# Operando Spectroscopy to Understand Dynamic Structural Changes of Solid Catalysts

Bidyut Bikash Sarma, Jan-Dierk Grunwaldt

► To cite this version:

Bidyut Bikash Sarma, Jan-Dierk Grunwaldt. Operando Spectroscopy to Understand Dynamic Structural Changes of Solid Catalysts. CHIMIA, 2024, 78 (5), pp.288-296. 10.2533/chimia.2024.288. hal-04593870

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Submitted on 30 May 2024

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Distributed under a Creative Commons Attribution 4.0 International License
# Operando Spectroscopy to Understand Dynamic Structural Changes of Solid Catalysts

Bidyut Bikash Sarma, Jan-Dierk Grunwaldt

► To cite this version:

Bidyut Bikash Sarma, Jan-Dierk Grunwaldt. Operando Spectroscopy to Understand Dynamic Structural Changes of Solid Catalysts. CHIMIA, 2024, 78 (5), pp.288-296. 10.2533/chimia.2024.288. hal-04593870

HAL Id: hal-04593870

https://hal.science/hal-04593870v1

Submitted on 30 May 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

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

Distributed under a Creative Commons Attribution 4.0 International License
# Operando Spectroscopy to Understand Dynamic Structural Changes of Solid Catalysts

Bidyut Bikash Sarma*a,b,c* and Jan-Dierk Grunwaldt*a,b*

**Abstract:** Solid materials like heterogeneous catalysts are highly dynamic and continuously tend to change when exposed to the reaction environment. To understand the catalyst system under true reaction conditions, *operando* spectroscopy is the key to unravel small changes, which can ultimately lead to a significant difference in catalytic activity and selectivity. This was also the topic of the 7th International Congress on *Operando Spectroscopy* in Switzerland in 2023. In this article, we discuss various examples to introduce and demonstrate the importance of this area, including examples from emission control for clean air (e.g. CO oxidation), oxidation catalysis in the chemical industry (e.g. oxidation of isobutene), future power-to-X processes (electrocatalysis, CO₂ hydrogenation to methanol), and non-oxidative conversion of methane. All of these processes are equally relevant to the chemical industry. Complementary *operando* techniques such as X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and Raman spectroscopy were utilized to derive the ultimate structure of the catalyst. The variety of conditions requires distinctly different *operando* cells that can reach a temperature range of 400–1000 °C and pressures up to 40 bar. The best compromise for both the spectroscopy and the catalytic reaction is needed. As an outlook, we highlight emerging methods such as modulation-excitation spectroscopy (MES) or quick-extended X-ray absorption fine structure (QEXAFS) and X-ray photon in/out techniques, which can provide better sensitivity or extend X-ray based *operando* studies.

**Keywords:** Cell design · Dynamic structure · *Operando* spectroscopy · Solid catalyst · Synchrotron methods

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

**Bidyut Bikash Sarma** is working as chargé de recherche (equivalent to assistant professor) at Laboratoire de Chimie de Coordination (LCC), CNRS, Toulouse, France since March 2024. He received his PhD in 2016 at the Weizmann Institute of Science, Israel under the supervision of Prof. Ronny Neumann. Afterwards, he started his postdoctoral work at the department of heterogeneous catalysis at the Max-Planck-Institut für Kohlenforschung, Germany, where he received the Max-Planck-Society postdoctoral fellowship (2016) and an Alexander von Humboldt postdoctoral fellowship (2017). He moved to Karlsruhe Institute of Technology (KIT), Germany in 2020 as a senior research associate. In 2023, he worked as a Marie Skłodowska-Curie postdoctoral fellow at the University of Cambridge, UK. He is also a guest lecturer at B. Borooah College, Assam, India.

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

**Jan-Dierk Grunwaldt** is a full professor and director at Karlsruhe Institute of Technology (KIT), Germany. He gave a plenary talk at the 7th International Congress on *Operando Spectroscopy* in Grindelwald in 2023. He obtained his PhD at ETH Zürich under the supervision of Prof. A. Baiker and worked as a project manager at Topsoe (Lyngby, Denmark), a company that has pi-

oneered *in situ* and *operando* spectroscopy. Afterwards, he acted as a group leader and lecturer between 2001 and 2007 at the Institute of Chemical and Bioengineering at ETH Zürich. Since then he has strongly collaborated both with the Swiss Light Source and many other synchrotron facilities around the world. In 2008, he started as full professor at the Department of Chemical and Biochemical Engineering, DTU, Lyngby, Denmark. Since 2010, he has been a full professor in chemical technology and catalysis at KIT. He is also an adjunct professor at DTU and has recently been a guest professor at the University of Padua.

## 1. Introduction

Understanding the catalyst structure during operation (frequently referred to as ‘*in situ*’ or ‘*operando*’) is one of the most challenging tasks in catalysis research, but has become possible with the development of modern spectroscopic tools.[1,2] The term ‘*operando*’ was introduced while ago[3–5] and refers to investigating the catalyst structure under the true reaction parameters (same temperature, pressure, reaction rates, selectivities, spatiotemporal profile, *etc.*) while simultaneously measuring activity and selectivity, as defined by Banares *et al.*[4,6] Studies that are not able to meet these criteria should be referred to as ‘*in situ*’ investigations. This led to the first International Congress on *Operando Spectroscopy* in Lunteren, Netherlands in 2003. 20 years later, the 7th International Congress on *Operando Spectroscopy* congress was held in Grindelwald in Switzerland (7th – 11th of May, 2023), organized by Davide Ferri and Maarten Nachtegaal (Paul Scherrer Institute, Villigen, Switzerland). Swiss groups are and have been very ac-

<footnote>*Correspondence: Dr. B. B. Sarma, E-mail: bidyut-bikash.sarma@lcc-toulouse.fr, Prof. Dr. J.-D. Grunwaldt, E-mail: grunwaldt@kit.edu;</footnote>

<footnote>†Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstrasse 20, 76131 Karlsruhe, Germany; ‡Institute of Catalysis Research and Technology, KIT, Hermann-von Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen, Germany; ©Laboratoire de Chimie de Coordination (LCC), UPR 8241 CNRS, Université de Toulouse, INPT, 205 route de Narbonne, 31077, Toulouse Cedex 4, France</footnote>
# Operando Spectroscopy to Understand Dynamic Structural Changes of Solid Catalysts

Bidyut Bikash Sarma*a,b,c* and Jan-Dierk Grunwaldt*a,b*

**Abstract:** Solid materials like heterogeneous catalysts are highly dynamic and continuously tend to change when exposed to the reaction environment. To understand the catalyst system under true reaction conditions, *operando* spectroscopy is the key to unravel small changes, which can ultimately lead to a significant difference in catalytic activity and selectivity. This was also the topic of the 7th International Congress on *Operando Spectroscopy* in Switzerland in 2023. In this article, we discuss various examples to introduce and demonstrate the importance of this area, including examples from emission control for clean air (e.g. CO oxidation), oxidation catalysis in the chemical industry (e.g. oxidation of isobutene), future power-to-X processes (electrocatalysis, CO₂ hydrogenation to methanol), and non-oxidative conversion of methane. All of these processes are equally relevant to the chemical industry. Complementary *operando* techniques such as X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and Raman spectroscopy were utilized to derive the ultimate structure of the catalyst. The variety of conditions requires distinctly different *operando* cells that can reach a temperature range of 400–1000 °C and pressures up to 40 bar. The best compromise for both the spectroscopy and the catalytic reaction is needed. As an outlook, we highlight emerging methods such as modulation-excitation spectroscopy (MES) or quick-extended X-ray absorption fine structure (QEXAFS) and X-ray photon in/out techniques, which can provide better sensitivity or extend X-ray based *operando* studies.

