# Understanding catalysts by time-/space-resolved operando methodologies

Miguel Bañares, Marco Daturi

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## To cite this version:

Miguel Bañares, Marco Daturi. Understanding catalysts by time-/space-resolved operando methodologies. Catalysis Today, 2023, 423, pp.114255. 10.1016/j.cattod.2023.114255. hal-04252478

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Submitted on 27 May 2024

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# Understanding catalysts by time-/space-resolved operando methodologies

Miguel A. Bañares<sup>a, *</sup>, Marco Daturi<sup>b</sup>

<figure><img src="image_1.png" /></figure>

Check for
updates

## ARTICLE INFO

### Keywords:

Operando

Heterogeneous catalysis

Structure-activity relationships

Time-resolved

Space-resolved

Review

## ABSTRACT

Heterogeneous catalysis has a central position in our everyday life and it is gaining importance with the changes in our economic model and with the progress of the energetic transition. New generation of catalysts are based on rational design, which is the fruit of thorough investigations in relevant conditions. Operando spectroscopy is the key to obtain the right information to understand the behavior of a catalytic material, as well as to unravel the reaction steps of a process. In the present contribution we have reviewed some of the most representative papers in the domain, focusing on energy-related investigations and on two extremely important aspects such as time and space resolution. We provide our point of view about the different methods and techniques, the shared vision about similarities in catalytic processes, as well as an opinion about the perspectives for future developments, notably in the field of industrial process monitoring.

## 1. Introduction

Heterogeneous catalysis is a central process for the generation, the conversion and the storage of energy under the form of chemical vectors. Catalysts can be used to both convert electric power into energy carriers (like hydrogen or hydrocarbons used as fuels) and to convert chemicals into electricity more efficiently than by combustion. As in all the cases where catalytic materials are involved, a deep knowledge of their global and local structure, not only at synthesis, but dynamically during the operating conditions, is necessary to assess their proper working way and durability, or a proper design in case of a scale-up. Spectroscopic studies are the most appropriate way to unravel structure and properties of the active site, to feature out its stability and to predict its resistance to poisons exposure [1,2].

The ultimate nature of the catalytic act is the same at atomic scale for any kind of catalysis, be it homogeneous, enzymatic or heterogeneous. In the end, it is an interaction between the reactant and the active site; we may call it the “catalytic trinity”, in the sense that the three aspects of catalysis are one. Catalysis occurs at the atomic scale; rearranging bonds in a dance that delivers new molecules. However, while the catalytic act takes place at the atomic scale, longer range constraints may apply. In homogeneous catalysis there is direct interaction with the molecules in the reaction media (diffusion limitations must be avoided); heterogeneous and enzymatic catalysis are bodies with larger dimensions. Large scale catalytic systems make gradients ubiquitous, and they have to be

known and steered to optimize catalytic performance minimizing diffusional limitations. Therefore, to understand catalysis and catalytic materials, adapted methodologies must be envisaged, considering the aforementioned nodal points.

Operando is a term which was introduced in the early 2000 years to differentiate in situ studies for catalysts under relevant reaction conditions, while measuring catalytic performances simultaneously [3]. It is not superfluous to mention (at the light of the numerous mistakes present in the literature) that the right way to name this methodology investigating catalysts at work is operando, the gerundive (present continuous tense) form of the Latin verb operare (to work, essentially). Expressions as “in operando” (probably contaminated by the expression in situ) should not been used, being grammatically incorrect. As a matter of fact, “in operando” is an ellipsis of “in operando conditions”, i.e., the researcher assumes the catalyst is “in working” conditions (“in operando”), but not necessarily working (operando); actual working catalyst is the key prerequisite to discriminate reaction in situ studies from operando studies. The operando methodology connects working catalyst site structure with its performance; this makes operando an invaluable tool to understand catalysis [4–19]. The operando reactor has to be designed to meet the requirements of both catalytic activity measurement and spectroscopic measurement. As such, it is a method that combines a characterization of a material at work with its performance. Since operando methodology spans from very fundamental to applied research, there is an increasingly evident need to engineer operando

<footnote><sup>a</sup> Spectroscopy and Industrial Catalysis, SpeiCat, Instituto de Catálisis y Petroleoquímica, ICP, CSIC, Marie Curie 2, E-28049 Madrid, Spain</footnote>

<footnote><sup>b</sup> Normandie Univ., ENSICAEN, UNICAEN, CNRS, Laboratoire Catalyse et Spectrochimie, 14000 Caen, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: banares@icp.csic.es (M.A. Bañares).</footnote>
# Understanding catalysts by time-/space-resolved operando methodologies

Miguel A. Bañares<sup>a, *</sup>, Marco Daturi<sup>b</sup>

<figure><img src="image_3.png" /></figure>

Check for
updates

## ARTICLE INFO

### Keywords:

Operando

Heterogeneous catalysis

Structure-activity relationships

Time-resolved

Space-resolved

Review

## ABSTRACT

Heterogeneous catalysis has a central position in our everyday life and it is gaining importance with the changes in our economic model and with the progress of the energetic transition. New generation of catalysts are based on rational design, which is the fruit of thorough investigations in relevant conditions. Operando spectroscopy is the key to obtain the right information to understand the behavior of a catalytic material, as well as to unravel the reaction steps of a process. In the present contribution we have reviewed some of the most representative papers in the domain, focusing on energy-related investigations and on two extremely important aspects such as time and space resolution. We provide our point of view about the different methods and techniques, the shared vision about similarities in catalytic processes, as well as an opinion about the perspectives for future developments, notably in the field of industrial process monitoring.

## 1. Introduction

Heterogeneous catalysis is a central process for the generation, the conversion and the storage of energy under the form of chemical vectors. Catalysts can be used to both convert electric power into energy carriers (like hydrogen or hydrocarbons used as fuels) and to convert chemicals into electricity more efficiently than by combustion. As in all the cases where catalytic materials are involved, a deep knowledge of their global and local structure, not only at synthesis, but dynamically during the operating conditions, is necessary to assess their proper working way and durability, or a proper design in case of a scale-up. Spectroscopic studies are the most appropriate way to unravel structure and properties of the active site, to feature out its stability and to predict its resistance to poisons exposure [1,2].

The ultimate nature of the catalytic act is the same at atomic scale for any kind of catalysis, be it homogeneous, enzymatic or heterogeneous. In the end, it is an interaction between the reactant and the active site; we may call it the “catalytic trinity”, in the sense that the three aspects of catalysis are one. Catalysis occurs at the atomic scale; rearranging bonds in a dance that delivers new molecules. However, while the catalytic act takes place at the atomic scale, longer range constraints may apply. In homogeneous catalysis there is direct interaction with the molecules in the reaction media (diffusion limitations must be avoided); heterogeneous and enzymatic catalysis are bodies with larger dimensions. Large scale catalytic systems make gradients ubiquitous, and they have to be

known and steered to optimize catalytic performance minimizing diffusional limitations. Therefore, to understand catalysis and catalytic materials, adapted methodologies must be envisaged, considering the aforementioned nodal points.

Operando is a term which was introduced in the early 2000 years to differentiate in situ studies for catalysts under relevant reaction conditions, while measuring catalytic performances simultaneously [3]. It is not superfluous to mention (at the light of the numerous mistakes present in the literature) that the right way to name this methodology investigating catalysts at work is operando, the gerundive (present continuous tense) form of the Latin verb operare (to work, essentially). Expressions as “in operando” (probably contaminated by the expression in situ) should not been used, being grammatically incorrect. As a matter of fact, “in operando” is an ellipsis of “in operando conditions”, i.e., the researcher assumes the catalyst is “in working” conditions (“in operando”), but not necessarily working (operando); actual working catalyst is the key prerequisite to discriminate reaction in situ studies from operando studies. The operando methodology connects working catalyst site structure with its performance; this makes operando an invaluable tool to understand catalysis [4–19]. The operando reactor has to be designed to meet the requirements of both catalytic activity measurement and spectroscopic measurement. As such, it is a method that combines a characterization of a material at work with its performance. Since operando methodology spans from very fundamental to applied research, there is an increasingly evident need to engineer operando

<footnote><sup>a</sup> Spectroscopy and Industrial Catalysis, SpeiCat, Instituto de Catálisis y Petroleoquímica, ICP, CSIC, Marie Curie 2, E-28049 Madrid, Spain</footnote>

<footnote><sup>b</sup> Normandie Univ., ENSICAEN, UNICAEN, CNRS, Laboratoire Catalyse et Spectrochimie, 14000 Caen, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: banares@icp.csic.es (M.A. Bañares).</footnote>
methodology [20,21], but also to make it more versatile and sensitive in time and in space [15,22–24].

The rise of *operando* methodology, along with significant advances in materials/reaction computational chemical modelling trigger a qualitative change in catalyst discovery [10], providing an alternative approach to conventional try-and-error, where the catalyst was mainly a “black box” that was characterized before and after catalytic reaction, i. e., before and after catalytic life. Such a characterization provided information hampered by multiple gaps (material, temperature, pressure) digressing from actual working conditions [12,25]. *Operando* methodology facilitated physicochemical characterization of the phases during operation. There is an entire arsenal of characterization techniques, which endow us with an eye with the sensitivity we need [16].

The use of diverse spectroscopies provides not only specific sensitivities, but also different time- and space- resolution of measurement. Space-time constraints are deeply entangled, and they are critical for implementing comprehensive *operando* mapping of catalysts. Higher space-resolution requires better time-resolution to be significant. Moreover, to fully overcome the “dimension gap”, besides reducing the size of the area to be probed, probing various areas of the solid is essential, in order to be able to identify inhomogeneities and their effect on the catalytic performance. Space-resolved *operando* is nowadays applied at the nano-, micro-, and macro-scale to study intra- and inter-reactor -catalyst and -molecule phenomena and their evolution. The progress in monitoring heterogeneous catalysis at different length and time scales has been greatly pushed and reviewed by the group of Weckhuysen [26–28] and others [15,29–31].

The development of microscopic methods and of capillary and fiber optic probes has greatly contributed to improve space-resolution [32], but also to perform multi-probe characterization. An interesting example is the *operando* study of the CrOx/Al₂O₃ extrudates deactivation in a pilot-scale propane dehydrogenation reactor. The experimental set-up combined UV/vis and Raman optical fiber probes to follow the formation of coke deposits on-line [33], which probe could be moved with respect to the catalyst bed for focusing purposes. This approach provided a monitoring tool to minimize aging due to thermal stress by decreasing temperature peaks on reoxidation and also to modulate the feed to keep the catalyst working around its optimum performance [33, 34].

For all catalyst characterization techniques, a premise remains fundamental: the chemical phase relevant to the catalytic process must be identified. Heterogeneous catalysis is an extremely surface-sensitive phenomenon; the interactions occur at the outermost layer of the catalysts, so that the characterization has to be related to the exposed surface (external or internal) of the solid to be representative of active sites. This circumscribes *operando* investigations: they must characterize the chemical structure that is relevant to the reaction and measure the activity that is derived from it. However, most spectroscopies are bulk sensitive or near-surface sensitive, while many characterization techniques present signals with contributions from both the bulk and the surface, being scarcely selective to the outermost layer. Therefore, the interplay between the bulk and the outermost domains can be critical for heterogeneous catalysis. For example, molecularly dispersed catalysts present the active phase totally exposed at the surface, so that their characterization corresponds to domains exposed to reactants, possibly relevant. Bulk catalysts possess an exposed surface and an unexposed bulk domain, with a surface to volume ratio that depends on the particle size: in nanomaterials the surface becomes dominant, whereas large particles will provide a signal from the bulk overwhelming that from the outermost active layer. The situation can even dynamically change during reaction, if sintering processes would aggregate small, dispersed particles, decreasing a higher surface-to-volume signal ratio.

Another point of paramount importance is the necessity to collect kinetic information to the highest degree of accuracy and relevance to provide true descriptions of the catalytic reactions. In this context, while it is important to remain within the conditions of the catalytic process,

transient experiments can provide additional evidence for intermediate species and relevant steps in the mechanism. Therefore, one of the most interesting approaches is to perform steady-state isotopic transient kinetic analysis (SSITKA) in order to keep the reaction conditions required by the investigated process, while introducing a perturbation detectable by spectroscopic analyzers thanks to isotopically substituted molecules. Specific examples will be provided in a dedicated section.

In summary, the *operando* spectroscopy methodology has been adopted by catalyst scientists even before, e.g., [35–43], but especially since the term was coined in 2002, when two research papers introduced the term and three reviews commented about who proposed the term [3, 13,14,44,45] and the interest and success of this methodology is exponentially increasing due to its ability to establish fundamental molecular structure-activity/selectivity relationships. The seminal impact of coining the term *operando* is that words create realities by fostering awareness; in this case on the relevance to design reaction in situ cells as if they were catalytic reactors. This is reflected in the literature on catalysis by circa 100 *operando* publications per year [46], the “*In-situ characterization of heterogeneous catalysts*” themed issue of Chemical Society Reviews guest-edited by B. Weckhuysen in 2010, and many other recent reviews [5,32,46]. Major research effort is currently devoted to investigate all kinds of catalytic systems, and thus new set-ups are being developed to obtain i) more realistic conditions not altered by the probe (P, T, environment, mass and heat transfer limitations, see for example [11,47]), ii) better time-resolution, [48] iii) better space-resolution in the solid [49] and fluid [50,51] phases, and iv) complementarity, coupling several techniques to obtain full characterization data [52].

In this review, instead of providing an exhaustive report of all the *operando* studies performed in the more than 20 years of this methodological approach, we will present some significant examples of energy-related investigations inserted in the context of the energetic transition that is taking place nowadays. In fact we have chosen contributions from the domains of steam reforming, Water Gas Shift, Gas To Liquid, and CO₂ hydrogenation oriented researches. But also studies in the domains of electrocatalytic conversion of energies and energy storage, which have adopted this approach, unexpectedly. We have shared the experimental tools into bulk- and surface-sensitive techniques. We have also focused on two aspects which, for us, appear of paramount importance: time and space resolution. Obviously, we will provide our point of view about the different methods and techniques too, as well as an opinion about the perspective for future developments, notably in the field of industrial process monitoring.

## 2. Electrochemistry and batteries

A field where *operando* is increasingly gaining importance is electrochemistry, a domain quite far from the original center of interest of the promoters of this approach, which was heterogeneous catalysis [14, 23,29,53]. This is a proof of the vitality of this methodology and of the inseminating strength of its scientific community.

### 2.1. Sensors

Also in the nearby field of sensors, *operando* spectroscopies have gained increasing importance [54,55]. For example, tetragonal BaTiO₃ spheroids were decorated with different percentages of Ag@CuO by wet impregnation, and their performances as sensitive layers in a micro-sensor for carbon dioxide gas was investigated. These metal nanocomposite-based sensors exhibit outstanding stability and sensitivity (6x) toward CO, very high repeatability (98%) and accuracy (96%) at 120 °C. Intermittent nanointerfaces of BaTiO₃ appear to endow high stability to Ag@CuO, accounting for improved gas-sensing performance at a very low concentration. The CO₂-sensing mechanism of was studied by DRIFT, showing that CO₂ reacts with BaO and CuO at the CuO/BaTiO₃ nanocomposite forming carbonates, which change the material
methodology [20,21], but also to make it more versatile and sensitive in time and in space [15,22–24].

The rise of *operando* methodology, along with significant advances in materials/reaction computational chemical modelling trigger a qualitative change in catalyst discovery [10], providing an alternative approach to conventional try-and-error, where the catalyst was mainly a “black box” that was characterized before and after catalytic reaction, i. e., before and after catalytic life. Such a characterization provided information hampered by multiple gaps (material, temperature, pressure) digressing from actual working conditions [12,25]. *Operando* methodology facilitated physicochemical characterization of the phases during operation. There is an entire arsenal of characterization techniques, which endow us with an eye with the sensitivity we need [16].

The use of diverse spectroscopies provides not only specific sensitivities, but also different time- and space- resolution of measurement. Space-time constraints are deeply entangled, and they are critical for implementing comprehensive *operando* mapping of catalysts. Higher space-resolution requires better time-resolution to be significant. Moreover, to fully overcome the “dimension gap”, besides reducing the size of the area to be probed, probing various areas of the solid is essential, in order to be able to identify inhomogeneities and their effect on the catalytic performance. Space-resolved *operando* is nowadays applied at the nano-, micro-, and macro-scale to study intra- and inter-reactor -catalyst and -molecule phenomena and their evolution. The progress in monitoring heterogeneous catalysis at different length and time scales has been greatly pushed and reviewed by the group of Weckhuysen [26–28] and others [15,29–31].

The development of microscopic methods and of capillary and fiber optic probes has greatly contributed to improve space-resolution [32], but also to perform multi-probe characterization. An interesting example is the *operando* study of the CrOx/Al₂O₃ extrudates deactivation in a pilot-scale propane dehydrogenation reactor. The experimental set-up combined UV/vis and Raman optical fiber probes to follow the formation of coke deposits on-line [33], which probe could be moved with respect to the catalyst bed for focusing purposes. This approach provided a monitoring tool to minimize aging due to thermal stress by decreasing temperature peaks on reoxidation and also to modulate the feed to keep the catalyst working around its optimum performance [33, 34].

For all catalyst characterization techniques, a premise remains fundamental: the chemical phase relevant to the catalytic process must be identified. Heterogeneous catalysis is an extremely surface-sensitive phenomenon; the interactions occur at the outermost layer of the catalysts, so that the characterization has to be related to the exposed surface (external or internal) of the solid to be representative of active sites. This circumscribes *operando* investigations: they must characterize the chemical structure that is relevant to the reaction and measure the activity that is derived from it. However, most spectroscopies are bulk sensitive or near-surface sensitive, while many characterization techniques present signals with contributions from both the bulk and the surface, being scarcely selective to the outermost layer. Therefore, the interplay between the bulk and the outermost domains can be critical for heterogeneous catalysis. For example, molecularly dispersed catalysts present the active phase totally exposed at the surface, so that their characterization corresponds to domains exposed to reactants, possibly relevant. Bulk catalysts possess an exposed surface and an unexposed bulk domain, with a surface to volume ratio that depends on the particle size: in nanomaterials the surface becomes dominant, whereas large particles will provide a signal from the bulk overwhelming that from the outermost active layer. The situation can even dynamically change during reaction, if sintering processes would aggregate small, dispersed particles, decreasing a higher surface-to-volume signal ratio.

Another point of paramount importance is the necessity to collect kinetic information to the highest degree of accuracy and relevance to provide true descriptions of the catalytic reactions. In this context, while it is important to remain within the conditions of the catalytic process,

transient experiments can provide additional evidence for intermediate species and relevant steps in the mechanism. Therefore, one of the most interesting approaches is to perform steady-state isotopic transient kinetic analysis (SSITKA) in order to keep the reaction conditions required by the investigated process, while introducing a perturbation detectable by spectroscopic analyzers thanks to isotopically substituted molecules. Specific examples will be provided in a dedicated section.

In summary, the *operando* spectroscopy methodology has been adopted by catalyst scientists even before, e.g., [35–43], but especially since the term was coined in 2002, when two research papers introduced the term and three reviews commented about who proposed the term [3, 13,14,44,45] and the interest and success of this methodology is exponentially increasing due to its ability to establish fundamental molecular structure-activity/selectivity relationships. The seminal impact of coining the term *operando* is that words create realities by fostering awareness; in this case on the relevance to design reaction in situ cells as if they were catalytic reactors. This is reflected in the literature on catalysis by circa 100 *operando* publications per year [46], the “*In-situ characterization of heterogeneous catalysts*” themed issue of Chemical Society Reviews guest-edited by B. Weckhuysen in 2010, and many other recent reviews [5,32,46]. Major research effort is currently devoted to investigate all kinds of catalytic systems, and thus new set-ups are being developed to obtain i) more realistic conditions not altered by the probe (P, T, environment, mass and heat transfer limitations, see for example [11,47]), ii) better time-resolution, [48] iii) better space-resolution in the solid [49] and fluid [50,51] phases, and iv) complementarity, coupling several techniques to obtain full characterization data [52].

In this review, instead of providing an exhaustive report of all the *operando* studies performed in the more than 20 years of this methodological approach, we will present some significant examples of energy-related investigations inserted in the context of the energetic transition that is taking place nowadays. In fact we have chosen contributions from the domains of steam reforming, Water Gas Shift, Gas To Liquid, and CO₂ hydrogenation oriented researches. But also studies in the domains of electrocatalytic conversion of energies and energy storage, which have adopted this approach, unexpectedly. We have shared the experimental tools into bulk- and surface-sensitive techniques. We have also focused on two aspects which, for us, appear of paramount importance: time and space resolution. Obviously, we will provide our point of view about the different methods and techniques too, as well as an opinion about the perspective for future developments, notably in the field of industrial process monitoring.

