Proton-exchange membrane (PEM) water electrolysis is a critical technology for hydrogen production. The oxygen evolution reaction (OER) kinetics at the anode significantly determines the electrolysis performance, requiring the development of active and stable catalysts for high conversion rates. Despite extensive experimental studies, it is still difficult to fully understand how the catalyst state, i.e., the structure, morphology, and oxidation state, which vary by synthesis conditions, affect the OER kinetics and free energies. In this study, we delve into the influence of catalyst calcination on the catalyst state and its relationship with the OER kinetics by a combination of experimental analysis and microkinetic modeling. Our results show that the increasing degree of crystallinity upon calcination and, thus, the reduced number of active sites are the main reason for the decreasing performance of Ir-oxide nanoparticles. Additionally, the water adsorption step becomes thermodynamically more favorable, CUS-mediated PCET and O2 release are modestly hindered, and the bridge-site redox contribution declines with increasing crystallinity. These subtle, systematic shifts help explain the nonlinear structure-activity relationships reported in the literature. This understanding of the interplay between catalyst synthesis conditions and the OER performance facilitates the tailored design and optimization of high-performance catalysts for more efficient electrocatalytic water electrolysis.

Many oxidation catalysts rely on ceria nanomaterials as efficient supports for platinum group metal nanoparticles, where morphology affects the catalytic performance. Rational exploitation of the chemical parameter space enables the tuning and control of these morphological features. This study presents a systematic investigation of conventional and microwave‐assisted hydrothermal syntheses of ceria nanorods and nanocubes. The specific role of Ce(III) counter‐ions in nanorod formation was systematically investigated, identifying chloride ions as essential for anisotropic growth. Microwave heating proved superior to conventional methods, yielding high‐aspect‐ratio rods in 10 min while preventing the surface area loss associated with thermal coarsening. For the nanocubes, an acid‐buffered route was developed to narrow the polydispersity. The reactor filling ratio in the microwave syntheses emerged as a key parameter to modulate solvent evaporation, allowing tuning of the nanocube size (from 14 to 26 nm) and truncation degree. Structural and morphological characterisation by X‐ray diffraction (XRD), transmission electron microscopy (TEM) and Brunauer–Emmett–Teller specific surface area analysis (BET) confirmed the formation of highly crystalline and porous nanomaterials, providing a rational framework for the synthesis of ceria nanostructures with controlled morphology via microwave‐assisted heating. The effect of autogenous pressure was rationalised by numerical calculations using a Python script developed by the authors.

The correlation of space- and time-resolved measurements of catalytic activity with catalyst state is an invaluable tool to advance the understanding and development of complex catalytic systems under conditions relevant to technical applications. Such an approach is employed here to investigate the catalytic deactivation in the reduction of NO by CO over Pt/Al2O3 on freshly reduced catalysts in a channel reactor and at concentrations typical for emission control. Planar laser-induced fluorescence (PLIF) is used to visualize the 2D concentration profiles and to derive space- and time-resolved NO conversion rates for different CO/NO ratios, temperatures and mass flow rates. The changes in catalytic activity are correlated with temporal and spatial changes in oxidation state determined by operando X-ray absorption spectroscopy (XAS) under the same conditions. The time scales of the changes in catalytic activity depend not only on the stoichiometry and temperature, but also on the position along the catalyst channel and differ significantly from the temporal changes of the oxidation state. The different time scales are discussed in the context of the known CO poisoning as well as the formation and storage of isocyanate on the support. Isocyanate formation temporarily counteracting CO poisoning could explain the differences in the observed time scales under different reaction conditions and at different locations on the catalyst.

A hydrothermally derived carbon support was synthesized from the sustainable feedstock chitosan, with optional subsequent pyrolysis at 600 degrees C and 1000 degrees C, to explore its potential as a catalyst support material for ruthenium (Ru). The catalysts were prepared through wet impregnation using Ru nitrosyl nitrate as the precursor. Their catalytic performances in ammonia decomposition were investigated under conditions of 5 % NH3 at 1 bar, within a temperature range of 300 degrees C to 600 degrees C, and a weight hourly space velocity of 15.000 mlN gcat analytical techniques employed in this study included elemental analysis, thermogravimetric analysis (TGA), gas adsorption measurements, Raman spectroscopy, X-ray diffraction (XRD), flame atomic absorption spectroscopy (F-AAS), scanning transmission electron microscopy (STEM), hydrogen-based temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS). They unraveled that non-pyrolyzed supports showed a strong tendency for Ru agglomeration, whereas pyrolyzed supports exhibited improved metal distribution, which correlated with enhanced catalytic activity exceeding 50 % ammonia conversion at 450 degrees C. The surface chemistry of the carbon support was modified by varying the pyrolysis temperature, which affected the concentrations and types of oxygen and nitrogen surface groups. These changes altered the interaction between Ru and these surface groups. During the decomposition of the Ru precursor and the reduction of Ru oxides, the partial breakdown of oxygen and nitrogen surface groups led to surface reconstructions of the Ru nanoparticles, thereby affecting their crystallinity. This phenomenon was also observed during the catalytic testing, which was more pronounced on the HC-600 support. Modifying the surface chemistry of hydrochar via pyrolysis affects Ru distribution, reducibility, and crystallinity, thereby enhancing the NH3 decomposition performance.

