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Abstract Restructuring of metal components on bimetallic nanoparticle surfaces in response to the changes in reactive environment is a ubiquitous phenomenon whose potential for the design of tunable catalysts is underexplored. The main challenge is the lack of knowledge of the structure, composition, and evolution of species on the nanoparticle surfaces during reaction. We apply a modulation excitation approach to the X-ray absorption spectroscopy of the 30 atomic % Pd in Au supported nanocatalysts via the gas (H2 and O2) concentration modulation. For interpreting restructuring kinetics, we correlate the phase-sensitive detection with the time-domain analysis aided by a denoising algorithm. Here we show that the surface and near-surface species such as Pd oxides and atomically dispersed Pd restructured periodically, featuring different time delays. We propose a model that Pd oxide formation is preceded by the build-up of Pd regions caused by oxygen-driven segregation of Pd atoms towards the surface. During the H2 pulse, rapid reduction and dissolution of Pd follows an induction period which we attribute to H2 dissociation. Periodic perturbations of nanocatalysts by gases can, therefore, enable variations in the stoichiometry of the surface and near-surface oxides and dynamically tune the degree of oxidation/reduction of metals at/near the catalyst surface.

Abstract Evolution of the Pd active centers in size and spatial distribution leads to an irreversible deactivation in many high-temperature catalytic processes. This research demonstrates the use of a defective alumina (Al2O3-x) as catalyst support to anchor Pd atoms and suppress the growth of Pd clusters in catalytic methane oxidation. A combination of operando spectroscopy and density functional theory (DFT) calculations provide insights into the evolution of Pd species and reveals distinct catalytic methane oxidation mechanisms on Pd single atoms, clusters, and nanoparticles (NPs). Among these Pd species, the cluster active centers are found to be the most favorable participants in methane oxidation due to their high dispersion, high content of Pd2+ oxidation state, and resistance to deactivation by carbonates, bicarbonates, and water. The Pd/Al2O3-x catalyst shows increased stability with respect to a Pd/Al2O3 counterpart during simulated aging in alternating reducing and oxidizing conditions due to stronger interactions with the support. This study demonstrates that defect engineering of non-reducible supports can constrain the evolution of active centers, which holds promising potential for widespread utilization across diverse industrial catalytic processes, including various hydrogenation and oxidation reactions.

Understanding structure–performance relationships are essential for the rational design of new functional materials or in the further optimization of (catalytic) processes. Due to the high penetration depth of the radiation used, synchrotron-based hard X-ray techniques (with energy > 4.5 keV) allow the study of materials under realistic conditions (in situ and operando) and thus play an important role in uncovering structure-performance relationships. X-ray absorption and emission spectroscopies (XAS and XES) give insight into the electronic structure (oxidation state, spin state) and local geometric structure (type and number of nearest neighbor atoms, bond distances, disorder) up to ~5 Å around the element of interest. In this mini review, we will give an overview of the in situ and operando capabilities of the SuperXAS beamline, a facility for hard X-ray spectroscopy, through recent examples from studies of heterogeneous catalysts, electrochemical systems, and photoinduced processes. The possibilities for time-resolved experiments in the time range from ns to seconds and longer are illustrated. The extension of X-ray spectroscopy at the new Debye beamline combined with operando X-ray scattering and diffraction and further developments of time-resolved XES at SuperXAS will open new possibilities after the Swiss Light Source upgrade mid 2025.

Structure insensitivity in catalysis has been empirically observed, but no satisfactory theoretical explanation could be given. By studying different nanoparticle sizes under dynamic catalytic conditions reaction-dependent particle size dependent restructuring was linked to the aforementioned.

It is commonly accepted that electrolyte alkali metal cations modify the catalytic activity for oxygen evolution reaction. Here the authors challenge this assumption, showing that the activity is actually affected by a change in the electrolyte pH rather than a specific alkali cation.

