Tables for
Volume I
X-ray absorption spectroscopy and related techniques
Edited by C. T. Chantler, F. Boscherini and B. Bunker

International Tables for Crystallography (2023). Vol. I. Early view chapter

Selected case studies in heterogeneous catalysis

Elisa Borfecchia,a Elena Groppo,a* Silvia Bordigaa and Carlo Lambertib,c

aDepartment of Chemistry, NIS Interdepartmental Center and INSTM Reference Center, University of Turin, Via Quarello 15, I-10135 Turin, Italy,bDepartment of Chemistry, CrisDi Centre for Crystallography, University of Turin, Via Giuria 7, I-10125 Turin, Italy, and cIRC `Smart Materials', Southern Federal University, 5 Zorge Street, Rostov-on-Don 344090, Russian Federation
Correspondence e-mail:

The role of a catalyst in improving the activity and selectivity of a given chemical reaction is defined, together with a discussion of the reasons why X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) are such important techniques in understanding the structure and electronic configuration of a working catalyst at the molecular level. A few selected examples are reported to support this thesis.

Keywords: catalysis; ex situ; in situ; operando; single-site catalyst; metal nanoparticles; Cu-zeolite; NH3-SCR; polymerization catalysts; Cr/SiO2; Phillips catalyst; palladium hydride; palladium carbide.

1. Introduction

Catalysis is the branch of science aimed at (i) increasing the rate of a chemical reaction A + BC (typically by finding a new path involving new intermediates characterized by a lower activation energy) and (ii) improving the selectivity towards the desired product C (usually, A + B may yield different products C, D, E … with different relative ratios; Chorkendorff & Niemantsverdriet, 2007[link]; Norskov et al., 2014[link]). This goal is achieved by inserting a substance called a `catalyst' into the reaction environment. Usually the catalyst undergoes multiple chemical transformations, but at the end of the cycle, unlike the reactants (A, B), it remains unchanged. Hence, the catalyst is not consumed and it can continue converting reactants A + B into C indefinitely. This chapter will describe how X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) can contribute to understanding the structure of the catalytic site and its modification throughout the whole catalytic cycle at the molecular level.

In most cases, catalysts do not exhibit long-range order. This is obviously the case for homogeneous catalysis (Tromp, 2015[link]), where the reactants, products and catalysts are in the liquid phase (Chadwick et al., 2011[link]; although much less common, gas–gas or solid–solid cases are also included in this definition). For heterogeneous catalysis (Bordiga et al., 2013[link]), in which the reaction is confined to the interface between the catalyst (a solid) and the reactants either in the liquid or the gaseous phase, the lack of long-range order still holds in most cases. In fact, in order to maximize such an interface, most of the solid supports are high surface-area materials (102–103 m2 g−1) that exhibit poor crystallinity [for example γ-Al2O3 (Muddada et al., 2011[link]), activated carbon (Furimsky, 2008[link]; Lazzarini et al., 2016[link]; Pellegrini et al., 2009[link]; Serp & Figueiredo, 2009[link]) and polymers (Bekturov & Kudaibergenov, 2002[link]; Groppo et al., 2010[link])] or amorphous metal oxides (for example silica; Groppo, Lamberti et al., 2005[link]). This implies that X-ray diffraction (XRD) usually cannot be used to determine the structure of the catalytically active centres, which are typically represented by isolated transition-metal ions (single-site catalysts) or dispersed metal nanoparticles. This makes extended X-ray absorption fine structure (EXAFS) the most suitable characterization technique to understand the structure of the catalytic sites (Bonino et al., 2015[link]; Bordiga et al., 2013[link]; Evans, 1997[link]; Frenkel, 2012[link]; Iwasawa et al., 2017[link]; Lamberti & van Bokhoven, 2016[link]; Newton, 2008[link]; Prins & Koningsberger, 1988[link]), while X-ray absorption near-edge structure (XANES; Fernández-García, 2002[link]; Guda et al., 2015[link], 2016[link]; van Bokhoven & Lamberti, 2014[link]) and XES (de Groot, 2001[link]; Glatzel & Bergmann, 2005[link]; Singh et al., 2010[link]) are informative on their electronic structure. Exceptions to this statement are made for three main classes of crystalline microporous catalysts, in which XRD has played an important role: zeolites, which are widely used industrially (Agostini et al., 2010[link]; Milanesio et al., 2003[link]), metal–organic frameworks (MOFs), which are of potential interest in the near future (Bordiga et al., 2010[link]; Borfecchia et al., 2017[link]; Butova et al., 2016[link]; Corma et al., 2010[link]), and, partially, metal nanoparticles, which exhibit poor crystallinity. Nonetheless, for crystalline or partially crystalline catalysts, a better understanding of the material was generally achieved when XRD data were complemented with an XAS study (Castillejos-López et al., 2017[link]; Clausen et al., 1998[link]; Martin et al., 2016[link]; Valenzano et al., 2011[link]).

The great benefit of the application of XAS and XES to catalytic studies is related to the high penetration depth of hard X-rays, which allows measurements on catalysts under operation conditions, i.e. in the presence of reactants and products from the gas or liquid phases. This requires specific experimental setups, as described in Agostini et al. (2023[link]). Due to space limitations, hereafter we will focus on applications of XAS/XES in heterogeneous catalysis, reporting representative examples of single-site catalysts and metal nanoparticle-based catalysts; this selection is based on the experience of the authors and is far from being exhaustive.

2. Single-site catalysts

2.1. Copper zeolites

In the 1990s, copper-exchanged zeolites were widely investigated after the discovery that Cu-ZSM-5 is active in the direct decomposition of NO to N2 and O2 (Iwamoto et al., 1990[link]; Lamberti et al., 1997[link]). This field obtained a new boost in 2010, when Cu-SSZ-13 (CHA framework and high Si:Al ratio) showed an outstanding performance in the NH3-assisted selective catalytic reduction (SCR) of NOx gases according to the reaction 4NH3 + 4NO + O2 → 4N2 + 6H2O (Beale et al., 2015[link]; Deka et al., 2013[link]; Kwak et al., 2010[link]). In situ and operando X-ray spectroscopies, both XAS and XES, often assisted by advanced density-functional theory (DFT) calculations, have played a crucial role in providing detailed information on the local environment and oxidation state of copper centres in different reaction conditions (Borfecchia et al., 2015[link]; Deka et al., 2012[link]; Giordanino et al., 2014[link]; Günter et al., 2015[link]; Lezcano-Gonzalez et al., 2015[link]; Paolucci et al., 2014[link], 2016[link]; Tyrsted et al., 2016[link]).

Combining XAS and XES, it was confirmed that upon heating in O2 the copper(II) centres in Cu-SSZ-13 undergo progressive dehydration, while interacting more closely with the framework, without a significant modification of their oxidation state (Borfecchia et al., 2015[link]). As-synthesized Cu-SSZ-13 zeolite exhibits XAS spectra (blue spectra in Fig. 1[link]) very similar to those of copper(II) ions in aqueous solution (Frank et al., 2005[link]). After dehydration in O2, features typical of copper(II) in a low-symmetry environment were observed in XANES, while EXAFS showed a marked decrease in the first-shell intensity due to loss of the coordinated water molecules (from the blue to the red spectra in Fig. 1[link]b). Conversely, as already observed for other zeolitic frameworks (Turnes Palomino et al., 2000[link]; Llabrés i Xamena et al., 2003[link]), upon activation in vacuum or in an inert atmosphere (for example helium; black spectra) copper(II) ions were reduced to copper(I), as shown by the disappearance of the copper(II) 1s→3d transition (inset in Fig. 1[link]a) and by the additional red shift of the edge in XANES. EXAFS revealed that the co­ordination of copper upon helium activation was further decreased compared with activation in O2. Coupled with the observation that the reduction in helium flow appears only at high temperatures (T > 250°C), while at lower temperatures the evolution of the spectra is identical to the O2-activation case, it indicates that a charged extra ligand is still coordinated to copper even at high temperatures in the case of O2 activation (Borfecchia et al., 2015[link]). This hypothesis was further supported by DFT-optimized structures of the relevant copper(II)/copper(I) coordination environments, which were used as input for EXAFS fits and XANES and XES simulations. The best overall agreement with the experimental data was obtained for models with copper in the 8-ring. While in the case of helium activation it was a bare copper(I) cation (Fig. 1[link]d), after O2 activation the OH ligand was found to be coordinated to the copper(II) ion (Fig. 1[link]c), confirming the previous assignment of the ν(OH) stretching mode at 3657 cm−1 (Giordanino et al., 2013[link]). Copper ions in the 8-ring position were also detected by X-ray powder diffraction (XRPD; Andersen et al., 2014[link]).