## 2. Electrochemistry and batteries

A field where *operando* is increasingly gaining importance is electrochemistry, a domain quite far from the original center of interest of the promoters of this approach, which was heterogeneous catalysis [14, 23,29,53]. This is a proof of the vitality of this methodology and of the inseminating strength of its scientific community.

### 2.1. Sensors

Also in the nearby field of sensors, *operando* spectroscopies have gained increasing importance [54,55]. For example, tetragonal BaTiO₃ spheroids were decorated with different percentages of Ag@CuO by wet impregnation, and their performances as sensitive layers in a micro-sensor for carbon dioxide gas was investigated. These metal nanocomposite-based sensors exhibit outstanding stability and sensitivity (6x) toward CO, very high repeatability (98%) and accuracy (96%) at 120 °C. Intermittent nanointerfaces of BaTiO₃ appear to endow high stability to Ag@CuO, accounting for improved gas-sensing performance at a very low concentration. The CO₂-sensing mechanism of was studied by DRIFT, showing that CO₂ reacts with BaO and CuO at the CuO/BaTiO₃ nanocomposite forming carbonates, which change the material
resistance [56].

Suzer et al. reported the use of operando XPS for chemical and electrical measurement of a CdS-based light-dependent resistor, providing analyses of states under actual working conditions [57], a report very relevant to electrochemical changes.

## 2.2. Electrocatalysis

More natural is the use of operando approaches in electrocatalysis. Jiang et al. have recently used operando XAFS measurement combined with XANES simulations to investigate the dynamic structural changes in the active site of an atomically dispersed Ni-N-C catalyst with a

<figure><img src="image_2.png" /></figure>

**Fig. 1.** (a) High-resolution N 1s XPS spectra of Ni-N-C and N-C. (b) N K-edge XAS spectra of Ni-N-C and N-C. (c) Ni K-edge XANES spectra of Ni-N-C, Ni foil, NiPc, and Ni(OH)2. The inset gives an enlargement of the pre-edge peaks of Ni-N-C and NiPc, where the colored areas represent peak intensity. (d) Wavelet transforms of the k3-weighted Ni K-edge EXAFS spectra of Ni foil, Ni(OH)₂, and Ni-N-C. (e) Comparison between the K-edge XANES experimental spectrum of Ni-N-C and the theoretical spectrum of a disordered pyrrole-type Ni-N₄O₂ moiety. The inset gives a schematic model of a disordered pyrrole-type Ni-N₄ moiety. Color scheme: Ni (light blue), N (blue), O (red), C (gray). D represents the distance from the offset Ni atom to the N4 plane. (Reprinted with permission from B. Mei, C. Liu, F. Sun, S. Lu, X. Du, X. Li, F. Song, W. Xu, Z. Jiang, Unraveling the Potential-Dependent Volcanic Selectivity Changes of an Atomically Dispersed Ni Catalyst During CO₂ Reduction, ACS Catal. 12 (2022) 8676–8686. Copyright 2022 American Chemical Society.)[58]
resistance [56].

Suzer et al. reported the use of operando XPS for chemical and electrical measurement of a CdS-based light-dependent resistor, providing analyses of states under actual working conditions [57], a report very relevant to electrochemical changes.

## 2.2. Electrocatalysis

More natural is the use of operando approaches in electrocatalysis. Jiang et al. have recently used operando XAFS measurement combined with XANES simulations to investigate the dynamic structural changes in the active site of an atomically dispersed Ni-N-C catalyst with a

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** (a) High-resolution N 1s XPS spectra of Ni-N-C and N-C. (b) N K-edge XAS spectra of Ni-N-C and N-C. (c) Ni K-edge XANES spectra of Ni-N-C, Ni foil, NiPc, and Ni(OH)2. The inset gives an enlargement of the pre-edge peaks of Ni-N-C and NiPc, where the colored areas represent peak intensity. (d) Wavelet transforms of the k3-weighted Ni K-edge EXAFS spectra of Ni foil, Ni(OH)₂, and Ni-N-C. (e) Comparison between the K-edge XANES experimental spectrum of Ni-N-C and the theoretical spectrum of a disordered pyrrole-type Ni-N₄O₂ moiety. The inset gives a schematic model of a disordered pyrrole-type Ni-N₄ moiety. Color scheme: Ni (light blue), N (blue), O (red), C (gray). D represents the distance from the offset Ni atom to the N4 plane. (Reprinted with permission from B. Mei, C. Liu, F. Sun, S. Lu, X. Du, X. Li, F. Song, W. Xu, Z. Jiang, Unraveling the Potential-Dependent Volcanic Selectivity Changes of an Atomically Dispersed Ni Catalyst During CO₂ Reduction, ACS Catal. 12 (2022) 8676–8686. Copyright 2022 American Chemical Society.)[58]</figcaption></figure>
disordered pyrrole-type NiN<sub>4</sub>O<sub>2</sub> configuration during electrochemical CO<sub>2</sub> reduction. The catalyst exhibited selectivity changes presenting a potential-dependent volcano-like curve. The *operando* XAS revealed that the best selectivity is reached when the distance between the Ni atom and the carbon basal plane is optimal, whereas it decreases as the distance changes, induced by a potential shift. This was justified by theoretical calculations as due to the distribution of the d band center of Ni modified by the local dynamic behaviors (Fig. 1). These findings provide clear indications for the design of single site catalysts [58].

Electrochemical Raman spectroscopy has been widely used to monitor structural changes of catalysts during CO₂ electroreduction. Among these, copper-ceria catalysts have delivered significantly different product distribution with ethylene yield higher than that of ethanol. The low CO coverage and the unfavorable adsorption of key intermediates (such as CH₃CHO* and CH₃CH₂O*) on the copper sites appear to determine this. An *operando* study shows that a key factor is related with surface oxide/hydroxide distribution. Hydroxide and oxide doping of the catalyst surface tunes the adsorbed hydrogen on copper. DFT calculations indicate that this would accelerate water dissociation changing hydrogen adsorption energy on copper. *Operando* Raman studies show that adsorbed hydrogen react with surface *HCCOH*, a key intermediate, whose steering towards ethanol rather than ethylene [59]. CuZn bimetallic catalysts synthesized by *in situ* electrochemical reduction exhibit a ZnO-shell/CuO-core structure. Zn atoms blend with Cu at the surface of this core-shell moiety, shifting selectivity towards ethanol. The *operando* Raman study shows that Zn modifies CO binding on Cu sites. Grätzel et al. propose that CO and adsorbed *CH₃ would combine into *COCH₃ intermediate to ethanol [60]. Catalyst porosity can deliver significant advantages to investigate multiproton/multielectron reactions. For instance, a detailed identification of adsorbed CO species was enhanced by the use of copper dendritic foam structures, which facilitates surface-enhanced Raman spectroscopy (SERS), allowing to identify two types of adsorbed CO [61,62]. Dutta et al. reported the use of a bismuth oxide nanofoam to analyze in detail the evolution of the oxide to metal though carbonates and substoichiometric carbonate stages, connecting this with the electrocatalytic performance, providing real-time evidence of CO₂ integration into the oxidic Bi₂O₃ matrix (forming a sub-carbonate) at low overpotentials prior to and during the CO₂ reduction reaction. Similar to systems commented above, intermediate oxide species affect the evolution of formate species and connect those to the evolution of bismuth phases [63].

## 2.3. Electrochemistry

In the electrochemical sector, in *situ* and *operando* diffraction techniques are mainly used to analyse electrodes and batteries at work [64]. Here the development of *in situ* and *operando* cells has made great progresses (Fig. 2) [65] and it is mainly targeted at searching the structure vs. performance parameters (see for example Fig. 3).

The influence of the preparation method and of the morphology of the BPO₄ matrix on the electrochemical properties of two Sn/BPO₄ composites was investigated modus operandi by combining both a long-range technique such as XRD and a molecular spectroscopy, i.e., Mössbauer. While the first technique evidenced the gradual amorphization of the starting tin in the composite and the impact of the reaction with lithium on the crystallinity of the BPO₄ matrix, the second allowed to follow in detail the gradual reaction of tin with lithium, including the formation of Li-Sn alloys increasingly rich in lithium. This provided important indications about the cycling behaviour of the composite and strategies to avoid deactivation [67]. The same approach allowed to uncover the alloying mechanism of formation of SnSb electrodes for Li-ion batteries and the reversible formation of different Li-Sn phases, the specific properties of their interfaces, as well as their failure mechanism for the long-term cycling stability [66]. Borkiewicz et al. developed a versatile, multi-purpose electrochemical cell suitable for *operando* and *in situ* measurements of battery materials during

electrochemical cycling using a variety of X-ray techniques, providing greater insight into the complex processes that occur in operating batteries thanks to time-dependent kinetic studies and the observation of transient species [68]. Next generation of highly performing rechargeable Li-S batteries was also investigated by performing XRD on the cells during operation, monitoring the entire charge/discharge cycle in real time, which demonstrated that the loss of polysulfides is the main reason for the reduction of the batteries lifetime and suggested efficient fallback solutions [69], while the active material changes constantly its form between solid and liquid phases [70]. The electrochemical behavior of sulfur-rich copolymers was also revealed by ATR-IR by monitoring the evolution of the C–S peak position and cyclic changes in the S–S bond stretching in sulfur-diisopropenylbenzene copolymers (the active material in Li–S batteries) at different potentials during cyclic voltammetry sweep. The IR response of various copolymers with different sulfur wt% indicated that the C–S bond in sulfur copolymers is not active in the voltage window of Li–S batteries, and that a significant change in electrochemical behavior of the copolymers depends on the sulphur content [71]. In the domain of post lithium-ion batteries, an *operando* attenuated total reflectance infrared spectroscopy method has been developed to visualise the changes within IR bands as a consequence of reduction/oxidation processes. *Operando* IR spectroscopy was first applied to the analysis of three different organic polymers in lithium batteries, then extended to the investigation of redox reaction mechanisms of poly(anthraquinonyl sulfide) in a magnesium battery, where a reduction of a carbonyl bond has been highlighted as a fundamental step of the electrochemical activity [72]. *In situ* ATR was also used to monitor both polysulfide speciation and triflate anion (electrolyte) coordination state while discharging/charging a full battery coin cell [73]. In this sense, important progresses were recently made by developing a novel dual-IR window/electrode *operando* infrared spectroscopy method which offers the advantage of accessing both, anode and cathode processes quasi-simultaneously by rotating the battery cell by 180° stepwise while keeping the same *operando* conditions. The proof-of-concept studies demonstrated clearly its investigative advantages in gaining broader insights into the chemistry of batteries during charging/discharging and beyond [74]. In the case of Mg-batteries, organo-aluminate electrolytes have been investigated by ATR-IR using two different spectroelectrochemical cell configurations: using a Pt gauze as a working electrode, or deposing a thin Pt film directly on the surface of the ATR crystal. The different spatial arrangements of the working electrode with respect to the ATR sampling volume induced substantial differences in the spectral response, so highlighting the crucial role of the cell design [10.1021/acs.analchem.2c02843]. *Operando* X-ray diffraction, together with X-ray absorption and DFT calculations was also at the base of the elucidation of the mechanism providing the superior properties of Fe-Na sustainable batteries. The formation of a superlattice originating from the Na⁺/vacancy ordering for Na<sub>0.5</sub>Fe<sub>1/2</sub>Co<sub>1/2</sub>O<sub>2</sub>, was proposed to involve a potential step in the charge/discharge profiles [75]. The complementarity of the data obtained from XAS, Raman spectroscopies, and XRD, all in *operando* mode, was crucial to understand the complex mechanism of the electrochemical lithiation of germanium, based on reversible formation of various crystalline and amorphous phases [76].

The efforts towards low-cost, non-precious metal, stable oxygen evolution reaction electrocatalysts, such as nickel-based, iron-based, or cobalt-based catalysts require detailed molecular understanding of the oxygen evolution reaction mechanism (OER). OER is the performance-limiting half reaction of water splitting, which can be used to produce hydrogen fuel using renewable energies. A number of transition metal oxides and oxyhydroxides have been developed as promising OER catalysts in alkaline media. The combination of electrochemical and isotopic *in situ* and *operando* methodologies have afforded mechanistic insights on OER; Hu et al. identified superoxide species as an intermediate, which forms at the same time as CoOOH oxidizes to CoO<sub>2</sub>, prior to oxygen evolution [77]. Engineering the catalyst structure allows
disordered pyrrole-type NiN<sub>4</sub>O<sub>2</sub> configuration during electrochemical CO<sub>2</sub> reduction. The catalyst exhibited selectivity changes presenting a potential-dependent volcano-like curve. The *operando* XAS revealed that the best selectivity is reached when the distance between the Ni atom and the carbon basal plane is optimal, whereas it decreases as the distance changes, induced by a potential shift. This was justified by theoretical calculations as due to the distribution of the d band center of Ni modified by the local dynamic behaviors (Fig. 1). These findings provide clear indications for the design of single site catalysts [58].

Electrochemical Raman spectroscopy has been widely used to monitor structural changes of catalysts during CO₂ electroreduction. Among these, copper-ceria catalysts have delivered significantly different product distribution with ethylene yield higher than that of ethanol. The low CO coverage and the unfavorable adsorption of key intermediates (such as CH₃CHO* and CH₃CH₂O*) on the copper sites appear to determine this. An *operando* study shows that a key factor is related with surface oxide/hydroxide distribution. Hydroxide and oxide doping of the catalyst surface tunes the adsorbed hydrogen on copper. DFT calculations indicate that this would accelerate water dissociation changing hydrogen adsorption energy on copper. *Operando* Raman studies show that adsorbed hydrogen react with surface *HCCOH*, a key intermediate, whose steering towards ethanol rather than ethylene [59]. CuZn bimetallic catalysts synthesized by *in situ* electrochemical reduction exhibit a ZnO-shell/CuO-core structure. Zn atoms blend with Cu at the surface of this core-shell moiety, shifting selectivity towards ethanol. The *operando* Raman study shows that Zn modifies CO binding on Cu sites. Grätzel et al. propose that CO and adsorbed *CH₃ would combine into *COCH₃ intermediate to ethanol [60]. Catalyst porosity can deliver significant advantages to investigate multiproton/multielectron reactions. For instance, a detailed identification of adsorbed CO species was enhanced by the use of copper dendritic foam structures, which facilitates surface-enhanced Raman spectroscopy (SERS), allowing to identify two types of adsorbed CO [61,62]. Dutta et al. reported the use of a bismuth oxide nanofoam to analyze in detail the evolution of the oxide to metal though carbonates and substoichiometric carbonate stages, connecting this with the electrocatalytic performance, providing real-time evidence of CO₂ integration into the oxidic Bi₂O₃ matrix (forming a sub-carbonate) at low overpotentials prior to and during the CO₂ reduction reaction. Similar to systems commented above, intermediate oxide species affect the evolution of formate species and connect those to the evolution of bismuth phases [63].

## 2.3. Electrochemistry

In the electrochemical sector, in *situ* and *operando* diffraction techniques are mainly used to analyse electrodes and batteries at work [64]. Here the development of *in situ* and *operando* cells has made great progresses (Fig. 2) [65] and it is mainly targeted at searching the structure vs. performance parameters (see for example Fig. 3).

The influence of the preparation method and of the morphology of the BPO₄ matrix on the electrochemical properties of two Sn/BPO₄ composites was investigated modus operandi by combining both a long-range technique such as XRD and a molecular spectroscopy, i.e., Mössbauer. While the first technique evidenced the gradual amorphization of the starting tin in the composite and the impact of the reaction with lithium on the crystallinity of the BPO₄ matrix, the second allowed to follow in detail the gradual reaction of tin with lithium, including the formation of Li-Sn alloys increasingly rich in lithium. This provided important indications about the cycling behaviour of the composite and strategies to avoid deactivation [67]. The same approach allowed to uncover the alloying mechanism of formation of SnSb electrodes for Li-ion batteries and the reversible formation of different Li-Sn phases, the specific properties of their interfaces, as well as their failure mechanism for the long-term cycling stability [66]. Borkiewicz et al. developed a versatile, multi-purpose electrochemical cell suitable for *operando* and *in situ* measurements of battery materials during

electrochemical cycling using a variety of X-ray techniques, providing greater insight into the complex processes that occur in operating batteries thanks to time-dependent kinetic studies and the observation of transient species [68]. Next generation of highly performing rechargeable Li-S batteries was also investigated by performing XRD on the cells during operation, monitoring the entire charge/discharge cycle in real time, which demonstrated that the loss of polysulfides is the main reason for the reduction of the batteries lifetime and suggested efficient fallback solutions [69], while the active material changes constantly its form between solid and liquid phases [70]. The electrochemical behavior of sulfur-rich copolymers was also revealed by ATR-IR by monitoring the evolution of the C–S peak position and cyclic changes in the S–S bond stretching in sulfur-diisopropenylbenzene copolymers (the active material in Li–S batteries) at different potentials during cyclic voltammetry sweep. The IR response of various copolymers with different sulfur wt% indicated that the C–S bond in sulfur copolymers is not active in the voltage window of Li–S batteries, and that a significant change in electrochemical behavior of the copolymers depends on the sulphur content [71]. In the domain of post lithium-ion batteries, an *operando* attenuated total reflectance infrared spectroscopy method has been developed to visualise the changes within IR bands as a consequence of reduction/oxidation processes. *Operando* IR spectroscopy was first applied to the analysis of three different organic polymers in lithium batteries, then extended to the investigation of redox reaction mechanisms of poly(anthraquinonyl sulfide) in a magnesium battery, where a reduction of a carbonyl bond has been highlighted as a fundamental step of the electrochemical activity [72]. *In situ* ATR was also used to monitor both polysulfide speciation and triflate anion (electrolyte) coordination state while discharging/charging a full battery coin cell [73]. In this sense, important progresses were recently made by developing a novel dual-IR window/electrode *operando* infrared spectroscopy method which offers the advantage of accessing both, anode and cathode processes quasi-simultaneously by rotating the battery cell by 180° stepwise while keeping the same *operando* conditions. The proof-of-concept studies demonstrated clearly its investigative advantages in gaining broader insights into the chemistry of batteries during charging/discharging and beyond [74]. In the case of Mg-batteries, organo-aluminate electrolytes have been investigated by ATR-IR using two different spectroelectrochemical cell configurations: using a Pt gauze as a working electrode, or deposing a thin Pt film directly on the surface of the ATR crystal. The different spatial arrangements of the working electrode with respect to the ATR sampling volume induced substantial differences in the spectral response, so highlighting the crucial role of the cell design [10.1021/acs.analchem.2c02843]. *Operando* X-ray diffraction, together with X-ray absorption and DFT calculations was also at the base of the elucidation of the mechanism providing the superior properties of Fe-Na sustainable batteries. The formation of a superlattice originating from the Na⁺/vacancy ordering for Na<sub>0.5</sub>Fe<sub>1/2</sub>Co<sub>1/2</sub>O<sub>2</sub>, was proposed to involve a potential step in the charge/discharge profiles [75]. The complementarity of the data obtained from XAS, Raman spectroscopies, and XRD, all in *operando* mode, was crucial to understand the complex mechanism of the electrochemical lithiation of germanium, based on reversible formation of various crystalline and amorphous phases [76].