While solar-driven water splitting may afford a renewable means to harvest energy, it is essential to understand how photocatalysts transform during catalysis. Here, authors study LaTiOxNy films by surface-sensitive techniques before and after photoelectrochemical water splitting.

Unlike the homogeneous Wacker process, understanding of the mechanism of the heterogeneous system remains superficial. Here, the authors investigate the mechanism of heterogeneous Wacker oxidation over Pd-Cu/zeolite Y through the synergistic combination of kinetic, spectroscopic and chemometric studies.

Two new operation modes, “pink beam” excitation, which has the energy bandwidth defined by X-ray mirrors and filters, and “broadband beam”, where the energy bandwidth is defined by the reflection from a multilayer monochromator, were implemented at the SuperXAS beamline of the Swiss Light Source (SLS). These setups allow measuring non-resonance X-ray emission spectra (XES) with 2–3 orders of magnitude higher incident flux than non-resonant XES measurements with a monochromatic incident beam. For the broadband beam mode, a Mo/Si multilayer structure was designed, with which the energy can be tuned in the 5–17 keV range. The multilayer demonstrates a relatively large energy bandwidth of 4 ± 0.2% through the whole energy range and a reflectivity of 23–60%, which increases with energy. We show that by using pink beam mode one can investigate the electronic structure of photocatalytic intermediates through time-resolved core-to-core XES experiments of diluted samples with a concentration of the element of interest of ~1 mM. Broadband beam mode is optimal for valence-to-core XES experiments and allow to avoid the excitation of additional KLβ satellites that can complicate the interpretation of spectra.

Palladium supported on zeolite is a highly active catalyst for complete methane oxidation, but its stability needs to be improved. Here, the authors design a highly active catalyst resistant to steam-induced sintering under reaction conditions by alleviating the high mobility of palladium nanoparticles and zeolite degradation.

Nanomaterials provide a route to efficient solid-state conversion between thermal and electrical energy. Here, the authors demonstrate that a combination of metal and semiconductor colloidal nanocrystals can produce thermoelectric nanocomposites with high performance.

The perovskite-type oxide Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) is known as a highly active and stable oxygen evolution reaction (OER) electrocatalyst composited out of non-noble metals. The possibility of using the scalable flame spray synthesis (FSS) technique for the production of BSCF nanoparticles intensified the interest in this material for a future application in an alkaline water electrolyzer. A possible scale-up would require the optimization of the synthesis parameters to maximize the production rate. To further understand the influence of the synthesis parameters of the tunable FSS on the OER activity of BSCF, a systematic study was carried out by producing BSCF with different total metal concentrations (CTM), flow rates of the precursor solution (FRPS) and of the dispersion gas (FRDG). This study reveals that all three parameters have a direct impact on the OER activity of BSCF—measured in a rotating disc electrode (RDE) setup—due to the controllability of the initial Co and Fe oxidation state—indicated by X-ray absorption spectroscopy (XAS) measurements—and with that also of the oxygen vacancy concentration in the as-synthesized BSCF. This controllability enables the optimization of the OER activity of BSCF and emphasizes the importance of having Co in a lower initial oxidation state for reaching a high electrocatalytic performance.

In this work, the synthesis of a stable supported Pd catalyst by aggregation of colloidal particles (CPs) of polyacrylonitrile (PAN) containing the noble metal is discussed. The metal is initially incorporated during the polymerization reaction (0.5 wt. %) and results to be well embedded inside the polymeric structure and anchored to the nitrogen atom of acrylonitrile. Moreover, aiming to provide free access towards the catalyst active sites, a KOH-activated heat treatment was performed on the PAN CP aggregates, thus creating pore sizes between 0.7 and 5 nm with surface area SBET = 1150 m2/g. After the harsh heat treatment, the amount of Pd remaining in the pyrolyzed polymer is 2.8 wt. %, as measured by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), which then corresponds to a total weight loss of 83% (assuming no loss of Pd). This result indicates a remarkable stability of Pd inside the polymer structure and that minimal Pd leaching can be expected during the catalyst use. X-ray absorption spectroscopic studies of pyrolyzed Pd-containing PAN and following extended X-ray absorption fine structure fitting showed no sign of bulk Pd(0), while a dominant proportion of Pd is present as dispersed Pd carbides. These results are consistent with the data obtained using X-ray absorption near edge structure. Finally, the catalyst was tested on a Suzuki coupling in the liquid phase and showed good activity, hence proving the accessibility of the active sites to reactants.