[Figure 1]

Figure 1

In situ static XAS data of the hydrated (RT), O2-activated and helium-activated (400°C) Cu-SSZ-13 catalyst. (a) Cu K-edge XANES spectra; the inset shows the background-subtracted pre-edge peak highlighted by the orange box in the main panel. (b) Magnitude of the phase-uncorrected FT EXAFS spectra obtained by transforming the k2-weighted χ(k) curves reported in the inset in the 2.4–12.4 Å−1 range. (c) DFT-optimized model of copper in the 8-ring that yielded the best agreement with the EXAFS data for O2-activated Cu-SSZ-13. Atom colour code: Cu, green; H, white; O, red; Al, orange, Si, grey; distances from Cu to the neighbouring atoms are given in Å in grey (DFT values) and black (EXAFS values). (d) As in (c) for helium-activated Cu-SSZ-13. Adapted from Borfecchia et al. (2015[link]).

In order to understand the reaction mechanism of SCR, great effort was invested in measuring XAS and XES spectra of this material under different reaction-relevant conditions. In the entire variety of experiments that were performed, two major approaches can be distinguished. The first involves experiments under truly operando conditions, in which the sample is exposed to the complete reaction mixture (including NO, NH3, O2 and H2O, and in some studies also NO2 and CO2) under controlled temperature conditions (Bates et al., 2014[link]; Kispersky et al., 2012[link]; Lomachenko, Borfecchia, Negri et al., 2016[link]; McEwen et al., 2012[link]).

The second strategy is to separately probe different steps of the SCR reaction to test independently developed hypotheses regarding the behaviour of the active sites under particular conditions. Such an approach allowed the decoupling of the oxidation and reduction steps of the suggested SCR cycle (Janssens et al., 2015[link]; see Fig. 2[link]). In particular, the authors tested the reducing capabilities of the individual SCR reactants, namely NO, NH3 and their mixture (Figs. 2[link]b and 2[link]c). The results clearly indicate that almost complete reduction only occurs after exposure of the catalyst to the mixture of NO and NH3. The resulting spectrum is characteristic of copper(I) species with linear geometry. Conversely, NO alone is not able to reduce copper(II), while exposure to NH3 alone only leads to partial reduction. Exposure of Cu(I)-SSZ-13 to a mixture of NO and O2 results in copper(II) species, thus closing the cycle. Subsequently, the same group investigated the nitrate–nitrite equilibrium in detail (bottom left in Fig. 2[link]a; Tyrsted et al., 2016[link]).

[Figure 2]

Figure 2

(a) NH3-SCR cycle obtained by combining XANES, EXAFS, XES, IR and EPR techniques and DFT calculations. (b, c) Catalytic cycle followed by XANES and EXAFS, respectively. Adapted with permission from Janssens et al. (2015[link]); copyright (2015) American Chemical Society.

Notably, with the development of specialized computer codes it has become possible to analyze XANES data not only by comparing the spectra with those of model compounds, but also by simulating them directly from a given structural model. Due to the high porosity of the structure and the significant anisotropy of the copper local environment, for Cu-CHA full potential codes are likely to be preferable to those that use the muffin-tin approximation, and show excellent results for densely packed systems (Rehr & Albers, 2000[link]). The main drawback of full-potential codes is that they are usually up to two orders of magnitude more demanding computationally than muffin-tin analogues such as the widely used FEFF code (Rehr et al., 2010[link]; Kas et al., 2020[link]). One of the most mature full-potential codes, FDMNES (Joly, 2001[link]; Bunău et al., 2021[link]), has been considerably sped up due to the use of sparse matrices and dedicated solvers (Guda et al., 2015[link], 2016[link]). The consequent 40-fold decrease in calculation time now enables studies which were prevously hardly feasible without huge computational clusters. To illustrate this, Figs. 3[link](a)–3[link](c) summarize the results of a structural investigation of [Cu(NH3)2]+ formed in Cu-SSZ-13 cavities upon exposure of the material to a mixture of NH3 and NO (Fig. 3[link]a). The simulated spectrum showed an underestimated gap between the first two XANES maxima, and therefore the Cu–N distances were decreased stepwise from the DFT value of 1.92 Å (Fig. 3[link]b). The best agreement with the experiment was reached at a Cu–N distance of 1.82 Å, which is significantly shorter compared with the DFT value (Fig. 3[link]c). The total calculation time for each of the reported spectra was around 24 h, whereas using the old version of FDMNES it would have required more than a month and twice as much memory, strongly hampering the feasibility of such a study. It is finally worth noting how valence-to-core XES (Fig. 3[link]d) clearly revealed the change in the first-shell ligands (from O to N) upon formation of the linear [Cu(NH3)2]+ complex by an O2-activated Cu-SSZ-13 catalyst.

[Figure 3]

Figure 3

(a) Fragment of the structural model of the [Cu(NH3)2]+ complex in the large CHA cavity above the six-membered ring. The variation range of the distances is shown in Å. (b) Theoretical Cu K-edge XANES spectra for the structureal model reported in (a) with different Cu–N distances. (c) Comparison between the experimental XANES spectrum of O2-activated Cu-CYA catalyst (red) and the best simulated spectrum (blue). The inset shows the discrepancy between the experimental and theoretical values of the gap between the first two XANES maxima as a function of the Cu–N distance. Adapted from Lomachenko, Borfecchia, Bordiga et al. (2016[link]) courtesy of K. A. Lomachenko. (d) In situ evolution of the Cu Kβ2,5 and Kβ′′ XES lines of O2-activated Cu-CHA (black) and after interaction with NH3. Reprinted with permission from Giordanino et al. (2014[link]); copyright (2014) American Chemical Society.

Finally, it is worth a reminder that Cu-zeolites have also been found to be attractive for the direct conversion of methane to methanol, which is a very challenging and important reaction. For this reaction XAS spectroscopy suggests that the active site is no longer a monomeric copper site but a μ-oxo copper dimer (Alayon et al., 2012[link], 2015[link]; Groothaert et al., 2005[link]; van Bokhoven & Lamberti, 2017[link]; Woertink et al., 2009[link]), although this topic is still highly debated in the literature.

2.2. Copper polymers

CuCl2 was molecularly immobilized inside a highly cross-linked P4VP matrix characterized by a permanent porosity that also allows catalysis in the gas phase (Groppo et al., 2008[link]). The immobilization procedure and the redox processes involving the copper sites were investigated by means of several complementary in situ techniques (UV–Vis, XANES and EXAFS), allowing determination of the structure of the system in all of the steps (Fig. 4[link]). Copper is immobilized inside P4VP as octahedral-like copper(II), with the first coordination shells containing two Cl anions, two N atoms of the pyridine rings of the polymer and two coordinated water molecules (Fig. 4[link]c and the blue spectra in Figs. 4[link]a and 4[link]b). The latter are easily lost upon mild thermal activation, resulting in copper(II) species with square-planar geometry (Fig. 4[link]d and the green spectra in Figs. 4[link]a and 4[link]b). Treatment with H2 at 450 K results in the loss of one Cl ligand (released as an HCl molecule), leaving reduced copper(I) with a high coordinative unsaturation (Fig. 4[link]e and the orange spectra in Figs. 4[link]a and 4[link]b). Such copper(I) species are highly reactive, being able to coordinate CO molecules and to reoxidize to copper(II) species upon interaction with O2 and H2O (spectra not reported here). The ability of a combined XAS/UV–Vis study to disclose the structure surrounding copper is a nontrivial result due to the amorphous nature of the host matrix. The detailed knowledge of the structural changes occurring during the copper redox reactions that was achieved in this study was a key prerequisite for exploring more complex catalytic processes. More recently, a similar redox ability has been observed for a CuCl2-functionalized UiO-67 metal–organic framework (MOF; Braglia et al., 2017[link]). The functionalization was obtained by substituting 10% of the standard 4,4′-biphenyl-dicarboxylate linkers by bipyridine-dicarboxylate moieties exhibiting metal-chelating ability and enabling the grafting of divalent metal ions into the MOF framework (Øien et al., 2015[link]).

[Figure 4]

Figure 4

(a) Effect of outgassing at RT (blue) and progressive reduction in H2 at 450 K (green and orange) of the CuCl2/P4VP sample, as monitored by XANES (main part) and UV–Vis (inset) spectroscopies (K-M, Kubelka–Munk). (b) As in (a) for the k3-weighted, phase-uncorrected FT of the EXAFS spectra, in both the modulus (main part) and the imaginary part (inset). (c, d, e) Representation of the copper environment during the grafting (c), dehydration (d) and reduction (e) processes followed by XANES, EXAFS and UV–Vis; blue, green and orange spectra, respectively, in (a) and (b). Cu, Cl, N, O, C and H atoms are represented in dark red, green, blue, red, dark grey and white, respectively. Adapted with permission from Groppo et al. (2008[link]); copyright Wiley-VCH.