**Keywords:** Cell design · Dynamic structure · *Operando* spectroscopy · Solid catalyst · Synchrotron methods

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

**Bidyut Bikash Sarma** is working as chargé de recherche (equivalent to assistant professor) at Laboratoire de Chimie de Coordination (LCC), CNRS, Toulouse, France since March 2024. He received his PhD in 2016 at the Weizmann Institute of Science, Israel under the supervision of Prof. Ronny Neumann. Afterwards, he started his postdoctoral work at the department of heterogeneous catalysis at the Max-Planck-Institut für Kohlenforschung, Germany, where he received the Max-Planck-Society postdoctoral fellowship (2016) and an Alexander von Humboldt postdoctoral fellowship (2017). He moved to Karlsruhe Institute of Technology (KIT), Germany in 2020 as a senior research associate. In 2023, he worked as a Marie Skłodowska-Curie postdoctoral fellow at the University of Cambridge, UK. He is also a guest lecturer at B. Borooah College, Assam, India.

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

**Jan-Dierk Grunwaldt** is a full professor and director at Karlsruhe Institute of Technology (KIT), Germany. He gave a plenary talk at the 7th International Congress on *Operando Spectroscopy* in Grindelwald in 2023. He obtained his PhD at ETH Zürich under the supervision of Prof. A. Baiker and worked as a project manager at Topsoe (Lyngby, Denmark), a company that has pi-

oneered *in situ* and *operando* spectroscopy. Afterwards, he acted as a group leader and lecturer between 2001 and 2007 at the Institute of Chemical and Bioengineering at ETH Zürich. Since then he has strongly collaborated both with the Swiss Light Source and many other synchrotron facilities around the world. In 2008, he started as full professor at the Department of Chemical and Biochemical Engineering, DTU, Lyngby, Denmark. Since 2010, he has been a full professor in chemical technology and catalysis at KIT. He is also an adjunct professor at DTU and has recently been a guest professor at the University of Padua.

## 1. Introduction

Understanding the catalyst structure during operation (frequently referred to as ‘*in situ*’ or ‘*operando*’) is one of the most challenging tasks in catalysis research, but has become possible with the development of modern spectroscopic tools.[1,2] The term ‘*operando*’ was introduced while ago[3–5] and refers to investigating the catalyst structure under the true reaction parameters (same temperature, pressure, reaction rates, selectivities, spatiotemporal profile, etc.) while simultaneously measuring activity and selectivity, as defined by Banares *et al.*[4,6] Studies that are not able to meet these criteria should be referred to as ‘*in situ*’ investigations. This led to the first International Congress on *Operando Spectroscopy* in Lunteren, Netherlands in 2003. 20 years later, the 7th International Congress on *Operando Spectroscopy* congress was held in Grindelwald in Switzerland (7th – 11th of May, 2023), organized by Davide Ferri and Maarten Nachtegaal (Paul Scherrer Institute, Villigen, Switzerland). Swiss groups are and have been very ac-

<footnote>*Correspondence: Dr. B. B. Sarma, E-mail: bidyut-bikash.sarma@lcc-toulouse.fr, Prof. Dr. J.-D. Grunwaldt, E-mail: grunwaldt@kit.edu;</footnote>

<footnote>†Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstrasse 20, 76131 Karlsruhe, Germany; ‡Institute of Catalysis Research and Technology, KIT, Hermann-von Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen, Germany; ©Laboratoire de Chimie de Coordination (LCC), UPR 8241 CNRS, Université de Toulouse, INPT, 205 route de Narbonne, 31077, Toulouse Cedex 4, France</footnote>
tive in this field of research.[7-9] Since the early times, *operando* spectroscopy has revolutionized the in-depth understanding of solid catalysts, which benefited both academia as well as many industrial processes.[10] One of the early catalytic systems studied with *in situ* and *operando* spectroscopy was the CO/CO₂-hydrogenation to methanol.[11] With *in situ/operando* spectroscopy and microscopy, the structural dynamics in the Cu/ZnO system for methanol synthesis were demonstrated.[12] Early research speculated that oxygen vacancies on the ZnO surface can lead to a stronger interaction of the copper particles with the ZnO surface during the reaction, which in turn leads to a morphological change.[13] Moreover, Nakamura *et al.* showed that Zn atoms from the ZnO support can migrate to the Cu surface and form a Cu-Zn surface alloy, which accelerates the CO₂ hydrogenation reaction.[14] The structure is not yet fully understood, but it is still believed that the interfacial Zn species play a major role during the reaction. The nature of these Zn species is not metallic but instead positively charged under industrial methanol synthesis conditions.[15-17] Another prominent example is the Au-catalyzed hydrochlorination of acetylene where *operando* spectroscopy shed light on the state of the Au species during the reaction.[18] It was shown that a single site redox pair of Au¹/³¹ Au³¹³ supported on carbon is a crucial factor during the reaction, and a highly oxidizing Cl₂ stream is responsible for maintaining the redox pair. This process, which was up to that point carried out at an industrial scale with hazardous HgCl₂/C catalysts[19] to produce over 40 million tons of vinyl chloride annually, has now been commercially replaced with the Au/C catalyst; thanks in part due to these *operando* studies.[20] Another example would be the selective catalytic reduction (SCR) of NOₓ by NH₃ over Cu/zeolite catalysts, which is one of the most crucial reactions concerning the reduction of emissions of harmful gases to the environment.[21] Recently, it was shown that the reaction occurs over isolated Cu¹¹ sites, which are located inside the zeolite structure.[22-24] Further work by Fahami *et al.* revealed that these isolated Cu sites interconvert between Cu¹¹ and Cu¹ during the reaction. They generate complexes with ammonia such as Cu(NH₃)₂⁺ and further form dimeric bis(μ-oxo)Cu species in oxidizing gas mixtures.[25] Becher *et al.* further investigated Cu-SSZ-13 catalysts by *operando* X-

ray spectrotomography at micrometer spatial resolution, which revealed the local structural changes of the Cu catalysts in 3D under operating conditions.[26] A final introductory example of the benefit of *operando* spectroscopy would be methane dehydroaromatization (DHA), a highly valuable process for converting methane to high-density products such as olefins or aromatic compounds over Mo catalysts.[27] Recent *operando* investigations showed that isolated Mo sites inside the zeolite were essential during the reaction.[28] The isolated MoOₓ species agglomerate during the reaction, resulting in molybdenum carbide formation, which reversibly converts back to the isolated site in the presence of oxygen. A more recent study showed that under non-oxidative conditions, methyl radicals form over the isolated Mo centers, which further transform to ethane and finally to ethylene after a dehydrogenation step.[29] This variety of selected studies demonstrates the potential of *in situ/operando* spectroscopy in unraveling the active site of complex systems. Therefore, the continuous development of more sophisticated techniques is inevitable. Further examples can be found in this special issue of *CHIMIA*. In a recent review, we discussed the implementation of synchrotron-based *in situ/operando* X-ray techniques to elucidate the dynamic structural changes of single-atom catalysts under operating conditions.[30] In the present article, we briefly outline the challenge of *operando* spectroscopy, in particular, finding the best compromise between running a catalytic reaction and performing spectroscopy at the same time, and then introduce various examples from our own research. Strategies to design appropriate cells to cope with the different reaction conditions, for example, in liquid-solid, gas-solid, photochemical, or electrochemical systems, or even at mixed interfaces are also highlighted in this review as well as in others from our group in the past.[31]

## 2. The Challenge of Operando Spectroscopy and Design of Appropriate Cells

In *operando* spectroscopy, the simultaneous structure determination and measurement of the catalytic performance are essential. Fig. 1, shows various spectroscopic techniques that can be adopted when light of different wavelengths interacts with the