We report on an element-selective study of the fate of charge carriers in photoexcited inorganic CsPbBr3 and CsPb(ClBr)3 perovskite nanocrystals in toluene solutions using time-resolved X-ray absorption spectroscopy with 80 ps time resolution. Probing the Br K-edge, the Pb L3-edge, and the Cs L2-edge, we find that holes in the valence band are localized at Br atoms, forming small polarons, while electrons appear as delocalized in the conduction band. No signature of either electronic or structural changes is observed at the Cs L2-edge. The results at the Br and Pb edges suggest the existence of a weakly localized exciton, while the absence of signatures at the Cs edge indicates that the Cs+ cation plays no role in the charge transport, at least beyond 80 ps. This first, time-resolved element-specific study of perovskites helps understand the rather modest charge carrier mobilities in these materials.

Perovskite oxides have been gaining attention for its capability to be designed as an ideal electrocatalyst for oxygen evolution reaction (OER). Among promising candidates, the layered double perovskite—PrBaCo2O6-δ (PBC)—has been identified as the most active perovskite electrocatalyst for OER in alkaline media. For a single transition metal oxide catalyst, the addition of Fe enhances its electrocatalytic performance towards OER. To understand the role of Fe, herein, Fe is incorporated in PBC in different ratios, which yielded PrBaCo2(1-x)Fe2xCo6-δ (x = 0, 0.2 and 0.5). Fe-doped PBCF’s demonstrate enhanced OER activities and stabilities. Operando X-ray absorption spectroscopy (XAS) revealed that Co is more stable in a lower oxidation state upon Fe incorporation by establishing charge stability. Hence, the degradation of Co is inhibited such that the perovskite structure is prolonged under the OER conditions, which allows it to serve as a platform for the oxy(hydroxide) layer formation. Overall, our findings underline synergetic effects of incorporating Fe into Co-based layered double perovskite in achieving a higher activity and stability during oxygen evolution reaction.

We report on the application of a short working distance von Hamos geometry spectrometer to measure the inelastic X-ray scattering (IXS) signals from solids and liquids. In contrast to typical IXS instruments where the spectrometer geometry is fixed and the incoming beam energy is scanned, the von Hamos geometry allows measurements to be made using a fixed optical arrangement with no moving parts. Thanks to the shot-to-shot capability of the spectrometer setup, we anticipate its application for the IXS technique at X-ray free electron lasers (XFELs). We discuss the capability of the spectrometer setup for IXS studies in terms of efficiency and required total incident photon flux for a given signal-to-noise ratio. The ultimate energy resolution of the spectrometer, which is a key parameter for IXS studies, was measured to the level of 150 meV at short crystal radius thanks to the application of segmented crystals for X-ray diffraction. The short working distance is a key parameter for spectrometer efficiency that is necessary to measure weak IXS signals.

The methanation of CO2 within the power-to-gas concept was investigated under fluctuating reaction conditions to gather detailed insight into the structural dynamics of the catalyst. A 10 wt % Ni/Al2O3 catalyst with uniform 3.7 nm metal particles and a dispersion of 21% suitable to investigate structural changes also in a surface-sensitive way was prepared and characterized in detail. Operando quick-scanning X-ray absorption spectroscopy (XAS/QEXAFS) studies were performed to analyze the influence of 30 s and 300 s H2 interruptions during the methanation of CO2 in the presence of O2 impurities (technical CO2). These conditions represent the fluctuating supply of H2 from renewable energies for the decentralized methanation. Short-term H2 interruptions led to oxidation of the most reactive low-coordinated metallic Ni sites, which could not be re-reduced fully during the subsequent methanation cycle and accordingly caused deactivation. Detailed evaluation of the extended X-ray absorption fine structure (EXAFS) spectra showed surface oxidation/reduction processes, whereas the core of the Ni particles remained reduced. The 300-s H2 interruptions resulted in bulk oxidation already after the first cycle and a more pronounced deactivation. These results clearly show the importance and opportunities of investigating the structural dynamics of catalysts to identify their mechanism, especially in power-to-chemicals processes using renewable H2.

