The intermetallic compound δ-Ni₅Ga₃ has emerged as a promising catalyst for CO₂ hydrogenation to methanol, offering high selectivity at low-pressure operation, and enhanced stability compared to conventional Cu/ZnO catalysts. However, the fundamental understanding of its active sites, reaction mechanisms, and deactivation pathways remains incomplete, hindering its further development. In this study, we utilize well-defined δ-Ni₅Ga₃ thin film model catalysts synthesized via magnetron sputtering to investigate these aspects under realistic reaction conditions. We investigate the evolution of the catalyst with temperature employing in situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) at 300 mbar, microreactor activity measurements, temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. Our experiments show the active catalyst as mostly metallic with only small amounts on oxidized gallium, which gradually reduces and gives way to an increased nickel-concentration at the surface at higher temperatures, accompanied by carbide-growth. We further observe the temperature-evolution of key intermediates, such as carboxyl, formate, and methoxy species. Based on these observations, we discuss distinct pathways for methanol synthesis and CO₂ methanation, with methoxy formation correlating directly with methanol activity, as well as the deactivation mechanism.

In this work, we introduce a modified dip-and-pull electrochemical X-ray photoelectron spectroscopy (ECXPS) approach that offers new mechanistic insight into the alkaline carbon monoxide reduction reaction (CORR) over a Cu(111) single crystal surface. We tackle two major unresolved questions in the CORR mechanism that persist in the literature. Firstly, we address the mechanism for methane formation on Cu(111) and show that the mechanism likely proceeds via atomic carbon, which subsequently couples, leading to the accumulation of amorphous carbon on the surface. Secondly, we provide insight into whether the mechanism for acetate formation occurs entirely on the surface or partially within the solution phase, showing that acetate is present on the surface, indicating a surface-based reaction. These insights into surface-based mechanisms provide a handle for designing future catalysts that can efficiently target the binding of specific intermediates. Furthermore, we expect that our modified approach to dip-and-pull ECXPS – in which we have changed the electrode geometry, the method of introducing the reactant gas and used hard x-rays – will significantly expand the technique's applicability, enabling studies of the CO(2)RR and beyond.

The surface chemistry of the Fischer-Tropsch catalytic reaction over Co has still several unknows. Here, we report an in-situ X-ray photoelectron spectroscopy study of Co(0001) and Co(), and in-situ high energy surface X-ray diffraction of Co(0001), during the Fischer-Tropsch reaction at 0.15 bar - 1 bar and 406 K - 548 K in a H₂/CO gas mixture. We find that these Co surfaces remain metallic under all conditions and that the coverage of chemisorbed species ranges from 0.4–1.7 monolayers depending on pressure and temperature. The adsorbates include CO on-top, C/-C_xH_y and various longer hydrocarbon molecules, indicating a rate-limiting direct CO dissociation pathway and that only hydrocarbon species participate in the chain growth. The accumulation of hydrocarbon species points to the termination step being rate-limiting also. Furthermore, we demonstrate that the intermediate surface species are highly dynamic, appearing and disappearing with time delays after rapid changes in the reactants’ composition.

The fragile-to-strong transition in supercooled water, where the relaxation dynamics shift from non-Arrhenius to Arrhenius behaviour, has been hypothesized to explain its anomalous dynamic properties. However, this transition remains unresolved, as previous ultrafast experimental studies of bulk water dynamics were limited to temperatures far from the proposed transition due to rapid crystallization. Here we use an infrared laser pump and an ultrashort X-ray probe to measure the structural relaxation in micrometre-sized water droplets, evaporatively cooled at timescales ranging from femtoseconds to nanoseconds. Our experimental data show a dynamic crossover at around 233 K. Below this temperature, the relaxation dynamics deviate from simple power-law fits and follow a shallower temperature dependence. Molecular dynamics simulations successfully reproduce our findings.

Iron-doped nickel oxyhydroxides, Nix(Fe1-x)OyHz, are among the most promising oxygen evolution reaction (OER) electrocatalysts in alkaline environments. Although iron (Fe) significantly enhances the catalytic activity, there is still no clear consensus on whether Fe directly participates in the reaction or merely acts as a promoter. To elucidate the Fe’s role, we performed operando X-ray spectroscopy studies supported by DFT on Nix(Fe1-x)OyHz electrocatalysts. We probed the reversible changes in the structure and electronic character of Nix(Fe1-x)OyHz as the electrode potential is cycled between the resting (here at 1.10 VRHE) and operational states (1.66 VRHE). DFT calculations and XAS simulations on a library of Fe structures in various NiOyHz environments are in favor of a distorted local octahedral Fe(III)O3(OH)3 configuration at the resting state with the NiOyHz scaffold going from α-Ni(OH)2 to γ-NiOOH as the potential is increased. Under catalytic conditions, EXAFS and HERFD spectra reveal changes in p-d mixing (covalency) relative to the resting state between O/OH ligands and Fe leading to a shift from octahedral to square pyramidal coordination at the Fe site. XES measurements and theoretical simulations further support that the Fe equilibrium structure remains in a formal Fe(III) state under both resting and operational conditions. These spectral changes are attributed to potential dependent structural rearrangements around Fe. The results suggest that ligand dissociation leads to the C4v symmetry as the most stable intermediate of the Fe during OER. This implies that Fe has a weakly coordinated or easily dissociable ligand that could serve to coordinate the O-O bond formation and, tentatively, play an active role in the Nix(Fe1-x)OyHz electrocatalyst.

The sustainability transition of the chemical industry hinges on the educated design of catalysts for reactions such as the CO hydrogenation, optimizing the materials for selectivity toward valuable products. So far, theoretical models have been used to predict reaction selectivity from the competition of elementary surface processes. Here, we provide an in situ experimental view of surface adsorbates during CO hydrogenation. We compare X-ray photoelectron spectra acquired at reaction conditions (200–325 °C, 150 mbar) over single crystals of Fe, Rh, Ni, Co, and Cu and infer which elementary steps decide the product distribution. We find that the chemisorption energies of C and O, as often used descriptors for catalytic activity, qualitatively predict the rate-limiting steps. They fail, although when reaction-induced carburization occurs on Ni and Fe, steering the selectivity toward methanation on Ni and hydrocarbon chain growth on Fe. For the noncarburized Co and Rh we show how the adsorbate distribution and the oxygen chemisorption energy allow for oxygenate production on Rh, but hydrocarbon chain growth on stepped Co. Ultimately, we show how in situ experiments provide a chemical and mechanistic understanding of CO hydrogenation selectivity, useful to tailor catalysts for a sustainable production of high-value chemicals.