benefiting from the continuous co-leaching of crystal water and Mo species, NiMoO4·xH2O is completely reconstructed only after 20-cycle CV. Based on the above results, the geometric/phase evolution is illustrated in Figures 3G–3J. Furthermore, the reconstruction mechanism is also proposed from the point of view of the crystal structure (Figure 3K). The reconstruction processes include bond breakage, co-leaching of crystal water and Mo species, and OH− contact and electro-oxidation, which will be discussed further below. In/ex situ technologies were further utilized to gain insights into the reconstruction mechanism of NiMoO4·xH2O. With the ex situ XRD measurement, disappearance of the representative peaks at ∼27.3° and 29.8°, which belong to NiMoO4·xH2O, demonstrates its structural crack and amorphization (Figure S11A). An in situ electrochemistry-Raman coupling system was applied to understand electrocatalytic reactions in liquid electrolytes because of the high molecular specificity and non-interference of water of Raman signals. 26 , 27 At potentials below 1.324 VRHE, the peak intensities of NiMoO4·xH2O decrease with increased potential, indicating gradual destruction of its crystal structure (Figure 4 A). At 1.424 VRHE, two well-defined bands at 474 and 554 cm−1 appear that belong to NiOOH, and such a potential is attributed to the Ni(II)/Ni(III) oxidation peak (∼1.37 VRHE). 28 Here, the oxide phase forms below 1.424 VRHE, which will be discussed later. The Raman peaks for NiOOH are kept as the applied bias voltage increases, indicating that it serves as an OER-stable catalytic species. Furthermore, the new peak at 900 cm−1 is assigned to MoO4 2− in alkaline solution, 29 which originates from dissolution of Mo species. Contrary to NiMoO4·xH2O, NiMoO4 shows unchanged Raman peaks under the same test conditions (Figure 4B). The undetected Raman peaks of NiOOH are attributed to its thin layer on the NiMoO4 surface, as shown in Figure 2H.It should be noted that the reconstruction process for NiMoO4·xH2O is complete and exhaustive rather than forming core-shell NiMoO4·xH2O@NiOOH as the final product. Generally, the reconstruction process is very common for the reported Ni-based OER catalysts. However, such a process is partial in these compounds, and only a thin layer of NiOOH/Ni(OH)2 forms on their surface. Here the origins of the complete reconstruction of NiMoO4·xH2O are analyzed. After soaking in 1 M KOH, NiMoO4·xH2O was gradually etched by alkali. As shown in Figure 4C, the intensity of three Raman peaks at 800–1,000 cm−1 assigned to the Mo-O-Ni stretching vibration 30 decreases and almost disappears after soaking for 1 h, suggesting breakage of the Mo-O-Ni bond. The newly formed peak at 900 cm−1 is assigned to MoO4 2−. The peak at 355 cm−1 assigned to MoO4 vibration shifts to a lower wave number after soaking for 800 s, which may be associated with its vibrational environment. The Mo species are dissolved during reconstruction of NiMoO4·xH2O (Figures S12A–S12C), and such a phenomenon also happens to the Ni-Mo nitride OER electrocatalyst reported by Yin et al.. 25 The NiMoO4·xH2O nanowire evolves to a nanosheet-assembled nanowire morphology after alkali etching (Figure S12D), and the etching reaction is further demonstrated by ex situ XRD patterns (Figure 4D). As a result, the washed product after soaking is Ni(OH)2, whereas the K2MoO4 phase is detected for the product without washing. Therefore, when NiMoO4·xH2O serves as pre-catalyst measured in 1 M KOH, the spontaneous etching reaction happens simultaneously. The multicomponent co-leaching results in a loose reconstruction layer and triggers its complete reconstruction.As discussed above, the several-nanometer-thick reconstruction layer is observed for the NiMoO4 pre-catalyst. Because the reconstruction reaction involves interaction with an alkaline solution, the limited reconstruction depth could be attributed to limited electrolyte penetration. A high-magnification STEM image of NiMoO4 after soaking in 1 M KOH verifies our speculation because it shows the dense etching layer of ∼3 nm (Figure 4E). However, for NiMoO4·xH2O, the fast etching rate makes the surface loose (Figure 4F), which facilitates electrolyte penetration for further etching. Therefore, we guess that the intrinsic properties of materials etched by alkali, which results in different etching structures, either loose or dense, are responsible for the two different results above. Even for the microns of Fe-doped cobalt molybdate hydrate, the loose etching structure enables its complete etching (Figure S13). It is not only difficult for alkaline solution to pass through the dense surface layer but also difficult for solution to pass through the crystal structure of the surface layer. Density functional theory (DFT) calculations confirm this difficulty by showing the high-energy barrier of 2.2 electron volt (eV) for OH− to pass through the NiOOH layer (Figure 4G). Besides, for reported pre-catalysts such as Ni2P 11 and Co4N, 14 their post-OER products show the dense reconstruction layer, which supports our points.CR catalysts feature an ultrasmall NPs-interconnected structure with evenly distributed gas-permeable pores, and the key is that etching and electro-oxidation reconstruction happen simultaneously. If NiMoO4·xH2O is soaked in 1 M KOH prior to electro-oxidation, CR-NiOOH with a nanosheet-assembled nanowire structure can be obtained (Figures S14A and S4B, denoted CR-NiOOH∗). This is because the etching reaction induces formation of monocrystalline/high-crystalline Ni(OH)2 nanosheets (Figures S14C and S14D), which further evolve to (oxy)hydroxide during the subsequent electro-oxidation. Because the smaller-size catalyst is endowed with more exposed active sites, CR-NiOOH shows much better OER catalysis (Figures S14E and S14F). Ni(OH)2 nanosheet arrays grown on nickel foam were also prepared but with poor OER activity (Figure S15), suggesting the effectiveness of structure engineering for better catalysis. To explain the unique structure of CR-NiOOH, identifying the formed amorphous intermediates shown in Figure 3D is important. The voltage range below the theoretical decomposition voltage is analyzed, which is helpful for understanding the effects of voltage bias during alkali etching. After CV at 0.924–1.224 VRHE, the nanowires mainly consist of ∼5-nm polycrystalline NiO NPs (Figure S16F). Furthermore, the Raman peak at 460 cm–1 is assigned to the Ni-O stretching mode of NiO, and other peaks may be assigned to molybdenum oxides, indicating phase separation and formation of amorphous oxide intermediates (Figures S16G and S16H). These results suggest that the simultaneous etching-reconstruction processes facilitate formation of an ultrasmall NP-interconnected structure with nanopores.The prerequisites for forming ultrasmall NP-interconnected CR catalysts can be summarized as follows: (1) achieving complete collapse of bulk materials, (2) promoting deep penetration of electrolytes for inner electro-oxidation as the loose reconstruction layer dominates, and (3) simultaneous reconstruction and etching. To demonstrate the universality of these results, other bulk alkali-sensitive pre-catalysts, such as NiMoO4·xH2O nanosheets, Ni-BTC (BTC = 1,3,5-benzene tricarboxylate) metal organic framework microspheres, CoMoO4·0.75H2O nanowires, and Co(CO3)0.5(OH)·0.11H2O nanowires, were also investigated (Figure S17). All of them can completely evolve to their corresponding hydroxides after alkali etching, which guarantees complete reconstruction of CR catalysts. Consequently, the CR catalysts of the (oxy)hydroxide phase are obtained with the ultrasmall NP-interconnected multilevel structure. Therefore, the complete reconstruction mechanism can be widely extended to various bulk alkali-sensitive pre-catalysts.Using a standard three-electrode system, NiMoO4·xH2O NWs/NF was directly employed as a binder-free working electrode to achieve activation and acquire the (oxy)hydroxide arrays. Before performance evaluation, the purity of KOH solution was examined. KOH reagent purity (Fe content of < 0.001%) and undetected Fe 2p XPS signals using an Mg source confirmed the absence of Fe impurities in the solution used. 19 CV curves of NiMoO4·xH2O ink coated on the carbon cloth for the initial cycles are provided in Figure 5 A. The small shift (∼20 mV) of the Ni(II)/Ni(III) oxidation peak is much below 50 mV, negating the influence of iron according to Klaus et al. 31 The chronopotentiometric response of fresh NiMoO4·xH2O shows a gradually decreased potential, but the potentials of NiMoO4 are almost unchanged, indicating fast reconstruction on the NiMoO4·xH2O surface during activation (Figure S18A). The simplex nickel foam shows the increased potentials, implying that the activity enhancement of NiMoO4·xH2O is independent of the substrate. After activation, the obtained CR-NiOOH was tested in 1 M Fe-free KOH, whereas surface-reconstructed NiMoO4@NiOOH and commercial IrO2/C served as control samples. Linear sweep voltammetry (LSV) curves were normalized by geometric area, electrochemically active surface area (ECSA), and catalyst mass, respectively (Figures S18B–S18D). CR-NiOOH has much higher OER activity than NiMoO4@NiOOH. For example, CR-NiOOH requires the lowest overpotential at 10 mA cm−2 (η10 of 278.2 mV), which is much lower than that of NiMoO4@NiOOH (353.6 mV). To reveal the advantages of the CR catalyst for the OER, the mass activity-related overpotential η10, m (η10, m is calculated as the ratio of η10 and mass of the loading catalysts) is compared. The η10, m of CR-NiOOH (289 mV mg−1) is even lower than that of the commercial IrO2/C (325.9 mV mg−1), indicating that it can serve as a superior catalyst. However, the η10, m value of NiMoO4@NiOOH is as high as 393.2 mV mg−1. In addition to a high-mass-activity OER, the CR catalyst also possesses the advantage of 97.4% Faradaic efficiency (Figure S18E). The 2.6% loss may be due to the dissolved gas in the solution and gas adsorbed on the electrode. 32 The reasons why CR-NiOOH is superior to NiMoO4@NiOOH were analyzed. First, CR-NiOOH possesses more OER-active species, whereas an ∼7-nm-thin layer of NiOOH on NiMoO4 serves as a catalytic species; thus, the former can achieve a higher mass activity toward surface-catalyzed reactions. Our as-prepared NiOOH is reconstructed completely and in situ from NiMoO4·xH2O, and the whole catalyst could perform the catalytic reactions with abundant catalytic sites. The higher intensity of the oxidation peak at ∼1.37 V for CR-NiOOH (Figures S18B–S18D) suggests larger amounts of active phases as well. 21 Second, CR-NiOOH possesses a sub-5-nm NP-interconnected structure. This unique multilevel structure is endowed with numerous pores accessible to electrolytes and conductive to gas diffusion. Although NiOOH has been shown to not be very active in the OER, 31 the low-crystalline characteristics and abundant defects in catalysts have been reported to accelerate the OER kinetics. 33 Therefore, the newly developed CR-NiOOH shows great potential as an IrO2-substituted oxygen evolving system. More importantly, CR-NiOOH exhibits a negligible change in potential after 1,350 h in the durability test, which demonstrates its potential for ultrastable electrolytic applications (Figure 5B). Electron microscope characterization after the durability test displayed an unchanged morphology and retained microstructure, suggesting a robust and stable nature of NiOOH in the face of corrosion (Figure S19). Its excellent stability is mainly attributed to the robust (oxy)hydroxide NP-interconnected structure with evenly distributed gas-permeable pores. To show the multiple applications of CR-NiOOH, the urea oxidation reaction (UOR) was also measured, which is essential for urea electrolysis and also has sluggish kinetics. 34 As a result, it shows decreased overpotential of 106 mV at 10 mA mg−1 compared with that of NiMoO4@NiOOH and good durability for 110 h at 0.48 VHg/HgO (Figure S20). In addition, CR-NiOOH can also provide stable OER catalysis at a high temperature of 52.4°C for 120 h, with an overpotential increase of only ∼10 mV (Figure S21).Fe impurities in the testing solution greatly enhance the OER activity of NiOOH, 35 which is attributed to formation of highly active nickel-iron (oxy)hydroxide. Here, the advanced iron-incorporated nickel (oxy)hydroxide (denoted Fe-NiOOH) was fabricated in situ in 1 M KOH solution containing a trace of Fe by adding an iron source. To achieve 10 mg cm−2, a small overpotential of 248 mV is required (inset in Figure 5C). The enhanced OER catalysis is attributed to its electronic tuning by Fe incorporation and enhanced electron conductivity. During a 20-day long-term chronopotentiometry measurement of Fe-NiOOH, the potential is almost unchanged, with a small potential change of only 10 mV (Figure 5C). These results confirm the well-retained ultrastable property after optimizing its electronic structure. To demonstrate the water electrolysis application, our reported heterostructured MoO2-Ni NWs/NF were chosen as hydrogen evolution reaction (HER) electrodes. 36 As shown in Figure 5D, the MoO2-Ni NWs/NF featured by interface catalysis exhibits excellent HER activity with a small decay of 0.155 mV h−1. When pairing Fe-NiOOH with MoO2-Ni arrays in a two-electrode alkaline water electrolyzer, the electrolyzer delivers 10 mA cm−2 at 1.48 V for over 580 h under fast-moving fluid condition produced by rapid stirring of fresh magneton (Figure 5E). Its electrolysis durability is superior to that in previous reports (Figure 5F; Table S3 ). These results highlight the potential of CR catalysts for ultrastable and high-efficiency catalytic applications.Most industrial alkaline electrolyzers are operated in a strong alkaline KOH solution (>20 wt %; Figure 5G). Therefore, we evaluated the half-reaction catalysis and AWE performance based on the abovementioned array system in a two-electrode cell in 30 wt % KOH, as illustrated schematically in Figure 5H. Under such harsh operating conditions, the Fe-NiOOH anode still performs stable OER catalysis at ∼0.5 VHg/HgO for over 210 h with activity decay of 0.075 mV h−1 (Figure 5I). Fe-NiOOH maintains its structural and component characterization after testing under such harsh conditions (Figure S22). For the cathodic MoO2-Ni array, it can catalyze HER for 300 h with activity decay of 0.21 mV h−1 (Figure S23). Here, the 10 μL solution containing 0.6 mg Fe(NO3)3·9H2O was also added to ensure the same test environment as for OER testing. As expected, the Fe-NiOOH//MoO2-Ni array system operated for 260 h (Figure 5J), indicating its potential practical applications. Performance evaluation of reported catalysts in industrial-concentration alkali has also been provided for comparison (Table S4).For the NiMoO4 pre-catalyst, the alkali etching rate on its surface is slow in 1 M KOH, and a dense reconstruction layer forms via surface reconstruction. This leads to quick termination of reconstruction. However, under harsh conditions of 30 wt % KOH, alkali etching is accelerated, and leaching of Mo species is promoted. This could result in a porous structure of the reconstruction layer and promote deep reconstruction in concurrent electro-oxidation processes. Therefore, NiMoO4 can also be completely reconstructed to (oxy)hydroxide in 30 wt % KOH, which is reflected by decreased potentials because of the enhanced number of active species (Figure S24). This result suggests that some pre-catalysts (such as phosphides, nitrides, and chalcogenides) may also be completely reconstructed to stable catalytic species in industrial alkali. Therefore, understanding reconstruction chemistry and evaluating performance under realistic conditions are necessary and meaningful, especially for pre-catalysts involved in reconstruction.In summary, we discovered different reconstruction results for hydrate/anhydrous molybdate pre-catalysts at oxidized potentials in 1 M KOH; i.e., complete/surface reconstruction. Such a difference depends on the microstructure characteristics (dense or loose) of the reconstructed layer, caused by different leaching species from pre-catalysts. The proposed reconstruction mechanism can be extended to other bulk alkali-sensitive pre-catalysts, resulting in a series of electrochemically formed CR catalysts. These CR catalysts display a unique structure interconnected by ultrasmall NPs endowed with abundant defects and pores accessible to electrolytes. Such an interconnected structure allows CR-NiOOH to perform ultrastable catalysis for 1,350 h. After iron incorporation, the obtained Fe-NiOOH exhibits remained structure and ultrastable catalysis. The coupled Fe-NiOOH and MoO2-Ni system was confirmed with excellent water electrolysis performance in 1 M and 30 wt % KOH. Furthermore, different reconstruction results of anhydrous NiMoO4 in industrial alkali were obtained, suggesting the importance of evaluating catalysts under realistic conditions. This work highlights fundamental reconstruction chemistry, CR catalysts with a unique structure and ultrastable catalytic properties, and different reconstruction phenomena in low-concentration and industrial alkali.Requests for further information and resources and reagents can be directed to the Lead Contact, Prof. Liqiang Mai (mlq518@whut.edu.cn).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 the Supplemental Information. All other data are available from the Lead Contact upon reasonable request.First, NiMoO4·xH2O nanowire/nanosheet arrays on nickel foam were fabricated following previous reports. 36 , 37 Next, the anodic oxidation process on NiMoO4·xH2O precursor was carried out in the standard three-electrode system in 1 M KOH and operated on an CHI760E electrochemical analyzer. A piece of NiMoO4·xH2O served as a working electrode, and the graphite rod and the unused Hg/HgO electrode served as a counterelectrode and a reference electrode, respectively. After carrying out CV tests in 0.924–1.724 VRHE at a scan rate of 50 mV s−1 for 30 cycles, black CR-NiOOH nanowire/nanosheet arrays were obtained with a mass loading of ∼1.2 mg cm−2. In addition, after calcination of NiMoO4·xH2O nanowire arrays at 550°C, NiMoO4 nanowire arrays with a mass loading of ∼2.6 mg cm−2 were fabricated.24 mmol Ni(NO3)2·6H2O and 24 mmol Na2MoO4·2H2O were dissolved into 360 mL deionized water and formed a transparent green solution. The solution was then transferred into a 500-mL Teflon-lined autoclave, and four pieces of nickel foam were added. After reaction at 120°C for 6 h, the nickel foam samples were taken out, washed, and vacuum dried, and NiMoO4·xH2O arrays were obtained.First, CoMoO4·0.75H2O nanowires, 38 Co(CO3)0.5(OH)·0.11H2O nanowires, 39 and Ni-BTC microspheres 40 were fabricated according to previous reports. Next, these compounds were transformed into the corresponding CR catalysts after CV activation at 0.924–1.724 VRHE at 50 mV s−1 for more than 30 cycles.Scanning electron microscope (SEM) images were collected with a JEOL-7100F microscope at an acceleration voltage range of 15–25 kV. Microscopy images, SAED patterns, elemental mapping, and linear scanning analysis were collected on JEM-2100F and Thermo Fisher Scientific Titan G260-300 scanning/transmission electron microscopes. Ex situ XRD patterns were obtained using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. In situ XRD patterns for alkali soaking experiments were recorded using a Bruker D2 Phaser X-ray diffractometer. Raman spectra and in situ Raman spectra for alkali soaking experiments were recorded using a HORIBA HR EVO Raman system. XPS measurements were carried out using an ESCALAB 250Xi instrument. Element content was determined using an inductively coupled plasma atomic emission spectrometer (ICP-AES) on a PerkinElmer Optima 4300DV spectrometer.All electrochemical measurements were carried out in fresh KOH (1 M or 30 wt %) on a CHI 760E electrochemical station using a standard three-electrode system. The test samples grown on the substrates (nickel foam or carbon cloth) served as a working electrode, and an unused Hg/HgO electrode was applied as a reference electrode; a graphite rod served as a counterelectrode. EIS was recorded in a frequency range of 0.01–100,000 Hz. Homogeneous ink was prepared by dispersing 8 mg commercial IrO2/C and 2 mg Vulcan XC-72R in 250 μL deionized water, 700 μL isopropyl alcohol, and 50 μL Nafion solution (5 wt %). Next, 9 μL ink was coated on glassy carbon with an area of 0.07069 cm2 for catalytic tests. All chronopotentiometric measurements were carried out by applying a constant current density of 10 mA cm−2. In 1 M KOH, the iR-corrected potentials were referenced to RHE based on the following equation: E RHE = E Hg / HgO + 0.059 × pH + E Hg / HgO o − iR. In situ electrochemistry-Raman measurements were recorded using a HORIBA HR EVO Raman system (633 nm laser) and an electrochemical workstation (CHI760E). The potential-dependent in situ Raman spectra were recorded with 150-s duration and a 50-s interval, and the LSV measurements were carried out in 0.924–1.824 VRHE at 0.25 mV s−1 in 1 M KOH during in situ Raman testing. Time-dependent in situ Raman spectra were recorded at 10 mA cm−2 with an interval time of 150 s.All DFT simulations were performed using Vienna ab initio simulation package (VASP) software. 41 The exchange-correlation interactions were described by generalized gradient approximation (GGA) 42 within the Perdew-Burke-Ernzerhof (PBE) function. 43 A plane wave basis set was adopted with a cutoff of 500 eV. Gaussian-type smearing with an energy window of 0.05 eV was used for optimization and frequency calculation. The energy convergence tolerance was 0.01 millielectron volt (meV). The force tolerance for the optimization task was 0.05 eV/Å. All calculations were performed with spin unrestricted, and initial magnetic moments of 2 Bohr magneton (μB) for Ni, 1 μB for K, and 0 μB for O and H were set. 1 × 1 × 1 K point was sampled. The GGA with Hubbard U parameter (GGA+U) method for Ni species was adopted with an Hubbard effective parameter (U-J) value of 6.6 eV, the same as in previous reports. 44 The DFT-D3 method was adopted for all calculations. The linear mixing parameter was set to 0.06, and the cutoff wave vector for the Kerker mixing scheme was set to 0.0001 to make electron state converge more stable than default settings.This work was supported by the National Natural Science Foundation of China (51521001 and 21890751), the National Key Research and Development Program of China (2016YFA0202603), the National Innovation and Entrepreneurship Training Program for College Students (WUT: 20191049701034), and the Foshan Xianhu Laboratory of the Advanced Energy Science and Technology Guangdong Laboratory (XHT2020-003). The S/TEM work was performed at the Nanostructure Research Center (NRC), supported by the Fundamental Research Funds for the Central Universities (WUT: 2019III012GX and 2020III002GX).X. Liu and L.M. conceived the idea. X. Liu, J.M., L.M., and R.G. designed the experiments, analyzed the results, and wrote the manuscript. X. Liu, B.W., and R.G. performed the experiments and analyzed the results. X. Liu, K.N., and X.W. performed the DFT computations and theoretical analyses. D.Z., J.W., and L.M. provided helpful suggestions and refined the manuscript. All authors read and commented on the manuscript and approved the final version of the manuscript.A patent application related to this work has been submitted in China (application number 201811648828.2).Supplemental Information can be found online at https://doi.org/10.1016/j.xcrp.2020.100241. Document S1. Figures S1–S24 and Tables S1–S4 Document S2. Article plus Supplemental Information

Fundamental investigations of reconstruction of oxygen evolution reaction (OER) pre-catalysts and performance evaluation under realistic conditions are vital for practical water electrolysis. Here, we capture dynamic reconstruction, including the geometric/phase structure, of hydrate molybdates at oxidized potentials. Etching-reconstruction engineering endows the formed NiOOH with a sub-5-nm particle-interconnected structure, as revealed by multi-angle electron tomography. The key to complete reconstruction is the multicomponent co-leaching-induced loose reconstruction layer, conductive to solution penetration and mass transport. This unique structure avoids particle agglomeration in catalysis and promotes complete exploitation of the catalyst with 1,350 h of durability to meet industrial requirements. Upon addition of iron during reconstruction, mainstream Fe-NiOOH with a retained structure forms. Coupled with MoO2-Ni arrays in a membrane-free and two-electrode cell, it achieves stable electrolysis in industrial-concentration KOH for 260 h. This work highlights the reconstruction chemistry of hydrate oxygen-evolving systems and their performance evaluation under industrial conditions.

The environmental pollution brought by automobiles emission is sharply increasing. The exhaust gas generated by the combustion of sulfur compounds in gasoline and diesel is a major source of pollution to acid rain and haze weather. To produce ultra-low sulfur transportation oil, one of the key measures is to develop efficient hydrodesulfurization catalyst for sulfur removal from the fuels (Sun and Prins, 2010; Escobar et al., 2018; Vatutina et al., 2016).Traditional crystalline alumina has excellent mechanical properties, suitable acid properties, and low price, especially the most widely used γ-Al2O3 (Moser et al., 2010; Santolalla-Vargas et al., 2015). But the specific surface areas of these alumina are usually less than 300 m2/g, and the pore size distributions are also relatively wide range (generally 3–15 nm). In recent years, researchers have focused on the synthesis of ordered mesoporous alumina materials with specific surface area greater than 300 m2/g, ordered pore channels and narrow pore distribution. The ordered mesoporous alumina (OMA) material is expected to be widely used in the fields of catalysis, adsorption and separation (Yuan et al., 2008; Wu et al., 2011; Liu et al., 2016). At present, the research on application of ordered mesoporous alumina is still in the experimental stage and the synthesis method is still complex and difficult. Thus, it is of great significance to develop and study new ordered mesoporous alumina material.It is generally believed that the interaction between the metal and the support plays an important role in sulfidation degree of the active component (Hu et al., 2020). The strong interaction between active metal Mo (W) and the traditional γ-Al2O3 support would result in the low sulfidation degree of MoS2 sulfide phase. Moreover, the metal Ni and γ-Al2O3 is easy to form the non-active spinel phase, leading to the low utilization rate of metal Ni and the formation of undesired active type I "Ni–Mo–S" active phase (Van Veen et al., 1993). Therefore, optimizing the active-support interaction is the focus of researchers to improve the catalytic activity of the bimetallic catalysts in the HDS reaction.Chelating agents, such as citric acid (CA) (Zhang et al., 2017; Li et al., 2011), ethylenediaminetetraacetic acid (EDTA) (Ortega-Domínguez et al., 2017; Badoga et al., 2012), aminotriacetic acid (NTA) (Lélias et al., 2009), and cyclohexanediaminetetraacetic acid (Cy-DTA) (Hiroshima et al., 1997) are commonly used as additives during the impregnation process to modify the active metal species. The hydrogenating catalysts prepared by using chelating agents have displayed excellent activity and stability, and the effects of complexing agents on the hydrogenating catalysts include the following three aspects: 1) To delay the vulcanization of the auxiliary metal. 2) To weaken the strong interaction between the active component and the support, thus contributing to form more highly active type II "Ni–Mo–S" active phases; 3) To adjust the dispersions of the active phases on the surface of the catalyst support. The MoS2 phase with a suitable sheet length and stacking number can effectively improve the HDS activity of the catalysts (Cattaneo et al., 2001; Nikulshin et al., 2014; Asadi et al., 2019).Based on above information, this research aims at synthesizing ordered mesoporous alumina (OMA) with high specific surface area, concentrated pore size distribution and outstanding hydrodesulfurization performance. The NiMoE/OMA series catalysts were prepared by adding different amounts of chelating agent EDTA. Dibenzothiophene (DBT) was served as the model reactant to evaluate the hydrodesulfurization activity of NiMoE/OMA series catalysts.Hydrothermal precipitation method is used to prepare ordered mesoporous alumina materials and the specific preparation steps are listed as follows: a certain amount of template PEG was added into the 0.6 mol/L aluminum nitrate solution. Then, 1.2 mol/L ammonium carbonate solution was dropwise added to the above mixed solution at a stirring rate of 400 r/min. Continue to stir for 3 h and then age for 6 h in a 70 °C water bath to obtain a white sol. After filtering, it needs to be dried at 70 °C for 12 h and calcined in a N2 atmosphere at 350 °C for 3 h, then heated up to 550 °C at a rate of 3 °C/min for 6 h. Finally, the series ordered mesoporous alumina materials were obtained through changing the ratio of PEG/Al (0.05, 0.1, 0.2), which were named as OMA-0.05, OMA-0.1 and OMA-0.2, respectively.The supported NiMo catalyst precursor was prepared by incipient wetness impregnation method with the mass fraction loadings of 3.5 wt% NiO and 15 wt% MoO3 respectively. NiMo/OMA catalyst precursor was obtained by dried at 80 °C for 5 h and calcined at 550 °C for 6 h.The modified catalysts were prepared the same as above, except that different proportions of EDTA chelating agent are added during the impregnation process. The synthesized catalysts were named as NiMoE(X)/OMA, where X represents the ratio of EDTA/Ni.X-ray powder diffraction (XRD) was used to perform phase analysis on the samples by Shimadzu X-6000 X-ray powder diffractometer (Cu Kα radiation, 40 kV tube pressure, and 30 mA tube current). Scanning range of 2θ is 0.5°–5°; wide angle range of 2θ is 5°–80°.Fourier Infrared Spectroscopy (FTIR) was used to analyze the skeleton structure of the sample by the DIGILAB FTS-3000 Fourier Infrared Spectrometer of Tianmei Technology Company. The sample and KBr were mixed and compressed at a mass ratio of 1: 100 with a resolution of 2 cm−1. Wavelength range: 400–1200 cm−1.Pyridine adsorption infrared spectrometer (Py-FTIR) was used to carry out qualitative and quantitative analysis of acidity in the samples by Digilab FT-IR from Bole Pacific Company. A vacuum (1 × 10−3 Pa) was used to purify the sample for 2 h at 350 °C, and then cooled to room temperature.Scanning Electron Microscope (SEM) was used to analyze the micro-morphology of the samples by the Quanta 200F scanning electron microscope of the Dutch company FEI.High resolution transmission electron microscopy (HRTEM) was used to calculate the dispersity, average length (L av) and stack number (N av) of the active phases on the sulfided catalysts by the JEM 2100 transmission electron microscope of Japan JEOL Company. The stacks number and average length of the MoS2 active phases on each catalyst were statistically calculated by at least 300 sticks according to the equations (a) and (b) (López-Benítez et al., 2017): (a) L av = ∑ i = 1 n n i l i ∑ i = 1 n n i (b) N av = ∑ i = 1 n n i N i ∑ i = 1 n n i where l i represents the length of MoS2 microstrips, n i is the numbers of MoS2 microstrips, N i refers to the layers stack of MoS2 active phases.The fraction of Mo atoms located on the edges of MoS2 clusters, which was denoted as f Mo, was calculated using Mototal (the total number of Mo atoms) and Moedge (the number of Mo atoms located on the edges of MoS2 particles). The values of f Mo were calculated by the equations (c) and (d). (c) f M o = M o e d g e M o t o t a l = ∑ i = 1 t ( 6 n i − 6 ) ∑ i = 1 t ( 3 n i 2 − 3 n i + 1 ) (d) n i = L 6.4 + 0.5 Where f Mo represents the ratio of the number of Mo atoms at the edge to the total number of Mo atoms on the active phase, n i refers to the number of Mo atoms on the side of the MoS2 crystal strips.Low temperature N2-adsorption and desorption experiment (BET) was used to determine the specific surface area, pore volume and pore size distribution of the samples by using Micromeritics ASAP 2010 adsorption analyzer. The sample was pretreated at 100 °C for 1 h under vacuum at a pressure of 15 μm Hg, and purified and degassed at 350 °C for 3 h, and then subjected to static adsorption analysis at −196 °C with N2.Raman Spectrum was used to study the state of the metal species on the surface of the sample by using Renishaw's Raman spectrometer. The laser light source has a wavelength of 325 nm and a power of 8 MW.X-ray Photoelectron Spectroscopy (XPS) was used to analyze the surface Mo species of the catalyst samples reduced by CS2. The XPSPEAK 4.1 software was used to Gaussian fit the Mo 3d XPS spectrum, and the relative content of Mo species in different valence states on the catalyst surface was calculated. The degree of sulfuration of Mo species and the utilization rate of Ni metal were calculated according to formula (e) and (f) respectively. (e) M o sulfurization = Mo 4 + Mo 4 + + Mo 5 + + Mo 6 + (f) N i NiMoS = NiMoS NiMoS + NiS x + NiO 1g of sieved 40–60 mesh catalyst is charged into the constant temperature section of the reactor. The two ends of the bed are filled with 20–40 mesh quartz sands, and each bed is separated by quartz cottons. 3.0 wt% CS2 cyclohexane solution was used to presulfide the oxidation state of the series catalysts. Firstly, the flow rate of the presulfiding solution was maintained at 60 mL h−1 for 20 min, and then adjusted to 5 mL h−1. After 4 h of presulfiding, the device was washed three times with pure cyclohexane, and the remaining presulfiding solution was washed out. After pretreatment, a 500 ppm DBT cyclohexane solution was passed as the raw material under the conditions of the hydrogen pressure of 4 MPa, the temperature of 340 °C, and the volume ratio of H2/Oil of 200. Fig. 1 (A) and (B) are the small-angle XRD and wide-angle XRD spectra of OMA materials prepared with different PEG/Al ratios respectively. As shown in Fig. 1 (A), a relatively strong diffraction peak appears in the range of 0.5-1°, indicating that the pore structure of the synthesized alumina material is ordered. It can be seen from Fig. 1(B) that there are no characteristic peaks of crystalline alumina, confirming that the synthesized alumina is an amorphous alumina material. Fig. 2 shows the pore size distribution (A) and N2 adsorption-desorption isotherm (B) of the alumina material under different PEG/Al ratios. It can be seen from Fig. 2(A) that the pore sizes of all the synthesized alumina materials are concentrated in the range of 5–10 nm, proving the relatively ordered pore size distribution of the support. Fig. 2(B) shows that the N2 adsorption-desorption isotherm of the synthesized OMA material belongs to a characteristic type IV adsorption equilibrium curve with a H2 hysteresis loop in the range of P/P0 = 0.5∼0.9. Table 1 summarizes the textural properties of OMA the series materials, among which the OMA-0.1 material shows an excellent pore structure with high specific surface area (328 m2 g−1), large pore volume 0.74 (cm3·g−1) and big average pore size (8.1 nm). Therefore, OMA material with PEG/Al ratio of 0.1 is chosen as the ideal support for hydrodesulfurization catalysts. Fig. 3 shows the wide-angle XRD patterns of NiMoE/OMA catalysts modified with different EDTA/Ni molar ratios. As can be seen from Fig. 3, NiMoE/OMA the series catalysts show a characteristic peak at 2θ = 67.30°, indicating that the support exists in amorphous structure rather than crystalline alumina. Moreover, no peaks of metal oxides in the spectrum are detected, which indicates that there are no large particle aggregations of Mo and Ni species. Fig. 4 shows the Raman diagrams of NiMoE/OMA oxidized state catalyst with different EDTA/Ni molar ratios. It can be seen from Fig. 4 that the diffraction peaks at 339 cm−1 and 850 cm−1 are weak, which are attributed to the characteristic bending vibration peak of the MoO bond of MoO4 2− tetrahedral species. The MoO4 2− tetrahedral species are the products of the strong interaction between the support and the active metal, which are difficult to be vulcanized into the MoS2 active phases in the pre-vulcanization stage (Xiao et al., 2018). In Fig. 4, the series NiMoE/OMA catalysts show obvious wide peaks at 953 cm−1 attributed to the stretching vibration of the MoO bond of two-dimensional polymer Mo7O24 6− species, which is easy to be vulcanized into MoS2 active phase in the pre-vulcanization stage (Parola et al., 2002; Wang et al., 2015). Therefore, the Raman result shows that the NiMoE/OMA catalysts modified by EDTA possess an optimal support-metal interaction force. The characteristic peak of the modified NiMoE(1.0)/OMA catalyst at 953 cm−1 is stronger than that of other catalysts, meaning that the MoS2 active phases are easier to be formed on the NiMoE(1.0)/OMA catalyst when a moderate amount of EDTA is added.To analyze the acid types and acid amounts, the Py-TR spectra of NiMoE/OMA series catalyst is shown in Fig. S1. Fig. S1(A) is the Py-TR spectra at 200 °C, representing the total acid amount, while Py-TR spectra at 350 °C represent the strong and medium strong acids in Fig. S1(B) (Barzetti et al., 1996; Zhang et al., 2008). It can be found that the diffraction peak intensity of EDTA-modified NiMoE/OMA series catalysts increases firstly and then decreases followingly as the increase molar ratio of the EDTA/Ni. Table 2 shows the calculated acid amounts of NiMoE/OMA series catalysts. The order of the quantities of total acidities and the medium and strong acidities is NiMoE(1.0)/OMA > NiMoE(1.5)/OMA > NiMoE(0.5)/OMA > NiMoE(0.2)/OMA > NiMo/OMA. Fig. S2 shows the Mo 3d XPS spectra of the series sulfided NiMoE/OMA catalysts. According to the peak splitting data in Fig. S2, the binding energy peaks at 228.9 ± 0.1 eV and 232.0 ± 0.1 eV are assigned to the Mo 3d5/2 and Mo 3d3/2 of Mo4+, and the binding energy peaks at 230.5 ± 0.1 eV and 233.6 ± 0.1 eV are ascribed to the Mo 3d5/2 and Mo 3d3/2 of Mo5+, whereas the binding energies of Mo 3d5/2 and Mo 3d3/2 of Mo6+ are 232.5 ± 0.1 eV and 235.6 ± 0.1 eV, respectively (José et al., 2002). According to Table 3 , the sulfidation degree of the NiMoE/OMA catalysts modified by EDTA is all higher than 0.6 compared to the unmodified NiMo/OMA catalyst. With the increasing molar ratios of EDTA/Ni, the sulfidation degrees of the corresponding catalysts are also increased (Li et al., 2019). Fig. S3 shows the spectra of Ni 2p XPS of the series sulfided catalysts. In Fig. S3, the existence forms of Ni metals in the support are mainly NiMoS, NiSx and NiO compounds, among which the corresponding binding energies are 856.2 ± 0.1eV, the 853.2 ± 0.2eV and 861.2 ± 0.2eV respectively (Lai et al., 2013). It can be found that Ni metals are mainly formed as NiMoS active phases, whereas the peak of NiSx is almost non-existent over NiMoE(0.5)/OMA and NiMoE(1.0)/OMA catalysts. As shown in Table 4 , the proportion of NiMoS phase increases from 0.54 over NiMo/OMA to 0.72 over NiMoE(1.5)/OMA, indicating that the addition of EDTA could significantly promote the formation of NiMoS active phases.HRTEM images of NiMoE/OMA sulfided catalysts and the distributions of stacking layers of MoS2 active phases are shown in Fig. 5 . The average length (L av), average number of layers (N av) and dispersion (f Mo) of MoS2 stacks are calculated and listed in Table 5 . With the increase of EDTA/Ni ratios, the average lengths of NiMoE/OMA series catalysts decreases from 3.5 nm of NiMoE(0.2)/OMA to 3.2 nm of NiMoE(1.0)/OMA and then increases to 3.3 nm of NiMoE(1.5)/OMA. Among all the catalysts, the NiMoE(1.0)/OMA catalyst exhibits short average length (3.2 nm) and suitable stacking layers (2.8). The f Mo value is closely related to the layer number and the average length of active phases, and the follows trend of NiMoE(1.0)/OMA > NiMoE(0.5)/OMA > NiMoE(0.2)/OMA > NiMoE(1.5)/OMA. Fig. 6 shows the DBT HDS performance of EDTA modified catalysts at different WHSVs. The HDS activity of NiMoE/OMA catalysts show a gradually increasing trend with the decrease of WHSVs from 100 h−1 to 20 h−1. The DBT HDS conversions of the series catalysts are in order of NiMoE(1.0)/OMA > NiMoE(1.5)/OMA > NiMoE(0.5)/OMA > NiMoE(0.2)/OMA, among which the highest desulfurization rate of NiMoE(1.0)/OMA catalyst is 97.7% at 20 h−1 WHSV. Compared with the unmodified NiMo/OMA catalyst, NiMoE/OMA catalysts modified with EDTA display higher hydrodesulfurization activities at each WHSVs.The cyclohexylbenzene (CHB) and biphenyl (BP) are the main products of DBT HDS detected from GC-MS, which are the main products of HYD and DDS reaction route respectively [24]. The product distributions of the series NiMoE/OMA catalysts are shown in Fig. 7 . As can be seen, the CHB selectivity over NiMoE/OMA catalysts increases as the ratio of EDTA/Ni increases from 0.0 to 1.0, whereas the BP selectivity decreases as the increasing amount of EDTA. Therefore, the appropriate addition of EDTA has great influence on the promotion of HYD reaction route of DBT HDS to some extent.Through homogeneous precipitation method, the synthesized ordered mesoporous alumina (OMA) has ordered mesoporous channels (8.1 nm), the high surface area (328 m2 g−1) and the large pore volume 0.74 (cm3·g−1), which could eliminate the diffusion resistance of the reactants and products through the mass transfer process and increase the accessibility of the reactants to the active metals. NiMoE/OMA catalysts synthesized by EDTA post-modification method show higher HDS activity of DBT compound compared with the unmodified NiMo/OMA catalyst. The HDS evaluation of DBT compound shows that as the molar ratios of EDTA/Ni increase, the DBT hydrodesulfurization conversions of the series catalysts increase at first and then decrease gradually, among which NiMoE(1.0)/OMA catalyst displays high DBT hydrodesulfurization activity, reaching 97.7%. The addition of EDTA has several following effects that are beneficial to the catalytic activity.The physicochemical properties of the OMA support contributes a lot to the diffusion process of DBT molecules. Through adjusting the ratio of PEG/Al during preparing ordered mesoporous alumina materials, the OMA with PEG/Al ratio of 0.1 exhibits high surface area (328 m2 g−1), large pore volume 0.74 (cm3·g−1) and big average pore size (8.1 nm). The well-ordered mesoporous structure and open pore channels of the OMA support can largely reduce the diffusion resistances of reactants and products compared to the traditional alumina materials. In addition, it can be seen from Fig. 3, no peaks of metal oxides are detected in the XRD spectra, which indicates that the outstanding pore structure of OMA supports can also avoid the aggregation of active metals.Secondly, the addition of EDTA is beneficial to the formation of Ni–Mo–S active phases. The XPS results show that NiMoE(1.0)/OMA and NiMoE(1.5)/OMA catalysts have the highest sulfidation degree of 0.65. The higher the sulfidation degree of the catalyst, the easier to form the type II “Ni–Mo–S″ active phase, which is more conducive to the HDS performance of the catalyst. As the molar ratio of EDTA/Ni increases, the proportion of NiMoS also increases, as shown in Table 4. It is well-known that NiMoS active phase is formed at the edge of MoS2 layer structure by horizontal bonding (Liu et al., 2020). And some non-active phases are easily formed between Ni and alumina support, which leads to low utilization rate of Ni active metal. Coulier et al., (2001) has reported that EDTA chelating ligand could stabilize nickel against sulfidation by forming a stable (NiEDTA)2- complex with the Ni promoter. The interaction between EDTA and Al cations acts as a driving force for decomposition of the (NiEDTA)2- complex so as to retard sulfidation of Ni to temperatures where Mo is completely sulfided in the form of MoS2 (Al-Dalama, Stanislaus, 2011). And Ni metallic species are combined with MoS2 at the corner positions of hexagonal configurations to form the highly NiMoS active phase, which greatly enhance the utilization rate of Ni active metals. Therefore, the presence of EDTA chelating ligands can achieve a good level of interaction between Ni and the MoS2 crystallites (Van Veen et al., 1993; Zepeda et al., 2016).Moreover, the chelating agent EDTA promotes the dispersion of both Ni(II) and Mo(VI) on alumina. It has been reported that the chelating ligands EDTA tend to isolate Ni from the environment, thus avoiding the formation of excessive bulk Ni sulfide (Rana et al., 2007; Zhao et al., 2006). Meanwhile, EDTA had a higher coordinating constant with the surface Al cations than Mo(VI). Free EDTA can compete with Mo(VI) on the alumina surface sites. Therefore, chelating agent EDTA can also limits strong interaction between the metal Mo ions and alumina. According to the HRTEM images, The NiMoE/OMA-1.0 catalyst has a short length of MoS2 active phase (3.2 nm), a moderate number of stacking layers (2.8), and a maximum dispersion degree (0.35). The Raman characterization results show that the stretching vibration of the MoO bond of polymer Mo7O24 6− species (which is easy to vulcanize) over EDTA-modified NiMoE/OMA series catalysts is higher than that of unmodified NiMo/OMA.The addition of EDTA can also change the acid properties of the modified catalysts so as to enhance DBT HDS activity. As shown in Py-FTIR characterization results, with the addition of EDTA increasing, the total acidities of NiMoE/OMA catalysts increase firstly and then decrease, among which the NiMoE(1.0)/OMA catalyst possesses the highest acid amount. It is well known that acidity plays an important role in improving the adsorption of DBT via its aromatic rings, thus finally enhancing HDS activity (Wang et al., 2016). The NiMoE(1.0)/OMA catalyst with the maximum total acid amount exhibits the high selectivity of CHB product, which indicates that the acid properties of the NiMoE/OMA catalysts have great influence on HYD and DDS pathways during the DBT HDS process.Based on the above analysis, the introduction of an appropriate amount of EDTA can promote the HDS performance of DBT and HYD reaction route due to the combination effect of the weak interaction between the metal support, the high sulfidation degree of NiMoS phase and suitable acid properties on HDS.Ordered mesoporous alumina (OMA) was successfully synthesized by homogeneous precipitation with aluminum nitrate as the inorganic aluminum source, ammonium carbonate as the precipitant and polyethylene glycol (PEG) as the template. The chelating agent EDTA was added to adjust the interaction between the support and the metal, so as to improve the sulfidation degree of Mo2S and enhance utilization rate of Ni metal. NiMoE/OMA catalysts synthesized by EDTA post-modification method displayed higher HDS activities of DBT compound compared with the unmodified NiMo/OMA. And among all the synthesized catalysts, NiMoE(1.0)/OMA catalyst showed the highest desulfurization rate of 97.7% and more HYD hydrogenation products of CHB.This research is financially supported by the National Natural Science Foundation of China (No. 21878330, 21676298) and the National Key R&D Program of China (2019YFC1907602) and the CNPC Key Research Project (2016E-0707).The following is/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.petsci.2021.11.005.

In this paper, ordered mesoporous alumina (OMA) support with the high surface area (328 m2 g−1) and the large pore volume 0.74 (cm3·g−1) was synthesized by homogeneous precipitation method. And the influence of EDTA on the physical and chemical properties of the modified catalysts was also studied. The characteristic results showed that the addition of EDTA could adjust the metal-support interaction and improved the acidity of the corresponding catalyst. Combined with the catalytic performance results, the EDTA-modified NiMoE(1.0)/OMA catalyst displays the highest DBT hydrodesulfurization conversion (97.7%).

Data will be made available on request.Polymerizations in aqueous heterogeneous media are characterized by good heat transfer, low amount of volatile organic solvents, low viscosity, and low toxicity [1], making them advantageous for commercial applications. [2] Moreover, conducting radical polymerizations in heterogeneous systems promotes easier access to high molecular weight (MW) polymers, because radical segregation and compartmentalization limit radical termination events. [3–6] In these dispersed and segregated systems, the small polymerization loci perform as “nanoreactors” [7] that enable the preparation of various nanoobjects. Polymerizations in microemulsion, miniemulsion, emulsion, and dispersion have been used to prepare polymer-based nanoparticles with various morphologies (e.g., core-shell, microcapsules, and multilayered particles), which have found applications in catalysis, coatings, and in the biomedical and diagnostic fields. [1,8–11].Reversible-deactivation radical polymerizations (RDRPs) are robust and versatile techniques for the synthesis of polymers with predetermined MW and low dispersity (Đ), starting from a wide range of monomers. [12,13] Among RDRP techniques, atom transfer radical polymerization (ATRP) [14–18], reversible addition-fragmentation chain-transfer (RAFT) polymerization [19], nitroxide-mediated polymerization (NMP) [20], and organotellurium mediated polymerization (TERP) [21] have been successfully developed in both homogeneous and heterogeneous media [22–29]. While limited advances in NMP in dispersed media were reported over the past 15 years [26], progress in TERP and particularly RAFT polymerization in dispersed media predominantly focused on exploiting the self-assembly of amphiphilic polymer chains prepared through emulsion polymerization processes, to produce nanoobjects with controllable morphologies. On the other hand, the implementation of ATRP in dispersed media has considerably advanced in the past 15 years, driven by the development of novel polymerization components and strategies to mediate ATRP systems [30], giving access to a broader range of building blocks and polymer architectures. In contrast to other RDRPs that require a stoichiometric amount of chain transfer or trapping agents, ATRP is a catalytic process, and the continuous evolution in catalyst design has enabled to prepare polymers with minimal catalyst contamination, which can additionally be removed through various methods.ATRP is based on radical generation by the “activator” form of a catalyst, which is typically a Cu complex with a polydentate amine ligand (L) in its lower oxidation state, i.e., CuI/L (Scheme 1 ). The CuI/L complex activates an alkyl halide initiator (R–X) or dormant chain end (Pn–X) via an inner sphere electron transfer (ISET) process, forming a propagating radical and a higher oxidation state, “deactivator” complex, X–CuII/L. [31–33] The high concentration of dormant species minimizes the fraction of terminated chains, in contrast to conventional radical polymerization. The fast initiation and rapid activation-deactivation equilibrium ensure that all chains grow concurrently, resulting in polymers with low dispersity and high chain-end functionality. [34].To compensate for the accumulation of CuII deactivator, which is generated by unavoidable radical termination, traditional ATRP methods required high concentration of Cu species. However, this caused issues related to catalyst solubility and removal. In the past decade, the loading of Cu catalysts was drastically decreased from over 10,000 parts per million (ppm, expressed as molar concentration relative to the monomer) to hundreds or less ppm, by implementing a variety of methods for the continuous regeneration of the CuI/L activator. These methods include the addition of a reducing agent in the polymerization system, as in activators re-generated by electron transfer (ARGET) ATRP [35,36], the addition of a thermal radical initiator as in initiators for continuous activator regeneration (ICAR) ATRP [37], and the use of metallic Cu in supplemental activator and reducing agent (SARA) ATRP [38–40], as well as the use of external stimuli such as electrical current, light, and ultrasounds in electrochemically mediated ATRP (eATRP) [41,42], photoATRP [43,44], and mechanoATRP, respectively. [45,46] These techniques are collectively called “ATRP with activator regeneration” or “low-ppm ATRP”, and they can provide polymeric materials with complex architectures, including decorated nanoparticles, networks and gels. [15] The residual small amount of catalyst could be left in the product, or removed by column filtration, electrodeposition, or other purification techniques [47–50], achieving a sufficiently low Cu contamination for most applications.During the past 15 years, low-ppm ATRP methods have been successfully implemented in dispersed media. Most initial works were carried out in miniemulsion, due to the advantage of conducting polymerizations in a “mini-bulk” environment, with minimal migration of polymerization components into the continuous phase. More recently, ATRP was expanded to ab initio emulsion, thanks to an improved understanding and engineering of catalyst location during the heterogenous polymerization process.This minireview describes the development of low-ppm ATRP in dispersed media. The outline and structure of this minireview is presented in Scheme 2 . Section 2 provides relevant background on the topics of polymerization in dispersed media and low-ppm ATRP. Section 3 discusses synthetic strategies involving engineered polymerization components, particularly surfactants and catalysts, as well as the different external stimuli that were used to trigger ATRP in (mini)emulsion systems. The unique features resulting from the combination of heterogeneous polymerizations and low-ppm ATRP enabled to prepare a broad variety of well-defined polymer architectures, which are presented in Section 4. Finally, relevant applications are reviewed in Section 5, while conclusions and perspectives are provided in Section 6.Heterogeneous polymerization processes involve multiphase systems where the starting monomer(s) and/or the resulting polymer are dispersed in an immiscible liquid. Typically, dispersed media are generated by using a surfactant and an external force to form a kinetically and hydrodynamically stable mixture, although the dispersion can also be thermodynamically stable in some cases. The dispersed droplets have a spherical shape, which enables to minimize the surface-to-volume ratio, and thus the surface energy. [51] Typically, oil-in-water (O/W) systems are employed, where “oil” refers to any water-insoluble liquid (monomer/polymer), and water is the continuous phase. Water-in-oil (W/O) systems are also possible.Heterogeneous polymerizations are generally categorized as suspension, emulsion, miniemulsion, microemulsion, dispersion, or precipitation. [51] However, the nomenclature and definitions are sometimes ambiguous in the literature. In this review, the different techniques of polymerization in dispersed media are distinguished by considering four features: (i) the initial state of the polymerization mixture; (ii) the kinetics of polymerization; (iii) the mechanism of particle formation; and (iv) the size of the final polymer particles. The following paragraphs describe the most relevant features of the different heterogenous polymerization techniques, which are then summarized in Table 1 . Suspension polymerization. To perform a suspension polymerization, the initiator is first dissolved in the monomer phase, which is then dispersed in the aqueous phase to form droplets. Water is a nonsolvent for both the monomer and the polymer. The mixture is stirred in the presence of a droplet stabilizer or suspension agent, such as poly(vinyl alcohol). Via thermal polymerization, the monomer droplets are converted directly to the polymer microbeads with no significant size change. Suspension polymerization is typically used to produce polymer beads with a size of 20 μm - 2 mm. [51]. Precipitation polymerization. This technique starts from a homogeneous solution of monomer, initiator, and stabilizers; the system quickly turns into a heterogeneous one, because the generated polymer is not soluble in the reaction medium beyond a critical MW. For example, in the polymerization of polyacrylonitrile (PAN), the polymer is insoluble in its own monomer (acrylonitrile), therefore it precipitates after reaching a critical chain length. After the first phase of particles nucleation, the monomer swells the particles, so the polymerization continues in the droplets/colloids. The polymer particles have a size in the range of 1–15 μm. [22]. Dispersion polymerization. This method is a subclass of precipitation polymerizations, with final particle dimension <10 μm, i.e., colloidal dimensions. The smaller particle size of a dispersion polymerizations is achieved by applying more effective dispersing agents in larger amounts than in typical precipitation polymerizations. Emulsion polymerization. A traditional emulsion polymerization system, also known as ab initio emulsion polymerization system, consists of a hydrophobic monomer, a water-soluble initiator, a surfactant, and water. At the beginning of the polymerization, a large portion of the monomer resides in large, surfactant-stabilized monomer reservoirs (≫ 1 μm), and only a small fraction of monomer molecules is in water and in surfactant-formed micelles (<10 nm). The initiator molecules are decomposed in water, where they initiate the growth of oligomeric radicals. Upon reaching a critical chain length, the hydrophobic oligomeric radicals enter the micelles rather than the monomer droplets, due to the much higher surface area of the smaller and numerous micelles relative to the larger but fewer monomer droplets. The monomer molecules diffuse from the reservoirs into the aqueous phase, and enter the micelles to propagate the radical chains, resulting in the formation and growth of polymer particles (Scheme 3 ). The final polymer particle size is 50–500 nm. [52]. Miniemulsion polymerization. Before polymerization, the hydrophobic monomer, oil-soluble initiator, and a hydrocarbon co-stabilizer form a macroscopic organic phase, while the surfactant is dissolved in a macroscopic aqueous phase (Scheme 4 ). The miniemulsion is generated by a vigorous homogenization process, such as by employing probe ultrasonication or a microfluidizer. The monomer droplets have a size of 50–500 nm, and they are stabilized by the surfactant and the co-stabilizer, which strongly limit the mass transfer of monomer during the polymerization. Thus, the polymerization proceeds in each droplet like in a “mini-bulk” polymerization and, as a result, the size of the final polymer particles is similar to that of the initial monomer droplets. [53]. Microemulsion polymerization. By using a large amount of an appropriate surfactant, the interfacial tension of a dispersed media can be ultra-low, leading to particle size <50 nm, and an optically transparent and thermodynamically stable system with high interfacial area. This system is termed microemulsion [54], and the polymerization proceeds similarly to a miniemulsion system.Other specialty configurations of polymerization in dispersed media are possible, some of which are described in the following paragraphs. Self-assembled non-spherical nanoparticles. Molecules of polymeric surfactants can self-assemble into structures that are not necessarily globular, resulting in a combination of spherical micelles, cylindrical micelles, vesicles, and even bicontinuous planar interfaces. Polymerization occurring inside these particles can lead to the formation of non-spherical latex particles. [55]. Pickering emulsion. A Pickering emulsion is an emulsion that is stabilized by solid particles (for example colloidal silica or proteins) which adsorb onto the interface between the water and oil phases. High Internal Phase Emulsions (HIPEs). Typically, for any polymerization mechanism, the volume percentage of the internal (i.e., dispersed) phase is 55% or less. Higher content of internal phase often results in very high viscosity. In fact, the maximum volume percentage occupied by uniform, non-deformable spheres packed in the most effective way is 74%. High internal phase emulsions (HIPEs) represent a special case where the internal (droplet) phase exceeds 74% of the total volume of the system. HIPEs are generally stabilized by large amounts of surfactants. Because of its high-volume fraction, the dispersed phase forms non-uniform interconnected spheres or polyhedral shapes. In such systems, the continuous phase is loaded with the monomer(s) and crosslinker(s), and polymerized to yield, upon purification, a highly porous material with interconnecting voids, which is called polyHIPE or pHIPE (Scheme 5 A). This emulsion templated method is convenient for the synthesis of porous polymers, as it can provide a wide variety of highly interconnected, highly porous monolithic systems (Scheme 5B). [56–58].The equilibrium constant of ATRP, K ATRP, is expressed by Equation (1), where P n ⦁ and P n -X are, respectively, the active and dormant chains. (1) K A T R P = [ P n ⦁ ] [ X − C u I I / L ] [ P n − X ] [ C u I / L ] Thus, the rate of ATRP polymerization, R p, can be expressed by Equation (2) (where M is the monomer, and k p is its propagation rate constant), and it depends on the relative amount of CuI/L activator and X–CuII/L deactivator. (2) R p = k p [ M ] [ P n ⦁ ] = k p K A T R P [ M ] [ P n − X ] [ C u I / L ] [ X − C u I I / L ] The dispersity Đ = M w/M n of the resulting polymer decreases by increasing the equilibrium concentration of the X–CuII/L deactivator, according to Equation (3), where DP n is the degree of polymerization, k deact is the deactivation rate constant, and p is the conversion. (3) M w M n = 1 + 1 D P n + ( k p [ R X ] 0 k d e a c t [ X − C u I I / L ] ) ( 2 p − 1 ) In low-ppm ATRP techniques, the polymerization kinetics follows the steady-state in radical concentration. [30,59] Thus, R p is expressed by Equation (4), whereby the numerator corresponds to the rate of CuI/L (re)generation. (4) R p = k p [ M ] [ P n ⦁ ] = k p [ M ] R C u I / L r e g e n e r a t i o n k t In ARGET ATRP, the CuI/L activator is (re)generated by means of a chemical reducing agent, such as the water-soluble ascorbic acid (AsAc) or oil-soluble SnIIR2 compounds. [35,36] The rate of polymerization depends on the amount or feeding rate of the reducing agent. In ICAR ATRP, a thermal radical initiator such as the water-soluble 2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (VA-044) or the oil-soluble azobisisobutyronitrile (AIBN) are employed to exploit their slow thermal decomposition to induce the reduction of the X–CuII/L deactivator. [30] The slow decomposition of the thermal radical initiator can cause the generation of a small fraction of new chains, thus ICAR ATRP can be non-ideal for the synthesis of well-defined block copolymers. SARA ATRP uses zero-valent copper as both supplemental activator and reducing agent [39,60], and the rate of polymerization is affected by the ratio of the surface area of Cu0 and the volume of the reaction. Metallic Cu can be reused for multiple polymerizations or can be periodically lifted from the reaction mixture to achieve temporal control. [61].Electrochemically mediated ATRP (eATRP) takes advantage of an applied current or voltage to continuously reduce CuII species. [41] No external chemicals are needed in eATRP. Since the rate of polymerization is affected by the ratio of CuI and CuII species, it can be tuned by the applied potential, according to the Nernst equation. [59] Temporal control over polymerization can also be achieved by adjusting the applied potential or current. eATRP was recently scaled up to a pilot scale. [62].In photoATRP, UV or visible light promotes the photo-excitation of X–CuII/L complexes followed by the reduction of excited X–CuII/L* to CuI/L in the presence of an electron-donor species, typically an aliphatic amine, which can be an excess of ligand. [43,44] Thus, the rate of polymerization is influenced by the ratio of ligand (or other electron donor) to copper. Similar to eATRP, the use of light as an external stimulus enabled temporal control over the polymerization. PhotoATRP can also be performed by employing other metal photocatalysts, which typically operate in the absence of an electron donor, as well as organocatalysts, such as phenothiazines, phenazines, and phenoxazines. [63,64] A procedure for scale up of photoATRP has also been recently disclosed. [65]. MechanoATRP and sonoATRP employ ultrasounds to (re)generate the activator. In the first technique, the sonication of piezoelectric materials induces an electron transfer to the CuII species [45,46], while in the second case, the application of ultrasound in aqueous media produces hydroxyl radicals from water molecules. [66] The hydroxyl radicals react with monomer or with alcoholic solvent, forming carbon based radicals that start the propagation.The implementation of ATRP in dispersed media requires the partitioning of each species among the aqueous and organic phases prior, throughout, and at the end of the polymerization. In a typical free radical polymerization in dispersed media the only components are the monomer and radical initiator, besides the surfactant and/or eventual co-surfactant, (co)stabilizers or dispersing agents (Table 1). Conversely, a low-ppm ATRP requires an alkyl halide initiator, the catalyst, i.e., a Cu-halide salt and the ligand, and either a reducing agent, a thermal radical initiator, metallic Cu, or an external trigger. All these elements should be located in the appropriate phase (water, oil, or their interphase) at any stage of the polymerization. For instance, at the onset of a miniemulsion ATRP, the catalyst and RX must reside in the surfactant-stabilized monomer droplets, and they should remain in the hydrophobic phase for the whole duration of the process. An eventual reducing agent or thermal radical initiator should also reach the hydrophobic phase, and the eventual external stimulus must be conveyed to the hydrophobic phase. Therefore, the design of low-ppm ATRP systems in dispersed media necessitates tuning the hydrophilicity of the catalyst, RX and eventual reductants, as well as appropriately selecting the surfactant and/or stabilizers. Low-ppm ATRP techniques have been effectively performed in various types of dispersed media by carefully designing the catalytic systems, as it will be explained in Section 3.An important advantage provided by the use of heterogeneous media is the possibility to reduce or suppress bi-radical termination events. In homogeneous ATRP, bi-radical termination is unavoidable and can result in decreased rate of polymerization, low chain-end functionality, and even gelation if a branching point exists. In dispersed media, radicals located in different particles are unable to terminate with each other (segregation effect), which is particularly relevant for the design of complex polymer architecture. [3] On the other hand, the confined space effect can result in enhanced reaction rate between two radicals located in the same particle as the particle size decreases. However, compartmentalization in dispersed media ATRP also enhances the rate of deactivation. Thus, depending on the catalytic system, lower polymerization rates and improved control were observed for particle volumes below a threshold value. [7,67] The latter is dependent on the particular system, and it generally increases with increasing the targeted degree of polymerization, i.e., decreasing the amount of initiating molecules (and thus of growing radicals) confined within each particle. [67].In dispersed media, each droplet acts as a “nanoreactor”, allowing the preparation of nanoobjects that cannot be easily prepared in other media, including crosslinked nanoparticles, nanocapsules, and core-crosslinked hairy nanoparticles. In Pickering emulsion system, Janus platelets with either a polymer grafted on a single side or different polymers on each side were synthesized. By adding the monomer and crosslinker to the continuous phase, porous polymer monoliths could be fabricated. The different polymer architectures prepared by low-ppm ATRP in dispersed media will be presented in Section 4, whereas their most relevant applications will be discussed in Section 5.Seminal ATRP in dispersed media with high loading of Cu catalyst generally employed non-ionic or cationic surfactants, which stabilized the latex without interfering with the cationic Cu complexes and thus with the ATRP equilibrium. [68,69] In contrast, anionic surfactants can interact with the Cu complexes used as catalysts, modifying their stability and catalytic activity. [22] The introduction of low-ppm ATRP methods was concomitant to the engineering of the surfactant, resulting in the use of anionic, ionic liquid, and reactive surfactants.The most important drawback of conventional surfactants is their tendency to remain in the final polymer, negatively affecting its electric, photonic, and surface properties. In conventional emulsion free radical polymerization, this issue was overcome by designing soap-free emulsion systems. In such systems, traditional surfactants are replaced by initiator or monomer molecules capable of anchoring onto the surface of the latex particles, leading to improved colloidal stability and the absence of surfactant leaching from the produced latexes. This immobilization strategy eliminates the need for surfactant removal after polymerization. In ATRP, soap-free emulsion polymerizations were developed by exploiting the concept of “reactive surfactant” [70], which is a multifunctional molecule that combines the function of a surfactant with an initiator, monomer, or catalyst/ligand. [71,72].For example, hydrophilic or amphiphilic macroinitiators prepared by RDRP were employed as reactive surfactants to simultaneously initiate polymerizations and stabilize micelles, forming amphiphilic polymers that behave as macro-emulsifiers (Table 2 ). In fact, poly(ethylene oxide) homopolymer with terminal α-bromoisobutyrate moiety (PEO-Br), and poly(ethylene oxide)-b-polystyrene (PEO-b-PSt-Br) block copolymer made by ATRP were used as macroinitiators and stabilizers in miniemulsion AGET ATRP of butyl acrylate (BA), generating polymer latexes with narrow particle size distribution. [73].Besides macroinitiator surfactants, small-molecule surfactants can be used to initiate an ATRP. When a polymerization initiating site is introduced in surfactant molecules the resulting construct is termed an “inisurf”, i.e., initiator-surfactant. [74] Both anionic [75,76], and cationic [77,78] inisurfs were employed for ATRP in dispersed media. Dextran derivatives bearing a phenoxy hydrophobic group were modified to introduce α-bromoisobutyrate sites for ATRP initiation, forming a multifunctional inisurf. Nanoparticles with hydrophobic cores and hydrophilic shells were formed during polymerization. [9] By grafting polymer from the inisurf, a “sterically-stabilized latex” was obtained, which was particularly resistant toward destabilization induced by high shear force, electrolyte addition, and freeze-thaw.The various types of reactive surfactants provided latexes with increased stability and eliminated the need for removing the surfactant after polymerization. Alternatively, surfactant monomers have been employed, that copolymerized within the main chain. Unsaturated molecules such as a methacrylic ester [79] or a cardanol ether [80], were covalently attached to a tetraalkylammonium cationic surfactant, forming surfactant-monomers exhibiting drastically decreased CMC values and enhanced stabilization capabilities.A complementary strategy consists of incorporating multidentate nitrogen groups into the surfactant to form a surfactant-ligand (SL). In ab initio emulsion ATRP, a SL “locked” the CuII on the surface of the droplet, eliminating the escape of CuII to water. [81,82] However, the immobilization of the catalyst on the surface of polymerizing droplets can restrict its diffusion rate and result in decreased degree of control. The addition of conventional ligand, dNbpy (4,4′-dinonyl-2,2′-bipyridine), narrowed the molecular weight distribution. [83].Ionic liquid surfactants represent a useful alternative to conventional ionic or neutral surfactants as they can be easily separated and reused. N-tetradecyl-N-methyl-2-pyrrolidonium bromide was employed in microemulsion AGET ATRP of methyl methacrylate (MMA). The ionic liquid surfactant had low toxicity and was recycled and reused for up to 5 times. [84].Besides ionic liquids, insoluble solids that could be partially wetted by both phases could significantly decrease the surface energy and stabilize the emulsion, giving, in this case, a so called Pickering emulsion. [85] An example of Pickering agent is cellulose nanocrystals (CNCs), which have been used for a photoATRP catalyzed by Eosin Y. [86] CNCs could be recycled and reused for Pickering emulsion polymerization multiple times. CNCs modified with α-bromoisobutyrate moieties could also stabilize inverted (W/O) or double (W/O/W) emulsions, and surface initiated ATRP (SI-ATRP) occurred from the surface of the CNCs. Thus, capsules, filled beads, and microporous polymers were directly prepared. [87] Similarly, silica nanoparticles modified with α-bromoisobutyrate groups stabilized Pickering emulsions and acted as initiators in SI-ATRP. [88,89] Biphasic grafting from both the aqueous and organic phase resulted in Janus particles, since the in situ formation of amphiphilic particles restricted their own rotation. [90].Common anionic surfactants were previously considered incompatible with ATRP, since they could displace the halide ion from the X–CuII/L deactivator, forming a CuII species that cannot deactivate radicals. [22,91] However, the detrimental displacement of X− was minimized by adding an excess of bromide ions in miniemulsion ATRP systems. This has opened up the possibility of employing inexpensive, effective, and readily-available surfactants, such as sodium dodecyl sulfate (SDS), instead of more costly, neutral surfactants, e.g., Brij 98, that were previously used in miniemulsion ATRP. [91].Catalysts are selected according to the type of dispersed media. In miniemulsion polymerizations, the hydrophobicity of the catalyst influenced the partition of the activator and deactivator between the organic and aqueous phases, thus affecting the rate of polymerization and degree of control. [92] The prevalent strategy toward well-controlled ATRP in miniemulsion involved the design of ligands for sufficiently hydrophobic Cu catalysts that resided within hydrophobic monomer droplets, where the polymerization proceeds. [93] Therefore, highly hydrophobic and highly active catalysts bearing hydrophobic alkyl chains in the polydentate amine were synthesized and effectively employed in low-ppm amounts in miniemulsion ATRP. [94,95] Only recently, the need for designing specific, hydrophobic ligands was overcome by demonstrating that the commercially available tris(2-pyridylmethyl)amine (TPMA) is a suitable ligand for miniemulsion ATRP when used in combination with an anionic surfactant, such as SDS.Interestingly, the strong interaction between dodecyl sulfate ions (DS−), and Cu-based ATRP catalysts was later exploited to identify a Cu complex that, in the presence of SDS, could lead to enhanced polymerization control. The interaction between SDS and Cu/TPMA formed a catalyst that was conveniently partitioned between aqueous phase, surface of hydrophobic monomer/polymer droplets, and inside the hydrophobic droplets to better control the polymerization within the hydrophobic phase. The presence of SDS affected the localization of hydrophilic Cu/TPMA that would have been otherwise present almost exclusively in the water phase. It was demonstrated that in miniemulsion systems composed of BA droplets stabilized by SDS, only 4% of Cu/TPMA was located in the continuous aqueous phase, while most of it (95%) was located at the interface of the droplets. Moreover, a small portion of Cu complex (1%), formed ion-pairs with DS− capable of entering the polymerizing particles (Scheme 6 ). This small amount of hydrophobic ion pair has high mobility inside the particle and can effectively deactivate growing radicals. Thus, the interaction between DS− and Cu/TPMA provided an “intelligent” catalyst that could control radical propagation from the interface and the inside of hydrophobic droplets. [96] In addition, after polymerization, a simple dilution of the system with water followed by centrifugation induced the migration of the hydrophilic Cu/TPMA back to the water phase, recovering polymers with residual Cu content as low as 0.3 ppm. [97].Beyond Cu-catalyzed systems, Fe catalysts, including N,N-butyldithiocarbamate ferrum [98,99], Fe/N,N,N′,N′-tetramethyl-1,2-ethanediamine [99,100], and Fe/ethylene diamine tetraacetic acid [101], were employed for ATRP in dispersed media, either in miniemulsion or microemulsion systems. Metal-free ATRP employing 10‐phenylphenothiazine as a photocatalyst was successful in microemulsion ATRP. [102].The monomers used in oil-in-water dispersed media ATRP are generally hydrophobic. A slight change in hydrophobicity (or hydrophilicity) results in different polymerization rates and control. The hydrophilicity of molecules, including monomers, is typically quantified by considering its partition coefficient (logP) in an octanol-water mixture. In emulsion ATRP, the monomer diffuses to the micelles under moderate stirring, thus, the solubility of the monomer affects the rate of diffusion, which in turn impacts the rate of polymerization and consequently the degree of control. Emulsion ATRPs of MMA, ethyl methacrylate (EMA), butyl methacrylate (BMA) and lauryl methacrylate (LMA) were carried out. Monomers with lower logP values, i.e., more hydrophilic monomers, revealed faster rate of polymerization yet lower degree of control. Additionally, more hydrophilic monomers and polymers migrated between the droplets, leading to reduced colloidal stability. [103] On the other hand, the most hydrophobic monomer, LMA, showed negligible conversion because it could not diffuse though water to reach polymerizing micelles. [104].Hydrophilic monomers were polymerized in inverse emulsion systems, i.e., water-in-oil emulsions. For example, oligo(ethylene glycol)methyl ether methacrylate (OEOMA) was polymerized in inverse miniemulsion and inverse microemulsion ATRP, forming well-defined brush-like structures. [105–107].ATRP is a versatile technique that enables the polymerization of monomers bearing various functional groups. For instance, glycidyl methacrylate (GMA) [108] and 2,2,3,3,4,4,4-heptafluorobutyl acrylate were polymerized in ab initio emulsion ATRP, with keeping intact the epoxy and fluorine functionalities, respectively. [109].As discussed in Section 2.1, traditional emulsion polymerization requires an aqueous initiation and nucleation phase, followed by polymerization within hydrophobic particles. Thus, in emulsion ATRP the Cu complex should “follow” the radicals from the aqueous phase to the organic phase to control the entire dynamic ATRP process. Thus, the partition of the catalysts is critical. However, several common ATRP catalysts are highly hydrophilic, resulting in the deactivating species, X–CuII/L, leaving the oil phase, negatively affecting control over the polymerization. At the same time, specifically designed hydrophobic catalysts also performed poorly because they mostly resided in the monomer reservoir and therefore could not control the polymerization in the aqueous phase. During the past decade, three main approaches were developed to perform well-controlled emulsion ATRP: (i) a microemulsion (or miniemulsion) “seed” approach, (ii) the use of a phase-transfer catalyst, and (iii) the engineering of the surfactant and ATRP catalyst.The seeded emulsion approach enabled to “bypass” the aqueous initiation and nucleation phase by introducing an initial microemulsion (or miniemulsion) step (Scheme 7 a). [110,111] Thus, first microemulsion (or miniemulsion) ATRP was performed using a tiny amount of monomer with the aim of encapsulating the hydrophobic catalyst complex into the polymer particles. Then, the latter served as “seeds” that were swelled by the addition of a large batch of monomer, during which the catalyst remained located in the particles. This two-step approach yielded well-controlled polymers and relatively uniform latex particles. Moreover, it could be further used to form a variety of structures, including block copolymers, hairy nanoparticles [112], and onion-like structures [113,114], as it will be described in Section 4. Seeded emulsion polymerization tend to have particle size with low batch-to-batch variability.Similar to the case of miniemulsion polymerizations, highly hydrophobic ligands were employed in emulsion systems to limit the escape of CuII deactivators from growing polymer particles. However, a large fraction of these hydrophobic catalysts resided in the oil phase rather than in the micelles during the first stage of the process, leading to a miniemulsion-like mechanism and poor polymerization control in the initial nucleation stage. The location of the catalyst could be modulated by using phase transfer catalysts in combination with shuttle molecules. Shuttle molecules were polar organic molecules, such as acetone, that was mixed with water to aid the solubility of several components in the continuous aqueous phase in an emulsion/miniemulsion ATRP. [115] Phase-transfer catalysts, typically organic ions, were used to transport the catalysts within the phases. For example, tetrabutylammonium bromide, TBAB, was shown to favor the mobility of the catalyst as well as the initiator and halide ions to the hydrophobic polymerizing particles (Scheme 7b). [115].To localize the catalyst within the micelles and surfactant-stabilized hydrophobic particles, a surfactant-ligand (SL) compound was specifically designed. The SL comprised a multidentate amine-based ligand for Cu centers, attached to an hydrophobic moiety (Scheme 7c) [83]. However, the diffusion of CuII species coordinated to the SL was relatively slow, likely due to the steric hindrance within the SL. Thus, the addition of a hydrophobic catalyst (e.g., CuBr2/dNbpy) was needed to promote sufficiently fast diffusion within the micelles.In a complex medium such as an emulsion, the key for excellent polymerization control is the presence of a dynamic catalyst that can react with propagating radicals throughout the life of a polymer chain, i.e., from aqueous nucleation to hydrophobic propagation inside monomer droplets. A simpler and scalable approach consists of exploiting the combination of interfacial and ion-pair catalysis provided by the Cu/TPMA-SDS system described in Section 3.1 to achieve ATRP in true ab initio emulsion. [27] The hydrophilicity of Cu/TPMA and the use of a hydrophilic alkyl halide initiator enabled initiation of the polymerization within the aqueous phase, which then seeded SDS-micelles (Scheme 8 ). The much higher total surface area of the micelles relative to the monomer reservoir caused the Cu/L complexes interacting with the surfactant to preferably reside on the surface or within the micelles. The anionic surfactant acted as a shuttle for the catalyst, promoting the localization of the catalyst at the interface of the hydrophobic particles, and to a lower extent inside the particles. This emulsion ATRP technique is facile, scalable, and it was successfully adapted to photoATRP. [104] Note that in this emulsion ATRP technique, pre-partitioning of catalyst and initiator was prevented by avoiding pre-mixing of the oil and water phases prior to starting the polymerization (Scheme 8). [27] A pre-emulsified monomer could potentially be fed as monomer reservoir, but this is not been tested yet.Prior to the development of low-ppm ATRP techniques, normal, reverse, simultaneous reverse and normal initiation (SR&NI), and activator generated by electron transfer (AGET) ATRP, were performed in various dispersed media using high loadings of Cu complexes. Low-ppm techniques such as ICAR and ARGET ATRP enabled to reduce the loading of Cu in dispersed media polymerization to hundreds of ppm. ATRP techniques based on external stimuli [116] such as light, electrical current/potential and ultrasound opened new possibilities for low-ppm ATRP in dispersed media, providing additional tuning of the polymerization rate and features by simple manipulation of the external stimulus.The main challenge hindering the implementation of eATRP in miniemulsion systems was identified as the physical disconnection of the working electrode (i.e., the electrode that provides the electrons for the reduction of CuII species) from the organic phase. [117] The electrode was instead in contact with the continuous aqueous phase. This hampered the regeneration of the hydrophobic catalysts generally employed in miniemulsion ATRP that dissolved in the organic phase. In fact, due to its “mini-bulk” feature, the mass transport in miniemulsion polymerizations is negligible. To solve this issue, a dual-catalyst approach was developed, whereby a water-soluble catalyst CuII/Laq and a hydrophobic catalyst CuII/Lorg were used at the same time, so that the first could shuttle the external stimulus from the working electrode to the droplets’ interface, while the second was controlling the ATRP equilibrium inside the hydrophobic phase (Scheme 9 ). By comparing several combinations of CuII/Laq and CuII/Lorg, it was determined that the hydrophilicity/hydrophobicity of the catalysts played a more important role on polymerization control relative to the activity of the catalysts (i.e. its standard reduction potential). Later, the development of the catalytic system based on Cu/TPMA interacting with SDS enabled to simplify miniemulsion eATRP by eliminating the need for a dual-catalyst system. [96].An electrochemically mediated ATRP approach was also used to prepare molecularly imprinted polymer (MIP) nanoparticles, through a precipitation polymerization system. eATRP of 4-vinylphenylboronic acid in the presence of ethylene glycol dimethacrylate (EGDMA) as crosslinker and sialic acid as template was conducted in water/methanol (1/4 v/v), catalyzed by Cu/TPMA. By tuning the applied potential, the polymerization yielded nanoparticles with hydrodynamic diameter ranging from 160 to 330 nm, capable of recognizing the sialic acid template. [118].The possibility to perform polymerizations in dispersed media through light irradiation could offer improved reaction efficiency, lower energy consumption, and increased safety. [119] However, photochemistry in dispersed-phase polymerizations is challenging because of the limited light penetration in turbid (mini)emulsion systems. Light absorption and scattering phenomena in miniemulsion photopolymerizations have been extensively studied by aid of theoretical modeling [120] and actinometry [121]. Droplet size played a crucial role, with smaller particles reducing the scattering coefficient and thus resulting in improved light penetration. [122] Despite these limitations, photoinduced miniemulsion free radical polymerizations have been successfully performed and even employed for the encapsulation of pigments in UV-cured nanoparticles. [10,123].After the application of the Cu/TPMA-SDS catalytic system (see Section 3.1) in miniemulsion eATRP and ARGET ATRP, photoATRP was investigated. Despite the turbidity of the heterogeneous media. photomediated miniemulsion ATRP was performed over a broad range of solid contents and particle sizes, achieved by tuning the surfactant amount. [124] In addition, excellent temporal control was achieved upon switching the UV light on and off multiple times. PhotoATRP was applied in ab initio emulsion polymerization of various methacrylate monomers, also by using an enzymatic degassing procedure. [104].Metal-free photoATRP was also conducted in heterogeneous media. For example, by using Eosin Y as a photocatalyst and triethylamine as electron donor, well-controlled PMMA was prepared via photoinduced electron transfer (PET)-ATRP of MMA in Pickering emulsions, stabilized by cellulose nanocrystals. [86].ATRP in dispersed media was performed by means of microwave irradiation and ultrasound. The use of microwave irradiation in ATRP generally results in significantly increased reaction rates and yields in comparison with other techniques. [125] The acceleration is attributed to the increased temperature and mass transport. In emulsion polymerization, microwave irradiation afforded nanoparticles with smaller average size and narrower size distribution compared to conventional heating, which typically yielded nanoparticles with size >100 nm. Conversely, combining microwave irradiation and emulsion ATRP, PEG-b-PSt nanoparticles with diameters in the range 30–50 nm were produced. [126].The use of ultrasound is a powerful strategy that enabled miniemulsion polymerization without chemical radical initiators or co-stabilizers, while maintaining fast polymerizations. [127] Acoustic cavitation provided radicals that sustained the polymerization, while the high shear forces limited the impact of Ostwald ripening. The interfacial and ion-pair catalyst system composed by Cu/TPMA-SDS was applied to miniemulsion sonoATRP. [128] Radicals generated in the aqueous phase by sonication effectively initiated and sustained the polymerization, which could be temporally controlled by switching ultrasound on and off.RDRP systems typically require a physical deoxygenation process, such as nitrogen bubbling or freeze-pump-thaw. The coupling of chemical deoxygenation reactions with RDRP simplified the reaction setup and allowed for conducting polymerizations in open-to-air conditions. [129] Glucose oxidase (GOx)-catalyzed oxygenation of glucose consumes oxygen efficiently, and thus it was applied to aqueous RAFT polymerization and ATRP. [130–135] The GOx-deoxygenation system was also implemented in miniemulsion and emulsion ATRP, leading to well-controlled synthesis of hydrophobic polymers. [104,136] Circular dichroism measurements demonstrated that the structure of GOx remained intact in the presence of anionic surfactant. [136] Lignin nanoparticles (LNPs) coated with chitosan and GOx enabled efficient stabilization of Pickering emulsions and simultaneous in situ enzymatic degassing of ATRP, without requiring hydrogen peroxide scavengers. [137] The enzymatic degassing eliminated the possibility of monomer evaporation during traditional degassing; moreover, the low cost of the stabilizer and deoxygenation reagents can favor the implementation in industrial settings, especially in the case of emulsion polymerizations.The biphasic nature of heterogeneous polymerizations can simplify catalyst removal thanks to the large surface area of the organic/water interface. In fact, in miniemulsion systems, the large interfacial surface facilitates the mass transport of the catalyst from polymer particles to the aqueous phase, provided that a sufficiently hydrophilic catalyst is used. At the end of a miniemulsion ARGET ATRP of BMA, the product was precipitated into methanol/water (1/1 by v/v) and analyzed by inductively coupled plasma-mass spectrometry (ICP-MS). Using the highly hydrophilic catalyst Cu/TPMA, the residual Cu in the polymer could be as low as 300 part per billion (ppb), which was 10 times less than the residual Cu obtained using the hydrophobic Cu/BPMODA* (BPMODA* = bis[2-(4-methoxy-3,5-dimethyl)pyridylmethyl]octadecylamine) catalyst. [97].Another strategy for Cu removal consists of destabilizing the ligand. After Cu/HMTA (HMTA = hexamethylenetetramine) catalyzed emulsion ATRP of MMA, HMTA was decomposed into NH3 and HCHO under acidic conditions (the optimal pH was 4–5) and CuII was released to the aqueous phase. [138].Moreover, Cu can be conveniently removed by electrolysis. This method is especially efficient in polymer latexes due to the large organic/water interface area. Electrolysis was used to remove Cu from emulsion ATRP systems that employed a surfactant-ligand (SL, see Section 3.1) After polymerization, >98% of Cu was collected by electrolysis of the system that employed the SL, while only 50% Cu was recovered from a similar system employing a conventional hydrophobic catalyst. The improved efficiency was attributed to the anchoring of Cu/SL on droplets’ surface, which decreased the diffusion resistance. Importantly, the latex stability was retained during the electrolysis. Purified polymer showed higher strength and higher antiaging performance than the untreated counterpart, as demonstrated in tensile tests. [50].Polymers and copolymers with different compositions and topologies were prepared by ATRP, owing to the retention of chain-end functionality and the possibility of introducing multifunctional moieties (i.e., crosslinkers, mono/multi-functional (macro)initiators, inimers, macromonomers). In dispersed media, the polymerization confinement in droplets of limited size contributes to decreased chain-termination events and can be exploited to tune the polymer architecture.Linear polymer chains represent the most common polymer architecture in ATRP. By tailoring the composition of monomer feed(s) and reaction conditions, statistical, block, and gradient copolymers could be synthesized in ATRP in dispersed media. Here we provide examples of block and gradient copolymers.In ATRP, block copolymers are typically prepared by first polymerizing the monomer that gives a more active chain end (e.g., polymethacrylates or polystyrene macroinitiators), followed by polymerization of less active monomers (e.g., acrylates), in order to assure a good initiation efficiency for the second block. However, this sequence cannot be followed when preparing certain A-B-A triblock copolymers. Therefore, halogen exchange is typically used as an efficient way to chain-extend from a less active macroinitiator (MI) to a more active monomer. With halogen exchange, the limitation of mismatching monomer reactivity can be circumvented by switching from C–Br to less active C–Cl chain ends. This has been achieved by using CuICl/L in equimolar amount to Pn-Br MI in the chain-extension step. [139–141] However, this approach cannot be effectively applied in systems based on activator regeneration, since they operate with ppm amounts of catalysts. Thus, catalytic halogen exchange (cHE) was developed [142] and later implemented in miniemulsion ARGET ATRP to chain-extend a less active PBA-Br MI with a more active MMA monomer, using a catalytic amount of Cu (Scheme 10 ). [143] Addition of 0.1 M NaCl or tetraethylammonium chloride to ATRP of MMA initiated by methyl 2-bromopropionate led to high initiation efficiency and polymers with low dispersity. Similar conditions were then employed in chain extension of PBA-Br MI with MMA to prepare P(BA-b-MMA) and P(MMA-b-BA-b-MMA). This technique allows for building various block copolymers with different structures and functionalities.Moreover, block copolymers could be synthesized via either in situ or stepwise chain extension. After miniemulsion eATRP of BA reached 78% conversion, in situ chain extension was achieved by dispersion of tBA into the miniemulsion under ultrasonication followed by N2 sparging and electrolysis, resulting in PBA-b-P(BA-co-tBA). [96] In stepwise chain extension, a solution of PBA-Br MI was isolated by precipitation from the miniemulsion system, then tBA was used in the organic phase of a second miniemulsion. The subsequent ATRP resulted in P(BA-b-tBA). [96].Copolymerizations of monomer mixtures by ATRP can result in statistical or gradient copolymers, depending on monomer reactivity [144,145], but also feeding rate, and hydrophilicity. For monomers with different reactivity, such as acrylates and methacrylates, spontaneous gradient copolymers could be prepared by miniemulsion ATRP. [146] Conversely, for monomers with similar reactivity it was necessary to feed one monomer into miniemulsion polymerization media to produce “forced” gradient copolymers. [146,147] On the other hand, in emulsion ATRP one can exploit the different water solubility of monomers with similar reactivity to obtain spontaneous gradient copolymers. The monomer with higher water solubility will diffuse more rapidly through the aqueous phase, therefore being incorporated first into the copolymer in comparison with the less water-soluble monomer. Thus, P(MMA-grad-BMA) and P(BMA-grad-LMA) (LMA = lauryl methacrylate) could be prepared by emulsion ATRP of the corresponding monomer mixtures, with no need for adopting a feeding strategy. [104,148].Compared to linear polymers, the distinct architecture and multiple chain-terminal groups of branched polymers endow higher solubility, lower solution/melt viscosity, less deformability, and more chain-end functionality. [149] In bulk and solution polymerizations, especially at high monomer concentrations, the increased number of initiation sites favors the occurring of crosslinking reactions, which can lead to macroscopic gelation. By switching from homogeneous systems to dispersed media, cross-termination reactions can be greatly decreased owing to the segregation of growing polymer chains (Scheme 11 a), thus high conversions and low polymer dispersity can be achieved more easily. This feature is especially beneficial for the synthesis of bottlebrush, star, and hyperbranched polymers (Scheme 11b), as it will be described in the next paragraphs.Molecular brushes, also known as bottlebrushes, comprise densely grafted side chains, allowing for decreased intermolecular entanglement and for the presence of multiple functionalities in the side chains. [150–153] This unique structure makes molecular brushes suitable for application as lubricants and surfactants, among many others. There are three methods to synthesize bottlebrush polymers by ATRP: “grafting-through”, “grafting-from”, and “grafting-onto”.“Grafting-through” refers to a process where an oligomer/polymer chain bearing a vinyl group at one end is polymerized by ATRP into a bottlebrush structure. For example, AGET ATRP of OEOMA475 (OEOMA with average MW 475) was initially conducted in aqueous solution. [105] Continuous feeding of AsAc and increasing monomer concentration resulted in higher conversion, but final polymers showed bimodal distribution of MW caused by bimolecular termination. Significantly improved polymerization control was obtained upon switching to an inverse miniemulsion system, where chain segregation effectively reduced the chances of termination reactions. The resulting polymers had desired MW and low dispersity.The “grafting-from” method employs a polymer backbone with multiple initiation sites (i.e., a multifunctional macroinitiator). ATRP was initiated from these sites to form densely packed side chains. However, in normal and AGET ATRP in solution, gelation typically occurred at 20–30% monomer conversion. Conversely, in miniemulsion systems eventual crosslinking occurs within the latex particles, with limited effect on the fluidity of the miniemulsion system even when monomer conversion reaches >80%. [97].Finally, “grafting-onto” could be conducted by attaching clickable functional groups to the backbone, followed by performing a click reaction. This approach was not yet used in dispersed media.Star polymers represent a class of branched architectures with linear “arms” connected to a central branching point, typically referred to as the “core”. [154–156] Similarly to the “grafting-from” approach for the synthesis of bottlebrushes, star polymers have been mainly prepared via the “core-first” approach. Compounds with multiple hydroxyl groups (e.g., cyclodextrin, glucose, tannic acid) can be transformed into (multi)functional ATRP initiators by substituting hydroxyl groups with C-X functionalities, typically a-bromoisobutyrate groups. Cyclodextrin-based ATRP initiators with 14 C–Br sites were used to prepare stars with PBA and PBMA arms in miniemulsion via ARGET ATRP, using the Cu/TPMA-SDS catalytic system. [97].Highly branched polymers, including dendrimers and hyperbranched polymers, possess highly compact structure, high branching density in the backbone, and numerous periphery groups, leading to many interesting properties, such as high solubility, low viscosity, high functional group density, and potential for cargo loading and release. [157] Dendrimers are regularly branched polymers with a dendritic, tree-like structure. [158,159] Hyperbranched structures do not necessarily have regular branches, however, in contrast to dendrimers, they are prepared via inexpensive one-pot synthesis while retaining highly branched architectures, high solubility, low viscosity and high functional group density. Hyperbranched structures with controlled number of branching sites were prepared by ATRP, through the copolymerization of an inimer (a monomer with an initiating group) with one or more conventional monomers. [160–162] The preparation of hyperbranched polymers via microemulsion ATRP enabled faster kinetics and the generation of polymers with higher MWs, narrower MW distributions and through faster polymerization processes than in typical solution polymerizations, where high dilution and low monomer conversions were needed to avoid gelation. [102].Microemulsion ATRP was also exploited to prepare hyperstar polymers, i.e., core–shell structured star polymers that contain a highly branched polymer as the core and densely grafted radiating arms. This required the polymerization of an inimer to form an hyperbranched core with abundant C–Br initiating sites for the subsequent growth of radiating arms through the addition of second monomer, in a one-pot microemulsion ATRP process. [163,164] When hydrophobic BA was employed as second monomer, BA molecules diffused into the latexes and swelled the hyperbranched polymers to form a seeded emulsion, minimizing hyperstar-hyperstar coupling events (Scheme 12 ). On the other hand, the use of a zwitterionic monomer, cysteine methacrylate, as second monomer enabled to stabilize the growing hyperbranched stars, owing to the electrostatic repulsion between charged arms and stars, which also avoided coupling at high conversion.The heterogenous nature of dispersed media has proven to be an excellent platform to produce nanostructured materials, generated either via (i) self-assembly of block copolymer surfactants, (ii) crosslinking polymers inside the dispersed phase, and (iii) exploiting the peculiar properties of the oil-water interphase.Post-polymerization self-assembly of pre-formed amphiphilic block copolymers with incompatible blocks typically occurs in highly diluted copolymer solutions to form microphase separated structures. Instead, polymerization induced self-assembly (PISA) in aqueous dispersion with relatively high solid content (up to 50 wt%) is characterized by simultaneous polymerization and self-assembly by using a soluble macroinitiator that also acts as a stabilizer as the polymerization of a second soluble monomer proceeds. Upon reaching a critical chain length, the second block becomes insoluble, driving the reorganization of the block copolymer into a variety of nanoobjects via a dispersion polymerization approach. [165] RAFT polymerization has been frequently coupled to a PISA approach. [166] In contrast, the application of ATRP in PISA is limited by the partitioning of Cu complexes in the complex dispersed system.ICAR ATRP with low Cu concentration was carried out using POEOMA-Br as the macroinitiator and stabilizer and poly(benzyl methacrylate) (PBnMA) as a core-forming block. [167] The system was homogenous in ethanol, but phase separated upon polymerization. Distinct architectures were obtained either at room temperature or at 65 °C, i.e., below and above the T g of PBnMA, respectively (Scheme 13 ). Another core-forming monomer, glycidyl methacrylate, underwent ring-opening reaction during PISA ATRP, allowing for the in situ formation of crosslinked nanoparticles. [168] Protein-polymer conjugates could be synthesized during ATRP induced self-assembly. The aqueous dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA) from a hydrophilic protein (human serum albumin, HSA) modified with ATRP initiating groups yielded HSA-PHPMA nanospheres and vesicles. During the PISA process, a model green fluorescent protein (GFP) was encapsulated in situ, and the polymeric architecture enabled enhanced intracellular GFP delivery. [169].Multi-layer polymer particles could also be prepared by means of hydrophobic multiblock copolymers with incompatible compositions prepared in dispersed media. Sequential miniemulsion ATRP and in situ seeded emulsion ATRP yielded poly(isobutyl methacrylate)-b-polystyrene P(iBMA-b-St) exhibiting a core-shell structure. Upon subsequent alternating addition of iBMA and St, an onion-like, alternating multilayered morphology was observed. [113,114] Similar polymer-vesicle latex particles were prepared by starved feeding of MMA to Br-modified vesicles particles. [170].Chemically crosslinked nanoparticles are more stable against external stimuli and mechanical processing compared to self-assembled structures. The most straightforward recipe to prepare chemically crosslinked nanonetworks via polymerization in dispersed media is to incorporate a crosslinker, e.g., EGDMA. 2-Hydroxyethyl methacrylate (HEMA), 4-vinyl pyridine (4-VP) and PEGDMA were copolymerized in an inverse emulsion, forming a pH-sensitive hydrogel that underwent multiple swelling-deswelling cycles. [171] Similar PHEMA-POEOMA nanogels with uniform and controllable sizes were prepared in inverse miniemulsion by AGET ATRP, where the hydroxy groups from the HEMA moiety allowed for introducing photoinitiation sites for subsequent photopolymerizations. [172].In comparison with crosslinked nanoparticles prepared by conventional free radical polymerization (FRP), those made by ATRP had narrower molecular weight distribution, sharper glass transition [173], and improved loading efficiency. [174] Degradable nano-networks were prepared by ATRP utilizing a disulfide-containing crosslinker. [175–177] Cross-linkable monomers such as allyl methacrylate were incorporated into a block copolymer, where the allyl group could crosslink under UV irradiation, forming network structures. [178] Alternatively, other techniques such as ring opening metathesis polymerization (ROMP) and click-chemistry were conducted simultaneously with heterogeneous ATRP, forming crosslinked networks in one pot. [179–181].Nanoparticles consisting of a solid core and some polymer chain “hair” attached to the core surface are termed “hairy nanoparticles”. Common core materials include metal/metal oxide, silica nanoparticles, and polymer networks. Polymer network nanoparticles, generally prepared by emulsion/miniemulsion polymerization and functionalized with ATRP initiating groups were re-dispersed in monomer solution to form hairy nanoparticles by SI-ATRP. For example, ATRP initiating groups were immobilized onto P(St-co-HEMA) microspheres, enabling the grafting of 2,2,6,6-tetramethyl-4-piperidyl methacrylate. [182] Hydrophilic polymer chains were grafted from hydrophobic cores, yielding amphiphilic hairy nanoparticles with poly(ethylene glycol) or polyzwitterionic-based corona (Scheme 14 ). [183].Nanocapsules have attracted much interest due to the encapsulation and controlled release capability provided by their hollow structure. [184,185] Nanocapsules are typically prepared in miniemulsion polymerizations, by confining the polymerization within the water/oil interface. [186] PEO-PBMA-Cl, an inisurf bearing an ATRP initiating group in the hydrophobic fragment, was used to initiate miniemulsion ATRP of BMA at the water/monomer droplet interface, with polymer chains slowly growing inward in a controlled manner. In the presence of a crosslinker and an organic solvent, polymeric nanocapsules with high stability and good dispersibility in organic solvents were prepared. [187] The outer surface of the nanocapsules was further modified when using a difunctional reactive surfactant. Using N3-PEO-PBMA-Cl yielded functional nanocapsules with N3 moieties on the surface, which allowed for attaching organic dye probes or additional polymers via click-chemistry. [188].Similar nanocapsules were fabricated via Pickering emulsion ATRP, initiated at the oil-water interface. Materials with different morphologies were formed via either FRP or ATRP in Pickering inverse emulsion stabilized by CNCs and initiator-stabilized CNCs, respectively (Scheme 15 ). [189] FRP formed beads, whereas ATRP resulted in hollow structures due to the initiation solely occurring from the oil/water interface. Similarly, when ATRP was performed on silica nanoparticles functionalized with initiator and immobilized at the interface of Pickering emulsions, a polymer network grew inwards, although it did not fill the whole available space of the polymerizing internal phase. [190] Therefore, the resulting polymer-inorganic hybrid has the shape of hollow capsules, which could be processed as semipermeable membranes that served as microdevices for drug or cell delivery.Polymer-inorganic composites and hybrid materials combine the functionality and flexibility of polymers with the high strength of inorganic materials. Polymer-inorganic composites are systems where polymer and inorganic phases are mixed, and they are typically both present in aggregates of large dimensions. However, simple blending of inorganic materials and hydrophobic polymers often leads macroscopic phase separation, leading to difficulties in processing, inadequate structure heterogeneity, and poor mechanical properties. [191].Conversely, polymer-inorganic molecular hybrids are materials in which chemical bonds are established between the constituents, so that mixing is effectively occurring at the molecular level, and aggregation and surface defects are minimized. [192] Polymer-inorganic composites have unique properties, such as good mechanical and thermal stability, gas barrier performance, and flame retardancy. [193–195].Polymer-inorganic hybrids can be formed via miniemulsion ATRP from the surface of inorganic materials, which are well-dispersed in monomer droplets thanks to high-shear sonication; this allows for encapsulating inorganic materials in polymer latexes via covalent bonds. [193,196–198] SI-ATRP is a powerful tool to fabricate polymer-inorganic hybrids with improved properties, however polymerizations in solution are generally limited to low monomer conversion (∼10%) and/or conducted in the presence of sacrificial initiators to avoid interparticle crosslinking and macroscopic gelation. [11,199,200] In contrast, the segregation of polymerization loci in miniemulsion SI-ATRP enables to reach high conversion without gelation. Polymers were grafted from CdS quantum dots [196], montmorillonite nanoclay [201], and silica nanoparticles [202] via miniemulsion SI-ATRP, yielding well-defined particle brushes. In emulsion ATRP, the electrostatic interaction between a negatively charged P(AA-co-BA)-Br macroinitiator and positively charged Gibbsite platelets facilitated the good dispersion and alignment of the platelets in the resulting polymer matrix, forming “muffin”-like encapsulated Gibbsite structures. [203–205].Janus structures are another example of complex polymer architecture, which consist of particles whose surfaces display two or more distinct physical properties. [206] Janus structures can be applied as particulate surfactants, imaging nanoprobes, and self-propelled colloidal materials capable of “smart” motion.Self-assembly of incompatible (co)polymers in emulsion is one strategy to prepare Janus nanoparticles (Scheme 16 a). For example, non-functional PMMA and functional P(St-BIEM) (BIEM = 2-(2-bromoisobutyryloxy)ethyl methacrylate, an ATRP initiator) were emulsified together and self-assembled into Janus composite particles during solvent evaporation. P(St-BIEM) accumulated on one side of the particle. Subsequently, poly(2-(dimethylamino)ethyl methacrylate) was grafted from the surface area occupied by localized C–Br initiation sites of P(St-BIEM), forming “mushroom” particles with controllable morphology. [207].In a Pickering emulsion, the Pickering agent itself can be transformed into a Janus particle via ATRP. Graphene oxide (GO) platelets, modified with an ATRP initiator, served as Pickering agents in a toluene/water emulsion. ATRP of toluene-soluble 2-(acryloyloxy)ethyl ferrocenecarboxylate (MAEFc) occurred only on the side of the platelet exposed to the monomer solution. Subsequently, grafting of polydopamine led to Janus GO nanosheets. [209] In an emulsion stabilized by silica-Br nanoparticles and containing an hydrophilic and an hydrophobic monomer in the two phases (Scheme 16b), different polymer brushes were simultaneously grafted from the nanoparticles via SI-ATRP, forming amphiphilic Janus colloids with advanced emulsification properties. [90,208] The stable colloid structure prevented the particles from rotating during polymerization, giving rise to a clean Janus morphology.Highly porous polymer monoliths have high surface area and thus can find application in separation, catalysis, and extraction. [210] pHIPE is a representative example of highly porous polymer monolith prepared through an emulsion templating approach. [57,58] The highly interconnected porosity and high surface area renders pHIPE materials suitable as liquid droplet elastomers and templates for molecular recognition materials. [211,212] Various technologies have been employed for pHIPE formation, including FRP, RDRP, step-growth polymerization, click reactions, etc. Among these approaches, RDRP (including ATRP and RAFT polymerization) provides more homogeneous network structures. [58].Typical HIPEs are water-in-oil emulsions, where the hydrophobic monomer(s) is dissolved in the continuous oil phase, while the internal phase represents over 74% of the total volume. The type and locus of ATRP initiation affects the macromolecular structure of pHIPEs. [213] When a conventional oil-soluble ATRP initiator was used by dissolving it in the monomer phase, the final pHIPEs presented rather spherical voids. When nanoparticles functionalized with ATRP initiators were employed, Pickering HIPEs were obtained, in which the type and locus of initiation affected the porous and macromolecular structure of the resulting pHIPEs. If a highly organic-soluble nanoparticle ATRP initiator was used, then a pHIPE with rather spherical voids was produced and no preferential diffusion of monomer molecules to the interface was observed. Conversely, when a water-soluble conventional radical initiator was introduced in the system, then interfacial polymerization occurred, and polyhedral voids were formed. During interfacially initiated polymerization, the monomer diffused toward the oil/water interface and the polymerizing macromolecules “locked-in” the nanoparticles at the interface, affecting both the wall and pore structure.Low density and degradable pHIPEs are also of interest. Materials with very low density (0.06 g/cm3) were prepared by using special star polymer surfactants by ATRP via an arm-first approach. The surfactants were active at a very low loading (<0.1 wt%). [214] Degradable pHIPEs were prepared by incorporating disulfide crosslinkers in the network. [215] Bis(2-methacryloylxyethyl)disulfide (DSDMA) crosslinker was copolymerized with 2-ethylhexyl methacrylate by AGET ATRP catalyzed by the hydrophobic catalyst CuBr2/BPMODA (BPMODA = bis[2-pyridylmethyl]octadecylamine). The material had a uniform crosslinked structure which was degraded by tributylphosphine (Bu3P), and the resulting degraded product had M n = 30,500 and Đ = 1.6. The low molecular weight and relatively low dispersity indicated the good degradability of the pHIPE.Similarly, ATRP was conducted in a medium internal phase emulsion (MIPE) system (where the internal phase is between 30% and 70%). The MIPE was stabilized by the synergy of Pluronic F127 and amphiphilic diblock glycopolymers. [216] The latter comprised a glycopolymer-block and a Br-terminated PSt block, thus serving as inisurf for AGET ATRP of styrene. The resulting pMIPE exhibited a biocompatible, homogeneous structure with bimodal pore size distribution, showing potential for use in catalysis and biomedical applications.The versatility of ATRP in dispersed media and the preparation of polymers with a broad range of architectures have opened the door to numerous applications. Polymers and copolymers made by ATRP in aqueous dispersed media have been tested for bio-related applications and for coatings. In the following paragraphs, several potential applications will be reviewed, including drug delivery, molecular recognition, bio-quantification, as well as the design of polymer coatings and films with electrocatalytic properties. These applications were supported by synthesizing smart (bio)materials with stimuli-responsive functionalities.Several complex polymer structures, (e.g., vesicles, nanogels, and hydrogels) hold promise as drug carriers and other biomaterials. Limited biocompatibility and hydrophobicity could be overcome by PEGylation of the nanoobjects. For example, upon incorporation of POEOMA chains by emulsion ARGET ATRP, the cytotoxicity of cationic nanogels was greatly decreased without compromising their antibacterial activity. [217].Polymer nanoparticles, including nanocapsules, nanospheres, and nanogels, are suitable materials for drug delivery. [218] Hydrophilic nanogels prepared by inverse miniemulsion/microemulsion ATRP have uniform network structure, and thus higher swelling ratios and better colloidal stability in comparison to analogous nanoobjects made by FRP. Moreover, they benefit from controlled degradability upon incorporation of functionalities with desired responsiveness. Nanogels made by ATRP in inverse mini/microemulsions could be loaded with star-branched polymer nanoparticles (via in situ covalent incorporation), with carbohydrates and proteins (via in situ physical incorporation), with fluorescent dyes, anticancer drugs, and gold nanoparticles (via physical incorporation). The versatility of cargos and the biocompatibility of the nanogels imparted great potential for targeted drug delivery applications. [219].On the other hand, hydrophobic molecules could be loaded into polymer particles made in O/W systems. Dispersion polymerization of PEG, 2-(diethylamino)ethyl methacrylate (DEAEMA), and tert-butyl methacrylate (tBMA) led to nanoparticles, where each monomer had a different function: the short hydrophilic PEG chains provided biocompatibility to the outer surface of nanoparticles, the hydrophobic PtBMA core facilitated the loading and release of hydrophobic fluorescein molecules, and the positively charged PDEAEMA could uptake negatively charged siRNA. Thus the cationic nanoparticles served as carriers for both nucleic acids and hydrophobic drugs. [173].The delivery of drugs could be achieved by incorporation of stimuli-responsive moieties, including thermo-responsive polymers, pH-responsive groups, degradable bonds, and magnetic Fe3O4. [220] Di(ethylene glycol) methyl ether methacrylate (M(EO)2MA) is a water-insoluble monomer, while its polymer has a lower critical solution temperature (LCST) of 25 °C. The transition temperature of thermoresponsive P(M(EO)2MA-OEOMA-EGDMA) microgels could be tuned by changing the network composition. [221] The drug-releasing profile was controlled by either tuning temperature or by chemical reduction of disulfide bonds in DSDMA (Scheme 17 ), used in place of EGDMA. The magnetically loaded microgels were guided to particular body parts for the delivery of anesthetic drugs. [222].The recognition of molecules through devices with high reusability, high selectivity, and low ageing is essential for sensing and removal of toxic compound. Molecularly imprinted polymers (MIP)s were used for effective molecular recognition. [223] MIPs were prepared by copolymerization of functional monomers and crosslinkers in the presence of a template (i.e., the target molecule or a dummy molecule with a similar structure to the target molecule), followed by the removal of template molecules to generate tailor-made recognition sites, which resemble the shape, size and functionality of the template. The preparation of MIPs in dispersed media polymerization is advantageous over surface/film imprinting and surface graft imprinting because of the low toxicity, good dispersibility, and large adsorption capacity in dispersed media.Indole MIP was synthesized by emulsion ATRP via copolymerization of 4-VP and vinyl modified SiO2 nanoparticles in the presence of EGDMA as crosslinker. Indole was incorporated by hydrogen bonding with 4-VP. The resulting indole MIP had high specific area with an equilibrium adsorption capacity of 34.5 mg/g, showing promise in removing indole from fuel oil. [224] Emulsion ATRP was used to synthesize superparamagnetic molecularly imprinted nanoparticles for selective recognition of tetracycline molecules from aqueous medium. Acrylate-modified Fe3O4 was copolymerized into the MIP, allowing for the recognition of tetracycline with a capacity as high as 12.1 mg/g; furthermore, the material could be reused multiple times. [225] MIPs prepared via emulsion ATRP from multifunctional initiation sites of yeast presented fast recognition of ciprofloxacin, high adsorption, and high reusability. [226] Precipitation eATRP of 4-vinylphenylboronic acid in the presence of sialic acid as template molecule yielded nano-MIPs with morphology and size tuned by varying the applied potential. [118].Glycopolymers bearing carbohydrates are highly effective in protein recognition due to the multifunctional carbohydrate side chains. Glycopolymer-modified colloidal particles prepared by emulsion ATRP methods allowed for the recognition of specific proteins. Chlorine-modified PSt nanoparticles were synthesized by emulsion polymerization, followed by SI-ATRP, to incorporate glycol-modified PSt chains on the surface. The resulting particles coagulated in the presence of lectins, including Concanavalin A (Con A) or peanut agglutinin (PNA) through specific glycol recognition sites. [92] Glycopolymer-grafted nanoparticles were also casted into a film to recognize proteins on a surface. For example, a glycosylated amphiphilic block copolymer P(HEMAGl-b-BMA)-Cl was used as an inisurf (see Section 3.1) for BMA emulsion polymerization to prepare glycolsilated core-shell particles. The resulting latexes were cast into polymer films with bioactive surface owing to the presence of PHEMAGl capable of specific binding of Con A. [227].ATRP in dispersed media was also applied to the preparation of highly selective filtration material. Hydrophilic Fe3O4 particles used as Pickering agents were modified by ligand exchange with Br-containing carboxylic acid on one side, and subsequent ATRP of MMA led to amphiphilic, superparamagnetic Janus nanoparticles with excellent performance in oil purification. [228].Precipitation polymerization of N-isopropylacrylamide (NIPAAm) has been used to quantitatively detect hemoglobin, which acted as a catalyst for the ATRP of NIPAAm. The polymerization was conducted at 37 °C (i.e., above the LCST of PNIPAAm), and simple measurement of the turbidity could reflect the amount of hemoglobin in the system (Scheme 18 a). The viability of the hemoglobin dose-turbidity formation rate assay both in solution and in physiological fluids proved the versatility of this method, which is also environmentally friendlier than established chemical assays for hemoglobin based on toxic reagents. [229].ATRP in dispersed media was translated from solution to surface polymerization for the preparation of polymer brushes. [231] A surface was functionalized with ATRP initiator and put in contact with an aqueous solution containing catalyst and hydrophobic monomer aggregates. Remarkably fast brush growth was observed, ascribed to the formation of monomer aggregates in the aqueous phase and to the beneficial role of hydrogen-bonding by interfacial water.Self-healing coatings via microencapsulation exploit the release of flowable material from a microcapsule to restore cracks or damages. These approaches were based on mixing ATRP components in a microreactor (i.e., the microcapsule) that induced polymerization upon release of its constituents. [232,233] Preparation of solvent-based microcapsules via Pickering emulsion templated interfacial ATRP (PETI-ATRP) involved the electrostatic deposition of a polyanionic ATRP initiator onto cationic nanoparticle surfaces, which yielded modified nanoparticles (Scheme 18b). [230] Subsequently, the microcapsules were synthesized by PETI-ATRP of N,N′-methylene bisacrylamide to form the shell wall. The method allowed encapsulation of core solvents (xylene, hexadecane, and perfluoroheptane) with different solubility properties, and the microcapsule wall-forming chemistry afforded the use of different vinyl monomers.In another application, switchable latexes were prepared and employed in coating compositions that responded to changes in the environment. Using 1,1-(diethylamino)undecyl 2-bromo-2-methylpropanoate as an amine-bearing inisurf (Scheme 19 ), the resulting PMMA polymer latexes could switch between aggregated and dispersed states using CO2 and argon as triggers. [234].Crosslinked nanoparticles obtained in dispersed media were used to prepare catalytic films on electrodes. For example, surface-protected P(AN-b-BA) polymer nanoparticles were pyrolyzed into individual nanoporous carbon spheres with electrocatalytic properties. [235] These PAN-based nanoparticles were cast as an electroactive material with high surface area, for application as supercapacitors or for the oxygen reduction reaction. Surface-protected P(AN-b-BA) self-assembled polymer nanoparticles prepared by miniemulsion ATRP were pyrolyzed into individual nanoporous carbon spheres with better performance for CO2 capture with a higher CO2/N2 selectivity, and increased efficiency in catalytic oxygen reduction reactions, as well as improved electrochemical capacitive behavior, as compared to regular nanocarbon monoliths. [235,236].The last 15 years have witnessed important developments and increasing interest in performing ATRP in aqueous dispersed media for preparing well-defined polymers and nanoobjects. The availability of low-ppm ATRP techniques triggered the development of novel synthetic approaches for heterogenous ATRP that used lower Cu loadings, facilitating the removal of Cu from the final latexes. Moreover, the design of reactive surfactants and catalytic systems capable of overcoming the limitations dictated by the partitioning of most Cu complexes among different phases enabled improved polymerization control. Noteworthy is the development of effective approaches for ATRP in emulsion, including the seeded-emulsion strategy, phase-transfer catalyst, surfactant-ligand complex, and a combination of ion-pair and interfacial catalysis via hydrophilic Cu complexes and anionic surfactants. Traditional ab initio emulsion is much easier to employ at large scale compared to miniemulsion or microemulsion methods, which require high shear forces and high surfactant loading, respectively. Moreover, oxygen scavenging strategies and catalytic halogen exchange simplified the reaction setup for a broader range of polymer compositions and architectures.When performing ATRP in dispersed media, the segregation of polymerization loci into droplets of 50–500 nm diminished interparticle radical coupling reactions, thus high viscosity and macroscopic gelation were avoided, enabling to reach higher monomer conversion compared to homogeneous ATRP. This benefited the preparation of polymer bottlebrushes, hyperbranched structures, and inorganic-polymer hybrids with excellent control over polymer dispersity even at high monomer conversion. Various polymer architectures and topologies were achieved by exploiting self-assembly or crosslinking approaches, and by introducing inimers or multi-functional surfactants. The tunable composition of polymers allowed for their use for molecular recognition and bio-quantification. The hydrophobic environment within polymer nanocapsules or nanogels and the incorporation of stimuli-responsive functionalities enabled the loading and delivery of hydrophobic drugs. Nanocapsules were designed for potential application in self-healing coatings.Innovations in dispersed media ATRP are expected to come from improved mechanistic understanding, as recently illustrated by the design of the Cu/TPMA-SDS catalytic system, which outperformed most catalysts in terms of polymerization control and ease of product purification. This advancement eliminated the need for developing hydrophobic Cu complexes or dual-catalyst systems. In-depth studies of catalyst partitioning and interactions with other components can guide the rational design of reactive surfactants, ligands, or phase-transfer complexes.Mechanistic analysis could be promoted by computational studies and simulations, although examples of computational studies of dispersed media polymerizations are still quite rare. PREDICI, a kinetic-based modeling tool, was successfully used for modeling ATRP under homogeneous conditions [237,238], even for bottlebrush preparation. [239] However, only one report used PREDICI for the simulation of (semi)batch emulsion ATRP of styrene. [240] A Design of Experiment (DoE) approach was used to assess the influence of five independent variables (catalyst, initiator, temperature, reducing agent and surfactant loadings) on monomer conversion, polymer average molecular weights, and dispersity in AGET emulsion ATRP. Analysis of 5 fractional factorial experiments showed that temperature was the most influential factor. [241].The translation of academic research on ATRP in dispersed media into industrial applications depends on the multidisciplinary collaboration among chemists and engineers for reaction scale-up, as well as with biologists and medical researchers for the preparation of biomaterials and efficient drug-delivery systems. Roughly half of commercial coatings are prepared by dispersed media polymerizations, and several polymer-based products, including paints, creams, and medical treatments are sold in a dispersed state. The industrial use of heterogenous ATRP systems is promoted by simple and low-cost reactions setups, as in the case of ab initio emulsion ATRP, which could be readily integrated into existing reactors for emulsion FRP. Indeed, emulsion ATRP was already scaled up to the 2 L volume. [242] Upon optimizing the reaction conditions, including temperature and reagent ratios, PMMA with dispersity of 1.17 was produced, without appreciable coagulation. [242,243] Despite the complexity of heterogenous emulsion ATRP systems, empirical models showed that temperature strongly affects the rate of polymerization, M n and Ð. Moreover, the surfactant amount and stirring speed affected the rate of emulsion ATRP, while the nature and amount of ligand mostly influences M n and Ð of the formed polymer. [244] Improved understanding of the influence of various parameters, achieved through modeling and high-throughput experimentations, as well as continuous advancement in reaction design, is expected strongly promote the industrialization of ATRP in dispersed media in the coming years.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: K Matyjaszewski reports financial support was provided by National Science Foundation.Fundings from the National Science Foundation (CHE 2000391 and DMR 2202747), the European Union – NextGenerationEU, and the University of Padua under the 2021 STARS Grants@Unipd programme “Photo-e-cat” are gratefully acknowledged.

Polymerizations in aqueous dispersed media benefit from good heat transfer, low viscosity, low content of volatile compounds, and established industrial use. Over the past two decades, the implementation of ATRP in dispersed media highlighted the possibility of achieving improved livingness and producing a variety of macromolecular architectures. With the introduction of several methods for activator regeneration in ATRP, catalyst loading has been greatly diminished, and the reaction setup has been simplified. The availability of ATRP techniques employing low-ppm (part per million) catalyst loadings enabled to access an even larger variety of polymer architectures and functional polymer particles, which have been applied for molecular recognition, drug delivery, bio-quantification, and advanced coatings. This minireview presents innovative synthetic approaches, polymer architectures, and relevant applications, as well as the challenges that remain to be overcome to promote the industrialization of ATRP in dispersed media.

Definition UnitDeactivation factor -Archimedes number -Carbon yield gC/gcat Weisz-Prater criterion -Effective diffusivity m/2Diameter of the catalyst particle mGravitational acceleration m/s2 Activation energy of kinetic parameters kJ/molKinetic rate constant mol CH 4 / atm CH 4 / g cat / min Adsorption rate constant of hydrogen 1/atm1.5 Adsorption rate constant of methane1/atmKinetic rate constants of deactivation factor Dependent on equationEquilibrium constant atmLifetime of the catalyst minConcentration of the reactant in bulk of gas mol / m gas 3 Order of reaction -Partial pressure of methane atmPartial pressure of hydrogen atmInitial reaction rate mol CH 4 / g cat / min Actual reaction rate mol CH 4 / g cat / min Reynolds number at u mf -Time minTemperature ° CSuperficial velocity of the gas m gas 3 / m reactor 2 / s Minimum fluidization velocity m/sSpace Velocity Ln/ gcat/ minEffectiveness factor -Density of the gas kg/m g 3 Density of the particle kg/m p 3 Viscosity of the gas Pa.sHeat of reaction kJ/mol CH 4 Heat of adsorption of components kJ/molThe destructive consequences of the climate change crisis followed by ongoing efforts toward emission-free technologies have instigated a growing interest in low CO2 and CO2-free hydrogen production (Amin et al., 2011; Borghei et al., 2010; Ashik et al., 2017; Nezam et al., 2021; Ra et al., 2020). Various approaches such as chemical looping reforming, steam methane reforming integrated with Carbon Capture and Sequestration processes (CCS), water splitting, thermal and thermocatalytic decomposition of methane have been studied for this purpose. Among those, thermocatalytic decomposition of methane (TCD) is one the most promising (Hadian et al., 2021). A major advantage of TCD is the potential capability of producing highly valuable carbon nanomaterials instead of CO2, next to hydrogen. The intrinsic characteristics of carbon nanomaterials make them suitable for many industrial applications such as building materials, semiconductors, catalytic materials and energy storage (Ashik et al., 2015; Ashik et al., 2017; Douven et al., 2011; Saraswat and Pant, 2013). In addition, TCD requires less complex down stream purification or separation units than conventional processes. These advantages make TCD an environmentally and economically attractive approach for CO2-free hydrogen production (Hadian et al., 2021).Methane is thermally decomposed to solid carbon and gaseous hydrogen in the absence of a catalyst or oxidizing agents at temperatures above 1300 ° C (reaction 1). Alternatively, in TCD, a catalyst facilitates the same reaction at a much lower temperature (500 °C-950 °C) with formation of nano-structured carbon materials. The structure of this material depends on operating conditions and foremost on the catalyst properties that are employed (Hadian et al., 2021). Nickel, iron, copper and carbon are the most studied active sites of the catalyst and among them nickel on silica support, Ni-SiO2, showed the highest methane decomposition activity (Pudukudy et al., 2016; Wang et al., 2000; Reshetenko et al., 2003; Avdeeva et al., 1996; Li et al., 2000; Guevara et al., 2010). (1) CH 4 (g) - > C(s) + 2 H 2 (g) Δ H ( 298 K ) = + 74.52 kJ / mol A considerable amount of literature has been published on the preparation of single or bimetallic or carbonaceous catalysts. Their performance in very small lab-scale units and under mild reaction conditions have been established. Srilatha et al. (2017) and Ashik et al. (2015, 2017) reviewed and compared these studies using carbonaceous and metallic catalysts, most of which have been employed in small-scale fixed bed reactors with up to 0.5g of catalyst at limited space velocities and low concentrations of methane. Since the size of the catalyst particle in TCD is increasing over time due to carbon build-up, fixed bed reactors suffer from serious drawbacks such as a high probability of clogging, particle crushing, increasing pressure drop and fracturing the body of the reactor. Therefore, fluidized bed reactors are preferred over fixed bed reactors for large-scale TCD. Indeed, there have been few studies that use fluidized bed reactors or high space velocities (ratio of the total flow rate at normal conditions per gram of catalyst initially loaded), SV. For instance Torres et al. (2012) performed experiments with 20g of fine catalyst particles in a fluidized bed; however, the SV did not exceed 0.2Ln/ gcat/ min. Suelves et al. (2009) used higher SV (2Ln/ gcat/ min) in a fixed bed reactor that contained no more than 0.05g of catalyst.Alongside experimental parametric studies on the performance of the reaction, kinetic studies on TCD in a fixed bed reactor and mild conditions and the mechanism investigations of reaction 1 over metallic catalysts have been performed. These studies revealed that the actual rate of TCD is not constant over time and can be described by Eq. 2, where r 0 is the maximum reaction rate and a ( t ) is defined as the deactivation factor (Borghei et al., 2010; Amin et al., 2011; Douven et al., 2011; Latorre et al., 2010). In an earlier contribution, the authors summarized the kinetic studies and the proposed kinetic models, including the maximum reaction rate and deactivation factor of the catalyst (Hadian et al., 2021). Several researchers (Amin et al., 2011; Saraswat et al., 2016) proposed a mechanism based on the molecular adsorption of methane followed by step-by-step dehydrogenation reactions until separate adsorbed atoms of carbon and hydrogen are obtained. The first dehydrogenation reaction was found to be the rate-limiting step. The remaining carbon atom of methane on the surface of metal active site, passes through the metal by diffusion and forms nano layers of carbon on the other side. If the decomposition step occurs faster than diffusion and construction rate of carbon nano-structures, carbon atoms accumulate on the surface of metal active site and deactivate it by encapsulation (Toebes et al., 2002; Henao et al., 2021). The maximum reaction rate is modelled by a Langmuir–Hinshelwood type equation that accounts for the thermodynamic equilibrium and the competition between hydrogen and methane adsorption over the active sites, as represented by Eq. 3. The semi-empirical deactivation factor expression is obtained from a species balance on the active sites of the catalyst, resulting in Eq. 4. (2) r ( t ) = a ( t ) × r 0 (3) r 0 [ mol CH 4 / g cat / min ] = k ( P CH 4 [ atm ] - P H 2 2 [ atm ] / K p ) 1 + K H 2 P H 2 1.5 [ atm ] + K CH 4 P CH 4 [ atm ] 2 (4) a = 1 1 - 0.5 k d k d , C + k d , CH 4 P CH 4 + k d , H 2 P H 2 0.83 t - 0.8 Although extensive research has been performed on catalyst preparation and reaction mechanism of TCD, very little is currently known about the feasibility of the TCD process in large scale industrial fluidized beds at the harsh operating conditions encountered. The performance of the catalyst and the fluidized bed reactor need to be thoroughly investigated by performing experimental and numerical studies. This performance can be expressed in terms of the maximum reaction rate, r 0 , the life-time of the catalyst, LT, and carbon yield (the ratio of the mass of produced carbon to the initial mass of catalyst used, Eq. 5), CY. The outline of this paper is as follows: in Section 2 we describe the experimental setup, materials as well as the adopted procedures. In Section 3 we introduce the reactor model used for the interpretation of the experiments, whereas the results and discussion is given in Section 4. Finally in Section 5 the conclusions are presented. (5) carbon yield ( CY ) = mass of produced carbon ( g ) initial mass of catalyst ( g ) In the experiments a commercial catalyst made by BASF (Ni 5256 E RS) was used. The catalyst is originally designed as a hydrogenation fixed bed catalyst that contains 56% nickel on silica support and was supplied as extrudates and in reduced and passivated state. Table 1 shows the characteristics of the fresh catalyst. All of the gases used in this study were at least 99.995% pure, supplied by Linde.The experiments were performed in a cylindrical quartz fluidized bed reactor equipped with a spherical free-board section, see Fig. 1 . The inner diameter and the height of the cylindrical part are 1cm and 10cm, respectively and the diameter of the free-board is 7.5cm. The spherical free-board reduces the chance of entrainment by lowering the gas velocity and at the same time acts as an expansion space for the growing catalyst particles. The reactor was placed in an electric oven and the desired feed gas composition and flow rate were adjusted by calibrated Bronkhorst mass flow controllers. The local temperature in the reactor can be measured with the help of thermocouples. The outlet gas is transferred to a SICK gas analyzer model GMS815P (three measuring modules: Thermor, Oxor-P and Multor) for gas composition measurement after cooling down, Fig. 2 .In each experiment 1g of crushed and sieved (500-600 μ m) catalyst was loaded into the reactor, unless otherwise stated (to examine the effect of the parameter). Prior to activation of the catalyst, the air in the porous catalyst particle was extracted by flowing 2Ln/ gcat/ min nitrogen to the reactor for 30min. Subsequently the temperature of the reactor was increased to 250 °C by a ramp of 5° C/min using 10vol. % H2 in the feed to actuate the catalyst. The catalyst was further reduced in 100vol.% H2 with the same ramp of temperature up to 500 °C. Afterwards, the catalyst was heated up further in nitrogen. Once the reaction temperature was reached, 4.5Ln/ gcat/ min of the predefined gas composition of methane, hydrogen and the inert gas (nitrogen) was fed to the reactor, and the outlet composition was measured typically until the catalyst was fully deactivated. Finally, the reactor was cooled down and the product was collected and weighed.BET surface area and pore volume measurements have been carried out for the used catalyst using Thermo Fisher Scientific Analyzer model Surfer. Oxidation temperature of the produced carbon was measured in air via Thermo Gravimetric Analysis (TGA) to characterize the products. Surface composition of the fresh and used catalyst was analyzed by X-ray Photoelectron Spectroscopy (XPS) measurements on Thermo Scientific K-Alpha XPS with an Al source (1486.6 eV). The structure of carbon nanomaterials were obtained by performing Transmission Electron Microscopy (TEM) imaging of the samples of the products on a FEI cryo TEM TITAN 300 kV.The composition of the gas entering the reactor is known and the same as the predefined values for each experiment. However, as the gas passes through the reactor methane is consumed and hydrogen is produced. Therefore, the composition of the gas, and consequently the reaction rate is varying along the height of the bed whereas only the outlet conversion can be compared with the experimental data. This conversion is calculated by a Plug-Flow Reactor (PFR) model, Eq. 6. See Fig. 3 -a. (6) dX dw [ g cat ] = - r 0 [ mol / min / g cat ] F A 0 [ mol / min ] , X ( 0 ) = 0 where X is conversion, F A 0 is the molar flow rate of methane to the reactor, and dw is a fraction of the bed, such that the reaction can be considered constant over the fraction. r 0 is replaced by Eq. 3. This differential equation is numerically integrated by Runge–Kutta fourth-order method (RK4). The equilibrium constant at temperature of reaction, K p in Eq. 3, is calculated by Eq. 7 proposed by Kuvshinov et al. (1998),Zavarukhin and Kuvshinov (2004). (7) K p [ atm ] = 5.0215 × 10 5 exp - 9.12 × 10 4 RT The local partial pressures of methane and hydrogen are updated by Eq. 8 and 9. Finally, the kinetic parameter of Eq. 3 was fitted by comparing the conversion of the last section of the fluidized bed with the maximum conversion obtained from each of the experiments. (8) P CH 4 = ( 1 - X ) P CH 4 1 + X (9) P H 2 = 2 XP CH 4 + P H 2 1 + X Over time as the carbon products are being formed, the catalyst particles grow in size and weight with different rates. This growth can reach a point where some of the particles become too heavy to be fluidized by the available gas flow rate. Therefore, they settle down at the bottom of the reactor. These segregated particles are exposed to the fresh feed entering the reactor with a higher concentration of methane and lower concentration of hydrogen compared to the upper parts of the reactor. As a result, they grow faster and they also deactivate quicker than the rest of the particles. Over time more and more particles are segregated and deactivated until the whole bed is deactivated.In order to predict the deactivation of the catalyst and determine the parameters of Eq. 4 it is crucial to model this complex behavior over time. In order to obtain a predictive model for the deactivation, the model described in Section 3.1 is run for each time step starting from the beginning of the reaction until full deactivation. At the end of each time step, the total consumed methane and produced hydrogen and solid carbon are calculated and used to update the particle size. Then the minimum fluidization velocity of the particles is calculated for fine and coarse particles with Eqs. 10 and 11, respectively (Kunii and Levenspiel, 2013). (10) Re p , mf = [ 33.7 2 + 0.0408 Ar ] 1 / 2 - 33.7 (11) Re p , mf = [ 28.7 2 + 0.0494 Ar ] 1 / 2 - 28.7 where Re p , mf is the Reynolds number of the particles, Eq. 12, and Ar is the Archimedes number calculated by Eq. 13. (12) Re p , mf = ρ g u mf d p μ (13) Ar = d p 3 ρ g ( ρ p - ρ g ) g μ 2 By growing the particles the minimum fluidization velocity increases and the ratio of gas velocity to the minimum fluidization velocity, u / u mf decreases. The moment that u / u mf is not sufficient to maintain fluidization ( u / u mf < 1.2 this ratio is dependent on the reactor and particles properties), the bottom part of the bed is separated from the rest of the reactor (Fig. 3-b), and the particles are not mixed with the top part anymore. The inflow of gas to the top part is higher due to production of 2 mol hydrogen for each mole of consumed methane in the segregated section. Therefore, the ratio u / u mf can be high enough for fluidization of the particles in the upper sections of the bed. Due to further growth of the particles, the segregated zone propagates along the reactor and eventually the entire bed becomes segregated as shown in Fig. 3-c.This is a phenomenological 1D model representing the evolving reactor and therefore radial difference, wall effect, channelization, and bubble formation are neglected. Before the particles in the first section start segregating, all the particles are fluidized and well mixed in the reactor. Therefore, the particles grow with the same rate at this stage. Segregation only occurs if the carbon yield is high enough (mostly in cases with lower temperature or if hydrogen is added to the feed). Due to segregation, particles are not mixed any more. The growth rate is higher at the bottom of the reactor but there the deactivation starts earlier.Many experiments were conducted by systematically altering the settings of operating temperature, gas concentrations, catalyst particle size and WHGV. All results were confirmed with at least one duplicate experiment. In these experiments depending on the settings lifetime varied from 5min to longer than 12h where the obtained carbon yield ranged between 0gC/gcat to more than 70gC/gcat (at 550 °C and 70vol.% CH4-5vol.% H2).The max. conversion of the reactor was limited to about 20% because of the very high SV. It was observed that although at lower SV fluidization occurs with fresh catalyst particles ( u mf ≈ 0.1 m / s ), the heavier and larger particles including the produced carbon ( u mf depends on the CY and in can exceed 2 m / s for the largest particles) cannot be fluidized and therefore leads to breaking the reactor. Therefore, the gas flow rate (and as a result SV) is chosen to be high enough for mobilization of the grown catalyst particles even after hours of accumulation of carbon on them.Since the diameter of the reactor is relatively small and the consumed heat by reaction is small compared to the heat supplied by the oven, temperature drop along the reactor was limited to a maximum of 17 °C (at maximum reaction rate at 600 °C and with a feed of 100% CH4). Fig. 4 shows that the maximum reaction rate increases as the temperature is increased as expected. On the other hand, as can be seen in Fig. 5 -bottom, a high temperature has a negative effect on the lifetime of the catalyst. These findings are in agreement with literature findings (Hadian et al., 2021; Amin et al., 2011). The carbon yield is a parameter that integrates both effects of maximum reaction rate and the lifetime of the catalyst. Therefore, as shown in Fig. 5-top from 550 °C to 650 °C a shorter lifetime can overcome the higher reaction rate and carbon yield is significantly lower. However, at lower temperatures the carbon yield is more affected by lower maximum reaction rate and there is an optimum temperature for carbon yield between 500-550 °C, balancing initial reaction rate and lifetime of the catalyst. Fig. 6 shows that the maximum reaction rate is directly dependent on the volumetric concentration of methane as the only reactant of the reaction. What stands out in this figure is that at 550 °C and lower, the maximum reaction rate slightly declines with an increase in methane concentration to 90vol.%. A possible explanation for this might be that adsorption of methane is the dominating step at this temperature and saturation of the active sites allow less neighboring active sites to facilitate the detachment of hydrogen molecules.One unanticipated finding was that the lifetime of the catalyst is shorter when the concentration of methane (and therefore the reaction rate) is lower, Fig. 7 , while some other researchers observed the opposite behavior (Latorre et al., 2010; Henao et al., 2021). This effect is stronger at lower concentrations or at higher temperatures. We believe that the key difference is the scale of the reactor. Small reactors can be considered as a differential reactor and all the catalyst particles are in an environment with the same concentrations as the feed, while in the larger reactors such as in this work, except for the small portion next to the gas inlet, catalyst particles are in contact with a gas mixture containing the produced hydrogen. Another difference is that the methane concentration range in this study is much higher than most of the literature studies where only mild conditions were tested (max. methane concentration of 7.5% and 42.9% was tested by Latorre et al. (2010) and Henao et al. (2021) respectively). High concentrations lead to higher reaction rate and larger temperature drop along the reactor that is in favor of longer lifetime. At higher methane concentrations, larger amounts of hydrogen are also produced and are present in the reactor. As mentioned in Section 4.1.3, the addition of hydrogen changes the chemical potential and enhances the reverse reaction and converts the accumulated carbon on the surface of active sites back to methane. This phenomenon prevents encapsulation of the active sites and renews them, which boosts the lifetime of the catalyst significantly. Fig. 7 also reveals that carbon yield is decreased as the methane concentration is lowered by dilution with nitrogen. This was confirmed by using argon instead of nitrogen that led to almost the same effect. Analyzing the gas outlet with a mass spectrometer and also the solid products with XPS tests confirmed that neither nitrogen nor argon are involved in any reaction and are indeed inert.Adding a small fraction of hydrogen to the feed decreases the maximum reaction rate by promoting the reverse reaction by Le Chatelier’s principle, Fig. 8 . This leads to higher refresh rate of the surface of active sites and as a result a higher lifetime of the catalyst. Fig. 9 shows that also the carbon yield is improved by introducing small amounts of hydrogen to the reactor. This behavior was also observed by other researchers such as Latorre et al. (2010).Three different catalyst particle sizes were tested to investigate the importance of mass transfer limitation in TCD process. It can be seen from Fig. 10 that the effect of changing the particle size from the average diameter of 350 μ m to 800 μ m on the maximum reaction rate is very small. This confirms previous findings in the literature (Hadian et al., 2021; Saraswat et al., 2016; Borghei et al., 2010). However, over time the catalyst particle become larger and the effect of mass transfer limitation becomes more important by decreasing both diffusion length scale and pore volume due to carbon formation (See Table 1). These findings are also confirmed by Weisz-Prater criterion, see the appendix. Fig. 11 shows that the carbon yield is directly affected by the initial size of the catalyst particle. Because, mass transfer limitation keeps the concentrations of methane and hydrogen inside the particles compared with smaller particles lower and higher respectively. Therefore, the lifetime of the catalyst is boosted and as the result carbon yield is also increased.SV was changed in the experiments while maintaining the volumetric flow rate at 4.5Ln/min by changing the amount of the catalyst in the reactor. Lowering the volumetric flow rate would change the fluidization regime and can lead to breaking the quartz reactor due to defluidization of the grown catalyst particles and increasing it would turn the reactor at the beginning of the reaction into a pneumatic riser. Fig. 12 shows that the reaction rate does not linearly increase with SV because the local reaction rate decreases along the height of the reactor. An increase in the reaction rate is due to larger amounts of methane and smaller amounts of hydrogen available per unit of catalyst. This also explains the shorter lifetime and lower carbon yield of the catalyst at higher SV, Fig. 13 .BET measurement results are presented in Table 1 and reveal a sharp decrease in the specific surface area and pore volume compared with the initial catalyst, suggesting the pores are filled up with carbon. The material density of the produced carbon including the catalyst is much lower than the fresh catalyst. As a result, since carbon is less dense than the catalyst material containing nickel the bulk density is also reduced by about 32%. XPS analyses of the deactivated catalyst showed that all the nickel was in the metallic form and covered by carbon graphene layers. It suggests that deactivation is happening due to encapsulation of the active sites which makes them inaccessible for the methane molecules. It was found that even in the case of less pure gases (99.5%) no byproducts (either solid or gas) were formed.The fraction of carbon that is produced in the form of desired graphene layer structures can be determined by evaluating the Derivative ThermoGravimetric (DTG) curve of oxidation temperature. The DTG is defined as the derivative of a TGA curve of the corresponding oxidation temperature. The highest temperature limit for oxidation of amorphous carbon reported in literature is 410°C (Luxembourg et al., 2005). However, Hu et al. (2003) and Li and Zhang (2005) reported 365°C and 350°C respectively for the oxidation of amorphous carbon and the carbon that is oxidized in temperatures above these limits is generally accepted to be nano-structured carbon. Fig. 14 illustrates the TGA and DTG curve of the carbon produced from a feed of 100% CH4 at 550 °C. Even with considering 410 °C as the limit temperature of oxidation of amorphous carbon, the lowest purity of the different tested samples was about 96% nano-structured carbon. Fig. 15 shows a TEM image of a cluster of produced carbon nanofibers, CNFs, in 100% CH4 at 550 °C. The diameters are in the range of 15-80nm. CNFs produced at the different operating conditions were in the form of stacked cones, see Fig. 16 . Stacked cones are also called a fish bone structure and were obtained also in the literature on the nickel catalyst supported by silica (Toebes et al., 2002; Lehman et al., 2011).The lifetime of the catalyst for some of the conditions at 650 °C is so short that the maximum reaction rate could not be measured reliably. At 500 °C the reaction was limited by thermodynamic equilibrium in some of the operating conditions. Therefore, only the experimental data obtained at 550 °C, 575 °C and 600 °C were used to find the exact kinetic parameters by the model described in Section 3.1. Table 2 presents the kinetic parameters of the TCD reaction rate, Eq. 3. The average and maximum relative error of Eq. 3, using parameters from Table 2, do not exceed 11% and 22% respectively.The deactivation factor was fitted to the experimental data at 550 °C, 575 °C and 600 °C and Table 3 shows the obtained values. The total carbon yield obtained from each experiment and the model were compared. The average and maximum relative difference were 13.0% and 28.7% at the highest. Figs. 17,18 illustrate three examples of the performance of the catalyst over time in the experiments and predicted by the model at different operating conditions. Segregation of the reactor as it is described in Section 3.2 is clearly visible by drops of the deactivation factor.In this study, the thermocatalytic decomposition of methane in a fluidized bed reactor was studied and the corresponding reaction kinetics were established. A commercial nickel on silica catalyst was used in this study and carbon yields of up to and in excess of 70gC/gcat were obtained. The carbon produced was mainly in the form of carbon nano fibers. Its purity was characterized by TEM, TGA and XPS tests. The produced carbon had at least 96% purity of fish bone structures.The effect of operating conditions has been investigated and it was found that at lower temperature, a larger amount of carbon was produced despite that the maximum reaction rate was lower. This was due to the delayed deactivation of the catalyst at lower temperature. Lowering the concentration of methane lowered the maximum reaction rate, lifetime and carbon yield. Our study has also revealed that although the presence of hydrogen decreases the maximum reaction rate, a higher carbon yield is achieved due to longer lifetime of the catalyst.A kinetic model describing the maximum reaction rate and the deactivation factor was developed. This model describes the reaction rate of TCD as a function of time in the temperature range of 550-600 °C with a reasonable accuracy and averaged error in initial kinetic rate of 10% and deactivation factors up to 17 %. This model together with the corresponding commercially available catalyst can be used for further study of TCD and reproduction of data. This kinetic model provides the basis to simulate a fluidized bed reactor for TCD with more detailed (i.e. CFD-based) models to facilitate reactor development and optimization.A very high SV was chosen in this study to facilitate the mobility of catalysts that have grown due to large depositions of carbon, and to prevent breaking the reactor’s wall. As a result, the conversion of the gas was limited. In an industrial scale, this can be overcome with a proper continuous reactor design and partial recycle of the gas stream.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 Advanced Research Center for Chemical Building Blocks, ARC CBBC, which is co-founded and co-financed by the Netherlands Organisation for Scientific Research (NWO) and the Netherlands Ministry of Economic Affairs.The authors acknowledge Rick Joosten (Center For Multi-scale Electron Micoscopy in Eindhoven University of Technology) for TEM imaging of the samples, and also Rim van de Poll (Inorganic Materials & Catalysis research group form Eindhoven University of Technology) for conducting XPS measurements.According to Weisz-Prater criterion, C WP , the mass transfer limitation can be neglected compared with reaction kinetics limitations, if Eq. 14 holds (Vannice, 2005). (14) C WP = Rate . ( dp / 2 ) 2 D eff . M b ⩽ 3 β (15) η = 1 - n β 4 where Rate is the observed rate, D eff is effective diffusivity, M b is the concentration of the reactant outside the porous particle, β maximum decrease in concentration gradient in pores, η is effectiveness factor, and n is the order of reaction. The reaction is considered first order (following Eq. 3) and and the observed rate of reaction is obtained from experimental results. Using the other parameters as is presented in Table 4 , it is found: C WP = 0.42 ⩽ 0.6 that confirms that the diffusional mass transfer limitation can just be neglected. However, with larger particles, it can become important as it has effect on the carbon yield and lifetime of the catalyst, see Section 4.1.4.

ThermoCatalytic Decomposition of methane (TCD) offers an interesting route to convert natural gas into hydrogen and functional carbon. In this study the reaction kinetics of TCD is studied for a nickel supported catalyst using a special fluidized bed reactor. The effect of operating conditions such as temperature, concentrations of methane and hydrogen and space velocity (SV) was studied on a commercial nickel catalyst on a silica support. The performance of the catalyst was evaluated in terms of three parameters: maximum reaction rate, lifetime and carbon yield. Values up to and in excess of 70gC/gcat and 12h (at 550 °C and 70vol.% CH4-5vol.% H2) have been achieved for carbon yield and lifetime, respectively. The carbon product has fish bone structure. Our study has revealed that at lower temperatures and in the presence of small amounts of hydrogen ( ≤ 10 % ) a higher carbon yield is obtained. Lower concentration of methane (higher concentration of the inert) lowers the reaction rate, the lifetime and therefore the carbon yield. A dual kinetic approach has been adopted to determine maximum reaction rate and the associated deactivation factor. The kinetic parameters were estimated for the temperature range of 550-600 °C.

