Uncovering Atomic-scale Dynamics in Solid Catalysts via X-ray-based Methods

: Deciphering the structural intricacies of catalysts is essential to advance their atomic-scale engineering. Solid catalysts are complex, with structural features spanning multiple length scales and involving dynamics, which possess challenges in understanding structure-performance relationships. However, advanced operando X-ray characterization techniques, including X-ray absorption spectroscopy (XAS), diffraction (XRD), and pair distribution function analysis (PDF) allow elucidation of structural features under working conditions, discovering transitions from supported nanocrystals to dispersed sites, from solid solutions to supported nanoparticles, or structural changes at the local level. In this mini-review, we discuss case studies exploring the structure of cata-lysts over different lengths and time scales under different applications, such as CO 2 hydrogenation to methanol or the dry reforming of methane, using a combination of operando XAS, XRD and PDF.


Introduction
To achieve global sustainability, it is essential to develop and implement innovative processes that effectively utilize renewable energy sources, raw materials and waste and to establish a circular economy in which products are recycled.Catalysis can play a crucial role in achieving this goal by facilitating chemical transformations required for the production of clean fuels, converting waste into energy, and reducing greenhouse gas emissions, and minimizing waste.Design guidelines for advancing catalysts require an in-depth understanding of the functioning of catalysts. [1]terogeneous solid catalysts are materials that contain a variety of atomic arrangements and chemical species.They have a complex structure that can be defined at multiple scales and undergo structural changes over a wide range of time scales, i.e. from sub-milliseconds to years.While catalysis takes place on the surfaces of the material, not the entire surface of the catalyst participates, but only specific centers or active sites.The activity of a catalyst's active site is influenced by geometric and electronic effects which in turn determine the binding strength of the molecular species at the catalyst's surface.Hence, a detailed understanding of the structure of the active sites, both electronic and geometric, is crucial for the design of highly active catalysts with high selectivity towards the desired product(s).However, the complex and dynamic nature of active sites challenges the establishing of structure-performance relationships.Therefore, to engineer active and selective catalysts, it is necessary to understand how the active sites are formed, how these sites respond to different reaction atmospheres and what are the deactivation routes. [1]he complexities inherent to heterogeneous catalysts call for multi-technique approaches, to probe catalysts at different scales and to enable the characterization of both electronic and geometric features under reactive atmospheres.Operando vibrational spectroscopies, such as infrared or Raman are useful to extract detailed information about adsorbed molecules on the catalyst's surface. [2]X-ray techniques such as scattering, absorption, or emission are valuable in studying the structure of catalysts under reaction conditions. [3]When coupled simultaneously with measurements of a catalyst's activity and selectivity, operando X-ray based studies allow the catalytic activity to be related to structural features (and changes thereof).In particular, synchrotron-based X-ray techniques have been instrumental in unveiling structural dynamics and addressing questions concerning the structure of complex catalyst systems. [4]Advancing operando methods allowing the study of materials with multiple techniques in their working state is a key aspect of catalysis research.
To probe the atomic structure of materials, the combination of X-ray absorption spectroscopy (XAS) and X-ray powder diffraction (XRD) is a powerful approach, irrespective of whether the two techniques are used in the same experimental setup or in two tive atmospheres.4a,c]