Theoretical modeling of X-ray absorption near-edge structure (XANES) spectra is often performed to interpret experimental spectra and to relate spectral features to the atomic and electronic structure of materials. The sensitivity of the XANES spectra to coordination environment, oxidation state and structural disorder is explored using FEFF and FDMNES simulation codes. With a suitable configuration of these instruments, one can also explore the quantitative characteristics of the studied materials, such as lattice constant, nanoparticle geometry, coordination numbers and longer-range ordering. However, fine-tuning the input parameters is often resource-consuming and non-intuitive. We test the application of the Bayesian optimization (BO) algorithm in finding the most optimal simulation parameters for the theoretical XANES spectra and compare the results using different spectrum similarity metrics. The BO method outperforms the random search technique by a factor of three in speed and shows that the correlation-based metrics provide better shape-level agreement than those defined as normalized distance.

Metal fuels such as iron are promising carbon-free energy carriers for a sustainable energy system, where energy release occurs via combustion in metal flames. Apart from the design of the appropriate burners, another challenge is to control the oxidation process and gain direct insight into the kinetics and the mechanism of metal oxidation in flames. Optimizing the combustion process to release the stored energy is crucial, but in situ analysis of metal flames remains a real chemical engineering and physical chemistry challenge. Here, we demonstrate an in situ approach with synchrotron X-ray radiation allowing identification and quantification of iron (oxide) phases during the combustion process that is adaptable for various combustion modes and conditions. Using quick scanning X-ray absorption spectroscopy, we were able to track the oxidation state of iron and thus the structure along the visual flame cross-section. Strong gradients in oxidation state and phase composition across and beyond the flame front were found. The flame front determined by visualization with an optical camera corresponds to the formation of FeO, while the main combustion product Fe 3 O 4 was mainly formed outside the visible flame zone.

GaPt nanocomposites are colloidally synthesized and purified using three washing procedures that directly affect the catalytic stability in propane dehydrogenation of supported catalytically active liquid metal solutions (SCALMS) fabricated thereof.

Cu/MgO/ZnO catalysts were prepared via single- and double-flame spray pyrolysis for CO 2 hydrogenation to methanol. Tuning the flame configuration alters metal–support interactions, affecting catalytic activity and stability.

Support engineering of oxide materials offers a powerful yet underexplored strategy for controlling the performance of supported metal catalysts. Here, phosphorus functionalization (1 – 5 wt% P) of alumina and silica provides a simple and modular strategy to tune the Pd-catalyzed selective hydrogenation of α,β-unsaturated aldehydes. Phosphorus incorporation enhances catalytic activity on both supports while inducing pronounced support- and substrate-dependent selectivity patterns. Using cinnamaldehyde and citral as model substrates, a design-of-experiments approach reveals that catalytic performance is governed primarily by support chemistry and reaction conditions, whereas phosphorus loading exhibits threshold-type behavior, consistent with discrete restructuring of Pd surface ensembles rather than gradual electronic perturbation. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and transmission electron microscopy (TEM) demonstrate that phosphorus suppresses extended Pd ensembles and stabilizes highly dispersed, low-coordinated Pd sites. X-ray photoelectron spectroscopy (XPS) further reveals a shift of the Pd toward more positively charged surface species upon phosphorus modification. These findings establish phosphorus-based support engineering as a simple, transferable strategy to control metal dispersion, active-site accessibility, and catalytic selectivity without the need for complex alloying or bulk phase formation.