Data will be made available on request.Global warming due to emissions of the greenhouse gases (GHGs) is a major environmental challenge today and carbon dioxide CO2 is the most potent contributor [1–3]. Consumption of fossil fuels worldwide in various sectors is the predominant cause of CO2 emissions and at a rate of 1.5 to 3 ppm per anum; from 2006 to 2020 alone, CO2 concentration has risen from 380 ppm to around 412 ppm globally [4–7]. Hence, a sustainable means of CO2 mitigation is required for the reduction of global warming as well as the clean production of renewable fuels. Indeed, carbon capture and storage (CCS) can be employed to capture CO2 from the atmosphere and subsequently store the captured CO2 [8,9]. However, the storage of large amounts of CO2 will require a lot of space in addition to increasing cost. In this regard, catalytic conversion of captured CO2 into methanol is ultimately more preferable and is highly desirable not only from environmental but also from economic perspectives [10]. This is because methanol can either be used directly as fuel additive (gasoline, dimethyl ether, methyl tertbutyl ether) and/or can be further converted to various useful chemicals, such as olefins, aromatics, formaldehyde and acetic acid [11,12]. Thus, creating a huge opportunity for utilization of CO2 for various finished products and commodities, such as plastics, polymers; consequently resulting in longer-term abatement of CO2 or into relatively greener fuels [13]. Carbon Recycling International operating in Iceland is a good example of focusing on methanol production from catalytic conversion of CO2 [14].Carbon dioxide hydrogenation to methanol is surface catalyzed reaction and occurs according to the following exothermic reaction: (1) CO 2 + 3H 2 → CH 3 OH + H 2 O , ΔH o 275 ° C = − 59 kJ mol − 1 However, the above reaction is accompanied by reverse water gas shift reaction (RWGS) and methanation reaction as given below, (2) CO 2 + 4H 2 → CH 4 + 2H 2 O , ΔH o 275 ° C = − 176 kJ mol − 1 (3) CO 2 + H 2 → CO + H 2 O , ΔH o 275 ° C = + 39.5 kJ mol − 1 Hydrogenation of CO2 to methanol has been subject of intensive study with various reports on noble and noble metal-based catalysts. Amongst these copper, indium and palladium-based catalysts have shown promise with relatively better activity, selectivity and stability [12,15–17]. Copper based ternary catalyst consisting of oxides of Cu, Zn and Al due to its lower cost and its use in commercial methanol synthesis via syngas have been extensively investigated; furthermore Cu catalyst have been supported and promoted with other metal oxide systems, such as Zn, Zr, Ce, Si, V, Ti, Ga, B, Cr and Mn, with the aim to improve overall activity and selectivity for CO2 to methanol [18]. However, due to thermodynamic and kinetic limitations as well as due to debatable reaction mechanism of CO2 hydrogenation until now no single catalyst system has been industrially optimized for methanol synthesis from pure CO2 feed. The key factor for the success of CO2 hydrogenation to methanol requires the development of highly active, selective and stable Cu-based catalysts.Indeed, it is generally established that particle size, particle size distribution, surface defects, composition of the catalyst and oxygen vacancies have considerable influence on the catalytic activity of CO2 hydrogenation catalysts [19,20]. This is because CO2 hydrogenation over Cu-based catalysts is believed to be structure sensitive in nature and with smaller CuO particle more active for CO2 hydrogenation process [21]. In spite of this, there is a limited reported literature on the effects of Cu-particle size on the catalytic performances for CO2 hydrogenation to methanol. Despite the presence of some relevant reports, there seems to be a lack of a systematic investigation of the dependence of CO2 hydrogenation over copper nanocrystallite size and a considerable debate and contradiction in conclusions of whether or not MeOH is structure sensitive exists. For example, according to some reports, CuO particles size in the size range of 8.5 nm–37.5 nm did not affect MeOH formation rates. Whereas, CuO particles with size bigger than >38-200 nm favored MeOH formation. Similarly, CuO with particle sizes lower than <6 nm, particles favored CO formation. The above mentioned behavior has been reported for methanol synthesis over Cu based catalysts from CO2/H2 mixture [22] and co-feed of CO2/CO/H2 [23]. By contrast, other reports suggest that the catalysts with CuO particle sizes in the range between 10 nm–27 nm, enhanced the formation rate of methanol [24]. Additionally, the catalytic studies of Cu/Zn/Zr catalysts with Cu particle sizes in the range between 2 nm–25 nm and with low percentage dispersion promoted higher methanol formation turnover frequencies [25]. In this regards, the preparative methodology have been reported to exhibit an enormous influence on the catalytic activities of Cu-based catalysts [26,27]. In the last decade, various synthesis methods such as sol-gel, co-precipitation, impregnation and strong electronic adsorption method have been reported for the preparation of Cu-based catalysts resulting in catalysts with desirable properties and believed to have a high catalytic activity for methanol synthesis from CO2 hydrogenation. However, to further increase the activity and selectivity of these catalysts the synthesis methodology should be improved. Amongst the synthesis techniques, the solution combustion synthesis (SCS) method has gained a lot of interest in recent years. This is because of attracting an attention for the bulk production of high quality nanomaterials and nanocomposites for various applications with synthesis of catalytic materials at the heart of it [28–30]. The SCS preparative methodology involves an exothermic and self-sustained reaction between oxidants and reductants/fuel. Oxidants are mainly nitrates of the metal/metals precursor salts whereas reductants/fuel used are glycine, hydrazine, citric acid etc. The SCS method results in the production of highly porous nanomaterials, with small and highly dispersed metal particle size and large active surface area. This is due to exhaust of large volume of gases during synthesis. In our previous work, we reported development of an active and stable nickel based catalysts for dry reformation of methane synthesized via the SCS method [29,31]. The Ni-based catalysts synthesized by the SCS method were comparatively more active and superior in stability than the catalysts prepared by conventional impregnation method. However, the physicochemical properties of the final products depends mainly on factors such as the amounts of fuel (glycine) and oxidant (nitrates) in the reaction mixture, type of the fuel and calcination temperatures [32,33]. For example, Bedekar and co-workers have demonstrated preparation of various phases of yttrium and chromium oxides at various fuel to oxidants ratios. The fuel enriched combustion system were reported to result in the formation of YCrO3 phase, by contrast the fuel deficit system favored the formation of YCrO4 [34]. Fuel deficit combustion system has also been utilized for the synthesis of single-phase oxide compositions of zinc, nickel, copper, iron and cobalt oxides. By varying the reductants to oxidants (F/O) ratios mixed oxide spinels of the aforementioned metal oxides were also synthesized [35]. However, it is worth to mention that even though SCS is an exothermic reaction, but due to heat losses and large volume of gases released the actual flame temperature in most cases has been found to be lower than the theoretically calculated values, thus making it difficult to theoretically determine properties of the final powder [36].It is clear from the above discussion that the development of efficient Cu-based catalysts for the hydrogenation of pure CO2 into value added chemicals remains a key factor and a challenging topic for researchers. Although, amongst the preparative methods, the SCS preparative method is regarded as a potential route to produce nanocatalysts with excellent catalytic properties. However, the SCS synthesis variables have greater influence in controlling various physicochemical properties, such as the phase, crystal size, crystal structure, oxidation states, mixed oxide phases and surface defects of the materials prepared. Thus, the aim of this study is to develop active and selective Cu-based catalysts for reduction of CO2 to methanol using SCS method. The work covers a systematic investigation of various key parameters of the SCS synthesis technique, such as glycine to nitrates ratio, effects of calcination temperature and effects of activation temperature. A correlation between the physicochemical properties of the Cu-based catalysts prepared at various G/O ratios and catalytic performance was carried out.The Cu-based catalyst, with composition of 30wt%CuO-49.65wt%ZnO/20.35wt%Al2O3, was synthesized via the preparative methodology of solution combustion synthesis (SCS). The samples were prepared at various glycine to nitrates ratios (nG/nO) of 0.105, 0.206, 0.258, 0.309, 0.618, 0.747, 0.804, 0.927, and 1.236 and were denoted as C1–0.1.5, C1–0.206, C1–0.258, C1–0.309, C1–0.618, C1–0.747, C1–0.927, and C1–1.236, respectively. Briefly, in this SCS method, calculated amounts of the precursor salts, Copper (II) nitrate hexahydrate (Cu(NO3)2.6H2O, BDH ≥98.5%%), Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Sigma Aldrich ≥98%%), and aluminum nitrate nonaahydrate (Al(NO3)3·9H2O, Sigma Aldrich, ≥99.9%) were dissolved in 100 ml of deionized water in a 500 ml beaker and stirred well to get a homogeneous mixture. The required amounts of glycine (Sigma Aldrich, 98.5%) as a fuel was then added to this solution under continuous stirring. The mixture was heated on a hotplate to a final temperature of 150 °C for combustion. The solution turned into soft gel before a self-sustained combustion reaction initiated. The synthesized powder was calcined in muffle furnace using static air at 600 °C at a heating and cooling rate of +1 °C/min and -1 °C/min for three hours. For studying the effects of calcination temperature, the C1–0.206 catalyst was calcined at 400 °C, 500 °C, 600 °C, and 800 °C. These catalysts were denoted as C1–400, C1–600, C1–400, and C1–800, respectively.The analytical technique of X-ray diffraction (XRD) analysis was used to study the bulk phase and crystallinity of the calcined catalysts. The XRD measurements were performed on a desktop X-ray diffractometer (Rigaku, MiniFlexII) having CuKa as radiation source at 30 kV and 15 mА. Morphology of the calcined catalysts in terms of average metal oxide particle size and particle size distribution was investigated by using a high-resolution transmission electron microscope (HRTEM). Samples for HRTEM analysis were prepared by dispersing around 10 mg of calcined powder into n-propanol by sonication. A drop of the dispersed sample was then placed onto a copper grid with a mesh size of 150-mesh. The HRTEM analysis was carried out using a JEO; 2010 F high-resolution microscope at an operating voltage of around 200kv. Chemical surface composition and oxidation states of various copper species over the surface of the calcined samples were studied by using X-ray Photoelectron Spectrometer (AXIS Ultra DLD, KRATOS). Prior to XPS analysis, the surface each sample was cleaned from adventitious carbon using the ion gun. Temperature programmed reduction (H2-TPR) analyses was used to study the reduction behavior of the catalysts using ChemiSorb2750 (Micromeritics) equipped with a thermal conductivity detector (TCD). For TPR analysis, around 100 mg of the calcined sample was loaded in the U-shaped quartz tube and degassed for two hours at 200 °C in presence of 30 ml/min of Ar. This step was followed by decreasing the furnace temperature to 40 °C in inert flow. The TPR was then recorded by switching the flow 30 cm3min−1 of 10vol%H2/Ar and heating from 40 °C to 900 °C at 5oCmin−1.The catalytic performances of the catalysts for CO2 hydrogenation was evaluated in a high-pressure lab-scale test unit (PID, Effi, Micromeritics). The unit is equipped with three gas lines controlled by high accuracy mass flow controllers and operates with a hastelloy fixed bed reactor (ID: 9.3 mm). The reactor tube is externally heated with a three-zone electric furnace. The exit stream of the reactor was cooled via a paltrier cold trap where liquid and gaseous products were separated. The reaction temperature was monitored with a thermocouple inserted in the catalytic bed. The gaseous products were analyzed on-line, with an online GC-TCD. The liquids were collected in a trap (5 °C) and were analyzed offline. The analysis was performed with a GC Agilent 7890A equipped with FID detectors. For catalytic tests around 0.5 cm3 of the palletized catalyst with sieve size of 75-150 μm was diluted with 1.0 cm3 of quartz powder with similar grain size was loaded into the reactor. Prior to reaction, the catalysts were activated at the required temperature (350 °C or 500 °C) in a stream of 30 ml/min of pure H2 with a dwell time of three hours. The reactor temperature was then cooled down to 250 °C followed by switching the reactants mixture (CO2/H2/N2) and increasing the pressure to the initial set point (60bars). (4) CO 2 Conversion % = F CO 2 in − F CO 2 out F CO 2 in x 100 4 (5) CH 3 OH Selectivity % = n CH 3 OH n Total products x 100 5 (6) CO Selectivity % = n CO n Total products x 100 (7) CH 4 Selectivity % = n CH 4 n Total products x 100 (8) CH 3 OH Yield = g CH 3 OH wt of catalyst g x h (9) CO Yield = g CO wt of catalyst g x h (10) Carbon balance % = n CO 2 in n CO 2 + n CH 3 OH + nCO + 2 nC 2 H 5 OH + 3 nC 3 H 8 OH out x 100 Where, F is flow of the gases in NL/h and n is number of moles.In order to study the effects of G/O ratio on the physicochemical properties of the catalyst, a series of the 30wt%CuO50wt%ZnO/Al2O3 (C1) catalysts with various glycine to nitrates (G/O) ratio were synthesized by the SCS method. The G/O ratio represents ratio of moles of glycine to the number of moles of total nitrates in the combustion mixture. Photographs of the as synthesized powder of different batches are shown in Fig. 1 . In general the sample prepared at a G/O ratio of <0.618 were porous, whereas samples prepared at a G/O ratio of >0.804 turned less porous and bulky. Moreover no combustion occurred for the catalyst prepared at a G/O ratio < 0.105. As shown in Fig. 1d, the combustion with a G/O ratio of 0.618 occurred in such a violent manner that almost all powder was blown away due to the aggressive combustion and exhaust of large volume of gases. This is because ratio of fuel to oxidants ratio (ϕ) was close to the stoichiometry and resulted in making the combustion violent. As shown in Fig. 1a and Fig. 1b, for the catalysts prepared at a G/O ratio of 1.23 and 0.927, a controlled combustion was observed. However, the synthesized powders were comparatively bulky and less porous. This behavior was attributed to the presence of excess fuel in the reaction mixture making oxidants (nitrates) limited. As can be seen in Fig. 1f, the catalyst prepared at a G/O ratio of 0.206 proceeded with a slow combustion. Although no real flame was observed but a large volume of gases kept exhausting until a complete combustion occurred. The as produced powder was more porous with some unburnt gel remaining on the walls of the beaker. As will be discussed in later section. This nanocatalyst exhibited exceptionally high catalytic performance in comparison with other catalysts tested for CO2 hydrogenation reaction. For sample synthesized with a G/O ratio of 0.105, since the combustion mixture was fuel deficit therefore negligible combustion occurred and almost no gases were exhausted. Interestingly, the combustion mixture with G/O ratio of <0.105 no combustion happened even upon increasing the hotplate temperature (Fig. 1h). This behavior was attributed to the unavailability of sufficient fuel to trigger combustion.XRD analysis of the uncalcined samples and after calcination at 600 °C were recorded to study a correlation between G/O ratio and textural properties. Average particle size was calculated using Scherer's equation and XRD diffraction patterns are shown in Fig. 2 . As shown in Fig. 2a, in the diffraction patterns of the unclaimed C1–0.206, C1–0.258 and C1–0.308, samples, a diffraction line corresponding to carbon at 2θ value of 15° was observed. This was expected as these catalysts were prepared at fuel deficit condition and small amounts of the uncombusted gel material remained. The XRD peaks of the uncalcined C1–0.206 and C1–0.258 samples were comparatively broader and distinct diffraction lines corresponding to various metal components were not observed. Broadening of the diffraction lines was attributed to smaller and/or well dispersed CuO nanoparticles. Average CuO particle size of uncalcined C1–0.206 and C1–0.258, samples were 3.5 nm and 5.6 nm, respectively. Diffraction patterns of the C1–0.309 were comparatively sharper and the average metal particle size was larger (12.6 nm). It is important to highlight that the diffraction lines of uncalcined samples prepared at G/O of 0.6 and > 0.6 were sharper and more intense. Especially for the catalysts samples prepared at G/O ratios of 0.8, 0.9 and 1.23, sharper and distinguishable diffraction lines were recorded. The average particle size of the catalysts also increased with larger particle size recorded for C1–1.23 (24.2 nm). The 2θ values at 32.5o, 35.9o, 38.6o, 48.6o, 53.3o, 58.2o and 61.4o were attributed to the presence of CuO particles [37]. Whereas the diffraction peaks located at 31.9°, 34.6°, 36.4°, 47.5°, 56.69°, 62.71°, and 68.13°, were assigned to the presence of ZnO nanoparticles [37,38]. A possible explain was that for the samples prepared with G/O ratio of >0.6 i.e. either with stoichiometric amounts of fuel to oxidants and/or fuel enriched systems a violent combustion occurred presumably with high flame temperature. Metal oxide particles might have agglomerated resulting in larger metal particles with less porosity. As shown in Fig. 2b, after calcination for the catalysts C1–0.206, C1–0.258, and C1–0.309, the diffraction lines corresponding to surface carbon disappeared indicating successful removal of surface carbon. As can be seen in Fig. 2c, in general the average particle of CuO increased with increase in G/O ratio from 0.105 to 0.804 and then remained constant at a G/O ratio of >0.6018. Moreover, for all samples metal oxide particles of the calcined samples were comparatively larger than that of the uncalcined samples.The results of the HRTEM analysis of the calcined samples affirmed the findings of XRD analysis results. Representative TEM images of the samples prepared with G/O ratio of 0.206, 0.6018 and 1.236 are demonstrated in Fig. 3 . The sample C1–0.206 exhibited smaller metal oxide particle size with narrow particle size distribution whereas the sample C1–0.618 was found to have larger metal oxide particles with broader particle size distribution. The average metal oxide particle sizes for samples C1–0.206, C1–0.6018 and C1–1.236 were16.14 ± 5.16, 21.46 ± 6.9, and 36.6 ± 11.70, respectively.The effects of G/O ratios on the surface chemical composition of the calcined catalysts was studied by means of X-ray photoelectron (XPS) analysis and the Cu2p3/2 core level region of the catalysts are represented in Fig. 4 . The XPS of the Cu(2p) core level region of the C1–0.206 catalyst (Fig. 4a) showed three Cu(2p3/2) peaks. The binding energies at 934.8 eV and 933.3 eV were assigned to Cu(2p3/2) strongly associated bCu2+ and loosely dispersed aCu2+ species, respectively [39,40]. The third peak at binding energy of 936.6 eV was assigned to highly ionized cCu2+ ions which were either more ionized and/or were experiencing stronger interaction than those associated with CuO. The appearance of high binding energy peak could possibly be due to the insertion of copper species into the ZnO/Al2O3 lattices resulting in the formation of surface defects and oxygen vacancies. The relative percentages (e.g. a/(a + b + c)) of the aCu2+, bCu2+ and cCu2+ ions were 25.6%, 41.3% and 33.1%, respectively. As shown in Fig. 4b, the Cu2p core level XPS spectrum of the C1–1.236 catalyst consisted of Cu species similar to that of the C1–0.206 catalyst. However, the relative percentage of highly ionized cCu2+ ions for C1–1.236 catalyst decreased to 27.7%. It was worth noticing the XPS of both the catalysts predominantly consisted of bCu2+ and cCu2+ ions whereas aCu2+ species for C1–0.206 and C1–1.236 were only 25.6% and 14.9%, respectively. The C1–0.206 catalyst revealed the highest percentage of highly ionized cCu2+ species and previous studies have reported positive influences of these species on the catalytic performance during CO2 hydrogenation to methanol [41]. As will be discussed in later section, the C1–0.206 catalyst due to smaller metal oxide particle size and presence of high percentage of cCu2+ species exhibited comparatively high catalytic activity than catalyst prepared at other G/O ratios.The calcination and/or annealing temperature is one of the most important parameter affecting various physicochemical properties of Cu-based catalysts and in turn affects the catalytic performance. Thus, identification of proper calcination conditions and/or temperature might results in catalysts with the desired properties. In order to study the effects of calcination temperature on textural properties as well as to make a correlation with the performance, the batch of the C1 catalyst prepared at a G/O ratio of 0.206 was divided into four portions and subsequently calcined at temperature of 400 °C, 500 °C, 600 °C and, 800 °C. These samples were and denoted as C1–400, C1–500, C1–600 and C1–800, respectively.As shown in Fig. 5a, the uncalcined sample revealed presence of diffraction lines at 2θ value of 15o corresponding to diffraction lines of carbon. As discussed in section 3.1, the C1–0.206 catalyst was prepared under fuel deficit conditions resulting in incomplete combustion and small amounts of the gel material remained even after combustion. XRD peaks of the uncalcined sample were broader and distinct diffraction lines did not appear. This is because the average metal oxide particle size was around 3.87 nm. In addition, broader diffraction lines also indicated that either the nanoparticles were well-dispersed or mixed oxide and/or induced phases were formed. The sample calcined at 400 °C exhibited diffraction lines very similar to that of the uncalcined samples. However, it is important to highlight that the diffraction lines corresponding to carbon disappeared. The average CuO particle size of the C1–400 catalyst was 4.46 nm suggesting that the calcination at 400 °C effectively removed surface carbon without having a significant impact on the metal oxide particle size and metal dispersion.It is worth to mention that for the C1–500 catalyst, the diffraction peaks became comparatively sharper and an increase in peak intensities was recorded. For the C1–600 and C1–800 catalysts sharper and distinguishable diffraction lines of CuO, ZnO and Al2O3 were observed. Especially, for the C1–800 catalyst, the diffraction peaks were not only intense but also clearly distinguishable from one another. For the C1–800 catalyst, the 2θ values at 32.5o, 35.9o, 38.6o, 48.6o, 53.3o, 58.2o and 61.4o confirmed the presence of copper oxide particles [37]. Whereas the diffraction lines located at 2θ degree of 31.9°, 34.6°, 36.4°, 47.5°, 56.69°, 62.71°, and 68.13°, were assigned to the presence of zinc oxide nanoparticles [37,38]. The average particle size of the catalyst C1–600 and C1–800 was 7.4 nm and 18.1 nm, respectively, which was larger than the other catalysts. Indeed, as shown in Fig. 5b, this behavior strongly suggested that at calcination temperature < 500 °C the metal particle size was smaller with good dispersion. However, for samples calcined at temperature > 500 °C agglomeration of nanocrystallites occurred resulting in larger copper oxide particles and presumably poor dispersion.HRTEM analysis results of the catalysts calcined at various temperatures affirmed the findings of the XRD analysis results. Representative TEM images of the samples are shown in Fig. 6 . The C1–400 catalyst exhibited smaller metal oxide particle size with narrow particle size distribution whereas the sample C1–600 was found to have larger metal oxide particles with broader particle size distribution. The average metal oxide particle sizes for samples C1–400 and C1–600 were 4.4 nm and 7.5 nm, respectively.The effects of calcination temperature on the surface chemical composition of the calcined catalysts was studied by means of X-ray photoelectron (XPS) analysis and the results are represented in Fig. 7 . As can be seen in Fig. 7a, the XPS of the Cu(2p) core level region of the C1–400 catalyst revealed presence of three Cu(2p3/2) peaks. The binding energies at 933.58 eV and 934.94 eV were assigned to Cu(2p3/2) strongly associated bCu2+ and loosely dispersed aCu2+ species, respectively [42]. The third peak appeared at binding energy of 936.44 eV and was assigned to highly ionized cCu2+ ions. The relative percentages (a/(a + b + c)) of aCu2+, bCu2+ and cCu2+ species were 42%, 30% and 28%, respectively. As can be seen in Fig. 7(b), the Cu2p core level XPS spectrum of the C1–600 catalyst revealed a shift in the binding energies to higher values. Moreover, the relative percentage of the bCu2+ and cCu2+ species also increased to 41.3% and 33.1%, respectively. It is worth noticing the XPS of the C1–800 catalyst predominantly consisted of bCu2+ (50%) and cCu2+ (42%) ions whereas the highly ionized aCu2+ species were only 7%.As shown in Fig. 8 , increase in calcination temperature resulted in proportional increase in the highly ionized cCu2+ ions as well as metal oxide particle size. For example, upon increasing the calcination temperature from 400 °C to 800 °C, metal oxide particle size increased from 4.4 nm to 18.1 nm and at the same time for the C1–400 and C1–800 catalysts, the relative percentages of cCu2+ ions were 28%, and 42%, respectively.Three catalysts i.e. the catalyst synthesized under fuel deficit conditions (C1–0.206), the catalyst prepared under stoichiometric amount of fuel (C1–0.618) and the catalyst prepared under enriched fuel conditions (C1–0.804) were tested for CO2 hydrogenation. Methanol, water, carbon monoxide and methane were the main products of the reaction. Traces of higher alcohols (ethanol and propanol) were also detected. As can be seen in Fig. 9 , the results indicated that the catalyst prepared at G/O ratio of 0.206 significantly improved the overall catalytic performance. As demonstrated in Fig. 9a, the CO2 conversion over the C1–0.206 catalyst was exceptionally higher than the C1–0.618 and C1–0.804 catalysts. For example at a reaction condition 1 (250 °C, 60bars, H2/CO2 = 3.43 and GHV = 7000 h−1) the CO2 conversion over the catalysts C1–0.6 and C1–0.8 was ∼3.1% and ∼ 2.2%, respectively, whereas the CO2 conversion over the C1–0.206 catalyst was ∼11.2%. This was higher by a factor of ∼5 than that of the C1–0.618 and C1–0.804 catalysts. In general, CO2 conversion over all catalysts increased with increase in reaction temperature and pressure. The maximum CO2 conversion of ∼30% over the C1–0.206 catalyst was obtained at temperature of 300 °C, P = 85b and a space velocity of 7000 h−1. Whereas, under these conditions the CO2 conversion over C1–0.616 and C1–0.804 was 17.60% and 16.4%, respectively.It is worth to mention that at reaction condition 1 (250 °C, 60 bar) and reaction condition 2 (250 °C, 85 bar), the methanol selectivity over the C1–0.206 catalyst was lower than the C1–0.618 and C1–0.804 catalysts. Whereas, the MeOH selectivities for the C1–0.618 and C2–0.804 catalysts were similar. For example, at 250 °C MeOH selectivities for the C1–0.206, C1–0.6 and C1–0.8 catalysts were 53%, 72% and 70%, respectively. Similarly CO selectivity for the C1–0.206 catalyst under these conditions was higher than that of the other two tested catalysts. Indeed, the reaction temperature had a remarkable impact on the products selectivity. Generally, for all three tested catalysts an increase in CO selectivity and a decrease in MeOH selectivity was recorded. However, increase in temperature from 275 °C and 300 °C resulted in a drastic decrease in MeOH selectivity. For example with increase in reaction temperature from 250 °C to 275 °C, MeOH selectivities over the C1–0.618 and C1–0.804 catalysts decreased by 19.1% and 37.5%, respectively. Whereas a decrease of only 7.8% in the MeOH selectivity over the C1–0.206 catalyst was recorded. It was interesting to note that with further increase in reaction temperature to 300 °C MeOH selectivities for all catalysts decreased. However, MeOH selectivity at an operating temperature of 300 °C for C1–0.206 catalyst was higher than that of the other catalysts. CO selectivity followed a similar trend where an increase in percentage selectivity with increase in reaction temperature was observed. The increase in CO selectivity and decrease in MeOH selectivity with increase in reaction temperature was due to enhancement of competing reverse water gas shift (RWGS) reaction. A similar behavior has also been reported by other researchers [43–45]. As shown in Fig. 9d and Fig. 9e, CO yield and MeOH yield over the C1–0.206 catalyst was higher than the C1–0.618 and C1–0.804 catalysts. The maximum MeOH yield of 0.24gMeOH/g-cat.h over the C1–0.206 catalyst was recorded under condition 3. The MeOH yield for the C1–0.618 and C1–0.804 catalysts was very similar. The higher yields of CO and MeOH was attributed to high CO2 conversions over the C1–0.206 catalyst. However, when reaction temperature was further increased to 300 °C a drop in the yield of MeOH due to decrease in MeOH selectivity was recorded.The results revealed that the C1–0.206 catalyst exhibited exceptionally high catalytic performance than the other catalysts. This exceptional high performance of the C1–0.206 nanocatalyst was related to three main differences; (a) Comparatively smaller and uniformly distributed metal oxide nanoparticles. As revealed from the analysis results of XRD and TEM, the C1–0.2 catalyst had an average metal oxide particle size of 6.4 nm. This was smaller than that of the C1–0.6 and C1–0.8 catalysts where CuO average particle size was 21.3 nm and 22.1 nm, respectively. It is clear that high catalytic performance was due to smaller CuO crystallite size of the C1–0.2 catalyst. Since the catalysts C1–0.6 and C1–0.8 had almost similar particle size, therefore to no surprise a quite similar behavior of catalytic activity was recorded. Indeed, Cu-based catalysts have been extensively studied for CO2 hydrogenation reaction and the structure sensitive nature of the catalysts for CO2 hydrogenation to methanol is still debatable [46–48]. The illustrated results with our catalysts provide a direct evidence that the CO2 hydrogenation reaction was structure sensitive in nature and the C1–0.2 nanocatalyst with competitively smaller metal oxide particles exhibited exceptionally high activity. (b) The high catalytic performance over C1–0.206 nanocatalyst might be related to the better dispersion of CuO nanoparticles as revealed by the XRD analysis, the diffraction lines of the C1–0.206 catalyst were broader with undistinguishable diffraction lines. By contrast, the XRD spectrum of C1–0.618 and C1O.804 were comparatively sharper with distinguishable diffraction lines of CuO and ZnO and alumina possibly due to agglomeration of the crystallites with poor dispersion. TEM analysis affirmed the findings from XRD analysis results. These findings were in accordance with previously reported literature [49–51]. (c) The high performance can also be attributed to the presence of surface defects, oxygen vacancies and induced phases. For example, the XPS analysis results the C1–0.206 catalyst revealed presence of around 34% of highly ionized and/or induced copper species (cC2+) whereas for the C1–1.26 catalyst the percentage of cC2+ ions decreased to around 26% only evidencing a direct relation of the presence of induced copper species with catalytic performance. Indeed, with increase in the number of induced copper species CO2 conversion and methanol production was expected to increase as previous studies [52–54] have also reported a similar correlation where the active sites of the CO2 hydrogenation were believed to be along with the Cu and support interferences and increase in induced phases improved the catalytic activity of the catalysts. Comparatively smaller and uniformly distributed metal oxide nanoparticles. As revealed from the analysis results of XRD and TEM, the C1–0.2 catalyst had an average metal oxide particle size of 6.4 nm. This was smaller than that of the C1–0.6 and C1–0.8 catalysts where CuO average particle size was 21.3 nm and 22.1 nm, respectively. It is clear that high catalytic performance was due to smaller CuO crystallite size of the C1–0.2 catalyst. Since the catalysts C1–0.6 and C1–0.8 had almost similar particle size, therefore to no surprise a quite similar behavior of catalytic activity was recorded. Indeed, Cu-based catalysts have been extensively studied for CO2 hydrogenation reaction and the structure sensitive nature of the catalysts for CO2 hydrogenation to methanol is still debatable [46–48]. The illustrated results with our catalysts provide a direct evidence that the CO2 hydrogenation reaction was structure sensitive in nature and the C1–0.2 nanocatalyst with competitively smaller metal oxide particles exhibited exceptionally high activity.The high catalytic performance over C1–0.206 nanocatalyst might be related to the better dispersion of CuO nanoparticles as revealed by the XRD analysis, the diffraction lines of the C1–0.206 catalyst were broader with undistinguishable diffraction lines. By contrast, the XRD spectrum of C1–0.618 and C1O.804 were comparatively sharper with distinguishable diffraction lines of CuO and ZnO and alumina possibly due to agglomeration of the crystallites with poor dispersion. TEM analysis affirmed the findings from XRD analysis results. These findings were in accordance with previously reported literature [49–51].The high performance can also be attributed to the presence of surface defects, oxygen vacancies and induced phases. For example, the XPS analysis results the C1–0.206 catalyst revealed presence of around 34% of highly ionized and/or induced copper species (cC2+) whereas for the C1–1.26 catalyst the percentage of cC2+ ions decreased to around 26% only evidencing a direct relation of the presence of induced copper species with catalytic performance. Indeed, with increase in the number of induced copper species CO2 conversion and methanol production was expected to increase as previous studies [52–54] have also reported a similar correlation where the active sites of the CO2 hydrogenation were believed to be along with the Cu and support interferences and increase in induced phases improved the catalytic activity of the catalysts.The catalyst pre-treatments parameters have been reported to improve the physicochemical properties of the catalyst surface [55,56]. Therefore, investigation of the correct pretreatment and/or activation parameters is very important step for the development of efficient hydrogenation catalysts. In this regards, the effects of the activation temperature prior to reaction over the C1–0.206 catalyst was also studied. In this study, the catalyst was activated either at 500 °C (Activation 2) or at 350 °C (Activation 1) in presence of 30 ml/min of pure hydrogen and the activity results are displayed in Fig. 10 . As can be seen, a significant performance difference between the activities of two activation treatments was recorded. As shown in Fig. 10a, there was a significant improvement in CO2 conversion with decrease in the activation temperature from 500 °C to 350 °C. For example at condition 1, an improvement of ͠ 17.9% in CO2 conversion was recorded for the catalyst activated at 350 °C compared to the activation at 500 °C. It is worth noticing that with the activation temperature at 350 °C a significant improvement in MeOH selectivity was also recorded. As shown in Fig. 10b and Fig. 10c, at an operating temperature of 250 °C with a pressure of 60 bars, selectivity for MeOH over the catalyst activated at 500 °C was 52.3%. Whereas, under similar reaction conditions for sample activated at 350 °C MeOH selectivity improved by 14%. At an operating temperature of 275 °C MeOH yield for the catalyst activated at 350 °C was 0.37 gMeOH/g-cat.h, which was notably higher than the catalyst activated at 500 °C (0.32 gMeOH/g-cat.h).Indeed, as was discussed in section 3.2, in light of the XRD and TEM analysis results it can be concluded that the activation temperature of around 350 °C resulted in reduction of Cu2+ to Cu0 without effecting the particle size. The decrease in overall activity with increase in the activation temperature to 500 °C was presumably because the increase in temperature during the pretreatment might result in agglomeration of the copper particles and thus giving rise to the increase in metal particle size and dispersion. This was in accordance with the results obtained for the effects of calcination temperature on metal particle size. It has been suggested that comparatively bigger copper particles have lower tendencies for hydrogen chemisorption and spill over and are therefore disadvantageous to methanol synthesis. Various researchers have reported similar trends of the effects of pretreatment on the catalytic activity during CO2 hydrogenation reaction [57–59].Previous studies have shown that calcination temperature exhibited profound effects on the catalytic performances of Cu-based catalysts [16,60–62]. Although there is a contradiction in the reported results, but in general, decrease in calcination temperature have been reported to result in an increase in overall catalytic performance. Moreover, as was discussed in section 3.2, calcination at a temperature of 400 °C was suitable to remove the residual surface carbon deposits with negligible impact on the metal oxide particle size. In addition, the catalyst activated at 350 °C was exceptionally more active than the catalyst activated at 500 °C. One can assume that the calcination temperature < 600 °C might further improve the overall catalytic performance. Inspired by the above mentioned insights the effects of calcination temperature on the activity of the C1–0.206 catalyst was also investigated. In this study, portions of the C1–0.206 catalyst were calcined at a temperature of 400 °C and 600 °C. These catalysts were denoted as C1–400 and C1–600 and were investigated for catalytic tests.As can be seen in Fig. 11a, the C1–400 catalyst exhibited exceptionally higher catalytic performance than that of C1–600 catalyst. Compared to C1–600, a significant improvement in CO2 conversion was recorded over the C1–400 catalyst. For example at condition 1 (250 °C, 60bars pressure), CO2 conversion over the C1–600 catalyst was 10% whereas for the C1–400 catalyst CO2 conversion increased by ͠ 100%. It is worth to mention that for the C1–400 catalyst a noticeable increase in MeOH selectivity and decrease in CO selectivity was also recorded. For example, at an operating temperature of 250 °C with a pressure of 60 bars, selectivities for MeOH and CO over the catalyst C1–600 were 47% and 52%, respectively. Whereas, under similar reaction conditions, the MeOH selectivity over the C1–400 catalyst increased to 57%, whereas CO selectivity decreased to 42%. At an operating temperature of 300 °C and pressure of 85bars, MeOH yield over the C1–400 catalyst was 0.52gMeOH/g-cat.h. This was significantly higher than that of the C1–600 catalyst where a MeOH yield of 0.35gMeOH/g-cat.h was recorded. Indeed, these findings were in accordance with the earlier test results presented in section 3.3. Since, the CuO particle size for C1–400 was 4.48 nm which smaller than that of the C1–600 catalyst where particle CuO was 7.4 nm and the CO2 hydrogenation over Cu-based catalysts is structure sensitive in nature [63–65], expectedly, the C1–400 catalyst out performed C1–600 catalyst during CO2 hydrogenation reaction. Moreover, C1–400 exhibited the highest number of surface defects and oxygen vacancies. Thus, it will be reasonable to conclude that the synergy between smaller CuO particle size and presence of higher number surface defects of C1–400 catalyst was responsible for higher catalytic activity.The effects of SCS synthesis variables such as G/O ratio, calcination and activation temperature on the catalytic performance of the Cu-based catalysts synthesized by SCS method was thoroughly investigated. The results suggest that the fuel deficit combustion mixture which was prepared at a G/O ratio of <0.618 resulted in porous materials with smaller metal oxide particle size, whereas the fuel enriched samples prepared at a G/O ratio of >0.804 turned less porous and bulky. Particularly, the catalyst prepared at a G/O ratio of 0.206 proceeded with a slow combustion, with no real flame but a large volume of gases kept exhausting until a complete combustion occurred. The highest catalytic performance for CO2 hydrogenation to methanol was achieved for the C1–0.206 catalyst prepared at G/O ratio of 0.206, calcined at 400 °C and activated in pure hydrogen at 350 °C. At an operating temperature of 300 °C, pressure of 85 bar, H2/CO2 ratio of 3.43 and GHSV of 7000 h−1 the CO2 conversion, CO selectivity and methanol selectivity over this catalyst were 30%, 38.60%, and 61.4%, respectively; whereas, methanol and CO production were 0.52gMeOH/g-cat.h and 0.33gCO/g-cat.h, respectively. The exceptional high catalytic performance of the C1–400 catalyst attributes to the smaller CuO particle size, better dispersion and to the presence of interfaces and/or oxygen vacancies. Sardar Ali: Conceptualization, Methodology, Data curation, Writing – original draft, Investigation. Dharmesh Kumar: Supervision, Writing – review & editing, Validation, Investigation. Kartick C. Mondal: Writing – review & editing. Muftah H. El-Naas: Supervision, Writing – review & editing.The authors have no competing interests to declare.This research work was made possible by the Qatar Shell Research and Technology Center (QSRTC) funded project QUEX-CENG-QSRTC18/19. The statements made herein are solely the responsibility of the authors.

This work investigates the effects of solution combustion synthesis (SCS) variables on the performance of copper-based catalysts for CO2 hydrogenation to methanol. The catalyst with a composition of 30wt%CuO50%ZnO/Al2O3 was prepared at various glycine to nitrates (G/O) ratios in the range between 0.1 and 1.23. A correlation of the effects of calcination and activation temperatures with catalytic activity was also studied. The catalyst synthesized at a G/O ratio of 0.206, calcined in air at 400 °C and activated in a stream of pure hydrogen at a temperature of 350 °C resulted in a significant improvement in the performance of the catalyst for CO2 hydrogenation. The exceptionally high catalytic performance of the catalyst was attributed to the synergic effects between small well-dispersed CuO nanoparticles and high number of induced copper phases. The highest activity of the catalyst was recorded at an operating temperature of 300 °C, a pressure of 85 bar and GHSV of 7000 h−1. The CO2 conversion, CO selectivity and methanol selectivity under these conditions were 30%, 38.60%, and 61.4%, respectively; whereas, methanol and CO yields were 0.52gMeOH/g-cat.h and 0.33gCO/g-cat.h, respectively.

The presence of pharmaceutical chemicals in natural water is a well-known phenomenon that poses a major threat to aquatic biota (Hong et al., 2021). Due to a lack of research on the environmental fate and behavior of lesser-known medicines, exposure assessments can be erroneous. Metoclopramide (MCP), a substituted benzamide and 4-aminobenzoic acid derivative well known as a dopamine antagonist and utilized as an antiemetic and analgetic, particularly in gastroenterology circumstances (Dabić et al., 2022), is one such chemical. Despite the fact that MCP is generally removed from the human body in its original form, its low biodegradability indicates that it is more sensitive to abiotic degradation in the aquatic environment (Wielens Becker et al., 2020).Over the past few decades, heterogeneous photocatalysis based on semiconductors has been recognized as the most powerful advanced oxidation process (AOPs) for efficiently degrading numerous detrimental organic chemicals and is an environmentally friendly, cost-effective, benign, and green process. Typical AOP techniques employ the generation of hydroxyl (•OH) and other free radicals as powerful oxidizing agents that are able to mineralize organic pollutants into CO2, H 2 O, and degrade some inorganic species (Husain Khan et al., 2022). For example, photo-excited semiconductors can produce electron/hole (e − /h + ) pairs, which then participate in reduction–oxidation (redox) reactions with dissolved oxygen and water molecules, to produce superoxide and hydroxyl radicals, respectively. These reactive radicals can degrade organic compounds ultimately leading to carbon dioxide and the mechanistic pathways have been discussed in the literature in detail (Aliyan et al., 2013; Fazaeli et al., 2014).Layered double hydroxides (LDHs) with the general formula [M 2 + 1−x M 3 + x (OH)2][A n− ] x/n zH 2O (M = metal, A = anion) represent a class of two-dimensional anionic clays having attractive features that include large specific surface area, high stability, tunable layer elements and environmental friendliness (Liang et al., 2015). The layered structures of these hydrotalcite materials are known to incorporate di- and trivalent metal cations, a wide range of organic or inorganic anions (e.g. OH − , CO 3 2 − etc.) and water molecules between the slabs. They have been carefully investigated in the context of catalysis (Sels et al., 1999), electroactive/photoactive materials (Lee et al., 2011) and molecular sieves (Villegas et al., 2003). The importance and versatility of LDHs and modified LDHs for the removal of differing organic pollutants from aquatic environments is well-recognized. Karim et al. (2022) Specifically, [NiFe]-LDH materials consist of sheets of edge-shared nickel oxide octahedral, with varying amounts of ferric iron substituting at nickel sites (Hunter et al., 2016). The excess positive charges of Fe3＋ substituting for Ni 2 + are balanced by interlayer anions and the hydroxide groups extend into the interlayer space, which also contain water (Duan and Evans, 2006). LDHs possess semiconductor properties, which facilitate the transfer of the photogenerated electrons on the surface of the photocatalyst and could provide great potential in many applications (Paušová et al., 2015).LDHs have recently been coupled with magnetic nanoparticles (MNPs), such as Fe3O4, to exploit their combined characteristics (Pengcheng et al., 2018). These adaptable composite materials are appearing in a variety of applications including targeted drug delivery, magnetic resonance imaging, photocatalysis and environmental remediation.A promising route to boost the photocatalytic activity of LDHs is to decorate the surface of these materials with metal nanoparticles. This process can improve the catalytic activity of LDHs and, by providing a support for the nanoparticles, address the issue of catalyst recyclability (Chaturvedi et al., 2012). Like other supports, Zhen and Sheng (2011) LDHs have been documented for successful immobilization of gold nanoparticles (Varade and Haraguchi, 2012) and bimetallic NPs, (gold-palladium: Au-Pd) (Sobhana et al., 2016), to develop efficient catalyst for organic reactions or photodegradation processes. In particular, plasmonic noble metals have been intensively studied for loading on semiconductors due to their unique characteristics (She et al., 2016). After combining with semiconductors, the strong surface plasmon resonance (SPR) of the noble metal nanoparticle can enhance the absorption of visible light (Samanta et al., 2014). Such visible-light-driven Z-scheme photocatalysts for degradation of aqueous pollutants (Hassani et al., 2021) Simultaneously, the photogenerated electrons can be transferred from the noble metal and trapped in the conduction band of semiconductor due to the Schottky barriers formed at the metal–semiconductor interface, while the holes can remain on the valence band (Wang et al., 2011). This combination can lead to the detachment of photogenerated electrons from the excitation site of both semiconductors and metals and prevent the recombination of charge carriers to provide a better opportunity for their consumption in photodegradation. Plasmonic metallic nanoparticles, particularly Pd, have attracted tremendous attention due to the highly efficient photodegradation achieved through the combination of Pd with g- C 3 N4. Previous studies have demonstrated that Pd particles can serve as sinks to transfer electrons from the conduction band of photocatalysts under simulated solar light irradiation (Li et al., 2016). Moreover, Pd particles can also exhibit local surface plasma resonance (LSPR) phenomenon, enabling enhanced visible light adsorption capability (Yin et al., 2020).The present work reports a new composite material Pd-Fe3 O4/NiFe-LDH, displaying a LDH with a combination of supported magnetic and noble metal nanoparticles. This novel material has been characterized by a broad range of structural and spectroscopic techniques. Furthermore, this material functions as a catalyst for the photodegradation of organic pollutant and is specifically effective for the photocatalytic degradation of metoclopramide (4-amino-5-chloro-N-(2-(diethylamino)ethyl)-2-methoxybenzamide, MCP), a model for pharmaceutical industry effluent.X-ray diffraction (XRD) patterns of the prepared samples were determined using an X-ray diffractometer model X’PertPro (with Ni-filtered Cu-Ka radiation source at 1.5406 Å, 40 kV, i 30 mA; Netherland). Fourier transform infrared (FTIR) spectrophotometer (Perkin Elmer Spectrum 65) was used for recording FTIR spectra. Ultraviolet–visible (UV–Vis) diffuse reflectance spectra (DRS) were recorded by a JASCO V 670 model instrument, Japan, against BaSO4 as reference. FESEM images were recorded by a MIRA3LMU scanning electron microscope (TESCAN Co., Czech Republic). A pH-meter (Jenway model 3505) was used for pH adjustment. A transmission electron microscope (TEM) Philips EM 208 s (100 kV) was used for recording the TEM images. A TOC analyzer (Analytik Jena, Germany) was used for the analysis of MCP samples before and after the photodegradation process. Brunauer–Emmett–Teller (BET) specific surface areas and pore volumes of the catalysts were determined by nitrogen adsorption–desorption at liquid nitrogen temperature using a Micromeritics TriStar II Series instrument. The samples were degassed at 623 K for 12 h at reduced pressure of 10−4 Pa before carrying out the adsorption measurements. A BAHR-STA-504 instrument was used to perform thermogravimetric analysis (TG) and differential thermal analysis (DTA). Thermal analyses were performed in the 25–800 °C range at a heating rate of 10 K min −1.The hydrothermal method was used to synthesize NiFe-LDH, however the hydrothermal method is energy-intensive due to requirement of elevated temperature. In a typical experiment, NiCl2. 6H2O and FeCl2. 6H2O in a 3:1 molar ratio were dissolved into a mixed solution of 10 mL of deionized water and 10 mL of absolute ethanol (metal-ion concentration 0.2 M) with 500 mg arginine; the resulting mixture was stirred continuously for 10 min. Then, 1 ml of NH3 H 2 O was added to the above mixture and further stirred for 10 min. The mixture was then transferred to a Teflon-lined steel autoclave and heated at 180 °C for 12 h in an oven. Once cooled down to room temperature, the resulting precipitate was collected and washed with deionized water and ethanol several times and subsequently dried at 60 °C to yield the hierarchical porous NiFe-LDH​ microspheres.In a 25 mL round-bottom flask, 20 mg of Pd(OAc)2 (0.05 mmol) and 0.1 g of NiFe-LDH microspheres were combined along with 5 ml of ethanol and the mixture heated to reflux for 24 h. At this point, 0.02 mmol of NaBH4 was added to the reaction mixture, which was again heated to reflux for 4 h. The reaction mixture was cooled to ambient temperature, filtered and washed with ethanol to remove any unreacted Pd complex and finally dried at 80 °C in vacuo (Fig. 1). The photocatalytic experiments were carried out in a photocatalytic reaction chamber using a moderate pressure Hg lamp (35 W, Philips, type G-line with maximum emission at 435.8 nm) which was positioned 10 cm above the reaction beaker that was equipped with a magnetic stirrer (100 rpm). The degradation of MCP was monitored by UV–Visspectroscopy. Direct photolysis was also studied under similar conditions on a solution in the absence of catalyst. The reaction suspension was centrifuged ( > 13,000 rpm) at regular times and the absorbance of the clear solution was recorded (at λ max = 212 nm for MCP). The values of the recorded absorbance, ( A 0 and A, for the samples before and after irradiation process, respectively) were used in Eq. (1) to calculate the change in concentration at time t and the percent decomposition, D. (1) D % = A 0 − A A 0 × 100 = [ C 0 − C C 0 ] × 100 The LDH starting material was prepared using a water/ethanol solution of NiCl 2 and FeCl2, in a 3:1 stoichiometric ratio, with the addition of arginine. Addition of aqueous ammonia produced a basic reaction mixture that was heated to 180 °C in a Teflon-lined steel autoclave for 12 h to yield a precipitate of NiFe-LDH microspheres Fu et al. (2017). The arginine plays a crucial role in this preparation by coordination to the metal ions and controlling the nucleation rate of the NiFe LDH (Fig. 1b); The interaction between arginine molecules through hydrogen-bonding and electrostatic interactions has also been proposed to be influential in the self-assembly of primary nanocrystals, as is illustrated in Fig. 1b. The Pd-nanoparticle functionalized NiFe-LDH was prepared by first adsorbing Pd(OAc)2 onto the NiFe-LDH microspheres in an ethanol suspension and then reducing the Pd(II) using NaBH4 in refluxing ethanol. Cooling, filtering, washing and drying the resulting solid yielded the Pd-Fe3 O4/NiFe-LDH. At this stage, Fe3O 4 nanoparticles (NPs) were formed on the NiFe-LDH microspheres as evidenced by the following characterization.The NiFe-LDH precursor displayed characteristic features of layered material, as observed in the PXRD patterns of NiFe-LDH precursor sample (Fig. S1), with strong, symmetric lines at low 2 θ values and weak, less symmetric lines at high 2 θ values. The average crystallite size and the d-spacing between the crystal lattice for NiFe-LDH are D = 12.36 nm and d = 2.41 Å, respectively.The powder X-ray diffraction pattern (PXRD) for the Pd-Fe3 O4/NiFe-LDH, along with the corresponding reflections for NiFe-LDH (ICDD-40-0216), Pd (ICDD-01-087-0641) and Fe3O4 (ICDD-01-089-4927) is shown in Fig. 2a The assignment of the individual components if based on the comparison of the sample PXRD with the standard materials. For example, the diffraction peaks at 2 θ = 11 .6°, 23.4°, 34.4°, 39.0°, 46.5°, 60.5° and 61.2° were assigned to the (003), (006), (012), (015), (018), (110) and (113) reflections of the layered structure of NiFe-LDH (Carvalho et al., 2015). Characteristic PXRD peaks for both Pd and Fe3O 4 are also displayed. The surface features and porous nature of the parent FeNi-LDH and the Pd-Fe3 O4/NiFe-LDH samples was measured by N 2 adsorption–desorption isotherms and are shown in Fig. 2b. In accordance with the International Union of Pure and Applied Chemistry (IUPAC) nomenclature, both samples show typical IV type isotherms and H1 type hysteresis looped at high relative pressures (Ross, 2019). The inset to Fig. 2b reveals that both samples exhibit pores of different sizes ranging from micro ( < 2 nm), meso (2−50 nm) to macro pores ( > 50 nm) and suggesting a hierarchical porous structure (Barret and Massalski, 1980). As expected, the pore size and pore volume of the bulk NiFe-LDH are generally larger than those of the Pd-Fe3 O4/NiFe-LDH (Table S2).The thermal analysis of NiFe-LDH and Pd-Fe3 O4/NiFe-LDH was carried out and the results are shown in Fig. 2c and Fig. S2. Thermogravimetric/differential thermal analyses measured over the range of 25–1000 °C revealed three events that all appear to be directly resulting from NiFe-LDH in the composite. The first (76–194 °C, about 6.16%), was assigned to the removal of water from internal gallery surfaces and the external non-gallery surface of LDH carbonate. The second event (194–283 °C, about 2.07%), was ascribed to the dehydroxylation of the brucite-like sheets and removal of interlayer anions with decomposition of arginine (Weiss et al., 2018). The third event (238–638 °C, about 9.54%) seems to correspond to several separate mass losses that have been assigned to loss of interlayer water, interlayer hydroxyl removal and interlayer anion decomposition (i.e. CO 3 2 − decomposition/release of CO2) (Luo et al., 2017).In order to probe the magnetic behavior of the Pd-Fe3 O4/NiFe-LDH composites, magnetic measurements were carried out and compared with similar measurements of Fe3O4 (Luo et al., 2016) as shown in Fig. 2d. At room temperature the saturation magnetization (Ms) values were 136.8 and 19.88 emu g−1 for Fe3O4 and Pd-Fe3 O4/NiFe-LDH, respectively. The smaller saturation magnetization of the Pd-Fe3 O4/NiFe-LDH nanocomposites was attributed to the existence of larger content of non-magnetic LDH phase (Yan et al., 2015).The FT-IR spectrum of the NiFe-LDH and Pd-Fe3 O4/NiFe-LDH are shown in Fig. 2e and is similar to those generally reported for hydrotalcite compounds (Pan et al., 2018). The bands around 3422 cm − 1 was ascribed to the stretching mode of the OH group with hydrogen bonding and of interlayer water molecules (Cheng et al., 2010). The band observed at 1630 cm − 1 was attributed to the bending mode of crystalline water (Sahu et al., 2013). The bands at approximately 829 and 530 cm − 1 arise from the metal–oxygen bond (M–O, M–O–M and M–OH) vibrations in the LDHs (Wang et al., 2014). The four weak bands at 830, 1024 and 1392 cm −1 are characteristic of carbonate ions (Hou et al., 2005). In addition, the preparation of this NiFe-LDH employed arginine and, therefore, the stretching modes of N-H from arginine were observed around 3150 cm − 1  (Kumar and Rai, 2010).The charge transfer between NiFe-LDH and the Pd and Fe3O4 particles was investigated by using photoluminescence spectroscopy (PL). The PL spectra analysis is essential to reveal the migration, transfer and separation efficiency of the photogenerated electron and hole pairs in various semiconductors (Xu et al., 2009). Fig. 2f shows the PL spectra of pure FeNi-LDH in comparison with the Pd-Fe3 O4/NiFe-LDH composite. The pure LDH material (NiFe-LDH) showed one strong PL emission peaks at around 359 nm, due to the trapping of charge carriers in the form of excitons on its surface, and another peak at 728 nm is PL emission due to the defect sites of LDH (Van Vugt et al., 2005). If the combination of Pd-Fe3 O4 with FeNi-LDH improves the e − /h + separation compared to the parent material, the photoinduced recombination rate would be reduced, which leads to a reduced peak intensity for the FeNi-LDH nanocomposite. The observed intensity reduction confirms that the photogenerated holes and electrons undergo less recombination and are well-separated after Pd-Fe3 O4 introduction.The scanning electron microscopy (SEM) image of the NiFe-LDH precursor shows a typical hexagonal nanoplatelet morphology with the edge length of about 100 nm (Fig. 3a,b and Fig. S3). The SEM images of the Pd-Fe3 O4/NiFe-LDH samples are shown in (Fig. 3c,d and Fig. S4) and revealed a hierarchical porous architecture with numerous nanosheets as the building blocks. Fig. 3e shows the particle size distribution of this material and indicated that the most common particle size in this composite material is < 50 nm. The scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) spectrum of Pd-Fe3 ​ O4/NiFe-LDH samples is shown in Fig. 3f and Fig. S5, which provided evidence of the expected elements Ni, Fe, and Pd in this composite. Interestingly, the peaks of Pd are distinct at 2.838 and 2.984 keV corresponding to Pd L α 1 and Pd L α 2 and confirmed the presence of metallic Pd consistent with Pd-NPs deposited on the NiFe-LDH surface. In addition, element mapping images of the Pd-Fe3 O4/NiFe-LDH material is shown in Fig. 3g and revealed a material that displayed uniform Pd, Fe and Ni distribution across the sample. The transmission electron microscopy (TEM) images and particle size distribution of the Pd-Fe3 ​ O4/NiFe-LDH particles, elucidated in Fig. 3h and i (different magnifications are shown in Fig. S6) suggested that Pd nanoparticles, with average size of 7 nm, are homogeneously distributed on NiFe-LDH. NiFe-LDH and Pd-Fe3 O4/NiFe-LDH showed absorption bands in both the UV and visible regions (Fig. 4a) and these absorption bands can be broken into three characteristic regions: 200–300, 300–600, and 600−800 nm. The intrinsic absorption band within 200−300 nm in the UV region were assigned to the LMCT from the O 2p → Ni 3d orbital, while the bands within the 300−800 nm region corresponded to d-d transitions and are characteristic geometry of Ni 2 + ions in an octahedral field (Wei et al., 2017). Another intense absorption band found at 476 nm was assigned to charge transfer for the transition of Ni 2 + –O–Fe3＋ to Ni + –O–Fe4＋, originating from the induced MMCT for the oxobridged bimetallic linkage (Nayak et al., 2015b). The absorption bands located at 383 and 750 nm corresponded to spin-allowed transitions of 3A 2g (F) → 3T 1g (P) and 3A 2g (F) → 3T 1g (F), which result from the characteristic d 8 configuration geometry of Ni 2 + ions in an octahedral field (Rudolf et al., 2014). Nanosized Pd in Pd-Fe3 O4/NiFe-LDH samples (Fig. 4b) was also reported to exhibit a LSPR, but it is mainly located in the UV region of spectrum (300 nm), which impedes the observation of this feature (De Marchi et al., 2020). Using a Tauc plot the UV–Vis analysis can provide a means to determine the band gap energy (Eg) of the crystalline Pd-Fe3 O4/NiFe-LDH by calculation using by Eq. (2) (Zeng et al., 2018): (2) (ahυ) 1 / n = A(hυ − Eg) where a, h, v , Eg and A are the absorption coefficient, Planck’s constant, light frequency, band gap energy, and a constant, respectively. In the case of an indirect optical transition of a semiconductor n = 1 / 2  (Nayak et al., 2015a). Therefore, the plot of ( α h υ )2 vs. h υ (Kubelka-Munk function as a function of light energy) gives the value of Eg = 3.56 eV corresponding to Pd-Fe3 O4/NiFe-LDH is shown in Fig. 4b.Photodegradation studies were carried out using metoclopramide (MCP) as a model pharmaceutical effluent. MCP is a benzamide derivative and is used as an anti-emetic in the treatment of some forms of nausea and vomiting and to increase gastrointestinal motility and this compound contains two substituents particularly reactive in radical reactions (chlorine, and amino group) (Herrero et al., 1998). The chemical structure and physico-chemical characteristics of MCP are summarized in Fig. S6(a). Hydrochloric acid (0.1 mol L −1) and sodium hydroxide solutions (0.1 mol L−1) were used for the adjustment of water pH-value. Fig. S6(b) presents typical UV–Vis spectra showing the three absorption bands for MCP (10 mg l−1) located at 212, 272, and 308 nm (Chaabanea et al., 2013).Before examining the ability of the nanocomposite Pd-Fe3 O4/NiFe-LDH to photocatalytically remove MCP several background measurements were carried out. The role of surface adsorption or dark reactions by the as-synthesized Pd-Fe3 O4/NiFe-LDH composite on removal of MCP were first investigated. When the nanocomposite was added to a solution of MCP and held in the dark, there was an approximate 6% removal of MCP molecules during a 7 min time interval with no significant further changes with increased time. As a result, prior to all photodegradation experiments, suspensions of Pd-Fe3 O4/NiFe-LD were shaken with MCP solutions at the dark for 7 min to allow the adsorption/desorption process to reach equilibrium (Fig. 5a). The potential for direct photolysis of MCP solutions in the absence of any added catalyst was next examined and resulted in a decrease of about 9% of MCP molecules over a 30 min. time interval. While this background effect confirmed that irradiation alone is not sufficient to degrade MCP, it was an important background correction for the subsequent photodegradation experiments The ability of the individual components of the Pd-Fe3 O4/FeNi-LDH composite to degrade MCP was next examined. Fig. 5a displays not only the background measurements but also the catalytic abilities of Fe3O 4 NPs, the support material FeNi-LDH, and two physically mixed materials Fe3O4/FeNi-LDH and Pd/FeNi-LDH. Each of these materials did demonstrate some level of catalysis for the destruction of MCP. However, substantially superior degradation efficiency was observed when the Pd-Fe3 O4/NiFe-LDH nanocomposite was used. The effect of Pd loading on the photocatalyst performance was measured and is shown in Fig. 5b. A clear and significant improvement of degradation was observed between 1 and 5% Pd loading. There was little improvement between 5 and 8% Pd loading. Further measurements were carried out with 7%Pd-Fe3 O4/FeNi-LDH (Fig. 5b).We examined a series of scavenging agents in order to probe the principle active species responsible for the photocatalytic degradation of MCP (Fig. 6a). Specifically we examined the effect of the following added reagents: isopropanol (IPA: an • OH scavenger); disodium salt of ethylenediaminetetraacetic acid (EDTA: a h + scavenger); benzoquinone (BQ: an •O 2 − scavenger); and H 2 O2 (an electron acceptor). The photodegradation efficiency was only marginally reduced when isopropanol (IPA) was added to the reaction cell as a • OH scavenger, demonstrating that the • OH radicals play no part in the degradation process. One reason may be that the holes in the photocatalyst VB were unable to oxidize OH − / H 2 O to • OH (OH − + h + → • OH, E o : 2.6 V vs. NHE). The addition of the other scavenging agents (i.e. EDTA, BQ, and H 2 O2) reduced the degradation efficiency, showing the importance of superoxide and holes as reactive species in the degradation of MCP molecules (Deng et al., 2017). We propose that the effectiveness of the full composite system is due to matching of the standard potentials between conduction ( C b ) and valence ( V b ) bands of the Pd and Fe3O4 with NiFe-LDH levels. A proposed mechanism for photocatalytic degradation of MCP by Pd-Fe3 O4/NiFe-LDH composite and the transfer pathway of charge carriers is schematically illustrated in Fig. 6b. Although in some particles both Fe3O 4 and NiFe-LDH semiconductors can produce e − /h + pairs, they may recombine and act as independent semiconductors. However, in some particles, the photogenerated electrons in Fe3O 4 − C b level migrate to that of NiFe-LDH because of its more negative potential (Li et al., 2018). This internal reduction–oxidation process extends the lifetime of the photogenerated e − /h + pairs and enhances the degradation efficiency (Fig. 15). Given the difference in the work function of metals and semiconductors, a Schottky barrier could be formed between Pd and Fe3O4–NiFe-LDH under UV–Vis illumination. In this model the Pd nanoparticles absorbed the resonant photons and the photogenerated electrons due to SPR would be transferred from Pd to NiFe-LDH until the two levels reached equilibrium to form a new Fermi energy level (Su et al., 2012). These photogenerated electrons would be transferred to the C b of NiFe-LDH. The complex heterojunctions between the two components facilitated the transfer of photogenerated electron–hole pairs during the course of photocatalysis (Seery et al., 2007). Kinetic analysis of this catalytic process was consistent with the Langmuir–Hinshelwood (L-H) model (Rezaei and Nezamzadeh-Ejhieh, 2020). Catalyst performance was quantified in terms of the apparent rate constant k a p p derived from the slope of ln( C 0 /C) versus time. C 0 /C was obtained from the absorbance at λ max ( A 0 /A), where A represents the MCP absorbance at time t. Fig. 7a presents the data as a function of catalyst dose on the rate of the degradation reaction. The plot of ln( C 0 /C) versus time revealed a linear dependence consistent with first order kinetics (Fig. 7a, inset). The rate constants as the function of the catalyst dose were obtained from the slopes of these linear curves and summarized in Table 2. The results illustrate an increase in the rate when the amount of the catalyst was increased from 0.1 to 0.4 g/L (k from 0.0196 to 0.0263 min −1) and thereafter decrease in the rate was observed. The increase in the rate was simply attributed to having more active sites of the catalyst as the amount of catalyst is increased. At higher doses, agglomeration and light scattering effects appear to lead to a decrease in rate of degradation, as has been illustrated in detail in the literature (Norouzi et al., 2021). Thus, the dose of 0.4 g/L was selected for the next experiments. As shown in Fig. 7b and Table 1, an increase in the MCP from 5 to 20 mg/L lead to an increase in the MCP photodegradation rate consistent with a first order dependence on the concentration of this reactant. Interestingly, at a high concentration of the MCP pollutant the degradation rate decreased (Yılmaz et al., 2015). This is likely due to a screening effect of MCP molecules resulting in a decrease in the photoexcitation of the catalyst. Thus, the MCP solutions with a concentration of 20 g/L and were selected for the next experiments. The effects of initial pH of MCP solution on the degradation efficiency are shown in Fig. 7c, for pH values from 3.5 to 9.5. The highest rate of degradation of MCP was obtained at pH 6.5. Estimation of pH PZC  (Dianat, 2018) (Fig. 7d) showed that the Pd-Fe3 O4/NiFe-LDH photocatalyst has a pH pzc of 3.8–4.0. At pH values <4.0, the surface of the catalyst was positively charged and repels the protonated dye molecules. This suggests that the negative charge of Fe3O4/NiFe-LDH photocatalyst at pH > 4.0, results in the protonated functional groups adsorbing to the surface of the catalyst and the attractive force between the cationic functional groups and the negatively charged catalyst adsorb MCP species and the degradation efficiency tends to increase (Dimitrakopoulou et al., 2012). These optimal conditions of catalyst dosing, MCP loading pH, were used for a set of photodegradation experiments with irradiation times ranged from 0 to 30 min. Based on the disappearance for the recorded absorbance for MCP, a typical Hinshelwood plot was constructed as shown in Fig. 7e. The apparent first order behavior yielded a rate constant of 0.0265 min −1 corresponding to a half-life of 26.25 min for the MCP photodegradation. The extent of mineralization of these solutions was determined by analysis of the chemical oxygen demand (COD) (Aliyan et al., 2013). The residual COD values are presented in Fig. 7f which confirmed a decrease in the COD from the photodegradation process. The inset of Fig. 7f shows the Hinshelwood plot obtained from the COD results. The COD measurements yielded a rate constant, k, of 0.0420 min −1 corresponding to t 1/2 = 16 . 50 min. The mineralization of MCP is about 1.59 times faster than its photodegradation extent. Generally, mineralization is a slower process associated with the formation of persistent transformation by-products. However, process performance is affected by several factors, namely irradiation time, photocatalyst type and loading, solution pH and the water matrix (Omrani and Nezamzadeh-Ejhieh, 2020). Thermodynamic functions The effect of the temperature on the MCP photocatalytic degradation by the Pd-Fe3 O4/NiFe-LDH nanocomposite was evaluated under the optimized conditions in the temperature range of 298–323 K. Fig. 8a shows a plot of ln( C 0 /C) for various reaction temperatures during the time interval of 5–30 min. From this data the apparent rate constant ( k app ) as a function of the reaction temperature was extracted and presented in Table 2. From a plot of ln k app versus 1/T (Harbourne et al., 2008) (Fig. 8b) the activation energy was obtained and is given in Table 2. The other thermodynamic parameters were calculated (Table 2) using the activation energy and apparent rate constant (Garsoux et al., 2004). Then, the plot of ln (k/T) versus 1/T was constructed (Fig. 8c) and the values of Δ H ‡ and Δ S ‡ was calculated from the slope and intercept, respectively. Chen and Ray (1998) reported that the increase in rate constant is most likely due to the increasing collision frequency of molecules in the solution that increases with increasing temperature. As shown, the MCP photodegradation by the Pd-Fe3 O4/NiFe-LDH photocatalyst was accompanied by a relatively high positive activation enthalpy and the Gibbs free energy values, indicating that a highly hydrated transition state complex was produced. These positive values also confirmed that for reaching this transition state complex needs enough energy and it cannot produce at ambient conditions (Gupta et al., 2015).The performance of the photocatalyst in different real water samples was tested and the results are shown in Fig. S8. The best photocatalytic activity was observed in the distilled water sample. Regarding other real examples, it can be said that the relative reduction of degradation is possibly due to the presence of other mineral compounds. The lowest rate of MCP degradation was obtained in the sewage water sample. Recyclability of the catalyst To further evaluate the performance of the Pd-Fe3 O4/NiFe-LDH composite, this material was subjected to five catalysis cycles. Before each run, the recycled catalyst was dried at 100 °C for 30 min to remove the adsorbed species, the specific performance (Table S3) revealed good stability and reusability.Moreover, the filtrate was examined by ICP-MS analysis, and after hot filtration, no trace of Pd was detected in the filtrate. In addition, FT-IR and XRD results of catalysts before and after recycling are shown in Fig. S9. No obvious change was observed, the spectral patterns of catalysts before and after use were identical, which is strong evidence of the stability of catalysts. However, after the fourth run, during successive uses of the catalyst, a small amount of leaching of Pd from the catalyst surface (approx. 0.05%, according to ICP analysis) was observed and may decrease the efficiency of catalyst. Conclusion In summary, a novel catalyst, Pd-Fe3 O4/NiFe-LDH, was prepared under hydrothermal conditions and characterized by XRD, BET, TG-DTG, VSM, FTIR, PL, SEM/EDX, TEM, and DRUV analysis. The photocatalytic activity of Pd-Fe3 O4/NiFe-LDH for the photodegradation of MCP was investigated under visible light irradiation. The results showed that the photodegradation efficiency of MCP by Pd-Fe3 O4/NiFe-LDH was 95.2% in 80 min, and the photodegradation rate was higher than that of pure NiFe-LDH or Fe3O4/NiFe-LDH. Furthermore, Pd-Fe3 O4/NiFe-LDH maintained good photocatalytic recyclability. It appears that the SPR effect of the Pd NPs accelerated the separation of photoexcited e − /h + couples. Furthermore, the porous interior cavities of this material may have created many reflections of the arriving photons, greatly lengthening the action time. Simultaneously, the ordered mesoporous opening structure of the catalyst, which greatly boosts the photocatalytic activity of the Pd-Fe3 ​ O 4 on the surface of the mesoporous silica, may efficiently enable the transfer of reactant molecules. The superparamagnetic Pd-Fe3 O4/NiFe-LDH particles may allow an avenue for easily separating the catalysts from the reaction medium using external magnetic fields. Finally, the use of this composite photocatalyst made of Pd-Fe3 O4/NiFe-LDH for the treatment of mineral processing wastewater, may lead to the development of photo-, photoelectro-systems for the treatment of genuine mineral processing of wastewater. Forouzan Shabib: Data curation, Writing – original draft, Visualization, Investigation, Software. Razieh Fazaeli: Conceptualization, Methodology, Supervision, Software, Writing – review & editing, Validation. Hamid Aliyan: Data curation, Writing – original draft, Visualization, Investigation, Software. Darrin Richeson: Conceptualization, Methodology, Supervision, Software, Writing – review & editing, Validation.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 gratefully thank Shahreza Branch, Islamic Azad University for financial support.Supplementary material related to this article can be found online at https://doi.org/10.1016/j.eti.2022.102515.The following is the Supplementary material related to this article. MMC S1

With the primary objective to develop efficient, environmentally benign, visible-light-driven heterogeneous catalysts for the degradation of metoclopramide (MCP), a waste-water pollutant, a newly prepared heterogeneous composite Pd-Fe3O4/NiFe-LDH catalyst is reported. This material has been characterized with a wide range of analysis methods (i.e. XRD, SEM-EDS, BET, FTIR, TG-DTG, DTA, DRUV and TEM analysis). When applied to aqueous solutions of MCP, Pd-Fe3O4/NiFe-LDH displayed a photocatalytic degradation of MCP with an efficiency of 95.2% in 80 min. The photocatalytic degradation rate for this composite material was higher than that of pure NiFe-LDH or Fe3O4/NiFe-LDH. The best photocatalytic activity was obtained at pH 6.5, with 0.4 g/L of the catalyst. Application of the Arrhenius equation yielded an activation energy for this process of 13.4 kJ/mol. A negative activation Δ S‡ (-0.25 kJ/mol) with the positive Δ H‡ and Δ G‡ values were obtained for this MCP photodegradation.

Data will be made available on request.Facing one of the acute environmental crises, i.e., global warming that is attributed mainly to the massive CO2 emission via the combustion of fossil fuels, more and more efforts from both industrial and academic researchers have been made to reduce atmospheric CO2 or, even better, to transform it into value-added chemicals and thus advocate a circular carbon economy. CO2 can be utilized as a promising carbon feedstock for various functional molecules, e.g., methanol [1,2], higher alcohols [3,4], dimethyl ether [5] and C2+ hydrocarbons [6–8]. The transformation of CO2 into methanol, which is well recognized as an important bulk chemical in industries [9,10], is of interest in this study. In general, the activation of CO2 molecules remains a challenge in CO2 conversion due to its notorious high thermodynamic stability ( Δ H f 0  = −393.5 kJ mol−1) [11]. Moreover, the methanol synthesis from CO2 is known from the literature to undergo a competition with the reverse water gas shift (RWGS) reaction forming CO and possibly the conversion of CO into methanol [12]. While RWGS is endothermic, the methanol synthesis is exothermic ( Δ H 0  = –49.5 kJ mol−1) and thermodynamically unfavored at high temperatures [10,13]. Therefore, catalysts are crucial to lower the activation energy and consequently enhance the yield of methanol in the thermocatalytic conversion of CO2. Alternatively, CO2 can be converted using various approaches, including molecular catalysis [14], photocatalysis, electrocatalysis [15] and hybrid approaches [16], which are out of the scope of this study.There have been numerous attempts to improve the catalytic activity via catalyst design. Regarding the active metals, three main groups have been studied extensively, namely, Au-, Pd- and Cu-based catalysts, besides other metals (Pt, Ni, Co, Ru, …) [5,12,14]. The selection of catalysts is further extended to bimetallic [17] and trimetallic catalysts [18] in order to advance the synergy of the multicomponent catalytic systems. Irrespective of active metals employed, the particle size and the dispersion of active metals were found crucial to the catalytic performance in the hydrogenation of CO2. In particular, the increase of Cu particle size (5–25 nm), in association with gradual raising Cu content (5–25 wt.%) supported on CeO2-ZrO2 materials, leads to a reduction in the number of active Cu sites exposed to reactants, as indicated by lower H2 consumption during temperature programmed reduction, and thus lower methanol formation activity (from 2.92 to 0.46 g CH 3 O H ∙ g Cu ∙ h - 1 ) [19]. Similar observations are also recorded for Cu-based catalysts supported on various types of materials, e.g., CeO2 [20], Al2O3 [21], ZnO [22], and ZrO2 [23]. Au-based catalyst is another classic example of the strong dependence between Au particle size and the corresponding catalytic activity [24–26]. The highly dispersed Au nanoparticles (≈1 nm) supported on amorphous ZrO2 via deposition precipitation method provided a much higher methanol formation rate than that of Au/ZrO2 (d Au≈50 nm) synthesized by impregnation, i.e., 2.1 > 0.4 g CH 3 O H ∙ g cat ∙ h - 1 in the methanol synthesis from CO2 (240 °C, 40 bar) [27]. It is suggested that the particle size of active metal is strongly associated to the number of interfacial surface area, at which the methanol synthesis has been repeatedly reported to take place [28,29]. Hence, the high specific surface area of interfacial sites can promote the hydrogenation of CO2 into methanol and thus lower the formation of CO.Besides, there are other factors affecting the catalytic performance of solids, in particular, reducibility indicated by the temperature at which most of the active metal is reduced, the chemical nature of active sites and the corresponding role of each site, and active sites dispersion determined by the specific surface area of support materials as well as the interaction between the metals and supports. Therefore, one of the approaches to improve the catalytic performance is via support materials.Regarding the support materials, there are a plethora of choices, among these, γ-Al2O3, ZnO, CeO2, ZrO2, SiO2, TiO2, and mixed oxides of various rare-earth and transition metals are widely studied in the hydrogenation of CO2. One of the key roles of these materials is providing high surface area and thus improving the dispersion of the active metals. Besides, the acid-base properties of the catalyst support are also required to influence the catalytic activity of CO2 conversion greatly. The catalysts supported on acidic materials (TiO2, ZrO2) are beneficial for CO2 conversion [30,31], which is contributed mainly by the RWGS reaction forming CO, whereas methanol formation is favored over catalysts exhibiting strong basic properties (CeO2, ZnO) coupled with low CO2 conversion [31–33]. The different behaviors were speculated to associate with the high basicity, which reinforced the adsorption of CO2 and CO, thereby stabilizing the formate intermediates to methanol and possibly hindering the production of by-product CO, respectively [34]. Moreover, acidity caused by Cu cations adjacent to oxygen vacancies was found to be linear with the methanol formation rate [35]. However, further understanding, particularly the influencing manner of these basic/acidic sites in CO2 hydrogenation, remains debatable. Furthermore, the combination of different metal oxides, particularly CeO2-ZrO2 mixed oxides, has shown numerous benefits, including greater dispersion, reducibility, and interaction between metallic sites, e.g., Cu and support [33,36–38]. Taking into consideration, for example, the Au-based catalysts on TiO2, due to the electronic polarization between Au and the support only observed in the presence of CeO2, both CO2 adsorption and activation were greatly improved [39]. Additionally, the surface hydroxyl groups available on metal oxides were found to participate in the surface chemical reaction and affect the selectivity of CO2 conversion into methanol. These groups were found to promote the adsorption of CO2 at the isolated Cu sites in proximity resulting in bidentate formate, the primary reactive intermediate of methanol synthesis, for urea-assisted hydrothermally synthesized Cu/SiO2 [40]. Another study combining experimental and theoretical results for SiC quantum dots revealed that the hydroxyl groups lowered the barrier to form formate intermediates and thus increased the methanol productivity [41]. This phenomenon was also observed in various catalytic systems, e.g., Rh-based catalysts supported on TiO2 [42] and Cu/γ-Al2O3 [43]. These are a few factors important for methanol synthesis, apart from the morphology of the supports [23,44–48], which are not discussed in detail. Nevertheless, the changes in catalytic activity seem to be under the influence of multiple factors. Thus, one should consider all possible reasons and their corresponding gravity to obtain a full spectrum based on which solid conclusions can be drawn.In our study, the objective was to exploit the advantages of Ce, i.e., great basicity and surface oxygen vacancy, in Au catalysts supported on amorphous ZrO2 (a-ZrO2) for CO2 hydrogenation. Owing to the great number of uniform acidic sites mainly contributed by Lewis acid sites and a fraction of surface hydroxyl groups as reported in [49], a-ZrO2 was chosen over monoclinic (m-) and tetragonal (t-) ZrO2, two commonly found phases of ZrO2. Furthermore, CeO2-ZrO2 mixed oxides were obtained by a simple and low-energy cost coprecipitation method, which is highly desired amid the current energy crisis [27]. Accordingly, a series of Au catalysts supported on CeO2-ZrO2 mixed oxides with a gradually increased molar ratio of Ce to Zr (from 0 to 0.1, with a step of 0.025) was prepared. The gradual increase of Ce content offers a systematic approach to investigating the influence of Ce on the properties of the resulting catalysts based on a-ZrO2. Furthermore, the catalytic activity of the Au-based catalysts supported on CeO2-ZrO2 mixed oxides in the hydrogenation of CO2 into methanol was examined and elucidated by considering various influential factors, e.g., reducibility, acid-base properties, chemical environment (of Au, Ce, Zr), and Au particles size of the catalysts.The ZrO2-CeO2 nanocomposites were synthesized following a precipitation method in a previously published work [27] with slight adjustments. Typically, 4 g of zirconium (IV) oxynitrate (ZrO(NO3)2·xH2O, Sigma Aldrich) and a predetermined amount of cerium (III) nitrate hexahydrate (Ce(NO3)3·6H2O, Sigma-Aldrich) was dissolved in 50 mL deionized water resulting in an aqueous solution of 0.1 M Zr4+ and 0.1x M Ce3+, respectively, with x denoted for the Ce to Zr molar ratios (x = n Ce/n Zr). To obtain high dispersion of Ce in ZrO2, x was varied in the low range from 0.000 to 0.025, 0.05, 0.075 and 0.100. The mixture was stirred at room temperature (RT) for 30 min. Subsequently, the precipitation was conducted by adding 5 wt.% ammonia solution (pH = 12) dropwise (approximately 10 mL) to the Zr4+ and Ce3+ aqueous solutions (pH = 1–1.5) until complete precipitation (pH ≈ 9). The mixtures were then placed in a dryer at 80 °C under static air for 12 h. Afterwards, the obtained mixtures were washed and filtered with deionized water. The solids were dried at 90 °C overnight and subsequently calcined at 300 °C under static air for 4 h. The obtained products are labelled as ZrCex with x referring to the nominal n Ce/n Zr ratio.In order to obtain small Au particle size, which is favored at low Au content, a nominal Au content of 1 wt.% was chosen for Au introduction. Specifically, 1 g of ZrCex was added into 50 mL of 1 mM gold (III) chloride trihydrate (HAuCl4·3H2O, Sigma-Aldrich) aqueous solution. The suspension was stirred at room temperature (RT) for 30 min. The deposition precipitation was conducted by adding 5 wt.% ammonia solution (approximately 3 mL) in droplets until complete precipitation (pH = 7). The suspension was then transferred to a dryer, and the temperature was kept at 80 °C under static air for 6 h. Afterwards, the solids were filtered and washed with water. The obtained materials were further dried at 120 °C for 2 h and named as Au/ZrCex (x = n Ce/n Zr = 0, 0.025, 0.05, 0.075 and 0.1).The structural properties of all investigated catalysts were characterized by a PANalytical X’Pert PRO MPD X-ray diffractometer (Cu Kα1 = 0.154 nm). The powder X-ray diffraction patterns were recorded at room temperature in the 2θ range from 5° to 90° with a step of 0.033°.The elemental content of Au, Ce, and Zr in synthesized catalysts was determined by optical emission spectroscopy with inductively coupled plasma (ICP-OES) using a Varian 715-ES ICP Optical Emission Spectrometer. In preparation for the analysis, 10 mg of the samples were dissolved in 10.0 mL HF and 1.0 mL HClO4 and diluted to obtain 50.0 mL aqueous solutions.N2 sorption isotherms were recorded on a volumetric adsorption analyzer Tristar 3000 (Micromeritics, Norcross (GA), USA) at 77 K. Prior to the measurements, the samples were evacuated at 180 °C for 10 h. The specific surface area (A BET) and total pore volume (V P) were determined using the Brunauer–Emmett–Teller (BET) model and single point (p/p0  = 0.98) method, respectively.Temperature-programmed reduction with H2 (H2-TPR) was performed using a Micromeritics AutoChem II 2920 chemisorption analyzer. 60 mg of the catalysts was inserted into a U-shaped quartz tube and oxidized at 400 °C under 5 vol% O2 in argon (Ar) (25 mL min−1) for 10 min with a heating rate of 10 K min−1. Subsequently, before the reduction of the samples, the gas line was switched to Ar (25 mL min−1) for O2 evacuation (50 °C, 10 min) followed by reducing the catalysts using the mixture of 5 vol% H2 in Ar (25 mL min−1) at a constant heating rate of 10 K min−1 up to 350 °C. Hydrogen consumption was determined using a TCD detector and a calibration using Ag2O (from Micromeritics) as a reference.Temperature-programmed desorption with CO2 (CO2-TPD) analyses were carried out using the aforementioned AutoChem II 2920 chemisorption analyzer (Micromeritics, USA) coupled with a mass spectrometer (Pfeiffer Vacuum, model ThermoStar). The samples were placed into a U-shaped quartz tube and reduced at 300 °C under 5 vol% H2 in Ar (25 mL min−1). The samples were then cooled down to 50 °C and flushed with Ar (25 mL min−1) for 15 min. At the same temperature (50 °C), the reduced samples were saturated with 80 vol% CO2 in Ar via 20 pulses of 0.532 mL. The average peak area of the last 15 pulses was used as the calibration for the quantification of CO2 desorption. Subsequently, the samples were heated with a heating ramp of 10 K min−1 up to 400 °C under Ar (25 mL min−1) for CO2 desorption. The amount of desorbed CO2 was determined via integration of the total area under the MS fragment m/z = 44.The pyridine adsorption-desorption experiments were carried out using a Perkin Elmer Pyris 1 TGA instrument to determine the acid site density of reduced catalysts. The as-synthesized catalysts were reduced externally in a tubular oven at 350 °C in H2 flow (50 mL min−1) for 4 h. Subsequently, the samples were placed in a sample pan and pretreated at 350 °C in nitrogen flow (50 mL min−1) for 30 min, then cooled down to 50 °C at which the sample weight was recorded and referred to as m0 (g). The pyridine saturation step was carried out by flushing with pyridine vapor in nitrogen flow (50 mL min−1) until recording a constant weight. The excess pyridine was removed by purging with nitrogen flow (50 mL min−1) at 50 °C for 2 h. The temperature was then increased to 450 °C, with a heating rate of 20 K min−1. The desorption of pyridine was recorded via the sample weight loss as a function of temperature/time. The weight loss during the desorption of pyridine while heating from 50 °C to 450 °C is Δm (mg). Assuming the stoichiometry between pyridine molecule and acid site is equal to 1, the acid site density (ASD) was, therefore, calculated as follows ASD = Δ m M pyridine ∙ m 0 (mmol⋅g−1) with Mpyridine = 79.1 g/mol.The carbon content of the fresh and spent catalyst samples were determined using a 2400 series II CHNS elemental analyzer (Perkin Elmer, USA).The crystallography, phase composition, morphology, and size of the samples containing Au nanoparticles were analyzed by transmission electron microscope (TEM, JEM-2100, JEOL), operating at 200 kV and equipped with a high-resolution slow-scan CCD camera (Orius SC1000, Gatan). The powdered samples pre-reduced in a tubular oven (in the H2 flow (40 mL min−1) at 350 °C for 4 h) were dispersed in absolute ethanol and sonicated to prevent agglomeration. The suspension was transferred onto Cu-supported amorphous carbon grids. The maximum Feret diameter was used as the size descriptor of Au nanoparticles, which was manually outlined and determined using ImageJ. To obtain a representative overview of Au particles, the TEM micrographs were recorded at various sites of interest for each sample, and the number of particles detected is up to 150 particles. Before the TEM investigation, the microscope image and diffraction mode were calibrated by the MAG*I*CAL® reference standard through all of the magnification ranges, with the overall uncertainty on the calibrated values Δt < 1.0 %.X-ray photoelectron spectroscopy (XPS) measurements were performed with a Supra plus instrument (Kratos Analytical, Manchester, UK) equipped with an Al Kα excitation source and a monochromator. The measurements were performed with a spot size of 700 × 300 µm. Pass energy of 160 eV and 20 eV were used to obtain a survey and high-resolution spectra, respectively. The binding energy scale was corrected using the C-C/C-H peak at 284.8 eV in the C 1s spectrum. The neutralizer was on during the spectrum acquisition. Data were acquired and processed using ESCApe 1.4 (Kratos, Manchester, UK). The background of the high-resolution spectra was subtracted according to the method of Shirley [50]. Reported atomic concentrations at the surface were normalized to 100.0 %.The hydrogenation of CO2 was carried out in a fixed-bed reactor (Microactivity Reference MA-Ref reactor from PID Eng&Tech, Madrid, Spain). Typically, an amount of ca. 200 mg of the catalysts was packed and sandwiched with quartz wool in a tubular reactor (I.D. = 9 mm, L = 305 mm) made of Hastelloy. During the packing step, the reactor was tapped frequently to ensure reproducible packing state of the catalyst beds. The catalysts were reduced internally using a mixture of H2 (30 mL min−1) and N2 (10 mL min−1) at 350 °C for 4 h. Afterwards, the reactor pressure was increased to 40 bar, and the temperature was reduced to 250 °C. The reactant mixture comprised of CO2 (24 vol%) and H2 (72 vol%) balanced in N2 (4 vol%). The gas flow rate was 40 mL min−1 (GHSV = 48000 h−1). The steady state was typically reached after 1 h marking the start of the catalytic experiments. In each experiment, the product mixture was sampled every 20 min in 2 h. The remaining reactants and gas products in the discharged gas stream were analyzed using an online Agilent 7890A chromatograph equipped with Porapak Q, HayeSep Q and molecular sieve 5A columns. The CO2 conversion ( X CO 2 ) and selectivity to methanol ( S CH 3 O H ) and CO ( S CO ) were calculated using the following equations: (1) X C O 2 = n C O 2 , i n - n C O 2 , o u t n C O 2 , i n ∙ 100 % (2) S CH 3 O H = n CH 3 O H n C O 2 , i n - n C O 2 , o u t ∙ 100 % (3) S CO = n CO n C O 2 , i n - n C O 2 , o u t ∙ 100 % where n C O 2 , i n and n C O 2 , o u t refer to the input and output molar amount of CO2. n CH 3 O H and n CO are the molar numbers of methanol and CO, respectively, in the product mixtures. Based on the 6 collected data points, the average and standard deviation of X CO 2 , S CH 3 O H and S CO were determined. Besides, the methanol formation rate ( r CH 3 O H ) was also used as a measure to evaluate the catalytic activity of the studied catalysts in the hydrogenation of CO2 into methanol. The methanol formation rate ( r CH 3 O H ) was calculated as follows: (4) r CH 3 O H = Q CO 2 ∙ X C O 2 ∙ S CH 3 O H ∙ M W CH 3 O H m Au [ g CH 3 O H ∙ g Au ∙ h - 1 ] where Q CO 2 , MW CH 3 O H and m Au denote the total flow rate of the reaction mixture, the molecular weight of methanol, and Au mass of the catalysts, respectively.The elemental composition of synthesized solids, namely, the content of Au, Ce, and Zr, was determined by the ICP-OES technique, and the results are displayed in Table 1 . All the catalysts exhibit a comparable Au content of 0.7 wt.% lower than the nominal Au content (1 %), which is probably associated with the hydration of hygroscopic Au precursor. Additionally, the Ce content gradually increased up to 4.6 wt.% rendering a gain in the Ce to Zr molar ratio (n Ce/n Zr) from 0 to 0.09, which agrees well with the nominal n Ce/n Zr. The chlorine content was determined in Au-containing catalyst samples, after digestion, by using in-house ion chromatography method. In the analyzed samples, the content of chlorine was below the level of quantification, i.e. <0.5 mg/g.The structural properties of all the catalysts were characterized by XRD analysis, from which the results are shown exemplarily for Au/ZrCex samples in Fig. 1 . Independently of Ce and Au content, all the samples exhibit almost identical XRD patterns featuring 2 broad signals centered at 2θ of 31° and 55°, which can be ascribed to a-ZrO2. No other phases, e.g., cerium oxides and Au-containing phases, were detected. This might be explained by the low content of Ce and Au, or they are present in very fine particles below the detection limit of XRD. The latter was later excluded by TEM analysis. Specifically, the deposited Au is evidenced by TEM and selected area electron diffraction (SAED), which are exemplarily shown for Au/ZrCe0.05 in Fig. 2 . The SAED pattern of the Au particle shows sharp and continuous rings corresponding to individual crystal planes. On the other hand, the ZrO2-based support material exhibits a halo ring typical for amorphous materials and blurred rings indicating a short-range order (Fig. 2 and Fig. S1). Additionally, separate images of ab-initio simulations of SAED included in Fig. S2, present the main differences between 2-phases, i.e., ZrO2-CeO2, and solid solution ZrO2/CeO2 (Fig. S2). The experimental SAED pattern of Au/ZrCe0.05 matches well with the corresponding simulated pattern, and thus confirms the formation of ZrO2/CeO2 solid solution. The presence of residual Ce species existing in the form of amorphous CeO2 is, however, not excluded.Furthermore, the morphology of Au particles visualized by TEM shows irregular shapes for all the Au particles (Fig. 3 ). The distribution of Au nanoparticle size is slightly uniform for Au/ZrCe0 catalyst and centered at ca. 35–40 nm, which accounts for 15–17 % of Au particles. In the meantime, a rather broad distribution was recorded for the particle size of Au particles in all the Ce-containing catalysts, which ranges from 10 to 150 nm. Noticeably, at the highest Ce content, Au/ZrCe0.1 catalyst exhibits the highest fraction of Au particles of ca. 70 nm, i.e., 10 %, as compared to 0–5 % for other samples (Fig. 3). This indicates that the introduction of Ce using the coprecipitation method did not improve Au dispersion in the amorphous ZrO2 support materials.Regarding the textural properties, minor changes were recorded by the N2 adsorption analyses as displayed in Fig. 4 . A Ib-type isotherm with a gradual N2 uptake over the low relative pressure range (p/p0 < 0.4) was recorded for all the samples suggesting a complex pore structure consisting of wider micropores and narrow mesopores [51]. Further analysis using non-local density functional theory model revealed the pore widths ranging from below 2 nm to 8 nm. Additionally, all the samples exhibit an H2b type hysteresis loop with a gradual delay on desorption branch indicative of a broader neck width distribution compared with the pore width distribution. The values of specific surface area and total pore volume of Au/ZrCex samples are listed in Table 1. The Ce-free catalyst Au/CeZr0, despite the slightly higher N2 uptake at the relative pressure of 0.6, exhibits a similar porous structure in comparison with the Ce-containing catalysts, ca. 200 ± 10 m2 g−1 specific surface area (A BET) and ca. 0.13 ± 0.01 cm3 g−1 total pore volume (V P).Concerning the redox properties, the results from H2-TPR profiles (Fig. 5 ) provide the very first influence of the introduction of Ce in the Au-based catalyst series. While no signal is visible in the TPR profile of Ce-free catalyst (Au/ZrCe0), the H2 consumption peak centered in the temperature range of 166–176 °C is recorded for all Ce-containing catalysts. Additional H2-TPR profiles were recorded for all supports (Fig. S3), in which the peak of interest is absent. The peak is, therefore, associated with the reduction of Auδ+ to Au0 [26,52], which is facilitated in the presence of Ce. This might be explained by the electron redistribution often known for metals supported on CeO2 due to the formation of surface oxygen vacancies, generated when Ce4+ cations are reduced to Ce3+ rendering to the oxygen transfer process between the metals and CeO2 support [53]. The interaction between Au and Ce-containing supports gradually increased as suggested by the linear correlation found between the Ce content and the H2 consumption shown in Table 2 . The absence of any reduction peak in Au/ZrCe0 sample is an indication of no positively charged Au in the sample [52,54], and the majority of the Au species is available in the metallic form, which is stable in the oxidation step conducted internally prior to the TPR measurements (see section 2.3). These observations suggest that the Au0 formation is more favored in pure ZrO2 sample, i.e., Au/CeZr0 catalyst, as reported in [54].Moreover, the synthesized solids were evaluated by XPS to obtain additional information related to the chemical environment of the catalysts. The survey spectra for the surfaces of the samples include O-, Zr-, Au-, and C-containing species indicated by the corresponding signals in Fig. 6 . The signal for Ce, i.e., Ce 3d, was detected and thus confirmed the presence of Ce in all the modified samples except for Au/ZrCe0 catalyst.To investigate the surface chemistry of the synthesized solids in detail, high-resolution XPS spectra were measured and are shown in Fig. 7 . The relative quantification of surface Au, Ce and Zr species obtained from XPS is displayed in Table S1.The surfaces of all samples consisted of C-containing species, i.e. C-C/C-H, C-O, and COO–/COOH located at the dashed lines designated in the C 1s spectra (Fig. 7a). These species originate from the adventitious carbonaceous species adsorbed on the surface after sample preparation and transport to the spectrometer. The presence of oxidized carbonaceous species is also confirmed in O 1s spectra, i.e., the spectral feature seen as a shoulder located at dashed line 2 in the O 1s spectra (Fig. 7b). The main peak at dashed line 1 in the O 1s spectra represents metal oxides. The doublet in the Zr 3d (the Zr 3d5/2 main peak at approximately 182 eV) and Au 4f (the Au 4f7/2 main peak located at 84.0 eV) spectra correspond to ZrO2 and Au, respectively. The Zr 3d and Au 4f spectra for all samples tested do not show significant changes in the binding energy (E B) and the shape of the spectra. This indicates that the Zr and Au environment is similar in all the samples. The observation of similar Au species for all the samples is, however, different from TPR results. The presence of H2 consumption for all Ce-containing samples, but not the bare ZrO2, is likely associated with changes in electronic properties induced by the addition of Ce in the ZrO2 structure.The spectra of Ce 3d are more complex because they contain many features. It is known that Ce(IV) contains the spectral feature at about 917 eV, which is absent in Ce(III) [55]. The spectra illustrated in Fig. 7e show the feature at approximately 917 eV marked with dashed line in Fig. 7e for all samples except for Au/ZrCe0 catalyst. Therefore, it is confirmed that Ce(IV) was present on the surface in all Ce-containing samples. On this basis, the spectra were fitted for the presence of CeO2 using the positions of the peaks from the previously reported reference spectra [55]. The Ce 3d spectra for CeO2 consist of two pairs of multiplets (2 pairs of 3 peaks) corresponding to spin-orbit splitting. However, the measured spectra do not fit well with this model probably due to partial reduction of Ce(IV) forming Ce(III) under vacuum conditions (ultra-high vacuum in XPS spectrometer) which is known for CeO2 [56]. Thus, the spectra of Ce-containing samples were fitted for both Ce(IV) and Ce(III) species (with two doublets [55]). The fitted results match well with all measured spectra suggesting the presence of these two oxidation states on the surface. The fitted Ce 3d spectrum for the sample Au/CeZr0.1 is exemplarily shown in Fig. 8 .The basic properties of all Au-containing catalysts were evaluated using CO2-TPD analysis and the obtained results are shown in Fig. 9 . The CO2 desorption of Au/ZrCe0 catalyst features a signal centered at ca. 100 °C, and end at ca. 280 °C, which is contributed mainly to the presence of weak basic sites. This peak was also recorded for the Ce-containing catalysts, but it is visible with a considerable broadening and coupled with an additional peak at higher temperatures above 300 °C, which is likely associated with the presence of Ce providing higher basicity than pristine ZrO2. However, no clear trend is observed for basic site density (BSD) when the catalysts gradually increases Ce content (Table 2). This suggests that in addition to Ce introduction, the changes in the number of basic sites on the surface of ZrO2 support are likely affected by other factors, e.g., thermal treatment causing dehydroxylation and formation of acid-base Zr4+/Ce4+-O2– pairs at various extents. Similarly, the influence of Ce introduction on the acid site density (ASD) occurs in a random fashion despite the gradual rise in Ce content as shown in Fig. S4. Interestingly, it was found that the acidic and basic properties of the Au-based catalyst series are closely related, as deducted from a linear regression observed between the acid and basic site density (Table 2). This might be related to the fact that a-ZrO2 is known to possess mainly coordinatively unsaturated Lewis acid-basic Zr4+/O2– pairs as well as surface hydroxyl groups [57].The Au-based catalysts supported on ZrO2-CeO2 mixed oxides were tested in the methanol synthesis from CO2 under various temperatures from 250 to 320 °C as displayed in Table 3 . In addition to CO2 conversion ( X CO 2 ), methanol selectivity ( S CH 3 O H ) and methanol formation rate ( r CH 3 O H ) are employed as measures to evaluate the activity for the methanol synthesis over the Au-based catalysts. Over the studied temperature range, CO2 conversion varies between 1 % and 11 %, with methanol selectivity up to 27 %, amounting to a methanol formation rate of 1.5 to 4.6 g CH 3 O H ∙ g Au ∙ h - 1 . Apart from methanol, CO and trace amounts of coke (except for Au/ZrCe0.1 catalyst), no other carbon-containing gas product was found in the discharged gas mixture, and the carbon mass balance is closed up to 85 % to 98 %. The catalytic activity obtained in this study is rather low, in particular, compared with the commercial methanol synthesis. Interestingly, the methanol selectivity of Ce-free Au/ZrCe0 catalyst is significantly lower, i.e., 20 % vs. 72.5 %, in relation to the Au/ZrO2 published in a previous work form our group despite the analogous Au content (0.7 wt.% and 0.5 wt.%, respectively) as well as reaction conditions (240–250 °C, 40 bar). This might be explained by the drastic discrepancy in the average Au particle size often reported to play a key role in the hydrogenation activity, i.e., ≈40 nm vs. 1.1 nm, respectively. The large size of Au particles might lead to low interfacial Au-support contact and thus facilitates the formation of undesired competive product CO generated by reverse water gas shift reaction. Moreover, the difference in other properties of the catalysts, e.g., phase composition of ZrO2-based support, acidic/basic properties, textural and redox properties, may also play a role.The reaction temperature, as expected, exhibits a positive effect in the CO2 conversion independent of Ce content but at the cost of the selectivity to methanol. This agrees well with literature reports [35,58,59] and is attributed to the higher sensitivity to temperature of the CO formation rate in relation to the methanol synthesis, which is evidenced by higher apparent activation energy calculated from Arrhenius plots, e.g., E A , C O =64 kJ mol−1 > E A , C H 3 O H =27 kJ mol−1, as observed in the case of Au/ZrCe0.025 catalyst. The values of EA for other catalysts are provided in Fig. S5. Besides, all studied catalysts exhibit similar apparent activation energy for methanol formation, suggesting that the introduction of Ce did not alter the hydrogenation mechanism of CO2 into methanol [49]. Consequently, the methanol synthesis via formate intermediate, as proposed in a previous study on Au/a-ZrO2 catalysts [27], was assumed for the Au-based catalysts in this study. Accordingly, the reaction starts with adsorption and activation of CO2 and H2 onto ZrO2-based support and metallic Au, respectively, which later migrate to the Au-support interface. The dissociated H species react with the activated CO2 forming mono- and bidentate formates, the two primary intermediates of methanol. Subsequently, the formate species undergo hydrogenation to H2COO*, and further terminal protonated to H2COOH*, which then cleaves at the C-O bond releasing OH* and H2CO*. The latter participates in hydrogenation, forming H3CO* and finally methanol. Among these steps, CO2 is simultaneously involved in the RWGS process forming undesired product CO.Furthermore, while the Ce-free catalyst Au/ZrCe0 exhibits the highest CO2 conversion (2.3 % at 250 °C) coupled with a 20 % selectivity to methanol, a slightly lower CO2 conversion was recorded for all the Ce-containing catalysts (Table 3). The decrease in CO2 conversion is more profound with increasing Ce content. Considering the comparable Au content as well as the chemical environment of Au, one of the other possible reasons for the decline in CO2 conversion most likely lies in the large size of Au particles generally accepted to be crucial for its catalytic activity, which was found larger for Ce-containing samples. Besides, catalyst deactivation caused by coke deposition in association with the presence of Ce, as reported by Pojanavaraphan et al. [52], might explain the lower activity in Ce-containing catalysts. However, it is noted that in comparison to the current study, the Ce content was much higher (≥25 wt.%) than that presented in this study, and consequently, the negative influence of Ce via boosting carbonaceous species is less likely. In fact, the drop in CO2 conversion is only considerable for Au/ZrCe0.1 catalyst exhibiting the highest Ce content (4.6 wt.%) and the highest extent of coke deposition (Table 3). On the other hand, the Ce-containing materials, as reported in the literature [25 30,32], exhibit a higher and stronger affinity towards CO2 molecules, which might lead to active sites poisoning. Additionally, it is worth mentioning that the addition of Ce makes H2O more favorably adsorbed on the surface [60] and contributes to a blockage of active sites and thus a decline in CO2 conversion.Unlike CO2 conversion, no clear trend was found for methanol selectivity in relation to the presence of Ce. Thus, the next part focuses on the influence of the acidic/basic properties on the methanol selectivity.In order to examine relation between methanol selectivity and the catalyst properties, the methanol selectivity S CH 3 O H was plotted against the density of base sites (BSD) and acid sites (ASD) as shown in Fig. 10 .Interestingly, volcano-like shaped dependencies were recorded for both BSD and ASD in relation to the methanol selectivity, which exhibited a critical point of 120 μmol g−1 (BSD) and 600 μmol g−1 (ASD) at which the highest methanol selectivity of 27 % is reached. The positive effect of the acidity of catalysts in the methanol synthesis was reported in previous studies using Cu/ZrO2 [55,56]. However, the correlation of the methanol selectivity with gradual changes in acidic/basic properties was not studied. To gain insight into this matter, the nature of acidic and basic sites should be considered. The majority of the acidity and basicity of the investigated catalysts is contributed to the Lewis acid-base pairs, i.e., coordinatively unsaturated Zr4+-O2–, which can reinforce the adsorption of CO2, particularly, the interaction between O and C atoms of CO2 molecules with Zr4+ and O2–, respectively [49]. Moreover, in the presence of Ce, the adsorption of CO2 forming monodentate carbonates, which was not observed for pristine ZrO2, is facilitated as suggested by density functional theory (DFT) results obtained for t-ZrO2 incorporated with ca. 2 wt.% Ce [60]. Monodentate carbonate is an important precursor to the more stable bidentate carbonate, which can be further converted into bidentate formate and subsequently to methanol via the formate reaction pathway. Besides, these Lewis acid-base pairs can also participate in the dissociation of water molecules generating surface hydroxyl groups, which can promote methanol synthesis via reacting with carbonate species, forming mono-/bidentate formates [61]. The eased formation of formates might hinder the competitive process, RGWS, as suggested by various studies [34,62], and thus increase the methanol selectivity. Alternatively, the methanol synthesis can also be facilitated via the hydrogenation of CO [57], which is, however, unlikely in this study as the CO adsorption was suggested to be hardly affected by Ce incorporation at low content [60]. Thus, the increase of the acid-base pairs in numbers, as well as strength, probably renders to the improved methanol selectivity (Fig. 10). Nevertheless, the interface between the multiple components of the catalysts, i.e., metallic sites, hydroxyl groups and the acid-base pairs, seemingly plays a crucial role in the methanol synthesis as depicted in Fig. 11 , which might explain the decrease in methanol selectivity when further increasing the number of acid/basic sites. The surface catalytic sites probably suffer catalyst deactivation due to blockage of active sites by a multilayer of adsorbed CO2 or intermediates.Considering the highest methanol formation rate among the Ce-containing Au-based catalysts, Au/ZrCe0.025 sample was selectively investigated in the catalytic stability carried out throughout 93 h under reaction conditions (280 °C and 40 bar) (Fig. 12 ). The obtained results suggest a stable performance through the whole experiment with a CO2 conversion of 3.5 % ± 0.2 % rendering to a methanol formation rate of 3.32 ± 0.25 g CH 3 O H ∙ g Au ∙ h - 1 . Noticeably, with respect to the start of the experiment, there is a slight loss of catalytic activity by 0.6 % in CO2 conversion and 0.47 g CH 3 O H ∙ g Au ∙ h - 1 in ( r CH 3 O H ) occurring in the first 3 h, which might be due to the considerable formation of water via RWGS causing saturation of the support surface due to high affinity to water in the presence of Ce [60]. Nevertheless, the reactivity of the catalyst (Au/ZrCe0.025) was steady for the next 90 h. Despite the long reaction time, the coke deposition is negligible as deducted from the marginal discrepancy in the consumed catalyst's C content, i.e., 0.7 wt.% (spent) vs. 0.6 wt.% (fresh).In summary, this study presents a systematic investigation of the influence of Ce in the Au catalysts supported on amorphous ZrO2 prepared via the simple coprecipitation method. The newly introduced Ce was present in the form of both Ce(III) and Ce(IV), which does not alter the textural properties compared to the pristine ZrO2 materials with a Ce content up to 4.6 wt.%. The Ce-containing catalysts show a slightly lower Au dispersion evidenced by large particles (ca. 40 nm) and a broad distribution ranging from 5 to 125 nm. Besides, the formation of metallic Au is less favored and coupled with increased Au cations with rising Ce content, probably due to the electron exchange between Au and CeO2. These findings might explain the decrease of CO2 conversion with increasing Ce content of Au catalysts supported on ZrO2-CeO2 mixed oxides in the hydrogenation of CO2. In addition to methanol formed via formate intermediates, only CO was observed as a side product under the studied reaction conditions (typically at 40 bar and 250–320 °C), irrespectively of the catalysts employed. It was proven that acid-base properties play a role in CO2 adsorption/activation and thus in tailoring the product distribution of the CO2 hydrogenation. The selectivity of the desired product methanol was found to be closely associated with the number of both acidic and basic sites of the catalysts in a volcano-shape fashion. In particular, there is a critical point of acid/base site density, at which the methanol selectivity reaches a maximum 27 % (at 40 bar and 280 °C), which is 120 and 600 μmol g−1, respectively. The Au catalysts on CeO2-ZrO2 mixed oxides exhibit excellent catalytic stability, for example, the methanol formation rate of 3.32 g CH 3 O H ∙ g Au ∙ h - 1 recorded for Au/ZrCe0.025 catalyst at 250 °C and 40 bar remained for up to 93 h TOS. Hue-Tong Vu: Investigation, Methodology, Visualization, Writing – original draft, Writing – review & editing. Matjaž Finšgar: Funding acquisition, Investigation, Writing – review & editing. Janez Zavašnik: Investigation, Writing – review & editing. Nataša Novak Tušar: Funding acquisition, Supervision, Writing – review & editing. Albin Pintar: Funding acquisition, Project administration, Conceptualization, Supervision, Visualization, 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 research was funded by the Slovenian Research Agency, grant numbers J7-3151, P1-0418 and P2-0118. The project is co-financed by the Republic of Slovenia, the Ministry of Education, Science and Sport and the European Union under the European Regional Development Fund. The authors thank Mojca Opresnik, Edi Kranjc, Špela Božič, Iris Štucin, Matevž Roškarič, and Gregor Žerjav from the National Institute of Chemistry (Ljubljana, Slovenia) for N2 sorption, XRD, CHN, ICP-OES analyses and their support in the labs, respectively. Janez Zavašnik acknowledges the support via ARRS P1-0417.Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc.2023.156737.The following are the Supplementary data to this article: Supplementary data 1

To exploit the potential of both ZrO2 and CeO2, the mixed oxides with Ce content up to 5 wt.% were prepared via the simple coprecipitation method, subsequently loaded with Au catalysts, and investigated in the hydrogenation of CO2. The obtained catalysts, namely Au/ZrCex (x = n Ce/n Zr = 0.0–0.1), exhibit similar Au content (0.7 wt.%), structural and textural properties, however, considerably different acidic/basic properties and great surface oxygen vacancy. The introduction of Ce leads to slightly decreased CO2 conversion, which was found proportional to the Ce content. Interestingly, it is evidenced that the methanol selectivity is closely related to the acidic/basic properties of the catalysts employed. Over the broad range of acid site density (ASD) ranging from 400 to 700 μmol g−1, the highest methanol selectivity of 27 % was recorded at 250 °C and 40 bar over the catalyst exhibiting an ASD of 600 μmol g−1, which decreased with further increasing ASD. The volcano-shape trend was also discovered for the base site density showing the critical point at 120 μmol g−1 over the range from 90 to 210 μmol g−1. These findings suggest that the acidic/basic properties can be tuned, e.g., via thermal treatment, to tailor the product distribution of the CO2 hydrogenation. Above all, the Au catalysts supported on ZrO2–CeO2 mixed oxides exhibit excellent catalytic stability, e.g., a methanol formation rate of 3.32 g C H 3 O H ∙ g A u ∙ h - 1 is remained over the reaction course of 93 h at 250 °C and 40 bar for Au/ZrCe0.025 sample.

Due to the industrial growth in emerging economies, extensive utilization of fossil fuels has been the major source of environmental pollution and energy crisis. Thus, transforming solar energy into chemicals through photosynthesis has been considered the favorable approach to tackle the situation [1,2]. Particularly, PEC water splitting effectively generates fuels without any harmful carbon dioxide emissions [3,4]. Material selection for PEC water splitting is critical, provided that photoelectrodes absorb incident photons and generate the charge carriers and need to be economical for practical applications [5,6,7–10]. Numerous n-type semiconductor metal oxides photocatalysts have been widely studied (TiO2, ZnO, Fe2O3, BiVO4, Bi2O3 etc.) [11-15]. Titanium dioxide (TiO2) is frequently utilized due to its chemical steadiness and photocatalytic capabilities [16-18]. However, due to the short-wavelength cutoff qualities of TiO2, only 6% of the solar energy reaching the earth's atmosphere can be exploited to drive photovoltaic and photocatalytic effects. More importantly, TiO2 has the latent capacity for full-water splitting reaction owed to its fortunate band-edge position [19,20]. In this context, two complicated multi-electron half-reactions are involving in the PEC water-splitting process (2H2O → O2 + 4H+ + 4e−, E = 1.23 VRHE; 4H+ + 4e− → 2H2, E = 0 VRHE). According to earlier reports, the reactions among photoinduced holes and water molecules primarily restrict the features of PEC reactions, as it usually happens at a substantially higher potential to remove 4 electrons and 4 protons from 2 water molecules to produce an O2 molecule [21-23]. However, the photoinduced valence band (VB) holes in TiO2 are considered to be kinetically inefficient, and thereby, it requires an additional anodic bias before water can be oxidized [24]. In this context, the combination of water oxidation catalysts with a photon-absorbing substrate represents an appropriate route to reduce the driving force of the electrolysis chemistry of the catalyst and thereby enrich efficiency. Significant effort has been put into emerging an appropriate oxygen evolution (OE) rection electrocatalyst for photoelectrode [25-31].In recent years, transition metal-based catalysts, such as oxides and hydroxides of Ni, Co, Fe, and Mo, as well as oxyhydroxides and phosphates, have been used as OER co-catalysts [32-35]. Though, all such co-catalysts hurt from low electrical conductivity [36]. Because of its abundant earth reserve properties, low cost, and ability to function under benign conditions, transition-metal phosphides (TMPs), namely FeP, CoP, MoP, and NiP is a superior catalyst for OE associated with precious metals (Ir, Ru) and metal oxides (RuO2, IrOx) catalysts [37-41]. Among the existing metal phosphates, Ni-based TMPs materials have been broadly examined for various kinds of electrochemical uses [42,43]. Notably, Ni-based TMPs show a substantial part in the electrochemical features, whereas P impacts a stable structure [44]. However, fabricating a trustworthy approach for incorporating TMPs with oxide-based electrodes in a convenient, scalable, and economical approach is still required. Liu et al. and Bu et al. integrated Fe-integrated Co2P and Ni2P nanoparticles as operative co-catalytic materials on Fe2O3-based electrodes to boost the water splitting system[45,46]. Thus, extensive research effort on the exact mechanisms of TMP-based electrocatalytic material for water oxidation reaction is still required. Ruifeng et al. reported that the NiPi modified of Pi-Fe2O3 photoanode showed enhanced photoelectrochemical activity for glycerol oxidation. With the addition of NiPi, the PEC features of Pi-Fe2O3 were increased by approximately twofold at 1.5 VRHE [47]. Schipper et al. have recently demonstrated the first successful use of FeMnP with rutile TiO2 as a co-catalyst for PEC solar water-splitting reactions [48].We demonstrated the PEC water splitting using NiPi nanoparticle-modified TNT array semiconductor photoanodes using simple two-step anodization and electrodeposition process.Fabricated TNTs comprised vertically stacked nanotubes arrays with a high surface area that allows high integration with NiPi nanoparticles. The TNTs/NiPi photoelectrodes displayed superior photocurrent density (0.759 mA/cm2) at 1.23 VRHE, representing nearly 3-fold enhancements than TNTs. Furthermore, the TNTs/NiPi demonstrated well-separated electron-hole pairs and improved the charge transfer process at the interface between the electrode and electrolyte, indicating the NiPi certainly accelerates the water oxidation rate and reduces the needed overpotential. These findings emphasized the multifunctional role of decoration of TNTs arrays with NiPi in enhancing PEC solar water splitting.TNT arrays were obtained via a two-step electrochemical anodization process involving Ti foil. Before reaction, a 0.25 mm Ti foil (>99.5%, Alfa Aesar) was ultrasonically washed with acetone and deionized (DI) water. Subsequently, Ti foil was assembled in a 2-electrode assembly with a Pt foil as the counter electrode in 0.12 M NH4F in a 5/100 (w/w) mixed solution of DI water and ethylene glycol (EG), at a continuous potential of 60 V. All through the anodization process, the foil was exposed to the solution, the first step was for 10 min, then ultrasonicated for 5 min, the second step was anodization for 20 min, and then cleaning by sonicated in ethanol for 10 sec then washed very well with DI water and then subjected to an annealing process at 450 °C for 2 h.TNTs/NiPi electrodes were fabricated by electrochemical deposition through an electrochemical bath comprised of 20 mM NiCl2 in DMSO. The deposition was executed in a 3-electrode cell comprising TNTs, Pt, and Ag/AgCl as working, counter, and reference electrodes. Afterward, the electrodeposition was carried out at −2.0 V vs. Ag/AgCl, and an optimization process was tuned by tuning the deposition charges varying from 1 to 10 mC/cm2. Then, the obtained film was exposed to an annealing at 450 °C for 1 h in the air (2.0 °C/min). Thus the optimal charge density was estimated to be 5 mC/cm2. Fig. S1 shows the different phases of the synthesis approach employed to obtain the TNTs/NiPi photoanodes.X-ray diffraction (XRD) investigations were done via X-ray diffractometer (Rigaku Miniflex 600). The UV–visible diffuse reflectance spectra (DRS) were acquired via Shimadzu UV-2600. Also, the surface morphologies of electrodes were performed through a field-emission scanning electron microscope (FE-SEM, JSM7600F, JEOL, USA). X-Ray Photoelectron (XPS) analyses were executed on the photoanodes with JEOL XPS-9030.All the PEC analyses were evaluated through an AutoLab potentiostat PGSTAT30. All the experiments were measured in a 0.1 M PBS (pH 7.5) solution with and without 1.0 M Na2SO3 as a hole scavenger. The applied bias photon-to-current efficiency (ABPE) of the TNTs, and TNTs/NiPi photoelectrodes under illumination were evaluated from the linear sweep voltammogram curves using the Eq. (1). (1) A B P E % = I × 1.23 v - V b P tot × 100 % where I = current density at certain potential Vb (mA/cm2) under illumination conditions (100 mW cm−2), and Ptot = power density of the incident light.In order to explore the source of PEC features of TNTs electrodes after optimal Ni incorporation, complete optical, crystalline, and morphological characteristics were carried out. Initially, the optimal Ni content over the bare TNTs was evaluated after evaluating different Ni incorporation by tuning the applied charge throughout the deposition method. Notably, the obtained results are presented in Fig. S2, assessing that the optimal charge agrees to 5 mC/cm2. Further, the crystalline nature of the TNTs, TNTs/Ni, and TNTs/NiPi composite nanotube arrays was dogged by XRD analysis (Fig. S2a, and Fig. 1 ). Also, the obtained diffraction peaks for all samples were at 25.3°, 46.7°, and 54.8°, respectively. The diffraction peaks belonging to the (101), (200), and (211) plans of the anatase phase have been identified [49]. This specifies that the TNTs crystal phase changed from amorphous to anatase only after annealing, but no rutile phase was observed. Moreover, an XRD analysis of TNTs decorated with different Ni loadings was performed (Fig. S2a,b). Remarkably, increasing the Ni incorporation over TNTs systematically increased the lattice constant, which is consistent with the bigger Ni2+ ion. It can be evidenced that the diffraction pattern of optimized TNTs/Ni, and TNTs/NiPi was identical to that of TNTs, then the intensity of the peak was comparatively decreased (Fig. 1a), due to Ni, and NiPi presence on TNTs. It must be noted that, due to the smaller quantity of Ni, the observed peak shifts in the XRD are not straightforward.Using Raman spectroscopy, we further examine the structure of unmodified TNTs, TNTs/Ni, and TNTs/NiPi samples. The spectra of bare and Ni/NiPi loaded TNTs photoanodes are shown in Figs. 1b, and S2c. The six different modes of anatase TNTs, such as 1A1g, 2B1g, and 3Eg, can be represented by the following Raman active modes: (1A1g + 2B1g + 3Eg) [50]. There are three waves with the numbers 143, 196, and 637 cm−1 associated with Eg mode in the anatase phase.The B1g mode is present at 396 and 516 cm−1, while the A1g mode can be found at 516 cm−1.A1g and B1g modes have a wavelength of 516 cm−1, hence they are unresolved doublet modes. In the resulting Raman pattern, a shift toward higher wavenumbers appears as the Ni concentration increases Fig. S2c, and NiPi is loaded (Fig. 1b). Similar trends are also seen in the XRD pattern (Fig. S2b, and Fig. 1a), where peak intensities above 10 mC shift towards high 2θ. The peak intensity increases as Ni and/or NiPi are deposited on TNTs. These findings were in concordance with our XRD measurements.In order to examine the impact of both additives (Ni and/or NiPi) on the behavior of the photoelectrodes under PEC, a detailed optical and structural characterization was performed (Fig. 2 a, Fig. S3). Fig. 2a shows the DRS methods applied to measure the bandgap and absorption of photoelectrodes.As can be seen, incorporating Ni and/or NiPi nanoparticles enhanced the visible-light optical density of the TNTs photoanodes by providing electron transitions at the band edges of anatase-scheelite phase TNTs. The band-to-band transition in TNTs causes them to show an intense band at wavelengths shorter than 400 nm. TNTs/Ni photoanode showed a red-shift compared to the undoped TNTs (Fig. 2b). Impurity levels can cause the photoexcitation energy to decrease and contribute to a substantial red-shift of the absorption edge [51]. Comparatively, in TNTs/Ni photoanode, there was an enhanced light absorption on the TNTs/NiPi photoelectrodes. The enriched light absorption is accredited to the excitation of electrons in the bandgap by localized P 2p. TNTs/NiPi samples exhibit a dramatic radiation absorption between 400 and 760 nm due to the additional crossovers between the Ti4+, VB, and conduction band (CB). Fig. 3 displays top-view and cross‐sectional FE-SEM photographs of two-step anodized TNTs, TNTs/Ni, and TNTs/NiPi photoanodes after the anodic oxidation process and annealing at elevated temperatures. As shown in Fig. 3, morphological observations reveal a homogenous distribution of highly ordered, vertically aligned, hollow TNTs in all samples (TNTs, TNTs/Ni, and TNTs/NiPi) with a diameter of around 95–115 nm, length of around 522 nm and a wall thickness of 18 nm. The TNTs still kept uniform nanotube arrays after the Ni (Fig. 3c, d), and NiPi modification process (Fig. 3e and f). No noticeable accumulation of NiO was observed on the surface of the TNTs; only tiny fragments made by cracking of the tube walls were present. Fig. 4 displays HR-TEM photographs of the fabricated TNT/NiPi electrodes. The TEM images in Fig. 4a and b specify the homogeneity and arrangement of the obtained nanotubes morphology in the TNT/NiPi electrodes. Furthermore, the acquired TNTs were greatly identical, with an outer diameter of 165 ± 2 nm and a 28 ± 2 nm wall thickness. Also, the characteristic lattice fringes of 0.351 nm seen in the photographs in Fig. 4c matching to the (101) plane of anatase TiO2 (JCPDS # 21-1272), suggesting the anatase phase of the TNTs. The TNTs/NiPi films were defined by the presence of crystalline particles in the range of 5–10 nm over the surface of TNTs (Fig. 4d). Further, the EDS spectrum of the TNT/NiPi in Fig. S4, where Ti, O, P, and Ni signals are observed, demonstrated a successful addition of NiPi layers over TNTs.The surface composition nature of the TNTs, and TNTs/NiPi composite electrode were explored by XPS, as presented in Fig. 5 . The survey XPS spectrum of TNTs/NiPi composite electrodes reveals the existence of all the elements (Ti, Ni, P, and O) without obvious contaminations (Fig. 5a). Further, Fig. 5b presents the deconvolution of the Ti 2p spectrum for TNTs, and TNTs/NiPi photoanodes. The Ti 2p spectra show the characteristic doublet of spin–orbit coupling (2p3/2, 2p1/2) for both photoanodes, with the highest intensity peak of the Ti 2p3/2 component at approximately 458.95 eV binding energy (BE) due to the presence of Ti4+ ions in TiO2 and a lower intensity peak at ca 460.10 eV due to the existence of Ti3+ ions in Ti2O3 [52,53]. This validates that both TiO2 and Ti2O3 are created in the TNTs crystals from the anodization route. It can be envisioned that the existence of Ti3+ in photoelectrodes could be a benefit for PEC uses under illumination since the ionic radius of Ti3+ (0.81 *) is near to Ni2+ (0.83 Å) than Ti4+ (0.75 Å) [48,54]. Fig. 5c shows the O1s spectra of different photoanodes. The high-resolution O 1s spectra show a more substantial peak at ∼530.18 eV and an observed shoulder peak at around 531.19 eV, implying the existence of two dissimilar O chemical states, with the crystalline lattice oxygen and hydroxyl oxygen with upsurging BEs [55,56]. Interestingly, in Ni 2p spectrum (Fig. 5d), the obtained peaks at 855.96 eV and 873.3 eV are allocated to Ni 2p3/2 and 2p1/2 of Ni2+, correspondingly [56-58]. Moreover, other peaks at 849.89 eV are allotted to Ni 2p3/2 for metallic Ni [59]. As seen in Fig. 5e, the BE of P 2p is at ∼133.53 eV, representing P in the phosphate group, endorsing that P subsists as the nature of the phosphate group [31].The PEC features of the unmodified TNTs, TNTs/Ni, and TNTs/NiPi electrodes toward water oxidation were examined under chopped and constant solar light irradiation in a 0.1 M PBS solution (pH ∼7.5). Beforehand the observed photocurrent of the TNTs/Ni photoanodes was optimized by tuning the total deposition charge for NiO from 1 to 10 mC/cm2 as shown in Fig. 6 a. Interestingly, the obtained photocurrent density upsurged from 0.533 to 0.621 mA/cm2 as the deposition charge of the Ni was upsurged from 2 to 10 mC/cm2, owing to boosted light absorption and, subsequently, improved carrier photo-generation. Though, the photocurrent density reduced when the deposition charge of the Ni extended by about 10 mC/cm2 owed to an increased carrier recombination rate (Fig. 6b). Thus, the optimal photocurrent of 0.621 mA/cm2 was succeeded at a charge density of 5 mC/cm2 and 1.23 VRHE under standard conditions. This can be simplified by the electrocatalytic OE reaction at the interface between the NiPi and electrolyte, and when NiPi integration at a higher deposition charge (>0.5 mC/cm2), the photoinduced holes must be moved amongst several NiPi molecules and NiPi/electrolyte interface, whereas considerably affected the reduced kinetics of the hole transfer, and successively, a lower photocurrent response is observed [60]. In addition to the deposition charge of the Ni, the morphological features and nanoarchitecture of the surface influenced the PEC features of photoelectrodes. Fig. 6c displays the chopped linear sweep voltammograms (LSV) plots of the TNTs, TNTs/Ni, and TNTs/NiPi electrode at 5 mV/s in 0.1 M PBS solution (pH ∼7.5). Upon irradiation conditions, the obtained photocurrent response of the electrodes at 1.23 VRHE declined as TNTs/NiPi (0.76 mA/cm2) > TNTs/Ni (0.621 mA/cm2) > TNTs (0.243 mA/cm2). The LSV plots for the PEC water oxidation in Figure 6d confirm that the optimized TNTs/NiPi photoanodes exhibited enhanced photocurrents than bare TNTs. The evaluated photocurrents, which we assign to OE reaction [61,62], are superior upon NiPi addition over TNTs photoanodes. Notably, the photocurrent density improved considerably with the applied bias and obtained ∼0.764 mA/cm2 at 1.23 VRHE, which agrees to a nearly 3.2-fold development associated with the bare TNTs(Table 1 ). After incorporating NiPi, the substantial reduction in onset potential and upsurge in photocurrent response revealed rapid water oxidation. Also, enhanced PEC features are caused by effective carrier separation and reduced rate of carrier recombinations. Notably, the recombination might occur in the bulk or the surface of the TNTs electrodes. Fig. 6e reveals the plots of the ABPE with respect to the applied bias. The bare TNTs electrodes display an optimal ABPE of 0.253% at ∼0.35 VRHE. Considerably, the TNTs/NiPi electrodes realize the highest ABPE of 0.46% at a quite lower potential of ∼0.31 VRHE (Table 1). Furthermore, >1.85 times enhanced ABPE at a smaller external bias directly determines that the addition of NiPi over TNTs is an accessible route to enrich the PEC features of TiO2. As debated above, the continued charge separation and transfer process of NiPi are the key factors for the enhanced PEC performance of TNTs/NiPi photoanode. The fabricated electrodes' comparative electrochemical surface area (ECSA) was assessed by the capacitive part of the cyclic voltammogram (CV). As presented in Fig. S5(a,b), CVs were executed at numerous sweep rates in the region of 10–100 mV/s in 0.1 M PBS. The ECSA was then evaluated by assessing the capacitive current related to double-layer charging from the sweep rate requirement of the CV. The double-layer capacitance (Cdl) was assessed from the association between ΔJ = (Ja-Jc) of RHE at 0.66 VRHE and the scan rate. As displayed in Fig. 6f, the linear slope is equivalent to twice the Cdl and can be applied to exemplify the ECSA (Table 1). Moreover, the linear slope of the TNTs/NiPi electrode is 2.98 times that of the TNTs electrode, which further proves that loading NiPi increases the specific surface area and enriches the active site.To better evaluate the efficiency of NiPi on the surface recombination, we applied Na2SO3 as a hole scavenger to exclude the injection barrier for holes [63]. The PEC measurements were executed in 0.1 M PBS mixed with 1.0 M of Na2SO3. Both TNTs and TNTs/NiPi electrodes show higher photocurrent density (Fig. 7 a) in the presence of Na2SO3, due to sulfite oxidation. Fig. 7b plots displayed the photocurrent produced by TNTs/NiPi electrode in the presence or absence of Na2SO3. Also, it clearly shows the TNTs/NiPi demonstrated an apparent upsurge of photocurrent and reduction of onset potential in the hole scavenger, signifying that Na2SO3 eliminated the surface carrier recombination and boosted the injection of holes to the electrolyte than TNTs. Moreover, to better recognize the charge transfer dynamics, surface charge transfer efficiency (ηsurface) of TNTs and TNTs/NiPi at various potentials were assessed in Fig. 7a, and the results are shown in Fig. 7c. Indeed, TNTsphotoanodes yield only <40% ηsurface , even at potentials as higher potentials, at which the larger electric field impedes surface carrier recombination. After incorporating NiPi, ηsurface of the TNTs/NiPi photoanodes is boosted to ∼80% at 1.23 VRHE, suggesting enriched charge transfer kinetics.As shown in the above PEC measurement results, sparse NiPi decoration can be used to enrich the PEC features of TNTs in neutral electrolytes. Additionally, we tested this positive electrocatalytic result with acidic and alkaline electrolytes. By adding concentrated H2SO4 or KOH to aqueous solutions, the pH values of four same 0.1 M PBS electrolytes were tweaked to 1, 4, 10, and 14, respectively, to maintain a similar ionic environment. Fig. 7d shows that all the photocurrent densities of the modified TNTs/NiPi photoelectrodes in various pH value electrolytes improved in the low bias potential region. In these studies, NiPi catalysts successfully promoted the PEC features of TNTs over a varied pH, ranging from 1 to 14. Since the NiPi modification can enhance light absorption across a wide pH range, it suggests applications for added light-absorbing substrates in numerous electrolytes.Finally, the durability of the TNTs, and TNTs/NiPi photoanodes were investigated using chronoamperometry at 1.23 VRHE acquired in 0.1 M PBS, under irradiation (Fig. 7e). The photocurrent-time profile of TNTs/Ni revealed better durability than bare TNTs; after 4 h of testing, 78.50% of its initial features were upheld (0.59 vs. 0.749 mA/cm2). This validated the part of the NiPi in boosting the durability of the TNTs by decreasing the carrier recombination or through the fast and complete OE reaction [64,65]. In order to explore the mass loss of TNTs throughout the J-t plots, the XRD patterns and FE-SEM photographs of TNT/NiPi photoanodes were acquired at four hours. As can be seen, the TNTs/NiPi had no discernible changes in XRD pattern or FESEM image (Figs. 7f and S6) after 4 h compared with samples obtained before 4 h. The PEC features of the TNTs/NiPi photoanode are considerably higher than the recently reported TNTs-based photoanodes (Table 2 ). The higher PEC activity of the TNTs/NiPi than TNTs could be explained by light absorption, the active sites, recombination of charge carriers, charge-transfer efficiency at the electrode/electrolyte interface, and oxidation kinetics of the water molecule on the surface of the electrode [61,66-80].To better recognize the interfacial charge transfer behavior in the TNTs and TNTs/NiPi photoanodes, their electrochemical impedance spectra (EIS) were evaluated. Fig. 8 a displays the Nyquist curves of the TNTs/NiPi and the TNTs electrodes evaluated in both dark and illumination response at 1.0 VRHE, and the corresponding equivalent circuit. As is evident on the EIS Nyquist plot, the diameter of arc radius under both dark and irradiation response was obviously much smaller for the TNTs/NiPi composite photoanodes than for bare TNTs, demonstrating rapid interfacial charge transfer across the interface, more active separation of photogenerated charge carriers, subsequent in superior PEC features [80]. Remarkably, NiPi-decoration improved the charge-carrier density and electronic conductivity, thereby reducing the resistance (Table S1). These results confirmed that the NiPi enhanced the separation of carriers in the TNTs/NiPi photoanodes, thereby contributing to their superior PEC features.The Mott-Schottky (M-S) plots of the TNTs/NiPi and bare TNTs electrodes are shown in Fig. 8b. M-S curves of unmodified TNTs, and TNTs/NiPi exhibited a positive slope, as anticipated for semiconductors in the n-type regime.Evidently, the obtained M-S plots noticeably verified that the TNTs/NiPiphotoanodes had a smaller slope than its bare TNTs, demonstrating the enhanced donor density and conductivity of the former [60]. We estimated the donor density of TNTs and TNTs/NiPi electrodes from the M-S plots. The estimated donor density ofTNTs/NiPiwas 2.4 × 1019 cm−3, superior to bareTNTs(7.90 × 1018 cm−3). Moreover, the greater donor density was attributed to the incorporation of NiPi, which decreased the recombination of carriers and thus contributed to the higher photocurrent response of the TNTs/NiPi photoanode. Also, by inferring the M-S plots to the potential axis, as revealed in Fig. 8b, the flat band potentials (EFB) of the bare TNTsandTNTs/NiPi electrodes were estimated to be 0.048 and 0.235 VRHE, respectively (Table 1). The EFB ofTNTs/NiPiwas more positively shifted and smaller than that ofTNTs, matching the anodic change of the overpotential OE reaction. The apparent positive shift of EFB inTNTs/NiPi enriched the band bending at the TNTs/NiPi and electrolyte interface, thus reducing the recombination of the photogenerated charge pairs and the overpotential in the OE kinetics of theTNTs/NiPi photoelectrode.Based on the earlier PEC examinations, a probable PEC splitting mechanism of TNTs/NiPi was proposed in Fig. 9 . The NiPi catalysts were loaded over the surface of and inside the TNTs. Interestingly, loaded NiPi catalysts in TNTs assist reaction sites and accelerate the transfer of photoinduced holes [74]. Also, the incorporation of NiPi enabled TNTs to absorb visible light. Under irradiation response, induced electrons in the VB of TNTs were agitated to form photoinduced carriers, and the electrons were moved to the CB of the tubes. Bearing in mind the Ni species of NiPi is an effective electrocatalyst for water oxidation reaction, a probable mechanism for the improved charge transfer is shown in Fig. 9, which comprised of the hole oxidation of Ni2+ to a higher valence state (Ni3+/4+), subsequently, it is reduced back to Ni2+ state with the instantaneous water oxidation to produce O2. The NiPi co-catalyst, as a greatly effective catalytic material for OE reaction, boosted the hole-trapping features to quicken the separation of photoinduced carriers.Herein, we have demonstrated a catalyst based on NiPi nanoparticles decorated over TNTs photoanode surface by anodization and photoelectrodepostion process. Decoration of the NiPi catalyst over TNTselectrodes revealed exceptional reaction features for OE reaction to produce oxygen gas. Thephotocurrentdensity was upsurged to 0.76 mA/cm2 with 3-fold enhancements at 1.23 VRHE for TNTs/NiPi related to the bare TNTs photoanode. The onset potential of TNTs/NiPi decreased to 0.06 VRHE, and the ABPE was 0.45% at 1.23 VRHE. The EIS spectrum and Cdl categorization specified that the fabricated NiPi particles delivered greater concentrations of the catalytically active region for the water splitting system. This study offers a new paradigm for designing nanostructured photoanode/donor density/hole transfer composite photoanodes for effective and highly stable solar fuel production, and the proposed material design strategy can be applied to other photoanodes.This project was funded by the National Plan for Science, Technology, and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award Number (3-17-02-001-0011).All data generated or analyzed during this study are included in this published article (and its Supporting Information files).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.Supplementary data to this article can be found online at https://doi.org/10.1016/j.jscs.2022.101484.The following are the Supplementary data to this article: Supplementary data 1

We present exemplary fabrications of controlled Nickel phosphate (NiPi)/TiO2 nanotubes arrays (TNTs) in phosphate buffer for boosted photoelectrochemical (PEC) water splitting. The TNTs/NiPi composite electrodes revealed a considerably enhanced photocurrent density of 0.76 mA/cm2, up to 3-time enhancements than bare TNTs, mostly because of the enhanced charge separation, decreased carrier recombination, and improving kinetics of the water oxidation. Also, we demonstrated that the NiPi can assist the PEC features of TNTs over a varied region of pH values from 1 to 14. Incorporation of NiPi over the TNTs surface advances the light absorption features of the electrode, resulting in an enhanced photogenerated charge carrier; and promotes the reactive sites for water oxidation, which was proved by the double-layer capacitance. The TNTs/NiPi photoelectrode exhibited excellent photostabilization under continuous illumination for 5 h, and the photoconversion efficiencies were 0.45%, 3-fold enhancements than with bare TNTs under the illuminations. Overall, this work might offer an innovative approach to fabricating and designing efficient electrodes with superior contact interfaces among photoanodes and numerous co-catalysts.

In an electrolyser device, the catalyst layer can contribute to losses in the overall running of the electrolyser due to inactive OER sites and low conductivity of the materials [1,2]. To achieve next generation inexpensive OER electrolyser catalysts, the catalysts themselves must be electrically conductive, mechanically and chemically stable under operating conditions, exhibit a high electrochemical surface area, and contain a high concentration of active sites for the evolution of O2. This has not been achieved to date for Proton exchange Membrane (PEM) and Alkaline Anion Exchange Membrane (AAEM) water electrolysis. One avenue to explore to make a catalyst that possesses all these characteristics would be to essentially combine different materials that exhibit these properties individually and make a ‘super’ catalyst.MXenes are a family of 2D materials, which are made up of transition metal carbides and nitrides, produced from MAX phases by various etching and delamination processes, Figure 1 a [3,4]. A MAX phase has the general formula of Mn+1AXn where the M is an early transition metal, the A is an element from group 13 and 14 of the periodic table, and the X represents a carbon or nitrogen [5]. During the etching process, done in a fluoride ion based solution, the element from group 13/14 is removed from the MAX structure causing the carbide layers to become terminated by OH−, O−, Cl− or F− groups which are subsequently called ‘surface groups or edge sites’ [5]. The resulting structure is known as a ‘MXene’ [5,6].MXenes are known to be highly conductive, hydrophilic, and tuneable which are all advantageous properties that could lead to improving pure metal oxides when combined in a catalyst layer for OER [6]. As metal oxide materials lack high conductivity, which adds to the overall losses in an electrolytic cell, the addition of MXene materials could provide a high conductive support network making the hybrid material into a superior OER catalyst. Additionally, the hydrophilic nature of the MXenes will allow for the full coverage of OH− ions on the surface of the catalyst from the electrolyte which should help in the formation of O2. Finally, being able to tune the various MXene materials could significantly improve the conductivity and the hydrophilic nature of the MXene, hence further improving the ability to evolve O2. However, to date MXenes are not known to contain active sites for the OER, as no MXenes with metals for promoting the OER (e.g. Ni, Ru, Ir, Co, Mn or Fe) have been successfully synthesised (however MXenes containing Mn and Fe have been theoretically reported) [7,8]. Interestingly, for the opposite water splitting reaction, the hydrogen evolution reaction (HER), MXene materials have been proven to be promising through computation calculations and experimentally methods [9–11].On the other hand, Transition Metal Oxides (TMOs) are an exciting group of materials, that possess various intriguing physical properties that can change depending on the oxidation state of the material and are known to be active OER catalysts, Figure 1b [12,13]. However, these TMO materials exhibit instability and dissolution during operation which renders these materials unsuitable for deployment in large-scale electrolyser devices [14].By combining inexpensive, active TMO catalysts with the conductive MXenes, most of the characteristics of the ‘super’ catalysts described previously, i.e. active site density due to the metal oxides and high conductivity due to the MXenes, in theory can be achieved [15]. The MXene materials may also provide a high surface area network for the TMO materials, similar to what has been attempted with carbon nanotubes (CNT) for OER composite materials, in order to improve electron transfer properties [16]. However, CNT based materials are known to contain metal impurities that enhance the OER (unaware to most) and corrodes under anodic OER potentials therefore are not the ideal composite materials with TMOs for the OER [17,18]. This high surface area network of a TMO/MXene composite will hopefully improve the operational stability of the catalyst layer due to improved mechanical properties.Finally, combining MXenes and the TMOs may even result in a lower loading of the TMOs, further lowering the catalyst costs. Hence, the combination of TMOs and MXenes into heterostructured layers or functionalising the MXenes with TMOs is a new and exciting avenue to be explored for the generation of highly active OER catalysts in PEM and AAEM electrolysers [19].To date only a hand full of papers have been published in this area, Figure 1c and Table 1 . All of these papers have the same overarching conclusion that the addition of MXenes to transition metal oxides improves the initial OER performance compared to the TMO or the MXene alone.For example, Lu et al. synthesised a hybrid MXene composite with Co3O4 decorated on Ti3C2Tx MXene flakes by a solvothermal reaction at 150 °C for 3 h [22]. The resulting Co3O4/Ti3C2Tx hybrid exhibited an OER overpotential of ∼300 mV at a current density of 10 mA cm−2 (the current benchmark used in literature when reporting the performance of OER materials) from linear sweep voltammetry measurements, Figure 2 a [22]. Under the same OER conditions, the authors reported that a Co3O4 only material reached the same current density at overpotentials of 390 mV, while the Ti3C2Tx can be deemed OER inactive as the current density at an overpotential of ∼400 mV is virtually zero [22]. Furthermore, in this study, Lu and co-workers investigated the effect of the ratio of metal oxide:MXene on the OER. The four ratios of metal oxide:MXene prepared were 1:0.1, 1:0.4, 1:1 and 1:10. The results showed that the lowest amount of MXene to metal oxide (i.e. 1: 0.1) exhibited the best OER results in terms of overpotential at a current density at 10 mA cm−2.Benchakar and co-workers have observed a similar phenomenon for a Co layered double hydroxide (LDH)/Ti3C2Tx material fabricated by a polyol and solvothermal process [23]. In this particular study, the Co LDH/Ti3C2Tx outperformed the unsupported Co LDH for the OER by 50 mV, Figure 2b [23]. The authors have also reported that the MXene structure can be preserved for oxidation during synthesis and the OER by the well distributed Co LDHs on the surface of the MXene. Interestingly, the authors also reported that this Co LDH/Ti3C2Tx hybrid synthesized by chemical routes outperformed a material which consisted of the Co LDH and the Ti3C2Tx mechanically mixed. This increase in performance of the chemically synthesized hybrid compared to the mechanically mixed catalyst was hypothesised to be due to the higher charge transfer resistance due to the close proximity of the Co LDHs and the Ti3C2Tx or the lower amount of active sites in the mechanically mixed material [23].Additionally, the integration of MXene materials into a composite with bimetallic TMOs has been shown to be advantageous for the OER. Yu et al. reported that the synthesis of an FeNi-LDH/Ti3C2Tx material, fabricated by the co-precipitation of Fe2+ and Ni2+ from metal salts with already exfoliated MXene flakes under reflux, which also outperformed its individual counterparts under OER conditions, Figure 2c. The authors attributed the superior OER performance of the composite material to the increase in the charge transfer properties from electrochemical impedance measurements, a shift in the Ni redox peaks to more anodic potential that may induce the OER earlier and an enhancement in the O binding strength of the FeNi LDH due to an electron extraction as a result of the coupling with the MXene [24].It is evident from literature that MXenes do significantly enhance the initial performance of TMO catalysts for the OER. However, MXene materials in a water-based solution are known to be unstable in air, Figure 3 a [28]. The edge sites/surface groups of the MXene materials will oxidise first (due to deoxygenated species) to produce TiO2 which will then lead to the whole flake becoming oxidized and hence decreasing the conductivity of the materials, Figure 3a. This can potentially be a huge problem for any electrochemical energy application including electrolysis.Unfortunately, the electrochemical instability of hybrid TMO/MXene materials have been already observed in the small amount of literature published in the area to date. For example, Lu et al. multi-cycled their Co3O4/Ti3C2Tx hybrid in the OER region for 2000 times and observed a significant decrease in activity over time, Figure 3b [22]. Furthermore, a FeNi-LDH/Ti3C2Tx material on Ni foam, synthesized by Yu and co-workers, also exhibited a decrease in activity over time during a chronopotentiometry test at 10 mA cm−2 for 60 h, Figure 3c.The instability of these materials at such low current densities presents a major drawback in the potential for MXenes to be incorporated as a component of an OER catalyst layer. If these hybrid materials are to be employed in large scale electrolysis devices, the materials must remain stable at high current densities of 1 A cm−2 and higher. In order to alleviate major instabilities in these hybrid materials, investigations using in-situ or operando measurements in conjunction with electrochemical techniques need to be carried out in order to determine the reasoning for the instability of the materials, which is likely to be related to the degradation of the MXene to TiO2 under the extreme oxidative conditions present during the OER. The controlled synthesis of hybrid materials that can hinder the instability of the MXene component by covering the edge sites with active materials for the OER is another avenue which can be explored. Benchakar and co-workers have reported that their polyol/solvothermal synthesis method did fabricate a TMO/MXene hybrid material that is OER stable by covering the MXene edge sites with Co LDHs, however no stability tests were conducted to determine the long-term stability, which is needed to learn about possible TMO/MXene instabilities if superior such composite materials are to be designed in the future [23].A second route which could be undertaken to improve the OER performance and stability of TMO/MXene hybrids is to explore the (continuously expanding) world of MXenes materials. In literature the only MXene which has been utilised to synthesise TMO/MXene materials is Ti3C2Tx. There are numerous MXene materials now available and all exhibit different chemical and physical properties [3,29,30]. These other MXenes may be more stable and/or active for the OER when compared to the most common MXene, Ti3C2Tx. For example, it is well known that the incorporation/presence of Fe based materials/impurities into metal oxides improves the parent oxide towards the OER [31]. Therefore if the theoretically proposed Fe2CT2 MXene materials could be synthesized [32], this may lead to an enhancement in the OER activity, while also attaining high conductivity and hydrophilicity of the combined TMO/MXene materials compared to when a non-Fe MXene (e.g. Ti3C2Tx) is present in the hybrid material.A third route which could be undertaken to improve the performance of the metal oxide/MXene hybrid is to further investigate what ratio of TMO:MXene is optimum for the OER. Lu and co-workers showed that a ratio of 1:0.1 metal TMO:MXene is the best ratio for the OER in their study. However, this 1:0.1 TMO:MXene material contained the lowest ratio of MXene. Therefore, further studies into lower amounts of MXene in the hybrid need to be undertaken to determine if even less MXene is favourable for the OER when combined with metal oxides.Furthermore, Gogotsi and co-workers have recently reported that MXene materials containing more than one metal can be fabricated with different stoichiometries and can all be tuned in respect to the chemical properties they exhibit [3]. If the chemical properties of the aforementioned mixed metal MXenes can be tuned, this opens up a huge space to investigate the suitability of these TMO/MXene hybrid materials for the OER.Due to the large volume of TMO/MXene combinations, Density Functional Theory (DFT) will also play a vital role in the screening of the most promising TMO/MXene OER catalysts, as it has done for the HER [11]. DFT calculations, such as free energy and surface Pourbaix diagrams, could be utilised to predict stable surface terminations under OER conditions which could help experimental synthesis of stable TMO/MXene OER catalysts.Finally, for these hybrid TMO/MXene catalysts to reach a stage of commercialisation, the TMO/MXene materials would need to outperform the current state-of-the-art materials in terms of activity and stability in actual electrolyser devices and not just in a conventional three-electrode cell used in OER studies throughout academia. The reason behind this, is that in a conventional three-electrode cell, it has been shown that OER catalysts do not behave the same as in an electrolyser device [21,33,34]. However, to reach a stage of testing TMO/MXene catalysts in an electrolyser device, first more fundamental issues must be tackled including the oxidation of MXenes under OER potentials, the optimum metal oxide to MXene ratio for OER and improving the overall activity of the hybrid catalysts by synthetic/in-situ characterisation feedback mechanisms.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.M.P.B. would like to acknowledge the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 884318 (TriCat4Energy). V.N acknowledges the support of SFI AMBER (12/RC/2278_P2) and the ERC CoG 3D2DPrint (681544).