2.3. The Cr/SiO2 Phillips catalyst for ethylene polymerization

The importance of the Cr/SiO2 Phillips catalyst in the industrial production of polyethylene (more than 30 million tons per year; McDaniel, 1985[link], 2010[link]; Weckhuysen & Schoonheydt, 1999[link]) and of its modified versions (Barzan et al., 2013[link]; Nenu et al., 2007[link]; Nenu & Weckhuysen, 2005[link]) makes Cr/SiO2 one of the most investigated catalytic systems using several experimental techniques (Groppo, Lamberti et al., 2005[link]; Weckhuysen et al., 1996[link]). The amorphous nature of silica prevents the use of diffraction techniques and, combined with difficulties in the treatment of open-shell systems, makes a computational approach not straightforward (Damin et al., 2009[link]). For these reasons, in situ XAS spectroscopy has extensively been used to investigate the different stages of the preparation, activation and operation of the catalyst (Demmelmaier et al., 2008[link], 2009[link]; Groppo, Prestipino et al., 2005[link]; Groppo et al., 2013[link]; Weckhuysen et al., 1995[link]).

The surface of the silica used for anchoring chromium(VI) is covered by hydroxyl groups (Si—OH) that react with the chromium precursor (chromium loading below 1 wt%) at a temperature above 750 K and under oxidizing conditions, thus acting as anchoring sites. The reaction results in tetrahedral-like hexavalent chromium species bound to the silica surface via two (Si—O—Cr) oxygen linkages (see the scheme in the black box in Fig. 5[link]) and characterized by a peculiar XANES spectrum with a strong pre-edge peak at 5993 eV due to A1E electronic transitions typical of d0 systems in Td symmetry (Bordiga et al., 1994[link], 2007[link]). Catalyst activation can occur via reduction in CO at 623 K, resulting in a highly un­saturated chromium(II) species (see the scheme in the red box in Fig. 5[link]), the XANES spectrum of which is characterized by a significant red shift of the edge (compare the red and black spectra in Fig. 5[link]). This reduction step is fully reversible and the tetrahedral-like hexavalent chromium species can be restored by exposure of the chromium(II) species to O2 at room temperature (Agostini et al., 2007[link]). These chromium(II) species are active in ethylene polymerization at room temperature, as followed by in situ XANES (blue curve in Fig. 5[link]).

[Figure 5]

Figure 5

Different steps in the preparation and activation of the Phillips catalyst followed in situ by Cr K-edge XANES spectroscopy: chromium(VI) grafted on silica (black), reduction by CO at 623 K resulting in a highly uncoordinated chromium(II) species (red) and during ethylene polymerization at RT (blue) or alternatively after interaction with CO at 100 K. Partially adapted from Gianolio et al. (2010[link]) with permission from the Royal Society of Chemistry.

The high coordinative unsaturation and the site flexibility of the chromium(II) species is shown by following the inter­action with CO at liquid-nitrogen temperature using XANES, EXAFS and IR spectroscopies (Gianolio et al., 2010[link]; see the evolution from the red to the green spectra in Fig. 5[link]). EXAFS and IR show that chromium tricarbonyls are formed. The ability of the chromium(II) species to coordinate three CO molecules is in line with their ability to coordinate both the growing polymer chain and one ethylene molecule for the next insertion (R and =, respectively, in the blue box in Fig. 5[link]) during polymerization. Also of interest is the elongation of the Cr—O bond with the silica surface from 1.86 ± 0.03 Å to 1.935 ± 0.007 Å upon the formation of Cr(CO)3.

The flexibility of the local environment of the active site in a catalyst is very important, as it guarantees the ability of the active site to maximize the coordination with ligands and reactants, thus improving its reactivity. EXAFS has been the technique of choice in highlighting such flexibility for many other different catalysts, such as the Cu-zeolites discussed above (Lamberti et al., 2000[link]; Prestipino et al., 2005[link]), TS-1 (Bonino et al., 2004[link]; Gallo et al., 2013[link]; Prestipino et al., 2004[link]; Signorile et al., 2016[link]), doubly anchored tetrahedral dioxo MoO4 units in mesoporous silica SBA-15 (Amakawa et al., 2013[link]) etc.

We conclude this section on the Phillips catalyst by mentioning that in situ XAS and XES techniques have also been very informative in the investigation of the Ziegler–Natta class of olefin-polymerization catalysts (Groppo, Seenivasan et al., 2015[link]; Seenivasan et al., 2013[link]).

3. Metal nanoparticles

Supported metal nanoparticle systems were among the very first catalysts to be investigated using the XAFS technique by Lytle himself in collaboration with researchers from Exxon (Sinfelt et al., 1978[link]; Via et al., 1979[link]). Particle shape and size are the most relevant parameters in determining the adsorption and reactivity properties of metal nanoparticles; therefore, a huge effort has been devoted to developing experimental tools that are able to provide statistically significant information on these two activity-determining properties. Atoms at the surface of nanoparticles exhibit a lower coordination number than atoms in the bulk. As a consequence, by experimentally determining the coordination numbers of the different shells with EXAFS, it is possible to estimate the average particle size assuming a certain particle shape (Agostini et al., 2009[link]; Agostini, Piovano et al., 2014[link]; Bezemer et al., 2006[link]; Frenkel et al., 2001[link]; Karim et al., 2009[link]; Miller et al., 2006[link]; Mostafa et al., 2010[link]; Newton, 2008[link]; Witkowska et al., 2007[link]; see Fig. 6[link]a). In this regard, the main strengths of EXAFS include (i) the ability to probe all metal atoms crossed by the beam (overcoming the limitation of XRPD, which only detects particles that are sufficiently large to give Bragg diffraction) and (ii) the large number of particles probed [overcoming the intrinsic statistical weakness of transmission electron microscopy (TEM) studies]. However, only average values can be obtained by EXAFS and therefore data analysis should be performed carefully and the EXAFS results should be compared with those obtained from other independent techniques such as XRPD, TEM, scanning TEM (STEM), small-angle X-ray scattering (SAXS), chemisorption, total X-ray or neutron scattering (Agostini, Lamberti et al., 2014[link]; Agostini et al., 2009[link]; Agostini, Piovano et al., 2014[link]; Briggs et al., 2015[link]; Cuenya et al., 2010[link]; Deganello et al., 2006[link]; Groppo, Agostini et al., 2015[link]; Groppo et al., 2012[link], 2014[link]; Lazzarini et al., 2017[link]; Pellegrini et al., 2011[link]) or supported by theoretical modelling (Bugaev et al., 2017[link]; Chill et al., 2015[link]).

[Figure 6]

Figure 6

(a) Comparison of the average coordination numbers of the first four shells for clusters with different shapes, cuboctahedron (CO), truncated cuboctahedron (TCO) and icosahedron (ICO), shown as solid, dashed and dotted lines, respectively. The data are plotted in terms of order m (bottom abscissa axis) and of the particle diameter in the case of palladium nanoparticles (top abscissa axis) with cell parameter 3.889 Å and first-shell Pd–Pd distance 2.75 Å. Reproduced with permission from Agostini, Piovano et al. (2014[link]); copyright 2014 American Chemical Society. (b) Determination of the model cluster sizes (shown for two different types of clusters) using the H:Pd ratio obtained from the measured Pd–Pd distances. The (111) model fits the data (shown by red arrows) for both the H:Pd ratio at 186 K and the particle size range, while the (001) model does not. Reproduced with permission from Wang et al. (2015[link]); copyright 2015 American Chemical Society. (c) Experimental Pd K-edge XANES spectra (left ordinate axis) and Δ-XANES (right ordinate axis) collected at 100°C on palladium nanoparticles in vacuum (black), in 100 mbar H2 (violet) and in 1000 mbar acetylene (orange). (d) As in (c) for a mixture of 650 mbar H2 and 350 mbar acetylene (red) and in 600 mbar H2 (blue). (e) Theoretical Pd K-edge Δ-XANES spectra for the PdHx phase in the 0 ≤ x ≤ 0.5 stoichiometry interval. (f) As in (e) for the PdCy phase in the 0 ≤ y ≤ 0.2 stoichiometry interval. Reproduced with permission from Bugaev et al. (2017[link]) with permission from Elsevier.

Through an accurate and unambiguous determination of the coordination number for the first four shells, interpretation of the EXAFS data collected on metal nanoparticles allows determination of both the particle size and shape (Agostini et al., 2009[link]; Frenkel et al., 2011[link]; see Fig. 6[link]a). This however holds for ideal cases only, i.e. for low-temperature data collections, narrow particle size and shape distributions, and symmetric bond-length distributions. The analysis becomes delicate when the catalysts are investigated under reaction conditions at high temperature and high reactant pressure. Further complications arise when the particles undergo modifications along the reaction such as changes in (i) size (sintering or disaggregation), (ii) shape or (iii) chemical composition (metal/oxide, metal/hydride or metal/carbide transformation). Hereafter, we will briefly discuss the case study of palladium-based catalysts, the active phase of which is subjected to chemical changes under reaction conditions.