From Nanocrystals to Atomically Dispersed Sites
The direct conversion of CO 2 to alcohols, such as methanol, is a promising route to produce platform chemicals and energy carriers.4c] Here, In 2 O 3 nanocrystals of 7 nm in diameter with a bixbyite type structure were synthesized via a colloidal route which acted as a model system.3b,8] The results showed that an initial activation stage involved the formation of oxygen vacancy sites associated with a partial reduction of indium, leading to the active state In 2 O 3−x .Subsequently, as the reaction progressed, reductive amorphization of the In 2 O 3 nanocrystals occurred, revealed through quantitative XANES, EXAFS, and XRD analyses (Fig 2a-d).A multivariate curve resolution-alternating least squares (MCR-ALS) analysis applied to the XANES data indicated that the most active catalyst state has an average oxidation state between +3 and +2 while the onset of deactivation coincided with the appearance of molten In 0 (Fig 2a-b).The over-reduction of In 2 O 3 with time on stream (TOS) led to a In 0 /In 2 O 3−x material of inferior catalytic activity.Indeed, the decrease in In-O and In-In coordination numbers determined by EXAFS were interpreted as a reductive amorphization of the In 2 O 3 nanocrystals (Fig 2c).XRD analysis provided further evidence for the amorphization processes and allowed the quantification of the degree of amorphization using BN as an internal standard (Fig 2c).Hence, these findings indicate that it is essential to implement strategies to prevent the over-reduction of indium oxide during reaction.
4a] The three different catalyst systems were probed by operando XAS-XRD to determine whether the different ZrO 2 phases affected the oxidation state and coordination environment of the In atoms.The goal of the operando study was to assess how structural differences between the catalysts correlate with their catalytic activity, selectivity, and stability.Interestingly, In 2 O 3 / m-ZrO 2 showed a significantly higher performance in terms of methanol yield and stability when compared to In 2 O 3 /t-ZrO 2 and In 2 O 3 /am-ZrO 2 .XAS-XRD analysis revealed that the phase of the ZrO 2 support played a crucial role in determining the structural evolution of the In 2 O 3 nanocrystals during reaction.In the In 2 O 3 /m-ZrO 2 system, In 2+ /In 3+ sites were effectively stabilized, as evidenced by XANES.EXAFS showed that the stabilization of the oxidation state was linked to a structural transformation that turned nanocrystals into In sites that were highly dispersed in the monoclinic ZrO 2 structure, forming a solid solution.The formation of a solid solution prevented the over-reduction of In 2+ /In 3+ sites to metallic In via the formation of highly active and stable In-Vo-Zr linkages (Vo is an oxygen vacancy).3b] XAS allows for the study of the local geometry and electronic state of an atom in a catalyst.This technique encompasses two different formalisms: X-ray absorption near-edge structure analysis (XANES) and X-ray absorption fine structure (EXAFS).3d] In addition, XRD via the analysis of Bragg peaks yields quantitative information on the periodic atomic arrangement of the atoms.Moreover, expanding beyond traditional XRD analysis, total scattering techniques such as the pair distribution function (PDF) analysis have emerged as important tools for probing the structure of materials across different length scales, i.e. from the local atomic environment to the mid-range structure (a fewAngstroms to several nanometres). [5]ig. 1 illustrates the complementary information accessible by XAS (XANES and EXAFS), XRD and PDF analyses under operando or in situ conditions.
In this mini-review, we will discuss how XAS, XRD, and PDF can provide information to understand in more detail the relationship between the structure and performance of solid catalysts, as well as elucidate their structural (activation or deactivation) dynamics.We will focus on research conducted in the Laboratory of Energy Science and Engineering at ETH Zurich, Switzerland which focuses on the examination of structural changes of catalysts under various reactive conditions.The investigations highlighted here have allowed a fundamental understanding of emerging catalyst systems that hold promise for applications in sustainable catalysis to be obtained, in particular in catalytic reactions that are concerned with CO 2 valorization.

Tracking Transformations in Solid Catalysts Using Combined XAS-XRD
4b] It was observed, that after 100 h of TOS at 700 °C, the exsolved catalysts deactivated by only ∼1% while a benchmark catalyst prepared by conventional wetness impregnation (denoted Ru/Sm 2 Ce 2 O 7 -Imp), lost ca.8% of the initial activity after only 48 h of TOS.
An effective exsolution process requires a precursor material in which Ru cations (Ru +4 ) are homogeneously incorporated into a metal oxide matrix forming a solid solution.Therefore, a vital step was the preparation of phase pure, Sm 2 Ru x Ce 2-x O 7 solid solutions (where x = 0, 0.1, 0.2, 0.4) and their in-depth structural characterization using XRD and XAS, in addition to complementary techniques, including ex situ scanning transmission electron microscopy (STEM) and Raman spectroscopy.The Sm 2 Ce 2 O 7 crystal structure is described as a C-type (space group Ia-3), which is a superstructure of a fluorite-type structure.The C-type atomic arrangement contains two oxygen positions (48e and 16c), with oxygen vacancies preferentially located at the 16c site.The structure contains two cation sites (8b and 24d) which are randomly occupied by Sm and Ce atoms.As Ru is gradually incorporated into the cationic sites of the solid solutions, the material transforms into a type of defective fluorite, in which oxygen vacancies are randomly located at the fluorite oxygen site (8c).This transformation was revealed through changes in the synchrotron-based ZrO 2 reduced to In 0 under reaction conditions explaining in turn their inferior performance.
These two examples show how a combination of advanced techniques such as operando XAS-XRD, is able to probe structural dynamics during CO 2 hydrogenation conditions, providing valuable insights into the active state and deactivation routes, which in turn allowed the development of stabilization strategies.