MXenes are a class of 2D/layered materials which are highly conductive, hydrophilic, have a large electrochemical surface area and are easily processible into electrodes for energy applications. Since the discovery of MXenes over ten years ago, these materials have been mainly used in the preparation of electrodes for batteries and supercapacitors. However, due to their aforementioned properties, MXenes could potentially be utilised as a component in the catalyst layer for the Oxygen Evolution Reaction (OER). This opinion piece will discuss some of the recent literature in the area of hybrid catalysts consisting of various Transition Metal Oxides (TMOs) and MXenes for the OER. We will also discuss current drawbacks and future outlook in this new area of research.

The majority of energy today is produced through the combustion of fossil fuels such as coal and petroleum. However, energy production is accompanied by the emission of greenhouse gases, including methane and carbon dioxide, resulting in global warming and other international environmental issues. Hydrogen, which offers high energy density, has emerged as an alternative fuel [1]. Methane has come under the spotlight as a resource for clean fuel production on the basis of its high H/C ratio, making it advantageous in hydrogen production, as well as large availability in natural gas, shale gas, landfills, and byproduct gases.Despite its less significant greenhouse effect, methane has a global warming potential that is about 25 times that of CO2. As such, many research groups have attempted to reduce carbon by using methane in reforming or direct decomposition [2].Among the various methane reforming reactions, steam methane reforming (SRM) is popular around the world as it is an affordable solution that offers high hydrogen yield. However, this method produces byproduct gases such as CO and CO2, adding to the cost of processing, and again contributing to greenhouse gases [3,4].The above issue was addressed through the thermal decomposition of methane (TDM), which produces hydrogen and solid carbon without any emission of CO and CO2 [5,6]. The cost of producing 1000 Nm3/h of hydrogen is about USD 2167 to 3764 under TDM, and USD 2639 under SRM, which highlights the need for the former to secure cost competitiveness [7].Catalytic chemical vapor deposition (CCVD) is recognized as a low-cost approach to the TDM process. Methane is thermally decomposed at temperatures higher than 1000 ℃, but its decomposition temperature can be lowered to 600–850 ℃ with catalysts. In addition, carbon of relatively low grades, such as amorphous CB, is obtained using the TDM process, but crystalline carbon nanotube(CNT) can be derived by applying the CCVD method [8–10].Recently, CNTs have been extensively studied as a conductive agent for the cathodes of lithium ion batteries [11–13], and they have vast applications covering sensors [14] to fuel cells [15]. With the increase in demand for CNTs, there has been active research on related production processes, among which the CCVD method has drawn considerable attention [16].The catalysts used in the CCVD method can be largely divided into carbon-based and metal-based catalysts. Carbon-based catalysts are affordable and stable at high temperatures, and do not require separate refinement of carbon products. Some examples of such catalysts are active carbon [17] and carbon black [18].Commonly used metal-based catalysts are transition metals with high carbon solubility, such as Co [19,20], Ni [21], and Fe [22,23] and bimetallic catalysts composed of Mo, Mn, and Cu. According to past studies, the Co-Mo catalyst offers higher catalytic activity compared to other bimetallic catalysts. Gamal, Ahmed et al. [24] examined the catalytic activity of cobalt catalysts including different promoters such as Fe, Mo, Cu, and Ni. A higher methane conversion rate of 6.3 % was observed when Mo was added as a promoter and allowed to react for 0.5 h. Chai, Siang-Piao et al. [25], who studied the effects of adding Cu, Fe, Mo, and Ni to Co, reported a methane conversion rate of 6.3 % and a carbon yield of 281 % for Mo with a reaction time of 1.5 h.Metal oxide supports such as Al2O3 [26–29] SiO2 [30], ZrO2 [31], and MgO [32–35] are also used to enhance the structural stability of catalysts, thereby maintaining catalytic activity. MgO supports are more easily removed in acidic solutions, minimizing the damage to products caused by acidic exposure [36,37]. The electrical conductivity of CNT is another important factor to be considered when using them as a conductive agent for cathodes of lithium-ion batteries. The use of MgO supports is favored since electrical conductivity is negatively affected by the introduction of a functional group to CNT under acidic conditions during catalyst removal [38].Catalytic activity is also influenced by the surface properties of supports [39]. Supports having a larger specific surface area (SSA) tend to have better catalytic activity due to the higher dispersion of active sites, which translates to a larger reactive surface area. In methane catalytic decomposition (MCD) reactions, the SSA of supports is expected to cause differences in carbon properties and catalytic activity.For this reason, various methods of synthesis of high surface area MgO have been examined [40–42]. Most methods require processing of precursors in various stages, as well as post-processing using expensive or toxic solvents.Meanwhile, Bartley, Jonathan K., et al. [43] proposed a method of synthesizing high surface area MgO via the thermal decomposition of different precursors without requiring separate post-processing, but research on the use of thermochemical processes as catalytic supports is still insufficient.This study examined the MCD of Co-Mo/MgO catalysts and changes in properties of carbon products in relation to the surface properties of supports. The temperature of synthesis of MgO supports was varied to fabricate supports having different SSA, and Co-Mo/MgO catalysts were obtained through impregnation of Co and Mo in the same ratio. To assess the effects of changes in surface properties of MgO supports on MCD, this study analyzed and evaluated the fabricated catalysts, MCD reactions, and properties of carbon products.Magnesium carbonate (MgCO3, Sigma-Aldrich, Lot No. BCCC2599), as MgO precursor was oxidized amount of 7 g for 2 h in a muffle furnace at varying temperatures of 400 ℃, 500 ℃, and 600 ℃, thereby preparing MgO supports having different surface properties. After oxidation treatment, the obtained MgO supports were 3.8 g, 3.6 g, and 3.2 g depending on the 400 °C, 500 °C, and 600 °C temperature, respectively. Commercial MgO powder (Sigma-Aldrich, Lot No. MKCH6857), which has very little SSA, was used as a control. The MgO specimens were named 4MgO, 5MgO, and 6MgO depending on the temperature of synthesis, and commercial MgO as C-MgO.Cobalt (Ⅱ) nitrate hexahydrate (Co(NO3)2·6H2O, SAMCHUN, Lot No. 012,920) and ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O, Alfa Aesar, Lot No. 10,223,334) were used as a precursor of cobalt and molybdenum, respectively. First, 5.1 g and 1.9 g, corresponding to 25 wt% Co and 25 wt% Mo, was respectively dissolved in distilled water, and impregnated via a rotary evaporator in four types of supports: C-MgO, 4MgO, 5MgO, and 6MgO. The catalysts were dried at 80 ℃ for 12 h and calcined at 400 ℃ under atmospheric conditions for 4 h, resulting in CM/C-MgO, CM/4MgO, CM/5MgO, and CM/6MgO.To synthesize CNT through MCD, 0.2 g of catalysts was placed in a horizontal reactor, having an internal diameter of 6 cm and length of 120 cm, equipped with a quartz boat. The temperature was increased to 800 ℃ (10 ℃/min, Ar 100 cc/min), and the active metal in the oxidized state was reduced at a flow rate of H2:Ar (20 cc:80 cc) for 30 min. Next, Ar (100 cc/min) purging was performed for 40 min to create inert conditions. After performing MCD at 50 cc/min for 2 h using a gas mixture with a mole ratio of CH4:N2 = 95:5, the catalysts were cooled to room temperature under argon. The reactor set up used in this study is described in Fig. S1.The used catalysts were denoted as U-CM/C-MgO, U-CM/4MgO, U-CM/5MgO and U-CM/6MgO.The physical and chemical properties of supports, synthesized catalysts, and used catalysts were characterized through various analytical techniques.The surface properties of the four MgO supports, synthesized catalysts, and used catalysts were examined by conducting an N2-adsorption desorption (TRISTAR 3020) analysis at 77 K after heat treatment at 474 K over 8 h.The crystal structure of the synthesized catalysts was analyzed by X-ray diffraction (Rigaku Ultima Ⅳ), with Cu Kα (λ = 1.5418 Å) as the X-ray source, current of 40 mA, voltage of 40 Kw, and 2θ = 10–90°. crystallite size was calculated by the Scherrer equation using the full width at half maximum (FWHM) of XRD peaks.An Inductively Coupled Plasma Mass Spectrometer (Agilent ICP-MS 7700S) was used to analyze the amount of impregnated active metal in catalysts.The distribution of surface elements on the catalysts was analyzed by X-ray photoelectron spectroscopy (Axis Supra) with monochromatic Al-Kα ( h υ = 1486.6 e V ) as the X-ray source.To determine the amount of hydrogen consumed in the reduction of metal oxides within catalysts and the extent of reduction, the specimens were pre-processed at 200 ℃ under Ar 50 ml/min for 1 h and analyzed by H2-TPR (AUTOCHEM Ⅱ 2920) using 10 % H2/Ar gas.After MCD, the catalysts were heated to 900 ℃ at 5 ℃/min under air conditions, and the oxidation temperature of carbon products and decrease in nitrogen were measured with a Thermo Gravimetric Analyzer (Thermo plus EVO Ⅱ).To compare the crystallinity of carbon products after MCD, a Raman Spectroscopy (Nanophoton Ramanforce) analysis was performed. The wavelength was 532.04 nm and the slit width was 50 ㎛.SEM (Scanning Electron Microscope, TESCAN MIRA3 LMU) analysis was carried out at an accelerating voltage of 10.0 kV to examine the shape and size of catalysts and products.TEM (Transmission electron microscopy) images were recorded using a FEI Talos F200X (ThermoFisher Scientific, USA) operating at a voltage of 200 kV.The electrical conductivity characteristics of the samples subjected to MCD were investigated using a powder resistance meter (HPRM-FA2, HANtech Co. Ltd., Korea), while increasing the pressure from 400 to 2000kgf at intervals of 400kgf.Lastly, to analyze the catalytic activity and hydrogen selectivity over time, the gases produced were analyzed using a Gas Chromatography (YL6500 GC). N2, H2 and CH4 were analyzed with a thermal conductivity detector (TCD) equipped with a packed column (ShinCarbon ST 80/100, RESTEK). Fig. 1 shows the N2 adsorption-desorption results for the four types of supports: C-MgO, 4MgO, 5MgO, and 6MgO.As can be seen from the N2 adsorption-desorption isotherms (Fig. 1(a)), 4MgO and 5MgO fall under IUPAC isotherm type Ⅱ, while 6MgO is type III [44]. Types Ⅱ and III are characterized by hysteresis loops, which occur due to capillary condensation within mesopores [45]. A hysteresis loop, classified as H3 by IUPAC, arises from the adsorption of non-polar gases associated with slit-shaped particles [46]. On the other hand, C-MgO is composed of particles that are almost non-porous, and thus does not exhibit N2 adsorption or desorption. Also, developed pore structure was observed in case of the synthesized MgO in TEM images (see Fig. S2). Fig. 1(b) shows the pore size distribution of the MgO supports: 4MgO has a bimodal distribution, comprised mostly of pores of 3 to 4 nm, while 5MgO pore size was around 7 nm. And the pore size of 6MgO were distributed from 10 to 100 nm. As a result, it was confirmed that 4MgO, 5MgO had mesopore structure and 6MgO had mesopore with macropore structure. Lastly, C-MgO support had non-porous structure. And the pore diameter of each support is presented in the Fig. S3. Table 1 presents the SSA, pore volume of C-MgO, 4MgO, 5MgO, and 6MgO supports. The SSA was found to be 0.481 m2/g, 170 m2/g, 131 m2/g, and 50.4 m2/g for C-MgO, 4MgO, 5MgO, and 6MgO, respectively. The specimens arranged in increasing order of SSA are as follows: 4MgO > 5MgO > 6MgO > C-MgO.The differences in surface properties of the synthesized 4MgO, 5MgO, and 6MgO assumed to the amount of oxidation of functional groups, such as –H2O, –CO2 remaining in precursors according to the oxidation temperature during MgO synthesis. Meanwhile, Bartley, Jonathan K., et al. [43] showed TGA analysis under an air atmosphere of MgCO3, as MgO precursor and found that H2O was removed at 250–400 ℃ and CO2 was removed at 325–550 ℃. Also, the SSA of supports were decreased with increasing MgCO3 oxidation temperature while the crystallinity was increased. Fig. 2 presents the TGA and DTG analysis results of the used catalysts obtained under atmospheric conditions after MCD. The weight decrease in air of the U-CM/C-MgO, U-CM/4MgO, U-CM/5MgO, U-CM/6MgO catalysts were 43.76 %, 60.12 %, 58.43 %, and 48.67 %, respectively. This decrease resulted from the oxidation of carbon products following MCD. Each catalyst produces a different amount of carbon products because of the varying dispersion of active metals, impregnated in the four supports, contributing to different surface areas involved in reactions.The DTG results showed that used catalysts experienced peaks in a temperature range of 500 to 600 ℃ due to oxidation of deposited carbon. Amorphous carbon is generally oxidized at temperatures below 400 ℃ [47], and MWCNT (Multi-Walled Carbon Nanotube) in the range of 500 to 700 ℃ [48]. As such, we can deduce that the carbon product obtained from the four types of catalysts after MCD is MWCNT. This was verified through the SEM images in Fig. 4.In addition, the weight of catalyst after MCD reaction and weight loss of used catalyst by TGA analysis were described in the Table S1. Fig. 3 (a) shows the results of Raman spectroscopy of the used catalysts. D, G band peaks were observed for all used catalysts. The D band peak at around 1350 cm−1 is caused by disordered carbon, such as amorphous carbon or defective graphite sheets [49]. The TGA results (Fig. 2) verified that the D band is induced by the disordered structure of MWCNTs, rather than amorphous carbon. The G-band peak at 1580 cm−1 corresponds to CC stretching vibrations that are characteristic of graphite [50]. The intensity ratio (IG/ID) of G band and D band is used to compare the crystallinity of carbon materials, where a higher ratio indicates higher crystallinity [51]. The intensity ratios IG/ID of MWCNTs on used catalysts were 2.15 ± 0.04, 1.72 ± 0.05, 1.40 ± 0.06 and 1.41 ± 0.04 for U-CM/C-MgO, U-CM/4MgO, U-CM/5MgO, and U-CM/6MgO, respectively. Since the IG/ID ratios of MWCNT grown on the used catalyst were higher than 0.98 ± 0.04 of conventional MWCNT (Sigma-Aldrich, Lot No. MKCM1457), it is considered that the synthesized MWCNTs using Co-Mo/MgO catalysts have high crystallinity.Deposited carbon on the U-CM/C-MgO catalyst showed higher crystallinity than that of MWCNTs produced by the supported catalysts on synthesized MgO. The formation of Carbon Nano Sheets (CNS) is deduced to have influenced the IG/ID ratio [51]. The growth of CNS can be seen TEM image. (Refer to Fig. S4.).In addition, it has been reported that the IG/ID ratio is closely related to the MWCNT diameter [52]. Based on this, Fig. 3(b) shows the correlation between the IG/ID ratio and the diameter of MWCNT according to SSA of support. In the case of U-CM/C-MgO, it is difficult to correlate due to the formation of a small number of CNS, but the IG/ID ratio and CNT diameter trend were consistent with the remaining catalysts mainly produced by CNTs. This was also consistent with the trend of the mean diameter of CNTs measured in the SEM images in Fig. 4 (a–d). Fig. 4 presents SEM images revealing the shapes of carbon products of each used catalyst. A diameter distribution histogram was derived from a bar graph of 100 CNT diameter values. The average diameter was 48.3 ± 12.9 nm for (a) the U-CM/C-MgO catalyst, 20.5 ± 7.8 nm for (b) the U-CM/4MgO catalyst, 34.3 ± 13.9 nm for (c) the U-CM/5MgO catalyst, and 36.5 ± 8.2 nm for (d) the U-CM/6MgO. In addition, active metals and CNT diameter of U-CM/C-MgO and U-CM/4MgO were confirmed through TEM images. (Refer to Fig. S5.).The mechanism by which methane is decomposed in contact with the Co0 surface (Sco) is as follows; i) CH4 + 2SCo ↔ CH3SCo + HSCo, ii) CH3SCo + SCo ↔ CH2SCo + HSCo, iii) CH2SCo + SCo ↔ CHSCo + HSCo, iv) CHSCo + SCo ↔ CSCo + HSCo, v) CSCo ↔ CCo + S, vi) 2HSCo ↔ H2 + 2SCo CH4 + 2SCo ↔ CH3SCo + HSCo,CH3SCo + SCo ↔ CH2SCo + HSCo,CH2SCo + SCo ↔ CHSCo + HSCo,CHSCo + SCo ↔ CSCo + HSCo,CSCo ↔ CCo + S,2HSCo ↔ H2 + 2SCo So, the difference in diameter of MWCNT produced by each catalyst after methane decomposition is affected by dispersion rate of active metal due to the surface properties of MgO supports. Fig. S6 shows the results of XPS analysis in the reduced state of CM/C-MgO and CM/4MgO. Through this analysis, it was confirmed that Co0 was formed in the reduced catalyst.In addition, the electrical conductivity at 2000 kgf of before and after acid treatment of used catalysts having different surface properties of the support are showed at Table S2. Before acid treatment of each used catalyst, U-CM/C-MgO, U-CM/4MgO, U-CM/5MgO, U-CM/6MgO, respectively, showed electrical conductivity values of 25.89, 55.54, 51.82, and 39.57 S/cm. Also, after acid treatment, 30.19, 67.55, 49.19, and 47.61 S/cm. A decrease in MWCNT diameter shows high crystallinity. As diameter becomes thinner, crystallinity and electrical conductivity are higher. Also, all used catalyst was increased electrical conductivity after acid treatment. It can be seen that U-CM/4MgO, which has high electrical conductivity, has a relatively thinner MWCNT [54]. Super P (Imerys Graphite & Carbon), which is used as a conductive additive for a cathode material for a lithium ion battery, exhibited an electrical conductivity of 29.43 S/cm under the same conditions.Through this, it was confirmed that the carbon material produced in the MCD reaction has electrical conductivity that can be used as the conductive additive. In addition, it was confirmed that the electrical conductivity characteristics of the carbon derived from MCD increased with increasing SSA of the MgO support. Fig. 5 shows the methane conversion over time on stream obtained from online GC. The initial methane conversion at 4 min on-stream was 68.9 %, 85.7 %, 84.3 %, and 83.4 % for U-CM/C-MgO, U-CM/4MgO, U-CM/5MgO, and U-CM/6MgO, respectively. After 108 min, the methane conversion rate was 0 %, 21.4 %, 17.6 %, and 14.9 %. The U-CM/C-MgO catalyst became inactive most rapidly, while the catalytic activity of U-CM/4MgO, U-CM/5MgO, and U-CM/6MgO decreased more gradually over time (Fig. 5).MgO supports with a larger SSA and greater dispersion have a broader area of contact between methane and active metal, which allows for a higher conversion rate. A larger SSA facilitates metal dispersion, enhancing interaction with supports and suppressing sintering.The difference in gas selectivity was also examined. The initial hydrogen selectivity was 72.76 %, 87.92 %, 86.94 %, and 86.32 % for catalysts synthesized using C-MgO, 4MgO, 5MgO, and 6MgO supports, respectively. All four catalysts showed a gradual decrease with reaction time (refer to Fig. S7). (1) CH 4 → C + 2 H 2 Where, 1 mole of methane is decomposed to produce 1 mole of carbon and 2 moles of hydrogen. Therefore, the higher the methane conversion rate, the higher the carbon production (Fig. 2) and hydrogen selectivity. And the hydrogen selectivity was also reduced according to the deactivation of the catalyst.CM/C-MgO, CM/4MgO, CM/5MgO, and CM/6MgO catalysts were fabricated with supports having different surface properties, and the catalytic activity was compared in relation to MWCNT properties after MCD reactions. The MWCNT yield and catalytic activity increased in the order of SSA of supports, namely, 4MgO > 5MgO > 6MgO > C-MgO. The resulting MWCNT also had a thinner diameter. A comparison of crystallinity with commercial MWCNT based on IG/ID ratio showed that MWCNT produced from the four catalysts had better crystallinity. Fig. 6 shows the XRD analysis of the fresh Co-Mo/MgO catalysts. The major diffraction peaks of MgO (JCPDS no. 45-0946) and CoO (JCPDS no. 65-0902) were observed for all catalysts at 2θ = 36.9°, 42.8°, 62.3°, 74.7°, and 78.6° [53,54]. In addition, the diffraction peaks of MgMoO4 and CoMoO4 (JCPDS no. 72-2153, no. 21-868) were observed at 2θ = 26.3° [55,56]. The results verify that the same crystal structure had formed in the four catalysts. As can be seen from the N2 adsorption-desorption of each support (Table 1), the impregnation of catalysts with C-MgO had sharp peaks, whereas those impregnated with porous supports such as 4MgO, 5MgO, and 6MgO had broader peaks, indicating that the crystals were smaller [57]. This was compared with the cobalt oxide crystallite size calculated at 42.8° using the Scherrer equation (2) for the peaks exhibited by the four catalysts. Where L is crystallite size (nm), λ is the X-ray wavelength in nanometer (nm), β is full width at half maximum (FWHM) and K is a constant related to crystallite shape, normally taken as 0.9. The θ can be in degrees or radians, since the cosθ corresponds to the same number. (2) L = K λ / ( β c o s θ ) The cobalt oxide crystallite size of catalysts was 162.17 nm, 11.74 nm, 13.94 nm, and 14.38 nm for CM/C-MgO, CM/4MgO, CM/5MgO, and CM/6MgO, respectively. In the case of 4MgO, 5MgO, and 6MgO porous supports, the crystallite size was relatively smaller due to the impregnation of active metals in pores allowing stronger interactions, thus resulting in greater dispersion. The cobalt oxide crystallite size of CM/C-MgO, which has smallest SSA, was much bigger than that of catalyst supported on synthesized MgO due to the clustering by weak interaction with support. Fig. 7 shows the results of a H2-TPR analysis, conducted to examine interactions between supports and active metals, and the temperature of reduction of compounds within catalysts. First, the fresh catalysts showed four peaks at similar temperature ranges. The first peak in the range of 300 to 360 ℃ is due to the reduction of Co3+ of Co3O4 to Co2+. Peaks at 500 to 750 ℃ are due to the reduction of Co2+ of Co3O4 to Co0 and compounds such as CoMoO4 and MoO3 in the catalysts [55,58]. The peaks at temperatures higher than 850 ℃ were due to the reduction of the CoO-MgO solid solution and compounds such as MgMoO4, MgCo2O4, and MoO2 [55,59].Compared to the catalyst prepared with C-MgO, the catalysts using synthesized MgO saw the reduction temperature shift to a higher range. Because the impregnation of active metals on porous supports reinforced the interaction between the active metals and supports. In case of the catalysts using synthesized MgO, the reduction temperature was decreased with increasing SSA of MgO support. It can be deduced that the dispersion rate of active metals was increased with increasing the SSA of MgO support. The intensity and area of CM/C-MgO reduction peaks were higher than the catalysts using synthesized MgO because of bigger size of active metal crystal. The tendency of the dispersion rate of active metal according to the SSA of the support was consistent with the crystallite size calculated from XRD. Table 2 shows the proportion of metals existing on the surface of catalysts based on the surface/bulk ratio of active metals impregnated in the fresh catalysts. And this formula was calculated from the XPS and ICP data. The CM/C-MgO which is supported on non-porous MgO had the highest Surface/Bulk ratio of metals. And also, Surface/Bulk ratio of fresh catalysts was decreased with increasing SSA of MgO supports. It can be inferred that the decrease of the Surface/Bulk ratio is due to the increase of active metal dispersion into the pores by the increase of interaction with support. XPS and ICP data of each catalyst are refer to Table S3.Next, the surface properties were evaluated after impregnation of active metals and MCD reaction. C-MgO and 6MgO, which had smaller SSA, saw an increase in SSA after active metals were impregnated. 4MgO and 5MgO, with larger SSA of 170 m2/g and 131 m2/g respectively, underwent a decrease in SSA. The increase in SSA of C-MgO and 6MgO can be traced to the active metals being impregnated on the external surface of supports. In case of CM/4MgO and CM/5MgO, the SSA was decreased because of blockage by active metals impregnated into pores. Specifically, the pore volume of the 4MgO support decreased from 0.4 cm3/g to 0.32 cm3/g, and that of the 5MgO support from 0.5 cm3/g to 0.32 cm3/g. Also, used catalysts showed an increase in SSA, attributed to the greater adsorption and desorption of N2 following the growth of CNT after MCD. This result could be confirmed through the data that each used catalyst was oxidized at 700 ℃ to remove the deposited carbon. (Table 2). Fig. 8 Shows the MCD behavior of Co-Mo catalysts supported on non-porous MgO (C-MgO; Fig. 8(a)) and porous MgO (4MgO, 5MgO, 6MgO; Fig. 8(b)). In the case of non-porous MgO, the active metals are present in large crystallite sizes with low dispersion on the support. The lowly dispersed large Co-Mo particles induced low conversion due to the low contact rate with CH4 gas. And also, the carbon products on large crystals were formed CNS or thick fibrous type by the carbon deposition mechanism on metal catalysts. In the case of a catalyst with a low dispersion of active metals, CNS or thick CNT were produced due to the horizontal carbon growth. On the other hand, in the case of a highly dispersed catalyst, a large number of CNT having a thin diameter were generated due to the vertical growth of carbon [47,51].So, comparatively high dispersed Co-Mo catalyst induced high methane conversion and thin MWCNT because of high rate of CH4 contact and vertical carbon growth. In conclusion, a catalyst with highly dispersed active metal can induce high methane conversion and carbon productivity, and can produce high crystalline/conductivity MWCNTs with a thin diameter.In this study, the porous MgO with high porosity was prepared by simplified oxidation process of MgCO3 with varying temperature. The SSA and pore size distribution of the prepared MgO were controlled by the temperature of the oxidation process. The improved pore structure of MgO enhanced the dispersibility of the Co active metal, thereby improving methane conversion and MWCNT production. The dispersion of supported active metal was increased with increasing porosity of MgO due to the enhanced improved interaction. The CM/4MgO catalyst using 4MgO support prepared by 400 ℃ oxidation, which had the largest SSA of 170 m2/g, showed highest methane conversion of 85.7 % with 60.12 % of MWCNT production. The produced MWCNT on CM/4MgO had highest crystallinity and electrical conductivity with average diameter of 20.5 ± 7.8 nm. Consequently, the 4MgO support induced the highest dispersilbility of Co, and the methane decomposition behavior and MWCNT properties were affected by dispersibility of active metal.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: This work was supported by the Technology Innovation Program (20010853, Development of natural gas based carbon material on graphite structure for high crystalline conductivity) funded By the Ministry of Trade, Industry & Energy (MOTIE, Korea).This work was supported by the Technology Innovation Program (20010853, Development of natural gas based carbon material on graphite structure for high crystalline conductivity) funded By the Ministry of Trade, Industry & Energy (MOTIE, Korea).Supplementary data to this article can be found online at https://doi.org/10.1016/j.jiec.2022.05.008.The following are the Supplementary data to this article: Supplementary data 1

To investigate the effects of porous properties of supports on the methane decomposition of Co-Mo/MgO catalysts, supports having different porous properties were prepared by varying the synthesis temperature of MgO. Co-Mo/MgO catalysts were prepared using the impregnation method. The CM/4MgO catalyst using 4MgO support, which had the largest specific surface area of 170 m2/g, showed highest methane conversion of 85.7 % with 60.12 % of MWCNT production. The produced MWCNT on CM/4MgO had highest crystallinity and electrical conductivity with average diameter of 20.5 ± 7.8 nm. Overall, the improved pore structure of MgO prepared by the oxidation process enhanced the dispersibility of the Co active metal, thereby improving methane conversion and MWCNT production.

Carbon nanotubes (CNTs) have been known as the miracle material in the material engineering sectors ever since been discovered in 1991 [1]. CNTs have been utilised in many sectors such as toughening agents in ceramic and metals [2,3], wastewater treatment [4,5], micro- and nano-electronics [6,7], energy scavenging [8,9], and drug delivery carrier [10,11]. CNTs were first synthesised through the arc-discharge technique [1], while today, CNTs can be mass synthesised through different methods, i.e. laser ablation [12], hydrothermal [13], electrolysis [14–17], gas phase growth [18] and chemical vapour deposition (CVD) [19–26].CVD is the most practical synthesis option for researchers and manufacturers to mass-produce CNTs at a lower operation cost. The supplied thermal energy from the heating filament catalysed the carbon nucleation on the catalyst surface or carbon dissolution into the catalyst core in the CVD system. Parameters control in the CVD system, e.g. the gas flow rate, is critical and it determines the CNTs’ quality and quantity. Lin et al. optimized the concentration of Ni catalyst (10 w.t.%) and the methane flow rate (100 sccm) during the CNTs growth on the TiB2 substrate [27]. Mamat et al. also presented a deposition temperature study on the CNTs growth using silica-supported NiFe2O4 catalyst. They claimed the highest CNTs yield with the best quality requiring 700 °C with the flowing H2/C2H4 gas mixture at a 50:50 ratio [28].Furthermore, Wang et al. produce MWCNTs with symmetric walls through an optimized fluidised CVD system [29]. They purged the gasified polyethylene to react with the fluidized NiCl2 catalyst (metallic salt) in an inert gas flow environment (Ar/H2) at 750 °C and grew symmetrical MWCNTs with diameters ranging between 30 and 40 nm on the catalyst. The ID/IG ratio of the fabricated MWCNTs is 1.12. Karaeva et al. derived Ni from nickelocene (metallocene) and grew straight CNTs, with resistivity 6.7 × 10−3 Ω•m, on Ni catalyst using floating catalyst CVD process at 1150 °C [30]. Researchers have made extensive refinements and optimization in the CVD system to produce large quantity CNTs with higher quality. However, there is always room for parameter optimization for the CVD technique to understand the best mechanism for obtaining a low-cost and straightforward technique to achieve mass production of CNTs.This study investigated the effect of the precursor flowing path (namely distance between precursor and catalyst) and the catalyst size, subjected to the synthesised CNTs’ quality and quantity from the CVD system. NiO particles and ethanol were selected as the catalyst and carbon precursor, while argon and hydrogen gas as the carrier gas and reducing agent, respectively.25.0 g nickel nitrate hexahydrate (Ni(NO3)2•6H2O, >97.0% purity, Systerm) was stir-mixed with 250 mL ethanol (C2H5OH, 95.0% purity, Systerm) homogenously, in which the stirring speed was set at 1300 rpm. The mixture was then dried at 100 °C (72 h) and oxidised at 400 °C (4 h) in the air. The initial oxidation temperature for bulk Ni is around 600 °C; however, the Ni powder oxidised at a lower temperature (398 °C) [31]. The oxidised powder was milled with different milling duration (0, 4, and 7 h) using high-energy ball milling (HEBM), labelled as M1, M2, and M3 (see Table 1).To investigate the effect of precursor flowing path (D) on the CNTs growth, M1 was fixed in the centre of the tubular furnace to conduct CVD (see Fig. 1a). The stainless steel tube acted as the carbon precursor channel, with a fixed internal diameter of 4 mm. The precursor flowing path (D) was adjusted by replacing the steel tube with different lengths (see Fig. 1b). The sample was labelled as N1, N2 and N3 after the CVD process, subjected to the distance, where D = 2, 7 and 12 cm. Ar gas was flowed into the furnace for 15 min before heating the metal catalyst to ensure an inert environment during the deposition. The metal catalyst was heated from room temperature to 600 °C at the heating rate of 10 °C/min, with the Ar flowing at 50 sccm. At 600 °C, the flowing Ar gas and H2 gas carried the vaporised ethanol (≈150 °C) into the CVD system. The deposition took 30 min (with flowing H2 gas) and cooled down to room temperature in the Ar only environment. The median cooling rate of this furnace is 1.94 °C/min (see the detailed furnace setting in Table 2 ). To further determine the effect of catalyst size onto the CNTs growth, the CVD process was repeated with samples M2 and M3 at the D = 12 cm, which showed the highest carbon yield. The abbreviations of the samples were listed in Table 3 .The crystal phases and compositions of the calcined powder and the sample were analysed with an X-ray diffractometer (XRD, PW 3040/60 MPD X'pert High Score Pro PANalytical, Philips, Almelo, Netherlands). The field-emission scanning electron microscope determined the morphology and element distribution of the samples (FESEM, NOVA NANOSEM 230, FEI, Oregon, United States of America) coped with energy-dispersive X-ray spectroscopy (EDX, Max 20, Oxford Instruments, Oxfordshire, England). The cross-section of synthesised CNTs was viewed under the transmission electron microscopy (TEM, JEM-ARM200F, JEOL Ltd., Tokyo, Japan), with 200.0 kV accelerating voltage and vacuum chamber pressure less than 2.5 × 10−5 Pa. Raman spectroscopy (NRS 3300, JASCO Inc, Japan) was employed to determine the quality of the synthesised carbon structure. The temperature profile of the tube furnace was measured using Infrared Thermometer (IR-750, Amprobe, Washington, United States of America).The computational fluid dynamics (CFD) within the tube furnace was performed using COMSOL Multiphysics 5.3a. The simulation uses creeping flow model instead of turbulence model. The stationary CFD equations were expressed as follows (see Eq. (1) and Eq. (2)).Continuity Equation of fluid flow [32] (1) ρ ∇ · ( u ) = 0 where ρ and u are the density of gas and velocity vector.Momentum equation [33] (2) 0 = ∇ · [ − p l + μ ( ∇ u + ( ∇ u ) T ] + F + ρ g where p, l, T, μ, F and g are the gas pressure, length, gas temperature, dynamic viscosity, applied body force and gravitational acceleration. Fig. 2 shows the FESEM micrographs of the oxidised and milled NiO powder, together with the size distribution. The M1 sample showed NiO particles with average size 98.99 ± 42.79 nm. For sample M2, the particles most likely experienced agglomeration due to the generated heat during the milling. The kinetically generated thermal energy melted the NiO particles partially and caused them to fuse with neighbour particles. For sample M3, it showed that the 7-hour milling process caused damage and crack on the NiO particles and agglomeration. Fig. 3 showed the XRD spectra of the oxidised and milled powder. The calcined powder showed the bunsenite crystal structure (NiO, cubic R 3 ¯ m , ICSD code: 61318). The cubic structure from XRD phase identification agreed to the structure observed under FESEM. The chemical conversion from Ni(NO3)2 to NiO was shown in Eq. (3). (3) N i ( N O 3 ) 2 · 6 H 2 O + C 2 H 5 O H ⟶ Δ N i O + C 2 H 4 + 7 H 2 O + 2 N O 2 + O 2 From the XRD spectra, both samples shared an identical plane but different FWHM (see detailed in Table 4 ). The crystallite size (τ) of the sample was determined by Scherrer equation, as presented in Eq. (4) [34], where k is the shape factor (0.9), β is the full width half maxima in radian, λ is the X-ray wavelength (1.542 Å), and θ is the Bragg angle in degree. The M3’s XRD spectra showed broader FWHM as the crystals were milled, and the resulted crystallite size were 45.36 nm (M1) and 27.05 nm (M3) [35]. (4) τ = k λ β cos θ Fig. 4 displayed the XRD spectra of the samples N1, N2 and N3. Nickel (Ni, cubic fm3m, ICSD code: 646090) and graphite (C, hexagonal P 63 / mmc , ICSD code: 76767) was detected in the samples. It agreed to the reduction of NiO to Ni with the presence of H2 at high temperatures. The graphite (002) plane was barely detected at 2θ = 26 ° in the samples, and the FWHM values were narrowed down to 0.3819 °. Fig. 4 also showed the graphite grain in sample N1 achieving the largest crystallite size yet lower intensity than the Ni phase (rel. int. = 0.46%). The flowing hydrocarbon (CxHx) crystallised rapidly once deposited on the Ni particles (for D = 2 cm), regardless of the number of deposited carbon atoms or the optimised precursor flowing path. Fig. 5 displayed the FESEM and TEM micrographs of samples N1, N2 and N3, with the stated average diameter of MWCNTs. The TEM image revealed the size of Ni particles decreased after the H2 etching, which agreed to Sheng et al. findings [36]. Worm-like nanostructure was identified as multi-walled CNTs with hollow structures. The external and internal diameters of the CNTs (dext and dint) were measured and further calculated for the average number of walls (Nwall) through Eq. (5) [37], displayed in Table 5. Ever since the diameter of the NiO catalyst is constant, the Nwall of the MWCNTs is in the range from 38 to 47 layers. This finding agreed to Ali et al., where the dint of MWCNTs was about 5 – 7 nm at the growth temperature between 600 and 700 °C [38]. (5) N w a l l = 1 0.34 n m ( d e x t − d int 2 ) + 1 Fig. 6 showed the XRD spectra of the as-synthesised samples with different catalyst sizes. Compared to the sample N1 – N3, the relative intensity of graphite (002) plane in the samples N4 and N5 are higher (N4: 10.60% and N5: 24.36%). The intensity increment of the (002) plane indirectly indicated that the carbon nucleation and crystallization on the NiO catalyst had increased. However, the calculated τgraphite from XRD spectra is much lower than samples N1 – N3. It was attributed to the existence of the nano-sized amorphous carbon, forming a mixed state with the crystalline graphite phase. Fig. 7 showed the morphology, fast Fourier Transform (FFT) pattern, and the line profile of the samples N4 and N5. The FESEM micrograph showed that the diameter of the nanotubes is in the range of 82.29 nm. The FFT image (Fig. 7a(iii)) showed the Ni diffraction, agreed with the XRD spectra of the hydrogen reduced NiO. Ni has the high oxidation rate (KP = 2.52 × 10−13 cm2/s, at 425 °C), meaning that the Ni oxidised easily at high temperature [39,40]. The graphite had encapsulated the Ni particles and prevent the particles from oxidizing. Subash et al. presented a similar work on the carbon encapsulated metallic Li to reduce the rapid oxidation [41]. Also, the line profile (Fig. 7a(iv)), representing the selected region in the TEM image, showed the interplanar spacing is 0.353 nm which agreed to the graphite (002) plane. In Table 5, sample N4 achieved the highest number of the wall (116 carbon layers) due to the smallest size of Ni catalyst (338.11 nm). Sample N5 achieved the smallest tube diameter in the range of 28.33 nm, which may contribute to the NiO cracked structure. The NiO cracked structure (see Fig. 2 M3) provided a smaller nucleation site during the carbon deposition, and it limited the graphite from rapid crystal growth. In the TEM micrograph (Fig. 7b(ii)), a blurry region (iii) was found and reconstructed into an FFT pattern. The pattern showed the amorphous carbon overlapped the Ni catalyst, where the Ni catalyst appeared as a darker background in the TEM graph due to high density. The line profile (iv) also showed the spacing between the planes in the range 0.345 nm, which also agreed with the graphite plane. Table 6 showed the carbon yield (or quantities) of the as-synthesised sample (N1 to N5). The carbon yield was calculated through Eq. (6), where Mi and Mf are the catalyst mass and the final mass of samples. The flowing Ar and H2 gas carried the vaporised ethanol (or the carbon precursor) into the system. When the gases flowed into the quartz tube, the thermal energy supplied the gas with an upthrust force, yet the gravity slowed down the carbon precursor and limited the upthrust force. It was suggested that a flow path for the ethanol gas at 600 °C, i.e. an approximately 12 cm parabolic flying path at the current gas flow before it deposited on the Ni catalyst. (6) Y i e l d = M f − M i M i × 100 % The quartz tube's real-time temperature profile and CFD simulation have been performed to support the statement mentioned earlier on the precursor-flowing path. Fig. 8a showed the temperature profile along the quartz tube during the deposition. We segmented the 1-metre quartz tube into three regions: the region exposed to the outer environment (L0 – L1 and L13 – L14), the region with Al2O3 blocks (L1 – L3 and L11 – L13), and the heating region (L3 – L11). Position L4 – L6 were the carbon feedstock introduced into the CVD reactor respected to D = 12, 7, and 2 cm. L7 is the position of the NiO catalyst. From the graph, the temperature increased sharply in the heating region (from 243.7 to 587.1 °C) after the position L3 (Al2O3 thermal insulation blocks). The temperature of L4 – L6 (or D = 12 to 2 cm) were slightly lower than the centre of the heating zone (587.1 °C), resulting in 507.4 °C (L4), 543.2 °C (L5), and 565.0 °C (L6). Fig. 8b showed the CFD simulation of carrier gas and carbon precursor within the tube at 600 °C. The gas entrance was estimated to achieve the maximum velocity, which was 4.165 × 10−2 cm/s, similar to the outlet. However, the gas that flew along the quartz tube was slower than the entrance (average velocity = 2.3 × 10−3 cm/s) due to the relatively larger diameter of the stainless steel tube and the boundary effect [44]. As a result, the gas flew slower for D = 12 and 7 cm (2.8 × 10−3 cm/s) before reaching the catalyst. Hence, the Ar/H2-carried carbon precursor was expected to have sufficient time to deposit on the Ni catalyst. Fig. 9 showed the Raman spectra of the samples. Sample N5 achieved relatively better quality in terms of the intensity ratio between D-band and G-band (ID/IG = 1.069), where D-band and G-band of the carbon layers located at 1349 and 1601 cm−1, respectively. The defect on the graphitic layers can be caused by H2 etching, which increases the ID/IG ratio [42]. The crystallite size of the synthesised carbon structure was calculated via Eq. (7), where λ is the laser wavelength during the analysis (532.08 nm) [43]. (7) L a = ( 2.4 × 10 − 10 ) λ 4 ( I D I G ) − 1 Fig. 10 displayed the element distribution and the crystallite size of the samples. It showed that the carbon deposition for samples N3, N4 and N5 is higher than 90%, according to the carbon yield in Table 6. Furthermore, the ID/IG-derived crystalline size increased with the milling time. This phenomenon is due to the fractured surface of the Ni particles lowered the activation energy by introducing a larger surface area. By considering the balance of quality and quantity of the as-synthesised MWCNTs in this work, the optimized precursor path is 12 cm and the milling time for the NiO catalyst was 7 h.The growth of CNTs on the NiO particles by CVD were demonstrated with ethanol as the carbon precursor. We identified two factors affecting the CNTs' growth by performing several parameter changes during CVD: the precursor flowing path and catalyst size. The precursor-flowing path (D = 12 cm) determined the highest carbon yield with the fixed gas flow rate. We suggested the precursor flowing path is a function of furnace temperature and gas flow rate. Also, the size of the synthesised NiO fluctuated as the milling time increased. The 7-h milled NiO particles resulted in a more fractured site, and it provided a smaller nucleation site and larger surface area for carbon deposition. Smaller nucleation sites catalysed the crystallisation of the CNTs. In this study, the optimal deposition conditions for MWCNTs growth on NiO particles were as follows: 12 cm distance between catalyst and precursor and 7 h milled NiO. Through the optimization of the parameter, the carbon yield of as-synthesised MWCNTs reached >50% and the ID/IG ratio is 1.069. Thus, our findings are noteworthy because they provide more information to researchers working to improve fabrication procedures to mass-produce high-quality MWCNTs.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 study was supported by Malaysian Ministry of Education (MoE) through Fundamental Research Grant Scheme (FRGS/1/2018/STG07/ UPM/02/3) No. 5540132 and Nanotechnology Platform Program <Molecule and Material Synthesis (S-19-NI-0041)> of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

Carbon nanomaterials have been found to have promising performance in various applications. However, the complexity and high operation cost during the fabrication still limit the mass production. In this study, multi-walled carbon nanotubes (MWCNTs) were grown on nickel oxide (NiO) via chemical vapour deposition (CVD) with ethanol as the carbon precursor. The NiO catalyst was fabricated from a nickel nitrate – ethanol mixture. The particle size of NiO was altered through high-energy ball milling for 0, 4, and 7 h. The influence of precursor flowing path (D) and NiO catalyst size during the MWCNTs growth have been investigated. The Raman spectra showed that the crystallite size of MWCNTs (La) increased from 16.97 to 18.00 nm as the NiO milling time increased. Furthermore, NiO-catalysed MWCNTs at D = 12 cm achieved the highest carbon yield (80.54%), with an ID/IG ratio of 1.134. Also, SEM and TEM revealed that the larger size of NiO catalyst produced fewer layers of MWCNTs. These findings are significant to aid researchers and manufacturers in optimising the CVD process towards large-scale MWCNTs fabrication.

Data will be made available on request.Solid oxide fuel cells (SOFCs) are considered as one of the most efficient electrochemical energy-conversion devices due to their high energy efficiency, low pollutant emission and high flexibility to utilize various fuels such as hydrogen, syngas and hydrocarbons [1–3]. Readily available hydrocarbon fuels with a low cost and good security are considered as promising alternatives to conventional H2 fuel for fuel cells [4]. Furthermore, liquid renewable hydrocarbon fuels such as methanol has high volume energy densities (1.6 × 104 kJ m−3), which is beneficial to fuel storage and the mobile applications at ambient pressure and temperature. Moreover, unlike other heavier alcohols like ethanol with the CC bond in the molecular structures, the cleavage of CH bonds is much easier for methanol through methanol thermal decomposition or steam reforming of methanol [5–7]. However, the utilization of hydrocarbon fuels in SOFC is hindered by serious carbon deposition on anode catalyst and the sluggish anode kinetics [8].Ni-based cermet with a high electrical conductivity is the most widely used anode material in SOFC research. Besides external and internal reforming, which first transforms the hydrocarbon feed to syngas, much effort has been focused on enhancing the activity and the resistance to coking of the Ni-based anode materials for the direct electrochemical conversion of hydrocarbon fuels. Cu with a high coking resistance has been used to replace Ni fully or partially, which leads to a low catalytic activity [9]. Yoon and Manthiram [10] found that the incorporation of 1 atom% W to Ni brings in surface hydroxyl groups through the reaction with water vapor, which facilitates the oxidation of carbon deposited. The addition of BaO and NbOx has similar effects [11–13]. The alloying of Ni with other metals, such as Co [14,15], Mo [16] and Fe [17,18], improves the anode activity and suppresses carbon deposition. Among the possible alternative alloys, Ni-Sn is potentially an excellent candidate for the anode catalyst [19,20]. The computation models suggest that Ni-Sn is more carbon-tolerant than Ni [21]. However, Li et al. [22] found that the addition of Sn in Ni decreases the anode activity slightly while improving the coking resistance remarkably with CH4 as fuel. Notably, intermetallic compounds (IMCs) gradually attracted more attention because they exhibit unique catalytic properties. Cabot et al. [23] prepared NiSn NPs with controlled stoichiometry and achieved excellent performance towards methanol oxidation reaction, meanwhile, significantly improved stability compared to single metal nickel. However, the effect of NiSn IMCs, as active sites on the anode of methanol fueled SOFC, on the catalytic performance of the anode has not been reported.The recent research on cermet anodes mainly focuses on the metal catalysts. Nevertheless, the ceramic support providing oxygen ions also have remarkable influence on the activity and coking resistance of the cermet anodes with hydrocarbon fuels [24,25]. The activity of a Cu-based anode with doped ceria as the support is much higher than that with yttria stabilized zirconia (YSZ) as support when CH4 is used as the fuel, attributing to the high oxygen storage capacity (OSC) of ceria [9]. The activity and coking resistance of cermet anode with Ce0.8Sm0.2O1.9 (SDC) as support are enhanced when Sm is partially substituted with other rare earth metals such as La, Pr and Nd due to the improved activity of surface oxygen species [26,27]. Among the very few examples in the literature, Sn doped CeO2 was used as a support in CO oxidation [28,29]. Sn improves the reducibility and OSC of CeO2, bringing about a high catalytic activity. However, Sn doped SDC has not yet been tested as anode material in SOFC.In this work, Sn doped SDC (Ce0.8−xSnxSm0.2O2−δ, x  = 0–0.15, SSnDC) is examined as the support in Ni-based cermet anode. We found that excessive Sn exsolves partially from the oxide phase and forms intermetallic compounds with Ni after reduction. The effects of Sn in both metal and the oxide phases are investigated. The dual-modified Ni-SDC anode material with Sn doping shows enhanced performance and stability with methanol as the fuel.SDC, Ce0.75Sn0.05Sm0.2O2−δ (SSn5DC), Ce0.70Sn0.10Sm0.2O2−δ (SSn10DC) and Ce0.65Sn0.15Sm0.2O2−δ (SSn15DC) powders were synthesized via a hydrothermal procedure [30,31]. All of the chemicals (A.R. in purity) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Stoichiometric Ce(NO3)3·6H2O, SnCl4·5H2O and Sm(NO3)3·6H2O were dissolved in deionized water to form a solution with a total metal ion concentration of 0.15 mol L−1. Urea was subsequently added to the solution under constant stirring to reach a concentration of 1 mol L−1. Then the precursor solution was transferred into a hydrothermal reactor and kept at 140 °C for 5 h. The precipitate was washed with deionized water until no Cl- was detected with 0.1 mol L−1 AgNO3 solution. The powder obtained was dried at 100 °C for 12 h and finally calcined in air at 700 °C for 2 h.NiO-SSnxDC (x = 0, 5, 10 and 15) powders with a Ni loading amount of 10 wt% were prepared through an incipient wetness impregnation technique with a Ni(NO3)2·6H2O aqueous solution. Then the powders were dried at room temperature overnight and calcined subsequently at 700 °C in air for 2 h. The sample was reduced in pure H2 at 700 °C for 2 h for characterization and are marked as Ni-SSnxDC. For comparison, a sample, denoted as Sn@Ni-SDC, was prepared as follows. Sn, equivalent to 50 wt% Sn in Ni-SSn10DC, was added into the NiO-SDC powder via impregnation with an aqueous solution of SnCl4·5H2O.A D8 Focus diffractometer (Bruker Corp., Cu Kα radiation, 40 kV and 200 mA) X-ray was used to record the diffraction (XRD) patterns at a scanning rate of 1° min−1. The microstructure of the samples was observed using a transmission electron microscope (TEM, JEM-200F, JEOL Inc., Japan). High-angle annular dark-field scanning TEM (HAADF-STEM) and energy-dispersive X-ray (EDX) analysis were carried out to mapping the element distributions. The surface property of the samples was analyzed with an ESCALAB 250 Xi X-ray photoelectron spectrometer (XPS, K-Alpha+, Thermo Fisher Scientific) using Al-Kα (hν = 1486.6 eV) as the X-ray source. The spectra obtained were referenced to the C 1 s binding energy (284.8 eV).The activity of oxygen species in the samples was evaluated with CH3OH temperature-programmed surface reaction (CH3OH-TPSR) in a quartz tube reactor. 80 mg of powder was reduced in hydrogen at 700 °C for 2 h. After cooling to 150 °C, the sample was treated in pure O2 (30 mL min−1) for 30 min and was purged subsequently with Ar (30 mL min−1) at the same temperature for another 30 min to sweep the weakly absorbed oxygen. Then about 10 vol% gasified CH3OH was added in Ar by bubbling Ar through liquid CH3OH at room temperature. The reactor was heated from 150 to 800 °C at a heating rate of 5 °C min−1, and the oxidation of CH3OH was monitored with measuring the amount of CO2 produced with an online mass spectrometer (HPR20, Hiden Analytic Ltd.). The reduced anode powder was treated with gasified anhydrous methanol (150 mL min−1, STP) at 700 °C for 4 h. The carbon deposited was observed with a scanning electron microscope (SEM, S-4800, Hitachi, Japan), and its amount was measured with thermogravimetric analysis (TGA) in oxygen atmosphere using a thermal analyser (NETZSCH STA449, Germany).70 wt% SDC and 30 wt% (Li0.67Na0.33)2CO3 composite was used as the electrolyte material [32], which was uniaxially pressed into disk-shaped pellets at 500 MPa and then sintered at 700 °C for 2 h. The diameter and the thickness of the electrolyte layer were 13 mm and 500 μm, respectively. The anode powder was mixed with an organic binder (V006, Heraeus Ltd.) to make a slurry, which was screen-printed on both sides of the electrolyte pellets, and subsequently calcined at 700 °C for 2 h to form symmetric cells. The geometrical area of the electrode was 0.64 cm2, and Ag paste was used as the current collector. The electrode layers were reduced in H2 at 700 °C for 2 h, and then the electrochemical impedance spectra (EIS) of the symmetric cell were recorded with an electrochemical workstation (VERSASTAT 3, Ametek) under various H2 partial pressures (with Ar as the balance gas).Electrolyte-supported single cells were fabricated via a similar screen-printing process [18]. The cathode consisted of 70 wt% composite electrolyte and 30 wt% lithiated NiO, which was screen-printed on one side of the electrolyte pellet. The anode slurry was printed on the other side of the pellet, followed by calcined at 700 °C for 2 h to form single cells. The anode was reduced in H2 at 700 °C for 2 h, and then the performance of the cell was measured using the electrochemical workstation with dry hydrogen and gasified anhydrous methanol (100 mL min−1, STP) as the fuels and O2 (30 mL min−1, STP) as the oxidant.The I-V and I-P characteristics of the single cells with dry H2 as the fuel at 700 °C are presented in Fig. 1 a. The open circuit voltages (OCV) of the cells are between 1.00 and 1.12 V. The maximum power density (P max) of the cells with Ni-SDC, Ni-SSn5DC, Ni-SSn10DC and Ni-SSn15DC anodes are 0.53, 1.15, 1.93 and 1.62 W cm−2, respectively. When the temperature drops to 650, 600 and 550 °C, the P max of the cell with the Ni-SSn10DC anode decreases to 1.56, 0.97 and 0.61 W cm−2, respectively (Fig. S1a). When methanol is used as the fuel, the OCVs are in the range of 1.00–1.08 V (Fig. 1b), and the cell with the Ni-SSn10DC anode shows the highest P max of 2.11 W cm−2, much higher than that of the cell with the Ni-SDC anode (0.78 W cm−2). When the temperature drops to 650, 600 and 550 °C, the P max of the cell with the Ni-SSn10DC anode decreases to 1.74, 1.18 and 0.83 W cm−2, respectively (Fig. 1c), which is similar to the trend of P max obtained at different temperatures with dry H2 as the fuel. The corresponding impedance spectra at the open circuit condition are presented in Fig. S1b, c. The good performance of the cell in the intermediate temperature range is attributed to the high catalytic activity and the low activation energies (E a) of the Ni-SSn10DC anode. Fig. 1d shows the short-term stability of the single cells with methanol as the fuel at a constant current density of 0.2 A cm−2 at 700 °C. The cell with the Ni-SDC anode exhibits a steady output voltage in the first 4 h, which then drops gradually probably due to the coking on the anode. On the contrary, the output voltage of the cell with the Ni-SSn10DC anode is stable for more than 12 h attributed to the improvement of the resistance to carbon deposition of the anode with Sn doping.The EIS curves of the symmetric cells with H2 and methanol at both sides at 700 °C are presented in Fig. 1e and Fig. S1d. The data are fitted with the equivalent circuit R 0(R 1 Q 1)(R 2 Q 2), in which R 0, R 1 and R 2 are resistances, while Q 1 and Q 2 are constant phase elements. The ohmic resistances of the cells shown as the intercepts of the Nyquist curves on the real axis in high frequency region are similar. The anode polarization resistances (R p) reflected by the arcs show an order of Ni-SDC > Ni-SSn5DC > Ni-SSn15DC > Ni-SSn10DC.The Bode plots of the cells are shown in Fig. 1f, which can be roughly devided into a high frequency (HF, 104–101 Hz) region and a low frequency (LF, 101–10−2 Hz) region. The R p in the LF region decreases remarkably with the doping of Sn in the anode support. Ni-SDC, Ni-SSn5DC and Ni-SSn10DC anodes exhibit similar R p in the HF region, while that of the Ni-SSn15DC anode is slightly larger.The EIS results of the symmetric cells under various hydrogen partial pressures ( P H 2 ) are plotted in Fig. S1e, in which the ohmic resistances are deduced for a better comparison. The R p of all the anodes increase with the decrease of P H 2 , and linear relationships between LogR p and Log P H 2 are observed (Fig. 1g). The slope values for Ni-SSnxDC are 0.24 ∼ 0.31.The Arrhenius plots of the R p of the anodes are presented in Fig. 1h. The E a of the electrochemical oxidation of H2 on Ni-SDC and Ni-SSn5DC anodes are 0.61–0.65 eV. The Ni-SSn10DC anode shows the lowest E a of 0.32 eV attributed to the acceleration of the surface steps, while the E a of Ni-SSn15DC increases to 0.43 eV due to the suppression of the bulk conduction of O2–.The cross-sectional SEM images of the single cell with Ni-SSn10DC anode before test are shown in Fig. 2 a and Fig. S2. The thicknesses of the anode, the electrolyte and the cathode layers are about 30, 500 and 40 μm, respectively. The anode surface exhibits a fine and uniform porous microstructure (Fig. 2b).The hydrothermally synthesized SDC powder shows a face-centered cubic fluorite structure (JCPDS#075–0158, Fig. S3a). With the partial substitution of Sn for Ce, the fluorite structure is maintained. Meanwhile, the XRD peaks shift gradually to higher angles, indicating the contraction of SDC lattice with the incorporation of Sn since Sn4+ (0.81 Å) is smaller than Ce4+ (0.97 Å) [33,34]. Furthermore, the characteristic peaks of SnO2 (JCPDS#041–1445) is observed in the XRD pattern of SSn15DC, implying a limited solubility of Sn in SDC [35,36]. The XRD results without obvious SnO2 peaks probably are caused by the low amount of SnO2 phase and its uniform distribution on the support. SDC and Ni (JCPDS#087–0712) phases are found in Ni-SDC after reduction (Fig. 3 a). With the addition of Sn in the SDC phase, the Ni peaks are weakened remarkably, while Ni3Sn and Ni3Sn2 phases are observed, indicating the partial exsolution of Sn and the formation of intermetallic compounds during the reduction. Ni-SSn10DC has the strongest Ni3Sn peaks, while more Ni3Sn2 phase is found in Ni-SSn15DC. Ni3Sn and Ni3Sn2 phases are also formed in the Sn@Ni-SDC after reduction. Rietveld refinement of XRD data for the Ni-SSnxDC anode is carried out and the results are shown in Fig. 3b and Fig. S2c–f. Fig. S4 depicts the unit cell structure of SSnxDC, with a cubic fluorite structure, drawn using the VESTA software. All materials have a tetragonal structure (space group I4/mmm). The lattice parameters of Ni-SSn10DC are a = b = 8.655 Å and c = 5.500 Å. These parameters are slightly smaller than those of Ni-SDC, which were a = b = 8.752 Å and c = 5.536 Å. These refinement results demonstrates the lattice volume shrinkage after Sn doping.The TEM micrograph of the NiO-SSn10DC anode powder before reduction is shown in Fig. 3c, revealing the nanoparticles with the size of 10–20 nm. The lattice fringes corresponding to NiO, SDC and SnO2 are observed in the HRTEM image (Fig. 3d). After reduction, the nanoparticles of Ni, Ni3Sn and Ni3Sn2 are formed (Fig. 3e–g). The HAADF-STEM image and the corresponding EDX elemental mappings of the reduced Ni-SSn10DC anode is shown in Fig. 3h. Ni exists in all the nanoparticles, while Ce is found only in the SDC phase. On the contrary, Sn is distributed in the whole anode, further proving the partial exsolution of Sn from SDC phase and the formation of Ni-Sn intermetallic compounds. The TEM results are consistent with the XRD characterization. The structure of the Ni-SSnxDC anode material is illustrated in Fig. 3i.The surface chemical states of the anode samples were further investigated with a XPS technique. Fig. 4 shows the XPS survey spectra and the corresponding fitting curves of the Ni 2p and Sn 3d spectra after reduction. The deconvoluted peaks at about 852.3 and 854.0 eV are attributed to Ni0 and Ni2+, respectively (Fig. 4a) [37,38]. Ni2+ is probably formed from the quick reoxidation of metallic Ni on the surface of the reduced anodes before the XPS test [39,40]. Meanwhile, the binding energy of Ni0 decreases slightly with the addition of Sn, indicating the electron transfer from Sn to Ni in Ni3Sn and Ni3Sn2 [41,42]. The Sn 3d spectrum of the Ni-SSnxDC in Fig. 4b could be deconvoluted into Sn0, Sn2+ and Sn4+components, revealing the coexistence of Sn, Sn2+ and Sn4+ species on the surface of Ni-SSnxDC.The CH3OH-TPSR results of the SSnxDC composite oxide powders are presented in Fig. 5 a. The weak CO2 peaks at about 300 °C are due to the reaction between CH3OH and oxygen species adsorbed weakly on the surface of the samples, while the strong peaks in 400–700 °C correspond to the oxidation of CH3OH by lattice oxygens [26]. The oxidation of CH3OH on SDC reaches the highest rate at about 666 °C. With the increase of Sn content in SDC, the oxidation temperature of CH3OH decreases gradually, indicating that the partial substitution of Sn for Ce enhances the activity of lattice oxygen in SDC. The CH3OH-TPSR curves of the Ni-SSnxDC anode powders after reduction are shown in Fig. 5b. The oxidation temperature of CH3OH on Ni-SDC (546 °C) is much lower than that on SDC, and the impregnation of Sn (Sn@Ni-SDC) further decreases the oxidation temperature significantly, demonstrating that the formation of Ni-Sn intermetallic compounds improves the catalytic activity towards CH3OH oxidation. These results suggest that the dual-modified Ni-SDC with Sn are more favorable to the CH3OH oxidation process.The SEM images of the anode powders after carbon deposition are shown in Fig. 6 . The surface of Ni-SDC is closely packed by filamentous carbon (Fig. 6a), while negligible carbon is found on Ni-SSn10DC (Fig. 6b). The TGA curves of the samples in the oxygen atmosphere are presented in Fig. 6c. The weight loss in 400–650 °C reflects the oxidation of carbon deposits. The weight losses of Ni-SDC, Ni-SSn5DC, Ni-SSn10DC and Ni-SSn15DC are 27, 4.5, 2.4 and 1.8 wt%, respectively. The addition of Sn results in the enhancement of oxygen activity of the support and the formation of Ni3Sn and Ni3Sn2, both bring about the remarkable improvement of the resistance to carbon deposition [43–45].The performance of the single cell, with a P max of 2.11 W cm−2 at 700 °C, is a significant improvement on previously reported cells operated below 800 °C [18,22,42–45]. It also exhibits superior operational stability when compared with similarly structured SOFCs in the literature [15,46], [47]. Previously reported data can be categorized based on two different strategies: Ni-alloy anodes and oxide doping to the anode support. Our strategy involves coupling the NiSn intermetallic compounds and Sn doped SDC as a composite anode. This composite approach is responsible for the excellent performance. This approach provides an effective example for high-performance hydrocarbon-fueled SOFCs, especially under intermediate operating temperature conditions.To design more efficient Ni-based SOFC anodes, it is essential to identify the underlying rate-limiting step of the fuel conversion process. The CH3OH-TPSR results demonstrate that the formation of Ni-Sn intermetallic compounds improves the catalytic activity towards CH3OH oxidation. However, the reaction mechanism of methanol fuel at SOFC anode is complicated, which involves many reactions such as CH3OH decomposition, partial/full oxidative reforming, steam reforming, dry-reforming, and electro-oxidation reactions [5]. Therefore, it is diffiticult to clarify the anode processes with CH3OH as fuel. Notably, the performance of the single cells exhibit a consistent trend for various anodes when fueled with CH3OH and H2. Therefore, a symmetric cell was examined in detail and the anode reaction mechanism are discussed with using H2 as the fuel.The electrochemical oxidation of H2 at the anode of SOFC starts from the dissociative adsorption of H2 on the active sites (step (1)), followed by the surface diffusion of the adsorbed H to the reaction site, i.e., the three-phase boundary (TPB, step (2)). Meanwhile, oxygen ions also transfer to TPB in the anode through the ceramic phase (step (3)), which then reacts with the adsorbed H, forming H2O and releasing electrons (step (4)) [18]. (step 1) H 2,g ↔ 2H ad (step 2) H ad ↔ H TPB (step 3) O O,bulk x + V O, TPB · · ↔ O O,TPB x + V O, bulk · · (step 4) 2H TPB + O O,TPB x ↔ V O, TPB · · + H 2 O TPB + 2e - The slope of the fitting straight line should be close to −1 if step (1) is the rate-determining step (RDS) in the anode steps, which will change to −0.5 when the RDS is step (2). The R p will not change with P H 2 if step (3) or (4) is the RDS. The Ni-SDC anode exhibits the lowest slope of −0.31, and the R p is more prominent in the LF region of the EIS result (Fig. 1f), implying that the rate of the anode reaction could be codetermined by the surface diffusion of the adsorbed H (step (2) and the surface reaction between lattice oxygen and the adsorbed H (step (4).The electron cloud density of Ni increases with the addition of Sn (Fig. 5a), which may weaken the adsorbing strength of H and accelerate the surface diffusion of H [18], resulting in the decrease of the R p in the LF region (Fig. 1f) and the increase of the slope (Fig. 1g). Meanwhile, the substitution of Sn for Ce in SDC improves the activity of lattice oxygen (Fig. 5a) and thus accelerates step (4). On the other hand, the doping of Sn may suppress the conduction of O2– in SDC (step (3), leading to a higher R p in the HF region (Ni-SSn15DC, Fig. 1f) and the further increase of the slope (Fig. 1g).Based on above analysis, Sn doped Ni-Ce0.8Sm0.2O2−δ anode clearly displayed improved performance and stability over the Ni-based anode. This fact has two different explanations: (1) The presence of Sn atoms within the Ni structure, forming Ni3Sn and Ni3Sn2 phase, certainly modifies the electronic density of Ni states, thus affecting its chemistry, which accelerates the surface diffusion of H. (2) The substitution of Sn for Ce in SDC improves the activity of lattice oxygen, which react with the adsorbed H, forming H2O and releasing electrons. Both the effects of Sn doping in metal and in oxide phases improve the carbon resistance.SSnxDC is hydrothermally synthesized and investigated as a catalyst precursor of a Ni-based cermet anode. Sn exsolves partially from the ceramic phase after reduction, and Ni3Sn and Ni3Sn2 intermetallic compounds are formed, in which the electrons transfer from Sn to Ni, weakening the adsorbing strength and facilitating the diffusion of H species on the surface of the anode. Meanwhile, the activity of lattice oxygen in the SDC phase, as the support in the composite anode, is also improved with the doping of Sn.A cell with the Ni-SSn10DC anode yields record high P max values, e.g., 1.99 and 2.11 W cm−2 at 700 °C with H2 and methanol as fuels, respectively. This remarkable performance is superior to the ever-reported Ni-based anodes. Meanwhile, it has been confirmed that the dual modified Ni-SSnxDC anode are highly resistant to carbon deposition. Such a strategy of anode design may have the great potential of application in the anode material design for non-hydrogen fueled SOFCs, which encounters the great challenge of carbon deposition when the temperature lowers to around 500 °C, around which selective and stable metal catalyst may be feasible for long term stability.The surface diffusion of the adsorbed H and the reaction between lattice oxygen and the surface H species are the probable RDS of the anode process, both of which are accelerated with the incorporation of Sn in both the support and metal phases. Ni-SSn10DC anode shows the lowest Ea of 0.32 eV for the electrochemical oxidation of H2.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 Program of Introducing Talents to the University Disciplines under file number B06006 and the support of the Program for Changjiang Scholars and Innovative Research Teams in Universities under file number IRT 0641 are gratefully acknowledged. The work has been also supported by the Start-up Fund of Suzhou University of Science and Technology.Supplementary data to this article can be found online at https://doi.org/10.1016/j.cej.2022.140692.The following are the Supplementary data to this article: Supplementary data 1

A crucial challenge in the commercialization of Ni-based materials as the anode of solid oxide fuel cell is the fast voltage drop due to carbon deposition and structural degradation during cell operation. Herein, Sn-doped Ce0.8Sm0.2O2−δ (SDC) supported Sn-Ni alloy anode is rationally designed and prepared, via a simple and convenient dual-modification strategy. The substitution of Sn of Ce in the oxide phase enhances the mobility of lattice oxygen in SDC. Meanwhile, Sn exsolves partially from the oxide phase and forms Ni3Sn and Ni3Sn2 intermetallic compounds with Ni after reduction. The composite anode thus formed achieves unprecedent activity in the electrochemical oxidation of H2 and CH3OH. The maximum power densities of a cell supported by 500 μm-thick Ce0.8Sm0.2O2−δ-carbonate electrolyte layer with the Ni-Ce0.7Sn0.1Sm0.2O2−δ (Ni-SSn10DC) anode reach 1.99 and 2.11 W cm−2 at 700 °C, respectively for using H2 and methanol as fuels. The doping of Sn also remarkably enhances the coking resistance of the anode. This work opens a path on the design of high-performance SOFC anode.

Volatile organic compounds (VOCs) as major atmospheric pollutants are typically defined as compounds with boiling points lesser than 260 °C and greater than 50 °C at atmospheric pressure by WHO. These compounds usually originated from indoor and outdoor sources. The emissions from outdoor are mainly composed of various industries, such as coal chemistry, organic chemicals, petrochemicals, painting, solvent, dyeing, and so on. And most indoor come from organic solvents for interior decoration materials, adhesives, cosmetics, cleaning agents, household products, etc. It has been widely recognized that VOCs are highly harmful to human health due to their toxic, carcinogenic, mutagenic and teratogenic effect. Besides, the VOCs are also highly contributed to the formation of ozone, photochemical smog and secondary aerosols, which are destructive to the environment. Thus, the restriction of VOCs emission has become one of the most important tasks for the protection of human health and environment, since the rapid urbanization and industrialization recently lead to the growth of VOCs pollutions. It is of great significance and urgency to employing efficient technology to reduce VOCs emissions.Currently, the existing manifold technologies can be mainly divided into two categories: adsorption (condensation, adsorption, absorption and membrane separation) and destruction (incineration, catalytic oxidation, photocatalytic degradation, and plasma technology). There are plenty of limitations to adsorption techniques. For example, the condensation and absorption efficiencies are mainly restrained by the concentration of VOCs, as well as the adsorption technology is restricted by the limitation of adsorption adsorbent and the potential risk of secondary contamination. Owing to the advantage of simple operation, the membrane separation exhibited good potential, but it was still limited by its expensive cost and maintenance charges. Adversely, the destruction technologies have wider applicability. Taking into removal efficiency, energy-saving and environment-friendly, catalytic oxidation seems to be an optimal strategy among these technologies because the VOCs can be fully degraded into CO2 and H2O at low temperatures. Therefore, the fabrication of suitable catalysts for VOCs elimination is crucial, but still a big challenge.Recently, numerous investigations have been considered on the development of catalysts for VOCs oxidation and these catalysts can be divided into noble-metal catalysts and transition-metal oxides. In general, the noble metal catalyst is an optimal choice for VOCs destruction due to its superior activity, but the nature of scarcity and poor stability still limit its further applications. On the other hand, owing to the abundant reserves and superior stability, more and more transition-metal oxides (including oxides of Co, Mn, Fe, Cu, Ni) are investigated to replace noble metal catalysts for VOCs purification. Although the catalytic activity of transition-metal oxides is not as good as noble metal catalysts so far, it has drawn increasing attention and deemed to be dominant in the future. Among these transitional metals, cobalt-based (Co-based) oxides usually exhibit promising performance in air purification reactions due to their excellent redox property, high activity and stability, which is one of the most promising candidates for VOCs oxidation.To date, lots of studies have been reported to optimize the catalytic activity of Co-based catalysts, and breakthroughs have been made for catalytic removal of VOCs at low-temperature (as low as room temperature for some VOC). Thus, based on recent researches, we prefer to give a comprehensive review to summarize Co-based oxides with various compositions for VOCs removal to offer a logical and systematic picture of this area. To be specific, single cobalt oxides involving different morphologies, crystal phase, structures and valence; cobalt-based compounds such as binary composites, multiple composites; common and unusual strategies to improve catalytic performance including acid treatment and doping strategy, have been coherently reviewed. Subsequently, the detailed mechanism of relevant catalysts is also summarized. Finally, considering the enormous challenges and opportunities in this field, we make some perspectives for future development directions for VOCs oxidation. We believe that this review will be instructive to establish a reference for catalyst design for the removal of VOCs.Cobalt oxides (CoOx) are one of the most active low-cost transition metal oxides, which were widely applied in the field of air purification. The superior activity of CoOx can be attributed to the oxidation states, various crystal phases, weak CoO bond strength, numerous structures and morphologies. Besides, the physicochemical property of CoOx can be also modified by the engineering of crystal facets, resulting in different catalytic performances. A brief summarize of the effect of these factors on catalytic performance was displayed in Table 1 .Owing to the multivalent oxidation state of cobalt species (Co2+, Co3+), the CoOx can be divided into various types, such as CoO, Co2O3, Co3O4. Simultaneously, the catalytic performance of VOCs oxidation over CoOx is mainly influenced by the adsorption, activation and breaking the CH bond, which is strongly related to the oxidation states of cobalt ions. For example, Zhao and Ye [10] reported Co3O4 nanosheets with rich Co3+sites exhibit excellent catalytic performance for toluene oxidation, which is even comparable to the state-of-the-art noble metal catalyst. And the enhanced catalytic activity can be assigned to the activation of CH bond by Co3+ sites. Similarly, Zhong and Ye [1] prepared Co3O4 catalyst with hierarchical morphology via alkali etching technology and explore its catalytic activity for toluene combustion, which also confirms the high low-temperature activity and selectivity are contributed by Co3+species. On the contrary, other researchers have found a good relationship between the catalytic performance and concentration of surface Co2+species during VOCs oxidation (propane, o-xylene, benzene) [11–13]. Thus, it seems that the influence of the valence state of different CoOx on catalytic performance is highly related to the type of VOCs. Taking toluene as an example, Jiang et al. [2] have studied the catalytic activity of Co2+and Co3+ for benzene oxidation by using the metal ion substitution method, as shown in Fig. 1 . The octahedrally coordinated Co3+ and tetrahedrally coordinated Co2+ sites were replaced with inactive ion species, such as Zn2+, Al3+, and their results indicated that the Co3+ is beneficial for breaking the benzene ring, leading to a superior catalytic activity. At the same time, partial Co2+ species located at octahedral sites were more easily oxidized to Co3+ species, which also contributed to catalytic performance.As reported, the different catalytic behavior was strongly associated with the activation of oxygen species, and that was originated from the formation of crystalline phases [12,14,15]. For instance, Li's group [3] used the MOF-templated approach to prepare M-Co3O4 catalysts with different crystallinity and explored their catalytic performance in toluene oxidation. It has been found that the calcination temperature was the main influence factor in the formation of structure and peculiarity, as well as the M-Co3O4–350 catalyst which calcined at 350 °C possess smaller nanosized crystal size and superior activity (T90 of toluene conversion is about 239 °C, 1000 ppm toluene with GHSV = 20,000 mL g−1 h−1). Such outstanding performance can be attributed to the smaller nanosized crystals formed, which raises the concentration of defect sites and active oxygen species. Similar results were also revealed by other researchers, the poor crystallinity offers a great number of structural defects and oxygen vacancies, thereby promoting the replenishment of oxygen species and the deep oxidation of VOCs [16–18]. Besides, the phase transformation of CoOx catalyst also plays a vital role in VOCs purification [19]. As displayed in Fig. 2 a-b, Jiang et al. [4] synthesized CoOCo3O4 mixed-phase catalyst by using citric acid complexation and C2 C5 diols, the relevant characteristic demonstrated the formation of mixed-phase could effectively reduce the strength of CoO bond, which elevated the mobility of oxygen species, resulting in best propane conversion activity (T90 =227 °C, 0.3 vol% propane with GHSV = 30,000 mL g−1 h−1). Such improvement by mixed phased construction could strengthen the interaction between different CoOx phases and bring better catalytic performance for VOCs abatement. However, the comprehensive building of the mixed-phase CoOx catalyst based on metal−oxide interaction was, to our knowledge, still pretty rare.The selective synthesis of Co3O4 with dominating crystal facets is of great significance for catalytic reactions [20–23], especially for air purification and practical applications [24,25]. In most cases, the high-energy facets with expected properties were buried inside the catalyst during rapid growth, which accompanied by the exposure of stable facets on the external surface, resulting in dissatisfactory catalytic performance [26]. Thus, exploring the exposure of reactive crystal facets seems to be a promising strategy to affiliate more active species for catalytic reactions. For instance, Liang et al. [5] developed a strategy for controllable construction of CoOx catalyst with numerous morphologies and specific crystal facets for propane combustion. The result demonstrated that the book-shaped Co3O4-B sample dominated with (110) facet, which endows favorable active oxygen species and good low-temperature reducibility, thus promoting the oxidation of propane (T90 = 278 °C, 0.2 vol% propane, GHSV = 120,000 mL g−1 h−1). Similarly, the catalytic activity of Co3O4 materials with different exposed crystal facets ((110), (100), (111)) was compared by He and coworkers [6], and the corresponding results also indicated the (110) facet with high-energy was favorable for the formation of oxygen vacancies and low coordination Co atoms, eventually induce the enhancement of propane oxidation (2500 ppm propane was fully converted into CO2 at 205 °C under WHSV = 30,000 mL g−1 h−1), as depicted in Fig. 2c. Obviously, the facet engineering of (110) plane appears to be an effective technical approach for VOCs removal.As a matter of fact, even for catalysts with the same components, the changing in microstructure always results in differences in the catalytic activity, which was caused by the variations in the exposure of active sites, the concentration of active species, low-temperature reducibility, and so on. Besides, the pore structure also plays an important role in VOCs oxidation, just as the 3D CoOx usually shows better catalytic performance than 1D catalyst due to its large specific surface area and rich porous structures, thereby promoting the mass transfer of reactant and providing more contact opportunities through complex channels. In an early study, Ye et al. [7] have prepared 1D-Co3O4 nanoneedle, 2D-Co3O4 nanoplate and 3D-Co3O4 nanoflower by using the template-free hydrothermal method. As illustrated in Fig. 2d, compared with 1D-Co3O4, the 3D nanoflower displayed much better catalytic activity, where T90 = 238 °C (WHSV = 48,000 mL g−1 h−1). A detailed investigation has revealed the large specific surface area with abundant active Co3+ sites and oxygen species were responsible for the improved oxidation ability. In addition to the three-dimensional transformation, the subtle structural variations also significantly affect the performance of the catalyst. Recently, Ren and Ye [27] fabricated 4 kinds of 3D hierarchical Co3O4 and explore their catalytic performance for toluene oxidation, where the sheet-stacked fan-shaped Co3O4 exhibits high efficiency due to its large specific surface area, highly defective structure and rich high valence Co species. Meanwhile, the CoOx catalyst with an orderly structure demonstrated great potential for VOCs abatement. 3DOM (Three-dimensionally ordered microporous) Co3O4 with surface area as high as 22.4 m2 g− 1 was prepared by PMMA template methods [28]. Compared with commercial 3D cobalt oxides, the 3DOM Co3O4 possesses rich adsorbed oxygen species, good low-temperature reducibility, leading to specific catalytic properties.The destruction of VOCs over cobalt oxides is dependent on the redox cycle of Co ions, which is likely to be influenced by its morphology (such as rods, tubes, spheres, sheets, flowers, cage, etc.), affecting the activity in total oxidation reactions [18,29–32]. The meso‑Co3O4 catalyst with urchin-like (U-80), shale-like (S-160) and mixture (US-120) morphologies were obtained and employed in toluene oxidation in our group [8], as displayed in Fig. 3 . It has been found that the morphologies can be tuned from urchin-like to shale-like by simply adjusting the hydrothermal temperature from 80 to 160 °C, and the S-160 sample possessed the best catalytic performance, where achieved the T50 of toluene conversion at 234 °C (toluene = 500 ppm GHSV= 60,000 mL g −1 h−1). And detailed investigation revealed the enhanced catalytic performance could be assigned as the rich active oxygen species, good redox property and dominated (110) planes, and these factors are closely related to its specific morphology. Apart from catalysts with common morphologies prepared by simple methods, there are plenty of CoOx with complex morphologies that have been obtained in recent years and possessed superior catalytic performance in VOCs abatement [33,34]. For instance, Lin et al. [9] reported the Co3O4 catalyst with nanorod interweaved lamellose structure by using MOF as a template, which exhibited better catalytic performance (toluene conversion T90 = 188 °C, under the conditions: toluene = 3000 ppm, WHSV = 30,000 mL g−1 h−1) than Co3O4 nanofiber and nanosheet. And such superior activity originated from the higher concentration of Co3+/Co2+ redox couples and abundant defects.As mentioned, it seems that the influence of the different designing strategies on catalytic performance is highly related to the type of VOCs. Thus, to compare the influence of different factors and consequently provide guidelines for the fabrication of highly effective catalysts, a preliminary analysis was conducted by using toluene as an illustration. As shown in Fig. 4 , several single cobalt oxides as mentioned before were selected and their catalytic activities were normalized as toluene conversion rates. It can be seen that all these single cobalt oxides follow the chemical formula of Co3O4, and this implies that the crystalline phase of Co3O4 favors the catalytic degradation of VOCs. Compared to bulk Co3O4 (M-Co3O4), the Co3O4 with well-designed 3D structure exhibited higher catalytic activity, which indicates the construction of 3D structure is beneficial for the transportation of VOCs molecules and the exposure of active sites, resulting in better catalytic performance. Besides, the Co3O4 with rich Co3+ species displayed superior catalytic activity and lower T90 temperature than bulk M-Co3O4, which also implies the exposure of Co sites with high valence states is of great help to prepare effective co-based catalysts. Besides, the modification of morphologies also plays a crucial role in the redox cycle of Co3+/Co2+, thereby promoting the catalytic performance, which can be certified by the excellent toluene conversion rates and lowest T90 temperature. Therefore, it can be concluded that designing Co3O4 catalyst with specific morphologies, three-dimensional structure and abundant cobalt sites with high valence states is promising in the field of VOCs elimination. In addition, all these works highlight the influence of oxygen mobility, oxygen vacancy density and the concentration of oxygen species, indicating modification of oxygen species is also important.Perovskite oxides (ABO3), as a promising catalyst for heterogeneous catalysis, contains the merit of relatively low price, excellent redox performance, high oxygen mobility, and good thermal stability [39]. Due to the modification of preparation conditions, the A and B metallic cations could exist as different valence states, leading to the redistribution of redox cycles, resulting in the alteration of the redox properties and catalytic activity [40]. Among these perovskites, the cobalt perovskite (ACoO3) is widely applied in air purification due to their nonstoichiometric composition and multivalent nature of cobalt species [41,42], and significant efforts have been made in current science to further improve the catalytic performance, as displayed in Table 2 . Representatively, the surface etching seems to be an efficient strategy to enhance the catalytic behavior of ACoO3 by creating more oxygen vacancies and providing more active sites. For example, Li et al. [35] reported well-designed LaCoO3 for toluene oxidation by employing the citrate sol-gel method and acid etching strategy. Compared to bulk LaCoO3 (LCO-0) catalyst (T90 = 263 °C), the catalytic activity of LaCoO3 treated with acetic acid (LCO-1) was significantly improved, where the T90 is approximately 223 °C under a WHSV of 60,000 mL g−1 h−1. The acetic acid etching was favorable to generate more small nanoparticles with large surface area and improve the Oads proportion, resulting in better catalytic performance. Simultaneously, the precisely controlled acid treatment also remains the structure of perovskite phase, allowing superior stability even after thermal treatment at 500 °C. Similarly, Chen and coworkers [43] prepared the Co-enriched LaCoO3 oxides with abundant surface defects by using selective acid etching. This approach is based on the La ions located at A-site being preferentially dissolved than cobalt ions located at B-site, which induces the re-dispersion of Co species on the surface of ACoO3. And such unique structure facilitates the electron transfer in the Co3+/Co2+ redox cycles and accelerate the activation of oxygen molecules, thus achieving optimum catalytic property (T90 = 206 °C under the condition of 1000 ppm toluene with GHSV = 15,000 mL g−1 h−1) and durability (remains stable over 10 h under the condition of 5 vol% water).Besides, the highly stable nature of ACoO3 also makes the appearance of structural defects possible via cation substitution, which produces more oxygen vacancies to enhance the catalytic activity in VOCs oxidation [39]. Thus caused the vast majority of works are focused on metal ion substitution over A or B site of perovskite to yield more defective structures. For instance, Liang et al. [44] had conducted A site substitutions over LaCoO3 catalyst by using Ag cations, which effectively raised the proportion of surface oxygen and low-temperature reducibility, hence promoting the catalytic performance. Similar conclusions were also found by Weng's group [45], in which the oxygen mobility and the redox properties of perovskite can be affected by the introduction of heteroatoms. The experimental results demonstrate the introduction of Ca2+and Mg2+ induces abundant oxygen vacancies and generates more Co cations with high valence states, respectively. Such synergistic effect introduced by dual-site substitution shows a relatively positive effect on toluene oxidation. Similarly, the combination of different modifications also exhibits unexpected catalytic properties. Wei et al. [46] fabricated a series of LaxSr1-xCoO3-δ catalysts via A site substitution together with acid treatment, as displayed in Fig. 5 . The investigation found that the combination of these two methods was not only conducive to generating more active oxygen species, but also favorable to eliminating the side effect on the catalytic performance that brought by the generation of SrO, which have resulted in catalytic activity improvements. As such, Liang et al. [36] employed two modified methods (Ca substitution and citric acid etching) to further improve the La-Co perovskites for toluene oxidation. The results show that the T90 value of Ca-substituted LaCoO3 and acid-etched was 220 °C and 215 °C, respectively, which are substantially lower than that bulk LaCoO3 (T90 = 305 °C). Such excellent catalytic behavior can be further improved by the combination of these two strategies, resulting in lower T90 value (202 °C). Corresponding characterization reveals that these modifications provide higher specific surface area, fluffy morphology, smaller crystallite size, more oxygen vacancies, less basic sites, thereby displaying the highest catalytic activity. All these studies have determined that the synergistic effect of different strategies is beneficial to design promising catalyst for practical application.In general, spinel-type catalyst displays better stability than single metal oxides and mixed metal oxides, thereby attracted more and more attention in the field of catalytic reactions. The typical AB2O4 spinel contains the A and B metals located in the tetrahedral and octahedral positions, respectively, and its catalytic performance can be optimized via the efficient charge transfer between adjacent cations (A and B) [38,75], thus the selection of appropriate A, B cations was really mattered. Among various spinel catalysts, the Co-based spinel (CoCo2O4) is usually regarded as the most functional material for VOCs abatement, which attracted extensive attention because of its high intrinsic activity [76]. Recent investigation reveals that to further improve the catalytic activity, the spinel with larger surface area, plenty of defects and abundant surface-active species was urgently needed, which is especially applicable to govern the spinel with Co cations as B sites [75,77]. For instance, Wang et al. [47] reported the solvothermal alcoholysis approach for the preparation of spinel MCo2O4 (M = Ni, Co, Cu) oxides with hollow mesoporous spherical structures towards acetone oxidation. The experimental results demonstrated that CuCo2O4 exhibits outstanding catalytic performance owing to the enriched surface Co3+ cations and abundant oxygen species, which is originated from the cation-substitution effect of Cu. While Han and Wang [48] also synthesized the spinel-type NiCo2O4 nanosheet by using hydrothermal method and discovered that E-NiCo2O4 (taking ethanol as solvent) shows satisfying catalytic performance (T99 =256 °C under the condition of SV = 10,000 mL h − 1 with 2538 ppm toluene) because of its unique mesoporous structure and specific crystal plane, enlarging the Co3+ active sites located at the octahedral position. Jiang et al. [37] proposed a facile time-saving method to design CuCo spinel catalyst by treatment of oxalic acid with different addition amounts and evaluated their catalytic activity of toluene elimination. The characterizations indicated that the appropriate addition of oxalic acid plays a crucial role in the enlargement of SBET and pore-creating via its thermal decomposition into CO2 and H2O, and it is also contributed to exposure of Co3+ cations and mobility of oxygen species by dissolving Cu2+ into Co3O4 lattice. According to the above investigations, it can be concluded that the intrinsic activity of Co3O4 spinel can be further elevated by A-site substitution method, as well as other surface embellish strategies.Besides, the fabrication of CoA2O4 spinel catalyst in which Co cations are located at A position has also attracted extensive attention. Li et al. [78] introduced Co3-xMnxO4 (x = 0.75, 1.0, 1.5) spinel with different Co proportion by using a controlled template-free route for toluene abatement, in which the porous flower structure provided an accessible approach for the reactant to get access to active sites. The Co2.25Mn0.75O4 catalyst with rich Co composition possessed the best catalytic performance (T100 = 239 °C, under the condition of 1000 ppm toluene with the total flow rate of 33.4 mL min−1), and such excellent property was raised from the integrated effects of rich surface oxygen vacancies, large surface area and porous structures via the formation of Mn-Co interaction. Dong et al. [38] compared the catalytic performance of CoMn2O4 (prepared by oxalic acid sol-gel method), single Co3O4, MnOx and mixed Co3O4/MnOx towards toluene combustion. It has been found that the CoMn2O4 sample with a large surface area and rich cationic defects demonstrate the highest catalytic activity and lowest activation energy (35.5 kJ mol − 1) compared to other metal oxides, where the 100% conversion of toluene is achieved at 220 °C even in the moisture situation (2.0 vol% water vapor). The well-designed TP experiments concluded the oxygen vacancy induced by Co substitution can accelerate the oxygen circulation, thereby elevating the rate of this reaction. Additionally, the in situ DRIFTS analysis also revealed the spinel-type catalyst has a strong ability to destroy the aromatic ring to generates anhydride species, which results in the rate-controlling step of toluene oxidation over CoMn2O4 is different from Co3O4/MnOx, thereby promoting its catalytic behavior. Hence, these investigations highlight the Co-based spinel was an efficient candidate which showing great promise in the field of VOCs elimination.The catalytic performance of Co-based perovskite and spinel catalyst was cross-compared by taking toluene oxidation as an example, as shown in Fig. 4. Compared to bulk Co3O4 (M-Co3O4), most of these Co-based perovskites and spinel catalysts displayed higher catalytic activity and lower T90 temperature, implying the fabrication of Co3O4 with specific structures is quite efficient to improve the catalytic property. It can be seen that the toluene conversion rates of these catalysts follow the order of CuCoOx > La0.9Ca0.1CoO3 = LaCoO3 > CoMn2O4, which is quite different from their rank of T90 temperature: CuCoOx > CoMn2O4 > LaCoO3 > La0.9Ca0.1CoO3. This result demonstrated the perovskite structure seems to be more reactive than spinel, endowing superior catalytic performance at relatively low-temperature. Therefore, we think it makes sense to use cobalt-based perovskite as the substrate for the forthcoming investigationsAlthough the single metal oxides possess the merit of both high reactivity and low cost, the downside of poor stability (both in chemical and thermal) usually causes particle aggregation, restraining its further application [26]. Thus, owing to the intrinsic nature of CoOx, it remains a challenge to improve the catalytic performance. Recently, the multi-metal combination provides a promising strategy to develop an efficient Co-based composite for VOCs elimination by instituting the synergistic effects between different metal oxides. This synergistic effect brings out new features by taking the complementary advantage of different metals that can promote the surface characteristic, electronic property and stability [79]. Specifically, the Co-based composite were shown to be more active than the individual cobalt oxides by similar methods, which can be mainly ascribed as the promotion of surface oxygen mobility, the improvement of low-temperature reducibility, the higher surface area, etc. [80–83]. Thus, much efforts have been devoted in the preparation of various composites, such as binary metal oxides (contains heterostructure of cobalt oxide, incorporation of CoOx with perovskite, the combination of cobalt oxides with other metal oxides and cobalt oxides coupled with carbon materials), multi-metal oxides and so on, which exhibits different catalytic properties. The catalytic performance of these composites was summarized in Table 2.To realize a better catalytic performance, fabricating transition metal-based heterostructures have emerged as an efficient strategy due to the optimal electronic properties and tunable structural morphologies [84]. Especially, for the compound oxides, the rationally designed interface of heterostructure enables to expose more defects and active sites, thereby facilitating the catalytic activity [85]. Thus, it is very important to choose appropriate cobalt oxides to construct the functional interface. For instance, Ye et al. [49] designed a series of Co3O4-based hetero-structured catalysts with monolithic core-shell structure by introducing different elements (Mn, Cu and Co) and evaluated their catalytic activity towards the co-oxidation of CO and toluene. Compared to single Co3O4 materials, the Co-based heterostructure catalyst displayed much better catalytic performance. While for composite oxides, the catalytic activity for the degradation of CO and toluene follows the order of Co3O4@Co3O4 > Co3O4@Co2CuO4 > Co3O4@Co2MnO4 > Co3O4@MnO2 > Co3O4, where the T99 value of Co3O4@Co3O4 reaches 230 °C under mixture conditions (1000 ppm toluene and 1.0 vol% CO, SV = 10,000 h − 1). Notably, the combination with catalytic and characterization results demonstrated that the functional interface with strong interaction was fabricated in the core-shell Co3O4@Co3O4 heterostructure, which promotes the exposure of Co3+ active species and causes rich lattice defects, contributing to the superior catalytic activity even in moisture environment.Besides, textural and redox properties of heterostructure catalyst were also shown to be affected by morphology transformation, the ratio of metal oxide with different crystal forms, etc. While these influencing factors have been extensively explored by taking manganese oxide as candidates. For example, Qu's group [86] developed a simple hydrothermal method to prepare α@β-biphases materials and displayed higher catalytic performance than single MnOx, which can be further improved by adjusting the ratio of α-MnO2 and β-MnO2. However, even though the catalyst with heterostructure possessed a bright prospect in the field of VOCs elimination, such investigation for CoOx heterostructure, to our knowledge, is still pretty rare. Thus, for cobalt oxides with heterostructure, in my opinion, the exploring of feasible preparation method, probing detailed interfacial mechanism toward VOCs abatement seems to be highly desired.As mentioned, the industrial use of single cobalt oxides is still not satisfied, mainly because of its poisoning and sintering trouble during the reactions. While the perovskite oxides possessed satisfactory thermal stability and excellent anti-poisoning ability due to their high temperature aged structure, which attracted widespread attention [87,88]. But for the same reason, compared to individual metal oxides, the catalytic activity of perovskite oxides is largely confined by its surface enrichment of lanthanide series cations and low specific surface area, which only provides limited actives sites. Thus, engineering binary metal oxides via the combination of perovskite oxide and cobalt oxide are proposed as an effective strategy to overcome these deficiencies. In fact, related researches demonstrated that the synergistic effects derived from CoOx and perovskite oxide are extremely useful to improve the catalytic performance [51]. For example, the Co3O4 with different content supported by three-dimensionally ordered microporous (3DOM) La0.6Sr0.4CoO3 was prepared via in situ PMMA-templating strategies by Dai and coworkers [50]. Of the catalysts researched, the 8 wt%Co3O4/La0.6Sr0.4CoO3 was found to be the most active catalyst for toluene oxidation, achieving 90% toluene conversion at 227 °C (reaction conditions: 1000 ppm toluene, GHSV = 20,000 mL g−1 h−1). Such superior performance was linked with the 3DOM architecture and the strong interaction between Co3O4 and La0.6Sr0.4CoO3, leading to high specific surface area, enriched oxygen adspecies and promoted low-temperature reducibility. Similarly, He et al. [89] fabricated 3D-ordered meso‑macroporous Co3O4/La0.7Sr0.3Fe0.5Co0.5O3 materials for 1,2-dichloroethane (DCE) oxidation via a one-step method by taking PMMA as template. The systematic investigation demonstrated the introduction of residual Co3O4 nanoparticle increase the generation of oxygen vacancies, which plays a vital role in the circulation of oxygen species and chlorine poisoning resistance. Along with the special meso‑macroporous structure of as-prepared perovskite favors the migration of 1,2-DCE to contact with the surface-active sites, further improving the total oxidation of 1,2-DCE. To reveal the synergistic effects and working principles between these two materials, LaOx-Co3O4 with varied La/Co ratios were synthesized via precipitation means by our group [90], as displayed in Fig. 6 . We found that the incorporation of La cations into Co3O4 facilitates the formation of LaCoO3 perovskite, as well as increases the concentration of cation defects and adsorbed oxygen species, all of which was beneficial to the low-temperature reducibility. Further investigation also demonstrated the variation of La/Co ratios have a great influence on the catalytic properties and the 80Co-20La catalyst present optimal catalytic efficiency, where the T90 of toluene oxidation was around 242 °C together with an impressive specific reaction rate (Rs = 2.0 × 10−3 mmol h − 1 m − 2). It is also revealed that the introduction of La cations in appropriate amounts could increase the interaction between CoOx and LaCoO3, promoting the re-dispersion of Co3O4 and decrease its crystal sizes, which result in the formation of lattice defects, leading to the improvement of catalytic activity. In addition to the common synthetic strategies, it is well known that the surface engineering strategy is able to tailor the textural and surface properties of catalyst, which is conducive to VOCs oxidation. For instance, Xiao et al. [51] proposed a convenient strategy to fabricate CoOx/LaCoO3 composites by using in situ H2O2 treatment over LaCoO3 materials, which showed a higher efficiency towards propane combustion than that of bulk catalyst counterparts, achieving T90 temperature around 312 °C (reaction condition: 1 vol% propane with the WHSV of 100,000 mL g−1 h−1). The authors declared that acidic treatment by H2O2 over LaCoO3 substrate not only result in porous structure but also leads to the re-dispersion of Co3O4 particles on the surface of LaCoO3, as well as causing the formation of oxygen vacancies via the selective removal of La cations, as a consequence, the propane combustion efficiency is greatly improved. Compared to powder counterpart, the monolith catalyst can offer better dispersion of active species and unhindered mass transfer, thus showing prospective in practical employment. For instance, CoOx/LaCeO2 powder was washcoated onto cordierite honeycomb by Vidal et al. [91] and applied in VOCs oxidation. The results indicated that the compositional property was well preserved upon being supported on cordierite monolith, while the catalyst powder was homogeneously dispersed, demonstrating good activity in the oxidation of toluene and ethyl acetate. This strategy is simple, reliable and sheds a light on catalyst designing for practical use.The shortcoming of individual cobalt oxides, such as limited absorption capacity, poor dispersity, low specific surface area, and cost issues limited its further application. In this regard, combining the cobalt oxides with non-metallic materials offer the possibility to fabricate new composites to overcome these drawbacks. Recently, various non-metallic materials with different chemical compositions, variable morphologies, and adsorption properties, such as carbon materials [92–96], silicon carbide [97], carbon nitride [52], silicon oxide [98] have been employed to modify Co-based oxides, which provides abundant adsorption sites, improved surface area and stronger stability, harvesting enhanced catalytic activity. Among these materials, carbon is being preferred due to its distinctive physicochemical properties, various structures and low price [92,99]. For instance, Huang et al. [92] employed the carbon modification strategy to prepare Co-based catalysts with small grain size and evaluate their feasibility of formaldehyde abatement. The result revealed that the as-prepared carbon/Co3O4 nanocomposite (CCo3O4) demonstrate stable removal efficiency (90%) for HCHO oxidation (1 ppm), by contrast, the bulk Co3O4 was rapidly deactivated. And the enhanced activity is linked to the formation and interaction between carbon and Co3O4 interface, which caused abundant lattice disorders, promoting the generation of oxygen vacancies, thereby improving the catalytic performance. In addition, the oxygen vacancy enriched surface is also conducive to continuously convert O2 and H2O into reactive oxygen species (ROS), consequently, giving the fast and deep oxidation of intermediates and preventing the carbonate cumulation, finally result in excellent stability. The carbon material is commonly used as support due to its low price and availability. Wey and coworkers [100] coated transition metal oxides (Cu, Co, Fe, Ni) on activated carbon (AC) via polyol method for VOCs elimination and revealed that the AC support shows positive effects on the catalytic performance due to its tremendous surface area. To screen the appropriate activated carbon (AC) support with low cost, Khorasheh et al. [99] selected three typical agricultural wastes including Iranian almond shells, walnut shells and apricot stones as precursors to prepare porous activated carbons. Compared to walnut and apricot stones approaches, the activated carbon prepared via the almond route displayed strong thermoresistance, rich oxygen groups and high hydrophobic surface area. These special properties of AC support offer multiple benefits for establishing efficient metal oxides/AC hybrids, such as enhanced surface area, high dispersion of active metal oxides, strong adsorption to VOCs and good water resistance, leading to better catalytic performance for the removal of toluene and cyclohexane. Fan et al. [97] adopted a two-step method to support CoOx on silicalite-1/SiC foam for isopropanol removal and found that the silicalite-1 layer could not only ensure the uniform growth of Co3O4 nanoflower but also provides acid sites to absorb oxygen species and isopropanol molecules, contributing to the improvement of catalytic efficiency.Additionally, charge transfer engineering has been noticed as a promising approach to improve catalytic activity. Ren et al. [52] well designed a series of supported CoOx catalysts by using different support, such as g-C3N4, SBA-15, γ-Al2O3 and AC to investigate the influence of support on the catalytic combustion of toluene. Compared to other formulations, the 10%CoOx/g-C3N4 exhibits the optimum catalytic performance, achieving T90 of toluene conversion at 279 °C (reaction condition: 1000 ppm toluene with a total flow rate of 130 mL min−1), as well as excellent durability (at least 36 h). Comprehensive digging revealed that the electron-rich nature of g-C3N4 and the well-formed interfaces enhance the charge transfer and promote the generation of active Co3O4 phase, which facilitates the reducibility of Co3+, along with the high surface Co3+ content and high density of active oxygen species, responding to the enhanced catalytic performance. Thus, it makes g-C3N4 good support for its intrinsic nature and deserves further development.The integration of Fe, Cu, Mn and other metal oxides with Co-based oxides have been previously investigated in the field of VOCs elimination and show significant improvement in catalytic performance [53,101–103]. And such enhancement mainly come from the synergistic effects of different elements, which usually promote elevated charge transfer between the multiple available energy levels of the metals and their associated oxygen anions, consequently resulting in better low-temperature reducibility and lavished oxygen species.For this reason, our group subsequently prepared a series of Co-M (M = La, Mn, Zr, Ni) composites using a co-precipitation strategy and found that their catalytic activity for toluene combustion varied quite significantly [104]. The performance observed for as-prepared catalyst follows the order as below: Co-La > Co-Mn > Co-Zr > Co-Ni, which is aligned with their low-temperature reducibility, specific surface area and pore volume results. Besides, among these materials, the Co-La sample possessed the most Co3+ active sites, excellent oxygen storage capacity (OSC) and sufficient oxygen species. All these feathers contribute to the optimal catalytic efficiency, where the 90% toluene conversion was obtained at 243 °C (reaction condition: 500 ppm toluene with a total flow rate of 100 mL min−1). And these results indicated the physicochemical property and catalytic performance are strongly influenced by the interaction of Co-M elements. Jia et al. [53] conducted the catalytic efficiency of CuO/Co3O4 binary oxide derived from in situ pyrolysis of Cu2+ partly-substituted ZIF-67 template. It has been found, compared to bulk CuO, Co3O4 and mix-CuO/Co3O4 (mechanical mixture), the CuO/Co3O4 composites were highly active and 90% of toluene could be fully converted into CO2 at 229 °C (the promotion is more than 40 °C under 1000 ppm toluene with the WHSV of 20,000 mL g−1 h−1). Meanwhile, it also demonstrated good stability and strong water resistance to high humidity (10 vol%) gas flow. The superior activity observed could be assigned to the porous structure, high Co3+/Co2+ ratios, good low-temperature reducibility and abundant oxygen vacancies, and all these characteristics originated from the formation of Co-Cu interaction. This strong interaction was closely related with the preparation process, the Co2+ cations of ZIF-67 frameworks was partly replaced by Cu+ ions, thus resulting in the enhanced interaction between CuO and Co3O4 in pyrolysis process.Introducing special morphology furnishes an efficient approach to synthesis bimetal oxides with considerable catalytic performance towards VOCs oxidation. For instance, well-structured CeO2@Co3O4 core-shell material was prepared by Chen and his coworkers employing a ZIF-67 as sacrificial template, while its feasibility of toluene abatement was also investigated [54]. The CeO2@Co3O4 was determined to display a higher catalytic activity (T90 = 225 °C) towards toluene oxidation than that of pure CeO2 and Co3O4, which was strongly linked with its unique morphology. The unique core-shell structure and the hierarchically wrinkled surfaces strengthen the synergistic effect between cobalt and cerium elements, which result in better low-temperature reducibility, endowing the improved catalytic performance. Similarly, Yan et al. [55] synthesized a series of Co3O4/CeO2 hybrids by using CeO2 with specific morphologies (plate, rod and cube) as substrate and studied their catalytic performance for dibromomethane (CH2Br2) elimination. They found the rod-like Co3O4/CeO2 material exhibited significantly improved catalytic performance than that of plate-like Co3O4/CeO2 and cube-like Co3O4/CeO2, where the T90 of CH2Br2 conversion is approximately 312 °C (under the condition of 500 ppm CH2Br2 with the GHSV of 75,000 mL g−1 h−1). The as-formed rod structure is not only conducive to the exposure of (100) and (110) planes with numerous Co3+ sites but also benefits to the improvement of surface adsorbed oxygen species, as well as the enhanced redox properties, making it superior for CH2Br2 abatement.Recently, compared to the construction of simple binary oxides, interfacial engineering has drawn extensive attention because the interface between different metal oxides usually possesses rich oxygen vacancies, which provides abundant active sites and a large number of oxygen species. For example, Ye et al. [56] designed a new bottom-down strategy to fabricate α-MnO2@Co3O4 nanocomposite via the in situ growth of ZIF-67 over 1D α-MnO2 substrate. Of the catalyst researched, the α-MnO2@Co3O4 demonstrated great catalytic activity with 90% of toluene conversion is fulfilled at 229 °C (1000 ppm toluene with WHSV of 48,000 mL g−1 h−1), which is 28 °C and 47 °C lower than those of pure Co3O4 and α-MnO2, respectively. It has been proposed the synergistic effect between Co3O4 and MnO2 is mainly reflected in their coupled interface, and the constructed interface is contributed to improve the content of surface adsorbed oxygen species, accordingly promote the mobility of oxygen species and accelerate the redox cycles of Mn and Co cations, finally result in enhanced catalytic efficiency. Besides, it was determined that the gaseous oxygen species is more tend to be activated into adsorbed oxygen species on the surface of α-MnO2@Co3O4, giving a better oxygen supplementation for the oxidation. To investigate the influence of establishing different interfaces on catalytic performance, Co3O4 was anchored on the surface of α-MnO2, β-MnO2 and γ-MnO2 by using a secondary hydrothermal method to prepare Co3O4/MnO2 binary oxides in our recent study [105], as displayed in Fig. 7 . We found that the catalytic performance increased in the order of Co3O4/β-MnO2 < Co3O4/γ-MnO2 < Co3O4/α-MnO2, where the toluene can be fully converted into CO2 at 260 °C over the most effective Co3O4/α-MnO2 material, revealing the existence of a remarkable interfacial effect. Subsequent work suggested such strengthened MnO2 Co3O4 interact not only favorable for the improvement of redox properties but also facilitate the generation of oxygen vacancies, both of these play crucial factors in the catalytic performance. Therefore, it can be concluded that interfacial engineering might provide a new way to prepare Co-based catalysts with good catalytic efficiency.As mentioned earlier, the integration of cobalt oxides and other metal oxides usually affect the structure, redox property, oxygen mobility, acid-base property of the composite, as a result, it offers a useful way to design catalysts with better catalytic activity. In this respect, the introduction of other metal elements to fabricate ternary or multicomponent composites is proposed to further enhance the capacity for VOCs oxidation. Thus, it is quite critical to understand the role of each element and the interaction between different elements to design efficient multicomponent catalysts. For the past few years, lots of elements, such as Fe, Mn, Cu, Ni, Ce, La have been employed to construct Co-based polymetallic oxides, in an attempt to improve the oxidative activity for VOCs removal [57–59].For instance, a highly efficient material with the composition of CuOCo3O4 CeO2 (Cu: Co: Ce = 10: 45: 45) was prepared by Domen's group [106], seeking proper means to control the large-scale VOCs emissions. It has been found that CeO2 and CuOCo3O4 are beneficial to reduce the risk of sintering and maintain structural strength during the oxidation process, respectively. As a result, the ternary oxides exhibited excellent activity and stability for the elimination of industrial exhaust gasses (including toluene, xylene, ethylbenzene, butyl alcohol and formaldehyde), where the most of VOCs was degraded to below 0.5 ppm (under the condition of SV =16,600 mL h − 1, reaction temperature = 280 °C, catalyst volume =188 mL). Jirátová et al. [107] prepared the Co-Mn-Al ternary composite and employed K doping strategy to further improve its catalytic behavior towards toluene and ethanol oxidation. It was determined that the low K additions (1%) could increase the acidity of the catalyst, as well as induce the generation of more active Co3+ species, resulting in catalysts with enhanced activity. Wang and his coworkers [57] reported the fabrication of CuyCo3-yFe1Ox catalyst with adjustable oxygen vacancies via LDHs precursors as the efficient catalyst for toluene elimination. Comparison with Co3Fe1Ox (289 °C) and Cu3Fe1Ox (304 °C) binary oxides, the 100% of both toluene conversion and mineralization over ternary Cu1Co2Fe1Ox are realized at approximately 241 °C, which is closely related to the as-obtained multi-phase interfaces. The systematic investigation demonstrated that good dispersion and close contact between different metal phases are conducive to the reinforcement of synergistic effect, which is responsible for the enhanced oxygen vacancies than binary oxides. Besides, it has been found that the ternary Cu1Co2Fe1Ox exhibited excellent stability and water resistance, and these factors are very crucial for practical application.Additionally, the precise design of multi-metal composite with specific structures was also investigated. Chen and coworkers [58] prepared a novel nanocage with the formula of MnCeOδ/Co3O4 by using ZIF-67 as a sacrifice template. Compared with bulk Co3O4 and Co3O4 nanocube, the as-prepared MnCeOδ/Co3O4 catalyst demonstrated the lowest apparent activation energy (56.10 kJ mol−1) and highest catalytic activity, where the T95 of toluene oxidation is around 230 °C (1000 ppm toluene, WHSV = 40,000 mL g−1 h−1). It can be concluded that the well-designed structure boosts the interactions between MnCeOδ and Co3O4, thereby providing an adequate specific surface area (100.40 m2 g − 1), high concentration of surface adsorbed oxygen and Co3+ species, excellent low-temperature reducibility, endowing the superior catalytic property. Similarly, the low-temperature elimination of 1,2-Dichlorobenzene (o-DCB) was investigated over 3D hollow nanoflower ball-like NiO@MnMOx (M = Co, Cu and Fe) materials by Zha and Tang [59]. They found the application of interfacial reaction over the surface of hollow substrate is an effective approach to prepare polymetallic oxide with specific structures and adjustable properties. The unique hollow flower structure of NiO@MnCoOx not only offers abundant adsorption sites, but also shows the merits of numerous active species (such as Co3+, Ni2+, and Mn4+), enriched active oxygen species, as well as the vast of acid sites. Based on these favorable conditions, the optimal NiO@MnCoOx exhibited outstanding catalytic activity and superior reusability towards the destruction of o-DCB.Creatively, a recent study of Sun's group [60] points out a new way to acquire polymetallic oxides by using spent lithium-ion cobaltate batteries. As displayed in Fig. 8 , the cathode of the spent lithium-ion batteries was treated by citric acid, subsequently, filtered out and finally calcined to reach the LiCoM (M includes Ni, Mn, Cu and Al elements) composite. Besides, the Li and Al ions were wiped out from the leaching solution via the treatment of oxalic acid, and the as-obtained solution was further calcined and marked as Co3-xMxO4 (M contains Ni, Mn, Cu). To make a comparison with the individual cobalt oxide doping by different elements, the corresponding catalysts were also prepared and named CoM-X. Of the catalyst prepared, the catalytic activity for toluene oxidation follows the trend of Co3-xMxO4 > Co3O4 > LiCoM, indicating the presence of aluminum and lithium elements is disadvantageous to the catalytic performance. Conversely, the introduction of manganese and copper displayed a positive effect on catalytic performance, which is conducive to the formation of stronger weak acid sites, abundant Co3+ and Mn4+ active sites, favorable Olatt/Oads ratios, together with the promotion of low-temperature reducibility. Thus, it shed a light on the employment of waste materials as precursors to prepare efficient catalysts for air purification.Differ from the integration of CoOx with other materials, the surface engineering strategy is another way to modify the redox property and the surface oxygen vacancy concentration of CoOx, which can facilitate oxygen mobility and consequentially improve the catalytic performance. Recently, among the different surface engineering strategies, doping method and acid treatment are two commonly studied strategies, which exhibits promising prospects in VOCs purification [26,69,72,73].Dopant was widely used to modify the surface oxygen property by varying the electronic and geometric nature of the host metal oxides. Generally speaking, the doping strategy can be divided into three categories according to the employment of different dopants, mainly including noble metal, non-noble metal, and non-metal dopant. As mentioned before, noble metal catalysts faced the disadvantage of sintering, poisoning and high cost, nonetheless, their outstanding catalytic efficiency is nonnegligible. Thus, the doping of noble metal (including Ag, Au, Pd, Pt, Ru, etc.) is commonly used to improve the catalytic performance of Co-based metal oxides [61,62,64,108].For this reason, Ge and Yu [61] prepared Ag/Co3O4 catalyst by using a one-pot solvothermal method and explored its catalytic behavior towards benzene elimination. It has been found that the as-obtained 2%Ag/Co3O4 displayed superior catalytic activity compared to the 2%Ag/Co3O4-I synthesized by simple impregnation, where the 90% of benzene can be oxidized into CO2 at 201 °C (under the condition of 100 ppm benzene with GHSV = 120,000 mL g−1 h−1). The characterization results demonstrated the doping of Ag by solvothermal strategy can induce more surface Co3+ species and oxygen vacancies, subsequently promoting the low-temperature reducibility and the amount of active oxygen species, respectively, contributing to the benzene elimination. He et al. [108] reported the catalytic oxidation of o-xylene over Pd supported on Co3O4 catalysts, and explore the influence of the Co3O4 supporter and Pd dopant. Depending on the carrier employed, the catalytic performance for o-xylene oxidation are as follows: Pd/Co3O4 (3D) > Pd/Co3O4 (B) > Co3O4 (3D) > Co3O4 (B). It was determined that the ordered mesoporous 3D Co3O4 support not only exhibit porous structure, but also benefits to the exposure of PdO active sites, and all these features would be responsible for its outstanding catalytic activity, resulting in the T90 value of o-xylene combustion was achieved around 249 °C (150 ppm o-xylene and WHSV = 60,000 mL g−1 h−1). Among the commonly used noble metal catalyst, the Pt have attracted extensive attention, which is attributed to its high efficiency at low temperature. It has been reported the introduction of Pt species over Co3O4 substrate may not only provides more active sites, but also increase the proportion of oxygen vacancies via its affinity to the 3d orbital of Co atoms [109]. Ye et al. [62] utilized a novel metal-organic templated conversion method to prepare Pt-Co3O4 catalyst and evaluated its catalytic performance for toluene oxidation. It was determined that the conversion from rhombic dodecahedron to nanosheet is favorable for the exposure of Pt nanoparticles, as well as the enhancement of strong metal-support interaction (SMSI), both of these have substantial effects on the catalytic activity, thereby achieving the optimal catalytic performance over Pt-Co(OH)2-O catalyst (T90 = 167 °C under the condition of 1000 ppm toluene with WHSV = 60,000 mL g−1 h−1). Besides, the further investigation indicates the existing SMSI effect could facilitate the electron transfer and weaken the CoO bond, which were conducive to catalytic activity.In addition, the co-doping of different noble metals also provides an efficient approach to further improve the catalytic efficiency. For example, Dai et al. [63] reported the preparation of 3DOM Co3O4 and x%AuPd/3DOM Co3O4 catalyst via polymethyl methacrylate-templating and polyvinyl alcohol-protected reduction routes, respectively. Of the catalyst researched, the 3DOM Co3O4 coupled with Au-Pd alloy performed much better than supported individual Au or Pd materials, and the best catalytic activity was achieved on 1.99%AuPd/3DOM Co3O4, where the T90 of toluene conversion is approximately 168 °C (GHSV = 40,000 mL g−1 h−1). It was found that compared to the supported single Au or Pd catalyst, the AuPd/3DOM Co3O4 possessed the lowest apparent activation (33 kJ/mol) and strongest ability in the activation of oxygen and toluene, thereby showing the better catalytic performance. Based on this research, they further prepared a meso‑Co3O4 support ternary AgAuPd alloy nanoparticles by using KIT-6 templating and NaBH4 reduction strategy [110]. It was found that the supported AgAuPd material outperformed the Au, Pd, Ag sample, which can be attributed to the strong interactions between alloy nanoparticles and meso‑Co3O4, resulting in the highest oxygen vacancy density, thereby improving the catalytic performance.However, owing to the high proportion of noble metal being used, the industrial application of these catalysts is still restricted by economic considerations. Thus, a lot of efforts have been made to tackle this problem. Recently, it has been found that the single-atom catalyst (SACs) is a promising way to reduce the cost of the noble metal catalyst, as well as improve the catalytic activity. For instance, a series of single atom Pt-Co/HZSM-5 composites were synthesized and used for the catalytic combustion of dichloromethane (DCM) by Liu and Han [64], as displayed in Fig. 9 a-b. Compared to individual Co/HZSM-5 material, the doping of trace amount of Pt species (0.01 wt.%) over Co/HZSM-5 support could result in tremendous promotion in catalytic performance. Further investigation revealed the construction of Co-Pt interaction was beneficial to anchor Pt atoms, which promote the high dispersion of single-atom Pt species, thereby increasing the concentration of oxygen vacancies and the redox properties of Co3O4, accelerating the deep oxidation of DCM, in turn, protect the Pt species from being poisoned. He et al. [109] presented an atomically dispersed 0.02%Pt1-Co3O4 catalyst and found that was exceptionally efficient for methanol oxidation. By combining the experimental investigation and density functional theory calculations, it has been found the single Pt atoms was anchored on the (111) planes of Co3O4, which exhibited high occupied electronic states, demonstrating significant electron transfer between Pt and Co, sequentially accelerating the regeneration of oxygen vacancies, leading to the effective oxidation of VOCs at last. Thus, it seems that the design of Co-based SACs catalyst is a promising way to fabricate catalyst with excellent activity and high cost-effective.Without the disadvantage of high cost and instability, the non-noble metal dopant has been extensively studied to improve the catalytic activity of individual cobalt oxides. For instance, Achraf et al. [65] synthesized cobalt spinel film doped with a trace amount of Cu by using a novel pulsed-spray evaporation chemical vapor deposition method to study its catalytic effect on C3H6 elimination. The as-formed catalyst displayed superior activity, and it is even comparable with the supported noble metal catalyst, which can be attributed to the enriched oxygen species and well-dispersed Cu particles. Mn decorated cobalt oxides with abundant surface defects were successfully prepared via a MOF-templated route by Ma and Yu [66]. It was determined that the incorporation of Mn into the surface lattice of cobalt oxide could cause the formation of lattice distortion, which is likely to expose more surface defects than bulk defects. Besides, owing to the efficient electron transfer between Co-Mn couples, more active Co3+ species were also exposed on the surface of catalyst. Both of these factors were conducive to the promotion of low-temperature reducibility and lattice oxygen mobility, contributing to the enhanced catalytic behavior rather than single Co3O4 catalyst. Fendler and coworkers [67] pointed out that the different metal dopants adopted for the preparation of M-Co composite had a significant influence on their activity for VOCs elimination. As depicted in Fig. 9c-d, the catalytic performance for toluene oxidation of these catalysts follows the order of NiCo > CuCo > MnCo > Co3O4 > FeCo, while their catalytic behavior towards propane combustion was quite different, in the order of MnCo ≈ Co3O4 > FeCo > NiCo > CuCo, indicating the doping of Mn seems to be ideal way to prepared Co-based catalyst with strong universality.In addition to the commonly used transition metal dopant (Cu, Mn, Fe, Ni, etc.), other metals, including rare-earth metal, alkali metal, etc. were also widely doped into Co-based materials to improve the activity. Recently, our group reported the Ce doped on Co3O4 lower the T90 temperature (238 °C- 257 °C) for toluene combustion as the Ce: Co ratio increased from 0:1 to 0.05:1 [68]. It has been revealed the introduction of modest cerium provide more surface Ce3+ species, which was highly related to the formation of oxygen vacancies, further improving the removal efficiency. Similarly, Xiao et al. [111] pointed out that the doping of La could induce the lattice distortion of Co3O4, thereby promoting the generation of surface defects, demonstrating improved performance. It is worth mentioning that the incorporation of La is not only conducive to the adsorption of toluene molecule, but also inhibits the accumulation of carbonate intermediates, as a result, contributing to the long-term reactions. Guo and Fendler [69] investigated propane combustion over Zr doped Co3O4 materials and found that the 1%Zr-Co3O4 exhibited the best activity, with T90 of propane conversion achieved at 241 °C. The performance of this material was dependent on several factors, including the smaller Co3O4 grain size, rich Co2+ and oxygen species, enhanced low-temperature reducibility, and all these features were triggered by the formation of Co-O-Zr species via Zr entering Co3O4 lattice. Schwank et al. [70] reported the indium with a large cation radius could affect the chemical status of oxygen species around Co cations, which demonstrated that the surface lattice oxygen tend to be more easily abstracted by C3H6 during the reactions, thus promoting its catalytic performance.However, doping technology was not always effective in improving catalytic properties. Gao et al. [71] investigated the doping effect of alkali metal (Na, K, Li) into Co3O4 catalyst, which revealed the introduction of alkali metals could significantly delay the VOCs combustion reactions. Such poisoning effect can be divided into aspects, including the locking effect on oxygen species and the growing adsorption of CO2, which result in poor oxygen mobility and diminished active sites, respectively.Alongside the traditional metal doping technology, the doping of nonmetallic elements has been employed in catalytic reactions, such as photocatalysis or electrocatalysis. It has been reported the introduction of nonmetallic dopants can optimize eg electron filling and improve the conductivity of oxygen ions, thereby raising the oxygen evolutions [112]. For this reason, Qian et al. [72] synthesized Co-based perovskite doped with P to investigate the catalytic effect on propane combustion. As shown in Fig. 9e-f, it was determined that the doping of P had a significant influence on the catalytic performance, where the T90 temperature for propane oxidation was achieved at 376 °C over the optimal LaCo0.97P0.03O3 catalyst under the condition of 0.8% C3H8 with GHSV = 60,000 mL g−1 h−1. The experiment results revealed the doping of P has several advantages, such as regulating the valence states of Co cations, improving active oxygen density, enhancing surface acidity, all of which results in better oxidation properties. Although this study provides a new strategy for designing efficient Co-based catalysts in the field of VOCs elimination, but as far as we know, similar work is still pretty rare.As mentioned in the section on Co-based perovskite, several researchers reported the mechanisms for instituting Co3+-rich perovskites via acid treatment, indicating the acid etching is quite effective in surface reconstruction. Thus, the influence of acid treatment on Co-based materials has been investigated in the field of VOCs degradation and found that was contributed to modify the valence states of Co ions, subsequently facilitating the oxidation reactions. For example, a series of mesoporous Co3O4-n (n represent the concentration of acid) catalyst was fabricated by using acid etching technology over Co3O4-P that prepared via hydroxycarbonate precipitation method [73], as displayed in Fig. 10 . Among these materials, an obvious enhanced catalytic performance was observed over Co3O4–0.01 catalyst, which exhibited outstanding activity (T90 =225 °C) and strong water resistance during the oxidation of toluene. It was concluded that after the treatment of acid, the Co3O4-n demonstrated rich Co2+ and adsorbed oxygen species, together with large surface area and more weak acidic sites, benefitting the improvement of catalytic behavior.Similarly, acid etching was applicable to the regulation of vacancy (including cation and anion) density. For instance, Liu et al. [113] reported the fabrication of LiCoO2 catalyst for benzene degradation via acid treatment of spent cathodes. They pointed out the Co3+ and Li+ cations were leached out due to the disproportion reactions caused by nitric acid treatment, inducing the formation of Lo, Co and O vacancies. And the existence of Li and O vacancies could promote the adsorption and activation of benzene species, while the presence of Co and O vacancies boosted the formation of abundant active oxygen species, accordingly, the catalytic performance of as-prepared LiCoO2 was significantly improved. Moreover, the recent investigation of Guo and coworkers [74] have proposed that the surface defects induced by HF modification can be utilized to strengthen the interaction between Co3O4 substrate and active RuOx, thus improving the dispersity and stability of active RuOx species, leading to the enhanced activity for the oxidation of vinyl chloride. All these works indicate the acid etching is an efficient method that can help to develop catalysts for VOCs elimination.The catalytic performance of as mentioned Co-based composite was normalized as toluene conversion rates to compare the effects of these strategies for the fabrication of highly effective catalysts. As shown in Fig. 11 , it was found that CeO2@Co3O4 showed better catalytic activity than catalysts prepared via the simple combination of cobalt oxide and other materials (perovskite, non-metal material), implying that Co-based binary oxides could be an effective path for VOC oxidation, where the choice of the second metal is very important. Besides, it can be observed the α-MnO2@Co3O4 exhibited extraordinary performance, even comparable to some noble catalyst at low temperature, which indicates the interfacial engineering strategies used in this work is incredibly effective, and this strategy may become one of the hot spots for future researchCompared with common cobalt-based binary oxides, the catalytic performance of cobalt-based ternary oxides (MnCeOδ/Co3O4) is not inferior or even better, suggesting that combining the advantages of multi-metals provides a useful way to design catalysts with better catalytic activity. In addition, as evidenced in Fig. 11, it is worth mentioning that surface engineering techniques are also an effective way to improve catalytic performance, and the doping strategy is of particular interest, where the toluene conversion rates of noble metal doping catalyst (Pt-Co3O4, AuPd/3DOM Co3O4) can reach high levels (2.68 and 1.79 mmol g−1 h−1) at very low temperatures (167 and 168 °C). Therefore, the future directions of cobalt-based noble metal catalyst development should be reducing the noble metal loading and improving the stability of noble metal by adopting noble metal alloy or single atom noble catalyst, so as to increase its industrial application potential. to investigate the catalytic effect on propane combustionWater, as one of the by-products often present in the flue gasses emitted by various industries, is also one of the products of catalytic oxidation of VOCs. The impact of water vapor on the activity of catalysts depends on a variety of factors, such as the type of VOCs, catalyst composition, reaction conditions, etc., which have been extensively studied and reported. Generally, the presence of water vapor is able to compete for the adsorptive sites of catalyst with VOCs molecules, leading to inadequate oxidation of VOCs. For instance, Li et al. [73] claimed that the water vapor had a significant negative effect on the performance of Co3O4 catalyst in toluene oxidation (toluene conversion decreased from 90% to 61% in the presence of 5 vol.% of water vapor at 225 °C), and this effect completely disappeared after removing the feeding of water vapor. Thus, the catalyst with high water resistance is one of the promising research directions. Zhan et al. [114] prepared a series of LaCoO3 catalysts via a citric acid sol-gel method to investigate the catalytic effect on propane combustion and CO oxidation. They claimed that the introduction of water inhibited the oxidation of CO over LaCoO3 catalyst due to a decreased activity under the humid atmosphere (3 vol.%) compared to the dry reaction conditions. Besides, they also found the acid etching was able to improve the water resistance of the catalyst, according to the stable catalytic behavior of LaCoO3-AE (treated by acid etching) in the humid reaction atmosphere. Hao and coworkers [115] reported the amorphous Co1Mn3Ox displayed great water resistance capacity (propane conversion slightly changes from 82% to 79% in the presence of 3.1 vol% water vapor), indicating the appropriate combination of bimetal will provide excellent water vapor tolerance. Similarly, Chen et al. [116] pointed out the construction of CuxCo3-xAl mixed metal oxides catalysts delivered strong water tolerance, which remains 93.8% of benzene conversion (94.5% in dry condition) with the presence of 1.5 vol.% water.Although water is generally considered to be a hindrance to VOCs removal, in some cases, the presence of water vapor may be beneficial. For example, the positive effect of water vapor on the catalytic removal of formaldehyde was reported by Huang and Shen [117]. They found that the Co@NC catalyst showed higher catalytic performance for formaldehyde removal in humid environments (relative humidity = 25%) than in dry conditions. Zhan et al. [118] proposed that the existence of water in the feed gas shows an obvious negative influence on the activity of Ru/CoANS (cobalt-doped alumina nanosheets) in propane oxidation attributed to the competitive adsorption of H2O and C3H8/O2 on the RuOx active sites. On the contrary, for Pd/CoANS catalyst, although the propane conversion instantaneously drops from 82% to 52% when water is first introduced, subsequently the propane conversion gradually increases to 100%, which is much higher than the initial conversion under dry conditions. Besides, such enhancement of activity could remain at least 8 h after the water vapor was switch off, demonstrating the re-activation phenomenon by water vapor. Further investigation revealed that the removal of Cl species remaining on the surface during catalyst synthesis by H2O was responsible for the re-activation effect. This phenomenon has also been reported for the catalytic elimination of CVOCs, which demonstrated the aggressive role of water in removing Cl− from the active site, preventing the catalyst from being deactivated. Zhang and Zhao [119] studied the catalytic durability of a RuCoOx/Al2O3 catalyst for vinyl chloride elimination and revealed that the introduction of 1 vol% water leads to a tremendous increase in VC conversion, which derives from the removal of surface Cl by H2O, leading to an increase in HCl yield. And after stopping the addition of water vapor, the conversion of VC gradually decreased to stabilization, further confirming the speculation.In conclusion, the contribution of water vapor in the catalytic removal of VOCs is quite complex. Therefore, the effect of water vapor should be considered while designing application-based catalysts.Generally, the mechanism of VOCs combustion can be divided into three categories, including Eley-Rideal (E-R), Langmuir-Hinshelwood (L-H) and Mars-van-Krevelen (MVK) models. And the applicability of each mechanism is strongly linked to the catalyst nature, as well as the properties of VOCs. Thus, the investigation of catalytic mechanisms over Co-based catalyst have been extensively researched, and it was demonstrated that the MVK model is the commonest for VOCs elimination [33,34,82].As shown in Fig. 12 , the MVK mechanism can be interpreted as a two-step redox model: Firstly, the absorbed VOCs molecules react with active lattice oxygen, which caused the partial reduction of metal oxide, resulting in the generation of oxygen vacancies; Subsequently, the as-formed oxygen vacancy was immediately replenished by gas-phase oxygen species in the airflow or oxygen from the bulk. The balance between oxidative and reductive rates is quite important for catalytic reactions. Thus, it indicates the improvement of oxygen mobility and defect density can be useful to design efficient Co-based catalysts.L-H model is also widely used in VOCs combustion, which assumed the reaction occurs between the absorbed VOCs molecules and the absorbed oxygen species [120]. Accordingly, the controlling step of this mechanism is the reaction rate between these two species. For example, Jiang et al. [82] reported the degradation of benzene over ACo2O4 spinel catalyst proceeded via both MVK and L-H mechanisms, in which the lattice oxygen takes part in the generation of carboxylates intermediate species and the absorbed oxygen species favor the oxidation of carboxylates species to final products. Thus, the L-H mechanism provides a reasonable explanation for the fact that those Co-based catalysts containing rich adsorbed oxygen species usually exhibit elevated catalytic performance.Unlike MVK and L-H model, the E-R mechanism demonstrated that the catalytic reaction happens between absorbed VOCs molecules and oxygen species in gas phase (or between the absorbed oxygen species and VOCs molecules in gas phase) [121]. The E-R model has very limited applications toward VOCs oxidation, which was only applicable to catalysts with inert or less reactive carriers (such as zeolites, activated carbon, etc.). For instance, Khorasheh et al. [122] found the E-R model was appropriate to elucidate the reaction kinetic of cyclohexane combustion over Co/AC catalysts.Cobalt catalyst is one of the most promising catalysts for VOCs purification in air, which is far more economical than noble metal catalysts, offering the possibility to perform the reaction at low temperatures. This article is aimed to provide a comprehensive overview on the catalytic oxidation of VOCs over the past few years, and the advantage of different Co-based catalysts was also summarized. Besides, the kinetic models of VOCs elimination over cobalt catalyst were all-sidedly discussed and summarized based on corresponding researches, which is beneficial for the discovery of VOCs theoretical degradation process over Co-based catalysts.Concerning single cobalt oxides, based on the present research, it can be concluded that the catalytic performance towards VOCs oxidation is strongly related to the oxygen vacancy density, bulk oxygen mobility, the exposure of reactive (110) facets, the concentration of surface oxygen species and active Co3+ sites, three-dimensional structure, as well as the specific morphologies.Regarding Co-based composites, a conclusion can be drawn that the preparation methods, the intrinsic nature of other components, and the application of surface decoration strategies seem to affect the interaction between cobalt oxide and other components, regulating the structural and textural properties of the catalyst. In specific, this interaction works as a function of adjusting the redox properties and constructing surface defects of the composites, contributing to significant improvement of catalytic efficiency. However, the internal mechanism of the interaction has not been completely addressed in recent studies. Hence, more work needs to be done in the future to understand more about the effect of interactions in Co-based composites.From the current results, it can be inferred that although much progress has been made on the elimination of different VOCs, but there are still lots of problems that need to be solved to meet the emission standards of VOCs. As concerns future perspectives, we thought the research direction of Co-based catalyst may have the following aspects: (1) To investigate the preparation method that satisfies the characteristics of simple methods, accurately controllable, large preparation scale, etc. (2) Designing efficient catalysts with highly dispersed active sites, abundant oxygen vacancies, highly exposed crystal planes, specific structures and morphologies. (3) Exploring the employment of cobalt oxides to support single-atom noble metal seems to be a promising way to reducing the cost of the noble metal catalyst, as well as improving the catalytic activity. (4) Developing simple and efficient surface modification strategies to further improve the performance of Co-based catalysts. (5) Deriving a greater understanding of the mechanism of interaction among three or more component catalytic systems, to establish the correlation between the interaction and its catalytic behavior. (6) Exploiting new technologies or integrating existing strategies, such as photothermal catalytic oxidation, non-thermal plasma catalytic oxidation, pre-adsorption-catalytic oxidation, to improve the catalytic efficiency of VOCs purification, simultaneously, reducing the cost of this reaction. (7) Developing efficient Co-based catalyst towards the degradation of multiple VOCs mixtures, furthermore, to reveal the different catalytic behaviors of the catalyst in the oxidation of single VOC and VOCs mixtures. To investigate the preparation method that satisfies the characteristics of simple methods, accurately controllable, large preparation scale, etc.Designing efficient catalysts with highly dispersed active sites, abundant oxygen vacancies, highly exposed crystal planes, specific structures and morphologies.Exploring the employment of cobalt oxides to support single-atom noble metal seems to be a promising way to reducing the cost of the noble metal catalyst, as well as improving the catalytic activity.Developing simple and efficient surface modification strategies to further improve the performance of Co-based catalysts.Deriving a greater understanding of the mechanism of interaction among three or more component catalytic systems, to establish the correlation between the interaction and its catalytic behavior.Exploiting new technologies or integrating existing strategies, such as photothermal catalytic oxidation, non-thermal plasma catalytic oxidation, pre-adsorption-catalytic oxidation, to improve the catalytic efficiency of VOCs purification, simultaneously, reducing the cost of this reaction.Developing efficient Co-based catalyst towards the degradation of multiple VOCs mixtures, furthermore, to reveal the different catalytic behaviors of the catalyst in the oxidation of single VOC and VOCs mixtures.The authors declare no competing financial interest.This work is supported by the National Natural Science Foundation of China (No.21872096), the Educational Department of Liaoning Province (LZ2019002, LQ2020011)

As the main contributor to air pollution, lots of volatile organic compounds (VOCs) were emitted into the atmosphere due to the rapid urbanization and industrialization, threatening environmental safety and human health. Catalytic oxidation has been verified as an efficient approach for VOCs elimination from industrial waste gas streams. Owing to the merits of cost-effective and high activity, cobalt-based catalysts have been considered as one of the most promising candidates for VOCs degradation. This review systematically summarized the developments achieved in the design of cobalt-based catalysts for VOCs removal over the past decade. Specifically, the fabrication of single cobalt oxides, cobalt-based binary oxides and cobalt-based composites, as well as the modified cobalt-based oxides by the surface engineering strategies, such as doping technology and acid etching method are coherently reviewed. Subsequently, the corresponding kinetic models and mechanisms are also discussed. Finally, considering the enormous challenges and opportunities in this field, the perspective with respect to future research on cobalt-based catalysts is proposed.

