Palladium hydride is a model system for studying metal-hydrogen interactions. Yet, its bulk electronic structure has proven difficult to directly probe, with most studies to date limited to surface-sensitive photoelectron spectroscopy approaches. This work reports the first in situ ambient-pressure hard X-ray photoelectron spectroscopy (AP-HAXPES) study of hydrogen incorporation in Pd thin films, providing direct access to bulk chemical and electronic information at elevated hydrogen pressures. Structural characterization by in situ X-ray diffraction and neutron reflectometry under comparable conditions establishes a direct correlation between hydrogen loading, lattice expansion, and electronic modifications. Comparison with density functional theory (DFT) reveals how hydrogen stoichiometry and site occupancy govern the density of occupied states near the Fermi level. These results resolve long-standing questions regarding PdH and establish AP-HAXPES as a powerful tool for probing the bulk electronic structure of metal hydrides under realistic conditions.

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 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.

In recent years, the multi-dimensional spectroscopy and inelastic X-ray scattering endstation MUSIX has become an extensively used instrument at the XUV/soft X-ray free-electron laser FLASH and the synchrotron source PETRA III at DESY in Hamburg. It was employed for static and time-resolved spectroscopy on quantum materials as well as for non-linear methods in the XUV/soft X-ray regime. Recently, the instrument was upgraded for more flexibility regarding the orientation of the sample and the spectrometer dispersion direction. These upgrades are described here, showcasing the scientific benefits. MUSIX is now even better suited for novel experimental schemes in the XUV and soft X-ray regime, especially at the upcoming facilities FLASH2020+ and PETRA IV.

The detection of inelastically scattered soft x-rays with high energy resolution usually requires large grating spectrometers. Recently, photoelectron spectrometry for analysis of x-rays (PAX) has been rediscovered for modern spectroscopy experiments at synchrotron light sources. By converting scattered photons to electrons and using an electron energy analyser, the energy resolution for resonant inelastic x-ray scattering (RIXS) becomes decoupled from the x-ray spot size and instrument length. In this work, we develop PAX towards high energy resolution using a modern photoemission spectroscopy setup studying Ba2Cu3O4Cl2 at the Cu L 3-edge. We measure a momentum transfer range of 24% of the first Brillouin zone simultaneously. Our results hint at the observation of a magnon excitation below 100 meV energy transfer and show intensity variations related to the dispersion of dd-excitations. With dedicated setups, PAX can complement the best and largest RIXS instruments, while at the same time opening new opportunities to acquire RIXS at a range of momentum transfers simultaneously and combine it with angle-resolved photoemission spectroscopy in a single instrument.