which has been performed using this technique deals with the study of surface restructuration in contact with different reacting molecules. For example, the impact of CO on Pt particles and the induced formation of metal clusters was well illustrated by variable pressure STM experiments [207]. Also exposing model industrial catalyst stepped surface nano-particles of Pt to carbon monoxide showed that a reversible restructuration of metal atoms into terraces depending on gas pressure occurred [208]. Similar results were found on copper (a catalytic active species for methanol synthesis) too, upon exposition to hydrogen [209]. Dramatic changes on Ni surfaces (relevant for hydrocarbon reforming) were similarly produced by CO contact and revealed by STM images [210]. But the most interesting results were provided by *operando* studies of CO oxidation on platinum: HP-STEM images proved that a PtOₓ surface is more active than a metallic one, due to the fact that highly active oxygen atoms on the superficial lattice quickly react with CO to form CO₂ through a Mars-Van Krevelen process (Fig. 13) [211].

In the domain of solid-state electrochemistry, major progresses have been obtained applying time-resolved *operando* electron energy-loss spectroscopy and scanning transmission electron microscopy to visualise the lithium-ion dynamics at the nanoscale in cathodes, anodes, solid electrolytes, and through their interfaces inside batteries submitted to charge/discharge cycles. Temporal and spatial resolution have been greatly improved applying image denoising and super-resolution via sparse coding. In particular, it has been observed that the lithium ions in the lithium cobaltite cathode are complicatedly extracted with diffusion through the lithium cobaltite domain boundaries during charging and that they move inside the cathode even in the open-circuit state [212].

## 11. Catalysts ageing, fouling and deactivation

Several studies investigate the loss of catalytic performance. In the investigation of aldehyde components in bio-oils, the gas-phase aldol condensation of propanal is a representative model. Hernandez-Gimenez et al. used a combined *operando* FT-IR & UV-Vis diffuse reflectance spectroscopy with on-line mass system on a cesium-exchanged X zeolite (Cs-X), a calcium hydroxyapatite (Ca-HA) and two alkaline metal-grafted ultrastable Y (Na- and Rb-USY) zeolites. The combined *operando* spectroscopy study showed the catalytic efficiency of the investigated materials (alkaline metal-grafted USY zeolites are the most selective catalysts towards aldol dimer product formation, while the hydroxyapatite, for successive aldol condensation). They show that the C-C coupling seems to be the rate-determining step during the surface reaction for Na-USY and Rb-USY, which is the limiting stage of the overall catalytic process. Conversely, the desorption appears to be the limiting step on the two more basic Cs-X and Ca-HA catalysts. The most relevant information is that they monitored the catalyst deactivation, due to the formation of carbon deposits. Cs-X and Ca-HA exhibit a rapid formation of carbonaceous deposits; (poly-)aromatics and highly conjugated structures, respectively. On the other hand, the strong basic sites and moderate acidity in Ca-HA limit its deactivation despite some coke is apparent. On the other hand, both USY catalysts were more efficient in suppressing coke formation likely due to the moderate strength of their active sites [213].

An inverse CeO₂/CuO catalyst was examined by XPS and DRIFTS spectroscopies under reactant mixtures relevant to the low temperature WGS process. It was shown that interfacial carbonate species poison the redox process, at the base of the catalyst activity. The presence of water promotes the decomposition of these species, leading to the generation of intermediates catalytically active in WGS [214].

There is significant understanding on catalyst deactivation using spectroscopic methods; e.g., Rojo-Gama et al. connect unit cell changes with zeolite deactivation in methanol to hydrocarbons conversion [215]. Periodic density functional theory could show that such change is induced by methyl substituted benzenes in the channel intersection in ZSM-5 pore structure. In a very practical work, they established a direct correlation between the difference in the length of the a- and b-unit cell

parameters and the total amount of coke, the remaining acidity, and the remaining surface area of the catalysts. They report an "a- minus b-" parameter that is a straightforward descriptor with the essential information regarding the degree of deactivation of ZSM-5 catalysts. This elegant approach can be done with routine diffractogram analyses. Implementing real-time monitoring on the states of catalyst and carbon deposits is, thus, of paramount importance. Optical spectroscopy methods is most convenient for real-time monitoring. Beato et al. report an *operando* Raman study of coke evolution during MTH in an *operando* fluidized bed reactor [216]. They report several systems; in the case of coke build-up in ZSM-5 and ZSM-22 zeolites, the use of UV laser prevents fluorescence limitations, while the use of a fluidized bed minimizes the effect of UV laser damage on the sample, in line with previous work by Stair's group [217]. Beato et al. showed that the Raman features of physisorbed methanol remain apparent while the bed is fluidized and no apparent damage is induced by the high-energy photons. Thus, they can analyze different evolution of surface carbon species during methanol reaction on either zeolite. This approach permits high-quality analysis of sp²-carbon species. Molecular insight will be more complete with complementary spectroscopies: Raman and infrared provide complementary information of carbon species, being the former more sensitive to polyolefinic structures, while the latter to oxygen-containing carbon deposits. A key to quantify carbon deposits may stand on UV-Vis absorption spectroscopy. Along this line, Bennici et al. reported a connection between coke (from propane dehydrogenation) build up molecular structure determined by Raman spectroscopy and amount, based on UV-Vis absorption [34]. This correlation paved the way for Sattler et al. to report a UV-Vis *operando* study of coke formation and burning on an industrial CrOx/Al₂O₃ (3 mm-sized particles) catalyst during propane dehydrogenation in a pilot-scale reactor (Fig. 14) [218]. They report the use of ad hoc made UV-Vis probes which are inserted in the pilot plant. This work uses UV-Vis absorption spectroscopy to monitor coke formation and combustion on shaped catalyst extrudates; the use of several probes along the catalyst bed enables mapping coke deposition and combustion along the bed.

## 12. Mapping catalysts and fluid phase in microreactors

Microstructured reactors possess a three-dimensional structure with inner channels of 10–100 µm; their main feature, in comparison to macroreactors, is the high surface-to-volume ratio [219]. Microreaction technology allow wider operating window condition and enable tighter process control due to their enhanced rate of heat and mass transfer [220]; this minimizes hot spots and minimize explosive regimes [221, 222], inhibiting lightoff and hysteresis effects. Microreactor design also allows for higher selectivity [223], as a whole conversion and selectivity are easily tuned with a broader range to control pressure and temperature [224]. Despite their reduced size, microreactors may exhibit chemical gradients in the catalyst and fluid phases [22,225]. Furthermore, microstructured channel reactors allow easy and modular integration of other functional units (e.g. mixing, separation) and also the incorporation of planar waveguides and optical fibres [219].

Most *operando* approaches focus on gas-solid reactions (Fig. 15), but there is also a major need to look at working catalysts in the liquid phase, inorganic and enzymatic. Catalytic science may take advantage from the high spatiotemporal resolution and sensitivity of fluorescence microscopy applied to the liquid phase [226].