<figure><img src="image_4.png" /><figcaption>Fig. 1. (a) Interaction of light with the catalyst under operating conditions, which can be exploited *via* various spectroscopic techniques, and (b) several *operando* cells designed in the earlier studies by our group. Adapted with permission from ref. [32].</figcaption></figure>
tive in this field of research.[7-9] Since the early times, *operando* spectroscopy has revolutionized the in-depth understanding of solid catalysts, which benefited both academia as well as many industrial processes.[10] One of the early catalytic systems studied with *in situ* and *operando* spectroscopy was the CO/CO₂-hydrogenation to methanol.[11] With *in situ/operando* spectroscopy and microscopy, the structural dynamics in the Cu/ZnO system for methanol synthesis were demonstrated.[12] Early research speculated that oxygen vacancies on the ZnO surface can lead to a stronger interaction of the copper particles with the ZnO surface during the reaction, which in turn leads to a morphological change.[13] Moreover, Nakamura *et al.* showed that Zn atoms from the ZnO support can migrate to the Cu surface and form a Cu-Zn surface alloy, which accelerates the CO₂ hydrogenation reaction.[14] The structure is not yet fully understood, but it is still believed that the interfacial Zn species play a major role during the reaction. The nature of these Zn species is not metallic but instead positively charged under industrial methanol synthesis conditions.[15-17] Another prominent example is the Au-catalyzed hydrochlorination of acetylene where *operando* spectroscopy shed light on the state of the Au species during the reaction.[18] It was shown that a single site redox pair of Au¹/³¹ Au³¹³ supported on carbon is a crucial factor during the reaction, and a highly oxidizing Cl₂ stream is responsible for maintaining the redox pair. This process, which was up to that point carried out at an industrial scale with hazardous HgCl₂/C catalysts[19] to produce over 40 million tons of vinyl chloride annually, has now been commercially replaced with the Au/C catalyst; thanks in part due to these *operando* studies.[20] Another example would be the selective catalytic reduction (SCR) of NOₓ by NH₃ over Cu/zeolite catalysts, which is one of the most crucial reactions concerning the reduction of emissions of harmful gases to the environment.[21] Recently, it was shown that the reaction occurs over isolated Cu¹¹ sites, which are located inside the zeolite structure.[22-24] Further work by Fahami *et al.* revealed that these isolated Cu sites interconvert between Cu¹¹ and Cu¹ during the reaction. They generate complexes with ammonia such as Cu(NH₃)₂⁺ and further form dimeric bis(μ-oxo)Cu species in oxidizing gas mixtures.[25] Becher *et al.* further investigated Cu-SSZ-13 catalysts by *operando* X-

ray spectrotomography at micrometer spatial resolution, which revealed the local structural changes of the Cu catalysts in 3D under operating conditions.[26] A final introductory example of the benefit of *operando* spectroscopy would be methane dehydroaromatization (DHA), a highly valuable process for converting methane to high-density products such as olefins or aromatic compounds over Mo catalysts.[27] Recent *operando* investigations showed that isolated Mo sites inside the zeolite were essential during the reaction.[28] The isolated MoOₓ species agglomerate during the reaction, resulting in molybdenum carbide formation, which reversibly converts back to the isolated site in the presence of oxygen. A more recent study showed that under non-oxidative conditions, methyl radicals form over the isolated Mo centers, which further transform to ethane and finally to ethylene after a dehydrogenation step.[29] This variety of selected studies demonstrates the potential of *in situ/operando* spectroscopy in unraveling the active site of complex systems. Therefore, the continuous development of more sophisticated techniques is inevitable. Further examples can be found in this special issue of *CHIMIA*. In a recent review, we discussed the implementation of synchrotron-based *in situ/operando* X-ray techniques to elucidate the dynamic structural changes of single-atom catalysts under operating conditions.[30] In the present article, we briefly outline the challenge of *operando* spectroscopy, in particular, finding the best compromise between running a catalytic reaction and performing spectroscopy at the same time, and then introduce various examples from our own research. Strategies to design appropriate cells to cope with the different reaction conditions, for example, in liquid-solid, gas-solid, photochemical, or electrochemical systems, or even at mixed interfaces are also highlighted in this review as well as in others from our group in the past.[31]

## 2. The Challenge of Operando Spectroscopy and Design of Appropriate Cells

In *operando* spectroscopy, the simultaneous structure determination and measurement of the catalytic performance are essential. Fig. 1, shows various spectroscopic techniques that can be adopted when light of different wavelengths interacts with the

<figure><img src="image_4.png" /><figcaption>Fig. 1. (a) Interaction of light with the catalyst under operating conditions, which can be exploited *via* various spectroscopic techniques, and (b) several *operando* cells designed in the earlier studies by our group. Adapted with permission from ref. [32].</figcaption></figure>
catalyst as well as some of the early designs of *operando* cells that have been developed by our group.[32] In order to find the best compromise for tracking the catalyst by *operando* spectroscopy with conditions as close as possible to working conditions and spectroscopy data with the highest quality, we propose to follow the well-known 10 commandments[33,34] for a catalytic experiment as shown in Fig. 2. Why are these commandments important? Let us consider the internal and external mass transport in solid catalysts, which if not appropriated for can ultimately lead to misinterpretation of data obtainable from *operando* studies.[32] In order to obtain the best compromise between optimal design for spectroscopic characterization and catalytic results, film diffusion ($\tau_{\text{film}}$), pore diffusion ($\tau_{\text{pore}}$) and the connected effectiveness factor (Thiele modulus) play an important role and need to be considered.[32] In addition, the shape of the catalyst, measurement geometry, reaction interfaces, and window materials for cells also need to be considered. To meet these criteria, some of the best beamlines to combine for *operando* studies are SuperXAS at PSI,[35] CATACT at KIT light source,[36] ROCK/SAMBA at SOLEIL,[37] and NSLS at Brookhaven National Laboratory,[38] where multiple spectroscopic techniques (XAS, DRIFTS, XRD, Raman spectroscopy etc.) can be implemented or combined.

<figure><img src="image_5.png" /><figcaption>Fig. 2. 10 commandments for *operando* spectroscopy covering a wide range of reaction conditions.</figcaption></figure>

Over the past two decades, our group has been continuously developing new tools for *operando* research covering a wide variety of reaction conditions.[30,31,39] Several different spectroscopic techniques have therefore been employed and some of the main information we have received with it is summarized in Fig. 3. In the following sections, we will explore some recent examples of these *operando* studies in more detail.

## 2.1 Selective Oxidation of Isobutene Using Operando Synchrotron-based X-ray Absorption Spectroscopy, Raman Spectroscopy and X-ray Diffraction

A nice and recent example for the challenges in *operando* spectroscopy is the oxidation of isobutene to methacrolein, which is an important intermediate in the production of polymethyl methacrylate (PMMA), commonly known as acrylic glass.[40] This reaction is catalyzed by a Bi-Mo multicomponent catalyst and the role of each component is strongly debated.[41] For example, why do 4-component Bi-Mo-Co-Fe-O systems perform better than the 2-component Bi-Mo-O systems? Even among the 2-component Bi-Mo-O systems, various phases, for example, α-Bi₂Mo₃O₁₂, β-Bi₁Mo₂O₉, and γ-Bi₂MoO₆ show different selectivity, requiring in-depth investigation. To answer those questions, we have synthesized the Bi-Mo-Co-Fe-O catalysts for *operando* investigations by flame spray pyrolysis (FSP), enabling a uniform nanocrystalline metal oxide with high surface area.[42] Three catalysts with different metal ratios were synthesized. While *operando* Raman spectroscopy provided insights into the different bulk phases of the catalysts which evolved during the reaction, *operando* XRD in combination with Rietveld refinement revealed the crystallinity and the quantitative composition of the metal oxide phases. The *operando* XAS measurements were carried out at Mo K-, Bi L₃-, Co K-, and Fe K-edges independently and simultaneously in a quartz micro-reactor. Synchrotron-based XRD techniques did not only provide information on the structural transformations of the crystalline phases in the catalysts but also showed the presence of several additional phases that could not be detected with laboratory-based XRD.