From Solid Solutions to Supported Nanoparticles
In addition to the hydrogenation of CO 2 to methanol, the dry reforming of methane (DRM) is also a promising route to valorize CO 2 .DRM converts methane (CH 4 ) and carbon dioxide (CO 2 ) into synthesis gas, i.e. a mixture of CO and H 2 which is an essential feedstock for various industrial processes, such as the production of synthetic fuels.4d,9] Noble metals (e.g.9b] Therefore, it is crucial to develop approaches that yield supported noble-metal-based DRM catalysts with a high surface to volume ratio enabling the minimizing of the noble metal content.In this context, the reductive exsolution has established itself as a promising approach to yield highly dispersed, metallic or bimetallic nanoparticles that are supported on a metal oxide matrix. [10]Reductive exsolution exploits the segregation of a metal from a host oxide when exposed to a reductive atmosphere.Typically, the host structure is a perovskite data (data collected at 50 °C), the first sphere Ru-Ru coordination number was determined as 6, which indicated the formation of Ru nanoparticles of ca.1-1.5 nm in diameter, in line with ex situ scanning transmission electron microscopy (STEM).Notably the Ru-Ru coordination number of a material prepared via a conventional impregnation approach (Ru/Sm 2 Ce 2 O 7 -Imp) was ca. 10 confirming that impregnation led to significantly larger Ru particle sizes (i.e.3-4 nm).An illustration of the reductive exsolution process is shown in Fig. 3e.
Under DRM conditions, operando XAS-XRD coupled with mass spectrometry confirmed the absence of changes in the oxidation state or structure of Ru over 1.5 h on TOS; further, no deactivation was observed by the off-gas analysis.However, EXAFS analysis (50°C) of the reacted catalysts indicated a slight increase in the coordination number from 6 to 7, a possible indication of a small growth of the Ru nanoparticles, yet this increase was within the experimental error.Although no detectable catalyst deactivation occurred, particle growth over longer TOS cannot be ruled out.Therefore, the cyclic reversibility of the exsolution-redissolution processes was investigated using an oxidative atmosphere (20 vol % O 2 in N 2 at 700 °C) as a regeneration route.4b] These studies demonstrate how XAS-XRD can aid in developing high-performance catalysts and reliable regeneration procedures.