Cao et al. report a Raman-GC *operando* study of methanol oxidation over silver in a silicon-glass microreactor, illustrating surface reconstruction of silver phase and monitoring the evolution of subsurface atomic oxygen, which appears directly related with formaldehyde formation [228]. Gänzler et al. monitored the oscillatory CO oxidation on Pt/Al₂O₃ combining IR thermography images with XANES/EXAFS profiles [176], illustrating the higher activity of the reduced platinum particles and how Pt oxidation state engages with the oscillatory CO conversion pattern. For methane partial oxidation in a capillary bed,
which has been performed using this technique deals with the study of surface restructuration in contact with different reacting molecules. For example, the impact of CO on Pt particles and the induced formation of metal clusters was well illustrated by variable pressure STM experiments [207]. Also exposing model industrial catalyst stepped surface nano-particles of Pt to carbon monoxide showed that a reversible restructuration of metal atoms into terraces depending on gas pressure occurred [208]. Similar results were found on copper (a catalytic active species for methanol synthesis) too, upon exposition to hydrogen [209]. Dramatic changes on Ni surfaces (relevant for hydrocarbon reforming) were similarly produced by CO contact and revealed by STM images [210]. But the most interesting results were provided by *operando* studies of CO oxidation on platinum: HP-STEM images proved that a PtOₓ surface is more active than a metallic one, due to the fact that highly active oxygen atoms on the superficial lattice quickly react with CO to form CO₂ through a Mars-Van Krevelen process (Fig. 13) [211].

In the domain of solid-state electrochemistry, major progresses have been obtained applying time-resolved *operando* electron energy-loss spectroscopy and scanning transmission electron microscopy to visualise the lithium-ion dynamics at the nanoscale in cathodes, anodes, solid electrolytes, and through their interfaces inside batteries submitted to charge/discharge cycles. Temporal and spatial resolution have been greatly improved applying image denoising and super-resolution via sparse coding. In particular, it has been observed that the lithium ions in the lithium cobaltite cathode are complicatedly extracted with diffusion through the lithium cobaltite domain boundaries during charging and that they move inside the cathode even in the open-circuit state [212].

## 11. Catalysts ageing, fouling and deactivation

Several studies investigate the loss of catalytic performance. In the investigation of aldehyde components in bio-oils, the gas-phase aldol condensation of propanal is a representative model. Hernandez-Gimenez et al. used a combined *operando* FT-IR & UV-Vis diffuse reflectance spectroscopy with on-line mass system on a cesium-exchanged X zeolite (Cs-X), a calcium hydroxyapatite (Ca-HA) and two alkaline metal-grafted ultrastable Y (Na- and Rb-USY) zeolites. The combined *operando* spectroscopy study showed the catalytic efficiency of the investigated materials (alkaline metal-grafted USY zeolites are the most selective catalysts towards aldol dimer product formation, while the hydroxyapatite, for successive aldol condensation). They show that the C-C coupling seems to be the rate-determining step during the surface reaction for Na-USY and Rb-USY, which is the limiting stage of the overall catalytic process. Conversely, the desorption appears to be the limiting step on the two more basic Cs-X and Ca-HA catalysts. The most relevant information is that they monitored the catalyst deactivation, due to the formation of carbon deposits. Cs-X and Ca-HA exhibit a rapid formation of carbonaceous deposits; (poly-)aromatics and highly conjugated structures, respectively. On the other hand, the strong basic sites and moderate acidity in Ca-HA limit its deactivation despite some coke is apparent. On the other hand, both USY catalysts were more efficient in suppressing coke formation likely due to the moderate strength of their active sites [213].

An inverse CeO₂/CuO catalyst was examined by XPS and DRIFTS spectroscopies under reactant mixtures relevant to the low temperature WGS process. It was shown that interfacial carbonate species poison the redox process, at the base of the catalyst activity. The presence of water promotes the decomposition of these species, leading to the generation of intermediates catalytically active in WGS [214].

There is significant understanding on catalyst deactivation using spectroscopic methods; e.g., Rojo-Gama et al. connect unit cell changes with zeolite deactivation in methanol to hydrocarbons conversion [215]. Periodic density functional theory could show that such change is induced by methyl substituted benzenes in the channel intersection in ZSM-5 pore structure. In a very practical work, they established a direct correlation between the difference in the length of the a- and b-unit cell

parameters and the total amount of coke, the remaining acidity, and the remaining surface area of the catalysts. They report an "a- minus b-" parameter that is a straightforward descriptor with the essential information regarding the degree of deactivation of ZSM-5 catalysts. This elegant approach can be done with routine diffractogram analyses. Implementing real-time monitoring on the states of catalyst and carbon deposits is, thus, of paramount importance. Optical spectroscopy methods is most convenient for real-time monitoring. Beato et al. report an *operando* Raman study of coke evolution during MTH in an *operando* fluidized bed reactor [216]. They report several systems; in the case of coke build-up in ZSM-5 and ZSM-22 zeolites, the use of UV laser prevents fluorescence limitations, while the use of a fluidized bed minimizes the effect of UV laser damage on the sample, in line with previous work by Stair's group [217]. Beato et al. showed that the Raman features of physisorbed methanol remain apparent while the bed is fluidized and no apparent damage is induced by the high-energy photons. Thus, they can analyze different evolution of surface carbon species during methanol reaction on either zeolite. This approach permits high-quality analysis of sp²-carbon species. Molecular insight will be more complete with complementary spectroscopies: Raman and infrared provide complementary information of carbon species, being the former more sensitive to polyolefinic structures, while the latter to oxygen-containing carbon deposits. A key to quantify carbon deposits may stand on UV-Vis absorption spectroscopy. Along this line, Bennici et al. reported a connection between coke (from propane dehydrogenation) build up molecular structure determined by Raman spectroscopy and amount, based on UV-Vis absorption [34]. This correlation paved the way for Sattler et al. to report a UV-Vis *operando* study of coke formation and burning on an industrial CrOx/Al₂O₃ (3 mm-sized particles) catalyst during propane dehydrogenation in a pilot-scale reactor (Fig. 14) [218]. They report the use of ad hoc made UV-Vis probes which are inserted in the pilot plant. This work uses UV-Vis absorption spectroscopy to monitor coke formation and combustion on shaped catalyst extrudates; the use of several probes along the catalyst bed enables mapping coke deposition and combustion along the bed.

## 12. Mapping catalysts and fluid phase in microreactors

Microstructured reactors possess a three-dimensional structure with inner channels of 10–100 µm; their main feature, in comparison to macroreactors, is the high surface-to-volume ratio [219]. Microreaction technology allow wider operating window condition and enable tighter process control due to their enhanced rate of heat and mass transfer [220]; this minimizes hot spots and minimize explosive regimes [221, 222], inhibiting lightoff and hysteresis effects. Microreactor design also allows for higher selectivity [223], as a whole conversion and selectivity are easily tuned with a broader range to control pressure and temperature [224]. Despite their reduced size, microreactors may exhibit chemical gradients in the catalyst and fluid phases [22,225]. Furthermore, microstructured channel reactors allow easy and modular integration of other functional units (e.g. mixing, separation) and also the incorporation of planar waveguides and optical fibres [219].