The findings from the *operando* studies are summarized in Fig. 4. The catalysts containing less than 50% Mo (FSP-Fe and FSP-Co) showed the formation of single metal oxides (Fe₃O₄, Co₃O₄, Bi₂O₃), which resulted in high conversion and formation of undesired (overoxidation) products, such as CO and CO₂. On the other hand, the catalysts containing 50% Mo (FSP-Ü) resulted in better selectivity via transformations into Mo structures such as α-Bi₂Mo₃O₁₂ and Fe₂Mo₃O₁₂. Additionally, the presence of β-CoMoO₄/β-Co₀.₇Fe₀.₃MoO₄, α-Bi₂Mo₃O₁₂ and Bi₃FeMo₂O₁₂ phases in the catalyst, FSP-Ü resulted in the best catalytic performance. XAS investigations proved that Fe III is converted to Fe II during the reaction. We further explored temperature and activity correlations by spatial reactor profile measurements, which will be combined with *operando* techniques in the future.[43]

We have recently discussed how various 1D, 2D and 3D advanced characterization techniques can be used to derive structural information on such Mo-based catalysts as shown in Fig. 5 in our concept article.[44] These results will further open up new possibilities for designing the next generation of catalysts for such an industrially relevant reaction.

<figure><img src="image_6.png" /><figcaption>Fig. 3. Schematics of *operando* spectroscopy and the information derived from a few *operando* techniques are highlighted in this review.</figcaption></figure>
catalyst as well as some of the early designs of *operando* cells that have been developed by our group.[32] In order to find the best compromise for tracking the catalyst by *operando* spectroscopy with conditions as close as possible to working conditions and spectroscopy data with the highest quality, we propose to follow the well-known 10 commandments[33,34] for a catalytic experiment as shown in Fig. 2. Why are these commandments important? Let us consider the internal and external mass transport in solid catalysts, which if not appropriated for can ultimately lead to misinterpretation of data obtainable from *operando* studies.[32] In order to obtain the best compromise between optimal design for spectroscopic characterization and catalytic results, film diffusion ($\tau_{\text{film}}$), pore diffusion ($\tau_{\text{pore}}$) and the connected effectiveness factor (Thiele modulus) play an important role and need to be considered.[32] In addition, the shape of the catalyst, measurement geometry, reaction interfaces, and window materials for cells also need to be considered. To meet these criteria, some of the best beamlines to combine for *operando* studies are SuperXAS at PSI,[35] CATACT at KIT light source,[36] ROCK/SAMBA at SOLEIL,[37] and NSLS at Brookhaven National Laboratory,[38] where multiple spectroscopic techniques (XAS, DRIFTS, XRD, Raman spectroscopy etc.) can be implemented or combined.

<figure><img src="image_5.png" /><figcaption>Fig. 2. 10 commandments for *operando* spectroscopy covering a wide range of reaction conditions.</figcaption></figure>

Over the past two decades, our group has been continuously developing new tools for *operando* research covering a wide variety of reaction conditions.[30,31,39] Several different spectroscopic techniques have therefore been employed and some of the main information we have received with it is summarized in Fig. 3. In the following sections, we will explore some recent examples of these *operando* studies in more detail.

## 2.1 Selective Oxidation of Isobutene Using Operando Synchrotron-based X-ray Absorption Spectroscopy, Raman Spectroscopy and X-ray Diffraction

A nice and recent example for the challenges in *operando* spectroscopy is the oxidation of isobutene to methacrolein, which is an important intermediate in the production of polymethyl methacrylate (PMMA), commonly known as acrylic glass.[40] This reaction is catalyzed by a Bi-Mo multicomponent catalyst and the role of each component is strongly debated.[41] For example, why do 4-component Bi-Mo-Co-Fe-O systems perform better than the 2-component Bi-Mo-O systems? Even among the 2-component Bi-Mo-O systems, various phases, for example, α-Bi₂Mo₃O₁₂, β-Bi₁Mo₂O₉, and γ-Bi₂MoO₆ show different selectivity, requiring in-depth investigation. To answer those questions, we have synthesized the Bi-Mo-Co-Fe-O catalysts for *operando* investigations by flame spray pyrolysis (FSP), enabling a uniform nanocrystalline metal oxide with high surface area.[42] Three catalysts with different metal ratios were synthesized. While *operando* Raman spectroscopy provided insights into the different bulk phases of the catalysts which evolved during the reaction, *operando* XRD in combination with Rietveld refinement revealed the crystallinity and the quantitative composition of the metal oxide phases. The *operando* XAS measurements were carried out at Mo K-, Bi L₃-, Co K-, and Fe K-edges independently and simultaneously in a quartz micro-reactor. Synchrotron-based XRD techniques did not only provide information on the structural transformations of the crystalline phases in the catalysts but also showed the presence of several additional phases that could not be detected with laboratory-based XRD.

The findings from the *operando* studies are summarized in Fig. 4. The catalysts containing less than 50% Mo (FSP-Fe and FSP-Co) showed the formation of single metal oxides (Fe₃O₄, Co₃O₄, Bi₂O₃), which resulted in high conversion and formation of undesired (overoxidation) products, such as CO and CO₂. On the other hand, the catalysts containing 50% Mo (FSP-U) resulted in better selectivity via transformations into Mo structures such as α-Bi₂Mo₃O₁₂ and Fe₂Mo₃O₁₂. Additionally, the presence of β-CoMoO₄/β-Co₀.₇Fe₀.₃MoO₄, α-Bi₂Mo₃O₁₂ and Bi₃FeMo₂O₁₂ phases in the catalyst, FSP-U resulted in the best catalytic performance. XAS investigations proved that Fe III is converted to Fe II during the reaction. We further explored temperature and activity correlations by spatial reactor profile measurements, which will be combined with *operando* techniques in the future.[43]

We have recently discussed how various 1D, 2D and 3D advanced characterization techniques can be used to derive structural information on such Mo-based catalysts as shown in Fig. 5 in our concept article.[44] These results will further open up new possibilities for designing the next generation of catalysts for such an industrially relevant reaction.

<figure><img src="image_6.png" /><figcaption>Fig. 3. Schematics of *operando* spectroscopy and the information derived from a few *operando* techniques are highlighted in this review.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 4. (a) Description of catalyst components synthesized by FSP and the corresponding metal oxide phases characterized by laboratory-based techniques (XRD and Raman) and (b) schematic representation of the isobutene oxidation with various catalyst compositions identified by synchrotron-based techniques under operating conditions. Green represents metal oxide phases favoring the selective reaction pathway to methacrolein (MAC); red represents metal oxide phases favoring the unselective reaction pathways (formation of CO and CO₂), and gray represents primary phases detected in all three catalyst systems. Catalyst compositions are FSP-Co (5% Bi, 35% Mo, 40% Co, 20% Fe), FSP-Fe (5% Bi, 35% Mo, 20% Co, 40% Fe), and FSP-U (4.2% Bi, 50% Mo, 33.3% Co, 12.5% Fe). Adapted with permission from ref. [42].</figcaption></figure>

## 2.2 Investigation of CO₂ Hydrogenation to Methanol via in situ DRIFTS and Operando XAS

Hydrogenation of CO₂ to methanol is one of the most promising routes to mitigate the CO₂ in the atmosphere and to tackle global warming.[45] As discussed in the introduction, Cu/ZnO is one of the most investigated catalysts for the CO₂ hydrogenation to methanol,[17,46,47] where Zn tends to migrate to the Cu surface during the reaction. Our first question is: How is Zn influenced by Cu? Many studies have addressed this question. Recently, we have synthesized an inverse ZnO/Cu/MgO catalyst and conducted in situ XAS and XRD studies.[48] The results showed that the ZnO fraction in direct contact with Cu is reduced first and afterwards, a Cu-Zn surface alloy formed above 300 °C. Above 400 °C, we observed bulk alloy formation. Finally, we found that the reduced Zn-species at the interface played a pivotal role during the methanol synthesis.

A further question was if the migration of Zn could be potentially controlled by adding another component, which would suppress formate decomposition to CO decreasing the H₂ dissociation energy, would this boost the methanol production rate?[49] Zirconia is one of the supports, which is known to enhance the methanol production rate. Therefore, we have synthesized a series of Cu-Zn-Zr catalysts (Cu-ZnZr, Zr-CuZn, Zn-CuZr, and CuZnZr) applying double-nozzle flame spray pyrolysis (DFSP) and tested them for CO₂ hydrogenation to methanol.[50] The various catalysts synthesized for this study, their catalytic activity, and the proposed reaction pathways are shown in Fig. 6.