Supported palladium catalysts are among the most used systems for the hydrogenation of hydrocarbons, such as alkynes and alkenes. Under reaction conditions, palladium nanoparticles may undergo phase changes to hydride and carbide phases, the nature of which affects the catalytic properties (Teschner et al., 2008[link], 2010[link]). Therefore, determining hydride and carbide formation during a catalytic process becomes an important problem that is also relevant to industry. Being the subject of numerous theoretical (Shabaev et al., 2010[link]; Zhdanov & Kasemo, 2008[link]) and experimental (Bugaev et al., 2014[link]; Davis et al., 1989[link]; Flanagan & Oates, 1991[link]; Lewis, 1982[link]; Mitsui et al., 2003[link]; Shegai & Langhammer, 2011[link]; Soldatov et al., 1993[link]; Tew et al., 2009[link]) studies, palladium hydride is among the most-studied metal hydrides.

The in situ formation of the PdHx phase in palladium nanoparticles supported on θ-Al2O3 has been investigated by in situ EXAFS in the 186–483 K range (Wang et al., 2015[link]). The experiments were performed in both hydrogen and helium atmospheres in order to disentangle thermal effects from modifications due to hydride formation in the first-shell EXAFS signal. By comparing the two data sets, the authors were able to isolate the effects of both the finite particle size and hydrogen absorption on the Pd—Pd bond length, to obtain hydrogen-intake values and to estimate the particle size, which agrees with independent ex situ STEM analysis. In more detail, they constructed two families of models of truncated hemispherical f.c.c. cuboctahedra of increasing size, with the support surface having a (001) or (111) orientation. The values of the H:Pd ratios were calculated for each cluster model (see Fig. 6[link]b). Calculations demonstrate that the estimated H:Pd ratio will be 0.52 for the (111)-oriented clusters and 0.65 for the (001)-oriented clusters of ∼2.1 nm in size. The latter value is far beyond the experimental value of 0.54 ± 0.02 at 186 K. Hence, the (111)-oriented clusters better match the experimental data. As a result, the particle size of 2.1 ± 0.4 nm obtained by STEM is in agreement with the particle size inferred from the experimentally determined H:Pd ratio of 0.54 ± 0.02. Subsequently, the experimental H:Pd ratio determined by expansion of the bond length was used to estimate the particle size by occupancy model calculation. It should be noted that this method is only effective for particle sizes below 3 nm because the H:Pd ratio varies strongly with size in this size range.

In contrast to the hydride, the structure and properties of the carbide phase are still under discussion (McCaulley, 1993[link]; Teschner et al., 2008[link]; Tew et al., 2011[link]). The formation of both the hydride and carbide phases is accompanied by an expansion of the palladium lattice, which can be followed by EXAFS, as discussed above, or by XRPD (Suleiman et al., 2003[link]; Vogel et al., 2010[link]). However, both techniques are scarcely sensitive to light atoms, such as carbon and especially hydrogen, due to their low scattering amplitudes compared with palladium. Thus, PdHx and PdCy phases are only indirectly observed by EXAFS and XRPD via Pd–Pd distance elongation or lattice expansion. During hydrogenation reactions, both H2 and alkynes (or alkenes) are fed to the catalyst so that PdHx, PdCy and PdHxCy phases could potentially be formed. Since all of them result in an elongation of the Pd–Pd distance, it will be very difficult to discriminate among the different phases using EXAFS or XRPD. Conversely, the formation of palladium hydride or carbide directly affects the shape of the Pd K-edge XANES spectra due to mixing of unoccupied states of hydrogen or carbon and palladium. As the mixing is different in the two cases, the perturbation of the XANES spectra upon hydride formation is different from that induced by carbide (Fig. 6[link]c), making XANES the technique of choice to distinguish the two cases under operando conditions (Bugaev et al., 2017[link]). In particular, XANES was able to prove that on feeding the palladium catalyst at 100°C with 350 mbar H2 and 650 mbar acetylene the active phase is a PdHx phase (see Fig. 6[link]d).

To quantify the PdHx and PdCy stoichiometry, the Rostov-on-Don group performed XANES calculations applying Monte Carlo simulations of H- and C-atom occupancy in the octahedral interstitials of the palladium f.c.c. lattice (Bugaev et al., 2017[link]). For each of the selected concentrations of x or y in PdHx or PdCy, they generated 1000 different geometries and averaged the resulting 1000 XANES spectra, which were subsequently used for the quantitative fitting of experimental XANES. Fig. 6[link](e) shows the evolution of the theoretical difference XANES spectra for the PdHx phase in the 0 ≤ x ≤ 0.5 range; Fig. 6[link](f) reports the analogous difference for the PdCy phase in the 0 ≤ y ≤ 0.2 interval. The fitting procedure was performed using a multidimensional interpolation approach implemented in the FitIt-3 code (Smolentsev & Soldatov, 2006[link], 2007[link], 2021[link]). In particular, the resulting stoichiometry of the active phase on the catalyst exposed to 100 and 600 mbar H2 at 100°C was PdH0.20±0.05 and PdH0.30±0.05, respectively. Analogously, the stoichiometry of the carbide phase obtained by exposing the catalyst to 1000 mbar C2H2 at 100°C was PdC0.13±0.05. It is clear that the results reviewed here (Fig. 6[link]) open up new opportunities for in situ XAFS investigations of palladium nanoparticle-based catalysts under reaction conditions where catalyst fragmentation, shape change and/or stoichiometry change may occur.

4. Conclusions

In this chapter, we have provided the necessary basis to understand the relevant role that XAS (and more recently XES) has played in disclosing the structural changes that are undergone by the active site of a working catalyst at the molecular level. Selected examples include copper zeolites for NH3-SCR and copper-functionalized polymers, the Phillips polymerization catalyst and supported metal nanoparticles.

We foresee that remarkable advances in the use of hard X-ray spectroscopy techniques in catalysis will be available in the future; the principal advances are listed below.

(i) The further improvement and more extended application of modulation excitation spectroscopy, which will potentially allow the selective detection of minority species that are sensitive to an external stimulus (Chiarello & Ferri, 2015[link]), thus allowing the discrimination of active sites from spectators.

(ii) Experimental setups allowing parallel IR, UV–Vis and Raman data to be collected simultaneously with XAS/XES data are expected to be a key area of technical development (Liu et al., 2017[link]).

(iii) Space-resolved (tomographic) techniques will provide a precise three-dimensional insight into the whole catalytic bed (hosted inside a capillary) and the individual catalyst grains to investigate effects such as the change in the reactants:products ratio occurring along the catalytic bed (Beale et al., 2010[link]; Buurmans & Weckhuysen, 2012[link]; Grunwaldt & Schroer, 2010[link]; Kalirai et al., 2016[link]).

(iv) The realization of new and improved secondary emission spectrometers at various beamlines (Kvashnina & Scheinost, 2016[link]) will allow oxidation state-specific EXAFS (to obtain separate EXAFS signals in samples containing the same element in different oxidation states; de Groot, 2000[link]) and spin-selective EXAFS spectra collection (Glatzel et al., 2002[link]).

(v) Hard X-ray Raman scattering will allow the collection of XAS-like signal from relevant light atoms such as, for exanple, C, O and N under catalytic working conditions (Bergmann et al., 2000[link]; Lamberti et al., 2016[link]; Mino et al., 2013[link]).

(vi) Laser pump/X-ray probe experiments, which to date have mainly been applied in studies related to photoinduced structural dynamics, may be employed to investigate photocatalysts, clarifying the structural and electronic rearrangements of the photocatalytic site just after (visible) photon absorption or other external stimuli (Borfecchia et al., 2013[link]; Chen, 2016[link]; Chergui, 2015[link]). Furthermore, in the immediate future, X-ray free-electron laser (XFEL) sources will revolutionize the physics and chemistry of time-resolved experiments (Gawelda et al., 2016[link]).

(vii) The simulation of XANES and XES spectra will be used more and more frequently to confirm or discard local structures hypothesized from the refinement of EXAFS or diffraction data (Borfecchia et al., 2012[link]; Gallo et al., 2014[link]; Guda et al., 2015[link]; Joly & Grenier, 2016[link]; Regli et al., 2007[link]).

(viii) Joint EXAFS/diffraction anomalous fine-structure (DAFS) studies will allows the exploration of biphasic systems and, for example, the contributions from two co-existing crystalline phases or from co-existing amorphous and crystalline phases to be disentangled (Groppo, Agostini et al., 2015[link]; Groppo et al., 2014[link]).

(ix) As far as metal nanoparticles are concerned, total scattering (PDF) experiments (Bozin et al., 2013[link]) will be able to bridge the gap between EXAFS, dominating the 0–30 Å diameter interval, and XRPD, which is informative in the 80 Å to bulk range (Newton et al., 2012[link]).

(x) X-ray magnetic circular dichroism (X-MCD; Rogalev et al., 2016[link]), coupled with more conventional visible-light MCD and electron paramagnetic resonance (EPR), will provide new insights in the investigation of transition metal-supported catalysts (Roa et al., 2010[link]) and in biocatalysis (Staniland et al., 2007[link]).