Most *operando* approaches focus on gas-solid reactions (Fig. 15), but there is also a major need to look at working catalysts in the liquid phase, inorganic and enzymatic. Catalytic science may take advantage from the high spatiotemporal resolution and sensitivity of fluorescence microscopy applied to the liquid phase [226].

Cao et al. report a Raman-GC *operando* study of methanol oxidation over silver in a silicon-glass microreactor, illustrating surface reconstruction of silver phase and monitoring the evolution of subsurface atomic oxygen, which appears directly related with formaldehyde formation [228]. Gänzler et al. monitored the oscillatory CO oxidation on Pt/Al₂O₃ combining IR thermography images with XANES/EXAFS profiles [176], illustrating the higher activity of the reduced platinum particles and how Pt oxidation state engages with the oscillatory CO conversion pattern. For methane partial oxidation in a capillary bed,
<figure><img src="image_17.png" /><figcaption>**Fig. 14.** Detailed sketch of the Linkam CCR1000 reactor (a) and schematic drawing of the sample holder with flow indications by arrows to illustrate the fluidization principle (b). Scheme of the pilot plant scale set-up for studying catalyst bodies under reaction conditions. Key to this set-up are the in situ UV-Vis probes, which can be inserted in different holes made within the reactor tube.</figcaption></figure>

(a) Reproduced with permission from P. Beato, E. Schachtl, K. Barbera, F. Bonino, S. Bordiga, *Operando* Raman spectroscopy applying novel fluidized bed micro-reactor technology, *Catal. Today*. 205 (2013) 128–133, copyright Elsevier 2013 [216] (b) Reproduced with permission from J.J.H.B. Sattler, I.D. Gonzalez-Jimenez, A.M. Mens, M. Arias, T. Visser, B.M. Weckhuysen, *Operando* UV-Vis spectroscopy of a catalytic solid in a pilot-scale reactor: deactivation of a CrOx/Al2O3 propane dehydrogenation catalyst, *Chem. Commun.* 49 (2013) 1518–1520, copyright Royal Society of Chemistry 2013 [218]

<figure><img src="image_18.png" /><figcaption>**Fig. 15.** Schematic drawing of the Raman *operando* reaction cell for monoliths (left). View of the cell in perspective (centre): 1—gas outlet, 2—water cooling system, 3—KBr window, 4—IR cell support, 5—monolith (in order to improve the IR signal, small circular holes were realized in the walls perpendicular to the beam, except for the central wall), 6—gas inlet, 7—oven. (b) Right: an exploded view of the monolith IR cell with the catalyst inserted in the analysis chamber.</figcaption></figure>

(a) (reproduced with permission from [22]), and schematic drawing of the *operando* cell for

monolithic-shaped catalyst characterisation by transmission FTIR (right). (reproduced with permission and adapted from S.B.Rasmussen, M.A. Bañares, P. Bazin, J. Due-Hansen, P. Ávila, M. Daturi, Monitoring catalysts at work in their final form: spectroscopic investigations on a monolithic catalyst, *Phys. Chem. Chem. Phys.* 14 (2012) 2171–2177 copyright Royal Society of Chemistry 2012 [227])

Grunwaldt's group mapped chemical profiles on Rh/Al₂O₃ [229] and Rh–Pt/Al₂O₃ [29] using *operando* XAS-QMS. The study illustrates a progressive reduction of Rh and Pt species as the flow passes along the bed.

Chemical profiles occur, not only on the catalyst phase, but also along the fluid phase. Fletcher et al., reported the spatial population distribution of reactant and product within a T-shaped channel micro-reactor where ethanol and acetic acid blend [230]. The contact between the two liquids and the presence of co-flowing laminar streams down the leg of the “T” can be clearly seen. The extent of ether formation correlates well with position along the channel. A very systematic study on fluidodynamics was reported by Aguirre et al., who optimized an attenuated total reflection (ATR) microfluidic cell to assess intrinsic kinetic parameters of reactions at the liquid/solid interface under chemical control [231]. This work shows a very elegant approach to obtain reliable spectra in a kinetically relevant flow cell.

Nanoreactors and nanofluidic systems can be realized by nano-confinement in porous catalysts too, which may induce reactant concentration gradients inside the pores due to local conversion. Single particle plasmonic nanospectroscopy has been used to directly visualize reactant concentration gradient formation inside model porous catalysts due to conversion of CO on single Cu nanoparticles. This approach can be extended to the general study of heterogeneous gas/solid catalytic interfaces [232].

## 13. Enzymatic catalysis

Enzymatic reaction can be considered as driven by a kind of single

site catalysis. Heterogeneous biocatalysis usually involves enzymes immobilized on mesoporous solid supports for the advantage of their large internal surface area. Unravelling enzyme behaviour under the confinement of a solid surface and its effect on the catalytic reaction present an order of complexity requiring state-of-the-art characterization techniques to be approached.

Bolivar et al. summarize the analysis of enzymes distribution on porous solid support using microscopic imaging methods and multiple spectroscopic characterizations to assess the protein conformation on the solid support; their immobilization in porous materials is key to enzymes to make them heterogeneous biocatalysts. They describe correlations between enzyme activity and reactivity in the context of immobilization and how this promotes the rational design of bio-catalysts, as well as the optimization and control of the related catalytic processes [233]. Crespilho et al. demonstrated that a catalytic reaction is intrinsically dependent upon both the electronic configuration and the tridimensional structure of the whole reactive site of the enzyme (i.e., the catalytic and bioelectro catalytic reaction within the enzymatic structure needs the optimal organization of both the active pocket and substrate molecule in the tridimensional space) by using *operando* XAS spectroelectrochemistry and theoretical investigations in the oxygen reduction reaction (ORR) of bilirubin oxidase (Fig. 16) Therefore, if a catalytic reaction occurs in a three-dimensional environment (3D-catalysis), the energies involved in the transition states during the electron transfer cannot be explained by adsorption kinetics. In contrast, there is the formation of a three-dimensional complex (TDC). Both the metallic cofactor electronic structure and the spatial disposition of its surrounding ligands on the electron transfer promote the biocatalysis
<figure><img src="image_19.png" /><figcaption>**Fig. 14.** Detailed sketch of the Linkam CCR1000 reactor (a) and schematic drawing of the sample holder with flow indications by arrows to illustrate the fluidization principle (b). Scheme of the pilot plant scale set-up for studying catalyst bodies under reaction conditions. Key to this set-up are the in situ UV-Vis probes, which can be inserted in different holes made within the reactor tube.</figcaption></figure>