Bimetallic PtNi/CeO2 catalysts were successfully synthesized via a mechanochemical approach, specifically ball milling, and evaluated for methane steam reforming (MSR). A fractional factorial design of experiments was employed to systematically explore the effects of key milling parametersmilling frequency, milling time, and ball-to-powder ratioon the catalysts' structural properties and catalytic performance. The catalysts were characterized by X-ray diffraction, H2 temperature-programmed reduction, transmission electron microscopy, and Raman spectroscopy. Catalytic activity tests were performed in a plug flow reactor under a high gas hourly space velocity (200,000 mL gcat -1 h-1) at a steam-to-carbon ratio of 2 between 700 and 950 °C. The mechanochemically synthesized catalysts were benchmarked against those prepared via incipient wetness impregnation. The most active milled catalysts achieved a methane conversion rate of ca. 22 mol CH4 gNi -1 h-1 at 700 °C (83.5% methane conversion for a PtNi/CeO2 mechanochemically synthesized), outperforming the impregnated counterpart (64% methane conversion under the same reaction conditions). Notably, increasing the milling intensity resulted in enhanced catalytic activity, with milling frequency emerging as the most influential factorcorrelating with the formation of smaller NiO particles. To elucidate the role of Pt addition, in situ X-ray absorption near-edge structure (XANES) and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) measurements were conducted on the most active milled catalysts under MSR conditions. NAP-XPS revealed surface segregation of Pt during MSR, alongside an inhibitory effect on solid carbon deposition, suggesting the potential for a coke-resistant catalyst. These findings highlight the power of mechanochemical synthesis in tuning catalyst properties, offering a scalable and efficient route to high-performance catalysts for methane reforming and hydrogen production.

Reduction of micron-sized Fe2O3 particles with hydrogen was investigated in a fluidized bed reactor (FBR), designed and developed as part of this study and equipped with X-ray transparent glassy carbon windows. Simultaneous in situ X-ray absorption spectroscopy (XAS) at a synchrotron source and quadrupole mass spectrometry (QMS) enabled correlated real-time monitoring of solid-phase transformations and gas-phase composition during isothermal reduction at 773 K in a 1:1 H2/N2 atmosphere. Time-resolved XAS revealed a sequential transformation from Fe2O3 to Fe3O4 and metallic Fe, while QMS detected a corresponding increase and subsequent decay of H2O. A systematic temporal offset between solid-and gas-phase signals indicates that the diagnostics probe slightly different particle ensembles: XAS captures particles present in the freeboard, whereas QMS reflects the integrated H2O formation from the entire reacting bed. Temporary trapping of H2O within the evolving pore network, previously reported for reducing iron oxide particles, may further contribute to the delayed gas-phase response. No evidence for a persistent FeO phase was observed under the investigated conditions. The reduction proceeds rapidly during the initial stage and subsequently slows down, likely associated with the formation of passivating Fe/Fe3O4 layers that limit hydrogen access and hinder the transport of water from the particle core. The present work establishes an experimental framework for synchrotron operando studies of iron oxide reduction in fluidized bed reactors, enabling correlated XAS-QMS measurements and providing the basis for upcoming systematic investigations of particle properties, morphologies and operating parameters. Novelty and significance statement: Braun et al. [1] investigated quasi-single iron oxide reduction in inert-particle-diluted fixed bed reactor cells under low hydrogen concentrations and varying temperatures. While providing kinetic insights, these experiments were limited to the solid phase and did not capture the multi-particle interactions, improved temperature control, or enhanced mass transport characteristic of fluidized bed reactors (FBRs). This work presents real-time tracking of iron oxide reduction with hydrogen in an X-ray absorption spectroscopy (XAS)-compatible FBR designed in this study and operated under practically relevant isothermal conditions. The combination of in situ XAS and quadrupole mass spectrometry (QMS) enables, for the first time, simultaneous monitoring of solid-and gas-phase transformations and reveals their correlated evolution during reduction. The XAS-QMS approach addresses the scarcity of operando studies in FBRs and is transferable to different reaction conditions, particle properties, and iron ores, extending its relevance from energy storage to steelmaking, materials, and catalysis.

The transformation of CO(2 )and green hydrogen into methanol presents a sustainable route for chemical and fuel production. Conventional methanol synthesis catalysts, such as Cu/ZnO/Al2O(3), employ Al2O(3) as a structural promoter, while Ga(2)O(3 )has recently emerged as a promising alternative. This study compares Cu-based catalysts supported on Al2O(3 )(CA) and Ga2O(3 )(CG), prepared via coprecipitation of layered double hydroxide precursors with identical molar Cu:M (M = Al or Ga) ratio of 70:30. Using in situ and operando X-ray absorption spectroscopy and X-ray powder diffraction, we investigate the structural and redox dynamics of Ga during activation and CO2 hydrogenation. Gallium from its precursor state undergoes several phase transitions. At elevated temperatures, Ga exhibits redox activity, transitioning from Ga(3+ )to metallic Ga0 and forming CuxGay alloys at 480 degrees C, followed by de-alloying and re-oxidation at even higher temperatures. Our results suggest that the beneficial role of Ga reported in literature arises from metal-oxide interfacial effects rather than bulk alloying. Excess Ga2O(3) leads to low conversion levels and pronounced deactivation compared to the Al2O3-supported Cu catalyst and thus should be prevented. These findings highlight the importance of controlling promoter loading and dynamic behavior in catalyst design to optimize activity, stability, and selectivity for CO2-to-methanol conversion.