Data will be made available on request.The acceleration of industrialization and excessive use of carbon-rich fossil fuel, oil, coal, and natural gases, the atmospheric CO2 concentration has reached an unprecedented high level (420 ppm), which results in global warming [1]. The advice of climate change experts on stabilizing surface temperature escalation below 2 °C compared to the preindustrial level in the 21st century to increase the CO2 emission approximately 450 ppm by 2100 represents the most stringent Representative Concentration Pathway (RCP) [2,3]. As a result, the environment and energy have emerged as the most pressing challenges of the 21st century. Due to this, researchers focus shifted to environmental mitigation and increasing the demand for greenhouse gas (CO2) utilization to value-added chemicals and fuels production [4,5]. However, the abundantly available CO2 can be used as a feedstock material to produce different kinds of important chemicals like formic acid, urea, methanol, and ethers. Atmospheric CO2 adsorbs and is used for valuable feedstock chemical synthesis [6]. Unfortunately, CO2 is a very rigid molecule that is kinetically and thermodynamically stable, with high bond energy (806 kJ mol−1). This led to a big challenge encountered in CO2 valorization [7]. Exploring and developing a heterogeneous catalyst for efficient CO2 upgrading have become one of the most active fields in catalysis [8]. In this context, significant progress in CO2 utilization has been made in recent years, yielding a variety of products such as urea, carbonate, salicylic acid, and polyols [9–11]. Carbon Recycling International (CRI) recently began the first commercial demonstration plant of methanol synthesis by using CO2 as a row material [12].Among all products, Formic acid (FA) is an important industrial product that can be obtained by reducing CO2 via various catalytic routes, including thermochemical, photocatalytic, and electrochemical reduction [13–15]. FA is mainly used in textiles, cleaning, preservatives, hydrogen storage, agriculture, pharmaceutics, and food additives [16,17]. Currently, FA is synthesized in the industry by using two steps: the first step involves carbon monoxide reacting with methanol to form methyl formate, which is then converted into formic acid through acidification with H2SO4 or hydrolysis with water. The hydrolysis step requires an excess amount of water and increases the production cost of formic acid. Formic acid is also produced as a by-product during the liquid phase oxidation of hydrocarbons to acetic acid. Nonetheless, this is not a atom-economy as well as not used renewable carbon feedstocks [18].The number of homogeneous catalysts has been reported for CO2 hydrogenation to FA synthesis. For instance, Inoue et al. used triphenylphosphine (PPh3), containing Rh, Ru, and Ir complexes for catalytic hydrogenation of CO2 to formic acid with high yield [19]. Yoon and research groups worked on developing iridium-containing homogeneous complexes and organic polymer ligands as a catalyst for CO2 to formate synthesis, achieving the highest 40,000 h 1 TOF with excellent selectivity [20,21]. Working on the same topic, Ertem et al. developed an amine-based iridium containing catalyst for CO2 hydrogenation to formate synthesis using a very low reaction temperature (25 °C) and gas pressure (0.1 MPa) H2/CO2 with achieving 198 h−1 TOF. The same report shows the reverse reaction of formic acid to H2 and CO2 generation with the same catalyst at 60 °C with good TON and TOF 118000 h−1 [22]. Also, a variety of transition metals, such as Pd, Ni, Cu, Fe, containing complexes with C-, N-, phosphorous ligand, N, N-chelated ligand, N-Heterocyclic carbine ligand (NHC), pincer complexes were also reported [23]. However, these catalysts have several drawbacks, including high energy requirements, high cost, catalyst separation from the reaction mixture, and other environmental issues [14].Keeping atom economy in mind, heterogeneous catalysts are the preferred choice for producing formic acid from renewable CO2. Thus, various mixed metal oxide (MMOs) catalysts such as Al2O3, MgO, CeO2, TiO2, ZnO, Cu2O, Ag2O, Co3O4, PbO, and SnOx have been used for the hydrogenation of CO2 to different chemicals [24–26]. Particularly, Mori and co-workers have used the impregnation method to synthesize PdAg nanoparticles supported on TiO2 for high-pressure hydrogenation of CO2 to formic acid, obtained > 99% selectivity of FA, with 748 h−1 TON [27]. Also, PdAg supported on a hydrophilic N-doped polymer-silica was used for the same reaction at mild reaction conditions [28]. The Pd/ZnO catalyst demonstrated CO2 hydrogenation to FA under base-free conditions and found that the crystal plane of ZnO plays a vital role in the catalytic activity [29]. Pandey et al. synthesized Cu dispersed on a TiO2 catalyst using a typical co-precipitation method and used for hydrogenation of CO2, achieving the highest TON 6 h−1 [30]. Furthermore, Chiang et al. investigated formic acid synthesis in a fixed bed reactor system using traditional CuZnO/Al2O3 and obtained 13.1% conversion of CO2 with 7.6% yield of formic acid [31].The literature reports indicate that the CO2 reduction reaction is still in its early stages, and a robust catalyst is required, which can be easily used in industrial production. Herein, we demonstrated that a lower percentage of Ir incorporation on Co3O4 and their catalytic application indirect hydrogenation of CO2 to HCOOH. The physicochemical properties of the synthesized composites were analyzed using analytical techniques to understand better the iridium role, morphology, and textural properties of the composites in the catalytic conversion of CO2 to formic acid.IrCl3.xH2O (99% Aldrich), Co(NO3)2.xH2O (99.5%, SRL India), Ammonia solution 25% (Finar, India). NaOH (96% SDFCL), N,N,N′,N′-tetramethylethane-1, 2-diamine (TEMDA >98%), KOH (85% SRL), Triethylamine (99.5% Loba Chemie), H2 and CO2 gas received from Raj sons Bhavnagar, India and used as received. A double distilled water prepared in the laboratory is used throughout the experiments.The different wt% of Ir on Co3O4 oxide were synthesized using co-precipitation, followed by the hydrothermal method. In a typical synthesis of 1 wt% Ir-Co3O4, 8 g of Co(NO3)2.6H2O was dissolved in 60 mL distilled water and stirred for 15 min at room temperature before adding 0.102 g of IrCl3.xH2O salt was added and stirred until complete dissolution of metal salt. Then 12.1 mL NaOH solution (6.3 M) was added dropwise as a co-precipitating agent and maintained the reaction pH between 9 and 10. The reaction mixture was then stirred at room temperature for 24 h. Then, the reaction mixture was transferred to a 100 mL Teflon-lined stainless-steel hydrothermal autoclave and placed in an oven at 120 °C for 5 h. Then the reactor was cooled to room temperature and the solid product was collected using simple filtration and washed several times with distilled water and methanol until the filtrate reached neutral pH. The obtained solid was dried in a hot air oven at 70 °C overnight before being calcined in a muffle furnace at 500 °C with 5 °C/min ramp rate for 5 h. The synthesized catalysts are donated as Ir-Co3O4-W, where W standards for wt% of Ir.The catalytic hydrogenation of CO2 into formic acid was carried out in a (300 mL Amar high-pressure reactor). The reactor was charged with 100 mL distilled water as a solvent, 0.2 g freshly prepared catalyst, and 10 mL N,N,N′,N′-tetramethylethane-1, 2-diamine (TEMDA) was used as a base. Then the vessel was tightly closed and purged with N2 gas three times before the reactor was pressurized by CO2 and H2 gas (1:1) up to 62 bars. The reactor was heated using a heating mental under slow stirring. The reaction was started by increasing the agitator speed and continue for 6 h at 120 °C. When pressure ceasing happened, the reaction stopped, and the reactor was cooled to room temperature to ambient conditions. The catalyst was filtered out, and the product solution was analyzed using high-performance liquid chromatography (JASCO, CO-2060 Plus, Intelligent column thermostat, MD-2015 Multiwavelength Detector, PU-2089 Quaternary Gradient Pump). The product analysis was done using the sSupelcogel C-610 H column. The 0.1 N H3PO4 as a mobile phase was passed through the column at a 0.6 mL/min flow rate. The samples were analyzed by selecting a 210 nm UV detector wavelength. The quantification of formic acid was carried out by plotting the standard calibration curve of formic acid by analyzing the formic acid solution of different known concentrations (2.5–400 mmol) in 0.67 molar TEMDA solution). Then TON and TOF are calculated based on metal loading obtained from the ICP result.The different Ir wt% loaded on Co3O4 catalysts were prepared by co-precipitation, followed by the hydrothermal method ( Scheme 1). Iridium chloride and cobalt nitrates are used as Ir and Co precursors to form an Ir-Co3O4 composite using the appropriate precipitation agent (NaOH) during continuous stirring. The reaction was carried out in a basic aqueous solution and resulted in the formation of a hydrated complex of both metal salts. The hydrothermal treatment was used to convert this aqua complex to crystal oxide because the hydrothermal process generates autogenous pressure and produces monomer, followed by nucleation and crystal growth during the calcination process at higher temperature ( Scheme 2) [32].The powder X-ray diffraction method was used to determine the purity of the formed mixed metal oxide Ir-Co3O4 phase after calcination ( Fig. 1). The sharp diffraction peak at 2θ= 19°, 31.3°, 37°, 39°, 44.9°, 48.3°, 55.7°, 59.5°, 65.4°, and 77°, which are indexed at corresponding diffraction plane (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), (4 2 2), (5 1 1), (4 4 0), and (5 3 3) of cubic phase of Co3O4 and are good agreement with JCPDS No. 42–1467 [33]. The diffraction peaks of iridium oxide were not observed due to very low wt% loading of IrCl3.xH2O salt on cobalt precursor, the spinal structure of Co3O4 selected the major plane (3 1 1) to determine the crystal size 6 nm and an average crystalline size of 1 wt% Ir-Co3O4 system is 1.88 nm determined by Debye Scherer’s equation (See in ESI S2).The surface elemental composition and chemical state of synthesized 1 wt% Ir-Co3O4 catalyst was studied by X-ray photoelectron spectroscopy (XPS), as shown in Fig. 2. Fig. 2 (A) survey spectrum confirmed the presence of Ir 4f, Co 2p, and O 1s species in the synthesized catalyst. The deconvoluted XPS spectra of Co 2p could split into two distinct peaks located with spin orbits at Co 2p3/2 and Co 2p1/2 and binding energy at 779.7 and 195.1 eV, respectively. The splitting characteristic of Co 2p3/2 and Co 2p1/2 indicate the presence of divalent and trivalent Co species in the cubic spinal structure of Co3O4. The fitting peak at 779.7 eV is a spin-orbit doublet of Co 2p3/2 deconvoluted into two peaks at 779.7 eV (FWHM 1.32) 781.3 eV (FWHM 2.4) which are assigned to Co3+ 2p3/2 and Co2+ 2p3/2 configurations respectively. While the shoulder peak of Co 2p1/2 at a binding energy of 795.1 eV could split into two distinct peaks at 795.1 eV (FWHM 1.8) and 797.0 eV (FWHM 2.5), these are attributed to the 2p1/2 spin-orbits of Co3+ and Co2+ respectively [34]. The typical two small shakeup satellite peaks of Co 2p3/2 are located at 790.0 (FWHM 2.1), and 804.5 (FWHM 2.5) eV are conformed to Co3+ and Co2+ species are present in Fig. 2 (B) spectra [35]. The Co2+ and Co3+ cations are found in Co3O4, with Co2+ being tetrahedrally coordinated ions and Co3+ being octahedrally coordinated cations [36]. Fig. 2 (C) shows that Ir 4 f high-resolution spectra are resolved in single doublet components of Ir 4 f7/2, indicating that Ir exists in two chemical states. The lower energy spin-orbit splitting Ir 4 f7/2 could be deconvolute into two distinct peaks with binding energies of 60.5 (FWHM 1.80) and 62.2 eV (FWHM 1.89), indicating that the iridium is present in the Ir+3 and Ir+4 chemical state. While the shoulder peak located with spin-orbit Ir 4 f5/2 at the binding energy 65.1 eV (FWHM 1.87) ascribed to the Ir+4 state present in the material. This is derived from the Ir+4 state in the form of IrO2 on the Co3O4 supporting material [37,38]. Fig. 2 (D) depicts the XPS spectra O 1s of the Ir/Co3O4 system and split into two peaks where the lower binding energy at 530 eV could correspond to the lattice oxygen O2-, and the higher binding at 531.8 eV can be attributed to the adsorped oxygen [39]. The considerable difference between O2- affections with the Co ion in the cubic spinal Co3O4 system may be single O three coordinated with three Co3+ and two coordinated O bonded with one Co2+ and Co3+ [40]. According to the results of the XPS study, the spinal structure of Co3O4 has more adsorption oxygen vacancies, as well as Ir+3 and Ir+4 species, and these results are in excellent agreement with surface area and pore size.N2 adsorption/desorption at liquid nitrogen temperature was used to determine the BET surface area and pore diameter of synthesized Ir-Co3O4 catalysts (See ESI Fig. S4). Catalysts were degassed at 300 for 4 h before analysis. The surface area of pristine Co3O4 was found to be reduced from 16 m2/g to 7.2 m2/g when 0.5 wt% iridium was added to Co3O4 but increased to 47.7 m2/g when 2 wt% iridium was added. In terms of pore size, the pristine Co3O4 has a pore diameter that was slightly reduced with the addition of Ir up to 1.25 wt%. When 2 wt% of Ir was incorporated into Co3O4, the pore diameter dramatically reduced to 388 Å; this indicates that the pores of the Co3O4 were occupied by iridium up to 1.25 wt%. Further increasing the amount of iridium, the cluster of iridium formed on the surface of Co3O4. The results of BET surface area analysis and pore size distribution are summarised in Table 1.The FE-SEM images of simple Co3O4 and 1 wt% of Ir-Co3O4 catalyst ( Fig. 3 A and B) show no distinct morphology. However, the elemental mapping displayed the uniform distribution of Ir contents on the Co3O4 surface (Fig. 3 C-F) with 1 wt% of Ir content was confirmed by SEM EDX (See ESI Fig. S2). The presence of the Ir and Co on 1 wt% Ir-Co3O4 were identified using the lattice fringes value shown in Fig. 3 (G) and (H). The lattice fringes value obtained through the SAED pattern 1 wt% Ir-Co3O4 catalysts were identical to the X-ray pattern as shown in (Fig. 1, JCPDS No. 42–1467). The composited solid crystalline sample reveals the various lattice fringes depicted in the selected different zones in Fig. 3 (G and H). The majority of separated lattice fringes are shown in Fig. 3 (G) and (H) 0.464, 0.24, 0.23, 0.28 nm correspond to (1 1 1), (3 1 1), (2 2 2), and (2 0 0) lattice indices, respectively, and exactly match with the XRD result d‐spacing of spinal cubic phase Co3O4 (JCPDS No. 42–1467). The active metal Ir oxide fringes are displayed in Fig. 3 (G) 0.258 nm correlate with (2 0 0) plane of Ir crystal and conform to the d‐spacing with (JCPDS 04–009–8479). The SEAD pattern also confirmed the plane, or d‐spacing value of Ir-Co3O4, see Fig. 3 (I).The surface acidity of 1 wt% Ir-Co3O4 oxide catalyst was quantified using temperature-programmed desorption (TPD) with NH3 and the results are shown in Fig. 4. Further ammonia adsorption was carried out at the heating rate of 10 °C/min by flowing 5% NH3/He stream (30 mL/min) from temperature 200–800 °C. The TPD result of the Ir-Co3O4 oxide catalyst indicates a broad distribution of strong acidic sites in the catalyst. Typically, acidic strength is determined by NH3 adsorption above > 400 °C [41]. Fig. 4 depicts the profile of NH3 TPD adsorption on the Ir-Co3O4 oxide catalyst. NH3 desorption shows two distinct regions, with a peak around 450–635 °C indicating a strong acidic site [42,43]. The quantitative analysis of NH3-TPD desorption of lower loaded Ir-Co3O4 oxide catalyst is estimated to be 0.963 mmol/g acidic sites are present in the material. The series of Ir metal loaded on the Co3O4 surface was confirmed by the Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The quantitative Ir concentration in the composite is quite analogous to the theoretical value of Ir, hence used for further study and determining the TON and TOF based on this result.To explore the hydrogenation of CO2 ( Scheme 3), initially, the hydrogenation reaction operates in the absence of a catalyst and base, there was no CO2 conversion was observed. ( Table 2, entry 1). Furthermore, in the presence of a base (TEMDA) without catalyst addition leads to no conversion of CO2. The results concluded that the hydrogenation of CO2 to formic acid required a catalyst to activate both CO2 and H2.The reaction was then carried out using only the supporting material Co3O4 as a base catalyst gave 7 mmol of FA. When a series of Ir-Co3O4 catalysts were used, initially, 0.5 wt% of Ir-Co3O4 catalyst gives 360.9 mmol of formic acid (Table 2, entry 4). Followed by 1 and 1.25 wt% Ir-Co3O4 catalyst gives excellent yield of formic acid 399 and 400 mmol, respectively (Table 2, entries 5, 6). However, no significant difference was observed when the reaction was carried out with a 2 wt% Ir-loaded catalyst (Table 2, entry 7).Furthermore, different types of bases were used with 1 wt% Ir-Co3O4 catalyst for hydrogenation of CO2 to formic acid. Initially, the hydrogenation reaction of CO2 does not show any catalytic activity without the addition of base ( Table 4, entry 1). The experiment concluded that base is necessary for the conversion of CO2 into other products. Further, the reactor was pressurized by an equal amount of CO2 and H2 (1:1) with 0.1 molar concentration of alkali bases like NaOH and KOH giving 5 and 73 mmol of formic acid. Additionally, compared the different amine-containing bases for CO2 hydrogenation. When 1 mole NH3 was used as a base 11 mmol of formic acid was observed (Table 4, entry 4). Then, investigated the activity of 0.6 molar piperidine, which provided 19 mmol yields of formic acid (Table 4, entry 5). The majority of the researchers used tertiary amine as a base for the synthesis of formic acid from CO2 because it efficiently coordinated with CO2 and converted to formic acid or formate. The 0.6 molar concentration of triethylamine yields 16 mmol of formic acid (Table 4, entry 6). When use N,N,N′,N′-tetramethylethane-1,2-diamine (TEMDA) as a base, the yield of formic acid (399 mmol) increased 25 times better than triethylamine (Table 4, entry 7).The higher catalytic activity of TEMDA, may be due to the presence of two active sites to co-ordination with CO2 and less hindered amine base. It can easily coordinate with CO2 molecules and form carbamate zwitterion intermediate species. The comparative results of various Ir based catalysts with Ir-Co3O4 are shown in Table 3 Most of the articles used trimethylamine and K2CO3 as a base, giving less formate yield than TMEDA. As a result, N,N,N′,N′-tetramethylethane-1,2-diamine base was chosen for further optimization study. The effect of temperature on the hydrogenation of CO2 to formic acid synthesis was investigated in the range of 80–140 °C (See Fig. 5 (A). The formic acid yield suggests that at the lower reaction temperature, the rate of CO2 conversion was also slow; the highest conversion of CO2 was obtained at 120 °C. When the reaction temperature was raised above 120 °C, the yield of CO2 hydrogenation did not increase. The reaction conducted at 140 °C, yield of formic acid was the same 397 mmol.The highest catalytic activity was observed in 0.67 M concentration of TEMDA; when decreasing the concentration of TEMDA to 0.33 M, the formic acid yield suddenly decreased to 210 mmol ( Table 5, entry 1). Further, increasing the concretion of TEMDA, there are no dramatic changes observed in the yield of formic acid, and 0.67 M TEMDA concluded for the further optimation studies (Table 5, entries 2–4). Fig. 5 shows how the rate of CO2 hydrogenation to selectively formic acid synthesis correlates with time (B). The CO2 conversion profile shows that the initial CO2 conversion rate was very high in one hour, yielding 281 mmol of formic acid and that the formic acid yield increased steadily up to 6 h, yielding 384 mmol. The CO2 to formic acid conversion rate was very slow after a 6–12 h reaction. The time variation study reveals that 6 h is enough to get the maximum yield of formic acid.Further, extend the optimization study of CO2 hydrogenation effect of gas ratio variation. To rule out the hydrogen source, CO2 hydrogenation was performed without H2 pressure and no conversion was obtained. This experiment confirms Ir-Co3O4 catalyst does not produce hydrogen from water ( Table 6, entry 1). As a result, a 62 bar CO2 pressure reaction also does not form formic acid or formate. When the reaction was carried out at a lower pressure, 7 bar CO2 and H2 gas respectively obtained 213 mmol yields of formic acid (Table 6, entry 2), when the pressure was increased to 21 bar CO2 and 41 bar H2 gas, 391 mmol of formic acid was obtained (Table 6, entry 3). Further, reduce of the H2 gas pressure to 21 bar and increasing the CO2 pressure to 41 bar also decreases the formic acid yield to 320 mmol (Table 6, entry 4). Then using a 31 bar (1:1) ratio of CO2 and H2 gases for the further subsequent reaction was obtained as a higher yield of formic acid 398.5 mmol (Table 6, entry 5). Further, increase the pressure of CO2 and H2 in equal amounts, but the yield of formic acid remains the same (398 mmol) (Table 6, entry 6). According to the gas ratio variation study, the synthesis of formic acid from CO2 is a temperature and pressure-dependent reaction.After each reaction, the catalyst was successfully recovered and washed with methanol before being dried at 200 °C for 2 h. The catalyst was used for the recycling experiment, as shown in Fig. 6. Initially, the first cycle catalyst yielded 403 mmol of FA, but the second and third cycles yielded only 8 and 13 mmol of formic acid yield decrease. Even after the fifth catalytic cycle, the 1 wt% Ir-Co3O4 catalyst demonstrated excellent catalytic activity with 372 mmol formic acid in 6 h, indicating the structural and catalytic stability of the Ir-Co3O4 metal oxide catalyst. The post catalyst was characterized by XPS analysis to understand the chemical changes of Ir and Co (see S7). No changes were observed in the chemical state of Ir, but in the case of Co the formation of CoO was observed; this may be due to the decomposition of Co3O4. At the same time, the decomposition of Co3O4 may be the reason for the slightly loss in the yield of FA.The formate ion formation was confirmed using 1H and 13C NMR, FT-IR, and quantified by HPLC. The experiment was conducted on NMR 600 MHz using D2O as an NMR solvent, which appeared peak at 4.8 ppm. The standard formic acid 1H proton appeared at 8.13 ppm, which acidic formic proton does not show due to hydrogen bonding with water and NMR solvent. Then the confirmation of the mixture of TEMDA and formic acid shows a 1H proton peak at 8.38 ppm because the base receives the acidic proton from FA and form formate, which causes the change in proton value shift from 8.13 to 8.38, indicating that it is the formate ion. The subsequent reaction was carried out in D2O as a reaction solvent; in this case, the source of transferred hydrogen to reduce CO2 by gases molecular hydrogen transfer from the metal surface rather than the solvent is shown in Fig. 7. (A). Fig. 7 (B) shows the FT-IR spectra and detects the reaction product formate ion instead of FA due to the experiment being performed in basic media, TEMDA used as a base. Initially, distilled water showed a broad band at the position 1645 cm−1. Then only TEMDA showed a significantly low intense band as compared to other samples. Fig. 7 (B) depicts the FT-IR spectra of a standard mixture of TEMDA and FA, in which TEMDA reacts with FA to form tetramethyethylendamine formate, but formic acid did not completely consume with base, resulting in a spectrum that shows the formic acid band at (1214 cm−1) as well as formate ion band 1351 and 1384 cm−1. In the reaction mixture, formate ions were observed through their most intense band located at 1351, 1384 and 1590 cm−1, respectively [50–52]. The standard formic acid stretching band is located at different positions 1216 and 1718 cm−1 than reaction mixture spectra. The FT-IR study concludes the presence of formate ions in the reaction mixture.Based on the characterization results and the combination of iridium and cobalt oxide catalytic performance, herein proposed a plausible mechanism of the catalytic route for the hydrogenation of CO2 to the formic acid formation ( Scheme 4). The initial step would be for gaseous CO2 molecules to dissolve in water and convert into carbonic acid in the reaction medium. When the reaction temperature is raised to 120 °C, the base TEMDA activates the CO2 molecule to form the carbamate zwitterion intermediate. As a Lewis acid, CO2 can easily co-ordinate with Lewis base (TEMDA) to form the carbamate intermediate. The formed carbamate zwitterion intermediate is very unstable and easily reacts with other species. Simultaneously, the active iridium nanoparticle participates in the activation of the H2 molecule on the catalyst surface and dissociates into the activated H species [53]. Particularly, Ir nanoparticles facilitated delocalized electron transfer to the vacant orbital of CO2 and promoted the CO2 hydrogenation.It can provide a more negative hydride, resulting in a more reactive nucleophilic attack on the electrophilic CO2 molecule [54]. The hydride (-H) is then transferred from the Ir interface side to the carbamate zwitterion, hydrogenated, and converted into a formate product, and upon acidification, it yields formic acid.In summary, the lower wt% of Ir loaded on traditional cobalt oxide demonstrated for the hydrogenation of CO2 to formate synthesis. The Ir/Co3O4 based mixed metal oxide catalyst was synthesized by typical co-precipitation followed by the hydrothermal method. The use of noncarbonated sources to directly hydrogenate CO2 for formic acid synthesis using TEMDA as a base over a spinal Ir-Co3O4 oxide catalyst. The synthesized cubic spinal Ir-Co3O4 catalysts are confirmed by various analytical tools. The Ir-Co3O4 catalyst selectively converted CO2 to formic acid via catalytic hydrogenation and obtained an excellent yield of (399 mmol) and TON 1916 h−1. While revealing excellent recyclability and producing the highest final formate concentration of 399 mmol within 6 h at milder reaction conditions, the formation of formic acid from CO2 was confirmed by NMR, FT-IR and quantified by HPLC. Its promising approach for direct CO2 hydrogenation and significant implications in the field of CO2 conversion chemistry are encouraged by the results of excellent catalytic performance, durability, simplicity, and high-pressure stability. Balasaheb D. Bankar: Conceptualization, Investigation, Writing – original draft, Data curation, Formal analysis. Krishnan Ravi: Data curation. Rajesh J. Tayade: Data curation. Ankush V. Biradar: 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.CSMCRI communication No. 209/2022. B. D. Bankar acknowledges to UGC government of India for the senior research fellowship. Dr. A. V. Biradar acknowledges MLP 0028, HCP 0009 CSIR India for the financial support. Also, Dr. P. S. Subramanian for discussion and encouragement. The analytical division provides the centralized instrumentation facility with all requisite instrumental analysis of CSIR- Central Salt and Marine Chemicals and Research Institute, Bhavnagar.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jcou.2022.102315. Supplementary material .

The utilization of abundantly available CO2 feedstock for the synthesis of high-value compounds and fuels is the main focus of catalysis research. Herein, we report the synthesis of different wt% Ir-Co3O4 oxide as a heterogeneous catalyst prepared by co-precipitation, followed by the hydrothermal method. The P-XRD analysis revealed the formation of a cubic phase with a high intense (3 1 1) plane with a 6 nm crystal size. The synthesized catalyst was used for direct hydrogenation of CO2 assisted by N,N,N',N' tetramethylenylenediamine. Under optimized reaction conditions, the 2.9 wt% Ir-Co3O4 catalyst demonstrated an excellent yield of formic acid (399 mmol). The formic acid yield in N,N,N',N; tetramethylenylenediamine is 25 times better than traditionally used trimethylamine. The outstanding performance of the Ir-Co3O4 catalyst was due to the stoichiometric amount of active Ir content uniformly distributed on Co3O4 support with a mixed Ir+3/4 oxidation state, which readily exchanges electrons between Co3O4 and Ir during the reaction. The catalyst was successfully recycled up to five times with negligible loss in the yield of formic acid.

Data will be made available on request.Climate change has emerged as one of the most pressing issues in politics and society as a whole. The emission of greenhouse gases, mostly CO2, has reached its highest level in human history and is a significant contributor to global warming [1,2]. A significant fraction of these emissions originate from the chemical industries which either depend directly on or utilize products from fossil fuels [3]. Moreover, demand for petrochemical products by the chemical industries is expected to increase in the coming years. However, some of these chemical intermediates can be produced by utilizing CO gas. For example, the hydrolysis of methyl formate to produce formic acid and the catalytic carbonylation of methanol to produce acetic acid are some of the vital chemical processes that utilize CO as a building block [4]. As a result, rather than using fossil fuels in these processes, green technologies such as solid oxide electrolysis cells (SOECs) could be utilized to produce CO through CO2 electrolysis. CO2 gases from different industrial outputs could be electrochemically reduced to CO thereby reducing the overall industrial carbon emission [5,6].Different competing green technologies are currently being investigated for CO production. Among the electrochemical CO2 reduction possibilities, three technologies easily stand out; low temperature electrolysis, molten carbonate electrolysis and the SOECs. Of the three technologies, the SOEC is the most advanced technology for CO2 electrolysis with years of operational hours [4]. In low temperature electrolysis, achieving high selectivity towards CO production is a non-trivial issue. Consequently, scarce catalysts such as IrO2 and noble cathodes (Au and Ag) must be utilized. On the other hand, in molten carbonate electrochemical cell, the rapid corrosion of the electrolysis cell container remains a major challenge [4,7,8]. The SOEC technology, however, presents higher performance efficiencies at an industrially relevant scale. Therefore, with respect to the different indices of comparison such as the operating efficiency, faradaic efficiency, cell voltage, and area-specific resistance, the SOEC outperforms the molten carbonate and the low temperature electrolysis modes [4,7,8].Despite the high efficiencies in SOECs, significant degradation, especially on the fuel electrode, has been observed. For example, considering the state-of-the-art Ni-YSZ fuel electrode materials, significant electrode degradation due to Ni migration and agglomeration has been reported [9–13]. Furthermore, noticeable degradation due to carbon deposition is observed when carbon-containing fuels are utilized [14–16]. Vanesa et al. [14] have investigated the formation of carbon on a Ni-YSZ electrode operating in fuel cell mode with a fuel mixture of 75% CO and 25% H2 at 1073 K. They reported a pronounced carbon deposition on the Ni-YSZ electrode, resulting in an increase in polarization resistance, decrease in porosity and deactivation of the electrochemical activity. Similarly, He et al. [15] have investigated the extent of carbon formation on a Ni-YSZ pellet after four hours of exposure to humidified methane fuel gas at a temperature range of 773–1073 K. They observed extensive carbon formation on the Ni-YSZ pellet. The deposited carbon was observed to dissolve into the bulk of the Ni particles leading to significant expansion of the Ni-YSZ pellet. Such expansion could lead to delamination and deactivation of the electrode in single cells. Yuefeng et al. [17] demonstrated that the Ni-YSZ fuel electrode is deactivated in pure CO2 electrolysis at 0.9 V and 700 °C. However, a better stable performance was observed at higher operating voltage of 1.3 V.These issues, i.e carbon deposition and electrode deactivation, could be minimized or even resolved by using electrode materials that have mixed ionic and electronic conducting (MIEC) properties as well as suppress carbon deposition. The MIEC properties ensure that the electrochemical reactions extend beyond the three-phase boundary. Therefore, for these reasons, attention has shifted to ceria containing electrodes [7,15,18]. Under a reducing atmosphere, doped ceria oxide has MIEC properties, exhibiting a mixed-valence of Ce3+ and Ce4+. As a consequence, the electrochemical reaction zone is extended from the three-phase boundary to the entire electrode surface. Furthermore, the electronic conductivity of ceria can partially compensate for Ni agglomeration and depletion thereby reducing the effect on performance [7]. With regards to carbon deposition, numerous works in literature have shown that the mixing of fuel electrode cermet with ceria reduces the amount of carbon deposition on the electrode [15,18–20]. For instance, He et al. [15] compared the amount of carbon deposition on Ni-YSZ pellet, with and without a ceria catalyst and observed less carbon on the pellet with a ceria catalyst than without a ceria catalyst. In general, the presence of localized electrons and oxygen vacancies in ceria electrodes has been observed to play a non-trivial role in preventing carbon deposition during CO2 reduction [7,18–20]. In line with this, complete replacement of the YSZ oxide phase (in Ni-YSZ) with the GDC oxide phase (Ni-GDC) is being pursued.In this work, the high temperature CO2 electrolysis on solid oxide cells (SOC) consisting of a Ni-GDC fuel electrode is examined in detail. The electrochemical activity of the electrode is investigated by using electrochemical impedance spectroscopy (EIS) at different operating conditions. Impedance measurements were obtained at different compositions of CO2 and CO, as well as at different temperatures (750–900 °C). Furthermore, a long-term stability test was performed to study the performance and durability of the electrode during operation. Finally, post-test analysis was carried out in order to understand the degradation behavior.For the electrochemical measurements, electrolyte-supported single cells were fabricated. The fuel electrode is made of commercial NiO-Ce0.9Gd0.1O0.95 (GDC) powder from Marion Technologies (NiO: GDC, 65:35 wt ratio), while the LSCF (La0.58Sr0.4Co0.2Fe0.8O3-δ) oxygen electrode powder was self-synthesized using a modified Pecheni method [21]. To prepare the electrode paste, NiO-GDC powder was mixed in a binder solution comprising 6 wt% ethyl cellulose (binder) dissolved in α-terpineol (dispersant). The slurry was then mixed using a planetary vacuum mixer (THINKY Mixer ARV-310) and subsequently homogenized for about 30 min by roll milling. A similar procedure was used to create the LSCF oxygen electrode slurry. Dense 8YSZ electrolyte supports from Kerafol® (d=20 mm, thickness 250 µm) were used to create the button cells. A thin layer (4–5 µm) of GDC was screen printed (EKRA screen printing Technologies) on one side of 8YSZ substrates and sintered at 1350 °C for 1 h under air to form a barrier layer for the oxygen electrode. After that, the fuel electrodes (15–18 µm) were screen printed on the opposite side of the electrolyte. Five different sintering temperatures were considered: 1150, 1200, 1250, 1300, and 1350 °C for 2 h at a heating rate of 2 °C‧min−1. Based on the polarization resistance, 1200 °C for 2 h was chosen as an optimized sintering condition. The LSCF layer was screen printed on the GDC barrier layer side and subsequently sintered at 1080 °C for 3 h. Finally, a NiO layer screen printed on the fuel electrode side was used as a current collector. The single-cell configuration before reduction is represented by NiO-GDC/8YSZ/GDC/LSCF. Following the same procedure, NiO-YSZ electrode was also fabricated and sintered at 1350 °C for 4 h, which is the optimized sintering condition for this electrode.For the electrochemical measurement, a two-electrode (four-wire) NorEcs Probostat™ set-up was used in the characterization of the single cells [22]. The cell was heated up to 900 °C (with 1 °C‧min−1), after which the nickel oxide cermet (NiO-GDC) was gradually reduced to nickel (Ni-GDC) as described by Foit et al. [23]. Following the reduction, IviumStat (Ivium Technologies) potentiostat/galvanostat devices were used to acquire the impedance spectra as well as the current density-voltage characteristics. The frequency range during the impedance measurement was varied from 110 kHz to 0.11 Hz with an AC amplitude of 50 mV and 21 frequencies per decade. Similarly, the current density-voltage (I-V) characteristics were obtained as previously described [23]. The quality of the impedance spectra was analyzed and validated through the Kramers Kronig transformation test [24]. Impedance measurements were taken at OCV under various temperature ranges (750–900 °C) and CO2 partial pressures. Long-term stability tests of the button cells were carried out at 900 °C with a current density of − 0.5 A‧cm2 for 1070 h. Impedance spectra were analyzed with both the complex non-linear least-square (NLLS) method as well as the distribution of relaxation times (DRT) transformation. A commercially available NLLS-fit program (RelaxIS® software, RHD-Instruments) was utilized in the fitting procedure and the DRT transformations.The morphology of the cells was examined with Quanta FEG 650 (FEI©) scanning electron microscope.Single cells of NiO-GDC were fabricated and analyzed based on their sintering temperature. Five different sintering temperatures were considered; 1150, 1200, 1250, 1300 and 1350 °C for 2 h. However, a tape test was performed on the electrodes showed that the electrodes sintered at 1150 °C showed poor adhesion to the electrolyte, hence it was not considered for the rest of the measurement. Fig. 1a-d shows the SEM images of the cell before the reduction process. The SEM images clearly show an increase in particle agglomeration with the increase in sintering temperature. The cell sintered at 1350 °C exhibits the most pronounced particle agglomeration while the 1200 °C sintered cell shows the least particle growth. Fig. 1e shows the impedance spectra of the cells obtained at 900 °C under OCV conditions. A decrease in polarization resistance (Rp) with decreasing sintering temperature is observed and the lowest Rp is observed for the cell sintered at 1200 °C. The result agrees with the microstructural observation from the SEM. An increase in particle agglomeration observed at higher sintering temperatures results in a decrease in the electrode surface area and thus, a decrease in the electrochemical reaction zone. Consequently, an increase in Rp is observed with increased particle agglomeration [25]. Optimized cells, sintered at 1200 °C were further used for the electrochemical characterization and the long-term degradation test.To characterize the cell performance, I-V characteristics as well as impedance measurements were compared to those of conventional Ni-YSZ cells. Fig. 2a compares the I-V characteristics of electrolyte-supported single cells containing Ni-GDC and Ni-YSZ fuel electrodes, respectively. Both single cells were prepared using 8YSZ electrolyte support with an LSCF oxygen electrode and measured in the same test rig. It can be seen that the Ni-GDC electrode containing single cell exhibits a higher current density of − 1.16 A cm−2 compared to the Ni-YSZ cell (−0.63 A cm−2) at 1.5 V and 900 °C. Fig. 2b shows the Nyquist plots obtained from the Ni-GDC cell in comparison to that of the Ni-YSZ cell. For the Ni-GDC cell, a lower Rp value of 0.23 Ω.cm2 is observed compared to the 0.44 Ω.cm2 observed for the Ni-YSZ cell at 900 °C. The higher performance of the Ni-GDC could be attributed to the enhanced electrochemical properties of the GDC as a result of the MIEC property [26–29].Further I-V measurements were obtained for the Ni-GDC cell under varying operating temperatures. Fig. 2c shows the I-V characteristics of the cell as a function of operating temperature (750–900 °C). The current density increases with an increase in temperature. Such a trend is expected due to the enhancement of electrochemical kinetics at higher operating temperatures. A maximum current density of − 1.16 A cm−2 is observed at 1.5 V and 900 °C. The continuity of the I-V curves across the OCV indicates that the Ni-GDC fuel electrodes can function as reversible SOCs [13]. In most of the cell measurements, the observed open circuit voltage was within 10 mV of the theoretical open circuit voltage according to the Nernst equation, which indicates sufficient cell sealing.To investigate thermally activated processes, impedance spectra were obtained and analyzed at different temperatures from 750° to 900°C in both OCV conditions as well as under polarization. Fig. 3a illustrates the Nyquist plots as a function of temperature at OCV. Two distinct arcs are easily identified in the impedance spectra; a low and a high frequency arc. While the low frequency arc is relatively unchanged with temperature variation, the high frequency arc shows a pronounced increase in magnitude with the decrease in temperature. This suggests that the high frequency electrochemical processes are thermally activated processes. Similar trend was also recorded under polarization (as shown in supplementary Fig. S1). In general, the decrease in temperature, from 900° to 750°C, caused an increase in the real and imaginary contributions in the Nyquist diagram of the impedance spectra. Such an increase is attributed to the reduction of ionic conductivity and electrochemical reaction kinetics in the electrodes with decreased temperature. The ohmic resistance (Rs) of the cell is determined from the intercept with the real axis at the high frequency in the Nyquist plot. Consequently, the activation enthalpy of the ohmic resistance is calculated from the slope of the Arrhenius equation as represented in Eq. (1) and illustrated in Fig. 3b. The determined value of 61 kJ/mol is consistent with the values of ionic conductivity of the 8YSZ electrolyte [30]. (1) ln R = − ln σ 0 + E A R g T The impedance spectra were analyzed with both the DRT transformation and the NLLS method. Fig. 4a shows the DRT representation of the impedance spectra as a function of temperature. Five peaks (P1, P2, P3, P4 and P4a) are observed in the DRT plot within the measured frequency range of 0.11 Hz to 110 kHz. The impedance spectra, however, were modeled with an equivalent circuit consisting of four time constants in series to a resistor and an inductor (LR-RQ-RQ2-RQ3-Ws) as shown in Fig. 4b. Furthermore, a comparison was made between the simulation of the fit and measured data. The comparison shows good agreement between the DRT of the proposed ECM and the DRT of the measured data (Supplementary Fig. S3). Also, the error plot showed non-systematic distribution around the frequency axis, which indicates that the proposed ECM can effectively reproduce the obtained impedance data across the measured frequency range (Supplementary Fig. S3).The time constants RQ1, RQ2 and RQ3 correspond to the processes P1, P2 and P3 on the DRT plot respectively. While the P4 peak corresponds to the infinite length Warburg short element (Ws) with the P4a peak interpreted as the satellite peak of the Ws [31,32]. The DRT reveals a significant dependence of the high/mid frequency peaks on temperature variation. P1, P2 and P3 exhibit an increase in magnitude with the decrease in temperature. On the other hand, P4 is almost independent of temperature variation. The measurements indicate that the mid frequency process (P3) dominates the electrode process at the lower temperature of 750 °C.The absolute values of the resistances were obtained from the NLLS fitting of the impedance spectra with the equivalent circuit model depicted in Fig. 4b. Fig. 4c illustrates the Arrhenius plot of the determined resistances. The resistances R1, R2 and R3 corresponding to processes P1, P2 and P3 respectively, exhibit a significant increase with a decrease in operating temperature, while Ws is relatively unchanged with the decrease in temperature. The trend shows good agreement with the observed DRT plot. R1 and R3 exhibit high activation energies of 109 ± 10 kJ mol−1 and 99 ± 2 kJ mol−1, respectively, while R2 shows an activation energy of 77 ± 10 kJ mol−1.Considering the electrochemical processes occurring in the electrode, different electrode reaction steps exhibit different temperature dependencies and these dependencies may identify the possible electrode process. For instance, gas diffusion processes exhibit an almost independent temperature dependency while charge transfer processes and processes from the transfer of ionic species show strong temperature dependency and high activation energies [33–36].Measurements under different compositions of the CO2 fuel gas were performed to further investigate the fuel electrode processes. The CO2/CO ratio was systematically changed from 90/10–50/50. The impedance spectra as well as current-voltage characteristics were obtained as a function of CO2/CO ratio. Fig. 5a illustrates the Nyquist plots obtained from the variation of CO2 content at OCV. An increase in the amount of CO2 in the fuel gas led to an increase in Rp. The mid and low frequency arcs exhibit a more pronounced dependence, increasing in magnitude with increasing CO2 content. This observation is contrary to what is expected when fuel gas is increased. In fact, in CO2 electrolysis mode at OCV conditions, there is lower electrochemical activity towards CO2 reduction than CO oxidation, hence an increase in Rp is observed with increasing CO2 content. Such observation could be attributed to preferential adsorption or higher activation energy of CO2 desorption on the active catalyst sites of the oxide phase and Ni metal [23,37]. This result is in agreement with a similar experiment by Foit et al. [23] on CO2 electrolysis in Ni-YSZ. They observed that the increase in CO2 content in the fuel gas composition led to an increase in ASR at OCV conditions. However, at higher current densities, increasing the CO2 gas compositions resulted in a decrease in ASR. They opined that at higher current densities, mass transport limitation dominates the overall reaction rate. Hence, decreasing the CO2 content resulted in a lesser amount of fuel gas for reaction leading to a decrease in the electrochemical reaction.The corresponding DRT plots of the impedance spectra are represented in Fig. 5b. Similar to the trend in the Nyquist plot, the low/mid frequency P4 and P3 peaks exhibit pronounced dependence on CO2 variation while P1 and P2 peaks are relatively constant. This suggests that the underlying contributing processes of P4 and P3 peaks are most likely fuel electrode processes while P1 and P2 could be oxygen electrode contributions. Furthermore, the P4 and P3 peaks are observed to exhibit the highest contribution to the electrochemical impedance. Fig. 5c illustrates the obtained resistance from the fitting of the impedance spectra using the equivalent circuit model. The trend is in agreement with the observation in the DRT plot, wherein Ws and R3 resistances show a significant increase with an increase in CO2 content. The significant CO2 content dependence of the low frequency P4 peak as well as the observed independent temperature behavior indicates that this process is most likely a diffusion process [34,35,38]. However, the GDC cermets have been reported to show a low-frequency peak resulting from the chemical capacitance caused by the variation in the oxygen nonstoichiometry of the GDC electrode [38,39]. Therefore, the low frequency P4 peak is attributed mostly to a gas diffusion process and possible contribution from the oxygen nonstoichiometry of the GDC electrode.Considering the frequency regime of the P3 process, reactions at the electrode/electrolyte interface can be ruled out since these processes typically exhibit high relaxation frequencies [33,40–42]. With a frequency range between 40 Hz and 250 Hz, this suggests a process towards the electrode sub-surface. Such mid frequency process could be attributed to gas-solid interaction such as adsorption, dissociation and desorption of the gas species [38]. Several authors [7,18,19] have attempted to suggest possible elementary mechanisms of CO2 reduction on ceria containing cermets. Chueh et al. [18] investigated the surface electrochemistry of CO2 reduction and CO oxidation on a ceria cermet. They opined that the electrochemical reduction of CO2 to CO occurs via two single-electron transfer steps, with carbonate ((CO3)2-) formation as an intermediate process. The formed carbonate further absorbs and saturates the electrode surfaces thereby reducing the overall kinetics of CO2 reduction. In general, the carbonate adsorption process is regarded as a major rate-determining step in CO2 reduction [7,18,19]. Therefore, following the variation of temperature and CO2 content, the P3 process has shown to exhibit the highest resistance and hence the rate-determining step in the CO2 electrochemical reduction. Therefore, this process could be inferred to be related to an adsorption process. The mid frequency P3 peak is therefore attributed to a possible surface electrode reaction process (adsorption/desorption) of the gas species in addition to a charge transfer process on the electrode surface.To investigate oxygen electrode processes, measurements under different partial pressures of oxygen (pO2) were performed from 0.1 to 1 atm. The obtained impedance spectra were analyzed with both the DRT method and ECM fitting. Fig. 6a and b represent the DRT transformation and the ECM fitting of the impedance spectra as a function of pO2 respectively. The P4 peak is independent of pO2, while P3, P2 and P1 show very slight changes with pO2 variation. From the DRT plot alone, it is arguable to attribute some of the peaks to the oxygen electrode process alone. However, this could indicate that the contribution of the oxygen electrode is minimal. The fitting results (Fig. 6b) of the impedance spectra with the equivalent circuit model showed that R1 (representing P1 peak) and R2 (P2 peak) resistances are mostly, contributions from oxygen electrode processes. However, following the inconsistency between the DRT representation and the NLLS fitting of the impedance, a further experiment was necessary to clarify the impedance contribution from the oxygen electrode. For this, impedance measurement was performed on a symmetrical half-cell containing LSCF electrodes (in two-electrode measurements). The obtained DRT was compared in Fig. 6a. The comparison shows that the P1 and P2 peaks are essentially oxygen electrode processes while P3 and P4 are mainly fuel electrode resistance contributions with slight contributions from the oxygen electrode. This is in agreement with the observed trend in the NLLS fitting. However, the focus of the current study is on the fuel electrode processes, hence the electrochemical processes of the oxygen electrode were inferred from the numerous literature on LSCF electrodes [40–43]. Overall, the obtained DRT representation of the LSCF impedance spectra (with their corresponding frequency range) for symmetrical half-cell is in good agreement with literature observation. [40,42]. Chen et [42] al. investigated the performance of LSCF symmetrical cells with different fabrication methods. From their analysis, they ascribed the low frequency peak between 1 and 10 Hz to gas diffusion process, the mid frequency peak between 10 and 500 Hz to surface exchange and ion diffusion process (which corresponds to the P3 and P2 in this report) and lastly, a high frequency peak (around 1000 Hz) to charge transfer process across the interface. A similar result was also observed by Leonide et al. [40] in the impedance study of LSCF and LSF electrodes; a low frequency (0.3–10 Hz) gas diffusion process, a mid frequency (2–500 Hz) oxygen surface exchange process followed by oxide diffusion in the bulk of the electrode and lastly a high frequency charge transfer process were observed. Therefore, following the high activation energy of R1 (as shown in Fig. 4c) as well as the high frequency range of the P1 peak (similar to ref [40,42]), there is no doubt that this is in good agreement and coincides with the charge transfer process of the oxygen electrode. Table 1 summarizes the possible electrochemical process contribution related to the individual polarization resistances.Long term stability test were performed to investigate the performance stability of the cells during long operating times. The measurement was performed at 900 °C at a current density of − 0.5 A‧cm−2 for up to 1070 h on two different cells with a fuel gas composition consisting of 80% CO2 and 20% CO. Fig. 7a illustrates the degradation rate of the cell, represented as an increase in cell voltage as a function of time. The cell degradation was obtained by evaluating the slope of the curve. A degradation rate of 31 mV‧kh−1 could be determined. In another cell, the evolution of the degradation mechanism was investigated during the long-term stability test by performing impedance measurements at OCV every 100 h. Fig. 7b and c illustrate the evolution of the impedance spectra as well as the ohmic (Rs) and polarization (Rp) resistances respectively as a function of operation time. The Rs increased from 0.48 to 0.53 Ω‧cm2 while the Rp increased from 0.28 to 0.35 Ω‧cm2 after the degradation test. It is known that the loss of electrode contact surface with the current collector could also lead to an increase in the ohmic resistance and thus an increase in degradation rate. Such ohmic resistance increase due to loss of contact surface is usually accompanied by a proportional increase in the Rp. However, in our case, the increase in Rp is higher and thus disproportionate to the ohmic resistance increase, indicating that ohmic resistance is most likely, not due to contact loss. Fig. 7d illustrates the equivalent circuit analysis of the spectra which shows that the degradation behavior is dominated by the high/mid frequency processes of R1, R2 and R3 while Ws is relatively unaffected. This implies that both the oxygen electrode and the fuel electrode contributed to the degradation mechanism.Post-test analysis was performed on the measured cell to investigate the morphology and the microstructure of the electrodes. The microstructure of the long-term measured cell was compared to that of a freshly reduced cell. Fig. 8a-c represents the fuel electrode of the reduced cell while Fig. 8d illustrates the microstructure of the corresponding oxygen electrode. Similarly, Fig. 8e-g shows the microstructure of the fuel electrode after long-term degradation test, while Fig. 8h represents the corresponding oxygen electrode. The secondary electron image of the electrode microstructure between Fig. 8a and e illustrates an increase in the Ni particle size after the degradation test. This indicates Ni particle agglomeration during CO2 electrolysis. The Ni agglomeration is further confirmed in the backscattered electron image of Fig. 8f-g when compared to Fig. 8b-c, where gray particles are Ni and white bright particles are GDC. In addition, Ni depletion and pore formation at the electrode/electrolyte interface are also visible after the long-term degradation test as compared to the reference cell (Fig. 8a,b and e,f). The observed microstructural changes would influence the measured impedance result in different ways. For example, the formation of a Ni-depleted layer as a result of Ni migration away from the electrolyte increases the electrolyte thickness thereby leading to an increase in ohmic resistance, as seen in Fig. 7b. Also, Ni agglomeration would effectively reduce the active surface area for electrochemical reactions, causing an increase in polarization resistance (Fig. 7c). Lastly, the continuous progression of these effects during cell operation would inevitably result in a decrease in cell performance over time due to an increase in area specific resistance. An extensive microstructural analysis of the electrodes, in relation to the observed electrochemical degradation processes, will be the objective of another paper.The impedance analysis also reveals that the high frequency processes, which are majorly oxygen electrode processes also contributed to the observed degradation. The secondary electron image comparison between Fig. 8d and h reveals no significant change in the LSCF microstructure. However, the mechanism of LSCF oxygen electrode degradation has been extensively studied [44–46]. One such mechanism is the formation of an insulating SrZrO3 layer due to the reaction between volatile SrO and the YSZ electrolyte [44,47]. To prevent this reaction, a GDC barrier layer between the electrolyte and the oxygen electrode is adopted. However, since the GDC barrier layer in Fig. 8d and h is not fully dense, this reaction cannot be entirely prevented. Monaco et al. [47] opined that the formation of an insulating SrZrO3 layer causes loss of Zr4+ in the 8YSZ electrolyte thereby reducing the ionic conductivity of the electrolyte and thus, leading to an increase in ohmic resistance of the cell. The increase in ohmic resistance entails a decrease in overall cell performance and thus, an increase in cell degradation [44–46].In this study, an electrolyte-supported single cell consisting of Ni-GDC fuel electrode and LSCF oxygen electrode was fabricated and analyzed under high temperature CO2 electrolysis conditions. Impedance measurements were carried out at different temperatures as well as under different CO2:CO fuel gas compositions and oxygen partial pressures. The obtained impedance spectra were evaluated with both DRT and NLLS fitting. Four time constants, representing four peaks in the DRT spectra, were used to fit the impedance spectra. The high frequency P1 peak corresponds to the oxygen electrode charge transfer process while the middle frequency processes of P2 and P3 consist of contribution from surface reaction processes from both the fuel and oxygen electrodes. Lastly, the low frequency process is assigned to the gas diffusion process in addition to the surface reaction in the fuel electrode. Long-term degradation analysis was performed at 900 °C and a current density of − 0.5 A‧cm−2. The cell shows a low degradation rate of 31 mV‧kh−1 during 1070 h of operation. Furthermore, analysis of the degradation mechanism showed that the high/mid frequency processes contributed more to the degradation rate. Microstructural evaluation of the measured cell with SEM revealed Ni particle agglomeration, increase in electrode porosity and Ni migration away from the electrolyte. Ifeanyichukwu D. Unachukwu: Methodology, Investigation, Formal analysis, Validation, Conceptualization, Data curation, Software, Visualization, Writing – original draft, Writing – review & editing. Vaibhav Vibhu: Methodology, Formal analysis, Validation, Conceptualization, Software, Supervision, Visualization, Writing – review & editing. Jan Uecker: Investigation, Formal analysis, Validation. Izaak C. Vinke: Methodology, Supervision, Validation, Project administration, Conceptualization, Resources, Software, Visualization. Rüdiger-A. Eichel: Supervision, Project administration, Resources. L.G.J. (Bert) de Haart: Methodology, Supervision, Validation, Project administration, Conceptualization, Resources, Software, Visualization.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 funding by the German Federal Ministry of Education and Research (BMBF) within the iNEW 2.0 Project: incubator sustainable and renewable value chains, under grant agreement number 03SF0627A.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jcou.2023.102423. Supplementary material .

The challenges of high degradation rate and significant carbon deposition, which are common with Ni-YSZ electrodes, have shifted attention to other electrode materials with enhanced performance in SOECs using carbon-containing fuels. In this study, the performance and electrochemical behavior of the Ni-GDC fuel electrode under CO2 electrolysis were investigated. The study was performed over a range of operating conditions, varying the operating temperature, the CO2 content of the fuel gas as well as the oxygen partial pressures in the oxygen electrode gas. Long-term stability test was performed up to 1070 h at 900 °C and a current density of − 0.5 A‧cm−2. The electrochemical impedance spectra obtained from the various measurement were evaluated with DRT as well as an equivalent circuit model consisting of 4 time-constant; (LR-RQ1-RQ2-RQ3-Ws). The low frequency Warburg (short) element (Ws) was attributed to gas diffusion and surface processes at the fuel electrode, the mid frequency processes of RQ2 and RQ3 are assigned to the combined contribution of fuel and oxygen electrode. The high frequency RQ1 was assigned to the charge transfer process at the oxygen electrode. A low degradation rate of 31 mV‧Kh−1 was observed during the long-term stability test. Furthermore, analysis of the degradation rate illustrates that significant contributions to the degradation were from the mid and high frequency processes, in addition to ohmic resistance. SEM analysis of the measured cell shows agglomeration of Ni particles, increase in electrode porosity as well as Ni migration away from the electrode/electrolyte interface.

Data will be made available on request.Metal–organic frameworks (MOFs) prepared from nitrogen-rich ligands are extremely versatile materials; over the last years, a great number of MOF materials with carboxyl and/or pyridine ligands have been designed and built [1]. Now, there is an increasing interest in using azolates as linkers to obtain MOFs, because result in strong metal-nitrogen bonds which endow high chemical and thermal stabilities to the frameworks [2–4]. N-based ligands include pyrazole, imidazole, triazole, and tetrazole that contain nitrogen atoms with Lewis basic activity which can act as coordinating and interactive sites to build MOFs. However, the tetrazolate ligands might be the most promising candidates to obtain versatile MOFs, due to their possibility of donor N sites to coordinate by different modes to the metal [1,5]. Tetrazole has similar pKa than carboxylate, which shows that the conditions to prepare the corresponding MOFs are similar. Tetrazole-based MOFs (Tz-MOFs) are robust and stable materials with good porosity, adequate topology, and adsorption characteristics comparable to those from carboxylate-based MOFs. Compared to bidentate carboxylate ligands, tetrazolate derivatives have strong coordination capabilities and different coordination modes. After deprotonation (partial or total), tetrazoles turn into azolate anions, which change the basicity characteristics of these linkers and therefore the coordination to metals [6]. In view of the good properties as stability and porosity, tetrazole compounds have applications in gas adsorption [7–10], magnetism [11,12], sensing [13,14], potential applications in optics [15–17] and are used to obtain energetic materials [18–21]. Additionally, in medicinal chemistry tetrazole compounds can be substituted for the carboxylate functions [2]. In general, MOFs with carboxylate ligands have been widely explored as catalysts [22–26], while MOFs prepared from azolate linkers and low-valent metals (Co2+, Ni2+, Zn2+, etc) have been less studied [27–30]. Thus, it is worth to synthesize tetrazole derivatives and exploring their potential applications.One of the most important current problems is the large emission of CO2. To mitigate this problem there are different strategies, being the valorization of CO2 to value-added products one of the most promising [31–33]. MOFs have been widely studied as heterogeneous catalysts for the addition of CO2 to epoxides to yield cyclic carbonates because of the Lewis or basic sites on their structures, thus, many MOFs are attractive due to the possibility to generate unsaturated metal sites by removing the coordinated solvent molecules. In particular, Co-MOFs (quite earth abundant and low-cost metal) have been used as catalysts for the oxidation of olefins due to their redox capability and as Lewis acid catalysts [34].One-pot cascade processes are greener, simpler and more efficient than general step by step reactions since it is not necessary to isolate or purify the intermediate products. However, there are a limited number of proper catalysts designed to be effective with a wide scope of substrates. Thus, the direct synthesis of cyclic carbonates from alkenes and CO2 is attracting growing importance [35–37].Based on the literature data and fascinating qualities of these ligands, we are interested in building new MOFs of cobalt from two interesting tetrazole-based ligands 2,6-di(1H-tetrazol-5-yl)naphthalene (H2NDTz) and 2,6-bis(4-(1H-tetrazol-5-yl)phenyl)naphthalene (H2NDPhTz) ( Fig. 1). Thus, besides the synthesis and characterization of the new MOFs, and due to coordinative unsaturated cobalt sites generated after thermal activation, we have demonstrated their catalytic efficiency in the epoxidation of alkenes, cycloaddition of CO2 to epoxides and in the synthesis of cyclic carbonates from styrene, through a one-pot tandem epoxidation-cycloaddition reaction. This tandem reaction has been rarely studied with Co-MOFs catalysts, thus one example has been reported so far [38].Details for the synthetic procedure of H2NDTz and H2NDPhTz ligands can be found in the supplementary information.A solution of CoCl26 H2O (22.5 mg, 0.12 mmol) in 0.5 mL of water was added to a 2.0 mL dimethylformamide (DMF) solution of H2NDTz (29.5 mg, 0.12 mmol) or H2NDPhTz (49.9 mg, 0.12 mmol). The 2.5 mL solution was poured into a 4 mL vial and heated at 90 °C for 1 d. Block-shaped pale-brown crystals were obtained and were washed using DMF, water and methanol and then dried in air to afford a total of 38.0 mg of a crystalline product in the case of Co-NDTz [Co3(NDTz)3(DMF)3(H2O)6] and 40 mg in the case of Co-NDPhTz [Co3(NDPhTz)3(DMF)3(H2O)6. Crystal data of both MOFs is provided in CIF format, accessible through CCDC numbers: 2180927, for Co-NDTz, and 2180928 for Co-NDPhTz.Co-NDTzs and Co-NDPhTz were thermally activated before catalytic experiments by heating them at 110 °C under vacuum for 10 h.In a typical reaction, styrene (4.8 mmol), t-butylhydroperoxide (TBHP, 5.5 M in decane, 7.2 mmol), activated Co-NDTz catalyst (0.2 mol% based on Co) were added into a Supelco glass microreactor (5 mL). The reaction mixture was heated at 50 °C and stirred. Aliquots were taken at different times and reaction evolution was followed by GC-MS. After reaction was complete, the catalyst was separated by centrifugation, thoroughly washed, dried under vacuum and after a new activation, it was reused.Epoxide (0.020 mol), biphenyl (internal standard (0.002 mol); nBu4NBr (0.06 mmol), and activated Co-NDTz catalyst (0.0114 mmol based on Co) were added into a picoclave Büchi reactor (10 mL). The reaction mixture was purged with CO2. Then, it was charged with 3 bar of CO2 and heated at 50 °C for 1–20 h. The catalyst was recovered by centrifugation and activated before reuse in a new cycle. Reaction products were monitored by GC-MS.Styrene (4.8 mmol), TBHP (5.5 M in decane, 7.2 mmol), nBu4NBr (0.018 mmol), and thermally activated Co-NDTz catalyst (0.2 mol% related to styrene) were introduced into a picoclave Büchi reactor (10 mL). The reaction mixture was purged several times with CO2, charged with 3 bar of CO2 and heated at 110 °C overnight. The catalyst was recovered by centrifugation, washed with acetone and activated again at 110 °C before the reuse in a new cycle. Reaction products were monitored by GC-MS.H2NDTz and H2NDPhTz ( Scheme 1) were prepared in two steps using 2-triflate,6-bromonaphthalene as common precursor. Thus, H2NDTz was synthesized according to a modified method to that reported [39,40]. It was obtained by reaction of brominated precursor with copper cyanide [41] and subsequent conversion of the cyano groups into the corresponding tetrazoles. H2NDPhTz was prepared by a Suzuki-Miyaura reaction between 2-triflate,6-bromonaphthalene and 4-cyanophenylboronic acid followed by reaction with sodium azide under the same conditions that the above ligand.Both ligands were obtained in good yields (>80 %) and the structures were confirmed by 1H NMR (Figs. S1 and S2) and elemental analyses (see supporting information).The synthesis of the cobalt-naphthalene tetrazole MOFs was carried out under solvothermal conditions. Co-NDTz was obtained by reaction of CoCl2.6 H2O with H2NDTz in a mixture DMF/H2O: 1/1 at 70–90 °C for 1 day. However, when ligand H2NDPhTz was employed, the corresponding Co-NDPhTz is only obtained when a mixture DMF/H2O: 1/10 ratio was used at 90 °C. Both materials have been fully characterized by elemental analysis, Fourier Transform Infrared Spectra (FT-IR), Thermogravimetric analysis (TGA), and powder X-ray diffraction (Fig. 1). FT-IR spectra show that the tetrazolate ligand is incorporated into the network with CN bands at ∼1600, 800 − 1300 cm−1; CO frequency due to DMF molecules appears at ∼1650 cm-1 (Fig. 1b).TGA under air atmosphere shows the decomposition patterns for both MOFs. Co-NDTz shows a degradation pattern in two steps, which occur at decomposition temperatures of around 300 and 380 °C respectively. However, Co-NDPhTz shows a unique degradation step with an initial decomposition temperature of around 340 °C. Besides, this thermogram shows a weight loss of around 100 °C attributed to solvent molecules entrapped within the network. From the residue obtained by TGA, assuming the formation of Co2O3, it was determined a Co content of 13.6 % for Co-NDTz and 10.8 % for Co-NDPhTz. These values and the N content obtained by elemental analysis showed us that both materials have a ratio ligand/Co: 1/1.The XPS survey spectra of Co-tetrazole-MOFs are shown in Figs. S3 and S4, indicating the presence of Co, N, C, and O elements in the materials. The Co 2p traces of both materials exhibit bands at ∼783.1, 783.5 and 798.3, 798.6 eV with a difference of ∼15.1 eV corresponding to Co2+ species 2p3/2 and Co 2p1/2 respectively; satellite peaks are also observed at 788.5 and 804.9 eV. XPS of C1s exhibited a predominant peak at 285.9, 286.4 eV (C-C, CN units) and N1s spectra showed a band at 401.5, 402.1 eV (CN) (Figs. S4 and S5) [42].The morphology of the new cobalt MOFs was examined by SEM (Fig. 1e). Co-NDTz showed elongated oval rough aggregates while Co-NDPhTz showed a plate-like morphology.It has not been possible to obtain single crystals of adequate size for the resolution of the structure by single-crystal X-ray diffraction, therefore, the structural elucidation was completed based on analysis of powder X-ray diffraction patterns (Fig. 1a) and computer modellization. Thus, the PXRD pattern of Co-NDTz was first indexed with a monoclinic primitive cell, with lattice parameters a = 13.00 Å, b = 14.95 Å, c = 6.48 Å, β = 92.16° (Table S1). A Pawley refinement was successfully completed, further supporting the feasibility of this cell. The short value of the c parameter strongly suggests the formation of a MOF with a rod-shaped secondary building unit (SBU), similar to previously reported structures including related pyrazole [43,44] or tetrazole [45,46] based linkers. Electron density maps were generated by applying the charge flipping method to the integrated intensities from the PXRD pattern, and used to build a crystal model in the P21 /c space group, consisting of inorganic SBUs running along the crystallographic c axis, formed by cobalt atoms coordinated to bridging tetrazole rings ( Fig. 2a). The model was geometrically optimized with force-field based energy minimization procedures coupled with Rietveld refinement cycles, using Biovia Materials Studio Software package [47]. In the refined structure, there is one crystallographically independent cobalt atom coordinated to four nitrogen atoms from the tetrazolate linkers, and to two oxygen atoms from additional water ligands that complete the octahedral environment of the metal centers. The tetrazolate rings are coordinating to the metal atoms through the nitrogen atoms at 2, and 3 positions, creating short bridges that extend the rod-shaped SBU, which are then connected through the organic linkers to produce a 3D framework (Fig. 2b). Based on this structure, an isoreticular crystal model was built up for the extended Co-NDPhTz MOF (Fig. 2c). The corresponding lattice parameters obtained after a Pawley refinement are a = 22.77 Å, b = 15.89 Å, c = 7.26 Å, β = 90.06° and the geometry optimization of the structure was therefore completed. We noticed that the relative intensities of some peaks in the experimental pattern are lower than the calculated one, which is possibly due to preferred orientation effects, expected for the plate like morphology of the crystals (Fig. 1e), and the diffraction data acquisition in reflexion geometry (Figs. S5 and S6).Nitrogen gas adsorption isotherms were measured after the evacuation at 373 K overnight to remove solvent molecules (under these conditions, PXRD pattern is maintained). The surface area calculated using Brunauer-Emmett-Teller (BET) method from the nitrogen gas adsorption data result in 43.0 m2g−1 for Co-NDTz (Fig. S7) and 4.47 m2g−1 for Co-NDPhTz. However, the CO2 sorption measured at 273 K (Fig. 2d) reveals for case of Co-NDTz a Dubinin-Astakhov [48] CO2 specific surface area of 636 m2g−1 and a CO2 uptake of 2.35 mmol·g-1; whereas for phenyl extended MOF, Co-NDPhTz, the Dubinin-Astakhov CO2 specific surface area was 308 m2.g-1 and the CO2 uptake of 1.31 mmol·g-1; which indicates a lower CO2 accessibility to the tetrazole group in Co-NDPhTz. This result suggests a blockage or collapse of the structure which could be attributed to the length and freedom of rotation of the bonds that make up this linker.The different N2/CO2 adsorption was recently observed for a different type of cobalt MOF, which exhibited a BET surface area of only 6.8 m².g−1 but a CO2 uptake of 2.26 mmol.g-1, attributed in this case to the interaction of the amide functional group of the framework with the polar CO2 [49]. Most of the cobalt MOFs reported exhibit CO2 uptake between 1.0 and 3.0 mmol.g-1 [50,51]. In our case, it is the presence of tetrazole groups, along with the rigidity of the linker NDTz, that made Co-NDTz exhibit a high CO2 uptake capacity. It is known that strong dipole-dipole and acid-base interactions are present between protonated and deprotonated forms of tetrazole ring and CO2 carbon dioxide [52].MOFs have been evaluated as heterogeneous catalysts for different types of reactions being effective and selective due to their properties as porosity (high surface area or the important number of active sites [53]. It is also known that for a MOF to be used as a heterogeneous catalyst, it is necessary that there exist coordinative unsaturated sites (CUSs) [54], and considering that the Co-MOFs, herein reported, have free and coordinated solvent molecules, the Co-MOFs should be thermal activated at 110 °C under reduced pressure for 4 h before each reaction. The PXRD pattern shows that activated Tz-MOFs maintain the crystallinity (Fig. S8). Encouraged by the availability of both Lewis acidic and potentially redox-active Co sites in thermally activated Co-NDTzs, we have investigated its catalytic performance in the tandem synthesis of cyclic carbonates from olefins and carbon dioxide. Before carrying out the tandem reaction, a preliminary evaluation of the novel catalysts in the separated reactions was carried out. Both, the epoxidation of olefins and the coupling of CO2 with epoxides to obtain cyclic carbonates were evaluated individually.The possibilities of MOFs as oxidation catalysts have been reported by different authors [55], now we have evaluated Co-NDTzs in olefin epoxidation and the results are presented in Table 1. The epoxidation reaction was initially examined under solvent-free conditions using styrene as substrate and either hydrogen peroxide or oxygen (4 bar) as oxidants, along with activated Co-NDTz (0.2 % mol), at 90 °C. Under these conditions, no conversion was achieved. When a solution of TBHP in decane (5.5 M), was employed as an oxidant, the styrene oxide was selectively obtained after 14 h with excellent yield (>99 %) (entry 1). Control experiments revealed the synergic effect between the oxidant and the MOF, since only 4 % of styrene oxide was obtained in absence of catalyst (entry 2). Co-NDPhTz seems to be less active than Co-NDTz (entry 3) since only 14 % conversion was observed. To explore the versatility of the catalyst in the epoxidation reactions, an internal alkene as cyclooctene was also oxidized, however only 34 % (64 % at 110 °C) of the epoxide is observed after 24 h of reaction (entries 4–5). Some examples of the CG-chromatograms obtained in this reaction are collected in the ESI.The removal of the catalyst by filtration after 4 h stopped the reaction, and the filtrate afforded nearly no additional conversion after stirring for another 8 h (Fig. S8b). These observations suggest that the catalyst is a true heterogeneous catalyst. Solids could be isolated from the reaction suspension by simple filtration. Recovered Co-NDTz was reused five times and, no significant loss of catalytic performance was observed (Fig. S9a). The structural integrity was verified by PXRD (Fig. S8).The cycloaddition of CO2 to epoxides to obtain cyclic carbonates is the second reaction explored with these catalysts.The reaction conditions were first optimized using a Picoclave Büchi glass reactor of 10 mL, Co-NDTz as catalysts and epichlorohydrin (ECH) as substrate ( Fig. 3), due to its reactivity and interest, because it can be produced from glycerine obtained from vegetable oil [56]. To optimize the reaction conditions, those reported so far for different Co-MOFs used as catalysts to promote this reaction, were tested [49,57–59]. Thus temperatures between 40 and 100 °C, CO2 pressures between 1 and 8 bar and different amounts of TBAB (from 2.5 to 10 mol %) were initially tested. From the above experiments, the most favorable conditions for this reaction with Co-NDTz as a catalyst were a temperature of 50 °C and 3 bar of CO2, but using a much lower amount of TBAB (0.3 mol %) ( Table 2). Moreover, with an ECH:Co ratio of 1750:1 a good conversion of ECH was achieved in 90 min (entry 1) and the corresponding cyclic carbonate was selectively obtained. It should be noted that the ECH:Co ratio used was also much higher than any of those reported so far for Co-carboxylate-MOFs, that do not exceed the ratio ECH:Co of 1000:1 [57–59].Using the same reactor, some other epoxides were used to evaluate the catalyst (Fig. 3). When a bulkier and less reactive epoxide such as styrene oxide (SO) was used (entry 2) a good conversion was obtained (65 %) taking into account the time employed (1.5 h). Usually, a good conversion of this epoxide requires longer reaction times (9–12 h) [57–59]. The same conversion was achieved with 1,2-epoxyhexane (HE) although it took 7 h of reaction (entry 3).Finally, an internal epoxide such as cyclohexene oxide (CHO) (entry 3) was evaluated. After 20 h of reaction, only 26 % of epoxide conversion was achieved. In order to check if the structure of the catalyst had an influence on this result, the reaction was carried out using Co-NDPhTz as catalysts (entry 5). In the same conditions, a slight increase in the conversion was obtained which indicated a great difficulty in opening this epoxide probably caused by the steric hindrance between the two rings as was observed in previous works [60,61].Two control experiments were carried out, using only TBAB to promote the reaction (entry 6) or using Co-NDTz in absence of TBAB (entry 7) in the same conditions as the above experiments and using ECH as substrate. In both cases, a lower conversion of ECH was observed after 90 min of reaction, which confirms the cooperative effect between Co-NDTz and TBAB.A scale-up of the reaction was carried out using ECH as substrate with a reactor of higher capacity that allowed the use of 6 times more epoxide than in the previous studies. For this experiment, the temperature was also of 50 °C, but a continuous pressure of CO2 at 3 bar was maintained and a lower amount of catalysts was employed, ECH:Co ratio of 3500:1. In these conditions (entry 8) a 63 % of ECH conversion was attained which involved a TON of 2205. The catalyst was removed by centrifugation, washed with methanol three times, and then, it was dried overnight at 110 °C under vacuum. The catalyst was used in a new reaction to investigate its recyclability. Thus, in a second run using fresh ECH and the same reaction conditions, a slightly lower ECH conversion was obtained although the corresponding cyclic carbonate was obtained as a unique product indicating a > 99 % of selectivity after the first recycling. Similar ECH conversions were obtained in the next runs (up to six runs) which confirms the good recyclability of this catalyst even using a big amount of substrate (Fig. S10a). CG-chromatograms of the fifth run are collected in the ESI.The catalyst was also removed by filtration after 30 min of reaction and no additional conversion after stirring for another 90 min was observed (Fig. S10b) confirming again the true heterogeneous character of this catalyst.The reaction was performed with the extended catalyst (Co-NDPhTz) (entry 9) obtaining also a good ECH conversion and excellent TON value.MOFs have been also evaluated as heterogeneous catalysts for cascade reactions by combining acid-base character, redox properties, and metal–organic nodes [62]. Because activated Co-NDTz demonstrated its ability as an effective catalyst in the epoxidation of olefins as well as in the cycloaddition of CO2 to epoxides, we were encouraged to study the one-pot oxidative carboxylation of styrene with carbon dioxide to obtain cyclic carbonates. Thus, activated catalyst, styrene, TBAB (as co-catalyst necessary to open the ring epoxide), TBHP (as an oxidant), and CO2 were introduced in a Picoclave Büchi glass reactor, when the reaction was performed at 50 °C, only the corresponding epoxide was obtained ( Table 3, entry 1); if the reaction mixture was heated to 110 °C, the epoxidation and the CO2 coupling could be completed with a single workup stage (Table 3, entry 2) The kinetic profile of this control reaction is shown in Fig. 4. The reaction performed with Co-NDPhTz yields selectively the epoxide and only traces of carbonate were detected.It has been reported that in the plausible mechanism for this tandem reaction, tert-butanol (TBOH) derived from TBHP might coordinate to the metal at the apical positions and inhibit the carboxylation step [63]. This is in agreement with our experimental observation, since it is required to thermally activate the MOF before using it to afford CUS, otherwise no conversion is observed in any reaction. Thus, when the reaction temperature is increased to 110 °C, the tert-butanol is thermally and in situ decoordinated, and the second step is enabled. To confirm this fact, the CO2 cycloaddition of epoxides has been performed in presence of tert-butanol under the standard conditions described in the above section (Table 2). It was observed that even after 5 h at 50 °C no conversion was observed, however, the same batch was subsequently heated to 110 °C and the carbonate was obtained in 97 % yield after 10 h Some examples of this CG-chromatograms are collected in the ESI.Based on the above mentioned, a possible mechanism for the tandem epoxidation-carboxylation reaction from styrene and CO2 is shown in Scheme 2. More into details: firstly, the reaction of unsaturated Co(II) sites with TBHP to form a Co(III)-peroxy complex that restores the Co(II) sites by releasing a t-butoxy radical. This radical reacts with styrene to generate a tert-butoxperoxy derivate that yields the styrene oxide and t-butanol which is coordinated to the cobalt. After activation at 110 °C, it is uncoordinated and the epoxide is activated by coordination to the cobalt active center. Then, the less hindered carbon of the epoxide is attacked by bromide from the TBAB, followed by the insertion of carbon dioxide into the Co-oxygen bond giving a cobalt-carbonate intermediate. The cyclic carbonate is obtained by a closure intramolecular of the cycle accompanied simultaneously by the regeneration of both Co-MOF and co-catalyst.The catalyst could be reused at least five times with moderate loss of activity (from 78 % to 70 % yield), although longer reaction times are needed (72 h) to achieve the same conversion (Table 3, Fig. S11). PXRD patterns, SEM images and FT-IR of the Co-MOFs recovered from the tandem reaction, revealed that the structures were mainly maintained in both cases (Figs. S8, S12-S13).The tandem epoxidation-carboxylation reaction has been studied with different types of catalysts, a comparison with recently reported metal-based catalysts along with some recent reviews can be found in Table S3. Higher CO2 pressures are usually needed for this transformation (from 5 to 100 in one case), and little attention has been paid to the recyclability of the system. For example, similar conditions were employed by Zhang and coworkers [64], with Ce-based MOF, and Lin and coworkers, with a Mn/Hf-MOF [65], in both the catalytic system was recycled 3 times. Besides, metal loadings are much higher compared to our system which only requires 0.2 mol %.We have obtained two new porous tetrazole-based MOFs: Co-NDTz and Co-NDPhTz using 2,6-di(1H-tetrazol-5-yl)naphthalene (H2NDTz) and extended 2,6-bis(4-(1H-tetrazol-5-yl)phenyl)naphthalene (H2NPhDTz) as linkers under solvothermal conditions. Based on the structural analysis, we can say that they both are isoreticular MOFs. After thermal activation, both compounds have a great number of coordinative unsaturated and redox Co(II) sites and proved to be excellent catalysts for the selective epoxidation of alkenes and in the CO2 cycloaddition to epoxides. Also, we have proved that activated Co-NDTzs result effective for the one-pot tandem epoxidation-carboxylation of styrene with CO2. This work opens a new via to develop nitrogen-rich tetrazole-MOFs as efficient heterogeneous bifunctional catalysts.Antonio Valverde: synthesis, characterization, formal analysis, investigation, and writing the original draft. M. Carmen Borrallo-Aniceto: characterization and catalytic experiments. Urbano Díaz: characterization and review. Eva M. Maya: catalytic experiments, formal analysis, investigation, and writing – review & editing Felipe Gándara: characterization and review. Félix Sánchez: review & editing. Marta Iglesias: conceptualization, funding acquisition and writing – review. All the authors discussed the results and commented on the 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.Authors acknowledge to Grants PID 2020-112590GB-C22 and PID 2020-112590GB-C21 funded by MCIN/AEI/10.13039/501100011033. A.V.G. thanks for FPU17/03463. We acknowledge Dr. Fátima Esteban from ICMM X-Ray Diffraction Facility for assistance in PXRD data acquisition.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jcou.2022.102298. Supplementary material .

The development of efficient catalysts that include the advantages of homogeneous and heterogeneous catalysts is a challenge that can be achieved with metal-organic frameworks (MOFs) since they can incorporate different functionalities in their structure that make them promising catalysts for different processes. Herein, two new isoreticular nitrogen-rich naphthalene cobalt-MOFs, Co-NDTz and Co-NDPhTz, were successfully prepared under solvothermal conditions from the corresponding linkers (H2NDTz=2,6-naphthaleneditetrazole and H2NDPhTz=2,6-bis(4-(1H-tetrazol-5-yl)phenyl)naphthalene). These Co-tetrazole-based MOFs combine Lewis acid and redox functionalities and good CO2 adsorption and after being thermally activated resulted to be excellent efficient catalysts for the epoxidation of alkenes and CO2 cycloaddition to epoxides yielding cyclic carbonates, reaching turnover numbers up to 2500. Furthermore, these two reactions take place following a highly desired one-pot tandem process and a cyclic carbonate was obtained from styrene and CO2 under solvent-free conditions. In addition, the heterogeneous catalysts are easily recycled without noticeable loss of catalytic activity and without important structural deterioration.

Data will be made available on request. Data will be made available on request.The production of wastewater containing acetic acid is common in chemical industries such as petrochemicals and wood pulp mills [1,2], and acetic acid has always been a by-product in some conventional wastewater treatment processes of macromolecular organic matter. Zhong et al. [3] reported several short-chain acids, such as acetic acid, as major by-products that formed during the ozonation degradation of humic acids, and Reisz et al. [4] found that acetic acid was formed when O3 was used for 2-propanol oxidation, because the α-position methyl group coordinated with the carboxyl group is not easy to be further oxidized [5]. Acetic acid-contaminated water not only causes great harm if ingested by livestock and to crop irrigation but also adversely affects the respiratory system and sensory organs of human beings [6,7]. Therefore, the effective degradation of acetic acid is of great significance for the complete mineralization of organic compounds in wastewater.Several physical approaches, such as adsorption and membrane separation methods, have been found to exhibit excellence performance of acetic acid removal [8,9]. However, acetic acid was not completely destructed and the adsorption materials required frequent regeneration. Besides, traditional chemical method using alkali to neutralize wastewater usually leads to an enormous load of total dissolved solid in the treated effluent for biodegradation. It was reported that almost twice amount of NaOH or CaCO3 were required to neutralize acetic acid in the wastewater for further biodegradation [10]. Biodegradation would be a very slow process, which was scarcely possible under the practical circumstance [11]. Advanced oxidation processes (AOPs) have been widely used to remove organic matter from aqueous environments [12]. The reactive species, such as ·OH and ·O, can effectively break down organic matter into harmless products [13]. Sannino et al. [14] found the heterogeneous photo-Fenton oxidation of acetic acid on LaFeO3, could reach 60% after 5 h oxidation. Cihanoğlu et al. [15] combined ultrasound with a catalyst to oxidize acetic acid wastewater, but the chemical oxygen demand (COD) degradation was only 25.5%. In addition to the drawbacks of incomplete degradation, the large scale of the removal unit and secondary contamination would restrain its practical application [16].By contrast, catalytic ozonation has been proven to be an effective method to produce reactive oxygen species and further degrade organic matter. Transitional metal oxides showed great potential for ozone decomposition and form more reactive oxygen species and hydroxyl radicals with higher redox potentials, thus further promoting the degradation efficiency of the target organic pollutants [17,18]. It has been found that nano-CeO2 greatly improved the oxidation efficiency of the H2O2/O3 system and promoted the degradation of acetic acid small molecules [19]. Peng et al. showed that the O3 oxidation efficiency of succinic acid reached 100% in combination with a Ni/Al2O3 catalyst [20]. Nevertheless, little attention has been paid so far to the degradation of acetic acid by catalytic ozonation, and the development of the corresponding catalysts is of great significance.In this work, a range of metal oxides (MnO2, Co3O4, Fe3O4, and CeO2) were loaded on γ-Al2O3 and their catalytic ozonation performance for acetic acid degradation were tested. The influencing factors (acetic acid concentration, O3 concentration, pH, and ozonation temperature) were also investigated. The physiochemical properties of the catalysts were characterized with X-ray diffraction (XRD), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS). Tert-butanol (TBA) and p-benzoquinone (PBQ) were used to explore reactive species for acetic acid oxidation. The mechanism of acetic acid ozonation on MnO2/γ-Al2O3 catalyst was further proposed based on the in situ diffuse reflectance Fourier transform infrared spectroscopy (in situ DRIFTS).The catalysts were prepared via wet impregnation method. The preparation method was as follows: Firstly, a certain amount of nitrates of manganese, cobalt, iron, and cerium was dissolved in the deionized water. Then the γ-Al2O3 powder (as support, 2000 mesh, 99%) was dosed into the nitrates solution and vigorously stirred for 1 h. The impregnated powder was dried at 100 °C for 5 h and finally calcined in air at 500 °C for 3 h to obtain the catalysts.The crystalline phases of the catalysts were determined using X-ray diffraction (XRD, Rigaku Ultima IV powder diffractometer, Japan) with a Cu Ka radiation. The morphological properties of the catalysts were measured using scanning electron microscopy (SEM, Supra 55, Zeiss, Germany) with energy-dispersive X-ray spectroscopy (EDS). The surface elements on MnO2/γ-Al2O3 were analysed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi+, Thermo, USA).The experimental set-up used for catalytic ozonation of acetic acid was shown in Fig. 1 . A pulse power supply (M10K-08, Suzhou Allftek, China) was used to drive the dielectric barrier discharge (DBD) reactor DBD to generate O3. The volume fraction of O3 was around 30.0–40.0 g Nm−3. The concentrations of acetic acid and oxidation products (CO and CO2) in the gases from the outlet of the ozonation reactor were on-line analysed using a gas chromatograph (GC) (GC2014, Shimadzu, Japan) equipped with two flame ion detectors (FIDs) and two columns: a 2 m Porapark-N column (Dalian Institute of Chemical, China) with a methanizer prior to the FID to analyse CO and CO2; and a capillary column (SH-Stabilwax-DA, Shimadzu, Japan) to analyse the acetic acid concentration in the water liquid solution. By-products from acetic acid oxidation were analysed using high-performance liquid chromatography (HPLC, Agilent 1260, USA) equipped with an Eclipse Plus C18 column and an ultraviolet (UV) detector. The mobile buffers of A (KH2PO4, 0.015 mol L−1) and B (Acetonitrile with 0.5 mL min−1) were used. The pH value of the acetic acid solution was modified using sodium hydroxide and measured using a pH meter (AZ 86505, Hengxin, China). The O3 oxidation mechanism of acetic acid was investigated by adding OH quencher tert-butanol (TBA, 362 mM) and the superoxide radical (· O 2 − ) quencher p-benzoquinone (PBQ, 1.1 mM) to the acetic acid solution.The degradation (X, %) was calculated using Eq. 1. (1) X = C 0 − C t C 0 × 100 % Where C0 is the initial concentration of acetic acid in g·L–1, and Ct is the concentration of acetic acid in g·L–1 at reaction time t (min).Mineralization (Yi+1 ) and energy efficiency (ηi ) (g·kWh–1) were defined as follows: (2) Y i + 1 = m g m 0 × 100 % (3) η i = m g P t i 60 × 100 % The power discharge (P, kW) was calculated by: (4) P = P a × f 1000 (5) P a = ∑ i ( V i + 1 + V i 2 ) ( I i + 1 + I i 2 ) ( t i + 1 − t i ) Where P a (J) is the energy injection in a pulse discharge period, and f (Hz) is the pulse frequency. Vi and Vi+1 was the discharge voltage at ti and ti+1 (s), respectively. Eq. 6 was used to calculate the amount of CO2 and CO generated from acetic acid oxidation. (6) m g = ∑ t = 0 t = t ( 60.5 F 22.4 ( [ CO ] + [ C O 2 ] ) t i + 1 − ( [ CO ] + [ C O 2 ] ) t i 2 ( t i + 1 − t i ) ) Where m0 is the initial amount of acetic acid in g. mg is the amount of acetic acid in g and calculated from the average concentration of [CO2] and [CO] at ti+1 and ti time (min) with the difference in oxidation time (ti+1 −ti ). P is the discharge power in W, 60 is the conversion factor between hour and minute, and 1000 is the conversion factor between W and kW. F is the flow rate of the bubbling gas, 0.100 L min–1; 22.4 is the molar volume (L) of a gas at standard state and 60.5 is the molecular weight of acetic acid. The discharge power (P) was calculated from waveforms of the pulse voltage supplied from the pulse power supply, using the voltage probe (P6015A, Tektronix, USA) and current probe (CP8030H, Cybertek, China) and oscilloscope (MDO 3022, Tektronix, USA).Firstly, the degradation, mineralization, and energy efficiency of acetic acid wastewater catalytic ozonation over MnO2/γ-Al2O3, Co3O4/γ-Al2O3, Fe2O3/γ-Al2O3, and CeO2/γ-Al2O3 catalysts were compared, and the results were shown in Fig. 2 . It is seen that MnO2/γ-Al2O3 achieved the best degradation among all the catalysts evaluated, and the degradation rapidly increased to 49.2% within 20 min and gradually to 88.4% at 300 min (Fig. 2a). By contrast, the degradation of acetic acid was only 39.9% with O3 bubbling and almost zero with O2 bubbling within 300 min. Besides, the degradation of acetic acid in the presence of catalysts were all much higher than that of O3 bubbling alone, which should be related to the promotion of O3 decomposition in the presence of the catalysts. The mineralization of acetic acid (oxidation to CO and CO2) is important to reflect the catalytic performance, which was also the key issues for acetic acid wastewater treatment. The mineralization of acetic acid is linear to ozonation time, and the slopes of the mineralization of acetic acid using MnO2/γ-Al2O3 were the largest (Fig. 2b). The energy efficiency using MnO2/γ-Al2O3 reached the highest level at 50 min, and the value was approximately 14.9 g·kWh−1 and also the highest in comparison with the other three catalysts (Fig. 2c). This demonstrated that MnO2/γ-Al2O3 was an excellent candidate for acetic acid wastewater treatment.Besides, the effects of MnO2 loading amount on the degradation, mineralization, and energy efficiency of acetic acid were also investigated. The highest degradation of acetic acid was 88.4% at 300 min of ozonation time over 1.0 wt.% MnO2/γ-Al2O3 catalyst (Fig. 2d). By contrast, the degradation of acetic acid only reached 56.3%. The mineralization over 1.0 wt.% MnO2/γ-Al2O3 was also the highest in general, which increased to a maximum level of 88.2% (Fig. 2e), with the best average energy efficiency 14.9 g·kWh−1 (Fig. 2f). Fig. S1 indicates that the CO2 was the primary product from acetic acid ozonation since the CO2 concentration was approximately 6.5–43 times as many as CO.After the optimal loading amount of manganese was determined, other primary factors influencing the catalytic performance, such as catalyst dosage, acetic acid concentration, O3 concentration, and ozonation temperature were further studied. The influence of the dosage of the 1.0wt.% MnO2/γ-Al2O3 catalyst on acetic acid degradation is shown in Fig. 3 . It was found that the both degradation and mineralization were increased as the dosage of 1.0 wt.% MnO2/γ-Al2O3 catalyst increased. However, there was no distinct difference between 30 and 40 g/L. Therefore, 30 g/L 1.0 wt.% MnO2/γ-Al2O3 catalyst was regarded as the optimum dosage with an overall consideration of the cost and effectiveness. The highest energy efficiency was also achieved when 30 g/L of 1.0 wt.% MnO2/γ-Al2O3 catalyst was dosed, reaching around 15 g·kWh−1. By contrast, the mineralization of acid acetic only reached 43.2% in the absence of the catalyst, with an energy efficiency of 5.1 g·kWh−1 (Fig. S2).Besides, the effects of initial acetic acid concentration on energy efficiency at various ozonation time were studied. As shown in Fig. 4 , the highest and lowest energy efficiencies were achieved when the initial acetic acid concentrations were 1 g·L−1 and 0.5 g·L−1, respectively. Since the ozonation of acetic acid related with the adsorption of acetic acid and O3 on catalyst surface, the presence of the highest initial concentration of acetic acid is possibly due to the optimum adsorption of acetic acid and O3 on catalyst surface under the experimental condition. Fig. 4b shows the mineralization of acetic acid at 300 min ozonation time as a function of initial acetic acid concentration. The mineralization reached 89%, 84%, 67%, and 50%, when the initial acetic acid concentration was 0.5, 1, 1.5, and 2 g·L−1, respectively. Fig. 4c shows the energy efficiency of acetic acid at 300 min ozonation time as a function of initial acetic acid concentration. The energy efficiency reached the highest around 15 g/kWh when the initial acetic acid concentration was 1 g·L−1. The mineralization decreased with increasing the initial acetic acid concentration, however, the amount of acetic acid mineralized increased (Fig. 4d). This finding implied that in order to get a large amount of mineralized acetic acid, a high initial acetic acid concentration is required. However, in order to get the highest energy efficiency, the ozonation should be carried out with optimized initial acetic acid concentration (1 g·L−1).In order to investigate the amount of O3 consumed for the catalytic ozonation of acetic acid, and the effects of O3 concentration on energy efficiency was also studied (Fig. 5 ). The O3 concentration was concisely adjusted by the pulse frequency of the DBD reactor. The O3 concentrations were 20.7, 35.6, 47.0, and 55.0 g Nm−3 when the frequencies were 50, 100, 150, and 200 Hz, respectively. In general, the higher O3 concentration, the higher acetic acid degradation, mineralization, and energy efficiency. The maximum energy efficiency (25.5 g·kWh−1) at 20 min was achieved when the frequency was 50 Hz. However, the energy efficiency decreased with increasing frequency (Fig. 5c). It was found that the O3 concentration drop was within 4.0–6.0 g/Nm3 after 140 min of ozonation time (Fig. 5d), although the O3 concentration increased with the frequency. This indicated that the amount of O3 used for acetic acid was limited, and most of the O3 did not take part in acetic acid ozonation and flowed away from the ozonation reactor.The reaction temperature also affects the catalytic ozonation results of acetic acid wastewater, and Fig. 6 demonstrates the degradation, mineralization, and energy efficiency of acetic acid catalytic ozonation in the range of 25-70 °C. It is seen that the catalytic ozonation reaction over MnO2/γ-Al2O3 catalyst was not very sensitive to reaction temperature. The degradation of acetic acid at 25 °C was much lower within the first 50 min, and this might be the weaker volatilization of acetic acid compared with higher temperatures (Fig. 6a). The mineralization of acetic acid was little affected by reaction temperature, indicating that the volatilized acetic acid would be finally oxidized into CO2 (Fig. 6b). In addition, the energy efficiency was also very close at various reaction temperature within 300 min (Fig. 6c). The highest degradation, mineralization, and energy efficiencies were achieved, and they were 85.2%, 95.3%, and 15.8 g·kWh−1, respectively, at 25 °C. In addition, the degradation (X) data at different ozonation temperatures were fitted with the first-order reaction. It is seen that ln(1−X) was linear to ozonation time and the linear coefficient R2 were between 0.96−0.99 (Fig. S3). This suggests that the acetic acid catalytic ozonation is exactly first-order reaction over 1.0 wt.% MnO2/γ-Al2O3 catalyst.The initial pH value of the acetic acid solution was adjusted with 0.1 M NaOH. Fig. S4 shows the energy efficiencies at different pH values as a function of ozonation time. When the pH was 3.42 (the original pH of acetic acid solution), the energy efficiency was at the maximum level of 14.9 g·kWh−1, higher than that when the pH value was 7.06 or 11.26. It has been reported that ·OH radical has a higher oxidation ability than other types of reactive oxygen species (such as O 2 − ), especially under acidic conditions, the similar pH effect was reported by Sahni et al. [21] during the degradation of polychlorinated biphenyls using liquid-phase discharge plasma. Therefore, ·OH radicals might also play a major role in acetic acid ozonation, and more details would be discussed in next section.The crystalline structures of γ-Al2O3 and MnO2/γ-Al2O3 with 0.5 wt.%, 1.0 wt.%, and 10 wt.% MnO2 loadings were characterized, and the XRD patterns are shown in Fig. 7a . The peaks ascribed to γ-Al2O3 were clearly observed, indicating that the crystalline structure of the support was well reserved after the addition of MnO2. However, the crystallinity of γ-Al2O3 greatly decreased with the increase of MnO2 loadings. It was noted that as the loading amount of MnO2 increased to 10 wt.%, new diffraction peaks at 28.68°, 37.32°, 42.82°, 56.65°, and 72.38° ascribed to MnO2 phase (JCPDS PDF#24-0735) were observed, and they were indexed to (110), (101), (111), (211), and (112) planes of MnO2, respectively. By contrast, when the MnO2 loadings were 0.5 wt.% and 1.0 wt.%, no characteristic peaks were found, indicating MnO2 was amorphous or highly dispersed on the surface of γ-Al2O3. The SEM images of 1.0 wt.% MnO2/γ-Al2O3 showed that MnO2 crystallites were in the form of nanoparticles, and the Mn and O elements were uniformly dispersed (Fig. 7b).The surface chemical states of the 1.0 wt.% MnO2/γ-Al2O3 catalysts before and after acetic acid ozonation were characterized with XPS. As shown in Fig. 7c, e and f, the Mn 2p3/2 spectra were deconvoluted into three peaks at 640.6, 641.7, and 643.0 eV, which were ascribed to the Mn2+, Mn3+, and Mn4+ species, respectively [22]. It is noted that Mn species on the surface of 1.0 wt.% MnO2/γ-Al2O3 catalyst mainly existed in the form of Mn4+, and the relative ratio of Mn4+ species accounted for 43.8% and 48.1% before and after acetic acid ozonation. Besides, the relative ratio of Mn4+ of 0.5 wt.% MnO2/γ-Al2O3 and 10 wt.% MnO2/γ-Al2O3 were calculated to be 35.4% and 36.5%, respectively, much lower than that of the 1.0 wt.% MnO2/γ-Al2O3 catalyst. A higher Mn4+ ratio for the manganese-based catalysts is typically strongly linked to a superior catalytic activity [23,24], and this also explained the best catalytic ozonation performance of the 1.0 wt.% MnO2/γ-Al2O3 catalyst. In addition, it was noted that the ratio of Mn2+ decreased from 27.5% to 16.2% after the catalytic ozonation process (Table 1 ), while the ratio of Mn3+ and Mn4+ increased. This was possibly due to a portion of Mn2+ being oxidized by O3.The O 1s spectra before and after ozonation were deconvoluted into the peaks at 531.7–531.8 eV that were assigned to surface chemisorbed oxygen (Oads) and at 530.8–530.9 eV that were ascribed to lattice oxygen (Olatt) (Fig. 7d) [25,26]. Due to the high reaction activity, surface chemisorbed oxygen played an important role in a series of organic substances oxidation reactions. The relative ratio of Oads/Ototal was 49.1%, 71.2%, and 32.7% for 0.5 wt.% MnO2/γ-Al2O3, 1.0 wt.% MnO2/γ-Al2O3, and 10 wt.% MnO2/γ-Al2O3 catalyst, respectively. Obviously, the relative ratio of Oads species at the surface of the 1.0 wt.% MnO2/γ-Al2O3 catalyst was greatly higher than that of 0.5 and 10 wt.% MnO2/γ-Al2O3, and it accounted for a much higher ratio than the Olatt species, corresponding to its superior catalytic ozonation performance. Besides, the relative ratio of Oads and Olatt species were not distinctly changed (Table 1), suggesting the excellent stability of 1.0 wt.% MnO2/Al2O3 catalyst in the process of acetic acid wastewater catalytic ozonation.In situ DRIFTS was carried out to investigate the mechanism of acetic acid catalytic ozonation over MnO2/γ-Al2O3 catalyst, and the results were shown in Fig. 8 . An acetic acid/water bubbler and O3 generator were used to feed the in situ cell with a gas mixture of acetic acid, water, and O3 and to simulate the gas-liquid-solid reaction on the MnO2/γ-Al2O3 catalyst surface. The absorption in the ν(OH) region (3200 cm–1) was attributed to the surface hydroxyl group, and the absorption peak at 3460 cm–1 was attributed to the hydroxyl of adsorbed water [27]. The absorption peak at 1345 cm–1 is caused by the δ(CH3) of the CH3C=O group, while the absorption peak at 1640 cm–1 was assigned to the δ(H2O) vibration. The absorption peak at 1563 cm–1 is attributed to ν(COO) of acetic acid [28]. The absorption peak at 982 cm–1 is due to peroxide O 2 2 − on the metal oxide (M- O 2 2 − ) [29]. As shown in Fig. 8d, the peak height of the reactive species M2+- O 2 − generated on the 1.0 wt.% MnO2/γ-Al2O3 after 80 min was greater than that on the 0.5 and 10 wt.% MnO2/γ-Al2O3. The M- O 2 2 − peak was ascribed to an oxygen molecule adsorbed on the surface of MnO2 that was associated with oxygen vacancies and would play an important role in catalytic oxidation reactions of acetic acid [30]. This explained why 1.0 wt.% MnO2/γ-Al2O3 exhibited much better acetic acid ozonation performance than 0.5 and 10 wt.% MnO2/γ-Al2O3. Besides, the COO peak height on the 0.5wt.% MnO2/γ-Al2O3 increased with time, while that of the 1.0 wt.% MnO2/γ-Al2O3 was relatively stable at a low level (Fig. 8e). This suggested that the COO species accumulated on the 0.5 wt.% MnO2/γ-Al2O3 surface and could not be further oxidized due to a lack of reactive oxygen species, leading to an undesirable catalytic ozonation performance. The peak height ratio of COO/ O 2 2 − could be regarded as an indicator to reflect the ability of a catalyst to inhibit the accumulation of COO (acetic acid), and the results shown in Fig. 8f demonstrated that 1.0 wt.% MnO2/γ-Al2O3 had the lowest COO/ O 2 2 − ratio. This implies that the best performance of the 1.0 wt.% MnO2/γ-Al2O3 was enhanced by the surface O 2 2 − peroxide.To determine the main active species during acetic acid ozonation, TBA and PBQ radical quenchers were added to the acetic acid solution, and the results were shown in Fig. 9 . It was noted that when the hydroxyl radical (·OH) quencher TBA (362 mM) was added, the degradation of acetic acid decreased from 88.1% to 15.7%. When the superoxide radical ( O 2 − ) quencher PBQ (1.1 mmol/L) was added, the degradation decreased from 88.1% to 63.2%. This finding suggests that the hydroxyl radical (·OH) and superoxide radical ( O 2 − ) species contributed to acetic acid degradation, and the ·OH species played a key role. This result was consistent with the results of the effects of pH values described above that a higher energy efficiency was obtained at a lower pH value. The by-products produced during the catalytic ozonation of acetic acid wastewater in the presence of MnO2/γ-Al2O3 was analysed using the HPLC. At 120 min, oxalic acid was found to be the main by-product, and a small amount of formic acid was also detected (Fig. 9c). This was similar to that of the experimental and theoretical research on the decomposition of acetic acid by pulsed DBD plasma reported by Matsui et al. [31].The adsorption configuration of acetic acid on metal oxides was a bidentate state [32], and the breakings of the C-H and C-C bands were the primary steps in acetic acid oxidation that produce oxalic acid and formic acid [33]. By combining the results of this study with the researches concerning catalytic ozonation and acetic acid oxidation reported previously [32,34,35,36], the reaction steps and catalytic ozonation mechanism of acetic acid wastewater over 1.0 wt.% MnO2/γ-Al2O3 catalyst were proposed and illustrated in Fig. 10 . O3 was firstly adsorbed on the surface of MnO2/γ-Al2O3 catalyst, and then decomposed into reactive oxygen species such as surface Mn- O 2 − and Mn2+- O 2 2 − Eq. 7-(9) [37,38]. Besides, O3 reacts with adsorbed water to form surface hydroxyl group Eq. 10 and (11) and ·OH radicals Eq. 10) [35], which also contributed to acetic acid oxidation (Eq. 13 and (14). The O 2 − species and ·OH radicals played an important role in the oxidation of acetic acid mineralization into CO2 and H2O. (7) Step 1: Mn-O + O3 → Mn-O-O3 (8) Step 2 : Mn - O - O 3 → Mn - O 2 − + O 2 (9) Step 3 : Mn + - O 2 − + Mn + - O 2 − → Mn 2 + - O 2 2 − + O surf (10) Step 4: Mn-OH2 + O3 → Mn-OH2-O3 (11) Step 5: Mn-OH2O3 → Mn-OH + HO3 (12) Step 6: HO3∙ → ∙OH+O2 (13) Step 7: Mn-OH + CH3COOH → Mn-OH2 + CH2COOH (14) Step 8 : CH 3 COOH + · OH / O 2 2 − → COOH + CO 2 + H 2 O The mineralization of acetic acid by catalytic ozonation was studied, the primary results are summarized as follows: Among the four metal oxides (MnO2, Fe2O3, Co3O4, and CeO2), MnO2 had the best catalytic performance for complete mineralization of acetic acid wastewater. The mineralization of acetic acid reached as high as 88.4% at 300 min with an average energy efficiency of approximately 14.9 g·kWh−1, when 30 g·L−1 of the 1.0 wt.% Mn/γ-Al2O3 catalyst was used to treat 100 mL of acetic acid at a concentration of 1.0 g·L−1 at 25 °C. A 100 Hz pulse frequency of the DBD reactor was appropriate to produce enough inlet ozone concentration to maintain the mineralization and energy efficiency at a relatively high level. The catalytic ozonation of acetic acid over Mn/γ-Al2O3 catalyst was not sensitive to the reaction temperature, and desirable mineralization of acetic acid could be achieved at an ambient temperature. The Mn/γ-Al2O3 catalyst was efficient for ozone converting into reactive oxygen species such as ·OH and O 2 − in the solution and O 2 2 − on the catalyst surface, and they were essential for the complete mineralization of acetic acid, and Mn/γ-Al2O3 catalyst was an excellent candidate for acetic acid wastewater treatment via catalytic ozonation for practical 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. Erhao Gao: Investigation, Funding acquisition, Writing – review & editing. Ruiyun Meng: Data curation, Formal analysis. Qi Jin: Formal analysis. Shuiliang Yao: Conceptualization, Funding acquisition. Zuliang Wu: Formal analysis. Jing Li: Investigation. Erdeng Du: Formal analysis.This research was supported by the National Natural Science Foundation of China (No. 12075037), and Leading Innovative Talent Introduction and Cultivation Project of Changzhou City (CQ20210083).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.chphi.2022.100149. Image, application 1 Image, application 2

The complete mineralization of acetic acid in wastewater through a biodegradation process is difficult due to the α-position methyl coordinated with the carboxyl group, and this work explored the oxidation performance of acetic acid by catalytic ozonation with metal oxides supported on γ-Al2O3. It was found that MnO2/γ-Al2O3 catalyst achieved superior mineralization performance to Co/Fe/CeOx supported on γ-Al2O3 for acetic acid wastewater treatment. The effects of MnO2 loading, catalyst dosage, acetic acid concentration, O3 concentration, ozonation temperature, and initial pH value of the acetic acid solution were investigated. Typically, the mineralization of acetic acid over 1.0 wt.% MnO2/γ-Al2O3 catalyst was as high as 88.4% after 300 min ozonation of 1.0 g·L‒1 acetic acid at 25 °C with the highest energy efficiency around 15 g·kWh‒1. By contrast, the mineralization of acetic acid could only reach 43.2% in the absence of the catalyst, with an energy efficiency of 5.1 g·kWh−1. Radical quenchers and indicated that ·OH radical, O 2 − species originated from ozone played an important role in the catalytic ozonation of acetic acid into CO2 and H2O. Besides, the catalytic ozonation mechanism of acetic acid over MnO2/γ-Al2O3 was carefully proposed based on the in situ DRIFTS results.

Data will be made available on request.Ethylene glycol (EG) is an essential product and versatile feedstock for the manufacturing of chemicals, fuels and polymers and so on [1–3]. Nowadays, EG is largely dependent on petroleum-derived ethylene in commercial synthesis. Fortunately, a promising approach for the preparation of EG, namely the plant of coal to ethylene glycol (CTEG) has been recently developed rapidly in China (Fig. S1) [4,5]. The process of CTEG includes a crucial step, namely control hydrogenation of dimethyl oxalate (DMO) to EG [6], which also has two main steps including DMO preliminary hydrogenation to methyl glycolate (MG) and then conversion of it to EG (Scheme 1 ) [7–10]. Moreover, EG can dehydrate further to ethanol and the final product should meet a rigid ultraviolet transmittance to serve as raw materials for polyester. Currently, DMO production from syngas (CO + H2) is already commercialized using Pd-based catalyst while the catalyst for DMO hydrogenation still possesses a lot of problems [11], such as short lifetime, toxic promoter and rigorous reaction conditions [12,13].The toxic chromium has been used as the preferred promoter to improve the stability of Cu-based catalysts in industry, which is detrimental to the health [14]. Therefore, strategies to enhance the copper-based catalyst lifespan mainly comprise doping with other elements (Pd, Au, Ag, Ni, Co or B) [12,15–22], and modifying the support by using specific oxides (TiO2, La2O3, Al2O3) [23–26] or molecular sieve (SBA-15, HZSM) [27,28]. However, these strategies usually lead to a higher costs, complicating fabrication process and even elevating reaction temperature, thus it is not suitable for large-scale production [29].We have previously found that dextrin modified Cu-SiO2 catalysts could largely increase catalytic performance for DMO hydrogenation [30]. The results suggested that the copper species, particularly the copper phyllosilicate (CuPS) and CuO species would be obviously affected by the amount of dextrin. Dextrin is an organic polymer with large molecular weight. However, β-cyclodextrin (β-CD) is a water-soluble cyclic oligosaccharide with different structural property, especially its inner hydrophobic cavity and outer OH-groups, may bring variational catalytic performance (Scheme S1B) [31]. It has been reported that the β-CD and SiO2 are compatible to generate a common phase and thus form mesoporous silica materials [32,33]. Moreover, the binuclear copper (II) complex with β-CD (Scheme S1C) could be readily prepared as an efficient catalyst for conversion of arylboronic acids [31,34]. Besides, combination of Cu-β-CD complex and SiO2 has been rarely reported to the vapor-phase hydrogenation reactions. Although we have previously reported two works on the introduction of β-CD to Cu-SiO2 catalysts for DMO hydrogenation [35,36], their preparation method, silica sources, catalyst morphology, catalytic performance as well as the mechanisms of β-CD effect on the catalyst were much different. The novelty in this work is to dig out the role of polyhydroxy molecular template in tuning the Cu-SiO2 based catalysts for DMO hydrogenation reactions.At present work, four Cu-SiO2 catalysts were synthesized by the ammonia evaporation method assisted with polyhydroxy molecular templates (Scheme 1). To further investigate the template effect, we also prepared 0.2PEG-Cu-SiO2 catalyst with polyethylene glycol (PEG, Average MW = 10,000, Scheme S1A) as template. Experimental results showed that PEG was coated in the catalyst precursors while β-CD was washed away during catalyst preparation, which was different from the results of dextrin. However, β-CD modified Cu-SiO2 catalyst was demonstrated to exhibit higher activity, whereas the 0.2PEG-Cu-SiO2 catalyst adversely deteriorated the catalytic performance.All samples (theoretical Cu loading: 25 wt%) were obtained with ammonia evaporation method. In brief, 10.6 g of copper nitrate trihydrate and the preset mass fraction (0.2 and 0.5) of β-CD were mixed into 150 ml of DI H2O with stirring and ultrasonication. Then the received aqueous ammonia solution (30 ml) was introduced and stirred. After this step, 21 g of 40 wt% colloidal silica was introduced and stirred. In the following step, the above mixture was heated at 90 °C to evaporate the ammonia. Then the retained products were filtered and washed with DI H2O. In the end, the samples were dried (120 °C, 24 h) and calcined (450 °C, 4 h). The obtained samples were named as CD-Cu-SiO2.For comparison, the pure Cu-SiO2 and 0.2PEG-Cu-SiO2 samples were prepared similarly to CD-Cu-SiO2 catalyst only except that none adding any template or using PEG instead of β-CD, respectively. All samples were named as X-Cu-SiO2 catalysts, where X indicates the used templates (PEG, β-CD, or none).The catalytic activity test was conducted in a fixed-bed reactor, as depicted in the Supporting Information. All the indispensable data about the reference Cu/SiO2 catalyst based on our previous work has been acknowledged [30].The X-Cu-SiO2 catalysts were characterized by inductively coupled plasma optical emission spectrometer (ICP-OES), elemental analyzer (EA), N2 adsorption-desorption method, N2O chemisorption, thermogravimetric analysis (TGA), transmission electron microscope (TEM), X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H2-TPR), hydrogen temperature-programmed desorption (H2-TPD), Fourier-transform infrared (FT-IR), and X-ray photoelectron spectroscopy (XPS). All the detailed characterization information is shown in the Supporting Information.To disclose the effect of different species and template's amount on the catalytic performance, the DMO hydrogenation over the X-Cu-SiO2 catalysts was examined. Fig. 1 and Table S1 present the results of vapor-phase DMO hydrogenation of DMO as variation of reaction temperature. It shows that the DMO conversion over the 0.2CD-Cu-SiO2 sample was better than other three catalysts at the same reaction condition, especially during the low temperatures of 160 and 190 °C. With increasing the temperature from 160 to 190 °C, the selectivity of MG drastically decreased while EG increased much (from 28.7% to 97.4%) for 0.2CD-Cu-SiO2 (Fig. 1C). Remarkably, the DMO conversion (Conv.DMO) and EG selectivity (Selec.EG) at 190 °C reached maximum value of 99.9% and 97.4%, respectively. Further increasing the temperature, the Selec.EG slightly decreased with the byproducts including ethanol (EtOH), 1,2-butanediol (1,2-BDO) and 1,2-propanediol (1,2-PDO) increasing. Besides, it is clear that the 0.2PEG-Cu-SiO2 sample has extremely low Conv.DMO at low reaction temperature (lower than 220 °C). The optimized reaction temperatures (210–270 °C) for the reference catalysts are much higher than 0.2CD-Cu-SiO2 (190 °C). We further optimized the other reaction parameters (such as time plots, space velocity, hydrogen pressure and the ratio of H2/DMO) for the optimal 0.2CD-Cu-SiO2 catalyst, and the results are shown in Fig. S2 and Fig. 2 . Fig. S2 indicates that the influence of WLHSVDMO and reaction pressure is not obvious while the effect of H2/DMO is very obvious when it is lower than 50. All the reaction parameters demonstrate that the DMO hydrogenation is beneficial to procced under high reaction pressure and high H2/DMO ratio as well as suitable WLHSVDMO, which is consistent the literature report [2]. If the hydrogen pressure and H2/DMO ratio were too low and WLHSVDMO were too high, the DMO conversion and EG selectivity would be dropped down and generated much MG product.Furthermore, there is no denying that the stability of catalyst is crucial for industrial application and academic research. Fig. 2 shows that the 0.2CD-Cu-SiO2 was much more stable than Cu-SiO2, and it can maintain 99.9% DMO conversion with above 95% EG selectivity within 200 h of reaction time. However, the reactivity and lifespan of Cu-SiO2 sample was poor under identical reaction conditions, and the catalyst began to deactivate only after 25 h. Many efforts have been spent on such similar deactivations of Cu-SiO2 catalyst [14,19,24], and the improved reasons would be further discussed below (see Section 3.6).As we all know, the surface composition, surface areas and metal dispersion have significant effects on catalytic performance, thus the physicochemical properties of the X-Cu-SiO2 catalysts are summarized in Table 1 . The actual Cu contents determined by ICP-OES for all the organic templates modified catalysts were slightly more than unmodified catalyst, but a little lower than the preset values of 25 wt%. This indicated that the Cu2+ ions weekly adsorbed on the support were lost during filtration and more copper species were coated by silica gel with organic template [19]. The elemental analysis of carbon for the dried catalyst precursors showed that carbon content was below than 0.3 wt% for both Cu-SiO2 and CD-Cu-SiO2 samples. Interestingly, the 0.2PEG-Cu-SiO2 precursor exhibited 4.17 wt% carbon loading, suggesting that β-CD was washed away while the PEG was successfully loaded in the catalyst precursors. Besides, Cu metallic surface area (SCu, 25.8 m2 g−1) and dispersion (22.5%) determined by dissociative N2O adsorption as well as specific surface area (SBET, 399.4 m2 g−1) were relatively increased for 0.2CD-Cu-SiO2 catalyst but dramatically decreased for the other organic additive modified catalysts. As mentioned above, the order of catalytic activities is similar to the variation trend of the surface areas. This confirms that SBET is essential for the vapor-phase DMO conversion, which is also proved by a number of reported papers [13,19,37].The N2 physisorption curves of the X-Cu-SiO2 samples are illustrated in Fig. 3 . It suggests that all the catalysts exhibited Langmuir type IV isotherms with H1-type hysteresis loops. The shape of the hysteresis loops did not change a lot when adding the template, suggesting that introduction of β-CD or PEG would not affect the silica pore shape. However, their corresponding pore size distributions changed clearly, as illustrated in Fig. 3B. The pore sizes of the 0.2CD-Cu-SiO2 catalyst focus around 4.8 nm while the 0.2PEG-Cu-SiO2 catalyst focuses around 11.1 nm. Therefore, a suitable template is beneficial for the formation of higher BET surface area with ordered channel structures. Fig. 4 shows TGA plots of the samples under N2 atmosphere from 30 to 900 °C. The pure β-CD lost a weight loss of 13.4% assigned to the water molecule from 30 to 120 °C (Fig. 4A). The molecular backbones of β-CD and PEG then dramatically collapsed before 435 °C. As shown in Fig. 4B, the β-CD modified samples released a little more water molecule before 120 °C. However, the 0.2PEG-Cu-SiO2 catalyst showed an obvious degradation of the PEG from 120 to 435 °C. Finally, it can be observed that a slight weight loss (7.69%) for pure Cu-SiO2 catalyst, a higher weight loss (11.69% and 10.77%) for CD-Cu-SiO2 catalysts, and the most weight loss (17.53%) for 0.2PEG-Cu-SiO2 sammple during the whole heating process. The weight loss was attributed to the lost of water and copper species. For example, a small amount of CuPS during calcination would decompose to CuO [7,38].Many works reported the viewpoint that the copper species such as CuO particles, Cu-O-Si layer, and CuPS could be influenced by many factors like promoter [13,24], copper loading, [39] copper precursors, [40] and ammonia-evaporation temperature [7]. In particular, Chen et al. reported that more copper phyllosilicate would facilitate the DMO hydrogenation [7]. Thus, the copper species were determined by characterizations including XRD, FT-IR H2-TPR, H2-TPD and TEM. Fig. 5 presents the XRD results of the pure templates and the catalysts in which the feature of 2θ of ∼22° was ascribed to amorphous SiO2. As shown in Fig. 5B, the absence of characteristic diffraction peaks that appeared in the pure templates (Fig. 5A) indicated that the dried catalysts synthesized by organic compounds still lack long range ordering of the structure [33]. It is also because that the amounts of the used templates were low and the crystallinity may also would be affected by adding aqueous ammonia solution [28]. After calcination at 450 °C in air, the XRD patterns shows that some small peaks at 2θ of 31.0°, 34.8° 57.2° and 63.3° were from the CuPS phase (JCPDS 00–003-0219) [41] exhibited in all the calcined catalyst precursors except that the 0.2PEG-Cu-SiO2 catalyst displayed strong diffraction peaks of CuO (JCPDS 05–0661) (Fig. 5C) [7,42]. After reduction in H2 atmosphere, the peaks at 2θ = 43.3°, 50.4°, 74.1° and 2θ = 36.4° are assigned to Cu (JCPDS 04–0836) and Cu2O (JCPDS 05–0667) species, respectively (Fig. 5D) [7]. This indicated that copper species of 0.2CD-Cu-SiO2 were uniformly dispersed in silica support while there were some large CuO particles in the calcined 0.2PEG-Cu-SiO2 catalyst.The FT-IR results are in accord with analysis from XRD pattern. Fig. 6A shows the FT-IR results of the pure PEG and β-CD in the 4000–400 cm−1 wavenumber range. The peaks at 2930 cm−1 and 1033 cm−1 can be ascribed to the antisymmetric CH vibration of -CH2 and C-O-C vibrations, respectively. The broad band at ∼3385 cm−1 is due to the hydroxy stretching vibration. It is interesting that we could not find the main FT-IR peaks of the pure organic template in the dried catalyst precursors from both XRD profiles and FT-IR spectra (Fig. 6B). The absorption bands at 1120, 800, and 470 cm−1 are attributed to the characteristic peaks of SiO2 [43]. Moreover, the presence of δ OH vibration (673 cm−1) and ν SiO shoulder peak (1030 cm−1) indicates the existence of CuPS [41,44], which were more stronger in 0.2CD-Cu-SiO2 sample but nearly disappeared in 0.2PEG-Cu-SiO2. On the contrary, the 0.2PEG-Cu-SiO2 sample exhibited a new band adsorption at 962 cm−1, indicating a new species of Cu-O-Si units [43]. Besides, it should also be mentioned that the peak of silica support (470 cm−1) would affect the vibrations of the CuO bond that appear at 460, 500, 575 cm−1 [45]. In short, the main copper species on the calcined 0.2PEG-Cu-SiO2 was large CuO particles and Cu-O-Si layer, while they were CuPS dominating on the other three catalyst precursors. After reduction in H2 atmosphere, all the samples had both copper and Cu2O species.The type and amount of copper species were further demonstrated by H2-TPR and H2-TPD profiles. Fig. 7A of the H2-TPR displayed a sharp reduction peak (190 °C) for pure Cu-SiO2 catalyst, which corresponds to comprehensive reduction of well dispersed CuO, CuPS, and Cu-O-Si units [7,38]. However, the H2-TPR peak shifts to a 194 °C and 201 °C with after using the template of β-CD, which maybe because enhanced chemical interaction between the metal species and SiO2 support [19,46]. A shoulder peak that appeared at 201 °C for 0.5CD-Cu-SiO2 may result from the reduction of larger particles of CuO [7]. In addition, the 0.2PEG-Cu-SiO2 exhibited a much higher reduction temperature (230 °C), which was assigned to the contribution of bulk CuO instead of well dispersed CuO [7]. Therefore, the H2-TPR results were in agreement with the XRD measurement.The H2 adsorption ability on the X-Cu-SiO2 catalysts were detected by the H2-TPD measurement. The catalysts were firstly activated at 350 °C, and showed a main peak centered at around 132 °C (Fig. 7B). For comparison, a H2-TPD test for the pure SiO2 displayed a weak H2 desorption peak in the 680 °C, indicating that strongly chemisorption H2 species on the SiO2 surface. [24] Therefore, the desorption peak located at 132 °C was assigned to chemisorption H2 on Cu species [47]. Besides, the peak at high temperature (737 °C) disappeared after adding temperate, suggesting that H2 was more easily released to take part in DMO hydrogenation reaction in the modified samples. The peak intensities of 0.5CD-Cu-SiO2 and 0.2PEG-Cu-SiO2 catalysts were clearly decreased. As identified by N2O chemisorption, the SCu and Cu dispersion for them were also much low even though their actual copper loadings were slightly >0.2CD-Cu-SiO2 sample. In a word, introduction of proper amount of β-CD into the Cu-SiO2 sample could obviously improve copper species dispersion and H2 activation.The morphology analysis based on TEM images are presented in Fig. 8 and Fig. S3. Many rod-like copper phyllosilicate with a lamellar structure can be found in pure Cu-SiO2 catalyst (Fig. 8A). After adding a suitable amount of β-CD during catalyst preparation, the 0.2CD-Cu-SiO2 catalyst has spherical supporting silica structures (light gray) with more dispersed copper species distribution (dark gray, Fig. 8C). However, with more β-CD or PEG as template, the copper particles became much larger and even crystallized (Fig. 8B and D). After reduction, both pure Cu-SiO2 and 0.2CD-Cu-SiO2 samples showed similar particle sizes (∼ 3.8 nm, Fig. 8E and F).The XPS was employed to analyze the surface metal species. The Cu 2p3/2 peaks at above 933.4 eV with the satellite peaks indicate that the chemical valence of Cu was +2 in the calcined samples (Fig. S4A). After deconvolution processing of the asymmetric Cu 2p3/2 envelope, two peaks at 934.2 and 936.4 eV are ascribed to dispersed CuO and CuPS, respectively [7,48,49]. Notably, it is important to emphasize that the proportion of CuPS is much more than CuO in the 0.2CD-Cu-SiO2 and inversely in the 0.2PEG-Cu-SiO2 catalyst. After reduction, the satellite peaks of Cu 2p disappeared (Fig. S4B). Fig. 9 and Table 2 show the Cu LMM XAES spectra and deconvolution results of the reduced catalysts to distinguish the surface Cu+ and Cu0 species. We have also noted that the Cu+/(Cu+ + Cu0) ratio (named as XCu+) increases (63.2% vs. 75.5%) with introduction of β-CD and decreases (63.2% vs. 49.8%) with using PEG as template. This result further proves that the interaction between the improved CuPS and the SiO2 carrier was increased significantly.Based on the above characterization results, it clearly indicated that the structure of β-CD modified Cu-SiO2 catalysts were different from dextrin coated Cu-SiO2 catalysts [30], which possessed smaller SBET, lower amount of CuPS and decreased metal dispersion. Due to the same preparation procedure, this phenomenon could be attributed to the different roles of polyhydroxy compounds in ammonia evaporation method. In our previous work, we have illustrated that dextrin could successfully coated copper nanoparticles during the catalyst preparation. However, this work has failed to generate Cu-β-CD complex, so that β-CD could be easily washed away during filtration. One reason was probably that the reported Cu-β-CD complex was formed in strong alkaline conditions like using NaOH instead of NH3·H2O [34,50]. Another reason was that the formation of Cu-β-CD complex was through a reversible reaction (Scheme 2 , eq. 0). When the ammonia evaporation proceeding, the OH− concentration decreased so that even formed Cu-β-CD complex would disassemble again. Matsui et al. disclosed a ligand-exchanged reaction of Cu-β-CD complex with EDTA to give Cu(EDTA)2− and β-CD, suggesting that Cu-β-CD complex was not very stable [50].However, compared with PEG, the β-CD was not simply added and it should have been interacted with copper species, as evidenced by the more ordered pore structure and the improved copper phyllosilicate morphology from TEM images (Fig. 8B). This could be ascribed to the special structure of β-CD [33,51]. Because of the exterior hydrophilic surface of β-CD, they form a sphere-like structure with the connection of hydroxyls. Then the Cu2+ ions were cooperated with the nanosphere β-CD [52]. Thus 0.2CD-Cu-SiO2 catalyst possessed nanosphere copper particles instead of rod-like copper phyllosilicate. At a deeper level, copper phyllosilicate is composed of the alternate layers of SiO4 tetrahedra and discontinuous layers of CuO6 octahedra [17]. The synthesis of CuPS would be affected by a lot of factors, such as the pH of the precursor solution [41], the solution/silica contact time [41], CuO loading [53], and the ammonia evaporation temperature or time [7]. This work showed that the polyhydroxy compounds also have effect on its formation, as proved by TEM, XRD and FT-IR results. It is reported that there are three equilibrium reactions for the formation of CuPS (Scheme 2, eq. 1–3): [17,41,53] 1) The silica sol was dissolved to yield silicic acid (Si(OH)4); 2) The copper ammonia complex was hydrolyzed to Cu(OH)2(H2O)4 as ammonia evaporation; 3) The heterocondensation reaction of silica acid with the Cu(OH)2(H2O)4. Finally, CuPS monomer would polymerize and chemically interact with the SiO2 surface. We speculated that some Cu2+ ions were coated by dextrin or PEG, decreasing neutral metal complex. While β-CD probably inhibited the growing of copper phyllosilicate monomer to form rod-like structure owing to that β-CD forming a sphere-like structure. Therefore, polyhydroxy compounds play a key role in ammonia evaporation method, especially for copper phyllosilicate morphology.The topic about DMO hydrogenation has been published in numerous literatures (Table S2). Nevertheless, many reported Cu-based catalysts exhibited good catalytic performance for EG synthesis under conditions of lower WLHSVDMO (0.6 vs. 1.0 h−1), higher H2/DMO molar ratio (80 vs. 50), copper loading (30 vs. 17.78 wt%), P(H2) (3.0 vs. 2.0 MPa) or reaction temperature (200 vs. 190 °C). In conventional Cu-based catalysts for EG synthesis, if the reaction temperature were below 200 °C, the intermediary MG would be largely generated, accelerating catalyst deactivation [23]. This synthetic methodology employing low cost Cu-based catalysts with β-CD could become useful in industry given its good activity and stability at low temperature.The reasons for the excellent performance of 0.2CD-Cu-SiO2 catalyst are analyzed below. One is elevated Cu metallic dispersion and SBET, as well as more ordered channel structures. Another cause is that the improved copper phyllosilicate morphology, resulting in a higher XCu+ ratio (75.5%). Zhang et al. have reported on the optimizing in the catalytic property of Cu/10-SiO2, which was because of the suitable SBET, Dp and larger Cu dispersion [37]. Both the SBET (399.4 m2 g−1) and SCu (25.8 m2 g−1) were increased much for the 0.2CD-Cu-SiO2 catalyst so that its activity was enhanced a lot (Table 1), especially at low reaction temperature of 190 °C. Moreover, the H2-TPR and H2-TPD results obviously showed that the chemical interactions between the copper species and the silica carrier were increased and H2 activation became more easily than pure Cu-SiO2 (Fig. 7). Thus, the sintering of copper species could be slowed during the reaction. Moreover, a little carbon deposit on the catalyst surface plays significant roles in eliminating some very active species, which maybe favorable for over hydrogenation to ethanol and carbon deposition during reaction. Just as the role of catalyst pretreatment by DMO hydrogenation at high reaction temperature (450 °C for 24 h, the common reaction temperature is 180–210 °C) to eliminate some very active species [19]. Therefore, the β-CD modified Cu-SiO2 catalyst presented better stability. When the mass fraction of β-CD was >0.2, the solvent was not enough to well dissolve it and finally the aggregation of copper nanoparticles happened (Fig. 7A and Fig. 8D). In addition, we also need to point out the reasons for the very poor activity of 0.2PEG-Cu-SiO2 sample. The large crystalline CuO bulk particles observed from the XRD, H2-TPR, and TEM images were the primary reason. As the carbon content was detected on the dried 0.2PEG-Cu-SiO2 precursor (Table 1), we propose that the copper species were coated by the PEG template instead of promoting the interaction between copper species and SiO2 carrier, resulting a poor SBET (258.5 m2 g−1) and SCu (9.3 m2 g−1) (Table 1). The lower copper surface area and larger copper nanoparticles resulted in a worse activity [54].Based on the XRD results, we know that all the copper oxides in the catalyst precursors were reduced to both Cu and Cu2O. The XPS spectra further demonstrated different Cu+/(Cu+ + Cu0) ratio among them. Much work so far has focused on the viewpoint that the synergic effect of Cu+ and Cu0 is effective in hydrogenation of DMO [7,19,39,55,56]. Generally, Cu+ sites bind and activate the ester and acyl groups while Cu0 sites absorb H2 molecules in ester hydrogenation [57]. The improved structure of 0.2CD-Cu-SiO2 was indicated to be capable of increasing the XCu+ (75.5%) than that on the pure Cu-SiO2 (63.2%). As the CuO species were easily reduced to Cu0, the XCu+ (49.8%) of 0.2PEG-Cu-SiO2 was decreased much [58]. Though the Cu+ species was also high for the 0.5CD-Cu-SiO2 sample (72.1%), it also displayed a poor activity. This is because that many other factors would also affect their catalytic results in addition to the XCu+ [38]. As shown in Table 1, although the actual Cu loading over 0.5CD-Cu-SiO2 was slightly higher than 0.2CD-Cu-SiO2, the Cu dispersion and SCu as well as SBET over 0.2CD-Cu-SiO2 was much higher than those over 0.5CD-Cu-SiO2. In addition, the Cu nanoparticles size was obviously increased from 3.7 to 6.8 nm over 0.5CD-Cu-SiO2 (Fig. 8). Therefore, the DMO hydrogenation on 0.2CD-Cu-SiO2 was more active than 0.5CD-Cu-SiO2.A green and facile route for the preparation of efficient Cu-SiO2 catalysts was demonstrated using low-cost β-CD as template. Unlike dextrin, the β-CD was not coated but successfully washed during catalyst preparation, leading to improving copper phyllosilicate morphology, increasing Cu dispersion and surface Cu+ species. Moreover, the modified structural properties of the 0.2CD-Cu-SiO2 could efficiently maintain the Cu dispersion without the deactivation. As a result, it possesses remarkable performance for DMO hydrogenation to ethylene glycol at 190 °C. However, the template of polyethylene glycol would obviously revise the distributions of copper species with formation of more Cu0 species and thus lower the DMO hydrogenation performance. Therefore, this synthetic strategy using β-CD for the modified Cu-based catalysts could become a suitable candidate in industry. Runping Ye: Conceptualization, Investigation, Writing – original draft. Chong Zhang: Software, Data curation, Formal analysis. Peng Zhang: Investigation, Data curation. Ling Lin: Conceptualization, Project administration, Writing – review & editing. Long Huang: Methodology, Investigation, Writing – review & editing. Yuanyuan Huang: Methodology, Investigation, Writing – review & editing. Tianyou Li: Methodology, Investigation, Writing – review & editing. Zhangfeng Zhou: Methodology, Investigation, Writing – review & editing. Rongbin Zhang: Methodology, Investigation, Writing – review & editing. Gang Feng: Writing – review & editing, Supervision, Funding acquisition. Yuan-Gen Yao: 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 National Natural Science Foundation of China (Grants No. 22005296, 22102186, 21875096, and 21763018), National Key Research and Development Program of China (2017YFB0307301, 2017YFA0206802, 2018YFA0704500), and Strategic Priority Research Program of the Chinese Academy of Sciences (XDA21020800). The authors are also thankful for and Shiyanjia Lab (www.shiyanjia.com) on the XPS analysis. Supplementary material Image 1 The following are the Supplementary data to this article. The introduction of plant of coal to ethylene glycol (Fig. S1), the influence of reaction parameters on the catalytic performance (Fig. S2), the details of catalyst characterizations, the details of catalytic performance test, the molecular structures of polyethylene glycol and β-cyclodextrin (Scheme S1), catalytic performance of DMO hydrogenation over X-Cu-SiO2 catalysts (Table S1), HRTEM images for X-Cu-SiO2 catalysts (Fig. S3), XPS spectra of X-Cu-SiO2 catalysts (Fig. S4), comparison of DMO hydrogenation over different CuSi catalysts (Table S2). Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2022.106586.

β-cyclodextrin (β-CD) was used to prepare remarkable and stable copper-based catalysts for hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG). The DMO conversion (95.6% vs. 99.9%), EG selectivity (53.6% vs. 97.4%) and lifetime (25 h vs. 200 h) were significantly increased on the 0.2CD-Cu-SiO2 sample compared with Cu-SiO2 at 190 °C. The prominent catalytic property was due to the significant roles of β-cyclodextrin to regulate surface dispersion of copper species along with their particle sizes, namely improved copper phyllosilicate morphology, increased BET surface area, improved Cu dispersion, and enhanced surface ratio of Cu+/(Cu+ + Cu0).