The operando XAS study carried out in a tailor-made stainless steel high pressure cell[51] revealed that during the reaction

highly dispersed Zn species are induced due to the strong interaction of Zn-Zr, which leads to a decrease in formate decomposition to CO and of the H₂ dissociation energy. Therefore, the methanol production was significantly enhanced. The in situ and

<figure><img src="image_8.png" /><figcaption>Fig. 5. Toolbox of advanced characterization techniques for deriving structural information of Bi-Mo-O-based catalysts for selective oxidation of olefins under operando conditions. Reproduced with permission from ref. [46].</figcaption></figure>
<figure><img src="image_7.png" /></figure>

Fig. 4. (a) Description of catalyst components synthesized by FSP and the corresponding metal oxide phases characterized by laboratory-based techniques (XRD and Raman) and (b) schematic representation of the isobutene oxidation with various catalyst compositions identified by synchrotron-based techniques under operating conditions. Green represents metal oxide phases favoring the selective reaction pathway to methacrolein (MAC); red represents metal oxide phases favoring the unselective reaction pathways (formation of CO and CO₂), and gray represents primary phases detected in all three catalyst systems. Catalyst compositions are FSP-Co (5% Bi, 35% Mo, 40% Co, 20% Fe), FSP-Fe (5% Bi, 35% Mo, 20% Co, 40% Fe), and FSP-U (4.2% Bi, 50% Mo, 33.3% Co, 12.5% Fe). Adapted with permission from ref. [42].

## 2.2 Investigation of CO₂ Hydrogenation to Methanol via in situ DRIFTS and Operando XAS

Hydrogenation of CO₂ to methanol is one of the most promising routes to mitigate the CO₂ in the atmosphere and to tackle global warming.[45] As discussed in the introduction, Cu/ZnO is one of the most investigated catalysts for the CO₂ hydrogenation to methanol,[17,46,47] where Zn tends to migrate to the Cu surface during the reaction. Our first question is: How is Zn influenced by Cu? Many studies have addressed this question. Recently, we have synthesized an inverse ZnO/Cu/MgO catalyst and conducted in situ XAS and XRD studies.[48] The results showed that the ZnO fraction in direct contact with Cu is reduced first and afterwards, a Cu-Zn surface alloy formed above 300 °C. Above 400 °C, we observed bulk alloy formation. Finally, we found that the reduced Zn-species at the interface played a pivotal role during the methanol synthesis.

A further question was if the migration of Zn could be potentially controlled by adding another component, which would suppress formate decomposition to CO decreasing the H₂ dissociation energy, would this boost the methanol production rate?[49] Zirconia is one of the supports, which is known to enhance the methanol production rate. Therefore, we have synthesized a series of Cu-Zn-Zr catalysts (Cu-ZnZr, Zr-CuZn, Zn-CuZr, and CuZnZr) applying double-nozzle flame spray pyrolysis (DFSP) and tested them for CO₂ hydrogenation to methanol.[50] The various catalysts synthesized for this study, their catalytic activity, and the proposed reaction pathways are shown in Fig. 6.

The operando XAS study carried out in a tailor-made stainless steel high pressure cell[51] revealed that during the reaction

highly dispersed Zn species are induced due to the strong interaction of Zn-Zr, which leads to a decrease in formate decomposition to CO and of the H₂ dissociation energy. Therefore, the methanol production was significantly enhanced. The in situ and

<figure><img src="image_8.png" /></figure>

Fig. 5. Toolbox of advanced characterization techniques for deriving structural information of Bi-Mo-O-based catalysts for selective oxidation of olefins under operando conditions. Reproduced with permission from ref. [46].
<figure><img src="image_9.png" /><figcaption>Fig. 6. (a) Schematic representation of the DFSP method, (b) Composition of the catalysts synthesized by DFSP method, (c) Methanol yield and selectivity obtained from CO₂ hydrogenation, and (d) Proposed mechanistic pathway towards CO₂ hydrogenation. DFSP and SFSP stand for double and single flame spray pyrolysis respectively. Adapted with permission from ref. [50].</figcaption></figure>

operando DRIFTS studies were carried out at a temperature of 230 °C and a pressure of 3.0 MPa using a dedicated reaction cell (PIKE Diffus IR<sub>TM</sub>). We observed that the Zn species, which formed during the reaction, promoted the selective conversion of CO₂ to methoxy species and subsequently to methanol via the strong formate adsorption. These results reveal that the nature of the Zn species is truly essential during methanol synthesis and can be modified with the introduction of other components such as Zr. With the help of operando spectroscopy we further found that the introduction of single-site Zr species in a Cu catalyst supported over amorphous SiO₂ promotes reverse water-gas shift + CO-hydro pathway, thus improving methanol production.[52]

In another recent high pressure operando study by our group, using a similar operando cell, we have conducted operando XRD and XAS investigations on Fischer-Tropsch synthesis at 30 bar and 250 °C, thereby bridging the gap between industrially relevant catalysts and fundamental science at synchrotron radiation facilities.[51] This turns out to be very important also in the field of renewable aviation fuels (see e.g. project CARE-O-SENE, https://care-o-sene.com). Pioneering research has been conducted here by the University of Cape Town and SASOL using especially in situ and operando magnetometry.[53,54]

## 2.3. Operando Electrochemical Oxygen Evolution Reaction (OER) – Overcoming the Influence of Bubble Formation

Production of green hydrogen via water splitting is one of the prominent ways to replace fossil based resources and to meet the future energy demand.[55] During water electrolysis, the oxygen

evolution reaction (OER) occurs at the anode whereas the hydrogen evolution reaction (HER) takes place at the cathode. It requires four electrons and high over potentials for the OER process. OER is best catalyzed by Ir- and Ru- based oxide catalysts. Abbott *et al.* with the help of operando XAS showed correlation between particle size, morphology, and the surface hydroxyl layer of the IrOₓ based catalysts during OER.[56] According to the report, IrO₂ synthesized at 350 °C, and consisting of 1.7±0.4 nm particles with a specific surface area of 150 m²g⁻¹, shows the highest OER activity. Reier *et al.* found that crystalline IrO₂ obtained at high temperature is detrimental, whereas amorphous Ir oxy-hydroxides present at low temperature are highly active and efficient catalysts for the OER.[57] Pfeifer *et al.* with the help of in situ XPS found that the surface of metallic Ir converts to a mixed valent species containing both O¹⁻ and O⁻, which is of high relevance during OER.[58] Following these pioneering works, our group has investigated the OER mechanism in-depth with the help of operando XAS and probed the high stability of IrO₂ as well as the Ir-Ir interaction during OER at high potentials.[59] As an alternative to in situ XPS,[58] modulation excitation spectroscopy (MES) in combination with XAS was implemented to achieve surface sensitivity and to understand the dynamic structural changes during the reaction. The XAS experiments were performed with the electrochemical flow cell setup at the SuperXAS beamline of the Swiss Light Source.[60] We found that oxygen vacancies in the IrO₂ are crucial in its stability and thus play an important role in the mechanism of the OER when changing from low to high overpotentials, as shown in Fig. 7. Relevant points to consider in the future are mass transport processes and improved electrocatalysts.
<figure><img src="image_9.png" /><figcaption>Fig. 6. (a) Schematic representation of the DFSP method, (b) Composition of the catalysts synthesized by DFSP method, (c) Methanol yield and selectivity obtained from CO₂ hydrogenation, and (d) Proposed mechanistic pathway towards CO₂ hydrogenation. DFSP and SFSP stand for double and single flame spray pyrolysis respectively. Adapted with permission from ref. [50].</figcaption></figure>

operando DRIFTS studies were carried out at a temperature of 230 °C and a pressure of 3.0 MPa using a dedicated reaction cell (PIKE Diffus IR<sub>TM</sub>). We observed that the Zn species, which formed during the reaction, promoted the selective conversion of CO₂ to methoxy species and subsequently to methanol via the strong formate adsorption. These results reveal that the nature of the Zn species is truly essential during methanol synthesis and can be modified with the introduction of other components such as Zr. With the help of operando spectroscopy we further found that the introduction of single-site Zr species in a Cu catalyst supported over amorphous SiO₂ promotes reverse water-gas shift + CO-hydro pathway, thus improving methanol production.[52]