(a) Reproduced with permission from P. Beato, E. Schachtl, K. Barbera, F. Bonino, S. Bordiga, *Operando* Raman spectroscopy applying novel fluidized bed micro-reactor technology, *Catal. Today*. 205 (2013) 128–133, copyright Elsevier 2013 [216] (b) Reproduced with permission from J.J.H.B. Sattler, I.D. Gonzalez-Jimenez, A.M. Mens, M. Arias, T. Visser, B.M. Weckhuysen, *Operando* UV-Vis spectroscopy of a catalytic solid in a pilot-scale reactor: deactivation of a CrOx/Al2O3 propane dehydrogenation catalyst, *Chem. Commun.* 49 (2013) 1518–1520, copyright Royal Society of Chemistry 2013 [218]

<figure><img src="image_20.png" /><figcaption>**Fig. 15.** Schematic drawing of the Raman *operando* reaction cell for monoliths (left). View of the cell in perspective (centre): 1—gas outlet, 2—water cooling system, 3—KBr window, 4—IR cell support, 5—monolith (in order to improve the IR signal, small circular holes were realized in the walls perpendicular to the beam, except for the central wall), 6—gas inlet, 7—oven. (b) Right: an exploded view of the monolith IR cell with the catalyst inserted in the analysis chamber.</figcaption></figure>

(a) (reproduced with permission from [22]), and schematic drawing of the *operando* cell for

monolithic-shaped catalyst characterisation by transmission FTIR (right). (reproduced with permission and adapted from S.B.Rasmussen, M.A. Bañares, P. Bazin, J. Due-Hansen, P. Ávila, M. Daturi, Monitoring catalysts at work in their final form: spectroscopic investigations on a monolithic catalyst, *Phys. Chem. Chem. Phys.* 14 (2012) 2171–2177 copyright Royal Society of Chemistry 2012 [227])

Grunwaldt's group mapped chemical profiles on Rh/Al₂O₃ [229] and Rh–Pt/Al₂O₃ [29] using *operando* XAS-QMS. The study illustrates a progressive reduction of Rh and Pt species as the flow passes along the bed.

Chemical profiles occur, not only on the catalyst phase, but also along the fluid phase. Fletcher et al., reported the spatial population distribution of reactant and product within a T-shaped channel micro-reactor where ethanol and acetic acid blend [230]. The contact between the two liquids and the presence of co-flowing laminar streams down the leg of the “T” can be clearly seen. The extent of ether formation correlates well with position along the channel. A very systematic study on fluidodynamics was reported by Aguirre et al., who optimized an attenuated total reflection (ATR) microfluidic cell to assess intrinsic kinetic parameters of reactions at the liquid/solid interface under chemical control [231]. This work shows a very elegant approach to obtain reliable spectra in a kinetically relevant flow cell.

Nanoreactors and nanofluidic systems can be realized by nano-confinement in porous catalysts too, which may induce reactant concentration gradients inside the pores due to local conversion. Single particle plasmonic nanospectroscopy has been used to directly visualize reactant concentration gradient formation inside model porous catalysts due to conversion of CO on single Cu nanoparticles. This approach can be extended to the general study of heterogeneous gas/solid catalytic interfaces [232].

## 13. Enzymatic catalysis

Enzymatic reaction can be considered as driven by a kind of single

site catalysis. Heterogeneous biocatalysis usually involves enzymes immobilized on mesoporous solid supports for the advantage of their large internal surface area. Unravelling enzyme behaviour under the confinement of a solid surface and its effect on the catalytic reaction present an order of complexity requiring state-of-the-art characterization techniques to be approached.

Bolivar et al. summarize the analysis of enzymes distribution on porous solid support using microscopic imaging methods and multiple spectroscopic characterizations to assess the protein conformation on the solid support; their immobilization in porous materials is key to enzymes to make them heterogeneous biocatalysts. They describe correlations between enzyme activity and reactivity in the context of immobilization and how this promotes the rational design of bio-catalysts, as well as the optimization and control of the related catalytic processes [233]. Crespilho et al. demonstrated that a catalytic reaction is intrinsically dependent upon both the electronic configuration and the tridimensional structure of the whole reactive site of the enzyme (i.e., the catalytic and bioelectro catalytic reaction within the enzymatic structure needs the optimal organization of both the active pocket and substrate molecule in the tridimensional space) by using *operando* XAS spectroelectrochemistry and theoretical investigations in the oxygen reduction reaction (ORR) of bilirubin oxidase (Fig. 16) Therefore, if a catalytic reaction occurs in a three-dimensional environment (3D-catalysis), the energies involved in the transition states during the electron transfer cannot be explained by adsorption kinetics. In contrast, there is the formation of a three-dimensional complex (TDC). Both the metallic cofactor electronic structure and the spatial disposition of its surrounding ligands on the electron transfer promote the biocatalysis
<figure><img src="image_19.png" /><figcaption>**Fig. 16.** Redox activity of Cu centers. a, Active site of BOD (PDB 2XXL) for ORR. b, Cyclic voltammogram of a BOD-based electrode with argon-saturated electrolyte. Black line is the raw voltammogram, and the blue line is baseline-corrected voltammogram; c, Cyclic voltammogram comparing the catalytic (red line: with oxygen) and non-catalytic (black line: with argon) conditions; d, Cu-K edge XAS spectrum of BOD-modified electrode under two different electrochemical potentials, showing the reduced (activated, orange line) and the oxidized (resting, black line) states

(Reproduced with permission from L.J.A. Macedo, A.A.E. Santo, G.C. Sedenho, A. Hassan, R.M. Iost, G.T. Feliciano, F.N.Crespilho, Three-dimensional catalysis and the efficient bioelectrocatalysis beyond surface chemistry, J. Catal. 401 (2021) 200–205, Copyright Elsevier 2021) [234]</figcaption></figure>

[234].

Apart from understanding the structure, we must also understand the chemistry. For redox, protein film electrochemistry, in which a protein is immobilized on an electrode surface provides functional insight into the redox mechanism. However, understanding requires electrochemical cycles synchronized with characterization. Thus, Ash et al. report protein film infrared electrochemistry, which reports infrared spectra in a flow cell with controlled pH and solute concentrations, to investigate redox enzymes observing long-lived intermediate species in the catalytic mechanism [235].

Time resolution is a challenge that require alternative approaches; among these, modulation excitation spectroscopy (MES) with phase sensitive detection (PSD). Modulated experiments enable qualitative and quantitative analysis of reaction mechanisms and dynamic behaviors. The use of phase sensitive detection (PSD) permits separating a weak response from a strong background. Thus, Toledo et al. use in situ infrared spectroscopic techniques coupled with molecular modeling and DFT calculations to provide the first report about the species formed between R/S-ketoprofen and the catalytic triad of lipase B from Candida Antarctica [236]. They provide experimental and theoretical evidence of the acyl-enzyme intermediate, characterized by an infrared mode at 1756 cm⁻¹. This intermediate forms via ester bonding between ketoprofen and the serine amino acid of the catalytic triad with the simultaneous release of one molecule of water. DFT calculations indicate that the released molecule of water interacts with the acyl-enzyme through H-bonding.