As the primary long-term drivers of climate change, the concentrations of the greenhouse gases CO2 and CH4 in the atmosphere are growing continuously, as reported by the Global Monitoring Division of the National Oceanic and Atmospheric Administration (NOAA)/Earth System Research Laboratory. Climate change makes it more attractive to reutilize CO2 and CH4 for the development of a carbon-neutral ecosystem. Great efforts are being directed at the chemical transformation of CO2 and CH4 into syngas [1–6], a mixture of CO and H2, that can be subsequently converted to fuels and chemicals using existing commercial processes [7–11]. However, this indirect route for CO2 and CH4 utilization is energy-intensive due to the high thermodynamic stability of the two molecules. Therefore, developing a single-step approach to convert CO2 and CH4 to fuels and chemicals under mild conditions, instead of syngas being the intermediary, is highly desirable.Non-thermal plasma (NTP) has been proved to be an effective way to directly convert CO2 and CH4 into high-value oxygenates such as alcohols, acids, ketones and aldehydes, under mild conditions through overcoming thermodynamic and kinetic limitations [12–14] and thus bypassing the process of syngas production. Earlier studies on converting CO2 and CH4 to oxygenates were carried out by Liu and co-workers using a dielectric barrier discharge (DBD) reactor. They found that the total selectivity of oxygenates, which included alcohols, acids, ketones, and aldehydes, changed with plasma parameters and reaction conditions, such as discharge gap, CH4/CO2 feed ratio, and starch as a dielectric layer. Pasting corn starch on the surface of the quartz tube reactor enhanced the formation of oxygenates, while inhibiting the generation of liquid hydrocarbons from CH4 and CO2 [15,16]. Tosi et al. investigated the synthesis of acids from a CO2-CH4 DBD plasma, and discovered that the ratio of propanoic-butanoic acids to acetic acid increased with discharge duty cycle. In addition, Ni and Cu electrodes produced more carboxylic acids, particularly formic acid, than stainless steel. Furthermore, the possible mechanisms for acid generation were investigated by density functional theory (DFT) calculations [17–19]. Torsten et al. revealed the importance of O2 in the formation of methanol and formaldehyde during the plasma conversion of CO2 and CH4 in a DBD reactor using He as a diluent gas [20]. Krawczyk et al. reported that only methanol and ethanol were produced in the plasma-catalytic reaction of CO2 and CH4 with and without packing materials (i.e., Al2O3, Fe/Al2O3, NaY and NaZSM-5) in a DBD reactor, and the highest selectivity of alcohols (< 3.5%) was obtained in the non-packing system [21]. Rahmani et al. discovered that oxygenates produced in the plasma reforming of CO2 and CH4 contained 10 liquid fuels such as alcohols, ketones, and light organic acids. The formation of oxygenates decreased when the Ar addition was higher than 33%, and the total yield of oxygenates represented 2–4% of the total mass of the products [22]. Shirazi et al. performed DFT calculations to investigate the pathway in the plasma conversion of CO2 and CH4 to methanol on a crystalline Ni(111) surface [23], and they found that aldehydes were preferably formed in a CH4/CO2 DBD reactor in comparison with alcohols using a one-dimensional fluid model [24]. In addition, Lambert et al. reported formaldehyde generation with a selectivity of 11.4% in the plasma-catalytic conversion of CO2 and C2H6 over BaTiO3 supported vanadia/alumina catalysts [25]. Similar studies were also carried out by Chen and co-workers, who found that alcohols, aldehydes, and acids could be obtained with a maximum total oxygenate selectivity of 12%, which agreed with the results of chemical kinetic modeling [26].Very recently, significant progress has been made in plasma-driven CO2 and CH4 conversion to oxygenate, with a focus on catalyst investigations. Li et al. reported that packing Fe/SiO2 or Co/SiO2 catalyst into a DBD reactor greatly increased the formation of oxygenates to 40% selectivity, with methanol and acetic acid being the main products, whereas syngas and C2-C5 gaseous hydrocarbons were generated as the main products in the non-packing mode [27]. They also found that Fe/SiO2 promoted the generation of alcoholic products, while Co/SiO2 favored the formation of acids and long-chain products. Song et al. investigated the effect of catalysts’ acidic properties on the total selectivity of oxygenates and carbon deposition in the plasma-catalytic conversion of biogas (a mixture of CO2 and CH4) [28], and the Pt/UZSM5 catalyst with a 100% ratio of weak acidic sites exhibited an oxygenates selectivity of up to 60%, including formaldehyde, methanol, ethanol, and acetone, implying that a high ratio of weak acidic sites benefits the formation of oxygenates. Shao and co-workers reported that packing Ni-based catalysts into the plasma region achieved around 30% selectivity of oxygenates, which is dependent on the microstructures, surface compositions, and reducibilities of the catalysts [29]. Our previous work achieved a liquid selectivity of 50–60% with acetic acid being the major liquid product through the development of a novel DBD reactor with a special water ground electrode [30].Despite great advances in the direct conversion of CO2 and CH4 to oxygenates using plasma technology, significant challenges remain. Firstly, the oxygenates that have been obtained to date consist of alcohols, acids, ketones, and aldehydes, and how to tailor the distribution of oxygenates is yet unclear. Second, current research on the catalyst design for this process is still in its infancy, and the correlation between catalytic active sites and specific oxygenate is still missing.Herein, a series of Cu-based catalysts were synthesized and tested in the conversion of CO2 and CH4 to oxygenates using a dielectric barrier discharge (DBD) plasma reactor at a reaction temperature of 60 °C and atmospheric pressure. The influence of the Cu-based catalysts, i.e., the electronic structure and acidity, on the formation of alcohols and acids was investigated. We use the support on which copper is anchored, i.e., Al(OH)3, Mg(OH)2, SiO2, TiO2 and HZSM-5 zeolite, to tune the valence state of copper species in the Cu-based catalysts, and use the preparation method of ion exchange and impregnation to regulate the valence state and Brønsted acid sites of the Cu/HZSM-5 catalyst. The correlations between acids/alcohols and copper valence state are well established. Cu2+ was found to be more favorable for the formation of alcohols, whereas Cu+ contributes more to the production of acetic acid. Moreover, in addition to Cu+, Brønsted acid sites of HZSM-5 can significantly improve the selectivity of acetic acid.The catalysts with 10 wt% Cu loading were synthesized using an incipient wetness impregnation method (IM). The HZSM-5 support was first calcined at 400 °C for 5 h to remove the impurities (e.g., adsorbed H2O). The precursor Cu(NO3)2·3H2O was dissolved in deionized water, followed by the addition of HZSM-5 powder under stirring. After 5 h of aging at room temperature, the sample was dried at 120 °C overnight. Finally, the sample was calcined at 540 °C for 5 h in air, and the catalyst is noted as Cu/HZSM-5. Other catalysts, i.e., Cu/Al(OH)3, Cu/Mg(OH)2, Cu/SiO2 and Cu/TiO2, were synthesized using the same method.The Cu/HZSM-5 catalyst was also prepared using an ion exchange method (IE). The HZSM-5 zeolite was first calcined at 400 °C for 5 h to remove the impurities (e.g., adsorbed H2O), and the precursor Cu(NO3)2·3H2O was dissolved in deionized water before adding HZSM-5 powder under stirring. The suspension was stirred at 80 °C for 2 h, before being filtered and washed with deionized water, and the process was repeated twice. Finally, the paste was dried at 120 °C for 12 h before being calcined in air at 540 °C for 3 h. The resulting sample is denoted as Cu/HZSM-5 (IE), while the Cu/HZSM-5 catalyst prepared by the incipient wetness impregnation method is denoted as Cu/HZSM-5 (IM). The as-prepared catalysts were used directly for plasma-catalytic conversion of CO2 and CH4 without any pretreatment.Conversion of CO2 and CH4 was carried out in a dielectric barrier discharge (DBD) catalytic reactor at low temperatures and atmospheric pressure, as illustrated in Fig. S1. The DBD reactor was made up of two coaxial glass cylinders with water circulating between the inner and outer cylinders, as well as two coaxial electrodes. The circulating water served as the ground electrode and was controlled at around 60 °C by a thermostat water bath. The inner high voltage electrode was a stainless-steel rod with an outer diameter (o.d.) of 2 mm, fitted along the axis of the inner glass tube (10 mm o.d. × 8 mm i.d.), which also functioned as the dielectric material. The discharge length was 40 mm, with a 3 mm discharge gap, and the catalyst (20–40 mesh) was packed into the entire discharge area. The temperature of catalyst bed was measured to be in the range of 60–140 °C using a thermal infrared camera, as shown in Fig. S2. The DBD reactor was powered by an alternating current (AC) high voltage power source with a maximum peak voltage of 30 kV and a variable frequency range of 7–15 kHz. In this work, the flow rate of CO2 and CH4 was controlled by mass flow controllers with a CH4/CO2 ratio of 1:1 at a total flow rate of 40 mL/min. The discharge frequency was set at 9 kHz, and the reaction lasted 2 h. A four-channel digital oscilloscope (Tektronix, DPO 3012) with a high-voltage probe (Tektronix, P6015) and a current probe (Pearson 6585) was used to record the electrical signals. A cold trap consisted of anhydrous ethanol and liquid nitrogen was connected to the exit of the reactor to condense liquid products, which were quantified using a gas chromatograph (Shimadzu GC-2014 C) equipped with a flame ionized detector (FID) with an FFAP column. The gaseous products were analyzed using an online gas chromatograph (Shimadzu GC-2014 C) equipped with a thermal conductivity detector (TCD) and an FID. The change in gas volume before and after the reaction was monitored using a flowmeter.In this study, the conversion of CO2 and CH4, as well as the selectivity of main gaseous products (CO, C2-C4 hydrocarbons) and major liquid products (acetic acid and C1-C4 alcohols) are used as performance indicators. The specific calculation methods are shown as follows. X CO 2 % = moles of CO 2 converted moles of initial CO 2 × 100 % X CH 4 % = moles of CH 4 converted moles of initial CH 4 × 100 % S CO % = moles of CO produced moles of CH 4 converted + moles of CO 2 converted × 100 % S C x H y % = x × moles of C x H y produced moles of CH 4 converted + moles of CO 2 converted × 100 % S oxygenates % = 100 % − ( S CO + S C x H y ) − ca . 10 % carbon deposition S CH 3 COOH ( % ) = 2 × moles of CH 3 COOH produced moles of total oxygenates produced × S oxygenates S C 1 − 4 OH ( % ) = carbon number × moles of alcohols produced moles of total oxygenates produced × S oxygenates The physicochemical properties of the as-prepared catalysts were analyzed using following techniques. The crystalline structure of the catalysts was determined by X-ray powder diffraction (XRD) using an X-ray diffractometer (Rigaku D-Max 2400) with Cu Kα radiation (λ = 0.15406 nm). The scanning range was from 5 to 80 (2θ), with a step size of 0.02 min−1 and a scanning speed of 10 min−1. The valence state and chemical environment of copper species of the catalysts were examined by X-ray photoelectron spectroscopy (XPS) using a Thermo ESCALAB Xl+ spectrometer. High-resolution transmission electron microscopy (HRTEM) images of the catalysts were recorded using a JEOL-JSM-2100 F (Tecnai G2 F30 S-Twin) with an energy dispersive X-ray spectrometer (EDXS) at an accelerating voltage of 200 kV. Scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) analysis was performed using the same apparatus as HRTEM. The redox properties of the catalysts were investigated by hydrogen temperature-programmed reduction (H2-TPR) using a chemisorption instrument (Quantachrome ChemBET 3000). TPR measurements were carried out in an H2/Ar mixture flow (10 vol% H2, 120 mL/min) from room temperature to 800 °C at a heating rate of 10 °C/min. A TCD detector measured the H2 concentration in the tail gas, which reflected the H2 consumption as a function of temperature. The acidity of the catalysts, i.e., the amount, strength, and type of acid sites, was studied by NH3 temperature-programmed desorption (NH3-TPD) and Pyridine adsorption FTIR (Py-FTIR). NH3-TPD was performed using the same system as for H2-TPR. The sample was purged with NH3 for 1 h at 100 °C, followed by purging with He to remove NH3 physically adsorbed on the catalyst. Finally, the sample was heated to 800 °C under the flowing He and the composition of the effluent gas was measured by a TCD detector. Py-FTIR spectra in absorbance mode were collected accumulating 64 scans with 4 cm−1 resolution on a Nicolet 6700 spectrometer equipped with an MCT cryodetector, cooled by liquid nitrogen. All samples were pressed into the form of self-supporting wafers and pretreated in situ for 4 h under vacuum conditions (10−3 Pa) in a homemade quartz IR cell with CaF2 windows at 673 K. Subsequently, pyridine vapor was exposed to the sample at room temperature for 30 min and then the system was outgassed at 200 °C for 30 min, before being cooled to room temperature for measurement.The emission spectra of the CO2-CH4 plasma at different conditions were recorded using a Princeton Instruments ICCD spectrometer (SP2758) in the 200–1200 nm range via an optical fiber facing the outside wall of the DBD reactor. The slit width and grating of the spectrometer were set to 20 µm and 300 g mm−1, respectively.Oxygenates, i.e., acetic acid (CH3COOH), alcohols (C1-C4OH), acetaldehyde (CH3CHO) and acetone (C3H6O), with acetic acid and alcohols being the major products were directly produced at mild conditions in this study, correspondingly denoted as R-COOH and R-OH. A small amount of acetaldehyde and acetone were also detected with selectivities less than 2% and 1%, respectively. Furthermore, no C5+ hydrocarbons were produced during this process. Therefore, our focus in this study is to get new insights into the formation of R-OH and R-COOH, as the formation of other liquid products is limited. Fig. 1 shows the effect of Cu-based catalysts and the corresponding supports on the generation of R-OH and R-COOH. Clearly, there is no rule regarding the effect of supports on the selectivity of oxygenates when packing pure support material into the DBD reactor. The distribution of R-OH and R-COOH selectivity, however, changed regularly after anchoring copper on the corresponding support. That is, the selectivity of R-OH using the Cu-based catalysts decreased in the order of Cu/Al(OH)3 > Cu/Mg(OH)2 > plasma only > Cu/SiO2 > Cu/TiO2 > Cu/HZSM-5 > HZSM-5, whereas the selectivity of R-COOH decreased in the opposite order. Therefore, Cu anchored to these materials can significantly tune the distribution of oxygenates. Moreover, when compared to the reaction using plasma only, the coupling of DBD with Al(OH)3, Cu/Al(OH)3 or Cu/Mg(OH)2 favored the production of R-OH, but inhibited the formation of R-COOH. This phenomenon was more pronounced for Cu/Al(OH)3. On the contrary, using Cu/HZSM-5 or HZSM-5 inhibited the formation of R-OH whilst enhancing the production of R-COOH. It is worth noting that packing Mg(OH)2 or TiO2 into the plasma area had a clear negative effect on the formation of oxygenates, but the generation of oxygenates was greatly enhanced by anchoring Cu.In addition, all of the cases gave a similar distribution of gaseous products, with CO and C2-C4 hydrocarbons being the most common gaseous products (Fig. S3). Moreover, Cu anchored on the supports slightly influenced the conversion of CO2 and CH4, as shown in Fig. 1(c) and (d). However, the CH4 conversion was almost twice the CO2 conversion in all cases, which agrees well with the results of the 0D chemical kinetic modeling of the CO2/CH4 plasma reaction [31]. A possible explanation for the lower CO2 conversion than CH4 is the regeneration of CO2 in plasma. Yao et al. reported that CO2 can be easily regenerated through the reaction CO + OH → CO2 + H in CO2/CH4 plasma [32], but Snoeckx et al. reported the charge transfer reaction, CO2 + + CH4 → CH4 + + CO2, contributes to 99% of the regeneration of CO2, which is considered to be the main reason for lower CO2 conversion than CH4 [31].To understand the difference in the selectivity between R-OH and R-COOH over Cu-based catalysts in Fig. 1(a), a series of techniques, such as XRD, XPS, HRTEM, H2-TPR and NH3-TPD, were used to characterize the as-prepared catalysts. As inferred from XRD profiles in Fig. 2(a), the as-prepared Cu-based catalysts show exclusively CuO phase with dominate characteristic peaks at 35.5° and 38.8° (JCPDS 48-1548), assigned to (11−1) and (111) plane of CuO, respectively [33]. Fig. 2(b) displays the reducibility of the as-prepared Cu-based catalysts changed when using different supports, reflecting the different interactions between copper species and support, i.e., Cu-O-M interfaces are formed (M is Al, Mg, Si, Ti and Al/Si). The Cu/Mg(OH)2 catalyst showed the highest reduction temperature, revealing that Mg(OH)2 has the strongest interaction with copper species. By contrast, Al(OH)3 and HZSM-5 exhibited weak interactions with copper species. Except for Cu/HZSM-5, other Cu-based catalysts showed a broad reduction peak, which can be assigned to the reduction of crystalline CuO since it follows a one-step reduction mechanism, i.e., CuO + H2 → Cu0 + H2O. Whereas the reduction of copper species on Cu/HZSM-5 is more complicated since it contains the reduction of both crystalline CuO and isolated Cu2+ species. In contrast to crystalline CuO, the reduction of isolated Cu2+ species is characterized by a two-step mechanism, i.e., Cu2+ + 1/2 H2 → Cu+ + H+ and Cu+ + 1/2 H2 → Cu0 + H+ [34–36]. Thus, the results of H2-TPR reveal the crystalline CuO is the major copper species in as-prepared catalysts, which is consistent with the CuO crystalline phase detected in XRD patterns.Furthermore, the HRTEM images of the catalysts showed the lattice fringes of CuO ( Fig. 3), with d-spacings of 0.25 nm and 0.23 nm, respectively, obtained from interference fringes, representing CuO (11−1) and CuO (111) plane, respectively. Apart from the cases of Al(OH)3 and SiO2 supports, the interfaces between CuO and catalyst supports can be observed in their HRTEM images, which confirms the formation of Cu-O-M interfaces as inferred from H2-TPR profiles,. Because both Al(OH)3 and SiO2 are amorphous, it is difficult to observe the interface between CuO and Al(OH)3 or SiO2 through the lattice plane in their HRTEM images.In addition to crystalline CuO as identified above, Cu 2p3/2 XPS fitting curves shown in Fig. 4(a) reveal that surface Cu species in all as-prepared catalysts exist in both Cu2+ and Cu+ forms [37,38]. However, the relative distribution of Cu2+ and Cu+ varied with the type of the support according to the deconvolution of the Cu 2p3/2 major peak, and the amount of Cu2+ species followed the descending order of Cu/Al(OH)3 > Cu/Mg(OH)2 > Cu/SiO2 > Cu/TiO2 > Cu/HZSM-5. Moreover, the amount of Cu2+ can also be estimated from the peak area of the Cu2+ satellite peak, since the Cu2+ species are featured by its large satellite peak, whereas satellite peaks are absent in the spectra of Cu+ or Cu0 species. As shown in Fig. 4(b), the peak area of the Cu2+ satellite peak, with binding energies between 940 and 945 eV, decreased in the order of Cu/Al(OH)3 > Cu/Mg(OH)2 > Cu/SiO2 > Cu/TiO2 > Cu/HZSM-5, which agrees well with the changing tendency of Cu2+ amount obtained by deconvolution of Cu 2p3/2 main peak. Among these as-prepared catalysts, the copper supported on Al(OH)3 had the most surface Cu2+ and the least surface Cu+, whereas the copper supported on the HZSM-5 zeolite showed the complete opposite results.In addition, Fig. 4(a) shows that the binding energy of the Cu 2p3/2 varied with support, with varying magnitudes. When comparing Cu/Al(OH)3 to Cu/Mg(OH)2, a noticeable shift to lower binding energy of Cu2+ was observed, which can be further confirmed by the down-shift binding energy of the Cu2+ satellite peak. Espinós et al. found that the binding energies of copper supported metal oxides are very sensitive to copper dispersion and the type of support on which copper is deposited, i.e., the nature of interactions between copper oxide and support [37]. In this study, all of the Cu-based catalysts had a comparable average particle size of CuO in the range of 22–25 nm calculated by Scherrer’s formula (Table S1). Thus, the variance in Cu 2p3/2 binding energy in this study was mainly caused by the varied chemical natures of the supports, as evidenced by the H2-TPR results (Fig. 2(b)). Obviously, the interaction of CuO with support varied depending on the type of support, with Mg(OH)2 having the strongest interaction with CuO, whereas Al(OH)3 showed the weakest interaction with CuO (Fig. 2(b)). This explains the huge difference in Cu 2p3/2 binding energy between Cu/Mg(OH)2 and Cu/Al(OH)3.It is worth noting that the correlations between the molar ratio of R-COOH/R-OH and the valence state of copper, indicated by the Cu+ percentage of the Cu+ and Cu2+, are well established, as shown in Fig. 4(c). The ratio of R-COOH/R-OH increased with the amount of Cu+, demonstrating an approximately linear relationship. This finding suggests that Cu2+ species are beneficial to the formation of R-OH, whereas Cu+ species are desirable to form R-COOH. Among these Cu-based catalysts, Cu/Al(OH)3 possessed the largest quantity of Cu2+. This explains why, in the case of plasma only, the distribution of oxygenates shifted significantly from a mixture of R-OH and R-COOH, towards almost exclusively R-OH when packing Cu/Al(OH)3 into the plasma zone, as shown in Fig. 1(a). By contrast, Cu/HZSM-5 with the highest Cu+ content demonstrated strong activity toward R-COOH formation. This finding suggests that the valence state of copper is one of the crucial factors to improve the distribution of oxygenated products in plasma-catalytic conversion of CO2 and CH4.The best catalysts for the formation of alcohols and acetic acid in this study were Cu/Al(OH)3 and Cu/HZSM-5, respectively. Thus, these two catalysts were analyzed by XPS after the reaction, as shown in Fig. S4. The Cu2+ content of the Cu/Al(OH)3 catalyst decreased after the reaction, but a visible peak of Cu2+ satellite peak emerged on Cu/HZSM-5, indicating a slight increase in Cu2+ content. Furthermore, there is no peak shift in Cu 2p2/3 spectra on Cu/HZSM-5 before and after the reaction.Interestingly, pure HZSM-5 support was found to be the most beneficial catalyst for the formation of acetic acid (Fig. 1(a)), but its activity toward acetic acid generation decreased after anchoring Cu, although Cu/HZSM-5 had the highest Cu+ amount. To be explicit, the Cu/HZSM-5 catalyst was also prepared using the ion exchange method (IE), and its catalytic performances are compared with those of Cu/HZSM-5 (IM) in Fig. 5.Apparently, the method for the preparation of Cu/HZSM-5 had a significant influence on the selectivity of R-OH and R-COOH. The Cu/HZSM-5 (IE) catalyst inhibited the formation of R-OH, but greatly enhanced the generation of R-COOH, resulting in higher total selectivity of oxygenates. Based on the XRF results (Table S2), the copper efficiencies of the Cu/HZSM-5 catalysts prepared by IE and IM methods, expressed by the mole numbers of the produced oxygenate per mole copper, are given in Fig. 5. The Cu/HZSM-5 (IE) clearly showed a much higher copper efficiency than the Cu/HZSM-5 (IM), especially in terms of R-COOH production. In addition to higher R-COOH selectivity, the conversion of CO2 was also enhanced, as shown in Fig. 5(b), and the gap between the conversion of CO2 and CH4 was narrowed by using the Cu/HZSM-5 (IM) catalyst. This finding in turn supports the increased selectivity of R-COOH over Cu/HZSM-5 (IE), since the stoichiometric ratio of CH3COOH formation from CO2 and CH4 molecules is 1:1, which means that the selectivity of CH3COOH approaches the theoretically maximum value only when the conversion of CO2 is equal to that of CH4.To understand the substantial differences in the catalytic activity of the Cu/HZSM-5 catalysts prepared by IE and IM methods, their chemical properties were comparatively investigated. As shown in Fig. 6(a), different from the Cu/HZSM-5 (IM) catalyst, no peak associated with copper species was observed in the XRD pattern of the Cu/HZSM-5 (IE) catalyst, indicating that copper species were highly dispersed on Cu/HZSM-5 (IE). This finding can be further confirmed by EDS element mapping images of Cu/HZSM-5(IE) in Fig. 7. Clearly, the Cu mapping image showed a homogeneous distribution of copper species, and the location of the Cu element appearing is almost always accompanied by the Al element but not for the Si element. This exhibits the feature of ion-exchange zeolite, i.e., Cu ion exchanges with H sites located at the Si-(OH)-Al unit of HZSM-5. However, where Al appeared, in turn, Cu was not always there by comparing the Al mapping with the Cu mapping in Fig. 7. These findings suggest that H sites were partially substituted by Cu ions, which is consistent with the low loading of Cu obtained on the Cu/HZSM-5 (IE) catalyst (Table S2) and the high dispersion of Cu ions on HZSM-5. These results reveal that copper species anchored on HZSM-5 mainly exist in the form of isolated copper species when using the ion-exchange method for catalyst preparation, but in the form of crystalline CuO when using the impregnation method (Figs. 2 and 3).XPS was then used to identify the electron structure of this type of isolated copper species, as shown in Fig. 6(b). The isolated copper species mainly exist in the form of Cu+ on Cu/HZSM-5 (IE) catalyst, according to the lack of the characteristic satellite peak of Cu2+ in Cu 2p3/2 XPS spectra. That is, the ion-exchange method is more favorable for Cu+ formation than the IM method. A similar result was also reported by Wu et al. [39]. Combined with the performance of Cu/HZSM-5 (IE) in Fig. 5, the high amount of Cu+ on Cu/HZSM-5 (IE) is one reason for the high activity toward CH3COOH formation, but low activity to C1-C4OH generation, which is consistent with the conclusion reached above, highlighting again that Cu+ enhances the formation of R-COOH.Furthermore, the reducibility of the Cu/HZSM-5 catalysts using different preparation methods was comparatively investigated by H2-TPR analysis, as shown in Fig. 6(c). Cu/HZSM-5 (IE) displayed two small H2-consumption peaks due to its low Cu amount (Table S2), i.e., the low-temperature and the higher-temperature peaks in the range of approximately 200–600 °C, which can be correspondingly assigned to the reduction of isolated Cu2+ to Cu+ and Cu+ to Cu0 according to the literature [34–36]. On the one hand, the H2-TPR results reveal that in addition to the main species of Cu+ confirmed by XPS in Fig. 6(b), the isolated Cu2+ species, exist in the Cu-exchange HZSM-5 (IE) catalyst, as evidenced by the typical two-step reduction of isolated Cu2+, i.e., Cu2+ to Cu+ and Cu+ to Cu0, whereas crystalline CuO has a one-step reduction mechanism from Cu2+ to Cu0, as described in Fig. 2(b). On the other hand, Cu+ species were further confirmed to be the main isolated copper species, not Cu2+, since the area of the higher-temperature peak was much higher than that of the low-temperature peak in the H2-TPR profile of Cu/HZSM-5 (IE). Otherwise, the area of the low-temperature peak should be approximately equal to that of the high-temperature peak, if Cu2+ is the unique isolated copper species. Although the H2-TPR results agree with the results of the XPS analysis in Fig. 6(b), the H2-TPR and XPS analyses gave different amounts of Cu2+. H2-TPR showed a certain amount of Cu2+ existed in Cu/HZSM-5 (IE), whereas only a very small amount of Cu2+ was obtained by XPS. This finding is mainly caused by the feature of the XPS technique that only copper species located at a depth lower than 10 nm can be detected.In contrast, an overlapping peak was observed in the H2 consumption profile of the Cu/HZSM-5 (IM) catalyst (Fig. 6(c)), which can be ascribed to the reduction of crystalline CuO particles, as supported by the XRD results (Fig. 6(a)) and HRTEM analysis (Fig. 3). The overlapped peak consists of four peaks (denoted as peak A-D) that were fitted using Gaussian deconvolution. According to the literature [40], isolated Cu2+ species may also exist in addition to the main reduction peak of crystalline CuO. Peak A, located at a low temperature, represents the reduction of isolated Cu2+ to Cu+; Peak B, located at a moderate temperature, being a major one, is assigned to the one-step reduction of CuO particles to Cu metal; Peak C, appeared at a high temperature, represents the reduction of isolated Cu+ to Cu0; Peak D may be related to the interaction of crystalline CuO and HZSM-5. In fact, the existence of isolated copper species on HZSM-5 is reasonable when using the IM method since the IM process is inevitably accompanied by ion-exchange with regard to HZSM-5. Therefore, these characterization results highlight that crystalline CuO is the dominant copper species on Cu/HZSM-5 (IM). On the Cu/HZSM-5 (IE) catalyst, however, only isolated copper species exist, i.e., isolated Cu+ and Cu2+ with Cu+ ions being the most abundant.In addition, the acidity of the Cu/HZSM-5 catalysts prepared by the IE and IM methods was determined using NH3-TPD and Py-FTIR, as shown in Fig. 6(d) and Fig. S5, using the pure HZSM-5 zeolite as a reference. The NH3-TPD profile of pure HZSM-5 displays two desorption peaks in the 150–300 °C and 300–600 °C ranges, representing abundant weak acid sites and a minor quantity of strong acid sites on HZSM-5, respectively (Fig. 6(d)). In addition, the Py-FTIR spectra of pure HZSM-5 shows that both Brønsted acid (H) sites and Lewis acid sites coexist in HZSM-5, with respective infrared bands at around 1540 and 1455 cm−1 probed by pyridine [41] (Fig. S5). When compared to HZSM-5 in Fig. 6(d), the peak area of the high-temperature desorption peak was significantly increased after anchoring Cu on HZSM-5 using the IM method, while the peak area of the low-temperature desorption peak was slightly lowered. This finding suggests that Cu anchored on HZSM-5 via the IM method increased the amount of strong acid sites while decreasing the number of weak acid sites. However, the corresponding Py-FTIR spectrum of Cu/HZSM-5 (IM) presented a significant drop in both H sites and Lewis acid sites (Fig. S5), with the H sites almost completely disappearing. The NH3-TPD and Py-FTIR studies of Cu/HZSM-5 (IM) reveal that the increased part of strong acid sites is mostly due to the addition of crystalline CuO (Fig. 6(a)) as strong Lewis acidic species. By contrast, the NH3-TPD profile and Py-FTIR spectrum of Cu/HZSM-5 (IE) were totally different, as shown in Fig. 6(d) and Fig. S5. The Cu-exchange HZSM-5 catalyst results in a decrease in both the acid amount and acid strength, as well as H sites of HZSM-5 due to the exchange of Cu ion with H sites in the Si-(OH)-Al unit of HZSM-5. It should be noted that a fraction of the H sites in HZSM-5 remained in the Cu/HZSM-5 (IE) catalyst, almost all of the H sites, however, disappeared in the Cu/HZSM-5 (IM) catalyst.Combing with the corresponding performance of pure HZSM-5, Cu/HZSM-5 (IM) and Cu/HZSM-5 (IE) as shown in Fig. 1 and Fig. 5, pure HZSM-5 possessed the highest amount of Brønsted acid sites and exhibited greatest activity toward R-COOH formation. Compared to pure HZSM-5, the H sites in Cu/HZSM-5 (IM) disappeared, and the associated selectivity of R-COOH decreased as well, despite the fact that Cu+ in Cu/HZSM-5 (IM) had a beneficial effect on R-COOH formation (Fig. 4(c)). Furthermore, Cu/HZSM-5 (IE) with a portion of H sites displayed higher activity toward R-COOH than Cu/HZSM-5 (IM) without H sites. Interestingly, although Cu/HZSM-5 (IE) had fewer H sites than pure HZSM-5, Cu/HZSM-5 (IE) showed comparable R-COOH selectivity, which can be attributed to the synergy of Cu+-H site as reported in the literature [42]. These findings indicate that Brønsted acid sites may be another important factor for tuning R-COOH selectivity in plasma-catalytic conversion of CO2 and CH4 in addition to the copper valence state. Song et al. found that a higher ratio of weak acid sites is beneficial for the generation of oxygenates, such as formaldehyde, methanol, ethanol, and acetone, in the plasma-catalytic conversion of CO2 and CH4, but the relationship between weak acid sites and the distribution of oxygenates was not demonstrated [28]. In this study, the roles of Brønsted acid sites in enhancing the formation of R-COOH will be discussed as follows.Different from catalytic reactions, CO2 and CH4 molecules in the plasma-catalytic system are pre-activated by energetic electrons into active species in Fig. S6, such as excited CO2* , CH4* and CO* , CH3, CH2, CH, H and O radicals [30,43]. According to the simulation results reported by De Bie et al. [44], CH3 radicals are much more abundant than CH species in the CH4-CO2 DBD. In addition, OH could be produced via three main reactions, i.e., CH + O → C + OH, H2 + O → OH + H and CO2(v) + H → CO + OH, which has been confirmed in detail in our previous studies [30].In this study, the proportion of CH3OH in the R-OH products exceeded 60%, followed by ethanol. That is, the enhanced formation of Cu2+ species in the production of R-OH can also be considered to promote the formation of CH3OH. Generally, Cu2+ species are accepted as the active sites in the oxidation of CH4 to CH3OH using O2 or N2O as an oxidant [45,46]. Herein, the active O species (777.5 nm, 3s5S0 → 3p5P; and 844.7 nm, 3s3S0 → 3p3P) [47], presented in Fig. S6, are highly efficient for oxidation. Therefore, it is reasonable to hypothesize that Cu2+ species act as the active sites for CH4 oxidation to CH3OH with O species produced from CO2 splitting in this study. As shown in Fig. 8, possible pathways are proposed for plasma-catalytic conversion of CO2 and CH4 to CH3OH.Methanol can be produced through the CH4 oxidation reaction. That is, Cu-CH3, as the important intermediate species, could be formed by CH4* dissociation or CH3 adsorption on copper sites. Subsequently, the resulting Cu-CH3 species react directly with gaseous OH to produce CH3OH in pathway ①. Alternatively, as shown in pathway ②, O radicals could insert into the Cu-C bond of Cu-CH3 to form Cu-OCH3 (methoxy) according to the literature [42] and followed by methoxy protonation with H radicals or nearby H adsorbed (Had) on the catalyst for the final synthesis of CH3OH. In addition, the recombination of gaseous CH3 and OH radicals can result in the formation of CH3OH [30].In addition, another way to generate methanol in this study is the CO2 hydrogenation reaction, i.e., the CO2 to CH3OH pathway, and the H atoms derive from CH4 splitting in plasma, which is supported by the existence of H radials (656.3 nm, 3d2D → 2p2P0) in the emission spectra of the CO2/CH4 plasma (Fig. S6) [48,49]. As shown in Fig. 8, CO (Angstrom bands at 451 −608 nm), formed in the plasma gas-phase reactions (Fig. S6), can be directly adsorbed onto the copper center, followed by stepwise hydrogenation with H radicals or Had to form CH3OH with HCOad, H2COad , and H3COad being the adsorbed intermediates in pathway ③. On the other hand, gaseous CO2 * could be adsorbed onto a copper center to form Cu-CO2, and subsequently, the Cu-CO2 is hydrogenated with H radicals or Had step by step via formate pathways, resulting in the formation of H3COad intermediate in the pathway ④. Finally, H3COad reacts with H radicals or Had to produce CH3OH. In our previous study, we described in detail the above surface-reaction pathways that lead to the formation of CH3OH during plasma-catalyzed CO2 hydrogenation [50]. Fig. 9 shows four main pathways proposed for plasma-catalytic conversion of CO2 and CH4 to CH3COOH. Pathway ①, Brønsted acid sites can enable CH3COOH generation directly through the CO2 protonation pathway even without Cu center in the catalyst, i.e., gaseous CO2* react with H sites in the Si-(OH)-Al unit of HZSM-5 to yield -COOH species [51], followed by direct C-C coupling with gaseous CH3 radicals to produce CH3COOH via the E-R process. In this way, the formation of CH3COOH is determined by the amount of Brønsted acid sites, and this pathway is well supported by the experimental results, i.e., pure HZSM-5, which possessed the highest amount of Brønsted acid sites, exhibited higher activity toward CH3COOH in the absence of a Cu center. Pathway ②, the resulting -COOH species can also couple with neighboring Cu-CH3 species, which is produced by CH4 dissociation on a Cu center [51–56] to form CH3COOH in the presence of a copper center. Pathway ③, gaseous CO2* is directly inserted into the bond of Cu-CH3 to form Cu-OOCCH3 (acetate species) [52–54] and subsequently, CH3COOH is formed via proton transfer from the adjacent Brønsted acid site to the Cu-OOCCH3 species. Alternatively, in Pathway ④, gaseous H radicals react directly with Cu-OOCCH3 to form CH3COOH, which accounts for CH3COOH generation on Cu-based catalysts lacking or having fewer Brønsted acid sites, such as Cu/SiO2, Cu/TiO2 and Cu/HZSM-5 (IM) in this study. In addition, surface -OH Brønsted acid sites can be generated by CH4 splitting on a metal center (M) into M-CH3 and M-H, with the H from M-H eventually bonding to the adjacent O site of zeolite or the surface O of oxides to form -OH [52–54], which maintains the catalytic cycle of Brønsted acid sites involved in Fig. 9.It is worth noting that both pathways ② and ③ reveal that the formation of CH3COOH results from a synergy of metal and Brønsted acid sites, which necessitates the development of a bi-functional catalyst. In this study, Cu-exchanged HZSM-5 serves as a bifunctional catalyst due to the co-existence of isolated Cu+ and Brønsted acid sites, enabling CH3COOH production through pathways ② and ③, as well as pathway ① due to accessible Brønsted acid sites. This explains why Cu-exchanged HZSM-5 showed fewer Brønsted acid sites than pure HZSM-5 while still exhibiting strong CH3COOH selectivity. The above findings suggest that acid sites are another important factor in tuning the selectivity of acetic acid, and the metal-Brønsted acid site synergy provides a potential strategy to enhance the formation of R-COOH. However, Brønsted acid sites are not required for R-OH formation.Tosi et al. found that direct coupling of gaseous CH3 with COOH species results in CH3COOH production in addition to the formation of CH3COOH on the surface of the catalysts, and the COOH species were produced in the gas phase via CO2 protonation with gaseous H radicals [19]. Our previous study also confirms the formation pathway of CH3COOH by recombination of gaseous CH3 radicals with COOH species [30].Direct conversion of CO2 and CH4 to high-value oxygenates such as alcohols (R-OH), acids (R-COOH), acetaldehyde and acetone with R-OH and R-COOH being the major oxygenates, was successfully achieved over the Cu-based catalysts driven by plasma at 60 °C and atmospheric pressure. For the first time, the correlations of oxygenate distribution versus copper valence state were experimentally confirmed. Cu2+ species exhibit superior activity towards the formation of R-OH products. Cu+ species, however, are critical for the generation of R-COOH. In addition to Cu+ species, Brønsted acid (H) sites on the Cu-exchange HZSM-5 catalyst also promote the synthesis of R-COOH via the CO2 protonation route, as well as the synergy of isolated Cu+ and H sites. These findings, i.e., the correlation between valence state/Brønsted acid and the catalyst activity, provide valuable insights into improving existing catalyst performance and designing cutting-edge highly selective catalysts for the oriented generation of oxygenates from plasma-catalytic CO2 and CH4 conversion. Yuezhao Wang: Conceptualization, Validation, Formal analysis, Resources, Data curation, Writing – original draft, Writing – review & editing. Linhui Fan: Conceptualization, Validation, Formal analysis, Resources, Data curation, Writing – original draft. Hongli Xu: Validation, Formal analysis, Resources, Data curation. Xiaomin Du: Validation, Formal analysis, Resources. Haicheng Xiao: Supervision, Funding acquisition. Ji Qian: Resources, Data curation. Yimin Zhu: Resources, Supervision. Xin Tu: Conceptualization, Formal analysis, Writing – review & editing, Supervision, Funding acquisition. Li Wang: Conceptualization, Validation, Formal analysis, Resources, Data curation, Writing – original draft, 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.We acknowledge financial support from the National Natural Science Foundation of China (No. 21908016), the PetroChina Innovation Foundation (No. 2019D-5007-0407) and the LiaoNing Revitalization Talents Program (No. XLYC1907008). X. Tu acknowledges the funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 813393.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcatb.2022.121583. Supplementary material . Supplementary material .

Direct conversion of CO2 and CH4 into value-added oxygenates under mild conditions is highly desirable since it has great potential to deliver a sustainable low-carbon economy and a carbon-neutral ecosystem. However, tuning the distribution of oxygenates in this process remains a major challenge. Here, the electronic structure and acidic properties of copper-based catalysts were exploited as strategies to tune the distribution of oxygenates (alcohols and acids) in the plasma-catalytic conversion of CO2 and CH4 at a reaction temperature of 60 °C and atmospheric pressure. We use support, on which copper is anchored, to regulate the distribution of Cu2+ and Cu+ in the Cu-based catalysts. Comprehensive characterization of the catalysts together with the reaction performances reveals that Cu2+ species are favorable to the formation of alcohols, whereas Cu+ species are critical to enhancing acetic acid production. Furthermore, the Brønsted acid sites of HZSM-5 significantly improved the selectivity of acetic acid, while the synergy of isolated Cu+ center and Brønsted acid sites, developed via Cu-exchange HZSM-5, exhibits potential for acetic acid formation. Finally, possible pathways for the formation of alcohols and acetic acid have been discussed. This work provides new insights into the design of highly selective catalysts for tuning the distribution of alcohols and acids in the plasma-catalytic conversion of CO2 and CH4 to oxygenates.

No data was used for the research described in the article.Over the years, the world has experienced climatic change due to increasing atmospheric levels of greenhouse gases (GHGs), which are responsible for global warming [1]. CO2 has been identified as the dominant contributor to global warming, accounting for over 65 % of the total annual GHG emissions [2]. The global CO2 emissions from combustion of fossil fuels for energy production and industrial processes have been increasing tremendously [3]. Although in 2020, there was an observed slight decline in CO2 emissions of about 8 % compared to 2019. This reduction was attributed to a sharp temporary cutback in global atmospheric CO2 emissions due to the COVID-19 pandemic and the corresponding restrictions resulting into limitations on the use of petroleum products. But this decline in CO2 emissions did not result into a significant reduction in the CO2 atmospheric levels due to the prevailing residual amount of this gas and emissions from other sectors. Moreover, recent figures show that a 6 % increase in CO2 emissions was observed in 2021 compared to 2020, which was partly attributed to the high energy demand during economy recoveries after easing the COVID-19 restrictions [3–5].Regulations and strategies to control the increasing atmospheric CO2 levels have been proposed by governments and the private sector [6]. Among the strategies designed to curb the CO2 problem, are the technologies for carbon capture and storage (CCS). Under CCS, CO2 is trapped at point sources such as industries and thermal power plants, and then stored in geological formations or under ocean beds. However, the volatile nature of CO2 and the high energy costs involved in these processes make them uneconomical. In this respect, effort has been directed towards the development of efficient carbon capture and utilization (CCU) technologies. The CCU approach, transforms the captured CO2 into value added chemical products such as fuels, polymers, among others [7,8]. Considerable effort has been made to design catalytic processes for CO2 conversion because it is an inexpensive, readily available, non-toxic, and renewable carbon resource [9,10]. However, due to the thermodynamic stability of CO2, it is usually challenging to activate in typical reactions and its utilization as a raw material on a large scale is still lacking. Nonetheless, some industrial processes utilizing CO2 as a chemical feedstock have been reported including synthesis of urea, salicylic acid, inorganic carbonates, organic carbonates and polymers [11,12].Recently, CO2 utilization as a feedstock in industrial polymer production has attracted a lot of attention, due to the long term storage potential of this greenhouse gas in the copolymers [13]. Polymers that contain CO2 groups are commonly known as polycarbonates and the aliphatic ones like poly(propylene)carbonate)PPC can undergo biodegradation at the carbonate linkages producing water and CO2 as byproducts [13]. This is in sharp contrast to the common non-biodegradable petroleum derived polymers such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), and polystyrene (PS) [14]. Currently, most polycarbonates are produced via an interaction between bisphenol-A and diphenyl carbonate or through a reaction of the sodium salt of bisphenol-A and phosgene to form poly (bisphenol-A carbonate) (BA-PC). However, this approach produces large amounts of phenolic by-products [15] not to mention the hazardous nature of phosgene [16]. An improved approach in the synthesis of BA-PC from Bisphenol A was reported by Asahi Kasei, where the phenol generated is recycled and the CO2 produced utilized as one of the starting materials, eliminating the need for use of phosgene [17,18].A sustainable approach to the preparation of polycarbonates was first demonstrated by Inoue and co-workers [19]. They reported an alternating copolymerization of propylene oxide (PO) with CO2 which resulted into an aliphatic poly(propylene)carbonate (PPC) as shown in Scheme 1 . Since its discovery, there has been growing interest in large scale production of biodegradable CO2-based polycarbonates. The CO2-based aliphatic polycarbonate plastics have excellent properties such as biodegradability, high transparency, UV stability, thermoplasticity and high young’s modulus [20]. This makes CO2-based polymers useful raw materials in the manufacture of barrier materials, plasticizers, plastic film products, medical plastics, reactive hot melt adhesives, binders, and many other products [13,21].In comparison to the current industrial processes for polycarbonate synthesis, the CO2 and epoxide copolymerization approach is advantageous because it lowers the amount of fossil fuel-based monomer raw materials incorporated in the polymer. When CO2 is used as the monomer, the production of PPC saves about 43 % by mass of petrochemical resources in comparison to poly(propylene oxide) (PPO) prepared by PO homopolymerization. Moreover, PPC formation usually follows chain growth mechanisms, which theoretically imply that the copolymerization of CO2 and PO has better control of molecular weight and selectivity, unlike BA-PC which follows step-growth mechanisms [22]. Moreover, the epoxide monomers involved in the process are not as toxic as phosgene that has been largely used in polycarbonate production processes [16].Nevertheless, the CO2-epoxide copolymerization process is still limited in its industrial application for polycarbonate synthesis, primarily due to CO2 thermodynamic stability. CO2 being in the most highly oxidized state of carbon, has a more negative standard Gibbs free energy (ΔG = −394.37 KJmol−1) in comparison to other C1 molecules. Thus, CO2 activation and utilization in chemical production requires a lot of energy [23]. The high energy barrier in CO2 activation can be overcome by either direct input of energy or via reaction of CO2 with chemically reactive species [24]. Due to the temperature-sensitive nature of the products of the CO2 based copolymerization, use of chemically reactive species in the presence of suitable catalysts is the preferred option [25,26]. Nonetheless, the advantages of copolymerization of CO2 with other monomers to form polycarbonates still outweigh the thermodynamic challenges involved in the process.Several copolymers have been synthesized from the reaction of CO2 and energy-rich molecules such as amines, epoxides, aziridines, episulfides and oxetanes [24,27]. However, the most widely studied copolymerization reaction is that of CO2 with PO to form PPC [28,29]. It should be noted that in the CO2 and PO co-polymerization process, the carbon dioxide molecule does not undergo reduction. The inert nature of CO2 is overcome by the energy (≈114 kjmol−1) released during the PO ring opening process [30]. The earlier work of the CO2/PO copolymerization successfully produced an alternating aliphatic copolymer of PPC with a high molecular weight [19]. Ever since, the copolymerization reaction triggered interest of several researchers to not only increase yields of the synthesized PPC but also to improve the selectivity towards CO2 insertion into the polymer chain. The PPC with high CO2 content possesses unique properties such as biodegradability and excellent oxygen barrier performance that has made it a potential raw material in substances used in food packaging, healthcare and automotive industries [13]. However, due to the amorphous nature of PPC and relatively low glass-transition temperature (Tg), its application in comparison to aromatic polymers like BPA-PC is still limited [21]. In this respect, more effort is still being sought to design efficient catalyst systems for the copolymerization of CO2 and epoxides to produce polycarbonates of desirable chemical and mechanical properties.In a typical CO2 and PO copolymerization reaction, there are two common side reactions that can occur. The first is the cyclization or back-biting reaction, and the second is the consecutive insertion of the epoxide monomer into the growing polymer chain. The chain back-biting reaction leads to the depolymerization of the polycarbonate to form the thermodynamically stable cyclic carbonate [31]. Consecutive insertion of the epoxide leads to the formation of ether bonds in the copolymer, which lowers the Tg of the polymer and compromises its material properties at room temperature [32].In the presence of a suitable catalyst, the CO2 and PO copolymerization reaction follows a coordination-insertion mechanism as illustrated in Scheme 2 . The metal center of the catalyst is coordinated to the epoxide, followed by a nucleophilic attack which results in ring opening to form a metal alkoxide. PO being an aliphatic epoxide, its ring-opening is usually favored at the least hindered CO bond, and the CO2 molecule is then inserted into the metal–oxygen bond resulting in the formation of a metal carbonate. Depending on the selectivity of the catalyst and reaction conditions, different pathways can be undertaken after this step. The metal carbonate can undergo cyclization or back-biting (path 1) to form the propylene carbonate by-product, regenerating the metal alkoxide which can propagate the reaction further [33]. On the other hand, propagation by multiple coordination and insertion of CO2/PO producing the polycarbonate chain (path 2) can also occur. This would result in the formation of a copolymer with alternating CO2/PO linkages. However, homopolymerization of epoxide can also occur and in this case, metal alkoxides successively attack the epoxide monomer instead of CO2 insertion (path 3) forming ether linkages in the copolymer [25].Moreover, chain transfer reactions can also take place when the copolymerization is conducted in the presence of protic compounds such as water (in the epoxide monomer), forming hydroxyl-terminated copolymers and metal alkoxides or hydroxide species [34]. The molecular weight (MW) of the formed copolymers is believed to be reliant on the amount of polymer formed in a living polymerization reaction. In this case, the degree of polymerization is dependent only on catalyst concentration which eventually results in a linear increase of polymer MW with monomer conversion [35]. However, experimental results generally show lower values of the MW for the polycarbonate than expected implying that MW is dependent on both chain transfer agents and catalyst concentrations [35,36].In PO and CO2 copolymerization, although PO ring-opening is favored at the methylene carbon–oxygen (–CH2-O-) bond due to lower steric hindrance, cleavage can also occur at the methine carbon–oxygen (–CH-O-) bond, giving rise to differences in the regioselectivity of the resultant polymers [37]. Spectroscopic studies of the copolymers show that the 13C NMR spectrum gives three signals in range of 154.0–155.2 ppm typical of carbonyl region for polycarbonates. These signals are usually assigned to head-to-head (HH), head-to-tail (H-T), and tail-to-tail (T-T) linkages in the ratio of 1:2:1 respectively as illustrated in Scheme 3 . The H-T linkages are formed by successive ring opening at the same carbon center, while the HH and T-T linkages are formed by sequential ring opening at the methine CO bond followed by ring opening at the methylene CO bond. Thus, the H-T linkages are the dominant linkages in the polycarbonate copolymers [37]. Additionally, different stereochemistries of the polycarbonates are also possible for the H-T linkages giving either isotactic (all the substituents have the same stereochemistry), or syndiotactic (alternating stereochemistries), or atactic (all substituents have random stereochemistry) polymer domains [13,25]. The catalyst systems utilized in the copolymerization reaction, exhibit differences in the regioselectivity of the formed polymer. For example, the catalyst developed by Quan et al., 2003, produced H-T linkages in higher concentrations of 70–77 % giving high Tg values of 37–42 °C, respectively [38]. Interestingly, some studies have shown that the regioregularity of polycarbonates is a key factor that influences material properties of the polymers [25].Ever since the successful copolymerization of CO2 and PO using an equimolar Et2Zn/H2O catalyst system [19], many catalyst systems of either heterogeneous or homogeneous nature [39], have been developed for the ring-opening copolymerization process (ROCOP). Several studies have investigated performance of homogeneous systems, owing to their high activities and selectivity for the CO2/PO copolymerization [40]. However, the use of these catalysts for chemical synthesis on an industrial scale is limited due to their complex synthesis, difficult separation of the catalysts from the product, and lack of recyclability [41]. In contrast, the ease of preparation, stability and recyclability have made heterogeneous catalyst systems attractive for large scale CO2/PO copolymerization. But the heterogeneous catalysts that have catalyzed this process with high polycarbonate selectivity, have often suffered low polymer yields while those with high activity usually exhibit low selectivity for PPC [25,26]. In this respect, significant research effort is still devoted to development of efficient heterogeneous catalysts for CO2 and PO copolymerization.In 1969, Inoue et al. utilized a ZnEt2/H2O (1:1 mol ratio) catalyst system for CO2 and PO copolymerization to form PPC. The reaction was carried out at 80 °C and 20–50 atm CO2 pressure for a period of 48 h, giving PPC with 88 % carbon content but in a low yield of only 4.2 g polymer/g catalyst [19]. When utilizing the ZnEt2/H2O catalyst system, the C2H5(ZnO)nH species produced from the hydrolysis of ZnEt2 was considered to be the active species responsible for the formation of the polycarbonate [42]. The DFT study done on the ZnEt2/H2O catalyst system by Pan et al.,2013 proposed a monometallic mechanism for the CO2/PO reaction. The low catalytic activity observed by this system was attributed to its monometallic mechanism unlike its homogeneous counterparts that usually follow a bimetallic mechanism [43]. Despite the poor activity of ZnEt2 /H2O catalyst system, it stimulated further research into similar systems utilizing different ZnEt2/active hydrogen-containing species. Studies have showed that compounds having two or more labile hydrogen atoms such as amines, alcohols or carboxylic acids coupled with ZnEt2 generated effective catalysts for the copolymerization of CO2 and PO [35,44]. On the other hand, ZnEt2/monoprotic sources of similar compounds only produced propylene carbonate [45].Kobayashi et al. studied the CO2 and PO copolymerization using ZnEt2 and diprotic phenolic compounds as catalysts in dioxane solvent at 35 °C and 30 atm [46]. Under optimal conditions, the best system was ZnEt2/resorcinol (1:1 mol ratio) which gave a higher copolymer yield of 3.5 g polymer/ g catalyst compared to the yield of 1.03 g/g catalyst obtained when using the Et2Zn/H2O (1:1) system under similar reaction conditions [46]. Using a similar approach, Kuran et al. investigated the CO2 and PO copolymerization catalyzed by ZnEt2 and di- or tri- protic compounds in 1,4-dioxane solvent at 35 °C and 60 atm of CO2. The ZnEt2/resorcinol catalyst was also among the most active systems giving a copolymer yield of 28 % with respect to PO used [47]. When utilizing Et2Zn/carboxylic acid systems, aromatic carboxylic acids displayed higher activities in the copolymerization reaction compared to their aliphatic counterparts [48]. The Et2Zn/primary amine (1:1) systems displayed comparable activities as the Et2Zn/H2O (1:1) systems while the Et2Zn/secondary amine systems hardly showed any activity in the copolymerization reaction [42]. Despite the interest and effort focused on the dialkylzinc based catalysts, these systems generally exhibited low turnover frequencies (TOFs) and give copolymers with very broad molecular weight distributions. These shortfalls could be partially attributed to the lack of optimal access to metal centers by monomers resulting into production of a wide range of polymers.The next major advancement in the CO2 and PO copolymerization reaction was reported by Soga et al. where the performance of metal salts of acetic acid [49], and air-stable salts of zinc hydroxide/dicarboxylic acid derivatives were investigated for this process [50]. Under solvent free conditions, at 80 °C and after a duration of 43 h the zinc acetate salt gave a copolymer yield of 2.3 g polymer/g catalyst with 100 % carbonate linkages (Fc), and weight average molecular weight (Mw) of 20,000 g/mol. Whereas the yield when using cobalt acetate was 0.89 g polymer/g catalyst (Fc = 100 %, Mw = 25,000 g/mol) after 70 h [49]. The copolymerization reactions performed using Zn(OH)2/dicarboxylic acids as catalysts showed that aliphatic dicarboxylic acids had superior activity compared to their aromatic counter parts [50]. The Zn(OH)2/glutaric acid (Zn(OH)2/GA) catalyst system turned out to be most active, giving a copolymer yield of 69.1 g polymer/g Zn, with a number average molecular weight (Mn) of 12,000 g/mol. The zinc glutarate (ZnGA) catalyst formed from Zn(OH)2/GA catalyst system gave an activity higher than that of the ZnEt2/H2O catalyst (23.8 g copolymer/g Zn) when tested under the same experimental conditions [50]. The ease of synthesis, non-toxicity, economic viability and high activity of ZnGA, have made it an attractive class of catalysts for industrial production of polycarbonates [32]. Extensive studies have been undertaken to improve the structure and surface properties of ZnGA using numerous zinc sources, combined with different synthetic approaches.In 1995, Darensbourg et al. synthesized ZnGA catalysts by addition of glutaric acid to zinc oxide (ZnO) in toluene which upon heating gave a white crystalline solid [51]. In their study, supercritical CO2 was found to be a suitable substitute for ordinary organic solvents in the CO2/PO copolymerization reaction. Under optimal conditions, a polymer yield of 15.9 g polymer/g Zn (Fc = 91 %, Mn = 26,783 g/mol) was attained when methylene chloride was used as a co-solvent [51]. Later, Ree et al. [52] studied the effects of different zinc compounds and glutaric acid derivatives on the synthesis of ZnGA, and performance of the subsequent catalysts. When ZnO precursor was used in ZnGA catalyst synthesis, it gave the best catalytic activity compared to other zinc sources. Under optimized reaction conditions this ZnGA catalyst, produced a high molecular weight copolymer (Mw = 343,000 g/mol, Mn = 143,000 g/mol, Mw/Mn (PDI) = 2.4), with a yield of 64.0 g polymer/g catalyst [52]. Although, the resultant PPCs were also amorphous, and exhibited a Tg of 38 °C and decomposition temperature (Td) of 252 °C under a nitrogen atmosphere [52].Studies also showed that the crystallinity and morphology of the ZnGA affected its activity. In 2002, Wang et al. successfully synthesized ZnGA catalysts using ultrasonic stirring. These catalysts exhibited high crystallinity and small particle size. They gave a higher catalytic performance compared to the ZnGA prepared using mechanical stirring methods. In addition, the PPC polymer obtained exhibited slightly high Tg (39.4 °C) and Td (278 °C) values compared to those reported in literature [53]. The ultrasonic stirring route was also used by Meng et al.[54], who reported high crystalline ZnGA catalysts with particle sizes in the range 0.2–0.3 μm, that produced PPC with a high molecular weight and in a good yield of 160.4 g polymer/g catalyst. In 2004, Eberhardt et al. successfully synthesized solid zinc glutarate catalysts with controlled amounts of Zn-ethyl sulfinate initiating groups [55]. When zinc ethyl sulfinate groups were incorporated in diethylzinc based carboxylates, the ZnGA catalysts that were formed showed enhanced catalytic activity up to a factor of 16 in the CO2/PO copolymerization reaction compared to those obtained with ZnO-based glutarates [55]. However, this catalyst modification had limited potential for industrial application because of its procedural complexity and need for use of expensive precursors.Investigations into the catalytic mechanism for the ZnGA system showed that there was adsorption of CO2 and PO onto ZnGA, but PO tended to be more easily adsorbed and inserted into the zinc oxide bond of the catalyst as compared to CO2. Implying that the catalyst surface played a key role in the process and could be modified by PO adsorption. This further implied that the reactivity of ZnGA in the copolymerization process is initiated by PO rather than CO2 [56]. Theoretical and experimental studies led to the proposal that a bimetallic catalytic pathway is followed for the CO2/PO copolymerization over the ZnGA system, involving successive insertions of CO2 and epoxide into Zn-alkoxide and Zn-carboxylate groups on the surface of the catalyst [57]. Further, the ideal separation between two adjacent Zn atoms which resulted in the optimal activation energy required for the copolymerization was suggested to be in the range 4.6–4.8 Å, as shown in the crystal structure of the synthesized ZnGA (Fig. 1 (a) [57]. In another study aimed at improving the frame work of ZnGA, a nanosized surface-etched ZnGA catalyst was prepared using a mild-HCl solution. This surface modified ZnGA exhibited increased productivity (∼83 %) in the copolymerization reaction with a turnover number (TON) of 132.1 g PPC/g catalyst superior to the standard-ZnGA [58]. Kim et al. 2004, also demonstrated that the catalytic activity of the ZnGA systems mainly originates from the outer surfaces of the Zn-dicarboxylates [59].Having found out that the activity of ZnGA is restricted to the outer surface of its particles, Padmanaban and Yoon also reported enhanced catalytic activity when ZnGA was treated with various metal chlorides to form surface-modified ZnGA-metal chloride catalysts [60]. The catalysts treated with iron (III) chloride (ZnGA-Fe) and zinc chloride (ZnGA-Zn) exhibited 25.6 % and 38.3 % increased performances respectively in comparison to the untreated ZnGA catalysts. Moreover, the surface-modified catalysts produced high-molecular-weight polymers [60].The catalytic activity of ZnGA in the copolymerization reaction was observed to depend mainly on the surface area of the catalyst, and in this respect, studies were also undertaken to utilize different supports to enhance the catalyst surface area. Catalyst supports that have been employed for this process include metal oxides (alumina, titanium oxide, magnesium oxide, zeolites), nonmetal oxides (silica, carbon), and polymers [61]. Zhu et al. 2002, reported the first PPC product in the copolymerization reaction catalyzed by supported ZnGA catalysts using a perfluorinated compound as the support [62]. The alternating PPC obtained under optimal reaction conditions was in very high yield (126 g polymer/g catalyst, Mw = 56,100 g/mol), with a high Tg (46.46 °C) and Td of 255.8 °C [62]. Meng et al. 2005, also reported a silica-supported ZnGA (SiO2/ZnGA) catalyst prepared by grinding ZnGA and SiO2 together in a planetary ball grinder under vacuum. The SiO2-ZnGA exhibited a high catalytic yield of 358.8 g polymer/g Zn [63].When ZnGA was dispersed onto the surface of acid-treated montmorillonite (MMT) in quinolone it gave a ZnGA–MMT catalyst with smaller crystal sizes. Under optimal conditions, ZnGA–MMT gave PPC with a high molecular weight and in a good yield (115.2 g polymer/ g ZnGA). The resultant PPC exhibited slightly high Tg (38 °C) and a Td > 250 °C which was attributed to the presence of residual MMT in the copolymer [64]. The MCM-41 supported ZnGA catalyst when utilized for CO2 and PO copolymerization, exhibited enhanced catalytic activity (89.5 g polymer/g catalyst) producing PPC with a high molecular weight under optimal conditions [65]. Gao et al. 2015, also reported an increase in polymer yield while utilizing a silica supported ZnGA catalyst (from 194 g polymer/g Zn over unsupported ZnGA to 392 g polymer/g Zn over ZnGA/SiO2). Moreover, a high carbonate content (>97 %) with Mn of>10,000 g/mol for the alternating PPCs were observed in comparison to previous studies [66].Double metal cyanides (DMCs) are another type of catalysts that have been widely investigated in the CO2/PO copolymerization reaction. These catalysts, also known as Prussian blues are a class of molecular salts made up of crystalline metal cyanide frameworks [67,68]. Their structures have two different metal centers, where one metal coordinates via the carbon atom of the cyanide (CN–) ligand and the other via the nitrogen atom. The general structural formula of DMCs is Tx[M(CN) y ]z, where T metal ions include Zn(II), Co(II), Fe(II) or Ni(II), whereas for M, cations such as Co(III), Fe(III), Cr(III), Fe(II) or Ir(III) are frequently used [69,70]. Typical divalent transition metal hexacyanometallates (III) exhibit structures based upon the cubic T3[M(CN)6]2 framework as showed in Fig. 2 , where [M(CN)6]2 ion complexes are linked via the octahedrally coordinated nitrogen bound T2+ ions [71].Several studies have demonstrated that the DMC catalysts are active in the CO2/PO copolymerization process. Kruper and Swart, 1995 demonstrated that the three dimensional Zn3[Fe(CN)6]2 based systems were mildly active for the random copolymerization of CO2/PO to produce PPC. The polymer product comprised of 2.8 g of PPC and about 0.53 g of polypropylene oxide (PPO). The total ratio of carbonate to the ether was 4.8 with 71 % PO conversion [73]. In 2004, Chen and co-workers, employed a Zn3[Co(CN)6]2 catalyst system (Fig. 2) for the CO2/PO copolymerization reaction. This system exhibited enhanced catalytic activity of over 1000 g copolymer/g of catalyst in comparison to its analog based on Zn3[Fe(CN)6]2 [74]. Under optimal reaction conditions (110 °C, 3.8 MPa of CO2,10 h), a PPC yield of 2000 g/g Zn3[Co(CN)6]2 was obtained exhibiting a 0.24 M fraction of CO2.Investigations on the structures of the DMC catalysts informed the possible reaction mechanism for the copolymerization process, concluding that an epoxide ring opening occurs at the Zn-OH group of the DMC in the initiation step of the reaction [75–77]. In typical CO2/epoxide copolymerization reactions over DMC catalysts, Dharman et al. [77] and Stahl and Luinstra [76], proposed a multistep reaction mechanism with competing routes as shown in Scheme 4. After epoxide ring opening and coordination to the catalyst, carbonate linkages are formed through CO2 insertion in a step-by-step polymerization process. On the other hand, ether linkages are also formed through a Lewis base assisted nucleophilic attack of an hydroxyl group at the coordinated PO in chain growth process [76,77].In this respect, the copolymerization process using DMCs as catalysts usually give polymers with a mixture of carbonate and ether linkages, as well as cyclic carbonate byproducts [79]. Significant efforts have been made to improve the activity and yield of PPC in the copolymerization process over DMC catalysts. In the 2005, Kim et al. performed the copolymerization of CO2 and epoxides using DMC synthesized from a zinc salt and K3Co(CN)6 in the presence of tert-butanol and poly(tetramethylene ether glycol) (PTMEG) as the complexing reagent. The study observed a high reactivity of DMC catalysts in the copolymerization of PO with supercritical CO2 (sCO2), giving a yield of 343 g of polymer/g Zn in 2 h compared to the homogeneous diethylzinc-based catalyst [51,80]. The polymer yields in the absence of sCO2 were 507 g and 535 g of polymer/g Zn at 50 °C and 80 °C reaction temperatures, respectively in 24 h. The copolymer carbonate fraction (FC) at a lower temperature was higher (22 % at 50 °C) than that at a higher temperature (13 % at 80 °C). Further, the catalyst system showed higher activity (526 g polymer/g Zn) for alicyclic epoxides like cyclohexene oxide (CHO) in the CHO/CO2 copolymerization (PCO2 = 140 psi) at 80 °C after 4 h in comparison to aliphatic epoxides like PO [80]. In 2006, Robertson et al. reported a series of anhydrous DMC catalysts of the formula Co(H2O)2[M(CN)4].4H2O (M = Ni, Pd, Pt) for the random copolymerization of CO2/PO [81]. These DMC catalysts exhibit a two dimensional structure as shown in the X-ray crystal structure for the (Co(H2O)2[Pd(CN)4].4H2O) catalyst displayed in Fig. 3 [81]. The reactions gave polymers with high number average molecular weights (Mn = 2.33–0.26 × 105 g/mol) and broad molecular weight distributions (MWDs = 5.8–2.3). The activity of these catalysts was significantly lower than that of Zn3[Co(CN)6]2 based systems, but they produced no cyclic propylene carbonate in the polymer [81].The Co[Ni(CN)4] catalyst showed better performance in the CO2/PO copolymerization (TOF = 1,860 mol PO (mol Co)−1h−1, at 130 °C and 54.4 atm CO2), but with low CO2 incorporation in the PPC polymer chain (FC = 20 %) [81].Studies have shown that modification of the coordination environment around the central metal (M) in the Znx[M(CN)y]z DMC catalyst, affects its catalytic activity in the CO2/PO copolymerization reaction. Zhang et al. reported that distortion of the octahedral coordination sphere by replacing one of the CN– ions with other anions, reduced efficiency in the copolymerization reaction [82]. In their study, low CO2 mole fractions (<0.36) in the polymer and high cyclic carbonate by-product yields (1.1–62.5 %) were observed. The low catalytic performance (<500 g polymer/g catalyst) of the resultant catalysts in comparison to the original DMC (1,466 g polymer/g catalyst) was attributed to the change in the electron-donating effect of the substituting groups. Thus, distorting the coordinative sphere around Zn affects the efficiency of the catalyst [82]. When Zn3[Co(CN)6]2 DMC catalyst was utilized in the absence of chain transfer agents, the catalytic activity in the CO2/PO copolymerization yielded 60,600 g polymer/g catalyst after 10 h [83]. However, the carbonate content in the obtained PPC ranged between 34 % and 49 %, although there was a decrease in the formation of the propylene carbonate by-product to below 1.0 %. Due to the dependence of PPC stability on the carbonate content, the Mn of the as-polymerized PPC with a carbonate content of 48 % reached 130,000 g/mol but decreased to 60,000 g/mol after 24 h of storage at 70 °C, and further dropped to 40,000 g/mol after 7 days [83].In another study, Varghese et al. reported a DMC catalyst where the potassium ion in the traditionally prepared DMC system was replaced with H+ from H3Co(CN)6) [84]. The hydrogen containing DMC catalyst exhibited enhanced catalytic performance in the CO2/PO copolymerization reaction giving a PPC yield of 1260 g polymer/g catalyst. The PPC displayed a carbonate fraction of 66 % and a polycarbonate selectivity of 97 %, which was a great improvement compared to the traditionally prepared DMC catalyst with a yield of 600 g polymer/g catalyst, carbonate fraction of 10 %, and PC selectivity of 92 % [84]. Furthermore, two active 2D- nanolamellar DMC catalysts (Zn–Ni DMC and Co–Ni DMC catalysts) synthesized via ball milling were reported by Guo et al. with an improved catalytic activity for the CO2/PO copolymerization reaction [85]. At optimal reaction conditions (60 °C, 24 h, 4 MPa of CO2), the Zn–Ni DMC catalyst displayed a higher activity. The PPC obtained had a higher molecular weight (Mn = 10,344 g/mol, PDI = 1.45) containing 83.5 % carbonate linkages and higher content of CO2 (42.7 %), while the Co–Ni DMC catalyst gave PPC (Mn = 8,478 g/mol, PDI = 1.44) with 73.7 % carbonate linkages, and lower CO2 content (35.9 %) [85]. Recently, Penche et al. 2021, reported a series of porous hexacyanometallate (III) complex catalysts for the ring-opening copolymerization of CO2-PO reaction. In their study, the catalysts displayed moderate activity giving copolymers exhibiting carbonate units in the range 16 to 33 %, coupled with fairly high molecular weights (Mw = 6,000–85,400 gmol−1) [86].Generally, DMCs have exhibited high catalytic activity in the CO2/PO copolymerization reaction. However, a major drawback of these systems is the low CO2 incorporation into the growing polymer chain causing high degree of ether linkages, due to significant homopolymerization of PO. In this respect, low percentages of carbonate linkages (20–40 %) in PPC are observed for the DMC systems. In addition, the attendant copolymerization reaction conditions are harsh, with temperatures in the range of 80 to 130 °C and pressures from 50 to 100 atm. Moreover, formation of the cyclic carbonate byproduct in significant yields is also reported for a number of catalysts [87–89].In an effort to further improve catalytic performance of DMC based catalysts in the CO2/PO copolymerization, Lu et al. 2013, designed multi-metal cyanides (MMCs) consisting of three different metals, which exhibited enhanced catalytic activity [90]. The catalysts were synthesized from different salts in varying ratios as shown in Table 1 .The polycarbonates produced using the MMCs catalysts, exhibited slightly higher carbonate linkages compared to those formed with the DMC catalysts. MMC-2 exhibited higher activity compared to MMC-1 and MMC-3. However, under optimal conditions and at 70 °C, the PPC produced by MMC-2 displayed slightly lower carbonate content and molecular weight compared to PPC formed using MMC-1 and MMC-3 which was attributed to the electron atmosphere around the central metal [90].In 1991, success in the CO2/PO copolymerization reaction was achieved by Chen et al. using ternary rare-earth metal coordination catalysts [91]. The ternary rare-earth metal catalyst system composed of yttrium phosphonates, triisobutyl aluminium, and glycerin (Y(P204)3-Al(i-Bu)3-glycerin) showed the highest activity [91]. The copolymers formed had a high molecular weight, narrow molecular weight distributions, and high thermal stability. Furthermore, the copolymerization process gave high polymer yields within shorter reaction times compared to the previous organometallic catalytic systems. However, the copolymers were randomly arranged with low carbonate linkages (30–40 %) [91]. This motivated further studies that led to the design of composite catalytic systems as an effort to improve the carbonate content in the polymer [92].In order to address the selectivity and activity challenges suffered by different catalysts in the copolymerization reaction, synergetic effects of combined catalysts were explored. An efficient composite catalyst of ZnGA/DMC (Zn3[Co(CN)6]2) for the CO2/PO copolymerization that produced PPC with high molecular weight and in high yield was reported by Meng et al. [93]. This composite catalyst containing a small amount of DMC, exhibited a higher activity, selectivity, and shorter reaction time than that of the traditional ZnGA catalyst. An alternating PPC with a high CU (97.7 %) and in high yield (508 g polymer/g cat) was obtained under optimal reaction conditions of 24 h, at 70 °C and 50 bar CO2 pressure. The resultant PPC (Mw = 380,000 g/mol) also showed good thermostability (Tg = 42.0 °C, Td (5%) = 253.4 °C) [93].Using the same composite catalyst system but with a higher ratio of DMC (DMC/ZnGA; 10:1), An et al. [94] reported an increased activity for the CO2/PO copolymerization reaction in the presence of a PPG initiator compared to the findings of Meng et al. [93]. This increase in activity was attributed to the high concentration of DMC in the composite catalyst. However, this compromised selectivity towards the carbonate insertion in the polymer resulting in low carbonate unit contents (45.1 %) in the polymer. The ZnGA catalyst in the composite improved the carbonate unit content in the copolymer and also transferred the propagating chain with more alternating structures compared to the DMC catalyst. Thus, the polymer chain growth was attributed to the DMC catalyst, while the increased CO2 insertion into the growing polymer chain was attributed to the ZnGA catalyst. Under optimal conditions, oligo (propylene‐carbonates) with Mn of 1,228 g/mol and a high yield of 1,689 g/g cat were obtained [94]. Despite the cost-effective synthetic procedures of these carboxylate/DMC composites, the catalyst reaction conditions are still harsh for the thermodynamically unstable PPC products formed.In an earlier study, Tan and Hsu, 1997 combined a yttrium rare earth ternary (RET) coordination complex with the diethylzinc catalyst for the CO2/PO copolymerization reaction [92]. The yttrium carboxylate-dialkylzinc glycerol catalyst system gave an alternating PPC with a high carbonate content (95.6 %), in high yield and within a short reaction time [92]. The polycarbonates generated had alternating arrangements with high Mn (100,000 g/mol) in a 12 h reaction period, unlike the polycarbonates formed with the other RET catalysts without ZnEt2 [91,92]. The resulting yield (4,200 (g/mol of Y)/h) was much higher than that obtained (2,451 g/mol of Y) for a 16 h run using only the RET catalyst system, reported by Chen et al. [91,92]. On the other hand, the yttrium carboxylate without the diethylzinc gave random copolymers with low carbonate units having inferior properties for industrial applications [92]. The RET/ZnEt2 composite offered a high activity in the copolymerization process due to the RET carboxylate and the enhanced generation of alternating PPC copolymers was attributed to the diethylzinc [92]. When this catalyst composite was combined with ZnCo-DMC as reported by Dong et al. in 2012, there was a reduction in the carbonate content in the PPC, though activity remained high [95]. The reduction in the selectivity for the CO2 was attributed to the DMC catalyst in the composite catalyst.Liu et al. 2003, developed a neodymium carboxylate-based complex composed of Nd(CCl3CO2)3, ZnEt2, and glycerine (Fig. 4 ), for the copolymerization process [96]. The catalyst system gave a high PPC yield with an activity as high as 6,875 g/mol of Nd in an 8 h run at 90 °C. The copolymers obtained had a Tg above 37 °C, though the metal residue in the polymer affected its stability [96].When yttrium-benzoate complexes (Y(RC6H4CO2)3 (R = H, OH, Me, or NO2) in combination with ZnEt2 and glycerol were tested for the copolymerization process, they gave alternating PPCs with up to 98.5 % carbonate linkages with productivities exceeding 100 (g/mol Zn)/h [37]. By varying the substituents at different positions of the aromatic ring in benzoate, the microstructure of PPC could be moderately adjusted and the head-to-tail linkage in polymer varied from 68.4 to 75.4 %. The polymers obtained by these catalysts had broad molecular weight distributions (Mw/Mn) which was attributed to the steric factor of the ligand in the yttrium complex, whereby substituents at the 2- and 4-positions are believed to affect the coordination or insertion of the monomer into the polymer chain. Although RET/ZnEt2 based composites have exhibited promising performance in the copolymerization process, the complex synthetic procedures of the RET coordination compounds limit their large-scale industrial application.Generally, the dialkylzinc based catalysts exhibited low catalytic activity in the CO2/PO copolymerization reaction, low turnover frequencies (TOFs) and gave copolymers with low and broad molecular weight distributions (Table 2 , entries 1 and 2). These shortfalls were attributed to lack of optimal access to metal centers by monomers resulting into production of a wide range of polymers. On the other hand, the double metal cyanides exhibited high catalytic activity in the copolymerization process, gave higher polymer yields in shorter reaction times. However, higher reaction temperatures and pressures were required for these systems (Table 2, entries 3 and 4). Moreover, the polymer products were marred with low carbonate linkages and broad to narrow molecular weight distributions. When carboxylates were utilized in the CO2/PO copolymerization process, the polymer yield was low but with high carbonate content, high molecular weight, and narrow molecular weight distributions (Table 2, entry 5). Use of composites, that employ more than one catalyst for the copolymerization reaction, resulted into improved performance due to favorable synergistic effects (Table 2, entries 7–9). The PPC obtained were in high yields, having high molecular weights and high carbonate content within a short reaction time.Polypropylene carbonate (PPC) is one of the most common CO2-based copolymers whose production not only contributes to mitigation of climate change through CO2 utilization, but this copolymer has several applications due to its attractive properties. However, PPC large scale production is still limited by the chemical inertness of CO2 and the lack of suitable catalytic systems that can give high turnover numbers combined with attractive properties of the resultant polymer. Over the past few decades, researchers in both academia and industry have made effort to develop efficient homogeneous and heterogeneous catalytic systems for PPC synthesis. However, the performance of the heterogeneous systems so far developed do not favorably compete with their homogeneous analogues, despite the several positive attributes enjoyed by the former.Dialkylzinc based catalysts were the first active systems designed for the CO2/PO copolymerization process. However, these catalysts exhibited low turnover frequencies (TOFs) and gave copolymers with very broad molecular weight distributions. These setbacks were attributed to the multinuclear nature of the catalysts and the lack of optimal access to metal centers by monomers producing a wide range of polymers. Improved performance in the CO2/PO copolymerization process was observed with zinc glutarate based catalyst systems. The easy synthesis, non-toxicity and high activity of the ZnGA systems, made them attractive for industrial application in polycarbonate synthesis. Despite the high catalytic activity and selectivity for polycarbonates exhibited by heterogeneous ZnGA, most of their structures were not well-defined and their TOFs were not as competitive as those of the homogeneous catalysts. Thus, more effort is still needed to improve the structure and surface properties of the ZnGA based catalysts. Double metal cyanides (DMCs) are another class of catalysts that have exhibited promising catalytic activity in the CO2/PO copolymerization reaction. However, the major drawback of these systems is low CO2 incorporation into the growing polymer chain causing high ether linkages due to PO homopolymerization. Moreover, DMCs require high reaction temperatures which degrades the PPC resulting in formation of the cyclic byproducts which are thermodynamically more stable.Rare-earth metal catalysts in the presence of co-catalysts have also exhibited enhanced catalytic activity in the CO2/PO copolymerization process. The polymers formed exhibit high molecular weight, narrow molecular weight distributions, and high thermal stability. Generally, the stability and high catalytic activity of these systems have made them potential candidates for industrial application in PPC synthesis. However, the complex and costly synthetic procedures required for these catalysts hinders their use on a large scale. In attempt to improve both selectivity and activity in the CO2/PO copolymerization process, catalyst composites have been designed by combining different catalysts under appropriate proportions. Typical catalyst composites comprising of DMCs and ZnGA catalysts showed enhanced catalytic performance in the copolymerization process due to favorable synergistic effects. The increase in activity of the composites was attributed to the DMC in the composite catalyst. Although this also led to low selectivity towards carbonate insertion in the polymer resulting in low carbonate unit contents in the polymer. On the other hand, the enhanced CO2 insertion into the copolymer was attributed to the ZnGA catalyst.Despite the promising achievements made in synthesizing active catalysts for PPC formation, still to date, a balance between catalyst activity and selectivity for the desired polymers with good properties is still lacking. Generally, catalysts with a high activity give PPC with poor material properties while several catalysts with comparatively low activities give PPC with promising polymer properties. In this respect, further research should be sought to develop efficient heterogeneous catalyst systems that can produce PPC copolymers in excellent yields without compromising the desired polymer properties. The use of composite catalysts offers favorable synergistic effects of different catalyst in the CO2/PO copolymerization reaction, since enhanced selectivity and activity has been observed for these systems compared to the individual catalysts. In this respect, further investigations into catalyst composites is likely to offer more efficient catalysts for the copolymerization process.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 express their sincere thanks for financial support from the International Programme in Chemical Sciences (IPICS), under the International Science Programme (ISP), Uppsala University, Sweden, and The German Academic Exchange Service (DAAD).

Chemical conversion of carbon dioxide (CO2) into value-added products is an attractive industrial process because it offers several economic and environmental advantages. This review presents advances and challenges in the CO2 and propylene oxide (PO) co-polymerization using heterogeneous catalysts to form poly (propylene carbonate) (PPC), an environmentally friendly polymer with several applications. In the co-polymerization process, CO2 is employed as a green carbon source, an alternative to the toxic phosgene which has numerous negative environmental impacts. However, this route of polycarbonate production, is hindered by the chemical inertness of CO2, and to overcome this, various catalysts have been developed. A number of heterogeneous catalysts including carboxylates, double metal cyanides and composites, have achieved varying success in activating CO2 in the production of polycarbonates. The effect of different reaction conditions including pressure, temperature and solvent has been explored. The limitations faced by various heterogeneous catalysts and improvements made over the past decades have been highlighted. Mechanistic insights for the production of PPC from CO2 and PO have been presented and the differences in both the regioselectivity and stereochemistry of the resultant polymers discussed.

Biomass is considered as a potential renewable feedstock for production of value added chemicals such as fuels and fine chemicals. Biomass, having low nitrogen and sulphur content is easily renewable and excellent source of energy with low CO2 emission [1,2]. The production of industrially important bio-chemicals using sugars has recently become a priority for many countries. 5-Hydroxymethylfurfural (HMF), is an important chemical, which can be produced by single step acid catalysed conversion of sugars. The production of 5-HMF over heterogeneous catalysts has many advantages including minimal catalyst deactivation, low waste, and cost-effective purification of products [3,4]. Among various solid acid catalysts, nanocrystalline zeolites with high surface area are widely used in different refining and petrochemical industries as heterogeneous catalysts [5]. Zeolites are also effective for the enhancement of hydrocracking activity [6,7]. The synthesis of heteropoly acids loaded zeolite catalysts like 12-Molybdophosphoric acid (PMo), Ni and Ni–PMo loaded HZSM-5 zeolites were well reported in the literature [8–10]. The conversion of sugars using Lewis acidic zeolites to produce useful chemicals is also studied [11]. Some researchers evaluated the conversion of C6 sugars like glucose into HMF using bi-functional catalyst systems [12,13]. A novel solid proton conducting material has been made by loading different weight percentages of heteropoly acids (HPA) onto Y-zeolite [14] (Scheme 1 ) [15].In the present work, a series of zeolite supported solid acid catalysts by loading different weight percentage of phosphomolybdic acid was synthesized. They were found efficient for conversion of C6 sugars into HMF, which is an important bio-based platform chemical.Natural zeolite (NZ) was repeatedly washed with double distilled water, dried, crushed, and then grinded in Agate mortar. These particles were then washed several times with water and dried in an oven at 100 °C for 1 h and then calcination was done at 400 °C for 4 h in muffle furnace. Thus obtained activated zeolite was grinded again in agate mortar to form smaller particle size.10 g NZ was loaded with 0.1 g, 0.3 g, 0.5 g, 0.7 g of phosphomolybdic acid (PMA) (for 1 wt%, 3 wt% 5 wt%, 7 wt% loading respectively) in 100 ml ethanol solution. The solution was stirred without heating in a 5 MLH magnetic stirrer at 350 rpm for 24 h. The prepared solution was aged for 48 h and then filtrated. The residue was dried at 100 °C for 4 h. The prepared PMA/NZ catalysts (PMA/NZ-1, PMA/NZ-3, PMA/NZ-5, PMA/NZ-7) were crushed and then stored in the desiccator.N2 adsorption-desorption was done by using Quantachrome (Model: Autosorb -Iq-Tpx) surface area analyzer. XRD patterns were recorded on X’pert Pro-3 powder, using Ni-filter and Cu Kα radiation (E = 8047.8 eV, λ = 1.5406A°) in 2θ range of 10°- 80° at a scanning rate of 1° /min. Morphology and surface topography was studied by FESEM (Tescan Model: MIRA-3 LMH). Thermo gravimetric analysis (TGA) of samples was carried out using TA InstrumentsSDT-Q-600, with a heating rate of 10 °C/min under nitrogen flow (50 cm3/min)..The reaction was carried out in a 100 ml stainless steel hydrothermal autoclave kept in silicon oil bath. To prepare the solution, 3 g. of reactants (glucose and fructose) were taken with 0.15, 0.2, 0.3 and 0.6 g. of PMA/NZ-1, PMA/NZ-3, PMA/NZ-5 and PMA/NZ-7 catalysts respectively mixed with 20 ml absolute ethanol, one at a time. The solution was poured into the autoclave and sealed pack the instrument. The whole assembly was then placed in oil bath and stirred at magnetic stirrer continuously at 140 °C for 4 h. A thermometer is kept in silicon oil bath to observe temperature time to time. Thus obtained solution was cooled at room temperature and kept out from the autoclave after 24 h and filtered using whatman filter paper.From the reaction it was found that among all the catalysts, PMA/NZ-3 showed maximum conversion percentage of glucose and fructose. So all the characterization studies were done for PMA/NZ-3 catalyst only. In Fig. 1 , the N2 adsorption desorption isotherm of synthesized PMA/NZ-3 catalyst exhibited a BET surface area of 7.2 m2 g−1 and pore volume of 0.005 cm3 g−1 and the pore diameter was determined to be 16.664 Å using BJH method. The BET surface area of natural zeolite is 99.096 m2 g−1 and pore volume is 0.024 cm3 g−1.In order to understand the phase symmetry of the prepared catalyst, a systematic study on the XRD was undertaken. The XRD pattern as shown in Fig. 2 expressess that the material dominantly contains a mordenite mineral phase ((Ca,Na2,K2) Al2Si10O24·7H2O) with major characteristic peaks at 2θ values of 9.79° and 26.27°. Other mineral phases such as quartz (SiO2) and hematite (Fe2O3), as well as other types of zeolites such as clinoptilolite ((Na,K,Ca)2-3Al3(Al,Si)2Si13O36·12H2O) and heulandite ((Ca,Na)2-3Al3(Al,Si)2Si13O36·12H2O) were also observed. Further, no peak corresponds to heteropoly acids were observed in the XRD structure of modified zeolite. These results imply that the heteropoly acid is well dispersed on the support surface as an amorphous or microcrystalline phase without altering the support phase.The prepared catalyst PMA/NZ-3 is subjected TGA analysis to evaluate the amount of physically adsorbed water removed from the catalyst surface. Two weight loss regions are obtained in the thermogram of the catalyst as shown in Fig. 3 . The first weight loss occurred between 150 and 250 °C, which corresponds to the loss of water: while the other weight loss obtained at 450 – 550 °C relates to decomposition of Keggin structure accompanied by water removal.In Fig. 4 , SEM-EDAX analysis of PMA/NZ catalyst showed the presence of Si, Al, O, Mo and P. The presence of molybdenum and phosphorus in the synthesized catalyst confirmed the loading of phosphomolybdic acid on the NZ suface.The percentage yield of 5-HMF produced by conversion of glucose and fructose was estimated by UV–Visible spectrophotometer. 5-HMF absorbs at 284 nm under UV–VIS region. A set of standard solutions of HMF has been prepared for calibration. The results are summarized in Table 1 .In this work zeolites loaded with phosphomolybdic acid has been synthesized and used in conversion of glucose into value added chemicals namely HMF. Characterization of the prepared material by XRD, SEM –EDAX and TGA confirmed the loading of PMA on zeolite structure. It is demonstrated that stable catalysts are possible to prepare which retain their structure and exhibit good catalytic properties. The activity is however increased initially marked upon loading till 3 wt% with heteropoly acid molecules and it decreases on further loading. The effect of loading is very clearly seen at low heteropoly acid loading (3 wt%). Sonal Gupta: Conceptualization, Methodology, Software, Visualization, Investigation, Data curation, Writing – original draft, Writing – review & editing. A.B. Gambhire: Software, Data curation. Renuka Jain: Conceptualization, Methodology, 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.We would like to thank Government College, Kota, Rajasthan for all guidance and CSIR, New Delhi for financial support.

Renewable biomass as a sustainable feedstock has been enormously explored for the manufacturing of high value-added products such as biofuels, commodity chemicals, and new bio-based materials. The present paper describes the synthesis of a series of natural zeolite based catalysts by loading different weight percentage of phosphomolybdic acid. The efficiency of catalysts were checked by conversion of carbohydrate (glucose and fructose) to 5-HMF, which is an important industrial chemical used in the production of various value-added chemicals, materials and biofuels.

Data will be made available on request.Biodiesel is regarded as a promising transmit solution to alleviate the environmental issue of burning fossil fuels [1,2]. However, the intensive development of the biodiesel industry has resulted in a huge accumulation of glycerol, a by-product from the biodiesel production process, which has led to abundant glycerol waste and a dramatically price drop in glycerol market that is further struck the economic profit of producing biodiesel [3,4]. Thus, converting glycerol into value-added products has become a promising approach to solve the problems of glycerol waste and improve the economics of biodiesel processes [4,5]. Among the various value-added products, glycerol carbonate (GLC) with the versatile properties can provide a wide range of applications [5,6], for example using as a solvent, a cosmetic ingredient, a laundry detergent, a building eco-composite, or a chemical intermediate [5–7]. Therefore, the value-added conversion of GL to GLC has attracted wide attentions, and four main methods have been developed to convert GL into GLC: (1) carbonization with phosgene or carbon monoxide [6]; (2) direct reaction with CO2 [8]; (3) glycolysis with urea [9]; and (4) transesterification with dimethyl carbonate (DMC) [7,10,11]. Comparing the pros and cons of each of these four methods — such as the toxicity of co-reactant, thermodynamic equilibrium limitation of the reaction, difficulties in by-product separation, and the reaction conditions — GL conversion with DMC (shown in Scheme 1 ) appears to be the most promising route to form GLC, as DMC has been considered as a green chemical and this conversion route can be conducted at relatively mild operational conditions.Converting GL with DMC to GLC requires the presence of catalysts, where homogenous catalysts, such as KOH, NaOH, and H2SO4, have been reported to achieve excellent catalytic performance, but this type of catalyst is difficult to separate from the reaction system [12]. Thus, heterogeneous catalysts have attracted more attention due to their efficient recyclability and good catalytic performance. Compared to acid catalysts, the presence of base catalysts can lead to relatively high yield and selectivity of glycerol carbonate, and also fast reaction rate in the transesterification of GL and DMC [12], which has been reported that the basic site of a catalyst is responsible for activating GL by cleaving its OH bonds [13,14]. Liu et al. [15] established a good correlation between catalytic activity and surface basicity for transition metal doped hydrotalcite catalysts. However, it is still debatable if tuning the amount and strength of catalyst basic sites can determine their catalytic performance in GL transesterification. For example, Hu et al. [16] reported that 15 wt% K/CaO catalyst calcinated at 700 °C shows higher glycerol conversion (99 %) than CaO (92 %) in the transesterification of GL and DMC, but the amount of basic sites of 15 % K/CaO-700 (30.37 mmol/g) is lower than that of CaO (33.93 mmol/g). MgO with a trapezoidal morphology has been synthesized and tested in glycerol transesterification, and it showed the highest glycerol conversion and GLC yield but with the lowest amount and weakest strength of basic sites compared to the MgO catalyst in a rod-like, spherical, flower-like, and nest-like structure [17]. Therefore, the surface basicity of a catalyst might not be the only factor that affects its catalytic performance in the glycerol transesterification.In addition, alkali and alkaline earth metals are high abundant in Earth’s crust [18], in particular calcium, sodium, magnesium and potassium, and their unit price is relatively cheap which provides great potential to be applied in industry. So, many alkali and alkaline earth metal based and modified catalysts have been studied in transesterification of GL and DMC, for example, introducing lithium to ZnO, La2O3 and ZrO2 support catalyst have been found significantly enhanced the conversion of glycerol to GLC from barely converted to over 90 % [7,19,20]. Moreover, the ionic radius and valence state of the alkali and alkali metals have been reported playing important roles on the doping location and coordination sphere, which further influence their catalytic abilities. Sugiura et al. [21] studied the alkali metal ion substitution on a layered calcium phosphate compound (octacalcium phosphate), and revealed that the difference in ionic radius between alkali metal and calcium affects the location of alkali metal ions in the layered compound. Ferreira et al. [22] used diffuse reflectance UV–vis spectra to analyse the coordination number changes of CeO2 after the addition of Ca and Mg, with Ca/CeO2 showing a lower coordination number (approximately 8) of Ce4+ ions. However, the effect of alkali and alkaline earth metals in improving the catalytic performance of metal oxides for glycerol carbonate production has not been fully understood. Additionally, no systematic investigation into the different combinations of the modifiers and base metal oxides currently exists, which has hindered the development of effective catalysts and efficient catalytic processes in glycerol value-added conversion.Therefore, this research work presents a systematically study of the promotional roles of alkali and alkaline earth metals on improving catalytic performance of La2O3 in transesterification of GL and DMC. The ionic radius and valence state for alkali and alkaline earth metals were found as dominant factors for improving the catalytic activity of La2O3, and other factors including molecular weight, surface composition, crystallinity, electron status, specific surface area and basicity of modified La2O3 samples were further elucidated.All chemicals used in this work are of analytical grade and without further purification. Lanthanum nitrate (La(NO3)3·6H2O), ammonium carbonate, magnesium nitrate (Mg(NO3)2), barium nitrate (Ba(NO3)2), N,N-dimethylformamide (DMF, 99 %), glycerol (99 %), methanol (99 %) and tetraethylene glycol (99 %) were purchased from Alfa Aesar. Lithium nitrate (LiNO3), sodium nitrate (NaNO3), potassium nitrate (KNO3), calcium nitrate (Ca(NO3)2), strontium nitrate (Sr(NO3)2), dimethyl carbonate (99 %), 4- (hydroxymethyl)-1,3-dioxolan-2-one (90 %) were purchased from Thermo Fisher Scientific Inc. Glycidol (96 %) was purchased from Sigma-Aldrich.The support material, La2O3, was synthesised using a modified version of the precipitation method from Li et al. [19]. The preparation process is as follows, 0.03 mol lanthanum nitrate and 0.12 mol ammonium carbonate were each dissolved in 120 ml deionised (DI) water. The obtained ammonium carbonate solution was slowly added into lanthanum nitrate solution under mechanical stirring at room temperature and continuously stirred for 6 hrs. Then the white precipitate was separated in a centrifuge (SIGMA® 2-16P) and washed with DI water. Finally, the solid paste was dried at 110 °C for 24 hrs and calcined at 800 °C for 6 hrs in a Muffle furnace (Carbolite® ELF 11/14B) to obtain final La2O3 catalysts.La2O3 doped by 25 mol% alkali and alkaline earth metals were prepared by wet impregnation method. A certain amount of M(NO3)n (where M = Li, Na, K, Mg, Ca, Sr, Ba; n = 1,2) was dissolved in DI water and then 0.5 g La2O3 powder was added. The suspension was magnetically stirred for 12 hrs. The resulting slurry was evaporated at 80 °C in a water bath to remove excess water. The solid residue was dried at 110 °C for 10 hrs and then calcined at various temperatures (400 °C − 800 °C) for 2 hrs.The samples were denoted as xM/La2O3T, where × represents the mass percentage or molecular percentage, M represents the alkali and alkaline earth metal, and T represents the calcination temperature. The default value for × and T are 25 mol% and 600 °C when the sample presented without ‘x’ and/or ‘T’.The crystal phases of pristine and modified La2O3 catalysts were characterised by powder X-ray diffraction (XRD) using a Bruker Phaser-D2 diffractometer with Cu Kα X-ray source. The scanning range (2θ) was from 10° to 90°, with a slit of 1° at a scanning rate of 10° min−1. The electron states for the samples were analysed via X-ray photoelectron spectroscopy (XPS), conducted on a Thermo ScientificTM K-AlphaTM+ spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV) operating at 100 W. All peaks have been calibrated with C1s peak where the standard binding energy (B.E.) is 284.8 eV for adventitious carbon source. The basicity of the samples was tested via CO2 temperature-programmed desorption (TPD). This was carried out by placing a 0.10 g sample into a U-shape reactor and pre-treating it under Helium flow (at 50 sccm) at 600 °C for 30 mins. CO2 was then introduced and adsorbed on the samples for 45 mins at room temperature. During the desorption process, the cell was heated up to 1000 °C with a ramping rate of 10 °C min−1 under the Helium flow (at 50 sccm).The amount of alkali and alkaline earth metal doped on La2O3 was analysed by inductively coupled plasma optical emission spectrometry (ICP-OES) operated on a Varian Vista Pro instrument with axial view. The samples were digested in aqua regia and then diluted to a certain concentration before the measurements. The surface area and pore size distribution of the samples was determined via the N2 physical adsorption–desorption experiments at 77 K in a chemisorption (& physisorption) gas sorption analyser (Quantachrome autosorb IQ). The samples were first degassed at 200 °C in vacuum for 2 hrs, and then their N2 isotherms were measured and analysed based on the Brunauer-Emmett-Teller (BET) equation theory. The morphology of the samples was determined by scanning electron microscope (SEM, JEOL JSM-6390A). Before measurements were taken, the sample was suspended in ethanol solution and dispersed in ultrasonic bath for 1 min. Then, the suspensions were added dropwise onto a copper tape for SEM analysis.The catalytic performance of La2O3 catalysts doped by alkali and alkaline earth metals were tested via glycerol (GL) conversion with dimethyl carbonate (DMC) in a stainless-steel reactor (Yanzheng® YZPR-100). The thermocouple was built in a stainless-steel blind tube inside the reactor for the temperature control. The reaction mixture was stirred with a magnetic stirrer during the reaction.The ratio 1:3 of GL and DMC was mostly claimed as optimal reactant ratio based on literature review [15,16,19,20,23–27], so in a typical experiment, 3.0 g of GL and 9.0 g of DMC were added into the reactor with 0.10 g of catalyst. After sealing the reactor, the mixture was continuously stirred and heated to a desired reaction temperature for a certain time. After this time was reached, the reactor was cooled down in an ice-water bath to stop the reaction. A certain amount of internal standard substance (ISTD), tetraethylene glycol, and DMF were added into the reaction mixture. Then the catalyst the catalyst was separated from the liquid phase in the centrifuge (SIGMA® 2-16P). The collected catalyst was retained and prepared for further recycling experiments. The obtained liquid phase was further analysed by gas chromatography (GC, Shimadzu GC-2010 plus), equipped with a flame ionization detector (FID) and a Stabilwax-MS (30 m × 0.25 mm) column. The qualitative analysis of the reaction products was carried via Shimadzu gas chromatography-mass spectra (GC–MS).In this section, the synthesis strategy of the wet impregnation method for doping alkali and alkaline earth metals on La2O3 catalysts is firstly illustrated in Section 3.1, then the catalyst characterisation obtained from the methods presented above for pristine La2O3 catalyst and La2O3 catalysts modified by 25 mol% alkali and alkaline earth metals are systematically discussed in Section 3.2. The catalytic performances of the pristine La2O3 and modified La2O3 catalysts in GL and DMC transesterification are then presented in Section 3.3, along with a discussion of the key characteristics which might affect the performance. The plausible mechanism was discussed based on modified La2O3 samples in Section 3.4. Finally, the optimal operating conditions for the transesterification of glycerol via doped La2O3 are shown in Section 3.5.The wet impregnation method was used for modifying La2O3 catalyst with alkali and alkaline earth metals. The principle behind this method is discussed based on synthesising Na/La2O3 catalyst which is shown in Scheme 2 . The synthesis procedure includes three main steps: impregnation, drying, and calcination. During the first step of impregnation, the pre-synthesised La2O3 powder is uniformly dispended in the NaNO3 aqueous solution containing the Na+, NO3 –, H+ and OH– ions, and part of La2O3 phase could be transferred into La(OH)3 and La2O2CO3 phases during contact of water and CO2 [28], and the reaction mechanisms are defined by Eq. (1) and Eq. (2) respectively. During the impregnation, Na+ ions might diffuse into the pores of La2O3 and be adsorbed onto the porous surface, and Na+ ions could also be adsorbed on the external surface of the support by forming the ion pair with its oxo/hydroxo-groups [29].During the drying procedure, the precursor of Na+ forms a homogenous distribution of the Nax(NO3)y(OH)z crystals on the surface of the La2O3 (or on La(OH)3 and La2O2CO3) crystal phases which is illustrated in Eq. (3),. As the solvent is removed during the drying process, it can result in a redistribution of the modified metal phase on the support material [30]. The dopants inside the pores which have a smaller size can more easily migrate out to the external surface and contribute to the formation of the Nax(NO3)y(OH)z crystals. In the calcination step, Nax(NO3)y(OH)z starts to decompose to NaNO3 at 100–200 °C, then NaNO3 is converted into molten salt at around 308 °C [31], where the molten salt phase can increase the mobility of Na which can lead Na to enter the lattice of the support material. As described in Eq. (4), the molten NaNO3 salt firstly starts decomposing to NaNO2 at 380 °C [31], and then the formed NaNO2 salt further decomposes to Na2O at around 600 °C [32–34]. In the meantime, La(OH)3 and La2O2CO3 phases can start converting into La2O3 at 600 °C and complete the conversion at around 800 °C [35]. The formation of surface defects also occurs during the calcination step [35,36]. (1) La 2 O 3 s + NaNO 3 a q + H 2 O → La 2 O 3 ( s ) + Na + ( a q ) + NO 3 - ( a q ) + H + ( a q ) + OH - ( a q ) (2) La 2 O 3 ( s ) + H 2 O + CO 2 → La 2 O 3 ( s ) + La ( O H ) 3 ( s ) + La 2 O 2 CO 3 ( s ) (3) La 2 O 3 s + Na + aq + NO 3 - aq + H + aq + OH - aq → Na x NO 3 y O H z ∙ La 2 O 3 s (4) Na x ( NO 3 ) y ( O H ) z → N a N O 3 → N a N O 2 + O 2 → Na 2 O + N O + N O 2 , Na x ( NO 3 ) y ( O H ) z ∙ La 2 O 3 s → ( N a ) La 2 O 3 s The synthesis strategy for doping other alkali and alkaline earth metals – Li, K, Mg, Ca, Sr, and Ba – on La2O3 is similar to that for Na doping La2O3, but the decomposition temperatures of their nitrates to oxides are different to that of NaNO3 which are summarised in Table S1.The bulk and surface composition of La2O3 samples are analysed by ICP-OES and XPS, and the results are listed in Table S2. The bulk composition of each element tested by ICP-OES is consistent with the designed composition where the amount of alkali and alkaline earth metals are around 25 mol% of La. While the surface compositions of Li and Na are around 68 % which are three times higher than their overall composition, that of Mg is about 48 % and two times higher than its bulk composition, and the surface composition for the other metals are similar to their bulk results. This result suggests that the majority of Li, Na and Mg are doped on the La2O3 surface, while K, Ca, Sr and Ba could form another phase along the bulk of La2O3. As illustrated in the synthesis strategy in Section 3.1, during the drying process, the dopants with smaller ionic radii more easily migrate out of the inner pores and adsorb on the surface. Zhang et al. [37] also found that the surface composition of Li and Na with smaller ionic radii are higher than K, due to Li and Na are more easily to migrate on the ZnO surface than K.The crystal structure of prepared La2O3 samples and the doping location of alkali and alkaline earth metals were studied via XRD analysis, and the results are shown in Fig. 1 . The XRD patterns of pristine La2O3 and the modified La2O3 samples followed the hexagonal structure lanthanum oxide phase (P-3 m1, JCPDS 83–1344), and the main diffraction peaks of La2O3 are observed at 2θ = 26.1°, 29.1°, 29.9°, 39.5°, 46.0° and 52.1°, corresponding to the (100), (002), (011), (012), (110) and (103) crystal planes, respectively. The blue stars in Fig. 1 represent for the La2O2CO3 phase (JCPDS 84–1963) which is inevitably formed when La2O3 sample is exposed to ambient atmospheric conditions [19,38]. In addition, no extra crystalline phases of alkali metal oxides were observed from their XRD patterns, therefore, the alkali metal could uniformly disperse on La2O3 surface [19,39,40]. Unlike surface doping, the bulk doping can influence the phase stability and crystal growth [40,41]. Diffraction peaks for alkaline earth metal doped La2O3 samples become much more weaken and broaden than that for alkali metal doped La2O3 samples, indicating the heavy doping of alkali earth metal inhibited the formation of La2O3 crystalline structure [41]. Nevertheless, the presence of CaO, SrCO3, and BaCO3 phase further confirmed that Ca, Sr, and Ba formed extra crystalline structure. These results suggest that most alkaline earth metals are incorporated in the bulk La2O3.X-ray photoelectron spectroscopy (XPS) analysis was used to clarify the electron environments for pristine and modified La2O3 samples. The XPS profiles for La 3d and O 1 s orbitals are shown in Fig. 2 and the corresponding binding energies of these orbitals are listed in Table S3. Due to spin–orbit coupling, the La 3d spectrum separated into two groups known as La 3d3/2 and La 3d5/2, respectively [19,42]. Each group can be further deconvoluted into one main peak (denoted as I), and two satellite peaks (denoted as II and III) [43,44]. The peaks of La 3d5/2 I for the pure La2O3 material are centred at 834.7 eV, and it is negatively shifted to between 834.2 eV and 834.4 eV for the La2O3 catalysts doped by alkali metals, while no peak shifting was observed for the samples doped by alkaline earth metals. This result suggests that alkali metals donate electrons to La, which makes the La in alkali metal doped La2O3 catalysts be able to donate more electrons to reactants [39].The peaks of O1s spectra are located at around 528.7 eV, 530.7 eV, and 531.4 eV, corresponding to the lattice O2− (OL), chemisorbed surface O− (OS) and weakly adsorbed OH– and CO3 2– (OA) species, respectively [15,39,42,44]. The binding energy of O 1 s spectra are positively shifted towards the higher energy field for both La2O3 catalysts doped by alkali metals and alkaline earth metals, with the latter elements being shifted more. This result indicates that a larger amount of electrons transferred from O sites to alkaline earth metals than to alkali metals, which indicates that alkaline earth metals have a strong interaction with O in La2O3 [45,46]. This is also consistent with the XRD result discussed in the previous section that an extra phase of alkaline earth metal oxide was formed on the La2O3 surface.The surface basicity of La2O3 catalysts was measured by CO2-TPD analysis and is shown in Fig. 3 , and the corresponding densities of their basic sites are calculated and listed in Table S4. The CO2 desorption peak for the pure La2O3 catalyst is centred at 400 °C − 500 °C, so the basic sites for pristine La2O3 sample can be denoted as strong basic site [19]. After doping alkali and alkaline earth metals on the La2O3 surface, the CO2 desorption peaks shifted to a higher temperature field compared to that of pristine La2O3 sample and are located at 600 °C – 800 °C, in which the basic sites for the modified samples can be denoted as extra strong sites [19 47]. As a result, the basicity of the La2O3 sample became stronger with introducing alkali and alkaline earth metals. Additionally, alkali metal doped La2O3 samples contains 4.56–5.60 μmol/m2 basic sites, higher than alkaline earth metal doped ones with 0.26–5.34 μmol/m2 basic sites, which is consistent with the XPS results that the electrons around O in alkali metal doped La2O3 samples are more dense than those in alkaline earth metal doped La2O3 samples [47].The average crystallite sizes of La2O3 and modified La2O3 were calculated via the Debyee-Scherrer equation, and the results are listed in Table S2, where the average crystal size of pure La2O3 is 68 nm, which is the largest of all the samples. The average crystal sizes of the alkali metal doped La2O3 samples were the next largest at about 42.2 nm-57.9 nm and the smallest were the alkaline earth metal doped La2O3 samples (at about 18 nm-34.3 nm). As a result, adding alkali and alkaline earth metal can hinder the growth of La2O3 crystal [30]. This is consistent with their specific surface areas measured by N2 isotherms which are shown in Fig. 4 a, and their specific surface areas, calculated by BET theory, are listed in Table S2. The surface areas of La2O3 samples doped by alkali metals are smaller than that of La2O3 samples doped by alkaline earth metals. The main pore size of all samples is distributed in the range of 2–4 nm, as shown in Fig. 4 b, while the pore width and total volume of modified La2O3 catalysts, especially in the range of 2–4 nm, became broader and higher than that of the pristine La2O3 catalysts. The surface morphologies of La2O3 and promoted La2O3 catalysts were individually determined by SEM analysis, and presented in Figure S1. The morphologies of the pure La2O3 sample and La2O3 samples doped by alkaline earth metals are similar and show a flake-like structure, while nanorod-like structures are shown in the La2O3 samples doped by alkali metals.In this section, the catalytic performance of pristine La2O3 catalyst and modified La2O3 catalysts by alkali and alkaline earth metals are presented in Section 3.3.1, followed by a thorough discussion on the potential factors of the dopants and catalyst characteristics in improving the catalytic performance of La2O3.The catalytic performance of the pristine La2O3 catalysts and the La2O3 catalysts modified by 25 mol% alkali and alkaline earth metals was examined in the transesterification of GL to GLC at 70 °C and 2 hrs and the results are presented in Fig. 5 a. The pristine La2O3 catalyst has barely any conversion of GL into GLC at 70 °C after 2 hrs, while doping alkali and alkaline earth metals on La2O3 catalysts significantly improved the catalytic performance of La2O3 catalyst in GL and DMC conversion. La2O3 catalysts doped by Li, Na and K achieved 48 %, 85 % and 40 % GL conversion, respectively, and the GL conversions for La2O3 catalysts doped by Mg, Ca, Sr and Ba were 13 %, 41 %, 21 % and 23 %, respectively. The GLC yield follows the similar trend as the GL conversion for alkali and alkaline earth metal doped La2O3 catalysts. Among all the modified La2O3 catalysts, Na doped La2O3 catalyst shows the best catalytic performance, and Ca doped La2O3 catalyst led to relatively higher GL conversion and GLC yield than the La2O3 catalysts doped by other alkaline earth metals. In addition, the alkali metal doped La2O3 catalysts showed relatively better catalytic performance than alkaline earth metal doped La2O3 catalysts.To rule out the effect of molecular weight for doping metals, a fixed mass ratio, of alkali and alkaline earth metals were doped on La2O3 and tested in the GL transesterification, where the results are presented in Fig. 5b. The 3.5 wt% mass ratio of Na/La is the equivalent mass ratio to the 25 mol% of Na/La catalyst, and the equivalent molar ratios (eq. mol%) for other dopants are listed in Table 1 . The catalytic performance of La2O3 based catalysts with the fixed mass ratio shows a similar trend as that for the catalysts with a fixed molecular ratio. As shown in Fig. 5 b, 3.5 wt% Na/La2O3 and 3.5 wt% Ca/La2O3 catalyst also achieved the highest glycerol conversion of 85 % and 37 %, respectively, among La2O3 catalysts doped by alkali metal and alkaline earth metal, and La2O3 catalysts doped by alkali metals showed better catalytic performance than the ones doped by alkaline earth metals. This result shows that the molecular weight of dopants is not the dominant factor that determines the ability of alkali and alkaline earth metals on improving catalytic performance of La2O3 catalyst.The catalytic performance of a catalyst is determined by its properties which can be tuned by the dopant added. So, in this section, the correlation between the catalyst properties and the catalytic performance are discussed, and the dominant effects of the dopants on improving the catalytic performance of La2O3 catalyst are revealed.Interestingly, the ionic radius of Na (1.02 Å) and Ca (0.99 Å), listed in Table 1, is similar to the cation radius of La (1.03 Å), and Na/La2O3 and Ca/La2O3 catalyst have showed the best GL conversion and GLC yield among the La2O3 catalysts modified by alkali metals and alkaline earth metal, respectively, so the similarity in their radius might be the dominant factor in affecting the dopant interaction with support material. Thus, a correlation between the ionic radius and catalytic performance was established and shown in Fig. 6 .For alkali metal promoted La2O3 samples, the ionic radius ratio of Na/La is 0.99 which indicates the radii of Na+ and La3+ are very similar, and the ionic radius ratio of Li/La and K/La are 0.74 and 1.34, indicating the radius of Li+ and K+ relatively smaller or larger than that of La3+, respectively. Interestingly, the GL conversion under La2O3 catalyst doped by Na, which has a similar ionic radius with La, is higher than that under La2O3 catalyst doped by metals with a dissimilar ionic radius with La, such as Li and K. This result indicates that the La2O3 catalyst doped by a dopant with a similar ionic radius to La can maximise the improvement of its catalytic performance in glycerol and DMC transesterification. The same trend was also found for the La2O3 catalysts doped by alkaline earth metals, with Ca having the most similar ionic radius to La, and having the highest GL conversion among the alkaline earth metals. Song et al. [7] and Kaur et al. [20] also claimed that ZnO and ZrO4 catalyst doped by lithium showed better catalytic performance than those catalyst doped by other alkali metals because Li has a similar ionic radius (0.76 Å) to Zn (0.74 Å) and Zr (0.72 Å). Therefore, the similarity of ionic radius for the dopant to its support material can be considered as one of the determining factors in improving the catalytic performance of the catalyst. The potential reason might be when the ionic radius of the dopant is smaller or larger than the support material, it could cause a significant structure distortion around the dopant, which might affect the stability of the active sites, so when the ionic radius is similar to the support cation, the formed active sites is more stable which leads to the better catalytic performance. Moreover, when comparing the dopants in the same period, the catalysts doped by alkali metals are superior to the ones doped by the alkaline earth metal in GL and DMC reaction, indicating that the valence state of the dopant also affects the catalytic performance of modified La2O3 catalysts.The surface concentration of alkali and alkaline earth metals was tested via XPS, as stated in Section 3.2.1, and the surface concentration of Li, Na and Mg is higher than their overall concentration, where that of K, Ca, Sr, and Ba is similar to their overall concentration. It has been reported that surface doping might be better than bulk doping [40], but in this work, the surface concentration of the dopant did not show a proportional effect on the catalytic performance of modified La2O3 catalysts. For instance, the surface concentration of Li and Na is similar, but 25 mol% Na doped La2O3 catalyst achieved 85 % GL conversion, much better than 48 % GL conversion for 25 mol% Li doped La2O3 catalysts at the same reaction conditions. In addition, 25 mol% Mg/La2O3 catalyst achieved less GL conversion than 25 mol% Ca/La2O3 catalyst, although the surface concentration of Mg as about two times higher than that of Ca. Thus, the surface concentration is not the dominant factor for La2O3 catalysts doped by alkali and alkaline earth metals showing different catalytic ability in GL conversion.The diffraction peaks for La2O3 catalysts doped by alkaline earth metals from XRD analysis as shown in Fig. 1 are much broader and weaker than those for the pristine La2O3 catalysts and the ones doped by alkali metals, which agrees with the previous study [49,50]. The broader and weaker peaks could result from the formation of a non-crystalline phase, the aggregation of particles [51], or an incomplete La2O3 crystal phase. As illustrated in the synthesis strategy, given in Section 3.2.2, La2O3 can easily transfer to La(OH)3 and La2O2CO3 phases when it comes into contact with water or carbon dioxide in the atmosphere, so during the drying and calcination process, the La(OH)3 and La2O2CO3 gains are decomposed into La2O3 and rebuild the crystalline structure. Castro et al. [41] reported that alkaline earth metals, especially Mg and Ca, could prevent host material reforming from the calcination process. So a higher calcination temperature 800 °C was used to synthesise the modified La2O3 catalysts, and their XRD patterns, as shown in Figure S2, indicate a better crystalline structure of hexagonal lanthanum oxide was formed. Conversely, the catalytic performance for the samples calcined at 600 °C is better than that for the samples calcined at 800 °C (as shown in Figure S3), despite the crystal structures being well formed at 800 °C. In addition, the catalyst calcined at 800 °C achieved a similar trend as the ones calcined at 600 °C in GL and DMC conversion, where Na/La2O3 and Ca/La2O3 showed the best catalytic performance among the La2O3 catalysts doped by alkali metals and alkaline earth metals, respectively, and the alkali metal doped La2O3 catalysts showed relatively higher GL conversion than alkaline earth metals doped La2O3 catalysts. Thus, these results further suggest that the crystallisation degree for La2O3 catalysts doped by alkaline earth metals is not responsible for improving the catalytic performance.The full analysis of electronic states of La2O3 catalysts modified by alkali and alkaline earth metals is given in Section 3.2.3, where alkali metals as dopants on La2O3 catalysts can donate their electrons to La but alkaline earth metals showed little effect on the electronic environment of La. This might be due to the alkali metals having a lower electronegativity than alkaline earth metals [52], meaning alkali metals are more likely to donate their electrons than alkaline earth metals. This phenomenon has also been reported on alkaline earth metals doped on ZrO2 [51] and Ni/La2O3 catalysts [53]. Additionally, alkali metals affect the electron distributions around O sites, while alkaline earth metals as dopants on La2O3 catalysts more strongly affect the electron distributions around O sites, which could be due to the extra phase of alkaline earth metal oxides formed [51]. This result is consistent with their catalytic performance where alkali metal doped La2O3 catalysts showed relatively better catalytic performance of alkaline earth metal doped La2O3 catalysts, which further reveals that the valence states of alkali and alkaline earth metals is one of dominant factors. However, for dopants within the same group, this cannot explain the trend in their catalytic performance.The basic sites have been reported to be an important factor for a catalyst to achieve high GL conversion, so the correlation between basic site density and catalytic activity of alkali and alkaline earth metals doped La2O3 is presented in Fig. 7 . The basic site density of La2O3 catalyst significantly increased after doping with alkali and alkaline earth metals as presented in Table S3, but the catalytic activities of La2O3 based catalysts are not proportional to the density of their surface basic sites. For instance, the basic site density of Na/La2O3 and Ca/La2O3 catalysts are very similar (5.60 μmol/m2 and 5.34 μmol/m2, respectively), but the GL conversion under Na/La2O3 catalysts is about 40 % higher than that under Ca/La2O3 catalyst. In addition, the basic site density of Li and K doped catalysts are also close to that of Na doped La2O3 catalyst, but Li and K modified La2O3 catalysts showed much lower GL conversion and GLC yield than Na/La2O3 catalyst. Therefore, these results imply that basic sites are important to the transesterification of GL and DMC, but it is not a determining factor.The plausible mechanism for the transesterification of GL and DMC on the La2O3 catalyst is proposed and shown in Fig. 8 . The carbonyl group of DMC and the hydroxyl group of GL are activated on the La site and the O site, respectively [25]. Then the activated GL anion attacks the carbonyl carbon of activated DMC to form a 1-(o-methoxy-carbonyl)glycerol complex (denoted as intermediate 1) and one molar methanol. The intermediate 1 then further cyclise to GLC with another molar of methanol. The addition of alkali metals on La2O3 catalysts can form the M−La centre that help activate DMC and cyclise intermediate 1, which leads to higher GL conversion and GLC yield (as shown in Fig. 5 a) than the pristine La2O3 catalyst. The extra phase formed by adding alkaline earth metals can provide more active sites and benefit the catalytic reaction of GL and DMC. The better catalytic performance of La2O3 catalyst promoted by alkali metal than by alkaline earth metals can illustrate that M−La active centre could be more effective in reducing activation energy of the reaction than extra phase of alkaline earth metal oxide.As La2O3 catalyst doped by 25 mol% Na achieved the highest GL conversion and GLC yield, Na/La2O3 catalyst was chosen for further investigation on the effect of dopant/support metal molar ratio, calcination temperature, catalyst dose, reaction temperature, reaction time and reusability. Each variable is considered independently using a standard set of conditions, and the best optimal condition for each variable is carried forward to the next variable analysis.The impact of Na/La molar ratio for modified La2O3 (xNa/La2O3) was studied in the GL conversion to glycerol carbonate. The catalytic activity of xNa/La2O3 calcined at 600 °C was studied under the reaction temperature of 70 °C for 2 hrs and the results are shown in Fig. 9 . The result indicates the GL conversion was significantly increased from 32 % to 85 % with increasing the amount of Na from 7 mol% to 25 mol% doping on La2O3 catalysts, and the GLC yield was increased from 25 % to 59 % respectively. With increasing the amount of Na further to 50 mol% of La, GL conversion slightly increased to 89 %, but the GLC yield dropped to 47 % in which the yield of glycidol (GLD) increased from the decarbonation reaction of GLC. Therefore, it can be inferred that the GL conversion and GLC yield can be boosted with increasing amount of Na to 25 mol% of La2O3 catalyst due to an increase in the number of active sites, but further increasing the amount of Na benefits the generation of the by-product. Thus, to balance the high GL conversion and GLC yield, 25 mol% Na/La ratio was chosen as the optimal doping ratio, which are used for further optimisation.To investigate the effect of calcination temperature on the catalyst activity, 25 mol% Na/La2O3 was calcined at various temperatures ranging from 400 °C to 800 °C, the obtained catalysts were tested in the GL and DMC conversion at 70 °C, and the reaction time was set for 2 h. The results are presented in Fig. 10 a. With the increase of calcination temperature from 400 °C to 600 °C, the GL conversion significantly increased from 5 % to 85 %, but then dropped from 85 % to 70 % with increasing the calcination temperature further to 800 °C. The crystalline structure of 25 mol% Na/La2O3 catalysts calcined at 400 °C to 800 °C was tested via XRD, and the result is presented in Fig. 10 b. When the 25 mol% Na/La2O3 catalyst was calcined at 400 °C and 500 °C, the catalyst was mainly in the La2O2CO3 phase, which then gradually transferred to the La2O3 phase with increasing the calcination temperature, and La2O3 phase was the only phase presented at the Na/La2O3 catalyst calcined at 800 °C, which is consistent with the literature work [35]. In addition, the crystallinity of the catalysts increased with increasing the calcination temperature. These results indicate that La2O3CO3 phase is not active in the conversion of GL into GLC. Increasing calcination temperature not only benefits to forming La2O3, but also promotes the NaNO3 decomposition to Na2O leading to better interaction with La2O3 catalyst. Although the La2O3 was also completely formed at 700 °C and 800 °C, the GL conversion and GLC yield are decreased with GLD yield increased which might be due to the aggregation of particles where Li et al. [19] reported a similar phenomenon of the decrease catalytic performance of Li/La2O3 catalyst calcined at 700 °C in GL and DMC conversion compared to the catalyst calcined at 600 °C. Thus, 25 mol% Na/La2O3 catalysts calcined at 600 °C were used for the further optimisation of reaction parameters.The amount of 25 mol% Na/La2O3 catalysts calcined at 600 °C was investigated in the range of 0.01 g to 0.20 g for 70 °C and 2-hour GL and DMC reactions. As shown in Fig. 11 a, with the amount of catalyst added from 0.01 g to 0.15 g, the production of GLC increases from 50 % to 60 % then drops to 40 % when further increasing the catalyst dose to 0.20 g, but the yield of by-product GLD also increased with the increase of catalyst dose. This result indicates that increasing the total number of active sites not only benefits the GL conversion to GLC, but also boosted the GLC decomposition to GLD. The catalyst performance peaks between 0.10 g and 0.15 g of catalyst, as the GLD yield is lower when 0.10 g 25 mol% Na/La2O3 catalyst was used, it seems to be more effective. Thus, 0.10 g of 25 mol% Na/La2O3 catalysts calcined at 600 °C was used for next optimisation of reaction temperature.The optimisation of the reaction temperature with 0.10 g of 25 mol% Na/La2O3 catalyst in GL and DMC conversion was carried out in the range of 50 °C − 90 °C for 2 hrs, and the result is presented in Fig. 11 b. The GL conversion significantly increases from 33 % to 96 % at a temperature increase from 50 °C to 80 °C. With temperature increasing further to 90 °C, the amount of glycerol converted is steady, but the yield of glycerol carbonate reduced. The maximum production of glycerol carbonate was 59 % when the reaction temperature was 70 °C. The glycerol transesterification to GLC is an endothermic reaction, therefore GL conversion increases with the reaction temperature increase [54]. However, the higher reaction temperature can drive GLC decomposed to GLD. Therefore, 70 °C was selected as the optimal reaction temperature.In the transesterification of glycerol and dimethyl carbonate, there is one primary product of glycerol carbonate and one by-product of glycidol. As shown in Fig. 11 c, with an increase in the reaction time from 0.5 hr to 4 hrs, the conversion of glycerol improved from 66 % to 95 %, and the yield of glycerol carbonate enhanced slightly from 58 % to 62 %. When the reaction time is beyond 2 hrs, the selectivity of GLC decreased and the amount of by-product GLD increased. So the optimal reaction time was selected as two hours.To investigate the reusability of the catalyst, the spent 25 mol% Na/La2O3 catalyst was recycled and reused in a fresh glycerol reaction operating at the optimal conditions (70 °C and 2 hrs), and the results are shown in Fig. 11 d. The catalyst activity slightly deceased after two cycles. The GL conversion dropped from 85 % to 71 % after the second cycle, and further dropped to 61 % after the fourth cycle. To reveal the reason of the deactivation, the following experiments were designed, and the results indicated that the catalyst decay is due to surface wearing which some active components lose in the solution as a fine powder because very high stirring speed (1000 rpm) was applied for the reaction system to ensure the catalyst well dispersing in the immiscible GL and DMC environment. In group A, to test whether the decay is caused in the process of diluting reaction media or cleaning the residue reactants and products from the catalyst, the fresh Na/La2O3 catalyst was washed in ethanol (A1) and DMF (A2), respectively, at room temperature and dried completely at 110 °C in the oven before adding into the reactor. The glycerol conversion and GLC yield under pre-treated Na/La2O3 catalyst, shown in Table 2 , are similar to that under fresh catalyst without treatment. Therefore, the solvents used in the regeneration process is not the reason causing the deactivation of the catalyst. In group B, the catalyst was separated from the reaction system via centrifuge after 1 hr reaction (B1), and after removing the catalyst particles the solution media continues the reaction for another 1 hr (B2). The results, listed in Table 2 for group B, show that the glycerol conversion and glycerol carbonate production increased after removing the catalyst particles from the reaction system, which revealed the some fine catalyst particles still remained in the solution and the deactivation of the catalyst likely resulted from wearing of active components from the catalyst surface. As the GL conversion continues after removing the La2O3 catalysts by centrifuge, one possible reason may be the leaching of Na ions from La2O3, and the leached Na promotes the reaction [7,19]. To clarify whether the deactivation is due to the leaching of Na ions from the doped La2O3, a certain amount of NaNO3 was added in GL and DMC, but no products were detected from gas chromatograph as shown in group C. So the reaction cannot be conducted with only the presence of Na+. As a result, the catalyst decay is not due to the leaching of Na+ into reaction system. Thus, the catalyst deactivation is mainly due to the surface wearing rather than the regeneration process or leaching.As the catalyst surface wearing is mainly caused by the friction between the magnetic stirrer and the bottom of the reactor, reducing the friction through increasing the volume of the solution and using mechanical agitator rather than a magnetic stirrer can be a potential solution to prevent the catalyst surface wearing. Moreover, using stronger support materials or coating the catalyst into stronger support materials, such as Al2O3 and ZrO2, or applying advanced synthesis methods, such as sol–gel and precipitation, have been reported as effective way to prevent and reduce the catalyst decay from catalyst surface wearing [55].Alkali and alkaline earth metal doped on La2O3 catalysts were successfully synthesised via a wet impregnation method, and their catalytic activities were tested in glycerol conversion to glycerol carbonate. Na was found to be the best dopant among the alkali and alkali earth metals for improving La2O3 catalytic performance, and 25 mol% Na doped La2O3 catalyst achieved highest GL conversion (85 %) and GLC yield (60 %) at 70 °C in a 2-hour reaction. La2O3 catalysts doped by alkali metals achieved relatively higher GL conversion and GLC yield than that doped by alkaline earth metals.Ionic radius and valence state of dopants play significant roles in affecting the catalytic performance of La2O3 catalysts. In general, alkali metals were well dispersed on La2O3 surface, while alkaline earth metals were formed an extra phase and aggregated on La2O3 surface. The electron distribution around La on La2O3 surface was affected by doping alkali metal, while doping alkaline earth metal on La2O3 surface affected activity of O sites. The similarity of ionic radius between the dopant and La was found as one of the determining factors for improving La2O3 catalytic performance. The dopant with an ionic radius close to La led to a larger improvement in La2O3 catalytic activity than other dopants with a smaller or larger ionic radius, and the dopant with a lower valence state showed a better enhancement for La2O3 catalytic activity. The basic sites on La2O3 surface were found important to the transesterification of GL and DMC, but the basic site density on modified La2O3 surface was not a determining factor for the catalytic performance of La2O3 catalysts. The findings about the effect of radius and charge of alkali and alkaline earth metals on La2O3 catalytic activity are expected to help to understand the promotional role of the dopant for designing more efficient and lower cost catalysts for glycerol value-added conversion to glycerol carbonate.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.Thanks to the financial support from the UK research council EPSRC (EP/V041665/1). The authors appreciate Mr Fergus Dingwall from School of Engineering in The University of Edinburgh for all his technical support; Dr. Laetitia Pichevin and Dr. Nicola Cayzer from School of Geoscience in The University of Edinburgh for their kind help on ICP-OES and SEM analysis; and Dr. Gary Nichol from School of Chemistry in The University of Edinburgh for providing XRD facility and valuable advice.Supplementary data to this article can be found online at https://doi.org/10.1016/j.cej.2023.141486.The following are the Supplementary data to this article: Supplementary data 1

Alkali metals and alkaline earth metals have been used to promote the catalytic performance of metal oxides in transesterification of glycerol and dimethyl carbonate, however, the promotional roles of the dopants in influencing the catalytic performance of the metal oxides have not been fully investigated which hinder the development of low-cost and high-efficiency catalysts in transesterification of glycerol and dimethyl carbonate. This paper, for the first time, systematically studied the influence of ionic radius and valence state of dopants, surface concentration of dopants and the basicity of the catalysts on the catalytic performance of La2O3 in transesterification of glycerol and dimethyl carbonate. Our results suggest that the ionic radius and valence state of the dopants are the determining factors, while the basic site density is not a crucial factor, although the basicity of catalyst surface is important in activating glycerol and dimethyl carbonate. Among alkali and alkaline earth metals, 25 mol% Na/La2O3 catalyst achieved the highest glycerol conversion (85 %) and glycerol carbonate yield (60 %) in the 70 °C and 2-hour reaction. After the detailed investigation, a plausible mechanism of glycerol and dimethyl carbonate transesterification on Na/La2O3 catalyst has been proposed. This research could help understand the promotional role of alkali metals and alkaline earth metals and the results may guide future design of metal oxide catalysts.