CL thanks the Russian Ministry of Education and Science for support (megagrant No. 14.Y26.31.0001). EB acknowledges the Innovation Fund Denmark (industrial postdoc No. 5190-00018B).


First citationAgostini, G., Gianolio, D. & Lamberti, C. (2023). Int. Tables Crystallogr. I. In the press.Google Scholar
First citationAgostini, G., Groppo, E., Bordiga, S., Zecchina, A., Prestipino, C., D'Acapito, F., van Kimmenade, E., Thüne, P. C., Niemantsverdriet, J. W. & Lamberti, C. (2007). J. Phys. Chem. C, 111, 16437–16444.Google Scholar
First citationAgostini, G., Lamberti, C., Palin, L., Milanesio, M., Danilina, N., Xu, B., Janousch, M. & van Bokhoven, J. A. (2010). J. Am. Chem. Soc. 132, 667–678.Google Scholar
First citationAgostini, G., Lamberti, C., Pellegrini, R., Leofanti, G., Giannici, F., Longo, A. & Groppo, E. (2014). ACS Catal. 4, 187–194.Google Scholar
First citationAgostini, G., Pellegrini, R., Leofanti, G., Bertinetti, L., Bertarione, S., Groppo, E., Zecchina, A. & Lamberti, C. (2009). J. Phys. Chem. C, 113, 10485–10492.Google Scholar
First citationAgostini, G., Piovano, A., Bertinetti, L., Pellegrini, R., Leofanti, G., Groppo, E. & Lamberti, C. (2014). J. Phys. Chem. C, 118, 4085–4094.Google Scholar
First citationAlayon, E. M., Nachtegaal, M., Bodi, A., Ranocchiari, M. & van Bokhoven, J. A. (2015). Phys. Chem. Chem. Phys. 17, 7681–7693.Google Scholar
First citationAlayon, E. M., Nachtegaal, M., Ranocchiari, M. & van Bokhoven, J. A. (2012). Chem. Commun. 48, 404–406.Google Scholar
First citationAmakawa, K., Sun, L., Guo, C., Hävecker, M., Kube, P., Wachs, I. E., Lwin, S., Frenkel, A. I., Patlolla, A., Hermann, K., Schlögl, R. & Trunschke, A. (2013). Angew. Chem. Int. Ed. 52, 13553–13557.Google Scholar
First citationAndersen, C. W., Bremholm, M., Vennestrøm, P. N. R., Blichfeld, A. B., Lundegaard, L. F. & Iversen, B. B. (2014). IUCrJ, 1, 382–386.Google Scholar
First citationBarzan, C., Gianolio, D., Groppo, E., Lamberti, C., Monteil, V., Quadrelli, A. E. & Bordiga, S. (2013). Chem. Eur. J. 19, 17277–17282.Google Scholar
First citationBates, S. A., Verma, A. A., Paolucci, C., Parekh, A. A., Anggara, T., Yezerets, A., Schneider, W. F., Miller, J. T., Delgass, W. N. & Ribeiro, F. H. (2014). J. Catal. 312, 87–97.Google Scholar
First citationBeale, A. M., Gao, F., Lezcano-Gonzalez, I., Peden, C. H. F. & Szanyi, J. (2015). Chem. Soc. Rev. 44, 7371–7405.Google Scholar
First citationBeale, A. M., Jacques, S. D. M. & Weckhuysen, B. M. (2010). Chem. Soc. Rev. 39, 4656–4672.Google Scholar
First citationBekturov, E. A. & Kudaibergenov, S. E. (2002). Catalysis by Polymers. Weinheim: Wiley-VCH.Google Scholar
First citationBergmann, U., Mullins, O. C. & Cramer, S. P. (2000). Anal. Chem. 72, 2609–2612.Google Scholar
First citationBezemer, G. L., Bitter, J. H., Kuipers, H., Oosterbeek, H., Holewijn, J. E., Xu, X. D., Kapteijn, F., van Dillen, A. J. & de Jong, K. P. (2006). J. Am. Chem. Soc. 128, 3956–3964.Google Scholar
First citationBokhoven, J. A. van & Lamberti, C. (2014). Coord. Chem. Rev. 277–278, 275–290.Google Scholar
First citationBokhoven, J. A. van & Lamberti, C. (2017). XAFS Techniques for Catalysts, Nanomaterials, and Surfaces, edited by Y. Iwasawa, K. Asakura & M. Tada, pp. 299–316. Cham: Springer.Google Scholar
First citationBonino, F., Damin, A., Ricchiardi, G., Ricci, M., Spanò, G., D'Aloisio, R., Zecchina, A., Lamberti, C., Prestipino, C. & Bordiga, S. (2004). J. Phys. Chem. B, 108, 3573–3583.Google Scholar
First citationBonino, F., Groppo, E., Prestipino, C., Agostini, G., Piovano, A., Gianolio, D., Mino, L., Gallo, E. & Lamberti, C. (2015). Synchrotron Radiation: Basics, Methods and Applications, edited by S. Mobilio, F. Boscherini & C. Meneghini, pp. 717–736. Berlin, Heidelberg: Springer.Google Scholar
First citationBordiga, S., Bonino, F., Damin, A. & Lamberti, C. (2007). Phys. Chem. Chem. Phys. 9, 4854–4878.Google Scholar
First citationBordiga, S., Bonino, F., Lillerud, K. P. & Lamberti, C. (2010). Chem. Soc. Rev. 39, 4885–4927.Google Scholar
First citationBordiga, S., Coluccia, S., Lamberti, C., Marchese, L., Zecchina, A., Boscherini, F., Buffa, F., Genoni, F. & Leofanti, G. (1994). J. Phys. Chem. 98, 4125–4132.Google Scholar
First citationBordiga, S., Groppo, E., Agostini, G., van Bokhoven, J. A. & Lamberti, C. (2013). Chem. Rev. 113, 1736–1850.Google Scholar
First citationBorfecchia, E., Braglia, L., Bonino, F., Bordiga, S., Øien, S., Olsbye, U., Lillerud, K. P., van Bokhoven, J. A., Lomachenko, K. A., Guda, A. A., Soldatov, M. A. & Lamberti, C. (2017). XAFS Techniques for Catalysts, Nanomaterials, and Surfaces, edited by Y. Iwasawa, K. Asakura & M. Tada, pp. 397–430. Cham: Springer.Google Scholar
First citationBorfecchia, E., Garino, C., Salassa, L. & Lamberti, C. (2013). Phil. Trans. R. Soc. A, 371, 20120132.Google Scholar
First citationBorfecchia, E., Lomachenko, K. A., Giordanino, F., Falsig, H., Beato, P., Soldatov, A. V., Bordiga, S. & Lamberti, C. (2015). Chem. Sci. 6, 548–563.Google Scholar
First citationBorfecchia, E., Maurelli, S., Gianolio, D., Groppo, E., Chiesa, M., Bonino, F. & Lamberti, C. (2012). J. Phys. Chem. C, 116, 19839–19850.Google Scholar
First citationBozin, E. S., Juhás, P. & Billinge, S. J. L. (2013). Characterization of Semiconductor Heterostructures and Nanostructures II, edited by C. Lamberti & G. Agostini, pp. 229–257. Amsterdam: Elsevier.Google Scholar
First citationBraglia, L., Borfecchia, E., Maddalena, L., Øien, S., Lomachenko, K. A., Bugaev, A. L., Bordiga, S., Soldatov, A. V., Lillerud, K. P. & Lamberti, C. (2017). Catal. Today, 283, 89–103.Google Scholar
First citationBriggs, B. D., Bedford, N. M., Seifert, S., Koerner, H., Ramezani-Dakhel, H., Heinz, H., Naik, R. R., Frenkel, A. I. & Knecht, M. R. (2015). Chem. Sci. 6, 6413–6419.Google Scholar
First citationBugaev, A. L., Guda, A. A., Lazzarini, A., Lomachenko, K. A., Groppo, E., Pellegrini, R., Piovano, A., Emerich, H., Soldatov, A. V., Bugaev, L. A., Dmitriev, V. P., van Bokhoven, J. A. & Lamberti, C. (2017). Catal. Today, 283, 119–126.Google Scholar
First citationBugaev, A. L., Guda, A. A., Lomachenko, K. A., Srabionyan, V. V., Bugaev, L. A., Soldatov, A. V., Lamberti, C., Dmitriev, V. P. & van Bokhoven, J. A. (2014). J. Phys. Chem. C, 118, 10416–10423.Google Scholar
First citationBunău, O., Ramos, A. Y. & Joly, Y. (2021). Int. Tables. Crystallogr. I, .Google Scholar
First citationButova, V. V., Soldatov, M. A., Guda, A. A., Lomachenko, K. A. & Lamberti, C. (2016). Russ. Chem. Rev. 85, 280–307.Google Scholar
First citationBuurmans, I. L. C. & Weckhuysen, B. M. (2012). Nat. Chem. 4, 873–886.Google Scholar