The efforts towards low-cost, non-precious metal, stable oxygen evolution reaction electrocatalysts, such as nickel-based, iron-based, or cobalt-based catalysts require detailed molecular understanding of the oxygen evolution reaction mechanism (OER). OER is the performance-limiting half reaction of water splitting, which can be used to produce hydrogen fuel using renewable energies. A number of transition metal oxides and oxyhydroxides have been developed as promising OER catalysts in alkaline media. The combination of electrochemical and isotopic *in situ* and *operando* methodologies have afforded mechanistic insights on OER; Hu et al. identified superoxide species as an intermediate, which forms at the same time as CoOOH oxidizes to CoO<sub>2</sub>, prior to oxygen evolution [77]. Engineering the catalyst structure allows
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Development of in situ X-ray diffraction cell designs (reprinted with permission from A.M. Tripathi, W.-N. Su, B.J. Hwang, In situ analytical techniques for battery interface analysis, Chem Soc Rev. 47 (2018) 736–851. Copyright Royal Society of Chemistry 2018)[65]</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Development of in situ X-ray diffraction cell designs (reprinted with permission from A.M. Tripathi, W.-N. Su, B.J. Hwang, In situ analytical techniques for battery interface analysis, Chem Soc Rev. 47 (2018) 736–851. Copyright Royal Society of Chemistry 2018)[65]</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** local changes in tin environments induced by the electrochemical reaction of SnSb with Li monitored using room temperature operando $^{119}$Sn Mössbauer spectroscopy. The Mössbauer spectra collected for two battery cycles are shown together with the corresponding voltage-composition profile
(P. Antitomaso, B. Fraisse, L. Stievano, S. Biscaglia, D. Aymé-Perrot, P. Girard, M.T. Sougrati, L. Monconduit, SnSb electrodes for Li-ion batteries: the electrochemical mechanism and capacity fading origins elucidated by using operando techniques, J Mater Chem A. 5 (2017) 6546–6555. Copyright Royal Society of Chemistry 2017) [66].</figcaption></figure>

steering the reaction mechanism from bifunctional to transfer steps
mechanism. FeOOH-NiOOH operates on a bifunctional mechanism first,
the rate-determining O-O bond formation via an the OH⁻ attack on a
Fe=O, coupled with a hydrogen atom transfer to a Ni³⁺-O site. The
highly active OER NiFe layered double hydroxide (LDH) operates by a
conventional mechanism of four consecutive proton-coupled electron
transfer steps. While nickel iron oxyhydroxide is the benchmark catalyst
for the OER in alkaline medium its reaction mechanism remains to be
fully disclosed. Wei at al. addressed the intrinsic mechanism at in-
terfaces combining operando ultraviolet-visible and Raman insight on
the surface reconstruction of FeOOH@NiFe layered double hydroxides
(LDH). This work revealed that FeOOH@NiFe LDH converted into
highly active FeOOH@ β-Ni(Fe)OOH phases during the OER. Operando
UV-vis data underscore a lower oxidation state of Ni during OER [78].
The role of Fe and Ni ions remains under debate; each species appears to
follow a different mechanism. Ni sites evolve O₂ through an active ox-
ygen species (Ni-O-O·) and with lattice oxygen participation, whereas Fe
sites evolve O₂ without lattice oxygen participation [79]. This should
lead to significantly different reactivities on each site. Hu et al. have
used oxygen isotopic labeling and operando Raman methodology to
discriminate the individual turnover frequency numbers (TOFs) of both
Ni and Fe sites for a series of Ni and NiFe layered double hydroxides
(LDHs). Fe sites have TOFs 20 – 200 times higher than those of the Ni
sites [80]. Increasing Fe contents leads to larger structural disorder of
the NiOOH host, as evidenced by the relative intensities of Raman modes

at 560 vs. 480 cm⁻¹. The peaks at 560 and 480 cm⁻¹ are associated with
the polarized A₁g mode (stretching) and the depolarized E_g mode
(bending) of g -NiOOH, respectively. These peaks reflect the changes in
local structure of nickel lattice disorder within sheets and interlayer
spacings. The decrease of activity upon Fe-content induced disorder on
NiFe LDH suggests structural ordering as a strategy to improve OER
catalysts, which was validated by mechanochemical generation of defect
and by the use of CsOH solutions [80].

Calcogenides also exhibit interesting features on OER as precatalysts. Thus, cobalt sulfide (CoSx) nanovesicles decorated with iron sulfide (FeSx) were prepared from a metal-organic framework precursor, resulting in a CoSx/FeSx with much higher OER activity than the pristine CoSx counterpart. Operando Raman and X-ray absorption show the electrocatalytically active species transformation in CoSx nanovesicles. The electrocatalytically active cobalt oxyhydroxides could more easily form on cobalt sulfide surface during OER and thus contributed to the highly improved performance [81].

2.4. *Photoelectrochemistry*

Photoelectrochemical water splitting is being explored by operando
Raman methodology, using UV-cut filters to prevent interference with
the Raman signal. Wei et al. have addressed the study on molybdenum
dichalcogenides to assess reaction intermediates for hydrogen evolution
reaction (HER). The study benchmarked ternary Mo-S-Se vs. its Mo-S
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** local changes in tin environments induced by the electrochemical reaction of SnSb with Li monitored using room temperature operando $^{119}$Sn Mössbauer spectroscopy. The Mössbauer spectra collected for two battery cycles are shown together with the corresponding voltage-composition profile
(P. Antitomaso, B. Fraisse, L. Stievano, S. Biscaglia, D. Aymé-Perrot, P. Girard, M.T. Sougrati, L. Monconduit, SnSb electrodes for Li-ion batteries: the electrochemical mechanism and capacity fading origins elucidated by using operando techniques, J Mater Chem A. 5 (2017) 6546–6555. Copyright Royal Society of Chemistry 2017) [66].</figcaption></figure>

steering the reaction mechanism from bifunctional to transfer steps
mechanism. FeOOH-NiOOH operates on a bifunctional mechanism first,
the rate-determining O-O bond formation via an the OH⁻ attack on a
Fe=O, coupled with a hydrogen atom transfer to a Ni³⁺-O site. The
highly active OER NiFe layered double hydroxide (LDH) operates by a
conventional mechanism of four consecutive proton-coupled electron
transfer steps. While nickel iron oxyhydroxide is the benchmark catalyst
for the OER in alkaline medium its reaction mechanism remains to be
fully disclosed. Wei at al. addressed the intrinsic mechanism at in-
terfaces combining operando ultraviolet-visible and Raman insight on
the surface reconstruction of FeOOH@NiFe layered double hydroxides
(LDH). This work revealed that FeOOH@NiFe LDH converted into
highly active FeOOH@ β-Ni(Fe)OOH phases during the OER. Operando
UV-vis data underscore a lower oxidation state of Ni during OER [78].
The role of Fe and Ni ions remains under debate; each species appears to
follow a different mechanism. Ni sites evolve O₂ through an active ox-
ygen species (Ni-O-O·) and with lattice oxygen participation, whereas Fe
sites evolve O₂ without lattice oxygen participation [79]. This should
lead to significantly different reactivities on each site. Hu et al. have
used oxygen isotopic labeling and operando Raman methodology to
discriminate the individual turnover frequency numbers (TOFs) of both
Ni and Fe sites for a series of Ni and NiFe layered double hydroxides
(LDHs). Fe sites have TOFs 20 – 200 times higher than those of the Ni
sites [80]. Increasing Fe contents leads to larger structural disorder of
the NiOOH host, as evidenced by the relative intensities of Raman modes

at 560 vs. 480 cm⁻¹. The peaks at 560 and 480 cm⁻¹ are associated with
the polarized A₁g mode (stretching) and the depolarized E_g mode
(bending) of g -NiOOH, respectively. These peaks reflect the changes in
local structure of nickel lattice disorder within sheets and interlayer
spacings. The decrease of activity upon Fe-content induced disorder on
NiFe LDH suggests structural ordering as a strategy to improve OER
catalysts, which was validated by mechanochemical generation of defect
and by the use of CsOH solutions [80].

Calcogenides also exhibit interesting features on OER as precatalysts. Thus, cobalt sulfide (CoSx) nanovesicles decorated with iron sulfide (FeSx) were prepared from a metal-organic framework precursor, resulting in a CoSx/FeSx with much higher OER activity than the pristine CoSx counterpart. Operando Raman and X-ray absorption show the electrocatalytically active species transformation in CoSx nanovesicles. The electrocatalytically active cobalt oxyhydroxides could more easily form on cobalt sulfide surface during OER and thus contributed to the highly improved performance [81].

2.4. *Photoelectrochemistry*

Photoelectrochemical water splitting is being explored by operando
Raman methodology, using UV-cut filters to prevent interference with
the Raman signal. Wei et al. have addressed the study on molybdenum
dichalcogenides to assess reaction intermediates for hydrogen evolution
reaction (HER). The study benchmarked ternary Mo-S-Se vs. its Mo-S
and Mo-Se binary counterparts. Mo-Se system exhibited the presence of Se-H stretching modes (2365 cm⁻¹) the presence of such an intermediate is consistent with DFT simulations. The ternary MoS₀.₉Se₁.₁ sample exhibits higher current density than its binary counterparts. Its *operando* Raman spectra exhibit the modes at 360, 384, 259, and 298 cm⁻¹, characteristic of S-Mo and Se-Mo; in addition, Se-H and S-H modes are apparent and increased their intensity with linear voltage sweep, which is followed by the redox reaction. The authors report an exponential relationship between the quantity of reactive electrons and the Raman intensity of intermediate species [82], which can be used to estimate photoelectrochemical H₂ production. The role of other chalcogenides in HER and OER as catalysts themselves or pre-catalysts is under debate. A recent work by Chen et al. uses *operando* and *in situ* X-ray absorption spectroscopy, liquid-phase transmission electron microscopy, and in situ Raman spectroscopy to assess the stability of P-substituted CoSe₂ electrocatalysts under both hydrogen and oxygen evolution reactions in an alkaline electrolyte. The work shows that this is a precursor and that the promoting effect of phosphorous is to trigger defects around cobalt sites facilitation the chalcogenide decomposition into its metallic and oxyhydroxide counterparts [83].

Concerning energy storage, *operando* spectroscopies are increasingly used to understand the mechanism of electrochemical reactions in batteries [65]. For example, ATR has been recently developed to visualise the changes of all infrared active bands that occur as a consequence of reduction/oxidation processes in different organic polymer materials in lithium batteries, and to investigate the redox reaction pathway of poly (anthraquinonyl sulfide) in a magnesium battery, where a reduction of carbonyl bond is demonstrated to be the mechanism for the electrochemical activity [72]. The group of Smotkin has deeply applied in situ and *operando* techniques to the components of electrochemical cells; his group has summarized their vision in a recent review [84]. For example, combining time-dependent IR with DFT calculations to the hydration/dehydration of Nafion membranes, they have identified bands associated to sulfonic acid and sulfonates, which can be used as a signal source for a low partial pressure water sensing [85,86]. *Operando* Raman of TO-LO phonons and synchrotron X-ray diffraction of silicon investigate the evolution of strain upon lithiation/delithiation in Li-ion battery anodes made of crystalline silicon nanoparticles (SiNP). Such cycles generate "crystalline core-amorphous shell" particles, which evolution is uncovered by a step-wise decrease of core size, as well as silicon strain induced by the shell [87]. *In situ* and *operando* multiple-internal-reflection infrared spectroscopy performed in an original and appropriate configuration efficiently extracted information on the surface and bulk phenomena affecting Li-ion electrodes during their operation in realistic conditions, giving an insight into the solid/electrolyte-interphase composition and dynamic behaviour [88].

A binder-free model Li/S cell N-doped reduced graphene oxide/carbon monolith cathode was studied by a combined electrochemical impedance spectroscopy (EIS) and X-ray radiography characterization during galvanostatical charge/discharge cycles [89] and they report the formation of sulfur dendrites. Imaging and electrochemistry blend in the scanning electrochemical microscopy (SCEM),[90]; Fermin and coworkers investigate hydrogen evolution reaction (HER) catalyzed by two-dimensional assemblies palladium nanoparticles, comparing their performance to its bulk metal counterpart [91].

It is thus clear that *operando* investigations occupy now a consolidated place in electrocatalysis, and contribute to the development of future batteries and components for fuel cells.

### 3. Bulk structures vs. performance

The structure of materials is currently determined using X-ray diffraction, consistently an *operando* approach has been developed also for this technique, in order to appreciate structural changes under duty as well. Obviously advanced diffraction techniques are of paramount importance also for the structural determination of heterogeneous

catalysts [92]. Recently, Newton et al. critically assessed the diverse possibilities available in the high energy X-ray domain as a result of the maturation of third generation synchrotron facilities and looked forward to the potential that forthcoming developments in synchrotron source technology may offer to the world of catalysis in the near future [93].

An interesting report of advantages and limitations of XAFS and XRD combined techniques under synchrotron radiation was published by Frenkel et al. [94]. Besides underlining the added value of combining the sensitivity of XRD to the long-range order within the material, and that of XAFS to the nearest environment around absorbing atoms, they also demonstrated how to reach a better understanding of the structure and kinetics of a catalyst during its chemical treatment. In particular, they studied the inverse Cu/ceria catalyst and revealed the formation of highly disordered, minority species along the two reaction routes: the CO reduction and O₂ reoxidation of the catalyst, showing that independent studies of such systems by either XAFS or XRD alone produce incomplete results. Moreover, they pointed out the exciting new possibility to study heterogeneous catalysis by in situ diffraction anomalous fine structure (DAFS). T. Li et al. have recently reviewed the combination of *in situ/operando* XAS and SAXS to provide high temporal and spatial resolution when monitoring the evolution of nanomaterials, both during their synthesis and at work [95]. The combination of XAS, XRD and Raman in a single experiment, while collecting catalytic data, highlighted the structural and compositional changes of the partially reduced Fe₂O₃ and 3% chromium oxide-modified iron oxide before, during, and after the water gas shift (WGS) reaction, associating a marked structural disorder and compositional heterogeneity with their catalytically active states. The role of Cr in stabilizing the low-temperature γ-Fe₂O₃ phase and the possible deactivation mechanism were also described [96]. A similar approach, but replacing Raman by DRIFT, was carried out on ceria support loaded with Pt and Ru; it assessed the activity and selectivity of the Pt-Ru alloy and the marginal role of formates observed on the catalyst surface [97]. Ce-based systems were also diffusely investigated for the steam reforming of ethanol: in situ XRD evidenced the interactions between Ni and ceria support, while *operando* DRIFT spectra provided indications on the role of ethoxy, acetate, carbonate and hydroxyl species in the reaction pathway [98]. Similar studies of ethanol and ethylene steam reforming were carried out on Co/CeO₂. In this case XRD allowed understanding the correlation between the particle size and the reactivity (Fig. 4): nanoparticles of ceria showed to be more active (even if less selective) and resistant to coking, whereas the catalysts supported on larger ceria particles suffered from coke formation. On another hand, *operando* DRIFTS uncover different surface species, including some surface species, which could be a precursor for ethylene. Pulse methanol oxidation experiments were performed to probe the surface sites of the catalysts. The results indicated a higher density of redox sites on the Co/CeO₂ nanoparticles catalyst, associated to a higher reducibility, as shown by *operando* EXAFS [99]. These techniques provided important support to the comprehension of CO oxidation as well. For example, in the case of Cu-ceria catalysts, the combination of XRD with Raman, DRIFT, HREM, XEDS, XPS and XAFS allowed to correlate the superior PROX performances to the degree of support-promoted reduction, achieved by the dispersed copper oxide particles under reaction conditions. However, it was also shown that catalysts which display higher CO oxidation activity are generally more efficient also for the undesired H₂ oxidation reaction, having structural differences proved by the characterisation techniques [100] and related to the copper-ceria interface [101], to the copper oxidation state [102] and to the exposed crystal planes [103]. Behm et al. identified the physical origin of the activation/deactivation and loss in selectivity of a Ru/TiO₂ methanation catalyst by XANES/EXAFS measurements, revealing that an initial activation phase is largely due to the reduction of oxidized Ru species, together with a distinct change in the Ru particle shape, until reaching a state dominated by metallic Ru species with the highest Ru mass normalized activity. The losses of activity and selectivity during the subsequent deactivation phase were
and Mo-Se binary counterparts. Mo-Se system exhibited the presence of Se-H stretching modes (2365 cm⁻¹) the presence of such an intermediate is consistent with DFT simulations. The ternary MoS₀.₉Se₁.₁ sample exhibits higher current density than its binary counterparts. Its *operando* Raman spectra exhibit the modes at 360, 384, 259, and 298 cm⁻¹, characteristic of S-Mo and Se-Mo; in addition, Se-H and S-H modes are apparent and increased their intensity with linear voltage sweep, which is followed by the redox reaction. The authors report an exponential relationship between the quantity of reactive electrons and the Raman intensity of intermediate species [82], which can be used to estimate photoelectrochemical H₂ production. The role of other chalcogenides in HER and OER as catalysts themselves or pre-catalysts is under debate. A recent work by Chen et al. uses *operando* and *in situ* X-ray absorption spectroscopy, liquid-phase transmission electron microscopy, and in situ Raman spectroscopy to assess the stability of P-substituted CoSe₂ electrocatalysts under both hydrogen and oxygen evolution reactions in an alkaline electrolyte. The work shows that this is a precursor and that the promoting effect of phosphorous is to trigger defects around cobalt sites facilitation the chalcogenide decomposition into its metallic and oxyhydroxide counterparts [83].

Concerning energy storage, *operando* spectroscopies are increasingly used to understand the mechanism of electrochemical reactions in batteries [65]. For example, ATR has been recently developed to visualise the changes of all infrared active bands that occur as a consequence of reduction/oxidation processes in different organic polymer materials in lithium batteries, and to investigate the redox reaction pathway of poly (anthraquinonyl sulfide) in a magnesium battery, where a reduction of carbonyl bond is demonstrated to be the mechanism for the electrochemical activity [72]. The group of Smotkin has deeply applied in situ and *operando* techniques to the components of electrochemical cells; his group has summarized their vision in a recent review [84]. For example, combining time-dependent IR with DFT calculations to the hydration/dehydration of Nafion membranes, they have identified bands associated to sulfonic acid and sulfonates, which can be used as a signal source for a low partial pressure water sensing [85,86]. *Operando* Raman of TO-LO phonons and synchrotron X-ray diffraction of silicon investigate the evolution of strain upon lithiation/delithiation in Li-ion battery anodes made of crystalline silicon nanoparticles (SiNP). Such cycles generate "crystalline core-amorphous shell" particles, which evolution is uncovered by a step-wise decrease of core size, as well as silicon strain induced by the shell [87]. *In situ* and *operando* multiple-internal-reflection infrared spectroscopy performed in an original and appropriate configuration efficiently extracted information on the surface and bulk phenomena affecting Li-ion electrodes during their operation in realistic conditions, giving an insight into the solid/electrolyte-interphase composition and dynamic behaviour [88].

A binder-free model Li/S cell N-doped reduced graphene oxide/carbon monolith cathode was studied by a combined electrochemical impedance spectroscopy (EIS) and X-ray radiography characterization during galvanostatical charge/discharge cycles [89] and they report the formation of sulfur dendrites. Imaging and electrochemistry blend in the scanning electrochemical microscopy (SCEM),[90]; Fermin and coworkers investigate hydrogen evolution reaction (HER) catalyzed by two-dimensional assemblies palladium nanoparticles, comparing their performance to its bulk metal counterpart [91].

It is thus clear that *operando* investigations occupy now a consolidated place in electrocatalysis, and contribute to the development of future batteries and components for fuel cells.

### 3. Bulk structures vs. performance

The structure of materials is currently determined using X-ray diffraction, consistently an *operando* approach has been developed also for this technique, in order to appreciate structural changes under duty as well. Obviously advanced diffraction techniques are of paramount importance also for the structural determination of heterogeneous

catalysts [92]. Recently, Newton et al. critically assessed the diverse possibilities available in the high energy X-ray domain as a result of the maturation of third generation synchrotron facilities and looked forward to the potential that forthcoming developments in synchrotron source technology may offer to the world of catalysis in the near future [93].

An interesting report of advantages and limitations of XAFS and XRD combined techniques under synchrotron radiation was published by Frenkel et al. [94]. Besides underlining the added value of combining the sensitivity of XRD to the long-range order within the material, and that of XAFS to the nearest environment around absorbing atoms, they also demonstrated how to reach a better understanding of the structure and kinetics of a catalyst during its chemical treatment. In particular, they studied the inverse Cu/ceria catalyst and revealed the formation of highly disordered, minority species along the two reaction routes: the CO reduction and O₂ reoxidation of the catalyst, showing that independent studies of such systems by either XAFS or XRD alone produce incomplete results. Moreover, they pointed out the exciting new possibility to study heterogeneous catalysis by in situ diffraction anomalous fine structure (DAFS). T. Li et al. have recently reviewed the combination of *in situ/operando* XAS and SAXS to provide high temporal and spatial resolution when monitoring the evolution of nanomaterials, both during their synthesis and at work [95]. The combination of XAS, XRD and Raman in a single experiment, while collecting catalytic data, highlighted the structural and compositional changes of the partially reduced Fe₂O₃ and 3% chromium oxide-modified iron oxide before, during, and after the water gas shift (WGS) reaction, associating a marked structural disorder and compositional heterogeneity with their catalytically active states. The role of Cr in stabilizing the low-temperature γ-Fe₂O₃ phase and the possible deactivation mechanism were also described [96]. A similar approach, but replacing Raman by DRIFT, was carried out on ceria support loaded with Pt and Ru; it assessed the activity and selectivity of the Pt-Ru alloy and the marginal role of formates observed on the catalyst surface [97]. Ce-based systems were also diffusely investigated for the steam reforming of ethanol: in situ XRD evidenced the interactions between Ni and ceria support, while *operando* DRIFT spectra provided indications on the role of ethoxy, acetate, carbonate and hydroxyl species in the reaction pathway [98]. Similar studies of ethanol and ethylene steam reforming were carried out on Co/CeO₂. In this case XRD allowed understanding the correlation between the particle size and the reactivity (Fig. 4): nanoparticles of ceria showed to be more active (even if less selective) and resistant to coking, whereas the catalysts supported on larger ceria particles suffered from coke formation. On another hand, *operando* DRIFTS uncover different surface species, including some surface species, which could be a precursor for ethylene. Pulse methanol oxidation experiments were performed to probe the surface sites of the catalysts. The results indicated a higher density of redox sites on the Co/CeO₂ nanoparticles catalyst, associated to a higher reducibility, as shown by *operando* EXAFS [99]. These techniques provided important support to the comprehension of CO oxidation as well. For example, in the case of Cu-ceria catalysts, the combination of XRD with Raman, DRIFT, HREM, XEDS, XPS and XAFS allowed to correlate the superior PROX performances to the degree of support-promoted reduction, achieved by the dispersed copper oxide particles under reaction conditions. However, it was also shown that catalysts which display higher CO oxidation activity are generally more efficient also for the undesired H₂ oxidation reaction, having structural differences proved by the characterisation techniques [100] and related to the copper-ceria interface [101], to the copper oxidation state [102] and to the exposed crystal planes [103]. Behm et al. identified the physical origin of the activation/deactivation and loss in selectivity of a Ru/TiO₂ methanation catalyst by XANES/EXAFS measurements, revealing that an initial activation phase is largely due to the reduction of oxidized Ru species, together with a distinct change in the Ru particle shape, until reaching a state dominated by metallic Ru species with the highest Ru mass normalized activity. The losses of activity and selectivity during the subsequent deactivation phase were
<figure><img src="image_5.png" /><figcaption>**Fig. 4.** correlation between material structure and reactivity obtained thanks to the *operando* approach (reproduced with permission from [1] I.I. Soykal, H. Sohn, U.S. Ozkan, Effect of Support Particle Size in Steam Reforming of Ethanol over Co/CeO2 Catalysts, ACS Catal. 2 (2012) 2335–2348. Copyright American Chemical Society 2012) [99].</figcaption></figure>

found as mainly due to slow Ru particle growth [104]. Carbon dioxide reduction is gaining increasing importance in energy-related applications and well understanding restructuring-induced catalytic activity is of paramount importance to design high-performance catalysts. The use of in situ and *operando* X-ray absorption spectroscopy shed a light on the reversible structural and oxidation state changes under working conditions of copper(II) phthalocyanine materials to form ~ 2 nm metallic copper clusters, catalyzing the carbon dioxide-to-methane. DFT calculations allowed to rationalize the restructuring behavior and attribute the reversibility to the strong divalent metal ion–ligand coordination in the copper(II) phthalocyanine molecular structure and the small size of the generated copper clusters under the reaction conditions [105]. A recent interesting study on a copper-zinc-alumina catalyst using *operando* X-ray absorption spectroscopy and X-ray diffraction displayed how the industrial precursor evolves into the working catalyst. Moreover, it highlighted two major phenomena that drastically altered the copper- and zinc-containing components during the activation, leading to the final active structure: the reduction of the starting copper(II) oxide, and the ripening and reoxidation of zinc oxide upon the switch to catalytic conditions. The synergy and counteraction between copper and zinc transformations, also accompanied by other gradual structural changes, determine the longevity and activity of the catalyst [106].