Aqueous phase reformingAttenuated total reflection-infraredBinding energyCO-Temperature programmed desorptionCO2-Temperature programmed desorptionDensity functional theoryDiffuse reflectance infrared Fourier transform spectroscopyExtended X-ray absorption fine structureFourier-transform infrared spectroscopyH2-Temperature programmed reductionIonic exchangeInternational Energy AgencyInfraredIncipient wetness impregnationMulti walled carbon nanotubesNH3- Temperature programmed desorptionProton exchange membrane fuel cellsPolyvinylpyrrolidoneSolution combustion synthesisSol-gel in acidic conditionsSol-gel in basic conditionsSingle wall carbon nanotubesTransmission electron microscopyTetraethylorthosilicateTurn over frequencyTime on streamTemperature programmed oxidationUrea matrix combustion methodWater gas shiftWeight hourly space velocityX-ray absorption near edge structureX-ray absorption spectroscopyX-ray photoelectron spectroscopyX-ray diffractionThe use of fossil fuels are considered the leading cause of global warming. For this reason, alternative and sustainable sources of energy are explored nowadays to overcome this issue. The use of biomass to substitute fossil oil to produce materials and energy brought to the development of the biorefinery concept. According to the International Energy Agency (IEA) definition, it is intended as the integration of different processes for the sustainable production of goods starting from biomass [1]. One of the advantages of biorefinery is the possibility of targeting small, decentralized plants; as a matter of fact, several drawbacks have been raised for large-scale implementation. One reason is due to the difficulty to reach a complete and effective exploitation of the organic content of the starting biomass. For example, in the biodiesel industry, 1 kg of undesired crude glycerol is produced together with 10 kg of the desired product, generating a considerable amount of waste [2]. Similarly, pyrolysis and hydrothermal liquefaction aim at producing a biofuel (namely bio-oil or bio-crude) [3], but a significant fraction of the carbon present in the feed is lost in the aqueous stream [4].Aqueous phase reforming (APR) process was proposed by Dumesic and coworkers to valorize oxygenated molecules and obtain a gas mixture rich in hydrogen [5]. It derives that it can be applied to carbon-laden wastewaters present in bio- and conventional refineries to increase the conversion efficiency of the plant, reducing the amount of waste that should be treated, and obtaining at the same time a valuable product. Most of the literature refers to the use of model compounds, such as alcohols (methanol, ethanol), polyalcohols (ethylene glycol, glycerol, xylitol, sorbitol) and represent a pillar of the present review since it focuses mainly on the development of the catalyst. Fewer works have been dedicated to complex matrixes (glucose, xylose, woody biomass) [6,7]. Finally, limited researches described the performance of multicomponent mixtures, close to an industrial application (e.g. wastewater from the brewery industry, food industry) [8–14].Previous reviews on aqueous phase reforming focused both on the influence of the reaction conditions and the catalytic systems. Davda et al. reported in the first review the fundamentals of their pioneering research [15]. Chen et al. reported the different reaction mechanisms among the substrates [16], while Coronado et al. summarized in their review a large number of issues, helping to compare various parameters [17]. Vaidya et al. classified the available literature on the base of the starting substrate [18]. Finally, very recently, El Doukkali et al. reviewed the research of effective catalysts for the valorization of glycerol through steam reforming, hydrogenolysis and, indeed, APR [19].Despite most of the works looked at the development of an effective catalyst, this subject has not been comprehensively reviewed so far in the APR field. For this reason, the present work aims to review the actual knowledge on the design of catalytic systems for the valorization of biomass-derived compounds via APR.Chapter 2 deals with a brief introduction to the process to show its possible applications and advantages. Thermodynamic and kinetic considerations are reported, with a particular focus to the latter, since its knowledge constitutes the basis for the rational design of the catalyst.The core of the work is based on the effect of different parameters on the performance of the process. The scientific contributions were extensively reviewed, starting from the pioneering works of the Dumesic's research groups up to the most recently published ones.Chapter 3 outlines the influence of the active site, both in monometallic and bimetallic systems, with specific attention to Pt-based bimetallic systems; moreover, structure sensitivity effects are discussed. Chapter 4 describes the impact of the support's choice on the APR performance. Chapter 5 deals with the effect of the preparation method as a key step for determining the properties of the catalyst and, in turn, the yield of the desired product. The open questions and challenges related to catalytic and technological subjects are discussed in Chapter 6.The research papers have been classified in the corresponding chapter according to the main aim of the study, although defining proper boundaries is not trivial. For example, bimetallic catalysts can alter both electronic properties (modifying the bond strength of reactants, intermediates and products) and structural properties (modifying the dispersion and, consequently, the number of available sites).Throughout the review, specific attention is put to the use of density functional theory (DFT), microkinetic modelling or in situ techniques to get information for the design of new catalysts. In fact, since the catalyst structure can be modified by the environment during the reaction itself, in situ characterizations can inspect connections between the catalyst structure and its performance. In conclusion, the primary outcomes are summarized and integrated to have some final messages for the design of an active, selective, and stable APR catalyst.The aqueous phase reforming is a process carried out at mild temperatures (220–270 °C) and pressures (30–60 bar), in the presence of a catalyst [15]. In these conditions, the aqueous solution remains in the liquid phase, leading to an energetic advantage thanks to the avoided vaporization. This is one of the potential benefits of APR compared to the conventional steam reforming process, that is performed typically at about 800 °C. The reaction stoichiometry is reported in equation (1). (1) C n H 2 y O n ↔ n C O + y H 2 In Fig. 1 , the influence of temperature on the Gibbs free energy for the reforming of alkanes and alcohols is reported. It can be observed that oxygenated hydrocarbons have a more favorable equilibrium, i.e., the hydrogen production can occur at lower temperatures than if obtained from alkanes.In the same temperature range, the water gas shift (WGS) reaction is favored as well (equation (2)). (2) C O + H 2 O ↔ C O 2 + H 2 This occurrence allows to generate in one reactor a gas mixture where CO is present in negligible percentage, making the gas stream a compatible feed for low temperature proton exchange membrane fuel cells (PEMFCs); contrarily, in the steam reforming plant, a double configuration with high- and low-temperature water gas shift reactors is necessary to maximize the hydrogen yield.Combining equations (1) and (2) we can derive the conventional reaction stoichiometry for APR (equation (3)) (3) C n H 2 y O n + n H 2 O ↔ n C O 2 + ( y + n ) H 2 It is important to recall here that, despite the advantageous thermodynamics for hydrogen production, another reaction involving hydrogen consumption is more favored: the methanation reaction (equation (4)). (4) C O 2 + 4 H 2 ↔ C H 4 + 2 H 2 O The APR mechanism is constituted by different steps, in which the interactions of the molecule with the active site, the support and their interface play a fundamental role to determine the product distribution. Knowing the possible steps involving the reacting molecule is essential to properly design the catalyst, tuning its structural, morphological, and textural properties to favor one pathway rather than the other. The possible reaction mechanism of glycerol APR is described in Fig. 2 . As will be reported in the next paragraphs, glycerol was chosen as model compound for several works, due to the importance of its valorization for the biodiesel value chain.The substrate can undergo dehydration (a) with the acid sites of the support, leading to hydroxyacetone, which can be subsequently hydrogenated (b) to propylene glycol. This route is undesired because hydrogen is consumed.If the molecule interacts with the metal active site, it is generally agreed that the dehydrogenation (c) of the molecule is the first step (glyceraldehyde intermediate). Afterwards, it can follow two routes. C–O bond breaking (d) can occur, leading to the formation of alkanes. In this case, since the C–H activation of alkanes is thermodynamically hindered at typical APR temperatures, their hydrogen content cannot be exploited. On the other hand, C–C bond breaking (e) can lead to the formation of carbon monoxide. This intermediate, while adsorbed on the active site, may interact with water that, once catalytically activated (i.e., H2O → OH + H), produces hydrogen and carbon dioxide via WGS (f). Another detrimental route is that CO undergoes methanation (or Fischer-Tropsch reaction), consuming hydrogen (g).Taking into consideration the reported possible reaction pathways, some key points should be considered by the catalyst designer to maximize the hydrogen production: • Dehydrogenation, C–C bond breaking, H2O activation, and water gas shift reaction should be favored; • C–O bond breaking, methanation/Fischer-Tropsch and dehydration should be avoided. Dehydrogenation, C–C bond breaking, H2O activation, and water gas shift reaction should be favored;C–O bond breaking, methanation/Fischer-Tropsch and dehydration should be avoided.As graphically reported in Fig. 3 , preparing the catalyst involves three main choices regarding the preparation, the metal and the support. In the next chapters, the available literature will be presented with the aim of systematically pointing out how each of these choices can be beneficial or detrimental to the activation of each of the reported steps, i.e., favor C–C bond breaking more than C–O breaking, or activate water dissociation without worsening the support stability. APR is strongly sensitive to the reaction conditions, such as nature and concentration of the feed, reaction temperature and pressure, catalyst amount, reactor configuration. It derives that fair comparisons of results between different works are difficult and will be limited in this review.In the present chapter, the effect of the active metal is reported. At the beginning, a comparison between monometallic systems is performed, both using first-principles and experimental methods. Afterwards, the synergy of bimetallic catalysts is reported, with a higher attention to Pt-based and Ni-based formulations. Specific paragraphs are dedicated to systems considered particularly interesting both for the obtained results and for the extent of characterization (i.e., Pt–Mo, Pt–Co, Pt–Ni, Pt–Re, Ni–Cu). Finally, the influence of the particle size is discussed.Platinum is the most studied metal for aqueous phase reforming since it combines high activity and moderate selectivity. These results have been primarily explained by several DFT investigations. Davda et al. studied the C–C and C–O bond cleavages of ethanol by DFT on Pt [20]. Firstly, the authors reported that Pt–C bonds are more stable than Pt–O bonds examining the stability of different isomeric species. Secondly, they suggested that the C–C bond cleavage should be faster than the C–O bond cleavage because the energy for the transition state of the former is one order of magnitude lower than the latter (4 vs 42 kJ/mol) [21]. It follows that, recalling Fig. 2, path e) is more favored than path d).In Fig. 4 the possible steps involved in ethanol reforming on Pt are depicted [22]. After being adsorbed on the active site, ethanol can follow two routes for the first dehydrogenation according to which H is abstracted (ethoxy CH3CH2O or 1-hydroxyethyl CH3CHOH); in the second step, this intermediate is dehydrogenated in acetaldehyde. Thereafter, the adsorbed acetaldehyde further dehydrogenates into acetyl (CH3CO), ketene (CH2CO) and ketenyl (CHCO) (please note that only the most plausible species were reported here among the possible pathways). It is just the ketenyl species that finally is subjected to C–C bond breaking to CO and methylidyne (experimentally confirmed). This outcome has been compared in a proper work in which DFT, microkinetic investigation and experiments have been combined [23]. The authors observed that the level of hydrogenation of the intermediates influenced the C–C or C–O cleavage barriers because of geometric and electronic effects. In this sense, more dehydrogenated species facilitated C–C bond breaking having the C–C bond parallel to the surface. The microkinetic model also highlighted that the formation of 1-hydroxyethyl via α C–H scission (CH3CHOH) is predominant compared to CH3CH2O after first ethanol dehydrogenation (bold arrows in the corresponding figure). Further theoretical works on platinum with different substrates were reported in Refs. [24–26], attaining analogous conclusions.On ruthenium, the activation energy for C–C bond cleavage is lower than on platinum, being 38 kJ/mol when the surface CH2CO species is formed (while it is 90.24 kJ/mol on Pt after CHCO), so it is more sensitive to the first dehydrogenation steps [27]. However, it is far more active towards methanation than Pt, decreasing the hydrogen selectivity [28].The decomposition of glycerol has been explored also on Pd, Rh, Cu, Ni, focusing on dehydrogenation, C–C and C–O cleavage [29]. Glycerol binds similarly on Pt, Pd, Rh, Cu, through the hydroxyl groups. The four possible mono-dehydrogenated species are reported, derived from C–H or O–H cleavage of the terminal or central carbon. Furthermore, the binding energies related to more dehydrogenated intermediates and C–O cleavage are reported as well. In agreement with previous studies, it has been reported that C–C breaking is favorable on Pt after several dehydrogenation steps, and it is more facile than C–O breaking. Despite the simplification of the models, the results were coherent with the experimental outcome. The pathway is similar on Pd, i.e., dehydrogenation up to C3H5O3 is necessary and C–C is more favorable, but the lowest C–C scission transition state energy is higher than on Pt, suggesting that Pt is more active. Despite dehydrogenation is always necessary, the situation is different on Rh and Ni, where the energies between C–C and C–O are more comparable. Moreover, the transition states are quite low in terms of activation energy, explaining the experimentally known high activity of Ni despite its low selectivity. Finally, on Cu the transition states show a quite high activation energy, highlighting the low Cu activity. It is worthy to point out that the model was further implemented considering the presence of adsorbed CO. Higher coverage of CO increased the free energy for glycerol dehydrogenation, emphasizing the pivotal role of the water gas shift reaction in the process to reduce the concentration of adsorbed CO.Davda et al. initially experimentally investigated Pt, Ni, Ru, Rh, Pd and Ir supported on silica for ethylene glycol APR [30]. In their kinetic study, the authors reported the promising ability of Pt to produce hydrogen, while Pd showed low activity and the highest selectivity (Fig. 5 ). Looking at non-noble metals, Ni was comparable to Pd, but suffered from low selectivity (alkane production) and it was more prone to deactivation. The use of a cheap material, such as Ni, would increase the cost-effectiveness of the process. However, its low stability and H2 TOF (ten-fold lower than platinum) would impede its utilization in a larger scale context.It has been largely reported that bimetallic catalysts perform better than monometallic in many different fields, and APR is part of this family [31]. Before entering in the core of the paragraph, it is worthy to cite the first outcomes derived from the works carried out in the Dumesic's research group.Huber et al. used a high-throughput system to study several catalyst formulations for ethylene glycol APR [32]. They found that Sn addition to Raney Ni catalyst greatly improved its performance. Sn was mainly present at Ni defect-sites and as an alloy (e.g. Ni3Sn), without affecting the Ni particle size. In this way, Sn hindered the CO methanation that, over Ni catalysts, preferentially occurs in the defects. This outcome was further explored in successive works. The addition of Sn, Au or Zn to Ni/alumina catalyst promoted the hydrogen selectivity, with Sn being the best promoter [33]. Being alumina not stable, Sn was added to Raney-Ni catalysts. With only 400:1 Ni:Sn atomic ratio, the methane production was halved, and eliminated at 18:1. The modified catalyst deactivated in the first 48 h by sintering, and afterwards by re-oxidation of the active site because of the interaction with water (no coke). Leaching of Ni was explained by formation of organometallic species after interaction of the catalyst with the feed. This phenomenon was reduced as well thanks to Sn addition. Finally, since Pt and Pd showed the best performance in monometallic systems, more than 130 Pt and Pd bimetallic catalysts based on these noble metals were screened using APR of ethylene glycol as probe reaction [34]. Four bimetallic catalysts were particularly interesting for their results: Pt–Ni, Pt–Co, Pt–Fe and Pd–Fe. The authors reported that alloying Pt with Ni, Co and Fe led to an electronic modification of Pt which caused the decrease in the binding energy (BE) of hydrogen and carbon monoxide and the increase in the dissociative adsorption energy of hydrogen. On the other hand, Fe addition in Pd-based catalyst promoted the WGS, which is considered the rate determining step on monometallic Pd.The next paragraphs report the outcome of several bimetallic systems, including the motivation that the authors proposed to explain the structure-activity relationship. The huge amount of information required a strong effort to rationalize and understand how to present it. They were not reported chronologically; rather, it was in the aim of the authors give a progressive insight into the phenomena, collecting different researches on similar systems to have common outputs. The paragraphs are divided according to the main actor of the system, e.g. platinum, nickel, etc., despite some of them could have been reported in more than one paragraph (i.e., the description of Pt–Ni could have been stated in the Pt or Ni section). Finally, ad hoc sub-paragraphs were dedicated to the most interesting Pt-based (Pt–Co, Pt–Mo, Pt–Ni, Pt–Re) and Ni-based (Ni–Cu) systems in order to go deeper with their comprehension. If not clearly stated, glycerol is considered as the feedstock.Godina et al. investigated the APR of xylitol over mono and bimetallic catalysts [35]. Among the platinum-based bimetallic catalysts, Pt–Re showed the highest conversion at each weight hourly space velocity (WHSV) investigated (Fig. 6 -A). This outcome was related to the higher water gas shift, promoted by Re. Low amount of erythritol was found in the liquid phase, suggesting that C–O cleavage was favored over C–C. Pt led mainly to propane, being active for C–C, while Pt–Re mostly led to butane and propane. Overall, because of the lower hydrogen selectivity of the bimetallic system than the monometallic one, the hydrogen turnover frequency of the former was lower than the latter (about 23 min−1 vs 26 min−1, at WHSV 9.5 h−1).Larimi et al. investigated different promoters (Rh, Cr, Re, Pd, Ru, Ir) for Pt-based catalysts supported on γ-alumina [36]. As reported in Fig. 6-B, while the conversion was approximately constant (about 82%), the hydrogen selectivity was minimum for monometallic Pt (69.9%) and maximum for Pt–Rh (89%). The variation of the lattice constant proved the formation of a solid solution. It was suggested that Rh helped the WGS, at the same time increasing the mobility and reactivity of surface oxygen atoms. The benefic addition of Rh was also reported in Ref. [37], where the use of MgO-supported nano-sheet catalysts allowed stability during 100 h TOS (time on stream), overcoming the limitations reported by Ref. [38].The addition of Rh can have different effects on different supports. When added to Pt/α-alumina, both the hydrogen yield and the stability increased, by preventing coke formation favoring methanation [39]. On the other hand, Pt/γ-alumina's performance was negatively influenced by Rh, with a slight decrease of the hydrogen yield and catalyst life span (46.3 h vs. 51.6 h).Guo et al. investigated the APR of glycerol on different Pt-based bimetallic catalysts [40]. The alloys were pre-synthesized and successively loaded on the support (γ-Al2O3) to exclude the influence of the particle size. Among the promoters (Ni, Co, Fe, Cu), Fe showed the highest carbon conversion to gas and hydrogen yield, with 1:1 considered as optimal atomic ratio. Water gas shift tests showed consistent results with the APR tests. The coherent results between APR and WGS confirmed that the latter has a strong influence on the overall result. To further investigate this aspect, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was utilized. It was showed that formate desorption is not easy on Pt/alumina, while it was easier for the Pt–Fe system. It was suggested that Fe promoted the water activation producing OH groups that can react with the proximate CO adsorbate on Pt. This is the reason why 1:1 atomic ratio led to the best results, because it is the one which presented the highest proximity between the two metals. Moreover, it favored the formate decomposition that causes otherwise the sites blocking. Further works on Pt–Fe bimetallic systems were reported in Refs. [41,42]. Moreover, further DRIFTS spectra showed new CO adsorption sites on Fe in Ref. [43].Dosso et al. studied bimetallic Pt based catalysts promoted with Ni or Co for APR of polyols derived from glucose degradation [44]. Pt and Ni are completely miscible, and this occurrence affected the electronic properties of Pt reducing the binding of adsorbate. This is in agreement with the results reported in Ref. [34]. X-ray photoelectron spectroscopy (XPS) showed that Pt was mainly in the oxidized state (Fig. 7 -A), likely because of the donation of electrons. The interaction of Pt with Co and Ni was suggested by the presence of a broad peak of reduction in the H2-TPR, as depicted in Fig. 7-B. Pt–Ni showed higher yield than Pt–Co. No direct comparison with the monometallic could be performed as the authors used also different preparation techniques, namely urea matrix combustion method (UCM) and incipient wetness impregnation (IWI). Being Ni not active for water gas shift, a high selectivity to CO was observed (almost 20-fold the one of Pt–Co). The authors finally highlighted the higher presence of coke on Pt–Ni compared to Pt–Co, not directly explained by its limited higher activity.Pendem et al. studied the effect of potassium promotion on Pt/Hydrotalcite catalysts for APR of glycerol [45]. Despite potassium addition decreased the surface area, it contributed to the increase of the surface basicity (measured by CO2-TPD) and stabilizing the Pt precursor, increasing the final dispersion of the active site. Adding 2.8% of K increased the glycerol conversion from 27 to 88%. Further addition up to 16.9% K decreased the activity likely for the covering of Pt particles (67% conversion). However, increasing the loading up to 28% changed the morphology improving further the dispersion and allowing to reach 83% conversion. Moreover, the increase of basicity caused an increase in the H2 selectivity. Finally, the catalyst showed stable performance after four runs, without appreciable leaching of both Pt and K. The effect of a second metal in modifying the acid-base properties of the catalyst was also reported in Ref. [46] for Mn. The results were comparable with [47] in the case of ethylene glycol APR, despite no leaching was observed in this case.Another way to improve the APR performance was reported adding Ru to Pt-based catalysts on carbon supports prepared by impregnation [48]. In the APR of wheat straw hydrolysate, Pt–Ru/Multi walled carbon nanotubes (MWCNT) showed the highest activity and hydrogen selectivity. It was attributed to the electron donor effect of Ru that increased the Pt electron density, filling its d-band and reducing the strength of adsorption of organics. The proximity of Ru and Pt clusters was the key to enhance the catalysis. In fact, the same did not happen using a different support, activated carbon. In this case, the catalyst was constituted by unpaired Pt and Ru atoms, which consequently were unable to express their synergy.Despite not performed in the context of APR, it is interesting to cite here that the addition of Ru improved also the sulfur tolerance of Pt monometallic [49]. The authors exploited the sulfur spillover concept, due to the higher resistance of ruthenium, and the hydrogen spillover that can regenerate the catalyst. Extended X-ray absorption fine structure (EXAFS) determination of the coordination number showed the presence of an alloy at the three investigated ratios (Fig. 8 - step 1). Sulfur species may interact with platinum (step 2) and then move to Ru by spillover (step 3), or directly interact with the latter (step 4). The authors reported that sulfur poisoning caused dealloying; afterwards, hydrogen spillover (step 5) could allow the removal of sulfur. Indeed, the regenerated sample showed similar spectra than the fresh one, through a re-alloying mechanism. In this sense, the atomic balance between the two metals is fundamental to favor both the sulfur-trap capacity (given by Ru) and the spillover at proximity.The last example reported in the paragraph regards the use of Zn as promoter. Lei et al. used atomic layer deposition to modify Pt/alumina catalysts by ZnO promotion [50]. A higher hydrogen production was obtained when Pt was deposited before than Zn. This is because it took advantage of the two interfaces Pt–ZnO and Pt–Al2O3. Thanks to X-ray absorption near edge structure (XANES) spectroscopy, the authors suggested that the significant charge transfer from Zn to Pt promoted the H2 selectivity. Moreover, the bimetallic system showed only limited sintering (the particle size increased from 0.9 nm to 1.2 nm), contrarily to the monometallic catalyst (from 1.0 nm to 2.4 nm). Table 1 summarizes the effects of the cited promoters on the performance of APR when added to Pt-based catalysts. It can be observed that the positive impact is commonly associated to the promotion of water gas shift, which can be obtained by different paths (increase of surface basicity, water activation, easier intermediates desorption). One typical drawback is that the increase of the conversion is (partially) counterbalanced by the decrease of the hydrogen selectivity, since methanation is promoted as well. It is important to observe that in some cases different results were obtained, despite similar catalytic systems and analogous substrates were used, such in the case of Pt–Rh on gamma alumina ([37,39]), or for Pt-Mn ([46,47]). On the other hand, similar results were obtained also with different substrates such as ethylene glycol or glycerol for Pt-Fe ([34,40]), or three different polyols in the case of Pt-Co ([44]).Dietrich et al. studied a Pt–Mo bimetallic catalyst supported on carbon prepared by IWI [51]. Transmission electron microscopy (TEM) showed that the bimetallic nanoparticles sintered during glycerol reforming, with the average size moving from 2 nm to 5.1 nm. X-ray absorption spectroscopy (XAS) showed that the Pt–Mo nanoparticles are metallic Pt-rich with the presence of Mo in different states (metallic when close to Pt, oxide when in isolated clusters, as reported in Fig. 9 -A). Even more interesting, operando XAS was carried out to evaluate if the state of the catalyst changes during the reaction. It was reported that Pt remained in the metallic state and its coordination number increased, coherently with the results reported by TEM. Overall, the basic structure remained the same, with a Pt-rich core and Mo in the near surface region of the nanoparticle. This is coherent with DFT calculations which predicts an analogous core-shell composition when starting from metallic Pt and MoO precursors.The comparison with monometallics and the extensive study on the role of Mo using operando spectroscopy and DFT was performed in a following work [52]. The authors reported that Pt was the real active site, while Mo acted as a promoter modifying the electronic properties of Pt and its interaction with the adsorbed molecules. DFT calculations showed that Mo decreased the Pt–CO binding energy; as a matter of fact, XANES showed that CO was the most abundant surface species in the case of Pt/C (about 0.6 fractional coverage), while it was under the detection limit for Pt–Mo/C. At the same time, it reduced the kinetic barrier for dehydrogenation and even more for C–O bond cleavage, leading globally to a reduction of the H2 selectivity (Fig. 9-B). Further DFT investigations in agreement with the ones reported here can be found in Ref. [53], where the role of Mo oxide is highlighted regarding the change of WGS mechanism and the bifunctionality of the catalytic system is suggested.The promotion of Co was studied on Pt supported on MWCNT by IWI [54]. Co raised by 4.6 times the glycerol activity normalized per Pt surface. The improvement was ascribed to the WGS promotion, so by removing the strong CO adsorbed on the Pt site facilitating the water activation. The hydrogen selectivity slightly increased compared to the monometallic Pt, in the whole range of glycerol conversion (Fig. 10 -A). Pt–Co and Pt showed an increase of the particle size from 1 nm to 2 nm (more than one-week test), indicating a good stability of the catalytic system. Pt–Co particles were found in three different configurations: Pt clusters (59%), Co core – Pt shell (30%) and Pt–Co alloy (11%). Therefore, despite only 40% of the particles was in the bimetallic form, still the performance improved. The high presence of Pt on the surface, contrarily to the case of the Pt–Mo where it was in the core, may explain the maintenance of high hydrogen selectivity. XANES outcome reported a downward shift in the d-band center compared to the monometallic Pt, which is related to a lower binding energy of CO.The same research group further evaluated the effect of Co addition to Pt-based catalysts performing operando X-ray absorption spectroscopy (XAS) investigation [55]. Adding Co increased the site time yield up to four-fold when the catalyst composition was 1Pt5Co (i.e., 1:5 Pt-to-Co molar ratio). Please note that monometallic Co showed negligible activity. The comparison between fresh and operando structures of the catalyst exhibited modification because of the hydrothermal environment, despite preserving the bimetallic configuration. The formation of an alloy was considered the reason for the improvement in the performance, rather than the nature of the particles (e.g. Pt shell/Co core). Being maintained the selectivity, the authors suggested that the role of Co was to exalt the Pt properties, at the same time reducing the CO binding energy and improving the WGS.Previously Wang et al. investigated by XAS Pt–Co nano-alloyed particle systems supported on single wall carbon nanotubes (SWCNT) [56]. XAS spectra showed a core (Co) and shell (Pt–Co) structure, which greatly enhanced the activity without decreasing the selectivity. The electron transfer from Co to Pt may perturbate the latter, probably affecting the reaction path.Co promotion on Pt based catalysts was also studied on CeO2–ZrO2 mixed oxides supports for ethylene glycol APR [57]. As reported in Fig. 10-B, the optimal Co:Pt molar ratio in terms of APR activity and WGS (CO conversion) was found equal to 0.5. The authors suggested that the oxophilic properties of cobalt favored the WGS and penalized the formation of coke (likely hindering acetic acid intermediate formation [58]). Further increase of Co loading decreased the ethylene glycol conversion. The interaction between the two metals was investigated via TPR. Adding the promoter, the strong metal-support interaction Pt–CeO2ZrO2 decreased. Other phenomena such as the formation of surface defects and higher dispersion of the bimetallic can contribute to the higher activity. As a matter of fact, the activity trend followed the results of the CO chemisorption. The promoter helped also to stabilize the platinum particles, which were less aggregated in the bimetallic catalyst compared than the monometallic.Platinum and nickel are likely the most studied active metals for APR, since they can be defined as representatives of two classes. Platinum is the most effective monometallic catalyst, but it suffers from a high cost; on the other hand, nickel is cheaper, and despite its performance are less enthusiastic than platinum, its use would increase the cost-effectiveness of the process. Ni/C exhibited much lower activity than Pt/C, particularly at low WHSV, and deactivated rapidly because of Ni leaching during APR of xylitol [35]. Even for Ni-based systems, as will be shown more extensively later, the addition of a promoter improves the performance. For example, it was cited that the presence of Sn in a Raney Ni catalyst led to a hydrogen production per unit volume equal to 350 μmol H2/cm3, comparable to the value obtained by Pt/Al2O3 (450 μmol H2/cm3) [32]. For this reason, several researchers investigated bimetallic systems which involved these two metals.He et al. studied the optimal ratio between Pt and Ni on MWCNT prepared by IWI for glycerol APR [59]. Adding the second metal allowed to increase the dispersion of Pt. The formation of Pt–Ni alloy modified the electronic properties of Pt, increasing the interaction with the support and thus the dispersion. Adding Ni also caused the highest increase of glycerol conversion (from 26% to 81%), while the carbon conversion approximately increased from 8% to 15% (Fig. 11 -A). The hydrogen yield increased from 1.83 mmol H2/g glycerol for the monometallic to a maximum 2.43 mmol H2/g glycerol, together with a strong increase of methane (approximately 3 folds higher). This is coherent with the known methanation activity of Ni. Looking at the effect of Ni loading, it was reported that 1:1 was the optimal atomic ratio, since excessive Ni may not form an alloy with Pt and rather cover it.Different Pt:Ni ratios (1% Pt and 3–18% Ni) on ceria doped alumina catalysts were also investigated in Ref. [60]. Two different types of surface alloys, NiPt and Ni3Pt, were identified. Adding Ni up to 6% reduced the crystallite size causing the highest activity (96% conversion) and H2 selectivity (approximately 83%), as depicted in Fig. 11-B. The shift in the Pt 4f7/2 XPS peaks highlighted the modification of the electronic environment by Ni addition. The synergy between the two metals is proved by the fact that physical mixture of the two catalysts did not perform similarly well. The improvement compared to monometallic Pt was referred to the improvement of WGS thanks to Ni addition, higher dispersion, and lower H2 and CO binding energy on Pt, making the sites more available. Looking at the catalyst stability, the authors showed that monometallic Ni severely deactivated (metal oxidation, carbon deposition and leaching), while 1Pt6Ni was stable on 85 h TOS (small particles aggregation but not carbon deposition was observed).Possible structural modification of bimetallic Pt–Ni catalysts under APR conditions were followed thanks to in situ EXAFS analysis [61]. Fig. 11-C shows the modification subjected by the Pt–Ni clusters under APR. When the catalyst is reduced, the core is Ni-rich, while the shell is Pt-rich. However, under APR conditions, the Ni–Pt particles restructures, with platinum diffusing to the core while Ni segregates to the surface. Importantly, this behavior was reversible prior subsequent re-reduction. The enhanced activity of the bimetallic catalyst compared to monometallic Pt was explained by this segregation. This is coherent with DFT studies which showed higher activity of Ni-terminated (i.e., Ni-rich surface) Pt–Ni nanoparticles due to the increase of oxygen binding energy which boosts the initial dehydrogenation rate [62].Pt–Re/C was initially studied for the gas-phase glycerol reforming, where it was noted that the H2 turn over frequency (TOF) increased by one order of magnitude compared to Pt/C [63]. In a simplified kinetic model, the authors observed that the CO adsorption equilibrium constant was 10 times lower for the bimetallic catalyst compared with the monometallic. Kunkes et al. carried out microcalorimetric measurements of CO adsorption and CO-TPD studies [64]. Pt/C catalyst reported 115 kJ/mol as heat of CO adsorption, while for Pt–Re/C catalyst it approached the one of pure Re (105 kJ/mol). Moreover, the partial oxidation of Re sites under the APR condition may form sites with lower binding energies. Avoiding that the sites are blocked by CO (or other intermediates) is of paramount importance because C–C bond breaking mainly happens with transition states multiply bonded to Pt sites, therefore the latter must be preferably free [21]. Thanks to attenuated total reflection-infrared (ATR-IR) in situ analysis, it was reported that CO desorption from catalyst surface was more facile on the bimetallic catalyst compared to the monometallic [65]. ReOx can interact with CO adsorbed on Pt, facilitating its desorption (CO spillover), contributing in this way to the increase of activity. Another reason for the higher activity can be related to the increased C–C and C–O bond breaking capacity, with the latter to a major extent. It means that the Re addition decreased the hydrogen selectivity because it favored hydrogenation reactions of dehydrated intermediates. This is because Re increased the acidity (Re-OH, Brønsted type) of the catalytic system.King et al. tested Pt–Re for the first time under APR conditions [66]. Re addition led to an increase of glycerol and H2 TOF. Re alone did not show any activity, so the effect was just as promoter. However, it decreased the H2 selectivity. As previously reported, the promotion was induced by multiple reasons: on one side, Pt–Re alloy can have higher activity by modification of the electronic properties that can affect the CO adsorption strength. Moreover, Re can favor water gas shift by water activation, which is limited on platinum [67]; in this way, higher OH surface coverage may be obtained, leading to a higher formation of COOH, which is a key intermediate for the reaction. This outcome was proved by CO stripping voltammetry in Ref. [68], where the authors observed that the Pt–Re had a lower onset potential of CO oxidation than Pt one, and it was ascribed to a higher concentration of OH species (stronger binding of oxygen species which promoted water activation). Finally, the acidic properties could explain the decrease in H2 selectivity. KOH addition showed further increase in liquid products, so that it can be assumed that base addition can compensate the surface acidity provided by ReOH.The correlation between the surface properties of the bimetallic catalyst and the product distribution was further investigated in Ref. [69]. XPS showed that Pt binding energy (BE) shifted, and it was higher with higher Re loading. The increase of the BE (electron deficiency) was due to the interaction with Re (possibly alloy formation). However, Re BE increased as well, ascribed to metal-support interaction. In order to simulate the interaction with water, the catalyst was treated with steam, leading to a change of oxidation in both platinum and rhenium. In particular, the presence of new Re-OH species increased the surface acidity, which in turn affected the product distribution favoring dehydration products via acid-catalyzed reactions. Interestingly, it was reported that the hydrogen selectivity increased with the conversion: this is due to the more difficult dehydration of smaller molecules compared with glycerol. Similar conclusions were reported in Ref. [70] and in the case of xylitol [71].In conclusion, it is worthy to highlight some of the outcome reported for these characteristic bimetallic Pt-based catalysts. Table 1 resumes the effects of Mo, Co, Ni and Re addition to Pt, as reported in the last sub-paragraphs. Commonly the promoters induced an electronic perturbation of platinum, which decreased the interaction with the adsorbates and increased the availability of free active sites. Apart from Co, each of the promoters caused a strong increase of the conversion but associated with a decrease of the H2 selectivity. In fact, often the C–O cleavage pathway is favored: for example, due to the enrichment of surface acidity in the case of Re, or due to methanation in the case of Ni. It is important to observe that tuning the reaction conditions, like adding a base, could overcome the reported drawbacks.In view of a possible industrial application of APR, the use of non-noble metals in the catalyst formulation may be a milestone for the success. For this reason, the use of transition metals-based catalysts, such as Ni, has been explored.Rahman investigated bimetallic Ni-based catalysts for APR of glycerol over multiwalled carbon nanotubes [72]. In the 1 Pt-xNi series, 1 Pt–3Ni was found the one with the highest activity (glycerol conversion higher than 99%) and hydrogen yield (Fig. 12 -A). Monometallic Ni was confirmed active towards methanation, reporting the highest yield for methane and the lowest glycerol conversion (44%). As depicted in Fig. 12-B, Ni influenced the electronic state of Pt (shift of the binding energy). While Ni monometallic suffered from deactivation after 65 h TOS, Cu and Pt addition allowed to maintain a stable hydrogen production in 100 h long runs.The influence of several promoters (Mg, Cu, Zn, Sn, Mn) on the APR of ethylene glycol was recently studied over Ni–Al hydrotalcites [73]. The screening reported that the conversion was not affected by the incorporated metal. On the other hand, the selectivity was improved in the case of Mg promoter. The authors explained this outcome because of the change in the electronegativity of Ni. Interacting with MgAlO, Ni became more electronegative, reducing the possibility that the feed may be dehydrated.Luo et al. studied the Ni:Co ratio for APR of glycerol on alumina support [74]. 1:3 was found the optimal ratio to maximize the hydrogen yield, thanks to the tradeoff between higher selectivity thanks to Co addition and lower activity due to the decrease of superficial Ni. The synergy between the two metals, proved by H2-TPR, was confirmed by the fact that both monometallics had much lower hydrogen yield. Slight addition of Ce further increased the hydrogen yield promoting the formation of NiO instead of Ni aluminate, improving the dispersion, and decreasing methane selectivity. Moreover, it avoided Ni sintering and stabilized the alumina support. Nevertheless, coke remained one of the causes for catalyst deactivation.The effect of cobalt addition to a bimetallic Ni–Fe catalyst was systematically studied thanks to Fourier-transform infrared spectroscopy (FTIR) characterization before and after APR of ethylene glycol [75]. The results are reported in Fig. 13 . Ni was found to be responsible for ethylene glycol activation (about 30% carbon conversion to gas for Ni alone vs 10% for Fe alone) thanks to the high activity of metallic Ni for C–C bond breaking. The bimetallic Ni–Fe further improved the conversion at 45%, also reaching higher hydrogen selectivity. Fe was present as Fe3O4 and was supposed to be involved in the WGS via a redox mechanism: Fe2+ was oxidized by water (producing hydrogen) to Fe3+; afterwards, the latter was reduced back by CO (producing carbon dioxide). The addition of Co strongly increased the conversion up to 95%, maintaining the hydrogen selectivity but at the same time cutting the alkane selectivity from 75% to less than 5%. It was suggested that Co increased the capacity to adsorb ethylene glycol at the same time decreasing the one for hydrogen. Finally, the authors highlighted the instability of the catalyst due to the re-oxidation of Fe and leaching of Ni.Further improvements of Ni catalyst were reported in Ref. [76]. A Ni–B amorphous alloy catalyst showed higher hydrogen production and stability than Raney Ni: the improvement was attributed to the boron oxides which, surrounding the hexagonal close-packed Ni, caused their stabilization and the higher resistance against sintering.Tuza et al. studied Ni–Cu catalysts supported on hydrotalcite-like compounds, with varying composition from 20% Ni monometallic to 20% Ni–20% Cu [77]. Ni monometallic showed the highest activity among the samples. The addition of copper increased the Ni dispersion and reducibility. The increase of hydrogen selectivity (250 °C) during 12 h TOS was ascribed to the lower particle size, since multiple Ni clusters are necessary for CO dissociation and methane production. At 270 °C a decrease of the hydrogen selectivity (defined by the authors as the molar fraction in the gas phase) with TOS was observed because of hydrogen-consuming reactions (acetol hydrogenation), with increase of CO2 selectivity.In a successive work, Manfro et al. looked also at surface acidity properties [78]. They observed that bimetallic catalysts were more acidic than monometallic, explaining the higher acetol formation. The absence of re-oxidation contrarily to other case reported in literature was motivated by the stabilization of the support. The slight sintering that occurred did not cause deactivation in 6 h TOS.Ni–Cu supported on MWCNT for glycerol APR was also run up to 110 h, in order to get information also on extended time on stream [79]. X-ray diffraction (XRD) spectra showed a shift in the Ni peak, indicating the higher interaction with Cu in the case of higher loading, with possibility of an alloy due to their complete miscibility. Moreover, the dispersion increased when Cu was added (further increase led to similar dispersion than the monometallic). At the same way, the 1Cu12Ni was the most stable against sintering. The catalytic tests showed superior performance of the bimetallic formulation, with 1Cu12Ni giving the highest hydrogen yield. It reported lower CO ascribed to the higher WGS activity; moreover, methanation activity of Ni decreased, attributed to the alloy formation and the reduction number of clusters with multiple Ni atoms which are necessary for CO hydrogenation (4-fold lower methane formation in the bimetallic catalyst compared to the monometallic one). Looking at the stability, 6Cu12Ni and 12Cu12Ni showed high sintering, while 1Cu12Ni did not, leading stable H2 yield along three consecutive 110 h TOS runs. Only minor sintering was observed, but no re-oxidation for Cu and Ni and leaching. WGS activity was assessed with a proper test.Park et al. studied different Ni-based catalysts supported on LaAlO3 with different promoters (Cu, Co, Fe) and found that Ni–Cu was the one with highest glycerol conversion and hydrogen selectivity [80]. It was the catalyst with the highest dispersion, while the monometallic Ni showed the worst dispersion. Furthermore, it showed less coke deposition (whisker type, less harmful because it grows on one side of the metal particle) than the monometallic (graphite type) and no sintering (contrarily to the monometallic). However, it was higher than the Ni–Co formulation. Table 2 summarizes the effects of the promoters on the performance of APR when added to Ni-based catalysts. Since the beginning of the APR research, Ni has been reported as one of the most interesting non-noble metals due to its high activity. The addition of the promoters like Sn, Mg, Co and Cu helped to increase the selectivity by different mechanisms (e.g. by blocking defect sites or decreasing the particle size). This outcome is a key improvement since the lack of selectivity due to the high tendency of methanation is a drawback of Ni-based catalysts. At the same way, most of the unsolved issues still regards the instability by re-oxidation, carbon deposition or leaching, which may prevent a long-term use.Apart from the catalytic systems previously reported, some others are worthy to be cited. In fact, despite they may be less frequently applied, their outcome may be interesting to stimulate further research. Therefore, in the following, formulations based on noble metals (Ir and Ru), bulk and metal-free catalysts will be reported.Liu et al. tailored the properties of a Ir-ReOx/SiO2 catalyst with a noble metal (Ru, Pd, Pt) to favor either the APR of glycerol (C–C breaking) or its hydrogenolysis (C–O breaking) [81]. Pt showed higher conversion compared to Ru and Pd, but lower selectivity to acetol and propylene glycol (1,2-PrD), due to its higher selectivity towards APR. Being the particle size similar, it cannot be ascribed to structure-sensitivity features. Increasing the conversion led to an increase in the C3 products selectivity. This is coherent with previous literature since hydrogen selectivity decreased with the reaction time due to parasite consecutive hydrogenation reactions. The properties of the Pt–Ir-Re system were investigated looking at the monometallic and combination of bimetallic (Fig. 14 ). Ir provided the highest conversion among the monometallic and the highest selectivity to acetol. Pt–Ir and Ir–Re showed higher conversion than the monometallic, with the former providing more APR activity: this is suggested by the higher presence of hydrogen and propylene glycol, meaning that more hydrogen was produced. This outcome showed the synergy among Pt and Ir since the corresponding monometallic catalysts reported very low activity. The authors suggested that new species were formed, with XRD showing a shift in the Pt and Ir peaks indicating the formation of alloys. The physically mixed catalyst had similar conversion level to the Ir-ReOx/SiO2 but lower than the trimetallic, underlying the importance of the proximity between the components, which cannot be obtained through a simple mixing. Adding ReOx allowed to reduce the metal particle size. Finally, it was observed that low loading of Pt (and likely smaller particles) favored the C–O hydrogenolysis.Espinosa-Moreno et al. studied the APR of glycerol over Ir-based bimetallic catalysts with Cu and Ni [82]. The authors observed that Ir–Cu led only to 0.9% of carbon conversion to gas, despite the high glycerol conversion (76.5% on La2O3 support). This result was not trivial from literature. In fact, Ir was initially suggested by Dumesic and coworkers as a plausible APR catalyst thanks to its high hydrogenolysis activity (referring to the work of Sinfelt [83]). The addition of Cu was not sufficient to overcome the WGS limitation and the possibility that CO poisoned it.The performance of Ru-based catalyst was improved as well thanks to the use of promoters, under multiple points of view. Its stability was improved thanks to the use of nitrogen, which avoided Ru sintering increasing the metal-support interaction, favoring at the same time the initial dispersion [84]. At the same time, higher glycerol conversion and hydrogen selectivity were obtained. Moreover, it exalted the basicity of the support. This feature helped to activate the water molecule, which is a necessary step for the WGS reaction: its facilitation allowed to improve the overall performance because the sites were less occupied by the CO.Novel unconventional types of catalysts can be finally reported, such as the use of cobalt aluminate spinel for the APR of glycerol [85] and metal-free catalysts [86]. Cobalt is not stable due to oxidation and leaching. However, the spinel formation with alumina may overcome these limitations. Thanks to a mutual protective effect, the spinel catalysts had surface area higher than cobalt oxide and alumina alone. Increasing the Co loading decreased the surface acidity (measured by NH3-TPD) because alumina, which is a Lewis acid, was substituted by cobalt oxide. Analogously, basic sites formation was favored by Co loading (measured by CO2-TPD). Reference tests with alumina and cobalt oxide alone gave respectively the highest and lowest selectivity for hydroxyacetone, which is acid-catalyzed; at the same time Co3O4 gave the highest H2 and CH4 selectivity, being methanation favored by basic sites. The spinel structure helped in reducing the methanation activity and the re-oxidation of Co (5% vs 10%), even in a reducing atmosphere. The results showed the predominance of C3 liquid products, highlighting the low activity for C–C breaking of Co. The conversion decreased with time on stream also due to coke and sintering of large particles.Esteve-Adell et al. used for the first time a metal-free catalyst to perform the APR of ethylene glycol, i.e. graphene, obtained by alginate pyrolysis [86]. In this context, the presence of defects is pivotal to activate reactions that commonly require metals to overcome kinetic limitations. Graphene allowed higher conversion compared to graphite-derived materials. One main point of concern was the low stability, that ended up in negligible activity at the third run. It was ascribed to the possibility that carboxylic acids by-products or hydrogen itself may react with the active sites (carbonyl groups and diketones). In particular, the author proposed that they consist of frustrated acid-base Lewis pairs, i.e., Lewis acid and bases close enough to interact with H2 but not enough to interact between each other, acting as dehydrogenation sites. IR spectra and thermogravimetric analysis showed the presence of adsorbate organics.Before moving on the third element chosen by the catalyst designer, the choice of the support, it is worthy to discuss one issue still related to the active metal category. It has been proven that aqueous phase reforming is a structure-sensitive reaction, i.e., its rate, normalized per exposed metal surface atom, changes with the particle size [87]. However, in the APR field, there is no apparent agreement if smaller of bigger particles are more benefic for hydrogen production. In the following, the main works analyzing this topic are reported. Despite in some cases TOF was not reported, valuable comments can be drawn.Lehnert and Claus reported for the first time that hydrogen selectivity increased with increasing particle size, without affecting the conversion (Fig. 15 -A) [88]. Increasing the particle size implies that the number of face atoms increases, while the number of edges and corner atoms decreases [89]. Therefore, it can be postulated that a greater extent of face atoms permits favorable adsorption of the oxygenates for the C–C breaking.On the other hand, Wawrzetz et al. reported different results. Despite the agreement on the fact that the conversion was slightly affected by the particle size, they reported higher hydrogen selectivity for smaller particles (Fig. 15-B) [90]. This result was ascribed to the higher concentration of metal sites that hindered the dehydration pathway (i.e., the catalytic chemistry favored by the support). Having smaller particles means highly coordinatively unsaturated metal atoms and at the same time higher concentration of metal atoms at the support-metal boundary. Both aspects may affect the mechanism followed by the molecule and its fate (hydrogen or alkane production).The latter outcome was supported by the results reported in Ref. [91], where small Pt particles favored C–C breaking more than C–O. Moreover, sintering happened during the first 60 min (from approximately 2 nm to less than 4 nm) and then it was constant up to 1440 min. Similar results were reported for Pt particles in Refs. [92–94], where quantum effects have been proposed to explain the change of TOF close to a critical diameter. Moreover, methanation is favored by bigger particles [36].Very recently, Vikla et al. prepared different Pt/Sibunit catalysts not only to study Pt size effects, but also looking at the influence of its distribution (uniform or egg-shell) [95]. The hydrogen production rate, normalized per unit of surface Pt, linearly increased with the mean particle size (up to 10.7 nm), in accordance with [88]. Furthermore, an optimal size for the agglomerated particles was found (about 21 nm).A change in the activity was reported in Ref. [96], where platinum nanoparticles with different particle size were synthesized for glycerol APR to obtain structure-activity relationship. Normalizing to the Pt surface area, it was showed that larger particles increased the conversion (the activity). Moreover, the product distribution was affected (the selectivity). It was reported that small particles (with higher concentration of edge sites – Pt(100)) favored the dehydrogenation, while larger particles (with higher concentration of facet sites – Pt(111)) favored the dehydration.A stability-related TOF was defined by Duarte et al., who looked at the influence of Pt loading on alumina supported catalysts (0.3–2.77 wt%) by IWI [97]. The authors reported that the amount of coke decreased when the number of surface Pt increased; in other words, larger particles caused less coke deposition. Moreover, sintering was observed for all the catalysts, and that was proved to occur during the initial phase of the reaction, since no decay in the performance was observed. The same research group previously studied the effect of Pt loading on xylitol APR in Ref. [98] and sorbitol [99].Higher H2 TOF for smaller particles were observed also for Ru-based catalysts [100]. The use of in-situ ATR allowed the authors to observe that on the catalyst with high loading (5 wt%) and large particles (4 nm) C–O breaking bond was favored, leading to acetylide intermediates and methane as the final product. On the other hand, the catalyst with low loading (0.5 wt%) and small particles (1 nm) did not lead to any acetylide. The difference was attributed to the fact that acetylide needs two bonds with the metallic site, therefore it was more likely to happen on flat metallic surface. Analogous outcome can be found in Ref. [101], where Ru catalysts were prepared at different loading to have particle size greater than 2 nm (representatives of high-coordination flat terraces) and smaller than 2 nm (representatives of low coordination step/edges). They observed that small Ru particles favored hydrogen selectivity, with CH4/H2 ratio less than one. At the same time, the C1/C2 products ratio is smaller, indicating less activity for breaking. This result is contrast with DFT studies, which foresee lower energy barriers on smaller particles rich in edges and steps. The discrepancy was ascribed to the possibility that more active smaller sites can be deactivated by CO blocking which is strongly adsorbed. To confirm this hypothesis, Fischer-Tropsch synthesis was carried out, which did not report alkane formation on small Ru particles because CO was not activated.Van Haasterecht et al. studied the influence of Ni particle size on different supports (carbon nanofibers, alfa and gamma alumina, zirconia, SiC) for APR of ethylene glycol [102]. Narrower peaks in the XRD of spent catalysts, together with TEM images, confirmed that sintering phenomena occurred for each of the supported catalyst in this order: CNF > ZrO2 > SiC > γ-Al2O3 > α-Al2O3. The difference was attributed to the different inter-particle distance (due to the different surface area) and the initial particle size. The authors reported that smaller particles grow faster and more than bigger particles; furthermore, they suggested that it was due to Ostwald ripening mechanism due to the high solubility of Ni and absence of the influence of the catalyst loading on the growth rate.In heterogeneous catalysis the function of the support is typically reported as a mean to increase the metal dispersion. However, its behavior may be active in determining the performance of the reaction. For example, the acid-base functionalities of the supports are well-known and exploited in many important industrial reactions, such as hydroisomerization. In the context of APR, the nature of the support has been reported as a mean for tuning the hydrogen production.The support can affect the quality of the hydrogenation sites [103] due to its electronegativity. In the case of APR, it has been reported that Pt/alumina is less hydrogenating because of the lower electronegativity of the support compared to Pt/amorphous silica alumina [104].Shabaker et al. screened different supports for platinum-based catalysts [105]. Looking at the H2 TOF reported in Fig. 16 , the ranking at 225 °C was TiO2 > black, carbon, Al2O3 > SiO2–Al2O3, ZrO2 > CeO2, ZnO, SiO2, with Al2O3, ZrO2 and TiO2 showing the highest hydrogen selectivity. The reaction temperature significantly influenced the ranking, with Pt-black being the one with the second worst TOF at 210 °C. SiO2 and CeO2 were reported to dissolve under the hydrothermal conditions.As reported in the introduction, the support has a strong role in modifying the hydrogen selectivity because of its acid-base properties. Wen et al. studied the effect of supports with different acidity with a Pt-based catalyst [106]. The authors showed that the scale of hydrogen yield was SAPO-11 < AC < HUSY < SiO2 < MgO < Al2O3, which is qualitatively coherent with the range of acidity (i.e., SAPO-11 and HUSY are zeolites, while MgO is a basic support). During 240 min TOS, each support was stable except for MgO and SAPO-11. Possible structure-sensitivity effects were excluded in Ref. [38] by pre-synthesizing platinum colloids that were then loaded on the supports through different techniques. It was pointed out that the scale of basicity (measured via CO2-TPD) was analogous to the range of hydrogen yield, i.e., MgO > Al2O3 > CeO2 > TiO2 > SiO2. The authors suggested that the support basicity polarized water, inducing its dissociation and facilitating the WGS step. Again, although MgO showed the best performance, it was not hydrothermally stable. Further screening of supports highlighted the importance of the basicity for Pt-based catalysts in Ref. [107].Liu et al. studied different supports for Pd-based catalysts in the APR of ethylene glycol [108]. Among NiO, Cr2O3, Al2O3, ZrO2, Fe2O3 and Fe3O4, the latter showed the best performance thanks to the promotion of the water gas shift, which is the rate determining step for Pd.Kim et al. investigated the effect of support on Pt–Re systems [109]. The authors reported that the activity increased in the order alumina < silica < activated carbon < CMK-3. The ordered mesoporous carbon was explained as the best thanks to the easier access of the active site for the reactants and escape of the products and higher dispersion. Similar outcome was obtained by CMK-9 in the case of Pt-Fe [41].Zirconia and boehmite supports were explored in the APR of hydroxyacetone [110]. The support alone (and zirconia notably) produced more coke than the platinum-supported catalyst (measured by CHN analysis and temperature programmed oxidation), indicating that the metal plays a role in preventing such deactivation via aldol condensation mechanism. Boehmite may have fewer coke thanks to its lower acidity. The in-situ ATR study allowed identifying the most present dimer between the two possible intermediates.It is interesting to observe also the influence that the support modification may have on the stability of the entire catalytic system. Stekrova et al. studied the influence of different Ce, Zr and La oxide supports for the APR of methanol [111]. Nickel catalysts are often subjected to deactivation due to re-oxidation and sintering of the metal particles. ZrO2 and CeO2 thanks to their oxygen storage and mobility are useful supports for WGS. Furthermore, oxygen vacancies can be increased if doped with lower-valence metal, like La, also increasing the metal-support interaction and, consequently, the Ni dispersion. The authors showed that Ni did not re-oxidize during the reaction. However, atomic absorption spectroscopy showed the occurring of Ni leaching. Ni sintering also occurred during the reduction step. While it was the same for pure zirconia and mixed ceria-zirconia oxide, it was lower in the case of mixed Ce–La, likely due to the strong interaction between Ni and La. Cerium carbonate was reported as the main cause for the deactivation of the WGS step, even if the activity was restored by thermal regeneration (300 °C, air flow). Looking at the performance of APR, they reported that the use of mixed oxides is beneficial for the activity. Surprisingly, pure CeO2 support showed the lowest WGS activity, despite its effectiveness in the gas-phase system. On the other hand, the highest WGS activity was reported by mixed 17Ce–5La–Zr support, linked to its highest basicity. Table 3 summarizes the main effects of the support on the performance of APR. The acid -base properties and the reducibility were mostly investigated since they are directly involved in the water gas shift step that, as reported throughout this work, is a key step to promote the hydrogen yield. Less attention has been put on the textural properties, despite it can be important to promote the selectivity. In fact, it can be favored not only playing with the nature of the active sites, but also avoiding that hydrogen, once produced, may contact other feed molecules/intermediated and hydrogenate them.Alumina is one of the most studied supports for APR. Different alumina supports for Pt catalysts were studied in the APR of ethylene glycol [113]. It was reported that the hydrogen yield decreased in the order alfa > delta > gamma thanks to the high dispersion obtained in the former. The authors reported that the absence of chlorine in the alfa sample (prepared with a higher temperature treatment compared with gamma) improved the dispersion because chloride facilitated the sintering. Moreover, it was suggested that Pt is more anchored on dry (e.g. alfa) alumina surface than on the hydroxylated one (gamma).Making an analogy with the active site, it has been reported that binary supports, i.e., mixed oxides, are able to increase the hydrogen production thanks to a synergy between the components. α-Al2O3 modified with CeO2 and ZrO2 improved by more than 50% the hydrogen yield compared to unmodified α-Al2O3 [91]. This is because CeO2 and ZrO2 participate in the water activation, therefore promoting the WGS reaction [114].Iriondo et al. modified alumina-supported Ni-based catalysts with Mg, Ce, Zr or La for glycerol APR [115]. La addition caused the highest increase in glycerol conversion, followed by Ce, Zr and Mg which was worse than the un-modified alumina. The supports did not influence the selectivity. Neither affected the dispersion, as there was not a clear trend. The authors suggested that geometric effects were due to the presence of the promoters on the Ni surface, similarly to Sn on Ni. All the samples showed deactivation ascribed to re-oxidation of Ni, while sintering was not reported.The influence of two different supports (alumina and nickel aluminate) have been studied for the aqueous phase reforming of methanol [112]. Pt supported on NiAl2O4 showed higher dispersion than on alumina (80% vs. 70%). The use of the spinel increased the methanol conversion from 26.5% to 99.9% and the hydrogen yield from 23.3% to 95.7%. Please note that in this case, Ni was present in an oxide state, so it was not able to activate the methanation reaction. Being NiAl2O4 not able to convert methanol, a synergy between the active site and the support was supposed to explain the improvement of the performance, due to several reasons. In situ DRIFTS was used to detail the CO formation process for the first time, showing that it is achieved via dehydrogenation of methanol to methoxy and formaldehyde species, followed by the decomposition of the latter to CO. Interestingly, the pathway was different for the alumina-supported catalyst. Indeed, it had mediocre dehydrogenation activity (the formation of formaldehyde and CO was observed at a longer time) and WGS (peaks of CO were already present, while they were absent for the spinel-supported catalyst). In the spinel support, platinum was more reducible, therefore more active for dehydrogenation (first reason for the synergy); the motivation was ascribed to the oxygen vacancy present in NiAl2O4. Furthermore, they contributed to carry out the water gas shift activating water via a redox mechanism which is faster than the associative mechanism observed for alumina. Summarizing, as reported in Fig. 17 , the first step (dehydrogenation) is performed in the same way on both catalysts: playing Pt a vital role for dehydrogenation, its characteristics affected the performance, and they improved thanks to the fact that it was more reducible. In the second step, the efficacy of WGS was further prompted by the redox mechanism, rather than the associative one.Moreover, the catalyst was stable along 600 h of time on stream, with only 10% of loss in conversion (no information on the possible change of selectivity were reported).Ceria support showed higher dispersion, WGS activity and resistance to coke than alumina [116]. Furthermore it suppressed methanation [117] by poisoning the responsible active sites. This mechanism would be analogous with the one proposed for the Sn-modified Ni Raney [32]. The effect of CeO2 addition to alumina supports for Pt catalysts in APR of glycerol was studied for the first time in Ref. [118]. 3% and 6% ceria doping increased the hydrogen and methane yield compared to unpromoted alumina, while 9% loading decreased the hydrogen yield. The catalysts showed similar hydrogen selectivity. The authors ascribed the improvements to the increase in the availability of metal surface area and reducibility of platinum precursor by adding ceria.Bastan et al. looked at the APR of glycerol over Ni-based catalyst supported on mixed alumina/MgO oxide, searching for a structure-activity relationship for the different Al/Mg ratio [119]. As reported previously, Ni suffers from sintering. MgO can be used to stabilize Ni particles. Increasing the Mg content also increased the Ni dispersion, while decreasing its reducibility. The A2M1 support showed the highest conversion because of the higher presence of surface metallic Ni. The spent catalyst showed re-oxidation of Ni and no carbon deposition, while no information on sintering was reported. Despite of the re-oxidation, the study of the performance showed stable hydrogen yield along 24 h of time on stream.Pt/SiO2–Al2O3 with different Si/Al molar ratio (range 0–1) was investigated to modify the surface acidity and influence the product distribution [120]. Maximum conversion and hydrogen production rate were reported at Si/Al ratio equals to 0.1. Increasing the ratio led to an increase of the methane selectivity and a decrease of ethane selectivity. The authors reported that increasing the Si loading led to an increase of the Brønsted/Lewis acidity ratio (because Si–OH species are present on the surface), as well as an increase of the weak acidic sites and a decrease of the strong (and total) acidic sites. This difference on the quality and quantity of acidic sites can alter the platinum dispersion as well (higher at higher Si/Al ratio). The authors correlated the conversion with the amount of strong acid sites and the hydrocarbon selectivity to the weak Brønsted acid sites. However, contrarily to most of the works reported in literature, the authors assumed that higher alkane selectivity can be attributed to both C–O and C–C cleavage.Despite the promising use of alumina, its transition into boehmite is well known under APR conditions, as reported in Refs. [121,122]. The use of silica deposition has been explored to overcome this limitation [123]. During the structural modification into boehmite, the metal particles may be encapsulated or sinter due to the losing contact with the support. One strategy to prevent this occurrence is increasing the support hydrophobicity, preventing the Al centers attack. The authors used tetraethylorthosilicate (TEOS). The total acidity decreased (measured by NH3-TPD), due to the substantial decrease of Lewis acidity (measured by pyridine adsorption) and a small increase of Brønsted acid sites. The silylation decreased the catalyst activity: the complete conversion was reached for pure alumina after 5 h, while the necessary time increased up to 7 h and 12 h with the increase of silylation time (0, 4, 8, 12 h). In Fig. 18 -A the modification of the hydrogen production rate is reported. The rise in acidity caused the increase in hydroxyacetone selectivity; the WGS remained effective (lack of CO). The boehmite formation was slowed down or prevented, while some minor sintering occurred. Globally, the support life-time increased from 12 h to 36 h, likely due to the removal of the protective layer.Similarly, Van Cleve et al. used alkyl phosphonate coatings to improve alumina hydrothermal stability [124]. The authors investigated phosphonic acids with different tail lengths, from C1 to C18. The initial surface area decreased due to this pretreatment likely because of the blockage of smaller pores. While alumina became boehmite if untreated, the alkyl phosphonate slowed down the transition, and it was slower the longer the tail length (Fig. 18-B). The reason may be ascribed to the hydrophobic properties or the density (the smaller the chain, the higher the loading) of phosphonate groups. The role of the tails was investigated after an oxidative treatment where the tails were removed. Since alumina remained stable, it was suggested that the key to the improvement was the interaction between the support and the head of the alkyl phosphonate. Nevertheless, the longer chain (C18) showed higher stability despite the lower P density. It may be since the high coverage of C1 led, on average, to lower coordination compared to C18, so that the support was less stabilized.The addition of cerium, yttrium and calcium was investigated to improve the redox capacity and basicity of zirconia-supported catalysts in methanol APR [125]. Base Ni/ZrO2 catalyst reported 48% of conversion and 40% hydrogen yield. Ce addition decreased the activity, in contrast with other works in the literature. Despite the improvement of Ni reducibility, Ce negatively influenced the quality of the surface basicity (decrease of weak basic sites, with the formation of intermediate strength ones). Therefore, it seemed that the basicity plays a more significant role than the redox properties. Ca and Y addition increased the total surface basicity equally, with the former acting mainly in the formation of weak basic centers, and the latter in the establishment of medium/strong basic centers. The Ca-doped catalyst showed better performance than the Y-doped one because strong basic sites can negatively affect the results by preventing the desorption of CO2. Finally, increasing the Ca loading from 4% to 14% worsened the activity, likely due to the decrease of Ni surface area (despite the higher amount of weak and intermediate basic sites).The addition of Ce is common on Zr support. We cite here its investigation on two examples, with Ni- and Pt-based catalysts. Bastan et al. investigated the effect of the Ce–Zr support composition for Ni-based catalysts prepared by co-precipitation for APR of glycerol [126]. XRD showed that the two oxides were present in a mixed form, not as separated clusters, and presented higher surface area than the single oxide support. Zr included in the Ce lattice lowered the crystallite size due to its lower ionic radius. The Ni reducibility changed in the mixed oxide, with the increasing of Zr loading leading to an increase of the reduction temperature peak, likely suggesting stronger metal-support interaction. Finally, the increase of Zr also allowed to increase the Ni dispersion. It is likely that this difference caused the increase of glycerol conversion for the mixed oxides, however the hydrogen selectivity decreased. The best catalyst (Ni/Ce0.3Zr0.7O2) was tested also for stability with 25 h TOS. No drop in the conversion was reported; moreover, commonly deactivation mechanisms such as Ni re-oxidation and sintering were excluded.The effect of cerium/zirconium molar ratio for Pt-based catalyst is studied in Ref. [127]. The activity and hydrogen selectivity were maximized when the Ce/Ce + Zr molar ratio was 0.4. The lowest CO concentration in this case suggests also more effective WGS. The authors reported that this ratio led to a higher abundance of oxygen vacancy sites which, being correlated with the platinum dispersion, may be considered the cause for the higher performance of this formulation thanks to strong metal-support interactions. Raman analysis was carried out to measure the defect sites. Mixed oxides have larger lattice strain and higher oxygen vacancy concentration, with the one at 0.4 ratio having the highest. H2-TPR showed improved reducibility in the case of mixed oxides, ascribed to the higher oxygen mobility. Finally, the sample 0.4 showed also less sintering compared to Pt supported on only ceria or zirconia.Larimi et al. investigated ternary Pt–Ce–Zr solid solutions prepared by controlled precipitation methods with different Ce/Zr ratio, looking at its influence on the oxidation state of Pt and its particle size (dispersion) [128]. The presence of a ternary solid solution was confirmed by XRD. When Ce/Zr ratio was equal to 1, the dispersion was the highest and it was the one with the highest hydrogen activity. Looking at the TOF, it was reported that face atoms (larger particles) are more active, while higher H2 selectivity was obtained with the smallest particles. XPS analysis showed the electronic interaction between Pt, Ce and Zr through the increase of Pt binding energies. Electron withdrawing from Pt should cause the decrease of the Pt–CO binding, enhancing in this way the WGS. Furthermore, the catalyst was proven stable in the APR condition, without agglomeration of Pt.Finally, Harju and coworkers reported a mild effect of zirconia particle size on butanol conversion, while the product distribution and the stability was negatively affected using bigger particles due to the onset of internal mass transport limitations [129]. In the 250–420 μm range, butyric acid selectivity increased, favoring the active metal (Rh) leaching and decreasing the hydrogen formation by favoring hydrogen-consuming reactions (e.g. hydrogenolysis).Several carbon supports (activated carbon, single and multi-walled carbon nanotubes, superdarco (i.e., methane and steam treated) carbon and graphene oxide) were studied for Pt-based materials [130]. Activated carbon reported the highest gas production among the supports while graphene oxide the highest hydrogen selectivity. Moreover, SWCNT performed better than MWCNT likely because of the larger pores that facilitate the reaction of large molecules present in the hydrolysate.Wang et al. studied the influence of surface functional groups in MWCNT used as support for Pt based catalysts in ethylene glycol APR [131]. HNO3 functionalization increased the Pt dispersion but decreased the turnover frequency. The authors ascribed this outcome to the presence of oxygen containing groups, which can create a competitive adsorption between water and ethylene glycol, due to the increase in hydrophilicity. An annealing treatment which removed these groups allowed to restore the original TOF.Finally, it is worthy to cite the effect that the carbon support morphology can have on APR. Meryemoglu et al. looked at three ranges of activated carbon: < 88 μm, 177-88 μm, 177–250 μm [132]. Smaller particles had higher surface area and pore volume. Pt size was similar, so the difference can be ascribed only to the support. It was showed that activity increased with decreasing particle size, as well as for narrower size distribution. Kim et al. investigated the importance of the configuration of the support looking at 3-D and 2-D ordered mesoporous carbon with hollow- and rod-type configuration [133]. The order of hydrogen production was different from the order of the metal dispersion, highlighting the importance of the structure of the support. It was noted that 3-d ordered mesoporous carbon (OMC) allowed higher resistance towards sintering for the Pt particles; furthermore, the mesoporosity of hollow-type framework configuration favored the transport of reactant and products to and from the active sites, respectively.The preparation of a catalyst involves different steps, and each of them can affect its final characteristics. First of all, the formation of the metal-support system is necessary. It may be carried out via impregnation, deposition, ion exchange, etc. During this stage, liquid/solid interfacial phenomena are important and can affect the behavior of the final catalyst. In the APR literature, most of the works deals with impregnation (wet or incipient wetness) and will not be reported in this paragraph, unless for the sake of comparison. Only limited information was reported on the procedures and methods themselves, since outside of the scope of the work. Then, the second stage involves (commonly) gas-solid reactions, firstly in the calcination (oxidation) step, and afterwards in the activation (hydrogenation, sulfidation) step. The modifications caused by this second stage strongly influence the structure of the catalyst as obtained after the first stage [134]. This is the reason why both of them are analyzed in the following.Alternative methods to the traditional impregnation can contribute to increase the reducibility of the metal, facilitate alloy formation or the interaction between two metals in case of bi-metallic systems [135,136]. However, most of the time they are developed to increase the dispersion, so that a high metallic surface area available for the reaction can boost the productivity. For example, it was reported that silica-supported platinum catalysts prepared via ionic exchange (IE) showed higher metal dispersion (60%) than the ones prepared via incipient wetness impregnation (20%) [91]. This result allowed to nearly double the hydrogen yield. Analogously, urea matrix combustion method (UCM) compared with IWI for the APR of different polyols allowed higher dispersion and, as a consequence, higher hydrogen yield [44].As far as the choice of the metal precursor is concerned, Lehnert and Claus evaluated the influence of different platinum precursors (amines, nitrates, sulfites) with IWI technique [88] They observed slight differences among the salts in terms of glycerol conversion and hydrogen selectivity, with tetrammine platinum (II)-nitrate giving the highest hydrogen rate of production.Lemus et al. developed a method to synthesize stable Pt size-controlled nanoparticles [137]. The novelty consisted in synthesizing the metal nanoparticles in situ, in the presence of the support, therefore leading to the immobilization on it (contrarily to methods where the nanoparticles are first prepared and later deposited, ex situ). The authors used polyvinylpyrrolidone (PVP) and NaBH4 as capping and reducing agent, respectively. Bigger particles were obtained by ex situ method, as well as with a reference method without PVP. However, the addition of PVP only after the contact with activated carbon led to a higher dispersion, as no competition was present between the nanoparticles and the capping agent.Apart from the activity and selectivity, also the stability can be affected by the preparation method. El Doukkali et al. compared the preparation of Pt and Ni catalysts on alumina via incipient wetness impregnation (IWI) and sol-gel method under basic conditions (SGB) [138]. Sol-gel created a material with higher surface area (375 vs. 140 m2/g) and pore volume (0.43 vs. 0.23 cm3/g). The basic agent leads to spherical cluster formation and a spongy material. The pore volume decreased in SGB after adding the metal, likely because they were incorporated in the pores network, while they were mainly in the outer surface in the case of IWI. SGB also increased the interaction of Ni with the support (leading to Ni aluminate) favoring the dispersion. The latter was confirmed by broader peaks in XRD. SGB method also stabilized Pt particles which otherwise sintered during the reduction step when prepared via IWI. The carbon conversion to gas increased with SGB catalysts. In a subsequent work, the authors added also the study of sol-gel preparation under acidic conditions (SGA) [139]. The differences in the preparation method are reported in Fig. 19 -A. SGA led to a material with fibrous and laminated morphology and high surface area, allowing an effective dispersion of Pt and Ni: the modification in the morphology facilitated adsorption of reactant and desorption of the products, whereas the higher dispersion increased the activity. The catalysts prepared by SGA had 50% higher surface area than IWI, but 50% lower than SGB. Looking at the performance, the activity was in the same trend (Fig. 19-B). Finally, SGB led to catalysts more resistant to sintering compared to the ones prepared by SGA.The same research group reviewed the deactivation mechanisms for the catalysts subjected to APR in Ref. [141]. No leaching was reported for Pt, Ni and PtNi catalysts both at 230 and 250 °C. The textural characterization showed that the decrease of surface area involved the materials prepared by sol-gel routes more than the ones by IWI. XRD showed higher transition to boehmite in the supports synthesized by sol-gel, likely due to their higher surface area that facilitates the incorporation of water molecule. Ni sintering occurred, and it was higher for those prepared by IWI. However, contrarily to literature, Pt did not sinter. XPS showed that monometallic Ni completely re-oxidized, while it was metallic only in the 11–25% in the bimetallic case (lower for the sol-gel because of the higher dispersion). Pt remained in the metallic form after the reaction. Finally, temperature programmed oxidation (TPO) showed the presence of carbonaceous species adsorbed on the catalysts.Roy et al. compared sol-gel method and solution combustion synthesis (SCS) combined with wet impregnation method for Ni/CeO2 catalysts [142]. The first one showed higher carbon conversion to gas and hydrogen selectivity, ascribed to the growth of Ni nanoparticles prepared by SCS during the reaction. Analogously, the same research group looked at Ni/γ-Al2O3 catalysts, revealing that in this case SCS samples outperformed the ones prepared by sol-gel in terms of activity (i.e., ethanol conversion), hydrogen selectivity and TOF [143,144]. The extensive structural and superficial characterization showed that it was not only due to the smaller particles obtained by SCS. In fact, SCS sintered in a lower extent, produced less coke and bulk spinel formation (promoting WGS) than SG.Other examples of better dispersion of the active phase in the case of sol-gel route compared to impregnation are reported in Ref. [145].Following the preparation steps, two further phases are commonly involved, calcination and reduction.Callison and coworkers used a colloidal synthesis procedure for the preparation of platinum particles [96]. The synthesis procedure was optimized varying the concentration of NaBH4 (reducing agent), reduction time of Pt precursor and immobilization time on the support. Afterwards, the reduction temperature was varied from 25 °C to 90 °C to change the particle size. As expected, it was shown that it increased with the increase of reduction temperature.Morales-Marin et al. used bulk nickel-aluminate catalysts reduced at different temperatures from 300 to 850 °C [146]. The authors observed that increasing the reduction temperature had a two-fold effect and the results of the catalytic tests are reported in Fig. 20 . On one side, the nickel dispersion increased because its migration from the spinel structure to the surface is favored. On the other hand, both surface acidity and (to a greater extent) basicity increased. Up to 450 °C, less than 5% conversion was reported due to the absence of the Ni0 active site. In this range, hydroxyacetone was the main product, deriving from dehydration reactions catalyzed by Lewis acid sites. As a matter of fact, this outcome is properly exploited in works where the aim is the hydrogenolysis of glycerol and the production of C3 products [147]. From 600 to 850 °C, the glycerol conversion and gas production gradually increased, with the maximum hydrogen yield at 850 °C. However, hydrogen selectivity partially reduced due to its consumption in parallel reactions. Nickel oxidation and sintering were identified as the leading cause for catalyst deactivation, while leaching and coke were excluded. Interestingly, sintering was observed mainly for the larger particles, in contrast with previous literature [102].El Doukkali et al. evaluated the influence of calcination temperature (550–750 °C) on the stability of alumina and Ni/Pt particles [148]. The Pt particle size was similar, independently from the calcination temperature. Moreover, in one case the active sites were incorporated during the SGB synthesis of alumina; in the second case, they were impregnated by IWI after sol-gel alumina preparation (SGI). Ni particles were bigger for SGI, but less sensitive to the calcination temperature, while sintering occurred with SGB, causing more deactivation. Bigger particles are more resistant to sintering (4-fold increase for SGB, two-fold for SGI) and re-oxidation, which are common deactivation causes for Ni. Increasing the calcination temperature decreased the activity but increased the stability.Irmak et al. studied different preparation and reduction methods of platinum on activated carbon, alumina and titania by IWI [149]. Thermal treatments were carried out under hydrogen and nitrogen: in the latter case the activity improved thanks to the lower particle size. Afterwards, reducing the precursor chemically (using NaBH4) rather than thermally, further improved the catalyst activity, thanks to the fast reduction process when it is added dropwise to the solution.Novel techniques, even if not applied for APR yet, are worthy to be mentioned. Keshavarz et al. used microemulsion systems to prepare Pt and Re based catalysts with controlled particle size for heptane reforming [150]. Two different microemulsions, neutral and acidic, were prepared. Re particle size was larger for neutral microemulsions, while Pt particle size was larger for acidic microemulsions, as well as for Pt–Re. Interestingly, the nature of the microemulsions did not affect the final acidic properties of the supported catalyst.The influence of surfactants on the synthesis of platinum nanoparticles via microemulsion method [151] was studied as well. In this method, nanosized water droplets in which the metal salt is dissolved work as a nanosized reactor during the reduction while dispersed into a continuous oil phase. Four surfactants were used: Sodium 1,4-bis(2-ethylhexoxy)-1,4-dioxobutane-2-sulfonate (AOT), cetyltrimethylammoniumbromide (CTAB), poly(oxyethylene) sorbitanmonooleate (Tween80) and poly(ethylene glycol) p-(1,1,3,3-tetramethylbutane)-phenyl ether (TX-100). It was reported that ionic surfactants allowed producing smaller nanoparticles than non-ionic ones (AOT < CTAB < Tween80 < T-X-100, i.e., anionic surfactant < cationic surfactant < nonionic surfactant) thanks to the influence of the different head group charge, as it affects the dynamic process of collision, nucleation and growth of the droplets. Other examples of microemulsion synthesis of nanoparticles can be found in Ref. [152], where the authors prepared NiPt bimetallic nanoparticles for methane dry reforming.Finally, Roy et al. reported the use of a radio-frequency plasma treatment to modify the surface of a Ni/alumina catalyst [153]. The plasma modification mainly influenced the metal-support interaction leading to a higher dispersion of the metal, leading to an increase of the catalytic activity.In Table 4 the influence of preparation method reported in the chapter is summarized. Among the others, sol-gel methods seemed particularly interesting for their simplicity and possibility to increase the dispersion; as usual, the trade-off is reported being affected the stability of the catalyst, which is particularly sensitive when alumina support is used. Once the support-active metal system is formed, the conditions used in the calcination and reduction steps can still play a role in modifying the final catalyst properties due to its dynamic structure.In the present work we collected the efforts of several research groups whose aim is the design of an effective catalyst for APR. The scientific outcomes have been classified according to the effect of three main steps: choice of the preparation method, choice of the metal, choice of the support. It is important to observe that the reality is not as simple as reported, and the boundaries are much more flexible. For example, glycerol conversion rate decreases in the order TiO2 > ZrO2 > CeZrO2 > CeO2 on Pt, but the ranking changes into TiO2 > CeZrO2 > ZrO2 > CeO2 on Pt-Re [154]. It means that changing the metal, apparently also changed the effect of the support. Similarly, changing the support affected also the ranking between preparation methods, as reported in the works of Roy et al., where CeO2 was more active prepared by sol-gel than SCS, but viceversa for Al2O3 [142–144]. Notwithstanding, an ideal combination of these ingredients may be proposed at the end of the present work. The active elements should likely rely on bimetallic systems. Nowadays, platinum appears inevitable due to its peculiar characteristics. Despite of its cost, the key point refers to avoiding its deactivation, which seems an affordable task, at least with model compounds. From the available literature, it should be accompanied by a promoter to increase mainly its water gas shift activity. Among the others, Re and Fe seemed the most suitable. If the metals could explicate the WGS effectively, the support may just play the role of dispersive medium. For this reason, mesoporous carbon, thanks to its inert behavior with respect to the aqueous phase and controlled pore size, may be a suitable support. Finally, the coupling of in situ formation of nanoparticles and their activation by chemical reduction methods could be a preparation method able to guarantee high dispersion and stability of the active phase.Apart from the characteristics of the catalyst, the hydrogen yields strongly depend on the nature of the substrate, due to severe selectivity issues that arise with the increasing complexity of the molecule. In other words, we can expect high yield for small molecules (methanol, ethylene glycol, glycolic acid), while it will decrease for glycerol, xylitol or sorbitol. For example, we observed with glycolic acid 65% hydrogen yield with Pt/C while it was 38% for sorbitol, at iso-conversion conditions [11,12].However, looking also to an industrial application, the TOF values could be even more interesting. Lange reported that its value should be in the range 0.033–16.7 tons of product per ton of catalyst per min [155]. For Pt-based catalysts, for example, it could reach 3 min−1 [95,156] and this figure is promising for the future. However, it remains to be proofed also for more complex systems (see paragraph 6.2). For example, Pt–Rh catalyst applied to the APR of pure glycerol showed 83.5% glycerol conversion and 89% hydrogen selectivity, while these values dropped respectively to 43.1% and 39% for crude glycerol [36].The future research should focus on each of the three cited topics to improve the performance of the catalytic system in the APR scenario. In the following, some points worthy of consideration are reported.In the field of the preparation method, innovative aspects such as the effect of the orientation of the active sites compared with unoriented Pt materials on graphene prepared by IWI, showed that the oriented material reported 2 order of magnitude higher catalytic activity expressed as TOF than unoriented ones [157]. Moreover, novel preparation techniques for bimetallic catalysts may be developed to handle the harsh conditions of the reduction treatment and reaction (for example, leading to stronger metal-support interactions) and stabilize the bimetallic clusters.Apart from experimental testing, the rational design of heterogeneous catalysts, thanks to the use of multiple tools such as DFT and micro-kinetic models, should benefit from further understanding of the reaction mechanism and lead to more effective catalysts. In this sense, the fact that APR occurs in liquid conditions is an obstacle, since the presence of a solvent that interacts with reagents, intermediates and products, modifies the energetic and reaction pathways [158]. The aqueous environment also affects the common knowledge in the reactivity of the catalytic system. It means that typical gas-phase WGS catalysts, such as Cu and CeO2, are not trivially effective catalysts also in the liquid phase.The use of a second metal to improve the activity, selectivity and stability of the primary active site has been proved to be commonly effective. However, despite the achievements, the complexity of the systems often requires further efforts. For example, it has been reported that the bimetallic structures modify under the reaction conditions: therefore, the development of in situ characterization techniques is necessary. Furthermore, new and cheap materials, such as tungsten, can be promising for future applications in the field [159].Much work needs to be done also on the support side, to clarify its role in the reactivity of the total system. For this reason, understanding phenomena such as charge transfer, spillover and perimeter activation may help in the design of new catalysts with tailored properties [160].Apart from the ones cited in this work, the catalyst will face new challenges when it goes towards the use of real wastewater streams. In fact, the complexity of the multi-components mixture may rise competitive adsorption phenomena [12]; moreover, the presence of inorganics or high molecular weight organics can lead to fast deactivation [161].Focusing on the latter, also the studies with model compounds, despite trying to assess the stability, are often referred to short runs, and so insufficient to probe the stability as demanded by chemical industry. Furthermore, studies on catalyst synthesis scalability and regeneration protocols should be developed.The literature cited herein applies the APR to simple model compounds, since its aim is the study and development of effective catalysts. However, the application of APR is devoted to the valorization of complex multicomponent mixtures, as it is the case of biorefinery wastewater streams. For this reason, in the last decade, the research started to investigate such feedstocks. Due to is versatility, APR could be applied to treat the water fractions derived from lignocellulosic biomass processing (e.g. not fermentable sugars post hydrolysis, aqueous phase from pyrolysis and hydrothermal liquefaction, etc.), aqueous effluents from food processing (e.g. breweries, cheese factories, etc.), crude glycerol from the biodiesel sector, and others [162]. Most of these works used simple catalytic systems (typically monometallic platinum catalysts), however they provide a range of hydrogen productivity into a more industrially relevant environment. For example, referring to the brewery wastewater, it was estimated that about 294 mL H2/g COD could be produced via APR, while anaerobic digestion could reach roughly half (150 mL H2/g COD) production [8]. Under the economic point of view, Larimi and coworkers showed that glycerol APR has lower production cost than glycerol steam reforming (3.55 vs 3.65 $/Kg), and this is competitive with other technologies which aim at a renewable hydrogen production (such as biomass gasification, dark fermentation, solar thermal electrolysis) [163,164]. Globally, the application of APR at industrial scale can be competitive if the cost of the feedstock is competitive as well. As a matter of fact, it can account for most of the production cost (e.g. up to 92% in the case of hydrogen from sorbitol syrup [165]).Aqueous phase reforming has been conceptualized as a strategic process for the valorization of biomass-derived compounds for hydrogen production. Since 2002, most of the literature focused on the pursuit of the optimal catalytic system that maximizes activity, selectivity and stability. Despite the efforts, the complexity of the reaction and the intercorrelation among the variables hindered, at the moment, the possibility to turn this process from the laboratory to the industrial scale. The aim of the present review was reporting, in a comprehensive way, the influence of several variables which can affect each of the three figures.Scope of the preparation method was mainly maximizing the dispersion to increase the number of available active sites. Alternative methods to the conventional impregnation techniques, such as ionic exchange, sol-gel and microemulsions reached this aim. Furthermore, they modified the electronic properties of the metal, for example via alloy formation or strong metal-support interaction, which in turn affected the reducibility, the tendency to CO binding or sintering.Theoretical investigation and first-principle methods, such as DFT, showed the intrinsic predisposition of metals to activate one or another pathway. Among the others, Pt showed higher tendency to C–C cleavage than C–O cleavage, maximizing the hydrogen production. The use of a promoter allowed to exalt or suppress some characteristic features of the monometallic catalytic form. Different promotion phenomena were reported. Ensemble (or geometric) effects were shown when Sn addition hindered the CO methanation on Ni-based systems; stabilizing effects have been attributed to Ru and Rh when protected Pt from coke deposition and sulfur poisoning, respectively; ligand (or electronic) effects were largely reported when the promoters decreased the interaction between carbon monoxide and Pt active site, favoring WGS (Re, Co, Fe, Mo). Overall, it has been widely documented that the second metal can tune the catalyst modifying the binding energy with reactants, intermediates or products, improving the reducibility of the first metal or its dispersion, changing the surface acid-base properties. Each of these modifications can have a different degree of importance, and it depends on the catalytic system as a whole. For example, it seemed that increasing the metal surface area is more important than the increase of (weak) basic properties, which in turn play a more important role than the metal reducibility. Trade-off is ubiquitous in the design of the optimal catalyst, as the example of the choice of the metal particles exemplifies. Despite results are not totally coherent, we can assume that the size of the particles mainly affects the selectivity, with the smaller ones favoring dehydrogenation and C–C cleavage, while the bigger ones favoring dehydration and methanation; if the conversion is affected, this is higher for larger particles, which caused less coke deposition as well.Finally, the choice of the support mainly affected the dispersion thanks to its surface area and favored (or not) dehydration acid-catalyzed reactions. Basic character of the support was linked to the promotion of water gas shift and, in turn, to higher hydrogen production. The hydrothermal stability, such as the case of Al2O3 and MgO, is a severe issue that can be overcome properly modifying the surface composition and morphology.Despite several challenges remain to be tackled, we strongly believe that the developments in the field of catalysis through innovative preparation methods, rational design and in situ characterization techniques can pave the way to the synthesis of effective catalysts for aqueous phase reforming and sustainable hydrogen production.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 project leading to this research has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement N 764675. Gianna Moscoso Thompson is gratefully acknowledged for the graphical abstract artwork.

The aqueous phase reforming (APR) is a catalytic reaction able to produce hydrogen from oxygenated compounds. The catalytic system plays a pivotal role to permit high conversion of the substrate, high selectivity towards hydrogen, and stability in the view of an industrial application. These figures of merit depend on several strategies taken by the researchers to properly design the catalyst, like the preparation method, the choice of the active metal together with possible promoters, the type of the support and so on. The available literature reports several studies where these parameters are evaluated and discussed. In this review, they were critically examined with the aim of finding correlations between the properties of the catalyst and the activity, selectivity and stability for the APR of carbon-laden water fractions. Both theoretical and experimental works have been included in the literature survey. When available, studies with the use of in-situ techniques allowed to increase the understanding of the catalytic phenomena involved in the reaction. Great attention was also reported to recently published works, so that the review could present the most up-to-date developments in the field. The most important outcomes regarding each parameter have been highlighted; moreover, the synergy among each of them has been pointed out, together with the trade-off that the researcher has to deal with in the pursuit of the optimum catalyst.

Data will be made available on request. Data will be made available on request.Specific surface area (cm-1)Concentration of species (M)Catalyst layerDiffusion coefficient (cm s-1)Diffusion driving force (m-1)Voltage (V)Faraday constant (A s mol-1)Force term (kg m-2 s-2)Gas diffusion layerHeight (m)Identity vectorCurrent density (A m-2)Mass flux (kg m-2 s-1)Permeability (m2)Constant of inter-diffusion coefficient fitted by Fuller, Schettler and GittingsHenry law constant (kPa)Length (m)Molar mass (kg mol-1)PtIr loading (kg cm-2)Number of electron transfer in reactionPressure (Pa)The sum of consumed or produced species in reactionUniversal gas constant (J K-1 mol-1)Area (m2)Temperature (K)Velocity (m s-1)Diffusion volume (m3 mol-1)Width (m)Molar fractionTransfer coefficientThickness (m)PorosityOverpotential (V)Contact angle (°)Dynamic viscosity (Pa s)Density (kg m-3)Conductivity (S m-1), or interfacial tension (N m-1)Electric potential field (V)Mass fractionAnodeCathodeFlow channelSpecies indexSpecies indexLiquid phasemembraneribAmong the energy generation technologies from renewable sources to conventional combustion, fuel cells have attracted much attention due to their high efficiencies and low emissions. Polymer electrolyte membrane fuel cells (PEMFCs) are formed by polymer electrolyte membranes as proton conductors and Pt-based nanomaterials as anode and cathode catalysts. The combined advantages of low operating temperature, high power density, light weight and readiness for mass production, make PEMFC a promising candidate for various applications, such as vehicle propulsion, stationary electricity generation and portable power. However, hydrogen is the currently used fuel that is difficult to be stored, transported and distributed. As compared with hydrogen and other fuels (e.g., methanol and ethanol) for fuel cells, ammonia is a high energy density carbon-free fuel that is readily liquified under ∼8 bar of pressure at ambient temperature [1].Direct ammonia fuel cells (DAFCs) utilize gaseous ammonia or its aqueous solution as a fuel which is oxidized to nitrogen and water within the anode catalyst layer (CL), coupled with a four-electron reduction of oxygen at the cathode. Comparatively, the ammonia oxidation reaction (AOR) kinetics is far inferior to the hydrogen oxidation reaction in PEMFCs. The AOR is considered to follow the Gerischer-Mauerer mechanism [2]. According to this mechanism, *NHx (x=2,1 and 0) adsorbates are created during the process of ammonia dehydrogenation. *NH and *NH2 adsorbates may dimerize and form N-N bonds and the products can be subsequently dehydrogenated to dinitrogen. However, nitrogen adatoms (*N) will not dimerize and are considered to form a surface poison and deactivate the Pt catalyst [3].The basic study on DAFCs with aqueous KOH as electrolyte and Pt as anode dates back to 1960s [4]. Afterwards, there were few reports on low-temperature DAFCs until Lan et al. adopted an alkaline anion-exchange membrane as the electrolyte for DAFCs [5]. Cr-decorated Ni (CDN) and PtRu/C were used as the anode catalysts and MnO2 as the cathode catalyst. They found CDN showed better catalytic activity than PtRu/C. However, the peak power density of those fuel cells is usually below 10 mW cm-2, limited by the unsatisfactory stability of alkaline membranes at high temperature, the sluggish kinetics of anode ammonia electrooxidation, and deactivation of the Pt-based cathode catalyst by ammonia crossover from the anode [6]. Furthermore, Silva et al. investigated the effect of different Pt: Au atomic ratios on catalytic activity in anode and found that PtAu/C 70: 30 presented better performance than Pt/C in DAFC while Au/C showed no activity [7]. A first-principles study by Herron et al. predicted Pt is the most active monometallic catalyst followed by Ir and Cu, whereas other metals such as Au, Ag, Pd, Ni, Co, Rh, Ru, Os and Re have significantly lower activity [8]. Song et al. studied the kinetics of Pt, Ir and PtIr alloy as electrocatalysts experimentally and found that Ir had a lower AOR onset potential, and lower peak current than Pt. Moreover, PtIr could combine the advantages of both metals and exhibit extraordinary catalytic performance among these metals [9]. Meanwhile, polymeric anion exchange membranes with high OH- conductivity, low ammonia permeability, mechanical robustness and chemical stability above 80 ºC under highly basic and chemically aggressive conditions are required to further improve the DAFC performance [10]. A series of poly(arylene piperidinium)s (PAPipQs) with the mentioned features have been available recently [11]. Based on a judicious choice of catalysts (PtIr anode and Fe-N-C cathode), PAPipQs-based membrane DAFCs demonstrated superior performance with a record peak power density of 135 mW cm-2 at 80 ºC [12]. Besides, Achrai et al. reported a DAFC with a KOH-free anode feed. This DAFC used Pt1Ir10/C as anode and Ag-based dispersed electrocatalyst as cathode of which the peak power density hit record for this type of fuel cell and could reach 180 mW cm-2 at 120 ºC [13]. To further improve the cathode's catalytic performance, Hu et al. synthesized Mn-Co spinel on three different carbon supports (BP2000, Vulcan XC-72R and multiwalled carbon nanotubes) which all showed good ammonia tolerance. Especially, compared with the Pt-C cathode, Mn-Co-BP2000 cathode paired with Pt-Ir anode could improve the peak power density of the cell to 128.2 mW cm-2 at 80 ºC under a 2 bar backpressure [14]. Jeerh et al. studied the ORR activity of the LaCoO3-δ based perovskites and found that the co-doping of Cr and Fe elements can significantly improve the ORR performance of the catalysts. Moreover, the DAFC based on LaCr0.25Fe0.25Co0.25O3-δ/C cathode achieved an open-circuit voltage (OCV) of 0.72 V and a maximum current density of ∼320 mAcm-2, which showed almost the same performance as the DAFC with commercial Pt/C cathode [15]. Although the study of low-temperature DAFCs is still at an early stage, ammonia is one of the clean and inexpensive energy sources which makes DAFCs promising power devices for the carbon-neutral economy [16]. However, the challenges, such as minimizing the ammonia crossover, developing suitable anode and cathode catalysts, maintaining mechanical stability of the alkaline exchange membranes and achieving high power density at low temperature, still need to be dealt with [17]. To the best of our knowledge, there is no theoretical simulation of DAFCs to guide the optimization of the cell performance at the membrane electrode assembly (MEA) level.The simulation of DAFCs involves various complex processes, such as species transport, momentum transport, and electrochemical reaction. Especially, the liquid-phase ammonia fuel is oxidized to gas-phase nitrogen within the anode CL, where the phase transition is similar to that at the anode of direct methanol fuel cells (DMFCs). Until recently, the numerical model of DAFCs has not been presented yet, so this work is developed mostly on the base of DMFC models. In several early investigations into DMFCs, Baxter et al. developed a one-dimensional single-phase model of DMFCs which considered kinetics of methanol oxidation and active specific surface area in the anode CL [18]. Then some other important factors, such as the methanol crossover [19], methanol concentration [20] and methanol mixed with air [21], began to be taken into account in other one-dimensional models. However, due to the limitation of the model dimension and the lack of considering two-phase flow in DMFCs, these models were not relatively accurate. In contrast, Wang et al. developed a two-phase, multicomponent model for liquid-fed DMFCs which considered diffusion and convection of liquid and gas phases in backing layers and flow channels [22]. Ge and Liu focused on using the Tafel equation to describe electrochemical kinetics and the effects of two phases in the anode and cathode sides of a DMFC model [23]. Many researchers also showed an interest in the effects of crossover, operation conditions and intrinsic parameters on cell performance. Yang and Zhao developed a two-dimensional, two-phase mass transport model of DMFCs. In this model, the effects of porosity and anode flow rates on cell performance and methanol crossover were studied [24]. Liu and Wang developed a three-dimension isothermal mixture multiphase flow model to investigate the interplay between local current density and methanol crossover [25]. Biswas et al. studied a DMFC anode model which could accurately predict the rate of methanol crossover affected by the geometric and material properties of anode layers [26]. García-Salaberri et al. developed a three-dimensional DMFC model which considered gas diffusion layer (GDL) compression and showed the optimal methanol concentration [27,28].These mentioned 2D/3D two-phase models accurately simulated multi-phase mixture properties and the effect of operating conditions or material properties on cell performance. The objective of this work is to develop a three-dimensional, two-phase multicomponent model for the anode CL based on the unique kinetics of DAFCs and the physical properties of involved species, and provide the insight into optimizing the anode CL structure by investigating the effect of its porosity, PtIr loading and thickness on cell performance at an MEA level. In our model, the Maxwell-Stefan model is applied to this DAFC anode in the porous region and flow channel, Darcy's law is used to describe the fluid dynamics in the porous region, and the Brinkman equation is applied in the flow channel [29]. The effects of CL material properties on cell performance and ammonia crossover are investigated. Fig. 1 shows the schematic diagram of the DAFC anode, which consists of a flow channel, GDL, CL and an alkaline anion-exchange membrane. The GDL and CL are assumed to be isotropic porous regions. The ammonia solution flows along the flow channel and diffuses through the GDL to the CL where it will be oxidized to nitrogen according to the AOR (2NH3 + 6 OH- → N2 + 6H2O + 6 e-). The nitrogen generated in the CL will aggregate and transport through the GDL to the flow channel. The ammonia liquid phase and nitrogen gas phase diffuse in opposite directions, which decelerate the mass transport of both phases and deteriorate the cell performance. All phases are assumed to be continuous. Nitrogen is assumed to be insoluble in liquid phase. The velocity of liquid-gas phase in the flow channel is assumed to be the same. The geometric parameters of the model are listed in Table 1 .Based on this model, several governing equations, including mass conservation equation, momentum conservation equation, and species conservation equation are used to describe the multiple processes such as mass transport and fluid dynamics.The Maxwell-Stefan model is used to describe the multicomponent flow. The model takes the collisions between different species, including ammonia, water and nitrogen into account and the corresponding equations are listed as Eqs. (1-3): (1) ∇ · j i + ρ ( u · ∇ ) ω i = R i (2) j i = − ρ ω i Σ k D i k e f f d k (3) d k = ∇ x k + 1 p [ ( x k − ω k ) ∇ p ] (4) D i k = k T 1.75 1 p ( v i 1 3 + v k 1 3 ) ( 1 M i + 1 M k ) 1 2 (5) D i k e f f = D i k [ ( 1 − s ) ε ] 1.5 (6) R i = M i ∑ m R i , m − ω i ∑ i M i ∑ m R i , m (7) R i = ν i i v n F (8) ν H 2 O = − 6 , ν N H 3 = 2 , ν N 2 = − 1 where ji is the flux of species i, ω and x refer to mass fraction and molar fraction separately and Dik is binary diffusivity between species i and k. The binary diffusivity is defined by Eq. (4). However, in porous medium with two-phase flow, Dik should be corrected by porosity and saturation. Therefore, the effective diffusivity is given by Eq. (5), as defined by Bruggeman correction. dk is the force exerted by species k. Ri in Eq. (1) is coupled with electrochemical reaction in Eq. (6) and given by Faraday's law in Eq. (7). νi is specified by Eq. (8). The Maxwell-Stefan model is applied for the entire flow in both flow channel and porous medium regions.Darcy's law is applied to both gas and liquid flows in porous medium. The law is given by Eqs. (9-13): (9) u l = − K k r l μ l ∇ p l (10) u g = − K k r g μ g ∇ p g (11) ∇ · ( ρ l u l ) = Q l , ∇ · ( ρ g u g ) = Q g (12) k r l = s 3 (13) k r g = ( 1 − s ) 3 where K is the absolute permeability of the porous medium and kr is a function of saturation that represents the relative permeability of one phase. μ and u denote the viscosity and velocity of gas or liquid phase separately. Q represents the sum of the consumed or produced species in the ammonia electrooxidation reaction.The two-phase property is defined by Eqs. (14-17): (14) p c = p g − p l = σ cos θ c ( ε / K ) 0.5 J ( s ) (15) J ( s ) = 1.417 s − 2.120 s 2 + 1.263 s 3 ( 90 ∘ < θ c < 180 ∘ ) (16) ρ = ρ l s l + ρ g s g (17) ρ u = ρ l s l u l + ρ g s g u g where the relation of liquid and gas phase pressure is connected by capillary pressure pc in Eq. (14). Furthermore, σ denotes interfacial tension, θc is the contact angle, J(s) lists the Leverette function. The average density and velocity of two phases defined by Eqs. (16)-(17) are utilized in the Maxwell-Stefan model.The liquid saturation is mainly determined by the mass fraction of nitrogen while the vaporizing of both ammonia and water is also considered. The relation is shown by Eqs. (18-20): (18) p N H 3 = k H x N H 3 , l (19) ω N 2 , g = M N 2 ( p − p H 2 O , s a t − p N H 3 ) ρ g R T (20) s = ρ g ( ω N 2 − ω N 2 , g ) ρ l ( ω N 2 , l s a t − ω N 2 ) + ρ g ( ω N 2 − ω N 2 , g ) where kH represents the Henry's constant. ωsat N2,l is saturated nitrogen mass fraction in liquid phase and is assumed to be 0.Electrochemical equations are used to describe the kinetics of the ammonia electrooxidation reaction in the anode. Since the anode reaction is much more sluggish than that in the cathode, the polarization only in the anode is taken into account for the potential loss of the whole cell. Butler-Volmer equation as shown in Eq. (21) is chosen, which is coupled with the effect of mass transport. (21) i l o c = i 0 [ c R e x p ( α a F η R T ) − c O e x p ( − α c F η R T ) ] (22) c R = ( c N H 3 / c N H 3 r e f ) γ c N H 3 > c N H 3 r e f , γ = 0 ; c N H 3 ≤ c N H 3 r e f , γ = 1 (23) i v = a v i l o c (24) i = ∫ ∫ ∫ i v d v S where iloc is the electrode reaction current density, i0 is the exchange current density. cR and cO are the mole concentration of reduction and oxidation separately, αa is the anode transfer coefficient, αc is the cathode transfer coefficient, av is the specific area, and iv is the current density. cR and cO are acquired by coupling with mass transport equations. The average current density in CL is defined by Eq. (24). av is defined by Eq. (25). mPtIr is the mass loading of PtIr, δCL the thickness of CL. (25) a v = m P t I r E C S A δ C L η is defined by Eq. (26). (26) η = Δ ϕ − E e q where Eeq is the thermodynamic equilibrium potential.Besides, part of the polarization is caused by Ohmic resistance of each component in the anode and the solution. According to Ohm's law, the potential loss is given by Eq. (27). (27) i k = − σ k ∇ ϕ k where σ is the conductivity and k denotes the electrolyte or solid part of the anode.The Brinkman equation is used to describe the fluid dynamics in the flow channel, which is given by Eq. (28) and Eq. (29). (28) 0 = ∇ · [ − p I + μ ( ∇ u + ( ∇ u ) T ) ] + F t (29) ρ ∇ · ( u ) = 0 where F t is the force term for the influence of gravity and other volume forces.In the membrane, not only hydroxide ions but water and ammonia can transfer to the other side. The ammonia crossover is the main cause of cathode voltage loss and catalyst deactivation. Based on the previous work of methanol crossover in DMFC models, the crossover mechanism can be explained by three parts: molecular diffusion, electroosmotic drag by proton and hydraulic transport. However, there is no proton transferring in the anion exchange membrane, so the effect of electroosmotic drag is neglected and the crossover is defined by Eqs. (30)-(31): (30) N N H 3 x o v e r = − D N H 3 , m e m ∇ c N H 3 − ( K m e m μ l Δ p l , c − a δ m e m ) c N H 3 (31) I x o v e r = 3 F N N H 3 x o v e r The physical and chemical processes within the DAFC anode are computed by the several modules in COMSOL Multiphysics 5.4: (1) Second Current Distribution is applied in both the CL and the membrane. ‘Porous Electrode’ is applied to the CL. (2) Darcy's Law is applied for two phases in both the CL and GDL. The Brinkman equation is applied in the flow channel. (3) Transport of Concentrated Species and the Maxwell-Stefan model are applied in CL, diffusion layer and flow channel. Half of the DAFC anode model is built due to the symmetrical structure and the symmetry plane of the anode model is set as ‘symmetry’ in Comsol Multiphysics. Hexahedral mesh is applied for the entire model. A probe is set in the CL to integrate the current source and get the average current density.In COMSOL Multiphysics, two end planes of the flow channel are set as inlet and outlet separately. The potential of the GDL upper surface is set as the operating potential while the potential of the membrane bottom is set as zero. Other outside surfaces of the model are all set as ‘no flux’ and ‘insulation’, so no species (such as ammonia) or current can get through these surfaces.To verify the accuracy of this model, the calculated polarization curve of the model is compared with the experimental polarization curve of DAFCs. The procedure for assembling the DAFC and its testing were similar to our previous work [14]. In particular, the MEA was composed of carbon cloth (W0S1009, Ce Tech Co., Ltd) supported by 3.4 mgPGM cm-2 PtIr/C (40% PtIr on Vulcan XC-72R, Pt/Ir = 1:1, Premetek Co.), 15 µm alkaline polymer electrolyte membrane (AEM, Alkymer W-211415, EVE Institute of New Energy Technology) and carbon paper (28BC, SGL Carbon) supported by 1.7 mg cm-2 MnCo-BP2000 (home-made cathode catalyst [14]). The anode catalyst ink was prepared by mixing PtIr/C, Nafion solution (Dupont, 5wt%) and isopropanol under ultrasonication for 1 h in an ice-water bath. The preparation of cathode catalyst (MnCo-BP2000) ink is the same as the anode. Both the mass ratio of the anode and cathode catalyst to Nafion is 3:1. The AEM was immersed in 2.0 M KOH for 12 h to replace the Cl- anion in the AEM to OH-, and then was washed three times with deionized water. Finally, it was pressed between carbon cloth and carbon paper to make the MEA.The DAFC was tested (G20, Greenlight Innovation Corp. Canada) under the conditions of 1 M/3 M ammonia in 3 M KOH as anode fuel (5.0 mL min-1, controlled by a peristaltic pump), humidified O2 as cathode oxidant (200 mL min-1, backpressure: 20 kPa) and the cell temperature of 80 ºC. The physicochemical properties and operation conditions of this model are listed in Table 2 .To optimize the DAFC anode model, the effects of structure parameters of the anode CL (e.g., porosity, thickness and PtIr loading) on cell performance are investigated, which can guide the optimization of electrode to achieve high performance DAFCs.As shown in Fig. 2 (a), the calculated polarization curve of this model is compared with the experimental polarization curve of DAFC. The theoretical open-circuit voltage (OCV) of the DAFC is 1.17 V [5]. In contrast, the experimental value of the OCV is only 0.59 V with 1 M ammonia and 0.66 V with 3 M ammonia, possibly attributed to the high onset potential of ammonia electrooxidation and mixed potential in the cathode caused by ammonia crossover. The OCV is set equal to the experimental value because the cathode overpotential is not considered in this anode model. Both the calculated and experimental results demonstrate that the cell voltage decreases with increasing the current density due to the polarization. In the whole current density range, good agreement is obtained between the predicted polarization curve and experimental data. The electrochemical polarization and concentration polarization produced in the oxygen reduction process in the cathode which is not as severe as in the anode and neglected in this model might account for the slight deviation. Fig. 2(b) presents the predicted rate of ammonia crossover as a function of current density. Because the ammonia concentration is the key factor which can cause crossover in Eq. (30), the parasitic current calculated for 3 M ammonia (3 M model) is higher than that for 1 M ammonia (1 M model) when the cells were operated under the same current density. In addition, the parasitic current decreases with the increase of the cell current density since more ammonia is consumed in AOR. The 1 M model in Fig. 2(a) shows severe concentration polarization at 390 mA cm-2, which means that most ammonia in the CL is consumed. This can be verified by the result in Fig. 2(b) where the parasitic current drops to nearly 0 mA cm-2 at the same current density and only a small amount of ammonia penetrates through the membrane to the cathode.In the 3 M ammonia model with 0.3 V cell voltage, the distribution of ammonia concentration and gas saturation in the anode cross-section are shown respectively in Figs. 3 (a) and (b). Fig. 3(a) shows that the ammonia concentration drops to 2 M in the flow channel region due to ammonia transferring to the GDL and the fluid density diminishing caused by nitrogen. Meanwhile, from diffusion layer to membrane, the concentration decreases sharply and reaches almost zero in the CL because most ammonia is consumed during the electrooxidation process. The gas saturation in Fig. 3(b) reflects the nitrogen volume fraction in the anode. The result shows that gas saturation is higher in the CL and GDL than that within the flow channel, because nitrogen is generated in the CL. As shown by Eq. (5), the diffusion rate is slower in porous media than that in channel. Nitrogen can easily aggregate in the CL and GDL and hinder the ammonia transport from flow channel to the CL, which explains why the ammonia concentration is much lower in the porous region than that in flow channel in Fig. 3(a) even though ammonia is consumed in the CL by AOR. Nitrogen flows with the ammonia solution stream and accumulates, causing higher gas saturation in the downstream region.The polarization and ammonia distribution of the DAFC model (1 M) are investigated under different anode CL porosity εcl (in Table 2). As shown in Fig. 4 (a), the porosity almost shows indiscernible effect on polarization when the current density is below 300 mA cm-2. In contrast, as the current density is higher than 300 mA cm-2, the concentration polarization becomes dominant and further turns more obvious with varying CL porosity. The current density decreases by 20 mA cm-2 at 0.1 V when the porosity increases from 0.4 to 0.5. Such difference can be further investigated by comparing the ammonia concentration distribution in the anode with the CL porosities of 0.4 and 0.5 when the cells are operated at 0.1 V, as illustrated in Fig. 4(b) and Fig. 4(c), respectively. The results indicate that the concentration polarization is severe and most ammonia is consumed to nearly zero in the CL. When focusing on the CL, a lower ammonia concentration is found in the case of porosity of 0.5, suggesting a slight decrease in cell performance. Obviously, higher porosity in the CL can enhance the diffusion of both ammonia and nitrogen. However, considering the low ammonia concentration in the CL, the porosity has a higher impact on nitrogen diffusion than ammonia diffusion since nitrogen has a larger diffusivity than ammonia in the feed solution. Therefore, nitrogen generated in the CL can easily diffuse to the GDL with increasing the porosity and in turn hinder the ammonia transport, which accounts for the decreased cell performance with increasing porosity within the high current density region.The polarization curves for various CL thickness (δcl ) ranging from 30 to 60 µm is shown in Fig. 5 (a). In the mediate current density region where the ohmic polarization has a major influence, voltage loss increases when the CL thickness increases from 30 to 60 μm. This is because the resistance is proportional to the layer thickness. However, as the current density is above 300 mA cm-2, the cell shows better performance and less concentration polarization upon increasing the CL thickness reaches. This can also be verified by the distribution of ammonia concentration in the anode at 0.1 V, as shown in Fig. 5(b) and Fig. 5(c) with the CL thickness of 30 and 60 μm, respectively. The results show that the ammonia in the 30 μm CL is almost consumed while some ammonia remains in the 60 μm CL. Since there is no enhancement in AOR kinetics with the increase of the CL thickness, the larger volume of a thicker CL can accommodate more ammonia and mitigate the concentration polarization.In addition to the porosity and thickness of CL, the effect of PtIr loading in the CL on cell polarization is also investigated. Due to the sluggish AOR kinetics, high anode catalyst loading is usually required to achieve high-performance DAFCs [13,36]. The PtIr/C loading of 3.4 mgPGM cm-2 used in our experiment is included in the finite element models. As shown in Fig. 6 , the increase of PtIr loading leads to the improvement in DAFC performance. The improvement in the high current density region is smaller than that in the low current density region. For instance, the current density at 0.42 V is tripled, namely increasing from 22 mA cm-2 to 66 mA cm-2, when the PtIr/C loading increases from 1.4 to 5.4 mgPGM cm-2. In contrast, the current density at 0.1 V is moderately improved from 359 mA cm-2 for 1.4 mgPGM cm-2 to 405 mA cm-2 for 5.4 mgPGM cm-2. Such results indicate that high PtIr loading can reduce the activation polarization of the DAFCs due to the increase of active AOR sites in the CL, but has a less significant effect on reducing the concentration polarization, which is mainly caused by the insufficient supply of NH3 and the accumulation of nitrogen in the catalytic layer.In summary, a three-dimensional two-phase multicomponent model of a DAFC anode incorporated with electrochemistry, mass transport and fluid dynamics has been developed and validated by experimental results. The modeling results indicate that the rate of ammonia crossover decreases with the increase of current density. By manipulating the parameters in the model including porosity, thickness and PtIr loading of the CL, their effects on cell performance and the distribution of ammonia and nitrogen are elucidated. Increasing CL porosity can degrade the cell performance at high current density due to the blockage of ammonia transport by nitrogen gas. The increase of CL thickness increases ohmic polarization in the middle current density region and provides larger volume for ammonia transport and oxidation, which reduces the concentration polarization at high current density. The electrochemical performance of DAFCs can be effectively improved by increasing the active sites of AOR in CL with higher PtIr loadings.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 financially supported by the National Key Research and Development Project of China (No. 2019YFB1504502).

Great progress has been made in recent years in the development of low-temperature direct ammonia fuel cells (DAFCs), motivated by the recognition that ammonia is a carbon-free hydrogen carrier with high energy density, low production cost, and ease in liquefaction at ambient temperature. However, the sluggish kinetics of ammonia electrooxidation and especially complicated mass transport in the anode catalyst layer hinder the further development of DAFCs. In this work, a three-dimensional two-phase multicomponent DAFC model considering the effect of ammonia crossover has been developed. Maxwell-Stefan model, Darcy's law and Brinkman equation are utilized to simulate the multicomponent fluid motion and transport. The predicted polarization curve simulates all experimental results well. The model shows the rate of ammonia crossover decreases with the increase of current density. Besides, the effects of the physicochemical property of the anode catalyst layer, including porosity, thickness and PtIr loading, on cell performance are investigated. The modeling results indicate that decreasing porosity and increasing thickness can slightly improve the electrochemical performance of DAFCs at high current density. Meanwhile, higher PtIr loading can effectively reduce voltage loss before approaching the limiting current density.

Data will be made available on request.Ethanol is one of the most versatile and available biomass-derived molecules, with an increasing industrial production in the last years [1]. It is an interesting building block molecule used in the manufacture of drugs, plastics, and other compounds, both by enzymatic and catalytic routes [2–4]. Nevertheless, its main use is as biofuel, providing sustainable energy with properties close to gasoline, but its poor lubricant properties can negatively influence the engine’s durability. Mixtures of ethanol with higher alcohols (mainly branched ones) can overcome this problem. The Guerbet reaction is the most promising route to obtain these second-generation biofuels [5,6]. The complex mechanism of this reaction, involving dehydrogenation, condensation and hydrogenation steps, hinders its industrial application, requiring more research to increase the selectivity. Most of the previous works have been performed in the gas-phase, with temperatures relatively high (>300ºC), defining the 1-butanol as the target compound, and reaching maximum yields lower than 30% [7–14]. Due to the extended number of side reactions, complex mixtures are obtained, being difficult to purify because of the similar physiochemical properties of most of these products.It is expected that working in condensed phase, at high pressure and close to the ethanol critical point, the activity changes significantly and the product distribution control increases. However, there are only few studies based on the optimum conditions obtained in the gas-phase. Thus, Riitonen and co-workers reached selectivities towards 1-butanol up to 70%, working with different γ-Al2O3 supported metal catalysts, at 250ºC with 100 bar, with conversions between 10% and 30% [15–17]. Similar selectivities are reported with more complex configurations, using microwaves-assisted reaction and Ni/Al2O3 catalyst [18] or combining metal catalyst with homogeneous bases [19]. At the same temperature but at 176 bar, a selectivity to butanol higher than 83% is reported by Ghaziaskar and Xu, using 8% Ni/γ-Al2O3, with 35% of ethanol conversion [20]. With these bifunctional catalysts, the acid sites promote the condensation as well as the CO hydrogenations by the Meerwein-Poondorf-Verley (MPV) mechanism whereas the metal nanoparticles enhances the dehydrogenation and CC hydrogenation steps. The main route competes with undesired acid-catalyzed additions and different acetals and acetates are also obtained. The relevance of these side reactions increases with the molecular weight of the compounds, whereas the acid-catalyzed condensation decreases. Consequently, the Guerbet mechanism is limited to the first condensation (C4).The co-presence of basic sites is expected to improve the initial dehydrogenation. This approach, scarcely studied, could be adapted to enhance the condensation step, increasing the size of the products to six and eight carbon atoms. Thus, mixed oxides have been considered, reaching selectivities of 1-butanol close to 70%, but with low ethanol conversions (<5%) [21]. Miller and co-workers propose a nickel supported mixed oxide (Ni/La2O3-γ-Al2O3), obtaining 41% of ethanol conversion with more than 70% of butanol [22,23].In the last years, the industry interest has shifted to these higher alcohols since they have a high value in the production of plasticizers, soaps, and fine chemicals, in addition to their properties as solvents and fuel additives [23,24]. However, the production of these heavy alcohols from ethanol has been poorly studied. Preliminary studies in gas-phase propose a sequential configuration (from ethanol to butanol and from butanol to 2-ethyl hexanol (2EH)) with different catalysts and reaction conditions [25–29]. This configuration is quite complex from the technical point of view, because of the low selectivity of the first stage. The limited literature in liquid phase (batch configuration) proposes a maximum of 32% of conversion with selectivities of 22% of hexanol and 60% of butanol, at 230ºC, using 7–10% Cu/MgAl, with a catalyst/reactant mass ratio close to 0.1 [6,30,31]. Despite these promising results, there is a lack of systematic study that allows identifying the relevant catalytic properties and the reaction conditions to improve the selectivity to higher alcohols (>C4) and their corresponding precursors.This work presents a comprehensive study of the liquid-phase ethanol self-condensation considering the production of heavy alcohols (C6-C8) as a one-step process. The activity of different catalysts (HAP, MgAl, MgZr, MgFe, MgCaAl) was studied analyzing the results as a function of their morphological and physic-chemical properties. These materials were chosen considering the previous literature for Guerbet gas-phase condensations, with well-recognized works highlighting the activity and selectivity of HAP [32–35] and different mixed oxides [14,21,31]. Despite the different structures of these materials, the same type of active sites (acid and base ones) as well as their hydrogenation capacity by the MPV mechanism are highlighted as the key parameters for the reaction, allowing the comparison. The reaction conditions (no solvent, low catalytic loading) were chosen to achieve a tight control of the activity to facilitate the identification of the catalytic behavior on each single step of the process. These results are analyzed to propose a mechanism, detecting similarities and differences with respect to the gas-phase configuration. Different metals were supported on the most promising materials, analyzing if the presence of nanoparticles with hydrogenation and dehydrogenation activity have a crucial role enhancing the production of the target compounds.A commercial hydroxyapatite (HAP) (Sigma Aldrich) is used in this work, whereas the different mixed oxides (MgZr, MgAl, MgFe and MgCaAl) were prepared in the lab. The details of each particular preparation method are included in the Supplementary Information.Bifunctional catalysts were prepared supporting Pt, Ni, Cu, Ru or Pd by the dry-impregnation method, using nitrate precursors. This was done by adjusting the volume of the metallic precursor solution (prepared to achieve the target 1 wt% of metal loading) to the pore volume of the support, ensuring the total impregnation and a high dispersion. The impregnated catalyst was dried for 24 h, calcined to 700 °C and reduced with a H2 flow of 20 mL·min−1, up to a temperature of 450 °C (ramp 5 °C·min−1), holding this temperature for 3 h.The catalytic morphology was determined by N2 physisorption, using a Micromeritics ASAP 2020 instrument, applying the BET and BJH methods to calculate the surface area, the pore diameter and volume. The crystalline phases were analyzed by X-ray diffraction (PANalytical X′Pert Pro), working with the Cu-Kα line (0.154 nm) in the range 2θ = 10 – 120°. These analyses were done with fresh and spent materials to identify possible changes in the structure during the reaction.The acidity and basicity quantifications (fresh and spent catalysts) were performed by a programmed temperature desorption (TPD) in a Micromeritics AutoChem II 2920, following the desorption of the probe molecules (NH3 or CO2) by a Pfeiffer Vacuum-300 mass spectrometer. A previous cleaning step with He ensures the absence of physisorbed compounds. The saturation was done for 20 min with a 20 mL·min−1 flow (2.5% NH3 in He or 99.5% of CO2). The desorption was monitoring from room temperature to 950ºC, with a slope of 5 ºC·min−1.The evolution of the catalytic surface with the reaction was analyzed by diffuse reflectance infrared spectra using a Thermo Electron Nicolet FTIR spectrometer equipped with a MCT/A detector. Spectra were recorded in the 4000–1200 cm−1 range, with a resolution of 4 cm−1, collecting 256 scans/spectrum. 20 mg of catalyst were used in each experiment, being placed inside a high temperature cell. Catalytic measurements were conducted at 230ºC under inert atmosphere (N2 flow of 20 mL·min−1) or under an atmosphere saturated in ethanol.The metal loading of the bifunctional catalysts was determined by Inductively Coupled Plasma Mass Spectrometer (ICP-MS) using a HP 7900 of Agilent. Approximately 50 mg of the sample were inserted into a microwave-assisted Teflon bomb; adding HCl (2.25 mL) and HNO3 (0.75 mL) to dissolve the sample. The metal dispersion and particle size distribution was quantified by transmission electronic microscopy (TEM) using a MET JEOL 1011. Histograms and average particle size were calculated by analyzing 100 particles in each sample, using the software Confocal ImageJ.The ethanol condensation was performed in a 0.5 L stirred batch autoclave reactor equipped with a PID temperature controller and a backpressure regulator (Autoclave Engineers EZE Seal). Firstly, 200 mL of ethanol (EtOH) (VWR, 100%) and the catalyst (0.5 or 2 g as a function of the experiment) was added to the reactor. The air was purged with N2, and condensation was carried out at 230 °C, under 30 bar of N2 (pressure at room temperature, increasing to 80 bar at 230 °C) under stirring (1000 rpm) for 8 h. This temperature is the average of those reported in the bibliography (from 200 to 250ºC) [6,22,23,31], considering the limit conditions allowed by the reactor. Based on these points, the conditions were selected as an equilibrium that guarantees the liquid state of all the compounds involved, the conditions that could promote the reaction to heavier compounds (>C4) as well as the minimum severity of the reaction, in good agreement with the desired sustainable character of the process.A filter placed inside the sampling port prevents the catalytic extraction during the sampling. The evolution of the different compounds involved in the reaction was analyzed by gas chromatography (GC) in a Shimadzu GC-2010 equipped with a FID detector, using a 30 m long CP-Sil 8 CB capillary column. Peak assignment was carried out by GC-MS in a Shimazdu GC/MS QP 2010 Plus Instrument, using a 30 m long TRB-5MS capillary column. EtOH and the majority peaks calibrations were done using commercial samples, whereas minority products calibration was carried out using the relative carbon concept proposed by Scanlon and Willis [36]. The analytical conditions (detailed in the Supplementary Information, Table S1) were chosen to guarantee that the ethanol signal does not saturate the detector, even in the case of initial samples (the highest signal expected). Each reported experimental point corresponds to the average value of at least two analyses. The maximum standard deviation of the reported values is 6%. The results were analyzed in terms of conversion, selectivities, and carbon balance, according to Eqs. (1), (2), and (3), where “ n i " corresponds to the number of carbons in each compound, and ” C i , t ” is the molar concentration of this compound at the time analyzed: (1) Selectivity : φ % = n i · C i , t 2 · EtOH 0 − EtOH t · 100 (2) Conversion : x % = EtOH 0 − EtOH t EtOH 0 · 100 (3) Carbon Balance : CB = ∑ n i · C i , t 2 · EtOH 0 At reaction conditions, the presence of solid deposits is discarded (assumption corroborated by TPO analyses). The differences in carbon balance closures are then attributed to the formation of light compounds, not detected in the liquid phase. This hypothesis was corroborated by the analysis of the gas phase, being recovered using a sampling bag. The gases accumulated were analyzed by GC-MS. Only a qualitative analysis is possible, the quantification and temporal evolution evaluation being not possible due to the accumulative sampling required.The accuracy of GC-FID analyses in these conditions was probed analyzing ethanol signal and the standard deviation of 12 repetitions of the same sample, as indicated in the Supplementary Information (see Table S2). The values indicated are congruent with the theoretical conversion required to obtain the products detected. However, this second methodology is discarded with the aim to compare the relevance of permanent gases produced in the reactions, analyses that could be not possible if the conversion is calculated based on products.Initial experiments were focused on the comparison between bulk materials to identify the catalytic properties that maximize the selectivity to higher alcohols, mainly C6 and C8, or their corresponding condensed precursors (in absence of metal particles with hydrogenation activity). In all the cases, anhydrous ethanol was used as reactant, setting the temperature at 230ºC. The evolution of ethanol conversion with time is analyzed in Fig. 1.The absence of an external solvent (high reactant initial concentration, 789 g·L−1) as well as the low catalytic loading (2.5 g/L) justifies the conversions obtained. Similar conversions are reported in most of the previous literature when working without external solvent, even with higher catalytic loading [14,17,21,23]. In fact, comparing the results in terms of mass of ethanol reacted per time and catalyst loading, the values obtained in this study (from 0.473 to 0.907 g/g·h with MgZr and MgAl (3/1), respectively) are better than most of those drawn from studies in absence of reduced metals (0.710 g/g·h [14,30], 0.647 g/g·h [21]), with the exception of Perrone and co-workers, who obtained values of 1.326 g/g·h with a partial substitution of Mg2+ and Al3+ by Cu2+ and La3+, respectively [31].Although higher conversions could be anticipated working with higher catalytic loading, reported conversions are more useful for gaining further understanding on reaction mechanism and the role of the different surface sites. MgZr shows the poorest activity, maximum conversion of 1.2% after 8 h, suggesting that longer times could rise this value because of the increasing trend observed. Low conversions (1.3–1.5%) are also reached with MgAl (2/1) and MgCaAl, but in these cases, the values remain almost constant after the first 2 and 3 h, respectively. On the other hand, conversions obtained with HAP, MgFe and MgAl (3/1) are significantly higher, with values from 2% to 2.3%. As in the previous case, three different trends are observed, with a constant conversion in less than 2 h with MgFe, a continuous evolution observed with HAP, and an intermediate behavior of MgAl (3/1), with a fast increase during the first 2 h and a second and slower increasing trend at longer times after an intermediate and flat step. Table 1 summarizes the product distributions after 8 h reaction time, in terms of absolute and relative selectivity of main families of compounds. Considering the large number of products obtained, the absolute selectivities are lumped by families, according to their number of carbons. Thus, C2 only involves the acetaldehyde, whereas C4 corresponds to crotonaldehyde, butanal, and 1-butanol, with a similar distribution for C6 and C8 families. Although quantitatively analyzed, side products (as esters) are not included in these families, being considered together as “undesired liquid by-products”. Reported carbon balance closure is calculated comparing the initial ethanol loading and the concentration of ethanol and all the desired and undesired reaction product analyzed in the liquid phase after performing the reaction. Although this carbon balance closure is very high, as the ethanol conversions are low, selectivity to gas products can be important. Thus, apparent selectivities to gaseous by-products have been estimated from ethanol conversion and carbon balance, being these values also reported in Table 1.As anticipated, the liquid-phase configuration promotes a different distribution than the gas-phase one, with a decrease of C4s in favor of a higher number of heavy compounds (C6 and C8), mainly observed with MgAl (2/1) and MgCaAl. These compounds are the heaviest detected in this study, concluding that subsequent dehydrogenations and/or aldolizations require more severe conditions. In the gas-phase reactions, only some traces of C6s are detected using bimetallic modified mixed oxides [31,43].More than 26% of the total compounds correspond to C6 and C8 when using MgCaAl, the best catalyst promoting condensation. In the case of MgAl (2/1), this percentage decreases to one half of this value. In terms of functional groups, MgAl (2/1) shows a high hydrogenation activity, alcohols representing almost 80% of the total. On the other hand, with MgCaAl, alcohols only correspond to 14%, suggesting that condensation prevails over hydrogenation. In both cases, all the C6-C8 alcohols are obtained following the same ratio as the carbon families. Regarding the aldehydes, both materials produce hexanal, which hardly condensates with other ethanol molecule, limiting the production of C8s.C6 are the heaviest compounds detected with MgZr (7.6%). A similar distribution of aldehydes and alcohols is obtained. The high concentration of crotonaldehyde (almost 30%) with respect to butanal (almost negligible) suggests that the CO hydrogenation is easier than the CC one, most of the butanal being directly converted into butanol.HAP, MgFe, and MgAl (3/1) do not show relevant activity for heavy condensations, with less than 3% of C6 and C8 compounds. In the case of HAP, the results are congruent with a lack of activity promoting the condensation of heavy compounds, obtaining a sample enriched in C4 (>40%) with a good balance between aldehydes and alcohols (35% of butanol). On the contrary, MgFe and MgAl (3/1) demonstrate a poor condensation activity, with total selectivities lower than 10%, producing almost 90% of undesired gases (ethylene and diethyl ether).To sum up, these materials reveal differences not only in terms of conversion but also in the product distribution. The instability of some of the catalysts at reaction conditions could be a possible justification of these discrepancies. IPC results indicate the absence of metals in the reaction liqueur since Mg, Al, Ca, Fe or Zr are not detected. The catalytic leaching is then discarded as a deactivation cause. On the other hand, the comparison between crystallographic phases of fresh and spent materials (XRD diffractograms shown in Fig. S1 and Table S3, discussed below) reveals a good correspondence between peaks before and after the reaction, prevailing the amorphous structure of these materials, without observing the parent hydrotalcite structure (crystalline one). Thus, the physical and morphological stability of these materials is also corroborated. In this context, a relevant role of their catalytic properties is suggested, promoting different steps of the main mechanism, preventing undesired lateral reactions, and minimizing the adsorption of different compounds. This discussion requires the analysis of the morphological and physico-chemical properties of these materials, main data being summarized in Table 2.Morphological results discard any relevant role of the external surface area or mass transfer limitations. In general, materials with a fast initial conversion (MgFe, MgAl (3/1), MgAl (2/1)) exhibit high concentration of acid sites (mainly weak and medium ones), whereas materials with a lower acidity require longer times. These results demonstrate the relevance of acid sites adsorbing the ethanol molecule. This conclusion was verified by DRIFT spectroscopy, observing more pronounced bands with MgAl (3/1), the most acidic material. The identification of all the DRIFT bands, based on previous literature [37,38], as well as their discussion, is detailed in Fig. S2.MgAl (2/1), MgCaAl, HAP and, in a small extent, MgZr have the highest capacity to promote the condensation, in agreement of their high concentration of medium and strong basic sites, releasing water. Previous literature indicates that some oxides derived from a hydrotalcite-type precursor undergo rehydration in presence of water, modifying their surface chemistry by the reconversion of the O2- basic sites to OH- ones (Brønsted sites responsible of aldol condensations) [39]. This hydration depends on their surface structure and can also occur with HAP [40]. This fact would be enough to increase its condensation capacity, not so relevant to observe differences in the crystallographic structure (surface phenomenon). This effect was corroborated by comparing the acidity and basicity of fresh and spent catalysts. Results obtained (shown in Figs. S3-S4 and Table S4) indicate the relative enrichment in medium-strength basic sites of MgZr and HAP, the materials that suffer reactivation, whereas the strength of acid sites remains almost invariable, resulting in stronger basic/acid pairs. According to the literature, these sites promote the dehydrogenation of ethanol via E1cb elimination mechanism [41], justifying the reactivation observed with these materials. This reactivation has not been reported in gas-phase reactions, suggesting that Brønsted sites are not stable at high temperatures.A similar reactivation and increase in the basic/acid sites strength is observed with MgAl (3/1), but it shows low condensation capacity (5% of >C2). This fact is justified by the lack of correlation between basic/acid pairs, prevailing the activity only catalyzed by acidity. In this case, water is released by undesired acid additions (yielding 1,1-diethoxyetane, selectivities up to 8%) and ethanol dehydrations, producing ethylene and diethyl ether in large amount (>84%). These two compounds were identified in the analysis of the gas phase by GC-MS. In good agreement with their presence, the carbon balance closure with this material (98.1%) is the lowest one. In other materials, a clear decrease in the acidity without a relevant change in the strength distribution is observed (43%, 61%, and 62% with MgAl (2/1), MgFe, and MgCaAl, respectively). With these materials, the expected rehydration effect is shielded by the adsorption of unsaturated intermediates, severely blocking the acid sites and hindering further reaction progresses.The different profiles of the two MgAl mixed oxides deserves special attention, suggesting a strong influence of the preparation method and the Mg/Al ratio. The minor differences in terms of weak and medium-strength acidity discard a different dehydrogenation capacity. In fact, the conversions during the first 3 h are very similar. XRD diffractograms (Fig. S1) illustrate relevant differences, consequences of the interaction between oxide phases during the preparation. In good agreement, a spinel (MgAl, JCPDS 00–021–1152) is detected in MgAl (2/1), in addition to periclase (MgO, JCPDS 03–065–0476), the only phase observed with MgAl (3/1), suggesting that Al is in an amorphous phase or in crystals too small to be identified with the resolution of this equipment. The different coordination state of Mg and Al on these crystalline phases (and the corresponding morphology of the acid sites) seems to play a key role in the interaction of reaction intermediates with the catalysts, producing opposite effects in the catalytic activity. DRIFT spectra (Fig. S2) visualize these differences. Thus, signals of ethoxide species (1460 cm−1 [42]) are significantly more evident in the case of MgAl (3/1) than in MgAl (2/1).The reaction products from the two materials with the lowest weak acidity (HAP and MgZr) are enriched in acetaldehyde, suggesting that these sites are involved in the condensation, directly by an acidic mechanism ore stabilizing the basic sites (basic/acid pairs). The high acidity justifies the poor results obtained with MgAl (3/1) and MgFe, with more than 84% of carbon as gaseous by-products. The sampling method for the analysis of these gases does not allow accurate and continuous quantification, but in both cases more than 90% corresponds to ethylene, with lower amounts of diethyl ether and other compounds in traces. These results contrast with those obtained with MgAl (2/1), material that prevents the formation of gaseous by-products. The high acidity of this material is well balanced with its basicity, suggesting the primacy of basic/acid pairs over isolated acid sites, promoting the main route of the Guerbet reaction.There is a good correspondence between the medium-strength basic/acid sites and the total selectivity to C6 and C8 compounds, indicating that these sites are the most relevant ones to promote condensations, as induced from the total C6-C8 selectivity of MgCaAl (25.8%) and MgAl (2/1) (12.7%). Even with this amount of C6 and C8, MgAl (2/1) produces the maximum amount of C4 (78.6%), followed by HAP (40.2%), being suggested as promising supports for next studies. These catalysts produce the maximum total alcohol selectivity, being enriched in butanol.Thus, the conversion is not always related with a high activity in the Guerbet reaction since also undesired ethanol dehydration and acid-catalyzed additions occur, obtaining light gases. MgFe and MgAl (3/1) produce selectivities to diethoxyethane at initial times higher than 9.7% and 10.4% (values that corresponds to relative weights of 39% and 24% of this compound in the products’ mixture), respectively. Only in the case of MgAl (2/1), this conversion corresponds to desired products, observing 1-butanol since the first samples.To explain these results, a separate experiment with MgAl (2/1) and pure butanol as reactant was done. The evolution of the main intermediates is detailed in Fig. S5. Less than 1.7% of conversion is reached after 8 h (99.8% carbon balance), with ethanol and butanal (16.3%) as the main reaction products and less than 1% selectivity for C6s and C8s (0.2% 3-hexen-1-ol, 0.4% of 1-hexanol, 0.2% of 2-etil-1-hexanol). Thus, the C6 adducts are suggested to be produced mainly by the condensation between crotonaldehyde and acetaldehyde, but not so easily from butanal (the selectivity of this compound is more than 20 times higher than when using ethanol). Thus, once crotonaldehyde is partially hydrogenated to butanal, the condensation capacity decreases. This hypothesis is congruent with the stability of the enol intermediate produced during the condensation in presence of CC double bonds. The quantification of acetaldehyde and ethanol suggests a partial reversible character of condensation not observed in gas phase or when ethanol is used as reactant since the reverse reaction is catalyzed by the same active sites than the direct one, the condensation prevailing in presence of aldehydes. According to this study, strong acid sites and basic/acid pairs are required to promote the butanol double dehydrogenation and condensation. Their low concentration as well as the adsorption of the C6 and C8 compounds with the subsequent active-sites blockage conditions the low second condensation ability and justifies the prevalence of hydrogenated C4 compounds.In all the cases, the hydrogenation capacity decreases with the size of the aldehydes. This is an anticipated result considering the absence of metal nanoparticles and the prevalence of the MPV route. According to this mechanism [8], the hydrogenation requires the co-adsorption of the aldehyde and an alcohol, on an acid site, obtaining a cyclic compound as the reaction intermediate. The stability of this intermediate decreases with the size, being the most unstable the one of six carbons (crotonaldehyde + ethanol). Thus, the ratio of butanol to the total C4 family of compounds reaches values higher than 99% with Mg/Al (2/1) whereas this percentage decreases to 45% when analyzing the C8 hydrogenation capacity.To establish a reaction mechanism, these results after 8 h must be analyzed together with the evolution in time of the different intermediates. The most relevant data are shown in Fig. 2, excluding MgFe and MgAl (3/1) because of their negligible condensation activity.Acetaldehyde is the first intermediate obtained with all the materials, with initial selectivities of 100% and a continuous decreasing trend with the time. This decreasing trend is more marked in those catalysts with higher condensation activity. According to these results, the ethanol dehydrogenation to obtain acetaldehyde is the starting point of the process, discarding the direct coupling between two ethanol molecules as a relevant step. Despite the lack of total agreement about the ethanol condensation in gas-phase, this mechanism prevails in the literature over the ethanol direct condensation [25,44].The typical evolution of a successive condensation C4-C6-C8 is observed with MgZr and, being not so marked, with HAP. With these two materials, C4s and C6s compounds reach a maximum in selectivity, slightly displaced in conversion in the case of C6s, according to their intermediate character. The high hydrogenation activity of MgAl (2/1) alters these curves, observing a high accumulation of C4s due to the stable character of butanol. The high condensation activity of crotonaldehyde is observed with MgCaAl, material with which C4s and C6s almost appear simultaneously, with a high proportion of the last ones, consuming most of the crotonaldehyde obtained in the first condensation.As to the evolution of each intermediate, a first hydrogenation of CC bonds is observed for the C4 family ( Fig. 3a), suggesting a consecutive production of butanal and butanol that is assumed to be extrapolated to heavier compounds (C6 and C8). The CO hydrogenation by the MPV mechanism as well as the subsequent condensations justify the low selectivity to these aldehydes, with a fast production of heavy compounds or alcohols once these intermediates are obtained. The slow but continuous hydrogenation activity of these materials (see Fig. 3b) suggests that longer times could enhance the selectivity of these target compounds with all the catalysts except MgFe and MgAl (3/1) because of their lack of condensation activity.According to these results, the basis of the mechanism in the liquid phase is based on the one in the gas phase but with some modifications. Different side reactions are observed (ethoxides not detected in gas phase and quite relevant in the liquid one). The softer conditions of this configuration and the absence of noble metals justify the slow rates of hydrogenations, increasing the opportunity to obtain heavier chemicals by the condensation of unsaturated compounds. The presence of isomers as well as partially hydrogenated derivatives rises the total number of compounds. Considering the experimental results, the scheme proposed for the liquid-phase ethanol condensation can be updated to a more complex one considering the different compounds detected with more than four carbon atoms, as shown in Scheme 1. In this scheme, the different isomers that could be simultaneously obtained are indicated, as well as the fact that C8s are produced by the sequential addition of an acetaldehyde molecule to crotonaldehyde, the direct condensation of two crotonaldehyde molecules or the condensation involving butanal being discarded as relevant at these conditions.The temporal evolution of reaction products also supports the previously mentioned required trade-off between hydrogenation and condensation activity, the fraction of heavy compounds reaching a maximum with those materials highly selective to alcohols, whereas observing a slow but continuous increasing trend with MgAl (2/1) and HAP, as shown in Fig. 4. These results indicate that higher selectivities can be obtained modifying the reaction conditions.According to the preliminary analysis, a higher catalytic loading is anticipated to have a positive influence on the product carbon-length if the condensation activity prevails over the hydrogenation one. To check this hypothesis, the results obtained with 2.5 g/L are compared with those reached using 10 g/L. Once the catalytic loading is increased, the conversion is 160% higher with MgAl (2/1), from 1.5% to 3.9% (with a continuous increasing trend during the 8 h), whereas a lower increase from 2.1% to 2.9% is observed with HAP (flat conversion after 5 h). However, the main differences are related to the selectivity distribution, compared in Fig. 5.There is an increase in the selectivity of the C6s, in detriment of the C4s and, in the case of the HAP, of acetaldehyde too. The amount of C8 compounds is negligible in both cases, suggesting that stronger sites are required to promote this step. The increase in the condensation activity is in detriment to the hydrogenation one. This is congruent with the decreasing stability of MPV intermediates with the size of the intermediates, as discussed before. Thus, hydrogenated compounds decreased from 79.1% to 35% MgAl (2/1), whereas the initial 34.9% reached with 2.5 g/L of HAP declines to 10.4% with 10 g/L. In both cases, these reductions are proportional to the increases in C6s. Butanol is the main alcohol in both cases, 30.9% and 9% with MgAl (2/1) and HAP, with only 2.9% and 1.4% of hexanol, respectively. No significant differences in terms of side products were observed with any of these materials (selectivities from 6% to 11%), being more than 80% due to the 1,1-dietoxiethane, the side product obtained by the ethanol dimerization. On the contrary, the control over the reaction decreases, observing 19.2% (MgAl (2/1)) and 55.4% (HAP) of undetected gases.Results obtained with MgAl (2/1) (>71% of target compounds) are significantly better than those reached with HAP, suggesting a good alcohols production by a second hydrogenation step. The poor increase of activity observed with HAP suggests that the positive effect of rehydration has a limited impact, and the results are mainly conditioned by other aspect, whose negative role is more evident as the reaction advances. In this context, previous literature suggests that water can play a double role, with a negative influence if the interaction with the catalyst occurs via adsorption on the strong sites [43].The analysis of the influence of water content is of great interest, from the catalytic point of view and to evaluate the technical-economic viability of this process. As to the mechanism, it allows to identify the rate determining step. As to the technical approach, the use of aqueous ethanol is preferred in terms of costs since anhydrous ethanol is more expensive due to the required additional dehydration steps. Simple separation processes such as distillation allow reaching a maximum ethanol purity of 95% (v/v), limited by the minimum-boiling ethanol-water azeotrope, requiring expensive technologies (azeotropic distillation with benzene or cyclohexane, distillation combined with adsorption) to fully remove water from ethanol.The results obtained in absence of water (anhydrous ethanol used as reactant) were compared to those introducing 2.5% and 5% (v/v) of water. Main results after 8 h are summarized in Table 3. Results in terms of ethanol conversion seem to be not very conclusive, with some materials for which the water promotes it (an increase in conversion of 44% observed with MgAl (3/1)), and materials with the opposite trend, more evident with HAP and MgFe (decreases of 52% and 60%, respectively). An intermediate situation is observed with MgAl (2/1), with almost constant conversion despite the water content.These results corroborate that water preferentially interacts with the catalysts via dissociative adsorption on the strong sites, producing the deactivation of the materials. This phenomenon has been previously reported by Miller and co-workers [45]. Thus, Lewis strong basic sites (O2-) are converted into weaker Brønsted sites (OH-), modifying the catalytic activity of these materials [6,14,46]. At the same time, Lewis acid sites are blocked by the adsorption of hydroxyl anions. In fact, the activity decreases in those catalysts that have the highest concentration of strong basic sites. The increase in the activity observed with MgAl (3/1) is explained by the lowest dissociation due to the lowest strong basicity (prevailing the molecular adsorption), and the subsequent lower blockage of the acid sites that promote the dehydrogenation.The condensation capacity is also altered, enriching the final mixtures in acetaldehyde (more than 60% in all the cases), observing a total disappearance of C8 compounds and a very significant decrease in C6 and C4 condensed ones. A reduction in the condensation capacity is observed with all the materials. This result is congruent with the adsorption of water on the condensation active sites, preventing the advance of the reaction. Thus, in presence of water, the Guerbet reaction is limited by the condensation step and, even in those cases in which the ethanol dehydration is promoted, there is not a clear advance to the target compounds.Acid sites are also involved in the hydrogenation by MPV mechanism. In good agreement, their blockage explains the almost total absence of alcohols even when feeding only 2.5% of water. This situation affects to all the fractions, observing only traces of butanol (lower than 1% in all the cases), with a total disappearance of C6 and C8 alcohols, even in those cases when the corresponding condensated adducts are still produced in significant amount, such as in the case of both MgAl materials.Water also promotes the production of 1,1-diethoxyethane, except for HAP, with a constant selectivity of 5.6% with and without water. This acetal is obtained by the reaction between an alcohol and an aldehyde molecule, and it has been observed in the literature with selectivities close to 40% in presence of acid materials [14]. With these basic-acid materials, its selectivity reaches a maximum of 14.1% with MgAl (3/1) and 5% of water. In all the cases, this compound represents more than 90% of the total undesired products detected.To sum up, a negative influence of free water is demonstrated, in agreement with the conclusions obtained in gas-phase, even with those catalysts showing reactivation in presence of the small amount produced during the reaction. Thus, the typical percentage of water presents in an azeotropic ethanol inhibits the reaction. Considering the increase in costs, the economic viability requires an improvement in the selectivity towards the target alcohols. The hydrogenation via the MPV mechanism is not enough to promote it, suggesting the use of bifunctional catalysts to activate the hydrogen produced during the dehydrogenation.Improving the dehydrogenation, the excess of acetaldehyde is expected to promote the condensations. Among the transition metals, the dehydrogenation activity of Cu is highlighted in the literature [47]. On the other hand, alcohols are produced by hydrogenation steps. The presence of noble metal nanoparticles could have a positive effect activating the hydrogen produced during the dehydrogenation, enhancing the hydrogenation activity [5]. This section analyzes the activity of different bifunctional catalysts (Cu, Ru, Pd, Pt) using MgAl (2/1) as support. Although most of the C4 obtained with MgAl (2/1) is butanol, the presence of dehydrogenation active metals could enhance the enolization of crotonaldehyde or butanal prevailing over the total hydrogenation of these intermediates. Improving the hydrogenation, not only the alcohols selectivity but also the conversion is expected to increase reducing the relevance of adsorption processes. In all the cases, a theoretical 1 wt% of metal loading is used, to limit the interference on the support properties and to guarantee the appropriate metal dispersion. Main results related to the catalytic characterization are summarized in Table 4 . The specific metal loadings measured by ICP-MS (>0.93) indicate a high similarity between materials, discarding any effect of this parameter in the discussion of their catalytic behavior. In the same way, TEM microscopy (Fig. S6) reveals the presence of metal particles in the range of 4–5 nm, with a very similar dispersion of Pd and Pt (29–30%), being slightly lower in the case of Cu (24%), and a bit higher with Ru (38%). As expected, the presence of metal particles partially modifies the acidity and basicity of the support. Thus, all the materials show a decrease in the acidity (more relevant in the cases of weak and medium sites), as well as the corresponding decrease (with the exception of Ru/MgAl) in the basicity. The slight increase in the strong acidity of some materials (Cu and Ru) is explained by the strong acid character of metal ions, suggesting the coexistence of some cations on the surface, together with the metal particles visualized by TEM. To sum up, the characterization of these materials corroborates a partial alteration in the morphological and chemical properties of the support, justifying the need of working with low amounts of metal (1%) to minimize these effects and guarantee a correct analysis of these effects. Fig. 6 compares the main results, in terms of ethanol conversion and product distribution. A clear improvement in the conversion is observed with Pd (3.4%) and mainly Cu/MgAl, reaching a final value of 6.8% (2.68 g of EtOH converted per g of catalyst and hour). This value is higher than those reported in the literature for systems with similar metal loading, even working with catalyst/reactant mass ratios four times higher [14,21]. The closest value published (2.46 g/g·h) corresponds to a catalyst involving Cu and Ni, two metals for dehydrogenation [48]. This supports the hypothesis of the high relevance of the first dehydrogenation on the global reaction.The conversion is proportional to the weak-strength basic/acid pairs (see Fig. 7a) for all the materials expect for the Cu/MgAl, material with which the conversion is almost double than the expected one. This result indicates that the presence of reduced metals (Pd, Pt, Ru) is not relevant for conversion, only affecting to the product distribution, whereas the well-known dehydrogenation activity of Cu plays a key role in the reaction.All the bifunctional catalysts produce mixtures enriched in acetaldehyde. This suggests that the condensation activity is affected by the metals. However, C6s and C8s reach more relevance, representing 14.9% with the parent support but 25% with Pt. These data, together with the continuous increasing trend observed in their profiles (shown in Fig. S7-10), indicate that the dehydrogenation activity is faster than the consumption by condensation of the acetaldehyde and longer times are expected to produce an enrichment in the heavy fractions. Lateral reactions are also favored by bifunctional catalysts, obtaining a ratio between the sum of esters and ethers and the sum of all the compounds involved in the Guerbet route that increases from 0.06 of the original MgAl (2/1) to 0.18, 0.28, 0.15, and 0.23, with Cu, Ru, Pd, and Pt, respectively.The total selectivity to target compounds (alcohols) decreases from 79.1% (MgAl (2/1)) to 49.7%, 12.3%, 42.6%, and 42.7% (Cu, Ru, Pd, and Pt). These values are justified since the presence of heavier compounds hinders the hydrogenation in absence of reducing atmosphere, using the hydrogen removed during dehydrogenation that is not desorbed from the liquid ethanol. Thus, the MPV hydrogenation mechanism prevails, the total selectivity of alcohols being proportional to medium-strength acidity, as illustrated in Fig. 7b. However, interesting conclusions can be extracted by analyzing the distribution of these alcohols, shown in Fig. 8.Butanol represents almost 100% of the alcohols observed with MgAl (98.7%), whereas the presence of metal nanoparticles produces a significant decrease in this percentage in favor of heavier fractions (in good agreement with the higher conversions). Thus, the alcohol distribution obtained with Cu/MgAl indicates 81.9% of butanol, with 15.7% of C6 alcohols and 2.4% of C8. The same analysis reports 88.5%, 8.3%, and 2.1% of butanol, C6, and C8 alcohols with Ru/MgAl; 77.6%, 18.3% and 4.1%, respectively, with Pd/MgAl; and 69.5%, 22.2%, and 8.3%, with Pt/MgAl. This last catalyst presents a very interesting hydrogenation capacity combined with the highest selectivity for unsaturated compounds. However, these results are not significantly different from those reached with Cu/MgAl, a catalyst that has a conversion 240% higher than Pt/MgAl. In global terms, and considering that Cu/MgAl produces the second higher alcohol total selectivity, this material is chosen as the optimum one among the tested in this screening.The activity of different catalysts in the ethanol liquid-phase condensation reveals a strong influence of acidity and basicity of the materials, global results being limited by the dehydrogenation activity. This limitation is more evident as the size of the molecule that must undergoes dehydrogenation increases, in such a way that dehydrogenation metal phases are required to promote the production of C6s and C8s and their subsequent alcohols. These compounds, not observed in gas-phase, are produced in the condensed one since the hydrogenation rate is significantly slower, promoting successive condensations.Some materials are reactivated by the rehydration of aluminum oxide phases with the water in situ produced. However, the competitive adsorption of water and ethanol on the acid sites produces a decrease in the dehydrogenation activity in presence of small percentages of free water (2.5, 5%), conditioning the complete evolution of the reaction.Cu is identified as the optimum metal observing a synergetic effect with the MgAl (2/1) support. 1% Cu/MgAl (2/1) allows a conversion almost five times higher than the one obtained with the parent material, producing almost 50% of alcohols with a selectivity distribution enriched in C6s and C8s (18.1%). These results involve a significant improvement in this field, supporting the liquid-phase production of heavy alcohols from ethanol with low catalytic loadings. Laura Faba: Methodology, Supervision, Data curation, Writing − original draft. Jennifer Cueto: Investigation, Data curation, Writing − original draft. Mª Ángeles Portillo: Investigation, Resources. Ángel L. Villanueva Perales: Formal analysis, Writing − review & editing. Salvador Ordóñez: Conceptualization, Supervision, Formal analysis, Writing − review & editing. Fernando Vidal: 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 has been carried out in the framework of the Project BIOC4+ (PY18-RE-0040) funded by Junta de Andalucía and European Union (ERDF funds). Authors would like to acknowledge the technical support provided by Servicios Científico-Técnicos de la Universidad de Oviedo.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcata.2022.118783. Supplementary material .

The production of higher alcohols (C4+) via ethanol liquid-phase condensation is studied in this work, screening catalysts with different acid/base properties, observing similarities but also relevant differences with respect to gas-phase reactions in the gas phase. The mechanistic analysis demonstrates the relevance of acidity, mainly to promote the dehydrogenation steps. In the same way, side reactions and hydrogenations have less relevance than in gas-phase, promoting the condensations and, subsequently, obtaining heavy compounds. The highest alcohol selectivity is reached with MgAl (2/1), with more than 79% of C4+ selectivity, but the activity of this material is conditioned by the low conversion obtained. The presence of water reduces the activity because of a competitive adsorption on the catalytic sites whereas the activity increases significantly when using bifunctional catalysts. The best results, obtained with 1% Cu/MgAl (2/1), allow rising the conversion up to more than 460% respect to the parent mixed oxide, with almost 44% of the alcohol mixture enriched in heavy compounds, mainly C6 and C8.