First citationCastillejos-López, E., Agostini, G., Di Michel, M., Iglesias-Juez, A. & Bachiller-Baeza, B. (2017). ACS Catal. 7, 796–811.Google Scholar
First citationChadwick, J. C., Duchateau, R., Freixa, Z. & van Leeuwen, P. W. N. M. (2011). Homogeneous Catalysts: Activity – Stability – Deactivation. Weinheim: Wiley-VCH.Google Scholar
First citationChen, L. X. (2016). X-ray Absorption and X-ray Emission Spectroscopy: Theory and Applications, edited by J. A. van Bokhoven & C. Lamberti, pp. 213–249. Chichester: John Wiley & Sons.Google Scholar
First citationChergui, M. (2015). Acc. Chem. Res. 48, 801–808.Google Scholar
First citationChiarello, G. L. & Ferri, D. (2015). Phys. Chem. Chem. Phys. 17, 10579–10591.Google Scholar
First citationChill, S. T., Anderson, R. M., Yancey, D. F., Frenkel, A. I., Crooks, R. M. & Henkelman, G. (2015). ACS Nano, 9, 4036–4042.Google Scholar
First citationChorkendorff, I. & Niemantsverdriet, J. W. (2007). Concepts of Modern Catalysis and Kinetics, 2nd ed. Weinheim: Wiley-VCH.Google Scholar
First citationClausen, B. S., Topsoe, H. & Frahm, R. (1998). Adv. Catal. 42, 315–344.Google Scholar
First citationCorma, A., García, H. & Llabrés i Xamena, F. X. (2010). Chem. Rev. 110, 4606–4655.Google Scholar
First citationDamin, A., Vitillo, J. G., Ricchiardi, G., Bordiga, S., Lamberti, C., Groppo, E. & Zecchina, A. (2009). J. Phys. Chem. A, 113, 14261–14269.Google Scholar
First citationDavis, R., Landry, S., Horsley, J. & Boudart, M. (1989). Phys. Rev. B, 39, 10580–10583.Google Scholar
First citationDeganello, G., Giannici, F., Martorana, A., Pantaleo, G., Prestianni, A., Balerna, A., Liotta, L. F. & Longo, A. (2006). J. Phys. Chem. B, 110, 8731–8739.Google Scholar
First citationDeka, U., Juhin, A., Eilertsen, E. A., Emerich, H., Green, M. A., Korhonen, S. T., Weckhuysen, B. M. & Beale, A. M. (2012). J. Phys. Chem. C, 116, 4809–4818.Google Scholar
First citationDeka, U., Lezcano-Gonzalez, I., Weckhuysen, B. M. & Beale, A. M. (2013). ACS Catal. 3, 413–427.Google Scholar
First citationDemmelmaier, C. A., White, R. E., van Bokhoven, J. A. & Scott, S. L. (2008). J. Phys. Chem. C, 112, 6439–6449.Google Scholar
First citationDemmelmaier, C. A., White, R. E., van Bokhoven, J. A. & Scott, S. L. (2009). J. Catal. 262, 44–56.Google Scholar
First citationEvans, J. (1997). Chem. Soc. Rev. 26, 11–19.Google Scholar
First citationFernández-García, M. (2002). Catal. Rev. 44, 59–121.Google Scholar
First citationFlanagan, T. B. & Oates, W. A. (1991). Annu. Rev. Mater. Sci. 21, 269–304.Google Scholar
First citationFrank, P., Benfatto, M., Szilagyi, R. K., D'Angelo, P., Della Longa, S. & Hodgson, K. O. (2005). Inorg. Chem. 44, 1922–1933.Google Scholar
First citationFrenkel, A. I. (2012). Chem. Soc. Rev. 41, 8163–8178.Google Scholar
First citationFrenkel, A. I., Hills, C. W. & Nuzzo, R. G. (2001). J. Phys. Chem. B, 105, 12689–12703.Google Scholar
First citationFrenkel, A. I., Yevick, A., Cooper, C. & Vasic, R. (2011). Annu. Rev. Anal. Chem. 4, 23–39.Google Scholar
First citationFurimsky, E. (2008). Carbons and Carbon Supported Catalysts in Hydroprocessing. Cambridge: Royal Society of Chemistry.Google Scholar
First citationGallo, E., Bonino, F., Swarbrick, J. C., Petrenko, T., Piovano, A., Bordiga, S., Gianolio, D., Groppo, E., Neese, F., Lamberti, C. & Glatzel, P. (2013). ChemPhysChem, 14, 79–83.Google Scholar
First citationGallo, E., Piovano, A., Marini, C., Mathon, O., Pascarelli, S., Glatzel, P., Lamberti, C. & Berlier, G. (2014). J. Phys. Chem. C, 118, 11745–11751.Google Scholar
First citationGawelda, W., Szlachetko, J. & Milne, C. J. (2016). X-ray Absorption and X-ray Emission Spectroscopy: Theory and Applications, edited by J. A. van Bokhoven & C. Lamberti, pp. 637–669. Chichester: John Wiley & Sons.Google Scholar
First citationGianolio, D., Groppo, E., Vitillo, J. G., Damin, A., Bordiga, S., Zecchina, A. & Lamberti, C. (2010). Chem. Commun. 46, 976–978.Google Scholar
First citationGiordanino, F., Borfecchia, E., Lomachenko, K. A., Lazzarini, A., Agostini, G., Gallo, E., Soldatov, A. V., Beato, P., Bordiga, S. & Lamberti, C. (2014). J. Phys. Chem. Lett. 5, 1552–1559.Google Scholar
First citationGiordanino, F., Vennestrøm, P. N. R., Lundegaard, L. F., Stappen, F. N., Mossin, S., Beato, P., Bordiga, S. & Lamberti, C. (2013). Dalton Trans. 42, 12741–12761.Google Scholar
First citationGlatzel, P. & Bergmann, U. (2005). Coord. Chem. Rev. 249, 65–95.Google Scholar
First citationGlatzel, P., Jacquamet, L., Bergmann, U., de Groot, F. M. F. & Cramer, S. P. (2002). Inorg. Chem. 41, 3121–3127.Google Scholar
First citationGroot, F. de (2001). Chem. Rev. 101, 1779–1808.Google Scholar
First citationGroot, F. M. F. de (2000). Top. Catal. 10, 179–186.Google Scholar
First citationGroothaert, M. H., Smeets, P. J., Sels, B. F., Jacobs, P. A. & Schoonheydt, R. A. (2005). J. Am. Chem. Soc. 127, 1394–1395.Google Scholar
First citationGroppo, E., Agostini, G., Borfecchia, E., Lazzarini, A., Liu, W., Lamberti, C., Giannici, F., Portale, G. & Longo, A. (2015). ChemCatChem, 7, 2188–2195.Google Scholar
First citationGroppo, E., Agostini, G., Borfecchia, E., Wei, L., Giannici, F., Portale, G., Longo, A. & Lamberti, C. (2014). J. Phys. Chem. C, 118, 8406–8415.Google Scholar
First citationGroppo, E., Agostini, G., Piovano, A., Muddada, N. B., Leofanti, G., Pellegrini, R., Portale, G., Longo, A. & Lamberti, C. (2012). J. Catal. 287, 44–54.Google Scholar
First citationGroppo, E., Lamberti, C., Bordiga, S., Spoto, G. & Zecchina, A. (2005). Chem. Rev. 105, 115–184.Google Scholar
First citationGroppo, E., Liu, W., Zavorotynska, O., Agostini, G., Spoto, G., Bordiga, S., Lamberti, C. & Zecchina, A. (2010). Chem. Mater. 22, 2297–2308.Google Scholar
First citationGroppo, E., Prestipino, C., Cesano, F., Bonino, F., Bordiga, S., Lamberti, C., Thüne, P. C., Niemantsverdriet, J. W. & Zecchina, A. (2005). J. Catal. 230, 98–108.Google Scholar
First citationGroppo, E., Seenivasan, K. & Barzan, C. (2013). Catal. Sci. Technol. 3, 858–878.Google Scholar
First citationGroppo, E., Seenivasan, K., Gallo, E., Sommazzi, A., Lamberti, C. & Bordiga, S. (2015). ACS Catal. 5, 5586–5595.Google Scholar
First citationGroppo, E., Uddin, M. J., Bordiga, S., Zecchina, A. & Lamberti, C. (2008). Angew. Chem. Int. Ed. 47, 9269–9273.Google Scholar
First citationGrunwaldt, J. D. & Schroer, C. G. (2010). Chem. Soc. Rev. 39, 4741–4753.Google Scholar
First citationGuda, A. A., Guda, S. A., Soldatov, M. A., Lomachenko, K. A., Bugaev, A. L., Lamberti, C., Gawelda, W., Bressler, C., Smolentsev, G., Soldatov, A. V. & Joly, Y. (2016). J. Phys. Conf. Ser. 712, 012004.Google Scholar