Iron is a promising energy carrier for sustainable energy storage via hydrogen-based reduction of iron oxides and energy release through oxidation with air. While it is well known that the cycling of iron powders affect their morphology and reactivity, the associated structural changes and the reaction mechanism remain poorly understood. Here, we investigate the solid-state cycling of iron oxide powder using complementary in situ techniques: synchrotron based Mössbauer spectroscopy, quick scanning X-ray absorption spectroscopy, and time-resolved synchrotron X-ray diffraction. With a time resolution better than 1 min, compositional changes during three consecutive oxidation–reduction cycles were monitored both with respect to changes in the oxidation state of iron and the transformation of the corresponding crystalline phases. During isothermal cycling at 560 °C, the rate of iron oxidation increased with each cycle number, whereas the corresponding reduction rates to metallic iron decreased. The oxidation proceeded initially via Fe → Fe3O4 → Fe2O3 and then involving wustite as reaction intermediate Fe → FeO → Fe3O4 → Fe2O3. Reduction occurred exclusively through Fe2O3 → Fe3O4 → FeO → Fe, though the observed reaction behavior changed in each cycle between initial and advanced reduction phase.

For increasing the noble metal efficiency and developing more robust emission control catalysts, an accurate understanding of their structural dynamics during operation is essential. This study systematically investigates the manifold interactions of Rh with various catalyst components in alumina/ceria-zirconia based washcoats by advanced in situ/operando characterization. Pronounced structural changes, which drastically affect the catalyst performance, were observed during short fuel-cut steps. Operando X-ray absorption spectroscopy and electron microscopy investigations revealed the formation of mobile metallic Rh particles under stoichiometric/rich reaction conditions. In contrast, Rh oxidation, volatilization and diffusion into the alumina surface/subsurface occurred above 950 degrees C during O2-rich steps. While the first phenomenon contributed to the redistribution of Rh on both supports, the partial relocation of Rh in low-loaded washcoats under lean reaction conditions was exclusively observed to occur from CeO2-ZrO2 to gamma-Al2O3. Overall, the intimate interaction between the washcoat components prevented the loss of the noble metal due to volatilization.

Pt through the gas phase in the form of volatile PtO2 has become a topic of interest within recent years due to its application in the production of single atom catalysts. It is furthermore important for oxidation reactions that take place at high temperatures, e.g., CH4 oxidation in order to track noble metal loss. Here, platinum migration is observed on the nanometer scale for mixed Pt/Al2O3 and CeO2 nanoparticles as grinded powders. Furthermore, Pt migration within a reactor in a dual bed of Pt/Al2O3 followed by a bed of sieved CeO2 particles is tracked in situ on the millimeter scale via time and spatially resolved X-ray absorption spectroscopy. It is observed that gaseous PtO2 is first captured at the beginning of the CeO2 bed. When the beginning of the bed appears saturated, PtO2 adsorbs further downstream. Such adsorption behavior has to our knowledge not yet been reported in the literature since it requires time and spatially resolved in situ tracking. Furthermore, preferential adsorption sites of Pt on CeO2 were identified using experimental extended X-ray absorption fine structure data and fitting based on models from Density Functional Theory calculations. They point to geometries as, for example, found in 4-fold hollow sites on CeO2 (110) with additional ligands for stabilization.