In another recent high pressure operando study by our group, using a similar operando cell, we have conducted operando XRD and XAS investigations on Fischer-Tropsch synthesis at 30 bar and 250 °C, thereby bridging the gap between industrially relevant catalysts and fundamental science at synchrotron radiation facilities.[51] This turns out to be very important also in the field of renewable aviation fuels (see e.g. project CARE-O-SENE, https://care-o-sene.com). Pioneering research has been conducted here by the University of Cape Town and SASOL using especially in situ and operando magnetometry.[53,54]

## 2.3. Operando Electrochemical Oxygen Evolution Reaction (OER) – Overcoming the Influence of Bubble Formation

Production of green hydrogen via water splitting is one of the prominent ways to replace fossil based resources and to meet the future energy demand.[55] During water electrolysis, the oxygen

evolution reaction (OER) occurs at the anode whereas the hydrogen evolution reaction (HER) takes place at the cathode. It requires four electrons and high over potentials for the OER process. OER is best catalyzed by Ir- and Ru- based oxide catalysts. Abbott *et al.* with the help of operando XAS showed correlation between particle size, morphology, and the surface hydroxyl layer of the IrOₓ based catalysts during OER.[56] According to the report, IrO₂ synthesized at 350 °C, and consisting of 1.7±0.4 nm particles with a specific surface area of 150 m²g⁻¹, shows the highest OER activity. Reier *et al.* found that crystalline IrO₂ obtained at high temperature is detrimental, whereas amorphous Ir oxy-hydroxides present at low temperature are highly active and efficient catalysts for the OER.[57] Pfeifer *et al.* with the help of in situ XPS found that the surface of metallic Ir converts to a mixed valent species containing both O¹⁻ and O⁻, which is of high relevance during OER.[58] Following these pioneering works, our group has investigated the OER mechanism in-depth with the help of operando XAS and probed the high stability of IrO₂ as well as the Ir-Ir interaction during OER at high potentials.[59] As an alternative to in situ XPS,[58] modulation excitation spectroscopy (MES) in combination with XAS was implemented to achieve surface sensitivity and to understand the dynamic structural changes during the reaction. The XAS experiments were performed with the electrochemical flow cell setup at the SuperXAS beamline of the Swiss Light Source.[60] We found that oxygen vacancies in the IrO₂ are crucial in its stability and thus play an important role in the mechanism of the OER when changing from low to high overpotentials, as shown in Fig. 7. Relevant points to consider in the future are mass transport processes and improved electrocatalysts.
<figure><img src="image_10.png" /><figcaption>Fig. 7. (a) Summary of the *operando* XAS results during OER over an IrOₓ catalyst, and (b) proposed adsorbate evolution mechanism (AEM) during OER in acidic medium. Adapted with permission from ref. [59].</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. (a) Normalized XANES spectra at the Pt L₃-edge of the Pt/CeO₂ (FSP) catalyst at various temperatures under He atmosphere, (b) *operando* XANES spectra at 975 °C in 90% CH₄/He atmosphere and (c) pulse reaction products at 975 °C. Adapted with permission from ref. [64].</figcaption></figure>

## 2.4. Operando XAS Investigation for the Non-oxidative Coupling of Methane – The Challenge of High Temperature

Direct conversion of methane to olefins and aromatics is one of the holy grails in comparison to the traditional multi-step processes of syngas conversion *via* Fischer-Tropsch (FT) synthesis or the methanol to olefin (MTO) process.[61,62] However, direct conversion of methane occurs at a much higher temperature (typically above 600 °C) than in MTO or FT synthesis processes, which makes it challenging to investigate the catalyst under operating conditions. In this regard, we have built a cell that can reach up to 1,000 °C, which makes it ideal for these kind of studies.[63] We have synthesized two sets of Pt/CeO₂ catalysts *via* FSP and incipient wetness impregnation (IWI).[64] The *operando* XAS experiments were carried out at 975 °C. The catalyst prepared *via* the FSP method showed atomically dispersed Pt, whereas the IWI method mainly resulted in Pt clusters on the CeO₂ support. Pulsed reactions with methane showed three stages of the reaction: (a) reduction of CeO₂ during activation; (b) induction phase; and (c) stable catalytic cycle. The *operando* X-ray absorption near edge structure (XANES) spectra at the Pt L₃-edge collected at various temperatures under He and CH₄/He flow at 975 °C with the products obtained are shown in Fig. 8.

*Operando* XAS, in combination with theoretical simulations of the extended X-ray absorption fine structure (EXAFS) and XANES spectra confirmed an increase in Pt-Ce interactions under reaction conditions (at 975 °C under CH₄ flow). The gas phase reaction intermediates were analyzed by synchrotron-based ultra-violet photoionization mass spectrometry (SVUV-PIMS), which

showed the formation of methyl radicals. This indicates that the strong Pt-Ce interaction probably leads to the activation of CH₄ *via* formation of methyl radicals suppressing the coke formation and ultimately enhancing the catalytic activity. These results revealed the structural evolution of the Pt/CeO₂ catalyst under industrially relevant conditions, which will help in further understanding the complex reaction pathways at such high temperatures.

## 2.5. Operando High-energy-resolution Fluorescence Detected (HERFD) XAS Investigation During CO Oxidation Over Pt Single-atom Catalyst (SACs)

Single-atom catalysts, an emerging field in heterogeneous catalysis, attract the interest of many researchers because they show the promise to achieve maximum theoretical efficiencies with minimum metal loadings.[65,66] Since the term SAC was coined, investigations on many reactions, which were believed to be catalyzed by nano-particles or clusters showed that SACs can be a viable alternative.[67-70] It is also worth mentioning here that when it comes to single-atom catalysts, complementary spectroscopic techniques such as Fourier transform infrared (FTIR)[71,72] are mandatory as X-ray based techniques have certain limitations.[73] We have recently shown that diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) can be a complementary spectroscopic method to X-ray based techniques for elucidating the local structural changes under operating conditions.[74] Jones *et al.* showed that Pt single atoms trapped inside CeO₂ supports at 800 °C in air are highly stable and active for the CO oxidation reaction.[75] With the help of *operando* XAS, our group has shown in the past that Pt nano-particles on CeO₂ are highly dynamic in nature when operated as emission control catalysts.[76] Following this, we have synthesized a single-site Pt/CeO₂ catalyst under hydrothermal conditions at 800 °C and tested it for CO (as well as C₃H₆ and
<figure><img src="image_10.png" /><figcaption>Fig. 7. (a) Summary of the *operando* XAS results during OER over an IrOₓ catalyst, and (b) proposed adsorbate evolution mechanism (AEM) during OER in acidic medium. Adapted with permission from ref. [59].</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. (a) Normalized XANES spectra at the Pt L₃-edge of the Pt/CeO₂ (FSP) catalyst at various temperatures under He atmosphere, (b) *operando* XANES spectra at 975 °C in 90% CH₄/He atmosphere and (c) pulse reaction products at 975 °C. Adapted with permission from ref. [64].</figcaption></figure>

## 2.4. Operando XAS Investigation for the Non-oxidative Coupling of Methane – The Challenge of High Temperature

Direct conversion of methane to olefins and aromatics is one of the holy grails in comparison to the traditional multi-step processes of syngas conversion via Fischer-Tropsch (FT) synthesis or the methanol to olefin (MTO) process.[61,62] However, direct conversion of methane occurs at a much higher temperature (typically above 600 °C) than in MTO or FT synthesis processes, which makes it challenging to investigate the catalyst under operating conditions. In this regard, we have built a cell that can reach up to 1,000 °C, which makes it ideal for these kind of studies.[63] We have synthesized two sets of Pt/CeO₂ catalysts via FSP and incipient wetness impregnation (IWI).[64] The *operando* XAS experiments were carried out at 975 °C. The catalyst prepared via the FSP method showed atomically dispersed Pt, whereas the IWI method mainly resulted in Pt clusters on the CeO₂ support. Pulsed reactions with methane showed three stages of the reaction: (a) reduction of CeO₂ during activation; (b) induction phase; and (c) stable catalytic cycle. The *operando* X-ray absorption near edge structure (XANES) spectra at the Pt L₃-edge collected at various temperatures under He and CH₄/He flow at 975 °C with the products obtained are shown in Fig. 8.