First citationGuda, S. A., Guda, A. A., Soldatov, M. A., Lomachenko, K. A., Bugaev, A. L., Lamberti, C., Gawelda, W., Bressler, C., Smolentsev, G., Soldatov, A. V. & Joly, Y. (2015). J. Chem. Theory Comput. 11, 4512–4521.Google Scholar
First citationGünter, T., Carvalho, H. W. P., Doronkin, D. E., Sheppard, T., Glatzel, P., Atkins, A. J., Rudolph, J., Jacob, C. R., Casapu, M. & Grunwaldt, J. D. (2015). Chem. Commun. 51, 9227–9230.Google Scholar
First citationIwamoto, M., Yahiro, H., Torikai, Y., Yoshioka, T. & Mizuno, N. (1990). Chem. Lett. 19, 1967–1970.Google Scholar
First citationIwasawa, Y., Asakura, K. & Tada, M. (2017). XAFS Techniques for Catalysts, Nanomaterials, and Surfaces. Cham: Springer.Google Scholar
First citationJanssens, T. V. W., Falsig, H., Lundegaard, L. F., Vennestrøm, P. N. R., Rasmussen, S. B., Moses, P. G., Giordanino, F., Borfecchia, E., Lomachenko, K. A., Lamberti, C., Bordiga, S., Godiksen, A., Mossin, S. & Beato, P. (2015). ACS Catal. 5, 2832–2845.Google Scholar
First citationJoly, Y. (2001). Phys. Rev. B, 63, 125120.Google Scholar
First citationJoly, Y. & Grenier, S. (2016). X-ray Absorption and X-ray Emission Spectroscopy: Theory and Applications, edited by J. A. van Bokhoven & C. Lamberti, pp. 73–97. Chichester: John Wiley & Sons.Google Scholar
First citationKalirai, S., Paalanen, P. P., Wang, J., Meirer, F. & Weckhuysen, B. M. (2016). Angew. Chem. Int. Ed. 55, 11134–11138.Google Scholar
First citationKarim, A. M., Prasad, V., Mpourmpakis, G., Lonergan, W. W., Frenkel, A. I., Chen, J. G. G. & Vlachos, D. G. (2009). J. Am. Chem. Soc. 131, 12230–12239.Google Scholar
First citationKas, J. J., Vila, F. D. & Rehr, J. J. (2020). Int. Tables. Crystallogr. I, .Google Scholar
First citationKispersky, V. F., Kropf, A. J., Ribeiro, F. H. & Miller, J. T. (2012). Phys. Chem. Chem. Phys. 14, 2229–2238.Google Scholar
First citationKvashnina, K. O. & Scheinost, A. C. (2016). J. Synchrotron Rad. 23, 836–841.Google Scholar
First citationKwak, J. H., Tonkyn, R. G., Kim, D. H., Szanyi, J. & Peden, C. H. F. (2010). J. Catal. 275, 187–190.Google Scholar
First citationLamberti, C., Bordiga, S., Salvalaggio, M., Spoto, G., Zecchina, A., Geobaldo, F., Vlaic, G. & Bellatreccia, M. (1997). J. Phys. Chem. B, 101, 344–360.Google Scholar
First citationLamberti, C., Borfecchia, E., van Bokhoven, J. A. & Fernández-García, M. (2016). X-ray Absorption and X-ray Emission Spectroscopy: Theory and Applications, edited by J. A. van Bokhoven & C. Lamberti, pp. 303–350. Chichester: John Wiley & Sons.Google Scholar
First citationLamberti, C., Turnes Palomino, G., Bordiga, S., Berlier, G., D'Acapito, F. & Zecchina, A. (2000). Angew. Chem. Int. Ed. 39, 2138–2141.Google Scholar
First citationLamberti, C. & van Bokhoven, J. A. (2016). X-ray Absorption and X-ray Emission Spectroscopy: Theory and Applications, edited by J. A. van Bokhoven & C. Lamberti, pp. 353–383. Chichester: John Wiley & Sons.Google Scholar
First citationLazzarini, A., Groppo, E., Agostini, G., Borfecchia, E., Giannici, F., Portale, G., Longo, A., Pellegrini, R. & Lamberti, C. (2017). Catal. Today, 283, 144–150.Google Scholar
First citationLazzarini, A., Piovano, A., Pellegrini, R., Leofanti, G., Agostini, G., Rudić, S., Chierotti, M. R., Gobetto, R., Battiato, A., Spoto, G., Zecchina, A., Lamberti, C. & Groppo, E. (2016). Catal. Sci. Technol. 6, 4910–4922.Google Scholar
First citationLewis, F. A. (1982). Platin. Met. Rev. 26, 20–27.Google Scholar
First citationLezcano-Gonzalez, I., Wragg, D. S., Slawinski, W. A., Hemelsoet, K., Van Yperen-De Deyne, A., Waroquier, M., Van Speybroeck, V. & Beale, A. M. (2015). J. Phys. Chem. C, 119, 24393–24403.Google Scholar
First citationLiu, L., Díaz, U., Arenal, R., Agostini, G., Concepción, P. & Corma, A. (2017). Nat. Mater. 16, 132–138.Google Scholar
First citationLlabrés i Xamena, F. X., Fisicaro, P., Berlier, G., Zecchina, A., Palomino, G. T., Prestipino, C., Bordiga, S., Giamello, E. & Lamberti, C. (2003). J. Phys. Chem. B, 107, 7036–7044.Google Scholar
First citationLomachenko, K. A., Borfecchia, E., Bordiga, S., Soldatov, A. V., Beato, P. & Lamberti, C. (2016). J. Phys. Conf. Ser. 712, 012041.Google Scholar
First citationLomachenko, K. A., Borfecchia, E., Negri, C., Berlier, G., Lamberti, C., Beato, P., Falsig, H. & Bordiga, S. (2016). J. Am. Chem. Soc. 138, 12025–12028.Google Scholar
First citationMartin, N. M., Nilsson, J., Skoglundh, M., Adams, E. C., Wang, X., Velin, P., Smedler, G., Raj, A., Thompsett, D., Brongersma, H. H., Grehl, T., Agostini, G., Mathon, O., Carlson, S., Norén, K., Martinez-Casado, F. J., Matej, Z., Balmes, O. & Carlsson, P. (2016). J. Phys. Chem. C, 120, 28009–28020.Google Scholar
First citationMcCaulley, J. A. (1993). J. Phys. Chem. 97, 10372–10379.Google Scholar
First citationMcDaniel, M. P. (1985). Adv. Catal. 33, 47–98.Google Scholar
First citationMcDaniel, M. P. (2010). Adv. Catal. 53, 123–606.Google Scholar
First citationMcEwen, J. S., Anggara, T., Schneider, W. F., Kispersky, V. F., Miller, J. T., Delgass, W. N. & Ribeiro, F. H. (2012). Catal. Today, 184, 129–144.Google Scholar
First citationMilanesio, M., Artioli, G., Gualtieri, A. F., Palin, L. & Lamberti, C. (2003). J. Am. Chem. Soc. 125, 14549–14558.Google Scholar
First citationMiller, J. T., Kropf, A. J., Zha, Y., Regalbuto, J. R., Delannoy, L., Louis, C., Bus, E. & van Bokhoven, J. A. (2006). J. Catal. 240, 222–234.Google Scholar
First citationMino, L., Agostini, G., Borfecchia, E., Gianolio, D., Piovano, A., Gallo, E. & Lamberti, C. (2013). J. Phys. D Appl. Phys. 46, 423001.Google Scholar
First citationMitsui, T., Rose, M. K., Fomin, E., Ogletree, D. F. & Salmeron, M. (2003). Nature, 422, 705–707.Google Scholar
First citationMostafa, S., Behafarid, F., Croy, J. R., Ono, L. K., Li, L., Yang, J. C., Frenkel, A. I. & Cuenya, B. R. (2010). J. Am. Chem. Soc. 132, 15714–15719.Google Scholar
First citationMuddada, N. B., Olsbye, U., Fuglerud, T., Vidotto, S., Marsella, A., Bordiga, S., Gianolio, D., Leofanti, G. & Lamberti, C. (2011). J. Catal. 284, 236–246.Google Scholar
First citationNenu, C. N., Groppo, E., Lamberti, C., Beale, A. M., Visser, T., Zecchina, A. & Weckhuysen, B. M. (2007). Angew. Chem. Int. Ed. 46, 1465–1468.Google Scholar
First citationNenu, C. N. & Weckhuysen, B. M. (2005). Chem. Commun., pp. 1865–1867.Google Scholar
First citationNewton, M. A. (2008). Chem. Soc. Rev. 37, 2644–2657.Google Scholar
First citationNewton, M. A., Chapman, K. W., Thompsett, D. & Chupas, P. J. (2012). J. Am. Chem. Soc. 134, 5036–5039.Google Scholar
First citationNorskov, J. K., Studt, F., Abild-Pedersen, F. & Bligaard, T. (2014). Fundamental Concepts in Heterogeneous Catalysis. Hoboken: John Wiley & Sons.Google Scholar