In the past decades, many environmental problems have emerged in rapid succession, and effective measures are urgently needed to deal with the situation. Among them, all kinds of sewage containing various refractory organic matter discharged from production and living cause more severe pollution of surface water and groundwater, which is more prominent in developing countries and regions [1–7]. These emerging pollutants (Eps), such as pharmaceuticals, personal care products, surfactants, endocrine-disrupting chemicals and sterols, are difficult to remove after entering the environmental water body, causing great harm to human production and life. So, it received wide attention and many related studies have been devoted to effectively eliminating such refractory organic pollutants. To date, the removal of Eps has been extensively studied, including adsorption [8], flocculation [9], centrifugation [10], coagulation [11], gravity separation [12], biodegradation treatment [13–15] and advanced oxidation processes (AOPs) [16–22]. Among them, AOPs can generate hydroxyl radicals (•OH, E0 ​= ​1.8–2.7 ​V), sulfate radicals (SO4 •−, E0 ​= ​2.5–3.1 ​V), superoxide radicals (O2 •−, E0 ​= ​0.94 ​V), singlet oxygen (1O2, E0 ​= ​0.65 ​V) and other reactive oxygen species (ROS), which can effectively realize the oxidation and even mineralization of refractory organic pollutants [23–28].Different advanced oxidation technologies use specific oxidants. Common oxidants (such as hydrogen peroxide (H2O2), peroxymonosulfate (PMS), ozone (O3), and persulfate (PS)) can be activated in a variety of ways to generate ROS. Activation methods include ultrasonic activation [29], thermal decomposition activation [30], microwave activation [31], UV photocatalysis [32–34], alkaline activation [35] and transition-metal catalyst activation [36,37]. Among these activation methods, the catalyst can effectively catalyze the reaction by reducing the reaction energy barrier. Especially transition metal activation (metal ions or metal oxides) is the most common and efficient way [38–41]. Many studies have confirmed that many transition metal-based catalysts (Fe, Cu, Co, Mn, Fe3O4, Co3O4, etc.) can effectively catalyze these oxidants for water treatment. Compared with traditional metal oxides, the emerging research hotspot metal sulfide has attracted much attention. It has appeared in many research fields, such as lithium-ion batteries, supercapacitors, oxygen generation reactions, CO2 reduction [42–47], etc. At the same time, many works have been published on the application of metal sulfides in the field of AOPs. Related studies have found that metal sulfide has higher electric conductivity, electrochemical activity, catalytic activity and other excellent physical and chemical characteristics than the corresponding oxides, making metal sulfide an ideal candidate to replace metal oxides [48–50]. However, metal sulfide synthesis, characterization and application still lack systematic summaries. Meanwhile, the catalytic mechanism and rules of different metal sulfides in AOPs also need to be further elucidated to help the future research direction of metal sulfides in treating water pollution in AOPs.The current research mainly focuses on applying metal sulfide materials in AOPs to treat water pollution. The organizational structure of this review article is as follows. After briefly comparing the similarities and differences between metal oxides and metal sulfides, the first section introduces the synthesis and characterization of metal sulfides in detail. Subsequently, the second part summarizes the application of different types of metal sulfide in AOPs (including catalysis, light and electricity). The third part discusses the application of heterojunction nanocomposites made of metal sulfide combined with other materials in AOPs. Section four concludes the reaction mechanism of metal sulfide in different reaction systems through density functional theory (DFT) calculation, and the active root of metal sulfide is revealed. Finally, some constructive suggestions on the feasibility and industrialization development of metal sulfide materials as catalysts for wastewater treatment were put forward. The review will help the advanced oxidation technology of metal sulfide play a more critical role in water pollution control and guide the development direction of this technology in the future.Metal oxide plays a vital role in the field of catalysis. It is widely used as the primary catalyst, cocatalyst and carrier [51]. As the primary catalyst, metal oxide catalysts can be divided into transition metal oxide catalysts and main group metal oxide catalysts. Sulfide is the simplest inorganic sulfur anion, metal sulfide is a sulfur anion combined with metal or semi-metal positive ions to form M x S y compounds, and bimetallic sulfide is A1-x B x S y , where x and y are integers [52,53]. To date, hundreds of metallic sulfide materials have been discovered, many of which have simple structures and a high degree of symmetry. In addition, they have excellent physical and chemical properties. Metal sulfide can be formed by the reaction of sulfur with metal to form binary compounds or by the reaction of hydrogen sulfide (or hydrosulfuric acid) with metal oxides or hydroxides. Metal sulfide is generally colored solid and insoluble in water. Only alkali metal sulfide and (NH4)2S are readily soluble in water, and a few alkali earth metals are slightly soluble in water, such as CaS, SrS, BaS and others soluble in water. In analytical chemistry, various sulfides can be identified and separated according to their solubility differences in water and their corresponding characteristic colors. Alkali earth metal sulfide is a specific solubility, such as N2S, K2S, ZnS, MgS, FeS, MnS are soluble in dilute acid, while PbS, CdS, Sb2S3, SnS, Ag2S, CuS, HgS are insoluble in dilute acid. Recently, researchers have devoted a great deal of energy to studying transition metal sulfide catalysis in many catalytic fields. So far, dozens of metal sulfides and metal sulfide nanocomposites have been reported for various catalytic fields. Fig. 1 shows the key keywords of metal sulfide application in the environmental field in recent years based on the Web of Science database, which represents the focus direction and route of current research. In addition, it is an excellent photocatalyst because the conduction band of metal sulfide catalyst is composed of d and sp orbitals, and the valence band is composed of S 3p orbitals, which is much more negative than O 2p orbitals [54]. The narrow band gap provides an excellent response to the entire solar spectrum compared to the oxide materials [55].It is well acknowledged that natural metal sulfides (e.g., pyrite, mackinawite, chalcocite, chalcopyrite, molybdenite, etc.) were extensively distributed on the earth. However, in targeted applications and academic studies, scholars are more inclined to choose synthetic metal sulfides for decontaminating the aquatic environment due to their high purity, high reactivity, and good dispersion. Compared with natural ores, synthetic metal sulfide is more beneficial for researchers to process and analyze. Currently, various methods have been employed to synthesize metal sulfides, including hydrothermal and solvothermal, template, precipitation, thermal composition, electrochemical, etc., summarized in Table 1 . Hence, the following sections will describe the available synthesis routes in more details.Hydrothermal processes can be defined as any homogeneous or heterogeneous chemical reaction performed under sealed high temperature and pressure conditions using water or organic solvents, in which the reactants can dissolve, and the resulting products are insoluble [56]. The method shows extraordinary potential for the preparation of advanced materials (bulk single crystals, fine particles, and nanoparticles) since the as-synthesized samples possess the characteristics of high purity, excellent dispersibility, good uniformity, and proper crystal shape.Zhang et al. [57] successfully fabricated the Co3S4 nanoparticles in the autoclave and kept the condition at 180 ​°C for 24 ​h. Similarly, the microwave hydrothermal method employing polyvinylpyrrolidone (PVP) as the surfactant was used to prepare doughnut-like CuS particles [58]. This strategy also applies to the preparation of other sulfides, such as CuFeS2, MoS2, NiCo2S4, and CuCo2S4 [45,59]. Meanwhile, the developments of multiple combination processing of catalysts, such as mechanochemical-hydrothermal, electrochemical-hydrothermal, sonar-hydrothermal, etc., have contributed significantly to the synthesis of many high-activity catalysts [60–62].The template method refers to the synthesis of nanoparticles by using mesoporous matrix materials as templates, which is the most widespread and successful strategy for developing advanced materials [63]. For example, Kim et al. [64] recently prepared the bulk MoS2 using an anodic aluminum oxide template with a hole size of 80 ​nm and neck width of 10 ​nm. Tang et al. [65] grew NiS2 on carbon cloth, which was used as an efficient 3D hydrogen evolution cathode in neutral solutions. Similarly, the soft template method is also deemed an effective strategy, which has been applied to fabricating various metal sulfides. Jiang et al. [66] synthesized the CuS hollow spheres through a facile microemulsion template route at room temperature using copper naphthenate as the metal precursor and thioacetamide as the source of S2−.However, hard and soft templates have many disadvantages, such as cumbersome preparation procedures, difficulty in eradicating the template, tedious preparation time and high cost, which limit the large-scale applications of samples to a certain extent [67]. Therefore, the self-assembly and template-free methods attracted much attention. For example, as shown in Fig. 2 (a), Tu et al. [68] reported that the hierarchically ZnIn2S4 nanosheet-constructed microwire arrays could be received via the template-free strategy. The samples were constructed by vertical nanosheets with about 1–5 ​μm diameters and more extensive than 10 ​μm in average length, which preferentially exposed (006) facets. In addition, Yu and co-workers [69] demonstrated a facile biomolecule-assisted one-pot route toward fabricating novel CdS/MoS2/graphene hollow spheres, regarded as the high-efficiency and low-cost photocatalysts for hydrogen evolution, owing to the unique hollow-shaped structure and enhanced charge separation ability. At the same time, as a new kind of porous crystal material, metal–organic ​frameworks (MOFs) are also widely used as precursors for synthesizing metal sulfide. Wu et al. [70] first synthesized the cobalt-containing MOFs precursor ZIF-67 by a simple agitation method and then successfully converted the MOFs precursor into a hollow amorphous CoS x hexagonal cage in situ by adding thioacetamide. The authors further utilized this strategy to synthesize bimetallic MOFs precursors by substituting sectional Co(II) for M(II) (M ​= ​Mn, Ni, Cu, Zn) [71]. Hollow bimetallic sulfide polyhedral were prepared by hydrothermal sulfurization in TAA solution.The mixture of metal salts should be continuously heated under certain conditions during the thermal decomposition process to obtain the target products. Deng et al. [72] prepared the CoS2/CC (commercial carbon cloth) nanoparticles by thermal composition method, in which the precursor Co3O4/CC and sulfur powder were placed in the backward position and upstream side of the porcelain boat, respectively. The temperature of the tube furnace was quickly elevated to 450 ​°C in 30 ​min and kept for 120 ​min in N2 atmosphere and the as-synthesized catalyst functioned as a 3D flexible electrode for water oxidation. Davar et al. [73] also fabricated CuS nanoparticles via a facile and low-temperature thermal decomposition method. In addition, as shown in Fig. 2(b), Vu and his colleagues [74] successfully prepared NiS by utilizing Ni(NO3)2 and thioacetamide as the precursors, and the materials were incorporated into an electrochemical sensor.The precipitation method has also been extensively used to synthesize solid catalysts. Both ion salt and precipitating agent are necessary for the precipitation, and the formed precipitate should be washed, dried and calcined under specific conditions. In the precipitation method, a series of factors will impact the properties of the precipitation products, such as the types of precipitating agent and their concentrations, precipitation temperature, precipitation pH, stirring speed and feeding order, so the operational conditions are essential to be optimized [75]. Kumar et al. [76] reported a route for the preparation of CdS nanoparticles with the precipitation method using the equimolar (0.1 ​M, 20 ​mL) solutions of cadmium acetate (Cd(OAc)2) and Na2S as precursors. Rafiq et al. [77] used 2-mercaptoethanol as a capping agent, homogenous solutions of 0.1 ​M Zn(NO3)·4H2O and Na2S·5H2O, to prepare ZnS quantum dots. They found that the synthesized ZnS quantum dots have excellent potential in dye degradation. The precipitation method attracted researchers due to some advantages, such as mild reaction conditions and simple reaction control.In this process, the powder particles are subjected to severe mechanical deformation by the collision with the ball in the stainless-steel container and constantly broken, cold welding and fracture, resulting in the solid-state reaction and mechanical chemical reaction in the powder blend. This low-cost, easily scalable mechanical-chemical route has prepared uniform 5 ​nm-sized CuS quantum dots (QDs) [78]. as shown in Fig. 2(c), CuS quantum dots have a high surface area with dislocation and planar defects, such as twinning and stacking defects, which are conducive to electrocatalytic and photocatalytic performance. In addition, several nickel sulfide powders, such as Ni3S2, Ni7S6, Ni x S6 and Ni3S4, were prepared by ball milling with NiS powder as raw material [79]. Ambrosi et al. [80] prepared a highly active MoS2 electrocatalyst using thin MoS2 slices. Ball milling improves the electrochemical and electrocatalytic properties of MoS2. In addition, CuCrS2 and NiCr2S4 were synthesized by mechanical alloying using ball milling technology [81], and the materials were sintered and treated to enhance electrical conductivity.The electrochemical method can electrodeposit the materials on the electrode surface by providing a constant voltage or current to the charged particles to induce directional movement of charged particles. The transfer of electrons usually accompanies the process of preparing materials, and the cost should be considered. Zhao et al. [62] reported that the ternary mixed metal Ni–Co–Fe sulfides based on three-dimensional (3D) nickel foam (NiCoFeS/NF) could be fabricated by a facile electrodeposition-solvothermal process, and the metal sulfides were used for efficient electrocatalytic water oxidation in alkaline media. Wang et al. [82] demonstrated a single-step potentiostatic method for the electrodeposition of Cu2S nanoparticles onto fluorine-doped tin oxide electrodes from CuCl2 and thiourea aqueous solution to develop counter electrodes for quantum-dot-sensitized solar cells.Microwave is a kind of electromagnetic wave, which, like high-frequency electromagnetic waves, is generated by the periodic change of electric and magnetic field energy in the electromagnetic oscillation circuit. Microwave radiation usually refers to an electromagnetic wave with a frequency of 300–300,000 ​MHz and a wavelength of less than 1 ​m. According to their wavelength, microwaves can be divided into decimeter waves, centimeter waves and millimeter waves. Microwaves are generated by magnetrons. Microwave radiation is much weaker than infrared radiation and needs to be processed before it can be received using a receiver. Microwave heating drivers can only accept limited 2.45 ​GHz frequencies. Through microwave radiation heating, the microwave generates regular interactions with the material to convert electromagnetic energy into heat energy. Heat is generated from within the material, as opposed to traditional heating methods, which transport heat from the outside to the inside. This internal heating shortens reaction times and saves energy. Therefore, compared with traditional methods, microwave irradiation is faster and more efficient. Chen et al. [83] prepared CdS, ZnS, CoS, PbS, CuS, Bi2S3, Sb2S3, Ag2S and other metal sulfides by microwave method. Glycol was used as the solvent. Finally, various metal sulfides were successfully prepared.The wet chemical method has also been applied to fabricate metal sulfides. Chen et al. [84] fabricated the ZnS rods via the wet chemical method under reflux conditions. They used Zinc acetate and thioacetamide as raw materials, and thioglycolic acid acted as a capping agent. Similarly, microwave irradiation is employed to prepare metal sulfides, providing a homogeneous heating process for rapidly synthesizing nanocrystals with controllable size and shape [67]. CuS/Graphene composite was obtained under microwave irritation, Cu(NO3)2 and Na2S2O3 were used as the copper source and the sulfur source, respectively [85]. The cation exchange method may also be an excellent choice, and the technique was used by Zhu et al. [86], who prepared (Ag, Cu)2S hollow spheres with a diameter of 700 ​nm to 1 ​m with the molar ratio of CuS to Ag+ 2:1 ​at room temperature.It is known that the different processes will result in various products with different shapes and catalytic performances. Table 2 displays the main differences among other preparation methods, all the preparation methods have certain limitations, and the structure of the catalyst highly depends on the preparation method, which is an essential factor that influences the catalytic activity of the established catalytic oxidation system.In general, even if the as-synthesized materials are the same, there will still have some differences in morphology, structure and catalytic activity due to different preparation conditions. Herein, we take CuS as an example to compare the differences in as-synthesized samples prepared under different conditions (seen in Table 3 ). As expected, the preparation conditions significantly impact the morphological structure of the materials, which may further affect the catalytic performance and reusability of the materials. Therefore, it is essential to utilize effective characterization techniques to help us better understand the basic structure and morphology of the catalysts and further clarify the mechanisms of the catalyst-induced decontamination reaction.The surface morphology, specific functional groups, chemical compositions, thermal properties and crystal phase of as-synthesized samples will directly or indirectly influence the catalytic performance of catalysts. The frequently used characterization methods for diverse metal sulfides are summarized in Table 4 . X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Brunauer-Emmett-Teller specific surface area (BET), Fourier transformed infrared spectroscopy (FTIR), photoluminescence (PL) spectra and thermogravimetric analysis (TGA) are the commonly used methods for identifying the chemical compositions, analyzing the phase structure, measuring the specific surface area and pore size, clarifying surface functional groups, determining the surface vacancies and transfer, as well as recombination of photogenerated electron-hole pairs and revealing the thermal properties, respectively. Thus, the following sections will focus on the six widely used characterization technologies to further understand the correlation between catalyst characteristics and catalytic performances.X-ray of specific energy is used as the excitation source to irradiate the surface of materials and excites photoelectrons, and then uses electron energy analyzer to detect photoelectrons according to different energy distributions. The composition and oxidation state can be obtained according to the deconvolution of the peak position and intensity of each element, and the relative content of elements can be determined. According to different studies, the peak positions of each bonding configuration can vary in a certain range, possibly owing to their various preparation environments [36,87–90]. From XPS analysis, the mechanism of catalyst activity and the reasons for its inactivation and poisoning can be acquired. However, XPS is not a destructive tool for the qualitative and quantitative study of the chemical state and composition of the elements, which exist on the thin surface of the samples (up to 5 ​nm) [67].XRD is one of the most powerful tools for accurately revealing the phase composition, grain size and crystal structure of catalysts. The diffraction pattern of the powder is usually obtained by Debye Scherrer and Guinier methods. The material composition can be qualitatively examined by comparing the position of the diffraction peak with the powder diffraction file. The grain size can be determined by fitting the peak widths based on the Debye-Scherrer equation, and the crystallinity can be received according to the diffraction peak shape and area. Besides, the Scherrer equation calculates the average size of the fresh and used catalysts. Thus, it is favorable to understand the reason for deactivation better. The phase structure of metal sulfides (single, double and ternary components) can be identified by XRD tests [91–95].It is well known that most heterogeneous catalysts are porous materials and their pore structure, size and pore volume highly depend on the catalyst preparation methods. The specific surface area represents the total surface area per gram of catalyst (m2/g), which is an essential parameter for evaluating catalytic properties. The gas adsorption Brunauer-Emmett-Teller (BET) methods are one of the most commonly used. It is a multi-molecular layer adsorption formula based on the classical statistical theory of BET. The specific surface area is related to particle size, shape, surface defects and pore structure, which affects the chemical and physical properties of the as-synthesized materials.The specific surface area is greatly affected by different preparation conditions. Generally, smaller particle sizes result in larger surface areas and enhance the surface adsorption ability and catalytic performance of as-prepared samples. Zhang et al. [57] found that the hydrothermal-synthesized nanosheets possessed a higher surface area (62.8 ​m2/g) than that of the Co3S4 nanoparticles (32.1 ​m2/g), which may be favorable for the improved H2 evolution on CdS/Co3S4. Chen et al. [84] revealed that the BET value of the ZnS increased as the thioglycolic acid content elevated. Furthermore, commercial mackinawite has been proven to degrade p-chloroaniline efficiently [87].Infrared spectroscopy is a common means to identify the molecular structure and determine the surface properties of catalysts. Quantitative analysis can also be carried out for individual components or mixtures of various components, especially for samples that are difficult to separate and cannot find significant characteristic peaks in the ultraviolet and visible regions. In addition, the structure of the unknown substance can be concluded according to the position and shape of the absorption peak in the spectrum. The intensity of the absorption peak can identify the content of each component for composites. Similarly, FTIR is beneficial for the investigation of catalytic mechanisms. For example, by ATR-FTIR analysis, Zhou et al. [84] studied the role of sulfur conversion in sulfate radical generation of PMS/FeS2 system. Meanwhile, Zhu et al. [96] found the graphene-supported hollow cobalt sulfide nanocrystals for PMS activation were highly efficient for bisphenol A elimination from the FTIR study.PL spectra have been extensively employed as a proven technique for studying the surface vacancies, transfer, and recombination of photogenerated electron-hole pairs of composites [97]. Ayodhya et al. [98] revealed that the synergistic effect of CuS and rGO would significantly restrain the recombination of hole-electron pairs and thus result in an enhanced separation of a photogenerated carrier by PL spectra analysis. Besides, the different emission phenomena from the PL spectra may also ascribe to the different morphology, sizes, and crystalline. Likewise, by comparing the PL spectra of CdS and CdS/rGO nanocomposite, Sagadevan et al. [99] found the immobilization of CdS nanopowders on the rGO sheets decreased the PL intensity, suggesting that the efficient charge separation process took place inside the composite matrix.Thermal analysis is defined as the technique for revealing the physical properties of a substance or its reaction product, which are measured as a function of temperature [100,101]. Differential scanning calorimetry (DSC), thermogravimetry (TG/DTG) and differential thermal analysis (DTA) are the most commonly used thermal analysis techniques, which have been successfully performed to investigate the thermal behaviors of heterogeneous catalysts, including the interaction between metal active components and carriers, the coordination state and distribution of the active metal ions, the deactivation mechanism of catalyst, the phase transitions, as well as the thermally induced chemical reactions and decompositions [43,50,102]. The thermal stability and structural changes of the prepared CuS and rGO-capped CuS composite were estimated through TGA in the range of temperature 30°C-1000 ​°C [98]. In addition, the thermal analysis method was successfully applied to choose the ideal calcination temperature of urchins-like cadmium zinc sulfide nanostructured particles [103].Electron microscopy technology can provide information about the interaction between electrons and target materials. Microscopic information about the catalysts can be obtained after data conversion, amplification, and other processing. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are frequently used techniques for comparative analysis of morphological changes of fresh and used catalysts, and the energy dispersive spectroscopy (EDS) is used to explore the change of element content of catalyst [102,104–110]. Besides, the atomic force microscopy (AFM) technique is regarded as a powerful tool that can also determine morphology (e.g., size, surface texture, and roughness) and electrical properties of as-prepared catalysts with high spatial resolution, thus becoming more and more widely utilized in material characterization.Moreover, Raman spectroscopy is a non-destructive method requiring no special sample preparation and nondestructive analysis and presenting analytical advantages such as speed, cost, sensitivity, repeatability and stability of analyses, which plays a vital role in the structural characterization of metal sulfides, mainly providing valuable information about phase composition, crystal symmetry and orientation, quality of the crystal and the total amount of the substance. Furthermore, compared with infrared absorption spectroscopy, the recording of Raman spectroscopy does not require special sample preparation and nondestructive analysis, the fiber optic probe brings the excitation laser into the sample and transmits the scattered light to the spectrometer, enabling remote detection of the Raman signal [111]. Xing et al. [44] confirmed the reduction capability of the exposed Mo4+ by analyzing the Raman spectra of MoS2 and Fe3+.Temperature-programmed desorption (TPD) involves heating a sample at raising the temperature and followed by detecting the released gas using a suitable device [112]. Quantitative analysis of TPD is widely employed to clarify the desorption strength of heterogeneous catalysts, particularly for the supported and multi-component metal catalysts, by performing experiments at different heating rates and then accompanied by theoretical treatment processing. For example, O2-TPD was introduced to investigate the effect of substitution-induced oxygen-ion vacancies, and H2-TPD was utilized to study active metal surface area on supported particles [113]. In addition, electrochemical analysis is occasionally used due to the excellent conductivity of metal sulfides. Cyclic voltammetry scan and electrochemical impedance spectra (EIS), as well as Tafel plot, are the most commonly used methods [43,45,47,49,74,89,114]. Understanding the electrochemical corrosion characteristics and electron and charge transfer ability of samples is helpful. For example, the excellent electrical conductivity of iron sulfides has been unveiled by the CV cycle profile and the typical Nyquist plots [115].Chiu et al. [116] investigated the heterogeneous interface's lattice, atomic structure and composition by using aberration-corrected scanning transmission electron microscopy (AC-STEM) at the interplanar junction of WS2/WSe2 and WSe2/MoS2. AC-STEM observations of heterojunction surfaces indicate that the in-plane heteroepitaxy growth is initiated by replacing sulfur atoms at the edge of the pre-grown transition metal dichalcogenides. Pan et al. [117] used synchrotron X-ray absorption near edge spectroscopy (XANES) to elucidate the electronic structure changes of CuCo2S4 NSs at unique core/shell electrodes. Furthermore, the corresponding Fourier transform (FT) k3χ(k) function of the extended X-ray absorption fine structure (EXAFS) spectroscopy further confirmed the successful vulcanization of CuCo2O4 NPs into CuCo2S4 nanosheets. Through atomic-resolution scanning tunneling microscope (STM) simulation images with energy wave function integration below 0 ​eV and 1.5 ​eV, Liu et al. [118] observed that two simulation images of single-layer MoS2 with 2S, 3S, and 4S vacancy chains showed almost the same black dot characteristics around the defects because any atoms do not occupy the defect.In addition, in situ characterization has become an indispensable characterization method in catalytic reactions, which not only helps us to further explore the catalytic mechanism but also makes outstanding contributions to the further design of various effective catalysts. In order to reveal the synergistic effect of 1T and 2H phases in MoS2 in the photo Fenton oxidation process, Chen et al. [119] detected the change of Mo chemical state under light irradiation by using solid EPR in operando. The EPR technique within operando has the potential to recognize the presence of Mo(III) instantaneously. By in-situ EPR, we demonstrate that the optimized 2H/1T heterojunction allows interfacial electron transport from the semiconductor 2H to the metal 1T phase and synchronizes partial reduction Mo(IV) to Mo(III) at the interface.Up to now, the catalytic mechanisms involved in metal sulfide-based AOPs have been exclusively revealed as the radical and non-radical pathways. Previous findings have suggested that metal sulfides could function as an effective activator for organic pollutants elimination in water and wastewater. According to the previous findings, there are two general mechanisms commonly proposed by metal sulfide-based AOPs. One is the improvement of the electron transfer efficiency resulting from the reductive S2− on the catalyst surface [45,46,87,120]; the one is the unsaturated S atoms on the surface of the metal sulfide can capture protons resulting in the formation of H2S and exposing metal active sites with reducing properties, thus accelerating the rate-limiting reaction step [44,91]. However, there are still some differences in various metal sulfide-based oxidation systems. As shown in Table 5 , some reported metal sulfide-AOPs are summarized. Therefore, a deep understanding of the catalytic performance and mechanisms of the target AOPs is beneficial to developing a more robust system for organic pollutants removal.In the following part, we will discuss the catalytic performance and the mechanism for mono-metal metal sulfides-based catalytic oxidation systems, such as Fe x S y , CuS, Co x S y , and MoS2 to reveal the superiority of the metal sulfides-based system.Pyrite (FeS2) is an abundant sulfide mineral containing structural Fe2+ and has been regarded as an electron donor for the removal of pollutants and an activator of either oxygen or peroxides. Feng et al. [121] studied the catalytic oxidation of 1,4-dioxane in FeS2/PMS system. They found that nearly 100% degradation of 1,4-dioxane (50 ​mg/L) was obtained after 40 ​min, and the removal efficiency was higher than the FeS2/PDS and FeS2/H2O2. They systematically studied the role of disulfides and activation sites and proposed the possible reaction mechanism of FeS2/PMS system (Fig. 3 (a)). The reaction mechanisms include the following aspects: 1) the direct activation of PMS by FeS2 can be ignored; 2) a synergistic effect was observed, which was attributed to the controlled activation of PMS by Fe2+ generated by the reaction of FeS2 with Fe3+; 3) electron transfer from S2 2− on pyrite surfaces to Fe3+ on or near the surfaces of pyrite; 4) PMS reacted with the S2 2− on the surface of pyrite to release Fe2+, which then activated PMS rapidly to generate radicals; 5) •OH and SO4 •− were involved in the pyrite-PMS system and •OH were the main active species. Thus, the combination of FeS2 and PMS could effectively remove target pollutants.FeS/PS system was also used to eliminate 2,4-dichlorophenoxyacetic acid (2,4-D) [122]. The authors found that 2,4-D could be efficiently removed and mineralized by FeS/PS system. The quenching experiments implied that both •OH and SO4 •− were responsible for 2,4-D degradation, but •OH was the dominant active species. The quenching tests and EPR analysis have proposed the possible mechanism of 2,4-D oxidation in FeS/PS. PS would decompose to yield SO4 •− by both Fe(II) and Fe2+, then the formed SO4 •− in the aqueous solution or on the surface of FeS could directly react with 2,4-D to generate •OH via the oxidation of H2O (or •OH). Subsequently, the hydrolysis of S2O8 would generate H2O2 and further lead to the Fenton reaction in the presence of Fe(II) (or Fe2+), then the generated SO4 •− and •OH would result in efficient degradation of 2,4-D.The formation of Fe sulfides usually occurs in anoxic deposits, where sulfides are produced by the metabolic activity of sulfate-reducing bacteria [120]. Yuan et al. [123] studied the degradation performance of p-chloroaniline (PCA) by persulfate activated with ferrous sulfide ore particles, FeS was proposed as an alternative electron donor, which could act as a continuous-releasing source of dissolved Fe2+ and Fe(II), leading to the formation of SO4 •− and then initiate a series of radical chain reactions. However, Fan and his co-authors [87] found that the FeS/PS system displayed excellent potential for PCA degradation and mineralization across an extensive initial pH range (3.0–11.0). The results were ascribed to the independent oxidations of Fe(II) and S(-II) and the regeneration of Fe(II) induced by S(-II) at the FeS surface, the clarified surface reaction catalytic mechanisms of FeS/PS system were different from Yuan et al. [123], because •OHfree, SO4 •− free diffusing from the FeS surface mainly contributed to PCA elimination.Pirgalıoğlu et al. [124] investigated the CuS/O3 system for treating aqueous single-dye solutions. They found that CuS increased the oxidation rates of ozonation side-products via a decrease in TOC values of the treated dye solution, and they discussed the catalytic ozonation kinetics and mechanism in detail in three parts: 1) the measurement of copper ions present in the liquid phase; 2) the homogeneous catalytic effect of liquid phase copper ions on ozonation efficiency; 3) the effect of CuS on the enhancement of ozone decomposition to generate more hydroxyl radicals. Nekoueia et al. [125] demonstrated that the synthesized CuS hollow nanospheres@N-doped CNCs hybrid composites had the outstanding potential for enhanced adsorptive removal and catalytic oxidation of ciprofloxacin (CIP) when PMS was present, and the mechanism of PMS activation was as follows: firstly, after PMS was added into the catalytic system, H2O molecules were adsorbed on the Cu(II) part of the as-prepared catalyst to produce ≡Cu(II)–OH according to Eq. (1), and then Cu(II) reacted with HSO5 − to produce •OH. Furthermore, the ≡Cu(II)–OH reacted with HSO5 − resulting in ≡Cu(II)−(OH)OSO3 − generation and further decomposed to SO4 •− via Eqs. ((2)−(3)). (1) ≡Cu(II)− –OH ​+ ​HSO5 −→ ≡Cu(III)+ SO4 2− ​+ ​•OH (2) ≡Cu(II)− –OH ​+ ​HSO5 − → ≡Cu(II)−(OH)OSO3 − ​+ ​OH− (3) ≡Cu(II)− (OH)OSO3 − → ≡Cu(III) ​− ​–OH ​+ ​SO4 •− Similarly, the CuS mineral was selected as an activator to persulfate (PS) for eliminating atrazine (ATZ), and 91.6% ATZ removal was obtained in 40 ​min under acidic pH [126]. They proposed the one-eight-lives method to evaluate the kinetic of CuS/PS system for ATZ degradation, which the following equation could conclude: −d[ATZ]/dt = (2.985 ​× ​10−4 mmol/(L·min)) [ATZ]0.023[PS]0·76318[CuS]0.80801. Based on the control experiments, they found that the CuS/PS system was superior to other systems (Cu2+/PS and CuO/PS system). Furthermore, the possible reaction mechanism of CuS/PS system was proposed as follows according to the radical scavenger tests and EPR study. Specifically, PS was first absorbed on the surface of CuS. Then PS was activated, followed by the generation of main ROS (SO4 •− and •OH) for ATZ removal. Simultaneously, the PS would induce the oxidation of CuS to produce Cu2+ and S(-II), S(-II) could not promote the degradation of ATZ, which was further oxidized to the different intermediate valence of sulfur species. Besides, the leached Cu2+ could also activate PS to produce SO4 •− and •OH in the liquid phase, but the role of the homogeneous process was insignificant.It was reported that the fabricated graphene nanosheet-supported hollow cobalt sulfide nanocrystals (Co3S4@GN-X and CoS@GN-X, X in accordance to the GO weight) via a template method (zeolitic imidazolate frameworks) were able to activate PMS to degrade bisphenol A (BPA) [96]. The mechanisms for enhanced BPA degradation efficiency are shown in Fig. 3(b). Firstly, PMS was activated by CoS to produce abundant SO4 •−. Secondly, for the pristine CoS with poor removal efficiencies and electron transfer, the formed SO4 •− was inclined to accumulate and then diffuse out of the catalyst surface after reaching saturation, as well as be converted into •OH by H2O/OH−. For CoS@GN-60, the existence of graphene provided adsorption territories and high-speed electron flow for BPA elimination. In contrast to •OH, SO4 •− was more likely to degrade BPA molecules through the charge-transfer mechanism. Therefore, once SO4 •− was generated, it would bound onto the catalyst by electrostatic force and then quickly capture electrons from the absorbed BPA across graphene since it functioned as the conductor. Thus, no excessive SO4 •− was released into the bulk solution to combine with DMPO when conducting EPR tests. Besides, the •OH generated by the side reaction of SO4 •− could combine with H2O/OH− of the bulk solution, which restrained SO4 •− transfer. Consequently, the combination of sulfides and the suitable carrier is more conducive to the degradation of target contaminants. In summary, sulfides combined with the excellent performance of the carrier are more conducive to removing target pollutants. In addition, another study also found a key role in hollow structures. Wu et al. [70] studied the effect of the hollow structure of the catalyst. A new hollow amorphous CoS x hexagonal cage catalyst was prepared by an aqueous solution-assisted solvothermal method, which activated PMS effectively to degrade antibiotics through advanced oxidation processes. The results show that the prepared hollow amorphous CoS x cage exhibits excellent tetracycline (TC) decomposition ability under PMS activation, which is far superior to the conventional Fenton reaction of solid Co3O4 and CoS. The excellent catalytic performance of PMS activated CoS x is due to the Co3+/Co2+ and S2−/S2 2− cycles and the existence of hollow structures.Ridruejo et al. [127] demonstrated that the complete removal of acid tetracaine could be achieved by electro-oxidation and photoelectron-Fenton processes with a boron-doped diamond anode at 100 ​mA/cm2. The results implied the viability of the manufactured CoS2-based cathode was highly suitable for water treatment since the use of an air-diffusion cathode containing CoS2 nanoparticles could enhance the electro-generation of H2O2. Moreover, Yin et al. [128] successfully established a strategy to prepare the quasi-single cobalt sites in the nanosized pores of SBA-15 (QS-CoS). Their findings suggested that the QS-CoS catalysts were highly efficient for PMS activation due to the confined space, and abundant silicon hydroxyl groups in the as-synthesized SBA-15 contributed to the generation of the resultant quasi-single cobalt sites in the form of Co–O–Si. Meanwhile, both SO4 •− and •OH were produced in QS-CoS/PMS system, SO4 •− was the dominant one responsible for phenol degradation. The possible mechanisms could be described as follows according to Eqs. ((4)−(7)). (4) Co2+–O–Si ​+ ​HSO5 − → Co3+–O–Si ​+ ​SO4 ˙ −  ​+ ​OH− (5) Co3+–O–Si ​+ ​HSO5 − → Co2+–O–Si ​+ ​SO5 ˙ −  ​+ ​H+ (6) SO4 •−  ​+ ​H2O → SO4 2− ​+ ​•OH ​+ ​H+ (7) SO4 •−  ​+ ​•OH ​+ ​C6H5OH → several steps → CO2 +H2O ​+ ​SO4 2− Xing and co-workers [44] revealed that commercial 2H-type MoS2 could function as an excellent co-catalyst to significantly enhance the decomposition efficiency of H2O2 by 47.2% and significantly decrease the consumption of H2O2 (0.4 ​mmol/L) and Fe2+ (0.07 ​mmol/L) in AOPs . The mechanism of the catalytic reactions used MoS2 as the co-catalyst is shown in Fig. 3(c). At first, the protons of the solution were captured by the unsaturated S atoms on the surface of MoS2 and led to the formation of H2S, which was clearly identified by EPR test, because the MoS2 powder showed a signal at g ​= ​2.0, implying the existence of sulfur vacancies. Subsequently, the surface Mo4+ was oxidized to Mo6+ as well as followed by the reduction of Fe3+ to Fe2+ (Eq. (8)), which significantly enhanced the original rate-limiting of Fe3+/Fe2+ conversion in conventional AOPs (Eq. (9)). In the Fenton reaction, Mo6+ was further reduced to Mo4+ with the assistance of H2O2 to ensure the catalytic cycling of MoS2 according to Eq. (10). (8) Fe3+ ​+ ​Mo4+ → Fe2+ ​+ ​Mo6+ (9) Fe3+ ​+ ​H2O2 → Fe3+ ​+ ​•O2H ​+ ​H+ (10) Mo6+ ​+ ​H2O2 → Mo4+ ​+ ​H2O ​+ ​O2 Furthermore, they pointed out that the efficiency of the AOPs involving metal sulfide co-catalysts could be further improved by visible light illumination due to the light-induced sensitization of selected pollutants. Remarkably, the reaction rate constant increased by 18.5 times compared with conventional AOPs, when MoS2 and rhodamine B were used as co-catalyst and target pollutants, respectively. In addition, they studied the co-catalytic performance of all the chosen sulfides (MoS2, WS2, Cr2S3, CoS2, PbS and ZnS) and the efficiency of reduction of Fe3+ to Fe2+ was as the following order: WS2 ​> ​CoS2 ​> ​ZnS ​> ​MoS2 ​> ​PbS ​> ​Cr2S3 ​> ​conventional Fenton. Moreover, the MoS2 has excellent stability and reusability even after ten cycles, and the newly developed H2O2-based AOPs not only shows outstanding potential for TOC degradation (90%) but also significantly decreased the COD value of actual wastewater from 10,400 ​mg/L to 360 ​mg/L at a record-low dosage of H2O2 and Fe2+. The above results indicated that the co-catalytic effect of the metal sulfides was universal, and it would make significant progress in the practical application of transition metals involved AOPs for environmental remediation. Similar results were also observed by Dong et al. [91]. They used the co-catalytic WS2 on the Fenton reaction to improve the decomposition of H2O2 for the reduction of Cr(VI) and remediation of Phenol synchronously, which further implies the general applicability of metal sulfides to the degradation of organic pollutants.Currently, metal sulfide catalysts studied in AOPs are usually nanomaterials, and it has been reported that the differences in the microstructure of metal sulfide greatly affect its catalytic performance. Zhu et al. [129] investigated the degradation of aromatic organic compounds by 3D-MoS2 sponge loaded with MoS2 nanospheres and graphene oxide (GO) in AOPs. Exposure of Mo4+ active sites on 3D-MoS2 can significantly improve the concentration and stability of Fe2+ in AOPs, so that Fe3+/Fe2+ is in a stable dynamic cycle, thus effectively promoting the activation of H2O2/PMS. More importantly, the authors found that 2D-MoS2 solids in powder form are difficult to recover and reuse after degradation, which may increase costs and cause secondary pollution to the environment. In addition, this shortcoming may prevent the widespread application of MoS2 in industrialization. However, 3D materials have a larger specific surface area and hierarchical pore structures, which is more conducive to the adsorption of organic pollutants and electron transfer in the advanced oxidation process. Compared with traditional 2D-MoS2, 3D-MoS2 sponge is more conducive to industrial production, and the feasibility of its industrial application is confirmed by pilot experiments.CdS is regarded as the classic IIeVI semiconductor, whose direct band gap at room temperature is 2.43 ​eV, which is regarded as the best semiconductor, and researchers have paid considerable attention to its potential application [130,131]. Yang and co-workers [132] developed an efficient visible-driven photo-Fenton system based on self-assembled CdS nanorods. The CdS/Fe2+ photo-Fenton system has highly degraded sulfamethazine (SMT) under visible light irradiation. The SMT of 20 ​mg/L is almost completely degraded within 90 ​min, and the pH range is 4–8. The large number of photoelectrons produced in the system can reduce the dissolved O2 to H2O2 by direct two-electron reduction pathways and accelerate the conversion of Fe3+ to Fe2+.ZnS, as an economical and efficient environmental protection material, administered due to its under ultraviolet (UV) light, has good catalytic activity and high-efficiency theory, which can rapidly generate electron-hole pairs (e−−h+) under photoexcitation, so it has been widely researched [133,134]. However, it has a wide and direct band gap (3.80 ​eV for wurtzite), a crucial factor inhibiting its visible light response. Tie et al. [135] synthesized the novel nitrogen-doped ZnS microspheres by a simple one-step method. The prepared nitrogen-doped ZnS catalyst showed excellent photocatalytic activity for removing organic pollutants under natural sunlight and producing hydrogen by water cracking. The prepared catalyst showed good photodegradation performance against many organic pollutants such as methyl orange, methylene blue, Rhodamine B, ciprofloxacin and sulfa. Nitrogen doping improves the visible light absorption capacity and electron transfer efficiency of ZnS, thus improving the photocatalytic performance of the catalyst.Bismuth sulfide has attracted extensive attention recently due to its high photocatalytic activity in degrading organic pollutants. Among these bismuth sulfides, Bi2S3 has been extensively reported in the field of photocatalysis due to its unique optical properties and band gap (1.3 ​eV) [136,137]. In recent years, the photocatalytic activity of Bi2S3-based NSs has attracted the attention of many researchers [138]. As shown in Fig. 3(d), Gao et al. [139] constructed an S-scheme heterostructure photocatalyst MoS2/Bi2S3/BiVO4 supported on 3D lignosulfonate modified poly(vinyl formal) sponges to enhance the synergic adsorption and photo-Fenton degradation of various fluoroquinolones in water. The synergistic adsorption of the carrier and the catalytic action of the photocatalyst significantly improve the applicable pH of the Fenton reaction (2.0–9.0) and reduce the dosage of Fe2+ to 0.014 ​mmol/L. Photogenerated electrons and the redox conversion of Mo(IV)/Mo(VI) and Bi(III)/Bi(V) accelerate the conversion of Fe3+ to Fe2+.Relative to the extensively investigated one-component metal sulfides, bimetallic sulfides have been demonstrated to be more remarkable owing to the excellent electrical conductivity and thermal stability as well as the synergistic effects of metal ions, which have been widely used as electrocatalysts for supercapacitor and OER [47,49,93,114]. However, only a few studies have been done on applying bimetallic sulfides in AOPs-based wastewater treatment. In the following part, we take sulfide carrollite (CuCo2S4) and chalcopyrite (CuFeS2) as examples to discuss the catalytic performance and the mechanisms in the catalytic oxidation system.As shown in Fig. 4 (a), The spinel sulfide carrollite (CuCo2S4) was introduced to activate PMS for the elimination of bisphenol S (BPS) in water [46]. It was verified that the catalytic activity and stability of hydrothermally fabricated CuCo2S4 during the oxidation process were superior to those of commonly used cobalt and copper oxides/sulfides. They also found that the neutral pH condition was most favorable for the degradation of BPS. Meanwhile, the synergistic surface redox couples of Cu(II)/Cu(I) and Co(III)/Co(II) played a vital role in the catalytic activation of PMS to produce SO4 •−, and SO4 •− was proved to be the dominant oxidant species for the elimination of BPA by quenching experiments and EPR study.Furthermore, the high-resolution XPS and turnover frequency (TOF) tests proposed a possible process for forming oxidant species. They studied the changes of high-resolution XPS spectra of fresh and used CuCo2S4 in detail. The ratio of Co(III)/Co(II) increased from 0.88 to 1.07 after the reaction, suggesting that electrons transferred from cobalt to PMS in the activation process. However, the ratio of Cu(I)/Cu(II) decreased from 1.25 to 0.92, indicating that a part of Cu(I) has been oxidized to Cu(II). The appearance of Cu(I) and Co(III) in fresh CuCo2S4 implied that Cu could represent carrollite (I)Co2 (III)(S4)(−VII) [140], and the reactions involved in CuCo2S4/PMS can be described in the following equations (Eqs. (11)−(15)), and the role of SO4 •− was excluded due to its much lower reducing potential (1.10 ​V). Eq. (13) was thermodynamically beneficial for (ECo(III)/Co(II) ​= ​1.84 ​V, ECu(II)/Cu(I) ​= ​0.15 ​V), proposing the synergistic effect between the two metal sites in CuCo2S4. Furthermore, TOF of as-prepared Co3O4, CuCo2O4 and CuCo2S4 were determined to be 1.31, 0.57 and 5.87 ​× ​10−3 ​s−1, respectively. The TOF of CuCo2S4 was 5 and 10 times greater than Co3O4 and CuCo2O4, respectively. It was reported that binary cobalt sulfides have much lower optical band gap energy and much higher conductivity than the corresponding oxides [45,93]. More importantly, the replacement of oxygen with sulfur would produce a more flexible structure because the electronegativity of S2− was lower than O2−. (11) ≡Cu(I) ​+ ​HSO5 −→ ≡Cu(II) ​+ ​SO4 •− ​+ ​OH− (12) ≡Cu(II) ​+ ​HSO5 −→ ≡Cu(I) ​+ ​SO5 •− ​+ ​H+ (13) ≡Cu(I) ​+ ​≡Co(III) → ≡Cu(II) ​+ ​≡Co(II) (14) ≡Co(II) ​+ ​HSO5 −→ ≡Co(III)+SO5 •− ​+ ​H+ (15) ≡Co(II) ​+ ​HSO5 − → ≡Co(III) ​+ ​SO4 •− ​+ ​OH− FeCo2S4, as a bimetallic sulfide, not only has a strong synergistic effect between Co and Fe bimetallic sulfide but also has more metal active sites, which is conducive to the occurrence of various catalytic reactions [141,142]. In addition, FeCo2S4 also has excellent electrical conductivity, which makes the material in electrochemistry, photoelectric chemistry, energy storage and other fields of wide attention [143,144]. As demonstrated in Fig. 4(b), Li et al. [145] prepared FeCo2S4–C3N4 nanomaterial as a novel multiphase catalyst for the activation and degradation of sulfamethoxazole (SMX) by PMS. 20 ​mg/L FeCo2S4–C3N4 and 0.15 ​mM PMS can effectively degrade SMX (91.9%, 0.151 ​min−1). Radical scavenger test and EPR analysis confirmed that the singlet oxygen (1O2) led nonradical pathway is the primary reaction mechanism of SMX degradation.Nie et al. [45] verified that CuFeS2-PMS system displayed excellent activity for BPA removal compared with Cu2S, FeS2, CuFeO2, and Co3O4. It was found that BPA was almost eliminated (99.7%) and the TOC removal reached 75% within 20 ​min. SO4 •− was confirmed to be the leading reactive species responsible for the BPA elimination by ESR and quenching tests. They pointed out that the as-synthesized CuFeS2 powders possessed rich active surface Cu+ and Fe2+, the sulfur species play a vital role in enhancing the reduction cycle of Cu2+/Cu+ and Fe3+/Fe2+. Thus, they proposed a possible mechanism for the CuFeS2-PMS system.As demonstrated in Fig. 4(c), Cu+ and Fe2+ on CuFeS2 surface react with PMS to form SO4 •− (Eqs. (16)–(17)). Based on the XPS spectra of the Fe 2p and Cu 2p, rich active Cu+ and Fe2+ species on fresh catalyst surface initiate PMS activation quickly, resulting in a quick degradation of BPA in the initial 1 ​min in the CuFeS2-PMS system. Simultaneously, Cu+ and Fe2+ would be oxidized to Cu2+ and Fe3+, and SO4 •− reacted with H2O to form •O2H (Eqs. (18)–(19)). Subsequently, the surface Cu2+ and Fe3+ were reduced and recycled to Cu+ and Fe2+ (ECu 2+ /Cu + ​= ​0.17 ​V, EFe 3+ /Fe 2+ ​= ​0.77 ​V) via strongly reductive sulfur species such as S2 2− (Eqs. (20)–(23)). Furthermore, the regeneration of Fe2+ active sites was also carried out by the electron transfer from Cu+ ​to Fe3+ (Eq. (24)). The regenerated Cu+ and Fe2+ active sites on the CuFeS2 surface were capable of activating PMS again, leading to the continuous formation of reactive species. Their findings indicated that bimetallic metal sulfides have outstanding potential for oxidizing organic contaminants. (16) Cu+ ​+ ​HSO5 − → Cu2+ ​+ ​SO4 •− ​+ ​OH− (17) Fe2+ ​+ ​HSO5 − → Fe3+ ​+ ​SO4 •− ​+ ​OH− (18) SO4 •− ​+ ​OH− → SO4 2− ​+ ​•OH (19) SO4 •−+ H2O → SO4 2− ​+ ​•OH ​+ ​H+ (20) 2S2− ​+ ​2Cu2+(Fe3+) → 2Cu+(Fe2+) ​+ ​S2 2− (21) 2S2 2− ​+ ​2Cu2+(Fe3+) → 2Cu+(Fe2+) ​+ ​Sn 2− (22) S n 2− ​+ ​2Cu2+(Fe3+) → 2Cu+(Fe2+) ​+ ​S0 (23) S0 ​+ ​2Cu2+(Fe3+) → 2Cu+(Fe2+) ​+ ​SO4 2− (24) Cu+ ​+ ​Fe3+ → Fe2+ ​+ ​Cu2+ Barhoumi et al. [146] developed a novel electrochemical advanced oxidation process with heterogeneous catalyst chalcopyrite for the degradation and mineralization of tetracycline (TC). They revealed that the performance of the electro-Fenton (EF)/chalcopyrite process was superior to conventional EF, acquiring nearly complete mineralization of the TC solution under optimized conditions after 360 ​min, which was attributed to the self-regulation of Fe2+ and Cu2+ content in the reaction medium, exerting a synergistic effect. Moreover, they noticed that oxalic and oxalic acids were more rapidly destroyed when using chalcopyrite, and the results could be accounted for the existence of Cu2+ ions, whose carboxylate complexes were more reactive towards •OH, thus led to an enhancement in their oxidation rate.In recent years, as a terylene chalcogenide, ZnIn2S4 with a typical layered structure has attracted significant interest due to its excellent electrical and optical properties, intense visible light capturing ability and excellent chemical stability [147,148]. More importantly, ZnIn2S4 has a matching bandgap structure with semiconductor materials such as g-C3N4 and TiO2, which makes it possible to construct binary heterojunctions based on ZnIn2S4 to improve the charge separation and migration efficiency of photocatalytic systems [149–151]. Shi et al. [152] synthesized a series of carbon quantum dots (CQDs) in different proportions as ideal cocatalysts to enhance the photocatalytic activity of ZnIn2S4 microspheres in the visible range. Meanwhile, the optimal CQDs(5)/ZnIn2S4 hybrid photocatalyst showed excellent methyl orange (MO) degradation ability (10 ​mg/L and 100% degradation within 40 ​min). The degradation rate was 2.34 times higher than that of ZnIn2S4 alone.Cu2FeSnS4 (CFTS) is one of the important ores in tin ore [153]. In addition, it has been proven to be an earth-abundant quaternary semiconductor and is an alternative material for solar energy conversion with excellent photovoltaic properties [154,155]. As a result, it is receiving increasing attention due to its potential applications in water decomposition, solar cells and pollutant degradation [156]. As demonstrated in Fig. 4(d), Kong et al. [157] prepared flower-like Cu2FeSnS4 nanomaterials for the activation of persulfate (PS) to degrade bisphenol A (BPA) in model industrial wastewater. The results showed that Cu2FeSnS4 catalyzed the decomposition of PS more effectively than mono-metal Cu/Fe/Sn sulfide and showed catalytic activity over a wide pH range. EPR spectroscopy, X-ray photoelectron spectroscopy and radical quenching experiments reveal the synergic effect of Cu, Fe and tin nutraceutical systems during PS activation. The intrinsic electron transfer between Cu, Fe and Sn, especially the Fe(II)∗ species formed on the surface of CFTS after Fe(II) is complexed by S, overcomes the inhibition of the M(n+1)+/Mn+ redox cycle.In view of the unsatisfactory performance of single-component catalysts, the combination of two or more catalytic materials to construct heterogeneous structures has become an effective strategy to improve catalyst activity. It can not only generate electron redistribution and achieve a synergistic effect at the interface by combining different components but also generate a new interface structure by changing the composition and crystal phase of the structure, to achieve efficient catalytic function. Metal sulfide-based heterojunction catalysts show outstanding performance in chemical reactions due to their beneficial interfacial properties. However, the rational design of heterogeneous catalysts with the required interface properties and charge transfer properties remains challenging. At present, many studies have reported the application of heterojunction catalysts constructed of metal sulfide and other excellent materials in the field of advanced oxidation.Guo et al. [158] reasonably designed the hollow flower-shaped polyhedron α-Fe2O3/defective MoS2/Ag Z-scheme heterojunction using the polyhedron α-Fe2O3 as template by a one-pot hydrothermal deposition process. Hollow floral polyhedral heterojunction uses multiple light reflections in the hollow structure to achieve the enhanced photocatalytic activity. As a link between α-Fe2O3 and Ag nanoparticles, defective MoS2 provides a large number of active sites and broadens the light response region. Importantly, the photocatalytic Fenton degradation of 2,4-dichlorophenol and salicylic acid by the hollow flower-shaped polyhedron α-Fe2O3/Defective MoS2/Ag is higher than that of α-Fe2O3 and α-Fe2O3/defective MoS2.As demonstrated in Fig. 4(e), Zhao et al. [159] successfully prepared for the first time a novel p-n heterojunction photocatalyst with core-shell structure n-BiVO4@p-MoS2 by in situ hydrothermal method. The shell thickness of MoS2 can be easily adjusted by changing the concentration of MoS2 precursor in solution. For the photocatalytic reduction of Cr6+ and the photocatalytic oxidation of crystal violet, BiVO4@MoS2 sample showed excellent photocatalytic performance. Its enhanced photocatalytic activity was mainly due to the high specific surface area and strong adsorption capacity of the catalyst for pollutant molecules. The core-shell geometry also increases the contact area between BiVO4 and MoS2 and promotes the charge transfer at the BiVO4/MoS2 interface.Density functional theory (DFT) studies the electronic structure of multi-electron systems and is a quantum mechanical method [160,161]. The main goal of DFT is to use electron density as a fundamental quantity to replace the wave function. Density refers to the number density of electrons. A functional says that energy is a function of electron density, which in turn is a function of spatial coordinates [162]. A function of a function is a Functional. DFT is a method to study the electronic structure of multi-electron systems by electron density [163]. Specifically, in operation, DFT simplifies difficult problems involving electron-electron interaction into non-interaction problems through various approximations, and then puts all errors into a single term (XC potential), and then analyzes this error. The most basic function of DFT is to calculate the electron density distribution and ground state energy of a system [164]. The core of the concrete calculation method is Hohenberg-Kohn theorem and Kohn-Sham equation. DFT is of great significance, which makes it possible to calculate the electronic structure of larger systems. The first-principles calculation does not rely on other empirical parameters, and only requires five basic constants (particle mass, electric quantity, Planck constant, light speed and Boltzmann constant). Starting from the chemical composition and crystal structure of the material, various ground state properties of the material can be obtained by solving the Schrodinger equation, such as band structure, optical properties, mechanical properties, thermodynamic properties and magnetic properties. In recent years, after continuous development, density functional theory occupies a mainstream position in material simulation, with advantages of small error and high efficiency, and prominent advantages in the calculation of transition metal atomic system and phonon system [165–168].DFT calculations are often combined with related experiments to supplement and extend the experimental discipline. The mechanism behind chemical reaction processes can be further explored by studying the structure of materials and small molecules (e.g., bond length, vibration). In recent years, much progress has been made in information about the surface binding positions of metal sulfide-based catalysts and oxidants by reliable DFT calculations. It mainly includes computational structural properties (3D, 2D, 1D, heterogeneous structure model, adsorption energy, binding energy), calculation of electronic properties (charge density, state density, band structure, orbital hybridization, charge projection, differential charge density, spin density), calculation of defect properties (formation energy, transition energy level, migration path and barrier) and calculation of adsorption and catalytic properties (adsorption configuration, free energy, molecular decomposition barrier).As shown in Fig. 5 (a), Fang et al. [169] synthesized a new P-doped CdS nanorods@NiFe layered double hydroxides (LDH) Z-scheme photocatalyst (P–CdS@NiFeLDH) by hydrothermal and calcination. By DFT calculation, it is found that phosphorus doping can form a new Fermi level, reduce the intermediate band gap, and thus extend the lifetime of photogenerated electron carriers. As a result, the degradation rate of bisphenol A by P–CdS@NiFe-LDH photocatalyst reached 98% within 160 ​min, and the photocatalytic degradation performance was significantly improved. Huang et al. [170] demonstrated that monodisperse iron atoms are confined to MoS2 nanosheets with dual catalytic sites as highly active and stable catalysts to catalyze the efficient oxidation of aniline through the activation of PS. The DFT calculation further explained the high catalytic performance of Fe0 · 36Mo0 · 64S2 by the low oxidation state of Fe and the strong metal-carrier interaction in Fe x Mo1-x S2. The Bader charge calculation results of Fe and Mo atoms in Fe0 · 36Mo0 · 64S2 showed that the perturbation of the electronic structure of MoS2 by Fe doping would trigger the catalytic activity of MoS2, and the charge transfer between the restricted Fe atom and the MoS2 carrier indicates that the metal-carrier interaction is vital. This interaction results in a rapid charge transfer between Fe and Mo during the catalytic process. As displayed in Fig. 5(b), Guo et al. [171] synthesized Co, S (pg-C3N4/Co3O4/CoS) using in situ template method and microwave vulcanization. DFT calculations show that electron migration from carbon nitride to CoS promotes the Co3+/Co2+ cycle. The calculation results of the projected density of state showed that the introduction of Co3O4 makes the Fermi level move towards g-C3N4 conduction band, and the formation of CoS on Co3O4 further makes the Fermi level enter the bottom of g-C3N4 conduction band. This means that g-C3N4 has better electrical conductivity, which is favorable for photogenic charge transfer. As shown in Fig. 5(c), Ye et al. [172] synthesized MIL-88 ​B(Fe)/Fe3S4 hybrid material with many unsaturated iron sites to treat the antibiotic trimethoprim by electro-Fenton processes. DFT calculations established the structure modeling, charge density and adsorption of H2O2. The results showed that the electron transfer from O to Fe promotes the transition from Fe(III) to Fe(II). Due to the presence of S atoms, more dense charge accumulation is concentrated near the Fe site, and the enhanced charge redistribution produces more electron-rich Fe active sites and accelerates the Fe(III)/Fe(II) redox cycle by increasing the rate of electron transfer. The adsorption energy of H2O2 on the catalyst showed that the adsorption of H2O2 on the catalyst was stable and conducive to the reaction.We have summarized the recently developed progress in synthesizing, characterization, and application of metal sulfides. Various methods have been extensively applied to prepare metal sulfides, including hydrothermal and solvothermal, template, thermal decomposition, and precipitation. The powerful characterization techniques (e.g., XPS, XRD, BET, FTIR, SEM, TGA, AC-STEM, XAFS and in situ characterization) are beneficial for us to understand the physicochemical properties of the catalyst better. The as-synthesized and natural metal sulfides can be introduced into various heterogeneous catalytic systems for wastewater treatment, and the performance and the mechanism of catalytic oxidation were discussed in detail. Although the mechanisms of catalytic oxidation were inconsistent to some degree, the following two conclusions were generally proposed by metal sulfide-based AOPs. One is the enhancement of the electron transfer efficiency caused by reductive S2− since S2− possesses lower electronegativity than O2−; the other is that unsaturated S atoms on the surface of the metal sulfide can capture protons to form H2S and expose the metal active sites with reducing properties, thereby accelerating the rate-limiting reaction step. In addition, we discussed the theoretical calculation of metal sulfides, and elaborated the reaction mechanism of metal sulfides in different AOPs from the perspectives of modeling, adsorption energy calculation and free energy calculation.Despite the excellent potential of metal sulfide-based AOPs for wastewater treatment, there are still some issues that need to be taken into consideration. (1) Preparation of metal sulfides. The preparation methods of functionalized polysulfide still face specific difficulties, particularly compared to the one-component metal sulfides. Although many synthetic methods were employed for the fabrication of metal sulfides, the yield and efficiency of these methods are not satisfied yet, thus limiting large-scale practical applications. (2) Roles of sulfur species. It is verified that metal sulfides have higher catalytic activity than the corresponding oxides. However, previous work only focused on the surface S of the catalyst, while it is far from completion because there is almost no concern about sulfur in aqueous water. Therefore, it is essential to further inspect the water chemistry of sulfur speciation, particularly the transformation and destination of sulfur. It is beneficial better to understand the mechanisms of metal sulfide-based AOPs. (3) Leaching of toxic metal ions. Many studies demonstrated metal sulfide's superiority in improving catalytic oxidation activity. However, the leaching of toxic metal ions cannot be overlooked. To counter this drawback, we should design novel supported catalysts. The material with high stability and superior electrical conductivity is preferably selected as the supporter, such as metal oxide and carbon-based materials, and then the leached toxic metal ions can be immobilized by the supporters. (4) Stability and reusability. The stability and reusability of as-prepared catalysts in a continuous operation need to be further investigated to be applied for practical wastewater treatment. It is essential to effectively prevent the loss of active components and avoid catalyst poisoning. (5) Laboratory research stage to engineering application stage. At present, most of the work in this research field adopts beaker experiments, which is quite different from the actual engineering application. Therefore, a great deal of effort is still needed to scale up these reaction processes to achieve practical applications of metal sulfide catalysts. It should be further explored in improving catalytic performance and reducing the use cost. Low-cost, abundant resources, stable and durable metal sulfide catalysts should be carefully selected. (6) In-depth exploration of the reaction mechanism. At present, there are many catalytic systems based on metal sulfides. However, for different reaction systems, the depth and breadth of theoretical calculation are still very limited, especially the lack of in-depth analysis of the change in the electronic structural properties of catalysts. Therefore, further work should thoroughly elucidate the root causes of the superior catalytic properties of metal sulfides through in-depth DFT calculations. Preparation of metal sulfides. The preparation methods of functionalized polysulfide still face specific difficulties, particularly compared to the one-component metal sulfides. Although many synthetic methods were employed for the fabrication of metal sulfides, the yield and efficiency of these methods are not satisfied yet, thus limiting large-scale practical applications. Roles of sulfur species. It is verified that metal sulfides have higher catalytic activity than the corresponding oxides. However, previous work only focused on the surface S of the catalyst, while it is far from completion because there is almost no concern about sulfur in aqueous water. Therefore, it is essential to further inspect the water chemistry of sulfur speciation, particularly the transformation and destination of sulfur. It is beneficial better to understand the mechanisms of metal sulfide-based AOPs. Leaching of toxic metal ions. Many studies demonstrated metal sulfide's superiority in improving catalytic oxidation activity. However, the leaching of toxic metal ions cannot be overlooked. To counter this drawback, we should design novel supported catalysts. The material with high stability and superior electrical conductivity is preferably selected as the supporter, such as metal oxide and carbon-based materials, and then the leached toxic metal ions can be immobilized by the supporters. Stability and reusability. The stability and reusability of as-prepared catalysts in a continuous operation need to be further investigated to be applied for practical wastewater treatment. It is essential to effectively prevent the loss of active components and avoid catalyst poisoning. Laboratory research stage to engineering application stage. At present, most of the work in this research field adopts beaker experiments, which is quite different from the actual engineering application. Therefore, a great deal of effort is still needed to scale up these reaction processes to achieve practical applications of metal sulfide catalysts. It should be further explored in improving catalytic performance and reducing the use cost. Low-cost, abundant resources, stable and durable metal sulfide catalysts should be carefully selected. In-depth exploration of the reaction mechanism. At present, there are many catalytic systems based on metal sulfides. However, for different reaction systems, the depth and breadth of theoretical calculation are still very limited, especially the lack of in-depth analysis of the change in the electronic structural properties of catalysts. Therefore, further work should thoroughly elucidate the root causes of the superior catalytic properties of metal sulfides through in-depth DFT calculations.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 the financial support from the National Natural Science Foundation of China (No. 52200105, 52070133) and Sichuan Program of Science and Technology (2023NSFSC0344, 2023JDZH0010).

Due to its unique physical and chemical properties, metal sulfide has been proven to be a promising ideal candidate for metal oxide catalysts and has been widely used in many catalytic fields. In recent years, advanced oxidation processes (AOPs), especially those based on metal sulfides, have been recognized as one of the most effective techniques for controlling water pollution due to their superior catalytic performance and stability. However, there is a lack of systematic summary and elaboration of the reported works on metal sulfide catalysts. This work reviews the synthesis, characterization and application of metal sulfide in AOPs for water decontamination. In addition, we further summarized the catalytic oxidation mechanisms of different metal sulfide-based AOPs and combined them with density functional theory (DFT) calculation to clarify the active root of the catalytic reactions of various metal sulfides. Finally, the application of metal sulfide is prospected, including the challenges in large-scale preparation, sulfur hydrochemistry and metal ion leaching, and the stability and reusability of metal sulfide. This review will help guide the future development of metal sulfide and further develop efficient and stable metal sulfide-based AOPs to better deal with water pollution.

No data was used for the research described in the article.A novel, sustainable, clean, and efficient energy is required to meet the ever-increasing energy demand [3,4]. The idea of generating H2 from renewable or non-renewable sources without harming the environment is considered the most promising solution for decarbonizing large sectors of the global economy [5]. Besides, H2 qualifies to work as an energy carrier/storage, which can upgrade the renewable energy sector [6]. Unfortunately, H2 is unavailability in nature and is usually obtained from fossil fuels because it is more cost-effective [7]. Shifting toward producing clean H2 at affordable prices requires novel, eco-friend, inexpensive production methods [8]. Indeed, the generation of “green” H2 from a solar-driven water splitting process (WSP) represents the trendiest clean energy source, which can diminish our addiction to fossil fuels and mitigate the problem of anthropogenic climate disasters [9,10]. Immense efforts have been devoted to develop the “ideal” photocatalyst material, which is still the main obstacle in this field. The majority of high-performance photocatalysts still need one of the following points or all of them, including (1) a noble co-catalyst, (2) a complex fabrication procedure, or (3) a “crutch” to secure hot electrons like PV-PEC model [11,12].In 2009, graphitic carbon nitride (g-C3N4), a fangled metal-free organic conjugated polymeric semiconductive (n-type) was presented, for the first time, as an active photocatalyst material [13]. Referring to Table 1 , the distinctive properties of g-C3N4 make it possessed research attention since then. However, virgin g-C3N4 can not harness long wavelength (400 ≤ λ ≤ 700 nm) and suffer from high recombination kinetics between photoinduced charges (e−/h+) [14]. Besides, the g-C3N4 bulk own low texture features, which also dwarf the photocatalytic performance [15]. More details are provided in Table 1 (cons). The aforementioned shortcomings can be alleviated by several means, such as structure re-engineering, doping method, framework defect engineering, substitution method, surface modification, nano heterojunction, etc. [12,16–19].Compared to non-nano counterparts, the fabrication of a nano-scale photocatalyst can improve the contact area with the reaction medium, enhance charge separation efficiency and modify the photoelectronic properties [37–40]. For instance, Xu and co-workers fabricated g-C3N4 hollow nanotubes (labeled as CNNTs) with the assistance of hydroxylamine sulfate. As expected, the CNNTs with distinguished texture-photoelectronic properties showed an impressive H2 evolution rate (HER) at 180 μmol/h with a ca. 19.5 folds improvement over bulk g-C3N4 [40].The essential role of a co-catalyst in photocatalyst material is to improve charge separation or/and increase active site abundance-accessibility. Therefore, presenting a cheap, abundant, eco-friendly co-catalyst with high performance is critical for potential applications. The NiO [41,42], NiS [43–45], MoS2 [46], Ag2O [47], Cu2O [48], Ni(OH)2 [49], Fe2O3 [50], and Co3O4 [50], belongs to the transition metal oxides/sulfides category. The NiO can be classified as a cheap, stable, and eco-friendly compound with good performance as a co-catalyst [41,42]. Historically, NiO was used for the first time as a co-catalyst in 1982 to improve the performance of SrTiO3 in water vapor decomposition [51]. Since 2012, nickel-based substances have been used as co-catalyst for g-C3N4, including, but not limited to, Ni(OH)2 [49], Ni [52,53], Ni2P [54], and NiSx [43–45]. The fall of the nobility: this report aims to proffer nickel oxide (NiO) as a cheap and efficient co-catalyst to offer an alternative to scarce extravagant noble metals (ex. Pt, Pd).To our knowledge, the NiO loaded over deficient, porous S-doped g-C3N4 nanofiber synthesis via the electrospinning process has not been reported yet. In light of this, it will be an exciting and valuable task to synthesize NiO nanoparticles from scratch and immobilize it over S-doped g-C3N4 nanofiber (x%NiO/S-g-C3N4-F) via an electrospinning process. This paper evaluated the performance of the H2 evolution rate from the water under simulated solar illumination (full spectrum) within a photoelectrochemical system (PEC, H type). In addition, this report scrutinized the impact of the NiO load amount (ca. 1–5 wt%) on the light absorption, charge separation, and photocatalytic activity of S-g-C3N4-F. Subsequently, the as-synthesized samples were characterized by various spectroscopic techniques and electron microscopy. Eventually, the results signalized that the inorganic-organic nanohybrid 2%NiO/S-g-C3N4-F photocatalyst could be a potential candidate for solar-to-fuel conversion applications.All the starting chemicals and solvents used to prepare or test the fabricated material are presented in Table 2 . All chemicals were used as received without any further purification.A former study showed that NiO particles could be prepared via a facile calcination method (see Fig. 1 ) [1]. In detail, Nickel (II) nitrate hexahydrate (5.0 g) in the solid phase was put in a crucible with a lid (200 mL) and calcined at 650 °C (5 °C/min) under an ambient atmosphere for 3 h and left to cool naturally overnight. As-obtained bulk NiO was added to 80 mL of distilled water and grinding using two homogenizer models. The first round with ULTRA-TURRAX (IKA® T25) for 10 min at 8500 RPM. Next, the 2nd round with ULTRA-TURRAX (Model: T10B) with a smaller blade for another 5 min at 6500 RPM. The obtained slurry was heated over a hot plate (85–90 °C) to remove the added water. Eventually, the collected samples were kept in sealed glass bottles for further use or characterization.The synthesis procedure of S doped g-C3N4 bulk (termed S-g-C3N4 bulk) by thermal treatment of TU, whereby 7.6 g (ca. 0.10 mol) was added to a crucible with led (300 mL) and calcinated at 575 °C/2 h, under an ambient atmosphere, with 5 °C/min as a ramping rate. After that, the prepared sample is left overnight in the oven to cool naturally. A similar procedure for product collection, grinding, and drying was followed as in the preparation of NiO.The synthesis procedure of NiO/S-doped g-C3N4 nanofibers (1D) comprised solution preparation, whereby a specific amount of NiO (10–50 mg) dispersed into 78 mL of solvent (40 mL DMF + 6.0 mL HAc + 32 mL ethanol) by using a homogenizer (6000 RPM) for 5 min. After that, 5.0 g of Thiourea (TU, NH2CSNH2) was dissolved in the solution with stirring (300 RPM) at room temperature for an hour. Next, 7.5 g of polyvinylpyrrolidone (PVP, Mw 1,300,000) was added and mixed (600 RPM) at room temperature for another hour until it became homogeneous. Then, the prepared solution was gently stirred (150 RPM) overnight at room temperature to remove any remaining bubbles.The electrospinning machine operated at a closed system (see Fig. S1) with a temperature of around 28–32 °C under controlled humidity (HUM ∼ 30%). An irradiation lamp (75 W) and dehumidifier device worked continuously to keep the humidity at the set point in the closed chamber (see Fig. S2). To keep the consistency in the experiment, the dehumidifier device was not turned off overnight to maintain the humidity at 30% for the following day. The spinning precursor was loaded into a plastic syringe (25 mL), and the stainless-steel needle size was 19 gauge in all experiments, as shown in Fig. S1. Pictures of the electrospinning setup with other details are provided in the supplementary material.The flow rate of the syringe pump was 0.8 mL/h for all electrospinning processes. Besides, the distance between the positive electrode and the ground collector was 20 cm, and the voltage difference was kept at 20 kV (1 kV/cm). One radiation heating unit (75 W) was operated continuously and located 50 cm higher than the middle point between the needle head and collector. Afterward, collected “green” fiber was thermally treated at 575 °C/2 h to polymerize g-C3N4 and completely remove the PVP and extra organic components [2,55]. A former study proved that PVP nanofiber fabricated via the electrospinning method is almost decomposed at 500 °C, as the TGA analysis showed [55]. The ramping rate was 5 °C/min, and the prepared sample was left to cool naturally in the oven overnight. For comparison, S-doped g-C3N4 nanofiber (labeled as S-g-C3N4-F) was prepared following the same procedure of NiO/S-g-C3N4-F without adding the NiO.A test run showed that the yield of pure S-g-C3N4-F synthesized at 575 °C/2 h without NiO is about 22%. Based on that, the added amount of NiO co-catalyst (10–50 mg) represents about 1–5 wt% of the total weight. The corresponding samples were denoted as x%NiO/S-g-C3N4-F, where x = 1, 2, 3, 4, and 5 wt%. Collected samples were kept in sealed glass bottles for testing and characterization purpose. More details about the journey from chemicals to target compounds through intermediate phases under thermal treatment are graphically presented in Fig. 1.The working electrode for testing the performance was prepared as follows procedure: (i) 100 mg of prepared photocatalyst material dispersed within Nafion solution (5 mL, 0.5 wt%), after that, (ii) the obtained slurry was loaded over indium tin oxide (ITO) glass (30 × 30 mm, 1850 Å thick, 10–15 Ω/in2) by slowly dripping, (iii) the as-prepared electrode was left at room temperature for 12 h, after that (iv) the as-prepared electrode was dried over a hot plate at ca. 100 °C for another 12 h.Different characterization techniques were used to identify the prepared material, quantify the structure features, investigate the photogenerated charges separation efficiency, and other purposes, as shown below in Table 3 .The photocatalytic hydrogen production system was performed in a photoelectrochemical cell (PEC, H-type) with a circle window from quartz (dia. 24 mm) to transfer the incident light efficiently, as shown in Fig. 2 . As prepared photocatalyst material loaded over ITO-coated glass was immersed in a methanol aqueous solution (20 vol%, 200 mL). After that, the photocatalytic system was exposed to lamp irradiation (Metal halide 400 W, full spectrum, 380 ≤ λ ≤ 780 nm). The light intensity of 400 W Metal halide was quantified at about 90 mW/cm2 [57].The distance between the irradiation device and the photocatalytic fuel cell (PEC) was kept constant in all the experiments at 20 cm. Nafion 117 (dia. 20 mm) was used as a replaceable ion transport membrane. Besides, a solid square of a platinum electrode (20 × 20 mm) was used for the hydrogen reduction reaction. A simplified sketch for the setup used to test photocatalysis material and an image of the actual setup are presented in Fig. 2 (a, b). The water displacement method was used to quantify the production rate of H2 [58–60]. The photocurrent response for prepared samples was also quantified without any bias potential via a digital multimeter with PC data logging. The photocatalytic fuel cell (PFC) was isolated in a closed box to eliminate any external light that may manipulate the results. The redox reactions for the water-splitting process are also presented in Fig. 2 (b).Powder X-ray diffraction patterns (XRD) were used to investigate the phase and composition of the as-prepared samples. Fig. 3 (a) shows the diffraction peaks of pure NiO at 2θ of 37.26°, 43.26°, 62.81°, and 79.36°, which were respectively indexed as (111), (200), (220), and (222) plane of cubic NiO structure according to JCPDS file no. 47–1049. Besides, Fig. 3 (b, c) show the XRD patterns of S-g-C3N4 bulk and NiO/S-g-C3N4-F following the JCPDS file no. 87–1526, where the two peaks (2θ) at 13.1 and 27.6° ascribed from (100) and (002) planes [20,52]. Besides, the slight shifting of the main peak for g-C3N4 can be related to injecting the S in the polymeric backbone (see Fig. 3 (b, c)) [61]. As shown in Fig. 3 (c)), one small peak of NiO was observed at 43.06° (plane 200) in the XRD of NiO/S-g-C3N4-F. The XRD of NiO/S-g-C3N4-F emphasized the presence of NiO, and a similar XRD pattern was captured in related reports (ex. Ni/g-C3N4 [53], NiO/g-C3N4 [62]). The average crystalline grain size (DXRD) for as-prepared samples was calculated from the full width at half-maximum (FWHM) of the XRD peaks using the Debye-Scherrer equation [63]. Accordingly, the average DXRD of NiO, S-g-C3N4 bulk, and S-g-C3N4-F were quantified as 25.1 and 3.64, and 0.58 nm, respectively. The DXRD of the NiO/S-g-C3N4-F series quantified around 0.60 nm (see Fig. 3 (c)), which indicates that the NiO did not impact the crystallinity, contrary to the structure. The DXRD error of the NiO/S-g-C3N4-F series was estimated at around 6.61% (ca. ± 0.03966 nm) [64]. Fabricated S-g-C3N4-F destroys the order in the long-range without effect on the short-range order, which creates an amorphous structure [65]. Referring to XRD of S-g-C3N4-F and NiO/S-g-C3N4-F series, the peak of 13.1° disappeared within XRD noise, and the main peak (002) became broader, which can be attributed to low crystallinity and effect of introducing the S as heteroatom into the framework [65–67]. To sum up, the XRD of NiO, S-g-C3N4-F, and NiO/S-g-C3N4-F confirmed the formation of the target materials without any impurities. Fig. 4 shows the morphologies of grinded NiO (a), 2%NiO/S-g-C3N4 ″green” fiber (c), and calcinated 2%NiO/S-g-C3N4-F (g, h). The diameter of 2%NiO/S-g-C3N4 ″green” fiber was analyzed via ImageJ software and quantified in a range of 131–1151 nm with an average of 540 nm over the whole tested area (Fig. 4 (d)). Besides, the EDX of 2%NiO/S-g-C3N4 ″green” fiber confirmed the presence of Ni with good distribution over the host material surface (see Fig. 4 (e, f-yellow)). Based on the EDX, there is a high properly that the small nanoparticles that decorated the “green"/calcinated S-g-C3N4-F surface are NiO nanoparticles.As shown in Fig. 4 (g, h), the building unit for 2%NiO/S-g-C3N4 nanofiber consists of short nanofiber (ca. 40–60 nm) that coalesce and fusion to construct the like-fiber structure with high porosity. The average diameter for calcinated 2%NiO/S-g-C3N4-F was quantified following the same technique as 463 nm (see Fig. 4 (i)). Interestingly, the calcinated 2%NiO/S-g-C3N4-F was carried on the flower leaf tip without even bending (Fig. 4 (j), indicating that the structure is filled with porous. Relied on the BET result (section 4.3), the density for 2%NiO/S-g-C3N4-F was quantified as ca. 9.17 mg/cm3. After thermal treatment, the distribution of NiO over electrospun PVP transferred from 7.5 g to about 1.0 g of S-g-C3N4-F, which increased Ni intensity as shown in EDX element mapping (Fig. 4 (k)). Referring to Fig. 4 (k), the EDX spectra of NiO/S-g-C3N4-F confirmed the presence of S as one of the constituent elements, which can be considered evidence of successfully injecting the S into the g-C3N4 matrix. Moreover, the excellent distribution of C and N elements emphasized that the S-g-C3N4 represents the main building units. In addition, the apparent concentration for EDX of 2%NiO/S-g-C3N4-F showed an approximate ratio of 1:1 for Ni to O, which is also considered solid evidence of the formation of the NiO component. To sum up, the synergetic effect of fabrication S-g-C3N4-F with porous nanofiber structure and the magnetic properties of NiO can improve the incident light absorption efficiency, enhance the photoinduced charges separation and material recoverability.Improvement in the g-C3N4 texture features can enhance the charge separation efficiency and harvesting capability of the incident light [12]. The texture properties for S-g-C3N4 bulk and 2%NiO/S-g-C3N4-F were analyzed by nitrogen adsorption–desorption experiment, and the result are presented in Fig. 5 (a). The typical structure of S-g-C3N4 bulk as a stacking layer offers humble texture features as 3.78 m2/g and 0.026 cm3/g for BET surface area (SSA) and pore volume (Vp), which deteriorate photoreactivity [15]. In contrast, the NiO/S-g-C3N4-F structure consists of short nanofibers (ca. 40–60 nm) that coalesce and fusion to construct the porous nanofiber structure. Consequently, the well-developed porosity for 2%NiO/S-g-C3N4-F showed a substantial improvement compared to S-g-C3N4 bulk with 65.5 m2/g and 0.143 cm3/g as SBET and Vp, respectively. Besides, a graphical comparison between S-g-C3N4 bulk and 2%NiO/S-g-C3N4-F in SSA and Vp is shown in Fig. 5 (b).The BET hysteresis loop for 2%NiO/S-g-C3N4-F (Fig. 5 (c)) is located at 0.85 < P/P0 < 1.00 (green area) and takes the shape of type IV (BDDT classification), which emphasizes that the structure is filled with mesoporous (2−50 nm) [68,69]. In addition, Fig. 5 (d) shows the pore size distribution for 2%NiO/S-g-C3N4-F concentrated in the range of 1.5–5 nm. A similar H3 hysteresis loop was captured in a former study for TiO2/g-C3N4 nanofiber, which was related to the coalescence of nanoparticles to construct the fiber structure that created the slit-shaped pores [70]. Indeed, the enhancement in the texture features for NiO/S-g-C3N4-F enhances the active site availability and accessibility, as well as the light harness capability. Furthermore, the charges separation efficiency for NiO/S-g-C3N4-F expect to be improved as a synergetic effect of (i) the presence of carbon vacancies (Vc) [56,71,72], (ii) the magnetic properties of NiO [53,73], and (iii) the impact of nanofiber structure (diam. 463 nm) on decreasing the hot charges transmission distance [12].The radiative recombination process for photogenerated charges (e−/h+) was studied by photoluminescence (PL) spectroscopy. The peak position for PL spectra of S-g-C3N4 bulk and NiO/S-g-C3N4-F centralized within the Vis-light zone (see Fig. 6 (a)), which inform that the optical band gap (Eg) is less than 2.70 eV [74]. Compared to the S-g-C3N4 bulk, the PL peak for S-g-C3N4-F was not detected, meaning the luminous recombination probability was efficiently suppressed. In addition, the convexity intensity in the x%NiO/S-g-C3N4-F series decreased with increased NiO content from 1 to 2 wt%, as shown in Fig. 6 (b). However, the overloaded of NiO (3 ≤ x ≤ 5 wt%) over the S-g-C3N4-F surface slightly facilitates the charges recombination rate, as shown in the overlap of PL spectra (Fig. 6 (b)).Grafted Sulfur as an electron-rich with smaller electronegativity (χ = 2.58) in the g-C3N4 matrix narrowed the bandgap and downshifted the Fermi level due to occurring spin polarization in the material, which increased overlap in the orbitals of C, N, and S atoms and the π states [66,75,76]. Moreover, using thiourea (TU) as a precursor source created defects within the polymeric backbones (see Fig. 1), which improved the excitons (e−/h+) separation efficiency and expanded the light-responsive range [56,72]. The black color for g-C3N4-based material was captured in former reports (ex. Ni/g-C3N4 [53], metal oxide/g-C3N4 hybrids [50]). The black color for S-g-C3N4-F with or without NiO (Fig. 7 (a)) can be related to the effect of changing the thermal treatment atmosphere during the decomposition of PVP (C6H9NO)n, which generated different gases (ex. N2, CO2, CO) [77]. Besides, both PL and XRD spectra support the supposition that S-g-C3N4-F based material owns an amorphous structure. A former study confirmed that the atomic arrangement impacted the band structure and charge recombination rate, which can justify the significant difference in the PL result between S-g-C3N4 bulk and S-g-C3N4-F [65]. A similar distinguished PL response was captured in a former study that fabricated amorphous g-C3N4 via thermal treatment (620 °C/2 h) under an Ar atmosphere. Based on the theory of band tails, formation of a proper disorder in the crystallinity of g-C3N4 narrowed the bandwidth (Eg) to 1.90 eV [65]. Based on that, the outstanding PL result for S-g-C3N4-F and NiO/S-g-C3N4-F series can be ascribed to the synergistic effect of the sulfur dopants and the defective-amorphous- nanofiber structure. (1) E g = 1240 / λ g ( e V ) The optical properties of S-g-C3N4 bulk, S-g-C3N4-F, and NiO/S-g-C3N4-F series were studied using UV–Vis diffuse reflectance spectroscopy (DRS). The absorption edge (λg) is quantified from the intercept between the tangent of the absorption curve and the abscissa coordinate, as shown in Fig. 7 (c) [78]. The bandgap energy (Eg) for as-prepared samples was quantified according to equation (1) [78].The bandgap (Eg) for fabricated g-C3N4-based materials was quantified in the following order: S-g-C3N4-F (1.77 eV) < 1%NiO/S-g-C3N4-F (1.73 eV) < 2%NiO/S-g-C3N4-F (1.76 eV) < 3%NiO/S-g-C3N4-F (1.84 eV) < 4%NiO/S-g-C3N4-F (1.95 eV) < 5%NiO/S-g-C3N4-F (2.12 eV) < S-g-C3N4 bulk (2.61 eV). Fig. 7 (b) graphically compares the capability of S-g-C3N4 bulk and 2%NiO/S-g-C3N4-F in harnessing solar energy. Indeed, the overload of NiO over S-g-C3N4-F negatively impacts the photoelectronic character. Former studies (ex. MoS2/CdS [79], Ni2P/g-C3N4 [54], Ni(OH)2/a-Fe2O [80]) stated that co-catalyst could lower the light absorption capability of the main photocatalyst material through the light-blocking effect. Former studies quantified the Eg for NiO as 3.5 eV, which consider another plausible explanation for increased Eg for NiO/g-C3N4 nanocomposite with increased NiO content [62,81]. Based on that, the co-catalyst amount and distribution must be optimized to maximize the photocatalysis performance.Photocatalytic H2 evolution rate for prepared samples was measured under simulated solar irradiation (400 W Metal halide, 380 ≤ λ ≤ 780 nm) using methanol (20 vol%) as a hole scavenger reagent. In the current study, no H2 production was detected before introducing the photocatalyst into the reaction medium or in the absence of the irradiation source. Based on that, the generated H2 was formed from photocatalytic reactions. Fig. 8 (a, b) shows the photocatalytic H2-production performance (HER) and photocurrent value of different g-C3N4-based materials, including the S-g-C3N4 bulk, S-g-C3N4-F, and x%NiO/S-g-C3N4-F series. The photocurrent value was quantified on top of every hour for 60 s over 4 cycles for each sample. The photocatalytic HER was quantified in the following order: S-g-C3N4 bulk < S-g-C3N4-F < 5%NiO/S-g-C3N4-F < 4%NiO/S-g-C3N4-F < 1%NiO/S-g-C3N4-F < 3%NiO/S-g-C3N4-F < 2%NiO/S-g-C3N4-F.Based on the collected data, the S-g-C3N4 bulk exhibits a modest hydrogen production rate (HER) at 22.2 μmol/h with about 0.08 μA as photocurrent value (see Fig. 8 (a, b)). The humble performance of S-g-C3N4 bulk can be related to the bulk structure features with a high recombination rate between photogenerated charges, and the inability to harness visible light. In comparison, the HER of pure S-g-C3N4-F reached 63.55 μmol/h with a 2.86-fold improvement over S-g-C3N4 bulk, suggesting the pivotal role of morphology in enhancing photocatalytic performance. As can be seen, introducing a proper amount of NiO (x = 2 wt%) as a co-catalyst over S-g-C3N4-F increased active site availability and further suppressed the charge recombination rate (Fig. 6 (b)). Consequently, the HER of 2%NiO/S-g-C3N4-F enhanced to 107.04 μmol/h, representing almost double the pure S-g-C3N4-F performance (see Fig. 8 (a)).The maximum apparent quantum yield (AQY) of the 2%NiO/S-g-C3N4-F and S-g-C3N4 bulk was calculated at about 1.51% and 0.31% at wavelength 420 nm. Unfortunately, the overload of NiO over the host material body harms the light-harvesting capability and facilitates the charge (e−/h+) recombination rate, as PL and DRS confirmed (Fig. 6 (b), Fig. 7 (c)). As a result, increased NiO content to 3–5 wt% over S-g-C3N4-F relapsed the HER with maintaining a better performance over virgin g-C3N4 in all cases present in this study (Fig. 8 (a)). A similar photocatalytic pattern was captured in documented reports for Ni/S-doped g-C3N4 [52] and NiO/g-C3N4 [62]. It is well-documented that the strong interaction between noble metals and host photocatalyst material facilitates charge flowability, which reinforces overall performance [12,54]. As well, creating a firm interface between the photocatalyst and the co-catalyst could afford the unobstructed charge-transfer channel [46,82]. Due to g-C3N4 tri-s-triazine (heptazine) ring structures, the transition metal ions can be readily absorbed on its surface [83]. Based on that, the significant difference in performance between S-g-C3N4-F and NiO/S-g-C3N4-F indicates the presence of an intimate interface between them.In addition, multiple runs of photocatalysis were carried out under identical conditions to test the stability of 2%NiO/S-g-C3N4-F in evolving H2 from the water dissociation process. After four consecutive cycles, the HER for 2%NiO/S-g-C3N4-F declined by only 9.3% from the initial performance (Fig. 8 (c)), indicating high stability [68,72]. The performance pattern for x%NiO/S-g-C3N4-F series and the significant enhancement for S-g-C3N4-F over S-g-C3N4 bulk is consistent with the PL and DRS results. Fig. 8 (d) displays the relationship between the HER rate and photocurrent value for fabricated g-C3N4. As expected, the HER for fabricated g-C3N4 and quantified photocurrent value showed harmonicity.A former study by Chaudhary et al. succeeded in loading g-C3N4 with Ni through thermal treatment with nickel carbonate (NiCO3) at 550 °C/2 h under an N2 atmosphere [73]. The fabricated Ni/g-C3N4 generated H2 at a rate of 40 μmol/h under natural “outdoor” sunlight. Moreover, the photocurrent value for Ni/g-C3N4 reached almost double the pure g-C3N4 value [53]. In this study, the fabricated noble-free 2%NiO/S-g-C3N4-F showed a competitive performance to many related reports (see Table 4 ). For example, the HER of 2%NiO/S-g-C3N4-F reached about 78.6% of Pt3%/S-doped g-C3N4 performance (136.0 μmol/h) [68]. The noble-free g-C3N4-based material that satisfies the thermodynamic requirements to split the water and form “green” H2 under the visible light spectrum has more potential to be used in potential applications [12]. To sum up, the distinguished texture properties of nanofiber morphology with the merit of adding NiO as a co-catalyst and the impact of injecting the S within the polymeric backbones consider the reasons behind the impressive photocatalytic performance of 2%NiO/S-g-C3N4-F.Nickel (Ni) and its derivatives (e.g. NiS, NiO) distinguish as cheap, abundant, and relatively effective co-catalyst [87]. Indeed, the magnetic properties of Ni can improve the separation efficiency of photogenerated carriers (e−/h+) by rapidly capturing and transferring hot electrons [53,73]. Besides, the NiO as a co-catalyst can act as an active site to lower the H2 evolution overpotential and facilitate electron transfer (see Fig. 9 (a)), which suppresses the recombination (bulk/surface) rate between generated charges (e−/h+), as PL spectra confirmed. Indeed, NiO can also represent a reactive site by acting as the hydride-acceptor and proton-acceptor centers, as shown in Fig. 9 (b)) [44].Under thermal treatment, the sulfur vaper self-gas foaming impacts the texture properties of the prepared g-C3N4. Besides, inserting a non-metallic element (ex., S, B, O) into a polymeric bone does not create mechanical barriers or structural limitations. And most importantly, the integration of S as an electron-rich non-metallic element into polymer backbones increased the orbital overlap and the π states as well as hybridized the S, C, and N orbital and created a sub-energy level below the conduction band (CB), as shown in Fig. 9 (c) [66,75]. Moreover, a former study stated that inserting S with smaller electronegativity (2.58) inside of N (3.04) altered the surface electronic properties by downshifting the CB (0.45 eV) and widened the VB by 0.12 eV, which positively impacted the mobility of hot h+ and enhanced redox reaction efficiency [76]. Based on that, the liberator electrons (e−) are expected to transfer from the valance band (VB) to the sub-energy level before moving to the conduction band (CB). This two-step mechanism for hot electrons can somewhat suppress the recombination rate. In addition, the carbon vacancies (Vc) in the S-g-C3N4 matrix can impact the photogenerated carriers transmission (Fig. 9 (d)) [71]. Former studies showed that formation vacancies within the g-C3N4 framework could impact textural-photoelectric properties [88–90] and extend charges (e−/h+) lifetime [89]. However, it usually requires a toxic, harmful treatment to create defects in the g-C3N4 matrix, like alkali-assisted (ex. KOH, NaOH, and Ba(OH)2) [88]. This study developed the carbon vacancies (Vc) in modified g-C3N4 as a sequence using the thiourea, a simple and efficient method (see Fig. 1) [56,72].The water-splitting process occurs in two steps: (1) two water molecules react with generated holes through an oxidation reaction, forming an oxygen molecule and four hydrogen protons (H+) (see Fig. 9 (a)), after that (2) two hot electrons (e−) react with two hydrogen protons (H+) over a platinum plate to form H2 that appears in tiny bubbles. The excess generated holes (h+) are consumed by reacting with methanol as a sacrificial reagent.This study fabricated and tested the inorganic-organic nanohybrid NiO/S-g-C3N4 nanofiber in evolution of hydrogen from water. The fabrication method for NiO/S-g-C3N4-F consists of the electrospinning method with simple thermal treatment. The significant enhancement in HER for NiO/S-g-C3N4-F can indicate the existence of intimate interfacial between constituent material. In the current study, there was no H2 production before introducing the photocatalyst into the reaction medium or in the absence of the irradiation source. Based on that, the generated H2 was formed from photocatalytic reactions. Amongst all fabricated samples in the study, the S-g-C3N4-F decorated with NiO (2 wt%) demonstrates the highest H2 production rate and photocurrent value at 107.04 μmol/h and 1.01 μA, which surpassed the S-g-C3N4 bulk by 4.82 and 12.62-folds, respectively. Due to the magnetic character, the fabricated NiO/S-g-C3N4-F can be collected from reaction media by a simple method. Moreover, the fabricated 2%NiO/S-g-C3N4-F shows high stability, as the H2 formation rate slightly declined (<10%) after 4 consecutive cycling tests. The improvement in the photocatalytic performance of NiO/S-g-C3N4-F was ascribed to the following reasons: (i) acted the NiO as an electron-trapping center, (ii) the merit of fabrication of amorphous nanofiber structure, and (iii) the red-shift of the absorption edge due to modify the electronic structure by injecting the S in the polymeric bones. This work demonstrates that NiO is an efficient co-catalyst, which can pave the way for upgrading the efficiency of photocatalyst material without harming economic viability. Eventually, the fabricated 2%NiO/S-g-C3N4-F as an efficient, stable, and low-cost photocatalyst can be a candidate for photocatalytic applications, especially Vis-light-driven H2 evolution and pollutants photodegradation.Conception and design of study: Suleiman M. Abu-Sari, Wan Mohd Ashri Wan Daud, Muhamad Fazly Abdul Patah, Bee Chin Ang. acquisition of data: Suleiman M. Abu-Sari, Wan Mohd Ashri Wan Daud, Muhamad Fazly Abdul Patah, Bee Chin Ang. analysis and/or interpretation of data: Suleiman M. Abu-Sari, Wan Mohd Ashri Wan Daud, Muhamad Fazly Abdul Patah, Bee Chin Ang. Drafting the manuscript: Suleiman M. Abu-Sari, Wan Mohd Ashri Wan Daud, Muhamad Fazly Abdul Patah, Bee Chin Ang. revising the manuscript critically for important intellectual content: Suleiman M. Abu-Sari, Wan Mohd Ashri Wan Daud, Muhamad Fazly Abdul Patah, Bee Chin Ang. Approval of the version of the manuscript to be published (the names of all authors must be listed): Suleiman M. Abu-Sari, Wan Mohd Ashri Wan Daud, Muhamad Fazly Abdul Patah, Bee Chin Ang.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 the Ministry of Higher Education Malaysia for the financial support through the Research Grant Scheme DP KPT (Project number: FRGS/1/2019/STG01/UM/02/5) and the University of Malaya through Impact-Oriented Interdisciplinary Research (Grant IIRG003B–2020IISS).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.rineng.2023.100952.

The energy dilemma is exacerbating. Artificial photosynthesis is a plausible blueprint for solar-to-fuel conversion applications to satisfy future energy demands. Developing a cheap, efficient photocatalyst material can be the watershed moment in this field. Herein, low-cost sulfur self-doped g-C3N4 nanofiber decorated by nickel oxide (denoted as x%NiO/S-g-C3N4-F) was obtained via electrospinning and one-step thermal treatment (575 °C). Outstandingly, the modified g-C3N4-based material could interact and harvest long wavelengths up to 706 nm. Moreover, the quantified specific surface area (SSA) for 2%NiO/S-g-C3N4-F is more than 17.3 folds larger than S self-doped g-C3N4 bulk (denoted as S-g-C3N4 bulk). As a result, the optimal photocatalytic property of 2%NiO/S-g-C3N4-F is almost five times as high as S-g-C3N4 bulk, achieving 107.04 μmol/h. The suggested photocatalysis mechanism was proposed and supported by the results. Significantly, loading a proper amount of NiO over modified S-g-C3N4 promoted performance as well as convenient recovery and reusability. According to the experimental and characterization results, the fabricated 2%NiO/S-g-C3N4-F consider a potential candidate for photocatalytic applications, especially Vis-light-driven H2 evolution.

Data will be made available on request. Data will be made available on request.The quest for green energy production and the search for environmentally friendly energy production is one of the top global priorities today. Along with the application and utilization of renewable energy sources, there is an urgent need to develop multifunctional energy storage and energy conversion technologies to balance the demands of supply/demand management of the produced clean and sustainable energy [1–3]. Supercapacitors are undoubtedly one of the most promising energy storage devices as they are safe to operate and demonstrate a potential for ultra-high power density storage, long cycle stability and fast response [4–6]. In supercapacitors, the physicochemical properties of electrode materials, such as morphology, microstructure and electrical conductivity govern their electrochemical performances [7–9]. For effective storage of electrical energy and its conversion to chemical energy, electrocatalytic water-splitting devices are considered to be a good option due to their high energy density and environmentally friendly processes [10–12]. Considering the requirements of successful implementation of electrolysis in practice, effective high performance electrode materials with capabilities for fast electron transfer and high electrochemical activity are solicited. In the past few decades, research on multifunctional energy storage and conversion materials has mainly focused on transition metal oxides, hydroxides and sulfides [13–16]. However, the relatively low electrical conductivity and poor long-term stability limit their wider applications. Therefore, it is highly desirable to design and develop multifunctional materials with enhanced activity, high charge storage capacity and long cycling stability for building next-generation supercapacitors and electrocatalysts [17].Transition metal phosphide, as a promising candidate material, has attracted much attention in recent years [18–21]. Due to the good thermal stability and electrical conductivity, they are beneficial both for energy storage and conversion applications [22–25]. Amongst numerous candidates, the phosphate nickel-cobalt bimetallic compound (P-(Ni, Co)) has a higher electrical conductivity, better electrochemical activity and charge storage capacity, rendering them suitable as a potential multifunctional electrode material for next-generation applications [26,27]. A few exploratory reports are available in the literature including the work of Chen et al. who reported three-dimensional (3D) hierarchical NiCoP@CoS tree-like core-shell nanoarrays designed for the one-dimensional core to act as “hyperchannel” for electron transfer and the two-dimensional vertical shell for fast ion diffusion; the composite exhibited battery-type electrochemical performance, including high specific capacitance, enhanced rate capability and good cycling stability [28]. Wang's group fabricated flexible NiCo2O4-P (phosphated nickel cobalt oxide) on carbon filaments, that exhibited improved oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) properties argued to arise from the heteroatomic doping and improved active sites (due to nanoparticles), leading to an improvement of conductivity thus promoting faster electron transfer during water splitting [29]. Lee et al. reported a branched spike-covered hollow structure of NiCo2Px with enhanced HER and OER performances in a wide range of pH [30]. Yan et al. explored complex p-n heterojunction nanorod arrays Co-Pi/Co3O4/Ti:Fe2O3 (cobalt phosphate/cobalt oxide/titanium-doped ferric oxide) that showed improved photocurrent density and water oxidation kinetics; this was reported to occur due to the faster removal of photogenerated holes, thus increasing the charge injection and bulk separation efficiency, leading to a facile charge transfer and separation of electron-hole pairs [31]. P-(Ni, Co) bimetallic compound has a prominent redox reaction site due to the synergistic effect of nickel and cobalt catalysts and is considered to be excellent electrode material for supercapacitors as well as a good electrocatalyst. Substitution by metal ions (Xn+) with greater electronegativity and lower pKa of [X(H2O)m] n + has been demonstrated to shift the formal redox potential of parent metal positively in transition metal complexes and (hydr-)oxides due to inductive effects leading to greater ORR/OER activity, attributed to optimized binding of the reaction intermediates on the surface in rate-limiting steps [32]. However, its further utilization is still limited due to low rate capability and poor cycling stability resulting normally due to the structural collapse during repeated fast redox processes.In general, substrates like nickel foam (NF) have been widely utilized due to their high electrical conductivity and strong mechanical flexibility [33–35]. Furthermore, electrode materials with large open frames, such as Prussian Blue (PB), have faster ion diffusion paths [36]. The PB crystal structure can be adjusted due to the characteristics of polynuclear metals that cause their internal metal ions to be replaced by alkali metal ions with different valence states [37]. Iron (Fe) atoms of Prussian blue can be replaced by metal ions like nickel (Ni), cobalt (Co), copper (Cu), zinc (Zn) and manganese (Mn), and the physicochemical characteristics such as morphology, crystals, and internal pore structure can be altered. Open framework structures like Co-Fe mixed oxides with retainable architectures have already been demonstrated to exhibit superior OER performance [38,39]. More importantly, Prussian blue analogues (PBA) can be synthesized by a very simple method leading to their wide application as multifunctional electrode materials for energy storage and water splitting [40–42]. Modification of PBAs as phosphates leads to further improvement in their electrochemical performances [43–45]. Therefore, the design starting from the growth of the first layer of 1D NiCo nanowires (NiCo NWs), followed by a layer of Prussian blue analogues with a third layer of phosphides on a 3D conductive NF substrate to form a 3D complex network seems to be a promising approach for enhancing the conductivity and structural stability. Thus, a 3D core/shell nanostructure of P(Ni, Co, Fe) on the NF substrate results in a self-supported binder-free multifunctional electrode with improved surface-active sites providing a transport path to more electrolyte ions thus effectively boosting electrochemical performances of the electrode.In this work, a hierarchical P(Ni, Co, Fe) nanoarrays electrode was obtained by phosphating NiCo NWs/NiCo-PBA core/shell nanoarray structures fabricated through an in situ self-sacrificial growth process. The NiCo NWs/NiCo-PBA core/shell nanoarrays were synthesized through a two-step process. The NiCo NWs core to boost the charge transport and improve conductivity was synthesized by hydrothermal method, whereas the NiCo-PBA shell that contributes to increasing the surface-active sites for redox reactions was obtained by a chemical bath method, utilizing the core materials as a self-sacrificial source for Ni and Co. This composite electrode exhibits superior specific capacitance, good rate capability and improved cycling stability. Furthermore, the hybrid structure electrode shows a promising application for OERs with excellent electrocatalytic activity. This work reveals a simple and effective solution for the fabrication and design of high-performance multifunctional electrode materials for supercapacitors and enhanced OER activities.NiCo NWs were prepared via a hydrothermal process (Scheme 1). Prior to the synthesis process, nickel foam was cleaned with 1 M HCl, ethanol, and deionized (DI) water under ultrasonication for 15 min, respectively, and then dried at 60 °C. For the synthesis of NiCo NWs, 1 mmol Ni(NO3)2·6H2O and 2 mmol Co(NO3)2·6H2O were dispersed in 50 ml DI water at room temperature with continuous stirring. Then 4 mmol urea was added to the above solution under strong magnetic stirring for 30 min. The derived solution was then transferred into a sealed autoclave containing a piece of clean nickel foam (NF, 3 × 5 cm2) and maintained at 120 °C for 6 h. After cooling down to room temperature, the obtained product was cleaned with absolute ethanol and DI water several times followed by overnight drying at 60 °C prior to further use.A piece of NF with NiCo NWs nanoarrays was put into 80 mL potassium ferricyanide (K3Fe(CN)6) (1 mM) solution, and then heated at 60 °C for 10 h in an atmospheric oven. After cooling down to room temperature in the ambient, the obtained product was washed and dried at 60 °C overnight.P(Ni, Co, Fe) nanoarrays were fabricated through a gentle phosphorization process. The mass ratio of NaH2PO2·H2O and NiCo NWs/NiCo-PBA nanoarrays was about 10:1. Typically, NaH2PO2·H2O powder was put at the upstream side and center of the tube furnace, while the sample was placed at the downstream side and about 2 cm away from the hypophosphite powder. Then, the furnace was heated to 350 °C at a heating rate of 5 °C min−1 and maintained for 2 h under a nitrogen atmosphere to obtain P(Ni, Co, Fe) nanoarrays upon naturally cooling down the samples to room temperature.The morphologies of the materials were observed by scanning electron microscopy (SEM, ZEISS, Ultra 55) equipped with Energy-dispersive X-ray (EDX) spectroscopy. The crystal structure was evaluated by an X-ray diffractometer (XRD, PANalytical X’ Pert PRO, Netherlands). Fourier transform infrared spectroscopy (FTIR, Nicolet iS10) was used to detect the functional groups on the prepared samples.All the electrochemical performances of the electrode were carried out in a three-electrode system by using an as-prepared electrode as the working electrode, platinum wire as the counter electrode, and saturated calomel electrode as a reference electrode (SCE) with 6 M KOH aqueous solution as the electrolyte. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance spectroscopy (EIS) and long-term cycling measurements were performed using Gamry Interface 1000 electrochemical workstation.Commercial activated carbon (AC) was used to prepare the negative electrodes as follows: AC powder, carbon black and polyvinylidene fluoride in N-methyl-pyrrolidone were mixed in a ratio of 8:1:1 to obtain the electrode slurry. Then, the slurry was coated on Ni foam substrate and dried overnight at 120 °C in a vacuum oven. Asymmetric devices were assembled with P(Ni, Co, Fe) cathode and AC anode. According to the charge balance, the mass ratio of the two electrodes is around 20:1.The specific capacitances (Cs , F g−1) of the electrodes from CV and GCD tests were calculated according to the following equations: (1) C s = 1 / ( m · υ · Δ V ) · ∫ V 0 V I ( V ) d V (2) C s ′ = ( I · Δ t ) / ( m · Δ V ) where m (g) is the mass of active electrode material, ν (mV s−1) is the scan rate, ΔV (V) is the potential window, ∫ V 0 V I ( V ) d V is the integrated area under the CV curve, I (A) is the current during the discharge process, and Δt (s) is the discharged time.The energy (E, Wh kg−1) and power (P, W kg−1) densities of the asymmetric supercapacitor were determined from Eqs. (3) and (4), respectively: (3) E = 1 / 3.6 · 1 / 2 · C S ′ Δ V ′ 2 (4) P = 3600 · E / Δ t where Cs’ (F g−1) is specific capacitance calculated from the GCD test, ΔV’ (V) is the operating voltage, and Δt (s) is the discharging time.For the determination of electrocatalytic performance, the working electrode was the catalyst material, the counter electrode was a platinum wire and the reference electrode used was mercury-mercury oxide (Hg/HgO) (1 M KOH). The electrolyte was an alkaline solution of 1 M KOH with a pH of 13.6. Prior to each test, the electrolyte was degassed to remove oxygen by bubbling nitrogen gas for 30 min. All the potential shown below were transformed to the reversible hydrogen electrode (RHE) by using the equation E R H E = E H g / H g O + 0.098 V + 0.0591 × p H .Linear sweep voltammetry (LSV) was carried out in the window potential of 1∼1.7 V vs RHE at a scan rate of 5 mV s−1. The overpotential (η = ERHE – 1.23 V) was calculated at current densities of 100 mA cm−2 and 300 mA cm−2, respectively. Tafel slope (b) was calculated according to the Tafel equation η = b l o g ( i ) + a , where i represent the current density. EIS was measured at 1.6 V (vs RHE) with an amplitude of 5 mV and a frequency range between 100 kHz to 0.01 Hz.Electrochemical surface area (ECSA) was estimated by considering a double-layer capacitance ( C D L ) according to Eq. (5) where C ″ S is a general specific capacitance with a typical value reported for the alkaline electrolyte of 40 µF cm−2 [46–48]. To determine the double layer capacitance, a series of CV curves were measured between 0.4 ∼ 0.9 V at scan rates of 10, 20, 30, 50, 80 and 100 mV s−1. Plotting half the charge current (at 0.52 V vs RHE) against the scan rate yields a straight line with the slope equaling the double-layer capacitance. (5) E C S A = C D L / C ″ S Chronopotentiometry was performed to evaluate the stability of the electrodes when they are under a current density of 100 mA cm−2 for 240 min. LSV measurement was recorded after the stability tests and the overpotential was compared before and after the testing of the stability of all the electrodes.As shown in Scheme 1, the hierarchical P(Ni, Co, Fe) nanoarray electrodes were obtained by phosphating NiCo NWs/NiCo-PBA core/shell nanoarrays structure fabricated through an in situ self-sacrificial growth process. The NiCo NWs/NiCo-PBA core/shell nanoarrays were synthesized through a two-step method. During the hydrothermal growth of NiCo NWs core, the Ni ions cooperate with Co ions to form the NiCo NWs. Then, the NiCo-PBA nanoparticles shell was formed in a chemical bath, with the former core serving as a self-sacrificial source of Ni and Co ions inducing the in-situ growth of the shell. Finally, a phosphating step induces an ultrathin layer of P(Ni, Co, Fe) on the surfaces of the first and second layers. This 3D hierarchical overlapped triple-layered composite can potentially possess more surface/interface, higher active sites and better electron transfer efficiency with a more effectively modulated electronic structure.The morphologies and microstructures of the synthesized materials were studied by scanning electron microscopy (SEM). As shown in Fig. 1 a and b, quasi-vertical densely packed nanowires are uniformly anchored on the NF substrate, which forms interconnected and overlapping charge transfer pathways in the nanowires. Anisotropic NiCo NW arrays of 1–3 μm length and ∼ 30 nm width are formed. The morphologies of the as-prepared NiCo NW nanoarrays synthesized at 120 °C for different reaction times are shown in Fig. S1(†ESI). The Morphology tends to change from nanoflakes to nanosheets, mixed nanosheets/nanowires to only nanowires, governed by the crystallization growth processes. Furthermore, the energy storage performance of the electrode fabricated for 6 h is obviously superior compared to the others reported here (Fig. S2, †ESI); thus, the optimal preparation condition of 120 °C/6 h was chosen for further modifications in this work. The in situ induced second layer NiCo-PBA nanocubes have a size of ∼100 nm encapsulated on each nanowire like Chinese candied haws (Fig. 1c and d). Fig. S3 (†ESI) shows the corresponding Energy-dispersive X-ray (EDX) mapping of the NiCo-PBA nanoarrays. The homogeneous distribution of the Ni, Co, Fe, C and N elements intuitively suggests that the core/shell nanostructure is composed of NiCo NW cores and PBA nanoparticle shells due to the nature of the synthesis of these materials.Furthermore, with the introduction of P, some of the interconnected nanowire and nanoparticle joints are welded by the conductive phosphate layer to form interconnected and overlapped clusters with the appearance of a rougher surface of the P(Ni, Co, Fe) nanoarrays. (Fig. 1e and f). Furthermore, at higher magnification, the nanowire skeleton can be clearly observed, indicating the preservation of the nanowire structure. The conductive and interconnected phosphate layer can effectively increase the electron transfer capability of the composite. The flexible coating layer can also relieve the stress from volume changes, thus improving the energy storage and conversion performances that require cyclic charging and discharging sequences. The corresponding EDX mapping (Fig. 1g-l) of the P(Ni, Co, Fe) nanoarrays shows the homogeneous distribution of the Ni, Co, Fe, C, N and P elements, while the presence of the Fe, C and N elements also indicates that the PBA nanoparticles are uniformly distributed. Therefore, it can be confirmed that this complex 3D hierarchical overlapped triple-layer composite may be composed of NiCo NWs core, NiCo-PBA nanoparticles second shell and the outermost phosphide layer as hypothesized.X-ray diffraction (XRD) was employed to study the crystal structure of the prepared materials. As shown in Fig. 2 a and S4a (†ESI), the three strong peaks located at 44.51°, 51.85° and 76.37° can be ascribed to the NF substrate (JCPDS card No. 04-0850) [49]. It can be seen that all the peak intensities seem to be very weak and the noise is high, making it difficult to distinguish the different phases except for the NF substrates. This is due to the strong signals from the NF substrates, the complex composition of the composite and the relatively low crystallinity. For clear comparison to eliminate the interference of the strong NF peak and confirm the crystal phase, the crystal structures of all kinds of powder materials by using the same synthesized methods without NF substrates were collected and shown in Fig. S4b (†ESI), which is in good accordance with the results shown in Fig. 2a. Combined with the analysis of all XRD results, it is known that the peaks located at 17.1, 26.5, 33.7, 39.5, 44.4, 47.1 and 62.1° can be indexed to (0 2 0), (2 2 0), (3 0 0), (2 3 1), (0 5 0), (3 4 0) and (4 5 0) planes of NiCo carbonated hydroxides (JCPDS card No. 16-0164 for nickel carbonated hydroxide and JCPDS card No. 48-0083 for cobalt carbonate hydroxide, respectively), which also demonstrates the successful formation of NiCo compound on the NF substrate [50]. In addition, the successful growth of the shell materials NiCo-PBA nanoparticles on the NiCo NWs core by the in situ induction is also demonstrated. The extra peaks located at 17.2, 24.3, 34.6 and 38.6° correspond to the (2 0 0), (2 2 0), (4 0 0) and (3 3 1) planes of PBAs (JCPDS card No. 46-0908 for Ni2Fe(CN)6·0.5H20 and JCPDS card No. 46-0907 for Co3[Fe(CN)6]2·10H20, respectively). After phosphating, the peaks located at 31.8, 41.1, 48.2 and 54.3° can be indexed to planes of phosphates (JCPDS card No. 13-0213 for NiP2, JCPDS card No. 29-0497 for CoP and JCPDS card No. 34-0996 for FeP4, respectively).In order to understand the bonding in the complex structure of the P(Ni, Co, Fe) nanoarrays, Fourier transform infrared (FTIR) spectra were recorded from 750 to 4000 cm−1 (Fig. 2b). The peaks located at 1379.8, 1067.4 and 827.8 cm−1 can be assigned to arise from symmetrical stretching vibration, stretching vibration and bending vibration of CO3 2−, respectively, confirming the existence of CO3 2− [51,52]. The peaks centered at 3598.5, 3496.3 and 1484.4 cm−1 can be ascribed to OH stretching and bending vibrations in water molecules [50]. Moreover, the weak peaks located under 1000 cm−1 reveal the existence of metal-OH bonds [53]. These results are consistent with the XRD analysis, confirming the compositions of NiCo NWs. Compared to the FTIR results from NiCo NWs, the peaks centered around 2091.9 cm−1 is the most prominent corresponding to the Fe-CN-M band in PBAs [54]. After phosphating, all the peaks are weakened and are broader, which suggests the complex composition and low crystallinity of the composite, which again is in agreement with the results obtained from XRD studies. Thus we have demonstrated the successful formation of the complex 3D hierarchical overlapped triple-layer P(Ni, Co, Fe) nanoarrays, which may possess high surface/interface active sites, high electron transfer efficiency, stable structure, and thus can be used as multifunctional electrodes.In order to evaluate the electrochemical performances of the electrodes, a series of comparative studies were designed with the binder-free NiCo NWs, NiCo NWs/NiCo-PBA and P(Ni, Co, Fe) electrodes in 6 M KOH aqueous solutions in a standard three-electrode system. Fig. 3 a presents the cyclic voltammetry (CV) curves of these electrodes with the potential range of 0 ∼ 0.6 V at 20 mV s−1. Obviously, the P(Ni, Co, Fe) electrode has the largest peak current and integration area, indicating the highest capacitance of the complex 3D hierarchical overlapped triple-layer composite. In addition, compared to a single crystal phase, the positions of the redox peak pairs of the multi-phased materials increase but shift, showing more redox reaction sites, leading to a higher capacitance. Fig. 3b describes the CV curves of the P(Ni, Co, Fe) electrode at different scan rates from 1 to 20 mV s−1. An obvious couple of redox peaks reveal the typical electrochemical behaviors generated from Faradaic reactions related to Ni-O/Ni-O-OH and Co-O/Co-O-OH [55,56]. With the increase in the scan rates, a slight shift of redox peaks can be observed, which indicates good electrochemical reversibility.The galvanostatic charge-discharge (GCD) curves at 5 mA cm−2 were measured as shown in Fig. 3c in order to further confirm the practical usefulness of these electrode materials. All the GCD curves have approximately symmetric charge and discharge time, which also indicates good reversibility. Obviously, the P(Ni, Co, Fe) electrode has the longest discharge time, demonstrating the highest specific capacitance. Fig. 3d and 3e show the GCD curves of the P(Ni, Co, Fe) electrode between 0 and 0.5 V at different current densities. The trend of the GCD curves follows same trend as results obtained in the CV measurements. GCD curves of NiCo NWs and NiCo NWs/NiCo-PBA electrodes are provided in Fig. S5 (†ESI) for comparison. Obviously, the P(Ni, Co, Fe) electrode exhibits a much higher capacitance as presented in Fig. 3f. The specific capacitance of the P(Ni, Co, Fe) electrode is calculated to be 1125.8 F g−1 (3.7 F cm−2) at a current density of 2 mA cm−2, which is approximately twice that of the NiCo NWs (524.9 F g−1, 1.8 F cm−2), and about 4 times higher than that of the NiCo NWs/NiCo-PBA (275.6 F g−1, 1.4 F cm−2). Moreover, a high specific capacitance of 565.1 F g−1 is maintained for the P(Ni, Co, Fe) electrode even at a high current density (20 mA cm−2), which suggests an excellent rate capability. These superior electrochemical performances of the P(Ni, Co, Fe) electrode are obtained due to the complex 3D hierarchical overlapped triple-layer nanoarrays structure. Fig. 4 a shows the corresponding relationship between the log (scan rate) and log (peak current) calculated from CV measurements (the inset in Fig. 4a, CV curves of the P(Ni, Co, Fe) electrode at scan rates from 0.2 to 2 mV s−1) to explore the energy storage mechanism of the electrochemical performances following the relationship as given in the equation: i=aνb , where a and b are constants, and i is the current. The b value can reflect the energy storage mechanism. When it is close to 0.5, it means the diffusion is the main process; when it is approximately next to 1, it stands for the capacitive behavior [7,57]. As shown in Fig. 4a, we obtain b values close to 1 and the good linear relationship both indicate that the as-prepared P(Ni, Co, Fe) electrode is not controlled by the ion diffusion, revealing the capacitive characteristics.Electrochemical impedance spectroscopy (EIS) was studied to determine the ion transport kinetics as shown in Fig. 4b. The inset shows the equivalent series circuit, and the corresponding resistance of each electrode is shown in Table S1 (†ESI). Fig. 4b shows that the Nyquist plot of the P(Ni, Co, Fe) electrode has the smallest semicircle in the high-frequency region, indicating the smallest charge transfer resistance at the interface between the electrolyte and electrode, which may be caused by the improved activity of the phosphate layer and the effective charge transfer efficiency, compared to the other electrodes studied here [58]. The 45° Warburg region at the middle frequency region is related to the diffusion of the electrolyte ions into the bulk of electrodes. Compared to the NiCo NWs electrode, the NiCo NWs/NiCo-PBA electrode shows no obvious increase, indicating that the diffusion resistance in the material is not significantly reduced due to the lower conductivity of the PBA nanoparticles [59]. Obviously, the P(Ni, Co, Fe) electrode presents the fastest migration and diffusion after phosphating due to the introduction of P and the complex 3D hierarchical nanostructures that enhance the electrical conductivity of the material. Of course, the highest slope of the P(Ni, Co, Fe) electrode at low frequencies indicates the best capacitive performance. As shown in Fig. 4c, the cycling stability of the P(Ni, Co, Fe) electrode is explored to evaluate its potential for application in practical devices. The P(Ni, Co, Fe) electrode possesses much better cycling stability with capacitance retention of 97.1% after 5000 cycles at 50 mA cm−2 and 89.9% after continuous 5000 cycles at 100 mA cm−2, which reveals the enhanced structural stability of the 3D hierarchical nanoarrays, and thus effectively relieving the volume expansion during the repeated charge-discharge process. The inset in Fig. 4c presents the GCD curves before and after long-term cycling, and the little-changed shapes indicate the fast rate capability and good stability. Moreover, the insert in Fig. 4d shows the SEM image of the complex composite after long-term cycling tests, which demonstrates the integrity of the synthesized electrode materials.To further confirm the usefulness of these materials for practical applications, the as-prepared P(Ni, Co, Fe) electrode was used as the cathode to construct an asymmetric supercapacitor (ASC) with activated carbon cloth as anodes. In Fig. S6a-d (†ESI) we have plotted the electrochemical performances of the AC anode in 6 M KOH. In Fig. S6e (†ESI) the comparison CV curves of P(Ni, Co, Fe) and AC electrodes at 20 mV s−1 were presented. Fig. S6f (†ESI) presents the CV curves of the ASC device at 10 mV s−1 under different applied voltages which allowed us to choose a working voltage of 1.6 V for further studies. In Fig. 5 a the schematic diagram of the device structure is presented while in Fig. 5b the CV curves of the ASC device at different scan rates are shown and in Fig. 5c the GCD curves at different current densities are plotted. It can be immediately observed that the CV curves have similar shapes, indicating fast electron transport in the devices. The capacitance is about 42.1 F g−1 at 0.1 A g−1 (Fig. 5d), which may be limited by the lower capacitance of the AC anode. As shown in Fig. 5e, the capacitance retention is 93.9% after 5000 cycles at 5 A g−1, demonstrating good cycling stability, which is further confirmed by the CV and EIS results before and after cycling (the insets in Fig. 5e). After a long-term charging/discharging cycling, there seems to be a slight upward trend in capacity, probably because of the improved wettability and permeability of the electrolyte into the bulk of the electrode. Moreover, from the Ragone plots as shown in Fig. 5f, the estimated energy density is 15.0 Wh kg−1, and the maximum power density is 4000 W kg−1. Two ASC devices were connected in series to successfully power different colored LED indicators after charging at 10 mV s−1, showing the promise for practical applications (inset of Fig. 5f).These electrodes were also evaluated for oxygen evolution reaction (OER) in alkaline media (1 M KOH aqueous solution). Linear sweep voltammetry (LSV) curves are shown in Fig. 6 a. It is possible to notice two extra oxidation peaks at 1.30 and 1.35 V before the OER takes place, corresponding to the oxidation of Co2+/Co3+ and Ni2+/Ni3+, respectively. These transformations are inevitable, and other works described such reactions enhancing the performance of the electrodes. Wu et al. found that the incorporation of trivalent nickel (Ni3+) was related to the improvement of OER activity and electrode stability [60]. Meanwhile, Menesez et al. found that Co3+ acts as an active site enhancing the electrophilicity of adsorbed O facilitating the formation of OOH species [61]. LSV curves also indicate a significant improvement in the electrocatalytic activity of the P(Ni, Co, Fe) electrode reaching an overpotential value of only 252 mV at 100 mA cm−2 Fig. 6b), which presents a better performance compared to the classical ruthenium oxide (RuO2) electrocatalyst for OER that shows an overpotential of 270 mV at 50 mA cm2 [62]. Moreover, the Tafel slope (Fig. 6c) shows that the overpotential needed to increase the production rate of oxygen is lower for P(Ni, Co, Fe) (68 mV dec−1), followed by NiCo NWs/NiCo-PBA and NiCo NWs with values of 106 and 113 mV dec−1, respectively. One of the main challenges of OER lies in its mechanism, which is a four-electron transfer through multi-step reaction pathways where several intermediates are formed. Researchers have proposed several paths explaining the mechanism of oxygen evolution [63]. Bockris and Otagawa studied the OER mechanism in perovskites, involving electron transfer steps and chemical steps (association and dissociation), establishing that the adsorption of the hydroxyl group on the electrode occurs as a first step followed by the rate-determining step (RDS) corresponding to the electrochemical desorption of OH forming hydrogen peroxide as an intermediate which decompose to yield oxygen [64]. Breaking the metal and oxygen group bond is involved in the RDS. The Tafel slope is closely related to the OER mechanism. From the literature, it is possible to find that when the Tafel slope is 120 mV dec−1 the RDE corresponds to a single electron transfer reaction, but when this value is 60 mV dec−1, RDE is the chemical reaction [65]. The most accepted OER mechanism in alkaline media is shown in Eqs. (6)-((10) where M corresponds to an active site and the place where the specie is adsorbed [66,67]: (6) M + O H − ↔ M − O H + e − (7) M − O H + O H − ↔ M − O + H 2 O + e − (8) M − O + M − O ↔ 2 M + O 2 (9) M − O + O H − ↔ M − O O H + e − (10) M − O O H + O H − ↔ O 2 + M + H 2 O + e − As was mentioned before changes in the Tafel slope have been associated with the RDS during the OER process, and smaller values suggest that the RDS is at the end of the electron transfer reaction, being an indication of a good electrocatalyst [65]. Mary et al. have associated a value of 40 mV dec−1 for Ni films with an RDS corresponding to the formation of peroxide intermediates (M-OOH) [68]. Doyle et al. indicated that a Tafel slope of 120 mV dec−1 is most likely associated with the adsorption of OH− as a rate-limiting step for iron oxide electrodes, and values of 64 mV dec−1 correspond to M-O as RDS for NiFe layered double hydroxide [69,70]. Therefore, in this work, the RDSs of NiCo NWs and NiCo NWs/NiCo-PBA are close to 120 mV dec−1 indicating that the limiting step for both electrodes is the formation of M-OH. Meanwhile, the P(Ni, Co, Fe) electrode with a Tafel slope of 68 mV dec−1 is closest to 64 mV dec−1 leading to the intermediate formation of M-O as the rate-limiting step for the OER process thus explaining the obtained better catalytic activity. Tafel slope corresponds well with the increase of overpotential from 100 to 300 mA cm−2 (Fig. 6b) where P(Ni, Co, Fe) only need an increase of 31 mV, unlike for the other electrodes. This attractive electrode performance may be due to the higher interfacial charge transfer kinetics as was mentioned above (Fig. 4b), and a better mass transfer due to the open structure of the electrode together with a high surface-active site density, which is comparable to that of most of high-end OER electrodes reported in the literature.The electrochemical surface area (ECSA) was evaluated by the electrochemical double-layer capacitance (CDL ) calculated at 0.52 V from CV curves (Fig. 6d and S7, †ESI) at different scan rates. As shown in Fig. 6e, the CDL of P(Ni, Co, Fe) is 49,160 µF, whereas that of the NiCo NWs/NiCo-PBA and NiCo NWs electrodes were 2010 and 688 µF, respectively. ECSA values calculated following Eq. (5) are summarized in Table S2 (†ESI). The P(Ni, Co, Fe) electrode shows a much higher specific area (83 m2 g−1) compared to the other electrodes fabricated in this study, indicating that the number of effective active sites for water oxidation increases after the phosphorization process leading to a better electrochemical oxygen evolution reaction activity. The electrocatalytic stability studied at 100 mA cm−2 (Fig. 6f) shows that the overpotential remains stable for 240 min under a continuous OER process for all the electrodes. A slight decrease in the working potential during the first 60 min for NiCo NWs and NiCo NWs/NiCo-PBA can be attributed to the activation of the catalyst with an increase in the active species participating in the OER, which is typically found in metal-hydroxide catalysts [71,72]. In contrast, the P(Ni, Co, Fe) electrode exhibits a lower and constant potential of 1.52 V during the chronopotentiometry measurement.During the chronopotentiometry test, the overpotential at 100 mA cm−2 changes until an equilibrium is reached. LSV curves were measured before and after the chronopotentiometry test as shown in Fig. 7 a. Overpotential increases by 41, 102 and 40 mV for NiCo NWs, NiCo NWs/NiCo-PBA and P(Ni, Co, Fe), respectively. The higher increase for NiCo NWs/NiCo-PBA is associated with the low stability of Prussian blue analogues in alkaline media [73,74]. Fig S8 (†ESI) shows a small increase in P(Ni, Co, Fe) Tafel slope after the chronopotentiometry test, suggesting the nature of the material and its operation mechanism slightly changed. Overpotential increase is in agreement with what is found in the literature for CoMnP and NiCoP, where the deactivation of the materials is associated with the formation of oxides like metal oxides (MOx), phosphate (POx), or phosphides (PO) on the surface [75,76]. Nonetheless, the P(Ni, Co, Fe) electrode still shows a much lower overpotential than the other two electrodes, revealing its excellent OER electrocatalytic activity.To explain the charge transfer process during OER, EIS measurements were performed at a potential of 1.6 V vs RHE. Nyquist plots in Fig 7b present a semicircle shape with a complete loop demonstrating that mass transport is not a limitation during the OER process. EIS data was modeled using the equivalent circuit inset in Fig. 7b. This circuit includes a solution resistance (Rs), two parallel constant phase elements (CPEf and CPEct), one resistance associated with the substrate surface (Rf) and a charge transfer resistance (Rct) representing the resistance caused by the OER process on the catalyst interface. Table S3 (†ESI) lists the values of the parameters for the fitting process. The inset figure in Fig 7b shows the charge transfer resistance, which demonstrates that the Rct value (0.196 Ω) of P(Ni, Co, Fe) is much smaller than the values for NiCo NWs (1.257 Ω) and NiCo NWs/NiCo-PBA (0.975 Ω) suggesting a more efficient charge transport.This multifunctional electrode comprising of 3D hierarchical overlapped triple-layer nanoarrays structured P(Ni, Co, Fe), presents superior electrochemical performances including high specific capacitance, high rate capability, and low overpotential, that arise from: 1) the unique interconnected and overlapped nanowire structure anchored on the 3D NF substrate, the in situ induced PBA nanoparticles, and highly conductive phosphate layer provide multidimensional and multiscale synergistic effects, boosting the electron transfer, ion diffusion and catalytic reaction kinetics; 2) the multidimensional hierarchical structure with close contact interface/surface provides more edges and defects, thereby generating more effective surface area and active sites leading to higher electrocatalytic activity and the adsorption capacity of electrolyte ions; 3) the introduction of the phosphorous enhances the electrical conductivity of the material, reducing electron losses during charge transfer process, thus facilitating the redox reaction, improving the specific capacitance, and increasing the electrocatalytic activity of OER.A simple growth strategy to obtain a complex 3D hierarchical overlapped triple-layered nanoarray structured P(Ni, Co, Fe) electrode materials was developed by phosphating NiCo-NWs/NiCo-PBA core/shell nanoarrays structures fabricated through an in-situ self-sacrificial growing process. Owing to the unique multidimensional structure, more surface-active sites, improved conductivity, and enhanced catalytic reaction kinetics, the promising electrodes exhibit superior specific capacitance of 1125.8 F g−1 (3.7 F cm−2) at 2 mA cm−2, good rate capability and improved cycling stability. The asymmetric device assembled with the hybrid electrode as cathode and activated carbon as anode show an energy density of 15 Wh kg−1 and a power density of 4000 W kg−1. Furthermore, the hybrid structure presents excellent oxygen evolution reaction performance with an overpotential of 252 mV at 100 mA cm−2 and low Tafel slope of 68 mV dec−1, and overall water splitting abilities with a low cell voltage of 1.52 V at 100 mA cm−2 and stability for 4 h. The strategy presented in this work is simple, effective and novel for designing highly conductive multifunctional electrode materials for energy storage and electrocatalysis. Xingyan Zhang: Conceptualization, Methodology, Formal analysis, Investigation, Visualization, Supervision, Writing – original draft. María Isabel Alvarado-Ávila: Formal analysis, Investigation, Visualization, Writing – original draft. You Liu: Software, Visualization. Dongkun Yu: Formal analysis. Fei Ye: Formal analysis. Joydeep Dutta: Resources, Supervision, Writing – original draft.The authors declare that they have no competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.X. Y. Zhang would like to thank Ragnar Holms Foundation for the Post-doctoral fellowship. M. I. Alvarado-Ávila would like to thank the National Commission for Scientific and Technological Research, (CONICYT) for the Doctoral scholarship “Beca Chile” 2018-72190682. D. K. Yu would like to thank the China Scholarship Council (CSC) for the Doctor scholarship (202006360037). J. Dutta acknowledges the support of the MISTRA Terraclean project (Diary No. 2015/31) and Vinnova (Diary No. 2021-02313).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2022.141582. Image, application 1

Highly efficient and environmentally friendly multifunctional electrode materials for application in supercapacitors to electrocatalysis are important for advances in the future of electrical energy storage and green hydrogen production. This work reports a simple growth strategy to obtain hierarchical P(Ni, Co, Fe) modified electrodes by phosphating a core/shell composite of nickel-cobalt (NiCo) Prussian blue analogues fabricated through an in situ self-sacrificial growth process. Due to the unique microstructure, abundant surface-active sites, and enhanced interfacial conductivity, the hybrid electrode exhibits specific capacitance as high as 1125.8 F g − 1 (3.7 F cm−2) at 2 mA cm−2, excellent rate capability and improved cycling stability (97.1% retention capacitance after 5000 cycles at 50 mA cm−2 and 89.9% after continuous 5000 cycles at 100 mA cm−2). Furthermore, the hybrid structure shows excellent oxygen evolution reaction performance with an overpotential of 252 mV at 100 mA cm−2 and 283 mV at 300 mA cm−2, with a low Tafel slope of 68 mV dec−1, and overall water splitting abilities with a cell voltage of 1.55 V at 100 mA cm−2. This work provides insights into the design of next-generation high-performance multifunctional electrode materials by controlling the surface/interface of multicomponent structures for enhancing their properties.