## 4. Bulk-surface interaction

As it is well shown in these examples, structural characterizations under reactant contacts are extremely important to understand the role of the structure and phase configuration active for a specific reaction. However, this cannot be the solely criterion: always these analyses are coupled with surface local methods to associate the correct structural parameters with the catalytic act in progress. EXAFS/XANES are characterization techniques increasingly popular in the field of *operando* studies, notably because they account for local structures of the investigated sites in the catalytic materials, therefore being more informative on the real state of the reaction centers in action [107]. A clear example of this approach can be observed in the report by Föttinger et al. [108], who combined time-resolved X-ray absorption spectroscopy (XAS) with online mass spectrometric activity measurements during methanol steam reforming, which shows that the PdZn alloy forms during reaction and accounts for the high selectivity to H₂ and CO₂. Such an alloying is reversible in the presence of oxygen, forming palladium metal and ZnO. An important aspect of this paper was the spatial resolution achieved, which showed how alloying started at the nanoparticle surface and proceeded "inwards" toward the particle center. In this regard, the paper by Grunwaldt and Baiker, monitoring with a beam of 1 mm 0.6 mm size along the axial position of a Pt-Rh/Al₂O₃ catalyst bed in a fixed-bed reactor during partial oxidation of methane, should be mentioned. In

fact, in a catalyst bed concentrations and/or temperature profiles may exist which can result in significant spatial variation of the catalyst structure. XAS showed that Rh-Pt-particles were reduced at the same time at the reaction temperature. However, the axial monitoring allowed discovering that Rh and Pt were still in oxidized state in the entrance region, whereas they were in reduced state in the zone at the end of the catalyst bed, with a gradual shift of the front of the oxidation state versus the temperature increase [29]. Again Grunwaldt et al. performed a long-time (> 300 h) *operando* XAS study on a commercial Fischer–Tropsch catalyst under realistic industrial conditions (30 bar and 250 °C), correlating the formation of the products and the material deactivation with structural changes and the appearing of surface poisoning species [109]. *Operando* XAS and EPR were used to track the structural variation of the active sites in a hydrogen photogeneration system consisting of semiconductor QDs supporting Ni¹¹ ions. The results showed that the Ni(OAc)₂ precursor transforms to Ni(H₂O)₆²⁺ and finally to Ni(OH)₂ in alkaline media. In the photoreaction process, Ni¹¹ obtained photoelectrons from the QDs and subsequently spilled out from the crystal lattice of Ni(OH)₂ produce the defective structure [Nix⁰/-Ni₁⁻ₓ(OH)₂], which acts as the real catalytic species for H₂ photo-generation [110]. X-ray absorption techniques were also applied to environmental relevant studies, which are not prioritized in this report, but are indissolubly associated with energy-related processes. *Operando* copper oxidation state and coordination in copper-exchanged chabazite (Cu-SSZ-13) during NOx selective catalytic reduction (SCR) with ammonia are combined with (DFT) and first-principles thermodynamics models. Four-fold-coordinated Cu(II) was found to be the dominant species under the so-called Fast and Slow SCR conditions, whereas under Standard SCR conditions (no NO₂ in the feed), mixed Cu(I) and Cu(II) oxidation states were observed along with a reduction in the average copper coordination. These results provided a deeper insight into SCR mechanism, highlighting the role of Cu redox chemistry for that reaction [111]. More quantitative and extended (also to SAPO-34 and ZSM-5 zeolites) studies were performed by Kipersky et al., finding coherent results and underlying the pitfall of in situ respect to *operando* analyses [112]. Synchrotron techniques also promoted progresses in identifying the active sites and surface reaction mechanisms for catalysts dedicated to fine chemistry [113]. *Operando* XAS was used to study the adsorbates and structural changes and their dependence on potential, existing during the ethanol oxidation reaction on carbon-supported Pt, PtRu, and PtSn anode catalysts, identifying nanoparticle structure and particle size and providing new insights into the role of OH for that reaction [114]. This approach can be applied to reactions in liquid phase as well, as reported by Mondelli et al. in the case of aerobic oxidation of benzyl alcohol on a Bi-Pd/Al₂O₃ catalyst [115]. Combining XAS with ATR-IR allowed to identify the effect of Bi on the evolution of surface species and on the structure of the catalyst, having as a major role that of
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** correlation between material structure and reactivity obtained thanks to the *operando* approach (reproduced with permission from [1] I.I. Soykal, H. Sohn, U.S. Ozkan, Effect of Support Particle Size in Steam Reforming of Ethanol over Co/CeO2 Catalysts, ACS Catal. 2 (2012) 2335–2348. Copyright American Chemical Society 2012) [99].</figcaption></figure>

found as mainly due to slow Ru particle growth [104]. Carbon dioxide reduction is gaining increasing importance in energy-related applications and well understanding restructuring-induced catalytic activity is of paramount importance to design high-performance catalysts. The use of in situ and *operando* X-ray absorption spectroscopy shed a light on the reversible structural and oxidation state changes under working conditions of copper(II) phthalocyanine materials to form ~ 2 nm metallic copper clusters, catalyzing the carbon dioxide-to-methane. DFT calculations allowed to rationalize the restructuring behavior and attribute the reversibility to the strong divalent metal ion–ligand coordination in the copper(II) phthalocyanine molecular structure and the small size of the generated copper clusters under the reaction conditions [105]. A recent interesting study on a copper-zinc-alumina catalyst using *operando* X-ray absorption spectroscopy and X-ray diffraction displayed how the industrial precursor evolves into the working catalyst. Moreover, it highlighted two major phenomena that drastically altered the copper- and zinc-containing components during the activation, leading to the final active structure: the reduction of the starting copper(II) oxide, and the ripening and reoxidation of zinc oxide upon the switch to catalytic conditions. The synergy and counteraction between copper and zinc transformations, also accompanied by other gradual structural changes, determine the longevity and activity of the catalyst [106].

## 4. Bulk-surface interaction

As it is well shown in these examples, structural characterizations under reactant contacts are extremely important to understand the role of the structure and phase configuration active for a specific reaction. However, this cannot be the solely criterion: always these analyses are coupled with surface local methods to associate the correct structural parameters with the catalytic act in progress. EXAFS/XANES are characterization techniques increasingly popular in the field of *operando* studies, notably because they account for local structures of the investigated sites in the catalytic materials, therefore being more informative on the real state of the reaction centers in action [107]. A clear example of this approach can be observed in the report by Föttinger et al. [108], who combined time-resolved X-ray absorption spectroscopy (XAS) with online mass spectrometric activity measurements during methanol steam reforming, which shows that the PdZn alloy forms during reaction and accounts for the high selectivity to H₂ and CO₂. Such an alloying is reversible in the presence of oxygen, forming palladium metal and ZnO. An important aspect of this paper was the spatial resolution achieved, which showed how alloying started at the nanoparticle surface and proceeded "inwards" toward the particle center. In this regard, the paper by Grunwaldt and Baiker, monitoring with a beam of 1 mm 0.6 mm size along the axial position of a Pt-Rh/Al₂O₃ catalyst bed in a fixed-bed reactor during partial oxidation of methane, should be mentioned. In

fact, in a catalyst bed concentrations and/or temperature profiles may exist which can result in significant spatial variation of the catalyst structure. XAS showed that Rh-Pt-particles were reduced at the same time at the reaction temperature. However, the axial monitoring allowed discovering that Rh and Pt were still in oxidized state in the entrance region, whereas they were in reduced state in the zone at the end of the catalyst bed, with a gradual shift of the front of the oxidation state versus the temperature increase [29]. Again Grunwaldt et al. performed a long-time (> 300 h) *operando* XAS study on a commercial Fischer–Tropsch catalyst under realistic industrial conditions (30 bar and 250 °C), correlating the formation of the products and the material deactivation with structural changes and the appearing of surface poisoning species [109]. *Operando* XAS and EPR were used to track the structural variation of the active sites in a hydrogen photogeneration system consisting of semiconductor QDs supporting Ni¹¹² ions. The results showed that the Ni(OAc)₂ precursor transforms to Ni(H₂O)₆²⁺ and finally to Ni(OH)₂ in alkaline media. In the photoreaction process, Ni¹¹² obtained photoelectrons from the QDs and subsequently spilled out from the crystal lattice of Ni(OH)₂ produce the defective structure [Nix⁰/-Ni₁⁻ₓ(OH)₂], which acts as the real catalytic species for H₂ photo-generation [110]. X-ray absorption techniques were also applied to environmental relevant studies, which are not prioritized in this report, but are indissolubly associated with energy-related processes. *Operando* copper oxidation state and coordination in copper-exchanged chabazite (Cu-SSZ-13) during NOₓ selective catalytic reduction (SCR) with ammonia are combined with (DFT) and first-principles thermodynamics models. Four-fold-coordinated Cu(II) was found to be the dominant species under the so-called Fast and Slow SCR conditions, whereas under Standard SCR conditions (no NO₂ in the feed), mixed Cu(I) and Cu(II) oxidation states were observed along with a reduction in the average copper coordination. These results provided a deeper insight into SCR mechanism, highlighting the role of Cu redox chemistry for that reaction [111]. More quantitative and extended (also to SAPO-34 and ZSM-5 zeolites) studies were performed by Kipersky et al., finding coherent results and underlying the pitfall of in situ respect to *operando* analyses [112]. Synchrotron techniques also promoted progresses in identifying the active sites and surface reaction mechanisms for catalysts dedicated to fine chemistry [113]. *Operando* XAS was used to study the adsorbates and structural changes and their dependence on potential, existing during the ethanol oxidation reaction on carbon-supported Pt, PtRu, and PtSn anode catalysts, identifying nanoparticle structure and particle size and providing new insights into the role of OH for that reaction [114]. This approach can be applied to reactions in liquid phase as well, as reported by Mondelli et al. in the case of aerobic oxidation of benzyl alcohol on a Bi-Pd/Al₂O₃ catalyst [115]. Combining XAS with ATR-IR allowed to identify the effect of Bi on the evolution of surface species and on the structure of the catalyst, having as a major role that of
controlling the supply of oxygen to the noble metal and prevent it from overoxidation. Operando synchrotron X-ray powder diffraction and modulated-excitation infrared spectroscopy were applied on a commercial Cu-ZnO-Al₂O₃ catalyst to gain insights into its morphology and surface chemistry during methanol synthesis promoted by optimal amounts of CO₂. These studies show all the interest of combining spectroscopic techniques with catalysis: as shown in Fig. 5, Cu and ZnO agglomerate and ZnO particles flatten under CO/H₂ and/or CO₂/H₂ flow, whereas under the optimal CO/CO₂/H₂ mixture, sintering is prevented and ZnO crystals adopt an elongated shape due to the minimal presence of the H₂O byproduct, enhancing the water-gas shift activity and thus the methanol production [116].

During oxidative dehydrogenation reactions, on supported oxides, typically, the supported oxide undergoes the redox cycle; ceria is an interesting system for it presents different behavior. Ceria is a particularly versatile oxide, which operando study deserves a story on its own [117]. Ceria is extremely reactive and tend to oxidize species near it, this helps minimizing carbon deposits during reaction, but it also stabilizes high oxidation states on nearby cations. A particularly remarkable case is that of ceria-supported vanadia. Preliminary work demonstrated that ceria stabilizes vanadia in its maximum oxidation state, to the point that vanadia does not reduce even under strong reducing conditions and thus, the catalytic cycle occurs on the ceria ions near vanadia, as indirectly determined from operando Raman and XANES studies along with

<figure><img src="image_6.png" /><figcaption>Fig. 5. a)Temporal evolution of the CO, MeOH, and H₂O concentrations during the hydrogenation of CO (top), CO₂ (middle), and CO+CO₂ (optimal mixture, bottom). Mass distributions of b)Cu particles of a given diameter (d) and of c)ZnO particles of a given base diameter (b) and prism height (h) and d)EDXS mapping of Cu and Zn for the catalyst at the end of each catalytic run. The insets in (b,c) depict models of the particles calculated based on the average dimensions (Reproduced with permission from O. Martin, C. Mondelli, A. Cervellino, D. Ferri, D. Curulla-Ferre, J. Perez-Ramirez, Operando Synchrotron X-ray Powder Diffraction and Modulated-Excitation Infrared Spectroscopy Elucidate the CO₂ Promotion on a Commercial Methanol Synthesis Catalyst, Angew. Chemie-International Ed. 55 (2016) 11031–11036) [116].</figcaption></figure>
controlling the supply of oxygen to the noble metal and prevent it from overoxidation. Operando synchrotron X-ray powder diffraction and modulated-excitation infrared spectroscopy were applied on a commercial Cu-ZnO-Al₂O₃ catalyst to gain insights into its morphology and surface chemistry during methanol synthesis promoted by optimal amounts of CO₂. These studies show all the interest of combining spectroscopic techniques with catalysis: as shown in Fig. 5, Cu and ZnO agglomerate and ZnO particles flatten under CO/H₂ and/or CO₂/H₂ flow, whereas under the optimal CO/CO₂/H₂ mixture, sintering is prevented and ZnO crystals adopt an elongated shape due to the minimal presence of the H₂O byproduct, enhancing the water-gas shift activity and thus the methanol production [116].

During oxidative dehydrogenation reactions, on supported oxides, typically, the supported oxide undergoes the redox cycle; ceria is an interesting system for it presents different behavior. Ceria is a particularly versatile oxide, which operando study deserves a story on its own [117]. Ceria is extremely reactive and tend to oxidize species near it, this helps minimizing carbon deposits during reaction, but it also stabilizes high oxidation states on nearby cations. A particularly remarkable case is that of ceria-supported vanadia. Preliminary work demonstrated that ceria stabilizes vanadia in its maximum oxidation state, to the point that vanadia does not reduce even under strong reducing conditions and thus, the catalytic cycle occurs on the ceria ions near vanadia, as indirectly determined from operando Raman and XANES studies along with

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** a) Temporal evolution of the CO, MeOH, and H₂O concentrations during the hydrogenation of CO (top), CO₂ (middle), and CO+CO₂ (optimal mixture, bottom). Mass distributions of b) Cu particles of a given diameter (d) and of c) ZnO particles of a given base diameter (b) and prism height (h) and d) EDXS mapping of Cu and Zn for the catalyst at the end of each catalytic run. The insets in (b,c) depict models of the particles calculated based on the average dimensions (Reproduced with permission from O. Martin, C. Mondelli, A. Cervellino, D. Ferri, D. Curulla-Ferre, J. Perez-Ramirez, *Operando Synchrotron X-ray Powder Diffraction and Modulated-Excitation Infrared Spectroscopy to Elucidate the CO₂ Promotion on a Commercial Methanol Synthesis Catalyst*, Angew. Chemie-International Ed. 55 (2016) 11031–11036) [116].</figcaption></figure>
in situ EPR analyses [118]. The strong interaction between vanadia and ceria accounts for the deactivation during alkane oxidative dehydrogenation reactions through the formation of ceria vanadate [118], the reducing stage in the redox cycle facilitate such a solid state reaction [119]. These data lead Bañares group to suggest that ceria cations at the interface with vanadia not only undergo the redox cycle but that ceria sites are reduced, being the V<sup>5+</sup>-O-Ce<sup>3+</sup> the active site of oxidative dehydrogenation reactions and for the reduction of the system. The interaction between ceria and vanadia is so strong that Ce<sup>3+</sup> sites remain present as high reaction temperatures, as confirmed by operando Raman spectra during oxidation reactions, leading to the formation of superoxide species (1121 cm<sup>-1</sup>) [120]. This hypotheses were presented to Sauer's group, which confirmed such a scenario through DFT calculations [121]. A direct experimental confirmation was reported by Bañares et al. on support-stabilized nanoscaled vanadia-ceria domains by operando XANES, which confirms the reduction of ceria sites at the interface with vanadia and how the redox cycle occurs between Ce<sup>3+</sup>/Ce<sup>4+</sup>, [122] while vanadium remains at is maximum oxidation state [118]. The redox role of ceria support for vanadia has recently been confirmed with several ingenious detailed experiments reported by Heß et al. [123,124].

Ceria has a remarkable role for catalytic combustion of volatile organic components [125]. Noble metals on ceria exhibit a strong oxidizing power of volatile organic compounds, in the case of gold/-ceria, the nature of adsorbed toluene molecules is completely different to that of vanadia/ceria catalysts. The latter adsorbs as a carbenium species on acidic vanadia Brønsted site while the former adsorbs via π-bonding, destabilizing toluene aromatic ring [120]. The nature of the reaction mechanism is affected by the metal on ceria, but also by the reaction temperature. Thus, *operando* Raman studies on 1D Pt@CeO₂ derived from Pt-MOF clusters results in illustrating a variable chemistry. The Pt-CeO₂ interface generates reduced ceria sites which form nearby adsorbed peroxide at low temperature; these oxidize toluene into benzoates by a Langmuir–Hinshelwood mechanism. As the reaction temperature increases, the role of lattice oxygen becomes important, producing CO₂ and H₂O mainly by the Mars-van Krevelen mechanism [126].

The nature of supported oxides has found several debates, particularly due to experimental discrepancies when spectra of a given sample are acquired with different equipments in difference cells; this may lead to experimental uncertainties. Different ambient humidity and temperatures do have a clear effect on vanadyl mode due to hydroxylation [127,128]. In an effort to combine infrared and Raman data Daturi and Bañares developed a simultaneous infrared-Raman *operando* reactor so that both spectra can be acquired on the same sample under the same experimental conditions [129]; these experiments demonstrated that the same infrared and Raman vanadyl mode is exhibited on zirconia under the same experimental conditions (Fig. 6), and that it is possible to investigate during reaction in an *operando* IR-Raman reactor drawing complementary information from the same experiment through two complementary vibrational spectroscopies [130].

**5. The use of *in situ* / *operando* techniques to investigate the surface of catalysts for energy relevant applications**

The study of material surface remains the decisive aspect to be developed to describe any heterogeneous catalyst [131]. Vibrational spectroscopies are particularly adapted to the analysis of a surface under duty, allowing to monitor active site and detect reaction intermediates [132].