First citationØien, S., Agostini, G., Svelle, S., Borfecchia, E., Lomachenko, K. A., Mino, L., Gallo, E., Bordiga, S., Olsbye, U., Lillerud, K. P. & Lamberti, C. (2015). Chem. Mater. 27, 1042–1056.Google Scholar
First citationPaolucci, C., Parekh, A. A., Khurana, I., Di Iorio, J. R., Li, H., Albarracin Caballero, J. D., Shih, A. J., Anggara, T., Delgass, W. N., Miller, J. T., Ribeiro, F. H., Gounder, R. & Schneider, W. F. (2016). J. Am. Chem. Soc. 138, 6028–6048.Google Scholar
First citationPaolucci, C., Verma, A. A., Bates, S. A., Kispersky, V. F., Miller, J. T., Gounder, R., Delgass, W. N., Ribeiro, F. H. & Schneider, W. F. (2014). Angew. Chem. Int. Ed. 53, 11828–11833.Google Scholar
First citationPellegrini, R., Agostini, G., Groppo, E., Piovano, A., Leofanti, G. & Lamberti, C. (2011). J. Catal. 280, 150–160.Google Scholar
First citationPellegrini, R., Leofanti, G., Agostini, G., Groppo, E., Rivallan, M. & Lamberti, C. (2009). Langmuir, 25, 6476–6485.Google Scholar
First citationPrestipino, C., Bonino, F., Usseglio, S., Damin, A., Tasso, A., Clerici, M. G., Bordiga, S., D'Acapito, F., Zecchina, A. & Lamberti, C. (2004). ChemPhysChem, 5, 1799–1804.Google Scholar
First citationPrestipino, C., Capello, L., D'Acapito, F. & Lamberti, C. (2005). Phys. Chem. Chem. Phys. 7, 1743–1746.Google Scholar
First citationPrins, R. & Koningsberger, D. C. (1988). X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES, edited by D. C. Koningsberger & R. Prins, p. 321. New York: John Wiley & Sons.Google Scholar
First citationRegli, L., Bordiga, S., Busco, C., Prestipino, C., Ugliengo, P., Zecchina, A. & Lamberti, C. (2007). J. Am. Chem. Soc. 129, 12131–12140.Google Scholar
First citationRehr, J. J. & Albers, R. C. (2000). Rev. Mod. Phys. 72, 621–654.Google Scholar
First citationRehr, J. J., Kas, J. J., Vila, F. D., Prange, M. P. & Jorissen, K. (2010). Phys. Chem. Chem. Phys. 12, 5503–5513.Google Scholar
First citationRoa, D. B., Barcelos, I. D., de Siervo, A., Pirota, K. R., Lacerda, R. G. & Magalhães-Paniago, R. (2010). Appl. Phys. Lett. 96, 253114.Google Scholar
First citationRogalev, A., Ollefs, K. & Wilhelm, F. (2016). X-ray Absorption and X-ray Emission Spectroscopy: Theory and Applications, edited by J. A. van Bokhoven & C. Lamberti, pp. 671–694. Chichester: John Wiley & Sons.Google Scholar
First citationRoldan Cuenya, B., Croy, J. R., Mostafa, S., Behafarid, F., Li, L., Zhang, Z. F., Yang, J. C., Wang, Q. & Frenkel, A. I. (2010). J. Am. Chem. Soc. 132, 8747–8756.Google Scholar
First citationSeenivasan, K., Gallo, E., Piovano, A., Vitillo, J. G., Sommazzi, A., Bordiga, S., Lamberti, C., Glatzel, P. & Groppo, E. (2013). Dalton Trans. 42, 12706–12713.Google Scholar
First citationSerp, P. & Figueiredo, L. J. (2009). Carbon Materials for Catalysis. Hoboken: John Wiley & Sons.Google Scholar
First citationShabaev, A., Papaconstantopoulos, D. A., Mehl, M. J. & Bernstein, N. (2010). Phys. Rev. B, 81, 184103.Google Scholar
First citationShegai, T. & Langhammer, C. (2011). Adv. Mater. 23, 4409–4414.Google Scholar
First citationSignorile, M., Damin, A., Bonino, F., Crocellà, V., Lamberti, C. & Bordiga, S. (2016). J. Comput. Chem. 37, 2659–2666.Google Scholar
First citationSinfelt, J. H., Via, G. H. & Lytle, F. W. (1978). J. Chem. Phys. 68, 2009–2010.Google Scholar
First citationSingh, J., Lamberti, C. & van Bokhoven, J. A. (2010). Chem. Soc. Rev. 39, 4754–4766.Google Scholar
First citationSmolentsev, G. & Soldatov, A. (2006). J. Synchrotron Rad. 13, 19–29.Google Scholar
First citationSmolentsev, G. & Soldatov, A. V. (2007). Comput. Mater. Sci. 39, 569–574.Google Scholar
First citationSmolentsev, G. & Soldatov, A. V. (2021). Int. Tables. Crystallogr. I, .Google Scholar
First citationSoldatov, A., Della Longa, S. & Bianconi, A. (1993). Solid State Commun. 85, 863–868.Google Scholar
First citationStaniland, S., Ward, B., Harrison, A., van der Laan, G. & Telling, N. (2007). Proc. Natl Acad. Sci. USA, 104, 19524–19528.Google Scholar
First citationSuleiman, M., Jisrawi, N., Dankert, O., Reetz, M., Bähtz, C., Kirchheim, R. & Pundt, A. (2003). J. Alloys Compd. 356–357, 644–648.Google Scholar
First citationTeschner, D., Borsodi, J., Kis, Z., Szentmiklósi, L., Révay, Z., Knop-Gericke, A., Schlögl, R., Torres, D. & Sautet, P. (2010). J. Phys. Chem. C, 114, 2293–2299.Google Scholar
First citationTeschner, D., Borsodi, J., Wootsch, A., Révay, Z., Hävecker, M., Knop-Gericke, A., Jackson, S. D. & Schlögl, R. (2008). Science, 320, 86–89.Google Scholar
First citationTew, M. W., Janousch, M., Huthwelker, T. & van Bokhoven, J. A. (2011). J. Catal. 283, 45–54.Google Scholar
First citationTew, M. W., Miller, J. T. & van Bokhoven, J. A. (2009). J. Phys. Chem. C, 113, 15140–15147.Google Scholar
First citationTromp, M. (2015). Phil. Trans. R. Soc. A, 373, 20130152.Google Scholar
First citationTurnes Palomino, G., Fisicaro, P., Bordiga, S., Zecchina, A., Giamello, E. & Lamberti, C. (2000). J. Phys. Chem. B, 104, 4064–4073.Google Scholar
First citationTyrsted, C., Borfecchia, E., Berlier, G., Lomachenko, K. A., Lamberti, C., Bordiga, S., Vennestrøm, P. N. R., Janssens, T. V. W., Falsig, H., Beato, P. & Puig-Molina, A. (2016). Catal. Sci. Technol. 6, 8314–8324.Google Scholar
First citationValenzano, L., Civalleri, B., Chavan, S., Bordiga, S., Nilsen, M. H., Jakobsen, S., Lillerud, K. P. & Lamberti, C. (2011). Chem. Mater. 23, 1700–1718.Google Scholar
First citationVia, G. H., Sinfelt, J. H. & Lytle, F. W. (1979). J. Chem. Phys. 71, 690–699.Google Scholar
First citationVogel, W., He, W., Huang, Q.-H., Zou, Z., Zhang, X.-G. & Yang, H. (2010). Int. J. Hydrogen Energy, 35, 8609–8620.Google Scholar
First citationWang, J., Wang, Q., Jiang, X., Liu, Z., Yang, W. & Frenkel, A. I. (2015). J. Phys. Chem. C, 119, 854–861.Google Scholar
First citationWeckhuysen, B. M. & Schoonheydt, R. A. (1999). Catal. Today, 51, 215–221.Google Scholar
First citationWeckhuysen, B. M., Schoonheydt, R. A., Jehng, J. M., Wachs, I. E., Cho, S. J., Ryoo, R., Kijlstra, S. & Poels, E. (1995). J. Chem. Soc. Faraday Trans. 91, 3245–3253.Google Scholar
First citationWeckhuysen, B. M., Wachs, I. E. & Schoonheydt, R. A. (1996). Chem. Rev. 96, 3327–3350.Google Scholar
First citationWitkowska, A., Di Cicco, A. & Principi, E. (2007). Phys. Rev. B, 76, 104110.Google Scholar
First citationWoertink, J. S., Smeets, P. J., Groothaert, M. H., Vance, M. A., Sels, B. F., Schoonheydt, R. A. & Solomon, E. I. (2009). Proc. Natl Acad. Sci. USA, 106, 18908–18913.Google Scholar
First citationZhdanov, V. P. & Kasemo, B. (2008). Chem. Phys. Lett. 460, 158–161.Google Scholar

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