# 14. Engineering operando methodology: shaped bodies

## 14.1. Reactor cells

Most operando reactor-cells follow the original idea [44] of obtaining kinetically relevant activity data, so that conversion and selectivity values are similar to those delivered by the original reactor [237]. Operando reactor-cells must comply with a minimum of characteristics to obtain kinetically relevant activity data, such as minimizing the dead volume, minimizing hot spots or avoiding reaction intermediates reabsorption phenomena. In early studies, some reaction cells had void volumes or preferential ways that can compromise the validity of the catalytic data. That may be the case of diffuse reflectance spectroscopy cells, which can be used for UV-vis, infrared or X-ray spectroscopy. Already in 1994, commercial DRIFTS reaction cells were optimized to minimized those limitations, optimizing catalytic measurements by providing spacing below (upstream) the fixed bed to distribute gas flow [238-240]. More recently, a very detailed analysis and fluid dynamics in DRIFT cells [11] or in other set-ups that provide structural as well as kinetic relevant information of high surface area catalysts [241] was reported. To perform experiments at high temperatures, microreactors made of quartz appear to be particularly advantageous for their resistance to the heat, transparency and easy design. An example can be found in [242].

Chemical engineering provides a phenomenological approach to optimize the performance of large reactors, global activity data connect with reaction parameters and reactor features. Structured catalysts, like honeycombs or foams, provide lower pressure drop, low axial dispersion and back-mixing, minimizing plugging and are easy to scale-up [243-247]. Catalyst shape and structure determines the fluid dynamic and physicochemical properties; these features have to be considered in the operando systems.

## 14.2. Shaped catalysts

Industrial catalysts are typically shaped. The implementation of new applications, which performance is enhanced by shaped materials, requires an accurate investigation on the chemistry occurring in their bodies.

An interesting case is the understanding of honeycomb catalysts, ranging from detailed analysis of the minimum expression of a honeycomb, a wall, that may be represented by a wafer to the actual shaped honeycomb. Thin wafers may represent a honeycomb wall allowing highly accurate spectroscopic studies, like X-ray absorption (XAS) [170, 248] or infrared absorption spectroscopy. Engineering of tiny wafers has to be considered in details to obtain data relevant to the shaped honeycomb counterpart. Thus, it must be free of diffusional limitations, both within the cell and within the wafer [47,249,250]. Regarding cell fluidodynamics, Daturi's group has reported a very accurate design to optimize gas flow in a cell for a wafer catalyst so that the flow regime in the 'sandwich' IR reactor cell is a combination of ideal plug-flow reactors both in series and parallel. An operando SSITKA study of methanol adsorption onto ceria validated this [47]. Regarding the flow of reactants within the wafer, it is critical to fine tune the pelletization process, which may induce diffusional processes. Pelletizing modifies the porosity of the powder sample, thus shaping them into a thin wafer will modify the porous structure, shifting pore volume distribution from macropores towards meso- and micro-pores. A systematic study demonstrates how important this is, and how the efficiency factors are affected, as a function of intrinsic rates, pelletizing pressure and wafer thickness [170]. The direct relationship between the Thiele modulus and the thickness of the pellet can be used as a simple procedure for evaluating efficiencies on the basis of pellet dimensions and solid phase characteristics [251].

The Fig. 17 shows the efficiency of a titania-supported vanadationsta catalyst pressed into a self-supported wafer for NOx selective
<figure><img src="image_21.png" /><figcaption>**Fig. 16.** Redox activity of Cu centers. a, Active site of BOD (PDB 2XXL) for ORR. b, Cyclic voltammogram of a BOD-based electrode with argon-saturated electrolyte. Black line is the raw voltammogram, and the blue line is baseline-corrected voltammogram; c, Cyclic voltammogram comparing the catalytic (red line: with oxygen) and non-catalytic (black line: with argon) conditions; d, Cu-K edge XAS spectrum of BOD-modified electrode under two different electrochemical potentials, showing the reduced (activated, orange line) and the oxidized (resting, black line) states

(Reproduced with permission from L.J.A. Macedo, A.A.E. Santo, G.C. Sedenho, A. Hassan, R.M. Iost, G.T. Feliciano, F.N.Crespilho, Three-dimensional catalysis and the efficient bioelectrocatalysis beyond surface chemistry, J. Catal. 401 (2021) 200–205, Copyright Elsevier 2021) [234]</figcaption></figure>

[234].

Apart from understanding the structure, we must also understand the chemistry. For redox, protein film electrochemistry, in which a protein is immobilized on an electrode surface provides functional insight into the redox mechanism. However, understanding requires electrochemical cycles synchronized with characterization. Thus, Ash et al. report protein film infrared electrochemistry, which reports infrared spectra in a flow cell with controlled pH and solute concentrations, to investigate redox enzymes observing long-lived intermediate species in the catalytic mechanism [235].

Time resolution is a challenge that require alternative approaches; among these, modulation excitation spectroscopy (MES) with phase sensitive detection (PSD). Modulated experiments enable qualitative and quantitative analysis of reaction mechanisms and dynamic behaviors. The use of phase sensitive detection (PSD) permits separating a weak response from a strong background. Thus, Toledo et al. use in situ infrared spectroscopic techniques coupled with molecular modeling and DFT calculations to provide the first report about the species formed between R/S-ketoprofen and the catalytic triad of lipase B from Candida Antarctica [236]. They provide experimental and theoretical evidence of the acyl-enzyme intermediate, characterized by an infrared mode at 1756 cm⁻¹. This intermediate forms via ester bonding between ketoprofen and the serine amino acid of the catalytic triad with the simultaneous release of one molecule of water. DFT calculations indicate that the released molecule of water interacts with the acyl-enzyme through H-bonding.

# 14. Engineering operando methodology: shaped bodies

## 14.1. Reactor cells

Most operando reactor-cells follow the original idea [44] of obtaining kinetically relevant activity data, so that conversion and selectivity values are similar to those delivered by the original reactor [237]. Operando reactor-cells must comply with a minimum of characteristics to obtain kinetically relevant activity data, such as minimizing the dead volume, minimizing hot spots or avoiding reaction intermediates reabsorption phenomena. In early studies, some reaction cells had void volumes or preferential ways that can compromise the validity of the catalytic data. That may be the case of diffuse reflectance spectroscopy cells, which can be used for UV-vis, infrared or X-ray spectroscopy. Already in 1994, commercial DRIFTS reaction cells were optimized to minimized those limitations, optimizing catalytic measurements by providing spacing below (upstream) the fixed bed to distribute gas flow [238-240]. More recently, a very detailed analysis and fluid dynamics in DRIFT cells [11] or in other set-ups that provide structural as well as kinetic relevant information of high surface area catalysts [241] was reported. To perform experiments at high temperatures, microreactors made of quartz appear to be particularly advantageous for their resistance to the heat, transparency and easy design. An example can be found in [242].