**5.1. Thermal catalysis**

An interesting mechanistic study on WGS reaction mechanism was performed by Shekhar et al. on gold nanoparticles supported on model Al₂O₃, and TiO₂. They discovered that the dominant active sites are the

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** (A) Raman and infrared spectra of the V=O mode on dehydrated vanadia on zirconia at different coverages. (B) position of the vanadyl Raman and infrared mode. [129]</figcaption></figure>

Reprinted with permission from [1] J.J.T. Hidalgo, M.O. Guerrero-Pérez, J. Rodríguez-Mirasol, T. Cordero, M.A. Bañares, R. Portela, P. Bazin, G. Clet, M. Daturi, *Operando* Reactor-Cell with Simultaneous Transmission FTIR and Raman Characterization (IRRaman) for the Study of Gas-Phase Reactions with Solid Catalysts, *Anal Chem.* (2020). https://doi.org/10.1021/acs.anal-chem.9b05473. Copyright 2020 American Chemical Society.

low coordinated perimeter and corner sites of the truncated cuboctahedron geometry of the gold nanoparticles; using *operando* FTIR experiments they determined that these sites are metallic in nature. This finding allowed to indicate how to further enhance the catalytic activity of Au catalysts, i.e. promoting new nanoparticle shapes that possess sites with Au–Au coordination number less than 4 and a higher fraction of low coordinated sites [133]. Important conclusions on the mechanisms of WGS and methanol steam reforming reactions was found by Kaftan et al. using DRIFTS coupled with online quadrupole mass spectrometry for a KOH-modified Pt/Al₂O₃ catalyst. They demonstrated the role of KOH coating and the formation of surface intermediates: during reaction, formates primarily develop on the uncoated catalyst, while a film of hydroxides and carbonates was present on the KOH-coated sample. The modes of CO adsorbed on Pt suggest a highly modified electronic structure due to K-coadsorption; the weaker C-O bond and stronger Pt-C bond increase CO₂ selectivity [134].

The long-standing problem of the methanation mechanism was studied by different teams. Westerman et al. investigated the behavior of 5%Ni-wt.-impregnated and 5%Ni-wt.-15%Ce-wt.-impregnated zeolites (of USY type) by in situ/operando IR spectroscopy, both for CO₂ adsorption and under methanation conditions (H₂:CO₂ = 4:1) reaction. CO₂ uptake was obviously prevalent over CeNiUSY, leading to the formation of a large amount of adsorbed carbonates on Ce sites. Infrared spectroscopic data clarified that under methanation conditions
in situ EPR analyses [118]. The strong interaction between vanadia and ceria accounts for the deactivation during alkane oxidative dehydrogenation reactions through the formation of ceria vanadate [118], the reducing stage in the redox cycle facilitate such a solid state reaction [119]. These data lead Bañares group to suggest that ceria cations at the interface with vanadia not only undergo the redox cycle but that ceria sites are reduced, being the V<sup>5+</sup>-O-Ce<sup>3+</sup> the active site of oxidative dehydrogenation reactions and for the reduction of the system. The interaction between ceria and vanadia is so strong that Ce<sup>3+</sup> sites remain present as high reaction temperatures, as confirmed by operando Raman spectra during oxidation reactions, leading to the formation of superoxide species (1121 cm<sup>-1</sup>) [120]. This hypotheses were presented to Sauer's group, which confirmed such a scenario through DFT calculations [121]. A direct experimental confirmation was reported by Bañares et al. on support-stabilized nanoscaled vanadia-ceria domains by operando XANES, which confirms the reduction of ceria sites at the interface with vanadia and how the redox cycle occurs between Ce<sup>3+</sup>/Ce<sup>4+</sup>, [122] while vanadium remains at is maximum oxidation state [118]. The redox role of ceria support for vanadia has recently been confirmed with several ingenious detailed experiments reported by Heß et al. [123,124].

Ceria has a remarkable role for catalytic combustion of volatile organic components [125]. Noble metals on ceria exhibit a strong oxidizing power of volatile organic compounds, in the case of gold/-ceria, the nature of adsorbed toluene molecules is completely different to that of vanadia/ceria catalysts. The latter adsorbs as a carbenium species on acidic vanadia Brønsted site while the former adsorbs via π-bonding, destabilizing toluene aromatic ring [120]. The nature of the reaction mechanism is affected by the metal on ceria, but also by the reaction temperature. Thus, *operando* Raman studies on 1D Pt@CeO₂ derived from Pt-MOF clusters results in illustrating a variable chemistry. The Pt-CeO₂ interface generates reduced ceria sites which form nearby adsorbed peroxide at low temperature; these oxidize toluene into benzoates by a Langmuir–Hinshelwood mechanism. As the reaction temperature increases, the role of lattice oxygen becomes important, producing CO₂ and H₂O mainly by the Mars-van Krevelen mechanism [126].

The nature of supported oxides has found several debates, particularly due to experimental discrepancies when spectra of a given sample are acquired with different equipments in difference cells; this may lead to experimental uncertainties. Different ambient humidity and temperatures do have a clear effect on vanadyl mode due to hydroxylation [127,128]. In an effort to combine infrared and Raman data Daturi and Bañares developed a simultaneous infrared-Raman *operando* reactor so that both spectra can be acquired on the same sample under the same experimental conditions [129]; these experiments demonstrated that the same infrared and Raman vanadyl mode is exhibited on zirconia under the same experimental conditions (Fig. 6), and that it is possible to investigate during reaction in an *operando* IR-Raman reactor drawing complementary information from the same experiment through two complementary vibrational spectroscopies [130].

**5. The use of *in situ* / *operando* techniques to investigate the surface of catalysts for energy relevant applications**

The study of material surface remains the decisive aspect to be developed to describe any heterogeneous catalyst [131]. Vibrational spectroscopies are particularly adapted to the analysis of a surface under duty, allowing to monitor active site and detect reaction intermediates [132].

**5.1. Thermal catalysis**

An interesting mechanistic study on WGS reaction mechanism was performed by Shekhar et al. on gold nanoparticles supported on model Al₂O₃, and TiO₂. They discovered that the dominant active sites are the

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** (A) Raman and infrared spectra of the V=O mode on dehydrated vanadia on zirconia at different coverages. (B) position of the vanadyl Raman and infrared mode. [129]</figcaption></figure>

Reprinted with permission from [1] J.J.T. Hidalgo, M.O. Guerrero-Pérez, J. Rodríguez-Mirasol, T. Cordero, M.A. Bañares, R. Portela, P. Bazin, G. Clet, M. Daturi, *Operando* Reactor-Cell with Simultaneous Transmission FTIR and Raman Characterization (IRRaman) for the Study of Gas-Phase Reactions with Solid Catalysts, *Anal Chem.* (2020). https://doi.org/10.1021/acs.analchem.9b05473. Copyright 2020 American Chemical Society.

low coordinated perimeter and corner sites of the truncated cuboctahedron geometry of the gold nanoparticles; using *operando* FTIR experiments they determined that these sites are metallic in nature. This finding allowed to indicate how to further enhance the catalytic activity of Au catalysts, i.e. promoting new nanoparticle shapes that possess sites with Au–Au coordination number less than 4 and a higher fraction of low coordinated sites [133]. Important conclusions on the mechanisms of WGS and methanol steam reforming reactions was found by Kaftan et al. using DRIFTS coupled with online quadrupole mass spectrometry for a KOH-modified Pt/Al₂O₃ catalyst. They demonstrated the role of KOH coating and the formation of surface intermediates: during reaction, formates primarily develop on the uncoated catalyst, while a film of hydroxides and carbonates was present on the KOH-coated sample. The modes of CO adsorbed on Pt suggest a highly modified electronic structure due to K-coadsorption; the weaker C-O bond and stronger Pt-C bond increase CO₂ selectivity [134].

The long-standing problem of the methanation mechanism was studied by different teams. Westerman et al. investigated the behavior of 5%Ni-wt.-impregnated and 5%Ni-wt.-15%Ce-wt.-impregnated zeolites (of USY type) by in situ/operando IR spectroscopy, both for CO₂ adsorption and under methanation conditions (H₂:CO₂ = 4:1) reaction. CO₂ uptake was obviously prevalent over CeNiUSY, leading to the formation of a large amount of adsorbed carbonates on Ce sites. Infrared spectroscopic data clarified that under methanation conditions
hydrogen is dissociated on Ni particles and reacts with hydrogen and bidentate carbonates, leading first to the formation of monodentate formates (detected in large amount since 150 °C), and then to methane, by further hydrogenation. Therefore formates were indicated as intermediates and their decomposition as a kinetic relevant step in CO₂ hydrogenation [135,136]. A study of CO₂ methanation at low temperature was performed on Rh/gamma-Al₂O₃ catalysts having different mean particle sizes. The intrinsic activity of the catalytic materials was found to do not depend on particle size at temperatures between 185 and 200 °C, whereas at lower temperatures larger particles favoured higher activity. Operando DRIFTS experiments showed CO₂ dissociation at 50 °C, giving rise to different Rh carbonyl and formates species, the first being identified as intermediates and the latter as spectators in the reaction path. Activation energy for CO dissociation is found to be similar to that of the overall reaction which suggests the importance of CO bond dissociation in the reaction path [137]. Takano et al. too found that bidentate carbonates are important intermediates for CH₄ formation during CO₂ methanation, whereas CO adsorption is not detected [138]. Aldana et al. revealed that the interactions between Ni and the ceria-zirconia support seem to be a key parameter for the methanation reaction. The use of IR operando spectroscopy uncovered that the main mechanism for CO₂ methanation does not require CO as reaction intermediate, whereas the most important step is CO₂ adsorption on weak basic sites [139]. Combining operando XANES, IR, and Raman with SSITKA experiments on Ru/CeO₂ and Ru/alpha-Al₂O₃ catalysts, allowed Wang et al. to discover that carbon dioxide methanation follows a route with formate intermediates in the presence of oxygen vacancies, whereas the CO route is favoured in their absence [140]. The proper use of operando DRIFT on Cu/ZnO catalysts for CO₂ hydrogenation to methanol has allowed Krossing et al. to deal with mechanistic aspects and reaction intermediates in deep for this reaction, showing the presence of oxygen surface defects correlated with the catalytic activity, a strong SMSI between Cu and the ZnO support, as well as a questionable role of formates [141]. By the way, it is important to insist on the fact that the use of DRIFT cells (popular because easily available) may present some problems of spectra interpretation, being not all the visible species chemically relevant. For this reason it is important to kinetically correlate the adsorbed species with the reaction products [doi-org inc. bib.cnrs.fr/10.1002/anie.201710301].

Concerning CO preferential oxidation in large amounts of hydrogen, we can mention the interesting study performed by Kydd et al. using 4 wt% Cu-CeO₂ catalysts. Surface redox properties of the catalyst were assessed using a series of TPR experiments, as well as operando infrared spectroscopy. CO and H₂ react at identical sites, CO₂ formation proceeds in parallel via three distinct Cuⁿ⁺-CO carbonyl species. The high selectivity towards CO below 150 °C appears due to the carbonyl stabilization on the catalyst surface; this competes with the undesired oxidation of hydrogen. Under non-selective conditions at higher temperatures, a gradual red-shift and loss of intensity in the carbonyl peak was observed, witnessing for Cu⁺ to Cu⁰ reduction, and the onset of an alternate redox-type oxidation mechanism where CO and H₂ compete for the oxidation sites [142].

## 5.2. Photocatalysis/photoelectrochemical catalysis

Another interesting study on CO₂ conversion into either renewable fuels or useful chemicals was reported on a titania cowrapped alpha-sulfur composite material, which, under a light-emitting diode source, affords the highest hydrogen evolution reported for any elemental material. In situ methanol from CO₂ photoreduction conversion was used as a sacrificial agent for the hydrogen evolution reaction. The effect of in situ methanol on hydrogen production was around 165 times stronger than the effect of ex situ methanol as a sacrificial agent. A plausible mechanism was proposed by combining the results from powder X-ray scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectrometry, X-ray photoelectron spectroscopy,

ultraviolet-visible diffuse reflection spectroscopy, Fourier transform infrared spectroscopy, and N₂ sorption studies. The formation of formaldehyde and methanol as intermediate products is established by in situ and operando FT-IR spectroscopy. Density functional theory calculations indicate the creation of new density of states in the band gap of the photocatalyst along with a shift in the conduction band to the energy potential for hydrogen formation [143].

Photoelectrochemical water oxidation has attracted a great interest for solar fuel production. Water oxidation on hematite is mediated by surface trapped holes, characterized to be the high valent -Fe=O species. Operando FTIR investigated the reaction intermediates for the rate-limiting O-O bond formation step by rate law analysis based on electrochemical impedance spectroscopy (EIS). The modes of surface superoxide species hydrogen bonded to adjacent hydroxyl groups confirm the water nucleophilic attack to -Fe=O to form the O-O bond, [144], in line with the trend suggested by reaction orders vs. pH.

Operando ATR-FTIR experiments performed on the surface of Ag thin films during the CO₂ electroreduction concluded on the formation of formate intermediates for such reaction [145].

## 6. Time resolution

When studying a catalytic reaction, the temporal dimension is of primary importance, essentially for two reasons: data collection should be performed at the time scale of the chemical phenomenon, in order to obtain the required elements constituting the process; the time sequence of the phenomena has to be reconstructed, in order to have a proper view of the reaction mechanism.

Commercial solutions already exist to cover the exigencies of high enough time resolution for understanding the main steps of a catalytic pathway, in many cases. For example, the team of Anderson has recently reported that IR rapid-scan operando in the milliseconds time frame is a tool powerful enough to study reaction mechanisms. As proof of concept, they applied this technical device to the study of the environmentally relevant NOx reduction by CO and hydrogen. By their experiments they confirmed the most accepted mechanisms for the SCR reaction and, for the first time, they elucidated the roles of hydroxyls on the catalyst surface [146].

Recently, a polymer electrolyte fuel cell has been designed to allow a time resolution of 5 s per spectrum during operando XAS measurements of catalysts. The cell has been developed to operate under standard fuel cell conditions, with relevant temperatures and humidity of the gas-phase, showing excellent performance respect to the currently used devices [147]. A further development has been obtained thanks to the realization of a quick operando ambient pressure hard X-ray photoelectron spectroscopy (HAXPES) system for the investigations of reaction kinetics in electrochemical cells under working conditions. This cell can reach a maximum time resolution of ~200 ms [148]. In the field of catalytic water splitting, using Pt as an active element, ultrafast infrared experiments opened the way to kinetic studies for this important reaction for hydrogen production. Examining the vibrational dynamics of Pt-H bonds (intermediate species appearing in water dissociation process) on a nanostructured platinum surface, three bands were observed at 1800, 2000, and 2090 cm⁻¹, assigned to Pt-CO in a bridged and linear configuration and Pt-H, respectively. Lifetime analysis revealed a time constant of 0.8 ± 0.1 ps for the Pt-H mode, considerably shorter than that of Pt-CO because of its stronger coupling to the metal substrate, and attributed to this parameter the high selectivity of this material for hydrogen production. Two-dimensional ATR-IR analyses (Fig. 7), in agreement with DFT calculations, provided additional evidence for the assignment based on the anharmonic shift, which is large in the case of Pt-H (90 cm⁻¹) [149]. Multiwavelength and time-resolved Raman spectroscopy with gas-phase FTIR and in situ UV-vis diffuse reflectance spectroscopy investigate the oxidative dehydrogenation of ethanol over a vanadia molecularly dispersed on silica. Raman and IR monitoring over several reaction cycles identified the active vanadia structures by
hydrogen is dissociated on Ni particles and reacts with hydrogen and bidentate carbonates, leading first to the formation of monodentate formates (detected in large amount since 150 °C), and then to methane, by further hydrogenation. Therefore formates were indicated as intermediates and their decomposition as a kinetic relevant step in CO₂ hydrogenation [135,136]. A study of CO₂ methanation at low temperature was performed on Rh/gamma-Al₂O₃ catalysts having different mean particle sizes. The intrinsic activity of the catalytic materials was found to do not depend on particle size at temperatures between 185 and 200 °C, whereas at lower temperatures larger particles favoured higher activity. Operando DRIFTS experiments showed CO₂ dissociation at 50 °C, giving rise to different Rh carbonyl and formates species, the first being identified as intermediates and the latter as spectators in the reaction path. Activation energy for CO dissociation is found to be similar to that of the overall reaction which suggests the importance of CO bond dissociation in the reaction path [137]. Takano et al. too found that bidentate carbonates are important intermediates for CH₄ formation during CO₂ methanation, whereas CO adsorption is not detected [138]. Aldana et al. revealed that the interactions between Ni and the ceria-zirconia support seem to be a key parameter for the methanation reaction. The use of IR operando spectroscopy uncovered that the main mechanism for CO₂ methanation does not require CO as reaction intermediate, whereas the most important step is CO₂ adsorption on weak basic sites [139]. Combining operando XANES, IR, and Raman with SSITKA experiments on Ru/CeO₂ and Ru/alpha-Al₂O₃ catalysts, allowed Wang et al. to discover that carbon dioxide methanation follows a route with formate intermediates in the presence of oxygen vacancies, whereas the CO route is favoured in their absence [140]. The proper use of operando DRIFT on Cu/ZnO catalysts for CO₂ hydrogenation to methanol has allowed Krossing et al. to deal with mechanistic aspects and reaction intermediates in deep for this reaction, showing the presence of oxygen surface defects correlated with the catalytic activity, a strong SMSI between Cu and the ZnO support, as well as a questionable role of formates [141]. By the way, it is important to insist on the fact that the use of DRIFT cells (popular because easily available) may present some problems of spectra interpretation, being not all the visible species chemically relevant. For this reason it is important to kinetically correlate the adsorbed species with the reaction products [doi-org inc. bib.cnrs.fr/10.1002/anie.201710301].

Concerning CO preferential oxidation in large amounts of hydrogen, we can mention the interesting study performed by Kydd et al. using 4 wt% Cu-CeO₂ catalysts. Surface redox properties of the catalyst were assessed using a series of TPR experiments, as well as operando infrared spectroscopy. CO and H₂ react at identical sites, CO₂ formation proceeds in parallel via three distinct Cuⁿ⁺-CO carbonyl species. The high selectivity towards CO below 150 °C appears due to the carbonyl stabilization on the catalyst surface; this competes with the undesired oxidation of hydrogen. Under non-selective conditions at higher temperatures, a gradual red-shift and loss of intensity in the carbonyl peak was observed, witnessing for Cu⁺ to Cu⁰ reduction, and the onset of an alternate redox-type oxidation mechanism where CO and H₂ compete for the oxidation sites [142].

## 5.2. Photocatalysis/photoelectrochemical catalysis

Another interesting study on CO₂ conversion into either renewable fuels or useful chemicals was reported on a titania cowrapped alpha-sulfur composite material, which, under a light-emitting diode source, affords the highest hydrogen evolution reported for any elemental material. In situ methanol from CO₂ photoreduction conversion was used as a sacrificial agent for the hydrogen evolution reaction. The effect of in situ methanol on hydrogen production was around 165 times stronger than the effect of ex situ methanol as a sacrificial agent. A plausible mechanism was proposed by combining the results from powder X-ray scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectrometry, X-ray photoelectron spectroscopy,

ultraviolet-visible diffuse reflection spectroscopy, Fourier transform infrared spectroscopy, and N₂ sorption studies. The formation of formaldehyde and methanol as intermediate products is established by in situ and operando FT-IR spectroscopy. Density functional theory calculations indicate the creation of new density of states in the band gap of the photocatalyst along with a shift in the conduction band to the energy potential for hydrogen formation [143].

Photoelectrochemical water oxidation has attracted a great interest for solar fuel production. Water oxidation on hematite is mediated by surface trapped holes, characterized to be the high valent -Fe=O species. Operando FTIR investigated the reaction intermediates for the rate-limiting O-O bond formation step by rate law analysis based on electrochemical impedance spectroscopy (EIS). The modes of surface superoxide species hydrogen bonded to adjacent hydroxyl groups confirm the water nucleophilic attack to -Fe=O to form the O-O bond, [144], in line with the trend suggested by reaction orders vs. pH.

Operando ATR-FTIR experiments performed on the surface of Ag thin films during the CO₂ electroreduction concluded on the formation of formate intermediates for such reaction [145].

## 6. Time resolution

When studying a catalytic reaction, the temporal dimension is of primary importance, essentially for two reasons: data collection should be performed at the time scale of the chemical phenomenon, in order to obtain the required elements constituting the process; the time sequence of the phenomena has to be reconstructed, in order to have a proper view of the reaction mechanism.

Commercial solutions already exist to cover the exigencies of high enough time resolution for understanding the main steps of a catalytic pathway, in many cases. For example, the team of Anderson has recently reported that IR rapid-scan operando in the milliseconds time frame is a tool powerful enough to study reaction mechanisms. As proof of concept, they applied this technical device to the study of the environmentally relevant NOx reduction by CO and hydrogen. By their experiments they confirmed the most accepted mechanisms for the SCR reaction and, for the first time, they elucidated the roles of hydroxyls on the catalyst surface [146].