*Operando* XAS, in combination with theoretical simulations of the extended X-ray absorption fine structure (EXAFS) and XANES spectra confirmed an increase in Pt-Ce interactions under reaction conditions (at 975 °C under CH₄ flow). The gas phase reaction intermediates were analyzed by synchrotron-based ultra-violet photoionization mass spectrometry (SVUV-PIMS), which

showed the formation of methyl radicals. This indicates that the strong Pt-Ce interaction probably leads to the activation of CH₄ via formation of methyl radicals suppressing the coke formation and ultimately enhancing the catalytic activity. These results revealed the structural evolution of the Pt/CeO₂ catalyst under industrially relevant conditions, which will help in further understanding the complex reaction pathways at such high temperatures.

## 2.5. Operando High-energy-resolution Fluorescence Detected (HERFD) XAS Investigation During CO Oxidation Over Pt Single-atom Catalyst (SACs)

Single-atom catalysts, an emerging field in heterogeneous catalysis, attract the interest of many researchers because they show the promise to achieve maximum theoretical efficiencies with minimum metal loadings.[65,66] Since the term SAC was coined, investigations on many reactions, which were believed to be catalyzed by nano-particles or clusters showed that SACs can be a viable alternative.[67-70] It is also worth mentioning here that when it comes to single-atom catalysts, complementary spectroscopic techniques such as Fourier transform infrared (FTIR)[71,72] are mandatory as X-ray based techniques have certain limitations.[73] We have recently shown that diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) can be a complementary spectroscopic method to X-ray based techniques for elucidating the local structural changes under operating conditions.[74] Jones *et al.* showed that Pt single atoms trapped inside CeO₂ supports at 800 °C in air are highly stable and active for the CO oxidation reaction.[75] With the help of *operando* XAS, our group has shown in the past that Pt nano-particles on CeO₂ are highly dynamic in nature when operated as emission control catalysts.[76] Following this, we have synthesized a single-site Pt/CeO₂ catalyst under hydrothermal conditions at 800 °C and tested it for CO (as well as C₃H₆ and
<figure><img src="image_12.png" /><figcaption>Fig. 9. (a) Schematic representation of the state of the Pt/CeO₂ catalyst under various reaction conditions.[76] (b) CO light-off curve after activation of the Pt/CeO₂ catalyst (single site and clusters) with different reductant, (c) HERFD XANES spectra at Pt L₃-edge at different temperatures under CO oxidation conditions, (d) fraction of Pt species derived from multivariate curve resolution-alternating least-squares (MCR-ALS) with linear combination analysis (LCA), and (e) scheme representing changes in the Pt single sites during CO oxidation shown in our previous work. Adapted with permission from ref. [77].</figcaption></figure>

CH₄) oxidation reaction.[77] The *in situ* IR and *operando* HERFD XAS studies demonstrated that under CO oxidation conditions, Pt single sites migrate from four-fold hollow sites, resulting in Pt clusters of a few atoms, which form the catalytically active centers. Hence, several atom clusters are the active species. The different states of the catalyst under varying reaction environments, the CO light-off curve, and the results of the HERFD XAS studies are summarized in Fig. 9. In Fig. 9 (c), the HERFD XANES spectra at the Pt L₃-edge clearly shows that the white line intensity significantly decreases under the reaction conditions indicating the formation of small Pt clusters, which was not detected in the conventional XAS experiment. This study demonstrates that *operando* spectroscopy (beyond conventional XAS) may be necessary to prove the fate of single-atom catalysts. Recently, with the help of *in situ/operando* XAS and DRIFTS we have shown that noble metals (Pt, Ir, Pd, Rh, and Ru) dispersed on CeO₂ are highly dynamic in their behavior already at room temperature under CO oxidizing conditions.[74] A spatiotemporal investigation[78] during CO and hydrocarbon oxidation over a Pt/CeO₂ catalyst deposited as a layer in a honeycomb structure showed different reaction rates at the first zone of the catalyst and presence of large Pt nanoparticles. In this work, we implemented combination of *operando*

infrared thermography and spatially and time-resolved XAS during CO oxidation reaction conducted on a fixed bed microreactor. Using IR thermography, the position and extent of hotspots was identified, which can be further extended with industrially relevant emission control catalysts. This concept has been recently further transferred to the Pd and Rh L₃-edge in vacuum tender X-ray emission spectrometer (TEXS) of beamline ID26 at European Synchrotron Research Facility (ESRF).[39]

### 3. Conclusion and Outlook

*Operando* spectroscopy is inevitable to elucidate the true nature of catalysts under operating conditions. In many of the examples discussed above, we have found that the structure of the catalysts is very dynamic and that even a small change in the catalyst structure can make a huge difference when it comes to activity and selectivity. In our first example of isobutene oxidation, synchrotron based techniques (XRD and XAS) provided much more structural information compared to the state-of-the-art laboratory-based methods, which suggests that synchrotron-based techniques will be more routine in the future when it comes to *operando* investigations. In addition, spatially resolved studies become important to understand phase cooperation effects. The
<figure><img src="image_12.png" /><figcaption>Fig. 9. (a) Schematic representation of the state of the Pt/CeO₂ catalyst under various reaction conditions.[76] (b) CO light-off curve after activation of the Pt/CeO₂ catalyst (single site and clusters) with different reductant, (c) HERFD XANES spectra at Pt L₃-edge at different temperatures under CO oxidation conditions, (d) fraction of Pt species derived from multivariate curve resolution-alternating least-squares (MCR-ALS) with linear combination analysis (LCA), and (e) scheme representing changes in the Pt single sites during CO oxidation shown in our previous work. Adapted with permission from ref. [77].</figcaption></figure>

CH₄) oxidation reaction.[77] The *in situ* IR and *operando* HERFD XAS studies demonstrated that under CO oxidation conditions, Pt single sites migrate from four-fold hollow sites, resulting in Pt clusters of a few atoms, which form the catalytically active centers. Hence, several atom clusters are the active species. The different states of the catalyst under varying reaction environments, the CO light-off curve, and the results of the HERFD XAS studies are summarized in Fig. 9. In Fig. 9 (c), the HERFD XANES spectra at the Pt L₃-edge clearly shows that the white line intensity significantly decreases under the reaction conditions indicating the formation of small Pt clusters, which was not detected in the conventional XAS experiment. This study demonstrates that *operando* spectroscopy (beyond conventional XAS) may be necessary to prove the fate of single-atom catalysts. Recently, with the help of *in situ/operando* XAS and DRIFTS we have shown that noble metals (Pt, Ir, Pd, Rh, and Ru) dispersed on CeO₂ are highly dynamic in their behavior already at room temperature under CO oxidizing conditions.[74] A spatiotemporal investigation[78] during CO and hydrocarbon oxidation over a Pt/CeO₂ catalyst deposited as a layer in a honeycomb structure showed different reaction rates at the first zone of the catalyst and presence of large Pt nanoparticles. In this work, we implemented combination of *operando*

infrared thermography and spatially and time-resolved XAS during CO oxidation reaction conducted on a fixed bed microreactor. Using IR thermography, the position and extent of hotspots was identified, which can be further extended with industrially relevant emission control catalysts. This concept has been recently further transferred to the Pd and Rh L₃-edge in vacuum tender X-ray emission spectrometer (TEXS) of beamline ID26 at European Synchrotron Research Facility (ESRF).[39]