Chemical engineering provides a phenomenological approach to optimize the performance of large reactors, global activity data connect with reaction parameters and reactor features. Structured catalysts, like honeycombs or foams, provide lower pressure drop, low axial dispersion and back-mixing, minimizing plugging and are easy to scale-up [243-247]. Catalyst shape and structure determines the fluid dynamic and physicochemical properties; these features have to be considered in the operando systems.

## 14.2. Shaped catalysts

Industrial catalysts are typically shaped. The implementation of new applications, which performance is enhanced by shaped materials, requires an accurate investigation on the chemistry occurring in their bodies.

An interesting case is the understanding of honeycomb catalysts, ranging from detailed analysis of the minimum expression of a honeycomb, a wall, that may be represented by a wafer to the actual shaped honeycomb. Thin wafers may represent a honeycomb wall allowing highly accurate spectroscopic studies, like X-ray absorption (XAS) [170, 248] or infrared absorption spectroscopy. Engineering of tiny wafers has to be considered in details to obtain data relevant to the shaped honeycomb counterpart. Thus, it must be free of diffusional limitations, both within the cell and within the wafer [47,249,250]. Regarding cell fluidodynamics, Daturi's group has reported a very accurate design to optimize gas flow in a cell for a wafer catalyst so that the flow regime in the 'sandwich' IR reactor cell is a combination of ideal plug-flow reactors both in series and parallel. An operando SSITKA study of methanol adsorption onto ceria validated this [47]. Regarding the flow of reactants within the wafer, it is critical to fine tune the pelletization process, which may induce diffusional processes. Pelletizing modifies the porosity of the powder sample, thus shaping them into a thin wafer will modify the porous structure, shifting pore volume distribution from macropores towards meso- and micro-pores. A systematic study demonstrates how important this is, and how the efficiency factors are affected, as a function of intrinsic rates, pelletizing pressure and wafer thickness [170]. The direct relationship between the Thiele modulus and the thickness of the pellet can be used as a simple procedure for evaluating efficiencies on the basis of pellet dimensions and solid phase characteristics [251].

The Fig. 17 shows the efficiency of a titania-supported vanadatungsta catalyst pressed into a self-supported wafer for NOx selective
<figure><img src="image_20.png" /><figcaption>**Fig. 17.** Pellet efficiencies as a function of pellet thickness and intrinsic activity for the 2, 6, and 10 tons/cm2 applied pressures during the pelletizing procedure. Red dot in the top left graphic is the conditions applied in our case study. (reprinted with permission from S.B. Rasmussen, S. Perez-Ferreras, M.A. Bañares, P. Bazin, M. Daturi, Does pelletizing catalysts influence the efficiency number of activity measurements? Spectrochemical engineering considerations for an accurate *operando* study, ACS Catal. 3 (2013). Copyright 2013 American Chemical Society [251])</figcaption></figure>

catalytic reduction in an *operando* infrared transmission reactor. The wafer thickness and the pressure applied to make it have a dramatic effect on the efficiency number. The absence of mass transfer limitations is fundamental to obtain correct information about the intrinsic reaction kinetics of a process, also in the case of *operando* spectroscopy analyses. Rodriguez-Garcia et al. have investigated the effect of mass transfer on the detection of active species at the catalytic solid-liquid interface using ATR-IR spectroscopy. To understand the mass transport within typical flow-through cells they have assessed the hydrodynamics as well as the transport of solutes within the liquid phase and the porous catalyst sample using a convection-diffusion model. They describe four design parameters that determine the mass transfer within the ATR-IR spectroscopy cells. They used the asymmetric hydrogenation of phenyl cinnamic acid on a chirally modified Pd catalyst as a model reaction to demonstrate the effects of mass transfer on the activity and selectivity [252].

Mass transfer limitations may significantly hamper the meaning of *operando* data. For this reason, Daturi's and Bañares' groups reported an *operando* FTIR-MS study of a V₂O₅-WO₃/TiO₂-sepiolite honeycomb in a system that is free of diffusional limitations, as commented above [227]. Because the study deals with a real sample, the presence of the sepiolite clay leads to saturation in several spectral ranges (3000-3500 cm⁻¹ and below 1250 cm⁻¹), hampering accurate analysis of adsorbed ammonia bands. The important information about ammonia species had to be extracted from the envelope band near 1600-1650 cm⁻¹ which is a linear combination of δOH and δ₃NH₃ modes; thus, a chemometric analysis was necessary (Fig. 18) [253]. This work illustrates that pre-adsorbed ammonia on V₂O₅-WO₃/TiO₂-sepiolite honeycombs evolves upon interaction with NO; both ammonia on Lewis acid sites and ammonium on Brønsted acid sites, appear involved in the reaction. The large honeycomb reactor did not allow for accurate discrimination of which acid site is critical. Thus, a SSITKA analysis on the wafer cell was used to provide higher time resolution on the Brønsted-Lewis interplay; this shows that SCR activity is much higher when adsorbed ammonia reacts with water formed during reaction to create adsorbed ammonium species. The very nature of supported vanadia sites changes with moisture:

<figure><img src="image_21.png" /><figcaption>**Fig. 18.** Transient from 20% O₂/Ar to SCR at 313 °C (second run) highlighting the formation of NH₃,ads and NH₄⁺. Top: FTIR spectra contour plot in the NeH bending region showing the growth of ammonia-derived species on the catalyst surface. Middle: surface species area profiles taken from the contour plot. Bottom: gas phase evolution of NH₃ (m/z 15, x20), H₂O (m/z 18), N₂ (m/z 28) and NO (m/z 30). Reprinted from Appl. Catal. B Environ. 224, S.B. Rasmussen, R. Portela, P. Bazin, P. Ávila, M.A. Bañares, M.</figcaption></figure>

(a) (reproduced with permission from [253]). (b) Daturi, Transient *operando* study on the NH₃ /NH₄⁺ interplay in V-SCR monolithic catalysts, 109–115, (2018) with permission from Elsevier.

the dissociative adsorption of water accounts for the generation of Brønsted acid sites at low [128] and high [127] vanadia coverage on titania. The most apparent spectroscopic effect of this is the redshift of the vanadyl mode upon hydroxylation. Some works have made attempts to correlate this with competitive adsorption of water and ammonia [254]. The scenario is complex since not only hydration, but some reduction of vanadia sites, may account for such a redshift [255]. Still, a higher time-resolution may be necessary to fully assess which is the critical site and the ultimate nature of the interplay of reacting species and intermediates.