The renewability and sustainability issues of producing chemicals, materials and fuels from depleted fossil resources have greatly accelerated and expanded the development of renewable energies and resources [1–3]. As a highly-accessible and renewable natural carbon source, biomass is currently gaining traction as a feedstock for the manufacture of valuable chemicals, functionalized materials and high-energy-density biofuels [4–6]. The replacement of fossil-derived products with bio-based ones is of paramount importance for human society to develop sustainably. In this regard, the chemical processing of cellulose, the most plentiful component of biomass on the planet, is one of the most promising and appealing approach to produce value-added chemicals. Because the resultant products display comparative or ever superior characteristics in many ways to fossil-based counterparts [7–9]. It is well known that the hydrolysis of cellulose through chemical or biological approach results in the formation of D-glucose, which can be subsequently transformed into 5-(hydroxymethyl)furfural (HMF), a key biomass-derived platform molecule, via successive isomerization and dehydration process [10–12]. HMF contains aldehyde and alcohol group at its 2 and 5 positions, both of which are pretty strong reactivity in oxidizing and reducing environment [13,14]. Therefore, HMF as well as its derivate are currently utilized to produce a wide range of high-value bio-based chemicals, which are expected to (partially) replace voluminously consumed petroleum-based chemicals for the manufacture of fine chemicals, bio-fuels and plastics [15–17].Among various HMF valorization routes, selective oxidation of the aldehyde and/or alcohol group(s) of HMF can fabricate several valuable and intriguing furanic chemicals (Fig. 1 ), such as 2,5-diformylfuran (DFF), 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), formyl 2-furancarboxylic acid (FFCA), 2,5-furandicarboxylic acid (FDCA) and furan-2,5-dimethylcarboxylate (FDMC). These products have been extensively described as monomers for novel polymers, pharmaceutical intermediates, precursors for sustainable dyes and fungicides as well as platform chemicals for other value-added chemicals [18–24] (Fig. 1). In particular, FDCA has a similar dicarboxylic acid structure to the fossil-derived terephthalic acid (TPA), thus it has been hailed as a renewable substitute to TPA for the manufacture of polyethylene terephthalate (PET)-analogous polymeric materials [25]. It is interesting to note that polyethylene furanoate (PEF), fabricated by the polymerization of FDCA and ethylene glycol, even discloses more attractive thermal properties concurrently with superior gases (O2, CO2, and H2O) barrier resistance with regard to PET, which enables it a promising packing material for food and beverage [26,27]. It has been estimated that an annual market volume for FDCA is approximately 50.5 million metric tons (MT) with an estimated value of $ 50.5 billion [28]. Recently, Avantium received a funding of 25 million Euros and plans to build an FDCA pilot plant with an annual output of 5000 tons from fructose in 2023 [29]. Novamont and Stora Enso also announced that they will invest 10 or 9 million Euros to build an FDCA pilot plant from fructose respectively [30,31].Recently, great efforts have been dedicated to developing new catalysts for the selective oxidation of HMF via thermo-catalytic, electro-catalytic or photo-catalytic approach [32,33]. There are several reviews documented the progresses in the selective oxidation of HMF [33–37]. In comparison with other methods, thermo-catalysis is the most widely investigated one because of its high catalytic efficiency, convenient operation and ample catalyst candidates. Especially, the heterogeneous catalyst holds prominent advantages over the homogeneous counterparts regarding the separation and reusability of the catalyst. It should be noticed that the selective oxidation of HMF to DFF, HMFCA, FFCA or FDCA over heterogeneous catalyst actually proceed via a similar reaction mechanism but with different oxidation degrees. In many cases, the same catalyst can be finely addressed to produce different oxidation products from HMF through manipulating the reaction parameters (reaction solvent, temperature, time and so on). For instance, a semicrystalline nanoporous multiblock copolymer matrix supported Au NPs (Au/sPSB) enabled the selective oxidation of HMF to DFF, HMFCA, FFCA, FDCA or FDMC with high yields respectively through judicious choice of the reaction condition and medium [38]. Thus, the design of efficient heterogeneous catalysts for the oxidation of HMF follows common principles in many cases no matter what the target product.In this review, we will present an overview of the established heterogeneous catalyst design strategy for the selective oxidation of HMF over transition metal (noble and cheap metal)-based catalyst as well as the metal-free catalyst. In contrast to the previously published reviews, we highlight the universal catalyst design strategy toward enhancing the catalytic performance of the catalyst for the oxidation of HMF. Particular attention will be focused on the following aspects: (1) the reaction mechanism of HMF oxidation over different catalysts; (2) the developed approaches aiming at boosting the catalytic activity and stability of the supported catalyst via manipulating the basic-acidic property, the redox ability, porosity, as well as the surface properties of the support; (3) enhancing the catalytic performance of the catalyst through introducing other constituents as the promoter to tailor the geometric as well as the electronic configurations of the active centers; (4) the reported catalytic systems for the one-pot oxidative esterification of HMF to FDMC.Noble metal-based catalysts, such as gold (Au), palladium (Pd), platinum (Pt), ruthenium (Ru) and silver (Ag) have been extensively studied for the selective oxidation of HMF for a long time because of their high activity for activating molecular oxygen (O2). Although these noble metal catalysts generally revealed high catalytic activity for HMF oxidation, their catalytic behaviors are quite diverse and strongly relate to the type of active metal species and the basicity of the reaction medium. For example, Ru-based catalysts were found to be more active for the conversion of HMF to DFF in the organic solvent than others [39,40]. A few effort has been devoted to investigating the mechanism of oxidation of HMF to DFF over Ru catalysts. Nie and co-workers well demonstrated that the oxidation of HMF to DFF over Ru/C in N,N-dimethylformamide (DMF) operated by the Langmuir–Hinshelwood mechanism [40] (Scheme 2 ), in which HMF and O2 were firstly adsorbed and dissociated to corresponding alcoholate (R–CH2O∗), hydrogen (H∗) and atomic oxygen (O∗) species over Ru/C surface, respectively. Then, a hydrogen of R–CH2O∗ was abstracted by O∗ species, resulting in the generation of the adsorbed DFF (R–CHO∗) and hydroxyl (OH∗) species. Subsequently, the resultant OH∗ species react with H∗ species to form the adsorbed water (H2O∗) species. Finally, R–CHO∗ and H2O∗ quickly desorb from the Ru/C surface to finish the catalytic cycle. Kinetic isotope effects (KIE) were also investigated in this study and proved that the activation of C–H bond is the kinetically-relevant step for the oxidation of HMF into DFF over Ru/C. A similar reaction process was also proposed by Sarmah and co-workers for the oxidation of HMF to DFF over Ru nanoparticle-supported H-beta catalyst (Ru/H-beta) [41].As for the consecutive oxidation of HMF to FDCA in aqueous solution with oxygen, two competitive reaction paths were reported according to the first step oxidation reaction, which can be dominantly proceeded either via oxidizing the aldehyde side chain of HMF to carboxyl group (Scheme 1, route 1) or converting its hydroxymethyl moiety to aldehyde group (Scheme 1, route 2). Then, the resultant intermediate (HMFCA or DFF) was oxidized to FDCA via FFCA. It has been widely observed that Au- and Pd-based catalysts offered higher catalytic activity for the oxidation of aldehyde group of HMF than that of hydroxymethyl side chain [33,34,42], thus the oxidation of HMF over Au- and Pd-based catalysts generally conducted via route 1 even under base-free conditions (Fig. 2 ) [43–45]. In contrast, Ru catalysts favored the oxidation of hydroxymethyl moiety instead of the aldehyde group of HMF in the first oxidation step, therefore the oxidation of HMF to FDCA over Ru-based catalyst followed route 2 [46,47]. Moreover, the reaction path for the HMF oxidation also has been influenced by the basicity of the reaction medium. Pt and AuPd alloy catalysts preferred the oxidation of the aldehyde group of HMF than hydroxymethyl moiety in the first oxidation step (route 1) might because of the basic medium favoring hydration of the aldehyde to the gem-diol [48,49]. Whereas different reaction pathway (route 2) was observed for the oxidation of HMF over Pt and AuPd alloy catalysts under base-free condition [50–52].Even though the reaction path for the oxidation of HMF to FDCA over noble metal catalysts relies on the active metal species and the basicity of the reaction medium, the role of water or oxygen during the HMF oxidation process was demonstrated to be the same in different catalytic systems by isotope labeling technology. In 2012, Davis and co-workers detected 18O incorporated HMFCA and FDCA products when HMF oxidation reaction was performed over Au/TiO2 and Pt/C catalysts in H2 18O under 16O2 pressure [53]. Whereas 18O atoms were not found to be incorporated into HMFCA and FDCA when analyzing the products of HMF oxidation under 18O2 pressure in H2 16O. These results indicated that aqueous-phase oxidation of the aldehyde proceeds through a geminal diol generated by the reversible hydration reaction of aldehydes and water in a basic environment (Fig. 2). Thus, water served as the direct oxygen source during the oxidation of the aldehyde side chain of HMF, DFF or FFCA to carboxyl group. Meanwhile, O2 was considered to play an indirect role during oxidation through scavenging the electrons deposited over the metal NPs to end the catalytic cycle. A similar reaction mechanism was also reported in many other works [46,54,55].To further elucidate the role of molecule oxygen during the HMF oxidation process, electron paramagnetic resonance (EPR) and density functional theory (DFT) calculations were utilized to detect and simulate the generated active oxygen species over noble metal NPs. Liu et al. found that different active oxygen species was produced over different facets of Pt NPs, such as Pt (100) surface prefer to generate ‧OH species whereas ‧O2 − species are largely formed on a Pt (111) surface, both of them could boost the dehydrogenation reaction by promoting the break of O–H and C–H bonds during HMF oxidation [48]. Lei and co-workers also revealed that oxygen could be hydrogenated to H2O2 over Pd (111) surface, which could remove the extra electrons in the HMF oxidation process and complete the catalytic cycle [56]. Thus, O2 participated in the reaction by forming active oxygen species, which are generally involved in the dehydrogenation process, or served as a scavenger to wipe out the deposited electrons over the metal NPs during the HMF oxidation process.However, there is a lack of studies that unravel the fundamental reasons for the different reaction paths for the oxidation of HMF over various noble metals, which is of great significance for the understanding of the reaction mechanisms. Investigating the adsorption behavior of the intermediates during the HMF oxidation process over different noble metal species may provide some useful information. Efforts are also supposed to be dedicated to gain more insights into the oxygen activation process over different noble metals.Stabilized metal nanoclusters (NCs) or nanoparticles (NPs) in the solution exhibit some peculiar advantages over the traditional supported metal clusters for the oxidation of alcohols because of their precisely controllable sizes and shapes as well as freely rotational and three-dimensional nature [57,58]. However, only a few works reported the selective oxidation of HMF over the dispersed noble metal catalytic systems. The first colloidal noble metal NPs for the HMF oxidation was reported by Siankevich et al. [55] They prepared a series of polyvinyl pyrrolidone (PVP) protected Pt NPs (Pt-PVP-GLY) with different sizes (1.5–5 nm) for the oxidation of HMF in the base-free aqueous solution. The activity of the Pt NPs decreases with the increasing of the size of NPs and Pt NPs with the smallest size of 1.5 nm afforded the highest FDCA yield of 95%. Similarly, size-dependent effect on HMF oxidation activity was also found in PVP stabilized Pd nanoparticles (Pd NPs) [59].The morphology of the NPs in the solution phase can be easily manipulated by regulating the preparation conditions, thereby allowing one to better understand the structure–reactivity relationship. Liu et al. studied the effect of different exposed facets on the catalytic activity of Pt nanocrystals for the oxidation of HMF [48]. Interestingly, oxygen was activated to different active oxygen species over different facets. Specifically, Pt (100) surface prefers to generate ‧OH species whereas ‧O2 − species are largely formed on a Pt (111) surface [48]. Density functional theory (DFT) calculations revealed that the energy barriers of O–H and C–H bond scission on ‧OH-precovered Pt (100) surface are much lower than that on ‧O2 −-precovered Pt (111) surface [48]. Therefore, Pt (100) surface displays a higher activity for the HMF oxidation than Pt (111). As for single-crystalline Pd nanocrystals, (111)-faceted nanooctahedra (Pd–NOs) disclosed 2.6 times higher turnover frequency (TOF) than (100)-faceted nanocubes (Pd–NCs) for the oxidation of HMF to FDCA [56]. Unlike the situation of Pt nanocrystals, DFT results indicated that oxygen tends to hydrogenate to H2O2 over Pd (111) surface, which can participate in the HMF oxidation process and complete the catalytic cycle [56]. In contrast, oxygen prefers to dissociate to the atom state over Pd (100) surface, which cannot serve as active oxygen species for the HMF oxidation reaction. These efforts provided some profound insights into the structure–reactivity relationship of noble metal nano-catalysts for the oxidation of HMF. However, some issues deserved to be further studied in the colloidal noble metal NPs systems for the oxidation of HMF, such as the lack of in-depth understanding of the effect of different stabilizer, the relatively high catalyst loading and the requirement of base additive to reach a high FDCA yield in some cases.The immobilization of noble metal NPs on a support not only can improve their stability and manipulate their spatial distribution but also can enhance their activity through tuning the support properties and the metal-support interactions. The manipulation of the basic-acidic property, the redox ability, porosity, as well as surface property of the support can effectively boost the activity of the metal nanoparticles. One should be noted that these characteristics of the support do not separate from each other but actually act together to make the catalyst more active. However, to make the story clear and understandable, we have summarized the developed strategies for engineering each property separately as an individual section. Furthermore, the fabrication of supported bimetallic NPs is also an important strategy for tuning the geometric as well as the electronic configurations of NPs. The developed supported bimetal catalytic systems are also discussed in this review.The base/acid properties of the support are well demonstrated to play a critical role in the catalytic activity of the supported noble metal catalysts for the oxidation of HMF. In general, the reaction rate of the alcohol oxidation reactions can be enhanced in an alkaline environment or the presence of basic support. Zhu et al. revealed that the strong basicity of Mg-Beta zeolites can promote the catalytic activity of Au/Mg-Beta catalyst for the HMF oxidation and reduce the dosage of additional base [60]. Because the basic environment could favor the formation of alkoxide intermediate as well as smooth the activation of C–H in alcohol oxidation process [61,62]. Therefore, we begin our discussion with catalytic systems based on basic supports. As summarized in Table 1 , typical basic support, including MgO, Mg(OH)2, hydrotalcite (HT) and MgAl2O4 etc. were widely employed for supporting Au, Ru, Pd and AuPd NPs for the oxidation of HMF. Interestingly, the employment of basic supports enabled the efficient oxidation of HMF to DFF or FDCA in the base-free condition in most cases (Table 1). Actually, the involvement of base additives during the HMF oxidation process raises many concerns regarding the product purification process and production costs. One should be noted that the activity of the catalyst strongly relates to the basicity of the basic support. Generally, strong basic supports (e.g. MgO, Mg(OH)2) afforded a relatively lower DFF or FDCA yield (selectivity) than that of supports with moderate basic sites (e.g. HT and MgAlO). Liu et al. attributed the relatively lower activity of Ru/MgO than Ru/Mg2AlOx to the strong basic nature of the MgO, which promoted the undesirable degradation and polymerization by-reactions of the susceptive HMF [40]. However, this is probably not the only reason for the inferior performance of Ru/MgO. Antonyraj et al. found that the surface area of Ru/MgO (73 m2 g−1) is much lower than Ru/MgAlO (200 m2 g−1) [63], and the larger Au particle size of Au/MgO (> 10 nm) than Au/HT (3.2 nm) was also observed [64], which are probably the other reasons for the inferior catalytic performance of Au/MgO for the conversion of HMF to FDCA.Even though high FDCA yields (95%–100%) were accomplished over the basic support catalytic systems under base-free conditions, the generated acidic products (FDCA, FFCA and HMFCA) may react with the basic support, resulting in the etch of active component and support, which leads to the downgrade of the catalyst activity and stability. In particular, 26%–38% mol/mol Mg of the support leached from Ru(OH)x/MgO and Ru(OH)x/HT catalyst after the reaction and the concentration of Mg2+ in the solution corresponded to the generated FDCA concentration, suggesting that the basic support may serve as a solid base to neutralize the produced FDCA and push the reaction forward [65]. Unexpectedly, HT supported Au [64], Pd [43] or Pt [50] catalysts showed similar FDCA productivity (5.7–6.9 molFDCA h−1 molmetal −1) and demonstrated to be relatively stable for the base-free oxidation of HMF to FDCA. An FDCA yield decrease of around 10% was observed after three or five runs with only a small amount of support leaching (around 3% Mg2+ etched after each run) for Au/HT [64]. That's to say, the transformation of HMF to FDCA can also proceed smoothly without stoichiometrical consumption of basic support, indicating that the basic support not just simply acted as solid base to react with FDCA to form FDCA salts. For example, Gupta et al. found that the basicity of HT support accelerated the transformation of aldehydes of HMF or FFCA to hemiacetals intermediate as well as the generation of metal alkoxide species during HMF oxidation process [64]. Subsequently, Ardemani et al. revealed that the moderate basic sites of HT support can react synergistically with the high fraction of surface gold to modulate the adsorption behavior of HMF and HMFCA intermediate, which afforded high activity for the oxidation of HMF under base-free condition (Fig. 3 a) [66].After gaining some insights into the role of basic support during the base-free HMF oxidation process, several strategies were developed to prevent the leaching of the support and improving the life of the catalyst. The restriction of the interactions between the carboxylic acid products (HMFCA, FFCA and FDCA) and basic metal components of the support can significantly relieve the leaching of metal species. Gao et al. reported that the introduction of La into AuPd/CaMgAl-layered double hydroxide (LDH) brings in new La3+-O2+ pairs on AuPd/LaCaMgAl-LDH catalyst, leading to the formation of abundant highly dispersed LaOx species over the support surface, which can enhance the stability of the catalyst by alleviating the strong interactions between acidic products (HMFCA, FFCA and FDCA) and metal species (Fig. 3b) [67]. Therefore, the leaching of Mg and Ca species were significantly suppressed in AuPd/LaCaMgAl-LDH catalyst (0.8 and 0.3 wt%) in comparison with that of in AuPd/CaMgAl-LDH catalyst (4.5 and 2.6 wt%) [67]. In addition, the incorporation of an acid-resistant phase into the basic support is another approach to avoid the potential metal leaching. For instance, the synergy between hydrotalcite (HT) and activated carbon (AC) endows HT-AC composite with the characteristics of these two components and prevents the leaching of HT in acidic environment [68]. The dilution of Au/MgO with inert MgF2 phase also enabled Au/MgF2–MgO to catalyze HMF oxidation under base-free condition with relatively high FDCA productivity of 15.8 molFDCA h−1 molAu −1 [45]. More importantly, the final pH value of the reaction solution was 3.8 whereas no Mg leaching was noticed, indicating the strong stability of Au/MgF2–MgO catalyst in an acidic environment. To eliminate the support leaching issue at the source, many novel basic supports without basic metal oxides or hydroxides were developed in recent year, such as magnesium-doped carbon (C–O–Mg) [69] and DOWEX 50WX2-100 resin [70]. Especially, DOWEX 50WX2-100 resin supported Pt catalyst (Pt@Dowex-Na) provided high FDCA yield (99%) and excellent stability in neat water under continuous flow (FDCA yield remained 99% after 43 h time-on-stream and no Pt leaching was detected) [70]. Although only a few works focus on the oxidation of HMF in continuous mode, continuous-flow oxidation of HMF is important for the large-scale utilization of HMF.Amphoteric, acidic and inert supports, such as TiO2, ZrO2, Al2O3 and active carbon, etc., with better chemical stability than basic supports especially under acidic conditions, were also extensively investigated for supporting noble metal NPs. Generally, as summarized in Table 2, a certain number of base additives (e.g. NaOH, Mg(OH)2 or NaHCO3) are required to achieve high FDCA yields (62%–99%) over TiO2, ZrO2 and Al2O3 supported noble metal catalysts. When HMF oxidation reaction was performed over non-basic supports supported catalysts under base-free conditions, a very low FDCA yield was usually offered. For example, only 2%–5% FDCA yields were obtained from HMF over Au/TiO2, Pt/ZrO2 [49] or Au/ZrO2 [45] in the absence of base additives. Note that ring-opening products of HMF such as levulinic acid (LA) and formic acid (FA) were detected over TiO2, ZrO2 and Al2O3 supported Ru(OH)x under base-free conditions [71]. In general, the basic sites of the support are supposed to favor HMF oxidation reactions while their acidic sites may promote the undesirable HMF ring-opening by-reactions. Hence, base additives are essential for accelerating HMF oxidation reactions and suppressing HMF ring-opening reaction in the presence of TiO2, ZrO2 or Al2O3 supported noble metal catalysts. Moreover, TiO2, ZrO2 and Al2O3 supported Au and Pt catalysts revealed higher FDCA yields (95–99%) than Ru and Pd catalysts (62–86%) in alkaline solution [44,74–76]. Especially, Au/Al2O3 offered the highest FDCA productivity of 24.8 molFDCA h−1 molAu −1 among TiO2, ZrO2 and Al2O3 supported noble metal catalysts at a relatively low reaction temperature of 70 °C [77].Notably, many studies also pointed out that the Lewis acid site of the support (e.g. CeO2) can serve as the active site to adsorb the alcohol, then favor the formation of alkoxide intermediates during the alcohol oxidation process [78,79]. In addition, Odriozola and co-workers found a positive correlation between the Brønsted acidity of CexZr1-xO2 (hydroxyl groups) and the corresponding FDCA yields of CexZr1-xO2 supported Au [80]. To be specific, CexZr1-xO2 support with higher acidity offered higher FDCA yields [80]. Recently, Ag NPs were found to be very active for the oxidation of HMF to HMFCA with unexpected selectivity [81–84]. In particular, the acidic ZrO2 supported Ag NPs offered the highest HMFCA yield of 92% among various supports with different acidic and basic properties (ZrO2, TiO2, CeO2 and MgO) [81]. Additionally, the modification of support with acidic groups (e.g. –HSO3) enabled the direct transformation of carbohydrates to FDCA [85] or DFF [86] in a one-pot process. For example, Rathod and co-workers realized the one-pot two-step production of FDCA from fructose with an overall yield of 64% over a sulfonated glucose-derived carbon-supported Pd catalyst (Pd/CC) [85].Although the inert active carbon with stable physical properties renders them ideal candidate support for the oxidation of HMF under base-free conditions, the catalytic activity of active carbon-supported noble NPs was lower than that of metal oxide one [39,40,42,74] [87]. The introduction of heteroatom dopant (e.g. N and P) into the carbon matrix can significantly regulate the physicochemical property of the support, thereby improving the catalytic activity of the supported noble metal NPs. Nitrogen-doped carbon is a type of material with basic sites and stable under acidic conditions. The N dopant of the carbon support could strongly coordinate with noble metal species (e.g. Au [88], Pt [52], Ru [89] and Pd [90]) to enhance its dispersity, rendering the formation of ultrafine metal NPs with boosted activity and stability. For example, Han et al. found that the activity of the nitrogen-containing carbon-supported Pt catalyst (Pt/C-EDA-4.1) was strongly related to the relative content of different types of nitrogen species, especially the concentration of basic pyridine-type nitrogen (N-6) [52]. Besides, it was reported that phosphorus can modify the 3 d electron density of noble metals and promote the activation of the substrate during the alcohol oxidation process [91,92]. Accordingly, highly porous nitrogen- and phosphorus-co-doped graphene sheets supported Pd catalyst (Pd/HPGSs) [93], P-decorated CNF supported Au–Pt NPs (Au–Pt/P-HHT-CNF) [94] and phosphorus-doped carbon-supported Ru catalyst (Ru/OMC-P0.56) [95] were reported to be more active for the oxidation of HMF than the catalyst without P-doping. However, in comparison with N-doped carbon supports, fewer efforts have been devoted to the study of the mechanism of P-doped carbon supported catalysts for the HMF oxidation, especially regarding the promotion effects of the P-doping for the oxidation of HMF.Reducible oxides are extensively employed for supporting noble metal NPs for the alcohol oxidation reactions because the oxygen vacancy (Ov) of the reducible oxides can effectively activate molecular oxygen, thereby working synergistically with noble metal NPs to boost the catalytic performance of the catalyst (Table 3). CeO2, Mn3O4, MnO2, NiO [100] and CoO are typical reducible oxides containing redox couples (e.g. Ce4+/Ce3+, Co3+/Co2+, Mn4+/Mn3+/Mn2+) and Ov. Casanova et al. revealed that Au–CeO2 [74] was very active for the oxidation of HMF to FDCA, offering an impressive FDCA productivity of 122.9 molFDCA h−1 molAu −1 (5 h, 130 °C, 10 bar Air), which is much higher than that of non-reducible oxides supported catalysts as reviewed in the 2.3.1 section. This work suggested that redox couples and Ov of the support were involved in the catalytic circles. Later, they further reported that the catalytic performance of the Au–CeO2 for the oxidation of HMF can be improved by the reductive pretreatment of the catalyst, which could increase the amount of Ce3+ and oxygen vacancies of the catalyst, thus contributed to the O2 activation and dehydrogenation process during the HMF oxidation reactions [74].In general, the doping of heteroatom with different valences in the reducible oxides can generate new surface defects to compensate for the charge imbalance, thus regulating the redox property of the support [101]. For instance, Miao et al. obtained an enhanced FDCA yield of 99% over Ce0.9Bi0.1O2-δ supported Au (Au/Ce0.9Bi0.1O2-δ) when comparing with Au/CeO2 (39%) under the same reaction conditions (2 h, 65 °C, 10 bar O2) [102]. It has been proved that introducing Bi3+ to substitute Ce4+ in the CeO2 lattice would lead to the improvement of the surface Ov concentration, thereby improving its oxygen activation capacity [102]. Moreover, the formed Bi–O–Ce linkages favored the hydrogen transfer process during the HMF oxidation reactions. A similar phenomenon was observed in Ce0.8Bi0.2O2-δ supported Pt NPs (Pt/Ce0.8Bi0.2O2-δ) [103]. Interestingly, an FDCA productivity of 392 molFDCA h−1 molPt −1 was achieved over Pt/Ce0.8Bi0.2O2-δ at 23 °C (0.5 h, 10 bar O2), which is the highest value among ever reported noble-metal-based catalysts. According to the same strategy, Gao et al. fabricated Mn–Ce mixed oxides with tunable redox property for supporting Ru NPs (Ru/Mn6Ce1OY) [47]. As shown in the hydrogen temperature-programmed reduction (H2-TPR) and oxygen temperature-programmed desorption (O2-TPD) profiles of the Mn–Ce oxides (Fig. 4 ), both the reduction and oxygen desorption peaks shift toward a lower temperature with the increasing of Ce content, indicating the enhancement of its higher oxygen mobility. It has been well demonstrated that the strong metal-support interaction between Ru NPs and Mn–Ce oxide afforded the catalyst an exceptional FDCA yield of > 99% under base-free conditions (15 h, 150 °C, 10 bar O2) and good catalytic stability against aggregation and oxidation of active Ru NPs [47].In addition to the heteroatom-doping method, regulating the morphology of the reducible oxides support is another way to modify the redox property of the support, which has been rarely studied for the catalysts in HMF oxidation. Li and co-workers compared the catalytic activity of Au/CeO2 with different CeO2 morphologies (rod, cube and octahedra) for the oxidation of HMF to FDCA, and found that the TOF value of Au/CeO2-rod (6.3 min−1) was 7–32 times higher than that of Au/CeO2-cube (0.9 min−1) and Au/CeO2-oct (0.2 min−1) [104]. The authors announced that CeO2-rod has the richest surface oxygen vacancies, which largely promoted the catalytic activity of Au/CeO2-rod by efficiently activating HMF and O2 [104]. Recently, Liao et al. obtained more than twice higher FDCA productivity over MnO2 supported atomic Pd catalyst (Pd–MnO2) than Pd NPs (PdNP–MnO2) [105]. The experimental and DFT calculation results revealed that the incorporated atomic Pd species worked synergistically with MnO2 framework to create more Ov and to promote the mobility of the surface lattice oxygen (OL), which favored the activation of oxygen and enhanced the HMF adsorption capacity. This finding presented a new way to design robust catalyst with strong metal-support interaction by regulating the redox property of the support toward the efficient production of FDCA.In comparison with bulk support, the utilization of porous materials as the support for noble metal NPs holds unique advantages not only for precisely tailoring the NPs size and morphology but also facilitating the accessibility of active sites to substrate molecules. Therefore, controlling the porosity of the support is an effective strategy to improve the activity and stability of the supported NPs (Table 4).Zeolites, as extensively industrialized catalysts, have been widely studied as support in many biomass conversion processes because of their rigid porous structure and high specific surface area [109]. Cai and co-workers found that cage-type Y zeolite supported Au catalyst (Au/HY) offered the best catalytic performance for the oxidation of HMF, whereas low FDCA yields of 1%–15% were achieved over H-MOR and Na-ZSM-5 zeolites supported Au catalysts (Au/H-MOR and Au/Na-ZSM) [110]. The authors concluded that the activity of the catalyst is strongly related to the Au NPs size and smaller Au NPs provide better catalytic performance. Especially, the unique small super-cage of HY zeolite resulted in the formation of ultra-small and uniformly dispersed Au NPs inside the HY cage with a diameter of 1.0 nm. A special hydrothermal approach was also developed to direct incorporate Pt NPs into the crystals of Beta zeolite, resulting in the formation of highly active Pt@Beta zeolite catalyst for the oxidation of HMF to FDCA thanks to the enhanced interaction between Pt and silica species [111].Besides, mesoporous metal oxides were also developed as supports for improving the activity of the supported noble metal NPs. Lolli et al. fabricated an ordered mesoporous CeO2 by using SBA-15 as a hard template for supporting Au NPs (Au/m-CeO2) for the oxidation of HMF to FDCA [112]. The small crystallites and high surface area of mesoporous CeO2 endowed the Au/m-CeO2 with better catalytic performance than commercial ceria-supported Au catalyst. Masoud and co-workers further emphasized the importance of the support pore structure for the stability of the Au NPs and the ordered mesoporous structure can minimize the growth of Au NPs during the HMF oxidation reactions [113]. Recently, Pichler et al. adopted a surface-casting method to manufacture high-surface-area ZrO2 (ZrO2 H-aero, 239 m2 g−1) [114]. The high surface area of the support endows the formation of ultra-fine Ru clusters (0.8–1 nm) in Ru/ZrO2 H-aero catalyst, which displayed superior activity for the oxidation of HMF to FDCA under base-free aqueous solution with a high FDCA yield of 97% [114].Apart from the zeolites and traditional mesoporous metal oxides, many novel metal-free porous materials were also developed as the support for noble metal NPs for the efficient oxidation of HMF. In particular, the pore structure and specific surface areas of the porous organic polymers can be fine-tuned by simply applying different monomers. Various covalent triazine frameworks (CTF), a class of highly stable polymers, were prepared in molten ZnCl2 as the support for Ru NPs and the resulting Ru/CTF gave DFF yield of 73%, which is higher than that of Ru/C, Ru/γ-Al2O3, Ru/hydrotalcite and Ru/MgO because of the high surface areas (2349 m2 g−1) and large pore volume (1.96 cm3 g−1) of CTF [115]. Furthermore, Ru/CTF also revealed a superior catalytic performance for the base-free oxidation of HMF to FDCA (78%) in water than Ru/C and Ru/γ-Al2O3 [116]. In addition, mesoporous poly-melamine-formaldehyde (mPMF) [117], activated chitosan carbon (PVP-ACS-800) [118] and hierarchical porous nitrogen-doped carbon (NC2) [119] were also employed as porous carbon material to confine small noble metal NPs for the oxidation of HMF and offered desirable FDCA yields.As we discussed above, one of the important targets of increasing the porosity of the support is to achieve better metal dispersity and reduce the size of metal NPs. The unique pore dimension and channel structure of porous materials endow an ideal space for the incorporation of metal NPs with tailed sizes and morphologies. The size effect of the NPs has been reported in many cases and higher dispersion of the supported active phases generally affords a better reaction rate [120,121]. Megías-Sayago et al. investigated the size effect of the active carbon immobilized Au NPs in the range of 4–40 nm for the oxidation of HMF to FDCA [120]. The authors claimed that the increase of Au NPs sizes resulted in the decrease of the exposed Au (100)/Au (111) facet ratio and an exponential decay trend was observed between the product selectivity and Au (100)/Au (111) exposure ratio (Fig. 5 ). Because the Au (100) face thermodynamically favored the oxygen reduction reaction and the smaller Au NPs exhibited higher Au (100) face concentration, which largely promoted the hydroxide ion concentration over the Au NPs surface, eventually boosted its catalytic activity for the oxidation of alcohol/aldehyde groups. This work provides new perspectives to gain insights into the size effect of the supported noble metal NPs, thereby may offer guidelines for developing efficient catalytic systems for the oxidation of HMF. However, rare reports focused on the interpretation of in-depth causes for the size effect of other supported noble metal catalysts (Pd, Pt, Ru and Ag).For the supported heterogeneous catalyst, the catalytic reactions generally occur over the catalyst surface, the surface nature of the support, such as the type and amount of the functional groups, hydrophilic and hydrophobic property as well as the coating property, thus plays a crucial role for the catalytic activity of the supported metal NPs. In this section, we will discuss the reported strategies for the modification of the support surface properties to enhance the catalytic performance of the loaded metal NPs for the oxidation of HMF, especially by introducing or altering the functional groups of the support and coating the support surface with another phase (Table 5).As discussed in section of 2.3.1, the introduction of basic functional groups (e.g. nitrogen-containing functional groups) on the carbon material support can increase the basicity of the support. In addition, nitrogen-containing functional groups are also frequently introduced onto the surface of the support as anchoring sites for soluble active metal species. For example, 3-aminopropyltriethoxysilane (APTES) functionalized silica-coated Fe3O4 NPs (Fe3O4@SiO2–NH2), poly (4-vinylpyridine)-functionalized carbon-nanotube (PVP/CNT) and bi-imidazole groups grafted SBA-15 (SBA-Im) were reported as ligand containing supports to immobilize Ru3+ for the selective oxidation of HMF to DFF [123–125]. The resultant supported Ru complex catalysts afforded high DFF yields of 86–94% in organic solvents [123–125]. Moreover, cumbersome procedures for the modification of the pristine support with nitrogen-containing functional groups impede their large-scale application.In addition to nitrogen-containing functional groups, Wan and co-workers revealed that modifying the oxygen-containing functional groups over carbon nanotubes (CNTs) can deeply influence the catalytic activity of CNTs supported noble metal catalysts for the base-free oxidation of HMF into FDCA [50,126]. For instance, CNT pretreated by 30 wt% H2O2 generated a high concentration of phenol and carbonyl/quinone groups over its surface, offering a desirable FDCA yield of 94% under base-free conditions [126]. And the increasing amount of these oxygen-containing groups greatly enhanced the absorption ability of CNT toward HMF and reaction intermediates, consequently boosting the catalytic activity of the supported Au–Pd NPs [126]. Interestingly, Chen et al. found that the optimal inter-site distance of acidic sites (-HSO3 species) and oxidation sites (Ru species) in sulfonic acid groups decorated reduced graphene oxide-supported Ru NPs catalyst (Ru/S-rGO-2) was determined to be 12.5 ± 2.2 nm and “closer” or “further” will promote the generation of undesirable by-products (such as humins and levulinic acid) in the one-pot conversion of fructose to DFF [86].The wettability of the catalyst support directly influences the adsorption capacity of the catalyst toward the substrates, thus wettability modulation of the support is an effective method to improve the performance of the supported metal NPs. However, a few works focus on studying the effect of the wettability of the catalyst on the oxidation of HMF. Wang and co-workers obtained an FDCA yield of 99% over hydrophilic mesoporous poly (ionic liquid) (MPIL) supported Au–Pd alloy [127]. Whereas the hydrophobic material supported Au–Pd alloy catalyst only provided a low FDCA yield of 36% under the same reaction conditions [127]. The authors revealed that hydrophilic support has a stronger affinity toward HMF than FDCA, which facilities the accessibility of HMF and leaving of the FDCA over the catalyst surface [127]. The establishment of facile and low-cost approaches to modify the wettability of the catalyst may be a promising method to enhance the activity of the catalyst for the oxidation of HMF.Iron-based magnetic materials are widely employed as catalyst support for both noble and non-noble metal NPs because of their low cost, facile separation and recovery features [128]. However, the self-interactions between magnetic nanoparticles make them easily aggregated, which deteriorates the catalytic performance of the catalysts. Surface encapsulation modification is a widely adopted strategy to avoid the aggregation of iron-based magnetic material and enhance their stability in basic and acidic environment [128]. Zhang and co-workers designed a series of hydroxyapatite (HAP), carbon or graphene oxide encapsulated γ-Fe2O3 or Fe3O4 supported Ru, Pd, CoOx or Mn3O4 catalysts for the oxidation of HMF to DFF or FDCA. Among them, γ-Fe2O3@HAP-Ru [129] and Fe3O4/Mn3O4 [130] provided high DFF yields of 82–89% under mild conditions. Pd/C@Fe3O4 [131], C–Fe3O4–Pd [132], γ-Fe2O3@HAP-Pd (0) [133] and nano-Fe3O4-CoOx [134] gave satisfactory FDCA yield of 69%–93%. It was also reported that Fe3O4 in the Fe3O4 decorated reduced graphene oxide supported Pt NPs (Pt/Fe3O4/rGO) can promote the dispersion of the supported Pt NPs [135]. More importantly, Fe3O4 can interact with the supported Pt NPs to modulate the exterior electron state of Pt and Fe atoms, leading to a more active Pt electron state with high catalytic activity, thus afford a high FDCA yield of 99% even in a base-free condition. Moreover, the FDCA yield increased from 71% in Pt/CeO2 to 100% under base-free conditions after the introduction of a nitrogen-doped carbon coating over the CeO2 support (NC–CeO2) [51]. Because the introduction of nitrogen-doped carbon coating leads to the generation of plentiful surface defects on the support and abundant electron-deficient metallic Pt species, as well as the increase of the basicity of the support [51].To circumvent the deactivation, instability and leaching issues of the supported single noble metal NPs, a second metal has been introduced as an additive to tailor the geometric as well as the electronic configurations of the active centers (Table 6).The combination of Au and Pd results in the formation of Au–Pd bimetal NPs, which is the most investigated bimetal catalyst for HMF oxidation. For instance, Villa and co-workers obtained a moderate FDCA yield of 80% over Au/C under mild reaction conditions whereas the yield of FDCA decreased to 60% after five recycling tests because of the irreversible adsorption of intermediates or aggregation of Au NPs [136]. It is interesting to note that Pd or Pt-modified Au/C catalyst revealed enhanced activity and stability than pristine Au/C for HMF oxidation. Especially, Au8–Pd2/AC catalyst provided a higher catalytic activity than Au8–Pt2/AC, offering almost quantitative FDCA yield and an impressive FDCA productivity of 99 molFDCA h−1 mol−1 noble metal under mild reaction conditions. Importantly, an FDCA yield of 99% can be maintained over Au8–Pd2/AC even after five runs, which proves that alloying Au and Pd overcomes the deactivation of Au/C for the oxidation of HMF. A similar promotion effect was also observed in Au–Pd/CNT [126], zinc hydroxycarbonate (ZOC) supported Au–Pd [137], Au6Pd1/TiO2 [138], Pd–Au/HT [139], porous bowl-like nitrogen-doped carbon-supported AuPd bimetallic (AuPd/pBNxC) nanoreactors [140] and Au0.5Pd@Co3O4 catalyst [141]. In addition, Xia and co-workers observed that the formation of PdAu alloy NPs reallocates the electron density of the NPs as a result of the different electronegativity of Au (2.4) and Pd (2.2), leads to a reduction of NPs size and promotes the dispersion of Pd species [139]. The observed geometric effects and electronic effects via alloying Pd with Au greatly improve the catalyst activity of the PdAu alloy NPs for HMF oxidation, especially promotes the oxidation of HMFCA [139]. Liao and co-workers further confirmed that the introduction of Pd into Au NPs induces the electron transfer from Pd to the 5d orbitals of Au in a Au0.5Pd@Co3O4 catalyst, which promoted the generation of highly active oxidation sites (Au+ species), eventually leading to an enhanced catalytic performance of AuPd alloy NPs (Fig. 6 ) [141].In addition to AuPd alloy NPs, the incorporation of budget non-precious metal into noble NPs not only can reduce the cost of the catalyst but also contributes to enhancing the catalytic activity of the NPs. TiO2 or CeO2-supported AuCu alloy NPs provided a better catalytic activity and stability than the monometallic sample, in which Cu species in Cu–Au alloy NPs actually serves as a promoter or dispersing agent to elevate the dispersity of the active Au species [142–144]. Gupta et al. further compared the promotion effect of introducing a second non-noble metal (Co, Cu and Ni) for the base-free oxidation of HMF over Pd/Mg(OH)2 [145,146]. Among various prepared bimetal catalysts, the Ni0.9Pd0.1/Mg(OH)2 catalyst revealed a better catalytic performance than monometallic Pd as well as Co and Cu-derived bimetallic catalysts. The authors interpreted that the incorporation of Ni into Pd NPs endows the formation of electron-rich Pd sites because of the electron transport between the two components, which favors the absorption and activation of oxygen molecule, thus providing better catalytic performance toward HMF oxidation.Several cheap metals, including Pb, Bi, Ni and Sn, were also introduced as the synergetic component to enhance the catalytic activity of the supported Pt NPs for HMF oxidation. A high FDCA yield of 99% was achieved over Pb–Pt/C at 25 °C in 1.25 mol L−1 NaOH aqueous solution [147]. The characterization of the recovered solid after the reaction revealed that the introduced homogenous Pb salts were deposited on the surface of the Pt/C during the reaction, resulting in the formation of bimetal Pb–Pt/C catalyst, which can be directly reused without the addition of extra Pb salts in the next catalytic cycle [147]. Apart from Pb, Bi was also introduced as a promoter for enhancing the reactivity of Pt/C catalyst for the oxidation of HMF [148]. The promotion effect of Bi-doping was ascribed to the interaction between bismuth and the π electrons of the furan ring. The oxophilicity of bismuth enhances the chemical adsorption of the geminal diol intermediate over the surface of Bi–Pt/C catalyst, which facilitates the dehydrogenation process on active Pt sites. In addition, the presence of Bi promoter in Bi–Pt/C catalyst can prevent the over-oxidation of the Pt NPs during the reaction, thereby improving the reusability of the catalyst. Recently, Shen et al. found that combining Pt and Ni in Pt–Ni/AC renders Pt NPs an improved CO absorption and oxidation ability, thereby offering the highest FDCA productivity of 25.3 molFDCA h−1 molnoble metal −1 under base-free conditions among noble metal catalysts [149]. Moreover, Sn-doping in 2-Pt1Sn1–H2 catalyst can also modify the electronic structures of Pt NPs and results in the formation of abundant Pt (0) species with high catalytic activity and stability [150]. Obviously, the incorporation of secondary metal into noble metal NPs is an efficient method to improve their catalytic activity and stability. More efforts should be devoted to figuring out the genuine active sites of the complex bimetal catalysts as well as the intrinsic function of the dopants for enhancing the catalyst activity.Basic supports can readily react with the generated acidic products (FDCA, FFCA and HMFCA) during HMF oxidation process, resulting in the etch of active component and support, which leads to the downgrade of the catalyst activity and stability [65,71,151]. Several strategies were developed to prevent the leaching of basic support, including the constraint of the interactions between the acid products [67], the incorporation of an acid-resistant phase [45,68] and replacing the basic metal oxides or hydroxides with novel basic supports [69,70,73,90]. For other types of supported catalysts, the reasons for the catalyst deactivation mainly include the absorption of byproducts [44], the over-oxidation of the metal active sites [75,81] as well as the sintering of metal particles [81,96,98]. In addition, the leaching of N species was observed during the HMF oxidation process over Pt/C-EDA-4.1 catalyst and resulted in the catalyst deactivation [52]. Interestingly, Pd/HPGS catalyst exhibited high stability for HMF oxidation and can maintain its activity after 20 consecutive cycles thanks to the strong metal-support interactions [93]. Therefore, the modulation of metal-support interactions can be an effective strategy to enhance the stability of the supported noble metal catalysts. Accordingly, as presented in Table 3 the reducible oxide-supported noble metal catalysts generally exhibited good catalytic stability and can be recycled several times (4–8) without activity deactivation. The existing robust metal-support interaction endowed the active noble metal species with high resistance to aggregation and oxidation during the oxidation process, thus revealing good recyclability [47,105]. Moreover, the encapsulation effect of the zeolite framework largely eliminates the leaching-induced deactivation of the catalyst, offering excellent catalyst stability [111]. The incorporation of secondary metal into noble metal NPs is another efficient method to improve the catalytic stability of the catalyst [136,138,142,148].As we summarized above, noble-metal-based catalysts exhibit excellent catalytic activity and recyclability toward HMF oxidation even in the absence of base promoter or under very mild reaction conditions. However, from an economic perspective, the implementation of these catalysts for the oxidation of HMF in a large-scale practical process certainly increases the production cost of the products because of the poor availability and exorbitant price of noble metals. Lately, the development of cost-effective non-precious metal-based catalytic systems for HMF oxidation is gaining momentum. A tremendous amount of non-precious metal oxides (e.g. V2O5, MnO2, CoOx and CeO2) were successfully applied for the efficient oxidation of HMF. It has been widely accepted that the oxidation of HMF over non-precious metal oxides operates by the Mars-van Krevelen (MvK) mechanism [152–156]. Generally, the MvK mechanism includes several elementary steps, (1) alcohols were absorbed and activated on the surface of the oxide and gave rise to the generation of alkoxy species and protonated OL species via the O–H bond dissociation; (2) the adsorbed alkoxide intermediates were subsequently converted into aldehydes and protonated OL species after deprotonation from the β-carbon by the vicinal OL, meanwhile, electron transfer from the substrates to the metal cations, leading to the reduction of the metal oxides; (3) the recombination of these formed protonated OL species leads to the formation of H2O and O2 as well as Ov; (4) the catalytic circle was finished after the reduced metal oxides were re-oxidized and oxygen vacancies were replenished by the dissociative chemisorption of aerial oxygen. Accordingly, this reaction mechanism comprises a metal oxide redox cycle, thus the reducibility and oxidizability of the catalyst play a key role in its catalytic activity for HMF oxidation [157,158]. The acid-base property of the catalyst was also reported could influence the dissociation of O–H bond [159–161].Kinetic parameters and reaction pathways are fundamental aspects to the interpretation of the catalytic reaction mechanisms. The selective oxidation of HMF to DFF involves the cleavage of βC−H and O–H bond, both of which was reported to be the rate-determining step during the HMF oxidation process over different catalysts. In a manganese oxide (OMS-2), KIE studies revealed that dissociating βC−H bond of the adsorbed alkoxide intermediates is a kinetically-relevant step in the catalytic cycle [157]. A kinetic ratio of (kH/kD) 4.19 was observed for the competing oxidation of HMF and its deuterated counterpart at the methylene group (R-CD2OH) [157]. This study pointed out that the dissociative chemisorption of O2 over the catalyst surface is also a kinetically-relevant step during HMF oxidation. For the oxidation of HMF to DFF over a vanadium oxide nanobelt-arrayed mesoporous microsphere (VOx-ms) [163], HMF molecules with a deuterated hydroxyl group (R–CH2OD) offered a unified kH/kD ration of 1.05 whereas R-CD2OH gave a kH/kD value of 2.21, which also suggested that βC–H bond cleavage is the rate-determining step during HMF oxidation. However, L. Suib and co-workers revealed that the energy for the break of O–H bond was higher than that of C–H bond in a mesoporous manganese incorporated cobalt oxide (meso Mn-CoOx) by the DFT calculations, implying that O–H bond dissociation is the rate-determining step during the HMF oxidation [156]. The DFT calculation results are in discordance with the previous KIE studies, thus the exact mechanism for the oxidation of HMF to DFF over non-precious metal oxides needs to be further studied.It is similar to the case of noble metal catalysts, HMF can be oxidized into FDCA over non-precious metal oxides proceeds via two reaction paths (Scheme 1. Route 1 and 2). The composition and structure of the catalyst, as well as the reaction medium, significantly affected the reaction route for HMF oxidation. Hara and co-workers investigated the rate-constants (k 1 -k 5 ) for the oxidation of HMF to FDCA over activated MnO2 and found that the oxidation of HMF to DFF in the first step gave a k 1 value of 2.4 × 10−3, which is almost ten times higher than that of conversion of HMF to HMFCA, indicative of that HMF was converted into FFCA mainly through DFF (route 2) [155]. Moreover, the k 5 value is obviously lower than that of k 1 -k 4 , suggesting that the oxidation of FFCA to FDCA is the rate-determining step during this catalytic circle. The same reaction path was also reported in many Mn-based catalytic systems [152,164,165]. However, the oxidation of HMF to FDCA over the (Fe, Co, Ni)-doped MnOx and holey 2 D Mn2O3 nanoflakes catalysts proceeded via route 2 (Fig. 7 ) [162,166]. The discrepancy reaction pathways over Mn-based catalysts may be ascribed to the different adsorption behaviors of hydroxymethyl and aldehyde moiety of HMF over different catalysts, which often receives cursory attention during the reaction mechanism investigations. Furthermore, Zhang and co-workers calculated the activation energy (Ea) of each step for the conversion of HMF to FDCA over Fe0.6Zr0.4O2 catalyst in [Bmim]Cl [167]. The results revealed that the oxidation of FFCA to FDCA possessed the highest E a value of 110.2 kJ mol−1, which is higher than that oxidation of HMF to HMFCA (82.7 kJ mol−1) and HMFCA to FFCA (86.4 kJ mol−1). Similarly, the same research group also reported that the oxidation of FFCA to FDCA is the rate-determining step for the oxidation of HMF to FDCA in [Bmim]Cl over heteropoly acid catalyst [168]. It should be noted that the oxidation of FFCA to FDCA was observed to be the slowest step during the HMF oxidation process over almost all the non-precious metal oxides catalytic systems, but a reasonable interpretation has not been proposed yet.In section 2.3.2, various kinds of non-precious reducible oxides were employed as the support to enhance the catalytic activity of noble metal NPs for HMF oxidation. Actually, non-precious transition metal oxide, such as V2O5 and MnO2 itself contains redox couple (e.g. V5+/V4+ and Mn4+/Mn3+/Mn2+) and OL, which are capable of oxidizing alcohols alone (Table 7). Several inorganic vanadium-containing materials, such as V2O5 [169] and VOPO4·2H2O [170,171], show high selectivity toward the oxidation of HMF to DFF in DMSO. To avoid the use of the solvent with a high boiling point (such as DMSO), Yan et al. fabricated a (010)-faceted vanadium oxide nanobelt-arrayed mesoporous microsphere (VOx-ms) for the selective oxidation of HMF to DFF, in which an excellent DFF yield of 89% was obtained within 1 h in aqueous solution [163]. DFT calculations revealed the OL of VO sites prefers to absorb the O–H group of HMF rather than –CHO moiety, thus guaranteeing high DFF selectivity [163].In comparison with vanadium-based catalysts, manganese oxides not only can selective oxidation of HMF to DFF but also enables the efficient formation of FDCA in basic aqueous solution. MnO2 is a typical manganese oxide with diverse crystal structures, but pure MnO2 displays offered DFF yields as low as 2%–42% in organic solvents [156,160,172–174]. Nitrogen-doped MnO2 (N–MnO2) afforded quantitative DFF yield was achieved at 25 °C within 6 h [175]. The authors demonstrated that nitrogen doping slightly elongates the Mn–O bonds and descends the Mn–O coordination numbers of MnO2, which creates more surface defect sites and coordinatively unsaturated Mn sites, eventually enhance the catalytic performance of the catalyst. Several polycrystalline MnO2 catalysts were also developed for the oxidation of HMF to DFF [174,176,177]. In particular, manganese oxides containing MnCO3, ɛ-MnO2 and Mn2O3 provided DFF yield of 88% in ethanol, in which MnCO3 and ɛ-MnO2 were believed to work concertedly to enhance the catalytic performance [176]. It should be noted that the most adopted solvent (DMF or DMSO) in vanadium or manganese-based catalytic systems is ranked as hazardous or problematic solvent according to the CHEM21 selection guide for organic solvent while ethanol belongs to recommended solvent [178]. The development of catalysts that enable the effective oxidation of HMF to DFF in eco-friendly and low boiling point solvent is an important direction for the production of DFF in the future.Even though pure MnO2 has low catalytic activity toward the oxidation of HMF to DFF in organic solvents, Hara's group achieved an FDCA yield of 91% over commercially available activated MnO2, which is much higher than that obtained over other manganese oxides (Mn2O3, Mn3O4, MnO and MnOOH) [153]. Later, the effect of MnO2 crystal structure (α-, β-, γ-, δ-, ε-, and λ-MnO2) on the HMF oxidation has been investigated [155,179]. DFT calculations demonstrated that the Ov formation energies at the planar oxygen sites in MnO2 crystal structure are generally higher than those at the bent oxygen sites (Fig. 8 ) [155]. Especially, the ratio of oxygen sites with relatively lower vacancy formation energies in β- and λ-MnO2 is higher than that of in α- and γ-MnO2, indicating the first two types are likely to be better candidates than later ones for the HMF oxidation [155]. Indeed, the succeeding experimental analysis shows that the activity of the manganese oxides for the oxidation of FFCA to FDCA decreases in the order of β-MnO2 > λ-MnO2 > γ-MnO2 ≈ α-MnO2 > δ-MnO2 > ε-MnO2. What’ more, increasing the surface area of β-MnO2 can further enhance its catalytic performance. Similarly, Bao and co-workers constructed holey 2 D Mn2O3 nanoflakes by the calcination of an Mn-based metal–organic framework (MOF) precursor, which provided almost quantitative FDCA yield thanks to its abundant surface pores structure [162]. Although satisfactory FDCA or DFF yield can be obtained over pure manganese oxides, high catalyst loading as well as a long reaction time are required, meanwhile the productivity of the product, especially for FDCA (0.08–0.3 mmolFDCA h−1 g−1 catalyst), is quilt low over these single V and Mn oxides.In addition to vanadium and manganese-based catalysts, CuO and Co3O4 revealed outstanding catalytic performance toward the oxidation of HMF to FDCA by using NaClO as oxidant, in which FDCA yields of 96%–100% were obtained under mild reaction conditions (40 °C, 2 h) [180]. However, the involvement of large amounts of oxidant (NaClO/HMF molar ratio = 45) for HMF oxidation made this process cost-intensive. Very recently, Lin's group developed an innovative NiOx catalyst for the efficient oxidation of HMF to FDCA [181], where a high FDCA yield of 97% was achieved by using only 6 equiv. NaClO as oxidant at 25 °C in a short reaction time of 30 min with a remarkable FDCA productivity of 24.3 mmolFDCA h−1 gcatalyst −1.In recent years, cheap binary and ternary metal oxides have gained boosting interest for HMF oxidation because of their higher catalytic activity and better recyclability than single cheap metal oxides (Table 8). The binary and ternary metal oxides are composed of two or three kinds of cheap metal species, in most cases, only one of them is the active site for HMF oxidation and other constituents serve as the promoter to enhance the activity of the active component. Moreover, the introduced second metal species can also interact with the original active species to form new active sites.Both vanadium oxides and phosphates display good catalytic activity toward the selective oxidation of HMF to DFF. However, DFF yields lower than 90% were obtained over single vanadium oxides or phosphates in extended reaction time (> 10 h in most cases). It was reported that the introduction of Cu or Fe can enhance the mobility of OL of the catalyst [182,183]. For example, Hou and co-workers demonstrated that binary α-CuV2O6 nanobelt offered almost quantitative DFF yield within 3 h in DMSO [182]. The introduction of Cu results in the formation of unique Cu–O–V units in α-CuV2O6 with excellent OL mobility due to the bimetallic synergistic effects.Various cheap metal species such as K, Fe, Co, Ce, and Cu were introduced into manganese oxides as the second or third component for modifying their redox or acid–base properties. The hydrothermal treatment of KMnO4 and MnSO4·H2O in acidic aqueous solution results in the generation of cryptomelane octahedral molecular sieves (K-OMS-2) with a composition of KMn8O16·nH2O, which revealed excellent catalytic activity for the selective oxidization of benzylic alcohols to aldehydes or ketones [184]. Inspired by this work, Fu's group successfully achieved a DFF yield of 99% from HMF over K-OMS-2 in DMSO whereas MnO2 only afforded a DFF yield as low as 4% [185]. Interestingly, the subsequent treatment of K-OMS-2 in 1 mol L−1 HNO3 solution produces an H–K-OMS-2 catalyst, which offered better catalytic activity than K-OMS-2 for HMF oxidation because of the presence of Brønsted acid sites over H–K-OMS-2 catalyst [185]. Subsequently, Nie and Liu further compared the catalytic performance of manganese oxides with different morphologies for HMF oxidation (OMS-1, OMS-2, OMS-6, OMS-7, γ-MnO2, amorphous MnO2 (AMO), birnessite-type MnO2 (Na-OL-1)), among which OMS-2 showed the best catalytic activity and offered a DFF yield of 97% in DMF within 1 h [157]. Note that the activities of the manganese oxides correlate well with their reducibility and oxidizability. The particular (2 × 2) tunnel structure of OMS-2 endows its high reducibility and oxidizability, thus provides high catalytic activity for HMF oxidation. OMS-2 only contains the oxidation sites for HMF oxidation, Sarmah et al. thus combined the H-Beta catalyst, which contains both of Brönsted and Lewis acidic sites, and OMS-2 for the transformation of carbohydrates into DFF, where excellent DFF yields of 97%, 95%, 93% and 91% were achieved from fructose, sucrose, glucose, and starch in a one-pot two-step process respectively [186].The superior catalytic performance of K-OMS-2 catalyst originates from its particular (2 × 2) tunnel structure formed in the presence of Mn and K in an acidic environment whereas the combination of manganese oxides with Fe, Co, Ce, La [187] and Cu [188] species boosts the activity of the catalyst mainly through modulating its redox or acid–base properties. Neatu and co-workers reported that the co-existence of multiple phases (bixbyite-Mn2O3, MnO2 and hematite) in Mn0.75/Fe0.25 catalyst works synergistically to enhance its catalytic activity through strengthening its medium basic sites (O2−) [161]. Recently, Lin's group further investigated the promotion effect of Fe2O3 for MnO2 catalyst for the oxidation of HMF, in which HMF conversion of 97% with DFF selectivity of 98% was achieved within 5 h [160]. It has been well demonstrated that the Fe2O3-doping greatly increased the content and activity of Mn4+-O2− acid-base pair in Mn6Fe1Ox, which was proved to be active sites for HMF oxidation.In addition to Fe-doping manganese oxides, Co has also been extensively introduced as a promoter to enhance the catalytic performance of the catalyst. Gui et al. observed that the synergy between Mn and Co species induces the formation of the dominant CoMnO3 phase in Mn0.50-Co0.50-O and spinel CoMn2O4 hollow spheres catalyst, which promotes the mobility of OL and also enriches its content, thus increasing its catalytic activity [172,189]. Interestingly, Xu and co-workers obtained an FDCA yield of 95% (Co–Mn-0.25, 120 °C, 1 MPa O2, 5 h) over Co–Mn oxides prepared by the solid-state grinding method, which outperformed the pure MnOx [165]. The excellent catalytic performance of Co–Mn-0.25 catalyst is ascribed to its high OL mobility and variable oxidation states of surface Mn species after the introduction of Co species. Recently, Lin's group further investigated the influence of the Ov concentration of the Mn–Co oxides on its catalytic performance for the oxidation of HMF [190]. Experimental and theoretical calculation results well proved that increasing the Ov amount not only can boost the OL reactivity of Mn–Co oxides by weakening the Mn–O bond intensity but also promote the adsorption and activation of HMF and O2 over the catalyst [190].On the other hand, the incorporation of Mn into cobalt oxide can also improve the mobility of OL of the catalyst by Jahn-Teller (J-T) distortion (Fig. 9 ) [156,191]. For example, L. Suib and co-workers designed a mesoporous manganese incorporated cobalt oxide (meso Mn-CoOx) for the oxidation of HMF, where 80% HMF conversion with DFF selectivity of 96% was obtained over 5% Mn-doped CoOx (meso 5% Mn-CoOx) [156]. Especially, the TOF value achieved over meso 5% Mn-CoOx catalyst is 300–391 folds higher than that of meso-CoOx or -MnOx samples. The substitution of Co3+ in meso-CoOx with Mn3+ species elongates the Mn–O bonds because of the J-T distortion, which improves the mobility of OL. It seems like that the key to enhance the catalytic performance of the metal oxides is to improve the mobility of OL; however, the deep reasons for this have not been clarified yet.Taking the excellent oxygen storage of CeO2 into consideration, Han and co-workers introduced CeO2 as a promoter to increase the OL transmission from Ce to Mn species to boost the activity of the catalyst for HMF oxidation [152]. H2-TPR and XPS results revealed that CeO2 in MnOx-CeO2 mixed oxide (MC-6) improves its oxygen mobility and surface Mn4+ and Ce3+ concentration. Mn4+ was assumed to be the primary active site for HMF oxidation over MnOx-CeO2 mixed oxide while the replenishment of OL was accomplished via transforming the OL of Ce species to MnO2 lattice. This mechanism indicates both Mn4+ and Ce3+ species are involved in the HMF oxidation process. Yu et al. further demonstrated that the intrinsic active sites in (Fe, Co, Ni)-doped MnOx catalysts are M3+O(-Mn4+)2 clusters, which can provide more active OL and stronger OL regeneration ability than Mn4+O(-Mn4+)2 clusters in pure MnOx catalyst [166]. These works suggested that the introduced dopant metal species can also interact with the original active species to form new active sites.Most of the above-listed multitudinous Mn-based oxides emphasized the important role of excellent OL mobility for their superior catalytic performance toward HMF oxidation. Actually, the modulation of the acid–base properties of the catalysts can also greatly improve their alcohol oxidation activity. For instance, Parvulescu et al. put forward that the introduction of Cu species into Mn–Al LDH greatly enhances its basicity, which weakens the intensity of the hydroxyl group of HMF, thus facilitating the oxidation of HMF into DFF under base-free aqueous solution [192]. Dibenedetto and co-workers also obtained 89% FFCA yield with impressive productivity of 8.3 molFFCA h−1 gcatalyst −1 over CuO·CeO2 mixed oxides in base-free aqueous solution [193]. Especially, the authors revealed that the balance between acidity or basicity of the catalyst plays a key role to achieve high FFCA selectivity. The same research group further revealed a volcano relationship between the ratio of strong basic and acid sites (nb/na) in Mg–Ce binary mixed oxides and their DFF selectivity [159]. The highest DFF yield of 96% was obtained over MgO·CeO2 with nb/na ratio of 1.1 in base-free aqueous solution. A higher or lower nb/na ratio than 1.1 leads to the formation of over-oxidized products FFCA and undesirable ring-opening by-products. To further increase the oxidation activity of CuO·CeO2 for the production of FDCA from HMF, MnO2 was included in the catalyst as a strong oxidant [164]. Different from the supported noble metal catalysts, in which the basic support generally provides better catalytic performance; the balance of the basic and acid sites of the non-noble metal oxides is important for the catalytic performance of the catalyst.Mo-containing Keggin heteropolyacids (HPMo12O40, HPAs) have already been experimentally and theoretically demonstrated as efficient catalysts for the production of DFF and FDCA from HMF or carbohydrates [154,168,194]. Especially, the partial replacement of Mo centers in HPAs by V (MVP-HPAs) can further enhance its oxidation ability through increasing its redox capacity [168,195]. However, HPAs function homogeneously in the majority of solvents, thus suffers from separation and recovery issues in the catalysis process. Note that the substitution of H+ in HPAs by Cs+ can generate insoluble cesium salt of HPAs (such as, CsxH3-xPMo12 and CsMVP-HPA), which can be employed as heterogeneous catalyst for the oxidation HMF and comparable products yield were achieved as the homogeneous HPAs [195,196]. Besides, Xu and co-workers found that CeCu(OH)6Mo6O18 heterogeneous trimetal catalysts shown excellent catalytic activity for the oxidation of HMF into DFF in p-chlorotoluene [197].Another active Mo-derived catalyst for the oxidation of HMF is molybdenum oxide (MoOx), but the lack of sufficient acidity for fructose dehydration limits its application for the one-pot conversion of fructose to HMF. Therefore, Yang and co-workers developed a series of 3D flower-like micro/nano Ce–Mo composite oxides with tunable redox and acidic properties, in which f-Ce9Mo1Oδ catalyst offered an DFF yield of 74% from fructose [198]. However, the fructose dehydration reaction was performed under N2 atmosphere while the HMF oxidation occurred under oxygen atmosphere, thus switching the inert gas to oxygen is inevitable to guarantee high DFF yield in this case. Interestingly, Zhao et al. accomplished the one-pot production of DFF from fructose by the integration of sulfonate zirconia and molybdenum oxide (MZS), where a satisfactory DFF yield of 74% was obtained from fructose over 10-MZS catalyst under oxygen flow [199]. An appropriate MoO3 content (10 wt%) in the catalyst is important to achieve a balance between its acidity and oxidability, thus promotes the fructose dehydration and HMF oxidation process while suppressing the direct fructose oxidation by-reactions under oxygen flow. Recently, Lei and co-workers revealed that the combination of zirconia and molybdenum oxides (ZrxMoyOδ) also enables the conversion of fructose into DFF in a one-pot process with a DFF yield of 61% under static air within 4 h [200]. Moreover, These Mo-based catalysts generally provided good recyclability after removing the absorbed organic impurities by a calcination post-treatment even if the substrate is carbohydrates.Iron-based materials were extensively employed as magnetic support to facilitate the recovery of the catalyst or promoter to enhance the catalytic activity of Mn-oxides, which were previously regarded inactive for the oxidation of HMF. Interestingly, Li and co-workers designed a series Fe-based catalysts by controlling the structure and crystal facets of Fe-oxides, which revealed excellent catalytic activity for the production of DFF from HMF or fructose [201–203]. In particular, the (111) crystal facet of octahedral Fe3O4 NPs was proved to be highly active for the selective oxidation of HMF to DFF because of the presence of negatively charged oxygen species in (111) crystal facet [202,203]. In addition to regulate the morphology of Fe oxides, the judicious selection of the reaction medium can also transform the inert Fe-based materials into active catalysts for HMF oxidation. Zhang and co-workers proposed a series of ionic liquid (IL)-promoted Fe-based catalytic systems for the base-free conversion of HMF or fructose to FDCA, among which FDCA yields of 38–61% was achieved from HMF or fructose over Ce0.5Fe0.15Zr0.35O2 [204] or Fe0.6Zr0.4O2 [167,205] in 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) without base additive. In contrast, only negligible FDCA was detected in other common solvents (e.g. H2O, methanol and 1,4-dioxane) over the same catalyst. The solvent effect seems to dominate the catalytic activity of the catalysts, but the original catalytic activity of the catalysts in IL remains unclear. Moreover, an effective strategy for the separation of FDCA and reuse of IL is urgently needed to improve the competitiveness of the IL/metal oxides catalytic system.Many soluble non-noble metal salts and organic complexes are reported to exhibit excellent catalytic activity toward the oxidation of HMF and even outperform the heterogeneous ones in some cases [208]. However, some flaws of homogeneous catalysts, such as difficulties in the separation of catalyst from the final product as well as the recycling of the catalyst, severely hinder their practical application and commercialization. Heterogenization of the soluble active components on stable support is a workable and widely employed solution for the above problems. Especially, encapsulation and tethering through a covalent bond or electrostatic interaction are the major adopted strategies to immobilize the homogeneous non-noble catalyst for HMF oxidation (Table 9).Encapsulation of the catalyst complex within the small pores of the support not only can avoid the catalyst leaching but also can maximally imitate the homogeneously-catalyzed reaction process. In 2003, Ribeiro and co-workers designed a bifunctional acidic and redox catalyst by encapsulating cobalt acetylacetonate into sol–gel silica (Co-gel), which afforded an overall FDCA yield of 71% from fructose via a one-pot process within only 1 h without base additive [209]. Lee and co-workers employed Cr-MIL-101-encapsulated phosphomolybdic acid (PMA-MIL-101) as a heterogeneous bifunctional catalyst for the production of DFF from HMF or fructose [210]. 2PMA-MIL-101 catalyst even offered a slightly higher DFF yield of 91% from HMF than that of homogeneous phosphomolybdic acid (88%). Due to the Mo species leaching (9%), the spent 2PMA-MIL-101 catalyst lost 10% DFF yield from fructose in the second run as compared to the fresh one. To avoid the leaching of metal active species, Wang et al. fabricated a mesoporous silica nanofibers-encapsulated H5PMo10V2O40 catalyst (HPMoV/SiO2) through surfactant-directed pore formation and electrospinning methods, which can be reused at least 10 runs for the oxidation of HMF to DFF without catalyst deactivation and metal leaching [211]. Recently, Wang and co-workers also developed a stable polyoxometalate-based mesoporous poly (ionic liquid) catalyst (PMoV2@CP-5.5-400) through a partial carbonization process (Fig. 10 ) [212].Comparing with the encapsulation approach, anchoring the soluble components in the support surface via a covalent bond or electrostatic interaction is a preferred methodology because of its better stability and general applicability. Many metal acetylacetonate complexes are known as active homogeneous catalysts for the oxidation of alcohols [213]. In recent years, various organic and inorganic supports, including organically functionalized SBA-15 mesoporous silica (SBA-Py-VO-2) [214], montmorillonite K-10 clay [215], hydroxyapatite encapsulated magnetic γ-Fe2O3 [216], polyaniline [217], were employed to heterogenise the soluble acetylacetonate complexes for HMF oxidation, and high product yields (around 90%, DFF or HMFCA) could be obtained.Actually, N atoms in many N-containing groups and complexes have lone pair electrons, which exhibit a high affinity toward various metal cations by coordinating with their free orbit, thus the immobilization of metal cation on the N-containing materials produces stable heterogeneous catalyst [218–223]. For example, Saha and co-woks prepared a stable porphyrin-based Fe(III)-porous organic polymer (FeIII–POP-1), which provided excellent productivity of 20.8 molFDCA h−1 gcatalyst −1 by using air as oxygen source under base-free condition [219]. N-containing basic materials can also tightly bond homogeneous heteropolyacid (HPA) through the acid-base interaction. Therefore, polyaniline [224], amino-functionalized CeO2 nanofibers [225] and chitosan nanofibers [226] were employed to immobilize polyoxometalates for the production of DFF from HMF or carbohydrates with good catalyst stability. In particular, chitosan nanofibers supported POM (HPMoV/CS-f (25)) provided excellent DFF yield of 94%, 62% and 31% from HMF, fructose and glucose respectively as a result of its unique coexistence of redox capacity and acid–base properties [226].Overall, immobilized catalytic systems not only addressed the separation and reusability issues of homogeneous catalysts but also present a promising opportunity to enhance the catalytic activity of the homogeneous counterparts through elaborately modulating the surroundings around the active sites. Nevertheless, the heterogenization methods usually involve a massive amount of organic solvents as well as lengthy and multi-step operations, more efforts should be made for the development of facile and low-cost approaches to immobilize the homogeneous catalyst.It's similar to the case of the supported noble metal catalyst, the catalytic activity of the supported non-noble metal oxides heavily depends on the properties of the support, such as the base-acid property, redox property as well as porosity of the support. Various strategies were developed to manipulate these properties of the support to boost the catalytic activity of the catalyst in many ways, like improving the dispersity of the active components as well as the support and metal oxides interactions (Table 10).It has been reported that increasing the Brønsted acidity of the support can promote the catalytic activity of the catalyst for the oxidation of HMF to DFF [80,158,227]. Riisager and co-workers found that V2O5/H-beta catalyst contained the lowest Lewis acidity but highest Brønsted acidity (derived from V2O5) comparing with H-ZSM-5, H–Y, and H-mordenite supported V2O5 catalysts, thus offering the highest V2O5 dispersity and highest DFF yield [227]. This result is similar to Odriozola's conclusion, in which increasing Brønsted acidity of the support was believed to promote the formation of the alkoxy intermediate during the HMF oxidation process [80]. The introduction of Brønsted acidity in the support also enables the formation of bi-functional catalysts for the one-pot production of DFF from fructose. DFF yields of 63–78% were obtained from fructose over protonated graphitic carbon nitride supported vanadium catalyst (V–g-C3N4(H+)) [228], protonated nitrogen-doped carbon-supported molybdenum trioxide (Mo-HNC) [229] and carbon sphere-supported molybdenum oxide (MoOx/CS) [230]. These works pointed out that the balance of the acid density and redox ability of the catalyst is important to achieve a high DFF yield in the one-pot process [228–230].Basic supports are also widely employed to enhance the catalytic activity of the catalyst for the oxidation of HMF. N-doped carbon materials are extensively adopted as support for loading non-noble metal species because the strong interaction between nitrogen and metal species can produce stable metal NPs, sub-nano clusters or even single-atom-catalysts [231,232]. Nitrogen-doped graphene confined Cu NPs (Cu/NG) was reported to be active for the oxidation of HMF to DFF with the aid of 2,2,6,6-tetramethyl-piperidin-1-oxyl (TEMPO) [233] or FDCA by using TBHP as an oxidant [234]. Zhang et al. further compared the catalytic activity of various non-noble metal nitrides (MNx/C-T, M = Fe, Co, Cu, Cr, and Ni; T represents the calcination temperature) for the oxidation of HMF, among which FeNx/C-900 offered the best catalytic performance with 97% DFF yield [235]. This research pointed out that Fe–N4 species (an iron moiety coordinated with four nitrogen groups) of the catalyst is the main active site for HMF oxidation and its concentration as well as chemical circumstances are crucial for the catalytic activity of the catalyst. Subsequently, several N-doped carbon supported Co catalysts were developed for the oxidation of HMF to FDCA, such as CoNx/C-900 [236], CoOx-MC [237], Co–Mn/N@C [238], nitrogen-doped carbon-supported single-atom cobalt catalyst (Co SAs/N@C) [239]. The presence of N species in the catalyst not only enhanced the basicity of the catalyst but also promoted the dispersity of the active components and even created new kinds of active sites by coordinating metal species.The employment of redox and porous materials as support generally boosts the catalytic activity of the non-noble metal oxides by means of promoting the dispersity of the active species and/or metal oxides–support interactions. The spontaneous redox reaction between VO3 − anions and the polyaniline-functionalized carbon nanotubes (CNTs) (VO2-PANI/CNT) afforded the formation of homogeneously dispersed VO2 NPs, which provided a high DFF yield of 96% from HMF [240]. The enhancement of the oxygen mobility of the support also greatly promoted the catalytic activity of the supported metal oxides for the oxidation of HMF [241,242]. For example, Fang et al. recently revealed that the catalytic activity of Mn–Co–O supported Co3O4 NPs can be improved by enhancing the oxygen mobility of Mn–Co–O support [242].By using the strong interactions between the metal oxides and zeolite, Wang and co-workers designed copper-containing mordenite zeolite (V2O5@Cu-MOR) [244] and high-silica MOR supported vanadium oxide catalyst (10V2O5@MOR (60)) [245] for the production of DFF from HMF or fructose, in which uniform and stable isolated V species were generated inside the pore channel of the zeolite. To further improve the dispersity of the active species, Fang and co-workers developed a universal method to fabricate mesoporous KIT-6 encapsulated ultrafine high-loading metal-oxides NPs (Co3O4, CuO, Fe3O4 and NiO) through calcinating a self-assembled MOF precursor inside the silica mesopores [243]. The authors concluded that the confinement effects of the KIT-6 mesopores enabled the generation of uniformly dispersed ultrafine metal-oxides NPs within the mesopores (Fig. 11 ), which thus provided high FDCA yields (80%–99%) and excellent productivities (40–50 molFDCA h−1 molmetal −1) under very mild base-free conditions.For the single non-noble metal catalysts, the absorption of organic impurities [160,164,172,189,193], the leaching of metal species during the oxidation process [228–230], the variation of the surface area as well as the redox cycle of the metal cation [153,162] are the common causes for the deactivation of the catalyst during the recycling experiments. The strong interaction between the metal species and N component can protect the active sites of the catalyst from leaching issue and guarantee a good reusability of the catalyst [175,233,234,238]. Especially, N–MnO2 catalyst can be reused at least 8 runs without decreasing its catalytic activity [175]. The established stable chemical bonds between the acetylacetonate complexes and support [215,217,218] as well as the robust interaction between polyoxoanion and N-containing groups [224–226] also enables the catalyst a good stability. In comparison with single metal-based catalysts, the introduction of a second or third component can also increase its stability, which may be beneficial from the optimized redox or acid–base properties [159,165,182,201,207]. The slight deactivation of some binary and ternary metal oxides could be contributed to the reversible transformation of the catalyst crystal structure [159]or the reduced redox properties [166].Taking into consideration the fact that metal-based catalysts generally suffer from the leakage issue of metal ions, high catalyst cost and environmental pollution problem, the development of metal-free catalysts can be an intriguing and promising approach to address these above drawbacks. In recent years, low-cost carbon-based materials bearing various heteroatom dopants or rich surface functional groups were developed as efficient catalysts for the oxidation of HMF (Table 11).In 2015, Watanabe and co-workers, for the first time, realized the conversion of HMF to DFF over a nitrogen-doped activated carbon catalyst (N-doped-AC-8), achieving a higher DFF selectivity (93%) than that of noble-metal catalysts (Ru/C and Pt/C, 0–14%) albeit at a relatively low HMF conversion (23%) [246]. Graphite-type nitrogen (Nc) species over the N-doped carbon surface are in charge of the selective oxidation of alcohols due to the good correlations between the conversion of alcohols and the amount of Nc species. Differentiating from the case of metal oxides, the oxidation of alcohol over N-doped carbon catalyst operates by a Langmuir–Hinshelwood process. As shown in Fig. 12 , oxygen was adsorbed and activated on a carbon site adjacent to the Nc and/or on the nitrogen atom to produce oxygen radicals, then alcohol, which might adsorb on Nc site, was oxidized to aldehyde by the formed active oxygen species. In addition, Nc species were considered as the active sites for the conversion of HMF to FDCA in a zeolitic-imidazole framework (ZIF-8)-derived nitrogen-doped nanoporous carbon (NNC) catalyst [247], nitrogen-doped graphene (NG-800) [248] and bamboo sawdust-derived nitrogen-doped carbon (NC) material [249]. Especially, the obtained FDCA yields over NNC catalysts prepared at various calcination temperature corresponds well with the Nc species contents of the catalysts, which further confirmed the key role of Nc species for HMF oxidation [247]. Recently, Tao et al. also verified that the graphitic N and pyridinic N species in N-doped graphene (NG) should be responsible for the activation of oxygen to generate active superoxide radicals (·O2 −) by combining DFT calculation and EPR experiment [250]. However, these above catalysts gradually deactivated with the increase of the recycling runs because of the decrease of the amount of Nc species during the oxidation process.To enhance the activity and stability of N-doped carbon catalysts, TEMPO [248] and HNO3 [251] were introduced as co-catalysts. Significantly increased DFF yields of 95–100% were observed with the aid of co-catalyst whereas only a trace amount of DFF was detected without co-catalyst [248,251]. The presence of TEMPO or HNO3 may generate reactive oxygen species over the catalyst, thus improving the DFF yield. The recyclability of these catalysts has also been greatly increased [248,251]. Interestingly, Cao and co-workers found that the introduction of P into N-doped carbon can significantly enhance the selectivity of DFF because of the high concentration of P–C and Nc species of the catalyst [252]. This work avoids the involvement of co-catalyst or second oxidant during the HMF oxidation process and inspires us to develop efficient carbon-based catalysts for HMF oxidation by a heteroatoms co-doping strategy.In addition to the heteroatom-doping strategy, the introduction of functional groups over the carbon surface can also create active sites for HMF oxidation [253]. Lv et al. revealed the carboxylic acid groups in the graphene oxide (GO) can oxidize HMF to DFF, affording a DFF yield of 90% in a reaction time of 24 h [254]. The little loss of GO reactivity during the recycling tests for the oxidation of HMF is largely related to the partial reduction of oxygen-containing functional groups [254]. Glucose-derived carbocatalyst (CC–SO3H–NH2) comprising of both acidic groups (-SO3H and –COOH) and basic groups (-NH2) was employed for the direct production of DFF from carbohydrates [255]. Accordingly, the basic sites of the CC-SO3H–NH2 catalyst are responsible for the isomerization process while its acid sites work as active sites for the dehydration and selective oxidation process. Anyway, green and sustainable metal-free catalytic systems will play a more important role in the oxidation of HMF and other biomass-derived compounds if the catalyst activity and stability can be further improved.In recent years, great progress has been accomplished for the production of FDCA and high FDCA yields can be obtained from HMF over noble or non-noble catalytic systems. Nevertheless, FDCA is a solid powder with a high boiling point (around 420 °C at normal atmosphere) and poor solubility in common solvents, which made it difficult to purify FDCA by conventional crystallization and rectification method [256]. The poor solubility of FDCA in water and major industrial solvents seriously encumbers its production at a high concentration. These limitations of FDCA have shifted research interest toward its methyl ester derivative, that is, furan-2,5-dimethylcarboxylate (FDMC). Benefiting from the good solubility of FDMC in methanol, the oxidative esterification of HMF to FDMC in high substrate concentration (> 20 wt%) with good yields (around 90%) has already implemented [257,258]. In addition, FDMC can be readily separated and purified from the reaction medium by sublimation as a result of its relatively low boiling point (around 140 °C at 10 Torr) [259]. More importantly, FDMC enables the production of PEF with high quality while a colored product of PEF is obtained in the case of FDCA due to the decomposition of FDCA during the polymerization process [260]. However, comparing with FDCA, a few studies focused on the production of FDMC from HMF. Only several heterogeneous catalysts, such as Au, PdCoBi and Co, were found to be active for this reaction (Table 12). It should be noted that a systematic review of this research area has not been published to date.In 2008, Taarning and co-workers, for the first time, accomplished the one-pot oxidative esterification of HMF to FDMC over Au/TiO2 catalyst with excellent yield (98%) and productivity (102.2 molFDMC h−1 molAu −1) in the presence of sodium methoxide [259]. Semicrystalline nanoporous multiblock copolymer matrix-supported Au NPs (Au/sPSB) also provides almost quantitative FDMC yield from HMF with the aid of a base promotor [38]. However, the involvement of base additives during HMF oxidation process makes these catalytic systems less attractive. Interestingly, CeO2 and ZrO2 supported Au catalysts can circumvent the use of alkali additives due to their appropriate acid–base properties, which enables the base-free conversion of HMF to FDMC with almost quantitative yield under mild reaction conditions [261,262]. Kinetic curves for the oxidation–esterification of HMF showed that the oxidation of alcohol to aldehyde over Au/CeO2 is the rate-limiting step and aldehyde can be rapidly converted into ester via hemiacetal intermediates [261]. However, Au/CeO2 catalyst was significantly deactivated in the second reuse because of the absorption of organic material and a calcination process (250 °C, air, 12 h) is required to regenerate the catalytic activity of the catalyst [261].To improve the activity and stability of Au NPs toward the oxidation esterification of HMF, Cu or Pd was introduced as the second component [263,264]. The γ-Al2O3 supported Au-CuOx nanohybrids (Au–Cu/γ-Al2O3) afforded much higher FDMC yield (98%) than that of γ-Al2O3 supported single Au catalyst (48%) [264]. The authors well demonstrated that the introduction of Cu induced a strong electron interaction between Au and Cu species, thus leading to the generation of abundant intensely interacted Au–CuOx interfaces as active sites for HMF oxidation. And the negatively charged Au species in Au-CuOx hybrids also improved the oxygen activation capacity of the catalyst [264]. Nevertheless, the interaction between Au and Cu species became weaker after the reaction, resulting in the deactivation of the catalyst during the recycling experiments. Similar to the case in the oxidation of HMF to FDCA [139], the introduction of Pd greatly promoted the activity of AuPd–Fe3O4 for the oxidation of alcohol group of HMF, rending FDMC yield of 92% at room temperature [263]. By contrast, a single Au–Fe3O4 failed to oxidize the hydroxyethyl group of HMF to aldehyde, thus affording the formation of 5-hydroxymethylfuroic acid methyl ester (HMFE) with a yield of 92%. In addition, AuPd–Fe3O4 sample revealed better catalyst stability than Au–Fe3O4 with significantly decreased metal leaching in the reaction solution thanks to the synergistic electron transfer between Au and Pd [263].Besides Au-based catalysts, Stahl and co-workers surprisingly found that the combination of Pd, Bi and Te with specific formulations rendered highly effective catalysts for oxidation-esterification of primary alcohols [265]. Recently, Fu and co-workers further revealed that FDMC yield of 93% was achieved from HMF when employing Pd/C as the catalyst and Co(NO3)2 and Bi(NO3)3 as promoters with the aid of K2CO3 [256]. Interestingly, the prepared heterogeneous catalyst (PdCoBi/C) afforded a slightly higher FDMC yield of 96% than the case with homogeneous promoters. However, it remains ambiguous for the nature of the promotion effects of additives (Bi and Co) in this above case. The noble metal catalysts, especially Au-based catalysts, revealed high catalytic activity for the oxidation esterification of HMF even under mild and base-free conditions, but the stability of the catalyst remains barely satisfactory.Recently, N-doped carbon-supported Co catalysts were developed as unique non-noble metal catalysts for the efficient oxidative esterification of alcohols in methanol [266,267]. Accordingly, Fu and co-workers attempted to employ CoxOy-N@C catalysts for the oxidative esterification of HMF in the presence of K2CO3. They found that CoxOy-N@C catalyst showed high catalytic activity for the conversion of aldehyde group to ester but weak catalytic activity for the oxidation of alcohol group to aldehyde [268]. Therefore, K-OMS-2 [268], α-MnO2 [269] and Ru@C [270,271] were introduced as the co-catalyst to promote the oxidation of hydroxymethyl moiety of HMF and HMFM, and thus high FDMC yields of 95–100% could be achieved over these multicomponent catalytic systems. During the oxidative esterification of HMF over N-doped carbon-supported Co catalyst, the pyridinic N species of the catalyst worked as Lewis base to abstract the acidic hydrogen of hemiacetal intermediate, forming pyridinic N+-H species. And the presence of Co3O4 species could facilitate the regeneration of pyridinic N by forming [OH−]ad to react with N+-H species. Lin's group recently further verified that substrate, oxygen and methanol were adsorbed and activated on nitrogen-doped carbon shells (C–N) of Co@C–N catalyst while Co species behaved as electron donator to enhance electron density of C–N material, improving its catalytic activity for oxidative esterification of HMF (Fig. 13 ) [272].In comparison with N-doped carbon-supported cobalt oxides, N-doped carbon-supported Co NPs, such as hollow yolk–shell Co@CN [258], porous cobalt/nitrogen co-doped carbons (Co@NCs) [273], hollow Co NPs embedded nitrogen-doped graphite (Co@CN) [274], successfully realized the base-free oxidative esterification of HMF, where FDMC yields of 89–95% could be achieved with a reaction time of 12–24 h. The enhanced catalytic performance of these catalysts may be related to the unique hollow structure, high specific surface area or optimized basic and acid sites of the catalyst [258,274]. In addition, Liu and co-workers surprisingly found that the catalytic activity of N-doped carbon-supported Co NPs (Co NPs-N@C) can be further boosted by reducing the size of Co NPs to single-atom Co species [275]. The isolated Co–N3C species of Co SAs–N@C catalyst served as the active sites during the alcohol oxidation process, which can significantly reduce the energies for O2 and alcohol activation as well as hemiacetal intermediate formation than in the case of Co NPs-N@C catalyst. It has been well documented that the oxygen was activated to · -O2 species by gaining electrons from the N-doped carbon-supported Co catalyst during the HMF oxidative esterification process [270,275]. Therefore, the enrichment of electron density of CoNx species may contribute to enhance the catalytic activity of Co catalysts. Indeed, metal NPs (Co or Cu NPs) were reported to be able to work synergistically with single CoNx sites by inducing the electron transfer from metal NPs to CoNx sites, which reduced the energy for O2 adsorption and favored the generation and release of active oxygen species, and thus promoting the oxidative esterification of HMF to FDMC [257,273,276]. Even though Co catalysts displayed satisfactory catalytic activity for the oxidative esterification of HMF, the Co catalysts generally gradually lose its catalytic activity during the recycling experiments because of the absorption of organic impurities, the aggregation and oxidation of Co NPs [257,274].It is beyond doubt that Co-based catalysts represent an important advance towards the efficient production of FDMC from HMF by using non-noble catalysts, but the Co-based catalysts were commonly prepared with expensive precursors (such as, 2-methylimidazole and dicyandiamide) and energy-consuming preparation process (≥ 800 °C with an inert atmosphere), which certainty hinders their industrial application. Therefore, the development of novel and facile strategy for the preparation of low-cost, efficient and stable Co-based catalysts are very necessary. In addition, the reason for the exclusive catalytic performance of Co-based catalysts toward the oxidative esterification of alcohols remains ambiguous. Thus, a clearer understanding on the reaction mechanism of the oxidation esterification of HMF based on elaborate experiments and computational simulations is of crucial importance.In this review, we systematically summarized the proposed universal catalyst design strategy toward the efficient selective oxidation of HMF. Overall, great progress has been achieved for the selective oxidation of HMF, many novel heterogeneous catalytic systems were developed and exhibited excellent catalytic activity even under mild reaction conditions. Nonetheless, to implement the large-scale production of downstream value-added products by selective oxidation of HMF, there is definitely a long way to go regarding how to address the following issues: (1) One of the challenges for commercializing the HMF oxidation processes is to design highly effective and stable but low-cost catalyst. The exorbitant prices as well as unsatisfactory stability of noble-metal catalysts severely hinder their practical application. Non-noble and metal-free catalysts seem to be economical alternatives for noble catalysts while their relatively low catalytic activity generally requires high catalyst loading to guarantee a high product yield and selectivity. On the other hand, non-noble metal oxides are less stable than noble metal NPs in an acidic environment. The fabrication of non-noble mixed-metal oxides (binary, ternary and even high-entropy metal oxides) and encapsulation of metal oxides with acid-resistant phase has emerged as workable approaches for designing highly durable nonprecious-metal catalysts. The catalytic activity of metal-free heteroatom (N, P, and S) doped carbon-based catalysts largely relies on the concentration of dopants as well as their coordination environment. Multiple heteroatoms co-doping strategy may herald the advent of a new avenue to fabricate highly effective and robust carbon-based catalysts for HMF oxidation. But, how to precisely regulate the number of dopants as well as their coordination environment should be considered. (2) Most previous works for the catalytic oxidation of HMF mainly focused on process optimization and scarce studies provide insightful information regarding the reaction mechanisms of HMF oxidation reactions. Convincing and evidential mechanism investigation is strongly encouraged to proceed through the combination of DFT calculations and advanced operando characterization techniques, which may be helpful to elucidate the relationship between the catalyst structure and activity. (3) Even though many recently developed novel catalytic systems afforded satisfactory catalytic performance for HMF oxidation, the catalyst preparation process often involved lengthy and multi-step operations as well as a massive amount of organic solvent, which is not suitable for large-scale industrial application. Therefore, in addition to excellent catalytic performance, the preparation method of the catalyst should be as simple as possible and inexpensive when designing a new catalyst for HMF oxidation. (4) Based on this review, the majority of the established catalytic systems only focused on the selective oxidation of pure HMF, whereas HMF currently has a similar price to its oxidation products. Thus, it's of significance to develop multi-functional catalysts bearing both acid sites and oxidation sites for the direct transformation of budget carbohydrates or even lignocelluloses into desirable products in a one-pot process. Especially, there are only several catalytic systems that have been reported for the production of FDCA from carbohydrates in a one-pot process with low catalytic efficiency [85,168]. To accomplish this goal, the developed catalysts should have good tolerance to the by-products such as humins. (5) Another limitation for the production of FDCA from HMF is the use of low substrate concentration solutions. The development of novel solvent systems with better FDCA solubility and providing better protection for the sensitive HMF molecule may be a solution for these issues. Moreover, the replacement of HMF with more stable HMF derivates can also realize the production of FDCA at high substrate concentration (10–20 wt%). Thus, the development of new catalysts, which can operate well at high substrate concentration solution and enable the efficient oxidation of HMF derivates, is an important research topic in the future. (6) The industrial-scale production of bulk chemicals generally operates under flow continuous conditions, which can offer better productivity and lower production cost. Thus, to close the gap between the laboratory investigation and industrial application of HMF oxidation processes, more efforts should be devoted to evaluating the catalytic performance (such as reaction pathways, optimal reaction conditions and kinetic parameters) of the catalyst under continuous flow conditions by employing fixed bed reactors. One of the challenges for commercializing the HMF oxidation processes is to design highly effective and stable but low-cost catalyst. The exorbitant prices as well as unsatisfactory stability of noble-metal catalysts severely hinder their practical application. Non-noble and metal-free catalysts seem to be economical alternatives for noble catalysts while their relatively low catalytic activity generally requires high catalyst loading to guarantee a high product yield and selectivity. On the other hand, non-noble metal oxides are less stable than noble metal NPs in an acidic environment. The fabrication of non-noble mixed-metal oxides (binary, ternary and even high-entropy metal oxides) and encapsulation of metal oxides with acid-resistant phase has emerged as workable approaches for designing highly durable nonprecious-metal catalysts. The catalytic activity of metal-free heteroatom (N, P, and S) doped carbon-based catalysts largely relies on the concentration of dopants as well as their coordination environment. Multiple heteroatoms co-doping strategy may herald the advent of a new avenue to fabricate highly effective and robust carbon-based catalysts for HMF oxidation. But, how to precisely regulate the number of dopants as well as their coordination environment should be considered.Most previous works for the catalytic oxidation of HMF mainly focused on process optimization and scarce studies provide insightful information regarding the reaction mechanisms of HMF oxidation reactions. Convincing and evidential mechanism investigation is strongly encouraged to proceed through the combination of DFT calculations and advanced operando characterization techniques, which may be helpful to elucidate the relationship between the catalyst structure and activity.Even though many recently developed novel catalytic systems afforded satisfactory catalytic performance for HMF oxidation, the catalyst preparation process often involved lengthy and multi-step operations as well as a massive amount of organic solvent, which is not suitable for large-scale industrial application. Therefore, in addition to excellent catalytic performance, the preparation method of the catalyst should be as simple as possible and inexpensive when designing a new catalyst for HMF oxidation.Based on this review, the majority of the established catalytic systems only focused on the selective oxidation of pure HMF, whereas HMF currently has a similar price to its oxidation products. Thus, it's of significance to develop multi-functional catalysts bearing both acid sites and oxidation sites for the direct transformation of budget carbohydrates or even lignocelluloses into desirable products in a one-pot process. Especially, there are only several catalytic systems that have been reported for the production of FDCA from carbohydrates in a one-pot process with low catalytic efficiency [85,168]. To accomplish this goal, the developed catalysts should have good tolerance to the by-products such as humins.Another limitation for the production of FDCA from HMF is the use of low substrate concentration solutions. The development of novel solvent systems with better FDCA solubility and providing better protection for the sensitive HMF molecule may be a solution for these issues. Moreover, the replacement of HMF with more stable HMF derivates can also realize the production of FDCA at high substrate concentration (10–20 wt%). Thus, the development of new catalysts, which can operate well at high substrate concentration solution and enable the efficient oxidation of HMF derivates, is an important research topic in the future.The industrial-scale production of bulk chemicals generally operates under flow continuous conditions, which can offer better productivity and lower production cost. Thus, to close the gap between the laboratory investigation and industrial application of HMF oxidation processes, more efforts should be devoted to evaluating the catalytic performance (such as reaction pathways, optimal reaction conditions and kinetic parameters) of the catalyst under continuous flow conditions by employing fixed bed reactors.There are no conflicts to declare.We are grateful for funding supported by the National Natural Science Foundation of China (Grant Nos. 22078275; 21978246), the National Key Research and Development Program of China (Grant No. 2019YFB1503903), the Key Area Research and Development Program of Guangdong Province (Grant No. 2020B0101070001), the Fundamental Research Funds for the Central Universities (Grant No. 20720190014), PetroChina Innovation Foundation (2019D-5007-0413).

The selective oxidation of 5-hydroxymethylfurfural (HMF), a versatile bio-based platform molecule, leads to the formation of several intriguing and useful downstream chemicals, such as 2,5-diformylfuran (DFF), 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), formyl 2-furancarboxylic acid (FFCA), 2,5-furandicarboxylic acid (FDCA) and furan-2,5-dimethylcarboxylate (FDMC). These products have been extensively employed to fabricate novel polymers, pharmaceuticals, sustainable dyes and many other value-added fine chemicals. The heart of the developed HMF oxidation processes is always the catalyst. In this regard, this review comprehensively summarized the established heterogeneous catalyst design strategy for the selective oxidation of HMF via thermo-catalysis. Particular attention has been focused on the reaction mechanism of HMF oxidation over different catalysts as well as enhancing the catalytic performance of the catalyst through manipulating the properties of the support and fabricating of multi-component metal nano-particles and oxides. The current challenges and possible research directions for the catalytic oxidation of HMF in the future are also discussed.

Carbon cycles are different processes which transfer carbon in many ways from energy production to atmospheric pollution. Burning of fossil fuels in daily life for the industrial operations and/or energy supply to human society has liberated most common greenhouse gases (GHGs) such as carbon dioxide ( ) and methane ( ) [1,2]. These are major contributors to the greenhouse effect, by trapping radiation or heat in the atmosphere. Increase in GHGs emissions has triggered climate change and global warming [3]. These emissions have had vital impact on our ecosystem, thereby initiating unaccountable anomalous weather conditions and natural calamities [3]. Therefore, the necessity of mitigation of these greenhouse gases has gained interest and search for the methods to produce clean energy. Reforming reactions are interesting methodologies that not only mitigate, but also convert these dreadful GHGs into value added products [4]. Among many reforming reactions, dry reforming of methane (DRM) has its own importance by converting two greenhouse gases into useful syngas ( ). The syngas ratio from DRM can be used as direct feed stock for F–T (Fischer–Tropsch) synthesis [5]. Although DRM has many advantages, it is a great challenge to design an active and suitable catalyst for it. Moreover, it is extremely difficult to activate highly stable bond in and bond in at low temperatures. Further, at low temperatures this reaction has been accompanied by side reactions, such as (i) reverse water gas shift (RWGS: ), (ii) methane decomposition (MD: ) and (iii) Boudouard reaction (BR: ) [4,6]. So for the low-temperature DRM the catalyst should suppress the side reactions, be sustainable in moist environment as well as resistant to coking from individual reactions of methane and . Several advantages of low-temperature dry methane reforming are demonstrated by a number of researchers [7–9]. Recently, Yabe et al. reported about the electricity generation and hydrogen storage from renewable resources by using catalytic low-temperature dry reforming of methane [10,11]. Noble metal and non-noble metal catalysts were extensively studied for DRM reaction [12–15]. Although noble metal catalysts showed high activity, industrialization through these catalysts is unimaginable for economic reasons. So in this scenario non-noble metal catalysts were considered as alternative source for DRM. Among them, based catalysts showed better performance and promising activity compared to noble metal catalysts. However, catalysts are highly prone to coking. nanoparticles of around 5nm and below are known to effectively reduce coking [16–18]. In order to enhance the resistance towards coking many supports and their modifications to improve oxygen storage capacity (OSC) have been taken into consideration. Cerium dioxide and ceria modified materials as supports have been receiving great attention and are extensively studied in catalysis from the past decades [4]. The generation of oxygen vacancies and OSC in can be achieved by enhancing the redox cycle with simple alterations [19]. Further, it is more advantageous to obtain that with shape controlled synthesis of ceria. The ceria nanocrystals with definite morphologies expose unique surface properties, for instance, ceria rods with prevailing {110}, {100} facets and cubes with {100} facets [20]. Previous literature reports of both theoretical and experimental studies on ceria nanocrystals revealed that the surface energy of these dissimilar crystal facets varies with definite morphologies [21,22]. The theoretical calculations carried out on nano ceria crystals revealed that the energy of formation of oxygen vacancy was surface sensitive and the stability of was closely related to the crystal facets. It was concluded that the rods with {110} and {100} facets have shown a more facile migration of lattice oxygen atoms from the bulk to the surface than cubes with {100} facets [20,22,23]. Furthermore, combination of a suitable support and metal yields improved catalytic activity. The strong synergy between and might generate a hybrid nanostructure that enhances the interfacial metal-support interactions [23]. These interactions were vital in improving the thermal stability as well as dispersion of active metal on support.A considerable number of publications show every year how significant advantages can be achieved by the application of microprocess technology in terms of product yield, purity and time required for chemical and biochemical conversions compared to equivalent bulk reactions [24]. Microreactor technology demonstrated the advantages of microfluidic devices for a very efficient performance of chemical and biochemical processes under controlled and repeatable conditions. Recently, new concepts such as continuous processing, flow chemistry, high-throughput screening and process intensification have been established to open novel pathways in process design and engineering. Process intensification with miniaturization provides insights into the different scales on which process intensification can be applied and enables the development of novel and sustainable plants that offer dramatic process improvements over the existing state of the art in terms of plant size, waste production and other factors [25]. As a result of the small size, the surface to volume ratio is much higher than in conventional reactors. This in turn affects other properties such as the flow regime and mass and heat transfer. Since high pressures and temperatures can be handled much easier on a very small scale, microreactors open up new process windows [26]. There is no doubt that process intensification through microreactor applications clearly has the potential to revolutionize chemical and biochemical synthesis. Between-two-plates reactors have recently gained some attention in the field, as it has been shown that they enable seamless scale-up possibilities, good process control and present a simple to build, as well as simple to assemble solution [27–30].Compared to homogeneous catalysis, heterogeneous catalysis is one of the key tools to increase the sustainability of the diversity of chemical syntheses by simplifying product processing and allowing easy separation and reuse of the catalyst. The incorporation of active, selective and stable solid catalysts in the form of immobilized coatings or micrometer-sized powders in microreactors offers additional advantages by providing highly specific surfaces for the catalysis of the normally demanding three-phase reactions (i.e. gas–liquid–solid or liquid–liquid–solid reactions) [31].Lattice Boltzmann (LB) methods have emerged in late 1980’s as an alternative approach to solving lattice-gas automata (LGA) [32]. They originally proved to be a computationally cheaper option of LGA solving of hydrodynamic problems, especially in low Reynolds number regimes. By the beginning of the new millennium the field has expanded to various applications such as: turbulence modeling, flow through complex geometries, heat and mass transfer, even reactive flows and others [33]. Some studies have focused on LB modeling gas flow in specifically in microchannels [34–37]. When considering gaseous reactive flows, combustion is an often studied topic [38,39], although non-combustive model reactions have also been studied [40]. Additionally, heterogeneous chemical reactions have been introduced in LB [41,42], as well as heterogeneous catalytic reactions [43–47]. The LB is showing great promise for use in modeling of packed- and fixed-bed reactors, among others, because of its easy implementation of complex geometries, therefore it is unsurprising that this has been a somewhat popular topic for its application [30,41,43–45]. In spite of all the development in the LB field, a suitable yet simple boundary condition for a surface-reaction which would incorporate some sort of reaction kinetics is lacking.In this study, deposited on ceria rods ( ) was studied in detail in the process of low-temperature dry reforming of methane conducted in two different reactors systems. The catalyst was well characterized by different instrumental techniques. The study showed promising catalytic activity of both conventional fixed-bed and micro-channel reactors. Furthermore, a modified bounce-back approach to modeling heterogeneously catalyzed reactions within the scope of LB is presented. The results of the computations are compared with experiments in two geometries: a conventional fixed-bed reactor and a between-two-plates microchannel fixed-bed reactor.Synthesis of nanorods was performed according to the following protocol: 53.8g of (Sigma Aldrich) was dissolved in 140mL of ultrapure (Elga Purelab, model Option Q) yielding solution A. A separate solution containing 84mL of ultrapure and 4.9g of (Sigma Aldrich) was prepared (solution B). Solutions A and B were mixed and stirred for 30min on a magnetic stirrer (IKA, model C-MAG HS7). Afterwards, they were transferred to a Teflon® clad stainless steel autoclave, where they were hydrothermally aged for 24h at 100°C. After this time elapsed, the autoclave was quench cooled to room temperature and the suspension centrifuged. This was followed by 3-times washing and centrifuging with water and final washing and centrifuging with absolute alcohol. The yellow powder was freeze dried (Christ, model Alpha 1-2 LDplus) and calcined for 4h at 450°C in air using a muffle furnace (Nabertherm, model P330) and a heating ramp of 5°C min-1.Nickel was deposited by dissolving in 100mL of ultrapure water, followed by addition of appropriate mass of nanorods. The suspension was mixed on a magnetic stirrer and a dropwise addition of diluted ammonia aqueous solution was used to raise the pH value of the aqueous suspension, facilitating deposition of . The pH value of the suspension was raised slowly over the course of 2 h. After the pH value reached 9.5–10 the resulting suspension was centrifuged, dried overnight at 60°C in a laboratory drier and calcined for 4h at 450°C using a heating ramp of 5°C min-1. A nominal 1wt.  % nickel content was deposited. The nickel concentration in the solution after its deposition was analyzed by means of a photometric analysis (Spectroquant by Merck). For the 1wt.  % nickel sample, deposition efficiency of 99% was measured, meaning that the nominal loading corresponds to the actual nickel content. physisorption analysis (Micromeritics, model TriStar II 3020) was carried out at −196°C on degassed catalyst samples to determine BET surface area, total pore volume and average pore diameter. In-situ XRD measurements consisting of a series of reduction and oxidation steps of the catalyst were collected on the PANalytical Empyrean diffractometer having K α  radiation ( λ = 1.54 Å ). At first, the room temperature measurements were recorded in a step wise increment of 0.045 ∘ with a count time of 0.5s and in 2 θ range of 10 to 80 ∘ . Soon after recording, the catalyst reduction was conducted in 5% balanced gas atmosphere at 300°C for 1h with a ramping of 5°C min-1; after completion of reduction, the measurements were recorded with similar procedure as mentioned above. Further, pure gas was used for purging in order to remove the atmosphere and then the compressed air was introduced into the chamber for 30min followed by cooling to 25°C . XRD diffractogram of the oxidized catalyst was again recorded and then the sample was purged with in order to create inert atmosphere. The second reduction was started from 25 to 500°C in flow with a ramping of 5°C min-1 for 1h and the XRD patterns were recorded at 500°C . gas was used to purge the chamber and then the second oxidation was carried out by using compressed air for 30min and then cooled down to 25°C with air atmosphere to record XRD diffractogram of the final oxidized sample.Micromeritics’ AutoChem II 2920 apparatus equipped with a TCD detector was used to conduct the temperature programmed reduction experiments. 50mg of a catalyst was loaded in a U-shaped reactor and subjected to pretreatment in 5% at 300°C for 30min. Then followed purging with gas for 30min to remove physisorbed from the catalyst surface. After 15min of purging, the catalyst was cooled down to 10°C and reduction started with 5% gas mixture to 300°C with a heating ramp of 10°C min-1. After 1h, the gas flow was again shifted to 5% and kept for 30min. The reactor was then cooled down to 10°C in the same atmosphere. Then the re-reduction of the same sample was carried out from 10 to 500°C . pulse chemisorption measurements were also conducted on Micromeritics’ AutoChem II 2920 instrument. The required amount of a catalyst was loaded in the U-shaped tube and reduced in 5% gas flow at 300°C for 1h and then the reactor was purged and cooled with . The consequent pulses were injected into the stream of until the saturation at 10°C . After that the reactor temperature was raised to 500°C for 1h. Then the same procedure was followed after performing reduction of the catalyst sample at 500°C .The DRM activity tests were performed at atmospheric pressure both in a conventional laboratory-scale fixed-bed reactor (differential reactor, I.D.: 9mm) made of quartz and a microchannel reactor made of stainless steel SS-316 plates separated by a graphite gasket (SGL Carbon, SIGRAFLEX®). From here onward the two systems will be referred to as “conventional” and “two-plate”, respectively. The two-plate channel dimensions were 100 mm × 10 mm × 0.5 mm . Schematic drawings of the two reactor systems are provided in a later section in Fig. 2. In the conventional reactor, 50mg of a pelletized, crushed and sieved catalyst sample (average particle diameter: 0.375-0.500mm) was employed. For the two-plate reactor, 100mg of the catalyst sample was mixed with 1.5g of material (irregularly shaped particles, average particle diameter: 0.5mm) and carefully arranged in a single layer on the bottom plate over which the top plate was placed. The catalyst was reduced at 500°C for 1h in flow, then the reaction gas mixture composed of pure and undiluted and streams ( CH 4 ∕ CO 2 = 1 :1) was fed into the reactor with three different total feed flow rates of 20, 40 and 60mLmin−1. The DRM reaction was carried out in the temperature range of 400–500°C . The outlet gas stream was analyzed online by using a micro gas chromatograph (Agilent, model 490) equipped with Porapak Q and molecular sieves (MS5A) columns. The diameter of the catalytic bed in the conventional fixed-bed reactor was 9mm, which is sufficient in order to avoid wall effects (the utilized reactor diameter to the average particle diameter ratio is above 20).The model used consisted of two domains: a mesoscopic domain, which modeled the reaction and flow patterns in the reactor bed, using the lattice Boltzmann method; and a macroscopic domain which modeled the mass transport in the space leading up to the fixed-bed. The two domains are sketched in Fig. 1. For the purpose of numerical simulation of the reactive flow inside the domain 2 of the two reactors, the isothermal lattice Boltzmann (LB) D 3 Q 19 model was used [48]. Although the reactions modeled are not isothermal processes, this assumption was made as the systems described are small in scale, and because of the large surface-to-volume ratio in such systems, no significant temperature gradients are expected. The central piece in the LB method is the LB equation: (1) f i s x → + e → i Δ t , t + Δ t − f i s x → , t = Ω i s ; s = CO 2 , CH 4 , H 2 , CO , H 2 O , where f i s is the distribution function of species s , pointing in direction i . f ’s are treated as the amount of substance at location x → at time t with lattice velocity e → i . The LHS of the equation represents the streaming of the f ’s in a time-step Δ t and the RHS represents the collision with the collision operator Ω i s . The collision model of choice here is the two-relaxation-time collision (TRT) [49]: (2) Ω i s = − f i s + − f i s + e q τ s + − f i s − − f i s − e q τ s − . τ s + and τ s − are the symmetric and asymmetric relaxation times of the substance s . When modeling fluid flow, one controls the fluid’s kinematic viscosity ν through the symmetric relaxation time ( ν s = 1 3 τ s + − 1 2 ) [49]. In such applications τ s − remains a free parameter. Specifically for gas-flow applications in microchannels the latter can be manipulated to give the correct slip-flow at solid walls [37]. However, when modeling advection–diffusion systems with TRT, the asymmetric relaxation time is used to determine the species’ molecular diffusion coefficient D ( D s = 1 3 τ s − − 1 2 ) [49]. In this case τ s + becomes a free parameter. As present study is dealing with a flowing mixture of gases, the two cases are combined: τ s + determines the species s ’s kinematic viscosity, while τ s − determines its molecular diffusivity coefficient. f i s + and f i s − in Eq. (2) represent the species s ’s symmetric and asymmetric link populations, respectively, which are computed as: f i s ± = 1 2 f i s ± f i ̄ s . i ̄ here represents the in-space opposite facing lattice direction ( e → i ̄ = − e → i , for further information on the D 3 Q 19 lattice see [48]). Similarly f i s + e q and f i s − e q represent such equilibrium link populations and are computed as: f i s ± e q = 1 2 f i s e q ± f i ̄ s e q . f i s e q is the equilibrium distribution function of the substance s in i th direction. f i s e q depends on the local pre-collision density of s , ρ s ⋆ , which is proportional to s ’s partial pressure and it also depends on the local flow velocity u → : (3) f i s e q = w i ρ s ⋆ 1 + 3 e → i u → + 9 2 e → i u → 2 − 3 2 u → 2 , with w i being the equilibrium weight in i th direction. u → is calculated by first computing all post-collision densities of s , ρ s : (4) ρ s = ∑ i f i s , and then by computing the sum of mesoscopic momenta: (5) u → = 1 ∑ s M s ρ s ∑ s ∑ i M s e → i f i s . Note that here the f i s ’s are multiplied by the dimensionless molecular weights of s , M s . This ensures mass conservation in the system. ρ s ⋆ , the pre-collision density, is computed by also including a macroscopic reaction term, which accounts for the RWGS reaction. This is included in the model as follows [41]: (6) ρ s ⋆ = ρ s + − k + ρ CO 2 ρ H 2 + k − ρ CO ρ H 2 O τ s + ; s = CO 2 , H 2 − k + ρ CO 2 ρ H 2 − k − ρ CO ρ H 2 O τ s + ; s = CO , H 2 O 0 ; s = CH 4 . k + is a second order reaction constant for the RWGS reaction, while k − is the constant of the reverse reaction. The equilibrium constant ( k e q = k + k − ) was set to 5 [50]. Other side reactions were neglected here.The catalytic DRM reaction is carried out at the solid nodes, where the ordinary half-way bounce-back boundary condition with a slight modification is used to describe the walls. Slip flow at solid walls was not controlled and the effects of slip-flow on model accuracy were not studied in this work. The bounce-back can be carried out in the normal way, where at non-catalytic walls the incoming f i s ’s get translated into f i ̄ s ’s or at catalytic walls they get translated to a modified f ˆ i s : (7) f i ̄ s = f i s ; wall f ˆ i s ; catalyst . At solid nodes marked as the catalyst, all reactants’ ( and ) f ’s get converted to products’ ( and ) f ’s. However, each site has a limited amount of available catalytic sites, which is controlled through a free parameter κ . κ represents the amount of available sites and it does not discriminate against any molecule type nor does molecule size affect how much space a molecule occupies on the catalytic site, i.e. all species have the same affinity towards the catalyst surface. Further parameters could be implemented into this model to control this, but such a model would require a molecular-level approach to determine these parameters. Because κ limits how much substance can get to the catalytic surface, there are two cases of how this boundary is handled. In case where ∑ s f i s > κ : (8) f ˆ i s = f i s − κ ˆ ; s = CH 4 , CO 2 f i s + 2 κ ˆ ; s = CO , H 2 f i s ; s = H 2 O , where κ ˆ = f ̄ ∑ s f i s κ , with f ̄ being either f i CH 4 or f i CO 2 , whichever is smaller in value (limiting reagent). In case where ∑ s f i s ≤ κ : (9) f ˆ i s = f i s − f ̄ ; s = CH 4 , CO 2 f i s + 2 f ̄ ; s = CO , H 2 f i s ; s = H 2 O . The factor 2 in both cases is added to keep the model in line with the stoichiometry of the DRM chemical equation. It was assumed that the reaction only takes place on the particle surface, so internal diffusion in particles was neglected.As the densities of s ’s (partial pressures) at the outlet were unknown, a constant density boundary condition could not be used to describe the outlet of the system. Instead a constant velocity boundary condition was used for the outlet. However, at the inlet the velocity was supposed to be set to a constant value (inlet velocity set in the experiments by the mass flow rate of gases), but more importantly the ratio of different species at the inlet needs to be set to 1:1:0:0:0 for , , , and , respectively. So a constant density boundary was used at the inlet. This could, however, not ensure that the velocity at the inlet would indeed match the inlet velocity set in the experiments. To address this issue a regulating algorithm was added to the inlet boundary condition, which ensured that the ρ s there were such, that the inlet velocity was correct. The boundary conditions used were the non-equilibrium boundaries by Guo et al. [51].Because the product gases are fast diffusing, a significant mass loss was initially observed at the inlet. This was due to products’ partial pressures being kept at 0 at the inlet. This issue could be solved by making the simulation domain (domain 2) longer, but this would increase the computational costs. The solution was to split the domain in two, as described in a previous section. The added domain (domain 1) was used only to model the transport of products with a macroscopic model. This eliminated the mass loss due to back-diffusion. A simple 1D model was used for this: (10) D s d 2 ρ s d x 2 = u x d ρ s d x ; 0 ≤ x ≤ L 1 , where u x is the average flow velocity in x -direction at domain 2 inlet and D s is the species’ diffusivity. 0 is the inlet to domain 1 and L 1 is the junction of the two domains. Shooting method was used to solve Eq. (10), by applying the following boundary conditions: (11) ρ s | x = 0 = 0 , (12) ρ s | x = L 1 1 = ρ s L 1 2 . ρ s L 1 is the species’ density at the domain 2 inlet. This value was then modified by a combination of d ρ s d x | x = L 1 1 (which was found by solving the model) and d ρ s d x | x = L 1 2 . Given the steady-state nature of this model, the dynamic-state solution was not necessarily physical, but the 1D model eventually “caught-up” with the LB model. The fluid dynamics in the 1D model were not considered in this study and thus the pressure drop in the domain 1 was neglected and the bulk reaction was also neglected in this domain.The computational domains for the lattice Boltzmann model (domain 2) were created with Blender [52]. Firstly four different shapes were designed by hand in Blender for both — catalytic and filler particles. Then the reactors’ channels were designed. Next the particles were multiplied to the amount that was expected in the experiments by accounting for particles’ average density and their average size, and sample masses. Then Blender’s physics engine was used to randomly load these particles in the channels. The geometries were then exported and turned into lattices, which could be used in the LB computations. The staircase shape of the walls that was obtained through this process further randomized the particles’ shapes.The computations used the model described above to compute (partial) pressure and velocity profiles inside the two-plate and conventional fixed-bed reactors. Total volumetric flow rates ( V ̇ ) studied in both systems were 20, 40 and 60mLmin−1. Because V ̇ ’s in the experiments were defined at room temperature, the inlet flow rate for the computations was estimated with the ideal gas law ( V ̇ ∝ T ) and mass conservation law ( m ̇ 1 = m ̇ 2 ). The viscosities of gasses as well as their molecular diffusivity coefficients were obtained from Engineering ToolBox web page [53,54]. The values of the latter had to be extrapolated to get an estimate of the values at the studied conditions (500°C ).To first determine parameters κ and k ± , the computations for the conventional fixed-bed system were run, as this was due to its smaller size computationally less expensive. The two parameters were at first chosen as an arbitrary guess and then adjusted to capture the behavior exhibited in the experiments. The same values of both parameters were then used to simulate the process in the two-plate system without further tuning.Specific surface area (SSA) of the and samples are depicted in Table 1. Catalysts showed SSA values of 98 and 93m2 g−1 for and solids, respectively. The near equal values of SSA indicate that the deposition on does not show any considerable effect on surface area of . Also, the total pore volume and average pore diameter showed similar values. Fig. 3 shows the X-ray diffractograms of calcined and solids as well as of reduced and re-oxidized catalyst samples. The calcined sample showed high intense patterns at 2 θ = 28 . 55 , 33.03, 47.48, 56.29 and 59.12 ∘ . These diffraction lines are well corroborated with the crystal planes (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) of cubic fluorite of as observed in JCPDS file 34-0394 [4]. In sample, all the high intense diffraction peaks are related to the fluorite and no peaks are observed. The addition of to showed a considerable shift in the diffraction peak from 28.55 to 28.68 ∘ . This higher 2 θ shift indicates incorporation of into lattice by the formation of solid solution [55,56]. The reduction of catalyst at 300°C showed considerable decrease in the intensity of peaks, indicating that the reduction exhibited a significant effect on the crystallite size (Table 2). The oxidation step conducted after reduction not only increases the peak intensity but also shows a gradual shift of the peak to its original position. This indicates that the oxidation of this material shows a reversible formation of solid solution. The re-reduction carried out at 500°C showed a little shift towards lower angle and also decrease in peak intensity indicating that the reduction process showed substantial changes in the lattice and crystallinity of the material. These shifts and intensities were clearly observed by a closer look (see Supplementary material, Fig. S1).Calculated lattice parameter values of (1 1 1) plane after considering the thermal expansion of and are presented in Supplementary material, Fig. S2. The lattice parameter value of was found to be equal to 5.4157Å, further the addition of decreased the lattice parameter to a value of 5.3910Å. As reported in literature, the decrease in lattice parameter can be referred as the incorporation into lattice in order to form a solid solution phase.Reduction of catalyst at 300°C showed a considerable increase in the lattice parameter value from 5.3910 to 5.4122Å. According to literature, during reduction the transformation of from 4+ to 3+ resulted in an increase of ionic radii. So, the lattice expansion is mainly due to these high ionic radii species. Another possibility of lattice expansion could be due to the exsolution of . During reduction, oxygen atoms were stripped from the support lattice thus the electrons resulting from this reaction caused the decrease of support oxidation state, thereby expanding the lattice [57]. Further oxidation of the reduced sample resulted in a similar lattice parameter value as the one belonging to the fresh catalyst, thus indicating that the reduction and oxidation cycles are reversible in . Re-reduction of oxidized catalyst at 500°C showed further enhancement in the lattice parameter value; the same behavior was also observed by other researchers [58,59]. It should be noted that the re-oxidation step following the reduction at 500°C also exhibited the lattice parameter value similar to the one of the fresh catalyst sample, thus further confirming the reversible red-ox cycles in solid. Fig. 4 shows the calculated lattice parameter values for different crystalline plane of . After the addition of to , one can clearly observe a decrease in lattice parameter values as well as the same trend for all planes, thus indicating the uniform influence of on lattice. When the catalyst was reduced under till 300°C , some planes showed higher lattice parameter values and some planes exhibited lower values than the support. This observation supports the expansion of lattice that results into distortion of cubic . In-situ oxidation promotes the lattice parameter values to their original states, which is in agreement with the previously explained observation that the reconstruction of lattice is reversible. The reduction step conducted at 500°C caused an additional expansion of the ceria lattice due to the enhancement in extent of reduction. However, re-oxidation conducted after the second reduction cycle was able to reproduce lattice parameter values very similar to the ones determined for the fresh catalyst sample. Fig. 5 shows the results of TPR analysis in which the reduction behavior of catalyst was examined by undergoing different reduction cycles. The fresh catalyst was first reduced in 5% gas stream from RT to 300°C and then kept at this temperature for 1h. The obtained TPR profile (marked as TPR 300 in Fig. 5) can be divided into four different reduction zones at 167, 237, 262 and 300°C , which were further denoted as α 1, α 2, β 1 and β 2 bands [11,56,60,61]. α 1 and α 2 bands are related to the adsorbed oxygen and/or surface active oxygen species that can be easily reduced at lower temperatures. β 1 band can be referred to the reduction of solid solution, while β 2 reduction band could be attributed to the reduction of isolated and/or ceria. On the other hand, reduction of the re-oxidized sample at 500°C (TPR 500 profile) shows the same bands, however, β 1 and β 2 bands are shifted slightly to higher temperatures. This high-temperature shift may be attributed to the highly interacted in the lattice. As observed in the XRD examination, expansion of the lattice might increase the possibility of reduction of trapped in the ceria lattice. The total hydrogen consumption during the TPR 300 run was 15.0mLg−1, while in the case of re-reduced sample at 500°C this value was equal to 15.6mLg−1. The extent of ceria reduction is 23 and 24% for TPR 300 and TPR 500 runs, respectively. This is in agreement with the results illustrated in Fig. 4 which indicate that the extent of reduction is enhanced at higher temperatures, thereby an increase in hydrogen consumption is noticed in the re-reduced sample. Fig. 6 shows the results of pulse chemisorption analysis for the catalyst undergoing different reduction cycles. The reported values of dispersion were found to be reproducible. The catalyst reduced at 300°C showed dispersion of 35%. Surprisingly, the catalyst reduced at 500°C showed higher dispersion (i.e. 44%) compared to the solid reduced at 300°C . This can be explained by means of TPR and XRD studies, where high-temperature reduction showed expansion of lattice that can fetch -containing ensembles trapped inside the lattice. Participation of trapped (i.e. re-surfaced) ensembles as an active phase might enhance the dispersion of after reduction at higher temperature. In addition to the above and as described during the explanation of results of XRD analysis, exsolution of from the solid solution is another possibility of lattice expansion. At high-temperature reduction, the increase in lattice expansion is due to the higher extent of exsolution from the lattice. Consequently, this increases the dispersion of after the reduction of the catalyst at 500°C . Further, after re-reducing the catalyst at 500°C (third cycle in Fig. 6) a slight decrease of dispersion was observed that may be attributed to sintering. Mean particle size of ensembles calculated from the pulse chemisorption study is 1.6, 1.3 and 1.4nm (as it derives from TPR 300 cycle 1, TPR 500 cycle 2 and cycle 3 analyses, respectively). Fig. 7 shows SEM and TEM images of fresh and samples. The rod-shaped morphology of is observed from the figure. Further, the morphology has still existed even after deposition.Results of the catalytic activity test conducted in the two-plate reactor with varying reduction conditions of the catalyst prior to the run are shown in Fig. 6. At T = 500 ° C , the equilibrium conversions of and are 15 and 23%, respectively; the conversions reported in this study are below the equilibrium levels. The catalytic activity increased with the increase of reduction temperature from 300 to 500°C ; an increase of methane conversion from 5.6 to 10.5% was observed. A similar increment (from 8.5 to 14.9%) was measured in the case of conversion. We believe that the observed behavior could be explained by taking into account an identical trend regarding dispersion of in the catalyst pre-reduced before the DRM reaction at different temperatures (black curve in Fig. 6). Fig. 8 shows the results of catalytic activity measurements conducted in both — two-plate reactor and conventional fixed-bed reactor varying the total gas flow rate from 20 to 60mLmin−1 with an increment of 20mLmin−1. The two reactor units showed promising activity: for instance, in the two-plate reactor conversions of and were found to be 10.5 and 15%, respectively, when utilizing the gas flow rate of 20mLmin−1. For the same gas flow rate, the conventional reactor showed conversions of and equal to 12.7 and 18.1%, respectively. Higher catalytic activity was observed in the conventional fixed-bed reactor compared to the two-plate reactor, although a double catalyst loading was used in the latter reactor system. The same trend was noticed for all the gas flow rates investigated. Furthermore, in the given range of experimental conditions enhanced production of water during the DRM reaction was observed in the two-plate reactor, whereas in the conventional reactor water formation was negligible. Water formed is mainly due to the RWGS reaction ( reacts with hydrogen and produces water and ). Higher conversion of over also indicates the co-existence of the RWGS reaction [62]. A close observation on the conversion vs. time profiles presented in Fig. 8 shows a noticeable difference in conversions as a function of time with an increase of gas flow rate. In the conventional fixed-bed reactor (Fig. 8b), and conversions decrease in the same manner with the change of gas flow rates, whereas in the two-plate reactor the decrease of conversions is not in accordance with the temporal conversion profile. Low differences in conversions when increasing gas flow rate in the two-plate reactor are mainly due to the participation in side reaction (RWGS). One can therefore conclude that participation in RWGS to produce water is highly favorable in the two-plate reactor compared to the conventional fixed-bed reactor. Formation of water slowly decreased with the increase of gas flow rate, indicating that the decrease of contact time decreases the extent of side reactions (i.e. RWGS reaction). In the two-plate reactor, single-layered catalytic particles were placed at nearly 10cm length; on the other hand, in the conventional fixed-bed reactor the catalytic bed was of 9mm diameter and 3mm height. As such, the possibility of to react with is much higher along the catalytic bed in the two-plate reactor than in the short catalyst bed in the conventional fixed-bed reactor. Consequently, the two-plate reactor favored the progress of side RWGS reaction which is evidenced by (i) low decreases of conversions when increasing gas flow rate (compare conversion vs. time profiles for both reactor systems in Fig. 8) and (ii) higher extent of water formation.The reaction rates calculated on the basis of and conversions are illustrated in Fig. 9. In the two-plate reactor, the reaction rate (based on conversion) increased from 1.08 to 2.24mmolg Ni −1 s−1 with the increase of gas flow rate from 20 to 60mLmin−1. On the other hand, the reaction rate (based on conversion of ) increased to a value of 0.86mmolg Ni −1 s−1 when increasing the gas flow rate from 20 to 40mLmin−1; further increase of gas flow rate exhibited zero effect on the reaction rate (based on conversion of ) indicating that the DRM reaction was conducted in the kinetic regime in the case of gas flow rates of 40 and 60mLmin−1. Very similar trend was observed also in the case of the conventional fixed-bed reactor. Because participates in side reactions (one should note that takes part in RWGS reaction which is much faster than methane decomposition), conclusions about the DRM kinetics based on the conversion of this reactant are not appropriate. The computations ran until a steady-state solution was reached, which varied from case to case. Final concentration profiles obtained are presented in Fig. 10. The transition between the two domains appears smooth, however, computing the data derivatives reveals some distortion in the transition (data not shown). From Figs. 10d–10f it is evident, that the omission of the transport in domain 1, i.e. only computing the transport through domain 2 and assuming the concentrations at the domain 2 inlet to be constant, would result in significant mass loss in the system. Domain 1 appears to be less necessary in the two-plate system, as can be assumed from Figs. 10a–10c. Although the range of flow rates was the same in both systems, the difference in back-diffusion appears because of the different cross-section geometries of the system, which result in different linear velocities, entering the system. Additionally, the catalyst was diluted in the two-plate system, which in turn meant that the concentration gradients of the products were possibly lower near the inlet and combined with faster convective transport, there was less potential for back-diffusion. The model results also show that about the same (non-negligible) amount of water vapor is produced in both systems. Fig. 11 is displaying the steady-state solutions of the velocity field as well as the partial concentration of along both channels. The velocity profile in Fig. 11a shows that in the two-plate system the bulk of the gas flow traveled over the fixed bed. This happened due to the bed height being uneven and the bed particles not being distributed evenly through the depth of the channel, which is a consequence of the bed particles being poured into an open reactor, which is then closed by pressing the two plates together. In turn this could affect the two-plate reactor’s performance. However, due to the advanced diffusive properties of gasses and reactor’s small dimensions, the extent of these effects is likely minimal. Fig. 11b is displaying the steady-state solution in the conventional reactor. Here the distribution of particles appears to be uniform and local spikes in flow velocity are observed due to gas being squeezed through narrow gaps in the bed. Tables 3 and 4 are comparing the results of computations with the experiments in both systems. There appear to be some differences in the reactants’ conversions, where the model mostly underestimates especially the conversion and the conversion gets lower with increasing flow rate faster in the model than with the experiments. The model predicts the ratio well for lower flow rates, however, at 60mLmin−1 it deviates from the experiments in both systems. The results of both — computations and experiments, are plotted together on one graph in Fig. 12. Here the conversion is adjusted to the amount of catalyst in each system (conversion of the two-plate system was divided by 2, to account for twice the amount of the catalyst) and is plotted against the Reynolds number in the reactors ( R e = D p u ̇ x ϵ ν ). Particle diameter ( D p ), average linear velocity ( u ̇ x ) and bed porosity ( ϵ ) used to calculate each R e were measured in the computations and assumed they were the same in experiments. This can give an insight into lower conversions of the two-plate system: as its cross-section dimensions are smaller the gases flow faster around the particles and the effective residence time of reacting species around catalytic sites is shorter than in the conventional system. Other differences may be setting the two systems apart however, as there is a noticeable “kink” in the pattern in the transition between the results of the two systems, especially with the conversions. Fig. 12 displays that the model successfully reproduced the reactant conversion trend in both systems. The model however struggled to correctly capture water vapor production shown in the experiments. In the experiments the conventional fixed-bed reactor produced significantly smaller amounts of water vapor than the two-plate system did. The model however predicts similar amounts of water vapor to be produced in both. From this it could be assumed that different residence times alone were probably not responsible for that.The synthesis of rods and their lattice modification with deposition was observed by means of XRD analysis. The lattice contraction of with deposition of illustrates that the incorporation of into the lattice results in the formation of solid solution. In-situ XRD examination conducted in the reductive atmosphere revealed that the expansion of lattice is enhanced with temperature. However, the lattice parameter values are quite similar to the untreated catalyst after oxidation, which evidences the reversible redox nature of lattice. TPR measurements showed an increase in consumption of content with temperature due to enhanced reduction. It was observed in low- and high-temperature TPR reduction cycles that trapping of ensembles in the lattice enhances dispersion of on the support.The promising catalytic activity in the low-temperature dry methane reforming is observed in both between-two-plates microchannel fixed-bed reactor and conventional fixed-bed reactor. The RWGS reaction is highly dominated in the former reactor system, which is identified with the enhanced amount of water formed during the reaction.The choice of TRT LB appears to have been appropriate, as the two collision parameters could be used to manipulate the gas viscosity as well as its diffusivity. Furthermore, the multiscale model significantly reduced the computational costs, by using the computationally more expensive LB method only in the more complex section of the domain. It performed well for finding steady-state solutions, but it needed a couple of iterations to start returning qualitatively smooth concentration profiles. The catalytic reaction model developed here was able to project the two systems’ behavior, however with limited accuracy. With more computing power κ and k ± could be tuned more precisely, which could possibly improve the results. Additionally, there appear to be further differences between the two studied systems other than their geometry, as the model predicts similar vapor formation in both, whereas the experiments showed that the conventional fixed-bed reactor produced significantly lower amounts of water. This suggests that the residence time alone is probably not the reason for differing performances.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 of the Slovenian Research Agency through PhD Grant MR-39080 (FS), Grants P2-0150, P2-0191 and projects J7-1816 and N2-0067 is acknowledged. The support through the H2020 project COMPETE, Slovenia (Grant No. 811040) is also acknowledged. The authors would also like to thank Dr. Janez Zavašnik and Dr. Gregor Žerjav for the TEM and SEM images, respectively. The graphite gasket was kindly provided by SGL Carbon.Supplementary material related to this article can be found online at https://doi.org/10.1016/j.cej.2020.127498.The following is the Supplementary material related to this article. MMC S1 .

Experimental and theoretical modeling on low-temperature dry methane reforming over Ni-containing CeO2 rods was studied. The catalyst was characterized by means of N2 physisorption, in-situ XRD, TPR and H2 chemisorption techniques. The characterization studies revealed the distortion of CeO2 flourite structure due to the Ni incorporation. Lattice expansion (due to reduction) and contraction (due to oxidation) suggest the reversible redox nature of CeO2. Ni–O–Ce solid solution formation was evidenced by both XRD and TPR studies. H2 chemisorption study revealed that the catalyst reduction temperature plays a significant role in Ni dispersion. The catalyst showed similar activity trends in two model geometries: a between-two-plates microchannel fixed-bed reactor and a conventional fixed-bed reactor. The activity tests were conducted in the kinetic regime, where conversions of CH4 were not influenced with the gas flow rate. A lattice Boltzmann model for mixed gas flow was developed along with a boundary condition for catalytic sites. The lattice Boltzmann model was used in a multiscale simulation of the studied reaction systems and produced data that qualitatively matched the experiments.

Triglycerides are long-chain organic compounds that are primarily present in different vegetable oils, animal fats. They are composed of carbon, hydrogen, and oxygen linkages, classified as glycerides (triglycerides, diglycerides, monoglycerides). In addition to glycerides, free fatty acids and phospholipids with metallic impurities are also present in the aforementioned feedstocks. These feedstocks produce biofuels via different catalytic and non-catalytic routes such as transesterification, hydrothermal conversions, or hydroprocessing route [1–16]. Newer ways for the production of sustainable aviation biofuels from biomass are being developed. Among different routes, the conversion of triglycerides to hydrocarbon fuel in single/multiple steps by hydroprocessing reactions is comfortable and well known to refinery engineers.Triglycerides are bulky in size, and for complete reaction, the molecules need to access the active sites on the catalyst surface through its porous structure. The catalyst activity, stability, life, and renewability are the crucial criteria for catalyst selection. Researchers have modified the conventional hydrocracking catalyst properties to achieve the desired yield and selectivity for hydroprocessing reactions. To complete triglyceride conversion to hydrocarbon fuel, changes in active metal composition and loading, different kinds of active metals, additives, etc., have been studied. Apart from conventional hydroprocessing catalyst supports, many other materials such as clays, carbon, oxides, and other mixed oxides derivatives have been tried and tested for hydroprocessing vegetable oil reactions.The encapsulated metal catalyst is gaining popularity as an advanced hydroprocessing catalyst with enhanced activity, chemoselectivity, and increased durability. Noble metal catalysts, for the hydroconversion of vegetable oil or oxygenated feeds, are not uncommon [1–3], but their use with feeds having high sulfur (>1000 PPM) is infrequent. Sulfur and carbon monoxides are known to poison noble metal catalysts [4,5]. Hydroprocessing of triglycerides and their co-processing with refinery streams have been studied and extensively published in the literature studies [1–16]. The hydroconversion parameters' effect has been evaluated for neat vegetable oil [17–25] or co-processing of vegetable oil with refinery streams [26–33]. Although noble metal catalysts have better hydrogenation than sulfided catalysts, they are more prone to deactivation due to CO or sulfur components present in the feed (typically in refinery feeds) [2–3]. One way to use noble metals in high sulfur or CO atmosphere is by encapsulating these metals inside some cage, e.g., zeolite cage, where bulkier components like CO, H2S, or other sulfur compounds may not enter the cage, while the smaller molecules, like H2, quickly enter the cage and adsorb on the metal [4].Choi et al. [8] have studied the effect of an encapsulated catalyst on the activity and selectivity for the oxidative dehydrogenation of methanol and iso-butanol. The prepared material, zeolite-like sodalite (SOD) material, which has a small pore size, having framework consisting of six-membered ring apertures of window size 3.6 Å and 5.2 Å and a cage diameter of 6–8 Å, encapsulated Pt nanoparticles of size < 1 nm could fit inside their cages [8–10]. Choi et al. [8] reported the maximum size of the encapsulated Pt nanoparticles as 0.5 nm (5 Å). These cages have dual benefits; they would protect the Pt clusters from sintering and prevent impurities like H2S and other 'S' containing compounds having a size more significant than that of the zeolite window to enter the cage [4,6–8]. Goel et al. [6] used a similar concept to encapsulate metal clusters inside small-pore zeolites. The prepared material activity was then tested for oxidative dehydrogenation of methanol and iso-butanol and hydrogenation of ethene and toluene. Wang et al. [5], using a similar concept by confining the Pd cluster inside silicalite for hydrogen production via formic acid decomposition. Studies by Choi et al. [4] and Juhwan et al. [7] provide a molecular-level understanding of encapsulation and hydrogen spillover mechanism. The authors suggested that surface hydroxyls, presumably Brønsted acidity, play a crucial role in hydrogen spillover and activity enhancement. The hydroxyl groups' role in hydrogen spillover was confirmed experimentally by changing the hydroxyl concentration in the encapsulated zeolites; this resulted in a considerable change in the hydrogenation activity. Srinivas and Rao [36] were the first to observe the H2 spillover phenomenon on Pt supported on the carbon surface. It is expected that encapsulated noble metal may provide extra spillover hydrogen [34–36] to nearby NiS and MoS2 sites, increasing hydrogenation [36–37]. Availability of more active hydrogen on active MoS2 and Ni3S2 sites would enhance the catalyst's hydrogenation function, which would help reduce naphthenes and aromatics produced during the hydroconversion of jatropha oil [38].Sibi et al. [4] used encapsulated Pt inside sodalite (SOD) cage with NiMo/ZSM-5 for the hydroconversion reactions of Jatropha oil and studied the effect of the process parameters on the hydrocarbon product yield and isomerization activity. Product yield pattern and isomerization activity were reviewed on this catalyst. Aviation fuel has stringent cold flow specification (<-47 °C. Since the iso-paraffins are better in cold flow properties than normal-paraffins, it is important to track the isomerization activity for each catalyst. The yield of kerosene and iso/normal hydrocarbon ratio was compared with the conventional hydroprocessing catalyst. The reported maximum kerosene range hydrocarbon yield for NiMo/Pt@SOD@HZSM-5 was six times higher than NiMo/HZSM-5 catalyst for >99% of triglycerides conversion.SiO2-Al2O3, a more economical catalyst support [21] for vegetable oil processing, is being used to support this study. As per the aviation fuel specification, it is necessary to limit aromatics and naphthenes in aviation range hydrocarbon under a specific range [9–13]. A noble metal, being more active for hydrogenation reactions, strongly affects the product composition like paraffin, naphthenes, and aromatics in the aviation range of hydrocarbon during vegetable oil conversion [4]. The composition of the aviation range hydrocarbons affects the specification and strongly affects the catalyst life. Naphthenes and aromatics are well-known precursors for coke deposition [14]. Cyclic hydrocarbons are more prone to coke formation compared to normal paraffin [14–15]. Though aromatics and naphthenes contribute to the coke formation, their presence in the aviation turbine fuel (ATF) range is essential. They improve the freezing point, lubricity, and fuel compatibility with engine parts where sealants are used [16].American Society for Testing and Materials (ASTM) specifies certain limits for naphthenes and aromatics for renewable and non-renewable based aviation fuels. Aviation fuel, meeting the specifications as per ASTM standards (ASTM D1655 for petroleum origin and ASTM D7566 for alternative routes), is suitable for aircraft worldwide. ATF obtained via hydroprocessing of esters, vegetable oils, animal fats are covered in Annexure 2 of ASTM D7566 document [10]. ASTM D7566 specifies the limits of a maximum 0.5% percentage of aromatics and 15 % for naphthenes. To meet the ASTM D7566 specification, keeping the aromatics and naphthenes for the aviation range hydrocarbons within these limits is necessary.Hydroconversion of vegetable oil produces mixed hydrocarbon ranges, namely gasoline, aviation, and diesel range hydrocarbons [17–28]. Hydrocarbons obtained via hydroconversion of vegetable oil consist of naphthenes, paraffin, and aromatics in the final product [21–22]. A higher percentage of aromatics in hydro-processed lipids is not desirable in ATF [21–22,25]. Xing et al. [25] explained the mechanism for the production of aromatics during hydroconversion of esters. At lower temperatures, it was reported that fatty acids are first saturated, and then a hydrodeoxygenation reaction occurs at the metal sites to form long-chain hydrocarbons. They also compared the mechanism of saturated and unsaturated feed. Unlike saturated feeds, unsaturated feed directly undergoes deoxygenation at acid sites before saturation. The long-chain hydrocarbons then crack into gases at Brønsted acid sites; and then undergo Diels–Alder reactions on the Lewis acid sites to form aromatics hydrocarbons (AHCs). Lewis acidity supports aromatics formation in the hydroprocessed product; at the same time, Brønsted acid is also responsible for hydroisomerization and hydrocracking reactions [25]. A trade-off between Lewis/Bronsted acid sites is required to maximize ATF range hydrocarbon and reduce aromatic content. It is expected that by increasing the hydrogenating function of the conventional hydrocracking catalysts, hydrocarbons with lower aromatics and cyclic, without compromising the product yield, could be produced.For better product (ATF) quality, a balance of acidic functionality and hydrogenation activity is an essential criterion in catalyst selection. Verma et al. [21,22] discussed the support effect on the product distribution for aviation range hydrocarbon using jatropha oil as feed. The catalyst was tailored by tuning the zeolitic acidity and porosity. Reported aviation range hydrocarbon yield was 40–45%, with high isomerization selectivity (iso/normal ratio) in the aviation range [2–6]. In another experiment, using Ni-Mo supported on high surface area semi-crystalline ZSM-5, very high aviation range hydrocarbon yield (77%) with iso/normal alkane ratio of 2.5 was reported. In addition to the catalyst acidity, hydrogenation strength also affects both qualities and the quantity of the aviation range of hydroprocessed products.The use of a noble metal catalyst along with a sulfided metal catalyst has been proposed in this work to enhance the hydrogenation activity of the catalyst for hydrogenation reaction. It is observed that high hydrogenation functionality reduces the aromatics and naphthenes present in the ATF range hydrocarbons to make the product better in quality (as per ASTMD-7566 specifications). In addition to limiting aromatics and naphthenes, high hydrogenating catalysts are also shown to have higher catalyst life due to reduced coke formation. With a desired Pt cluster size inside sodalite cage and optimized Pt@SOD to NiMo/SiO2-Al2O3 ratio, the desired product quality (reduced aromatics and naphthenes in aviation range hydrocarbons) could be achieved.Anhydrous sodium aluminate, NaAlO2 (Riedel-de-Haën, ≥99.95%); Sodium hydroxide, NaOH (Sigma Aldrich; ≥99.8%); Fumed silica, SiO2 (Sigma Aldrich 99.8%); Tetraammineplatinum(II) nitrate, [Pt (NH3)4](NO3)2 (Acros, 99%); tetraethyl orthosilicate, TEOS (Sigma Aldrich, 99.999 %), tetrabutylammonium hydroxide, Tetrabutylammonium hydroxide, TBAOH (Acros, 10% of TBAOH in water); Octadecyl dimethyl (3-(trimethoxy-silyl) propyl) ammonium chloride, (ODAC, 60 % diluted in methanol from Gelest Inc.), and deionized water were used during synthesis. Ammonium heptamolybdate and nickel nitrate precursors were obtained from Sigma Aldrich.Pt encapsulated sodalite was prepared by the hydrothermal crystallization method as described in the literature [6]. The gels' composition is as follows: 20 Na2O: 1.0 Al2O3: 1.5 SiO2: 160·H2O (mol/mol). NaAlO2 and NaOH were dissolved in DI water (deionized water) and mixed with fumed SiO2. Metal precursor, [Pt (NH3)4](NO3)2, was dissolved in 10 ml water, and the prepared solution dropwisewas added to the gel at the rate of 0.08 ml/min. The gelobtained was then transferredinto a polypropylene container (125 ml), sealed, and vigorously stirred homogenized, the final mixture for 10 min. The gelformed was stirredin at 400 rpm and 100 °C for 7 h. An oil bath maintained the temperature. The solid product was then collected using a fritted funnel, and the filtrate was washed with DI water repeatedly till the pH was 7–8. The sample was then dried in ambient air overnight (14 h) at 100 °C. It was then heated in air at 100 ml/min at 350 °C for 3 h and treated in 9% H2/He (100 ml/min) at 650 °C for 2 h.To prepare Pt@SOD-NiMo-SiO2-Al2O3 catalyst, 3.0 g of SiO2-Al2O3 supportwas well mixed with 1.0 g ofPt@SOD support (Pt = 1.0%)by reducing the size in a mortar pestle. 4% NiO, 18% MoO3/SiO2-Al2O3 was synthesized by the incipient wetness impregnation method described in the literature [25]. After the metal impregnation, the resultant mixture was molded into pellets. The pellets of size 1–2 mm size were used for reactor loading.N2 adsorption–desorptionisotherms were used to examine the physical properties like surface area, pore-volume, and a pore radius of the catalysts at −196 °C (BelsorbMax, BEL, Japan). The catalyst aciditywas measured by the ammoniaadsorption–desorptiontechnique using an instrument equipped with a thermal conductivity detector, model Micrometrics 2900 instrument (USA). 0.25 g of the sample was saturated with NH3 at 120 °C and flushed with helium at the rate of 100 ml/min to remove physically adsorbed NH3.The desorption of NH3 was carried outin the helium flow condition at the heating rate of 10 °C/min.XRD analysisto confirm the crystallinityof the sodalitematerialwasdone usinga Bruker D8 diffractometer (step size 0.002° and scanning rate 1°/min) using Cu Kα radiation (40 kV and 40 mA).A field emission scanning electron microscope(FEI Quanta 200F) was used to obtain the surface images of the catalyst. ETD detector and lanthanum hexaboride (LaB6) doped in tungsten filament was used as an X-ray source, under high vacuum condition. Secondary electrons of acceleration voltage in the range of 10–30 kVwere used. Samples were prepared by spreading them on glued carbon tape. Energy-dispersive X-ray (EDX) coupled with SEM was used for the surface elemental analysis. TEM imageswere takenby the TECNAI electron microscope operated at 75 kV. Sample preparation was done by dispersing it in ethanol, and it was then sonicated for 15 min. The dried sample was then placed on a carbon-coated copper grid.Inductively coupled plasma (ICP) emission was used to quantify the Pt loading on the catalyst. Metal dispersions andtemperature-programmed reduction (TPR) and were determined by hydrogen chemisorption using Micromeritics 2720 equipment. The catalyst was reduced by heating to 650 °C (at 2 °C min−1) in H2 (99.999%) and held foronehour andthen evacuated for 1 h at 650 °Cto remove any chemisorbed hydrogen. Pt dispersion and total hydrogen uptake were measured by manual injection. The catalyst was washed in naphtha and dried at 100 °C before the elementalanalysis.Agilent 7890A, Refinery gas analyzer (RGA), equipped with 2- thermal conductivity detector (TCD) detectors, 1-FID, and seven columns (Column 1 HayeSep Q 80/100 mesh, Column 2 HayeSep Q 80/100 mesh, Column 3 Molsieve 5A 60/80 mesh, Column 4 HayeSep Q 80/100 mesh, Column 5 Molsieve 5A 60/80 mesh, Column 6 DB-1, Column 7 HP-PLOT Al2O3) and PCM: Electronic pneumatics control (EPC) module was used to analyze the gases.Varian 3800-GC withVt-5 ms column (30 m * 0.25 mm, 0.25 µm) was used to analyze hydrocarbon products formed during the reaction. The vegetable conversion was observed to confirm constant activity. The GC oven temperature program was: 35–150 °C (rate of heating 3 °C/min; hold time: 5 min), 150–300 °C (rate of heating 12 °C/min; hold time: 5 min), and 300–320 °C (rate of heating 15 °C/min; hold time: 15 min). The experimentswere repeated,andstandard deviationswerealso plotted for each product component.The liquid hydrocarbon products selectivity was calculated on a relative percentage basis considering the entire range of hydrocarbon products formed as 100%. The liquid products analyzed on liquid GCwere reportedas relative percentages of lighter components (<C9), middle-range hydrocarbons (C9-C15), heavier range hydrocarbons (C16-C18), and oligomers (>C18). Complete conversion of jatropha oil hasbeen observedin all the experimental runs.Jatropha oil, which consists of main triglycerides, is used as the feedstock in all the experimental runs [8]. The vegetable oil is spiked with a 0.025% dimethyl-disulfide (DMDS) spiking agent to reduce leaching chances. The lipid composition of Jatropha curcas has been reported in the literature [38]. Approximately 90% of the jatropha oil consists of triglycerides. Other components are mono-glycerides, diglycerides, polar lipids, sterols, and sterol esters [38]. The scheme followed over the catalyst surface during hydroprocessing of vegetable oil (jatropha oil) is explained in Fig. 1 . Liquid feed was mixed with hydrogen and fed to the reactor. Temperature, pressure, H2/Liquid-feed, and liquid hourly space velocity are major controlling parameters for the product pattern and composition. The gaseous product obtained consists of unreacted hydrogen along with H2S, CO, CO2. Condensable gases include water vapor, propane, and other hydrocarbons (<C6). The liquid products obtained could be grouped as lighter components (<C9), Mid-range (C9-C15 or kerosene range), Heavier range (C16-C18, Diesel range), and Oligomers (>C18 hydrocarbons).The prepared catalystswere loadedinside stainless-steel tubular fixed bed reactors (1.3 cm ID and 30 cm in length). Pt@SOD-NiMo--SiO2-Al2O3(4 g and 3 g) were loaded inside the reactor. Jatrophaoil was usedas feed. Ceramic beads were loaded at the top part and bottom part of the reactor. The hydrogen pressure inside the reactorwas controlledby a back pressure regulator (TESCOM). The inlet gas flowrate was maintainedby a mass flow controller (Brooks). The catalyst bedtemperatures were measuredby K-type thermocouples connected with a microprocessor-based temperature controller system.A high-pressure liquid pump (HPLC pump, Eldex made) was used to maintain the reactor's desired liquid flow rates. Reaction conditionswere variedover a wide range of temperaturesbetween 350 and 450 °C, the pressurebetween 60 and 100 bar, H2/feed ratio between 1500 and 3000 NL/L. Liquid hourly space velocity (LHSV) of 0.5–3 h−1 was maintained during the runs. All the necessary measuring componentswere calibratedbefore theexperiments.The reactor outlet stream was sent to the gas–liquid separator. The gas–liquid separator has a much higher surge volume than the inlet lines; hence due to sudden expansion, the gaseous fractions (containing hydrogen and lighter hydrocarbons with small quantities of H2S, CO, CO2) were separated from the liquid product. The gaseous products were sent to an alkali solution and then vented through a gas meter. Gas was collected at the outlet of the gasometer for analysis using gas chromatography (Refinery Gas Analyzer, Agilent India Pvt). The liquid product was obtained by draining it from thehigh-pressureseparator, and it was then analyzed using gas chromatography (Varian 3800-GC with Vt-5 ms column).Experiments were carried out to compare the activity of Pt encapsulated NiMo/SiO2-Al2O3 catalyst with NiMo/SiO2-Al2O3 catalyst. The reaction scheme followed is shown in Fig. 1. Hydrogen is taken in excess in all the experiments to increase hydrogen solubility in liquid Jatropha feed. Pt encapsulated sodalite was prepared by hydrothermal crystallization method as reported in the experimental section. The preparation of Pt@SOD has been discussed in detail by Sibi et al. [4]. Pt@SOD, prepared, was characterized for its physical characteristics. After the incorporation of Pt inside the sodalite cage, the powder obtained was intimately mixed with SiO2-Al2O3, and then NiO and MoO3 were impregnated on the support. The final catalyst was then sulfided and reduced for reactivity test using Jatropha as a liquid feed. Product characterization includes product distribution (lighter, middle distillates, heavier hydrocarbons, and oligomeric compounds. Hydrogen activity was differentiated based on the product's componential analysis (PNA) obtained on these two different catalysts.Periodic, all-electron DFT calculations were performed using the double numerical plus polarization function (DNP) basis set of DMol3 module of Material Studio 8 (Biovia, San Diego) [39]. Revised Perdew Burke Erzenhof (RPBE) [40], generalized gradient approximation (GGA) exchange–correlation functional was used for all the calculations. Convergence criteria for structure geometry optimization were set to 0.05 eV/Å, 0.005 Å, and 0.0001 eV, respectively, with respect to atom displacement, force, and energy. The smearing value of 0.005 au was used to improve the SCF convergenceThe TS search method available in DMol3 was used to find the transition state (TS) for each reaction step, the linear synchronous transit/quadratic synchronous transit (LST/QST) method was used to calculate the TS search. Obtained geometries TS search method further iterated using the “TS optimization” using the DMol3 module to complete refined TS structures. The vibrational frequencies of the transition state (TS) were analyzed. TS confirmed all the vibrational frequencies to be real, except one, in the reaction coordinate direction. The spin-polarized setup is used for both geometry optimization and TS calculations. The adsorption energy of the intermediate species (Eads) was calculated from the following equations, E ads = E mol + P t - S O D - E Pt - S O D - E mol Emol+Pt-SOD, EPt-SOD, and Emol are the energies of the molecule adsorbed at Pt3 encapsulated inside SOD, Pt3 encapsulated inside SOD, and molecule in the gas phase respectively.The activation energies (Ea) for spillover of hydrogen was calculated by the difference in the energy of the transition (ETS) and initial states (EIS) E a = E TS - E IS The initial structures of the SOD zeolite, having the chemical formula Na6Al6Si6O24, were adapted from the material studio database (shown in Fig. 2 (a)). Pt3 triangular cluster is encapsulated inside the SOD cage, as shown in Fig. 2(b). Considering the reported maximum size of the encapsulated Pt nanoparticles as 5 Å by Choi et al. [8], the Pt3 cluster was chosen, having a diameter of 4.2 Å, for this study. The Pt3 triangular cluster is encapsulated inside the SOD cage, as shown in Fig. 2(b). The geometry optimized SOD encapsulated Pt3 (Pt3@SOD) is used to study the chemisorption of H2 and the H spillover from Pt3 cluster to the SOD zeolite.The prepared catalyst was characterized by SEM, TEM, XRD for physicochemical characteristics. Support crystallinity is an essential aspect during the encapsulation of noble metal. The crystalline nature of the material (sodalite) was confirmed by XRD, SEM, and TEM analysis, as shown in Figs. 3, 4 . XRD analysis of Pt@SOD is shown in Fig. 3. Peaks at 2θ values of 140, 240, 320, and 370 correspond to sodalite's crystal structure (JCPDS 11-0401). SEM-EDX of the sodalite surface confirms the absence of Pt on the surface (Fig. S1); however, the Pt particles in the sodalite were confirmed by ICP analysis (Table S1).Since the noble metal is being used along with the Ni and Mo oxide catalyst and the sulfidation, reducing the Pt is crucial before the catalyst hydrogenation activity runs. TPR was performed at different temperatures to finalize the reduction temperature. Fig. S1 shows the TPR of Pt@SOD; the prominent peak observed was in the range of 450–650 °C. This peak may be attributed to the reduced state of Pt(0) from Pt (II) [4]. Peaks observed beyond 800 °C, may be due to the collapse of sodalite, causing the inner Pt to get exposed and reduced at this higher temperature. The SEM image of Pt@SOD reduced at 900 °C (Fig. 5 ) confirms the collapsed sodalite structure at a higher temperature (900 °C). The thermal stability of the SOD cage was also studied. The reduced catalyst retains its crystallinity was confirmed by XRD analysis for Pt@SOD reduced at temperatures 350, 450, 550, and 650 °C. TPR shows that the catalyst crystalline nature is retained until 650 °C of Pt@SOD, as shown in Fig. S2.Study the effect of the temperature on product yield and quality; the reactor temperature was varied from 380 to 440 °C. Other parameters were kept constant for the catalyst, liquid hourly space velocity 1 h−1, H2/feed ratio as 2200 (vol/vol), and pressure at 100 bar. Since hydrodeoxygenation of triglycerides to hydrocarbon is spontaneous and highly exothermic, catalyst activity was compared in terms of lighter, midrange, and heavier hydrocarbons. The complete conversion has been observed for both the catalyst in the given range of temperature variation. The effect of temperatures on the yield% for lighter components (<C9) in Fig. 6 (a), mid-range (C9-C15 or kerosene range) in Fig. 6(b), heavier range (C16-C18, diesel range) in Fig. 6(c), and oligomers (>C18) in Fig. 6(d) over NiMo/SiO2-Al2O3 and Pt@SOD-NiMo-SiO2-Al2O3 catalyst has been shown. Cracking was observed to be 6–17 times higher for NiMo/SiO2-Al2O3 than that of Pt@SOD-NiMo-SiO2-Al2O3 catalyst. Light hydrocarbon (<C9) increased rapidly from 5 to 50% with an increase in temperature, from 380 to 440 °C on NiMo/SiO2-Al2O3 catalyst, while the rise in same range hydrocarbons was only from 1 to 6% for similar temperature range on Pt@SOD-NiMo-SiO2-Al2O3 catalyst. The higher increase in lighter hydrocarbons over NiMo/SiO2-Al2O3 catalyst was due to the catalyst's high acidity and increased acidic activity. The reason for the lower acidity of Pt encapsulated NiMo/SiO2-Al2O3 catalyst is that for the same weight of catalyst bed, Pt@SOD-NiMo-SiO2-Al2O3 has 33% lower SiO2-Al2O3 group present per grams of a catalyst than NiMo/SiO2-Al2O3 catalyst. Fig. 6(b) shows that mid-range hydrocarbons (C9-C15) increased slowly for both the catalysts; a maximum was observed around 420 °C over NiMo/SiO2-Al2O3. Conventional NiMo/SiO2-Al2O3 catalyst showed a 2–3-time higher yield when compared to Pt@SOD-NiMo-SiO2-Al2O3 at different temperatures. Lower yield over Pt@SOD-NiMo-SiO2-Al2O3 may be attributed to lower acidity per gram of total catalyst in the case of Pt encapsulated catalyst.In Fig. 6(c), the diesel range hydrocarbons (C16-C18) yield was observed to be higher for Pt@SOD-NiMo-SiO2-Al2O3 catalyst compared to NiMo/SiO2-Al2O3 in the temperature range of 380–400 °C. Variation of diesel yield with temperature was much significant over NiMo/SiO2-Al2O3 due to a higher acidic function, which shows higher cracking ability at higher temperatures. Diesel yield obtained over Pt@SOD-NiMo-SiO2-Al2O3 was 1.4–5 times higher than that of NiMo/SiO2-Al2O3 catalyst.A maximum yield (11%) of oligomeric hydrocarbons (>C18) (Fig. 6 (b)) was observed over NiMo/SiO2-Al2O3 catalyst at 420 °C; it decreased to <4% at a higher temperature 440 °C on NiMo-SiO2-Al2O3 catalyst, while over Pt@SOD-NiMo-SiO2-Al2O3 catalyst maximum yield of oligomeric hydrocarbons (>C18) was observed at higher temperatures 420–440 °C. The lower yield of oligomers (6–7%), were observed over a new catalyst (Pt@SOD-NiMo-SiO2-Al2O3), compared to conventional NiMo/SiO2-Al2O3 catalyst. Lower yield may be attributed to higher hydrogenation activity in the presence of the Pt, which provides active hydrogenation for the reaction [7]. In the case of Pt encapsulated catalyst, high hydrogenation function would remove most of the unsaturation and hence the precursors for oligomer formation.From the studies on sulfided metal oxide catalysts during the hydroconversion of triglycerides (jatropha oil) [8–13], it has been reported that the isomerization activity of hydrocracking catalysts increases with an increase in the acidity of the catalyst [6]. In the case of Pt@SOD-NiMo-SiO2-Al2O3, which has lower acidity, it is expected that the presence of strong hydrogenation sites would hydrogenate the triglyceride chains [9], which eventually decreases the overall isomerization selectivity. In the temperature range of 380–440 °C, the isomerization activity was 2–30 times higher for NiMo/SiO2-Al2O3 than that of Pt@SOD-NiMo-SiO2-Al2O3 catalyst (Fig. 7 ). Maximum isomerization was observed at the lowest temperature for NiMo Pt@SOD-/SiO2-Al2O3 catalyst while it was highest at maximum temperature (440 °C) for NiMo/SiO2-Al2O3 catalyst. An optimum acidity is required for the required isomerization activity and better catalyst life [11]. Since isomers have better freezing characteristics than their linear counterpart; the catalyst acidity improves the isomerization of the hydroprocessed product, which improves the freezing point of mid-range hydrocarbons. Though literature shows that acidity also increases coke formation during hydrocracking reactions, optimum catalyst acidity is essential to check cracking ability and coking reactions [39]. Increased hydrogenation function limits naphthenes and aromatics, as evident in the subsequent results. Fig. 8 shows the influence of temperature on naphthenes and aromatics distribution along with H2 consumption for C9-C15 range hydrocarbons on NiMo/SiO2-Al2O3 catalyst. Aromatics concentration varied between 0.2 and 3% in the temperature range of 380–440 °C. The minimum aromatics (0%) was observed at 400 °C for NiMo/SiO2-Al2O3 catalyst. Paraffins (Fig. S3), varying from 94 to 78% from 380 to 440 °C. Similar to the aromatics trend, a minimum was observed for naphthenes at 400 °C. Naphthene content increases more rapidly (3–19%) than aromatics in the range of 400–440 °C. Poly-aromatics was less than <1% in the entire temperature range on the NiMo/SiO2-Al2O3 catalyst. Polyaromatics observed was much lower for Pt@SOD-NiMo-SiO2-Al2O3 catalyst due to high hydrogenating functionality present due to the noble metal-based catalyst (Fig. S4). Fig. 9 shows the influence of temperature on naphthenes and aromatics distribution for C9-C15 range hydrocarbons along with H2 consumption on Pt@SOD-NiMo-SiO2-Al2O3 catalyst. Mono-aromatics was <0.6% in the entire range of temperature. Low aromatics were expected, as noble metals are more active for hydrogenation than sulfided Ni and Mo. In the mid-range product, 5–15 times reduction in aromatics and 3–15% reduction in naphthenes in the entire operating range of temperature was observed for Pt@SOD-NiMo-SiO2-Al2O3 catalyst. Since noble metal catalysts are more hydrogenating, as expected, there were 1–2 times excess hydrogen consumption compared to conventional NiMo-SiO2-Al2O3 catalysts. Noble metal catalysts have a high hydrogenating function. Hence they adsorb more hydrogen compared to sulfide catalysts. In the case of the encapsulated Pt@SOD-NiMo-SiO2-Al2O3 catalyst, the adsorbed hydrogen is expected to be transferred to nearby sites via a spillover mechanism [7]. TPR data shown in Table 1 shows that the hydrogen adsorbed on Pt@SOD-NiMo-SiO2-Al2O3 catalyst is more significant than combined chemisorbed hydrogen on Pt@SOD and NiMo-SiO2-Al2O3 catalyst. The only possible excess hydrogen adsorbed could be explained by the spillover phenomenon. Maximum naphthene yield was observed at 440 °C for NiMo/SiO2-Al2O3 catalyst. The product's higher paraffinic yield was observed (Fig. S3) when the triglyceride was hydro-processed over NiMo-SiO2-Al2O3 catalyst than Pt encapsulated catalyst. In both the cases' aromatics and naphthene content, a minimum was observed at 400 °C temperature due to the kinetic and thermodynamic limitation of hydrogenation function at lower and higher temperatures [40]. In the case of Pt@SOD-NiMo-SiO2-Al2O3 catalyst, the naphthene and aromatic content were within the range of ASTM limits for aviation hydrocarbons obtained by hydroconversion of vegetable oil (ASTM D7566) [11].Juhwan et al. [7], studies hydrogen spillover mechanism on the metal oxides, nature of spillover species, migration mechanism, and theoretical catalytic functions. They studied that surface hydroxyl groups, especially Brønsted acid sites, play a crucial role in hydrogen migration via spillover from active metal to nearby support. The role of hydroxyl groups in hydrogen spillover was confirmed experimentally by changing the hydroxyl concentration in the encapsulated zeolites; this resulted in a considerable change in the hydrogenation activity. The activation energy for each step of migration of H2 via spillover has also been calculated by DFT [7]. The H-spillover's overall activation energy on the zeolite surface was reported to be <125 kJ/mol by Choi et al. [8].Pt promotional effect in hydrogenation reactions, over Pt@SOD-NiMo-SiO2-Al2O3 catalyst, the dissociative chemisorption of H2 molecule over Pt3 cluster encapsulated inside the SOD cage, and the spillover of H from the Pt cluster to the SOD zeolite Si-O-Al site are studied. The energies of the physisorbed molecular H2, dissociated chemisorbed H2, Transition state (TS) for H spillover, and the final geometry after the H spillover are given in Table 2 . Molecular H2 is physisorbed at the Pt3@SOD through bonding with two Pt atoms, as shown in Fig. 2(a). The Pt-H bond distances, Pt(1)-H(1) and Pt(1)-H(2), are measured to be ∼1.85 Å. The internal H-H bond distance is measured as 0.74 Å, indicating molecular adsorption of H2. The molecular H2 is then dissociated over the Pt3 cluster, where the H atoms are adsorbed to different Pt atoms, as shown in Fig. 10 (b). The internal H-H bonds are entirely broken, as observed from the H(1)-H(2) bond distance of 1.62 Å. The Pt(1)-H(1), Pt(3)-H(1), Pt(2)-H(2), and Pt(3)-H(2) bond lengths are calculated to be 1.68 Å, 1.78 Å, 1.66 Å and 1.82 Å, respectively. The dissociative chemisorption of H2 is an exothermic process with reaction energy −69 kJ/mol. The H2 dissociative chemisorption reaction is highly exothermic in nature has been experimentally seen in the high H2 chemisorption value of the Pt incorporated catalyst in Pt/NaA and Pt/SiO2 catalysts [7].For the catalyst to be able to hydrogenate the bulky naphtha molecule, the spillover of chemisorbed H atoms from the Pt cluster to the zeolite and from zeolite to the NiMo catalyst is indeed essential. Juhwan et al. [7] have extensively studied the H spillover over the zeolite-A and found the process to be fast under the reaction condition with an overall activation energy ∼95 kJ/mol in the presence of surface –OH group. However, the spillover from the Pt cluster to the zeolite Si-O-Al backbone has not been studied. As the H is strongly chemisorbed at the Pt cluster, the spillover of H from Pt to the SOD backbone is an important step and was analyzed here using the DFT method, as shown in Fig. 10(b, b' and c). The transition state obtained for the H spillover reaction is shown in Fig. 10b', whereas the final state after the spillover with one H is transferred to the zeolite Si-O-Al site is shown in Fig. 10c, forming a –OH group at the sodalite cage. Both the Pt(2)-H(2) and Pt(3)-H(3) bond distance increases from 1.66 Å and 1.82 Å in the initial state (dissociated chemisorbed H2, Fig. 10(b)) to 2.11 Å and 2.78 Å in the transition state (Fig. 10(b')). At the same time, the O(z)-H(2) bond distance decreases from 3.91 Å in the initial state (Fig. 10(b)) to 1.35 Å in the transition state (Fig. 10(b')), indicating the transfer of H atom from the Pt3 cluster to the O atom of the SOD framework. The H is completely transferred to the zeolite O atom in the final state, as shown in Fig. 10(c), with O(z)-H(2) bond length is measured as 1.06 Å. The complete table for the important bond lengths is given in the SI, Table S2. The activation barrier for H spillover from the Pt3 cluster to the SOD Si-O-Al site is calculated to be 111.5 kJ/mol, whereas the reaction energy for the H spillover process is endothermic by 95.8 kJ/mol. DFT results obtained in this study indicate that the activation barrier for H spillover from the Pt3 cluster to the zeolite is lower compared to the H- spillover over the zeolite surface (<125 kJ/mol) as reported by Choi et al. [8]. This suggests that for the Pt@SOD-NiMo(S)-SiO2-Al2O3 catalyst, the H2 molecules will preferably dissociate over the metallic Pt3 cluster and then spillover first to the zeolite Si-O-Al site and then to the outer surface NiMo(S) active site, where it will hydrogenate the hydrocarbon fragments.In Fig. 11 , the Pt@SOD-NiMo-SiO2-Al2O3 catalyst showed stable activity for >900 h. C9-C18 yield remained >85% throughout the run. The catalysts were active and showed stable activity. The yield of aviation range hydrocarbon was 2–4 times in the entire run for conventional NiMo/SiO2-Al2O3 catalyst compared to Pt@SOD-NiMo-SiO2-Al2O3 catalyst. Even though NiMo/SiO2-Al2O3 showed better activity for aviation range hydrocarbons, but due to the presence of higher aromatics and naphthenes in this range of hydrocarbon, there is a need for a secondary step to reduce the contents to the recommended level (ASTM D7566 sets the limit for aromatics <0.5% and naphthene as <15% in sustainable aviation fuel obtained from non-petroleum sources originating from hydroprocessing of esters and fatty acids). Pt@SOD-NiMo-SiO2-Al2O3 catalyst showed better product quality (<0.6% aromatics, <5% naphthenes) but lower product yield and isomerization activity. It is expected with further optimization of catalyst, specially Pt@SOD percentage in final catalyst and acidity, a better yield with desired product quality can be obtained for hydroconversion of triglycerides.The state-of-the-art reported catalyst Pt@SOD-NiMo(S)-ZSM-5 had limitations in terms of catalyst life (Table 3 ; stable activity only for 350 h)4. Faster deactivation is correlated to Oligomeric (>C18) products, which tend to adhere to catalyst surface and contribute to rapid deactivation. The undesirable coke precursors (>C18 oligomers) are nearly 3 times more on Pt@SOD-NiMo(S)-ZSM-5 than the current catalyst. A much lower deactivation rate as evidenced by much longer stable activity (660 h) makes this catalyst superior to all the catalysts reported to date for this reaction. Higher acidity of ZSM-5 (0.95 mmol−1) and mixed micro-mesoporosity (pore sizes: 0.6 nm, 13 nm) compared to the current catalyst supported on mesoporous SiO2-Al2O3 (acidity: 0.77 mmol g−1; pore size: 8.6 nm), makes the former more susceptible to deactivation during hydrocracking reaction.Sustainable aviation fuel (mid-range product) with reduced aromatics and naphthene was achieved in the temperature range of 380–440 °C with the encapsulated noble metal-based catalyst combined with NiMo/SiO2-Al2O3 hydroprocessing catalyst. Pt@SOD reduced the aromatics by 5–15 times and naphthenes by 3–15 times when compared with conventional NiMo/SiO2-Al2O3 catalyst. However, the isomerization activity was observed to be much lower with the Pt@SOD-NiMo-SiO2-Al2O3 catalyst. Although better product quality was obtained with Pt@SOD-NiMo-SiO2-Al2O3, the mid-range hydrocarbon yield was almost half of the yield obtained on NiMo/SiO2-Al2O3 catalyst. With the optimized, Pt@SOD loading and acidity, a trade-off between naphthenes and aromatics percentage in the aviation range vs. aviation range hydrocarbon yield and isomerization activity may be achieved. Theoretical DFT studies suggest that encapsulated Pt inside sodalite cage would provide extra spillover hydrogen to nearby sites for jatropha conversion. Noble metal along with sulfided catalyst was used in a high sulfur condition (>1000 PPM sulfur), and the activity was stable (>500 h) for the hydroconversion reaction using jatropha oil as liquid feed along with hydrogen as gaseous feed. An optimized ratio of encapsulated noble metal and acidity would improve the product quality without affecting the yield.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 Council of Scientific & Industrial Research-Indian Institute of Petroleum Dehradun for research funding. The author also acknowledges staff members of the biofuel division and analytical division for help and support.

Encapsulation of noble metals inside the zeolite cage has attracted much attention in the area of catalysis due to unique properties, i.e., shape selectivity, higher activity, and product yield. In the combined work, encapsulated Platinum (Pt) inside the sodalite cage combined with sulfided nickel and molybdenum supported on silica-alumina (NiMo(S)/SiO2-Al2O3) is used to improve the catalyst hydrogenation function by providing spillover hydrogen to the nearby active sulfided NiMo sites. Encapsulation ensures the protection of the noble metal sites from poisoning due to the sulfur-containing compounds. They have a higher hydrodynamic diameter than that of the sodalite window cage. Experimental results showed platinum encapsulated sulfided nickel and molybdenum supported on silica-alumina (Pt@SOD-NiMo(S)-SiO2-Al2O3) had a substantial effect on the product composition, yield, and hydrogen consumption compared with the conventional NiMo(S)/SiO2-Al2O3 catalyst for the hydroconversion reaction of jatropha oil to produce carbon–neutral green fuels. Sustainable aviation fuel with lower aromatics and naphthene content (<1%) was produced over the bi-functional Pt encapsulated Ni-Mo(S)/SiO2-Al2O3 catalysts in the reaction temperature range of 380–420 °C. Aromatics and naphthalene obtained in the aviation range product were as much as 15 times lower than the conventional NiMo(S)/SiO2-Al2O3 catalyst. The Pt@SOD-NiMo(S)-SiO2-Al2O3 catalyst showed stable activity for >600 h of the run, whereas state-of-the-art reported catalyst Pt@SOD-NiMo(S)-ZSM-5 had limitations in terms of catalyst life (stable activity only for 350 h).