### 3. Conclusion and Outlook

*Operando* spectroscopy is inevitable to elucidate the true nature of catalysts under operating conditions. In many of the examples discussed above, we have found that the structure of the catalysts is very dynamic and that even a small change in the catalyst structure can make a huge difference when it comes to activity and selectivity. In our first example of isobutene oxidation, synchrotron based techniques (XRD and XAS) provided much more structural information compared to the state-of-the-art laboratory-based methods, which suggests that synchrotron-based techniques will be more routine in the future when it comes to *operando* investigations. In addition, spatially resolved studies become important to understand phase cooperation effects. The
operando studies carried out for CO₂ hydrogenation, Fischer-Tropsch synthesis, and non-oxidative conversion of methane demonstrate that we can even reach harsh reaction environments in our operando studies enabled by appropriate design of cells, thus closing the gap between fundamental science and industrially relevant processes. The operando HERFD XAS studies prove that conventional XAS sometimes has certain limitations when it comes to the distinction between single-site and ultra-small clusters. Hence, more complementary techniques are required to aid in elucidating the true structure of the catalyst. The design of the corresponding operando spectroscopic cell must be well-thought, as displayed in various examples in this special issue (e.g. mass transport in liquid-phase reactions including electrochemical reactions). Operando tomography, a rapidly emerging area will have significant impact when it comes to visualization of the catalyst structure in 3D under operating conditions. Modulation excitation spectroscopy (MES) can also be utilized for transient experiments, which are otherwise challenging to observe, increasing the response of some spectroscopic signals, for example, during the electrochemical oxygen evolution reaction (OER). For changes that occur in a fraction of a second, conventional XAS is not ideal. Alternatively, quick-XAS (also known as QEXAFS) is an advanced XAS technique, which makes it feasible to acquire XAS spectra at a high scan rate (1s/spectrum or faster). By utilizing QEXAFS, one can either observe phenomena that take place on a very fast scale or eliminate the noise in the measurements by averaging a large number of spectra. Moreover, it enables spatially resolved XAS studies. For enhanced chemical sensitivity and time resolutions, superXAS beamline at PSI is ideal. Finally, we would like to stress that data initiatives like DAPHNE (Data from PHoton and Neutron Experiments), which is a part of the German Research Foundation (DFG) for a National Research Data Infrastructure (NFDI), are extremely important. Through DAPHNE, we are able to record metadata at various synchrotron light sources in a systematic way, establishing a community repository of processed data, and making it publicly available to all users. Many more aspects of operando spectroscopy are given in this special issue of the very active Swiss community in this field.

## Acknowledgments

We would like to thank Dr. Maarten Nachtegaal and Dr. Davide Ferri from PSI, Switzerland and Dominik Neukum and Dr. Vera Truttmann from KIT, Germany for fruitful discussion during the preparation of the article. We would like to acknowledge various synchrotron light sources across Europe (SLS/PSI, PETRA III/DESY, BESSY, ESRF, SOLEIL, KIT Light Source) and other parts of the world (APS, SSRF) for providing us with beamtime for our studies. In addition, JDG would like to thank colleagues at Haldor Topsoe (particularly Dr. Bjarne S. Clausen), at ETH Zürich (Prof. Alfons Baiker), DTU, KIT and all collaborators for the exciting discussions, beamtimes and new results. We thank former and present projects like CRC1441 ('TrackAct', Project-ID 426888090), DAPHNE4NFDI (DFG, project number 460248799), SPP2080 (GR3987-13/2), further DFG-grants (GR 3987/9-1, INST 121384/73-1), the BMBF-project 'CARE-O-SENE' (03SF0673B), ROCK-IT, 3D-Process (03EN2065D) and especially present and former colleagues of the 'Grunwaldt' group. J.-D. Grunwaldt photograph copyright, KIT, Magali Hauser.

Received: March 16, 2024

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operando studies carried out for CO₂ hydrogenation, Fischer-Tropsch synthesis, and non-oxidative conversion of methane demonstrate that we can even reach harsh reaction environments in our operando studies enabled by appropriate design of cells, thus closing the gap between fundamental science and industrially relevant processes. The operando HERFD XAS studies prove that conventional XAS sometimes has certain limitations when it comes to the distinction between single-site and ultra-small clusters. Hence, more complementary techniques are required to aid in elucidating the true structure of the catalyst. The design of the corresponding operando spectroscopic cell must be well-thought, as displayed in various examples in this special issue (e.g. mass transport in liquid-phase reactions including electrochemical reactions). Operando tomography, a rapidly emerging area will have significant impact when it comes to visualization of the catalyst structure in 3D under operating conditions. Modulation excitation spectroscopy (MES) can also be utilized for transient experiments, which are otherwise challenging to observe, increasing the response of some spectroscopic signals, for example, during the electrochemical oxygen evolution reaction (OER). For changes that occur in a fraction of a second, conventional XAS is not ideal. Alternatively, quick-XAS (also known as QEXAFS) is an advanced XAS technique, which makes it feasible to acquire XAS spectra at a high scan rate (1s/spectrum or faster). By utilizing QEXAFS, one can either observe phenomena that take place on a very fast scale or eliminate the noise in the measurements by averaging a large number of spectra. Moreover, it enables spatially resolved XAS studies. For enhanced chemical sensitivity and time resolutions, superXAS beamline at PSI is ideal. Finally, we would like to stress that data initiatives like DAPHNE (Data from PHoton and Neutron Experiments), which is a part of the German Research Foundation (DFG) for a National Research Data Infrastructure (NFDI), are extremely important. Through DAPHNE, we are able to record metadata at various synchrotron light sources in a systematic way, establishing a community repository of processed data, and making it publicly available to all users. Many more aspects of operando spectroscopy are given in this special issue of the very active Swiss community in this field.

## Acknowledgments

We would like to thank Dr. Maarten Nachtegaal and Dr. Davide Ferri from PSI, Switzerland and Dominik Neukum and Dr. Vera Truttmann from KIT, Germany for fruitful discussion during the preparation of the article. We would like to acknowledge various synchrotron light sources across Europe (SLS/PSI, PETRA III/DESY, BESSY, ESRF, SOLEIL, KIT Light Source) and other parts of the world (APS, SSRF) for providing us with beamtime for our studies. In addition, JDG would like to thank colleagues at Haldor Topsoe (particularly Dr. Bjarne S. Clausen), at ETH Zürich (Prof. Alfons Baiker), DTU, KIT and all collaborators for the exciting discussions, beamtimes and new results. We thank former and present projects like CRC1441 ('TrackAct', Project-ID 426888090), DAPHNE4NFDI (DFG, project number 460248799), SPP2080 (GR3987-13/2), further DFG-grants (GR 3987/9-1, INST 121384/73-1), the BMBF-project 'CARE-O-SENE' (03SF0673B), ROCK-IT, 3D-Process (03EN2065D) and especially present and former colleagues of the 'Grunwaldt' group. J.-D. Grunwaldt photograph copyright, KIT, Magali Hauser.

Received: March 16, 2024

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## License and Terms

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This is an Open Access article under the terms of the Creative Commons Attribution License CC BY 4.0. The material may not be used for commercial purposes.

The license is subject to the CHIMIA terms and conditions: (https://chimia.ch/chimia/about).

The definitive version of this article is the electronic one that can be found at https://doi.org/10.2533/chimia.2024.288
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License and Terms

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

This is an Open Access article under the terms of the Creative Commons Attribution License CC BY 4.0. The material may not be used for commercial purposes.

The license is subject to the CHIMIA terms and conditions: (https://chimia.ch/chimia/about).

The definitive version of this article is the electronic one that can be found at https://doi.org/10.2533/chimia.2024.288