Recently, a polymer electrolyte fuel cell has been designed to allow a time resolution of 5 s per spectrum during operando XAS measurements of catalysts. The cell has been developed to operate under standard fuel cell conditions, with relevant temperatures and humidity of the gas-phase, showing excellent performance respect to the currently used devices [147]. A further development has been obtained thanks to the realization of a quick operando ambient pressure hard X-ray photoelectron spectroscopy (HAXPES) system for the investigations of reaction kinetics in electrochemical cells under working conditions. This cell can reach a maximum time resolution of ~200 ms [148]. In the field of catalytic water splitting, using Pt as an active element, ultrafast infrared experiments opened the way to kinetic studies for this important reaction for hydrogen production. Examining the vibrational dynamics of Pt-H bonds (intermediate species appearing in water dissociation process) on a nanostructured platinum surface, three bands were observed at 1800, 2000, and 2090 cm⁻¹, assigned to Pt-CO in a bridged and linear configuration and Pt-H, respectively. Lifetime analysis revealed a time constant of 0.8 ± 0.1 ps for the Pt-H mode, considerably shorter than that of Pt-CO because of its stronger coupling to the metal substrate, and attributed to this parameter the high selectivity of this material for hydrogen production. Two-dimensional ATR-IR analyses (Fig. 7), in agreement with DFT calculations, provided additional evidence for the assignment based on the anharmonic shift, which is large in the case of Pt-H (90 cm⁻¹) [149]. Multiwavelength and time-resolved Raman spectroscopy with gas-phase FTIR and in situ UV-vis diffuse reflectance spectroscopy investigate the oxidative dehydrogenation of ethanol over a vanadia molecularly dispersed on silica. Raman and IR monitoring over several reaction cycles identified the active vanadia structures by
<figure><img src="image_8.png" /><figcaption>**Fig. 7.** Normalized 2D ATR IR spectrum of the Pt–H band at a pump–probe delay time of t = 0.3 ps. The inhomogeneously broadened GSB/SE signal of Pt–H (blue) is accompanied by the ESA signal (red) that is anharmonically redshifted along the probefrequency axis by ~90 cm⁻¹.</figcaption></figure>

Paleček, G. Tek, J. Lan, M. Iannuzzi, P. Hamm, Characterization of the Platinum–Hydrogen Bond by Surface-Sensitive Time-Resolved Infrared Spectroscopy, J Phys Chem Lett. 9 (2018) 1254–1259. https://doi.org/10.1021/acs.jpclett.8b00310. Copyright 2018 American Chemical Society [149]. The corresponding bands from Pt–CO* reveal a much smaller anharmonic shift of ~20 cm⁻¹, Reprinted (adapted) with permission from [1]D.

uncovering how ethoxy species opening V–O–Si and V–O–V bonds upon adsorption. The trends are consistent, Raman spectra show the concomitant redshift of the V=O mode and larger oligomeric VOx structures break into smaller moieties. This data show that the V=O bond is not involved as the critical site for ethanol oxidation [150]; such observation is in line with the trends exhibited by molecularly dispersed vanadium oxide for alkane oxidative dehydrogenation and for its reducibility by molecular hydrogen [151,152].

Increasing time resolution helps better discrimination of spectators; the approach reported by Thibault et al. used fast pressure jumps and microsecond infrared spectroscopy to monitor adsorbed CO as a probe molecule [153]. This was used to investigate the accessibility and evolution of platinum nanoparticles supported on ZSM-5; they identified different sintering mechanisms in the mesopores and the zeolite external surface. Time-resolved optical emission spectroscopy (OES) was used to provide information modus operandi on the species present in a spark from as early as approximately 100 ns after the initiation of the discharge. The emission spectra of the spark were used to identify, among others, the main stages of material erosion and to calculate the quenching rate of the generated metal vapour [154].

Important progresses on time-resolution for vibrational spectroscopy investigations has been made recently by coupling a rapid-scan FT-IR spectrometer with tuneable quantum cascade lasers (QCLs). This has allowed to maintain the information potential of a standard FT-IR instrument while boosting the time resolution to a few µs to continuously detect fast phenomena, which risk to be missed at a lower spectra recording frequency (Fig. 8) [155]. This ultrafast device has been applied to the detection of short lifetime reaction intermediates in the methanol photooxidation [156].

## 7. Time resolution: transient operando studies

Steady-state reaction conditions are typically used in kinetic investigations, although further insight can be obtained under transient conditions. Step/pulse-response experiments where one variable is

modified (concentration, temperature, irradiance, etc.) help to increase the detection sensitivity and selectivity. Especially useful for operando studies is the steady-state isotopic transient kinetic analysis (SSITKA), because the microstructure and reactivity are not altered by the isotopic exchange. SSITKA studies allow demonstrating mechanistic aspects, such as the role of species as real reaction intermediates or simple spectators. The combination of mass spectrometry and DRIFT analysis during SSITKA-type experiments allowed Goguet et al. to discriminate between active and spectator species in the case of the RWGS reaction over a Pt/CeO₂ catalyst, and to quantify the number of active sites. The formate species were discarded as main reaction intermediates, even if it was recognised that CO was (at least partially) formed by formate decomposition. The main reaction pathway was proposed to proceed via surface carbonate intermediates and a direct reoxidation of the support by CO₂ [157]. In our opinion, the main merit of this paper is to propose an innovative spectrokinetic approach to the study of catalytic reactions, combining quantitative IR analysis with isotopic labelled molecules and kinetic calculations. Rodriguez et al. concluded on the prevalence of a redox or associative mechanism as well, extended to Cu, Au or Pt metals supported on several metal oxides [158]. Also Polo-Garzon et al. found the redox mechanism as prominent for the WGS reaction on the industrial-type high-temperature CuCrFeOₓ catalyst, by combining spectroscopic, computational, and kinetic evidence [159]. The WGS reaction was also addressed by Silberova et al.: basing on DRIFTS spectra and operando transient experiments, they deduced that the adsorptive mechanism of the WGS reaction consisting of the CO adsorption on Au particles, the formation of carbonates and bicarbonates at the gold-iron oxide interface, and their decomposition to CO₂ prevail at room temperature. They also excluded a role of formates in the mechanism, mainly based on a reduction/reoxidation cycle, as shown by experiments performed at higher temperature, where gold promotes the support reduction and the reoxidation involving water as the reactant, which finally yields hydrogen [160]. Combining SSITKA and DRIFT studies, the group of Martinez-Arias identified those carbonates as bi- or tri-dentate species present on ceria and reacting at the same speed as hydrogen was produced [161]. Keturakis et al. used a combination of *in situ* and *operando* IR, Raman, and XAS spectroscopies during WGS/RWGS reactions to determine the real composition of the phases present during the catalytic activity, as well as the reaction pathway. Isotopic labeling established that the RWGS reaction proceeds via the redox mechanism, where only O* sites from the surface of the chromium iron oxide catalysts participate. This work demonstrates that the number of redox O* sites has traditionally been undercounted by nearly an order of magnitude. These sites affiliated with iron oxide are active for WGS/RWGS; chromia is a textural promoter that increases the number of catalytic active sites without any chemical promotion effect [162]. Another impressive work recently published on CO₂ conversion provides very important evidences on the hydrogenation mechanism: *operando* transmission FTIR/SSITKA experiments (Fig. 9) demonstrate that CO₂ methanation and RWGS are not parallel reactions during CO₂ reduction over Pd/Al₂O₃ catalysts; they actually share the initial steps and intermediates (from bicarbonates to formates). The rate-determining step for CO formation is the conversion of adsorbed formate, whereas the hydrogenation of adsorbed carbonyls is the rate-determining step for methane formation. Thus, the balance of the hydrogenation kinetics between adsorbed formates and carbonyls steers the reaction to CH₄ and CO. In fact, considering that carbonyls and formates mainly coordinate onto metal (Pd) and support (alumina), respectively, a balance could be easily tuned to achieve the desired CH₄ and CO selectivities by optimizing the loading of the metal and the surface area of the support [163]. Combining *operando* XANES, IR, and Raman, as well as SSITKA on a Ru/CeO₂ catalyst allowed Wang et al. to reveal the generation process of Ce³⁺, surface hydroxyl, oxygen vacancies and their structural evolutions under CO₂ methanation conditions. Furthermore, they demonstrated that methanation undergoes formate route over a reducible support, while the formate dissociation to methanol catalyzed by
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Normalized 2D ATR IR spectrum of the Pt–H band at a pump–probe delay time of t = 0.3 ps. The inhomogeneously broadened GSB/SE signal of Pt–H (blue) is accompanied by the ESA signal (red) that is anharmonically redshifted along the probefrequency axis by ~90 cm⁻¹.</figcaption></figure>

Paleček, G. Tek, J. Lan, M. Iannuzzi, P. Hamm, Characterization of the Platinum–Hydrogen Bond by Surface-Sensitive Time-Resolved Infrared Spectroscopy, J Phys Chem Lett. 9 (2018) 1254–1259. https://doi.org/10.1021/acs.jpclett.8b00310. Copyright 2018 American Chemical Society [149]. The corresponding bands from Pt–CO* reveal a much smaller anharmonic shift of ~20 cm⁻¹, Reprinted (adapted) with permission from [1]D.

uncovering how ethoxy species opening V–O–Si and V–O–V bonds upon adsorption. The trends are consistent, Raman spectra show the concomitant redshift of the V=O mode and larger oligomeric VOx structures break into smaller moieties. This data show that the V=O bond is not involved as the critical site for ethanol oxidation [150]; such observation is in line with the trends exhibited by molecularly dispersed vanadium oxide for alkane oxidative dehydrogenation and for its reducibility by molecular hydrogen [151,152].

Increasing time resolution helps better discrimination of spectators; the approach reported by Thibault et al. used fast pressure jumps and microsecond infrared spectroscopy to monitor adsorbed CO as a probe molecule [153]. This was used to investigate the accessibility and evolution of platinum nanoparticles supported on ZSM-5; they identified different sintering mechanisms in the mesopores and the zeolite external surface. Time-resolved optical emission spectroscopy (OES) was used to provide information modus operandi on the species present in a spark from as early as approximately 100 ns after the initiation of the discharge. The emission spectra of the spark were used to identify, among others, the main stages of material erosion and to calculate the quenching rate of the generated metal vapour [154].

Important progresses on time-resolution for vibrational spectroscopy investigations has been made recently by coupling a rapid-scan FT-IR spectrometer with tuneable quantum cascade lasers (QCLs). This has allowed to maintain the information potential of a standard FT-IR instrument while boosting the time resolution to a few µs to continuously detect fast phenomena, which risk to be missed at a lower spectra recording frequency (Fig. 8) [155]. This ultrafast device has been applied to the detection of short lifetime reaction intermediates in the methanol photooxidation [156].

## 7. Time resolution: transient operando studies

Steady-state reaction conditions are typically used in kinetic investigations, although further insight can be obtained under transient conditions. Step/pulse-response experiments where one variable is

modified (concentration, temperature, irradiance, etc.) help to increase the detection sensitivity and selectivity. Especially useful for operando studies is the steady-state isotopic transient kinetic analysis (SSITKA), because the microstructure and reactivity are not altered by the isotopic exchange. SSITKA studies allow demonstrating mechanistic aspects, such as the role of species as real reaction intermediates or simple spectators. The combination of mass spectrometry and DRIFT analysis during SSITKA-type experiments allowed Goguet et al. to discriminate between active and spectator species in the case of the RWGS reaction over a Pt/CeO₂ catalyst, and to quantify the number of active sites. The formate species were discarded as main reaction intermediates, even if it was recognised that CO was (at least partially) formed by formate decomposition. The main reaction pathway was proposed to proceed via surface carbonate intermediates and a direct reoxidation of the support by CO₂ [157]. In our opinion, the main merit of this paper is to propose an innovative spectrokinetic approach to the study of catalytic reactions, combining quantitative IR analysis with isotopic labelled molecules and kinetic calculations. Rodriguez et al. concluded on the prevalence of a redox or associative mechanism as well, extended to Cu, Au or Pt metals supported on several metal oxides [158]. Also Polo-Garzon et al. found the redox mechanism as prominent for the WGS reaction on the industrial-type high-temperature CuCrFeOₓ catalyst, by combining spectroscopic, computational, and kinetic evidence [159]. The WGS reaction was also addressed by Silberova et al.: basing on DRIFTS spectra and operando transient experiments, they deduced that the adsorptive mechanism of the WGS reaction consisting of the CO adsorption on Au particles, the formation of carbonates and bicarbonates at the gold-iron oxide interface, and their decomposition to CO₂ prevail at room temperature. They also excluded a role of formates in the mechanism, mainly based on a reduction/reoxidation cycle, as shown by experiments performed at higher temperature, where gold promotes the support reduction and the reoxidation involving water as the reactant, which finally yields hydrogen [160]. Combining SSITKA and DRIFT studies, the group of Martinez-Arias identified those carbonates as bi- or tri-dentate species present on ceria and reacting at the same speed as hydrogen was produced [161]. Keturakis et al. used a combination of *in situ* and *operando* IR, Raman, and XAS spectroscopies during WGS/RWGS reactions to determine the real composition of the phases present during the catalytic activity, as well as the reaction pathway. Isotopic labeling established that the RWGS reaction proceeds via the redox mechanism, where only O* sites from the surface of the chromium iron oxide catalysts participate. This work demonstrates that the number of redox O* sites has traditionally been undercounted by nearly an order of magnitude. These sites affiliated with iron oxide are active for WGS/RWGS; chromia is a textural promoter that increases the number of catalytic active sites without any chemical promotion effect [162]. Another impressive work recently published on CO₂ conversion provides very important evidences on the hydrogenation mechanism: *operando* transmission FTIR/SSITKA experiments (Fig. 9) demonstrate that CO₂ methanation and RWGS are not parallel reactions during CO₂ reduction over Pd/Al₂O₃ catalysts; they actually share the initial steps and intermediates (from bicarbonates to formates). The rate-determining step for CO formation is the conversion of adsorbed formate, whereas the hydrogenation of adsorbed carbonyls is the rate-determining step for methane formation. Thus, the balance of the hydrogenation kinetics between adsorbed formates and carbonyls steers the reaction to CH₄ and CO. In fact, considering that carbonyls and formates mainly coordinate onto metal (Pd) and support (alumina), respectively, a balance could be easily tuned to achieve the desired CH₄ and CO selectivities by optimizing the loading of the metal and the surface area of the support [163]. Combining *operando* XANES, IR, and Raman, as well as SSITKA on a Ru/CeO₂ catalyst allowed Wang et al. to reveal the generation process of Ce³⁺, surface hydroxyl, oxygen vacancies and their structural evolutions under CO₂ methanation conditions. Furthermore, they demonstrated that methanation undergoes formate route over a reducible support, while the formate dissociation to methanol catalyzed by
<figure><img src="image_9.png" /><figcaption>**Fig. 8.** IR absorbance at 1435 cm⁻¹ through QCL diagnostic (left axis) and standard rapid-scan FT-IR spectroscopy (right axis) as a function of time upon diffusion of (a) 8 Torr, (b) 60 Torr, (c) 80 Torr, and (d) 110 Torr of ammonia into H-ZSM-5 at 350 °C. The kinetic modelling of the physical phenomenon strictly depends on the spectra recording frequency, i.e. Schnee, P. Bazin, B. Barviau, F. Grisch, B.J. Beccard, M. Daturi, Coupling a Rapid-Scan FT-IR Spectrometer with Quantum Cascade Lasers within a Single Setup: An Easy Way to Reach Microsecond Time Resolution without Losing Spectral Information., Anal Chem. 91 (2019) acs.anal-chem.8b04621. https://doi.org/10.1021/acs.analchem.8b04621.</figcaption></figure>

Copyright 2019 American Chemical Society)[155]. On the time resolution (Reprinted with permission from [1]J.

oxygen vacancies is the rate-determining step. In contrast, they showed that CO₂ methanation proceeds through a CO route over Ru surface in a non-reducible alumina support (in the absence of oxygen vacancies), demonstrating an active site dependent catalytic mechanism for this reaction. Additionally, they showed out that the presence of oxygen vacancies catalyzes the rate-determining step with a much lower activation temperature than in their absence [159]. Despite the harsh reaction conditions of the Fischer-Tropsch synthesis, Efstathiou et al. have been able to carry out a very thorough and pioneering study on important kinetic parameters of the methanation reaction on a commercial Co/γ-Al₂O₃ catalyst, highlighting the participation of more than one kind of active –CHₓ intermediates in FTS and the independence of CO adsorption dynamics on the H₂ pressure thanks to the ¹²CO/H₂ → ¹³CO/H₂ SSITKA step-gas switch [164].

An increasingly important approach is the combination of concentration-modulation excitation spectroscopy (MES) with phase-sensitive detection (PSD). Since under reaction stream many phenomena concerning various species in different phases occur simultaneously, the interpretation of the resulting spectra appears difficult. Therefore, applying MES, i.e. the periodic perturbation of a given system, in combination with PSD analysis, the noise is significantly reduced, and become possible distinguishing between active and spectator species, extracting kinetic information as well [165]. This technique allows sensitive and selective spectroscopic detection and monitoring of dynamic behaviours especially when applied to infrared spectroscopy, helping to monitor gas-solid interface. For example, it was applied to understand the partial and reversible inhibition of the CO oxidation on gold nanoparticles supported on Ce-Zr-O [166]. Collins' work shows that deactivation cannot be due to structural changes of the catalyst, like

sintering of the gold particles, or poisoning by CO or O₂. They used isotopic transient analysis (¹³CO/¹²CO₂) monitored by mass spectrometry and by time-resolved and concentration-modulation spectroscopy in DRIFT mode to assess this reversible deactivation. CO adsorption allowed them to state on the oxidation degree of the gold active site, even if the concomitant presence of an electronic transition band of the ceria-zirconia support (2120 cm⁻¹) makes the spectral assignments complex in that area. Ferri and coworkers have reported a Raman operando reactor for the application of MES during selective catalytic reduction on titania-supported vanadia catalysts, diffuse reflectance-Vis Spectroscopy on Cu-SSZ-13 and DRIFTS on Fe-ZSM-5 [167].

## 8. Spatial resolution

### 8.1. Mapping solids at the sample scale during operando studies

Heterogenous catalysts are not uniform; bidimensional and three-dimensional mappings are becoming increasingly powerful and with increasing time resolution. Grunwaldt et al. summarized time-resolved and spatially resolved spectroscopic studies during preparation of catalysts and during operation (as in Fig. 10) [168]. They also highlighted the chemical state of copper species in 3D, in a Cu-SSZ-13 washcoated monolith catalyst during NOx reduction, combining operando X-ray absorption spectroscopy with microtomography. Gradients in copper oxidation state and coordination environment were revealed at micro-metre spatial resolution, and assigned to an interplay of NOx reduction with adsorption-desorption of NH₃ and mass transport phenomena [169].

Combination of surface-sensitive (DRIFTS) and bulk-sensitive
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** IR absorbance at 1435 cm⁻¹ through QCL diagnostic (left axis) and standard rapid-scan FT-IR spectroscopy (right axis) as a function of time upon diffusion of (a) 8 Torr, (b) 60 Torr, (c) 80 Torr, and (d) 110 Torr of ammonia into H-ZSM-5 at 350 °C. The kinetic modelling of the physical phenomenon strictly depends on the spectra recording frequency, i.e. Schnee, P. Bazin, B. Barviau, F. Grisch, B.J. Beccard, M. Daturi, Coupling a Rapid-Scan FT-IR Spectrometer with Quantum Cascade Lasers within a Single Setup: An Easy Way to Reach Microsecond Time Resolution without Losing Spectral Information., *Anal Chem.* 91 (2019) acs.anal-chem.8b04621. https://doi.org/10.1021/acs.analchem.8b04621.</figcaption></figure>

Copyright 2019 American Chemical Society)[155]. On the time resolution (Reprinted with permission from [1]J.

oxygen vacancies is the rate-determining step. In contrast, they showed that CO₂ methanation proceeds through a CO route over Ru surface in a non-reducible alumina support (in the absence of oxygen vacancies), demonstrating an active site dependent catalytic mechanism for this reaction. Additionally, they showed out that the presence of oxygen vacancies catalyzes the rate-determining step with a much lower activation temperature than in their absence [159]. Despite the harsh reaction conditions of the Fischer-Tropsch synthesis, Efstathiou et al. have been able to carry out a very thorough and pioneering study on important kinetic parameters of the methanation reaction on a commercial Co/γ-Al₂O₃ catalyst, highlighting the participation of more than one kind of active –CHₓ intermediates in FTS and the independence of CO adsorption dynamics on the H₂ pressure thanks to the ¹²CO/H₂ → ¹³CO/H₂ SSITKA step-gas switch [164].