## 15. Conclusions

The *operando* methodology enables assessing structure-activity relationships at a molecular scale. Since its conception, the *operando* methodology has expanded to multiple functional materials, and covered almost every aspect in catalysis science; it is poised to settle into catalysis engineering. *Operando* is becoming increasingly resolved in time and space, to get closer to the catalytic act and uncover the physical phenomena determining it. Time resolution combined with transient studies, like SSITKA or modulation experiments, can uncover important mechanistic details of the chemical act occurring during catalysis. From this deep comprehension, the design of new generations of advanced catalysts become possible.

Average or too local *operando* readings may miss or average phenomena occurring, and thus hamper its applicability for further
<figure><img src="image_22.png" /><figcaption>**Fig. 17.** Pellet efficiencies as a function of pellet thickness and intrinsic activity for the 2, 6, and 10 tons/cm2 applied pressures during the pelletizing procedure. Red dot in the top left graphic is the conditions applied in our case study. (reprinted with permission from S.B. Rasmussen, S. Perez-Ferreras, M.A. Bañares, P. Bazin, M. Daturi, Does pelletizing catalysts influence the efficiency number of activity measurements? Spectrochemical engineering considerations for an accurate *operando* study, ACS Catal. 3 (2013). Copyright 2013 American Chemical Society [251])</figcaption></figure>

catalytic reduction in an *operando* infrared transmission reactor. The wafer thickness and the pressure applied to make it have a dramatic effect on the efficiency number. The absence of mass transfer limitations is fundamental to obtain correct information about the intrinsic reaction kinetics of a process, also in the case of *operando* spectroscopy analyses. Rodriguez-Garcia et al. have investigated the effect of mass transfer on the detection of active species at the catalytic solid-liquid interface using ATR-IR spectroscopy. To understand the mass transport within typical flow-through cells they have assessed the hydrodynamics as well as the transport of solutes within the liquid phase and the porous catalyst sample using a convection-diffusion model. They describe four design parameters that determine the mass transfer within the ATR-IR spectroscopy cells. They used the asymmetric hydrogenation of phenyl cinnamic acid on a chirally modified Pd catalyst as a model reaction to demonstrate the effects of mass transfer on the activity and selectivity [252].

Mass transfer limitations may significantly hamper the meaning of *operando* data. For this reason, Daturi's and Bañares' groups reported an *operando* FTIR-MS study of a V₂O₅-WO₃/TiO₂-sepiolite honeycomb in a system that is free of diffusional limitations, as commented above [227]. Because the study deals with a real sample, the presence of the sepiolite clay leads to saturation in several spectral ranges (3000-3500 cm⁻¹ and below 1250 cm⁻¹), hampering accurate analysis of adsorbed ammonia bands. The important information about ammonia species had to be extracted from the envelope band near 1600-1650 cm⁻¹ which is a linear combination of δOH and δ₃NH₃ modes; thus, a chemometric analysis was necessary (Fig. 18) [253]. This work illustrates that pre-adsorbed ammonia on V₂O₅-WO₃/TiO₂-sepiolite honeycombs evolves upon interaction with NO; both ammonia on Lewis acid sites and ammonium on Brønsted acid sites, appear involved in the reaction. The large honeycomb reactor did not allow for accurate discrimination of which acid site is critical. Thus, a SSITKA analysis on the wafer cell was used to provide higher time resolution on the Brønsted-Lewis interplay; this shows that SCR activity is much higher when adsorbed ammonia reacts with water formed during reaction to create adsorbed ammonium species. The very nature of supported vanadia sites changes with moisture:

<figure><img src="image_23.png" /><figcaption>**Fig. 18.** Transient from 20% O₂/Ar to SCR at 313 °C (second run) highlighting the formation of NH₃,ads and NH₄⁺. Top: FTIR spectra contour plot in the NeH bending region showing the growth of ammonia-derived species on the catalyst surface. Middle: surface species area profiles taken from the contour plot. Bottom: gas phase evolution of NH₃ (m/z 15, x20), H₂O (m/z 18), N₂ (m/z 28) and NO (m/z 30). Reprinted from Appl. Catal. B Environ. 224, S.B. Rasmussen, R. Portela, P. Bazin, P. Ávila, M.A. Bañares, M.</figcaption></figure>

(a) (reproduced with permission from [253]). (b) Daturi, Transient *operando* study on the NH₃ /NH₄⁺ interplay in V-SCR monolithic catalysts, 109–115, (2018) with permission from Elsevier.

the dissociative adsorption of water accounts for the generation of Brønsted acid sites at low [128] and high [127] vanadia coverage on titania. The most apparent spectroscopic effect of this is the redshift of the vanadyl mode upon hydroxylation. Some works have made attempts to correlate this with competitive adsorption of water and ammonia [254]. The scenario is complex since not only hydration, but some reduction of vanadia sites, may account for such a redshift [255]. Still, a higher time-resolution may be necessary to fully assess which is the critical site and the ultimate nature of the interplay of reacting species and intermediates.

## 15. Conclusions

The *operando* methodology enables assessing structure-activity relationships at a molecular scale. Since its conception, the *operando* methodology has expanded to multiple functional materials, and covered almost every aspect in catalysis science; it is poised to settle into catalysis engineering. *Operando* is becoming increasingly resolved in time and space, to get closer to the catalytic act and uncover the physical phenomena determining it. Time resolution combined with transient studies, like SSITKA or modulation experiments, can uncover important mechanistic details of the chemical act occurring during catalysis. From this deep comprehension, the design of new generations of advanced catalysts become possible.

Average or too local *operando* readings may miss or average phenomena occurring, and thus hamper its applicability for further
progress. Single-point operando analyses may miss the chemistry occurring around; this is particularly important for laboratory- and pilot-scale reactors. On a more fundamental fashion, this may also apply within the atomic nearby range.

Even when catalysis occurs locally, at a single site, it is not isolated from the rest of the catalytic material. This applies at all ranges. Long and middle range result in gradients, which affect the state of the catalytic material, the reactants and the fluid phase. As reaction progresses, the stream progressively changes from unconverted reactant to increasingly converted ones, which in turn induce gradients on the catalyst states. Mapping both the catalyst state and reactivity, along with temperature, is key information to optimize catalyst invention, design and control. For the very short range around the site, a complete molecular comprehension of structure-activity relationships, can be affected by the very near atomic range environment. Enzyme catalysis is characterized by allostery, in which reactions occurring nearby communicate within nanometer range distances. Current art illustrates that a formal allostery also rules in nanocatalysts domain. The mechanism is different but related to reactant-induced modifications in the solid, which shows effects at nearby sites, thus affecting reactivity. Somehow, this reinforces the vision of the “catalytic trinity”: the ultimate nature of catalysis is related to the chemical act between the reactant and the site, be it a homogeneous, enzymatic or inorganic solid catalyst.