In this short review, we describe the complementary experimental capabilities for catalysis research at two beam lines available to the Swiss community, SuperXAS at SLS (Swiss Light Source, Villigen) and SNBL (Swiss Norwegian Beam lines) at ESRF (European Synchrotron Radiation Facility, Grenoble). Over the years, these two facilities have been developed to provide powerful techniques for structural studies under in situ and operando conditions. These techniques, X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and X-ray emission spectroscopy (XES) in combination with Raman or infrared spectroscopy provide new avenues for structure–performance studies of catalysts. Several exemplary studies are used to demonstrate the capability of these facilities.

The conversion of methane to value-added liquid chemicals is a promising answer to the imminent demand for fuels and chemical synthesis materials in the advent of a dwindling petroleum supply. Current technology requires high energy input for the synthesis gas production, and is characterized by low overall selectivity, which calls for alternative reaction routes. The limitation to achieve high selectivity is the high C–H bond strength of methane. High-temperature reaction systems favor gas-phase radical reactions and total oxidation. This suggests that the catalysts for methane activation should be active at low temperatures. The enzymatic-inspired metal-exchanged zeolite systems apparently fulfill this need, however, methanol yield is low and a catalytic process cannot yet be established. Homogeneous and heterogeneous catalytic systems have been described which stabilize the intermediate formed after the first C–H activation. The understanding of the reaction mechanism and the determination of the active metal sites are important for formulating strategies for the upgrade of methane conversion catalytic technologies.

Pre-edge spectral features below the main X-ray absorption K-edge of transition metals show a pronounced chemical sensitivity and are promising sources of structural information. Nevertheless, the use of pre-edge analysis in applied research is limited because of the lack of definite theoretical peak-assignments. The aim of this study was to determine the factors affecting the chromium K-pre-edge features in trivalent chromium-bearing oxides and oxyhydroxides. The selected phases varied in the degree of octahedral polymerization and the degree of iron-for-chromium substitution in the crystal structure. We investigated the pre-edge fine structure by means of high-energy-resolution fluorescence detected X-ray absorption spectroscopy and by 1s2p resonant X-ray emission spectroscopy. Multiplet theory and full multiple-scattering calculations were used to analyze the experimental data. We show that the chromium K-pre-edge contains localized and nonlocalized transitions. Contributions arising from nonlocalized metal−metal transitions are sensitive to the nearest metal type and to the linkage mode between neighboring metal octahedra. Analyzing these transitions opens up new opportunities for investigating the local coordination environment of chromium in poorly ordered solids of environmental relevance. [ABSTRACT FROM AUTHOR]

The catalytic partial oxidation of methane to synthesis gas over ruthenium catalysts was investigated by thermogravimetry coupled with infrared spectroscopy (TGA-FTIR) and in situ X-ray absorption spectroscopy (XAS). It was found that the oxidation state of the catalyst influences the product formation. On oxidized ruthenium sites, carbon dioxide was formed. The reduced catalyst yielded carbon monoxide as a product. The influence of the temperature was also investigated. At temperatures below the ignition point of the reaction, the catalyst was in an oxidized state. At temperatures above the ignition point, the catalyst was reduced. This was also confirmed by the in situ XAS spectroscopy. The results indicate that both a direct reaction mechanism as well as a combustion-reforming mechanism can occur. The importance of knowing the oxidation state of the surface is discussed and a method to determine it under reaction conditions is presented. [ABSTRACT FROM AUTHOR]