An increasingly important approach is the combination of concentration-modulation excitation spectroscopy (MES) with phase-sensitive detection (PSD). Since under reaction stream many phenomena concerning various species in different phases occur simultaneously, the interpretation of the resulting spectra appears difficult. Therefore, applying MES, i.e. the periodic perturbation of a given system, in combination with PSD analysis, the noise is significantly reduced, and become possible distinguishing between active and spectator species, extracting kinetic information as well [165]. This technique allows sensitive and selective spectroscopic detection and monitoring of dynamic behaviours especially when applied to infrared spectroscopy, helping to monitor gas-solid interface. For example, it was applied to understand the partial and reversible inhibition of the CO oxidation on gold nanoparticles supported on Ce-Zr-O [166]. Collins' work shows that deactivation cannot be due to structural changes of the catalyst, like

sintering of the gold particles, or poisoning by CO or O₂. They used isotopic transient analysis (¹³CO/¹²CO₂) monitored by mass spectrometry and by time-resolved and concentration-modulation spectroscopy in DRIFT mode to assess this reversible deactivation. CO adsorption allowed them to state on the oxidation degree of the gold active site, even if the concomitant presence of an electronic transition band of the ceria-zirconia support (2120 cm⁻¹) makes the spectral assignments complex in that area. Ferri and coworkers have reported a Raman operando reactor for the application of MES during selective catalytic reduction on titania-supported vanadia catalysts, diffuse reflectance-Vis Spectroscopy on Cu-SSZ-13 and DRIFTS on Fe-ZSM-5 [167].

## 8. Spatial resolution

### 8.1. Mapping solids at the sample scale during operando studies

Heterogenous catalysts are not uniform; bidimensional and three-dimensional mappings are becoming increasingly powerful and with increasing time resolution. Grunwaldt et al. summarized time-resolved and spatially resolved spectroscopic studies during preparation of catalysts and during operation (as in Fig. 10) [168]. They also highlighted the chemical state of copper species in 3D, in a Cu-SSZ-13 washcoated monolith catalyst during NOx reduction, combining operando X-ray absorption spectroscopy with microtomography. Gradients in copper oxidation state and coordination environment were revealed at micro-metre spatial resolution, and assigned to an interplay of NOx reduction with adsorption-desorption of NH₃ and mass transport phenomena [169].

Combination of surface-sensitive (DRIFTS) and bulk-sensitive
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** The responses of reactants, products and surface intermediates after the steady-state isotopic switch. a, b Normalized mass spectrometry signals of gases in the effluent and c FTIR contour plot collected as a function of time. The feed gas was switched at 0 s from ¹²CO₂/H₂/Ar to ¹³CO₂/H₂ over 5% Pd/Al₂O₃ at 533 K. Ar gas was used as an inert tracer (reproduced with permission from X. Wang, H. Shi, J. Szanyi, Controlling selectivities in CO₂ reduction through mechanistic understanding, Nat. Commun. 8 (2017) 513. Copyright SpringerNature 2020; [163])</figcaption></figure>

(Raman) time-resolved detection at three different catalyst-bed positions (2.5 mm distance) during NOₓ storage and reduction reaction on Pt-Ba/CeO₂ illustrated the delay in the formation of nitrite and nitrate surface species, and also a decrease in their surface coverage downstream [6].

X-ray techniques are increasingly used to assess chemical structure-reactivity relationships during real operation [170,171]. 3D profiles of Ni/γ-Al₂O₃ extrudates from μ-XRD-CT (computed tomography) combined with μ-absorption-CT during methanation illustrate the stability of Ni fcc in the 200–450 °C range [172]. A 3 mm Co/γ-Al₂O₃ catalyst was studied by μ-XRD-CT/μ-PDF-CT during Fischer–Tropsch reaction [173]. They monitor the progressive reduction of cobalt oxide nanoparticles, which -as expected- are less reducible as their interaction with the support increases. During reaction, agglomerated small fcc Co metal nanoparticles oxidize to CoO/Co₃O₄; this correlates with a decrease in CH₄ selectivity and increased water-gas shift activity.

Synchrotron infrared micro-spectroscopy investigate the formation of olefins from dimethylether in single crystals of the silicoaluminophosphate STA-7; these are characterized for exhibiting a uniform and homogeneous distribution of acidic hydroxyl groups. Hydrogen-bonded dimethylether dissociates forming surface methoxy groups above 473 K, and aromatic hydrocarbon species above 573 K. Time-resolved infrared-MS correlate alkene formation with a direct reaction of dimethylether with surface methoxy groups [174].

Transient spectroscopic surface-chemistry experiments in combination with spatially resolved kinetic Monte Carlo (KMC) simulations offer great potential to discriminate spectators, which typically dominate in steady-state conditions. In this case, Hess et al. have designed an experiment sensitive to the relative activity of different oxygen species on RuO₂ oriented crystals; they saturated the surface with loosely bound oxygen so that CO probe molecule may adsorb only at single vacancies and recombine with oxygen. *In situ* reflection-absorption infrared spectroscopy (RAIRS) and ab initio based KMC simulations tackled the long-standing controversy about the nature of the active oxygen species in RuO₂(110) for CO oxidation: both surface O species (O-br and O-ot) appear equally active, although their adsorption energies differ by more than 150 kJ/mol [175].

Another way to identify the active zones on a catalyst is the use of infrared thermography. A typical case study was reported by Gaenzler et al., who applied this technique, coupled with spatially and time-resolved XAS and online mass spectrometry to the study of oscillatory CO oxidation over a Pt/Al₂O₃ oxidation catalyst having small Pt particles under relevant reaction conditions. IR thermography recorded the hot spots, so identifying the catalyst-bed zone responsible for the oscillatory behaviour, related to the dynamic structural change of the metal particles. Interestingly, it was demonstrated by this way that the oscillations of the Pt oxidation state and the hot spot regions evolved simultaneously and moved from the end toward the beginning of the catalyst bed with increasing reaction temperature [176].

The polarization response of a coplanar electrochemical capacitor covered with an ionic liquid as the electrolyte has been examined using a combination of XPS and SEM. The spatiotemporal distribution of the ionic liquid surface potential was evidenced through SEM high spatial resolution and speedy imaging, together with the application of a data mining algorithm. The results demonstrate the power of a synergistic combination of the two techniques, yielding similar polarization relaxation time constants, and pave the way to a deeper understanding of liquid/solid interfaces and for performance evaluation and diagnostics of electrochemical devices [177].

Another interesting technique for getting 3D chemical information with sub-nm resolution is atom probe tomography (APT). Coupled with in situ UV–vis spectroscopy and microscopy it has provided the nano-scale coking behavior of zeolite SSZ-13 and its relation to bulk coke formation on the macro-/micro-scale during the methanol-to-hydrocarbons (MTH) process [178].

## 8.2. Mapping solids at the reactor scale during operando studies

Spatial resolution is important both in the micro- and in the macro-scale. The use of spectroscopic microreactors was recently reviewed by Hartman et al. [21].
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 9.** The responses of reactants, products and surface intermediates after the steady-state isotopic switch. a, b Normalized mass spectrometry signals of gases in the effluent and c FTIR contour plot collected as a function of time. The feed gas was switched at 0 s from ¹²CO₂/H₂/Ar to ¹³CO₂/H₂ over 5% Pd/Al₂O₃ at 533 K. Ar gas was used as an inert tracer (reproduced with permission from X. Wang, H. Shi, J. Szanyi, Controlling selectivities in CO₂ reduction through mechanistic understanding, Nat. Commun. 8 (2017) 513. Copyright SpringerNature 2020; [163])</figcaption></figure>

(Raman) time-resolved detection at three different catalyst-bed positions (2.5 mm distance) during NOₓ storage and reduction reaction on Pt-Ba/CeO₂ illustrated the delay in the formation of nitrite and nitrate surface species, and also a decrease in their surface coverage downstream [6].

X-ray techniques are increasingly used to assess chemical structure-reactivity relationships during real operation [170,171]. 3D profiles of Ni/γ-Al₂O₃ extrudates from μ-XRD-CT (computed tomography) combined with μ-absorption-CT during methanation illustrate the stability of Ni fcc in the 200–450 °C range [172]. A 3 mm Co/γ-Al₂O₃ catalyst was studied by μ-XRD-CT/μ-PDF-CT during Fischer–Tropsch reaction [173]. They monitor the progressive reduction of cobalt oxide nanoparticles, which -as expected- are less reducible as their interaction with the support increases. During reaction, agglomerated small fcc Co metal nanoparticles oxidize to CoO/Co₃O₄; this correlates with a decrease in CH₄ selectivity and increased water-gas shift activity.

Synchrotron infrared micro-spectroscopy investigate the formation of olefins from dimethylether in single crystals of the silicoaluminophosphate STA-7; these are characterized for exhibiting a uniform and homogeneous distribution of acidic hydroxyl groups. Hydrogen-bonded dimethylether dissociates forming surface methoxy groups above 473 K, and aromatic hydrocarbon species above 573 K. Time-resolved infrared-MS correlate alkene formation with a direct reaction of dimethylether with surface methoxy groups [174].

Transient spectroscopic surface-chemistry experiments in combination with spatially resolved kinetic Monte Carlo (KMC) simulations offer great potential to discriminate spectators, which typically dominate in steady-state conditions. In this case, Hess et al. have designed an experiment sensitive to the relative activity of different oxygen species on RuO₂ oriented crystals; they saturated the surface with loosely bound oxygen so that CO probe molecule may adsorb only at single vacancies and recombine with oxygen. *In situ* reflection-absorption infrared spectroscopy (RAIRS) and ab initio based KMC simulations tackled the long-standing controversy about the nature of the active oxygen species in RuO₂(110) for CO oxidation: both surface O species (O-br and O-ot) appear equally active, although their adsorption energies differ by more than 150 kJ/mol [175].

Another way to identify the active zones on a catalyst is the use of infrared thermography. A typical case study was reported by Gaenzler et al., who applied this technique, coupled with spatially and time-resolved XAS and online mass spectrometry to the study of oscillatory CO oxidation over a Pt/Al₂O₃ oxidation catalyst having small Pt particles under relevant reaction conditions. IR thermography recorded the hot spots, so identifying the catalyst-bed zone responsible for the oscillatory behaviour, related to the dynamic structural change of the metal particles. Interestingly, it was demonstrated by this way that the oscillations of the Pt oxidation state and the hot spot regions evolved simultaneously and moved from the end toward the beginning of the catalyst bed with increasing reaction temperature [176].

The polarization response of a coplanar electrochemical capacitor covered with an ionic liquid as the electrolyte has been examined using a combination of XPS and SEM. The spatiotemporal distribution of the ionic liquid surface potential was evidenced through SEM high spatial resolution and speedy imaging, together with the application of a data mining algorithm. The results demonstrate the power of a synergistic combination of the two techniques, yielding similar polarization relaxation time constants, and pave the way to a deeper understanding of liquid/solid interfaces and for performance evaluation and diagnostics of electrochemical devices [177].

Another interesting technique for getting 3D chemical information with sub-nm resolution is atom probe tomography (APT). Coupled with in situ UV–vis spectroscopy and microscopy it has provided the nano-scale coking behavior of zeolite SSZ-13 and its relation to bulk coke formation on the macro-/micro-scale during the methanol-to-hydrocarbons (MTH) process [178].

## 8.2. Mapping solids at the reactor scale during operando studies

Spatial resolution is important both in the micro- and in the macro-scale. The use of spectroscopic microreactors was recently reviewed by Hartman et al. [21].
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Schematic setup of the plug flow reactor and local X-ray absorption spectroscopy around the Pt L3 and the Rh K-edge at the entrance zone and the end of the fixed-bed at 319 °C (after steady-state was reached).
(reproduced with permission from J.-D. Grunwaldt, B. Kimmerle, A. Baiker, P. Boye, C.G. Schroer, P. Glatzel, C.N. Borca, F. Beckmann, Catalysts at work: From integral to spatially resolved X-ray absorption spectroscopy, Catal. Today. 145 (2009) 267–278. Copyright Elsevier 2009 [168].)</figcaption></figure>

Macroscale spatial resolution in integral reactors is a need due to composition and conditions gradients associated with extensive conversion of reactants along the catalyst domain; thus, not only the catalyst body profiles, but also de fluid composition and temperature profiles become important and are deeply connected. This scenario adds significant complexity to the experimental setup since typically several probes may be necessary [32].

A new experimental reactor concept was developed for *operando* and spatially resolved DRIFTS surface analysis over a microstructured catalytic reactor. Both gas and surface adspecies concentration profiles could be established along the channels coated with a Pt/CeO₂-Al₂O₃ catalyst, which was studied for the oxidation of CO from 25° to 300°C, as a demonstrative application. For these nearly isothermal conditions, a continuous increase in conversion along the microchannels of the microstructured reactor was observed. In contrast, an important thermal effect with characteristic light-off was obtained in the fixed bed reactor, underlining the unique behaviour of the microstructured system. The new device allowed the modelling of CO conversion and surface adspecies concentration vs. temperature along the channels, and kinetic parameters (in good agreement with the literature) depending on the catalyst composition were obtained too [179].

The work by Signorile et al. addresses the role of reactor configuration for *operando* spatiotemporal study of catalyst deactivation for MTH reaction on SAPO-34, underlining the relevance of reactor type on the deactivation pattern, thus determining data interpretation [180], such a feature is at the core of the *operando* concept, were genuine catalytic data are critical for a clear insight.

## 8.3. Single molecule operando imaging

An ultimate *operando* scale is the single-molecule monitoring during reaction; such is possibly based on the high sensitivity and fast acquisition time of fluorescence microscopy, which allows monitoring (electro)catalytic reactions with nanometer space resolution. These studies provide unprecedented insights into catalytic mechanism, reactivity, selectivity, and dynamics in spite of the inhomogeneity and temporal variations of catalyst structures [181,182]. There is major background in biomedical research that is now been translated to catalytic research [183]. The biology background may account for the fact that early applications had a bias towards biocatalysis [184]. This is particularly

interesting for mesoporous materials for they enable an in depth understanding of the interaction of guest molecules within the porous matrix that hosts them (Fig. 11). This provides a real-time understanding of the interactions between nano-structured mesoporous materials with reactants to deliver invaluable structure-performance relationships knowledge [185]. Spatial resolution taken to the channel of zeolite microcrystals provides fundamental insight on the genesis of active sites, like is the formation of Brønsted acid sites upon steaming of a ZSM-5 zeolite [186]; steaming at 500°C results in a fourfold increase in local turnover frequencies for the oligomerization of furfuryl alcohol; however, deactivation due to severe dealumination occurs upon steaming at 700°C with a decrease of orders of magnitude in the local turnover frequencies.

Monitoring molecules with high spatiotemporal resolution also deliver means to quantify local diffusion phenomena; Hendriks et al. use single-molecule fluorescence microscopy to analyze self-diffusion of probe molecules with nanometer accuracy in the macro pores of a micrometer-sized, fluid catalytic cracking (FCC) particle [187]. They observe a population distribution in the molecules, where 12% of them had mobility, which diffusion coefficients were calculated. Thus, it is possible to quantitatively map heterogeneities in diffusional properties within complex hierarchically porous materials.

Fluorescence spatial resolution has also been correlated with SEM images, like Andoy et al. report [188]. They show that corner regions in gold nanoplates have the highest activity, followed by the edge regions and then the flat surface facets. This work illustrates the deep entanglement between catalytic activity at the nanoscale as well as the interplay amid nanocrystal growth, morphology, and surface defects. Beyond this, recent reports evidence cooperative interaction among nanocatalysts, as was reported for single Pd and Au nanocatalysts [189]. Chen's group uses spatiotemporally resolved single-molecule catalysis imaging to report that catalytic reactions on a single Pd or Au nanocatalyst can communicate with each other. Such a phenomenon occurs within spatiotemporal ranges up to hundreds of nanometers and up to hundreds of seconds. Generalization of these long-range intra- and interparticle catalytic communication mechanisms may introduce a novel conceptual framework for understanding nanoscale catalysis. The amazing impact of this study is that those trends formally mimic the allosteric effects in enzymes; making nanocatalysis, in some manner, biomimetic.
<figure><img src="image_15.png" /><figcaption>**Fig. 10.** Schematic setup of the plug flow reactor and local X-ray absorption spectroscopy around the Pt L3 and the Rh K-edge at the entrance zone and the end of the fixed-bed at 319 °C (after steady-state was reached).
(reproduced with permission from J.-D. Grunwaldt, B. Kimmerle, A. Baiker, P. Boye, C.G. Schroer, P. Glatzel, C.N. Borca, F. Beckmann, Catalysts at work: From integral to spatially resolved X-ray absorption spectroscopy, Catal. Today. 145 (2009) 267–278. Copyright Elsevier 2009 [168].)</figcaption></figure>

Macroscale spatial resolution in integral reactors is a need due to composition and conditions gradients associated with extensive conversion of reactants along the catalyst domain; thus, not only the catalyst body profiles, but also de fluid composition and temperature profiles become important and are deeply connected. This scenario adds significant complexity to the experimental setup since typically several probes may be necessary [32].

A new experimental reactor concept was developed for *operando* and spatially resolved DRIFTS surface analysis over a microstructured catalytic reactor. Both gas and surface adspecies concentration profiles could be established along the channels coated with a Pt/CeO₂-Al₂O₃ catalyst, which was studied for the oxidation of CO from 25° to 300°C, as a demonstrative application. For these nearly isothermal conditions, a continuous increase in conversion along the microchannels of the microstructured reactor was observed. In contrast, an important thermal effect with characteristic light-off was obtained in the fixed bed reactor, underlining the unique behaviour of the microstructured system. The new device allowed the modelling of CO conversion and surface adspecies concentration vs. temperature along the channels, and kinetic parameters (in good agreement with the literature) depending on the catalyst composition were obtained too [179].

The work by Signorile et al. addresses the role of reactor configuration for *operando* spatiotemporal study of catalyst deactivation for MTH reaction on SAPO-34, underlining the relevance of reactor type on the deactivation pattern, thus determining data interpretation [180], such a feature is at the core of the *operando* concept, were genuine catalytic data are critical for a clear insight.

## 8.3. Single molecule operando imaging

An ultimate *operando* scale is the single-molecule monitoring during reaction; such is possibly based on the high sensitivity and fast acquisition time of fluorescence microscopy, which allows monitoring (electro)catalytic reactions with nanometer space resolution. These studies provide unprecedented insights into catalytic mechanism, reactivity, selectivity, and dynamics in spite of the inhomogeneity and temporal variations of catalyst structures [181,182]. There is major background in biomedical research that is now been translated to catalytic research [183]. The biology background may account for the fact that early applications had a bias towards biocatalysis [184]. This is particularly

interesting for mesoporous materials for they enable an in depth understanding of the interaction of guest molecules within the porous matrix that hosts them (Fig. 11). This provides a real-time understanding of the interactions between nano-structured mesoporous materials with reactants to deliver invaluable structure-performance relationships knowledge [185]. Spatial resolution taken to the channel of zeolite microcrystals provides fundamental insight on the genesis of active sites, like is the formation of Brønsted acid sites upon steaming of a ZSM-5 zeolite [186]; steaming at 500°C results in a fourfold increase in local turnover frequencies for the oligomerization of furfuryl alcohol; however, deactivation due to severe dealumination occurs upon steaming at 700°C with a decrease of orders of magnitude in the local turnover frequencies.

Monitoring molecules with high spatiotemporal resolution also deliver means to quantify local diffusion phenomena; Hendriks et al. use single-molecule fluorescence microscopy to analyze self-diffusion of probe molecules with nanometer accuracy in the macro pores of a micrometer-sized, fluid catalytic cracking (FCC) particle [187]. They observe a population distribution in the molecules, where 12% of them had mobility, which diffusion coefficients were calculated. Thus, it is possible to quantitatively map heterogeneities in diffusional properties within complex hierarchically porous materials.

Fluorescence spatial resolution has also been correlated with SEM images, like Andoy et al. report [188]. They show that corner regions in gold nanoplates have the highest activity, followed by the edge regions and then the flat surface facets. This work illustrates the deep entanglement between catalytic activity at the nanoscale as well as the interplay amid nanocrystal growth, morphology, and surface defects. Beyond this, recent reports evidence cooperative interaction among nanocatalysts, as was reported for single Pd and Au nanocatalysts [189]. Chen's group uses spatiotemporally resolved single-molecule catalysis imaging to report that catalytic reactions on a single Pd or Au nanocatalyst can communicate with each other. Such a phenomenon occurs within spatiotemporal ranges up to hundreds of nanometers and up to hundreds of seconds. Generalization of these long-range intra- and interparticle catalytic communication mechanisms may introduce a novel conceptual framework for understanding nanoscale catalysis. The amazing impact of this study is that those trends formally mimic the allosteric effects in enzymes; making nanocatalysis, in some manner, biomimetic.
<figure><img src="image_14.png" /><figcaption>**Fig. 11.** Fluorescence-based visualization of catalytic sites and events. (a) A fluorescent product (F) is formed by transformation of a fluorogenic reactant (FG) or by cleavage of a covalently bound quencher (Q). (b) Cycling of a metal catalyst (M) between two redox states causes quenching and dequenching of a fluorescent reporter (F). (c) Mapping of basic sites on a layered double hydroxide crystal with acid probes: (i) Imaging of time-dependent sorption of a perylene monoimide carboxylic acid. Even at the ensemble level differences between crystal faces can be distinguished (ii). At the single-molecule level, the probe hops between individual basic sites.
(reproduced with permission from M.B.J. Roeffaers, G. De Cremer, H. Uji-i, B. Muls, B.F. Sels, P.A. Jacobs, F.C. De Schryver, D.E. De Vos, J. Hofkens, Single-molecule fluorescence spectroscopy in (bio)catalysis, Proc. Natl. Acad. Sci. U. S. A. 104 (2007) 12603–12609 [184]).</figcaption></figure>

## 8.4. Single site spectroscopy

A metal catalyst supported on an inert substrate may consist of both metal nanoparticles and singly dispersed metal atoms. The support may impinge unique properties to the singly dispersed metal atoms; how those properties make the single metal atoms are active may allow developing new catalytic properties. In the case of Rh atoms on silica, Zhang et al. have explored that effect for NO reduction by CO. His approach was to assess if singly dispersed Rh atoms could perform a complex multi-step catalytic reaction differently from larger supported metal nanoparticles. *In situ* IR show that single Rh atoms reduce nitric oxide with carbon monoxide via two reaction pathways; these are different from those of Rh nanoparticles. CO and NO coadsorb on a single Rh atom to form N₂ and CO₂ through –NCO or N₂O intermediates [190].