Operando methodology is poised to major progresses in time and space resolution, which successful implementation paves the ground to understand catalysis, industrial catalysis and fundamental catalysis. In either field, the amount of data will grow exponentially, which will result in a major emergence of chemometrics and intelligent data handling, management to overcome scenarios of data-drowning/ information-starvation. This combined effort will allow understanding the ultimate nature of the catalytic act and engineer it. From the knowledge of the local elements (material sites or reaction steps) the global view of the catalytic system and process will be straightforwardly obtained.

Declaration of Competing Interest

The authors declare no conflict of interest in this review paper.

References

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[15] V. Calvino-Casilda, M.A. Bañares, Recent advances in imaging and monitoring of heterogeneous catalysts with Raman spectroscopy, in: Catal, Vol. 24, The Royal Society of Chemistry, 2012, pp. 1–47, https://doi.org/10.1039/9781849734776-00001.

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progress. Single-point operando analyses may miss the chemistry occurring around; this is particularly important for laboratory- and pilot-scale reactors. On a more fundamental fashion, this may also apply within the atomic nearby range.

Even when catalysis occurs locally, at a single site, it is not isolated from the rest of the catalytic material. This applies at all ranges. Long and middle range result in gradients, which affect the state of the catalytic material, the reactants and the fluid phase. As reaction progresses, the stream progressively changes from unconverted reactant to increasingly converted ones, which in turn induce gradients on the catalyst states. Mapping both the catalyst state and reactivity, along with temperature, is key information to optimize catalyst invention, design and control. For the very short range around the site, a complete molecular comprehension of structure-activity relationships, can be affected by the very near atomic range environment. Enzyme catalysis is characterized by allostery, in which reactions occurring nearby communicate within nanometer range distances. Current art illustrates that a formal allostery also rules in nanocatalysts domain. The mechanism is different but related to reactant-induced modifications in the solid, which shows effects at nearby sites, thus affecting reactivity. Somehow, this reinforces the vision of the “catalytic trinity”: the ultimate nature of catalysis is related to the chemical act between the reactant and the site, be it a homogeneous, enzymatic or inorganic solid catalyst.

Operando methodology is poised to major progresses in time and space resolution, which successful implementation paves the ground to understand catalysis, industrial catalysis and fundamental catalysis. In either field, the amount of data will grow exponentially, which will result in a major emergence of chemometrics and intelligent data handling, management to overcome scenarios of data-drowning/ information-starvation. This combined effort will allow understanding the ultimate nature of the catalytic act and engineer it. From the knowledge of the local elements (material sites or reaction steps) the global view of the catalytic system and process will be straightforwardly obtained.

Declaration of Competing Interest

The authors declare no conflict of interest in this review paper.

References

[1] W.D. Michalak, G.A. Somorjai, Catalysis in energy generation and conversion: how insight into nanostructure, composition, and electronic structure leads to better catalysts (perspective), Top. Catal. 56 (2013) 1611–1622, https://doi.org/10.1007/s11244-013-0096-0.

[2] Y. Zhang, D. Fu, X. Xu, Y. Sheng, J. Xu, Y. Han, Application of operando spectroscopy on catalytic reactions, Curr. Opin. Chem. Eng. 12 (2016) 1–7, https://doi.org/10.1016/j.coche.2016.01.004.

[3] M.A. Bañares, I.E. Wachs, Molecular structures of supported metal oxide catalysts under different environments, J. Raman Spectrosc. 33 (2002) 359–380, https://doi.org/10.1002/jrs.866.

[4] M. Hunger, J. Weitkamp, In situ IR NMR, EPR, and UV/Vis spectroscopy: tools for new insight into the mechanisms of heterogeneous catalysis, Angew. Chem. -Int. Ed. 40 (2001) 2954–2971, https://doi.org/10.1002/1521-3773(20010817)40:16-2954:AID-ANIE2954>3.0.CO;2-#.

[5] A. Urakawa, Trends and advances in operando methodology, Curr. Opin. Chem. Eng. 12 (2016) 31–36, https://doi.org/10.1016/j.coche.2016.02.002.

[6] A. Urakawa, N. Maeda, A. Baiker, Space- and time-resolved combined DRIFT and Raman spectroscopy: monitoring dynamic surface and bulk processes during NO (x) storage reduction, Angew. Chem. -Int. Ed. 47 (2008) 9256–9259, https://doi.org/10.1002/anie.200804077.

[7] J.A. Dumesic, H. Topsøe, Mössbauer spectroscopy applications to heterogeneous catalysis, in: D.D. Eley, H. Pines, P.B. Weisz (Eds.), Adv. Catal., Academic Press, 1977, pp. 121–246. (http://www.sciencedirect.com/science/article/pii/S0360056408600711).

[8] R. Burch, In situ methods in catalysis, Catal. Today 9 (1991). R9–R9.

[9] B.S. Clausen, H. Topsøe, R. Frahm, Application of combined X-ray diffraction and absorption techniques for in situ catalyst characterization, in: D.D. Eley, W. O. Haag, B. Gates, H. Knozinger (Eds.), Adv. Catal., Vol 42, Elsevier Academic Press Inc, San Diego, 1998, pp. 315–344.

[10] M.A. Bañares, Operando spectroscopy: the knowledge bridge to assessing structure-performance relationships in catalyst nanoparticles, Adv. Mater. 23 (2011) 5293–5301, https://doi.org/10.1002/adma.201101803.

[11] F.C. Meunier, The design and testing of kinetically-appropriate operando spectroscopic cells for investigating heterogeneous catalytic reactions, Chem. Soc. Rev. 39 (2010) 4602–4614, https://doi.org/10.1039/b919705m.

[12] B.M. Weckhuysen, Preface: recent advances in the in-situ characterization of heterogeneous catalysts, Chem. Soc. Rev. 39 (2010) 4557–4559, https://doi.org/10.1039/c0cs90031a.

[13] B.M. Weckhuysen, Snapshots of a working catalyst: possibilities and limitations of in situ spectroscopy in the field of heterogeneous catalysis, Chem. Commun. (2002) 97–110, https://doi.org/10.1039/b107686h.

[14] M.A. Bañares, M.O. Guerrero-Perez, J.L.G. Fierro, G.G. Cortez, Raman spectroscopy during catalytic operations with on-line activity measurement (operando spectroscopy): a method for understanding the active centres of cations supported on porous materials, J. Mater. Chem. 12 (2002) 3337–3342, https://doi.org/10.1039/b204494c.

[15] V. Calvino-Casilda, M.A. Bañares, Recent advances in imaging and monitoring of heterogeneous catalysts with Raman spectroscopy, in: Catal, Vol. 24, The Royal Society of Chemistry, 2012, pp. 1–47, https://doi.org/10.1039/9781849734776-00001.

[16] M.A. Bañares, In situ to operando spectroscopy: from proof of concept to industrial application, Top. Catal. 52 (2009) 1301–1302, https://doi.org/10.1007/s11244-009-9313-2.

[17] J.W. Niemantsverdriet, Spectroscopy in catalysis. Completely Revised and Enlarged Edition, third ed., Wiley, 2007.

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