Power-to-liquid processes are considered a crucial route to substitute petroleum-based processes for the production of valuable chemicals and liquid fuels, opening the possibility for the production of sustainable aviation fuels. Two selected catalysts, 20wt.%Co/Al2O3 (CA) and 20wt.%Co/TiO2-SiO2 (CTS), were tested in the Fischer-Tropsch synthesis (FTS) reaction under industrially relevant conditions. The CTS catalyst showed 10% higher activity, improved C5+ selectivity, and lower methane formation than the CA catalyst. The components of these complex catalysts were characterized, and the structural evolution of cobalt during activation and reaction was monitored by various in-situ and operando methods. H2 temperature-programmed reduction, X-ray diffraction, magnetometry, and X-ray absorption spectroscopy revealed a decrease in the temperature of stepwise cobalt reduction in CTS catalysts compared to CA. The reduction behavior of Co during activation was dependent on the support nature and strength of the metal-support interaction. The formation of composites with the support was proposed based on the results obtained by bulk probing XAS and more surface-sensitive NEXAFS methods. XRD, magnetometry, and XAS revealed that the size of the activated Co particles was dependent on the support chosen. The DFT modeling results confirmed the higher affinity of cobalt for titania, which might explain the smaller Co crystallite size and higher stability observed for the CTS catalyst. The difference in activity between the two commercially relevant cobalt-based catalysts on different supports could be linked to the difference in pore sizes and the strength of the metal-support interaction.

Electron transfer-oxygen transfer reactions of vanadium polyoxomolybdates have been investigated in the homogeneous catalytic oxidation of xanthene and biomass-derived compounds. At room temperature, we observed the formation of a stable adduct between the polyoxometalate and xanthene following the initial electron transfer in acetonitrile. The high energy resolved fluorescence detected X-ray absorption near edge structure spectroscopic investigations revealed that vanadium occupies a distorted octahedral position within a defect site in the polyoxometalate framework. In situ X-ray emission spectroscopy further revealed that, at first, vanadium is reduced from VV to VIV and there are changes in the coordination around the vanadium during the electron transfer with significant changes in the valence to core Kβ″ and Kβ2,5 lines. This fundamental investigation of molecular catalysis tackles some of the key questions regarding electron transfer-oxygen transfer reactions using state-of-the-art spectroscopic techniques.

In light of the increasing relevance of alternative energy carriers, the established domain of emission control is confronted with novel challenges. Among these carriers, ammonia (NH3) has emerged as particularly promising due to its high energy density and carbon-free nature. This study focuses on the selective catalytic oxidation (SCO) of NH3 to elemental nitrogen (N2) under lean conditions. While conventional platinum-based catalysts exhibit high activity, they suffer from poor N2 selectivity and significant formation of nitrous oxide (N2O), a potent greenhouse gas. As an alternative, palladium-based catalysts are investigated due to their inherently higher selectivity toward N2. Although palladium exhibits limited activity under static feed conditions, its performance is markedly enhanced under forced dynamic operation (FDO), wherein short, intermittent reducing pulses are introduced through oxygen cut-off. This operational mode not only boosts catalytic activity beyond that of platinum but also preserves palladium's superior selectivity. Operando time-resolved X-ray absorption spectroscopy (XAS) and ex situ Raman spectroscopic analysis of spent catalyst samples indicate that FDO promotes the formation of metallic palladium, which exhibits higher activity compared to palladium oxide that is the predominant phase under lean conditions. Operando XAS uncovers that a finite fraction of metallic palladium is retained at the catalyst inlet, enhancing low-temperature activity and likely contributing to improved long-term stability as long as 300 degrees C is not exceeded for extended periods. These findings correspond directly to axially resolved concentration profiles of gas phase species, which reveal the development of a highly active front zone during FDO, where the majority of NH3 conversion occurs selectively to N2. This phenomenon is observed under both dry and humid conditions, underscoring the robustness of the approach. The results suggest that FDO enables the use of significantly shorter monolithic converters for selective NH3 oxidation, i.e., a reduction of length by up to 50% in realistic humid conditions, thereby offering substantial reductions in both reactor volume and noble metal demand for practical applications.

Ni-based catalysts for CH4 steam reforming require fine-tuning to withstand deactivation under dynamic operation conditions relevant to emerging H2-driven technologies. This study investigates the impact of the Mg:Al ratio and Pt presence in catalyst composition on the activity and stability of industrially relevant Ni-based monoand bimetallic catalysts during simulated daily start-up and shut-down cycles in various gas atmospheres. The evolution of the catalyst structure during extensive testing procedures was investigated in detail by complementary electron microscopy, in situ/operando XAS and XRD. The results obtained revealed that catalyst deactivation is promoted at high Mg:Al ratios and especially affects the monometallic catalysts. By converting CH4 at lower temperatures, Pt regulates the extent of Ni oxidation and its incorporation into MgO lattice. Further prevention of catalyst deactivation was achieved by optimizing the reactor shut-down procedure to minimize the simultaneous exposure to high temperatures and H2O vapors. By flushing the reactor with N-2 only around the reaction extinction temperature, a fraction of Ni species is maintained in metallic state, which is beneficial for the long-term activity and reaction operation economy.