A very interesting approach has been recently developed by Brückner and Briois by coupling operando EPR, XANES/EXAFS, UV-Vis and ATR-IR spectroscopic methods in the same experimental setup to investigate debated mechanistic aspects and active sites in the selective aerobic oxidation of benzyl alcohol by a Cu/TEMPO catalytic system. It was found that an initially in situ formed tetracoordinated (bpy)-(NMI) Cu¹-complex, is firstly converted to two different Cu¹¹ species: an EPR-active mononuclear (bpy)(NMI)(CH₃CN)Cu¹¹-OOH species and an EPR-silent dinuclear (bpy)(NMI)Cu¹¹(m-OH)₂Cu¹¹(bpy)(NMI)(CH₃CN) complex. Then the dinuclear species is cleaved during reaction into (bpy) (NMI)(OOH)Cu¹¹-TEMPO monomers that are also EPR-silent (due to

antiferromagnetic interaction between Cu¹¹ and TEMPO) but govern the catalytic activity in due course. Though the Cu¹¹ dimer obviously participates in the initial phase of the reaction, it is definitely.

not a major active species nor a resting state since it is irreversibly dissolved into monomers. This provides two parallel catalytically active Cu¹/Cu¹¹ redox cycles (contributing almost equally to the catalytic activity), one detectable by EPR and the other not, which differ in their mode of coordinating TEMPO. Therefore, the added value of the simultaneous coupling of EPR/UV-Vis/ATR-IR spectroscopies with XANES/EXAFS is clearly demonstrated: while XAS and UV-Vis spectroscopy can detect both Cu¹¹ monomers and dimers as well as both valence states of Cu, their ability to reflect the precise coordination geometry of Cu is limited, in particular for XAS without complete full multiple scattering simulations of XANES. But, this limitation is compensated by EPR, at least in the case of Cu¹¹. On the other hand, EXAFS can overcome the drawback of EPR, which can neither detect Cu¹ nor distinguish between mono- and dinuclear Cu¹¹ when both Cu¹¹ ions interact only weakly without forming a S= 1 spin system, while antiferromagnetic coupling prevents detection of Cu(II) completely [191].

## 9. Microscopy

Local catalyst structures can be efficiently identified by Environmental Transmission Electron Microscopy (ETEM) as well. HP-STEM and ETEM are complementary techniques, since the latter cannot easily image adsorbates on surface of a catalyst particle, the lateral
<figure><img src="image_16.png" /><figcaption>**Fig. 11.** Fluorescence-based visualization of catalytic sites and events. (a) A fluorescent product (F) is formed by transformation of a fluorogenic reactant (FG) or by cleavage of a covalently bound quencher (Q). (b) Cycling of a metal catalyst (M) between two redox states causes quenching and dequenching of a fluorescent reporter (F). (c) Mapping of basic sites on a layered double hydroxide crystal with acid probes: (i) Imaging of time-dependent sorption of a perylene monoimide carboxylic acid. Even at the ensemble level differences between crystal faces can be distinguished (ii). At the single-molecule level, the probe hops between individual basic sites.
(reproduced with permission from M.B.J. Roeffaers, G. De Cremer, H. Uji-i, B. Muls, B.F. Sels, P.A. Jacobs, F.C. De Schryver, D.E. De Vos, J. Hofkens, Single-molecule fluorescence spectroscopy in (bio)catalysis, Proc. Natl. Acad. Sci. U. S. A. 104 (2007) 12603–12609 [184]).</figcaption></figure>

## 8.4. Single site spectroscopy

A metal catalyst supported on an inert substrate may consist of both metal nanoparticles and singly dispersed metal atoms. The support may impinge unique properties to the singly dispersed metal atoms; how those properties make the single metal atoms are active may allow developing new catalytic properties. In the case of Rh atoms on silica, Zhang et al. have explored that effect for NO reduction by CO. His approach was to assess if singly dispersed Rh atoms could perform a complex multi-step catalytic reaction differently from larger supported metal nanoparticles. *In situ* IR show that single Rh atoms reduce nitric oxide with carbon monoxide via two reaction pathways; these are different from those of Rh nanoparticles. CO and NO coadsorb on a single Rh atom to form N₂ and CO₂ through –NCO or N₂O intermediates [190].

A very interesting approach has been recently developed by Brückner and Briois by coupling operando EPR, XANES/EXAFS, UV-Vis and ATR-IR spectroscopic methods in the same experimental setup to investigate debated mechanistic aspects and active sites in the selective aerobic oxidation of benzyl alcohol by a Cu/TEMPO catalytic system. It was found that an initially in situ formed tetracoordinated (bpy)-(NMI) Cu¹-complex, is firstly converted to two different Cu¹¹ species: an EPR-active mononuclear (bpy)(NMI)(CH₃CN)Cu¹¹-OOH species and an EPR-silent dinuclear (bpy)(NMI)Cu¹¹(m-OH)₂Cu¹¹(bpy)(NMI)(CH₃CN) complex. Then the dinuclear species is cleaved during reaction into (bpy)(NMI)(OOH)Cu¹¹-TEMPO monomers that are also EPR-silent (due to

antiferromagnetic interaction between Cu¹¹ and TEMPO) but govern the catalytic activity in due course. Though the Cu¹¹ dimer obviously participates in the initial phase of the reaction, it is definitely.

not a major active species nor a resting state since it is irreversibly dissolved into monomers. This provides two parallel catalytically active Cu¹/Cu¹¹ redox cycles (contributing almost equally to the catalytic activity), one detectable by EPR and the other not, which differ in their mode of coordinating TEMPO. Therefore, the added value of the simultaneous coupling of EPR/UV-Vis/ATR-IR spectroscopies with XANES/EXAFS is clearly demonstrated: while XAS and UV-Vis spectroscopy can detect both Cu¹¹ monomers and dimers as well as both valence states of Cu, their ability to reflect the precise coordination geometry of Cu is limited, in particular for XAS without complete full multiple scattering simulations of XANES. But, this limitation is compensated by EPR, at least in the case of Cu¹¹. On the other hand, EXAFS can overcome the drawback of EPR, which can neither detect Cu¹ nor distinguish between mono- and dinuclear Cu¹¹ when both Cu¹¹ ions interact only weakly without forming a S= 1 spin system, while antiferromagnetic coupling prevents detection of Cu(II) completely [191].

## 9. Microscopy

Local catalyst structures can be efficiently identified by Environmental Transmission Electron Microscopy (ETEM) as well. HP-STEM and ETEM are complementary techniques, since the latter cannot easily image adsorbates on surface of a catalyst particle, the lateral
arrangement of catalyst atoms on different terraces of the topmost sur-
face of the catalyst particle, and the packing of atoms at step edges,
whereas it can provide a clear view of the subsurface and a plane pro-
jection of tridimensional structures [192]. This technique has shed a
light on different aspects of catalysis, starting from material preparation:
for example, Ni dispersion on titania and metal-support interactions
were studied during the impregnation process [193,194]. ETEM also
unravelled the role of Ru as cobalt reduction promoter on a bimetallic
Fisher-Tropsch catalyst [195]. In the same way, it was shown that
copper promotes nickel reductibility in a bimetallic CuNi/TiO₂ catalyst
[196]. Important information was also provided about the material
structural evolution under gas reactants. It was typically the case for
gold nanoparticles supported on different metal oxides and changing
their shape and geometry upon exposure to hydrogen, CO and/or O₂
(Fig. 12); the use of operando microscopy clearly shows here its interest
for the correct visualization of the active sites and data interpretation
[197,198].

An evident advantage of ETEM is the monitoring of catalyst deactivation by particle sintering. Pd coalescence in industrial catalysts supported on α-alumina was investigated by Liu et al. [199], while DeLaRiva et al. Demonstrated the primary contribution of the Ostwald ripening of Ni/MgAl₂O₄ under hydrogen and water at high temperature [200], then generalised to a variety of metal/support systems. ETEM gave a contribution to the study of the catalyst deactivation by coke too, when the carbon deposit acquired a defined structure, similar to grapheme, as it has been the case for a Pt/MgO system exposed to isobutene at 475 °C [201]. The improvements in gas-analysis techniques now allow even operando TEM analyses to be performed for gas-phase catalytic reactions, even if the transfer of the residual gas phase from a large volume as it is inside the TEM analysis chamber to a gas analyser represents a challenge. An alternative is represented by EELS as a way to detect and quantify reaction products. This technique was for example used to analyse the products of CO oxidation on a Ru-based catalyst

loaded in the ETEM [202]. An overview of the recent research progress
of in situ and *operando* electron microscopy techniques applied to
energy-related catalytic reactions is presented in [203].

To facilitate the use of microscopy for characterising catalysts at work, a modular, homebuilt gas feed system for *operando* TEM studies has been developed directly coupled to a quadrupole mass spectrometer. The main interest of this setup (able to support different gas environments, temperatures and pressures and over time by combining traditional TEM techniques with MS data) is that it is compatible with conventional, commercially available gas cell TEM holders, making it widely accessible and reproducible by the community [204].

Being HP-STEM and ETEM complementary techniques, their integration can potentially provide a much deeper understanding of the complex processes taking place on the surface of a catalyst [192]. However, it should be mentioned that these techniques may suffer of the intrinsic defect of the microscopy methods: while providing a view of the investigated catalyst at the atomic level, they are focussed on individual objects, without any specific information on the surrounding entities. Therefore, without averaging on a huge number of observations and a statistical treatment of the imaging, they are submitted to the subjectivity of the operator and might take to a biased view of the material under observation.

Another source of problems are the frequent samples damages
induced by the microscope beam, notably in the presence of reacting
gases, a problem that can be mitigated only by operating at cumulative
doses not exceeding the threshold for a given gas [205].

10. Electron-based *operando* studies

Surface of catalytic materials can also be investigated using electron-
based surface analytic techniques. Their application to in situ or oper-
ando studies is more challenging than for the photon-in-photon-out
techniques, because scattering of electrons by molecules in the gaseous

<figure><img src="image_15.png" /><figcaption>**Fig. 12.** Frames from insitu ETEM observations of a GNP, during CO oxidation in a) 1 vol% CO/air (1mbar), and b) O2 (1mbar). In O2 gas, the GNP exhibited {111}, {100}, and {110} facets dynamically, while in CO/air the major {111} and {100} facets were stable. The electron flux was estimated to be 4.6 Å cm⁻² (reproduced with permission from T. Uchiyama, H. Yoshida, Y. Kuwauchi, S. Ichikawa, S. Shimada, M. Haruta, S. Takeda, Systematic Morphology Changes of Gold Nanoparticles Supported on CeO2 during CO Oxidation, Angew. Chemie-International Ed. 50 (2011) 10157–10160) [197].</figcaption></figure>
arrangement of catalyst atoms on different terraces of the topmost sur-
face of the catalyst particle, and the packing of atoms at step edges,
whereas it can provide a clear view of the subsurface and a plane pro-
jection of tridimensional structures [192]. This technique has shed a
light on different aspects of catalysis, starting from material preparation:
for example, Ni dispersion on titania and metal-support interactions
were studied during the impregnation process [193,194]. ETEM also
unravelled the role of Ru as cobalt reduction promoter on a bimetallic
Fisher-Tropsch catalyst [195]. In the same way, it was shown that
copper promotes nickel reductibility in a bimetallic CuNi/TiO₂ catalyst
[196]. Important information was also provided about the material
structural evolution under gas reactants. It was typically the case for
gold nanoparticles supported on different metal oxides and changing
their shape and geometry upon exposure to hydrogen, CO and/or O₂
(Fig. 12); the use of operando microscopy clearly shows here its interest
for the correct visualization of the active sites and data interpretation
[197,198].

An evident advantage of ETEM is the monitoring of catalyst deactivation by particle sintering. Pd coalescence in industrial catalysts supported on α-alumina was investigated by Liu et al. [199], while DeLaRiva et al. Demonstrated the primary contribution of the Ostwald ripening of Ni/MgAl₂O₄ under hydrogen and water at high temperature [200], then generalised to a variety of metal/support systems. ETEM gave a contribution to the study of the catalyst deactivation by coke too, when the carbon deposit acquired a defined structure, similar to grapheme, as it has been the case for a Pt/MgO system exposed to isobutene at 475 °C [201]. The improvements in gas-analysis techniques now allow even operando TEM analyses to be performed for gas-phase catalytic reactions, even if the transfer of the residual gas phase from a large volume as it is inside the TEM analysis chamber to a gas analyser represents a challenge. An alternative is represented by EELS as a way to detect and quantify reaction products. This technique was for example used to analyse the products of CO oxidation on a Ru-based catalyst

loaded in the ETEM [202]. An overview of the recent research progress
of in situ and *operando* electron microscopy techniques applied to
energy-related catalytic reactions is presented in [203].

To facilitate the use of microscopy for characterising catalysts at work, a modular, homebuilt gas feed system for *operando* TEM studies has been developed directly coupled to a quadrupole mass spectrometer. The main interest of this setup (able to support different gas environments, temperatures and pressures and over time by combining traditional TEM techniques with MS data) is that it is compatible with conventional, commercially available gas cell TEM holders, making it widely accessible and reproducible by the community [204].

Being HP-STEM and ETEM complementary techniques, their integration can potentially provide a much deeper understanding of the complex processes taking place on the surface of a catalyst [192]. However, it should be mentioned that these techniques may suffer of the intrinsic defect of the microscopy methods: while providing a view of the investigated catalyst at the atomic level, they are focussed on individual objects, without any specific information on the surrounding entities. Therefore, without averaging on a huge number of observations and a statistical treatment of the imaging, they are submitted to the subjectivity of the operator and might take to a biased view of the material under observation.

Another source of problems are the frequent samples damages
induced by the microscope beam, notably in the presence of reacting
gases, a problem that can be mitigated only by operating at cumulative
doses not exceeding the threshold for a given gas [205].

10. Electron-based operando studies

Surface of catalytic materials can also be investigated using electron-
based surface analytic techniques. Their application to in situ or oper-
ando studies is more challenging than for the photon-in-photon-out
techniques, because scattering of electrons by molecules in the gaseous

<figure><img src="image_17.png" /><figcaption>**Fig. 12.** Frames from insitu ETEM observations of a GNP, during CO oxidation in a) 1 vol% CO/air (1mbar), and b) O2 (1mbar). In O2 gas, the GNP exhibited {111}, {100}, and {110} facets dynamically, while in CO/air the major {111} and {100} facets were stable. The electron flux was estimated to be 4.6 Å cm⁻² (reproduced with permission from T. Uchiyama, H. Yoshida, Y. Kuwauchi, S. Ichikawa, S. Shimada, M. Haruta, S. Takeda, Systematic Morphology Changes of Gold Nanoparticles Supported on CeO2 during CO Oxidation, Angew. Chemie-International Ed. 50 (2011) 10157–10160) [197].</figcaption></figure>
environment makes generation of electron beams of a constant energy or the collection of elastically scattered electrons difficult. However, impressive developments of sophisticated instrumentations have been performed in the last years, allowing a reliable use of these techniques also in the presence of representative reactants [192]. The determination of local catalytic structures, at the atomic scale, can be obtained using high pressure scanning tunnelling microscopy (HP-STM) and environmental transmission electron microscopy (ETEM).

Scanning Tunnelling Microscopy is based on the quantum mechanics principle that an electron can “tunnel” through a potential barrier although its energy is inferior to that of the barrier itself. This effect is exploited, in practice, using a tip which is moved over the surface of a sample [206]. The so engendered tunnelling current is proportional to the local density of states of the sample surface, allowing to identify differences in local density of states of different atoms or of adsorbates, which can thus be distinguished. One of the most interesting application

<figure><img src="image_16.png" /><figcaption>**Fig. 13.** (Upper panel) Mass spectrometer signals of O₂, CO, and CO₂, measured directly from the reactor cell. The mass spectra were recorded simultaneously with the STM movie. Labels (A)–(H) correspond to the STM images in the lower panel. Rₗlow and Rₗhigh denote the low and high CO₂-production rate branches. Pₗth indicates the threshold value of the CO pressure at which the rate switched from Rₗlow to Rₗhigh and the surface changed simultaneously from smooth to rough. (Lower panel) STM images of 210 × 210 nm² from an STM movie of a Pt(110) surface at a temperature of 425 K in a 3.0 ml/min flow of mixtures of CO and/or O₂ at 0.5 bar (65 s/image, Iₜ = 0.2 nA, Vₜ = 80 mV). The images were differentiated to enhance the contrast. Images (A), (B), (E), (F), and (H) show flat terraces separated by steps of the Pt lattice. This corresponds to the metallic, CO-covered surface. Image (C) shows the change of the surface during the step in activity (the image was built up from bottom to top). The bright scan lines and the change in slope are the result of extra thermal drift due to the increased, exothermic reaction. Images (D) and (G) show the rough surface consisting of protrusions, with heights of 0.2–0.4 nm, and pits (see inset). These are formed during the high activity stage. There is a direct correspondence between the buildup of the roughness and the total amount of produced CO₂.

(Reprinted with permission from B.L.M. Hendriksen, J.W.M. Frenken, Phys. Rev. Lett., CO oxidation on Pt(110): Scanning tunneling microscopy inside a high-pressure flow reactor 89, 46101 (2002). Copyright (2002) by the American Physical Society) [211]</figcaption></figure>
environment makes generation of electron beams of a constant energy or the collection of elastically scattered electrons difficult. However, impressive developments of sophisticated instrumentations have been performed in the last years, allowing a reliable use of these techniques also in the presence of representative reactants [192]. The determination of local catalytic structures, at the atomic scale, can be obtained using high pressure scanning tunnelling microscopy (HP-STM) and environmental transmission electron microscopy (ETEM).

Scanning Tunnelling Microscopy is based on the quantum mechanics principle that an electron can “tunnel” through a potential barrier although its energy is inferior to that of the barrier itself. This effect is exploited, in practice, using a tip which is moved over the surface of a sample [206]. The so engendered tunnelling current is proportional to the local density of states of the sample surface, allowing to identify differences in local density of states of different atoms or of adsorbates, which can thus be distinguished. One of the most interesting application

<figure><img src="image_18.png" /><figcaption>**Fig. 13.** (Upper panel) Mass spectrometer signals of O₂, CO, and CO₂, measured directly from the reactor cell. The mass spectra were recorded simultaneously with the STM movie. Labels (A)–(H) correspond to the STM images in the lower panel. Rₗlow and Rₗhigh denote the low and high CO₂-production rate branches. Pₗth indicates the threshold value of the CO pressure at which the rate switched from Rₗlow to Rₗhigh and the surface changed simultaneously from smooth to rough. (Lower panel) STM images of 210 × 210 nm² from an STM movie of a Pt(110) surface at a temperature of 425 K in a 3.0 ml/min flow of mixtures of CO and/or O₂ at 0.5 bar (65 s/image, Iₜ = 0.2 nA, Vₜ = 80 mV). The images were differentiated to enhance the contrast. Images (A), (B), (E), (F), and (H) show flat terraces separated by steps of the Pt lattice. This corresponds to the metallic, CO-covered surface. Image (C) shows the change of the surface during the step in activity (the image was built up from bottom to top). The bright scan lines and the change in slope are the result of extra thermal drift due to the increased, exothermic reaction. Images (D) and (G) show the rough surface consisting of protrusions, with heights of 0.2–0.4 nm, and pits (see inset). These are formed during the high activity stage. There is a direct correspondence between the buildup of the roughness and the total amount of produced CO₂.</figcaption></figure>

(Reprinted with permission from B.L.M. Hendriksen, J.W.M. Frenken, Phys. Rev. Lett., CO oxidation on Pt(110): Scanning tunneling microscopy inside a high-pressure flow reactor 89, 46101 (2002). Copyright (2002) by the American Physical Society) [211]