Probing Changes at the Nanoscale Using PDF Analysis
Traditional crystallographic methods have limitations in providing detailed and quantitative structural information on small nanoparticles and nanocrystals.This is because the Bragg diffraction peaks broaden due to the short coherence length and frequent-diffraction patterns, showing that the relatively weak peaks due to superstructures in the C-type phase gradually vanished upon the substitution of Ru into Sm 2 Ce 2 O 7 (Fig. 3a).
Further, Ru K-edge XAS was utilized to probe the local environment around the Ru atoms in the solid solutions.EXAFS analysis revealed the presence of an Ru-O shell in the solid solution, while no higher coordination spheres were observed.This contrasted the EXAFS data of RuO 2 or the benchmark catalysts Ru/Sm 2 Ce 2 O 7 -Imp, which exhibit well-defined second Ru-metal spheres (Fig. 3b).4b,6c] The gradual reduction of Ru +4 to Ru 0 was evidenced by the decrease in the white line intensity and the shift of the absorption edge to lower energies (Fig. 3c).Principal component analysis (PCA) and MCR-ALS applied to the in situ XANES data allowed the evolution of the oxidation state of Ru during reduction to be resolved in time (Fig 3b).This analysis evidenced that Ru exsolves from the parent solid solution through an intermediate Ru δ+ state, that is Ru 4+ →Ru δ+ →Ru 0 .EXAFS analysis revealed that the Ru-O interatomic distances increase from 1.96 to 2.02 Å during the formation of the intermediate Ru δ+ state.Moreover, the EXAFS analysis evidenced a developing Ru-Ru sphere due to Ru 0 nanoparticle formation starting at ca.  ly defective structures.As a result, diffuse scattering dominates the scattering patterns.5b,c] It provides the probability of finding pairs of atoms separated by a distance r, and thus offers direct insight into the short-and intermediate-range order of materials.PDF bridges the gap between the structural length scales accessible by diffraction methods (> ca. 5 nm) and EXAFS (< ca.6 Å).PDF data is derived experimentally from X-ray total scattering patterns (optimized for high scattering vector with Q > 15 Å -1 , using high energy photons) via a Fourier transformation.PDF can probe structures at multiple length scales from a few Å to several nm, making it an ideal method to study the structures of catalysts at multiple scales and complementing XAS.5b,12] In nanocrystalline materials, the high surface energy and strain associated with the high density of grain boundaries can stabilize metastable phases.For instance, metastable γ-Ga 2 O 3 nanocrystals have received increasing interest for different (electro)catalytic applications.12b,13] The disorder arises from a distortion of the Ga-O polyhedra, which are randomly oriented within the nanocrystal.Castro-Fernández et al. using in situ time resolved PDF, showed that the transformation of the γ-Ga 2 O 3 nanocrystals towards the thermodynamically stable β-Ga 2 O 3 polymorph occurs in different structural domains. [13]The study revealed the appearance of sub-nanometric β-Ga 2 O 3 domains at approximately 300 °C, while the bulk γ→β-Ga 2 O 3 transition occurs at a much higher temperature (600-750 °C), which may have implications for their photo/thermal catalytic performance.
Moreover, PDF can be used to probe the structure of supported nanoparticles or films; an important aspect, because in catalytic applications the active phase is often dispersed on high surface area materials (supports) to prevent or at least minimize their agglomeration and sintering during operation.5a] For example, the d-PDF approach has been used to characterize the structure of SiO 2 -supported GaO x catalysts that were synthesized using different methods, viz.colloidal based approaches, wetness impregnation of aqueous Ga(NO 3 ) 3 solutions, and atomic layer deposition. [14]14b] 3.2 Bimetallic Nanoparticles and the Cooperativity of Alloyed and Oxidic Species A versatile strategy to tune the catalytic properties of a metal nanoparticle is to add a second metal.4g] For example, Ni-based catalysts are very effective in the methanation reaction, while the addition of Ga to Ni has shown a shift in its selectivity towards methanol. [16]immerli et al. conducted research to understand the role and structure of Ga as a promoter in Ni-based catalysts for the hydrogenation of CO 2 to methanol. [17]To this end, a series of Ni-Gabased catalysts with different Ni:Ga ratios (Ni 100 /SiO 2 , Ni 75 Ga 25 / SiO 2 , Ni 70 Ga 30 /SiO 2 and Ni 65 Ga 35 /SiO 2 ) were synthesized using surface organometallic chemistry (SOMC) [18] and the structure of the catalysts were interrogated using operando d-PDF and operando XAS. [17]n this research d-PDF analysis was critical to provide atomiclevel insight of the structure of the very small (2 nm) nanoparticles formed (Fig. 4 a,b).It was found that when activated at 600 °C in H 2 , nanoparticles of an Ni x Ga y alloy with an fcc structure and a size of approximately 2 nm formed.Using the cell parameters determined by d-PDF refinements, it was possible to determine the composition of the alloys (Fig 4 c).Additionally, the quantity of GaO x species were estimated by taking into account the total Ga and Ni contents determined by inductively coupled plasma optical emission spectroscopy (ICP-OES).The results obtained by in situ XAS at the Ga K-edge confirmed the presence of oxidic Ga species while Ni K-edge XANES indicated that the electronic structure of Ni is changed upon the addition of Ga, viz.there is a charge transfer from Ga to Ni (Ga δ+ /Ni δ− ).Additional operando d-PDF and XAS experiments of the catalysts under CO 2 hydrogenation conditions (20 bar CO 2 :H 2 :N 2 = 1:3:1, 230 °C) showed that the structure of the catalysts formed after activation was maintained under reaction conditions, i.e. the quantities of the alloyed Ga and GaO x species remained constant with TOS.
When correlating catalytic activity and structural descriptors, it was discovered already that the alloying of very small amounts of Ga with Ni (Ni:Ga ratio = 82:12) resulted in a significant selectivity shift from methane to methanol under CO 2 hydrogenation conditions.The most active and selective catalyst of the series studied (i.e.Ni 65 Ga 35 /SiO 2 ) contained an alloy with a Ni:Ga ratio of 75:25 and GaO x species (0.14 mol GaOx mol Ni -1) (Fig. 4d-e).In addition, monitoring the catalysts by operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) provided insight into the surface species under reaction conditions.It is worth noting that only the most active catalyst showed bands due to formate species.Combining the findings of operando d-PDF, XAS and DRIFTS, the authors concluded that the alloying of Ni with Ga is crucial for achieving high methanol selectivity, while the presence of oxidized Ga species appreciably enhances the rate of methanol formation.Alloy nanoparticles with a Ni:Ga ratio of 75:25 result in the high methanol activity and selectivity, considerably surpassing the performance of Ni-richer alloys, while the presence of GaO x further increases the rate of methanol formation.
To summarize, in this work the combination of operando Xray-based techniques and infrared spectroscopy provided atomicscale insight in the geometric and electronic structure of ultrasmall nanoparticles (2 nm) that is crucial for the advancement of Ni-Ga-based bimetallic catalysts for CO 2 hydrogenation.Further, the study highlighted that regulating the quantity of both alloyed Ga and GaO x species is crucial in achieving high methanol selectivity and a high rate of methanol formation.

