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 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 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.

Operando probing by x-ray photoelectron spectroscopy (XPS) of certain hydrogenation reactions are often limited by the scattering of photoelectrons in the gas phase. This work describes a method designed to partially circumvent this so called pressure gap. By performing a rapid switch from a high pressure (where acquisition is impossible) to a lower pressure we can for a short while probe a remnant of the high pressure surface as well as the time dynamics during the re-equilibration to the new pressure. This methodology is demonstrated using the CO2 and the CO hydrogenation reaction over Rh(211). In the CO2 hydrogenation reaction, the remnant surface of a 2 bar pressure shows an adsorbate distribution which favors chemisorbed CHx adsorbates over chemisorbed CO. This contrasts against previous static operando spectra acquired at lower pressures. Furthermore, the pressure jumping method yields a faster acquisition and more detailed spectra than static operando measurements above 1 bar. In the CO hydrogenation reaction, we observe that CHx accumulated faster during the 275 mbar low pressure regime, and different hypotheses are presented regarding this observation.

We have used in situ X-ray photoelectron spectroscopy to obtain information about the chemical state of a Fe single-crystal catalyst upon addition of CO₂ in the syngas feed during Fischer–Tropsch synthesis (FTS) between 85 and 550 mbar. We found that at certain temperatures, the ternary mixture of CO, CO₂, and H₂ yields a chemical state which is resemblant of neither the CO hydrogenation nor the CO₂ hydrogenation reaction mixtures in isolation. The addition of CO₂ to a CO + H₂ reaction mixture mostly affects the chemical state at low-temperature FTS conditions (i.e., below 254 °C). In this temperature span, the ternary reaction mixture resulted in a carburized surface, whereas the CO + H₂ reaction led to surface oxidation. We propose a hypothesis, where a carbonate intermediate produced by CO₂ interaction with Fe oxide aids the reduction of the Fe oxide, paving the way for the carburization of the Fe by dissociated CO. Very small differences in the spectra of the CO + H₂ and the CO + CO₂ + H₂ reaction mixtures were observed above 254 °C, suggesting that the CO₂ is a spectator in these conditions. Changing the total pressure of both the CO hydrogenation and ternary reaction mixture causes quantitative changes in the spectra at both low- and high-temperature FTS conditions, the degree of oxidation/carburization was affected in the low-temperature-FTS regime, and the degree of hydrocarbon build-up was affected in the high-temperature-FTS.

Oxide-derived metals are produced by reducing an oxide precursor. These materials, including gold, have shown improved catalytic performance over many native metals. The origin of this improvement for gold is not yet understood. In this study, operando non-resonant sum frequency generation (SFG) and ex situ high-pressure X-ray photoelectron spectroscopy (HP-XPS) have been employed to investigate electrochemically-formed oxide-derived gold (OD-Au) from polycrystalline gold surfaces. A range of different oxidizing conditions were used to form OD-Au in acidic aqueous medium (H₃PO₄, pH = 1). Our electrochemical data after OD-Au is generated suggest that the surface is metallic gold, however SFG signal variations indicate the presence of subsurface gold oxide remnants between the metallic gold surface layer and bulk gold. The HP-XPS results suggest that this subsurface gold oxide could be in the form of Au₂O₃ or Au(OH)₃. Furthermore, the SFG measurements show that with reducing electrochemical treatments the original gold metallic state can be restored, meaning the subsurface gold oxide is released. This work demonstrates that remnants of gold oxide persist beneath the topmost gold layer when the OD-Au is created, potentially facilitating the understanding of the improved catalytic properties of OD-Au.

The Haber-Bosch process produces NH₃ from N₂ and H₂^1,2, typically with Fe and Ru³.  HB has been proposed as the most important scientific invention in the 20th century⁴. The chemical state during reaction has been proposed as oxides⁵, nitrides², metallic, or surface nitride⁶. The proposed rate-limiting step has been the dissociation of  N₂^7–9, reaction of adsorbed nitrogen¹⁰, or desorption of NH₃¹¹. Due to the vacuum requirement for surface-sensitive techniques, studies at reaction conditions are limited to theory computations^12–14. We determined the surface composition, during NH₃ production, at pressures up to 1 bar and temperatures as high as 723 K on flat, stepped Fe, and stepped Ru single crystal surfaces using operando X-ray Photoelectron Spectroscopy¹⁵. We found that all surfaces remain metallic. On Fe only a small amount of adsorbed N remains, yet Ru’s surface is almost adsorbate free. At 523 K, high amines (NH_x) coverages appear on the stepped Fe surface. The results show that the rate-limiting step on Ru is always N₂ dissociation. Still, on Fe the hydrogenation step involving adsorbed N atoms is essential for the total rate, as predicted by theory¹³. If the temperature is lowered on Fe, the rate-limiting steps switch and become surface species’ hydrogenation.

Accurate discrimination between metallic copper (Cu0) and cuprous oxide (Cu2O, Cu+) in electron spectroscopy commonly relies on the Auger electron spectroscopy (AES) Cu L3M4,5M4,5 transitions, as the X-ray photoelectron spectroscopy (XPS) Cu core-levels do not provide large enough binding energy shifts. The kinetic energy of the AES Cu L3M4,5M4,5 electrons is ∼917 eV, which leaves the AES electron susceptible for efficient scattering in the gas phase and attenuation of the signal above near-ambient pressure conditions. To study copper-based materials at higher pressures, e.g., the active state of a catalyst, Auger transitions providing electrons with higher kinetic energies are needed. This study focuses on AES transitions involving the Cu K-shell (1s electrons) that exhibit discernible kinetic energy shifts between the oxidation states of Cu. It is shown that the AES Cu KL2M4,5 transition, with kinetic energy of ∼7936 eV, provides a large enough kinetic energy shift between metallic copper and Cu2O. AES signal is demonstrated in an ambient of 150 mbar CO2.