Operando Spectroscopy to Understand Dynamic Structural Changes of Solid Catalysts

: 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 7 th 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, CO 2 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.


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 a while ago [3][4][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 7 th International Congress on Operando Spectroscopy congress was held in Grindelwald in Switzerland (7 th -11 th of May, 2023), organized by Davide Ferri and Maarten Nachtegaal (Paul Scherrer Institute, Villigen, Switzerland).Swiss groups are and have been very ac-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 MoO x 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 synchrotronbased 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]

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.[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/CO 2 -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 CO 2 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.[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 Au I / Au III supported on carbon is a crucial factor during the reaction, and a highly oxidizing Cl 2 stream is responsible for maintaining the redox pair.This process, which was up to that point carried out at an industrial scale with hazardous HgCl 2 /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 NO x by NH 3 over Cu/zeolite catalysts, which is one of the most crucial reactions concerning the reduction of emissions of harmful gases to the environment. [21][24] Further work by Fahami et al. revealed that these isolated Cu sites interconvert between Cu II and Cu I during the reaction.They generate complexes with ammonia such as Cu(NH 3 ) 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- 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-Bi 2 Mo 3 O 12 , β-Bi 2 Mo 2 O 9 , and γ-Bi 2 MoO 6 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 L 3 -, 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 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]e 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.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 mis-interpretation 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 (τ film ), pore diffusion (τ 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.
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.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 H 2 dissociation energy.Therefore, the methanol production was significantly enhanced.The in situ and 2.2 Investigation of CO 2 Hydrogenation to Methanol via in situ DRIFTS and Operando XAS Hydrogenation of CO 2 to methanol is one of the most promising routes to mitigate the CO 2 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 CO 2 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 H 2 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 CO 2 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 red represents metal oxide phases favoring the unselective reaction pathways (formation of CO and CO 2 ), 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].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 TM ).We observed that the Zn species, which formed during the reaction, promoted the selective conversion of CO 2 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 SiO 2 promotes reverse watergas shift + CO-hydro pathway, thus improving methanol production. [52]n 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]

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 hydroxo layer of the IrO x based catalysts during OER. [56]According to the report, IrO 2 synthesized at 350 °C, and consisting of 1.7±0.4nm particles with a specific surface area of 150 m 2 g -1 , shows the highest OER activity.Reier et al. found that crystalline IrO 2 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 O II-and O I-, 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 IrO 2 as well as the Ir-Ir interaction during OER at high potentials. [59]s 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 Su-perXAS beamline of the Swiss Light Source. [60]We found that oxygen vacancies in the IrO 2 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.showed the formation of methyl radicals.This indicates that the strong Pt-Ce interaction probably leads to the activation of CH 4 via formation of methyl radicals suppressing the coke formation and ultimately enhancing the catalytic activity.These results revealed the structural evolution of the Pt/CeO 2 catalyst under industrially relevant conditions, which will help in further understanding the complex reaction pathways at such high temperatures.

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][69][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 CeO 2 supports at 800 °C in air are highly stable and active for the CO oxidation reaction. [75]ith the help of operando XAS, our group has shown in the past that Pt nano-particles on CeO 2 are highly dynamic in nature when operated as emission control catalysts. [76]Following this, we have synthesized a single-site Pt/CeO 2 catalyst under hydrothermal conditions at 800 °C and tested it for CO (as well as C 3 H 6 and 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]e have synthesized two sets of Pt/CeO 2 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 CeO 2 support.Pulsed reactions with methane showed three stages of the reaction: (a) reduction of CeO 2 during activation; (b) induction phase; and (c) stable catalytic cycle.The operando X-ray absorption near edge structure (XANES) spectra at the Pt L 3 -edge collected at various temperatures under He and CH 4 /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 CH 4 flow).The gas phase reaction intermediates were analyzed by synchrotron-based ultraviolet photoionization mass spectrometry (SVUV-PIMS), which  CH 4 ) 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 L 3 -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 CeO 2 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/CeO 2 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 L 3 -edge in vacuum tender Xray emission spectrometer (TEXS) of beamline ID26 at European Synchrotron Research Facility (ESRF). [39]

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-theart laboratory-based methods, which suggests that synchrotronbased 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  [76] (b) CO light-off curve after activation of the Pt/CeO 2 catalyst (single site and clusters) with different reductant, (c) HERFD XANES spectra at Pt L 3 -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].
operando studies carried out for CO 2 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 wellthought, 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.

License and Terms
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

Fig. 1 .
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].

Fig. 5 .
Fig. 5.To olbox 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].

Fig. 6 .
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 CO 2 hydrogenation, and (d) Proposed mechanistic pathway towards CO 2 hydrogenation.DFSP and SFSP stand for double and single flame spray pyrolysis respectively.Adapted with permission from ref. [50].

Fig. 7 .
Fig. 7. (a) Summary of the operando XAS results during OER over an IrO x catalyst, and (b) proposed adsorbate evolution mechanism (AEM) during OER in acidic medium.Adapted with permission from ref. [59].

Fig. 9 .
Fig. 9. (a) Schematic representation of the state of the Pt/CeO 2 catalyst under various reaction conditions,[76] (b) CO light-off curve after activation of the Pt/CeO 2 catalyst (single site and clusters) with different reductant, (c) HERFD XANES spectra at Pt L 3 -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].