Conclusions and Outlook
Advanced operando X-ray-based characterization techniques play a crucial role in advancing our understanding of the complex dynamics of solid catalysts at the atomic-and nano-scale.In this mini-review, where we highlight recent advances of our group with regards to operando synchrotron XAS, XRD and PDF analyses, we present a number of diverse case studies of catalysts for the hydrogenation of CO 2 to methanol, dry reforming of methane, and propane dehydrogenation.These case studies demonstrate the value of XAS, XRD, and PDF to formulate correlations between a catalyst's structure and its catalytic performance.Operando and in situ XAS and XRD techniques provide structural information over multiple length scales, ranging from the atomic to the nmscale.Specifically, XAS provides element-specific information of the local and electronic structure around the element being investigated, while XRD yields insight concerning the average structure that extends to several nanometers or even micrometers.In addition, PDF analysis bridges the gap between XAS and XRD and provides structural information (although not being element specific) across several length scales and is particularly suited for amorphous materials or very small nanoparticles.In situ, operando and transient experiments using X-ray-based techniques are valuable for deciphering structural dynamics and hence to elucidate catalyst activation and/or deactivation mechanisms.
In summary, the cooperative use of XAS, XRD, and PDF techniques will contribute significantly in untangling the complex relationships between structure and performance in solid catalysts.In turn, this understanding can be utilized to improve catalyst formulations and mitigate their deactivation.In the future, it will be important to advance further the development of tailored setups that combine these techniques under reaction conditions, offering analyses that are time resolved and integrated with additional methods such as small-angle X-ray scattering to gain insight into the nanoparticle morphology and changes thereof.

Fig. 1 .
Fig. 1.Schematic overview of operando complementary X-ray-based techniques and the underlaying information that can be extracted from them.

Fig. 2 .
Fig. 2. a) Methanol production rate; b) MCR-ALS analysis and c) fitted In-O and In-In coordination numbers d) fraction of crystalline bcc-In 2 O 3 and e) an illustration of the transformation of In 2 O 3 nanocrystals during an operando CO 2 hydrogenation experiment as a function of TOS.Adapted with permission from ref. [4c] Copyright 2019 American Chemical Society.f) XANES, g) Fourier transformed EXAFS data and h) an illustration of the transformation of In 2 O 3 /m-ZrO 2 during CO 2 hydrogenation.Adapted with permission from ref. [4a] Copyright 2020 American Chemical Society.

2
Ru x Ce 2-x O 7 solid solutions did not show any second shell peak because the backscattering signals from Sm and Ce atoms interfere destructively and point to a local disorder around Ru in these solid solutions.In line with the EXAFS analyses, XANES data shows features in Sm 2 Ru x Ce 2-x O 7 that are distinct with respect to the reference RuO 2 suggesting a unique environment of Ru in Sm 2 Ru x Ce 2-x O 7 .
500 °C.The XRD data further shows the emergence of Ctype reflections due to an oxygen vacancy ordering in Sm 2 Ce 2 O 7 , indicating that the exsolution of Ru from Sm 2 Ru 0.2 Ce 1.8 O 7 triggers a fluorite-to-C-type transition.By modelling of the EXAFS

Fig. 4
Fig. 4. a) d-PDF data fitted to a fcc-Ni y Ga (100-y) alloy and the corresponding agreement factors (R w ) at 1 bar H 2 , 230 °C, after in situ activation.b) Zoom into the region r = 2-4.75Å of the d-PDF data and the fitted position of the metal-metal pair correlation labelled A as a function of x ICP in the catalysts.c) Lattice parameter as a function of the fraction of Ni. d) Product formation rates over Ni x Ga (100-x) /SiO 2 and e) methanol formation rate and methanol selectivity as a function of x ICP in Ni x Ga (100-x) /SiO 2 as determined by ICP.Reproduced from ref. [17].